%n. 3, 1 4 $2 EX*LIBRIS Dr. W.W. Lindahl 23 o 8 Bigelow Avenue North Seattle, Washington 98109 , . . “ --'•V- : ' u / y\ 'Ltf/Hj 1 i ( ^ ° Q PRINCIPLES OF PHYSIOLOGY, GENERAL AND COMPARATIVE. WITH THREE HUNDRED AND TWENTY-ONE WOOD-ENGRAVINGS. BY WILLIAM B. CARPENTER, M.D, F.R.S., F.G.S., EXAMINER IN PHYSIOLOGY AND COMPARATIVE ANATOMY IN THE UNIVERSITY OF LONDON ; PROFESSOR OF MEDICAL JURISPRUDENCE IN UNIVERSITY COLLEGE, ETC. ETC. (Station. LONDON : JOHN CHURCHILL, PRINCES STREET, SOHO. MDCCCLI. Oa Sf- pe/ff&r TO SIR JOHN F. W. HERSCHEL, BART., K.n. F.E.S. L. AND E. ETC., THIS VOLUME IS MOST RESPECTFULLY DEDICATED, AS A TRIBUTE DUE ALIKE TO HIS HIGH SCIENTIFIC ATTAINMENTS, AND MORAL WORTH, AND AS AN EXPRESSION OF GRATITUDE FOR THE BENEFIT DERIVED FROM HIS “DISCOURSE ON THE STUDY OF NATURAL PHILOSOPHY,” BY THE AUTHOR. Digitized by the Internet Archive in 2016 https://archive.org/details/principlesofphys01carp PREFACE. “ SCIENCE IS THE KNOWLEDGE OF MANY, ORDERLY AND METHODICALLY DIGESTED AND ARRANGED, SO AS TO BECOME ATTAINABLE BY ONE.” — Si r John F. W. Herscllel. The Second Edition of his “ Principles of General and Comparative Physiology” having been for some time out of print, the Author com- menced, about two years ago, the task of preparing for the re-appearance of that work ; in the expectation that, although much addition and modification would be requisite, in order to embody in it the most important of those discoveries in which the Physiological researches of recent times have been so fertile, yet that it would be found possible to interweave these with the original matter, in such a mode as to preserve the unity of the whole. On proceeding with his task, however, he soon perceived that it would be impossible to do it the justice he desired, without putting aside his former work altogether, and entirely reconstructing the Treatise ; only employing the materials of the original, where they might chance to be appropriate to his purpose. And thus it happens that the present work may be considered as essentially a new one; since, out of the 1080 pages of which it consists, not above 150, or less than one-seventh, belong to the previous Edition. The Author hopes that this fact may be admitted as evidence of his desire to spare no pains, in order to render his Treatise a faithful exposition of the present state of Physiological Science; and that it will be deemed a sufficient justi- fication of the slight change, by which he considers that the Title has been made to represent more appropriately the present character of the work. It has been with considerable satisfaction, however, that, notwithstand- ing this almost complete renewal of the materials of his Treatise, the Author has found no important modification to be requisite in its plan; for, in consequence of his having originally selected the simplest Vegetable organism, consisting of a single cell, as the starting-point from which to trace out the essential character of each Physiological action, through its successive complications in the higher forms of Vegetable and Animal life, the ‘ cell-doctrine ’ of Schleiden and Schwann, perfected and extended by the labours of the numerous observers who have subsequently prosecuted Vlll PREFACE. the same line of research, has fallen (so to speak) into its appropriate place, and now serves as a complete confirmation of the validity of the principles which the Author had previously propounded. In regard to certain points on which his opinions have undergone modification, the Author refers with satisfaction to the following passage in the Preface to his former edition : — “ Truth is his only object; and, even if his own doctrines should be overthrown by more extended researches, he will rejoice in their demolition, as he would in that of any other error. The character of the true philosopher as described by Schiller, — one ivho has always loved truth better than his system, — will ever, he trusts, be the goal of his intellectual ambition.” In attempting to embody in a Systematic Treatise the general aspect of Physiology or any other Science of like comprehensiveness, it will be obvious that an Author, however extensive his own range of acquirement, must largely avail himself of the labours of others; and that the scientific character of such a treatise must depend, not so much on the amount of original matter it may contain, as on the degree in which “ the knowledge of many ” has been “ orderly and methodically digested and arranged, so as to become attainable by one.” It is by this standard that the Author desires his work to be tried ; and he cheerfully leaves the verdict to the judgment of those, who are qualified by their own knowledge of the subject to pronounce it. He feels it due to himself, however, to state that he has devoted considerable time and attention to the verifica- tion of the statements of other observers, especially on points under dispute, — a kind of labour which is but little appreciated by those, who contemptuously designate works like the present as ‘ mere compilations ; ’ and that a large amount of materials, drawn from his own original enquiries, are scattered through the work. It would have been easy for him to bring these last into greater prominence, had he been so disposed; but as his constant aim has been, to work out his general plan har- moniously and methodically, rather than to force any one portion of it into undue prominence, he has generally preferred to allow his own contributions to pass undistinguished, rather than to be continually obtruding his originality upon the attention of his readers. — Since, how- ever, he has sometimes found himself referred to, as the authority for views or statements whose merit or demerit legitimately belongs to otheis, he thinks it well here to specify the most important of the facts and doctrines which he regards as more particularly his own. I. The mutual connection of the Vital Forces, and their relation to the Phy- sical (chap, iii., Sect. 1, and elsewhere): this doctrine is more fully developed in a paper on “ The Mutual Relations of the Vital and Physical Forces,” in the Philosophical Transactions for 1850. PREFACE. IX II. The general doctrine that the truly Vital operations of the Animal as well as the Vegetable organism are performed by the agency of untransformed cells (§ 135) ; which was first developed in an Essay “ On the Origin and Functions of Cells,” published in the Brit, and For. Med. Rev., vol. xv., 1843. III. The Organic Structure of the Shells of Mollusca, Echinodermata, and Crustacea (§§ 195-199) ; of which a full account is contained in the Reports of the British Association for 1844 and 1847. IV. The application of Van Bar’s Law of Development from the General to the Special, to the interpretation of the succession of Organic forms presented in Geological time (§ 345) ; here first brought forwards. V. The relation between the two methods of Reproduction, that by gemma- tion, and that by sexual union (§§ 647, 648), with the application of this doctrine to the phenomena of the so-called ‘ Alternation of Generations first developed in the Brit, and For. Med.-Chir. Rev., Jan. 1848, and Oct. 1849. VI. The relation between the different methods of Sexual Reproduction in Plants (§ 650) ; first developed in the Brit, and For. Med.-Chir. Rev., Oct. 1849. VII. The application of the doctrine of Reflex Action to the Nervous System of Invertebrata, especially Articulated Animals (chap, xx.) ; first developed in the Author’s “ Prize Thesis,” published in 1839. VIII. The Functional relations of the Sensory Ganglia to the Spinal Cord on the one hand, and to the Cerebral Hemispheres on the other (chap, xx.) ; first sketched out in the Edinb. Med. and Surg. Journ., No. cxxxii. (1837), and more fully developed in the Brit, and For. Med. Rev., vol. xxii. (1846), and in the Brit, and For. Med.'-Chir. Rev., Jan. 1850. In the selection of his materials, the Author has endeavoured to avail himself of the best and most recent information he could procure upon each department of the subject ; it is scarcely to be expected, however, that he should be equally well-informed upon every point ; and those who have followed particular departments into detail, will doubtless find scope for criticism in what they may regard as deficiencies, or even as errors.* Here, again, the Author must beg that his work may be estimated by its general merits ; and rather by what it does, than by what it does not contain. It would have been far easier to expand it by mere compilation to twice its present dimensions, than it has been found to compress the collected materials within the space they even now occupy. It has been the Author’s endeavour, wherever practicable, to draw the materials, both for his text and its illustrations, direct from original Trea- tises and Monographs ; and thus to avoid the errors which too frequently arise from second-hand transmission. To have attempted, however, to assign each individual fact to its original discoverer, each doctrine to its * As the printing of the work has been necessarily spread over a space of a year and a half, faults of omission, in regard to recent discoveries, may, it is probable, be especially noticed in the earlier portions ; but the Author has endeavoured to correct, in the latter part, such as he deemed of most importance. X PREFACE. first enunciator, would have at least doubled the bulk of a volume which has already far outgrown the dimensions originally intended ; and while most desirous to avoid taking credit for what is not his own, the Author has felt himself compelled to limit his references, for the most part, to those new facts and doctrines, which cannot be yet said to have become part of the common stock of Physiological Science. The Illus- trations not his own are referred to their originals, in the list at the commencement of the volume ; and this list, together with the occasional foot-notes, will serve to indicate the principal sources on which the Author has relied for his information, as to those points into which he has not personally examined. From the mode in which these materials are combined and arranged, he believes that he has frequently been able to develope relations between facts and phenomena, which, in the isolated form in which they were originally stated, had only that value which all statements of truth possess ; and that he has thus been able to impart to them a new and unexpected significance. The Author thinks it but due to himself to state, that as the almost constant occupation of his time in other indispensable avocations, leaves him much less opportunity than he would desire for the prosecution of those original researches in which he feels the greatest satisfaction, so he trusts that the same plea may in some degree avail him, in regard to any deficiencies which may be found in the present work. The whole of his disposable time has been devoted to it, for nearly two years, not merely without the anticipation of the slightest pecuniary reward, but under the certain loss involved in the relinquishment of other literary engagements of a remunerative character ; and the sale of the entire edition will not do more than remunerate his liberal Publisher, for the very large outlay which has been incurred in the production of the work, and more especially for the beautiful series of illustrations with which it is embellished, most of them having been executed expressly for it by Mr. Bagg. The Author cannot bring his task to a conclusion, without expressing his particular obligations to Prof. Owen, not merely for the readiness which he has at all times manifested to afford him such information as he might seek ; but also for his kindness in revising those portions of Chapter VII., in which an outline view of the ‘Vertebral Theoiy ’ has been embodied. This outline may, it is hoped, serve to convey an idea of this beautiful system to those wdio are not disposed to pursue it into detail ; and may advantageously prepare the professional student for its application to the Osteology of Man. Regent’s Park, London, May 1 , 1351 . TABLE OF CONTENTS. BOOK I. GENERAL PHYSIOLOGY. CHAPTER I. PAGE On the Nature and Objects of the Science of Physiology . . 1 CHAPTER II. Of the General Characters of Organised Structures . 7 CHAPTER III. of the nature and conditions of vital phenomena. 1. Of Life, or Vital Activity . . . . . . .21 2. Of the Material Conditions of Life ; — Food, Water, and Oxygen . . . 37 3. Of the Dynamical Conditions of Life; — Light, Heat, and Electricity . . 44 4. Of Dormant Vitality . . . . . . 74 5. Of Death . . . . . . . . .79 CHAPTER IV. OF THE COMPONENT STRUCTURES OF ORGANISED FABRICS. 1. General Considerations ....... 86 2. Of the Primary Tissues of Plants . . . . . . . 88 3. Of the Primary Tissues of Animals . . . . . .103 4. Transfonnation of Tissues . . . . . 1 76 CHAPTER V. Of the Distinctive Characteristics of the Vegetable and Animal Kingdoms 181 CONTENTS. xii CHAPTER VI. GENERAL VIEW OF THE VEGETABLE KINGDOM. PAGE PAGE I. Cryptogamia 194 Hepatic® 219 Protophyta 196 Mosses 222 Algse 198 Ferns 224 Charace® 207 II. Phanerogamia 229 Lichens 209 Monocotyledons 233 F ungi 211 Dicotyledons . 235 CHAPTER VII. GENERAL VIEW OF THE ANIMAL KINGDOM. General Subdivisions 239 IV. Articulata . 340 I. Protozoa 243 Entozoa . 345 Polygastrica . 246 Rotifera 358 Rhizopoda 252 Annelida . 365 Porifera 255 Myriapoda 374 11. Radiata 259 Insects 378 Polypifera 262 Crustacea 390 Acalephse 277 Cirrhipoda 410 Echinodermata 283 Arachnida 415 III. Mollusca 294 V. Vertebrata 426 Bryozoa . 298 Fishes 447 Tunicata 301 Amphibia 468 Conchifera 310 Reptiles 470 Gasteropoda . 316 Birds 497 Pteropoda 326 Mammalia 517 Cephalopoda . 329 CHAPTER VIII. On the General Plan of Organic Structure and Development . 559 BOOK II. SPECIAL AND COMPARATIVE PHYSIOLOGY. CHAPTER IX. General View of the Functions of Animated Beings, and of their Mutual Relations ....... 589 CHAPTER X. of aliment, its ingestion and preparation. 1. Sources of the Demand for Aliment . . . . . .600 2. Nature of the Alimentary Materials . . . . . 606 3. Ingestion and Preparation of Aliment in Vegetables . . . .618 4. Ingestion and Preparation of Aliment in Animals . . . . . 619 CONTENTS. X1U CHAPTER XI. OP THE ABSORPTION OF NUTRITIVE AND OTHER MATTERS. PAGE 1. General Considerations . . . . . . 644 2. Absorption in Vegetables ....... 651 3. Absorption in Animals . . . . . . . . 657 CHAPTER XII. OF THE CIRCULATION OF NUTRITIVE FLUID. 1. General Considerations . . 669 2. Circulation in Vegetables . 670 3. Circulation in Animals CHAPTER XIII. 677 OF RESPIRATION. 1. General Considerations . . 725 2. Respiration in Vegetables . . 728 3. Respiration in Animals CHAPTER XIV. . 735 OF THE EXHALATION OF AQUEOUS VAPOUR. 1. General Considerations . . 767 2. Exhalation in Vegetables . . 767 3. Exhalation in Animals . CHAPTER XV. . 774 OF NUTRITION. 1. General Considerations . . 77 9 2. Nutrition in Vegetables . . 788 3. Nutrition in Animals CHAPTER XVI. . 793 OF SECRETION. 1. General Considerations . . 805 2. Secretion in Vegetables . . 806 3. Secretion in Animals CHAPTER XVII. . 807 OF THE EVOLUTION OF LIGHT, HEAT, AND ELECTRICITY. 1. General Considerations ........ 837 2. Evolution of Light . . . . . . . . 838 3. Evolution of Heat ........ 844 4. Evolution of Electricity . . . . . . . 856 XIV CONTENTS. CHAPTER XVIII. OF THE REPRODUCTION OF ORGANISED BEINGS 1. General Considerations ...•■• 2. Reproduction in Vegetables .... 3. Reproduction in Animals ..... CHAPTER XIX. OF THE SENSIBLE MOTIONS OF LIVING BEINGS CHAPTER XX. OF THE FUNCTIONS OF THE NERVOUS SYSTEM. 1. General Considerations ...... 2. Comparative View of the Structure and Actions of the Nervous System, in the principal groups of the Animal Kingdom . . . . .1 000 CHAPTER XXI. OF SENSATION AND THE ORGANS OF THE SENSES. 1. General Considerations ........ 1050 2. Of the Sense of Touch ....... 1052 3. Of the Sense of Taste . . . . ■ • • • 1055 %. Of the Sense of Smell . . . . • • • • 1 056 5. Of the Sense of Hearing ........ 1058 6. Of the Sense of Sight ......•• 1064 CHAPTER XXII. OF THE PRODUCTION OF SOUNDS BY ANIMALS 1074 PACE 866 875 905 989 997 LIST OF ILLUSTRATIONS. FIG. PAGE 1. Simple isolated Cells (xxxix.) . . • • 25 2. Section of Leaf of Agave ( xxix.) . . . • .88 3. Circulation of fluid in hairs of Tradescantia Virginica (lxvii.) . . 89 4. Various stages of development of Hcematococcus binalis (xxx.) . .91 5. Process of cell-multiplication in Conferva glomerata (lii.) . • 92 6. Development of Achlya prolifera (lxxv.) . • • .94 7. Various examples of Vegetable Cells (lxvii.) ... 96 - 8 Spongiform Parenchyma, composed of stellate cells (xxix.) . . 96 9. Cells from the gritty tissue of the Pear (xxxv.) ... 98 10. Elongated cells, constituting Woody Fibre (lxvii.) . . .99 11. Formation of Vegetable Ducts (lxvii.) .... 100 12. Longitudinal Section of stem of Italian Reed (lxiv.) . . ■ 101 13. Laticiferous vessels (lxvii.) . . . . . 102 14. Multiplication of Cartilage-cells by duplication (xli.) . . . 104 15. Parent-cells of Cancerous growths, containing secondary cells (xxxix.) . 105 16. Fibrous membrane from egg-shell . . . . .107 17. White fibrous tissue . . . . .108 18. Yellow fibrous tissue ...... 109 19. Development of fibres from cells (xxxix. ) . . . Ill 20. Ideal section of a joint . . . . . .112 21. Blood-vessels of Intestinal villi (hi.) . . . . 114 22. Blood-vessels of follicular Mucous Membrane (hi.) .. .114 23. Blood-vessels of Skin of finger (in.) . . . 115 24. Colourless corpuscles of Blood . . . . .116 25. Red Corpuscles of Frog’s Blood (xx.) . . . . 117 26. Red Corpuscles of Human Blood (xx.) .... 117 27. Oblique section of Epidermis ( xxxi.) .... 121 28. Cells from Pigmentum Nigrum of Eye (xxxi.) . . .121 29. Pigment-cells from tail of Tadpole (xxxi.) . . . 122 30. Hair of Muslrdeer and Sable . . . . .123 31. Pavement-Epithelium cells (xxxix.) .... 124 32. Diagram of Intestinal mucous membrane (xxvm.) . . .125 33. Ciliated Epithelium-cells (xliv.) . . . . 125 34. Shell-membrane, illustrating the progressive development of cells . .127 35. Areolar and Adipose tissue (xliv.) .... 129 36. Section of the branchial Cartilage of Tadpole (lxv.) . .131 37. Section of Fibro-cartilage (xxxix.) . . 131 38. Dilated loops of nutritive vessels of Cartilage (lxxiii.) . 132 xyi LIST OF ILLUSTRATIONS. FIG. PAGE 39. Portion of shell of Echinus, with section highly magnified . . 133 40. Prismatic cellular structure of Shell of Pinna .... 135 41. Tubular shell-structure of Anomia . . . • 136 42. Lacunae of Osseous substance (xliv.) . . • .138 43. Section of bony scale of Lepidosteus .... 139 44. Vertical section of Bone, showing network of Haversian canals (xxvn.) . 140 45. Minute structure of Bone, as seen in transverse section (lxxxh.) . 141 46. Vertical Section of Cartilage at the seat of Ossification (lxxxii.) . .145 47. Sections of teeth of Lamna and Pristis (lvi.) . . • 147 48. Oblique Section of Dentine of Human Tooth (l vi.) . . • 1-18 49. Vertical Section of Human Molar Tooth (xliv.) . . . 149 50. Capillary network around Fat-cells (hi.) . . . .153 51. Capillary network of Muscle (hi.) .... 153 52. Capillary circulation in the web of the Frog's foot (lxxx.) . .153 53. Web of Frog's foot, slightly magnified (lxxx.) . . . 154 54. Formation of Capillaries in tail of Tadpole (xxxvii.) . . • 155 55. Fasciculus of striated Muscular Fibre (xliv.) . . . 156 56. Striated Muscular Fibre, separating into fibriHae . • • 156 57. Muscular Fibre splitting into discs (vm.) .... 157 58. Transverse section of Muscular Fibre (viii.) .... 157 59. Structure of ultimate fibrillae of striated Muscular Fibre . . 157 60. Non-striated Muscular Fibre (x.) ..... 158 61. Muscular Fibre from Foetus, treated with tartaric acid (lxxxii.) . 159 62. Primitive Nerve-fibres, and Ganglionic globules (lxxx.) . . ’ . 167 63. Dorsal ganglion of Sympathetic nerve of Mouse (xxvu.) . . 168 64. Arrangement of motor Nerves in Muscle (xxvu.) . . .170 65. Various stages of development of ITcematoeoccus sanguineus (xxx.) . 198 66. Successive stages of multiplication of Botryocystis morum (xxxviu.) . . 199 67. Successive stages of development of simpler Alga (xxxviu.) . . 200 68. Frond of C/uetophora tuberculosa (xxxviu.) . . . .201 69. Various stages of the history of Zygnema quininum (xxxviu.) . . 202 70. Structure of Carpocaulon mediterraneum (xxxviu.) . . . 205 71. Structure of Sargassum coarctatum (xxxviu.) . . . 206 72. Portions of Nitella flexilis, natural size, and enlarged (lxvii.) . . 208 73. Structure oi Parmelia acetabulum (xxxv.) . . . 210 74. Different stages of vegetation of Torula cerevisii . . . 212 75. Structure of lower forms of Fungi ; Botrytis, and Mucor (lxiii.) . 213 76. Structure of Agaricus campestris (lxiii.) . . . .217 77. Fructification of Marchantia polymorpha .... 220 78. Gemmiparous conceptacles of Marchantia polymorpha (li.) . . 221 79. Structure of Mosses (xxxv.) ..... 223 80. Tree-Fern (xxxii.) ...... 225 81. Fructification of Ferns (xxxv.) .... 226 82. Ideal Plant (lxiv.) ...... 280 83. Diagram of the structure of Monocotyledonous stem . . . 231 84. Diagram of the structure of Exogenous stem .... 235 85. Diagram of the first formation of an Exogenous stem . . 237 86. Different forms of Amoeba princeps (xxn.) .... 245 87. Different forms of Polygastric Animalcules (xxu.) . . . 247 88. Group of Vorticella nebulifera (xxn.) .... 248 LIST OP ILLUSTRATIONS. xvii FIG. PAGE 89. Structure and development of Volvox globator (xxii.) . • 250 90. Different forms of Rhizopoda (xxii.) . 253 91. Shell of Nonionina germanica, with its contained animal (xxiv.) • 254 92. Structure and development of Spongilla (xii.) . • • 257 93. Hydra fusca (lxxiv.) . 264 94. Gemmation of Hydra fusca (lxxix.) . 265 95. Portions of Campanularia gelatinosa, natural size, and magnified (lxxvi.) 268 96. Development of Medusa-buds in Syncoryna Sarsii (lxii.) . • 269 97. Actinia, open and closed, attached to a pebble ( xix.) . • 270 98. Diagrammatic section of Actinia (lxvi.) . . • .271 99. Portion of the stem of Dendropliyllia (xix.) . . • 273 100. Multiplication of polypes of Astrcea by subdivision (xix.) . .274 101. Polypidom of Alcyonidium elegans (xlv.) .... 274 102. Portion of branch of Alcyonidium elegans, enlarged (xlv.) . . 275 103. Internal structure of polypes of A Icyonidium elegans (xlv.) • . 275 104. Portion of young branch of Alcyonium stellatum (xlv.) . . .276 105. Transverse section of polypidom of Alcyonidium elegans (xlv.) . 276 106. Structure of Cyanoea aurita ( xxm.). . . . .279 107. Cydippe pileus, and Beroe Forshxlii (xxvi. and xvn.) . . 280 108. Structure of Velella limbosa (xxxiv.) .... 282 109. Physalia Atlantica (xlii.) ..... 282 110. Structure of A sterias aurantiaca (lxxii.) . . . .288 111. Anatomy of Echinus lividus (xvii.) .... 290 112. Holotliuria (Cucumaria) pentactes (l.) .... 292 113. Anatomy of Holotliuria tubulosa (xvii.) .... 293 114. Laguncula repens ( lxxvi.) ..... 299 115. Eschara cervicornis (xvu.) . . . . . 301 116. Anatomy of Ascidia (Cynthia) microcosmus (xvii.) . . . 302 117. Internal anatomy of Ascidia (Cynthia) microcosmus (xvii.) . . 304 118. Compound mass of Amaroiaium proliferum (xlvi.) . . . 305 119. Anatomy of Amaroucium proliferum (xlvi.) . . . 305 120. Botryllus violaceus (xlvi.) ..... 306 121. Anatomy of Botrylloides rotifera (xlvi.) . . . 307 122. Salpa maxima (xvii.) . . . . . .308 123. Terebratula (Atrypa) psittacea (l.) . . . . 310 124. Anatomy of Mactra (l.) . . . . . .315 125. Chitonellus and Cldton (xm.) ..... 319 126. Doris Jolmstoni, showing the tuft of external gills (i.) . . . 320 127. Interior of Aplysia (xvi.) . . . . . 321 128. Anatomy of Paludina vivipara (xvi.) .... 322 129. Carinaria, in its natural position when swimming . . . 326 130. Hyalaa, Criseis, and Clio (xm.) ..... 327 131. Octopus (l.) ...... 329 132. Ammonite (l.) ...... 330 133. Diagram of the structure of Nautilus pompilius (l vii.) . . 332 134. Onychoteuthis (l.) . . . . . .334 135. Anatomy of Octopus (xlvii.) ..... 336 136. Belemnite restored (lvii.) ..... 338 137. Centipede (l.) . . . , . 340 138. Section of the trunk of Melolontha vulgaris (lxix.) . . . 344 b xviii LIST OF ILLUSTRATIONS. PIG. PAGE 139. Tania solium (l.) ...... 349 140. Anatomy of Tania solium (vi.) .... 350 141. Structure of Polycelis lavigatus (lx.) .... 353 142. Anatomy of Fasciola hepatica (vi.) .... 354 143. Anatomy of Strongylus gigas (vi.) ..... 356 144. Rotifer vulgaris (xxn.) ..... 360 145. Stephanoceros Eichorni (xxii.) ..... 364 146. Noteus quadricornis (xxii.) ..... 364 147. Nephthys ITombergii (xvii.) ..... 366 148. Cluster of Serpula (xvir.) ..... 368 149. Anatomy of Arenicola piscatorum (xlvui.) . . . . 371 150. lulus (l.) ...... 375 151. External skeleton of an Insect (l.) . . . . .379 152. Ideal section of Sphinx ligustri (lv.) .... 382 153. Ideal section of Larva of Sphinx ligustri (lv.) .... 388 154. Ideal section of Pupa of Sphinx ligustri (lv.) . . . 388 155. Under side of AstacusjluviatiUs (l.) .... 393 156. Anatomy of Cancer pagur us (l.) . 396 157. Metamorphoses of Carcinus mcenas (xv.) . . • .401 158. Ammothea pycnogonoides (lix.) .... 403 159. Limnadia (l.) ....-• 405 1 60. Lemcea in young and adult states (l.) .... 409 161. Anatifa lavis (xv n.) ...... 411 162. Digestive apparatus of Mygale (xvii.) . . . • 417 163. Heart of Mygale (xvii.) ..... 418 164. Interior of Mygale ( xvii. ) ..... 419 165. Cephalo-thorax of Mygale (xvii.) ..... 420 166. Nervous system of Androctonus (lv.) . . . • 421 167. Elements of Vertebra (lvlii.) ..... 431 168. Human Sacrum (lxxxii.) ..... 432 169. Human Cervical Vertebra (lxxxii.) .... 432 170. Diagram of Archetype Vertebral skeleton (lviii.) . . . 435 171. Diagram illustrating the Nature of Limbs (lviii.) . . • 439 172. Diagram of the Anatomy of Amphioxus (lvii.) . . . 449 173. Skeleton of Perch (l.) ...... 456 174. Anatomy of Pish (xvii.) ..... 464 175. Lepidosiren paradoxa (l.) ..... 469 176. Skeleton of Cistudo Europaeus (l.) .... 479 177. Plastron of Cistudo and Chelone (xxi.) .... 480 178. Skeleton of Pterodactylus restored (l.) .... 486 179. Draco volans (l.) ...... 486 180. Anatomy of Coluber (l.) ..... 491 181. Circulating apparatus of Lizard (l.) ..... 492 182. Lungs of Bipes, Bimanus, and Coluber (xxv.) . . . 493 183. Nervous centres in Frog (xliii.) ..... 494 184. Skeleton of Vulture (l.) ..... 499 185. Bones of wing of Falcon ..... 502 186. Digestive apparatus of Fowl (l.) .... 507 187. Pulmonary apparatus of Pigeon (xxv.) .... 509 188. Generative apparatus of Fowl (xxv.) .... 515 LIST OF ILLUSTRATIONS. XIX FIG. PAGE 1 89. Skeleton of Camel (l.) ...... 520 . 190. Skeleton of Bat (l.) ..... 529 191. Front view of Os hyoides (lxxxii.) .... 536 192. Ideal section of Mammalian Heart (l.) . 546 193. Galaopithecus (l.) ...... 561 1 94. Ascidia of Sarracenia, Nepenthes, and Cephalotus (xxxn.) . . 618 195. Digestive apparatus of Rhizostoma (x vn.) .... 623 196. Compound Stomach of Sheep . . . . . 631 1 97. Section of part of the Stomach of Sheep (xxv.) . . . 632 198. Digestive Apparatus of Annelida (xvii.) .... 635 199. Digestive Apparatus of Carnivorous Beetle (i.) . . . 636 200. Diagram of Lymphatic gland (xxvm.) .... 661 201. Circulating Apparatus of Terebella (xl viii.) .... 685 202. Circulating Apparatus of Eunice (xlviii.) . . . 686 203. Circulating Apparatus of Arenicola (xlviii.) .... 687 204. Circulating Apparatus of Scolopendra (lv.) . ■ . 689 205. Circulating Apparatus of Buthus (lv.) .... 693 206. Circulating Apparatus of Salpa (xvii.) .... 699 207. Circulating Apparatus of Pinna (xlvii.) .... 700 208. Diagram of the Circulation in Fishes (l.) . . . 703 209. Diagram of the Circulation in Reptiles (l.) . . . .706 210. Respiratory Circulation in the Tadpole (l.) . . . 707 211. The same in a more advanced state (l.) .... 707 212. The same in the perfect Frog (l.) .... 707 213. Diagram of the Circulation in Birds and Mammals (l.) . . . 710 214. Vascular area of Fowl's egg (lxxx.) . . . . 716 215. Diagram of formation of great Arterial trunks . . . .718 216. Diagram of Arterial trunks arising from Aorta . . . 724 217. Diagram of different forms of Respiratory apparatus . . .738 218. Gill of Doris Johnstoni (i.) ..... 742 219. Branchial arch and leaflets of Fish (xvii.) . . . .746 220. Tracheal system of Nepa (l.) .... 749 221. Respiratory organs of Frog (xxv.) ..... 756 222. Section of Lung of Turtle (vii.) .... 757 223. Capillaries and Air-cells of Human Lung . . . .759 224. Vertical section of Leaf of Lilium album (xi.) . . . 768 225. Under surface of Leaf of Lilium album (xi.) . . .769 226. Surface and section of frond of Marchantia polymorpha (li.) . . 769 227. Sudoriparous gland from Human hand (lxxx.) . . . 774 228. Capillary network around follicles of Parotid Gland (hi.) . . 809 229. Glandular follicles of Stomach (lxxx.) . . . .810 230. Mammary Gland of Ornithorhyncus (liii.) . . . 810 231. Rudimentary Pancreas, from Cod (liii.) . . . .811 232. Lobule of Parotid Gland of Human Infant (lxxx.) . . 811 233. Biliary tubuli of Musca carnaria (xli.) .... 816 234. Hepatic caecum of A stacus (xli.) .... 817 235. Lobules of Liver of Squilla (liii.) . . . . .818 236. Glandular cells of Liver . . . . . 818 237. Arrangement of biliary ducts of Human Liver (xxx vi.) . . .819 238. Arrangement of blood-vessels in Human Liver (xxxvi.) . . 820 XX LIST OF ILLUSTRATIONS. t FIG. PAGE 239. Early stage of development of Liver of Fowl (mi.) . . . 821 240. Kidney of fcetal Boa (liii.) ..... 826 241. Section of Kidney of Coluber (liii.) .... 826 242. Section of Kidney of Bird (liii.) .... 827 243. Section of Human Kidney (lxxxii.) .... 827 244. Tubulus uriniferus, with epithelial cells (lxxx.) . . . 828 245. Distribution of vessels of Kidney (ix.) .... 828 246. Corpora Wolffiana, from Cliiclc (liii.) .... 829 247. Electric Organs of Torpedo (l.) ..... 862 248. Multiplication of Coccocldoris cystifera (xxx.) . . . 877 249. Conjugation of Euastrum oblongum (lxi.) .... 878 250. Conjugation of Eunotia turgida (lxxi.) . . . . 879 251. Development of Sporangia in A ulacoseira (lxxi.) . • • 880 252. Fructification of Sporochnus adriaticus (xxxvm.) . . • 883 253. Development of spore of Pieris serrulata (xl.) .... 887 254. Pro-embryo of Pteris serrulata (xl.) ... 888 255. Antheridia and phytozoaires of Pteris serrulata (xl.) . . . 889 256. Pistillidium of Pteris serrulata (xl.) .... 890 257. Development of embryo of Polypodium aureum (xl.) . . . 891 258. Development of embryo of CEnotheraceee (xxxm.) 259. Embryoes of Potamogeton and Amygdalus (xxxv.) . . . 898 260. Germination of embryoes of Zanichellia and Acer (xxxv.) . . 900 261. Constituent parts of Mammalian Ovum (xiv.) . . . 912 262. Segmentation of yolk of Ascaris (n.) .... 915 263. Segmentation of yolk of Mammalian Ovum (xiv.) . . . 954 264. Development of Ovum of Coregonus palaca (lxxviii.) . . 916 265. Fissiparous multiplication of Chilodon cucullulus (xxii.) . . . 917 266. Ovarial Chamber of Actinia coriacea (lxx.) . . . 920 267. Development of polype-bud of Campanularia (lxxvi.) . . . 922 268. Development of Medusa-buds from Pcrigonimus (lxii.) . . 923 269. Medusiform gemmae of Campanularia (lxxvi.) . . . 924 270. Strobila ( Hydra tvha ) propagating by gemmation (xvm.) . . 927 271. Group of Strobiles or Medusan lares (xvm.) .... 928 272. Development of Medusa-buds from Strobila (xvm.) . . 928 273. Further progress of development of Medusa: (xvm.) . . . 929 274. Gemmiparous multiplication of Cytceis (lxii.) . . . 930 275. Development of ovum of Echinaster (lxii.) .... 934 27 6. Bipinnaria asterigera, or larva of Star-fish (liv.) . . . 936 277. Embryonic development of Echinus (liv.) .... 937 278. Origin of Ophiura (liv.) ..... 938 279. Gemmiparous extension of Laguncvla repens (lxxvi.) . . . 940 280. Development of embryo of Amaroucium (xlvi.) . . . 942 281 More advanced embryo of Amaroucium (xlvi.) . . 943 282. Development of embryo of Acteon (lxxix.) . . 948 283. Generative apparatus of Fasciola (xvn.) .... 953 284. Generative organs of Nais (xvii.) ... 956 285. Development of larva of Terebella (xlix.) .... 95 7 286. Latter stages of segmentation of yolk of Mammalian ovum (xiv.) 969 287. Germ and surrounding parts, from Uterine ovum (xiv.) . 970 288. Ovum of Coregonus palcea, in course of development (lxxviii.) . 971 LIST OF ILLUSTRATIONS. XXI FIG. PAGE 289. Incipient formation of Amnion (lxxx.) .... 972 290. Embryo of Coregonus palcea (lxxviii.) . . . . 973 291. More advanced embryo of Coregonus (lxxviii.) . . 974 292. Further development of Amnion in Human ovum (lxxx.) . . 975 293. Incipient formation of Allantois in Human ovum (lxxx.) . . 975 294. Further development of Allantois in Human ovum (xiv.) . . 976 295. Formation of vascular tufts of Chorion in Human ovum (xiv.) . .976 296. Extremity of Placental villus (xxviii.) .... 977 297. Embryo of Dog , viewed sideways (iv.) . . . .978 298. The same, viewed in front (iv.) .... 978 299. Nervous system of Solen (v.) ..... 1005 300. Nervous system of Argonauta argo (xvii.) . . . 1008 301. Nervous centres of Octopus (xvii.) ..... 1009 302. Nervous centres of Sepia (xvii.) .... 1009 303. Nervous system of larva of Sphinx ligustri (tv.) . . .1011 304. Portion of ganglionic tract of Polgdesmus ( lv.) . . . 1012 305. Parts of nervous system of Centipede and Sphinx ligustri (lv.) . . 1015 306. Nervous System of lulus terrestris ( lv.) . . . . 1017 307. Nervous centres of Frog (xliii.) .... 1021 308. Brains of Fishes (xliii.) ..... 1023 309. Brain of Turtle (lxviii.) ..... 1026 310. Brain of Buzzard (lxviii.) ..... 1027 311. Brain of Human Embryo at twelfth week (lxxx.) . . . 1027 312. Brain of Squirrel (lxviii.) ..... 1028 313. Brain of Babbit (xliii.) ..... 1028 314. Capillary Loops in fungiform papilla of the Tongue (hi.) . . 1055 315. Diagram of the Human Ear (lxxxii.) .... 1062 316. Head and Compound Eyes of Bee .... 1065 317. Section of the Compound Eye of Melolontlia vulgaris (lxix.) . . 1066 318. Longitudinal Section of the Globe of the Eye (lxxxii.)- . . 1068 319. Diagram of the course of the rays in the Eye .... 1070 320. View of the Larynx from above (lxxxi.) . . . 1077 321. Artificial Larynx (lxxxi.) ..... 1078 TREATISES AND MEMOIRS REFERRED TO IN THE LIST OF ILLUSTRATIONS. i. Alder and Hancock , Monograph of the British Nudibranchiate Mollusca. (Pub- lished by the Ray Society.) li. Bagge, De Evolutione Strongyli et Ascaridis. hi. Berres, Anatomie der Mikroskopischen Gebilde des Menschlichen Korpers. iv. Bischoff, Entwickelungs-geschichte des Hunde-Eies. v. Blanchard, Observations sur le Systeme Nerveux des Mollusques Acephales Tes- taces. (Ann. des Sci. Nat., Juin, 1845.) vi. Recherches sur l’Organisation des Vers. (Ann. des Sci. Nat., 1847-50 and xvn.) vii. Bojanus, Anatome Testudinis Europeae. viii. Bowman, On the Minute Structure and Movements of Voluntary Muscle. (Phil. Trans., 1840.) ix. On the Structure and Use of the Malpighian Bodies of the Kidney. (Phil. Trans., 1842.) x. (and Todd), Physiological Anatomy and Physiology of Man. xi. Brongniart, Sur la Structure, etc. des Feuilles. (Ann. des Sci. Nat., 1830.) xii. Carter, On the Freshwater Sponges of Bombay. (Ann. of Nat. Hist., Aug. 1849.) xiii. Chenu, Lemons Elementaires d'Histoire Naturelle. Paris, 1847. xiv. Coste, Histoire Generate et Particuliere du Developpement des Corps Organists, xv. Couch, On the Metamorphosis of the Decapod Crustaceans. (Report of Cornwall Polytechnic Society for 1843.) xvi. Cuvier, Memoires sur les Mollusques. xvii. — — Regne Animal. (Edition accompagnee de Planches gravees, par une reunion de Disciples de Cuvier.) xviii. Dalyell, Rare and Remarkable Animals of Scotland. xix. Dana, Structure and Classification of Zoophytes. xx. Donn6, Cours de Microscopie (Atlas). xxi. Dum&ril (et Bibron) Histoire Naturelle complete des Reptiles. xxii. Ehrenberg, Die Infusionsthierchen. xxiii. Des Leucthen des meeres. (Abhaldlungen der Koniglichen Akademie der Wissenschaften zu Berlin, 1835.) xxiv. On the Animals of the Chalk formation still found in a living state. (Taylor’s Scientific Memoirs, Vol. hi.) xxv. Flourcns, Memoires d'Anatomie et de Physiologie Comparees. xxvi. Forbes, On Beroe pileus. (Ann. of Nat. Hist., Vol. u.) xxvii. Gerber, Elements of the General and Minute Anatomy of Man and the Mam- malia. TREATISES AND MEMOIRS REFERRED TO IN ILLUSTRATIONS. XX111 .xxviii. Goodsir, Anatomical and Pathological Observations, xxix. Hartig, in Mulder’s Chemistry of Animal and Vegetable Physiology. xxx. Hassall, History of the British Freshwater Algae. xxxi. Henle , Allgemeine Anatomie. xxxii. Henslow, Principles of Descriptive and Physiological Botany. xxxiii. Hoffmeister , Snr la Fecondation chez les CEnothdrees. (Ann. des Sci. Nat., Fevr., 1848.) xxxiv. Hollard, Sur l’Organisation des Velelles. (Ann. des Sci. Nat., Avril, 1845.) xxxv. Jussieu, Cours Elementaire de Botanique. xxxvi. Kiernan, On the Structure of the Liver. (Phil. Trans., 1835.) xxxvii. Kolliker, Sur le Developpement des Tissus chez les Batraciens. (Ann. des Sci. Nat., Aout, 1846.) xxxviii. Kutzing, Phycologia generalis. xxxix. Lehert, Physiologie Pathologique. xl. Leszczyc-Luminski, Zur Entwickelungs-Geschichte der Farrnkriiuter. xli. Leidy, On the Comparative Structure of the Liver, and on the Intimate Structure and History of the Articular Cartilages. (Amer. Journ. of Med. Sci., Jan., 1848, and April, 1849). xlii. Lesson, Histoire Naturelle des Acalephes. xliii Leuret, Anatomie Comparee du Systeme Nerveux. xliv . Mandl, Manuel d’Anatomie Generale. xlv • Milne- Edwards, Recherches sur les Polypes. XL vi. Observations sur les Ascidies Composees des Cotes de la Manche. xlvii. — Observations sur la Circulation chez les Mollusques. (Ann. des Sci. Nat., Juin, 1845, et Juillet, 1847.) xlviii. — Observations sur la Circulation chez les Annelides. (Ann. des Sci. Nat., 2ieme Ser. Tom. x.) xlix. - Observations sur le Developpement des Annelides. (Ann. des Sci. Nat., Mars, 1845.) l. Elemens de Zoologie. (See also xvii.) li. Mirbel, Sur la Structure et la Developpement de la Marchantia polymorpha. (Nouv. Ann. du Musee, Tom. in.) lii. Mold, Vermischte Schriften botanischen Inhalts. liii. Milller, De Glandularam secernentium Structura penitiori. liv. Ueberdie Larven und die Metamorphose der Echinodermen. (Abhaldlungen der Koniglichen Akademie der Wissenschaften zu Berlin, 1846 und 1848.) lv. Newport, Memoirs on the Structure of Insects, Myriapoda, and Macrourous Arachnida (Phil. Trans. 1839 — 1843.) lvi. Owen, Odontography. lvii. Lectures on Comparative Anatomy. lviii. On the Homologies of the Vertebrate Skeleton. lix. Quatrefages, Memoire sur l’Organisation des Pycnogonides (Ann. des Sci. Nat., Aout, 1845.) lx. Memoire sur quelques Planariees marines. (Ann. des Sci., Sept. 1845.) lxi. Ralfs, Monograph of the British Desmideae. lxii. Sars, Fauna littoralis Norvegiae. lx in. Schleiden , Principles of Scientific Botany. lxiv. The Plant, a Biography. lxv. Schwann, Mikroskopische Untersuchungen iiber die Uebereinstimmuno- in der Structur und dem Wachsthum der Thiere und Pflanzen. lxvi. Sharpey, Art. Cilia (Cyclop, of Anal, and Physiol.) xxiv LX VII. LX VIII. LXIX. LXX. LXXI. LXX II. LXXIII. LXX1V. LXX V. LXXVI. LXX VII. LXXVI1I. LXXIX. LXXX. LXXXL LXXXII. TREATISES AND MEMOIRS REFERRED TO IN ILLUSTRATIONS. Slack, On the Elementary Tissues of Plants, and on Vegetable Circulation. (Trans, of Soc.of Arts, Vol. xxxix.) Solly, The Human Brain, its Structure, Physiology, and Diseases. Strauss- Durckheim, Considerations Generates sur l’Anatomie comparee des Ani- maux Articules. Teale, Anatomy of Actinia Coriacea. (Trans, of Phil, and Lit. Soc. of Leeds, Vol. i.) Thwaites, On the Conjugation of the Diatomaceae. (Ann. of Nat. Hist., 1847 and 1848.) Tiedemann, Anatomie der Rohrenholothurie, &c. Toynbee, On the Organisation and Nutrition of the Non-vascular Animal Tissues. (Phil. Trans., 1841.) Trembley, Memoires pour servir a l’Histoire d’un genre de Polype d'Eau douce. Unger, Recherches sur PAchlya prolifera. ("Ann. des Sci. Nat., Juillet, 1844. ) Van Beneden, Memoire sur les Campanulaires de la Cote d’Ostende. (Mem. de l'Acad. Roy. de Bruxelles, Tom. xvu.) Recherches sur les Bryozoaires de la Cote d’Ostende. (Mem. de l’Acad. Roy. de Bruxelles, Tom. xvnt.) Vogt, Emhryologie des Salmones. Recherches sur l’Embryologie des Gasteropodes. (Ann. des Sci. Nat., Juillet, 1846.) T Vagner, leones Physiological. Willis, On the organs of Voice. (Trans, of Cambridge Phil. Soc., Vol. iv.) Wilson, Anatomist's Vade-Mecum. BOOK I. GENERAL PHYSIOLOGY. CHAPTER I. ON THE NATURE AND OBJECTS OF THE SCIENCE OF PHYSIOLOGY. 1. The most superficial observer can scarcely fail to recognize, in the world around him, many evident distinctions between living beings and inanimate bodies. A scientific comparison is not required to discover such obvious differences in their external Form, as are usually sufficient for their ready discrimination. A very cursory examination of the inter- nal Structure which they respectively present, adds many new indications of their fundamental diversity. And daily experience teaches, scarcely less satisfactorily than the refined analysis of the laboratory, that there is a contrast no less remarkable in their Chemical Composition. 2. But it is in their Mode of Existence, that we find this disagreement the most complete and the most obvious. In the kingdom of Inorganic matter, Permanence is the rule, change the exception. The particles of the Mineral mass remain coherent, and their primal constitution (whether elementary or compound) is retained, so long as no external forces over- come by their superior potency the existing attractions and affinities. The “ everlasting hills ” which rear their summits to the clouds, unaf- fected by the lapse of ages which have rolled by since their first elevation, bear testimony through countless generations to the vastness of the forces which first upheaved them — perennial monuments of Nature’s power ; and even the edifices erected by the hand of Man frequently convey to a remote posterity the attestation of the grandeur of a people, which may have subsequently relapsed into barbarism and insignificance, or which may have altogether disappeared from among the nations of the earth. But in the domain of Life it is altogether different. Here, Change is the rule, permanence the exception ; and this change does not so much pro- ceed from the agency of external forces, as from those inherent in and peculiar to the body itself. To be bom— to grow — to arrive at maturity — to decline— to die — to decay, is the sum of the history of every being that lives, from the highest to the lowest, from the most transient to that of most protracted duration. “Our life,” to use the expressive simile of the sacred writer, “ is as a vapour, which appeareth for a little time, and then vanisheth away.” 3. Yet amidst this constant change and succession of individuals, we observe the form and character first impressed upon each race by the Creator of all, uninterruptedly transmitted from parent to offspring ; so that, as “ one generation passeth away,” “ another cometh ” similar to it in structure and actions, and, like it, destined to give place in due time to a successor. The lofty stem of the Date is even now hailed from afar B 2 GENERAL PHYSIOLOGY. by the thirsty traveller in the Red Sea desert, as a welcome indica- tion of the proximity of water, just as when the palm-trees and wells of Elim refreshed the weary Israelites on their route to Sinai. The Mistle- toe flourishes on the oak of our own forests, not otherwise than when it was an object of superstitious veneration in the hallowed groves of our Druidical ancestors. The Bee still constructs her comb with the same wonderful regularity, and stores it with the same materials, as when her beautiful works attracted the notice of the poets and philosophers of classic ages. And Man, however various his degree of civilization, how- ever great the development of his physical powers, however lofty the elevation of his spiritual nature, is yet born in the same dependent state, with the same dormant faculties of body and mind, as the first offspring of our original parents. 4. And when we examine more attentively into the succession of changes exhibited by each individual, we see that the history of the fabric as a whole is repeated in every portion of it. Each one of that assemblage of diverse components, of which the higher forms of living beings — whether Plants or Animals— are made up, has its own inde- pendent powers of growth and development, its own peculiar endowments when fully evolved, and its own limit to the term of their exercise ; and when this has been completed, it is either annihilated by death and decay, or it passes into the inactive condition of inorganic bodies, to which it becomes assimilated in structure and composition. Hence there is a con- stant change in the materials of which any living being is composed, even while its structure aud constitution remain essentially the same ; and as the continuity of the race depends on the replacement of the departing generation by a young and vigorous progeny, so is the maintenance of the life of the individual dependent on the continual replacement of its decaying components by newly-developed tissue. Notwithstanding the constancy of this change, however, the foim, structure, and endowments of the entire fabric are preserved unaltered, except iu so far as they arc modified in accordance with the progress of its own life ; aud its indivi- duality is maintained continuously, from its first origin to its final decay, although not a single particle may remain among its constituents during any considerable portion of its term of existence. There is nothing, per- haps, in the inorganic world, which affords so apt an illustration of this marvellous truth, as the continuity of the mighty cataract, without the least perceptible change in the form presented by its descending torrent, whilst every instant witnesses the renewal of its component particles, — the delivery of those which have performed them descent into the gulf below, and them replacement by those perpetually supplied from the source above. 5. Even a cursory glance, then, at the ever-varying conditions of the animated world, serves to impress us with the idea of regularity aud arrangement ; and the more attentively we investigate the succession of changes which it exhibits, the more firm becomes our assurance, that all these changes take place according to a definite and stable plan ; or (to use the current phraseology of science) that they are governed by laws * * The only legitimate use, in my apprehension, of the term Law of Xaturo, is as an expression of that uniformity observed by the philosopher in the phenomena of the Universe, which indicates the stability of the original plan. Thus when we speak of the “ law of NATURE AND OBJECTS OF PHYSIOLOGICAL SCIENCE. 3 as determinate as those which regulate the movements of the heavenly bodies, or control those molecular changes in the inorganic world of which the Physicist and the Chemist take cognizance. To arrive at the know- ledge of these laws, and, having attained them, to trace their application to all the countless variety of phenomena presented by the myriads of living beings whose beautiful forms people this globe, is the object of the science of Physiology, using that term in its most extended sense, to which the designation Biology is perhaps rather applicable. That the most advantageous plan of studying it is that inductive method which has been successful in other sciences, will not perhaps now be disputed : yet the prevalence of a contrary system has long retarded its pro- gress ; and it is only within a recent period that the ends to be attained have been generally understood, and the most satisfactory means of pur- suing them fully determined. 6. In this, as in the Physical Sciences, the first object of the philosophic enquirer is to collect a body of facts, from the comparison of which the general truths common to all may be elicited. Now the facts which the observation of living beings brings under our notice, are obviously of two kinds ; — one class having reference to their structure, the other to their actions or functions. The investigation and comparison of the former class of particulars, is the object of the science of Anatomy; whilst by the gravitation,” we simply refer to the general fact, that all masses of matter attract each other in a certain determinate ratio, from which we never witness any departure. But it must he remembered that all our so-called “ laws ” are but human expressions, based upon a very limited amount of observation, and liable to be modified, therefore, by the extension of our knowledge of the entire scheme of Creation ; and to suppose that the Deity is in any way tied down to those “ laws ” which Man supposes that he has discovered, is equivalent to making Man rather than God the Legislator of the Universe. — The true idea appears to me to be this. The material universe was constructed in the first instance on a plan so perfect, that no departure from that plan has been required for the maintenance of its existence, and for the welfare of its sentient inhabitants. (To this statement, the occurrence of those apparent departures from the “ order of nature ” which we term Miracles, does not constitute a real exception ; for every believer in the Divine Omniscience must admit that they were as much foreseen and provided for in the original plan, in order to produce a special influence upon the mind of Man, as were those occurrences which impress him with the idea of uniformity and fixity, and which thus give rise to his notion of immutable laws.) But whilst we reject the idea of continual interposition, or change of plan, as inconsistent alike with our belief in the Divine perfections, and with the results of our observation of His agency, we cannot do other- wise, in my apprehension, than recognize his constant sustaining action as a necessary condition of the continued existence of the Universe. For if we are led to believe, by the indications of a Designing Mind, which are so abundantly presented to every one who seeks for them, that the first existence of matter was but an expression of an Intelligent Will, to what can its continued existence be attributed, but to the sustaining action of that same Will ? Reason and Revelation here seem to me in perfect harmony. Further, I can conceive of no agency intermediate between an Infinite Deity and His works. Either all the phenomena of the material universe are the immediate results of His will, or they have no dependence upon it whatsoever. In the former case, the “ laws of Nature ” are simply expressions of what we know or imagine as to the mode of operation of that Will. In the latter they are nothing else than concise statements of comprehensive truths established by observation. In neither case can the “ laws ” be conceived to have any real force or agency in themselves ; such as is attributed to them by those who speak of the Deity as framing laws for the Universe, and then leaving them to their own independent operation. This mode of viewing the subject has probably originated from the supposed analogy of human legislatures ; but this analogy, when carefully examined, not only fails to support such a doctrine, but sustains that for which I have argued. For a human “law” is nothing else than an expression of the will of the governing power ; and its action upon the community entirely depends upon the constant though silent operation of that will. Let the governing power be overthrown by political convulsion, and the laws no longer exert any controlling agency. 4 GENERAL PHYSIOLOGY. collection and generalisation of the latter, the science of Physiology (strictly so called) is built up. 7. The obstacles which interpose themselves to the prosecution of these sciences, result more from that difficulty in the ascertainment of facts and the observation of phenomena, which is occasioned by the peculiar condi- tions of living beings, than from any incapability on the part of these facts and phenomena to be comprehended within laws as stable and as definite as those of the physical sciences. Thus, although the structure of the human body has been carefully and minutely examined by so many thou- sands of Anatomists, many points are still uncertain, and much still re- mains to be discovered. Yet this structure is but one of those groups of instances (the Baconian term for 'phenomena') that must be collected from the many hundred-thousand species of Plants and Animals, which the Naturalist knows to exist on the present surface of the globe, or whose remains are disinterred from its depths by the Geologist, before we can form anything more than a limited and imperfect idea of the wondrous plan, on which it has been thus peopled with living inhabitants. 8. The difficulties that present themselves in the observation of the facts which it is the object of the science of Anatomy to ascertain and gene- ralise, are as nothing to those which beset the path of the Physiological enquirer, who has to study the changes which all classes of living beings perform and undergo diming the whole period of them existence. The sum of all the phenomena which occur diming the Life of a single organ- ised structure, and which result from the actions of that structure, is, in like manner, to be regarded as a collection of facts ; of which each must be stated in a separate and concise form, before it cau be made the subject of any general expression, founded upon the comparison of similar facts derived from the study of other living beings. Now the great difficulty in physiological investigation results from the complexity of the combinations in which vital phenomena present themselves, and from their dependence upon one another to a degree that almost entirely precludes their separate examination. Were we able to ascertain the changes which take place in the interior of the living body, with the same ease that the Astronomer watches the motions of a planet, or the Chemist observes the formation of a precipitate, — the very multiplicity of these changes, and the variety of the conditions under which they occur, would be of essential service in the determination of them laws, instead of being, as at present, sources of doubt and embarrassment. The Chemist, when desirous of establishing to which of the ingredients in a given mixtime a particular effect is due, places each separately in the conditions required to produce the result : but the Physiologist finds that the attempt to insulate any one organ, and to reduce the changes performed by it to definite experimental investiga- tion, necessarily destroys, or considerably alters, those very conditions under which alone its functions can be normally performed. Take away an important and essential part of a living being, and it ceases to exist as such ; it no longer exhibits even a trace of those properties which it is our object to examine ; and its elements remain subject only to the com- mon laws of matter. We cannot, like the fabled Prometheus of old, breathe into the lifeless clay the animating fire; we cannot, by a judicious and skilful arrangement of those elements, combine them into new and artificial forms so as to produce new and unexpected phenomena ; and NATURE AND OBJECTS OF PHYSIOLOGICAL SCIENCE. 5 almost all our knowledge of the laws of Life must therefore be derived from observation only. Experiment can conduct us very little further in this enquiry, than the determination of the dependence of the functions upon one another, and upon the external agents, Heat, Light, &c., by the action of which upon the organism the phenomena of Life are produced. But a judicious and careful system of Observation will almost supply the place of experiment ; for the ever-varying forms of organised beings by which we are surrounded, and the constantly-changing conditions in which they exist, present us with such numerous and different combinations of causes and effects, that it must be the fault of our mode of study if we do not arrive at some tolerably definite conclusions as to then - mutual rela- tions. To use the language of Cuvier with some extension, all the different forms of living beings may be regarded as “ so many kinds of experiments ready prepared by Nature, who adds to or deducts from each of them different parts, just as we might wish to do in our laboratories, showing us herself at the same time their various results.” 9. From such considerations as these, it will be evident that the laws of Life can only be searched for, with a probability of success, by investi- gating their operations wherever presented to us ; and that the study of Physiology can only be scientifically prosecuted (the attainment of these laws being regarded as its ultimate object), by embracing within its range the examination of the phenomena exhibited by all classes of living beings. It is a great mistake to suppose that there is anything fundamentally different in the character of the vital operations, as performed in the Animal and in the Vegetable structures, or in the simpler and more complicated organisms of either kingdom. An enlarged view of their functions, not based upon the observation of their conditions in one or two instances only, but derived from an extended examination of their performance wherever manifested, will recognise an essential conformity throughout, wherever those which are really analogous are compared. There is an obvious advantage, therefore, in commencing the study of Physiology by an enquiry into the simplest manifestations of each of those functions, which in the higher organisms are so complicated in their nature. From no such enquiry should the consideration of the Vegetable Kingdom be excluded ; for those vital functions which are performed by Plants in common with Animals, are presented by the former in a state of greater simplification than is ever exhibited by the latter ; since all the changes necessary to the support of the individual and the continuance of the species, are performed without the influence or interference of those powers which are possessed in a greater or less degree by the whole Animal kingdom. Hence the Physiologist may advantageously resort to the study of Vegetable life, for the explanation of many of the proximate causes of those phenomena, which are complicated in the higher forms of organised beings by so great a variety of secondary influences. In the pursuit of his science on this plan, therefore, the student will learn what are the essential conditions of life ; he will see the changes indispensable to its support manifested in their simplest circumstances ; he will be able to ascertain what structures are necessary to their performance, and what additions and modifications these may undergo to suit the various purposes of them existence ; and thus he will be saved the necessity of unlearning many erroneous notions, which he would almost unavoidably imbibe from the study of the complex 6 GENERAL PHYSIOLOGY. phenomena exhibited by the living human organism, if prosecuted without such guidance. Moreover, in those departments of Physiology which are capable of being elucidated by experiment, recourse may generally be had with advantage to the lower classes of Animals, and to Plants ; since that bond of union which links together, in the higher Animals, all the changes concerned in the maintenance of the vitality of the system, is in them much less close and decided ; so that particular organs may be insulated, and the study of the conditions of then actions prosecuted to a much greater extent. 1 0. The mass of facts which is gradually being accumulated relative to the structure of living beings, and to the vital phenomena they exhibit, must be classified and arranged, before they can become subservient to the pmposes of science; and this object is accomplished in different ways, according to the nature of the laws of which the philosophical enquirer is in search. Thus the Anatomist and Physiologist, whose object it is to discover the peculiarities of organised structures, then adaptations to par- ticular uses, and the conditions of the functional changes to which they are subservient, analyse, as it were, the group of facts which each living being exhibits (§ 7, 8) : pursuing then enquiries through an extensive range of objects, they classify these individual results according to their simi- larity with each other, and then obvious tendency to the same end ; or, to speak in less abstract language, they compare the individual organs and functions through all the forms of animated beings in which they are ma- nifested : and their final aim is the attainment of some comprehensive formulce, which shall express the essential characters of each of these groups of phenomena and of the sum of the whole. The object of the Naturalist, on the other hand, is to ascertain the plan according to which the combination of the separate organs into living fabrics, and their adap- tation to different modes of existence have been effected : he, therefore, viewing each organism in its totality, arranges similarly formed beings into the same group, placing as the character common to the whole the points in which they agree, and leaving the subordinate differences to be added to this common character, in order to express the qualities of an individual. This classification is but a step towards the discovery of those general laws, — expressing the manner in which the organs are combined and adapted to each other, the relative development or simplicity of each, the modifications which the typical form of each group may undergo accord- ing to the circumstances in which the being is to be placed, and various other conditions of its formation, — which it is the ultimate object of the Naturalist to ascertain; and any mode or system of classification which he may adopt, is valuable in proportion as it keeps the determination of these laws in view, and facilitates the accumulation of the knowledge by which they may be reached. The connexion between these two branches of investi- gation is so intimate, that neither can be pm-sued with any probability of success, without a considerable knowledge of the data and principles upon which the other is founded ; and he will evidently be the most likely to arrive at the discovery of important general truths in either, who includes the whole of the phenomena of Life in one extensive sm-vey. The Physio- logist refers to the Naturalist for instances in which a function is performed on the same general plan, but under a great variety of circumstances, as manifested by the adaptation of the structure of the organ to the medium GENERAL CHARACTERS OF ORGANISED STRUCTURES. 7 of existence, ( e . g. the formation of the respiratory membrane into lungs or gills) ; whilst the Naturalist refers to the Physiologist to assist him, by the examination of the history of the development of an organ, in determining its real character, to which the consideration of its form and structure alone might not lead him. It has been by this conjoint kind of investigation that the most important of the recent advances have been effected, towards a more intimate acquaintance with the plan, on which it has pleased the Creator to construct and maintain the Organised World. 11. Although the special object of the present treatise is the exposition of purely Physiological principles, it seems desirable to preface these by such an outline of the general structure and arrangement of the organs on which the phenomena of Life are dependent, as may render subsequent details respecting their functions more intelligible. We shall first con- sider, therefore, what there is peculiar in the chemical composition and physical arrangement of the particles of which organised structures are composed, and in the forms which such fabrics present. The principal varieties of the primary or elementary tissues, of which the more complex organs of Plants and Animals are constructed, will next be described and compared with one another. And thirdly, the general characters of the principal groups in each of the animated kingdoms of Nature will be pointed out, the mode in which then - individual organs ai’e arranged and combined will be explained, and their relative positions assigned. Although such knowledge is readily accessible to the student of Natru'al History, the embarras des richesses may not be a little perplexing to such as seek only that extent of it, which will enable them to enter upon the study of Physiological Science, without being immediately checked by the want of this kind of information. CHAPTER II. OF THE GENERAL CHARACTERS OF ORGANISED STRUCTURES. 12. The changes which constitute the Life of every animated being, take place through an instrumentality which is peculiar to the bodies that exhibit such changes, and which is entirely different from anything we observe in the surrounding universe. These bodies are invariably com- posed of what is termed organised structure, a designation implying that it consists of separate parts or organs, each of which is adapted to perform some distinct part in the Vital economy. The whole organised structure of any living being is termed its organism ; and the word organisation is used to imply that peculiar process by which the organism is constructed out of the materials supplied by the inorganic or mineral world, and some- times also to indicate the state or condition of the matter upon which this process — one of the most remarkable of all the vital actions — has been effected. When we come to enquire into the nature of the functions which these organised structures perform, it will be seen that they all tend towards a common object — the maintenance of the integrity of the fabric. And it may be regarded therefore as the peculiarity of an organism, that its distinct parts or organs are destined thus to subserve, each in its own particular GENERAL PHYSIOLOGY. way, some general purpose. This, indeed, is one of the peculiarities which distinguish organised structures from inorganic matter ; for in a mineral, every particle possesses a separate individuality, and whatever changes the whole may undergo in obedience to physical agencies, such changes are the result of their separate and independent influence on each integer • whilst in a living being, the actions of all parts of the machine are so con- nected together, that whatever influences one single particle of the organism on which these actions depend, will more or less affect the entire system. Thus, we may suppose a mass of gold alloyed with a small quantity of silver, and immersed in nitric acid ; this chemical agent will affect every particle of the silver as completely as if the mass consisted of nothing else, and will leave the gold in its previous condition, having of itself no power of dissolving it. On the other hand, a similar chemical agent applied to an organised structure, will not only destroy the integrity of the part itself, but will produce a disturbance of the general functions proportional to the importance of the organ which has been injured ; and when we come to trace the influence of any of the external agents on which life is dependent (see chap, iii.) we shall find that this is exercised not only on the parts with which they are in immediate relation, but through them on the entire organism. 13. But it may be said that this is no more than takes place in any engine of human construction, or in the complicated machine of the universe, — that in these, as well as in living bodies, there is an adaptation of parts to each other, and of their actions to some general purpose, — and that all forms of matter are possessed of properties, by whose mutual influence changes may be produced, as regular, as ceaseless, and as con- nected, as those which living beings exhibit. Thus, the uniform motions of the heavenly bodies, the alternation of the seasons, the continual alter- ations which the crust of the earth is undergoing, the earthquakes and volcanic eruptions which so remarkably excite our attention to those alterations, and which may be regarded as more prominent indications of the same latent causes, the successive evaporation and condensation of the w T atery covering of the globe, the perpetual variations in the force and direction of the wind, the occasional recurrence of those violent meteoro- logical changes which spread terror and desolation wherever they occur, but which serve such important purposes in the economy of Nature, — all these phenomena, and many more which might be instanced, result, no less than the constant changes exhibited by the animated creation, from that adaptation of parts to a whole, which is so characteristic of Design in the universe at large. Hence some philosophers have gone so far as to embody these phenomena into a general expression — the Life of the World ; and as far as the abstract meanino- of the term is concerned, it would be difficult to show that a single piece of mechanism, or the entire universe, is not organised as completely as any animated structure. But a little consideration will show that, in the construction of a machine, man avails himself only of the ordinary or physical properties of matter ; and that, in the most complicated arrangement of its parts, each single element possesses only the same capabilities as it would have if separated from the rest. Thus, the moving power of a clock is given by the gravitation of a weight, — that of a watch by the elasticity of a spring, — that of a steam- engine by the expansion and condensation of watery vapour ; and all the GENERAL CHARACTERS OF ORGANISED STRUCTURES. 9 rest of the mechanism is contrived only to give effect to these agencies, by employing them in the manner most advantageous for the designed end. In the phenomena of the universe, again, we see nothing but the agency of the ordinary physical properties of matter. Thus the motions of the solar system are all dependent on that universal property of matter — gravitation — which, originally balanced against other forces, will continue to produce the same effects as long as may be consistent with the designs of the Creator. By these motions are produced the variations of climate and season in our own globe ] and from the same property which causes them, results the constant movement of the waters of our ocean. By the heat of the central luminary, again, are probably occasioned, either directly or indirectly, most of the atmospheric changes which are of such conse- quence to the well-being of the plants and animals that people this earth ; and the same agent has a most important and direct influence on the phenomena of their growth and development. But these changes are only exhibited, when different material bodies are brought into such a relation with one another, as shall call their respective properties into action ; and so long as any mass of inorganic matter is placed out of the pale of their influence, it may remain perfectly unchanged for an indefinite period. 14. When, therefore, we compare the constant activity which we encounter in living organised beings, with the passive condition of inor- ganic matter, we are compelled to conclude, that to whatever extent the forces which control the latter contribute to the actions going on in the former, there must be additional forces resulting from the operation of properties, to which we know nothing analogous elsewhere. The degree in which these superadded forces harmonise or interfere with those com- mon to other forms of matter, constitutes a fair and highly interesting branch of enquiry which will hereafter be pursued, (chap, iii.) But it is at present sufficient to state that, since these properties are never exhibited by any forms of matter except those usually denominated ‘ organised,’ our notion of an organised structure is founded not only upon the adaptation of its parts to one another, but upon the indisputable possession by each part, of independent properties, by which it is enabled to execute actions for which physical laws will by no means account. And the process of organisation implies, therefore, not only the conversion of the homogeneous materials into regular and complex structures, but the simultaneous development of vital properties in the organisms thus generated. 15. Although in most Animal and Vegetable fabrics there are many different kinds of organised tissue, differing from one another both in structure and properties ; and although these present differences according to the class of beings in which the examination is made ; — yet there are certain general peculiarities by which all are seen to be characterised, when contrasted with mineral or inorganic bodies ; and these peculiarities are as manifest in the humblest and simplest member of the animated creation, as in the most elevated and complex. It has been a favourite attempt with some naturalists, to trace a regular gradation or scale through the whole material universe ; and not only to prove that the line of separation is indistinct between the Animal and Vegetable kingdoms, but to show that there is not such a complete division between the Organised and Inorganic world as physiologists think themselves justified in erecting. It is doubtless true that the discoveries of modern science are constantly 10 GENERAL PHYSIOLOGY. bringing to light connecting links, which were previously deficient in many parts of the chain ; and that, in particular, an increased acquaintance with the various races of Animals and Vegetables which formerly inhabited this earth, through many successive epochs, seems likely to fill up what- ever chasms are left open between the groups at present existing. But it is no indication of a philosophical spirit to attempt to discover relations where none can by possibility exist. The simple cells which constitute the humblest forms of Vegetation (§ 36), as well as the most minute and apparently least complex Animalcule, exhibit, when carefully examined, all the characteristics of organised structure, as well as all that can be regarded as peculiar in vital actions. They grow from a germ, increase, reproduce then 1 kind, die, and decay, as regularly as any of the higher members of their respective kingdoms ; and they present the same peculiar and definite arrangement of particles, the same combination of fluid and solid materials, the same mutual adaptation of organs, as the latter possess. 16. However close, therefore, may be the links by which the Animal and Vegetable kingdoms are connected together, the relation is confined to them ; and between Organised fabrics, and the products of Crystallisa- tion, (or of any other mode of aggregation by which Inorganic matter is held together, in masses great or small,) there is a total want of resem- blance, either in structure or in properties. — To show in what the state of Organisation essentially consists, it will be necessary to contrast, in more detail, the peculiarities common to all beings which exhibit it, with the condition of Inorganic or Mineral bodies. 17. Wherever a definite form is exhibited by Mineral substances, that form is bounded by straight lines and angles, and is the effect of the pro- cess termed crystallisation. This process results from the tendency which evidently exists in particles of matter, especially when passing gradually from the fluid to the solid state, to arrange themselves in a regular and conformable manner with regard to one another ; and there is, perhaps, no inorganic element which is not capable of assuming such a form, if placed in circumstances adapted to the manifestation of this tendency among its particles. The mineralogist is conversant with an immense variety in the forms of crystals j these, however, may all be reduced to a few primary types, from which the mathematician can deduce the rest. But, on the other hand, if the particles be not placed in circumstances favourable to this kind of union, and the simple molecular attraction, or attraction of cohesion, is exercised in bringing them together without any general control over their direction, an indefinite and shapeless figure is assumed. — Neither of these conditions finds a parallel in the Organised creation. From the lowest to the highest of living beings, the shape is determinate for each species or race ; and this shape is such, that, instead of being cir- cumscribed within plane surfaces, right lines, and angles, organised bodies are usually bounded by convex surfaces, and present curvilinear outlines. It is true that, in the Vegetable kingdom, and to a certain extent among Animals also, we find a considerable difference in the external configuration of individuals of the same species ; but this difference is restrained within certain limits, and may usually be referred to the influence of external causes. (§ 71.) It is generally of the lowest tribes of both kingdoms, that we find the typical form of each species least definite, and the ten- GENERAL CHARACTERS OF ORGANISED STRUCTURES. 11 dency to irregular departures from it the greatest ; and there seems reason to believe that among these the same germ may assume a variety of dis- tinct forms, according to the circumstances under which it is developed ; and thus is presented an approach, on the one hand, to that entire inde- finiteness which is characteristic of uncrystalline mineral masses ; and, on the other, to that variety of crystalline forms which the same mineral body may present, according to the conditions under which its crystal- lisation takes place. 18. With regard to size, again, nearly the same remarks apply. The magnitude of Inorganic masses is entirely indeterminate ; being altogether dependent upon the number of particles which can be brought together to constitute them. On the other hand, the size of Organised structures is restrained, like their form, within tolerably definite limits, which may nevertheless vary to a certain extent among the individuals of the same species. These limits are least obvious in Vegetables, and in the lower classes of Animals. A forest-tree may go on extending itself to an almost indefinite extent ; certain species of sea-weed attain a length of many hundred feet, and their growth does not appear to undergo any check ; and the same may be said of those enormous masses of coral, of which so many islands and reefs in the Polynesian Archipelago are composed, or of which the debris seem to have constituted a large proportion of the cal- careous rocks of ancient formation. But in these cases the increase is produced, not so much by the consecutive development of the individual, as by the continued production of new individuals, which remain in con- nection with the original and with each other, — the older portion of the mass having not unfrequently perished, whilst the younger is in a state of active growth. Thus, each bud of a tree may be regarded as a distinct individual, because, if placed under favourable circumstances, it can main- tain its life by itself, and perform all the actions proper to its species ; and consequently the almost indefinite extension of a tree by the multiplication of buds is not so much an act of growth as of reproduction. The case is nearly the same with regal’d to the extension of a sea-weed ; for although it does not present the same obvious separation into distinct elements as we notice in the tree, yet almost every part of its lengthened frond is but a repetition of other similar' parts, and may continue to exist independ- ently of them. In the coral-mass the resemblance to a tree is more com- plete ; for its extension is effected by the multiplication of polypes by a process of budding from the original ; and thus the entire structure is rather to be regarded as an assemblage of individuals, than as a single body. Hence the form and size of the aggregate mass may present very wide extremes of variation, while that of the individuals may be as con- stant as it is among the higher orders of organised beings. 19. It is, however, in the internal arrangement or disposition of the parts, of which Organised structures and Inorganic bodies are respec- tively composed, that we find the difference between the two most strongly marked. Every particle of a Mineral body (in which there is no mere admixture of ingredients) exhibits the same properties as those possessed by the whole ; so that the Chemist, in experimenting with any substance, cares not, therefore, except as a matter of convenience merely, whether a grain or a ton be the subject of his researches. The minutest atom of carbonate of lime, for instance, has all the properties of a crystal 12 GENERAL PHYSIOLOGY. of this substance, were it as large as a mountain. Hence we are to regard a mineral body as made up of an indefinite number of constituent par- ticles, similar to the whole and to each other in properties, and having no further relation among themselves than that which they derive from their juxtaposition. Each particle may be considered, therefore, as having a separate individuality. The Organised structure, on the other hand, whether of a Plant or Animal, is made up of a number of dissimilar parts or organs, each of which has a peculiar texture and consistence j and it derives its character from the wdiole of these collectively. By their action with each other, and with external agents, the Life of the entire fabric is sustained ; and hence there is a relation between its elementary consti- tuents much closer than that of proximity only, namely, that of mutual dependence ; so that, as no one part can continue to exist without the rest, it cannot be regarded as possessing that separate individuality which belongs to the whole system alone. Thus, in the perfect Plant which has roots, stem, and leaves, the entire destruction of any one set of organs is fatal to its life ; although, as many of these organs are but repetitions of each other, some of them may be removed without permanent injury to it, provided enough are left to maintain its present existence. 20. In the lower Animals, as in Vegetables, we find a marked tendency to the repetition of similar parts, which cannot themselves exist indepen- dently of the entire fabric, although a part of them may be removed without serious detriment to it. This is especially the case among the Radiated and the lower Articulated tribes. It is not uncommon, for example, to meet with specimens of the common five-rayed Star-fish, in which not only one or two, but even three or four, of the arms have been severed, without the destruction ot the animal’s life ; and this is the more remarkable, as the arms are not simply organs of locomotion or prehen- sion, but contain prolongations of the stomach. And in the Tape-worm, a large number of the joints of the body, each containing a portion of the alimentary canal and of the generative apparatus, may be detached with- out any apparent injury to those which remain in connection with the head. In the bodies of the higher Animals, however, there are few or no such repetitions, the diversity of structure and function existing among their several organs being much greater ; hence the mutual dependence of their actions upon one another is much more intimate, and the loss of a single part is much more likely to endanger the existence of the whole. Such structures are said to be more highly-organised than those of the inferior tribes ; not because the whole number of parts is greater, for it is frequently much less ; but because the number of dissimilar parts is larger, and the consequent adaptation of the whole to a variety of purposes is more complete, — the principle of division of labour, in fact, being carried much further, a much larger range of purposes being kept in view, and a much greater elaborateness in the accomplishment of them being provided for. 21. Keeping in view, then, what has just been stated in regard to the divisibility of a Tree or a Zoophyte into a number of parts, each capable of maintaining its own existence, we may trace a certain gradation from the condition of the Mineral body to that of the highest Animal, in regal'd to the character in question. Thus, the individuality of a Mineral sub- stance is sometimes said to reside in each molecule, for every one is an ex- GENERAL CHARACTERS OF ORGANISED STRUCTURES. 13 act repetition of the rest • that of a Plant or a Zoophyte in each member that possesses, or can develope, the organs essential to the maintenance of life ; and that of one of the higher animals in the sum of all the dissimilar components. The distinction is much greater, however, between even the lowest Organised fabric and any Mineral body, than it is between the highest and the lowest organised structures ; for the existence of even the least diversity of structure in the several parts, and of the slightest mutual dependence between their actions, is sufficient to indicate an essential difference between organisation and mere aggregation ; whilst the differ- ence between the highest Animal and the lowest Plant is essentially one only of degree, as will appear from the details hereafter to be given. 22. Between the very simplest Organised fabric and every form of Mineral matter, there is a marked difference in regard to intimate structure and consistence. Inorganic substances can scarcely be regarded as possess- ing a structure ; since (if there be no admixture of components), they are uniform or homogeneous throughout, being composed of similar, particles, which — whether simple or compound, solid, liquid, or gaseous, — are field together by attractions which affect all alike. Far different are the characters of Organised structures ; for in the minutest parts of these may be de- tected a heterogeneous composition,— a combination of solid and liquid elements, which are so intimately blended as to impart peculiarities to the tissues, even in regard to their physical properties, of a kind which we never encounter amongst Mineral substances. In the latter, solidity or hardness may be looked upon as the typical condition ; whilst of the former, softness (resulting from the large proportion of fluid components) may be considered the distinctive quality, being most obvious in parts that are most actively concerned in vital operations. 23. The greatest solidity exhibited by Organised fabrics is found where it is desired to impart to them the simple physical property of resistance ; and this is attained by the deposition of solid particles, usually of a mineral character, in tissues that were originally soft and yielding. When mineral matter is thus introduced into the fabric, it appears in some instances to enter into actual combination with the organic base, as seems to be the case in bone ; whilst in other instances it is deposited per se in the interstices of organised tissue, and is then sometimes amorphous, sometimes crystalline, in its aggregation, — the former being the case with the particles of carbonate of lime which are dispersed through the fibrous basis of the shell of the fowl’s egg (§ 160), whilst the latter condition is seen in the solid columns of the same material which occupy the prismatic cells of the shells of many Mollusks (§ 197). Most of the higher Plants have a greater or less proportion of carbonate and oxalate of lime diffused through their tissues ; the latter most commonly in the form of rapliides, or long needle-shaped crystals, which are collected in bundles in the interior of particular cells ; the former being generally dispersed in rhombohedral crystals in the interstices of the tissues.* A dense incrus- tation of carbonate of lime is found on the exterior of the tubular cells of many of the Cliaracece, which hence derive then- popular name of ‘ stone- * The oxalate of lime is converted into the carbonate by calcination, as are also the compounds of lime with vegetable acids which are dissolved in the juices of the living plant ; hence we are not to regard the amount of carbonate of lime in the ash as any measure of the quantity which existed in the solid form in the living fabric. 14 GENERAL PHYSIOLOGY. worts.’ But the most remarkable example of consolidation of vegetable texture by this material is presented by the true Corallines and Nuttipores, the greater part of whose fabric is rendered of stony hardness, by the large amount of carbonate of lime deposited in then - tissues. In the Grasses and Equisetacece, a large quantity of silicious particles, retaining then- crystalline form, is deposited beneath the epidermis, imparting great strength to the integument. In Animal structures, earthy depositions are usually more concentrated in particular spots, especially where the loco- motive powers are considerable ; since it is obviously essential to the exercise of those powers, that, whilst the frame-work which gives attach- ment to the organs of propulsion should be solid and unyielding, these organs themselves, as well as other parts of the fabric, should be capable of great freedom of action. In the higher animals, therefore, we find carbonate and phosphate of lime deposited in special situations, so as to give a firm basis for the attachment of softer structures ; the former ingredient predominating where the skeleton is massive and external, serving chiefly for the protection of an inert body, as in the Mollusca in general ; whilst the latter, of which certain mineral forms are among the hardest substances known, is chiefly met with where the skeleton is enclosed within the softer parts, as in Vertebrated animals, and where concentration of bulk without diminution of strength is therefore an important object. But there are some among the lowest, in which the adaptation for locomotive power is no object ; and here we find the struc- ture even more universally penetrated with calcareous matter, than that of vegetables. Thus, the masses of Coral, which were long supposed to be constructed as habitations by the Polypes found in connection with them, are now known to be produced by the deposition of calcareous matter in the soft jelly-like substance which constitutes the real flesh of the animal, and are therefore to be considered as properly belonging to the organised fabric itself (§ 194). 24. The pai’ts in which depositions of this kind take place, however, are those which participate least in the operations which are essentially concerned in the maintenance of life. They may be from the first de- veloped simply for the physical uses already adverted to ; as is the case with the bones, shells, &c., of Animals. Or they may be tissues which have served for a time some more important purpose in the economy, but which, instead of then wasting by decay, are consolidated by interstitial deposit, and are thus rendered subservient to a mechanical purpose, at the very time that they cease to participate in the proper Life of the system. This is the case, for example, with the woody structure; which, when first formed, and in the condition of ‘ sap-wood,’ assists in transmitting the nutritious fluids through the stem ; but which, when condensed into ‘ heart-wood,’ and acting as the skeleton of the fabric, can no longer serve that purpose. So the soft succulent tissue, which, when newly formed at the extremities of the rootlets, is known as the spongiole, and serves as the special organ for the absorption of fluid, gradually becomes condensed into the hard substance of the root, and henceforth merely takes a share in the mechanical office of fixing the stem, its original function being per- formed by newer tissue generated beyond it. This is the case, too, with the Corallines and Nullipores, the growing parts of which are always soft ; and also with the Corals, whose soft fleshy matrix is continually GENERAL CHARACTERS OF ORGANISED STRUCTURES. 15 extending itself above, whilst it is progressively converted into an almost homogeneous stony mass below.— In contrast with these, we may advert to that texture of animals, whose functions are the most important, and the fiu’thest removed from anything which we meet with in the domain of Physics, namely, the Nervous matter ; this is the softest and most decomposable of all the tissues of the body, and is continually wasting by decay in proportion to the demands upon its functional activity, whilst in the healthy fabric it is regenerated no less constantly and ra- pidly. — As a general rule, then, it may be stated, that wherever organised fabrics approach mineral bodies in solidity and homogeneity of structure, they approach them also in the purely physical nature of their endow- ments, and in the want of those properties which are distinctive of living bodies. Whilst, on the other hand, these vital properties are most remarkably exhibited by textures and organs, which, in the heterogeneous- ness of their structure, present the greatest dissimilarity to mineral bodies. 25. Organised structures are further distinguished from Inorganic bodies, by the pecidiarity of their chemical constitution. This peculiarity does not consist, however, in the presence of any elementary srdistances which are not to be found elsewhere ; on the contrary, all the elements of which organised bodies are composed, exist abundantly in the world around. When the vast differences in the endowments of animated and of inert matter are considered, it appears at first sight surprising that the same elements should present themselves in both ; but a little consideration will show, that this fact is a necessary result of the mode in which organised fabrics are generated. For the parent communicates to its offspring, not the structure itself, but the power of forming that structure out of the materials supplied to it from external sources, — these materials being- derived by Plants directly from the Mineral world, whilst Animals obtain them through the intermediation of Vegetables. The Chemist enume- rates sixty-one elementary or undecomposed bodies as existing in the solid crust of the globe, and as forming, by their union in different propor- tions, the various mineral substances of which it is composed. Of these, not more than eighteen or nineteen have been detected in organised struc- tures ; and several of them appear to be rather accidentally than neces- sarily present there. In fact, although the known Organic compounds are at least equal in number to those of the Inorganic world, and surpass them in variety of properties, yet the number of elementary substances which appear essential to then- composition is not above seven or eight. Again, the bulk of the Mineral kingdom consists of the metals and their com- pounds, which form the earths, alkalies, and some of the acids ■ and these, when present in living bodies, rarely form part of their tissues, but are either held in solution in their juice for some purely chemical pur- pose, or are deposited (as already explained) in a completely unorganised condition to serve some mechanical object. On the other hand, the chief components of the organised tissues of Plants and Animals, and of the products of their activity, are the four non-metallic substances, Oxygen, Hydrogen, Nitrogen, and Carbon ; and of these the first three in their "un- combined state have a gaseous form, whilst the last is never met with in nature as a solid body, except under circumstances which lead us to regard it as derived from Vegetable or Animal substances. The two substances 16 GENERAL PHYSIOLOGY. next in importance as constituents of living bodies, are Sulphur and Phosphorus, also non-metallic. And Lime and Iron, which belong to the class of Metallic substances, may be regarded as the lowest in importance of all the components essential to organised structures ; their presence being much less universal than that of the preceding, and the purposes to which they appear to be subservient having a close relation to physical and chemical agencies. Thus Lime, even when it enters into organic union with the animal basis of bone, is only destined to impart solidity to the structure ; and Iron, whose chief seat is in the red corpuscles of the blood (§ 176) may be removed from the substance of which it is a compo- nent, without any other obvious change in its properties than chemistry can account for.* 26. In considering the distinctive Chemical characters of the sub- stances, formed at the expense of the elements just enumerated, which are met with in the Vegetable and Animal fabric, it is proper to bear in mind the conditions under which they occur in the organism. — Thus in the circulating juices of the Plant, or stored up in the neighbourhood of its growing parts, we meet with certain nutritive materials, as Gum, Sugar, and Starch; substances which have a close chemical relation to each other, and which are all capable (as it would appear) of undergoing organisation, and of being converted into living tissue. So, again, in the blood of the Animal, and set apart in the egg, we find the substance termed Albumen; which appeal’s to hold a similar relation to its organised fabric, being the pabulum at whose expense its tissues are formed. These must be regarded as purely Chemical compounds, in which no properties of a truly Vital nature are yet discernible ; and they are, as such, proper subjects of analytical investigation. — But, secondly, the living organised tissues of Plants and Animals must be regarded in a veiy different light. So long as they retain their distinctive attributes, they do not appeal* subject to the laws of ordinary affinity; and it is only when these attributes have ceased to manifest themselves, that the Chemist can bring them within the sphere of his research. Then* constitution as living tissues, therefore, cannot be expressed by formulae, nor can their pro- perties be accounted for by analysis; and their chemical relations can only be judged of by the nature of the materials at whose expense they grow, and by the products of their ultimate decomposition. In this manner it seems to be determined that the walls of the cells, vessels, and hollow fibres of Plants are composed of a substance, to which the name of Cellulose has been given, closely related in composition to gum, sugar, and starch. And in Animals a large proportion of the tissues formed upon the cellular type (§ 156) may be regarded as composed of Fibrin, which seems nothing else than organised albumen, and which presents itself in an incipient state in the blood ; whilst the tissues formed upon the simple fibrous type (§ 161) are resolvable into the substance termed Gelatin, which, though not to be detected in the nutritious fluids, is undoubtedly * It is a fact of some interest, that the Combining Equivalents of all these substances are, according to the latest and most exact researches, exact multiples of that of Hydrogen. Thus, Hydrogen being taken as 1, Oxygen is 8, Carbon 6, Nitrogen 14, Sulphur 16, Phosphorus 32, Calcium 20, and Iron 28. Out of the forty-eight remaining elementary substances whose combining equivalents are known, there are not above seven of which the same can be stated even with probabilitj* ; these are, Arsenic, Cerium, Lanthanum, Mercury, Potassium, Silver, and Uranium. GENERAL CHARACTERS OF ORGANISED STRUCTURES. 17 formed at the expense of Albumen. — But from the original nutritive materials, in the third place, there is formed a class of bodies which are never destined to undergo organisation, but which are designed for various ulterior purposes in the economy of the being itself or of some other, and are deposited in the interior of the cells, or in the interspaces of the tissue. Such are the liguine, which gives solidity to the woody tissues of Plants; the oils, resins, alkaloids, acids, colouring-matters, &c., which they generate and set apart; as well as a large share of those albuminous compounds, which seem to be prepared by the Vegetable kingdom expressly for the maintenance of the Animal world, the pro- portion which answers any direct purpose in the economy of the plant being apparently very minute. So in the Animal body, we find olea- ginous compounds laid up in certain cellular textiues for special purposes ; whilst other cells contain homy matter, in others are found colouring- particles, and so on. All these substances are formed at the expense of those of the first class ; and there is no reason to consider them as having any other than purely chemical relations. They have not entered into the composition of the living tissues, and they do not appear capable of undergoing organisation; and the progress of research renders it highly probable, that the agencies by which they are elaborated are of a purely chemical nature, acting under that peculiarity of conditions which a living body can alone supply. Hence, like the substances of the first class, they rightly belong to the domain of the Chemist. — But, fourthly, we encounter even in the living structure, more especially in that of Animals, certain substances which cannot be looked upon in any other light than as the products of the continual decomposition of the nutritive fluids and of the solid textures. These, however, are never destined to be retained within the body, but are always met with on their way out of it. .Among the most characteristic of such substances are urea, kreatine and kreatinine, ammonia, and carbonic acid. The last of these is the only one which appeal’s to be regularly thrown off as a product of decom- position from Vegetable structures. Of the purely chemical character of all these, there can be no doubt whatever; the existence of all of them being the result of the tendency of the higher organic compounds to pass into simpler and more stable forms, when entirely freed from the influence of vitality, and subjected to the ordinary conditions of matter; and most of them being capable of synthetical production in the laboratory of the Chemist. 27. Of the substances belonging to the first three classes, it may be remarked that their atomic constitution is far more complex than that of Inorganic compounds generally; in regard alike to the number of the elements which they contain, and to the number of atoms or combining- equivalents of these, which exist in a single combining equivalent of the compound. Thus all the substances included in the first and second classes, and all of the third also, except a few whose place might rather seem to be in the fourth, consist at the least of three elements, Oxygen, Hydrogen, and Carbon; another, Nitrogen, is present in a large propor- tion of them ; and in addition, Sulphur and Phosphorus appear in many instances to be essential constituents. Again, in Sugar, which is one of the simplest of all organic bodies in its composition, we find 12 eq. of Carbon united with 10 of Oxygen and 10 of Hydrogen; a proportion 18 GENERAL PHYSIOLOGY. which is common also to gum, starch, and cellulose. In the nitrogenous compounds, the number of combining equivalents is much greater ; thus the proportion of the four principal constituents of albumen is repre- sented by Liebig and Mulder as follows; — Liebig.. .. 49 Carbon, 36 Hydrogen, 6 Nitrogen, 14 Oxygen, Mulder ... 40 Carbon, 31 Hydrogen, 5 Nitrogen, 12 Oxygen: and if the equivalents of sulphur and phosphorus, which are present in comparatively minute proportions, are to be represented by whole numbers, then it will be requisite to multiply the preceding formulae by twenty throughout. So far as is at present known, these very complex combinations cannot be resolved into others of a simpler nature, without such a complete disruption as precludes that re-formation of the original product by the synthesis of its components, which can be always effected in the case of inorganic compounds. Thus, whatever may be the mode in which the atoms of Sulphur, Oxygen, and Potassium are disposed in the salt which we call Sulphate of Potash (concerning which Chemists are not at present in accordance), we may separate this salt into two substances of opposite characters, sulphuric acid and potass, by bringing which together, we reproduce the salt ; and each of these substances may be again resolved into its component elements, the sulphuric acid into sulphur and oxygen, the potass into potassium and oxygen, and each may be regenerated by the synthesis of its elements. Nothing of this kind can be accomplished in regard to the saccharine, albuminous, or gelatinous compounds. Thus although sugar might be said to be made up of carbon united with the elements of water, no method of analysis with which we are acquainted could separate the carbon and the water, without the addition of elements which shall form new products whose sum does not represent the original ; still less can the synthetical method enable us to obtain sugar, either by the direct combination of carbon with water, or by the union of its elements under any other form. So, again, the utmost that has been yet accom- plished with regard to the nitrogenized compounds, has been to produce various new substances by decomposing them with powerful reagents ; and no reunion of these substances, or of the elementary components into which they are resolved by ultimate analysis, can reproduce the original. 28. With regai’d, however, to the substances comprised in the third class, the Chemist has been more successful ; many of them being arti- ficially producible from those of the first class or from each other ; and many being capable of separation into two or more components, from which the original may be reproduced by synthesis. Thus from Salicine, the alkaline base of willow-bark, may be obtained the fragrant essential oil of Spiraea ulmaria or “ queen of the meadows,” by treating it with bichromate of potass and sulphuric acid, whereby it receives two additional equivalents of oxygen, and is resolved into oil of Spiraea and grape sugar. The latter product, by a further oxidation, is converted into formic acid (naturally secreted by the red ant as a defensive fluid,) carbonic acid, and water. — So, again, valerianic acid, to which Valerian-root owes its pungent foetid odour, may be obtained by the action of caustic potass on the hydrated oxyde of arnyle, which is an analogue of alcohol, obtained during the distillation of spirit from fermented grain or potatoes. — The essential oil of Mustard may be converted into that of Garlic, by dropping into the former a piece of potassium, which subtracts from it two atoms of carbon, GENERAL CHARACTERS OF ORGANISED STRUCTURES. 19 one of nitrogen, and one of sulphur; so as itself to become sulphocyanide of potassium, whilst the residuary atoms undergo a new arrangement in virtue of which they form oil of garlic. — And the essential oil of the Gualtheria procumbens (a beautiful ericaceous plant of New Jersey), which has become an article of commerce on account of its beautiful perfume, combining the odours of hyacinth and cinnamon, may be obtained by distilling salicylic acid (which may be procured by heating either indigo, salicine, or the oil of spiraea, with caustic potass) with pyroxylic spirit or wood naphtha, and a little sulphuric acid.* — There appears strong reason to believe that even vegetable alkaloids may be artificially generated after a similar fashion. Thus when bran is treated with sulphuric acid, an oily substance termed Furfurol may be distilled over ; and this, when brought into contact with ammonia, becomes transformed into a solid slightly crys- talline substance, which is easily decomposable again into ammonia and furfurol. But when this substance is boiled with weak caustic potass, a rearrangement of its atoms appears to take place without any change in their relative number ; and it now becomes a stable and powerful alkaline base, forming a regular series of salts with acids. — Again, it has been found that Piperine, the peculiar constituent of pepper, may be regarded as a neutral combination of a nitrogenous acid with the alkaline base Aniline which may be obtained from indigo, gas-tar, and a great variety of sub- stances ; piperine being producible synthetically by the union of the acid and the base. In like manner, Narcotine may be separated into a non-nitrogenous acid and a peculiar - base. Little doubt is entertained by Chemists, that all the alkaloids having high equivalents will thus be separated into simpler compounds which may be common to many of them ; and it is anticipated that some of the rarer and more valuable among them, such as quinine, morphine, Ac., may be generated from othera which are more abundant or less serviceable in themselves. — Still in regard to all such products, it must be borne in mind that they cannot be generated by the synthesis of their elements, in any mode with which the Chemist is acquainted, or by any means of which he can at present even conceive ; it being requisite that he should have, as the basis of Iris operations, some compound which has been formed by the agency of a living body. 29. The substance of the fourth class stands in a much closer relation to inorganic compounds, than do any of the preceding ; as regards alike the simplicity of their composition, the readiness with which they may be resolved into compounds of a yet simpler nature, and their capability of being formed by synthesis. Thus Urea, -which is one of the products of the decomposition of the muscular tissue, and which is being conti- nually excreted in large amount from the animal body, is found by ulti- mate analysis to be a compound of 2 eq. of Carbon, 2 Nitrogen, 4 Hydro- gen, and 2 Oxygen. When strongly heated, however, it is resolved into cyanic acid and ammonia ; and by the recombination of these two sub- stances with an equivalent of water (CyO + NH 3 + HO), an artificial urea is produced. It has not yet been found possible thus to form kreatine or kreatinine (which also are products of decomposition, excreted in the urine) by the artificial union of their elements ; but the comparative * See Dr. Golding Bird’s Lectures on Organic Chemistry in connection with Therapeutics, in Medical Gazette, 1848. 20 GENERAL PHYSIOLOGY. simplicity of their composition, their crystalline aggregation, and their behaviour with reagents, encourage the belief that they too may be found to be binary compounds, capable of being reproduced by synthesis. Such products of decomposition as Ammonia, Cyanogen, and Carbonic acid, stand on the border-line between Organic and Inorganic Chemistry, belonging alike to both domains. With regard to Cyanogen, however, it may be remarked that it has been only of late that chemists have succeeded in procuring it by the direct union of Carbon and Nitrogen ; in all the previous methods of generating it, the presence of a nitrogenous organic compound having been required. But the Carburets of Hydrogen, which are among the products of the decomposition of vegetable sub- stances, have not yet been found capable of formation by synthesis ; although the apparent simplicity of their composition encourages the belief that this will ere long be effected. 30. There is an important difference amongst the chemical compounds peculiar to organised bodies, in regard to their tendency to spontaneous change or decomposition. As a general ride, those which include nitrogen are much more prone to such changes than those which are composed of oxygen, hydrogen, and carbon alone ; and we shall hereafter see that one very important end of the presence of azotized substances in the vegetable organism, is that they may, whilst themselves undergoing decomposi- tion, effect catalytic changes in the ternary compounds (§ 39). But it is also a general rule, that the compounds belonging to the third and fourth of the classes just enumerated are for the most part much less prone to change than those of the first. Tlius horny matters and urea, which are the products of the metamorphosis of albuminous compounds, may be preserved unchanged for any length of time ; whilst albumen can only be kept from speedy decomposition, either by completely drying it, or by entirely excluding it from air, or by subjecting it to a veiy low temperature, — all these conditions being the very opposites of those under which it exists in the living body. So, again, the hgnine which gives to the heart-wood of trees its remarkable durability, oils, resins, and many other products of vegetable chemistry, are more durable in their nature than the saccharine or gummy fluids at whose expense they are elaborated. It is true that, when reduced to the solid form by drying, sugar and gum approach inorganic substances in their freedom from disposition to spon- taneous change ; but this is not the condition under which they exist in the living plant, the juices of which are prone to fermentation at mode- rate temperatures, especially when any azotized matter is present. How far the actual components of living tissues (forming our second class of organic substances) are subject to decomposition, is a question which will be more fitly considered in the next chapter (§ 46). OP LIFE, OR VITAL ACTIVITY. 21 CHAPTER III. OF THE NATURE AND CONDITIONS OF VITAL PHENOMENA. 1. Of Life, 07' Vital Activity. 31. There is no department of Philosophy around which so much unnecessary mystery has been cast, as the investigation of the character and laws of the Phenomena of Life. And this veil of mystery can only be removed by the rigorous adoption of those methods of research, which have been proved to be most effectual in the study of Physics and Chemistry. To designate any of the actions of a living body as vital, or as effected by the vital principle, was long regarded as a sufficient account of them, and had the effect of placing a complete check upon further inquiry. The history of Physical Science shows, however, that it was once labouring under the same restraint ; and that, until the true objects of investigation were understood, scarcely any advance was made. Thus, in past ages all the movements of the heavenly bodies were attributed to the operation of some vague “principle of motion,” the laws of which it was considered impracticable to attain ; and even after the empirical ingenuity of Kepler had led him to the discovery of some of these laws, it was still maintained that the powers concerned in the celestial motions could have nothing in common with terrestrial forces, — a doctrine which the genius of Newton subverted, by proving the universal applicability of the laws of motion, and the existence of the attractive force between all masses of matter. Again, the simple optical fact — that when the sun’s light passes through a hole, the bright image, if formed at a considerable distance from it, is always round, instead of imitating the figure of the aperture, — was attributed by Aristotle to the “ circular nature ” of the sun’s light ; whilst the simple consideration that the rays of light travel in straight lines, would, if properly applied, have explained this phenomenon, not only in regard to the sun, but in the case of any other round luminous body placed at a sufficient distance. To attribute all the operations of Life, which cannot be referred to Physical or Chemical laws, to a “ Vital Principle,” is in reality to proceed just as unphilosophically as the ancients did in the cases just quoted ; since a strict examination into their character will show that, although not identical with physical phenomena, they are analogous to them, in so far as they take place according to a regular plan, and present themselves under fixed conditions, a definite acquaintance with which would give to Physiological Science the same kind of precision and comprehensiveness as it is the aim of the Physical philosopher to attain in his department of study. The laws of Vital phenomena (§ 5), in fact, are as open to investigation as those which comprehend the phenomena of Gravity, Electricity, or Chemical Affinity ; and although the intricacy of the combinations under which these phenomena are usually presented to our observation (§7, 8), renders their laws more difficult of attainment, the success which has attended the philosophical method of enquiry of late pursued by scientific Physiologists, is a most 22 GENERAL PHYSIOLOGY. satisfactory proof that they are not beyond the reach of persevering and well-directed research. 32. Few terms have been employed in a greater variety of significations, or more frequently without any definite meaning at all, than the word Life. The older philosophers regarded it as a distinct entity or substance residing in certain forms of matter, and as the cause both of their organ- isation and of the actions exhibited by them.* We have seen that the tendency to rest satisfied with vague hypotheses of this kind, operated in the retardation of Physical Science ; and had it not been for the compara- tive prominence and simplicity of its phenomena, it is not improbable that, even at the present day, we should hear employed in it such terms as the ‘ Vital Principle,’ ‘ Organic Agent,’ or any other equally unphilosophieal refuge of those physiologists, who neglect the substance to grasp at the shadow. — To the term ‘principle’ no very definite meaning can be at- tached. It has been remarked that “this word, characteristic of a less advanced state of science, has been generally employed (as the final letters of the alphabet are used by algebraists) to denote an unknown element, which, when thus expressed, is more conveniently analysed.” Thus, it has been customary to speak of the ‘principle’ of Gravity, the ‘principle’ of Electricity, or of the ‘ principle ’ of Magnetism, as the unknown causes of certain phenomena as yet imperfectly comprehended. In so far’, however, as the laws of these phenomena are understood, they are really nothing else than generalised expressions of the conditions under which they occur - ; oiu- conviction of the uniformity of Nature (that is, of the unchange- ableness of the Divine plan of government, § 5, note ) leading us to expect the same result whenever the same conditions are presented. Thus, the “ law of Gravitation” is nothing else than a statement of the fact, that all masses of matter attract one another in a certain ratio ; the essential condition being, that two or more masses should exist; whilst the other conditions of relative bulk and distance determine the amormt of force exerted by each mass upon the other. So the “law of Definite Proportions” in Chemistry expresses the universal fact, that when different substances combine with each other, they do so in one out of several definite ratios ; the essential condition here being, that two or more substances capable of uniting should be brought into the requisite proximity with each other ; whilst the particular’ ratio in which they will unite is determined by a variety of other considerations, of which the Chemist seeks to arrive at some precise and comprehensive expression. And as it is the whole aim of the scientific Physiologist to attain similar general and definite expressions of the conditions under which Vital phenomena occur, it seems obvious that nothing can be really gained by the use of the term ‘Vital principle;’ whilst the employment of it is really injurious * Every sect had its own notion of the origin and nature of this entitj- ; some regarding it as a kind of fire ; others as a kind of air, or ether, or spirit ; and others, again, as merely a kind of water. The fable of Prometheus embodies this doctrine in a mythological form, the artist being described as vivifying his clay statues by Fire stolen from the chariot of the Sun. And whatever was the idea entertained as to the character of this agent, all regarded it as universally pervading the World, and as actuating all its operations in the capacity of a Life or Soul ; whilst a special division of it — divines particula aura — regulated all the concerns of each individual organism. The vestiges of these ideas may be traced in the expressions 1 vital spark,’ ‘ vital spirit,’ ‘ breath of life,’ and others which are still prevalent. OF LIFE, OR VITAL ACTIVITY. 23 in so far as it tends to limit the range of enquiry into the phenomena of Life. 33. Still more opposed to the progress of Physiological Science, is the doctrine propounded by one of the most distinguished philosophers of our age,* that Vital phenomena are to be attributed to the operation of “dis- tinct intelligent agents, superior to, and possessing the power of direct- ing and controlling, the common forces of matter.” Of these “ Organic agents” it is supposed that a series exists in every organised body ; each agent “ possessing more or less control over all the agents below itself, and having the power of appropriating their services, till at length in the combined operation of the whole series of agents at the top of the scale, we reach the perfection of organic existence.” This hypothesis, in favour of which not a single fact can be advanced, may perhaps be best disposed of by carrying out the analogy of an army, which is suggested by the phraseology just cited. As an army is at present constituted, there is large room for the operation of intelligence in the subordinate officers or delegated powers ; because it is upon them that the carrying-out of the will of the commander-in-chief entirely rests, his knowledge and skill being confined to the general plan ; and they have not only to choose the best means of effecting this, but to modify and accommodate it to circum- stances, sometimes even acting in opposition to it, should it prove danger- ous or impracticable. But if the supreme commander had a perfect control over every component individual of a large army, and possessed the means of immediately operating upon the minds of every soldier so as to cause him to execute his will without any intervening command ; and if he himself possessed the knowledge and skill requisite, not only to guide the general evolutions of the masses composing his army, but to direct the movements of every man, in the most advantageous manner ; — would he trouble himself with the intermediate machinery of subordinate generals, colonels, captains, lieutenants, sergeants, and corporals, in order to convey his will and direct the operations of his troops 1 To imagine, then, that anything is gained by the interposition of ‘ agents,’ intelligent or non- intelligent, bet-ween the Deity and the materials upon which He operates, is either to set limits to His knowledge and power, or to give to those agents an office purely nominal. For if they have any of that inde- pendence which is possessed to a certain extent by the officers of a human army, then the Almighty intelligence is not directing the operations of matter in their detail ; whilst if that detailed direction has a real exis- tence, the function attributed to them is a nonentity. 34. Any such idea of a delegated control over the actions of living- beings is not only quite useless and unnecessary in the explanation of facts, but is totally unsupported by the analogies of Nature, and by what we know of the Divine Government in general. No reflecting mind has any doubt that this earth and its inhabitants form a system, of which every part is perfectly adapted to the rest, (so that w T e might almost call it an organised one, if the idea of a particular structure were not involved in the term, § 13,) and of which all the actions and changes, however in appearance independent or even contrary, have one common tendency, the ultimate happiness of the creatures of Infinite Benevolence. It cannot be regarded as an improbability that the other spheres and systems, — whose * Dr. Prout, in Bridgwater Treatise, 3rd ed. p. 396 et seq. 24 GENERAL PHYSIOLOGY. countless multitudes, revealed by the aid of science, impress our minds with the nearest conception of infinity, of which our finite comprehension is capable,— are peopled with beings, if not similar in structure with ourselves, a least equally worthy of the Creator’s care. In the government of our own planet, itself but a point in the vast universe, we are able to recognise, to a small extent, the laws according to which its material operations are conducted ; and we discern indications of those which affect the moral condition of sentient beings. So far as we can understand the mutual adaptation of these laws, we everywhere see them working to the same end ; and we entertain the highest anticipations of that beauty and harmony which will be revealed to us, when our imperfect glimmer- ings of knowledge shall be extended and corrected by the light of Eternal Truth. Should we not consider it degrading to the dignity of Infinite Wisdom, to suppose that, at the creation of each world, He had found it necessary to delegate to a subordinate the control over its working, — instead of so devising the original plan, in all its details as well as in its general arrangements, that the various changes which it was His intention to produce, should be harmoniously effected by the uniform operation of the same all-sustaining energy ? It cannot be said that the adaptation of means and ends is less evident in the scheme of the Universe at large, than it is in the structure and actions of a single Organised being. And if it be the aim of the Physical Philosopher to discover the simple but majestic plan, of which this wonderful adaptation is the residt, why should not the Physiologist directly refer all the operations of the living body to the same primary agency, and seek, in like manner, to comprehend the scheme of its government 1 35. If the application of the term ‘ Life ’ to some independent agent, which is assumed to be the immediate cause of the peculiar attributes of Plants and Animals, be found useless or injurious, it may be reasonably in- quired what is to be understood by it. Even our most elementary notion of a living being, based upon the phenomena presented by the simplest organism that can maintain an independent existence, is so complex, as to render it impossible to express it in the form requisite for a concise definition 3 and although various attempts have been made to convey in words the essential ‘ idea ’ which it involves, none has been yet successful enough to satisfy the scientific Physiologist.* It has been already pointed out that there is a class of phenomena, to which we give the designation Vital, which is even more different in its character from those of Physics and Chemistry, than they are from each other ; and that these ‘vital’ phenomena are only manifested by bodies of that peculiar structure which we term ‘ organised.’ By the term ‘ Life,’ then, we most appropriately designate the state or condition of a being that exhibits vital actions : and it is thus placed in opposition to the term * One of the latest of these attempts is put forth in a posthumous work by S. T. Cole- ridge, entitled “ Hints towards the Formation of a more Comprehensive Theory of Life.” (London, 1848.) lie considers Life as the ‘principle of individuation,’ that is to say, as a power which discloses itself from within, combining many properties into one individual thing. In his view, however, the term must be extended to Inorganic matter; each indivi- dual molecule being endowed with a low degree of this principle, the higher forms of which are manifested in Plants, and the highest in Animals. (See § 21.) Hence the term ‘ Life’ as thus employed is no longer a distinctive one; and it would be requisite to invent some other to designate the peculiar activity of Organised bodies. OF LIFE, OR VITAL ACTIVITY. 25 Fig. 1. ‘ Death,’ which implies the state of a being in which those actions have altogether ceased, and whose structure is subject to no other forces than those of inorganic matter, which speedily effect its decomposition : whilst both are distinguished from the state which may be designated as that of ‘ Dormant Vitality,’ in which the whole vital activity of the being has been temporarily suspended, without any such impairment of the struc- ture or composition of the organism, as shall prevent the resumption of that activity when the requisite conditions are supplied. 3G. The distinctive characters of Vital actions will be best understood, if we take a general view of the series of phenomena presented by a living being of the simplest kind, from its origin to its final decay. For this purpose, we must have recourse to the humblest forms of Cryptogamib Plants, which consist of mere aggregations of cells, every one of which may be regarded as a distinct individual, since it is perfectly independent of the rest, and performs for and by itself, all the functions of Growth and Reproduction. We shall find in the operations of the simple cell, an epitome, as it were, of those of the highest and most complex Plant; which again bear a close correspondence with those that are immediately concerned in the Nu- trition and Reproduction of the Animal body. The functions peculiar to Animals will not enter into the present enquiry, not being common to all organised beings. — A Cell, in Physiological language, is a closed vesicle or minute bag, formed by a membrane in which no definite structure can be discerned, and having a cavity which may contain matters of variable con- sistence. Every such cell constitutes an entire organism in such simple plants as the Protococ- cus nivalis (Red Snow), or Palmella cruenta (Gory Dew) ; for although the patches of this kind of vegetation, which attract our notice, are made up of vast aggregations of such cells, yet they have no dependence upon one another, and the actions of each are an exact repetition of those of the rest. In such a cell, every organised fabric, however complex, originates. The vast tree, almost a forest in itself, — the zoophyte in which we discover the lowest indications of animality, — and the feeling, thinking, intelligent man — each springs from a germ, that differs in no obvious particular from the permanent condition of one of these lowly beings. But whilst the powers of this latter are restricted, as we shall see, to the continual multiplication of new and distinct individuals like itself, those of the former enable it to produce new cells which remain in closer connexion with each other ; and these are gradually converted, by various trans- formations of their own, into the diversified elements of a complex fabric. The most highly-organised being, however, will be shown to consist in great part of cells that have undergone no such transformation, amongst which the different functions performed by the individual in the case just cited, are so distributed, that each cell lias its particular object in the general economy, whilst the history of its own life is essentially the same as if it were maintaining a separate existence. 37. In the first place, then, the cell takes its origin from a germ, which may be a minute molecule not visible without a microscope of high Simple isolated cells, contain- ing reproductive molecules. 26 GENERAL PHYSIOLOGY. power.* This molecule appears, in its earliest condition, to be a simple homogeneous particle, of spherical form ; but it gradually increases in size ; and a distinction becomes apparent between its transparent exterior and its coloured interior. Thus we have the first indication of the cell- wall, and the cavity. As the enlargement proceeds, the distinction becomes more obvious; the cell- wall is seen to be of extreme tenuity and perfectly transparent, and to be homogeneous in its texture ; whilst the contents of the cavity are distinguished by then- colour, red or green according to the species. At first they, too, seem to be homogeneous; but a finely-granular appearance is then perceptible; and a change gradually takes place, which seems to consist in the aggregation of the minute granules into molecules of more distinguishable size and form. These molecules, which are the germs of new cells, seem to be at first attached to the wall of the parent-cell ; afterwards they separate from it and move about in its cavity ; and at a later period, the parent-cell bursts and sets them free. Now, this is the termination of the life of the parent-cell ; but it is the commencement of a life of a new production ; since every one of these germs may become developed into a cell, after precisely the foregoing manner, and will then in its turn propagate its kind by a similar process. 38. By reasoning upon the foregoing history, we may arrive at certain conclusions, which will be found equally applicable to all living beings. — In the first place, we witness the origin of the cell in the reproductive action of a pre-existing organism ; a phenomenon peculiarly characteristic of Life. There is no sufficient reason to believe that there is any exception to this rule. So far as we at present know, every Plant and every Animal is the offspring of a parent, to which it bears a resemblance in all essential particulars ; and the same may be said of the individual cells, of which the composite Animal and Vegetable fabrics are composed. — It does not always seem requisite, however, that a definite germ-particle should have been prepared by the parent-cell ; for there appear to be cases in which new cells originate in the midst of a homogeneous ‘ protoplasma, ’ in which no distinct germs can be detected (§ 143). Still, that fluid must have been elaborated by cell-agency ; the first stage of cell-formation consists in the appearance of aggregations of molecules, forming nuclei from which cells grow ; and these new cells bear the same relation to those which prepared the ‘ protoplasma,’ as if they had originated in distinct germs or reproductive molecules. 39. The first stage in the development of the germ or nucleus thus furnished, into a new cell, is the production of the requisite pabulum or material. This, however, is effected only in the Vegetable kingdom ; for even the simplest Animal cell appears entirely dependent for its means of growth upon a ready-prepared supply of its pabulum, elaborated either directly or indirectly by the agency of Plants. The materials which the Plant requires are simply Water, Carbonic Acid, and a small quantity of Ammonia ; for the first two supply it with Oxygen, Hydro- gen, and Carbon, in the condition requisite for the production of saccharine or gummy matter ; whilst the Ammonia furnishes azote for the albu- minous matter, which (according to late researches) is required by every * We here purposely restrict our history to one form of cell-development, that from Zoospores, seen in the Conferva (§142); other modes will hereafter be noticed (§139-141). OF LIFE, OR VITAL ACTIVITY. 27 Vegetable cell as an essential material for its development. — Now this combining process is probably of a purely Chemical nature (§ 26), the product being a mere chemical compound ; and analogous cases are not wanting, amongst the phenomena of Inorganic Chemistry, in which one body exerts an influence upon other bodies, so as to occasion their union or their re-arrangement, without itself undergoing any change. Thus Platinum, in a finely-divided state, will cause the union of Oxygen and Hydrogen at ordinary temperatures ; and finely-powdered Glass will do the same at the temperature of 572°. This kind of action is called Cata- lysis. — A closer resemblance, perhaps, is presented by the act oi fermenta- tion ; in which a new arrangement of particles is effected in a certain compound, by the presence of another body which is itself undergoing change, but which does not communicate any of its elements to the new products. Thus, if a small portion of animal membrane, in a certain stage of decomposition, be placed in a solution of sugar, it will occasion a new arrangement of its elements, which will generate two new products, alcohol and carbonic acid. If the decomposition of the membrane have proceeded further, a different product will result ; for, instead of alcohol, mannite and lactic acid will be generated. — There appears no improba- bility, then, in the idea, that the influence exerted by the germinal mole- cule is of an analogous nature ; and that it operates upon the elements of the surrounding water and carbonic acid, according to purely Che- mical laws, uniting the Carbon with the elements of Water, and setting free the Oxygen. This result of the nutritive operations of the simple cellular plants may be easily verified experimentally, by exposing the green scum, which floats upon ponds, ditches, &c., and which consists of the cells of a minute Cryptogamic Plant, to the influence of light and warmth beneath a receiver ; it is found that oxygen is then liberated, by the decomposition of the carbonic acid contained in the water. We shall presently have to return to the consideration of the Chemical phenomena of living beings ; and shall pass on, therefore, to consider those to which no such explanation applies. 40. The next stage in the process of cell-development consists in the appropriation of the new product thus generated to the enlargement of the living cell-structure ; — a phenomenon obviously distinct from the pre- ceding. It is well to observe, that this process, which constitutes the act of organisation, may be clearly made out in the higher Plants and Ani- mals to consist of two stages ; the first of these being the further prepara- tion or elaboration of the fluid matter, by certain alterations whose nature is not yet clear, so as to render it organisable, or fit to undergo organisation ; the second being the act of organisation itself, or the conversion of the organisable matter into the solid texture, and the development in it of the properties that distinguish that texture. Thus, for example, we do not 'find that a solution of Dextrin (or starch-gum) is capable of being at once applied to the development of Vegetable tissue, although it is identical in composition with Cellulose ; for it must first pass through a stage, in which it possesses a peculiar glutinous character, and exhibits a tendency to spontaneous coagulation, that seems like an attempt at the production of organic forms. And in like manner, the Albumen of Animals is evi- dently not capable of being applied to the nutrition of the fabric, until it has first been converted into Fibrin ; which also is distinguished by its 28 GENERAL PHYSIOLOGY. tenacious character, by its spontaneous coagulability, and by the fibrous structure of the clot. Now, in both these cases, there is probably some slight modification in Chemical composition, that is, in the proportions of the ultimate elements ; but the principal alteration is evidently that, which is effected by the re-arrangement of the constituent particles ; so that, without any considerable change in their proportions, a compound of a very different nature is generated. 41. Of the possibility of such changes, we have abundant illustrations in ordinary Chemical phenomena ; for there is a large class of substances, termed isomeric, which, with an identical composition, possess chemical and physical properties of a most diverse character. It is by no means sufficient, however, to attribute the production of Fibrin from Albumen, the organisable from the unorganisable material, to the simple operation of the same agencies as those which determine the production of the different isomeric compounds ; for the properties of Fibrin are much more vitally distinct from those of Albumen, than they are either chemically or physically ; that is, we find in the one an incipient manifestation of Life, of which the other shows no indications. The spontaneous coagulation of fibrin, which takes place very soon after it has been withdrawn from the vessels of the living body, is a phenomenon to which nothing analogous can be found elsewhere ; for it has been clearly shown not to be occasioned by any mere physical or chemical change in its constitution ; and it takes place in a manner which indicates that a new arrangement of particles has been effected in it, preparatory to its conversion into a living solid. For this coagulation is not the mere homogeneous setting, which takes place in a solution of gelatine in cooling ; nor is it the aggregation of particles in a mere granular state (closely resembling that of a chemical precipitate), which takes place in the coagulation of albumen : it is the actual production of a simple fibrous tissue by the union of the particles of fibrin in a determinate manner, bearing a close resemblance to the similar process in the living body (§ 158). We say, then, that the coagulation of Fibrin, and the production of a fibrous tissue, are the result of its vital properties, rather than of chemical or physical agencies; because no substance is known to perform any such actions, without having been subjected to the influence of a living body; and because the actions themselves are altogether different from any which we witness elsewhere. This production of an organisable or partially vitalised sub- stance, from an unorganisable one, possessing none but chemical proper- ties, and therefore as yet inert, so far as the living body is concerned, may be termed Assimilation ; and it may be conceived, as we have seen, to consist of a new arrangement of the particles of the substance thus changed, analogous to that which occurs when one isomeric product is converted into another by some ordinary chemical agency, — Heat or Electricity for example; but not identical with it, because it cannot be produced by any other agency than that furnished by a living structure. 42. We now come to the act of Organisation itself: which seems to consist of a continuation of the same kind of change, — that is, a new arrangement of the particles, producing substances which differ both as to structure and properties from the materials employed, but which may be so closely allied to them in chemical composition, that the difference OF LIFE, OB VITAL ACTIVITY. 29 cannot be detected. Thus, from the dextrin of plants is generated, in the process of cell-development, the membrane which constitutes the walls of the cells : chemically speaking, there seems to be no essential distinction between these two substances ; and yet between the living, growing, reproducing cell, and the inert, unchanging, starch-grain, how wide the difference! So in the animal body, we find that the composition of the fibrin of muscular fibre scarcely differs, in regard to the proportion of its elements, from the fibrin, or even from the albumen, of the blood ; and yet what an entire re-arrangement must take place in the particles of either, before a tissue so complex in structure, and so peculiar in pro- perties, as muscular fibre, can be generated ! Both in the Plant and the Animal, we find that tissues presenting great diversities both in structure and properties, may take their origin in the same organisable material ; but in most cases (at least after the fabric has attained its full develop- ment) the new tissue of each kind is formed in continuity with that previously existing. Thus in the stem of a growing tree, from the very same glutinous sap or cambium, intervening between the wood and the bark, the wood generates, in contact with its last-formed layer, a new cylinder of wood ; whilst the bark produces, in contact with its last- formed layer, a new cylinder of bark ; — the woody cylinder being charac- terised by the predominance of ligneous fibre and ducts ; and the cortical by the predominance of cellular tissue. In like manner we find that, in animals, muscle produces muscle, bone generates bone, nerve develops nerve, in continuity with it — all at the expense of the materials supplied by the very same blood. 43. The Nutrition of tissues, by the organisation of the materials con- tained in the nutrient fluid with which they are supplied, may be super- ficially compared, therefore, to the act of crystallization, when it takes place in a mixed solution of two or more salts. If in such a solution we place small crystals of the salts it contains, these crystals will pro- gressively increase by their attraction for the other particles of the same kind, which were previously dissolved ; each crystal attracting the par- ticles of its own salt, and exerting no influence over the rest. And if one of the angles of a crystal be broken off, it will be reproduced by a new deposit, so as to preserve the typical form. But it must be borne in mind, that such a resemblance goes no farther than the surface ; for the growth of a crystal cannot be really regarded as in the least analogous to that of a cell. The crystal progressively increases by the deposit of particles upon its exterior ; the interior undergoes no change ; and what- ever may be the size it ultimately attains, its properties remain precisely the same as those of the original nucleus. On the other hand, the cell grows from its original germ by a process of interstitial deposit ; the substance of which its wall is composed, extends itself in every part ; and the new matter is completely incorporated with the old. Moreover, as the increase proceeds, we see an evident distinction between the cell-wall and its cavity; and we observe that the cavity is occupied by a peculiar matter, different from the substance of the cell-wall, though obviously introduced through it. Of the essential difference which may exist in composition, between the cell-wall and the contents of the cavity, we have a remarkable example in the case of the simple Cryptogamic plant, which constitutes Yeast, and which differs in no essential part of the history of 30 GENERAL PHYSIOLOGY. its growth from the species already referred to. The substance of its cell- walls is nearly identical with ordinary Cellulose; whilst the contents of the cells are closely allied iii composition to the Albumen of Animal fluids. Again, in the fat-cells of Animals, the cell-wall is formed from an albuminous compound; whilst the oily contents agree, in the absence of nitrogen, and in then - general chemical relations, with the materials of the tissues of Plants. It is evident, then, that one of the inherent powers of the cell, is that by which it not only combines the surrounding materials into a substance adapted for the extension of its wall, but that which enables it to exercise a similar combining power on other materials derived from the same source, and to form a compound, — of an entirely different character, it may be, — which occupies its cavity. Now this process is as essential to our idea of a living cell, as is the growth of its wall ; and it must never be left out of view, when considering the histoiy of its development. 44. Every kind of cell has its own specific endowments ; and generates in its interior a compound peculiar to itself. The nature of this com- pound is less immediately dependent upon the nutrient materials which are supplied to the cell, than upon the original inherent powers of the cell itself, derived from its germ. Thus we find that the Red Snow and Goiy Dew invariably form a peculiar red secretion ; and that they will only grow where they can obtain, from the ah’ and moisture around, the ele- ments of that secretion. Again, the Yeast Plant invariably forms an albuminous secretion ; and it will only grow in a fluid which supplies it with the materials of that substance. Hence the Red Snow would not grow in a fermentible saccharine fluid ; nor would the Yeast Plant vegetate on damp cold surfaces. Yet there is little difference, if any, between their cell- walls, in regard to chemical composition.-— So, also, we shall find hereafter, that one set of cells in the animal body will draw into them- selves, during the process of growth, the elements of bile ; another, the elements of milk ; another, fatty matter ; and so on : — the peculiar endowments of each being derived from their several genus, which seem to have an attraction for these substances respectively, and which thus draw them together ; whilst the cell-wall appeal’s to have a uniform com- position in all instances. — The term Secretion, or setting-apart, is com- monly applied to this operation, to distinguish it from Nutrition or growth ; but it is obvious from what has now been stated, that the act of secretion is in reality the increase or growth of the cell-contents, just as the process of enlargement is the increase or growth of the cell-wall ; and that the two together make up the whole process of Nutrition, which cannot be properly understood, unless both are taken into account. It is to be remembered, however, that the contents of the ceU may not be destined to undergo organisation ; indeed we shall find hereafter, that the main use of certain cells is to draw off from the circulating fluid such ma- terials as are incapable of organisation ; and the operation may be so far attributed, therefore, to the agency of Chemical forces. But we shall find that, in other instances, the cell-contents are destined to undergo organisa- tion, and this either within the parent-cell, or after they leave it : here, then, we must recognise a distinct vitalising agency, as exerted by the cell upon its contents. 45. This organising or vitalising influence must be exerted .upon a eer- OF LIFE, OR VITAL ACTIVITY. 31 tain portion of the contents of every cell that is capable of reproducing itself ; for it is in this manner that those germs are produced, in which all the wonderful properties are inherent, that are destined to manifest themselves, when they are set free from the parent-cell. This power of Reproduction is one of those, which most remarkably distinguishes the living being ; and we shall find that, in the highest Animal, as in the humblest Plant, it essentially consists in the preparation of a cell-germ, which, when set free, gradually develops itself into a structure like that from which it sprang. The reproductive molecules or cell-germs are formed, like the tissue and the contents of the parent-cell, from the nu- trient materials which it has the power of bringing together and com- bining ; and in their turn they pass through a corresponding series of changes ; and at length produce a new generation of similar molecules, by which the race is destined to be continued. Notwithstanding the mystery which has been supposed to attach itself to this process, it is obvious that there is nothing in reality more difficult to understand in the fact, that the parent-cell organises and vitalises the nutriment which it has elaborated, so that it should form the germ of a new individual pos- sessing similar properties with itself ; than in its incorporating the same material with its own structure, and causing it to take a share in its own actions. 46. Finally, the parent-cell having arrived at its full development, having passed through the whole series of changes which is characteristic of the species, and having prepared the germs by which the race is to be propagated, dies and decays ; — that is, all those operations, which distinguish the living organised structure from inert matter, cease to be performed ; and it is subjected to the influence of chemical forces only, which speedily occasion a separation of its elements, and cause them to return to their original foims, namely, water and carbonic acid. There is evidence, how- ever, that interstitial death and decay are incessantly taking place during the whole life of the being ; and that the maintenance of its healthy or nor- mal condition depends upon the constant removal of the products of that decay, and upon their continual replacement. If, on the one hand, those products be retained, they act in the manner of poisons ; being quite as injurious to the welfare of the body, as the most deleterious substances introduced from without. On the other hand, if they be duly carried off, but be not replaced, the conditions essential to vital action are not ful- filled, and the death of the whole must be the result. — Now it is to be observed that, as Plants obtain the materials of their growth from water and carbonic acid, taking the carbon from the latter and setting free the oxygen, so do they require, as the condition for their decay , the presence of oxygen, which may reunite with the carbon that is to be given back to the atmosphere. If secluded from this, the vegetable tissues may be pre- served for a long time without decomposition. Generally speaking, in- deed, they are not prone to rapid decay, except at a high temperature ; and hence it is that we have so little evidence, in Plants, of the constant interstitial change, of which mention has just been made. Its existence, however (at least in all the softer portions of the structure), is made evi- dent by the fact, that a continual extrication of carbonic acid takes place, to an amount which sometimes nearly equals that of the carbonic acid decomposed, and of the oxygen set free, in the act of Nutrition (§ 73). 32 GENERAL PHYSIOLOGY. The latter operation is only effected under the stimulus of sun-light ; the former is constantly going on, by day and by night, in sunshine and in shade ; and if it be impeded or prevented by want of a due supply of oxygen, the plant speedily becomes unhealthy. Now this extrication of the products of interstitial decay is termed Excretion. It is usually confined in Plants to the formation of carbonic acid and water, by the union of the particles of their tissues with the oxygen of the air, — a process identical with that, which occurs after the death of the entire structure. But in Animals it is much more complicated ; owing to the larger number of constituents in their fabric, and to the much greater variety in the proportions in which these are combined ; hence the pro- ducts of interstitial decomposition are much more numerous and varied, and several distinct modes are devised for getting rid of them. Moreover, as the Animal tissues are much further removed than the Vegetable from the composition of Inorganic bodies, they are subject to much more rapid and constant decay ; and we shall find that this decay is so considerable in amount, as to require on the one hand a very complex excretory appa- ratus to carry off the disintegrated matter, and on the other a large supply of nutrient material to replace it. 47. The preceding history may be thus summed up. — i. The Vegetable cell-germ draws to itself, and combines together, certain inorganic ele- ments ; and thereby produces a new and peculiar compoimd, as the pabu- lum for its further development. This compound, however, exhibits no properties that distinguish it from others, in which ordinary Chemical agencies have been concerned ; and we may, therefore, regaixl the first act of the cell-germ as of a purely chemical nature. — n. The cell-geim then exerts an agency upon the pabulum thus prepared ; by which a new arrangement of its particles is produced. This new arrangement is evi- denced by the new and peculiar qualities of the fluid, which show that it is something more than a mere chemical compound, and that it is in the act of undergoing the process of organisation. — hi. The Organisation of this elaborated pabulum then takes place ; its materials are withdrawn from the fluid, and incorporated with the solid texture ; and in thus be- coming part of the organised fabric, they are caused to exhibit its own peculiar properties. — iv. At the same time, another portion of this pabu- lum is gradually prepared to serve as the germ of a new cell, or set of cells, by which the same properties are to be exhibited in another genera- tion. — v. By a Chemical operation resembling that concerned in the first preparation of the pabulum, a certain secretion, more or less differing from it in character, but not destined to undergo organisation, is formed in the cavity of the cell. — vi. A decomposition or disintegration of the organised structure is continually going on, by the separation of its elements into simpler forms, under the influence of purely Chemical agencies ; and the setting-free of these products by an act of excretion, is thus incessantly restoring to the inorganic world a portion of the elements that have been withdrawn from it. — vii. When the term of life of the parent-cell has expired, and its reproductive molecules are prepared to continue the race, the actions of nutrition cease ; those of decomposition go on unchecked ; and the death of the structure, or the loss of its dis- tinguishing vital properties, is the result. By the decomposition which then takes place with increased rapidity, its elements are restored to the OF LIFE, OR VITAL ACTIVITY. 33 inorganic world • presenting the very same properties as they did when first withdrawn from it ; and becoming capable of being again employed, by any number of successive generations of living beings, to go through the same series of operations. 48. The germ, however, can in no wise appropriate the materials of its growth, nor develop itself into a cell, unless it be brought under the in- fluence of certain Physical Agents. The first production of the pabulum out of water, carbonic acid, and ammonia, requires the agency of Light ; this operation, however, is limited to the Vegetable kingdom, and (as already remarked, § 39) is probably rather of a chemical than of a vital nature.* But that a certain amount of Heat is necessary to all vital activity, is a fact of universal applicability ; as is also the fact that, ceteris paribus, the amount of activity increases with the temperature, until this reaches an elevation which becomes injurious, either by over-stimulating the vital properties of the tissues, or by effecting some physical change. These truths will be more fully developed hereafter (Sect. 3) ; but it is necessary here to state them, in order that the relation which we shall presently attempt to demonstrate between Vital and Physical phenomena may be comprehended (§ 54). 49. The solitary cell which constitutes the entire organism of the hum- blest of living beings, is also the simplest organic element of the most elaborate and heterogeneous fabrics. We shall hereafter see (chap, iv.) that all the Vegetable tissues originate from similar cells, and that all the truly vital operations are performed, even in the fully -developed plant or tree, by tissues retaining their primitive cellular character ; and notwith- standing that, at first sight, the Animal fabric appears to have very little in common with that of the Plant, we shall find that even in it the same general facts are demonstrable, both as to the origin of all the tissues (directly or indirectly) in cells, and as to the instrumentality of cells in the performance of all the truly vital actions of the organism. Hence we are justified in affirming that, as the simple cell presents us with the type of Organised structure (§ 15), so our idea of what is essential to Life should be founded upon those vital phenomena which every cell exhibits in the course of its history as an individual, and upon the distinctive attributes which we must consequently regard it as possessing. Putting aside, then, such portions of its series of actions as we appear justified in referring to Chemical agencies, we find that the incorporation of the pabulum into the substance of the cell itself, which is taking place not only during its first development but throughout the whole period of its vital activity, must be considered as its most important manifestation of life ; on this all sub- sequent phenomena are dependent ; and whatever may be the endow- ments which the cell is to possess in its perfect state, they are inseparably connected with this process of Organisation. 50. These endowments are extremely various ; and their diversities * The following experiment of Schleiden’s confirms this view. Having allowed some grains of Oats to germinate in darkness until the buds were four inches long, he cut them down,' washed them and dried them between blotting paper, and then placed them upon white paper to be dried in the sun. They were previously of a pale yellow colour ; but after six hours’ exposure to the solar rays, during which the plants were almost perfectly dried, they had all become of a perfectly green colour. It can scarcely be supposed that any peculiarly vital agency was concerned in the transformation under such circumstances. D 34 GENERAL PHYSIOLOGY. constitute, in fact, the differences between the properties of the several tissues. Thus we find certain cells whose agency is exerted in developing a preliminary organisation or vitalisation in the liquid pabulum (§ 39) ; which, when thus prepared, is to be yielded up for appropriation by other cells in process of development. This process we have designated by the term Assimilation. — Again, there are other cells which are distinguished for their power of Reproduction; their function as individuals being apparently confined to the preparation of the germs of a new generation. — Others, again, are characterised by the change of form which they undergo, after having attained their development as cells ; this change of form, rendering them subservient to some new purposes in the economy, origi- nates entirely in their own vital activity, and must be considered as indicating a power of Morphological Transformation. — In many other instances, the endowments of the cells are manifested in their power of effecting Chemical T ransformcdions, of a kind which we do not meet with under any other circumstances. — And both in Animals and Plants, but in the former more especially, we find a number of cells endowed with the power of giving rise to Mechanical Motion ; either by a sudden and tem- porary change of form, as in the contraction of a muscle, and in most of the sensible movements of plants ; or by the vibratile action of certain filaments attached to the exterior of the cells, and termed cilia (§ 183). — Other cells in the Animal body have it as their special endowment to generate that peculiar power which we call Nervous Agency ; the opera- tion of which is more nearly allied to that of Electricity than to that of any other power with which we are acquainted, though not identical with it (§ 246). 51. Now all these endowments may be regarded as related to each other through their common parentage ; being all manifestations of the power existing in the living cell, and consequently of the power by which that cell was formed. We may best express the distinctive character of Vital Activity, therefore, by describing it as essentially manifested in Cell- formation.* And it will be convenient to employ the term Vital Force as an expression of the agency by which this is accomplished — That the various operations, enumerated in the preceding paragraph as effected by cell-agency, are to be regarded (however dissimilar in themselves) as so many manifestations of this Vital Force, acting through different instill- ments, appeal’s probable from this consideration ; — namely, that each endowment is so far inconsistent with the rest, that it would seem as if the expenditure of the vital force of the cell upon that one purpose unfitted it for any other. — Thus, the Assimilating cells do not exercise chemical transformation ; they do not undergo change of form ; they do not exert mechanical or nervous power; and it is doubtful whether they reproduce themselves. So, again, the cells which are specially endowed with the Reproductive power seem to possess no other ; whatever may he the peculiar endowments of then’ progeny. And the cells which are operative in Chemical transformations are destitute of the power of * The terra formation is here intended to include all that we separately express by the terms development, growth, and maintenance ; — the first expressing the progressive evolution of a new form ; the second, its simple increase ; and the third, its continued nutrition after the attainment of its complete development and full dimensions. See Mr. Paget's Lectures on Nutrition, &c., in Medical Gazette for 1847. OF LIFE, OE VITAL ACTIVITY. 35 exciting mechanical movement, as they are also of reproductive force. And the same is true conversely of the cells concerned in Mechanical Movement, which are incapable of exercising chemical transformations, as they are of continuing their race. We shall hereafter meet with a variety of exemplifications of these general statements ; and shall at present only refer to the production of the secreting cells within a glandular follicle (§ 157), to that of the cells of the muscular fibrillse within the tubular fibre (§ 226), to the difference of endowment between the ciliated and the secretory epithelium-cells (§ 183), and to the cessation of ciliary action in the zoospores of the Algae when they begin to exercise chemical transform- ations (§ 1 85), as illustrations of the argument. 52. Another argument for the mutual relation here contended for, is furnished by the analogy of the Forces operating in the Inorganic world. Reasons have been lately given* for the belief, that Physical Forces of every kind, — namely, Heat, Light, Electricity, Magnetism, Chemical Affinity, and that which produces or resists Motion, — are all 4 correlative,’ or have a relation of mutual dependence ; each being capable of producing any one of the rest, either directly or through the medium of some other ; and each being consequently producible by any of the remainder. And we may observe that this conversion of one force into another is due to its passage through some material body. Thus the Motion of a body, retarded or resisted by friction, gives rise to Heat ; and conversely, heat applied to any form of matter produces its expansion, i. e. Motion. The friction of two dissimilar bodies produces not merely heat but Electricity ; and Heat itself, when made to act on certain combinations of metals, also produces Electricity ; whilst, on the other hand, the Electric current may produce Heat, Light, Magnetism, or Motion, according to the nature of the substances through which it is transmitted. Light, Heat, and Elec- tricity, again, are closely related to Chemical Affinity, which is often specially excited by them (§ 68), and which can, in its turn, generate these Forces ; a material substratum being required in both cases. So in the beautiful experiments by which Faraday has brought Magnetism to act upon Light, the effect is only produced when the ray is being transmitted through certain material substances. 53. Now the relation which exists among the several endowments peculiar to Organised Structures, and therefore distinguished as Vital, appears to be most appropriately expressed in the very same form ; — they may be said to be mutually correlated, in such a manner that each can either directly or mediately produce the other. — The most universal and characteristic of these forces has been shown to be that from which ‘ Cell- formation ’ proceeds ; and out of this arise all the other agencies. Thus, the tissue of the Muscle is constructed solely with a view to its manifes- tation of contractile power ; and the development of Nervous matter has reference entirely to the peculiar agencies in which it is to be concerned. The contraction of a Muscle, and the production and transmission of a Nervous impression, are phenomena of a distinctly vital character ; and it will hereafter appear that, with the manifestation of the peculiar attri- butes of these tissues, they cease to maintain their existence as living structures, — as if the general Vital force which they previously possessed, See Prof. Grove on the “ Correlation of Physical Forces,” 1 846. 3G GENERAL PHYSIOLOGY. had been expended by its conversion into the Muscular and Nervous powers. Here, too, we see that a certain material substratum is requisite for the conversion of one force into another ; the vital force only being able to produce contractile action, by means of a certain variety of organ- ised structure which we term Muscle ; whilst it can only develope itself as nervous agency, through that peculiar form of tissue which is called Nerve. Again, we find that the nervous force can evoke the muscular, and this in a degree proportional to the intensity of its own excitement ; and nervous agency would seem capable of influencing cell-formation, in such a manner as to justify the idea that it is actually re-converted into the form of vital force concerned in it. 54. If this view be considered to have a certain claim to reception as (to say the least) a probable hypothesis of the mutual relation of the various forms of Vital activity, we have next to consider whether any similar ‘correlation’ can be traced between the Vital and the Physical forces ; since, if such can be shown to exist, the probability of the hypothesis will be greatly increased. Now it will be shown (Sect. 3) that all Vital activity is dependent upon the constant agency of one at least of the Physical Forces already enumerated — viz. Heat; and that Light and Electricity, operating through Chemical Affinity, are also concerned in sustaining particular forms of it. These agents, indeed, which have long been known under the term ‘ Vital Stimuli,’ may perhaps be more justly considered as the real Forces concerned in the production of Vital phe- nomena, modified in their operation by the organised structure through which they are exerted; just as Heat becomes Electricity when it passes through a certain combination of metals. And this idea is borne out by the fact, that the Vital Forces appear capable of being in their turn re- converted into the Physical ; as in those numerous cases already referred to (§ 50), in which the whole energy of one set of cells is directed to the performance of some chemical transformation, and that of another to the production of mechanical movement; as well as to other instances, in which it seems nearly certain that the liberation of Electricity, and perhaps also of Heat and Light, are due to Nervous power operating through a particular structure (chap. xvii.). Indeed, when we come to consider the manifestations of Nervous agency in more detail, we shall find that it is at least as closely con-elated to Electricity, as Electricity is to Light or Heat (§ 246); and the intimacy of this relationship, existing between one of the most special or peculiar of all the Vital forces and one of an exclusively Physical nature, strongly confirms the idea just expressed in regard to the mutual connection of the remainder. 55. In the consideration of this or any other Theory of Life, it is very important to keep constantly in view, that it has relation only to the agencies concerned in the formation and conservation of the material organism. The phenomena of mind belong to a category altogether distinct, and cannot be considered as bearing any necessary relation to the former. — It seems also requisite to point out, that in speaking of forces as possessing an absolute existence, it is not intended on the one hand to imply that they are anything else than ‘ affections of matter ;’* nor on the other to regard them in any other light than as the direct operation of the primal all-sustaining Cause. We can form no conception of matter, * See Grove on the “ Correlation of Physical Forces,” p. 7. OF THE MATERIAL CONDITIONS OF LIFE. FOOD. 37 except as possessing properties, which when in action give rise to powers or forces; whilst on the other hand, we cannot think of forces, except as ope- rating through some form of matter, of whose properties they are the mani- festation. The existence of matter, and the action of the forces to which material phenomena (whether physical or vital) are attributable, are alike the expressions of the Divine Will; and our aim must be limited to the discovery of the plan, according to which it has pleased the Creator to develop and maintain the existing condition of the Universe we inhabit. (See § 5, note). 2. Of the Material Conditions of Life. — Food, Water, and Oxygen. 56. Every organised structure requires to be placed in certain con- ditions, in order that its Vital properties may be manifested in action ; and although this, as we have seen, is no more than is true of every form of Inorganic Matter, in regard to its Physical and Chemical properties, yet the conditions of vital activity are more numerous, and require to act in a more complex combination. It will simplify our consideration of them to arrange them into two groups ; the first comprehending the material substances, of which a continual supply is required by all living- beings; the second including certain forces brought to bear upon these substances, without the agency of which they could not be appropriated by the organism. There is an important difference in the relation of these two sets of conditions to Vital activity ; for whilst that activity cannot be increased by any augmentation of the material supplies beyond the amount normally required, it can be promoted by a more energetic operation of these forces, as well as retarded by then- diminution ; and the demand for the former will be modified accordingly. — A simple analogy may render these distinctions obvious. For the development of the power of the Steam-engine, two things are essential; the material sub- stance, Water; and the force of Heat, by which the particles of the water are made to repel each other, and a powerful mechanical agency thereby obtained. Now if the Steam-engine be supplied with as much water as is required to maintain the proper amount in the boiler, no increase of that supply can of itself produce the least augmentation of the power of the machine; but if increased heat be applied, the elastic force of the steam is augmented, and the power of the machine proportionably raised; whilst as the water is thereby more rapidly converted into steam, a larger supply of it is required for the maintenance of that power. 57. The material conditions essential to Life are three;— Food, or substances capable of being converted into the solid portion of the organised fabric; Water, for the maintenance of the due proportion of liquid in its constitution; and Oxygen, for promoting various changes in the assimilated matter, by which it is applied to a greater variety of purposes, and also for uniting with the particles destined for excretion, so that they may pass off in the form in which they can be most readily got rid of. 58. Of Food. — It has been shown that all Vital Action is dependent in some way or other upon Cell-formation (§ 51); and it is obvious that the supply of materials for that formation is essential to its continuance. These materials, in the case of Plants, are exclusively supplied by the 38 GENERAL PHYSIOLOGY. Inorganic world ; for even when organic substances are used to promote vegetable growth (as in the case of manures), they act only by setting free the carbonic acid and ammonia, which, with water, constitute the staple food of Plants, and at the same time, it may be, by supplying certain mineral constituents of importance to their nutrition. It is only among the lowest tribes of the Vegetable kingdom, however, that every cell directly obtains its food from the Mineral world ; for in all but the most homogeneous organisms, we find certain portions of the structure set apart for the purpose of effecting the requisite combination of Oxygen, Hydrogen, Carbon, and Nitrogen, and of thus preparing a pabulum, which shall serve not merely for their own growth and maintenance, but also for the formation of new parts elsewhere, which are destined for purposes of a different kind, and are not themselves capable of obtaining the materials of their growth from external sources. Thus we shall find that in the perfect Plant, the requisite water is taken in by the spongioles of the roots ; whilst the carbonic acid and ammonia are in part imbibed in solution in that liquid, but are in part also derived (the former more especially) from the atmosphere, through the agency of the cells of the leaves and green surfaces ; and the compounds formed in these cells serve as the pabulum for the growth of every other part of the organism, — the new leaf- and flower-buds being developed, and the increase of substance taking place in the solid framework of the stem, roots, and branches, entirely at its expense. The food of Animals entirely consists, as formerly stated, of compounds which have been already generated by the agency of Plants; but these compounds have first to be reduced to a condition in which they can be applied, by the various component tissues of the fabric, to then formative processes ; and it is in the nutrient fluid thus prepared and circulated through the body, that each individual cell finds the materials of its growth. 59. The various sources of the demand for food in the living Plant and Animal, and the circumstances under which this demand varies from time to time, will hereafter have to be fconsidered in detail (chap, x.) ; at present it will be sufficient to state the general principle — as one in which theory and experience alike coincide, — that the demand for Food is proportional to the Vital Activity; and this alike in the organism as a whole, and in each individual component of it. We may refer, by way of illustration, to the fact that cold-blooded animals require a mucb smaller amount of food, and are able to sustain a deprivation of it for a much longer period, at low than at moderately high temperatures ; their vital activity being at its minimum in the former case, and at its maximum in the latter (§ 97). In this respect, also, the hybernating Mammal corre- sponds with the cold-blooded Reptile, to which it is for a time assimilated in its whole physiological condition (§ 95). 60. The following curious fact, well known to Naturalists, affords proof that the hind of food supplied may not only influence the ordinary pro- cesses of nutrition, but may produce an important modification in the development of the organism. In eveiy hive of Bees, the majority of individuals consists of neuters , which have the organs of the female sex undeveloped, and are incapable of reproduction; that function being restricted to the queen, who is the only perfect female in the community. If by any accident the queen be destroyed, or if she be purposely OF MOISTURE AS A CONDITION OF VITAL ACTIVITY. 39 removed for the sake of experiment, the bees choose two or three from among the neuter eggs that have been deposited in their appropriate cells, which they have the power of converting into queens. The first operation is to change the cells in which they lie into royal cells, which differ from them considerably in form, and are of much larger dimen- sions; and when the eggs are hatched, the maggot is supplied with food of a very different nature from the farina or bee-bread which has been stored up for the nourishment of the workers, being of a jelly-like con- sistence and pungent stimulating character. After the usual transforma- tions the grub becomes a perfect queen, differing from the neuter bee into which it would otherwise have changed, not only in the development of the reproductive system, but in the general form of the body, the pro- portionate length of the wing's, the shape of the tongue, jaws, and sting, the absence of the hollows on the thighs in which the pollen is carried, and the loss of the power of secreting wax. 61. Of Moisture . — Independently of the utility of Water as an article of food, supplying two of the most important materials (oxygen and hydrogen) for the construction of the tissues, there can be no doubt that a certain supply of moisture is requisite as one of the conditions without which no vital actions can go on. It has been already remarked, indeed, that one of the distinguishing peculiarities of organised structures is the universal presence of solid and fluid parts (§ 22) ; and this in the minutest portion of the organism, as well as in the aggregate mass. And for all the vital as well as chemical actions in which these structures are concerned, the presence of liquid is essential. All nutrient materials must be reduced to the liquid form, before they can be assimilated by the solids ; and, again, the solid matters which are destined to be carried off by excretion, must be reconverted to the fluid state, before they can be thus withdrawn from the body. In all these operations, water is the liquid employed; and no other could be substituted for it. Taken as a whole, the tissues of Animals contain a much larger proportion of water than those of Vegetables ; but this is because a considerable part of the fabric of the latter is usually destined for a long-continued existence, and is removed by consolidation out of all participation in vital actions (§ 24). With such structures we might compare that of Coral among Animals ; in which the proportion of organic matter is smaller than in any plants, and liquid can scarcely be said to exist ; whilst, on the other hand, the tissues of many rapidly -growing Fungi are as soft, and contain as large a proportion of water, as any of those most actively concerned in the maintenance of Animal life. As a general rule, the amount of water contained in an organised tissue bears a proportion to its vital activity; but this rule is not without numerous exceptions. For although we never find any active vital operations carried on by organised structures in which the solid elements predominate, yet there are numerous cases in which a great preponderance of fluids is obvious, without any correspond- ing indication of vital energy. Thus many succulent Plants, such as the Sedums of our own country, the Cacti and Euphorbias of the tropics, are rather remarkable for their slow than for their rapid growth; the suc- culence of their tissues having relation to their scanty opportunities of obtaining moisture, and being designed to furnish a store for the wants of the organism, when external sources are deficient. So, again, among 40 GENERAL PHYSIOLOGY. Animals, we find an entire group, that of Acalephce (§ 293), characterised by the extremely small proportion that the solid matter bears to the water of their tissues ; nevertheless their vital activity is extremely low ; and the purpose of the arrangement does not seem obvious, unless it be to endow them with a bulk altogether disproportionate to the amount of organic material consumed in its production. 62. When we take a general survey of the Vegetable kingdom, we find that there are some Plants which are capable of adapting themselves to a great variety of situations, differing widely as to the amount of Moisture which then inhabitants can derive from the soil and atmosphere ; and we may generally notice a marked difference in the mode of growth, when we compare individuals that have sprung up under opposite circumstances. Thus a plant from a dry exposed situation shall be stunted and hairy, whilst another, of the same species, but developed in a damp sheltered situation, shall be rank and glabrous. But in general there is a certain quantity of moisture congenial to each species; and the excess or deficiency of this condition has, in consequence, as great an influence in determining the geographical distribution of Plants, as the amount of light and heat. Thus the Orchideie and Tree Ferns of the tropics grow best in an atmosphere loaded with dampness; whilst the Cactus tribe, for the most part, flourishes best in dry situations. The former become stunted and inactive, if limited in their supply of aerial moisture ; whilst the latter, if too copiously nourished, become dropsical and liable to rot. Among the plants of our own country, we find a similar limita- tion ; a moist boggy situation being indispensable to the growth of some, whilst a dry exposed elevation is equally essential to the healthy de- velopment of others. There is a beautiful species of exotic Fern, the Trichomanes speciosum, the rearing of which has been frequently at- tempted in this country and elsewhere, without success ; but which only requires for its healthy development an atmosphere saturated with damp- ness, being easily reared in one of Mr. Ward’s closed glass-cases. In this, as in similar examples, it is only necessary to imitate as closely as possible the conditions under which the species naturally grows; and sometimes this can only be accomplished by surrounding the plant with small frees and shrubs, so as to give it a moister atmosphere than it cotdd otherwise attain. Professor Boyle mentions the growth under such circumstances, of a fine specimen of the Xanthochymus dulcis, one of the Guttiferce or Gamboge-trees, in the garden of the king of Delhi ; this tree is naturally found only in the southern parts of India ; and the success of its cultivation in this northerly situation is entirely due to its being sheltered by the numerous buildings within the lofty palace wall, surrounded by almost a forest of trees, and receiving the benefit of perpetual irrigation from a branch of the canal which flows through the garden. 63. In regard to the influence of external moisture upon Animal life, there is much less to be said ; since the mode in which fluid is received into the system is so entirely different. It may be remarked, however, that Animals habitually living beneath the water, like submerged Plants, are usually incapable of sustaining life for any length of time when removed from it, in consequence of the rapid loss of fluid which they undergo from their surface. It is, however, by the desiccation of the OF MOISTURE AS A CONDITION OF VITAL ACTIVITY. 41 respiratory surface, preventing the due aeration of the blood, that the fatal result is for the most part occasioned ; since we find that when there is a special provision to prevent this, as in the case of certain Fishes and Crustacea, the animals can quit the water for a great length of time. The amount of Atmospheric Moisture is one of those conditions, which are collectively termed Climate, and which influence the geographical distribution of Animals, no less than that of Plants. But it is difficult to say how far the variations in moisture act alone. There can be no doubt, however, of their operation ; for every one is conscious of the effect, upon his health and spirits, of such variations as take place in the climate he may inhabit. The two principal modes in which these will operate, will be by accelerating or checking the exhalation of fluid from the skin and from the pulmonary surface ; for when the air is already loaded with dampness, the exhaled moisture cannot be carried off with the same readiness, as when it is in a condition of greater dryness ; and it will consequently either remain within the system, or it will accumulate and form sensible perspiration (chap. xv). 64. Although, as already stated, no vital actions can go on without a reaction between the solids and fluids of the body, yet there may be an entire loss of the latter, in certain cases, without necessarily destroying- life ; the structure being reduced to a state of dormant vitality (§ 114), in which it may remain unchanged for an unlimited period ; and yet being capable of renewing all its actions, when moisture is again supplied. Of this we find numerous examples among both the Vegetable and the Ani- mal kingdoms. Thus the Mosses and Liverworts, which inhabit situations where they are liable to occasional drought, do not suffer from being, to all appearance, completely dried up ; but revive and vegetate actively, as soon as they have been thoroughly moistened. Instances are recorded, in which Mosses that had been for many years dried up in a Herbarium, have been restored by moisture to active life. There is a Lycopodium inhabiting Peru, which, when dried up for want of moisture, folds its leaves and contracts into a ball ; and in this state, apparently quite devoid of animation, it is blown hither and thither along the ground by the wind. As soon, however, as it reaches a moist situation, it sends down its roots into the soil, and unfolds to the atmosphere its leaves, which, from a dingy brown, speedily change to the bright green of active vegetation. The Anastatica (Bose of Jericho) is the subject of similar- transformations ; contracting into a ball, when dried up by the burning sun and parching ah - ; being detached by the wind from the spot where its slender roots had fixed it, and rolled over the plains to indefinite distances ; and then, when exposed to moisture, unfolding its leaves, and opening its rose-like flower, as if roused from sleep. And there is a blue Water-Lily, abound- ing in several of the canals at Alexandria, which at certain seasons become so dry, that then- beds are brunt as hard as bricks by the action of the sun, so as to be fit for use as carriage roads ; yet the plants do not there- by lose their vitality ; for when the water is again admitted, they resume their growth with redoubled vigour. 65. Among the lower Animals, we find several of considerable com- plexity of structure, which are able to sustain the most complete desicca- tion. This is most remarkably the case in the common Wheel-Animalcule ; which may be reduced to a state of most perfect dryness, and kept in 42 GENEHAL PHYSIOLOGY. this condition for any length of time, and which will yet revive immedi- ately on being moistened (§ 310). Experiments have been carried still further with the allied tribe of T ar digrades ; individuals of which have been kept in a vacuum for thirty days, with sulphuric acid and chloride of calcium (thus suffering the most complete desiccation the Chemist can effect), and yet have not lost their vitality. It is singular that in this desic- cated condition, they may be heated to a temperature of 250°, without the destruction of their vitality ; although, when in full activity, they will not sustain a temperature of more than from 112° to 115°. Some of the minute Entomostracous Crustacea, which are nearly allied to the Rotifera, appear to partake with them in this curious faculty. Many instances are on record, in which Snails and other terrestrial Mollusks have rerived, after what appeared to be complete desiccation ; and the eggs of the Slug, when dried up by the sun or by artificial heat, and reduced to minute points only risible with the Microscope, are found not to have lost their fertility, when they are moistened by a shower of rain, or by immersion in water, which restores them to their former plumpness. Even after being treated eight times in this manner, the eggs have been hatched when placed in favourable circumstances ; and even eggs in which the embryo was distinctly formed, have survived such treatment without damage. — That such capability should exist in the animals and eggs just mentioned, shows a remarkable adaptation to the circumstances in which they are destined to exist; since were it not for then- power of surviving desiccation, the races of Wheel-Animalcules and Entomostraca must speedily become extinct, through the periodical drying-up of the small collections of water which they inhabit ; and a season of prolonged drought must be equally fatal to the terrestrial Mollusca. 66. It would seem that many Cold-blooded animals are reduced, by a moderate deficiency of fluid, to a state of torpidity closely resembling that induced by cold ; and hence it is, that during the hottest and driest paid of the tropical year, there is almost as complete an inactivity, as in the winter of temperate regions. The common Snail, if put into a box with- out food, constructs a thin operculum or partition across the orifice of the shell, and attaches itself to the side of the box : in this state it may remain dormant for years, without being affected by any ordinary changes of temperature ; but it will speedily revive if plunged in water. Even in their natural haunts, the terrestrial Mollusca of oiu own climates are often found in this state during the summer, when there is a continued drought ; but with the first shower they revive and move about. In like manner it is observed that the rainy season, between the tropics, brings forth the hosts of insects, which the drought had caused to remain inactive in their hiding-places. Animals thus rendered torpid seem to have a tendency to bury themselves in the ground, like those which are driven to winter- quarters by cold. Mr. Darwin mentions that he observed with some sur- prise at Rio de Janeiro, that, a few days after some little depressions had been changed into pools of water by the rain, they were peopled by numerous full-grown shells and beetles. — The torpidity consequent upon drought is not confined to Invertebrated animals. There are several Fish, inhabiting fresh-water, which bury themselves in the mud when them streams or pools are dried up, and which remain there in a torpid con- dition until they are again moistened. This is the case, also, with the OF OXYGEN AS A CONDITION OF VITAL ACTIVITY. 43 curious Lepidosiren (§ 323), which is an inhabitant of the upper parts of the river Gambia, which are liable to be dried up during much more than half the year ; and the whole of this period is spent by it in a hollow which it excavates for itself deep in the mud, where it lies coiled up in a completely torpid condition, — whence it is called by the natives the sleeping-fish. When the return of the rainy season causes the streams to be again filled, so that the water finds its way down to the hiding-place of the Lepidosiren, it comes forth again for its brief period of activity ; and with the approach of drought, it again works its way down into the mud, which speedily hardens around it into a solid mass. In the same manner, the Proteus (§ 323), an inhabitant of certain lakes in the Tyrol, which are liable to be periodically dried up, retires at these periods to the underground passages that connect them, where it is believed to remain in a torpid condition ; and it thence emerges into the lakes, as soon as they again become filled with water. The Lizards and Serpents, too, of tropical climates appeal' to be subject to the same kind of torpidity, in consequence of drought, as that which affects those of temperate regions during the cold of winter. Thus Humboldt has related the strange acci- dent of a hovel having been built over a spot, where a young Crocodile lay buried, alive though torpid, in the hardened mud ; and he mentions that the Indians often find enormous Boas in the same lethargic state ■ and that these revive when irritated or wetted with water. — All these examples show the necessity of a fixed amount of fluid, in the animal structure, for the maintenance of vital activity : whilst they also demon- strate, that the preservation of the vital properties of that structure is not always incompatible with the partial, or even the complete, abstraction of that fluid ; the solid portions being then much less liable to decomposition by heat, or by other agencies, than they are in their ordinary condition. 67. Of Oxygen. All living beings have constant access to Oxygen, in a free or uncombined state, either as a constituent of the Atmosphere, or as diffused through the Water which they inhabit. It does not appear - , however, that this free oxygen is ever appropriated by Plants, as one of the materials of the primary organic compounds which they generate from the Inorganic elements ; for it seems to be an essential condition of the union of these elements in such peculiar modes, that they should be in a nascent state, — that is, in the act of being set free from a previous com- bination. When these compounds have been once formed, however, they may undergo further modifications within the living organism, whether Vegetable or Animal (§ 28) ; and for some of these modifications the addi- tion of a further measure of oxygen seems needed, which is absorbed from the free oxygen of the atmosphere. Such a use of oxygen, however, pro- perly falls under the head of F ood ; since this element, when thus introduced into the organism, becomes a constituent part of it. — The chief dependence of Vital Activity upon the access of Oxygen is quite of another character ; for it results from the necessity that exists for the continual getting rid of the effete matter, which has served its piupose in the organism, and which would be injurious if retained there. By this action of free oxygen on the Vegetable tissues, carbonic acid and water are being continually produced, even during their state of greatest vital activity (§ 46 ) ; and thus the materials which they have drawn from the Inorganic world are returned to it again in the same form. The same excretions are constantly 44 GENERAL PHYSIOLOGY. set free, by the very same agency, from the Animal body ; but in addition to these, other azotized compounds are thrown off, for the production of which free oxygen introduced from without is required. — This source of demand for Oxygen is common to all living beings, and its energy is pro- portioned to their vital activity. There are other purposes, however, of a less universal character, to which it is subservient ; thus the manifestation of the Muscular and Nervous power of Animals appeal's to be essentially dependent upon the union of oxygen with the elements of then- tissues (§§ 236 and 243) ; and the maintenance of the standard heat of Warm- blooded animals is dependent upon a process of combustion in the in- terior of their bodies, to which the presence of free Oxygen is as essential as it is to the burning of hydrogen or carbon elsewhere. This process, however, will require special consideration on a future occasion (chap, xvi. Sect. 3). 3. Of the Dynamic Conditions of Life ; — Light, Heat, and Electricity. 68. No organised fabric, however perfect in its structure and composi- tion, and however amply supplied with the materials requisite for its growth and maintenance, can perform a single Vital Action without the concurrence of certain other agencies, of the class commonly termed Phy- sical Forces. These afford the stimulus or provocative to activity ; and the germ could no more develope itself into a cell without the operation of one (at least) of them, than it could do so with then - assistance but with- out the supply of the alimentary materials. Hence these agents, — Light, Heat, and Electricity, — when viewed with reference to their participation in the phenomena of Life, have been termed Vital Stimuli. Then' operation on Organized structures is not altogether without analogy to the influence they possess over inorganic matter, which is exerted in the production of Chemical phenomena. Thus the union between two substances, which previously showed no tendency to combine, although in close contact with each other, may be frequently caused to take place, with various manifes- tations of energy, by the momentary influence of one of these agents. When Chlorine and Hydrogen are mingled together in a bottle, and this is placed in darkness, they may be kept for any length of time with- out change. But the agency of Light brings their previously-dormant affinities into a state of activity ; for if they be subjected to the direct rays of the sun, an instantaneous explosion takes place, with production of Hydrochloric acid; whilst, if exposed for some time to diffused day- light, they unite more slowly. The same effect may be produced by Heat ; for a union of the two gases occurs, with a violent explosion, when any incandescent substance is introduced into the mixture. And a like result is produced by Electricity ; the transmission of a spark through the mixture producing an explosion, with a generation of hydrochloric acid. In neither of these cases can we consider that Heat was excluded ; for although a high temperature was required to bring about the union when no other agent was in operation, yet neither Light nor Electricity could have effected the union, unless the elements had been in the gaseous condition which they present at all ordinary temperatures, in virtue of the latent heat they contain. 69. In like manner wo shall find that, of these three agents, Heat is PHYSICAL AGENCIES NECESSARY TO LIFE. 45 the one which is most universally concerned in the production of Vital phenomena. The action of Light is indispensable for certain purposes, and we shall see that upon it essentially depends the first production of Organic compounds; but it would possess no efficacy whatever, without the simultaneous operation of Heat ; and a very large proportion of the vital operations of Animals have but little direct dependence upon it. Of the operation of Electricity, we have little certain knowledge, although there is reason to believe that it occasionally exerts an important in- fluence.* In regard to all these Vital Stimuli it may be observed, that the dependence of the vital actions of an Organism upon their constant influence varies with its degree of organisation (§ 20) ; so that beings of simple organisation are capable of enduring a deprivation of these stimuli, which would be fatal to those higher in the scale. This will be partly understood, when it is borne in mind that the higher the development of the living being, the more complete is the distribution of its different actions amongst separate organs, — the more close, therefore, is their mutual dependence, — and the more readily, in consequence, are they all brought to a close by the interruption of any one. But there is no doubt, that the actions of even the individual parts of the higher organisms require for their excitement a greater supply of these stimuli, than the similar actions of the corresponding parts in the lower : whilst if these stimuli be exerted upon the lower with the intensity that is required for the higher, they destroy the vital properties of the tissues altogether, by the excess of their action. This distinction is most obvious in regard to the relative influence of Heat upon warm-blooded and cold- blooded animals ; of which examples will be given hereafter. 70. It may also be observed of the influence of these, as of that of other stimuli whose agency is less general, that it is rather relative than absolute; being frequentty dependent upon the degree of change, rather than upon the actual measure of the amount of the stimulus. This constitutes a marked difference between the influence of these stimuli on mere chemical compounds, and their operation on bodies endowed with vitality. In the former case, their action is always uniform ; thus the same amount of Heat, the same exposure to Light, the same charge of Electricity, would be required to produce a given Chemical effect, how often soever the action might be repeated. But this is not the case with living bodies ; since an increase or diminution in the intensity of the vital stimuli, which, if made suddenly , would be scarcely compatible with the continuance of Life, may be so brought about as to produce no marked change in its phenomena, — the organism possessing a certain power of adapting itself to conditions which are habitual to it, and thus allowing great changes in these conditions to be gradually effected, without any serious disturbance. — Thus of two individuals of the same species, one may become torpid at a temperature of 60°, because it has been accustomed to a temperature of 70° ; whilst another, habituated to a temperature of 60°, would require to be cooled down to * Some allusion might here be expected to the influence of Magnetism on the living body; since the researches of Prof. Faraday have proved that even organized structures are subject to its agency. There is as yet, however, no proof whatever, nor even any indication, that the Magnetic force has any operation, either direct or indirect, in calling forth the ordinary phenomena of life. 46 GENERAL PHYSIOLOGY. 50°, in order to induce torpidity ; — the influence of temperature upon the vital condition being proportioned, more to the variation from the usual standard, than to the actual elevation of that standard Yet the first of these individuals might be gradually habituated to live in the same tem- perature with the second ; and to require the same amount of further depression to induce torpidity. (See § 99.) 71. It is' a very curious fact that, whilst the lower classes of living beings are more capable than the higher of bearing the deprivation of these Vital stimuli, they are at the .same time more liable to alterations in their own structure and development, in consequence of valuations in the degree of the agency of these, or of other causes external to themselves. Thus the forms of the lower tribes of Plants and Animals are liable to be greatly affected by the conditions under which they grow ; and these also modify their degree of development. It seems as if the formative power were less vigorous in the lower, than in the higher classes ; so that the mode in which it manifests itself in the former is more dependent upon external influences ; whilst in the latter it either predominates over them, causing the regular actions to be performed, or gives way altogether. — The same principle applies to the early condition of the higher organisms ; their embryos, like those beings of permanently-low type which they resemble in degree of development, being liable to be affected by modifying causes, which the perfect beings of the same kind are able to resist. When the organism has so far advanced in its development, that its several parts have assumed different characters, the influence of the Vital Stimuli upon its development becomes more restricted. A remarkable example of this will be presently given in regard to Light (§ 78). 72. Of Light. — The importance of this agent, not only to the Vegetable but to the Animal World, is not in general sufficiently estimated. Lender its influence alone can the first process be accomplished, by which inor- ganic matter is transformed into an organic compound, adapted by its nature and properties to form part of the organised fabric. The following is an example of the simplest phenomenon of this kind, which demon- strates the influence of Light the more clearly on account of that sim- plicity. “ If we expose some spring-water to the sunshine, though it may have been clear and transparent at first, it presently begins to assume a greenish tint ; and, after a while, flocks of green matter collect on the sides of the vessel in which it is contained. On these flocks, whenever the sun is shining, bubbles of gas may be seen, which, if collected, prove to be a mixture of oxygen and nitrogen, the proportion of the two being variable. Meanwhile the green matter rapidly grows ; its new pans, as they are de- veloped, being all day long covered with air-bells, which disappear as soon as the sun has set. If these observations be made upon a stream of water, the current of which runs slowly, it will be discovered that the green mat- ter serves as food for thousands of aquatic Insects, which make their habi- tations in it. These insects are endowed with powers of rapid locomotion, and possess a highly-organised structure ; in their turn they fall a prey to the Fishes which frequent such streams.” * Such is the general succession of nutritive actions in the Organised Creation. The highest Animal is either directly dependent upon the Vegetable Kingdom for the materials of its fabric, or it is furnished with these by some other Animal, this again * Prof. Draper, on the “ Forces which produce the Organization of Plants,” p. 15. OF LIGHT AS A CONDITION OF VITAL ACTIVITY. 47 (it may be) by another, and so on ; the last in the series being always necessitated to find its support in the Vegetable kingdom, since the Ani- mal does not possess the power of causing the Inorganic elements to unite into even the simplest Organic compound. This power is possessed in a high degree by Plants ; but it can only be exercised under the influence of Light, which is, therefore, the prime agent on which all the phenomena of Life depend. — We shall now examine more in detail the conditions of this influence, both in the instance just quoted, and in others drawn from the actions of the higher Vegetable organisms. 73. The “green matter of Priestley” (as it is commonly called), which makes its appearance when water of average purity is submitted to the action of the Sun’s light, and which also presents itself on the surface of walls and rocks that are constantly kept damp, is now known by Botanists to consist of cells in various stages of development, — the early forms, it may be, of several different species of Confervse. That these cells all originate from germs, and not from any direct combination of the inor- ganic elements, appears not only from general considerations, but also from the fact that, if measures be taken to free the water entirely from any possible infusion of organic matter, and to admit into contact with it such air alone as has undergone a similar purification, no green flocks make their appearance, under the prolonged influence of the strongest sunlight. We find, then, that the presence of a germ is one of the conditions indis- pensable to the new production in question. — It may be asked how it can be certainly ascertained that light and not heat is the essential condition of this process ; seeing that the two agents are combined in the solar beam. To this it may be replied, that a certain moderate amount of heat is undoubtedly necessary ; but that no degree of heat without light will be effectual in producing the change, as is easily proved by exposing the water to warmth in a dark place. Moreover, when a certain measure of light is afforded, variations in the amount of heat make very little differ- ence ; but we shall presently see that under the same degree of heat, the amount of the change is directly proportional to the intensity of the light. Although, therefore, heat furnishes an essential condition, it cannot be questioned that light is the chief stimulus to that process, by which the germ brings into union the elements to be employed in the development of its own fabric. — These elements are present in all water that has been exposed to the atmosphere, and has absorbed from it a certain amount of its constituent gases ; their relative quantities, in a given measure of water, being proportional to the facility with which they are respectively absorbed by the liquid. Thus carbonic acid is most readily absorbable, oxygen next, and nitrogen least so. From the experiments of Prof. Draper it would appear, that notwithstanding the very small proportion of car- bonic acid contained in the atmosphere (usually not more than 1-2 000th part), this gas forms as much as 29 per cent, of the whole amount of air expelled from water by boiling. The absolute quantity of this water-gas contained in any measure of water, is subject to variation with the tem- perature ; the quantity being diminished as the temperature rises. — Now when water thus impregnated with carbonic acid, oxygen, and nitrogen, -and containing the germs of simple aquatic plants, is exposed to the sun’s light, so that a development of vegetable organisms takes place, it will be found that the carbonic acid is diminishing in amount, and that oxygen is 48 GENERAL PHYSIOLOGY. being evolved. The growing mass increases in volume and weight ; and after a time exhausts the whole carbonic acid originally contained in the water. If then it be prevented from receiving an additional supply, the process stops ; but as conducted naturally, there is a free exposure to the atmosphere, through which carbonic acid is diffused ; and hence, as fast as it is removed by decomposition, it is restored by absorption. 74. The process whose conditions we have thus examined, is canned on in the individual cells that compose the highest and most complex Plants, precisely as in those which constitute the entire forms of the lowest. Thus if a few garden-seeds of any kind be sown in a flower-pot, and be caused to germinate in a dark room, it. will soon be perceived that although they can continue growing for a time without the influence of light, that time is limited ; the weight of their solid contents diminishes, although their bulk may increase by the absorption of water ; their young leaves, if any should be put forth, are of a yellow or gray-white colour, and they soon fade away and die. But if these plants are brought out sufficiently soon into the bright sun-light, they speedily begin to turn green, they un- fold their leaves, and evolve their different parts in a natural way ; and the proportion of their solid contents goes on increasing from day to day. If the fabric be then subjected to chemical analysis, it is found to contain oxygen, hydrogen, carbon, and azote ; united in various proportions, so as to form compounds, many of which differ in the various species, though some are common to all. If plants be made to grow in closed glass ves- sels, under such circumstances that an examination can be accurately made as to the changes they are impressing on the atmosphere, it is discovered that they are constantly decomposing its carbonic acid,— appropriating its carbon, and setting free its oxygen, — so long as they are exposed to the influence of sunshine or bright daylight. They also appropriate a part of the minute quantity of ammonia which is diffused through, the atmosphere ; extracting its nitrogen to employ it in the production of then- azotised compounds. It is capable of being demonstrated by experiment, that these changes are confined to the green surfaces of plants, and therefore to the leaves or leaf-like organs, the young shoots, and the stems of herbaceous plants or of those in which (as in the Cactus tribe) the leaves are wanting and the enlarged succulent stem supplies their place. When these sur- faces cease to become green, the decomposing action also ceases ; carbon is no longer fixed, and oxygen set free ; but, on the contrary, carbonic acid is exhaled : this is the case when the leaves change colour, previously to their fall, in the autumn. The compounds which are thus generated in the green surfaces, are conveyed, by the circulation of the sap, to the re- mote parts of the fabric, and become the materials of then - nutrition : and thus the green cells of the leaves have exactly the same function, in minis- tering to the growth of the largest tree, which the green cells of the humble Conferva perform in regard to themselves alone. 75. Yarious experiments have been recently made, with the view of determining more precisely the conditions under which Light acts, in pro- ducing the chemical changes that have been now discussed. These expe- riments for the most part agree in the veiy interesting result, that the amount of carbonic acid decomposed by plants subjected to the differently- coloured rays of the solar spectrum, but otherwise placed in similar cir- cumstances, varies with the illuminating power of the rays, and not with OF LIGHT AS A CONDITION OF VITAL ACTIVITY. 49 their heating or their chemical power. The method adopted by Prof. Draper (op. cit. p. 177), which seems altogether the most satisfactory, con- sisted in exposing leaves of grass, in tubes filled with water which had been saturated with carbonic acid (after the expulsion of the previously dissolved air by boiling), to the influence of the different rays of the solar spectrum, dispersed by a prism ; these were kept motionless upon the tubes, for a sufficient length of time to produce an active decomposition of the gas, in the tubes which were most favourably influenced by the solar beams ; and the relative quantities of the oxygen set free were then mea- sured. It was then evident that the action had been almost entirely con- fined to two of the tubes, one of them being placed in the red and orange part of the spectrum, and the other in the yellow and green. The quantity of carbonic acid decomposed by the plant in the latter of these, was to that decomposed in the former, in the ratio of nine to Jive ; the quantity found in the tube that had been placed in the green and blue portion of the spectrum, would not amount, in the same proportion, to one ; and in the other tubes, it was either absolutely nothing, or extremely minute. Hence it is obvious that the yellow ray, verging into orange on one side, and into green on the other, is the situation of the greatest exciting power possessed by Light on this most important function of plants ; and as this coincides with the seat of the greatest illuminating power of the spectrum, it can scarcely be doubted that light is the agent here con- cerned ; more especially as the place of greatest Heat is in the red ray, and that of greatest Chemical power is in the blue, both of which rays were found to be nearly inert in the experiment just quoted. It must not be supposed from this experiment, however, that the yellow ray, and those immediately adjoining it, are the only sources of this power in the Solar spectrum ; since it proves no more than that, when the leaves were exposed to a highly carbonated atmosphere, they could only decom- pose it under the influence of these rays. It is certain, from other experiments, that plants will grow, in an ordinary atmosphere, under rays of different colours ; and it appears that the amount of carbon they severally fix, bears a constant proportion to the illuminating powers of the respective rays. 7 6. Although this fixation of carbon by the decomposition of carbonic acid, is the most essentially-dependent, of all the processes of the Vegetable economy, upon the influence of Light, yet it is not the only one, especially among the higher Plants, in which that agent becomes an important con- dition. The Exhalation of superfluous fluid by the stomata of the leaves (chap, xv.) is also very much affected by it ; and through its influence upon these two important operations, Light affects, more or less directly, almost every process in the Vegetable economy. The effect of its complete and continued withdrawal from a growing plant, is to produce an etiolation or blanching of its green surfaces ; a loss of weight of the solid parts, owing to the continued disengagement of carbon from its tissues, unbalanced by the fixation of that element from the atmosphere ; a dropsical distension of the tissues, in consequence of the continued absorption of water, which is not got rid of by exhalation ; a want of power to form its peculiar secretions, or even to generate new tissues, after the materials previously stored up have been exhausted ; in fine, a cessation of all the operations most necessary to the preservation of the vitality of the structure, of which 50 GENERAL PHYSIOLOGY. cessation its death is the inevitable result. A partial withdrawal of the influence of light, however, is frequently used by the Cultivator, as a means of giving an esculent character to certain Plants, which would be otherwise altogether uneatable • for in this manner their tissues are ren- dered more succulent and less stringy, whilst their peculiar secretions are formed in diminished amount, and communicate an agreeable flavour instead of an unwholesome rankness of taste. 77. There is one period in the life of the Flowering Plant, however, in which the influence of Light is injurious instead of beneficial ; this is during the first part of the process of germination of seeds, which is decidedly retarded by its agency. This forms no exception, however, to the general rule ; since the decomposition of the carbonic acid of the atmosphere, and the fixation of carbon in the tissues, do not constitute a part of the process : on the contrary, the chemical changes taking place in that substance of the seed, which has been stored up for the nutrition of the embryo, involve the opposite change, — the extrication of carbon, which is converted into carbonic acid by uniting with the oxygen of the atmosphere. It is obvious, then, why light should not only be use- less, but even prejudicial, to this process ; since it tends to fix in the tissues the carbon which ought to be tin-own off.— The non-luminous chemical rays, however, are stated by Mr. Hunt to favour this process ; so that seeds will germinate in the soil beneath blue glass, — which allows these to pass freely, whilst keeping back a large proportion of the lumin- ous rays, — even more readily than usual ; whilst, on the other hand, ger- mination is retarded by the use of yellow glass, which obstructs the chemical or ‘ actinic ’ rays, whilst it allows free passage to the luminous and calorific. 78. The influence of Light upon the direction of the growing parts of Plants, the opening and closing of flowers, &c., is probably due to its share in the operations already detailed. Thus the green parts of plants, or those which effect the decomposition of carbonic acid (such as the leaves and stems), have a tendency to grow towards the light ; whilst the roots, through whose dark sm-faces carbonic acid is thrown out by respiration, have an equal tendency to avoid it. That the first direction of the stems and roots of plants is very much influenced by this agent, appears from the fact, demonstrated by Schultze, that, by reflecting light upon germi- nating seeds, in such a manner that it shall only fall upon them from below, the stems are caused to direct themselves downwards, whilst the roots grow upwards. — The periodical movements of the flowers of certain plants appear to be partly under the control of light, although without doubt subject to other influences. Some flowers open in the morning and close at evening ; others, on the contrary, expand at night, and fold toge- ther early in the day. When withdrawn for a long time from the stimu- lus of light, these movements cease and the flowers remain closed ; but by artificial illumination, they may be made to recommence ; and Decandolle found that, by preserving plants in a cellar, which was at night lighted with lamps, and by day kept dark, then- natural times could be reversed. This, however, could only be effected when then- original tendency had been subdued by the continued deprivation of the stimulus ; and the experiment was not found to succeed with all plants. — Light has been found to have a still more direct influence on the development of particular OF LIGHT AS A CONDITION OF VITAL ACTIVITY. 51 organs in certain Vegetables. Thus when the gemmae * of the Marchantia polymorpha (§ 275) are in process of development, it has been shown, by the repeated experiments of Mirbel, that stomata are formed on the side exposed to the light, and that roots grow from the lower surface ; and that it is a matter of indifference which side of the little disk is at first turned upwards, since each has the power of developing stomata, or roots, accord- ing to the influence it receives. After the tendency to the formation of these organs has once been given, however, by the sufficiently prolonged influence of light upon one side, and of darkness and moisture upon the other, any attempt to alter it is found to be vain ; for if the siu'faces be then inverted, they are soon restored to their original aspect by the twist- ing growth of the plant. 79. The same amount of this stimulus is not requisite or desirable for all Plants. Nature has provided in the constitution of each for the periodical changes, which the alternation of day and night, and the succes- sion of the seasons, produce in the degree of illumination afforded to it. We find some adapted to exist only where they can be daily invigorated by the powerful rays of a tropical sun • and others, whose energies, after remaining dormant during the tedious winter of the arctic regions, are aroused into a brief activity by the • return of the luminary on whose cheering influence they depend. Neither race could flourish if transferred to the external conditions of the other ; for to each are the circumstances of its existence natural, its original constitution being framed to take advantage of them. Generally speaking, the succulent thick-leaved Plants require the largest amount ; then - stomata are few in number ; and the full influence of light is requisite to induce sufficient activity in the exhaling process : accordingly we find them growing, for the most part, in exposed situations, where there is nothing to interfere with the full influ- ence of the solar rays. On the other hand, plants with thinner and more delicate leaves, in which the exhaling process is easily excited to an excessive amount, evidently find a congenial home in more sheltered situations ; and there are some which can only develope themselves in full luxuriance, in the deep shades of a plantation or a forest. By a fur- ther adaptation of the same kind, some species of Plants are enabled to live and acquire their green colour, under an amount of deprivation which would be fatal to most others ; thus in the mines of Freyberg, in which the quantity of light admitted must be almost infinitesimally small, Hum- boldt met with Flowering Plants of various species ; and Mustard and Cress have been raised in the dark abysses of the collieries of this country. Generally speaking, however, the Cryptogamia would seem to be better adapted than Flowering Plants, to carry on their vegetating processes under a low or very moderate amount of this stimulus. Thus Humboldt found a species of Sea-weed near the Canaries, which possessed a bright grass-green hue, although it had grown at a depth of 190 feet in the sea, where, according to computation, it could have received only 1-1 500th part of the solar rays that would have fallen upon it at the surface of the ocean. Many Ferns, Mosses, and Lichens seem as if they avoided the light, choos- ing the northern rather than the southern sides of hedges, buildings, &c., * These gemmse are analogous to the buds of higher plants; and they consist of little collections of cells, arranged in the form of flat disks ; which are at first attached by foot- stalks to the parent plant, but afterwards fall off, and are developed into new individuals. 52 GENERAL PHYSIOLOGY. for their residence ; so that the former often present a luxuriant growth of Cryptogamic vegetation, whilst the latter are comparatively bare. It must not be supposed, however, that they avoid light altogether, but only what is to them an excessive degree of it. The avoidance of light seems to be much stronger in the Fungi, which grow most luxuriantly in very dark situations ; and the reason of this is probably to be found in the fact, that, like the germinating seed, they form rather than decompose carbonic acid ; their food being supplied to them from the decaying sub- stances on which they grow ; and the rapid changes in then- tissues giving rise to a high amount of Respiration, — a change exactly the converse of that, on which, as we have seen, Light exerts such a remarkable stimu- lating power. 80. In regard to the influence of Light upon the functions of Animals, comparatively little is certainly known. It is evident that the influence it exerts on those chemical processes, which constitute the first stage of Vegetable nutrition, can have scarcely anyplace in Animals ; because they do not perform any such acts of combination, but make use of the pro- ducts already prepared for them by Plants. Hence we do not find that the surface of Animals undergoes that extension, for the purpose of being exposed to the solar rays, which is so characteristic a feature in the Vege- table fabric, and so important in its economy. — Still there can be no doubt, that the degree of exposure to light has a great influence upon the colours of the Animal svu-face ; and here we seem to have a manifestation of Chemical agency, analogous to that which gives colour to the Vegetable surface. Thus it is a matter of familiar experience, that the influence of light upon the skin of many persons, causes it to become spotted with brown freckles ; these freckles being aggregations of brown pigment-cells, which either owed their development to the stimulus of light, or were enabled by its agency to perform a chemical transformation which they could not otherwise effect. In like manner, the swarthy hue, which many persons acquire in warm climates, is due to a development of dark pig- ment-cells diffused through the epidermis ; and an increased development of the same kind gives rise to the blackness of the Negro-skin. There can be no doubt that the prolonged influence of light upon one generation after another, tends to give a permanent character to this variety of hue ; which will probably be more easily acquired, in proportion to the pre- viously-existing tendency to that change. Thus it is well known that a colony of Portuguese Jews, which settled at Tranquebar about three cen- turies ago, and which has kept itself distinct from the surrounding tribes, cannot now be distinguished as to colour from the native Hindoos. But it is probable that a similar colony of fair-skinned Saxons would not, in the same time, have acquired anything like the same depth of colour in their skins. — -There can be no doubt that the brilliancy of colour, which is characteristic of many tribes of animals in tropical climates, especially of Birds and Insects, is in great part dependent, like the brightness of the foliage and fruit of the same countries, upon the intensity of the light to which their sm-faces are exposed. When birds of warm climates, distin- guished by the splendour of then’ plumage, are reared under an artificial temperature in our own country, it is uniformly observed that they are much longer in acquiring the hues characteristic of the adidt ; and that these are never so bright, as when they have been produced by the influ- OF LIGHT AS A CONDITION OF VITAL ACTIVITY. 53 ence of the tropical sun. And it lias been also remarked, that if certain Insects (the Cockroach for example), which naturally inhabit dark places, be reared in an entire seclusion from light, they grow up almost as colour- less as Plants that are made to vegetate under similar circumstances. 81. There is abundant proof, that Light exercises an important influ- ence ou the processes of development in Animals, no less than in Plants. Thus, the appearance of Animalcules in infusions of decaying organic matter is much retarded, if the vessel be altogether secluded from it. The rapidity with which the small Entomostracous Crustacea (Water-Fleas, &c.) of our pools, undergo their transformations, has been found to be much influenced by the amount of light to which they are exposed. Even when full-grown, they continue to exuviate their horny casing every two or three days ; and this change is necessary for their welfare, since, if it does not take place, their bodies become clothed with minute plants, which attach themselves to their surface, impeding their motions through the water, and preventing them from breathing with freedom. If they be secluded from light, they are not able to renew their shells with the usual frequency ; and cannot so often get rid, therefore, of their troublesome incumbrances. Further, it has been ascertained that, if equal numbers of Silk-worms’ eggs be preserved in a dark room, and in one exposed to com- mon daylight, a much larger proportion of larva are hatched from the latter than from the former. — The most striking proof of the influence of Light on Animal development, however, is afforded by the experiments of Dr. Edwards. He has shown that, if Tadpoles be nourished with proper food, and be exposed to the constantly-renewed contact of water (so that their respiration may be freely carried on, whilst they remain in their Fish-like condition), but be entirely deprived of light, their growth continues, but their metamorphosis into the condition of air-breathing animals is arrested, and they remain in the condition of large tadpoles. It is interesting to remark that the Proteus anguineus (§323 ) an animal which closely cor- responds in its fully-developed condition with the transition-stage between the Tadpole and the Frog, finds a congenial abode in the dark lakes of the caverns of Styria and Camiola, and in the underground caverns that con- nect them ; thus showing its adaptation to a condition, which keeps down to the same standard the development of an animal, that is em- powered under other circumstances to advance beyond it. — Numerous facts, collected from different sources, lead to the belief that the healthy development of the Human body, and the rapidity of its recovery from disease, are greatly influenced by the amount of Light to which it has been exposed. Thus it was remarked by Dr. Edwards, that persons who live in caves or cellars, or in very dark and narrow streets, are apt to pro- duce deformed children ; and that men who work in mines are liable to disease and deformity, beyond what the simple closeness of the atmo- sphere would be likely to produce. Part of this difference is doubtless owing to the relative purity of the atmosphere in the former case, and the want of ventilation in the latter ; but other instances might be quoted, in which a marked valuation presented itself, under circumstances otherwise the same. Thus, it has been stated by Sir A. Wylie (who was long at the head of the medical staff in the Russian army), that the cases of disease on the dark side of an extensive barrack at St. Petersburgh, have been uniformly, for many years, in the proportion of three to one, to those on 54 GENERAL PHYSIOLOGY. the side exposed to strong light. And in one of the London Hospitals, with a long range of frontage looking nearly due north and south, it has been observed that residence in the north wards is much less conducive to the welfare of the patients, than in those on the south side of the build- ing. On the other hand, the more the body is exposed to the influence of light, the more freedom do we find, ceteris paribus, from irregular action or conformation. Humboldt has remarked that, among several nations of South America who wear very little clothing, he never saw a single indi- vidual with a natural deformity ; and Linnaeus, in his account of his tom 1 through Lapland, enumerates constant exposure to solar light as one of the causes, which render a summer journey through high northern lati- tudes so peculiarly healthful and invigorating. 82. These facts being kept in view, it is easy to perceive that there must be differences among the various species of Animals, as among those of Plants, in regard to the degree of light which is congenial to them. Among the lowest tribes, in which no special organs of vision exist, there is evidently a susceptibility to the influence of light, which appears scarcely to deserve the name of sensibility, but which seems rather analogous to that which is manifested by Plants ; thus among those Polypes which are not fixed to particular spots, and amongst Animalcules, there are some species which seek the light, and otheis which shun it. And it appears from various observations upon the depths at which marine animals are found, especially from the extensive series of facts collected by Prof. E. Forbes,* that there is a series of zones, so to speak, to be met with in descending from the surface towards the bottom of the ocean, each of which is characterised by certain species of animals peculiar to itself, whilst other species have a range 'through two or more of the zones ; — the extent of the range of depth, in each species, bearing a close correspondence with the extent of its geographical distribution. Now there can be no doubt, that the restriction of particular species to particular zones is due in great part to the degree of pressure of the surrounding medium ; but there can be as little doubt, that the variation in the degree of light also exerts a most important influence, the solar rays in their passage through sea water being subject to a loss of one half for every seventeen feet. From the results of Prof. Forbes’s researches, it appeal’s that no species of Invertebrated animals habitually live at a greater depth than 300 fathoms ; and although Fishes have been captured at a depth of from 500 to 600 fathoms, it is probable that they had strayed from their usual abodes. That an almost infinitesimally small amount of Light, however, is a sufficient stimulus to the vital activity of some animals, appeal’s from the fact mentioned by Capt. Scoresby, that various species of Star-fish, of the most brilliant and beautiful markings, were brought up at the end of a line of 250 fathoms. 83. Of Seat . — The most perfectly organised body, supplied with all the other conditions requisite for its activity, must remain completely inert, if it do not receive sufficient stimulation from Heat. For each Vital action requires a certain amount of Caloric for its performance, and can only continue within a particular range of temperature ; between the limits of which, it is excited by the additional application of the stimulus, and depressed by its abstraction. Different species of Animals and Vegetables * Report on the Invertebrata of the iEgean Sea, in Transactions of British Association, 1843. OF HEAT AS A CONDITION OF VITAL ACTIVITY. 55 exhibit great varieties in the limitations of temperature which they require, aud in their power of adaptation to extreme conditions ; and hence in part arises their diversity of geographical distribution. As a general rule it may he stated, that the greater the amount and variety of vital action, the more immediate is the dependence of the individual upon the maintenance of its usual temperature. We shall hereafter see (Chap, xvii.) that, whilst the higher Animals possess within themselves the means of generating the heat necessary to sustain their vital activity, the lower Animals, as well as Plants in general, are almost entirely dependent upon the medium they inhabit for the necessary supply of caloric ; it being- only during one or two periods of the existence of the more perfect kinds, that any sensible degree of heat is generated by them, and this only for local purposes. But, being thus dependent, then’ vital actions are so adjusted as to be carried on within wide extremes of heat and cold ; and it is in them that we can most clearly trace the influence of Caloric upon vital activity. This is attested, on the large scale, by the striking contrast between the dreary barrenness of polar regions, and the luxuriant richness of Tropical countries, where almost every spot to which moisture is sup- plied teems with Animal and Vegetable life. And the alternation of Winter and Summer in temperate climates may be almost said to repeat under our own view the opposite conditions of these two extreme cases. The Vegetable world, in the depth of winter, is reduced to a state of death-like torpor ; all the actions of growth are suspended below the freezing point, not merely in the tree completely bared of its foliage, but also in the £ evergreen ’ that presents more of the semblance of life ; and this condition is prolonged until an elevation of temperature takes place. Even a low degree of warmth suffices to produce a renewal of vital activity ; and this not merely in evergreens, but in trees unclothed with leaves, a movement of sap taking place in their stems and branches during every mild and sunny day in winter, and the leaf-buds being gradually prepared to start into full development with the steadier warmth of the returning spring. The influence of Heat upon Vegetation is easily made apparent by experiment ; in fact, experimental illustrations of it, upon a large scale, are in daily progress. For the gardener, by artificial warmth, is not only enabled to rear with success the plants of tropical climates, whose constitution would not bear the chilling influence of our winter ; — but he can also, in some degree, invert the order of the seasons, and pro- duce both blossom and fruit from the plants of our own country, when all around seems dead. 84. To every species of Plant there is a temperature which is most con- genial, from its producing the most favourable influence on the actions (whether physical or vital) concerned in the maintenance of its life • and by this admirable provision we find that each zone is endowed with the power of sustaining a vegetation of its own, which would not flourish elsewhere. Thus in the countries lying near the Equator, the vegetation consists in great part of dense forests of leafy evergreen trees, Palms, Bamboos, and Tree-ferns, bound together by clustering Orchidese and strong creepers of various kinds. There are no verdant meadows, such as form the chief beauty of our temperate regions ; and the lower orders of Vegetation are extremely rare. It is only in this torrid Zone, that Dates, Coffee, Cocao, Bread-fruit, Bananas, Cinnamon, Cloves, Nutmegs, Pepper, Myrrh, Indigo, 56 GENERAL PHYSIOLOGY. Ebony, Logwood, Teak, Sandal-wood, and many other of the vegetable products most highly valued for them flavour, their odour, their colour, or their density, come to full perfection. — As we recede from the Equator, we find the leafy Evergreens giving place to trees with deciduous leaves 3 rich meadows appear, abounding with tender herbs 3 the Orchidese no longer find in the atmosphere, and on the surface of the trees over which they cluster, a sufficiency of moisture for their support, and the parasitic species are replaced by others which grow from fleshy roots implanted in the soil 3 but aged trunks are now clothed with Mosses 3 decayed vegetables are covered with parasitical Fungi 3 and the waters abound with Conferva?. In the warmer parts of the temperate regions, the Apricot, Citron, Orange, Lemon, Peach, Fig, Vine, Olive, and Pomegranate, the Myrtle, Cedar, Cypress, and Dwarf Palm, find then - congenial abode. — These give place, as we pass northwards, to the Apple, the Plum, and the Cherry, the Chesnut, the Oak, the Elm, and the Beech. Going further still, we find that the fruit-trees are unable to flourish, but the timber-trees maintain their ground. Where these last fail, we meet with extensive forests of the various species of Firs 3 the Dwarf Parches and Willows replace the larger species of the same kind 3 and even near or within the arctic circle, we find wild flowers of great beauty, the Mezereon, the yellow and white Water-Lily, and the Globe-flower. 85. Where none of these can flourish, where trees wholly disappear, and scarcely any flowering plants are to be met with, an humbler Crypto- gamic vegetation still raises its head, in proof that no part of the Globe is altogether unfit for the residence of living beings, and that the empire of Flora has no limit. It is for the most paid, among the Cryptogamic tribes (the Ferns, Mosses, Liverworts, Fungi, Lichens, and Algae) that the greatest power of growing under a low temperature exists 3 and we accordingly find that the proportion of these to the Phanerogamia, or Flowering Plants, increases as we proceed from the Equator towards the Poles. It has been estimated by Humboldt, that, in Tropical regions, the number of species of Cryptogamia is only about one-tenth that of the Flowering Plants ; in the part of the Temperate zone which lies between Lat. 45° and 52°, the proportion rises to one-half ; and the relative amount gra- dually increases as we proceed towards the Poles, until, between Lat. 67° and 7 0°, the number of species of Cryptogamia equals that of the Phane- rogamia. Among the Flowering Plants, moreover, the greatest endurance of cold is to be found in those, which approach most nearly to the Cryp- togamia in the low degree of them development 3 thus the Glumaceous group of Endogens, including the Grasses, Bushes, and Sedges, which forms about one-eleventh of the whole amount of Phanerogamic vegetation in the Tropics, constitutes one-fourth of it in the Temperate regions, and one-third in the Polar 3 and the ratio of the Gymnospeimic group of Exogens, which chiefly consists of the Pine and Fir tribe, increases in like manner. Still the influence of a high temperature is evident even upon the Cryptogamia and then allies 3 for it is only under the influence of the light and warmth of tropical climes, that the Ferns, — the liighest among the former, — can develope a woody stem, and assume the character of trees 3 and it is only there that the tall Sugar-Canes, and the gigantic Bamboos, which are but Grasses on a large scale, can flourish. 86 . But distance from the Equator is by no means the only element. OP HEAT AS A CONDITION OP VITAL ACTIVITY. 57 in the determination of the mean temperature of a particular spot. Its height above the level of the sea is equally important ; for this produces a variation in the amount of heat derived from the Sun, at least as great as that occasioned by difference of latitude. Thus it is not alone on the summits of Hecla, Mont Blanc, and other mountains of arctic or tempe- rate regions, that we find a coating of perpetual snow ; we find a similar covering on the lofty summits of the Himalayan chain, which extends to within a few degrees of the tropic of Cancer ; and even on the higher peaks of that part of the ridge of the Andes, which lies immediately beneath the Equator. If, then, Temperature exert such an influence on Vegetation as has been stated, we ought to find on the sides of lofty mountains in tropical climates the same progressive alterations in the characters of the Plants that cover them, as we encounter in journeying from the equatorial towards the polar regions. This is actually the case. The proportion of Cryptogamia to Flowering Plants, for example, is no more than one-fifteentli on the 'plains of the Equatorial region ; whilst it is as much as one-fifth on the mountains. In ascending the Peak of Teneriffe, Humboldt remarked as many as five distinct Zones, which were respectively marked by the products which characterize different climates. Thus at the base, the vegetation is altogether tropical ; the Date-Palm, Plaintain, Sugar-Cane, Banyan, the succulent Euphorbia, the Dracoena, and other trees and plants of the torrid zone, there flourish. A little higher grow the Olive, the Vine, and other fruit-trees of Southern Europe; there Wheat flourishes ; and there the ground is covered with grassy herbage. Above this is the woody region, in which are found the Oak, Laurel, Arbutus, and other beautiful hardy evergreens. Next above is the region of Pines ; characterised by a vast forest of trees resembling the Scotch Fir, intermixed with Juniper. This gives place to a tract remarkable for the abundance of Broom ; and at last the scenery is ter- minated by Scrofularia, Viola, a few Grasses, and Cryptogamic plants, which extend to the borders of the perpetual snow that caps the summit of the mountain. 87. There are some examples of the adaptation of particular forms of Vegetable life to extremes of temperature, which are interesting as showing the extent to which this adaptation may be carried. In hot springs near a river of Louisiana, of the temperature of from 122° to 145°, there have been seen to grow not merely Confervse and herbaceous plants, but shrubs and trees ; and a hot-spring in the Manilla islands, which raises the ther- mometer to 187°, has plants flourishing in it, and on its borders. A species of Chara has been found growing and reproducing itself in one of the hot- springs of Iceland, which boiled an egg in four minutes ; various Con- ferva), &c., have been observed in the boiling-springs of Arabia and the Cape of Good Hope ; and at the island of New Amsterdam, there is a mud-spring, which, though hotter than boiling-water, gives birth to a species of Liverwort. — On the other hand, there are some forms of Vege- tation, which seem to luxuriate in degrees of cold, that are fatal to most others. Thus the Lichen, which serves as the winter food of the Rein- deer, spreads itself over the ground whilst thickly covered with snow ; and the beautiful little Protococcus nivalis, or Red Snow, reddens extensive tracts in the arctic regions, where the perpetual frost of the surface scarcely yields to the influence of the solar rays at Midsummer. 58 GENERAL PHYSIOLOGY. 88. Of the particular processes of Vegetation, those of Flowering and Germination appear to he most dependent upon a specific amount, of heat. Thus we generally find that a degree of warmth which is barely sufficient for the development of leaves, will keep back that of flowers ; whilst, on the other hand, flowers are rapidly withered by the scorching rays whose in- fluence can be borne by leaves with impunity. And we occasionally find in Flowers a provision for generating heat independently of external sources. — This is still more remarkable in the germination of seeds, which can only take place within a comparatively limited range of temperature ; a circumstance that greatly contributes to limit the geographical range of plants. We have a striking example of this in the case of the Cereal ia (corn-plants), which civilized Man endeavours to cultivate wherever climate and soil admit of their growth. The limit, in regard to heat at least, is rather set by the necessity for a moderate soil-temperature in the stage of germination, than by the effect of the solar rays or of a heated atmosphere upon the growing plant. Thus it has been found by experiment that Wheat will not germinate in water at a higher temperature than 95°, that which is most suitable to the process being about 65° ; and we at once see, therefore, why it cannot grow in the torrid zone, where the tempera- ture of the soil is not unfrequently 120°, sometimes even rising to 140°. But Maize will germinate in water at 113° ; and hence it will grow in situa- tions too hot for wheat. On the other hand, the limit to the germination- temperature of Barley is lower than that of wheat, and it will consequently fail to grow in a soil whose heat can be borne by wheat ; whilst, in virtue of the same property, it will thrive in climates too cold to produce wheat. 89. The effects of Temperature on Vegetation are not only seen in its influence upon the Geographical distribution of Plants, that is, in the limitation of particular species to particular climates ; for they are also shown, perhaps even more remarkably, in the variation in the size of indi- viduals of the same species, when that species possesses the power of adapting itself to widely-differeut conditions, which is the case with some. Thus the Cerasus Virginiana grows in the southern states of North America as a noble tree, attaining one hundred feet in height ; in the sandy plains of the Saskatchawan, it does not exceed twenty feet : and at its northern limit, the Great Slave Lake, in Lat, 62°, it is reduced to a shrub of five feet. — Another curious effect of heat is shown in its influence on the sexes of certain Monoecious flowers ; thus Mr. Knight mentions that Cucumber and Melon plants will produce none but male or staminiferous flowers, if their vegetation be accelerated by heat ; and all female or pis- tilliferous, if its progress be retarded by cold. 90. The influence of excessive Heat in destroying life, can sometimes be traced through the direct physical or chemical changes which it occa- sions in the vegetable tissues and fluids. Thus it has been ascertained that grains of corn will vegetate, after exposure to water or vapour pos- sessing a considerable degree of heat ; provided that heat do not amount to 144° in the case of water, and 167° in that of vapour. At these tem- peratures, the structure of the seed undergoes a disorganising change, by the rupture of the vesicles of starch which form a large paid of it ; and the loss of its power of germinating is therefore readily accounted for. So, again, when a growing plant is killed by immersion in boiling water, it is easy to conceive that the loss of vitality is due to the coagulation of OF HEAT AS A CONDITION OF VITAL ACTIVITY. 59 these albuminous matters, which are - actively concerned in the most important functions of the organism. The mode in which excessive heat usually operates, is to increase the aqueous evaporation, and thus to carry off the water that is requisite for the due performance of the various chemical and physical changes on which life depends. If the supply be adequate to the demand, the effect of heat will be to stimulate all the vital operations of the plant, and to cause them to be performed with increased energy ; though this energy may be such as to occasion a pre- mature exhaustion of its powers, by the excessive luxuriance which it occasions. But if the supply of water be deficient, the plant is burnt up by the continuance of heat in a dry atmosphere ; and it either withers and dies, or its tissues become dense and contracted without losing their vitality. Thus it has been remarked, that shrubs growing among the sandy deserts of the East, have as stunted an appearance as those attempt- ing to vegetate in the Arctic regions ; their leaves being converted into prickles, and their leaf-buds prolonged into thorns instead of branches. 91. It scarcely admits of doubt, that the destructive influence of a very low temperature upon the Vitality of Plants, is immediately exerted through its chemical and physical effects upon the tissues and their con- tents. Thus it will produce congelation of their fluids ; and the expansion which takes place in freezing will injure the walls of the containing cells, — distending, lacerating, or even bursting them. The same cause will probably occasion the expulsion of air from some parts which ought to contain it ; and the introduction of it into other parts which ought to be filled with fluid. And a separation will take place, in the act of freezing, between the constituent parts of the vegetable juices ; which will render them unfit for discharging then - functions, when returning warmth would otherwise call them into activity. Hence we are enabled in some degree to account for the differences in the power of resisting cold, which the various species of Plants, and even the various parts of the same indivi- dual, are found to possess. For, other things being equal, the power of each plant, and of each part of a plant, to resist a low temperature, will be in the inverse ratio of the quantity of water contained in the tissue ; thus, a succulent herbaceous plant suffers more than one with a hard woody stem and dense secretions ; and young shoots are destroyed by a degree of cold, which does not affect old shoots and branches of the same shrub or tree. Again, the viscidity of the fluids of some plants is an obstacle to their congelation, and therefore enables them to resist cold ; thus it is, that the resinous Pines are, of all trees, those which can endure the lowest temperature. The dimensions of the cells, too, of which the tissue is composed, appear to have an influence ; the liability to freeze being diminished by a very minute subdivision of the fluids. And when the roots are implanted deep in the soil, where the temperature does not fall so low as that of the surface by many degrees, the fluidity of the sap may be maintained, in spite of an extremely cold state of the atmosphere. It is in Cryptogamic plants, that the greatest power of sustaining cold exists ; as might be inferred from what has been already stated in regard to their geographical distribution. The little Fungus ( Torula Cerevisice ) which is one of the principal constituents of Yeast, does not lose its vitality by exposure to a temperature of 7 6° below zero, though it requires a somewhat elevated temperature for its active growth. It woidd appear 60 GENERAL PHYSIOLOGY. that Seeds are enabled to sustain a degree of cold, without the loss of their vitality, which would be fatal to growing plants of the same species ; thus grains of corn, of various kinds, will germinate after being exposed for a quarter of an hour to a temperature equal to that of frozen mercury. It is not difficult to account for this, when the closeness of then’ texture, and the small quantity of fluid which it includes, are kept in view. The act of Germination, however, will only take place under a rather elevated temperature ; and we find in the Chemical changes which it im*olves, a provision for maintaining this, when the process has once commenced. 92. The influence of Heat upon the vital activity of Animals, is quite as strongly marked as we have seen it to be in the case of Plants ; but the mode in which it is exerted is in many instances very different. In those Animals which are endowed with great energy of muscular movement, and in which, for the maintenance of that energy, the nutritive functions are kept in constant activity, we find that a provision exists for the deve- lopment of heat from within, so as to keep the temperature of the body at a certain uniform standard, whatever may be the climate in which they live. Their energy and activity are, in fact, so dependent upon the steady maintenance of a high temperature in then’ bodies, that, if this be not kept up nearly to its regular standard, a diminution or even a complete cessation of vital action takes place, and even a total loss of vitality may result. In these warm-blooded animals, as they are -termed, we do not so evidently trace the effects of Heat, because they are constantly being exerted, and because external changes have but little influence upon them, unless these changes be of an extreme kind. But if those internal operations, on which the maintenance of the temperature is dependent, are from any cause retarded or suspended, the effect is immediately visible, in the depressed activity of the whole system. In the class of Birds, whose muscular energy, and whose general functional activity, are greater and more constant than those of any other animals, the tempera- ture is pretty steadily maintained at from 108° to 112;° and we shall presently see, that a depression of the heat of the body to about 80° is fatal. Among Mammalia, the temperature is usually maintained at from 98° to 102° ; and it seems that here too a depression of about thirty degrees is ordinarily fatal. In the different tribes of Birds and Mammals, we find a very diversified power of generating heat ; and on this depends their adaptation to various climates. Where the usual tem- perature of the atmosphere is but little below the normal standard of the body, a small amount of the internal calorifying power is required ; and accordingly we find that animals which naturally inhabit the torrid zone, cannot be kept alive elsewhere, except, like the Plants of the same regions, by external heat. On the other hand, the animals of the colder- temperate and frigid climes are endowed with a much greater internal calorifying power ; and then’ covering is adapted to keep in the heat which they generate. Such animals (the Polar Bear for example) cannot be kept in health, in the summer of our own country, unless means be taken for then - refrigeration. The constitution of Man seems to acquire, by habituation to a particular set of conditions through succes- sive generations, an adaptation to differences of climate, of which that of few other animals is susceptible ; and thus we find different races of human beings inhabiting countries, which are subject to the extremes of OF HEAT AS A CONDITION OF VITAL ACTIVITY. 61 heat and cold. — There is a small number of species, which, being endowed with a similar adaptiveness to temperature, can accompany Man in all his migrations. 93. The conditions on which the power of maintaining the heat of the body, in despite of external cold, is dependent, will become the subject of inquiry hereafter (chap. xvii.). It is sufficient here to state, that this power is in great part the result of numerous Chemical changes going on within the body ; and especially of a process analogous to combustion, in which carbon and hydrogen taken in as food are made to unite with oxygen derived from the atmosphere. It is dependent, therefore, as to its amount, upon the due supply of the combustible material on the one hand, and of atmospheric air on the other. If the former be not fur- nished either by the food, or by the fatty matter of the body (which acts as a kind of reserved store laid up against the time of need), the heat cannot be maintained ; and it is in part for want of power to digest and assimilate a sufficient amount of this kind of aliment, that animals of warm climates cannot maintain their temperature in colder regions. On the other hand, if the supply of oxygen be deficient, as it is when the respiration is impeded by diseased conditions of various kinds, there is a similar depression of temperature. — Now if, from either of these causes, the temperature of the body of a Bird or Mammal (except in the case of the hybernciting species of the latter, to be presently noticed) be lowered to about 30° below its usual standard, not only is there a cessation of vital activity, but a total loss of vital properties ; in other words, the death of the animal is a necessary result. This occurrence is preceded by a gra- dually-increasing torpidity ; which shows the depressing influence of the cooling process upon the functions in general. The temperature of the superficial parts of the body is, of course, soonest affected ; the circulation is at first retarded, causing lividity of the skin ; but, as the temperature becomes lower, the blood is almost entirely expelled from the surface, by the contraction of the vessels, and paleness succeeds. At the same time, there is a gradually-increasing torpor of the nervous and muscular systems, which first manifests itself in an indisposition to exertion of any kind, and then in an almost irresistible tendency to sleep. At the same time, the respiratory movements become slower, from the want of the stimulus that should be given by the warm current of blood to the medulla oblon- gata, which is the centre of those movements ; and the loss of heat goes on, therefore, with increased rapidity, until the temperature of the whole body is so depressed, that its vitality is altogether destroyed. — But when there is a deficiency of the proper animal heat, the vital activity of the system may be maintained by caloric supplied by external sources. This fact is of high scientific value, as giving the most complete demonstration of the immediate dependence of the vital functions of warm-blooded animals upon a sustained temperature ; and its practical importance can scarcely be over-rated. It rests chiefly upon the recent experiments of Chossat, who had in view to determine the circumstances attending death by Inanition or starvation. He found that, when Pigeons were entirely deprived of food and water, their average temperature underwent a toler- ably regular diminution from day to day ; so that, after several days (the exact number varying with their previous condition), it was about 4J° lower than at first. Up to this time, it seems that the store of fat laid up 62 GENERAL PHYSIOLOGY. in the body supplies the requisite material for the combustive process ; so that no very injurious depression of temperature occurs. But, as soon as this is exhausted, the temperature falls rapidly, from hour to hour ; and as soon as the total depression has reached 29i° or 30°, death supervenes. Yet it was found by M. Chossat, that when animals thus reduced by star- vation, whose death seemed impending (death actually taking pjlace in many instances, whilst the preliminary processes of weighing, the applica- tion of the thermometer, &c., were being performed), were subjected to artificial heat, they were almost uniformly restored, from a state of insen- sibility and want of muscular power, to a condition of comparative acti- vity. Their temperature rose, their muscular power returned, they took food when it was presented to them, and them secretions were renewed ; and, if this artificial assistance was sufficiently prolonged, and they were supplied with food, they recovered. If the heat was withdrawn, however, before the time when the digested food was ready, in sufficient amount, to supply the combustive process, they still sank for want of it. 94. There is a certain group of warm-blooded animals, however, which offers a remarkable peculiarity in respect to heat ; — their power of generating it being for a time greatly diminished or almost completely suspended ; the temperature of them bodies following that of the all- around, so that it may be brought down nearly to the freezing-point ; then- general vital actions being carried on with such feebleness as to be scarcely perceptible ; and yet the vital properties of the tissues being retained, so that, when the temperature of the body is again raised, the usual activity returns. This state, which is called hybernation, appeal's to be as natural to certain animals, as sleep is to all ; and it corresponds with sleep in its tendency to periodical return. No account can be given of the causes to which it is due : but the condition of the animals presenting it offers several points of much interest. There are some, as the Lagomys, in which it appears to differ but little from deep ordinary sleep ; they retire into situations which favour the retention of them warmth ; and they occasionally wake up, and apply themselves to some of the store of food, which they have provided in the autumn. In other cases, a great accu- mulation of fat takes place within the body in autumn, favoured by the oily nature of the seeds, nuts, Arc,, on which the animals then- feed ; and this serves the purpose of maintaining the temperature for a sufficient length of time, not indeed to the usual standard, but to one not far below it. The state of torpor in these animals is more profound than that of deep sleep, but it is not such as to prevent them from being easily aroused ; and their respiratory movements, though diminished in fre- quency, are still performed without interruption. But in the Marmot, and in animals which, like it, liybemate completely, the temperatime of the body (owing to the want of internal power to generate heat) and the general vital activity, are proportionally depressed ; the respiratory move- ments fall from 500 to 14 per hour, and are performed without any consi- derable enlargement of chest ; the pulse sinks from 150 to 15 beats per minute ; the state of torpidity is so profound that the animal is with dif- ficulty aroused from it ; and the heat of the body is almost entirely dependent upon the temperature of the surrounding air, not being usually more than a degree or two above it. When the thermometer in the air is somewhat below the freezing-point, that placed within the body frills to OF HEAT AS A CONDITION OF VITAL ACTIVITY. 63 about 35° ; and at this point it may remain for some time without any apparent injury to the animal, which revives when subjected to a higher temperature. When, however, the body is exposed to a more intense degree of cold, the animal functions undergo a temporary renewal ; for the cold seems to act like any other stimulus in arousing them. The respiratory movements and the circulation increase in activity, so as to generate an increased amount of heat ; but this amount is insufficient to keep up the temperature of the body, which is at last depressed to a degree inconsistent with the maintenance of life ; and not only the suspension of activity, but the total loss of vital properties is the result. 95. Now the condition of a hybernating Mammal closely resembles that of a cold-blooded animal, in regal’d to the dependence of its bodily tem- perature upon external conditions. There is this important difference, how- ever ; — that the reduction of the temperature of the former to 60° or 50° is incompatible with a state of activity, which is only exhibited when the heat of then bodies rises to nearly the usual Mammalian standard ; whilst a permanently low or moderate temperature is natural to the bodies of most cold-blooded animals, whose functions could not be well carried on under a higher temperature. Thus all the muscles of a Frog are thrown into a state of permanent and rigid contraction, by the immersion of its body in water no warmer than the blood which naturally bathes those of the Bird ; and we find, accordingly, that cold-blooded animals which can- not sustain a high temperature, are provided with a frigorifying rather than with a calorifying apparatus. — Although we are accustomed to rank all animals, save Birds and Mammals, under the general term cold-blooded , yet there exist among them considerable diversities as to the power of generating heat within themselves, and of thus rendering themselves independent of external variations. These diversities, which will be par- ticularly considered, in their proper place (chap, xvii.), will usually be found to have reference to the nervous and muscular activity by which the species or tribe is characterized ; as is seen especially in the classes of Fishes and Insects. 96. But there is abundant evidence of the influence of Heat, not merely upon the movements -of cold-blooded animals, but also upon the processes of nutrition and development. Thus the time of emersion of Insect-larvse from their eggs, — or in other words, the rate at which the previous formative processes go on, is entirely dependent upon the tempe- rature. In the case of the Bird we find that, if the temperature be not sufficient to develope the egg, chemical changes soon take place, which involve the loss of its vitality ; or if the temperature be reduced so low as to prevent the occurrence of those changes, the loss of heat is in itself destructive of life. But this is not the case in regard to the eggs of cold- blooded animals in general ; for, like the beings they are destined to produce, they may be reduced to a state of complete inaction by a depres- sion of the external temperature ; whilst a slight elevation of this renews their vital operations, at a rate corresponding to the warmth supplied. Hence the production of larvse from the eggs of Insects may be accele- rated or retarded at pleasure ; and this is, in fact, practised in the rearing of Silk-worms, in order to adapt the time of then’ emersion from the egg to the supply of food which is ready for them. The same may be said in regard to the eggs of other cold-blooded animals ; those, for 64 GENERAL PHYSIOLOGY. example of the minute Entomostracous Crustacea which people our ditches and ponds. In many of these, the race is continued solely by the eggs which remain dormant through the winter ; all the parents being destroyed by the cold. The common Daphnia pulex produces two kinds of eggs ; from one, the young are very speedily hatched ; but the others, which are produced in the autumn, and enveloped in a peculiar covering, do not give birth to the contained young until the succeeding spring. They may be at any time hatched, however, by artificial warmth. — Phe- nomena of an equally interesting and instinctive character may be observed in the history of the Pupa-state of Insects ; which, in those that undergo a complete metamorphosis, may be almost characterised as a re-entrance into the egg, for the attainment of a higher grade of deve- lopment (§315). This process of development is remarkably influenced by external temperature ; being accelerated by genial warmth, and retarded by cold. There are many Larvae, which naturally pass into the Pupa state during the autumn, remain in it during the entire winter, and emerge as perfect Insects with the return of spring. It was found by Reaumur, that Pupae, which would not naturally have been disclosed imtil May, might be caused to undergo then metamorphosis during the depth of winter, by the influence of artificial heat ; whilst, on the other hand, their change might be delayed a whole year beyond its usual time, by the prolonged influence of a cold atmosphere. A very curious provision exists in the economy of the Social Bees, in order to hasten the development of their pupae (chap. xvii. Sect. 3). — It has been remarked by Mr. Paget that the processes of development seem to require a higher degree of vital force than those of simple growth; and the facts just stated, when viewed in this light, afford an interesting confirmation to the hypothesis advanced in the early part of this chapter, as to the relation between the Vital Force and Heat (§54). 97. The influence of variations in the Heat of the body upon its vital activity, is further manifested by the very remarkable experiments of Dr. Edwards ; who has shown that Cold-blooded animals live much faster (so to speak) at high temperatures, than at low ; so that they die much sooner, when deprived of other vital stimuli. Thus when Frogs were con- fined in a limited quantity of water, and were not permitted to come to the surface to breathe, it was found that the duration of their lives was inversely proportional to the degree of heat of the fluid. When it was cooled down to the freezing-point, the frogs immersed in it lived during from 367 to 498 minutes. At the temperature of 50°, the dura- tion of their lives was from 350 to 375 minutes ; at 72°, it was from 90 to 35 minutes ; at 90°, from 12 to 32 minutes; and at 108° death was almost instantaneous. The prolongation of life at the lower tempera- tures is not due to torpidity, for the animals perform the functions of voluntary motion, and enjoy the use of their senses ; but it is occasioned by then- diminished activity, which occasions a less demand for ah*. On the other hand, the elevation of temperature increases the demand for air, and causes speedier death when it is withheld, by increasing the general agility. The natural habits of these animals are in correspond- ence with these facts. During the winter, the influence of a sufficient amount of aerated water upon their exterior serves to maintain the required amount of respiration through the skin, so that they are not OF HEAT AS A CONDITION OF VITAL ACTIVITY. 65 obliged to come to the surface to take in air by the mouth. As the sea- son advances, however, their activity increases, a larger amount of respi- ration is required, and the animals are obliged to come frequently to the surface to breathe. During summer, the yet higher temperature calls forth an increased energy and activity in all the vital functions ; the respiration must be proportionably increased • the action of the air upon the cutaneous surface, as well as upon the lungs, is required ; and if the animals are prevented from quitting the water to obtain this, they die, as soon as the warmth of the season becomes considerable. — The result of experiments on Fishes, in regal’d to the deprivation or limited supply of the air contained in the water in which they are immersed, is exactly similar ; the duration of life being inversely as the temperature. And precisely the same has been ascertained with respect to hybernating Mam- mals ; which, as already remarked, are for a time reduced, in all such respects, to the level of cold-blooded animals. 98. The energy of the reparative actions in higher animals appears to be much influenced by temperature ; as might be inferred from what has been just said of their nutritive and developmental operations. Thus in Frogs, w T e find that wounds are healed much more rapidly in summer than in winter ; and it has been recently stated that the Triton (water-newt) possesses the power of reproducing its lost limbs between the temperature of 58° and 75°, but cannot do so without the stimulus of that degree of warmth. * 99. We have seen that the animals termed cold-blooded are greatly influenced as to the temperature of then’ bodies, by the temperature of the surrounding medium ; although many of them are endowed with the power of keeping themselves a certain number of degrees above it. Now the consequence of this is, that all of them which are subject to any con- siderable and prolonged amount of cold, pass into a state of more or less complete inactivity during its continuance ; which state bears a close correspondence with the hybernation of certain Mammalia. Among the Reptiles of cold and temperate countries, this torpid state uniformly occupies a considerable part of the year ; as it does also with Insects, terrestrial Mollusks, and other Invertebrated animals, which are subject to the influence of the cold. On the other hand, Fishes, Crustacea, and other marine animals, do not usually appear to pass into a state of tor- pidity ; the temperature of the medium they inhabit never undergoing a degree of depression, nearly so great as that of the atmosphere. The amount of change necessary to produce this effect, or on the other hand to call the animals from a state of torpidity to one of active energy, differs for different species ; and there is probably a considerable difference even among individuals of the same species, according to the temperature under which they habitually live. Thus one animal may remain torpid under a degree of warmth, which will be sufficient to arouse another of the same kind, accustomed to a somewhat colder climate ; because the stimulus is relatively greater to the latter. It was observed by Mr. Darwin, that at Bahia Blanca in South America, the first appearance of activity in animal and vegetable life, a few days before the vernal equinox, presented itself under a mean temperature of 58°, the range of the thermometer in the middle of the day being between 60° and 70°. The plains were ornamented * Mr. Higginbottom, in Proceedings of the Royal Society, March 18, 1847. F GG GENERAL PHYSIOLOGY. by the flowers of a pink wood-sorrel, wild peas, evening primroses, and geraniums ; the birds began to lay their eggs ; numerous beetles were crawling about ; and lizards, the constant inhabitants of a sandy soil, were darting about in every direction. Yet a few days previously, it seemed as if nature had scarcely granted a living creature to this dry and arid country ; and it was only by digging in the ground that then- existence had been discovered, — several insects, large spiders, and lizards, having been found in a half torpid state. Now at Monte Video, four degrees nearer the Equator, the mean temperature had been above 58° for some time previously, and the thermometer rose occasionally during the middle of the day to 69° or 70°; yet with this elevated temperature, nearly equivalent to the full summer heat of our own country, almost every beetle, several genera of spiders, snails, and land-shells, toads and lizards, were still lying torpid beneath stones. We have seen that at Bahia Blanca, whose climate is but a little colder, this same temperature, with a rather less extreme heat, was sufficient to awake all orders of animated beings ; — showing how nicely the required degree of stimulus is adapted to the general climate of the place, and how little it depends on absolute tem- perature. 100. We may leam much from the Geographical distribution of the different species of cold-blooded animals, in regard to the influence of tem- perature on Animal life. No general inferences of this kind can be founded upon the distribution of warm-blooded animals ; since them own heat-evolving power makes them in great degree independent of external warmth. And it is probably from the distribution of the marine tribes, whose extension is least influenced by local peculiarities, that the most satisfactory deductions are to be drawn. In regard to the class of Crus- tacea, which is the one that has been most fully investigated in this inspect, the following principles have been pointed out by M. Milue Edwards ; and they are probably more or less applicable to most others. I. The varieties of form and organisation manifest themselves more, in 'proportion as we pass from the Polar Seas towards the Equator. — Thus on the coast of Norway, where there is frequently a vast multiplication of individuals of the same species, the number of species is very small ; but the latter increases rapidly as we go southwards. The number of species of Crustacea of the two highest orders, for example, known to exist on the coast of Norway and in the neighbouring seas, is only 16 ; but 82 are known to be inhabitants of the western shores of Britain, France, Spain, and Portugal ; 114 are known in the Mediterranean Sea; and 202 in the Indian Ocean. A similar increase may be observed in following the coast of the New World, from Greenland to the Caribbean Sea. II. The differences of form and organisation are not only more numerous and more characteristic in the warm than in the cold regions of the globe : they are also more important. — The number of natural groups which we find represented in the polar and temperate regions, is much smaller than that of which we find types or examples in the tropical seas. In fact, nearly all the principal forms which are met with in colder regions, also present themselves in warm ; but a veiy large proportion of the latter have no representatives among the former. Of the three primary groups composing the Class, indeed, one is altogether wanting beyond the 44th degree of latitude ; and in the other two there are whole Orders, as well OF HEAT AS A CONDITION OF VITAL ACTIVITY. as numerous subordinate divisions, which are as completely restricted to the warmer seas. III. Not only are those Crustacea , which are most elevated in the scale, deficient in the Polar regions ; hut their relative number increases rapidly as we pas§ from the Pole towards the Equator. — Thus the Brachyoura, which must be considered as the most elevated of the whole series, are totally absent in some parts of the arctic region ; and we find their place taken by the far less complete Edriophthalma, with a small number of Anomourous and Macrourous Decapods. In the Mediterranean, however, the Decapods surpass the Edriophthalma in regard to the number of species ; and the Brachyourous division of the former predominates over the Macrourous, in the proportion of two to one. And in the East and West Indies, the species of Brachyoura are to those of Macroura, as three, four, or even five, to one. Again, the Land Crabs, which are probably to be regarded as taking the highest rank among the Brachyoura (and there- fore in the entire class), are only to be met with between the tropics. Moreover, of the fiuviatile Decapods (inhabiting rivers, brooks, and fresh- water lakes,) a large proportion belong, in tropical regions, to the elevated type of the Brachyoura ; whilst all those found in the temperate and arctic zones (the River Cray-fish and its allies) belong to the Macrourous division. IV. When we compare together the Crustacea of different parts of the world, we observe that the average size of these animals is considerably greater in tropical regions, than in the temperate or frigid climes. — The largest species of the arctic and antarctic seas are far smaller than those of the tropical ocean ; and they bear a much smaller proportion to the whole number. Further, in almost every natural group, we find that the largest species belong to the equatorial regions ; and that those which represent them (or take their place, as it were) in temperate regions, are of smaller dimensions. V. It is where the species are most numerous and varied, and where they attain the greatest size, — in other words, where the temperature is most elevated, — that the peculiarities of structure which characterise the several groups, are most strongly manifested. Thus the transverse development of the cephalo-thorax, which is so remarkable in the Brachyourous Decapods (the breadth of the carapace or arched shell in the typical Crabs being much greater than its length from back to front), is carried to its greatest extent in certain Crustacea of the Equatorial region ; and the same might be said of the characteristic peculiarities of most other natural groups. Fur- ther, it is in this region that we find the greatest number of those anomalous forms, which depart most widely from the general structure of the Class. VI. Lastly, there is a remarlcable coincidence between the temperature of different regions, and the prevalence of certain forms of Crustacea.— T\ms there are few genera to be met with in the West Indian seas, which have not their representatives in the East Indian, — the species, however, being usually different. The same may be said of the genera inhabiting the temperate regions of the globe ; — similar generic forms being usually met with in the corresponding parts of the Old and New World, and of the Northern and Southern Hemispheres, although the specific are almost invariably different. G8 GENERAL PHYSIOLOGY. 101. Now although, as appears from the foregoing general statements, the number of species of Crustacea inhabiting the colder seas bears a very small proportion to that which is found within the tropics ; and although the species formed to inhabit cold climates are so far inferior, both as to size, and as to perfection of development ; yet it does not follow that the same proportion exists in regard to the relative amount of Crustacean life in the two regions, for this depends upon the multiplication of individuals. In fact it may be questioned whether there is any inferiority in this respect ; so abundant are some of the smaller species in the Arctic and Antarctic, as well as in the Temperate seas. Thus we see that a low range of temperature is as well adapted to sustain their life, as a higher range is to call forth those larger and more fully-developed forms, which abound in the tropical ocean. There is an obvious reason why the seas of the frigid zones should be much more abundantly peopled than the land ; the mean temperature of the former being much higher. And it would almost seem as if Nature had intended to compensate for the dreariness and desolation of the one, by the profuseness of life which she has fitted the other to support. 102. The influence of Temperature in modifying the size of individual Animals of the same species, is not so strongly marked as it is in the case of Plants ; for this reason, perhaps, that an amount of continued depres- sion or elevation, which might be sustained by a Plant, but which would exert a modifying influence upon its growth, would be fatal to an Animal formed to exist in the same climate. Instances are not wanting, however, in which such a modifying influence is evident ; and these, as might be anticipated, are to be met with chiefly among the cold-blooded tribes. Thus the Bidimus rosaceus, a terrestrial Mollusk, is found on the moun- tains of Chili of a size so much less than that which it attains on the coast, as to have been described as a distinct species. And the Litton na petrosa found on the south side of Plymouth Breakwater, acquires, from its superior exposure to light and heat (though perhaps also from the greater supply of nutriment which it obtains) twice the size common to individuals living on the north side within the harbour. — The following circumstance shows the favourable influence of an elevated temperature, in producing an unusual prolificness in Fish ; which must be connected with general vital activity. Three pairs of Gold-fish were placed, some years since, in one of the engine-dams or ponds common in the manufacturing districts, into which the water from the engine is conveyed for the purpose of being cooled ; the average temperature of such dams is about 80°. At the end of three years, the progeny of these Fish, which were accidentally poisoned by verdigris mixed with the refuse tallow from the engine, were taken out by wheelbarrows-full. It is not improbable that the unusual supply of aliment, furnished by the refuse grease that floats upon these ponds (which would impede the cooling of the water, if it were not consumed by the Fish), contributed with the high temperature to this unusual fecundity. 1 03. Although a very low temperature is positively inconsistent with the continuance of vital activity, in Animals as in Plants, yet we find that even very severe Cold is not necessarily destructive of the vital properties of organized tissues ; so that, on a restoration of the proper amount of heat, their functions may continue as before. Of this, the case of frost- bitten limbs furnishes an example; but the fact is much more remarkable. OF HEAT AS A CONDITION OF VITAL ACTIVITY. 69 when considered in reference to the whole body of an animal, and the com- plete suspension of all its functions. Yet it is unquestionably true, not only of the lowest and simplest members of the Animal kingdom, but also of beings so high as Fishes and Reptiles. — In one of Captain Ross’s Arctic Voyages, several Caterpillars of the Laria Rossii, having been exposed to a temperature of 40° below zero, froze so completely, that, when thrown into a tumbler, they chinked like lumps of ice. When thawed, they re- sumed their movements, took food, and underwent their transformation into the Chrysalis state. One of them, which had been frozen and thawed four times, subsequently became a Moth. — The eggs of the Slug have been exposed to a similar degree of cold, without the loss of their fertility. — It is not uncommon to meet, in the ice of rivers, lakes, and seas, with Fishes which have been completely frozen, so as to become quite brittle ; and which yet revive when thawed. — The same thing has been observed in regard to Frogs, Newts, Ac. ; and the experiment of freezing and subse- quently thawing them, has been frequently put in practice. Spallanzani kept Frogs and Snakes in an ice-house for three years ; at the end of which period they revived on being subjected to warmth. — It does not appear, however, that the same capability exists, in regard to any warm- blooded animals; since if a total suspension* of vital activity take place in the body of a Bird or Mammal for any length of time, in consequence of the prolonged application of severe cold, recovery is found to be impos- sible. The power which exists in these animals, however, of generating a large amount of heat within their bodies, acts as a compensation for the want of the faculty possessed by the cold-blooded tribes ; since they can resist for a great length of time (if in then - healthy or normal condition) the influence of a temperatiu-e, sufficiently depressed to produce a com- plete suspension in the activity of the classes below them. 104. It only remains to say a few words regarding the degree of Heat which certain Animals can sustain without prejudice, and which even appears to be genial to them. Among the higher classes, this range seems to be capable of great extension. Thus many instances are on record, of a heat of from 250° to 280° being endured, in dry air, for a considerable length of time, without much inconvenience ; and persons who have become habituated to this kind of exposure can (with proper precautions) sustain a temperature of from 350° to 500°. In all such cases, however, the real heat of the body undergoes very little elevation ; for, by means of the copious evaporation from its surface, the external heat is prevented from acting upon it. But if this evaporation be prevented, either by an insufficiency in the supply of fluid from within, or by the saturation of the surrounding air with moisture, the temperature of the body begins to rise ; and it is then found, that it cannot undergo an elevation of more than a few degrees, without fatal consequences. Thus in several experiments which have been tried on different species of warm-blooded animals, for the purpose of ascertaining the highest temperature to which the body could be raised without the destruction of life, it was found that as soon as the heat of the body had been increased, by continued immersion in a limited quantity of hot air (which -would soon become charged with mois- ture), to from 9° — 13° above the natural standard, the animals died. In * bi the case of hybernating Mammals, the suspension is not total ; and if it be rendered such, the same result follows as in other instances. 70 GENERAL PHYSIOLOGY. general, Mammals die, when the temperature of their bodies is raised to about 111°; the heat which is natural to the bodies of Birds. The latter are killed by an equal amount of elevation of bodily heat above their natural standard. 105. Hence we see that the actual range of temperature, within which vital activity can be maintained by warm-blooded animals, is extremely limited; a temporary elevation of the bodily heat to 13° above the natural standard, or a depression to 30° below it, being positively incon- sistent, not merely with the continuance of vital operations, but also with the preservation of vital properties : and a continued departure from that standard, to the extent of only a very few degrees above or below it, being decidedly injurious. The provisions with which these animals are endowed, for generating heat in their interior, so as to supply the external deficiency, and for generating cold (so to speak), when the external temperature is too high, are therefore in no respect superfluous : but are positively necessary for the maintenance of the life of such animals, in any climate, save one whose mean should be conformable to their stand- ard, and whose extremes should never vary more than a veiy few degrees above or below it. Such a climate does not exist on the surface of the earth. 106. The range of external temperature, within which cold-blooded animals can sustain their activity, is much more limited, as well in regal’d to its highest as to its lowest point ; notwithstanding that the range of bodily heat, which is consistent with the maintenance of their life, is so much greater. In those which, like the Frog, have a soft moist skin that permits a copious evaporation from the surface, a considerable amount of heat may be resisted, provided the air be dry, and the supply of fluid from within be maintained. But immersion in water of the tem- perature of 108°, is almost immediately fatal. In many other cold- blooded animals, elevation of the temperature induces a state of torpidity, analogous to that which is produced by its depression. Thus the Helix pomatia (Edible Snail) has been found to become torpid and motionless in water at 112° ; but to recover its energy when placed in a colder situa- tion. It would seem to be partly from this cause, but partly also from the deprivation of moisture, that the hottest part of the tropical year brings about a cessation of activity in many tribes of cold-blooded animals, as complete as that which takes place during the winter of temperate climates. The highest limit of temperature compatible with the life of Fishes has not been certainly ascertained : and it appears probable that there are considerable variations in this respect amongst different species. Thus it is certain that there are some which are killed by immersion in water at 1 04° ; whilst it is also certain that others can not only exist, but can find a congenial habitation, in water of 113°, or even of 120°; and examples of the existence of Fishes in thermal springs of a much higher temperature than this, have been put on record. Various fresh-water Mollusca have been found in thermal springs, the heat of which is from 100° to 145°. Kotifera and other animalcules have been met with in water at 112°. Larvae of Tipuke have been found in hot springs of 205°; and small black beetles, which died when placed in cold water, in the hot sulphur baths of Albano. Entozoa inhabiting the bodies of Mammalia and of Birds must of coiu’se be adapted to a constant temperature of from OF ELECTRICITY AS A CONDITION OF VITAL ACTIVITY. 71 98° to 110° ; and they become torpid when exposed to a cool atmosphere. These lowly-organized animals seem more capable of resisting the effects of extreme heat, than any others ; — at least if we are to credit the state- ment, that the Entozoa inhabiting the intestines of the Carp have been found alive, when the Fish was brought to table after being boiled. — In all such cases, it is to be remembered, the heat of the animal body must correspond with that of the fluid in which it is immersed ; and we have here, therefore, evident proof of the compatibility of vital activity, in cer- tain cases, with a very elevated temperature. Additional and more exact observations, however, are much wanting on this subject. 107. Of Electricity. — Much less is certainly known as to the ordinary influence of this agent, than in regard to either of the two preceding ; and yet there can be little doubt, from the effects we observe when it is power- fully applied, as well as from our knowledge of its connexion with all Chemical phenomena, that it is in constant though imperceptible operation. Electricity differs from both Light and Heat in this respect ; — that no manifestation of it takes place so long as it is uniformly diffused, or is in a state of equilibrium; but in proportion as this equilibrium is disturbed, by a change in the electric condition of one body, which is prevented, by its partial or complete insulation, from com- municating itself to others, in that proportion is a force produced, which exerts itself in various ways according to its degree. The mechanical effects of a powerful charge, when passed through a substance that is a bad conductor of Electricity, are well known ; on the other hand, the chemical effects of even the feeblest current are equally certain. The agency of Electricity in producing Chemical change is the more powerful, in proportion as there is already a predisposition to that change ; thus, as already remarked, the largest collection of oxygen and hydrogen gases, or of hydrogen and chlorine, mingled together, may be caused to unite by the minutest electric spark, which gives the required stimulus to the mutual affinities that were previously dormant. Hence it cannot but be inferred, that its agency in the Chemical phenomena of living- bodies must be of an important character ; but this may probably be exerted rather in the way of aiding decomposition, than of producing new organic combinations, to which (as we have seen) Light appears to be the most effectual stimulus. Thus it has been shown that pieces of meat, that have been electrified for some hours, pass much more rapidly into decomposition, than similar pieces placed under the same circumstances, but not electrified. And in like manner, the bodies of animals that have been killed by electric shocks, have been observed to putrefy much more readily than those of similar animals killed by an injury to the brain. It is well known, moroever, that in thundery weather, in which the electric state of the atmosphere is much disturbed, various fluids containing organic compounds, such as milk, broth, &c., are peculiarly disposed to turn sour ; and that saccharine fluids, such as the wort of brewers, are extremely apt to pass into the acetous fermentation. 1 08. The actual amount of influence, however, which Electricity exerts over a growing Plant or Animal, can scarcely be estimated. It would, perhaps, be the most correct to say, that the state of Electric equilibrium is that which is generally most favourable ; and we find that there is a pro- vision in the structure of most living beings for maintaining such an equili 72 GENERAL PHYSIOLOGY. brium, — not only between the different parts of their own bodies, but also between their own fabrics and the surrounding medium. Thus a charge given to any part of a Plant or Animal, is immediately diffused through its whole mass ; and though Organized bodies are not sufficiently good conductors to transmit very powerful shocks without being themselves affected, yet a discharge of any moderate quantity may be effected through them, without any permanent injury, — and this more especially if it be made to take place slowly. Now the points on the surfaces of Plants appear 1 particularly adapted to effect this transmission ; thus it has been found that a Leyden jar might be discharged by holding a blade of grass near it, in one third of the time required to produce the same effect by means of a metallic point ; and an Electroscope furnished with Vegetable points has been found to give more delicate indications of the electric state of the atmosphere, than any other. Plants designed for a rapid growth have generally a strong pubescence or downy covering ; and it does not seem improbable that one purpose of this may be, to maintain that equilibrium between themselves and the atmosphere, which would otherwise be disturbed by the various operations of vegetation, and espe- cially by the process of evaporation, which takes place with great activity from the surface of the leaves. 109. There appears to be sufficient evidence, that, chuing a highly electrical state of the atmosphere, the growth of the young shoots of certain Plants is increased in rapidity ; but it would be wrong thence to infer that this excitement is useful to the process of Vegetation in general, or that the same kind of electric excitement universally operates to the benefit or injury of the Plant. From some experiments recently made by Messrs. Solly and Sidney, it would appeal' that potatoes, mustard and cress, cinerarias, fuchsias, and other plants, have their development, and, in some instances, them productiveness, increased by being made to grow between a copper and a zinc plate, connected by a conducting wire • while, on the other hand, geraniums and balsams are destroyed by the same influence. The transmission of a series of moderate sparks through plants has been found, in like manner, to accelerate the growth of some, and to be evidently injurious to others. It is not unreasonable to suppose, that, as a great variety of chemical processes are constantly taking place in the growing plant, an electric disturbance, which acts as a stimulus to some, may positively retard others ; and that its good or evil results may thus depend upon the balance between these individual effects. This would seem the more likely from the circumstauce, that, in the process of Ger- mination, the chemical changes concerned in which are of a simpler cha- racter, Electricity seems to have a more decided and uniform influence. The conversion of the starch of the seed into sugar, which is an essential part of this change, involves the liberation of a large quantity of carbonic and of some acetic acid. Now as all acids are negative, and as like elec- tricities repel each other, it may be inferred that the seed is at that time in an electro-negative condition ; and it is accordingly foimd that the pro- cess of germination may be quickened, by connexion of the seed with the negative pole of a feeble galvanic apparatus, whilst it is retarded by a similar connexion with the positive pole. A similar acceleration may be produced by the contact of feeble alkaline solutions, which favour the liberation of the acids ; whilst, on the same principle, a very small OF ELECTRICITY AS A CONDITION OF VITAL ACTIVITY. 73 admixture of acid in the fluid with which the seed is moistened, is found to produce a decided retardation. 110. It is well known that Trees and Plants may be easily killed by powerful electric shocks ; and that, when the charge is strong enough (as in the case of a stroke of lightning), violent mechanical effects,— as the rending of trunks, or even the splitting and scattering of minute frag- ments, — are produced by it. But it has also been ascertained, that charges which produce no perceptible influence of this kind, may destroy the life of Plants ; though the effect is not always immediate. In parti- cular it has been noticed, that slips and grafts are prevented from taking- root and budding. There can be little doubt that, in these instances, a change is effected in the chemical state of the solids or fluids ; although no structural alteration is perceptible. The coagulation of the layer of albuminous matter which lines every growing cell, would be quite suffi- cient to account for the result. 111. In regard to the influence of Electricity upon the Organic func- tions of Animals, still less is certainly known • but there is evidence that it may act as a powerful stimulant in certain disordered states of them. Thus in Amenorrhea, a series of slight but rapidly-repeated electric shocks will often bring on the catamenial flow ; and it is certain that chronic tumours have been dispersed, and dropsies relieved by the excitement of the absorbent process, through similar agency. Again, it is indubitable that a higlily-electric state of the atmosphere produces very marked effects on the general state of many individuals ; and brings on in some a degree of languor and depression, wdiich cannot be accounted for in any other way. An instance is on record, in which the atmosphere was in such an extraordinary state of electric disturbance, that all pointed bodies within its influence exhibited a distinct luminosity ; and it was noticed, that all the persons who were exposed to the agency of this highly-elec- trified air, experienced spasms in the limbs and an extreme state of lassitude. 112. Animals, like Plants, are liable to be killed by shocks of Electri- city ; even when these are not sufficiently powerful to occasion any obvious physical change in their structure. But there can be no doubt that such Mechanical changes may be produced in their delicate parts, as are quite sufficient to account for the destruction of their vitality, even though these may be only discernible with the Microscope. The production of changes in the Chemical arrangement of their elements, is, however, a much more palpable cause of death ; since it may be fully anticipated beforehand, and can easily be rendered evident. To take one instance only ■ — it is well known, that albumen is made to coagulate, i. e. is changed from its soluble to its insoluble form, under the influence of an electric current ; and it cannot be doubted that the production of this change in the fluids of the living body (almost every one of which contains albumen), even to a very limited extent, is quite a sufficient cause of death, even in animals that are otherwise most tenacious of life. “ I once discharged a battery of considerable size,” says Dr. Hodgkin, “through a common Earth-worm, which would in all probability have shown signs of life long- after minute division. Its death was as sudden as the shock ; and the semi-transparent substance of the animal was changed like Albumen which has been exposed to heat.” — It has been noticed that the bodies of 74 GENERAL PHYSIOLOGY. animals killed by Electricity pass into decomposition with unusual rapidity ; a fact which affords an additional indication that the chemical condition of the organic components of the fabric has been affected. 113. Electricity possesses, in a remarkable degree, the power of exciting the Contractility of Muscular fibre, and the Nervous force ; but this series of phenomena will be more fitly described, when the properties of those tissues are under consideration (§§ 231 and 246). 4. Of Dormant Vitality. 114. There are many organized beings, whose vital activity is occasion- ally suspended, either wholly, or as regards some portion of their structure, without being altogether destroyed : and this may result either from the withdrawal of the stimidi necessaiy for the maintenance of the operations thus suspended, in which case the suspension may be brought about or terminated at any time ; or it may proceed from some change in the organism itself, whereby its capability of responding to those stimuli is for a time diminished or lost, in which case we generally find it presenting a tendency to 'periodical recurrence. — This suspension of vital activity cannot long continue without the entire disintegration of the tissues, and consecpient loss of then- vital properties, through that breaking-up of their peculiar composition, which is continually taking place even when they are in the state of greatest vital activity (§ 46), and which would now go on unchecked and uncompensated; unless at the same time the conditions to which the organism is subjected, be such as are unfavourable to the play of chemical affinities. Thus if a man fill into the water, or receive a concussion of the brain, and be reduced to a state of death-like torpor without vitality being actually destroyed, he can only remain in that condition for a short period ; for if means be not employed to restore the circulation, and thus to renew the other functions, death will speedily ensue, either from the lowering of the temperature below the point at which vitality departs (§ 93), or, if the bodily warmth be artificially maintained, from the decomposition which will then speedily ensue. But, on the other hand, if we subject a cold-blooded animal to a temperature of 32°, we reduce it to a state of torpidity in which it may remain for an unlimited period (§ 1 03) ; the reduction of its temperature not having in itself the power of destroying the vitality of the tissues, whilst it effec- tually prevents their decomposition ; so that, when they are again acted on by warmth, the tissues are precisely in the condition in which they were when their activity was suspended, and are ready for the perform- ance of their several functions. 115. Beings placed in this condition can scarcely be said to be alive, if Life be regarded as synonymous with vital activity ; on the other hand, they cannot be rightly designated as dead, since this implies their incapa- bility of ever resuming their previous state ; and they may be most appropriately said to be in a state of dormant vitality, since they retain their peculiar attributes without any manifestation of them, just as a sleeping man possesses mental power although its operation is for a time suspended. — Some of the most remarkable examples of it are presented by the spores or reproductive germs of the Fungi, which appear to be diffused in incalculable multitudes through the air, ready to staid into develop- OF DORMANT VITALITY. ID ment whenever the fitting conditions are afforded them (§ 271). We have no means, however, of ascertaining the length of time during which these almost infinitesimally minute particles can retain their vitality whilst floating about in the air ; but with respect to the seeds of Flowering- Plants, we have evidence that the term may be so prolonged as to be really unlimited. 116. One of the most interesting cases of this kind on record is thus related by Dr. Lindley. “ I have now before me,” he says, “ three plants of Raspberries, which have been raised in the gardens of the Horticultural Society, from seeds taken from the stomach of a man, whose skeleton was found 30 feet below the surface of the earth, at the bottom of a barrow which was opened near Dorchester. He had been buried with some coins of the Emperor Hadrian, and it is probable, therefore, that the seeds were sixteen or seventeen hundred years old.” — Instances are of very frequent occurrence, in which ground that has been turned up, spontaneously pro- duces plants dissimilar to any in their neighbourhood. There is no doubt that, in some of these, the seed is conveyed by the wind, and becomes developed in particular spots which afford congenial soil ; and this fact has a very interesting bearing upon the question of the production of Ani- malcules in infusions of decaying organic matter. Thus, it is commonly observed that clover is ready to spring up on soils, which have been ren- dered alkaline by the strewing of wood-ashes, or by the burning of weeds ; and it is stated by Professor Graham that, after any hill-pasture in Scot- land has been laid dry and limed, and the surface broken, white clover always makes its appearance. But there are many authentic facts, which can only be explained upon the supposition, that the seeds of the newly- appearing plants have lain for a long period imbedded in the soil, at such a distance from the surface as to prevent the access of air and moisture ; and that, retaining their vitality under these circumstances, they have been excited to germination when at last exposed to the requisite condi- tions. — Several cases of this kind are related by Dr. Prichard,* on the authority of Professor Graham ; amongst them is the following : — “ To the westward of Stirling there is a large peat-bog, a great part of which has been flooded away by raising water from the river Teith, and discharging it into the Forth, the under-soil of clay being then cultivated. The clergyman of the parish standing by while the workmen were forming a ditch in this clay, which had been covered with 1 4 feet of peat-earth, saw some seeds in the clay which was thrown out of the ditch : he took some of them up and sowed them ; they germinated and produced a crop of Chrysanthemum septum. What a period of years must have elapsed while the seeds were getting their covering of clay, and while this clay became buried under 14 feet of peat-earth !” 117. For the following very interesting case, not elsewhere published, the author is indebted to his valued friend, the late Dr. Tuckerman, of Boston, N. E. “About 25 or 30 years ago, Judge Thacher, then one of the Judges of the Supreme Court of Massachusetts, told me that he knew the fact, that in a town on the Penobscot river, in the state of Maine, and about 40 miles from the sea, some well-diggers, when sinking a well, struck, at the depth of about 20 feet, a stratum of sand, which strongly excited curiosity and interest, from the circumstance that no similar sand * Researches on the Physical History of Mankind, vol. i., p. 39, &c. 76 GENERAL PHYSIOLOGY. was to be found anywhere in the neighbourhood, and that none like it was nearer than the sea-beach. As it was drawn up from the well, it was placed in a pile by itself ; an unwillingness having been felt to mis it with the stones and gravel which were also drawn up. But when the work was about to be finished, and the pile of stones and gravel to be removed, it was found necessary to remove also the sand-heap. This, therefore, was scattered about the spot on which it had been formed, and was for some time scarcely remembered. In a year or two, however, it was perceived that a large number of small trees had sprung up from the ground over which the heap of sand had been strewn. These trees became, in their turn, objects of strong interest, and care was taken that no injury should come to them. At length it was ascertained that they were Beach- Plum trees ; and they actually bore the Beach-Plum, which had never before been seen except immediately upon the sea shore. These trees had, therefore, sprung up from seeds which were in the stratum of sea-sand that had been pierced by the well-diggers. By what convulsion of the elements they had been thrown there, or how long they had quietly slept beneath the surface of the earth, must be determined by those who know very much more than I do.” 118. In these and other similar instances, the complete seclusion of the seed from moisture and oxygen, and the maintenance of a uniform tem- perature, were sufficient to preserve its organic structure and composition entirely unchanged ; so that after the lapse of thousands or even of tens of thousands of years, its vital properties were still found capable of being aroused into vigorous activity. If the seclusion from ah’ and moisture be not complete, and the temperature be not sufficiently high for germination, a slow decay takes place in the seed, by which its germinating power is destroyed ; so that we perceive that it is only by the withdrawal of all those conditions under which its decomposition can possibly take place, that its vitality can be preserved. — Various examples of the reduction of Plants and Animals to a state of dormant vitality, in consequence of the withdrawal of the Heat or Moisture requisite to maintain their activity, have already come before us (§ 103) and (§ 65) ; in all of which the same general fact holds good, that this state, if complete, can only be prolonged by such a reduction of temperature, desiccation of the tissues, or complete exclusion of oxygen, as must necessarily prevent the disintegration of the tissues by the play of Chemical Affinities. In the hybernating Mammal, however, this state is not complete ; for the temperature of the body cannot be reduced below about 35° without the destruction of its vitality ; and we find that a feeble amount of vital activity continues during the whole period, rather, it would seem, for the purpose of getting rid of the products of decomposition as fast as they are set free, than for the purpose of adding anything to the structure. The same is not improbably the case in regard to the torpidity of Reptiles, &c., in the hot and dry season (§ 66). 119. That this state of Dormant Vitality does not always result solely from changes in the external conditions on which Life is dependent, but is connected in part with alterations in the constitution of the animal itself, which make it a part of its regular history, is rendered particularly obvious by a consideration of the phenomena attending the pupa stage of Insects. This stage is one in which foi'mative changes of a most important character take place, notwithstanding the appearance of complete torpor : OF DORMANT VITALITY. 77 but the activity of these changes varies in different cases, as they are per- formed in some instances in a few days or even hours, whilst in others they are spread over almost as many months ; and a considerable portion of the latter period must be passed in a state of complete inactivity. The influence of artificial heat or cold, in accelerating or retarding the trans- formation, has been already noticed (§ 96) ; but we have now to show that there are other causes, inherent in the animal itself, which tend to prolong the period of inactivity. — Thus in the Papilio Machaon there are two generations every year ; for the butterfly that comes forth in the early summer lays eggs, which rapidly come to maturity and produce another set of eggs later in the season, whose larvae pass into the pupa state before the winter. Now the pupa-stage of the first brood, which passes into it in Jidy, lasts only thirteen days ; but that of the second brood, which passes into it in September, lasts nine or ten months, the butterfly not appearing until the June following. The difference of tem- perature is obviously insufficient of itself to account for this remarkable diversity between the two periods. — The following, moreover, are cases in which a similar diversity exists, without any difference in external condi- tions. If a number of the pupae of Eriogaster lanestris, a moth whose larvae are common on the blackthorn in June, be selected at the same time* and placed under the same circumstances, the greater number of them will disclose the perfect insect in the February following ; but some wall not produce it until the February of the year ensuing ; and the remainder not before the same month in the third year. The same has been observed of the Arctia menclica, of which thirty-six pupae, grown from eggs laid by the same parent, produced twelve perfect insects in each of the three following seasons.* — The purpose of this curious adaptation is probably, as suggested by Mr. Kirby, to secure the race from being cut off by unfavourable seasons, or by some extraordinary increase of its natural enemies ; but the cause of it can only be sought for in the state of the organism itself, since there is here no variation in the external conditions to which the several individuals of the three broods have been subjected. 120. That such a predisposition really exists, appears also from the periodical manifestation of the tendency to pass into a state of dormant vitality more or less complete, wTiich is seen in a large proportion of Plants and Animals. Thus the ‘winter sleep’ (as it has been termed) of Plants is not referable solely to the depression of external temperature (§ 83) ; for it is easily shown that, in the trees of temperate climates, there is a tendency to its annual recurrence, arising out of their own succession of vital changes. Thus, the exuviation of the leaves is not the cause of their death, but its consequence ; when the cells of wliich they are composed have accomplished their term of life, they cease to perform the function for which they are created, they begin to undergo decay, and the dead and useless members are thrown off ; and this takes place whatever may be the temperature. Until new leaves shall have been developed, therefore, there must be a great reduction in the vital activity of the organism ; and it is by its agency in hastening or retarding this development, that exter- nal temperature exerts so powerful an influence over the duration and in- tensity of the torpor. Thus in the trees of tropical climates, we frequently see a new generation of leaf-buds called forth, in the course of a few weeks * Kirby and Spence’s Entomology, vol. iii. p. 266. 78 GENERAL PHYSIOLOGY. after the tree has been completely bared by the fall of the preceding set of leaves ; and two or even three generations may thus replace each other every year. In those of the coldest regions that can produce them, we see, on the other hand, a prolongation of the torpor until the year has far advanced ; and then with the lengthening days of summer, into which the dreary winter seems suddenly transformed, every form of vegetation is rapidly excited to activity. The constitution of the tropical and of the arctic Plant is adapted to its own periodicity ; so that neither can be charged by artificial means, without manifest injury. — So, again, in regard to Animals, it is certain that the tendency to hybernation (§ 94) results from a periodical alteration in the bodily constitution of the animal, which corresponds with the seasonal change, but is not dependent upon it. For although it is possible to reduce a Marmot or a Dormouse to this condi- tion, even in summer, by the application of artificial cold, yet the external temperature must then be lowered to a point much below that, at which hybernation naturally commences on the approach of winter. 121. The diurnal sleep of Vegetables and Animals is a phenomenon of the same general character. In all the higher Plants, there is a tendency to diminished activity during a part of the cycle of twenty-four horn's : which, though really affecting their entire series of nutritive operations, is chiefly manifested in the folding or drooping of the leaves and flowers. It appears from experiments made with a view to determine the conditions of this phenomenon, that it cannot be attributed solely to alterations in those external physical agents which most influence the processes of vegeta- tion, — such as light, heat, and moisture • but that, although capable of being modified by them, its occurrence is essentially dependent on a periodically-recurring change in the vital condition of the plant itself. — The sleep of Animals differs essentially in its chai’acter from that of Plants ; for it consists, not in a state of diminished energy of the nutritive functions, but in a cessation of the sensorial activity, dependent upon a suspension of the functional power of certain parts of the nervous system, during which there is reason to believe that the nutritive and reparative operations of those organs go on with even augmented rapidity. Still this complete suspension of the peculiar activity of the nervous substance is obviously a form of dormant vitality ; and it is one in which we can trace, better than in most other cases, the influence of preceding changes in the organism itself in bringing it about. For the occasional suspension of sensorial activity is requisite for the reparation of the destructive effects of that activity (§ 243) ; so that, however unfavourable may be the external circumstances, sleep will supervene as a necessary result of exhaustion when this has been carried very far. Thus, sleep may come on amidst the roar of artillery, and the excitement of a battle ; and it is related that Damien slept during his protracted tortm-es on the rack. The diurnal periodicity of sleep, — which is a fact common to most animals, although some are active during the repose of others, — is certainly favoured by the alternation of darkness with light in every twenty-fom- hours ; but after we have subtracted all that can be fairly attributed to this, or to acquired habits, there still remains a periodical character about this condition, which, if not absolutely determinate in itself, accommodates itself to the period of the earth’s rotation, more readily than to any other. * * See Art. Sleep, in the Cyclopaedia of Anatomy and Physiology. OP DEATH. 79 5. Of Death. 122. The limited duration of the vital activity of organised structures lias been several times referred to, as one of then - most distinctive charac- ters. We have seen that this activity may he suspended, even for lengthened periods, without the loss of the power of renewing it ; hut when the very organisms which appear thus tenacious of their vitality are called into active Life, they become subject, like all others, to that com- plete and final loss of their peculiar attributes, which we call Death. The duration of vital activity in different organised structures is extremely various ; depending partly upon the peculiar attributes of the structures themselves, the arrangement of their chemical components, the propor- tion of their solid and watery constituents, &c. ; and partly upon the activity of their Life — their duration being, ceteris paribus, inversely to the rate at which the various changes incident to their living condition take place in them. Hence that duration may be greatly affected by those external agencies which modify the vital activity ; and on the whole it would be more correct to say, that the natural death of each organism takes place when it has accomplished the whole series of changes it was adapted to perform, whether these have been executed at a more or at a less rapid rate, than to attempt to assign any definite limit of time to its duration as a living being. 123. The state of Death differs from that of Dormant Vitality in this, — that the suspension of vital activity which is temporary in the latter condition, is permanent and complete in the former. Thus a seed which will not germinate under any treatment, must be regarded as dead ; whilst another which does not differ from it in any perceptible character, but which retains its capability of development, must be considered as endowed with vitality. The idea of decay or decomposition is usually con- nected with that of death ; but there is no essential relation between them. For we have seen that decomposition is continually taking place during life ; whilst, on the other hand, decay does not necessarily super- vene upon death. There are many structures both in the Plant and Animal, which, when once consolidated by then- processes of growth, may resist decay for an unlimited period (§ 21) ; and this as well when they are detached from the living structure, as whilst they remain connected with it. Thus the heart-wood of a Tree may be preserved longer when used as timber, and placed in a dry situation, than when remaining in the trunk of which it forms the solid support, and subjected to various agencies that favour its decay (§ 125). — Even the softer azotized tissues of Animals, which much more readily pass into spontaneous decomposition under ordinary cireumstances, may be kept from it by the complete exclusion of oxygen, as when meat is preserved in air-tight tin cases, or by the entire expulsion of their water, as in the desiccation of the Rotifera (§ 310). 124. The phenomena of Death, like those of Life, are displayed to us in the simplest form in those humble Plants in which eveiy cell is really a distinct individual ; since we have not there to make the distinction which becomes necessary in the case of the higher organisms, between the death of the individual parts and that of the fabric as a whole. And the case is nearly the same with those that consist of a homogeneous mass of 80 GENERAL PHYSIOLOGY. cells, which are developed so rapidly as to be almost of the same age, and which all perish together, — as is the case with the softer Fungi. Here we find that, as soon as vital activity has finally ceased, the whole struc- ture decays ; its components being resolved, by a new arrangement of their molecules in combination with oxygen derived from without, into carbonic acid, water, &c. — But with forms of vegetable structure some- what higher in the scale, the case is different; for each of these consists of an aggregation of cells, every one of which has its own independent vitality, goes through its own series of changes, and dies at its own time, without reference to the rest ; and yet by the development of new cells in continuity with those which are themselves about to undergo dissolution, the life of the mass may be prolonged almost indefinitely. Thus, as remarked by Schleiden, we might yet find floating about, on the great Fucus bank of Cervo and Flores, plants of Sargassum which were cut into strips by the bark of Columbus ; and the masses of Fungous vegetation which form the progressively-extending fairy rings or magic circles of the grassy meadow, may be said to be several years of age, although, as fast as new tissue is generated on the exterior of the ring, that of the central side dies and decays, so that no individual part has more than a brief duration. 125. Passing upwards to the higher Plants, we find that with the in- crease of heterogeneousness in the structure and of variety in its functions, the bond of mutual dependence becomes more intimate ; so that the death of one part has a tendency to destroy the vitality of the whole, by inter- rupting the actions which are requisite to maintain it. This is best seen in the herbaceous plants whose duration is limited to a single season ; for to such the destruction of any entire set of organs, such as the leaves or roots, is necessarily and immediately fatal ; and the whole fabric dies together, as soon as the seeds of a new generation have been matured. A perennial Tree, on the other hand, may be regarded as a composite struc- ture, made up of a number of integral plants ; and its continued existence depends upon the successive development of new individuals, as the old ones die and decay. This may take place to an unlimited extent. We have numerous examples of trees whose age is calculated at from one to five thousand year’s ; and there is no proof that, under favourable circum- stances, any such tree ever died from the ‘ weakness of old age.’ In most cases, death takes place as a consequence of mechanical injuries. A branch is broken off by a storm, and the broken surface is exposed to the action of rain-water ; this causes decay of the woody structure, and the decay gradually spreads inwards to the heart-wood of the trunk ; finally, a new storm casts the whole tree to the ground, separates the leaves from the roots by the fracture of the trunk, and the death of the whole is the inevitable result. — If no such interference take place, however, the deve- lopment of new leaf-buds, and the formation of new layers of wood and bark, may take place as long as the cambium-layer (§ 28 0) retains its activity ; and as this is being continually regenerated, there does not seem any reason why the structure should perish from any causes inherent in itself. The periodical exuviation of the leaves is an obvious indication, that the life of the fabric as a whole is perfectly consistent with the death of certain of its constituent parts. Ou the other hand, the continued life of a bud or olf-shoot, which we have detached from the stock and placed in circumstances favourable to its separate development, affords a clear proof OP DEATH. 81 that the life of the individual segment does not depend upon its con- nection with the entire structure, — all that is requisite being that it should possess the organs essential to the maintenance of its own life, or should have the power of developing them, or should be able to make use of those of another plant. Thus a leaf of the Bryophyllum (one of the ‘ air-plants’) will of itself develop roots, when detached and laid upon damp earth, and null in time grow into a new plant ; on the other hand, a detached leaf- bud generally thrives best when it is enabled to obtain its nutriment through the roots and stem of another plant, on which it is fixed by ‘grafting,’ and with which it speedily forms an organic union. 126. The condition of Zoophytes and of the lower Mollusca and Articu- lata, which are of a similar composite nature with the higher Vegetable forms, corresponds so closely with that of the latter in regal’d to the topic now under consideration, that it is not requisite to notice them sepa- rately ; and accordingly we shall pass on at once to those higher types of Animal existence, in which the heterogeneousness of structure and variety of function are the greatest, and in which, therefore, the bond of mutual dependence is the closest. We here still find that every part of the organism has its own limited term of vitality ; and that there is a conti- nual succession of new cell-life, in those parts more especially in which active vital changes are taking place. In fact, the periodical exuviation of the leaves of Plants is a phenomenon which is daily repeated in the body of every oue of the higher Animals ; the duration of life being extremely short in many parts of the fabric of the latter, and the pro- duction of new tissue taking place with wonderful rapidity. Even the most solid portions of the animal fabric have a limited term of existence as living structures ; for if they become so condensed and mineralised as to possess the character of permanence (which is the case with Shells and Corals, § 24), they are then so completely cut off from participation in the Vital Activity of the organism, that they cannot be said to be alive ; whilst, on the other hand, if they contain a larger proportion of organised tissue, and this remains capable of taking any share in the nutritive ope- rations (as is the case with Bones, and to a less extent with Teeth), their duration in the living organism is limited, — although they may be pre- served in their dead state, under favourable circumstances, for an indefi- nite period. Thus nearly the whole of the Animal fabric is undergoing continual change, by the death and decay of its integral parts ; and this not merely during the period of growth, when it is requisite that the structure originally laid down should be progressively extended, by the removal of the parts first deposited, and the deposition of new material elsewhere; but even during the whole term of its existence. For there is no doubt that the exercise of the truly Animal powers, — which are the endowments of the Nervous and Muscular tissues, — involves the death and disintegration of those tissues, as its necessary condition ; so that even to the end of corporeal life, every sensation, every movement, and proba- bly every act of mind, takes place at the expense of the vital energy of a certain amount of organised structure, which thereby gives up its own Life and becomes subject to decay. Hence if it were not for the continual reconstruction of these organs by the Nutritive or Vegetative processes, the Animal fabric would speedily perish ; and we accordingly find that after this has attained its full growth and development, the whole energy G GENERAL PHYSIOLOGY. of these processes is directed towards its maintenance ; and, as we have just seen, even their performance involves the continual death and disin- tegration of certain parts of the fabric, in order that the materials may be prepared for the development of others of more permanent character. 127. The continual series of changes, however, to which the Animal fabric is subject in virtue of its essential nature, is harmonized and co- adapted with such perfection, that the molecular death, which is perpe- tually taking place in some or other of the component parts of the organism, shall contribute to the maintenance of the Life of the entire being. But this is only the case, so long as each important organ remains in its due functional activity ; for if an interruption occurs in the vital actions of any one, those of the remainder are necessarily checked, in con- sequence of the relation of mutual dependence which exists among them. Thus, for example, the growth of every tissue in the body, and conse- quently its vital activity, is dependent upon a supply of blood ; hence, if the circulation of the nutritive fluid be interrupted, eveiy action of Life must speedily cease. The interruption is first seen in the Animal func- tions ; since the Nervous tissue requires, as the condition of its activity, not so much nutritive material (for this is applied to its regeneration in the intervals of its action, § 236), as oxygen ; and this can only be sup- plied to it by the movement of the blood, which conveys to it this element from the respiratory organs where it is introduced into the body. The vital activity of Muscle may continue for a longer period, especially in cold-blooded animals ; — thus the heart of a Frog, when removed from the body, will pulsate for some hours ; and it has been asserted that the heart of a Sturgeon, cut out and hung up to dry, continued alternately contracting and dilating, until the movement produced a crackling noise in consequence of the desiccation of the texture ; — but its persistence in warm-blooded animals is comparatively brief, and it is necessarily extin- guished after a time in all. The processes of Nutrition and Secretion may continue for a certain time after the suspension of the circulation, the requisite materials being supplied by the blood remaining in the ves- sels ; thus Mr. T. Bell mentions * that in dissecting very carefully and minutely the poison-apparatus of a large Rattlesnake which had been dead for some hours, he found the yellow poison continually secreted by the gland, so fast as to require being occasionally dried off with a sponge. We find, again, that blood retains its vitality for some time, so as neither to coagulate nor to putrefy, when withdrawn from the current of the circu- lation, if it remain in contact with living tissues. Hence it is that pails which have been completely separated from the body may be reunited with it, if they were previously in a healthy state, and too much time nas not elapsed ; thus, there are many cases on record, in which Angel's, foes, noses, or ears, that have been accidentally severed, have been made to adhere and grow as before, by bringing the cut surfaces into contact. It is evident, then, that the parts so separated cannot have lost then vitality : since no treatment could produce union between a dead mass and a living body. And we are fully justified in assuming that, in cases where attempts at such reunion have not been successful, the death of the sepa- rated part has resulted from the too-prolonged interruption of its regular nutritive operations, whereby chemical and physical changes have taken * History of British Reptiles, p. 61. OF DEATH. 83 place in it, which have destroyed the peculiar structure and composition of its several parts. 128. The cessation of the Circulation, which supplies the nutritive material for the various components of the fabric, and which also in warm- blooded animals sustains the temperature of the body, may be considered as constituting Somatic Death* or the death of the organism as a whole. This must be followed at no distant interval by Molecular Death, or the death of every integral portion of the organism. — Somatic and Molecular death may, however, be coincident. Thus when an animal dies of pure ‘ old age,’ there is a gradual diminution in the vital power of each part of the organism ; so that, when the heart’s action ceases, activity of every other component part of the body terminates at the same time. And the same thing happens when an animal is killed by a powerful electric shock, by concussion of the brain, by the introduction of certain poisons into the blood, or by any other cause which has an injurious action (direct or indirect) upon the whole frame alike. — But it more generally happens that Somatic death is the result of the suspension of some or other of the functional changes necessary to the Life of the organism as a whole ; and this may be a consequence of the molecular death of a very small part of it. Thus if the muscular power of the heart should fail, the circulation of the blood is thereby brought to a stand, and death necessarily ensues, notwithstanding that every other part of the body may be in the most perfect condition. So it will be shown hereafter, that the failure in the vital activity of a very small portion of nervous matter (situated at the summit of the spinal cord, just within the skull) involves the cessation of the movements concerned in respiration ; that the aeration of the blood will be thereby checked ; that its movement in the capillaries of the lungs will be consequently stagnated ; and that the whole current of the circu- lation will be thus brought to a stand. And again, if the liver or the kidneys fail to perform their office of removing from the blood the matters with which it is continually being impregnated by the decay of the tissues, these matters accumulate in the circulating fluid, and act as poisons, which more or less affect the powers of the whole organism, but those of the nervous system more especially. 129. It will be obvious, from the considerations already adduced, that the dependence of Somatic death upon the Molecular death of any indivi- dual organ, will depend upon the degree in which the actions of that organ form a necessary part of the chain or connected series of operations con- cerned in the maintenance of the Life of the entire organism. Thus even in the higher animals, the death or removal of the limbs, although they may constitute (as in Man) a large proportion of the fabric, is not neces- sarily fatal ; because it does not involve any interruption in the nutritive operations of the viscera, or of the sensorial functions of the brain. On the other hand, the destruction of a certain minute portion of the nervous centres, or an injury of the heart which would be trivial elsewhere, will be the occasion of immediate death. It sometimes happens that life may be prolonged, after the cessation of the activity of an important organ, by the power which some other possesses of discharging its function ; thus we find that in Man, thekidneys occasionally take upon themselves the elimi- nation of bile from the blood ; whilst in the Frog, the skin can perform * See Art. Death , by Dr. Symonds, in tbe Cyclopaedia of Anatomj' and Physiology. g 2 84 GENERAL PHYSIOLOGY. part of the office of the lungs, so as to effect to a certain extent the aeration of the blood. Of course the more this is the case, the greater is the power which animals possess of sustaining injuries of great severity without the loss of Life : and the power is further augmented by the capability which some of them possess, of re-forming the organs of which they have been deprived. The greatest amount of this appears to be exhibited by the Hydra (§ 289) ; almost every part of whose fabric can regenerate every other part ; so that its life cannot be annihilated by any agencies, ex- cept such as effect the destruction of the entire organism. 130. It has been maintained, however, by those who consider Vitality as something superadded to an Organised Structure, essentially indepen- dent of it, and capable of being subtracted from it, that Death frequently takes place under circumstances which leave the organism as it was ; so that “ the dead body may have all the organisation it ever had whilst alive.” But for such an assumption, there is not the least foundation. In nearly all cases in which death takes place as a result of disease, the connexion between changes of structure and composition, either in the tissues or in the blood, and such a loss of the vital properties of some part or organ as is sufficient to bring the Circulation to a stand, is so palpable as to require no proof ; and in by far the greater number of cases in which it is not at once obvious, a more careful scrutiny will reveal it. It must be confessed on both sides, that our means of investi- gation, and our knowledge of the normal structure and composition of the tissues and the blood, are not yet sufficient to enable us to detect minute shades of alteration, nor to assert what extent of change is incon- sistent with the continuance of life. But as no one has yet shown, by the careful and precise microscopical and chemical examination of the solids and fluids of a dead body, that it has the exact organisation and composition it had whilst alive, the assertion above quoted is totally un- warranted by experience, and is contradicted by all our positive know- ledge of the matter. 131. But it has been urged, that Death may result from the sudden operation of some agency of an immaterial character, which leaves no trace behind it, — such as a powerful electric shock, or a violent mental emotion. Here, too, the argument entirely fails. It is scarcely possible that a powerful electric shock could be transmitted through a mass like the animal body, composed of elements in such a loose state of combination that they are continually undergoing decomposition, without producing important chemical changes in it (§ 112); and its imperfect conducting power renders it equally liable to physical disturbances. Thus it has been ascertained, that when Eggs in process of development have had their vitality destroyed by an Electric shock, the minute vessels of the vascular area (chap, xviii.) have been ruptured. — V or is it more difficult to explain the immediate cause of death, as a result of Mental emotion. In some cases, an obvious physical change has been produced, by the too violent action of the heart, the movements of which are stimulated by the emotion ; thus, even in a healthy person, rupture of the heart or aorta has been known to take place, — an occurrence to which those affected by previous disease of that organ are much more liable. Where there is any disorder in the heart’s action, resulting from thickened valves, narrowed orifices, &c., the physical influence of mental emotion can be easily OF DEATH. 85 accounted for. But it must be admitted that cases have occurred, in which no such explanation can be offered ; sudden death having taken place without any perceptible structural cause. We are not obliged, however, to have recourse, for an explanation of even these cases, to any hypothesis which is not borne out by ample analogy. For it is well known that mental emotions exert a powerful influence over the composi- tion of the fluids of the body, and are capable of instantaneously altering these. Thus in many human beings, and still more in the lower animals, alarm or agitation will occasion the immediate disengagement of power- fully odorous secretions, which must have resulted from new combinations suddenly formed. And there can be no question that a fit of passion may immediately occasion such a change in the milk of a nurse, as renders it a rank poison to the infant. There is no reason to doubt, therefore, that the blood itself may undergo changes of analogous character from the same cause ; and that it may become a violent poison to the individual himself, instead of being the source of wholesome nutriment, or the stimulus to vital activity. 132. To conclude, then; — we only know of Life or Vital Activity as exhibited by an Organised Structure under the influence of certain Physical Forces. And we only know of Vitality , the peculiar state or endowment of the being which exhibits that activity, as consequent upon that new arrangement of the molecules of matter entering into its composition, which we term Organisation. The conditions of that endow- ment, therefore, must be found in the properties of the original elements of the structure composing it, and in the forces by which they are brought into play. When w r e ‘ magnetise ’ a piece of steel, by the agency of another piece already magnetic, or by the transmission of an electric current across it, we are but calling forth the properties which were previously dormant or inactive in the metal. So when the cell-germ, under the influence of Light and Heat, effects the combination of the inorganic elements into new and peculiar compounds, and applies these to the formation of its own structure, it does nothing else than call forth the dormant properties of those forms of matter. For the existence of the properties which thus enable certain elements to become component parts of Organised structures and to perform Vital actions, we can assign no other cause than the Will of the Creator. They are ultimate facts in Physiology, just as Attractions and Repulsions constitute the ultimate facts in Physics, and as do Affinities in Chemistry. The constancy of the actions which result from them, when the conditions are the same, — that is, their conformity to a fixed plan, or (in the language commonly employed) then- subordination to Laws, — indicates the constancy and unchangeableness of the Divine Will, as well as the Infinity of that Wisdom by which the plan was at first arranged with such perfection, as to require no departure from it in order to produce the most complete harmony in its results. (See § 5, note . ) 86 GENERAL PHYSIOLOGY. CHAPTER IV. OF THE COMPONENT STRUCTURES OF ORGANISED FABRICS. 1. General Considerations. 133. Having thus taken a general survey of the characters which are common to all Organised, beings, and which distinguish them from Inorganic matter, — having also enquired into the conditions that are essential to the occurrence of all Vital phenomena, and enquired into the relations subsisting between the peculiar agencies to which we must attribute them, and those which are known as the Physical Forces, — we are prepared to examine, in more detail, into that combination of diverse elements, and that succession of mutually-related actions, which every living being presents to our observation. It is a fact now well-established by microscopic investigation, that just as the Chemist resolves the count- less substances found in Nature or producible by Art into a comparatively small number of ultimate elements, each having its distinctive properties ; so can the Anatomist resolve the fabrics of Plants and Animals, whatever may be their dimensions, into a limited number of ‘ Elementary Tissues,’ each having a structure peculiar to itself ; and in like manner, the Physiologist can reduce the most complicated series or combination of actions to its individual components, and can assign each of these to some form of the elementary tissues as its own special and peculiar instrument. Thus, then, by an examination into the structure and endowments of these ‘ elementary ’ or ‘ primary tissues,’ we shall obtain that kind of pre- paration for the study of the more complex phenomena of Life, which the Chemical student seeks to acquire, when he investigates the properties of each elementary substance, before endeavouring to acquaint himself with the changes which this undergoes in its combination with others, or with the multitudinous variety of phenomena which arise out of such combinations. 134. It must be freely conceded that investigations into the structure and properties of the Elementary parts of which Organised structures are composed, are often attended with much difficulty and liability to error. The minuteness of the objects which are the subjects of our examination, and the production of changes in them by the preparation they are necessarily made to undergo before being submitted to Microscopic inspection, not to mention the deceptions arising from imperfection of the instrument itself or from ignorance of the right mode of employing it, have led to much discrepancy in the statements of different observers, which has brought the Microscope into undeserved disrepute. Too often the descriptions given have not been of what has been actually seen, but of what has been imagined ; and thus, without any intention of falsifying them, they have been shaped according to the preconceived notions of the inquirer. Hence, an examination of the characters of the primary tissues, whether of Plants or Animals, requires not only considerable manual skill and dexterity in the use of the microscope, but an acquaintance with all the fallacies arising from the difference between the image presented to COMPONENT STRUCTURES OF ORGANISED FABRICS. 87 the eye, and the object as it exists in its natural situation ; besides— what is even more important — a perfect readiness to give up preconceived notions, when they are inconsistent with observation, and a determination to consider nothing as proved until every mode of investigation has been employed with the same result. The improvements which have been recently effected in the optical powers and in the mechanical construction of the Microscope, together with a more general diffusion of the know- ledge ‘ how to observe,’ have gone far to remove these discrepancies ; and there is now so general an agreement among those who have studied the structure of the ‘ primary ’ or ‘ elementary ’ Tissues of Plants and Animals, in regard to their most important attributes, that no reasonable doubt can be any longer entertained as to the accuracy of their descrip- tions. There are, of course, many phenomena in the interpretation of which there is room for a difference of opinion ; but this is the case, more or less, in all Sciences ; and it is the almost invariable tendency of the progressive extension of knowledge, to clear up such obscurities. 135. There is no department of Anatomical or Physiological inquiry, in which more elucidation has been obtained from an extension of the survey to all forms of Organised structure, than that which concerns the Primary Tissues. Botanists had long been aware that the Cell is the type of Vegetable structure ; — that the simplest Plants are composed of individual cells ; — that those a little higher in the scale are made up of aggregations of Cells not very different from each other in form or endow- ments ; — that where other structures are present, cells still constitute the essential part of the fabric, being the special instruments of its vital actions ; — and that these superadded structures are still closely related to the cellular, and are formed in the first instance by metamorphoses of the cells, of which the original embryonic structure, and the tissue of all young- parts, are exclusively composed. It was known also that certain tissues of Animals, especially the adipose or fatty, resemble those of Plants in their simple cellular character ; and as it had been established by observation of their embryonic structure, that even the most complex fabric takes its origin in a mass of cells, it was anticipated that a more complete analysis of the Animal organism would show that many other parts of it are formed upon the same type. It is to the industrious and skilful researches of Schwann, however, that we owe the first establishment of the general fact, which has been since more fully developed by the researches of other Microscopists,* — that the Animal like the Vegetable tissues originate, directly or indirectly, in cells ; many of them retaining their original cellular character through life ; whilst in others a trans- formation takes place, by which this is more or less completely obscured. And it seems to have been further established, as the aggregate result of the labours of many observers, that, in Animals as in Plants, all the parts in which active vital changes are taking place essentially consist of cells, which may be regarded as the real instruments of these operations, the tissues with which they are blended having no other purpose than to supply the physical conditions requisite for them.+ These two general * Mikroscopische Untersuchungen liter die Uebereinstimmung in der Structur und dem Wachsthum der Thiere und Phlanzen. Berlin, 1839. + The Author believes that he may himself claim the credit of this second generalization ; which he first put forth in regard to the organic functions, in his Report on the Origin and 88 GENERAL PHYSIOLOGY. facts lie at the foundation of the whole fabric of modem Physiological science ; and it will therefore be requisite to go at some length into the exposition of them. As we proceed, it will become apparent how valuable is the light which the study of £ cell-formation ’ in one class of organised beings can throw upon its phenomena in others ; and how comprehensive is the expression of these phenomena, which the Physiologist can now give ; since, basing his account of the process upon that which he sees in the simplest form of Cryptogamic Vegetation, he finds it to hold good, as to all essential particulars, in regard alike to the original germ-cell of the most complex Plant and of the highest Animal, and to each of those individual cells which enter in such incalculable numbers into the compo- sition of the fully-developed organisms of both. Fig. 2. 2. Of the Primary Tissues of Plants. 136. Formation of Cells . — The general idea of the structure and history of a Cell, which has already been given (§ 47), has now to be more specially applied and more fully developed ; and we have first to enquire, therefore, in what essential particulars the Vegetable cell differs from the Animal, and what is the mode in which the former comes into existence. Although we have hitherto spoken of the cell-wall as a simple membrane, yet it is now well known to be made up, in most if not in all instances, of two layers of very different composition and properties. The inner of these layer's, which lias received the name of ‘ primordial utricle,’ appears to be the one first formed, and most essential to the existence of the cell ; it is extremely thin and delicate, so that it escapes attention so long as it remains in contact with the external layer, and is only brought into view when circumstances occa- sion its separation from this (Fig. 2) ; it seems to consist of an azotised compound, pro- bably of an albuminous na- ture ; and it appears to par- ticipate actively in the vital operations of the cell. The external layer, on the other hand, though commonly re- garded as the proper ‘cell-wall,’ seems to be generated on the external surface of the primor- dial utricle, after the latter has completely enclosed the cavity and its contents, so that it can- not be regarded as essential to the cell ; it is usually thick and strong in comparison with the other, but it may possess various degrees of consolidation, from mere mucus to a firm and tenacious substance ; it is composed of cellulose, a Section of leaf of Agave, treated with dilute nitric acid, showing the primordial utricle contracted in the interior of the cells: a, epidermic cells; b, boundary cells of the stoma; c, cells of parenchyma ; d, their primordial utricles. Functions of Cells in No. xxix. of the British and Foreign Medical Review. Jan. 1843; and which, by ihe discovery of the cellular structure of the muscular fibrilla in 1846, he was enabled to extend to the animal functions. VEGETABLE TISSUES ; CELLS. 89 substance nearly identical witli starch ; and it does not appear to take any active share in the vital operations of the cell, its principal office being to protect, locate, and isolate the matter it contains.* This external layer may consist of many laminae, the results of successive deposits from the surface of the primordial utricle (Fig. 9) ; but it still usually remains readily permeable to fluid, although no pores can be distinguished in it, even under the highest magnifying power. 137. The granular matter contained in the interior of the cell, being usually coloured, has received the name of endochrome ; it is this, with its containing sac or ‘ primordial utri- cle,’ which essentially constitutes the cell ; and all the vital phenomena which the cell exhibits have their seat in it. Among these phenomena, there is one which, not being as yet certainly known to be common to all cells, has not been previously adverted to. This is the existence of a continuous motion in a certain part of the fluid contents of the cells, as marked by the revolution of the float- ing granules which are carried along in the stream. The motion appears to be confined to the somewhat viscid layer (probably of albuminous matter) which immediately lines the primor- dial utricle. In most plants of the aquatic families of Characece, JVaya- dacece, and Hydrocharidacece, and especially in the long tubular cells of the Characece (§269) we may observe a single broad current, sweeping up one side of the cell and down the other, and returning continuously into it- self ; and this current is strong enough to carry along with it granular masses of starch, chlorophyll, and albumi- nous matter, of considerable size. The ascending and descending cur- rents do not come into absolute con- tact, being separated by a clear space in which no motion can be detected. If the cell be carefully tied across, the current is in short time re-established in each segment, as if the cell had naturally subdivided itself. The vigour of the movement increases and diminishes in accordance with the general vital activity of the cell ; as appears, not merely from observation of its ordinary growth, but also from the exciting or depressing effects of physical agents. Although this * See the admirable Memoir by Mr. Thwaites on the Cell-Membrane of Plants, in the Annals of Natural History, Vol. xvm., to which the reader is referred for facts and argu- ments in support of the above position. Circulation of fluid in hairs of Tradescantia Virginica : — a , portion of cuticle with hair attached ; a , b , c, Successive cells of the hair ; d. cells of the cuticle ; e. stoma : — b, joint of a beaded hair, show- ing several currents ; a , nucleus. 90 GENERAL PHYSIOLOGY. movement may be advantageously studied in the plants of the families just named, it is by no means confined to them • for it may be readily detected in the hairs of Flowering-plants, many of which afford very beautiful examples of it, and it has been observed also in the young cells of leaves, fruits, and other parts ; so that it may with probability be regarded as existing in every vegetable cell at a certain stage of its deve- lopment. The current is never so broad, however, as in the instances previously cited : and usually presents itself as a thread-like stream passing through a stratum of motionless matter. This stream may be single, passing up one side of the cell, and down the other, as is usually the case in long and tubiform cells (Fig. 3, b, c ) : but several distinct currents may exist in the same cell, and these are observed to have a common point of departure and return, — namely, a collection of granular matter in one mass attached to the wall of the cell, which is termed the nucleus (Fig. 3, a, b, a). Here, too, it would appear, that the currents are connected with the general activity of the life of the cell, and that their cessation indi- cates the cessation of its formative powers ; and from the manner in which they are connected with the ‘ nucleus,’ it would seem that this, where it exists, is to be regarded as the centre of the vital activity of the cell. Although such a nucleus is to be found, however, in a very large propor- tion of Vegetable cells, yet its absence in some instances is a matter of equal certainty with its presence in others ; and as these do not manifest any inferiority of vital power, the nucleus is obviously not essential. Perhaps we should be correct in regarding it as concentrating in itself a portion of the forces which are elsewhere diffused through the entire ‘ endochrome ’ and ‘ primordial utricle,’ rather than as having any powers peculiar to itself ; and this idea is confirmed by the facts presently to be stated in regard to the production and multiplication of cells. 138. There is another phenomenon of occasional occurrence, which is indicative of the existence of properties in certain Vegetable cells, that have not yet come under our notice. This is the rapid change of form which may be sometimes witnessed in them, and which produces obvious movements ; these being in some cases spontaneous, that is, origi- nating in no other causes than those which are concerned in the forma- tion of the cells ; whilst in other cases they are excited by mechanical or chemical stimulation. Of the former, we have a very curious example in the Oscillator ice, a tribe of Plants of the simplest possible construction, consisting of filaments or elongated cells, which continue in a state of rhythmical vibration throughout the greater part if not the whole of their lives ; many of these are remarkable as possessing no proper cell- wall, so that the cause of the movement must reside in the primordial utricle and endochrome. Of the latter we have examples in the Sensitive Plant, Diomea, Berberry, &c., certain parts of which are put in motion by the contraction of tissues that are designated as 1 irritable ;’ these contrac- tions being due to a change of form in the component cells of those tissues, which takes place when certain stimuli are applied to them (chap. xix.). 139. We have now to enquire into the origin and mode of multiplication of the Vegetable cell. — It may be stated as a general fact, that every cell owes its origin in some way or other to a pre-existing cell ; but the mode in which new individuals are generated, is by no means constant ; and there are some forms of the process which are not yet clearly understood. VEGETABLE TISSUES J FORMATION OF CELLS. 91 That which is already known, however, leaves no room for doubt, that in every instance a portion of the endochrome of the parent cell is the starting point of its successor : and that, although the primordial utricle may be also concerned, the outer cell-wall is entirely passive in the matter. The most common method of multiplication in Plants is the subdivision of the rinal cell into two halves, such as Fig. 4. Various stages of development of Hcematococcus binnlis : — a, a, simple rounded cells ; b, elongated cells, the endochrome preparing to divide ; c, c , cells in which the division has taken place; d , is seen very characteristically in the Hcematococcus binalis and others of the humblest forms of vegetation, in which every cell, being capable of existing by itself, may be regarded as a distinct individual. The cells of this little plant are originally of a globular form (Fig. 4, a) ; and the first step in the process of subdivi- sion is their elongation into an oval shape, and the appearance of a slight constriction round them, as seen at b. This constriction indicates the ten- dency of the endochrome to separate into two halves, each included, it would appear, in an envelope of its own, so that two young cells are now included within the external wall of cluster of four cells formed by a repetition of the the parent ; and after this has been same proccss- effected, the contiguous portions of the two ‘primordial utricles’ appear to develop or secrete a thick partition between them, so that the two young cells are now completely divided, as seen at c, c. An increase of a some- what similar but less condensed secretion on their exterior forms a mass of mucus, in which the cells are imbedded ; and these are frequently carried, by the interposition of this new substance, to a considerable distance from each other, as is shown at d, where each of the first pair of ‘ twin ’ cells has itself undergone a similar subdivision. — This process may also be well studied in the Confervce, which are filamentous aquatic plants, each filament composed of a single file of cells, adherent to each other, end to end ; and we shall derive from the examination of one of these a further insight into some stages of the process. The first step is here seen to be the subdivision of the endochrome, and the inflexion of the primordial utricle around it (Fig. 5, a, a ) ; and thus there is gradually formed a sort of hour-glass contraction across the cavity of the parent-cell, by which it is divided into two equal halves (b). The two surfaces of the infolded utricle produce a double layer of permanent cell-membrane between them ; and the formation of this may be seen to commence even before the complete separation of the cavities of the twin-cells (d). This deposi- tion is not confined, however, to the contiguous surfaces of the young cells, but takes place over the whole exterior of the primordial utricle ; so that the new septum is continuous with new layers that are formed throughout the interior of the original parent-cell (c). 140. The foregoing is the method according to which the extension of Vegetable structures from cells already in existence most commonly takes place. Among the lower Algse, for example, we find the single cell giving 92 GENERAL PHYSIOLOGY. rise to an amorphous cluster, to a prolonged filament, or to a flattened leaf- like expansion (Fig. 67), according to the mode in which the subdivision takes place, and to the degree in which the new cells remain attached to Fig. 5. A Process of cell-multiplication in Conferva glomeruta. a, portion of filament w ithin complete partition at a and complete partition at b. b, commencement of the formation of a partition ; c, more advanced stage; d, the separation nearly complete ; e, the two primordial utricles com- pletely detached, and cell-membrane deposited between them ; f, successive layers of cell-mem- brane, making up the thickness of the partition. In the last five figures, a indicates the primordial utricle, b, the endochrome; c, the cell-membrane; and d, the mucous investment. each other. It is iu this manner, again, that the cell-multiplication first takes place in that germinal mass, in which not merely the higher Plants, but the highest Animals also, have their origin (chap, xyiii.) ; and the process continues to take place on the same plan, until certain parts of that mass begin to undergo transformation into heterogeneous struc- tures, in which case we commonly find it superseded by some other method. Of its persistence in certain individual parts, even of the highest Animal, we shall presently meet with a veiy characteristic example, in the multiplication of Cartilage-cells by division (§ 157). — It does not appear that in this process the ‘ nucleus ’ performs any essential paid ; for we find it taking place where no nucleus can be distinguished, in virtue, it may be surmised, of a sort of mutual repulsion between the two halves of the endochrome, which leads to then- spontaneous separation. Where a nucleus is present, this also undergoes subdivision at the same time with the endochrome, so that half of it is appropnated by each of the ‘ twin-cells.’ 141. But we sometimes observe that new cells originate in little bud- like prominences on the sm-face of the parent-cell, which are developed in continuity with that from which they arise. This is well seen iu the Conferva glomerata, a common species, which increases not merely (like other Confervae) by the repeated subdivision of the cells at the extremity of its filaments, but by the origination of new cells from every part of their surface. A certain portion of the primordial utricle seems to undergo VEGETABLE TISSUES ‘ CELL-FORMATION. 93 increased nutrition ; for it is seen to project, carrying the outer cell- wall before it, so as to form a protuberance, which sometimes attains consider- able length before any separation of its cavity from that of the parent-cell begins to take place. This separation is gradually effected, however, by the infolding of the primordial utricle, just as in the preceding case ; and thus the endochrome of the young cell is completely severed from that of its parent, and its separate individuality may be said to begin from that time. We may consider this process of ‘budding’ as differing from that of ‘ subdivision ’ only in this, — that whilst in the latter case the indivi- duality of the parent-cell is lost by the equal subdivision of its cavity into two similar parts, it is retained in the former through the unequal division of the cell, of which only a small portion is pinched off (so to speak) to form the new cell, whilst the greater part remains unaltered.* Of the extent to which the multiplication of cells by this budding process takes place among plants, we have as yet no certain knowledge. It is obviously the regular method of growth among the Cliaraceoe (§ 269), in which the long tubiform cells that form the axis give off, at their points of junction with each other, circular rows of buds, from each of which is developed a whorl of lateral branches. And the same mode of increase is observable among the ‘ferment-cells’ (§271), which, whilst rapidly multiplying under favourable circumstances, shoot forth little buds from one or even both extremities, from each of which a new cell is developed. There is no reason to believe that in this process the ‘ nucleus’ takes any direct share ; since the evolution of buds may take place from cells destitute of nuclei ; and even where nuclei exist in budding cells, they do not seem to be specially connected with the process, since the buds are not observed to originate in or near them. 142. In cases where a very rapid production of new cells is requisite, it would seem to be effected by the primary separation of the endochrome into numerous parts, each of which acquires for itself a covering of cell- membrane ; so that a whole brood of young cells may thus be at once generated in the cavity of the parent-cell, which subsequently bursts and sets them free. Of this plan we have the most characteristic examples in the formation of the ‘zoospores’ of the inferior Algte (§ 267). Thus in Adilya prolifera, a plant composed of tubiform cells, which grows parasitically upon fish, the end of the filament dilates into a large cell, the cavity of which is cut off from the rest by the formation of a partition ; and within this dilated cell, an irregular circulation of granular particles can for a time be distinguished. (Fig. 6, c). Very speedily, however, it appears that the endochrome is being broken up into a large number of distinct masses, which are at first in close contact with each other and with the walls of the cell (a), but which gradually become more isolated, each seeming to acquire a proper cell-wall ; they then begin to move * We have here, in fact, in their very simplest forms, the two methods of fissiparous and gemmiparous reproduction, which we shall hereafter find carried out not merely as regards the individual components, but in the entire fabric of many Plants and Animals. Thus the ‘fissiparous’ method of reproduction, in which the body is cleft (as it were) into two equal halves, each of which may be regarded as a new individual, is exhibited in the lower tribes of Infusorial Animalcules (§ 284) in a form almost as simple at that under which we have traced it in the humblest Algae ; and the ‘ gemmiparous ’ plan is manifested in Zoophytes (§ 288) and in the lower Articulata and Mollusca, just as in almost the entire Vegetable Kingdom. 94 GENERAL PHYSIOLOGY. about within the parent-cell ; and, when quite mature, they are set free by the rupture of its wall (b), to go forth and form new attachments, and to acquire for themselves the materials of development into tubiform cells resembling those from which Fig ‘ 6 ‘ they sprang. A similar pro- cess may be observed in the production of the ‘ zoospores’ of Confervse and Algae in ge- neral ; usually taking place, however, upon a smaller scale, and the number of new cells being generally less, than in the instance just quoted. And there ap- pear to be some cases among the higher plants, in which parent-cells give origin to a new brood in then- interior, by a process somewhat simi- lar, without the successive duplication which is certain- ly the more usual method of the production of ‘cells within cells,’ or ‘endogenous multiplication.’ This seems the case, for example, in the ‘ embryo-sac’ of Flowering- Plants (chap, xviii.); which at one time contains only a mixture of albuminous and starchy matter, but which is afterwards filled up by a mass of cells, that have incorporated these materials into their own substance, forming the ‘ endosperm.’ According to Nageli, who has recently investigated this process with much care, the following are the essential points in its history. Minute globular particles of perfectly homogeneous matter, varying in diameter from 1 to 4-1000ths of a line, seem to be first formed in the midst of the mucilaginous contents of the HR mm || % Development of Adilya proliferu : — a, dilated extremity of a filament b , separated from the rest by a partition a , and con- taining young cells in progress of formation ; — b, conceptacle discharging itself, and setting free young cells, a, b,c ; — c, por- tion of filament showing the course of the circulation of gra- nular contents. embryo-sac ; larger globular bodies are apparently produced by the aggregation of other (nitrogenous'?) particles around these, forming the nuclei of the future cells. Each of these nuclei seems to attract around it a greater or smaller quantity of the contents of the parent-cell ; and over this a membrane is subsequently generated.* Tlius the history of such a formation is very nearly the same with that which we have traced in the inferior Cryptogamia ; and it may be that even in the latter, the formation of nuclei, round which the contents of the parent-cell group themselves, may be really the first stage in the production of the mass of secondary cells. 143. The first visible stages of the development of new cells, however, * See Nageli's Reports on the Formation of Vegetable Cells, in the Reports and Papeis on Botany published by the Ray Society, 1845 and 1849. VEGETABLE TISSUES j FORMATION OF CELLS. 95 do not always take place in the interior of a pre-existing generation ; for cells sometimes appear to originate de novo, in that mixture of starchy and albuminous fluids, which, being the appropriate pabulum for Vegetable cells, has been denominated protoplasma. This protoplasma, however, must have always been elaborated by cell-agency ; so that, even if the young cells appear to be developed quite independently in its substance, they must really be regarded as the offspring of the cells which formed it. In some instances which have been regarded in this light, it is probable that the protoplasma was contained in a cellular parenchyma which escaped observation through its extreme delicacy : whilst in other cases, there can be no doubt that definite cell-germs had been prepared and set free with the protoplasma, escaping observation on account of their minuteness. 144. There is evidence that the process of cell-production may take place with a rapidity almost inconceivable. Extensive tracts of snow, in alpine and arctic regions, have been seen to be suddenly reddened by the cells of the little Protococcus nivalis; and there are some minute blood-red Fungi, which occasionally make their appearance in almost equal multi- tudes upon the surface of every organic substance. Almost every one is familiar with the appearance of certain more elevated forms of Fungous vegetation, which shoot up in the course of a single night, and seem to melt away before the morning sun. A specimen of Bovista giganteum, a large Fungus of the puff-ball tribe, has been known to grow in a single night from the size of a mere point to that of a huge gourd ; and from a calculation of the average size of the cells, and of the probable number contained in the full-grown plant, it has been estimated that they must have been generated at the rate of four thousand millions per hour, or more than sixty-six millions per minute. In all such cases, the amount of solid matter present in the tissues bears a very small proportion to the fluids. — A very rapid growth of leaves may be occasionally noticed ; thus the leaf of Urania speciosa has been seen to lengthen at the rate of from 1 1 to 3 1 lines per hour, and even as much as from four to five inches per day. It is doubtful, however, how much of this result may be attri- buted to the formation of uew cells, and how much is due to the enlarge- ment of those of which the leaf was previously composed. 145. Cellular Tissue. — Such cells as those which we have been describing, present themselves not only as the constituents of those simplest Vegetable organisms, in which every cell may be accounted a distinct individual ; but also as the chief and frequently the sole com- ponents of fabrics of much higher rank, and of much greater complexity of structure. Thus we do not meet with any other forms of tissue than those referable to the simple Cellular type, in any of the Algse, Lichens, Fungi, or Mosses : whilst in Ferns and Flowering Plants we find this tissue not merely in the soft substance of the leaves, flowers, and fruits, but also in the stem, branches, and roots ; uniformly making up the chief part of the organs most actively concerned in the vital operations, and being almost the sole component of young and growing structures. The simplest condition of this tissue is that in which the cells retain their primitive spheroidal form, and have simply membranous walls (Fig. 7, a). The rounded form is only exhibited when the cells are but loosely aggre- gated together, and it is then that the distinctness of their sides is most 96 GENERAL PHYSIOLOGY. evident. When the tissue is more solid, the sides of the vesicles are pressed against each other, so as to become flattened, and to be in close apposition (b) ; and sometimes they adhere in such a manner, that the Fig. 7. A. b c D E F Various examples of Vegetable Cells: — a, spheroidal membranous cells; b, membranous cells with flattened walls ; c, cells with internal spiral fibre : d, cells in which the fibre is obscured by its coalescence with the cell-wall ; e, dotted cells exhibiting traces of spiral fibre; f, dotted cells without vestiges of spiral. partition between two adjacent cells seems to be but a single instead of a double membrane. If the pressure to which the vesicle is subject be . equal in all directions, the form it will assume is that which is mathema- tically termed a rhomboidal dodecahedron ; that is to say, a twelve-sided solid with all its faces equal, and showing an hexagonal section when cut across. Each cell will thus be in contact with twelve others, which completely surround it without leaving interstices. 146. It is not very often, however, that this form is displayed with such extreme regularity • since there is usually, in the growing plant, a disposition to elongation in the direction of increase, and to compression in the transverse one, so that the cells are found to have rather a prolonged form ; such cells are especially found in the lower tribes of plants, which have no other kind of tissue, and are destitute of vessels, the function of which is partly performed by them ; and we also meet with them in the ‘medullary rays’ of Exogenous stems (§ 280), in which this elongation takes place horizontally, so as to favour the transmission of fluid, through their means, from the exterior to the interior of the trunk. Not unfre- quently the cells are found to possess a cubical or prismatic shape ; and their arrangement is occasionally such as to give them the appearance of bricks in a wall, in which case the tissue is said to be ‘ muri- form.’ One of the most curious varieties of form which Vege- table cells present, is that which is delineated in Fig. 8, and which constitutes the stellate cell. This kind of metamorphosis is of great importance, as illus- trating the formation of some of the Animal tissrres. It is obvious that, if the radiating prolongations of these cells were to coalesce at the points where they come into mutual contact, so as to throw together the cavities of the entire series, a network of vessels would be produced ; this, however, does not seem to occur in Plants : in Fig 8. Spongiform Parenchyma composed of stellate cells. VEGETABLE TISSUES ; FIBRE-CELLS. 97 whose fabric this stellate tissue appears to be generated for the purpose of forming a loose spongy parenchyma. — The dimensions of the component vesicles of Cellular Tissue are extremely variable ; they are usually from l-300tli to l-500th of an inch in diameter, but they are occasionally as much as l-30th of an inch across, whilst in other instances they do not measure the hundredth part of that amount. They seem to be held toge- ther by an intercellular substance which intervenes between them, and which is analogous to the mucus-layer that surrounds the isolated cells of the simple Algae. This analogy becomes obvious, when we trace the gradual aggregation of isolated cells into such fabrics as we meet with in the higher Algae ; between whose component cells a considerable thick- ness of this mucus is still to be found (§ 267, 268). 147. The walls of the vesicles of Cellular Tissue, however, do not always consist of simple membrane alone ; for these are occasionally strengthened by fibre that lies in contact with their internal surface ; or, as more frequently happens, are thickened by a deposit of hard matter secreted from the juices of the plant some time subsequently to the original formation of the cell. Of the first of these methods we have an example in Fig. 7, c, which shows cells with a fibre spirally coiled in their interior. This kind of Fibre, which may be termed elementary , is of extreme minuteness, its diameter frequently not exceeding 1-12, 000th of an inch ; it is solid, and, when first formed, is usually veiy elastic. Of this elasticity we have a very interesting example in the fibre-cells which form the external coating of the seeds of Collomia, of Salvia verbenaca (Wild Clary), and of many other plants ; for the walls of these cells are so imperfectly consolidated, that they are readily softened by water ; and the spiral fibres suddenly elongate themselves, like springs which have been compressed, so soon as they are set at liberty by the rupture of the cell-wall which held them together. The elaters of the Jungermanniaceoe (§ 275) are constructed upon a like plan; being fibrous cells, whose mem- brane gives way when mature and dry, so that they rupture with a jerk that disperses the spores among which they lie. There are a few plants which are almost entirely made up of fibrous cells ; this is the case, for example, with the Sphagnum (Bog-moss), in which we meet with this fur- ther peculiarity, that the cells occasionally communicate with one another by apertures large enough to allow of the passage of Wheel-Animalcules, which find a congenial abode in their interior. — The spire may wind in either direction, but towards the right is the most common ; sometimes we find two spires winding in the same direction in the same cell ; but there is no satisfactory example of two spires crossing each other in con- trary directions. 148. Of the origin of this Spiral Fibre, we have at present no definite knowledge ; it would seem to result, however, from a general tendency to the arrangement of the components of the cell in a spiral direction ; for we see the endochrome presenting more or less of this disposition in many of the simpler plants, but more especially in the Zygnema; and the cell-wall itself is sometimes found to tear most readily in a spiral direction. — Occasionally the fibre becomes so adherent to the cell-wall that it cannot be separated ; and the distinctness of its edges is lost, so that it almost forms an inner lining to the cell (Fig. 7, d). There would seem, in fact, to be a great tendency in any additional deposits which are H 98 GENERAL PHYSIOLOGY. Fig 9 . applied to the lining of the cell, to arrange themselves upon the spiral fibre ; and thus it is increased in breadth and thickness, and the regularity of its spires disappears, in consequence of the coalescence of its several turns at various points. There are usually some parts of the cell-mem- brane, however, which are left uncovered by the secondary deposit ; these can sometimes be clearly distinguished to be spaces intervening between the spiral fibres, as seen at e, Fig. 7 ; but more frequently all trace of the spire is obscured in these porous cells (Fig. 7, f), by the irregular mode in which the new matter is deposited. Such transition-forms as the pre- ceding, however, serve to indicate their origin, which we shall presently see to be a point of some interest (§ 152). The secondary deposit often forms numerous layers, which present themselves as concentric rings when the cells containing them are cut through (Fig. 9) ; and these layers are sometimes so thick, as almost to obliterate the original cavity of the cell. In the long cells of Ligneous tissue, we usually find each layer continued over the whole interior of the cell ; but in other cases we frequently meet with a continuation of the same arrangement as that which shows itself in the first layer of the dotted cell ; each de- posit being deficient at certain spots, which correspond in the successive layers, so that a series of passages is left, by which the cavity of the cell is extended at some points to its membranous wall. Where several cells of this kind are aggregated together, as happens in the ‘ stones’ of fruit, the gritty „ „ , ... .. „ tissue of the pear, &c., the points at which divided transversely at a, longitudinally the deposit IS Wanting On the Walls of two contiguous cells are coincident, so that the membranous partition is the only obstacle to the communication between their cavities. — Such deposits are not found in cells which are actively contributing to the vital operations of the plant ; but are rather intended to give strength and durability to the tissues in which they occur ; the radiation of the cell-cavity being chiefly found in those cases, in which it is necessary that the tissue thus consolidated should still have the power of conveying fluid. It would seem probable, although it cannot be stated with certainty, that the deposit is formed on the exterior of the ‘ primordial utricle,’ which pro- gressively contracts as it secretes new matter from its surface ; so that at last, in such cells as those represented in Fig. 9, this utricle has a stellate form, resembling that depicted in Fig. 8. The secondary deposit, which thus gives firmness to the tissue which was originally tender and succulent, is sometimes termed sclerogen ; it does not appear, however, that in its purest condition it is essentially different from Cellulose ; but it is fre- quently intermingled with resinous and other matters. 149. Woody Fibre. — A large proportion of the denser fabric of the higher plants is made up of what is usually termed Ligneous Tissue or Woody Fibre. This, however, can only be regarded as a very simple variety of the Cellular ; for it is composed of cells which have undergone elongation at one or both extremities, so as to become fusiform or spindle- shaped (Fig. 1 0, a, b). and which have a peculiar tendency to the production of an internal consolidating deposit (d). The elongation does not seem to at b, showing concentric layers of internal deposit, with passages radiating from the central cavity. VEGETABLE TISSUES ; WOODY FIBRE. 99 Elongated cells constituting Woody Fibre: — a, fusiform cells elongated at both ends; b, cells elongated in one direction only; c, dotted fibre from Pine; n, transverse be the result of pressure, but of the unequal nutrition of different parts of the cell-wall. It is obvious that a tissue consisting of elongated cells, adherent together by their entire length, and strengthened by internal deposit, must possess much greater tenacity than any tissue in which the cells depart but little from the primitive spherical form ; and Fig - 1 °- we accordingly find this tissue introduced, wherever it is re- quisite that the fabric should possess not merely density, but the power of resistance to ten- sion. In the higher classes of the Vegetable kingdom, it con- stitutes the chief part of the stem and branches, where these have a firm and durable cha- racter ; and even in more tem- porary structures, such as the herbaceous stems of annual plants, and leaves and flowers of almost every tribe, this tissue forms a more or less important constituent, being especially _ ' o X xJ uneCLiUIl umy ; uuucu mnc uum x xxxo y it, luiuoviiioc found in the neighbourhood of section of a fasciculus of fibres, showing concentric internal the spiral vessels and ducts, to deposits ' which it affords protection and support. Hence the bundles or fasciculi composed of these elements, which form the skeletons of leaves, and which give ‘stringiness’ to the tissues of various esculent Plants, are commonly known under the name of fibro-vascular tissue. In their young and uncon- solidated state, the ligneous cells seem to conduct fluids with great facility in the direction of their length ; and in the Coniferous tribe they afford the sole channel for the ascent of the sap, their stems and branches being- destitute of vessels. But after their walls have become thickened by internal deposit, they are no longer subservient to this function ; nor, indeed, do they then appear to fulfil any other purpose in the Vegetable economy, than that of affording mechanical support. It is this which constitutes the difference between the alburnum or ‘sap-wood,’ and the duramen or ‘heart-wood,’ of Exogenous stems (§ 280). 150. A peculiar set of markings, seen on the woody fibres of the Coni- fer a; , and of some other tribes, is represented in Fig. 10, c ; in each of these spots, the inner circle appears to mark a deficiency of the lining deposit, as in the porous cells of other plants ; whilst the outer circle indicates the boundary of a lenticular cavity, which intervenes between the adjacent cells at this point, and which contains a small globular body that may be some- times detached. Of the purpose of these minute bodies interposed between the wood-cells, nothing is known; there can be no doubt, however, from the definiteness and constancy of their arrangement, that they fulfil some im- portant object in the economy of the plants in which they occur; and there are varieties in this arrangement so characteristic of different tribes, that it is sometimes possible to determine, by the microscopic inspection of a minute fragment, even of a fossil wood, the tribe to which it belonged. 100 GENERAL PHYSIOLOGY. a and b, rows of cells coalescing into ducts, the partitions being still indicated at a , b, c,d; c, elongated spiral cell, or spiral vessel; d, portion of the same enlarged ; e, fibrous duct, spiral at a, b , annular at c, spiral again at d. 151. Spiral Vessels. — The spiral cells are sometimes elongated in a similar manner, and then become what are commonly known as Spiral Vessels * (Fig. 11, c, d). These, like the cells out of which they are transformed, are characterized by the presence of a spiral fibre winding from end to end, remaining distinct from the cell-wall, and retaining its elasticity ; this fibre may be single, double, or even quadruple, — this last character present- ing itself iu the very large elongated fibre-cells of the Nepenthes (Chinese pitcher-plant). These cells in their perfect state contain air only ; they are especially found in the delicate mem- brane surrounding the pith of Exogens, and in the midst of the woody bundles occurring in the stem of Endogens ; thence they proceed in each case to the leaf-stalks, through which they are dis- tributed to the leaves. By careful dis- section under the microscope, they may be separated entire ; but their structure may be more easily displayed by cutting round, but not through the leaf-staik of the strawberry, geranium, SSy-.-5fc? -J •. , ' — their cavity being obliterated ; their origin is indicated, however, by the nucleus in the centre of each. This flattening appears to result from the gradual desiccation or drying-up of the contents of the cells, which results from their exposure to the air. Thus each cell of the Epidermis is developed from the nucleus on the surface of the basement-membrane (§ 160), and is gradually brought to the surface by the development of new cells beneath, and by the removal of the super- ficial layers ; whilst at the same time it is pro- gressively changed in form, until it is converted into a flattened scale (Fig. 27). This change in form is accompanied by a change in the Chemical composition of the tissue, which seems to be due to the metamorphosis of the contents of the cells into a product identical with that of which hair, horn, nails, hoofs, cind. OI chcinges cells are flattened into lamellge, of temperature. We perceive the value of this forming the exterior portion of the -L x epidermis at protection, when the Epidermis has been acci- dentally removed. It is very speedily replaced, however ; the increased determination of blood to the Skin, which is the consequence of the irritation, being favourable to the rapid production of Epidermic cells on its surface. 178. Mingled with the Epidermic cells, we find others which secrete colouring matter instead of horn ; these are termed Pigment-cells. The most remarkable development of Pigment-cells in the higher animals is on the inner surface of the Choroid coat of the eye, where they have a very regular arrangement, and form several layers, known as the Pigmentum nigrum. When examined separately, they are found to have a polygonal form (Fig. 28, a), and to have a distinct nucleus (6) in their interior. The black colour is given by the accumulation, within the cell, of a number of flat rounded or oval granules, of extreme minutenes, which ex- hibit an active movement when set free from the cell, and even whilst enclosed within it. The Chemical natm’e of this black pigment has not yet been made evident ; it has been shown, how- ever, to have a close relation with that of the Cuttle-fish ink or Sepia, which derives its colour from the pigment-cells lining the ink-bag ; and Ce:is from . p ’g mentum Ni- to include a larger proportion oi Carbon than concealing the nucleus; b, the most other organic substances, — every 100 parts nucleus distinct - containing 58J of this element. The pigment-cells are not always of a simple rounded or polygonal form ; they sometimes present remarkable stellate prolongations, under which form they are well seen in the skin of Fig. 28. 122 GENERAL PHYSIOLOGY. Fig. 29. the Frog (Fig. 52, c, c.) The gradual formation of these prolongations may be traced in the pigment-cells of the Tadpole during its metamor- phosis (Fig. 29). Similar varieties of form are to be met with in the pigmentary cells of Fishes and some Crustacea, which also present a great variety of hues. On the surface of the shell of the common Crab, there is a pigmentary layer composed of regular polygonal cells, resembling those of the choroid (Fig. 28), but containing red colouring matter ; and similar cells are found in the margin of the mantle of many shell-forming Mollusca, com- municating various hues to the shell, by setting free their coloured secretion to be incorporated with its substance (§ 197). That the development of the Pigment-cells, or at least the formation of their peculiar secretion, is in some degree due to the influence of Light, seems evident from the facts already mentioned (§ 80). 179. The Epidermic Appendages, as they are commonly termed, — namely, hoofs, nails, claws, scales, &c., — seem to partake of the same character. They were formerly regarded, like the epideimis itself, in the light of an inorganic exudation from the time skin, — a sort of hardened glue — which could undergo no subsequent change. It has now been ascertained, however, that they are produced in the first instance by the growth of cells, the contents of which gradually evaporate, so that the walls are left to approximate with each other. Iu the most recently-formed portion of the hoof, the separate cells of which it is composed may be distinctly recognised ; they are somewhat flattened against each other, but retain their general rounded form. At a subsequent period, however, there seems to be a deposit of horny matter on the interior of the cell, by which the membranous walls, originally thin aud smooth, are thickened as well as roughened. The cells afterwards change their form iu a con- Pigment-cells from tail of Tadpole : — a, a, simple forms of recent origin; b, b , more complex forms subsequently assumed. siderable degree, and their walls approximate to each other so closely, that their several boundaries can be scarcely distinguished. The nails of the new-born infant have been found to exhibit a similar structure. The thin horizontal laraime into which they may be split, consist of cells resembling those of the epithelium, which afterwards become much extended and flattened, whilst horny matter is at the same time deposited within them, just as sclerogen is iu the cells and woody fibres of Plants (§ 148). When this is effected, no further change appeal's to take place in the tissue ; so that it may be regarded as a dead and almost inorganic substance. The cellular character may be traced with perfect distinct- ness in the spongy portion, or medullary centre, of the stem of the feather, which closely resembles the parenchyma of plants ; it is also very evident in the soft interior of the Porcupine’s quill. In the outer hard envelope, or cortical portion, however, an indistinctly fibrous structure may be seen. The Epidermis aud all its appendages are completely extra- vascular ; that is, they are not traversed by nutritious and absorbent vessels. They have little tendency to spontaneous decomposition ; and ANIMAL TISSUES ; EPIDERMIC APPENDAGES ; HAIR. 123 Fig. 30. therefore they do not require that constant interstitial change, which is so characteristic of the tissues that are actively employed in the performance of the vital functions (§ 46). In this respect, then, they closely resemble the heart-wood of the tree, and the horny sheaths or solid stony axes of the Polypifera ; which are originally formed by a consolidation of living- tissue, but which, when once fully developed, undergo little or no subsequent change. 180. Under the same head it is proper to consider Hair, which is truly an £ epidermic appendage,’ although not developed upon the external surface, but in the interior of a follicle formed by a depression of the true skin. This follicle is lined by a continuation of the Epidermis, the cells of which are developed in peculiar abundance from a spot at its deepest portion ; the dense exterior of the cluster thus formed being known as the bulb of the hair, while the softer interior is termed its pulp . — Although the Hairs of different animals vary considerably in the appearances they present, we may generally distinguish in them two elementary parts, corresponding with those -which we meet with in the stem of a feather ; namely, a cortical or investing substance, of a dense horny texture ; and a medullary, or pith-like sub- stance, usually of a much softer character, occupy- ing their interior. The fullest development of both substances is seen in the spiny hairs of the Hedge- hog, and in the quills of the Porcupine, which are but hairs on a magnified scale ; they are very charac- teristically seen, however, in some of the hairs of the Sable (Fig. 30, b), whose medulla is composed of large rounded cells, whilst the cortical substance is obviously composed of flattened epidermic scales, arranged in an imbricated manner. In some in- stances, almost the entire hair seems made up of cortical substance ; whilst in others the medullary predominates, as in those of the Musk-deer (Fig. 30, a). In the hair of the Mouse and other small Rodents, the cortical substance forms a tube, which we see crossed at intervals by partitions that are sometimes complete, sometimes only partial ; these are the walls of the single or double line of cells, of which the medullary substance is made up. In the Human hair, we do not find these structures so characteristically developed ; they are still readily distinguishable, however, in a transverse section ; the medullary matter presenting a granular arrangement and often containing pigmentary molecules ; to which the cortical sheath forms a thin but dense and apparently homogeneous envelope, its origin in epi- dermic scales being still indicated by wavy lines crossing its surface irregularly. The hair of Man has been commonly reputed to be tubular; but this is proved by microscopical examination to be seldom or never the case. The mistake seems to have arisen from a misinterpretation of the ap- pearance of a dark band in the interior of the hair, when viewed by Structure of Hair: — a, hair of Musk-deer, consist- ing almost entirely of poly- gonal cells; b, hair of Sable, showing large rounded cells in its interior, covered by imbricated scales or flattened cells. 124 GENERAL PHYSIOLOGY. transmitted light ; this band being really due in part to the refraction of light by the cylindrical surface, and in part to the presence of pigmentary matter in the central portion of the shaft. — The cortical envelope of hairs is a continuation of the outer scaly layers of the epidermis that lines the follicle ; whilst the medullary is derived from the deeper stratum, whose cells are produced in unusual abundance at its ccecal extremity : > and it is by the constant development of new cells at this point, that the continual growth of the hair is kept up. 181. The Shells of Mollusca, also, are to be regarded in the light of ‘ epidermic appendages and were formerly supposed to be formed, like the Epidermis itself, by an exudation from the surface of the true skin. It has been demonstrated, however, by microscopic inquiry, that their structure is as much organised as that of the Epidermis is now known to be ; and that the substance of the shell is formed by the consolidation of epidermic cells by calcareous matter which the cells imbibe into their interior. — It will be more convenient, however, to consider this structure in connection with that of other foims of Shell, and of Bone (§ 197). 182. The Epithelium may be designated as a delicate cuticle, covering all the free internal sui’faces of the body ; and apparently designed, in some instances, simply for their protection ; whilst in other cases, as we shall presently find, it serves purposes of far greater importance. The Epithelial cells, being always in contact with fluids, do not dry up into scales like those of the Epideimis ; and they differ from them also in regard to the nature of the matter which they secrete in their interior. In this respect, however, the Epithelial cells of different parts are unlike one another, fully as much as any of them are unlike the cells of the Epi- dermis ; for we shall find that all the secretions of the body are the pro- duct of the elaboration of Epithelium-cells ; and consequently there are as many varieties of endowment in these important bodies, as there are varieties in the result of their action. — The Epithelium covering the Serous and Synovial membranes, and forming the lining of the blood-vessels, is composed of flattened polygonal cells (resembling those shown in Fig. 31), lying in apposition with each other, so as to form a kind of pavement ; hence this form is termed pavement- or tesse/afecT-Epithehum. There is no reason to believe that it possesses any active endowments in these situa- tions : since it does not appear to be concerned in the elaboration of any peculiar secretion (§ 167). The Epithelium of the Mucous membranes and their prolongations are found under two principal forms, the tesselated, and the cylindrical. An example of the Tessel- ated form is shown in Fig. 31, which shows the flattened epithelium-cells of the mucous mem- brane of the mouth, as they are frequently met with in saliva. The Cylinder-epithelium is veiy differently constituted. Its component cells are cylinders, which are arranged side by side ; one extremity of each cylinder resting upon the base- ment-membrane, whilst the other forms paid of the free surface. The perfect cylindrical form is only shown, when the sm-face on which the cylinders rest is flat or nearly so. " When it is convex, the lower ends or basements of the cells are of Fig. 31. Separated Epithelium-cells, a, with nuclei, b, and nucleoli, e, from mucous membrane of mouth. ANIMAL TISSUES ; — CILIATED EPITHELIUM. 125 Fig. 32. much smaller diameter than the upper or free extremities ; and thus each has the form of a truncated cone rather than of a cylinder, as is well seen in the cells which cover the villi of the intestinal canal (Fig. 32, a). The two forms of Epithelium pass into one another at various points, and various transitional forms are then seen. 183. Both these principal forms of Epithelial cells are frequently observed to be fringed at their free margins with delicate filaments, which are termed Cilia ; and these, although of extreme minuteness, are organs of great importance in the animal economy, through the extra- ordinary motor powers with which Diagram of mucous membrane of jejunum, when Absorption is not going on ; a, protective epithe- lium of a villus b, secreting epithelium of a follicle; c, c , c , primary membrane, with its ger- minal spots or nuclei, d, d ; e , germs of absorbent vesicles ; f, vessels and lacteals of villus. Fig. 33. they are endowed. The form of the ciliary filaments is usually a little flattened, and tapering gradually from the base to the point. Their size is extremely variable ; the largest that have been observed being about l-500th of an inch in length, and the smallest about 1-1 3,000th. When in motion, each filament appears to bend from its root to its point, returning again to its original state, like the stalks of corn when depressed by the wind ; and when a number are affected in- succession with this motion, the appearance of progressive waves following one an- other is produced, as when a cornfield is agitated by successive gusts. When Ciliated Epithelium a, nucleated cells, resting i • i • on their smaller extremities ; b, cilia. the ciliary motion is taking place m full activity, however, nothing whatever can be distinguished but the whirl of particles in the sm-rounding fluid ; and it is only when the rate of movement slackens, that the shape and size of the cilia, and the manner in which their stroke is made, can be clearly seen. The motion of the cilia is not only quite independent (in all the higher animals at least) of the will of the animal, but is also independent even of the life of the rest of the body ; being seen after the death of the animal, and proceeding with perfect regularity in parts separated from the rest. Thus, isolated epithe- lium-cells have been seen to swim about actively in water, by the agency of their cilia, for some hours after they have been detached from the mucous surface of the nose ; and the ciliary movement has been seen fifteen days after death in the body of a Tortoise, in which putrefaction was far advanced. In the gills of the River-Mussel, which are among the best objects for the study of it, the movement endures with similar pertinacity. 184. The purpose of this Ciliary movement is obviously to propel fluids over the surface on which it takes place ; and it is consequently limited in the higher animals to the internal surfaces of the body, and 126 GENERAL PHYSIOLOGY. always takes place in the direction of the outlets, towards which it aids in propelling the various products of secretion. The case is different, how- ever, among animals of the lower classes, especially those inhabiting the water. Thus the external surface of the gills of Mollusks, Tadpoles, Ac., is furnished with cilia ; the continual movement of which renews the water in contact with them, and thus promotes the aeration of the blood. In the lower Mollusca, and in many Zoophytes, which pass their lives rooted to one spot, the motion of the cilia serves not merely to produce currents for respiration, but likewise to draw into the mouth the minute particles that serve as food. And in the free-moving Animalcules of various kinds, the cilia are the sole instruments which they possess, not merely for producing those currents in the water which may bring them the requisite supply of air and food, but also for propelling their own bodies through the liquid. This is the case, too, with many larger animals of the class Acalepha (Jelly-fish), which move through the water, sometimes with great activity, by the combined action of the vast numbers of cilia that clothe the margins of their external surfaces. In these latter cases it would seem as if the ciliary movement were more under the control of the will of the animal, than it is where concerned only in the organic functions. In what way the will can influence it, however, it does not seem easy to say ; since the ciliated epithelium-cells appeal’ to be perfectly disconnected from the surface on which they lie, and cannot, therefore, receive any direct influence from them nerves. — Of the cause of the move- ment of the Cilia themselves, no account can be given ; they are usually far too small to contain even the minutest fibrilhe of muscle ; and we must regard them as being, like those fibrillse, organs sui generis, having their own peculiar endowment, — which is, in the higher animals at least, that of continuing in ceaseless vibration, during the whole term of the life of the cells to which they are attached. The length of time during which the ciliary movement continues after the general death of the body, is much less in the warm-blooded than in the cold-blooded animals ; and in this respect it corresponds with the degree of persistence of muscular irritability, and of other vital endowments. 185. The simplest office which the Epithelium-cells of Mucous mem- branes perform, appears to be that of elaborating their protective Mucus (§ 169); but where these membranes are prolonged into the follicles or tubuli of which glandular structures are essentially composed (chap, xyi.), we find that the epithelial cells of these follicles (Fig. 32, l>) are the real agents in the process of Secretion which the glands carry on ; drawing from the blood, as the materials of their own growth, the substances which the glands are destined to separate ; and setting free these products, by their own death and decay, at the appropriate time and place. The epithelial cells of different glandular surfaces differ widely from one another, in regard to the kind of matter which they appropriate and assemble in their cavities, although the nature of their walls is probably the same throughout. Thus we find biliary matter and oil, easily recognisable by their colour and refracting power, iu the cells of the liver ; milk in the cells of the mam- mary gland ; sebaceous or fatty matter iu the cells of the sebaceous follices of the skin ; and so on. All these substances are derived from the blood ; being either contained in it previously, or being elaborated from its constituents by a simple process of transformation, — as, for example. ANIMAL TISSUES ; AGGREGATED CELLS. 127 that which converts the albumen of the blood into the caseine of milk. Hence they may be considered as the peculiar aliments of the several groups of cells ; whose acts of nutrition are the means of drawing them off, or secreting them, from the general circulating fluid. When they have attained their full growth, and accomplished their term of life, their walls either burst or dissolve away, and thus the contents of the cells are delivered into the cavity, or upon the surface, at which they are required. Now as all the canals of the glands open either directly outwards upon the surface, or into cavities which communicate with the exterior, it is evident that the various products of the action of these epithelial cells must be destined to be cast forth from the body. This we shall find to be the case ; some of them, as the bile and urine, being excretions, of which it is necessary to get rid by the most direct channel ; whilst others, like the tears, the saliva, the gastric fluid, the milk, &c., are separated from the blood, not so much for its purification, but because they are required to answer certain purposes in the economy. — We shall find, too, that the cells in which the Spermatozoa are developed for the fertilisation of the ova (chap, xviii.), are the offspring of the epithelial cells lining the tubes or follicles of the Spermatic gland. 186. Aggregated Cells, forming Permanent Tissues. — We now pass onto consider those Cells, which enter as component elements into the solid and permanent fabric of the body, and which do not take so active a part in its vital operations. These we shall find to be usually more or less closely connected together, either by a general enveloping membrane, or by an intercellular substance, which is interposed between their walls, and holds them together by its adhesive properties. This generally presents no distinct traces of organisation ; and usually consists of Gelatine, or of a substance allied to it in composition. The proportion which this substance bears to the cells may vary in different cases ; and very different characters may thus be presented by tissues made up of the same elements. Thus the subjoined figure represents a portion of one of the animal layers included Fig. 34. Portion of shell-membrane, showing the origin of cells in the midst of horny intercellular sub- stance ; a, nuclei ; b, incipient cells ; c, the same further advanced, but separated by intercellular substance; d } the cells become polygonal by mutual pressure. between the calcareous laminae of a bivalve shell ; in which we see on the one side (a) a number of nuclei or incipient cells, scattered through a bed of homogeneous intercellular substance, and bearing but a veiy small propor- tion to it ; while the opposite end (d) exhibits a set of polygonal cells, in close 128 GENERAL PHYSIOLOGY. contact with each other, the intercellular substance being only represented by the thick dark lines, which mark the boundaries of the cells, and which are rather thicker at the angles of the latter. Between these two extremes, we observe eveiy stage of transition. — The presence of a very large amount of intercellular substance, through which minute cells are scattered at considerable intervals (Fig. 34 a), is characteristic of various forms of Cartilage ; and more particularly of that soft semi-cartilaginous structure, of which the Jelly-fish are for the most part composed. In other forms of cartilage, we find the cells more developed, and in closer proximity to each other, the proportion of the intercellular substance being at the same time diminished (as seen at 6 and c, Fig. 34) ; but it is not often, save in embryonic structures, that we find the cells in such close proximity, and the intercellular substance so nearly wanting, as at d. Such examples do occasionally present themselves, however, even in the soft tissues. Thus the chorda dorsalis, which replaces the vertebral column in the lowest Fishes, and of which the analogue is found in the embryos of the higher Vertebrata, is made up of a structure of this kind. The true Skin, in the Short Sun-fish, is replaced by a similar layer of cellular tissue, which extends over the whole body, varying in thickness from one-fourth of an inch to six inches. And in the Amphioxus (§ 321), a considerable portion of the fabric is made up of a similar parenchyma. 187. Now we shall find that one method, by which the requisite firm- ness and solidity are given to the animal fabric, consists in the deposition of earthy substances in the interior of such cells, by a peculiar secreting action of their own. Thus in Shell, we find them completely filled up with carbonate of lime ; and in the enamel of Teeth, chiefly with phosphate of lime. When this is the case, there is often a tendency to a coalescence of the cells, by the removal of the whole intercellular substance from between them, and then by the obliteration of the partitions themselves ; so that the solid mass appeal’s homogeneous, and its cellular origin can only be detected by watching its development, and by observing the various stages of transition. This is the case, for example, in regard to the shells of many Mollusca, especially those of a porcellanous character ; in which the amount of animal matter is very small. 188. We often find, however, a coalescence between the cavities of cells, as in Plants (§ 152), for the production of straight or anastomosing tubes. Thus the smaller Blood-vessels of Animals seem to originate in rows of cells, the cavities of which have run together by the obliteration of the transverse partitions ; and the persistent nuclei of such cells may be generally brought into view in the walls of the capillaries. And the same appears to be the origin of the tubular fibres of Muscular and Nervous tissue, which contain the elements characteristic of those tissues ; these elements, — the fibrillse of muscle, and the granular pith of the nerve-tube, — being evidently the secondary products of parent-cells, which seem to remain as their investing tubuli, the original nuclei being often to be seen in their walls (§ 226 and 238). — Besides these changes, the original cells may often undergo marked alterations of form ; and this quite independently of any pressure to which they may be subject. Thus the pigment-cells, as already mentioned (§17 8), frequently exhibit a curious stellate form ; arising from the development of radiating pro- longations, which are put forth from the original spheroid. And we ANIMAL TISSUES j FAT CELLS. 129 Fig. 35. sometimes find cells undergoing such an elongation, that they first become fusiform or spindle-shaped, and are at last converted into solid fibres (§ 166). 189. We now proceed with the description of the various tissues in the Human body, which are composed of cells united or transformed in the foregoing manner ; and we shall commence with Adipose or Fatty tissue, which may be considered as a sort of link, connecting the permanent tissues with those which are more actively concerned in the processes of Nutrition, Secretion, &c. The Adipose tissue is composed of isolated cells, which have the power of appropriating fatty matter from the blood, precisely in the same manner as the secreting cells appropriate the elements of bile, milk, &c. These cells are sometimes dispersed in the interspaces of the Areolar tissue ; whilst in other cases they are aggregated in distinct masses, — constituting the proper Adipose tissue. The indi- vidual fat-cells always present a nearly spherical or spheroidal form ; sometimes, however, when they are closely pressed together, they become somewhat polyhedral, from the flattening of their walls against each other. Their intervals are traversed by a minute net-work of blood-vessels, from which they derive their secretion ; and it is probably by the constant moistening of their walls with a watery fluid, that their contents are retained without the least transudation, although they are quite fluid at the temperature of the living body. If the watery fluid of the cell-walls of a mass of Fat be allowed to dry up, and it be kept at a tem- perature of 100°, the escape of the contained oily matter is soon perceptible. By this provision, the fatty matter is altogether prevented from escaping from the cells of the living tissues, by gravitation or pressure ; and as it is not itself liable to undergo change when secluded from the air, it may remain stored up, apparently unaltered, for a period only limited by the demand for it in the system. — The consistency, as well as the Chemical constitution, of the fatty matter contained in the Adipose cells, varies in different animals, according to the relative proportions of three component substances, which may be distinguished in it, — Stearine, Margarine, and Oleine. The two former are solid when isolated, and the latter is fluid ; but at the ordinary temperature of the warm-blooded animal, they are dissolved in it. These fatty matters are abundantly supplied by the Vegetable kingdom, and are found to exist largely in substances which were not previously supposed to contain them ; and it is not requisite, therefore, to suppose that Animals usually elaborate them by any transforming process from the elements of their food. The portion of fatty matters separated from the circulating fluid to form the Adipose tissue, is only that which can be spared from the other purposes to which they have to be applied ; and hence the production of this tissue depends in part upon the amount of Fatty matter taken in as food. This is not entirely the case, however, as some have maintained ; for there is sufficient evidence that animals may produce fatty matter by a process of chemical transformation, from Areolar and Adipose tissue; a, a , fat-cells ; b, b , fibres of areolar tissue. 130 GENERAL PHYSIOLOGY. the starch or sugar of their food, when there is an unusual deficiency of it in their aliment (chap. x.). 190. The development of Adipose tissue in the animal body appears to answer several distinct purposes. It fills up interstices, and forms a kind of pad or cushion for the support of moveable parts ; and so necessary does it seem for this purpose, that, even in cases of great emaciation, some fat is nearly always found to remain. It also assists in the retention of the animal temperature by its non-conducting power ; and we accordingly find a thick layer of it, in those warm-blooded mammals that inhabit the seas, either immediately beneath their skin, or incorporated with its substance. And it also serves as a reservoir of combustible matter, at the expense of which the respiration may be maintained when other materials are deficient : thus we find that the respiration of hybernating animals is kept up, during the period when they cease taking food (§ 94), by the consumption of the store of fat which was laid up in their bodies, previously to their passing into that state ; and it is also to be noticed that herbivorous animals, whose food is scanty during the winter, usually exhibit a strong tendency to such an accumulation, during the latter part of the summer, when their food is most rich and abundant, iu order to supply the increased demand created by the low external temperature of the winter season. Other circum- stances being the same, it appears that the length of time during which a warm-! flooded animal can live without food, depends upon the quantity of fat in its body ; for the rapid lowering of its temperature, which is the immediate cause of its death (§ 93), takes place as soon as the whole of this store has been exhausted. It has been shown by the experiments of Matteucci, that fatty matter will pass by Endosmose through animal membrane, when a slightly alkaline liquid is on the other side ; and it would not seem improbable that it is iu this mode that the contents of the fat-cells are progressively taken back into the blood, iu proportion as the quantity of fatty matter which it usually contains is expended. 191. In the simpler forms of Cartilage, we have an example of a tissue of remarkable permanence, composed entirely of cells scattered through an inter-cellular substance. This substance has a close resemblance to Gelatine in composition and properties, but is not identical with it ; and it has received the distinguishing appellation of Chondrine, which marks it as the solidifying ingredient of Cartilage. It requires longer boiling than Gelatine for its solution in water ; but the solution fixes into a jelly in cooling, and dries by evaporation into a glue that cannot be distin- guished from that of gelatine. When a pure Cellular Cartilage is examined microscopically, its cells are seen to lie, sometimes singly, and sometimes in clusters of two, three, or four, in cavities excavated in the intercellular substance ; and these occur at very variable distances. From the various appearances which may be observed in the same cartilage, at different stages of its growth, it would appear that the component cells multiply by the doubling process already described (§ 157) ; that they then separate from one another, each of them drawing towards itself (as it were) an envelope of intercellular substance ; and that, by the repetition of the same process, the number of cells in the cartilage may be indefinitely multiplied. Various stages of this history are shown in the accompany- ing figure (Fig. 36), which is taken from a section of the cartilaginous ANIMAL TISSUES ; CARTILAGE. 131 branchial ray of the tadpole of the Rana esculenta, or Edible Frog. This primitive cellular organisation is retained in some Cartilages through the whole duration of their existence ; as, for example, in most of the articular cartilages which cover the ends of bones where they enter into joints. — Fig. 36. Section of the Branchial cartilage of Tad- pole; a, group of four cells, separating from each other; b, pair of cells in apposi- tion ; c , c, nuclei of cartilage cells; d, cavity containing- three cells. Fig. 37. Section of Fibro-Cartilage ; showing disposition of Cartage- cells in areolae of fibrous tissue. But the intercellular substance, instead of being homogeneous, may have a fibrous character; the tissue called Fibro-Cartilage is thus produced, and this may be either elastic or non-elastic, according as the yellow or the white form of fibrous structure prevails (Fig. 37). In some instances, the fibrous structure is so predominant over the cellular, that the tissue has rather the character of a ligament than of a cartilage. The white fibrous structure is seen in all those cartilages, which are destined not merely to sustain pressure, but also to resist tension ; this is the case especially in the substances, which intervene between the vertebrae, and which connect the bones of the pelvis. The yellow-fibrous tissue is intro- duced where more elasticity is required, as in the epiglottis, and in the concha of the ear. 192. It is certain that the substance of the permanent cellular Carti- lages is not permeated, in a state of health, even by the minutest nutrient vessels ; none such being brought into view under the highest mag- nifying power. They are, how- ever, surrounded by vessels, which form large dilatations at their edges, or spread over their surfaces (Fig. 38) ; and it is by the fluid which is drawn from them by the Cartilage-cells, that the latter are nourished. The nutrition of a mass of Cartilage thus seems to bear a strong resemblance to that of the thick fleshy Sea-weeds, which are in like manner composed entirely K 2 Fig. 38. Dilated loops of capillary vessels between Articular Cartilage and attached Synovial membrane. 132 GENERAL PHYSIOLOGY. of cells, with intercellular substance disposed between them in greater or smaller amount (Figs. 69, 70). The cells in nearest proximity to the nutrient fluid, draw from it the requisite materials, and transmit these to the cells in the interior of the mass, receiving a fresh supply in their turn from the source in their own neighbourhood. — The Cartilaginous tissue, however, appears to be more removed than almost any other in the body, from the general tide of nutritive action. Its properties are simply of a physical character ; and they are not impaired for a long time after the death of the tissue, its tendency to decomposition being very slight, so long as it is exposed to ordinary temperatures. It is pro- tected by its toughness and elasticity from those mechanical injuries, to which softer or more brittle tissues are liable ; and consequently it has little need of any active power of reparation. 193. With the subject of Cartilage, that of Bone seems naturally to connect itself. We shall find this, however, to be a fabric of great com- plexity, peculiar to Vertebrated animals; being especially adapted, in its mode of growth, to the changing dimensions of their internal skeletons. And it will be convenient to consider, in the first instance, the principal modes in which the tissues of Invertebrated animals are condensed by deposits of solid matter, in order that they may become subservient to those purposes of mechanical support and protection, which are required in various degrees by the different tribes of that division of the Animal Kingdom. 194. Commencing with Zoophytes, we find that in some of their' more delicate forms the requisite support and protection are derived from the consolidation of the external or tegumentary membrane by a deposit of horny matter ; this is the case, for example, more or less, with most of the compound Hydroida (§ 290). In other instances, we find this homy deposit taking place in the centre of the stem, as in the common Gorgon ia .(Sea-Fan), Antipathes (Black Coral), &c. There are many Zoophytes of the Alcyonian group (§ 292) which have a spongy structure ; and in these, as in true Sjoonges, we find the tissue strengthened by the presence of a number of detached crystalline spicules, sometimes composed of carbonate of lime, sometimes of siliceous matter, which remind us of the raphides of Plants (§ 23). These spicules present a great variety of forms, and are interspersed without any obvious arrangement among the elements of the softer tissues ; thus we even find them in the soft flesh which clothes the horny stems of Gorgonia and its allies. The stony Corah are formed, however, by the consolidation of the soft animal tissue with a deposit of carbonate of lime ; and this, again, may be either external or internal, the two opposite conditions presenting each other in animals which are othei’wise closely allied. Thus in the group of Alcyonian Zoophytes (§ 292), we find the Organ-pipe Coral ( Tubipora musica) to be composed of a series of calcareous tubes formed by the deposition of carbonate of lime in the integument of the polypes ; whilst the common Red Coral, is formed by a similar consolidation of the central axis which connects them. So, again, in the Flustra, Eschara, and others of the Bryozoa (§ 298), we find the coral chiefly formed by the calcification of the whole external integument ; whilst in the massive corals formed by Actiniform Polypes (§ 291), it is only in the soft tissue at their base, and in that which intervenes between the polypes, that the consolidation takes ANIMAL TISSUES ; SHELLS OF ECHINODERMATA. 133 place. Thus we see that among these lowest members of the Animal kingdom, neither the material nor the position of the skeleton is by any means constant ; and we may consider both the external skeletons of the Articulata and Mollusca, and the internal skeletons of Vertebrated animals, as here pre-figured or sketched out. These last seem especially foreshadowed by the Isis hippuris, in which the coral stem acquires a jointed character from the alternation of calcareous and of homy deposit in the soft tissue which forms its basis. 195. In the class of Echinodermata, the skeleton is generally dermal; being formed by the union of the fibrous tissue of the true skin with calcareous matter. Although usually ranked as an external skeleton, it is not altogether so ; for it is covered with a contractile membrane, by the agency of which the movements of its different parts (where they are capable of motion), are effected. Thus in the Echinus (§ 295) the body is enclosed in a shell of somewhat globular form, having an opening at each pole, one for the mouth and the other for the anus ; this shell is made up of a number of plates more or less regularly hexagonal (Fig. 39, a), on the outer surface of with gelatinous fibres (§ 1^^)) which aie SO Portion of the shell of the Echinus, showing at a the constituent disposed as to leave plates; with the calcified areolar tissue, of which they are composed, multitudes of areolce at *' or minute interspaces freely communicating with each other (Fig. 39, b). The arrangement of this calcareous network in the spines, is most varied and elaborate ; and causes thin sections of them to be among the most beautiful of all microscopic objects. The external and internal surface of each plate, in the shell of the living Echinus, is covered with a membrane from which its nutrition is derived ; this membrane, also, dips down into the spaces between the adjacent plates. A similar membrane covers and encircles the spines ; and it also connects these with the shell, being con- tinuous with the membrane that envelops the latter. The substance of which the shell is composed, although of extreme delicacy, is of great per- manence, and does not exhibit the slightest tendency to decay, however long it is preserved ; so that, when once consolidated, it appears to undergo no further change in the living animal. The growth of the animal, how- ever, requires a corresponding enlargement of its enveloping shell ; and this is provided for by the simple process of adding to the surface, through the subdivision of the whole shell into component plates. For by the addition of new matter at the edge of each plate, through the consoli- which are little tuber- cles for the articula- tion of the spines, which serve as instru- ments of locomotion and defence. The sub- stance of the shell and of the spines is exactly alike, being a sort of calcified areolar tissue, formed by the union of carbonate of lime Fig. 39, 134 GENERAL PHYSIOLOGY. dation of a portion of the soft membrane that intervenes between the adja- cent plates, the whole shell is enlarged, without losing its globular form ; and at the same time it is strengthened in a corresponding degree, by the consolidation of the soft tissue covering the surface of each plate. In like manner, the spines are enlarged and lengthened by the progressive forma- tion of new layers, each on the exterior of the preceding ; so that a trans- verse section exhibits a number of concentric rings, like those of an Exogenous tree (§ 280). — Thus even in the growth of this complex and elaborate structure, we recognize the principle of superficial deposit, which we shall find to be universal amongst the hard parts of the Invertebrata ; notwithstanding that, at first sight, it would have appeared impossible to provide on this plan for the gradual enlargement of a globular shell, com- pletely enclosing the animal, and therefore required to keep pace with the latter in its rate of increase. 196. Among the Mollusca we find the body sometimes altogether destitute of solid organs of support, protection, or locomotion, — as is the case, for example, in the Slug : and the movements are feeble and the habits inert, the muscles having no fixed points for their attachment, and acting without any of the advantages of leverage. In other cases, we find the body more or less completely protected by a Shell ; which is suffi- ciently large in some instances to cover the soft parts completely ; whilst in others, it affords them only a partial investment. The plan on which this shell is formed, however, is veiy different from that which has just been described, being much less complex. The Univalve shells, or those formed in one piece, are always of a conical form : the axis of the cone being some- times straight, as in the Limpet ; in other cases being spirally coiled, as in the Snail : and through the open base of this cone the animal can project its moveable parts. When its increasing size requires additional accommoda- tion, it is obvious that an addition to the large end of the cone will increase its diameter and its length at the same time ; so as to afford the required space, without any alteration in the form or dimensions of the older and smaller portions of the cone. This last, indeed, is frequently quitted by the animal, and remains empty ; being sometimes separated from the later portions, by one or more partitions thrown across by the animal, — as is seen especially in the Nautilus and other chambered shells. Besides the new matter added to the mouth of the shell, a thin layer is usually formed over its whole interior surface ; so that the lining of the new paid is continuous with that of the old. — In the Bivalve shells, we trace this mode of increase without any difficulty ; especially in such shells as that of the Oyster, in which the successive larnime remain distinct. Each lamina is interior to the preceding, being formed ou the living surface of the animal ; but it also projects beyond it, so as to enlarge the capacity of the shell ; and as the separation of the valves affords free exit to those parts of the animal, which are capable of being projected beyond the shell, there is obviously no need of any other provision to maintain the shell in its natural form. — Thus in the shells of the Mollusca, increase takes place at the surfaces and edges only. 197. The proportion of organic and calcareous matter in Shell differs considerably in the various tribes. The former is sometimes present iu such small amount, that it can scarcely be detected ; and the condition of the calcareous matter then obviously approaches that of a crystalline ANIMAL TISSUES J SHELLS OF MOLLUSCA. 135 deposit. But in other instances, the animal basis is very obvious ; remain- ing as a thick consistent membrane, after all the calcareous matter has been dissolved away by an acid. This membrane is formed of an aggre- gation of cells arranged with great regularity (Fig. 40, a) ; the cavities of which are filled with carbonate of lime in a crystalline state. The form of the cells approaches the hexagonal ; their diameter varies in different shells from 1-1 00th to 1 -2800th of an inch ; their thickness also is extremely Prismatic cellular structure of shell of Pinna ; a , surface of lamina; b, vertical section. variable, even in different parts of the same shell. Thus we sometimes meet with a lamina of such tenuity, as not to measure a hundredth of an inch in thickness ; whilst in other instances, a single layer may have a thickness of half an inch, or even (in certain large fossil species) of an inch or more. In this case, the cells, instead of being thin flat scales, like the pavement-epithelium (§ 182), are long prisms, with their walls flat- tened against each other ; and the appearance which is then presented by a vertical section of them, is represented in Fig 40, b. The long pris- matic cells are there seen to be marked by delicate transverse strife ; and these, taken in connection with other indications, appear to show, that every such prism is in reality formed by the coalescence of a pile of flat cells, resembling those which are seen in the very thin laminae just described ; so that the thickness of the layer depends upon the number of the cellular lamime, which have coalesced to form its component prisms. This character is of interest, as representing on a magnified scale a corre- sponding appearance in the enamel of human Tooth, which we shall pre- sently find to be formed upon the very same plan (§ 214). — We are to regard this kind of shell-substance, therefore, as formed by the secreting action of the epithelial cells covering the mantle of the animal ; which membrane, though it answers in position to the skin, has the soft, spongy, glandular character of a mucous membrane. These draw calcareous matter into their cavities, as a part of their own process of growth ; this matter being supplied from the fluids of the vascular surface beneath. Now when these calcigerous cells are separated by intercellular substance (Fig. 34), they remain distinct through the whole of their lives, and they form by their cohesion a tenacious membrane, that retains its consistency after the removal of the calcareous matter. But this is only the case in certain 136 GENERAL PHYSIOLOGY. groups of shells, chiefly belonging to the bivalve division. When the intercellular substance is wanting, and the cells come into close contact, their partitions become indistinct, on account of then' extreme tenuity ; and not unfrequently a f usion of the whole substance appears to take place, by the dissolution of the original cell-walls, so that it becomes more or less homogeneous, — traces of the original cellular structure being here and there distinguishable (§ 187). 198. Sometimes where this fusion has taken place, so as to obliterate the original cell-structure, we find the almost homogeneous substance tra- versed by a series of tubuli ; these not being arranged, however, on any very definite plan, but forming an irregular net-work. The tubes vaiy in size from l-2000th to 1-20, 000th of an inch; but their general diameter, in the shells in which they most abound, is 1-45 00th of an inch. Although it might be supposed that this structure is destined to convey nutrient fluid into the substance of the shell, yet there is no evidence that such is the fact ; and, on the contrary, there is ample evidence, that, even in shells most copiously traversed by these tubuli, no processes of interstitial increase or removal take place. The permanent character of the sub- stance of all Shells, when once it is folly formed, is as remarkable as that of Coral ; and as the adap- tation of their size to that of the animals to which they belong, is entirely effected by additions to their surfaces and edges, no inter- stitial deposit can have a shai'e in producing it. Still there is a difference, well known to the Conchologist, between a dead shell and one which remains in connection with the living animal ; so that it is probable that the organic basis of shell has its composition preserved by communication with the nutrient fluid of the mantle that lines its inner surface, so long as the latter can supply it. — Such are some of the principal forms of Shell-tissue ; there are, however, very numerous varieties in the structure of the shells of different groups of Mollusca, and these varieties are occasionally so characteristic as to allow of the determination of the tribe to which the shell belonged from the microscopic examination of a minute fragment.* 199. Among the Articulated classes, we still find that the skeleton is altogether external, and belongs therefore to the cutaneous system ; but it is formed upon a very different plan from the shells of the Mollusca, being closely fitted to the body, and enveloping eveiy part of it ; conse- quently it must increase in capacity, with the advancing growth of the contained structures. Moreover it is destined not merely to afford sup- port and protection to these, but to serve for the attachment of the mus- cles by which the body and limbs are moved ; and the hard envelopes of the latter serve, like the bones of the Vertebrata, as levers by which the motor powers of the muscles are more advantageously employed. Again, * See the Author’s Researches on the Microscopic Structure of Shells, in “ Reports of the British Association ” for 1844 and 1847. Fig. 41. Tubular Shell-structure, from Anomia. ANIMAL TISSUES ‘ SKELETONS OF ARTICULATA. 137 the hard envelopes of the body and limbs are not formed of distinct plates, like those of the Echinus-shell ; but are only divided by sutures at the joints, for the purpose of permitting the requisite freedom of motion. It might have been thought that here, if anywhere, a process of interstitial growth would have existed, to adapt the capacity of the envelopes to the dimensions of the contained paids, as the latter increase with the growth of the animal ; but, true to the general principle, that epidermic structures are not only extra-vascular, but that they undergo no change when they are once fully formed, we find that the hard envelopes of Articulated ani- mals are thrown off, or exuviated, when the contained parts require an increase of room ; and that a new covering is formed from their surface, adapted to their enlarged dimensions. This is well known to occur at cer- tain intervals in Crabs, Lobsters, and other Crustacea ; which thus exuviate not merely the outer shell, with the continuation of the epidermis over the eyes, but also its internal reflexion, which forms the lining of the oesopha- gus and stomach, and the tendinous plates by which the muscles are attached to the lining of the shell. A similar moulting may be observed to occur in some of the minute Entomostracous Crustacea of our pools, every two or three days, even after the animals seem to be full grown. During the early growth of Insects, Spiders, Centipedes, etc., a similar moult is frequently repeated at short intervals ; but after these animals have attained their full size, which is the case with Insects at their last change, no further moulting takes place, the necessity for it having ceased. — This moulting is precisely analogous to the exfoliation and new forma- tion of the Epidermis; in Man and most other Vertebrata ; differing from it only in this, that the latter is constantly taking place to a small extent, whilst the former is completely effected at certain intervals, and then ceases. We have examples of a periodical complete moult in Vertebrata, however, among Serpents and Frogs.— The structure of the hard envelopes of Articulated animals corresponds with that of the Epidermis and its appendages in Man. The firm casings of Beetles, for example, are formed of layers of epidermic cells, united together, and having their cavities filled by a horny secretion. The densest structure is found in the calcareous shells of the Crustacea ; which consist of a substance precisely analogous to the Dentine of Teeth (§ 212) ; covered on the exterior with a layer of pigment-cells. The calcareous matter consists chiefly of carbonate of lime ; but traces of the phosphate are also found. The animal basis has a firm consistent structure, resembling that of teeth. 200. Now the condition of the osseous skeleton of Vertebrated animals is altogether different. It forms a part of the internal substance of then- bodies ; and as these grow in every part, and not merely by addition to this or that portion, so must the Bones also, in order to keep pace with the rest of the structure. Hence we find them so formed, that the pro- cesses of interstitial deposition may be continually going on in their fabric, as in that of the softer tissues ; and the changes in their substance do not cease, even when they have acquired their full size. The continuance of these changes appears destined, not so much to repair any waste occasioned by decomposition, — for this must be very trifling in a tissue of such solidity, — as to keep the fabric in a condition, in which it may repair the injuries in its substance occasioned by accident or disease. The degree of this reparative power is proportional to the activity of the normal changes 138 GENERAL PHYSIOLOGY. which are continually taking place in the bone ; and is thus much greater in youth than in middle life, and greater in the vigour of manhood than in old age. — The presence of Bone is not solely confined, however, to the in- ternal skeleton of Vertebrata, which, being especially destined for the pro- tection of then’ Nervous System, is sometimes termed the neural skeleton ; for there are many instances in which time osseous structure is found in the dermal scales, which form a more or less complete external skeleton. This is more especially the case with Ganoid and Placoid Fishes (Fig. 43), whose neural skeleton is generally but imperfectly ossified ; but true bone is also met with in the scaly armour of some Reptiles and Lacunae of Osseous substance, magnified 500 diame- ters ; — a, central cavity ; b, its ramifications. Mammals. 201. In describing the structure of Bone, it will be desirable to com- mence with the simplest form of osseous tissue, such as is presented in a very thin natural plate of bone, — the scapula (blade-bone) of a Mouse, for example ; — since the real essentials of the tissue are thus better com- prehended. Such a lamella is found by microscopic research to be composed of a substance apparently almost homogeneous, in the midst of which a number of dark spots are to be observed, the form of which is very peculiar. In their general outline they are usually somewhat oval ; but they send forth numerous radiating prolongations of extreme minute- ness, which may be frequently traced to a considerable distance (Fig. 42). These spots, now known as the bone-cells or lacunae (formerly termed the Purldnjean corpuscles, after the name of their discoverer), are highly character- istic of the true osseous structure ; being never deficient in the mi- nutest parts of the bones of the higher animals, although those of Fishes are occasionally destitute of them. They were formerly supposed, from their dark appearance, to be opaque, and to consist of aggregations of calcareous matter which would not transmit the light : but it is now quite certain that they are open spaces, and that the radiating prolongations from them, which are far smaller than the minutest capillary vessel, are canaliculi or delicate tubes. Of these canaliculi, some may be seen to interlace freely with each other, whilst others proceed towards the surface of the bony lamella; and thus a system of passages, not by any means wide enough to admit the blood-corpuscles, but capable of transmitting the fluid elements of the blood, or matters selected from them, is esta- blished through the whole substance of the lamella. — Thus the nutrition of the ultimate osseous texture seems to be canted on upon the same plan with that of Cartilage; being effected by the imbibition of nutrient matter from the surface, through the agency of cells. But it differs in this ; — that there is a provision in Bone for the ready transmission of nutrient matter through its texture, by means of minute channels, which does not exist in Cartilage ; — a difference obviously required by the greater solidity of the substance of the former, which does not allow of the diffused imbibition that is permitted by the softer and moister nature of the latter. We shall presently find that these channels are only formed ANIMAL TISSUES ; — STRUCTURE OF BONE. 139 at a late stage of the development of bone, when the remaining tissue is acquiring its completest consolidation. 202. The size and form of the lacunae differ considerably in the several Classes of Vertebrata, and even in some instances in the Orders; so as to allow of the determination of the tribe to which a bone belonged, by the microscopic examination of even a minute fragment of it. The following- are the average dimensions of the lacunae/ in characteristic examples drawn from the four principal classes, expressed in fractions of an inch. Man Ostrich Turtle Conger-eel Long Diameter. 1-1440 to 1-2400 1-1333 to 1-2250 1-375 to 1-1150 1-550 to 1-1135 Short Diameter. 1-4000 to 1-8000 1-5425 to 1-9650 1-4500 to 1-5840 1-4500 to 1-8000 Fig. 43. The lacunae of Birds are thus distinguished from those of Mammals by their somewhat greater length and smaller breadth ; but they differ still more in the remarkable tortuosity of the canaliculi, which wind backwards and forwards in a very irregular manner. There is an extraordinary increase in length in the lacunae of Reptiles, without a corresponding- increase in breadth; and this is also seen in some Fishes, though in general the lacunae of the latter are remarkable for their angularity of form, and the fewness of their radiations, — as shown in Fig. 43, which represents the lacunae and canaliculi in the bony scale of the Lepidosteus (one of the few existing re- presentatives of the once numerous tribe of Sauroid fish), with which the bones of the internal skeleton per- fectly agree in struc- ture. The dimensions of the lacunae in any bone do not bear any re- lation to the size of the animal to which it belonged ; thus there is little or no perceptible difference between their size in the enormous extinct Iguanodon, and in the smallest Lizard now inhabiting the earth. But they bear a close relation to the size of the blood-corpuscles in the several classes ; and this relation is particularly obvious in the perennibranchiate Batrachia (§ 323), the extraordinary size of whose blood-corpuscles has been already noticed (§ 174). Section of the bony scale of Lepidosteus; — a, showing the regular distribution of the lacunae and of the connecting canaliculi; b, small portion more highly magnified. Proteus Siren . Menopoma . Lepidosiren . Pterodactyle Long Diameter. 1-570 to 1-980 1-290 to 1-480 1-450 to 1-700 1-375 to 1-494 1-445 to 1-1185 Short Diameter. 1-885 to 1-1200 1-540 to 1-975 1-1300 to 1-2100 1-980 to 1-2200 1-4000 to 1-5225 It will be seen from the preceding table, that the lacunte of the Proteus, Siren, and Menopoma differ from those of ordinary Reptiles in their great breadth ; and that in this respect the lacunte of the Lepidosiren agree 140 GENERAL PHYSIOLOGY. with them, as do the dimensions of their blood-discs. The lacunae of the Pterodactyle agree in dimensions with those of ordinary Reptiles, and may be at once distinguished from those of Birds, to which class frag- ments of their wing-bones have been occasionally referred.* 203. When the calcareous matter has been dissolved away from a very thin lamella of bone, and the remaining substance is carefully examined, it is found to consist, not (as commonly stated) of Cartilage, but of a substance made up of indistinct fibres interwoven with each other. These fibres correspond in appearance and composition with those of the White Fibrous tissue (§ 162); and it seems probable that the solid mass of fully- formed bone is formed by the calcification of this tissue. The presence of lacunae and canaliculi, however, is essential to perfect Bone ; and these appear to have their origin in cells, interspersed in the midst of the fibrous basis, which shoot out radiating prolongations (analogous to those of pigment cells, § 178), insinuating themselves through the fibrous substance at the time it is undergoing consolidation, and thus forming the canali- culi.t 204. Although the Osseous texture is not itself penetrated by blood- vessels, yet it would seem essential to its nutrition that no part of it should be far removed from a vascular surface, from which its canaliculi Fig. 44. may absorb the fluid they are destined to convey. Hence we find a special arrangement requisite, for carrying blood-vessels into the neigh- bourhood of every portion of the substance of the larger bones. The solid osseous texture, which forms the cylindrical shafts of the long bones, and of the thick external plates of the denser flat bones, is penetrated by a series of large canals, termed the Haversian (after Clopton Havers, their discoverer), which form a net-work in its in- terior (Fig. 44), and which serve for the trans- mission of blood-vessels into its substance. These canals, in the long bones, run for the most part in a direction parallel to the central cavity ; and they communicate with this, with the external siu-face, and with each other, by frequent transverse branches ; so that the whole system forms an irregular network, per- vading every part of the solid texture, and adapted for the establishment of vascular com- munications throughout. The diameter of the Haversian canals varies from 1-2 5 00th to 1-2 00th of an inch, or more ; then - average diameter may be stated at about l-500th of an inch. They are lined by a membrane, which is continuous with that of the external surface, and which carries this inwards (so to speak) to form the lining membrane of the central cavity, and of the cancelli. On this membrane, a plexus of blood-vessels is distributed, Vertical section of tibia, showing the net-work of Haversian canals. * See Mr. J. Quekett’s Researches, in Trans, of Microsc. Soc. vol. ii. + This view of the origin of the lacunae and canaliculi, which the author's own observa- tions had led him to entertain, has been recently advocated by an observer of distinguished ability. Dr. Leidy of Philadelphia. See American Journal of Medical Sciences, for April, 1849, p. 53(5. ANIMAL TISSUES j STRUCTURE OF BONE. 141 Fig. 45. where the size of the canal admits it ; — otherwise, the tube encloses a single twig of an artery or vein. Thus we may consider the whole Osseous texture as inclosed in a membranous bag ; on which blood-vessels are minutely distributed ; and which is so carried into the bone, by involutions and prolongations, that no part of the latter is ever far removed from a vascular surface. 205. The arrangement of the elementary parts of the osseous substance around the Haversian canals, is very interesting and beautiful. When a transverse section of a long bone is made, the open orifices of the longitudinal canals present themselves at intervals, sometimes connected by a transverse canal, where the section happens to traverse this. Around these orifices, we see the osseous matter arranged in the form of cylinders, which appear to be marked by concentric circles (Fig. 45). Now when one of these circles is minutely examined, it is found to be made up of a series of lacunae, analogous to those already de- scribed ; these, however, are seldom or never so continuous as to form a complete circle. The long sides of the lacunae are directed, the one towards the Haversian canal in the cen- tre, the other towards the cir- cular row next beyond it. And when the course of the canali- culi is traced, it is found that they converge on the inner side towards the central canal, inosculating with those of the series next within, whilst those of the outer side pass outwards in a radiating or diverging di- rection, to inosculate with those of the series next external. Thus a complete communica- tion is formed, by means of this system of radiating canaliculi and intervening lacunse, between the central canal, and the outermost cylindrical lamella of bony matter ; and each of these lamellae derives its nourishment from the vessels of the central canal, through the lamellae which intervene between itself and the vascular membrane lining that tube. — Thus every one of the Haversian canals is the centre of a cylindrical ossicle, which is complete in itself as far as its elementary structure is concerned, and which has no dependence on, or connexion with, other similar ossicles. These are arranged, however, side by side, like sticks in a faggot ; they are bound together by a thin cylinder of bone, on the exterior of all, which derives its nourishment from the periosteum or enveloping membrane ; in like manner, the hollow bundle is lined by a similar cylinder, which surrounds the great medullary cavity, and is nourished by its vascular membrane ; and the spaces that here and there intervene between the ossicles, are filled up Minute structure of Bone, as seen in transverse section ; magnified 300 diameters. 1, an ossicle surrounding an Haversian canal, 3, showing the concentric arrangement of the lamellae; 2, the same, with the lacunae and canali- culi ; 4, portions of the lamellae parallel with the external surface. 142 GENERAL PHYSIOLOGY. with lam i me, (Fig. 4-5, 4), which are parallel to those of the external and internal cylinders, and which seem to derive then’ nutriment through them. In this manner, the whole structure acquires great density and solidity. — The structure of the outer and inner tables of the skull, and of other thick solid layers of bone, is precisely similar ; except that the Haversian canals have no such definite direction, and form an irregular network. — In the extremities of the long bones, and between the solid layers of the flat bones, we find what is termed a cancellated texture ; that is, a sort of spongy substance composed of osseous lamellse and fibres interwoven together (like those of areolar tissue, on a larger scale) so as to form a multitude of minute chambers, or cancelli, freely communicating with each other and with the cavity of the shaft ; the whole being enclosed in a thin layer of solid bone. These cancelli, like the Haversian canals, are lined by prolongations of the investing membrane, accompanied by blood- vessels. The Haversian canals and cancelli are repetitions of each other in all essential particulars, then- form alone being different. 206. In the long bones of Man and of most Mammalia, we find a central cavity, which may be considered as an enlarged Haversian canal, and which is filled with the fatty matter known as marrow. This cavity does not exist in the bone in its early state ; but is formed by the removal of a portion of the cancellated osseous tissue first developed in its interior ; and the primitive condition is retained in the bones of the Sloths and then - great extinct congeners, the Mylodon and Megatherium (which, of all Mammalia, present the nearest approach to Reptiles in the inertness of then - habits ) ; and also in the bones of the Cetacea and Seals, which may be regarded as the representatives of Fishes. In some of the bones of the head, even in Man, but still more in the Giraffe, Mylodon, and Elephant, we find the cancelli of the bones of the skull enlarged and occupied with air instead of marrow ; but these have no direct communi- cation with the lungs. — Among Birds, however, we meet with a very different condition ; the central cavity being usually unoccupied by marrow, and communicating with the lungs, so that the membrane lining it becomes an auxiliary organ of respiration (chap, xiii.), whilst the lightening of the bones thus produced diminishes their specific gravity. In the Swift, and other birds of peculiarly rapid flight, this condition extends throughout the entire skeleton ; even the ordinary cancellated structure being permeated with ah’. But in birds of inferior powers of flight, the proportion of bones receiving ah’ is di m i n ished ; and in the completely wingless Apteryx and aquatic Penguin, not a single bone receives air, though a medullary cavity still exists in all bones except those of the last-named bird. — In Reptiles we meet with considerable diversity as regards this character. The Samians and most of the Batrachians usually have their long bones hollowed in the centre, the cavity being occupied by marrow ; the same is the case with the ribs of the larger Serpents ; and in the bones of the extremities of the gigantic Iguanodon and Megalosaurus (the highest genera among Reptiles, so far as can be ascertained from their osseous conformation) the medullary cavities are as capacious as those of any Mammal. On the other hand, in the aquatic Crocodiles the medullary cavities are small ; and in the Turtle tribe they are reduced to the condition of cancelli. — In Fishes we usually find that the osseous structme presents a still less advance upon the original ANIMAL TISSUES ] — STRUCTURE OF BONE. 143 condition ; the outer portion of the bones, in many instances, being the only parts that have undergone consolidation, and the interior remaining filled vdth the original cartilage (so that in dried specimens there may be the semblance of a medullary cavity) ; and no regular system of Haver- sian canals and cancelli being established, — the large irregular lacunse and canaliculi apparently serving to convey the plasmatic fluid, through dis- tances which it could not otherwise traverse. The bones of some Fishes, however, closely approximate to those of Reptiles in their general struc- ture.* 207. The principal features of the Chemical constitution of Bone are easily made evident. After all the accessory parts have been removed, and nothing remains but the real Osseous texture, this may be separated, by simple processes, into its two grand constituents, — the animal basis, and the calcareous matter. The latter may be entirely dissolved away, by maceration of the bone in dilute Muriatic or Nitric acid ; and a substance of cartilaginous appearance is then left, which, when submitted to the action of boiling water for a short time, is almost entirely taken up by it, and the solution forms a dense jelly on cooling. This same sub- stance, Gelatine, may be obtained by long boiling under pressure, from previously unaltered bone ; and the calcareous matter is then left in a friable condition.— By submitting a bone to a heat sufficient to decompose the animal matter, without dissipating any of the earthy particles, we may obtain the whole calcareous matter in situ ; but the slightest violence is sufficient to disintegrate it. The bones of persons long buried are often found in this condition ; their form and position being retained, until they are exposed to the air, or are a little shaken • when they crumble to dust. — The proportion of the earthy matter of Bones to the animal basis may be differently stated ; according as we include in our estimate of the latter, the contents of the medullary cavity, the Haversian canals, and the cancelli ; or confine ourselves to that portion only of the animal matter, which is united with the calcareous element in the proper osseous tissue. According to the experiments of Dr. Stark, t the relative amount of the two elements, in the latter estimate, is subject to veiy little variation, either in the different classes of animals, or in the same species at different ages ; the animal matter composing about one-third, or 33| per cent. ; and the mineral matter two-thirds, or 66§ per cent. The degree of hardness of bone does not altogether depend, therefore, on the proportion of mineral deposit they may contain ; for the flexible, semi-transparent, easily- divided bones of Fish contain as large a proportion of earthy matter, as the ivory -like leg-bones of the deer or sheep. It seems to be influenced in part by the proportion of phosphate to carbonate of lime, which is greater in the higher animals ; in part, by the quantity of water that may he united with the osseous matter ; and in part, by the degree in which the Haversian canals and cancelli become filled up with bony substance. It is from this last circumstance, that the bones of old people have a greater density than those of young or middle-aged persons ; for after the full dimensions of the bones have been attained, they are progressively con- solidated during the whole remainder of life, by the formation of new * See Prof. Owen’s Lectures on Vertebrate Animals, vol. i., p. 33, ct seq. + Edinburgh Medical and Surgical Journal, April, 1845. 144 GENERAL PHYSIOLOGY. layers of osseous substance within the preceding, so as to contract the calibre of the Haversian canals. 208. The first development of Bone is commonly preceded by the formation of a Cartilaginous structure, which occupies the place the bone is afterwards to take ; and it has been commonly considered that the bone is formed by the ossification of cartilage. This, however, does not appear to be the case ; for none of the peculiar substance of the cartilage — Chondrine (§ 191) — can be found in perfect bone; and microscopic examination of the process shows, that although a calcareous deposit does in the first instance take place in the intercellular substance of cartilage, the structure thus formed has not the character of true bone, and is destined but for a temporary purpose. — The process of bone-formation always commences in the immediate neighbourhood of blood-vessels, which pass down into canals excavated in the substance of the cartilage and lined by a continuation of its investing membrane ; hence the spots where these vascular canals are especially developed are termed centres of ossification. We usually find one of these in the centre of the shaft of a long bone, and one at each end, with an additional one for any consider- able projection or ‘process;’ in the flat bones there is generally one in the middle of the surface, and one in each of the principal ‘processes.’ Up to the period when the bone attains its full dimensions, the parts which contain distinct centres are not connected by osseous union, but only by cartilage, so that they fall apart when this decomposes ; the purpose of this is, to allow of an increase in the size of the bone, by the growth of cartilage between its detached portions, which cartilage may give place to bony structure when there is no further need of increase. There is a remarkable difference in this respect, however, between the warm-blooded and the cold-blooded Vertebrata; for the bony pieces which arise from separate centres of ossification have a much greater tendency to remain distinct in the latter than in the former ; so that the adult skeleton is composed of a much larger number of bones. On the whole, a close correspondence exists between the number of ossific centres, in the early condition of the skeleton of all Vertebrated animals ; and it is on the basis of this conformity, that the Philosophical Anatomist forms his idea of the fundamental plan of the Vertebrate skeleton (§ 320) ; but this conformity is often obscured by the multiplication or suppression of ossific centres, in order to meet certain peculiar requirements in the economy of one or another class. Thus, contrary to the general rule, we find the femur (thigh-bone) of Man and the Mammalia developed from four distinct centres, — one for the shaft, one for each of the extremities, and one for the great trochanter ; — whilst in Birds and most Reptiles, it is developed from a single centre. This peculiarity seems to have refer- ence to the use which is to be made of these bones in the Mammalia, the voung of which soon begin to run and leap upon the ground, in a manner that would produce a jar or shock to their yet delicate fabric, especially to their nervous centres, if it were not for the interposition of layers of cartilage between the shaft of the bone and its extremities. And this view is confirmed by the fact, that in the Frog, which, unlike other Reptiles, moves by leaps, the extremities of the thigh-bones are developed from distinct centres as in Mammalia.* On the other hand, we not * See Prof. Owen’s Lectures on Vertebrate Animals, vol. i., p. 36, et scq. ANIMAL TISSUES ; FORMATION OF BONE. 1 45 unfrequently meet with a reduction in the number of ossific centres below their typical amount : thus the regular number of distinct bones in the carpus (wrist) and tarsus (instep), as deduced from an extensive comparison, is undoubtedly ten; but these are reduced in the Human carpus to eight, and in the tarsus to seven, each having but a single centre ; arid the reduction is still greater in the hoofed Mammalia. * 209. The structure of the temporary Cartilage, which occupies the place to be subsequently taken by Bone, is essentially the same with that of the permanent Cartilage already described (§191). If we make a section of it, however, in a direction vertical to the plane of ossification, we find the cartilage-cells undergoing multi- plication longitudinally (§157), so as to form cylindrical piles or columns, which ai’e sepa- rated from each other by intercellular sub- stance (Fig. 46). It is in this substance that the first calcareous deposit takes place ; this deposit forming a sort of network, in the meshes of which the lower ends of the columns are imbedded. The further stages of the process have not been precisely made out ; but it appears that the cartilage-cells speedily degenerate and die, leaving the spaces void which they previously occupied ; and that of the first calcified network (which, as it contains no lacunae or canaliculi, has scarcely a title to be designated true bone) the greater portion soon breaks down, leav- ing larger open areolae of no very definite form. These spaces become filled, from the neighbouring vessels, with a formative blas- tema containing cells; and it is probable that the production of all the succeeding layers of bone, which line these spaces, and gradually contract them into the definite o c tt • i j it Vertical Section of Cartilage at the seat IOrm OI Haveisian canals and. cancclll, ai6 of ossification, showing the arrangement generated by the consolidation of this bias- ? f the cartilage-cells in columns; at the . J „ . i-i -| lower part, ossific deposit has begun to tema into a fibrous tissue, which subse- take place between these, whence arises quently becomes calcified, whilst the inter- the opacity ' spersed cells are metamorphosed into lacume and canaliculi (§ 201). 210. Although, in a large proportion of the Skeleton, the formation of Bone is thus preceded by that of Cartilage, yet this is by no means necessarily the case ; for it is certain that true bone may be developed in the substance of Fibrous Membrane. This may be seen in the original production of the tabular bones forming the roof of the skull ; these having usually only a centre of cartilage, beyond which the ossifying process extends in membrane only. But it is equally well displayed in the successive additions which are made to the surfaces of all other bones, subsequently to their first development ; these additions having their origin in the progressive calcification of the inner layers of the periosteum (or fibrous membrane covering the bones), whilst new fibrous tissue is * See Prof. Owen’s Discourse on the Nature of Limbs, p. 24, et seq. L Fig. 46. 146 GENERAL PHYSIOLOGY. formed upon their exterior. It is not difficult to trace the consolidation of these fibres, on the growing surface of a long bone ; and the gradual extension of the canaliculi from the originally spherical cells included in their meshes, which become the lacunae. The first rude outline of the Haversian canals is produced by the development of the bony fabric in longitudinal ridges ; and these gradually rise up and arch over, so as to meet and include irregular lengthened spaces, which are gradually contracted by the subsequent formation of new laminae in then interior, whereby the new osseous layer is progressively consolidated.* 211. The growth of Bones, like that of Shells, is thus in reality effected by the addition of new tissue to the part already formed ; but this addition may take place in three modes, namely, — by the development of new bone in the cartilage intervening between the parts yet remaining separated, sometvhat as in the enlargement of the shell of the Echinoder- mata (§ 195), — by the development of new bone in the membrane covering the surface, as in the addition of new layers to the shell of Mollusca, — and by the interstitial formation of new layers within the Haversian canals of the osseous fabric already developed, which is a process peculiar to Vertebrata. But it is not by addition only, that we find that ‘ moulding ’ of the bones effected, which is requisite for the maintenance of their form whilst their dimensions are undergoing increase. For this purpose, a removal of osseous tissue already formed is also necessary ; and of such a removal we have a very remarkable example in the first formation of the central cavity in the long bones of the higher Vertebrata (§ 206), and in its continued enlargement during their growth. — The perfect reparation of Bone, after severe injuries, is one of the most remarkable features in its history ; and it seems to be for the effectual perforance of this, in the event of its being demanded, that the complex nutritive apparatus which has been described is kept up after the bone has attained its full dimensions; since but very little interstitial change can be requisite, in a tissue so free from tendency to spontaneous decay. The reparation of bone is effected by the development of new osseous tissue in the substance of membrane or cartilage formed in the seat of injury ; and the new fabric gradually becomes perfectly continuous with the old, progressively acquiring the form and solidity of the original bone. The reparative power is even more remarkably displayed, when the original bone has been destroyed by disease, than when it has been the seat of mechanical injury. 212. We next proceed to the Teeth, which are organs of mechanical attrition, developed in the first part of the alimentary canal, for the pur- pose of comminuting the food conveyed into it. Their place of origin is altogether different from that of Bone, as they commence in little papillary elevations of the mucous membrane covering the jaw ; but they are very analogous to Bone in their structure and composition. It is in the inferior Vertebrata that this resemblance is the strongest ; and it is there, too, that * For a fuller account of the structure and development of Bone, the Author would more especialfr' refer his readers to the admirable researches of Professor Sharper, as detailed in his Introduction to the fifth edition of Dr. Quain’s Elements of Anatomy. His own researches have led him to concur with Professor Sharpey on every point save the origin of the lacunae and canaliculi, of which he thinks that Professor Sharpey has given an imperfect account in regarding them as mere interspaces in the fibrous tissue. ANIMAL TISSUES ; STRUCTURE OF TEETH. 147 the history of the development of Teeth may he most conveniently studied. Thus, in the foetal Shark, the first appearance of the tooth is in the form of a minute papilla on the mucous membrane covering the jaws ; the substance of this papilla is composed of spherical cells, which are imbedded in a kind of gelatinous substance resembling that of incipient cartilage ; whilst its exterior is composed of a dense, structureless, pellucid mem- brane. The cellular mass is not at first permeated by vessels ; but a small arterial branch is distributed to each papilla, and spreads out into a tuft of capillaries at its base. The papilla gradually enlarges, by the formation of new cells at the part immediately adjacent to the blood- vessels, which supply the material requisite for their development ; and when it has acquired its full size, the process of calcification takes place, by which it is converted into Dentine or tooth-substance. In the Shark tribe, as in many other Fishes, the general structure of the Dentine is extremely analogous to that of Bone ; the tooth being traversed by numer- ous medullary canals, which are continuous with the Haversian canals of the subjacent bone, and receive blood-vessels from them (Fig. 47, a) ; Structure of the Teeth of Fish : — a, perpendicular section of tooth of Lamna moderately en- larged, showing network of medullary canals ; b, transverse section of portion of tooth of Pristis, more highly magnified, showing orifices of medullary canals, with systems of radiating and inos- culating tubuli. and each of these canals being surrounded by a system of tubuli (b), which radiate into the surrounding dense and apparently-homogeneous substance. These tubuli, however, do not enter lacunae, nor is there any concentric lamellar arrangement around the medullary canals ; but each system of tubuli is continued onwards through its own division of the tooth, the in- dividual tubes sometimes giving off lateral branches, whilst in other instances their trunks bifurcate. — In the teeth of the higher Vertebrata, however, we usually find the centre excavated into a single cavity (Fig. 49), and the remainder destitute of vascular canals ; but there are intermediate cases (as in the teeth of the great fossil Sloths) in which the inner portion of the dentine is traversed by prolongations of this cavity, conveying blood- vessels, which do not pass into the exterior layers. The tubuli of the ‘non- vascular’ dentine, which exists by itself in the teeth of nearly all Mam- malia, and which in the Elephant is known as Ivory, all radiate from the 148 GENERAL PHYSIOLOGY. central cavity, and pass towards the surface of the tooth in a nearly parallel course (Fig. 48). Their diameter at their largest part, averages 1-1 0,000th of an inch ; their smallest branches are immeasurably fine. Fig. 48. It is impossible that even the largest of them can receive blood, as then- diameter is far less than that of the blood-discs ; but it is pro- bable that, like the canaliculi of bone, they may absorb nutrient matter from the vascular surface upon which their inner extremities open. — There is a greater predominance of mineral matter in Dentine than in bone ; but still the former has a definite animal basis, which retains its form when the calcareous matter has been removed by maceration in acid. In every 100 parts, the animal matter forms about 28 ; and of the mineral portion, phosphate of lime constitutes about G4j pails, carbonate of lime 54- parts, and phosphates of Oblique section of Dentine of hu- man tooth, highly magnified, showing < the caicigerous tubuii, and the out- magnesia and soda, with chloride of sodium, lines of the original cells (?). about 2 j parts. 213. That the Dentine has its origin in the calcification of the original cellular pulp, and is not an excretion from its surface as formerly sup- posed, is now generally admitted. Observers are not agreed, however, upon the mode in which this conversion takes place, and we shall there- fore not attempt to describe it. The inner portion of the pulp still remains uncalcified, and occupies the central cavity of the tooth ; hut this sometimes becomes consolidated at a later period into a substance resem- bling vascular dentine, or even into bone. — In those simple teeth which consist solely of Dentine, the consolidation of a papilla upon the mucous membrane of the mouth is all which is requisite ; and the tooth thus formed may remain attached only to the mucous membrane, which is the case in the Shark, or it may grow downwards, by the addition of new dental structure at its base, until it comes in contact with the bone of the jaw. Where it is only attached to the mucous membrane, it is very liable to be torn away ; but a new tooth, formed from a distinct papilla, is ready to replace it ; and this process is continually repeated, the development of new papillie being apparently unlimited. On the other hand, where the root of the tooth comes in contact with the jaw, it may completely coalesce with it, which is the case in many Fishes, the Haversian canals of the bone being continued as medullary canals into the dentine ; or it may send long spreading roots into the hone, which are united to it at their extremities. 214. It is obvious that there is no provision, in the simple calcification of the dental papilla, for any variations of density, other than those which may result from the different degrees of hardness in the substance of the Dentine itself. Now in the teeth of Man and most other Mammals, and in those of many Reptiles and some Fishes, we find two other substances, one of them harder, and the other softer, than Dentine ; the former is termed Enamel; and the latter Cementum or Crusta Petrosa. For the development of these, a peculiar modification of the apparatus is requisite. — The Enamel is composed of long prismatic cells, exactly resembling ANIMAL TISSUES J STRUCTURE OF TEETH. 149 Fig. 49. those of the prismatic shell-substance formerly desm'ibed (§ 197), but on a far more minute scale ; the diameter of the cells not being more, in Man, than 1-5 6 00th of an inch. The length of the prisms corresponds with the thickness of the layer of enamel ; and the two surfaces of this layer pre- sent the ends of the prisms, which are usually more or less regularly hex- agonal. The quantity of animal matter in the enamel of the adult tooth is extremely minute, — not above 2 parts in 100; and it is only at a very early age, that the true character of the organic structure can be distinctly seen. The course of the prismatic cells is more or less wavy ; and they are marked by numerous transverse stria), resembling those of the pris- matic shell-substance, and probably originating in the same cause, — the coalescence of a series of shorter cells, to form the lengthened prism. — Of the 98 parts of mineral matter in the enamel, 88 J consist (according to Berzelius) of phosphate of lime, 8 of carbonate of lime, and 1 J of phosphate of magnesia. In density and resisting power, the Enamel far surpasses any other organised tissue, and approaches some of the hardest of mineral substances. In Man, and in Carnivorous animals, it covers the crown of the tooth only, with a simple cap or superficial layer of tolerably uniform thickness (Fig. 49, a), which follows the surface of the dentine in all its inequalities ; and its component prisms are di- rected at right angles to that sin-face, their inner extremities resting in slight but regular depres- sions on the exterior of the dentine. In the teeth of many Herbivorous animals, however, the Enamel forms (with the Cementum) a series of vertical plates, which dip down into the sub- stance of the dentine, and present their edges alternately with it, at the grinding surface of the tooth ; and there is in such teeth no continuous layer of enamel over the crown. The purpose of this arrangement is evidently to provide, by the unequal wear of these three substances, — of which the Enamel is the hardest, and the Cemen- tum the softest, — for the constant maintenance of a rough surface, adapted to triturate the tough vegetable substances on which these animals feed. — The Enamel is the least constant of the Dental tissues. It is more frequently absent than pre- sent in the teeth of the class of Fishes ; it is wanting in the entire order of Ophidia (Serpents) among existing Reptiles ; and it forms no part of the teeth of the Edentata (Sloths, &c.) and Cetacea (Whales) amongst Mammals. 215. The Cementum , or Crasta Petrosa, has the characters of true Bone; possessing its distinctive stellate lacunae and radiating canaliculi. Where it exists in small amount, we do not find it traversed by medullary canals; but, like Dentine, it is occasionally furnished with them, and thus re- sembles bone in every particular. These medullary canals enter its sub- stance from the exterior of the tooth, and consequently pass towards those which radiate from the central cavity towards the surface of the dentine, where this possesses a similar vascularity, — as was remarkably the Vertical section of human mo- lar tooth; — a , enamel; b, ce- mentum or crusta petrosa ; ' c, dentine or ivory; cl, osseous ex- crescence, arising from hypertro- phy of cementum ; e, pulp-cavity ; /, osseous cells at outer part of dentine. 150 GENERAL PHYSIOLOGY. case iii the teeth of the extinct Megatherium. In the Human tooth, however, the Cementum has no such vascularity ; but forms a thin layer, which envelops the root of the tooth, commencing near the termination of the capping of Enamel (Fig. 49, b). This layer is veiy subject to have its thickness increased, especially at the extremity of the fangs, by hypertrophy, resulting from inflammation ; and sometimes large exostoses are thus formed (Fig. 49, d), which very much increase the difficulty of extracting the tooth. When the tooth is first developed, the Cementum envelops its crown, as well as its body and root ; but the layer is very thin where it covers the Enamel, and being soft, it is soon worn away by use. In the teeth of many Herbivorous Mammals, it dips down with the Enamel to form the vertical plates of the interior of the tooth ; and in the teeth of the Edentata, as well as of many Reptiles and Fishes, it forms a thick continuous envelope over the whole of the surface, until worn away at the crown. 216. The development of these additional structures is provided for by the enclosure of the primitive papilla, from which the Dentine is formed, within a Capsule, which, at one period, completely covers it in : between the inner surface of the capsule and the outer surface of the dentinal papilla, a sort of epithelium is developed, by the calcification of which the Enamel is formed ; and the Cementum is generated by the conversion of the fibrous capsule itself into a bony substance. The Cap- sule is at first formed by the sinking-in of the Mucous Membrane around the papilla, so that this lies at the bottom of an open follicle, analogous to those which are common in mucous membranes elsewhere (§ 169), or to those in which Hair originates (§ 1 80). After a time, however, the edges of the follicles are lengthened into little valve-like processes or opercula, which are destined to meet and form covers to the follicles, so as com- pletely to enclose the pulps withiu. Before the calcification of these pulps commences, a provision is made for the production of a subsequent set of teeth ; for a bud or offset springs from the upper part of each capsule ; aud this, although at first in the condition of an open follicle communicating with the primitive capsule, is gradually detached from it and closed in, so as to form a new and independent capsule. — From the inner surface of the capsule, which is in a villous and vascular condition, a copious formation of epithelial cells takes place, so as to occupy the space left between the surface of the dentinal papilla and the fining of the capsule ; this is termed the enamel-pulp ; and its conversion into enamel- prisms probably takes place iu the same manner as the formation of the prismatic shell-substance (§ 197). The ossification of the capsule itself, so as to form the investment of Cementum, does not take place until the rest is completed. — The layers of Enamel and Cementum, which are interposed between the plates of Dentine in the molar teeth of the Elephant and of Herbivorous animals generally, are formed from pro- longations of the opercula of the capsules, which dip down and are received into depressions of the deutiual pulp before its calcification begins. 217. The history of the first development of the higher forms of Teeth may thus be divided into three stages, the papillary, the follicular, and the saccular. — The papillary corresponds precisely with the complete mode of dental development in the Shark and some other Fishes, as already mentioned. — The follicular, which commences with the enclosure of the ANIMAL TISSUES j FORMATION OF TEETH. 151 papillae in open follicles, and terminates when the papillae are completely hidden by the closure of the mouths of those follicles, has also its per- manent representation in the development of the teeth of many Reptiles and Fishes; the primitive papillae of which, though enclosed in follicles, are never covered-in at the summit, and thus free themselves from their envelopes by simply growing upwards through their open mouths. But in Man, and in all other animals which agree with him in going on to the saccular stage, there must also be an eruptive stage, which consists in the biu’sting-forth of the tooth from the enclosing capsule ; the summit of the tooth being carried against the lid of the sac, by the growth of its root. By the continuance of the same growth, the teeth are caused to penetrate the gum, and are gradually raised above its surface. — The duration of the first-formed teeth is limited in all Animals. They die and are cast forth ; and are then replaced by others, which originate either in buds derived from the primitive capsules, or in newly-formed papillie. The teeth of the second set, in Man and the Mammalia, are comparatively permanent, being destined to last through the whole of life ; and we do not usually find that new capsules are derived from them by gemmation, except in the case of the posterior molars, which were not represented in the first set. Thus in Man, behind the last temporary molars, which are replaced by the permanent bicuspids, three permanent molars are to be developed, on either side of each jaw. The first of these is formed, like the milk-teeth, from a primitive capsule ; but is not completed imtil the seventh year - . The capsule of the second is formed at a later period from that of the first, by a process of budding exactly analogous to that by which the other permanent capsules are formed from the corresponding temporary ; and at a still later period, the capsule of the third permanent molar is formed as a bud from that of the second. The evolution of this molar does not usually take place until the system has acquired its full development ; and the process of budding then ceases in Man, being- limited to a single act of reproduction in the case of the ordinary Milk- teeth, and to a double one in that of the first permanent Molar. In those Fishes and Reptiles, however, whose dentition goes on to the follicular stage, this reproductive process continues through the whole of life with- out any limit ; the newly-formed teeth usually taking the places of those of the previous set, instead of coming up at their sides like the second and third permanent molars of Man. The analogy between the continued succession of teeth in the lower Yertebrata, by the gemmiparous repro- duction of their capsules, and the development of the capsules of the per- manent teeth of Man from those of the temporary set, is made further evident by the fact, that a third set occasionally makes its appearance in persons advanced in life ; the development of which would not be intelli- gible, if we could not refer it to the continuance of the same process in the other capsules, as that which regularly takes place to a limited extent in the permanent molars in Man, and which goes on without limit through the whole lives of the lower Yertebrata. — Iu those Fishes, however, in which the dental development stops at the papillary stage, the successive teeth are formed from new and independent papilla). 218. There are certain cases, however, in which the teeth are not changed more than once, but continue to gro,w through the whole remainder of life from persistent pulps ; as for instance, the tusks of the 152 GENERAL PHYSIOLOGY. Elephant, the front teeth of the Rodentia, and the grinders of the Edentata. In such teeth, the base of the pulp remains uncalcified, and a new development of cells is continually taking place in that situation • these new cells are in then turn converted into dentine, in continuity with that previously formed ; and thus the tooth or tusk is continually lengthening at its base, in a degree which compensates for its usual wear at its summit. If anything should prevent that wear, — as when the opposite tooth has been broken off,— there is an absolute increase in the length of the tooth, from the continued growth at its base ; which may become a source of great inconvenience to the animal. There is nothing, in the human subject, at all analogous to this mode of development from persistent pidps ; the process being checked by the closure of the root around the base of the pulp, which obstructs the supply of blood it receives. 219. We have seen that the Teeth are formed, in the first instance, upon the surface of the Mucous membrane of the mouth ; and conse- quently they really form a part of the external or dermo-skeleton, and not of the internal or osseous skeleton. They correspond, therefore, with the external skeletons of the Invertebrata ; and thus the analogy which has been pointed out, between the Enamel of teeth and the prismatic cellular substance of the shells of Mollusca, and between the Dentine and the shells of the higher Crustacea, holds good also in regal'd to the situation of these bodies. Great diversities exist amongst the teeth of different tribes of Yertebrata, not only in regard to the general arrange- ment of then' component tissues, but also with respect to the minute characters which these severally present ; and a knowledge of these diversities enables the Microscopist, in a great number of instances, to determine from a minute fragment of a tooth, not merely the class and order, but sometimes even the genus and species, to which the owner of it bejonged. * 220. Simple Tubular Tissues. — In all but the veiy simplest forms of Animal structure, we find a system of canals for the conveyance of fluid from one part of the fabric to another. These canals frequently seem to be nothing else than channels excavated in the midst of the other tissues, like the ‘ intercellular passages ’ of Plants (§ 154); such appeal's especially to be their character in the lower tribes of animals, and in the early embryonic condition of the higher. In the more advanced condition of the latter, however, we find that all the canals for the conveyance of the nutritive fluids, — whether blood, chyle, or lymph — have definite walls ; which are present equally in the largest and in the minutest vessels. The vessels of Animals do not, like the ducts of Plants (§ 152), pursue a straight and independent course ; for the plan of the Animal circulation requires that they should be distributed on a ramifying arrangement : — those which convey blood towards any part being progressively subdivided and diverging from each other, like the branches of a tree from its trunk : whilst those which collect and return the blood which has traversed the part, converge and reunite into trunks, like the roots of a tree or the somces of a river. Between these two sets of vessels, of which the former * See the “Odontography” of Prof. Owen, and the “Recherches sur les Poissons Fossiles" of Prof. Agassiz. ANIMAL TISSUES ; FORMATION OF TUBULES. 153 are termed Arteries, and the latter Veins, intervenes a network of extremely minute tubes, usually so narrow as only to allow the blood- corpuscles to pass in single file ; and it is when traversing these, which are termed Capillaries, that the blood undergoes the various changes to which it is subservient in the processes of Nutrition. The form of the meshes of the Capillary network is adapted to that of the elementary parts of the tissue between which they pass ; thus in Fig. 50 is seen the Fig. 50. Fig. 51. Capillary network around Fat-cells. Capillary network of Muscle. capillary network around the Fat-cells; and in Fig. 51 the arrangement of these vessels in a fasciculus of Muscular fibres. The disposition of the Fig. 52. Capillary circulation in a portion of tlie web of a Frog's foot, magnified 1 1 0 diameters ; a , trunk of vein; b, b, its branches; c, c , pigment cells. Capillaries, with the blood-corpuscles in their interior, as seen in the web of the Frog’s foot, is shown on a larger scale in Fig. 52. Into this network 154 GENERAL PHYSIOLOGY. the blood is conveyed by the ramifications of the arteries on one side ; and it is collected and returned by the rootlets of the veins on the other ; as is shown in Fig. 53. Fig. 53. Web of Frog’s foot, stretching: between two toes, magnified 3 diameters; showing the blood- vessels, and their anastomoses : a, a , veins ; b, b , b, arteries. 221. The trunks and branches of the blood-vessels appear' to be formed, like the ducts of Plants, by the coalescence of cells arranged in linear series ; those of moderate size taking their origin in single or double files of such cells, whose coalesced walls form the primitive simple mem- branous tubes of these vessels ; whilst the principal trunks, like the heart, are formed out of aggregations of cells, of which those in the interior liquefy to form the cavity, whilst those on the periphery are metamor- phosed into the fibrous and other tissues of which their more substantial walls are composed. The true Capillaries, however, seem rather to be formed by the coalescence of the prolongations of stellate cells, much resembling those of Plants (Fig. 8), or the pigment-cells of Animals (Fig. 29); these prolongations come into contact with the walls of vessels already existing, or with each other (Fig. 54); and then’ cavities having coalesced, they begin to receive fluid from the vessels, then enlarge, and finally appear as continuations of them, — their cellular - origin, however, being still indicated by the presence of nuclei in the walls of the capil- laries thus formed. The development of the minutest Absorbeut vessels takes place in a mode essentially the same ; the chief difference being, that the cells unite more linearly with each other and with the previously- formed vessels, so as to form simple continuous tubes, instead of sending oft’ prolongations to form a network.* * See the observations of Kolliker in the “ Annales des Sciences Naturelles, Zool., Aout, 1846 and a summary of them, with confirmatory observations by Mr. Paget, in the “ Supplement to Muller’s Physiology,” p. 101. ANIMAL TISSUES J MUSCULAR £lBRE. 155 222. Compound Tubular Tissues. — We now come to two tissues of the highest importance in the Animal fabric ; the presence of which is, indeed, its distinguishing characteristic. These are the Muscular, and the Nervous tissues. The former is the one, by which all the sensible movements of the body are effected ; and the latter serves as the instrument, by which sen- sations are received, and by which Fl(}> the will excites the muscles to action, — besides serving as a medium for other operations, in which motion is produced, without the intervention of either sensation or will. These tissues, with the apparatus of bones and joints on which the muscles act, constitute the purely animal portion of the fabric ; and if a being could be constructed, in which they should be capable of continued activity with- out any other assistance, it would be in all essential particulars an Ani- mal. But, as we shall presently see, the plan on which these tissues are formed, in fact the very conditions of their existence and activity, are such, that they require constant nu- trition and re-formation ; so that the Animal cannot exist, without an ap- paratus for preparing, circulating, and maintaining in constant purity, a fluid by which nutrient operations may be effected, and which shall also be the means of carrying off the pro- ducts of the waste consequent upon the action of those tissues. This ap- paratus constitutes the Vegetative portion of the frame ; the elementary parts concerned in which have been already noticed. — Now it might not have been unreasonable to expect, that tissues altogether so peculiar in their endowments, and in the purposes to which they are destined, should have had a structure departing widely from the type of the simple Cell. Such, however, is not the case. We shall find the Muscular Fibre (as it is commonly called) to be a tube formed by a coalescence of cells, and distinguished by the nature of its contents, which, in the most perfect form of the tissue, are composed of numerous linear series of very minute secondary cells. And in like manner, the Nervous Fibre is a tube having a similar origin in cells ; whilst that portion of the Nervous matter by which its most active functional changes are effected, retains its original cellular character without any change except in form. 223. When we examine an ordinary Muscle with the naked eye, we Formation of Capillaries in tail of Tadpole : — a , a , capillaries permeable to blood ; b, b, fat gra- nules attached to the walls of the vessels, and con- cealing the nuclei ; c , hollow prolongation of a ca- pillary ending in a point; d, a branching cell, with nucleus and fat-granules, communicating by three branches with capillaries already formed ; e, blood- corpuscles, still containing granules of fat. 156 GENERAL PHYSIOLOGY. observe that it is made up of a number of fasciculi or bundles of fibres ; which are arranged side by side with great regularity, in the direction in which the muscle is to act; and which are united by areolar tissue. These fasciculi may be separated into smaller parts, which appeal’ like simple fibres ; but when these are examined by the microscope, they are found to be themselves fasciculi, composed of minuter fibres bound toge- ther by delicate filaments of areolar tissue. By carefully separating these, we may obtain the ultimate Muscular Fibre. This fibre exists under two forms, the striated and the non-striated. The former is chiefly distin- guished by the transversely-striated appearance which it presents, and which is due to an alternation of light and dark spaces along its whole extent (Fig. 55, d ) ; the breadth and distance of these strife vary, how- ever, in different fibres, and even in different pails of the same fibre, according to its state of contraction or relaxation. The Fig. 55. longitudinal strice, which are also frequently visible ( b ), are rather due to a partial separation between the com- ponent fibrillse, into which the fibre may be broken up. Neither class of striae shows itself in the non-striated fibre (Fig. 60) which more resembles that of the simple fibrous tissues. — When a fibre of the foimer kind is more closely examined, it is seen to consist of a delicate tubular sheath, quite distinct on the one hand from the areolar tissue which binds the fibres into fasciculi, and equally distinct from the internal substance of the fibre. This membranous tube, which has been teimed the Myolemma , is not perforated either by nerves or capillary vessels ; and forms, in fact, a complete barrier between Fasciculus of striated the real elements of Muscular structure and the sur- at theTtransverse strlte^ rounding parts. These elements appear to be very and at 6, the longitudinal min ute cylindrical particles with flattened faces of nearly stria?, more distmctij . un jf orm ar:) •■•jiucii/ i f- r [Cl!Ii USSjtf mtiatamo/ ® ® I' 11 w rrj [r, 1 ] r, n (Til © I ism 00 can «««**>/ g® fiuwsra ©*0®,S( 1 mr*. Successive stages of development of simpler Algse: — a, individual cells of Protococcus viridis ; b, c, clus- ters formed by their multiplication; d, filament of Schizogonium murale ; e, a similar filament, sub- dividing laterally, which constitutes the early form of the Ulvacece; f, g, portions of the expanded thallus of Ulva furfur acea, formed by the continuance of the same process of transverse subdivision. GENERAL VIEW OF THE VEGETABLE KINGDOM. ALGiE. 201 a proper cell-wall of its own, and thus becomes an incipient cell. The young cells thus formed in the interior of their progenitor, may be seen for a time to move about within its cavity, and to strike against its walls ; an orifice then forms, by which they are set free ; and they then swim forth with an active movement, and disperse themselves through the surrounding liquid. Their motion seems to depend, in some cases, upon the presence of cilia (§ 183) on the exterior of the cell; whilst in other instances it appears due to the undulation of one or two filaments, of larger size, attached to the cell. During their state of activity, which is not of long duration, the ‘ zoospores ’ (as they are termed) may be easily mistaken for Animalcules. The history of their development, in the case of Achly a prolifer a, has been already detailed (§ 142) ; where it has been shown that the formation of these ‘ zoospores ’ is to be regarded as merely a variation of the ordinary multiplication by binary subdivision ; the principal difference consisting in this, that the young cells are detached from the parent fabric, and are endowed with locomotive power, wdiereby they may be dispersed to a distance so as to originate new colonies, instead of remaining to develope themselves in connection with it. This propagation by ‘zoospores’ has been hitherto noticed chiefly in the Confer- void Alg'se ; their development taking place in some instances within every one of the cells of which the plant is composed ; whilst in other cases they are formed within speci- ally-enlarged cells, developed either at the sides or the ends of the filaments (Fig. 68). — An entirely distinct method of propaga- tion, however, has been observed in certain forms of the inferior Algie ; and recent ob- servations appear to indicate that it is not confined, as was formerly supposed, to a single genus, but occurs throughout the whole series, at some period or other in the existence of every tribe. One of the most characteristic illustrations of the pro- cess is furnished by the Zygnema, a common confervoid plant, inhabiting fresh waters, which derives its name from the ‘ conjuga- tion ’ of the filaments of wdiich it is com- posed. The filaments are developed sepa- rately, like those of ordinary Confervas (Fig. 69, a), increasing in length by the subdivi- sion of then’ component cells after the usual d ' la ‘ e , d eel l s s P rm s in s from the sid es of l . the filaments. manner ; as they approach to maturity, the endochrome generally assumes a spiral arrangement, as shown at b, and the process of ‘ conjugation ’ then takes place. This consists in the approximation of two filaments, and the formation of protuberances from the corresponding cells of each, which meet and adhere ; the partitions which still separate the cavities of the cells thus joined, disappear after a time, so that the endochromes of the two may freely intermingle ; and by Fig. 68. Chcetophora tuberculosa , a branching confervoid plant, each filament consisting of a linear series of cells, and having its zoospores developed within capsules or 202 GENERAL PHYSIOLOGY. their junction a new mass is formed, which is called the sporangium. In the Zygnema, the endochromes of all the cells of one filament (Fig. 09. c a) pass over into the corresponding cells of the other filament (b), and the sporangia are consequently formed in the cavities of the latter • but Fig. 69. A. Various stages of the history of Zygnema quirtinum : — a, three cells a, b, c, of a young fila ment, of which b is undergoing subdivision; b, two filaments in the first stage of conjugation, showing the spiral disposition of their endochromes, and the protuberances from the conjugating cells; c, completion of the act of conjugation, the endochromes of the cells of the filament a having entirely passed over to those of filament b, in which the sporangia are formed. there are other cases (some of them to be noticed hereafter), in which both endochromes are discharged from the conjugating cells, so that their inter mix ture does not take place within the cavity of either ; and there are others, again, in which the endochromes meet, and the sporangium is formed, within a dilatation of the passage that unites them. In these and other varieties, however, the fundamental nature of the process remains the same ; for it essentially consists in the reunion of the endochromes of two cells, the product of which is a new and peculiar body, apparently answering to the seed of the higher plants, and giving origin to an entirely new generation. It will hereafter be shown (chap, xviii.) that this is the simplest type of the proper generative process ; that it represents in all essential particulars the ‘ sexual ’ propagation of the higher plants and animals ; and that it is the very converse of propagation by gemmation . — whether this take place by the continuous extension of the original fabric, as by the formation of new branches in the Cha-topliora (Fig. 68), — or by the detachment and dispersion of Zoospores, as in the multiplication of Achlya prolifer a (Fig. 6). 268. In the higher tribes of this group, we meet with forms which strongly remind us of those of the more perfect plants ; the thallus being frequently evolved in the likeness of a distinct stem, with roots, leaves, and fruit (Figs. 70, A, / and 71, a). This resemblance, however, is only superficial ; for there is none of that difference in internal structure, which is characteristic of these parts in the Phanerogamia, the whole substance, even of the highest Algae, being made up of cells which nowhere depart widely from the typical form (Fig. 70, b, p. Fig. 71, c. p) ; GENERAL VIEW OF THE VEGETABLE KINGDOM. ALGiE. 203 whilst, again, there is but little difference in function, each part of the plant for the most part living for and by itself, and being but little dependent upon the rest. The chief advance is in the more complete separation of the reproductive from the nutritive apparatus ; and in the higher type which the latter presents. — The substance of which the higher Algae are composed is often remarkably tenacious, having in some in- stances the toughness of leather, in others the firmness of wood ; and there is one remarkable tribe, that of Corallines and Nullijpores (long- ranked in the Animal kingdom, on account of the resemblance of its members to various forms of Zoophytes), in which the tissue is so com- pletely consolidated by carbonate of lime as to possess a stony hardness, although, when this is removed by the action of an acid, it has all the characters of that of the ordinary Algae. The component cells, even in the highest Algae, are generally separated from each other by a large quantity of mucilaginous intercellular substance ; and a layer of this covers their surface also, giving it that peculiar ‘ sliminess ’ through which these plants are generally at once distinguished by the touch, from the Zoophytes to which many of them bear a close resemblance in form and aspect. The greatest heterogeneousness of structure which is anywhere to be .met with in this group, is seen in some of the larger Fucoids ; in which a section of any division of the thallus shows four distinct portions concentrically arranged, foreshadowing in some degree the concentric arrangement of the pith, w x ood, and bark, of Exogenous stems (§ 280). The central portion, which occupies fully a third of the diameter of the branch, is composed of densely -packed, longitudinal, parallel fibres, or strings of cellules, firmly cohering into one compact mass. Outside this is a much less dense layer, of a paler colour, composed of cells elongated into the semblance of fibres, branching out and anastomosing with each other, so as to form a network. This, again, is surrounded by a third and much denser layer, composed of elongated cells, lying parallel to the surface, and very closely packed together. Outside this portion, and forming the external coating of the thallus, is a very thin layer of small cells, which is often but loosely attached, and separates much in the manner of an epidermis. In other cases, however, the whole central portion is composed of large and loosely-aggregated cells, whilst the outer portion is very dense (Fig. 70, d), thus prefiguring the Endogenous type of stem-structure in Phanerogamia (§ 279) ; whilst in certain genera, as the Laminaria, we find the stem hollow, foreshadowing that of the Grasses. Most of the higher Sea-weeds have a root-like organ at the base of the stem ; this sometimes consists of a simple disk, whilst in other cases it is formed of thick clasping fibres, which sometimes extend for a consider- able distance upon or around the body to which they attach themselves. There can be no question that this root chiefly serves to anchor the plant in its place, and that it cannot be regarded as a special organ for the absorption of nutriment, like the root of the higher Plants. Some species, indeed, as the Sargassum bacciferum or Gulf-weed (which forms vast fields floating in the Gulf-stream), do not seem ever to possess roots ; whilst others are attached by them in their young state, but are afterwards car- ried about freely by the waves. These floating Sea-weeds are almost universally provided with air-vesicles (Fig. 71, a, d) ; and these are found also in many of the rooted species, such as the common Fucus vesiculosus, 204 GENERAL PHYSIOLOGY. and serve to keep the upper portion of the plant near the surface of the water. Many of the smaller and more tender kinds of AlgEe are of rapid growth and quickly perish ; appearing, for instance, early in the spring, and passing away before the summer is far advanced. Others live through the greater part of the year ; and there are many which partly die down on the approach of winter, but send out fresh growths from the old stumps with the return of spring. In others, again, the entire plant may endure for several years, and may continue to extend itself by addi- tions to its extremity, or by sending out new branches, according to its type of growth ; and thus, as the older portion dies away at one end, whilst new tissue is formed at the other, the life of the aggregate fabric seems capable of almost indefinite prolongation, notwithstanding the change continually taking place in its individual components (§ 124).* In the Laminaria, however, the new growth begins at the base, and pushes the older portion before it ; as is most remarkably seen in the L. digitata, in which the growths of successive years can be distinguished by their diversity of form. This development of the individual is some- times carried on to an enormous extent ; thus plants of Macrocystis pyri- fera of the South Seas are found to range from 500 to 1500 feet in length, though then stems are not thicker than the finger, whilst the upper branches (which are buoyed up with aiivvesicles) are as slender as common pack-thread ; and the M. luxurians is said by Dr. Hooker to range from 200 to 700 feet in its horizontal growth at the surface of the ocean, lining the beach at the Falkland Islands for miles with entangled cables, much thicker than the human body. In some instances, the diverging root-fibres send up new stems at a distance from the principal trunk ; whilst in other cases, portions of the branches spontaneously detach themselves, and become separate plants. 268 a. Besides these methods of propagation, however, we find in most of the higher Algae, as in the inferior tribes, two more special provisions for the multiplication and renovation of the race ; the one consisting in the simple production of cells, by a slight variation of the ordinary process of ‘ subdivision,’ which are destined to be set free from the parent strue- ture, and to originate new plants ; whilst the essential featiue of the other is the ‘ reunion ’ of the contents of two cells, to form a new and peculiar body, the sporangium, with the production of which an entirely new generation must be considered as originating. The reproductive bodies of the first class, wliich are analogous to the 1 zoospores ’ of the inferior Algae* are known in the higher division of the class under the title of £ tetra- spores,’ in consequence of their occurring in groups of four (Fig. 7 0, c, b ) ; they are sometimes found imbedded in the general substance of the tliallus, whilst in other instances they are developed only at the extremi- ties of the branches, or in other cases, again, are produced in special receptacles or thecae. They are set free by the rupture of the parent-cell, * It may be said that in this way the identity of the individual is lost; since, after the lapse of years, no part of the original structure remains. But it must be borne in mind that (as will he shown hereafter) this is equally true of the most highly organised animals, such as Man ; the materials of whose fabric are so constantly being changed by decay and replace- ment, that probably no single particle of the body of the infant remains in the child after its second dentition ; and nothing which the child possesses, save the teeth, enters into the com- position of the body in adult age ; yet the individuality of the whole is universally admitted to he maintained throughout this continual change of component parts. GENERAL VIEW OP THE VEGETABLE KINGDOM. ALGiE. 205 by the subdivision of which they were generated ; but when they have made their escape, they have no power of spontaneous movement, their dispersion being effected by the action of the waves and currents. These ‘ tetraspores,’ like the Fi(j ^ ‘ zoospores ’ of the in- ferior tribes, must be regarded in the light of detached gemmce or buds * and we shall find that this method of propagation is by no means restricted to Algae, but that it occurs also through- out the Cryptogamic series, and is very fre- quent among Zoo- phytes. — The proper fructification in the higher Algae is usually restricted to some par- ticular portion of the plant ; being either developed at the ex- tremities of the ordi- nary branches, or in branches expressly modified to evolve it (Fig. 71, a c, b, c), ra- in special capsules still more completely separated from the vegetative portion of the thallus. From the recent researches of MM. Decaisne and Thuret * it appears that the fructification is not simple, as was formerly supposed, but that it consists of two classes of cells, which differ essential- ly from each other, but the conjunction of whose contents is necessary for the production of a fertile embryo. Of these cells, those from which the new plant is subsequently to proceed may be termed the ‘ germ cells ;’+ they are commonly found in the middle of a Structure of Carpocaulon mediterraneum: — a, entire plant; b, longitudinal section of branch, with tetraspores ; c, tetraspores, a , within parent-cell, b separated ; d, transverse section of stem. * Annales des Sciences Naturelles, Botan., tom. iii. 1845. + The terms ‘germ-cell’ and ‘sperm-cell,’ which will be frequently employed in this treatise, are adopted from Prof. Owen (Lectures on Parthenogenesis, 1849). 20G GENERAL PHYSIOLOGY. cavity lined with filamentous cells, as seen in Fig. 71 c, in which are shown the successive stages of development of the germ-cells in the fruit-bearing branch of Scirgassum. The contents of the filamentous cells (a) seem to be gradually absorbed into the germ-cells, so that the former are emptied of their endochrome ( b ), and may at last disappear altogether (c, d). But Fro. 71. Structure of Sargassum coarctatum: — a, portion of the plant, showing a stem, b leafy portion, c a fruit-bearing branch, d an air-vesicle; b, fruit-bearing branch, with sporangia, enlarged ; c, section of fruit-bearing branch, showing successive stages, a, b, c, d , of the evolu- tion of the sporangia; d, section of leafy portion, passing through its central part at a , and showing one of the tufts of hairs at b. besides these sterile cells, there are others whose contents are altogether peculiar ; their endochrome arranging itself into the form of a spiral filament, which lies coiled up within the cavity of the cell, until set free by its rupture, when it moves freely through the waters for some time, by the agency of the cilia with which it is furnished. These * sperm-cells,’ as they may be appropriately termed, are sometimes contained within the same conceptacles as the ‘ germ-cells in other instances the two classes of cells are formed in different organs, and those containing the * sperm- cells ’ are then termed antheridia * whilst those containing the ‘ germ- cells ’ are termed pistillidia. In some species the antheridia and pis- tillidia are found on the same plant ; in others, on distinct individuals :+ * The term antheridia is applied by MM. Decaisne and Thnret (loc. r.it.) to the ‘sperm- cells ’ of the Fuci ; hut it appears to the Author much more appropriate to employ it, here as elsewhere, to designate collections of sperm-cells within a common receptacle. ■f It is very curious to find such a marked variation in this respect, between species that are otherwise closely allied. Thus in the Fucus canaUcidatus, the sperm-cells and the germ- cells are mingled with each other throughout the interior of the same conceptacles ; in the F. tuberculatus the upper portion is occupied by the sperm-cells, the lower by the germ-cells ; in the F. vesiculosus, the sperm-cells and germ-cells are sometimes mingled in the same conceptacles, but are more commonly developed in distinct antheridia and pistillidia, which are seated upon different individuals ; in the F. nodostis, the two organs are always separate, but are sometimes developed on the same, sometimes on distinct individuals ; whilst in the GENERAL VIEW OF THE VEGETABLE KINGDOM. CIIARACJLE. 207 and thus we have, even in this lowest class of Plants, the ‘ monoecious ’ and f dioecious,’ as well as the ‘ hermaphrodite ’ arrangements of the repro- ductive apparatus, which are more distinctly seen in the Phanerogamia. It is from the ‘ germ-cell,’ when fertilized by the agency of the ‘ sperm- cell,’ that the new generation originates. The process of ‘ fertilization ’ appears to consist in the passage of certain of the contents of the sperm-cell into the £ germ-cell, being thus essentially the same with that of the ‘ conjugation ’ of the inferior Algte ;* and the product, though commonly termed a ‘ spore,’ would be more correctly designated a sporangium, since it is homologous with the product of conjugation in Zygnema, he., whilst it is essentially different, not only from the ‘ zoospores ’ and £ tetraspores’ of the Algse, but also (as will hereafter appear) from the £ spores ’ of the higher Cryptogamia. 269. We may now rank with the Algve a very interesting little group of plants, termed the Characece, in which we have a vegetative apparatus as simple as that of the Confervee, whilst the reproductive organs are more highly developed than those of the Fucaceee. Each plant is composed of an assemblage of long tubiform cells, placed end to end ; with a distinct central axis, around which the branches are disposed at intervals with great regularity (Fig. 72, a). In one of the genera, Nitella , the stem and branches are simple cells, 'which sometimes attain the length of several inches ; whilst in the true Chara, each central tube is surrounded by an envelope of smaller ones. Some species have the power of secreting carbonate of lime from the water in which they grow, if this be at all impregnated with calcareous matter ; and by the deposition of it beneath their teguments, they have gained their popular name of ‘ Stone- worts.’ These humble plants have attracted much attention, in conse- quence of the facility with which the ‘ rotation,’ or movement of fluid in the interior of the individual cells, may be seen in them. Each cell, in the healthy state, is lined by a layer of green oval granules, which cover every part except two longitudinal lines that remain nearly colourless (Fig. 72, b) ; and a constant stream of semi-fluid matter, containing numerous jelly-like globules, is seen to flow over this green layer, the cur- rent passing up one side, changing its direction at the extremity, and flowing down the other side, the ascending and descending spaces being- bounded by the transparent lines just mentioned. That the currents are in some way directed by the layer of granules, appears from the fact recently noticed by Mr. Varley,+ that if accident damages or removes them near the boundary between the ascending and descending currents, a portion of the fluids of the two currents will intermingle by passing the boundary ; whilst if the injury be repaired by the development of new granules on the part from which they had been detached, the circulation resumes its regularity, no part of either current passing the boundary. In the young cells, however, the circulation may be seen, before their granu- F. serratus , the antheridia and pistillidia seem to be always found on separate individuals. The antheridia of the Fucoids may be generally distinguished by their yellow colour, whilst the pistillidia are of an olive-green. * The actual process of fertilization has not yet been observed in the Algae; but there can be little doubt, from the great development of their sperm-cells, and from the analogical evidence hereafter to be stated (chap, xviii.), that it takes place as above stated. t Transactions of the Microscopical Society, vol. ii. p. 99. 208 GENERAL PHYSIOLOGY. lai lining is formed. The rate of the circulation is affected by anything which influences the vital activity of the plant ; thus, the movement is accelerated by moderate warmth, whilst it is retarded by cold, and mav be at once checked by a slight electric discharge through the plant. The moving globules, which consist of starchy matter, are of various sizes ■ being sometimes very small, and of definite" figure, whilst in other instances Fig. 72. Nitella flexilis : — A, stem and branches of the natural size ; a, b , c, d, four verticils of branches issuing from the stem ; e,f, subdivision of the branches; — b, portion of the stem and branches enlarged; a , b, joints of stem; c, d, verticils ; e, /, new cells sprouting from the sides of the branches ; g , h, new cells sprouting at the extremities of the branches. they are seen as large irregular masses, which appear to be formed by the aggregation of the smaller particles. The production of new cells, for the ex- tension of the stem or branches, or the origination of new whorls, is not here accomplished by the subdivision of the parent-cell, but takes place by the method of out-growth (Fig. 72, b, e,f g, h) which, as already shown (§ 141), is nothing but a modification of the usual process of cell-multiplication. In this manner, the extension of the individual plant takes place with con- siderable rapidity ; but no provision seems to exist for the multiplication of individuals by the spontaneous detachment either of buds or of spores, although parts of the plant artificially separated will continue to vegetate under favourable circumstances, and will develope themselves into the typical form. — The true process of ‘ generation ’ in the Characeae appears to be effected by the conjoint action of two different bodies, the ‘globules.' and the ‘ nucules,’ both of which grow from the bases of the branches. The GENERAL VIEW OF THE VEGETABLE KINGDOM. LICHENS. 209 peculiar structure of each of these will be hereafter explained more in detail (chap, xviii.) ; and at present it will be sufficient to state that the ‘globules’ are antheridia, each containing vast numbers of ‘ sperm-cells whilst the ‘nucules’ are pistillidia, each containing but a single ‘germ-cell,’ from which, when fertilized by the sperm-cells, the young plant springs up. 270. The contrast between the conditions under which the typical Lichens and Algm are respectively developed, is not more remarkable than that which exists between their general aspect and mode of growth j and yet we shall find that both the lower and the higher forms of the two groups are closely connected by intermediate links. — In the simpler Lichens, the primordial cell gives origin, by the usual process of subdi- vision, to a single layer of cells, which usually spreads itself over the surface to which it is attached, in a more or less circular form ; and one or more additional layers being afterwards developed upon its free surface, a thallus is formed, which has no very defined limit, and which, in conse- quence of the very slight adhesion of its component cells, is said to be ‘ pulverulent.’ There are some tribes, however, in which gelatinous matter is interposed among the cells, and which thus connect these simpler Lichens with the inferior Algae ; such is the case with the genus Collerna, in which we find a ‘ membranous ’ thallus essentially consisting of green spherical cells, arranged in bead-like filaments, and imbedded in a firm gelatine ; whilst these are tied together (as it were) by cylindrical threads formed of elongated cells, which are closely interwoven at the surface, so as to form a sort of cuticle.* Passing on towards the higher tribes of Lichens, we find these interlaced filaments forming a tough envelope to both surfaces ; whilst from the under side of the thallus, which is then said to be ‘ crustaceous,’ thread-like fibres are put forth, which serve for the attachment of the plants, and probably also for the absorption of nu- triment. One of the highest forms of thallus seen among the Lichens, is that which is termed the ‘ foliaceous ’ or leaf-like ; since there is here the greatest approach to regularity and definiteness of form (Fig. 73, c) as well as the greatest complexity of internal structure (a, b). Three principal layers are here easily distinguishable in a vertical section ; namely, the ‘ medullary’ layer (b, c to), and the upper and under ‘ cortical ’ layers (c c). The former consists of long, thin, dry, and usually branching cells, loosely interwoven together ; and the latter of shorter cells, more closely packed together, and firmly connected by intercellular substance. On the con- fines between the medullary and the upper cortical layers, is found a layer of rounded green cells, sometimes nearly continuous, hi other cases more interrupted, the cells being aggregated in little masses of variable size, which are termed gonidia. These green cells probably constitute the true vegetative portion of the plant, being to the medullary and cortical tissues of the foliaceous Lichens what the bead-like rows of green cells in Collema are to the gelatine and filamentous substance in which they are enveloped, t These green cells sometimes increase in particular spots, and make their way through the cortical layer, so as to appear as little masses of dust upon the surface, which are termed soredia ; and when dispersed and separated, they are capable of originating new plants. — A more special reproductive apparatus exists, however, in the higher Lichens ; consisting * See Thwaites in Annals of Natural History, March, 1849. t Thwaites loc. cit. V 210 GENERAL PHYSIOLOGT. of a series of asci or spore-cases (Fig. 73, b, t t ), each containing a small number of spores (from four to sixteen, but always a multiple of two), and congregated in groups, sometimes immediately upon the sur- face of the thallus,, sometimes upon an intermediate substance. The groups of asci are often confined to certain projecting portions of the thallus ; and these have received various names according to then- form, being termed ‘ shields ’ or apothecia in the Parmelise (c, s). The asci are imbedded in a mass of straight elongated cells or filaments, which are termed paraphyses (b, p). The fructification appeal's to oiiginate in the medullary layer of the thallus (probably in its green cells), and to burst through the upper cortical layer, in order to make its way to the surface, and expand into the shield. — Nothing is yet known as to the existence of any process, among the Lichens, analogous to the ‘conjugation’ of the lower Algse, or to the fertilisation of the ‘ germ-cell ’ by the ‘ sperm-cell ’ in the higher ; but an- alogical considerations afford a strong probability that such a conjugation does take place between certain pairs of the green cells, and that the asci with their contained spores are its product. In this case, we must look upon the sored ia t as the representatives of the ‘ zoospores ’ and ‘ tetraspores ’ of the Algse ; whilst the asci are the homologues of the spo- ys rangia, which frequently give origin in the Algse to four or moi'e reproductive cells, of which every one is capable of developing itself into a perfect plant. — It is by their fructification, that Lichina and certain other genera inhabiting the sea-shore between the tide-marks, are determined to belong to this group ; although both in their general appear- ance, and in the gelatinous character of their thallus, they so closely resemble the higher Algse, that they have been frequently ranked among them. — The tribe of Lichens is of peculiar interest, as the one by whose agency vegetation is spread over the bare surfaces of newly-formed rocks. Pulverulent Lichens are first observed, as minute specks upon the pumice or lava of the volcano : these are succeeded by the crustaceous forms ; these again by the foliaceous kinds ; and to these follow Liverworts and Mosses. Many Lichens form oxalic acid as one of the products of their vital chemistry : and this acts upon the substance of the rock (especially if it be calcareous), forming a hollow which retains the decaying particles of the plant, when its term of Structure of Purmtlia acetabulum ; — a, vertical section of the apothecium, a , with the layer t p composed of asci and paraphyses ; b, portion of the same more highly magnified, showing cc cortical layers, cm medullary layer, t asci, and p paraphyses; c, upper surface of the growing plant, showing 1 1 the lobed folia- ceous thallus, and s the shields in different stages of development. GENERAL VIEW OP THE VEGETABLE KINGDOM. FUNGI. 211 life is concluded ; this hollow is deepened by the growth of successive generations, which leave their debris behind them ; and the moisture which is caught in them, finding its way into the crevices of the rocks, assists in their disintegration (especially when expanded by freezing), and thus continually adds to the forming soil. This is no sooner adequate to the support of higher forms of vegetation, than we find such developing themselves upon it, their seeds being conveyed by the winds, by the agency of birds, or by other means ; and by the growth and decay of successive generations of these, a sufficient thickness of soil is produced for the nourishment of the luxuriant herbage, and the support of the lofty forest- tree. And thus, by the labours of these apparently insignificant plants, the naked and desolate rock is gradually fitted to afford sustenance to various tribes of animals, and at last to become the dwelling-place of Man. 271. In the simplest forms of Fungi, we return to the lowest type of Vegetable existence, namely, the single cell ; whilst in the higher divisions of the group, we meet with fabrics of much greater complexity, in which there is a considerable degree of heterogeneousness of structure and specialization of function. The most distinctive physiological character of the entire series of Fungi, as already remarked, is their dependence upon dead or decomposing organic matter as the material of their growth. They seem to be for the most part, if not entirely, deficient in the power of decomposing carbonic acid under the influence of light ; accordingly, they generally seem to grow in dark situations by preference, their vege- tative portion being frequently buried beneath the ground, even when their reproductive apparatus raises itself in the air, as in the common Agarics, which rank among the highest members of the group ; and they give out carbonic acid, derived (no doubt) from the decomposing matters on which they grow, and thus increase the amount of that component of the atmosphere, instead of diminishing it after the manner of other Plants. Further, their cells usually contain a large proportion of azotised sub- stances, identical in composition, or nearly so, with the albuminous compounds of animals ; and thus they approximate more closely in com- position to the animal tissues, than do any other entire Plants. The character of their texture is usually soft ; it is frequently very rapid in its growth (§ 144), but it is also prone to pass with nearly equal rapidity into a state of decay ; so that there are probably scarcely any plants, whose entire term of existence is so brief as that of many Fungi. There are instances, however, in which the texture possesses an almost woody hardness ; and it is then comparatively persistent and of slo-w growth. Nothing like real woody structure, however, exists in this group ; the whole fabric being composed of cells, whose forms are often strangely modified by the outgrowth of processes in different directions, although they still preserve their independence, no coalescence into tubes being as yet observed. — Of one of the simplest forms of Fungi, we have a good example in the Torula cerevisii, which so abounds in Yeast that this substance may be said to be almost entirely made up of it. When a small quantity of yeast is placed under the Microscope, it is found to be full of globules, which are clearly cells ; and these cells vegetate, when placed in a fermentable fluid containing some form of albuminous matter in addition to sugar, in the manner represented in the accompanying figure. Each cell puts forth one or two projections, which seem to be 212 GENERAL PHYSIOLOGY. young cells budding off from their parents (§ 141) ; these, in the course of a short time, become complete cells, and again perform the same process ; and in this way the single cells of yeast develope themselves, in Different stages of the vegetation of Turula Cerevisii, or Yeast Plant ; — a, single cells of which it at first consists ; b, cells with buds ; c, the same more advanced,- d, rows of cells produced by continuance of the budding process. the course of a few hours, into rows of four, five, or six, which remain in continuity with each other whilst the plant is still growing, but which separate if the fermenting process be checked, and return to the condition of those which originally constituted the yeast. Thus it is that the quantity of yeast first introduced into the fermentable fluid is multiplied six times or more, during the changes to which it contributes.* This little plant is very tenacious of its vitality ; its power of growth not being destroyed by exposure either to the heat of boiling water, or to a cold of 76° below zero, or by being completely dried up. It is as yet uncertain whether the processes just described constitute its entire growth, or whether they are anything else than the early stages of its development ; for there is reason to believe that if its vegetation be not checked, it will o*o on to produce a continuous filament of some length, bearing a regular fructification analogous to that of the plants next to be described. — Accord- ing to Schlei den’s observations, many of the simpler Fungi (such as the ‘ moulds’ which appear on cheese, fruits, preserves, &c.), consist of single cells, which, instead of increasing in number by subdivision or gemmation, augment in size, and send out ramifying prolongations, wliich give to the plant a complex form, whilst its structure remains in the simplest possible condition, t This seems to be the case, for example, with regard to the Botrytis, a little plant of which some species are parasitic upon leaves (Fig. 75, a), whilst another developes itself in the bodies of silk-worms. In the accompanying figure, we see the lower part of the tubular cell subdividing into root-like prolongations, which insinuate themselves among * Much discussion has taken place, as to whether the vegetation of the Yeast-plant or ferment-cells is to he regarded as the essential cause of fermentation. Some explain this process altogether on chemical principles, and deny that the vegetation has any connection with it ■ whilst others assert that fermentation is entirely caused by the decomposition effected by the Fungus, and cannot take place without it. The truth appears to lie between the two extremes. The elements of the saccharine fluid are arranged in such a manner, that they may be easily induced to enter into new combinations; and these may be formed under a o-reat variety of conditions, several of which are examples of the catalytic action already noticed (§ 39). The introduction of the germs of the Yeast-plant appears to be the most favourable of all conditions ; the fermentation which is occasioned by its vegetation being more active and complete than that which can be produced in any other way. It appears to act upon the fluid in the same way as common Mould does upon a sweet preserve ; and this action is not very dissimilar to that of a germinating seed upon the starch laid up for it by its parent. See on this subject the Comptcs fiend us for August 20, 1838 ; Meyen’s Report on Vegetable Physiology for 1837, p. 83—87 ; and Liebig’s Organic Chemistry. f Principles of Scientific Botany, p. 153. GENERAL VIEW OF THE VEGETABLE KINGDOM. FUNGI. 213 the cells of the leaf oil which it is parasitic ; the stem, which is a part of the same tubular cell, rises through the stomate, or cuticular orifice, of the leaf ; and it then divides into branches, each of them an extension of the same individual, and each bearing a ‘ spore ’ or reproductive cell at its termination. This spore is at first de- veloped within the extremity of the branch ; as it gra- dually increases in size, it distends its envelope without rupturing it ; and when mature, it detaches itself from the branch with the integument it has thus derived from the parent cell. The stages of this process are well seen in the fructi- fication of the _ , , , -nr t of Allium, which is shown in section;— b, a Mucor, showing a the whole common lMUCOV or plant; b, head of the spore-case with a few of its spore-bearing protube- e mould ■’ in which rances » c > earlier condition of such a protuberance with the spore originating , * from it ; d , protuberance with mature spore. the spores are form- ed on a spherical enlargement at the extremity of the stem (Fig. 75, b, a), which is furnished with minute pear-shaped processes, projecting in all di- rections (6). Within the extremity of each of these processes, a ‘spore’ or reproductive cell originates, as seen at c ; and this, when mature, is seen to be attached to the lower part of the process (whose extremity is expanded over it) by a narrow neck, the rupture of which ultimately sets it free. Each spore in its development, seems to expand into similar irregularly branching cells ; and in its turn developes a multitude of new spores after the same method. — Thus we see that even in these simplest forms of Fungous vegetation, as in the highest, the nisns displays itself especially in the development of the reproductive apparatus ; and the knowledge of this fact, together with the excessive minuteness of the spores, renders it not difficult to account for the apparently universal diffusion of these bodies, and the readiness with which various forms of Fungi spring up, wherever the proper soil exists for their development. 272. The conditions under which that large variety of forms is gene- rated, which presents itself among the inferior Fungi, constitute a subject of inquiry of great interest alike to the Physiologist and to the Systematic Botanist ; since it is the business of the former to determine how far the diversified influence of external circumstances may modify the characters of plants produced from similar germs ; whilst the latter lias to ascertain, on the basis of the data thus obtained, what among these forms are to be regarded as real species, and what must be considered as mere varieties. That much is yet to be learned on these subjects, and that we are only in the infancy of our knowledge as to the influence of external conditions Fig. 75. 214 GENERAL PHYSIOLOGY. upon the development of these low forms of vegetation, may be judged of from the following facts, in addition to that already mentioned (§ 266 ) as to their occasional confervoid aspect. It has been shown by Professor Henslow* that the ‘ rust ’ of com ( Uredo rubigo) is only an earlier form of the ‘ mildew ’ ( Puccinia graminis ) ; the one form being capable of develop- ment into the other, and the fructification characteristic of the two sup- posed genera having been produced from the same individual. And it is asserted by Fries, that out of a single species of Thelephora, more than eight genera have been constructed by various authors. Hence it is evident that a very considerable variety of forms may be developed as the off- spring of a single plant, according to the circumstances under which the evolution of the germ takes place ; and we seem justified, therefore, in partly attributing the constancy with which particular forms of Fungi make their appearance in particular situations,! to the influence of the conditions peculiar to each situation in developing similar genus into one or another form. Still there is ample reason to consider that this mode of accounting for such diversities is only applicable in a limited degree ; and that an immense number of germs of species really distinct and inconvertible are constantly floating about in the atmosphere, ready to develope themselves wherever and whenever the fitting conditions of food and temperature are afforded them. The difficulty which may be felt in regard to such an admission, ceases to present itself with any force, when it is borne in mind how vast is the development of reproductive particles iu this group of Plants. Of this, any one may convince him- self by examining a puff-ball in a state of maturity. It has been well remarked by Fries on this point, that the sporales are in such vast numbers (in a single individual of Reticularia maxima he has estimated that ten millions must be present), are so subtle (being invisible to the naked eye except when collected in masses, and appealing as a thin smoke when diffused in vast multitudes through the air), are so light (being raised, perhaps, by evaporation into the atmosphere), and are dispersed in so many wavs (by winds, insects, elasticity, &c.), that it is difficult to conceive a place from which they can be excluded. There can be no reasonable doubt, then, that notwithstanding the rapidity and constancy with which particular forms of Fungi make their appearance under par- ticular combinations of circumstances, they all arise from germs produced by a pre-existing generation ; and that, although there may be a consider- able degree of latitude as to the precise form into which any particular germ may evolve itself, so that the new plant may not be an exact copy either of its parent or of any one of its fellows of the same generation, * Armais of Nat. Hist. vol. vi. p. 379. 1 Thus, no Puccinia but the P. rosce is found upon rose-bushes, and this is seen nowhere else ; (Inygena eotigua is said to be never seen but upon the hoof of a dead horse ; and Isaria felina has only been observed upon the dung of cats deposited in humid and obscure situations. It is affirmed by Dutrochet, that whilst a solution of albumen in distilled water may be kept for a year without giving rise to any fungoid production (portions of such productions, introduced from without, not developing themselves in it), this same solution, if acidulated, is speedily found to contain a species of Monilia; whilst, if a small quantity of alkali be added, a kind of Botrytis developes itself in it. On the other hand, a solution of fibrin with alkali produced Monilia; and water distilled over lettuce and acidulated with phosphoric acid gave rise to Botrytis \ so that the form which the fungus assumed was not due to the direct influence of acids or alkalies respectively. GENERAL VIEW OF THE VEGETABLE KINGDOM. FUNGI. 215 still there is a certain fundamental type for each assemblage of forms, the amount of departure from which is limited. Now as this is true of every species of Plant or Animal, whether of low or high organisation, — there being probably no one which is not capable of being in some degree modified in form and manner of development by external conditions, — the latitude in this respect which seems to be characteristic of the lower Fungi is only a peculiarity in degree, and does not by any means place them on a different footing from other Plants. An ingenious attempt has been made to show, that certain reputed F ungi which develope themselves in the substance (usually near the surface) of the higher plants, are not independent structures of extraneous origin, but are the result of morbid alterations in the tissues of the plant in which they are found, analogous to the eruptive diseases of animals. But the latest investigations have shown, that although morbid growths do occasionally present themselves in Plants," which present a certain superficial resemblance to fungous vegetation, yet that in the case of Uredo, Puccinia, JEcidium, and other genera whose appearance it has been attempted to explain in this manner, no such idea is tenable, since they have a distinct fructification of the regular fungous type, and then - whole development may be traced from the spore in which they originate. Still it would appear probable, from the obvious influence exerted upon their multiplication by the condition of the plant upon which they vegetate, that when the higher forms of vegetation are attacked by ‘ rust,’ ‘ smut,’ ‘ mildew,’ or other like forms of ‘ blight,’ an unhealthy condition of the plant itself is usually the first and most necessary condition of the growth of the fungi ; their spores, which are probably everywhere diffused, not developing themselves, except where they meet with then' appropriate 'pabulum in the decomposing products afforded by the juices of an unhealthy plant. t 272 a. It is not only in the substance or in the cavities of Plants, that several of the inferior Fungi are disposed to become parasitic ; for there are many instances in which they develope themselves in the interior of the Animal body. Thus the Silk-worm breeders of Italy and the South of France, especially in particular districts, have been subject to consider- able loss by a disease termed Muscardine, which sometimes attacks the caterpillars in large numbers, just when about to enter the chrysalis state. This disease has been ascertained to be due to the growth of a minute Fungus, nearly resembling the common mould, within their bodies. It is capable of being communicated to any individual from one already affected, by the introduction, beneath the skin of the former, of some par- ticles of the diseased portion of the latter ; and it then spreads in the fatty mass beneath the skin, occasioning the destruction of this tissue, * Such, for example, as the ‘oak-spangles’ and other minute ‘galls,’ which owe their origin to the puncture of various species of Cynips. •f The Author has been recently informed by Mr. John Marshall, that in the course of some investigations made to determine the frequency of the presence of the Uredo segetis in wheaten bread, he has detected a few sporules near the apex of every grain in very fine samples of wheat ; thus fully confirming the idea of the universal presence of the germs of such fungi, and of their dependence for the conditions of their development upon an unhealthy state of the plant on which they are lodged. The spores, although thus deposited in the ears, seem to remain dormant, unless a cold and wet season, by inducing a morbid state of the growing wheat-grain, favours their evolution at the expense of the food supplied by its decomposing juices. 216 GENERAL PHYSIOLOGY. which is very important as a reservoir of nutriment to the animal, when it is about to pass into a state of complete inactivity. The Fungus spreads by the extension of its own stems and branches ; and also by the produc- tion of sporules, which are taken up by the circulating blood, and earned to distant parts of the body. The disease invariably occasions the death of the Silk-worm ; but it seldom shows itself externally until afterwards, when it rapidly shoots forth from beneath the skin. The Caterpillar, Chrysalis, and Moth, are all susceptible of having the disease communi- cated to them by the kind of inoculation just described ; but it is only the first which usually receives it spontaneously. By a careful investiga- tion of the circumstances which favour its propagation, the breeders of Silk-worms have been able greatly to diminish the mortality.* — Again, it is not at all uncommon in the West Indies to see individuals of a species of Polistes (a wasp-like insect) flying about with plants of their own length projecting from some part of their surface, the germs of which have been probably introduced through the breathing-pores at their sides, and have taken root in their substance, so as to produce a luxuriant vegeta- tion. In time, however, this fungous growth spreads through the body and destroys the life of the insect ; and it then seems to grow more rapidly, the decomposing tissue of the dead body being still better adapted than the living structure to afford it nutriment. A similar growth of different species of fungi of the genus Sphceria takes place in the bodies of certain caterpillars in New Zealand, Australia, and China ; the Chinese species is valued as a medicinal chug. Even Man is not free from the invasion of these parasites ; for simple forms of fungi have been found growing in the stomach and in the lungs ; in certain diseased conditions of the cuta- neous surface, named Porrigo favosa and Sycosis menti, a considerable development of fungous vegetation takes place ; and the same has been recently discovered to be true of these white patches [Aphtha) on the lining membrane of the mouth in children, which are known as thrush, these being found to contain filamentous forms of fungi, with distinct fruc- tification. In all these cases, however, it seems most probable that a cer- tain morbid condition of the animal fluids must exist, in order that the germs of the fungus may develope themselves ; so that this condition, rather than the presence of the fungus, must be looked upon as the essence of the disease. 273. The principal advance which is seen in the tribe of Fimgi towards the higher types of vegetation, consists in the separation of the nutritive from the reproductive apparatus. In many of the lower forms of the group, even when the entire plant is made up of an aggregation of distinct cells, instead of consisting of a single one (as in the instances just described, Fig. 75), we still find each cell capable of originating reproduc- tive cells or spores ; but in other cases, the fertile or spore-producing cells are collected at the extremities of the stalks or branches. In higher forms we find the sporiferous cells clustered together in a mass of tolerably determinate figure, and enclosed in a distinct envelope, as in the Lyco- perdon or puff-ball j but this mass of fructification, termed the sporan- gium, arises from a filamentous expansion, which is the real vegetating portion of the plant, and is termed the mycelium. Even in the highest * See Atmales des Sci. Nat. 2me Sdrie. Zool. tom. viii. GENERAL VIEW OF THE VEGETABLE KINGDOM. FUNGI. 217 Fig. 76. forms of Fungi, this mycelium — which is, in fact, the real plant, the part of the fungus that appears above ground being only its reproductive apparatus — still retains its simple condition ; being composed of a sort of flocculent mass of elongated cells, loosely interlaced with each other, and sending out prolongations which extend themselves through the soil ; it is this which constitutes the ‘mushroom-spawn,’ from which the edible mush- rooms are usually grown. The highest form of reproductive apparatus among the Fungi is that of the Hymenoviycetous group, of which the Agarics may be taken as the type. In these, when the fructification is fully developed, we see a dome-shaped body, termed the pileus (Fig. 76, A, a), surmounted upon a stipes or stem (c), which rises from the mycelium (cl) that constitutes the nutritive por- tion of the fabric. The pileus is com- posed of two mem- branes ; of which the upjier and outer is simple and sterile like the cortical layer of Lichens ; whilst the inner and lower, which is termed the hyme- nium, bears the spores. The surface of this membrane is usually extended by duplication or involution ; thus in the Agarics it forms vertical plates termed lamellce or gills, which radiate from the stipes towards the circumference of the pileus (Fig. 76, a, b, and B, a) ; in Boletus and Polyporus it lines a mass of vertical tubes arranged like the cells of a honeycomb ; and in Hydnum it covers the exterior of a simi- larly arranged series of solid columns. In all these cases, however, the mode in which the fructification is produced on the surface of the hymenium is essentially the same; the spores being developed from projecting bodies termed basidia, each of them usually having four prolongations, in every one of which a spore is developed in precisely the same manner as in the lower forms of Fungi (Fig. 7 6, b, b, c, d, e). Among the basidia are found a number of other vesicular bodies of large size, which do not bear spores ; these, which have been termed cystidia, have been supposed to have the nature of antheridia ; but this is very doubtful, more especially as they are not universally present ; and they may be more probably considered as analogous to the paraphyses or sterile filaments which lie among the asci of Lichens (§ 270). If anything like a conjugation takes place in this group, of which no evidence has hitherto been discovered, it probably occurs between the cells of the part of the mycelium from which the Structure of Agaricus campestris ; — a, longitudinal section of the entire plant; showing a, the pileus ; b, the lamellae covered by the hymenium; c, the stipe ; d, the mycelium: — b, portions of the fructification enlarged; a, section through the lamellee, showing the investing hymenium, and the spores clustered upon it ; b , portion of hymenium with basidia in four dif- ferent stages of formation ; c , basidium somewhat more developed, one of the processes showing the spore in its first stage, the other more advanced ; d, basidium with four processes and as many half-developed spores ; e, the upper part of a basidium with a fully-developed spore oil one of its processes. 218 GENERAL PHYSIOLOGY. apparatus of fructification is evolved. The first stage of its evolution which has been yet traced, consists in the development of a small round hollow body called the volva, within which a spherical body is found, attached to the base by a short stalk. The fructification is at" first deve- loped in the interior of this body, as in the lower forms of F ung i • but whilst its upper wall increases in thickness, so as to form the solid disk of the pileus, the lower part of it gives way, so as no longer to enclose the hymenium, and allows the pileus to expand ; its central portion some- times remaining traceable, however, as a membranous fringe round the base of the stipes. Whilst these parts are being developed, they break through the volva in which they were at first enclosed, and this usually decays and disappears very speedily. — There are certain forms of Fungi usually ranked in the Hymenomycetous group, whose fructification con- sists of thecae, or elongated cells, usually containing eight free spores, closely resembling the asci of the Lichens (§ 270) ; these obviously con- nect the two groups, being fungoid in then - mode of growth, whilst they are lichenoid in their fructification ; and we ought perhaps (with Schleiden) to transfer these tribes of thecaspori to the group of Lichens, on the same principle that this is now considered to include the genus Lichina and its allies, in spite of their marine habitation. — One of the great purposes of the group of Fungi in the economy of Nature, appeal’s to be the removal of refuse and decaying matter, which, if left to decompose, would prove inju- rious to animal life. Their vapour-like germs seem to float about in the atmosphere, in countless myriads, waiting only for a soil adapted to afford them nutriment ; in which, so soon as it is presented to them, they at once develope themselves, by the appropriation of the products of decom- position which would otherwise be diffused through the air. The more rapid the decomposition, and the greater the quantity of noxious exhala- tions which would thus be given off, the greater is the tendency to luxu- riant growth and rapid multiplication in these humble plants, which may not unappropriately be designated by the title more expressly appropriated to the class of Insects, that of “ Scavengers of Nature.” 274. Having thus studied the principal characters of the three im- portant groups which constitute the inferior subdivision of the Crvpto- gamic series, we proceed to those more elevated tribes, which present a manifest approach in their general conformation to the highest types of Vegetable structure. Their comparative elevation is most obviously displayed in the distinction which now shows itself, more or less com- pletely, between stem, root, and leaf; that is, in the separation and specialisation of the several parts of the nutritive apparatus. The separa- tion of the leaf from the stem is that which first occurs, being commenced in the Hepaticce, and fully carried out in the Mosses, in neither of which groups are true roots present ; indeed it is only in the Ferns and their allies, that we meet with that downward prolongation of the stem which corresponds with the true roots of Phanerogamia, the radical fibres of the inferior tribes being merely elongated cells, and rather comparable to the hairs with which the surfaces of leaves are frequently beset. The growth of the stem is always limited to its extremity, the part already formed being subject to very little increase, if any ; and from this character it has been proposed (by Dr. Lindley) to designate this higher division of the Cryptogamic series by the title Aerogens, significant of growth at GENERAL VIEW OF THE VEGETABLE KINGDOM. HEPATICjE. 219 their points. — The advance in complexity of outward form is accompanied by a corresponding elevation in the character of the internal organi- sation. Throughout the whole series, we find a cuticle or enveloping membrane, very distinct from the subjacent tissue ; and this is perforated by stomata or breathing pores, which are frequently of very complex structure, and which often communicate with air-chambers of very regular form, that are excavated in the parenchyma. There is a tendency, also, to a higher development of the individual cells, as manifested in the first instance by the formation of spiral fibres in then- interior (§ 147) ; such spiral cells are for the most part confined to the reproductive apparatus of the Hepaticae, where they answer a special purpose ; but we find them occasionally constituting nearly the entire plant in the Mosses. In neither of these tribes, however, do we meet with true woody fibre or spiral vessels ; the existence of these tissues being confined to the Ferns and their allies, which are the first plants in the ascending series in which we meet with a true woody stem composed of ligneous and vascular bundles regularly arranged. And it is in the same group that we first meet with continuous ducts for the conveyance of fluid ; their introduc- tion into the vegetable fabric being obviously related to the separation between the absorbent and the foliaceous organs which here takes place, the former being carried downwards by the extension of the root, whilst the latter are borne upwards by the elongation of the stem. The absence of woody tissue from the leaves of Mosses forms a marked distinction between them and the leaves of the Ferns, which have a regularly ‘ veined’ skeleton of fibro-vascular tissue, like that of the leaves of higher plants ; in the distributions of the £ veins,’ however, there is this marked distinc- tion, that those of Fern-leaves do not unite again after their subdivision, so that none of that reticulation occurs, which is generally so remarkable in the leaves of Phanerogamic, and especially in those of Dicotyledonous plants. — In the reproductive apparatus of Acrogens, we see but a very remote approximation to the type which distinguishes the Phanerogamia. Although it sometimes appears to be (as in Mosses) very distinct from the nutritive apparatus, the spores being enclosed in an elaborately constructed ‘ urn ’ which is borne aloft ppon a long stalk, yet we shall find reason to believe that the essential part of the generative operation is performed, in the whole of this series, whilst the organs concerned in it are as yet un- distinguished from the simply vegetative portion of the plant, instead of in organs which are already obviously set apart for it, like the flowers of Phanerogamia. 275. The group of Hepaticae, or Liverworts, forms a very natural and complete transition between the Lichens and the Mosses ; its lowest subdivision, the Hicciacece or Crystal-worts, being nearly allied to the former • while its highest, the Jungermanniacece or Scale-mosses, approxi- mates equally closely to the latter. — It is in the Marchantiacece, or ordinary Liverworts, that we see the characters of the group in their most distinctive form. These are humble plants, growing on the earth or on the surface of trees, where they can obtain an adequate supply of moisture ; they generally exhibit a definite axis of growth, but the dis- tinction between leaf and stem is not clearly manifested (Fig. 77 ) ; and from the whole of the under surface of the frond (or foliaceous expansion) radical fibres are put forth. This frond is composed of a soft green 220 GENERAL PHYSIOLOGY. parenchyma, which contrasts strongly with the hard dry crusts of the Lichens. In the common Marchantia polymorpha we find the surface divided into minute diamond-shaped areolse (Fig. 78, a, a), which mark the boundaries of chambers excavated in the substance of the frond. These chambers are enclosed by very regular walls, and are covered-in by the cuticle, which is perforated by stomata of remarkable complexity of structure, hereafter to be described (chap. xiii.). The development of this frond takes place by the continuous multiplication of cells, originating in the spore or reproductive cell detached from the parent, very much as in the case of the Ulvce (§ 267). The ‘spores’ or reproductive cells are produced within theca} or spore-cases, which are clustered together in the substance of an umbrella-like disk (termed the pelta or shield) that is mounted upon a long stalk, which rises from the centre of the frond (Fig. 77). With the spores, we find a number of curious bodies teimed Fig. 77. Portion of a frond of Marchantia polymorpha in fructification. elaters, each of which consists of a double spiral thread resembling that of a spiral vessel, but more elastic, and destitute of an enveloping cell-membrane. The elaters are developed, however, as ovoidal spiral cells, lying in the midst of the parent-cells within which the spores are generated ; when the cell has attained its maturity, the cell-membrane gives way, and the spires no longer confined by it have a ten- dency to elongate themselves : this they cannot do, however, so long as they are packed with the spores in the interior of the spore-case ; but, when both are set free by the opening of the thecse (which usually takes place by irregular fissures), the elate re spring out suddenly, and jerk forth the spores which may be adherent to them coils, thus assisting in their dis- persion. Besides these organs, the Marchantiacese produce antheridia, or clusters of sperm-cells containing peculiar self-moving filaments (closely resembling those of the Characece, § 269), included within special envelopes : these also are generally collected in disk-like bodies, which sometimes are but little elevated above the frond, whilst in other cases they are borne on stalks like those of the pelta;. In some instances the antheridia are borne on the same shields with the spore-cases ; thus in Marchantia androgyna , we find the upper sm-face of one half of the pelta developing antheritfla, whilst the under surface of the other half bears thecie. The contents of the antheridia are mature before those of the spore-cases ; and it seems probable, from considerations hereafter to be stated, that the entire theca with its contents is to be regarded as the product of an act of fertilisation (consisting in the union of the filament of the ‘sperm-cell’ with a special ‘ germ-cell’), which takes place previously to its development. — There is, however, another very curious provision for the multiplication of these plants ; consisting in the production of little disk-like bodies, which may be distinguished as ‘ bulbels,’ within basket-like conceptacles ; these may commonly be seen in different stages of growth, on various parts of the surface of the frond (Fig. 78, a, b). These ‘ bulbels’ must be regarded as gemmce or buds, having the power of independent existence ; they are at first developed upon little footstalks which rise from the base of the receptacle (c, d) ; and becoming spontaneously detached from these, they GENERAL VIEW OF THE VEGETABLE KINGDOM. HEPATICiE. 221 are commonly washed out by rain and carried to different parts of the neighbouring surface, on which they grow very rapidly when well supplied with moisture ; sometimes, however, they may be found growing whilst still contained in their receptacle, and seen to graft themselves (as it were) on the parent plant. — The principal departure from the type of the Marchantiacese pi’esented by the Ricciacece, which consist of irregular Fig. 78. Gemmiparous conceptacles of Marchantiapolymorpha : — a , conceptacle fully expanded, rising from the surface of the frond a a, and containing disks already detached : — b , first appearance of the conceptacle on the surface of the frond, showing the manner in which its fringe is formed by the splitting of the cuticle : — d, section of a conceptacle in progress of expansion, showing a, a, its thin fringed edges, b, b, the thicker walls of its lower portion, and c its base whence spring d the bulbels : — c, portion of the base more enlarged, showing the bulbels in various stages of development; a, pedicel, b, b , bulbels in their earliest condition, ^1 manner ot a sail. Beneath the air-bag is the digestive cavity, which is entirely distinct from it ; this has no single mouth, but receives its supplies of aliment through a number of GENERAL VIEW OF THE ANIMAL KINGDOM. ECHINODERMATA. 283 flask-shaped cirrhi which hang down beneath, each having an orifice at its extremity, surrounded by a sucker. The suctorial cirrhi are entirely distinct from the long contractile tentacula, which are employed for grasping prey, at the same time paralysing it by their peculiar stinging- power. These tentacula can be drawn up to within half an inch of the air-bladder ; and may then be suddenly shot forth to a length of eighteen or twenty feet ; and by means of these instruments, which are furnished with glandules secreting a highly irritating fluid, the Physalise can attack small fishes, &c. at a considerable distance, and can then draw them within reach of their suctorial appendages. Besides the suctorial cirrhi and the prehensile tentacula, there are other appendages, covered with vibratile cilia, which have been supposed to be respiratory organs. A glandular apparatus exists in the immediate neighbourhood of the diges- tive cavity ; and this may probably be regarded as a liver. No gen- erative organs have been hitherto found in these curious beings ; and nothing- is yet known of the history of their development. There can be little doubt, however, that their early form, whatever it may be, must be very different from that of the animal whose peculiar structure has been just described. And the same remark may be made with respect to certain other forms still more aberrant, which are referred to the Acalephse chiefly because they have no claim to admission into any other class. 295. The class of Echinodermata (or ‘ prickle-skinned ’ animals) derives its name from the spiny integ-ument with which some of the best known species belonging to it — such as the Star-fish and Sea-Urchin — are covered. The term, however, is far from being generally appropriate, for the spiny surface is only found in three out of the six orders of which the class is composed. A more universal character is the presence of cirrhi or tendril-like suckers, capable of being put forth from different parts of the surface of the body, and subservient to progression rather than to the prehension of food. But a still more constant character is the existence of a calcareous skeleton, which, however different in the general arrangement of its parts, is composed of the peculiar ‘ calcified areolar tissue ’ formerly described (§ 1 95), which is found in no other tribe of animals. Of this material there is formed in the Encrinite a branching stem, bearing a more or less massive body, which it does not entirely enclose ; in the Star-fish, a regular frame-work of detached plates, lying just beneath the skin ; and in the Echinus, a complete globular shell, armed on its exterior with spines, which are sometimes mere prickles, but which sometimes attain considerable length and massiveness. On the other hand, in the Holothuria, the skeleton is merely represented by a few scattered plates, which still, however, present its characteristic structure ; whilst in the aberrant Sipunculus, which forms the transition to the Annulosa, it is altogether wanting. The class is made up of six groups, which are so closely connected, one with another, that there can be no doubt as to the propriety of their being thus associated ; and yet, as in the preceding case, it is difficult to find any general characters which shall include them all under one definition. On the whole, how- ever, they are much more elevated in the scale than any of the forms of Animal life which have been yet brought under notice. A very distinct nervous system exists in them all ; and some of them exhibit a high 284 GENERAL PHYSIOLOGY. degree of complexity and elaborateness in the arrangement of their mus- cular structure, and in the manner in which the muscles are made to give motion to the hard parts. The most remarkable example of this is found in the dental apparatus of the Echinus. It is only in the Star-fish, how- ever, that any organs of special sense have been yet detected. — The digestive apparatus here no longer presents the obscurity which charac- terized it in some of the preceding class ; but consists of a definite stomachal cavity, with distinct membranous walls, opening by a single mouth : this mouth, in some instances, also serves as the anal orifice ; whilst in other cases the alimentary canal is continued as an intestinal tube, which terminates by a separate aperture, usually at the opposite extremity of the body. The stomach is in some instances extended into the remoter parts of the body, as in most of the Acalephac ; but it is more commonly restricted to the central portion ; and we here for the first time meet with a proper vascular apparatus, consisting of a system of anastomosing tubes which are not continuous with the digestive cavity, but which take up by absorption the nutritious fluid prepared by the digestive process, and carry it by a regular circulation through the system. The circulation, however, is not carried on by the agency of any muscular organ of impulsion, or heart ; the only approach to such being a pulsatile enlargement upon some of the vascular trunks. Distinct respiratory and glandular organs make their appearance in this class, as will be presently seen. In the higher Echinodennata, we entirely lose the capability of gemmiparous or fissiparous propagation ; some approach to it being shown in the remarkable power of regenerating parts of the body, which is possessed by the Star-fish, Holotliuria, &c. The repro- ductive process, therefore, is entirely performed by a sexual apparatus ; and so far as is yet known, the male and female organs are possessed 1 »y distinct individuals. In the development of the embryo of some of the orders, several curious phases are presented ; the most remarkable of which will be hereafter described (chap, xyiii.). We shall now take a rapid survey of the principal featimes of each of the six orders, into which the existing forms of this class have been distributed by Prof. E. Forbes, and which he has characterised according to the structure of then- loco- motive organs ; and shall also notice a seventh, now altogether extinct, which is of peculiar interest to the Palaeontologist. 296. Of the first order, Crinoidea, the existing forms are few ; but it was extremely abundant in former periods of the earth’s history ; and its remains form no inconsiderable portion of the solid crust of the glol >e. Its characteristic structure is best known from the Comat id a ; an animal which, at first sight, does not depart very widely from the type of the Star-fish, but which differs from this in several important particulars. It consists of a central disk with arms, which is attached during the early period of its life to solid bodies, by means of a jointed stalk pro- ceeding from the under side of the disk ; this stalk, in the ordinary Crinoidea, is permanent ; and in the freedom of its adult life, the Coma- tula may be regarded as forming a transition from them towards the unattached forms of the class. The digestive apparatus is confined to the central disk, and consists of a stomach opening by a mouth in the centre * “ History of British Star-fishes.' GENERAL VIEW OF THE ANIMAL KINGDOM. ECHINODERMATA. 285 of the disk, with an intestinal tube which passes off from its side, winds round the body, and opens externally by a projecting vent, situated between the month and one side of the body. The arms arising from this disk are solid, being composed of the calcareous framework already described, covered with a thick, soft, contractile integument ; they are five in number, but they speedily subdivide, each usually separating into four. To the central stem of each arm, jointed lateral appendages of a similar structure are attached ; and these also are clothed with the fleshy integument, which extends on either side in a sort of fin-like expansion. By the simultaneous movement of the arms, and the stroke of these numerous pinnae, or lateral appendages, against the water, the Comatula swims through the ocean very much after the manner of a Medusa; hence the order is distinguished by the name Pinnigrada. * Sometimes, however, the Comatula attaches itself to sea-weeds or other floating bodies, and employs its long arms in bringing food to its mouth. The position of the ovaries constitutes one of the most curious features in the structure of this animal ; for they are dispersed through the integument of the arms, in which they form many thousand distinct spots. The Comatula, in its young and attached state, has been described as the Pentaerinus Puropanis, and has all the characters of a true crinoid. The stem is made up of the same kind of calcareous framework as the rest of the skeleton, and is covered with the same irritable integument, by the contractions of which the head may be made to turn in any direc- tion ; the attachment of its base to solid bodies is effected by means of a suctorial disk. The pinnae are not develojied, until near the close of the period of attachment — A much larger Pentaerinus ( P . Caput-Medusae) has been found, however, in the West Indian Seas ; which probably passes its whole life in the attached condition, and is thus a truer repre- sentative of the vast assemblage of extinct Crinoidea. The disk and arms are formed like those of the Comatula ; the latter are very nume- rous, and are thickly set with jointed pinnse. The stem is more than a foot long, and is composed of a large number of pieces similar to those of the arms ; from this stem there arise, at regular intervals, several verticils of secondary arms, which do not subdivide, and are destitute of lateral appendages. The ovaries are not so dispersed as in the Coma- tula ; but they are still external to the central disk, being situated on the arms near their base. In some of the fossil species of Pentaerinus, which are especially abundant in the Lias formation, the subdivision and ramifi- cation of the arms is carried to a much greater extent than in either of the existing forms ; and the number of pieces in the skeleton thus be- comes very large. Thus in the P. Briareus it has been calculated that at least 100,000 separate joints exist, exclusive of those of the lateral appendages, which are probably more than 50,000 additional. The base of the stem of the recent P. Caput-Medusae has not yet been obtained ; so that its mode of attachment to solid bodies cannot be positively stated ; but from the circumstances under which fossil remains of other species are sometimes met with, there is strong reason to believe that the animals of this genus were not permanently adherent to solid bodies, but had the power of occasionally detaching themselves, and * This designation, however, is open to the objection that it is applicable to only a very small part of the order ; the character of which, as a whole, is its attached condition. 286 GENERAL PHYSIOLOGY. moving freely from place to place in search of a new and more appro- priate basis. — The bulk of the order, however, is made up of the very numerous tribe of Encrinites, of which we have no existing representa- tives, but which seems to have been in many respects of lower organiza- tion, connecting the free Echinodermata with true Zoophytes. The body and jointed stem exhibit a rounded instead of a pentagonal form ; the latter is usually destitute of secondary arms ; and the principal aims do not ramify with the same minuteness as in the Pentacrinus. The stalk seems to have been attached by a sort of spreading root, resembling that of many Corals ; and we must believe this tribe of Crinoidea, therefore, to have been entirely fixed during the whole of life. It contains a nume- rous series of forms, which vary considerably in the degree in which the body is enclosed by the calcareous skeleton. In some instances it is nearly shut in, on its upper as well as its under side, by calcareous plates, so that it almost resembles a stalked Echinus ; whilst in other cases, the under side of the body alone seems to have been supported by the solid disk, which is formed in the shape of a cup, more or less deep, from the margin of which the arms spring, its upper portion or opening having been apparently enclosed by soft integuments alone, as is the case with the existing species of this order. 296 a. The second order, Ophiurida, receives its name from the long serpentiform arms, which are appended to the round depressed urchin- like bodies. These animals, too, are commonly associated with the Star- fish, but are constructed upon a very different type. As in the Crinoidea, the viscera are entirely confined to the central disk, and the arms are solid, being composed of a jointed calcareous framework, covered with a contractile integument, by which they are moved with great freedom. On the other hand, they differ from the Crinoidea, and agree with the Asteriada (or true Star-fish), in having but a single orifice to the digestive cavity, which is nearly as simple in its structure as that of the Actinia (§ 291). In the position of then - ovaries, the Ophiurida differ from both these orders ; for these bodies are situated within the disk, but discharge their contents by separate orifices near the base of the arms. The disk is never attached, except in the early embryonic state of these animals, which in their adult condition are among the most active of the whole class ; their arms being very efficient instruments of locomotion. These are sometimes simple and undivided from their base to then - free ex- tremity, gradually tapering to a point, as in the ordinary Ophiurce ; whilst in Empale they ramify minutely, dividing regularly into branches, which again sitbdivide, so as to form a complex series of appendages. Their surface is usually beset with spines, or covered with rough scales : and these appear to be of great use in the movements of the animals, giving to the arms the power of resting against points of the surface over which they are moving, from which they can propel themselves in any direc- tion. Hence, as the Ophiuridce mainly depend upon them spines for locomotion, they have been termed Spinigrada. The body and aims in this order are furnished with cirrhi ; but these are not sufficiently de- veloped to assist in locomotion ; although those near the mouth are enl arsed into tentacula, which seem to draw the food towards that orifice. Most of the animals of this order are remarkable for then - brittleness, and their tendency to separate into a number of pieces when laid hold of. It GENERAL VIEW OF THE ANIMAL KINGDOM. ECH1NODERMATA. 287 is certain that the body can reproduce the arms ; and it has been affirmed that if a portion of the disk remain attached to a separated arm, the entire individual may be reproduced. Whether in this manner two or more individuals can be developed by the subdivision of a single one, or whether this process of fission ever takes place spontaneously, has not yet been ascertained. The ordinary mode of propagation is undoubtedly by sexual generation ; but the young animal, on its emersion from the egg, bears a form which differs so completely from that of the adult, that it would not be recognized as having any relation with it. The larval or embryonic structure, which has received the distinctive name of Pluteus, and from which the Ophiura is subsequently to be developed, is a transpa- rent gelatinous body, conical above, and prolonged into eight branches below, thus having somewhat the form of an easel with eight supports instead of three ; it is beset with rows of cilia, by whose agency it is freely propelled through the water, after the manner of the ciliograde A calephse (§ 294a), for one of which it might readily be mistaken, were it not for the presence of calcareous rods and spicula in the body and branches. A more particular description of this curious larva, and of the mode in which the Ophiura is developed from it, will be given here- after (chap. XVIII.). 2966. We now come to the Asteriada, or true ‘Star-Fishes,’ in which the real arms disappear ; what are commonly known as such, but more properly termed rays, being lobes or divisions of the body itself. In some instances there is scarcely any central disk, the body being almost entirely divided into rays ; whilst in other cases there is scarcely any appearance of rays, the central disk being but slightly indented at its edges. The skeleton of these animals is composed of calcareous plates, covered with a dense integument ; and this is very commonly raised into prickles, which are sometimes horny, but are sometimes calcareous in their interior, like rudimentary spines. The calcareous plates have usually no great regularity on the dorsal surface of the body and rays ; but on the lower surface of the rays, they are arranged with the most exact symmetry, in such a manner as to afford protection and support to the tubular cirrhi, which now become the principal instruments of pro- gression (Fig. 110 e f). These cirrhi are tubes composed of a contractile membrane, and furnished with suckers at their extremities] every one of them is connected internally with a vesicle filled with water, by the contraction of which the tube can be projected through a passage that is left between the calcareous plates ; whilst, on the other hand, by the general contraction of the membrane of the tube itself, it can be shortened and drawn in, or, by its contraction on one side more than on the other, it may be turned in any direction. The vesicles are themselves con- nected with a vascular canal which surrounds the mouth. The Asteriada possess a considerable amount of locomotive power, but it is for the most part sluggishly exerted. Their progression is accomplished entirely by means of the tubular cirrhi ; which, when extended, take an attachment to some fixed object, and wdien retracted, draw the body towards it ; and hence they are appropriately termed Cirrhigrada. The rays, too, in many instances, are very flexible, so that they can be drawn up towards the mouth, or moved from side to side towards each other ; and in this manner food can be brought to the entrance of the stomach, and the 288 GENERAL PHYSIOLOGY. body can be insinuated through narrow apertures. The mouth of the Asteriada is situated in the centre of the disk, and opens at once into the stomach, which occupies a considerable part of the cavity. The surface on which the mouth opens, and which is that whence the cirrhi are put forth, is designated as the lower surface ; the Star-fish and its allies habitually living in what would be (as compared with other animals) an inverted position. From the central stomach (Fig. 110, a), which is thin Fig. 110 . Aster ias aurantiaca , with the upper side of the hard envelope removed : a, central stomach : b, cceca upon its upper surface (salivary glands?); c c, ccecal prolongations of the stomach into rays; c ' , the same empty; d , the same laid open; e, the under surface, seen from within after the removal of the cceca, showing the vesicles of the tubular cirrhi ; /, the same in a contracted state, showing the skeleton between them. and membranous, a pair of coecal prolongations passes out into each ray (c, c ) ; these are considerably subdivided and ramified, and then- walls are beset with numerous follicles, which are not improbably to be regarded as a rudimentary form of a liver. Above the central stomach, in some species, are seen five other coeca (b, b), which appear to be secreting organs, and may be the analogues of the salivary or pancreatic glands of higher animals. No separate anal orifice exists ; the indigestible matters being voided by the mouth. A regular system of blood-vessels, for the ab- sorption of the products of the digestive process, and for the conveyance of GENERAL VIEW OF THE ANIMAL KINGDOM. ECH1NODERMATA. 289 them to the different parts of the fabric, exists in . the Asteriada ; its arrangement being conformable to the very regular radial symmetry which exists in the body generally. No special respiratory apparatus, however, yet exists ; but the surrounding water is freely admitted into the general cavity of the body, and bathes the surfaces of the viscera, which are covered with vibratile cilia. A distinct nervous system is here found, arranged upon the most regular radiated type (§ 282 a) ; and many species possess ocelli at the extremities of the rays. — These animals possess a remarkable capability of reproducing lost parts ; but the pre- servation of the disk seems essential to their power of doing so ; and hence, although three or four of the rays may be removed without the destruction of the disk, which gradually regenerates them, the separated rays cannot form a new disk ; so that no multiplication of individuals can ever take place by this process. The true generative apparatus is very obvious in them at the breeding season ; consisting of a set of ramified tubes, which are arranged in pairs in each ray, and which seem to open by minute pores in the circle of calcareous plates that surrounds the mouth. These tubes, in the male, are filled with sperm-cells, and in the female with ova ; the sexes being thus distinct. The ova, when matured and fertilized, are retained (in some species at least) in a kind of marsupium formed around the mouth of the parent by a peculiar change in its form ; and there the embryo undergoes the early phases of its development, attached at its base by a sort of footstalk divided into three lobes, which contracts as the disk is developed, and is at last entirely withdrawn into the body of the animal, remaining only traceable as the ‘ madreporiform tubercle’ which is found on one of the plates of the dorsal surface of the body. 296 c. Of the order Echinida, the type is the well-knowui Echinus or ‘ Sea-Urchin,’ which seems to have received its name from the resem- blance presented by its prickly sui'face to that of the hedgehog or ‘ urchin.’ This animal has a shell of nearly globular form, composed of polygonal plates (Fig. 39) accurately fitted together, so as to constitute a very complete investment to the body. The surface of the shell is beset with tubercles, on which are implanted a vast number of spines, having regular sockets at their bases, enabling them to be readily moved in any direction by the contractile integument which invests the shell, and which is attached to the bases of the spines, round which it is considerably thick- ened. These tubercles, however, are not equally present on all the component plates of the shell ; for we find rows of smaller plates, passing from pole to pole, the tubercles of which are comparatively few and small, whilst they are themselves perforated with apertures for the tubular cirrhi. The structure of these organs is precisely the same as in the Asteriada ; but their development is greater, for they are always capable of being projected further than the spines (which are frequently several inches in length), and of taking an attachment to fixed objects beyond. By their agency, the shell can be drawn onwards in any direction ; the movement being effected by the contraction of the tubes, whilst the body is supported upon the spines ; and from this compound mode of progres- sion, the Echinida may be appropriately termed Cirrhi-Spinigrada. In all the Echinida, the alimentary canal possesses two distinct orifices; which, in the Echinus and its nearest allies, are both of them central, being u 290 GENERAL PHYSIOLOGY. Fig. 111. situated at the two poles of the globular shell. Iu the Clytpeaster and Scutella, however, we have an approach towards the Asteriada ; the shell being more or less flattened and divided at its margin, so as to resemble the body of a Star-fish; whilst the anus leaves its central position on the upper surface, and approaches the mouth, which still retains its central position below. In the Spatangus and its allies, on the other hand, the radiated form is considerably departed from ; the shell being oval instead of globular, and the mouth and anus being neither of them central ; hi fact, the radiating arrangement shows a tendency to give place to a bi-lateral symmetry ; and in this and some other particulars, the Spatar- gaceae may be considered as leading towards the next order. — Notwith- standing these variations, however, the general plan of structure in the Echinida is nearly the same. The mouth (Fig. Ill, a), situated at the lower pole of the globe, and surrounded in the Echinus and its allies 1 y a complicated apparatus of teeth set in a calcare- ous framework («,c), leads by an oesophagus ((/) to the most dilated portion of the alimentary canal (e) which may be con- sidered as the stomach. F rom this passes off the in- testinal tube (g, k), which makes two turns round the shell, and terminates at the anal orifice (i). The alimentary canal is attached to the interior of the shell by a fold (/) of the membrane which lines the cavity ; and this ‘ mesenteric’ membrane contains distinct blood- vessels, by which (as in the Asteriada) nutriment is absorbed from the di- gestive cavity, and distributed to other parts of the body. Respiration is chiefly provided for, as in them, by the admission of the surrounding water into the cavity of the shell ; the membrane lining which is clothed with cilia, whose action occasions a constant renewal of the liquid. The water is introduced into the cavity by ten respiratory tubes, which are placed around the mouth, and which bear a strong resemblance to the tubular tentacula of the Actinia. In the neighbourhood of the mouth, also, are glandular structures, which would seem to be salivary organs. No distinct traces of a liver have yet been detected. The nervous system is formed upon the same plan as that of the Asterias ; but no organs of special sense have yet been detected. — No trace of the fissiparous or gemmiparous inodes of reproduction seems to remain among the Echinida ; and it is not yet known what extent of reparative power they possess. Their proper generative apparatus, however, is highly developed ; and consists Anatomy of Echinus lividus, laid open from the underside; a, buccal apparatus turned to one side ; b, portion of tegumentary membrane surrounding the mouth; c, calcareous jaws; d, oeso- phagus ; e. commencement of intestine ; f mesenteric membrane ; g, duplicature of the intestine, which forms a second convolution, //, along the course of the first, and terminates at the anal orifice i, around which are seen the five oviducts . j, ovaria ; k k, ambulacral vesicles. GENERAL VIEW OF THE ANIMAL KINGDOM. ECHINODERMATA. 291 of five large sacculi, which open by as many separate orifices in the plates that surround the anus. These sacculi contain spermatic cells in the male, and ova in the female. No provision here exists for the protection of the embryo during its immature state ; but it comes forth from the egg in the condition of a free-moving ‘pluteus,’ and it is only by a long process of de- velopment, during which the animal is supported upon nutriment obtained and assimilated by itself, that the typical form of the Echinus is evolved. 296 d. The recent discovery of an entirely new series of forms of Echinodermata, which abounded in the early ages of the earth’s history, but which seems to have become entirely extinct before the Pentacrinites were called into existence, has rendered it necessary to institute a new order, that of Cystidea, the place of which seems to be intermediate between the Crinoidea, the Ophiurida, the Asteriada, and the Echinida ; for it combines within itself, in the most remarkable manner, some of the distinctive characters of each of these groups. “ The Cystidea are more or less spherical bodies covered with polygonal plates, varying in number according to the genus, closely fitting together so as to invest the entire surface with a compact coat of mail, except at four points ; viz. inferiorly, where the body unites with a stem ; centrally, or above the centre on one side, where there is an opening closed by valves, supposed with good reason to be the orifice of the reproductive system ; and superiorly, where the mouth is found, usually if not always with a small perforation, supposed to be a vent, alongside of it. These parts, viz. the plates investing the body, the three orifices (for the fourth perforation, that of the base, is continuous with the canal of the stem where the latter is well developed), and probably the stem, are common to all Cystidea. There are certain other parts, apparently of great consequence in the organisa- tion of the animal, which are common only to certain members of the order. These are, the brachial appendages (arms and tentacula), and certain curious organs or appendages connected with the plates, to which the name of ‘pectinated rhombs’ may be appropriately given.” * — Thus iu the attachment of the body by a stem, the Cystidea resemble the Crinoidea, and some of the aberrant forms of the two orders come into very close approximation with each other : their types differ essentially, however, in the relative position of the ovaries. In the complete enclosure of the body by polygonal plates, they correspond with the Echinida ; but differ from them in the attachment of the body by a stem, in the single outlet of the reproductive apparatus, and in the possession (in some instances) of arms. In the division of the body of certain genera into lobes, again, they approach the Aster iada ; but differ from them in the characters just enumerated. The arms, where they are present, are more allied in structure and origin to those of Ophiurida, than to those of Crinoidea' ; and it is through this group that the Cys- tidea show the nearest approach to the free-moving forms of Echinoder- mata; certain genera having five ovarial orifices, instead of a single one, and these orifices being situated near the base of the arms, as in the Ophiura and its allies. In the singleness of the generative apparatus, however, which is the ordinary character of the group, it may even be thought to manifest a tendency towards the Holothuriada. * See Prof. E. Forbes on the British Cystidea, in Great Britain,” vol. ii. “ Memoirs of the Geological Survey of u 2 292 GENERAL PHYSIOLOGY. 296 e. In the order Holothuriada, we find united in a very curious manner the radiated arrangement of the exterior, and especially of the oral appendages, which is characteristic of the Echinodermata in general, with that elongation of the body and bi-lateral symmetry of the internal Fig. 112. organs, which are marked features in the structure of Articulated animals ; and some of them, moreover, exhibit obscure indications of a transverse division of the body into segments. We here take leave almost entirely of that calcareous skeleton, which has been so remarkable a feature iu the structure of all the preceding orders ; for we find the body inclosed within a soft distensible and contractile integument, unsupported, save (in some species) at the base of the tentacula and in a few other situa- tions, by calcareous plates ; and these disposed in such a manner as to allow the utmost freedom of motion, so that both the size and form of the body can undergo great variation. The tubular cirrhi, however, still exist (Fig. 112); and are sometimes found in distinct rows, extending from one pole of the body to the other, whilst in other cases they are scattered irregularly over the surface, or are limited to one side of it on which the animal usually crawls. The progressive movement of the body is partly effected by the cirrhi, but partly also by the extension and contraction of the body itself, as in the vermiform tribes : hence this order has been designated by the term Cirrhi-vermigrada. The general form of the body is not unaptly expressed by the vernacular name of ‘ sea-cucumber,’ which is given to some of the commonest Holothuriae of British coasts ; but in some species it is more globular, approaching that of the Echinus, whilst in others it is more prolonged.— -In their internal structure, the Holothuriada present us with the highest type anywhere encountered in this order. The mouth and anus are always placed at the opposite extremities of the body, and never show any tendency to approximate. The mouth (Fig. 1 13,6) is surrounded by ramifying ten- tacula (Fig. 112), which maybe regarded as highly developed cirrhi; these can be completely drawn back into the mouth, and are sometimes not put forth for days together. Within the mouth is a dental appara- tus, strongly resembling that of the Echinus ; to which is appended a set of glandular coeca ( t'), which are probably salivary organs. The oesopha- gus passes through this, and opens into a stomach, the walls of which GENERAL VIEW OF THE ANIMAL KINGDOM. ECHINODERMATA. 29J are sometimes dis- Fig. 113. tinctly muscular, but which is gene- rally not clearly distinguished from the intestinal canal. From the stomach proceeds the intes- tine (Fig. 113, i), supported by its mesentery m, to terminate in the funnel-shaped cloa- ca c. In the mes- entery is seen a co- pious distribution of blood-vessels ; and this forms part of a complex vas- cular system, which conveys the blood through all por- tions of the body. Several parts of its main trunks seem contractile; but the impulsive power appears essentially concentrated in one organ, a contrac- tile vesicle ( p ), which is the rudi- ment of a heart. A distinct and ela- borate respiratory apparatus here ex- ists, consisting of a pair of arborescent cceca (?’ r), -which originate in the cloaca, and ex- tend and ramify throughout the body ; and water appears to be alter- nately introduced into these organs, and expelled from them, by the ac- tion of the power- ful muscles which Anatomy of Holothuria tubulosa; — a , anus ; b, mouth, surrounded by 20 tentacula; c, cloaca, surrounded by muscular dilators c' ; i, intestinal tube; m, mesentery, ml, ml, longitudinal muscles; mt , transverse muscles lining the entire inner surface of the integument; o, ovary; ' ap , coecal appen- dages, probably seminiferous ; p, contractile vesicle, probably a heart ; r, r, respiratory apparatus, originating in the cloaca ; t, oral tentacula ; t', coecal reservoirs ; va, annular vessel surrounding the mouth and supplying the ten- tacula ; ve, external intestinal vessel, giving off a large anastomotic branch va' which enters another part of the same trunk; vi, internal intestinal vessel, with contractile dilatations; vl, longitudinal tegumentary vessel, giving off transverse branches vl', seen by removing the longitudinal muscles ; vm, mesenteric vessels, connecting the branches of the external intestinal vessel with those of the respiratory system of vessels, vr. 294 GENERAL PHYSIOLOGY. form the walls of the visceral cavity. These muscles, which are disposed with great regularity [ml, mt), appear to be put in action by a nervous system, which is formed upon the general plan of the group, consisting essentially of a gangliated ring that surrounds the mouth. Among the most curious facts in the economy of these animals, is then tendency to eject the entire mass of viscera by a kind of convulsive action ; this they are usually said to do in alarm when captured ; but Professor E. Forbes states that he has never seen such an occurrence. He has met with speci- mens of many species, however, in which there was not a trace left of any of the viscera, although the creature was taken alive and seemed healthy. According to Sir J. G. Dalyell, the viscera are all regenerated in the course of a few months. It is affirmed by the same observer, that some species of this order divide spontaneously through the middle into two.or more parts, each becoming ultimately perfect by the development of new organs. This self-division would seem especially to occur in the most vermiform tribe of the group (that of Synaptce ) ; and it is to be regarded rather as a character of approximation to the lower Annulosa, in some of which we shall find it remarkably manifested, than as representing the gemmipa- rous multiplication of the lower Radiata. The ordinary mode of repro- duction in the Holothuriada is by a true generative apparatus, which differs from that of all the preceding orders now existing, but agrees with what seems to have been the plan of conformation among the Cystidea, in being a single organ, instead of being repeated five or more times 3 round a common centre. This organ (Fig. 113 c) is a cluster of coeca, j which opens, by a canal of its own, near the mouth ; and it contains spermatic cells, or ova, according to the sex. Of the development of the embryo in this order nothing whatever is yet known. 296/! In the order Sipunculida, the radiated arrangement still more completely gives place to the annular. In their external appearance, the animals composing it are Worms; they have no calcareous plates, nor cirrhi ; the oral tentacula are wanting, or are irregular in number and disposition ; and their progression is entirely accomplished, like that of worms, by the contraction of then’ integuments ; whence they may be designated F erm i- grada. Their internal structure is still imperfectly understood ; but it appears probable, from the latest enquiries, that their true place will be found rather in the Articulated than in the Radiated series A 297. The range of Animal forms comprehended in the Sub-kingdom Mollusca is so great, that it is difficult to include them in any positive definition that shall be applicable to them all. The lowest group which the series comprehends, presents so strong a resemblance to Zoophytes, both in its general plan of structure, and in its habit of life, that it was until recently associated with them ; whilst, on the other hand, in the highest and most individualized Mollusks, there is a decided approximation to- wards the Vertebrated series, — not so much, however, in then - general type of conformation, as in the structure of then separate organs. The zoophytic Mollusks for the most part exhibit, more or less distinctly, that circular arrangement of the parts about the mouth, which is the rno-t obvious character of the Radiated classes ; but it is comparatively seldom that we meet with that exact bilateral symmetry of external form, which is nearly an invariable characteristic of Articulated and Vertebrated animals. * See Blanchard, in Ann. des Sci. Nat. Juillet, 1849. GENERAL VIEW OF THE ANIMAL KINGDOM. — MOLLUSCA. 295 With the exception of the Chiton (Fig. 125) and its allies, which form a connecting link between the Molluscan and Articulated series, we do not find any trace of that division of the external skeleton into segments, which is the distinctive feature of Annrdose animals ; and even in those highest Mollusks, which present the rudiment of an internal skeleton for the support and protection of the nervous centres, we never perceive the least indication of that segmental division of this skeleton, wdiich is the characteristic feature in the structure of the Vertebrated series. — Taking them as a whole, the Mollusca are characterized rather by the absence, than by the possession, of any definite form -; and there is a cor- responding absence of any regular organs of support, by which such a form could be maintained. The shell, where it exists, is to be regarded rather in the. light of an appendage, designed for the mere protection of the body, and deriving its shape from it, than as a skeleton giving attachment to muscles, and l’egulating the form of the whole structure, which is the case with the skeletons both of Articulated and of Vertebrated animals. It is in no instance a fixed point for the muscles of locomotion ; and it is only, indeed, where the body is uncovered by a shell, or where the locomotive organs can be projected beyond it, that any active move- ments' can be executed. In a considerable proportion of the series, no shell exists : and it is a clear indication of the non-essential character of this appendage, that we find it absent in the one, and present in the other, of two groups which are formed in every other respect upon the same general plan, the Slugs and the Snails. Leaving the shell, therefore, out of consideration, and attending merely to the general characters of the body, we notice, in the first place, that this, as the designation of the group implies, is altogether soft, and that even where locomotive appen- ' dages exist, they are as destitute of firm supports, either internal or external, as is the body itself. The whole fabric is enclosed in a thick fleshy glandular skin, which is called the mantle ; it is on the surface of this envelope that the shell is formed, by the development and subsequent calcification of epithelial cells, as already explained (§ 197) ; and it is by the muscular fibre which the mantle contains, that the movements of the animal are chiefly effected. The muscular structure is frequently concen- trated, as it were, in some particular part or parts of the mantle, so as to constitute a single or multiple locomotive organ or foot ; thus in the Gas- teropoda (§ 302), it forms a sort of flattened disk, by the alternate con- traction and extension of different parts of which, the animal can slowly crawl onwards ; in the fine moving Conchifera (§ 301) it is a tongue-like organ, capable of being projected from between the valves of the shell, and serving to propel the body by more sudden movements ; whilst in the Cephalopoda (§ 306), we find the prolonged and contractile tentacula (or oral appendages) to be the chief instruments of locomotion. The disposi- tion of the nervous centres is chiefly remarkable for its want of constancy (§ 282 a), and for the absence of either circular or longitudinal repetition. In the lowest forms of this group, we are presented with the simplest known type of a nervous system, namely, a single ganglion, with afferent and efferent fibres ; and this is adequate to call the muscular mantle into contraction upon the application of stimuli, which is the only movement that such animals have occasion to perform. But with the advance in com- plexity of structure, and more especially with the development of sensoiy 296 GENERAL PHYSIOLOGY. organs and of special locomotive appendages, we find the number of ganglia increasing; still, however, without any approach to a definite arrange- ment, save that they are for the most part disposed in the neighbourhood of the mouth. — From this absence of any uniform plan in the arrange- ment of the nervo-muscular apparatus, and from the want of any solid framework which might afford fixed points of attachment to the muscles and enable them to act at a mechanical advantage, it is obvious that the development of the proper animal portion of the fabric of the Mollusca is quite subordinated to that of the organic or vegetative, which we shall find to present a remarkable elaborateness and complexity. 297 a. The alimentary canal of the Mollusca invariably possesses two distinct orifices, an oral and an anal ; and it usually presents an obvious separation between the oesophagus or gullet, the stomach or digestive cavity, and the intestinal tube. The mouth, or entrance to the oeso- phagus, is not situated, in the lower Mollusca, on a prominent paid of the body, nor is it surrounded by organs of special sense ; in some instances, indeed, it is buried deeply amidst other organs ; but in the higher classes, it is situated on a head distinct from the trunk, which is also fur- nished with well-developed eyes, and with rudimentary organs of smell and hearing. In the lower Mollusca, again, the mouth leads directly to a funnel-shaped oesophagus, down which a current of water is continually driven by ciliary action ; and it is from the animalcules and other minute organic particles which the water may contain, that these beings derive their support. But in the higher members of the group, the mouth is furnished with instruments for the mechanical reduction of solid food, which are put in action by a complex muscular apparatus ; and they have locomotive power enough to go in search of that food, instead of being dependent upon such as the currents in the water they inhabit may chance to bring them. The alimentary canal occupies in the Bryozoa, as in Polypes, a large part of the general cavity of the body; but in all the higher Mollusca it is more limited in its dimensions, and is completely isolated from other organs. In the curious group of Xudibranchiate Gasteropods, however, it is prolonged into a multitude of tubular appen- dages, which radiate into almost every part of the body (§ 303 d), in a manner that reminds us of the extension of the gastric coeca into the rays of the Star-fish (§ 2966). Throughout the whole series, the liver is very distinctly developed. The form in which it presents itself iu the Bryozoa is quite rudimentary ; but even in the higher Acephala. such as the Mussel or Cockle, it constitutes a large part of the mass of the body ; and in the Cephalopoda it is constructed upon a plan nearly as elevated as that of Fishes. Other secreting structures, also, present themselves in these classes, in a more or less distinct form ; such as salivary glands, pancreas, and urinary organs. In all but the very lowest Mollusca, we find a set of vessels adapted to absorb from the walls of the alimentary canal the nutrient fluid which has been prepared by the digestive opera- tion ; and to convey this into the parts of the body remote front the visceral cavity. In the class in which these vessels first present them- selves, we find that, as in the Holothuria (§ 296 c), the heart is little more than a pulsating dilatation of the principal vascular trunk ; but as we ascend the series, we find it becoming more and more distinct from the vessels with which it is connected, presenting a division into two or GENERAL VIEW OF THE ANIMAL KINGDOM. MOLLUSCA. 297 more cavities, and acquiring distinct muscular walls. The blood is inva- riably colourless, and contains but few floating corpuscles. In all but the very lowest, again, we find a distinct respiratory apparatus, to which the blood is transmitted for aeration. With the exception of the terres- trial Gasteropods, which constitute but a small proportion of the entire series, we find this apparatus uniformly constructed with a view to aquatic respiration ; and in all but the Cephalopods, in which streams of water are propelled over it (as in Fishes) by muscular action, we find that the renewal of the water in contact with the surface of the gills is chiefly due to ciliary movement. The propagation of the lower Mollusca is effected, like that of Zoophytes, by gemmation as well as by the true generative process ; the gemmae being sometimes detached, so as to make their way freely through the water ; but more frequently remaining con- nected with the parent structure and with each other, so as to form aggre- gate or composite fabrics, strongly resembling those of Zoophytes, for which some of them have been mistaken. In all instances, however, a true generative apparatus is very distinctly developed. In the lowest classes, the two sets of organs are united in the same individual, in such a manner that its ‘sperm-cells’ may impregnate its ‘germ-cells’ and produce fertile ova. In certain higher forms, although both sets of organs are present in each individual, yet the congress of two is requisite, the germ-cells of each being fertilized by the sperm-cells of the other. But in a large proportion of the higher classes of the Molluscous series, the sexes are completely distinct ; and the power of propagation by gemmation is altogether wanting. — Thus it appears that the predomi- nant character of the group consists in the high development and func- tional activity of the apparatus of organic or vegetative life, in compari- son with that of the locomotive and sensorial organs. A considerable number of the lower Mollusca are fixed to one spot during all but the earliest period of their lives ; the movements of one part of the body upon another are of the simplest possible kind ; and their food is obtained by an action of the same kind as that which serves for the alimentation of Amimalcules. And thus we find beings possessed of an elaborate digestive apparatus, of a complex system of blood-vessels with a distinct contractile heart, and of special respiratory organs of great extent and completeness, furnished with no organs of sense but such as Zoophytes seem to possess in common with themselves, and leading a life almost as inactive (save in regard to the movements of the cilia which clothe their internal surfaces) as that of Plants. Even among the Gasteropod Mol- lusca, which may be regarded as the types of the whole group, although the body is free to move, yet the term ‘ sluggish,’ derived from one of the best-known forms of the class, is applicable to the locomotion of the whole of it ; and there is every reason to believe that the sensations are as obtuse as the movements are slow, the use of both these kinds of animal power being limited to the search for food, which, when found, is voraciously devoured. It is only in the Ceplialopod Mollusks, which con- stitute an aberrant group, leading us in many respects towards the Verte- brated series, that we meet with very active powers of locomotion, or with evidence of acuteness of sensation. — The distinctive characters of the principal classes into which this Sub-kingdom is divided, will be now more particularly described. * 298 GENERAL PHYSIOLOGY. 298. In the class of Bryozoa, we have a remarkable adaptation of the Mofcscan type of structure to the Zoophytic mode of life ; and it is not surprising, therefore, that, so long as the former was ill understood, and the latter received almost exclusive attention, the group should have been associated with that which contains the true Polypes. That its true place, however, is in the Molluscan series, is now generally admitted by those most qualified to appreciate its affinities, and will become sufficiently obvious when the close alliance between this class and the next shall have been shown by the comparison of the essential features of both. — The animals of the Bryozoa, in consequence of their universal tendency to multiplication by gemmation, are seldom or never found solitary, bur form clusters of various kinds; these are sometimes connected by a stolon or creeping stem, as in the Laguncula repens (Fig. 114), but sometimes spread out in every direction over a plane surface, as in the ordinary Flustrce, Lepralia ?, etc., and not unfrequently form (by their arbores- cent mode of growth, and by the consolidation of the connecting tissue) a coral-like mass, such as that of JEschara cervicornis (Fig. 115). Eacli individual is composed externally of a sort of sac, of which the outer or tegumentary layer is either simply membranous, or is homy, or in some instances calcified ; and this is always continuous with that of other indi- viduals, either by direct contiguity, or by the intermediation, of the stem, or of a connecting tissue which may be either spongy or solid. This tegumentary sac is lined by a more delicate membrane, which closes its orifice, and which then becomes continuous with the wall of the alimen- tary canal ; this lies freely in the visceral sac, floating (as it were) in the liquid which it contains. The further details of the anatomy will be best understood from the examination of a characteristic example, such as the Laguncula repens ; which is shown in the state of expansion at a, Fig. 114, and in the state of contraction at b and c. The mouth is surrounded by a circle of tentacula, which are clothed with vibratile cilia ; these tentacula, in the species we are considering, vary from ten to twelve in number ; but in some other instances they are more numerous. By the ciliary investment of their tentacula, the Bryozoa are at once distinguish- able from those Hydraform polypes to which they bear a superficial resemblance, and with which they were at one time confounded ; and accordingly, whilst still ranked among the Zoophytes, they were charac- terized as Ciliobrachiata. The mouth, situated in the centre of this circle of tentacula, leads to a funnel-shaped cavity, or pharynx, b, which is sepa- rated from the oesophagus cl by a valve at c ; and this oesophagus opens into the stomach e, which occupies a considerable part of the visceral cavity. In the BowerbanJcia, and some other Bryozoa, a muscular - stomach or gizzard, for the trituration of the food, intervenes between the oeso- phagus and the true digestive stomach. The walls of the stomach h have considerable thickness ; and they are beset with minute follicles, which seem to have the character of a rudimentary liver. This, however, is more obvious in some other members of the group. The stomach is lined, especially at its upper part, with vibratile cilia, as seen at c g : and by the action of these, the food is kept in a state of constant agitation during the digestive process. From the upper part of the stomach, which is (as it were) doubled upon itself, the intestine i opens, by a pyloric orifice /, which is furnished with a regular valve ; within the intestine are GENERAL VIEW OF THE ANIMAL KINGDOM. BRYOZOA. 299 seen at Tc particles of excrementitious matter ; which are discharged by the anal orifice at l. No circulating apparatus here exists ; hut it is probable that the liquid which fills the visceral cavity is not mere water, but con- tains the nutritive matter which has been prepared by the digestive opera- tion, and which has trans- uded through the pari- etes of the alimentary canal ; a few corpuscles of irregular size are seen to float in it. We shall hereafter see that this sac, in several other in- stances, may he regarded as a sort of diffused form of the circulating appa- ratus, and even consti- tutes a part of it when it is more specialized. The visceral sacs of the different individuals put forth from the same stem, appear to communicate with each other. No other respiratory organs exist than the tentacula, which are hollow, their cavities being connected with that of the visceral sac ; and into these it is probable that the nutri- tive fluid is sent for aera- tion. — The sperm-cells are developed in a glan- dular body, the testicle m, which lies beneath the base of the stomach ; when mature, they rup- ture, and set free the spermatozoa q q , which swim freely in the liquid of the visceral cavity. The ova, on the other hand, are formed in an ovarium n, which is lodged in the membrane lining the tegumentary sheath, near its outlet; Fig. 114. Laguncula repens , as seen in its expanded state at a, and in its contracted state, in two different aspects, at b and c. The same references answer for each figure : — a a , tentacula clothed with vibratile cilia; b, pharyngeal cavity; c, valve separating this cavity from d the oesophagus ; e, the stomach, with /its pyloric valve, and g the circle of cilia surrounding that orifice ; h, wall of the stomach with biliary follicles; i , the intestine, containing k excrementitious matter, and terminating at l the anus ; m, the testicle; n, the ovary; o, an ovum set free from the ovary; p, openings for the escape of the ova; q, spermatozoa freely moving in the cavity that surrounds the viscera ; r, retractor muscle of the angle of the aper- ture of the sheath ; s, retractor of the sheath ; t, retractor of the tentacular circle ; u, retractor of the oesophagus ; v , retractor of the stomach; w, principal extensor muscle; x, transverse wrinkles of the sheath ; y, fibres of the sheath, themselves probably muscular ; z , muscles of the tentacula ; a (at the base of the tentacular circle in a) nervous or oesophageal ganglion ; /3, stem.— d. a portion of the tentacular circle shown separately on a larger scale : a a, the tenta- cula clothed with cilia ; b b, their internal canals ; c, muscles of the tentacula ; d, transverse muscles forming a ring at the base of the tentacula; e, muscles of the tentacular circle. the ova, having escaped from this into the visceral cavity, as at o, are fertilized by the spermatozoa which they there meet with and 300 GENERAL PHYSIOLOGY. are finally discharged by an outlet at p, beneath the tentacular circle. Besides this mode of propagation, however, we find a provision for the multiplication of individuals by gemmae ; these are not here put forth from the bodies of the individuals previously completed, but from the stem. In those cases, however, in which the tegumentary sheaths are in close contiguity, instead of being united by a stem or by other connecting tissue, the gemmae are put forth from the individuals already formed, as in Flustra and its allies. — These beings possess a considerable number of muscles, by which their bodies may be projected from then- sheaths or drawn within them ; of these muscles, r, s, t, u, v, w, x, the direction and points of attachment sufficiently indicate the uses ; they are for the most part retractors, serving to draw and double up the body, to fold together the circle of tentacula, and to close the aperture of the sheath, when the animal has been completely withdrawn into its interior. The projection and expansion of the animal, on the contrary, appears to be chiefly accomplished by a general pressure upon the sheath, which will tend to force out all that can be expelled from it. The teuta- cula themselves are furnished with distinct muscular fibres, by which their separate movements seem to be governed ; the arrangement of these is seen at n. At the base of the tentacular circle, just above the anal orifice, is a small body (seen at a, a), which is probably a nervous gan- glion ; this being the situation of the single ganglion in the Tunicata. As yet no branches have been distinctly seen to be connected with it in this species ; but its character is less doubtful in some other Bryozoa. — The general structure of the animals whose sheaths are imbedded in a coralline fabric (formed by the deposit of carbonate of lime in their connecting tissue), is essentially the same ; as shown in Fig. 115. When this fabric is decalcified by the action of dilute acid, it leaves a much more definite residuum than that which is obtainable from the ordinary corals ; and it is a curious circumstance, which shows that it possesses a power of growth inherent in itself, that, as the individuals lodged in its hollows become less active, the mouths of their sheaths gradually contract around them ; and at last, when the minute inhabitants have ceased to live, the cavities in which they were imbedded are completely closed by the growth of new tissue from the border of their orifices, so that no trace of these at last remains. — If we scrutinize these characters, we shall find that the most important of them are Molluscan, rather than Zoophytic. In the first place, all true Polypes use their tentacula to grasp then- food and convey it to the mouth ; and these tentacula are destitute of cilia : whilst, on the other hand, in all the Acephalous Mollusca, the nutritive matter is drawn in by a ciliary current, which also serves to aerate the fluids. Now, the latter, as we have just seen, is the case with the Bryozoa ; and thus, although then- arms very commonly present a circular disposition around the mouth, they may be considered as representing, in then- rela- tion to the economy of the animal, the ciliated branchial sac of the Asci- dians (§ 299). But they do not by any means constantly present this radial symmetry : thus, in the Plumatella, a beautiful fresh-water geuus of Bryozoa, the ciliated arms are set upon two lobes or projections, one > on either side of the mouth. The structure of the alimentary canal, again, removes the Bryozoa from the zoophytic series. In no true polype is there a separate intestine and anal orifice; nor does the whole apparatus GENERAL VIEW OF THE ANIMAL KINGDOM. BRYOZOA. 301 hang freely in the visceral cavity ; and the existence of a gizzard-like organ (foreshadowing that of Cephalopods, § 307), and of a rudimentary liver (closely resembling that found in the lowest Tunicata), are also characters of elevation. The most important of all the single characters furnished by the anatomy of these animals, is their nervous system ; which, as already pointed out, is distinctly Molluscan in its type. The absence of a heart and a distinct circulating system is, it is true, a Zoo- phytic character ; but we shall presently find that even in the Tunicata, which are true Mollusks, the character of the circulating apparatus is extremely degraded. The propagation by gemmation, although formerly Fig. 115. A Eschara cervicornis; a, portion of its coralline fabric, of the natural size; b, portion of a young branch enlarged, to show the arrangement of the superficial apertures ; c, an individual withdrawn from its cell, and highly magnified ; a, mouth; b, pharyngeal cavity; c, secreting tubuli (?) ; d , digestive stomach; e, intestinal canal;/, anus; g , retractor muscles. supposed to be a character exclusively Zoophytic, is now known to belong also to the greater part of the Tunicated Mollusks ; and from this, there- fore, no argument can be drawn in favour of the Zoophytic nature of the Bryozoa. And although many of their composite fabrics have a stony density, and closely resemble the solid polypidoms of the Helianthoid and Asteroid polypes, yet in others, especially amongst the fresh- water species, we find a very close resemblance to the gelatinous bed or leathery crust in which the Compound Ascidians are lodged ; and if we imagine calcareous matter to be deposited in this bed or crust, we should have a fabric closely resembling that of many Bryozoa. In their power of projecting their bodies from their sacs, the Bryozoa must be admitted to resemble Polypes rather than Tunicata ; but this is a character of no particular importance, and some approaches to it are seen among the compound Ascidians. 299. The class of Tunicata contains a series of forms which are closely allied to each other in the most important features of their structure, and 302 GENERAL PHYSIOLOGY. which constitute a chain of connection between the Zoophytic Bryozoa on the one hand, and the more truly Molluscous ConcTivfera on the other. Its lower tribes are allied to Bryozoa, not merely in the general plan of them conformation, but also in their tendency to produce composite struc- tures by gemmation ; they are distinguished, however, by the absence of the ciliated tentacula which form so conspicuous a feature in the external aspect of the Bryozoa, by the presence of a distinct circulating apparatus, of which no traces have as yet been found in that class, and also by the special modification of the pharyngeal cavity for respiratory action. J; In them habits, too, they are more inactive ; exhibiting scarcely any thin , comparable to those rapid movements of expansion and retraction, which it is so interesting to watch among the Bryozoa. The highest forms of the class, however, are quite solitary ; for in those which form ge mmae , the gemmae are detached before they have advanced far in them develop- ment ; whilst in many other cases, no gemmation takes place, the repro- duction being effected only by the sexual process : and it is among these that we find the closest approximation, in general structure, to the type of the Mollusca contained within ‘ bivalve’ shells. — Of the characters of the class, as a whole, we shall derive our best idea from an examination of the structure of one of the ‘solitary’ Ascidians, such as the Cynthia micro- cosmus (Figs. 116, 117). Its body, like that of Tunicata in general, is completely enclosed within a general integument or ‘tunic’ (Fig. 116, A Fig. 116. B Anatomy of Ascidia ( Cyntlda ) microcosmus. — At a, a portion of the external tunic, a, a, having been removed, the muscular tunic b is shown; c, the oral orifice; d, the anal orifice. — At b, the muscular tunic a having been divided, the cardiac cavity a is shown, partly excavated in its substance; b, the oral orifice; c, oral tentacles surrounding the entrance to the branchial sac: d, branchial membrane; e, ventral furrow, corresponding to the great thoracic sinus; f, dorsal fold, corresponding to the dorsal sinus; g, oesophageal orifice; h , extremity of the intestine, situated in the cloaca ; i, anal orifice. a a) possessing two orifices, a mouth (c), and a vent (c/) ; this tunic, in the Ascidians, usually possesses some degree of toughness, frequently being leathery or even cartilaginous in its texture ; and it is occasionally ren- dered still more resisting by the agglutination of grains of sand, gravel. * It may be thought that these are not characters sufficient for the separation of ilie Bryozoa from the Tunicata, and the erection of the former into a distinct class; but the Author would again request his readers to bear in mind, that he is only endeavouring to make them acquainted with the characters of the groups which are most Ph ijsiolotjii'aliy distinct from each other, without aiming to determine their relative Zoological value. I he rank of the Bryozoa will not improbably prove in the end to be that of the lowest order of Tunicata. GENERAL VIEW OF THE ANIMAL KINGDOM. TUNICATA. 303 &c., to its external surface, so that it almost takes the place of a shell. Within the tunic or ‘ test ’ we find the muscular ‘ mantle ’ (a b, b a), which is adherent to the tunic only near the orifices, and which is able by its contraction to produce a general compression of the cavity it encloses, or a constriction of either of the apertures. A large part of the cavity is occupied by the branchial or respiratory sac (b cl), which may be consi- dered as a dilated pharynx, and which fills the whole of the space not taken up by the nutritive and reproductive organs. This branchial sac is lined with cilia ; and by the agency of these, a continual current is drawn in through the oral aperture, part of it traversing the respiratory surface, whilst another part is driven through the alimentary canal, the whole being finally ejected through the anal orifice. The digestive organs of the Tunicata present but a slight advance in their plan of structure upon those of the Bryozoa. In all instances, the alimentary canal has two orifices ; these may Tie situated near to each other, as in most of the Asciclidns (Tig. 116 a, e) ; or they may open at the opposite extremities of the body, as in the Salpce (Fig. 124). The oral orifice is often furnished with short ten taenia (Fig. 116 b, c), which appear to be sensory organs ; their purpose apparently being to indicate the presence of any such particles, in the stream that enters the orifice, as may be unfit for admission into the branchial cavity ; and to call forth a reflex contraction of the muscular tunic, wdiich serves not only to prevent the entrance of the offending par- ticle, but also to eject a gush of -water that shall convey it to a distance.* The stomach and intestinal tube are formed, in the inferior Tunicata, upon the plan of those of the Bryozoa, the lower part of the pharyngeal (bran- chial) sac narrowing to form the entrance to the stomach ; and the liver still consists of a set of isolated follicles imbedded in the walls of the latter. In the higher forms, however, we find a short oesophagus com- mencing by a distinct orifice (Fig. 117 a, c ) at the bottom of the pharyn- geal chamber ; this oesophagus leads to the stomach ( d ), which receives the terminations of a number of separate biliary canals, that proceed from a liver (b, k) altogether distinct from the stomach, but composed of nu- merous lobules that cluster around it ; and from the stomach, a short wide intestine (a, e) passes off, which soon reaches (at/) the cloaca, or common receptacle of the various matters that are to be ejected through the anal outlet. — All the Tunicata possess a distinct circulating apparatus ; and in all of them, too, the movement of blood seems to take place upon a very peculiar plan. The heart is very simple in its structure, being sometimes a mere contractile dilatation of the principal trunk (Fig. 119, o), and some- times appearing as an excavation in the muscular tunic itself (Fig. 116, B, a) ; in neither case, however, is there any division into an auricle or receiving cavity and a ventricle or impelling cavity. The course of the blood is not uniform ; for after the heart has received it from one set of vessels for a short time, and projielled it into another, its contractions become slower and the current feebler, and after a while they cease; recommencing, after a short pause, in the opposite direction. After conti- nuing for a further period in this reversed direction, the current returns again, in the same manner, to its original course. This remarkable phe- nomenon has been witnessed in species from each of the principal divi- * This action is exactly analogous to that of coughing in higher animals. 304 GENERAL PHYSIOLOGY. sions of the class ; and may, therefore, be considered as a peculiarity common to the whole of it.— The Tunicata all possess distinct generative organs, which are usually united in the same individual ; and from the ova thus produced and fertilized, are developed embiyoes, which, in their early state, are extremely unlike the form they are ultimately to attain, and which possess a power of free locomotion, even in the species most completely destitute of it in the adult condition. These embiyoes, after swimming through the water for a time, so as to disperse themselves to wide distances from the parent and from each other, become stationary, attach themselves, and go through a series of phases of development, which end in their assumption of the perfect form. The varieties in the methods according to which the extension by gemmation takes place in Fir,. 117. A B Internal anatomy of Ascidia ( Cynthia ) microcosmus. — At a, the external tunic has been removed, and the internal opened in such a manner as to display the alimentary canal: a, mus- cular tunic : b , oral orifice; c, cesophageal orifice at the bottom of the branchial chamber: d, stomach and hepatic orifices: c. intestine; /, its termination in the cloaca; g, valves dividing the cloaca; h, anal orifice; i, orifice of the generative canal;; j, glandular organ supposed to be a testicle; j, branchial membrane: k, liver. — At b, the branchial membrane has been removed to show the arrangement of the thoracic chamber and its contained organs; a , external integument : b, oral orifice; c, oral tentacula; d , d, portions of the branchial membrane turned back; e, ven- tral furrow: /. dorsal furrow, with gangliform tubercles; g, cloaca; h, oesophageal orifice; i, termination of intestine; j, orifice of generative canal; k, liver; l, portion of the thoracic cham- ber lodging the organs of generation; m , m , channels for the passage of water to the cloaca; n, portion of cloaca on which are seen the fibres of the muscular tunic. the different subdivisions of the class, will be presently more particularly explained. — The nervo-muscular apparatus in this class seems to present its simplest possible form • the nervous portion of it consisting of but a single or double ganglion (Fig. 117, B,/), connected by cords of commu- nication with the sensoiy tentacula that guard the entrance of the pharyngeal sac, and with the muscular tunic which envelopes it ; whilst the muscular system consists only of the general envelope, composed of fibres crossing in various directions, and of the sphincters formed by the circular disposition of these fibres around the orifices (Fig. 117, a a). Its functions appear to be limited to the forcible ejection of the contents of the branchial sac through one or both orifices ; the contraction of the muscular tunic being excited either by the stimulus of the contact of some hard body with the external integument, or by an impression made upon the oral tentacula, or by distension of the respiratory sac. It is only by this ejection of water, that the nervo-muscular apparatus takes a share in the general locomotion of the body of the free floating Salp;e ; and the movement, being simply respiratory, cannot be regarded as dis- GENERAL VIEW OF THE ANIMAL KINGDOM. TUNICATA. even of consciousness, still less Fig. 118 . tinctly indicative volition. 299 a. The Tunicata most nearly allied to Bryozoa, are those which are designated Compound Ascidiaus or Botryllidce; being constructed upon nearly the same plan with the ‘solitary’ Ascidians, of which some account has already been given ; but presenting themselves in composite masses, each made up of numerous animals which appear to have been developed from a single individual by the process of gemmation. Of these we have a characteristic example in Amaroiu- cium 'proliferum ; of which the form of the composite mass is shown in Fig. 118, whilst the anatomy of a single individual is displayed in Fig. 119. The clus- ters of these animals present themselves on rocks, sea-weeds, &c., very commonly between the tide- marks. They appear almost completely inanimate, exhibiting no very obvious movements when irritated; but if they be placed when fresh in sea-w r ater, a slight pouting of the orifices will soon be perceptible, and a constant and energetic series of currents will Compound mass of Amaroucium proliferum. be found to enter by one set and to be ejected by the other, indicating that all the machinery of active life is going on within these apathetic bodies. In the tribe of Polyclinicins, to which this genus belongs, the body is elongated, and may be divided into three regions, the thorax (Fig. 119, a) which is chiefly oc- cupied by the respiratory sac, the abdomen (b) which contains the digestive apparatus, and the post-abdo- men (c) in which the heart and generative organs are lodged. At the summit of the thorax is seen the oral orifice c, which leads to the branchial sac e; this is perforated by an immense number of slits, which allow part of the water to pass into the space between the branchial sac and the muscular mantle, where it is especially collected in the thoracic sinus/! At k is seen the oesophagus, which is continuous with the lower part of the pharyngeal cavity ; this leads to the stomach l, which is surrounded by biliary tubuli ; and from this passes off 1 the intestine to, which terminates at n in the cloaca. The long post- abdomen is principally occupied by the large ova- rium j h which contains ova in various stages of de- velopment. These, when matured and set free, find 30-j as an evidence of Fig. 119 . Anatomy of Amaroucium proliferum : — a, thorax; b, abdomen; c, post-abdomen : — c, oral orifice ; e, branchial sac ; f thoracic sinus ; i, anal orifice; projection over- hanging it ; j, nervous gan- glion ; k, oesophagus; l, sto mach surrounded by biliary tubuli; m, intestine; ^ter- mination of intestine in cloaca; o, heart; o', pericar- dium ; p, ovarium ; p', egg ready to escape; q, testis; r, spermatic canal; r' , termi- nation of this canal in the cloaca. X 30G GENERAL PHYSIOLOGY. their way into the cloaca ; where two large ova are seen (one marked p>', and the other immediately below it), waiting for expulsion. In this posi- tion they receive the fertilizing influence from the testis cp which discharges its products by the long spermatic canal r, which opens into the cloaca at r'. At the very bottom of the post-abdomen, we find the heart o, enclosed in its pericardium o '. — In the tribe we are now considering, a number of such animals are embedded together in a sort of gelatinous mass, and covered with an integument common to them all. We are probably to regard this common integument as representing the ‘ tunic ’ or ‘ test ’ of the solitary Ascidians, since the individuals of the cluster are destitute of it ; and both in the compound and solitary species, we find the gelatinous matter intervening between the test and the muscular tunic or proper ‘ mantle,’ to contain cellulose, which is, however, probably derived imme- diately from the food of these animals, and not generated by them (§ 263). The mode in which new individuals are developed in this mass, is by the extension of ‘ stolons ’ or creeping stems from the bases of those previously existing ; and from each of these stolons several buds may be put forth (as in Fig. 121 a, tt ), every one of which may be evolved into the likeness of the stock from which it proceeded, and may in its turn increase and multiply after the same fashion. A communication between the circulating systems of the different individuals is kept up, through their connecting stems, during the whole of life ; and thus their relation- ship to each other is somewhat like that of the several polypes on the polypidom of a Campanularia (§ 290). — In the family of Didemnians, the post-abdomen is absent, the heart and generative apparatus being placed by the side of the intestine in the abdominal portion of the body. The individuals are frequently arranged in star-shaped clusters, the anal orifices being all directed towards a common vent which occupies the Botryllus violaceus : — A, cluster on the surface of a Fucus: Cavities, all the 01 guns beil! r — B, portion of the same enlarged. bl'OUght together in 01)0, which must be considered as thoracic. In this respect, there is an evident approximation towards the solitary species. — The approximation is still closer, however, in the ‘ social ’ Ascidians, or Clavelinidce ; in which tbe general plan of structure is nearly the same, but the individuals are simply connected by their stolons, instead of being included in a coninn >n A Fig. 120. centre. — This is still more remarkably the case, how- ever, with the family of Bo- try Ilians, whose beautiful stellate gelatinous incrusta- tions are extremely common upon sea-weeds and sub- merged rocks (Fig. 120). The anatomy of these ani- mals, as will be seen from the accompanying figures (Fig. 121, a, b), is very simi- lar to that of the Amarou- cium already described : with this exception, that the body exhibits no distinction of GENERAL VIEW OF THE ANIMAL KINGDOM.— TUNICATA. 307 investment, so that their relation to each other is very nearly the same as that of the individuals of a Laguncula (Fig. 114), the chief difference being that a regular circulation takes place through the stolon in the former case, such as has no existence in the latter. — Between the fore- going and the ‘solitary’ Ascidians, or A scidiadce, there are no very important structural differences ; save that in the latter, the two sets of sexual organs are always in distinct individuals, instead of being combined in the same. In these, how- ever, we entirely lose sight of the power of gemmation ; the only means of propagation amongst them being- through the true ge- nerative process, ex- cept in one or two instances (as Cynthia aggregata), in which the power of putting forth buds from the root - fibres appears still to be retained, although the general plan of conformation is that of the solitary Onp nf flip ora * orifice; e, branchial sac ;/, thoracic sinus ; h, cloaca ; i, anal ori~ optJOItJb. Ulie 01 me fi ce; i' } projection overhanging it ; k, oesophagus ; l, stomach : m, first chief Structural differ- portion of the intestine, or duodenum; in', second portion of the intes- , , tine, or chylific ventricle ; in", third portion of the intestine, or rectum ; eUCeS exhibited. by n, termination of the intestine in the cloaca; o, heart; q, testis; r, fViot'o 1c in +1-.^ Ki’on spermatic canal , t, root-like appendage ; t', reproductive buds ; x, glan- mese ism me oran- d \ llar mass (Uver?) . ^ its excretory duct, dual or pharyngeal sac, which is often folded into longitudinal plaits (Fig. 116 b), so as to increase its extent of surface, thus in some degree prefiguring the branchial lamellae of Conchifera. The muscular tunic, too, seems to be more employed in the ordinary acts of respiration, than it is in the ‘ compound ’ Ascidians ; for whilst in the latter the respiratory currents are maintained by ciliary action alone, there appeal’s in some of the solitary species to be a regular alternate movement of expiration and inspiration ; the former, which is sudden and energetic, being produced by the contraction of the muscular sac ; whilst the latter, which is more gradual, is apparently dependent in part upon the elasticity of the external tunic, which tends to return to the dilated state, and in part upon the distending influence of the ciliary current. In both cases, however, a part of the water that is introduced through the oral orifice escapes through the slits in the branchial membrane into the surround- ing space, from which it is conducted by special channels (Fig. 117 B, m m) to the cloaca ; whilst another portion is driven through the Fig. 121. Anatomy of Botrylloides rotifera ; a, front view; b, side view: — c> 308 GENERAL PHYSIOLOGY. alimentary canal by the cilia with which it is lined, and reaches the cloaca through the anal orifice of the intestinal tube. From observation of some of the microscopic species, it would appear that the ciliary action is specially arranged in such a manner as to drive down all solid pairicles towards the oesophageal orifice, allowing liquids only to pass through the branchial fissures. 299 b. The animals composing the other principal division of the Tunicata, that of Salpidce, differ from the Ascidians as well in their habits as in their structure. They are not attached to solid bodies, but habitually swim through the waters of the ocean, sometimes singly, some- times in chains or clusters ; their movement being due to the respiratory currents, which are in great part maintained by the alternate contraction and relaxation of the pharyngeal sac. In form (Fig. 122), they resemble short but rather wide tubes, with an opening at each end ; the tube is composed of a delicate transparent tunic, lined by the muscular coat, which here forms bands adherent to the tunic and running across the body. The oral orifice is wide, and unprovided with tentacula, but it is Fig. 1 22. Salpa maxima; a, oral orifice; b, vent ; c, nucleus, composed of the stomach, liver, &*c. ; d. branchial lamina ; e, the heart, from which proceeds the longitudinal trunk /, sending trans- verse branches across the body ; g g, projecting parts of the external tunic, serving to unite the different individuals into a chain. furnished with a valve that permits the free ingress of water whilst it prevents its return ; the vent ( b ) is narrower and somewhat conical, and the stream that issues from it impinges against the surrounding water, with sufficient force to propel the body in the opposite direction. The pharyn- geal sac, as a whole, does not appear to be subservient to the respiratory process ; but a more special organ is here developed, consisting of a double riband-like fold of its wall, minutely furnished with blood-vessels, and extending through the entire length of the body (d). This distinctly prefigures the branchial apparatus of the Lamellibranchiata (§ 301). The viscera are collected into a single mass at the posterior extremity of the body (c) ; and this mass or nucleus is usually very conspicuous, owing to the brilliant orange, brown, or reddish hue of the liver, of which the external part of it is composed, and to the transparency of the investing membranes. — The Salpa; are met with in two states ; the ‘ solitary.' and the ‘aggregated.’ The latter are simply adherent to each other by little suckers, instead of being organically united like the Compound Asci- dians ; this adhesion is so strong in some species, that it is easier to tear the bodies of the animals, than to separate them from each other : in other species, however, it is less powerful, so that when a mass is placed in a vessel of water, the sides of which are smartly struck, the individuals GENERAL VIEW OF THE ANIMAL KINGDOM.— TUNICATA. 309 fall asunder. The ‘ aggregate ’ and the ‘ solitary ’ Salpse, as appears from the researches of Chamisso and Krohn, are related to each other in a most remarkable manner ; for each solitary Salpa may give origin to an aggregate chain or cluster, not by the ordinary process of external gemmation (as in the Compound Ascidians), but by the formation of an internal stolon, which seems to take the place of the true ovarium, and to produce chains or clusters of buds ; these become detached from the stolon, but remain adherent to each other j and in this condition they swim forth from the cavity of the parent, and maintain an independent existence in the aggregate state. Each of these ‘ aggregate ’ Salpse pos- sesses true sexual organs ; the ovaria and testes, however, are not united in the same individuals, and are not even present in the different individuals of the same chain ; but the ova produced by one set are fertilized by the spermatic fluid of another, and from each of these fertile ova a ‘ solitary’ Salpa is produced, which in its turn developes another chain of aggre- gate individuals. The ‘solitary’ and ‘aggregate’ forms thus alternate in continuous succession ; although they differ so much from each other, as to have been regarded as distinct species until their true relationship was known. There are many forms, however, of both ; but it would appear that every ‘solitary’ species has a corresponding ‘aggregate’ form, and vice versa* The differences in conformation between the aggre- gate and the solitary individuals of the same species, have reference chiefly to their adhesion to one another. — Among the ‘ aggregate ’ Salpse, there is one curious group, that of Pyrosomidce, which seems to unite this tribe with the Botryllidce. Each composite mass is a hollow cylinder, closed at one end ; and this cylinder may be regarded as consisting of a pile of star-shaped clusters, resembling those of Botrylli (Fig. 120, b) ; the oral orifice of each individual being on the outer margin, while its vent opens into the central channel of the cylinder. As this is closed at one end, the water forced into it by the respiratory currents of the component individuals is obliged to issue from the other ; and the continual stream which is thus ejected causes the entire mass to move in the opposite direc- tion. — It is a curious fact, that Sal'pce are sometimes found making their w T ay through the sea by the rhythmical contractions of their muscular sac, after having been deprived of their nuclei by birds or fishes ; but whether, like the Holothuriada (§ 296 e), they can in time regenerate the important organs they have thus lost (those of digestion, circulation, and reproduction), has not yet been ascertained. 299 c. In addition to the connection which is so obviously established by the Bryozoa between Compound Ascidians and Zoophytes, we find the * This method of propagation is one of the most characteristic examples of what has been termed the “alternation of generations in which, (it is affirmed) generation A ( e.g . a ‘ soli- tary’ Salpa) produces generation B, a being of a different form (e. g. an ‘ aggregate ’ Salpa) ; and the offspring of B resembles generation a; so that each individual is unlike its own immediate parents and offspring, but resembles its grandparents and grandchildren. In this mode of stating the doctrine, however, the fact is too much lost sight of, that the two modes of reproduction alternate, as well as the two forms produced. In fact, the real nature of the process is essentially the same as that by which the clusters of compound Ascidians are produced; the only difference being that the ‘stolon’ projects into the cavity of the body, instead of being prolonged from its exterior (very much as if the finger of a glove were turned inwards so as to project into the cavity of the hand), and that the gemmae formed from it spontaneously detach themselves, instead of remaining in connection with the parent. This subject, however, will be more fully considered hereafter (chap, xviii,). 310 GENERAL PHYSIOLOGY. Tunicata related to the Echinodermata through the medium of the very curious genus Pelonaia ; which in its general form, and in some points of its structure, exhibits an obvious approximation to the Sipunculida (§ 296 /) ; whilst in other respects there is (as in that group) a decided approximation to the Annulose division, more especially in the bi-lateral symmetry of the organs, and the transverse wrinkling of the external tunic.* 300. From the Tunicata we pass on to the Mollusca inhabiting bivalve shells, which are sometimes distinguished as Conchifera. These, like the preceding, are acephalous, or headless ; that is, the mouth is not situated upon a prominent part of the body, nor is it assisted in its choice of food by organs of special sensation in its neighbourhood. In then’ general organisation, the Conchifera present a certain degree of advance upon the Tunicata ; but this is not very strongly marked ; and the principal dif- ferences in structure which we shall encounter, have reference more to the general plan of conformation, than to the comparative elevation of the former in the scale. This plan presents itself under two very dissimilar aspects, the Conchiferous Mollusca being capable of arrangement, accord- ing to well-marked peculiarities of structure, into two subdivisions, which are perhaps as dissimilar to each other as either is to the Tunicata, and which should rank, therefore, as distinct classes.— The one which will be first noticed, as being on the whole of inferior organisation to the other, is that of Brachiopoda ; which receives its name from the two long arms, fringed with filaments, that spring from either side of the mouth in most of the animals composing the class, and occupy a considerable paid of the cavity of the shell (Fig. 123, a). These arms, however, do not appear to be instruments of prehension ; for in most of the existing Brachiopoda Fig. 123. a, Terebratula (Atrypa) psittacea, showing the arms; b, Perforated valve of Terebratuhi australis ; c, Carriage-spring framework of Terebratula australis. they are incapable of being extended, being attached along them entire length to a very curious and delicate framework, composed of shelly sub- stance, -which is attached to one of the valves (c). Nor do they appear to be respiratory organs; for a careful examination of their structure shows that the blood-vessels are not distributed to them in a degree that would * See Forbes and Goodsir, in Edinb. New Philos. Journ. vol. x.xxi. p. 29. GENERAL VIEW OF THE ANIMAL KINGDOM. JBRACHIOPODA. 311 warrant our attributing to them any direct participation in this function. It is probable that their special purpose is to create currents of water, by the agency of the cilia that clothe their surfaces, which shall convey food to the mouth, and shall at the same time aerate the blood, which is trans- mitted for this purpose to the extended surface of the mantle. Each valve of the shell is lined by half of the mantle, or general investment of the body ; and the two halves are completely unattached, except near the hinge at which the two valves meet, so that, by this splitting-open of then containing sac, the viscera that lie between them are freely exposed to the contact of the water, which enters the cavity of the shell. Of this cavity, however, the viscera occupy a very small proportion ; being limited to the central part of it, so as only to fill (in the Terebratulse) the space surrounded by the ‘ carriage-spring ’ framework of the arms, which is closed-in by a firm membrane that fills up the space between its loops. The entrance to the stomach thus lies buried deeply between the lobes of the mantle ; the digestive apparatus is itself constructed nearly upon the plan of that of the Ascidia (§ 299) ; and, as in that animal, the short intes- tine terminates not far from the mouth. The principal distribution of blood-vessels is upon the lobes of the mantle, which apparently perform the functions of a respiratory or aerating organ ; and the heart or organ of impulsion is here double, consisting of a dilatation of the trunk that con- nects the visceral vessels with those of the mantle, on each side of the visceral mass. In this, as in the conformation of the shell, arms, and various other parts, there is a remarkable manifestation of bi-lateral sym- metry. — We lose, in this group, all trace of the power of gemmation ; and the continuance and multiplication of the race is altogether effected by proper sexual process. The ovaries in the female, and the spermatic organs in the male, are spread out in an arborescent form over the surface of the mantle, accompanying its great veins. — All the Brachiopoda are fixed to one spot, except during the earliest period of their development ; but the mode in which they are attached presents a considerable diversity. In the Terebratula, the shell, which is chiefly composed of carbonate of lime,* is affixed to solid bodies by means of a tendinous peduncle, which is attached to the valve that bears the framework, and passes through a hole in the beak-shaped prolongation of the other valve (Fig. 123, b). These valves fit together with somewhat of a hinge-like arrangement, and are further connected by a complex muscular apparatus, which draws them closely to one another. In many species, the ‘ carriage-spring ’ framework is so constructed, that the valves cannot be completely closed together -without somewhat compressing it ; to this compression its elasti- city allows it to yield without breaking ; but in this state it will tend to force the valves again apart; and one of its purposes would thus seem to be, to keep the shell usually somewhat open. This, in the living animal, must be its ordinary condition ; currents of water, created by ciliary action, being thus continually drawn through the mouth, and transmitted * It is remarkable that in all the true Terebratulae now existing, the shell is perforated by a multitude of minute canals which extend through its thickness, these canals giving lodg- ment to glandular coeca, the use of which is at present unknown : in many of the extinct species, however, such perforations did not exist; but these species constitute a group, which is distinguished from the perforated species in several other particulars. See the Author’s “ Reports ” to the British Association for 1844 and 1847. 312 GENERAL PHYSIOLOGY. over the surface of the mantle. But when the animal is alarmed, the valves are probably drawn together, as in other Bivalves ; this action being due to an impression which is made through its sensory organs upon its ner- vous system, and thence reflected upon the muscles. A distinct nervous system has been found, in the form of a collar round the oesophagus, with slight ganglionic enlargements : but no special organs of sense have been yet detected. — In Lingula, the shell is of homy texture, and neither of the valves is perforated ; but the peduncle, which is longer, is attached to both alike, and serves to hold them together. There is no framework for the arms, which are unattached except at their bases, and may be extended beyond the shell ; and the animal further departs from the ordi- nary type, in having the mantle raised into fringes, which may be consi- dered as rudiments of the special branchiae of the Lamellibranchiata. It is probable, from the circumstances under which the Lingulae are found, that they possess more locomotive power than the Terebratulae ; the former live near the surface, whilst the latter are inhabitants of very deep water. — The Orbicula, like the Lingula, has a homy shell ; this is at- tached, however, not by a long peduncle, hut by a short muscle which is affixed to one valve and passes through a fissure in the other.— These are the principal forms of existing Brachiopoda ; and the variety of con- formation which they present is not considerable. There is ample evi- dence, however, that in fomier periods of the Earth’s histoiy, the class contained a much larger number of more diversified types than it does now ; and that these constituted a far larger proportion of the Bivalve Mol- lusca, than do the Brachiopoda of the existing epoch. 301. The class of Lamellibranchiata is chiefly distinguished from the preceding by the presence of special respiratory organs, or branch ice, which, in nearly every case, form four riband-shaped lamelke, two of them attached to each lobe of the mantle, and lying within the marginal portion of the shell. The variety of forms which we encounter in this class is very considerable ; and the modes of life of the different animals which it includes are no less dissimilar. Some of them, as the Oyster, are fixed by the adhesion of then’ valves during all but the earliest period of life ; others, as the Pinna and the Mussel, are attached to solid bodies by a tendinous cord or byssus ; others, as the Pecten, float freely through the water, and are said to propel themselves by a flapping movement of then' valves : others, as the Cockle, can move by leaps over hard surfaces, by means of their fleshy foot ; others, as the My a, Solen, Ac., bore deeply into sand or mud, and live in the excavations they have formed ; and others, as the Teredo and Pholas, make their way into wood, or even into stone. In all but the small and peculiar group which may be considered to connect this class with the Tunicata, we find the bivalve shell forming the chief external feature of the structure. The valves are connected by a more or less per- fect hinge, which is often beset with ‘ teeth ’ that lock into one another : and this joint is sometimes very perfect, and so peculiar in its character, that, even when the shells are dry, it allows free motion of the valves without permitting them to be separated. In general, however, the re- tention of the valves in apposition to each other, is due to a ligament attached to the hinge in such a mftnner, that its elasticity keeps the valves somewhat apart, unless counteracted by the action of the muscle iu the interior of the shell, which draws them together. This is a very beautiful GENERAL VIEW OP THE ANIMAL KINGDOM. L AMELLI BRAN CHIA TA. 313 provision for the performance of the animal functions without difficulty or effort ; for when the creature is undisturbed, the ligament keeps the valves open ; but when danger is apprehended, or circumstances require it, the ‘adductor’ muscle contracts, overcomes the resistance of the hinge, and shuts the valves close, until they may be opened with safety. One of the earliest signs of the loss of vitality in Conchifera, is the unusually wide gaping of the shell ; this arises from the continuance of the elasticity of the ligament (which does not depart as long as its structure is un decom- posed), unbalanced by the vital contractility of the muscle. The valves are formed and increased by the calcification of the epithelial layer of the mantle, by which the shelly substance is generated (§ 197). There are usually two distinct layers in the shell ; the inner one being often more or less pearly or ‘ nacreous ’ in its character ; whilst the outer is usually duller in aspect, and contains a larger proportion of animal matter, in which the original cellular constitution is more distinctly recognised. The augmentation of the shell, in accordance with the growth of the contained animal, is effected by additions made to the previously-existing valves ; but this is done in a different manner for the two valves ; for whilst the outer layer is simply extended by a new growth proceeding from its margin, an entirely new inner layer is formed in the interior of the old one and pro- jecting beyond it. This is easily seen in shells, in which the successive laminse are not closely adherent, as in the common Oyster ; but it is not less true of those, in which the boundaries of the successive additions can- not be detected by an ordinary scrutiny.* It is not only for the augmen- tation of the shell, that new layers are thus formed ; for it would seem as if the animal were impelled by a necessity of its nature to contract its cavity, when this is too large for its body. Thus an Oyster kept without food will frequently expend its last energies in secreting a new pearly layer, at a distance from the old internal surface of the concave valve ; so as to reduce the cavity to the dimensions to which its body has shrunk during its fast. In certain species of Spondylus, the new layers are always formed at a considerable interval from the old ones ; so that each valve, when cut across, presents a tolerably regular chambered structure. In the Pholas, the valves have ‘ accessory pieces,’ constituting what is called a multivalve shell ; these accessory pieces may probably be regarded as a rudiment of the shelly tube of the Teredo. In the Teredo and its allies, the respiratory siphons, as well as the proper mantle, form shell on their surface ; and thus, in addition to the two valves, we find a long shelly tube, lining the whole burrow of the animal. In certain other animals formed on the same plan, the valves are scarcely discoverable, being- imbedded (as it were) in the substance of this tube, which thus seems itself to constitute the shell. — The position of the animal in the shell, as determined by the place of the mouth and anus, is not the same in the Lamellibranchiata, as in the Brachiopoda. In the latter, the two valves may be regarded as forming the back and belly of the animal (the per- forated valve of the Terebratula being the ventral, and the non-perforated the dorsal ) ; whilst the two halves of each valve correspond to the two sides of the animal, which, as already remarked, are usually symmetrical. But in the former, the two valves are the two sides of the animal, being some- See the Author’s “ Report ” to the British Association for 1847. 314 GENERAL PHYSIOLOGY. times symmetrical and sometimes not ; whilst each valve has an anterior extremity, towards which the mouth is directed (Fig. 119, o), and a pos- terior, in the neighbourhood of which is the anus (a) ; and it thus has also a dorsal margin, of which the hinge forms part, and a ventral margin, along the side opposite the hinge. 301 a. Each valve, in the ordinary Lamellibranchiata, is lined by one of the lobes or divisions of the mantle ; and these, in the Oyster and other genera which approach most nearly to Brachiopoda, are completely dis- united except near the hinge, so that the water has free access to the internal surfaces of the mantle, to the gills, and to all the viscera included between them. In the Mussel, however, the two lobes are adherent along the posterior margins of the valves, except where a slit is provided for the egress of the water, which is drawn in through the fissure that separates its lobes elsewhere. In the Cockle , again, the lobes of the mantle are adherent around the greater part of the margin of the shell ; a large fissure being left, however, for the passage of the fleshy foot, at the part opposite to the hinge ; and two openings being left, one for the ingress and the other for the egress of water, at the posterior margin. These last, in many other Bivalves, are prolonged into tubes or siphons (Fig. 122, y, z), which are sometimes of considerable length, especially when the animal makes for itself a deep excavation in sand, mud, wood, &c. ; that through which the water enters (y) is termed the oral siphon, whilst that through which it escapes (z) is termed the anal siphon. In the Solen we find the body elongated, and the mantle entirely closed except at the two extremities of the shell ; the opening for the foot, at the anterior extremity, being only just large enough to allow it to pass ; and no water being admitted to the interior, or passing out from it, except by the oral and anal siphons pro- longed from the posterior extremity. The inclosure of the animal is still more complete in some of the Cuvierian family of Inclusa ; and we are thus conducted hack into close proximity with the higher Tunicata. For if the membranous integument of a Salpa (whose branchial leaflets approach those of the Lamellibranchiata, § 299 b) were to form a layer of shell, and the two orifices of its mantle were in proximity (as in the Ascidians), we should have a structure resembling that of the Inclusa in the most important particulars ; the presence of a foot, in the latter, which is the strongest point of difference, not being essential, since many Lamelli- branchiata are destitute of it. — The general organisation of the Lamelli- branchiate bivalves will be understood from the accompanying represent- ation of the structure of the animal of J Lactra (Fig. 124). The mouth, or rather the entrance to the oesophagus (o), is btuied deep between the two lobes of the mantle, and looks towards the anterior extremity of the shell ; it is furnished with four long sensory tentacula, of which one is seen at p. The short oesophagus leads to the stomach (A), which has a long coecal appendage v. From the stomach passes off a convoluted intes- tine ( t ), which, after making several turns, runs towards the dorsal margin of the shell (e), and is continued along it to the posterior extremity, where it terminates in the anal orifice (a), which discharges itself into the anal siphon (z). The stomach and intestine are partly invested by the liver ( k ). which now assumes a very distinct form, and attains a considerable size. The rudiment of a kidney, also, is discoverable in the higher Bivalves. The heart (i) lies along the dorsal margin of the shell, and is frequently perforated GENERAL VIEW OF THE ANIMAL KINGDOM. LAMELLIBRANCH1ATA. 315 by the intestine ; a distinction now becomes apparent between the auricle or receiving cavity, to which the blood is transmitted from the branchial lamellae ( w ), and the ventricle or propelling cavity, from which it is sent forth to the general system ; and the auricle is not unfrequently double, a distinct cavity appertaining to the branchiae of each lobe of the mantle. Between the heart and the posterior extremity is seen the reproductive Fig. 124. Dorsal Margin. i, heart; A - , liver; l, anterior or oesophageal ganglia; m, anterior adductor muscle ; n, nervous filaments ; o, mouth ; p, one of the oral tentacula ; q x , mantle ; r, margin of the shell ; s, foot ; w, branchial lamellae ; y, oral siphon ; z, anal siphon. organ (d), which is an ovary in the female, and a spermatic gland in the male. In the Cyclas, the two sets of sexual organs are combined ■ but in other bivalves they are set apart in distinct individuals, so that the ova developed by one are fertilized by the spermatozoa set free from another. This fertilization must be usually effected, however, before the ova are sent forth from the body of the female ; for the early development of the embryo takes place in some instances, and probably in all, within the shell of the parent. — The muscular apparatus of the Lamellibranchiata is very simple. The muscle most constantly present is the adductor, by which the valves are drawn together ; this is sometimes single, but is more com- monly double, one portion of it (m) being situated near the anterior extremity, and the other (d) near the posterior extremity of the shell, on both valves of which its attachments are plainly distinguishable. But the greater number of this class have a fleshy tongue-like organ (s), termed the foot, which is capable of being put forth from between the ‘valves, and of being applied to a variety of purposes. In some of those in which it is least developed (as the Mussel), its principal office appears to be to form the byssus or cord of horny filaments by which the animal attaches itself to rocks, &c. But in others it is an instrument of active locomotion ; thus, 316 GENERAL PHYSIOLOGY. the common Cockle can take considerable leaps, by suddenly extending this organ, which was previously bent at an acute angle ; and the first specimen of Trigonia discovered by Mr. S. Stutchbury on the coast of New South Wales, having been placed in the stem of the boat from which he was dredging, leapt over the gunwale, a height of about six inches, into the sea. And in others, again, it is the instrument for excavating and perforating : although upon the mode in which it is used for these pur- poses, there is still some difference of opinion. Besides these muscles, most of the Bivalves furnished with long respiratory siphons possess an arrangement of muscular fibres in then - walls, by which they may be pro- jected or drawn in. In accordance with tliis advance in the specialisation of the motor apparatus, we find the nervous system presenting a greater complexity ; for in addition to the single or double branchial ganglion (c). situated near the posterior adductor muscle, and obviously corresponding with the solitary ganglion of the Tunicata, we find a pair of small ganglia (l) situated near the oesophagus, and specially connected with the sensory tentacula that surround its entrance ; from this proceed branches (n) to the anterior portion of the body, whilst a cord of communication (y) passes backwards to the posterior ganglion. In those Bivalves which have a foot, a third ganglionic centre is always to be found at or near its base, con- nected with the oesophageal ganglia. And -where the respiratory siphons are muscular, these muscles are supplied from a small ganglion of their own, situated at the point of junction of the tubes. The tentacula pro- longed from the mouth are the most constant organs of sense possessed by these animals ; but some of the more active among them, such as the Pecten, have red spots which appear to be ocelli or rudimentary eyes, disposed around the margin of their mantle ; and a rudimentary organ of hearing has also been detected in the substance of the anterior ganglion of Cyclas. — We thus find the Lamellibranchiata advancing, in several particulars, towards the higher type of the Mollusca ; but the predomi- nance of the organic over the animal life is still very strongly marked. Some of them acquire a very considerable size and weight ; thus the Pin na sometimes attains a length of four feet (and some of the extinct species appear to have been much larger), whilst the Tridacne (or Giant Clamp- shell) has been known to weigh 600 lbs. Those which are most bulky are the least active, their lives being passed almost entirely in the same situation, and scarcely any movement (save the ciliary action which sup- plies food to the stomach and water to the gills) being exhibited by them, except the closure of then valves which takes place when the animal is in any way irritated.* 302. The class of Gasteropoda, which receives its name from the situa- tion of the muscular disk on which the animal creeps, may be regarded as the type of the whole -Molluscous series ; for it presents in then most * It is by taking advantage of this action, that the natives of the East Indian Archipelago obtain the shells of the Tridacne. In their young state, the animal is anchored by a byssus; but as it approaches adult age, and the shell acquires considerably increased weight, the byssus disappears, and the groove in the shell is filled up by a solid growth. When it is thus lying free at a depth of ten or fifteen feet in the water, a long pole is pushed down between the opening of its valves, which then close upon it (just as the valves of an Oyster do upon the knife inserted between them) with so strong a hold, as to allow the massive shell to be lifted by the pole out of the water. GENERAL VIEW OF THE ANIMAL KINGDOM. GASTEROPODA. 317 distinct and complete form all its characteristic features, and is further removed than any other class from either of the remaining sub-kingdoms. Moreover, it contains by far the greatest number of species ; and these present a remarkable variety in their plan of conformation, whilst the grade of development is nearly the same in all. A large proportion of them possess a shell, within which the body can be entirely retracted ; and this shell, being formed in one piece, is said to be univalve. Its typical form may be considered to be a cone with a broad open base, as we see in the common Limpet ( Patella ) ; in the Pileopsis, the point of the cone is prolonged and somewhat turned to one side, so as to resemble a ‘ fool’s cap,’ thus presenting the rudiment of a convolution. The increase of this tendency to deflexion in the axis of the cone produces a regular spiral shell, such as that of the Planorbis, in which all the whorls or convolutions are on the same plane. But if these whorls do not revolve in the same plane, the axis of the cone then forms a helix instead of a simple spire, as is the case in the common Snail, and in the greater proportion of the shells of this class. From this form we may return to the straight cone by the Scalaria, in which the coils of the spire touch each other only by their ribs ; and by the Vermetus and Magilus, in which the commence- ment only of the shell presents a helical form, the remainder being pro- longed into a tube which is but slightly curved, thus approaching the Dentalium, the shell of which is an elongated cone, whose axis is almost a straight line. Where the whorls revolve around a vertical line, instead of remaining in the same plane, a sort of central pillar is formed, which is termed the columella ; this is frequently grooved at its lower part, for the passage of water to the respiratory organs. — The substance of univalve shells generally contains less animal matter than that of bivalves ; and we frequently find it extremely hard or porcellanous, exhibiting little indica- tion of its organic origin. There can be no doubt, however, that it is formed as a calcified epithelium from the surface of the mantle, as in other cases. The size of the shell is augmented, in accordance witli the growth of the animal, by the extension of the margin or opening of the cone ; this, of comse, at the same time enlarges its diameter, and increases its length ; so that as, in a helical shell, the number of whorls increases with age, so does also the size of the successive whorls. As in the Bivalves, however, with every such addition, a new shelly layer is formed within the whole interior of the previously-existing shell ; but this layer is so thin, except near the mouth, that it cannot be detected except by microscopic examina- tion. — When the full growth of the individual has been attained, a thickened rim or lip is formed around the aperture, which serves to strengthen the whole fabric ; but in several marine species, which attain to large dimensions, such a lip is formed after each extension ; so that the shell has a ribbed appearance, each rib having been at one time the boundary of the aperture. This margin is sometimes fringed with spines formed from prolongations of the mantle, as in the Mur ex ; and the dis- similar number of these has led to the establishment of many distinct species, which, when the habits of the animal came to be better known, have proved to be but different forms of the same. For it now appears that the animal has not only the power of forming new spines, but of removing old ones, especially such as would interfere with the continued growth of the shell. How the absorption of shelly matter from their 318 GENERAL PHYSIOLOGY. base, which causes them to drop off, is effected, is still unexplained ; various analogous phenomena may be witnessed among other species, portions of the shell first formed being wholly or partly removed. Sometimes the walls of the older portion are thinned for the purpose of lightening the shell, and the same object seems to be attained by other inhabitants of helical shells in a different manner ; these withdraw their bodies from the highest part of the cone, throw a partition across the cavity, and then allow the point (which, not being internally supported, is brittle, and ap- pears to have been purposely thinned) to be broken off, leaving the shell decollated as it is termed.* It must be borne in mind, however, that the changes thus effected in the shell are not the consequence of any interstitial absorption, such as takes place in the osseous structures of Vertebrata, — but result from the same kind of power of superficial absorption, as appears to be exercised by many Gasteropoda upon calcareous rocks, which they perforate for their habitations, as well as upon the shells of other species upon which they feed. — Several of the aquatic species of this class form not merely a spiral shell, but an accurately fitted cover to its mouth, so attached to the body, that, when the latter is entirely withdrawn, the oper- culum, as it is called, completely shuts it in. Sometimes this is homy, but not unfrequently calcareous ; and occasionally it bears so large a pro- portion to the shell, as almost to appear like a second valve, such as is characteristic of the Conchifera. Some of the land species also possess an operculum ; but in general they are destitute of it, and form during hybernation a temporary closure to the mouth of the shell by a viscid secretion, which becomes hard and includes a bubble of air ; behind this a second and even a third similar partition are occasionally found, as in the common Snail — The presence of a shell, however, is by no means uni- versal in this group ; as many Gasteropods are ‘ naked ’ or shell-less, and many more have shells too small to allow their bodies to be completely withdrawn into them. Of these we find some, especially among the terres- trial species, that are so closely allied to the proper testaceous Gastero- pods, that it is evident that the presence or absence of the shell is a feature by no means essential ; thus between the Slug and the Snail, the former a ‘naked,’ and the latter a ‘testaceous’ Mollusk, there is a very close alliance ; and the two extremes are connected by a very complete series of intermediate forms, many Slugs having small shells upon their tails, and many Snails having shells far too small to include their bodies. There is one marine order, however, that of Nudibranchiata (§ 30-1 c), in which the absence of the shell is a characteristic feattue, being connected with the extension of the respiratory apparatus over so huge a part of the external surface, that the investment of the body by a shell would ob- struct its functions. It is a very remarkable fact, however, that in their young state the Nudibranchiate, as well as (it is believed) all other marine Gasteropod Mollusca, possess a shell. — The most remarkable departure from the general type of conformation in the shell of this group, is pre- sented by the Chiton; the animal of which is more closely allied to the Limpet than to any other Gasteropod ; while the shell, instead of being a simple cone, is composed of a number of pieces jointed to each other in such a manner as to present a remarkable resemblance to the covering of * Mr. S. Stutchbury has informed the Author, that he has seen the Bulimus forcibly strike the apex of its shell against a stone, as if for the purpose of decollating itself. GENERAL VIEW OF THE ANIMAL KINGDOM.— GASTEROPODA. 319 an Articulated animal (Fig. 125, b). This resemblance is not confined to the shell ; for its different portions are connected together by a complex muscular apparatus, which enables the animal to move them one upon another in such a manner as to roll itself up ; and there are other points in the internal structure of the ani- mal, such as the symmetrical arrange- ment of the generative apparatus, and the resemblance of the heart to the ' dorsal vessel ’ of an Annelide rather than to the heart of an ordinary Gasteropod, which induce the Philo- sophic Naturalist to regard the Chiton as a connecting link between the Mol- luscous and the Articulated series.* It is worthy of note that in the Chi- tonellus (Fig. 125, a) we find the shell not sufficiently developed to cover the animal, yet its several ru- dimentary pieces are arranged at regular intervals along the back, precisely as in the Chiton, thus strongly marking the tendency to seg- mentation in the body. 303. Excluding the Chitons, for the reasons just stated, the class of Gasteropoda may be characterised as a whole by the presence of a head, on which is situated the mouth, with organs of sense, more or less numerous ; by the possession of a locomotive disk, formed by a fleshy expansion from the posterior part of the head ; by the want of symmetry in the viscera generally, and especially in the generative apparatus ; and by the tendency to a spiral disposition of the organs, which almost invariably manifests itself in the embryo, and frequently continues through the whole period of life. The class thus defined must be sepa- rated, however, into two sub-classes ; namely, the True Gasteropoda, in which the fleshy disk is broad and flattened horizontally, lying beneath the visceral mass, and serving to enable the animal to creep over solid surfaces, and in which the abdominal region of the body is fully de- veloped ; and the Heteropoda, in which the locomotive organ is flattened into a vertical fin (Fig. 129), whereby the animal swims freely through the water but cannot crawl, and in which the abdominal portion of the body is rudimentary, whilst the cephalo-thoracic is enormously developed. The latter must be considered as an aberrant group ; and the species which it contains are few in comparison with those of the true Gaste- ropods. — Putting these on one side for the present, the sub-class of True Gasteropoda may next be divided into the Branchiferous and Pul- monated sections ; the former including all that breathe by gills and inhabit the water ; whilst the latter comprehends those which breathe air by means of a pulmonary sac, and which are for the most part inhabitants of land. There are other important differences between these two sections, which will be noticed hereafter. — In the Branchiferous Gas- * See the admirable Memoir on the Classification of the Gasteropoda, by Prof. Milne Edwards, in the Ann. des Sci. Nat., Ser. hi., Zool. vol. ix. p. 102. Fig. 1 25. a a a, Chitonellus . — b, Chiton. 320 GENERAL PHYSIOLOGY. teropods, moreover, we find two dissimilar types of structure, which are designated by M. Milne-Edwards,* in accordance with the position of the gills, as Prosobranchiata, and Opistobranch iata. In the former, of which we may take Paludina vivvpara (Fig. 128) as an example, the body is always protected by a shell of sufficient dimensions to include the whole of it ; the mantle is prolonged forwards, and forms behind the head an arched chamber, in which are found the orifices of the excretory canals, and in which the gills are usually lodged. The gills (Fig. 128, g) are composed of a series of straight and simple lamellae, inserted along a vascular stem, and having a pectinated arrangement (that is, resembling the teeth of a comb) ; they are usually situated in front of the heart, so that the blood flows backwards from them towards the heart ; and even when they are prolonged to the hinder part of the body, the course of the vessels by which the blood is conveyed from them to the heart is such that it flows into that organ from the front. Further, these animals are dioecious, the male and female organs of generation not being possessed by the same individuals. In the Opistobranchiata (Fig. 127), on the other hand, the shell, although existing in the larva, becomes rudimentary, or even disappears altogether, in the adult ; the gills are usually arborescent in their form (Fig. 126), and are never included within any special cavity, being always more or less obvi- ously displayed on the back or hinder paid of the animal ; the blood flows from them forwards to the heart ; and the reproduc- tive apparatus is hermaphro- dite, the male and female organs being combined in the same in- dividuals. — The general struc- ture of the Pulmonated Gaste- ropods most nearly corresponds with that of the Prosobranchiata (Fig. 128), with the exception that the blood is not transmitted for aeration into a pectinated branchial apparatus immersed in water, but is distributed over the walls of an internal cavity to which air is admitted. The pre- sence or absence of the shell in this group does not seem to be a matter of as much importance as it is among the branchiferous Gasteropods ; as we find the naked Pulmonata corresponding in all essential respects with the testaceous. 304. We shall now take a general survey of the structure and arrange- ment of the principal organs of the body, in these different groups, commencing with the Gasteropoda proper. The mouth is almost invari- ably furnished with instruments for cutting or rasping solid substances these animals not being dependent, like the Acephalous tribes, upon the minute particles brought by their ciliary currents, but being able to go in search of their own food. This they separate, in many cases, by means of a pair of horny jaws, fitted for the division of vegetable or animal sub- stances, and worked by a muscular apparatus of some complexity (Fig. 128, a). In the Patella (Limpet), however, the food is obtained by means of a tongue, which, when extended, exceeds the body in length, * Loc. cit. Fig. 126. Doris Johnstoni, showing the tuft of external gills. GENERAL VIEW OF THE ANIMAL KINGDOM. GASTEROPODA. 321 and which is furnished with sharp, recurved, horny teeth, that render it a very efficient instrument for rasping the sea-weeds which constitute the nourishment of this animal. In the Buccinum (Whelk) and other Fig. 127. Aplysia laid open, to show the arrangement of the viscera: — «, upper part of the oesophagus ; b, penis ; c, c, salivary glands ; d, superior or cephalic ganglion ; e, e, inferior or subcesophageal ganglia; f, entrance of the oesophagus into g, g, the first stomach or crop ; h , the third or true digestive stomach ; i, the second stomach or gizzard ; k, intestine ; l , l, l , liver ; m , posterior or branchial ganglion ; n, aorta; o, hepatic artery ; p , ventricle of heart ; q, auricle ; r, s, branchiae ; t, testis; u, lower part of intestine; v, ovary; w, anus. carnivorous species, again, a tongue of a very similar description, but shorter, is inclosed within a proboscis-like organ (Fig. 128, p), which is capable of considerable elongation, or of being entirely retracted within Y 322 GENERAL PHYSIOLOGY. the cavity of the body, and which is vertically cleft at the extremity into two lips, the inner surface of each being covered with recurved spines ; by the actions of this apparatus, the Buccinum can perforate the shells of other Mollusks, and extract the soft parts of the animals which they contain.- — Behind the mouth there is usually found a pair of salivary glands (Fig. 127, c c, Fig. 128, v v), between which passes down the oesophagus (Fig. 127, /, Fig. 128, q) towards the stomach. In the Apiysia and some other Gasteropods, the oesophagus first dilates into a sort of crop or ingluvies (Fig. 127, g g), in which the food is macerated with the salivary secretion before it passes into the true stomach ; froa this it is passed onwards, to be subjected to the triturating action of a gizzard (i), the interior of which is beset with firm horny teeth, or with hard calcareous plates, fitted for rasping or crushing ; and it is then transmitted to the true stomach ( h ), in which the actual process of digestion takes place. But in the Paludina, as in the carnivorous tribes generally, this Fic;. 128. Paludina vivipara, withdrawn from its shell and laid open ; — a, foot; b , operculum ; c, one of the tentacula with its ocellus ; d , siphon ; /, border of the mantle ; g, pectinated branchiae: h, li . the oviduct dilated for the retention of the ova ; h ", portion of it situated within the spire of the shell ; i, termination of the intestine ; l, canal for the urinary (?) secretion ; n. heart ; o, liver ; p. proboscis ; q , q', oesophagus ; r, stomach ; s, s', s", intestine, lying at t within the branchial cavity ; u> u , cephalic ganglia ; v, v, salivary glands : the principal muscular nerve. complex apparatus is wanting ; and the stomach dilates at once into a stomach of simple form (Fig. 128, r), which suffices to perform the di- gestive operation of the whole. The intestinal tube (Fig. 128, s, s’, s ’ j Fig. 127, k, u), which is most prolonged in the herbivorous species, usually makes several convolutions in the substance of the liver (which orgfin here attains a high degree of development, Fig. 127, /. I, l. Fig. l-' s o ) ; and at last generally turns upon itself, so as to open behind the GENERAL VIEW OF THE ANIMAL KINGDOM. GASTEROPODA. 323 head (Fig. 128, i), although in some of the naked Gasteropods, it termi- nates near the posterior part of the body (Fig. 127, to). The heart (Fig. 128, n ) is more muscular than in the Acephalous Mollusca, and is always distinctly divided into an auricle and a ventricle (Fig. 127, n, p) ; the auricle, which receives the blood from the respiratory organs, being sometimes double. From the ventricle proceeds the aorta (Fig. 127, o), which transmits the blood to the system in general ; the fluid, in its return thence, however, is not confined within distinct vessels, but meanders through sinuses or passages excavated in the tissues ; and through these it reaches the branchial apparatus (Fig. 127, r, s, Fig. 128, g) or the pulmonic cavity, whence it is transmitted back to the heart through the branchio-cardiac canals. Besides the liver and the salivary glands, we find other glandular organs in this class. Tlius the rudiment of a pancreas distinctly presents itself, in some instances, in connection with the stomach ; and a follicular gland is frequently to be found discharging a mucous secretion, which contains uric acid, and is therefore to be considered as urine, through a duct that opens near the anus (Fig. 128, l). Secretions of colouring matter, as well as of other special products, are effected by distinct follicular glands in particular cases. Thus the whole apparatus of organic life displays a considerable advance upon the type presented in the Acephala. 304a. A similar advance is manifested in the apparatus of Animal life ; the Gasteropods being generally possessed of various organs of spe- cial sense, and of considerable locomotive power, although their actions are slow and apparently feeble. The number of ganglionic centres is increased, in accordance with the number of organs to whose functions they minister; and there is a greater tendency to a concentration of them in the neighbourhood of the mouth, so as to form a sort of collar around the oesophagus. Of this collar, the upper and lateral part, swhich some- times forms a single ganglion (Fig. 127, d) but is more commonly double (Fig. 128, u, u), is composed of the ganglionic centres of the organs of special and general sense, which are situated in the head ; whilst the lower (Fig. 127, e, e) is usually formed by the ganglionic centres of the mantle and foot, which are sometimes combined, sometimes distinct. Besides these, there is a separate ganglionic centre for the respiratory apparatus (Fig. 127, m), which is always found in its neighbourhood, and whose position, therefore, varies with that of the gills ; and there is also gene- rally one that is connected with the muscles of mastication and deglu- tition. Nearly all the Gasteropoda possess one or two pairs of sensory tentacula ; and where, as in the Snail, two pairs are present, one of them (the smaller) usually bears the eyes, which in other instances are situated at the base of the principal pair. These eyes never exceed tw T o in number, and they are generally well developed ; but in some species they are minute, and in a few are entirely wanting. A rudimentary organ of hearing exists, imbedded in the posterior part of the oesophageal nervous collar ; and there is reason to believe that the terrestrial Gasteropods (to say the least) enjoy the sense of smell, although no distinct organ for this purpose can he detected. The tongue, where it exists, is a mere mechanical instru- ! ment, and does not appear to minister to the sense of taste ; but this may have its seat in the soft membranes of the mouth and pharynx. The softness of the general surface of the body might naturally lead to the v 2 ;24 GENERAL PHYSIOLOGY. supposition that it is a delicate organ of touch ; but the fact that the ter- restrial Gasteropoda show uo signs of pain w T hen their integument is being devoured by their carnivorous enemies, would seem to prove that its sen- sibility must be extremely low. It seems probable, however, both from the copious supply of nerves and blood-vessels to the foot, as well from the consideration of the purposes to which that organ is applied, that it possesses more tactile sensibility, as well as more muscular power, than the remainder of the mantle, of one part of which it may be regarded as an augmented development. It is interesting to observe that a symme- trical arrangement prevails in the organs of sense, and in other parts which are specially concerned in the functions of animal life ; that sym- metry being usually deficient in the posterior part of the body, which simply encloses the apparatus of organic life, but extending also to it (so far as external form is concerned) when the locomotive powers are greatest, as is seen especially in the Nudibrancliiata (Fig. 126). 3046. Of the power of reproduction by gemmation, there is no known example in the Gasteropoda ; although we observe its traces in their capa- bility of repairing injuries, which extends to the regeneration of the head, tentacula, and eyes, when the cephalic ganglia have been left uninjured. The proper generative apparatus is usually developed largely. The male and female organs are combined in the same individuals in the Pulmouated Gasteropods and in the Opistobranehiata ; but the sexes are distinct in the Prosobranchiata. It is uncertain whether any of the hermaphrodite Gas- teropods are self-impregnating, like most of the inferior Mollusks ; but it is certain that the pulmouated tribes are not, the congress of two indivi- duals being requisite, each to fertilize the ova of the other. We find in this class a more elaborate provision for the accomplishment of this act, than we have hitherto seen ; namely, an intromittent organ, or penis (Fig. 127, b), frequently of considerable dimensions, which conveys the sper- matic fluid of the male organs into the immediate neighbourhood of the ova which it is to fecundate ; instead of this fluid being simply diffused through the surrounding water, and left to find its way to the ova by that medium. The ovarium usually occupies a considerable space in the pos- terior part of the body (Fig. 127, v) ; but the orifice of its oviduct is gene- rally behind the head. This oviduct, iuthe Paludina (Fig. 128, h, h', k ) and some other Gasteropods, is greatly dilated at its termination, and serves to retain the eggs until they are hatched, so that the young are produced alive. — There is an important difference between the condition of the embryos of the Branchiferous, and that of the Pulmonated Gaste- ropods. All the former, whether naked or testaceous in their adult state, are provided, at their emersion from the egg, with a minute spirally-convo- luted and transparent shell, from the mouth of which is protruded a rudi- mental head, in which the eyes are distinctly perceptible ; but instead of the proper disk-like foot which is characteristic of the class, we find a bi- lobed expansion fringed with cilia, by the agency of which the embryo is propelled through the water it inhabits ; and the viscera which should occupy the interior of the shell are as yet scarcely distinguishable, that space being chiefly filled up by the mass of yolk, which is not yet con- verted into the fabric of the embryo. In the latter, the shell is not formed in the embryo, except where it is to be retained in the adult, and even then it is not formed until a period which is comparatively much GENERAL VIEW OF THE ANIMAL KINGUOM. HETEROPODA. 325 later ; and the ciliated lobes are altogether wanting, although ciliary move- ment is seen in the embryo before its emersion from the egg, especially on the portion of the surface which may be considered as the rudiment of the pulmonic organ. 304 c. A remarkable departure from the ordinary Gasteropodous type is presented by a portion of the Nudibranchiate section of Opistobran- chiata, consisting of the Eolis and its allies, which some have considered as constituting a distinct order, in virtue of the peculiar conformation of their digestive and respiratory organs. The stomach of these animals seems to be extended, by a series of ramifying prolongations, throughout the entire body ; these prolongations terminating in numerous papillae, which arise from the dorsal surface, and which are themselves sometimes arborescent in their form (as in Dendronotus). But a careful examination of these terminations shows that they are surrounded by hepatic secreting cells ; so that the whole of this ramifying apparatus must be regarded, not as a prolongation of the digestive cavity, but as an unfolding and extension (so to speak) of the liver.* The external surface of these papillte is co- vered with cilia ; and from this circumstance, as well as from the manner in which the blood is transmitted to it, there seems little doubt that it is specially adapted for the respiratory fimction, there being no other bran- chial apparatus. In some of the most degraded forms of this group, however, these papillae are absent, notwithstanding that the same ramifica- tion of the biliary apparatus through the body still presents itself ; and the general integument here constitutes the only respiratory sm-face. Through such Mollusks as these, w r e may trace a relationship with the Planaria and its allies, which are usually ranked among the Entozoa (§ 309 d). — The generative apparatus of these Mollusca is enormously developed, the ova- ries occupying a considerable part of the body ; and it would seem as if this enlargement of one set of organs necessitated a different distribution of the other, on wdiose place it encroaches. 304 d. The Heteropoda , although separated from the Gasteropoda by some Naturalists, seem properly to belong to the same class, of which, however, they constitute an aberrant section ; corresponding with the true Gasteropods in their grade of development, but differing from them in the arrangement of the organs, and in the proportional size of the several parts of the body. Thus the expanded discoidal foot is converted into a ver- tical fin-like expansion (Fig. 129,/), the edge of which in some species bears a sort of sucker (y), the only rudiment of the flattened disk of the remainder of the class. The body has an elongated form, and is symme- trical through its whole length, and the principal part of it is composed of a pellucid gelatinous substance, included in a thin muscular layer, which is itself sufficiently transparent to allow the organs contained within it to be distinctly seen. The mouth, which is situated at the anterior extre- mity, is furnished with a muscular mass (£), and with a rasplike tongue ; * By M. de Quatrefages, and some other French Naturalists, it has been supposed that the circulating apparatus of these Mollusks is very imperfect, and that, in fact, this extension of the alimentary canal answers the purpose of a venous system ; and from this supposed combination of the venous and intestinal cavities, the term Plilebenterata has been applied to | the group. The researches of Messrs. Alder and Hancock and other British Zoologists, how- ! ever, have shown that the circulating system is as complete in itself as is that of other I Gasteropoda ; and have established the hepatic nature of the ramifying prolongations of the alimentary canal. 326 GENERAL PHYSIOLOGY. and the head (l) bears two tentacula (i), at the base of which are usually found distinct eyes (A). The alimentary canal passes directly backwards from the mouth towards the posterior and superior part of the bodv, where it leaves the trunk to enter the visceral mass, which is borne upon the back, as if detached from it ; in its course this canal undergoes a dilatation, which must be considered as the stomach ; and thus the ante- rior portion constitutes the oesophagus, whilst the posterior forms the Fig. 129. Carinaria , in its natural position when swimming; — a, situation of the anus ; b. branchia ; c, shell; e, liver; /, fin-like foot; g, sucker; h, eye; i, tentacula; k, proboscis; l, head. intestine. The body (which, as already remarked, must be regarded as formed by an extraordinary development of the thoracic portion of the animal) does not contain any other parts of the nutritive apparatus : the heart, the branchiae ( b ), the liver (e), and the generative organs, being aggregated together in a small mass, which reminds us of the nucleus of the Salpse (§ 299 b), except that it is external to the remainder of the body, instead of being imbedded within it. This nucleus, in the Cari- naria, is protected by a delicate shell (c), strongly resembling that of the Argonaut ; but in other Heteropods the shell is wanting. The sexes are believed to be disjoined in some forms of this group, but united in others. The nervous system is formed upon the type of that of the Gasteropoda in general ; consisting of a pair of cephalic ganglia, at the sides of the oesophagus, which receive the nerves of the organs of sense, and supply the muscular apparatus of the mouth ; with a large four-lobed ganglion near the foot, which may be considered as combining within itself the pedal and palleal ganglia. These animals swim freely through the water, by the strokes of their fin-like foot, and of the vertically-flattened tail : those which possess the sucker at its margin occasionally anchoring them- selves (it is presumed) to solid bodies, either fixed or floating. 305. Another small group which should perhaps be rather regarded ;> an aberrant subdivision of the Gasteropod class, than as ranking as a class of itself, is that of Pteropoda, which is so named from the presence of a GENERAL VIEW OF THE ANIMAL KINGDOM. PTEEOPODA. 327 wing-like membranous expansion on each side of the neck (Fig. 130), by the agency of which the animals swim through the water with greater activity than do most of the tribes to which they seem most nearly allied. Their zoological position is a matter of considerable interest. By some eminent Zoologists they have been regarded as ranking above the Gastero- poda, and as connecting them with Cephalopods. But there can be no doubt that on the whole they are inferior in organisation to most Gastero- pods, and that in many particulars, and especially in that which is their peculiar characteristic, they bear a striking resemblance to the embryonic condition of that class. Some of them, as the Hyalcea and Criseis (Fig. 1 30, a and b) have the hinder part of the body encased in a delicate shell ; whilst others, as the Clio c, are desti- tute of any such pro- tection. The shell, where it exists, may present many different forms,- one of the most remarkable is that of the Hyalaea, which is like a bivalve shell cemented together at the hinge, and having a narrow fissure in front and at the sides, from which the ante- rior part of the body projects ; that of the Criseis , again, is a straight cone; and that of Limacina is a con- voluted cone. In all instances, however, the shell, like the body, is perfectly symmetrical laterally; whilst it usually presents some indication of a division into a dorsal and ventral valve ; thus resembling, as respects its relation to the contained animal, the shell of the Brachiopoda (§ 300), whose long ciliated arms may be considered to be represented by the short but expanded lobes which occupy a somewhat similar position in the Pteropoda.* Thus the Ptero- pods may perhaps be considered as bearing the same relation to the Brachiopoda, that the Gasteropocls do to the Lamellibranchiata ; that is, as carrying on the same plan of organisation into a more advanced grade of development. They have always a more or less distinct head, this being usually most prominent in the naked species ; but although this is usually furnished with a complex apparatus for the prehension and mastication of food, the organs of sense are rudimentary. In Hyalaea and many other Pteropods, there are no cephalic tentacula whatever. In Clio there are six tentacula, which seem to prefigure the arms of Cephalopods, being * See Owen, Lectures on the Comparative Anatomy and Physiology of the Invertebrata, p. 292. Fig. 130. A, Hyalcea; b, Criseis; c, Clio. 328 GENERAL PHYSIOLOGY. covered with minute suckers, the whole number of which is estimated at 360,000 ; and there are also two slender tentacula, which appear to be instruments of the tactile sense. In Cymbulia, there are two small sensory tentacula, with a rudimentary eye at the base of each. A rudimentary organ of hearing has also been detected. The digestive apparatus is formed upon the general plan of that of Gasteropods. The mouth is usually furnished either with homy jaws or with a rasp-like tongue ; in Clio, both are present. The stomach varies in its degree of complexity, some genera being provided with curiously-armed gizzards, whilst others are destitute of them, and have merely a simple globular cavity ; the intestine usually forms three or four coils in the midst of the voluminous liver, and terminates at the right side towards the anterior part of the mantle. The structure of the heart resembles that of Gasteropoda ; it is situated in front of the branchial apparatus, which is a network of blood- vessels, developed on the exterior of the body in the naked species, hut enclosed in a cavity in the testaceous.— The muscular apparatus, inde- pendently of that which is connected with the mouth, chiefly consists of the fibres that give motion to the lateral fin-like expansions, by means of which these animals swim with considerable activity. The cephalic ganglia are relatively smaller than those of Gasteropods, as might be expected from the infei’ior development of the organs of sense; the pedal and palleal ganglia are both double, and are symmetrically disposed beneath the oeso- phagus. The sexual organs are combined in the same individuals ; hut it is probable that, although hermaphrodite, they are not self-impregnating, but that, as among the Pulmonated Gasteropods, the spermatic fluid of one individual fertilizes the ova of another. Of the history of the develop- ment of these animals, little is yet known. — Although the number of distinct forms contained in this class is comparatively small, yet the multi- plication of individuals is frequently enormous. Thus the Clio borealis in the Arctic seas, and the Clio australis in the Antarctic, often present them- selves in such vast numbers, that, when the weather is calm, the surface of the water is covered by them for miles together ; and these little animals are among the most important articles of food to the Whale, which is enabled, by the curious sieve-like apparatus in its mouth, to strain them (as it were) out of the water which it gulps-in. 305 a. The group of Pteropocls may he regarded as representing among the Mollusca the class of Insects in the Articulated series, and that of Birds in the Vertebrated; all these being characterised by the highest degree of locomotive power that is to be found in their respective sub- kingdoms, by the exact bilateral symmetry of then- bodies, and by the possession of lateral appendages, put in action by muscles, as instruments of progression. The highest manifestation of the same plan of construction in the Radiated division, is perhaps to be found in the Ciliograde Acaleplice (§ 294 a); in which, as we have seen, there is an obvious tendency to bilateral symmetry, and to the formation of wing-like expansions, whilst their movements are more rapid and energetic than those of any other Radiata. It is interesting to remark, further, that whilst the class of Insects, which carries into fullest development what is the most character- istic tendency of the whole Articulated series, is by far the largest class, not only in it, but in the whole Animal kingdom ; — and whilst the class of Birds, which is the exponent (as it were) of a similar tendency, extremely GENERAL VIEW OF THE ANIMAL KINGDOM. CEPHALOPODA. 329 well marked, if not absolutely the most characteristic, in the Vertebrated series, is also very numerous ; — the group of Pteropods, which, in its mani- festation of this tendency, runs counter to the general ‘ idea ’ of Molluscous organisation, is the smallest class (if class it is to be considered) in the whole sub-kingdom ; — whilst the Ciliograde Acalephse, whose organisation and habits are still more alien to the plan of structure and to the mode of life of Radiated animals generally, constitute but a subordinate and still more restricted subdivision of that series. 306. The class of Cephalopoda, so named from the attachment of the feet or principal locomotive organs to the head, includes a series of Molluscous animals, of which the lowest are not far removed above the Gasteropoda, whilst the highest present, as regards the development of their individual organs, a certain degree of approximation to Vertebrated animals. As regards their general conformation, however, we do not find any consider- able departure from the ordinary Molluscous type ; and the only indica- tion of an approach to the plan of structure peculiar to Vertebrata, is given by the presence of cartilaginous plates within the body, disposed in such a manner as to support and protect the nervous centres. The feet, which are disposed in a radiating manner around the mouth, so as to remind us of the arms of the Polypes (§ 289), must be regarded as highly-developed Fig. 131. Octopus , or Poulp. tentacula, and have nothing in common with the locomotive organs of any other animal. They are sometimes connected at their bases by a circular fin ; and by the alternate expansion and contraction of this, the animals which possess it can swim through the water with considerable rapidity. In other instances, however, the body is more elongated, so as to possess somewhat of the form of a Fish • and it is provided with lateral fin-like expansions (Fig. 134), by the strokes of which the animal can propel itself through the water with considerable velocity. We do not find any trace j muscular disk or foot of the Gasteropods, except in the Nautilus, ' which approaches more nearly to that class in its general structure, than 330 GENERAL PHYSIOLOGY. Fig. 132. does any other living Cephalopod. This animal is the only existing type of a section of the class, which was once extremely numerous, and which appears to have contained a great number of diversified forms; all of them, however, being now extinct except this genus, of which only three species remain, out of the many scores which are known to the Paleontologist. In the Nautilus and its allies (Fig- 133), the body is protected by a firm shell, the cavity of which is divided by transverse septa or partitions, into chambers, their number depending upon the age of the individual ; it is in the last or outer chamber alone, that the soft parts are contained ; but a membranous tube, termed the ‘siphon,’ runs backwards from this, through the successive septa, even to the most remote chamber. These ‘ chambered’ shells, like the simple shells of Gasteropods, may be regarded as formed upon the type of a cone, which is prolonged, and at the same time widened, by successive additions to its base ; when these additions are made in such a manner that the axis remains a straight line, the shell is a simple cone, the cavity of which is divided by transverse septa, as in the Orthoceratite and Baculite ; but if the axis of the cone wind round a centre, the convolutions still remaining in the same plane, a spiral chambered shell is produced, as that of the Nautilus or Ammonite (Fig. 132) ; whilst if the axis be obliquely coiled, so that the successive convolu- tions do not lie in the same plane, a helical shell is formed, as that of the Turrit ite. Between these, as in the Gasteropoda, there are inter- mediate forms; some shells, as the Lituite and Samite, being spiral at one period of their growth, and straight at another ; others, as the Toxoceras, having a gentle curvature, like that of a bow, through their whole length ; and others, as the Crioceras, being spirally curved, like the Ammonite, but not having the turns of the spire in contact with each other. — On the other hand, in that section of the Cepha- lopod class, which includes all the existing forms except the Nautilus, the presence of the shell is by no means con- stant ; and even where it does exist, it does not serve as a complete invest- ment or protection to the body. Begular chambered shells, both straight and spiral, — as those of the extinct Belemnite and its allies, and of the existing Spirula, — are not wanting ; but these are internal and sup- porting, not external and protecting. They are not included, however, within the soft tissues of the animal, after the manner of a true internal skeleton ; but are simply enfolded by prolongations of the mantle. This is the case, too, with regard to the so-called ‘bone’ of the Cuttle-fish, which is a rudimentary chambered shell, with its last or outer chamber largely developed on one side only ; and it is the case, also, with what seems an obvious exception to the general rule, namely, the delicate shell of the Argonauta, commonly but erroneously termed the Paper Nautilus. This shell, in the ordinary state of the animal, is nearly or entirely covered over by the two expanded arms ; the animal is not attached to its interior by any muscular connection ; and it seems to be wanting in the male, so that it is probably to be regarded less as a proper shell, than a." a receptacle for the eggs. In many of the Cuttle-fish tribe, the internal Ammonite. GENERAL VIEW OF THE ANIMAL KINGDOM. CEPHALOPODA. 331 shell is reduced to a mere horny rudiment, whilst in others it is absent altogether ; and it is in the latter that we find the greatest flexibility of the body, the greatest general activity of movement, and the highest con- dition of the nervous system, as w T ell as of that rudiment of an internal skeleton which is developed for its special support and protection. The naked species, whether possessed of an internal shell or not, are furnished with an ‘ ink-bag,’ which can discharge a black pigment, that renders the surrounding water quite opaque ; and this is an important means of passive defence to the animals that are provided with it, since, under cover of the inky cloud which thus hides them from their enemies, they can make their escape. The ink-bag appears to be invariably present when the external chambered shell is wanting, and to be deficient when the testaceous covering exists ; and these two conditions are respectively characteristic of the two principal sections of the class, which have been designated by Prof. Owen as Dibranchiata and Tetrdbranchiata, according to the number of the gills, which are two in the one case, and four in the other. 307. The order Tetrdbranchiata, being the one whose conformation approaches most nearly to the Gasteropod type, is that which we should first consider. Although the order contains only a single recent genus, the true Nautilus, yet we seem justified in referring to it all those numerous extinct forms of siphoniferous chambered shells, whether straight, curved, or spirally or helically convoluted, in which the size of the outer chamber evidently adapted it for the reception of the body of the animal, in which the external surface does not present any indications that it was once invested by soft vascular membranes, and in connection with which no traces of an ink-bag have ever been discovered ; and there is a strong pre- sumption that the animal inhabitants of such shells did not depart very widely from the type of conformation wdiich we shall find in that genus. The body in Nautilus is an oblong mass, adapted in its general form to the shape of the cavity in which it lies ( a a, Fig. 133), but presenting some indication of a division into an anterior portion, which includes the organs of sense and motion, and a posterior, which encloses the viscera. The posterior segment, or abdomen, is completely enveloped by the mantle, which is thin and membranous ; and lies in close contact with the testaceous walls of the cavity ; and from the deepest portion of this cavity, a tubular prolongation of the mantle, the ‘ siphon,’ passes off through an aperture in the septum, into the next chamber, and thence through the whole succession of chambers, penetrating their successive septa, which not only give it passage, but also afford it a shelly covering for about a quarter of the distance between one septum and another.* * In some of the fossil chambered shells belonging to this group, the membranous tube appears to have been enclosed in a complete shelly covering throughout its entire length; and this fact disproves the idea that the distention or contraction of the siphon could have had any connection with that alteration of the specific gravity of the animal, by which it raises or sinks itself in the water. The most probable use of the siphon, in the author's estimation, is that assigned to it by Mr. Searles Wood ; who considers that its office is to maintain the vitality of the portion of the shell most removed from the soft parts. A small artery and vein have been found by Professor Owen to pass inwards with the membranous tube; and he has also pointed out that the septa and walls of each chamber continue to be lined by a delicate membrane. The vitality of this membrane can only be kept up by vascular com- munication with the animal, of which it forms a remote part ; and that of the shell must be dependent upon that of the animal membrane in contact with it. There are many cases, in which we find an erosion of shelly structures to take place, when they are far 332 GENERAL PHYSIOLOGY. Towards the front of the abdominal portion, however, the mantle increases in thickness, becomes more muscular, and extends freely out- wards, forming a wide fold (e) which is reflected over part of the involuted whorl; whilst it elsewhere presents a sinuous margin, thickened by horny matter, so as to form a ring of adhesion to the internal surface of the shell, with -which it is further connected by a pair of muscles that take a firm attachment to the sides of the latter. Towards the lower part, we find the mantle perforated for the passage of the muscular expiratory and excretory tube (i), which is termed the 'funnel.’ The anterior portion, which may be considered as the head, though not Fig. 133. Diagram of the structure of Nautilus pompilius; — a , a, outer chamber of the shell ; b, b, b , its internal chambers ; c, calcareous siphon perforating the septa : d, membranous siphon, passing back through the whole series ; e , fold of the mantle, reflected over the convexity of the involuted whorl; f, muscular hood; g y digital tentacula; //, labial tentacula ; £, mantle forming the funnel i ' ; k, l, horny mandibles ; m, mouth ; n, oesophagus ; o, crop ; p , gizzard ; q, globular coecal appendage (rudimentary pancreas) ; r, r, intestine; s, heart; t, t, branchiae; u, ovary; v , oviduct; x x, liver ; y, aorta ; z z, nervous collar around oesophagus. apparently capable of being projected beyond the shell, is covered at its back or upper side with a fleshy expansion of the mantle (/'), which fr termed the ‘ hood this organ appears to serve in part for the closure of the entrance to the shell ; but it probably serves also, like the foot of the Gasteropods, to enable the animal to creep over a solid surface. The head is furnished with a large number of tentacula, amounting in all to about ninety ; of these, about thirty-eight arise directly from its muscular envelope, and are named by Prof. Owen digital tentacula ; whilst about forty-eight arise from the circular fleshy lip that surrounds the mouth, and are hence designated as ‘ labial and four others, again, which spring removed from a living surface ; and there can be no question that a dead shell is much more brittle than a living one. There seems a peculiar necessity for the preservation of the shell un- injured in these Cephalopods ; since the fracture or loss of any part of it would probably destroy the hydrostatic balance of the animal ; and to such fracture or loss they must be peculiarly liable, from their habit of swimming backwards. See Mr. Edwards's Monograph on the Cephalopoda of the London Clay, published by the Palteontographical Society, p. 12. GENERAL VIEW OF THE ANIMAL KINGDOM. CEPHALOPODA. 333 from the immediate neighbourhood of the eyes, are termed £ ophthalmic.’ These numbers, however, are not constant. None of these tentacida are furnished with suckers, as are the fewer but more powerful arms of the Dibranchiata ; but they seem extremely well adapted both for prehension and sensation. This multiplication of similar parts in the tentacula, as well as in the gills, is a character which of itself indicates the inferiority of the Tetrabranchiate to the Dibrancliiate Cephalopods. This inferiority, however, is not strikingly apparent in the apparatus of organic life. We find the mouth, in the Nautilus, furnished with a pair of horny mandi- bles (k, l), resembling those of certain Fishes, Reptiles, and Birds ; and, as in those Vertebrata, opening vertically instead of laterally, which is not the case in any other Invertebrata furnished with analogous instru- ments. These jaws are moved by powerful muscles, and inclose a fleshy tongue, of which part is clothed with horny spines, whilst another part is covered with sensory papillae. The oesophagus (n) dilates into a capacious crop (o), and this again contracts into a short canal which leads to the muscular gizzard (p). The intestine (r r) commences near the cardiac orifice of the stomach ; and it soon communicates with a small globular pouch ( q ) through which the biliary secretion arrives at the intestine ; this pouch itself contains secreting cells, and is probably to be regarded as a rudimentary pancreas. The liver ( x x) is a bulky gland, extending on each side along the crop, as low down as the gizzard. The intestinal tube, after making two abrupt bends, terminates in the branchial cavity, close to the funnel. The heart ( s ) is situated at the posterior and inferior part of the body, near the base of the branchiae ( t , t) of which there are two pairs ; it has no distinct auricle ; but the blood returning from the gills passes at once to the ventricle (s), and from this it is transmitted forwards by the aorta (which is seen l'unning towards y), and by other arteries. From the general system it returns by venous sinuses and channels in different parts of the body, of which the peritoneal lining of the visceral cavity itself forms a part ; by these it finds its way to the gills, at the base of wdiich we find certain peculiar organs of a glandular character, of which the use is unknown ; where also, at its entrance into the proper branchial arteries, it may possibly receive a fresh impulse from the contractile fibres of what appear to be rudiments of the branchial hearts in the Dibranchiate Cephalopods. The blood which is distributed by the branchial arteries to the leaflets of which the gills are composed, obtains its aeration from the water admitted into the branchial cavity through a wide fissure at the base of the head ; and this water, after passing over the branchite, is ejected through the funnel ( i , i‘), along with the matters voided by the intestine, and with the products of the generative organs, which open into the same cavity. 307, a. The sexes are distinct in all Cephalopods ; and the male and female organs of the Nautilus probably occupy a corresponding position at the back of the abdomen. The ovary is shown at u, lying in the immediate neighbourhood of the gizzard, but in a separate division of the peritoneal cavity ; its products are discharged by the wide oviduct v, which termi- nates very near the anus. In the immediate neighbourhood of this orifice is found a peculiar glandular apparatus ; the office of which is probably to afford a ‘ nidamentum ’ or protective envelope to the ova when discharged, similar to that which is found in many Gasteropods 334 GENERAL PHYSIOLOGY. and in the higher Cephalopoda. Of the development of the embryo, nothing is yet known. — The cephalic portion of the nervous system is considerably larger, relatively to the remainder of the body, than it is in the inferior Mollusca ; but the increase in its dimensions is seen rather in the inferior ganglia of the oesophageal ring ( z ' z 7 ), than in the superior portion (y), which is still little else than a connecting band or com- missure ; and its chief departure from the type of the Gasteropoda is seen in the distinctness of the optic ganglia, which now present themselves as separate masses appended to the sides of the supra-cesophageal ring, in accordance with the higher development of the eyes, which present a near approach to those of Vertebrata. The oesophageal ring is double a‘ its lower side ; the anterior portion supplying the tentacula, whilst the posterior sends its nerves backwards to the viscera. Besides the great cephalic centres, we find other smaller ganglia supplying certain parts of the tentacular and pharyngeal apparatus ; and others again in connection with the branchiae. The cephalic ganglia are supported by a true internal cartilage ; and in this is found a cavity on either side, which has been supposed to be an organ of hearing, but this is doubtful. In front of the mouth is a curious laminated structure, copiously supplied with nerves : which is regarded by Prof. Owen, with great probability, as an organ of smell. The conformation of the tongue and of the interior of the mouth are such as to indicate that the Nautilus possesses the sense of taste ; and the numerous tentacula, especially those proceeding from the circular lip, are doubtless instruments of very acute touch. The move- ments of this animal through the water, however, can scarcely be supposed to be very active ; since it appears to be destitute of all other means of progression than that supplied by the respiratory current proceeding from the funnel, the con- stant ejection of which will tend to give it a backward motion. The air included in the chambers of its shell, however, gives it such a degree of buoy- ancy, that the whole, with the animal, is almost exactly equal in weight to its own bulk of water ; and thus the animal will be made to rise or fall by a very slight change in its specific gravity. This change it certainly has the power of making; as the living Nautilus ‘has frequently been seen floating on the sur- face of the ocean, and the difficulty of its capture arises from the suddenness with which it sinks when alarmed. 307 b. In describing the Dibranchiate Cephalopods, it will be sufficient to indi- cate the chief points of difference which exists between them and the preceding order. These have reference, in the first place, to the appendages of the head, which are never more than ten in num- ber, and frequently only eight. Where there are ten, one pair is frequently Fig. 134. GENERAL VIEW OF THE ANIMAL KINGDOM. CEPHALOPODA. 335 prolonged to a considerable extent beyond the rest, as is the case with the Onychoteuthis (Fig. 134), and in a less degree w r ith the common Sepia (Cuttle-fish). In the Octopod group, we frequently find the bases of the arms connected by a circular fin (Fig. 131) by the alternate expansion and contraction of which, these animals are enabled to swim backwards through the water with considerable rapidity. In the Argonaut a , which belongs to this division, two of the arms are extended into broad flattened expansions 3 these were formerly supposed to act as sails which the animal might spread to the wind 3 but it is now known that they are never em- ployed as such, but that they are applied to the exterior of the delicate shell (which, not being chambered, and having no muscular attachment to the body, has been consequently supposed by many Naturalists to belong to some other animal, and to be only parasitically tenanted by the Argo- naut,) and that they serve not merely to hold it in its place, but also to extend it, or to repair it when injured, by a new formation from their epithelial surface. In all instances, the arms are covered on their inner sides with suckers, which enable them to take a very firm grasp of the object which they seize ; and in the Onychoteuthis we find the two long arms furnished also with horny hooks, by which the grasp is rendered still more powerful.— The mouth is furnished, as in Nautilus, with a pair of strong horny parrot-like jaws, enclosed in a mass of muscles (Fig. 133,/) that serves to put them in action 3 and it receives the secretion of two pairs of salivary glands (g, h). The oesophagus (i) dilates (in the Octopods, but not in the Decapods) into a crop (j), and then enters the gizzard-like stomach (7r), from which passes off the intestine (to, to') 3 this is connected, near its commencement, with a spirally-convoluted appendage (l), which is obviously the rudiment of a pancreas, more highly developed than that of Nautilus. Into the same part is discharged the biliary secretion, formed by a large liver, which (though removed in the figure, in order to show the other parts) occupies a similar position to that which it has in the preceding order. The intestine terminates in the branchial chamber, near the base of the funnel (e), as in the Tetrabranchiata; and lying by the side of its latter portion, and opening in the same situation, we find the ink-bag. In the structure of the heart, we see a decided advance upon that of Nautilus. The systemic heart ( p ), placed at the base of the aorta ( 7 ), is still the principal impelling cavity 3 but the blood returning from the body at large by the venous canals (of which one is seen at ?’), then enters a pulmonic heart or impelling cavity (.s) which is situated at the base of each gill 3 and by its agency, the branchial circulation is ren- dered more energetic. Although the branchiae ( 0 , 0 ) are single instead of double, on each side, yet the single gill, from the extent of surface which it presents, and the more active movement of blood through it, is a more efficient apparatus for aeration than the double gill of the Nautilus. At the base of the pulmonic hearts, we find the same glandiform appendages (r) as in Nautilus. The blood impelled through the branchial arteries (s') by the pidmonic hearts, returns by the branchial veins ( t ) 3 and these, in their passage towards the heart, present dilatations (u, u ) which might be almost regarded as auricles. In this division of the impelling power (so to speak) between the commencement of the systemic and that of the respiratory circulation, we see a transition between the plan of the circu- lating apparatus characteristic of the Mollusca in general, which have the 33G GENERAL PHYSIOLOGY. heart entirely systemic, and that hereafter to be described in the class of Fishes, which have the heart entirely pulmonic. — The nervous system of the Dibranchiate Cephalopods presents a considerable advance upon that of the Tetrabranchiate, especially in the development of the superior por- tion of the oesophageal nervous collar, and in the still lai’ger dimen- sions of the ganglia Fig. 135. of sense, although in other respects formed upon the same gene- ral plan. The cephalic portion is completely enclosed in a thick cartilage ; and in the basal portion of this is an excavation on each side, which is evi- dently a rudimentary organ of hearing. The eyes are of consider- able size, and present some remarkable pe- culiarities of structure hereafter to be de- scribed (chap. xxi). No distinct traces of a special organ of smell can be discovered in the Dibranchiata; but there is reason to be- lieve that the olfactive sense is possessed by them ; and the exter- nal lips may proba- bly be its seat. The mouth, as in Nautilus, contains a highly or- ganised tongue; which is probably in part Anatomy of Octopus , the animal being laid open on the ventral side, SllbsOrvicilt to the and the inferior wall of the abdominal cavity, with the liver, having ?p .. Qp fhp been removed; a, a , base of the tentacula, with the suckers a' a ' on v -“ t Uloie. their inner surface; b, head; c, eye; d, d, mantle turned back; e, SG11S6 of touch, tllG funnel; /, fleshy mass surrounding the mouth ; g, salivary glands of the ^ 1 i . i first pair; h, salivary glands of the second pair, with their suspensory &nilS cll’6 QOUOllCNS ligaments h' , and their excretory canal h"; i, oesophagus;./, crop: tllG DGCuliilT SG&t GSP6- A% stomach ; l , spirally convoluted coecal appendage (rudimentary pan- • i-f 1 * , . creas?); m, commencement of intestinal tube, with biliary canal on each CiallY towards tllGir side; m', intestinal convolution; m", anal extremity, turned down- -i p +pv n -nnnfnniw * wards and to one side \ n, ovarium; n\n', oviducts, of which one is ttriii «- in its natural position, and the other turned downwards ; o, o, branchiae; blit tbGSG are l'lltllCl' p, heart; q, ascending aorta; r, venous trunk passing towards the . pulmonary heart; r', its glandiform appendage ; s, pulmonary heart; s', lUStrUlUGlItS 01 pit- branchial artery ; branchial vein ; w, w, bulbous dilatations of branchio- bGiision ; and tlio ;id- cardiac veins. . > . ", liGsiYG power ot tiie suckers does uot require seusibility for its exercise, as is sliowu by the fact that it will continue in operation when the arms are entirely detached from GENERAL VIEW OF THE ANIMAL KINGDOM. CEPHALOPODA. 337 the body. The general surface is usually beset with papillae, which appear to be tactile ; and these are especially developed in some species about the head and eyes, taking the place in some degree of the numerous tentacula of the Nautilus. The movements of the Dibranchiata are generally very active. Their principal instruments of locomotion are their powerful ten- tacula, by the agency of which they can swim backwards ; and this retro- gression will be aided by the respiratory current, which is constantly issuing forth from the funnel. The shorter-bodied Octopi can also crawl over solid surfaces, with the body upwards, and the mouth downwards, in the position represented in Fig. 131 ; whilst, on the other hand, the long slender-bodied Calamaries seem to propel themselves through the water, like Fish, by the tin-like expansions at the posterior extremity of the body, and are even said to raise themselves above the surface, and to dart through the air for a short distance, like Flying-fish, by the force of the strokes they can make with these organs. In all their movements, the Dibranchiate Cephalopods seem guided by their very acute sense of vision : being enabled, by the degree in which their eyes project from the sides of their head, to see in every direction. — In the structure of their generative apparatus, the Dibranchiata do not present any essential departure from the type of the Nautilus. The ovary of the female ( n ) is single, and lies at the posterior part of the body ; but in the Octopus the oviduct (n' n') is double, though it remains single in the Sepia ; in both cases, however, it passes through nidamental glands, from which the ova receive an additional investment, before terminating in the branchial chamber. The testis or spermatic organ of the male lies in the same situation as the ovary of the female ; and its efferent canal passes through a glandular organ, in wdiich the spermatozoa become enclosed in peculiar cases, having a power of spontaneous movement, which are termed the ‘ filaments ’ or ‘ animalcules of Needham,’ after their first discoverer. The spermatic fluid containing these is discharged through a penis : it does not appear, however, to be conveyed by this into the body of the female ; but seems to be rather applied to the masses of ova, at the time when they are being deposited by the female. The ova, enclosed in their nidamental capsules, are aggregated together in large masses, almost every species having its own peculiar arrangement ; and it would appear to be for the pxu’pose of penetrating these clusters, that the filaments of Need- ham are endowed with their very curious motor powers ; as they burst and set free their contained spermatozoa, within a short time after they have themselves come into contact with water. The young of the Dibran- chiate Cephalopods do not undergo any metamorphosis after coming forth from the egg, but already possess at their emersion the general form of the adult, whilst all their organs are sufficiently developed to enable them to obtain their food and to escape from their enemies. 307 c. The differences which have been shown to separate the ordinary Dibranchiate Cephalopods from the Tetrabranchiate order, are not less characteristically displayed by those which possess chambered shells, not- withstanding the very close resemblance which these appendages bear to the shells of Nautilus, and its extinct allies. And it is a remarkable proof of the justice of the grounds on which the separation of these orders was proposed by Professor Owen, that the recently-acquired knowledge of the animals of Spirula and Belenmite, of which the former possesses a z 338 GENERAL PHYSIOLOGY. spirally-convoluted, and the latter a straight conical chambered shell, has shown them to be, in all essential points, what the prediction founded upon a small number of data had asserted that they would prove. — The shell of the Spirula is enclosed between two lobes of the mantle, at the posterior end of the body of a Decapod ; the animal has eight short arms, beset with numerous minute suckers promiscuously arranged, and two long tentacula terminating in club-shaped enlargements ; it possesses an ink-bag ; its gills are single on each side, and have pulmonic hearts at their base ; and it has a distinct auditory apparatus, excavated in the cephalic cartilage ; in all these characters conforming to the regular Dibranchiate type, as had been predicted from the conformation of its cephalic appendages.* — So in regard to the Belemnite, the fact that it is an internal shell having been guessed from the markings upon its external surface, and from its possession of an ink-hag (the fossilized remains of which are not unfrequently preserved in its last chamber), it followed upon the same train of reasoning, that the animal must have been of the Dibranchiate order, rather than allied to that of Nautilus, and that it must have possessed not more than ten arms ; whilst the elongation of the chambered portion of the shell in a straight line (Fig. 136,5), and still more the elongation of the solid calcareous cone (c) that includes it (which, from its crystalline texture, has been termed the ‘ spathose guard ’), naturally led to the inference that the form of the animal must also have been elongated, like that of the existing Calamaries. The recent discovery of specimens of Belemnites with the soft parts sufficiently pre- served to allow their form to be clearly made out, has fully justified this prediction ; as will be seeu from Fig. 136, which exhibits Professor Owen's restoration of the entire animal from the materials thus obtained, t The arms, instead of being fur- nished with suckers, were provided with hooks, like those of the long tentacles of the Onychoteu- this ; and the body had a pair of small lateral fins (f /) at about the middle of its length. From the weight of its dense internal shell, the Belem- nite may be supposed to have commonly main- tained a vertical position ; but, like the Nautilus, it probably had the power of ascending and de- Belemnite restored ;. «, o, outer spending in the water with facility. It would chamber of conical shell ; o, o J , . chambered cone; c, spathose rise swiftly and stealthily, to fix its claws 111 the f, u /( d frns kookeiTarms • belly of a fish swimming near the surface above; i, funnel. and would then descend, perhaps as swiftly, and devour its prey at the bottom. We cannot doubt that, like the hooked * See Prof. Owen’s description of the Anatomy of Spirilla, in the Zoology of the Yojage of the Samarang : — Mollnsca, p. 6. t See Prof. Owen's Memoir on the Belemnite, Phil. Trans. 1844. GENERAL VIEW OF THE ANIMAL KINGDOM. CEPHALOPODA. 339 Calamaries of the present seas, the ancient Belemnites were the most formidable and predaceous of their class. 307 d. In now quitting the Molluscous type, we may notice that not- withstanding the high development of their organs of nutrition, there are yet strongly-marked peculiarities in the arrangement of these organs in that series ; and thus, notwithstanding their individual resemblance to the corresponding organs of the lower Vertebrata, there is still an entire absence of any general approximation to the latter type. Thus we find in all Cephalopods, as in Mollusca generally, the intestinal tube doubling back upon itself, so as to terminate near the mouth ; instead of passing, as it invariably does in Vertebrated animals, to the posterior extremity of the trunk. And we find in Cephalopods, as in most Gasteropods, the branchial apparatus to be in connection with the anal termination of the alimentary canal; instead of being, as in all Vertebrata, in relation with its oral com- mencement. So again, although we find the assemblage of cephalic ganglia, which are the centres of the organs of sense, approaching the lower forms of the brain in Fishes, — although we find these included in a car- tilage which may be likened to a rudimentary cranium, — and although we find in the instruments of sight, hearing, smell, and taste, a similar approxi- mation to those of Vertebrata; — yet no such resemblance exists between the ganglia connected with the locomotive apparatus of Cephalopods and the spinal cord of even the lowest Vertebrata, the locomotive apparatus itself, as we have seen, being constructed upon an entirely different plan in the two series. The muscles which move the various parts of the body and aims of the Cephalopod may be considered as merely a development of particular parts of the general muscular envelope ; they have no fixed points of attachment, and have no levers to act upon ; and they do not present the least approach to that segmental arrangement, which is charac- teristic of the locomotive apparatus of Vertebrata, in common with that of Articulata. Consequently, as there is no succession of segments in the body, each endowed with a muscular apparatus of its own, there is no trace of that corresponding series of ganglionic centres which forms the gan- gliated cord in the Articulata, and which is consolidated into the spinal cord of Vertebrata. Hence, whilst the Cephalopods show no rudiment of that segmental division of the integument, which we have seen in the Chiton (Fig. 125), they are equally far from possessing any traces of the ‘vertebral column ’ which is the distinguishing characteristic of the Verte- brata, and which, in its full development, affords support and protection not merely to the principal nervous centres, but also to the viscera in general, together with fixed points for the attachment of the muscles, and levers by which their power may be most advantageously applied. These differences, however, will come before us hereafter ; and they are only here alluded to, for the sake of bringing into clear view the points on which the Cephalo- pod Mollusca do, and those on which they do not, approximate to the Vertebrata. — It is particularly instructive to see how the higher develop- ment of the organs of animal life, and the more active locomotive powers of the Cephalopoda, involve a much more complete lateral symmetry than we meet with (except in a few comparatively rare exceptions, and these among the inferior forms of the class) in the Gasteropoda. This is seen, ,not merely in the general contour of the exterior, but also in the conform- ation of the apparatus of organic life ; thus the respiratory organs in the 340 GENERAL PHYSIOLOGY. Cephalopods are always double and symmetrical ; and the ovaries show a tendency to become so. There is also an almost exact bi-lateral symmetry in the nervous centres. — On the whole we may say that the group of Cephalopoda presents us with as close an approximation to the Yertebrated sub-kingdom, as it could well do without a departure from the general Molluscan type, and consistently with the plan of structure peculiar to itself. 308. The characters of the sub-kingdom Articulata are much more definite than those of the Molluscous series, and are for the most part in remarkable contrast with them. One prevailing plan of construction runs through all the classes associated under this designation ; and this is gene- rally easy of recognition, although it is subject in particular cases to consi- derable modification, and is sometimes partially obscured. This plan, the idea of which is conveyed by the terms ‘ articulated ’ and ‘ annulose,' may be defined as follows. The body is made up of an assemblage of segments, which succeed each other longitudinally ; it is completely inclosed in an integument, consisting of a series of rings of firm texture, jointed or ‘ articu- lated ’ one to another, and held together by a flexible membrane, which allows considerable freedom of motion ; and there is a very complete bilateral symmetry, the two sides of the body being exactly similar, not merely in their external form, but for the most part also in then - internal structure. In the lowest grades of this type, the successive segments are nearly exact repetitions of each other, so that each can maintain an independent existence and can reproduce the rest, thus almost deserving to be regarded as distinct individuals. Ascending higher, we still find the segments maintaining their equality, and presenting a close resemblance to each other externally (Fig. 137); but the organs they contain are fused together (so to speak) into an apparatus that is subservient to the life of the whole, and no part can maintain a separate existence. In the highest forms of the series, such as Insects, Crustacea, and Araclmida, we no longer meet with this equality of the segments ; some being developed in excess whilst others remain in an almost rudimentary con- dition, and then endowments being alike dissimilar ; s, ■ that the fabric (according to the general principle of eleva- tion in the empire of Life, § 20) loses its homogeneity, and acquires more special powers. — So, again, in the lower forms of the group, we find the integument altogether so soft, that the intervals of the articulations are scarcely distinguishable from the rings themselves ; the whole 1 >ody is thus extremely flexible ; and its progression is effected by the movement of its parts one upon another. In certain parts of the highest, on the other hand, the hard segments of the skeleton are often so closely united together, that the line of separation between them is not apparent ; hence these seg- ments have no power of motion one upon another ; and the propulsion of the body is delegated, more or less completely, to members developed for this especial purpose. These members, in the lowest tribes in which they make their appearance, are all similar and equal one to Fig. 137 . Centipede. GENERAL VIEW OF THE ANIMAL KINGDOM. ARTICULATA. 341 another, and are developed alike from every segment of the body (Fig. 137) ; but in the higher Articulata, we find them restricted to certain parts of the trunk, especially to those which are most firmly united toge- ther ; this very union, it would seem, being subservient to the power of the limbs, by giving a firmer attachment to the muscles that move them. These members, like the body itself, are enclosed in an articulated enve- lope, or external skeleton ; and this, so far as regards the mechanical action of the muscular apparatus, answers essentially the same purpose as the in- ternal skeleton of Yertebrata, giving to the muscles fixed points of attach- ment, and furnishing a system of inflexible levers for the advantageous application of their contractile power. — A large proportion of the fabric of the Articulata is made up of the Muscles by which the several segments and then - various appendages are put in action (Fig. 138) ; and the deve- lopment of the organs of nutrition would seem to be for the most part subservient to that of the locomotive apparatus, their function being chiefly to supply the nerves and muscles with the aliment necessary to maintain then’ vigour, and with the oxygen required for their functional activity. The power of these muscles is so great in proportion to the size of the ani- mals, that in energy and rapidity of movement, some of the Articulated tribes surpass all others. Their locomotive actions are directed by organs of sensation, which, though developed on a very different plan from those of Mollusca, are evidently very acute in their powers. It is curious to observe that even in these we have the principle of repetition strikingly manifested ; the eyes of all, save the highest Articulata, being £ compound,’ that is, being made up of an assemblage of ocelli, sometimes to the number of several thousand. There are very few instances in which Articulated animals are in any way restrained as regards freedom of motion ; and these occur, for the most part, in a single group, the Cirrhipocla or Barnacle tribe (§ 311), which connects this sub-kingdom with the preceding. In general they roam freely abroad, whether in search of prey, or for sexual purposes ; and in their various actions we trace an obvious, and frequently a most remarkable adaptation to particular purposes. This adaptation, however, seems rather to be the result of the instinctive propensities of the animals, than of intelligent will ; these animals apparently acting like machines, adapted by the Designing Mind which constructed them to execute a certain set of operations ; and doing this blindly, without any choice or purposive intention of their own, yet producing results which Man, by the highest efforts of his reason, could not surpass in the per- fection of their adaptiveness to the required end. 308 a. All the Articulata, save a few of the very lowest species, possess a distinct head at one end of the body, furnished with organs of special sensation, and also (except in those which live by suction) with lateral jaws for the prehension and reduction of food. The alimentary canal is almost uniformly provided with a second orifice, which is situated towards the posterior extremity of the body ; and it frequently passes towards this in a perfectly straight line, exhibiting an exact bi- lateral symmetry both in its own structure and in that of its glandular appendages. In most of the higher Articulata, however, the intestine is more or less convoluted, so that this symmetry is in some degree lost ; but it still manifests itself in the early development of the apparatus, and in the glandular appendages connected with it (Figs. 153, 154). In 342 GENERAL PHYSIOLOGY. regard to the development of these, which for the most part represent the liver, we have a remarkable variety, that seems to have a relation to the medium which the animals are destined to inhabit. In the aquatic Articulata, the hepatic apparatus, at first consisting merely of secreting cells lodged in follicles of the stomach, comes at last (in the Crustacea) to form a large glandular mass, which occupies, as in the Molluscs, a considerable part of the abdominal cavity (Fig. 156,/). In Myriapods, Insects, and Spiders, on the other hand, all of which breathe air, the liver consists of but a small number of tubuli, containing secreting cells ; and although these are sometimes considerably elongated, yet they never occupy any large proportion of the visceral cavity. — In all save the very lowest Articulata, we find a distinct provision for the Circulation of nutritive fluid. This consists of a set of blood-vessels, which, in the inferior tribes, are nothing else than channels or interspaces among the tissues, receiving the nutritious fluid which transudes from the walls of the alimentary canal ; but which we find, as we pass towards the higher, to assume the characters of regular vessels, of which a paid are distributed upon the alimentary canal, and act as absorbents, whilst another division conveys the absorbed fluid to the system at large, and to the respiratory organs. In the lower forms, moreover, there is no central organ of impulsion ; but the blood meanders through its canals, partly under the influence of forces developed during its circulation (chap, xii.), and partly by the agency of the continual restless movements of the body itself, one paid upon another. As we pass upwards, we frequently meet with a numerous succession of organs of impulsion ; the tendency obviously being to the development of one or more for each segment. In the higher classes, however, these separate centres are fused together (so to speak), in such a manner as to form a continuous dorsal vessel, which occupies the median line, and propels the blood from behind forwards ; this vessel still presenting indications of its composite origin, in the valvular partitions which correspond to the successive segments. It is only in the Spiders and Crabs, which, in the shortness of then- bodies and concentration of their organs, present the greatest departure from the ordinary Articulated type, that we find any approach to the compact heart of the Mollusca (Fig. 156, c). The blood is usually colourless, as in other Invertebrata ; but it contains a larger number of corpuscles than are seen in that of most of the Mollusca. 308 b. The Respiratory apparatus of Articulated animals everywhere presents a similar tendency to repetition of parts. In the lowest forms of this type, as in the inferior Radiata, we have no distinct provision for the aeration of the blood ; the demand for oxygen not being great, and the integument being so soft as to allow the free transmission of gases through every part of its substance, so that all the blood sent to it is subjected to the respiratory process. As we pass upwards, however, we meet with distinct organs for this purpose, which are in the fist instance developed equally in every segment : in the aquatic tribes they consist of leaf-like or arborescent gills, which are developed from the exterior of the body, so as to expose the blood transmitted to them to the influence of the air contained in the water which bathes them ; whilst in the air-breathing classes, they are so constructed as to introduce atmo- spheric air into the interior, the action of each organ, however, being GENERAL VIEW OF THE ANIMAL KINGDOM. ARTICULATA. 343 limited to its own segment. Even in the highest Articulata, we still find much of this repetition of the respiratory organs ; but the portions belonging to the different segments are brought into more combined action ; and the whole apparatus attains such an extension in Insects, as to give it an amount of functional power that is nowhere surpassed. In all instances, the respiratory organs are perfectly symmetrical. — In regard to the development of other Secreting organs, such as salivary glands, pancreas, and kidneys, there is little to be said. In the lower classes of the series, no rudiment of them can be traced ; and even in the highest, there is by no means the same approximation to the more elevated type of these organs, that we have seen in the Mollusca. In fact, although salivary glands, in the form of long coecal tubes, are very commonly found in Insects and Crustacea that masticate their food, it is not by any means certain that even the rudiment of a pancreas exists ; and the urinary organs even in Insects, Spiders, and Crustacea, are never more developed than are the biliary tubuli of Insects, which they strongly resemble, except in position. — The Reproductive process, in some of the lowest Articulata, as in the lower Radiata and Mollusca, takes place after a double method ; that is, by the multiplication of the individual, as well as by true sexual generation. The former operation, however, is not so frequently accomplished by a process of ‘ gemmation,’ or the formation of an offshoot from the parent fabric (which we have seen to be the usual method of multiplication in Zoophytes and the lower Mollusca), as by the subdivision of the latter into two or more parts, of which every one is capable of reproducing the entire structure. This subdivision takes place spontaneously in some species ; in others w T e only find the multiplication effected, when it has been artificially performed ; and in others, again, although considerable power of reproducing new parts is retained, it is limited to only one of the divisions of the body, so that no multiplication can be thus effected, all but that one of the severed pieces ceasing to exist. This power of reproducing lost parts is retained to a considerable extent even in the highest Articulata. In the lowest mem- bers of the Articulated series, and especially in those which have com paratively little freedom of locomotion (such as the Entozoa), we find the two sets of sexual organs (which are almost invariably sym- metrical) united in the same individual, which has the power of self- fertilization, and these organs are sometimes repeated in every segment ; in all save these, however, the sexes are distinct, and a sexual congress is required, in order that the ova may be fertilized by the spermatic fluid of the male, whilst yet within the body of the female. In a few instances, as in certain Mollusca, the eggs are hatched within the maternal oviduct, so that the young are born alive ; but generally speaking, the embryos come forth in a very immature state, and have to pass through a long- series of changes of form and condition, before they attain the type of their parents. The nature of these changes, however, differs considerably in the several classes ; and no statement can be made respecting them that shall be even generally applicable. 308 c. Reference has already been made, on more than one occa- sion, to the high development of the Muscular apparatus of the Ar- ticulata generally. In the lowest forms of the series, however, no distinct muscular fibres can be detected ; and the movements of the body appear 344 GENERAL PHYSIOLOGY. to be produced by the contractility of an almost homogeneous tissue resembling that of the Hydra (§ 289). Passing on to the higher forms, we find that where the segments of the body are similar to one another in form and general endowments, the muscular apparatus of each is. in like manner, a repetition of that of the rest, as is the case in the Centipede 3 but where, as in the Insect, the locomotive appendages are restricted to only one portion of the body, and the segments are dissimi- lar, the muscular apparatus is modified accordingly, its greatest develop- ment being of course in those segments to which the members are attached (Fig. 138). — The same general principle applies also to the Fig. 138. Section of the trunk of Melolontha vulgaris (Cockchafer), showing the complexity of the muscular system. The first segment of the thorax 1 2 « is chiefly occupied by the muscles of the head, and by those of the first pair of legs. The second and third segments (3 and a i contain the very large muscles of the wings, and those of the two other pairs of legs. The chief muscles of the abdomen are the long dorsal and abdominal recti, which move the several segments one upon another. Nervous system. It is not distinguishable in some of those beings, which, as presenting the Articulated type in its most degraded form, must be placed in this series, although but little elevated above the Protozoa. Where it first presents itself, it is little more than a double thread, running from end to end of the body, with scarcely any ganglionic enlargement. As the segmental division of the body, however, becomes more distinct, and the locomotive apparatus more developed, so do the ganglionic enlargements on the ventral cord in- crease ; and wherever the locomotive apparatus is uniformly repeated in the successive segments, there do we find its ganglionic centres (which may be regarded as so many repetitions of the pedal ganglion of the higher Mollusca) repeated likewise. But with the concentration of the locomotive apparatus in a particular division of the body, we find the ganglia of the ventral cord undergoing a corresponding enlargement in that portion, and sometimes even coalescing into one mass (Fig. 101). The size and relative importance of the cephalic ganglia bear- a constant relation to the development of those organs of sense of which the head is the seat. These variations are traceable, not merely as we pass from one group to another, but as we follow the history of development in any one GENERAL VIEW OF THE ANIMAL KINGDOM.— ENTOZOA. 345 of the higher Articulata, such as an Insect (Figs. 152 — 154) : for it is uniformly to be observed that equality of the segments, and of all they contain, is the first condition ; and that the differences in the endowments of these, arising from variations in the degree of development of particular organs, are of subsequent origin. The respiratory organs of the higher Articulata usually possess, as in the Mollusca generally, distinct gan- glionic centres of their own ; and where these organs are repeated in successive segments, the ganglia are repeated also. 308 d. Thus we perceive that the Articulated series, in its com- prehensive acceptation, includes an extensive range of forms ■ the lowest of which are not more highly organized than the inferior Radiata, and differ from them chiefly in the longitudinal in place of the circular repetition of their parts ; whilst the highest approximate the Verte- brated series in the development of their nervo-muscular apparatus, and in the disposition of their viscera, without losing the essential pecidiarity of the group, namely, the articulated external skeleton. The peculiar confirmation of this skeleton involves a special mode of adapting it to the increasing dimensions of the animal during the period of growth, such as we have not yet elsewhere met with. Although this skeleton is made up of a number of pieces, yet it could only be increased longi- tudinally by additions to the edges of these ; for, as each ring com- pletely encircles the body, an augmentation iu its diameter would be impossible, without an interstitial growth, of which these skeletons are not capable. The requirement is met by the exuviation or ‘ moulting ’ of the entire tegumentary skeleton, which takes place during, the period of increase of the body, as often as this becomes too large for its invest- ment ; preparation being made for the formation of a new skin or shell, before the older one is cast off. It is only when these animals have attained their full growth (as in the case of perfect Insects), that this exuviation ceases to be performed ; and in many tribes of Crustaceans it appears to take place occasionally through the whole of life. 309. The lowest forms of the Articulated type are found in the class which has received the designation of Entozoa, from the circumstance that most of the animals which it includes are parasites of other animals ; their habitat being iu some instances the intestinal canal, whilst in others it is the substance of the muscles, glands, &c. ; and their nourishment being derived by suction from the juices of the bodies which they thus infest. In this respect, therefore, they bear a striking analogy to the parasitic Fungi (§ 271, 272). But uo such peculiarity of habit would of itself justify the association of the animals displaying it into a distinct zoological group, which must be founded on peculiarities of structure common to all : and taking these characters as our proper guide, we find that some of the parasitic suctorial Articulata are really aberrant forms of Crustacea ; whilst with the remainder, which present a general accordance in structure and mode of development, rve must associate j other forms which are not parasitic, but which agree with the true En- tozoa in their general plan and grade of organisation. It is very difficult, however, to determine what are are to be regarded as the true distinctive i characters of the class ; for, like other groups which occupy a low place in their respective series, it contains a much wider variety of forms than i we find in those of more elevated grade ; and these forms are such as 346 GENERAL PHYSIOLOGY. connect it closely with all the principal types of the Animal Kingdom, except the Vertebrated series. We shall find in it a manifest approxima- tion to the Protozoa, to the Radiata, and to the Mollusca, as well as to the higher Articulata ; and in fact, it is only because the most numerous and characteristic forms of Entozoa possess a distinctly articulated struc- ture, that the class as a whole is best referred to the last-named series. It has been proposed to break it up, aud to distribute it amongst the other types to which it is related ; but such an arrangement would have the effect of widely separating from each other a number of beings, which, notwithstanding the variety of structure exhibited by their most dissi- milar forms, are brought into mutual approximation by such a succession of intermediate grades, as manifestly liuks them into a continuous series. We may, on the whole, most philosophically regard the class as' occupying an intermediate position between the Protozoa, Radiata, Mollusca, and Arti- culata ; its closest affinity, however, being with the last-named series. And the best idea of its distinctive peculiarities will be acquired from the exa- mination of a succession of forms, which will manifest these relations, and which, at the same time, will display the typical characters of its principal groups. Any attempt at the systematic arrangement of the Entozoa (and many such attempts have been made), must be regarded as merely provisional ; since there is doubtless much still to be learned, not merely with regard to their anatomy, but also with respect to the his- tory of their development ; and what is already known on this last point leads to the suspicion, that many forms which are so dissimilar as to have been ranked in distinct orders, are really but different stages of evolution of one and the same being. 309 a. The simplest form of Entozoa at present known, is that which has received the name of Gregarina, from the aggregation of large num- bers of individuals in the same situation. It infests the alimentary canal of insects, worms, rief one, being continued only so long as the store of nutriment supplied by the egg remains unexhausted. — Although the number of eggs in these animals is so small, yet the rapidity with which the whole process of their development and maturation is accomplished renders the multiplication of the race very rapid. The egg of the Hydatina is extruded from the cloaca within a few hours after the first rudiment of it is visible ; and within twelve hours more the shell bursts, and the young animal comes forth. In the Rotifer and several other genera, the development of the embryo takes place whilst the egg is yet retained within the body of the parent, and the young are extruded alive ; whilst in some other instances, the eggs, after their extrusion, remain attached to the posterior extremity of the body, until the young are set free. In general it would seem that, whether the rupture of the egg-membrane takes place before or after the egg has left the body, the germinal mass within it is developed at or.ee into the form of the young animal, which resembles that of its parent : no preliminary metamorphosis being gone through, nor any parts deve- loped which are not to be permanent. The transparency of the ecg- membrane, and also of the tissues, of the parent Rotifer, allows the process of development to be watched, even when the egg is retained within the body ; and it is curious to observe, at a very early period, not GENERAL VIEW OF THE ANIMAL KINGDOM. ROTIFERA. 363 merely the red eye-spot of the embryo, but also a distinct ciliary move- ment. The multiplication of Hydatina (in which genus three or four eggs are deposited at once, and their development completed out of the body) takes place so rapidly, that, according to the estimate of Prof. Ehrenberg, nearly seventeen millions may be produced within twenty-four days from a single individual. This extraordinary rate of increase, taken in connection with the power which many species possess of sustaining the complete desiccation of their bodies without loss of their vitality (§ 65), removes all difficulty in accounting for the extent of the diffusion of these animals, and for their occurrence in incalculable numbers in situations where, a few days previously, none were known to exist. For their entire bodies may be wafted in a dry state by the atmosphere, from place to place ; and their return to a state of active life, after a desicca- tion of unlimited duration, may take place whenever they meet with the requisite conditions, — moisture, warmth, and food. It is probable that the ova are capable of sustaining treatment even more severe than the fully- developed animals can bear ; and that the race is frequently continued by them, when the latter have perished. Even in those species which usually .hatch their eggs within their bodies, it appears that a different set of ova is occasionally developed, which are furnished with a thick glutinous investment : these, which are extruded entire, and are laid one upon another, so as at last to form masses of considerable size in proportion to the bulk of the animals, seem not to be destined to come so early to maturity, but very probably remain dormant during the whole winter season, so as to produce a new brood in the spring. — Not- withstanding that, in most of the Rotifera, the embryo at its emersion from the egg possesses all the most important characteristics of the parent, yet according to the observations of Dr. Mantell,* the embryo of the Stephanoceros first comes forth from the egg in a form and condition much resembling that of the Stentor (one of the Yorticellinse), and only gradually attains the characteristic form and structure of its parent. Should any such resemblance present itself in other cases, it will natu- rally lead to the suspicion of a closer relationship between these groups, than is at present known to exist. 310 c. We shall now briefly notice some of those aberrant forms, by which the class of Rotifera is connected with other groups. As already remarked, there are certain Wheel-Animalcules, which, in the general simplicity of their structure, and the form of their rotatory organs, present a decided approximation to the Vorticellince ; and in the genus Chcetonotus, the cilia are no longer arranged so as to form a distinct rotatory organ, hut cover the whole body, whilst at the same time the internal structure is very indistinct, so that it stands on the confines of the two classes of Rotifera and Polygastrica. — Again, in Stephanoceros (Fig. 145) and other Rotifera which have the external wall separated from the body so as to form a cell, there is a manifest approximation to the Polypifera and Bryozoa ; and the resemblance to the latter group is increased in Ste- phanoceros, by the conversion of the rotatory organ into a set of ciliated tentacula. However, the internal structure of these animals still remains conformable to the ordinary rotiferous type, in every respect save the * Thoughts on Animalcules, pp. 68, 69. 364 GENERAL PIIYSIOLOGY. dilatation of the pharynx into a sort of crop, which may remind us of the dilated pharynx of the Bryozoa. In Melicerta, the exterior of the cell is strengthened by the agglutination of foreign particles, which are arranged with great regularity; reminding us of the sheaths of some of the Tubi- colous Annelida (§ 311 b ). — The passage between the Rotifera and the Annelida, however, is more directly established by a very curious group, that of Tardigrada, which, with the general organisation of the former, possesses the segmental division of the body, the setigerous appendages, Fig. 145. Fig. ] 4fi. Stephanoceros Eichornii . Noteus quadricornis ; a, dorsal view: b, side view. and the mode of progression, characteristic of the latter. The number of segments is usually small, rarely exceeding five or six ; and in this and other characters, the Tardigrades present a strong resemblance to the early embryonic condition of the true Annelides. — Lastly, in the enclosure of the body of some of the higher forms of this group within a shell-) ike envelope (Fig. 146), and in some points of their internal organisation, there appears to be an approximation towards the simpler forms of En tomoslracous Crustacea (§ 317). GENERAL VIEW OF THE ANIMAL KINGDOM. ANNELIDA. 365 311. The class Annelida includes the higher types of the Vermiform subdivision of the Articulated series, — that in which the body is more or less worm-like in aspect, and is destitute of articulated members. We find in it, however, a foreshadowing of the presence of such members ; most of the segments of the body, in the greater part of the class, being furnished with projecting setce, or bristles, which serve to assist in progression. The body in this class is usually very long, and nearly always presents a well-marked segmental division ; the segments are for the most part equal and similar to each other, it being the head alone that is different. In the lower forms, however, such as the Leech and its allies, the segmental division is very indistinctly seen, on account of the general softness of the integument ; and in these we scarcely find the cephalic segment ostensibly different from the rest. In their general organisation, they present a considerable advance upon the higher Tre- niatoid and Nematoid Entozoa, to which they seem most nearly allied ; the head being provided with special organs of sense, and the mouth having an apparatus for the reduction and prehension of food ; the intestinal canal, though sometimes simple, being frequently furnished with complicated glandular appendages ; the circulating system being much increased in extent and complexity, and a special respiratory apparatus being almost invariably developed ; and the nervous system being formed upon a higher plan, the two ventral cords being closely approximated to each other on the median line, and being studded with a succession of distinct ganglia which correspond with the segments. In the general structure of the different orders of this class, however, there are some important dif- ferences, which require that they should be separately noticed. 311a. The order which may probably be considered as the type of the group, is that of Dorsibranchiata ; so named from the attachment of the respiratory appendages to the dorsal sin-face of the body, along the greater part or the whole of its length (Fig. 147). The animals which it includes are all aquatic, worm-like in their appearance, and active in their habits, crawling with facility and swimming rapidly ; and from this last circumstance they are sometimes designated as Errantia, in order to distinguish them from the succeeding group, in which an opposite mode of life prevails. The head of these worms is always quite distinct from the trunk, and is furnished with one or two pairs of rudimentary eyes, which are seen as black or reddish specks upon its upper side, and also with one or more pairs of tactile appendages, resembling the antennse of Insects, &c., except in being soft and unjointed. The body is frequently of very considerable length, some of the tropical Nereids being not less than four or five feet long, and possessing four or five hundred segments. These segments are externally similar to each other excepot in size ; and their appiendages, also, are almost precisely repleated from one extremity of the body to the other. These appoendages vary in the different genera ; but we usually observe that there are the rudiments at least of two fleshy tubercles on either side of each segment (thus making four for the entire segment), from which these appendages proceed ; and these are termed the ‘dorsal oars’ (d, a') or the ‘ventral oars’ (s'), according as they project from the upper or the lower half of the segment. Each oar usually possesses a long soft cylindrical appendage, or cirrhus (d, a, e ), analogous to the antenniform appendages of the cephalic segment, which proceeds 3GG GENERAL PHYSIOLOGY. from the base of the tubercle ; whilst its summit bears a tuft of setae, or bristles, which serve as instruments of locomotion when the animal is crawling over solid surfaces. In the ordinary Nereids, each oar has also a membranous lobe (that of the dorsal oar is shown at l), which is the Fig. 147. instrument of propulsion in water. All these probably serve as organs of respiration, exposing to the air dif- fused through the surrounding water the blood transmitted to them ; hut there is a more special respirato'-.- organ (d) developed from the under side of each dorsal oar, consisting either of a branching tuft, or of a flattened vesicle, to which the blood is minutely distributed ; this, however, is not always continued through the entire length of the animal. — The mouth is usuaEy situated at the extremity, or rather on the under surface, of the head ; it is armed in some species with one, two, or even three pairs of jaws, opening sideways; whilst others have a proboscis (Fig. 147, b, c) which is capable of being projected from the head, and everted in such a manner as to bring into play a set of homy teeth which are at other times withdrawn within it. In most of the Nereids, which consti- tute the principal part of this order, there is no distinction between sto- mach and intestine, but the aliment- ary canal proceeds with little altera- tion in size from the mouth to the posterior extremity ; it is, however, frequently furnished with ccecal ap- pendages, apparently of a glandular character. In all of them there is a complex vascular apparatus ; and we find this to be provided, not merely with a contractile impelling cavity in the front of the body, but with a multitude of pulsatile organs, which are situated at the bases of the branchiae, and which propel the blood through these organs. The blood is red in these as in the greater num- ber of Annelida ; but this is not a character of much importance, since the hue of the fluid does not depend upon the presence of red corpuscles (§ 173) floating in it, but is derived from colouring matter diffused through the plasma. — The nervous system 4 a, Nephthvs Hombergii ; b, its proboscis; c, the same laid open, to show the horny teeth it contains; d, one of the feet, showing a', dorsal oar; b', ventral oar ; a, dorsal cirrhus ; b, mem- branous lobe of dorsal oar; c, tentaculiform ap- pendage ; d y branchial appendage ; e, ventral cir- rhus ; /, membranous lobe of ventral oar. GENERAL VIEW OF THE ANIMAL KINGDOM.— ANNELIDA. 3G7 of the Dorsibranchiata presents the highest type which it anywhere attains in this class ; the cephalic ganglia being much superior in size to the ganglia of the ventral cord, in virtue of the development of organs of special sense with which they are connected ; and indications being- observable of a set of nerves and ganglia appertaining to the digestive and respiratory organs, distinct from the proper ventral gangliated cord which is subservient to motion alone. Moreover, the two halves of the latter are closely approximated on the median line ; but them ganglia are still usually bilobed. The multiplication of these ganglia is conformable to that of the segments ; so that as many as five hundred double ganglia, repeating each other precisely in function and connections, are sometimes met with. The complex muscular apparatus of each segment is repeated in like manner.— Notwithstanding this complex organisation, we find even in the highest Annelida a remarkable power of regenerating portions of the body which have been cut off ; and in certain species, which undergo spontane- ous fission, the whole body is reproduced from the paids that are detached. This power is obviously related to the original mode of development of these animals, which, like the Taeniae, come forth from the egg without the least rudiment of a body ; the formation of this being subsequently commenced by a process of gemmation from the posterior part of the cephalic segment (which at first almost resembles a ciliated animalcule), and being continued by the subdivision of the last segment that has been developed. This process would seem to go on, in some species at least, without any precise limitation ; and in some of the Nereids we find the same detachment of egg-bearing segments from the posterior part of the body, which we have seen in the Tienia (§ 309 c); the chief difference consisting in this, that whilst in the Tienia these segments are detached singly, and are unprovided with organs of locomotion, those now alluded to are detached in groups, each of them having a head and eyes, with the ordinary apparatus of motion, but their digestive apparatus being so imperfect, that these moving egg-sacs do not seem able to assimilate food for themselves. The resemblance of these Annelida to the Cestoid Worms, as regards their reproductive operation, is further borne out by the universal diffusion of their true generative apparatus ; the sexes not being- distinct, and both male and female organs being repeated in every segment except the cephalic, and each discharging its products by an orifice of its own. 3116. The order Tubicolce consists of Annelida which closely resemble the preceding in general structure, but which have the respiratory organs developed from the first or cephalic segment alone. The name of the order is derived from the fact, that the animals composing it are for the most part inhabitants of tubes ; and to this habit then’ structure is most beautifully adapted, since it is only by such a transposition of the branchial tufts, that the blood of animals thus enclosed could be effectually aerated. The materials of these tubes, however, vary con- siderably. In the Serpula (Fig. 148), a true shell is formed, agreeing in all essential respects with that of Mollusca, and hardly in fact to be distinguished (when the animal has been removed) from that of the Gasteropod Vermetus. The tubes of Serpuke are generally found cluster- ing in masses, and attached to the surface of stones, shells, or other bodies which have been immersed for any length of time in the sea ; they 368 GENERAL PHYSIOLOGY. are usually more or less contorted in form, varying in this respect accord- ing to the position in which they grow ; but they are always closed at one end, which tapers to a point, whilst the wide open mouth gives exit to the head of the inhabitant with its branchial appendages. The SaleUa appears to have some power of exuding calcareous matter from its surface, but strengthens its shell by the agglutination of particles of fine sand, clay, or mud to its exterior. The Terebella forms its tube entirely by the agglutination of grains of sand, of pieces of shell, endra, as in the Nereids, are distinct from each other; but they are more numerous, of higher organisation, and clustered in closer proximity to each other. In the lulus they coalesce into a composite mass on each side, closely resembling the composite eye of Insects and Crustacea. The antennae are, without doubt, instruments of tactile exploration ; and they may possibly be subservient also to the reception of olfactive or auditory impressions. — In regard to the structure of the generative apparatus, a remarkable difference exists between the lower and the higher tribes of Myriapods ; for whilst in the former it is arranged upon the plan of that of Annelida, in the latter it more re- sembles that of Insects. The sexes, however, are always distinct, the two sets of organs never being united in the same individual. In the lulus, both the male and female organs extend through a great part of the body; but instead of being separately repeated in each segment, they coalesce into continuous elongated sacs. In each sex, however, the generative opening is near the anterior part of the body. In the Scolopendra, on the other hand, the generative apparatus is entirely restricted to the posterior paid; of the body, where it occupies seven segments ; it is remarkable, however, that in the structure of the spermatic organs there is more of segmental distinctness than in the lulus. The history of the development of the embryo of Myriapoda presents a remarkable resemblance to that of the true Annelida ; for the embryo, at the time of its emersion from the egg, possesses but a small number of segments ; and these continue to increase , by the repeated subdivision of the penultimate segment, until the number characteristic of the species has been attained. The original number of GENERAL VIEW OF THE ANIMAL KINGDOM. MYRIAPODA. 377 segments rarely exceeds nine ; but it is sometimes multiplied, during the period of growth, to eight times that amount. In the lulus, the larva at its emersion from the egg seems entirely destitute of legs, these mem- bers not making their appearance until after the first moult ; they may be found, however, beneath the outer skin when this is stripped off. Whilst the progress of development is still going on, a considerable power of regenerating lost parts is exhibited by these animals, the antennae and legs being reproduced after being cut off ; but it does not appear that any portion of the body which has been separated can be renewed, or that it can itself regenerate the other parts ; and the reparative power appears to be lost, as soon as the animals have attained their full development. 313. In the highest division of the Articulated series, which is chiefly made up of the three principal groups of Insecta, Crustacea , and Aracli- nida, we find the general tendency, of which we have seen an indication in the superior Myriapoda, — that, namely, to concentration and spe- cialization, — more completely carried out. The number of segments in the body is considerably reduced ; they are more intimately united one with another ; they no longer present the same repetition of similar parts, but differ considerably in their respective endowments ; the motion of the body is almost entirely delegated to the appendages ; the number of j which is reduced, whilst a greater variety of conformation is observable among them ; and the internal organs exhibit a more or less complete restriction to particular divisions of the body, instead of being uniformly diffused through the whole series. It is peculiar to the history of the development of the animals included in the three groups just named, that at the time of the emersion of the embryo from the egg, it has nearly or completely attained the number of segments characteristic of its perfect state ; and that in the changes which it may subsequently undergo, therefore, there is little or no multiplication of segments, but rather a higher elaboration of the structure of some or of the whole of them. Although presenting this general accordance, however, in regard to their nisus of development, these classes are constructed upon very different types, and each of them includes an extensive range of forms ; so that it is difficult to say which should be considered as the most elevated, and which as the lowest, in the series. The animals of two of these classes, namely, Insects and Spiders, are atmospheric in their respiration ; breathing by means of air introduced into passages or cavities in the interior of their bodies. But between these two there is a considerable difference. The body of the Insect, in its perfect form, manifests a division into three distinct parts, head, thorax, and abdomen ; in the Spider, on the other hand, the head is united to the thorax, forming a large ‘ cephalothorax.’ The Insect in its perfect state never possesses more than six legs ; whilst the animals of the Spider tribe have never fewer than eight. The Insect, in its typical form, possesses also two pahs of wings ; and of these not a rudiment exists among the Spiders or their allies. The Insect usually comes forth from the egg in the condition of an Annelide, and only attains its characteristic form and structure after a series of metamorphoses, in which its original consti- tution is greatly modified ; in the greater part of the Spider tribe, the young is completely formed at the time of its emersion, and in none does | h undergo any real metamorphosis. — On the other hand, the class of 378 GENERAL PHYSIOLOGY. Crustacea includes a more varied series of forms ; some of which may lie regarded as the aquatic representatives of Insects and Spiders, and agree with them in the general elevation of their plan of structure ; others would seem, in like manner, to represent the Myriapods ; whilst in others we find a very inferior grade of organisation and a much wider departure from the characters of the class, so that the true affinities of these animals are chiefly to be determined by the history of them development. Thus it has been ascertained that certain reputed Entozoa * and a group which has received the designation of Epizoa, must be considered as aberrant Crustacea ; whilst even the Cirrhipoda, whose claim to a place in the Articulated series was long overlooked, must be regarded as finding then true place on the borders, if not actually within the limits, of the same class. — In many parts of their organisation, the highest Crustacea and Arachnida approach the Yertebrated series much more closely than Insects do; espe- cially in the concentration of them nervous centres, of their circulating apparatus, and of their respiratory organs, and (in the Crustacea) in tin high development of the liver. But, on the other hand, we find in Insects the highest perfection of the Articulated type of structure ; the characters which are most peculiar to the series, taken as a whole, being most com- pletely developed in that class. Thus it is in Insects that we find the greatest activity of locomotion, the largest predominance of the nervo- muscular apparatus over the nutritive, the most acute sensations, and the most extraordinary display of instinctive power, unmodified ly intelligence. And it is in Insects, as already stated (§ 281), that we find the greatest multiplication of distinct forms ; the number of species in this class being probably at least twenty times that of all the other classes of Animals put together. 314. In considering the general characters of the class of Insects. we find, that, putting aside these differences in the conformation of the mouth, which serve as the chief foimdation for the subdivision of the class into orders, there is on the whole a remarkable conformity to one general type. The tegumentary skeleton of Insects, which in some tribes (especially the Coleoptera or beetles) possesses a re- markable thickness and resisting power, is composed of epidermic cells, consolidated partly by an albuminous deposit, but still more by a peculiar horny substance, to which the distinctive appellation of chlt'nu has been given. This differs from ordinary horn in not being soluble in caustic potass, and in not being softened by heat ; so that parts com] '< seJ of it retain then - form even after being incinerated. With this substance a small quantity of phosphate of lime appears to be usually blended. The regular number of segments is thirteen ; the first being the head : the thorax being constituted of the three following ; and the remaining nine forming the abdomen. Any departure from this number is on the side of diminution rather than of excess, the last segment, or even two segments, being sometimes imperfectly developed, and not distinctly separated from the preceding ; but where this is the case, the last ganglion of the ventral cord is of unusual size, and is sometimes bilobed. as if formed by the coalescence of two distinct centres. — The cephalic segment bears a pair of jointed appendages, the antenna f, which an * The Acanthotheca have been recently proved by M. Van Beneden to be Crustacean allied to Lerncea. See Ann. des Sciences Nat., Aout, 1849, p. 67. GENERAL VIEW OF THE ANIMAL KINGDOM.— INSECTS. 379 frequently of considerable length, and which present a great variety of form. — From the thoracic region are developed the proper locomotive appendages; each of its three component segments having its pair of legs, and the second and third segments each bearing a pair of wings in addition. The legs consist of several pieces, which, from the analogy they have been supposed to present to the limbs of Vertebrated animals, have received the names of coxa or ‘hip’, femur or ‘ thigh ’, tibia or ‘shank’, and tarsus or ‘ foot’, — terms, however, which are by no means , strictly applicable. The wings have not the least analogy, except in Fig. 151. Antennae E y es V \ Head. 1st pair of legs 1 st pair of wings 2nd pair of legs 2nd pair of wings 3rd pair of legs Tibia Tarsus External skeleton of an Insect. function, to the wings of Vertebrata, — these last (when adapted for actual flight) being formed by a modification of the anterior pair of limbs for the •support of an expanded surface,— but are, in fact, extensions of the super- ficial tegumentary membrane (which is usually very thin and transparent) over a framework formed by an extension of the denser portion of the integument ; and this framework is penetrated, throughout its ramifica- tions, by ‘ tracheae ’ or air-tubes, communicating with those of the interior of the body. The essential structure of the wings, in fact, corresponds olosely with that of the external respiratory organs with which the iquatic larvae of certain Insects (as the Libellula, or ‘ dragon-fly ’) are provided : and they have hence been not inappropriately designated by Oken as ‘aerial gills,’ a term of which the applicability becomes more apparent, when the method in which the first expansion of the wings is iccomplished (§ 315 g) is understood. The texture of the wings varies in different orders ; but the variation depends chiefly upon the degree in viiich the denser layer of the integument of the body is continued into I i 1 380 GENERAL PHYSIOLOGY. the wings, and in which their surface is covered by epidermic scales. Thus in the Neuroptera and Hymenoptera, the wings are composed of two layers of an extremely thin membrane, extended over a light frame- work of the horny tissue with which the body is enclosed ; and their surface is nearly smooth, bearing only a few scattered hairs. But in the Lepidoptera, the wings, themselves of similar texture, are covered with a layer of epidermic cells or ‘ scales,’ arranged in an imbricated manner . and it is to these scales that the colours of the wings of butterflies, moths. &c., are entirely due. In the Coleoptera, the wings of the posterior pair are simply membranous ; but those of the anterior are entirely consolidated by the amount of horny tissue they contain, and are converted into elytra or ‘wing-covers’, not being employed in flight, but being only of sufficient size to cover the back when closed together, and thus serving as protectors of the time wings which are folded beneath them. In the orders Orthoptera, Hemiptera, and Homoptera , we have various intermediate gradations between these two extremes ; one or both pah's of wings being sometimes of a coriaceous or parchmenty texture throughout ; whilst in other cases, the wings are dense at their base and membranous towards their extremities. In the Diptera, only one pah’ (the anterior) of these organs is developed ; the posterior being represented by a pair of small club-shaped appendages, which are termed the ‘ balancers.’ In nearly all the orders of Insects, certain species are to be met with, in which one or both sexes are apterous or destitute of wings ; but there is a considerable group, made up chiefly of parasitic insects (such as the Flea, Louse, lire.) in which the absence of wings is a constant and important character. — The relative development of the segments of the thorax bear's a close relation to that of the wings ; and it is frequently so unequal, that the regular segmental division cannot easily be discovered, this being further obscured by the partial or complete union of one piece with another, whereby a more secure attachment is given to the powerful muscles that move the legs and wings. Thus, in many cases the first segment is extended so far backwards, on the dorsal surface of the thorax, as completely to cover-in the other two ; so that these can only be recog- nised by examining the ventral stuface, where they may be distinguished by observing the intervals between the origins of the three pail's of legs. — The segmentation of the abdomen, however, is usually very regular ; the divisions of the tegumentary skeleton being simple rings, which dii'er in little else than in size. The chief exception is in regard to the last segment, which is frequently modified in its development for particular pm-poses. These abdominal segments, in the perfect Insect, are never furnished with the least rudiments of locomotive appendages. 315. There is an immense variety in the class of Insects, as to the mode in which food is taken into the mouth ; but we may take the oral apparatus of the Coleoptera and the Lepidoptera as presenting us with its two principal types, which are designated as the mandibulate and the haustellate respectively. In the Coleoptera and other 1 mandibulate ' insects, we find an apparatus adapted for the division and prehension of solid substances ; and this consists of two pairs of jaws opening laterally, the one above and the other below the entrance to the cavity of die mouth, the former being designated as the mandibles, and the latter as the maxillae- ; with an upper lip or labrum, and a lower lip or labium. GENERAL VIEW OF THE ANIMAL KINGDOM. INSECTS. 381 This labium is elsewhere seen to be composed of a pair of elongated pieces which constitute the tongue, and of a central piece on which these are supported, which is termed the mentum or chin. The maxillae usually bear one or two pairs of small jointed appendages, termed palpi ; and another pair of palpi is borne by the labium. These palpi appear to be sensory organs for taking cognizance of the food when it is near the mouth ; and they also serve to hold the food between the mandibles, whilst these divide it. The maxillae are sometimes enormously developed, forming a large pair of pincers on the top of the head, as in the Stag- Beetles. — The ‘ haustellate ’ Insects, on the other hand, are adapted to imbibe their food by suction ; and where, as is the case with the Lepi- doptera, there is no occasion for any apparatus for the division of the tissues whose fluids are to be drawn up (these insects imbibing the juices of flowers which are already poured forth), we find the most complete deficiency of all the parts of the mouth, save those which enter into the formation of the haustellium or ‘ proboscis.’ This organ is in reality composed of the two maxillae, which are excessively prolonged, and united on the median line ; and each maxilla being channelled on its internal margin, the union of the edges of the two channels forms a complete tube. At the base of the proboscis is seen a small membranous piece, which is the representative of the labrum ; and on each side a small tubercle, ! which is the only vestige of the mandible. Rudiments of maxillary palpi are also discoverable ; whilst behind the proboscis is a small triangular labium, having two very large labial palpi, composed of three joints, and nearly always clothed with scales. In the Hgmenoptera (or Bee and Wasp tribe) we find the mouth presenting a curious adaptation of the mandibulate type to the suctorial mode of obtaining the food. The labrum and the mandibles are constructed as in mandibulate insects ; ; and the mandibles serve as the instruments with which the remarkable fabrics of these animals are constructed, or with which the insects, Ac., are killed, whose juices serve for their food. The maxillae and the tongue, however, are greatly elongated ; the former being so shaped as to inclose the latter and form a tube when they are brought together, but not being- adherent along the line of junction ; and the tongue having a pair of lateral appendages of its own. The labial palpi, also, are of unusual size. In the suctorial Hemipteva (Bugs, Cicadse, Ac.), the mandibles and maxillae are converted into lancet-shaped organs, which serve to puncture the vegetable or animal tissues whence these insects imbibe the juices on which they live; whilst it is through the grooved labium, with which the labial palpi seem to have coalesced, that these juices are drawn into the mouth. — Such are some of the principal methods in which Insects obtain their food ; the more detailed scrutiny of them would still further demon- strate the wonderful variety, which is produced by different modes of development and combination of the same elementary organs. 315 a. The structure of the alimentary canal presents, as might be , expected, a similar variety ; its most complex forms, however, are found in the mandibulate insects, especially such as live on coarse vegetable food ; whilst its simplest condition is seen in those which live upon animal juices. In the former, the mouth is furnished with salivary glands, whose secretion assists in the act of mastication ; the oesophagus is generally wide, and is frequently dilated into a ‘ crop from this, the i 382 GENERAL PHYSIOLOGY. food passes into a muscular stomach or gizzard, the walls of which are frequently armed with homy plates or teeth, which assist in the commi- nution of the contents ; and below this is the true digestive stomach (Fig. 152, cl), whose texture is soft and delicate, and which often has a number of minute follicles (for the elaboration of the gastric secretion) projecting from its walls. From the stomach proceeds the intestinal tube (e), which is usually of much smaller diameter than the preceding portions of the canal, and which is frequently much convoluted ; this receives, close to its commencement, the terminations of the secreting Fig. 152. Ideal section of Sphinx ligustri (Privet Hawk-moth), to show the relative position of its organs : — 1-13, the successive segments; a, a, dorsal vessel; b, b, ligamentous bands holding it in its place ; c, oesophagus ; d , stomach ; e, intestinal canal ; f, biliary tubuli ; g, ccecum . h, cloaca; i, ovary; — the gangliated nervous cord is seen to commence in the large cephalic ganglia beneath 1, and to pass backwards along the floor of the thorax and abdomen. " tubuli (/), which constitute the only rudiment of a liver possessed by these animals ; whilst, lower down, it sometimes receives another set of tubuli, which seem to he urinary in their character. Not unfiv- quently the intestine dilates in its course into a ccecum (g); its anal orifice is always at the posterior extremity of the body, where we find « sort of cloaca m), which receives the termination of the intestine, and is also the outlet of the generative organs. — The circulation of the blood in Insects is carried on in part by means of distinct vessels, and in part by channels excavated in the tissues. Its central organ is the 1 dorsal vessel ’ (a, b), which, as in Myriapods, is segmentally divided ; the compartments are separated by valves, which do not allow the blood to pass in any other direction than from behind forward. This segmertal division, however, in the perfect insect, does not extend into the thorax : the dorsal vessel in that region being converted into an aortic trunk, which carries the blood onwards, that it may be distributed to the head and thoracic appendages. From these it returns backwards along the limbs and body, to re-enter the dorsal vessel, either by veins which open into its several chambers, or by larger vessels that collect the whole to convey it into the posterior chamber. In its course, however, it is brought into very close relation with the air that is conveyed through the whole interior of the body by the complex tracheal apparatus ' for it appears from recent observations * that the blood not only bathes the exterior of the air-tubes, but moves through that space between * Blanchard, in Ann. des Sci. Nat., Juin, 1848. GENERAL VIEW OF THE ANIMAL KINGDOM. INSECTS. 383 the outer and inner membranes, in which a spiral filament winds (as in the spiral vessels of Plants, § 151) to keep them from being closed by lateral pressure. — These air-tubes form a complex system, which is distributed with the most elaborate minuteness throughout the body, commencing from lateral stigmata or breathing-pores, of which each (Segment normally contains a pair, though some of them are frequently closed up, so that the number is considerably reduced ; between all the parts of this system there is the freest communication ; and in some parts of it, especially in Insects of most rapid flight, we find the air-tubes dilated into large air-sacs, which both serve as reservoirs of air, and contribute to diminish the specific gravity of the body. By this extra- ordinary development of the respiratory apparatus, the apparent imper- fection of the circulating system is compensated ; since the chief demand for a very rapid movement of the blood, in animals which (like Birds) put forth a vast amount of muscular energy and activity, arises not so much out of the demand for nutrition, as from the necessity for a constant supply of oxygen to the tissues, which is here provided for by the pene- ration of the air itself into their substance. 315 b. The nervous system consists, as in other Articulata, of a series )f ganglionic centres corresponding to the successive segments, united by i pair of fibrous cords, and disposed along the ventral surface of the j jocly ; but we no longer meet with the same uniformity which we have ;een in the lower classes of this sub-kingdom. The cephalic centres here possess a much greater proportionate size, in accordance with the high levelopment of the eyes and other sensory organs ; they are placed mtirely above the oesophagus (Fig. 152 ) ; and a cord of communication Passes down on either side, to unite in the first infra-oesophageal ganglion, rom which the ventral cord is continued backwards. The thoracic entres, from which the nerves of the legs and wings proceed, are much larger 'than those of the abdomen, which do not supply any locomotive ■rgan ; and the. former are sometimes found to have coalesced into a iugle mass of large dimensions, whilst some of the latter seem altogether ranting. The respiratory and stomato-gastric systems of nerves, also, ttain a higher development ; the ganglia of the former being distinctly raceable along the ventral cord, intermediate between the locomotive entres ; whilst those of the latter are found in front of and behind he great cephalic ganglia, and also among the viscera to which the erves are distributed. There are also indications of the existence of set of nerves comparable to the ‘ sympathetic system’ of Yertebrated nimals. — The development of the organs of sense, and especially of the isual apparatus, attains a much higher elevation in Insects than in nv of the inferior classes. The clustered eyes of the Myriapods are ere aggregated into large compound masses, usually of nearly lremi- rherical form, which are so large as to occupy a considerable part of the des of the head. The structure of each individual eye (which is of parly cylindrical form) seems most perfectly adapted to bring to a >cus the rays which impinge upon it in the direction of its own axis ; rd by the mode in which the single eyes of each hemispheric mass are psposed, the range of vision is extended in every direction, although Pe eyes themselves are perfectly motionless. This multiplication of lindrical eyes precisely similar to each other, to gain an end which is 384 GENERAL PHYSIOLOGY. answered in Vertebrated animals by a single globular eye, endowed with the power of motion, on either side, is in remarkable accordance with tin general plan of structure characteristic of the Articulata (§ 308), and h sometimes carried to a most wonderful extent ; the number of single eyes, in the common House-Fly being 4000, in the Cabbage-Butterfly 1 7,000. in the Dragon-Fly 24,000, and in the Mordella Beetle 25,000. Beside' the great compound eyes, most insects have a few minute simple eyes, disposed upon the top of the head, in the narrow space between the aggregate masses ; and these appear to be of considerable use in directing their upward flight.- — There can be no doubt that Insects are possessed of the sense of hearing ; for although the precise organ which is sub- servient to it has not been determined, there is ample evidence that they are guided and influenced by sounds, one of the most striking indications of which is, that the males of many Insects (such as Cicada-. Crickets, &c.) possess the power of emitting peculiar sounds, which attract the females to them. These sounds, however, are produced by purely mechanical means, and cannot be regarded as vocal. It would seem probable, from various considerations, that some part of the base of each antenna is the auditoiy organ. — There is similar evidence that Insects possess the sense of smell ; and there is a strong probability that this is exercised by the soft sm-faces of some of the appendages to the mouth. Thus, in some cases, the palpi are swollen at their extremities into club-shaped dilatations, composed of a soft membrane which shrivels when dry ; and the insects may be seen to apply these to particles of food and liquid, as if taking cognizance of their odorous properti..- before taking them into the mouth. In the common house-fly, the same use is made of the sheath of the proboscis. It is certain that other actions of insects are guided by the sense of smell ; thus the flesh- fly deposits its eggs in the thick fleshy petals of the Stapelia (cairion- flower), deceived by its odour which resembles that of tainted meat— The nicety of selection shown by many Insects in their choice of food seems further to indicate that they are possessed of the sense of taste how far the tongue is the instrument of this sense, however, remains t< be ascertained. — As the enclosiue of the body of Insects in a dense integument must prevent any considerable exercise of the sense of touch by the general surface, we find particular parts of it specially modified for the exercise of the tactile function. That such is the principal purpose c ' the antenme, notwithstanding that they are themselves usually covered by an integument as firm as that of the rest of the structure, cannot foi a moment be doubted by any one who watches the exploratory actions oi these organs ; the extremities of these antennae are frequently fumishei with plumes, tufts, or delicate laminated expansions, which must peculiarly fit them for such purposes ; and they are always copiously supplied witl nerves. In many cases, too, we find that the extremities of the feet art furnished with peculiar organs, such as soft cushions, or delicate expands suckers, that are apparently adapted to receive tactile impressions iron the surfaces to which they may be applied. 315 c. The muscular system of Insects (as already remarked. § 308 c) i- highly developed; being entirely made up of the striated muscular fibr- in its most perfect form (§ 223) ; and consisting not only of muscles to the contraction and elongation of the trunk by the approximation ■ GENERAL VIEW OF THE ANIMAL KINGDOM. INSECTS. 385 separation of its segments ; but also of numerous muscles which give motion to the legs and wing’s (Fig. 138). In insects of rapid and power- ful flight, these last are so highly developed as almost to fill the cavity of the thorax. The joints are for the most part so constructed, however, as to admit hut of two kinds of movement, namely, flexion and extension ; and the muscular apparatus has consequently not that variety of action which is seen where the ‘ ball-and-socket joint,’ which permits movements of circumduction, takes the place of the simple ‘hinge-joint.’ Neverthe- less there are no animals which surpass Insects in command over the ' organs of flight. Even the swallow, as Prof. Owen remarks, cannot match the dragon-fly ; “ this insect has been seen to outstrip and elude its swift pursuer of the feathered class : nay, it can do more in the air than any bird, it can fly backwards and sidelong, to right or left, as well as for- wards, and alter its coiu-se on the instant without turning.” When, moreover, we compare the actual space traversed by an Insect hi a given time, with the dimensions of its body, we find that it vastly exceeds the similar ratio in the Bird ; and thus we perceive that the locomotive powers of Insects are far higher than those of any other animals whatever. This power is most remarkably developed in the orders Neuroptera (Dragon-flies, Termites, &c.), and Hymenoptera (Bees, Ants, &c.) ; and it is remarkable that these are the very orders in which we find the most I extraordinary manifestation of those instinctive tendencies, the high de- velopment of which, with an almost complete absence of intelligence, is a striking characteristic of the Articulated series in general (§ 308), and of the class of Insects in particular. These tendencies, as will be hereafter [shown (chap, xx.), may be considered as dependent upon an association between sensory impressions and muscular movements, which arises from the original constitution of the nervous systems of these animals ; and they may thus be regarded as necessities of their nature, not in the least indicative of intelligence, design, or voluntary choice on their own parts, -but rather indicating the wise adaptation, by which they have been con- structed to work out plans of most admirable elaborateness, with the most wonderful perfection. Now 7 these and all other instinctive actions have or their object the maintenance of animal life ; as distinguished, on he one hand, from the mere organic life of Plants ; and, on the other, rom the mental or psychical life of higher being’s. And thus if we con- icier the Animal kingdom, as holding an intermediate position between he Vegetable world on the one side, and the domain of Mind on the other, re should be led to regal’d the class of Insects, and especially the orders vleuroptera and Hymenoptera, as its type. 315 d. Insects are endowed with extraordinary powers of multiplica- ion ; but this is accomplished, with only one exception, by means of the 'exual process of generation. The exception referred to is that of the 1 phis, which is capable of propagation by a process that appears to be nalogous to the gemmation of the Salpce (§ 299 6) ■ the new individuals eing budded off, so to speak, from internal stolons, instead of being eveloped from ova provided by the female and fertilized by the male, ’his method of propagation may be several times repeated ; the in- jividuals thus generated being all apparently of the female sex, and enerating others like themselves. At the end of the season, however, erfect winged males and females are developed ; and these concur to c c 386 GENERAL PHYSIOLOGY. produce ova, which retain their vitality through the winter, and give birth to a new generation in the spring, long after the parents have perished. Each viviparous Aphis is capable of producing a hundred repetitions of itself ; and as the process may be repeated ten times in the season, it has been calculated that, if no destructive agencies were at work, the last brood would number 1,000,000,000,000,000,000 individuals. The con- ditions under which this curious phenomenon takes place will be more particularly examined hereafter (chap, xviii.) ; at present we must be satisfied with noticing it as a remarkable fact, that such an extraordinary power should he confined (so far as is yet known) to a single genus among Insects, no traces of it having been elsewhere observed ; and that a method of propagation which is restricted to the lower classes in the Radiated and Molluscous sub-kingdoms, should be met with at so high a place in the Articulated. The sexes are distinct in all Insects ; and the males and females are frequently distinguished by diversities in size, con- figuration, or colour, as well as by the difference in their generative organs. The spermatic glands of the male present numerous varieties in arrangement, in the different tribes of Insects ; being sometimes united together on the median line, and forming but a single globular mass : sometimes elongated and double ; and sometimes multiplied on each side. Whether they be single or multiple, however, their product is at last poured into a single seminiferous duct, from which it is ejected, through a penis or intromittent organ (which is a modification of the one or two last segments of the abdomen), into the female organs. The ovaries of the female present as many varieties in their conformation, as do ti 3 testes of the male; in general, however, they consist of one or more cascal tubuli on each side, in which the eggs are found lying in regular rows, those nearest the outlet of the tube being in a state of most advanced development. These tubuli are often extremely elongated, and are laid in coils •which are closely approximated to each other. In many instances, the masses of eggs reach an enormous size, which is to be especially seen where the sexual organs are only developed in a few perfect females, the others remaining sterile, and acting as ‘ workers, which is the case with Bees, Wasps, Ants, Ac. ; the most remarkable example of this is probably the female Termes (white-ant), whose abdo- men, at the time she commences laying, is from 1500 to 2000 time;- larger than the rest of the body, the eggs being subsequently deposited at the rate of 80,000 a day, for several successive weeks. The oviducts of the two sides rniite into one canal on the median line ; and on this we find, near its outlet, a vesicular dilatation for the reception and retention of the spermatic fluid, which is thus brought into contact with the eggs as they are successively extruded. The last segment of the body of the female is usually modified to perform the functions of an ovipositor : and this, in some tribes of insects (especially the Ich neumon idee), is pro- longed to a length considerably exceeding that of the animal’s body, thus enabling it to lay its eggs in situations to which it could not itself gum access. The ova are usually deposited whilst the embryo is as yet very immature ; and frequently they remain in this condition during a period of many weeks, months, or even years (§§ 96 and 116). In a few instances, however, the eggs are retained until their included embryoes are ready tc escape, so that these are sent forth alive into the world, either in the larval GENERAL VIEW OF THE ANIMAL KINGDOM. INSECTS. 387 stage of development, as is the case with the common Musca vomitoria (flesh-fly), or in the condition of pupae, as is seen in the Hippobosca (forest-fly). 315 e. No Insects come forth from the egg in their perfect condition ; and their state in many cases at the time of emersion is quite embry- onic; so that it is usually not until a series of very considerable changes have taken place in external configuration and internal structure, to- gether constituting what is known as the ‘ metamorphosis,’ that the com- plete development of the specific type is attained. The amount of this metamorphosis, and the mode in which it is accomplished, vary con- siderably in the different orders of insects ; but three stages are usually marked out, more or less distinctly, in the life of each individual. The term Larva, in the ordinary language of Entomology, is applied to the insect from the date of its emersion from the egg, up to the time when the wings begin to appear ; the term Pupa is in like manner employed to mark the period during which it is acquiring wings ; and from the time when these and other organs characteristic of its perfect state are com- pleted, it is spoken of as the Imago. The grade of development, how- ever, at which the insect comes forth from the egg, is very different in the several orders and families ; and it is consequently very unphilosophical , to associate under the same designation beings which are in conditions essentially diverse. In all cases the embryonic mass within the egg is first converted into a footless worm, resembling the higher Entozoa or the inferior Annelida in its general organisation, but possessing the number of segments — thirteen — which is typical of the class of Insects. Such, in the Diptera and Hymenoptera, and in some of the Coleoptera, is : the condition of the larva at the time of its emersion from the egg ; and it is remarkable that many of the larvae of the first of these groups resemble Entozoa in their parasitic habits. The head, in larvae of this kind (which are familiarly known as ‘maggots’), differs but little from the segments of the body; the eyes in many instances not being developed, and the mouth being furnished -with a mere suctorial disk. In the I^epidoptera and most of the Coleoptera , however, the larva at the time of its emersion possesses the rudiments of the three pairs of thoracic legs, although they are little else than simple claws (Fig. 153), save in the carnivorous Beetles ; whilst in addition to these, several of the abdominal segments are furnished with fleshy tubercles or pro-legs (generally to the number of fcmr or five pairs), which are peculiar to the larva-state. In such larvae (which are commonly designated as ‘caterpillars’) we observe a remarkable equality in the different segments, both as to size, form, and plan of con- struction, which strongly reminds us of the Annelida. The alimentary banal occupies nearly the whole of the cavity of the body, and passes vithout flexure from one end of it to the other. The compartments of he dorsal vessel, the respiratory organs, the nervous centres, and the nuscular bands, are repeated with great regularity ; and there is as yet io distinction between the thoracic and abdominal portions of the trunk Fig. 153). The head, however, is usually protected by a horny covering, md is provided with simple or clustered eyes like those of the higher Annelida and Myriapoda ; and the mouth is furnished with powerful cut- ing jaws for the division of the food, which is usually vegetable in its lature. In the Orthopterous and Hemipterous orders, however, these 388 GENERAL PHYSIOLOGY. stages of development are passed through within the egg ; and as the young Insect does not emerge thence until it has attained a higher grade, in which it presents a close resemblance to its parents in almost every particular save the want of wings, it cannot be regarded as having the characteristics of a real larva. This is the case, too, with some of the Coleoptera, in which order we find a considerable variety as regards the stage of development at which the embryo quits the ovum. — In the true Larva condition, the whole energy seems concentrated upon the nutritive functions; the quantity of food devoured is enormous ; and the increase in the bulk of the body is very rapid. According to Lyonnet, the comparative weight of the full-grown caterpillar of the Goat-Moth, and that of the young one on its emersion from the egg, is as 70,000 to 1 ; and the maggot of the Flesh-fly is said to increase in weight 200 times in the course of twenty-four hours. During this rapid growth, the caterpillar throws off and renews its epidermis several times ; but the larvae of the Hymenop- tera and Diptera do not undergo this exuviation until they pass into the pupa state, their integument being soft enough to yield to the distension from within. The sexual organs are but little developed during the larval life ; but their rudiments may be detected. The activity of growth, however, seems to supersede in the larva the progress of development : for the tissues remain in an embryonic state, and the organs retain their original condition with little or no essential change except in size, until a sufficient store of nutriment has been taken into the system, to serve as the pabidum for all the subsequent developmental operations, by which the fabric of the perfect Insect is to be completed. Fig. 153. Fig. 154. Ideal sections of Larva and Pupa of Sphinx Ligmtri (Fig. 152), shoving the relative state anil position of tlieir organs: — 1-13, the successive segments; a, a, dorsal vessel; b, b, ligamentous bands holding it in its place; c, oesophagus; stomach; e, intestinal canal ; f, biliary tubuli : g j c tecum ; h, cloaca; i, sexual organ ; k, cephalic ganglion; — the ventral gangliated cord is seen passing backwards from this along the floor of the thorax and abdomen. 315/. Of the mode in which the Insect enters the Pupa state, and of its condition in that state, no general statement can be made ; since they correspond for the most part with the grade of development which, the larva has previously attained. Where it already possesses the general form and structure of the Imago, and little else is required for its completion than the development of the wing's and of the sexual organs. GENERAL VIEW OF THE ANIMAL KINGDOM. INSECTS. 389 this is usually effected without any cessation of its activity ; and after the first moult, which is regarded as marking the commencement of the pupa state, it continues to move about and to take food as usual, whilst its wings are sprouting and gradually becoming elongated beneath the next skin. This, again, is exuviated, and the wings as yet unexpanded are seen on the exterior of the body. After a third moult, the wings which were previously short, thick, and soft, are caused to expand, probably by the injection of ah 1 into their trachea; : and no change : subsequently takes place. This is the case with the Orthoptera , Hemip- tera, and some Neuroptera. It is curious to observe that in the viviparous broods of Aphides (§315 d), neither metamorphosis nor moult- ing takes place ; but that the process of gemmation is performed in a condition which corresponds with that of the larvae of the perfect insects that are developed at the end of the season ; as if the vital force which would otherwise be directed to the complete evolution of the individual, is here employed in the multiplication of the race. — In the Coleoptera, Lepidoptera, Hymenoptera, Diptera, and some Neuroptera , however, the pupa-state is one of complete inactivity as regards all the manifestations of animal life; although the formative processes are carried on with extraordinary energy. The imperfect larvae of these i orders, as we have already seen, are truly embryonic in their condition ; 1 and the processes of development which were commenced within the egg, and which were then only carried far enough to enable the larvae to come forth and obtain their own nutriment, are now continued at the expense of the food which they have collected and stored up within their bodies ; so that the passage into the pupa-state, in such cases, may be fairly likened to a re-entrance into the egg. The pupa is enclosed in the last skin exuviated by the larva, which, instead of being thrown off, dries up and remains to encase the proper skin of the pupa that is formed beneath it ; and in addition to this, it is frequently protected by a silken ‘ cocoon,’ the construction of which was the last act of larval life. The duration of the pupa condition, and the rate at which the developmental changes take place, vary considerably in different cases ; some Insects remaining in this state for years, whilst others pass through it in a few days or even in a few hours ; in both cases, however, we perceive that an important influence is exerted by external temperature (§ 96). As the state of the Pupa is one of rapid transition, it cannot be said to have any characteristic organisation ; the intermediate condition of its structure, however, between that of the Larva on the one hand, and that of the Imago on the other, is shown in Fig. 154 ; and the nature of the changes which its principal organs undergo, will be hereafter explained in more detail (chap, xviii). 315 g. The assumption of the Imago or perfect type of Insect life, is xlways marked by an exuviation of the integument w'hich covered the •pupa ; and with this are cast off all the vestiges of the organs peculiar ;o the larva-state, while the wiugs, the true legs, the compound eyes, the mtenuEe, the complete masticating or suctorial apparatus, and many >ther organs, are now revealed for the first time in all those whose pupa condition was inactive. The wings, however, are seldom ready for use at he time of the Insect’s emersion from the pupa-case ; being usually soft 390 GENERAL PHYSIOLOGY. and moist, hanging loosely at the sides of the body, and having none of that rigidity which is requisite to give them the power of serving as organs of impulsion in air. It is not until the Insect has forcibly injected their trachese with ah', — by taking several full inspirations, and then making an expiratory effort whilst the spiracles are closed, — that the wings are expanded ; they then soon become dried up by exposure to the air, and by the cessation of the circulation which previously took place within them ; and from that time they are the chief instruments of locomotion in all the Insects in which they are fully developed. The nutritive apparatus of the Imago is far less developed relatively to tie muscular, nervous, and sexual organs, than it is in the preceding condi- tions ; and its subordination to the offices of these is shown by the fact, that many Insects take no food whatever after then' last change, the sole purpose of their existence, in their perfect state, being the propagation of the race by the generative process. In many instances, the duration of the Imago state is very brief, even where that of the preparatory periods has been very long ; as in the case of the Ephemera (day-fly), which usually dies within a few hours after its last change, although the term of its previous life as a larva and an active pupa has not been less than two or three years. And even where the length of the life of the perfect insect is much greater, as in Bees, Wasps, &c., it seems to have a special relation to the nurture of the offspring, which are tended and supplied with food during the whole of their larva state. This duty, in the ‘ social ’ Insects, is performed by the ‘ neuters ’ or ‘ workers,’ which are females whose sexual organs have not been evolved ; then - sexuality being proved (in the case of the Hive Bee at least) by the fact that they may be developed into perfect females by a different treatment during the larva state (§ 60). In the Ant tribe, the neuters do not acquire wings ; and some of them, which are two or three times the size of the rest, and are somewhat differently formed, are characterized as ‘soldiers,’ their special office being the defence of the nest rather than the nurture of the young. Among the Termites (white-ants), however, the ‘soldiers' appear to be pupae arrested in their development ; whilst the ‘ work el's’ have the characters of permanent larvce. — In the Apterous orders of Insects, we find some tribes undergoing a regular metamorphosis, which is complete in every respect save the non- development of the wines. Thus the larvae of the Pulex (flea) are footless worms, which afterwards pass into the pupa state, spinning for themselves a silken cocoon ; in this they remain inactive for about twelve days, after which the Imago comes forth, having the rudiments of wings attached to the second and third segments of the body, though without any proper distinction of thorax and abdomen. The Pediculus (louse), Podura (spring-tail), and some other Aptera, however, undergo no metamorphosis ; coming foith from the egg in the condition in which they remain all their lives ; and this being far from the type of the perfect Insect. 316. The class of Crustacea includes all those Articulated animals which are provided with articulated limbs, but which are furnished with organs fitted for aquatic instead of for atmospheric respiration. Thus it may be considered as containing within itself the aquatic representatives of all those Annulose animals, which are characterized, like it. by the possession of articulated members, but which are adapted for living in the GENERAL VIEW OF THE ANIMAL KINGDOM. CRUSTACEA. 391 air : and we find such a representation to he really indicated by the diver- sity in form and structure presented by the members of this class ; some of which (the Isopoda), in the equality of their segments and in their general organization, form a group that may be regarded as the parallel of that of Myriapods ; whilst the Lobster may be considered as a gigantic Marine Insect ; and the Crab in many points of its organization may be looked upon as a Spider adapted for a predaceous life along the shore, instead of for pursuing or entrapping its prey upon land. The classes of Crustacea and Arachnida, indeed, are very closely connected by a curious little group, that of Pi/cnogonidce (§316 course is determinate, partly towards the heart and partly to the gills. At the base of the gills we sometimes find them provided with muscular walls, and serving by their pulsation as accessory hearts, whereby the blood is propelled through the respiratory circulation, as in many Anne- lida. — The Respiratory organs differ greatly, as we have seen, in their position ; being generally attached externally, either to the abdominal or to the thoracic members ; whilst in the Decapod Crustacea they are enclosed within the general envelope, and seem disconnected with the locomotive appendages. They differ also in their form ; being sometime-! membranous vesicles, as in Lcemodipoda ; sometimes simple expanded plates, as in Amphipoda ; sometimes plates dividing into filaments, so as GENERAL VIEW OF THE ANIMAL KINGDOM. CRUSTACEA. 397 in some degree to resemble a feather, as in Stomapoda ; whilst in the Decapoda they have the form of long slender quadrangular pyramids (from 9 to 22 on either side), each of them consisting of a central stem, to which are attached a multitude of thin plates or cylindrical filaments (Fig. 156, i, i). Further, the interchange of the current of water in con- tact with these organs is dependent, in the lower tribes, upon the general movement of the limbs; in the Amphipoda we have seen that certain pahs of the abdominal appendages are specially adapted to maintain this current by their repeated strokes upon the water ; but in the Decapoda we find what seems to be a peculiar apparatus, which serves not merely to maintain a current of water through! the branchial chamber, but also to keep the branchial filaments or lamella} apart from each other, so as to facilitate the action of the water ujion them. A careful examination of the development of the respiratory organs, however, even in this their highest condition, shows that they are here (as elsewhere) to be regarded as appendages of the legs ; that the chamber which encloses them is formed by the gradual folding-in (so to speak) of the carapace around them ; and that the parts by whose action the stream of water is maintained and brought into contact with every individual filament, are nothing else than parts of the regular organs, specially adapted by a peculiarity of conform- ation for this purpose. (See chap. xiii.). Neither in the terrestrial Isopods (such as the ‘ wood-louse ’) nor in the terrestrial Decapods (such as the ‘ land-crab ’), is the type of the respiratory apparatus changed ; for all that is necessary to adapt it for atmospheric respiration, is to keep the gill-filaments constantly moist, for which these animals have a special provision. — The only apparatus known to exist in Crustacea, which can at all be regarded as a urinary organ, consists of a set of simple, long, unbranched, slender tubes, which open into the intestinal canal of some of the higher tribes ; but it has not yet been shown that the contents of these have a urinous character ; and as their place of discharge into the intestine is high up, near the termination of the biliary ducts, it would seem by no means impossible that they are rather to be regarded as a rudimentary pancreas. 316 c. The condition of the Nervous System in Crustacea presents a regular series of gradations, between the type on which it is constructed in the Annelida and Myriapoda, and one of higher concentration than exists in any Insect. The former is seen in those whose bodies display the greatest equality of segmental division, and in which there is the greatest similarity in the endowments of the several members ; for in such we find the ganglia of the ventral cord corresponding in number with the segments of the body, nearly equal to each other in size, and placed at uniform distances from each other ; and in Talitrus we find the distinctness of the two lateral halves unusually obvious, the two strands of the ventral cord being separate along their whole length, each having its own series of ganglia, and the ganglia which are thus arranged in pairs in the succes- sive segments being connected transversely by commissural bands. In the genus Astacus (lobster and cray-fish), on the other hand, we find (as in Insects) the thoracic ganglia developed to a greater size than those of the abdomen, that the locomotive members may receive their due supply of nervous influence ; but as the tail is a powerful swimming organ in these animals, the difference is not so great as it would have been if the 398 GENERAL PHYSIOLOGY. thoracic members had been the sole instruments of locomotion ; and we find the last ganglion, situated above the anus, and radiating nerves to the swimming plates of the tail, particularly large, being apparently made up by the coalescence of the sixth and seventh abdominal ganglia. In Pcilemon (prawn) and Palinurus (rock-lobster) we find a coalescence of the thoracic ganglia into a long elliptical perforated nervous mass ; but this coalescence reaches its greatest degree in the Crab, in -which all the ganglia of the ventral cord are blended into one large oval ganglion, with a perfora- tion in its centre, which is situated near the middle of the under surface of the body, and from which nerves radiate to all parts of the trunk, to the legs, and to the short tail. The relative development of the cephalic ganglia varies with that of the organs of sense and motion which are situated in the head. They usually appear to be made up of those of the four anterior segments of the head, from which proceed nerves that connect them with the eyes, the two pairs of antennae (the second containing the organ of hearing), and the mandibles ; whilst those of the two posterior segments ordinarily coalesce with the first ganglion of the ventral cord, to form the great sub-oesophageal ganglion, whence proceed the nerves to the feet-jaws. There is not, as in Insects, a distinct system of respiratory nerves, these being blended with those of the general sensori-motor apparatus ; but the ‘ stomato-gastric ’ system is here well developed, and its relations to the visceral system of Vertebrata become apparent. — In regard to the develop- ment of the sensorial apparatus in this class, it may be stated generally that, with the exception of the visual organs, no paid of it attains a high standard. The investment of the entire body in a dense crustaceous envelope must necessarily almost completely destroy its tactile sensibility ; and the antennae, being themselves enclosed in a similar casing, cannot be delicate instruments of touch, although they may readily convey impres- sions produced by the mere contact of hard bodies, to the nerves that supply them, just as an inflexible rod will be the means of conveying such impressions to the hand that holds it. The palpi, which are appended to the mandibles and feet-jaws, probably serve (as in Insects) to take cogni- zance of the qualities of the substances which are being employed as food, and may possibly minister to the sense of smell. For taste there is no special organ ; and whatever be the degree of this sense which these animals may possess, it must depend upon the endowments of the mem- brane lining the mouth and oesophagus. For hearing there appears to l>e a special organisation, in the highest Crustacea at least ; the first joint of the second or larger pair of antennae being excavated into a cavity with a round orifice closed by a membrane, and this cavity containing a vesicle filled with fluid, upon which the antennal nerve expands. It is in the Crabs that this apparatus presents its most complex condition. — The visual organs, in all the higher Crustacea, are elaborately constructed : and it is chiefly by the indications which they afford, that these animals are guided in their movements. These organs are formed upon the same plan as in Insects ; each of the large ‘compound eyes’ being made up by the aggregation of a number of ocelli or single eyes, every one of which is a separate instrument of vision, perfect and complete in itself. In the highest division of the group, the compound eyes are mounted upon jointed peduncles, by the flexure of which they can be turned in any direction ; but still, except in a few species, they are protected by the GENERAL VIEW OF THE ANIMAL KINGDOM. CRUSTACEA. 399 over-arching of the carapace, which forms a sort of orbital cavity for their lodgment. It is interesting to remark, that the compound structure of the eyes is one of the most unequivocal indications of the Crustacean nature of the extinct Trilobites, which might otherwise have been asso- ciated with the Chitons (§ 302) ; this structure being so beautifully pre- served, notwithstanding the very early period at which these animals became fossilized, that it even serves to indicate the particular division of the Crustacean class to which they should be referred. — The muscular apparatus attains a great development in this class, forming masses whose size renders the larger Crustacea available articles of food. They are attached to the shell by means of strong tendinous plates, which are con- tinuous with the animal substance of the latter, and which are thrown off with its exuviation. Still there is no great variety in their actions ; those which move the trunk and the limbs being alike limited to the simple movements of flexion and extension of the joints, which are usually so constructed as to allow of motion in only one plane. 316 c?. Although no propagation by spontaneous fission or gemmation takes place in the Crustacea, yet they possess a power of regenerating lost parts which is truly astonishing, the complexity of their organisation being considered. The mode in which this takes place will be hereafter explained in more detail (chap, xviii.) ; at present it will be sufficient to state that it extends to the regeneration of all the limbs, and that it seems essentially connected with the ‘ moulting’ process, the new limb making its first appearance when the shell has been cast off. The sexes are distinct throughout the entire class ; and the generative apparatus of each sex presents this remarkable difference from that of Insects, that the two lateral halves of it do not unite on the median line so as to have a common outlet, but that the orifices of the seminal organs and of the ovaria are separate on the two sides. A progressive complication in their structure presents itself, as we ascend from the lower to the higher forms of this division of the Crustacea ; for in the inferior tribes the testes consist of a small number of vesicles opening into a common tube on either side, whilst in the higher they are made up of a mass of minute convoluted tubes (Fig. 156, /), opening into a common duct, which is oftei^ dilated into a receptacle for the seminal secretion ; and the ovaria, also, are a pair of simple sacculi in the inferior tribes, but in the superior show a division into the proper ovaries (which are long branching cteca communicating with each other on the ttvo sides), the oviducts, and the spermothecce or ‘copulatory pouches’ within which the spermatic fluid is stored up. The orifices of the male organs are usually stated to be in the first joints of the last pair of thoracic limbs; and it has been supposed j that the first and second pairs of abdominal appendages, or false legs, in the male serve the purpose of exciting organs. From recent observations, however, it appears that the spermatic duct of each side passes on as a membranous tube from the base of the thoracic limb to the first or . second abdominal appendage, and runs to the extremity of this, which thus becomes a true penis or intromittent organ.* The spermatic fluid, received and stored' up in the spermothecm, may remain there for a con- I siderable period, and may fertilize ova which are far from maturity at the * See Mr. Spence Bate’s “ Notes on Crustacea,” in the ‘ Annals of Natural History ’ for August, I 860 . i 400 GENERAL PH YSIOLOGY. time of copulation ; and it even appears that, in many instances, the spermatic fluid itself is far from being ready for its office at the time that it is deposited in the female organs, the final development of the sperma- tozoa within the spermatic cells having yet to take place,* — a striking example of the independent life of cells, which can be thus transferred from one organism to another, without any interruption to then- develop- ment. The separation and independence of the lateral halves of the sexual apparatus in the Crustacea, appear to have some relation with the frequency of what is termed ‘ lateral hermaphrodisnT in this class ; mon- strosities being of no unfrequent occurrence, in which the sexual organs present the male character on one side, and the female on the other. — The ova, after their extrusion from the oviduct, are usually canned about by the female parent, supported either by the thoracic or the abdominal appendages, until the embryo is nearly ready to come forth from its envelope ; and special provisions for their protection, made by the peculiar development of these appendages, are found in a considerable number of Crustacea, as well as in the Mysis (§ 316 a). The relation which the young animal, at the time of its emersion from the egg, bears to the adult, seems to differ very greatly in different tribes, and even in genera which are otherwise closely allied. Thus in the A stacus fluviatilis (cray-fish), according to the observations of Rathke, the embiyo passes through what may be considered as its successive stages of metamorphosis, whilst yet within the egg, and comes forth in a form which corresponds in all essential particulars with that of its parent. The same would appeal- to be the case (from the observations of Mr. Westwood) with the young of the Gecarcinus (land-crab). On the other hand, several competent observers have confirmed the discovery originally announced by Mr. V. Thompsont as to the metamorphosis of the common Crab ; which comes forth from the egg in a condition so unlike the adult, that in its early or larval condition it was accounted a distinct genus, Zoea, and placed among the inferior Crustaceans ; and which only acquires the characters of the adult by a series of progressive modifications, which present themselves at the successive exuviations of the shell. Similar metamorphoses have been observed in Astacus marinus (lobster), Palinurus (rock-lobster), Palcmon (prawn), Crangon (shrimp), and many other species among the higher Crustacea ; so that the absence of any such change would seem to be the exception rather than the ride. Generally speaking, a strong resemblance exists among the young of all the species which undergo a metamorphosis ; the head and thorax being included within a large cara- pace, while the abdominal portion is so much developed as to make up the chief part of the length of the body ; and the thoracic segments being furnished with £ fin-feet’, whilst the abdominal segments are destitute of appendages, the last, however, being flattened out into a large tail-fin. Iu the Macrourous (long-tailed) species (as the lobster, prawn, he.), meta- morphosis chiefly consists in the replacement of the thoracic ‘ fin-feet’ by true locomotive appendages and by a special apparatus of respiration, and in the development of abdominal appendages; but in the Brachyourous (short-tailed) Crabs and their allies, a much more complete change of * See Mr. H. D. S. Goodsir’s paper on “ The Testis and its Secretion iu the Decapoduiis Crustaceans,” in “Anatomical and Pathological Observations.” + Zoological Researches, vol. i. GENERAL VIEW OF THE ANIMAL KINGDOM. CRUSTACEA. 401 form takes place, the thorax being gradually developed at the expense of the abdomen, which becomes rudimentary. The following description of this remarkable series of changes in the young of Carcinus mosnccs (small edible crab), is condensed from that given by Mr. E. Couch.* — The young, when it first escapes from the egg, is of very small size, rarely exceeding half a line in length. The most bulky portion of its body is formed by the almost hemispherical dorsal shield, which covers the head and thorax, and has a long spine projecting from the middle of its upper part (Fig. 157 , a) ; the tail (formed by the six abdominal segments) is long, rounded, and slender, and its extremity is forked, the lateral portions being attached to the last ring by joints. The eyes are large, and situated under the front of the dorsal shield, but are not mounted on peduncles ; two pairs of flattened appendages are seen in their neighbourhood, behind which are three pairs of claws, each composed of three joints, and ter- minating in four long slender hair-like appendages ; whilst at the posterior part of the shield are two pairs of ‘ fin-feet.’ These little creatures are extremely active in the first stage of their lives, swimming freely by con- tinual flexions and extensions of their tails, and repeated beating motions of their claws ; but as the shell becomes more solid, they retire to the bottom of the water, and become more stationary. The progressive solidification of the shell is curiously seen in the dorsal spine, which is quite soft within the egg, being doubled back upon the shield ; for some little time after the emersion of the animal from the egg, the movement of blood can be seen to proceed as far as its apex ; but as the consolidation advances, the circulation becomes more and more limited in its extent, and is finally confined to the base. When the first shell has been exu- viated, the animal presents itself under a second form (b) ; the dorsal Fig. 157. ?pine has disappeared ; the shield is more depressed ; the eyes are mounted on footstalks, and the appendages in their neighbourhood are converted into antennae ; the first pair of claws becomes stouter than the rest, and is armed with a pair of nippers on each side ; the other claws resemble those of the lobster, except that the posterior pair are branched | fl ear the base, and one of the branches ends in a brush-like tuft. The Report of the Cornwall Polytechnic Society for 1843. D D 402 GENERAL PHYSIOLOGY. tail is greatly diminished in its relative size and proportions, and is some- times partially bent under the body, but is more commonly extended. In this state, the active swimming habits of the earlier larva are still pre- served. When this shell has been cast, the animal comes forth under a third, form (c), which presents a nearer approximation to that of the adult. The cephalo-thoracic shield is now flattened, and somewhat squarer in its form ; the tail also is flattened, and is provided with false feet ; the first pair of claws is still more developed in relation to the rest, approaching more nearly to the chelae of the adult ; and the remainder of the thoracic limbs are well developed for walking. In this condition, the swimming habits of the larva are but little preserved ; the tail is usually bent under the body ; and progression is chiefly accomplished by the thoracic members. After the next moult, the animal comes into the world under a fuur'h form (d), which is that of the adult, although its size is still very minute. 316 e. In the very curious tribe of Pycnogonidce, we have the higher Crustacean type reduced to its extreme of simplification. The general form of the body (Fig. 158) reminds us of that of the Crab ; the abdomen being entirely deficient or represented only by a small tubercle ; the head also being very small, and generally fused (as it were) into the thorax ; so that the last-named region, wdth its appendages, constitutes almost the whole bulk of the animal. In the complete obliteration of the abdomen, a resemblance is presented to the Cyamus and other Lannodipoda (§ 316 a); but the departure from that type is manifested in every organ of the body. The mouth is formed for suction, and the parts of which it is elsewhere composed appear to be either rudimentary or fused together ; the only appendages to the head, in several genera, being a single pair of antenme and one pair of feet-jaws. The thorax bears four pail’s of legs, each composed of several joints, and terminated by a hooked claw; and these legs all radiate, as it were, from one centre. The mouth opens by a narrow orifice at the extremity of a trank-like prolongation of the head, and it seems to be furnished with vibratile cilia, by the instrumentality of which the liquid aliment is drawn in. It leads to a very narrow oesophagus (Fig. 158, a), which passes backwards to the central stomach (b) situated in the middle of the thorax; and from the posterior part of this cavity, a narrow intestine (c) passes off, to terminate at the posterior extremity of the body. From this central stomach, however, five pairs of csecal prolongations radiate, one pair (d) entering the feet- jaws, and the other four ( e , e) penetrating the thoracic legs, and passing along them as far as the last joint but one ; when these caeca are distended with fluid, they present a series of contractions and dilatations, which correspond with the articulations of the limbs. That these curious organs are veritable extensions of the stomach into the limbs, and not mere secreting caeca, is proved by the curious peristaltic action which the transparency of the body permits to be observed in them ; for they all contract and dilate alternately, so as to produce a continual flowin’ the contents of this curious digestive cavity from one part into another. Their membranous walls are covered, however, with a layer of brownish- yellow granulations ; and these, which are scarcely perceptible on the wall of the central stomach, and not at all upon that of the intestine, may probably be regarded as a diffused and rudimentary condition of the liver. In the fluid which is transmitted from one portion of the digestive cavity GENERAL VIEW OP THE ANIMAL KINGDOM. CRUSTACEA. 403 to another, minute granular particles can be seen, which are probably of a fiecal nature. The digestive sac is freely suspended in the general solid cavity of the body, by occasional fibrous bands; and nothing intervenes between its ex- ternal surface and the Fig. 158. muscular lining of the crustaceous envelope. Thus a wide lacuna is left around it, occupying the greater part of the thoracic cavity, and also the hollow of the legs ; and this lacuna is filled with a transparent liquid, in which are seen float- ing a number of minute transparent corpuscles of irregular size. This fluid is kept in continual mo- tion, not only by the general movements of the body and limbs, but by the more special and con- stant action of the differ- ent parts of the digestive sac ; for when the caecum in any one of the legs undergoes dilatation, a part of the circumambi- ent liquid will be pressed out from the cavity of the limb, either into the thorax or into some other limb whose caecum is contracting ; and when, in its turn, the first caecum contracts, the space around it will be again filled with liquid forced into it by the dilatation of the central stomach or of some other caecum. There is no other circulating apparatus whatever ; and it cannot be doubted that the liquid which fills the great cavity of the body must be regarded as the nutrient material derived from the digestive organs, that is, blood ; and that its movement from one part of the body or limbs to another is to be regarded as a great lacunar circulation, no part of it being as yet circumscribed within distinct vessels, and no special organ of impulsion being developed. There is no special organ of respiration ; but the aeration of the circulating fluid must be accom- plished by exposure to the surrounding medium through the crustaceous envelope, which does not possess great density in any of these animals. — The nervous system consists of a supra-cesophageal or cephalic ganglion, and of four ganglia in the thoracic region ; the latter are adherent to each other, so as to form one continuous mass ; and this is connected with the cephalic ganglion in the usual mode, by a cord that passes round the oesophagus on either side. The front of the cephalic ganglion expands into a sort of tubercle, on which are set four eyes at a little iistance from each other, resembling in their fewness and in their Ammothea pycnogonoides ; — a , narrow oesophagus ; b, stomach; c, intestine ; d, digestive caeca of the feet jaws ; e e, digestive caeca of the legs . 404 GENERAL PHYSIOLOGY. separate condition the eyes of the Annelida. No other organs of sense have been observed. Each of the thoracic ganglia sends off a pair of nerves to one of the pairs of legs. The movements of these animals arc very sluggish. They have no swimming power, hut crawl over the surfaces of Sea-weeds, &c., by means of their long flexible legs, taking an attachment to some fixed point by means of the hooked claw, and then drawing themselves towards it. The reproductive apparatus of the Pycnogonidce is as yet unknown.* 317. Besides the Crustacea to which the descriptions given in the preceding paragraphs are more or less completely applicable, the class includes a great variety of simpler forms, all of them purely aquatic, but many of them inhabiting fresh water ; some of these having append- ages fitted for swimming, and having their mouths furnished with a masticating apparatus; whilst others attach themselves as parasites to the bodies of different aquatic animals, especially Eishes, and live by the imbibition of their juices through a mouth adapted for suction. The former of these groups, all the existing representatives of which arc animals of minute size, is commonly known under the designation of Entomostraca ; from the circumstance that the body is in many instances completely enclosed within a horny casing, which sometimes closely resembles a bivalve shell in form and in the mode of attachment of its pieces to each other, whilst it sometimes forms but a single piece like the hard casing of certain Rotifera (Fig. 146). There is no limitation in the number of segments of the body in this group; and it almost always exceeds that which is characteristic of the higher Crustacea, being some- times increased to an extraordinary extent. Thus in Eranchipus stag- nalis (a very common little Entomostracan of stagnant waters), the number of thoracic segments is eleven, and that of the abdominal, nine : the segmentation of the head, however, being altogether undistinguish- able. In Apus, the thorax and abdomen together contain thirty seg- ments, and the appendages to these present a remarkable multiplication in the number of their parts ; thus the first pair of legs gives off a set of branches, whose articulations altogether amount to about 150, and the caudal segment gives off two long cylindrical filaments, each of which has no fewer than 480 joints ; and each segment of the body bears two pairs of legs, so that the number of these members is sixty pain. — the total number of articulations in this creature being (according to Latreille) not less than two millions. In the greater part of these animals, the number of proper locomotive appendages is small, the anterior pairs of thoracic appendages alone being specially modified for this function, and the remainder (if present at all) being converted into ‘ fin feet’, which are subservient rather to the aeration of the blood than to locomotion ; this is well seen in Limnadia (Fig. 159), and other Branchio- podous (or gill-footed) Crustacea. Sometimes the ‘ fin-feet ’ constitute the principal organs of progression ; and the movement is then of an easy gliding character, the animals frequently swimming on their backs (as is the case with the Eranchipus ), or moving with equal facility on the back. * The above account of this very curious group is given from the Author's own observa- tions, which were made previously to the publication of the excellent paper of M. de Quatre- fages, “ Sur [’Organization des Pycnogonides,” in the Ann. des Sci. Nat. for 1845. He lias found M. de Quatrefages’ observations accordant with his own, in every particular. GENERAL VIEW OF THE ANIMAL KINGDOM. CRUSTACEA. 405 belly, or sides, as is done by the Artemia salina, or ‘brine-shrimp.’ The action of the fin-feet also serves to bring a current of water to the mouth, and thus to supply the animal with food, limbs and their appendages are chiefly used for locomotion, as in Apus, Limna- dia, Daphnia (the common ‘ water-flea’) and many others, the animal moves in a series of jerks ; and this is the case also with the Cypris and other Entomostraca of the order Ostrapoda, in which there are no proper respiratory appendages whatever, and only a small number of pairs of legs. — In the general formation of their nutritive apparatus, the Entomo- straca present a type of structure much simpler than that which characterises the higher division of the class. The mouth usually has a pair of mandibles, with only one or two pairs of feet-jaws ; and the alimentary canal presents very little dilatation from one end to the other, nor do the biliary organs rise above the condition of simple caeca. The heart is elongated into a dorsal vessel ; and this, in some species, may be observed to be partially divided into a series of distinct chambers (as in Insects), no fewer than nineteen having been counted in Branchipus. The movement of the circulating fluid does not seem to take place, in any great degree, within distinct vessels, but rather through sinuses or lacunae excavated among the various tissues and organs ; and there is no special respiratory circulation, the blood being aerated in its course through the ordinary appendages of the body. In regard to the structure and arrangement of the nervous centres, little is certainly known ; but that of the visual organs presents a variety of interesting conditions, all gradations being traceable, from the moveable single eye made up of a small number of ocelli aggregated together, to the large compound eyes which very closely approximate to those of the higher orders of the class. In some instances, too, single eyes are met with, in addition to the compound masses. 317 a. Some of the most interesting points in the natural history of this group, lie in the peculiar modes in which the generative function is performed, and in their tenacity of life when desiccated, in which last respect they correspond with many Rotifera (§310 b). This provision is obviously intended to prevent them from being completely exterminated, as they might otherwise soon be, by the drying up of the pools, ditches, , aud other small collections of water which constitute their usual ‘ habitats.’ It does not appear, however, that the adult animals can bear a complete desiccation, although they will preserve their vitality in mud that holds the smallest quantity of moisture ; but their eggs are more tenacious of life, and there is ample evidence that they will become fertile on being- moistened, after having continued for a long time in the condition of fine dust. Most Entomostraca, too, are killed by severe cold, and thus the whole race of adults perishes every winter ; but their eggs seem unaffected : by the lowest temperature, and thus continue the species which would 1 otherwise be exterminated. It is a curious fact that the Artemia salina, But where the anterior thoracic Fig. 159. Limnadia, one of the valves of the carapace having been removed. 406 GENERAL PHYSIOLOGY. or ‘ brine-shrimp,’ should never make its appearance in large numbers, except in water that is highly charged with salt, as in the ‘ salt pans ’ in which sea-Avater is undergoing concentration by evaporation; the proportion of salt which is found to be most favourable to its presence, being as much as 4 oz. to a pint of water. — Again, we frequently meet in this group Avith that reproduction by ‘ gemmation,’ of which so remarkable an example has been brought under notice in the class of Insects (§ 315 d). In many species there is a double mode of mrdtiplication, the sexual and the non- sexual. The former takes place at certain seasons only, the males (which are often so different in conformation from the females, that they would not be supposed to belong to the same species, if they were not seen in actual congress) disappearing entirely at other times ; whilst the latter continues at all periods of the year, so long as warmth and food are supplied, and is repeated many times (as . among the Aphides), so as to give origin to as many successive ‘ broods.’ Further, a single act of impregnation serves to fertilize not merely the ova which are then mature or nearly so, but all those subsequently produced by the same female, which are deposited at considerable intervals. In these two modes, the multiplication of these little creatures takes place Avith great rapidity, the young animal speedily coming to maturity and beginning to propagate ; so that, according to the computation of Jru’ine, founded upon data ascer- tained by actual observation, a single fertilized female of the common Cyclops quadricornis may be the progenitor in one year of 4,442,189,1 2u young. The eggs of some Entomostraca are deposited freely in the water, or are carefully attached in clusters to aquatic plants ; but they are more frequently carried for some time by the parent in special receptacles developed from the posterior part of the body ; and in many cases they are retained there until the young are ready to come forth, so that these animals may be said to be OA' 0 -\ T iviparous. In the common Daphnice, the eggs are received into a large cavity between the back of the animal and its shell, and there the young undergo almost their whole development, so as to come forth in a form nearly resembling that of them parent. Soon after their birth, a moult or exuviation of the shell takes place ; and the egg-coverings are cast off Avith it. In a very short time afterwards, another brood of eggs is seen in the cavity, and the same process is repeated, the shell being again exuviated after the young have been brought to maturity. At certain times, however, the Daphnia may be seen with a dark opacue substance within the back of the shell, which has been called the epkippium from its resemblance to a saddle. This, when carefully examined, is found to be of dense texture, and to be composed of a mass of hexagonal cells ; and it contains two oval bodies, each consisting of an ovum covered with a horny casing, enveloped in a capsule which opens like a bivalve shell. The first traces of the £ ephippium ’ are seen after the third moult, as a green matter in the ovaries, which differs both in colour and appear- ance from that of the eggs ; after the fourth moult, this green matter passes from the ovaries into the matrix or open space on the back, and there becomes developed into the ephippium ; and at the fifth moult this is thrown off, and the ephippium, with the two eggs inclosed, floats on the water until the next spring, when the young are hatched Avith the returning warmth of the season. This curious proAnsion is obviously destined to afford protection to the eggs which are to endure the severity of Arinter GENERAL VIEW OF THE ANIMAL KINGDOM. CRUSTACEA. 407 cold ; and some approach to it may be seen in the remarkable firmness of the envelopes of the ‘winter eggs’ of some of the Rotifera (§310 b). It has been ascertained by Dr. Baird, that the young produced from the -epliippial eggs have the same power of continuing the race by non-sexual reproduction, as the young developed under ordinary circumstances. In most Entomostraca, the young at the time of their emersion from the egg differ considerably from the parent, especially in having only the thoracic portion of the body as yet evolved, and in possessing but a small number of locomotive appendages ; the visual organs, too, being frequently wanting at first. (See Fig. 160, b, c.) The process of development, however, takes place with great rapidity ; the animal at each successive moult (which process is very commonly repeated at intervals of a day or two) presenting some new parts, and becoming more and more like its parent, which it very early resembles in its power of multiplication, the female laying eggs before she has attained her own full size. Even when the Entomostraca have attained their full growth, they continue to exuviate their shell at short intervals during the whole of life ; and the purpose which seems to be answered by this repeated moulting, is the preventing the animal from being injured, or its movements obstructed, by the over- growth of parasitic Animalcules and Confervee ; weak and sickly individuals being frequently seen to be so covered with such parasites, that their motion and life are soon arrested, apparently because they have not strength to cast off and renew their envelopes. The process of development appears to depend in some degree upon the influence of light, being retarded when the animals are secluded from it ; but its rate is still more influenced by heat ; and this appears also to be the chief agent that regulates the time which elapses between the moultings of the adult, which, in the Daphnia, take place at intervals of two days in warm summer weather, whilst several days intervene between them when the weather is colder. The cast shell carries with it the sheaths of the limbs and plumes, and of the most delicate hairs and setse which are attached to them. If the animal have previously sustained the loss of a member, it is generally renewed at the next moult, as in higher Crustacea.* 317 b. All the Entomostraca at present existing are animals of minute size, many of them being scarcely discernible by the unassisted eye, and the largest of them (such as the Branchipus) scarcely exceeding an inch in length. The exuviie of even the smaller species, however, may accumulate to such an extent, as to form masses of no inconsiderable size. Thus in certain fresh-water strata of the Secondary and Tertiary formations, we find layers, sometimes of great extent and thickness, which are almost entirely composed of the fossilized shells of a species of Cypris, none of them much exceeding a line in length ; and even in the marine Cretaceous deposit, considerable masses are sometimes to be found, composed of the remains of bivalve Entomostracous shells, probably belonging to the genus Cytherina. It was in the earlier ages of the earth’s history, however, that the Entomostraca seem to have attained them highest development. * The statements of experimenters upon this subject do not agree with each other, some asserting that little or no reparative power of this kind exists among the Entomostraca. But it is probable that, as in other instances, this power may be greater in the young animal than in one which is nearer the close of its life, and that it may be partly dependent upon the temperature (§ 98). 408 GENERAL PHYSIOLOGY. During the whole of the Palaeozoic series of formations, we meet with the fossils termed Trilobites (from the division of the body longitudinally into three lobes), which seem to have been essentially Entomostracous, although resembling the Isopod order among the higher Crustacea in certain points of their conformation. The absence of all vestiges of appendages of any kind to the under side of the body, makes it certain that, whatever was their form, these must have been so soft as to be readily decomposable ; since other parts, even to the facetted structure of the compound eyes, are found in most perfect preservation. It seems probable, therefore, that they must have belonged to the group of Branchiopoda ; to some existing members of which they seem most nearly allied in the structure of their eyes also, as well as in other points of their organisation. The cephalic shield of the Trilobites did not extend far back over the body ; but the segments of the thorax and abdomen were enclosed in a succession of distinct rings, of which the dorsal portion (which is alone preserved) was of firm consistence ; and the last segment also frequently bore an expanded caudal shield. The segments of the thorax were from ten to fifteen in number, and those of the abdomen, in some species at least, eight ; so that in this essential particular the Trilobites corresponded with the lower rather than with the higher division of the Crustacean class. Generally speaking, the segments of both thorax and abdomen were nearly similar to each other in form, and gradually diminished in size from before back- wards ; sometimes the abdomen narrowed into a tail, which was bent under the body like that of Crabs. The flexibility of the body, in a large pro- portion of the group at least, has been made evident by the fact that the remains of many species have been found coiled up like ‘ wood-lice.’ Evidence has recently been obtained that these animals came into the world in a condition very unlike that of their parents ; and that they only attained this after a long succession of exuviations, some progress being apparent at every change.* — It is interesting to remark that the Limulus (§ 316 a), which maybe regarded in some respects as a gigantic Entorno- stracan, seems to have been introduced at nearly as early a period as the Trilobites, its remains being coeval with theirs in the Carboniferous strata ; and that the earliest forms of this genus present characters of approxima- tion to the Trilobites, w r hich are also shown in the immature states of the existing Limuli. 317 c. The most complete and remarkable departure from the ordinary Crustacean type is presented by the sub-class of Suctorial Crustacea, iu which the mouth, instead of being furnished with mandibles and maxillae adapted for the division of solid aliment, is prolonged into a kind of proboscis adapted only tor the reception of liquid substances. As in Insects (§ 313), * M. Barrande of Prague, who is preparing a work on the Silurian system of Bohemia, in studying the numerous Trilobites which he has collected in that country, has traced for the first time the development of a Trilobite from its embryonic state to its adult condition ; and has observed twenty successive stages, during which this one species undergoes very remark- able changes of organisation, passing from a simple disk-like body to a fully formed trilobite with seventeen free thoracic segments and two caudal joints. This discovery diminishes the number of the so-called species; by showing that as many as ten genera, including eighteen species, have been made out of the successive fonns of one and the same trilobite (Athenu-um, July 7, 1849). There can be no doubt that a similar reduction in the number of species will take place in many other cases, alike amongst existing and extinct groups, when the whole history of each species shall have been made known. GENERAL VIEW OF THE ANIMAL KINGDOM. CRUSTACEA. 409 however, the parts which compose the mandibulate mouth of the preceding groups may be recognized in that which is modifiedfor suction ; the proboscis itself being formed by a prolongation of the upper and lower lips, and being usually accompanied by a pair of long appendages that represent the mandibles and serve to make the punctures in which the proboscis is inserted ; whilst other appendages, representing the feet-jaws, are furnished with hooks by which these parasites attach themselves to the animals from whose juices they derive their nutriment. Many of the Suctorial Crustacea bear a strong resemblance, even in their adult condition, to certain Ento- mostraca ; but more commonly it is between the immature forms of the two groups that the resemblance is the closest, most of the Suctoria under- going such extraordinary changes in their progress towards the adult condition, that if their complete forms were alone attended to, they might be excluded from the class altogether, as has (in fact) been done by many Zoologists. As an example of this curious transformation, we may take the Lerncea, a parasite which infests the gills and eyes of Fish. The form of the adult female is represented in Fig. 160, A ; where we see its long- suctorial proboscis at a, its short thorax at b, bearing the single pair of legs, c, by which the animal is attached, the large abdomen, d, and the two egg-capsules, e ; the legs are imited to each other at their extremities, and bear a kind of sucker, /, which is applied to the surface of the animal attacked. The integument of the whole body and of the limbs is soft, and scarcely displays a trace of segmentation ; the animal has hardly any locomotive power, its whole anterior portion being usually buried in the substance of the part from whose juices its nutriment is derived. But however dis- similar the form and characters of such a creature may appear to those of the ordinary Crustacea, its connection with them is esta- ! blished in two ways. In the first place, there are numerous parasitic species which depart much less from the ordinary type, forming a gradual transition from the Entomostraca to the most aberrant Suctoria ; so that, if the latter are to be excluded, it would be difficult to say -where the line is to be drawn. And, Lerncea and its allies, so strongly resemble other Entomostraca, that it would be impossible to point out any obvious distinction between them. It is interesting to observe how the activity of the young animal, in most or all of these parasitic Crustacea, compensates for the comparative immobility of the adult, which shows scarcely any other sign of life than taking food and pro- ducing eggs ; thus effecting the dispersion of the progeny, which, if they were all to attach themselves to one individual, would soon destroy it, and would then become themselves extinct from the want of food. The two large egg-sacs, which, when filled with ova, give such a peculiar aspect to these animals, are only a higher development of the receptacles which are ! Fig. 160. A Lerncea in adult state (a), and in larva conditions (b and c) : — a, sucto- rial proboscis ; b, thorax , c, single pair of legs, serving for attachment by the sucker /; cZ, abdomen ; e, egg- capsules. again, the early forms of those of the Cyclops and 410 GENERAL PHYSIOLOGY. possessed by many of the Entomostraca, and of which we trace the rudi- ments in some Rotifera. Within these egg-sacs the embryoes veiw com- monly undergo their whole development, up to the point at which they are ready to come forth from the egg, and to enter upon active life. At this period they are of an oval shape, having a large eye situated in the centre of the anterior and upper part of the body, and two pail’s of swimmin" feet, with a pair of jointed antennae (Fig. 160, b). After the first moult- ing, the body is seen to manifest a division into two parts, the anterior of which is furnished with three pairs of hooked feet, and the posterior with two pairs of swimming feet (c). Not only the greater number of the feet, but also the eye, subsequently disappear, when the animal becomes attached ; but the precise grade of development at which this takes place has not yet been accurately determined. As in the Entomostraca geu ■- rally, the adult male differs from the adult female in many important characters, although the larval state of the two seems to be precisely the same. Instead of the single pair of united feet, provided with a suctorial disk, he has two pairs of strong hooked feet ; and the relative propor- tions of the several parts of his body are very different ; the whole being much smaller than that of the female, sometimes not above a fifth part of her size. It is curious, too, that the males bear a strong resemblance to each other, even in different genera, of which the females are very dis- similar. — Among those suctorial Crustacea which present a nearer approach to the ordinary Entomostracous type, may be specially mentioned the Argulus foliaceus, which attaches itself to the surface of the bodies of fish, and is commonly known under the name of the £ fish louse.’ This animal has its body covered with a large firm oval shield, which does not extend, however, over the posterior part of the abdomen. The mouth is armed with a pair of styliform mandibles ; and on each side of the proboscis there is a large short cylindrical appendage, terminated by a curious sort of sucking disk, with another pair of longer jointed members, terminated by prehensile hooks. These two pail’s of appendages, which are probably to be considered as representing the feet-jaws, are followed by four pairs of legs, which, like those of the Branchiopoda, are chiefly adapted for swimming ; and the tail, also, is a kind of swimmeret. This little animal can leave the fish upon which it feeds, and then swims freely in the water, usually in a straight line, but frequently and suddenly changing its direc- tion, and sometimes turning over and over several times in succession. The stomach is remarkable for the large csecal prolongations which it sends out on either side, immediately beneath the shell ; for these sub- divide and ramify in such a manner as to be distributed almost as minutely as the c£ecal prolongations of the stomach of the Planarice (Fig. 141). The proper alimentary canal, however, is continued backwards from the central cavity of the stomach, as an intestinal tube, which terminates in an anal orifice at the extremity of the abdomen. 318. From the parasitic suctorial Crustacea, the transition is not rc-nlly so abrupt as it might at first sight appear, to the class of Cirrhipoem. consisting of the Barnacles and then allies, which, like many of the Suc- toria, are fixed to one spot dining the adult portion of their fives, but come into the world in a condition that bear’s a strong resemblance to the early state of many of the true Crustacea. The departure from the ordinary Crustacean type in the adult, is, in fact, so great, that the GENERAL VIEW OF THE ANIMAL KINGDOM. CIRRHIPODA. 411 animals belonging to this group were formerly ranked among the Mol- lusca, to which they bear many points of resemblance. Thus, the body and its appendages are themselves quite soft j and the skin has the loose, spongy, muscular character which is characteristic of the ‘ mantle ’ of the Mollusks (§ 297). The shell, which is formed upon its surface, corresponds closely with that of Mollusks in its general characters ; and although it is ‘ multivalve,’ or composed of several pieces, yet these pieces are not arranged in such a manner as to bear any resemblance to the dermal skeleton of an articulated animal, — such a resemblance, in fact, being much more strongly presented by the shell of the Chitons (Fig. 125) whose general structure is essentially Molluscous. Further, the shells of the Cirrhipoda are either themselves firmly united at their bases to rocks or other solid masses (not unfrequently to the bodies of living animals) ; or they are attached by a long peduncle or foot-stalk (Fig. 161, a); no one species being free in its movements in any but the early period of its life : so that the conditions in which these animals exist, closely resemble Fig. 161. Anatifa Icevis a, group of animals of different ages ; as attached in the living state : — b, in- terior structure, enlarged, as shown by the removal of the valves of one side; a , peduncle; b, mantle; c , cephalic portion of the body; d, mouth; e, articulated members; /, flabelliform appendages (or branchiae) ; g, abdominal appendage. those to -which we find the Mollusca especially adapted. — On the other hand, when we examine the animals themselves, we find them present- ing a perfectly symmetrical form, with indications of a segmental division of the body into six parts (b) ; whilst from each of these segments there arises a short pah’ of feet, obscurely divided into three joints, and bearing a pair of long, slender, many-jointed cirrhi, or tendril-like appendages ( e ) 412 GENERAL PHYSIOLOGY. fringed with delicate filaments, and curled towards the mouth. These appendages, from which the class derives its name, are furnished with cilia, the continual vibration of which produces currents in the water, that bring food to the mouth, and also aerate the blood transmitted to the respiratory organs ; hence they may be likened, on the one hand, to the ci- liated legs of many Entomostraca (§ 317), whilst, on the other, they bear no inconsiderable resemblance to the spirally-coiled ‘ cirrhi ’ of the Brachio- poda, which seem to perform a similar function (§ 300). — The anterior extremity (c) of the body is so modified as to form the organ of attach- ment of the animal ; being expanded into a broad disk or basis of adhe- sion in the sessile tribes, such as the B alarms or ‘acom-shell’ and its allies, which attach themselves directly to the body that affords them sup- port, by the entire base of their somewhat conical shells ; but being prc - duced into a long and flexible stalk (a) in the pedunculate forms of t his class, as the common Anatifa or ‘barnacle,’ which float more freely in the water, being generally attached to some object that has motion through it, such as a log of timber or a ship’s bottom. The general organisation is very nearly the same, however, in both these tribes, not- withstanding that they differ widely, not merely in the mode of their attachment, but also in the structure of their shells. The shell of the ‘ pedunculate ’ species (in some of which this protection is very imperfect) is formed upon the plan of that of bivalve Mollusca, with certain acces- sory pieces : and the two valves can be drawn together, so as completely to enclose and protect the animal, by means of an adductor muscle. The peduncle, also, can be shortened by the contraction of its muscular coat , whilst it is lengthened again by the elasticity of its external fibrous coat, when the contractile force is removed. The structure of the shell in the ‘sessile’ species, however, is usually much more complicated. It consists of a nearly circular base, from which rise six A-shaped plates with the base of their triangle downwards and the apex upwards ; whilst between these are intercalated a corresponding series of V-shaped plates having the base upwards and the apex downwards. The dorsal piece con- sists of one of the erect or A-shaped plates ; on each side there are two pieces made up by the adhesion of one of the erect, and one of the inverted plates ; and the ventral piece is composed of an erect plate in the centre, with an inverted plate on either side of it. Each plate consists of two layers of shell, with ‘cancellated’ structure (§ 2 05) between them ; and the mantle sends prolongations into the sutures, so that the edge of each plate may receive additions, whereby the whole envelope may be enlarged, and thus be adapted to the increase of the animal, without changing its form. The mouth of the shell is closed by an operculum of two or more shelly plates of similar texture, which can be precisely fitted to the aperture by means of the muscles that act upon it, so as to afford a complete protec- tion to the animal within. The shell of one of the sessile Cirr bipods, Tubicinella, which is parasitic upon the whale, is developed as a nearly cylindrical tube of considerable length ; whilst, on the other hand, in the Coronula, which is also parasitic upon the whale, the shell is a flattened cone, its base being extended by the enormous develop ment of the cancel- lated structure which intervenes between its inner and outer wall, and its cavity being small in comparison with its external diameter. Scarcely a trace of this cancellated tissue is to be found in the shells of Mollusks : GENERAL VIEW OF THE ANIMAL KINGDOM. CIRRHIPODA. 413 and its presence in very large amount in the curious extinct group of Rudistes (including the Hippurite, Sphcerulite, and their allies), taken in connection with other characters presented by them, is an indication that this group had very intimate relations with the Cirrhipods, which it probably served to connect with the Bivalve Mollusks. 318 a. Turning now to the internal structure of these animals, we find, on the under side of the cephalic portion of the body (c), the mouth (d), which is provided with a broad upper lip, with two palpi or feelers, and with three pairs of jaws, toothed at their edges, and opening laterally as in all Articulata. The marine animalcules, brought by the currents that are kept up by the action of the cirrlii, after passing through the mouth, are transmitted by a short oesophagus to a dilated stomach lying beneath the cephalic hood ; into this stomach the ducts of two long glandular fol- licles, probably salivary, discharge themselves ; and from its walls are developed numerous hepatic caeca, which constitute a rudimentary form of liver. From the stomach there passes off an intestinal tube, which follows a slightly sinuous course, and terminates by an anal orifice at the base of the long, jointed, caudal appendage ( g ), which is seen at the oppo- site extremity of the body. The circulating system has not been tho- roughly' made out ; but its organ of impulsion is a £ dorsal vessel,’ which transmits the blood through the body, and to the respiratory organs. Although its circulation through the cirrhi, in which a double current can be seen, probably contributes to the aeration of the blood, yet there seem to be also special respiratory organs, though of a diftei’ent character in the two groups respectively. In the ‘ pedunculate ’ Cirrhipods they resemble those of the Crustacea, in being attached, like flabelliform appendages (/) to the base of the maxillary feet, and to that of some of the cirrhigerous legs ; whilst in the ‘ sessile ’ tribes, the branchiee are formed by duplica- tures of the mantle, as in the Mollusca generally. — The nervous system of this group has the truly articulated character. The cephalic or supra- cesophageal ganglia are but little developed, in accordance with the almost entire deficiency of organs of special sense j but a pair of cords passes backwards from them, diverging to surround the oesophagus, at the two sides of which we find the ganglia of the first segment of the body, meeting again below in the coalesced ganglia of the second segment, and then proceeding along the ventral surface of the body to its caudal extre- mity, with a pair of ganglia for every segment. No traces of eyes or of auditory organs can be found in the adidt ; and the oidy indication of sensation which these animals exhibit, consists in the retraction of their cirrhi and the closure of their shells, at the sound, or more probably at the vibration, of an approaching footstep, or even, in some of the sessile species, at the atmospheric movement caused by the approach of the hand within some inches’ distance — The nature of the generative organs in these animals has not yet been clearly elucidated. All the Cirrhipods with which we are at present acquainted possess ovaria and deposit fertile ova ; and these must consequently either possess male in addition to female organs, or the male must exist separately under some entirely dif- ferent form, which conceals its relations to the class. A large glandular organ is found in the pedunculated species, covering the viscera immedi- ately beneath the muscular tunic of the body, and extending from the mouth to the anus ; its numerous ducts unite into three or four principal 414 GENERAL PHYSIOLOGY. trunks, which terminate in a lateral receptacle at the side of the intestine ; and from this a single or double canal is prolonged to the very extremity of the extensile tail (Fig. 161, g), where it terminates. This organ is by some supposed to be an ovarium, whilst by others it is considered as a testis. Certain it is, that ova are found in the hollow peduncle j and those who regard the organ just described as a testis, believe that the ova are developed in that situation ; whilst those who consider that organ as an ovarium, believe that the ova are conveyed by the long tail or ovipo- sitor to the peduncle, whose hollow serves as a receptacle in which their early development may take place. In whatever way the ova are ferti- lized (and the idea of a distinct, smaller, and differently formed male, is the one conformable to the analogy of the Entomostracous and Suctorial Crustacea to which these animals are nearly allied), it is certain that the early history of the young Cirrhipods presents phenomena of the most interesting character. So strange, indeed, are the metamorphoses which these animals undergo, that when they were first made known by Mr. V. Thompson,* his account of them was scarcely credited by Zoologists, who were not then aware, as now, in how large a number of instances, pheno- mena of the same order present themselves among the Invertebrated classes. — The young of Balanus, at the time of its emersion from the egg, is enclosed in a bivalve shell, of which the valves are united by a hinge along the back, and open along the lower margin, for the protrusion of a large and strong anterior pah - of prehensile limbs, provided with an ad- hesive sucker and hooks, and of six pairs of posterior legs, adapted for swimming. The form and general condition of the animal thus bears a strong resemblance to that of the Daphnia (common ‘ water-flea ’) ; and its movements, too, are similar, consisting of a succession of jerks. A pair of compound eyes, supported on a peduncle, is seen on the anterior part of the body. Nearly all these organs are thrown off together ; and the animal assumes an entirely different form and condition. It first attaches itself to a neighbouring surface by a layer of calcareous matter, which is subsequently to become the base of its new shell ; the valves that surround its body are gradually developed on its envelope or mantle ; the new ciliated arms begin to grow within ; and the eyes disap- pear, the animal being henceforth destitute of visual organs. — The form of the young Barnacle, however, is different, having more resemblance to that of Cyclops. The animal is not enclosed in a bivalve shell, but : ts back is covered by an expanded shield like that of Argulus (§ 317, c) ; it has a single eye, not pedunculated ; and three pairs of members, of which the anterior is simple, while the posterior are bifid.— Hence if then - early state be alone considered, the Cirrhipods might be regarded as an aberrant oi - der of Crustacea ; but the complete change, not merely of form, but of condition, which they undergo in their passage to then - adult state, and the peculiar transition which this presents towards the Mollusean type, seem to remove them to a far greater distance from the true Crustaceans, than that at which the Lernsea and its allies may be placed • and may be held to justify the erection of this group into a distinct class, — unless, indeed, it should prove that separate males exist, retaining more of the ordinary Crustacean conformation. Zoological Researches, Fourth Memoir, 1830; and Philosophical Transactions, 1833. GENERAL VIEW OF THE ANIMAL KINGDOM. ARACHNIDA. 415 319. Returning now to the air-breathing Articulata, we find in the class of Arachnida, which includes Spiders, Scorpions, and Mites, much of that concentration of structure which has presented itself to our notice iu the highest members of the Crustacean class, and which, whilst it is a departure from the typical condition of Articulata, may be considered as in some degree indicating an approximation to the Vertebrated series, — an approximation, however, not so much in form, as in character. The Arachnida are nearly related to Insects in many points of their organisa- tion ; hut are usually distinguishable from them at the first glance by the general form of their bodies, and by the number of their limbs. Instead of the three regions which may always be distinguished in perfect Insects, — the head, thorax, and abdomen, — the Arachnida have only two, the head and thorax being (as it were) fused together into one mass, which is termed the cephalo-thorax : whilst instead of the six legs which charac- terise the perfect Insect, the Arachnida have always eight, all of them attached to the cephalo-thorax. Of wings there is not the least trace. The antennae, of which Insects possess one pair, and the higher Crustaceans two, are here entirely wanting ; but the palpi, which are appendages to the maxillse (or lower pair of jaws), are usually developed to a large size in the Arachnida, so as often to resemble the chelae or pincers of Crabs and Lobsters. The tegumentary skeleton of Arachnida varies consider- ably in its degree of firmness, in the different members of the class ; and tbe segmentation of the body is least apparent in those which have the softest integument. Thus in Spiders and Acari, the integument is generally soft, especially in the abdomen, which is a soft spheroidal mass without any external trace of segmental division ; whilst in the ‘ cephalo- thorax’ and limbs it is a little harder, and some degree of articulation may be perceived. In the Scorpions, on the other hand, it is almost as dense as among Beetles, and (as in that tribe) is hardened by chitine (§ 314); and not only is the segmentation of the ‘ cephalo-thorax’ and of the limbs there quite distinct, but the abdomen also, the posterior part of which is very slender and much prolonged, is obviously divided into several rings. — This class may be distributed into two principal groups, distinguished by several important differences in their internal structure, and therefore requiring separate consideration. The first of them, which is that of Pidmonaria, containing the Spiders and Scorpions, is espe- cially characterised by the condition of their respiratory apparatus, which consists of pulmonary cavities instead of tracheae (as in Insects), and by the completeness of their circidating system ; they are also distinguished from some of the second group, which greatly resemble them in external appearance, by the number of their eyes, which is six, eight, or even more. The two great tribes of Araneidce (Spiders) and Scorpionidce (Scorpions), which make up this group, differ rather iu the form of their bodies, and in certain special modifications of particular organs, than iu then general plan of conformation ; thus we find in Spiders a soft abdo- men of more or less spheroidal form, provided with £ spinnerets ’ at its extremity, and the palpi are comparatively small : whilst Scorpions have a prolonged and slender abdomen, which is not furnished with spin- nerets, but usually bears a poison-apparatus in their place ; and the palpi are large, equalling or exceeding the legs in size, extended like arms, and furnished with pincers. These two groups are connected by the family 416 GENERAL PHYSIOLOGY. Thelyphonidoe (inhabiting tropical regions only), in which the general form of the body is that of the Spider, but the spinnerets are wanting, whilst the palpi are developed into large members as in the Scorpion. — The second principal subdivision of the Arachnida is composed of the Trachearia, so named from the mode of respiration, which is carried on by tracheae (as in Insects), instead of by pulmonary sacs. In this group the circulating apparatus is very incomplete, and in some members of it the digestive cavity is much extended, as if to compensate for the want of vessels. Altogether the plan of conformation is much lower ; and although the form of some of the members of this group frequently pre- sents a close resemblance to that of the Spiders and Scorpions, they may be at once distinguished by the absence or by the fewness of the eyes, the number of which never exceeds foiu\ The typical family of this group is that of Acaridce (Mites), which presents many characters of approximation to the wingless parasitic Insects. — The Arachnida in general differ from Insects, not merely in the structure of their respective adult forms, but in the mode in which those forms are attained ; for the higher members of this class undergo no metamorphosis, but come forth from the egg in a form and condition resembling, in all essential parti- culars, that of their parents, to the production of which form, the whole course of evolution within the egg manifestly tends ; whilst in Insects, as we have seen, the first process of evolution tends to produce a worm-like being, which only attains the higher type by the subsequent development of the organs that characterise it. The Acaridce, however, undergo a slight change of form, coming forth from the egg with only six legs, nmi not acquiring the additional pair until after the first moult ; and in certain of the aquatic and parasitic species of this family, a greater modi- fication takes place, reminding us of that which is seen among the Suctorial Crustacea (§ 317 c). 319 a. The mouth, in all the higher Arachnida, is formed upon the Mandibulate type ; for although many of these animals live exclusively upon the juices of others, they do not obtain these by imbibing them as parasites from living bodies, but by first destroying then victims and then availing themselves of the liquids which their dead bodies afford. The mandibles, or anterior pair of jaws, situated at the front of the head (Fig. 165, a), are long in Spiders, and are furnished with a moveable hook, which has a small orifice near its extremity ; and from this orifice a poisonous liquid is poured out, which is secreted by a gland situated in the preceding joint, and which almost immediately proves fatal to the small animals that serve as their usual prey, although too weak to be injurious to Man. Iu Scorpions, on the other hand, the mandibles are shorter and are terminated by a pair of pincers ; and the poison-apparatus is removed to the opposite end of the abdomen. The maxillae, or second pair of jaws (Fig. 164, b, and Fig. 165,/), are in themselves usually simple ; and the maxillary palpi, which are developed to such an extra- ordinary extent as instruments of prehension in the Scorpions, are compa • ratively short in the Spiders, in the males of which, however, they take a remarkable share in the generative function, being (in fact) the intro- mittent organs. No labial palpi present themselves in ordinary Arach- nida; but in the genus Gcdeodes (one of the family of Pseudo-Scorpiomda : , which connects the Scorpionidse and the Acaridse) we find the head GENERAL VIEW OF THE ANIMAL KINGDOM. ARACHNID A. 417 Fig. 162. unusually distinct from the thorax, and it supports the first pair of legs, which nearly resemble the maxillary palpi in form, being short ancl slender, and destitute of the terminal hooks ; whence it appears that the first pair of legs in the ordinary Arachnida really represent the labial palpi of Insects. In this genus, rudimental antenme are present, thus present- ing another interesting approximation to the Insect type ; and as they are found attached to the mandibles, it is probable that they are to be regarded as ordinarily confluent with them, which the disposition of the nerves of the latter organs would seem to indicate. — The Alimentary canal of the Scorjiionidce is very simple in its character, extending in a direct line, without any dilatation into a stomach, or convolution of the intestinal portion of the tube, from the mouth to the anus. In the thoracic region, however, it sends off five short and straight caecal pouches, which are probably to be regarded as constituting the liver ; whilst at the beginning of that portion of the tube which is continued into the tail-like abdomen, it receives two delicate secreting tubidi on each side, which may perhaps be considered as renal organs. In the Araneidce we meet with a conformation of the digestive apparatus, which reminds us of that of the Pycnogonidce (§317 e). The aperture of the mouth (Fig. 165, g) is very narrow ; since it only needs to receive the juices which are pressed from the body of the victim by means of the maxillary plates (/) • and in several instances the cavity of the mouth communicates with the oesophagus by two or more narrow pharyngeal apertures (Fig. 162, b). The oesopha- gus passes backwards through the nervous collar, and then enters the stomach, which is situated in the middle of.-the cephalo-thorax, and sends off l ine > receiving biliary vessels ; ciecal prolongations (c), which extend into the legs largement, receiving the urinary as far as the first joint, and also into the maxil- ' e “ cl5 ’ /i ’ *• ary palpi. These do not seem, however, to be connected with any secreting structure, but appear destined to increase the capacity of the gastric cavity )y their dilatation. From the stomach a straight intestine passes backwards, which is narrow at first where it enters the abdomen (at d), but then dilates '*) to receive four ducts on either side, proceeding from a large biliary appa- ratus that occupies a considerable part of the abdominal cavity (Fig. 164) ; oelow these it again contracts (/), and forms two short convolutions, after vliich it dilates into the csecal enlargement (g), receiving two long secret- ug tubuli ( h , h), which are probably uriniferous. Thus it appeal's probable hat the caeca of the intestinal tube of the Scorpion are the representa- ives. not of the gastric but of the hepatic caeca of the Spider ; their place Ii the cephalo-thorax being determined merely by the extreme contraction ef the post-abdomen, which throws forward (so to speak) the appendages to he alimentary canal. — The centre of the Circulation is formed upon the lan of the dorsal vessel of the lower Articulata, being a prolonged trunk, E E cated origin of oesophagus; stomach with lateral caeca pass- ing into the limbs and palpi ; d, portion which traverses the abdo- minal peduncle ; e, dilated intes- 418 GENERAL PHYSIOLOGY. divided by valvular folds into a succession of chambers ; but its walls are much thicker and more muscular than they are in Insects ; the multiplica- tion of its chambers is not so great ; and (in the Scorpion idee at least) the valvular partitions are not complete. In the A raneidee, we find the anterior portion of the dorsal vessel, which lies in the cephalo-thorax, contracted into an arterial trunk ; and the pulsatile chambers exist only in its abdominal portion (Fig. 163), where they are reduced in number to four or five. In the Scorpionidce, on the other hand, the pulsatile portion of the dorsal vessel occupies nearly the whole of the body (that is, of the cephalo-thorax, and anterior portion of the abdomen), and contains eight chambers ; whilst in the post-abdomeu or tail, it is contracted into an ai-terial trunk. Each chamber, on its dilatation, receives aerated blood from the respiratory organs, by the large venous trunks (Fig. 163, b, c) ; and from each chamber also proceed systemic arteries, through which a part of this blood is impelled by its contraction to the various organs of the corresponding segment. The remainder is propelled forwards, in the Spider tribe, by the successive contractions of the pulsatile cavities, through the great dorsal trunk a, to be distributed to the cephalo- thorax and its members ; whilst in the Scor- pions, whose segmental heart lies much more in front, the propulsion seems to be partly forward and partly backward, the post-abdo- seis of the anterior, and e, vessels of men requiring a supply, as well as the head posterior pulmonic apparatus. and thoracic members/ The arterial system supplying the body of the higher Arachnida appears to he very complete and minute in its distribution, as will be more particularly explained hereafter (chap, xii.) ; but it seems probable that the canals by which the blood is returned, have rather the character of sinuses or lacuna channelled out among the tissues, than of true vessels. By this system of canals, the blood is conveyed to the Respiratory organs, which, from their peculiar conformation, have been termed ‘ pulmonary brand i;e. — combining, as they do, the characters of pulmonic cavities and of branchial expansions, especially when the latter are enclosed in cavities of their own, as is the case in the higher Crustacea (§ 316 d). These organs are usually four in number on each side, and they open by separate orifices or stigmata, although communicating freely with each other by a connecting tube, so that uniformity in the respiratory action throughout the whole series is seemed. Each cavity, covered in by an opercular plate (Fig. 164, d, d'), contains a number of lamella; 0 formed by folds of its lining membrane ; and over these, the blood which is brought by the respiratory system of vessels is distributed for aeration, to be then returned to the dorsal heart. In certain genera of Spidei”. however, this pulmono-branchial apparatus is less complete ; and respira- tion is partly carried on, as in Acaridans and Insects, by means of trachece, which pass from the pulmonic cavities into the body and limbs Fig. 163. GENERAL VIEW OF THE ANIMAL KINGDOM. ARACHNIDA. 419 Fig. 1()4. — The character and position of the biliary and urinary organs having- been already noticed, it remains to speak of the special organs of Secre- tion, of which either one or two are found in all the higher Aracknida. The poison-apparatus of the Scorpions is situated at the extremity of the post-abdomen or tail, the slenderness and flexibility of which seems to have special reference to the wielding of this weapon ; it consists of a sharp recurved sting, with a minute slit near the point., into which open the ducts of two secreting vesicles which are situated in the last dilated joint of the tail. In the Spiders, on the other hand, the perforated sting- forms the second joint of the mandible ; and the elongated ovoid vesicle which forms the poison-gland, is lodged in the basal joint, sometimes extending into the cephalo- thorax. This apparatus corresponds in position with the salivary organs in Insects, which in the Silk-worm are modified to produce the silken thread for the cocoon. The spinning apparatus of the Araneidm, on the other hand, consisting of the ‘ spin- nerets,’ and of the glandular vesicles in which the fluid is elaborated, takes the place of the poison-apparatus of the Scor- pions. The usual number of the spinnerets (Fig. 164, li) is six, two of them being- longer than the rest, and somewhat differ- ently constructed ; they are all, however, mammillary projections, of which the apex is perforated, not by one simple aperture, but by a multitude of microscopic pores ; and through these the glutinous Secretion showing the thoracic and” abdominal ca^ . ~ 0 . 0 vities laid open; — a, mandibles; o, max- illae, bearing palpi ; c, first joints of legs, issuing from the thorax, within which is seen the large thoracic ganglion; d , d', opercular plates, covering-in the pulmonic a ~ i • . i organs ; e, anterior pulmonic cavity laid llDlG Ol GclCll SpmilGrGt IS COHipOSGd. OI £1 open; e' , posterior pulmonic cavity, partly bundle of as many minute fibrils as there cove / ed b ? tbe ‘““g^dinai muscle /; . J . g, transverse furrow, m the middle or are pores 111 tile spilllieret ? cind tile nurils which is the slit forming the entrance to is forced out in a liquid state, drying iuto a tenacious thread, however, immediately on coming into contact with ah-. Thus the Interior surface of Mygale (maconte), of all six sjiinnerets may unite to form a single thread ; so that each thread is com- posed of many thousand fibrils adherent to the vulva; from the left-hand corner of this proceeds the oviduct, leading to the ovarium which fills a large part of the abdominal cavity ; the chief part of the remainder is occupied by the liver, and by ! each other. 'The size and complexity of gland for the secretion of the silk, . I J seen just above the spinnerets h. tne secreting glandiuse vary with the de- mand for then- product occasioned by the habits of the spiecies ; thus in the Spiders which are most remarkable for the large size and the regu- larity of then- webs, they occupy, when in full activity, as much as one- fourth of the abdominal cavity, and are composed of slender branching- tubes, whose length is increased by numerous convolutions ; whilst in | those which have only occasional use for their threads, the secreting organs ' are smaller, and are either short and simple follicles, or undivided tubes 1 moderate length. There is some reason to think that the character of 420 GENERAL PHYSIOLOGY. the secretion formed by these several g'landulse is not precisely the same in all ; for differences are perceptible between the circular and the radiating threads in the common Spider’s web, which make it probable that thev issue from different spinnerets, and that their material may be provided by different glandulse.* 319 b. The Nervous system of the higher Arachnida exhibits a high degree of concentration, and is very analogous (especially in Spiders) to that of Crabs (§ 316 c) ; the cephalic ganglia, however, being in vet closer relation to the subcesophageal mass. The former are fused together into a single bilobed gan Fig. 1G5. Cephalo-thorax of My gale, viewed from the left side and considerably en- larged; — a, mandible; b, carapace; c, eyes; d, section of plastron ; e, section of lip ; /, camerostoma ; g , entrance to the mouth; h, oesophagus; i, com- mencement of stomach ; j, cephalic ganglion, receiving the optic nerves k, and sending off the mandibular nerve l ; m, great thoracic ganglion ; n, its terminal cord. glion (Fig. 165,./). lying above the oesophagus (A), a little behind the mouth ; this is con- nected anteriorly with the eyes (c) by as many nerves (k) as there are oceiii ; and it sends off two great nerves (k) to the mandibles ; whilst beneath, it gives off two peduncles, which closely embrace the oesophagus (A), and * The purposes to which these curious threads are applied, vary greatly in the different families of the Spider tribe. Some (as the Cluliones) construct for themselves silken tubes, with which they line a cylindrical or conical retreat, formed sometimes of a coiled-up leaf : and from this tube, which has an outlet at both extremities, they issue forth in search of prey, which they take means to entrap by sending forth loose glutinous threads, like so many fishing lines. Others (as the Segestrice ) form similar tubes, of five or six inches in length, in the fissures of walls, or other corresponding situations. The Mygale oementarea, or mason-spider, again, burrows deeply into the ground, forming a long tortuous gallery, which it lines with a silken tube ; and also constructing with the same fabric a very curious trap door, which effectually closes and conceals the entrance of the burrow. The Argyrop.eta aqucttica, or diving-spider, constructs a nest for itself beneath the water; spinning a ch-se hemispherical web, of which the threads adhere to each other so completely that the fabr'e is impervious to air or water; this web, which remains open at the bottom, is gradually rilled with air by the successive descents of the little spider, which carries down a few minute bubbles at each descent, entangled in the hairs beneath its body; and when this operation is complete, this Spider (which, notwithstanding its atmospheric respiration, is aquatic in its habits) takes up its abode in this diving-bell, rears its young there, and passes the winter within it in a state of torpidity. The Clotho, again, fabricates a kind of tent, composed ot several layers of the most delicate silken web, beneath which the female lives and rears her young.- — The Spiders which make such large use of their silken threads in the construction of their habitations are not those which employ it the most for the purpose of entrapping their prey:. The webs spun by the latter vary greatly in their degree of regularity and complica- tion ; some being little else than long threads of silk rudely matted together; others being clos- in their texture and without any definite arrangement; whilst the highest type is seen in those beautiful webs, of an open and regular texture, which are widely spread out for the entangle- ment of flying insects ; these spiders either clinging to the centre of their webs, or lyhta >r. wait in the neighbourhood, until warned that their net has caught a victim, by the agu.nion of a thread passing from it to their hiding-place. — But besides these purposes, which have reference to the well-being of the individual, Spiders employ their silken webs for another object, the protection of the eggs (§ 319 c). GENERAL VIEW OP THE ANIMAL KINGDOM. ARACHNIDA. 421 connect it with the great stellate mass (to), which is formed by the fusion of all the thoracic ganglia. (The form of this mass, as seen from below, is shown in Fig. 164). From this mass, five pairs of nerves are given off on each side ; the first to the maxillae and their great palpi, and the remainder to the four pairs of legs ; whilst posteriorly, a double cord (n) is sent back- wards towards the abdomen, where it soon subdivides (as seen in Fig. 164) into a bundle of nerves, which radiate to the several parts of the abdominal mass. In Mygale, an additional small ganglion is found upon the cord ante- riorly to its subdivision ; but this seems to be wanting in other Spiders. — Iu Scorpions, however, the nervous system is less concentrated, as might be anticipated from the prolongation and more complete segmentation of their bodies ; and it bears somewhat the same relation to that of Spiders, that the nervous system of the Ma- crourous Decapods hears to that of the Brachyourous (§316 c). The ganglia of the tho- racic and of part of the abdominal re- gion coalesce into one stellate mass (Fig. 166, e), as in Spiders ; and this supplies the thorax and its members, and the anterior portion of the ab- domen, including the pulmonary branchiae; the ven- tral cord of the pos- terior part of the abdomen, however, has ganglia of its own (Fig. 166, 10— 16), which are very _ _ . t . v . Nervous system of Androctonus (Scorpion): — a, antennal nerves ; b, cepha- , small 1EL Its cauclal lie ganglia; c, principal optic nerves ; d, lateral ocelli and nerves; e, sub- prolonontion The oesophageal ganglion; /, coxa; g , femur; h, tibia; i, basal joint of tarsus; 1 o cl 2, 3, second and third joints ; k, terminal nerves to double claw ; l, m , n y o, general distribll- nerves to abdominal branchiae; p, q, r , s, nerves to the segments ; t, u , firm nf Ao - nerves of the caudal ganglia; v, terminal nerves; w, fifth joint of tail; nun 01 one neives Xy anal collar; y , z y termination of the cord in the extremity of the sting; proceeding from 1 t0 nerve-trunks from the great subcesophageal ganglion; 10 to 16, gan- ,i & t glia of the ventral cord. these ganglia will , be seen in the accompanying figure. — The Eyes of the higher Arachnida : exhibit a very interesting intermediate condition between the articulate | and the vertebrate type. We nowhere meet with the large compound eyes, composed of multitudes of clustered ocelli, which are so characteristic 422 GENERAL PHYSIOLOGY. of Insects and Crustaceans ; but in their stead we find six or eight simple eyes, sometimes all clustered together in one group at the anterior part of the cephalo-thorax (Fig. 165, c), sometimes divided in such a manner that some of them are on the median line, while the remainder form two lateral and symmetrical groups. In them internal structure, these eyes have at least as much resemblance to those of Vertebrated animals, as to the single ocelli which make up the large composite eyes of Insects ; but it is interesting to remark that the simple eyes, placed at the top of the head in many members of the latter class, represent the more perfect organs of the same kind in the Arachnida. Thus on the borders of the sub-kingdom Articulata, we find the peculiar visual organs, which are on the whole so characteristic of the group, entirely disappearing, and replaced by others in whose reduced number and isolation from each other, as well as in whose internal structure, there is an obvious fore- shadowing of the Vertebrated type. There can be no doubt that the visual sense is very acute in these animals, especially in such as do not spin webs for the capture of their prey, but lie in wait for it, and pounce upon it as soon as their sight informs them of its sufficient proximity. — It is obvious from the effect of sounds upon them, that Spiders have the sense of Hearing ; but no special organ of this sense has yet been dis- covered. The same may be said of the sense of Smell. It is possible that the membrane lining the mouth and pharynx may have sufficient gustative sensibility to influence these animals in their selection of food ; but their predaceous operations appear to be principally directed by the sense of sight. The softness of the general integument of Spiders pro- bably allows them to possess a considerable share of the Tactile sense over the whole surface, and particularly in that of the anterior limbs, which in many Spiders are used rather for palpation than for locomotion ; and this may probably be the explanation of the absence of the antennce, which seem to be the special organs of touch in the higher Articulata generally. — The Muscular system is not so greatly developed in Arachnida, relatively to the size of these animals, as it is in Insects ; and this might be antici- pated from the want of firm points of support and attachment for the contractile organs, consequent upon the softness of them dermo-skeleton. Hence their movements are much less active and vigorous than those of Insects ; but them deficiency in locomotive power is amply compensated by the possession of instincts, which lead them to employ the greatest address in the pursuit and capture of their victims, and which thus enable them to prey upon creatures of apparently far greater capabilities than their own. Although the class of Insects has been spoken of (§ 315 c' as presenting the highest manifestation of instinctive tendencies not compli- cated by the operation of intelligence, yet it can scarcely be said that thi Arachnida are inferior to them in the former respect ; for whether we look at the construction of them habitations, the various devices to which they have recourse in order to secure their prey, or the care and protection which they afford to their eggs, it is difficult to point to any tribe ot Insects that surpasses them, — the economy of the social Hymenoptera and Neuroptera being wonderful rather as regards the combination of the operations of multitudes of individuals, than as regards the actions them- selves, which are perhaps equalled, in the elaborateness of them adaptation to particular ends, by those of many Spiders. There is every reason t< GENERAL VIEW OP THE ANIMAL KINGDOM. ARACHNIDA. 423 believe, that, neither in the one case nor in the other, is this adaptation designed by the animal ; both the Insect and the Spider appear, from the extraordinary uniformity of their operations, to be working out (so to speak) the law of then respective natures ; but there are indications, in the Spider tribe, of the occasional operation of a degree of intelligence , which Insects do not possess. This is especially manifested in their edu- cability ; many instances being on record, in which Spiders have learned to recognize the individuals who cherished them, and have been trained by them to actions foreign to their nature. In this respect, then, as in several others that have been mentioned, do the higher Arachnida depart from the general characters of the Articulate series, and exhibit a certain degree of approximation towards the Vertebrata. 319 c. Like the other classes which form the higher group of Articulata, the Arachnida possess only one method of propagation ; that, namely, which is accomplished by sexual union ; and there are no known excep- tions to this rule, such as present themselves among Insects (§ 315 d) and Entomostracous Crustacea (§ 317). They retain, however, some vestiges of the method of multiplication by gemmation, in the power of reproduc- tion of lost parts, which they seem to possess in about an equal degree with the higher Crustacea (§ 316 d). If either of their limbs be broken off, it is replaced by a new growth ; but this must take place from the articulation nearest the body, and the remainder of the limb is cast off at that joint. The new limb is not seen externally until after the next moult of the integument, and it is then of small size ; after the succeeding moult, however, it presents its usual dimensions. — The condition of the sexual : apparatus in the higher Arachnida presents certain very remarkable peculiarities. The sexes are always distinct, and the male (among the Araneidae at least) may generally be distinguished from the female by his ! smaller size, longer limbs, and brighter colours. The spermatic organs of the male Spider are two long worm-like tubes, which commence at the posterior end of the abdomen, either by a simple decal termination, or by j an oblong vesicle ; and which terminate, either by a single or double orifice, ; on a slit in the integument of the under side of the abdomen, near its ■ anterior extremitj 7 , in a position corresponding to that of the vulva of the female (Fig. 164). The palpi, however, contain the organs by which the spermatic fluid is introduced ; each of these being furnished with a tubular projection, terminated by a horny appendage, which, when not in use, is : retracted within the last joint of the palp. No connection whatever can be traced between the organs which prepare the spermatic fluid and these intromittent instruments ; nevertheless it has been ascertained by repeated observations, that no closer sexual congress takes place, than the introduction I of these appendages of the palpi within the vulva of the female ; and it would seem probable that the male himself applies these appendages to his abdominal aperture, and charges them with the fertilizing fluid, before the act of intromission. The ovarium of the female Spider is a simple ] elongated vesicle, closed at one extremity, and communicating at the other with a slender oviduct, that terminates at the corresponding side of the transverse sexual fissure, which, as in the male, is situated between the anterior pirlmonic cavities ; when this ovarium is dilated with ova, it occupied a considerable part of the abdominal cavity (Fig. 164). The peculiar mode of sexual congress just described seems to have relation to 424 GENERAL PHYSIOLOGY. the remarkable instinct which prompts the female spider to attack and devour the male, as soon as the fertilization of her ova is accomplished ; which her superior size and strength enable her to do, if he remain within her reach. It is obvious that if the orifice of his sexual canal were applied to hers, his danger would be augmented with this closer approximation. It is curious that, with such an instinct towards the opposite sex. the attachment displayed by the female towards her offspring should be of most extraordinary strength. The ova are generally enveloped in a soft and warm silken cocoon, which she guards with the most jealous care. Some species carry this about with them ; others attach it to trees, c: hide it in empty snail-shells. Within this the young remain until then- development is completed, and they then make their way out. The mother generally foregoes all nourishment during her watch ; and if th i young are prevented by the coldness of the weather from coming forth at the accustomed time, she will die of hunger, rather than quit her post. — In the Scorpionidce, the testes and ovaria are formed upon the same general plan as in the Araneidse, but are more complex in their structure ; the outlet by which they terminate is situated at the middle of the under side of the last segment of the thorax ; and as the palpi are destitute of the peculiar appendages which they possess in the Spiders, it is probable that they do not take any share in the sexual operation. The ova are retained in a dilatation of the oviduct, until the young- are mature ; so that the egg-case is ruptured within the body of the parent, and the young are born alive. — In both of these tribes, the embiyo attains the form and the organisation of the parent, previously to its emersion from the egg ; and it is remarkable that not even in its course of intra-oval development does it pass through stages which can be said to repeat, with any degree of closeness, those which are permanent in the lower Articulata ; the whole tendency of its development being at once to evolve the Arachnoid type, as will be hereafter shown (chap, xviii.). 3 1 9 <1. The class of Arachnida is not limited, however, to animals of the high organisation first described ; for, like that of Crustacea, it includes beings of a structure so simple as to connect it with the inferior division of the Articulate series. Of the tracheary Arachnida (§ 319) there are some that approach the Spiders and Scorpions in then- general organisation ; but in the tribe of Acaridce (mites), and especially in those which are parasitic, we meet with a general degradation of structure which leads us towards the Entozoa, whilst yet there is a remarkable character of elevation in the type of then respiratory apparatus. In the bodies of these animals, there is a more complete absence of segmentation, and a greater fusion of the principal divisions, one into another, than even in the Araneidse ; the head, thorax, and abdomen forming but one mass, of which the cephalo-thoracic portion is but very little developed in relation to the abdominal ; and the integument being extremely soft throughout, s.. that the segmentation is only marked by the points of origin of the limbs. The integument of the members, however, is generally more firm and horny, and them joints more distinct ; though in the curious parasitic Demodex, which infests the sebaceous sacs and ham-follicles of the human skin, the legs are of the simplest and most rudimental kind, bearing a strong resemblance to the pro-legs of a caterpillar. In most of the true Acaridce, they are terminated either by adhesive suckers, or by bristle-like GENERAL VIEW OF THE ANIMAL KINGDOM. ARACHNIDA. 425 filaments, instead of by curved claws. The structure of the mouth varies in the different members of the group. In some, the mandibles are forci- pated like the claws of Scorpions ; in others they are provided with -recurved hooks, as in the Spiders ; whilst in the parasitic Acari, again, they are usually prolonged into piercers, for puncturing the integument of the animals whose juices they imbibe. When formed after the pattern of those of the Spider, the mandibles of the Acari are provided with what appears to be a poison-apparatus. The maxillae present a corresponding series of transitional states, leading to the production of a prolonged channel for the imbibition of the juices, not unlike that of some of the parasitic Insects ; and the mouth of those Acaridae which are formed for suction, has its cavity surrounded by a distinct layer of muscular fibres, the contraction of which will augment the capacity of the pharynx. Behind this pharynx, however, it has hitherto been found impossible to trace any distinct alimentary tube ; the digestive cavity being to all appearance a mere excavation amongst the soft tissues of the body, every- where surrounded by a mass of cells, which, from their brownish colour, appear to form the biliary secretion. This cavity occupies a considerable part of the interior of the body, and penetrates to nearly the extremities of the limbs, as in the Pycnogonidce (Fig. 150). As in that group, the unusual extension of the alimentary cavity renders any special circulating- apparatus unnecessary ; whether any movement of fluid takes place, how- ever, in spaces around that cavity, has not been ascertained. — In the lowest parasitic A car idle (such as the Sarcoptes, or ‘itch-insect’) there is no respiratory apparatus ; so that the aeration of the fluids must take place through the general surface. In the higher members of the group, how- ever, there is a regular tracheal system, nearly resembling that of Insects ; and between these two extremes there are several gradations, of which that which is seen in Trombidion and in Hydrachna (water-mite) and its allies, is the most curious. The whole tracheal system (the branches of which are destitute of the usual spiral filament) diverges from two large trunks, which pass forwards to an orifice situated at the base of the mandibles ; it appears, however, that this merely serves for expiration, the introduction of air into the body being accomplished by a different means. A network of passages, excavated in a transparent homogeneous substance, is found beneath the skin ; and the integument is covered, in Trombidion, by plumose hairs, by which its surface is increased ; whilst in Hydrachna it is pierced with a number of orifices (resembling the stomata of the leaves of plants) each of them covered by a very thin membrane, and in communication with the lacume beneath, whilst by its side is constantly found a hair, the movements of which seem to contribute to the renewal of the water in contact with the surface of the body, and thus promote the introduction of air into the passages beneath. The air, thus taken in by the general surface, appears to be distributed through the aeriferous canals of the body, and then to be expelled by the tracheal apparatus through the single expiratory orifice, the position of which presents us with the nearest approach anywhere seen in Invertebrated animals, to that connection of the respiratory passages with the alimentary mouth, which is so characteristic of the Vertebrated series. — The condition of the nervous system in the group of Acaridae is as singular as the other parts of their structure already described ; for the only ganglionic , centre 426 GENERAL PHYSIOLOGY. which it appears to possess is a single large globular ganglion, which receives the optic nerves, and gives off trunks both anteriorly and pos- teriorly, without forming a collar round the oesophagus ; as if the cephalic- ganglia of the Spider were fused with its thoracic mass. The eyes are usually four in number ; but sometimes they are reduced to two, or even to one ; and in some of the parasitic Acari they are entirely absent. — There seems reason to believe that in some, at least, of the Acaridce, the sexes are united ; and in the larger proportion of this group, there appears to be no definite ovarium, the ova being lodged in the general substance of the tissues, as in many Entozoa (Fig. 141). In the genus Oribahs. the young are born alive ; but in other Acaridse the ova are deposited and left to themselves, the young coming forth in due time, in a form resem- bling that of their parents, except that they have only six legs instead of eight, the deficient pair being supplied at the first moult.* 320. We now arrive at the sub-kingdom Vertebrata, which unques- tionably ranks as the highest division of the Animal Kingdom, since it contains those classes which display the greatest perfection of organic structure, as manifested in the special adaptation of each paid of their organism to some different purpose, and in the consequent number and variety of their faculties. It must not, however, be left out of view, that there are members of the Vertebrated series, which must be accounted lower in then' general organization than some of the higher Invertebrata ; but these are aberrant forms, in which (as we shall hereafter see) the Vertebrated characters are but very imperfectly developed, and which correspond in many respects to the embryonic conditions of the proper types of the group. — It is in the high grade presented by the A< rvo- Musculctr system, and in the obvious subordination of the whole structure to its purposes, that we see the most characteristic difference between the Vertebrata and the sub-kingdoms already passed in review. In the latter, the functions of this system are obviously restricted to the main- tenance of the nutritive and reproductive operations, and to the guidance of the individual, by means of its sensory endowments, in its search for food, or for an individual of the opposite sex; the locomotive portion of the apparatus being predominant in the Articulata, whilst the organs of sense attain a higher relative development in the Molluscs. Iu the Vertebrata, on the other hand, we find both these elements iu a state of much greater concentration and completeness ; and we meet with supc - added organs obviously ministering to the Intelligence, which seems almost exclusively restricted to this group of animals, and which is, perhaps, its most characteristic endowment. The nervo-muscular ap- paratus thus becomes no longer the mere instrument of sustaining animal life ; it now ministers to higher endowments ; and the intelli- gence, called into activity by impressions received through the organs of sense, and executing its voluntary determinations by the apparatus of motion, has an existence independent of either. It is in Man that we see the most complete subordination of the entire organism to the purpioses of psychical existence ; but through the whole Vertebrated series, this direction of development may be easily traced ; and it is in no * Most of the facts stated in this paragraph are derived from the Memoir of M. Dujaidib, ‘ Sur les Acariens,’ in the Annales des Sciences Naturelles, Troisieme Serie, Zool., tom. :ii. p. 5. GENERAL VIEW OF THE ANIMAL KINGDOM. VERTEBRATA. 427 particular more obvious, than in the peculiarities which the vertebratecl skeleton presents, in contrast with the skeletons of the various classes of Invertebrata. - 320 cl Wherever a proper skeleton exists among Invertebrata, that skeleton — with very few and unimportant exceptions — is tegumentary or dermal : and like the epidermis and its appendages, it is non- vascular, and can only be increased by additions to its edges (§§ 193-199). This hard insensible armour serves alike to prevent the animal from being- cognisant of hurtful agencies, and to protect it from their injurious operation ; and whilst it affords fixed points of attachment to the muscles that move the body and limbs, and may even serve as a framework of levers on which they may act with mechanical advantage, these muscles are always attached to its interior, and are enclosed by it, as are the viscera in general. In scarcely any instance among the Invertebrata, do we find that the skeleton has any special relation to the nervous centres ; and these are far from presenting such a fixity in their arrangement, as could serve to govern its development. Thus the number and situa- tion of the nervous ganglia vary in the Mollusca with the development of the locomotive organs and of different portions of the muscular layer of the mantle, and with the position of the branchiae ; all of which condi- tions are subject to so much diversity, that scarcely any two genera are alike in the arrangement of their nervous centres. And in Insects, Crustacea, and other Articulata, although the ventral chain of ganglia is usually disposed on a more uniform plan, their number varies with that of the segments of the body, and their relative size with that of the development of the parts of the muscular apparatus with which they are respectively connected, whilst their degree of concentration is most diverse, and has not yet been shown to follow any fixed plan whatever. The nervous system, in the Invertebrata generally, receives only the same amount of support and protection from the skeleton, as the other tissues possess;* and its comparatively low importance in the life of these animals is shown by the severe injuries which it may sustain, without fatal results necessarily ensuing. 320 b. On the other hand, in the Yertebrated series, the Skeleton is for the most part interned; and is composed of true bone, whose relations with the vascular system are such, that its form and dimensions may be changed not merely by deposition but by removal ; so that its configura- tion can be adapted to that of the organs which it encloses and supports, not only during the period of growth and increase, but during the whole of life (§§ 200-211). This internal osseous skeleton is essentially connected with the Nervous system, which is formed upon one uniform plan throughout the entire sub-kingdom, its scattered centres being collected (with the exception of those of the sympathetic system) into a linear series, the cerebrospinal axis, in which they always occupy the * It has been imagined that certain internal projections in the dermo-skeleton of the Insect and Crustacean were specially adapted to the protection of the nervous cord ; but, as Prof. Owen remarks, there is no conformity between these and the number of ganglia, and they are so situated as to inclose the communicating cords rather than the ganglia themselves; so that their relation to the nerve-trunks seems merely accidental, their special purpose being to afford attachment to the muscles. It is only in the Cephalopoda, especially the dibrancliiate order (§ 307 J), that we meet with a true rudiment of an internal skeleton, specially designed to support and protect the nervous centres. 428 GENERAL PHYSIOLOGY. same order, and differ only in their relative development ; and this axis is included in a vertebral column, which constitutes the fundamental portion of the skeleton, and the number of whose pieces is in constant relation with the number of pairs of nerves given off from the nervous axis. The vertebral column is expanded at one end into the cranium. which affords protection to the cephalic portion of the nervous axis, and to the organs of sense connected with this ; and although in the higher animals the relation of the cranial bones to the vertebrae of the spinal column is obscured by the high development of the portion that arches over the brain, and by the adaptation of other parts for special purposes, this relation becomes obvious, when the comparison is made in those forms of Vertebrata in which the cephalic ganglia are not largely deve- loped in proportion to those of the spinal cord. The vertebral column, at its opposite extremity, is usually contracted instead of dilated, forming a tail, or the rudiment of one, from which the nervous centres are entirely withdrawn ; and the alteration which the vertebrae there undergo, by the loss of one element after another, is scarcely less in degree (though more easily traced) than that to which they are subjected in the cranium by the excess of development of some of these elements. But besides the canal for the lodgment of the nervous or ‘ neural ’ axis, which lies on the upper side of the central osseous column, there is another (less constantly present, however), for the lodgment of the great vascular trunk, which may be called the ‘ haemal ’ canal. This is dilated, in the trunk of the body, to an enormous size ; being made to include the great centres of the circulation, the digestive and respiratory organs, and in fact the whole visceral apparatus concerned in the preparation and purification of the blood. The walls of this cavity are chiefly formed by the ribs and sternum, which, though commonly regarded as distinct bones, are really to be considered, in a philosophical sense, as parts of the vertebral segments. In addition to the bones of the head, trunk, and tail, the Vertebral skeleton commonly contains a set of bones that give sup- port to the two pairs of locomotive organs, or ‘ extremities,’ which are developed with greater or less completeness in by far the larger part of the series. The bones proper to each pan* of limbs are connected with the vertebral column by a peculiar arch, of which the anterior is termed the ‘scapular’, and the posterior the ‘pelvic.’ Even these bones are reducible (as will be presently shown) to the general type of the vertebra ; the scapular and pelvic arches being modifications of the haemal arches which elsewhere serve to protect the visceral cavity ; whilst the bones of the limbs themselves may be regarded as originating in an extraordinary development of certain ‘diverging appendages’ of those haemal arches, which are elsewhere adapted to a very different purpose. When it is added that the ‘ hyoid ’ bone, which is detached from the remainder of the skeleton (lying between the summit of the larynx and the root of the tongue), is also a ‘ haemal arch ’ of one of the cranial vertebrae, all the parts of the proper vertebral skeleton will have been (so to speak) accounted for. 320 c. In many Vertebrated animals, however, a very distinct dermal skeleton is present, notwithstanding that the internal skeleton is fully developed ; and although this dermo-skeleton is sometimes only horny in its texture, yet there are several instances in which it is composed of true GENERAL VIEW OF THE ANIMAL KINGDOM. VERTEBRATA. 429 bone. This is the case, for example, in the Lepidosteus and Ostracion among existing Fishes, as well as in the whole of the great group of Ganoids now almost extinct ; in the Crocodiles, many of the dermal plates present almost the same complete ossification ; and even among Mam- malia, the tesselated armour of the existing Armadilloes and gigantic extinct Glyptodons affords a striking example of the co-existence of a well- developed bony envelope with a complete osseous endo-skeleton. Even where no such complete osseous envelope exists, bones are occasionally found, especially among Fishes and Reptiles, which, though intercalated amongst those of the vertebral skeleton, do not really belong to it, but are derived from the dermal system. Such are, in Fishes, the ‘rays’ upon which the dorsal fin is supported (Fig. 173); whilst among Reptiles, the Turtle presents a remarkable example of this derivation, in the bones forming the margin of the carapace (Fig. 176), as well as in part of both the dorsal and ventral shields themselves (§ 324 /). Besides the osseous elements of the vertebral and dermal skeletons, certain other bones occasionally present themselves in Vertebrate d animals, which seem to fall naturally into a distinct category ; being developed to afford a special support and protection to particular parts of the visceral apparatus. We find some rudiment of such structures, even in the Invertebrata ; thus in the Lobster , the walls of the stomach contain a calcified framework, supporting the gastric teeth, and giving attachment to the muscles that work them ; and in the Bulla, there is a pair of calcareous plates imbedded in the walls of the muscular stomach or gizzard, which adds considerably to the crushing and triturating powers of the organ. Now in most air-breathing Vertebrata, we find the larynx, trachea, and bronchial tubes more or less supported by a carti- laginous framework, which sometimes becomes ossified ; in Fishes and in the tadpole state of Batrachians, the gills are supported upon a carti- laginous or osseous framework developed independently of the vertebral skeleton ; and in many Mammalia, the heart contains a bone that serves as a support to its muscular and ligamentous fibres. These, with some other isolated bones whose presence is only occasional, may be regarded as constituting the ‘ splanchno-skeleton ;’ and to this we are probably to refer the osseous cups or plates which are found in the outer coat of the eye of many Birds and Reptiles, — the hard bony envelope which sur- rounds the internal ear, and which becomes incorporated with the neural skeleton in most Vertebrata, forming what is commonly known as the ‘petrous portion of the temporal bone’, — and the turbinate or spongy bones of the nose, with the cartilages which enclose some parts of the nasal cavity. It is quite plain, from the history of their development, that these ‘ sense-capsules ’ are originally independent of the vertebral . skeleton, however closely they may be subsequently incorporated with it. The teeth of Fishes, Reptiles, and Mammalia, which originate in papillae developed from the surface of the mucous membrane lining the mouth (§212), must be regarded as belonging to the splanchno-skeleton ; and in their well-known mode of implanting themselves in the jaws, we have a good example of the process by which parts of the dermo- and splanchno- skeletons elsewhere form an intimate connection with the vertebral. 320 d. Thus the skeleton of Vertebrata has to be regarded as consisting of the endo-skeleton, osseous or cartilaginous ; which is composed of a 430 GENERAL PHYSIOLOGY. series of vertebrse, and their appendages, developed in different modes and degrees, but always especially related to the cerebro-spinal axis ; — of an exo-skeleton, more or less complete, which may even envelope the entire body in a bony casing, or may be reduced to the condition of hairs, quilk or horny scales, or may be entirely wanting; — and of a splan chn o-skeleton . the various parts of which have no other relation to each other, than that of being developed with express reference to the visceral system. It is not always easy to discriminate the class to which a particular bone should be referred ; indeed, a knowledge of its form and connections in a great variety of animals, and of the history of its development, is frequent 'v necessary to enable this classification to be satisfactorily carried out. “ As in all arrangements of natural objects,” observes Prof. Owen, “ where nature is followed in selecting their characters, so in classifying the parts of the general skeleton of Vertebrata, the primary groups blend into one another at their extremes, and make it difficult to draw a well-defined boundary line between them. But we must not on that account abandon the advantage of arrangement and classification, in acquiring an intelli- gible and tenable knowledge of a complex system of organs, when typical characters clearly indicate the general primary groups.” This analysis lias been pursued by Prof. Owen far more satisfactorily than by any other Philosophical Anatomist; and in the more particular description which will be now given of the vertebral skeleton, and of its modifications in the several classes, his views of its constitution will be adopted and explained so far as space will permit.* 320c. A complete ‘typical vertebra,’ — “one of those segments of the endo-skeleton which constitute the axis of the body and the protecting canals of the nervous and vascular trunks,”- — consists, according to Prof. Owen, of the parts represented in the accompanying figures, of which a is a mere ideal diagram, whilst b shows one of the modes in which they are actually seen united in nature. In the former, the parts which are fully shaded are those which are usually developed from distinct and independ- ent centres, and which are termed by Prof. Owen ‘autogenous’ elements : those, on the other hand, which are left in outline, shoot out as ‘ processes ' or projections from some of the other’s, and are termed ‘ exogenous." These elements may be united with each other in various modes and degrees, sometimes remaining almost entirely disjoined even in the adult animal, whilst in other cases they are united into a single piece, so that their real distinctness is only to be recognized by tracing the history of their development. In a large proportion of instances, some one or more of these elements may be entirely deficient ; whilst in other cases, ore element or set of elements is developed in so much larger proportion ti- the rest, as to throw them (so to speak) into the shade. Thus in the Mammalian cervical vertebra (Fig. 1 69) we have a mere trace of the haemal arch ; whilst in the thoracic vertebra (Fig. 167, b) we find it expanded h- * These views are set forth in detail in Prof. Owen’s admirable works on the “ Horn" - ”ies of the Vertebrated Skeleton,” and on the “ Nature of Limbs a general account of them, with their special application to the class of Fishes, being also contained in his “ Lectures on Comparative Anatomy,” vol. ii. These works are examples of that rare combination of logical appreciation of facts, with originality in the conception of ideas, which distinguishes the true philosopher from the rash speculator on the one hand, and from the mere plodding observer on the other. GENERAL VIEW OF THE ANIMAL KINGDOM. VERTEBRATA. 431 many times the size of the neural arch, in order that it may include and protect the great centre of the circulating system, with the respiratory organs. It is in the cranial segments, however, that the vertebral ele- Elements of a Vertebra , according to Prof. Owen: — a, ideal typical vertebra: — b, actual thoracic vertebra of a Bird : — c , centrum, giving off d , d. the diapophyses, and p,p, the parapo- physes; the neural arch, enclosing the spinal cord*, is formed by n , n, the neurapophyses, and ns, the neural spine; the hsemal arch, enclosing the great centres of the circulation, is formed by h i Ji, the heemapophyses, and hs, the heemal spine. From both the neurapophyses and hcemapophyses may be given off the zygapophyses, z, z. The lateral arches, which may enclose the vertebral arteries o o, are completed by the pleurapophyses, pi ; these in b are bent dowD- wards, so as to form part of the haemal arch, and give off the diverging appendages a, a. ments undergo their most remarkable transformations ; and it is only by tracing them through their simplest to their most complicated forms and arrangements, that the true nature of the latter can be elucidated. 320/. The centrum (c, Fig-s. 167, and 170) forms the axis of the verte- bral column, and most commonly serves as the central bond of union between the peripheral elements of the vertebra. In some instances it retains its primitive soft state, when some of the other elements have been completely ossified ; and it is remarkable that such should have been the condition of the vertebral column of all the Fishes, whose fossil remains have been preserved to us in strata earlier than the Permian epoch, whilst among existing Fishes it is comparatively rare. Towards the extremity of the caudal prolongation of the vertebral column, the segments are often found reduced to the body or centrum alone. Though it may present a great variety of shapes, its normal form may be regarded as cylindrical ; the two extremities of the cylinder being either flat, more or less concave, or convex, or one concave and the other convex. The individuality of the bodies of the vertebrae is frequently lost by their coalescence with each other ; of this the ‘sacrum’ of Man (Fig. 168) and of Mammals and Birds generally, is one of the most common examples. The centrum supports the ‘neural axis’ or spinal cord (Fig. 167, *) on its upper side, the vertebral column being supposed to be in a horizontal position) ; whilst on its under side it is in relation with the ‘ luemal axis ’ (o) or l 432 GENERAL PHYSIOLOGY. Fig. 168. principal trunk of the vascular system. — The neurapophyses, which consti- tute what are commonly known as the ‘ laminae ’ or ‘ arches ’ of the vertebra (Fig. 169, 2) are constantly found protect- ing the neiu’al axis wherever this passes through the vertebral segment, and are even more constant as osseous or cartilagi- nous elements than the centrum itself : it being only near the extremity of the tail, along which neither the neural axis nor its prolongations into nerve-trunks extend, that they do not present themselves. They very frecpiently lose their individuality by coa- lescing with the centrum at their base, and with the neural spine and with each other at their apex. When this is the case, they form a complete osseous ring (as seen in any one of the human vertebra, Fig. 169), ' ' ' ' " ' ' Be- Human Sacrum , seen upon its anterior four pieces ; 2, 2, the anterior sacral fora- mina; 3, the promontory of the sacrum; 4, the ear- shaped surface which articulates surface;— 1,1, thetransverse lines marking through which tllC UCUral axis passes, the original constitution of the bone of ° n , , x tween eveiy pair of vertebral segments, a pair of nerve-trunks makes its exit from with the ilium ; 5, the sharp edge to which the central axis, sometimes through the in- the sacro-ischiatic ligaments are attached; 6, the vertebral articular surface; 7> the broad triangular surface which supports the psoas muscle and lumbo-sacral nerve; 8, the articular process of the right side ; 9. the inferior extremity, or apex, of the sacrum; 10, one of the sacral cornua; 11, the notch which is converted into a foramen by the coccyx. Fig. 169. terspaces between the contiguous neurapo- physes, sometimes through apertures formed by the junction of notches in the margin of each (Fig. 169, 3). It occasionally happens that the neurapophyses, like the bodies, of several successive segments coalesce, so as to form a continuous bony canal ; but then’ original distinctness is always marked by the apertures for the exit of the nerve-trunks (Fig. 168, 2, 2). — The neural-spine commonly retains, in the trunk of the body, the long-pointed form which its name im- plies, and constitutes the ‘ spinous process ' of the ordinary vertebra (Fig. 169,4) which is specially adapted for the attachment of muscles ; occasionally, however, it is flat- tened and greatly increased in breadth and solidity, so as even to coalesce with die pleurapophyses. It is seldom altogether absent, but it frequently loses its individu- ality by coalescing with the neurapophysc s. — The foregoing elements constitute, when united, the ‘ neural arch,’ which is the part Human Cervical Vertebra; — I, the 0 f -(pie vertebra not Onlv the lUOSt Constant body; 2, one of the lamrHse; 3, inter- . . , , J .. vertebral notch; 4, spinous process; 1U its presence, DUt aiSO the least disposed to vary widely from a common type. From 8, inferior articular process — n.b. This some part of this arch, we general 1\ find figure must be considered as inverted , , 1 , ,, , , v , , ,• V, „ > with reference to Fig. 167 , the neural arch developed the oblique 01’ ‘articulating being here below the centrum. processes (Fig. 169, 7, 8 ), which Serve to connect the vertebra together ; these, which are termed by Prof. Owen the ‘ zygapophyses,’ are never developed as distinct elements, but shoot out from the neurapophyses. SI GENERAL VIEW OF THE ANIMAL KINGDOM. VERTEBRATA. 433 320 g. Projecting from tlie centrum, we find a pair of processes on either side, between which is a canal that serves for the passage of a blood- vessel. The ‘ diapophyses,’ which project above this canal, are nearly always £ exogenous ’ elements, shooting forth from the centrum or from the neurapopliyses ; the ‘ parapophyses,’ which form the lower boundary of the canal, are £ autogenous ’ or independent elements in Fishes, but are developed from the centrum in all the higher Vertebrata. These elements never attain any high development in Mammals, Birds, or Reptiles ■ in which classes they serve merely to enclose the vascular canal, and to support the ‘ pleurapophyses.’ Thus in the Human cervical vertebra, they form the two roots of the ‘ tranverse process,’ surrounding the foramen for the passage of the vertebral artery (Fig. 169, fl ) ; whilst in the thoracic vertebra (Fig. 167, b), the diapophyses (d, d) form the £ transverse processes,’ and the parapophyses ( p, p), reduced to mere rudiments, constitute the articulating surfaces with which the £ heads ’ of the ribs come into contact. In Fishes, however, they are more highly developed, and in the Cod-tribe are even longer and broader than the £ pleurapophyses ’ or true ribs. The ordinary function of these lateral processes is to give support to the pleurapophyses, to afford attachment to muscles, and to protect the lateral vascular trunks. — Usually connected with the foregoing, where they exist, but not unfrequently absent, and sometimes directly connected with the centrum, are the £ pleurapophyses’ (pi, pi), whose development varies extremely in different parts of the same trunk, as well as in different animals. Thus in the cervical vertebim of Man, they form the short bifid £ transverse processes’ (Fig. 169, 5 ), which are anchylosed at their base to the two £ roots,’ the diapophyses and the parapophyses, which surround the vertebral canal. In the thoracic segments, on the other hand, they are developed separately and constitute the ‘ribs,’ which form the greater part of the boundary of the haemal arch. The ribs of Birds usually possess certain peculiar £ diverging appendages ’ (a, ci), each of which projects backwards over the next rib ; their special function appears to be, to give increased solidity to the thoracic frame- work, by thus connecting the ribs with each other (§ 325 a). These ap- pendages to the ribs of birds, however, will be hereafter shown to be the rudiments of the elements, which, in a state of higher development, con- stitute the £ extremities ’ or ‘ limbs f these bearing the same relation to the pleurapophyses of the scapular’ and pelvic arches, as do the connect- ing pieces of the ribs of birds to the ribs themselves (§ 320 k). Similar diverging appendages, of much larger relative size, are found proceeding from the ribs of many Fishes. 320 h. On the under side of the centrum, we find, in a complete ver- 1 tebra (Fig. 167, a), the £ haemal arch,’ formed of two ‘ haemapophyses ’ (h), and of a hiemal spine ( hs ), and thus answering to the neural arch. The entire htemal arch, however, is very frequently wanting, as in the cervical and lumbar vertebrae of Man and Mammalia ; but in the tail of ; some Mammals and of Reptiles, a haemal arch exists, and presents a remarkable similarity, both in form, size and composition, to the neural arch. It is made up of two bony lamime, the £ hsemapophyses,’ which form by them junction with each other at their apex a set of Y-shaped bones, but which retain their primitive independency at their bases, where they simply articulate with the centrum ; these lnemal arches protect the caudal F F 434 GENERAL PHYSIOLOGY. artery and .vein. In tracing the tail from its attached towards its free ex- tremity, we find the haemal arch first disappearing, and then the neural, the centrum alone being left. It is in the thoracic region of Mammalia, Ends, and Reptiles, that we find the greatest expansion of the haemal arch, which is made, by the articulation of the hiemapophyses with the extremities of the ribs, instead of with the centrum, to surround the entire visceral cavity (Fig. 167 , b). In Man and the Mammalia, the haemapophyses, remain- ing unossified, are known as the £ cartilages of the ribs but they are ossified in Birds and Reptiles, and are there known as the ‘ sternal ribs.' The ‘ hiemal spine’ presents a much greater variety in form than tie neural, and is much less constantly present. In the thorax of Mammalia it constitutes the flattened ‘ sternum which is originally formed (even in Man) of as many distinct segments as there are costal cartilages con- nected with it ; but which may be at last made, by the coalescence of these, to appear as a single bone. The sternum of Buds, on the other hand, usually presents a prominent keel on the median line, so that ; ts transverse section more resembles that of the neural spine (Fig. 167, b). In most Reptiles, again, the haemal spine (sternum) is flattened out laterally as in the Mammalia ; and in the Turtles this lateral extension takes place to an unusual extent (§ 324 li ) ; thus offering an interesting analogy to the expansion and flattening of the neural spine in the cranium of large-brained animals. The haemapophyses and hamial spines (sternum and sternal ribs), which are wanting in the abdo- minal region of Mammalia and Birds, are usually continued backwards in the Lizard tribe ; although, for want of the pleurapophyses, the haemal arch is not completed. In Serpents, on the other hand, the haemal arches are wanting along the whole trunk, the ends of the ribs being everywhere free. In Fishes, also, the haemapophyses and haemal spines are altogether absent or unossified ; but a haemal canal is fre- quently formed by the inclination of the parapophyses towards each other ; and in the herring a sort of sternum is formed of dermal plates, articulated to the ends of the ribs. 320 i. The number of the segments entering into the skull has been a subject of much discussion among those who admit the ‘ vertebral theory ’ of its composition. By Oken, who first (in 1807) distinctly enunciated the idea, the number was fixed at three, although he recognized the rudiment of a fourth anteriorly, which he has since adopted into his system ; Spix and Boganus also made out four ; but Geoffrey St. Hilaire maintained the existence of seven; whilst Cams would find sir. Prof. Owen (in whose determinations on this point the author has entire con- fidence) returns to the original number, four, as corresponding with that ef the four primary divisions, succeeding each other in a linear series, which are distinctly marked in the early development of the Brain ; as well as wffth that of the nerves of special sense, which issue from this paid of the neural axis with the same regularity that the ordinary sensori-motur nerves do elsewhere. These segments are named (proceeding from behind for- wards) the Epencephalic, the Mesencephalic, the Prosencephalic, and the Rhinencephalic, according to the divisions of the brain to which they respec- tively correspond; and from the Epencephalic segment proceed the amlii'o'y nerves ; from the Mesencephalic, the lingual or gmtative; from the Pro-en- cephalic, the optic ; and from the Rhinencephalic, the olfacti ve. In the skull GENERAL VIEW OF THE ANIMAL KINGDOM. VERTEBRATA. 435 of the Fish, it is not at all difficult to make out the vertebral elements of the neural arches of these vertebra, the re- lative position of which is nearly the same as it is represented in the ideal “Archetype Ske- leton” (Fig. 170), al- though the forms are much modified. Thus in the ‘ epencephalic’ segment, we have the ‘basi-occipital’ (1), re- presenting the cen- trum ; the ‘ par-occi- pital ’ (4) on each side, representing the para- pophysis ; the ‘ exocci- pital’ (2) on each side, representing the neu- rapophysis ; and the ‘ supra-occipital ’ (3) on the summit, re- presenting the neural spine. All these bones coalesce in the single ‘ occipital bone ’ of Man, in which, how- ever, most of these elements are distinct from each other at an early period of de- velopment ; and this segment is thus con- veniently known as the occipital vertebra. — Of the £ mesence- phalic’ segment, the centrum is formed by the ‘ basi-sphenoid ’ (5) ; the parapophyses are represented by the ‘ mastoid’ bones (8) ; the neurapophyses by the ‘ali- sphenoids’ (6); and the neural spine by the junction of the parietals (7). In the adult human cranium, 436 GENERAL PHYSIOLOGY. the ‘ parietals ’ attain an enormous relative development, in accordance with that of the cerebral hemispheres over which they arch, so that this vertebra is most appropriately termed the parietal ; and the bad- sphenoid and ali-sphenoids are united (with other elements) into one bone, the ‘ sphenoid,’ of which the former constitutes the ‘ body,’ whilst the latter are known as the ‘ great wings.’ The acoustic cavity is always formed by an excavation in the parietal and occipital vertebrae, which are disposed around it in the same manner as the elements of the frontal vertebra are arranged to form the orbit of the eye, or those of the ethmoidal to form the nasal cavity. The organ of hearing has usually a bony capsule of its own, forming part of the splanchno-skeleton ; and this, which is called the ‘ petrosal’ bone on account of its hardness, coalesces, in the higher animals, with the ‘mastoid’ and other elements, to form the ‘ temporal ’ bone. — The centrum of the ‘ prosencephalic ' segment is formed by the ‘ pre-sphenoid ’ (9) ; which, though developed from the same centre of ossification with the basi-sphenoid, may be regarded as distinct from it ; the parapophyses are represented by ihe ‘ post-frontals’ (12) ; the neiunpophyses by the ‘ orbito-sphenoids ’ (10) : wdiilst the ‘ frontal’ (11) obviously constitutes its neural spine. The £ pre-sphenoid ’ and the ‘ orbito-sphenoid ’ enter into the composition « if the ‘ sphenoid ’ bone of man and the higher animals ; the former consti- tuting the anterior part of its ‘ body,’ and the latter its orbitar portion. The ‘ frontal ’ and ‘ post-frontals ’ coalesce to form the single ‘ frontal bone,’ which, like the parietals, is enormously developed, and from which this vertebra is appropriately termed the frontal. It is chiefly by the elements of this segment, that the ‘ orbit ’ of the eye is formed. The capsular bones of the eye itself, where they exist, are always distinct from the vertebral elements which form the general protective envelope to the organ. — Of the ‘ rhiuencephalic ’ or nasal vertebra, the centrum is formed by the ‘ vomer’ ( 1 3), which in fishes is a broad flat bone ; the parapophyses are not present as distinct elements in this segment ; the neurapophyses are represented by the ‘ pi'e-frontals ’ (14); and the neural spine, by the single or double £ nasal ’ bone (15). In Man and the higher animals, the vomer is a thin vertical plate very unlike the centrum of a vertebra ; and the pre- frontals form the orbital portion (os planum) of the £ ethmoid ’ bone.* 320 k. In tracing out the ‘haemal’ arches of the four cranial ver- tebrae, the Philosophical Anatomist is more impeded by the extraor ti- nary changes of form and position which these elements undergo, and his determinations are consequently more open to objection, than in the case of the neural arches. These changes of form and position have reference to the special wants of the animal, and cannot, therefore, be reduced to any general expression. In some instances, there is not only an extraordinary enlargement, but an absolute repetition of similar parts ; whilst in -ether cases, there is as remarkable a contraction or complete deficiency. It is only from a most extensive and careful comparison of the facial and other bones connected with the cranial segments in different animals, that Trot. * The body of the ‘ethmoid’ bone, which has been regarded by many anatomists as the centrum of the anterior vertebra, is considered by Prof. Owen in the light of a ‘ sense- capsule’ to the olfactive organ; a view which its position and condition in Fishes appears fully to sanction. It does not there (as in the higher Vertebrata) form part of the door of the cranial cavity, and give support to the olfactive ganglia, or ‘ rhinencephalon but feints its anterior wall, and gives protection to the olfactive organ. GENERAL VIEW OF THE ANIMAL KINGDOM. VERTEBRATA. 437 Owen has felt justified in the following determinations. — The hsemal arch of the ‘ occipital ’ segment is in reality the scapular arch, to which the anterior extremities are attached ; its detachment from the cranium, in the fully developed condition of the higher animals, masking its original connection with it, which is obvious enough in the class of Fishes, where the scapular arch is as distinctly articulated with the cranium at its hinder part, as the lower jaw is in front. The several parts of which this arch consists (Fig. 170, 50, 51, 52, 53), 'will be more particularly described in the next paragraph.— The hsemal arch of the £ parietal ’ segment is the hyoidean, which is suspended from the mastoid bones, and consists, when complete, of a considerable number of independent pieces. The pleura- pophyses are represented by two bones on each side ; the ‘ stylo-hyals ’ (38), and the ‘ epi-hyals ’ (39) ; the luemapophyses by the £ cerato-hyals ’ (40) ; and the hsemal spine by the £ basi-liyal ’ (41 ), to which the £ glosso- hyal ’ (42) is added in front, and the ‘ uro-hyal ’ (43) behind. The £ diverging appendages ’ of this hsemal arch present themselves in Fishes as a set of simple elongated rays, projecting from the outer margins of the lisemapo- physes, and supporting the membrane which closes the gill-chamber exter- nally, whence they are termed £ brancliiostegal ’ rays ; their number and size vary considerably, their most common number being seven, as in the Cod. The hyoid arch is reduced to a very small relative size in Man and the higher Vertebrata, and is comparatively incomplete (§ 326 I, Fig. 191). — The hsemal arch of the ‘ frontal ’ segment is the iympano-mandibular, consisting of the lower jaw, with the tympanic pedicle which it has in all oviparous vertebrata, connecting it with the centrum of that segment. The ‘tympanic’ bone (38), which is subdivided in most Fishes into four bones articulated end to end, constitutes the pleurapophysis on either side ; the articular portion of the lower jaw (29), which is subdivided into numerous pieces in Reptiles, constitutes the hsemapophysis ; whilst the body of the jaw (32), on which the teeth are implanted, is the haemal spine completing the arch. A remarkable set of bones is developed in Fishes, on either side of the head, as the diverging appendage of the pleurapophysis of this arch ; those, namely, which form the osseous por- tion of the gill-cover, and are known as operculars .* — The most anterior of the haemal arches, that belonging to the £ nasal ’ vertebra, is termed the palato-maxillary ; it forms the palate and upper jaw, and is always much more developed in point of size than the neural arch of the same segment. The pleurapophyses are represented by the £ palatine ’ bones (20), which are suspended from the vomer ; the haemapophyses by the ‘maxillary’ (21) ; and the hsemal spine by the ‘premaxillary’ or ‘inter- maxillary ’ bone (22). This arch frequently has diverging appendages developed from its pleurapophysis, constituting the ‘pterygoid’ (24), and ento-pterygoid (23 ) ; whilst the hsemapophysis also has an articulating appendage which brings it into connection with the posterior segment. * Philosophical Anatomists have long since repudiated as utterly untenable the strange doctrine propounded by Geoffroy St. Hilaire, that the opercular bones are the homologues of the minute ossicula auditus (bones of the ear) of higher animals, — a doctrine which was an apt illustration of what genuine philosophical anatomy is not. By some, as once by Prof. Owen himself, these bones are regarded as belonging to the dermal skeleton ; but his present determination seems fully borne out by the size, importance, and tendency to multiplication of parts by subdivision, which the ‘ diverging appendages ’ elsewhere present in the class of Pishes. 438 GENERAL PHYSIOLOGY. In Man and the higher Yertebrata, the pterygoid bones coalesce with the sphenoid ; and the articulating appendage is developed into the malar bone and the zygomatic and squamous portion of the temporal. The pre- maxillary bone in Man usually coalesces with the maxillary. On the other hand, in Fishes, the bones entering this arch are multiplied, and are so disposed as to appear like three parallel and independent arches. This arrangement seems to have reference to the peculiar mobility which the lips of fish, in the absence of other prehensile organs, require to pos- sess ; and is one of the most striking examples yet brought under notice, of a departure from the regular type of conformation, for the sake of answering a specific purpose. (320 l). We have now to enquire into the nature of those appendages to the trunk, which are commonly known as ‘ members ’ or ‘ limbs,' — to consider how far they can be regarded as offsets from the vertebral column, or as peculiar developments of any of its elements, — and, if such be their nafrare, to ascertain from what segments they are put forth. In this enquiry, it is obviously necessary to take as the guide those simplest or most elementary forms, in which there is the least depar- ture from the primitive fundamental type ; and a most beautiful example of this kind is furnished by the Lepidosiren (Fig. 171, a, c), in which the extremities are severally formed of a single series of bones, articulated to each other end to end. Now in this animal, the scapular arch, with which the anterior pair’ of extremities is connected, departs in no considerable degree from the haemal arch of the typical vertebra. We trace in it (Fig. 171, a) the rib-like pleurapophyses bpl, 51) which answer to the ‘ scapula of higher animals ; and from the extremities of these hang down the lnema- pophyses ( li , 52), which represent the ‘ coracoids,’ meeting on the median line ; the haemal spine is alone wanting to complete the parallel, and this we know to be frequently deficient. The single jointed ray, which consti- tutes the rudiment of the bones of the extremities (a, 53-57), is obviously the ‘ diverging appendage ’ of this arch ; its connection with the hsema- pophysis, instead of with the pleurapophysis, being but a trivial displace- ment. Now, this haemal arch hangs down from the occipital vertebra in the Lepidosiren ; and is obviously required to complete the segment, whose neural arch is formed by the bones that support and cover-iu the epencephalon. Consequently, the scapular arch of the Lepodisiren must be regarded as the haemal arch of its occipital vertebra ; and the jointed rod-like anterior extremities as its diverging appendages. But the Lepi- dosiren is not an exceptional instance of this connection and relation. The pectoral fins of all osseous and of some cartilaginous Fishes, whatever their form and size, are supported by an inverted bony arch, which is as closely connected with the centrum and neural arch of the occipital ver- tebra, as it is in the Lepidosiren ; and which must consequently be regarded (notwithstanding its somewhat wider departure from the original simple type) as the luemal arch of the first cranial segment, being even present when the pectoral fin itself is not developed. And the pectoral fin is formed by a mere multiplication of parts resembling the single elements of the spiny appendage in the Lepidosiren ; just as the branchiostegal rays are the multiplied or subdivided diverging appendages of the frontal vertebra. — When we go to the lower Batrachian Reptiles, as the Amphinma , we find the scapular arch disconnected from the occipital vertebra : but GENERAL VIEW OP THE ANIMAL KINGDOM. VERTEBRATA. 439 when these parts are again brought together (as in Fig. 171, b), the origi- nal relation between them is no less evident than in the preceding case. The bones of the extremities now begin to exhibit the arrangement char- acteristic of terrestrial animals ; the distinction of humerus (53), ulna and radius (54, 55), carpus (56), and digits (57) being now apparent ; hut the latter are as yet only two in number. The humerus (or origin of Fig. 1 71. A B Diagram illustrating the Nature of Limbs : — a, posterior view of the occipital vertebra of Lepidosiren annectens : — b, posterior view of the occipital vertebra of Amphiuma didactylum : — c, posterior view of the pelvic vertebra of Lepidosiren : — d, posterior view of the pelvic vertebra of Pro tens angumus. In the several diagrams, the following references indicate corresponding parts; c, centrum; n, neurapophyses ; 5 , neural spine ; pi, 51 , pleurapophysis of the occipital vertebra, or scapula; 7i, 52 , haemapophysis of the occipital vertebra, or coracoid bone; a, 53 - 57 , diverging appendages of the occipital vertebra, or anterior limbs ; pi, 62, pleurapophysis of the pelvic vertebra, or iliac bone; h, 63 , haemapophysis of the pelvic vertebra, or ischiac bone; a, 65-69, diverging appendages of the pelvic vertebra, or posterior limbs. the diverging appendage) is here articulated with the haemal arch at the junction of the pleurapophysis and haemapophysis, both the scapula and coracoid entering into the composition of the shoulder-joint ; and this is the arrangement which we meet with in all oviparous vertebrata, in which the clavicle is a subordinate bone. In the Mammalia, however (except among the non-placental orders), the coracoid bone is reduced to a mere rudiment, being known as a ‘ process ’ of the scapula ; and its function (that of keeping the shoulders apart) is taken-on by the £ clavicle ; ’ which is regarded by Prof. Owen as the haemapophysis of the ‘ atlas ’ or first cervical vertebra (Fig. 170, 58), the portion of the e sternum ’ (properly the episternum) to which it is attached, being the haemal spine of the same arch. — Now although to such as have not studied Anatomy with a view to the determination of the original or primitive relations of the compo- nent parts of the structure, it may be diffioult to conceive of the scapular arch in Man and the air-breathing Vertebrata, as being thus detached from its original segment and carried backwards, so as to be implanted on the trunk below the neck instead of at its summit ; yet a more careful survey of the phenomena of Nature shows that the possibility or even probability 440 GENERAL PHYSIOLOGY. of such a displacement must be freely admitted. For, not to mention other instances in which it is less striking, we find that in Fishes the abdominal fins, which obviously answer to the posterior extremities of higher Vertebrata, may be carried forwards to any part of the trunk, and may even occupy a position anterior to that of the pectoral fins. There would be as much reason, then, for regarding these organs as no longer the same with the normal abdominal fins, when they are tlms displaced forwards, as there would be for contending that the scapular arch and anterior extremities of air-breathing Vertebrata are not the representatives of the occipital appendages of Fishes, because they aie displaced backwards ; and as no Anatomist has ever suggested a doui a upon the former point, no difficulty can be reasonably urged upon the latter. In each case, it is obvious that the displacement is an adaptation of the original plan, to meet the particular wants of the animal; the abdominal fins being brought forwards in those fishes in which the power of readily ascending and descending in the water is specially required ; and the pectoral extremities being earned backwards from the front to the hinder end of the neck, in order to give advantageous support to the trunk of those animals which are to be modified for progression on land. 320 m. That the pelvic arch, however greatly it may be modified, is the representative of the scapular, — the ‘ilium’ answering to the scapula, the ‘ ischium ’ to the coracoid, and the ‘ pubis ’ to the clavicle, — is not doubted by Anatomists who have studied then - relations in different animals. Such being the case, the pelvic arch must be regarded as the haemal arch of one or more of the pelvic vertebrae ; and such it plainly appears to he, when it is studied in its simpler forms. Thus in the Lepidosiren (Fig. 171, c), we find the posterior jointed spines attached to a pair of bones (63), which hang beneath the pleurapophyses (02 ) of the pelvic vertebra, and which are obviously its hsemapophyses. In common with the whole class of Fishes, the hsemal arch is here divided, the pleurapophyses being articulated with the centrum, whilst the luema- pophyses are detached from them ; and this arrangement obviously has reference to the mobility of the arch in this class. In the lower Batnv chian Reptiles, however, we find, with a very little wider departure horn the primitive type, that arrangement of the pelvic arch which generally prevails in air-breathing animals, in which the situation of the posterior extremities is constant. Thus in the Proteus (Fig. 171, d) we see '.lie rib-like ‘ ilium ’ (62), articulated as a pleurapophysis with the par-apo- physis of the centrum ; whilst the arch is completed below by the ‘ isebium (63), which is evidently the hsemapophysis. The posterior extremity, like the anterior, is here connected with the pleurapophysis and the hsemapophysis at their junction ; both these entering into the compo- sition of the hip-joint. The extremities themselves, in the Proteus, show a slight advance upon their condition in the Ampliiuma ; for the tarsal bones (68) are now distinct, and the digits (still only two in number) are divided into metatarsus and phalanges (69). The ‘pubis,’ which does not exist in Fishes, makes its appearance in Reptiles as an arch altogether distinct from the ischiatic ; and this condition it retains in Buds, where it evidently holds the same relation to the ischium in the pelvic arch, that the clavicle does to the coracoid in the scapular, and would thus appear to be the hsemapophysis of another vertebral segment. In Man and .the GENERAL VIEW OF THE ANIMAL KINGDOM. VERTEBRATA. 441 Mammalia, the ischia do not meet on the central line, the arch being com- pleted by the pubis. — Thus it appears that the position of the posterior extremities is constant and normal in the air-breathing Vertebrata, whilst it is exceptional in Fishes. On the other hand, the position of the anterior extremities is constant and normal in Fishes alone, being exceptional (that is, departing from the fundamental type) in Reptiles, Birds, and Mammalia. If the former exception be allowed, there need be no diffi- culty about the latter, since it is obviously due to the operation of the very same principle, — that of special adaptation to a particular mode of existence, involving a departure from the ‘archetype’ or primitive design. 320 n. The condition of the vertebral skeleton, and its degree of development, vary greatly in different parts of the series. In the lowest Fishes, as we shall presently see, it retains through life its embryonic condition ; no bone, nor even true cartilage, being formed around the neural axis ; the indications of segmental division being very slight ; and no members or appendages being developed ; the progression of these animals being accomplished, like that of Worms, by the flexion of the body itself. In proportion, however, as distinct members are developed, and the power of locomotion is delegated to them, we find the firmness of the spinal column increasing, and its flexibility diminishing ; of this we have the most remarkable example in the class of Birds, in which (as in Insects) it is requisite that the muscles of the organs of flight should have very firm points of support, and in which the internal skeleton of the trunk (like the tegumentary skeleton of the thorax of Insects) is so closely knit together as to form an almost unyielding framework. As a general rule, then, the firmness and consolidation of the vertebral column, and the locomotive power of the extremities, vary in a like proportion. The number of these extremities never exceeds four ; and two of them are absent in certain members of each Vertebrate class; whilst in the entire group of Serpents, and in some worm-like Fishes, both pairs are wanting. We find them adapted, by variations in their mode of development, to a great variety of purposes ; thus, in Fishes, aquatic Reptiles, and Mammals, they are modified for the mere propulsion of the body through water, and not for giving it support ; in the terrestrial quadrupeds, whether Reptilian or Mammalian, they are adapted to support the body on the solid ground, as well as to give it motion over its siufface ; whilst in Birds, as also in certain members of each of the other classes, the anterior pair of extremities is developed into wings, by whose agency the body is (more or less completely) supported and propelled through the air. Notwithstanding the variety of locomotive power possessed by Verte- brated animals, however, its energy must be regarded as inferior to that which is seen in the Articulata ; the swiftest Bird or Fish not traversing nearly so many times its own length in a given period, as a large propor- tion of the Insect tribes pass through with ease. It is far greater, however, than is enjoyed by the Moilusca, among which only a few of the Cephalopoda can rival Fishes in the rapidity and energy of their move- ments ; and there is no species that is fixed to one spot, without the power of changing its place. In the segmental division of the body, the I articulation of the limbs, and the general arrangement of the locomotive i apparatus of the Vertebrata, we trace a strong resemblance to the plan which we have seen to be pursued in the Articulata ; with the important 442 GENERAL PHYSIOLOGY. and typical difference in the position of the skeleton, on which we have already so fully dwelt. This resemblance is especially apparent in the bilateral symmetry , which, with very few exceptions,* is as constantly exhibited in the entire external configuration of Vertebrated, as in that of Articulated animals ; and also in the repetition of the ganglionic centres of the nerves of the body and limbs, though these unite in Vertebrata into a continuous tract — the spinal cord, — instead of remaining disjoined as they are in the ventral cord of Articulata generally. Consequently, in regard to the nervo-muscular apparatus of locomotion, we may consider the Vertebrata as carrying upwards the type of the Articulata. It is interesting to remark that the muscular fibre of the whole apparatus of locomotion in the Vertebrata is of that higher type, presenting transverse striae (§ 223), which is found throughout nearly the whole of the Arti- culated series ; wdiilst that of the apparatus of organic life resembles that generally found even in the locomotive organs of the Mollusca, in being of the lower or non-striated kind (§ 225). 320 o. It is upon the type presented by the highest Mollusca, however, that we find the development of the organs of special sense, and of the ganglionic centres with which they are connected, proceeding in Verte- brated animals. Thus the eyes of Fishes hear a much closer resemblance to those of the Octopus (Fig. 131), than they do to the compound eyes of Articulata ; and there is never, in any Vertebrated animal, more than a single pair of these organs, — the only vestige of that extraordinary multi- plication of them which is such a characteristic feature of the Articulata (§ 308) being seen in the curious Andbleps , in which the cornea and pupil are double on each side, though the whole of the posterior part of the eye is single. So, again, the rudimentary apparatus of hearing in Gasteropods and Cephalopods bears a very close resemblance to that of even the highest Vertebrata at an early period of its development. All the four special senses — sight, hearing, smell, and taste — appear to be possessed by most Vertebrated animals ; but it is not uncommon for the visual organs to remain undeveloped, when the circumstances in which the animal is adapted to exist are such as would render eyes of no use to it. There are Fishes, Reptiles, and even Mammals, which caunot possess the power of sight, even if they have the perception of light : but there are no known Birds that are destitute of this sense. The organ of hearing is more uni- versally present, not being known to be deficient iu auy vertebrated anim u save the Amphioxus (§ 321). It is difficult to determine in some cases how far the senses of smell and taste are present ; but it is probable that they are never entirely deficient. The sense of touch is more generally * The upper and under surfaces of the Pleuronectidce (Turbot, Sole, &c.), although differently coloured, are in reality the two sides of their bodies ; and the external conformity between what are popularly regarded as their right and left halves, is produced by the similarity in form, and in the osseous framework, of the dorsal and ventral regions. The principal departure from the bilateral symmetry of the skeleton, is seen in the configuration of the head, in which both the eyes are brought to one side. — In the Dolphin and Physeter, the right nasal orifice is much the larger, and the right nasal orifice is pushed to one side, the nasal bones lying rather behind than by the side of each other. — In the Narwhal, only one of the two long tusks is usually developed, and the half of the upper jaw which bears it is longer and larger than that on the other side, the same want of symmetry extending its-lf also to the lower jaw.' — -The position of the anal orifice is unsymmetrical in Amphio.rus (§ 321) and Lepidosiren (§ 325 a). GENERAL VIEW OF THE ANIMAL KINGDOM. VERTEBRATA. 443 diffused over the bodies of most Vertebrated animals, owing to the absence or low development of their dermal skeleton, than it is in the greater number of Invertebrata ; and we find it rendered subservient in the former, by its remarkable perfection in particular organs, to the attain- ment of a much more complete knowledge of the conditions of external objects, than it can afford to the latter. 320 p. The fundamental elements of the Nervous system of the Verte- brata, then, may be looked upon as consisting of a set of locomotive ganglia, arranged in a continuous longitudinal series (as in the Articulata), forming the Spinal Cord ; and of a set of sensory ganglia, more resembling those of the Mollusca, which may be regarded as constituting the essential part of the Encephalic centres, being the parts most universally present, and being those first developed at the commencement of embryonic life. Interposed between these, however, in the portion of the neural axis termed the Medulla Oblongata, we find the ganglionic centres which answer to the respiratory and pharyngeal ganglia of Invertebrated animals, and which in them are usually disunited from the general sensori-motor apparatus, being connected with the cephalic ganglia alone by communicating cords. This incorporation of so many distinct centres into one system, would seem destined in part to afford to them all the protection of the vertebral column, and in part to secure that consentaneousness of action, and to afford the ready means of that mutual influence, which are peculiarly requisite in animals in whom the activity of the nervous system is so predominant. — Although the sensory ganglia may be regarded as funda- mental elements of the brain of Vertebrata, yet they do not, except in the I lowest Fishes, constitute the whole of it ; for it is one of the most constant and remarkable of the general characters of the animals composing this sub-kingdom, that they possess an additional nervous mass, the Cerebrum, j superimposed (as it were) upon the sensoiy ganglia ; not itself receiving the central terminations of the nerves of sense, nor giving off the central origins of those of motion ; but being connected with both, through the medium of their proper ganglionic centres. The development of the Cerebrum in the Vertebrated series bears so close a relation to that of the ' intelligence, which has been already mentioned as their special character- istic (§ 320), that little doubt can exist of its being the instrument of those peculiar psychical endowments, which may be conveniently grouped together under this designation. It is this organ, more than any other, which is especially characteristic of the Vertebrated sub-kingdom • for if any vestige of it exist among the inferior classes (which is by no means certain), it is only found to be in such as present some degree of general approximation to the Vertebrated type f and whilst it is a mere rudiment iu those lowest Fishes which present but little that is distinctive of the Vertebrata in their general conformation, it manifests a regular progressive development in the ascending series, until it reaches its culminating point in Man, in whom (more than in any other animal) we find that highest grade of development of the corporeal organism, which is exhibited in the ^specialisation of each part — even of such as most closely resemble one janother — -for distinct uses. Besides the Cerebrum, we find in Vertebrata another superadded nervous centre, which is equally distinct from the * By tliis is meant not the vertebral conformation itself, so much as the general condition of the group. 444 GENERAL PHYSIOLOGY'. ganglionic centres of the nerve-trunks. Of this organ, the Cerebellum, the function is less certainly known. Its development is less uniformly progressive in the different classes ; and it seems to hear the closest relation to the number and variety of muscular actions, which it is neces- sary to combine in the movements of the animal, as if it were the instru- ment for the automatic selection and combination of the various single operations, of which a considerable number is involved in almost even- movement that is executed by a determination of the will. The Cerebro- spinal axis, formed by the imion of these several centres, is wholly lodged in the neural canal of the vertebral column, and in its cranial expansion : and it lies entirely above the alimentary canal, the cords which connect the cephalic ganglia with those of the trunk (the fibrous strands of the medulla oblongata ) not being separated from each other, as they are both in Articulata and Mollusca, for the passage of the oesophagus between them. — But besides the cerebro-spinal nervous system, which is the instru- ment of the animal life of Vertebrata, we find another set of ganglionic centres and nerve-trunks especially connected with the viscera, and con- stituting what is known as the Sympathetic system, or nervous system of organic life. Very distinct approaches to this system are presented by some of the higher Invertebrata ; but it is not so completely isolated in them, as it is in Vertebrated animals, from the nervous system of animal life. 320 q. The principal modifications presented by the various parts of the apparatus of Organic life in Vertebrated animals, all seem to hear more or less directly upon the exercise of their psychical endowments ; and cannot, therefore, be regarded as so characteristic of the class, as are those pecu- liarities of the apparatus of animal life, to which we have given the first :i nd fullest consideration. — The Digestive cavity invariably possesses two orifices, a buccal and an anal : of these the former is usually situated at the most anterior part of the head, and is a transverse fissure provided with a pair of jaws opening vertically (the mouth of the Amphioxus and of tin Cyclostome Fishes being the only exception to this general statement) : whilst the latter is placed at the posterior extremity of the trunk, on the median line, beneath the origin of the tail where this organ exists. The alimentary canal usually exhibits a distinct division into oesophagus, stomach, and intestinal tube ; and it is for the most part disposed, with its glandular appendages, in the posterior or abdominal portion of the cavity of the trunk. In all Vertebrata, the liver is a very impoitant organ, and presents not only a large size, but a high grade of develop- ment ; it has also a special circulation of its own, a part of the venous him ,1 on its return from the system being transmitted through its capillaries, before it reaches the heart. This organ is usually non-sym metric a 1. from the unecpial development of its two halves, so that it occupies more of tin- right side in Mammals, and of the left side in Fishes ; but in an early stage of its development, its symmetry is exact in all Vertebrata : ami this character is retained in Birds, in which the internal symmetry is as great as that of the Articulata. The pancreas, which made its first appeuranee in the Cephalopods, is obviously an organ of considerable importance in the higher Vertebrata; and its function seems to be specially related, by the peculiar property of the secretion which it forms, to the assimilation of those fatty constituents of the food, which are the chief sources of theii GENERAL VIEW OF THE ANIMAL KINGDOM. VERTEBRATA. 445 treat-producing power. It is in Vertebrata, again, that we first meet with the peculiar organ termed the spleen ; though some approaches to it may be seen in the higher Mollusca. The most important peculiarity, how- ever, in the apparatus of ‘ sanguification,’ exhibited in the Vertebrata, consists in the mode in which the crude alimentary materials are taken into the current of the circulation. We no longer find this effected by the agency of the blood-vessels alone ; for, although these are still con- cerned in the process, a special set of absorbent vessels is found, commenc- ing, like so many rootlets, in the walls of the intestine, and uniting into one or more main trunks, which discharge into the systemic veins the fluid they have taken up, there to mingle with the circulating blood. The fluid of the ‘ lacteals,’ known as ‘ chyle,’ is especially characterized by the presence of fatty matter in a state of very minute subdivision, which seems to perform an important part in the subsequent processes of organisation. The ‘ absorbent system ’ is not confined, however, to the walls of the intestinal tube, but is distributed throughout the entire body ; the ‘ lymphatics,’ as these interstitial absorbent vessels are termed, being especially adapted to carry back into the current of the circulation such nutritive material as has escaped from it without being appropriated, or such as, having once formed a part of the living structure, has been set free by its death, without having been rendered unfit for re-assimilation by a change of composition. The current of blood is always put in motion, save in the anomalous Amphioxus (§ 321), by a single compact muscular organ, the heart ; which is situated in the anterior or thoracic part of the cavity of the trunk. The bloodvessels form a complete dosed system ; the blood being returned by veins, within which it is entirely confined, instead of escaping from the vessels into the great cavities of the body or the interstices of the tissues, as it does in most Invertebrata. The blood (save in the Amphioxus) is reddened by the presence of ‘ red corpuscles’ (§ 173), which, being exclusively present in Vertebrated 1 animals, impart to their blood a distinctive character. The colourless blood in the Invertebrata may, in fact, be regarded as resembling the chyle of Vertebrated animals more than it resembles their blood, which is a fluid of much higher elaboration. 32 Or. The apparatus for the depuration of the blood presents a very high development in the Vertebrated classes. The Respiratory organs, whether formed for aquatic or atmospheric respiration, are always placed in immediate connection with the heart ; and of the venous blood, which has been returned to it by the systemic veins, either the udiole or a part is sent directly to the aerating surface, before being again propelled through the systemic arteries to the body at large. The respiratory cir- culation is therefore carried on at a much greater advantage, than when it is a mere offset from the venous portion of the general or systemic circulation, as it is in most of the Invertebrata. The efficiency of the respiration is further provided for by this, — that the apparatus is every- where connected with a set of muscles, which, by an automatic action of the nervous system, keep up a regular inspiratory and expiratory move- ment ; whereby the aerating medium, whether air or water, in contact with the respiratory surface, is continually renewed. The respiratory , organs are always so connected with the mouth, that the inspiration takes place through it ; the expiration takes place in Fishes, and in the perenni- 44G GENERAL PHYSIOLOGY. branchiate Batrachia (§ 323), throng'll one or more fissures on each side of the neck (which fissures present themselves, up to a certain period of embryonic development, in all the higher classes) ; but in all the air- breathing Yertebrata, the expiratory as well as the inspiratory current passes through the mouth. — In no organs is the principle of concentration more remarkably manifested, than it is in the Lungs ; which possess a much greater functional activity in the Mammalia than they do in Rep- tiles (notwithstanding their much larger relative size in the latter), in virtue of their very minute subdivision, and the energetic working of those subsidiary arrangements, whereby the blood and the air which they contain are continually renewed. — The Kidneys, of which scarcely a vestige can be traced in the greater number of Invertebrata, are here constantly present ; and from their size, as well as their high grade of glandular development, may be considered as organs of essential import- ance in the depuration of the blood, — a conclusion which we shall here- after find to be confirmed by physiological evidence. In the cold-blooded Yertebrata, they partake, with the liver, in the special circulation just described as diverting a portion of the venous current on its return to the heart ; but in Birds and Mammals, they are supplied exclusively with arterial blood from the general circulation. — In the mucous membrane of the alimentary canal, and in the skin, we find an immense number of minute glandulse, apparently destined for the elimination of other effete matters from the blood ; the constant maintenance of whose purity is especially necessary, in animals that are destined to support a life of continual activity. 320 s. It is in the higher Vertebrata alone, that we meet with a pro- vision for the constant maintenance of an elevated temperature. The calorific power is possessed to a certain degree by particular tribes of Insects (chap, xvii., sect. 2) ; but it depends for its exercise upon the state of activity of the individual, the temperature of the animal when in repose being scarcely above that of the surrounding medium. In Birds and Mammals, however, we find a provision for the maintenance of a constantly- elevated temperature, which is scarcely at all dependent upon the general activity of the individual, being sustained up to the regular standard during repose, though somewhat augmented by continued muscular exer- tion. This provision is connected with the energy of the respiration of these animals, and with the large number of red corpuscles in then - bleed : it serves to render them almost entirely independent of moderate changes in external temperature, enabling them to keep up the same amount of activity, both in the organic and nervo-muscidar functions, through all variations of the seasons ; and it answers another important purpose, also, in promoting the development of the embryo. 320 1. In the Vertebrate series, we find the power of reproduction by ‘ gemmation ’ limited to the reparation of lost or injured pails, and not extending to the multiplication of individuals ; and in no known inst nice does this multiplication take place in any other way, than by true sexual generation. This function is always performed by the congress of two individuals ; the sexes being invariably separate. In both sexes the outlet of the generative organs is situated near the posterior extremity of the body, in close proximity with the anal and renal orifices; and where the organs are of moderate size, they are restricted to the posterior part of the abdo- GENERAL VIEW OF THE ANIMAL KINGDOM. VERTEBRATA. 447 urinal cavity, though they may extend far forwards in certain cases. The ova are not fertilized by the contact of the spermatic fluid until after they have escaped from the body of the female, in a considerable proportion of the class of Fishes, and in the lower Reptiles ; but in the higher Fishes and Reptiles, and in all Birds and Mammals, the ova are fertilized whilst yet within the oviducts, and are retained for a shorter or longer period after- wards, so that the embryo undergoes the early processes of development before the deposition of the ovum. In the higher Cartilaginous and in some Osseous Fishes, and in a few Reptiles, the ova are thus retained even imtil they are hatched, and the young are produced alive ; in the greater num- ber of Reptiles, however, as in Birds, they are deposited before their deve- lopment has made any great advance ; but in the latter class the process of evolution is completed under the influence of heat supplied by the body of the parent. In the Mammalia, the embryo is retained within the body of the mother, and continues to receive from her not only warmth but nutriment, its development taking place in a dilated portion of the oviduct, known as the uterus. — In nearly all Vertebrated animals, the young animal, when it first comes forth into the world, has the characters of the class to . which it belongs ; and those of its order, family, genus, and species, if not at once distinguishable, speedily become so ; consequently there is not anything like true ‘ metamorphosis,’ but only a gradual evolu- tion of a special type from one more general. In one remarkable group, however, a true metamorphosis takes place ; the members of the group of Amphibia, or Batrachian Reptiles, coming forth from the egg in the form and condition of Fishes, and gradually assuming that of Reptiles, by a series of changes in which every paid of their organism is concerned (§ 323). Still, as we shall find hereafter, these changes are not more remarkable than those which take place in the embryoes of all Vertebrata during their development within the ovum ; and the chief peculiarity of the Batrachia consists in them emergence from the ovum at a comparatively early period, j and in their adaptation to maintain their own existence at a grade of development, at which other embryoes are dependent upon the supply of nutriment afforded to them, directly or indirectly, by their parents. In this particular, there is an exact correspondence between the metamorphosis of Batrachia and that of Insects (§ 315e). 321. In the class of Fishes, we have, on the one hand, the lowest deve- lopment of the Vertebrated type of structure, not only in the skeleton, but in the entire fabric ; and, on the other hand, a variety of special modifications, whereby a Vertebrated animal is enabled, not merely to inhabit the water and to move through it with rapidity, but also to use that medium for the purpose of respiration, extracting from it the oxygen which it contains. Before proceeding, however, to our general survey of the characteristic structure of this important class, it will be desirable to bestow some attention on the anatomy of an animal, -which serves to connect together the Vertebrated and Invertebrated series in a most remarkable manner ; linking the lower tribes of Fishes, not with the highest, but with some of the lowest forms both of Mollusca and Articulata. This mimal is the Amphioxus* or Lancelet; of which it has been justly * The history of our knowledge of this extraordinary creature is worth a short notice in his place. The first mention of it in any scientific treatise appears to have been that of rallas, the eminent Russian Naturalist; who, in his Spicilegia Zoologica , described a single 448 GENERAL PHYSIOLOGY. remarked, that “it presents a combination of characters which must excite the wonder and interest equally of the Physiologist aud the scientific Naturalist.” Its usual length is about 2 inches ; its greatest breadth from the dorsal to the ventral margin is about 1— 5th of an inch ; and its thickness, from one side to the other, about 3-20ths of an inch. It tapers gradually, however, towards the two extremities, the head being somewhat more rounded than the tail ; and it also thins away towards its dorsal and ventral margins, the former of which is bordered along its whole length by a membranous fin-like expansion, whilst on the under side of the body there are two strong lateral folds, extending from the mouih nearly to the abdominal pore, but giving place behind this to the medim fin which extends to the end of the tail.* On the head, which is scarcely distinguishable from the body, there are no external indications of the existence of organs of sense ; and the mouth, situated on its under surface is a longitudinal slit, fringed on each side by a row of cirrhi or tendril- like filaments (Fig. 172, //), which can close-in towards each other ami clasp together, so as to protect the oral orifice. These cirrhi are attached at their base to a cartilaginous framework (/<), formed of separate pieces, of which each sends a prolongation into one of the filaments, t About half way between the posterior extremity of the mouth and the tail, is the abdominal pore (o d), which opens at once into the abdominal cavity, and serves as an outlet for the water that has passed over the branchial surface, as also for the genital products. At some little distance behind this is the anus {a s), which is not situated precisely on the median Hue. but on the left side of the caudal fin. The body is marked externally ' >y a succession of oblique striae, which are the indications of the lateral mus- cular bundles, seen through the translucent skin. — Such are the external characters of this curious little animal, in which there is an almost entire want of those peculiar to Verteb rated animals, whilst some of them (par- ticularly the unsymmetrical position of the anus) remind us of Molluscn and others (especially the longitudinal direction of the oral fissure, with the specimen, which he had received from Cornwall during the latter part of the last century, under the designation of Limax lanccolaius ; Pallas having believed it to be a naked Mollusk. though he remarked in his description of it, on the resemblance of some of its characters to those of a fish. It seems to have been lost sight of for more than half a century : when, in 1834, it was rediscovered by Costa on the Neapolitan shores, and described by him as a Fish, under the name of Branchiostoma. Although this name has the priority to that of Amphioms, conferred by Mr. Yarrell in 1836, yet as the former designation is founded on an erroneous conception of the office of the filamentous appendages with which the mouth is guarded, the author thinks with M. De Quatrefages (from whose admirable Memoir on the anatomy of this animal, in the Ann. des Sci. Nat. for 1845, his account of it is chiefly condensed), that it would be better to abandon it for Amp/iiorus. In 1836, Mr. Yarrell, without being acquainted with Costa’s account of the animal, described it in his ‘ British Fishes." from a specimen obtained by Mr. Couch on the coast of Cornwall ; and about the same time it was taken by several naturalists on the coast of Sweden. It has been subsequently found in considerable numbers on the west coast of Scotland, as well as in several other localities : and has been the subject of many admirable Anatomical Memoirs. The Author is informed by Prof. Owen that it is regularly brought to market by certain fishermen on the Neapolitan coast, to whom it has been known from time immemorial, being used, as we employ shrimps, in sauce to larger fish ; and it is not a little remarkable that an article in such common use should have so long escaped the attention of the Italian Naturalists. * These lateral folds, being mistaken by Pallas for the margins of a ventral disk, were among the characters which led him to rank it as a Gasteropodous Mollusk. + These filaments, which are obviously analogous to the cirrhi of Cyclostome fishes, were erroneously supposed by Costa to be branchiae. GENERAL VIEW OF THE ANIMAL KINGDOM. VERTEBRATA. 449 lateral movement of the cartilaginous framework that bounds it) indicate a relation to the Articulata. We shall see that, in the internal structure, the Vertebrated type is most distinctly manifested, although in a most .rudimentary condition ; whilst approximations to the Mollusca on the one hand, and to the Articulata on the other, are also very clearly exhibited. Fig. 172. Diagram of the Anatomy of Amphioxus, or Branchiostoma (Lancelet) ; — ch, ch , chorda dorsalis; n , n, fibro-membranous wall of neural canal ; ol, olfactory capsule ; op, optic nerve ; ob, trigeminal nerves ; md, md, neural axis ; bv, branchial vein ; ba, branchial artery ; /, /, hepa- tic Cceeum, opening at lib ; py , cardiac orifice ; h, renal organ ; y, caudal extremity of neural axis; as, anus; i, i, intestine ; od, abdominal pore ; ou, cardiac sinus ; r, r, r, dilated oesophagus, with branchial fissures ; ph , pharyngeal orifice ; g, g, vascular intra-buccal processes ; h, carti- laginous arch, supporting/,/, the ciliated labial tentacula. 321a. Almost the only trace of the vertebral column in the Amphioxus, is to be found in the fibrous and cellular structures surrounding and sup- porting the neural axis. Passing along almost the entire length of the animal, with but little change in its dimensions from one extremity to the other, is a nearly cylindrical tube (n, n), formed of a delicate fibrous membrane ; this encloses the neural axis with the ‘ chorda dorsalis’, and sends off lateral fibrous prolongations, which pass between the muscular bundles, and constitute the only representatives of ribs. The neural axis is nearly uniform in size from one extremity to the other ; consisting of a series of ganglia, obviously distinct from each other, although in close approximation ; and not presenting any such enlargement at its cephalic termination, as could be denominated a brain. The only vestiges of organs of special sense consist of rudimentary eyes, which, though possessed of a distinct crystalline lens with pigmentary matter, are lodged in the substance of the fibrous investment of the neural axis ; and of another peculiar body resembling the rudimentary organ of smell in the embryoes of other fishes : no trace has yet been detected of an organ of hearing. The neural axis rests upon the chorda dorsalis ( ch , ch), a cylinder composed of a membranous envelope, filled with a soft cellular substance i which resembles cartilage in its earliest grade of development. In this ■cylinder, which represents the column formed by the ‘bodies’ of the vertebrae if higher animals, there is but the most indistinct indication of subdivision mto segments. The arrangement of the lateral muscles, which intervene yetween the costal prolongations of the fibrous axis, is precisely that of he intercostal muscles of fishes ; besides these, there is a set of longi- udinal muscles running along the under side of the abdominal cavity which, unlike the intercostals, are composed of non-striated fibre ) ; and here are also special groups of muscles connected with the oral ring and dth the branchial apparatus. The movements of the animal are some- G G 450 GENERAL PHYSIOLOGY. what snake-like, and are rapid and powerful ; but it seems to pass a large part of its time either buried in sand, or lying flat upon its surface. — In all the foregoing particulars, the structure is obviously that of a Yerte- brated animal ; resembling that of higher Yertebrata at a very early period of development, and being (so to speak) a further degradation of the already low type which is presented in the Myxinoid fishes. The Avant of an osseous or even a cartilaginous skeleton is a simple result of ‘arrest of development and the same may be said of the rudimentary condition of the brain and organs of sense. In the distinctness of the ganglionic segments of the neural axis, we are reminded of the Articulate ; but this, also, may be regarded as a mere result of the want of that coa- lescence which ordinarily takes place in higher Yertebrata ; and it d< >es not indicate a real approximation to the Articulated sub-kingdom, from which the neural axis differs in its dorsal position (lying above the alimen- tary canal along its entire course) and in its relations to the chorda dorsalis. The arrangement of the lateral muscles more resembles that of the Fish than that of the Worm ; and their attachment is to the rudi- ment of an internal, not to any trace of an external skeleton. 321 b. It is in the arrangement of the organs of nutrition, however, that the greatest approximation to Invertebrate types is displayed. The oral cavity is lined with cilia ; and these are especially disposed upon a set of digitate prolongations of the mucous membrane (g, g), which have been denominated £ rotatory organs,’ from their resemblance to the ciliated lobes of the Rotifera (§ 310). It opens at the back, by the orifice ph, into a Avide pharyngeal cavity (r, r), the walls of which are strengthened by a sort of cage, formed of delicate, transparent, hair-like cartilaginous arches, seventy or eighty in number ; whilst between these are a series of clefts, the edges of which are fringed with cilia. This appeal's to serve as the principal respiratory apparatus ; a considerable part of the water which is taken in by the action of the oral ciliated lobes, and which enters the pharyngeal orifice (ph), being driven through these slits into the abdominal cavity, from which it makes its exit by the abdominal pore (od). The remainder of the water passes backwards through the cardiac orifice ( p>/) into the digesth'e cavity, which consists of a short and nearly straight intestine (i), having no distinct dilatation into a gastric cavity at its anterior part, but being there connected with a peculiar caecum (/. I) which extends forwards, and which is probably to be regarded as a rudi- mentary liver, being lined by a layer of greenish cells, which extends, however, along the intestine also. The intestinal tube, which terminates in the anal orifice (as), is lined throughout by vibratile cilia ; and these appear, as in the lower Invertebrata, to be the sole instruments by which the alimentary as well as the respiratory current is sustained. The blood, wdiich contains no red corpuscles, and only a few colourless corpuscles of irregular size and form, moves for the most part through a regular system of closed vessels, though these appeal’ to give place in some parts to lacunae ; its movement, however, is not due to the action of any single propelling organ, or heart, but is accomplished by the agency of several pulsating dilatations, which are developed on different parts of the vascular system. As in Articulata, we find two principal trunks, a dorsal and a ventral ; these are connected together by arches at the two ex- tremities of the body, and also by transverse vessels xvliich pass along the GENERAL VIEW OF THE ANIMAL KINGDOM. VERTEBRATA. 451 walls of the branchial sac ( r , r, r), between its fissures. In these vessels, however, the blood moves in a direction contrary to that in which it flow's in the Articulata ; for it passes backwards along the dorsal trunk ( ba ), receiving in its course the blood which has been aerated on the branchial surface ; it is then distributed to different parts of the body, whence it is collected by veins wdiich carry it forwards towards one of the principal contractile dilatations (or) situated close to the under surface of the branchial sac ; and by tins it is propelled, partly through the branchial vessels which deliver it into the dorsal trunk, and partly through the anterior continuation of the ventral trunk. The latter portion finds its way through the aortic arches at the anterior extremity of the body, which are themselves pidsatile ; and by their agency it is distributed to the head, and especially through the vessel bv to the ciliated appendages g, g (which are considered by Prof. Owen as the chief respiratory organs), whence it is collected by the dorsal vessel (ba), and transmitted backwards, mingling with that wdiich has crossed the pharyngeal sac. At the base of each of the branchial arteries given off from the principal heart (or), is a small pulsating dilatation ; there is another on the large vein (the vena portse) w'hich collects the blood from the intestinal tube and conveys it to the hepatic caecum ; and one more upon the principal venous trunk, the vena cava. These several pulsating organs do not contract together, but alternate with each other, one filling as another empties itself; and in this manner a constant flow is maintained, which is, however, far from rapid, the interval between the successive pulsations of the principal branchial heart (ov) being as much as a minute. — It only remains, for completing the description of this curious animal, to mention that the upper part of the abdominal cavity contains a peculiar organ ( h ) apparently glandular in its character, and probably performing the office of a kidney ; whilst the ovaries or testes lie along its floor, and discharge their products into its cavity, ■whence they make their exit by the abdominal pore. 321 c. In looking at the structure and relations of the digestive and respiratory organs of the Amphioxus, we are at once struck with their resemblance to those of the Ascidian Mollusks (§ 299). In these, as in the Amphioxus, the respiration is carried on through a pharyngeal sac whose conformation is almost identically the same in both cases, that of the Amphioxus being strengthened by a splanchno-skeleton of delicate cartilaginous arches ; in both alike is the water drawn into the oral orifice by ciliary action, and thus propelled partly along the alimentary canal, and partly through the pharyngeal slits into the visceral cavity ; in both alike does the expiratoi’y current pass out through the same orifice with the genital products ; and the only important difference lies in the cir- cumstance, that the intestinal tube terminates in a separate external orifice, instead of opening into a common cloaca with the preceding. On the other hand, the plan of the circulating apparatus is distinctly Anne- lidan (§ 311); and in the peculiar disposition of the separate pulsating cavities, it bears a very close resemblance to that which will be hereafter described in the Eunice (chap. xii.). — When we examine, on the other hand, into the relations of these peculiarities to the structure chai’acteristic of the respiratory and circulating organs in ordinary Fishes, we shall see that the departure is not so great as might be at first supposed. For if we suppose the pharynx of Amphioxus to be considerably shortened, the 452 GENERAL PHYSIOLOGY. number of its arches to be greatly reduced, the slits to be fringed with fila- ments suspended to the cartilaginous arches, and the surrounding space to be shut off from the abdominal cavity, and to communicate externally by an orifice or set of orifices of its own, we have the ordinary branchial apparatus of Fishes ; which further differs, however, in this — that the passage of the water over the branchial filaments is accomplished in them by muscular contraction, instead of by the vibration of cilia. So, again, we may regard the dorsal trunk as the representative of the aorta of Fishes, which receives its blood from the branchial arteries and transmits it backwards along the body ; the posterior part of the ventral trunk in like manner corresponds with the vena cava ; the principal pulsating cavity, situated at the origin of the branchial arteries, with their proper heart ; whilst even the anterior arches, which establish a communication between this heart and the aorta, independently of that which is formed by the branchial circulation, find their representatives in certain Fishes whose respiratory organs depart in some degree from the regular type. — Thus we have, in this most interesting little creature, the following ano- malous combination an internal skeleton, formed on the Yertebrated type, but in so rudimentary a condition as not even to show a trace of separation into vertebrm, although its segmental division is marked in the rib-like intermuscular fibrous bands : a nervous system foimed on the Spini-Cerebrated type, but destitute of any distinguishable brain, and having the spinal cord composed of a succession of ganglia ; an oral orifice -which is rather that of an Articulated than of a Yertebrated animal ; a ciliated alimentary canal, such as most closely resembles that of the lower Mollusca ; an anal orifice on one side of the median plane, as in the higher Mollusca ; the circulating system of an Annelide ; the colourless blood of Invertebrata generally ; and the respiratory apparatus of an Ascidian. There can be no question that it is to be regarded as essential!;/ a Fish ; and whilst some of its peculiarities are those by which embryonic Fishes are distinguished from those whose development is complete, others obviously link this class to the lower members of the two great parallel series of Mollusca and Articulata. It can scarcely be thought improbable by the Philosophic Naturalist, that this curious being is “the relic of some great order of Fishes, which in their organisation brought down the verte- brated series to a parallel with the lower forms of Mollusca, and which became extinct in the earlier epoch of the world’s geological history, and from the unpreservable nature of their bodies, and the absence of hard parts, left ‘ not a wreck behind. ’ ” 322. The class of Fishes has been usually divided, according to tne material of the skeleton, into two principal groups, the Osseous and the Cartilaginous. Of the latter division, however, some are inferior to the former in their general organisation, whilst others are much superior : and it cannot, therefore, be regarded as a natural assemblage. In our general account of the skeleton of Fishes, however, it will be convenient to adopt it. — Among the inferior members of the Cartilaginous group, con- stituting the order Cyclostomi (or circular-mouthed), the characteristic structure of Vertebrata is but little more developed than in the Amphi- oxus. Thus in the Myxine, or Hag, the neural axis still rests upon a ‘chorda dorsalis’, which is a soft and flexible cylinder, semifluid towards its centre, and not presenting the least indication of segmental division. GENERAL VIEW OF THE ANIMAL KINGDOM.- FISHES. 453 In the Lamprey, a series of proper cartilaginous rings is developed upon the chorda dorsalis at intervals ; these are the rudiments of the bodies of the vertebrae, the neural arches of which begin to make their appearance in the form of cartilaginous laminae, in the substance of the fibrous envelope of the neural axis. There are no corresponding rudiments, however, of haemal arches ; the cartilaginous framework, which supports the seven rows of gills, not being composed of true ribs, but being a part of the splanchno-skeleton, like that of the branchial arches both in the Osseous Fishes and in the larvae of Batrachia; and no members or appendages of any kind being developed. In the head of the Myxine are a set of cartilages, protecting the cephalic ganglia, and closely resembling, in their degree of development, the first rudiments of the cranium in the embryo of higher fishes ; whilst with these are connected a set of rudimentary haemal arches, specially modified to give support to the suctorial lips and to the rasp-like tongue. This animal presents several interesting approxi- mations to the Amphioxus ; being, like the latter, destitute of eyes ; its mouth being guarded by cirrhi, which (being eight in number, and arranged in a circle) strongly remind us of the oral tentacula of the Cephalopods ; and the branchial orifices, through which the respiratory current passes out, being situated far back, nearly in the situation of the abdominal pore, though double (one on each side) instead of single.* In the head of the Lamprey, the cranial cartilages are more developed ; as are also those supporting the buccal apparatus, which consists of a flat circular disk formed by the expanded lips, and covered with horny tooth- like processes that gradually increase in size towards the centre of the mouth. 322 a. In the higher Cartilaginous Fishes, w r e might trace (did space permit) the progressive steps of the development of the vertebral column, from the first indication of segmentation in the chorda dorsalis, to its complete replacement by a pile of vertebral ‘ centra ; ’ and from the first appearance of neurapophyses, to the complete development of the neural and haemal arches. A very general account, however, must suffice. In the Sturgeon, we meet with firm cartilaginous rings, formed in the outer part of the ‘ chorda dorsalis,’ increased in thickness, and evidently forming the bodies of a series of vertebrae, with which the peripheral elements are articulated ; these elements, though themselves also cartilaginous and disconnected from each other, form complete neural arches, with the addition of haemal arches in some parts. Amongst the Sharks, the pro- gression may be traced from this point, up to the complete development of the bodies of the vertebrae, as a succession of cylindrical pieces, partly osseous and partly cartilaginous, having deep conical excavations at each end, so that, when fitted together, bi-conical spaces are left between them. These spaces, occupied by the unconsolidated portions of the ‘chorda dorsalis,’ communicate with each other, in the inferior Sharks, by a portion of the unconsolidated ‘ chorda’ which still remains in the middle of each vertebral ‘centrum ;’ but as we pass through the ascending series, we find this canal gradually diminished in size by the development of the vertebral body towards its centre ; until, in great Selache (basking-shark), nothing- remains in its place but a small impervious cord, the intervertebral spaces * This current, however, does not pass into the visceral cavity, but makes its exit by a special canal leading from the branchial chambers. 454 GENERAL PHYSIOLOGY. being now occupied by a series of disconnected elastic capsules, filled with gelatinous fluid.* The neural arches are here rendered firmer by the partial ossification of their elements 3 the pleurapophyses of the dorsal vertebrae, or ribs, are also partly ossified ; whilst in the posterior part of the vertebral column, the haemal arches are nearly as complete as the neural. In these Fishes we find several fins, or locomotive appendages, some of them upon the median line, others in pairs on the two sides. The former, which are known (according to them position) as the dorsal , anal, and caudal (Fig. 173, d, a, c), are supported upon a set of cartilaginous or osseous rays, belonging to the dermo-skeleton, but intercalated amongst the neural or haemal spines, in the manner shown. Of the latter, the pectoral fins, or anterior members (Fig. 173 , p), are not, as in Osseous Fishes, connected with the occipital vertebra, but are suspended to a firm scapular arch, which encircles the body behind the branchial apparatus, and is connected with the vertebral column. In the Rays, these pectoral fins are immensely expanded, and are supported upon a remarkable number of digits 3 these sometimes amounting to a hundred, every one of which bifurcates towards its extremity. It is interesting to remark, that, in this group, the vertebrae of the anterior part of the spinal column are immovably fixed together by an incrustation of earthy matter, that forms a kind of tube or sheath! in which they are encased 3 thus giving a much firmer support to the muscles of the great wing-like fins, in accordance with the general principle already laid down (§320 n), that the consolida- tion of the spinal column takes place pari passu with the delegation of the locomotive power to the extremities. The posterior or ventral fins are seldom highly developed as swimming organs 3 but portions of them in the male are converted into £ claspers’, to be used in the sexual congre^ of these Fishes 3 the pelvic arch is less perfect than the scapular, not being connected with the vertebral column. The tail, in all these Carti- laginous fishes, is formed upon a different plan from that of most Osseous ; for whilst, in the latter, the framework of the caudal fin consists of dermal rays, intercalated amongst the neural and haemal spines of a number of vertebrae whose bodies have coalesced together (Fig. 173), the caudal tin of the former is supported upon a prolongation of the vertebral column itself, which passes on to the termination of the tail, as in all other Verte- brata. The first of these methods of conformation is termed hoinocemd, from the equality between the upper and lower lobes of the caudal fin ; whilst the latter is termed heterocercal, the vertebral column being con- tinued into the upper lobe of the tail, which is consequently the largest. + * “ There are few examples in the animal economy,” says Prof. Owen (Lectures on Comparative Anatomy, Fishes, p. 55), “ in which the smallest possible quantity of earthy matter is arranged according to such beautiful and clearly-manifested mechanical principles, for affording the greatest amount of strength, and that degree of resistance which the necessarily light semi-ossified vertebra of a gigantic Shark, maintaining itself near the surface by muscular exertion, without help from a swim-bladdei;, must have to sustain during the vigorous inflexions of the vertebral column, producing the violent compressions >4 their interposed elastic balls.” + It is very interesting to remark that the Heterocercal conformation is found in a!I Fishes at an early period of their development ; giving place to the Homocercal, in the Osseous Fishes, in proportion as their other peculiarities manifest themselves. AVe may consequently regard the heterocercal tail as the most general type, whilst the homocercal is a modification of this, peculiar to those of the class of Fishes which may be regarded as its more typical members. It is further to be mentioned, that all the earlier Fishes whose remains are GENERAL VIEW OF THE ANIMAL KINGDOM. FISHES. 4 55 In their retention of the heterocercal tail, as well as in many other parts of their conformation, the higher Cartilaginous Fishes show a decided approximation to the class of Reptiles ; the transition to which is also estab- lished by the Lepidosteus (or gar-pike) an Osseous Fish having many Rep- tilian characters ; by certain extinct Saurian Reptiles, which had many important Ichthyic characters; as well as by the Lepidosiren* (Fig. 175), a Batrachian Reptile, which retains in very many points of its organisation the low grade of development that is generally characteristic of Fishes.— Of the skull of the higher Cartilaginous Fishes it must here suffice to remark, that it is formed on the same general plan with that of the Osseous ; but as the cartilage of which it is composed is consolidated rather by a general granular deposition of osseous matter, than by regular ossification from distinct centres, there is no well-marked division into separate pieces ; so that, although the situation of the different bones may be pointed out under the guidance of the skull of an Osseous Fish, to define their limits is impossible. Not only are the component parts of the brain-case thus consolidated into one mass, but the bones of the face are united with this, — the great maxillary ring, however, composed of the upper and lower jaw-bones articulated together, being but loosely connected with the rest. 322 b. In describing the skeleton of the Osseous Fishes, we shall for the most part confine ourselves to a notice of its chief departures from the ‘ Archetype’ already surveyed; as in this mode will the special modifi- cations of the common vertebrate type to the requirements of the life of the Fish, be best brought into view. — The bodies of the vertebrae, with few exceptions, retain their cupped extremities ; and these work as sockets over a set of doubly-convex intervertebral capsules (the remnants of the ‘ chorda dorsalis’, and the representatives of the ‘intervertebral cartilages’ of higher animals), in such a manner as to give great freedom of motion i to the entire column. The number of these vertebrae varies greatly in the different families of the class, being greatest in the long serpent-like Fishes, and least in those whose spinal column possesses comparatively little extension longitudinally. Thus in the Conger-Eel the number of vertebrae is 162, and in the Gymnotus 236 ; whilst in the Mackerel there are but 31, and in the Diodon no more than 17. The distinction into regions, which is so well marked in most of the higher Vertebrata, cannot be made in Fishes ; for as the pectoral fins are attached to the occipital segment of the skull, and as the formation of the expanded haemal arches for the protection of the visceral cavity commences imme- diately behind it, there is no neck or cervical region, but the dorsal portion of the vertebral column must be considered as commencing with the very first vertebra ; again, as the pelvic arch has no bony attachment to the spinal column, so that there is no sacrum,- and as its position is inconstant, it does not mark out the separation between the dorsal and preserved to us, possessed the heterocercal tail ; these Fishes having either belonged to the Placoid order (Agassiz) which comprehends all the Sharks and Rays, or to the Ganoid, a large series, now nearly extinct, some members of which carried onwards the same general type of conformation into closer connection with the Reptilian class. . * In ranking the Lepidosiren amongst Batrachia, rather than in the class of Fishes, the Author ventures to dissent from his friend Prof. Owen, on whose authority he generally places the firmest reliance ; and to express his accordance with the views of several other eminent Anatomists, who regard its general characters as indicating an affinity to the former group, notwithstanding the ichthyic (or fish-like) condition of its skeleton. 456 GENERAL PHYSIOLOGY. caudal portions of the spine, which is usually determined merely by the extent of the visceral cavity, those vertebrae whose hsemal arches are expanded for its protection being regarded as dorsal, and those in which they are again narrowed, so as merely to enclose the great vascular trunk, being reckoned as caudal ; there is, therefore, no proper lumbar region in Fishes. In most Fishes, but especially in those of the Eel kind, the caudal portion forms a considerable part of the whole length of the body ; and it is in this, that the principal multiplication in the num- ber of vertebral presents itself. A mere glance at the vertebral column suffices to show that the successive segments resemble one another far more closely than they usually do in the higher animals ; or, to use Prof. Owen’s phrase, that the principle of ‘ vegetative repetition ’ is more obviously manifested in Fishes than in higher Vertebrata. In nearly all cases, the neural spines are strongly developed, from the head to the end of the tail ; and in the caudal region, the haemal arches are similarly prolonged in a vertical direction. In this manner, the movement of the spinal column is limited in a vertical direction, whilst its lateral flexion is not interfered with ; and the lateral surface of the body is so augmented, that its stroke against the water (which is the chief means whereby the fish is propelled through the liquid) is rendered far more effectual. We generally meet with a further augmentation of the surface with which the water is struck, in the various median fins which are implanted upon the summits of the neural and hsemal arches. These are named the dorsal (Fig. 173, d 1 , d-) when they are erected upon the back ; the Fig. 173 . d\ Skeleton of Perch ; — d 1 , d 2, dorsal fins ; c, caudal fin ; a, anal fin ; c, ventral fin ; p, pectoral fin. caudal (c) when they are supported upon the caudal vertebrae ; and the anal (a) when they project from the under side of the body in the imme- diate neighbourhood of the anus. As already remarked, the rays (some- times osseous, sometimes cartilaginous) upon which these median fins are supported, belong to the dermo-skeleton ] and the intercalary bones upon which they rest are probably to be regarded in the same light. In the dorsal region, we usually find each vertebra made up of its centrum or body, with a complete neural arch : whilst the haemal arch is only partially formed by the expansion of the pleurapophyses (or ribs), which are articulated to the parapophyses, over the upper part of the visceral GENERAL VIEW OF THE ANIMAL KINGDOM. FISHES. 457 cavity ; its lower part being unprotected by those haemapophyses and haemal spines, which we find constituting the sternal ribs and sternum of higher Vertebrata. The pleurapophyses often bear ‘ diverging appendages,’ in the form of simple undivided bony rays, sometimes of considerable length, analogous to those of the ribs of Birds. The haemal arches of the caudal region are completed in a manner that constitutes a very marked de- parture from the archetype, and distinguishes Fishes from higher Ver- tebrata ; for they are formed by the bending-down of the ‘ parapophyses ’ until they meet on the median line, the proper hsemapophyses and haemal spines being here also deficient, and the pleurapophyses being either absent altogether, or so articulated to the parapophyses as to take no part in the formation of the arch. At the extremity of the caudal region, the bodies of a number of the vertebra are usually found to have coalesced ; but their neural and haemal arches and spines remain distinct, and give support to the caudal fin, which is equally developed above and below, or ‘ homocercal,’ in nearly all the Osseous Fishes of the present epoch. 322 c. The looseness of the framing of the vertebral column of Fishes, which so greatly favours their free movement through the water, obvi- ously incapacitates the body from deriving support from its lateral appen- dages ; but no such support is required, since, when immersed in a fluid of equal specific gravity with itself, every part of the body is borne up by it. The functions of these appendages, therefore, in regard to locomotion, are limited to the balancing of the body, and to the change in the direction of its movement, especially from the horizontal to the vertical. In the pectoral fins (p), which are always situated immediately behind the head, and connected with the occipital segment, where their power can be most advantageously exerted, we notice that the arm and fore-arm are scarcely developed, and are hidden within the trunk, from which the hand alone projects, its movement being confined to the wrist-joint ; and the ex- tent of surface required to strike the water is given by the elongation and multiplication of the fin-rays, which represent the carpus and digits. The same is observed in the ventral fins (r), which are generally of less size and importance, are not connected with the vertebral axis, and are very 1 variable in situation ; being usually placed near the anus in the long- bodied and small-headed fishes, where their chief use is to serve as accessory balancers ; whilst in most of the shorter and larger-headed fishes which enjoy a considerable range of depth, they are brought forwards (as in the Perch) to assist in raising the head, being sometimes placed beneath the throat, even anteriorly to the pectorals. In the ‘ abdominal’ fishes, in which the pelvic arch may be considered as occupying its normal position, this is connected with the pleurapophysis of a vertdbra at the extremity of ' the dorsal region, by a fibrous band on either side ; whilst in the ‘jugu- lar’ fishes, hi which it is moved forwards to the neighbourhood of the scapular arch, it is sometimes connected in like manner with this. Both pectoral and ventral fins are frequently modified for special purposes ; thus j the former are occasionally furnished with tactile appendages, are some- times developed into wing-like organs, capable of supporting the fish in the air, and may even be adapted for an imperfect progression over the I sm-face of the earth ; whilst the latter are often reduced to the condition ■ of mere filamentary ‘ feelers,’ and may even be modified in such a manner • as to form a suctorial disk or organ of adhesion. 458 GENERAL PHYSIOLOGY. 322 d. In the conformation of the skull, we find several remarkable departures from its ordinary structure in higher Vertebrata, which adapt it to the particular conditions of ichthyic existence, independently of those which mark its low grade of development. The latter is especially shown in the want of consolidation of the vertebral elements, which remain for the most part as distinct bones, whose edges overlap each other, without being united by ‘ sutures but the former are manifested in the extraordinary size and figure which some of these elements exhibit. That which first strikes the observer, on looking at the head of most Osseous Fishes, is its large size, and the absence of neck ; the head being, as it were, the anterior rounded termination of the large extremity of tee body, which gradually tapers off behind towards the tail. These pecu- liarities are favourable to free and rapid movement through the water ; for where the propulsive power comes from the opposite extremity, it F mechanically advantageous that the parts in advance should be the 1 largest, and able to bear the most pressure. The large external size of the skull does not involve a large cranial cavity ; it being the face and nut the brain-case which is thus expanded, and the greater part of this ex- pansion being due to the extraordinary development of the whole buccal apparatus. The limbs of Fishes have no prehensile power ; the mouth may be propelled and guided by them to the food, but the act of prehen- sion must be performed entirely by the jaws ; and these are frequently endowed with the power of protrusion and retraction, as well as of opening and shutting to a very wide extent. This peculiar adaptation for the prehension of food, is further displayed in the disposition of the Teeth, in the interior of the mouth. These appendages are usually extremely numerous, and are seldom confined (except in the Sharks) to the upper and lower jaws. There is, in fact, scarcely one of the bones which forms part of the walls of the cavity, that is not, in some tribe or other, beset with teeth ; and in some instances, we find teeth disposed on nearly all these at once, so as to hue the mouth as completely as possible. The intermaxillary and premandibular bones, the palatines, the vomer, the lingual bones, the branchial arches, and sometimes also (though more rarely) the pteregoid and the sphenoid bones, are found carrying teeth, very commonly of different shapes, and adapted for different purposes (§ 322,/, g ). — But further, the peculiar mode in which the act of Respira- tion is performed in Fishes, necessitates an extraordinary adaptation of the osseous framework for this purpose ; and parts which are elsewhere rudimentary, are here developed in their most complicated form. This is the case with the diverging appendages of the tympano-mandibu 1 ar arch, here developed into the broad flat ‘ opercular bones ’ which support the valvular flaps that cover-in the external orifices of the gill-chamber : and with the whole of the hyoidean arch, which is the chief point of suspen- sion of the branchial arches (a part of the splanchno-skeletou) that support the gills ; whilst its diverging appendages, the ‘ branchiostegni rays,’ give support to the brancliiostegal membranes that regulate the course and exit of the respiratory currents. The last pair of branchial arches is reduced to the capacity of the pharynx which it surrounds ami is converted into an accessory pair of jaws, which, by means of the teeth implanted in them, aid in reducing the food in its passage from the mouth towards the stomach. In no other Vertebrate animals is the moutli thus GENERAL VIEW OF THE ANIMAL KINGDOM. FISHES. 459 provided with maxillary instruments at its posterior as well as at its anterior aperture ; and it is very interesting to see lrow essentially this peculiarity is connected with the special modification of its bony skeleton for aquatic respiration. 322 e. The development of the dermo-skeleton varies greatly in this class, and bears no constant relation, either direct or inverse, to that of the endo-skeleton. Thus in the Amphioxus and the Cyclostome fishes, in which the vertebral skeleton is rudimentary, the dermo-skeleton is rudimentary also ; the surface being entirely unprotected by scales, and the median fins not being supported by dermal bones. In the Lepi- dosteus, again, the body is completely enclosed in a casing of bony scales, which are developed into 'large plates on the head; whilst the internal skeleton is highly developed and completely ossified. In the Sturgeon, on the other hand, the dermo-skeleton is developed in the form of large osseous plates, completely enclosing the body and head, whilst the internal skeleton is for the most part cartilaginous ; and such appears to have been the condition of a large proportion of the fishes which lived in the earlier epochs of the Earth’s history, — their dermo-skeletons, consist- ing of bony scales or plates, having been perfectly preserved to us, whilst scarcely any trace exists of their vertebral column. In the Sharks and Rays, again, whose neuro-skeleton is but slightly ossified, the dermo- skeleton is also very imperfect ; and of the vast numbers of this tribe which must have existed in the ancient seas, the greater part are known to us only by their teeth and by osseous fin-rays belonging to the dermal skeleton, every portion of the neural skeleton, and every other part of the dermal and visceral, having disappeared by decay. In most Fishes, the dermal skeleton affords (as we have seen) the chief support to the median fins ; in the Herring and its allies, it supplies a sort of sternum with sternal ribs, which have been supposed (but erroneously) to be parts of the neuro-skeleton ; and in many other cases it undergoes an unusual development for some special purpose. — It is, however, in its protective relation, as a part of the general integument, that we have now more especially to view it. With the exception of the Amphioxus and the Cyclostomi, all the existing Fishes have the skin strengthened by the development of cartilaginous or osseous ‘ scales ’ in its substance ;* these scales being of various shapes and sizes, in some instances merely studding the skin at intervals, sometimes coming into close contact wfith each other at their edges, sometimes partially covering each other in an imbricated manner, and sometimes being developed into large plates firmly united into a sort of cuirass. Each of these scales is developed within a distinct capsule, and grows by its own inherent powers of nutrition ; and where, as in the ctenoid or comb-like scales (that of the Sole for example), it has a set of spiny projections from its edge or surface, each of these has a separate capsule, and attaches itself to the scale in the progress of its deve- lopment, like a tooth to the jaw which supports it. The structure of the cartilaginous scales is best seen in those cases in which (as in the Eel) they are very thin, being composed of only a single layer of cells held * The scales of Fishes are not, like those of Reptiles, epidermic formations of horny composition; hut are always inclosed within the substance of the true skin, and are covered by its outer layer, as well as by the epidermis ; and though sometimes containing horny matter, are essentially constituted of cartilage. 460 GENERAL PHYSIOLOGY. together by intercellular substance ; when the scales are thicker, the cells are arranged in several layers, and their under part has more of a fibrous than of a cellular arrangement. They are usually marked by a set of radiating and of concentric lines ; the former are the indications of furrows or canals, which probably lodge nutrient vessels ; the latter are due to the peculiar transformation undergone by some of the concentric rows of cells on the outer surface of the scale, these assuming an elongated form so as to touch each other at them ends, and becoming filled with homy matter.* Nearly all the existing Osseous Fishes have cartilaginous scales ; and according to the form of these scales, they are grouped by I J r< >f. Agassiz under the two heads of Cycloid and Ctenoid, — the former com- prehending those in which the scales have a more or less regular circular form with smooth edges, as in the Carp ; whilst the latter includes those, such as the Perch, whose scales have the posterior margin serrated, or furnished with comb-like teeth. t The simplest condition of the bony scales is seen in the Shark tribe, in which the skin is beset with a number of little hard points or tubercles, giving to it the roughness which is well known in ‘ shagreen ; ’ these, when examined microscopically, are found to consist of miniature teeth, developed in distinct capsules near the surface of the skin, and differing in no essential particular, save in size, from the teeth which are developed in the mucous membrane cover- ing the jaws (§ 212). In some cases, these small tooth-like scales are replaced by much larger bony plates, which are not, however, covered by enamel, and do not come into close contact with each other. The Fishes whose scales present either of these conditions, agree amongst, themselves in their general structure (constituting the Plagiostome tribe of Cartilaginous Fishes in Cuvier’s arrangement), and are designated by Prof. Agassiz as Placoid. — The most perfect condition of the demo- skeleton of Fishes, is that in wdiich the bony scales or plates come into close contact at their edges, and are covered externally with a layer of a peculiar lustrous substance which has received (though improperly i the designation of ‘ enamel.’ The Fishes thus protected were ranked by Prof. Agassiz under the single order Ganoid ; but the few existing species which have an envelope of ganoid scales or plates, are too diverse in their internal structure to be thus associated, and require to be grouped in at least three orders. To that of True Ganoids, however, of which the Sturgeon and the Lepidosteus are the principal existing representatives, the fossil remains of all those Fishes of the Palaeozoic period and of nearly the whole of the Secondary, which were not Placoid, appeal - to be re- ferable, — the Cycloid and Ctenoid orders making then first appeal anee at the Cretaceous epoch. The structure of the Ganoid scales, though commonly spoken of as osseous, and corresponding essentially with that of true bone (Fig. 43), presents several remarkable peculiarities, which * For additional information on this subject, see the Memoir of Dr. Mandl on the Scales of Fishes, in his Anatomie Alicroscopique, or in the Ann. des Sci. Xat. 2ieme Ser e, tom. xi. T Trivial though such a character may seem, it is found to correspond very closely with the distinctive peculiarities of the internal structure of these groups, the Cycloid Fishes being nearly identical with the Malacopterygii of the Cuvierian arrangement, and the Ctenoid n ah the Acantliopterygii ; and thus it is of essential service in the determination of the fossil remains of Fishes, whose scales are often better preserved than the internal skeleton. GENERAL VIEW OF THE ANIMAL KINGDOM. FISHES. 4G1 are characteristic of particular families and genera.* It is interesting to remark that the Placoid fishes of the earlier epochs, whose prey consisted of Ganoids invested by a dense bony armour, had teeth of very different conformation from those of the existing Sharks, which are destined to restrain the undue multiplication of the soft-scaled Ctenoids and Cycloids ; for whilst the latter are generally thin and lancet-shaped, the former had broad flat surfaces, fitted for crushing and grinding, like those of the existing Cestracion Philippi, or Port Jackson Shark. 322/. The Teeth may be considered (as already remarked) either as portions of the dermo-skeleton, or as constituting part of the splanchno- skeleton. To the former they would seem to belong in virtue of their origin from papillae developed on the surface of the mucous membrane of the mouth, which is the internal continuation of the dermis ; being thus analogous to hair, feathers, &c. With the latter they may be associated in consideration of their instrumentality as parts of the digestive apparatus. Although there are a few Fishes in which the teeth are entirely wanting, and a few more in which they are reduced to a very small number (the Myxinoids, for instance, having only a single pointed tooth on the roof of the mouth, and two serrated dental plates on the tongue, — and the Tench having but a single dental plate in each jaw-bone, with two small denticles on the nasal bone), yet as a general rule the teeth are extremely numerous in this class ; being sometimes so small, and so closely crowded together over nearly the whole internal surface of the mouth, as to be almost countless. In such cases they repeat each other almost precisely in size and form, thus presenting another example of the Sinciple of vegetative repetition which so extensively prevails in this group. They are found to present great diversities, however, in the several tribes constitut- ing the class, according to the purposes for which they are adapted. Thus in some instances they are nothing more than minute, slender, sharp pointed filaments, closely set together like the ‘ pile ’ of velvet (the ‘ dents en velours ’ of Cuvier) ; sometimes equally fine and numerous, but longer (the ‘ ciliiform ’ teeth of Prof. Owen) ; sometimes long and slender, but a little stronger, like the bristles of a brush (the ‘ dents en brosse ’) ; some- times of 1 larger size, and either straight or recurved cones (the ‘ dents en rape,’ or ‘ en cardes ’) ; and sometimes depressed into broad flat plates, with the surfaces roughened, for crushing and grinding. — The structure of these teeth, the history of their development and succession, and their mode of connection with the bones on which they are set, offer several interesting peculiarities that are very characteristic of the class. They are almost entirely composed of various modifications of ‘ dentine f that form of it which is penetrated (like bone) by vascular canals, and hence called ‘vascular dentine’ (§ 212), constituting a far larger proportion of the dental structures in this class, than in any other. In fact we might say that the typical structure of a Fish’s tooth is for its central portion to consist of vascular-dentine, and its peripheral or superficial of hard or , non-vascular dentine. In some instances the former predominates (as in the tooth of Lcimna, Fig. 47, a), and the latter is reduced to a thin layer of almost enamel-like hardness, which covers the exterior of the tooth. On the other hand, the non-vascular dentine occasionally forms the greater * See the admirable memoir on this subject by Mr. W. C. Williamson in the Philosophical Transactions for 1849. 462 GENERAL PHYSIOLOGY. part, or even the whole, of the dentinal portion of the tooth ; and this is covered externally by a layer of £ cementum.’ the crown being even occasionally protected by a thin coating of a substance resembling 1 enamel.’ • The most complex dental organisation in any existing Fish is that presented by the Scaru-s (parrot-fish), which is adapted to browse upon the branches of the stony Corals, deriving its food from the gelatinous bodies of the soft polypes to which these corals serve as the skeleton (§ 291 a) • for in its pharyngeal teeth there is a very remarkable resem- blance, both in their structure and in their mode of successional develop- ment, to the molars of the Elephant (§ 326 p). On the whole, however, the teeth of Fishes may be considered as bearing a much closer resemblance to bone in their structure, than they do in higher animals ; and this resemblance is peculiarly remarkable in certain cases, in which the entire tooth is made up of a bundle of ‘ denticles,’ each having its own medullaiT cavity or canal, and its own system of radiating tubuli (as in the Pristis, or saw-fish, Fig. 47, b), and is thus nearly allied to an ordinary long bone made up of its bundle of ‘ossicles’ (§ 205, Fig. 45). In fact, when teeth become connected with the jaws by continuous ossification, which is not uncommon in Fishes, the vessels of the Haversian canals of the bone pass on into the medullary canals of the tooth ; and sometimes these medullary canals are surrounded by concentric lain elite, as in bone ; thus forming the peculiar modification of tooth-substance, which has been termed by Prof. Owen ‘ osteo-clentine.’ 322 g. The development of the teeth in Fishes takes place in a mode that is strictly comparable with the early part of the process by which the teeth of higher animals are developed (§ 216, 217) ; and it is very inter- esting to trace its arrestment, in different members of the class, at different stages of the process. Thus in the Shark tribe, the develop- mental process is stopped at the ‘ papillary ’ stage ; and it is in these, as already pointed out, that the largest proportion of the tooth is made up of the ‘vascular dentine’ which constitutes the lowest form of dental tissue. As Prof. Owen remarks, “the simple crescentic maxillary plate, ■with the open groove behind containing the germinal papillae of the teeth, offers in the Shark a magnified representation of the earliest condition of the jaws and teeth in the human embryo.” In the Pike, and many other Fishes, the process goes on to the ‘ follicular ’ stage ; the follicle does not become invested, however, by the bony socket of the jaw, but the to ah completes its growth within the substance of the vascular gum. In the Batistes (file-fish), Scarus (parrot-fish), and some others, in which the peculiarly dense nature of the food necessitates a higher elaboration of the dental structure, the process of development goes through all the stages which characterise it in the higher Vertebrata ■ for the orifice of the follicle into which the papilla sinks, becomes completely closed so as t< > form a ‘ capsule,’ which, again, is included in the ‘ alveolus ’ or socket formed by the growth of the jaw around it ; in those which are to possess enamel, a distinct ‘enamel-pulp’ (§ 216) is developed : and the eruptive process takes place just as in Mammalia. — In all Fishes, the greater number of the teeth are shed and renewed, not once only (as in Mammals), but frequently, during the whole course of their lives ; the exceptions to this rule being extremely few. In by far the larger proportion of the class, this successional development takes place from new and independent GENERAL VIEW OF THE ANIMAL KINGDOM. FISHES. 463 papillae ; the germs of the replacing teeth being produced from the free surface of the buccal membrane, and advancing in their regular course of development in proportion as the teeth previously formed are ‘ exuviated’ (that is, cast off as parts that have completed their term of life), or are torn away or worn down in the predatory operations of the animal. In those, however, in which the teeth are developed within capsules, the new or replacing teeth have their origin in germs which are budded-olf (so to speak) from the capsules of those which preceded them, just as in higher Vertebrata (§ 217 ) ; sometimes coming up side by side with their predecessors, sometimes displacing them in a vertical direction, as in Man. In this endless succession and decadence of the teeth of Fishes, we have another example of the principle of ‘vegetative repetition,’ as it manifests itself in the introduction of new organs in the ascending series of the animal kingdom. — The most common mode of attachment of the fully-formed teeth of Fishes, is by a continuous ossification between their expanded bases and the jaw on which they rest. Sometimes, however, the connection remains in the ligamentous condition in which it is before this ossification of the intervening substance has taken place. Even when the tooth is received into a distinct socket, it is almost invariably anchy- losed to some part of the surrounding bony wall, rarely exhibiting that complete freedom which we see in the teeth of Mammals. And although, i as in the latter group, the teeth of Fishes may be thus implanted by distinct ‘ fangs,’ yet these fangs never diverge from each other. In the powerful teeth of Dendrodus, an extinct Sauroid fish, the broad base sub- divides into many root-like processes, which are implanted in the coarse osseous substance of the jaw ; but these become so completely continuous with the bony tissue, that the line of separation cannot be traced.* 3 22 h. The digestive apparatus of Fishes is generally formed upon a very simple plan, their food being usually animal, and very easy of digestion. They do not masticate their food, and there is consequently no need of salivary glands ; but it passes at once, with little or no divi- sion, through the short w r ide oesophagus, into a capacious stomach (Fig. 173, 2), the walls of which are generally thin, although sometimes so dhick and muscular as to give to the organ the character of a gizzard. The intestinal tube (4, 5) is usually short, and makes but a small number of convolutions in the visceral cavity ; the real extent of the internal canal, however, is greatly augmented in the higher Cartilaginous Fishes, by a spiral fold of membrane which winds from one extremity of it to the other. The liver (6) is a large and compact gland, usually rich in oil ; and it pours its secretion by a single duct into the intestinal tube near its commencement. The pancreas is commonly represented in Osseous Fishes by a set of large caeca (3), which seem to be prolongations of the stomach ; in the Cartilaginous Fishes, however, it is composed of a mass of smaller caeca, clustered together into a glandular mass, and conveying its secretion into the intestine by a single duct, — some approach to which structure is seen in the pancreas of the Cod (Fig. 231). A spleen is found, under some form or other, in nearly all Fishes ; it usually presents itself as a flattened highly vascular body, lying between the folds of the mesen- tery ; and the large quantity of blood which it receives, is transmitted, after circulating through it, to the vena portae. The ‘ absorbent system’ of * For further information on this subject, see Prof. Owen’s ‘ Odontography.’ 464 GENERAL PHYSIOLOGY. vessels in Fishes is abundantly distributed through the body and on the walls of the intestines ; and it discharges its contents into the general venous system at several points. No lymphatic or mesenteric glands present themselves upon its trunks. — The heart of the Fish is composed of a single auricle and ventricle, like that of the Gasteropod Mollusks (§ 394) ; but it is placed at the commencement of the branchial circula- tion, instead of at the root of the great systemic artery, and the blood is consequently propelled through the respiratory organs with its fullest energy. The auricle (Fig. 174, a) receives the blood which has beeu returned by the great systemic veins ( [b'b "), and collected in the great venous Fig. 174. Anatomy of the Circulating apparatus, and other viscera, of a Fish : — a , auricle: a', ventricle; a ", bulbous dilatation of the branchial trunk c; b, sinus venosus; b\ trunk and sinus of the cephalic veins ; b" b" , great venous trunks from the locomotive organs ; b"\ venous trunks from the digestive organs, the liver, kidneys, generative apparatus, and air-bladder: c, branchial artery, giving off a branch to each branchial arch ; d, branchial veins, whose union forms the aortic trunk that supplies the body with arterial blood, the head and heart beins supplied by arteries, d' d', which originate immediately in the branchial veins ; e, visceral branch of the aorta; e ' , spinal trunk, supplying the apparatus of locomotion; 1, oesophagus : 2. stomach: 3, pancreatic caeca ; 4, small intestine ; 5, large intestine and rectum : 6, liver ; 7, renal organs : 8, urinary bladder ; 9, organs of generation ; 10, swimming bladder. a. Diagram of the heart, to show the course of the blood through it ; the references as above. sinuses ( bb ) ; and from this the blood is transmitted into the ventricle (■: that the fluid which it contains can be readily thrown into vibration by external impulses; these vibrations are strengthened by the calcareous bodies or otolithes which are suspended in the cavity, but there is as yet no vestige of the tympanic apparatus of higher Vertebrata. An organ of Smell is almost invariably present, in the form of a pah' of cavities in the front of the snout, to which the surrounding water is admitted by nasal orifices ; then- lining membrane, on which the olfactive nerve is distributed, is commonly thrown into folds whereby the surface is increased ; the ‘posterior nares,’ however, which in air-breathing animals establish a com- munication between the nasal cavities and the back of the mouth, are here wanting. There is reason to believe that the olfactive sense is very acute in many Fishes, serving to guide them to then food. How far they possess the sense of Taste cannot be determined, but it is probably low ; for although the tongue is frequently large, it is furnished with prehensile teeth, rather than with sensitive papillae ; and the food seems to be swallowed as soon as it has been taken into the mouth. The sense of Touch cannot be generally acute in Fishes that are covered by a scaly armour ; but it would seem to be specially possessed by the parts about the mouth, and by the pectoral fins which are used by many Fishes for the pmpose of exploration. In the Gurnards and some other Fishes having a hard covering on their cheeks, certain digitate appendages are developed from the pectoral fins, which appear to be specially modified as tactile organs.- — The Muscular apparatus of Fishes, which constitutes a large proportion of the bulk of the body, is for the most part disposed in two great lateral masses running from the head to the tail ; the arrangement of whose elements partakes of the simplicity and uniformity which characterise the vertebral skeleton, the muscles of the different segments repeating each other with great precision. These either bend the tail from side to side, giving impulses whereby the body is propelled directly onwards through the water or even raised above it ; or, the trunk being made a fixed point, they act upon the head, moving it rapidly from side to side, so that it may deal powerful blows with the weapons with which it may be armed, such as the serrated pectoral spines in the Siluroids, mid the opercular spines in the Perch and the Coitus (Bull-head). But there are also special sets of muscles connected with the anterior and posterior members, and with the respiratory apparatus ; and those belonging t< ■ the head have a complexity of arrangement, which corresponds with that of the parts of the cranium itself. 32 2 A The Generative apparatus of Fishes is developed upon a very low type, although it usually attains an enormous size at the period of irs functional activity ; these animals being by far the most prolific of GENERAL VIEW OF THE ANIMAL KINGDOM. FISHES. 467 Vertebrata, and bearing comparison in this respect with Insects. In the Osseous fishes, the ovaries of the female and the testes of the male occupy a corresponding position at the posterior part of the visceral cavity, extending far forwards, however, when distended with their mature contents. The ovary is a capacious membranous bag, the interior of which is thrown into folds or festoons ; and the ova, at first developed in the substance of these, fall into the central cavity, and are expelled from it (in most Osseous Fishes) by an oviduct which terminates externally behind the anus and in front of the urinary orifice. In like manner, the seminal fluid, formed within the reticulated tubuli of the testes, is carried out of the body by an excretory duct which terminates in the same situa- tion. In the greater number of Osseous Fishes, there is no sexual congress ; but the ova deposited by the female are fertilized by the seminal fluid which the male diffuses through the surrounding water. In a few cases, however, the ova are fecundated whilst yet within the body of the female, the male being provided with an external prolongation of the orifice of the seminal duct, by which its products are conveyed into the oviduct of the other sex ; and in some of these instances, the ova are retained within the female passages until the embryonic development is complete, and the young then come forth alive. In a few Osseous Fishes, as in Carti- laginous Fishes generally, we find an approach towards the mode in which the ova are set free in the higher Vertebrata. In the Eel and the Lamprey, the ovarium, instead of having an internal cavity and an excretory duct, is a solid mass, composed of membranous folds attached beneath the spine ; the ova developed within the substance of these, escape into the abdominal cavity, within which they are loosely dispersed ; and they make their exit through a pair of simple orifices, placed one on either side of the anal opening. The seminal fluid of the males of these Fishes is discharged in precisely the same manner. In the higher Cartilaginous Fishes, however, we find the external orifices prolonged inwards, so that each forms a tube with a funnel-shaped dilatation at its extremity ; into this the ova are received when they are detached from the ovarium, and by it they are conveyed towards the outlet. A regular sexual congress takes place in these Fishes, the males of which are provided with ‘ claspers’ developed from the ventral fins, for the purpose of holding the female ; the seminal fluid is introduced within the female oviduct, so as to fertilize the ova soon after they have quitted the ovary ; and these ova (which are far less numerous than those of most Osseous Fishes) undergo their early embry- onic development before they are discharged, or are retained until the embryoes are mature ; — the ova, in the former case, being enclosed in peculiar protective capsules ; whilst in the latter, a continuous supply of nutriment is in some instances afforded to the young, by means of an ap- paratus specially developed for the purpose, foreshadowing the ‘ placenta ’ of Mammalia. Thus in then* Nervous and Generative systems, — the two which most characteristically mark the position of any group or individual, — as well as in many other particulars, we find the Sharks and their allies presenting a decided superiority to Fishes in general. The development of the embryo Fish within the egg takes place in such a manner, that at no time does it present the characters of any inferior animal ; and whether the egg be hatched before or after its emersion from the oviduct, the young- fish is already so far advanced as distinctly to manifest the characters of H h 2 468 GENERAL PHYSIOLOGY. its class, although its subordinate distinctions generally make their ap- pearance at a later period. For some time, however, after it comes fortli into the water, it lives upon the store provided for it in the yolk-bag, which still hangs beneath the abdomen ; its tissues and organs present quite an embryonic condition ; and its usual state is by no means one of activity, until it begins to seek food for itself. 323. The transition from the class of Fishes to that of Reptiles, which is the lowest of the air-breathing Vertebrata, is accomplished by a pecu- liarly interesting group, which is ranked by some Naturalists as the Batrachian order of Reptiles, by others as a distinct and independent cla.-s under the name of Amphibia. The peculiar character of this group is derived from the ‘metamorphosis’ which its members undergo; all of them coming forth from the egg in the condition of Fishes, having the respiration and circulation, the digestion and nutrition, the locomotion and sensation, of that group ; and gradually advancing towards the state of true Reptiles, which some attain much more completely than others. This metamorphosis is most complete in the Frog and its allies, which constitute in their adult state the anourous or taildess family of Batrachia. Its tadpole, or larva, which is an inhabitant of the water exclusively, has the large head and vertically-expanded tail of a Fish ; it is entirely desti- tute of locomotive members, but is urged through the water by the lateral undulations of its body. As its growth proceeds, two pairs of limbs are developed, and the tail is proportionally atrophied ; and at last, the power of progression is entirely committed to the member’s, which are formed for movement upon the solid ground. Whilst this change is taking place, the whole organic apparatus is undergoing important modi- fications. The branchial fringes of the Fish give place to the pulmonic cavities of the Reptile, and the aquatic respiration is exchanged for the aerial ; the heart acquires an additional auricle, and the whole course of the circulation is changed ; the intestinal canal undergoes considerable modification, in accordance with the change in the alimentation of the animal ; the skeleton is raised from the type of that of the Fish to that of the Reptile by various essential changes, such as the ossification of the inter- vertebral capsules, and the union of these with the bodies of the vertebrae, so that each is convex at one end and concave at the other, and is con- nected with those before and behind it by a ball-and-socket joint at each end ; and the nervous and muscular systems undergo a corresponding advance. The true Reptilian condition is also attained by the Batrackians of the Salamander tribe ; in which, however, the limbs are not developed to the same degree, and the tail is retained, the habits of these animals for the most part continuing aquatic. And there is another curious tribe, that of the Ccecilia and its allies, which attains the true Reptilian state by a metamorphosis, and represents the Serpents in form. These are ;dl distinguished from ordinary Reptiles, however, by the simple extenrd character of the smoothness of then - skin and the absence of scales : and this, as we shaU presently see, corresponds with certain peculiarities in internal structure, by which (as well as by their metamorphosis) the Batrachian Reptiles are distinguished from Lizards, Serpents, Turtles, Aw. 323a. Besides those animals, however, which attain the true Reptilian condition by a complete metamorphosis, this group contains several whose development is arrested, as it were, in an intermediate or transition state ; GENERAL VIEW OF THE ANIMAL KINGDOM. AMPHIBIA. 469 their adult form presenting a remarkable mixture of the characters of the two classes which they thus connect ; and their respiration being partly aquatic and partly atmospheric,— lungs being developed, without the gills ceasing to act. Of this group (which is known as that of the perenni- brancldate Batrachia, this name indicating the retention of the gills) may be especially mentioned the Proteus, which inhabits the waters of the underground lakes of the Tyrol, and has a long serpent-like body, with fom - short and slender limbs on which it crawls ; the Axolotl, an inhabitant of the lake of Mexico, which is nearer in form to ordinary Fishes, and whose limbs are more adapted for swimming ; the Siren, which is found in the marshy grounds of Carolina, and has no posterior extremities, the anterior pair being short feeble rudiments ; and finally the curious Lepi- dosiren (Fig. 175), of which oue species inhabits the Gambia and other rivers of Western Africa, whilst another is found in some of the South American rivers, and which presents such a remarkable combination of the characters of the Fish and the Reptile, that some of the most eminent Zoologists are at issue (as already remarked) with regard to its proper place in the series. In the general form and aspect of its body, it bears a considerable resemblance to some of the ‘ perennibranchiate’ Batrachia; but its skin is beset with cycloid scales, and its dorsal and caudal fins are supported by soft elastic rays, which rest upon the extremities of the neural and haemal spines of the vertebral column, as in Fishes. Its peculiar rudimentary extremities, which are single slender-jointed rods, bear more resemblance to the limbs of the Proteus reduced to a state of further simplification (Fig. 171), than they do to the simplest fins of Fishes, which always present a great multi- plication of digits, and they are used by the animal to crawl along the bottom of the water it inhabits ; but the anterior pair is connected with the occipital segment, as in Fishes (§ 322 c); whilst in the pelvic arch we find the same absence of completion by attach- ment to the vertebral column, as in that class. It is curious to observe in this animal, as in the Amphioxus, a want of symmetry in the position of the anal orifice, which is situated a little to one side of the median plane. The vertebral column presents an organisation which is found in no existing true Reptile; for the central elements of the vertebrae are not more consolidated than they are in the Sturgeon, whilst the neural arches are completely ossified. In the condition of the cranium, the Lepidosiren is stated by Prof. Owen to bear a resemblance to the higher Cartilaginous Fishes, as regards the confluence of its elements ; but in this respect the Siren also resembles them. It has, however, a pre-oper- cular bone, which is not found in any of the Perennibranchiates ; and the composition of the lower jaw corresponds rather with that of the Osseous Fishes, than with that of the Amphibia. In its respiratory and circu- lating apparatus, on the other hand, it certainly bears a closer resemblance Fig. 175. Lepidosiren paradotvu. 470 GENERAL PHYSIOLOGY. to the Pererm ibrai i ch iata than to any existing Fishes ; its lungs being as much developed, and as much used for breathing, as those of the Siren : and the general arrangement of the bloodvessels being the same, although the additional auricle of the heart is wanting. The brain presents cha- racters of resemblance to both groups, and it would be difficult to say which predominates. As in Fishes, the nasal sacs, though possessing a double orifice on either side, have no connection with the back of the mouth by ‘ posterior nares on the other hand, the Reptilian alliance of the Lepidosiren is indicated by the large size of its blood-corpuscles, which nearly approach in dimensions and appearance the remarkable blood- disks of the Proteus and Siren (§ 174). From recent enquiries into the anatomical relations of the Lepidosiren, not only to the Perennibrancliiata, but also to the tadpoles of the Frog tribe (especially the gigantic tadpole of the Eana paradoxa) in their transition state, its correspondence with these appears to be much closer than it is with any Fishes. — Whatever be the view taken, however, in regard to its Zoological position, its in- terest to the Physiologist remains the same, as an animal in which the characters of Fish and Reptile are so blended, that it completely bridges over (so to speak) the hiatus between the two classes. 324. The class of Reptiles is oviparous and cold-blooded like that of Fishes ; but the animals belonging to it are formed to breathe air, and to inhabit the surface of the earth, along which they for the most part creep or crawl, rather than run or leap ; the few which are adapted to make the water their dwelling, being obliged to come to the surface at intervals to breathe. Although they breathe air, however, their respiration is much less ener- getic than that of Fishes, and their blood is for the most pail less per- fectly aerated, — that which is transmitted along the systemic arteries being a mixture of the oxygenized blood just returned from the lungs, with venous blood brought back by the systemic veins and sent again into the circulation without aeration. Reptiles present a remarkable contrast both to Fishes and Birds, in regard to the general inactivity both of their animal and their organic functions. Their perceptions are obtuse, and their movements usually sluggish. Their demand for food is not frequently repeated, and they can sustain a long privation of it ; then- demand for oxygen, also, is by no means urgent, and they can exist in a much less pure atmosphere than that which higher air-breathing animals require. Altogether they may be said to live veiy slowly ; but their rate of life and their degree of general activity (of which the amount of oxygen c< >n- sumed by them may be regarded as the exponent) vary remarkably with the temperature to which they are subjected, then- whole vital activity being increased by warmth — within certain limits — and diminished ly cold (§ 97). Not only in all the foregoing particulars, but in the grade of development of the several parts of the vertebral skeleton, and in the condition of the nervous centres and organs of sense, there is a very close agreement between the four most diverse groups of Reptiles, — -Bat-rack ia (or the Frog tribe), Ophidia (Serpents), Sauria (Lizards) ; and Chelonia * The possession of a second orifice to each nasal sac (of which the posterior was at first overlooked by Prof. Owen, through the partial decomposition of the specimen examined by him,) has been advanced as a Reptilian character; hut both the orifices open within the lip, instead of externally to it, whilst neither of them communicates with the proper cavity of tnc mouth, still less with the respiratory passages ; so that this character must be regarded as essentially Ichthyic. GENERAL VIEW OF THE ANIMAL KINGDOM. REPTILES. 471 (Turtles) ; notwithstanding the strongly-marked differences which exist between them, both in regard to external form and general configuration, and with respect to their adaptation for entirely different habits of life. These differences, however, are not so great as those which present them- selves, when we come to include not merely the existing but the extinct forms of Reptilian life ; for among the remains which represent the latter, we find some which combine many of the characters of Fish with a general organisation essentially Saurian, whilst others exhibit to us the Saurian organisation adapted to the mode of life of Birds, and others again show a nearer approximation to the Mammalian type than is mani- fested by any existing species. In order to include these, it is necessary to add three orders to those which have been already enumerated ; and it has been also found advantageous to separate the Crocodiles and their allies from the Saurian order, and erect them into a distinct group inter- mediate between the Sauria and Chelonia. — The distinctive peculiarities of the Reptilian skeleton, and the chief modifications which it undergoes in the different orders, will be now briefly noticed ; and for this purpose it will be convenient to begin with that of the Crocodile. 324 a. In the skeleton of the Crocodile, and of Lizards in general, the distinction between the neck, trunk, and tail is well marked, as it is in higher animals ; the anterior extremities being now removed backwards from the head, and the intervening space being considered as the neck ; whilst the posterior have now a fixed position at the back of the visceral cavity, the pelvic arch being attached to the spinal column, so as to divide the tail from the trunk posteriorly, as the neck is marked off anteriorly. The total number of vertebrae is frequently very great ; but the multipli- cation is chiefly in the caudal region. Thus in the neck we never find more than 7 ; those of the trunk do not exceed 25, except in the curious group which forms the transition to the Ophidian order; whilst those of the tail are 36 in the Crocodile, 72 in the Iguana, and 115 in the Monitor. — The neural arches of the spinal column are everywhere completed (save towards the extremity of the tail, where they are deficient), by the adhesion of the neurapophyses to the bodies of the vertebrae ; but the connection is esta- blished by ‘ suture ’ only, so that they remain as really distinct bones, instead of by continuous ossification as in the higher Vertebrata. The vertebrae of the neck are furnished in the Crocodile with short pleurapo- physes (cervical ribs) which are articulated with the transverse processes ; these lie over those of the adjoining vertebrae in such a manner as to limit the movements of the spinal column in a lateral direction, giving that strength to the head and neck which is required by the predacious habits of the Crocodile, but preventing the animal from easily turning to j cue side. These cervical ribs, however, are wanting in many Lizards, or are only developed as continuations of the transverse processes. No por- tion of the haemal arch, strictly so called, is developed in the neck ; but behind the Scapular arch we find the pleurapophyses (ribs) greatly elon- , gated, and coming into connection at their extremities with the haemapo- phvses (sternal ribs) and haemal spines (sternum) ; the whole being so , highly developed, as to enclose a capacious visceral cavity. The pleurapo- I physes are articulated to the transverse processes merely, having no direct ! connection with the bodies of the vertebrae! as in Mammalia ; but in the ' Crocodile we see the double articulation foreshadowed (as it were) by the 472 GENERAL PHYSIOLOGY. presence of two articulating surfaces between the rib and the transverse process, one of them corresponding to the ‘ head ’ and the other to the ‘ tubercle ’ of the rib. Between each ossified spinal and sternal rib, a cartilaginous piece intervenes, which may be regarded as an unossific-d portion either of the pleurapophysis or of the haemapophysis, — probably of the former, as it bears a ‘ diverging appendage ’ in the shape of a some- what expanded plate. The sternum varies considerably, among different Sauria, in its degree of ossification. The anterior portion of it is gene- rally T-shaped, having a central median portion and two lateral bars ; these last probably represent the distinct ‘ epistemal ’ pieces which arc so remarkable in the scapular arch of Enaliosaurians (§ 324 h) and Monotiv- mata (§ 326 c). Posteriorly the sternum is cartilaginous in some Sauna, osseous in others, being generally compounded of several distinct pieces which do not unite ; and these extend, with their sternal ribs, far back over the abdominal cavity, even where the spinal ribs are not sufficiently developed to meet them and thus to complete the haemal arch. The presence or absence of distinct pleurapophyses determines the distinction between ‘dorsal’ and ‘lumbar’ vertebrae, as in Man. The two vertebrae to which the pelvic arch is attached, are usually immoveably united together to form a ‘ sacrum,’ which is attached to the ilium by long trans- verse processes ; the pelvic arch itself, as already explained (§ 320 m), is the haemal arch of one of these vertebrae, and probably includes that of another. In the vertebrae of the tail, the haemal arches are carried back- wards in nearly the same form as the neural, the haemapophyses being articulated with the bodies of the vertebrae instead of with the ends of the pleurapophyses (as they are in the trunk of the body), and uniting below into a haemal spine ; the presence of these arches, which, when detached from their bodies, are known as the Y-shaped or ‘ chevron bones,’ consti- tutes a marked distinctive feature between the caudal vertebrae of a Saurian and those of nearly all Mammals having the same general form (§ 326 c). Both neural and haemal arches gradually become rudimentary as we approach the extremity of the tail ; the bodies only of the vertebra being left. In the existing Sauria, as in Reptiles in general, one sin-face of each centrum is concave and the other convex ; the gelatinous capsules that intervene between the vertebrae of the Fish having become replaced la- bone, which has united itself in some instances with the anterior extremity of the body, so that all the convexities look forwards, but more commonly with the posterior, so that the convexities look in the opposite direction. This, with several other differences between the skeleton of the Sauiiau Reptile and that of the Fish, obviously tends to the greater consolidation of the osseous framework, which is now constructed to bear up the body on four points of support, instead of to allow every part of it free play in a liquid element whose pressure is equably distributed over its whole surface. — The bones of the extremities now present very nearly the same general aspect as those of higher Quadrupeds, and are developed in proportion to the weight which they have to sustain, and to the use that is to be made of them in the act of progression. We have in the anterior extremity an arm with its ‘humerus,’ a fore-arm with its ‘radius’ and ‘ ulna,’ a ‘ carpus’ composed of several distinct bones, and live digits, each with its ‘ metacarpal’ and three ‘phalangeal’ bones ; and in the posterior extremity a thigh with its ‘femur,’ a leg with its - tibia' and ‘fibula, a GENERAL VIEW OF THE ANI MA T, KINGDOM. REPTILES. 473 i ‘tarsus’ composed of several distinct bones, and five digits with their ‘me- tatarsal’ and ‘phalangeal’ bones. This number of digits is not exceeded in any existing Reptile. The limbs, in certain of the Lizards which approach most nearly to Serpents (as in the genus Seps ), are but very feebly de- veloped ; and one of the pairs of extremities may even be entirely deficient ; the anterior alone being present in Bimanus, and the posterior in Bipes. — • Notwithstanding the general similarity between the bones of the extremi- ties of Lizards and those of Mammalian Quadrupeds, the composition | of the scapular’ and pelvic arches presents a marked difference ; for in the first of these we find the connection of the humeral articulation with the sternum established by the large ‘ coracoid’ bones, the true clavicles being either deficient or but slightly developed, abutting on the median line against each other, instead of upon the sternum, and not entering into the composition of the shoulder-joint ; whilst in the second we have the proper pelvic arch completed below by the ‘ischia,’ the pubic bones forming a separate arch in front of this, and not entering into the com- position of the ‘acetabulum’ or articular cavity of the femur. 324 b. It is in the Cranium, perhaps, more than in any other part of the skeleton, that the inferior condition of the Reptile as compared with the Mammal is most strongly manifested ; for the vertebral elements, although not as distinct from each other as in the Fish, undergo much less coales- cence than in warm-blooded animals ; and the number of separate bones is therefore very considerable. Thus in the Crocodile the ‘ occipital ’ bone, or neural arch of the occipital segment, is made up of four pieces, which remain permanently distinct ; although they all coalesce to form the single ‘occipital bone’ of Man. The ‘sphenoid bone’ of Man, again, is represented by six distinct bones ; its body by the ‘ anterior ’ and ‘ pos- terior ’ sphenoids, whilst its great wings remain separate as the ‘ ali- spkenoids,’ as do also the ‘ pteregoids,’ which form part of its haemal arch. In the ‘ frontal ’ we find three pieces ; namely, the principal ‘ frontal ’ on the central line, and the post-frontals. And in the ‘ temporal ’ on each ' side we have four pieces ; the ‘ petrosal ’ which forms the auditory cap- side, the ‘ tympanic ’ which not merely gives support to the membrana tympani but forms a pedicle for the attachment of the lower jaw, the ‘mastoid’ which enters into the composition of the second or parietal neural arch, and the ‘ squamous ’ bone which is considered by Prof. Owen as part of the diverging appendage from the htemal arch of the next vertebra. Among the bones constituting the upper part of the face, we find no greater complexity than in Man ; but in the front of the upper jaw, we find the ‘intermaxillary’ remaining permanently separate on each side of the median line ] and the lower jaw presents us -with a peculiarity : of composition, to which we elsewhere find no parallel. Each lateral half of it is subdivided into at least five and generally six distinct pieces, which are united together by suture ; the purpose of this arrangement being apparently to diminish the risk of fracture, which would otherwise attend I the snapping-together of these long jaws.'" Whilst in the bones of the face and lower jaw, we have usually a high development of the maxillary i I * See Dr. Buckland’s Bridgwater Treatise, p. 176. — It is very interesting to see how the l necessity for a particular adaptation, to meet a special purpose, has here prevailed over the I tendency to adhesion to the ‘ archetype’ or fundamental plan. For the lower jaw does not ; contain any other vertebral elements than the hcemapophyses and the haemal spine of the 474 GENERAL PHYSIOLOGY. and mandibular arches, which belong to the frontal and nasal vertebra, the hyoidean arch, which belongs to the parietal vertebra, and attains to such an extraordinary development in Fishes, is here greatly reduced iu size and completeness, but comes into connection with the laryngeal apparatus, the support of which, as well as of the tongue, is its chief office in air-breathing Vertebrata. Of the scapular arch, which belongs to the occipital vertebra, mention has been already made. — In the con- nection of the skull with the spinal column, we find an arrangement that is peculiar to the scaly Reptiles (Saurians, Chelonians, and Ophidians) ; for the articulation is formed by a single condyle projecting from the 1 > ■?. si- occipital bone, which is received into a hollow in the body of the ‘ atlas ' (or first vertebra of the neck), just as the convexity at the back of each ordinary vertebra is received into a concavity of the vertebra behind it. A portion of the body of the atlas is developed out of connection with the rest, and coalesces with the body of the second to form the ‘ odontoid process,’ round which the first vertebra and the head can be made to rotate. 324 c. Such are the chief peculiarities in the vertebral skeleton of the Saurian order ; and we have now to notice briefly the condition of their dermal skeleton. This is represented, in the ordinary Lizards, simply by a covering of horny scales, which are truly ‘epidermic appendages but we find in some species of the genus Scincus, beneath the external scaly layer, a firm armour of bony scutes developed in the substance of the true skin. In the Crocodiles and their allies, the presence of this defensive armour is a very important and distinctive feature. The dermal bones along the midde line of the back serve not merely for protection, but also to support the crest which rises from the neural spines of the body and tail, and acts as a sort of fin iu swimming ; and on either side of this ridge are several rows of thick flat bony scutes, which possess great resisting power. Hence these animals have received the distinctive appel- lation of Loricata ; and, as we shall presently see (§ 324 h), the high development of this part of then- dermo-skeleton is a character iu which they present a decided approximation to the Chelonian order. — All existing Saurians are furnished with numerous teeth, implauted on the margins of the jaws. These, in the Crocodile, are fixed in distinct bony ‘ alveoli’ or sockets ; but in many tribes of Lizards, they have a much less complete mode of attachment. Some of their most interesting pe u- liarities will be described hereafter (§ 324 o, p). 324 d. When we turn from the skeleton of a Lizard to that of a Frog, the contrast is very striking. Whilst the former is remarkable for the great multiplication of vertebrae, and for the small size of the extremities in proportion to the trunk, the latter presents us with a remarkable dimi- nution in the number of vertebrae, whilst the extremities (especially the hinder pah') are immensely increased in dimensions, and take upon them- selves the whole locomotive function. In the common Frog there arc but nine moveable segments in the spinal column, and this number is reduced in the genus l'i pa to seven ; behind these is the single sacral vertebra to which the pelvic arch is attached ; and to the posterior extremity of this mandibular arch ; yet these two parts are multiplied or subdivided into six, and this not merely by simple repetitions of each other (like those which we see in the digital bones of a Fish’s fin), but in such a manner as to form a very complex arrangement. GENERAL VIEW OF THE ANIMAL KINGDOM. REPTILES. 475 i is fixed a long sword-sliaped bone, which may be regarded either as a second sacral or as a caudal vertebra. The bodies of the vertebras are concavo-convex; their neural arches are everywhere complete, and their articular processes (zygapopliyses) are well developed ; but their pleura- pophyses are rudimentary, the ribs being reduced to the condition of transverse processes ; and of the haemal arches of the trunk, no parts are nresent save those that go to make up the sternum, which bone is largely developed, and serves to give extensive attachment to the muscles of the abdomen. In the scapular arch we find the scapula made up of two distinct portions, the proper ‘ scapula’ and the ‘acromial bone;’ and the latter unites with both the coracoid and the clavicle to form the glenoid cavity of the shoulder joint. The clavicle is more developed than in the Saurians, but is still inferior in size to the coracoid, which is a broad expanded bone, meeting its fellow on the median line, besides being- articulated to the sternum. In the anterior limbs we have the usual number of bones ; but those of the fore-arm are united together.* The posterior extremities are enormously developed, and are connected with the trunk by a pelvic arch of very unusual dimensions, the chief part of which, however, is formed by the extension of the iliac bones. Like their representatives in the scapular arch, the pubic bones take a share in the formation of the articulating cavity for the reception of the head of the femur. The two bones of the leg, like those of the fore-arm, coalesce together, and their original distinctness is more completely lost. The hind foot of the Frog presents a peculiarity which especially fits it for leaping ; not merely the digital bones, but those forming the first row of the tarsus, being extremely elongated, as in the foot of the Kangaroo. — The head of the Frog retains in many respects more of the fisli-like character, than we see in the higher Reptiles ; and it is remarkable for the great lateral extension of the bones of the face, especially of those which form the jaws. The occipital segment is connected with the atlas by a louhle articulating surface or ‘condyle the Batrachia differing in this respect from other Reptiles. The lower jaw is not furnished with teeth in the Frog, although they are present in it, as well as in the upper jaw, in )other Batrachia ; but, as if to make up for the deficiency, the vomers have iach a transverse row of teeth, as in some Fishes. 324 e. The conformation of the skeleton in the inferior Batrachia, how- ever, and especially in the Perennibranchiate group, by no means corre- sponds with that which it presents in the Frog ; some of the differences consisting in diversities in the proportions of its component parts, adapting hem to different purposes ; whilst others result from the inferior advance n development. Thus if we look at the proportion between the spinal 'lolumn and the limbs in the Scdamandridoe (water newts) we shall find (hat it is much the same as in Lizards ; they have 15 or 16 vertebrie in he trunk, and from 25 to 30 in the tail, which becomes their chief instru- ment of progression ; they possess short ribs, although the sternum is but fiery imperfectly developed ; and the extremities are only so far developed * An interesting departure from the usual type presents itself in the mode in which the pead of the humerus of the Frog is articulated with its glenoid cavity; for it is provided with ; he ligamentous connecting cord, which is elsewhere confined to the hip-joint. The use of jhis provision becomes apparent, when it is remembered that although the Frog makes its 'eap with its hind-legs only, it alights upon its fore-limbs, which might be dislocated by the hock, if it had not been for this unusual attachment. 476 GENERAL PHYSIOLOGY. as to aid their movements in the water, and to enable them to crawl upon land. Their general form and proportions thus bear so close a resem- blance to those of Lizards, that it is not surprising that they should be popularly associated with them ; they are readily distinguishable, however, on the slightest inspection, by the want of scales upon their shins. — In the Ccecilia, again, the Batrachian type is so modified as to present an extra- ordinary resemblance to the group of Serpents, in which this genus was long placed, notwithstanding the nakedness of its integument. The verte- brae are greatly multiplied, their number in one species reaching even 200 : nearly all of them possess short ribs ; aud there are no traces either of sternum or of extremities. Yet the structure of the skull is distinctly Batrachian ; and this animal has been found to possess gills during the early part of its life, and to undergo subsequently the characteristic meta- morphosis of the Batrachia. — It is iu the Perennibranchiate division, more especially, that we meet with those variations from the structure of the Frog, which depend upon general arrest of development. Among these is one which most obviously marks their approximation to Fishes ; namely, the persistence of the bi-concave form of the bodies of the vertebrae, the in- tervertebral capsule remaining unossified. And in the head we find, not merely the large proportional development of the hyoid arch in connection with the branchial apparatus, but the ready separability of the bones of the skull, and many other characters, which show a yet nearer approxima- tion to the type of Fishes in the skulls of these animals, than in those of such as have advanced, by the completion of their metamorphosis, to the true Reptilian condition. As already remarked (§ 323 a), the Lepidosinu completes the transition between the two classes ; its osteology presenting a still closer conformity with that of Fishes.* 324/. Turning now to the order Opkidia or Serpent tribe, we meet with another extraordinary modification of the Reptilian skeleton, effected, by the multiplication of some parts, and the non-development of '.'titers. The vertebral column here presents us with the greatest number of segments which it attains in any vertebrated animal, the number of ver- tebrae being no less, in the Python (or Boa of the Old World) than 422 ; whilst, on the other hand, there is uo external appearance either ■ t anterior or of posterior extremities. About six-sevenths of the vertebrae possess ribs, which are articulated to their bodies by a kind of ball-and- socket joiut that allows them free play; whilst their opposite extremities are nearly unattached, not the least vestige of a sternum being present, and the sternal ribs only existing as short cartilages tipping the ends , t the spinal ribs, which are connected with the abdominal scuta of the integument. In virtue of their unusual mobility, the ribs can be employed * It is scarcely possible, in surveying this order, not to be struck with its remarkable representation of the other existing orders of Reptiles. The Cecilia is so like a .S / eiii. that it was long regarded as an Ophidian, even by scientific Naturalists. The Salamandride , in like manner, represent the Suurians. And although it might appear difficult to find any analogy between Frogs and Turtles, yet in the delegation of the locomotive power w the extremities, in the relative shortness of the vertebral column, and in the conformation of the scapular and pelvic arches, there are many points of resemblance. In the genus P pa. tins resemblance is also seen in the conformation of the cranium, and in the condensation of tilt skin, which is beset with horny tubercles.. And in another South-American Batrachian, tks condensation proceeds so far in the skin of the back, as to protect the body in a kind ot carapace. (Ann. des Sci. Nat. Deuxieme Serie, tome iii. p. 318) GENERAL VIEW OF THE ANIMAL KINGDOM. REPTILES. 477 in some degree as instruments of progression, their extremities moving backwards and forwards beneath the skin, like the slender legs of the lulus (Fig. 150); but the remarkable locomotive power which these animals enjoy, is chiefly due to the extraordinary flexibility of the spinal column, and to the power of the muscles by which movement is given to it. “ By the endless combinations and inflections of their long spine,” remarks Prof. Owen, “ the absence of locomotive extremities is so compensated, that the degraded and mutilated Serpent can overreach and overcome animals of far higher organisation than itself ; it can outswim the fish, outrun the rat, outclimb the monkey, and outwrestle the tiger ; crushing the carcase of the great carnivore in the embrace of its redoubled coils, and proving the simple vertebral column to be more effectual in the struggle than the most strongly developed fore-limbs with all their exquisite rotatory me- chanism for the effective and varied application of the heavy and formid- ably armed paws.”* Although no true limbs can be seen externally in the typical Serpents, yet in the Python we have a trace of a pelvic arch, with ‘ diverging appendages,’ terminated by horny hooks which appear exter- nally and serve as ‘ claspers.’ In some members of the order, however, which present slight deviations from its ordinary characters (such as the genera Amphislxena, Tortrix, and Typhlops) there is a rudiment of a | scapular as well as of a pelvic arch. And in the Anguis fragilis (com- monly known as the ‘ blind- worm ’ or ‘ slow-worm ’), whose form is that of a Snake, but which approaches more nearly to the Lizards in some other important characters, we find not merely a distinct though minute scapular and pelvic arch, but also some traces of limbs as well as of a sternum. Through this animal, the order is connected with those serpent-like Lizards, in which the body is extremely elongated, and one of the pairs of extremities deficient ; or in which they are both present, but almost in a rudimentary condition. 324 g. The head of Serpents is chiefly remarkable for the extraor- dinary dilatability of the entrance to the month. This results in part from the want of union between the two halves of the lower jaw on the median line, so that they can be opened out from one another ; and in part from the peculiarity of the method by which it is connected with the cranium, — the £ tympanic pedicle ’ to which it is articulated being not only moveable in itself, but having a moveable base ; for the ‘ mastoid ’ bone is here itself lengthened out into a pedicle, and united to the rest of the cranium only by ligaments and muscles. The bones of the upper jaw, too, are less closely approximated to each other than usual ; and being only connected by ligaments, possess some degree of mobility upon each other. In the non- venomous Serpents, the palate-bones as well as the upper and lower jaws have a row of teeth of simple conical form, pointing backwards ; these are not strong enough, however, to serve any other pur- pose than that of prehension of the prey. In the venomous Serpents, the ■' poison-fangs ’ with which their deadly wounds are inflicted, are two large teeth situated in front of the superior maxillary bone, which does not bear any other teeth at its margin : these teeth are either simply grooved for the passage of the poison-duct, or the groove is converted into a complete canal (especially near the point of the tooth) by the arching-over and meeting of its walls. The poison-fangs of some venomous serpents are * Homologies of the Vertebrate Skeleton, p. 182. 478 GENERAL PHYSIOLOGY. immoveably fixed in the jaw ; but in those which are most dreaded on account of the fatality of their bite (as the Rattlesnake, or the Cobra di Capello), the poison-fangs when not in use lie flat upon the roof of the mouth, and are erected by special muscles when the animal makes its attack, the poison-gland being at the same time compressed by the action of the muscle that closes the jaw, so as to force out some of its secretion through the duct which deposits it in the wounds made by the teeth. — The dermo-skeleton of Serpents consists of epidermic horny scales, which are sometimes of small size and arranged in an imbricated manner over the whole surface, but which, in the typical Snakes, present themselves along the ventral surface as broad plates arranged transversely to the axis of the body. The purely epidermic character of this formation is remark - ably shown in its frequent exuviation ; for the whole of the scaly tegu- ment of Snakes is thrown off at least once a year, sometimes mom frequently ; in some instances coming away piecemeal ; but in other case- being detached entire, and even carrying with it (like the exuviae of the Crustacea, § 316) the outer layer of the transparent cornea of the eye-* 321 h. The most remarkable modification of the archetype vertebral skeleton presented to us in the class of Reptiles, is that which we meet with in the order Chelonia, which includes the Tortoises and Tui'tles. t\Y here find the trunk of the body enclosed within a bony casing, into which the head, tail, and the four extremities can be withdrawn in most species. This casing is composed of two shields, covered with homy plates ; the upper one, which is more or less highly arched, is termed the carapace . whilst the lower, which is usually nearly flat, is called the piastre When we look at the carapace on the exterior, it is seen to be composed of a longitudinal series of ‘ central ’ plates united together by sutures ; on each side of these is a corresponding series of ‘lateral’ plates, which form the principal part of the expanded arch ; and the edges are bounded by a series of ‘ marginal’ plates, articulated with each other and with the edges of the lateral plates. On looking at the under side of the carapace (Fig. 1 7 6), these plates are seen to be intimately connected with the vertebral skeleton ; for the ‘median’ plates are supported by the neural arches of the dorsal vertebrae (e) with which they have coalesced ; and the lateral plates (/), in like manner, appear’ like expansions of the ribs. It is not surprising, then, that Comparative Anatomists of the greatest emiuenec- (such as Cuvier) should have regarded each ‘median plate’ as nothing else than a modification of the ‘ spinous process’ (neural spine) of the dorsal vertebra on which it rests, and each ‘ lateral plate’ as an expansion of the rib (pleurapophysis) ; and further, that the ‘marginal plates' (g) should have been likened to the sternal ribs (liasmapophyses) of other Reptiles. This mode of viewing the composition of the carapace has been generally adopted, notwithstanding the suggestion of Cams that the carapace is partly formed by the coalescence of dermal bones with those of the vertebral skeleton ; but Prof. Owen, having investigated the history of its * Of the exuviation of the common Ringed Snake of this country, Mr. Bell says : — “ It i- a mistake to assign a particular period to this process ; some have stated it to occur once, some twice in the summer ; but I have found it to depend upon the temperature of the atmosphere, and on the state of health, and the more or less frequent feeding, of the animal. I have known the skin shed four or five times during the year. (British Reptiles, p. d'J.) GENERAL VIEW OF THE ANIMAL KINGDOM. REPTILES. 479 development in the existing Chelonia, and taken a comprehensive survey if its various modifications, not merely in existing but in extinct species, ias recently demonstrated that the greater part of the carapace is really lermal, — the ‘median plates’ being dermal bones, representing the median lerrnal scutes of the Crocodile’s back-shield, Fig^176. lut developed in connec- ion with the neural ipines of the vertebrae .vhich bear them ; the lateral plates ’ in like nan ner representing the nedio - lateral rows of lermal scutes in the Cro- ;odile, but never being listinct from the ribs, vhich become (so to n ipeak) more or less com- iletely buried in them ; e vhilst the ‘ marginal / plates’ are wholly der- o Inal, the representatives u >f the sternal ribs being * 0 be found in the plas- ron.* — The plastron, or loor of the thoracico- Mominal chamber, con- ists in all Chelonia of line pieces, of which one Fig. 177, a) is median, vhile the others (bb, cc, I'd, ee ) are arranged in i ; lairs. Their develop- Skeleton of Cistudo Europceus (Common Tortoise) with the nont irorido nnncirlm'nhLr plastron removed a, cervical vertebrae ; A, scapula; c, clavicle; neni vanes consiueiaoiy A coracoid bone . e> dorsal vertebra:; /, ribs, incorporated with a different sections of dermal plates ; g, marginal plates ; h, pubic bones ; i, femur ; 1 mi • ii tibia; l. fibula. he group, rhus m the jand-Tortoises (a), the plastron covers the ventral surface as completely s the carapace protects the dorsal ; being rendered entire and solid hrough its whole length by the extension of its component plates into ontact with each other, and by their adhesion to each other at their dges • whilst it is connected laterally by cartilaginous union with the Marginal pieces of the carapace, leaving only an anterior fissure for the massage of the head and fore-limbs, and a posterior fissure for the tail and ind-limbs. And in those which are knowm as ‘ box-tortoises,’ the plas- ,ion is divided by transverse articulations into three portions, of which lie central one alone is united immoveably with the carapace, whilst the nterior and posterior move upon it as upon a hinge, in such a manner * See Prof. Owen’s masterly paper on the “ Development and Homologies of the Carapace ;nd Plastron of the Chelonian Reptiles,” in the Philosophical Transactions for 1849; and is Memoir on the “ Chelonia of the London Clay,” published by the Palaeontosraphical Society in 1849. 480 GENERAL PHYSIOLOGY. that they can completely close the fissures, after the head, tail, and lim! have been drawn within their secure protection. On the other hand, h the Marine Turtles (b), these nine pieces do not by any means form a con tinuous shield, hut only afford an incomplete osseous protection to tin under surface, which is uncovered both in the centre and in parts of tb sides, save by the external envelope supplied by the skin and by its Lorre Plastron of Chelonia; a, Cistudo ; b, Chelone: — a, entosternal, or median piece, representing the true sternum ; b b, episternals ; c c, the so-called hyosternals, dd , hypostemals, and, re, xiphysternals, of GeofFroy. epidermic plates. The plastron was considered by Geoffroy St. Hilain (and, following him, by many other Comparative Anatomists) as compose of the sternum, immensely extended laterally ; others have considered i as altogether furnished by the dermal skeleton ; whilst other's, again, he regarded its elements as derived from both sources. By Prof. Owen, how ever, the median piece (entosternal, a) with perhaps the first pair of later; pieces (episternal, bb), which together make up the T-shaped sternum i the Lacertian Reptiles and in Monotrematous Mammalia, ai'e regarded a the only parts of the plastron which represent the true sternum or ‘ haem; spine;’ whilst the other lateral or parial elements (the hyosternals, c< lxyposternals, dd, and xiphysternals, ee, of Geofh'oy) represent the stem', ribs or haemapaophyses ; all these parts being extended by the developmeu of portions of the dermal skeleton in continuity with them. 324 i. Iu the piosition and mode of attachment of the extremities Chelonian Repatiles, we have peculiarities which at first sight appear quit exceptional. Whilst in all other Vertebrated animals the scapular arch outside the walls of the visceral cavity, we here find it within ; the scajaui lying on the under side of the pleurapophyses, instead of ou their upap>e This displacement could be scarcely accounted for, did we not hear i mind that this arch is really part of the occipaital segment (§ 320 ! ). an that in being (as it were) drawn backwards, it may be carried under rl dorsal paleurapophyses, without a much greater departure Rom the arch typre, than if it were carried over them as it usually is. The pelvic arch, al- is included between the carapace aud plastron ; but this is simply effeett by the great extension of the bony plates of which these are compose- GENERAL VIEW OF THE ANIMAL KINGDOM. REPTILES. 481 whereby they are made to cover-in many segments of the vertebral column posterior to those from which they are developed. — As in the structure of the trunk of the Chelonia we have the most complete consolidation of which its vertebral segments are capable (presenting in this respect the condition most strikingly opposed to that which has been just described in Serpents), the locomotive power is delegated to the extremities in a greater degree than in most other Reptiles ; and we accordingly find, at least in the aquatic members of the group, a greater development, not merely of the bones of which they are themselves composed, but also of the arches by which they are connected with the vertebral column. The ‘scapula’ (Fig. 1766), which is suspended by ligaments from the second vertebra of the carapace, is a narrow slender bone ; but on the under side of the arch we find not merely a very broad and strong coracoid bone (<7), but a well developed clavicle (c), which is not separated, however, from the scapida, and was regarded by Cuvier as an elongated ‘ acromion pro- cess.’ In like manner, the pelvic arch, which is attached by somewhat narrow ilia to two sacral vertebrae, is completed below by the very broad pubic arch ( h ), as well as by the union of the ischiatic bones. In the Marine Turtles, the bones of the hand and foot are greatly elongated, but the digits are not separated ; the whole of each extremity being enclosed in a continuous skin, which is strengthened with a covering of horny plates ; and constituting a very efficient ‘ oar,’ or instrument of aquatic propulsion, which presents the nearest approach that is shown by any existing animal to the ‘paddles’ of the Enaliosauria. In the proper Land Tortoises, on the other hand, the bones of the extremities are very much reduced in length, the digits of the anterior member being almost rudimentary, whilst those of the posterior are somewhat longer, and are furnished with claws that serve for digging in the ground. The Common Tortoise belongs to the group of E my dee or Mud-Tortoises, the conforma- tion of whose members presents a condition intermediate between these two extremes (Fig. 176). — The skull of the Chelonia does not present us with any such considerable departure from the general Reptilian type, as to require special description ; except in the replacement of the teeth by a horny beak, and in the peculiar osseous covering which is furnished to the temporal fossa in certain members of the group. The horny beak, which invests the margin of the jaws along their whole extent, is extremely firm in its texture ; in the carnivorous species it is raised into a sharp cutting edge, that of the lower jaw shutting within that of the upper, so that the two act against one another like the blades of a pair of shears ; whilst in the vegetable-feeders, its working surface is variously sculptured, and is adapted for bruising as well as cutting. According to Prof. Owen, the development of this horny sheath commences “ from a series of distinct papillae, which sink into alveolar cavities, regularly arranged (in Trionyx) along the margins of the upper and lower jaw-bones : these alveoli are indicated by the persistence of the vascular canals, long after the originally separate tootli-like cones have become confluent, and the horny sheath completed.”* Thus it appears that the horny beak is really the representative of the two rows of teeth which most other Rep- tiles possess ; and it is a confirmation of this view to find that a similar Cyclopaedia of Anatomy, Art. Teeth, vol. iii. p. 882. 482 GENERAL PHYSIOLOGY. beak is possessed by the larvae of Batrachia, in most of which it is replaced by teeth when they have assumed the Reptilian form ; the casting-off of the beak being analogous to the shedding of the deciduous or milk-teeth in Mammalia. In the Marine Turtles, we find the temporal fossa (in which is lodged the temporal muscle) covered-in externally by a complete arch, which is prolonged on either side from the cranial vault ; this arch is formed by the combination of unusual expansions of the parietal, posterior frontal, malar, squamosal, and mastoid bones ; and its purpose appears to be, to furnish a peculiarly firm attachment to the temporal muscle, which is lodged in the temporal fossa, and is the prin- cipal elevator of the lower jaw. It is but little developed in the Emydoe, and still less in the Land Tortoises.— Although the Chelonia appear so completely isolated from other Reptiles by the peculiarities of then- organisation, yet the transition to the Loricated Samians is by no means abrupt. For in the Emydosaurus of North America (known as the ‘ alligator tortoise’ or ‘ snapping turtle’), the shell is so small in proportion to the long neck, tail, and limbs, that these cannot be retracted into it : whilst in its general habits, the animal has at least as much of the Samian as of the Chelonian. And in the Trionyx (or ‘ soft tortoise’) not merely the plastron but the carapace is partly destitute of bony support, the ribs not reaching to its edges, and its marginal pieces being formed of flexible cartilage. Thus we pass, without any very wide hiatus, to the Loricated Samians (Crocodiles, Alligators, &c.) in which the dermal portion of the Chelonian carapace is represented by the rows of dermal bones which protect the back. — The epidermic portion of the dermo-skeleton of Chelonia is generally remarkable for the large size and thickness of the plates which cover the carapace and plastron, and which either fit together at their margins, as in ordinary Tortoises and Turtles, or have an imbricated arrangement, as in the Chelonia imbricata (hawksbill turtle) which yields what is known as ‘ tortoiseshell.’ The neck, tail, and limb- are usually covered with scales not unlike those of Lizards and Serpents. In the Sphargis (leathery turtle), however, and in the Trionyx, the surface is everywhere destitute of scales. In the latter genus, the skin is prolonged at the sides, beyond the margins of the carapace, and serves ns a sort of lateral fin, which helps these animals to shuffle along beneath the sand or mud at the bottom of the rivers in which they reside. 324 k. This general account of the principal varieties in the conforma- tion of the skeleton in the principal groups of Reptiles would be by no means complete, without a notice of three extinct orders, which seem to have occupied a very conspicuous place among the Yertebrated animals of former epochs. — Through the whole of the ‘ Secondary Period." com- mencing with the Triassic Series, and ending with the Chalk, we find the remains of a remarkable tribe in which the characters of Fishes and Whales are most curiously blended with those of Lizards, these animals having been obviously adapted to an exclusively aquatic life, and dwellers in the ocean rather than in rivers or lakes. Hence this group has received the name of Enaliosauria or Marine Lizards. The greater part of the species it includes belong to the genera Ichthyosaurus and Plesio- saurus ; the most obvious distinction between which lies in the size of die head, and in its mode of connection with the trunk. In the Ichthyosaurus (or fish-lizard) the head is very large, and is joined-on (as it were) to the GENERAL VIEW OP THE ANIMAL KINGDOM. REPTILES. 483 trunk, without any intermediate neck, as in Fishes and Cetacea. In the Plesiosaurus, on the other hand, the head is comparatively small in pro- portion to the size of the body, and is carried at the end of a long swan- like neck. The large size of the head of the Ichthyosaurus is due rather to the elongation of the jaws, and to the extension of the bones to which their muscles would be attached, than to any other unusual development ; and it is here that we best see the obvious purpose of that peculiar sub- division of the bones of the jaws, which has been noticed as characteristic of Reptiles (§ 324 b ), — the shock occasioned by the sudden closure of a pair of jaws 7 feet long (for to such a length do w r e find them occasionally extending) being quite sufficient to snap asunder bones that might be put together with less regard to elasticity and tenacity. These jaws are fur- nished, along then- whole extent of margin, with a set of conical teeth, the peculiarity of whose implantation will be noticed hereafter (§ 324^). The eye of the Ichthyosaurus was extremely large, the diameter of the orbit being sometimes as much as 1 8 inches ; and the eyeball was pro- tected by a complete osseous capsule, formed of several plates articulated together. The nostrils were placed, as in the Cetacea, nearly at the top of the head ; and thus the animal could breathe 'whilst the entire body and nearly all the head were immersed. — The spinal column was com- posed of numerous vertebrae, usually from 100 to 120. Each vertebra was biconcave, as in Fish, and must have moved over a soft and elastic intervertebral capsule ; and the freedom of movement must have been further increased by that want of ossific union between the bodies of the vertebrae and the neural arches, which is characteristic (as we have seen) of the highest Cartilaginous Fishes. From these, however, they differ in the completeness of the ossification of their bones, which has caused them to be preserved to us with the most remarkable perfection. The cervical vertebrae of the Ichthyosaurus are thin, and closely united together, as in the Whales ; on the other hand, the tail is greatly elongated and produced to a point, as in the Saurians. Its vertebrae, however, do not bear any vestige of either neural or haemal arches for any great distance behind the posterior paddles ; but from a slight lateral compression which they exhibit, and from the circumstance that the tail is broken off and twisted round at nearly the same part in most of the specimens in which it is preserved, it has been inferred by Prof. Owen that it was furnished with a caudal fin, resembling that of the Cetacea in having no bony support, but corresponding with that of Fishes in being vertical instead of horizontal. The ribs are very numerous, commencing at the second vertebra of the neck, and extending backwards as transverse processes along even a portion of the tail. Those of the anterior portion of the trunk are not united to a sternum, but are connected to each other by a set of ‘ costo- sternal arches,’ each of which contains five pieces ; of these pieces the central one obviously represents the ‘ haemal spine,’ whilst one of the lateral pairs is equivalent to the sternal ribs or heemapophyses, and the other is an intercalated bone resembling that which intervenes between the sternal and spinal ribs in Crocodiles. 324 1. In the form of its paddles, the Ichthyosaurus seems to have | resembled the Cetacea ; but in the number of their component bones, which sometimes amount to two hundred in each member, and in the multiplication of the rows in which they are disposed, it presents a much i i 2 484 GENERAL PHYSIOLOGY. nearer approach to Fishes. The mode of attachment of the anterior extremities to the trunk, however, by the scapular arch, is essentially Reptilian, whilst it presents peculiarities found in no other Reptiles. The broadest part of the arch is formed by the two expanded coracoids, coming into contact with each other on the median line, as well as with the sternum, which is here the mere keystone of the scapular arch, not entering at all into the formation of the thoracic cavity ; the clavicles form an arch of their own, uniting together on the central line, so as to form a single bone resembling the ‘ furcula ’ of Birds ; but besides these, the upper portion of the sternum sends off a lateral prolongation on either side (corresponding with the epistemal pieces of the plastron of Cheloniu, Fig. 177, b b) which extends itself along the curve of the clavicular arch, and comes into close proximity with the margin of the scapula. To this cm’ious arrangement, the nearest approach presented among existing animals is that which is shown in the scapular arch of the Ornithorhyncus (§ 326 d), which in many features of its organisation reminds us of the Rep- tilian type. The posterior extremities, on the other hand, which are gene- rally smaller than the anterior, are not attached to the trunk by a pelvic arch of corresponding firmness ; the iliac bones not being connected with sacral vertebrae, but being detached from the spinal column, like the arch which supports the ventral fins of Fishes. The skin seems to have been for the most part naked, like that of existing Cetaceans ; although, from the regularity of certain markings on the paddles, the integument of those organs appears to have been divided into a number of shield-like com- partments, analogous to those of the paddle of the Turtle. — The Plesio- saurus (as its name imports) approaches more nearly than the Ichthyo- saurus to the ordinary Saurian type ; and this is especially apparent in the conformation of the head, and in the reception of the teeth into distinct sockets, as in the Crocodile. The chief elongation of the verte- bral column here exists in the neck, which in some species contains us many as thirty segments, — a considerably larger number than that which we find in the neck of the Swan. The surfaces of the vertebrae of the trunk are nearly flat, as in Mammals and some land Reptiles, and are locked together by the articulating processes, in such a manner as t" increase the strength of the spine, whilst diminishing its flexibility ; but still the neural arches are unconnected with the centra. The costo- sternal arcs are each composed of seven pieces, as if to allow of a great dilatation of the cavity of the trunk. The scapular arch is more con- formable to the ordinary Reptilian type, but is still of most extraordinary strength ; the pelvic arch is more closely connected with the spine, and the members it supports are usually of the same size with the anterior, sometimes even larger ; the tail is shorter, and was obviously less used to propel the body, but probably served rather to steer it, the locomotive power being chiefly delegated to the extremities. The Enaliosauria wore still more closely connected with Crocodilian Reptiles (the most aquatic in their habits of all existing Sauria) by the extinct genus Phosa’irus. which combined in some degree the characters of both the genera already mentioned, with those of the higher orders. — From the number of remains of distinct species of Enaliosauria, preserved in the several formations ot the middle and later Secondary Period, and from the frequency with which these remains present themselves in strata whose mode of deposit GENERAL VIEW OF THE ANIMAL KINGDOM. REPTILES. 485 was favourable to their entombment and preservation, it may be inferred without improbability that they were then among the most numerous of the larger tenants of the seas ; as their great size, predaceous habits, and great activity rendered them among the most formidable. It is impossible to contemplate their skeletons, without perceiving that they were altogether framed for rapid motion through the water ; and that these animals must have rather participated in the speed and energy so characteristic of Fishes, than in the comparatively slow and feeble progression which is ordinarily seen amongst Reptiles. It is not improbable that the peculiar mode in which the haemal arches are completed, allowing great distensi- hility of the cavity inclosed by them, may have been destined to afford space for unusually capacious lungs. Another purpose, however, may perhaps be assigned to it, — that of allowing the trunk to particijiate in the mobility of the spinal column, and to take upon itself the form most favourable for rapid progression through the water, when the animal was urged to its highest speed.* 324 m. It need not much surprise us to meet, in the group just con- sidered, with Saurian Reptiles, in many respects degraded (so to speak) to the type, of Fishes ; because w'e have been led, by the succession of forms presented to us in the Batrachia, to view the classes of Fishes and Reptiles as very nearly related to each other. But Reptiles and Birds may be regarded almost in the light of physiological antipodes ; and an animal which should in any degree unite their seemingly incompatible qualities, must necessarily present a peculiar interest for the Zoologist and Physio- logist. Such a combination is presented by the extinct genus Pteroclac- tylus, which is a Saurian Reptile so modified as to enjoy the powers and to lead the life of a Bird. The Reptilian character of this remarkable animal is at once demonstrated by the structure of its skull, which approaches, not only in its general proportions, but even in the details of its different parts, that of the Crocodile. Thus we find the lower jaw composed of six bones on each side ; both jaws are furnished with teeth, and these are implanted in distinct sockets. The orbits, as in the Ichthy- osaurus, are very large ; but there is no trace of any bony capsule for the eye. The neck only contains the usual number of cervical vertebrae, 7 ; but these are of great length, and also of unusual size in relation to the body. The head was partly supported by stout tendinous cords running along the sides of the neck ; and these are even found partially ossified in some species. The number of vertebrae in the back is much greater than in Birds ; the ribs are narrow and almost thread-like, and are destitute of the peculiar ‘diverging appendage’ possessed by the pleurapophyses of Birds ; the tail is short and rudimentary, as in Birds. — It is in the structure of the organs of flight, however, that the great peculiarity of * The author learns from Mr. J. Scott Russell, well known for his admirable researches on the forms of Waves, and upon the best construction of vessels in regard to speed, that if a boat be built of slight planks, without any supports to determine a particular form, and be then drawn rapidly through the water, she will be moulded (as it were), by the pressure of the medium through which she moves, into that which he designates as the ‘ wave form,’— i that is, the form of least resistance, adapted to move through the water with the slightest possible propelling force. It is well known that certain ships will sail best when their framing is loose or slack, apparently because they can then accommodate themselves in some 1 degree to the ‘wave form.’ Is not this looseness in the framing of the Enaliosauria a provision of the same nature ? 48G GENERAL PHYSIOLOGY. the Pterodactylus consists, from which it derives its name. The bones of the arm and forearm, especially the latter, are of considerable length : but it was not upon these bones, which give the chief support to the wings of Birds (Fig. 185), nor upon the outstretched and lengthened hand that supports the mem- branous wings of Bats (Fig. 190), that the Pterodactyle cvas borne through the air ; for its wings were extended by a single finger, deve- loped out of all pro- portion to the rest (Fig. 178). The re- maining digits of the anterior members seem to have been left perfectly free : and may have either served for walking, when the wings were folded, or, more probably, were used for climbing, or for suspending the animal in the mode that bats hang by the hooked nail of the thumb. Of these Firing Lizards a number of species have been fomid, sufficient to show that they were once probably the most numerous vertebrated inhabitants of the air; if indeed they were not, at the time when they most flourished, the only vertebrated animals capable of flight. And if it should prove that certain wing-bones found in the Chalk formation, whose microscopic structure seems decidedly Reptilian, were those of a Pterodactyle, vre shall have to extend our notion of the spread of wing of the largest species of this genus, from four or five feet to twelve or fourteen. — The only existing Reptile that presents the least approximation to the Ptero- dactyle in the development of organs of flight, is the little Draco colons Fig. 179 . Draco Volayis. (Fig. 179), which is provided with a membranous wing-like expansion on each side of its body, that serves it as a kind of parachute, whereon h can sustain itself in the air whilst it leaps from branch to branch. These expansions, however, instead of being sustained and put in motion bv the GENERAL VIEW OF THE ANIMAL KINGDOM. REPTILES. 487 anterior members, are altogether independent of them, being supported by the first six false ribs, extended horizontally in a straight line ; and they cannot be moved with sufficient force to sustain the animal in the air for any length of time, far less to propel it in the manner of true wings. 324 m. We have lastly to notice an order of Reptiles that has been formed by Prof. Owen for the reception of a group of fossil Saurians, which present, with a mixture of the Lacertian and Crocodilian types, a closer approximation than either of them exhibit towards the class of Mammalia. Of this order, which has been denominated Dinosauria from the gigantic size of the Lizard-like Reptiles which it includes, the principal genera are the Megalosaurus, the Iguanodon, and the Hylceo- saurus ; of which the first was undoubtedly carnivorous, and the second herbivorous. The principal points in which these Saurians departed from the ordinary Reptilian type, and approached the Mammalian, were the much greater size of the extremities (especially the posterior) in pro- portion to the trunk, so that their heavy foodies were raised above the ground at least as much as those of a Rhinoceros ; the connection of their ribs with the spinal column by a double articulation, as in Mammalia (§ 326 6), instead of by a single one, as in ordinary Reptiles (§ 324 a) ; the consolidation of at least five vertebrae into a sacrum as in many Mammalia, instead of two as in Reptiles, in order to give firmer support to the pelvic arch ; the flatness of the surface of most of the bodies of the vertebrae ; and the hollowness of the bones. In the structure of the teeth of the Iguanodon, moreover, are peculiarities adapting them for crushing and grinding vegetable food, which are found in no other Reptiles, and to which the nearest approach is presented in the gigantic extinct Sloths of South America ; and it is curious that a further resem- blance to these animals should be presented by the Iguanodon, in the absence of teeth from the front of the lower jaw, as well as in the massive proportions of the body, and the great development of the posterior extremities ; so that it would not seem improbable that their general mode of life was the same, and that the Iguanodon performed the same part in restraining the inordinate extension of arboreal vegetation during the later Secondary period, as the Mylodon and Megatherium did in part of the Tertiary (§ 326 A).* 324 o. Having thus sketched the chief peculiarities in the neuro- and dermo-skeletons of Reptiles, it remains for us to speak of their Teeth ; from a brief survey of which we shall naturally pass to the consideration of the Digestive apparatus. — The proportion of the Reptilian class in which there is an entire absence of teeth, is much greater than that of Fishes ; for the whole order Chelonia is destitute of them ; in addition to which they are absent in the Bufonidce, or Toad family of the Batra- chian order ; and although they are found in all existing Sauria, yet certain extinct genera, which present a remarkable combination of Che- lonian characters with a predominant Saurian organisation {e. g. the * For a vast body of information respecting the Fossil Reptiles, and for the grounds upon wnich the order Dinosauria has been constituted, the reader may be especially referred to Prof. Owen’s Report on the British Fossil Reptiles in the “ Reports of the British Associa- tion ” for 1841, and to the various Memoirs contributed by him to the “Transactions of the Geological Society ; ” also, to the Memoirs contributed by Dr. Mantell to the successive volumes of the “ Philosophical Transactions.” 488 GENERAL PHYSIOLOGY. Rhynchosaurus), were destitute of them. The larvae of the Batrachia in general are without teeth, and their jaws are covered with a continuous horny sheath like that of the Chelonia ; this is shed, during the meta- morphosis, in all these animals save the Siren, by which it is retained even though true teeth are subsequently developed. Dissimilar as the horny sheath of the jaws of Turtles and Birds may appear to the proper dental organisation, yet it would really seem, as already pointed out (§ 324 i), to be the representative of this. — The teeth are situated upon the jaws alone, in most Saurians ; and it is in that group that they attain their highest development, and show the closest approximation to Mam- malian teeth. In some Lizards, however, they are found upon the pteregoid bones also ; in most Serpents, they are implanted on the pala- tine as well as the pteregoid bones ; and in the Batrachia, they are commonly met with on the vomer in addition. In no existing Reptile furnished with teeth, is the number less than 30. The extinct Dicynodon, however, which, though a Saurian in its general organisation, had only two long tusks in front of the upper jaw, growing through life from persistent pulps, presents a remarkable departure from the ordinary type ; and it is probable that the teeth which are wanting' at the sides of the jaws, were replaced by a horny sheath. The greatest number of teeth is found in the Batrachian order, where the small size and great multi- plication of the teeth remind us of Fishes, though the teeth are nowhere so numerous as in some of the last-named class. It is rarely that the number of the teeth is so fixed and determinate in any Reptile, as to be character- istic of the genus or species. The form of the teeth, with few exceptions, is that of a simple cone, more or less acute at its point, but sometimes compressed, so as to form sharp edges. They are adapted rather for seizing and holding prey, than for dividing and masticating their food. The most remarkable departure from the typical form is presented by the herbivorous Iguana, and by the extinct Iguanodon which resembled it in regimen ; in the teeth of these animals, the crown is expanded, both in length and breadth, and the margins are notched or dentated ; and those of the Iguanodon are further marked by longitudinal ridges, presenting the most complicated external form anywhere met with in this class. A trenchant edge is formed, on the outer side, by a layer of enamel ; and the dentine is softer as it recedes from the enamelled edge ; so that the crown of the tooth is worn away in a regular oblique plane. 324y>. The varieties of dental structure are few in Reptiles, as compared either with Fishes or with Mammals. The teeth are most commonly com- posed of ‘ ordinary ’ or ‘ non-vascular dentine,’ ‘ enamel,’ and ‘ cementum ; and, in addition to these, we occasionaRy meet with a substance closely resembling bone, in the interior of certain teeth, where it is formed by the calcification of the remains of the pulp. The inner portion of the dentine of the Iguanodon is traversed by medullary canals extending from the pulp-cavity ; but these are not, as in the ‘ vascular dentine’ of Fishes, centres of radiation for their own set of tubuli. This, however, is the nearest approach to that texture, which the class of Reptiles presents. The most constant of the foregoing elements are the dentine and the cementum; these constitute the entire substance of the tooth in Ophklia, the body of dentine being invested with a thin layer of cemen- tum ; in some Batrachia and in most Sauria , the enamel usually forms a GENERAL VIEW OF THE ANIMAL KINGDOM. REPTILES. 489 thin coat over the crown ; and it is in the latter order, that we occasionally find a deposit of coarse bone in the interior. The peculiar conformation of the poison-fang of the venomous Serpents has been already noticed (§ 324 g) ; and the only other important variety of structure is one which is presented by certain of the lower Reptiles in common with some among the higher Fishes ; namely, an implication or doubling-in of the dentinal cylinder that surrounds the pulp cavity, so as to form a series of vertical folds, which are usually marked by ‘ flutings’ upon the surface, : as in the lower part of the teeth of the Ichthyosaurus* — The development of the teeth of Reptiles is intermediate, as to the grade to which it pro- ceeds, between that of the teeth of ordinary Fishes, and that of the same organs in Mammalia. In all cases, the process passes beyond the ‘papil- lary’ stage to the ‘follicular’ and ‘capsular;’ and in some Reptiles the capsule becomes more or less surrounded by true bone ; but it is never so far invested by it, that the emergence of the tooth can be considered as a true ‘eruptive’ stage. The teeth of Reptiles, like those of Fishes, are ‘ successional ; ’ that is, new teeth are constantly in progress of develop- ment, whilst the old are as continually undergoing exuviation. The new teeth are not here developed from independent germs ; but each forma- tive pulp is derived as an offset from the follicle of the preceding tooth, and its development takes place in a cavity at the side of its base. As the tooth acquires hardness and size, it presses against the base of the contiguous attached tooth, causes a progressive absorption of that part, and finally undermines, displaces, and replaces its predecessor. The rapid succession of tooth-germs, which stamps the impress of decay upon their predecessors, often before the growth of these is completed, is most strikingly manifested in the Crocodiles ; in which three, and sometimes even four generations of teeth, sheathed one within the other, are con- tained in the same socket. The process of replacement is subject to certain variations, in accordance with the diversities in the mode of attachment of the teeth which are characteristic of particular tribes of Reptiles ; but it is performed in every case in a manner essentially the same, the extinct Dicynodon constituting the only known exception. — The simplest mode of attachment of the teeth of Reptiles to the jaws that bear them, is that in which the tooth becomes anchylosed by its base to the margin of the jaw, or to a conical process of bone rising above it ; this, which is the case in the Mososaurus, corresponds with the condition of the human jaw at that early period of embryonic development, when as yet not even the ‘primitive dental groove’ has been formed. In the greater number of ordinary Lizards and in Frogs, the teeth are partly imbedded in a groove formed by the elevation of an alveolar ridge on the external margin of the jaw, and to this ridge they adhere by anchylosis at the outer side of their base ; to this stage of the formation of the primitive dental groove, we have the analogue in the human embryo at about the seventh week. In the Ichthyosaurus, as in the slightly more advanced human embryo, the dental groove is completed by the elevation of a ridge on the inner margin of * According to Prof. Owen, the wonderfully-complicated structure of the tooth of the .extinct Lahijrinthodon (one of the earliest known Reptiles in order of geological time, which : was essentially a Batrachian with some Saurian affinities,) is referable to this type ; by Mr. Tomes, however, it has been recently explained as resulting from the aggregation of a series of denticles, — the only known example of such a composite structure in the class of Reptiles. 490 GENERAL PHYSIOLOGY. the jaw, and indications of a transverse subdivision into sockets may be perceived; here the tooth does not become anchylosed to its socket, but remains free. In most Ophidia and some Batrachia the tooth is sur- rounded by a shallow socket, to which its base becomes adherent ; and in the extinct M egalosaurus and Thecodon, this socket was moderately deep, and the tooth was adherent to its walls. In the Plesiosaurus and the whole Crocodilian group, the teeth are enclosed in sockets, and remain free ; a condition which approaches towards that of Mammalian teeth. — the difference being that in Reptiles the base of the tooth is seldom closed and contracted to a ‘ fang,’ and there is never any implantation of Reptilian teeth by diverging fangs. * 324 q. The structure and arrangement of the several parts of the Nutritive apparatus in Reptiles do not offer nearly the same important diversities, as we have seen to exist in the Osteology of these animals ; for although the form of the organs may vary, in accordance with that of the body, yet their essential characters remain unchanged. Thus in Serpents, we have an extraordinary elongation of most of the viscera (Fig. 180), the general arrangement of which strongly reminds us of the disposition of parts in the Nematoid Entozoa (Fig. 143). Although the food of Reptiles is generally seized by their jaws, armed as these usually are with prehensile teeth, or furnished with a sharp cutting horny ridge, yet the tongue is the chief instrument by which it is obtained in some of the smaller Reptiles of insectivorous habits, such as the Chameleon, Frog, and Toad ; among which we find a provision for its being suddenly darted out to a consider- able distance, and either attaching the victim to its point by the glutinous saliva with which it is furnished, or grasping it between the points of its bifid extremity. Among very few Reptiles does it seem that anything like a proper mastication takes place ; yet a considerable salivary secre- tion is formed by a glandular apparatus, which is sometimes disposed in the tongue itself, sometimes along the margin of the jaws ; and the purpose of this secretion would seem to be chiefly to lubricate the prey, which In- frequently to be forced down the oesophagus with considerable effort. The oesophagus is usually capacious, and opens into a stomach, which is sometimes a scarcely perceptible enlargement of the alimentary canal (as in Serpents, Fig. 180, i), but which in other instances presents itself as a large sac of variable form ; being in both cases susceptible of very great dilatation. The greatest departure from the usual type is shown in Croco- diles, in which the stomach has strong muscular walls, the fibres of which radiate from a central tendon, in a manner that reminds us of the con- formation of the gizzard of Birds. The pancreas now presents more of the character of the ordinary glands, and its secretion, with that of the liver, is poured into the upper part of the intestinal tube ; this is usually short, but is divided into two portions, representing the small and the large intestines respectively, by a prominent valve analogous to the iieo ciecal valve in man ; but it is very interesting to remark, that the intes- tine of the Enaliosauria must have had the spiral fold, which augments the extent of the intestinal tube in the Cartilaginous Fishes (§ 322 h), as is shown by the preservation of its markings on the ‘ coprolites < t * For full information on this subject, see Prof. Owen’s c Odontography,' from which the preceding statements are derived. GENERAL VIEW OF THE ANIMAL KINGDOM.— REPTILES. 491 fossilized faeces, found abundantly in the strata in which the remains of these animals are entombed. — The absorbent system appears to be very highly developed in Reptiles, the trunks being unusually large and numerous, forming extended plexuses, and sometimes being furnished Fig. 180. Anatomy of Coluber ; — l, tongue and glottis ; oe , ce, oesophagus, divided at oe' to show the heart, &c. ; t , trachea; ac , ac, carotid arteries ; ad, right aortic arch ; ag, left aortic arch ; c, left auricle; c', right auricle, vt, ventricle; v, vena cava superior; vc , vc, vena cava inferior; p, p, principal lung; p ' , rudimentary lung; /, liver; i, stomach; bit, intestine; cl, cloaca; an, anus ; o, ovary; o', ova. with pulsating dilatations or lymphatic hearts ; but they are still desti- tute of those glandulse or rather ‘ ganglia ’ (knots), which, in the higher Vertebrata, concentrate a great length of tube within a small space ■ and they are also unpossessed of valves. Moreover, they terminate by nu- merous communications with the venous system in different parts of the body. 324 r. It is, however, by the peculiar arrangement of then- Circulating and Respiratory apparatus, that Reptiles are most remarkably charac- terized. Like Birds and Mammals, they breathe air ; but like Fishes, 492 GENERAL PHYSIOLOGY. Fig. 181 . they are cold-bloocled ; that is, their respiration is not sufficiently active to enable them to sustain a fixed standard of temperature, above that of the surrounding medium. This arises from the veiy slight degree of subdivision that exists in their lungs, whereby the aerating surface is but little extended ; and from the arrangement of the pulmonary with reference to the general circulation, from which it hap- pens that the body is not svp- plied, as it is in all other Verte- brata, with blood of which the whole has just been transmitted through the respiratory organs, but with a mixed fluid, of which part alone has been aerated, while the other part consists of the venous blood that has just returned from the sys- temic circulation. The heart, in all save the Crocodiles and their allies, consists of but three cavities, one ventricle and two auricles (Fig. 181); of these auricles, one ( b ) receives tie blood from the vena cava or great systemic vein (/), the other from the pulmonary veins (p) ; both of them discharge them contents into the single ventricle (s), where the venous and the aerated fluids are min- gled; and this half-aerated blood is sent forth by its contraction, both to the system through the aorta (a, d, i), and to the lungs (o), by the pulmonary arteries (e,e) ; the pulmonary system of vessels being thus a mere offset from the general circulation. The venous blood which has passed through the capillaries of the stomach («) and of the intestines (k), is not at once discharged into the systemic- veins : but is collected by the vena porta (/), which conveys it for distribution to the kid- neys ( h ), and to the liver (g) ; and it is not until after it has passed through the capillary system of these organs, in which it is subjected to their secreting action, that it returns to the heart through the vena cava. — The lungs in most Reptiles an Circulating: Apparatus of Lizard a, a', arches of the aorta; b, right auricle ; c, vena cava superior; d, ventral aorta; e, e, pulmonary arteries; /, vena cava inferior ; g, liver and hepatic vein ; h, kidneys and renal vessels ; i, ven- tral aorta ; k, intestines ; l, vena portae ; ni, gastric vein ; n, stomach; o, lung; p, pulmonary vein ; q, brachial artery; r, single ventricle ; s, left auricle ; t, carotid artery. GENERAL VIEW OF THE ANIMAL KINGDOM. REPTILES. 493 Fig. 182. V -A very capacious ; but in their internal structure they present, among the inferior forms of the class at least, but little advance above the type which is exhibited in the air-sacs of some of the higher Fishes. There is but little subdivision of the cavity by those partitions, which, in the higher Yertebrata, separate it into air-cells ; and consequently the blood is only exposed to the influence of the air by being distributed upon the walls of the cavity, and upon the extension of the surface which is gained by the duplications or inflexions of these ; so that the capillary network is in relation with the air on one side only. In the lungs of the Chelonici, however, and in the upper part of those of some of the higher Sauria, we find a degree of subdivision which leads us towards the more complicated arrangement presented by the lungs of warm-blooded Yertebrata. The relative inferiority of the re- spiratory apparatus of Rep- tiles is further shown in the absence of those means for effecting a continual inter- change in the gaseous con- tents of the lungs, which we see in Birds, and still more in Mammals. In Batrachia and Amphibia, the lungs can only be filled with air by an action that resembles swal- lowing ; and in Serpents and Lizards, although the move- ments of the ribs can in some degree effect such a dilata- tion of the pulmonary cavity as shall occasion the ingress of air, yet this is a slow and imperfect process compared with the descent of the dia- phragm in the Mammalia. It is a curious circumstance that in Serpents the lung is single (Fig. 182, c), that of one side (c) being developed to a great length, whilst that of the other side (c') remains in a iti dimentary condition. And it is further remarkable that the lungs of those Sau- rians, which approach the Ophidian type in the great elongation of their bodies and the imperfect development of their limbs (§ 324 a), present an approach to the same inequality (Fig. 182, a, b). — In regard to the other excreting organs, it will be sufficient to mention that the urinary apparatus now manifests an advance in its condition, which is really as considerable as that of the respiratory organs ; the permanent Corpora Wolffiana of Fishes being replaced by true — Lungs of the two sides, in a, Bipes lepidopus ; in b, Bima- nus canaliculatus ; in c, Coluber natrix ; — a, trachea: b, bron- chial tubes ; c, right lung; c\ left lung; dL , pulmonary artery. 494 GENERAL PHYSIOLOGY. Kidneys. These organs are generally placed far back in the abdominal cavity, and are sometimes found even within the cavity of the pelvis. In Serpents, however, as in Fishes, the urinary organs are greatly elongated, and advance towards the head ; and it is curious to observe that the want of symmetry, which prevails in the pulmonary apparatus of these animals, shows itself also in their urinary organs, the right kidney of Serpents being placed in advance of the left, as if for the convenience of packing. The two ureters, by which the secretion is collected and conveyed away, open into the cloaca (Fig. 180, cl), which is a common vent for the discharge of the faeces, of the urinary secretion, and of the genital products. A sac is found in some Reptiles in connection with the cloaca, which has received the name of urinary bladder ; but although its mode of development is essentially the same with that of the urinary bladder of Mammalia (being the lower portion of the sac of the allantois), yet it does not appear to discharge the functions of that viscus, but ministers (in Batrachia at least) to an entirely different purpose, being a reservoir of fluid to supply the continual evaporation which takes places from their surface. — The hard dermal or epidermal envelopes with which the body is covered in the scaly Reptiles, prevent their skin from being rendered in any con- siderable degree an excreting organ ; in the naked soft-skinned Batrachia, however, the tegumentary surface takes a very important share in the respiratory function, and it also sets free a large quantity of fluid in the state of vapour. This fluid, however, would seem to be little else than water, and not to contain other excreted matters in any large amount. 324 s. In the nervous centres of Reptiles, we find a decided advance upon the type which they present in Fishes, in the much larger size of the Cerebral Hemispheres in relation to the Sensory Ganglia : still the latter make up a considerable propor- tion of the Encephalon ; and the entire brain is small in relation to the bulk of the body and to the Spinal Cord. The Cerebellum is for the most paid of veiy small size in Reptiles, being relatively less developed than in the higher Fishes ; so that it does not always cover-in that fissure between the two halves of the Medulla Oblongata, which is known in higher animals as the ‘ fourth ventricle.' (Fig. 183). — The structure of the Spinal Cord presents some very interesting variations, which are precisely ac- cordant with the arrangement of the locomotive apparatus. Thus in Serpents, which have no members, and in which every segment of the body takes a similar part in tno actions of locomotion, the spinal cord is of uniform size throughout, like the ganglionic cord of the Annelida and Myriapoda, only differing from these in the continuity of the ganglionic tract, which is not broken up into distinct centres, although its segmentation is marked by the uui- Nervous Centres fortuity of the exit of the successive pairs of spinal nerves. Uvfgan-iTaV nfee- which come off between the vertebra:. On the other hand, rebrai hemispheres ; in Frogs and Turtles, in which the locomotive power is cerXeTium S not col chiefly delegated to the extremities, there are distinct gau- venu-f k wh/ h^is §'^ ou ' c enlargements in the spinal cord, at the parts front seen below it. which their nerves issue ; and this is particularly the case Fig. 183 . GENERAL VIEW OF THE ANIMAL KINGDOM. REPTILES. 495 in the Frog in the 'posterior segment of the spinal cord, in accordance with the greater use which this animal makes of the posterior extremities. In regard to the development of the Organs of Sense, we see, as in the case of the respiratory apparatus, an advance in type, without a corre- sponding advance (indeed it may even be with a positive diminution) in functional power. Thus the nasal cavity, in all Reptiles, as in higher Vertebrata, has a posterior communication with the mouth, and through it with the air passages ; so that, of the air drawn in by the inspiratory movement, a part, at least, shall pass through the Olfactive organ. But in this organ we find no provision for extending the surface on which the nerve is distributed ; and there is every reason to believe that the sense of smell is very obtuse. — The Eye is always present in Reptiles, except in the Proteus, in which it is a mere rudiment, quite unfitted for visual use, It presents no remarkable peculiarity, except in the rudimental existence, among certain Chelonia and Sauria, of that osseous capsule which is so largely developed in the extinct Enaliosauria, and which performs a most important office in the eyes of Birds. We here first meet, however, with a special arrangement of the integuments of the face for the protection of this delicate organ ; for whilst in Serpents the skin of the head passes continuously in front of the eyes, merely becoming transparent 'where it covers the cornea, it is doubled in most other Reptiles into two folds, constituting the upper and lower eyelids, which can be drawn together by a sphincter muscle ; and we also find a rudiment of a third eyelid, formed by an additional fold of membrane at the inner angle, which is so completely developed in Crocodiles as to form a ‘ nictitating membrane,’ that can be drawn completely across the eye, as in Birds, by a muscle specially adapted for the purpose. In the Batrachia, although the upper and lower eyelids are nearly motionless, the eye can be covered by the third eyelid, as in the Crocodile. — The organ of Hearing presents a great advance in the higher members of this class, a tympanic apparatus and chain of bones being interposed between the entrance of the vestibule and the outer air ; the purpose of which is obviously to cause the sonorous vibrations to act more effectually upon the fluid in the auditory cavity. This tympanic apparatus is very imperfectly formed, however, in Serpents and Frogs ; and it is entirely wanting in the Perennibranchiate Batrachia, the condition of whose organ of hearing presents but very little advance upon that of Fishes, save that the membranous labyrinth is now entirely imbedded in bone, by the complete ossification of the capsule by which it is surrounded. It is only in the Crocodile and its allies, that the rudi- ment of a cochlea first makes its appearance ; and throughout the class, the auditory capsule contains the ‘otolithes’ which are so characteristic of the inferior grades of this organ. — The tongue in Reptiles would seem to be less an organ of Taste than of Touch ; being also modified, in certain cases, for some special purpose, as the capture of prey, or the formation of the salivary secretion (§ 324 m). The firmness of the dermal skeleton, in the scaly Reptiles, obviously unfits their general surface for the exercise of any acute tactile power ; and although in the naked species there is no mechanical obstacle to the sensibility of the skin, yet the general want of true sensory papillae seems to indicate that its feeling cannot be acute. 324 1. Notwithstanding the general advance in the development of the Nervous system of Reptiles, its functional activity would seem to be kept 49(3 GENERAL PHYSIOLOGY. down by the imperfect oxygenation of their blood ; their movements being usually slow, and their perceptions obtuse. And even when, as in the case of Serpents, we see unusual manifestations of nervo-muscular energv and of acuteness of sensation in the pursuit and capture of prey, such manifestations alternate with long periods of almost torpid repose, during which these animals seem as if they were recovering from the fatigue of such unwonted exertions. From their want of power to sustain an inde- pendent temperature, and from their exposure to a degree of winter cold which is incompatible with the maintenance of their activity, the Reptiles of temperate climates pass a considerable part of the year in a state of profound torpor (§ 99) ; and even those of tropical regions are not exempt from this periodical inactivity, the immediate cause of which, however, appears in their case to be the deficiency of moistme occasioned by extreme and continued heat (§ 66). 324 m. Reptiles are uniformly oviparous ; and, being also cold blooded, can afford but little of that assistance in the development of the ova, which is so remarkably imparted by Birds. Nevertheless we observe a decided advance in the character of the generative function, in this respect, that a sexual congress of some kind always takes place ; so that the fertilisation of the ova is not left (as in most Osseous Fishes) to the chance-contact of the seminal fluid that may have been dispersed through the neighbouring water, but is either accomplished before they have been expelled from the female passages, by the introduction of the seminal fluid within these, or in the very act of expulsion. The latter is the case in the Frog and its allies ; the former, in most of the higher Reptiles, many of which possess a penis or intromittent organ. The testes or spermatic organs of Frogs show an approach to the structure which is characteristic of higher Yerte- brata, being composed of an aggregation of numerous short caeca, the representatives of the tubuli seminiferi ; but they discharge their secretion by several excretory ducts, which open into the ureters, instead of uniting into a distinct spermatic canal or ! vas deferens.’ The ovaries of the female are composed of duplicatures of a vascular membrane in which the ova are developed, and these lie in the posterior or pelvic portion of the visceral cavity ; but, on the other hand, the oviducts are prolonged for- wards nearly as far as the heart, and there terminate by open fimbriated extremities ; the ova, therefore, set free in the abdominal cavity by dehiscence from the surface of the ovary, must travel horn the posterior towards the anterior extremity of the visceral cavity, before they can enter the dilated orifice of the oviduct which is to convey them forth. — In the higher Reptiles, the testes exhibit a decided advance in development, iu the greater length and convolution, and in the diminished multiplication, of the tubuli seminiferi ; and the products of each testis are collected into a single excretory duct or ‘vas deferens,’ which does not open into the ureter, but either discharges the spermatic fluid into the cloaca, or is con- tinued as a groove along the penis, and conveys the fluid to its extremity. The internal orifices of the oviducts in the female approach more closely to the ovaries ; and these organs present a more compact structure. The oviducts invariably open externally into the cloaca, and the ova are usually discharged soon after fertilization, if not before. In Batraehia, Optudia, and Sauria, however, there are certain species which retain then ova in a sort of uterine cavity formed by a dilatation of the oviduct near its extre- GENERAL VIEW OF THE ANIMAL KINGDOM. BIRDS. 497 j mity, until the development of their contained embryo is so far advanced that the enveloping membrane ruptures in the very act of the expulsion of the ovum, or even previously, so that the young are born alive. There is not, however, in any Reptile, the least trace of that provision for afford- ing a continued supply of nutriment to the embryo during its develop- ment, which is characteristic of true viviparous animals, and to which we have seen a slight approach in the higher Cartilaginous Fishes ; and the proper designation of this method of generation is ovo-viviparous . — In all Reptiles, save in the Batrachia (the phenomena of whose metamorphosis have been already considered, § 323), the young come forth from the egg in a condition which does not essentially differ from that of their parents, and are so far advanced towards their complete evolution, that on their entrance into the world they are at once qualified to obtain their own support, and are but little, or not at all, dependent on assistance from their parents. With this higher development, the greater relative size of the eggs in Reptiles seems to have an important connection j for their in- creased bulk, as compared with that of the eggs of Fishes, is given by the larger provision of nutrient material which they contain • and we accord- ingly find that whilst, in the higher Reptiles, the number of eggs is com- paratively small, the Batrachia alone, whose young come forth at an earlier grade of development than those of other orders, and whose eggs do not contain yolk enough to serve for the evolution of the animal on the perfect Reptilian type, deposit ova at all comparable numerically with those of Fishes. 325. From Reptiles to Birds, the transition would seem rather abrupt, since the latter class is in its general characters the almost precise oppo- site of the former. But we have seen in the class of Reptiles a special modification of the Saurian type to the life and habits of a Bird ; and the class of Birds presents us with certain forms, which are deficient in some of the distinctive attributes of the group, and which show an obvious degradation towards the Reptilian type. Taken as a whole, however, the members of this class exhibit a remarkable conformity to one general type, presenting in this respect a marked contrast to the diversity which we have seen to prevail amongst Reptiles ; and this conformity is both struc- tural and physiological, being shown alike in the plan of organisation, and in the working of the organs. — Birds have been denominated, and not inappropriately, “the Insects of the Yertebrated series.” As in the ani- mals of that class, we find the whole structure peculiarly adapted for motion, not in water, nor upon solid ground, but in the elastic and yield- ing air. It is impossible to conceive any more beautiful series of adapta- tions of structure to conditions of existence, than that which is presented to us in the conformation of the Bird, with reference to its intended mode of life. In order to adapt the Yertebrated animal to its aerial residence, its body must be rendered of as low a specific gravity as possible. It is further necessary that the surface should be capable of being greatly extended ; and this by some kind of appendage which should be extremely light, and should at the same time possess considerable resisting power. Tiie degree of nervo-muscular energy required for support and propulsion ; through the ah-, involves the necessity of a very high amount of respira- tion (§ 67); and as the general activity of the vital processes depends greatly upon the high temperature (sometimes reaching 112°) which this K K 498 GENERAL PHYSIOLOGY. energetic respiration contributes to maintain, a provision is required for keeping in this heat, and not allowing it to be carried away by the atmo- sphere, through which the Bird is rapidly moving. Xow of these four objects, the lightening of the body and the extension of the respiratory surface are beautifully attained, in a mode which will be found to corre- spond closely with the plan adopted for the same purpose in Insects (§ 315 a). The air which enters the body is not restricted to a single pair of lungs placed in the upper part of the visceral cavity, but is transmitted from these organs to a series of large air-sacs, disposed in the abdomen and in various other parts of the body ; and even the interior of the long bones is made subservient to the same purpose, their shafts being hollowed out, and lined with a delicate membrane over which the blood-vessels are minutely distributed. In this manner the respiratory surface is greatly extended ; whilst by the large quantity of air introduced into the mass, its specific gravity is diminished.* Again, the extension of the external sur- face, and the retention of the heat within the body, are accomplished by the same beautiful and refined contrivance, the covering of feathers. Like hair or scales, feathers are to be regarded as epidermic appendages to the cutis. The stem is formed by an apparatus which may be likened to a £ hair-follicle ’ on a very large scale (§ 180) ; but there are some additional parts for the production of the laminae that form the vane of the feather, which are joined to the stem during its development. These laminae, when perfectly formed, are connected by minute barbs at their edges, which hook into one another, and thus give to the entire vane the necessaiy con- tinuity of resisting surface. The substance of which feathers consist, is a very bad conductor of heat ; and when they are lying one over the other, small quantities of air are included, which still further obstruct its trans- mission by their non-conducting power. In the two most aberrant forms of the class, however, one of which leads us back towards Reptiles, whilst the other presents certain approximations to the Mammalia as well as tc Reptiles, we meet with remarkable modifications in the typical structure of feathers. In the Penguin , those which cover the wings have a strong resemblance to scales ; and the wings are not employed to raise this bird in the air, but only to propel it through water (as fins would do) by their action on the liquid. On the other hand, in the Ostrich tribe, which consists of Birds adapted simply for running on the ground like Mammals, the laminae of the feathers are so separated from each other, as no longer to form a continuous surface ; in the Emeu they resemble mere branching hairs, of which two arise from each root, one of them being the ‘accessory plume ’ of other feathers developed to the same size with the principal shaft : and in the Cassowary some of them are developed as shafts without any vane, so as almost to resemble Porcupine’s quills. * The degree in which the bones of Birds are thus connected with the respiratory function, and their interior occupied with air instead of with marrow, bears a very exact proportion to their powers of flight (§ 206). In the Ostrich and its allies, the cavity of the shaft of the long bones is partly occupied by medullary cancelli; and it was from finding this cavit; in the fragment of a femur of the extinct Dinornis first submitted to him, still more divided into cancelli than that of the corresponding bone in the Ostrich, that Prof. Owen ventured to predict, with true philosophic sagacity, that the Struthious Bird to which that bone belonged must have been still more wingless than the Ostrich itself, — a prediction which has been amply verified by the result, since remains of many hundreds of individuals have now been collected, without the least trace being discovered of even such rudimentary wing-bones as the Ostrich possesses. GENERAL VIEW OF THE ANIMAL KINGDOM. BIRDS. 499 325 a. The bony framework constituting the Bird’s vertebral skeleton, presents many remarkable adaptations to the same purposes. In the first place it is to be remarked, that the faculty of locomotion is here most com- pletely taken from the spinal column and delegated to the extremities ; so that the skeleton of the trunk must be consolidated, in proportion to the power with which the members are to be endowed, as we have seen in numerous other cases. The relative length of the neck to that of the body varies considerably, and seems to follow no constant rule ; thus we find it very great in the long-legged Waders and Struthious birds, as if to enable their heads to reach the ground without flexure of their limbs ; but it is equally great in the Swan and in some other short-legged aquatic birds, in which its purpose is obviously to allow the bill to be plunged deep into the water in search of food, whilst the body is floating upon the surface. The tail, on the other hand, is always very short, or even almost rudimentary ; and it serves no other purpose than to support the tail-feathers, by whose agency the birds of active flight are steered (as by a rudder) in their rapid course through the air. — In each of the vertebras of the * neck of Birds, whose num- /■ her ranges from 9 to 23, 1 we find the rudiments of all the principal elements of the m neural, haemal, and lateral arches, consolidated into a single bone, and including four canals, — the superior, or neural, for the passage of the spinal cord ; the inferior, or haemal, for the passage of the two carotid arteries, here brought together on the median line: and the two Inf '1 f , ±\ p Skeleton of Vulture; — a, cervical vertebrae; b , clavicle: lttlGlcll IOr tllG p&SSclgG Ol c, carpus ; d, phalanges; e, sternum; f, tibia; g, tarsus ; h , the vertebral arteries whilst ^ones fbre-arm ; i, humerus ; /<*, sacral vertebrae ; l , femur ; , ? m, caudal vertebrae. they also enclose the trunks of the sympathetic nerve. The articulating surfaces of these vertebrae are connected, like those of other joints, by regular synovial capsules, the bodies of the vertebrae not being held together by intervertebral fibro- cartilage ; so that great mobility is provided for, whilst the requisite tenacity is given by the numerous muscles and ligaments which con- nect the vertebrae with each other. In the posterior part of the cervical region, we find the pleurapophyses elongating, and becoming detached from the ‘transverse processes,’ so as to constitute ‘cervical ribs ’ ; and in some Birds one or two pairs of such ribs attain considerable length. The proper dorsal portion of the spinal column, which is considered to commence with the first vertebra whose ribs are attached to the sternum, presents a remarkable contrast to the cervical in regard to K k 2 500 GENERAL PHYSIOLOGY. the mode of connection of its component segments ; for the dorsal ver- tebrae, which never exceed 11, and are commonly no more than 7 or 8 in number, are so united as to provide for the greatest fixity which is compatible with the degree of mobility that the osseous frame- work is required to possess for respiratory and other purposes. This is especially seen in the Birds of most rapid and powerful flight, in which the bodies of a part of the dorsal vertebrae are ‘ anchylosed ’ (or united by continuous ossification) to each other ; some one or more of the articula- tions, however, being usually left free, so that the body can be inflected from side to side, which is especially necessary in such as seize their prey whilst on the wing. But the union of the bodies of the vertebrae is not the only mode of consolidation adopted ; for the spinous processes (neural spines), also, are frequently anchylosed to each other ; whilst the transverse processes (diapophyses) send off splint-like prolongations, which overlap those of the contiguous vertebrae. In the Struthious birds and Penguins, on the other hand, which cannot fly, all the dorsad vertebrae are moveable one upon another. The anterior cervical ribs are connected only with the diapophyses (Fig. 167, j >ed from independent centres ; and retain their individuality through life in some, as the Apteryx and Penguin, in which they are even provided with proper muscles for their elevation and depression ; so that them relation to the limbs, through such rudimental forms of the latter as are presented by the Lepidosiren, Proteus, etc. (§ 320 Z), becomes apparent. The ster- num (Fig. 183, e ) is more remarkably developed in Birds than in any other class of Yertebrated animals,* in accordance with the enormous size of the muscles for the depression of the wings, to which it furnishes the '■hie* attachment. Its peculiar character is given to it, not merely by such an enormous increase in its length and breadth that it covers the ventral surface of a great part of the abdominal as well as of the thoracic cavity (extending in some cases even to the pubis), but also by the projection of * We have seen that of the ‘plastron’ of the Chelonian Reptiles, which is commonly described as a sternum whose elements are extended lateral!}-, only a small part is > insti- tuted by the true sternum, the remainder bein? made up of sternal ribs and dermal bones ($ 324 h). GENERAL VIEW OF THE ANIMAL KINGDOM. BIRDS. 501 its central portion as a keel or ridge, the degree of prominence of which bears a regular proportion to the strength of the pectoral muscles, and consequently to the power of flight. This is well seen in those exceptional forms, whose wings are merely rudimentary ; for the Ostrich, Emeu, &c., have a comparatively small sternum, entirely destitute of keel. The ribs are developed (except iu some aquatic birds in which the last pair does not present itself) from all the vertebrae intervening between the scapular and pelvic arches ; although the last pair is seldom continuously connected with the sternal ribs and sternum. Hence all these vertebrae rank as ‘ dorsal,’ and the usual ‘ lumbar ’ region would seem to be wanting in Birds : but it has been pointed out by Prof. Owen, that we are probably to regard as lumbar vertebrae those that form the anterior part of the sacrum, and are connected at their sides with the ilia ; which bones are prolonged very far forwards, and are connected with a far larger number of segments than we elsewhere find giving support to them. The vertebrae which are ancliylosed together to form the sacrum of Birds (Fig- 183, X - ) are scarcely ever fewer than 10 in number, and are sometimes as many as 19; the greatest number presenting itself, as might be expected, in Birds whose posterior extremities are those chiefly concerned in the propulsion and support of the body.- — The caudal portion of the spinal column (Fig. 183, m) is made up of from 7 to 9 vertebrae, gradually dimi- nishing in size from before backwards, but terminating in a singularly formed bone, like the body of a vertebra in size, but compressed laterally so as to form a sharp edge above and sometimes below. It is with this bone that the tail feathers are more particularly counected, whose function is the important one of guiding the bird in its movements through the ah. 325 b. The scapular arch is still formed upon the type which we have seen to prevail through the Oviparous Yertebrata in general ; for the scapula, a long narrow bone lying upon the thoracic ribs, is chiefly connected with the sternum by the strong coracoid bone, which is firmly articulated to the scapula where it helps to form the glenoid cavity, and with the sternum at its opposite extremity : whilst the clavicles, united with each other on the central line so as to form a single bone (known as th efurcula or ‘ merry-thought’), constitute an independent arch, which is much less closely connected with the shoulder-joint, and of which the centre or keystone is connected either by ligament or by osseous junction with the summit of the sternal keel. It is interesting to remark that, in the Ostrich tribe, the two clavicles are separated from one another at their median extremities, but are ancliylosed to the scapula ; a condition which exists in the Turtles, and thus indicates a certain relationship, however slight, between these tw T o groups. The clavicles are sometimes entirely wanting, or are merely rudimentary, especially in certain birds which live much upon the ground, or are climbers among trees, having little power of active flight ; whilst they are strongest, and their arch most open, in birds of great force of wing, in which their function is to oppose the forces that would tend to drag the shoulder-joint towards the median plane, in the downward stroke of the wing. Notwithstanding the modification which the anterior extremity has externally undergone, in order to con- vert it into an instrument of flight, it is not difficult to recognise its osseous elements as corresponding to those of other Yertebrata. The 502 GENERAL PHYSIOLOGY. humerus (Fig. 185, a) varies considerably in its relative length, even in birds of great powers of flight, being short in the Swifts and Humming-birds, whilst it is two-thirds the length of the body in the Albatross and other sea-birds. At the elbow-joint (i), we sometimes find a detached ‘ sesamoid bone,’ analogous to the ‘ patella’ of the lower extremity ; this in Mam- malia is usually anchylosed to the ulna, of which it forms the ‘ olecranon process.’ The two bones of the fore-arm, the ulna and radius, are some- Bones of the wing of Falco gyrfalco (Jerfalcon) ; ted together and to the metUCRlpal generally discoverable. In the Struihious birds, the bones of the anterior extremity are reduced to then smallest dimensions ; still affording support in most cases to wing-feathers, and being moved by muscles which enable these rudimentary wings to give some assistance to the animals in their rapid course over the ground ; whilst in the Apteryx they do not appear to give any assistance in the act of locomotion, but serve only to support a horny spur which it uses as a defensive weapon.— The pelvic arch is not merely remarkable for the great number of sacral vertebrae which are auchylosed together to afford it support ; but it contrasts very strongly with the scapular arch in this respect, — that whereas in the latter tin' greatest strength is found at its centre or keystone, where resisting power is most required, in the former there is an incompleteness at that very part, neither the ischial nor the pubic elements of the two halves of the pelvic arch meeting on the median line. This incompleteness seems to have inference to the relatively larger size of the eggs of Birds ; since to allow their passage through a complete bony ring, would have involved an Fig. 185. times nearly equal in size, but where there is a marked difference the ulna (6) is always the strongest ; they are so connected with each other and with the humerus, that scarcely any movement of rotation is permitted in the fore-aim, the firm hinge-like mo- tion of the elbow-joint being what is alone required in the act of flight. The bones of the hand are extended in length, but drawn together (as it were) laterally ; and the movements I 111 of the wrist-joint (ii) are limited to the hinge-like action requisite for the folding and extension of the wing. The carpal bones are only two in v number, and these support two prin f cipal metacarpal bones (c) anchylosed together at both extremities, with another separate bone (o) which is to be regarded as a rudimentary thumb. Of the digits, only one (1) is con- siderably developed, possessing two el- even three phalanges, which are uni- of another digit, or even of more, are GENERAL VIEW OF THE ANIMAL KINGDOM. BIRDS. 503 increase in its dimensions, which would have been unsuitable to the general conformation of the posterior part of the trunk in birds of flight. And it would seem as if the elongation of the sacrum was intended, by giving a firmer attachment to the corresponding elongated ilium, to afford the requisite fixation to the hip-joint, in default of the support which it usually receives from below. The ilium is not merely very much length- ened, but it is even anchylosed to the sacral vertebrae ; and as the ischial and pubic bones are anchylosed to it (forming an ‘os innominatum’ as in Man), the entire pelvis presents a remarkable degree of firmness, not- withstanding the incompleteness of its descending arch. This consolidation of the pelvis takes place to the most extraordinary extent in the Struthio- nidce, in which the iliac bones arch over and meet each other on the dorsal surface of the sacrum, whilst in some instances the ischiac bones do the same on its ventral surface, so that the sacrum is enclosed between a pair of bony walls formed by the ossa innominata. In the Ostrich, too, we find the under side of the pelvic arch completed by the slender pubic bones, which arch downwards towards one another, and meet on the median line ; this junction of the pubic bones, whilst the ischial remain, separate, is a decidedly Mammalian character. In the con- formation of the lower extremities, we have chiefly to notice that the femur (Fig. 1 84, l) remains short, even wdien the general elongation of the limb is the greatest ; this elongation being given by the extraordinary development of the tibia and metatarsus. The fibula is rarely present as a separate bone, being usually anchylosed with the tibia (/), and sometimes being almost undistinguishable. The tarsal bones are anchylosed with the metatarsal at an early period in the life of most birds, though rudi- ments of them may often be subsequently detected. The metatarsus is represented in most Birds by a single bone (y) in each leg ; but this bone is usually grooved, in such a manner as to show that it is made up of tlnee bones fused together ; and these bones actually remain separate in the Penguin through a considerable part of their length. The bones of the foot are usually developed considerably, so as to afford to the bird the extent of support necessary for the maintenance of its equilibrium ; in those which inhabit marshy places, the toes are not only elongated, but radiate widely from each other, so as to render these birds less likely to sink into the oozy ground on which they tread ; whilst in the aquatic birds, they support the web by whose agency the body is propelled along the surface of the water. The typical number of true digits is four ; but in Gallinaceous Birds we find the metatarsal bone bearing a ‘ spur ’ on its inner side, which is perhaps to be regarded as the rudiment of an addi- tional toe ; whilst in the Ostrich, on the other hand, the number of toes is reduced to two. The number of phalanges in these digits increases regularly from within outwards ; being two in the innermost of the ordi- nary toes, which ranks as the first ] three in the second ; four in the third ; and five in the fourth or outermost. Of the four moveable toes possessed by all save the Struthious Birds, the innermost is turned backwards, while the other three are usually directed forwards ; the prehensile power of the foot is increased, however, in the Scansores (Parrot and Woodpecker tribe) by the opposition of two of the toes to the other two. 325 c. In the structure of the Cranium of Birds, we trace, more than in any other part of their Osteology, the striking contrast between this 504 GENERAL PHYSIOLOGY. group and the preceding ; for whilst the skull of Reptiles is characterized by the number of pieces of which it remains permanently composed, that of Birds is remarkable for the closeness of the union which is earl) established between the different parts ; in this respect differing as much from Mammalia, as Mammalia differ from Reptiles. If the skull of a Bird be examined at a sufficiently early period, the several elements of the neural arches of its cranial vertebrae may be readily distinguished ; but the ossifying process advances so rapidly, that the sutures soon become obliterated, and the whole brain-caffe seems formed of but one piece. — the tympanic portion of the temporal bone, which forms a part of the maxillary arch, being the only element that remains detached. The boues of the face, however, do not undergo the same consolidation ; for they are always so connected with those of the cranium, as to possess considerable mobility ; and their union amongst each other is seldom complete, hi their number and relative position, they correspond with those of Mam- malia ; but they differ greatly in their forms and proportions. The principal part of the elongation of the upper jaw, constituting the palato-maxillary arch, is due to the great development of the intermax- illary bone (haemal spine), of which the two halves usually coalesce at an early period of development ; with this, the slender maxillaries and the palatine bones are connected by bony union ; and the three pairs together constitute the most anterior of the haemal arches (the palato- maxillary), which is in great part detached from the other bones of the face, and is so connected by the malar or zygomatic bone with the tympa- nic or ‘ os quadratum,’ that the movements of the latter are in some degree transferred to it. This tympanic bone, as in all the Vertebrata already con- sidered, is interposed between the cranium and the lower jaw ; being, iu fact, the pleurapophysis of the frontal vertebra, with which the proper haemal arch that constitutes the jaw is articulated. In Mammalia it is always anchylosed with the temporal bone ; and thus its presence as a separate element is an invariable mark of the Oviparous cranium. It is curious to observe, in the Rodent Mammalia, among several other points of degradation towards Ovipara, that the tympanic bone long remains detached from the temporal, although it does not enter into its articula- tion with the lower jaw ; whilst, on the other hand, in some of the Stru- thionidse, the tympanic bone is so fixed that it can add very little by ifs own movements to the mobility of the jaws, thus presenting an approxi- mation to the Mammalian type. The lower jaw, which, with the tym- panic bone, constitutes the temporo-mandibular arch, is originally com- posed, like that of Reptiles, of six pieces on each side, making twelve in all ; but the number of these is soon diminished by then - mutual coalescence. This begins in the two pieces, one on each side of the median line, which form the body of the jaw ; for these, like the intermaxilhuies of the upper jaw which they represent in their own segment (being, ! ; ke them, haemal spines) coalesce completely at the symphysis ; and an union of other portions follows, until there are only two pieces left on either side of the jaw, and these are very firmly connected by suture with each other and with the central or symphyseal element. The traces of the original separation of these parts remain longest in those aquatic birds which shew the closest approximation to the Reptilian type ■ whilst, on the other hand, the sutures themselves are partly obliterated by complete osseous union GENERAL VIEW OF THE ANIMAL KINGDOM. BIRDS. 505 in the Struthionidce. The hyoidean arch, which was found to be so highly developed among Fishes in connection with their branchial appa- ratus, and which is so nearly rudimentary in Reptiles, again rises into importance in this class, in which it is subservient to the actions of the complicated laryngeal apparatus. It is considered by Prof. Owen, how- ever, that a large part of what is usually reckoned as the os hyoides of Birds really belongs to the visceral skeleton ; for we find, in addition to the small cerato-hyals (Fig. 170, 40) and basi-hyal (41), which form the hasmapophyses and haemal spine, with the glosso-hyal (42), projecting for- wards, and the uro-hyal (43) passing backwards, a pair of long branches, analogous to the ‘ great cornua ’ of the human hyoid (Fig. 191,2), but very much more elongated, especially in the Humming-birds and Woodpeckers, in which they pass round the back and top of the head, to be inserted into a pair of canals on the sides of the upper mandible (§ 325 i). These are stated by Prof. Owen to consist of retained portions of the branchial arches, -which appear at an early period in the development of the Bird, and of which every other portion speedily vanishes. On the other hand, the stylo-hyal and epi-hyal elements (together making up the pleurapo- physes of this segment) are not developed into bone, but remain ligamentous. The skull of Birds is articulated to the spinal column by a single tubercle, which projects from the occiput, and is received into a hollow, on the body of the ‘ atlas ’ or first cervical vertebra. This arti- culation allows much greater extent and freedom of motion to the head, than exists in the Mammalia. In the Penguin, however, part of the lateral or condyloid portions of the occipital bone are included with the basilar in the articulating tubercle, as in the Chelonian Reptiles ; whilst in the Ostrich, there is a vertical fissure in the tubercle, which indicates an approach to that separation of the articulating surface unto two halves, which is characteristic of the Mammalia. 325 d. The covering of feathers, which so remarkably distinguishes Birds from all other Vertebrated animals, constitutes their sole dermal skeleton ; with the exception of the homy scales that invest the lower part of the legs and feet, and the horny sheath which encases the bony jaws, and forms the 1 bill.’ This last is developed into various forms, in accordance with the uses to which it is to be applied by the Bird in the search for, or the prehension of, its appropriate food ; and accordingly its shape is so far an index of the general habits of the species, that it fur- nishes characters of great importance to systematic Ornithologists. These, however, need not here detain us ; it being sufficient for our present pur- pose to know that the horny bill serves the purpose of teeth, which it replaces in this group (as in Chelonian Reptiles, § 324 i, o), in seizing, cutting and tearing the food ; but the act of trituration or mastication it is entirely unable to perform. In the Parrot tribes, the strong hooked beak is also used as an instrument for locomotion ; the bird frequently raising itself in climbing, by employing it like a third foot. 325 e. As might be expected from the analogy of Birds with Insects, the development of their organs of nutrition (excepting that of the respi- ratory organs) is much less striking in point of size, than is that of the locomotive apparatus ; and we find the abdominal portion of the visceral cavity small in size in comparison with the bulk of the body. But what is wanting in size is made up in activity of function • and the demand for 506 GENERAL PHYSIOLOGY. food is more active in Birds than in any other animals. As their food is not subjected to any mechanical reducing process within the cavity of the mouth, the salivary secretion (except in some special cases) is only required for its lubrication previously to being swallowed. Accordingly, the salivary glands are but little developed ; and it is interesting to observe that in certain Birds the secreting follicles constitute part of the tongue itself, as in some of the Reptilian class. In nearly all the mem- bers of this class, the food is delayed in a receptacle formed by the dila- tation of some part of the mouth, pharynx, or oesophagus, before it passes into the stomach ; and in this receptacle it sometimes undergoes a sort of preliminary digestion. In the Pelican we find it in the form of a wide pouch suspended from the lower jaw, the tw T o halves of which bone are not united at the symphysis, so that the pouch is enormously dilatable : n the Swift, and other birds that catch insects on the wing, a similar dis- tensibility exists in the membranous wall of the ‘ fauces,’ or upper part of the throat ; in the Cormorant and other fishing birds, it is the wide oesophagus which serves as the receptacle ; but in the grauivor ms birds, as well as in many others which take in a large quantity of food at once, we find a pouch, termed the £ crop ’ (Fig. 186, b), deve- loped from the side of the gullet, into which the food passes when it is first swallowed, and in which it lies for some time; during which it is moistened by a secretion copiously poured out from glandular follicles in its walls, and is thus prepared for the further stages of the digestive pro- cess. Below the crop, the oesophagus is usually again dilated, before its termination in the gizzard, into another cavity, known as the ‘ proveutri- culus’ (c), from the walls of which, the true gastric fluid is poured out upon the food that is delayed in it ; this may be considered as the firtt or cardiac portion of the true gastric cavity ; whilst the ‘ gizzard" («') or muscular sac which succeeds it, corresponds with its second or pyloric portion. The gizzard is usually a somewhat lengthened sac, having its intes- tinal orifice, as well as that by which it communicates with the proven- triculus, at its upper part. In those whose food requires the aid of mecha- nical reduction for its solution, the walls of the gizzard are very thick and muscular, and its fibres radiate from two central tendons situated on opposite sides of the cavity, which is lined by a horny epithelium of remarkable toughness. The triturating power of the gizzard is frequently much increased by the presence of hard angular stones, which the bird occasionally swallows, and which are retained in its cavity. In Birds, how- ever, whose food consists of flesh, fish, succulent fruits, or other substances easily reduced, the muscular structure is so little developed, that the ordinary character of the £ gizzard’ is entirely wanting. The intestinal tube varies considerably in length, and also in the degree of defmitene's of the division between the small and the large intestines. At no great distance from the stomach, it receives the secretions of the liver (e) and of the pancreas (A) ; the former gland, which is of moderate size, possesses a gall-bladder (/), in which the bile can be stored up when not required for use, and yet has a second excretory duct (g ) by which the bile can he poured direct into the intestinal tube ; the latter, which is now much increased in size, and has attained a higher grade of glandular developmcni, still pours forth its products, sometimes by two ducts, in other cases by three. After a larger or smaller number of convolutions, the intestinal GENERAL VIEW OF THE ANIMAL KINGDOM. BIRDS. 507 Fig. 186. I tube terminates in the cloaca (o) ; first, however, receiving the products secreted from one or two csecal appendages (k), whose opening into the canal is considered to mark the boundary between the small and the large intestines (l and V). — The Absorbent system of Birds, al- though not so extensive in ap- pearance as that of Reptiles, is really developed upon a higher i type, since we find its vessels not merely provided with valves, but in some parts possessed of the so- called ‘ glandulse,’ which may be more compared to knots formed by the convolution of the tube, and in which (as will be shown hereafter, chap, xi.) it is probable that the elaborating function of this appa- ratus is performed with peculiar activity. . The greater part of the fluid taken up by the Absorbents is collected into two dorsal trunks, or ‘thoracic ducts,’ which pour their secretion into the veins of the neck, as in Man ; but we do not yet lose either the ‘ lymphatic hearts’ of Reptiles, or the com- munications of the absorbent sys- tem with the venous in the pos- terior part of the body. The Spleen is of very small size. 325/ The Circulating appara- tus of Birds is formed upon what is called the ‘ complete double ’ type ; the pulmonary and systemic circulations being entirely distinct from each other, each having an organ of impulsion at its com- mencement ; and the relation of the two being such, that every part of the blood which has passed through the system must be transmitted through the lungs, before being again sent into the body. Thus we have the systemic heart of the Mollusca superadded (so to speak), to the respiratory heart of Fishes ; a super- addition of which we saw some traces in the Cephalopoda, whose systemic and respiratory hearts, however, were detached from each other (§ 307 b). In the warm-blooded Vertebrata, the pulmonic and systemic hearts are brought together, in accordance with that principle of concentration which elsew T here prevails, so as to form but one organ ; and this con- tains two auricles or receiving cavities, and two ventricles or propelling- cavities (Fig. 192). The blood returning from the system is discharged by the vena cava into the auricle of what may be termed the pulmonic Digestive apparatus of Fowl : — «, asophagus ; b , crop ; c , ventriculus succenturiatus ; d , gizzard ; e, liver ; /, gall-bladder; g , hepatic ducts; //, pancreas; i, duo- denum; k , caecum; l, small intestine; l', large intes- tine; m , ureter; n, oviduct; o, cloaca; p , anus. 508 GENERAL PHYSIOLOGY. heart, which is situated on the right side ; from this it is emptied into the ventricle, and by its contraction is propelled, through the pulmonarv artery, to the capillaries of the lungs. After undergoing aeration in those organs, it is returned by the pulmonary veins to the auricle of the left or systemic heart ; and being discharged by it into the ventricle, it is pro- pelled by the latter along the great systemic trunk or aorta, by the ramifications of which it is carried into the capillary network of the whole body, to be returned again by the veins. Now it is peculiar r Birds among Yertebrated animals, that the exposure of the blood to the influence of the atmosphere takes place, not merely during the pulmoiurv. but also during the systemic circulation ; for the vessels which ramify on the walls of the air- sacs dispersed over different parts of the body, and on the membrane lining the hollow bones, are supplied from the neighbour- ing trunks, and not by prolongations of the pulmonary system. Thus wo are carried back to that intimate union of the systemic and respiratorv circulations, which exists in the class of Insects (§ 315 a). — The trachea or windpipe, by which the air receives admission to the lungs, is furnished at its upper part in Birds with a ‘ larynx,’ which is obviously the equivalent, both in position and structure, of that of Mammalia ; this larynx, how- ever, seems rather to be connected with the regulation of the supply of air to the lungs, than with the production of vocal tones ; for the organ of voice in Birds is a second larynx, which is peculiar to the class, and is situated at the lower end of the trachea, just above its bifurcation into the two bronchial tubes which proceed to the right and left lung*. The structure of this apparatus will be described hereafter (chap. xxii). The Lungs of Birds present us with an immense advance in their type of structure, upon that which is to be found even amongst the highest Reptiles ; being so minutely subdivided, as entirely to lose the character of hollow air-sacs, and to present instead a somewhat spongy texture. Still we shall find hereafter, that, in their minute structure, they corre- spond rather with the lungs of Reptiles than with those of Mammals : each lung of the bird being composed, as it were, of an aggregation of small ‘lunglets,’ every one of which is furnished with its own bronchial tube, and is a miniature representative of the lung of the Frog. From the lungs of Mammalia, again, those of Birds differ in not lying hoc in the thoracic cavity, but in being bound down on their dorsal aspect t- its walls, as in many Reptiles ; and the thoracic cavity itself differs fi < on that of Mammals in this essential point, that it is not closed-in below by a complete diaphragm, but is only separated from the abdominal cav it y by the serous sacs which respectively line the two, and by the partial extension of the tendinous expansion of the imperfect diaphragm over the bases of the lungs, on which it loses itself. The pleural sac which sur- rounds the lungs is itself made part of the respiratory surface, the air being admitted into its cavity by bronchial tubes (Fig. 187,5). whose open orifices ( d ) may be seen upon the surface of the lungs ; and from the pleural sac it passes into the peritoneal, through perforations in them walls which also pierce the diaphragm. It is by reflected prolongations of these membranes, that the ‘ air-cells ’ are formed, which occupy, when distended, a large part of the visceral cavity. These communicate freely with each other, with the cavity of the long bones, and with a set of tegumentary air-cells, which seem to take the place of the ‘ bursae mucosae GENERAL VIEW OF THE ANIMAL KINGDOM.— BIRDS. 509 that are elsewhere found to be interposed among the muscles and tendons, for the purpose of preventing them from pressing unduly upon one another. Several mechanical advantages seem to be gained by the peculiar arrangement of these air-cells, which is adapted in each case to the particular wants and habits of the species ; but such uses are all sub- servient to the essential purpose of these cavities, which is to contain a supply of air that may at once diminish the specific gravity of the body and contribute to the aeration of the blood. The mechanism Fl<3, 1 ' ’ ' ' by which the interchange of the air in the lungs and visce- ral air-cells is effected, is as peculiar as are the provisions we have already noticed for the energetic performance of this function. From the elasticity of the bony framework by which the thoracico-abdominal cavity is surrounded, the state of fulness is natural to it, and that of emptiness is forced. When the lungs and air-cells are filled with air, they are partly emptied by the agency of muscles which draw the sternum nearer to the spinal column, and which consequent- ly diminish the capacity of the visceral cavity ; but when these muscles are no longer caused to contract, the sternum springs Olltwards again, the cavity is Pulmonary apparatus of a Pigeon, as seen on removing restored to its original dimen- the anterior \vall of the thorax;- a, trachea; b, bronchi; P c, lungs ; a , apertures of communication with air cells. sions, and the air rushes m to fill the lungs and air-cells. The distension of the lungs is further aided by the contraction of the diaphragm, which, by its attachment to their bases, draws these downwards in the act of inspiration. This muscle is more developed in the Struthionidce, than in any other birds ; and it is most interesting to see how closely, as regards the respiratory functions, some of this tribe approximate to Mammals. The small sternum of these birds cannot take the same share in the process of filling and emptying their pulmonic cavities, that it performs in the rest of the class ; and some other provision is requisite. This we find most complete in the Apteryx ; in which the abdominal cavity is entirely separated from the thoracic by a musculo-tendinous diaphragm, capable by its contraction of enlarging the capacity of the latter, as in Mammalia ; and although the air passes into the pleural cavity, it does not proceed elsewhere, neither into the abdominal cavity, nor into the bones, which are occupied by marrow ; so that, although the amount of respiration in this bird is doubtless much less than is usual in the class, the function is performed upon a plan which shows a decided advance towards the Mammalia. — It 510 GENERAL PHYSIOLOGY. is interesting to observe that a more exact symmetry prevails in the arrangement of the circulating and respiratory apparatus of Birds, than we find in the Mammalia ; and this is evidently connected with the neces- sity involved in the peculiar mode of locomotion of the former, of an exact balance between the weight of the two sides of the body, and of a perfect equality in their functional power. The lungs and air-sacs arc precisely similar in size and situation on the two sides ; consequently the heart is placed on the median plane ; and the mode of origin, from the aortic arch, of the trunks supplying the head and upper extremities, is alike on the two sides, — there being on each an ‘ arteria innominate,’ that subdivides into the carotid and subclavian arteries, the latter of which is chiefly distributed on the great pectoral muscles, the vessel that repi e- sents the ‘brachial artery’ of Man being quite a subordinate branch instead of constituting the main trunk. 325 <). The great nervo-muscular activity of Birds, which involves the necessity for a large and constant supply of food, for a rapid circula- tion, and for an energetic respiration, also requires that ample provision shall be made for carrying off that part of the waste of the system, which it is not the function of the liver and lungs to eliminate ; and we accord- ingly find that the Kidneys of birds are very large, and their functional activity great. A large part of these organs lies within the pelvic cavity, where they occupy deep depressions on the internal surface of the pro- longed ilia ; bitt they extend anteriorly as far as the lungs. The urinary secretion, which contains very little water, but is loaded with mate of ammonia, is discharged from each kidney through several excretory ducts ; but these unite to form the ureter on each side, and the two ureters terminate separately (Fig. 186, m), very near to the orifices of the genital ducts, in a portion of the cloaca which has hence received the designation of the ‘ urethro-sexual canal.’ Between the termination of the large intestine and this urethro-sexual canal, the cloaca generally exhibits more or less of a pouch-like dilatation, which is bomided, both above and below, by valvular folds ; and this, which is obviously the rudiment of a urinary bladder, attains its greatest development in the Ostrich and its allies, thus presenting another character of approximation to the Mammalia. — The skin would not appear to contain, in this class, any large amount of excreting glandulse. It is obvious that there will be little need of cutaneous exhalation, as a means of keeping down the temperature of the body, where every means is adopted to maintain this at a high fixed standard ; whilst, again, as Birds drink so little that even their urinary secretion is almost solid, there is but little superfluous water in their bodies to be thus eliminated. One superficial gland, however, known as the Glandula Uropygii, which nearly all Birds possess, deserves special mention ; this is situated above the last caudal vertebra, and its office is to secrete an oily fluid, which is applied to the feathers by the bill, and thus preserves them from being wetted by water. However short may be the neck of Birds, it is always long enough to enable the bill to reach this gland, and to carry its product to every part of the surface ; and, as might be expected, the gland is the largest, and its secretion most abun- dant, in the aquatic Birds, whose plumage is thus enabled to keep *he surrounding medium from contact with the skin, which would otherwise be chilled by it. GENERAL VIEW OF THE ANIMAL KINGDOM. BIRDS. 511 3 25 h. The condition of the Nervous System in Birds shows a decided advance upon the Reptilian type, in the larger proportion which the Encephalic centres bear to the Spinal, and in the greater development of the Cerebrum and Cerebellum in relation to that of the Sensory Ganglia. The optic ganglia, however, still form a very conspicuous part of the brain, the cerebral hemispheres not extending sufficiently far back to cover them completely when viewed from above ; and we find that the instinctive propensities still take the lead in prompting and guiding the actions of the animal, although they are often modified (in a manner and degree which strikingly contrast with the fixity of the methods followed by Insects) by the operations of their superior intelligence, in which we can indubitably trace an adaptation of means to ends which springs from their own design or purpose, not being provided for in the ordinary in- stinctive habits of the species. The proportionate development of this intelligence, and of the instinctive propensities, varies considerably in the different orders and families of Birds ; and it is interesting to remark that whilst the latter predominate among the Insessores, which may be regarded as the typical Birds — possessing in the highest degree the dis- tinctive attributes of the class, — the former is most strikingly manifested in the Parrot tribe, which are not only (as every one knows) the most educable of all Birds, but which are pre-eminently distinguished for the hand- dike use which they make of the inferior extremity (§ 325 b). This variety in the relative development of the intelligence in Birds, bears a close cor- respondence -with the relative development of the Cerebrum. On the other hand, the development of the Cerebellum seems to bear a like conformity to the variety and complexity of the movements habitual to different tribes of Birds. The relative development of the anterior and posterior ganglionic enlargements of the Spinal Cord, again, bears a constant pro- portion to the relative use that is made of them in the act of locomotion. In Birds whose power of wing is very great, and the use made of the legs and feet very small, except for support in ‘ perching,’ the anterior enlarge- ment equals the posterior in size ; and would probably exceed it, if it were not that the posterior enlargement supplies not only the legs but the tail, which is an important accessory organ of flight. In the Struthious birds, on the contrary, whose progression is almost entirely performed by the legs, the posterior enlargement is by far the greatest ; and an approach to this extreme inequality is presented by the wading and aquatic birds, in which these members take a large share in the general movements. — The decided inferiority in the nervous system of Birds to that of Mammals, is marked, not only by the relatively low development of the Cerebrum and Cerebellum, but by the deficiency of those commissural bands, which unite the lateral halves of the nervous centres on the median line. Thus in the Cerebrum we have no trace of the corpus callosum , whilst in the Cerebellum w r e find no pons varolii ; and at the lower extremity of the Spinal Cord, there is a wide divergence between its columns, leaving a space (termed the sinus rhomboidalis ) that resembles the fourth ventricle in the medulla oblongata (§ 324 o). — The Sympathetic system of nerves in Birds bears a close resemblance to that of Mammalia; Its chief peculiarity consisting in the lodgment of its cervical chain of Iganglia, along with the vertebral arteries, in the canals formed by the lateral arches of the cervical vertebrae (§ 325 a). i 512 GENERAL PHYSIOLOGY. 325 i. The class of Birds is as much distinguished by the acuteness of its sensations, as that of Reptiles is by the opposite condition. This is especially to be noticed in regard to the sense of Sight, which is that whereby the active locomotion of these animals must be especially guided. Accordingly we find the visual organs never absent, and generally of remarkably large size. The tough fibrous investment of the globe of the eye is very commonly strengthened in front by a circle of bony plates, usually from twelve to fifteen in number ; and among other purposes which these plates appear to serve, is that of giving a fixed point of action to a peculiar muscular apparatus, by which the adaptation of the eve t- > distances is effected. The eye of Birds is also remarkable for the presence of a peculiar body, termed the ‘pecten’ or ‘ marsupium,’ which projects forwards from the fissure of entrance of the optic nerve, into the vitreous humour ; it is chiefly composed of a layer of blood-vessels, constituting an ‘ erectile tissue,’ folded in numerous plications, the size, form, and number of which vary considerably in different Birds, even such as are otherwise closely allied ; and the whole is covered by a continuation of the black pigment which lines the choroid. The uses of this organ are not certainly known ; it may, perhaps, serve more than one purpose, but its action seems not improbably to be connected with the adaptation of the eye to distances, the relative positions of the crystalline lens and of the retina being altered when its vessels are distended with blood. The Apteryx is the only bird, in whose eye the marsupium is known to he absent. In Birds, as in the higher Reptiles, the front of the eye is pro- tected by three eyelids ; of these, the third, or 4 nictitating membrane,’ B thin and semitransparent, and is employed not so much for the exclusion of light, as for sweeping off impurities from the surface of the eye, being continually drawn rapidly across it by a special muscular apparatus; whilst of the two which move vertically, the under one is the largest, and is alone furnished with a tarsal cartilage. — The organ of Hearing is not yet marked, except in a few of the Owl kind, by any trace of those external appendages, which, in most of the Mammalia, constitute such important features in the general appearance of the head ; iu other respect*, how- ever, it presents a decided advance upon the Reptilian type, the tympanic apparatus being more complete, and the rudiment of a cochlea being considerably developed, the otolithes being almost entirely absent, and the surface upon which the auditory nerve is distributed considerably aug- mented. — The Nasal cavity of Birds is of considerable size, and its lining membrane is greatly extended by prolongation over the ‘turbinated bones,’ which project into its cavity. The degree of this extension depends, however, upon the amount of convolution of the turbinated bones, which varies considerably in different tribes of Birds ; but ii never seems to approach that which is presented in the corresponding part of those Mammals which are distinguished for their acute scent : and it seems probable, from experiments made for the purpose, that many of the actions commonly regarded as indicative of great acuteness in the oltaetive sense (in Vultures more especially) are really to be attributed to the guidance of sight. In Birds, as in the preceding classes, the nostrils are mere perforations leading to the nasal cavity ; and are not fur- nished with moveable cartilages and muscles for dilating and con- tracting their apertures, such as are possessed by most Mammalia. — GENERAL VIEW OF THE ANIMAL KINGDOM. — BIRDS. 513 The sense of Taste appears to be but little developed in Birds. The tongue, as in the preceding class, seems to be organised rather for the capture and prehension of food, than for taking cognizance of its gustative qualities ; and the full examination of these is further pre- vented by the circumstance, that the food is not retained in the mouth for mastication, but is swallowed as soon as it is received into the cavity. The tongue of most Birds is nearly or entirely destitute of true gustative papillae, and is invested in a homy sheath ; and it is by the prolongation of this sheath into filamentous processes, that the brush-like appendage is formed at its extremity, in the Lorikeets, which serves to take up the nectar of flowers upon which they feed. In the Humming-birds, the tongue presents a most peculiar conformation, which strongly brings to mind the hanstellium of Lepidopterous Insects (§ 315) ; for it is composed of two musculo-fibrous tubular cylinders, which are adherent to each other (like the two barrels of a double-barrelled gun) until near their extremity, where they diverge, and open out into a pair of spoon-like terminations, well fitted to lick up honeyed exudations, which are trans- mitted from front to back of this tubular tongue by the contraction of its muscular walls, as well as to seize upon the small insects which form part of the food of these birds. In the Humming-birds, Woodpeckers, and some other birds, the central part of the hyoidal apparatus, upon which the tongue is fixed, is rendered very moveable by the extraordinary prolongation and flexibility of its lateral cornua (§ 325 c) ; and by a muscular arrangement specially adapted for the purpose, the tongue can be instantaneously projected out of the mouth to nearly the length of the body, and as suddenly drawn in. — The exercise of the sense of Touch must of course be limited in Birds, by the entire want of digital sub- division in the anterior extremities (which are elsewhere its chief instru- ments), and by the character of the investments of the skin, which must greatly diminish its power of receiving external impressions. The only part which would seem to possess any special tactile sensibility, is the soft skin with which the bill is covered in the Duck tribe, and in other birds which, like these, obtain their food by digging into soft mud ; this skin is copiously supplied with branches of the fifth pair of nerves, and lias a regular papillary structure. 325 k. The disposition of the Muscular system in Birds may be, for the most part, easily anticipated from what has been already said of their mode of locomotion, and of the conformation of their bony skeleton. In all Birds distinguished by their powers of flight, the pectoral muscles — attached at one end to the keel and expanded surface of the sternum, and at the other to the humerus, so as to draw the latter downwards by their contraction, — are those whose development chiefly attracts attention j and there are some instances in which the ‘pectoralis major’ alone weighs as much as all the rest of the muscles of the body put together. On the other hand, in proportion as the anterior extremities are reduced in size, and the posterior take a larger share in locomotion, do we find the pectoral muscles dwindling down, and those of the legs presenting the chief devel- opment ; and this, of course, is most remarkably seen in the Struthious tribe. The muscles of those Birds which habitually rest on ‘perches,’ rather than on the ground, are so disposed as to cause the foot to grasp the perch during sleep, and thus to keep the bird in its position, without L L 514 GENERAL PHYSIOLOGY. any muscular effort ; for besides those which ordinarily connect the thigh with the body, and put in motion the lower part of the member, we find one of the thigh-muscles (whether the representative of the rectus femorU or of the gracilis in Man is not yet determined) which arises from the superior part of the pubis, passing continuously over the convexity of the knee-joint, and then over the projection of the heel, into the foot, where it becomes connected with a flexor of the toes, in such a manner that when the knee and ankle-joints give way to the weight of the body, the toes must be drawn firmly round any object within their grasp. — The most remarkable of the special modifications of the muscular apparatus of Birds, consists in the development of a vast number of small muscular fasciculi in the substance of the skin, in such connection with the feathers as to be able to move them in different directions. These are best seen in the larger birds, such as the Goose, in which as many as four or five of these slips can be traced to each feather ; and as this bird has about 3000 quill-feathers on its whole body, the number of its quill-muscles must be at least 12,000. 325 l. Notwithstanding the higher grade of embryonic development which Birds present, as compared with Reptiles, at the time of their entrance into the world, this is not dependent upon any decided advance in the conformation of the genital organs, which retain the type charac- teristic of the preceding class, with very little essential modification. The ‘ testes ’ or spermatic organs of the male, are compact bodies of a globular or elongated form, situated close to the upper pail of the kidneys ; they are composed, as in the Mammalia, of long convoluted tubuli ; and they undergo a remarkable periodical development, their size at the period of sexual activity being from twenty to fifty as great as it is at other times. The left testis is generally the largest ; but we never find it alone developed, and the right testis sometimes equals it in size. The two seminal ducts discharge themselves into the cloaca by two distinct orifices ; and a pair of papillary elevations in which these terminate, constitute, in most Birds, the sole rudiment of a penis or intromittent organ. In the Ostrich and some other Birds, however, a distinct penis, furnished with erectile tissue, is present ; and the seminal ducts are made to open into a groove upon its upper surface, which, though not completed into a canal, serves by the apposition of its lips to convey the fluid to its extremity, and thus to deposit it within the female passages. When rot in a state of erection, the organ is entirely concealed within the cloaca. — The genital apparatus of the female is usually remarkable for its unsym- metrical development ; the right ovary and oviduct remaining unde- veloped (Fig. 188,/), like the left lung of Serpents. Up to a certain period of embryonic life, however, these organs are developed with perfect symmetry on the two sides ; and in some of the Raptorial birds, they present an equal development even in adult age. The ovarium, which is attached to the superior or anterior extremity of the right kidney, presents an approach to the structure of this organ in Mammalia ; boms composed of a ‘ stroma’ or bed of compact fibrous tissue, in the midst of which the ova are developed. These, as they increase in size, begin to protrude from its surface ; and at last, before they are set free, hang merely by foot-stalks, composed of prolongations of the investing mem- branes of the ovary. At the time of greatest functional activity, especially GENERAL VIEW OF THE ANIMAL KINGDOM. BIRDS. 515 i in the domestic Fowls, in which the number of eggs is not limited, and their stages of development different, the surface of the ovary presents an appearance like that of a cluster of fruits of various sizes (Fig. 188, a). When set free, by the bursting of their membranous capsules, the eggs are received into the dilated extremity (c) of the oviduct (6) which con- veys them towards the cloaca. In their way, however, they receive an important addition ; for each ovum quits the ovarium in the condition of a ‘yolk-bag’ only, and this is surrounded, as it passes along the oviduct. , with layer over layer of albu- men poured out from its walls Fig. 188. and forming the ‘ white ;’ and over all is formed the peculiar fibrous membrane (Fig. 16), of which the outer layer is con- solidated, by the deposition of chalky particles in its meshes, into the calcareous shell, whilst the inner remains as its lining- membrane. The last of these formations takes place in a di- | latation at the lower part of the oviduct (opposite to g, Fig. 188), which is called the ‘ calci- fying segment.’ The oviduct opens at its lower extremity into the cloaca ; which, in the females of the species whose males possess a penis, contains a ‘ clitoris,’ or organ of sexual excitement. — The ovum of Birds chiefly differs from that ! of the lower Vertebrata in its great size ; and this is due to the presence of a very large store of yolk within the yolk- bag, and to the subsequent addi- tion of a large amount of albu- men, which answers the same purpose, being taken into the yolk-bag as the contents of the latter are exhausted. Hence it is that the num- ber of eggs laid by Birds, in their natural state, is but small in proportion to that which is deposited by Fishes and Insects, and does not even approach the number deposited by the higher Reptiles. The peculiar deficiency in the pelvic arch already noticed (§ 325 b) would seem to have reference to the large space required for the exit of the egg ; and the non-development of ■ the ovarium on the right side may very probably be intended to prevent injury to the eggs from mutual pressure, during their development and 1 their passage down the oviduct. The special provision which we find in ' the parental instinct of the Bird, for the care and maturation of the ova, j and for the subsequent nurture of their young, has no parallel in any other class of oviparous animals, save in Insects. The eggs are deposited L L 2 Generative apparatus of the common Fowl ;—a, single ovary, situated nearly on the median line ; b, left oviduct ; c, its funnel-shaped opening ; d, cloaca; e, rectum ; f, right oviduct, atrophied ; g , g ' , ureters. ! 516 GENERAL PHYSIOLOGY. in an artificially-constructed nest, which frequently displays the most ingenious and elaborate workmanship ; and the heat necessary for the development of the embryo is imparted to it by the body of the parent. Enabled by the continued agency of this developmental force (§ 96) to appropriate the large store of nutriment laid up in the egg, the embryo of the Bird does not stop short in the process of evolution at the phase which marks the reptilian type ; but advances to a far higher grade before quitting the shell. It comes forth into the world with all the attributes of a Bird ; and although many of its distinctive characters are at first 1 ait slightly manifested,* yet these are very speedily evolved, under the influ- ence of the high temperature which the body possesses, this being partly the result of its own calorific power, but still in part derived from ti e parents. The degree of the dependence of the young upon the warmth imparted to them by their parents, varies greatly, however, in the different orders of Birds. Thus among the Raptores and Insessores, the young usually come forth from the egg in a blind and naked state; and it is only by the warmth supplied by the parents, and by the obstacles presented to the dissipation of then own heat by the shelter of the nest and the mutual contact of their own bodies, that their temperature can be kept up to its standard, even during the heat of summer. On the other hand, the young of most birds that live on the ground or on the water, belonging to the orders Rcisores, Grallatores, and Natatores, come forth from the egg in a state which is relatively much more advanced ; being capable of running or swimming almost as soon as they quit the shell, and having the power of sustaining their own heat with but little assistance from the parent. As already remarked, the complete ossification of the skeleton of Birds takes place at a very early period of their lives ; and the development of the wings is soon sufficiently advanced, even in the Raptorial and Inses- sorial orders, to enable the young birds to sustain themselves upon them for short flights in the air. During the period which elapses before these are able to maintain themselves by them own exertions, they are nurtured and tended by their parents, who in some instances not only select but even prepare the food for them offspring, by macerating it in the crop, and then returning it to the mouth, from which it is transferred to the mouth of the young bird. In the Pigeon tribe, the ‘ crop/ during the breeding season, secretes a milky fluid, which is mingled with the ma- cerating grains, and is returned with them to the mouth, to be imparted to the yormg. This constitutes the nearest approach seen among Birds to the iacteal secretion of the Mammalia. The young of those inferior orders, however, which live upon the ground or on the waters, are iar less dependent upon their parents for their early sustenance ; being able to provide for themselves almost from the time of quitting the egg. — Here, as in many other instances, we notice that the prolongation of the period during which the early development is aided by parental nurture, is closely related to the elevation in the grade of development which the being is destined ultimately to attain. * It is in the downy character of its plumage, and the consequent imperfect development of the feathery expansions which constitute the acting surface of its wings and tail, that the young Bird chiefly differs from its parent. These were the very characteristics of the now extinct Dodo , of which (as remarked by Mr. Strickland) we may form a very good idea, by imagining a young duck or gosling enlarged to the dimensions of a swan. GENERAL VIEW OF THE ANIMAL KINGDOM.— MAMMALIA. 517 326. The Mammalia are universally regarded as the highest group iu the Animal kingdom ; not only as being that to which Man belongs, but also as possessing the most complex and heterogeneous organisation, — that is, having a structure made up of the greatest number of dissimilar parts, which are severally and specially adapted for the performance of distinct operations. It is in this group also, even putting Man out of view as belong- ing to it, that we have the greatest manifestation of intelligence which we any where meet with in the Animal series. This is intimately connected with the high development of the Cerebrum, which presents a rapidly increasing- complexity of structure, as well as augmentation of relative size, as we trace it from the lower to the higher Mammalia. And this, again, is closely related to the greatly-prolonged connection between the parent and the offspring, which we find to be a characteristic of this class ; for it is a general rule, to which reference has already been several times made, that the higher the development an organized being is ultimately to attain, the longer is it dependent, during the early stages of its evolution, upon the fostering aid of its parent. Now it is the essential characteristic of the Mammalia, considered as a class, that the young are brought into the world alive ; and this not merely, as in certain Reptiles and Fishes, by the retention of the egg within the oviduct, until the development of the embiyo has so far advanced that it is ready for hatching before the depo- sition of the ovum ; but by the formation of a new connection between the embryo and its parent, while the former lies within the maternal cavities, so that the means of its continued development is provided, not (as in Birds) by the accumulation of a large store of nutriment in the ovum itself, but by a supply of freshly elaborated material continually furnished by the maternal blood. The ovum of Mammals, owing to the very small quantity of its yolk, is of extremely minute size ; and its con- tents only serve to carry on the very earliest portion of the developmental process, which is especially directed towards the formation of the tem- porary nutrient apparatus. This consists, in the first instance, of a set of root-like tufts, which are prolonged from the surface of the ovum, and insinuate themselves among the maternal vessels, without, however uniting with them ; and which absorb from the maternal fluid the ingredients necessary for the support of the embryo, whilst they convey back to it the effete particles of the growing structure, to be eliminated by her secreting operations. Up to this stage, the connection of the mother and foetus is essentially the same in all Mammalia ; but in the lower subdi- vision of the class (which includes the orders Marsupialia and Monotre- mata) the connection of the ovum with the parent ceases before it becomes more intimate, and the young are brought into the world in a condition which (as compared with the young of other Mammalia) is one of extreme immaturity. But in the higher division which constitutes the great bulk of the class, the ovum is delayed for a longer period, within a cavity formed by the dilated union of the two oviducts, termed the uterus, which only exists in a rudimentary condition in the two orders just named ; and here the embryo forms a new and more direct connection ' with its parent. The tufts proceeding from the chorion become especially .developed at one point, and root-like prolongations of the foetal vessels insinuate themselves into them ; these vascular tufts dip down (as it .were) into a chamber formed by an extension of the inner lining of the 518 GENERAL PHYSIOLOGY. uterine vessels ; and thus the fostal blood is brought into such intimate relation with the maternal — without any intercommunication hy open orifices — that a ready transudation of elements can take place from one liquid to the other. The peculiar organ thus formed is termed the 'placenta ; and its foetal portion may be considered as bearing a resem- blance alike to the roots of plants and to the branchiae of aquatic animals, absorbing from the maternal blood the materials required for the nourish- ment of the embryo, and aerating the blood of the foetus by exposing it to the influence of that of the parent. The two sub-classes into which the Mammalian group is divided, are thus designated as ‘placental' and ‘ implacental,’ according as this organ is or is not developed ; and v e shall presently see that they differ in many remarkable particulars of their organisation, the Implacentalia occupying an intermediate position between the Oviparous Vertebrata and the Placental Mammals. In both alike, however, do we find that a provision exists for nourishing the animal, during the early period of its extra-uterine life, by a lacteal secretion formed in external mammary glands ; and as this is found in no other class, it is the character by which, in a zoological arrangement, the entire group is most positively defined. 326 a. Taken as a whole, this class cannot be said to be so much characterized by the possession of any one particular faculty, like the power of flight which is so typical of Birds, as by the combination "f different powers which we find in the group generally, and also (for the most part) in each individual. Thus whilst, in the greater number of Mammalia, the body is supported upon all four extremities, and is adapted for progression along the surface of the earth, this typical structure un- dergoes such a modification in the Bat tribe, as to adapt these animals to the mode of life characteristic of Birds. So again in the Whale tribe, we find the ordinary Mammalian type so modified as to adapt these animals to the life of Fishes ; the locomotive power being almost completely taken from the extremities, and given back to the trunk ; and the form of the body being such, as to enable it to move rapidly through an aquatic medium. In neither case, however, do we find that these modifications involve any essential departure from the mode of organisation character- istic of the class, or any important variation from its usual physiological conditions. The wing of the Bat does not represent that of the Bird in any other respect than its analogous use ; but is formed upon a plan of its own, the extended skin being chiefly stretched over the elongated fingers of the anterior member (Fig. 190). And the expanded tail-tin ot the Whale is not extended vertically upon the neural and haemal spines of its caudal vertebrae ; but is flattened out horizontally, and supported by cartilages that do not form a part of the vertebral skeleton. Turning, again, to the endowments of the individual members of the class, we find that whilst there are none that can rival Birds in rapidity of locomotion, there are few that cannot perform several different kinds of progress'; >n : and whilst there are probably none that can compete with Birds in acute- ness of sight, there are few that do not possess a greater range of special sensibility. — The inferior energy of muscular movement in the Mammalia is accompanied by an inferior amount of respiration ; but we shall find the type of the respiratory apparatus to be decidedly higher than that of Birds. The diminished amount of respiration, again, involves the pro- GENERAL VIEW OF THE ANIMAL KINGDOM. MAMMALIA. 519 duction of a lower degree of animal heat (the temperature of this class seldom rising above 104°) ; and there is consequently the less need of means for effectually confining their caloric ; especially, too, as their greater average size causes their radiating surface to be much less, in proportion to their hulk, than it is in Birds. Accordingly we find them provided only with a covering of hair or fur ; and even this is sometimes very scanty, or is almost entirely wanting. In the Cetacea, the same end is answered by another provision, the interposition of an immense quantity of oleaginous matter in the meshes of their enormously-thickened skin, — thus forming the ‘blubber,’ which not only constitutes an admirable badly-conducting septum between the warm body of the animal it incloses ' and the cold water of the surrounding ocean, but serves, by its peculiar resisting power, to antagonize the enormous pressure to which the surface is subjected when the animal sinks into the depths of the sea. — The inferior nervo-muscular energy of Mammals as compared with Birds, involves not merely a less active respiration and an inferior calorifying power, but renders it unnecessary that the nutritive functions in general should be carried on with that extraordinary activity which characterizes the former class. Accordingly we find that the demand for food is less constant, the digestive process less rapidly accomplished, and the circu- lation slower, than in Birds ; whilst, on the other hand, the ‘ waste ’ of the body, as indicated by the amount of the excreta, is less considerable. 326 b. In the skeleton of the ordinary four-footed Mammalia, we find a return to the same general plan as that on which the skeleton of the Saurian Reptiles is constructed, the method of locomotion being essen- tially the same in both cases ; but we shall observe in many points a considerable advance in the grade of osseous development, not manifested by any conspicuous change of external form. This is remarkably the case with regard to the intimate structiue of the bones themselves, which retain, in many Reptiles, the characters which they present in the higher Mammalia during only the early period of their development (§ 206) ; and it is the case also -with regard to the degree of consolidation which the skeleton undergoes, by the union of parts originally developed from distinct centres, of which several examples have already been brought under notice. Now although Mammalia might be considered as inferior to Birds in both these respects, — the bones being filled with oil like those of the Reptile, instead of being penetrated by air, and the separate osseous elements not being consolidated to nearly the same extent as in the preceding class, — yet this inferiority is only apparent ; for it is obvious that the peculiarities of the Bird’s skeleton must be considered as special modi- fications having reference to its mode of life, and not as naturally pre- senting themselves in the general progress of development, as intermediate conditions between Reptiles and Mammalia. In fact, through the lower or non-placental group of Mammalia, and in some degree also through the gigantic extinct Sloths (which seem to have approximated the herbivorous Dinosauria of a still older epoch in many points of their organisation), the connection of the Mammalian and Reptilian groups is better established without the intermediation of Birds ; which must be considered as on the- whole more separated from both of them, than they are from each other. In our general view of the osteology of Mammals, therefore, we shall take the Saurian skeleton as oiu standard of comparison, and make little 520 GENERAL PHYSIOLOGY. reference to that of the Bird. — In regard to the conformation of the vertebral column we have first to remark, that the number of vertebra in the different regions is more constant than we elsewhere find it ; seldom departing much from that which Man possesses, excepting in the caudal region. The typical number of cervical vertebrae (Fig. 189, vc) is 7 ; and however long the neck may be (as in the Giraffe), or short (as in the Whale), this number is almost uniformly observed ; the only known exceptions on the side of diminution being two of the whale-like Puchy- derms ( Manatus and Rytina ) which have but 6 ; whilst an excess has only been met with in the A i or three-toed Sloth, which has 9. In casts where little or no more power of moving the head upon the trunk is required, the cervical vertebrae are very thin, and are more or less closely united by the anchylosis of their bodies and arches ; this we especially set in the Cetacea. The cervical vertebra are composed of bodies and complete Fig. 189 . Skeleton of Camel ; — vc, cervical vertebrae ; vd , dorsal vertebrae ; vl, lumbar vertebrae ; vs, sa- cral vertebrae; vq , caudal vertebrae; o, scapula; h , humerus; cu , radius; ca , carpus; me, metacarpus; ph, phalanges ; c, ribs ; fe, femur ; ro, patella; ti, tibia ; ta, tarsus; mt, meta- tarsus. neural arches, with ‘transverse processes’ which are formed by the union of the diapophvses and parapophyses with the rudiments of pleui -apo- physes, so as to form the vertebral canals (Fig. 169). These pleunipo- physes are very seldom developed as distinct elements ; we find them, however, in the two lower cervical vertebrae of the Ai, where they consti- tute true ‘ cervical ribs,’ differing only from those of the dorsal region iu not being sufficiently developed to reach the sternum : aud they are occasionally seen as a ‘ monstrosity’ in Man. No other trace of the haemal arch, save the clavicles (§ 326 d), exists in the cervical vertebra of Mammalia. The number of dorsal vertebra, from which distinct ribs are developed (Fig. 189, v, cl), is usually from 11 to 20 : the greatest number GENERAL VIEW OP THE ANIMAL KINGDOM. MAMMALIA. 521 known is in the two-toed Sloth, which has 23. In the upper part of this region, the haemal arch is completed by the elongation of the pleura- pophyses (ribs), and by the development of the hsemapophyses and haemal spine. Excepting in the M onotremata, the ribs of Mammals always have a double articulation with the spinal column, one by their ‘ heads ’ with the bodies of the vertebrae, and the other by their ‘ tubercles ’ with the transverse processes ; and it is curious that the greater number of the ribs should thus articulate in part with the bodies of other vertebrae than those to which they properly belong, each head being received into a cavity partly formed in the centrum of its own vertebra, and partly in the one anterior to it. The haemapophyses generally remain cartilaginous, and hence are not commonly known by their proper appellation of ‘ sternal ribs,’ but are designated as ‘ costal cartilages they show a marked ten- dency to ossification, however, in advancing life ; and when fractured, they are commonly repaired by bone. The haemal spines of the successive segments, which are always distinct in the foetus, continue so in some Mammalia, constituting so many subdivisions of the ‘ sternum ;’ they frequently coalesce, however, to such a degree as to form only three dis- tinct bones (which process takes place during the period of adolescence in Man) ; and even these may lie at last united into one, as we occasionally find in advanced age. The central portion of the sternum forms a projecting keel, as in Birds, in certain cases in which it is necessary that the anterior members should be endowed with unusual strength ; and we meet with this conformation, not merely in Bats (which, as the represen- tatives of Birds, might be most expected to present it), but also in the Moles and Armadilloes, whose fore-feet are used as spades in burrowing. The lower portion of the sternum, which usually remains cartilaginous in Man until a late period of life (being known as the £ ensiform cartilage’), is more developed in the Edentata, and is prolonged nearly to the pubis ; thus reminding us of the abdominal sternum of Reptiles. In other Mammalia, however, this portion of the sternum is only represented by the fibrous aponeurosis which stretches from the sternum to the pubis, intervening between the abdominal muscles on the median line ; and the 1 sternal ribs, by the transverse aponeurotic bands dividing the ‘ recti ’ muscles into distinct segments. In some of the posterior segments of the dorsal region, the haemal arch remains incomplete, by the non-develop- ment of the haemal spine and of part of the haemapophyses ; so that the ribs are either connected to those in front of them by the adhesion of their cartilages, or they are altogether free at their sternal extremities. The proportion of these ‘ false ’ ribs is very great in the Cetacea, which have only one or two true ribs ; in this respect presenting a very curious reversion to the conformation of Fishes, which have no sternum, and in which all the ribs are consequently free (§ 322 b). — In the lumbar vertebrae (Fig. 189, vl), we again lose the pleurapophyses as distinct elements; and the rudiments of the abdominal sternum and sternal ribs, just alluded to, usually constitute the only representatives of the haemal arches of the lumbar segments. The number of these vertebrae varies from 2 to 9 ; , but it is commonly either 4 or 5. In some of the larger Quadrupeds, the lower lumbar vertebrae are joined to each other and to the sacrum by bony union of the transverse processes ; a condition which sometimes presents itself in advanced age in Man. 522 GENERAL PHYSIOLOGY. 326 c. The sacral vertebrae (Fig. 189, vs), which unite to form the ‘sacrum’ (Fig. 168) and to support the pelvic arch, are by no means con- stant in number ; and we perceive in the conformation of this part of the Mammalian skeleton, a variety which becomes very striking when we consider its character and importance. In Man the number is 5, and in the Mole, 6 ; but in other placental Mammalia it is diminished to 4, and even to 3 ; whilst in Marsupialia and Monotremata it is reduced, as in Reptiles, to 2 • and in the Ornithorhyncus these two remain permanently separated from each other. In Cetacea, as in Fishes, there cannot be said to be any sacrum ; the rudimentary pelvis being but loosely connected with a single vertebra, which is not united with those before or behind it. — The number of caudal vertebra (Fig. 189, vq) varies more widely than that of the segments of any other region ; for whilst in Man and the higher Apes they are no more than 4 or 5, these being abortive and be- coming anchylosed with each other (forming the ‘ os coccygis ’) or even with the sacrum, the number rises in other Mammalia to 27 (Whale), 31 (Long-tailed Monkey), 36 (Long-tailed Opossum), and even 40 (Long-tailed Ant-Eater). It is interesting to remark that the highest numbers occur in members of those orders, — the Marsupialia and the Edentata, — which present the greatest number of other approximations to the Reptilian type of structure ; but it must be recollected that the tail is a part of the ver- tebral column, whose development or non-development seems to vary according to the special use to be made of it, rather than according to any general plan. Thus we find it in the Kangaroo serving as a third leg of the tripod, on which this animal rears itself erect ; and there can be no doubt, from the conformation of their skeletons, that the gigantic Sloths of former ages were enabled to support their massive bodies in the' same manner. In the American Monkeys, on the other hand, as well in some of the Opossums inhabiting the same Continent, and in a few other ani- mals, the tail is prehensile ; being capable of coiling round branches of trees with such a firm grasp, as to support the whole weight of the body without difficulty. In the Cetacea, again, as in Fishes, the tail is the chief instrument in the propulsion of the body through the water ; and in some other Mammals (as the Squirrel) it seems to steer the animal in its leaps through the air, just as it steers Birds in their flight, being converted moreover into a kind of parachute by the wide lateral expansion of its bushy hair. The tail of the Beaver, again, which is flattened horizonta’ly and covered with scales instead of with hair, is the principal instrument of propulsion in water ; it has also been said, but erroneously, to serve as a trowel for laying on the mud with which the habitation is plastered.- - The form of the caudal vertebra, and the number of elementary parts discoverable in them, vary in accordance with the purposes to which the member is to be applied. In some instances, the pleurapophyses may he traced in the anterior vertebra ; but they coalesce with the diapophyses to form the elongated transverse processes, and do not bend down to enter into the haemal arch. In the Kangaroo, and some other Marsupials, how- ever, some of the caudal vertebrae have proper haemal arches, formed (as in Reptiles) of the haemapophyses and haemal spine, which are anchylosed to the under side of the bodies of the vertebra?, leaving a haemal canal through which the caudal artery passes. The terminal vertebra? are reduced to the ‘ centra ’ alone. — In the conformation of the tail of Mam- GENERAL VIEW OF THE ANIMAL KINGDOM. MAMMALIA. 523 malia, we have thus a very remarkable instance of that specialization which has been pointed out as a distinguishing feature of the structure of this class ; the same instrument being modified in various ways, so as to lie adapted to a number of purposes very distinct from each other, including all those for which it is employed elsewhere, with others which are pecu- liar to the Mammalian group. 326 d. This specialization is carried to a still greater extent in the con- formation of the extremities ; in which a very remarkable degree of variety is presented. The scapular arch, in nearly all Mammalia, differs from that of Oviparous Yertebrata in this ; that its coracoid element is not sufficiently developed to reach the sternum, or to meet its fellow on the median line ; but that where the scapula has any osseous connection with the sternum, that connection is established by the ‘ clavicle,’ which is pro- bably to be regarded as the hsemapophysis of another segment (possibly that of which the ‘ atlas ’ constitutes the centre with its lateral and neural arches). The clavicle itself, however, is very frequently wanting ; and we find it absent in some Mammals, whilst present in others very nearly allied to them (as in the group of Rodentia). It is obvious that, taking the whole Vertebrated series together, the completion of the scapular arch by coracoid is the regular or typical plan ; whilst the introduction of the clavicle always seems to have a view to some special purpose. As a general rule it is present in those Mammalia in which the anterior extre- mities are drawn inwards (towards the median line), as well as downwards or backwards, in the act of locomotion ; and it serves the double purpose of giving attachment to the muscles by which this movement is accom- plished, and of keeping the shoulder-joints from being drawn together when they are put in action. It is also present when the anterior members are unguiculated, and are used for prehension ; as in Man, the Quadrumana, most Carnivora, and in part of the Rodentia. On the other hand, it is usually absent in the ‘ hoofed ’ Quadrupeds, in which the anterior extre- mities are simply concerned in supporting the weight of the body, and in propelling it forwards by their own backward movement. The coracoid bone is for the most part reduced to a mere rudiment, and is anchylosed with the scapula, of which it ranks as a ‘process.’ In the Monotremata, however, with many other Reptilian affinities, this most remarkable one is presented ; that the scapula is connected with the sternum by a fully- developed coracoid, as well as by a clavicle ; that, in addition, the clavicles do not (as in Birds) unite on the median line, but articulate with a bone which is placed upon the summit of the ordinary sternum ; and that the lateral prolongations of - this T-shaped bone (termed the £ episternum ’) pass outwards parallel with the clavicles, just as in the Ichthyosaurus (§ 324 /b), and thus give additional assistance in the fixation of the shoulder- joint. — The structure of the pelvic arch presents a much greater constancy iu the majority of Mammalia, although its form is liable to undergo great modification. The three bones of which it is composed on each side, — the ilium, ischium, and pubis, — generally coalesce together, as in Man, at an early period of life ; but in the Monotremata, as in Reptiles, they remain separate much longer. And just as we have found that it is generally characteristic of the scapular arch of Mammalia, to be closed- in by the clavicle instead of by the coracoid, so is it characteristic of their pelvic arch to be completed by the pubic instead of the ischiac bones. 524 GENERAL PHYSIOLOGY. In most Mammals, the symphysis, or inferior median junction of the two lateral halves of the pelvis, is entirely formed by the rami of the pubis ; but in the Implacental sub-class, the ischium also has a share in the junction, thus carrying us back towards the Eeptilian type. It is very in- teresting to see how strongly the conformation of the pelvis in the Bats and Whales resembles that of the groups which they respectively represent ; for in the former it is greatly elongated, and neither pubic nor ischiac bones unite to form a symphysis, so that the lower front of the arch remains open as in Birds ; whilst, on the other hand, in the latter the pelvis is reduced almost to a minimum, consisting merely of a simple haemal arch (not unlike the hyoid bone of Man), formed by the convergence of the hsemapophyses of the single sacral vertebra, and by their junction with each other, either directly, or through the mediation of a haemal spine. The pelvis of the Implacental sub-class presents this further peculiarity : that from the pubic symphysis two additional bones project forwards, being articulated at their bases with the pubis, whilst their free extremi- ties are connected with the tendons and aponeuroses of the abdominal muscles. These additional bones have been termed ‘ marsupial,’ from their supposed function in supporting the ‘ marsupium ’ or pouch, whose pre- sence characterises the principal of the two orders that make up the group. As they are present, however, in both sexes of .Marsupials, and also in animals of the order Monotremata which do not possess the pouch, it seems likely that they must be subservient to other purposes. They are not, it would appear, to be considered as part of the vertebral skeleton ; but are rather adventitious bones, developed in the substance of the abdominal tendons, as the ‘ sesamoid bones ’ (of which the patella is a gigantic exam- ple) are in some of the powerful tendons of the limbs. 326 e. The anterior extremities are never deficient in Mammalia, although they present very diversified modes of conformation in the several orders of the class, according to the purposes for which they are designed. Where they are adapted merely as instruments of support and locomotion on land, as in the Ungulata or ‘ hoofed ’ Quadrupeds, the extremity is reduced to its simplest condition ; only a pair of digits, or even a single one, being developed in each member, and these being entirely enveloped in hard homy casing's ; the bones of the fore-arm being consolidated into one, so as entirely to prevent any rotation of the hand ; and all the articulations, even that of the shoulder-joint, being so forme 1. as to limit the movements of the limb to that one plane (backwards and forwards) in which its actions are required for the onward propulsion of the body. The opposite extreme is where (as in Man) a thin nail covers only one side of the extremity, leaving the other possessed of all its tactile delicacy ; where several such fingers exist, capable of being used independently, one of them being opposed to the rest, so as to render prehension more perfect, and to adapt the member for a great variety of actions ; where the plane of the whole hand can be turned in any posi- tion, by the rotation of which the fore-arm is capable ; and where the form of the humeral articulation, and the arrangement of its muscles, confer upon the arm, as a whole, the power of free and extensive moti n. Between these two extremes, however, there are many intermediate grada- tions ; and some of the more important of these will be now considered. — Starting with the anterior extremity of Man, as presenting us with it." GENERAL VIEW OF THE ANIMAL KINGDOM. MAMMALIA. 525 highest type of conformation as a prehensile organ, we find it composed of a humerus, articulated at its upper extremity by a ball-and-socket joint with the scapula, and at its lower with the radius and ulna ; of wdiich the latter alone enters into the proper ‘hinge’ of the elbow-joint, the radius being kept out of that articulation in order that it may freely rotate (carrying the hand with it) around the axis of the ulna. The radius alone forms the wrist-joint, with the carpal bones ; and this joint has more of the ‘ ball-and-socket ’ than of the ‘ hinge ’ character ; so as to allow great freedom of movement to the hand. The ‘ carpal ’ bones in Man are eight in number, forming two rows ; but the typical number is probably ten (as seen in the Turtle), its reduction being the result of the coalescence of four bones into two, the ‘ unciform ’ and ‘ scaphoid,’ the latter of which is double in many Quadrupeds. The carpal bones sup- port the five ‘ metacarpal,’ which are included in an undivided integu- ment ; that of the thumb, however, being free to move independently of the rest. The metacarpal bones support the ‘ phalanges,’ of which each digit possesses three, the thumb alone having two ; and these are all per- fectly independent, so that either phalanx of each digit may be flexed or extended without any movement of the others. The chief peculiarity of the hand of Man, as distinguished from that of those higher Quadrumana which are like him possessed of an opposable thumb, consists in the much greater length of this thumb, and in the power which Man alone pos- sesses of bringing it into forcible opposition to the extremities of each one of the fingers. — Passing from the hand of Man to that of the Quadru- mana, we see a much nearer approximation, than in him, to a similarity between the anterior and posterior extremities. The possession of ‘ four hands ’ is not, as might be supposed from the name, a character which raises them in the least degree above Man; for none of these four hands are adapted to the same variety of actions of which his are capa-. ble ; and all of them are in some degree required for support. In this respect their character approaches much nearer to that of the extremities of the inferior Mammalia ; and there are several among them, in which, the thumb being rudimentary, and its opposable power deficient, there is no very marked distinction between the so-called ‘ hand,’ and the ‘ foot ’ of some Carnivora. The arm of the Ape has as wide a range of motion as that of Man, so far as its articulations are concerned ; but its freedom of movement can only be fully exercised when the animal is in the erect position, which is unnatural to it. — The next degree of degradation is seen in the Carnivora ; in which the anterior extremity is used, almost as much as the posterior, for support and progression ; but in which it is still em- ployed as an instrument of prehension, though in a comparatively limited degree. Here we find the fore-arm still composed of two distinct bones ; but these have little capability of rotation ; and the carpal articulation does not give the hand any freedom of movement in more than one plane. The fingers are inclosed in a common integument, as far as their last phalanx, and cannot be used independently of each other ; and the first or innermost digit, representing the thumb of Man, is but very little deve- loped, or is altogether wanting. The homy appendage to the extremities of the fingers is no longer a mere ‘nail,’ covering only one side of their last phalanx ; but is a strong ‘ claw ’ embracing its whole termination. In the Insectivora and ‘plantigrade ’ Carnivora, we find a gradual transition 526 GENERAL PHYSIOLOGY. in the conformation of the anterior extremity, from the type which it pre- sents in the lower Quadrumana, to that which characterizes the ‘ digitigrade’ portion of the order, of which the F elines are the type ; the carpal and metacarpal portion of the hand being elongated at the expense of the pha- langes, and the latter alone resting upon the ground ; an arrangement which is in beautiful conformity with the ‘ springing ’ habits of the order, but one that diminishes the prehensile power of the extremities. The clavicles also disappear ; and in the shortening of the humerus, its inclu- sion within the integuments of the trunk, and the greater limitation of the movement of the shoulder-joint to the anteroposterior direction, the Fe- line shows the complete reduction of this member from the quadruwnznoMi to the quadrupedal type. — The same reduction is earned still further in the group of Rodentia; in some of which we find the bones of the fore-arm, though still distinct from each other, incapable of rotation ; whilst in others they are partially fused together. And among the Edentata (Sloths, &c.), we find a reduction in the number of digits to three (as in the Ai) el- even to two (as in the U nau) ; whilst in the gigantic extinct forms of this order we have a transition to the hoofed quadrupeds, which they resem- bled in their general proportions, — two of the five digits of the fore-foot of the Mylodon having been furnished with flattened callous coverings to their stunted terminations, so as evidently to serve as points of support, and the other three having been provided with long sharp claws adapted for digging. — In the TJngulated Quadrupeds, we meet with a consider- able variety in the degree of consolidation of the anterior extremity. Thus in the Elephant, the five digits are present, but their terminations are all included together in a callous skin, which takes the place of a hoof : in the Hippopotamus there are only four digits (the first being deficient), but these are inclosed in separate hoofs ; in the Hog there are also four, but the two middle ones are longer than the others, and it is on these alone that the body rests : in the Rhinoceros and Tapir, the number of digits is reduced to three, by the non-development of the fifth or outer one : in the entire Ruminant group, the third and fourth digits are enormously developed, and rudiments only of the second and fifth are present (Fig. 189) ; whilst in the Horse and other Solidungula, the third digit alone is fully developed aud furnished with a hoof, the rudiments of the second and fourth being also present, forming what are known as the ‘splint-bones.’ * This progressive reduction in the number of digits, and the increased development of some of those which remain, may be con- veniently represented by the following table, in which the figures 1,2. 3. 1, 5. used for Man and the Elephant, indicate the presence of these re- spective digits in a condition of nearly equal development, the smaller figures expressing the rudimentary condition of the digits they repre- sent, and the largest indicating an extraordinary excess in then- relative size. * This statement is made on the authority of Prof. Owen, who opposes the ordinary notion that the great single digit of the Horse is formed by the coalescence of the two digits cf the Ruminants ; affirming that the extremity is rather constructed upon the plan of that of the Rhinoceros, the central digit of its tridactyle foot becoming enormously developed, whilst the others are reduced to the rudimentary condition. The extinct Palaotlmrium presents the transitional link between the tridactyle and the apparently monodactyle extremities. See his “ Discourse on the Nature of Limbs,” pp. 31, 3'2. GENERAL VIEW OF THE ANIMAL KINGDOM. MAMMALIA. 527 Man, Elephant Spider-Monkey Hippopotamus, Pig Bandicoot (Marsupial) Ox (Ruminant) Rhinoceros Horse With this reduction, in the number of the digital extremities, and the limitation of their functions to those of mere support and progression, we find a corresponding simplification in the other portions of the member. The bones of the carpus are reduced in number, only six presenting themselves in the Horse. The two bones of the fore-arm, where distinct from each other, have no power of rotation ; and both of them enter into the composition of the elbow and wrist-joints; but they are generally connected with each other by continuous ossification, especially at their carpal extremity ; and in the Ruminants and Solipeds, the radius is much more developed than the ulna, and forms the greater part of the elbow- joint, as well as the carpal articulation. The humerus is shortened in proportion to the lower portion of the extremity, and its articulation with the scapula has almost the character of a hinge-joint, allowing it only to play in the antero-posterior direction ; whilst the clavicles are altogether deficient, whereby considerable mobility is allowed to the scapulae. All this modification seems destined to contribute towards that fleetness in running, which is the especial attribute of the Ruminant and Solipede Mammalia, and which can only be maintained by the complete sacrifice of the prehensile power which the anterior extremity elsewhere pos- sesses. 326/. In the posterior or pelvic extremities of all the Mammalia alluded to in the preceding paragraph, there is a very close resemblance to the anterior. The difference is greater in Man, than it is in any other case ; in consequence of the special adaptation of these extremities for the support of his body in the erect posture. Thus we find the two bones of his leg, which represent those of the fore-arm, arranged more after the fashion of the bones of the fore-arm in some Herbivora : the weight of the body being almost entirely thrown upon one bone, the ‘ tibia,’ which corresponds with the radius ; whilst the £ fibula,’ which represents the ulna, takes no share whatever in the knee-joint, but only guards the exterior of the ankle-joint ; and there is consequently no power of rotating the foot upon the leg. The ‘ os calcis ’ is extraor- dinarily developed, so as to project far backwards, and to serve for the .attachment of the powerful ‘ extensor’ muscles which keep the leg erect upon the foot ; and it is regarded by Prof. Owen as representing two bones of the carpus (the ‘ cuneiforme ’ and £ pisiforme ’), thus accounting for the inequality in number between the carpal and tarsal bones.* The * There are other differences, however, in the relative positions of the bones constituting the first row of the carpus and tarsus, which make it impossible to understand their true delations without the aid of Comparative Anatomy. Indeed it is only in this way, that the 'mutual correspondence of the other bones of the anterior and posterior extremities in the [Human subject, can be satisfactorily determined. Thus Vicq D’Azyr, who was the first anatomist who attempted to work out these ‘ serial homologies ’ (or ‘ homotypes, ’ as they are 1 2 3 4 5 i 2 3 4 5 2 3 4 5 2 3 4 5 2 3 4 5 2 3 4 2 3 4 528 GENERAL PHYSIOLOGY. tarsal and metatarsal bones make up the principal paid of the length of the foot, the phalanges being comparatively short ; and the great toe i> not separated from the rest (though still distinguished by its possession of only two phalanges), and is not capable of being opposed to them. — Now in the Quadrumana, this dissimilarity is greatly diminished ; the posterior and anterior extremities approximating much more closely (as already mentioned) to a common type ; and thus conducting us towards the ordinary Quadrupeds, in which the hind-foot undergoes a series of modifications, that repeat, with scarcely any considerable departure, the various conditions already described as presented by then' anterior exire- mities. The reduction in the number of digits by the disappearance of some, and the excessive development of others, go on for the most part pari passu in both pairs of extremities ; whilst the coalescence of ihc tibia and the fibula almost always takes place to about the same degree as that of the radius and irina. It is curious to observe among many of these, that the fibula is much larger in proportion to the tibia than it is in Man, and that its analogy to the ulna thus becomes more obvious. This analogy, however, is most completely presented among the Impla- cental Mammalia ; for there are many species in that group, in which the hind-foot possesses a rotatory movement in virtue of the rotation of the tibia round the fibula ; aucl the latter bone has a prominence at its upper extremity, which distinctly represents the olecranon. In the Opossums and Phalangers, there is even an opposable thumb on the hind foot : whilst in the fore-foot the first digit is parallel with the rest, as in the foot of Man ; hence these animals have been termed Pedimana (foot- handed), from their reversal of the ordinary relations between the ante- rior and posterior extremities. 326 g. We have now to consider some of the most remarkable modifi- cations which the extremities undergo, for special purposes not compre- hended in the foregoing general description. These modifications pre- sent themselves for the most part in the anterior extremity ; which, in the Bat, is made to answer the purpose of the wing of the Bird ; in the Whale it is modified into the likeness of the fin of the Fish ; whilst in now termed by Prof. Owen), though correctly interpreting the correspondence between the scapula and the ilium, the humerus and femur, the bones of the hand and those of the loot, was misled by the different proportions of the ulna and fibula in the human skeleton, to :he adoption of the belief that the tibia is the representative of the ulna, and the fibula of the radius, the olecranon being supposed to be represented by the patella ; so that each anterior extremity is not paralleled or repeated by the posterior limb of the same side, but by that of the opposite side. This notion was adopted by Cuvier. A still more absurd hypothesis was put forth by Cruveilhier ; who made the upper end of the tibia the representative of tiie upper half of the ulna, and its lower end the representative of the lower end of the radius : whilst the fibula in like manner represents the radius above and the ulna below. “ Nature, however (as Prof. Owen justly remarks), when rightly interrogated and propitiated by due observant service, extricates us from these complex involutions and alternations of serial homology, and makes the simple truth plain.” The true view, put forth by Dr. Barclay and M. Flourens, has been still further elucidated by Prof. Owen ; who has removed the diffi- culty that had been felt in regard to the olecranon, by showing that it is not represented by the patella, which is developed even where (as in the Wombat) the fibula presents the broad, flat, high process which obviously corresponds with the olecranon of the ulna ; wh 1st in the'' elbow- joint of the Bat, the true representative of the patella presents itself, not (y has been supposed) in its detached olecranon, but (according to Prof. Owen) in a sesamoid I. me contained in the tendon of the biceps. See Prof. Owen’s “ Discourse on the Nature et Limbs,” p. 19. GENERAL VIEW OF THE ANIMAL KINGDOM. MAMMALIA. 529 the Mole and other burrowing animals, it is metamorphosed into a most efficient instrument for the excavation of earth. — The skeleton of the Bat is remarkable, in the first place, for the high development of all the bones connected with the movement of the anterior extremity ; for the breadth of the scapula, the strength of the clavicles, and the projecting keel of the sternum. The humerus (Fig. 190, li ) is not of disproportionate length; the bones of the fore-arm, however, are unusually long, and we find that, as no movement of rotation is required, the ulna (cu) is for the most part in a state of coalescence with the radius (r), as in the Horse. The bones of the carpus are small, but they serve to support the metacarpal bones, which are of extraordinary length, and are capable of a wide divergence from each other. These metacarpal bones support the phalangeal or true Fig. 190. 7J^/> Skeleton of Bat ; — o, scapula; cl, clavicle; h, humerus; cu, ulna; r, radius; ca , carpus; po, thumb; me, metacarpus; ph , phalanges; f, femur; ti, tibia. finger-bones, the number of which is the same in Man ; and it is over these, with the assistance of the hind-legs and tail, that the membrane of the wings is extended. The thumb, however, does not partake of this extension in length, nor does it assist in the support of the wing- membrane ; but it is short and free, and is terminated by a hooked claw, with which the animal can suspend itself when at rest. — It is marvellous to see how the same elements are adapted to such widely different pur- poses, as the support and propulsion of the body through the yielding air, and the excavation of a passage through the solid earth ; yet in the burrowing trowel of the Mole we have the same elements as those which present themselves in the expanded wing of the Bat, developed (as it were) in precisely the opposite direction ; all that is elongated and attenuated in the bat being shortened and thickened in the mole, witli the exception of the scapula, which is here a long and narrow but very strong bone. The clavicle has almost a cubical form ; the humerus is nearly as broad as it is long, its extraordinary expansion being not merely for strength, but also for giving an extended surface for muscular ^attachment ; the radius and ulna are likewise short and very thick bones, yet are so constructed as to be capable of admitting the rotation of the 530 GENERAL PHYSIOLOGY. fore-arm ; the carpal bones form the usual series, with the addition of a large sabre-shaped ossicle, which strengthens the digging edge of the broad palm ; and these support the usual number of metacarpal and phalangeal bones, nearly all of them as broad and thick as they are long, and all five digits being buried, as far as the strong digging claws, in a sheath of tough skin. With such rapidity does the Mole accomplish its work of excavation, when the soil is light, that it has been figuratively said to ‘ swim through the earth.’ — In the Cetacean group, on the other hand, we find the anterior extremities converted into paddles for assisting in the propulsion of the body through the water ; and in the general arrange- ment of their parts, we trace a close analogy to the pectoral fins of Fishes, of which they are the representatives. Thus, in consequence of the thinness of the cervical vertebrse (§ 326, b), and the consequent rudimentary condition of the neck, they are placed immediately behind the head, although not connected with the occipital segment ; the arm and fore-arm are very short, and in great part included within the muscles of the trunk ; and the expansion of the fin is almost entirely supported by the bones of the hand. Still we trace in the conformation of all these parts the essentially Mammalian type, so that the skeleton of the anterior extremity of the Whale bears far more resemblance to that of the arm of Man, than it does to that of the pectoral fin of the Fish ; the two bones of the fore-arm, although partly anchylosed together, have their normal relations ; the carpal bones are all determinable as the representatives of those of the terrestrial Mammalia ; and the only real approximation to an inferior type consists in the multiplication of the phalangeal bones, in which respect the Cetacean paddle slightly resembles that of the Enaliosauria (§ 324, l). Still the number of digits never exceeds five ; and the conformity to the Mammalian type is curiously shown in this, that whatever be the number of phalanges, the first digit (representing the thumb) has a smaller number than the rest. 326 h. With regard to the posterior extremities, there is comparatively little to remark. In the Seals, which are ordinary Carnivora partially modified for the aquatic life of Fishes, we find the posterior extremities carried backwards in a horizontal direction, and supporting two broad expanded fins, by which they swim with great facility and strength : their motions on land, which are chiefly effected by the anterior extremities, being proportionally awkward and shuffling. In the Cetacea, which ire adapted to an exclusively aquatic habitation, the posterior extremities are altogether deficient, only a rudiment of the pelvic arch being discoverable (§ 326 d) ; but their place is taken by the horizontally-expanded tail, which still more effectually answers the purpose of a propelling instru- ment ; and the power of movement on land is so completely lost, that a Whale which is ‘ stranded ’ on the shore cannot get itself off On the other hand, there are several Mammalia in which the posterior extremities are, as in Frogs, the chief instruments of locomotion ; and this not. as in Man, by their alternate movement in walking or running, but by their combined action in leaping. When this is the case, they are developed to a much greater size than the anterior members ] and the strength of the body is chiefly shown in the pelvic arch which supports them, and in the muscles by which they are put in action. We see an approach to this type of conformation in many of the ordinary Bodentia, such as the GENERAL VIEW OF THE ANIMAL KINGDOM. MAMMALIA. 531 Hare and Babbit, which, though four-footed in their ordinary progression, can move by leaps in which the posterior extremities are the chief agents ; in the Beaver and Squirrel, the disproportion is still more strongly marked, the body being very commonly supported upon the posterior extremities, whilst the anterior are employed for prehension by opposition with each other ; and in the Jerboa, the difference is still more striking, the power of progression being almost entirely committed to the posterior members. It is in the Kangaroo, however, that this peculiar type of conformation is most strikingly exhibited ; for, although this animal partly supports itself upon its fore-paws when browsing upon the herbage, its movements are entirely accomplished by its hind-limbs and tail, this last member, which is of enormous thickness and strength, affording important assistance in them. The extraordinary length of the feet chiefly depends upon the elongation of the metatarsal bones, especially that of the fourth toe ; the first or innermost toe being generally absent altogether, the second and third being present only as rudiments, and the fifth being of large dimensions but inferior in size to the fourth. The under surface of the foot is covered with a callous sole along its whole length ; whilst the fourth toe is furnished with an elongated hoof, which is sharp- pointed and three-sided like a bayonet, and serves as a powerful weapon of defence. The anterior extremities, though comparatively small, are highly developed as prehensile instruments ; approaching in this respect to those of some other Marsupials, in which we almost see the Quadrumanous type repeated. — Somewhat of the same remarkable con- trast, having reference, however, to a very different purpose, is presented between the anterior and posterior extremities of the great extinct Sloths ( Mylodon , Megatherium, &c). For whilst the former are constructed (as already noticed) partly as excavating instruments, and partly for support, the whole conformation of the posterior extremities, of the pelvic arch which supports them, and also that of the tail, points to these members as having possessed the capability (as in the Kangaroo) of supporting the principal weight of these enormous animals in the semi-erect position ; and thus leads, with other data derived from the structure of their teeth and the general conformation of their skeleton, to the almost irresistible conclusion, that whilst these giants of a period long gone by must have been supported upon food of precisely the same nature as their pigmy representatives of the present time, — namely, the leaves and soft shoots of forest-trees,— they obtained that food, not, like the existing Sloths, by climbing the stems and clinging to their branches, which would not have sustained their weight, but by excavating the soil from around and beneath their roots with their scraper-like fore-feet, and then, by rearing themselves upon their hind-legs and tail, placing their fore-legs against the trunk, and applying to it the w T hole force of their monstrous bodies, laying it prostrate upon the earth, to browse upon it at their leisure.* 326 i. The skull of Mammalia, so far as regards the number and arrangement of its individual bones, presents a general conformity to one type, which is highly characteristic of the class ; and the greatest departure * See Prof. Owen’s ‘Memoir on the Mylodon;’ a masterpiece of philosophical reasoning, built upon the most careful and exact comparison of the peculiarities of conformation of this singular group of extinct animals, with those of all existing forms to which it has any alliance. mm2 532 GENERAL PHYSIOLOGY. from this presents itself (as might be anticipated) in the lowest order of the Implacental sub-class, the cranial conformation of the Monotre- mata presenting many curious points of resemblance to that of Birds and Reptiles ; whilst the next degree of modification is perhaps exhibited by the Cetacea, in which the skull has certain characters that remind us of Fishes. Excluding these, however, we find that the ordinary Placental Mammalia do not present any wider departure from the common type in one direction, than Man exhibits in the other ; and it will be convenient to take his cranium as the standard of comparison, in pointing out the chief peculiarities which distinguish the Mammalian skull from that cf other Vertebrata, and the principal varieties which present themselves in the conformation of the several orders of the group. The most impor- tant distinctive peculiarities of the skull of Mammalia are the following : — the cranial cavity, which is of a rounded form, is shut in by a set of bones connected with each other by sutures, which usually remain visible during the whole of life, although they are sometimes obliterated with age ; the number of these bones is much less than in Reptiles (the differ- ence being at once seen in the single ‘ occipital ’ bone, which represents the four bones forming the neural arch of the occipital vertebra in tLe Crocodile) ; the articulation of the cranium with the spinal column is now no longer effected by the ‘ centrum ’ of the occipital vertebra, but is formed by two prominent ‘ condyles,’ one on either side of the ‘ foramen magnum’ or neural canal, which represent the ‘ zygapophyses’ or articu- lating processes of ordinary vertebrae, and are received into corresponding depressions in the broad zygapophyses of the ‘ atlas’ or first vertebra ; the bones of the face are all (with the exception of the lower jaw) immoveably articulated to each other and to those of the cranium ; and the lower jaw articulates directly with the cranium, having a single convex ‘condyle' on either side, which is received into a ‘ glenoid cavity ’ at the base of the skull, instead of being connected with it (as in all oviparous Vertebrata ; by the intermediation of a moveable ‘os quadra turn’ or tympanic bone. — The principal variations, on the other hand, which we encounter in the crania of different orders of Mammalia, have reference chiefly to the rela- tive development of the cranial and facial bones, the adaptation of the jaws to different kinds of food, and the support of organs specially deve- loped in particular groups, such as the horns, tusks, proboscis, er part of the occipital, for the protection of the cerebral mass beneath. We do not, in the Mammalia, find the ‘ parapophyses ’ of this segment developed as distinct elements; they are represented by the ‘ external angular processes’ of the frontal bone, which shoot forth by exogenous growth from the proper frontal, instead of constituting independent ele- ments like the ‘ postfrontals ’ of Fishes. — Of the Ethmoid bone, which completes the category of bones usually ranked as forming the cranium proper, only the ‘os planum’ (according to Professor Owen) forms part of the vertebral skeleton; the remainder being a true ‘ sense-caps' do, though developed in the higher animals in an unusual degree, aud form- ing most intimate relations with the bones among which it is intercalated GENERAL VIEW OF THE ANIMAL KINGDOM. MAMMALIA. 535 (§ 320 i, note). The ‘ossa plana,’ supported upon the vomer, represent the ‘ neurapophyses ’ of the Nasal or rhinencephalic vertebra. In their turn they support the Nasal bones, which together represent the ‘ neural spine ’ of the same vertebra ; these, each developed from a single centre, coalesce into a single bone in certain Mammals, like the frontals and parietals. The ‘ centrum ’ of this vertebra is formed by the Vomer, whose form and relations in Man would seem to give it no title to such a distinction, but whose conformation in Fishes is altogether different, and marks its true character (§ 320 i). When we consider the modi- fications which the terminal caudal vertebrae undergo, by the suppression of some of their elements, and by the peculiar conformation or arrange- ment of others, it is not surprising that this, the segment which forms the anterior termination of the body, should be subject to similar irregu- larities. Neither in the Human subject, nor in the lower Vertebrata, do we find any distinct representative of its parapophyses. 326 l. Turning now to the bones which represent the haemal arches of the cranial vertebrae, it will be convenient to proceed first with those which constitute the face ; and in this manner we shall be carried from before backwards, as in the enumeration of those of the cranium we have passed from behind forwards. — The Superior Maxillary bone is more simple in its composition, than its form in Man would at first seem to indicate. It appears to be developed from several distinct centres on either side ; but most of the parts thus formed coalesce at a very early period ; and each bone consists essentially of two portions, the true ‘ maxillary,’ and the ‘ intermaxillary,’ of which the former represents the ‘ hsemapophysis,’ and the latter a ramus of the ‘ hsemal spine.’ In the Human subject, the inter- maxillary bone usually coalesces with the maxillary long before birth ; but it remains distinct in most of the lower Mammalia, and attains an enormous development in the Rodentia and also in the Elephant, in which it bears the peculiar ‘ tusks ’ growing from persistent pulps, which are characteristic of these animals ; and it is sometimes found to continue separate even in Man. — The Palatine bones, which, notwithstanding the complexity of their form and connections, are developed from single centres, are the ‘ pleurapophyses ’ of this segment ; and it is very interesting to see how completely this view of their character harmonizes with, and even (so to speak) explains, their very peculiar connections. Thus we see them coming into connection above, not merely with the ‘ centrum ’ (vomer) but with the ‘ neurapophyses ’ (ossa plana of the ethmoid), and also with the ‘ sense-capsule ’ (cellulse ethmoidese) of this segment ; whilst below they articulate with the ‘ luemapophyses ’ (maxillary bones) and with the ‘ diverging appendages ’ (pterygoid bones) which belong to them. — The Malar bone is another ‘ diverging appendage ’ of this segment, passing backwards (like the brancliiostegal rays of Fishes) towards the temporal region, and there expanding, as it were, into the squamous por- tion of the temporal bone. Thus we have, in this anterior segment, the highest development of the hsemal arch, with the lowest condition of the neural. — The Inferior Maxilla, or lower jaw, of Man, and of the Mamma- lia generally, presents a striking contrast, in its simplicity of conforma- tion, with the extraordinary complexity which the tympano-mandibular arch exhibits in the lower Vertebrata. It is always developed in two dis- tinct halves ; and these remain separate during life in a large proportion 536 GENERAL PHYSIOLOGY. of the inferior Mammalia, being only united at the symphysis by liga- mentous connections. Each half is developed from at least two nuclei in the human subject, one producing the ‘ articular ’ portion that forms the joint, the other forming the ‘ dentarium ’ which contains the teeth ; and of these, the former constitutes the ‘ haemapophvsis,’ and the latter a ramus of the ‘haemal spine,’ of the Frontal vertebra. The ‘pleurapo- physes ’ of this segment enter into the composition of the temporal bone, forming its tympanic element, and no longer participating in the articula- tion of the lower jaw. This tympanic portion is far more largely deve- loped in some orders, than it is in Man ; thus in the Cetacea it forms an almost solid mass of ivory, of considerable size, which is scarcely at all connected by bony union with the mastoid. — Thus we have accounted for all the component bones of the Human skull, save the ‘ lachrymals,’ the ‘turbinate bones’ of the nose, the ‘bones of the internal ear,’ and the ‘ teeth.’ The lachrymals do not in reality belong to the neuro-skeleton, but are portions taken in from the dermal skeleton, and connected with the proper vertebral elements ; just in the same way that the mar- ginal and other bones of the carapace of Turtles are introduced and blended with the spinal column and ribs (§ 324 li). The turbinate bones constitute part of the ‘ sense-capsule ’ of the olfactive organ, being ana- logous to the petrosal element of the temporal bone, which is the ‘ sense- capsule ’ of the organ of hearing ; there are not in Mammalia any bony plates in the fibrous sclerotic, forming a sense-capsule to the eye. The bones of the internal ear, and the teeth, belong, like the sense-capsules, to the ‘ splanchno-skeleton.’ — We have as yet noticed,, however, the consti- tuents of the haemal arches of only the two anterior cranial vertebra;. That of the third, of which the greater part is known as the Hyoid bone, is yet more reduced than the second ; yet can we clearly trace its principal elements. In the Hyoid bone, simple as its form is in Man, there are no fewer than five centres of ossifi- cation, one for the ‘ body,’ one for each of the ‘ great cornua,’ and one for each of the ‘ lesser cornua.’ The body, or ‘ basi-hyal,' is obviously the ‘haemal spine’ of this arch : the lesser cornua, or ‘ cerato-hyals,’ which are connected by ligaments to the styloid processes of the temporal bon>\ are its ‘ haemapophyses ;’ the greater cornua, or ‘ thyro-hyals,’ developed as appendages to the larynx, are its ‘diverging appendages;’ and the styloid processes of the temporal bone, as already mentioned, are its ‘pleur- apophyses.’ This styloid process is the last to coalesce with the other ele- ments which unite to form the temporal bone. In several of the lower Mammalia, the cerato-hyals are considerably longer than the thyro-hyals, instead of being far inferior to them in size, as they are in Man ; and in some cases they form a continuous bony arch with the styloid bones to which they are articulated. 326 m. The dermo-skeleton, in the greater number of Mammalia, is reduced for the most part to the condition of Hair (§ ISO) ; a form of tegumentary appendage which is peculiarly characteristic of this class, being rarely or never entirely absent, and not presenting itself in the Fig. 191. GENERAL VIEW OF THE ANIMAL KINGDOM.— “-MAMMALIA. 537 same form elsewhere.* An exuviation or moulting of the hair, which falls off and is replaced by a new growth, takes place to a considerable ex- tent in most Mammalia provided with this covering ■ sometimes with, and sometimes without, an alteration of colour. This change usually takes place either in spring or autumn, or at both periods. The degree in which the hair covers the body, varies greatly in the several orders and genera of the class ; and it is interesting to observe that its deficiency is most marked in those which in other respects represent the squamigerous (scale-bearing) Ovipara. Thus in the Cetacea (Whales), which are (so to speak) Mammalian Fishes, we have an almost entire absence of hair, without, however, any approach to a scaly covering in its place ; one of the great purposes which the hairy investment usually serves, — that of keeping in the heat of the body, — being here provided for by the thickening of the skin itself, and the deposition of an enormous quantity of fatty matter in its areolar interspaces, forming the ‘blubber.’ On the other hand, in many of the Edentata (the group which, of all placental Mammals, approaches most nearly to Reptiles), we have the hairy covering in great part replaced by horny scales or by osseous plates. The former is the case in the Manis, or ‘ scaly ant-eater,’ whose body is almost entirely covered with dense horny scales which have an imbricated arrangement ; these scales, how- ever, seem as if composed of a congeries of hairs ; and in certain parts on which they are deficient (generally speaking, the under surface of the head, trunk, and tail, and the inner surface of the limbs), the skin is furnished with scattered hairs in their stead. When alarmed, the Ma- nis rolls itself into a ball, wraps its tail over its head, and raises all its pointed and sharp-edged scales in such array, as to defy the attack of almost any possible enemy. In the remarkable Chlamyphorus, the upper side of the body is covered by a tesselated cuirass, composed of a series of plates, in apposition with each other at their edges, but each enclosed in a membranous sac of its own ; so that these are probably (like the scales of Fishes) dermal rather than epidermic ; the under side of the body of this animal is covered by soft silky fur, resembling that of a Mole. In the Armadillo, the body is much more completely inclosed by its dermal cuirass, which invests the same parts as those covered by scales in the Manis, with the addition of the under side of the tail, which member is completely enclosed in a succession of bony rings. The bony armour consists, first, of a triangular or oval plate on the top of the head, which projects backwards over the neck ; secondly, of a large cuirass or buckler which covers the shoulders ; thirdly, of a series of transverse bands, somewhat moveable one upon another, which nearly encircle the body, then- number varying with the species ; fourthly, of another cuirass which covers the haunches ; and fifthly, of the series of bony rings investing the tail. Each of these distinct segments consists of a number of small pieces united to each other, like the separate portions of a mosaic, or of a tesselated pavement ; and this arrangement allows of a certain degree of yielding at every part of this coat of mail, although it is only at the joints between the segments that any proper movement of one part upon another takes place. The limbs, which are short and thick, are almost * The so called ‘hairs’ of Insects and Annelida are not properly comparable with those of Mammals ; being, in fact, only elongated epidermic scales, and not being developed from cutaneous follicles. 538 GENERAL PHYSIOLOGY. entirely concealed by the edges of this projecting shield ; and these ani- mals, when alarmed, can so roll themselves together, and draw the head and limbs within the protection of the cuirass, as to present nothing to their enemies but a bony coat of mail enveloping them on every side, — an arrangement which strongly reminds us of that which exists in the ‘ box-tortoises ’ (§ 324 h). The skin of the parts unprotected by the bony armour is tough, and is beset with long scattered hairs, of which some are seen to issue forth between its joints. In the gigantic extinct Glyptodon, which was formed upon the plan of the existing Armadillo, the bony cuirass was of still greater density, and formed a vast undivided dm sal shield entirely composed of tesselated plates united together at their edges, without any joint through its entire extent, and overhanging the sides of the body, without, however, covering any part of its under surface. Tne tail was completely enclosed in a similar envelope. It is obvious that tins animal could not have possessed the means of passive defence which the puny Armadilloes of the present time enjoy, whilst its vast dimensions were in themselves a sufficient protection ; for if it crouched, by the flexure of its short limbs, in such a manner as to allow the edges of the shield to rest upon the ground, and then withdrew its head beneath the front of the cuirass, none but a beast of far greater power than any Car- nivora of the present day could have overthrown it, so as to get at the unprotected parts beneath. 326 n. The other modifications of hair, which depart less from its ordi- nary character, such as the spiny bristles of the Hedgehog, and the quills of the Porcupine, have been already noticed (§ 180) ; and the structure of the horny substance that forms the nails, claws, and hoofs, which are organs of great functional importance in the Mammalia, constituting part of their dermo-skeleton, has also been described (§ 179). It remains, however, to speak of the horns, the possession of which is a remarkable feature of the Ruminant group, though some approaches to it are seen elsewhere. With the exception of the Camels and Musk Deer, the male' (and sometimes the females) of all the species of this order bear upon their frontal bones one and occasionally two pairs of bony prominences ; which are sometimes simple cones, more or less elongated and twisted ; but which in other cases branch and spread out to a very great distance from their base. The bony protuberances may be covered by the skin of the forehead, which does not differ at this part from that of the rest of the body, and which does not undergo any change ; such is the character of the horn in the Giraffe, in which the bony core is articulated with the frontal bone, and not an outgrowth from it. In other instances, the bony core has more the character of an out-growth from the frontal bones, and is covered with a horny sheath, which grows with it during life and never frills off; this is the case with that division of the group which includes the Oxen, Sheep, Goats, and Antelopes. In the Deer tribe, on the other hand, the bony portion of the horns is at first covered with a soft bony skin, termed the ‘velvet,’ and never becomes invested with horn : tins skin dries up and wears away, so that the bone becomes bare ; and at the same time, the vascular canals of the interior of the bone become gradu- ally blocked up by the progressive consolidation of its substance. At last the nutrition of the bone ceases completely ; it dies and frills off, like a dead leaf; and is very speedily renewed by the growth of a new horn. GENERAL VIEW OF THE ANIMAL KINGDOM. MAMMALIA. 539 which is usually larger, and possesses more branches, than that -which it replaced. The horns of the Deer, being thus purely bony, are properly called ‘ antlers ;’ and their exuviation and renewal usually take place in the spring of every year. It is only in the Reindeer, that the female, as well as the male, possesses antlers. — To describe these bony ‘antlers’ as a part of the dermo-skeleton, rather than of the endo-skeleton, may appear inconsistent with the history of their development, which seems to take place by exogenous growth from the frontal eminences. But their annual exuviation is a very marked indication of their dermal character, no such exuviation taking place in any portion of the neural skeleton, whilst it is a very ordinary occurrence in the dermal ; and they seem naturally to fall into the same category with the dermal portions of the carapace and plastron of the Chelonia (§ 324 h), which are developed in continuity with the parts of the vertebral skeleton that support them. — The single or double horn which the Rhinoceros bears upon its snout, is a prmely epi- dermal formation, and may be compared to a bundle of hairs adherent together. In this animal, as in many other Pachydermata, the epidermis covering the general surface of the body is of extraordinary thickness and of almost horny density ; whilst the hairy covering is very scanty. A similar local development of a horny epidermis in unusual amount, forms the callous pads on the under side of the feet of most Unguiculated Quad- rupeds, the callosities on the buttocks of the Quadrumana, &c. 326 o. The Teeth of Mammalia constitute so characteristic a feature in their organisation, and afford such important assistance in classification, as to require a much more special mention than was necessary in the case of Reptiles and Fishes. There are few animals belonging to this class, in which these organs are altogether absent ; and these animals belong to the groups which have been already alluded to as the most aberrant. No rudiments of teeth are found either in the true Ant-eaters, which belong to the Edentate order, or in the Echidna (or ‘spiny ant-eater’ of New Hol- land), one of the members of the Monotrematous order ; and in the Orni- thorhyncus, which is the only other existing Monotreme, the teeth are only represented by horny projections on the surface of the bill-like sheath with which its jaws are covered, the purpose of these projections, as of those which are found in many piscivorous Birds, being simply to increase the prehensile power of the bill. In the Whalebone-Whales, no bony teeth ever appear above the gum, although their rudiments are found already calcified in the jaws of the embryo. These are the only exceptions to the general rule of the presence of teeth in this class. — The number of teeth m Mammalia is usually much more restricted than in the lower Verte- brate, Through a considerable proportion of the class, it is the same as in Man, namely 32 ; but the typical number is considered by Prof. Owen as 44. Now it is curious that here, again, the greatest departure, both in the way of reduction and excess, should be found among the Fish-like Cetacea. In two genera of this order ( Ilyperoodon and Zipliius ) only two teeth are developed ; in both cases from the lower jaw. In the male Jfarwhal, although the rudiments of two teeth are formed and sometimes developed, one of them is usually abortive, whilst the other grows to an enormous size, this unequal development being accompanied with a want of symmetry in the jaw-bones themselves (§ 32 On) ; hence this genus received from Linnaeus the name of Monodon (single-toothed), which is 540 GENERAL PHYSIOLOGY. not in reality much more appropriate than its common name of ‘ Sea- Unicorn.’ On the other hand, the Cachalot (Spermaceti Whale) has upwards of 60 teeth, although most of them are confined to the lower jaw; the Porpoise has between 80 and 90 teeth ; and the Dolphin as many as 190. The only other similar example of great excess is presented in one of the Reptile-like Armadilloes, which has 98 teeth. In all these cases of unusual multiplication, the teeth are, like those of most Reptiles, com- paratively small, and nearly similar to each other in size and shape. This similarity extends also to the teeth of other members of the E dent. if e order, in which these organs present themselves in then - simplest and least specialized condition ; such, as the ordinary Armadilloes, the Orycterope, and the three-toed Sloth. In the two-toed Sloth, we perceive the fust approach to specialization, in the increased size of the two anterior teeth of the upper jaw, which are adapted for cutting and tearing, whilst the remainder are only fit for crushing substances of no great hardness. In nearly all the other Mammalia, we find the principle of specialization remarkably carried out in the teeth ; diversified forms being given to them, whereby they are adapted to different uses. The front teeth usually termi- nate in a thin cutting edge ; and being intended simply to effect the first division of the food introduced into the mouth, are termed incisors. On the other hand, the back teeth, which are adapted for the further division or comminution of the aliment, are termed molars, or grinding teeth : a term, however, which is by no means appropriate to the molars of the Carnivora, whose action is simply cutting. Between these, we very com- monly find certain teeth of conical form, projecting beyond the rest, which are not adapted either for cutting or grinding the food, but for taking a firm hold of it, whereby the animal may tear it asunder ; these, from being well developed in the Dog, are called canine teeth, and are the especial attribute of the Carnivorous order, though developed in some Pachyder- mata (as the Boar) and in many Quaclrumana, as weapons of attack and defence. The first and the last of these kinds of teeth may be deficient ; the molars, however, are never wanting. 326 p. In the mode in which the teeth of Mammalia are implanted in the jaw, we find a marked distinctive character of the class. In all save those which grow from persistent pulps (§ 218), we find the dental cavin' closed-in at its lower part, and the base of the tooth prolonged into a ‘ fang,’ which is implanted into a proper ‘socket’ formed by a projection of the bony substance of the jaw that grows up to invest it. Each tooth has its particular socket, and the exterior of the fang, which is formed of the ossified capsule of the tooth (§ 215), adheres firmly to its periosteal lining ; there is never, however, such a continuous ossification of the tooth to the jaw, as constitutes its mode of attachment in many Fishes (§ 213). The most imperfect form of the regular mode of connection between the teeth and jaws of Mammalia, shows itself in some of the Cetacea, in which the sockets, at the posterior part of the jaw, are wide and shallow, and the teeth adhere more strongly to the gum than to their periosteal lining ; so that, when the gum has been stripped away, in the Cachalot, it has brought all the teeth away with it. The simplest form of the fang, in the higher Mammalia, is that which is presented by the incisors and canines, in which it is for the most part a simple cone, sunk in a corresponding conical socket. In the molar teeth, however, the fang GENERAL VIEW OF THE ANIMAL KINGDOM. MAMMALIA. 541 is usually subdivided into two, three, or even four portions, which diverge more or less from one another ; and as the sockets are moulded (as it were) upon these diverging processes, it is obvious that this method of implantation gives to the jaw a most secure hold of the teeth. Although some approach to the implantation of the teeth in distinct sockets, by simple fangs, is seen in Fishes and in Crocodilian Reptiles, yet there is no known Fish or Reptile, existing or extinct, in which teeth are thus implanted by bifid fangs ; and this character, therefore, appears suffi- cient to determine the Mammalian nature of a jaw, or even a fragment of 1 a jaw, in which it occurs, such as that of the Amphitherium and Phascolo- therium of the Stonesfield Slate, whose Reptilian nature was advocated by many zoologists, but which are undoubtedly to be ranked as extinct Mammals. — True teeth implanted in sockets are found nowhere else in the Mammalian class, than in the margins of the upper and lower jaws ; they may be confined to either one or the other of these, as we see in the Cetacea; they may be wanting in the intermaxillary bone, as in the Ruminantia, which have incisor teeth developed only in the lower jaw, although their germs may be detected in the upper ; or they may be wanting in the front of both jaws, as in the Sloths. Generally speaking, they include the three kinds of tissue which have been described as making up the complete tooth ; namely, the Dentine, which is usually non- vascular, the Enamel, and the Cementum (§ 212). The first of these is the most constant element, and makes up the bulk of the teeth ; the second is absent in the Sloths (both extinct and fossil) and in some of the Cetacea, thus presenting another character of degradation in the dental apparatus of these orders ; the third is present in all instances around the lower part of the tooth, and originally forms a thin layer over the crown (which, however, is soon worn away) in Man, Quadrumana, and Carnivora ; but it forms a large part of the substance of the molar teeth of many Herbivorous Quadrupeds, in which it is imbedded, with vertical plates of enamel, in the dentine of the crown during its development (§ 21G), in such a manner as to form an uneven surface for trituration, the rough- ness of which is constantly preserved, however much the tooth may be worn down, by the unequal density of these tissues. In the molar teeth of the Sloths, the only provision for the maintenance of a grinding- surface consists (the enamel being absent) in the difference between the wear of the hard or non-vascular dentine which forms the outer part of each tooth, and the softer or vascular dentine which occupies its centre (§ 212). The teeth of the Orycterope present us with a very interesting- reversion to the inferior type, being of a composite nature, like that of the teeth of many Fishes, which are made up by the coalescence of several little ‘ denticles,’ each having its own medullary canal (the representative of the pulp-cavity) and its system of radiating tubuli. 326 q. The Mammalia further present us with certain marked pecu- liarities in the succession of their teeth. We have seen that in Fishes and Reptiles the teeth have not a permanent character ; but that, like most epidermic formations, they are continually being cast off and renewed ; the new development taking place either from new and independent papillae, or from offsets from the follicles of the previous teeth. In Mam- malia, on the other hand, this process is restricted in a much less de- gree j the shedding and renewal not taking place, except in some peculiar 512 GENERAL PHYSIOLOGY. cases, more than once during the whole of life. In the Cetacea , the inferiority of whose dental organisation has been already noticed' the follicles of the teeth first formed never give off gemmae for the production of successors ; so that when once completed, and worn down or torn away, these teeth are not replaced. In those Edentata, too, which are provided with teeth (Sloths and Armadilloes), this germinal power seems to be deficient ; but its want is here compensated by the persistence of the matrix (or tooth-pulp), and by the uninterrupted growth of the teeth. In most other Mammalia, we find the entire series of teeth first developed (which are commonly known as the ‘ deciduous ’ or ‘ milk-teeth’) replaced at an early period of life by a second or ‘permanent’ set ; the latter having a size and form as suitable to the jaws of the adult, as the former were to those of the young animal. Of these permanent teeth, all that occupy the places of the milk-teeth are developed from offsets detached from their follicles (§ 217) ; but the number of the ‘ permanent ’ set is greater than that of the ‘ deciduous,’ and all the additional teeth are molars, which come up behind the •premolars (known in Man under the name of ‘ bicuspids ’) that replace the molars of the milk set. The fore- most of these true molars originates (as in Man) in a new and independent matrix, and is therefore like a milk-tooth retarded in its development, and endowed with unusiral permanence of character. It further resembles the milk-teeth in its power of producing another by gemmation ; this , how- ever, does not come up in place of it, but is developed posteriorly t< > it. and forms the second true molar ; and from this, again, another follicle is budded off, which gives origin to the thud true molar that comes up at a later period of life behind the second. In most of the Placental Mam- malia, the process of gemmation stops short at this point, so that ouly three true molars are ever developed ; but in many of the Marsupials, it advances one more stage, and produces four true molars. It is remarkable that, in the Kangaroo, the first of these molars should be shed before the last is developed ; and it is by a still more complete change of the same kind, that the curious succession of molars takes place in the jaw of the Elephant, which was long regarded as an altogether exceptional pheno- menon. Each of these molars is a large composite tooth, occupying a considerable part of the lateral segment of each jaw; and it is formed of a succession of vertically -alternating plates of enamel, dentine, and cemen- tum. The chief wear of these teeth takes place in front ; and a reprod ac- tion of new teeth is continually going on behind ; so that each tooth, as it wears down, is pushed forward by the new tooth behind it, which comes to occupy its place. The jaw almost always contains two such teeth ( a either side ; and sometimes three may be met with, — one that has been nearly worn away, another in full maturity, and a third in an early stage of advancement. The molars are thus changed six or eight times ; each tooth is of greater dimensions, and contains a larger number of vertical plates, than the one which preceded it ; and the last formed, which is the largest of all, remains in its place for the rest of the animal s life. In the ‘ tusks ’ of the same animal, and of its extinct allies, we have a gigantic example of a very different mode of development ; namely, the continued growth of a persistent tooth, whose formative pulp remains uninclosed (through the absence of root) during the whole of life, and is continually undergoing conversion ; these tusks are known, by their GENERAL VIEW OF THE ANIMAL KINGDOM. MAMMALIA. 543 implantation in the intermaxillary bones, to be really ‘ incisors.’ In the extinct genus Mastodon, the lower as well as the upper jaw possessed tusks, in the young state, of dimensions inferior to those of the others ; and one or both of this second pair were retained by the males in adult age, although both were shed by the females without being renewed. The mode of development of the molar teeth, also, w T as strikingly different in these two genera 3 for the first and second deciduous molars in each jaw of the Mastodon were replaced by permanent molars, which pressed upon them not from the bach but from the base of the socket, as in ordinary Mam- mals. In the gigantic Dinotheriiim, the tusks were developed from the lower jaw alone. In several other Pachydermata, as the Hog, Babi- roussa, and Hippopotamus, the canines are developed as ‘tusks,’ con- tinuing to grow from persistent pulps ; this is the case also in the Musk- deer, which in some degree connect this order with the Ruminants ; and we meet with it again in the Walrus among the Carnivora. This peculiar arrangement, in the foregoing cases, would seem rather to have reference to the special habits of the animal, than to follow any regular plan. But in the Rodentia, the entire order is characterised by the growth of the long incisors from persistent pulps, whereby the teeth are pushed up by additions to their base as fast as they are ordinarily worn away at their cutting edges ; and in some of this order, as also in the Edentata, the molars also are developed from persistent pulps, so that in proportion as their surfaces are worn away by the trituration of the tough vegetable substances that constitute their food, they are kept up to their level by new growth at the base. — It is from the great variety of such special modifications as these, of the ordinary type of dental structure, in accordance with peculiarities in the food of the several animals that present them, and in the mode in which they are organized to obtain it ; and from the constancy in the form, number, position, structure, and succession of the teeth, in the greater number of families of Mammalia ; that these organs furnish characters of such value to the Zoologist for the construction of his systematic arrangements. They cannot be taken, however, as the sole or even as the principal basis of such arrangements, without serious infractions of the natural affinities of the several groups ; as happened when Cuvier associated the Monotrematous Echidna and Ornithorhyncus with the ordinary Ant-eaters, in the order Edentata. No arrangement can be really valuable, which is not based upon the -whole collection of the characters of each tribe, and -which is not in conformity with the phenomena of development ; and the value of the characters furnished by the teeth, therefore, entirely depends upon the degree in which they are exponents of the general type of organisation. This is especially the case in the typical Carnivora, and in the Ruminant Quadrupeds which may be regarded as the typical Herbivora. On the other hand, the characters furnished by the teeth especially fail in the aberrant Edentata, in which we have to associate the Sloths, Armadilloes, and Ant-eaters, on account of marked features of accordance in then- gene- ral structure and physiological condition, although they differ widely in the nature of their food and in the mode in which they obtain it ,* and * It is very interesting to note the curious representations of the Reptilian class that occur in this order, which, of all ordinary Mammalia, presents the closest approximation to it. The Manis , or Scaly Ant-eater, in its general form and scaly covering reminds us of the Sauria ; 544 GENERAL PHYSIOLOGY. also in the Cetacea, which, in their dental organisation so closely resemble Fishes.* 326 r. The Digestive apparatus acquires its greatest completeness in this group ; its several parts here attaining their highest development, and the combinations which they present being more elaborate than those elsewhere met with. It is evident, from the increased size and fleshiness of the tongue, and from the large supply of nerves sent to its papillary surface, that the sense of Taste is more acute than in the inferior classes, and that it becomes an important guide in the selection of the food. Its introduction into the mouth is usually accomplished by a pair of fleshy lips, the Ornithorhyncus being the only Mammal whose lips are homy and inflexible, and even its lips being soft at an early age, when it draws its support from the mammary glands of its parent. For the pi’ehension of the food, however, whereby it may be brought within the reach of the lips, we find a variety of provisions, some of which are special to this group, whilst others carry into a higher development the plan which has already been shadowed forth in other classes. Thus of the former kind are the 'trunk’ of the Elephant, formed by a prolongation of its nose ; the elongated proboscis of the Tapir and Desman ; the prehensile tongue of the Giraffe ; and the prolonged upper lip of the Ruminants generally, the pressure of which against the incisor teeth of the lower-jaw enables them to crop the herbage on which they feed. But the highest foim of prehensile instrument is undoubtedly that which is constituted by the special modification of the anterior extremities for this purpose. This modification is most complete (as we have seen) in Man, in whom the anterior members are not used for support or progression ; but we observe that these limbs are constantly employed in the prehension of food by the Quadrumana, and in a less degree by the Carnivora and Rodentia. — The food, when received into the mouth, is almost always submitted, in greater or less degree, to the masticating process ; which in Carnivorous animals, however, consists in little else than the division of the food by the sharp- edged molar teeth ; whilst in Herbivora, it subjects the food to a crushing and triturating action, either before it is swallowed, or, in the Ruminant order, on its return to the mouth after being macerated in the fluids of the first stomach. In all Mammals to which the masticating process is impor- tant, the salivary glands are highly developed ; and their secretion appears not merely to moisten the food, but to accomplish a change in cert.' m elements of it, which is the first act of the digestive process. The cavity of the mouth is sometimes dilated in a very remarkable manner into ‘ cheek- pouches ’ for the storing-up of food ; these are met with in most of the Simiadce, or Monkeys of the Old World, and also in some Cheiroptera : but they attain their highest development in certain Rodentia, as the the Armadillo presents us with the closest Mammalian analogy to the Chelonia ; and the great extinct Sloths ( G-ravigrada of Prof. Owen) had much in common with the herbivorous Dinosauria. * Ample details upon the Teeth of Mammalia will be found in the elaborate ‘ Odonto- graphy’ of Prof. Owen ; to which the Author has been chiefly indebted for the materials of the preceding sketch of their dentition, as for the account previously given of the teeth of Reptiles and Fishes. Prof. Owen’s latest views on the succession of Teeth in Mammalia, as the guide to the determination of their homologies in different groups, will be found in ms article Teeth in the ‘ Cyclopaedia of Anatomy and Physiology;’ and in his Memoir on the Teeth of the Wart-Hogs in the Philosophical Transactions for 1850. GENERAL VIEW OF THE ANIMAL KINGDOM. MAMMALIA. 545 Hamster, in which they extend deeply down the neck, and are compressed by peculiar tegumentary muscles. 326 s. The oesophagus is usually a long and narrow canal, passing down through the thoracic cavity until it reaches the Stomach. The form of the latter organ varies remarkably in the different orders of Mammals ; being, for the most part, a simple saccular dilatation of the alimentary tube, in those which are most purely carnivorous in their diet ; but being- rendered more or less complex, in these which feed on vegetable substances of various kinds, by the subdivision of its cavity into two or more chambers, in the first of which (as in the proventriculus of Birds) the food undergoes a preparatory maceration, whilst it is in the last that the true process of digestion takes place. It is in the Ruminant tribe, as will be described hereafter (chap, x.), that the most complex organisation of the stomach is met with. The length of the Intestinal canal, again, is subject to great variations ; and between these, also, and the food of the animal, some relationship may be generally traced. Thus in the purely Carnivorous tribes, the intestine is not above 3 or 4 times the length of the body ; whilst in the frugivorous Monkeys, it is 8 or 10 times ; in the Ruminants it is very commonly 20 times, and in the sheep it is as much as 28 times, the length of the body. The Liver and Pancreas attain a very high development in this class. The secretion of the former is poured into the intestinal tube, at a short distance from the stomach, usually by a single duct, which is generally furnished with a dilated receptacle, the gall-bladder, for the storing-up of the secretion in the intervals of the digestive process. The secretion of the latter, also, is convejmd by a single duct, which frequently discharges itself into the intestine at the same point with the hepatic duct. The respective offices of these secretions in the digestive process will be considered hereafter (chap, x.) — The second portion of the intestinal canal is usually much more dilated than the first, and is known as the ‘ large,’ the first part being distinguished as the ‘ small ’ intestine. At the termination of the small intestine, there is frequently a very considerable dilatation, answer- ing to the caecum of Man ; this is very conspicuous in the Ruminants, and still more in the Horse ; but it is in the Rodentia that it attains its greatest development, being usually, in that order, many times larger than the stomach. In this part of the alimentary canal, the food appears to undergo a sort of secondary digestion ; the secretion formed in its walls being of an acid character, like the gastric juice. The walls of the small intestine usually contain a large number of peculiar glandulse, known as the Peyerian from the name of their discoverer ; these, which are some- times solitary, sometimes arranged in large clusters, probably serve more as excreting than as secreting organs, the matters which they separate from the blood being rather destined to be directly carried out of the system, than to perform (like the biliary and pancreatic secretions) any purpose subservient to the digestive function ; and there is reason to believe that their office is the elimination of the peculiar putrescent matter which characterises the fmces, and which is evidently not derived from the undigested residue of the food. This putrescent excretion is found more abundantly in the fieces of Mammalia, than in those of other classes. The anal termination of the intestinal canal is quite distinct from the outlets of the urinaiy organs, in all Mammalia, save in the Mmiotremala, N N 546 GENERAL PHYSIOLOGY. in which, as in Birds, the intestinal and urinary passages open into a common cloaca . — The Absorbent system of vessels attains its highest development in the Mammalia ; for although the length and apparent c< in- volution of the tubes are not nearly so great as in Beptiles, yet the glandular which are developed in their course present the same elements in a form of much greater concentration. The contents, both of the lacteals and lymphatics, are for the most part collected into a single tube, the thoracic duct, and are discharged by it into the venous current, at the angle between the jugular and subclavian veins on the left side ; a smaller duct which terminates at a corresponding point on the right side of the venous system, receives and discharges the contents of the lymphatics of the right side of the head and upper part of the trunk. 326 t. The Circulating apparatus of Mammals is constructed upon the same plan as that of Birds (§ 325/) ; the heart possessing four cavities (Fig. 192) ; and the whole of the blood returned from the system being transmitted for aeration to the lungs, before d-:. Fig - 192 - being again propelled through the systemic 1 “ b ' arteries. The chief peculiarities of this ap- > ; paratus in Mammals consist in the diver- sified arrangements for the distribution of the blood, and more particularly in the de- velopment of enormous extensions both of the arterial and venous systems in the Cetacea and other diving animals. The most important of these modifications will be described hereafter (chap, xii.). — The Respiratory organs, however, present a very different arrangement. The lungs are more minutely subdivided into air-cells, so that they must be considered as attaining a i k higher development than those of Birds, Ideal Section of Mammalian Heart:— although, in the activity of the 1'eSpiratOl'V a, arch of aorta ; b, b' , pulmonary arteries; ° r i ^ e, superior vena cava; a, d\ pulmonary iunction, Mammals are on thewhole interior. veins; e, right auricle ; /, tricuspid valves ; r |’] ie thoracic CUvitv, in which thev lie. is Se- g, inferior vena cava; h, right ventricle; * 7 , , i, septum ventriculorum; k, descending paratGCL 11*0111 the 0.00.01111110.1 by O COlllplCtC nHeft aurfcie' ea ‘ r * cle ’ “ lt,al valve; muscular diaphragm; by the action of which, concurrently with that of other muscles which act upon the ribs, a continual exchange of the air contained in the lungs is provided for. There is no extension of the respiratory surface be- yond the cavity of the chest ; but the large supply of oxygen required by the most active and energetic Mammals, and the free removal of the carbonic acid which they abundantly generate, are amply provided for by this arrangement. For, supposing the expiratory movement to have been just performed, the cavity of the chest, reduced to its smallest dimensions by the ascent of the diaphragm, the descent of the ribs, and the falling-back of the sternum, is enlarged in all its dimensions by the contraction of the diaphragm, which diminishes the convexity of its arch, and by the elevation of the ribs, which brings them more nearly into a straight line with their cartilages, and thus pushes forwards the sternum, whilst it separates the middle points of the opposite ribs from each other laterally, so as to increase the breadth of the thoracic cavity. A vacuum is thus created. GENERAL VIEW OF THE ANIMAL KINGDOM. MAMMALIA. 547 which can only be filled by the expansion of the lungs ; and they conse- quently become distended with atmospheric air, which rushes dowm the trachea, and makes its way through the ramifications of the bronchia} tubes, to the ultimate air-cells. When the inspiratory movement has beeli com- pleted, the diaphragm relaxes, and is pushed up by the abdominal Viscera, under the pressure of the abdominal muscles which are now called into contraction ; whilst the ribs, no longer sustained by the contraction of their elevators, are drawn down again, partly by the contraction of another set of muscles, and partly by the elasticity of their own cartilages ; so that the thoracic cavity is reduced in all its dimensions, and a portion of the air contained in the lungs is expelled from them. There is usually an inequality in the size of the lungs in Mammalia, the heart being situated rather to the left side of the median line, so that a larger space remains for the right lung than for the left ; in the Musk-deer this inequality is very considerable, the right lung being double the size of the left. The Larynx, which is situated at the upper extremity of the trachea, serves the double purpose of regulating the ingress and egress of air, and of produc- ing vocal tones ; two functions which appear to be separately performed in Birds by the upper and the lower larynx. The laryngeal cavity is in- creased in many of the Quadrumana, by the development of large sacculi in connection with it ; and in the “ howling monkeys ” these sacculi are 1 enclosed by an extraordinary expansion of the laryngeal cartilages and even of the hyoid bone, which concur to form a resonant apparatus, by which the power and volume of the voice are most remarkably increased. 326 u. The Kidneys of Mammalia are for the most part of moderate size, but possess a very compact structure, and are copiously supplied with arterial blood, — not being connected, as in oviparous Vertebrata, with the portal circulation, which is here restricted to the liver only. In some Mammalia they possess a lobulated structure, which is observable in all of them in their foetal condition. What they want in size appears to be made up by their functional activity, to which the large quantity of water taken in by Mammals obviously contributes. The urinary excretion of Birds and Reptiles is nearly solid ; but that of Mammals consists chiefly of water, in which the greater part of the solid excretory matters are held in solution, so that their transudation appears comparatively easy. All Mammals are furnished with a urinary bladder, in which the excretion may accumulate so as only to require being discharged at intervals. The urethra, or outlet of this receptacle, usually coalesces with the genital canal, both in the male and in the female ; being continued in the former into the penis, and receiving the spermatic ducts in its course ; whilst in che latter, it opens into the vagina. The Monotremata, as will be here- after explained (§ 32 6 z) present a curious exception to this general statement, in which we see an approximation towards the oviparous Ver- tebrata ■ and an exception of an opposite kind is met with in a few cases in which the three orifices are distinct in the female, the urethra being ccn- 'tinued onwards through the clitoris, instead of discharging itself into the vagina. — The skin, in most of the Mammalia, is an important excreting ‘organ, being usually furnished with a multitude of glandulae, which dis- charge then- products upon its surface. The products of these glandulse differ very considerably in different animals, and in different parts of the same animal. The ordinary perspiratory glands draw off watery fluid 548 GENERAL PHYSIOLOGY. from the blood, with a small quantity of excretory matter closely re- sembling that which is eliminated by the kidneys ; and their action, which is specially augmented by external heat, serves to resist its undue influence, the vaporization of the perspiratory fluid abstracting warmth from the surface, and thus keeping it cool. Besides these, however, many of the cutaneous glandulae secrete a sebaceous or fatty matter ; which serves to lubricate the skin, and to prevent it from being dried up by exposure to the sun and air ; these are generally restricted, how- ever, to particular situations. In most Mammalia, too, an odoriferous secretion, characteristic of the particular species, is formed by glandule which pom - out their product on some part of the surface, their situa- tion, however, being extremely variable in the different tribes ; this secre- tion is usually formed most abundantly at the period of sexual excite- ment, and appears to be concerned in the attraction of the sexes towards each other. A peculiar gland is found in the male Ornithorhyncus and Echidna on the outer side of the leg, and an excretory duct proceeds from it to the extremity of the peculiar homy ‘ spur ’ which these animals possess ; it does not seem, however, that the spin is used as a weapon of offence or defence, so that the gland connected with it is probably not to be considered as a ‘ poison-gland,’ but rather as an accessory genital organ. 326 v. It is in the development of their Nervous Centres, that the chief superiority of Mammals to all other Vertebrata is shown ; yet even here, we find that the transition from the type of the Ovipara is by no means abrupt, — the Encephalic ganglia of the Implacental Mam- mals, and even of the Rodents, not presenting any great advance m development, as regards either size or complexity of arrangement, upon that of Birds. The Cerebrum of the lower Mammalia, the size of which does not bear any large proportion to that of the sensory ganglia, cannot be considered as representing the whole cerebrum of M an on a diminished scale, but only the anterior portion of it. The corpus callosum, or great commissure between the cerebral hemispheres of the two sides, is but very little developed in the Rodentia, and scarcely at all in the Marsupialia : its place is in some degree supplied, however, by the higher development of the anterior commissure, whose connection, however, is not so much with the cerebral hemispheres, as with the corpora striata. As we ascend the series, however, we find the Cerebrum increasing in size, and be- coming more and more elongated posteriorly, by the development of the middle lobes ; coincidently with which the corpus callosum is developed. No vestige of the posterior lobes is seen, however, until we ascend as far as the Carnivora ; and even in that group they are present in merely a rudimentary form. This rudiment is somewhat enlarged in the Qua- drumana ; but it is only in Man that the posterior lobes attain such a development, as to cause the Cerebrum to cover-in, not merely the Sensory Ganglia, but also the Cerebellum. It will be hereafter shewn that the evolution of the Cerebrum in the Human foetus presents a series of phases, which are fairly comparable with the foregoing. — Tin inferior development of the Cerebrum in the lower orders of Mam- malia, is marked also by the deficiency of those ‘ convolutions.' which, in the higher tribes, greatly increase the surface of grey matter, and aM allow the blood-vessels to come into contact with it over a much larger extent. These are entirely absent in the brains of the Implacental GENERAL VIEW OF THE ANIMAL KINGDOM. MAMMALIA. 549 Mammals ; the furrows present themselves onlj- as a few shallow de- pressions in the Rodentia ; their number increases in the Carnivora, being greater in the Dog than in the Feline tribe ; among the Herbi- vorous Mammals, it is most considerable in the Horse and the Elephant ; whilst it is in the higher Quadrumana, that the convoluted arrange- ment of the surface presents the closest approximation to that of Man. — The Sensory Ganglia are thrown (so to speak) into the shade, by the enormous relative development of the Cerebrum ; but they are not really smaller in proportion to the bulk of the body, than they are in the inferior classes ; and if (as is probable) we are to regard the tlialami optici as the ganglionic centres of the sense of Touch, their relative bulk would really on the whole be greater than it is even in Birds. — The dimen- sions of the Cerebellum, also, and its complexity of structure, present a very marked increase, as we pass from the lower to the higher Mam- malia ; the increase being especially seen in the augmented size of its hemispheres as compared with its central portion, and in the amount of grey matter which it contains, as well as in the peculiar lamellated arrangement whereby the extent of communication between the grey and the white matter is increased. It is in the semi-erect Apes, that the condition of this organ approximates most nearly, both in form, size, and structure, to that which it presents in Man. — On the whole it may be affirmed that a very close correspondence may be traced between the relative development of the Cerebrum (as marked not merely by its size, but by its complexity of structure, the depth of its convolutions, &c.), and that of the Intelligence, in the different orders of Mammalia ; whilst the development of the Cerebellum appears to be related rather to that power of combining a diversity of muscular movements, not only in acts of progression, but more especially in the maintenance of the equilibrium of the body, which is required in animals that are endowed with such a variety of powers as we have seen to be the special characteristic of the Mammalian class, and more especially in those which habitually sustain themselves without any assistance from the anterior extremities, in the erect or semi-erect position. — The Sjoinal Cord, as in the other classes, presents enlargements that correspond with the issue of nerves to the extremities ; the relative size of these enlargements bearing a constant correspondence to the development of the limbs, and to the use which is made of them in the act of progression. In the Cetacea, as might be expected, the posterior enlargement is entirely deficient. Generally speaking, the spinal cord extends as far as the sacrum ; beyond this point, the spinal canal contains only a bundle of nerves (cauda equina) which is prolonged from it ; and this is the case even in the Cetacea, in which the tail is the chief organ of propulsion. 326 w. The organs of Sense in the Mammalia are remarkable for their generally high development ; although perhaps that of Vision may be considered as inferior in power to that of Birds, its function in directing the movements of Mammals being partly taken-on by the organs of Smell and Hearing. The eye, except in the Ornithorhyncus, is not pro- tected by a circlet of bony plates, but only by a tough fibrous membrane ; and there is reason to believe that the range of distance to which it may he accommodated, is not so great as in Birds. In its general plan of structure, it departs little from the eye of Man, though there are several 550 GENERAL PHYSIOLOGY. minute variations in the different orders ; thus in the Cetacea, the crys- talline lens is nearly globular, as iu Fishes ; and in many Mammalia, a portion of the choroid is lined with a pigment-layer of metallic lustre, silvery in the Carnivora, blue-green in the Herbivora, which is called the tapetum. In addition to the six muscles by which the eye is moved in Man, this organ, in most of the lower Mammalia (except the Quadrumana), has an additional muscle, the ‘ suspensor ’ or ‘ retractor,’ which is of conical form, embracing the back of the eye and the optic nerve, and serving to draw it back into the orbit. Most Mammalia possess the third eye’id or ‘ nictitating membrane,’ of which the semi-lunar membranous fold at the internal angle of the eye of Man is a rudiment ; this covering, however, is not moved across the eye by the special muscular apparatus which it possesses in Birds, being merely projected by the pressure of the globe, drawn back by the retractor muscle, against the cartilage which it contains. — The organ of Hearing, in Mammalia generally, corresponds closely in its structure with that of Man ; and its chief advance in con- formation, beyond the type which it presents in Birds, may be considered to lie in the more complete development of the cochlea, and in the great augmentation in the size of the external ear, as well as of the cancellated structure contained in certain bones which surround the auditory appa- ratus, whereby its resonance appears to be increased. The external ear is least developed in the aquatic Mammals, in most of which it is entirely wanting ; it attains its highest development in point of size, in the noc- turnal Cheiroptera, whose movements are in great degree regulated by impressions received through the hearing, and through their delicate apparatus of touch ; whilst in many Herbivora, as the Horse, it is remarkable for its extreme mobility, and for the number of muscles by which it may be acted upon, so as to be directed to any point whence sound proceeds. It seems as if the want of external ears is compensated, in the Cetacea, by the extraordinary development of the extensions of the tympanic cavity, which penetrate into the cranial bones, but are partly covered with membrane only. It is remarkable, however, that in this order the semicircular canals should be of very small size ; not being so large in the Whale, the largest of Mammalia, as in the Field-mouse, which is the smallest. The cochlea always forms a spiral, except in the Ornithorhyncus, whose cochlea resembles that of a Bird ; the number • T turns of this spiral varies from 1 1 to 5, the fewest being found in the Cetacea (in which order, however, this organ is veiy large in proportion to the semicircular canals), and the most numerous occurring in the Rodentia. — The organ of Smell is for the most part highly developed, the nasal cavity being large, and the surface over which the olfactive nerve is distributed being augmented by the convolution of the tur- binated bones ; of these, the lowest is the one that is usually most deve- loped ; and in the Carnivora, to which animal this sense is of the greatest importance in guiding them to their prey, this bone is divided into a number of irregular lateral laminae, so that a transverse section of it lias an arborescent form. The nasal cavity is usually more or less extended by sinuses, which communicate with large cancelli in the surroundy increasing, and the ‘ angles’ diminishing in proportion ; until the original division is completely lost sight of, except in the slight dilatation of the cavity at the points at which the Fallopian tubes enter it. In most Mam- malia the female possesses a clitoris, which resembles a rudimentary penis, and which in some instances attains a considerable size. The urethral canal generally terminates near the base of this organ ; but in some of the Marsupialia, this canal seems to be continued as a groove along its under side ; and in the Lemming, the Mole, and some of the lower Qoa- drumana, it is completed into a tube, which passes along the whole length of the clitoris, just as the urethra of the male traverses the penis. The GENERAL VIEW OF THE ANIMAL KINGDOM. — -MAMMALIA. 557 J. larsupialia are remarkable for the peculiar addition to their generative apparatus, which consists of a sac or pouch upon the front of the pelvis, formed by a pair of folds of the integument, and covering-in the mammary glands and nipples ; and the function of this ‘ marsupium ’ is obviously to afford protection to the young animal, in that state of extreme immaturity which characterizes it at the date of its emersion from the uterus and for some time afterwards ; during which period it is clinging to the nipple, at first apparently almost unpossessed either of sensibility or of power of movement, but gradually acquiring the ability to sustain an independent existence. — The possession of mammary glands, as already remarked, is the exclusive attribute of the class Mammalia ; and they are universally present throughout the series, being even found in the Ornithorhyncus. In the greater part of the class, they are found in close connection with the generative apparatus, their orifices in the Cetacea being very near the outlets of the rectum and vagina, whilst in many other groups (as the Ruminants) they are only a little removed from these ; but as we ascend towards Man, we find them occupying a more and more advanced position on the trunk, until they are limited, as in him, to the pectoral region. In complexity of structure, also, the advance is alike gradual : for the Mammary glands of the Ornithorhyncus are of the most simple conforma- tion, consisting of a mere cluster of isolated follicles without any nipple (Fig. 230) ; those of the Cetacea are but little above them in structure, and the nipples are buried in a cleft of the integument ; whilst in the higher Mammalia, these glands are formed upon a very elaborate type, and the nipples are prominently developed, so as to be received into the mouth of the offspring, which draws forth the secretion by suction. The most remarkable development of the nipple, however, is found in the Marsupialia, in which it serves for the attachment of the ‘ marsupial foetus,’ and for the conveyance into its oesophagus of the secreted fluid, which is here expelled from the mammary gland by the action of a com- pressor muscle. A similar expulsor action seems to be exercised by the abdominal muscles of the Ornithorhyncus ; for although (as Prof. Owen has shown) the young possesses flexible lips instead of a horny bill, and a tongue adapted for suction, these can scarcely act effectually without a nipple, and the nutriment is probably imparted and received by both actions conjointly. 326 bb. Although the class of Mammalia, as a whole, is essentially viviparous, yet the condition in which the young are brought into the world varies greatly in its different orders ; and the subsequent attain- ment of that of the adult does not always depend upon growth or increase aloue, but usually involves a certain alteration in foi’m, as well as the development of certain organs which are only rudimentarily present at the time of birth. The young Mammal, however, never presents the least approximation to any inferior class, in those special modifications of the general plan of Vertebrate organisation by which they are respectively characterized ; and, as will be shown hereafter, the whole course of its development, before as well as after birth, is destined to the evolution of the Mammalian type, and of no other, out of that which, at a certain grade of the process, is common to this class with Birds, Reptiles, and Fishes. — The condition of the young of the Monotremata at the time of their birth, has not yet been ascertained ; but at the earliest subsequent 558 GENERAL PHYSIOLOGY. period at which they have been met with, they are entirely destitute of hair, the body is doubled up so that the head and tail approximate each other, the limbs are but very little developed, and the eyes are completely covered by the integument ; so that it would appear that for some time after their entrance into the world, they can scarcely possess more loco- motive power than that which belongs to the human foetus at about haif its term of uterine gestation. The young Marsupial, at the time of its exit from the maternal vagina, is extremely small in proportion to the size of its parent, the new-born offspring of a Kangaroo that measures 7 feet in height (when standing erect on the hind feet and tail), not being more than an inch in length. Like the young Ornithorhyncus, it is blind and destitute of hair, its body is bent forwards, its hind-legs (which are so enormously developed in the adult) are smaller than the fore-legs, and the short tail is tucked in between them. It is probably conveyed to the mar- supium by the mouth of the mother ; and there it attaches itself to the very elongated nipple, its mouth being so framed as not merely to receive this, but to hold firmly to it, whilst its larynx is continued upwards into such close proximity with the posterior nares, that the air-passage is kept distinct from the alimentary tube, and the milk ejected by the mammary gland of the mother passes into the oesophagus, without interfering with the act of respiration. In this condition it remains for some months, gradually increasing in size and advancing in development ; it then no longer suspends itself to the nipple, but lies freely in the pouch, occasionally protruding its head from its aperture, aud cropping the grass at the same time that the mother is browsing ; and after a further period of marsupial gestation, it quits the pouch, still occasionally returning to it for shelter and supply of milk, until it has attained a weight of ten pounds. — In no other Mammalia do the young quit the uterus at so early a grade <1 development ; their relative size at birth being far greater, and their organisation in every respect more advanced. Still we fiud that the young of the Carnivora and of most Rodent ia are brought forth in a very feeble condition, their eyelids being adherent, their movements being feeble, and their calorifving power being insufficient for the independent maintenance of their heat, for some time after birth. On the other hand, in the Ruminants and several other orders, the young possess the sense of vision from their first entrance into the world : they speedily acquire considerable locomotive power ; and they are from the first capable ■ f sustaining their normal temperature, in an atmosphere not too much below this, without the calorifying assistance of the parent. In all cases, however, the young are for some time entirely or chiefly dependent upon the maternal parent for their supply of food ; and the teeth, by wk >se instrumentality they are afterwards to prepare it for themselves, are not developed until near the close of the period of lactation. The generative organs are the last to undergo their full development, aud to come into functional activity ; this not taking place until the dimensions of the adult have been nearly attained. The period required for this purpose varies considerably, among the different orders of Mammalia, in its pro- portion to the whole term of life ; being usually shortest iu the lower orders, such as the Rodentia, in which the evolution of the nervous system is checked at a very early period, whilst it is longest in Man and iu those that approach him. GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT. 559 CHAPTER VIIT. ON THE GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT. 327. There are few things more interesting to those who feel pleasure in watching the extraordinary advancement of almost every department of knowledge at the present time, than the rapid progress of philosophical views, in sciences which have hitherto been too much confined to mere observation. The laws of Life were long considered beyond the reach of human investigation ; and the mind shrunk from attempting to analyse the complex and varied phenomena, which, though constantly under observation, must be reduced to their simplest form, before any inductive reasoning can be founded upon them. It is recorded, however, of Newton, that, whilst contemplating the simplicity and harmony of the plan accord- ing to which the Universe is governed, as manifested in the relations which his gigantic mind developed between the distant and apparently unconnected masses of the solar system, his thoughts glanced towards the organised creation ; and reflecting that the wonderful structure and arrangement which it exhibits, presents in no less a degree the indica- tions of the order and perfection which can result from Omnipotence alone, he remarked, “ I cannot doubt that the structure of animals is governed by principles of similar uniformity.” (“ Idemque dici possit de uniformitate ilia, quae est in corporibus aniuialium.”) “ Why,” asks Cuvier in his eloquent discourse on the revolutions of the globe, “ should not Natural History some day have its Newton ? ” — Although the labours of the Naturalist and Comparative Anatomist have not yet unveiled more than a small part of that general plan, the complete discovery of which may perhaps be reserved for another Newton, many subordinate prin- ciples have been based on a solid foundation, and many more, which were at first doubtful, are daily receiving fresh confirmation. Several of these laws are alike important from their extensive range, and interesting from the unexpected nature of the results to which they frequently lead ; and though their application may sometimes appear forced, and incon- sistent with the usual simplicity of Nature, further investigation will generally show that the difficulty is more apparent than real, — frequently arising solely from our own prejudices, and diminishing in proportion as we fix our attention upon that combination of unity of plan with variety of purpose, by which is produced the endless diversity united with har- mony of forms, so remarkable in the animated world. 328. In comparing phenomena of any kind, for the purpose of arriving at a principle common to them all, it is necessary to feel certain that they are of a similar character. Indeed the sagacity of the philosopher is often more displayed in his discovery of that relation amongst his facts, which allows of their being compared together, than in the inferences to which such comparison leads him. The brilliancy of Newton’s genius was shown in the perception that the fall of a stone to the earth, and the 560 GENERAL PHYSIOLOGY. motion of the moon around it, were analogous phenomena, subject to the same law ; not in the mere deduction of the numerical law from the ratios supplied by those facts. In the sciences which have Life for their subject, the dissimilarity of the facts which are made the object of com- parison, often prevents the true relation between them from being readily detected. Here it is that the mental training which the previous culti- vation of Physical science affords, becomes peculiarly valuable to the Physiologist. “The most important part of the process of Induction;' says Professor Powell,* “consists in seizing upon the probable connectiu- relation, by which we can extend what we observe in a few cases to all. In proportion to the justness of this assumption, and the correctness ■■{ our judgment in tracing and adopting it, will the induction be successful. The analogies to be pursued must be those suggested from already-ascer- tained laws and relations. Thus, in proportion to the extent of the inquirer’s previous knowledge of such relations subsisting in other parts of Nature, will be his means of guidance to a correct train of inference in that before him. And he who has, even to a limited extent, been led to observe the connection between one class of physical truths and another, will almost unconsciously acquire a tendency to perceive such relations among the facts continually presented to him. And the more extensive his acquaintance with Nature, the more firmly is he impressed with the belief that some such relation must subsist in all cases, however limited a portion of it he may be able actually to trace. And it is by the exercise of unusual skill in this way, that the greatest philosophers have been abk to achieve them triumphs in the reduction of facts under the dominion of general laws.” — If, as was formerly stated (chap, i.), the true objects of Physiological investigation are only now beginning to be understood, it is no less certain that the true mode of pm-suing them has not long been followed. From the time of Aristotle downwards to the commencement of the present century, Anatomists have been in the habit of regarding similarity of external form, and of evident purpose, as indicating the analogies between different parts. Now this mode of estimating the character of organs is perfectly correct when they are considered as instrumental structures; that is, when we are inquiring into the conditions of the function performed by them. But it totally fails, when we are examining into the laws according to which the development of the organs has taken place ; since it is frequently found that two structures, which are not unlike in external form, and have corresponding functii ns in the system, originate from elements entirely different, and are therefore fundamentally dissimilar. 329. If, for example, we take a cursory glance at the organs of support or motion in the air with which different Animals are furnished, we shall observe a community of function, and a general similarity of external form, concealing a total diversity of internal structure and of essemial character. Amongst all the classes which are adapted for atmospheric respiration, we encounter groups of greater or less extent, in which the resistance of this element becomes the principal means of progression : and even among aquatic animals, there are many instances in which the function of locomotion is partly dependent upon the same agency. * Connection of Natural and Divine Truth, p. 33. GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT. 561 Wherever true wings exist among the Vertebrata, some modification of the anterior member serves as their basis ; but there is considerable variety in the mode in which the apparatus is constructed. Thus, in the Bat (§ 326 e ) the required area for the surface of the wing is formed by an extension of the skin over a system of bones, of which those of the hand form a very large part ; and this membrane is extended also from the posterior extremity, and is attached to the whole length of the trunk, as well as to the tail where one exists. In the Bird (§ 325 b), on the con- trary, the wing is formed by the skin and its appendages attached to the anterior member alone ; and here the bones of the hand are developed in a comparatively slight degree, those of the arm and fore-arm being the principal support of the expansion. From what is preserved of th e Ptero- dactylus (§ 324 m), it seems that the wing of this extraordinary animal was extended, not over the whole member, as in the Bird, — nor over the hand, as in the Bat, — but over one of the fingers only, which was im- mensely elongated in proportion to the rest. In the Flying-fish, again, the pectoral fins may be regarded as, in some sort, its wings ; though it does not appear that the animal has the power of raising itself by means of their action on the air, the impulse being given at the moment of quitting the water. These fins evidently represent the anterior members of higher Vertebrata ) but the bones of the arm and fore-arm are scarcely developed, while the hand is expanded, and joined immediately, as it were, to the trunk. 330. A very different structure prevails among those imperfect wings, which serve rather to support the animals which possess them, in their movements through the air, than to propel them in that medium. Thus, in the Flying Squirrels, Flying Lemurs, and Pha- langers or ‘ flying opossums,’ there is an extension of the skin between the fore and hind legs (Fig. 19 3), which, by acting as a p>arachute, enables these animals to descend with safety from considerable heights. In the Draco Volans (Fig. 179), on the other hand, the wings are affixed to the sides of the back, being supported by prolongations of the ribs, and are quite independent of the extremities. Here we have still the same function and general form ; but it would evidently be absurd to say that the organs are of the same structural cha- racter. — Among the Invertebrated classes, there is still greater variety in the construction of the organs, which make use of the resistance or im- pelling power of the air as a means of locomotion. Details on this subject have already been given in various sections of the preceding chapter ; and it will suffice to refer to what was there stated in regard to one of the most remarkable kinds of propelling organs met with in these classes, — the wings of Insects (§ 314), — namely, that there ! now seems adequate reason to consider them as essentially appendages to the respiratory system.* * That they bear no real analogy to the wings of Vertebrata, would appear almost self- , evident, when their structure is compared ; and yet there are Entomologists who have O 0 Fig. 193 . Galceopithecus , or Flying Lemur. 562 GENERAL PHYSIOLOGY. 331. These instances will show the caution which must be exercised in deciding upon the relations of the organs of different Animals,, from correspondence in external form and function merely. Many similar examples might readily be adduced from the Animal kingdom ; but the Vegetable world affords them in even greater abundance. To take a very simple case ; — the tendril is an organ developed to serve a particular purpose, that of supporting the plant by twining round some neigh- bouring prop ; but this varies much in its real character, being in the Vine a transformation of the peduncle or flower-stalk, in the Pea a prolongation of the petiole or leaf-stalk, in Gloriosa the point of the leaf itself, whilst in the singular genus Strophauthus, it is actually the point of the petal which becomes a tendril and twines round other parts 332. In all these instances we might with perfect propriety found any inquiries regarding the function of the organs respectively compared, upon them external analogy. For example, we might estimate the respective rates at which Birds, Bats, and Insects could be propelled through the air, by ascertaining the superficial extent of their wings, and the force and rapidity with which these are moved ; although, as just shown, the wing are really formed upon a different plan in each case. But to maintain that the same laws of formation cau apply to them, is evidently absurd. The organs are not related in this respect ; and no such com- parison can be erected between them, as could help to the determination of the fundamental principles of which we are in search. Although in each case the extent of surface is given by an expansion of the general integument, that expansion is supported in the Bat and Bird by one part of the osseous system ; in the Draco Volans by another ; and in the Insect by prolongations of the respiratory apparatus. The Philosophical Anatomist, who seeks to determine the structural relation of these organs, first considers their internal conformation, and examines into the elements of which they are composed. In the cases just alluded to, he would find these elements nearly the same in the wings of the Bat and Bird, con- sisting of the bones, muscles, etc. of the superior extremity ; whilst he would discover nothing corresponding with these in the Insect, the ‘ nerves’ of whose wings are composed of trachem and vessels. — He will then trace their connections ; and he will thus gain a clearer insight into the nature of the respective parts. It is a pjrinciple of very extensive applicati n. that similar parts are connected with similar parts, in different Animals of the same type. Thus, we never find a hand or foot springing directly from the spinal column of Vertebrata ; but always through the medium of other bones, which, however different them size and shape, are never -wholly wanting. This principle, as we shall presently see, may be applied pretty rigorously within the limits of each principal type of structure ; and the application of it often assists the Anatomist in ascertaining the real character of an organ. He will tints see that the principal bones supporting the wing of the Bat are those of the hand ; whilst iu the Bu d they are those of the arm. — But the most certain of all the means of discovering the structural relations of organs, is the study of their development ; and this, if carefully pursued, will scarcely ever fail to clear maintained that the wing of an Insect is a modification of its leg. A very little attention to the relative positions of these parts, and to the history of their development, will disprove this doctrine ; and the true homology of the wings of Insects must be sought in the lower Artieelata. GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT. 563 up -whatever doubts may be left by other modes of investigation. It is in this manner, as already shown, that the true solution has been at last j attained, of many of the most difficult and controverted questions in the i Osteology of Vertebrated Animals ; such as the nature of the spiny rays l supporting the median fins of Fishes (§ 322 b), and the composition of the bony carapace and plastron of the Chelonia (§ 324 h). 333. We may have recourse to the Respiratory system for another instance, which will bring the difference between functional similarity, or j analogy, and structural correspondence, or homology ,* into clear view. An uninstructed observer would scarcely perceive any resemblance between the gills of a Fish and the lungs of a Quadruped, or between the elegant tufts on the body of a Sand-worm, and the air-tubes ramifying through the structure of an Insect ; and those who are in the habit of forming- exclusive notions upon a hasty survey, might be led to deny that any real analogy could exist. When the character of the function is investi- gated, however, with the structure which it requires for its performance, it becomes evident that, in order to bring the circulating fluid into the due relation with the atmosphere, all that is needed is a membrane which shall be in contact with the air on one side, and with the fluid on the other. And this key, applied to the examination of all the forms of j respiratory apparatus which exist in the Animal kingdom, shows that they all possess the same essential character as instrumental structures, and that their modifications in particular instances (which will hereafter be specially described, chap, xiii.) are only to adapt them to the conditions of the organism at large. There is therefore, functionally considered, a relationship of analogy amongst all these organs ; although they are not really the homologues of one another. Thus the gills of the Fish, and the branchial tufts of the Sand-worm, are external prolongations of the tegumen- tary surface ; whilst the tracheae, of the Insect, and the lungs of air-breathing Vertebrata are internal reflexions of that surface ; and farther, the two former sets of organs, as the two latter, differ from each other in regard to the part of the surface from which the prolongation or inversion takes place. In the Perennibranchiate Batrachia (§ 323), both kinds of organs are present ; and their essential difference of character is most apparent, whilst their correspondence as instruments of the same functional opera- tions is equally evident. Now in the air-bladder of the Fish, we have an apparently anomalous organ ; the only known use of which is to assist in locomotion. What is its real character % It is certain, from a com- parison of its most developed forms with the simplest pulmonary sacs of Amphibia, that it is to be regarded as a rudimentary lung (§ 322 li) ; and the study of its development leads to the same conclusion. — Here, then, we have a structural analogy, affording a very interesting addition to the * In the former editions of this work, the terms functional and structural analogy were used to express the mutual relations of parts, on the one hand as instrumental structures, on the other as fundamentally or organically correspondent. It will be found convenient to limit (as Prof. Owen has done) the use of the term Analogy to functional resemblance, and to employ Homology as indicative of structural correspondence. Thus by Analogue we now understand “ a part or organ in one animal, which has the same function as another part or organ in a different animal : ’’ whilst by Ilomologue is implied “ the same organ in different animals under every variety of form and function.” (Prof. Owen’s Hunterian Lectures, Vol. I. Glossary.) Thus, for example, the wings of an Insect are the analogues of those of a Bat or Bird, but not the homologues ; whilst the latter are homologues with the inns of Man, the fore-legs of a Quadruped, and the pectoral fins of a Fish. o o 2 5G4 GENERAL PHYSIOLOGY. collection of instances upon which the laws of formation are based, and nevertheless totally valueless as regards the laws of the respiratory function. It would he easy to adduce parallel examples from the Vege- table kingdom ; where organs which correspond in structure, connections, and development, and which are therefore homologous, are observed to assume the most varied forms, and to perform the most different func- tions ; whilst the same functions are often performed, as just shown (§ 331), by organs which are really different, though analogous as regards the form they assume and the uses to which they are applied — These illustrations, however, must at present suffice. 334. Much controversy has taken place among Physiologists, in regard to the general laws founded on the comparison of different plans of orn to that of the lowest and simplest member of either kingdom, than it is to the germ of its own kind in the earliest periods of its evolution ; and GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT. 5 75 in fact, when we go back to the very commencement of the process, we observe that the most general type of organised structure,— the simple cell, — is that in which every living being commences. The evolution of the germ commences in the multiplication of this cell, precisely after the fashion of the multiplication of the simplest Protophyta and Protozoa j and it is not until this has proceeded to a considerable extent, that it could be stated with certainty, from the examination of the germ alone, whether it is that of a Plant or of an Animal. At the time, again, when the distinctive characters of animality first present themselves, it could not be predicated whether the germ is that of a Radiated, Molluscous, Articulated, or Vertebrated animal ; the special characters of these sub- kingdoms not being evolved until a later period. These, however, are the next to appear, but still the distinctive peculiarities of the class are wanting ; the germ of a Vertebrated animal (for example) being at first destitute of anything that can mark it out as a Fish, Reptile, Bird, or Mammal. When the distinctive characters of the class have been made manifest by the further progress of development, those of the order still remain indeterminate ; these are evolved in their turn ; and then those of the family, genus, species, sex, and individual, in succession.* This, at least, is the result of observations made in a considerable number of cases ; and where such an accordance does not exist, the want of it is probably due to imperfections in the system of classification with which the comparison is made. — Thus we see that in watching the history of the development of any one of the higher forms of organised structure, we find the realization of that ideal evolution of the more special characters from the more general , which it is the object of the Philosophic Naturalist to bring into view by the methods of proceeding already pointed out. 342. It is maintained, indeed, by many distinguished Naturalists, that the information derivable from the history of Development, in regard to the relative value of characters and the affinity of groups, is so much more certain and satisfactory than that of any other kind, that it ought to furnish the fundamental data for a truly scientific classification ; those tribes being considered as most nearly related to each other, whose em- bryonic development advances furthest along the same course without divergence ; whilst those are considered as most fundamentally dissimilar, whose directions of development are distinct from the earliest period. This principle may be admitted as one which deserves to be fully taken into account, in any attempt at a systematic arrangement on philoso- phical principles ; but to adopt it to the exclusion of all comparison of forms in their state of perfect and complete evolution, would be to deprive the changes which may occur at a comparatively late period of develop- * Although certain approximations had been previously made to this general truth, yet the first definite and complete statement of it, with its application to Classification, will be found (the Author believes) in two papers contributed by Dr. Martin Barry to the “ Edinburgh Philosophical Journal” for 1837. It has been subsequently developed in a very admirable manner by Prof. Milne-Edwards in a Memoir on the Principles of the Natural Classification of Animals (“ Annales des Sciences Naturelles,” 18-14), which bears evidence of having been written without the knowledge of what Dr. Barry had put forth ; the principle, in fact, hav- ing been advanced in a more limited form by Prof. Milne-Edwards himself, in a Memoir on the Changes of Form exhibited by various Crustacea during their development, read by him to the French Academy in 1833, and published in the “Annales des Sciences Naturelles,” Ser. i., tom. xxx., Ser. n., tom. iii. 576 GENERAL PHYSIOLOGY. merit, of what appears to he their due claim to consideration. An illus- trative example will perhaps make this apparent. — In the class of Crus- tacea, as long since observed by Prof. Milne-Edwards, the young of such as come forth from the egg in an early period of development, and have many changes to undergo, resemble one another very closely. As tliev increase in size, however, the peculiarities of the respective tribes to which they may belong gradually manifest themselves, partly through an alter- ation in the rate of development of different parks, and partly bv the evolution of new and special organs. Thus in one case, it is the thorax which grows more rapidly than the abdomen, and greatly preponderates ; in another, it is the abdomen which presents the greatest increase in irs dimensions ; in other instances, again, an extraordinary development is seen in certain extremities, or even in certain articulations of these extre- mities. So, again, it is not uncommon for certain parts which were possessed by the embryo Crustacean, to become atrophied and to disap- pear ; thus still further tending to specialize the particular form, in its progress towards its complete development. In these and other modes, the larva, which, at the time of its emersion from the egg, may have presented no characters that serve to distinguish it from the lame of numerous other very dissimilar forms, gradually comes to present in succession the characters of its tribe, genus, species, and sex.* — If we turn, however, to the Cirrhipods, we find that whilst they agree with Crustacea in their larval condition, and must, if the principle of develop- ment be followed as the sole guide, be placed at no great distance from the Entomostracous sub-class, if not as actual members of it, they undergo such extraordinary metamorphoses at a later stage, that their character is changed in a degree sufficient to exclude them from any definition, how- ever comprehensive, which may be framed for that class ; whilst they are brought into much closer approximation with the Mollusca, than is ex- hibited by any other group of the Articulated series (§ 318). 343. But although the application of this general principle to the purposes of classification may require more limitation in particular cases, than some of its advocates are willing to admit, yet there can be no question that it is itself an expression of fact ; and that in the Develop- ment of all living beings, to use the words of Von Bar, who first enunci- ated the doctrine, t — “A heterogeneous or special structure arises out of one more homogeneous or general, and this by a gradual change. ' In tae subsequent portion of this treatise, there will be foimd such ample illus- trations of this law, in the progressive complication of structure, witnessed alike in the ascending scale of Vegetable and Animal life, and in the advance from the earliest embryonic condition towards the perfect form of the higher types, that it need not be more fully dwelt on at present. 344. It is peculiarly interesting to remark, that the same general plan appears to have been followed, in the organisation of the vast series of living forms which have successively appealed on the face of this globe. The entombment of the remains of many of these, in the strata in pro- gress of formation at the time of their existence, has enabled the Paheon- tologist to reconstruct, to a certain extent, the Fauna and Flora of each * See the Memoir of Prof. Milne-Edwards already cited ; also his ‘ Histoire des Crustaces.' and his article Crustacea in the ‘ Cyclopaedia of Anatomy and Physiology,’ vol. i. f Uber Entwickelungsgeschichte der Thiere, Konigsberg, 1828-37. GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT. 577 of those great epochs in the Earth’s history, which are distinctly marked out in Geological time, both by extensive disturbances in the earth’s crust, and by striking changes in the structure and distribution of the living beings which dwelt upon it. Each of these epochs was character- ized by some ‘peculiar forms, or combinations of forms, of Animal and Vegetable life, which existed in it alone ; and the farther we go back from the existing period, the wider are the diversities which we meet with, both in that general aspect of these kingdoms of nature which depends upon the relative proportions of their different subordinate groups, and in the features and structure of the beings composing these groups. The attempt has been made to prove, that these changes might be re- duced to a law of ‘ progressive development ; ’ meaning by this, that the lowest forms of Vegetable and Animal life were first introduced, that those of the least degree of elevation next presented themselves, and so on consecutively, until we reach Man, who, as the highest in the series, was the last to make his appearance upon the globe. Further, it has even been surmised that an actual ‘ transmutation’ of the lower forms into the higher took place in the course of geological time ; so that, from the germs first introduced, or from others which have since originated in combinations of inorganic matter, the whole succession of organic forms, from the simplest Protophyte np to the Oak or Palm, from the Protozoon up to Man, has been gradually evolved ; not, however, in a single series, but from several distinct stirpes, whose development has taken different directions.* The facts of Geological science, however, do not seem to bear out the first of these doctrines ; and the facts of Physiology lend no real support to the second. For it is easily capable of being shown, that although the doctrine of ‘ progressive development ’ as just stated may be true in some of its main features, — Radiata, Mollusca, and Articulata having perhaps existed before any Vertebrated animals left traces of their existence, Fishes having been abundant before we have any evidence from the remains of Reptiles that the latter had been introduced, and Reptiles having been for a time the sole air-breathing Vertebrata, and having- occupied the place (so to speak) of Birds and Mammals, when as yet these had been either very scantily produced or -were altogether wanting, — yet that when we come to apply it more closely, it altogether fails. And even if the doctrine of progressive development , in its usual form, were true in every particular, it would afford no ground whatever for the doctrine of transmutation, which is not only opposed to all our experience, but which fails to account for the intimate nexus that so commonly unites together, not merely the higher and the lower forms of each series, but the members of different series with each other. The question how far any real support is afforded to this hypothesis by the physiological facts advanced in its behalf, and which have been supposed to prove the possi- bility of such transmutation, will be more advantageously considered hereafter (chap, xviii). * See the “ Vestiges of Creation,” in which this hypothesis is put forth and sustained with great ingenuity. The writer of that remarkable work has drawn some of his materials from the former edition of this treatise; but the Author does not regard them as legitimately bearing the construction which has been put upon them, and dissents altogether from the conclusions above stated. He does not think it desirable, however, to enter upon a length- ened discussion of them in this work ; which is not designed to examine and criticise error, but to develope what the Author believes to be established or probable Truth. P P 578 GENERAL PHYSIOLOGY. 345. A more satisfactory account of the succession of Organic Life on the surface of the globe, may probably he found in the general plan which has been shown to prevail in the development of the existing forms of organic structure • namely, the passage from the more general to the more special. Many indications present themselves, that the types of each principal group first introduced were not the lowest, but that they pre- sented in combination those characters which are found to be separately distributed, and more distinctly manifested, among groups that subse- quently made their appearance. One of the most curious exemplifica- tions of this principle in the Radiated division of the Animal kingdom, is to be found in the history of the class Echinodermata ; for the group which seems to have attained a high development at the earliest period, is not that of Crinoidea (§ 296), by which the class in question is most closelj connected with Zoophytes, but that of Cystidea (§ 296 d), which (there xs reason to believe) was much superior to this in general organisation. Now this order seems to have presented, as fonnerly pointed out, a most extraordinary combination of the distinctive character's of the remaining groups ; of which some appear not to have existed, and the rest to have presented a very limited range of forms, at the time when it was predo- minant. Thus, the Crinoidea of the Palseozoic pei’iod, though veiy numerous, exhibit but little variety of type ; and in the complete enclo- sure of the body by polygonal plates, they present a closer approximation to the Cystidea, than do the Crinoidea of the Secondary period, in which the variety of forms is much greater. So, again, the Asteriada and Ophiurida of the Palseozoic period appear to have represented only a small paid of the forms which those groups have since included. It is probable that the true Echinida did not exist at all in the Palseozoic pei’iod ; * and although we are unfortunately not likely ever to obtain proof or disproof of the exist- ence of Holothuriada, it cannot but be thought probable that they, too, were as yet absent. In the Secondary pei’iod, on the other hand, when the ( y.?- tidea had ceased to exist, we have evidence that they were replaced by all the orders just named (save the Holothuriada, the softness of whose bodies would be likely to prevent then’ preservation) ; and these soon came to present a very high degree of development, dividing among them (so to speak) the cha- racters possessed by the Cystidea, and carrying these out separately as the distinctive peculiarities of their respective structures. — So among the higher Mollusca, we find that a prominent place in the earlier formations was occupied by that group (§ 307) which presents the least development of the distinctive characters of the Cephalopod class, and which has much in common with the testaceous Gasteropods. Now’ there is no evidenc' of the existence of the higher order of 1 Dibranchiate ’ Cephalopods, at that early date in the Palaeozoic period at which the tetrabranchiate order had acquired an extraordinary multiplication and variety of forms ; and so far it might seem that w’e have a progression from the lower to the higher. But the paucity of remains of typical Gasteropods, at the same period, is almost as remarkable ; and some of those forms which are most abundant {e.g. Euomphalus and Bellerophon) present indications of close proximity to Cephalopoda. So that it would seem as if the Xautiloid * The genera Palcechinus and Paleeocidaris, which have been usually referred to tens group, are considered by Prof. E. Forbes as connecting links between the Cystidea and true Ecliinida,, approximating most nearly to the former. GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT. 579 type is really to be regarded as having occupied the place at that period, not merely of the order above, but also (in part) of the class below ; its decline and almost complete disappearance, during the Secondary epoch, being coincident with the multiplication of forms of the more typical Gasteropods, and of the higher Cephalopods. — Again, among the Fishes which were the earliest of the Vertebratecl inhabitants of the globe, we find a remarkable assemblage of characters ; some of them presenting, in the extraordinary development of the dermo-skeleton, and in the softness and probably rudimentary condition of their vertebral skeleton, an evi- dent leaning towards the Invertebrated series ; whilst others seem to have foreshadowed the class of Reptiles, an approach to which is presented, not merely by the Sharks and their allies, but by the Sauroid tribe of Osseous fishes, which was extremely abundant towards the end of the Palaeozoic period. The Ctenoid and Cycloid orders, which (on a review of the whole class) may be undoubtedly considered as comprehending the most typical Fishes, did not make their appearance (so far as can be determined from the evidence of their fossil remains) until the Cretaceous period. The universal possession of the homocercal tail (§ 322 a) by the earlier Fishes, is to be considered on this view, not as an essentially embryonic character, but as the most general character of the class, which is pre- sented in every member of it at an early stage of development, but which subsequently gives place in certain cases to a special modification. — Turning to the air-breathing Vertebrata, again, we find that during the Secondary period, this series was chiefly represented by the class of Reptiles, which then attained its greatest importance, and included groups which represented Fishes, Birds, and Mammals respectively • thus having a more general character than the class at present exhibits. These groups subsequently gave place to the more special forms, which carry out most exclusively the Reptilian type. And when we look at the earliest forms of Reptilian life, of which we have any cognizance, we find them to pre- sent very remarkable combinations of the characters which are now distributed among different groups. Thus, the Labyrinthodon (§ 324 jo) of the Triassic formation, appears to have been essentially Batrachian in its structure, but to have possessed some characters of the Crocodilian order. And the same formation contains remains of Reptiles, which, while essentially Saurian in their general structure, had the horny man- dibles, and probably many other characters, of the Chelonia. — In the early history of the class Mammalia, so far as known to us, the same general plan may be traced. The only order that is distinctly recognizable by the remains preserved in the Secondary strata, is that of Marsupialia, which has much in common with the Oviparous Vertebrata. Near the com- mencement of the Tertiary epoch, remains of Pachydermata are abundant ; but these were for the most part different from those of the present epoch, containing combinations of characters which are now distributed among several distinct families, and presenting also a closer approximation to the Herbivorous Cetaceans on the one hand, and to the Ruminants on the other, than is exhibited by any existing species of the order. So among the early Edentata, we meet with a group now entirely extinct, which connected that order with the massive Pachyderms. 346. In regard to the Geological history of the Vegetable kingdom, it must be admitted that our knowledge is still very imperfect ; in con- 580 GENERAL PHYSIOLOGY. sequence of tlie small number of cases in which the internal structure ami fructification of the earlier plants have been preserved, in a condition that allows of the exact determination of their characters and affinities. So far as our present information extends, however, it is fully in harmony with the above doctrine ; the characteristic Flora of the Coal-formation appearing to have been chiefly composed of Coniferas, which constitute a connecting link between the Phanerogamia and Cryptogamia ; and of these Conifer®, while some may have been nearly allied to existing forms, the great majority ( Sigillarice , Lepidodendra, Catamites, &c) appear to have presented such a combination of the characters of the Conifer® with those of the higher Cryptogamia, as no existing group exhibits (§ 277 b). 347. So far as at present known, therefore, the general facts of Palae- ontology appear to sanction the belief, that the same plan may be trace ! out in what may be called the general life of the globe, as in the indi- vidual life of every one of the forms of organised being which now people it ; and that in the successive introduction of the several groups com- posing the Animal and Vegetable kingdoms respectively, the progression was not so much from the lower to the higher forms, as from the more general to the more special, — from those which were in closest relation- ship to each other, to those that are most isolated as types of their re- spective groups. And thus it has happened, that, as eveiy Palaeontologist must be ready to admit, a large proportion of the extinct forms of Animals and Vegetables must rank, in any philosophical system of classification, as osculant or transitional forms ; connecting together the groups which seem naturally to assemble round existing types, and seldom standing as centres round which existing forms should be arranged. — It would be premature and presumptuous to assert that such was the plan, on which the progressive evolution of the great scheme of Organic Creation has proceeded ; but the foregoing indications may be thought sufficient to justify the assertion that such may have been the plan ; and the a priori evidence in its behalf, as a manifestation of that great Unity of Design which we have been attempting to trace in the history of the living beings that now exist, may be urged as entitling it to candid considera- tion. If it have a foundation in truth, the full development of the principle in all its completeness must be left for the time, when Palaeon- tology shall possess, as the result of the accumulated labours of many generations (it may be) of industrious explorers, a collection of informa tion respecting the past distribution of Animal and Vegetable life upon our globe, in some degree comparable to that to which the Natural History of the present time is rapidly attaining. 348. The general principle of Von Bar affords the real explanation of those resemblances which are sometimes discernible, between the trans- itory forms exhibited by the embryoes of the higher beings, and the permanent conditions of the lower. When these resemblances were first observed in the study of Animal Embryology, an attempt was made to generalize them in the assertion, that the higher animals, in the progress of their development, pass through a series of forms corresponding vitli those encountered in ascending the animal scale. But this statement was hasty and unphilosopMcal ; and it is only now referred to, for the sake of showing what amount of real truth there is in it. — No animal as a whole passes through any such series of changes, except where it comes forth GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT. 581 from the egg at an early period of development, hut in a condition that enables it to sustain its own existence, and to lead the life of a class below, from which it is afterwards raised by metamorphosis. This is the case, for example, with such Insects as resemble Annelida in their larva condition, and with Batrachian Reptiles, which are essentially Fish during the early period of their lives. But in neither of these instances, is the condition of the Larva that of the perfect animal which it represents in form and grade of organisation : for the condition of its tissues and organs is altogether embryonic ; so that the Caterpillar bears a much closer accordance with the embryonic than with the adult Annelide, while the Tadpole is more nearly related to the embryonic than to the perfected Fish. These and other cases of the same kind must be regarded as special modifications of the general plan to meet a particular purpose ; and while they present nothing discordant with that plan, they cannot be taken as examples of the usual mode in which it is followed out. On studying the development of any one of the higher animals, which remains within the ovum until it has attained the form characteristic of its class, we find that its entire structure does not present at any time such a resemblance to either of the classes beneath, as would justify the slightest analogy ; thus, the Human embryo is never comparable with a Fish, a Reptile, or a Bird, much less with an Insect or with a Mollusk. In its very earliest grade, indeed, it might be likened to the cells, or clusters of cells, of which the Protozoa are constituted ; but so soon as the multipli- cation and conversion of these has proceeded to such an extent, as to give it a form and structure in which a resemblance can be traced to any higher animal, it is to the Vertebrated type that we shoidd at once assign it. Now, whilst it is passing through this condition, a close correspon- dence may be traced between the several parts of its structure and those of any other vertebrated embryo at a similar grade of development ; — there is, for example, no essential difference between the vertebral column of the early embryo of Man, and that of an embryo Fish ; the evolution of the nervous centres begins in both upon the same plan ; so also does that of the circulating apparatus. And as the progress of development is arrested in the lower tribes, at the stages thus indicated in the transitional conditions of the higher, a mutual resemblance iu the condition of par- ticular organs may most assuredly hence arise. Thus, in the Cyclostome and higher Cartilaginous Fishes, we find permanently represented the various stages in the development of the vertebral column, which may be detected in the embryo of higher Vertebrata. But each of these animals in its adidt condition presents a special adaptation of the general plan to its own organism ; and this special modification is not represented in the human embryo. Thus, whilst the cranium of the human embryo is developed from a great number of distinct centres, which represent the bcnes that remain permanently separate in the skull of the Fish, — so that there is a correspondence in the condition or grade of development, as presented in the two cases respectively, — yet it never exhibits those peculiar characters, which distinguish the skull of the Fish from that of all other Vertebrata. Or, to take an illustration from another soiu’ce, the circulation is canned on, at an early period in the development of all vertebrated animals, by a system of blood-vessels distributed upon the same plan as that which is met with in the adult Fish. It is not, how- 582 GENERAL PHYSIOLOGY. ever, correct to affirm, that the circulating apparatus of Man ever passes through the condition of that of a Fish; for although the ‘branchial arches’ are developed in all Vertebrated animals, so that their presence may be considered as the most general fact in the history of their arterial system, yet the twigs which they give off in the adult Fish for the supply of the branchial filaments are never developed, except in animals that are to be adapted for aquatic respiration ; so that the blood flows onwards continuously through the branchial arches, and is delivered by them into the aorta, instead of being distributed amongst the gills, to be returned from them by a distinct set of vessels, the branchial veins. Hence, how- ever close may be the resemblance between the embryo of Man and the embryo Fish, there is no real correspondence between the embryo of Man and the completed Fish ; since every departure from the general plan or ‘ archetype,’ which gives to the embryo Fish the special characteristics of its class, does in reality diminish the resemblance borne to it by the embryo of either of the higher classes. Thus, whilst there is at an early period a very close correspondence between the embryoes of all classes of Vertebrata, in virtue of the general principle of Yon Bar, each one of these, as it proceeds in its course of development, takes a direction that separates it from the rest ; and the mutual divergence consequently becomes greater and greater, in proportion as the perfected form and condition, that are characteristic of each class respectively, are approximated. 349. Now although the life of all Organised beings commences in the simplest and most general type of organic structure, so that there is no perceptible distinction between their germs, yet we see that each germ must have a certain capacity of development peculiar to itself ; since it is a general law of Organic Development that like produces like. However varied may be the series of forms through which the parent passes, the offspring repeats these with the greatest exactness ;* and the whole scheme of development may be described, as one in which the primordial cell is tending towards the attainment of the perfect form and condition of its parent. In proportion to the mutual resemblance of the parents, will b<_ the conformity of the processes by which then respective forms are at- tained ; in proportion to the dissimilarity of their adult conditions, will be the divergence of their directions of development : thus the development of the heart of the Bird and of the Mammal proceeds up' m a method essentially the same, the single ventricle being divided first, and the single auricle subsequently (the septum remaining imperfect in the Mammal until birth) ; but in the Beptile, the auricle is first divided, its circulation being carried on upon a plan, to which the embryo Bird and Mammal never present anything comparable. And in accordance with the degree of proximity of each complete form to the general model or ‘ archetype ’ of the entire series, will be the degree in winch it will be represented in the transitional states of the higher forms : thus the vertebral skeleton of the Fish as a whole departs much less from the archetype, than does that of the Bird ; and consequently, that of the embryo Mammal is much more nearly related to the former, than it ever is to the latter.— These examples will serve, it is hoped, to show the dis- tinction between the fundamental principle of development, first enun- * It will be shown hereafter (chap, xviii.) that the phenomena ranked under the term ' Alternation of Generations ’ do not constitute a real exception to this rule. GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT. 583 ciated by Yon Bar, which is applicable (as the author believes) to all the facts hitherto ascertained, and that crude and illogical generalization which has brought discredit upon Philosophical Biology, and has led to a host of erroneous inferences. 350. We have hitherto spoken of organisms and their development, in relation rather to their structural composition, than to their functional character as instruments for particular ends. Under the latter aspect it is perhaps more difficult to trace a fundamental Unity, concealed as this is by that extraordinary variety, which is the chief source of the diversity of forms presented by living beings. It cannot be doubted that this variety is itself the expression of a systematic plan ; but at present we have but very faint glimpses of such a plan, and can only trace its lead- ing outlines. In place of looking at the origin and connections of the parts which we compare functionally, we must now regard them only in reference to the acts to which they are subservient ; and must endeavour to seize upon the essential or fundamental character of that relation. If in this manner we compare the organs which are common to all living beings, Plants as well as Animals, we shall perceive amongst them a more general analogy, than any that we can trace by the guidance of evident structure only. The simplest Plant, for example, differs from the most complex, principally in this, — that the whole external surface of the former participates equally in all the operations which connect it with the external world, as those of Absorption, Exhalation, and Respiration, — whilst in the latter we find that these functions are respectively confined to certain portions of the surface. Although, therefore, the leaves and roots of a Vascular plant are distinct organs, they both have a functional analogy with the same simple membrane of the lowest species of Proto- phyta. So, again, in the highest Animals, we find that the organs specially adapted to the functions of Absorption, Exhalation, Respiration, Secretion, and Reproduction, are all essentially composed of a membrane which is a prolongation of the general surface, as may be proved, not only by its continuity with this, but by its development from it during the evolution of the embryo ; and this general surface is the sole instrument for the performance of these functions in the lowest Animals, and shows no special adaptation for one or for another of them. — Where the greatest specialization of function presents itself, the particular arrangement of the organs will depend upon the general plan of conformation, and on the circumstances in which the being is destined to exist. Thus, whilst the absorbing organs of Plants are prolonged externally into the soil, they are distributed in Animals upon the walls of a cavity fitted to retain and pre- pare the food, in order that there may be no impediment to locomotion. Still, the same fundamental unity exists ; and the spongiole of the vascular Plant, and the origin of the absorbent vessel in the Animal, have precisely the same essential character with the membrane which consti- tutes the general surface of the Sea-weed or Red Snow. It may, then, be enunciated as a general truth, that, “ throughout the tvhole animated Creation, the functional character of the organs which all possess in common, remains the same ; whilst the mode in which that character is manifested varies with the general plan upon which the being is constructed.'" The latter part of this law may be rendered more intelligible by another illus- tration. The respiratory surface of Plants is always prolonged externally ; 584 GENERAL PHYSIOLOGY. as they have no means of introducing air into cavities, and effecting that constant change in it which is necessary for the function. The same is found to be the case in nearly all aquatic Animals, the gills of which are evidently analogous to the leaves of Plants. In terrestrial AnimaLs, on the contrary, the respiratory membrane is prolonged internally, so as to form tubes or cells exposing a large amount of surface. The different cavities which we find adapted to this office, are all lined by a membrane which is either derived immediately from the external surface, as in Insects, terrestrial Mollusks, &c., or from that inversion of it which forus the digestive cavity, as in Yertebrata ; and this membrane is everywb~"e functionally or instrumentally the same, although the organs of which it forms a part are not homologous, those of one kind often existing in a rudimentary state, where another is fully developed. — Throughout the second division of the work, illustrations will be found of tins essential unity in the functional character of the different organs common to all : and it need not, therefore, be further dwelt upon in this place. 351. Now the great principle of progression from the more general to the more special, appears to hold good as well in regard to the functional character of organs, as with respect to their structural and developmeutal conformity ; as may be seen in proceeding from the lower to the higher forms of organised being, and in following the successive stages of deve- lopment of any one of the higher organisms. If we compare the forms which the same instrumental structure presents in different parts of the series, w y e shall always observe that it exists in its most general or diffused form in the lowest classes, and in its most special and restricted in the highest ; and that the transition from one form to the other is a gradual one. The function, therefore, which was at first most general, and so com- bined with others performed by the same surface as scarcely to be distin- guishable from them, is afterwards found to be confined to a single organ, or to be specialized by separation from the rest ; these also, by a similar change, having been rendered dependent on distinct organs. Thus, to refer again to the provisions existing in different Plants for the Absorption of fluid ; we find that this action is performed by the entire surface of the lower tribes, which may be thus said to be all root ; as we ascend through the series of Cryptogamia, it becomes more and more limited to certain parts of that surface ; until in Flowering plants it is chiefly per- formed by its special organs, the £ spongioles,’ or succulent extremit.es of the root-fibres, which draw-in fluid far more energetically than any portion of the general surface can do. And so in the early development of Phanerogamia, the germ absorbs by its whole surface the nutriment in which it lies embedded ; it continues still to absorb by a part of that surface, even after the expansion of its plumula and the extension of its radicle into the soil ; and it is not until the store laid up by the parent has been nearly exhausted, that the proper root-fibres are evolved, winch are henceforth to be its special instruments of imbibition. Abundant illustrations of this principle will be found in the subsequent portions of this treatise. — But it would seem as if, even in the most complex being's, the specialization of function seldom proceeds so far, as to incapacitate the more general instrument for taking some part in it ; for observation >f the functions of the more complex forms of animated beings leads to the knowledge of another law, which is simply an expression of this tact : GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT. 585 namely that, in cases where the different functions are highly specialized, the general structure retains, more or less, the primitive community of function which originally characterized it* As this principle, also, -will he copiously illustrated in subsequent chapters, it is unnecessary here to do more than point out its mode of application ; and we shall again refer to the Absorbent system for this purpose. As, in the simplest or most ho- mogeneous beings, the entire surface participates equally in the act of im- bibition, so, in the most heterogeneous, every part of the surface retains some connection with it ; since, even in the highest Plants and Animals, the common external integument admits of the passage of fluid into the interior of the system, especially when the supply afforded by the usual channels is deficient. In the same manner we find, that whilst, in the lowest Animals, the functions of Excretion are equally performed by the whole surface, there is in the highest a complicated apparatus of Glan- dular organs, to each of which some special division of that function is assigned ; but as all these glands have the same elementary structure, and differ only in the peculiar adaptation of each to separate a particular constituent of the blood, it is a necessary result of the law just stated, that either the general surface of the skin, or some of the special secreting organs, should be able to take on, in some degree, the function of any gland whose duty is suspended ; and observation and experiment fully bear out this result, as will hereafter appear (chap. xvi.). 352. In comparing the different forms of Plants and Animals with each other and with the ‘ archetype,’ we observe that many of their differences depend upon diversities in the proportional development of the organs which all possess in common. The attempt has been made to bring these diversities under a general expression, — the balancing of organs , — which is nothing else than that which is alluded to by Paley and other authors as the “ principle of compensation.” Like many other generalisations, this has been earned too far by many writers who have dwelt upon it. Thus, it has been stated in the following most objection- able form ; — that the extraordinary development of one organ occasions a corresponding deficiency in another, and vice versa. It is perfectly true that, in a great majority of cases, the extraordinary development of one organ is accompanied by a corresponding deficiency of development in another ; but the development and the deficiency are both parts of one general plan, and neither can be regarded as the cause or the effect of the other. Thus, in the human cranium, the elements which form the covering or protection of the brain are very largely developed, whilst those which constitute the face are comparatively small. Iu the long- snouted herbivorous Mammals, Reptiles, and Fishes, on the other hand, the great development of the bones of the face is coincident with a very small capacity of the cerebral cavity. In the Bat, we find the anterior extremity widely extended, so as to afford to the animal the means of rising in the air ; whilst the posterior is very much lightened, so as not to impede its flight. In the Kangaroo, on the other hand, the posterior members are very large and powerful, enabling the animal to take long- leaps ; whilst the fore paws are proportionally small. The Mole, again, requires for its underground burrows the power of excavating with its * See Edinburgh Philosophical Journal, July, 1837. 586 GENERAL PHYSIOLOGY. fore-feet, whilst the hind legs are used for propulsion only; and the relative development of these members follows the same proportion as in the Bat, although the plan in the two cases is widely different. Moreover it is obvious that, from the peculiar habits of this animal, eyes would be of little or no use to it ; and accordingly we find them merely rudimentary, and no cavity in the skull for then reception ; whilst to compensate for the want of them, the organ of smell, and its capsule, the ethmoid bone, are amazingly developed. In other classes of animals, similar illustrations abound ; thus, the Birds of most active and energetic flight usually have the smallest and feeblest legs ; and the Struthious birds, in which the legs are enormously developed, have only rudimentary wings. So, again, among Reptiles, we find the vertebral column most lengthened, and the tail especially developed, in those whose limbs are feeblest, or altogether deficient, as among Serpents, and serpent-like Sauria and Batrachia . whilst, if the limbs are the principal instruments of locomotion, as in Frogs and Turtles, the vertebral column is shortened and the tail con- tracted. And in Fishes the same general rule holds good. In no class, however, is this rule without its exceptions ; and it must be taken rather as an expression of facts, possessing a certain empirical value, than as entitled to the character of a ‘law’ of development which some would claim for it. 353. Another principle, propounded by Cuvier, and supported by those who have adopted the ‘ functional’ rather than the ‘homological’ relations of organs as their guide, is that of the harmony of forms, or the coexistence of elements. It implies that there is a necessity, arising out of the c< mdi- tions of organic existence, for the combination of organs according to their several actions ; that there is a constant harmony between organs v liich are functionally connected ; and that the altered form of one is inva- riably attended with a corresponding alteration in the others. — That this statement is true as far as it goes, no one can deny ; and the researches which have been based upon it have been most successful in repeopling the globe, as it were, with the forms of animals which have long been extinct, but which can be certainly predicted even from minute frag- ments of them. A general comparison of the skeleton of the Carnivor- ous with that of an Herbivorous quadruped, will show the manner in which this enquiry is pursued. The Tiger, for example, is furnished with a cranial cavity of considerable dimensions, in order that the size of the brain may correspond with the degree of intellect which the habits < f the animal require. The face is short, so that the power of the muscles which move the head may be advantageously applied. The canine teeth are large and pointed ; whilst the molars have sharp edges, adapted only for cutting, to which purpose they are most effectively applied by the scissors- like action of the jaw. The lower jaw is short, aud the cavity in which its condyle works is deep and narrow, allowing no motion but that of opening and shutting ; the fossa in which the temporal muscle is imbed- ded, is very large ; and the muscle itself is attached to the jaw in such a manner as to apply the power most advantageously to the resistance. The spinous processes of the vertebrae of the back and neck are very strong and prominent, giving attachment to powerful muscles for raising the head, so as to enable the animal to carry off his prey. The bones of the extremities are disposed in such a manner, as to allow the union of strength with free- GENERAL PLAN OF ORGANIC STRUCTURE AND DEVELOPMENT, 587 dom of motion ; the head of the humerus is round, and the articular sur- faces on the fore-arm indicate that it possesses the power of pronation and supination. The toes are separate, and armed with claws, which are retracted when not in use by a special apparatus that leaves its mark upon the bones.— On the other hand, in the conformation of the Herbi- vorous quadruped, we are at first struck with the diminished capacity of the cranium, and the increased size of the bones of the face. The jaws are long, and have a great degree of lateral motion, the glenoid cavity being broad and shallow : and whilst the pterygoid fossa, in which the muscles that rotate it are lodged, is of large size, the temporal fossa is comparatively small, no powerful biting motions being required by the nature of the food or the mode of obtaining it. The front teeth are fewer and smaller ; but the surfaces of the grinding teeth are extended, and are keq>t constantly rough by the alternation of dentine and enamel. The extremities are more solidly formed, and have but little freedom of motion, the shoulder being scarcely more than a hinge-joint ; the extre- mities are encased in hoofs, which are double if the animal ruminates, and either single or multiple if it does not. The whole body is heavier in proportion, the nutritive system being more complicated ; and the muscles which enable the tiger to lift considerable weights in his mouth, are here necessary to support the weight of the head itself. 354. A little consideration, however, will show that the existence of this adaptation of parts is nothing more than a manifestation of the general plan of development, and gives us no information of the nature of that plan. It is evident that, if it were deficient, the race must speedily become extinct, the conditions of its existence being no longer fulfilled ; and that what- ever be the laws of development, they must operate to this end, in order that the world may be peopled wfith life. An animal with the carnivorous propensity of the Tiger, for instance, and the teeth or hoofs of a Horse, could not remain alive, from the want of power to obtain and prepare its aliment ; nor would a horse be the better for the long canine teeth of the tiger, which would prevent the grinding motion of his jaws required for the trituration of the food. The statement above given cannot, therefore, be regarded as a law ; since it is nothing more than the expression, in an altered form, of the fact, that, as the life of an organised being consists in the performance of a series of actions, which are dependent upon one another, and are all directed to the same end, whatever seriously interferes with any of these actions must be incompatible with the maintenance of its existence. The splendid discoveries of Cuvier and other anatomists, who have succeeded in determining, from minute fragments of bones, the cha- racters of so many extraordinary species of remote epochs, have residted from the sagacious appreciation of this truth, and from the use made of it iu the laborious comparison of these remains with the similar parts of animals now existing. Until that comprehensive Plan shall have been discovered, of which these are so many individual results, no briefer process can be adopted. — In seeking to discover the plan of Organic Structure, then, the Philosophic Anatomist does not regard the ostensible object or function of a particular organ as a sufficient account of its lexistence, but follows it through all its varieties of form and condition, (that he may learn what are its real elements, and its fundamental ■elations to other parts. Of this kind of comparison we have traced out 588 GENERAL PHYSIOLOGY. the general method, in the exposition of the views of Prof. Owen as to the homologies of the Vertebrate Skeleton. — On the other hand, in seeking to determine the laws of Vital Action, the Philosophic Physiologist must in like manner disregard the ‘ homological ’ relations of organs, and must look at them simply in then- ‘ teleological ’ or purposive capacity ; and it will be to this that the reader’s attention will be directed in the re- mainder of the present treatise. BOOK II. SPECIAL AND COMPARATIVE PHYSIOLOGY CHAPTER IX. GENERAL VIEW OF THE FUNCTIONS OF ANIMATED BEINGS, AND OF THEIR MUTUAL RELATIONS. 355. It has been stated (§§ 5-9) to be the ultimate object of the Physio- gist, to ascertain the laws in accordance with which the Vital Actions of Organised beings take place ; and it has been pointed out that in order to arrive at any certain general conclusions respecting them, he must col- lect and compare all the facts of similar character, with which the study of the animated creation furnishes him. The changes which occur during the Life of any one being, are of themselves inadequate to fur- nish the required information : since this presents us only with a group of dissimilar phenomena, incapable of comparison with each other, or permitting it but to a low degree. Were we to derive all our notions of Physiology from the history of one of the simple Cellular Plants, we should obtain but very vague ideas as to the character of its different nutritive processes ; since we cannot separate these from one another, and investigate them apart. And, on the other hand, we should be apt to form very erroneous conceptions of the essential conditions of these processes, were we to study them only in that specialized condition, which they present in the most complete Animal, and reason thence as to their dependence upon particular kinds of structure. It is only, then, from a comprehen- sive survey of the whole organised creation, — embracing the unobtrusive manifestations of life which Nature presents at one extremity of the scale (as if to show the simplicity of her operations), as well as those obvious changes which she every moment displays to' us in her most elaborate works, — that any law's possessing a claim to general application can be deduced. 356. In every living Organism of a complex nature, w 7 e can readily distinguish a great variety of actions, resulting from the exercise of the different powers of its several component parts ; and these actions are said to be the functions of the structures by whose instrumentality they are performed. Thus it is the function of a Muscle, to contract on the appli- cation of a stimulus ; and that of a Nerve, to receive and convey sensory and motor impressions. When we look at such an organism, however, as a whole, we perceive that these changes may be associated into groups, in accordance with their mutual relations to each other ; each group consist- ing of an assemblage of actions, which, though differing among themselves, concur in effecting some determinate and important purpose. To these groups of actions, also, the name of Functions is applied, but not in the same sense as before. Thus when we speak of the “ Function of Respi- ration,” we imply that assemblage of actions w'hich concur in effecting the 590 SPECIAL AND COMPARATIVE PHYSIOLOGY. aeration of the nutritious fluid, by exposing it to the atmosphere, or to the gases diffused through water, so as to effect a certain change in its com- position. Simple as this operation appears, many provisions are required in the higher organisms for its effectual performance. In the first place, there must be an aerating simface, consisting of a thin membrane permeable to gases ; on one side of which the blood may be spread out, while the air is in contact with the other. Secondly, there must he a provision for continually renewing the fluid which is brought to this surface, in order that the whole mass of it may be equally benefited by the process. And thirdly, the stratum of air must also be renewed, as frequently as its con- stituents have undergone any essential change. In each of these sub- divisions, a great many diverse actions are involved ; yet all of them are included under the same general term, since they concur to the same fundamental purpose. — Now if we analyse that series of phenomena, which constitute the “Function of Respiration” in one of those higher Animals (Man, for example) in which it presents its greatest complexity, we shall find that whilst some of these are indispensable to the continuance of life, and can only be performed under the conditions supplied by the organised system, others are merely superadded for the purpose of facili- tating them ; and these, if from any cause not performed by the mechanism contrived for their production, may be artificially imitated, with a degree of success exactly proportional to the perfection of the imitation. The essential part of that function being the aeration of the blood, all the other changes which are associated together as partaking in it must be regarded as non-essential, sharing in it only by contributing to this — the real con- stituent of it. Thus the alterations in the capacity of the chest, which are effected by the actions of the diaphragm, and have for then - object the renewal of the quantity of air in contact with the aerating surface, are really a part of the functions of the Muscular and Nervous systems ; and are only associated under that of Respiration, on account of their obvious tendency towards its essential purpose. They have no share in the production of the aeration of the blood, except by supplying its conditions ; aud if these conditions can be supplied independently of them, the essential part of the function will be performed as when they were concerned in it.* 357. By an analysis of this kind applied to the other functions, similar conclusions might be arrived at respecting their essential character : for it will appear in every one of them, that some of the changes which are thus grouped together are essential, and others superadded. But these conclu- sions do not possess the same certainty as if they were founded upon a broader basis; nor are they so easily attained. For, to revert to the instance just quoted, observation alone of the vital phenomena of the lower * Thus in Asphyxia , the deficient supply of arterialised blood to the brain soon paralyses its functions ; and the nervous stimulus required for the respiratory movements being with- held, those movements cease. But, if the chest be artificially inflated, and emptied again by pressure, and these alternate movements be sufficiently prolonged to re-excite the circulation of the blood through the lungs, by aerating that which had been stagnated there, the whole train of vital actions may be again set in motion. Or, if the cessation of the respiratory movements result from a cause primarily affecting the nervous system- — as when narcotism is induced by poisoning with opium — and the blood be, in consequence, stagnated in the iungs bv the want of aeration, this change, so essential to the continuance of vitality, may be prolonged by artificial respiration, until the narcotism subsides, unless the dose have been too powerful. GENERAL VIEW OF THE FUNCTIONS. 591 animals, will reveal what could only be determined in the higher by experiment. Until an experiment (the insufflation of the chest) had been found successful in continuing the aeration of the blood, it could not be certainly known that the respiratory movements had not some further share in the function, than that of mechanically renewing the air in apposition with the circulating fluid. But when the conditions of the function are examined in the lower animals, it is found that these are varied (the essential part being everywhere the same) to suit the respec- tive circumstances of their existence. Thus, many Reptiles, having no dia- phragm, are obliged to fill the lungs with air by a process which resembles swallowing. In Fishes and other aquatic animals, to have introduced the necessary amount of the dense element they inhabit into the interior of the system, would have occasioned an immense expenditure of muscular power ; and the required purpose is answered by sending the blood to meet the water, which is in apposition with the external surface. And in those simple creatures, in which the fluids appear equally diffused through the whole system, their required aeration is effected by the mere con- tact of the water with the general surface : the stratum in immediate apposition with it being renewed, either by their own change of place ; or, if they are fixed to a particular spot, through the means they possess of creating currents, by which their supply of food also is brought to them. And, going still further, we find the essential part of the function of Re- spiration performed in Plants without any movement whatever ; the wide extension of the surface in contact with the atmosphere affording all the requisite facility for the aeration of the circulating fluid. — It is by such a mode of analysis, then, that we are most certainly enabled to distinguish the essential conditions of vital phenomena, from those which are super- added or accidental ; and it is this which will form the subject of the remainder of the present Treatise. 358. When we examine and compare the several Functions, or assem- blages of Vital Actions grouped together according to the principle just set forth, we find that they are themselves capable of some degree of classification. Indeed the distinction between the groups into which they may be arranged, is one of fundamental importance in Physiology. If we contemplate the history of the life of a Plant, we perceive that it grows from a minute germ to a fabric of sometimes gigantic size, generates a large quantity of organic compounds which it appropriates as the mate- rials of its own structure, and multiplies its species by the production of germs similar to that from which it originated ; but that it performs all these complex operations without (so far as we can perceive) either feeling or thinking, without consciousness or the exertion of will. All its vital operations, therefore, are grouped together under the general designation of Functions of Organic or Vegetative life ; and they are subdivided into those concerned in the maintenance of the structure of the individual, which are termed functions of Nutrition ; and those to which the Repro- duction of the species is due. — The great feature of the Nutritive operations in the Plant, is their constructive character. They seem as if destined merely for the building-up and extension of the fabric ; and to this extension there seems in some cases to be no determinate limit. It is important to remark, however, that the growth of the more permanent parts of the fabric is only provided for by the successive development, 592 SPECIAL AXD COMPARATIVE PHYSIOLOGY. decay, and renewal of parts whose existence is temporary ; the growth of the dense, durable, but almost inert woody structure being dependent upon the continual production of new leaves, composed of a soft, transitory, but active cellular parenchyma. — Sooner or later, however, the life of the individual must come to an end ; and the race itself must become extinct, were it not for the special provision whicli is made for its continuance, in the Generative function. This consists in the evolution of germs, which, becoming detached from the parent, are able to support an independent existence, usually at the expense, however, in the first instance, of nutr- ment in some way provided by the being that gave them origin : tbev gradually become developed into its likeness, perform all the vital opera- tions characteristic of it, and in their turn originate a new generation be a similar process. 359. Now, it may be observed, before proceeding further, that there is a certain degree of antagonism between the Nutritive and Reproductive functions, the one being executed at the expense of the other. The reproductive apparatus derives the materials of its operations through the nutritive system, and is entirely dependent upon it for the continuance of its function. If, therefore, it be in a state of excessive activity, it will necessarily draw off from the individual fabric some portion of the aliment destined for its maintenance. It may be universally observed that, where the nutritive functions are particularly active in supporting the individual, the reproductive system is in a corresponding degree unde- veloped, — and vice versa. Thus, it has been stated that in the Algae, the dimensions attained by single plants exceed those exhibited by any other organised being ; and in this class the fructifying system is often obscure, and sometimes even undiscoverable. In the Fungi, on the other hand, the whole plant seems made up of reproductive organs ; and as soon as these have brought their germs to maturity, it ceases to exist. In the Flowering-Plant, moreover, it is well known that an over-supply of nutriment will cause an evolution of leaves at the expense of the flowers, so that what actually would have been flower-buds, are converted into leaf-buds ; or the parts of the flower essentially concerned in reproduction, namely, the stamens and pistil, are converted into foliacious expansions, as in the production of ‘ double’ flowers from ‘ single’ ones by cultivation : or the fertile florets of the £ disk ’ in composite species, such as the Dahlia, are converted into the barren but expanded florets of the ‘ ray.’ And the gardener who wishes to render a tree more productive of fruit, is obliged to restrain its luxuriance by pruning, or to limit its supply of food i >y trenching round the roots. — The same antagonism may he witnessed in the Animal Kingdom ; but as a third element (the sensori-motor appara- tus) here comes into operation, it is not always so apparent. It appears to be a universal principle, however, that diming the period of rapid growth, when all the energies of the system are concentrated upon the perfection of its individual structure, the reproductive system remains dormant, and is not aroused until the comparative inactivity of the nutri- tive functions allows it to be exercised without injury to them. Thus, in the Larva condition of the Insect, the assimilation of food and the increase of its bulk seem the sole objects of its existence (§ 315 e ) ; its locom< rive powers are only adapted to obtain nourishment that is within easy reach, to which it is directed by the position of its egg, and by an unerring GENERAL VIEW OF THE FUNCTIONS. 593 instinct that seems to have no other end. The same is the case, more or less, with all young animals ; although there are few in which voracity is so predominant a characteristic. In the Imago, or perfect Insect, on the other hand, the fulfilment of the purposes of its reproductive system appears to be the chief and often the only end of its being. The increased locomotive powers which are conferred upon it, are evidently designed to enable it to seek its mate ; its instinct appeal’s to direct it to this object, as before to the acquisition of food ; it now shuns the aliment it previously devoured with avidity, and frequently dies as soon as the foundation is laid for a new generation, without having taken any nutriment from the period of its first metamorphosis. In the adult condition of the higher Animals, again, it is always found that, as in Plants, an excessive activity of the nutritive functions indisposes the system to the performance of the reproductive ; a moderately-fed population multiplying (cieteris paribus) more rapidly than one habituated to a plethoric condition. 360. On analysing the operations which take place in the Animal body, we find that a large number of them are of essentially the same character with the foregoing, and differ only in the conditions under which they are performed ; so that we may, in fact, readily separate the Organic functions, which are directly concerned in the development and maintenance of the fabric, from the Animal functions, which render the individual conscious of external impressions, and capable of executing spontaneous movements. The relative development of the organs destined to these two purposes, differs considerably in the several groups of Animals, as we have already in part seen (chap. viii.). The life of a Zoophyte is upon the whole much more ‘vegetative’ than ‘animal;’ and we perceive in it, not merely the very feeble development of those powers which are peculiar to the Animal kingdom, but also that tendency to indefinite extension which is charac- teristic of the Plant. In the perfect Insect, we have the opposite extreme ; the most active powers of motion, and sensations of which some (at least) are very acute, being combined with a low development of the organs of nutrition. In Man and his allies, we have less active powers of locomotion, but a much greater variety of Animal faculties ; and the instruments of the organic or nutritive operations attain their highest development, and their greatest degree of mutual dependence. We see in the fabric of all beings, in which the Animal powers are much developed, an almost entire want of that tendency to indefinite extension, which is so charac- teristic of the Plant ; and when the large amount of food consumed by them is considered, the question naturally arises, to what purpose this food is applied, and what is the necessity for the continued activity of the Or ganic functions, -when once the fabric has attained the limit of its de- velopment. — The answer to this question lies in the fact, that the exercise of the Animal functions is essentially destructive of their instruments ; every operation of the Nervous and Muscular systems requiring, as its necessary condition, a disintegration of a certain part of their tissues, probably by their elements being caused to unite with oxygen. The dura- tion of the existence of those tissues (as formerly stated, § 236 and 243) varies inversely to the use that is made of them ; being less, as their func- tional activity is greater. Hence, when an Animal is very inactive, it re- quires but little nutrition ; if it be in moderate activity, there is a moderate demand for food ; but if its Nervous and Muscular energy be frequently Q Q 594 SPECIAL AND COMPARATIVE PHYSIOLOGY. and powerfully aroused, the supply must be increased, in order to main- tain the vigour of its system. In like manner, the amount of certain products of excretion, which result from the disintegration of the Nervous and Muscular tissues, increases with their activity, and diminishes in pro- portion to their freedom from exertion.* 361. In the Animal fabric, then, among the higher classes at least, the function or purpose of the organs of Vegetative life is not so much the extension of the fabric, for this has certain definite limits ; as the mainte- nance of its integrity, by the reparation of the destructive effects of the exercise of the purely Animal powers. Thus, by the operations of Diges- tion, Assimilation, and Circulation, the nutritious materials are prepared and conveyed to the points where they are required ; the Circulation of Blood also serves to convey oxygen, which is introduced by the Respiratory process ; and it has further for its office to convey away the products of the decomposition of the Muscular and Nervous tissues, that results from their functional activity, — these products being destined to be separated by the Respiratory and other Excreting operations. In the performance of the Organic functions of Animals, as in those of Plants, there is a con- tinual new production, decay, exuviation, and renewal, of the cells, by whose instrumentality they are effected ; which altogether effect a change not less complete than that of the leaves in Plants. But it takes place in the penetralia of the system, in such a manner as to elude observation, except that of the most scrutinizing kind ; and it has been in bringing this into view, that the Microscope has rendered most essential service in Physiology. 362. The regular maintenance of the functions of Animal life is thus entirely dependent upon the due performance of the Nutritive operations. But there also exists a connection of an entirely reverse kind, between the Organic and Animal functions ; for the conditions of Animal existence render the former in a great degree dependent on the latter. In the acquisition of food, for example, the Animal has to make use of its senses, its psychical faculties, and its power of locomotion, to procure that, which the Plant, from the different provision made for its support, can obtain without any such assistance. Moreover, the propulsion of the food along the alimentary canal of the former, requires a series of operations, in which the Nervous and Muscular systems are together involved at the two extremes, simple muscular contractility being alone employed through the greater part of the intestinal canal ; and thus we see that the change in the conditions required for the ingestion of food by Animals, has rendered necessary the introduction of additional elements into the apparatus, to which nothing comparable was to be found in Plants. The same may be observed, as already pointed out, in the operations of the Respiratory apparatus. And it may be stated as a general rule, that the more exalted is the animality of any particular being (or, in other words the more complete the manifestation of characters peculiarly animal), the more closely are the organic functions brought into relation with it.t * This doctrine, though propounded in general terms by previous writers, was first point- edly stated by Prof. Liebig in his Treatise on Animal Chemistry. -f A simple illustration will render this evident. In certain of the lower tribes of animals, whose locomotive powers are feeble and general habits inactive, the circulation of nutritive fluid is carried on nearly in the same manner as in plants ; there is no central organ for GENERAL VIEW OF THE FUNCTIONS. 595 3G3. From what has been said, then, it appears that all the functions of the body, among the higher Animals, are so completely bound up together, that none can be suspended without the cessation of the rest. The pro- perties of all the tissues and organs are dependent upon their regular Nutrition, by a due supply of perfectly-elaborated blood ; this cannot be effected, unless the functions of Circulation, Respiration, and Secretion, be performed with regularity, — the first being necessary to convey the supply of nutritious fluid, and the two latter to separate it from its impurities. The Respiration cannot be maintained, without the integrity of a certain part of the nervous system : and the due action of this, again, is dependent upon its regular nutrition. The materials necessary for the replacement of those, which are continually being separated from the blood, can only be supplied by the Absorption of ingested aliment ; and this cannot be accomplished, without the preliminary process of Digestion. The intro- duction of food into the stomach, again, is dependent, like the actions of Respiration, upon the operations of the muscular apparatus and of a part of the nervous centres ; and the previous acquirement of food necessarily involves the purely Animal powers. On the other hand, the functions of Animal life are even more closely dependent upon the Nutritive actions, than are those of organic life in general ; for many tissues will retain their several properties, and their power of growth and extension, for a much longer period after a general interruption of the circulation, than will the Nervous structure ; the action of which is instantaneously affected by a cessation of the due supply of blood, or by a depravation of its qdality. 364. Yet, however intimate may be the bond of union between the Organic and Animal functions, the former are never immediately dependent upon the latter ; although, as already shown, they generally depend upon them for the conditions of their maintenance. There is no good reason to believe that “ nervous agency” is essential to the processes of Nutrition and Secretion in Animals, any more than to the corresponding processes in Plants. This is a question which may be more certainly determined by observation, than by any possible experiment. That these processes are very readily influenced by changes in the condition of the nervous system, is universally admitted ; and it is the intimacy of this connection, which has given rise to the idea of a relation of dependence, and which prevents that idea from being experimentally disproved. In order to cut off all nervous communication from any portion of the organism — a gland for example, — so violent an operation is required (involving no less than the complete division of the blood-vessels, on which a plexus of ganglionic nerves is minutely distributed), that it is impossible to say that the dis- turbance of the function may not be owing to the shock produced on the general system. Observation, however, shows us that these processes are performed in the most complex and elaborate manner by Vegetables, in which all the attempts that have been made to prove the existence of a nervous system have signally failed (these attempts seeming to have been propelling it through the vessels, and ensuring its regular and equable distribution; and its motion appears dependent upon the forces created in the individual parts themselves. In the higher classes, on the other hand, the comparative activity of all the functions, and the pecu- liar dependence of those of animal life upon a constant supply of the vital fluid, require a much more elaborate apparatus, and especially a central power, by which the movements of that fluid through the individual parts may be harmonised, directed, and controlled. q <1 2 596 SPECIAL AND COMPARATIVE PHYSIOLOGY. only excited by an indisposition to admit the possibility of any vital actions being independent of “ nervous influence ”) ; — that the lowest Animals appear equalfy destitute of a nervous apparatus destined to influ- ence them ; — that in the higher classes there are many tissues into which nerves cannot be traced, and which yet exhibit as much vital activity a s th c se which are in most intimate relation with nerves ; — and that in their early embryonic condition, the formative operations are performed with their greatest energy, at a period when it is quite certain that no nerves have yet been developed. The intimacy of the connection which seems to be established between processes essentially independent of each other, by the instrumentality of the Nervous system, and especially by that part of it known as the ‘ Sympathetic,’ seems to have relation to the necessity which arises, from the complication and specialisation of the Organic functions, for their being harmonised and kept in sympathy with each other, and with the conditions of the Animal system, by some mode of communication more certain and direct than that afforded by the circu- lating apparatus, which is their only bond of union in Plants.* If the views already offered, respecting the ‘con-elation of the vital forces’ (§ 53), be accepted, they offer a definite expression of the relation of ‘ nervous influence’ to ‘ cell-growth ’ and other organic functions; for just as an electric current, set in motion by certain chemical reactions, is capable of influencing the chemical reaction of substances submitted to its agency, so may it be fairly anticipated that the nervous force, itself the result of a certain class of nutritive operations (of which it may be considered the highest product), should be able to modify the course of those operations elsewhere. 365. The Absorption of alimentary materials is the first in the train of strictly Vital operations, and is common to Plants and Animals, although performed under somewhat different conditions in the two Kingdoms. The Plant derives its support immediately from the surrounding element' : it is fixed in the spot where its germ was cast ; and it neither possesses a will to move in search of food, nor any locomotive organs for so doing. By the peculiar properties of its roots, however, it is endowed with some power of obtaining aliment not immediately within its reach (§ 421). The Animal possesses a recipient cavity, in which its food, consisting of matter previously organised, undergoes a certain preparation or Digestion, bef'ou- it is taken up by the absorbent vessels w hich are distributed ou its sides. The introduction of food into this cavity, or its ingestion, seems more and more dependent upon the animal functions, in proportion as we ascend the scale. The ciliary movements of the lower classes of animals, which produce rapid currents in the water that surrounds them, aud thus bring a supply of food to the entrance of the digestive cavity, are probably to be regarded as of the same involuntary character as those which exist in the higher (§ 184). In animals of more complex structure, the process of obtaining food requires a much greater variety of movements, which are evidently dependent on the Muscular aud Nervous systems ; but these * This, it may be safely affirmed, is all that has yet been proved of the functions of the Sympathetic or Ganglionic system of nerves in Vertebrata, and any hypothesis which pre- sumes further, must be regarded as unphilosophical, because unnecessary to explain tacts. The onus probandi certainly rests with those who maintain what is contrary to the important analogy just adduced, and not with those who frame their opinions in harmony with it. GENERAL VIEW OF THE FUNCTIONS. 597 may still be regarded as not involving changes of a psychical character. Rising still higher, however, we find the psychical endowments of the animal evidently concerned in procuring its support ; and in Man, in whom these exist in their highest perfection, the reliance upon them is necessarily the greatest, his bodily organisation not being adapted for the supply of his physical wants, except under the direction of his Intelligence. 366. The alimentary materials taken up by the absorbent vessels, are carried by the Circulation into all parts of the fabric. This movement, so important in the highest classes of Plants and Animals, becomes less ne- cessary in the lower, where the absorbent surface is in more immediate relation with the parts to be supplied w r ith nourishment (§ 266). Besides affording a continued supply of nutrient material for the maintenance of the formative operations, the circulating system of Animals is usually the means of conveying to their nervo-muscular apparatus the oxygen whose presence is a necessary condition of its vital activity. It also serves to take up the effete matters which are set free by the ‘w r aste’ of the system, and to convey these to the organs provided for their elimination. The Circulation in Animals, as in Plants, is entirely independent of the will, cannot be in the least degree controlled by it, and, in its usual con- dition, is even unaccompanied with consciousness. The Muscular appa- ratus is concerned in it, only to give to it the energy and regularity wbich the conditions of animal existence require (§ 362 note) ■ and Nervous agency merely brings it into sympathy wdth other operations of the corporeal and mental systems. — In the higher Animals, we find that the absorption of crude alimentary materials is not only effected by the blood-vessels, but also by a special set of Lacteal A bsorbents spread over the walls of the digestive cavity ; these discharge the matters they have taken up, into the current of the circulation ; and it is probable that in this ‘ absorbent system’ that function of Assimilation commences, by which the crude material is pre- pared for taking part in the formative processes. There is another division of this £ absorbent system,’ which extends itself through the body, and which seems destined to collect the superfluous nutritive material which may have escaped from the blood-vessels, together (perhaps) with such as may have served its purpose in the system, and has died without decom- posing, so that it may be again employed as an alimentary substance ; and this Lymphatic sj'stem of vessels, also, would seem to partake in the Assimilative operation, of which w r e have next to speak. 367. The alimentary materials first taken up by the absorbent process, must undergo various changes by Assimilation, before they can become part of the organised fabric. There is much difficulty, however, in tracing these with precision, either in the Animal or the Vegetable system. The first step which is perceptible in the latter, is the formation of organic compounds by a new combination of the elements supplied by the food, w T hich appears to commence as soon as these elements are absorbed. In the former, these organic compounds are directly supplied by the food ; and this pre- liminary operation is consequently not required. From whichever source they are derived, hownver, these organic compounds appear to be subjected, ■whilst yet circulating in the liquid form, to a vital influence, exerted either by the living tissues through which they flow, or by cells that are floating in the stream, or perhaps by both ; for we find them exhibiting peculiar properties, without any detectible change in ultimate composition, which 598 SPECIAL AND COMPARATIVE PHYSIOLOGY. mark the transition from the crude form in which they are received into the body, towards the condition of living tissue. The ‘ protoplasma ’ of the Plant, and the ‘ liquor sanguinis ’ of the Animal, are very different from mere admixtures of gum, albumen, water, and other organic compounds, but show a capacity for becoming organised, which these do not possess ; hence their peculiar ingredients are designated as organisahle or plastic substances. From these materials the individual tissues of the fabric are developed and renewed, by the process of Nutrition ; the elements of each tissue deriving from the plastic fluid that portion which them composition requires. This process is influenced in the Animal, through the Nervous system, by conditions of the mind or of the general fabric ; but it does not seem to be maintained by any such influence (§ 364). It is of course dependent, however, upon the continued supply of blood, and cannot long continue if the Circulation be brought to a stand ; there is evidence, however, that it may go on for a time, at the expense of the blood con- tained in the vessels of a part, after the current has ceased to flow ; and a retardation of the current is often seen, where the formative processes are most actively going on. 368. In order to preserve the circulating fluid in the state required for the due performance of its important functions, means are provided for separating and carrying out of the system whatever may be superfluous or injurious in its constituent parts ; as well as for elaborating from it certain fluids having a destined use in the economy. These changes may be com- prehended in the general term Secretion, the former constituting the special function of Excretion. This is one no less important to the wel- fare of the system, than the absorption of aliment ; and in proportion to the complexity of the structure, and to the diversity of its actions, do we find a multiplication of the excreting organs, as well as a variety in then- products. The elimination of fluid by Exhalation, and of carbonic acid by Respiration, are constant, however, in all living beings. These changes seem to have no more immediate dependence upon the Nervous system, than have those of nutrition ; and they will take place, to a certain extent, after the final extinction of the animal powers. Wherever a proper Cir- culation exists, however, they are most intimately dependent upon its maintenance, and soon come to an end if it ceases ; but it is probable that, in particular cases, they are kept up by the capillary circulation, after the general propulsive force is extinct. 369. The essential difference between the function of Reproduction and that of Nutrition consists in this, — that in the latter case the alimentary materials are appropriated in renovating the structures of the individual . — whilst in the former they are applied to the production of a new structure, which, although for some time a part of the parent being, subsequently becomes a new individual. In the lowest classes of Plants and Anim;ds. we often find these two functions completely blended together : thus, in the Protophyta and the Protozoa, each new cell may be regarded either as a part of the parent structure, or as a distinct individual, being capable of living, growing, and multiplying by itself. Even in higher members of both kingdoms, we have seen that a multiplication of individuals is effected by the development of gemmae, by a process which corresponds in every essential particular with that of ordinary Nutrition, and which is no more dependent than it is upon the activity of the Animal functions. In GENERAL VIEW OF THE FUNCTIONS. 599 the true generative function, the concurrent action of two sets of organs, the ‘sperm-cell’ and the ‘germ-cell,’ is necessary; but this we see taking- place in Plants without any interference of will, or excitement of conscious- ness, on the part of the individual ; the two organs being sometimes united in the same being (as in hermaphrodite flowers), or, if separated (as in monoecious species), their functions being made to concur by external influences, as when the pollen of one flower is conveyed to the stigma of another at some distance, by the agency of the wind, insects, t- ence appears to be the elaboration of an organised fabric from the ele- ments furnished by the inorganic world. In Man, the being that possesses the largest share of these capabilities, the whole apparatus of Organic life would seem destined but to serve for the maintenance of the Animal functions. The processes of Nutrition are here chiefly directed towards perfecting the Nervous and Muscular systems, and bringing their functions into most advantageous operation ; whilst in many of the lower animals this would seem quite a subordinate object, the extension of the organs of vegetative life being the direction taken by their development, and their organic functions being performed under conditions nearly the same (except as regards the nature of the food) with those which prevail in Plants. CHAPTER X. OF ALIMENT, ITS INGESTION AND PREPARATION. 1. Sources of the Demand for Aliment. 372. We have seen that all Vital Action involves a change in the con- dition of the Organised Structure which is its instrument. — The vital activity of the Plant is chiefly manifested in its increase, development , and reproduction ; in the multiplication of its component cells, in the meta- morphoses which these cells undergo, and in the formation of germs which are destined to be cast off by it, and to originate new organisms elsewhere. These operations can only be performed, however, when the plant is supplied with such alimentary substances as can be converted by it into the proximate materials of its own tissues ; which materials are then appropriated by these, as the pabulum, at whose expense their growth takes place. In those simple Cellular Plants whose structure is nearly homogeneous throughout (§ 266), every act of cell-multiplication conduces directly to the growth of the entire mass ; since the new cells thus pro- duced remain as constituent parts of it, so long as the organism holds together. In the higher tribes of Plants, however, this is not the case : for we find, as we have seen, that certain organs are periodically developed, the term of whose existence is comparatively brief, so that they only form constituent parts of the structure for a short time, being cast off as soon as their term of life is over, to be replaced by another set possessing attri- butes of precisely the same kind. It is, however, through the agency of these temporary organs, — namely, the leaves and the flowers, — that the means of increase and reproduction are provided for the more permanent parts of the organism, which could neither grow nor regenerate its kind without their agency. Thus it would seem to be a general rule, that wherever true woody tissue (the production of which seems to be the highest exertion of the vital force of Plants) is produced, the Plant shall be furnished with a set of organs whose peculiar function it is to prepare SOURCES OF DEMAND FOR ALIMENT. 601 and elaborate its materials, and which accomplish this with such vigour and activity, that their own vital energy is soon exhausted, so that they die, and are cast off in a state of decay ; the most permanent portion of the highest vegetable fabrics being thus built up through the instrumen- tality of the most transient part of their organisation, and the growth of the wood-cells being entirely dependent upon the vital activity of the leaf-cells. So again, in the higher Plants, we find that instead of that simple liberation of germs from the interior of certain cells, more or less distinctly set apart from the general structure, which is the common method of reproduction in the lowest, a complex apparatus is provided for their evolution ; and that this apparatus, — consisting in fact, of elements which might be developed into leaves, and which, though metamorphosed into the parts of the flower, still present an accordance with the general laws of leaf-growth, — is itself of yet more transient duration than the leaves, being cast off as soon as the germs which it has brought into existence are capable of being separated from the parent structure. Further we have to note, that it is one part of the office of the apparatus of fructification, to prepare and store up a supply of nutriment for the early development of the germ ; and this store, which makes up the chief bulk of the seed, is derived from the materials provided by the roots aud leaves of the parent plant. 373. Thus, then, if we look at the sources of demand for Aliment in the Vegetable Organism, we shall see that they may be reduced to the following heads. — I. The extension of the individual fabric, by the multi- plication and development of its component parts ; which parts may themselves possess the same degree of duration as the rest of the fabric, and may in their turn be the subject of further multiplication ; or may be destined only for a temporary activity, replacing for a time those that have already expended their powers and passed through their term of life, and themselves going through the same phases of existence, which have as their ultimate object the extension of the more permanent parts of the structure. — II. The production of germs for the continuance of the race ; which not only incorporate in their own substance a certain amount of elaborated aliment, but which, in the higher plants, carry with them a further supply, at the expense of which their early development takes place. — It is, then, by the activity of the functions of growth and repro- duction, that the demand for food is alone determined in the Plant ; and we shall find that, in their turn, the activity of these functions is dependent upon, and in some degree determined by, the supply of nutriment afforded to it. Thus it is when the plant is most abundantly supplied with appro- priate materials by its roots and leaves, that it will most tend to throw out new leaf-buds, to form new wood, and thus to grow as an individual ; whilst, on the other hand, it is when more sparingly supplied with food, that it will produce the greatest number of flower-buds, and develope the most numerous progeny. In the life of the simpler plants, there may thus be said to be no 4 waste’ whatever ; for so long as their tissues are growing and multiplying, so long do they resist the influences which tend to their decomposition. But in the life of the higher plants, a source of ‘ waste ’ arises from the temporary natru'e of some of their organs ; but even this is connected, as we have seen, with the constructive, not with the destructive class of operations, the successional development and death of the leaves 602 SPECIAL AND COMPARATIVE PHYSIOLOGY. being subservient to the building-up of the individual fabric, and to the regeneration of the race. Thus a very large proportion of the aliment taken into the Vegetable system is directly appropriated to these purposes ; the amount of carbonic acid and of other excretory matters given off during the period of growth, being very small in proportion to that which is taken in ; and the fall of the leaves, which restores to the condition of inorganic substances a certain quantity of matter that has undergone the organising process, not taking place, until by their instrumentality a considerable addition has been made to the solid fabric of the tree. To these pro- cesses of extension and reproduction, there would not seem to be, — ar least in those more complex Vegetable structures, in which one part after another may die and decay, without destroying the life of the organism in its totality, — any very definite limit (§ 124, 125). 37 4. The case is very different, however, in the Animal. In it, also, the activity of the functions of growth and reproduction becomes a source of demand for food. But the period of increase is limited. The full size of the body is usually attained, and all the organs acquire their complete evolution, at a comparatively early period. The continued supply of food is not then requisite for the extension of the structure, but simply for its maintenance ; and the source of this demand lies in the constant ! waste,’ to which, during its period of activity, it is subjected. We have seen that every action of the Nervous and Muscular systems involves the death and decay of a certain amount of the living tissue, as indicated by the appearance of the products of that decay in the Excretions ; and a largo part of the demand for food will be consequently occasioned by the neces- sity for making good the loss thus sustained. Hence we find that the demand for food bears a close relation to the activity of the ‘ animal ' or destructive functions ; and thus the Birds of most active flight, and the Mammals which are required to put forth the greatest efforts to obtain their food, need the largest and most constant supplies of nutriment : whilst even the least active of these classes stand in remarkable contrast with the inert Reptiles, whose slow and feeble movements are attended with so little waste, that they can sustain life for weeks and even months with little or no diminution of their usual activity, without a fresh supply of food. — This waste and decay, however, do not affect the muscular and nervous tissues alone ; for as in the Plant we have found that the higher parts of the structure are developed by the instrumentality of the vital activity of the lower, so do we find in the Animal, that the exercise of those constructive operations, by which the materials for the first growth and the subsequent maintenance of the fabric are prepared and kept in a state of the requisite purity, involves the agency of a set of organs, which maybe said to be entirely ‘vegetative’ in their character, and in which, as in the higher Plants, a continual renewal of the cells that constitute their essential structure, seems necessary for their functional activity. Thus all the glandular and mucous surfaces are continually forming and throwing off epithelial cells, whose formation requires a regular supply of nutri- ment ; and only a part of this nutriment (that which occupies the em it it of the cells) consists of matter, that is destined to serve some other purpose in the system, or that has already answered it ; the remainder (that of which the solid walls are composed) being furnished by tin nutritive materials of the blood, and being henceforth altogether lost to SOURCES OF DEMAND FOR ALIMENT. G03 it. — Thus every act of Nutrition involves a waste or decay of Organised tissue. 375. We may observe a marked difference, however, between the amount of aliment required, and the amount of waste occasioned, by the simple exercise of the nutritive or vegetative functions in the building-up and maintenance of the animal body, and that which results from the exercise of the animal functions. The former are carried on, with scarcely any intermixture of the latter, during foetal life. The aliment, in a state of preparation, is introduced into the foetal vessels ; and is conveyed by them into the various parts of the structure, which are developed at its expense. The amount of waste is then very trifling, as we may judge by the small amount of excretory matter, the product of the action of the liver and kidneys, which has accumulated at the time of birth ; although these organs have attained a sufficient development to act with energy when called upon to do so. But as soon as the movements of the body begin to take place with activity, the waste increases greatly ; and we even observe this immediately after birth, when a large part of the time is still passed in sleep, but when the actions of respiration involve a con- stant employment of muscular power. — In the state of profound sleep, at subsequent periods of life, the vegetative functions are performed, with no other exercise of the animal powers, than is requisite to sustain them ; and we observe that the waste, and the demand for food, are then diminished to a very low point. This is well seen in many animals, which lead a life of great activity during the warmer parts of the year, but which pass the winter in a state of profound sleep, without, however, any considerable reduction of temperature ; the demand for food, instead of being frequent, is only felt at long intervals, and the excretions are much reduced in amount. And those animals which become completely inert, either by the influence of cold, or by the drying-up of their tissues, do not suffer from the prolonged deprivation of food ; because not only are their animal functions suspended, but their nutritive operations also are in complete abeyance ; and the continual decomposition of their tissues, which would otherwise be taking place, is checked by the cold or by the desiccation to which they are subjected ; so that the whole series of changes which goes on in their active condition, is completely at a stand. 376. But there is another most important source of demand for food, amongst the higher Animals, which does not exist either amongst the lower Animals, or in the Vegetable kingdom. We have seen (§ 92) that Mammals and Birds, and to a certain extent Insects also, are able to sus- tain the heat of their bodies at a fixed standard, and thus to be inde- pendent of variations in external temperature. This they are enabled to do, as will be explained hereafter (chap, xvii.), by a process analogous to ordinary combustion ; the carbon and hydrogen which are directly supplied by their food, or which have been employed for a time in the composi- tion of their living tissues and are then set free, being made to unite with oxygen introduced by the respiratory process, and thus giving off as much heat as if the same materials were burned in a furnace. And it has been experimentally proved, that the immediate cause of death in a warm- blooded animal from which food has been entirely withheld, is the in- ability any longer to sustain that temperature, which is requisite for the performance of its vital operations (§ 93). Hence we see the necessity for 604 SPECIAL AND COMPARATIVE PHYSIOLOGY. a constant supply of aliment, in tlie case of warm-blooded animals, for this purpose alone ; and the demand will be chiefly regulated by the difference between the external temperature and that of the animal’s body. When the heat is rapidly carried off from the surface, by the chilling influence of the surrounding air, a much greater amount of carbon and hydrogen must be consumed within the body, to maintain its proper heat, than when the air is nearly as warm as the body itself ; so that a diet which is appropriate to the former circumstances, is superfluous and injurious in the latter ; and the food which is amply sufficient in a warm climate, is utterly destitute of power to enable the animal to resist the influence of severe cold. Again, the Bird, whose natural temperature is 110° or 112°, and whose body exposes a large surface in proportion to its bulk, must con- sume a greater quantity of combustible material for the maintenance of its normal heat, than is required by a Mammal, whose natural tempera- ture is 100°, and whose body, being of much greater bulk, exposes a much smaller proportional surface to the cooling influence of the sur- rounding medium. 377. Thus we find that, in the Animal body, aliment is ordinarily re- quired for four different purposes ; the first two of which are common to it and to the Plant, whilst the others are peculiar to it. — I. The first construction or building-up of the organism, by the multiplication and development of its component parts. II. The production of genus for the continuance of the race ; and, in addition, in the female, the provi- sion of the store of aliment required by these germs during their early development. III. The maintenance- of the organism, both during its period of growth, and after its attainment of its full size, notwithstanding the ‘ waste ’ occasioned by the active exercise of the nervous and muscular systems. ' IV. The supply of the materials for the heat-producing process, by-which the temperature of the body is kept up. — The amount required for these several purposes will vary, therefore, not only with the general activity of the nutritive processes, but in accordance with the conditions of the body, as regards exercise or repose, and external heat or cold. It is also subject to great variation with difference of age. During the period of growth, as might be anticipated, a much larger supply of food is re- quired in proportion to the bulk of the body, than when the full stature has been attained ; but this results, not so much from the appropriation of a part of the food to the augmentation of the fabric (the proportion of its whole amount which is thus employed being extremely small), as from the much greater rapidity of change in the constituents of the body of the young animal, than in that of the adult, which is evidenced by the large proportional amount of the excretions of the former, by the rapidity with which the effects of insufficiency of aliment manifest themselves in the diminution of the bulk and firmness of the body, by the short duration of life when food is altogether withheld, and by the readiness with which losses of substance by disease or injury are repaired, when the nutritive processes are restored to their full activity. The converse of all this holds good in the state of advanced age. The excretions diminish in amount the want of food may be sustained for a longer period, losses of substance are but slowly repaired, and everything indicates that the interstitial changes are performed with comparative slowness ; and, accordingly, the demand for food is then far less in proportion to the bulk of the body, than SOURCES OF DEMAND FOR ALIMENT. G05 it is in the adult. This contrast is most remarkably shown in the Insect tribes, which, as we have seen, are far more voracious in the larva than in the imago state ; many species, indeed, taking no food whatever, after their last metamorphosis ; and most others taking very little, except such as they may be preparing to apply to the sustenance of their progeny. 378. The influence of the supply of food upon the size of the individual, is very evident in the Vegetable kingdom ; and it is most strikingly mani- fested, when a plant naturally growing in a poor dry soil is transferred to a rich damp one, or when we contrast two or more individuals of the same species, growing in localities of opposite characters. Thus, says Mr. Ward,* “ I have gathered, on the chalky borders of a wood in Kent, perfect specimens in full flower of Erythraea Centaurium (Common Cen- taury), not more than half an inch in height ; consisting of one or two pairs of most minute leaves, with one solitary flower : these were growing on the bare chalk. By tracing the plant towards and in the wood, I found it gradually increasing in size, until its full development was attained in the open parts of the wood, where it became a glorious plant, four or five feet in elevation, and covered with hundreds of flowers.” On the other hand, by starvation, naturally or artificially induced, Plants may be dwarfed, or reduced in stature : thus the Dahlia has been di- minished from six feet to two j the Spruce Fir from a lofty tree to a pigmy bush | and many of the trees of plains become more and more dwarfish as they ascend mountains, till at length they exist as mere underwood. Part of this effect, however, is doubtless to be ■ attributed to diminished temperature ; which, as formerly remarked (§ 89), concurs with deficiency of food in producing inferiority of size. 379. Variations in the supply of food would not appear to be effectual in producing a corresponding variety of size in the Animal kingdom : this is not, however, because Animals are in any degree less dependent than Plants upon a due supply of food ; but because such a limitation of the supply, as would dwarf a Plant to any considerable extent, would be fatal to the life of an Animal. On the other hand, an excess of food, which (under favourable circumstances) would produce great increase in the size of the Plant, would have no corresponding influence on the Animal ; for its size appears to be restrained within much narrower limits, — its period of growth being restricted to the early part of its life, and the dimensions proper to the species being rarely exceeded in any great degree. Even in the case of giant individuals, it does not appear that the excess of size is produced by an over-supply of food ; but that the larger supply of food taken-in is called for by the unusual wants of the system, — those wants being the result of an extraordinary activity in the processes of growth, and being traceable rather to the properties inherent in the system, than to any external agencies. The influence of a diminished supply of food, in producing a marked inferiority in the size of Animals, is most effectually exerted during those early periods of growth, in which the condition of the system is most purely ‘vegetative.’ Thus it is well known to Entomo- logists, that, whilst it is rare to find Insects departing widely from the average size on the side of excess, dwarf-individuals, possessing only half the usual dimensions, or even less, are not uncommon ; and there can be On the Growth of Plants in closely glazed Cases,” p. 7. 606 SPECIAL AND COMPARATIVE PHYSIOLOGY. little cloubt that these have suffered from a diminished supply of nutri- ment during their larva state. This variation is most apt to present itself in the very large species of Beetles, which pass several years in the larva state ; and such dwarf-specimens have even been ranked as sub-species. Abstinence has been observed to produce the effect, upon some Cater- pillars, of diminishing the number of moults and accelerating the trans- formation ; in such cases, the Chrysalis is more delicate, and the size of the perfect Insect much below the average. A most characteristic example of the influence of particular kinds of food in modifying the processes of development, seen in the economy of the Hive-Bee, has been already cited (§ 60). That insufficiency of wholesome food, continued through successive generations, may produce a marked effect, not merely upon the stature, but upon the form and condition of the body, even in the Human race, appears from many cases, in which such influence has operated on an extensive scale. Of these cases, some of the most remarkable are those of the Bushmen of Southern Africa, and the aborigines of New Holland ; whose low physical condition appears to be in great part due to imperfect nutrition. 2. Nature of the Alimentary Materials. 380. Amongst the general differences between the Animal and Vegetable kingdoms, none are more striking, as already pointed out (chap. v.). than those existing between the aliments whereon they are respectively sup- ported, and the mode of their ingestion or introduction into the system. The essential nutriment of Plants appears to be supplied by the inorganic world, and to consist chiefly of the elements of water , with certain saline impregnations, carbon, and nitrogen. The water is partly derived from the fluid which percolates the soil and is absorbed by the roots ; and partly from the moisture of the atmosphere, which is imbibed by the leaves. The carbon is principally obtained from the carbonic acid of the air : but most plants are assisted in their growth by its introduction through the roots also. In all soils of moderate richness, there exists a large quantity of the remains of organised structures, the upper layer of which is con- stantly undergoing some degree of decomposition by contact with the atmosphere, so that carbonic acid is formed in it. The water which traverses such a soil, therefore, will become charged with this gas ; and this state of solution appears to be that in which carbon may be most advantageously introduced into the vegetable system. It may be regarded as certain, that the organic matter which rich soils contain is not itself applied to the nutrition of the plant without this previous decomposition ; for it is found that those soils which afford the most steady and equable supply of carbonic acid, are the most favourable to vegetable growth ; and that this end may be answered, not merely by an admixture of decom- posing organic matter, but by the introduction of substances, such as gypsum or powdered charcoal, which have the property of condensing carbonic acid from the atmosphere. The opinion that air and water furnish the essential food of plants, is confirmed by the fact, that not only will the simpler forms of Lichens appear on barren rocks in the midst of the ocean, increasing by absorption from the atmosphere alone, and preparing by their decomposition a nidus for the reception of the NATURE OP THE ALIMENTARY MATERIALS. 607 germs of higher orders of vegetation ; but that many, even of the more highly organised species, will grow in circumstances where no other kind of nutriment is accessible to them. The small amount of earthy or saline matter contained in the tissues of such plants, must be derived from the atmosphere, which is known to hold such particles in suspension. A constant supply of the mineral ingredients naturally found in the tissue of each species, is more important than is generally supposed ; and the fertility of a soil, and the efficiency of a manure, will often depend as much upon this, as upon any other cause. It is only within a recent period, that the dependence of all Vegetable growth upon a due supply of nitrogen has been ascertained ■, but it is now known that, although usually existing in only a small proportion, its presence in the vegetable tissues is peculiarly important at the time of their greatest formative activity ; the ‘ primordial utricle,’ which is the seat of the most active vital operations (§ 136), being- composed of albuminous matter, in which nitrogen is an essential ingre- dient. The small quantity of nitrogen which the usual rate of growth of ordinary Plants causes them to require, appears to be derived from the minute proportion of ammonia existing in the atmosphere ; this being- condensed from it in rain or dew, or absorbed in the gaseous state by porous soils, so as in either case to find its way to the roots in the liquid which they imbibe. But the growth of most plants is powerfully stimu- lated by an additional supply of ammonia, such as they derive from the introduction of decaying animal substances into the soil, as manures ; and the efficacy of these is peculiarly manifested in the large increase of the amount of azotized compounds, then generated by such Plants as naturally produce them in considerable proportion, — such, for example, as the corn- grains. To the fertility of a soil, then, it is essential that it yield a suffi- cient and regular supply of moisture, carbonic acid, and ammonia ; the two latter being either attracted from the atmosphere, or evolved by its own decomposition. — The only class of Plants which even seems to be dependent for its support upon matters already organised, is that of Fungi (§ 271 — 273) ; but it is probable that this dependence only arises from the peculiarly large and constant supply of carbonic acid and ammonia, which they require as the condition of their growth ; as well as (perhaps) from their being only able to appropriate these compounds in the ‘ nascent ’ state. There is no reason to believe that they can make use of organic compounds in any other than a state of decomposition ; and hence it is that their great utility in the economy of Nature arises, the products of decay, which might otherwise have poisoned the atmosphere, being con- verted into living and growing tissues. Fungi present us with two curious analogies to the Animal kingdom ; both resulting, no doubt, from the mode in which they receive their aliment. The large quantity of carbonic acid with which their absorbent apparatus furnishes them, prevents the necessity of their drawing any additional supply of it from the atmosphere ; but on the contrary, like animals, they have only to get rid of what is superfluous. And again, the proportion of azotized matter contained in their tissues is much greater than in any other vegetable ; so that their substance, if capable of being digested, is almost as nutritious as animal flesh. 381. It is a general law of vitality, that the materials of nutrition can only be introduced into the living system in the fluid state ; and although 608 SPECIAL AND COMPARATIVE PHYSIOLOGY. the ingestion of solid aliment by the higher Animals might seem to con- tradict such a principle, a little examination into the character of their nutritive organs will show that they are framed in conformity with it. In addition to the absorbing organs with which Plants are furnished, and which form the medium of communication between the organic life and i he external world, nearly all Animals are provided with cavities for the recep- tion of their food, and for its reduction to a state fit to enter the vessels. The necessity for these cavities arises out of the nature of the aliment re- quired by Animals, which usually pre-exists in a form more or less solid ; and also from the occurrence of intervals between the periods at which it is obtained. Whilst the roots of Vegetables are fixed in the soil, and ramify through it in pursuit of their nutriment, Animals, whose locomotive powers are necessary for the search after the food they require, may be said to carry their soil about with them ; for their absorbents are a distributed on the walls of the digestive cavity, just as those of Plants are externally prolonged into the earth. This cavity is in all instances formed by a reflexion of the external sur- face, of which the Hydra (§ 289) may be regarded as present- ing us with the simplest example. It is merely a bag with one opening (a), which may be regarded as all stomach, A higher form is that in which the cavity has two orifices, and thus becomes a canal (b) ; and all the complicated intestinal apparatus of the higher animals may be considered as a more extended development of this simple type. That the presence of the stomach, however, is not an essential character of the Animal (as taught by some Physiologists), but is rather a special adaptation of their organism to the peculiarity of their food, which may be dispensed with under peculiar circum- stances, has been already pointed out (§ 262). — The food which is introduced into this cavity, is acted upon mechanically 1 y the motion of the walls, and chemically by the secretions poured from their surface ; so that the nutritious parts of it are separated from those which may be rejected, and are reduced to a fluid form.— That the process of Digestion in Animals is really of no higher a character than this, and that it has nothing to do with ‘organising’ or ‘vitalizing’ the materials submitted to it, appears from a priori considerations, and from experiment also. For the substances contained in the alimentary canal, and in contact with that reflexion of the external integument which con- stitutes its lining, are really as much external to the living body, as if they were placed in contact with the skin ; we cannot regard them as introduced into it, until they have been absorbed ; and up to that period, they hold precisely the same relation to the absorbent vessels, as the fluid diffused through the soil bears to the roots of Plants that ramify upon the surface of that Earth, which has been expressively said to be their ‘ com- mon stomach.’ All the experiments which have been performed upon artificial digestion, have precisely the same bearing ; since it appeal’s from them, that if the food be subjected to the action of the same solvent fluids, with the same assistance from heat and from mechanical movement, ti e result is the same out of the stomach as in it. 382. The particular articles which constitute the food of the different races of Animals, are as various as the races themselves. Some appear to NATURE OP THE ALIMENTARY MATERIALS. 609 draw their nutriment from the Inorganic world ; but this is not the case in reality. Thus the Spatangus (§ 296 c ) and Arenicola (§ 311 c) fill their stomachs with sand, but really derive their nutriment from the minute animals which it contains. The Earth-worm and some kinds of Beetles are known to swallow earth ; but they only derive from it the particles of organised matter which it includes, and reject the rest.* Some tribes in almost every division of this kingdom are maintained solely by Vegetable food ; and wherever Plants exist, we find Animals adapted to make use of the nutritious products which they furnish, and to restrain their luxuri- ance within due limits. Thus, the Dugong browses upon the submarine herbage of the tropics ; whilst the Hippopotamus roots up with his tusks the plants growing in the beds of the African rivers ; the Giraffe is enabled by his enormous height to feed upon the tender shoots which are above the reach of ordinary quadrupeds ; the Rein-deer subsists during a large part of the year upon a lichen buried beneath the snow; and the Chamois finds a sufficient supply in the scanty vegetation of Alpine heights. Many species of Animals, especially among the Insect tribes, are restricted to particular Plants ; and, if these fail, the race may for a time disappear. But there is probably not a species of Plants which does not furnish nutriment for one or more tribes of Insects, either in their larva state or perfect condition, by which it is prevented from multiplying to the exclu- sion of others. Thus, on the Oak not less than 200 kinds of Caterpillars have been estimated to feed ; and the Nettle, which scarcely any beast will touch, supports fifty different species of Insects, but for which check it would soon annihilate all the plants in its neighbourhood. The habits and economy of the different races existing on the same plant, are as various as their structure. Some feed only upon the outside of the leaves ; some upon the internal tissue ; others upon the flower or on the fruit ; a few will eat nothing but the bark ; while many derive their nourishment only from the woody substance of the trunk. It is very curious to observe that many plants injurious to Man afford wholesome nutriment to other animals ; thus, Henbane, Nightshade, Water-hemlock, and other species of a highly poisonous character, are eaten greedily by different races of quadrupeds. Some cattle, again, will reject particular plants upon which others feed with impunity. 383. Every class of the Animal kingdom has its Carnivorous tribes, also, adapted to restrain the too rapid increase of the vegetable-feeders, by which a scarcity of their food would soon be created, — or to remove from the earth the decomposing bodies, which might otherwise be a source of disease or annoyance. The necessity of this limitation becomes evident, if we consider the rapid multiplication which the prolific tendency of the Herbivorous races would speedily create, until checked by the famine that would necessarily result from their inordinate increase. Thus, the myriads * Among the human race, some savage nations are in the habit of introducing large quan- tities of earthy matter with their food ; and this sometimes through ignorant prejudice, but more frequently to give bulkiness to the aliment, so that the stomach may be distended, — as among the Kamschatdales, who mix saw-dust or earth with their train-oil. It has been until recently supposed that the siliceous earth, which has been employed in Lapland in times of scarcity, admixed with flour and the bark of trees, merely answered this purpose; but recent microscopic examination has shown, that it consists of the exuvia of Infusoria (§ 284), and contains a large portion of animal matter. If the latter be dissipated by incineration, the earth loses about 20 per cent, of its weight. R R 610 SPECIAL AND COMPARATIVE PHYSIOLOGY. of Insects which find their subsistence on our forest trees, if allowed to increase without restraint, would soon destroy the life that supports them, and must then all perish together ; but another tribe (that of the insectivorous Birds, as the Woodpecker), is adapted to derive its subsist- ence from them, and thus to keep within salutary bounds the number of these voracious little beings. Sometimes, however, they increase to an enormous extent. The pine-forests of the Hartz mountains have been several times almost destroyed by the ravages of a single species of Beetle, the Bostrichus typographic, which is less than a quarter of an inch in length. The eggs are deposited beneath the bark ; and the larvae, when hatched, devour the alburnum and inner bark in their neighbourhood. It was estimated that, in the year 1783, a million and a half of pine-trees were destroyed by this insect in the Hartz alone ; and other forests in Germany were suffering at the same time. The wonder is increased when it is stated, that 80,000 larvte are sometimes found on a single tree. Their multiplication is aided by their tenacity of life. It is found that, even if the trees infested by these larvae be cut down, floated in water, kept for a length of time immersed either in water or snow, or even placed upon ice, they remain alive and unhurt. In the pupa state, however, they are more susceptible ; and vast numbers perish in this condition from the influence of unfavourable seasons, which operate as the principal check to their multiplication.— A very curious instance of the nature of the checks and counter-checks, by which the “ balance of power ” is main- tained amongst. the different races, is mentioned by Wilcke, a Swedish naturalist. A particular species of Moth, the Phalcena strohilella, has the fir-cone assigned to it for the deposition of its eggs ; the young cater- pillars, coming out of the shell, consume the cone and superfluous seed ; but, lest the destruction should be too great, the Ichneumon strohilella lays its eggs in the caterpillar, inserting its long tail in the openings of the cone until it touches the included insect, for its body is too large to enter. Thus it fixes upon the caterpillar its minute egg, which when hatched destroys it.* 384. The alimentary value of the various substances used as food by the several races of Animals, is not so different as, from the diversity of the sources whence it is drawn, we might be led to suppose. It depends, in the first place, upon the quantity of solid matter they respectively contain : being of course the greater, as the solids form the larger proportion of the entire weight. Many esculent vegetables contain so large a quantity of water, that the nutriment they afford is very slight in proportion to their bulk. — Next, it depends upon the proportion of digestible matter which the solid parts include ; for it is not every substance containing the requisite ingredients, that is capable of being reduced to a state which enables it to be absorbed. Thus, woody fibre is composed of the same elements as starch-gum ; but it passes out of the intestinal canal of the higher animals unchanged, and therefore affords them no nutriment : yet there are many tribes of Insects, which seem to draw their supply of nutriment exclusively from wood, and this even in its driest condition. * The Chapters on the “ Economy of Nutritive Matter,” in Dr. Roget's Bridgxvati : Treatise, and those of Mr. Lyell’s Principles of Geology, on the “ Equilibrium of Species, may be referred to for a more extended view of this interesting subject, than the limits ot the present work permit. NATURE OF TIIE ALIMENTARY MATERIALS. 611 So, again, the horny tissues of animals, though nearly allied to albumen in them composition, are completely destitute of nutritive value to Man and the higher animals, because not capable of being reduced by their digestive process ; though certain Insects appear capable of living exclu- sively upon them. — But when the watery and indigestible parts of the food are put out of consideration, and our attention is directed only to the soluble solids, we find most important relations in the chemical compo- sition of the several alimentary materials, whether furnished by the Animal or the Vegetable kingdom, which render them more or less appropriate to the different purposes that have to be answered in the nutrition of the body. It has been already pointed out (§ 259), that Vegetables possess the power of combining the elements furnished by the Inorganic world into two classes of compounds, — the ternary, consisting of oxygen, hydrogen, and carbon, — and the quaternary, which consist of these elements, with the addition of azote or nitrogen. These two classes are hence termed the non-azotized, and the azotized. 385. Now the azotized compounds which are formed by Plants, are essentially the same with those Albuminous substances which are furnished by the flesh and by the nutritious fluids of Animals ; and are equally adapted with the latter for the reparation of the waste of the muscular tissue, and for the general nutrition of the body. The quantity of these, however, which Plants yield, is usually in small proportion to that of the non-azotized ; being considerable only in the Corn-grains, and in the seeds of Leguminous plants, which the universal experience of ages has demon- strated to be the most nutritious of Vegetable substances. But, unless the food contain a sufficient proportion of these compounds, the body must be insufficiently nourished, and the strength must diminish, even though other elements of the food be in superabundance ; and consequently, if the food be of a kind which contains but a small proportion of albuminous matter, a very large amount of it must be ingested, to afford the requisite supply of the essential ingredients. We see a provision for this require- ment, in the capacity of the alimentary canal of Vegetable-feeding Animals ; which is almost invariably far greater than that of the carnivo- rous members of the same groups. — There is another azotized compound, Gelatine, that is furnished by Animals, to which nothing analogous exists in Plants ; and this, although it cannot sustain life by itself, and is not an essential article of food, is a valuable adjunct to the albuminous com- pounds. For as the gelatinous tissues (§ 163) suffer waste, in common with the others, it is evident that if the gelatine be supplied already- prepared, it may be at once applied to their nutrition ; and thus the proportion of albuminous material, which they would otherwise require, is not demanded, and the labour of transformation is also saved. But there is no evidence that Gelatine can ever be transformed into an albuminous compound, and can thus be applied to the nutrition of the muscular and other fibrinous tissues ; and the presumption, derived from the results of various experiments, is veiy strong the other way. 386. The non-azotized compounds supplied by Plants, exist under various forms ; of which the principal are starch, sugar, and oil. The two former may be regarded as belonging to one class, the Saccharine or Farinaceous ; because we know that starch, and the substances allied to it, may be converted into sugar by simple chemical processes, and that this trans- it. R 2 612 SPECIAL AND COMPARATIVE PHYSIOLOGY. formation takes place readily both in the Vegetable and Animal economy. On the other hand, the Oily matters are usually ranked as a distinct group of alimentary substances ; and it has been maintained that, under no cir- cumstances, has the Animal the power of elaborating fatty matter from starchy or saccharine compounds. But this is now known to be an unfounded limitation ; since the transformation of a saccharine into a fatty compound takes place in the case of Bees, 'which form wax when fed upon pure sugar ; and it has been recently shown that it may be effected in the laboratory of the Chemist, butyric acid (the characteristic fatty acid of butter) being one of the products of the ‘lactic fermentation’ of sugar’, ex- cited by animal substances. Thus, then, whether derived from Vegetable or from Animal bodies, the non-azotized substances available as food are essentially the same. The former kingdom supplies them chiefly in the saccharine or farinaceous form, the latter chiefly iu the oleaginous ; but a considerable quantity of oil is furnished by certain Plants ; and there are Animals which incorporate the cellulose of plants with their own sub- stance (299 a). The only Animal tissue to which the non-azotized com- pounds apparently serve as the appropriate pabulum, is the Adipose ; the peculiar character and offices of which have been already explained (§§ 189, 190). There is reason to believe, however, that oleaginous matter performs a most important part in the incipient stages of Animal nutri- tion ; and that its presence is not less essential to the formation of cells, than is that of the albuminous matter which forms their chief component. If such be the case, it is not surprising that oil in some form, or substances capable of conversion into it, should be such universal constituents of the food of Animals. — Besides serving these purposes, however, the non- azotized compounds have a most important use in warm-blooded Animals ; that of supporting the respiratory process, and thus maintaining the temperature of the body. We have seen that, in the compounds of the Saccharine group (in which Starch is included), the amount of oxygen is no more than sufficient to form water with the hydrogen of the substance (§ 27) ; so that the carbon is free to combine with the oxygen taken in by the lungs, and thus becomes a source of calorifying power. Again, in the Oily matters taken in as food, the proportion of oxygen is far smaller ; so that they contain a large quantity of surplus hydrogen, as well as of carbon, ready to be burned-off in the system, and thus to supply the heat required. The extraordinary power of oleaginous substances to impart heat to the system by the combustive process, is indicated by the experi- ence of the Human inhabitants of frigid zones, who feed upon whales, seals, and other animals loaded with fat, and who devour this fat with avidity, as if instinctively guided to its use. It is by the enormous quantity of this substance taken in by them, that they are enabled to pass a large part of the year in a temperature below that of our coldest wintt r, spending a great portion of their time in the open air ; as well as to sustain the extremes of cold, to which they are occasionally subjected. And in consequence of its being more slowly introduced into the system than most other substances, a larger quantity may be ingested at one time, without palling the appetite • whilst its bland and non-irritating character favours its being retained until it is all absorbed. In this manner, the Esquimaux and Greenlanders are enabled to take in 20 or 30 pounds of blubber at a meal ; and, when thus supplied, to pass several days without NATURE OF THE ALIMENTARY MATERIALS. 613 food. — On the other hand, among the inhabitants of warm climates, there is comparatively little disposition to the use of oily matter as food ; and the quantity of it contained in most articles of their diet is comparatively small. 387. The greatest economy in the use of Aliment is therefore exercised, when the diet contains a sufficient proportion of azotized substances to repair- the ‘ waste ’ of the albuminous and gelatinous tissues ; and a suffi- cient amount of non-azotized compounds, to develope (with the aid of other processes) the requisite amount of heat by combination with oxygen. Now in the Milk, which is the sole nutriment of young Mammalia during the period immediately succeeding their birth, we usually find an admix- ture of albuminous, saccharine, and oleaginous substances ; which seems to indicate the intention of the Creator, that all these should be employed as components of the ordinary diet. The Caseine, or cheesy matter, is an albuminous compound ; the Butyrine of butter is but a slight modification of the ordinary fats ; and the Sugar differs from that in common use, only by its larger proportion of water. The relative amount of these ingre- dients in the milk of different animals, is subject, as we shall hereafter see, to considerable variation ; but they are constantly present in the milk of the Herbivorous Mammalia, and of those which, like Man, subsist upon a mixed diet. It has been recently found, however, that the milk of the purely Carnivorous animals is destitute of Sugar, consisting, like their food, of albuminous compounds and fatty matter only ; though even their milk is found to contain sugar, when saccharine or farinaceous compounds have formed part of their diet. — No fact in Dietetics is better established, than the impossibility of long sustaining health, or even life, upon any single alimentary principle. Neither pure albumen or fibrine, gelatine or gum, sugar or starch, oil or fat, taken alone for any length of time, can serve for the due nutrition of the body. This is partly due, so far as the non- azotized compounds are concerned, to their incapability of supplying the waste of the albuminous tissues. This reason does not apply, however, to the azotized compounds ; which can not only serve for the reparation of the body, but can also afford the carbon and hydrogen requisite for the sustenance of its temperature. The real cause is to be found (partly at least) in the fact, that the continued use of single alimentary substances excites such a feeling of disgust, that the animals experimented on seem at last to prefer the endurance of starvation, to the ingestion of them. Conse- quently it is quite impossible to ascertain, by such experiments, the nutri- tive power of the different alimentary principles ; no animal being- capable of sustaining life upon less than two of them. The same dis- gust is experienced by Man, when too long confined to any article of diet which is very simple in its composition ; and a craving for change is then experienced, which the strongest will is scarcely able to resist. The natural combinations in which the alimentary substances present them- selves, appear to be those which are best adapted for the healthful nutri- tion of the animal body. 388. The Organic Compounds, which have been enumerated as sup- plying- the various wants of the system, would be totally useless without the admixture of certain Inorganic substances, which also form a consti- tuent part of the bodily frame, and which are constantly being voided in the excretions, especially in the urine. These substances have various 614 SPECIAL AND COMPARATIVE PHYSIOLOGY. uses in the system. Thus common Salt, or the Chloride of Sodium, appears to afford, by its decomposition, the muriatic acid which is con- cerned in the digestive process, and the soda which is an important con- stituent of the bile. Its presence in the serum of the blood, also, and in the various animal fluids which are derived from this, probably aids in preventing the decomposition of the organic constituents of these fluids. — Phosphor us seems to be chiefly requisite as one of the materials of the Nervous tissue (§ 243) ; and also, when acidified by oxygen, to unite with Lime in forming the bone-earth by which bone is consolidated. — Sulphur exists in small quantities in several animal tissues ; but its part appears to be by no means so important as that performed by Phosphorus. — Iron is an essential constituent of Hsematine (§ 176) ; and is consequently required for the production of the red corpuscles of the blood in Verte- brated animals. — Lime is required for the consolidation of the bones of Vertebrata, and for the shells and other hard parts that form the skele- tons of the Invertebrata ; and it exists in the animal body in combination either with carbonic or with phosphoric acid. The Carbonate would seem principally destined to mechanical uses only ; and w r e find it predomi- nating, or existing as the sole mineral ingredient, in those non-vascular tissues of the Invertebrated animals, which give support and protection to then’ soft parts (§§ 194-199). The degree of development of these tissues depends in great part upon the supply of carbonate of lime which the animals receive. Thus the Mollusca which inhabit the sea, find in its waters the proportion of that substance which they require ; but those which dwell in streams and fresh-water lakes, that contain but a small quantity of lime, form very thin shells : whilst the very same species in- habiting lakes, which, from peculiar local causes, contain a large impreg- nation of calcareous matter, form shells of remarkable thickness. The curious provision by which the Crustacea, which periodically throw off then calcareous envelope, are enable to renew it with rapidity, has been already noticed (§ 316). The large amount of carbonate of lime winch is required by the laying Hen, is derived from chalk, mortar, or other sub- stances containing it, which she is impelled by her instinct to eat ; and if the supply of these be withheld, the eggs which she deposits are soft on their exterior, — not being destitute of shell, as commonly supposed, but having the fibrous element of it unconsolidated by the intervening deposit of calcareous particles (§ 160). 389. These substances are contained, more or less abundantly, in most of the articles generally used as food ; and where they are deficient, the animal suffers in consequence, if they be not supplied in any other way. — Common Salt exists, in no inconsiderable amount, in the flesh and fluids of animals, in the milk, and in the substance of the egg ; it is not so abundant, however, in Plants ; and the deficiency is usually supplied to herbivorous animals in some other way. Thus, salt is purposely min- gled with the food of domesticated animals ; and in most parts of the world inhabited by wild cattle, there are spots where it exists in the soil, and to which they resort to obtain it. Such are the “ buffalo-licks " of North America. — Phosphorus exists also, in combination with albuminous compounds, in all animal substances composed of these ; and in the stale of phosphate, combined with lime, magnesia, and soda, it exists largely in many vegetable substances ordinarily used as food. The phosphate of NATURE OF THE ALIMENTARY MATERIALS. 615 lime is particularly abundant in the seeds of the grasses • and it also exists largely, in combination with caseine, in Milk. — Sulphur is derived alike from vegetable and from animal substances. It exists, in union with albuminous compounds, in flesh, eggs, and milk ; also in several vegetable substances ; and, in the form of sulphate of lime, in most of the river and spring water used as drink. — Iron, also, is very generally diffused, in small quantity, through the tissues of plants ; but it exists in much larger pro- portion in the flesh, and more particularly in the blood, of animals ; and it appears from comparative analyses of the blood of Carnivorous and Herbivorous Mammals, that the proportion of red corpuscles, and conse- quently of iron, is greatest in the former ; which circumstance seems partly attributable to the nature of their diet. — Lime is one of the most universally diffused of all mineral bodies ; for there are very few Animal or Vegetable substances, in which it does not exist. The principal forms in which it is an element of Animal nutrition, are the carbonate and phosphate. Both these are found in the ashes of the grasses, and of other plants used as food ; the phosphate of lime being particularly abundant in the corn-grains. Phosphate of lime unites readily with albumen, caseine, &o. ; and a large quantity of it is found in the milk of the Mammal, and in the egg of the Bird, for the promotion of the ossifying process in the bones of the young animals which they are respectively to nourish. 390. The dependence of Animal life upon a constant supply of aliment, is more close in some cases than in others. As a general rule it is the most immediate, when the vital processes, particularly those of Nutrition, are being most actively performed. Thus, we find that young animals are never able to bear the deprivation of food to the same extent with older ones of the same species ; and that the warm-blooded Vertebrata — viz. Mammals and Birds, — are usually less capable of abstinence than Reptiles and Fishes. Even of the first of these classes, however, many species pass several months without eating, during the state of hybernation (§ 94) ; whilst among many cold-blooded animals, the period of abstinence from food may be indefinitely prolonged, under the influence of those agencies which keep them in a state of complete torpidity or ‘ dormant vitality ’ (§ 118). — When we carry our enquiries further, however, it becomes diffi- cult to give any general explanation of the varieties which we meet with in this respect, among the different species of animals. Thus, it has been observed by Flourens and Duges,* that the Mole perishes, when in a state of confinement, if not fed every day, or even more than once a day ; whilst the Dog has lived without food for 36 days, the Antelope and the Wild Cat for 20, and the Eagle for five weeks. It is in Reptiles, that the power of abstinence appears to exist to the greatest extent among Vertebrata. Putting aside those cases in which the natural period of torpidity has been artificially extended, we find numerous instances in which these beings have performed all the functions of life for many months together, with- out the ingestion of food. Tortoises, Lizards, Serpents, and Batrachians seem to agree in this respect. It is to be borne in mind, however, that a large supply of food is frequently ingested at once by these animals ; and that, owing to the slowness of their digestive process, the introduction of the Aliment into the system is protracted over a very long period, — as is Physiologie Comparee,” tom. ii. p. 288. 616 SPECIAL AND COMPARATIVE PHYSIOLOGY. seen, for example, in the case of the Boa constrictor, which occupies a month in the digestion of a single meal. Little is known regarding the powers of abstinence possessed by Fish ; but it has been stated that some of this class, such as the Perch, naturally take food but once a fortnight." — It is perhaps among the Insect tribes, that we find the power of sustain- ing a deprivation of aliment the most remarkably evidenced. The Scor- pion has been known to endure an abstinence of three months, the Spider of twelve months, and the Scarabceus beetle of three years, without incon- venience or loss of activity ; the Melasoma, also one of the beetle tribe, has lived for seven months pinned down to a board. We notice in the class of Insects a very striking illustration of the general fact already stated, respecting the difference between the old and the young animal of the same species. The Larva is not only extremely voracious, but is usually in- capable of sustaining a long abstinence ; whilst in many tribes the Imago never eats, but dies as soon as its share in the propagation of the race is accomplished. — From what is known regarding the power of abstinence in the Mollusca, it may be stated generally, that they are not capable of maintaining their activity if not frequently supplied with food, in this respect corresponding with the larvae of Insects ; but that, when reduced to a state of torpidity, whether by cold or by the deprivation of food, they may continue without any aliment for a veiy protracted period. 391. Some have attempted to show that Herbivorous animals are more dependent upon a constant supply of food, than Carnivorous species ; and that domesticated animals less easily sustain a deprivation of it, than wild ones. But these statements, though generally true, are found to be want- ing in accuracy when applied to individual cases. It would probably be more correct to state, that, in proportion to the facility with which each species usually obtains its food, will be the directness of its dependence upon this, and its inability to sustain a protracted abstinence. In accord- ance with this principle, we observe that, though vegetable-feeders have in general their food within reach, and are very dependent upon its constant supply, some, as the Camel, are enabled to sustain abstinence better than many carnivorous species ; and that some carnivorous animals, being enabled from the nature of their food to obtain it with little difficulty, are comparatively unable to bear the want of it. On the same principle it is evident that domestication may induce a change in the character of the animal, in this respect, as in others, by causing it to become accustomed to frequent or constant supplies of food. The same adaptation may be found among the Larvae of Insects. Those which feed upon vegetables or upon dead animal matter, speedily die if out of the reach of their ali- ment ; whilst those that lie in wait for living prey, the supply of which is uncertain, are able to endure a protracted abstinence, even to the extent of ten weeks, without injury, t 392. The general facts which appear - to have been substantiated, in regard to the mode in which the Animal body is nourished by Food, may be thus summed up. * Gold-fish have been known to live and thrive in small vessels of water, without any perceptible nutriment, for two or three years. But, if they be not fed in any other way, :t is requisite that the water they inhabit should be frequently changed ; and the minute quan- tity of organic matter which it holds in solution is probably the source of their aliment. -f Lacordaire, “ Entomologie,” tom. ii. p. 152. NATURE OP THE ALIMENTARY MATERIALS. 617 I. The ‘waste’ of the tissues of which Albumen is the basis, must be supplied by Albuminous compounds, whether these be derived from the Animal or from the Vegetable kingdom ; the amount of this ‘waste,’ and consequently the demand for albuminous aliment, depends essentially upon the degree of vital activity which has been put forth, and especially upon the exercise of the nervo-muscular apparatus ; and therefore, coeteris paribus, it is greater in cold-blooded animals in proportion to the elevation of the external temperature. II. The ‘ waste ’ of the tissues of which Gelatine is the basis, may be supplied either by Albuminous or by Gelatinous compounds, since there is no doubt that albumen may be converted into gelatine, although the reverse process cannot be performed. As Gelatine does not exist in Plants, it must be formed in Herbivorous animals at the expense of the albuminous elements of their food ; whilst in Carnivorous animals, it is probably derived immediately from the gelatinous components of the bodies on which they prey. III. The materials of the Adipose tissue, and the oleaginous particles which seem requisite in the formative operations of the system generally, are derived, in the Carnivorous races, from the fatty substances which the bodies of their victims may contain ; whilst the Herbivorous not only find them in the oleaginous state in then- food, but have the power of pro- ducing them by the conversion of farinaceous and saccharine matters. IV. The foregoing statements are applicable to all tribes of Animals, ‘ cold-blooded ’ as well as ‘ warm-blooded ; ’ we have now to consider the special case of the latter. — In the Carnivorous tribes, the ‘ waste ’ of the tissues is so great, in consequence of the restless activity which is habitual to them, that it appears to furnish a large proportion of the combustible material required for the maintenance of their proper temperature. The remainder is made up by the fat of the animals upon which they feed ; and it is to be observed that the amount of this is much greater in the bodies of animals inhabiting the colder regions of the globe, than in the inhabitants of tropical climates. — In the Herbivorous tribes, the case is different. They are, for the most part, much less active ; and the ‘ waste ’ of their tissues consequently takes place in a less rapid manner, and is far from supplying an adequate amount of combustible material, especially in cold climates. Their heat is in great part sustained by the combustion of the saccharine and oleaginous elements of their food, which are appro- priated to this purpose without having ever formed part of the living tissues ; and the demand for these will be larger in proportion to the de- pression of the external temperature, a greater generation of caloric being- then required to keep up the heat of the body to its proper standard. V. Hence ‘ cold-blooded ’ animals can usually sustain the privation of food longer than warm-blooded ; and this more especially when they are kept cool, so that they are made to live slowly ; and death, when at last it does ensue, is consequent upon the general deficiency of nutrition. On the other hand, ‘ warm-blooded ’ animals, whose temperature is uniformly high, must always live fast ; and deprivation of food is fatal to them, not only by preventing the due renovation of their tissues, but also by de- stroying their power of sustaining their heat (§ 93). The duration of life under these circumstances depends upon the amount of fat previously stored up in the body, and upon the retardation of its expenditure by G18 SPECIAL AND COMPARATIVE PHYSIOLOGY. external warmth, or by the enclosure of the body in non-conducting sub- stances ; and there is evidence that, if this be duly provided for, and all unnecessary waste by nervo-muscular activity be prevented, the life even of a warm-blooded animal may sometimes be prolonged for many weeks without food. 393. Although, as already explained, the Vegetable world as a whole is supported by the introduction of the alimentary materials, derived from the earth and air, into the organism, without any preliminary alteration, yet there are particular cases in which adaptations of structure are met with, that appear to be subservient to the reception and preparation of nutritive materials ; and some of these it would not be easy to exclude from any definition we might frame of a ‘ stomach.’ Concavities in dif- ferent parts of the surface, fitted for the collection of the moisture caught from rain or condensed from dew, may frequently be observed ; and these vary in the completeness of their structure, from the simple hollow formed in the leaf of the TiUandsia (wild pine of the tropics), or of the Dipsacm (teasel), to the extraordinary ascidia of the ‘ Pitcher-plants.’ The exact method in -which the fluid thus obtained is applied to the nutrition of the plant, is not always evident. Sometimes the channelled leaves seem to convey it to the roots, by which it is absorbed in the usual manner. The function of the pitcher of the Nepenthes or ‘Chinese pitcher-plants’ (Fig. 191, 6), has not been certainly determined; it is difficult to ascertain, how Different forms of Ascidia (pitchers); — a, pitcher of Sarra- would liave great difficult) IU cenia; b, of Nepenthes ; c, of Cephaiotus. obtaining moisture through the stem, a sufficient supply is provided by the pitchers, which store up the fluid collected from the occasional rains. “ The cavity of the bag,” says Dr. Wallich,* “is narrow, and always contains a dense tuft 3. Ingestion and Preparation of Aliment in Plants. Fig. 194. much of the fluid which it contains is collected from the atmosphere by the downy hairs that line its interior, and how much is secreted by the plant itself ; and it is certain that the young pitcher contains fluid, se- creted from an organ within it, before its lid first opens. The object of the pitchers of the IJischidia is, however, less doubtful, and their structure far more compli- cated. This curious plant grows by a long creeping stalk, which is bare of leaves until near its summit ; and as, in a dry tropical atmo- sphere, the buds at the top * “ Plant* Asiatic* rariores,” vol. ii. p. 35. ALIMENT, ITS INGESTION AND PREPARATION. G19 of radicles, which are produced from the nearest part of the branch, or even from the stalk on which the bag is suspended, and which enter through the inlet by one or two common bundles. The bags generally contain a great quantity of small and harmless black ants, most of which find a watery grave in the turbid fluid which frequently half fills the cavity, and which seems to be entirely derived from without.” Thus it would seem as if the failure of the ordinary means of support in this curious Plant, has been compensated by the addition of an organ, which, like the stomach of Animals, serves as a receptacle for the supplies it may occasionally obtain. According to Mr. Burnett,* in the pitcher of the Sarracenia (Fig. 194, a), a process still more like that of animal digestion goes on ; for it appears that the fluid it contains is very attractive to insects, which, having reached its surface, are prevented from returning by the direction of the long bristles that line the cavity. The bodies of those which are drowned seem, in decaying, to afford a supply of nutri- ment that is favourable to the growth of the plant ; like the similar process on the leaves of the well-known Dioncea muscipnla (Venus’ fly trap), to the health of which a supply of animal food appears to be essential. — Although such instances as these may seem to contradict the general statement, that Plants derive the materials of their nutrition from the inorganic world, yet they probably do so more in appearance than in reality. In all cases where previously-organised matter influences their growth, it is only whilst in a decomposing state, during which it is sepa- rated into its ultimate elements or very simple combinations of them. In Animal digestion, on the contrary, the proximate principles contained in the food appear to be immediately subservient to the formation of others of a higher order ; and whatever tendency to disunion their elements might have previously manifested, this is immediately checked by the antiseptic qualities of the gastric fluid. 4. Ingestion and Preparation of Aliment in Animals. 394. In considering the various organs which Animals possess for the ingestion and digestion of their food, it is right to take notice, in the first instance, of those cases in which there appears to be an absence of that provision for its reception, which is on the whole so characteristic of the beings included within the limits of that kingdom. There are several examples, even amongst animals of high organisation, in which, during the last stage of their evolution, there is an entire absence of any power of receiving nourishment into the system. These are principally met with in the class of Insects ; among which there are many (as already stated, § 315, g) which take no food in their perfect ox Imago state, the duration of this being very short, and serving merely for the performance of the re- productive act. In some of these cases, the mouth appears to be actually closed ; although the digestive apparatus remains, but in an atrophied condition. In all such examples, however, the appropriation of food has been actively performed during a previous stage of the animal’s existence ; but we have now to notice a peculiar case, in which an animal seems to take-in no nutriment, from the time of its quitting the egg, until its death. The animal referred to is the male of the curious Notommata described by * “ Brande’s Journal,” vol. vi. 620 SPECIAL AND COMPARATIVE PHYSIOLOGY. Mr. Dairy rriple (§ 310 a), which does not possess either oral apparatus or digestive cavity, and must be entirely incapable of deriving support from the nutriment upon which the female subsists.* Its organism is built up at the expense of the store prepared for it in the egg ; and it is probable that the duration of its life is extremely limited, and that it ceases to exist soon after it has performed its part in the reproductive functiou. Such instances, then, are not really so exceptional as they at first sight appear ; we have now to consider those cases, in which the growth and development of beings included in the Animal kingdom takes place at the expense of aliment appropriated by themselves ; and in which there is, nevertheless, no digestive cavity for its reception. 395. The group of Protozoa includes many forms which can scarcely but be reckoned among Animals ; when we consider, on the one hand, their close alliance to tribes which unquestionably belong to that king- dom, and with which they are connected by a continuous series of links ; and when we bear in mind, on the other, the probable evidence we possess, that they derive their nourishment from organic compounds which have previously been prepared by the Vegetable kingdom, and not, like Plants, from those which they have themselves elaborated from inorganic elements. It is true that some eminent Naturalists, who still hold to the doctrine that a stomach is a necessary constituent of an Animal, are ready to affirm the presence of this organ in the veiy lowest members of the series ; but the Author regards it as much more philosophical to believe, that a stomach may be absent in the simplest forms of Animal life, as in the earliest period of the embryonic life of the highest ; and when the facts of the case are carefully examined with an unprejudiced eye, he believes that they will be found in perfect accordance with this view. For although the simple cell of the Amoeba (§ 283 a), or the Sp>onge-proteus (§ 286 may appear to receive into its interior the solid particles from which ic derives its nutriment, yet there is strong reason to believe that they are really external to it, and that the fluid which they yield is imbibed through the cell-wall ; notwithstanding that, by folding around them, it may form a sort of extemporaneous stomach. And the most careful examination of the anatomy of many of the Rhizopoda (§ 285) has failed to discover in them a mouth, stomach, or intestinal canal ; and seems to justify the con- clusion, that their pseudopodia are really to be regarded as ‘radical fila- ments ’ like those of Plants, and that the masses of cells of which tbeir bodies are composed, are nourished by imbibition, partly through these special prolongations, and partly by their general surface, to which the surrounding liquid gains access by means of the porosity of the shell. So again, the Gregarina (§ 309 a), which seems to be the simplest known form of Entozoon, appears undoubtedly to be nourished by absorption through its external surface alone ; and even the Cystic and Cestoid Entozoa (§ 309, b, c) seem to possess no mouth, though an alimentary canal may be dis- tinctly made out in them. 396. In the group of Sponges (§ 286) we seem to have the transition from this condition to that of higher forms of Animal life : for whilst, cj the one hand, we may regard each mass as an aggregation of Amoeba-like cells, held together by a common integument, and supported by a frame- * “ Philosophical Transactions,” 1850, p. 342. ALIMENT, ITS INGESTION AND PREPARATION. 621 work which all participate in forming ; we may, on the other hand, trace in the higher types of the group a manifest approximation to the Alcyonian Zoophytes (§ 292). And thus whilst each of the component cells, like the elements of the tissues of higher animals, lives for and by itself, they all derive their nutriment from the fluid which is imbibed through the pores of the surface, and which circulates through its canals. These canals, if considered with reference to the entire body, must be regarded in the light of an alimentary cavity, which is so diffused through the mass, as to supply to every part of it the materials of its growth, without any special circulating apparatus.* We shall presently find that in many Animals of far more complex organisation, the digestive cavity is extended through the body in a manner essentially the same as this. 397. Before proceeding to the description of the principal forms under which the Digestive apparatus presents itself, in those organisms of which it forms a definite and limited part, something must be said of its condition in the (so called) Polygastrica, in the true members of which group there is undoubtedly a reception of food into the interior through an oral orifice, although it is very questionable whether it passes into any special recep- tacle, or into the general cavity of the body. The doctrine of Prof. Ehrenberg, that each of these Animalcules contains a number of distinct sacculi, sometimes opening externally by distinct orifices at the same part of the body, sometimes communicating with an alimentary canal that is common to them all, has been already stated (§ 284) ; and some of the principal forms of the apparatus, described by him, have been represented (Fig. 87). Other observers, however, have been far from confirming the correctness of these statements ; and the Author’s own investigations have led him to place little reliance upon them. The belief in the existence of a number of distinct sacculi, opening from a common intestinal tube, is founded upon the appearance of Animalcules which have been fed with coloured particles, and which exhibit numerous coloured globules in their substance (Fig. 87, b, c, e, g, h). These globules have been regarded as consisting of aggregations of the coloured particles within the spherical sacs, into which they have been received through the intestinal tube. Not only, however, has no such tube been ever seen, but no separate par- ticles have been traced in their passage from one part of the body to the other along such a canal ; and, what appears a still more fatal objection, the globules themselves have been seen to perform a kind of circulation through the body, changing their relative positions to each other, and sometimes escaping by the anal orifice. Moreover, many Animalcules have been seen to receive as prey, into the cavity of their bodies, other animalcules of nearly their own size ; which could scarcely be, if the intestinal tube were of the nature described by Ehrenberg. — An account of the nature of the digestive system of these animals, which appears much more consistent with observed facts, has been given by Meyen. He states that behind the oral orifice is a small globular cavity ; and that the * Since the account of this group (§ 286) was printed, ciliary movement has been discovered by Mr. Bowerbank in the canals of Grantia compressa ; whence it seems probable that the motion of fluid in other Porifera is due to the same agency. The extreme minute- ness of the cilia, and the difficulty of making observations upon these bodies in their living state with the high magnifying power requisite for their discernment, furnish the explana- tion of the cilia having so long remained undetected. 622 SPECIAL AND COMPARATIVE PHYSIOLOGY. particles of the food which are drawn into the mouth by ciliary action, are here aggregated into a globular mass, and pressed into a degree of con- sistence, sufficient to cause them to hold together. 'When a ball has thus been formed, it is expelled into the general cavity of the body ; and the formation of a second globular mass commences. The general cavity is undivided by any membranous partitions, and is chiefly occupied by a semifluid gelatinous substance, in which no distinct organisation can be discovered. Into the midst of this, the globule is propelled by the con- traction of the first cavity ; and, as one after another is thus forced in, each, as it is introduced, pushes on the rest, and a kind of circulation of the globules is occasioned, as already described, — those first formed making their escape (after yielding then nutritive materials) by the second orifice.* Besides the globular masses of colouring particles, the cavity of the body is often seen to contain peculiar transparent vesicles ; and as these are much more obvious when no colouring matter whatever has been intro- duced, it has been supposed by Prof. Ehrenberg that they are unfilled stomachs. It is scarcely questionable, however, that these vesicles are closed and unattached ; for if the body of any Polygastric Animalcule in which they are distinctly visible, be caused to rupture by pressure, they are forced out entire, and are seen to be destitute of either aperture or peduncle ; and this result may also be obtained by the application of liquor potassse,+ which causes the vesicles to undergo a perceptible enlarge- ment whilst still within the general investment. The vesicles thus dis- charged are seen to contain numerous minute molecules in active motion ; and .it would seem not improbable that they are ‘assimilating cells.’ whose function it is to select from the materials introduced into the general cavity, and to prepare for the nutrition of the body at large, such constituents as may be appropriate to that purpose. 398. The food is usually introduced into the digestive cavity of higher animals, through a single orifice ; the only known exception to this statement being in the case of the curious family of Rhizostome (root- mouthed) Acalephse (§ 291). In these animals, the single oral orifice of the ordinary Medusae is replaced by a multitude of pores at the sides and extremities of their tentacula (Fig. 195); from each of these pores a narrow tube commences, which terminates in a canal running upwards through the centre of the tentacle ; and the canals of the eight tentacles unite in pairs, so as to form four large trunks, which then discharge them- selves into the cavity of the stomach. It has been shown by the experi- ments of Prof. Milne-Edwards, that colouring particles are absorbed through the pores ; but they are too small to allow any save very minute Animalcules to enter.— The relative size and form of the single oral orifice that is common to all other animals, differ very greatly, according to the nature of the food upon which the species is destined to live. The Inver- tebrate division contains many groups, that are supported by suction of the juices of higher animals or of plants ; and in them we usually find the oral orifice narrowed and sometimes immensely prolonged. As such a means of obtaining food usually involves the necessity of locomotive powers, whereby the animal may go in search of it, we find, as might be expected, that it is most common in the Articulated sub-kingdom ; all the * See “ Armais of Natural History,” vol. in. pp. 100 and 170. b See Dr. Addison's “ Experimental Researches on the Functions of Cells,” p. 44. ALIMENT, ITS INGESTION AND PREPARATION. 623 principal classes of which, — Entozoa, Annelida, Insects, Crustaceans, and Spiders, — contain groups of greater or less extent, which are especially adapted for obtaining their food by suction. The Pycnogonidce (Fig. 158) present a characteristic example of the most simple form of suctorial mouth, in which the aperture is merely narrowed and somewhat pro- longed;* but the most re- markable modifications for Fig ' this purpose are to be met with in the class of Insects (§ 315), especially in the Lepidopterous and Dipterous orders. Of the first of these groups, a considerable pro- portion feed upon the juices of flowers, which the large expanse of their wings pre- vents them from entering ; and their long haustellium, which is coiled up beneath the head when not in use, can be so extended as to suck up the honey from the bottom of a deep blossom, while the insect rests upon its outer edge. Of the latter, however, a considerable pro- portion feed upon the juices of animals ; but there are some that have recourse to the honeyed exudations of flowers ; and among these the Nemestrina longirostris (a Dipterous insect of the Cape of Good Hope) deserves Rhizostoma injected with coloured fluid, to show the central especial mention, the length digestive cavity, and the canals branching off from it to ramify fT • . , . , . , & , in the arms and in the disk. ol its proboscis being about 3 inches, whilst that of its body is only about 8 lines ; so that it is enabled to feed upon the juices of a flower whose long tubular corolla equals that of its trunk. Many animals whose mouths are adapted for suction, possess some means of attaching themselves to the spots in which they can best obtain the nutriment they require ; thus we fiud the Cystic and Cestoid Entozoa very commonly provided with a set of hooklets on their heads (Fig. 140, a) ; the Trematocla usually have the mouth surrounded by a circular sucker (Fig. 141, a) ; the Suctorial Annelida (§ 311 d) have not merely a sucker for attachment, but an incising apparatus for making- incisions into the blood-vessels of the animals whose juices they suck ; the Suctorial Crustacea usually have a sucker formed by the peculiar develop- ment of one of the pairs of members (Fig. 160,/), whilst the mouth is elongated into a proboscis armed with penetrating instruments ; and the * From the situations in which these creatures are commonly found, the Author would infer that they draw their nourishment from the mucus that covers the surface of sea-weeds. 624 SPECIAL AND COMPARATIVE PHYSIOLOGY. Suctorial Insects, which seldom attach themselves permanently in anv one situation, but make a fresh puncture whenever their appetite incites them to feed, are for the most part destitute of any special organs of adhesion, but have a most elaborate set of instruments for incising the skin of the animals they attack (§ 315). — The Cyclostome Fishes (§ 322) are the only Vertebrated animals, which present any similar adaptation to a suctorial mode of nutrition. 399. When the food consists of solid matters, which is the case in by far the larger proportion of the Animal kingdom, we find the entrance to the digestive cavity of much greater pi'oportionate size ; and it is seldom prolonged forwards, hut far more frequently has somewhat of a f unn el shape, so that the aliment may be more readily drawn within it. The means by which the introduction of food is accomplished, are extremely various. The simplest plan is one which carries us back to the type of the Infusoria, although it presents itself in a much more elaborate form. Among many aquatic animals, whose food consists of minute particles diffused through the water, we find that the action of cilia situated around and within the entrance of the alimentary canal, or upon organs in im- mediate connection with it, whereby a cim-ent of water may be driven into the stomach, is the sole means by which aliment is introduced. The most remarkable examples of entire dependence upon this agency are furnished by the Acephalous Mollusks ; for in the Bryozoa (§ 298) the polype-like arms are unfitted for prehension, and effect the introduction of alimentary matter solely by the ciliary currents they produce ; and the T unicat a (§299), Brackiopoda (§ 300), and Lamellibranchiata (§ 301), are all supplied with aliment in a manner essentially the same. Among Radiated animals, recourse seems less frequently had to this plan, which is obviously one that is unsuitable to those forms of the digestive cavity that have hut a single orifice, since the requisite current could not be effectually main- tained through such ; but among the Ciliograde Acalephce (§ 294 a), which possess an anal orifice, the ciliary action appears to be the means of intro- ducing aliment, as well as of propelling the body through the ocean. In the Articulated classes, too, this ciliary action is seldom the means of intro- ducing aliment into the stomach ; most of them feeding upon solid bodies of comparatively large dimensions, and being enabled by their locomotive powers to go in search of their food. But in those which resemble tfie Molluscous tribes already alluded to, in their habits of life, we find tfie same method of obtaining nutriment ; thus it appeal's to be by the currents set in motion by the cilia clothing the respiratory tufts which arc seated on then - heads, that the Serpulce (Fig. 148) and then- allies draw in their food ; and in the Rotifera (§ 310) and the Cirrhipoda (§ 318) we find special ciliated organs developed, which appeal' subservient both to the purposes of respiration and to the ingestion of aliment. It is probable that the Amphioxus (§ 321 b) is nourished in a similar manner ; but that curious animal affords the only known example among the Vertebrata, of the employment of ciliary action for this purpose. It is curious to observe, however, in the Greenland Whale, a mode of ingestion which is essentially the same, though accomplished by a different instrumentality : for this huge animal gulps enormous volumes of water into its a- pacious mouth, and then ejects these through its blow-holes, straining out, through its whalebone-sieve, tlie small Medusm, Pteropods, Crustacea, ALIMENT, ITS INGESTION AND PREPARATION. 625 Fishes, or other free-swimming marine animals which the water may contain ; it being upon such alone that it is adapted to subsist. — The foregoing plan is one that seems intermediate, in regard to the kind of nutriment for which it is adapted, between the suctorial method, which is fitted only for the introduction of liquid aliment, and the one w r e have next to consider, which is appropriate to the ingestion of solid masses. 400. The organs with which the digestive apparatus is provided, for the introduction of solid food into its cavity, vary greatly in complexity in the different tribes of animals which derive their subsistence from aliment of this kind. Thus we shall find that in the lowest and simplest types, these organs are in immediate connection with the stomach ; but that they are partly superseded in the higher by the addition of an oral apparatus, with prehensile appendages for laying hold of the food, which, after passing- through the oral cavity, is drawn downwards into the stomach ; and that in the highest of all, the introduction of food into the mouth is itself aided by accessory organs, wdiich do not form part of its own organization. Of the first of these types, we have a characteristic example in the Hydra (§ 289), the entrance to whose stomach is surrounded by ‘tentacula,’ which,, as formerly remarked, appear homologous with the oesophageal muscles of higher animals. The tentacula of the Actinia (Fig. 97) cor- respond with these in character and mode of action ; and it may be said that the same type prevails through the whole class of Polypifera. The Medusae (Fig. 106) and their allies are formed upon a plan -which must be considered as essentially similar, the tentacula being there, also, placed around the entrance to the stomach ; and it is by the adhesion of the edges of these tentacula, that the ‘ proboscis ’ is formed in those of the Pulmograde Acalephie which possess it (§ 294). It is interesting to observe in the class of Echinodermata, a gradual transition towards a more elevated type. Thus in the Asteriada (§ 296 b), which have no oral apparatus, we find the food brought to the stomach, not by tentacula, but by the flexion of the lobes of the body, commonly known as their ‘ arms.’ “ Starfishes,” says Prof. E. Forbes,'" “ are not unfrequently found feeding on shell-fish; in such cases they enfold their prey within their arms, and seem to suck it out of its shell with their mouths pouting out the lobes of the stomach. They can project the central parts of their stomachs in the manner of a proboscis.” Whether the true ‘ arms ’ of the Crinoidea (§ 296), and Ophi- urida (§ 296 a) serve to bring food to the stomach, is not certainly known ; but their position in the former of these orders, at least, would lead to the inference that they are subservient to this purpose. In the Echinida (§ 296 c) and Holothuriada (§ 296 e), however, we find that the entrance to the alimentary canal is no longer the entrance to the stomach ; but that the former becomes a true ‘ mouth,’ being furnished with a prehensile apparatus of its own, and being separated from the stomach by the inter- vention of the oesophagus. The mouth is provided, in the typical Echinida, with a dental apparatus (Fig. Ill, a), which is capable of being projected beyond the oral orifice ; whilst in the Holothuriada it is sur- rounded by prehensile tentacula, which are here to be regarded as labial, being extensions of the lips, rather than a divided pharynx. — Turning to the Articulated series, we find that in a considerable proportion of the * “ History of British Echinodermata,” p. 86. S S 620 SPECIAL AND COMPARATIVE PHYSIOLOGY. Annelida (§311 a), the month is furnished with powerful jaws, sometimes to the number of three pairs, opening laterally ; or with a proboscis which is capable of being everted, and which then displays an armature of teeth on its exterior. In the Mandibulate Insects (§ 315), and in the greater part of the Crustacean class (§ 316 a), we find the jaws highly developed, and taking the place of any other kind of apparatus ; the former group always possessing two pairs of these organs, which open laterally, one above and tjie other below the oral orifice ; whilst in the latter, besides the regular mandibles and maxillae, there is a variable number of feet-jaws. — In the higher paid of the Molluscous series, again, we find the mouth usually provided with jaws ; these animals being adapted to go in search of their food, and to select and divide it for them- selves. In many of the Gasteropoda, we find a single pair of homy jaws, opening laterally ; but there are several whose buccal apparatus rather consists of a proboscis-like organ, somewhat like that of the Annelida, which can be everted, and which then displays a set of powerful teeth, adapted to abrade substances of even shelly hardness. The Cephalopoda , on the other hand, possess a pair of horny jaws opening vertically, like those of Yertebrata (§ 306) ; and the oral orifice is surrounded with ‘ arms,’ which, though resembling those of the Polypes in uses, and appa- rently also in position, are not really homologous with them, being the representatives of the labial tentacula, which are possessed by many of the inferior Mollusks, as well as by several Fishes. The difference between two sets of organs, which are ostensibly so like one another, will become apparent when it is remembered that the ‘ arms ’ of the Polype surround the entrance to the stomach ; whilst those of the Cephalopod surround the oral orifice, which is separated from the stomach by the jaws, buccal cavity, pharynx, and oesophagus. The latter w r ould seem to be more truly prefigured by the oral tentacula of the Holothuriada. — Throughout the Vertebrated series, it is almost invariably by the action of the jaws and lips, that food is introduced into the mouth ; the suctorial Cyclostomi already referred to, and the Syngnathklae or ‘pipe-fishes,’ which have the jaws prolonged and adherent together, and which seem to draw up worms, small mollusks, and crustaceans, and the ova of other fishes, through the tube thus formed, being the only cases of even partial reversion to the lower type. 401. But among the more elevated forms of the Articulated and Yer- tebrated sub-kingdoms, we find a special provision for the prehension of food, by organs which have no immediate connection with the digestive apparatus ; the aliment being thus brought within reach of the mouth and of its proper appendages. Some approaches to such an arrangement we have seen even in the lower Echinodermata, in which the ‘ arms ’ and lobes of the body are made to bring the food to the entrance of the stomach ; and in the Echinida it seems not improbable that the spines and ‘ cirrhi,’ especially the latter, may be occasionally used in subservience to the ingestion of food, as well as in locomotion. — In most of those higher Articulata which possess numerous well-developed members, we find that those nearest the mouth are used to a greater or less extent in the prehension of food ; and that in many instances one pair is specially developed for this purpose. The transition from the general to the special form of the prehensile apparatus is well seen among the Decapod Crustacea ALIMENT, ITS INGESTION AND PREPARATION. 627 (§ 316 a) ; in some of which (as the Cray-fish) we find all the true legs of nearly equal size, and most of them terminated by forcipated appendages ; whilst in others (as the Crab) the first pair are enormously developed as prehensile organs, and are furnished with powerful pincers, whilst the remainder are restricted to locomotive action. — In the Vertebrated series, it would appeal - as if the restricted number of members, and the necessity for employing these in locomotion, placed a greater limitation upon the use of them as prehensile organs. Among Fishes and Reptiles, the pre- hension of food is solely accomplished by the jaws, save where the tongue (as in the Chameleon) is subservient to this function. In Birds, it is only in a few cases that the foot is employed for this purpose, the anterior members being never modified into subserviency to it, and the tongue (as in Reptiles) being the only instrument that is ever specially developed as a prehensile organ. Among Mammals, as has been fully pointed out already (§ 326 e), the special modification of the anterior extremities for prehension is limited to those few groups in which they are comparatively little needed for locomotion : thus, among most of the Ungulated quad- rupeds which have the power of standing erect upon the hind legs, as the Rodentia, Plantigrade Carnivora, and many Marsupialia, we find the anterior extremities employed to clasp objects between them ; in the Quadrumana, the prehensile power is exercised by a single extremity, which can hold an object of moderate size in the grasp of the digits ; but it is in Man that this prehensile power is carried to its fullest extent, by the peculiarity of conformation of the thumb, which brings its extremity into ojiposition with those of each of the fingers. — Some of the Mammalia whose extremities are not adapted for the prehension of food, are furnished with other organs for the purpose, which are special modifications of those elsewhere existing under the ordinary type ; it is sufficient to refer to the proboscis of the Elephant, the prolonged snout of the Tapir, the pre- hensile tongue of the Giraffe, and the very extensible tongue of the Ant- eater. 402. Thus, between the simple stomach furnished with prehensile ten- tacles, of which the Hydra consists, and the complex apparatus for the prehension of the food which is placed in front (so to speak) of the cardiac orifice of the stomach in Man, a regular gradation may be traced. The tentacula are brought together to form a pharyngeal tube ; a buccal cavity is placed at the entrance of this ; jaws and lips are developed at the orifice of the buccal cavity ; and external members are developed or modified, for the purpose of bringing the food within their reach. Yet the development of these more special organs does not supersede the necessity of those more generally diffused through the Animal kingdom. Although the hand of Man brings his food to his mouth, yet it is by his bps and jaws that his food is taken into its cavity, as in the Herbivorous Mammalia ; and notwithstanding that the muscular apparatus of the mouth propels the food backwards into the pharynx, it only thereby carries it within reach of the pharyngeal constrictors, which then lay hold of it and draw it into the oesophagus, by the muscles of which it is carried down into the stomach, precisely as by the tentacula in the Hydra. And it is curious to observe, that just as the tentacula of the Hydra no longer make any attempts to grasp an object that touches them, when the stomach is already gorged with food, so does Man experience a difficulty s s 2 628 SPECIAL AND COMPARATIVE PHYSIOLOGY. in tiie act of swallowing at the end of a full meal, which seems to result from a like want of readiness to contract on the part of the pharyngeal constrictors. — In the progress of the food along the alimentary canal of higher Animals, we may observe the gradual removal of it from connec- tion with the functions of animal life. To procure it in the first instance, is one important office of these functions ; and the highest exercise of the locomotive, sensorial, and intellectual powers is often required for this purpose. Its introduction into the mouth is an act of pure volition in Man ; whilst the masticatory movements to which it is there subjected, may be regarded as essentially automatic (§ 244) in their character (though capable of being controlled and directed by the will), and as closely corresponding with the instinctive actions of the lower animals. The act of ‘ deglutition,’ or swallowing, is of a purely reflex nature (chap. xx.), being the result of a nervous influence in which neither the will nor sensation is concerned : for when the solid or fluid contents of the mouth are brought in contact with the surface of the pharynx, the impression made upon the nerve distributed on it is transmitted to the upper part of the spinal cord ; and an automatic impulse is propagated along the motor fibrils, by which the muscular movements requisite for the action of swal- lowing are produced. A similar action assists the propulsion of food down the oesophagus, and the movements of the stomach seem to be in paid excited in the same manner ; but the proper fibres of the oesophagus itself are not dependent for their power of action upon nervous influence, nor are those of the stomach. Beyond the stomach, the connection of the motions of the alimentary canal with the nervous system almost wholly ceases, the ordinary peristaltic movements of the intestines appearing to depend upon the stimulus directly applied to then - muscular coat by the contact of food ; although they may be in some degree controlled by a system of muscles disposed around the outlet of the canal, which are, like those at its entrance, partly involuntary, and partly under the direc- tion and restraint of the will. — So, in descending the Animal scale, we find that the introduction of food into the digestive cavity is first removed from the agency of the intellect and will, and provided for by the instincts alone ; this is probably the case in even the highest Invertebrata, aud likewise in the lower Vertebrata, as it undoubtedly is in the infantile condi- tion of Man. When we descend to Zoophytes, we find strong reason to believe that the movements of the tentacula, by which the food is grasped and drawn into the stomach, are not merely involuntary, but are not even dependent upon sensation (chap, xx.) ; and there is still stronger reason to believe that such is the case, in those inferior Mollusks in which the supply of food is obtained by ciliary currents. And thus we may perceive a regular gradation between those beings, in which the supply of food has been made (for a wise purpose) dependent upon the highest exercise of the functions of Animal life, and those in which the operation is as purely Organic as it is in Plants. 403. One more class of accessory organs has to be noticed, before we proceed to the account of the essential part of the Digestive apparatus, and of the operations to which it is subservient. In order that solid food may be more easily dissolved in the stomach, it is frequently submitted in the first instance to a process of mechanical reduction by a triturating apparatus ; and either at the same time, or at some other, it is iucorpo- ALIMENT, ITS INGESTION AND PREPARATION. 629 rated with a Salivary fluid, the admixture of which not only renders the subsequent process of solution more easy, but also appears to effect a change in the alimentary matters themselves. It is in animals that are destined to feed upon vegetable substances, and especially upon such as are of tough consistence, that we find this reducing apparatus most power- ful and efficient ; for as the flesh of animals is of comparatively easy solubility, there is less occasion for any such preliminary preparation. This reduction is not by any means constantly performed, however, in the mouth ; for almost any part of the first division of the alimentary canal may be the seat of the apparatus. Among the Radiata, there are none, save the typical Echinida, which possess such a reducing apparatus ; but it is remarkable that it should attain in that group an extraordinary com- plexity, and should occupy a situation corresponding to that which it possesses in the highest Vertebrata. The ‘lantern of Aristotle’ (Fig. Ill, a), as it is sometimes termed, is a five-sided conical mass, composed of five ‘jaws’ in apposition with one another, each of them having the form of a triangular pyramid ; two of their sides, which are flattened, look towards those of the neighbouring jaws, and have their surfaces roughened by grooves like those of a file ; whilst the thud, which is somewhat convex, is external, and serves for the attachment of the muscles that fix the apparatus to the interior of the shell, which is furnished with projections round the oral orifice developed for this purpose. Each jaw contains a long pointed tooth of remarkable hardness, which projects to a considerable length from its socket, and this appears (like the ‘ tusks ’ of Mammalia) to be continually renewed by new growth at its base, as it is worn away at its apex. The whole is moved by a most complex set of muscles, which can bring the whole ‘ lantern ’ nearer to the oral orifice, and can cause the points of the teeth to project beyond it ; which can separate the jaws, and consequently the points of the teeth, from one another, and then draw them together, so as to enable the latter to grasp and divide any hard substance adapted for food ; and which can subject this to trituration, not merely between the points of the teeth, but also between the flattened file-like surfaces of the jaws, which can be made to work against each other. — Among the Acephalous Mollusks, no reducing apparatus seems usually to be required ; their food being in a state of very fine division at the time it is introduced ; nevertheless in the Bowerhankia and some other Bryozoa (§ 298), a ‘gizzard ’ or muscular stomach, apparently sub- servient to this purpose, intervenes between the oesophagus and the true digestive stomach. A gizzard of considerable power, closely resembling that of Birds, is found in many Gasteropods, especially in those whose mouth is unfurnished with a reducing apparatus j this is the case, for example, with Aplysia (Fig. 127), whose gizzard (i), situated between the crop (y, g ) and the true digestive stomach ( h ), is lined with firm horny teeth ; and in some instances, these teeth are replaced by firm calcareous plates, adapted to crush the shells of the smaller Mollusks that are de- voured by these animals, which is the case in Bulla* But in the greater number of instances, the reduction of the food is accomplished by some part of the buccal apparatus ) either by the cutting jaws with which some Gasteropods are furnished ; or, as in Buccinum, by the teeth which form a * The Author once met with an entire shell of Pandora roslrata in the gizzard of a Bulla. G30 SPECIAL AND COMPARATIVE PHYSIOLOGY. sort of palatal lining to the proboscis, and which are earned outwards by its eversion ; or, as in Patella and some other phytophagous Gasteropods, by the long rasp-like tongue. In the Cephalopoda, notwithstanding the presence of jaws for the division of the food, its reduction seems to be chiefly accomplished by a muscular gizzard closely resembling that of Birds (Fig. 133, p, and 135, k ). — In most of these instances, we find salivary glands pouring their secretion into the mouth or pharynx ; and where a gizzard exists, there is usually also a ‘ crop,’ or ‘ first stomach ’ (which is rather to be regarded as a dilatation of the lower part of the oesophagus), into which the food is received in the first instance, and in which it is probably macerated in the salivary fluid, before it is subjected to the trituration of the gizzard. 403 a. In the lower Articulata, there is seldom any special reducing appa- ratus j in the Potifera, however, which obtain their food (like the Bryozoa) by ciliary action, we find, in place of a gizzard, a curious pair of jaws (Fig. 144, e), furnished with sharp hard teeth, and worked by a complex muscular apparatus ; these are situated in the pharynx, and serve to divide the larger particles as they pass downwards to the stomach. Owing to the transparency of these animals, the movements of their jaws can be distinctly seen, when the ‘ wheels ’ are in action, and are driving downwards the particles which have entered the mouth. — In the J landi- bulate Insects, the reduction of the food, as well as its division and inges- tion, is partly performed by the jaws, and the mouth is usually furnished with salivary glands ; but many insects are also provided with a gizzard (Fig. 199, d), which, as in the cases already alluded to, does not act upon the food until it has been received into the crop (c). In many of the higher Crustacea, notwithstanding then- complex buccal apparatus, the stomach is furnished with a powerful set of teeth, affixed to a complicated framework which is worked by powerful muscles, so as to constitute very efficient reducing instruments. Among the mandibulate Articulata, sali- vary glands are almost invariably found opening either into the mouth, pharynx, or oesophagus ; but sometimes the stomach itself is clothed with caeca (Fig. 199, e), that appear to have this character. — Of that dental reducing apparatus, which is so especially characteristic of the Vertebrata, a full account has already been given in the description of the respective classes of that sub-kingdom (Fishes, § 32 2 f Beptiles, § 324 o, Mammals, § 32 G o) ; and it is unnecessary to do more here than refer to the substi- tute for it in the class of Birds, which is altogether destitute of the means of reducing the food in the mouth, but carries us back, in the modification of the stomach for the purpose, to the plan so common among the In- vertebrata. There is this curious distinction, however, that the gizzard succeeds, instead of preceding, the proper gastric cavity ; the food being not only macerated in the salivary and other secretions furnished by the crop, but also impregnated with the proper gastric fluid, before it is sub- jected to the triturating action of the gizzard. Although in the Mammalia the reducing apparatus is limited to the mouth, yet the stomach frequent ly presents a complex arrangement, of which the purpose seems to be to favour the mechanical reduction of the food, and its impregnation with fluid, before it is subjected to the true digestive action. The most remarkable example of this kind is presented in the group of Ruminantia , in which the stomach is subdivided into four distinct cavities ; the first ALIMENT, ITS INGESTION AND PREPARATION. 631 Fig. 196 . two of these, however, being rather dilatations of the oesophagus, than parts of the stomach itself. The food, on its first passage down the oesophagus in a crude unmasticated state, enters the large cavity termed the ingluvies or ‘paunch’ (Fig. 196, b), which, like the crop of Birds, serves as a temporary receptacle for it, and moistens it with the fluid secreted from its walls ; the liquid swallowed, on the other hand, seems to be specially directed into the second cavity (c), which is termed the reticulum or ‘ honeycomb-stomach ’ from the reticulated appearance of its interior, occasioned by the irregular folding of its internal membrane. It is here that the peculiar provision of ‘ water-cells ’ is found, for which the Camel has long been so celebrated, but which exists in a greater or less degree in all the Ruminants. These cells, which are the spaces between the deepest reticulations, are bounded by muscular fasciculi, by the con- traction of one set of which their orifices may be closed and their contents retained ; whilst by that of another set, the fluid they contain may be expelled into the general cavity of the stomach. After remaining there until it is sufficiently impregnated with fluid, the solid matters which have passed into the first and second stomachs are returned at intervals, in the form of little balls or pellets, to the mouth ; where they undergo a thorough mastication, and are completely incorporated with salivary fluid. When finally swal- lowed, the food is directed, in the manner to be pre- sently described, in- to the third sto- mach (d), the oma- sum, commonly termed the ‘ many- plies ’ from the pe- culiar manner in which its lining- membrane is dis- posed ; this pre- sents a number of folds, lying nearly close to each other like the leaves of a book, but all directed by their free edges to the centre of the tube, a narrow fold intervening between each pair of broad ones. The food, now reduced to a pulpy state, has therefore to pass over a large surface, before it can reach the outlet of that cavity ; which leads to the abomasum or fourth stomach (e), commonly called the reed. This is the seat of the true process of digestion, the gastric fluid being secreted from it alone; and it is from this part of the calf’s stomach, that the ‘rennet’ is taken, which derives its extraordinary power of coagulating milk from the organic acid it contains. In the suckling animal, the milk which forms its nourishment passes directly into this stomach ; the aperture leading to the first and second being closed, and the folds of the third adhering together so as to form a narrow undivided tube. The direction of the food into one or another of these cavities appears to be effected without any voluntary effort on the part of the animal itself, but to result simply from the very peculiar endowments of the lower part of the oesophagus. This does not entirely terminate at its opening into the first Stomach of Sheep ; — a, oesophagus ; b, paunch ; c, second, or honeycomb- stomach ; d, third stomach, or inany-plies ; e, fourth stomach or reed ; /, pylorus. 632 SPECIAL AND COMPARATIVE PHYSIOLOGY. stomach or paunch, but it is continued onwards as a deep groove with two lips (Fig. 197) : by the closure of these lips it is made to form a tube, which serves to convey the food onwards into the third stomach ; but when they separate, the food is allowed to pass either into the first or the second stomach. When the food is first swallowed, it has undergone but very little mastication ; it is consequently firm in its consistence, and is brought down to the termination of the oesophagus in dry bulky masses. These separate the lips of the groove or demi-canal, and pass into the fust and second stomachs. After they have been macerated in the fluids of these cavities, they are returned to the mouth by a reverse peristaltic action of the oesophagus ; this return takes place in a very regular manner, the food being shaped into globular pellets by compression within a sort of mould formed by the ends of the demi-canal, drawn together, and these being conveyed to the mouth at re- gular intervals, apparently by a rhyth- mical movement of the oesophagus. After its second mastication, it is again swallowed in a pulpy semi- fluid state ; and it now passes along the groove which forms the continu- ation of the oesophagus, without open- ing its lips ; and is thus conveyed into the third stomach, whence it passes to the fourth. Now, that the condition of the food as to bulk and solidity is the circumstance which determines the opening or closure of the lips of the groove, and which consequently regulates its passage into the first and second stomachs, or into the third and fourth, appears from the expert ments of Flourens ; who found that when the food, the first time of being swallowed, was artificially reduced to a soft and pulpy condition, it passed for the most part along the demi-canal into the third stomach, as if it had been ‘ ruminated,’ — only a small portion finding its way into the first and second stomachs. 404. We have now to trace out the principal forms, under which the proper Digestive apparatus presents itself in the different classes of animals ; and the mode in which it operates. Putting aside that of the Protozoa, which has been already considered, the simplest of all its conditions is that in which we find it in the Hydra, Actinia, and other solitary Zoophytes. Thus in the Hydra (§ 289), it will be remembered, the stomach occupies the whole of the cavity enclosed by the outer walls : and the membrane with which it is lined is so completely identical with Fig. 197. Section of part of the Stomach of the Sheep , to show the demi-canal of the oesophagus; the mu- cous membrane is for the most part removed, to show the arrangement of the muscular fibres. At a is seen the termination of the oesophageal tube, the cut edge of whose mucous membrane is shown at b. The lining of the first stomach is shown at c, c ; and the mucous membrane of the second stomach is seen to be raised from the subjacent fibres at d. At e, e , the lips of the demi-canal are seen bounding the groove, at the lower end of which is the entrance to the third stomach or man 3 r -plies. ALIMENT, ITS INGESTION AND PREPARATION. 633. that which forms the external integument, that the one may be made to take the place of the other without any injury to the animal. Yet it is obvious that a powerfully-solvent fluid is secreted from the avails of the gastric cavity ; for the soft parts of the food which is drawn into it (usually in a living state) are gradually dissolved, and this without the assistance of any mechanical trituration ; whilst the hard parts are ejected again by the orifice through winch they were introduced. The product of this operation appears to be absorbed from the lining of the digestive sac, by the w r hole of the tissue which forms it ; and to be con- veyed into the tentacula (which are the only parts not in immediate relation with the digestive cavity) by interstitial channels left for that purpose. — The same general condition presents itself in the Actinia; the only difference being, that the digestive sac does not occupy the whole of the cavity of the body, but is surrounded by the ovarian chambers (§ 291). — In the compound Hydroida, on the other hand, the lower part of the digestive sac of each polype is prolonged into a tube, which communicates with similar prolongations from other polypes, so that the stem and branches of the entire arborescent structure contain a continuation of the gastric cavities of the several polypes which they bear (Fig. 95) ; and the fluid that has been prepared by the digestive opera- tions of the latter, passes through the aperture at the lower extremity of each (which is guarded by a sphincter muscle), and is received into the system of ramifying tubes, which may be considered as an extension of the digestive cavity throughout the general structure, for the purpose of conveying to every portion of it, and especially to such parts as are undergoing increase by the formation of new branches or polype-cells, the materials prepared for nutrition. Through this system of canals, the contained fluid has been observed to move with considerable regularity, and in a manner that cannot be accounted for by any mechanical agency communicated by the contraction of the stomachs of the polypes. Thus, when the stem and branches of a living Sertularia are examined with a sufficiently high magnifying power, a current of granular particles is seen moving along the axis ; which, after continuing one or two minutes in the same direction, changes and sets in the opposite one, in which it continues about as long, and then resumes the first ; thus alternately flowing down the stem to the extremities of the branches, and back again. The change of direction is sometimes immediate ; but at other times the particles are quiet for a while, or exhibit a confused whirling motion for a few seconds, before it takes place. The current extends into the stomachs of the polypes ; in which, as well as in their orifices, a continual agitation of particles is perceptible. When these particles are allowed to escape from a cut branch, they exhibit for a time an apparently spon- taneous motion. No contraction of the tube, any more than of the stomach, seems concerned in the production of the currents ; and then- rapidity and constancy appear intimately connected with the activity of the nutritive processes taking place in the parts towards which they are directed, the ‘ set ’ of the current being especially toiuards any portion which is undergoing development, and away from any part which exhibits a diminution or loss of vitality. In the Tubularia, in which the polype- bearing stem ramifies but little, or not at all, and is sometimes divided by nodes or partitions somewhat resembling those of the Chara (§ 269), 634 SPECIAL AND COMPARATIVE PHYSIOLOGY. the movement of fluid very strongly resembles that which is seen in that plant ; for the current passes down one side, crosses the septum, and then ascends the other side, following a somewhat spiral line of particles deposited on the walls of the tube. — The same prolongation of the gastric cavity through the whole of the mass on which the polypes are seated, is characteristic of the Asteroid Zoophytes (§ 292); and here, also, this extension of the digestive sac through the entire body, serves to convey to every part the materials of its growth, which it can at once absorb for itself without the mediation of a special circulating apparatus. 405. In the lowest members of the Articulated series, we find the same general type of conformation of the digestive apparatus still retained, though its form is very different. The gastric cavity, in the lower Entozoa, is excavated (as it were) in the soft tissues of the body, so that, although provided with walls of its own, these are blended ext era ally with the surrounding parts ; and further, the cavity is extended through the entire body, being repeated in eveiy segment of it, so that there is but little occasion for any separate system of vessels for the conveyance of nutritive fluid. We have seen this to be the case in the Taenia (Fig. 1 40, c), in which, with its allies, the gastric cavity appeal’s to be filled by imbibition through its walls, instead of by suction through an oral orifice (§ 309 b, c ) ; and it is also the case in the Planarice (Fig. 141), Fasciolae (Fig. 142), and the other Trematoda, in which the aliment is received through a suctorial orifice, but in which no anal opening exists. —The same type is repeated under a higher form and more complete de - velopment, in some of those more elevated members of the Radiated series, which constitute the class Echinodermata. In the Ophiurida (§ 296 a), and Aster iada (§ 296 b), the gastric cavity is still a single sac, occupying the centre of the body, and furnished with but one orifice, through which food is drawn in, and imdigestible or faecal matter is ejected. But the sac is now suspended freely in the general cavity of the body, its walls being quite distinct from the surrounding tissues. In the caecal prolong- ations of the central stomach of the Asterias (Fig. 110), and in the abund- ance of the cells which line them, we find the first manifestation of special glandular organs for the elaboration of the fluids required in the digestive processes, though the precise nature of these is as yet unknown. The proper digestive apparatus appears to be limited in the Star-fish, as in the Ophiura, to the central cavity; and being removed by a considerable interval from other parts of the body, it does not here directly convey the nutritious fluid to the tissues which make use of it ; and the inter- vention of a set of vessels becomes requisite, for its absorption and trans- mission to distant parts. — On nearly the same grade with the preceding . as regards the conformation of its digestive apparatus, we may place the curious Eotiferous Animalcule formerly noticed (§ 310 a) as possessing no intestine nor anal orifice, and as ejecting the indigestible particles of food through its mouth. This is the only known instance among the Arti- culata, save in the class of Entozoa, in which the digestive cavity has but one orifice. No conformation of a similar kind is ever met with in Molluscous and Vertebrated series. 406. We have now to glance at those higher foims of the digestive apparatus, in which a second orifice or ‘ anus ’ is present, for the discharge of indigestible or faecal matters ; and this, as we shall see, is by far the ALIMENT, ITS INGESTION AND PREPARATION. 635 Fig. 198. most general plan of conformation. Although this orifice may externally he situated in the neighbourhood of the mouth, yet it always communi- cates with the part of the digestive cavity that is most remote from it ; this cavity being usually more or less prolonged into a tubular form. The lowest of the animals which present this great advance in the type of conformation, are the Acaleplice ; in most of which we find the digestive cavity provided either with a single or a multiple outlet, the former being- seen in the Beroe (Fig. 107, a), the latter in the Medusae (Fig. 106). This class is further remarkable for the absence of true blood-vessels ; their place being supplied by the tubular prolongations of the gastric cavity, which extend themselves into the mass of the tissues, and which convey to them the materials for their growth and renovation (Figs. 106 and 195). In the Beroe we see the first indication of that divi- sion between the ‘stomach ’ and ‘ intestinal canal,’ which be- comes so much more obvious and important in the higher classes.- This division is by no means well marked, however, even in the highest Radiata ; the alimentary canal, which commences with the mouth and .terminates at the anus, being of nearly uniform size through- out, and apparently of similar endowments ; as we see in the Echinus (Fig. Ill), and the Holothuria (Fig. 113). In the Nematoid Entozoa, again, the same uniformity presents itself (Fig. 143) ; and it is curious to observe this almost exactly re- peated in the Serpenttx'Ate, whose organisation is so much higher ; but the prolongation and uni- formity of whose body seems to necessitate a somewhat similar conformation of the alimentary canal. Even in the Annelida, the same general plan is con- tinued with but little modifica- tion; the alimentary canal pass- ing in a straight line from one extremity of the body to the other, without distinction of stomach or intestine (Fig. 198); but from its sides we find csecal prolongations extending, sometimes as a single pair of prolonged tubes (b, c, c, h), sometimes as mere sacculi in the walls of the stomach (b, b), and sometimes as a multiplied and extended series of appendages (a, cl), which seem, like the radiating cieca of the Digestive apparatus of Annelida : — a, Aphrodite acu- leata ; a, mouth ; b , fleshy proboscis ; c, central portion of digestive tube, representing the stomach: d, lateral ap- pendages ; e , anus: — b, Hcemopis vorax ; a, mouth; b, lateral sacculi of the stomach; c, two large terminal cEeca ; d, intestine: — c, Aulostoma nigrescens ; a, mouth; b, caeca ; c , intestine. 636 SPECIAL AND COMPARATIVE PHYSIOLOGY. Fig. 199. Asterias, to be rather glandular in their character, than destined to admit the passage of food into them. 407. From these forms we might gradually ascend towards the more complex digestive apparatus of Insects and Crustacea ; but we shall first revert to that of the lowest Mollusca, as the simplest case in which the division of the canal into stomach and intestine is clearly marked out. This is well seen in the Bryozoa (§ 298), in which, as in the Hydra, the whole process of digestion may be watched, owing to the transparency of the tissues of the animal. The digestive stomach is here separated from the comparatively narrow intestinal tube, by a valvular orifice, or true ‘ pylorus ; ’ and whilst the whole process of solution takes place in the former division of the canal, whose walls are beset with secret- ing follicles that pour forth a bile-like fluid, the latter seems to have little to do, save to convey to the anal orifice at the mouth of the polype-cell the excre- mentitious particles which are to be ejected from it. The same essential type is retained through the whole of the Molluscous series : the principal ad- vance being shown in the higher deve- lopment of the liver, by the withdrawal of its follicles from the parietes of the stomach, and by their aggregation into distinct glands which discharge them se- cretion into the upper part of the intes- tinal tube ; in the development of a rudimentary pancreas (in the Cephalo- poda) ; and in the increased length given to the intestinal portion of the tube, which usually makes several ‘ convolu- tions ’ before it finally terminates at the anus. — The same is the mode of advance which presents itself in the higher Arti culata ; among which the digestive ap- paratus of the Crustacea presents the greatest resemblance to that of Mollusca whilst that of Insects (Fig. 199) is re- markable for the very low grade of de- velopment of the liver, which only makes its appearance in the form of a small number of c fecal tubuli (/) discharging their contents into the intestine. The numerous short c fecal follicles which cluster round the cliylific stomach (e), are perhaps to be regarded as pancreatic. As yet no distinction between the ‘ small ’ and the ‘ large ' intestine presents itself; this being only manifested fully in the higher Vertebrata. — It is interesting to observe that, among the higher tribes of both the Molluscous and Articulated sub-kingdoms, we should find Digestive apparatus of Carnivorous Beetle ; — a, pharynx ; b, oesophagus ; c, crop ; d , gizzard ; e, cliylific stomach ; f, biliary ves- sels ; g , small intestine ; h, special secreting organs ; £, anus. ALIMENT, ITS INGESTION AND PREPARATION. 637 examples of reversion to that lower type of conformation of the digestive apparatus, which is manifested in its extension into csecal prolongations that radiate into parts of the body remote from the proper digestive sac, and in the diffusion of the glandular apparatus over the whole or the greater part of their surface. This is the case with the Nudibranchiate Gastero- pods (§ 304 c), in which the alimentary canal sends off as many caeca as that of the Planariae (Fig. 141), each one of them, however, being sur- rouuded at its termination by a group of hepatic cells, the liver being thus subdivided (so to speak) amongst them all. So among the Crustacea, we find the curious group of Pycnogonidae (§ 316 e) especially character- ized by the extension of the digestive cavity into the articulated members, almost to their extremities (Fig. 158), and by the spreading-out of the hepatic cells over the entire surface of these caeca ; and it is peculiarly worthy of note, that whilst the alimentary matter passes into these caeca, and is conveyed by them (as microscopic observation shows) into the im- mediate neighbourhood of every portion of the body, the special circulating apparatus presents its very lowest grade of development. An approxima- tion to the same structure is seen in the caecal prolongations of the digestive cavity, which pass into the thoracic members of the Arachnida (§ 319 a). These, in the Araneidce (Fig. 162), would not seem to possess any special secreting function, true biliary vessels being found in the abdomen ; in the Scorpionidce, however, they seem to be the only hepatic organs : whilst in certain Acaridce (§319 d) we return to a condition even lower than that of the Pycnogonidae, the parietes of the caecal prolonga- tions of the digestive cavity not being distinctly separable from the tissues around, and not even the rudiment of a proper circulating apparatus being distinguishable. 408. The digestive apparatus of Vertebrated animals maybe considered as carrying onwards the general plan which has been seen to prevail in the Molluscous sub-kingdom. Thus in Fishes, there is usually a well- marked distinction between the stomach and intestinal tube ; the liver attains a high development, and is completely withdrawn from the parietes of the alimentary canal ; and the pancreas, also, begins to appear as a distinct gland, instead of being a mere collection of caeca clustering around the digestive cavity. In the Amphioxus, however, we see a rever- sion to a very inferior type (§ 321 b) ; the digestive portion of the alimen- tary canal being a short tube of nearly uniform size throughout, which commences behind the pharyngeal respiratory apparatus, and runs back- wards almost in a straight line to the anal orifice, receiving in its passage the fluid discharged by the simple large caecum, which is the only rudiment of the liver. In the Cyclostomi the stomach is merely a dilated portion of the tube, which lies in a straight line between the oesophagus and the intestine ; but in nearly all the higher Fishes, both Osseous and Cartilaginous, it forms a large cavity which lies out of the course of the alimentary canal, and is adapted to retain the food while the process of solution is being carried on. It is usually separated from the intestine by a narrow ‘ pyloric ’ orifice ; and it is also generally divided from the oesophagus by a ‘ cardiac ’ valve, the aperture of which, however, is very wide, so as to admit the food which is swallowed in an undivided state, and does not completely prevent its regurgitation. Indeed it seems common for Fishes to disgorge the shells and other indigestible parts of G38 SPECIAL AND COMPARATIVE PHYSIOLOGY. their food, through the mouth, like Polypes ; and there are some (especi- ally of the Carp tribe) in which a sort of rumination takes place (§ 403), the food being sent back to the pharynx to be masticated by the pharyn- geal teeth, and then returning to the stomach to undergo its final digestion. The intestinal tube is usually short and almost destitute of convolutions, as well as of neaidy uniform diameter throughout, so that the distinction between the small and the large intestine is only marked (and this not constantly) by the ileo-colic valve. The sin-face of the mucous membrane lining the canal is considerably extended, however, by being thrown into rugae or wrinkled folds, more or less projecting ; these in the Osseous Fishes have seldom any great regularity of arrangement ; but by the great projection and spiral continuity of one of these folds, the real length of the intestinal tube is greatly increased in the higher Cartilaginous Fishes, just as a spiral staircase is much longer than the cylindrical cavity within which it winds. — In the omnivorous tadpoles of Batrachicin Reptiles, the alimentary canal is of great length, and forms numerous convolutions in the abdominal cavity ; but the stomach is narrow and elongated, the intestinal tube is of nearly equal size throughout, and the boundary between the small and the large intestine is not distinctly marked. This condition is retained in many of the Perennibranchiata (§ 323 a) ; but in the adult condition of the Frog and its allies, a nearer approach is exhibited to the character presented by the alimentary canal in the higher Reptiles, most of which are carnivorous. In these we usually find the gastric dilatation larger, and more completely distinguished from the intestinal tube ; although the intestine is relatively much shorter, yet the extent of its mucous surface is augmented by rugae and villi (§ 169) ; and the small intestine is now more constantly separated from the colon, by a valvular constriction. It is in the vegetable-feeding Turtles, that we find the intestinal tube possessing the greatest length, and presenting the greatest difference of diameter in its two divisions ; and in most of these, as well as in some other Reptiles, the large intestine has a csecal dilatation at its commencement. In Birds, the length of the alimentary canal varies greatly in the different families, in accordance with their habitual diet, being greatest in the granivorous and fi-ugivorous tribes, and least in the predaceous ; the intestinal tube is always distinctly separated from the stomach by a pyloric valve ; and the large intestine is somewhat more dilated relatively to the small, and is frequently furnished at its com- mencement with a pan of long csecal appendages (Fig. 186). The mucous membrane is much more regularly plicated, and its villi are more highly developed, than in Reptiles. It is in the Ostrich and its allies, that the intestinal canal presents the closest resemblance to that of Mammals. 408 a. The length and complexity of the alimentary canal in the Mam- malia, as in Birds, vary in accordance with the diet on which the several tribes are adapted to exist ; and no paid exhibits a greater diversity of conformation than does the stomach, — even putting aside that peculiar apparatus which has been already described (§ 403 a) as concerned iu the preparation of the aliment in the Ruminantia. Thus in the carnivorous and insectivorous Mammals, whose food is easy of solution, and con- sequently requires to be but little delayed for the digestive operation, the stomach is usually simple in its form, being a mere dilatation of the ALIMENT, ITS INGESTION AND PREPARATION. G39 alimentary canal, and lying nearly in the direction of its course ; but in proportion as the composition of the food departs more widely from that of the body itself, and it is less capable of reduction by the gastric fluid, do we find (in most cases at least) the dimensions of the stomach in- creasing, and its form undergoing alteration, so that it has more and more the character of a diverticulum, into which the aliment is turned aside, and in which it is retained during the time required for its subjec- tion to the solvent power of the gastric juice. The difference in form is principally given by the increased development of the large or left- hand extremity of the stomach, which thus forms a sort of csecal dila- tation off the line that connects the cardiac and pyloric orifices ; and not uufrequently this portion of the stomach is separated from the othei', by an incomplete partition. In addition to this, numerous smaller cseca are frequently developed in connection with the principal cavity ; whilst in many Cetacea we find a succession of constrictions interven- ing between the principal gastric caecum and the pyloric orifice, which partially divide the entire stomach into numerous cavities, all of them to be traversed by the food during its passage from the oesophagus to the intestine. The small intestine is chiefly remarkable for the great length to which it extends in some of the herbivorous Mammalia, and for the increase of its mucous surface by ‘ valvulse conniventes,’ which are deep folds of membrane, succeeding one another at tolerably regular intervals, but not embracing above two-thirds of the circumference of the tube ; towards the lower extremity of the smaller intestine, also, we usually meet with the ‘ glands of Peyer,’ which are either altogether absent, or are but slightly developed, in the lower Vertebrata. In nearly all Mammals, save in the Cetacea (which exhibit a return to a fish-like inferiority in this respect), a well-marked distinction between the small and the large intestine is manifested, not only by the presence of the ileo-caecal valve, but also by that difference of diameter from which these two divisions of the canal derive their names. The large intestine is here not only larger in proportion to the small, but is longer, than in the lower Vertebrata ; and in many Mammals, as in Man, a sacculated character is given to the colon by the peculiar arrangement of its muscular bands, so that the extent of its lining membrane is greatly increased. The caecal enlarge- ment, in which the ileum terminates and the colon commences, is fre- quently of great size, especially in herbivorous animals ; being sometimes larger than the stomach, as is the case in the Horse, or even many times larger, as may be seen in some of the Eodentia. The villi, which in all the lower Vertebrata are dispersed over the surface of the large as well as of the small intestine, are now limited to the latter ; whilst from the great number of glandulse contained in the former, and from the change in the character of the contents of the alimentary canal which takes place when they arrive there, it seems probable that whilst the small intestine is the part of the canal in which absorption specially takes place, the large is particularly destined for excretion. 409. Thus then, we see, that although the development of the Digestive apparatus is subjected, perhaps more than that of any other portion of the apparatus of organic life, to variations which are immediately connected with the purposes to which it is to be subservient, a gradual specialization can be most distinctly traced, when we take a general comparative survey G40 SPECIAL AND COMPARATIVE PHYSIOLOGY. of the succession of forms presented by the principal groups of animals. F or, in its lowest condition, we have seen it to consist of a simple cavity excavated in the tissues, for the reception of alimentary substances, which, when reduced to the state of solution, are taken up from every part of its walls, and are carried, without any special system of canals, into the parts of the body most remote from it : whilst the indigestible matters are ejected by the same orifice as that by which the food was taken in. Next, we find the digestive cavity furnished with parietes distinct from the surrounding tissues ; and the dissolved nutriment is taken up from its walls by a set of vessels, which carry it to the most distant organs. Next, we find the cavity changed into a canal, and pro- vided with a second orifice, through which the indigestible and faecal matters are ejected. Next, we find the stomach more or less distinctly separated from the intestinal tube ; and the former becomes possessed of special endowments which are not shared by the latter, the gastric fluid being secreted by its walls alone, and the secretion of the liver being either poured into it, or into the first portion of the intestine. Next, we find the mucous surface of the intestinal canal considerably extended by folds and duplicatures ; and these are thickly set with villi, each of which not merely contains a copious net-work of blood-vessels (Fig. 21), but also includes the commencement of one of the special absorbents or ‘ lacteals ’ peculiar to Vertebrated animals. Next, we find the distinction between the small and the large intestine gradually becoming more distinctly marked, and each part becoming characterised by its own peculiarities ; whilst at the same time, the concentration of structure is still further indicated by the greater extension of the mucous sirnface within the canal, by the larger number of villi in the small intestine, and by the increase of the glandulie and follicles in the large. — Now in its general outlines, the history of the embryonic development of the digestive cavity in the higher animals closely corresponds with this ; but the entirely different mode in which the alimentation of the embryo is provided for, involves a considerable variation in the particular method adopted. The embryonic cell, when it is first distinguishable in the ovum, lies in the midst of a store of nutri- ment (the yolk) provided for it by the parent ; and this it begins to draw in, like the simple cells of the Protozoa which it resembles in grade of development, by imbibition through its cell-wall The successive gener- ations produced by fissiparous multiplication, are at first nourished after the same fashion ; but these soon tend to spread themselves in the form of a membranous expansion around the yolk, and thus to include it com- pletely within the embryonic cavity. This cavity may be likened to the stomach of the Hydra, in every respect save that it has no orifice : but this is not needed, since it is already filled with the requisite supply of aliment. The further introduction of this into the embryonic tissues, is accomplished at first by the cellular layers of the germinal membrane : but blood-vessels are soon developed, which thenceforth are the special channels for its reception and conveyance. This cavity, however, serves but a temporary purpose, — in this respect corresponding with the cotyle- dons, by which nutriment is received into the embryo of the higher plants ; and the permanent digestive apparatus commences in the more advanced embryo, as a small portion of the temporary vitelline sac. that is gradually detached from it, like the stomach of the budding Hydro, from ALIMENT, ITS INGESTION AND PREPARATION. 641 that of its parent (§ 289). The form which this detached portion at first exhibits, is that of a simple tube, closed at both extremities, whilst its middle portion remains connected with the yolk-bag. An oral and an anal orifice are then formed ; and the tube thus comes to present the characters of the alimentary canal of the lower Articulata. The next grade in development is the evolution of the stomach, which first shows itself as a projection of the tube towards the left side ; and the accessory glands, the liver and pancreas, soon make their appearance. The short straight tube gradually increases in length, and is thrown into convolu- tions ; that part being most increased in length, which remains of the smallest diameter, and thus arises the difference between the small and the large intestine. The folds of the mucous membrane, the villi, and the glandular’ apparatus, which are the parts of the apparatus most restricted to higher animals, are the last to appear in the course of their develop- ment. 410. We have now to consider the proper function of Digestion, which may be said to commence with the introduction of the food into that part of the alimentary canal, in the walls of which is secreted the fluid destined for its solution. In the higher animals, this secretion is restricted to the gastric cavity or stomach; and hence it is named the ‘gastric juice.’ The only chemical change which the food appears to have undergone, before being submitted to this, is that which is effected by the admixture of saliva in the mouth ; a peculiar animal principle contained in this fluid having the power, like the diastase in Plants, of converting starch into sugar. The process thus commenced in the mouth, is checked in the stomach, the acid character of the gastric fluid being unfavourable to it ; it is recommenced, however, in the intestinal canal, after that acid has been neutralized by the alkali of the bile and pancreatic fluid. The great purpose of the gastric digestion appears to be, to dissolve the albuminous and gelatinous constituents of the food ; and, by the withdrawal of these, the remainder are usually reduced to a state of fine division, in which they are afterwards more easily acted upon. In regard to the constitution of the gastric fluid, there is at present much diversity of opinion ; and it does not seem improbable that it may vary in different animals. It appears essentially to consist, however, of a free acid (either the hydro- chloric, acetic, or lactic) which is the real solvent ; and of an animal principle in a state of change, which acts as a ferment , and disposes the organic compounds to solution. Water slightly acidulated with these acids, is capable of dissolving albuminous compounds with the aid of a high temperature ; but if a solution of ‘ pepsin ’ (which is the animal matter obtained by macerating the stomach of a pig in cold water, after it has been repeatedly washed) be added, the acid solvent will then be able to act efficaciously at the ordinary temperature of the body. The solvent action of the gastric fluid is aided by the movements of the walls of the stomach, which are produced by the successive contractions and relax- ations of their muscular fibres. The purpose of this motion is obviously to keep the contents of the stomach in that state of constant agitation, which is most favourable to their chemical solution ; and particularly to bring evei’y portion of the alimentary matter into contact with the lining of the stomach, so as to subject it to the action of the solvent fluid which is poured forth from it. Thus, little by little, the reduction of the alimentary T T 642 SPECIAL AND COMPARATIVE PHYSIOLOGY. materials to the homogeneous pulpy mass termed chyme is accomplished ; this escapes into the intestine, as fast as it is formed, through the pyloric orifice, which closes itself to solids, but allows liquids to pass ; whilst the solid residue is continually subjected to the same action, until its solution lias been effected. There is no doubt, however, that a portion of the nutritious matter dissolved by the gastric fluid is at once absorbed into the blood-vessels of the stomach, and never passes into the intestinal tube, nor into the special lacteal system of vessels. — That the action of the gastric fluid is in all respects one of a purely chemical nature, there can be no longer any question. When drawn direct from the stomach of Man or of the lower Mammalia, it is found to possess the power of dissolving various kinds of alimentary substances, provided that these are submitted to its action at a temperature equal to that of the body, and are fre- quently agitated. The solution appears to be in all respects as perfect as that which naturally takes place in the stomach ; but a longer time is required to make it, — a difference which is easily accounted for, when the impossibility of fulfilling all the conditions under which gastric digestion takes place, is borne in mind. The quantity of food which a given amount of gastric fluid can dissolve, is limited ; precisely as in the case of the acidulous solution of ‘ pepsin,’ or ‘ artificial gastric juice,’ whose solvent power is chiefly regulated by the quantity of acids which it con- tains, the same quantity of pepsin being capable of disposing the solution of many successive amounts of the substance to be acted on, provided that acid be added as required. 411. The Chyme which passes into the intestinal tube, is commonly a greyish, semifluid, and homogeneous substance, possessing a slightly acid taste, but being otherwise insipid. When the food has been of a rich oily character, the chyme possesses a creamy aspect ; but when it h;is contained a large proportion of farinaceous matter, the chyme has rather the appearance of gruel. The state in which the various alimentary prin- ciples exist in it, has not yet been accurately determined ; the following, however, may be near the truth. The albmninous compounds, whether derived from the Animal or from the Vegetable kingdom, whether previously possessing the form of fibrin, of casein, of gluten, Ac., are ;dl reduced by solution to the condition of Albumen ; but a paid of these compounds may still remain undissolved in the chyme of the intestine. Gelatine will be dissolved, or not, according to the previous condition of the substance containing it ; for if this be a tissue which does not readily yield gelatine to hot water, the gastric fluid will have little influence upon it. The gummy matters of plants are dissolved, when they exist :n a soluble form ; but starch is not dissolved unless it has been previously acted upon by the saliva, save in the case of the Granivorous Birds and Buminant Mammals, in which the provision for the mechanical reduction of this element of the food is greater than in other tribes. Sugar, whether introduced as such, or formed by the transformation of starch, is undoubtedly reduced to a state of complete solution, and is probably taken up by the blood-vessels as fast as it is dissolved. Oily matters, whether of animal or vegetable origin, are reduced to minute particles, and these are dispersed through the other constituents of the chyme. Most other substances, as the woody fibres, all the firmer cell- walls, and the resinous matters, of Plants, the horny matter, yellow fibrous tissue. ALIMENT, ITS INGESTION AND PREPARATION. 643 epidermic and other thick-walled cells, of Animals, pass unchanged from the stomach, and undergo no subsequent alteration in the intestinal canal, and form part of the fecal matter which is discharged from it. — On passing into the small intestines, the Chyme soon becomes mingled with the biliary and 'pancreatic secretions, which appear to effect important changes in its character, though the nature of their respective agencies has not yet been clearly made out. It may, however, be pretty certainly stated, that by the excess of alkali which they contain, they neutralise the acid of the gastric juice, so that thenceforth there is no further solution of albuminous compounds, but the conversion of the starch into sugar, which was interrupted in the stomach, now recommences, and is probably carried on while the food is passing through the small intestines; and farther, that either by their separate or their combined actions, the fatty matter of the chyme is reduced to that state of fine division which is known as an ‘ emulsion,’ and that it is thus rendered capable of being received into the absorbents. Thus, then, during the continued passage of the food along the upper part of the alimentary canal, the process of reduction and solution are still carried on ; those operations being gradually performed in the intestine, which would have interfered with the digestion of albu- minous matters if performed in the stomach ; and the prolongation of this process being also the means of preventing the too rapid entrance of those saccharine and oily matters into the blood, which are only taken into it with a view to being speedily eliminated by the respiratory process, for the maintenance of the animal temperature. It is chiefly in warm-blooded animals, that we find the farinaceous or starchy substances, supplied by- plants, entering largely into the composition of the food ; and notwith- standing the evidence we possess that they are actually received into the vessels, yet there is a difficulty in detecting them under any form in the blood or chyle, in the healthy condition of the system. It is obvious, then, that their removal from the intestinal canal by the process of absorption must take place slowly ; and thus we seem to have the expla- nation of the prolongation of the intestinal tube in herbivorous animals. It has been supposed that the contents of the intestinal canal are subjected to a sort of second digestion in the caecum, for the purpose of dissolving any albuminous matters which have not been previously reduced, especially in those animals in which the ciecum is of large size in proportion to the stomach ; and this idea seems to derive confirmation from the fact, that the secretion of the caecum has been found to possess an acid reaction. It cannot, however, be at present regarded as more than a probable hypo- thesis. — The residuary undigested portions of the food, mingled with the excrementitious portion of the biliary and pancreatic secretions, together form the principal part of the fecal matters which are discharged from the large intestine ; but there is strong reason to believe, that the putrescent matter, which, in the Mammalia especially, gives the peculiar odour and appearance to the excrement, is not derived from either of these sources, but is a real excretion, separated from the blood by the glandufe that are so thickly set in the intestinal walls, and especially at the lower part of the small intestine. 644 SPECIAL AND COMPARATIVE PHYSIOLOGY. CHAPTER XI. OF THE ABSORPTION OF NUTRITIVE AND OTHER MATTERS. 1 . General Considerations. 412. The process by which the alimentary materials, whether in their original fluid form (as in the case of Plants), or after they have been reduced to that form by the digestive process (as in Animals), are intro- duced into the living body, is termed Absolution. Before considering the particular conditions under which this operation is performed in the different classes of Organised beings, it will be right to consider what is its essential character, and how far Physical principles can be applied to its elucidation. — It was formerly the general opinion that Absorption is always effected by vessels, the open mouths of which, being in contact with the fluid, might imbibe it by capillary attraction, suction, or other like agencies ; and hypothetical ‘ absorbent vessels ’ were inferred to exist in all beings, even in the simplest Cellular Plants, as the channels where- by fluids are received at the surface and conveyed into the interior. But it has been shown by minute Anatomical inquiry, that in no one instance are absorbent vessels thus brought into immediate relation with the fluid to be received by them, but that the transmission always takes place in the first instance through some tissue of a membranous character ; and further, that in a great number of cases, vessels are not concerned in the process at all, the fluid being at once received into the tissues which it is destined to nourish. Thus, we shall find that neither in the roots of Plants, nor in the walls of the alimentary canal of Animals, do the absorbent vessels commence in open mouths ; but that all the fluid which enters them, must first traverse the membrane which covers their ex- tremities ; whilst, in the lowest members of both kingdoms, and in the early embryonic condition of the higher, the external investment, or that reflexion of it which lines the digestive cavity (§ 381), has an equal power of imbibition throughout its entire surface, and communicates a portf n of that which it has taken up to the tissues in contact with it, whence it is progressively transmitted to the more remote parts,— and this without the aid of any system of vessels, either for the Absorption or for the Circula- tion of nutritive fluid. And even in the most elaborately-constructed organisms, we find that a considerable portion of the tissues is noiu’islied by the same kind of imbibition, — not, however, from the external surface, or from the walls of the digestive cavity, but from nearer sources of supply. Thus, as we have seen, the interior of a mass of proper cellular Cartilage is entirely nourished by imbibition from the blood-vessels which bring the nutriment to its surface and edges (§ 192) : the dense tissue of Bones and Teeth is in like manner transversed by nutritious fluid, which is drawn from the vessels of the nearest vascular canal or cavity (§ 2<*4) ; and the Epidermic and Epithelial tissues must derive their nourishment from vessels ramifying on the opposite side of the basement-membrane GENERAL CONSIDERATIONS. 645 whereon they rest (§ 177). — Thus it appears that the first introduction of fluid into the organism by Absorption, is but a particular case of that Imbibition, which takes a most important part in its subsecpient diffusion through the system, even when the most complete vascular apparatus- exists ; and it will be right, therefore, to consider in the first instance what are the physical conditions of Imbibition. 413. When any porous substance (not already saturated) is brought in contact with a liquid which has such a molecular attraction for its particles as to be capable of ‘ wetting ’ it, the liquid is imbibed by it, and, if the force of imbibition be strong enough, is speedily diffused through the whole mass. This force depends in part upon the degree of attraction subsisting between the particles of the solid and those of the fluid ; and in part upon the size of the capillary pores or canals. Thus it was found by Professor Matteucci,* that when glass tubes of about § of an inch in diameter were filled with fine sand, previously dried, and introduced without pressure, and w r ere immersed at their lower ends into the following liquids, the action of imbibition (which took place at first rapidly, then more slowly, and then ceased after about ten hours) raised the liquids in the tubes to the following heights respectively : — Solution of carbonate of potash, 85 millimetres. Solution of sulphate of copper, 75 99 Serum of blood. 70 9’ Solution of carbonate of ammonia, . 62 Distilled water, .... 60 99 Solution of common salt, .... 58 99 Milk, ..... White of egg diluted with its own volume of water, 55 35 99 When thick solutions of gum or starch, or fixed oils, were employed, scarcely any imbibition took place ; and it was but little more, when strong saline solutions were used. — The degree in which imbibition is affected by the peculiar attractions subsisting between the solids and the liquids employed, is further illustrated by the following experiment. Three tubes, respectively filled with sand, pounded glass, and sawdust, were immersed at their lower extremities in water, and three similar tubes in alcohol ; the following were the comparative results : — Sand. Pounded Glass. Sawdust. Alcohol . . 85 millirn. 175 millim. 125 millim. Water . . 175 ,, 182 „ 60 „ Thus we see that, whilst the imbibition of the tw T o liquids took place into the pounded glass in nearly an equal degree, the quantity of water drawn up by the sand w T as more than double that of the alcohol, whilst precisely the reverse effect obtained in the case of the sawdust, the quantity of alcohol imbibed being more than double that of the water. — That the force of imbibition is dependent in part upon the size of the capillary channels, was proved by the fact, that when the experiment was tried with two tubes, both holding pounded glass, but one of them containing twice as much as the other (the powder being finer, and the capillary channels being consequently more minute), water was found to rise in the fullest “ Lectures on the Physical Phenomena of Living Beings,” translated by Dr. Pereira, pp. 21, et seep 646 SPECIAL AND COMPARATIVE PHYSIOLOGY. tube to 170 millim., in the same time which it occupied to rise to 107 millim. in the other. — Temperature, also, was found to have a remarkable influence upon the result ; for two tubes, similarly prepared with sand, having had their extremities immersed in water, and having been kept, the one at 131° Falir., and the other at 59° Fahr., the water rose in eleven minutes to 175 millim. in the former, whilst in the same period it only rose to 12 millim. in the latter. Hence we see that an elevation of tempe- rature has not only a direct influence in augmenting vital activity, but that it assists in supplying an essential condition of that activity, by promoting the transmission of nutritive fluid through the textures. 414. But further, when two liquids capable of mixing together freely, without chemical decomposition, are allowed to do so, there is a tendency in each to a uniform diffusion through the other. This phenomenon of the ‘ Diffusion of Liquids ’ has been recently studied by Prof. Graham (the discoverer of the law of the £ diffusion of gases ’) ; and the following are some of the results obtained by him, which bear most directly upon the present subject.* — The phenomenon was studied by means of a simple apparatus, consisting of an open phial to contain the liquid to be diffused, which was immersed in a large jar of pure water. The diffusion was stopped, generally after seven or eight days, by closing the mouth of the phial with a plate of glass, and then raising it out of the water-jar ; and the quantity of the liquid which had found its way into the water-jar (or the ‘ diffusion-product ’ ) was then determined by evaporating it to dry- ness. The characters of ‘ liquid diffusion ’ were first examined in detail in reference to common salt ; and it was found that when the amount of diffusion from solutions containing 1, 2, 3, and 4 per cent., was compared, it varied nearly in the same proportion, being in each case about one- eighth of the whole amount, after a period of eight days. But here, too, temperature has a remarkable influence ; for an elevation of 80° Fahr. was found to double the quantity of salt diffused in a given time. — Different substances possess this property of di {fusibility in very varying degrees ; thus when solutions of the following substances were employed, of the strength of 20 parts to 100 of water, the relative quantities diffused in a given time were as follows : — Chloride of sodium, 58-68 Crystallized cane-sugar. 26-74 Sulphate of magnesia, 27-42 Starch-sugar (glucose). 26-94 Nitrate of soda, 51-56 Gum-arabic, 1 3-24 Sulphate of water. 69-32 Albumen, 308 The low diffusibility of albumen is very remarkable ; and it is further to be noticed that if common salt, sugar, or urea (which is as highly diffusil >lo as chloride of sodium) be added to the albumen under diffusion, they diffuse away from this as readily as from their aqueous solutions, leaving the albu- men behind in the phial. So, when solutions of two salts are mixed in the phial, they diffuse out into the water-atmosphere separately and indepen- dently of each other, according to then - respective individual diffusibilit Vs. In comparing the diffusibility of different salts, it was foimd that equal weights of £ isomorphous ’ compounds, dissolved in ten times their weight of water, diffused themselves to the same amount. And further, one -all. such as nitrate of potash, will diffuse into a solution of another salt, such * “ Philosophical Transactions,” 1850, p. 1, et seq. GENERAL CONSIDERATIONS. 647 as nitrate of ammonia, as rapidly as into pure water ; the solutions appear- ing to have that mutual diffiisibility, which gases are known to possess (§ 495). — The properties of different substances in regard to then- relative diffusibility, cannot but have an important influence on the course of the vital operations. Thus the low diffusibility of albumen obviously tends to the retention of the serous fluids within the tissues ; whilst the high diffusibility of imea will favour its escape from them. 415. Both of these agencies, — the imbibition of fluids by porous solids, and the mutual diffusion of miscible liquids, — seem to be concerned in the production of the cui’ious process, to which the term Endosmose was given by its discoverer, Dutrochet ; and this phenomenon bears so close a resemblance to a vast number of the operations continually taking- place in the living body, that it is scarcely possible to doubt that the same causes are in operation in both cases. The following is a general account of the process in question.— If into a tube, closed at one end with a piece of bladder or other membrane, be put a solution of gum or sugar, and the closed end be immersed in water, a passage of fluid will take place from the exterior to the interior of the tube, thvough the membranous septum ; so that the quantity of the contained solution will be greatly increased, its strength being proportionably diminished. At the same time, there will be a counter-current in the opposite direction ; a portion of the gummy or saccharine solution passing through the membrane to mingle with the exterior fluid, but in much less quantity. The first current is termed Endosmose, and the counter-current Exosmose. The increase on either side will of course be due to the relative velocity of the currents ; and the changes will continue, until the densities of the two fluids are so nearly alike, as to be incapable of maintaining it. The greater the original difference (provided that the denser fluid be not actually viscid, but be capable of mixing with the other), the more rapidly and powerfully will the process be performed. The best means of experi- menting upon these phenomena is afforded by a tube narrow above, but widely dilated below, so as to afford a large surface to the membrane, compared with that of the superincumbent column, which will then increase in height with great rapidity. By bending this tube into the form of a syphon, and intro- ducing into its curve a quantity of mercury, the force as well as the rapidity of the endosmose between different fluids may be estimated with precision. In this way it was ascertained by Dutrochet, in some of his experiments, that fluid might be raised against a pressure of no less than 4 1 atmospheres, or nearly 7 0 pounds to the square inch.* — Although it is not universally true that the activity of the process depends upon the difference in density of the two fluids (for in one or two cases the stronger current passes from the denser to the lighter), it seems to be so with regard to particular solutions, as those of gummy or saccharine matter. No endosmose takes place between fluids which will not mingle, such as oil and water ; and very little between such as act chemically on each other. Although an organic membrane forms the best septum, yet it has been found that thin laminae of baked * For further information on this curious subject, see the Article Endosmosis in the “ Cyclopaedia of Anatomy and Physiology,” Dutrochet’s “ Memoire3 Anatomiques et Physiolo- giques,” tom. i., and Matteucci’s “ Lectures on the Physical Phenomena of Living Beings.” 618 SPECIAL AND COMPARATIVE PHYSIOLOGY. pipe-clay will suffice for the evident production of the phenomenon ; and that porous limestones possess the same property in an inferior degree. 116. Although it may not as yet be possible to explain all the pheno- mena of Endosmose upon physical principles, yet these will go so far towards it, that the general conditions of the process may be considered as well understood. Supposing that two mutually diffusible liquids are on the opposite sides of a porous septum, which is not equally penetrable by them, then the one which is most readily imbibed will tend to occupy the capillary passages of the septum, and will thus be brought into contact with the liquid on the opposite side. This contact will permit the diffusion of that which has passed through the pores of the septum ; and as fast as that which occupies these pores is removed by diffusion, so fast will it be renewed from the other side, — -just as oil continues to ascend through the capillary channels in the wick of a lamp, so long as it is being dissipated by the combustive process at its summit. In this way, then, an Endosmotic current is produced, the force of which will depend upon the diffusion-powers of the two liquids, and upon the difference of the attractive power which the capillary tubes of the septum have for the two respectively. Thus, when a solution of sugar or gum is on one side of the septum, and water on the other, the water is the most readily imbibed : and consequently, the chief mixture and diffusion of the liquids, the one through the other, takes place at the surface of the septum in contact with the more viscid liquid. But at the same time, this liquid is tending to diffuse itself through the water which occupies the capillary channels of the septum ; and, as it is not repelled by the septum, but is only attracted by it in a less degree than the water, a portion of it finds its way in a direction opposed to the principal current, and diffuses itself through the water on the other side, thus constituting Exosmose. — Thus it happens that the direction of the principal current, or Endosmose, will be deter- mined by the attractive power of the septum for one or other of the liquids ; though the diffusion-power of the liquids through each other will help to determine its force. When alcohol and water, for example, are separated by a septum composed of animal membrane, the endosmotic current will be from the water towards the alcohol, because the former liquid most readily ‘ wets ’ the membrane, and consequently tends most strongly to occupy its capillary passages : but, on the other hand, when the separation is made by a thin lamina of caoutchouc, the endosmotic current is from the alcohol towards the water, because the former is most readily imbibed by the septum. 417. It has further been ascertained by the experiments of Matteucci, that, when an organic membrane is employed as a septum, the rapidity of transmission is considerably affected by the direction in which the endos- motic current traverses the membrane. Thus when the skin of the Torpedo was employed, with a solution of sugar on one side of it, and water on the other, although there was always an endosmotic current from the water to the sugar, yet this current was strong enough to raise the in- terior liquid to 80 degrees, when the water was in contact with the internal surface of the membrane, in the same time that was occupied by its rise to 20 degrees, when the external surface of the membrane was turneJ towards the water. Again, when the mucous membrane of the stomach of a dog was used as the septum, and its external (or muscular) surface GENERAL CONSIDERATIONS. 649 was placed in contact with alcohol, the passage of water from the other side took place with such rapidity as to raise the liquid in the tube to 130 degrees ; whilst if the internal (or mucous) surface of the membrane was placed in contact with the alcohol, and the muscular surface with water, the current was only sufficient to raise the liquid 6 degrees in the same time ; so that it is evident that the transudation of water takes place much more readily from the mucous lining of the stomach towards the outer side of the viscus, than in an opposite direction, in virtue simply of the physical properties of the membrane. In fact, according to Prof. Mat- teucci, the cases are very rare in which, with fresh membranes, endosmose takes place with equal readiness, whichever of their two sides is exposed to the water. The direction which is most favourable to endosmose through skins, is usually from the internal to the external surface, with the excep- tion of the skin of the frog, in which the endosmotic current, in the single case of water and alcohol, takes place most readily from the external to the internal surface. But when stomachs and urinary bladders are em- ployed, the direction varies much more, according to . the nature of the liquids employed. This variation appears to have some relation to the physiological conditions in which these membranes are placed in the living animal ; thus, the direction most favourable to endosmose between water and a saccharine solution, is not the same for the stomach of a Ruminant as for that of a Carnivorous animal : as yet, however, no positive state- ment can be made on this subject. When membranes are employed, that have been dried or altered by putrefaction, we either do not observe the usual difference arising from the position of the surfaces, or endosmose no longer takes place ; thus affording another indication, that it is to the physical condition of the perfectly-organised membrane that we are to look, for many of the peculiarities which are noticeable in the transudation of fluids through them. — The Exosmotic current does not bear any con- stant relation to the endosmotic, as may be easily comprehended from the preceding explanation ; for if the liquids have a strong tendency to mutual diffusion, and the difference in the attractive power which the septum has for them respectively is not great, each may find its way towards the other, and a considerable exosmose may ensue, -with very little change of level. The amount of the exosmotic as of the ewcfosmotic current, varies with the direction in which it traverses the membrane ; thus when sugar, albumen, or gum was employed in solution, its transudation towards water took place most readily from the internal towards the external surface of all the skins examined by Matteucci ; — a fact which is not without its significance, when it is remembered that it is in this direction that the secretion of mucus takes place on the skins of fishes, frogs, &c. 418. Applying these considerations to the phenomena of imbibition of liquids into the tissues and canals of the living body, we shall have to enquire how far they are capable of being accounted for on the physical principles which have been now brought forwards. It has been maintained by some that Absorption is a purely vital operation, because it does not occur save during the continuance of life. But this is not true ; since imbi- bition will take place into dead tissues, though more slowly than into the i same parts when living • and the difference of rate seems to be fully ; accounted for, by the difference of the condition between a mass of tissue all whose fluids are stagnant, and another in which an active circulation 650 SPECIAL AND COMPARATIVE PHYSIOLOGY. is taking place. Tims, as Matteucci has shown, if the hind-legs of a frog recently killed be immersed for some hours in a solution of ferrocyanide of potassium, it mil be found that eveiy part of the viscera is so pene- trated with the salt, that by touching it with a glass rod moistened with a solution of the chloride of iron, a more or less deep blue stain is the result. Now the same effect is produced much more speedily in a living frog; and it is easily proved that the imbibition takes place, in the latter case, into the blood-vessels, and that the salt is conveyed to the remoter parts of the body by the circulation, instead of having slowly to make its way by transudation through the tissues, as in the dead animal. — But further, not onty does the movement of blood iu the vessels promote the diffusion of liquid which has been already absorbed ; it also increases the rapidity of the absorption itself, in a very extraordinary degree. Thus, if a mem- branous tube, such as a piece of the small intestine or of a large Yein of an animal, be fixed by one extremity to an opening at the bottom of a vessel filled with water, and have a stopcock attached at the other extre- mity, and be then immersed in water acidulated with sulphuric or hy- drochloric acid, it will be some time before the acid will penetrate to the interior of the tube, which is distended with water ; but if the stopcock be opened, and the water be allowed to discharge itself, the presence of the acid will be immediately discovered (by tincture of litmus) in the liquid which flows out, showing that the acid has been assisted in its pene- tration of the walls of the tube, by the current traversing its interior. Thus the continuance of the Circulation is obviously one of the most potent t f all the conditions of Absorption ; and the difference in the rate of the process in dead and living organisms placed under the same circum- stances, may be accounted for in great part, if not entirely, by the stop- page of the circulation in the former. All the circumstances which are laid down by Physiologists as favouring Absorption, are in strict accord- ance with the Physical principles which have been now explained. These circumstances are : — 1. The ready miscibility of the liquids to be absorbed with the juices of the body. 2. The penetrability of the tissue through which the absorption takes place. 3. The absence of previous distensi< >u in the tissues or canals towards which the flow takes place. 4. The ele- vation of the temperature, within certain limits. 5. The vascularity of the tissue, and the rate of movement of the blood through the vessels. —And the results of experiments upon recentlv-dead membranes, which retain almost exactly the same physical conditions as those which they possessed during life, but have entirely lost then- vital properties, seem most decidedly to indicate, that the relative facility with which differeat substances are absorbed, and the direction most favourable to then pas- sage through the tissues, are determined in great part by the physical relations of those tissues (and of the vessels which traverse them) to the liquid which is seeking to enter them. In this way, then, many of the phenomena of selective absorption are probably to be explained, espe- cially in Plants and the lower Animals ; we shall hereafter see. however, that the special absorbent vessels of Vertebrata seem to possess properties which can scarcely be thus accounted for. ABSORPTION IN VEGETABLES. 651 2. Absolution in Vegetables. 419. In the lowest orders of Plants, we find this function performed under its most simple conditions. Their substance is composed of vesicles more or less firmly united to each other, and but slightly altered from their original spheroidal form ; and the envelope which surrounds them can seldom be regarded as a distinct structure, as it generally differs but little from the remainder of the cellular tissue. In all the Algae (§§ 267— 269), the whole surface appears to be endowed with the power of absorp- tion to nearly an equal degree ; and although the semblance of a stem and roots presents itself in the higher orders, yet these seem to have no other function than to give the means of attachment to the frondose or leaf-like expansion, which performs not only the nutritive but the repro- ductive function on all parts of its surface. The preference of particular species of Algrn for particular rocks, cannot be fairly considered as indi- cating that any absorption of the mineral particles takes place through their roots ; it is much more probably due to the fact, that the materials of these rocks are in some degree diffused by solution through the neigh- bouring water ; and something may be also attributable to the mechanical qualities of the rocks, as affording advantageous surfaces for attachment. — The difference in the situation inhabited by the Lichens (§270) appears to involve a separate appropriation of portions of their surface to the nutritive and reproductive functions, and a certain specialization of the absorbing organs. The upper surface of these plants, being exposed to the sun and air, becomes hard and dry, a condition which seems to favour the evolution of the fruit ; whilst it is mostly by the lower surface, which is usually soft and pale, that the nutriment is introduced into the system. The latter is not unfrequently furnished with hair-like prolongations, which not only serve to fix the plant, but appear to be much concerned in the absorption of its aliment; being so much developed in some Lichens, which are located upon the ground, as almost to resemble roots. — In the Fungi we find the same evolution of the more special organ from the more general type. The lower forms of this group (§ 271) seem to im- bibe their aliment by their whole surface ; but in the more complex structures, in which the reproductive system is separated from the nutri- tive by the intervention of a stalk (as in the Mushroom, § 273), the my- celium at its base is prolonged into filaments, whereby the decaying matter, that constitutes the food of this remarkable group of plants, is introduced into the system. In some species, too, the whole surface is covered with hair, which may assist their very rapid development by absorption of fluid from the atmosphere. — In the Mosses and their allies (§ 276), we find a somewhat higher form of the same structure. From the base of the stem there usually proceed slender radical filaments, which sometimes ramify through the soil to a considerable extent ; and other similar filaments are frequently developed from the sides of the stalk, and from the lower sur- face of the leaves. In Mosses that exist on rocks, however, these fila- ments are but little developed, and appear to serve rather for mechanical support, than for absorption of nourishment, which must in such circum- stances be derived from the atmosphere through the leaves. These, as is well known, are very permeable to fluid, so that the application of moisture 652 SPECIAL AND COMPARATIVE PHYSIOLOGY. will cause Mosses to recover the appearance of life after being long dried : and the same property enables these beautiful little plants to vegetate rapidly during a moist season, whilst them tenacity of life enables them to withstand a subsequent drought. — In ascending through these tribes of Cryptogamia, then, we may trace a gradual development of separate absorbent organs, and may observe the specialization of the function, bv its restriction to one particular part of the surface, instead of being diffused over the whole. The absorbent filaments, however, are veiy inferior in them structure to true £ roots,’ being little else than elongated cells, resembling the hairs with which other parts of the surface are covered : and they seem to absorb fluids equally throughout them entire length. Further, we find that, when these special organs are not developed, or are insufficiently supplied 'with nutriment, the general simface can take on its original function, and thus supply the deficiency. 420. It is in the Ferns (§ 277) that we first meet with a regular ‘de- scending axis ’ of growth, from which the absorbent fibres are given off ; and this is evolved in its completest form in the Phanerogcimia. In these, moreover, it seems to be through the newly-formed succulent extremities alone, that fluid is admitted ; and the function is, of course, more actively performed by them, in proportion to the diminution in the amount of surface they expose. The ‘ root ’ presents a great variety of forms in different plants ; there are, however, some parts which are essential, aud others that are merely accessory. The simplest form, as well as the most essential part, consists of single fibres ; these occasionally exist alone (as at the base of ‘bulbs’), but more often proceed from ramifying branches of woody texture (as in most trees and shrubs), or from ‘tubers’ (as that of the turnip). Each radical fibre is of a structure far more complex than the absorbent filaments just mentioned as existing in Cellular plants ; for it is a cylinder, in whose axis lies a bundle of fibro-vascular tissue, whilst its exterior is composed of firm cellular parenchyma ; at its free termination, however, the extremities of the vessels are covered with loosely-formed cellular tissue, through which the fluid passes into them. The spongiole , as this point has been termed, is sometimes spoken of as a distinct organ : but it is nothing more than the growing point of the root, which, with a few exceptions, lengthens only by additions to its extremity. The soft lax texture of the newly-formed part causes it to possess, in an eminent degree, the power of absorption ; but as the fibre continues to grow, and additional tissue is formed at its extremity, that which was formerly the spongiole becomes consolidated into the general structure of the root, and loses almost entirely its peculiar properties. That it is to the spongioles that the principal absorbing power of the root is due, was fully proved by the experiment of Senebier. Having fixed two roots in such a manner, that tbe extremity of one was in contact with water, whilst of the other every part was immersed except the extremity, he foimd that the first root absorbed nearly as much as usual, whilst the second scarcely took up a sensible quantity. It is not improbable that the relative absorbent power of the spongioles and of the general surface of the root, may vary in dif- ferent plants, according to the character of the texture of each, and tbe situation in which it grows ; but it appears to he a general fact, that, in Vascular plants, the spongioles are the organs specially destined for intro- ducing the fluid nutriment into the system. ABSORPTION IN VEGETABLES. 653 42 1 . There are evident limits to the supply of alimentary materials to the roots of Plants, as long as they remain in the same spot ; and some change must take place to ensure its continuance. As the Plant cannot remove itself to a new situation, its wants are provided for by the simple elongation of its radical fibres ; and their extension takes place, not by increase throughout their whole length, but by addition of fresh tissue to then - points. This addition, being made in the direction of least resist- ance, enables the fibrils to insinuate themselves into the firmest soil, and even to overcome the obstacle presented by solid masonry ; for however narrow the crevice may be into which the filament enters, the subsequent expansion of the tissue by the infiltration of fluid is so great, as to enlarge the opening considerably, and even to rupture masses of stone. This tendency to increase in the direction of least resistance, null also evidently cause the root to grow towards a moist situation ; and by keeping this in view, many of the facts regarding the so-called instinct of plants, which at first sight appear so remarkable, may be satisfactorily explained. Thus, it was noticed, when the water of the New River was conveyed through wooden pipes, that if these pipes were carried within thirty yards of trees, they were very likely to be in time obstructed by their roots ; which ‘found ’the joints, and then spread out in ‘ foxtails ’ of fibres, two or three feet long. It is well known to the agriculturist, that the course of large drains, even at a considerable depth in the ground, is liable to be interrupted by the extension of roots, not only from trees, but also from apparently insignificant plants. Thus at Saucethorpe, in Lincolnshire, a drain nine feet deep was filled up by the roots of an elm-tree, which was growing at upwards of fifty yards from the drain : a deep drain outside the garden- wall at Welbeck was entirely stopped by the roots of some horse-radish plants, which grew seven feet into the ground : and at Thoresby Park, a drain fourteen feet deep was entirely stopped by the roots of gorse growing at a distance of six feet from it.* — In other cases, we must attribute the result to the dispersion of vapour through the atmosphere in a particular direction. Thus, in a case which fell under the Author’s cognizance, a lime-tree, which grew at the distance of about fifteen feet from the shaft of a well, sent a single long root through the soil, in a direct line towards a point of the shaft at which there was a small aperture left by the deficiency of a brick ; this aperture was at a height of eleven feet above the usual level of the water in the well ; and having passed through it, the root divided into a brush-like mass of fibres, which descended into the water, and formed a large mass in the lower part of the well. Again, in a peculiar case known to the Author, in which one tree grew upon the trunk of another, having originated from a seed deposited at about twelve feet above the ground, one of the large roots which it sent down, subdivided about two feet above the surface of the ground, instead of proceeding directly down to it, as did all the rest. Now this subdivision took place above a large stone, on the centre of which the root would have impinged if it had continued to grow directly downwards ; and it would appear as if its division, half proceeding to one side, and half to the other, was due to the direction given to its growth by the ascent of vapour from the soil beneath. On the same principle * “ Journal of the Royal Agricultural Society,” vol. i. p. 364. 054 SPECIAL AND COMPARATIVE PHYSIOLOGY. we are probably to explain tire following case. “ Near the waterfall at the head of the river Leven, in the Western Highlands, is the trank of a decayed oak, rotten within, but alive on some parts of the outside. From one of these, a shoot grows out, about fifteen feet from the ground ; and this shoot has protruded from its lower part a root, which, after having reached the ground (a bare rock), runs along the rock in a horizontal position, about thirty feet further, till it reaches a bank of earth in which it has embedded itself.” * 422. The absorbent power of the Spongioles appears limited by the size of their pores ; for if the roots be immersed in coloured solutions, they take up the most finely divided particles, leaving behind the larger molecules, which are only absorbed w hen the spongioles have been da- maged. The pores are liable to be blocked up by fluids which are of too viscid or glutinous a consistence to pass readily through them ; and if the roots be immersed in a thin solution of gum or sugar or neutral salts, the watery particles are absorbed in the greatest degree, so that the portion which is left contains a larger proportion of the ingredient in solution. The power of selection, however, would seem to extend beyond this : since of two substances equally dissolved, some plants will take one, and some the other ; whilst some neutral salts are rejected altogether. It docs not appear that the selecting power is employed to prevent matter, which is capable of exerting a deleterious influence upon the plant, from being introduced into its tissue ; for many substances are taken up by the roots, which speedily put a stop to vital action, if opportunity be not afforded for their excretion. From the little that is at present known or, the subject, it seems a reasonable inference, that the rejection of any par- ticular ingredient of the fluid in contact with the roots, results either from an organic change effected by it on their delicate tissue, such as is proved by the experiments of M. Payen t to occur when tannin enters into the solution, even in very minute proportion ; or from the want of molecular attraction between its particles and the substance of the spongioles. That some kind of relation between the living tissue and the crystalline cha- racter of the salt, is concerned in the selection, appears from the interest- ing results of the experiments of Dr. Daubeny on the absorption of mineral substances by Plants. He has found, that, if a Plant naturally absorbs the compounds of any particular base, it will also take up those of another base which are isomorphous with them : for instance, most vegetables will absorb the salts of Lime and Magnesia writh equal readi- ness ; but salts, however soluble, which have a crystalline arrangement different from theirs (such as those of Strontia), are not absorbed. — The quantity of fluid absorbed, and the force with which it is propelled up- wards in the stem, vary not only in different species and individuals, but in the same plant at different periods of the year, and even of the day. The former seems intimately connected with the activity with which the other processes of vegetation are being carried on, and especially to depend upon the quantity of vapour transpired from the leaves (cHAr xiv.) ; all the causes which increase exhalation, may therefore be considered as stimidants to absorption also. The force of the roots in the propulsion of the sap is sufficiently proved by the celebrated experiments of Hales on * “ Gardeners’ Magazine,” Oct. 1, 1837. + “ Annales des Sciences Naturelles,” Deuxieme Serie, Botan., tom. iii. p. o. &c. ABSORPTION IN VEGETABLES. 655 the vine. By gages affixed to the stem during the “ bleeding season,” when the sap rises rapidly, he found that a column of mercury 20 inches high, equal to a column of water of nearly 31 feet, might be supported by the propellent force of the absorbent organs ; but if the upper part of the plant was cut off, this pow T er soon diminished, and after a time ceased altogether. 423. There would seem much reason to believe, that the mere act of Absorption, in this and other cases, is due to the physical property already referred to as possessed by many organised tissues, — viz. the capa- bility of producing Endosmose (§ 415). The succulent extremities of the spongioles serve as the medium required for this process ; but it may be reasonably enquired whence the other condition is furnished, namely, that difference in density of the fluids on the opposite sides of the septum, which is necessary for the commencement and continuance of the action. This is supplied, in the first instance, by the store of nutritious matter obtained by the embryo from its parent, and contained within its tissues ; and, possibly, at a later period, "when the plant is supporting an indepen- dent existence, by the admixture of a portion of the dense elaborated sap, with the crude and wutery ascending fluid. If this be the true explanation of the phenomenon, a counter-current ought to exist, and an exosmose of the fluids within the system should take place into the sur- rounding medium. That this is actually the case, would appear from the fact, that an excretion of the peculiar products of the species may be often detected around the roots of the plant ; a fact of very important practical application. The cessation of this action of admixture (a change evidently depending upon other vital actions) at the death of a plant, fully accounts for the non-continuance of endosmose, which is also checked if the superincumbent column of fluid be not drawn off by the leaves. It has been very justly remarked by Professor Henslow, that, “ if we suppose the plant capable of removing the imbibed fluid as fast as it is absorbed by the spongioles, then we may imagine the possibility of a supply being kept up by the mere hygroscopic property of the tissue ; much in the same way as the capillary action of the wick in a candle maintains a con- stant supply of wax to the flame by which it is consumed.” * 424. It is an axiom in Vegetable Physiology, which has been laid down by De Candolle, “ that when a particular function cannot, according to a given system of structure, be sufficiently carried into effect by the organ which is ordinarily appropriated to it, it is performed wholly or in part by another.” This is a single case of the general principle which has been already laid down (§ 351) ; and the reason that it is more evident in the Vegetable than in the Animal kingdom, is simply, that in the former the specialization of function is nowhere carried so far as in the latter ; so that any part of the general surface of a plant can perform in a considei’able degree the functions of all the rest. We might then a priori expect, that whilst the roots are, in the usual condition of the perfect plant, the organs by which its fluid nutriment is absorbed, and the leaves its organs of transpiration and respiration, some traces of the primitive community of function enjoyed by the general surface of the simpler tribes, would be found in the capacity of each of these organs to perform in a certain * “ Cabinet Cyclopaedia : Botany.” p. 177. 656 SPECIAL AND COMPARATIVE PHYSIOLOGY. degree, if required, the function of the other. Thus, it is evident that when the roots are either absent or imperfect, or are implanted in an arid or barren soil, serving merely to fix the stem (as happens with many Orcliidece and the generality of aerial parasites), the plant must derive its chief supply of nutriment through the absorption performed by the leaves, or, in leafless plants (as the Cacteas), through the general surface. And it must be obvious to all who have observed the manner in which plants, faded by the intense action of light and heat, are refreshed by the natural or artificial application of moisture, that absorption takes place, in these instances also, by the general surface, as well as by the roots. — Various experiments have been devised, with the view of determining the relative extent to which the plant is supplied by these two channels ; but the pro- portion appears to depend upon the circumstances of its growth. Thus, Bonnet took some specimens of Mercurialis, and immersing the roots of part of them in water, he placed others so that only their leaves touched the fluid. A small shoot of each plant was kept from contact with water ; and after the experiment had proceeded for five or six weeks, those which had derived all then- nutriment through the leaves were nearly as vigorous as those which had imbibed it by the roots. It is by the under surface of the leaf, where the cuticle and cellular tissue beneath are least compactly arranged, that absorption is performed with the greatest rapidity ; and the downy hams with which some plants are plentifully furnished, seem to contribute to this function, acting like so many rootlets. These prolon- gations of the surface are usually wanting in such plants as grow in damp shady situations, where moisture already exists in abundance ; but in hot, dry, exposed localities, where it is necessary that the plant should avail itself of every means of collecting its food, we find the leaves thickly set with them ; and this diversity may be observed in different individuals of the same species of plant, according to the soil and climate in which they exist, and even in the same individual if transplanted. 425. In tracing the gradual evolution of the special Absorbent appara- tus of the more perfect Plants, we may observe many interesting relations between the progressive stages of its development, and the permanent forms of the system in the lower orders. Thus, the embryo at its first appearance within the ovule (chap, xviii.) is nothing but a single cell, like that of the Protococcus, in the midst of the store of semifluid nutri- ment prepared by its parent, which it gradually absorbs by its whole surface, just as do the simplest cellular plants. At the time of the ripening of the seed, we find the rudiment of the future root, which is developed during germination ; but in the early stages of this process, the radicle simply prolongs itself into the ground, and appeal's to be equally capable of imbibing moisture through its whole length, like that of the Liverworts or Mosses. It is not until the true leaves are evolved, that the root begins to extend itself by ramification ; then first protruding perfect fibrils, composed of woody fibre and vessels, and terminated by spongioles. — Thus, then, in the development of the Absorbent system of Vegetables, the first which we have been called upon to study in detail, we have per- ceived the application of the laws which have been already enunciated (chap, viii.) ; for it has been found that, whether we trace its various forms through the ascending scale of the different tribes of Plants, or watch the progress of its evolution in the more perfect orders, it is con- ABSORPTION IN ANIMALS. 657 stantly to be observed that the special structure and function arise by a gradual change out of one more general ; and that, even where the special organ is most highly developed, the general structure retains, in some degree, the primitive community of function which originally charac- terised it. 3. Absorption in Animals. 426. It has been shown in the preceding chapter, that the conditions under which the function of Absorption is performed in Animals, are so far different from those which obtain in Plants, that a preparatory process of Digestion becomes necessary in the former, for the reduction of the food to the fluid form required for its entrance into the system. This process is effected in cavities of the body, which are bounded by a continua- tion of its external surface, modified, by its secreting power, to supply the means necessary for the solution of the aliment, and, by its absorbent faculty, for the selection of the part of it capable of contributing to the nutrition of the fabric. But, so long as this aliment remains unabsorbed, it cannot be regarded as introduced into the system ; since it merely holds the same relation with the absorbent vessels, as the nutritious fluid in which the roots of plants may be immersed, bears to the ducts which they enclose. This is brought into clear view by the remarkable fact, that the poison of the most venomous Serpents, and the Woorara-poison of the South American Indians, of either of which a very small quantity will produce death when it is introduced by the minutest puncture or scratch into the current of the circulating fluid, are perfectly innocuous when taken into the stomach ; the mucous membrane of the alimentary canal apparently having a peculiar inaptitude for allowing them to penetrate by imbibition, either into the blood-vessels or the absorbents. Some experi- ments recently made upon the latter substance, by MM. Bernard and Pelouze, are peculiarly interesting, as confirming the statement already made (§ 422), that the absorption of particular substances, however favourable their condition may appear, may be prevented by the simply- physical conditions of the membrane which they have to traverse.* — It * That the absence of poisonous effects from the Woorara poison, when it is simply intro- duced into the stomach, does not arise from any modification effected in its properties by the agency of the gastric juice, is shown by the fact that the poison, after digestion in that fluid for 24 or 48 hours, remains as virulent as ever ; whilst the gastric fluid to which Woorara has been added, loses none of its solvent power. The various secretions which make up the intestinal juices, have been experimented on with the same results. Hence it appears that the cause of the innocuousness of the poison, under these circumstances, must be looked for in the gastro-intestinal mucous membrane, which will not give passage to the active prin- ciple of the poison, soluble as this is. Experiment proves this to be the case. If the gastric mucous membrane of a recently-killed animal be adapted to an endosmometer, so that the mucous surface looks outward, and the endosmometer containing sugared water is then placed in a watery solution of woorara, endosmose will have been found to have taken place in three or four hours, for the liquid will have risen in the tube, and yet this will contain no trace of woorara, as may be ascertained by inoculating with it. If the experi- ment were allowed to go on for a much longer time, the endosmosis of the poison might occur ; but we should then find that the mucous membrane had undergone modification, the mucus and epithelia covering it being altered, so that imbibition and endosmosis of the woorara becomes possible ; and if, in place of taking a quite fresh mucous membrane, we take one that has undergone some change, the endosmosis of the poisonous fluid occurs instantly. As it was interesting to ascertain whether other mucous membranes possessed this resist- ing power, those of the bladder, nasal fossae, and eyes were tried, and constantly with the same results. An injection was retained in the bladder without inconvenience, for from six U U 658 SPECIAL AND COMPARATIVE PHYSIOLOGY. has further been shown that the introduction of the nutritive material into the system, is effected in the lowest animals by simple imbibition into the tissues that surround the digestive cavity, and by percolation through them towards the more remote parts ; and where such is the case, the digestive cavity either itself occupies a very large part of tLe body, as in the Polypifera, or prolongations of it, in the form of canals, extend to the parts remote from the principal cavity, as in many of the Acalephse. In the higher Radiata, however, as well as in all the Articu- lata (save the lowest Entozoa) and Mollusca, "we find that the conveyance of nutriment to the organism at large is specially accomplished by the agency of the circulating system, some part of which is in direct relation with the alimentary canal. Thus, in the Holothuria (Fig. 113), one of the principal vascular trunks runs parallel with the intestinal canal for a con- siderable part of its length, and receives from it the liquid imbibed by the venous ramifications which are copiously spread out upon its surface ; whilst on the other hand, in the Pycnogonidce, nearly the whole alimen- tary canal is included in what may be considered as a great venous sinus, into -which the nutritive material transudes from its cavity, and by which this is conveyed throughout the body (Fig. 158). In most Inverte- brated animals, these two plans seem to exist in combination, one or the other being predominant in different groups ; but in all cases, the nutri- tive fluid, when withdrawn from the alimentary canal, is introduced at once into the general current of the circulation ; and this introduction would appear to be effected simply by endosmotic imbibition. 427. In the Vertebrata, however, it is by vessels alone, that the nutri- tive fluid is removed from the alimentary canal, the walls of which do net allow it to transude into the surrounding cavity. And we here find pro- vided, in addition to the blood-vessels that are copiously distributed upon the coats of the gastr o-intestih al canal, a special set of Absorbent vessels, which seem to be set apart for the introduction of nutritive materials into the system, and for submitting these to a certain degree of preparation, before they are admitted into the current of the circulation. The ‘ Ab- sorbent System’ of vessels consists of two principal divisions, which may be compared to two sets of roots proceeding from a common trunk ; one of these commences upon the walls of the intestines, and is termed the ‘ lacteal’ system, from the milky character of the ‘ chyle’ which it con- tains ; -whilst the other takes its origin in various parts of the substance of the organism at large, especially in the skin and subcutaneous textures, and is known as the ‘ lymphatic’ system, from the transparent watery aspect of the liquid it conveys. — Although the walls of the whole gastro- intestinal canal are furnished with Absorbent vessels, in common with other musculo-membranous parts, yet it is on those of the small intestine, below the point at which the liver and pancreas discharge their secretion, that the Lacteals especially abound • and they seem, in the higher 5 ertc- brata at least, most commonly to originate in the interior of the villi (§ 169), where they are surrounded by the plexus of capillary blood- vessels that lies immediately beneath the external surface of these fila- to eight hours, by a dog ; but the urine it passed after this time had all the toxical proper- ties of rvoorara. One mucous membrane alone is excepted from this immunity, the pulmo- nary ; for the poison, when applied to it, produces the same effects as when introduced into the subcutaneous cellular tissue. — “ L’Union Medicale,” 1850, No. 1'25. ABSORPTION IN ANIMALS. 659 mentous processes (Fig. 21). In Fishes, however, the villi are few or altogether absent, and the lacteal trunks receive their supplies through a coarse plexus situated in the walls of the intestinal canal. Such a plexus is seen also in Reptiles, in which villi are developed ; and it seems probable that it exists even in higher Vertebrata, in which the mucous membrane is much more thickly set with villi, and in which it appears to be chiefly through the lacteals contained in these, that the chyle gains admission into the larger trunks. The precise mode in which the lacteals commence near the free extremities of the villi, cannot be stated wdth certainty ; but it is probable that they form loops by anastomosis with each other (Fig. 32), so that there is no proper free extremity in any case. It is beyond all doubt, however, that the lacteals never commence by orifices upon the internal surface of the intestine, as was formerly imagined. When these vessels are turgid with chyle, the extremity of each appears to be imbedded in a collection of globules, presenting an opalescent appearance, which give to the end of the villus a mulberry-like form ; and it is supposed by Prof. Goodsir, by whom this circumstance was first observed, that these globules are cells, which are developed during the act of absorption, from granular germs that may be seen in the same situation during the inter- vals of the process (Fig. 32) ; and that these cells, drawing to themselves during their growth certain of the nutritive materials contained in the intestines, are thus the real agents in the selection of the substances that are to be introduced into the lacteals, delivering them to these, by the rupture or deliquescence of their walls, as soon as their term of life is completed. * It is further held by Prof. Goodsir, that the epithelial cells which cover the extremities of the villi (Fig. 32) fall off during the process of absorption, so as to leave the villi more free to imbibe the fluids in contact with their surface ; and thus that a new set of absorbent cells is developed with every recurrence of the act of absorption, and a new set of protective epithelial cells in the subsequent interval. These views have been partly confirmed by other observers ; but there is still considerable doubt whether the supjiosed ‘ absorbent cells’ are anything else than oil- globules : and it is maintained by Prof. E. Weber and others, that, so far from the epithelium falling off as useless or obstructive during the act of absorption, it is by these epithelial cells that the selection is first made, the chyle which they have absorbed being afterwards transferred to the cells within the villi, or at once to the lacteals. Some, indeed, have argued that the liquid contained in the lacteals has been first absorbed by the blood-vessels, and has been eliminated from them by a process resem- bling glandular secretion ; and this view would seem to derive confirmation from the fact, that if the circulation be stopped in the blood-vessels of any part of the intestinal canal, the introduction of chyle into the lacteals is checked also. But if a process of cell-development be required for the selection of the materials to be received by the lacteals, a sufficient reason is furnished for the dependence of the absorption of chyle upon the con- tinuance of the circulation ; since the growth of the cells within the villi could scarcely take place, without a continual supply of elaborated blood to form their walls and maintain their vital activity, although their con- tents are drawn from the intestine. — The whole question of the precise * “ Anatomical and Pathological Observations,” pp. 5 — 10. u u 2 660 SPECIAL AND COMPARATIVE PHYSIOLOGY. mode in which the contents of the lacteals find their way into them, must be regarded as at present sub judice. 428. The Lymphatic vessels are distributed in the greater number of tissues and organs which possess vessels for the conveyance of blood : but to this general statement, there are some remarkable exceptions ; for they appear to be entirely wanting in the substance of the brain and spinal cord] although they are found in then - investing membranes ; they have not been detected in the interior of the eye, nor in the placenta aud umbilical cord ; and it is doubtful whether they accompany blood-vessels in the Haversian canals and cancelli of bone. That they are wanting in cartilage, the epidermic appendages, and other non-vascular parts, follow* naturally from the general principle just stated. It appears to be in the skin and subcutaneous textures, at least in Man, that the lymphatics are most plentifully distributed ; and they seem there to originate, like the lacteals of the intestinal walls, in plexuses of which the meshes are veiy close. According to Prof. Kolliker, the lymphatics in the tail of the Tadpole do not form a network, but branch out like rootlets, their ulti- mate extremities, or rather their commencing radicles, having free but closed ends, running out into fine points;* it may be doubted, however, whether this is not the result of a want of completeness in their develop- ment, and whether these ramifications would not meet and inosculate in the fully-developed Frog. Like the capillary blood-vessels (§ 221), they take their origin in stellate cells, which send forth long projections ; but these branches do not inosculate with each other in any other way than to form continuous tubes ; and if they subsequently constitute a plexus, it must be by the development of connecting arches at a later period. — It has been maintained that the minute lymphatics communicate with the capillary vessels in their neighbourhood ; and these communications have been supposed by some to allow the direct transmission of the lymph into the sanguiferous system ; whilst by others it has been inferred that the liquor sanguinis, or fluid portion of the blood, (which, when diluted, closely resembles the contents of the lymphatics in its composition) finds its way into the absorbents. It is nearly certain, however, that no such apertures exist ; and that when any direct passage of fluid does take place from one set of vessels into the other (as is often the case in artificial injections, and was noticed by Prof. Kolliker in watching the circulation in the tail of a Tadpole which had been injtued), it is through an abnormal opening. 429. The walls of the Absorbent vessels (whether lacteals or lymphatics) are extremely thin, so that the character of their contained fluid can be readily discerned through them. Those which form the ultimate ramifi- cations of the system appear to be limited only by a very delicate homo- geneous membrane ; and it is not certain that even this is universally present in the absorbents of Fishes. A similar membrane, covered with a layer of pavement-epithelium upon its inner or free surface, constitutes the lining of the mid-sized and larger trunks ; but these possess, iu addition, a middle or fibrous layer, in which non-striated muscular fibres may be distinguished, and an external sheath of areolar tissue. Iu the higher Vertebrata, the Absorbents are furnished with valves, which allow the passage of their contents in only one direction, namely, from their origin towards their termination in the sanguiferous system : these valves. * “ Annales des Sciences Naturelles,” Zool. (1846), tom. vi., p. 98. ABSORPTION IN ANIMALS. 661 however, seem to be wanting in the ‘ plexuses of origin,’ which are filled by mercury injected into any part of them. In Fishes and Reptiles, however, in which this system is obviously developed upon an inferior type, the valves are few or are altogether wanting. — In the higher Vertebrata, again, certain small solid bodies are found in the course of the lacteals and lymphatics, which are termed ‘absorbent glands.’ The structure of these, however, does not correspond with that of ordinary glands ; and they have been more appropriately named ‘ganglia,’ for they are essentially com- posed of plexuses of absorbent vessels, convoluted (so to speak) into knots, and dilated into larger cavities, amongst which capillary blood-vessels are minutely distributed ; the wdiole being bound together by areolar tissue, and invested in a capsule of the same. These blood-vessels have no direct communication with the interior of the lacteals, but are separated from them by the membranous walls of both sets of tubes ; so that what- ever passage of fluid normally takes place from one set of vessels to the other, must be accomplished by transudation through these.* According to the observations of Prof. Goodsir, the absorbents, when they enter a gland, lay aside all but their internal coat and epithelium ; and the latter, in place of retaining its pavement-like character, presents itself as an irregular layer of spherical nucleated corpuscles, measuring about l-5000th of an inch in diameter (Fig. 200, b) ; which layer is thickest in the dilated lymphatics which form the ‘ cells ’ of the centre of the gland, and Fig. 200 . a . b A, Diagram of a lymphatic gland, showing the intra-glandular network, and the transition from the scale-like epithelia of the extra-glandular lymphatics, to the nucleated cells of the intra- glandular. — B, Portion of intra-glandular lymphatic, showing along the lower edge the thick- ness of the basement-membrane, and upon it, the thick layer of glandular epithelial cells. becomes gradually thinner towards the periphery, where it is continuous with the epithelium of the afferent and efferent vessels (a). The inner layers of the central epithelium appear to have no tenacity ; so that the component cells may be readily detached from one another, and carried off in the fluids which traverse the cavity.— Having thus considered the general structure of the Absorbent system, we shall proceed to notice its more special peculiarities in the different classes of Vertebrata. 430. The proper Absorbent system is exhibited in its simplest and most diffused form in Fishes, the lowest class in which its existence lias been demonstrated. Where it consists of distinct vessels, their walls are very thin and distensible. The Lacteals commence in a somewhat coarse net- work of canals, that seem channelled-out (as it were) beneath the mucous * It has been asserted by many anatomists, that free apertures exist, by which the con- tents of one set of vessels can pass directly into the other ; but these statements are founded on the results of injections, which can be very easily forced to make such apertures ; and the most careful examination has failed to detect them, when no such procedure has been employed. 662 SPECIAL AND COMPARATIVE PHYSIOLOGY. lining of the intestinal canal, and cannot be shown to possess definite walls ; from these, however, the chyle is conveyed away by proper vessels, which form capacious plexuses in the mesentery, along the course of the alimentary canal. The lymphatics are distributed extensively through both the superficial and deep-seated parts of the body ; and they, also, by the convolutions and anastomoses of their trunks, form numerous plexuses in different parts of the body, especially around the veins, which may be regarded as the first indications of the so-called £ glands ’ that are pre- sented in the higher classes. Some of these tranks, moreover, dilate into sinuses, which appear to be contractile, and pre-figure the ‘ lymphatic hearts ’ of Reptiles. Such a sinus may be seen in the tail of the Eel ; and another is found on each side of the cranial cavity of many Fishes, externally to the jugular veins. Although a considerable proportion of the lymphatic trunks unite with the lacteal vessels, to form principal canals (corresponding with the thoracic duct in higher animals), which empty their contents into the systemic veins near the heart, there are many other communications between the two systems, as Fohmann appeal's to have sa- tisfactorily demonstrated. Thus, the caudal sinus of the Eel discharges its contents into the caudal vein, the orifice being provided with a valve. 431. The conformation of the Absorbent system presents several inter- esting peculiarities in the class of Reptiles. As in Fishes, the vessels are generally destitute of valves, though these may occasionally be observed in the larger tranks ; but they everywhere seem to possess distinct walls. When compared with that of Birds and Mammals, the absorbent system of Reptiles seems to possess an* enormous extension ; large and capacious lymphatic plexuses being developed around the great veins, and the length of the trunks being often augmented by doublings and convolutions. But this extension is rather apparent than real ; for there is still an absence of the ‘ glands,’ which seem to concentrate, as it were, the assimilating power of a long series of tubes ; and the relation of the Absorbent system of Rep- tiles to the more concentrated apparatus of Birds or Mammals, thus comes to resemble that which the extended tracheal system of the Insect bears to the lungs of the higher Yertebrata. — The lacteals in Reptiles, as in the classes above them, partly commence in the ‘ villi ’ of the intestinal canal ; but, as in Fishes, there is also a very coarse plexus of absorbents beueuth its mucous lining. The fluid which they absorb is collected into a rect r- taculum chyli, situated at the root of the mesentery ; and from this it passes by two or more ducts, which also receive many of the lymphatic trunks, into the great systemic veins. The Lymphatic portion of the system is furnished, in most Reptiles, with certain pulsating dilatations, or lymphatic hearts, which aid in the propulsion of the contents of the vessels ; the walls of these contractile cavities are formed of striated muscular fibres. In the Frog there are two pairs of these ; one situated just under the skin, through which its pulsations are readily seen in the living animal, immediately behind the hip joint ; while the other pair is more deeply seated at the upper part of the chest. The former receive lymph from the posterior part of the body, aud pour it into the veins proceeding from the same part, by orifices furnished with valves ; the latter collect that which is transmitted from the anterior part of the body and head, and empty their contents in like manner into the jugular vein. Their pulsations are totally independent of the heart and of the acts ot ABSORPTION IN ANIMALS. 663 respiration, since they continue after the removal of the former, and for an hour or two after somatic death and the complete dismemberment of the animal. Neither are they synchronous with each other on the two sides of the body, nor always performed in the same space of time ; for the pulsations are not only generally irregular, but sometimes exhibit long and frequent intermissions ; when in constant action, they occur about sixty times in a minute. From the observations of Volkman, however, it appears that these movements are dependent upon the connection of the lymphatic hearts with the corresponding segments of the spinal cord ; since they cease when these are destroyed, although all other parts may be left uninjured j and continue so long as this connection remains perfect, although all other parts of the nervous centres be destroyed.* — A pair of vesicles similar to the posterior pair in the Frog, has been detected in Salamanders , Lizards, and Crocodiles, in which they are situated near the root of the tail, and are connected, in like manner, with the veins of the lower extremity ; they have also been discovered in Serpents, where they lie under the last rib. It was for some time believed that the Chelonia formed an exception to the general fact of the existence of these pulsating receptacles ; but it has been shown by Muller that they are particularly large in that group, in which they lie behind the superior extremity of the iliac bones, receiving the lymph by capacious trunks from the posterior extremities, and pouring it into veins that discharge themselves into the reno-portal system.! 432. In Birds, we find the Absorbent system existing in a more perfect form ; its trunks being everywhere provided with valves, and the diffused plexuses being partly replaced by ‘ glands ’ or ‘ ganglia,’ which may be probably considered as performing the same function by an organisation of more concentrated character. The lacteals, which are not furnished with these glands, all converge towards a receptaculum chyli, from which proceed two thoracic ducts, one on either side, to terminate in the angles formed by the junction of the jugular and subclavian veins. These ducts receive, also, most of the lymphatic trunks ; but the lymphatic system has two other communications, as in Reptiles, with the veins of the lower extremity. These are connected with two large dilatations of the lymph- atic trunks, which are evidently analogous to the lymphatic hearts of Reptiles, but which do not seem to have any power of spontaneous movement. In the Goose, they are about the shape and size of a kidney- bean, and are situated in the angle between the tail and the thigh. They were supposed by Panizza to possess an automatic power of alternate con- traction and dilatation ; but this motion has been shown by Muller to be due to the respiratory actions, — being synchronous with them, and ceasing when they are interrupted. 433. In the Absorbent system of the Mammalia, we witness its most concentrated and highly-developed form ; the vessels are copiously pro- vided with valves ; and their parietes are firmer than in the lower classes. Instead of the extensive plexuses of Fish, we find small dense ‘ glands ’ disposed in different parts of the system ; these are more numerous than in Birds, and present themselves on the lacteals as well as on the lym- phatics, being known in the one case as the ‘ mesenteric,’ and in the other as ‘ lymphatic ’ glands. In some Mammalia, especially of the order * “ Muller’s Archiv.,” 1844. f Ibid. 1840. 664 SPECIAL AND COMPARATIVE PHYSIOLOGY. Carnivora, the mesenteric glands cluster together into a single mass, named the pancreas Asellii, which lies at the root of the mesenteiy. The lacteals all discharge their contents into the receptaculnm chyli, which is situated in the lumbar region, close to the spine ; into the same receptacle many lymphatic trunks pour the fluid which they have collected from the pos- terior part of the trunk and extremities ; and from it arises the thoracic duct of the left side, which passes forwards along the spine, receiving other lymphatic trunks in its course, and terminates at the junction of the left jugular and subclavian veins. A smaller trunk on the right side receives the lymphatics of the right side of the head and upper extremity, with those of the right lung and right side of the liver ; and this terminates, in like manner, at the junction of the right subclavian and jugular veins. It is a beautiful instance of mechanical adaptation, that as the angle formed by the convergence of two veins is a point of much less resistance than any other part of the walls of the vessels, the easiest possible entrance is thus provided for the fluid discharged from the thoracic ducts into the current of the circulation. Although these are the only two canals by which the Absorbents usually communicate with the veins in Man, their number is greater in many species of Mammalia ; they all terminate, however, in the same part of the venous system. Thus, the left thoracic duct often resembles rather a plexus of vessels than a single tube ; branches proceeding from it and then reuniting, and at last terminating in the veins by several apertures. Sometimes it consists throughout of two tubes, which anastomose with each other and with the duct on the right side, and terminate separately in the veins ; and in the Pig a branch of communication is sent off to the vena azygos, which is a small trunk running in proximity with it along the spinal column. All these modes of distribution occur as irregularities of conformation in the human subject, the former not being uncommon ; the last, however, is rare. 434. We have now to enquire into the cause of the onward movement of the contents of the Absorbent vessels in the higher Yertebrata, which do not possess £ lymphatic hearts ; ’ and also of the flow of fluid towards these pulsating cavities in the animals which possess them, situated (as they are) close to the points where the fluid of the absorbents is discharged into the veins. A part of the movement may be attributed to the vis a tergo, pro- duced by the continual entrance of fresh fluid into the rootlets (so t-i speak) of the vascular tree ; and although it may be thought, from the extreme distensibility of the walls of the absorbents, that this force will be rather expended in dilating them, than in pushing onwards the column of liquid which they contain, yet it must be remembered that they are sur- rounded by tissues, whose tonicity, during the living state, gives to them much more resisting power than they possess after death. Further, in all the moveable parts of the body, assistance is doubtless afforded by the occasional pressure which will be exercised upon the absorbents by the surrounding tissues ; for while this pressure is operating, it will tend to empty them of their contents, which are only permitted by then valves to pass in one direction ; and when the pressure is relaxed, they will be re-filled from behind. But it seems probable that the regular propulsion of the fluid mainly depends upon an alternate contraction and dilatation of successive portions of the vessels, slowly repeated at intervals ; such alternations having been witnessed by Prof. Kolliker in the tail of the ABSORPTION IN ANIMALS. GG5 tadpole ; and it being apparently by such contraction, without subsequent dilatation and re-filling, that the absorbents are emptied after death, and this with considerable rapidity. 435. We have now to enquire into the relative parts which are per- formed in the function of Absorption, by the proper Absorbents, and by the Blood-vessels ; and although these cannot yet be said to be precisely definable, yet there can be little doubt that we are in possession of the general truth with regard to them. — From the time when the Lacteal vessels were first discovered, down to a comparatively recent period, it was supposed that they constitute the channel through which all fresh nutri- tive material is taken into the body ; and this idea seemed to derive con- firmation from the great uniformity in the composition of the chyle, which, though different as a whole from that of blood, appeared adequate to supply those substances to the latter, which are most constantly being eliminated from it by the nutritive and respiratory operations. Various considerations, however, would lead to the conclusion, that the nutritive materials do not enter through the lacteals alone ; but that the most soluble portions of them, together with other substances not nutritious, find their way directly into the blood-vessels. — In the Invertebrata, it will be recollected, no special Absorbent system exists, and in all those which possess blood-vessels, it is by them alone that alimentary matters are in- troduced into the system ; it would not seem likely, therefore, that this most general method of performing the function should be entirely super- seded in Vertebrata by the more special one. But further, when the extraordinary vascularity of the whole gastro-intestinal membrane is con- sidered, together with the peculiarity of the special distribution of the capillaries in the villi ; and when it is remembered also that the rapid move- ment of blood through these, creates the condition most especially favour- able to the passage of liquids into them from the outside (§ 418), it might be almost certainly affirmed that endosmose must take place between the contents of the alimentary canal and the blood in the vessels. — This conclusion has been confirmed by numerous experiments. Thus it was ascertained by MM. Tiedemann and Gmelin, that when various substances were mingled with the food, which might be easily detected by their colour, odour, or chemical properties, — such as gamboge, madder, cam- phor, musk, asafoetida, and various saliue substances, — they were seldom found in the chyle, though many of them were detected in the blood, and some had even passed into the urine. So, again, it was found that if any of these substances be introduced into a portion of the intestine separated by ligatures from the remainder, and all the vessels of that portion be divided or tied save its artery and vein, the substance may speedily be detected in the blood, and if it be poisonous its effects are manifested nearly as soon as usual ; whilst, if the blood-vessels be tied, the lacteals being left entire and uninterrupted, a long period elapses before there is any evidence of absorption. It cannot be doubted, then, that ali- mentary substances in a state of solution, such as albumen, gelatine, or sugar, may pass into the blood-vessels by simple endosmose, when the relative densities of the blood and of the intestinal liquids are such as to favour the inward current. Conversely, it might be inferred that if the liquid in the intestines be of a nature to determine the endosmotic current in the contrary direction, some of the constituents of the blood would be 666 SPECIAL AND COMPARATIVE PHYSIOLOGY. drawn from them into the alimentary canal. Now it has been shown by the experiments of Poigseuille, that an endosmotic current takes place through animal membranes from, the serum of the blood towards certain saline solutions ; and thus it happens that when these are taken into the alimentary canal, they produce a copious exudation of fluid from its walls, which fluid contains a considerable quantity of albumen.— It appears, then, that an interchange between the contents of the blood-vessels and those of the alimentary canal takes place, in either direction, in a manner which is in all respects conformable to physical principles ; so that Ab- sorption must be effected through the blood-vessels of the higher ani- mals, as of the lower. 436. On the other hand, the Lacteals, as already pointed out, would appear to receive only substances of a particular class, more especially fatty matters in a state of more or less fine division, with which albumi- nous compounds are intimately mixed. These, whether received at once into the lacteals by the instrumentality of epithelial and other cells at the extremity of the villi, or first imbibed by the blood-vessels which enclose the lacteals, and then transferred to the latter, are doubtless obtained almost directly from the food • for the quality of the Chyle depends upon that of the aliment last digested, it being of an opaque white if that food contained much oily or fatty matter, and of a more transparent aspect if such matters were deficient. The experiments of Sandras and Bouchards t have shown, that particular kinds of fatty substances with which animals may have been fed, are recognisable in the chyle ; and on the whole it may be considered, that the special fimetion of the lacteals is to take up the oleaginous portion of the food, and to bring it into that intimate rela- tion with albumen which seems to be requisite for its subsequent assimi- lation (§ 560). Still it appeal’s by no means certain that the blood-vessels also may not absorb oleaginous substances, as they do in Invertebrated animals ; particularly as it has been shown by the experiments of Prof. Matteucci,* that if an emulsion be made by shaking a few drops of oil in water to which a little alkali has been added, and an endosmometer filled with a weak alkaline solution be then immersed in this emulsion, the oil penetrates in a short time through the membranous partition, and makes its appearance in the interior of the endosmometer. Now as the blood is slightly alkaline, and as its density favours the imbibition of fluid, there seems no reason why fatty matters should not thus find their way into the blood-vessels, when they have been reduced to a state of fine divisi n- veying back the ‘ elaborated sap’ from the leaves to the stem. Its trans- mission appears to be simply a process of imbibition, taking place between contiguous cells, whereby each communicates to the rest a portion of the nutritive materials with which it is charged ; every one making that use of them which is in accordance with its own endowments. Additional evidence in favour of the view here advocated, will be given hereafter (chap. xv.). 448. It appears from microscopic observation, however, that a special circulation of peculiar juices does take place in certain parts of plants, through that system of anastomosing vessels, which has been termed * It is difficult to understand, however, how this process can he effected by the cells of the interior of the stem and roots, secluded as they are from the direct influence of lirt' ; without which (we have every reason to believe) no direct production of organic compounds from inorganic elements can take place. CIRCULATION IN VEGETABLES. 67 5 laticiferous (§ 154). This has been especially observed in plants with ‘ milky’ juices, that is, in those in which the ‘latex’ is rendered opaque by the presence of floating particles of resin, caoutchouc, or other sub- stances, which enable the movement of the fluid to be distinguished. There is reason to doubt, however, whether it is really confined to such ; and whether it might not be discerned in many plants with transparent juices, if their movement could be in any way made evident. When the laticiferous vessels are cut or broken across, a flow of fluid takes place from the wounded part; and if a piece of the bark or leaf of a ‘milky’ plant be cut out and placed under the microscope, the fluid contained in the vessels will be seen in rapid movement throughout. This, however, has no relation to the true circulation, or cyclosis, which was first described by Prof. Schultz as taking place in the laticiferous vessels of an uninjured part.* The movement seems to take place in all directions, the currents often running contrariwise in contiguous vessels. Sometimes one of these currents may be observed to stop, its cessation being preceded by a tem- porary oscillation ; it afterwards recommences, or a new current is established in a contrary direction. The rate of movement is greatest in parts which are in progress of development, other things remaining the same ; it is also accelerated, within certain limits, by warmth ; and is retarded or entirely brought to a stop by cold, recommencing on the renewed application of warmth. A strong electric shock puts an end to it immediately. 449. The resemblance between this cyclosis in Plants and the capillary circulation in Animals, makes it a point of peculiar interest and importance to determine the nature and source of the forces by which the former is sustained. It is quite obvious that the movement cannot be due to any vis a tergo ; both because it is far from being constant in its direction in particular vessels, and because there is no organ to supply a propelling force, * The statements of Schultz have been called in question by many distinguished Botanists, amongst others by Prof. Schleiden, who have regarded the movements described by him as the result of injury to the vessels, permitting the discharge of their contents, and conse- quently establishing a current towards the point of exit. So many competent observers, however, have satisfied themselves that such is not a sufficient explanation, that the existence of a cyclosis in certain plants must, in the Author’s opinion, be regarded as an established fact, whatever be its degree of generality. The latest recorded observations on this point are those of Prof. Balfour ; which are to the following effect. — “ From observations made last summer, I am disposed to agree with Schultz’s statements. It is true, as Mohl remarks, that any injury done to the part examined causes peculiar oscillatory movements, which speedily cease. Thus if the young expanded sepal of the Celandine is removed from the plant and put under the microscope, or if the inner lining of the young stipule of Ficus elaslica be treated in a similar manner, very obvious motion is seen in the granular con- tents of the vessels, and this motion is affected by pricking the vessels or by pressure. In order to avoid fallacy, however, I applied the microscope to the stipules of Ficus elastica, while still attached to the plant, and uninjured ; and I remarked that, while pressure with any blunt object on the stipule caused a marked oscillation in the vessels, showing their con- tinuity, there could, nevertheless, be observed a regular movement from the apex towards the base, independent of external influences, when the stipule was simply allowed to lie on the field of the microscope without any pressure or injury whatever. This movement continued for at least twenty minutes during one of the experiments, and I have no doubt might have been observed longer. It is of importance to distinguish between those molecular move- ments which are caused by injury and pressure, and those which depend on processes going on in the interior of the living plant. My experiments are by no means complete ; but they lead at present to the adoption of Schultz’s opinion relative to the existence of the cyclosis.” — “Manual of Botany,” 1849. x x 2 676 SPECIAL AND COMPARATIVE PHYSIOLOGY. which could extend itself through such a complex system of anastomosing canals. Nor, again, can we attribute it to any vis a f route, like that which takes part in producing the ascending current from the roots towards the leaves. It is certain, too, that no such contraction of the laticiferous vessels themselves takes place, as could be effectual in propelling the fluid through them. Further, the movement continues for some time in parts that have been completely detached from the rest, and on which neither vis a tergo nor vis a f route can have any influence. On the other hand, the facts stated in the preceding paragraph all indicate that, like the ‘rotation’ within the individual cells of Plants (§§ 137, 269), the move- ment of fluid within the laticiferous vessels (whatever may be its purpose iu the vegetable economy) is intimately connected with the formative operations of the part, and is dependent upon forces which arise out ot these. The manner in which they become so, is the next object of oiu enquiry; and on this subject, some views have been put forth by Prof. Draper,* which seem to help towards an explanation of the phenomena. — It is capable of being shown, by experiments on inorganic bodies, that, if two liquids communicate with each other through a capillary tube, for the walls of which they both have an affinity, but this affinity is stronger in the one liquid than in the other, a movement will ensue ; the liquid which has the greatest affinity being absorbed most energetically into the tube, and driving the other before it. The same result occurs when the fluid is drawn, not into a single tube, but into a network of tubes, perme- ating a solid structure ; for if this porous structure be previously saturated with the fluid for which it has the less degree of attraction, this will be driven out and replaced by that for which it has the greater affinity, when it is permitted to absorb this. Now if, in its passage through the porous solid, the liquid undergo such a change that its affinity be diminished, it is obvious that, according to the principle just explained, it must be driven out by a fresh supply of the original liquid, and that thus a continual movement in the same direction would be produced. — Now this is precisely what seems to take place in an organised tissue which is permeated by a fluid, between whose particles, and those of the tissue which it penetrates, affinities exist, which are concerned in the formative changes that take place during its circulation. Hence these affinities are continually being newly developed by acts of growth, as fast as those which previously existed are satisfied or neutralised by the changes that have already occurred ; and thus in the circulation of the nutritive fluid, there is a constant attraction of its particles towards the walls of tire vessels, and a continual series of changes produced in the fluid as the result of that attraction. The fluid, which has given up to a certain tissue some of its materials, no longer has the same attraction for that tissue ; and it is consequently driven from it by the superior attraction then possessed by the tissue for another portion of the fluid, whicli is ready to undergo the same changes, to be in its tmu rejected for a fresh supply. Thus in a growing part, there must be a constantly-renewed attraction for that portion of the nutritive fluid which has not vet traversed it ; whilst, on the other hand, there is a diminished attraction * “ On tiie Forces which produce the Organisation of Plants,” by John William Draper, M.D., New York, 1844 ; pp. 29 el seq. CIRCULATION' IN ANIMALS. 677 for that which has yielded up the nutritive materials required by the particular tissues of the part ; and thus the former is continually driving the latter before it. But the fluid which is thus repelled from one part, may still be attracted towards another, because that portion of its con- tents, which the latter requires, may not yet have been removed from it ; and in this manner the current may be maintained through the whole capillary network, until the liquid has been entirely taken up by the tissues which it permeates. The source of the movement being thus attributable to the formative actions to which it is subservient, it is obvious that it must be affected by any external agencies which quicken or retard these ; and it is thus that the influence of heat, cold, and electricity upon the rate of the flow seem most readily explicable. — These principles will be hereafter shown to have a most definite applica- tion to the phenomena of the ‘ capillary circulation’ in Animals (§ 481). 450. The development of the Circulating system during the growth of Vascular Plants, has not yet been made an object of special attention ; the general facts with which we are acquainted, however, correspond exactly with the principles which have been previously stated. As the absorption of nutriment by the embryo within the ovule (§ 278 a) appears to take place through the whole surface, there is no transmission of fluid from one portion to another ; nor do we find, even at the period of the ripening of the seed, any distinct vascular structure. As far as its circulating system is concerned, therefore, the young plant, at the commencement of germi- nation, is on a level with the simpler cellular tribes. During the rapid longitudinal development, however, which then takes place in the stem and root, there is of course a peculiar transmission of fluids in those directions ; and this appears to be at first performed, as in the stem of the Fungi, by elongated cells and intercellular passages. It is not until the tine leaves are expanded, that we find a distinct formation of woody or vascular structure ; and it is very interesting to remark, that the ducts of young plants often present the appearance which is characteristic of the Ferns, having the spiral fibre more or less regularly disposed within them (Figs. 11, e, 12, 1 , 2); whilst, after the stem has ceased to increase rapidly in length, these canals are converted into dotted ducts (Fig. 12, 3) by the process already described (§ 152). The anastomosing vessels of the latex in like manner originate from cells of less regular form, which open into one another at several points, and not, as in the formation of ducts, by their extremities alone (§ 154). This change is represented in progress iu Fig. 13, a. 3. Circulation in Animals. 451. In tracing the evolution of the Circulating system through the Animal scale, it will be easy to discover its conformity to the same gene- ral plan, as that which has just been traced out in the Vegetable kingdom. In proportion as the power of absorbing aliment is restricted to one part of the surface, whether external or internal, does it become necessary that means should be provided for conveying the nutritious fluid to distant i organs, — not merely that it may furnish the supplies, which they are con- stantly requiring for the maintenance of their respective structures, and for the manifestation of their vital properties, but also that it may itself. 678 SPECIAL AND COMPARATIVE PHYSIOLOGY. undergo certain changes, which are essential to the continuance of its cha- racteristic qualities. Not only does the circulation of fluid through the system enable the new materials to be deposited in their appropriate situa- tions, but it also takes up and removes the particles, which, having mani- fested a tendency to disorganisation, are no longer fit for the offices which they previously contributed to perform ; so that, by the various processes of secretion, these may be separated from the general mass, and be either appropriated to some other purpose in the economy, or altogether carried out of the structure. The excretion of carbonic acid by the Respiratory apparatus is one of the most considerable and important of these processes ; and it will be found that the distribution of the circulating apparatus ha-; always an express relation to the conditions under which it is performed. In fact, so peculiar is this adaptation in the higher Animals, that many have considered the vascular system under two heads, — that belonging to the general circulation of nutritious fluid through the system, — and that w T hich performs the respiratory circulation, conveying the blood, which has been rendered impure by the changes it has previously undergone, to the organs where its physical and vital properties are to be renewed by contact with the air. Respiration differs not in kind, however, from the other functions of Excretion, but only in its relative importance ; and although in warm-blooded Vertebrata, whose nervo-muscular energy can only be maintained in full vigour by a constant supply of oxygenated blood, its cessation even for a short time is fatal, there are many amongst the lower classes, in which it can be suspended for a considerable period with impunity, and in which the increased amount of other secretion-! appears to counterbalance the diminution in its products. We find too, even in the highest Vertebrata, peculiar modifications of the circulating apparatus in connection with other secreting organs, as the liver in Mam- malia, and the kidneys in Birds : so that it should rather be stated as a general fact, that, in proportion to the variety of the organs, and the im- portance of the functions they perform, is the complexity of the circu- lating apparatus which supplies them, — than that it undergoes modification according to the conditions of the respiratory system alone, as Cuvier maintained. In proportion as the function of Absorption is restricted to one part of the surface, that of Respiration will be limited to another . and the processes of Nutrition, and the formation of Secretions, will go on in parts of the structure distant from both ; and all these must be brought into harmony by vascular communication, the arrangement of which will evidently vary from the most simple to the most complicated form, accord- ing to the number and variety of the actions to which it is subservient, aud the vigour with which these are performed. Still, in most of tLo-e animals whose nervo-muscular energy is the greatest, the arrangement ol the Circulating apparatus, and the rate of the movement of blood through it, appear to have special reference to the demand for oxygen created m the discharge of the Animal functions, and to the necessity for the removal of the carbonic acid generated in the same processes ; whilst there is evi- dence that the Organic operations of growth and development might be carried on by means of a much less rapid flow, and by a less perfectly oxygenated blood. A remarkable confirmation of this view is presented by the condition of the Circulation in the class of Insects, in which the activity of the animal functions is relatively greater than in any other CIRCULATION IN ANIMALS. 679 group ; for we find the movement of their nutritive fluid, which is sub- servient in them to nutrition alone, to be comparatively slow and feeble ; the very active aeration of their nervo-muscular apparatus being accom- plished, not so much by the medium of the blood, as by the penetration of air-tubes into the tissues themselves (§ 520). 452. The Circulating apparatus of Animals, among the higher tribes at least, differs in one important particular from that of Plants. We have seen that, in the latter, the sap which has been elaborated in the leaves is dispersed through the fabric, giving up its nutritive constituents to the parts to which it finds its way ; and that if any of it mixes with the ascend- ing current, and circulates a second time through the system, the amount of this is comparatively small (§ 447). In the higher Animals, on the contrary, we observe that the same fluid is repeatedly transmitted through the body ; and that the alterations which are effected in one part of its course by certain processes of Nutrition, are counterbalanced in others by different nutritive operations, as likewise by those of Respiration and Secretion, and by the continual admixture of new alimentary materials. These, as we have seen, are wholly taken up, in the Invertebrata, by the blood-vessels which ramify on the sides of the intestine ; whilst in Verte- brated animals they are partly absorbed by a special set of vessels, which after a time empty them into the general circulating current. The variety of changes, which the blood undergoes in different parts of the body, requires the admixtime of the portions returned from each, that a uniform fluid may be again sent forth into the system. This is provided for, by the mode in which the apparatus is constructed. In all but the simplest forms of it, we find a central reservoir, the Heart, into which the whole fluid returned from the body is poured, and on whose contractile force the maintenance of the current chiefly depends. From this it passes out by one or more large trunks, which convey it to the several organs and tissues of the body ; these are called A rteries. The arteries gradually subdivide into ramifying vessels, which, repeatedly undergoing the same change (Fig. 53), terminate in a complex system of anastomosing (inter- communicating) tubes, of nearly uniform size, which are termed Capil- laries (§ 220). It is in these only, that the blood comes into sufficiently intimate relation with the tissues which it supports, or by which secretions are elaborated from it, for the performance of chemical or vital reactions between them ; so that we may consider the function of the arteries to be the simple conveyance of the nutritive fluid from the general reservoir to this network of capillaries, which exists in most of the living tissues of the body, and in near proximity to the remainder. Even the walls of these trunks are furnished with a distinct set of branches (the vasa vasorum), proceeding from neighbom’ing vessels, for their own nutrition. After tra- versing the capillaries, the blood is received into another series of vessels, formed by their reunion, which are termed Veins ; and these, gradually coalescing into larger trunks, return it to the central reservoir. — The walls of the Arteries and Veins are formed of several layers of tissue, which, however, constitute three principal ( coats.’ The inner coat is a pellucid structureless membrane, continuous both with that which lines the heart, and with that which forms the sole boundary of the capillaries ; and this is covered on its free or internal surface by a layer of epithelial cells. The outer coat is simply protective, and is formed of condensed areolar 680 SPECIAL AND COMPARATIVE PHYSIOLOGY. tissue. The middle or ‘ fibrous ’ coat, however, which is much thicker iu the arteries than in the veins, is partly composed of yellow elastic tissue : and partly of non-striated muscular fibre. By the agency of the forme) - , which is especially abundant in the larger arteries that receive the blood direct from the heart, the intermitting jets in which the fluid is at first propelled by its contractions, are gradually converted into a continuous stream. The latter, on the other hand, is more abundant in the smaller arteries, and appears to supply a propulsive force in some degree comple- mentary to that of the heart ; but its main purpose seems to be, to regu- late the diameter of the vessels, in doing which it is probably influenced in some degree by the Sympathetic system of nerves that is minutely distributed upon it. Of a rapid alteration in diameter, which, beinp emotional in its source, we can only regard the nervous system as the instrumental agent in producing, we have a characteristic example in the act of ‘ blushing.’ 453. The description above given, however, is by no means applicable to the entire Animal kingdom ; for in many of the simpler tribes, there is no circulation of a proper nutritive fluid ; and in many other’s, the vascular system is far from possessing the completeness of organisation which it exhibits in the higher groups. Thus we shall see that in many of the lower Articulata, there is no central cavity for the reception and propulsion of the blood ; but the principal vessels are of nearly uniform diameter throughout, and are everywhere in some degree contractile j whilst the fluid within them does not appeal’ to move in a determinate direction, but seems to be transmitted irregularly from part to part by undulating contractions, extending themselves through the tubes. In many Articulata, again, and still more in the Mollusca, we find the blood, for at least a part of its course, not contained within distinct vessels, but dispersed through the lacunae, which, like the ‘intercellular passages ol Plants, intervene between the tissues ; and even the visceral cavities (such as that of the pericardium and peritoneum) sometimes form part ol the spaces which it traverses, from the time it leaves the heart until its return to it. Indeed, there are some instances in which it seems to be in the general visceral cavity only, that any movement of nutritious fluid, dis- tinct from the contents of the alimentary canal, takes place ; this lu - s been shown to be the case, for example, in the Bryozoa (§ 298), probably also in the Rotifer a (§ 310), and also in the small tribe of Pycnogonidos (§ 316 e) • and it would appear to be in great part true also of the larvae of many Insects, in which, however, the elongated visceral cavity seems connected at its two extremities with a contractile ‘ dorsal vessel, whereby a constant movement of its contents in a determinate direction is secured. In the Vertebrated classes, the only trace that seems to remain of this lacunar circulation, is the commencement of the lymphatic system of Absorbents, which, as already mentioned (§ 437), seems to take up, for return into the circulating current, the nutritious fluid that has escaped from the capillaries into the interstices of the tissues.* 454. In the Protozoa, and in the lower Rcidiata, as already shown, the ex- tension of the digestive cavity into all parts of the fabric does away with the * See the admirable Memoir of Prof. Milne-Edwards on the “ Circulation of Nutritive Fluid,” in the ‘Ann. des Sci. Nat.,’ Mai, 1845. CIRCULATION IN ANIMALS. 681 necessity for a special circulation of nutritive fluid ; and as the materials escaping from this cavity at once penetrate the tissues by which it is surrounded, there is no opportunity for that dispersion of it through the general visceral sac, which is so remarkable in the lower Mollusca and Articulata. — In the class of Eehinodermata, however, which presents us with the highest development of the Radiated type, a true circulation of nutritive fluid, that has been absorbed through the walls of the alimentary canal, unquestionably exists ; and this takes place, through the greater part of its course at least, in vessels having distinct parietes. At some part of the vascular system, there is usually found a pulsating dilatation which seems the first rudiment of a heart ; but this is neither capacious nor powerful enough to possess much influence on the movement of the blood through the long series of trunks, branches, and capillary plexuses, which it must traverse after leaving this organ, before returning to it again. The arrangement of the vascular system differs in the several orders of Eehinodermata, with the general diversity in their type of con- formation ; it has been especially studied in the Asterias, Echinus, and Holotkuria . — In the Asterias, in which the digestive cavity is prolonged into the- ‘rays’ or lobes of the body (Fig. 110), a vascular trunk is found lying on the surface of each of the radial caeca, and receiving minute branches from its subdivisions ; and the several trunks, converging from the rays to the central disk, unite with other branches from the stomach, to form a circle or vascular ring round the upper part of the disk. This is connected with a similar ring surrounding the entrance to the stomach on the lower surface, by means of a vertical descending vessel, which Tiedemanu found to possess muscular irritability, and regards as the rudi- ment of a heart ; whilst, from the lower ring, other vessels proceed which are distributed through the disk and rays. The vessels first men- tioned probably act as absorbent veins, and convey the nutritious fluid to the central rings, which are furnished with numerous dilatations ; whence it is propelled through the second set of vessels, -which may be regarded as arterial trunks, to the system at large. No communication has yet been detected, hoivever, between the terminations of the second set of ves- sels and the commencement of the first ; and it seems probable that this is only established by lacunae. Moreover, the supposed course of the vital fluid has not been verified by observation, being merely conjectured from the distribution of the vessels. As no special respiratory organs appear . to exist in the Asterias, so is there no special respiratory circulation. — In the Echinus, the vessels which arise from the sides of the alimentary canal unite into a trunk (apparently analogous to the mesenteric vein of nigher animals) ; and this conveys it to a vascular ring, which surrounds the anal orifice, and which may be likened to the upper ring of the Aste- rias. From this circle, which also receives the venous blood of the ovaries, five trunks (branchial arteries) pass off, distributing the blood over the mesenteric membrane that lines the visceral cavity, to -which water is admitted for aerating the blood, and also to the vesicles at the base of the tubular cirrhi, which are probably in part subservient to the same pur- pose. Here, therefore, we find an express modification of the circulating apparatus for the purpose of carrying into effect the second of its princi- pal objects ; and it is interesting to trace this modification so low down in the scale, before a heart is distinctly evolved, and whilst the motion of 682 SPECIAL AND COMPARATIVE PHYSIOLOGY. fluid in the vessels seems dependent upon the changes which it under- goes in them. After ramifying on the membrane which lines the shell, and being there submitted to the respiratory process, the circulating fluid is carried back by a set of branchial veins to another ring formed round the anal outlet of the alimentary canal ; from this the ovaries are sup- plied with arterialized blood, whilst the remainder of the aerated fluid enters a canal, which proceeds directly to the oesophageal extremity of the alimentary tube, and there dilates into a short oval sac, which, pos- sessing muscular parietes, and exhibiting during life sloiv but distinct con- tractions, is regarded as a heart. From this canal and its pulsating dila- tation, which seems to represent the vertical contractile trunk of the Asterias, arterial trunks pass forth, which form a ring round the mouth, and distribute the blood to the several parts of the complex buccal ap- paratus ; and the venous blood, returned from these to a venous ring round the mouth, thence proceeds to the intestinal vein first described, mingling with the newly-absorbed alimentary materials, and passing on with them to the respiratory system of vessels. — Such, at least, appears the most probable account of the course of the blood through the vessels ; but the information hitherto obtained is not sufficient to enable it to be affirmed with certainty that its direction is not precisely the reverse. 455. In the Holothuria, as already stated (§ 296 e), a transition may be perceived from the Eadiated to the Annulose form ; and this is in some degree manifested in the arrangement of its circulating apparatus. The vessel which seems best to answer to the mesenteric vein of the Echinus, is the trunk which rims along the external border of the intestinal canal (Fig. 113, ve ), receiving the venous branches which have collected the blood distributed over it, together with the fresh nutritive materials which they have absorbed. From this trunk, the blood is again sent forth, to be dis- tributed by means of the ‘ mesenteric,’ but rather ‘ branchial,’ vessels (vm), upon the respiratory apparatus. This, as formerly shown, differs entirely from that of the Echinus, in being formed much more upon the plan of that of Insects, though destined for aquatic respiration : and the blood is distributed in a beautiful capillary network over the walls of the curious ‘ respiratory trees ’ (rr), into which water is introduced from the cloaca After being aerated by this means, the circulating fluid is collected 1 >y branchial veins, which unite into a trunk (vi) that runs along the internal border of the intestine ; this trunk presents numerous dilatations, and these appear to be contractile, thus foreshadowing the dorsal vessel of Insects. From it the blood is again transmitted to the mesentery (in ) by numerous branches, and thence over the intestinal surface, to find its way through the capillary network into the intestinal vein (ve). Besides tins system of vessels, however, there is another, consisting of an annular trunk (ya), which surrounds the mouth (as in Echinus), and semis off branches to the buccal apparatus and the tentacula, as also to a set of tegumentary vessels (vl, vl'), which supply the general surface. In on nection with the oesophageal collar is found a saccular dilatation (/<), which seems to represent the pulsatile vessel of the Echinus ; but whether it possesses a similar contractile power, is not known. The connection of this set of vessels with the preceding has not yet been made out. — It is scarcely possible to conceive that the impulse which the blood receives, in any of the three preceding cases, from the central propelling organs, can be CIRCULATION IN ANIMALS. 683 sufficiently strong to convey it through all the complex ramifications that have been described, and through a double or even triple system of capil- lary tubes, back to the centre from which it was originally distributed ; especially since the pulsations which have been witnessed in those organs are slow and feeble. That the passage of the blood through the respira- tory apparatus is independent of it, there seems good reason to believe ; and the very complete analogy, which exists between this circulation and that of the higher plants, affords a striking confirmation of this view. As in plants, the nutriment taken up by the absorbent vessels enters at once into the general circulating system, by which it is conveyed to the respi- ratory organs ; and, after leaving these, it is distributed to the fabric at large, to supply to it the materials for its nutritive operations. We have seen that in plants, the motion of the fluid in the laticiferous canals appears to be entirely independent, not only of the propulsion of the roots, but of every direct mechanical impetus (§ 449) ; and there seems no difficulty, but, on the contrary, the highest probability (if consistent with other facts, as it will hereafter appear to be), in supposing that the same causes, whatever may be their nature, are in operation in this instance also. 456. The Articulated classes are usually regarded as inferior to the Mol- lusca in the evolution of their circulating apparatus ; and it certainly never presents the same concentrated condition in the former group, as in the latter. That which is chiefly remarkable, however, in the vascular system of the Articulata, is the multiplication of propelling cavities, in accordance with the segmentation of their bodies ; it being only in the very highest tribes of the series, that we find all the blood transmitted to a single receptacle, and distributed by branches proceeding from it alone. It may also he remarked, that a very exact bi-lateral symmetry almost uniformly prevails in the conformation of the central organs, and in the distribution of the vessels ; and that the plan of circulation which may be regarded as typical of the entire group, is for the blood to pass along the body from behind forwards through a dorsal trunk in which the propelling power is concentrated, and then to return from before backwards through lateral or ventral trunks. The principal variations from this plan have reference to the arrangement of the Respiratory organs, the great func- tional importance of which, in the Articulated series, almost invariably necessitates some special provision for the distribution of blood to them ; and thus, with every change in their position and character, a new modi- fication of the circulating apparatus is presented. 457. Putting aside the cases already referred to (§ 453), in which there is no proper vascular circulation, the distribution of the nutritious fluid being altogether lacunar , we find that the simplest condition of the circulating apparatus is presented to us in the lower Entozoa. Thus in the Tcenia (§ 309 c) we have seen that four minute trunks pass from end to end of the body, which are of nearly uniform size throughout, and com- municate by transverse branches ; and in neither of these would any special propulsive power seem to exist. In the Fasciola, on the other hand, the capillary network that traverses the body communicates with a single trunk (Fig. 142) which passes along the dorsal region on the median line, and which is evidently analogous to the ‘ dorsal vessel ’ of higher Articulata. In the Planaria, again, the capillary network is chiefly formed by the ramifications of a pair of trunks which traverse the body longitu- 684 SPECIAL AND COMPARATIVE PHYSIOLOGY. dinally at some distance from the median line ; and these possess, at their anterior extremities, a pah - of pulsatile cavities, which imite on the median line, and surround the cephalic ganglia, giving off smaller vessels to the anterior part of the body (§ 309 d). And lastly, in the St-ronyylm and other Nematoidea, we seem to have a combination of this plan with that which is presented to us in the Taenia ; for a pair of trunks run along the length of the body on either side, of which those that are nearest to the median line communicate with each other freely by trans- verse branches, whilst at the anterior extremity they arch towards each other, and a small vesicle is developed on the median line at their point of junction. This vesicle is supposed to perform the functions of a bean, and to send the blood along the internal trunks, which may perform the functions of arteries, whilst those which accompany them may possibly act as veins. But on this subject nothing is certainly known ; and it may be doubted whether there is any determinate sanguineous current in these comparatively simple beings. None of them have any distinct respiratory apparatus ; and consequently we do not find any special arrangement of the blood-vessels for the aeration of the blood. 458. The arrangement of the Circulating apparatus among the different tribes of the class Annelida is found to present many remarkable varieties : some of these have relation to the form and situation of the respiratory organs, which are no less variable ; but for the purpose of others it seems difficult to account. This class has long been noted for the redness of the blood of the animals composing it, — a character by which they are distinguished from others of the Articulated series, as well as from the Mollusca ; and which, combined with the transparency of their bodies, allows the circulation to be distinctly seen. This shade does not, however, exist throughout the class ; for in a few species, the blood is nearly colourless ; and in one, at least, it is of a green tint, approaching olive. It is not dependent upon the presence of coloured corpuscles, hut is proper to the plasma or liquid element of the blood. The condition of the vascular system in the Leech and its allies appears to approach more nearly to that which has been just pointed out as characteristic of the Entozoa, than does that of any other Annelida ; as might be expected from the fact that the order Suctoria forms the connecting link between the two classes (§311 d). It essentially consists of four longitudinal vessels, a dorsal, ventral, and two lateral trunks, together with a multitude of communicating branches. The dorsal vessel receives the blood posteriorly from the intestinal canal, and from the dorsal portion of the integument, and carries this forwards towards the head ; arrived near the mouth, it subdivides into a pair of arches which pass round the oesophagus, uniting below to form the ventral trunk ; and this runs backwards towards the posterior sucker, giving off branches in its course, which are chiefly distributed on the alimentary canal. The lateral vessels pass along the two sides of the body in a wavy course ; they communicate with each other at both extremities, and likewise at intervals along then whole length, by transverse branches ; and their ramifications are principally distributed to the integuments and to the so-called ‘ pulmonary vesicles,’ which appear, however, to he merely receptacles for the mucous secretion. The branches of all four sets of vessels freely inosculate with each other. The walls of the main trunks are furnished with muscular fibres disposed CIRCULATION IN ANIMALS. 685 Fig. 201. circularly, and it appears to be chiefly by the successive or peristaltic contractions of these, that the blood is kept in motion ; no determinate course, however, can be assigned to it ; as it seems to have an undulating movement, sometimes in one direction, sometimes in another, — reminding us of the want of constancy in direction which is so remarkable in the Tunicated Mollusks (§ 466). The lateral trunks, from their connection with the tegumentary system, would seem to be the part of the apparatus that is chiefly subservient to the respiratory function. 459. In the typical Annelida (§ 311 a, b), the most constant parts of this apparatus are the dorsal and ventral trunks ; the lateral vessels, however, often exist, although they are seldom of the same relative importance as in the Suctoria ; and the dorsal and ventral trunks are them- selves sometimes double along a part or the whole of their course, running on the two sides of the median line, or at a little dis- tance from it, and thus reminding us of the Entozoa. Moreover, we generally find special contractile dilatations on some part of the vascular system ; sometimes only a single one exists ; but more commonly they are greatly multiplied. Our acquaintance with the circulation in this group, has chiefly resulted from the skilful observations of M. Milne-Edwards." — In the Terebella , whose gills are filamentous, and situated round the head (Fig. 201, 7c, 7c), there lies in the anterior part of the body, on the dorsal aspect, a large trunk ( l ), which receives at its posterior extremity, from a venous sinus (n) surrounding the oesophagus, the con- tents of an extensive vascular plexus, that has ramified on the walls of the intestine, and on the muscles, integuments, &c. This trunk, or dorsal vessel, propels forwards the blood, which it receives from behind, by irregular contractions. At its anterior extremity it subdivides into numerous branches, of which the principal enter the respiratory organs (7c, 7c), whilst others pass to the head and tentacula ( b , b) ; so that a large proportion of the blood is aerated, be- fore it is again circulated through the sys- tem. The vessels that return it from the gills reunite into a trunk (o, o), which passes down the ventral surface of the body, giv- ing off a pair of transverse vessels to each segment, and then returning along the Circulating apparatus of Terebella con - chilega: — a, labial ring; b, b, tentacula; c, first segment of the trunk ; d, skin of the back ; e , pharynx ; /, intestine ; a, longitudinal muscles of the inferior sur- face of the body ; h, glandular organ (liver?); i, organs of generation; j, feet; k, Ic, branchiae ; l, dorsal vessel acting as a respiratory heart ; m, dorso-intestinal vessel ; n, venous sinus surrounding oeso- phagus ; n', inferior intestinal vessel ; o, o, ventral trunk ; p, lateral vascular branches. * See his elaborate Memoir in the 66 Ann. des Sci. Nat.” 2ieme Serie, Zool. tom. x. i 686 SPECIAL AND COMPARATIVE PHYSIOLOGY. Fig. 202. inferior side of the intestine ( n ) ; this trank is of course to he regarded in the light of an artery. The blood is conveyed back into the venous sinus, both from the intestine and from the parietes of the body, by the dorso-intestinal vessels ( m ), which must be considered as veins. The pro- pulsion of blood into the gills seems principally due to the contractions of the dorsal vessel, which may here be regarded as a sort of respiratory heart ; but its motion through the arterial trank would appear to he partly owing to contractions of the gills themselves, which are occasionally seen to take place. The irregularity of these, however, requires some supple- mental force, such as that which has been already described in the inferior tribes, for the maintenance of a steady current ; and there are many allied species, in which the blood circulates as energeticallv, without any such evident propelling agents. — In the Eunice (Fig. 202) we find the same general distribu- tion of vessels, but an important change in the position in the respiratory organs, which involves a complete alteration in the character of the different parts of the system. The branchial organs (it) are not concentrated round the head, but are disposed in tufts along the whole body. The dorsal trank (l ) receives from the dorso-intestinal vessels (I), as in the former case, the blood which has ramified on the intestines ; but this fluid, as will presently appear, is as much arterial as venous. The contractions of this trunk are not so regular and powerful as in the Terebella, and seem to be but little concerned in maintaining the circulation. The vessels into which it dirides anteriorly, are distributed only to the head and neighbouring parts ; and, by the reunion of the vessels which return the blood so distributed, the ventral trank (q) is formed, which here possesses a venous character. The general distribution of its ramifications is very similar to that described in the Terebella, except that it transmits blood to the branchiae as well as to the general system : but each transverse branch (t) presents a dilatation or bulb (t) near its origin, which seems to propel the blood that enters it, by regular contractions, partly through the branchial tufts, and partly upon the intestines, cuta- neous surface, muscles, &c., after permeating which it re-enters the dorsal vessel. This multiplication of propelling organs is very interesting, when viewed in reference to the general tendency to repetition of parts manifested in this class. It is found that, in the Annelida which possess it, the vitality of portion? of the body is preserved during a very long time after the sub di vis! a < >f the animal. In higher tribes, however, this multiplication is restricted to a particular division of the body, as will he presently seen in the Earth- Circulating apparatus of Eunice sanguinea; — a. b, c, antennas ; e, first segment of the body; /, feet; g, pha- rynx ; g', mandibular mus- cles; i, intestine; Z', dorsal vessel ; Z, superior intesti- nal vessels ; s, their lateral branches; q, ventral vessel ; t, its lateral branches; t ' , contractile bulbs on these branches ; u , branchiae ; .r, subcutaneous vessels of the back. CIRCULATION IN ANIMALS. 687 Fig. 203. - 460. In the Arenicola (Fig. 203) we observe another interesting variety in the arrangement of the vascular system, which partly resembles the forms already noticed, and partly conducts us to others which would at first sight appear entirely different. The dorsal vessel (o,o) traverses almost the entire length of the body posteriorly, and it receives, as befoi’e, the blood which has circulated on the intestine and ex- ternal surface, as well as some directly transmitted by the branchite. It terminates, however, at about the anterior fourth of the body, in a kind of contractile ventricle ( n ), which answers the purpose of a heart ; but it first sends forward branches ( x ) to the head, the vessels returning from which enter the ventral trunk (i f) that passes backwards from the propelling- cavity. The branches of this trunk are almost entirely distributed to the gills ; and the blood which is re- turned from them is partly transmitted to the intestine by the lateral intestinal vessels (p), partly to the in- teguments, and partly to the dorsal vessel direct. The branchise here, as in the Terebella, seem to have a direct propelling power on the blood which passes through them. — In the Lumbricus or ‘ Earth-worm,’ again, we find the dorsal vessel communicating with the ventral trunk, not by one contractile cavity at its anterior extremity, but by several loop-like dilated canals, which seem to exercise a similar propelling- agency. The waves of blood can be distinctly seen, if the animal be kept without food for a time, until it has discharged the black earth which usually fills the intestinal canal. The blood which is thus forcibly pro- pelled into the ventral trunk, is conveyed backwards along the body, and distributed to its different organs, especially to the secreting follicles which appear to re- present the respiratory sacs of higher air-breathing Articulata (§ 311 c) ; and from these it is returned to the dorsal vessel, its aeration having been effected through the medium of the general surface.—' The great extent and importance of the capillary system in the Annelida, compared with the feebleness of the central propelling powers, is an interesting feature in the cha- racter of their vascular apparatus, and shows that we have not yet arrived at a condition of the circulating system very far removed from that which it presents in Plants. 461. In the Circulating apparatus of the Myriapoda, as in other parts of their structure, we meet with the condition which may be regarded as typical of the Articulated series ; for this class presents the full evo- lution of that multiple heart, which is developed (so to ; speak) out of the principal dorsal trunk usually found in the lower Articulata ; whilst it also exhibits that uniformity in its structure, and in the distribution of the vessels proceeding- from it, -pr ■-•A Circulating apparatus of Arenicola piscatorum , as seen from one side : — a , proboscis ; b , pharynx ; d, dilated oesophagus ; e, CEecal appendage; f, stomach; n, heart ; o, o, dorsal vessel ; p , lateral intestinal vessels; f, ven- tral trunk ; u, u\ cuta- neous vessels coming olf from the branchial ; x, lateral pharyngeal ves- sels ; z y labial vascular ring. (See also Fig. 149.) 688 SPECIAL AND COMPARATIVE PHYSIOLOGY. which are obscured in the higher Articulata by the unequal develop- ment of the successive segments, and by the specialization of particular organs. This change of type, however, is by no means abrupt. It appears from the researches of Mr. Newport,* to whom we owe the greater part of our knowledge of the vascular system of this class, that in the Iulidce (§ 312), which form the connecting link between the Worms and the higher Annelida, the walls of the dorsal vessel are veiy thin, and the valvular constrictions between its successive segments, which are formed by reduplications of the muscular tunics, are by no means complete ; further, the number of these chambers, which corresponds with that of the moveable segments of the body, is very large, being no less in one genus than seventy-five. A greater multiplication even than this is seen at an early period in the life of the Iulidee ; for each of then’ move- able segments, to which two pairs of legs are attached, is foimed by the coalescence of two original segments ] and eveiy one of the latter at first possesses its own subdivision of the dorsal trunk, the indications of which are seen, even in the adult animal, in the duplication of the cardiac muscles and of the arterial trunks on each side, whilst the cavities have completely coalesced. — In some of the Geophilidce, the number of distinct chambers is even greater than in the Iulidae, being no fewer than a hundred and sixty in one species ; but the whole apparatus presents a higher type of structure. — In the Scolopendridce, the number of chambers is reduced, in accordance with that of the segments of the body, being never more than twenty-one, and sometimes as small as fifteen ; the muscular portion of them walls is much more developed ; and the valvular partitions which isolate the successive segments from each other, as well as the valves which guard the orifices of the vessels, are much more complete than in the lower tribes. The anterior and posterior portions of this dorsal vesst !. and the parts in immediate connection with it, are shown in Fig. 204 ; in which the figures 1, 2, indicate its first and second chambers, and 17, 18. 19, 20, 21, those of the corresponding segments at the opposite extremity. The walls are formed of two layers of muscular fibres, some of them annular and others longitudinal ; and similar fibres may be traced upon the principal systemic arteries. The pimpose of these fibres is obviously to produce contraction of the chambers] their dilatation being accom- plished by the bands of alar fibres,/,/, which extend to a considerable distance from either side of the dorsal vessel, and are inserted into the dermo- skeleton of them own segment. Each chamber has a pair of apertures guarded by valves, which is probably for the entrance of venous blood : but the source from which they receive it has not been clearly made out . it is probably, however, the great sinus formed by the pericardial sac which surrounds the dorsal vessel. From each chamber, also, a pair of systemic arteries, h, h, is given off, which are especially distributed to the organs of the upper side of them own segment, but inosculate with those of other segments. From the most anterior chamber (1) is given off a pair of large arches, pr, pr , which encircle the oesophagus, to meet again upon its under side in the ventral trunk, xy ; but it also sends forwards a median trunk, q, that gives off two smaller pairs or arches, u, v, which in like * “ On the Nervous and Circulatory Systems of Myriapoda and Macrourous Arachnida.” Philos. Transact., 1843. CIRCULATION IN ANIMALS. 689 manner meet in the anterior continuation of the ventral trunk, w. From the median trunk are also given off the vessels which supply the cephalic segment and its organs of sensation ; the lateral branches, s, t, which supply the mouth and its Fig - 204 - appendages, being furnished by a trunk that comes off from the principal aortic arch on each side. The distribution of the arterial branches of the dorsal vessel is extremely minute ; even its own parietes being furnished with distinct nutrient branches, z. The ventral trunk, which lies upon the ganglionic cord, and is principally formed by the union of the arches that surround the oesophagus, may be regarded as representing the ‘ aorta’ of Vertebrated animals ; it is of large dia- meter in the anterior portion of the body, but gives off a pair of arterial branches in each seg- ment ; and at its posterior extremity is reduced to a conrparatively small size, there subdividing into two principal branches, which supply the last pair of legs. The branches successively given off from this aortic trunk chiefly proceed to the muscles of the legs ; but one set of them is dis- tributed to the tracheae ; and it is worthy of note that the tracheae and blood-vessels are mixed up together in a remarkably intricate manner in the peritoneal membrane. — The venous system has not been made out by Mr. Newport ; but there is a strong probability from analogy that it is alto- gether lacunar, and that the blood which lias cir- culated through the system, together with that which has been sent to the tracheae for respira- tion, finds its way to the great pericardiac sinus, and thence re-enters the dorsal vessel through the apertures in its successive chambers. A por- tion of this mixed fluid will be at once sent out- wards by the contraction of each chamber, into the arterial trunks proceeding from it ; but the principal portion will be transmitted from behind forwards by the peristaltic action of the cham- bers, those of the anterior segments dilating whilst those of the posterior contract ; and thus the current will be directed through the aortic arches into the ventral trunk, and also through the arterial branches supplying the head. Be- sides the dorsal and ventral trunks, we find a pair of lateral trunks, o, which are specially con- nected with the hepatic organs ; the branches of these inosculate with those of the ventral trunk ; but in what way the blood is propelled through them is unknown. — In the family of Scutigeridce, an interesting transition is presented to the structure which the dorsal vessel possesses in Insects; for Y Y Circulating apparatus of Scolo- pendra: — a, cephalic segment; b, basilar segment ; c, foot-jaws ; l , 2, anterior cardiac chambers ; 17, 18, IQ, 20, 21, cardiac cham- bers of the posterior extremity of the body ; a, antennae ; b, c, cephalic ganglia; d, eyes ; e, ex- tremity of foot-jaws ; /, /, alar muscles ; ff, <7, valvular partitions ; h,h, systemic arteries; i , k, l, m, n, their subdivisions, some of them inosculating with the hepa- tic vessels 0 ; p, p , aortic arches; q , median trunk continued from the dorsal vessel ; r, r, aortic arches reuniting, to form the ventral trunk ; s, mandibular ar- tery ; t, cephalic artery ; u, v , secondary arches ; w , anterior continuation of the ventral trunk x, y, z, nutrient arteries of the dorsal vessel. 690 SPECIAL AND COMPARATIVE PHYSIOLOGY. every alternate chamber is much smaller and shorter than the one before and behind it, and receives very little blood from its auricular orifices, so that, the total number of chambers being sixteen, the number of the prin- cipal chambers is no more than eight ; whilst at the same time, the whole organ is shorter and of more compact form. 462. It is not a little remarkable that Insects should have been long regarded as unpossessed of a proper circulation ; the peculiar provision for the conveyance of air through the interior of then’ bodies having been supposed to render the movement of blood unnecessary. Such, however, is by no means the case ; for the circulation in Insects is at least as actit e as it is in other Articulata ; and it is only the extraordinary rapidity in the flow of blood, that would have been required by animals of such won- derful energy, if their respiration had been localised in one organ, which is rendered unnecessary by the universal diffusion of that function. — We find in Insects a dorsal vessel (Figs. 152-4, a a), formed upon the same plan as that of Myriapoda ; but the division into chambers is restricted to the abdominal portion of the body, the vessel being continued through the thorax to the head, as a simple contractile trunk. Thus it happens that the number of distinct chambers never exceeds eight ; and it appears that in many Insects it is even less. The muscular walls of these chambers are considerably developed, and their valvular apparatus is veiy complete. The dorsal vessel is surrounded, as in Myriapoda, with a pericardial sac, in which blood has been seen to move ; this may, therefore, be regarded as a venous sinus, from which the blood is received into the several cham- bers, by a pair of valvular orifices within each. The fluid is propelled from behind forwards, through the successive chambers ; and is then driven onwards to the head, through the trunk which is the anterior continuation of them. Several branches have been detected, into which this subdivides for the supply of blood to the parts of the head ; but no pair of aortic arches for the conveyance of blood to the under side of the body, has yet been made out, although a ventral trunk has been discovered by Mr. New- port, lying upon the gangliated nervous column, as in Myriapoda. The course of the circulation has been chiefly watched in transparent Larva, and in Pupae during their development ; and it has appeared as if, when the currents had once passed out of the dorsal vessel, they made their way rather through lacunae among the tissues, than through distinct vessels. The principal stream of blood from before backwards does not seem to flow along the ventral trunk, but through two lateral passages which lie near the ventral surface ; and it is from these that the secon- dary currents diverge, which pass into the wings and legs, and then return back to the main stream. These currents, too, seem to be rather ‘ lacunar,’ than restrained by distinct parietes ; but it is probable that the walls of the passages may be more complete in the perfect Iusect. The mode in which the aeration of the blood is provided for in Insects, has not until recently been made out ; but from the recent enquiries of M. Blanchard * it appears that the trachea are ensheathed, even to their minutest ramifications, by prolongations of the sanguiferous canals, and * “ Annales des Sciences Naturelles,” June, 1848. — Tire statements of M. Blanchard have been called in question by several Anatomists, more especially by M. Joly (“Ann. des Sci. Nat.,” Nov. 1849); he has replied to them, however, in his Memoir on the Circulation in the Arachnida, to be presently referred to. CIRCULATION IN ANIMALS. 691 that the blood is therefore aerated wherever the traphese penetrate. This view is in perfect accordance with the fact long since observed by Mr. Bowerbank* and others, that the current of blood in the ‘ nerves ’ of the wings moves in a space which completely surrounds the tracheae. — So far, then, as the course of the circulation in Insects is yet known, it may be probably considered to be as follows. The blood impelled forwards by the dorsal vessel, is transmitted to different parts of the body, either by dis- tinct vessels, or by lacunae ; after passing through the tissues, it finds its way by imbibition into the outer sheaths of the minuter tracheae, which thus serve to collect it for aeration ; and from these it is transmitted, whilst undergoing exposure to the air contained in the tracheae, towards their external orifices, where it is collected from their sheaths by a system of canals which convey it back into the great pericardiac sinus, whence it enters the dorsal vessel. t — The dorsal vessel of the Larva is far less perfect in its . structure, than is that of the Imago ; being sometimes almost as destitute of valvular partitions, as is that of the Annelida ; and very commonly presenting no higher a development, than that of the lower Myriapoda. But in its advance towards the perfect state, a gradual thickening of its walls, and a completion of its valvular partitions, maybe noticed ; and at the same time the whole organ becomes contracted in length, and presents a more concentrated condition. In many aquatic larvae, especially of the order Neuroptera, there are leaf-like appendages affixed to the tail, in which the circulation may be distinctly seen, the streams passing off in loops from the main trunks, and entering them again, so as to be conducted to the posterior extremity of the dorsal vessel. Previously to the metamorphosis, the currents cease in these organs ; and this is for the most part true also of the currents in the wings, which may be uniformly observed in these organs in the Pupa state, but which are very seldom to be seen for any length of time after the last metamorphosis, although they may be frequently seen in individuals that have recently emerged. This cessation of the circulation in the wings, is obviously the cause of their complete deficiency in reparative power ; no losses of sub- stance in them being ever made good, so that old bees may always be distinguished from young ones by the chipped indented edges of these organs, resulting from the accidental injuries to which they have been subjected. — The completely lacunar condition of the circulation in certain aquatic larvae, especially of the Gnat tribe, has been already noticed (§ 453). The venous sinus surrounding the dorsal vessel seems to fill the entire visceral cavity ; and the movement of blood, which can be most distinctly seen, on account of the multitude of corpuscles which it contains and the transparency of the bodies of these larvae, appears to take place from behind forwards in the dorsal vessel, and from before backwards in the great venous sinus, without any diverging currents ; not only the parietes of the body, but all their contents, being in such immediate relation with the circulating fluid, that no further provision is necessary. 463. The Circulating apparatus of the Arachnida presents us, at least in the higher forms of the class, with a much greater completeness than * “ Entomological Magazine,” April, 1 833, and October, 1836. + Such, at least, appears to the Author to be the most probable interpretation of the facts ascertained by M. Blanchard’s injections, taken in connection with those observed in the living animal. 692 SPECIAL AND COMPARATIVE PHYSIOLOGY. that of Insects, as might be anticipated from what has already been said of the higher development of most of the organs in this class (§ 319) ; and the greater localisation of the respiratory organs involves the addition of a set of vessels, which shall be specially subservient to the aeration of the blood. Nevertheless there is no essential departure from the plan of structure already described as prevailing in Myriapods and Insects ; and between the higher members of the former class, and the Macrourous Arachnida, such as the Scorpion, the conformity in the arrangement of the vascular system is extremely close. The following are the most important of the facts ascertained by Mr. Newport, in his minute investi- gations into the Circulating apparatus of the Scorpionidce .* — The dorsal vessel runs continuously from the posterior extremity of the tail as far as the cephalo-thorax, and may be described as consisting of a cardiac portion (Fig. 205, h, h, 1-7) which occupies the abdomen, a dorsal artery , a, which is the anterior continuation of this, and a caudal artery, q, q, which is its posterior portion. The structure of the cardiac portion, which con- tains eight chambers (the posterior one, however, being very imperfect), is similar in most respects to that of the dorsal vessel of the higher Myria- pods and perfect Insects ; but it differs in this, that the valvular’ partitions between them are much less complete ; a difference which probably has reference to the fact, that the cardiac portion has to send a great arterial trunk backwards as well as forwards. The blood enters the cardiac cham- bers from the surrounding sinus, through the auricular orifices u, u ; and a portion of it is sent forth again, without being propelled into the adjacent chambers, through the systemic arteries p, p. The anterior continuation, or dorsal trunk, a, passes through the septum x, which divides the cephalo- thorax from the abdomen ; and soon afterwards gives off a pah - of large branches, which pass round the oesophagus, like the aortic arches of Myria- poda, to reunite below it into the ventral trunk i, i. It then gives off large branches, 1, 2, to the two posterior pairs of legs, and a smaller one, 2*, to the thorax ; after which it separates into three principal trunks, of which one, f, is median, and runs above the (esophagus, giving off branches (6-1 4) to the cephalic ganglia and organs of sense, whilst the others, g, pass forwards at the sides and below the oesophagus, to supply the two remaining pairs of legs (3, 4) and the great prehensile claws (5). In this arrangement, there is a veiy marked conformity to the type of the Scolo- pendra (§ 461). Besides these vessels, the dorsal trunk gives off proper visceral branches, z, which proceed backwards along the anterior portion of the alimentary canal, inosculating with branches from the systemic arteries p, p. The posterior continuation of the multiple heart, constituting the caudal artery, g, g, passes backwards to the extremity of the tail, giving off in its course the systemic arteries l, l, and also the lateral branches r, r, which communicate with the portal trunk s, s . — The ventral trunk, i, i, which lies upon the upper surface of the gangliated nervous cord, extends backwards from its commencement in the aortic arches, nearly to the termination of the tail ; gradually diminishing in diameter, as it gives off minute vessels for the nutrition of the cord, and successive pairs of branches, k, k, which pass downwards to communicate with the portal trunk ; its terminal branches in the last caudal segment being distributed Philosophical Transactions," 1843. CIRCULATION IN ANIMALS. G93 Fig. 205 . with the terminal nerves proceeding from the ganglionic cord. — Beneath the ganglionic column we find another trunk, designated by Mr. Newport as the 'portal ; the purpose of which appears to be, to collect the blood from the systemic vessels, and to transmit it for aeration to the pulmonary bran- chiae, c, c. This trunk is formed by the coalescence of branches from various sources ; but es- pecially, in the abdominal re- gion, from the ventral trunk, and in the tail, from the caudal artery. Its branches are almost entirely distributed upon the respiratory organs. — Such is the general distribution of the proper ‘vascular’ portion of the circulating apparatus, which seems to be altogether arterial in its character ; the venous circulation in the body at large would seem to be altogether ‘lacunar whilst the return of the blood from the pulmonary branchiae to the heart probably takes place by definite canals. — The course of the blood, there- fore, would seem to be as fol- lows. Of that which is re- turned from the respiratory organs to the chambers of the multiple heart, one part is sent forth by the systemic trunks proceeding from each chamber which it has just entered, an- other part is transmitted for- wards from chamber to cham- ber into the dorsal trunk, whilst a third portion seems to be propelled backwards into the caudal trunk. The dorsal trunk distributes the blood to the cephalo-thorax and its organs of sensation and motion ; whilst a portion of it is carried back- wards through the aortic arches and ventral trunk, partly for the nutrition of the ganglionic cord, and partly for transmis- sion into the portal system. Diagram of the Circulatory and other organs in Buthus : — a, antennal claws; b, eyes; c, c, pulmono-branchiie ; d, alimentary canal; e, anal orifice; f, poison-glands; «, anterior continuation of the dorsal trunk, giving off 1, 2, 2*, branches to the posterior legs, besides the two great arches which form the ventral trunk ; b, cephalic ganglia ; c, optic nerve; d, great subcesophageal ganglion ; e, ocelli; /, supraoesophageal artery, giving off 6 — 14, branches to the cephalic ganglia and organs of sense ; g, lateral trunks of subcesophageal arteries, giving off 3, 4, 5, branches to the anterior members; h, h , chambers of the heart, 1 — 7; h h ventral artery ; A*, Ic , branches given off in each segment ; 7, 7, branches of caudal artery; m, m , ganglionic cord; n , n, portal system of vessels ; o, o, its branches ; p, p, systemic arteries given off from the cardiac chambers ; q , q , great caudal artery ; r, r, its branches of communication with the portal trunk s, s; t, termination of cardiac portion in anterior dorsal trunk; u, u , auricular openings into cardiac chambers ; «r, diaphragm dividing cephalo-thorax from abdomen ; y , anterior pair of systemic arteries dis- tributed on this; z, visceral arteries. So, again, the caudal trunk conveys the 694 SPECIAL AND COMPARATIVE PHYSIOLOGY. blood backwards to the extremity of the tail, giving off nutrient branches to the various parts of that organ, and also transmitting a part of its contents directly into the portal system. The great portal trunk, (which may perhaps be considered as made up by the coalescence of the two lateral canals that exist in Insects and many Articulata) receiving blood from these sources, and collecting that which has been distributed through the tissues by the systemic branches of the dorsal and ventral trunks, transmits this fluid for aeration to the pulmonary branchiae, from which it is returned by a set of ‘ branchio-cardiac canals’ to the great cavity surrounding the heart. — The circulation has not been studied with the same minuteness in the A raneidce ; but from the researches of M. Blanchard "' it appears that the coruse of the blood is essentially the same as in Insects, but that it is distributed by more perfect vessels. The dorsal vessel forms a multiple heart of four or five chambers in the abdo- minal region (Fig. 163) ; but the partitions between these chambers are often scarcely perceptible, so that the cardiac cavity is really single but elongated, as in the Stomapod Crustacea (§ 464). The blood is sent forth from this organ, partly by systemic vessels directly proceeding from its segments, but chiefly by the dorsal trunk which forms its anterior con- tinuation ; and from this, on its entrance into the cephalo-thorax, numerous branches are given off to the various organs of sensation and motion, and also to the viscera contained in that division of the body. The blood thus distributed through the system is stated by M. Blanchard to find its why to the pulmonary branchiae by lacunar passages, neither ventral trunk nor a portal system of vessels having been yet made out ; and from the respi- ratory organs it is carried back to the heart by distinct branchio-cardiac canals, which discharge themselves into the several chambers of the mul- tiple heart. The more diffused the respiratory function is rendered, by the prolongation of the pulmonary branchiae into tracheae, the more does the circulation resemble that of Insects in the predominance of the lacunar over the vascular type. 464. It is among the Crustacea, that we find the sanguiferous system presenting the most developed form under which it exists in the Articu- lated series. In the lower orders, however, the segments of whose bodies are nearly alike throughout, the contractile portion of the dorsal vessel is elongated, and the distribution of its branches is nearly uniform in ea .-ii segment ; but the advance towards a higher type is shown in the order Stomapoda, the members of which have usually an elongated fusiform heart, developed as a muscular dilatation of a part of the dorsal vessel, which, towards its anterior and posterior extremities, possesses the charac- ter of a sanguiferous trunk. The same is the case also in the Limulvs. But in the order Decapoda, which includes the most elevated forms of this class, we find the heart contracted into a short fleshy sac, possessed of con- siderable muscular power, and concentrating in itself the propellent force which is diffused in the lower tribes through a large part or the whole length of the dorsal trunk. This organ in the Crab and Lobster is situated on the median line, at the posterior part of the cephalo-thorax (Fig. l-36.o) : from its anterior part is given off a large cephalic trunk (d) which passes forwards, and soon subdivides into branches for the supply of the eyes. * “ Annales des Sciences Naturelles ; ” November, 1849. CIRCULATION IN ANIMALS. G95 antennae, stomach, and neighbouring organs ; whilst from its posterior extremity is given off the abdominal or caudal artery (e), which is, of course, of largest dimensions in the ‘ macrourous’ Decapods. These two trunks, with the heart, obviously represent the entire dorsal vessel of the lower Articulata ; the transitional form being shown in the Scorpion. Besides these trunks, a pair of large hepatic arteries is given off from the sides of the heart, which are exclusively distributed to the liver ; whilst beneath the caudal artery, a large sternal trunk originates, which bends down towards the ventral aspect of the body, where it divides into an anterior and a posterior branch, the former of which supplies the thoracic members, whilst the latter distributes blood to the under surface of the abdomen. This may probably be regarded as homologous with the ventral trunk of the Scorpion, though it has not yet been shown to have any connection with the cephalic by aortic arches. The blood conveyed to the body by these arteries, appears to return through the lacunae of the tissues into two sets of large venous sinuses ; of which one consists of a series of flattened cavities, freely communicating with each other, which lies immediately beneath the shell of the back, covering the upper surface of the heart and dorsal trunks, and obviously representing the pericardiac sinus which incloses the dorsal vessel in other Articulata ; whilst the other series lies at the bases of the branchiae, on each side of the inferior surface of the thorax. The former collects the venous blood from the dorsal and caudal portions of the body, and carries it back to the heart, which it enters by two pairs of orifices, guarded by semilunar valves. The latter (which apparently corresponds to the ‘portal’ trunk of the Scorpion) collects the venous blood from the maxillie and legs, and distributes it to the branchiae for aeration. From these organs it is again collected by veins, which all coalesce in a pair of large trunks, the hranchio-cardiac canals, that convey the aerated blood back to the heart. Hence, in that central organ, the venous blood received from a portion of the body is mingled with the arterial blood that is transmitted direct from the gills ; and the fluid which is propelled through the systemic arteries is hence of a mixed character, as in Reptiles, — a class to which the Crustacea have many points of analogy. Thus the localisation of the respiratory organs in the higher Crustacea involves the existence of a special respiratory circulation. In the lower orders, however, the blood is aerated in its progress through the general system, as in Insects ; and in many of them, the arterial as well as the venous portion of the system appears to be alto- gether ‘lacunar.’ In one of the most degraded forms of the class, we revert to the simplest possible type of the Circulating apparatus ; even the dorsal vessel, which is so characteristic of the Articulata, being apparently deficient in the Pycnogonidce (§ 316 e). 465. From the view which has thus been taken of the Circulating ap- paratus in the Articulated series, it will be seen that, notwithstanding the diversity of detail, there is a very general conformity to a definite plan ; and that the tendency, as we ascend the series towards the higher forms of this apparatus, is to a concentration or specialisation in a single organ, of that impulsive power, which, in the lower tribes, w T as diffused through various parts of the system. Now when w r e follow a similar course with regard to the Mollusca, it will be seen that almost in the lowest forms of that series, the central organ is as powerful, and the circulation as much carried G96 SPECIAL AND COMPARATIVE PHYSIOLOGY. on through distinct vessels, as in the highest Crustacea. The explanation of this general inferiority of the circulation in the Articulated series is partly to be found, as we have already seen, in the general diffusion of the respiratory apparatus ; hut it seems to be in part connected with the mechanical conditions of the system, the movements of whose several parts furnish an additional means of propulsion to the blood. Even in the highest animals, the alternate contractions and relaxations of the muscular apparatus assist in maintaining the flow of blood through the veins, the position of which renders them peculiarly subject to this kind of influence : for in every instance in which muscular contraction takes place, a portion of the veins of the part must undergo compression ; and as the blood is prevented by the valves from being forced backwards, it is driven onwards towards the heart ; whilst, as each set of muscles is relaxed, the veins that were compressed by it fill out again, to be again compressed by a renewal of the force. Thus it is that the general muscular movements, even in Man, have an important influence in promoting the venous circulation ; so that continued exercise, involving the alternate con- traction and relaxation of several groups of muscles, sends the blood more rapidly towards the heart, and thus increases the rapidity of its pulsations ; whilst the sudden and simultaneous action of a large number of muscles after repose (as in rising up from the sitting or lying, to the standing posture), drives the blood to the centre of the circulation with a powerful impetus. Applying these facts to the case of Articulated animals, it becomes obvious that the general muscular movements, which are usually so active in them, may well be an important auxiliary to the circulation of then nutritive fluid, or may even be the principal means of maintaining it. " — In most of the instances in which we have hitherto found an organ of propulsion materially affecting the current of the cir- culation, it has transmitted the blood which it has received from the venous sinuses or canals, into the principal ‘ systemic arteries,’ and may therefore be designated as the systemic ventricle. Among the Annelida, however, the impelling cavities are frequently situated at the commence- ment of the branchial circulation, and are to be considered as representing the pulmonary ventricle of higher animals. In the Mollusca generally, we find superadded to the ventricle, an auricle or contractile cavity, adapted to receive the blood transmitted to the heart, and to propel '.t into the ventricle ; and the existence of these two cavities constitutes the * To use the language of Dr. Grant (Lectures on Comparative Anatomy), “ It is the rest- less activity of the Worm and of the Insect, that makes every fibre of their body as it were a heart to propel their blood and circulate their fluids. They require no complicated apparatus to accelerate the ever-active current of their blood, and hence the imperfect development ot the great centre of their vascular system. Indeed it lias been shown by Ehrenberg and by Nordnann, that, in the simplest of these animals, the trematoid Entozoa, the blood flows through the system by the mere motions of the body, without the least motion or impulse from the vessels which contain it. How differently circumstanced are the Mollusca ! 1 he inert Tunicata, the lowest of the Molluscous classes, fixed like plants upon the sea-beaten cliffs, and in which we can scarcely discover a trace of life, enclose in their motionless carcase a heart as highly developed as that of the Crustacea, the highest of the Articulated c.usr -s : and if they did not, their blood would soon stagnate in the complicated labyrinth of vessels and organs through which it has to pass. The slow-creeping snail that feeds upon the tu r f, has a heart as complicated as that of the red-blooded vertebrated fish which bounds with such velocity through the deep. It is because the fish is muscular and active in every point, that it requires no more heart than a snail to keep up the necessary movements of its blood. CIRCULATION IN ANIMALS. 697 typical character of the heart through nearly the whole of that sub- kingdom, although many variations are presented in their form and situ- ation. Notwithstanding this higher development of the heart, the rest of the circulatory apparatus remains in a state of relative incompleteness ; for in the lower classes of this series, the distribution of blood to the general system is almost entirely effected by lacunae, of which the visceral cavity forms part, it being only in the respiratory organs that it moves through distinct vessels ; and even in the higher tribes, in which the arterial part of the systemic circulation is generally truly vascular throughout, the venous portion of it is still in some degree lacunar. 466. Putting aside the Bryozoa,— in which there is no proper Circulation, the nutritive fluid that has transuded through the walls of the alimentary canal, being diffused through the general visceral cavity and passing into the tentacula (§ 298), — we commence our survey of the Circulating apparatus of the Mollusca with an examination of the condition which it presents in the class Tunicata. We hei’e find a distinct heart, whose parietes although veiy thin are muscular, usually of a somewhat elon- gated form, and generally situated in the neighbourhood of the ovarium ; but it is not yet divided into auricular and ventricular cavities • and the passages with which it communicates, are rather sinuses than proper vessels. In the long-bodied Polyclinians, the heart is situated at the extremity of the post-abdomen (Fig. 119, o) ; but in the Botryllians, which have no proper abdominal cavity, it is brought into much closer approximation with the pharyngeal sac (Fig. 121, o) and in the Saljxe it holds nearly the same relative position (Fig. 206, a, e). In the soli- tary Asddians, however, it usually lies between the pharyngeal sac and the ventral surface of the body, partly excavated in the muscular tunic (Fig. 116, B, a). In the curious osculant genus Pelonaia, there seems to be no heart, although the vascular portion of the circulating apparatus is more complete than usual ; but it is not improbable, that, as in many Articulata, to which that genus is related, the dorsal trunk is contractile throughout, and supplies the place of this organ. — The most curious feature in the action of the Circulating apparatus in this group, is the alternation which presents itself in the direction of the flow of blood ; and this renders it impossible to designate either set of vessels or passages as arterial or venous, since both are arterial, and both venous, in their turns. The contraction of the heart, in all the species which are sufficiently translucent to allow it to be observed during life, is somewhat peristaltic in its character, proceeding from one extremity of its cavity towards the other ; after the pidsations have continued in either direction for a minute or two, a short intermission takes place, during which the current of blood comes to a stand ; and the peristaltic contraction then recommences in the opposite direction, the flow of blood being now first directed to- wards those parts from which it had last returned. If the course of the circidation be watched in an individual of the Amaroucium (Fig. 119) separated from the common envelope, it will be seen that the blood is not propelled towards the thorax in distinct vessels, but that it passes through the space left between the viscera and the lining membrane of the cavity ; chiefly, however, along either its dorsal or its ventral aspect. When the ascending current takes the latter direction (which corresponds to its j course in the higher Acephala), it enters a large vertical canal, which lies G98 SPECIAL AND COMPARATIVE PHYSIOLOGY. in front of the branchial sac. and which is designated as the great thoracic or ventral sinus. From this sinus, a great number of vessels pass off transversely around the branchial sac ; and these are connected together by numerous communicating twigs, which pass between the branchial fissures : so that a minute network is thus formed, in which the blood is exposed for aeration to the water that enters the sac. From this network the aerated blood is collected into another sinus lying on the dorsal aspect of the body ; and this sinus also receives blood which has been trans- mitted to it, direct from the thoracic sinus, by means of a vessel that passes round the branchial orifice, so that the fluid which it contains is of a mixed character. From the dorsal sinus the blood returns to the heart, bathing the viscera in its course. When the reversal of the circulation takes place, the blood is first transmitted along the dorsal side of the body to the dorsal sinus ; thence it is distributed over the branchial sac ; and it finally returns to the heart by the ventral sinus. — The circulation in all the Tunicata is performed upon a plan essentially the same ; but its peculiarity in certain compound Clavellince deserves a special notice. In the Perophora (as first observed by Mr. Lister),* the several individuals are not included in a common ‘ test,’ but grow by footstalks from a common ‘stolon.’ Each of these stalks contains a double channel for the passage of blood, and these channels communicate in the common stem with those proceeding from other individuals ; so that a vascular com- munication exists among them all, and this extends also to the undeveloped buds which sprout forth from the stolons. One of the channels in each peduncle enters the heart of the animal which it supports ; and of the blood, when thus received into the propelling cavity, a great paid is transmitted along the ventral canal over the branchial surface, whilst the remainder is distributed to the visceral apparatus and the mantle. Both these currents reunite in the dorsal sinus, which then conducts the blood, not back to the heart again, but into the peduncle, in which it may be seen to flow towards the principal stem. As in other cases, however, a reversal takes place about every two minutes; the blood then flowing towards the body in the peduncular channel that communicates with the dorsal sinus, and re- turning from it in that which is continuous with the heart. Alien one of the animals was separated from its peduncle, the circulation was at first distiubed, but soon regained its usual regularity ; a new communication being apparently formed, by which a free passage of blood could take place between the dorsal sinus and the heart. — In the Salpidce, which, as formerly remarked (§ 299 6), present the nearest approximation to the bi- valve Acephala, we find a much more complete system of vessels, adapted to diffuse the blood over the general surface of the mantle, as well as upon the special respiratory organ. From the heart (Fig. 206, a. e) proceeds the ventral trunk (/'), which passes towards the anterior extre- mity of the body, giving off numerous branches at right angles on eitLer side, from which arise numerous smaller branches, the whole forming a minute net-work over the entire body (b). A separate set of vessels dis- tributes blood to the branchial membrane, where it undergoes aeration. From all these vessels it is collected into the dorsal sinus, which conveys it back to the heart. Its movement among the viscera, however, is entirely lacunar. The same alternation may be witnessed in the direction * “ Philosophical Transactions," 1834. CIRCULATION IN ANIMALS. G99 of the flow, in the Salpse, as in other Tunicata ; but here, as elsewhere, it has been noticed that what may be considered as the direct current, — i.e. the forward movement of the blood from the heart, along the ventral trunk, — is of much longer duration than the reverse, in which the blood is propelled from the heart through the dorsal trunk ; the number of pul- sations being usually about twice as great for the former as for the latter. Fig. 20G. Circulating apparatus of Salpa maxima, as seen from the side at a, and from the ventral sur- face at b ; — a, oral orifice ; b, vent ; c, nucleus, composed of the stomach, liver. &c. ; d, branchial lamina; e, heart, from which proceeds the longitudinal trunk f, sending tranverse branches across the body; g, g , projecting parts of the external tunic, serving to unite the different individuals into a chain. 467. In the Conchifera we find the circulation carried on upon the same general plan, but the central organs are more highly developed, and the systemic vessels are more complete. The ventricle or impelling cavity of the heart (Fig. 207, a) is a distinct sac, of which the walls are formed by muscular fibres interlacing in every direction, and even projecting into the interior. From this centre, which is situated between the adductor muscle and the rectum, the blood is sent by two principal arterial trunks, an anterior (d) and a posterior (e) to the system at large ; and thence it is returned, not by distinct veins, but by a system of lacunae in the substance of the foot (b), of the labial tentacula (e), of the adductor muscle (h), of the glandular organs, and of the viscera generally ; the contents of which are collected, in great part by a furrow (i) along the free margin of the mantle, into the afferent vessels ( h , li), by which it is conveyed to the branchiae. After being exposed in these organs to the action of the air contained in the surrounding water, it repasses to the heart by two large branchio-cardiac canals, which do not enter the ventricle, but terminate in auricles ( b ), of which one is usually placed on each side. The branchio- cardiac canals also receive the blood brought back by the venous sinuses from the general surface of the mantle, which still continues to act as a respiratory organ, although the proper branchiae of the LamelUbranchiate bivalves are undoubtedly the special instruments of this function. In the 700 SPECIAL AND COMPARATIVE PHYSIOLOGY. Brachiopoda, in which these special instruments are not developed, there are of course no proper branchial vessels ; and the blood is aerated by the Fig. 207. Pinna marina laid open, to show the arrangement of the Circulating apparatus : — a, mouth : b, foot; c, digitiform appendage to the foot; d, byssus ; e, labial tentacula ; f, f, branchiae, those of the right side being in place, those of the left being divided near their anterior extre- mity, and turned back, so as to expose their anterior surface; G, g, the mantle, of which the left lobe has been detached and folded back ; n, posterior adductor muscle; i, first stomach, covered by the liver; k, retractor muscles of the foot; l, anus; m, glandular organ, probably urinary- between this organ and the branchiae of the right side is seen the second stomach ; a. aortic ventricle; a', anterior portion of this ventricle, passing round the rectum ; b. one of the auricles turned back, the other being seen in its natural position on the opposite side of the ventricle; c, one of the branchio-cardiac canals ; the other is seen in front of the adductor muscle h ; d, anterior aortic trunk ; e, posterior aortic trunk; f f, palleal veins, proceeding to empty them- selves into the branchio-cardiac canals at g ; h, h, afferent vessels of the branchia ; t, canal of communication between these last and the general lacunar system of the abdomen. medium of the palleal vessels and sinuses alone. — On returning from these, it spreads itself through a large sinus that surrounds the visceral mass ; and from this it is received into the auricle on either side.* — * See Prof. Owen’s account of the Circulating Apparatus of Brachiopoda in the “Annaies des Sciences Naturelles,” Mai, 1845. It is from the admirable series of Memoirs on the Circulation in Mollusca, contributed by Prof. Milne-Edwards to the same Journal, that the sketch in the text is for the most part condensed. CIRCULATION IN ANIMALS. 701 Although the auricular cavity is double in most Bivalve Mollusca, it is not to be regarded as representing the two auricles of warm-blooded animals, of which one receives the blood from the system at large, and the other that which is transmitted from the lungs ; since here the two auricles have the same function, being both respiratory, and being merely repetitions of one another, separated for the sake of convenience. In the Oyster, in fact, they are united into a single cavity; whilst in another tribe, the Archidce, the ventricle is divided like the auricle, in conformity with the breadth of the back of the animal, and the consequent separation of the gills from one another ; and in the Brachiopoda, also, there are two ventricles at a considerable distance from each other. 468. Among the Gasteropoda, we find the same general arrangement of the circulating system ; but the situation of its centre, and the distribution of its vessels, present great variations in different orders (§ 303). The heart is composed of a ventricular and of an auricular sac (Fig. 1 2 7, p, q ) ; the former of which is usually provided with walls of considerable firmness, having muscular bands interwoven with its coats and projecting slightly into its cavity. In some Gasteropoda, as in Conchifera in general, the auricle -is double ; and in a few genera, even the ventricle is partly divided by the intestine which passes through it. The blood which is propelled from the ventricle by one principal systemic artery, or aorta (u), is dis- tributed by its branches to the various organs of the body ; in these it is dispersed through a capillary network possessing distinct parietes ; and from this network it passes, not into a proper venous system, but into a set of intercommunicating lacuna;, which even pour it into the visceral cavity, so that it bathes the external surface of the alimentary canal. From a part of this lacunar system, it is collected into the afferent trunks which distribute it to the respiratory organs, whether branchial or pulmo- nary ; and after having been aerated in these, it is returned to the auricle by the branchio-cardiac canals. Into these, however, is also poured the venous blood that is returned by a portion of the lacunar system, without passing through the respiratory organs ; so that the general plan of the circulation strongly resembles that which we have seen in the Crustacea (§ 464), only a part of the blood which is sent forth from, the heart, being subjected to the aerating process before returning to it again. The proportion which thus escapes aeration, however, seems to differ considerably in the several orders and genera of the class. — Many curious varieties in the arrangement of the vascular system of Gasteropoda might indeed be enumerated ; but it must here suffice to mention that they principally have reference to variations in the condition and position of the respiratory organs, and do not present any essential departures from the type just described. Thus in Haliotis and Patella, certain parts of the arterial system present the lacunar condition ; and in Thethys, the branchio-cardiac canals unite into an immense venous sinus, which occu- pies the dorsal region, and which is only separated from the visceral cavity by a thin membrane.*— In Gasteropod Mollusks, as in the higher * The attempt has been made by M. de Quatrefages to show that the Nudibranchiate Mollusca (§ 304 c) differ from other Gasteropoda in the deficiency of a proper venous system ; but this has been negatived, on the one hand, by the discovery that in the Gaste- ropoda in general the systemic venous circulation is lacunar ; whilst, on the other hand, the Nudibranchiata have been proved to possess, like other Gasteropoda, a proper branchio- cardiac canal. 702 SPECIAL AND COMPARATIVE PHYSIOLOGY. Crustacea, we find the liver very largely developed, and supplied with blood by a large arterial trunk (Fig. 127, o). We shall subsequently find that the function of the liver is in part supplementary to that of the respiratory organs ; and that the former organ is in general the largest, when the changes for which the latter are adapted, are least actively performed. 469. Hitherto we have usually found the respiratory organs, whether branchial or pulmonary, interposed between the capillaries of the system and the central propelling cavity ; the canals which collect the blood fr-in the different organs of the body, uniting only to distribute it again, wirh- out any fresh impulse being given to their contents. The only instance of the interposition of any thing like an impelling cavity, between the systemic and the respiratory vessels, was seen in certain Annelida, which are provided with branchial hearts (§ 459). Now in the class of Fishes, the heart is entirely respiratory, the arterial trunk which proceeds from it being distributed at once to the gills, and the blood which has been aerated in them being returned into a systemic artery or aorta, whence it proceeds to the body at large • and in the class of Cephalopoda we meet with a condition of the circulating apparatus, which manifestly establishes the transition between that of the Mollusca in general, and that which is peculiar to Fishes. The systemic heart of the Octopus consists of only one cavity or ventricle (Fig. 135, p). which is usually of a nearly globular form, tolerably strong and muscular, and exhibiting on its internal surface bundles of fibres ( cameos columned) interlacing with one another, as well as distinct valves protecting the orifices by which the blood enters it. The aorta (q), with other arteries which proceed from it, distributes arterial blood to the general system ; and this is returned by means of a regular system of veins possessing distinct parietes, of which one is seen at r. Of these, however, one pair opens directly into the visceral cavity, which thus, as in the inferior Mollusca, is made to take paid in the venous circulation. By the union of the systemic veins on either side, the blood is conveyed, not immediately to the gills, as in the other Mollusca, but to two super- added impelling cavities, or branchial hearts (s), one of which is situated in connection with each row of gills. These branchial ventricles are less powerful than that which propels the blood through the system ; but they are still sufficiently muscular and contractile to accelerate the circulation through the respiratory organs, and thus to prepare the blood for the maintenance of the increased muscular exertions which are required for the superior locomotive powers of these animals, as well as for the general activity of the functions of then- highly-organised bodies. The blood that has been thus impelled through the branchial arteries (s' ), is returned in an aerated condition by the branchial veins (t), which unite into a single branchio-cardiac canal on each side. These canals, before entering the ventricle, present dilatations (u, u), which have been usually regarded as mere sinuses ; but they haA T e been observed to be distinctly contractile, and must therefore be considered as representing the double amides of the lower Mollusca. Here, therefore, we have, sketched-out as it were, the complicated form of the vascular system in warm-blooded annuals possessed of a complete double circulation ; the trunks which convey the blood to the respiratory organs being furnished, like that which distributes it to the system at large, with an impelling cavity, by which a constant CIRCULATION IN ANIMALS. 703 and regular current is maintained. This structure, however, is peculiar to that order of Cephalopoda, which, in the general superiority of its organisation, its deficiency of external shell, and its possession of a rudi- mentary internal skeleton, exhibits many points of resemblance to Fishes (§ 307 b). In the Nautilus tribe (§ 307), on the other hand, whose general structure is more analogous to that of the inferior Mollusks, the vascular system presents nearly the same arrangement as in the Gastero- poda ; for the veins that return the blood from the system enter a common sinus, which has not, however, a distinctly muscular character, and does not seem to possess contractile powers ; and from this proceed the four branchial trunks which distribute the blood to the two pairs of gills, whence it is conveyed back to the heart or systemic ventricle by brancliio- cardiac canals. — From various parts of the systemic venous trunks, both in the Tetrabranchiate and Dibranchiate orders, a curious series of folli- cles or little sacs is seen to proceed, forming spongy masses, sometimes of considerable size (Fig. 135, r). The use of these is not certainly known. Their glandular aspect, and the distribution of arterial branches over their surface, joined with the peculiar character of the fluid found in them, has caused them to be regarded as secreting organs, destined to purify the circulating fluid ; and it has been thought probable that they are really Kidneys. 470. Although in Fishes we find the same simple conformation of the heart as that which exists in the Molluscous classes, the alteration in its position, relatively to the other parts of the vascular system, occasions its influence on the function of the cir- culation to be greatly modified. The blood expelled from the single ventricle (Fig. 174, a'), the cavity of which possesses firm muscular parietes, is carried at once to the gills through an arterial trunk, c, which presents a bulbous enlargement, a", at its origin ; this bulbus arteriosus, as it is termed, will be hereafter shown to exist at an early period of deve- lopment in the higher Vertebrata, and to conduce towards the formation of the two principal trunks, which are subsequently to arise from the heart (§ 488). This trunk subdivides into four or five branches on each side (F, c', c', c', Fig. 174, a), which run along the branchial arches, sending ramifications to every filament. After being thus aerated, the blood is collected by the branchial veins, d, into the great systemic artery or aorta , which then distributes it to the different organs of the body ; and thence it is returned to the auricle by the systemic veins (b', b", b "‘ ), which, before entering it, unite in a large 1-1 , , . /7 v ° . - ° me spiciu , tucuuc it is CU1- auatation, the sinus venosus (6). ihe circle just de- lected, and returned to the scribed, of which a plan is given in Fig. 208, appears simple in character ; but it possesses one peculiarity which is worth notice, as foreshadowing more important modifications in higher classes. Two or three small arteries are usually seen passing off from the branchial arches, so as to convey the pure aerated blood directly Diagram of the Circulating apparatus of Fishes ; — a, the auricle; b, the ventricle; c, the trunk supplying the bran- chial arteries, d; the aerated blood returning from the gills is conveyed by e, e, the branchial veins, to /, the aorta, which distributes it to the system ; thence it is col- 704 SPECIAL AND COMPARATIVE PHYSIOLOGY. to the head, instead of transmitting it to the general systemic trunk. It will be hereafter shown that a similar provision exists in the Crocodile, and has a very important pur-pose in its economy ; and that the same condition is manifested up to the termination of the embryo state of the higher Vertebrata, including the Human species. — Of the blood which is being returned by the veins from the systemic capillaries, a pail is diverted into another chamrel before reaching the heart ; for the veins of the digestive and generative organs, together with a part of those proceeding from the posterior part of the body and tail, reunite into trunks which convey the blood to the liver and kidneys ; and it is minutely distributed through these organs, in order that it may undergo purification by the elimination of their respective secretions. After this process has taken place, the blood is conveyed to the heart by the hepatic and renal veins, which enter the vena cava, or proceed direct to the sinus venosus. Thut the portal circulation, as it is termed, holds precisely the same relation to the general circulation in Fishes, as did the respiratory circulation in Crus- tacea ; being interposed, for the purification of the blood which has circu- lated through the system, between a part of its capillaries and the heait. 471. The foregoing is the general plan upon which the Circulating apparatus of Fishes is constructed ; but there are some remarkable departures from it, which require special notice. The most important of these is that which is presented in the AmpTiioxus (§321 a), the condition of whose sanguiferous system almost carries us back to the type of Eunice (§ 459) ; for the impelling power is not here concentrated in a single organ, but is distributed among a number of separate pulsatile dilatations developed upon the vascular trunks. Thus we have not only a principal branchial heart (Fig. 172, ov), which corresponds in its position to the heart of the higher Fishes; but there is also a minute contractile bulb at the origin of each branchial artery, so that there are from twenty-five to fifty of these bulbs on either side. The arterial arches, also, at the ante- rior extremity of the body, appear to be contractile. The venous system, too, is furnished with its own pulsatile dilatations ; for a venous heart is developed upon the vena cava or great dorsal vein, and another upon the trunk of the vena porta, which runs along the ventral side of the intestine. — Some traces of a similar arrangement may be seen in other Fishes, especially in those of the Cartilaginous group. Thus in Myxine there is a portal heart, which contracts regularly, and assists in maintaining the portal circulation ; in Chimcera, which has no bulbus arteriosus, each of the pair of large arteries given off from the aorta to the pectoral fins, has a contractile bulb at its origin ; and a pulsating dilatation is found at the extremity of the tail of the Eel , where it receives the blood from the delicate veins of the end of the caudal fin, and pz-opels it into the caudal vein. — There is another set of modifications, however, in the Circulating apparatus of Fishes, which conducts us towards the Reptilian type. For there are numerous instances in which the filaments on one or more of the branchial arches remain undeveloped ; so that the artery of that arch, instead of subdividing into capillaries, carries on the blood at once into the aorta ; and thus the fluid transmitted through that trunk to the system lias only in part undergone aeration, the head, however, being always supplied with pure arterial blood from the branchial veins. Con- currently with this change, we find some provision for atmospheric respira- CIRCULATION IN ANIMALS. 705 tion ; either in the advanced development of the air-bladder into a rudimentary lung, to which air gains access through a tracheal canal, as .in the Lepidosteus and Polypterus, the two existing representatives of the great Sciuroid family so abundant in the earlier periods of the Earth’s history ; or in the development of peculiar organs, analogous to these in function, but not homologous in structure, being saccular prolongations of the lining membrane of one of the gill-chambers, such as are found in the Cuchia, an eel-like fish of the Ganges, and in some others of the same tribe (§ 524). The blood is sent to these pulmonary organs by prolonga- tions of the branchial arteries, and is returned from them in an aerated state into the aorta. In some of the Fishes whose Reptilian affinities are the greatest, there is a slight indication of that subdivision of the bulbus arteriosus into two distinct trunks, which takes place during the develop- ment of higher animals (§ 488). 472. Quitting now those classes which are modified for existing in water, and passing on to the air-breathing Vertebrata, we find that very important modifications of the Circulating system are necessary, to adapt the animal to the conditions of atmospheric respiration. It is evident that the blood will be aerated much more rapidly when exposed to the air itself, than when merely submitted to the small quantity which is diffused through the watery element. If, therefore, the whole amount of the cir- culating fluid be thus exposed, the changes which it undergoes will be performed with such increased energy, that, if the other vital processes be made to conform to them, a “warm-blooded” animal is produced at once. But as Reptiles are intended to lead a life of comparative inertness, and to exist in circumstances which would be fatal to animals of higher organiza- tion, the respiratory process is reduced in amount, by the peculiar arrange- ment of the sanguiferous system now to be described. The ventricle of the heart is either single (which is the case in Batrachia) or it is divided by an imperfect septum (as in most of the higher orders), so that the blood which is poured into it from its two sides can mingle more or less freely iu its cavity. From this ventricle, a single truncus arteriosus is given off in Batrachia, which distributes the blood, by a series of arches very closely resembling the branchial arches of Fish, — partly to the system, through the cephalic arteries (Fig. 212, o, t t ) which come off from the first and second branchial arches [hr, 1 , 2), and through the aorta (av) which is formed by the union of the second pair with branches from the third, — and partly to the lungs through the pulmonary arteries (a/p). The dis- tribution of the blood in the higher Reptiles is very nearly the same in effect ; but the pulmonary and systemic trunks are kept distinct at their origin, by the division of the ‘ truncus arteriosus the former arising from the right, and the latter from the left side, of the partially divided ventri- cular cavity. Still the general appearance of branchial arches is preserved, as is shown in Fig. 181 ; and a part of the blood expelled by each con- traction of the ventricle, is sent to supply the requirements of the nutri- tive system, and a part is separated for aeration. The pure arterialised blood which returns from the lung is conveyed to the left auricle, whilst the venous blood which is transmitted by the systemic veins enters the right ; these two auricles are hence not repetitions of one another, but have distinct functions. Both empty themselves into the ventricle, in which the blood derived from these different sources is mixed, and from z z 706 SPECIAL AND COMPARATIVE PHYSIOLOGY. Fig. 209. which one part is again sent to the body, and another transmitted to the lungs, according to the plan represented in Fig. 209. The portal system of Reptiles corresponds •with that of Fishes, in the circumstance that the kidneys are supplied by it with venous blood, as well as the liver ; and also in deriving paid of its supply from the veins of the tail, posterior extremities, and genital or- gans, instead of (as in higher animals) from the veins of the digestive organs alone. The degree in which the renal, hepatic, and portal circula- tions are united, however, and in which they are supplied from the systemic veins, differs consider- ably in the several orders ; the closest approxima- tion to Fishes being presented (as might he anticipated) in the order Batrachia. 473. Although the foregoing may be regarded as the general type of the Circulating apparatus in the class of Reptiles, yet there are some very curious modifications of it, which connect it very closely with that of Fishes on the one hand, and with that of Birds and Mammals on the other. These are shown in the several genera of Peren- nibranchiate Amphibia, which, as formerly ex- plained (§ 323 a), present us with a very com- plete series of transitional forms connecting the two classes : and also in the progress of the meta- morphosis of the Batrachia, which in their tadpole or larva condition must be regarded as Fishes in eveiy essential particular of their organisation. Their circulation is for a time performed exactly upon the same plan as in that class ; the blood being transmitted from the simple bilocular heart to the branchial arches, then being propelled through the branchial filaments, and after aeration being circulated through the system. The mode in which the transition is effected from this condi- tion of the vascular organs, to that which they present in the perfect Reptile state of the animal, described in the preceding paragraph, will be rendered intelligible by the accompanying figures. In Fig. 210 is seen the arrange- ment of the parts before the metamorphosis has commenced. Three branchial trunks {ab, ab) pass off on each side of the heart, terminating in a minute capillary network which is contained in the branchial arches (br, 1, 2, 3), and by which the blood is aerated during the aquatic exist- ence of the animal ; from this network the returning vessels take then- origin, which unite into trunks (vb), one for each gill; and of these the first gives off the main arteries ( 1 1) to the head, while the second and third join to form the great systemic artery (av), as in Fishes. But. besides these vessels, there arc some small undeveloped branches (1, 2, 3), which establish a communication between each branchial artery and the returning trunk that corresponds with it. There is also a fourth small trunk (up) given off from the heart, which unites with another small branch from the aorta, to be distributed upon the (as yet) rudimentary lungs. After the metamorphosis has begun, however, by which the animal from a Fisli has to be converted into a Reptile, the branches that connect the arteries ot Diagram of the Circulation in 'Reptiles a, single ventricle, re- ceiving the aerated blood from b, the pulmonary auricle, and ve- nous blood from c, the systemic auricle; and propelling part of this mixed fluid to the pulmonary capillaries d, and part to the sys- temic capillaries, e. CIRCULATION IN ANIMALS. Fig. 210. 1 tot o vb 3 a op av c ab 2 Respiratory circulation of the Tadpole, in its first state : — a, arterial bulb, giving off three pairs of branchial arteries, ab, to supply the gills hr 1, br 2, br 3, from which return the branchial veins vb ; av, aortic trunk, formed by the union of two arches c, each of which receives the veins of the second and third pairs of branchiae ; t, t, arterial trunks proceeding to the head, derived from the first pair of branchial veins ; 1, 2, 3, communicating branches, between the branchial arteries and veins, as yet but little developed; ap, pulmonary arteries, almost rudimentary; o, orbital artery. the gills with their returning trunks are much increased in size, so that a large part of the blood flows continuously through them without being- sent to the gills at all, and the branchial vessels are themselves relatively diminished (Fig. 211); at the same time, the pulmo- nary trunk, which was before the smallest, becomes the largest, so that an increased proportion of blood is sent to the lungs. By a continu- ance of these changes, the branchial vessels gradually become obliterated, and the communicating branches, which were at first like secondary or irregular channels, now form part of the continuous line of the circulation (Fig. 212) ; the upper one sending off the cerebral vessels, the second and third uniting to supply the trunk, and the fourth passing as before to the lungs. — Turning from these to the Perennibranchiata, we find in the Lepidosiren a plan of circulation but little elevated above that which has been just described as existing in certain Fishes that present approxi- Fig. 211. br 2 3 ap av ap The same in a more advanced state ; the communicating branches and the pulmonary arteries being now greatly enlarged in proportion to the branchial vessels. Fig. 212. hr 3 The same in the perfect Frog ; the pulmonary arteries and the communicating branches being now fully developed, and the branchial vessels atrophied. z z 2 708 SPECIAL AND COMPARATIVE PHYSIOLOGY. mations to tlie Reptilian class. The ventricular cavity is single, and gives off but one primary trunk, which is furnished with a ‘ bulbus arteriosus.’ This trunk subdivides into six pairs of branchial arteries ; of which the 1st, 4th, 5th, and 6th are distributed to the branchial fringes; whilst the 2nd and 3rd are not thus distributed, their arches having no gills attached to them, but reunite to form the aortic trunk, first giving off, however, the pulmonary arteries. Thus the blood which finds its way into the aorta, consists partly of that which has been aerated in the branchiae, and partly of that which has passed to it direct from the heart. But that which is transmitted from the heart is itself of mixed quality, as in Reptiles ; for the pulmonary vein passes through the systemic auricle, and discharges itself directly into the ventricle, where its aerated blood is mingled with that returned by the systemic veins. — In the Proteus, the arrangement of the vascular system permanently resembles that which has been represented as intermediate between the larva and perfect condition of the Frog. This animal is provided with lungs slightly developed, as well as with permanent gills ; and the blood which is expelled from the ventricle is partly transmitted through the gills, partly finds its way directly into the aorta by means of the communicating branches, and a small quantity is transmitted to the lungs ; the latter is returned perfectly arterialised to the auricle here developed upon the pulmonary vein, and is afterwards mixed in the ventricle with the venous blood transmitted from the sys- temic auricle. 474. In many of the higher Reptiles, on the other hand, we find the cavity of the ventricle more or less perfectly divided into two ; and the pulmonary circulation more completely separated from the systemic. Thus, in the Lacerta ocellata (spotted lizard), whose ventricle is partly divided, the right side of it, into which the systemic auricle discharges itself, gives off the pulmonary trunks, so that a large proportion of the venous blood returned from the system is transmitted to the lungs for aeration ; and this being returned to the pulmonic auricle, is discharged into the left side of the ventricle, from which the systemic arteries pro- ceed. As long as there is any direct communication, however, between the two sides of the heart, it is obvious that a part of the blood returned from the systemic veins may be sent immediately into the aortic trunks, without being previously arterialised ; and in proportion to the degree in which the septum is complete, will be the approach of the animal towai ds the condition of the warm-blooded Vertebrata. The distribution of the vessels has a considerable effect upon the character of the fluid with which individual organs are supplied ; for in Reptiles which manifest this sepa- ration to a considerable extent, a part of the blood transmitted to the system has still a venous character, whilst that which is furnished to the brain and upper part of the body is purely arterial. This difference arises from the fact, that of the two arches which unite to form the aortic trunk, one is connected with the right and the other with the left side of the ventricle ; the latter receives chiefly the arterial blood from the left or pulmonary auricle, and this gives off branches which convey it without admixture to the head ; while the main trunk passes on to unite with the second aortic arch that arises from the right side of the heart, and conse- quently is supplied with blood almost entirely venous, which has been discharged from the system into the right auricle. This second arch, CIRCULATION IN ANIMALS. 709 before its union with the first, however, gives off a large branch, which is distributed to the intestines and other viscera, and which, therefore, contains venous blood with little admixture of arterial ; and the common aortic trunk, formed by the union of the two arches, conveys mixed arterial and venous blood to the remainder of the trunk and members. It is beautiful to observe how by these simple contrivances the greatest economy of material is obtained, whilst each organ is supplied with blood suf- ficiently oxygenated to maintain its functions. — The Crocodile presents us with a condition of the vascular system still more allied to that of warm-blooded Vertebrata ; the ventricular septum being complete, and the circulation, as far as the heart is concerned, being truly double. Still, however, whilst the principal aortic trunk arises from the left ventricle, which contains nothing but arterialised blood, a second arch arises from the right (or venous side) along with the pulmonary artery, of which it might almost be considered a branch ; and this, after giving off its intes- tinal branches, enters the first trunk, which has already furnished the cerebral arteries with pure arterial blood, and transmits the mixed fluid to the rest of the system. There is another communication between the trunks arising from the two sides of the heart, by means of an apertiu-e which passes through their adjoining walls just after their origin ; so that although the blood in the heai’t is entirely venous on one side and arterial on the other, it undergoes admixture in the vessels, according to the character of the functions to which it is to minister. We shall pre- sently see a remarkable analogy to this distribution of the vascular system, exhibited in the foetal condition of Birds and Mammalia (§ 489). 475. In the highest form of the Circulating system, that possessed by the ‘warm-blooded ’ Vertebrata, Birds and Mammals, there is a complete double circulation of the blood ; each portion of it, which has passed through the capillaries of the system, being aerated in the lungs, before being again distributed to the body. This is effected by a form of the vascular apparatus, of which a sketch was presented in the Cephalopoda (§469), and to which a near approach is exhibited by the higher Reptiles. The heart consists of four cavities, two auricles and two ventricles ; those of the right or venous side having no direct communication with those of the left or arterial side ; and the vessels proceeding from them being entirely distinct, and having no connection whatever except at their capillary terminations. The blood transmitted by the great veins of the system to the right auricle or receiving cavity, passes into the ventricle or propel- ling cavity, and is transmitted by it through the pulmonary arteries to the lungs of the two sides. After being there arterialised by exposure to the atmosphere, it is brought back to the left auricle ; and having been poured by it into the corresponding ventricle, is transmitted through the great systemic artery or aorta to the most distant parts of the body (Fig. 213). The heart is therefore completely duplex in structure, and, so far as its functions are concerned, might be regarded as consisting of two distinct portions ; for economy of material, however, these are united, the partition of the ventricles serving as the wall to each. In the Dugong (one of the aquatic Pachydermata ), however, the heart is bifid at its apex, and thus presents a partial division into two separate organs, not only functionally but structurally. — The portal circulation is limited in Mammalia to the liver, the kidneys being supplied with arterial blood only. In Birds, how- 710 SPECIAL AND COMPARATIVE PHYSIOLOGY. Fig. 213 . ever, we find a trace of that arrangement of this peculiar off-set from the general circulation, which has been pointed out as existing in Reptiles and Fishes ; for the great portal trunk receives its blood, not only from the veins of the digestive apparatus, as in Mammalia, but also by branches from those of the pelvis and posterior extremities : and it still communicates with the renal circu- lation, although this connecting branch seems rather destined to convey blood from than towards the kidney. 476. Various peculiarities in the distribution of the vascular system, which are presented by different orders of Birds and Mammals, would be worthy of notice if our limits permitted. Of these one of the most remarkable is the modification both of the venous and arterial trunks, existing in the Cetacea and other diving animals, which are occasionally prevented from respiring for some time, and in which, therefore, the arterial- isation of the blood is checked. Various arteries of the trunk are here found to assume a ramified and convoluted form, so that a large quantity of blood may be retained in the reservoirs formed Diagram of the Circulating ap- paratus in Mammals and Birds: — a , the heart, containing four cavities ; b, vena cava, delivering venous blood into c, the right , auricle; d , the right ventricle by these plexuses ; Wllllst the VeilOllS trunks eX- hibit similar dilatations, capable of being dis- capiiiaries of the lungs; g, the tended with the blood which has been transmitted left auricle, receiving the aerated blood from the pulmonary vein, and delivering it to the left ventricle, h, which propels it, through the aorta, i, to the sys- temic capillaries, j, whence it is collected by the veins, and car- through the system, so as to prevent the heart being loaded with the impure fluid, whilst the lungs have not the power of arterialising it. In some diving animals, this object is effected, not lied back to the heart through so much by a number of venous plexuses, as by a single great dilatation of the vena cava before it enters the heart, resembling the ‘ sinus venosus’ of Fishes. — In other in- stances, the force with which the blood is sent to particular organs seems to be pinposely diminished, by the division of the trunk that conveys it into a number of smaller vessels, which, after a tortuous course, unite again and are distributed in the usual manner. A struc- ture of this kind is found in the arteries of the long-necked grazing animals, to which the blood would be transmitted with too great an impetus, owing to the additional influence of gravitation, were it not retarded by such a contrivance. A similar distribution of the arteries is found in the trunks supplying the limbs of the Sloths, and of other animals which resemble them in tardiness of movement. In other cases, the arterial canals are specially protected from compression by surround- ing organs, in order that there may be no obstruction to the passage of blood through them, and that they may be guarded from injury ; thus, in the fore leg of the Lion, where all possible force and energy is to be attained, the main artery is made to pass through a perforation in the bone, as if to secure it from the pressure of the rigid muscles, which when in a state of contraction, might otherwise altogether check the current through it. In most Mammals, as in Man, the right anterior extremity is CIRCULATION IN ANIMALS. 711 more directly supplied with blood from the aorta than the left ; so that the superior strength and activity of this limb would seem to be not alto- gether the result of habit and education, as some have supposed ; in Birds, however, where any inequality in the powers of the two wings would have prevented the necessary regularity in the actions of flight, the aorta gives off its branches to the two sides with perfect equality. Some further peculiarities in the distribution of the arterial system will be hereafter noticed (§ 492). 477. Having now traced the Sanguiferous system to its highest form, it is proper to inquire how far this differs functionally from that simple condition which it presents in the lowest forms in which it exists. There can be no doubt that, in the higher animals possessed of a distinct muscular heart, this is the chief agent in keeping up, by its successive contractions and dilatations, the motion of the blood through the vessels. But a careful survey of all the phenomena of the circulation would seem to lead to the conclusion, that the impulse of the heart is not the only means by which the motion of the blood is sustained ; but that an additional impulse is given by the contraction of the muscular walls of the arteries upon the jets of blood successively impelled into them by the heart ; and that the changes which this fluid undergoes in the capillaries have some share in its production, and have at any rate a very consider- able modifying effect upon the quantity transmitted through the indi- vidual organs. We have seen that in Vegetables the laticiferous cir- culation is entirely capillary ; that in the Radiata and lower Articulata it is chiefly so, the pulsatile action of some arterial trunk being the only force which can extend itself through the system, and this being too feeble to have much influence ; whilst even in the higher Articulata and in all but the Cephalopod Mollusca, so large a part of the systemic circu- lation is £ lacunar,’ that it seems impossible to imagine that the action of the heart can impel the blood through the branchial vessels which succeed. Some more ‘ diffused ’ forces, therefore, would seem to be in operation ; and the following are among the facts which appear to support the con- clusion, that, even in the highest animals, these most general forces are not obliterated, but are merely superseded by the energy of the special organ, which is developed as the centre of the whole circulation, and is endowed with an amount of power sufficient to govern and harmonise the numerous actions going on in different parts of the system. 478. In many warm-blooded Vertebrata, and still more in the cold- blooded Reptiles (amongst which the vitality of individual parts much longer survives injury to the general system), motion of blood in the capillaries has been seen to continue for some time after the heart has ceased to act, or has been removed, or after the great vessels have been tied ; and this motion may be immediately checked by certain applica- tions to the parts themselves. After most kinds of slow natural death, the arterial trunks and left side of the heart are found to be almost or even completely empty, and the venous cavities full of blood. This effect has been ascribed to the contraction of the arterial tubes after the heart has ceased to beat ; but it seems impossible that it can be entirely due to that cause, since their calibre is not found to have diminished in a pro- portional degree ; it must be partly attributed to a continuance of the capillary movement, after the general systemic circulation has ceased. 712 SPECIAL AND COMPARATIVE PHYSIOLOGY. The continuance of various processes of secretion and even of nutrition, subsequently to general or somatic death, affords an excellent proof of this lingering vitality ; and it is scarcely possible that these can be maintained without some degree of capillary circulation. There are some kinds of sudden death, however, in which the vitality of the whole system appears to be simultaneously destroyed, and the blood remains in the vessels as it was at the moment of decease. — Further, a careful examination of the circulation in the living animal discloses many irregularities in the rate of the capillary currents, which it is impossible to attribute to an influence derived from the heart or from the vessels that supply them ; and sue] > variations may present themselves, either in the capillary network of a part, or in a portion of it ; the circulation taking place with diminished rapidity in one part, and with increased energy in another, though both are supplied by the same trunk. The change sometimes extends to a complete reversal of the direction of the movement, in certain of the transverse or communicating branches ; this movement taking place, of course, from the stronger towards the weaker current : and not un fre- quently an entire stagnation, of longer or shorter duration, precedes the alteration of the direction. Irregularities of this kind are most frequent, when the heart’s action is enfeebled or partially interrupted ; and it would thus appear, that the local influences by which they are produced, are overcome by the propelling power of the central organ, when this is acting with its full vigour. When the whole current has nearly stagnated, and a fresh impulse from the heart renews it, the movement is seldom uniform through the entire plexus supplied by one trank ; but is much greater in some of the tubes than in others, — the variation being in no degree connected with their size, and being very different in its amount at short intervals. 479. Amongst the most remarkable proofs of the influence of forces generated in the capillary circulation, on the general distribution of the blood, is one derived from the observation of organs which undergo changes in activity that are quite independent of alterations in the heart s action. Thus, when the uterus begins to develope itself during pregnancy, the unusual activity of the capillary circulation occasions an increased demand for blood, which is supplied by an increase in the diameter of the trunks that transmit fluid to the organ ; and this is entirely independent of any increased energy in the heart’s action, which wovdd have affected the whole system alike. The same may be said of the occasional develop- ment of the mammae for the secretion of milk ; of the rush of blood through these organs during the act of suckling ; and of similar changes in other parts, of which the activity is not constant or uniform. In certain diseased states, also, of particular portions of the system, which do not occasion any appreciable alteration in the heart’s action, the quantity of blood sent to the part is much increased, and the pulsation of the arterial trunk leading to it is evidently stronger than that of the corre- sponding vessels on the outside of the body. These phenomena, and many others which might be mentioned, are evidently analogous to one formerly stated as having been ascertained by experiments on Plants (§ 44(3) : and. when taken in connection, they seem to indicate without much doubt, that the quantity of blood sent to individual organs, and the force with which it is transmitted through them, are augmented with any increase "f CIRCULATION IN ANIMALS. 713 energy in the vital processes taking place in them, the vis a tergo derived from the impulsive power of the heart remaining the same. — Additional evidence of the influence of the forces generated in the capillaries on the general circulation, is derived from cases in which the normal changes to which the capillary circulation ministers are suspended, and in which it then appears that the heart’s impulse is not alone sufficient to maintain the current of blood. One of the most conclusive of these proofs is drawn from the phenomena of A sphyxia, or suffocation ; since it now seems distinctly ascertained, that the check given to the circulation, and thence to all the other functions, arises from the stagnation of the blood in the capillaries of the lungs, consequent upon the cessation of the reaction be- tween that fluid and the air.* So, again, cases of spontaneous gangrene of the lower extremities are by no means of unfrequent occurrence, in which the local stagnation of the circulation has been clearly dependent upon the cessation of the nutrient actions to which it was subservient ; it being- found, by examination of the limb after its removal, that both the larger tubes and the capillaries were pervious throughout, so that no mechanical impediment existed, to jirevent the propulsive power of the heart from transmitting the blood through them. The influence of the prolonged application of Cold to a part, may be referred to in support of the same general proposition ; for although the calibre of the vessels is diminished by this agent, yet their contraction is not sufficient to account for that complete cessation of the flow of blood through them, which precedes the entire loss of their vitality. — A periodical retardation or suspension of the circulation in particular portions of the body, unaccompanied by any other ostensible change, and not dependent upon any failure of the heart’s power, is by no means an uncommon phenomenon. It frequently presents itself, for example, in one of the fingers ; and a curious case is recorded by Dr. Graves,! in which the whole of one leg was thus affected, with remark- able periodicity, for about twelve hours out of the twenty-four ; whilst in the intervals the circulation in the limb was unusually active, the action of the heart being quite natural throughout, and the circulation in the rest of the body not being in the least affected. 480, In the development of the embryo of the higher Yertebrated ani- mals, moreover, there is a period at which a distinct movement of red blood is seen, before any pulsating vessel can be detected to possess an influence over it (§ 484). Further, instances not very unfrequently occur, of foetuses having attained nearly their full development, which have been- unpossessed of a heart, and in which the circulation has been, as it were, entirely capillary : and although in most, if not all, of these cases, the monster has been accompanied by a perfect child, the heart of which may have been suspected to have influenced its own circulation, yet, in one of those most recently examined, the occurrence of this lias been disproved. From a careful examination of the vascular system, it ap- peared impossible that the heart of the twin foetus could have caused * For a fuller discussion of this part of the subject than the limits of this Treatise per- mit, see the Author’s 41 Human Physiology,” and his Article on Asphyxia in the “ Library of Practical Medicine.” See also the very conclusive experiments of his late valued friend Dr. John Reid, in the “ Edinb. Med. and Surg. Journal ” for April 1841 ; and Dr. Reid’s *’ Physiological, Pathological, and Anatomical Researches,” chap. ii. t “ Lectures on Clinical Medicine,” second edition, vol. i. p. 73. 714 SPECIAL AND COMPARATIVE PHYSIOLOGY. the movement of blood in the imperfect one ; and this must, therefore, have been maintained by forces arising out of the nutritive changes occurring in the capillaries.* 481. All these circumstances inchoate that the movement of blood through the Capillaries is very much influenced by local forces ; although these forces are not sufficiently powerful, in the fully developed state of the higher animals, to maintain it by themselves. And from other facts it appears, that the conditions necessary for the energetic flow of blood through these vessels, are nothing else than the active performance of the nutritive and other operations, to which its movement is subservient, — The principle already noticed (§ 449) as having been developed by Prof. Draper, seems fully adequate to explain these phenomena. It will be convenient to take the Respiratory circulation as an example of its application ; since the changes to which this is subservient are more sim- ple than those which take place elsewhere. The venous blood transmitted to the lungs, and the oxygen in the pulmonary cells, have a mutual attraction, which is satisfied by the exchange of oxygen and carbonic acid that takes place through the walls of the capillaries ; but when the blood has become arterialised, it no longer has any such attraction for the air. The venous blood, therefore, will chive the arterial before it, in the pul- monary capillaries, whilst respiration is properly going on : but if the supply of oxygen be interrupted, so that the blood is no longer aerated, no chance in the affinities takes place whilst it traverses the capillary network ; the blood, continuing venous, still retains its need of a change and its attrac- tion for the walls of the capillaries ; and its egress into the pulmonary veins is thus resisted, rather than aided, by the force generated in the lungs. In the Systemic circulation, the changes are of a much more complex nature, every distinct organ attracting to itself the peculiar sub- stances which it requires as the materials of its own nutrition ; and the nature of the affinities thus generated will be consequently different in each case. But the same law holds good in all instances. Thus, the blood conveyed to the liver by the portal vein, contains the materials at the expense of which the bile-secreting cells are developed ; consequently the tissue of the liver, which is principally made up of these cells, possesses a certain degree of affinity or attraction for blood containing these mate- rials ; and this is diminished, so soon as they have been drawn front it into the cells around. Consequently the blood of the portal vein will drive before it, into the hepatic vein, the blood which has already tra- versed the capillaries of the portal system, and which has given up. m doing so, the elements of bile to the solid tissues of the liver. 482. We are now prepared, therefore, to understand the general prin- ciple, that the rapidity of the local circulation of a part will depend in great measure upon the activity of the functional changes taking place in that part, — the heart’s action, and the state of the general circulation, * For the details of this interesting case, which was communicated by Dr. Houston cl Dublin to the British Association in 1836, see the “British and Foreign Medical Review, vol. ii. p. 596, and the “ Dublin Medical Journal ” for 1 837. An attempt was made by Dr. Marshall Hall (“ Ediub. Monthly Journal,” 1843) to disprove Dr. Houston’s inferences . out a most satisfactory reply was made hv Dr. H. in the “ Dublin Journal ” for Jan. 1844. See also the “ Edinb. Med. and Surg. Journal,” July, 1844. — A similar case is recorded by Dr. Jackson, of Boston (N.E.) in the “ American Journal of the Medical Sciences.” Feb. 1838. CIRCULATION IN ANIMALS. 715 remaining the same. When, by the heightened vitality, or the unusual exercise, of any organ, the changes which the blood naturally undergoes in it are increased in amount, the affinities which draw the arterial blood into the capillaries are stronger, and are more speedily satisfied, and the venous blood is therefore driven out with increased energy. Thus a larger quantity of blood will pass through the capillaries of the part in a given time, without any enlargement of their calibre, and even though it be somewhat diminished • but the size of the arteries by which it is conveyed soon undergoes an increase, which adapts it to supply the increased de- mand. Any circumstance, then, which increases the functional energy of a part, or stimulates it to increased nutrition, will occasion an increase in the supply of blood, altogether irrespectively of any change in the heart’s action. This principle has long been known, and has been ex- pressed in the concise adage “ Ubi stimulus, ibi fluxus which those Physiologists, who maintain that the Circulation is maintained and governed by the heart alone, cast into unmerited neglect. 483. The development of that Circulating system which has been described as peculiar to the higher classes of Vertebrated animals, is not completed until the moment of birth ; and the progressive changes which the heart and vascular apparatus undergo, in the evolution of the foetus of Birds and Mammals, afford a most beautiful illustration of the principles already laid down (chap, viil), respecting the amount of correspondence between the transitory stages of each system in the higher animals, and the forms permanently exhibited by the lower. It has been seen that in the organs of Circulation, as well as in all others, the tendency, as we rise from their lowest to their highest condition, is one of specialization. In the simplest Animals, as in Plants, whatever motion of fluid takes place is effected by each individual part by and for itself ; -whilst in the complex and highly-developed structures that occupy the other extremity of the scale, the development of a powerful organ of impulsion, the influence of which extends over the wdiole system, has superseded the diffused agency by which the circulation was previously maintained. This progress from a more general to a more special type is equally manifested in the vascu- lar system of the embryo ; and the analogy which thus arises between the forms it presents at different epochs of its development, and those pre- sented by the lower tribes of animals, is not superficial only, but extends even to minute particulars. The egg of the Bird affords the best opportu- nity for studying the early changes which it undergoes, and these have been described with great minuteness ; but such a sketch of them only can here be given, as wall serve to illustrate the principles alluded to. The preliminary stages of the process will be described in their proper place. 484. At an early period of incubation, the yelk is found to be enveloped by a “germinal membrane,” composed of distinct cells, which is divisible into three layers ; and a thickened portion of this is easily distinguishable, at which the embryo will be subsequently evolved. The middle layer gives origin to the Circulating system, and is therefore termed the vascular layer. The thickened portion of this that surrounds the germ, soon be- comes studded with numerous irregular points and marks of a dark yellow colour ; and as incubation proceeds, these points become more apparent, and are gradually elongated into small lines, which are united together, first in small groups, and then into one network, so as to form what is 716 SPECIAL AND COMPARATIVE PHYSIOLOGY. Fig. 214. called the Vascular area (Fig. 214). A large dark spot of a similar kiud is seen in the situation to be subsequently occupied by the heart. These dark points and lines are formed by collections of blood-corpuscles, which originate in the transformation of the cells of the embiyo and of the ger- minal membrane ; and the rows and masses of blood-disks seem at first to lie in mere channels, the walls of the heart and blood-vessels that subse- quently enclose them, being of later formation. From the first, however, a definite plan is perceptible ; the network of capillaries that is formed over the vascular area, being sup plied with blood by the ramifica- tions of a pah - of arterial trunks, ff; whilst the blood is collected from them by the circular venous sinus which bounds the area, and is returned to the embryo by the venous trunks g, g. In the blood- vessels which are first observed in the body of the embiyo, as well as in the vascular area, no difference is at first perceived between the characters of the arteries and tin >se of the veins ; and these are oniy to be distinguished by the direc- tion of the currents of blood cir- culating through them. But at about the fourth or fifth day of incubation, the coats of the arte- ries begin to appeal - thicker than those of the veins, and the dis- tinction between them soon be- comes evident. After the principal vessels are formed, the develop- ment of new ones appears to take place in two modes, according as they are to occupy the interspaces existing among those previously formed, fl- are to extend themselves into out-growing pails. In the first of these cases, the new capillaries appear to be formed, like the original ones, fr< m stellate cells, whose prolongations meet the vessels in which blood is already circulating, coalesce with them, and thus receive the current into their own cavities, to transmit it to some other vessel (§ 221, Fig. 54). But in the second, the new vessels are formed entirely by extension from those already existing. This takes place in the following mode. " Sup- pose a line or arch of capillary vessel passing below the edge or surface of a part to which new material has been superadded. The vessel will at first present a slight dilatation in one, and coincidently, or shortly after, in another point, as if its wall yielded a little, near - the edge or surface. The slight pouches thus formed gradually extend, as blind canals or diverticula, from the original vessels, still directing their course towards the edge or surface of the new material, and crowded with blood-corpus- cles, which are pushed into them from the main stream. Still extending, they converge, they meet; the partition- wall, that is at first formed by the meeting of their closed ends, clears away ; and a perfect arched tube is formed, through which the blood, diverging from the main or former Vascular Area of Fowl's egg, at the beginning of the third day of incubation ; — a, a , yelk; b, b , b, b, venous sinus bounding the area; c , aorta ; d, punctum saliens, or incipient heart; e, e, area pellucida ; /,/, arteries of the vascular area; g,g, veins; h, eye. CIRCULATION IN ANIMALS. 717 stream, and then rejoining it, may be continuously propelled.”* — This last process may be seen in the growing parts of the tail of the Tadpole, in the development of the filamentous gills and legs of the Water-Newt, in the first evolution of the extremities of the embryoes of higher animals, and in the formation of new structures in the fully developed organism, either for the repair of injuries, or as the result of morbid processes. In some instances it would appear that the wall of the newly-forming vessel gives way, and that the blood-corpuscles escape from it into the paren- chyma, at first collecting in an undefined mass, but soon manifesting a definite direction, and coming into connection with another portion of the arch or with some adjacent vessel. Thus, then, a channel and not a vessel is formed ; and it is probably in this way that those passages are exca- vated, which take the place of distinct vessels in many of the lower tribes of animals, and also, according to Mr. Paget, in some of the softer and least organised morbid growths in Man.t 485. The first rudiment of the heart appears about the 27th hour, and is a mass of cells, of which the innermost soon break down, so as to form a tubular cavity ; for some time it is simple and undivided, extending, how- ever, through nearly the whole length of the embryo ; but the posterior part may be regarded as corresponding with the future auricle, since prolonga- tions may be perceived extending from that part into the transparent area, which indicate the place where the veins subsequently enter. Although the development has proceeded thus far at about the 35th hour, no motion of fluid is seen in the heart or vessels until the 38th or 40th hour. When the heart, which may be considered as analogous at this period to the dorsal vessel of the Annelida, first begins to pulsate, it contains only colourless fluid mixed with a few globules. A movement of the dark blood in the circumference of the vascular area is at the same time perceived ; but this is independent of the contractions of the heart, and it is not until a sub- sequent period that such a communication is established between the heart and the distant vessels, that the dark fluid contained in them arrives at the central cavity, and is propelled by its pulsations. This fact, which we have just seen to possess a very important bearing on the theory of the circulation, and which has been denied by some observers, appears to have been positively established by the latest researches of Yon Baer, j; — The contraction of this dorsal vessel (for so it may be termed) begins, as in the Annelida, at its posterior extremity, and gradually extends itself to the anterior ; but, between the 40th and 50th hours, a separation in its parts may be observed, which is effected by a constriction round the mid- dle of the tube ; and the dilatation of the posterior portion becomes an auricular sac, and that of the anterior a ventricular cavity. Between the * See Mr. Paget’s “ Lectures on Reproduction and Repair,” Medical Gazette, 1849. + Op. cit. + He says that there is no doubt of the blood being formed before the vessels. The formation of the blood goes on in every part of the body ; and when formed, it is put in motion by some unknown cause that impels it in the proper direction, until at length it reaches the central formation of blood, around which is developed a tubular canal afterwards to be further modified and changed into a heart. The first motions of the blood are towards the heart, and consequently the first vessels formed are veins; a fact of itself sufficient to disprove the hypothesis that the motive power which presides over the circulation resides exclusively in the ventricles of the heart. “ Uber Entwiekelungsgeschicte der Thiere,” &c. Konigsberg, 1837, part ii. p. 126. 718 SPECIAL AND COMPARATIVE PHYSIOLOGY. •50th and GOtli hours, the circulation of the blood in the vascular area becomes more vigorous, and the action of the ventricle is no longer con- tinuous with that of the auricle, but seems to succeed it at a separate period. At the same time the tube of the heart becomes more and more bent together until it is doubled ; so that this organ now becomes much shorter relatively to the dimensions of the body, and is more confined to the portion of the trunk to which it is subsequently restricted. The convex side of the curve which the tube presents, is that which subse- quently becomes the apex or point of the heart : aud, between the 60tli and 70th hours, this is seen to project forward from the breast of the embryo, much in the situation it subsequently occupies. About the same time, the texture of the auricle differs considerably from that of the ven- tricle ; the auricle retaining the thin and membranous walls which it at first possessed, while the ventricle has become stronger and thicker, both its internal and external surfaces being marked by the interlacement of muscular fibres, as in the higher Mollusca. About the 65th hour, the grade of development of the heart may be regarded as corresponding with that of the Fish, the auricle and ventricle being perfectly distinct ; but their cavities are as yet quite single. — The heart of the Dog at the 21st day bears a great resemblance to that of the chick at the 55th or 60th horn ; it consists of a membranous tube twisted on itself and partially divided into two principal cavities, besides the bulb or dilatation which at this period is found at the commencement of the aorta, and which con-e- sponds with the bulbus arteriosus of Fishes. 486. Having thus traced the evolution of the heart of the Chick up to the grade which it presents in Fishes, we may uow enquire what is the condition of the other parts of the vascular system at the same time. At the end of the second day, the primitive arterial tmnk (Fig. 215, a) is seen to have divided into two canals (1, 1',) which separate from one an- other to enclose the pharynx, and then unite again to form the aortic tmnk, a, which passes down the spine. During the first half of the third day (about the 60th hour), a second pair of arches, 2, 2', is form- ed, which encompasses the pharynx in the same manner ; and towards the end of the third day, two other pail’s of vascular arches, 3, 3' and 4, 4', are formed ; so that the pha- rynx is now encompassed by four pail’s of vessels, which unite again to supply the general circulation. These evidently correspond with the bran- chial arteries of Fishes, although no respiratory apparatus is connected with them ; and in fact the distri- bution of the vascular svstem of Fig. 215. Diagram of the formation of the great Arterial trunks in the Chick : — a, descending aorta; a', ascending aorta ; p, p, pulmonary arteries ; r', their common trunk ; a, truncus arteriosus ; b, b ', subclavian arteries; c, c', carotid arteries; 1, l', 2 , 2 ', 3 , 3 ', 4 , 4 ', 5 , 5 ', five pairs of branchial arches; 6,6', 7,7', communicating branches which become the trunks of the carotids; 8,8', communicating branches afterwards obliterated; 9, ductus arte- riosus; 10, aortic arch of the left side. CIRCULATION IN ANIMALS. 719 the Bird, on the fourth and fifth days, exactly resembles that presented by many Cartilaginous Fishes, as well as by the tadpoles of the Batrachia. The first pair of arches is obliterated about the end of the fourth day ; but a pair of vessels, which is sent from it to the head and neigh- bouring parts, and which afterwards remains as the carotid arteries, c,c', continues to be supplied through a communicating vessel, 6, 6', from the second arch. While the first pair is being obliterated, a fifth, 5, 5', is formed behind the four which had previously existed ; and proceeds, in the same manner as the fourth, from the ascending to the descending- aorta, On the fourth day, the second arch also becomes less, and on the fifth day is wholly obliterated ; whilst the third and fourth become stronger. From the third arch, now the most anterior of those remaining, the arteries are given off which supply the upper extremities, b, b' ; and the vessels of the head, c, c', are now brought into connection with it, by means of the communicating branches, 7, 7', which previously joined the third with the second arch. When these vessels are fully developed, the branches, 8, 8', by which these arches formerly sent their blood into the aorta, shrink and gradually disappear ; so that, by about the thirteenth or fourteenth day, the whole of the blood sent through the two anterior arches (the second and third) is carried to the head and upper extremities, instead of being transmitted to the descending aorta as before. There now only remain the fourth and fifth pair of branchial arches, the develop- ment of which into the aorta and pulmonary arteries will be described in connection with the changes, which are at the same time going on in the heart. 487. During the fourth day, the cavities of the Heart begin to be divided, for the separation of the right and left aiu-icles and ventricles. About the 80th hour, the commencement of the division of the auricle is indicated externally, by the appearance of a dark line on the upper part of its wall ; and this, after a few hours, is perceived to be due to a contrac- tion, which, increasing downwards across the cavity, divides it into two nearly spherical sacs. Of these the right is at first much the larger, and receives the great systemic veins ; the left has then the aspect of a mere ap- pendage to the right, but it subsequently receives the veins from the lungs, when these organs are developed, and attains an increased size. The sep- tum between the auricles is by no means completed at once ; a large aper- ture (which subsequently becomes the foramen ovale ) exists for some time at its lower part, so that the ventricle continues to communicate freely with both auricles. This passage is afterwards closed by the prolongation of a valvular fold, which meets it in the opposite direction ; it remains pervious, however, until the animal begins to respire by the lungs, and sometimes is not convpletely obliterated even then. The division of the ventricle commences some time before that of the auricle, and is effected by a sort of duplicature of its wall, forming a fissure on its exterior and a projection on its interior ; and thus a septum is gradually developed within the cavity, which progressively acquires firmness, and rises higher up, until it reaches the entrance to the bulb of the aorta, where some communication exists for a day or two longer. At last, however, the division is complete, and the inter-ventricular septum becomes continuous with the inter-auricular, so that the heart may be regarded as completely a double organ. The progressive stages presented in the development of 720 SPECIAL AND COMPARATIVE PHYSIOLOGY. this septum are evidently analogous to its permanent conditions in the various species of Reptiles (§ 473-4) ; but it must not be lost sight of that in all Reptiles the inter-auricular septum is first developed, and that it is completely formed in many instances in which the inter-ventricular septum is absent or imperfect. — The changes which occur in the heart of the Mammalia are of a precisely similar character ; and, as they take place more slowly, they may be watched with greater precision. Soon after the septum of the ventricles begins to be formed in the interior, a corresponding notch appears on the exterior, which, as it gradually deepens, renders the apex of the heart double. Tins notch between the right and left ventricles continues to become deeper until about the eighth week in the Human embryo, when the two ventricles are quite separated from one another, excejjt at their bases ; this fact is very interesting, from its rela- tion with the similar permanent form presented by the heart of the Dugong (§ 475). At this period, the internal septum is still imperfect, so that the ventricular cavities communicate witli each other, as in the chick on the fourth day. After the eighth week, however, the septum is complete, so that the cavities are entirely insulated ; whilst at the same time their external walls become more connected towards their bases, and the notch between them is diminished ; and at the end of the third month the ventricles are very little separated from one another, though the place where the notch previously existed is still strongly marked. 488. Returning again to the distribution of the arterial tranks, we are now prepared to follow their final modification, by which they are adapt td to the existence which the individual is soon to commence as an air- breathing animal.— The first, second, and third branchial arches have been shown to be replaced by the brachial and carotid arteries, and to have lost all communication with the primitive arterial trunk (Fig. 215, a) except at its commencement, where the third pair of arches arises with tlw other trunks from its dilated bulb. This remains as a single cavity even after the ventricles have been separated ; but towards the end of the fifth or the beginning of the sixth day, in the Chick, the bulb becomes flattened, and the opposite sides adhere together, so as to divide it into two tubes running side by side. Of these, one communicates with the left, and the other with the right ventricle. The former, which subsequently becomes the ascending aorta (a'), is continuous with the fourth branchial arch ( 1 ) on the right side only ; but from this the carotid and brachial arteries arise by two principal trunks. This arch becomes gradually larger, so as to form the freest mode of communication between the heart and the descending aorta ; it subsequently becomes, in fact, the arch of the aorta The trunk (p'), which is connected with the right ventricle, on the other hand, and which subsequently becomes the pulmonary artery, transmits its blood through the fourth arch of the left side, 4', (the two primary tubes twisting round each other) and the fifth arch, 5, of the right : but the fifth arch on the left side, 5', now ceases to convey blood. From the two trunks, 4', 5, which still discharge their blood into the descending aorta, the pulmonary vessels, p, p, branch off as the lungs are developed ; and the prolongation, 10, of the former of these, which previously consti- tuted one of the arches of the descending aorta, soon afterwards becomes impervious. The original prolongation, 9, of the latter trunk, which meets the descending aorta, still remains ; so that a portion of the blood CIRCULATION IN ANIMALS. 721 sent from the right ventricle is transmitted through this communicating branch directly into the descending aorta, just as in the adult Crocodile. After the first inspiration, however, the whole of the blood transmitted through the pulmonary artery passes into the lungs, and does not enter the aorta until it has been returned to the heart ; and this communicating- vessel, which is termed the ductus arteriosus, soon shrinks and becomes impervious. Thus the third pair of branchial arches becomes converted into the two arterice innominatce, or common trunks, from each of which, in Birds and in some Mammals, the carotid and subclavian arteries of one side originate. The fourth branchial arch of the right side becomes the arch of the aorta. And the fifth branchial arch of the right side, with the fourth of the left, become the right and left pulmonary arteries. — The general plan of the changes which occur in the vascular system of the Mammalia, is the same as that which has been described in Birds ; the differences being only in detail, — as for instance, that the aortic arch is formed, not from the right, but from the left branchial arch. 489. Up to the period of the hatching of the egg in Birds, and the separation of the fretus from the parent in the Mammalia, the circulation retains some jieculiarities, characteristic of the inferior type which is per- manent in the Reptile tribes. Of the blood which is brought by the venous trunks to the right auricle, part has been purified by transmission to the respiratory organ (the allantois in Birds, and the placenta in Mam- mals), whilst a part has been vitiated by circulation through the system. The former is brought from the abdomen by the ascending vena cava, mixed with the blood which has circulated through the lower extremities ; whilst the descending cava brings back that which has passed through the capillaries of the head and upper extremities, and which, having received no admixture of arterial blood, is not fit to be again transmitted in the same condition. It will be recollected that a communication still exists between the two auricles, the ‘foramen ovale’ yet remaining pervious; and by a fold of the lining membrane of the right auricle, forming the Eusta- chian valve, the ascending and descending currents are so directed, that the former (consisting of the most highly-arterialised blood) passes at once into the left auricle, whilst the latter flows into the right ventricle. From the left auricle, the arterial blood is propelled into the left ventricle, and thence through the arch of the aorta to the vessels of the head and upper extremities, a comparatively small part finding its way into the descending- aorta. The venous current is propelled through the pulmonary artery ; but the lungs not yet being expanded, little of it is transmitted to these organs, and the greater part finds its way through the ductus arteriosus into the descending aorta, where it mixes with the remainder of the first- mentioned portion. This trunk not only supplies the viscera and lower extremities (which are thus seen to receive, as in Reptiles, blood of which only a portion has been oxygenated), but sends a large proportion of its con- tents to the umbilical vessels, by which it is conveyed to the oxygenating organ, and returned again to the venous trunk of the abdomen. The peculiar course taken by the blood through the heart, which was suspected from anatomical investigation, has been actually demonstrated by means of coloured injections by Dr, J. Reid.* — Another peculiarity in the foetal * “ Edinb. Med. and Surg. Journ.,” vol, xliii. pp. 11 and 308. 3 A 722 SPECIAL AND COMPARATIVE PHYSIOLOGY. circulation is the mode in which the blood passes through the liver. The vena portse in the foetus receives not only the venous blood of its abdominal viscera, but the arterial blood sent from the respiratory surface ; and as it would not be desirable that the whole of this should pass through the liver before being transmitted to the heart, an immediate passage into the vena cava is provided for a part of it, in the ductus venoms, which, net being required after birth, shrivels into a ligament, A similar communica- tion exists permanently in Fishes, and to a less degree in other oviparous Vertebrata : and it seems there intended to transmit directly to the heart whatever proportion of the blood, brought to the vena portae, may be at the time superfluous as regards the function of the liver. 490. Thus we have traced, in the development of the Circulating apparatus of the higher Vertebrata, the same progressive advance from a more general to a more special condition, as that which we have witnessed in ascending the Animal series ; and -when considered analogically rather than homologically (§ 333), the correspondence is extremely close. For in the state of the circulating system in the early embryo, when the heart is as yet but a pulsating enlargement of one of the principal trunks, and the walls of the vessels are far from being complete, we have the represen- tation of its condition in the higher Radiata, and in the lower Articulata and Mollusca. In the subsequent division of the cardiac cavity into an auricle and a ventricle, an advance is made corresponding to that which we encounter in passing from the Turricata to the higher Mollusca. And when the branchial arches are formed, which enclose the pharynx and meet in the aorta, the type of the Fish is obviously attained. But it will be observed that, notwithstanding this similarity, the Yertebrated embryo never presents any of those features of the circulating apparatus, which are characteristic of the other sub-kingdoms respectively ; thus, it does not exhibit that radiated distribution of the vascular trunks, which is seen in the Echinodermata (§ 454) ; nor does the heart, even when most like a ‘ dorsal vessel,’ ever present the least approach to that transverse division into successive segments, which is typical of the Articulata (§ 461) : and in its position, and connections, being situated in the immediate neigh- bourhood of the pharynx and sending its primitive trunks around it, the heart of every Vertebrated animal differs from that of the Mollusca, whose relations are with the opposite extremity of the alimentary canal (§ 466). In the subsequent progress of the Circulating apparatus, from the grade of the Fish, through that of the Reptile, to that of the Bird or Mammal, vre have a characteristic illustration of the principles formerly laid down (§ 348) ; for although the branchial arches are formed in all Vertebrated animals, yet it is only in Fishes and Batrachian Reptiles that they give origin to branchial tufts ; and although at a subsequent period the con- dition of the heart and great vessels presents a strong general resemblance to that of the typical Reptiles, yet that resemblance is wanting in the essential feature of the complete separation of the auricles, and the mix- ture of arterial and venous blood in the single ventricle. It is obvious that this want of conformity has reference to the difference in the seat of the respiratory p&Tcess ; the pulmonary vessels in the embryo being developed for future use, but the actual aeration of the blood being per- formed elsewhere. 491. The knowledge of these different stages of the development of the CIRCULATION IN ANIMALS. 723 Circulating apparatus, enables us to explain many of the malformations which it occasionally presents in Man. One of the most common of these gives rise to the malady termed Cyanosis; for this results from the foramen ovale, which establishes a communication between the auricles, remaining open after pulmonary respiration has been established • so that a considerable portion of the blood transmitted to the right cavity passes into the left, without having been previously arterialised by passage through the lungs. Persons thus affected have always a livid aspect, from the quan- tity of venous blood circulated through the arteries ; they are deficient in muscular energy and in power of generating heat, and they are seldom long-lived. A consequence partly similar would probably have resulted from a curious malformation mentioned by Kilian, had the infant re- mained alive : in this case, the aortic arch had not been developed, so that the primary aortic trunk gave off only the vessels to the head and upper extremities ; whilst the communicating branch between the pulmo- nary artery and descending aorta, which usually is of a secondary charac- ter, constituting the ductus arteriosus, was here the only means by which the blood could be transmitted to the latter ; so that the circulation through the lower part of the trunk and extremities would have been entirely venous. A malformation of this kind in a diminished degree has not been found incompatible with the continuance of life ; several cases being on record, in which the ductus arteriosus has remained pervious, and has brought part of the blood from the pulmonary artery to the descending aorta. Cyanosis is of course, as in the former instance, the result of this imperfect arterialisation ; and the individual is reduced, as far as his vascular system is concerned, to the condition of the Crocodile. An arrest of development at an earlier period may cause still greater imperfections in the formation of the heart. Thus, the septum of the ventricles is sometimes found incomplete, the communication between the cavities usually occurring in the part which is last formed, and which in most Reptiles remains open. In other cases it has been altogether want- ing, although the aorta and pulmonary artery were both present, and arose side by side from the common cavity ; and this form of the circu- lating apparatus is evidently analogous to that presented by Reptiles in general. A still greater degradation in its character has been occasionally evinced ; for several cases are now on record, in which the heart has presented but two cavities, an auricle and a ventricle, thus correspond- ing with that of the Fish ; and in one of these instances the child had lived for seven days, and its functions had been apparently but little disturbed. The occasional entire absence of the heart has already been noticed ; and coexistent with this, there is always great deficiency in the other organs, the brain, and sometimes the liver and stomach, being unde- veloped. The bifid character of the apex, which presents itself at an early period of the development of the heart, and is permanent in the Dugong, sometimes occurs as a malformation in the adult human subject ; evidently resulting, like the others which have been mentioned, from an arrest of development. On similar principles, some occasional peculiarities noticed in the distribution of the vessels may be accounted for, of which a striking example will be presently given. The Vena Cava is occasion- ally observed to consist of two parallel trunks, which are sometimes partially united, and then separate again ; a similar condition is permanent 724 SPECIAL AND COMPARATIVE PHYSIOLOGY. in some Cartilaginous Fishes, and the explanation of it is to be sought for in the history of the development of the venous system in general. Wt have seen that in many of the lower animals, such as the Crustacea, where the arteries are perfect canals having distinct coats, the veins seem to be merely channels through the tissues, having no definite walls : in like manner, at an early period of the foetal development of the higher animals, several small vessels are found where one vein subsequently exists ; and, if the coalescence of these has been from any cause checked, they will remain permanently separated to a greater or less extent. 492. Several interesting varieties have been detected in the ar-range- ment of the principal tranks given off from the Aorta : and though we cannot account for them on the principles already mentioned, it is not a little curious, that nearly all of these irregular forms possess analogues in the arrangements, which are peculiar to some or other of the Mammalia. The mode in which the cephalic and brachial vessels usually arise in the Human subject, is shown in the subjoined figure, a, where a b is the arch Fig. 216. ABCDE F GH a b a b a b a b a b a b a baba Diagram of the principal varieties in the origin of the cephalic and brachial trunks from the arch of the aorta; a, Man; b, Elephant; c, some Cetacea; d, Bat, &c. ; e, Carnivora, Sec. ; f, Seal; g, Ruminants; h, Reptiles: — 1, right subclavian; 2, right carotid; 3, left carotid, 4, left subclavian; 5, vertebral; a, ascending aorta; ft, descending aorta. of the aorta, 1 and 2 the trunks of the light carotid (which supplies the head) and of the right subclavian (which is distributed to the upper ex- tremity), arising by a common trunk — the arteria innominata ; while the left carotid, 3, and the left subclavian, 4, arise separately. At b is seen a distribution which is rare in the human subject, the two carotids arising by a common trunk, and the right as well as the left subclavian beiug given off separately ; this is the regular arrangement of branches in the Elephant. It is not so unusual for all the branches to arise from single trunks, as at c ; and this appears to be the regular- type in some of the Cetacea. Sometimes, again, there is an arteria innominata on each side ; subsequently dividing into the carotid and subclavian, as at D ; and on this plan the branches are' distributed in the Bat tribe, and also in the Porpoise. A not unfrequent variety in the human subject, is for both carotids to arise with the right subclavian from a single trunk, os .a r. the left subclavian coming off by itself ; this is observable as the regular form among many animals, being common among the Monkey tril e, the Carnivora, the Rodentia, &c. Another variety which is not unfrequent is shown at f, the vertebral artery on the left side, o, which usually arises from the subclavian, springing directly from the aorta ; it is on this plan that the branches are given off in the Seal. A form which is very uncommon in man is that represented at G ; here the aorta divides at RESPIRATION. GENERAL CONSIDERATIONS . 725 once into an ascending vessel, from which the two subclavian and two carotid arteries arise, and a descending trunk ; this is the regular distri- bution of the vessels in Ruminating animals, and appears to be most general in Mammalia possessing a long neck. Lastly, at h, is seen a form which evidently results from an arrest of the usual changes in the arterial trunks described in §§ 486, 488 ; the aoi’ta continuing to possess a double arch, from the ascending part of which the subclavian, external carotid, and internal carotid arteries are given off on each side, the single descending trunk being formed by the union of the two original branches. This, it will be recollected, is the normal type of formation in Reptiles.* CHAPTER XIIT. OF RESPIRATION. 1. General Considerations. 493. The function of Respiration essentially consists in the evolution of carbonic acid from the fluids of Organised beings, and the absorption of oxygen from the surrounding medium, usually in a nearly equivalent proportion. This process is performed by Plants as well as by Animals ; and it may be regarded as arising out of the same general requirements in both kingdoms, although it answers some special purposes in the latter, which render it more immediately essential to the maintenance of their vital activity, than it seems to be in the former. For we shall hereafter find that the imperious necessity for the continual introduction of oxygen and exhalation of carbonic acid, which requires a most active performance of the respiratory function, and causes even a brief suspen- sion of it to be fatal, in the higher Animals, is consequent upon the energetic exertion of their peculiarly animal powers, and upon the per- formance of that combustive operation by which their high temperature is maintained ; whilst, on the other hand, when we pass to those tribes which are most remarkable for the inertness of their habits, and for their entire want of power to sustain an independent temperature, the demand for oxygen is greatly diminished, and the exhalation of carbonic acid may be checked for a time without injury. The amount of Respiration, then, which is required for the performance of the organic or constructive functions of Animals, is comparatively small ; and it is not surprising that the existence of this function should have been long overlooked in Plants, in which its effects on the atmosphere are masked by a change of an entirely opposite nature, that is subservient to the introduction of alimentary material into the system, — namely, the decomposition of the carbonic acid of the air, under the influence of light, the fixation of its 'carbon in the vegetable tissues, and the consequent liberation of its * In the foregoing account of the development of the Vascular system, the Author has (wailed himself freely of the valuable papers of Dr. Allen Thomson, in the “ Edinb. Philos, loumal,” vols. ix. and x.; in the sketch of the malformations of the heart, he has made use if the paper of Dr. Paget in the “Edinb. Med. and Surg. Journal,” vol. xxxvi. ; and the last mragraph, with the accompanying figures, has been entirely derived from the magnificent vork of Tiedemann on the Arteries. 726 SPECIAL AND COMPARATIVE PHYSIOLOGY. oxygen. To this last process the term Respiration has been commonly applied ; and the Respiration of Plants is ordinarily spoken of as anta- gonistic to that of Animals. This statement is perfectly true, if under the term Respiration be included the sum-total of the changes produced in the air by the growth of a Plant ; but it will be presently shown, that whilst Animal life gives rise to but one set of changes in the atmosphere (namely, the removal of a portion of its oxygen, and a replacement of this by carbonic acid), Vegetable life produces two sets of changes, which ought to be kept quite distinct from each other in a scientific description of them, their nature and their sources being alike different ; and that it is only on account of the excess of one set of these changes beyond that which it antagonises, that it alone has received general attention, and has been commonly regarded as the proper respiration of Plants. 494. Restricting the meaning of the term Respiration, then, to the removal of carbonic acid from the living system in a gaseous form, and the introduction of oxygen into it, we have to enquire what are those most general sources of demand for this action in the vital economy, which are common to Plants and Animals. These appear to be two-fold ; one arising out of the disintegrating changes which are always going on in the living system ; the other being consequent upon some of those chemical operations, which necessarily participate in the constructive functions. The former seem to be the most general ; the latter are rather of a special character, and manifest themselves most strongly, as we shall see hereafter, at certain periods in Vegetable life (§ 503). — All organised bodies, as already explained, are liable to con- tinual disintegration, even whilst they are most actively engaged in per- forming the actions of life ; in fact, a succession of organs whose indi- vidual duration is short, but whose functional energy is great, seem> necessary for the maintenance of the life of the more permanent parts of the organism (chap, x., Sect. 1). The necessary result of this disintegra- tion is decay ; and one of the chief products of that decay is carbonic acid. A large quantity of this gas is set free, during the decomposition of almost every kind of organised matter, the carbon of the substance being united with oxygen supplied by the air. Hence we find that the formation and liberation of carbonic acid go on with great rapidity after death, both in the Plant and in the Animal ; and that it is then but me continuation (so to speak) of that which has been taking place during life. Thus in Plants, as soon as they become unhealthy, the extrication of carbon in the form of carbonic acid takes place in greater amount tb m its fixation from the carbonic acid of the atmosphere ; and the same change normally takes place during the period that precedes the exuvia- tion of the leaves, their tissue being- no longer able to perform its proper functions, and its incipient decay giving rise to a large increase in the quantity of carbonic acid set free. In some of these cases it would seem that the carbon of the decomposing tissue unites with the oxygen con- tained in the fluids of the system, and that carbonic acid is thus generated, the extrication of which contributes to the introduction of fresh oxygen (§ 495) : in other instances, however, the oxygen may be more directly derived from the atmosphere. — The other source of demand for Respi- ration, which is common alike to Plants and Animals, arises out of the chemical transformations which are always going on in their systems. RESPIRATION. GENERAL CONSIDERATIONS. 727 as a part uf their nutrient operations. These are as yet but very little understood ; but enough is known to justify the belief, that in many of them the presence of oxygen is essential, and that carbonic acid is among their products. Some examples of such transformations, drawn from the Vegetable kingdom, have been already cited (§ 28) ; but in addition to these it may be remarked that the conversion of starch into sugar, a change that takes place in the neighbourhood of many growing parts, involves the combination of carbon with oxygen to a considerable amount ; and that, in general, the production of the vast multitude of organic compounds yielded by Plants, from the gum, starch, and sugar which are first generated by them at the expense of the inorganic elements, requires a series of chemical changes, in many of which oxygen is taken in and carbonic acid given forth. Although the number of organic compounds generated by Animals is much less than that which we find in Plants, yet there can be no doubt, from a comparison of their atomic constitu- tion, that oxygen must be taken into combination, and carbonic acid given off, in many of the chemical transformations which take place in the living body ; and among these, the metamorphosis of Albumen into Gelatine has been pointed out by Dr. Prout as one of the most constant and important. 495. Besides the evolution of Carbonic acid and the absorption of Oxygen, it would appear that the exposure of the circulating fluid to the air is the means of keeping the Nitrogen of the system at its proper standard ; and the whole series of reactions which take place between the living body and the air that surrounds it, or that is contained in the water in which it lives, may be couveniently included under the general term A eration. This aeration would appear to be, like absorption, a change dependent on physical agencies, and taking place in conformity with their laws, when the requisite conditions are supplied by the structures of an organised being, and by the functional alterations which the living state involves. — All gases of different densities, which are not disposed to unite chemically with one another, have a strong tendency to mutual admixture. Thus, if a vessel be partly filled with hydrogen, and partly with carbonic acid, the latter, which is 22 times heavier than the former, will not remain at the bottom, but the two gases will be found in a short time to have uniformly and equably mixed ; and it is on this principle, that the con- stitution of the atmosphere is everywhere the same, although the gases which compose it are of very different specific gravities. So strong is this tendency to admixture on the part of different gases, that it will take place when a membrane or other porous medium is interposed between them. This interchange, therefore, evidently resembles the endosmose and exosmose of fluids (§ 415) ; and although the tendency to admixture of the two gases is the fundamental cause of then’ movement, the nature of the septum has so much influence over the phenomenon, as sometimes to reverse the results. When plaster-of-paris is employed as the medium of diffusion, the exchange will take place with simple relation to the relative densities of the gases ; and a general law has been ascertained t>y Prof. Graham, which applies to all instances,- — that the ‘replacing’ or ‘ mutual-diffusion volumes ’ of different gases vary inversely as the square- roots of their densities. Thus, if a tube, closed at one end with a plug of plaster-of-paris, be filled with hydrogen, the gas will soon be entirely 728 SPECIAL AND COMPARATIVE PHYSIOLOGY. removed, and will be replaced by something more than one-fourth of its bulk of atmospheric air; the density of hydrogen being about l-14th that of the atmosphere. But when organic membranes are employed, the result is much influenced by the relative facility with which each gas permeates the septum. Thus, carbonic acid passes through moist bladder much more readily than hydrogen ; and, in consequence, when a bladder of hydrogen is placed in an atmosphere of carbonic acid, a certain quantity of hydrogen will pass out ; but a much larger proportion of carbonic acid will enter, so as to distend the bladder even to bursting. Further, it is found that, if a fluid be charged with any gas which it readily absorbs (as, for example, water with carbonic acid), it will speedily part with it when exposed to the attracting influence of another gas, such as atmo- spheric air ; and the more different the densities of the two gases, the more rapidly, and with more force, will this take place. As in the former instance, this attraction null go on with little interruption through a porous membrane ; and part of the exterior gas will be absorbed by the fluid (if of a nature to be so imbibed), in place of that which has been removed. 496. These simple phenomena will be found a key to the explanation of the changes which take place in the aeration of the circulating fluid by exposure to air ; for it seems a universal fact, that carbonic acid existing in that fluid is exhaled, and is replaced by absorbed oxygen ; and that an exhalation and absorption of nitrogen take place in animals, and perhaps also in plants. 2. Respiration in Plants. 497. Under the above designation have been associated two distinct changes, both neai’ly constant throughout the Vegetable kingdom. The atmosphere being the chief source whence Carbon is supplied to the living plant, the introduction of that element has been confounded with the contrary change, which is also necessary for the continued health of the structure, and which corresponds exactly with the respiration of Ani- mals. The introduction of carbon is effected by the power which the green surfaces of Plants possess, of decomposing, under the stimulus of Light, the carbonic acid contained in the air or in the liquids supplied to then: ; and of retaining or fixing the carbon, whilst they set free the oxygen (§§ 72—75). In the Phanerogamia, the green surfaces of the leaves, and other appendages to the axis, are those by which this fixation of carbon, which may be considered as a process of alimentation, is chiefly, if not entirely effected ; and where, as in the Cactus tribe, the leaves are defi- cient, but the stems are succulent and then surfaces green, it is obvious that these perform the same function. In the Ferns, J fosses, Ac. there is the same separation of parts as in the Flowering plants ; and the process is here also, without doubt, performed by the green parts of the surface. Of the inferior Cryptogamia, however, we know very little. The Fvngi would not seem to depend upon the atmosphere for any part of their supply of carbon, which is altogether furnished by their peculiar aliment (§ 380) ; and these plants scarcely ever present any green surface, and flourish most in situations to which light has but little access. The same may be said of the Cuscuta (dodder) and other leafless parasitic plants of RESPIRATION IN PLANTS. 729 more complex structure, that live upon the prepared juices they derive from the plant to which they attach themselves. There can be no doubt that Lichens ordinarily obtain the carbon which enters into their structure, entirely from the atmosphere ; and, that the Algos are supported, in like manner, by the carbonic acid contained in the circumambient water ; but experiments are yet wanting to ascertain the precise conditions under which its assimilation is effected. Few Lichens have any green surfaces ; and although many of the Algae are very brilliantly coloured, yet we find them occasionally existing at such depths, as make it difficult to believe that light is the only stimulus under which they can attain this appear- ance. The simpler forms of Algae, especially the Confervce, which inhabit fresh water, appear to exercise an important influence in maintaining it in a state fit for the support of animal life ; since it seems probable that they absorb the products of the decomposition of that foul matter by which all ponds and streams are constantly being polluted, and at the same time yield a supply of oxygen to the water. It is a notorious fact that Fishes are never so healthy in reservoirs destitute of aquatic plants, as in ponds and streams in which these abound. 498. The entire mass of Vegetation upon the surface of the globe is thus mainly dependent upon the minute proportion of carbonic acid con- tained in the atmosphere, which is not above 4 parts in 10,000. This is probably as much as Plants in general, under the feeble illumination which those of them are liable to receive whose ‘ habitat ’ is in variable climates, could advantageously make use of ; and a larger proportion would probably have been injurious to them, as well as to Animals. But it has been ascertained by direct experiment, that Plants will thrive in an atmosphere containing six or eight per cent, of carbonic acid, or even more, so long as they are exposed to strong sun-liglit ; and it would appear that in climates where the solar light is less obscured by clouds than it is in our own, the growth of plants may be favoured by an unusual supply of this alimentary substance. Thus the floating islands which are constantly being formed on the lake Solfatara in Italy, exhibit a striking example of the luxuriance of cryptogamic vegetation in an atmosphere impregnated with carbonic acid. These islands consist chiefly of Confervse and other simple cellular plants, which are copiously supplied with nutriment by the carbonic acid that is constantly escaping from the bottom of the lake, with a violence which gives to the water an appearance, of ebullition.* Dr. Schleiden mentions that the vegetation around the springs in the valley of Gottingen, which abound in carbonic acid, is very rich and luxuriant ; appearing several weeks earlier in spring, and continuing much later in autumn, than at other spots in the same district, t — A very ingenious hypothesis has been raised by M. Brongniart upon the fact, that an increased quantity of carbon may, under particular circumstances, be assimilated by Vegetables. He supposes that, at the epoch of the growth of those enormous primeval forests which supplied the materials of the coal-formation, the atmosphere was highly charged with carbonic acid, as well as with humidity ; and that from this source, the Ferns, Lycopodiacese, and Coniferm of that era were enabled to attain their gigantic development. He imagines that they not only thus con- * Sir H. Davy’s “ Consolations in Travel,” 3rd ed., p. 116. 4 “ Wiegman’s Archiv.” iii. 1838. 730 SPECIAL AND COMPARATIVE PHYSIOLOGY. verted into organised products an immense amount of carbonic acid which had been previously liberated by some changes in the mineral world, but that, by removing it from the atmosphere, they prepared the earth for the residence of the higher classes of Animals. The hypothesis is a very interesting one, and well deserves consideration. It may be regai ded as an almost absolute certainty, that the whole of the carbon now solidi- fied in the coal-deposits of various ages, must have previously existed iu the atmosphere ; and if we were acquainted with the extent" of these, it would be a simple matter of computation to determine, whether, if all tlr- carbon were reconverted into carbonic acid, it would sensibly affect the proportion of that ingredient in the atmosphere. — The recent experiments of Dr. Daubeny * to a certain extent justify the hypothesis of M. Brou- gniart, by showing that the existing Plants and Animals most allied to those of the Carboniferous period can exist without injury in an atmo- sphere containing nearly 5 per cent, of carbonic acid • and if such a difference of climate then prevailed (either in consequence of a different distribution of land and water, or as a result of the internal heat of the globe), as would enable the solar rays to act with more constancv and power than they do in Britain at the present time, there seems no reason for asserting that such might not have been the case.t 499. The change which, strictly speaking, constitutes the Respiration of Vegetables is not, like that we have been describing, an occasional one : but is constantly taking place during the whole life of the plant, and ap- pears to be more immediately necessary to its healthy existence. This consists in the disengagement of the superfluous carbon of the system, either by combination with the oxygen of the air, or (which is more likely) by replacing with carbonic acid the oxygen that has been absorbed from it ; and it does not cease by day, by night, in sunshine, or iu shade. If the function be checked, the plant soon dies, — as when placed in an atmosphere with a large proportion of carbonic acid, and without the stimulus of light which enables it to decompose the deleterious gas. Plants which are being ‘ etiolated ’ by the want of light (§ 7 6), absolutely diminish in the weight of their solid contents, owing to the continued excretion of carbon by the respiratory process, although their bulk may be much increased by the absorption of water ; and if the proportion of carbonic acid in the surrounding ah’ be augmented by its accumulation, they become sickly and die, from the impediment to them respiration. * “ Report of the British Association” for ] 849 ; p. 56. f The Author would suggest it as a point worthy of further consideration, whether there may not have been a special relation between the luxuriant growth of the plants that furnish those carbonaceous deposits, which are found especially to be intercalated amongst the Car- boniferous, Oolitic, Wealden, and Cretaceous formations ; and the deposit of those vast calcare- ous beds with which they are so remarkably associated. The latter, there is strong reason to believe, are almost entirely of animal origin ; the carbonate of lime having been drawn by Zoophytes, Echinoderms, and Mollusks, from the waters of the ocean, just as the carbon of the Vegetation of those periods was drawn from the carbonic acid of the atmosphere. Now if we imagine that during the progress of these deposits, there was an unusual discharge of carbonate of lime, held in solution by free carbonic acid, by submarine springs issuing from the interior of the earth (like the Solfatara and other calcareous springs, which furnish the ‘ travertine’ deposits, and at the same time promote the growth of plants, at the present day), we seem to have a probable account of the extraordinary abundance of carbonate of lime in the ocean-waters, and of carbonic acid in the atmosphere, at those periods ; since the greater part of the latter would have escaped into the air, as soon as it became free to do so by tiie removal of the pressure which previously restrained it. RESPIRATION IN PLANTS. 731 The parallel, therefore, between Plants and Animals appears to be com- plete, as regards the influence of carbon upon their growth ; for to both it is deleterious when breathed, and to both it is invigorating to the digestive system when absorbed as food ; and whenever Plants, or parts of Plants, derive their nutriment, like Animals, from organic compounds already prepared for them, there do we find the true respiratory process taking place, without the counterbalancing fixation of carbon as aliment. This is the case, for example, with Fungi in general; it is the case, too, with the leafless Phanerogamic parasites, which draw their materials from the elaborated juices of other plants, instead of preparing them for them- selves (§ 558) ; it is the case also with the growing embryo, whose food is derived from the store laid up in the seed, and whose action upon the air is contrary to that of the developed plant, until it has exhausted this store, and has unfolded its leaves to the light so as to take in fresh carbon from the atmosphere ; it is the case, again, with all the growing parts which derive their nutriment from the leaves (not being themselves able to decompose carbonic acid), and especially with the flowers ; and it is the case, too, even with the leaves themselves, when their functional activity is diminishing, and the decomposing changes in their tissue are com- mencing. 500. It becomes a question of much interest to determine the relative amount of carbon thus absorbed and excreted by Vegetables. Since a large part of the solid material of their tissues is derived from the atmo- sphere, it is evident that the whole quantity of carbonic acid in the air must be diminished by their growth ; but as a certain proportion of that carbonic acid is taken in by the roots, which are supplied with it through the absorbent agency of the soil (§ 380), the agency of the leaves requires to be tested by experiment. Such experiments have been repeatedly made by very careful and experienced observers ; and the general result of them is, that, so long as the leaves continue in healthy action, and are exposed to the influence of light, they are actively engaged in taking in carbon from the atmosphere ; * and that, when entire plants, consisting not only of leaves, but of stems and other parts, are confined in the same portion of air, day and night, and are duly supplied with carbonic acid gas during sunshine, they will go on adding to the proportion of oxygen pre- sent, so long as they continue healthy ; the slight diminution of oxygen and increase of carbonic acid which take place during the night, bearing no considerable ratio to the degree in which the opposite effect occurs by day.t The balance of nutrition, therefore, between the Animal and Vege- table kingdoms, is thus maintained in a very perfect and interesting manner (chap. v.). 501. With regard to the changes effected by vegetation upon the prin- cipal constituent of the atmosphere — Nitrogen — no very certain or definite statement can be made. We have seen that this element enters largely into the composition of Plants ; but there is reason to believe that all which they require is derived by them, not directly from the atmo- sphere, but by the decomposition of the ammonia absorbed by the soil, * See the experiments of Mr. Pepys in the “Philosophical Transactions” for 1843, p. 329. + See Dr. Daubeny's letter to Prof. Lindley, in his “ Introduction to Botany,” vol. ii. P 297. 732 SPECIAL AND COMPARATIVE PHYSIOLOGY. and taken up in solution by the roots (§ 380) ; indeed proof is wanting that the free nitrogen of the air has any concern with Vegetable Respira- tion ; for the few experiments which have been performed with express view to this subject, lead to the belief that azote is more frequently exhaled than absorbed. 502. In the Fungi among Cryptogamia, however, and in the ‘ leafless parasites/ such as the tribe of Orobanchece, among Phanerogamic i, we have examples of the performance of the true respiratory process, without the antagonizing action of the alimentative ; the only change which the growth of these plants induces in the smTounding air, being the replace- ment of its oxygen by carbonic acid. Thus from the experiments of Marcet upon Fungi, it appeared that growing Agarics absorbed from the air a large quantity of oxygen ; a portion of which appears to combine with the carbon of the plant, and thus to form the carbonic acid which replaces it ; whilst the remainder seems to be retained in its structure. The recent experiments of M. Lory upon the respiration of the Oroban- chese * are peculiarly interesting. He found that in every stage of their vegetation, all the parts of these plants, whether they are exposed to solar light, or whether they are in the dark, absorb oxygen and give out car- bonic acid ; the volume of the carbonic acid generated being nearly equal to that of the oxygen which disappears. But that the production of carbonic acid is not a result of a mere union of carbon excreted by the plant, with atmospheric oxygen, but takes place in the interior of the plant as a part of the changes in which its growth consists, appeal's from the fact, that the disengagement continues for a time when the plants are surrounded by au atmosphere of pure hydrogen. (A parallel fact will here- after be cited in regard to the respiration of Animals, § 534.) The amount of carbonic acid exhaled is augmented by warmth, which increases the activity of the nutrient operations ; and, as in other plants, it is peculiarly great during the period of flowering. The contrast between the action of one of these leafless parasites upon the atmosphere, and that of the plants from which they draw their nutriment, was curiously dis- played by placing in two receivers of the same capacity, a portion of the stem of an Orobanche, and a portion of the leafy stem of the Teucrium on which it grew, each piece being of the same weight ; the atmosphere surrounding them was composed of six volumes of common air mingled with one of carbonic acid ; and they were exposed to light from 9 a. v. until 3 p. m. of the succeeding day. At the end of this time, the atmo- sphere of the jar containing the Teucrium did not present a trace of carbonic acid, whilst that in which the Orobanche had been immersed exhibited such an augmentation of carbonic acid, that whilst its original proportion to the oxygen was as 20 to 25, it was then as 36 to 9. — The explanation of this difference is doubtless to be found in the mode in which parasitic plants obtain then nourishment. Being supported, like Animals, upon organic compounds that have been already elaborated by the agency of other Plants, they are entirely destitute of the power of decomposing the atmospheric carbonic acid for the purpose of alimenta- tion ; and the sole change which they produce in the surrounding air ;s of the same kind with the respiration of animals. As already remarked, * “ Annales des Sciences Naturelles,” Botan., Sept., 1847. RESPIRATION IN PLANTS. 733 there is every reason to believe that the same change takes place in all other Plants, and that carbonic acid is continually given off from their interior, whilst oxygen is absorbed ; although this is masked by the opposite change, which is effected by their green surfaces during the influence of light upon them. For so soon as the light ceases to act upon the latter, the respiratory change makes itself manifest ; and it has been shown by the experiments of Saussure, that through the dark portions of plants, this change is continually taking place. 503. Further, there are certain processes in the life of all Plmnero- gamia, in which the function of Respiration seems to go on with remark- able activity, and in which its manifestation is not concealed by the converse operation. One of these is Germination, or the development of the young plant from seed (§ 278 a), which requires that the starch laid up by the parent for the support of the embryo should be converted into sugar, the latter being the form in which it is applied to the purposes of nutrition. This conversion involves the liberation of a quantity of carbon, which is disengaged by means of its combination with the oxygen of the surrounding atmosphere ; and the young plant may then be regarded as living under the same conditions as the parasitic tribes just referred to, its nutriment being supplied to it without the necessity for the alimenta- tive operation which it will be afterwards required to perform. Germi- nation takes place most readily in the dark, since the most essential part of the change — the extrication of carbon — would be antagonised by the influence of light. The young plant is, therefore, much in the condition of one which is being etiolated ; and it is accordingly found that, during the early period of germination, the weight of the solid contents of the seed diminishes considerably, though its bulk increases by the absorption of moisture. This is its state until the cotyledons, or seed-leaves, have arrived at the surface, and temporarily perform the functions of leaves. It is an interesting fact that, after many trials, germination has been found to take place most readily in an atmosphere consisting of 1 part oxygen and 3 parts nitrogen, which is nearly the proportion of the air we breathe. If the quantity of oxygen he much increased, the carbon of the ovule is abstracted too rapidly, and the young plant is feeble ; if the proportion be too small, carbon is not lost in sufficient quantity, and the young plant is scarcely capable of being roused into life.- — The changes which take place during Flowering, are very similar to those occurring in germination. A large quantity of oxygen is converted into carbonic acid by the action of the flower ; and it is believed that the starch, previously contained in the disk or receptacle, is changed by this process into saccha- rine matter adapted for the nutrition of the pollen and young ovules, the superfluous portion flowing off in the form of honey. It is remai'kable that this analogy between germination and flowering holds good, not only in their products, but in the conditions essential to their develop- ment. Neither will commence except in a moderately warm tempera- ture ; both require moisture, for flowers will not open unless well supplied with ascending sap ; and the presence of oxygen is in each case necessary. It has been well ascertained that the carbonisation of the air bears a direct relation to the development of the glandular disk, and that it is principally effected by the essential parts of the flower, or organs of fructification. Thus, Saussure found that the Arum Italicum, whilst in 734 SPECIAL AND COMPARATIVE PHYSIOLOGY. bucl, consumed in twenty-four hours 5 or 6 times its own volume of oxygen ; during the expansion of the flower, 30 times ; and during its withering, 5 times. When the floral envelopes were removed, the quantity of oxygen consumed by the remaining parts was much greater in proportion to their volume. In one instance, the sexual apparatus of the Arum Italicum consumed in twenty-four hours 132 times its hulk of oxygen. Saussure also observed that double flowers, in which petals replace sexual organs, vitiate the air much less than single flowers in which the sexual organs are perfect. (See also § 616.) — The same is the case, also, during the development of leaf-buds from parts in which (as in the tuber of the Potato) a supply of starchy matter has been laid up as the material for their evolution, until they are so far expanded that they can obtain carbon from the atmosphere. Here, too, the growing bud is in the condition of a parasite, being supported upon materials provided for it by other agencies than its own ; and here, again, we find that the conversion of the starch into sugar is the process to which the production of carbonic acid appears to be chiefly due. •504. Besides the means of aeration which the transmission of the nutri- tive fluid to the external surface affords, the more highly organised Plants seem to have the power of admitting air into cavities existing in the leaves (especially beneath their inferior cuticle, Fig. 224), through their stomata ; and in this manner a much larger extent of membrane is exposed to its influence. The peculiar organisation which is probably subservient to this purpose will be hereafter described (§§ 540, 541) under the head of exhalation, for which function it appeal’s more particu- larly designed. But, superadded to this, we find in the Phanerogamia a system of tubes apparently intended to connect the interior of the structure with the external air. These are the spiral vessels (§ 151). which, in their perfect form, are never found to contain any but gaseous fluids. In Exogens they usually exist in only one part of the stem, being confined to the ‘ medullary sheath ’ (§ 280) ; in Endogens they are more universally distributed through the stem, forming part of every bundle of fibro-vascular tissue (§ 279). In each case, however, they traverse the stem for the purpose of entering the leaves ; and they seem to communicate with the intercellular passages, and, through their medium, if not more directly (as some have supposed), with the external air. We have already noticed the curious analogy between these respi- ratory tubes and the tracheae of Insects (§ 153) ; and although their exact office is not fully ascertained, there can be little doubt that they contri- bute in some way to the aeration of the internal fluids. It has been found that they contain a larger quantity of oxygen, by 7 or 8 per cent., than that which exists in the atmosphere.- — In a great number of the aquatic tribes, both among the simpler and the more highly organised plants, we find cavities expressly adapted for the inclusion of air; but these would seem designed rather to give buoyancy to the structure, than to take any share in the Respiratory function. The air which they contain, however, is seldom identical in composition with that of the atmosphere. 505. Regarding the progressive evolution of the Respiratory system m Plants, much might here be said, which will perhaps be more advanta- geously deferred to the account of their general development (chap, xviil). RESPIRATION IN ANIMALS. 735 It may be remarked, however, that the early form of the embryo of the Flowering plants resembles, in its want of special organs, the simple vege- tation of the cellular Cryptogamia, although it differs in regard to the mode in which nutriment is supplied ; the latter deriving it by their unassisted powers from the surrounding elements, whilst the former is pro- vided with it by the parent. At the first period of the germination of the seed, a close analogy exists, as we have seen, between the growing embryo and the tribe of Fungi. Both are supplied with nutriment previously organised, the one by its parent, and the other by the decay of animal or vegetable matter ; both are developed most rapidly -when supplied with warmth and moisture, and in the absence of light ; and both liberate carbon to a large amount, without assimilating any from the atmosphere. By the time, however, that the cotyledons have risen to the surface and acquired a green colour, the plant has advanced a stage in its growth, and the respiratory system has now arrived on the level of the Marchantia (§ '215), possessing, like it, stomata and intercellular spaces, but being- destitute of spiral vessels. These do not appear until true leaves are * evolved ; and by the time that this last stage in the development has taken place, the cotyledons, which may be regarded as temporary respir- atory organs, decay away. — When we have traced the evolution of the respiratory system of Animals in a similar manner, we shall observe a most interesting correspondence between the consecutive phenomena, as they occur in the two kingdoms. 3. Respiration in Animals. 506. The dependence of the life of Animals upon the constant perform- ance of the Respiratory function, is more immediate than that of Plants ; and this arises, not merely from the circumstance that there are sources of demand for oxygen and of production of carbonic acid, in the former, which do not exist in the latter ; but also from the fact, that from those which are common to both (§ 494), the amount of carbonic acid gene- rated, as well as of oxygen required, is far larger in the Animal than in the Plant. The more active are the organic functions, and the softer and more prone to decomposition are the tissues, the more considerable will be that constant decay to which all organised fabrics are exposed, even during life ; and thus in ‘ warm-blooded ’ animals, the high temperature of the body, which favours the vital activity of its component organs, and causes them to live fast, will accelerate their decay, and thus give rise to a more rapid production of carbonic acid, and a greater demand for oxygen ; whilst in ‘ cold-blooded ’ animals, so long as the temperature of their bodies is low, the c waste ’ of the tissues from this source is kept down. But when the temperature of the Reptile is raised by external heat to the level of that of the Mammal, its need for respiration increases, owing to the augmented waste of its tissues. When, on the other hand, the warm-blooded Mammal is reduced, in the state of hybernation, to the level of the cold-blooded Reptile, the waste of its tissues diminishes to such an extent, as to require but a very small exertion of the respiratory process to get rid of the carbonic acid which is one of its chief products. And in those animals which are capable of retaining their vitality when frozen (§ 103), or when their tissues are completely dried up (§ 65), the decom- 736 SPECIAL AND COMPARATIVE PHYSIOLOGY. position is for the time entirely suspended, and consequently there is no carbonic acid to be set free. 507. But another source of Carbonic acid to be set free by the Respi- ratory process, and one which is peculiar to Animals, consists in the rapid changes which take place in the Muscular and Nervous tissues, during the period of their activity. Every development of muscular force is accom- panied by a change in the condition of a certain amount of tissue, to which change the presence of Oxygen is essential ; and one of the products of the union of oxygen with the elements of Muscular fibre, is carbonic acid. The same may probably be said of the Nervous tissue. Hence it may be stated as a general principle, that the peculiar waste of the Mus- cular and Nervous substances, whicli is a condition of then - function. ! activity, and which is altogether distinct from the general slow decay that is common to these tissues with others, is another source of the generation of carbonic acid, and of the demand for oxygen in the animal body ; and that the amount of the one gas produced, and of the other gas required, will consequently depend upon the degree in which these tissues are exercised. In animals which are chiefly made up of the organs of vege- tative life, in whose bodies the nervous and muscular tissues form but a very small part, and in whose tranquil plant-like existence there is but very little demand upon the exercise of these structures, the quantity of carbonic acid thus liberated will be extremely small, and the dependence upon a supply of oxygen by no means close. On the other hand, iu animals whose bodies are chiefly composed of muscle, and whose life is au almost ceaseless round of exertion, the quantity of carbonic acid thus liberated is very considerable, and the demand for oxygen is incessant : so that vital activity is speedily suspended, if the respiratory function be not performed. We are enabled to trace the connection between the amount of muscular exertion, and the energetic performance of the act of respiration, in the class of Insects, better than iu any other. They have no fixed temperature to maintain ; and they are consequently not in the condition of warm-blooded animals, in which the quantity of carbonic acid set free is kept up to a more regular standard by the provision to be presently noticed. On the other hand, they are pre-eminent among all animals, in the energy of then- muscular power as related to the bulk of their bodies, and the waste of muscular tissue during their state of activity must therefore be very great : and we shall hereafter find that :he amount of carbonic acid generated in a given time bears a close corre- spondence with this. 508. Besides these sources of demand for Respiration, which are common to all Animals, there is another, which appears to be peculiar to the two highest classes, Birds and Mammals. These are capable of maintaining a constantly-elevated temperature, so long as they are supplied with a proper amount of appropriate food ; and their power of doing so appears to depend upon the direct combination of certain elements of the food, with the oxygen of the air, by a process analogous to combustion (§ 376). The quantity of carbonic acid that is thus generated, seems to vary consider- ably in different animals, and in different states of the same individual ; but the principal source of difference lies in variations of external tempe- rature ; for the energy of the Respiration increases with its diminution, as more heat must then be generated : and diminishes with its increase. — In RESPIRATION IN ANIMALS. 737 all cases, if a sufficient supply of food be not furnished, and the store of fat be exhausted, the animal dies of cold (§ 93). 509. To recapitulate, then ■ the sources of the production of carbonic acid, and of the demand for oxygen, in the Animal body, are fourfold. — 1. The continual decay of the tissues ; which is common to all organised bodies ; which is diminished by cold and dryness, and increased by warmth and moisture ; which takes place with increased rapidity at the approach of death, whether this affect the body at large, or oidy an indi- vidual part • and which goes on unchecked, wdien the actions of nutrition have ceased altogether. — 2 . The various changes in composition that take place in the tissues in the progress of their organic construction, many of which involve a liberation of carbon and a higher oxygenation. — 3. The metamorphosis that is peculiar to the Nervous and Muscular tissues ; which is the very condition of their activity ; and which therefore bears a direct relation to the degree in which they are exerted. — 4. The direct conversion of the carbonaceous materials of the food into carbonic acid ; which is peculiar to warm-blooded animals ; and which seems to vary in quantity, in accordance with the amount of heat to be gene- rated. 510. The organs appropriated to the performance of the function of Respiration, in the various classes of the Animal Kingdom, appear at first sight so very different, that a superficial observer would hardly trace any relation between them (§§ 333, 350). A little reflection, however, will show, that all their forms are reducible to the simple element of which the respiratory organs are constructed in the Vegetable kingdom ; — an extension of the external surface, peculiarly adapted, by its permeability to gases, for the interchange of ingredients, between the circulating fluid brought in contact with one side of it, and the atmosphere which it touches on the other. Considered, therefore, under this instrumental character, there is a complete ‘ unity ’ amongst them all ; but when con- sidered with reference to the general plan of structure, we find them to be ‘ homologically ’ diverse. Thus, the extension may take place from very different parts of the surface, so that its connections with other organs may be altogether dissimilar in the several tribes of Animals. Again, it usually takes place internally or externally, according as the animal is to be an inhabitant of the air or of the waters. In animals modified for atmospheric respiration, the air enters the system to meet the blood : a peculiar set of movements, more or less complicated, being appointed for its constant renewal by successive inhalation and expulsion. In those adapted to an aquatic residence, a different plan is required. The small quantity of air contained in the water, is all that the respiratory system employs; and it would have been a useless expenditure of muscular exertion, to have provided means for the constant inspiration and expi- ration of a large amount of so dense a fluid. Inmost aquatic animals, therefore, the aerating surface is extended outwardly, instead of being prolonged inwards ; and the blood is propelled through it so as to come in relation with the surrounding medium ; the portion of which in appo- sition with it is continually being renewed, either by the natural move- ments of the animal, or by others more expressly contrived for the pur- pose. The relation between these organs will, perhaps, be made more ipparent by a simple diagram. Let a b represent the general external 738 SPECIAL AND COMPARATIVE PHYSIOLOGY. surface of the body ; then at a is shown the character of a simple out- ward extension of it, forming a foliaceous gill, such as is seen in the lower Crustacea ; and in like manner, h may represent a simple internal prolongation or reflexion, such as that which forms the pulmonary sac of Fig. 217. the air-breathing Gasteropods. A higher form of the branchial apparatus is shown at c, the respiratory surface being extended by the subdivision of the gill into minute folds or filaments, as we see in Fishes ; and a more elevated form of the pulmonary apparatus is seen at d, the membranous surface being extended by subdivision of the internal cavity, as we find to be the case especially in Birds and Mammals. Lastly, at e is shown a plan of one of the 1 pulmonary branchiae ’ of the Arachnida, which form? a kind of transition between the two sets of organs ; the extent of sur- face being given by gill-like plications of the membrane lining the inte- rior of a pulmonic cavity. — Putting aside such modifications, however, as are destined to suit the particular conditions under which the function is to be performed, and looking simply at the essential characters of the Respiratory organs, we shall observe, on tracing them upwards through the principal classes of animals, the same gradual specialisa- tion which has been noticed in the other systems ; for, beginning with the lowest, it will be seen that the general surface is the organ of re- spiration as well as of other functions ; whilst, in the highest, the aera- tion of the blood is almost entirely effected in one central apparatus adapted to it alone, although the general surface is not altogether destitute of participation in it. 511. In the simpler forms of Animal structure, we do not find any special provision for the performance of this function; the aeration of their fluids being accomplished solely by their exposure to the sur- rounding medium, through the thin membrane which forms their external integument. Such is the case, for example, with all the Protozoa, and also with the Zoophytes among the Radiated tribes, and with the Entozua. the lower Annelida, and the Rotifera, among the A rticulated. These animals are all aquatic ; and the only atmospheric air with which they come into relation, is that which is diffused through the liquids they inhabit. Many of them are provided with the means of renewing the stratum of liquids in immediate contact with their bodies, by ciliary action ; the same mechanism thus serving for the acquirement of food (§ 399), and for the aeration of their fluids. Passing to the higher classes of the Articulated series, we find that among the more inert and RESPIRATION IN ANIMALS. 739 inferiorly-organized Crustacea, such as the Pycnogonidce (§ 316 e), and even in some of the lowest Arachnida, such as the simplest Acaridas (§319 d), there are no special respiratory organs ; hut it is a curious mark of the superiority of the Mollusca in regard to the development of their apparatus of organic life, that in scarcely any instance is there such a complete deficiency of special respiratory organs in the higher parts of that series, the only case of the kind being among the most degraded forms of the Nudibranchiate order (§ 304 c). In the Vertebrated classes they are never wanting. 511 a. The first indication of a special provision for the aeration of the fluids, presents itself among the Badiata, in animals which have no proper circulating apparatus. Thus in many of the Pulmograde Acalephse, we find the digestive cavity sending out prolongations, which form by their inosculation a close network along the margin of the disk (Fig. 195) ; and as this margin is furnished with cilia, a constant renewal of the water in contact with its external surface is provided for. In the Ciliograde members of the same class, we find a provision of a similar nature ; the canals prolonged from the digestive cavity being extended beneath the rows of cilia which give movement to the body (Fig. 107, b) ; and thus imparting the materials of nutrition to the tissues which they penetrate, whilst they expose the fluids which they contain to the influence of the aerated water that bathes their surface.— In the class Echinodermata, however, we find a more special provision for the performance of the respiratory function ; which is required, not merely by the increased nervo-muscular energy of these animals, and by the development of a special circulating system, but also by the condensation of their external tegument, which is no longer capable of serving, as in the classes we have been considering, for the aeration of the fluids contained within the tissues it invests. Contrary, however, to the general principle which has been just stated,— that in aquatic animals the circulating system is prolonged outwardly, bringing the blood to meet the air contained in the dense element, — we find that the respiratory apparatus for the most part consists of the membrane lining the cavity which intervenes between the viscera and the interior of the shell ; into this cavity, the water from without is freely admitted, probably in all the member's of this class, save the Holothuriada ; and a network of blood-vessels is mi- nutely distributed upon its walls. The water seems to be introduced into its interior, in the Asteriadci, by means of an immense number of short, conical, membranous tubes, which pass between the pieces of the shelly framework, aud project externally in little tufts ; each of these tubes has a minute orifice at its extremity, corresponding with that existing at the extremity of each of the tentacula of the Actinia (§ 291), which they much resemble ; and through this multiplied series of apertures, the water appears to be drawn in and expelled, by means of the cilia that clothe the respiratory surface. In the Echinida, the pieces of whose shell are in such close apposition as not to allow the passage of such tubes, we find that they are all collected into ten bundles, which pass through the flexible membrane surrounding the mouth ; and it is possible that water may also find its way into their visceral cavity, through the inter- spaces of the teeth. The respiratory tubes may themselves act as external branchiae ; and it does not seem improbable that the admission of water 3 b 2 740 SPECIAL AND COMPARATIVE PHYSIOLOGY. into the tubular cirrhi or ‘ feet’ (§ 296, b, c) may also be subservient to this process, since we find the blood-vessels distributed in a minute net- work on the vesicles at their base. The visceral cavity of the Echinoder- mata is obviously homologous with the peritoneal cavity of Vertebrata ; and it is remarkable that in many Fishes, and even in some aquatic Rep- tiles (as the Crocodile), a provision should exist for admitting water to its interior, apparently for the purpose of aiding in the aeration of the blood. — In the Hololhuriada we find a very remarkable development of a special respiratory apparatus ; for instead of the admission of water into the visceral cavity, we find that it is drawn into a long membranous sac, which opens externally in the cloaca, and extends nearly as far as the mouth. This sac is sometimes single and undivided ; but in all the higher forms of the order it is double, and presents a series of beautiful and minute ramifications, forming what are known as the ‘ respiratory trees’ (Fig. 113, r,r). Both the stem and branches of this organ contain distinct circular and longitudinal muscular fibres, which contract on being irritated, and which expel the water it contains at tolerably regular inter- vals (such as once, twice, or three times in a minute), its re-introductiou being apparently accomplished by ciliary action. The contraction of the muscular fibres of the external integument may probably assist in the expulsor action ; but it is not required, since the respiratory movement continues even after the sac has been laid open. The nearest approach to this curious form of respiratory apparatus, that is presented by any other class of animals, is that which we meet witli in the tracheal system of Insects (§ 520) ; but although there is an obvious relation of analog)/, yet they are not really homologous, for the ‘ respiratory tree ’ of the Holo- thuriada is an offset from the cloacal termination of the intestinal tube, and might thus perhaps be likened to the allantois (chap, xviii.) more correctly than to any other organ of higher animals. 512. In the various classes of the Molluscous sub-kingdom, we find the respiration provided for by the adaptation of special organs for the pur- pose ; but there is a remarkable difference in the mode in which this adaptation is brought about in the several groups. With the exception of the Pulmonated Gasteropods, however, they are all aquatic ; and the aeration of the blood is therefore provided for, by bringing it into relation with the surrounding water ; a continual interchange in the particles of the circulating fluid being seemed by its regular movement through the vessels of the respiratory surface, and a like interchange in the aerating medium being effected by the action of the cilia with which it is clothed as well as, in certain cases, by the muscular contraction of the sac into which it may be received. — In the Bryozoa, however, we find a deficiency of special respiratory organs ; but this deficiency is easily understood. when it is remembered that they have no distinct circulation. Still, the aei a- tion of the nutritious fluid that occupies the visceral cavity, seems to be provided for by its transmission along channels in the interior of the ciliated tentacula (Fig. 114, d, b, b) ; and it has also been observed that the wide pharynx is sometimes dilated with water, which is expelled again without passing into the stomach, as if it had been taken in for the aeration of the fluids of the body. This pharyngeal cavity, in the Tunicata. con- stitutes the special respiratory organ, and is now known as the ‘ branchial sac ; ’ in all those which are formed upon the Ascidian type it is divided RESPIRATION IN ANIMALS. 741 by an immense number of fissures (Figs. 119 and 121), through which a part of the water received into it by the oral orifice passes to the inter- space between its outer side and the muscular mantle ; in the Salpians, however, there are no such apertures, and the respiratory function seems to be chiefly performed by the longitudinal fold of the branchial sac (Fig. 206, cl), to which the blood-vessels are minutely distributed. In the Ascidians, again, the renewal of the aerating fluid is chiefly provided for by the action of the cilia lining the branchial sac ; but the cavity is occasionally emptied by the sudden contraction of its muscular walls, which ejects the contained water with considerable force. In the Salpians, on the other hand, the respiratory current is chiefly sustained by the rhythmical contractions and relaxations of the muscular sac, which also effect the propulsion of their bodies through the sea (§ 299 b). — Turning from these to the Brachiopoda, we find that the mantle which lines their bivalve shells is the principal organ of respiration ; the surrounding water being freely admitted to its internal surface by the fissure which passes along the whole margin of the valves, and thus coming into the closest possible relation with the blood contained in the vessels which are minutely distributed upon it. But as the water also bathes the external surface of the viscera, as well as the ciliated arms, the blood of these parts also must be acted on by the air which it contains ; so that they may be regarded as accessory organs of respiration. — In the LameUibrcinchiata, or ordinary 1 Bivalves,’ the internal surface of the mantle lining the valves, is doubled (as it were) into four riband-like folds, which are delicately fringed at their edges, and which have, in fact, the same essential structure as the gills of higher animals (Fig. 207). To these the blood is transmitted, when it has been rendered venous by traversing the tissues of the body generally ; and in these it is exposed, through a surface which is greatly extended by the minute division of the fringes, to the action of water introduced from without, and constantly renewed by ciliary action. In many of these animals, as in the common Oyster, the two lobes of the mantle are so completely separated, that the water can still enter freely between the valves ; but in general, they are more or less united, so that the cavity in which the gills lie is partially closed. There is always a provision, how- ever, for the free access of water from without, by means of two apertures, one for its entrance and the other for its ejection ; and in certain species which burrow deeply in sand or mud, these apertures are furnished with long tubes, or siphons (Fig. 124, y, z), which convey the wmter from nearer the entrance of the burrow, and carry it thither again. In these also, a continual flow of water over the respiratory surfaces is maintained by the vibration of the cilia, with which they are clothed. 513. The position of the gills in the Brcmchiferous Gasteropoda, is extremely variable. In the Nudibvanchiata they are disposed, without any protection, upon various parts of the general surface of the body ; sometimes forming tufts of delicate leaf-like or arborescent appendages (Fig. 218), which are collected into one cluster (Fig. 126) ; sometimes, on the other hand, presenting themselves as papillie which are scattered over the whole external surface ; whilst in a few of the inferior species of this tribe, these special organs are undeveloped, and the function is per- formed by the general surface alone. In all these cases, the renewal of the water in contact with the external surface is effected by ciliary movement 742 SPECIAL AND COMPARATIVE PHYSIOLOGY. only. In most of tlie other branchiferous Gasteropods, the gills are more or less covered by the shell ; and even where this is but little developed, as in the Aplysia, we find it specially devoted to then- protection. The order Pectimbrancliiata, which is the highest and most numerous subdivision of the class, receives its name from the peculiar pectinated or comb-like arrangement of its gills (Fig. 128), which are lodged within a cavity formed by the arching of the mantle behind the head; and into this cavity the water is admitted by a special channel or siphon.— The highest development of the branch- ial apparatus presented by the Mollusca, is that which we find in the Cephalopoda. In these animals, the gills are very large, especially in the Dibranchiate order (Fig. 135, o, o) ; and they are lodged in a cavity formed by the folding-over of the mantle, to which water is admitted through a wide fissure at the base of the head ; whilst cesses, forming the gills of Doris the current, after passing over them, is ejected ju /ms tom, separated and en- through the funnel (e). The respiratory surface is not here covered with vibratile cilia ; but the respiratory current is sustained entirely by the alternate contractions and dilatations of the muscular parietes of the cavity ; whilst in the presence of special branchial hearts for the propulsion of the blood through the gills (§ 469), we have another provision for the increased vigour of the respiratory function, required by the active habits of these animals, which present a remarkable contrast to the sluggish inert character of the Mollusca in general. — In these classes, taken as a whole, the respiration is low in its amount. The blood contains no red corpuscles, excepting perhaps in the highest class ; and the change in its composition, which is effected by the air, is confined, therefore, to the fluid plasma, or liquor sanguinis. And as it is not exposed directly to the air, except in a few species, but to the ah- contained in the water inhabited by the animals, this change cannot be very energetically performed. But as the life of these animals is cliiefly vegetative, as their movements, except in the highest classes, are few and feeble, and as they maintain no independent heat, there is comparatively little need of the interchange which it is the object of the Respiratory process to effect ; and they can sustain the complete suspension of it for a long time. 514. Among many of the Articulated tribes whose habitation is aquatic, the respiration is carried on upon a similar plan. It is in the Annelida that we first meet with a special provision for tins function ; the blood being transmitted, in the course of its circulation, to a series "f gill-tufts, which are composed of a delicate membrane prolonged from the external surface of the body, and which sometimes have the form <>f branching trees, and sometimes of delicate brushes made up of a bundle of separate filaments. In either case, the filaments are traversed by blood-vessels, and are adapted to bring the blood into close relation with the surrounding water ; and the continual interchange of the latter is provided for by the restless movements of the body, as well as by the Fro. 218. RESPIRATION IN ANIMALS. 743 action of the cilia covering the respiratory organs. The tufts are some- times arranged along every segment of the body, and their multiplication prevents them from individually attaining any considerable size, this is the case in the Nereidans and other Dorsibranchiata (Fig. 147). In other instances, they are disposed at intervals, and are then larger, being- less numerous, as in the Arenicola (Fig. 149). Their most beautiful development is where they are present on the head only, the rest of the body being enclosed in a shelly or sandy tube, as in the Serpulce and Terebellce (Figs. 148, 201). The gill-tufts then frequently present the appearance of a flower, endowed with the most brilliant and delicate hues.* In many animals of this group, as we have seen in the preceding- chapter, there are special provisions for the propulsion of the blood through the gills, for which the feeble action of the dorsal vessel would not furnish sufficient power.— The higher Articidated classes are, for the most part, adapted to atmospheric respiration, on the plan to be presently explained ; but there is one class, that of Crustacea, whose respiration is still carried on through the medium of water. In the lowest forms of this group, there is no special respiratory apparatus ; the general surface being soft enough to admit of the required aeration of the fluids through its own substance, and the animal functions being performed with so little activity, that a very small amount of interchange is required. In the higher orders, however, whose bodies are encased within a harder envelope, a special respiratory organ is almost invariably found. In some of these (as the Branchiopoda) the last joints of all or of the greater number of the legs are flattened out into a surface which is soft and vascular, and wdiich, by its continual strokes upon the water, appears calculated to facilitate the influence of the air upon the nutritious fluid. Proceeding higher (to the order Amphipoda ) we find a particular portion only of the extremity, the flabellar appendage, devoted to respiration ; but this is developed to an increased extent, and the water in contact with its surface is incessantly renovated by currents set in motion by the abdominal members. The next stage in the specialisation of this func- tion, is the restriction of the branchial apparatus (as in the order Isopoda) to the abdominal members, which are entirely devoted to it, and cease to have other uses. In a still higher order ( Stomapoda ), the gills have assumed more of the character which they present in Fishes and some Mollusca ; the laminated or leaf-like form which they at first possessed, having given place to one in which the surface is greatly extended by minute subdivision into delicate filaments. The most developed form of respiratory apparatus possessed by Crustaceans, is that which exists in Crabs, Lobsters, and other Decapods. In this order, not only is the function thrown upon particular organs entirely set apart for the purpose, but these organs are lodged and protected within a special cavity • and the renewal of the water necessary to their operation is secured by * There are few sights more striking to the observer of nature in tropical regions, than the unexpected view of a bed of coral in shallow water, having its surface scattered with the brilliant tufts of the Serpulae which have formed their habitations in it ; the glowing and variegated tints of which, when lighted up by the mid-day sun, and contrasted with the sombre hues of the surrounding rocks, present an appearance compared to which the most beautiful garden of carnations (which flower the animals much resemble in form) sinks into insignificance. 744 SPECIAL AND COMPARATIVE PHYSIOLOGT. the motion of distinct appendages. This cavity is formed by a redupli- cation of the external tegument, and is provided with two orifices, one for the introduction and the other for the expulsion of the fluid. Through these orifices a constantly-renewed supply of water is made to pass, by the agency of a large valve-like organ, placed in the efferent canal, which, by its movements, drives a continual current from behind forwards, and thus occasions a constant ingress through the afferent opening ; this organ is nothing else than the flabelliform appendage of the second pair of feet-jaws, specially developed to answer this purpose. The perfect contact of the water with the respiratory surface is further provided for by the actions of the flabelliform appendages of other maxillary or am- bulatory members, which, in most Decapods, penetrate into the branchial cavity, and incessantly sweep the surface of the branchiae, apparently for the purpose of combing out their filaments (so to speak) by means of the stiff hairs with which they are furnished. In those Crustacea which are adapted to live for a time on land, the orifices of the branchial cavity are very small, so that but a trifling amount of evaporation can take place from them ; and it appears that, in all the species, the gills can be subservient to aerial as well as to aquatic respiration, provided their surface be kept moist, — the asphyxia of the animals in a dry atmosphere being due to the desiccation of the membrane, and its consequent unfitness for the per- formance of its functions. There are other species, known as Laud- Crabs, which not only live habitually out of water, but are infallibly drowned if kept long immersed in that fluid. The membrane lining their branchial cavities is sometimes disposed in folds capable of serving as reservoirs for a considerable quantity of water ; and sometimes pre- sents a spongy texture equally well adapted for storing up the fluid, which is necessaiy to keep the organs of respiration in the state of humidity required for the performance of them functions. Land-crabs are never known to remove far from damp situations ; and this humidity may be either derived from the atmosphere, or may be secreted, as in higher animals, from the circulating fluid. It can scarcely be doubted that the spongy lining of the branchial cavity in these Crustacea is peculiarly subservient to aerial respiration ; and it appears to be owing to the check given to its activity, that the land-crabs are drowned when plunged under water. 515. The stages in the development of the branchial apparatus of the Astcicus fluviatilis (Cray-fish) have been so beautifully traced by M. Milne-Edwards, in connection with the various forms of the same in aduk species of different tribes, that it seems advantageous to notice them here for the sake of ready comparison, rather than to defer the account of them to the general description of the progressive evolution of the system in the embryo of higher animals. — In the earlier periods of embryonic life, no trace of branchiae can be discovered ; so that the embryo may then be considered as on a level with those inferior Crus- taceans, whose respiration is entirely earned on by the general surface. When these organs are first evolved, during the process of incubation, they consist of simple laminated expansions, representing the flabelliform appendages of the three pairs of maxillary member's, and thus cor- responding with the branchial feet of the Branchiopoda. These soon subdivide, and one part assumes a cylindrical form, and seems no longer RESPIRATION IN ANIMALS. 745 to belong to the apparatus, whilst branchial filaments begin to appear on the other, which are subsequently prolonged into complete gills ; during this interval the thoracic extremities have made their appearance, and they also become furnished with branchial appendages ; and thus in possessing respiratory organs which are quite distinct from the instru- ments of locomotion, but are altogether external, the embiyo Decapod corresponds with the higher Stomapoda. At a subsequent period, a narrow groove or furrow is seen along the under edges of the thorax, the margins of which, in no long time, are prolonged so as to meet each other and enclose the gills ; openings being left for the entrance and exit of water, which are at first large, but subsequently become contracted to the proper size. It is thus evident that the lining membrane of the cavity, as well as that which covers the filaments of the branchiae, is but a prolongation of the external tegument. 516. Although the respiratory apparatus in Fishes retains the type which characterised it in the inferior aquatic classes, it undergoes great increase both in extent and importance. In order to keep up with the rapid advance in the development of the other systems, the respiration requires to be conducted, though by means of an aquatic element, with great velocity and effect. For this purpose, it is not sufficient that Fishes should have merely filamentous tufts hanging loosely at the sides of the neck ; but it is requisite that they should possess the means of rapidly and constantly propelling large streams of water over their surface, and of forcing the whole blood of the system through the respiratory apparatus, to be submitted to the action of the air that is contained so scantily in the water. The former of these ends is effected by the connection of the gills Avith the cavity of the mouth, the muscles of which send a rapid cur- rent of water through the branchial passages ; and the latter, by the alteration in the position of the heart, which is placed so as to propel the blood through the respiratory organs before it proceeds to the system at large (§ 470). The gills in most Fishes are disposed in fringed laminse, the iamellie of which are set close together like the barbs of a feather, and are attached on each side of the throat, in double rows, to the convex margins of four or five long bony or cartilaginous arches, which are very similar to the ribs (Fig. 219, a). The extent of surface exposed by these gills is very great, and the network of capillaries spread over them is extremely minute (b). In the Osseous fishes, the gills are lodged in a capacious chamber, which communicates internally with the pharynx by a distinct orifice for each interspace between the branchial arches ; whilst , externally it opens by a single large orifice on each side, which is furnished with a valvular bony operculum, that allows free passage to the currents ejected from the mouth, but serves for the protection of the delicate organs beneath it. In the higher Cartilaginous fishes, the gill-chamber is more completely subdivided ; the branchial lamellae being attached, not merely ,at their bases to the branchial arches, but by their opposite extremities to the membrane lining the chamber ; and each branchial interspace has its own external orifice for the discharge of the water. We shall hereafter find that the branchial fissures in the neck, which thus form a direct com- munication between the pharynx and the external surface, may be seen in all the higher Vertebrata at a certain stage of development. The curious variation from the ordinary type of the respiratory apparatus of 746 SPECIAL AND COMPARATIVE PHYSIOLOGY. Fishes, which presents itself in Amphioxus, and which carries us hack to the type of the Tunicated Mollusca, has been already pointed out (§ 32 1 b). Another variation, which in some degree reminds us of the rudimental respiratory apparatus of the Leech and Earth-worm, is presented in the Cyclostomi, in which we find six or seven pairs of gills on each side, and these not attached to cartilaginous arches, but developed as folds from Fig. 219. a, Branchial arch, with its boodvessels ; cc' } -branchial artery, diminishing in size from c to c' , in proportion as it furnishes the twigs c" c" to the branchial lamella: dd, the branchial rein, which augments in the same proportion, as it receives the venous twigs d' d'. — b, Capillary net- work of a pair of leaflets of the gills of the Eel : — a, a, branches of the branchial artery convey- ing venous blood ; b ? b , branches of branchial vein, returning aerated blood. The disappearance of the dark shading in the net- work, as it traverses the gill, is designed to indicate the change in the character of the blood, as it passes from one side to the other. ^ the lining membrane of as many distinct sacculi ; and to understand the relation between these and the gills of ordinary fishes, we must suppose (as Prof. Owen has remarked") each compressed sac of the J [yxine to be split through its plane, and each half to be glued by its outer smooth side to an intermediate septum, which would then support the opposite halves of two distinct sacs ; if then these vascular surfaces be prolonged into lamellae, tufts, or filaments, and an intermediate basis of support be devel- oped, we have the branchial arch of one of the higher cartilaginous Fishes, which is the homologue, not of a single gill-sac, but of the contiguous halves of two distinct gill-sacs, in the Myxine. The first paid of this change is seen in the Lamprey ; in which we also find the branchial sac- culi communicating with the pharynx, not directly by a separate fissure for each, but through the medium of a tube which comes off from the pharynx on either side, and conveys the water by a distinct branch to each gill-sac. In most of the Cyclostomi, as in the Lamprey, each sac- opens externally at the side of the neck by a separate orifice ; but in the Myxine the efferent current is conveyed by a special tube, which collects it from all the sacculi of one side, and passes backwards to discharge itself near the middle line of the ventral surface, — thus presenting a connecting link between the provision for the discharge of the respiratory current in Amphioxns, and that which prevails in ordinary Fishes. Some even or the Osseous fishes present a departure from the ordinary type in the pro- longation, and contraction of the efferent canal ; and this is particularly remarkable in the Eel tribe, in which we find the two orifices approxi- * “ Hunterian Lectures,” vol. ii. p. 259. RESPIRATION IN ANIMALS. 747 mating each other on the under side of the neck, until, in Synbranchus, they actually meet, so that the respiratory current is discharged from both sides by a single pore on the median line. — During the embryo con- dition of both of the principal divisions of Fishes, the gills may be seen hanging loosely from the back part of the neck ; for, in Osseous fishes, they have attained considerable development before the prolongation of the integument has been formed into the valve which covers them ; and I in the Cartilaginous fishes, the branchial openings are at first large, and the filaments of the gills are prolonged much beyond them,— other fila- ments also, which subsequently disappear altogether, being produced from their edges. 517. The mechanism of respiration is very complex in Fishes; and is evidently adapted to produce the most effectual aeration possible. The mouth is first distended with water ; and its muscles are then thrown into contraction, in such a manner as to expel the fluid, through the apertures on either side of the pharynx, into the gill-cavity. At the same time, the bony arches are lifted and separated from each other, by the action of muscles especially adapted to this purpose ; so that the gill- fringes may hang freely, and may present no obstacle to the flow of the i water between them. When they have been thus bathed with the aerat- ing liquid, and their blood has undergone the necessary change, the water is expelled through the external apertures by muscular pressure, its return to the pharynx being prevented by the valves with which the orifices are furnished. It is well known, that most Fishes speedily die when removed from the water ; and it can be easily shown, that the deficient aeration of the blood is the immediate cause of their death. But as it might have been expected that the atmosphere would exert a much more energetic influence upon the blood contained in the gills, than that which is exercised by the small quantity of air contained in the water, the question naturally arises, how this deficient aeration comes to pass. It is chiefly due to the two following causes ; — the drying-up of the membrane of the gills themselves, where it is exposed to the ah - , so that the aeration of the blood is impeded ; — and the flapping-together of the filaments of the gills, -which no longer hang loosely and apart, but adhere in such a manner as to prevent the exposure of the greater portion of their surface to the air. Those among ordinary fishes can live longest out of water, in which the external gill-openings are very small, so that the gill-cavity may be kept full of fluid : but there is a particular family, that of Labyrinthibranchii, in which the anterior branchial arches give origin to a curious lamellated apparatus, in whose interspaces water may be retained for a considerable length of time, so as to keep the gills moist ; and by this provision, analogous to that which exists in the Land- Crab (§ 514), such fishes are enabled to remain for a considerable time out pf water, performing long migrations over land in search of food, and even (it is said) ascending trees. — The respiration of Fishes is much more bnergetic than that of any of the aquatic Invertebrata ; and this is partly lue to the great extension of the surface of the gills, partly to the pro- dsion just explained for maintaining a constant flow of fresh water over "heir surface, and partly to the position of the heart at the base of the nain trunk that conveys the blood to the gills, by which the energetic impulsion of that fluid through these organs is secured. Their blood, 748 SPECIAL AND COMPARATIVE PHYSIOLOGY. too, is furnished with red corpuscles, which seem to give important aid iu conveying oxygen from the gills to the remote tissues of the body, and in returning the carbonic acid to be excreted. The proportion of these, however, varies considerably in the different species of the class : being very small in those that approach most nearly to the Invertebrata, whilst they are present in large numbers in the blood of certain Fishes, which have great muscular activity, and can maintain a high independent temperature. 518. The branchial apparatus of the larvae of Batrachia and of the Perennibranchiate division of the same order, is constructed in all essentia! particulars upon the plan of that of Fishes. In the first instance the branchiae are external, hanging like tufts from the sides of the neck ; and these external gills, like the branchiae of Invertebrated animals, are covered with cilia, whose actions are of considerable importance in re- newing the stratum of water in contact with them. This state continues in most of the Perennibranchiata through the whole of life ; but iu the Amphiuma and Menobranchus, the external gills disappear, without heine replaced by internal organs of a like kind, and nothing remains of their branchial apparatus but the fissures leading from the pharynx and the surface of the neck. In those Batrachia, however, whose development proceeds further (as Frogs and Water Newts), the branchiae are subse- quently more or less enclosed by a fold of the skin, which forms a mem- branous valve, analogous to the bony operculum of Fishes ; and in rhe tadpole of the Frog, the branchial cavity thus formed is closed com- pletely on the right side, the water which has passed through it being ejected through the opening that remains on the left. 519. Having thus traced the organs of aquatic Respiration, from their simplest and most general, to their most elaborate and most specialised forms, we have to follow the same course with those which are provided for atmospheric respiration. None such are met with among the Badiatnl classes, which are all aquatic ; and the Pulmonated Gasteropods (§ 303) are the only Mollusca which are adapted for breathing air. This group includes not merely the Snails, Slugs, and other terrestrial Gasteropods : but also several, which, although habitually living beneath the water, receive air (taken-in at the surface) into a pulmonic cavity, instead of respiring through the medium of branchiae. This pulmonic cavity is a simple undivided sac, on the walls of which the blood-vessels are minutely distributed ; it lies beneath the dorsal portion of the mantle, and com- municates with the external air by a single orifice on the right side of the neck, provided with a sphincter muscle by which it can be occasionally closed. — In a large proportion of the A rticulated series, however, we find the apparatus for atmospheric respiration presenting a high degree o! development; but, like the branchial organs of the lower members "I that series, it exhibits a tendency to repetition in successive segments, which is in striking contrast with the conformation just described. Tin air-breathing Annelida , such as the Earth-worm, may be regarded a- presenting us with the lowest type of this apparatus, each segment <>f tin body being provided with a pair of sacculi, which open outwards by ori- fices corresponding in position to those of the stigmata of Myriapods an. Insects ; but it does not seem that either in the Earth-worm or in tin Leech (which has a like series of vesicles), these organs are subservient t< RESPIRATION IN ANIMALS. 749 respiration, as they are always found to contain mucus ; and it would appear that the aeration of the blood must be accomplished through the medium of the general surface of the body. In Myriapoda, however, we find a regular transition presented by the respiratory apparatus of the different families, from that just described, to that which is characteristic of Insects ; for whilst in the lower Chilognatha the stigmata lead only to a double series of sacculi presenting no ramifications, the higher mem- bers of that group have a set of ramifying tracheae issuing from each sacculus, and proceeding to the various organs of its own segment ; and in the Chilopoda, the tracheal systems of different segments come into connection with each other by mutual inosculation, some even possessing the two longitudinal canals connecting together the whole series of stig- mata and the main trunks of the tracheae, as in Insects. 520. In studying the Respiratory system of Insects, we shall have occasion to observe several peculiar modifications which it undergoes for particular purposes, whilst its 9on essential character remains un- altered ; and we shall have also an opportunity of noticing the h varieties of form and function which the same apparatus may c ' present at different periods of life, and under changes in ex- ternal conditions. The mus- cular energy required for the d locomotive powers of the per- fect Insect, and the general ac- / tivity of its organic processes, necessarily involve a large amount of communication be- tween the nutritious fluid and the atmosphere ; but, on the other hand, the low develop- ment of the circulating system would prevent the aeration ' J from being accomplished with sufficient rapidity, by the trans- h mission of the blood through one particular organ. The dif- ficulty is obviated by the intro- duction of the vivifying agent into every part of the body, .by means of a complex and minutely-distributed system of tubes ] which ramify through even the smallest and most de- , , , , I • , , , , Tracheal system of Nepa (Water- Scorpion ) ; — a, head; LlCcltG organs, and. tniis bring’ b,. 1st pair of legs; c, 1st pair of wings; d, 2nd pair of the air into immediate relation T’ inss; 2 . nd pair le 6 5 : s, tracheal trunk; . , . h, one oi the stigmata. with all their tissues, whilst they also dilate in certain parts into sacculi, which sometimes attain i considerable size (Fig. 220). The proportional size of these saccidi 750 SPECIAL AND COMPARATIVE PHYSIOLOGY. varies greatly, however, in different families. They are usually most developed in those that sustain the longest and most powerful flight (such as the Bee tribe), which are generally those whose larva condition has been most imperfect, and in which there has been originally no appear- ance of these enlargements ; on the other hand, they are almost entirely absent in the Insects destined to live upon the ground, or in them are little larger than the slight expansions found in the early conditions ot such as undergo no complete metamorphosis. There can be little doubt that one use of these cavities is to diminish the specific gravity of the Insect, and thus to render it more buoyant in the atmosphere ; bul it would not seem improbable that they are intended to contain a store of air for its use while on the wing, as a part of the spiracles are at tl at time closed, so that less can enter from without. — The extent of respira- tory surface thus created is such, that the amount of the aerating changes performed by an Insect in a state of activity, is not less in proportion tv its bulk, than that effected by the most energetic of the Vertebrata. It is impossible to view this subject philosophically, without being struck by the fact, that this very high degree of respiratory power is given, not by a sudden advance to a more complicated and perfect system of organs, such as exists in the Vertebrated classes of animals, but by an extension of the comparatively simple plan, of which we observed the first traces in the Annelida ; thus affording a beautiful example of the great law of regular progression in the development of organs, which has few apparent and perhaps no real exceptions. Nor would it be easy for any reflecting mind to contemplate the manner in which the ah - is thus brought int" contact with the blood' in the minutest textures of the body, without a feeling of admiration at the contrivance shown in the compensation of the limited circulation of the fluids by the extensive distribution of the respiratory apparatus ; and at the means by which the necessary lightness elasticity, buoyancy, and muscular energy are imparted to the bodies of these beautiful and interesting inhabitants of the ah’. 521. In the Larva condition of such aerial Insects as undergo a com- plete metamorphosis, and are therefore most different in their early state from their ultimate character, the structure of the respiratory system closely corresponds with the type it had attained in the higher Myriapoda. We find it entirely consisting of ramifying trac/iece (§ 153), connected with the external air by the stigmata that open on either side of the ventral surface of the body ; and freely communicating with each other, espe- cially by the two longitudinal tubes which traverse its length, and into which the stigmata open by short straight passages. Just as the Larm is passing into the Pupa state, the larger tracheae exhibit dilatations at intervals, which are subsequently developed into expanded sacs that some- times attain considerable size. The efforts which the animal makes at the moment of transformation, to rupture its skin by the distension of its body, appear to contribute towards the expansion of these sacs, the formation of which had previously commenced.* One remarkable portion of the tracheal system, also, the incipient evolution of which may hi detected in the Larva state, now shows an increased tendency to prolon- gation ; — that, namely, which forms the wings. It may be regarded as * Newport on the Respiration of Insects, “ Philosophical Transactions,” 183(1. RESPIRATION IN ANIMALS. 751 absurd to maintain that the wings of Insects are a part of the respiratory apparatus ; but that such is their real homology is shown by their perfect structure (§ 314), and by the history of their development (§ 315/) g). During the first metamorphosis of the Sphinx ligustri, as observed by Mr. Newport, the wings, which, at the moment of slipping off the larva skin were scarcely as large as liemp-seeds, have their tracheae distended with air ; and, at each inspiration of the insect, are gradually prolonged over the trunk by the propulsion of the circulating fluid into them. The enlargement of the tracheae may also be observed in the antennae, which just before the change were coiled up within the sides of the head, but are now extended along the sides and abdomen. The complete development of the respiratory apparatus only takes place, however, at the time of the last metamorphosis ; when the wings become fully distended with air, and prepared for flight by the active respiratory movements of the body ; and the expansion of the pulmonary sacs proceeds to a greater extent. It may frequently be noticed that, for some hours or even days after the perfect Insect has emerged from the pupa state, it makes no effort to fly, but remains in almost the same torpid condition with that it has quitted ; when stimulated to move, however, it makes a few deep inspirations, its wings rapidly become fully expanded, and it soon trusts itself in the element which was intended for its habitation. ■5 22. The various provisions which are made for the respiration of such Insects as inhabit the water, are of a nature too interesting to be passed by. In those aquatic Larvie which breathe ah’, we often find the last segment of the abdomen prolonged into a tube, the mouth of which remains at the surface while the body is immersed. The larva of the gnat may often be seen breathing in this manner, which calls to mind the elevation of the trunk of the elephant, when crossing rivers that entirely conceal his head and body. Sometimes this air-tube, which is to be regarded as a prolonged spiracle, is several inches in length, and its mouth is furnished with a fringe of setae (or bristles), which entangle bubbles of air sufficient to maintain respiration when the animal descends entirely to the bottom ; and the large trachem proceeding from this tube convey the air through the body in the usual way. Most aquatic Larvie which are unpossessed of such an air-tube, have their spiracles situated only at the posterior extremity of the body, and may be seen apparently hanging from the surface, whilst taking in the necessary supply. There are some Larvae, however, more particularly adapted to aquatic respira- tion, by the development of the tracheal system externally into branchial plates or tufts ; the object of which is not so much to carry the circu- lating fluid into contact with the water, as to absorb from that element the air which it contains, and which is then carried into the internal respiratory apparatus. Sometimes the membrane which covers the bran- chiie, and which is a prolongation of the external surface, is continuous, so that the gills have a foliaceous appearance like that of the wings ; but in other cases, it is divided, so that the branchiie more resemble the filamentous tufts of the Nereis. Their position is constantly varying ; \ sometimes they are attached to the thorax, sometimes to the abdomen, i sometimes even situated within the intestine ; but in every case they have an important relation with the movements of the animal, and are frequently the sole organs of progression with which it is furnished. 7 52 SPECIAL AND COMPARATIVE PHYSIOLOGY. Thus, the sudden darting motion of the larva of the Libellula (dragon- fly) is caused by the violent ejection from the intestine, of the -water which has been taken in for the supply of the gills it contains, whence air is imbibed into the tracheal system. A very little examination inio the structure of the wings will show, that it is essentially the same as that of the expanded gills of aquatic larvae ; each consisting of a pro- longation of the superficial covering of the body over a system of rami- fying nerves or ribs, which are principally composed of tracheae in connection with those of the interior of the fabric. Hence the designa- tion of aerial gills, which was given by Oken to the wings of Inserts, however ridiculed by succeeding writers on Entomology, and however absurd it may seem to those who look at the organs in them functional capacity alone, is really supported by the strictest analogies in structure, situation, and development. It is only by taking an extensive view of comparative organisation, that we can have any hope of arriving at accurate results ; and great care is necessary to dismiss from the mind all prejudice in favour of a particular structure as the standard or type of the rest (§ 330, note). All perfect Insects (with the exception of a Neuropterous insect, Pteronarcys, described by Mr. Newport as retaining its larval branch he in the imago state) being adapted for tracheal respiration only, many curious contrivances may be witnessed among such as inhabit the water, for carrying down a sufficient supply of oxygen to aerate their blood whilst under the surface. Some, for example, enclose a large bubble beneath the elytra (wing-cases), which, not being closely fitted to the exterior of the body, leave a cavity into which the spiracles open ; whilst others have the -whole under surface of the body covered with dowu, which entangles minute bubbles of air, in such large quantity, as to render the insect quite buoyant, and to oblige it to descend by creeping along the stem of a plant, or by a strong muscular effort. 523. In the lower tribes of the Arachnida, such as the Acarida. the respiratory apparatus is usually constructed on the plan which prevails among Insects, being composed of a system of trachea;, ramifying through the body, and opening externally by stigmata ; this apparatus, however, would seem to be in some cases very imperfectly developed (§ 319 d). In the higher Arachnida, however, such as the Spider, Scorpion, Ac., the circulating system is more complete, and there is no longer occasion for such universal aeration of the individual parts, that of the nutrient fluid being sufficient. Accordingly the respiratory apparatus exists in a more concentrated state, -which approximates nearly to that which has been described as possessed by the higher Crustacea ; but, being adapted for aerial respiration only, it must be regarded as belonging rather to the pulmonary than to the branchial system. The stigmata in these animals, usually four on each side, instead of opening into a prolonged set of rami- fying and anastomosing tubes, enter at once into distinct sacs, disposes! along the sides of the abdomen, to which the air has therefore ready access. The interior of these cavities is not smooth, however, like that of the pulmonary sacs of Insects, hut is prolonged into a number of duplica- tures or folds ; these lie close to each other like the lamina; of gills, ana may be regarded either as analogous to them, or as rudiments of die partition of the cavity into minute cells, like those of the lungs of higher animals. Each of these laminae is described by Mr. Newport as consisting RESPIRATION IN ANIMALS. 753 of an exceedingly delicate and apparently structureless double membrane, which includes within it a parenchymatous tissue formed of single cells aggregated together, their arrangement having a degree of regularity in some parts, but being quite irregular in others. The blood is brought to this organ by a vessel which runs round its convex margin ; but it must be distributed over its surface by a lacunar circulation between the com- ponent vesicles of the parenchyma.* — Before quitting the Invertebrated classes, whose Respiratory apparatus, both atmosjiheric and branchial, has now been described, it is worthy of note that in no instance do their respiratory organs communicate with the mouth, which is an organ solely appropriated to the reception and subdivision of the food. It may also be remarked, that it is only in the higher tribes that any muscular move- ments are performed, with express reference to the maintenance of the respiratory process by the renewal of the air or water which is required for it ; this renewal, in the lower tribes, being accomplished by ciliary action, or by the general locomotive actions of the body. 524. We now pass on to the consideration of the pulmonary apparatus of Vertebrated animals ; a rudiment of which is found in most members of the Osseous division of the class of Fishes, notwithstanding that the aeration of their blood is otherwise provided for. This rudiment is com- monly known as the ‘ air-bladder which in many Fishes, as in the embryo of Mammalia, is a simple sac, placed along the middle of the back (Fig. 174, 10) ; whilst in others, its cavity is divided by one or more membranous partitions ; and this division proceeds to such an extent in some instances, as to give to the organ the character of the lung of a Reptile. The air-bladder, in a large proportion of the Fishes which possess it, is closed on every side, its cavity having no connection with any other organ ;t this is the case, for example, with that of the Perch, Cod, Mackerel, and other Acantkopterygii of Cuvier. But in most of the Malacopterygii, as the Herring, Salmon, Pike, Carp, Eel, &c., the air-bladder communicates with some part of the alimentary canal near the stomach, by means of a short wide canal termed the ductus pneumati- cus ; and in the Cyprinidce (Carp tribe) this communication is formed with the oesophagus. The true relations of this organ are most remark- ably shown in the Lepidosteus or ‘bony-pike’ of the North American lakes. This curious fish, which presents many Reptilian affinities, has the air-bladder divided into two sacs that possess a cellular structure ; the trachea which proceeds from it opening high up in the throat, and being- surmounted -with a glottis. Another Fish may be mentioned as presenting an apparatus adapted for atmospheric respiration, which is rather a peculiar development of a portion of the branchial apparatus, than the rudiment of the lung of air-breathing Vertebrata ; this is the Cuchia, the peculiarity of whose circulating system has been already noticed (§ 471). Plere, as in the Synbranchus, there is but a single branchial orifice, which is situated under the throat ; and this leads by a passage on each side to the gills, of which there are only two rows, and these but slightly developed. The principal organs of respiration are two vascular sacculi, prolonged * “ Philosophical Transactions,” 1843, p. 296. + According to V on Bar, the air-bladder is developed as a process or diverticulum from the upper part of the alimentary canal ; so that, when it forms a closed sac, the original communication must have been obliterated. See Note to § 531. 3 c 754 SPECIAL AND COMPARATIVE PHYSIOLOGY. from the branchial chamber, and placed on either side of the head ; these communicate with the cavity of the month by two orifices, each of which is provided with a sort of constrictor muscle that serves to contract or entirely close it, and which thus resembles a glottis ; and into these air is received for the oxygenation of the blood which is distributed upon their walls. From what has been said of the anatomical structure of this cu- rious animal, it is obvious that it possesses the circulation of Reptiles, and the respiration partly of that class and partly of Fishes. Its blood will, therefore, be less oxygenated than in the regular types of either class : since the respiratory organs are less adapted for its aeration than those of Reptiles, and only a part of the blood is sent to them, instead of the whole as in Fishes. To this deficiency we may attribute the obtuseness of its senses, and sluggishness of its movements, which form a striking contrast to the vivacity of the Eel. It is generally found lurking in holes and crevices, on the muddy banks of marshes or slow-moving rivers. The power which the animal possesses of distending the respiratory sacs with air, while on land, and the necessity it is under of rising to the surface of the water for the same purpose, prove beyond a doubt that they perform the function of lungs ; and lead us to the conclusion, therefore, that the Cuchia is amphibious in the strict sense of the word — forming a connect- ing link between the Ophidian Reptiles and the Synbranchus among Fishes.* — In some other Fishes, especially such as naturally inhabit small collections of fresh water whose temperature is liable to be c< n- siderably raised during the heat of summer, the mucous lining of the alimentary canal appears to serve as an additional organ of respiration ; for such fishes are frequently seen to rise to the surface, and to swallow au, which is subsequently discharged by the anus with a large quantity of carbonic acid substituted for its oxygen. This is the case, for example, with the Cobitis (loach) ; and it would seem as if, under these circum- stances, some such supplemental means is required for carrying on the respiratory process with unusual activity (§ 538). 525. It does not seem improbable that in those Fishes which possess a short and wide ductus pneujnaticus, the air-bladder may serve as au acces- sory organ of respiration ; atmospheric air being taken in, and carbonic acid ejected, through the alimentary canal, in the manner just described. But in those whoso air-bladder is a closed sac, it seems evident that u cannot in any way conduce to the aeration of the blood ; and this seems one of the instances, of which many might be pointed out both in the Vegetable and Animal kingdoms, where the rudimentary form of an organ, that attains its full development in other classes, is adapted to dis- charge some office quite different from that to which it is destined in its perfect state. The gas which the air-bladder contains, is composed of the same elements as atmospheric air, namely oxygen, nitrogen, and carbonic acid ; but these are mixed in proportions that are very liable to variate u. It has been said that oxygen is deficient in the contents of the air-bladder of fresh-water fishes, and is predominant in that of fishes which inhabit considerable depths in the sea. This organ is altogether absent in fishes accustomed to remain at the bottom, and, whose movements are slow, such as the Pleuronectklcc, or ‘ flat-fish ; ’ whilst it is of large size m Taylor, in “ Brewster’s Journal,” 1831. RESPIRATION IN ANIMALS. 755 those remarkable for vehement and prolonged movements, especially in Flying-fish of various species. It is generally supposed that the fish is enabled by means of the air-bladder to alter its specific gravity, by com- pressing the bag or permitting its distension ; but experiment shows that, after the organ has been removed, a fish may still retain the power of raising or lowering itself in the water. — In many Fishes, especially such as have either no ductus pneumaticus, or a narrow one, the blood-vessels of the air-bladder are developed into vascular tufts, which are sometimes spread over its interior, but are more commonly aggregated into one mass, which is spoken of as a ‘ vascular gland ’ or ‘ vaso-ganglion ; ’ the use of this organ is altogether unknown. 526. The members of the family of Perennibranchiata (which is the only true amphibious group) all possess lungs more or less developed ; those of the Proteus being very similar to the air-bags of Fishes, w'hilst those of the Lepidosiren and of the Siren exhibit saccular - depressions upon their internal surface, which are the first indications of a subdi- vision of the cavity into air-cells. The tube by which they open into the mouth bears a greater resemblance to the ‘ ductus pneumaticus ’ of Fishes, than to the trachea of higher animals, being simply membranous without an appearance of rings ; and the glottis in which it terminates is a mere slit in the throat. Thus, the transition from the simple closed sac of Fishes, to the more complex subdivided lung of the Frogs, is perceived to be very gradual ; whilst, at the same time, the point of connection be- tween the respiratory cavity and the alimentary tube may be observed to ascend, by similar gradations, from the stomach or some neighbouring part, to the oesophagus, and at last to the pharynx. Although all the animals which retain their gills, at the same time that they acquire lungs, are more or less adapted both to aerial and aquatic respiration, the relative degree of the two varies with the comparative development of their organs. Thus, in the Siren, the pulmonic respiration is more extensive and important than the branchial ; but the reverse is the case in the Proteus. Even the Siren, however, dies, if its branchial respiration be prevented by dryness of the gills. In the Ampkiuma and Menopoma (whose early condition is not known) the lungs are more developed ; and the animals seem to breathe by them alone, no branchial filaments being- present. 527. The lungs of the several orders of Reptiles are for the most part formed upon one type, being capacious sacs, the extent of whose vascular surface is but little augmented by sacculi developed in their walls ; in the Loricata and Chelonia, however, there is an incipient subdivision of the principal cavity, which foreshadows, so to speak, that which is pre- sented by the lungs of Birds and Mammals. The lungs of the Frog (Fig. 221) present a good illustration of the inferior type of pulmonated structure in the class of Reptiles. On laying them open, each is found to consist of a capacious cavity, in which the bronchus of its own side terminates, and on the walls of which the pulmonary vessels are distri- buted ; these walls at the lower part are thin and membranous, but at [the upper part they are strengthened by prolongations of the cartila- ginous framework of the trachea, which forms a ring (b) at the root of the lungs ; and these prolongations produce by their inosculation a carti- laginous netwoik, in the interspaces of which are minute sacculi, whose 3 c 2 756 SPECIAL AND COMPARATIVE PHYSIOLOGY. lining membrane is crowded with blood-vessels. In this manner a set of ‘ air-cells ’ is formed in the wall of the upper part of the lung, which communicates with the general cavity, and very much increases the extent of surface by which the blood comes into relation with the air introduced into the pulmonary organs ; but each air-cell has its own capillary network on its walls, and consequently the blood is only exposed to the ah’ on one side of that network, instead of on both as in Mammals (§ 529). — In Serpents, we usually find the pulmonary apparatus to consist of a single long cylindrical sac (Fig. 180), simply mem- branous at its lower part, but furnished at its upper extremity with a cartilaginous reticidation projecting into its cavity, and inclosing in its interstices several layers of ‘air-cells’ with minutely vascular walls, which communicate through each other with the general cavity. The lung of the left side is generally undeveloped. From the great capacity of the respiratory sac, the mobility of their ribs, and the power of their intercostal muscles, Serpents are ca- pable of rapidly inspiring and expiring a large quantity of ah, by which the want of an extensive surface is compensated, and energy is imparted to their muscular exertions. It is the prolonged expulsion of the air after the lung has been fully inflated, that gives rise to the continued hissing sound by which these animals sometimes alarm their prey. In the aquatic Serpents, the large volume of air contained in the body serves to render it buoyant, and at the same time supplies the wants of the animal during a prolonged immersion.— In the Saurian Reptiles, we still find a very imperfect subdivision of the pulmonary sacs : but they are equally developed on both sides of the body, excepting in those genera which approach the preceding order (§ 324 r). In the lower forms of these organs, there is scarcely any appearance of cells, and they are usually much prolonged, frequently extending through the whole trunk, as in the Chameleon, as well as in many other Lizards ; and their fulness or emptiness of air gives rise to the plump or lean appear- ance, either of which these animals have the power of assuming by the simple processes of inspiration or expiration. But when we have ad- vanced upwards to the Crocodile, we find the lungs, though externally small, subdivided to a great degree of minuteness by internal part it ions : and we also find the lungs more restricted to the thoracic region, v ith some indications even of a diaphragm, which is entirely wanting in all the inferior genera. It is not a little curious that in the Crocodile two openings are found near the cloaca, leading from the external surface to the interior cavity of the abdomen, which is lined by the peritoneum. This structure is evidently similar in character to that which has been described in the Echinus (§511 a) ; whether it is adapted to the same purpose, is not yet fully ascertained. It has been supposed by Geoff. St. Fig. 221. apparatus ; b , cartilaginous ring at the root of the lungs; c , pulmonary sacs, co- vered with vascular ramifications. RESPIRATION IN ANIMALS. 757 Fig. 222. Hilaire, that the superior energy of the Crocodile when immersed in water is due to the penetration of that fluid into the abdominal cavity, and the consequent conversion of the peritoneum into an additional respira- tory surface. — The structure of the lungs in Turtles and other Chelonia is very similar to that exhibited by the Crocodile tribe ■ the sacs have their cavities subdivided by incomplete partitions (Fig. 222 ) ; but they are still very capacious, and materially assist, by the quan- tity of air they contain, in buoying up the heavy trunk of these animals when sailing on the surface of the water. — The very imperfect character of the mechanism by which air is dra\\ r n into the lungs in Reptiles, has been already noticed (§ 324 r) ; and it will here suffice to add, that, taken as a whole, this class is remarkable for the feebleness of its respiratory actions, and for the length of time during which the process can be suspended without injury. The demand for aeration is very much regulated, however, by the tempe- rature to which the animal is subjected (§ 97). 528. The respiratory apparatus of Birds, notwithstanding its extraor- dinary extension, is intermediate, in its grade of development, between that of Reptiles and that of Mammalia. In this class, as in Insects, it extends through a great part of the body ; large sacs connected with the lungs being contained in the abdomen, and even continued beyond the cavity of the trunk, as under the skin of the neck and extremities (§ 325/).* Even the bones are made subservient to this function ; for though at an early period they possess a spongy texture, like those of Reptiles, and are filled with thin marrow, they subsequently become hollow, and their cavities communicate with the lungs ; in the aquatic Section of the Lung of the Turtle. * Various surmises have been formed as to the particular uses of these air-sacs in the economy of the Bird ; and it does not seem improbable that, besides contributing to the function of Respiration by the extension of surface they afford, they have some subsidiary purposes. One of the most evident is that of rendering the body specifically lighter, as in Insects ; and this will be obviously assisted by the great heat of the system, which rarifies the contained air. Again, the distension of the air-cells assists in keeping the wings out- stretched ; as is shown by the fact that inflation of those situated in the neighbourhood of their muscles is followed by their expansion ; this must be a most important economy of muscular action, in birds which hover long in the air. Their evident analogy to the pul- monary sacs of Insects (§ 520) is confirmed by their relatively larger dimensions in Birds of long-continued and rapid motion, than in the slow-moving tribes which are almost confined to the earth or waters. It has been remarked in addition, that “ the same air which exerts its renovating influence upon the blood, supports all the more delicate structures which it reaches and surrounds, as a cushion of the most perfect softness and elasticity ; so that by the most rapid motion, and the most violent twitches which the body receives in the changes and turnings of that motion, there can be no concussion of the parts more immediately necessary for the life of the birds.” It would scarcely seem improbable that the large air- cells, which are found extending beneath the integument of the whole surface, especially the under side, of the Pelican and Gannet, serve to deaden the concussion which the body must experience, when the bird, after raising itself to considerable height in the air, lets itself suddenly fall upon the water in pursuit of its finny prey. 758 SPECIAL AND COMPARATIVE PHYSIOLOGY. species, however, the original condition is retained through life. In those Birds of which the bones are thus permeated by air, the trachea may be tied, and the animal will yet continue to respire by an opening made in the humerus or even in the femur. The general structure of the Respi- ratory apparatus of Birds, and the mechanism of its action, having been already sufficiently explained (§ 324/), it is unnecessary here to do more than describe the minute structure of the lungs, which presents features of great interest. The entire mass of each lung may be considered as subdivided into an immense number of £ lobules,’ or ‘ lunglets,’ each of which has its own bronchial tube (or subdivision of the windpipe), and its own system of vessels, which have but little communication with those of other lobules. Every lobule has a central cavity, which closely re- sembles that of the Frog in miniature, its walls being strengthened by a network of cartilage derived from the bronchial tube, in the interstices of which are openings leading to sacculi in their substance. But these sacculi are not, as in Reptiles, bounded by a distinct membrane, pro- longed from that of the general cavity ; for, with the exception of the part nearest the latter, the whole thickness of the wall may be considered as made up of a very close plexus of blood-vessels, between the meshes of which the air penetrates freely without any limitation ; and thus eveiy capillary is in immediate relation with air on all sides/ — a provision which is obviously very favourable to the complete and rapid aeration of the blood which it contains. Of all animals, Birds are most dependent upon a constant renewal of the ah’ in then - lungs, and upon the purity of that with which they are supplied. Most Birds will die in ah’ which has been but slightly charged with carbonic acid, and which can be respired by Mammals without immediate injury ; whilst a greater degree of irn- . purity, which is at once fatal to Mammals, can be sustained for a long period by Reptiles. — It is beautiful to observe that in Birds, as in Insects, the great extension of the respiratory surface is given by a simple increase in the capacity and prolongation of the sacs, and not by that concentra- tion of it into a small bulk, which is effected by the minute partitioning of their cavity, and which indicates the highest form of the respiratory organs. Another analogy presented by their respiratory system to that of Insects, is this : in the latter the whole of the aeration is effected, 1 y bringing the air in contact with the blood actually circulating through the system ; whilst, in the higher air-breathing animals, possessed of a more centralised apparatus (whether consisting of lungs or giUs), the blood is transmitted through it by a special adaptation of the vascular’ system, in the intervals of its circulation through the body. In Birds we find a curious adaptation of the latter more elevated type to the Insect-like conditions of their existence ; for, whilst the air introduced into the lungs acts upon the blood transmitted by the pulmonary vessels, that which fills the air- cells and cavities of the bones comes into relation (as in Insects) with the capillaries of the system at large. 529. The respiration of Mammalia is not, like that of Birds, extended through the system, but is restricted to the lungs ; and as a perfect diaphragm is now developed, which completely separates the thoracic from the abdominal cavity, these organs are confined to the former. * See Mr. Rainey’s description of the ‘ Minute Anatomy of the Lung of the Bird.' in the “ Medico-Chirurgical Transactions” for 1849. RESPIRATION IN ANIMALS. 759 Although their bulk is proportionally so much smaller than that of the pulmonary sacs of Reptiles, the actual amount of surface over which the blood is exposed to atmospheric influence, is beyond comparison larger, owing to their very minute subdivision into cells. The want of capacity, too, is compensated by the active movements of inspiration and expiration, which constantly and most effectually renew their contents (§ 326 t). The 'lungs are greatly developed in all the more powerful Mammalia, especially in the Carnivorous species ; but they are comparatively smaller in their extent of surface, in the feeble and inferiorly-organised Herbivora ; aud the red corpuscles are much more numerous in the blood of the former, than in that of the latter. The varieties in the conformation of the lungs presented by the different order of Mammals, relate chiefly to their exterior divisions, and to their greater or less capacity ; the plan of struc- ture being nearly the same in all. The whole interior of the lungs of Man, and of Mammals generally, is divided into minute ‘air-cells,’ which freely communicate with each other, and with the ultimate ramifi- cations of the bronchial tubes. The partitions between these cavities, the diameter of which in Man varies from l-200th to l-70th of an inch, are formed by double folds of basement-membrane, between which is a capillary plexus ar- ranged in a single layer (Fig. 223); so that the blood in these capillaries is exposed to the air contained in the cells on both sides of it, but is not, as in Birds, brought into relation with the atmosphere without the intervention of a basement- membrane.* It has been cal- culated that the number of these air-cells grouped round the termination of each bron- chial tube, which cluster re- * Arrangement of the Capillaries of the air-cells of the presents a ‘ lobule’ of the lung Hum “" lMn s- of the Bird, and the entire lung of a Frog, — is not less than 18,000; and that the total number in the Human lungs is not less than six hundred millions. Some idea may be formed from this estimate, of the vast extent of surface which is thus provided for bringing the blood into relation with the air. 530. From the preceding sketch of the progressive evolution of the Respiratory system in the Animal scale, it will have been seen that the fundamental character of the respiratory organs is everywhere the same, however different their external form ; and that it is only the disposition of their parts that is varied, in accordance with the circumstances in which their function is to be performed. The progressive specialisation of the function has been traced in ascending the series, by marking the * It is stated by Mr. Rainey (loc. cit. ), that the lung of the Kangaroo, and even that of some Rodentia, presents a condition intermediate between that of Birds and that of the higher Mammalia ; the air-cells, in the parts most remote from the surface, being very small and being but imperfectly bounded by a basement-membrane. Fig. 223. 7G0 SPECIAL AND COMPARATIVE PHYSIOLOGY. evolution of a particular apparatus for its exercise, and the restriction of it to that apparatus ; in no instance has any sudden change in character been witnessed ; but, in the classes adjoining those in which a new organ was to be introduced, has been found some adumbration of it ; yet even where the function is most highly specialised, the general surface is found to retain in some degree its participation in it. The function of Respira- tion is not confined to the lungs, even in animals which possess them in their most developed form. The blood which circulates through the capillaries of the skin is aerated by communication with the atmosphere, wherever there is no impediment offered by the density of the tegumei. tary covering, In Batrachia, especially Frogs, the cutaneous respiration is of such importance to the animal, that, if impeded by covering the skin with oil or other unctuous substance, death will take place almost as soon as if the lungs were removed ; and the animal may be supported for a considerable time by it alone, if the temperature be not too high (§97). In such circumstances, it is found that carbonic acid is generated in an atmosphere of hydrogen, as by pulmonary respiration. In like manner, if Birds or Mammalia be enclosed in vessels out of which their heads pro- trude, carbonic acid will be found to replace a portion of the oxygen ; and the same result has been obtained by the similar enclosure of a limb of the Human body. 531. We shall now briefly trace the evolution of the Respiratory appa- ratus in the embryo of the higher Yertebrata ; reserving, as before, the account of the earliest changes in the ovum to a future period (chap, xviii.), and leaving until then the description of the organs which are peculiar to the foetal condition, and which serve only to assist in the conversion of the nutriment that is supplied from the parent system, as during the germination of seeds.* — At about the third day of the development of the Chick, four pairs of clefts or tranverse slits are observable behind the mouth, in the situation of the branchial apertures of Fishes • and at the same time, the branchial vessels are developed from the aorta, as already described (§ 486). One the apertures is inter- mediate between each pair of vascular arches, just as in the gills of Fishes and Tadpoles. Nothing like branchial tufts, however, are developed : and the appearance described is very transitory, the vessels changing their direction and condition within two days. The development of perfect gills would have been useless, as the animal is not destined to be for a time an inhabitant of water like the tadpole, but has the aeration of its blood provided for, until the time of the perfect evolution of its respiratory system, by an apparatus specially evolved for the purpose. The lung is developed, like the air-bladder of Fishes, as a diverticulum co- process from the upper part of the alimentary canal. Soon after the middle of the third day, two minute wart-like projections are seen upon the tube, which are foimd to be hollow, and to communicate with its * It may here he remarked, however, hy way of completing the series of analogies, that the germinal membrane of the ovum, -which serves as its first respiratory organ, may be regarded with much probability as corresponding with the ‘ mantle ’ of the Mollusca ; and that the allantois seems to be the representative of the ‘ respiratory tree ’ of the Holothuria. The gills of many of the Gasteropods are developed immediately around the anus (Fig. 1'26); and they may thus he considered as an external prolongation of that part of the surface, which in the Holothuria is extended inwards. RESPIRATION IN ANIMALS. 761 cavity.* These gradually increase in size ; and the channels of commu- nication become elongated into tubes. A little later, the tubes partly coalesce into one, and enter the pharynx by a single aperture. This is what we observe in the Proteus, and, as in that animal, the sacs are still simple undivided bags ; after a little time, however, they send out pro- longations in various parts, which again put forth others, so that the cavity becomes gradually more complex. The larynx and glottis are not per- fectly formed until a late period.— The history of the evolution of these or- gans in Mammalia is precisely analogous. It is usually at about the sixth of the entire period of uterine gestation, that the rudiments of the branchial apparatus are seen, as marked by the shortness and thickness of the neck, the penetration of the sides of the pharynx by the branchial clefts, and the division of the aorta into vessels corresponding in number and distribution with the branchial arteries of fishes. These general features have been observed in the embryoes of most orders of Mammalia, not excepting Man himself ; and they are probably common to all. A few days after the appearance of the fifth arch, which is the last developed, the neck begins to elongate, the apertures are closed gradually on the outside, while the vascular arches undergo those changes by which the permanent arterial trunks arising from the heart are formed. The_ lungs in Mammalia are developed much in the same manner as in Birds. They are not discernible before the period when the branchial apertures begin to close ; a single mass is first perceived, which is soon divided into the rudiments of a right and left lung by a longitudinal groove ; and the trachea and bronchi are subsequently developed, as in Birds. 532. Scarely a more beautiful illustration of the Unity of Design, manifested in the creation of different classes of animals, could be ad- duced, than this hidden but not obscured correspondence. Nor is the analogy confined to Animals alone ; for it is impossible to compare the stages of the evolution of the perfect respiratory apparatus in the higher forms of the two kingdoms, without being struck with their essential correspondence. In the Flowering Plant we have seen a temporary respiratory organ, the cotyledon, first developed, like the branchiae of a tadpole ; and disappearing altogether, when the evolution of the perma- nent aerating apparatus renders it unnecessary. And just as the system which is the permanent one of the lower tribes of animals, is transiently indicated in the early development of the higher, so will it hereafter be shown (chap, xviii.) that the foliaceous expansions of the inferior stemless Cryptogamia are to be regarded as the homologues of the cotyledons of Flowering plants, which continue, in the inferior tribes, to perform their functions diming the whole of life, like the gills of aquatic ani- mals. That which has been said of the correspondence of the essential * The account in the text is given on the authority of Von Bar. Many subsequent observers, however, agree in stating, that the bud-like process in which the lungs originate, is not hollow but solid ; being produced by a multiplication of cells of the external layer of the alimentary canal, into which its external tunic is not prolonged. According to this view, the downward extension of the cellular mass in which the lungs originate, is the consequence of the progressive multiplication of cells in that direction ; and the production of the trachea and bronchial tubes is the consequence of the fusion of the cells in particular lines, like the development of the great vessels. Viewed in this aspect, the absence of the ‘ ductus pneu- maticus’ in many Fishes is a result rather of non-development than of subsequent oblitera- tion ; and such would certainly seem to be by far the most probable account of it. 762 SPECIAL AND COMPARATIVE PHYSIOLOGY. structure of the Respiratory apparatus, through all its varieties of external form, will apply with equal truth to its function also ; for, in whatever tribe of Animals the changes composing it have been investigated, they are found to be of a very uniform character. The object of these changes appears to be, in all instances, the liberation of carbonic acid from the blood, the replacement of it by oxygen, and the exchange of nitrogen on one side or the other. It will be more convenient to enquire into the particular character of these changes, in the distinct form in which they are presented to us in the higher Animals, before proceeding to investi- gate their more obscure manifestations in the inferior tribes. Those changes may be examined, either in the circulating blood, or in the ah to which it has been exposed. 533. The most obvious difference between the Blood brought to the lungs for aeration after passing through the capillaries of the system, and that which has undergone the process — or, in short, between ‘venous’ and ‘ arterial ’ blood — is its colour, which is dark purple (sometimes called black) in the former, and bright red in the latter. The alteration in colour may be produced by agitating venous blood with oxygen, or even by exposing it for a time to the atmosphere ; in the latter case, however, only the surface acquires the ai-terial tint. The bright scarlet colour may also be given by the admixture of neutral salts ; whilst the addition of acids renders it still darker and prevents the change. It has been supposed until recently, that these effects are due to a chemical change produced by these agents in the hsematine of the red corpuscles. But such would not appear to be the case ; for when the hsematine has been separated and diffused through water, it is neither darkened by carbonic acid, nor brightened by oxygen, unless some corpuscles be floating in the solution. Moreover, it is found that the action even < t distilled water will darken an arterial clot. Taking all these circum- stances in connection with the facts already stated (§ 173), in regard t'» the changes of form to which the red corpuscles are liable from the influ- ence of reagents, it appears probable that the immediate effect of oxygen, like that of saline solutions, is to contract the corpuscles and thicken their walls, and thus, by altering their mode of reflecting light, to make them appear bright red ; whilst carbonic acid, like water, may be seen to occa- sion a dilatation of the corpuscle, aud a thinning of its walls (which are at last dissolved by it), in a degree that may be well supposed to produce the darkening of the mass formed by their aggregation.* — In what state of combination these gases exist in the blood, or whether they are present in a state of simple solution, has not yet been clearly determined. When venous blood is placed under the vacuum of an air-pump, a small quantity of carbonic acid gas is given out ; but a larger amount, sometimes one- sixth of the whole volume, is evolved when the blood is agitated with atmospheric air, hydrogen, or nitrogen. Gas may be extracted also from arterial blood, by means of an air-pump capable of producing a very com- plete vacuum ; and this is found to consist of a larger proportion of oxygen. From the experiments of Magnus, the latest and most satisfac- tory on the subject, it appears that the Oxygen in arterial blood * For a full discussion of this subject, see Scherer’s Reports in the recent volumes : “ Canstatt’s Jahresberichte,” and the works and memoirs to which he refers. RESPIRATION IN ANIMALS. 763 amounts to about one-half of the quantity of Carbonic acid which it contains, whilst in venous blood it is only about one-fifth ; for whilst about 10 per cent, of Oxygen, and 20 per cent, of Carbonic acid, may be extracted from arterial blood, the quantity of Oxygen removable from venous blood is diminished to 5, whilst that of Carbonic acid is increased to 25. The relative quantity of Nitrogen is extremely variable. 534. The changes in the Air which has been respired, are capable of being examined with greater accuracy. They may be considered under four heads : — 1. The disappearance of Oxygen which is absorbed. 2. The presence of Carbonic acid, which has been exhaled. 3. The absorption of Nitrogen. 4. The exhalation of Nitrogen. — It was formerly supposed that the Oxygen which disappears, is the precise equivalent of the Car- bonic acid which is generated, the latter gas containing its own bulk of the former. But it is now known that the amount of Oxygen that disap- pears, is usually more than that which is contained in the Carbonic acid expelled, so that the surplus must be actually absorbed into the system. The amount of this surplus varies in such proportion, that it sometimes exceeds the third part of the carbonic acid formed, and is sometimes so small that it may be disregarded, — the difference depending, not only on the constitution of the species, but on the comparative degree of develop- ment, and more especially on the nature of the diet. This last fact has been established by the very careful experiments of MM. Regnault and Reiset,* who have shown that if the same animals be fed at one time upon flesh, and at another upon farinaceous food, the quantity of oxygen which is absorbed into the system is much greater in the former case, than in the latter. Thus it would seem evident, that the demand for oxygen in the system is partly connected with the metamorphoses which the alimentary materials undergo, in their passage through the body. Of the nature of these metamorphoses, our information is still very imperfect. — There is now quite sufficient evidence to prove that the i generation of carbonic acid is not due, as -was formerly supposed, to the union of carbonaceous matter brought by the blood, with the atmospheric 'Oxygen introduced into the lungs ; since carbonic acid is not only found to exist in venous blood, but in the products of the respiration of gases entirely free from admixture with oxygen. Such an experiment can only be performed on animals, which can sustain for a time the absence of the stirpulus of oxygen. That Snails confined in hydrogen will generate car- bonic acid, was long ago shown by Spallanzani ; but the later experiments of Edwards, Muller, &c. upon Frogs are more satisfactory, both from their superior accuracy, and from their freedom from the objection which might be raised against the others, on the ground of the low place of their subjects in the Animal scale. It appears that, when confined in hydrogen, frogs will give out carbonic acid, for a time at least, as rapidly as in atmospheric air ; and that the quantity generated in nitrogen is not much inferior. These results are evidently conformable with the prin- ciples formerly stated as regulating the mutual diffusion of gases. Owing ;.o its energetic reaction with carbonic acid (occasioned by its great difference in specific gravity), hydrogen removes it from the blood with greater force than any other gas ; so that venous blood will give off * “ Annalcs de Chiraie,” 1849. 764 SPECIAL AND COMPARATIVE PHYSIOLOGY. carbonic acid when exposed to an atmosphere of hydrogen, even after it has been submitted to the exhausting power of a vacuum. It is obvious, however, that, for the continued generation of carbonic acid, oxygen must be supplied from without, as there is no superfluity of it in the system. — The following, therefore, appears to be the history of the changes which the blood undergoes in its passage through the body. In the capillaries of the lungs it becomes charged with oxygen, which it carries into those of the system ; in the course of the actions which there occur between the nutritious fluid and the textures it supports and stimulates, pail of the oxygen disappears, and carbonic acid takes its place ; the venous blood, therefore, returns to the lungs, holding this in solution, together with the unabsorbed oxygen ; and, in the capillaries of the lungs, the former gas is removed by the atmosphere, and replaced again by oxygen, — the inter- change being entirely in accordance with the physical principles already stated.* 535. With regard to the absorption and exhalation of Nitrogen, it appears probable that both these processes are constantly going on, but that their relative activity varies in different species, at different parts of the year, and under different circumstances. It appeared from the experiments of Dr. Edwards, that an increase in the volume of nitrogen in the respired air took place in most young animals, and during the summer mouths ; but that, in the autumn and winter, there is a con- siderable absorption when adult animals are employed. According to the recent experiments of MM. Regnault and Reiset (op. cit.), the exhalation of nitrogen is more common than its absorption ; since warm-blooded ani- mals in general, when subjected to then- ordinary regimen, increase the amount of nitrogen in the atmosphere, in the proportion of from 1 to _ j i cent, of the oxygen consumed. But when food is withheld, or animals are fed upon a diet to which they are unaccustomed, an absorption of nitrogen takes place ; and this was found to take place to a considerable extent in hybernating Mammals, whose production of carbonic acid does not go ou at more than 1-5 Oth of the ordinary rate ; so that them absorption of oxygen and nitrogen even exceeds the loss of carbon and that sustained by perspiration, and the animal actually increases in weight, without taking food, until it voids its excretions. 536. Animals whose respiration is aquatic do not decompose the rate.' they breathe, but merely abstract the oxygen from the air contained ii it ; for if one of this class be placed in a limited quantity of water, from which it speedily exhausts the air, or in water from which the air has heei expelled by boiling, it dies almost as soon as would an animal whost respiration is aerial, when placed in a vacuum. If, however, the surfao * This view of the function of Respiration was given in a paper which the Author pul lished in the “ West of England Journal ” in 1835, as that which best accorded with the fact then known. It has been fully confirmed by subsequent experiments, especially those Magnus; and it is the one now generally received. Notwithstanding that the proportion between the oxygen absorbed and the carbonic acid exhaled are so inconstant, that they cat not be reconciled with the numerical ratio of 117-1 of the former to 1000 of the latter, with would constantly prevail if the force of ‘ mutual diffusion’ were the only one that regular the exchange, yet there seems to the Author no adequate reason for doubting that the proof is mainly effected b3 r the agency of that force ; the minuter variations being determined 1 other conditions, — in part, not improbably, by the relative amount of the gases existing in t blood, and the tenacity with which it may hold them. RESPIRATION IN ANIMALS. 765 of the water be in contact with the atmosphere, it will absorb air from it ; and the life of the animal will be longer, the more fully the quantity thus obtained compensates for that which is consumed. When a Fish, in a limited quantity of aerated water, has reduced the proportion of air until its respiration has become difficult, it rises to the surface, and takes in ah’ from the atmosphere ; and, if prevented from doing so, it dies much sooner. The air thus taken in probably acts upon the lining membrane of the intestines ; for, after being expelled, it is found to contain a large proportion of carbonic acid. — The respiration of some of the inferior aquatic tribes, such as Crustacea, Mollusca, and Annelida, has been examined with similar results. According to the researches of Humboldt and Gay Lussac, the air contained in water is richer in oxygen than that of the atmosphere ; the proportion being 3 2 per cent, in the former, and but 21 in the latter. 537. The respiration of Insects has been made the subject of accurate research by Mi’. Newport f and the results which he has obtained cor- respond in a remarkable manner, with those of Dr. Edwards’s experiments on Yertebrated animals under different conditions. In those tribes which undergo a complete metamorphosis, the proportion of air con- sumed by the larva is much smaller than that which the perfect Insect requires, when their relative bulk is allowed for, and their condition is the same as to rest or activity. If a larva of the common Butterfly, for instance, has arrived at its full size at the time of making the observa- tion, it appears to respire in a given time more than the perfect insect ; but the result is liable to this fallacy — that the former is at least two- thirds larger than the latter, and is almost always in a state of activity, whilst the latter is frequently in a state of quiescence. This fact is evidently analogous to one ascertained by Dr. Edwards, that in the higher animals, a greater quantity of oxygen is required in the adult , state, in proportion to the size of the respiratory apparatus, than in the infant condition. Again, many larvae can support a degree of privation of oxygen which would be fatal to the perfect Insect ; thus, there are 1 some which inhabit the bodies of other insects, or are buried deeply in the soil, or seek their subsistence in noxious and unaerated places, all of which situations would be soon destructive to life in their advanced condition. This, too, finds its parallel in the history of the Yertebrated classes : for Dr. Edwards found that puppies, soon after birth, wall recover after submersion in water for 54 minutes, thus bearing the priva- tion of oxygen much better than the adult animal. The amount of respiration in the perfect Insect depends chiefly upon its state of activity ,or excitement. When its movements are rapid and forcible, the aeration of the tissues must be performed to a greater extent than when it is at rest ; and the difference is manifested, as well by the respiratory motions, as by the amount of oxygen consumed. Thus, the number of respirations in an Humble Bee (Bombus terrestris), while in a state of excitement soon after its capture, was from 110 to 120 in a minute ; after the lapse of an hour they had sunk to 58, and subsequently to 46. Moreover a sjiecimen of the same insect, confined in a limited quantity of air, produced in one hour after its capture, whilst still in a state of great activity, about l-3rd “ Philosophical Transactions," 1837. 766 SPECIAL AND COMPARATIVE PHYSIOLOGY. of a cubic inch of carbonic acid ; yet during the whole twenty-four hours of the succeeding day, the animal evolved a quantity absolutely less. The amount of respiration in the Pupa state is much less than in any other condition of the insect, which will readily be undei’stood when its complete inactivity is remembered ; the state of the animal at that time being comparable (as far as its respiration at least is concerned) to that of the hybernation of warm-blooded Vertebrata. 538. In cold-blooded animals in general, the activity of respiration is increased with elevation of the temperature of the surrounding medium. This has been shown in a very striking degree with regard to the Amphi- bia, by the researches of Dr. Edwards (§ 97). The influence of tem- perature is seen also on the existence of Fishes in limited quantities of water ; and the degree of heat which obliges frogs to increase their respiration by quitting the water entirely, causes fishes to take-in air from the surface, as may be frequently witnessed during the summer, espe- cially in small collections of water (§ 524). The amount of carbonic acid generated by Insects, also, increases with the surrounding temperature, other things being equal. — The activity of respiration in warm-blooded animals, on the other hand, is increased by a depression of the external temperature. Thus it was found by Letellier, that when small Birds and Mammals were enclosed in limited quantities of air, the quantity of carbonic acid produced in a given time was in close accordance with the difference between the temperature of the air and that of their bodies. The following were the amounts generated in an hour, by the respective animals named, at the temperature of 32°, at one near to that of their own bodies, and at an intermediate grade : — A Canary A Turtle-dove . Two Mice A Guinea-pig . Temp. 86° — 106°. Grammes. 0T29 0- 366 0-268 1- 453 Temp. 59° — 68°. Grammes. 0-250 0-684 0-498 2-080 Temp, about 32°. Grammes. 0-325 0-974 0-531 3-006 Thus it would appear that the quantity of carbonic acid exhaled by Birth between 86° and 106° is not much more than half of that which i' exhaled between 59° and 68° ; and is only about two-fifths of that which is given off at 32°. In Mammals, the quantity exhaled at 86° — 106 is about half that which is given off at 32°. It is easy to understand the meaning of this difference in the effect of external temperature upon warm and cold-blooded animals, when viewed in its relations to their economy. For in the latter, the increase of respiration consequent upon the elevation of temperature is simply an exponent (so to speak) of the general vital activity of the animal, which heat tends to augment. But in the former, an elevation of external temperature decreases, whilst a depression increases, the demand for that internal combustive process, by which the standard heat of the body is maintained (chap, xyii., Sect. 3). EXHALATION IN PLANTS. 767 CHAPTER XIV. EXHALATION OF AiQUEOUS VAPOUR. 1. General Considerations. 539. As all the alimentary materials taken into living bodies, for the nutrition of their solid tissues, are in a fluid form, being either dissolved or suspended in water, it is evident that a large quantity of that liquid must be superfluous, and that means must be provided for carrying it out of the system. This is partly accomplished, in Animals more especially, by its combination with various other ingredients, — which have either been introduced in greater quantity than the processes of nutrition require, or have already served their pm'pose in the vital economy, — into the fluid excretions, for the separation and deportation of which various arrange- ments are provided (chap. xvi.). But besides the means thus afforded for the diminution of the superfluous fluid of the system, we find that the external surface has this function peculiarly imposed upon it, and that the disen- gagement of nearly pure aqueous vapour, though really the effect of simple evaporation, is principally dependent upon a special arrangement, by which its liberation from the circulating fluid is facilitated. This is most evident in Plants, where the quantity of fluid absorbed bears a much larger proportion to the amount of the solid matter contained in it, than it does in Animals ; and wdiere, from the little opportunity which there is for the introduction of superfluous nutriment, and the slight tendency to decomposition in the solid structures, the necessity for a constant excre- tion of other ingredients unfit to be retained is much less. 2. Exhalation in Plants. 540. The soft and succulent tissues of Vegetables, if freely exposed to the atmosphere, would soon lose so much of their fluid as to be incapable of performing their functions ; and in all plants, therefore, which are subject to its influence, w 7 e find a provision for restraining such injurious effects. In the Algae , however, and other tribes constantly immersed in (rater, or in a very moist atmosphere, no such loss can take place in their natural condition, and no means are required to prevent it. The outer .ayer of cells composing their integument, differs but little from those ■vhich it holds together, except in density (Figs. 70, 71) ; and it is accord- ugly found that such plants, when exposed to a dry air, speedily desiccate, ill but the simplest Cellular Plants, whose natural residence is the air, m the other hand, are covered with a membrane of peculiar character, yhich is termed the cuticle * (Fig. 224, a, h). This is composed of cellular * Notwithstanding the usage of many distinguished Botanists, who apply the designation pidermis to this membrane, the Author has preferred retaining the term ‘ cuticle,’ not only s being the one in common use in this country, but likewise as being the more appropriate, ’or by the term epidermis is implied, a membrane lying upon the dermis or skin ; and there- Ire it can only be appropriately employed to designate the external pellicle presently to be escribed. . 768 SPECIAL AND COMPARATIVE PHYSIOLOGY. tissue, the vesicles of which are flattened, and arranged with great regu- larity, in close contact with each other ; and they differ from those of the parenchyma beneath, not only in form, but also in the nature of then- contents. The form of these vesicles is different in almost every tribe of plants; thus in the cuticle of the Iris and of many other Monocoty tedons, they are elongated, and possess straight walls and regular angles ; whilst in that of the Fig. 224. Apple and Lily (Fig. 225) their boundaries have a sinuous character. The cells of the cuticle are filled with a colour- less fluid; and their walls are usually thickened by secondary deposit, espe- cially on the side nearest the atmosphere ; this de- posit is usually of a waxy nature, and conse- quently renders the mem- brane very impermeable to liquids. In most Eu- ropean plants, the cuticle contains but a single row of cells, which are more- over thin-sided ; whilst in the generality of tropi- Vertical section of the Leaf of Lilium album ; a, cells of the upper cal SpeciGS, there exist cuticle; 6, cells of the under cuticle; c. d, cells of the parenchyma; . e e e, stomata. two, three, or even fern- layers of thick-sided cells. as in the Oleander, the cuticle of which, when separated, has au almost leathery toughness. This difference in conformation is obviously adapted to the respective conditions of growth ; since the cuticle of a plant indigenous to temperate climates would not afford a sufficient pro- tection to the interior structure, against the rays of a tropical sun ; whibi the diminished heat of this country would scarcely overcome the resistance afforded by the dense and non-conducting tegument of a species formec to exist in warmer latitudes. — Externally to this membrane, there usualh exists a very delicate transparent pellicle, without any decided traces <> organisation, though occasionally somewhat granular in appearance, anc marked by lines wdiich seem to be the impressions of the junction of tin cells with which it was in contact. This membrane is obviously formec by the agency of the cells of the cuticle ; and is either a secretion fron their exterior (as maintained by Schleiden, Payen, and others), anal -gou to that which forms a thick gelatinous layer on the surface of the Alga and which elsewhere constitutes the ‘ intercellular substance ;' or itsel consists of the external layers of the thickened walls of the cuticular cell which have coalesced with each other, and which become detached five the subjacent layers by maceration (as affirmed by 5 1 old and Henfiey). 541. In the Cuticle of most Plants which possess such a structure di- tinctly formed, there exist minute opening's termed Stomala (Fig. 224. ■ ■ EXHALATION IN PLANTS. 769 £■*#> '4ww MM:: which ape bordered by cellules of a peculiar form, distinct from those of the cuticle, and more resembling iu character those of the tissue beneath. These boundary cells are usually somewhat kidney-shaped (Figs. 225, b, b), and the opening between them oval ; but, by an alteration in their form, the opening may be contracted or completely closed. They are sometimes more numerous, however, and the opening angular ; and in Fio. 225. the curious Marchantia poly- morplia, their structure is ex- tremely complicated. The openings in the cuticle of this plant (Fig. 226, a, b, b) are sur- rounded by five or six rings placed one below the other, so as to form a kind of funnel or chimney, each ring being com- posed of four or five cellules (b, h). The lowest of these rings (i) appears to regulate the aperture, by the contrac- tion or expansion of the cells which compose it. Wherever stomata exist in the cuticle, hey are always found to open nto cavities in the tissue beneath, which are thus brought into immediate ■elation with the external air. In the Marchantia these chambers are very arge, and are surrounded by regular walls (c, c) ; whilst in the leaves of jrigher plants they exist simply as intercellular spaces, left by the deficiency >f the tissue. Stomata do not exist where there is no regular cuticle ; and Under surface of the leaf of Lilium album ; a a, cells of the cuticle; bb, stomata; ee, cells of the parenchyma in contact with the cuticle. Fig. *226. a, Portion of foliaceous expansion of Marchantia polymorpha , seen from above; a a , lozenge- shaped divisions ; b b> stomata situated in the centre of the lozenges; c c, greenish bands sepa- rating the lozenges ; — b, vertical section of the foliaceous expansion, showing a a, parenchymatous cellular tissue, forming the floor of the cavity; 6 6, superficial layer, covering-in the cavity; c c , cellular tissue forming the walls of the cavity; d , air-cavity, partly occupied by loose cells ff; y> stoma divided perpendicularly; h, layers of cells forming its wall ; i, cells forming the obtu- rator ring. ey are consequently not found upon the lower cellular plants, and but ry rarely on Mosses, in which group they seem to be restricted to the psules or ‘ urns,’ and to their ‘ setrn ’ or footstalks, on which parts the tide is more distinct from the subjacent tissue than it is elsewhere. iey are not formed upon the cuticle of any plants growing in darkness, 3 D 770 SPECIAL AND COMPARATIVE PHYSIOLOGY. nor upon the roots, nor the ribs of leaves ; but they exist in general on all foliaceous expansions, and on herbaceous stems, especially on those of which the siu’face performs the functions of leaves, as in the Cad fa:. They are most abundant on the under surface of leaves, except when these float on water, and then they are found on the upper side alone ; hut they exist equally on both surfaces of erect leaves, as in the Lily tribe and Grasses. It has been estimated that no fewer than 160,000 are contained in every square inch of the under surface of the leaves oi Hydrangea, and of many other plants ; and the greatest number seems to present itself in species, in which the upper surface of the leaf is absolutely destitute of these organs. As a general fact, they are least abundant on succulent plants whose moisture is to be retained in the system ; and they arc frequently so imperfectly formed, as not to have any tendency to open, especially on the leaves of those adapted to exist in hot and dry situa tions. In all instances in which perforations exist, the tissue beneath i- very loosely arranged, and contains many intercellular spaces ; in tin greater number of leaves, therefore, the most closely-packed cells will bt found on the upper side (Fig. 224, c), whilst the parenchyma of the lowei part ((/) only comes into contact with the cuticle at intervals (Fig. 225, r. c) and it is to this arrangement that the darker hue generally possessed In the superior surface is principally due. If a leaf be placed in water, ant the pressure of the air above be taken off, a number of minute globule wall be seen to escape from these cavities, and to stud its exterior wit I brilliant points. 542. The loss of fluid from the surface of Plants may take place, as ha been said, by simple Evaporation, or by proper Exhalation. The quau tity of the former will be regulated by the degree of moisture in th tissue exposed to the atmosphere, and by the compactness of its Arras.-, ment. Thus, although the simpler terrestrial cellular plants, such a Lichens, have no true cuticle distinct from the subjacent tissue, the; external layer of cells is generally of so dense a consistence, as to h almost impervious to water ; so that their moisture is very slowly evajx rated. The process is one quite independent of vitality, and is, indeci the means by which dead plants are dried up, and by which the gradvu loss of weight takes place from fruits, tubers, &c., that undergo no othi alteration. It will, therefore, be influenced by those obvious extern, causes, under the control of which the process is universally performed namely, variations in temperature, and in the humidity of the sunv-ur, ing medium. — Exhalation, on the other hand, is a change which on! continues during the life of the plant, and appears to be more closely eo; nected with the performance of its other vital functions. If a piece 1 glass be held near the upper surface of a vine-leaf in full growth iu a ho house, it is scarcely dimmed after some time ; but if in proximity wii the lower surface of the same leaf, it is speedily bedewed with meistui which accumrdates in a short time so as to form drops. This rap transpiration of fluid appears to take place through the stomata, as it now satisfactorily proved that it bears a strict relation (other things beii equal) with the number of stomata in the plant, or on the particular pa of it made the subject of examination. Various experiments have he* made at different times, with the view of ascertaining the quantity water thus transpired from different plants, and the circumstances me EXHALATION IN PLANTS. 771 favourable to the process. With regard to quantity, the results obtained by Dr. Woodward* are among the most worthy of attention, although probably the earliest on record. Four plants of Spearmint were placed with then’ roots in water, and in a situation fully accessible to light, during 56 days (from June 2nd to July 28th) ; and the following table exhibits the quantity of water which each plant absorbed (proper allow- ance being made for the evaporation from the surface of the fluid), and its increase in weight at the end of the experiment. The difference must of course be the quantity exhaled, and would scarcely express the whole amount of it, as part of the increase in weight would be due to the fixation of carbon from the atmosphere. Original Weight. Gain. Water expended. Difference. No. 1. 127 grs. 128 grs. 14,190 grs. 14,062 grs. Nos. 3 and 4 were No. 2. 110 grs. 139 grs. 13,140 grs. 1 3,001 grs. immersed in water No. 3. 74 grs. 168 grs. 10,731 grs. 10,563 grs. with a little earth No. 4. 92 grs. 284 grs. 14,950 grs. 14,666 grs. at the bottom. Total 403 grs. 719 grs. 53,01 1 grs. 52,292 grs. These experiments afford satisfactory evidence of the very large proportion ; )f the absorbed fluid which is given out again by transpiration ; and, 'oined with others by the same individual, they show that the activity of his function is much greater in summer than in the autumn. — A valuable eries of experiments, communicated by Guettard to the Academic Koyale i 1740, confirms this conclusion. He stated that transpiration is so nich less active during the winter than at other parts of the year, even i evergreens, that a Laurel parts with as much fluid in two days of sum- ier, as during two months in winter. He also maintained that transpi- ition goes on much more rapidly under the influence of light and a oderate degree of heat, than at a high temperature and without light. — fie experiments related by Hales in his essays on ‘ Vegetable Statics’ will rer remain, like those which he performed on the Animal Circulation, a onument of his skill and perseverance. The results which he obtained im the accurate observation of a specimen of Ilelianthus annuus (Sun- wer) during 15 days, are those most frequently quoted by succeeding tfiors ; but there are many others scarcely less interesting. This plant s 34 feet high, weighed 3 lbs., and the surface of its leaves was estimated : 5616 square inches. The mean perspiration during the whole period ' s found to be 20 oz. per day ; but on one warm dry day it was as much 30 oz. During a dry warm night it lost 3 oz. ; when the dew was - :sible though slight, it neither lost nor gained ; and by heavy rain or ^ it gained 2 or 3 oz. — The following table shows the results of similar t ieriments on other plants. ubject. Surface. Mean Transpiration. Greatest Transpiration. abbage . 2736 sq. in. 19 oz. 25 oz. ! ine . 1820 sq. in. 5J oz. 6 g oz. pple . . 1589 sq. in. 9 oz. 1 5 oz. pmon . . 2557 sq. in. 6 oz. 8 oz. antain . 2024 sq. in. 5 oz. llo oz. : last column shows the mean quantity of water transpired from equal “ Philosophical Transactions,” 1699. 3 d 2 772 SPECIAL AND COMPARATIVE PHYSIOLOGY. areas in the different plants (its depth being stated in parts of an inch) for the sake of ready comparison. That of the Sun-flower would be 1-1 65th ; and is shown, therefore, to be less than half that of the Cabbage. The Lemon may be remarked to have exhaled far less than any of the others ; and the same observation seems true with regard to evergreens in general. An experiment performed by Bishop Watson will assist in giving an idea of the extraordinary amount of change effected by this function in Plants. He placed an inverted glass vessel, of the capacity of 20 cubic inches, on grass which had been cut during a very intense heat of the sun, and after many weeks had passed without rain ; in two minutes it was filled with vapour, which trickled in drops down its sides. He collected these drops on a piece of muslin, which he carefully weighed ; and, repeating the experi- ment for several days between twelve and three o’clock, he estimated a the result of these inquiries, that an acre of grass land transpires in 21 hours not less than 6400 quarts of water. This is probably, however, an exaggerated statement ; as the amount transpired during the period of the day in which the experiment was tried, is far greater than at any other. 543. The water exhaled is very nearly pure, so that what is furnished by different species varies but little in taste or odour. Duhamel re- marked, however, that fluid thus obtained sooner becomes foul than ordinary water. Senebier analysed the liquid which he had collected from the exhalation of a vine at the commencement of the summer, and found that 40 oz. contained scarcely 2 grains of solid matter ; and in a similar experiment on fluid collected at the end of the summer, 1 05 oz. gave but little more than 2 grains, or about 1-25000 part of solid matter. This minute impregnation does not indicate that any vital process of secretion is concerned in the process ; since it is well known that the vapour of all water carries with it a small proportion of whatever substance the liquid may have held in solution. 544. All experiments point to the conclusion, that Light has a mo- important influence on the function of Exhalation. Thus it was sliowi by Senebier that if Plants, in which the process is being vigorously per formed, are carried into a darkened room, the exhalation is immediately stopped ; and that the absorption by the roots is checked almost a completely, as if the plant had been stripped of its leaves. Again, fron the experiments of Dr. Daubeny,* it appears that exhalation is stimulate by the coloured rays of the solar spectrum in proportion to their ilbnn netting not to their heating power, these two being separated by th prism ; and Dr. D. further states that exhalation is not promoted by tl) most intense degree of artificial light, in which he contradicts the opimo expressed by Decandolle. Still, it is certain that heat also, especial] when combined with dryness of the atmosphere, has a greater effect upe the loss of fluid than light alone. It would not seem improbable the that the effect of Light is confined to the opening of the stomata, wlucn is known to perform ; and that the large quantity of fluid diseharg' from them, may be due to the effect of simple evaporation, from tl extensive surface of succulent and delicate tissue, which is thus brougl into relation with the air, and to the constant supply of fluid from with Philosophical Magazine,” May, 1836. EXHALATION IN PLANTS. 773 by which it is maintained in a moist condition. — If Plants are exposed to a light of too great intensity, especially if they are not at the same time well supplied with water, their tissue becomes dried up by the increased exhalation which then takes place, and which is not sufficiently counter- balanced by absorption, so that their vegetation is materially checked ■ — a fact of which we see abundant evidence in dry sandy soils and exposed situations. If, on the contrary, the leaves are shaded, and the roots take up much moisture, the growth of the plant is active and luxuriant, but its tissue is soft ; — an effect partly owing to the retention of fluid, and partly to the diminution of the quantity of carbon fixed from the atmo- sphere. If a plant be kept for some time in total darkness, so that it becomes etiolated (§ 7 6), its texture is soft and succulent, and its tissue is distended with the moisture it has absorbed, and with which it cannot part ; and if this state be allowed to continue too long, the leaves dis- articulate and chop off, and the plant dies of dropsy. Succulent plants naturally require most light ty secure for them a regular discharge of moisture ; and there are some of this character which possess so few stomata, that they may be preserved out of the ground for many days and even -weeks, without perishing from want of moisture. The thickness of their cuticle and their deficiency of stomata render it very difficult to dry them, even with the aid of heat ; and it sometimes happens that Seduins and other such plants push considerable shoots, when placed under pressure whilst being prepared for the Herbarium. 545. The quantity of fluid lost by Exhalation, though ultimately de- pendent upon the degree of moisture supplied to the roots, does not appear to be increased by the propellent force of the sap ; and this, obseives the sagacious Hales, “ holds true in animals, for the perspiration in them is not always greatest in the greatest force of the blood ; but then often least of all, as in fevers.” On the contrary, it is chiefly : by the activity of Exhalation, that the amount of fluid absorbed is deter- mined ; just as the rate of ascent of oil in the wick of a lamp is dependent on the rapidity of combustion at its summit. Plants which exhale largely, therefore, must also absorb largely ; and thus they are enabled to take in and to appropriate, in considerable quantity, whatever nutrient materials the soil may contain. These plants, moreover, have usually a great extent of green surface, by which they can obtain a large additional supply of carbon from the atmosphere ; and thus their vegetative func- tions are performed with great activity, and they rapidly add to their growing parts. On the other hand, the plants which exhale slowly absorb hut little from the soil ; and their fixation of carbon from the atmosphere Is usually performed by a comparatively small surface, and at a com- paratively slow rate ; so that their whole vital activity is very inferior, and the amount of solid tissue added to the fabric in a given time is far less. This is the case especially with succulent plants, whose soft and pulpy tissues are enclosed in a cuticle of remarkable density, that pre- vents the evaporation of their fluids ; and whose life may be said to be remarkably slow. It is the case, also, to a certain degree, with most Evergreens ; whose exhalation is slow, although their leaf-surface may be large, so that they fix a great quantity of carbon from the atmosphere ; and it seems to be from this peculiarity, that their leaves possess a re- markable degree of firmness of texture, whilst they have a much longer 774 SPECIAL AND COMPARATIVE PHYSIOLOGY. duration of life than those of deciduous trees. — That by regulating the rate of Exhalation, through the influence of light and heat, we can affect the rate of vital activity, and consequently the duration of life (k 122), appears from the well-known fact, that the freshness of a bouquet of flowers may be preserved for many days longer than usual, by keeping them in the dark. 3. Exhalation in Animals. Fig. 227 . ,d Sudoriferous gland from the palm of the Human hand , magnified 40 diam.; a, a, convoluted tubes, composing the gland, and forming two excretory ducts, h, b, which unite into one spiral canal, that perforates the epidermis at c, and opens on its surface at d; the gland is imbedded in fat-vesicles, which are seen at e, e. 546. The loss of fluid which is constantly taking place from the surface of all Animals inhabiting the air, or at least from some part of it, appears due, like the exhalation of plants, partly to its physical, and partly to its vital conditions. There can be no doubt that from all soft moist surfaces. evaporation will take place in a warm and dry atmosphere ; and the quantity of fluid lost in this manner will be in strict relation with the temperature of the surrounding medium, and the rapidity with which this medium passes over the evaporating surface. The process will of course be impeded by a humid state of the atmosphere, and entirely checked by contact of water, whether warm or cold, with the part which previously effected it. — But there is another process by which fluid is exhaled from the surface, and which is more closely connected with the vital actions of the economy ; this is the transudation from the blood of a wateiy fluid, usually containing a small quantity of saline and animal matter in solution, through the medium of a set of minute glands imbedded in the substance of the cutis or true skin. Each of these little bodies consists of a convoluted tube, in the neighbourhood of which the blood- vessels ramify minutely ; this tube is con- tinued to the surface of the skin as uu excretory duct (Fig. 227), traversing the remaining thickness of the cutis aud epi- dermis in a spiral manner, and opening by a veiy minute pore on the exterior of the latter, passing through it so obliquely that a kind of valve is formed by the membrane over its orifice. When the transudation ol the sweat or sensible perspiration is ob- served with a glass, as it occurs on tlu palms of the hands or the tips of the Angel's the first drop from each pore will be seer to be preceded by an elevation of this littk EXHALATION IN ANIMALS. 775 valve. The ducts are visible in the form of delicate fibres passing from the cutis to the epidermis, when the latter is torn off. 547. The special object of the disengagement of fluid in the form of vapour, from the surface of the bodies of Animals, appears to be the re- duction of the temperature. Animals which inhabit the watel’ have no need of any special provision for keeping down the temperature of their bodies within a certain limit ; since the rapidly-conducting power of the medium is sufficient to reduce any superfluous amount of caloric which may be generated. The tenants of the deep, indeed, have very little power of maintaining a temperature above it, unless they are provided, like the Whale tribe, with a layer of non-conducting fat, or, like diving Birds, "faith a downy covering possessed of a similar property (§ 325). Moreover, the vicissitudes of temperature in large collections of water are never great, so that there is no demand from this source for a means of regulating the temperature of the individual inhabitants. But an Animal living upon the surface of the earth, exposed to constant and extensive atmospheric changes, and deprived of the power of rapidly parting with its heat, when superfluous, by mere contact with a conducting medium, has need of some special means, not only of generating caloric, but also of getting quit of it. The former will be hereafter described in detail (chap, xvii.) ; the latter is simply effected by the watery transudation from the surface, which, being poured out of the perspiratory ducts in a fluid form, and carried off as a vapour by the atmosphere, necessarily renders latent a large quantity of caloric, and thus diminishes the sensible heat of the exhaling body. 548. There is no evidence that either evaporation or transpiration takes place in aquatic animals ; for whilst simple evaporation from their sur- face is of course prevented by the contact of water, the secretions which are formed in their integument would seem to be purely protective. When exposed to the air, all those which are formed of soft tissue, unpro- I tected by a hard envelope, are rapidly desiccated, and usually perish ; and it is evident that such Animals are, when exposed to the atmosphere, in the same condition with the Algie among plants, which lose weight very rapidly owing to the softness of their tissues and the want of a cuticle. Even amongst those which are provided with a hard envelope, there is always a peculiar tendency to evaporation from some part of the surface ; thus, a veiy rapid evaporation of fluid takes place from the gills of the Crustacea, which would speedily offer a fatal impediment to the perform- ance of their functions, if a special provision were not made for preserving their membrane in a humid condition (§ 514). From the experiments of Br. Edwards on Fishes, it appears that the loss of fluid by evaporation from the general surface of the body and from the gills, when the animal is exposed to the air, is so great as to be one of the chief causes of its death. Sometimes the impediment to respiration, which is produced by desiccation of the gills, is the immediate cause of death ; but where this is prevented, and the action of these organs continues during life, the sur- face parts with so much fluid by evaporation, that the body becomes stiff and dry, and previously to death loses from 1-1 4th to l-15th part of its weight. Further, if the lower part only of the body be immersed in water, no absolute diminution in the weight of the whole takes place, and life is prolonged, although death at last results, seemingly from the un~ 776 SPECIAL AND COMPARATIVE PHYSIOLOGY. favourable influence of dry air upon the branchial apparatus ; but if, on the other hand, the head and gills be immersed and the trunk suspended in air, life may be almost indefinitely prolonged, although the drying of the sm-face of the part of the trunk exposed to the air is as marked, as in the case where these animals are entirely exposed to the atmo- sphere, and where they die after a considerable diminution in weight. 549. It is among terrestrial Animals, that the process of exhalation assumes a higher rank amongst the vital functions ; and, even in tie lowest classes, we find it exercising a very important influence on the condition of the system. Thus, in Insects, it has been ascertained by Mr. Newport, that the transpiration of fluid takes place to a considerable extent ; and this not only in the species which have a soft external tegu- ment, but among those which have the body encased in a dense homy envelope, such as the Beetle tribe. It is of course difficult to ascertain what proportion of the loss of fluid takes place in each case from the ex- ternal surface, and from the prolongation of it that lines the air-passages, which in this class are so extensive and minutely ramified ; probably it is from the respiratory membrane, as in the Crustacea, that the principal liberation of it occurs. These observations show that Insects have the power of reducing their temperature, by this means, when it has been excessively raised by a continuance of rapid movements, or when the heat of the surrounding medium is too great.— It is among the Batrachia, however, that the exhalation of fluid from the sm-face is carried on to the most remarkable degree, and seems to answer the most important purpose in the economy ; and it is here, therefore, that its conditions may be most advantageously studied. Of the numerous experiments performed by Dr. Edwards on this subject, the following are the general results. He found that, when a Frog was placed in a dry calm atmosphere, the loss of weight during different succeeding hom-s varied considerably, but with a marked tendency to progressive diminution : that is to say, the more fluid the animal had lost, the less actively did exhalation go on. The actual quan- tity lost was influenced by various external agents, such as the rest or movement of the air, its temperature, and degree of humidity. Thus, frogs, hung in the draft of an open window, lost double, triple, or quad- ruple the amount exhaled by others placed at a closed window in the same room. The influence of the humidity of the air was tested by placing animals of the same kind in a glass vessel inverted over water : and it was ascertained that exhalation, if not then entirely prevented, was reduced to its minimum. On the other hand, when the dryness of the air was maintained by quicklime during the progress of the experiment the diminution of weight was found to be increased, the perspiration being from five to ten times greater in dry air than in extreme humidity, according to the duration of the experiment. The influence of tempera- ture is shown principally in increasing the transudation, or Secretion irorn the skin ; since the amount of fluid lost in a heated atmosphere differs but little whether the medium be humid or diy, and increases in much more rapid proportion than mere evaporation would do. When Frogs were placed in an atmosphere saturated with humidity, by which mere evapo- ration would be almost or entirely suppressed, the loss by transudation between 32° and 50° was very slight, as also between 50° and 6S C : but between 68° and 104° it was so great, that at the last-named degree its EXHALATION IN ANIMALS. 777 amount was 55 times that at 32°. The secretion is not even altogether suppressed by immersion in water. When Frogs are exhausted by excessive transpiration, and are placed in water, they speedily repair the loss, by absorption from the surrounding fluid (§ 440) ; and the quantity thus gained sometimes amounts to one-third of their entire weight. 550. From his experiments on warm-blooded Animals, Dr. Edwards obtained results of a similar kind ; but the influence of changes in exter- nal conditions w ? as not quite so marked. The distinction between the simple evaporation which takes place in accordance with physical laws, and the transudation which is the result of a secreting process, must be kept in view in order to account for their effects under different circumstances. It might, at first sight, appear to correspond w r ith that between insensible or vaporous, and sensible or liquid transpiration ; but this is not altogether true, since the secretion of the skin, if not very abundant, may pass off in the same form with the vapour which arises from its surface. The degree of evaporation from the skin of warm-blooded Yertebrata is modified, as in the.Batrachia and other cold-blooded animals, simply by the tempera- ture, degree of humidity, movement, or pressure of the surrounding medium. Wholly to suppress it, the air must not only be of extreme humidity, but also at a temperature not inferior to that of the animal ; since, if the air be colder, it will be warmed by contact with the body, and thus be capable of holding an additional quantity of aqueous vapour in solution. Although cold, therefore, diminishes or even altogether suppresses transudation, evaporation will continue to a certain extent. In Man, as in the Batrachia, it seems probable that heat alone stimulates the function of secretion from the skin ; so that, at moderate temperatures and in ordinary states of the atmosphere, the quantity transuded is not more than one-sixth of that which is evaporated : whilst at an elevated tempe- rature, especially if the air be already humid, the amount of secretion will much surpass that lost by evaporation ; but if the air be dry and sufficiently agitated, evaporation may increase nearly in the same ratio. — Of the quantity of fluid which may be set free by exhalation within a short time, an idea may be formed from the observations of Dr. South- wood Smith upon labourers at the Phoenix Gas Works.* These men are employed twice a day in emptying and charging the retorts and making- up the fires, for about an hour on each occasion ; the labour is performed in the open air, but is attended with much exposure to heat. On a foggy and calm day at the end of November, when the temperature of the external air was 39°, and the men continued at their work for an hour and a quarter, the greatest loss observed was 2 lb. 15 oz., and the average of eight men was 2 lb. 1 oz. On a bright clear day in the middle of the same month, when the temperature of the surrounding air w-as 60°, with much wind, the greatest loss was 41b. 3oz., the average being 31b. 6oz. And on a very bright and clear day in June, when the temperature of the external air was 60°, without much wind, the greatest loss (occurring in a man who had worked in a very hot place) was 51b. 2oz., and the average of all was 2 lb. 8 oz. — From the experiments of Lavoisier and Seguin it appears, that the average amount of aqueous exhalation passing off insen- Philosophy of Health,” vol. ii. p. 322. 778 SPECIAL AND COMPARATIVE PHYSIOLOGY. silily from the human body is about 3J lbs. daily ; the maximum being 51b., and the minimum l|lb. Of this, however, a considerable pro- portion is set free from the pulmonary surface • the pulmonary being to the cutaneous exhalation, according to the estimate of Seguin, as 7 to 11. There is no reason to believe that the pulmonary exhalation is liberated in any other way than by evaporation , under the peculiarly favourable circum- stances afforded by the delicacy and permeability of the respiratory mem- brane, its constant supply of fluid blood, and the frequent renewal of the air in contact with it. It is obvious that changes in the external condi- tions will have much less influence upon its amount, than upon the quantity evaporated from the skin ; since the temperature of the air in the pulmonary cells will be nearly uniform under all circumstances (in the healthy state at least), and its movements are uninfluenced by the variations of the atmosphere. If, however, the external ah- be saturated with moisture, and be of the same temperature with the body (so as to be unable to acquire by its heat an increased capacity for vapour), it is obvious that the evaporation from the lungs, as well as that from the skin, will be entirely checked. This, indeed, appears to be the explana- tion of the peculiar feeling of oppression, which is consequent upon exposure to an atmosphere that is not only hot but moist. Under such circumstances, the temperature of the body is no longer kept down to its proper standard by vaporisation from its surface ; and at the same time the capillary circulation of the skin and lungs is probably disturbed, by the obstruction to the elimination of fluid which should normally take place through these organs. 551. From the foregoing facts it would appeal- that we may look upon the process of Exhalation as essentially physical in its character. That portion of it which consists in simple evaporation from the cutaneous surface, is obviously so ; and as to that which is eliminated by the agency of the sudoriparous glaoduke, it seems probable that this also may arise from the mere exudation of the watery parts of the blood through their thin-walled capillaries. AVe shall hereafter see that the escape of tbe superfluous water of the blood through the Kidneys, may be looked upon in precisely the same light (§ 600) ; and as we know that any medicinal agent which specially determines the blood towards those organs, will produce an augmentation in the wateiy portion of the urine, so does ii seem probable that the stimulus of external Heat, which specially deter- mines the blood to the cutaneous vessels, should occasion an increased pressure from within, and consequently an augmented passage of fluid through the walls of the capillaries. That the wateiy portion alone should pass, whilst the albuminous and saline matters are kept back, can be easily comprehended, from the facts already cited (§ -ill:) in regard to the very low tendency to £ diffusion ’ exhibited by albumen, and the special power of retaining it possessed by animal membranes. NUTRITION. GENERAL CONSIDERATIONS. 779 CHAPTER XY. OF NUTRITION. 1. General Considerations. 552. The function of Nutrition may be regarded as consisting of that series of changes in the Alimentary materials, by which they are converted into Organised tissues, and are caused to manifest Vital properties. The new tissues thus generated may either constitute an augmentation of the previous fabric, as is the case with Plants even to the end of their lives, with the Zoophytic and other forms of Animals whose increase by gem- mation is indeterminate, and with all Animals during their period of immaturity ; or they may take the place of tissues of an inferior order, without any necessary increase of bulk ; or they may simply rejdace, by a similar production, the loss by disintegration which the fabric at large, or any particular portion of it, may have undergone from the various causes formerly enumerated (§§ 373, 374), as in Animals which, having attained their full dimensions and complete organisation, exhibit no change in size, form, or intimate structure during long periods of time. In the first case, the Nutritive operation manifests itself in Growth ; in the second, in Development ; in the third, in Maintenance : in all cases, however, it appeal's to be essentially the same, and may be considered as the end and aim of all the other Organic functions, in so far as they are concerned in maintaining the life of the individual. For it is dependent for its continuance, in the first place, upon the Absorption of alimentary materials, and hence (in Animals), upon the preliminary process of Digestion ; secondly, it requires the maintenance of the Circulation, for the transmission of the absorbed fluid from the points where it is taken in, to those at which it is to be applied ; thirdly, it cannot long be effected without the continuance of Respiration, the vital properties of the fluids being only maintainable by free communication with the atmo- sphere ; fourthly, it requires the separation of whatever is superfluous or injurious to the process, and thus becomes dependent on the due per- formance of the process of Exhalation in Plants, and on that of Excretion in Animals. 55 3. It has been already shown that every component part of each organ- ism has an individual Life of its own, whilst contributing to support the Life of the entire being, which last is dependent upon the due performance of the vital actions of all the subordinate parts. But the degree of this dependence differs greatly in the different ranks of Organised beings ; and thus the conditions under which the Nutritive function is performed, present a very remarkable diversity, which influences the apparent character of the result, notwithstanding that the essential nature and tendency of the operation is the same in every case. Thus in the lowest forms of Organised beings, the entire fabric is made up of repetitions of the same simple elements, and every part can perform its functions inde- pendently of the rest. In such, every act of nutrition is in fact an act 780 SPECIAL AND COMPAKATIVE PHYSIOLOGY. of growth ; for the tissues never advance beyond the simple cellular character which is common to every part of these homogeneous bodies : and it consists merely in the multiplication of cells by the act of subdi- vision (§ 139), each cell thus produced living for itself alone, and going through all its operations without influence or assistance from the rest. Precisely the same is the case with the early embryo of all the higher forms of organisation ; for this, too, is composed of a homogeneous mass of cells, increasing in number by subdivision, and having no dependence upon one another for the conditions of their vital activity. But as we ascend the series, we find that the simple cellular tissues give place in greater or less degree to others, which are evolved out of them hv a process of ‘histological transformation thus, from the simple cellular Plants we ascend to those in which woody fibre and various forms of ducts are combined with cells ; and in the Animal series, we meet, very low in the scale, with fibrous and membranous, muscular and nervous, homy and calcified tissues, which depart more or less from the original cellular type. Here, then, the act of nutrition does not consist in growth alone, but involves development, not merely in the first evolution of the organism from the embryonic state, but also in every extension which it subsequently undergoes. And it is in proportion to the heterogeneous- ness of this development, as already shown, that the several parts of the organism become more and more dissimilar to each other in structure and function, and consequently more dependent upon one another for the conditions of their vital activity ; so that at last the integrity of the whole structure, and the complete performance of its entire series of actions, comes to be necessary for the continued maintenance of any one. Bat notwithstanding that the life of the individual pails seems to be thus merged (so to speak) into that of the body at large, it is so more in appearance than in reality ; for although a bond of mutual dependence is created by the ‘ division of laboiu - ’ which has been established in the organ- ism, yet the integrity of each individual part remains as much dependent upon the due discharge of its own vital activity, as it is in those simpler forms of organic existence, in which eveiy cell lives for and by itself alone. 554. As every component part of the most complex Organised fabric has an individual life of its own, so must each have a limited duration of its own, quite irrespective of the condition of the fabric at large, except in so far as this may tend to increase or to diminish its functional activity. We find, however, that this duration varies greatly in the different kinds of organised tissue ; and they may be classified in relation to this attri- bute under the following heads. I. Cells may be generated, which have a very transient existence, and which die and disappear again without undergoing any higher develop- ment. This is the case, for example, in the course of the formation of the pollen-grains, and at a certain stage of that of the ovule, of Flowering- Plants ; in which the production of temporary cells, that afterwards liquefy again, appears to take place as a preparation for the formation of higher and more permanent structures. In Animals we have very numerous examples of the same general fact. The ‘ germinal vesicle ’ of the ovum seems to become filled with cells before its disappearance, and no trace of these cells can be afterwards detected ■ so that it would appeal - as if they NUTRITION. GENERAL CONSIDERATIONS. 781 too had deliquesced, leaving their component particles to perform some further part in the process of development. The special £ absorbent cells,’ if such there really be (§ 427), the ‘assimilating cells’ of which an account will be given iu the present chapter, the greater part of the ‘secreting cells’ (§ 576), and also the red blood-corpuscles (§ 173), ap- pear, in like manner, to have a very transient existence, in the warm- blooded animal at least ; their allotted functions being all performed within a few hours or days, anti their term of life being brought to a close as soon as this is the case. II. There are certain component parts of both Vegetable and Animal organisms, whose duration is not less determinate, although it is more extended. This is the case, for example, with the reproductive organs of a large number of Plants (as the flowers of Phanerogamia, the urns of Mosses, and the pileus of Agarics), and with the capsules which seem to represent them in the Hydroid Zoophytes ; for these parts are developed only to serve a particular purpose, which is speedily accomplished, aud they then die and are cast off. So, again, the leaves of Plants, and the polypes of the compound Hydroida, which are the organs most actively concerned iu preparing nutritive material for the permanent fabric, have a limited though longer duration ; and in the higher Animals, we find a considerable number of structures which undergo a periodical exuviation, these being for the most part epithelial or epidermic in their character. Thus, in a considerable proportion of the Articulated tribes, the external integument is thrown off many times during life, and is replaced by a new covering ; a similar repeated moulting of the whole epidermis at once takes place in Frogs, Serpents, and other Reptiles ; in Birds, the feathers are thus periodi- cally cast off and renewed ; in Mammalia, generally, the hair is regularly shed at certain parts of the year, whilst iu Man there is a continual exuviation of the outer layers of the epidermis, and in the Deer tribe even the massive antlers are cast off aud renewed every year. Even the Teeth, as we have seen, are limited in their duration among Reptiles and Fishes, being continually cast out to be renewed again ; and a similar limit exists in the case of the deciduous or milk-teeth of Mammalia, which are shed at a determinate period of life. Now in most of these cases, it is capable of being clearly proved that the exuviation is consequent upon the death of the component elements of the part thrown off, not the death upon the exuviation. For with respect to the leaves of Plants, we have seen (§ 494) that they cease to perform their peculiar vital operations, and that the changes characteristic of decomposition commence, whilst they are still connected with the axis. So it has been pointed out by Mr. Paget,* that the death of the hair-bulb precedes the falling-off of the hah ; and it is a familiar observation that the absorption of the fang of the deciduous tooth, which could not take place without a previous de- generation, very commonly occurs before it is pressed upon by the per- manent tooth which is rising up to occupy its place. So, also, it may be shown by acetic acid or potash, that the outer layers of epidermis are in a state of degeneration : for these reagents produce very little effect upon them, whilst they render the younger cells of the interior layers trans- parent, as they do other tissues in a state of active growth. The ova, too, * ‘ Lectures on Nutrition,' &c. “ Medical Gazette,” 1847. 782 SPECIAL AND COMPABATIVE PHYSIOLOGY. as Mr. Paget remarks, exhibit a remarkable fixity in their term of exist- ence. “ These attain their maturity in fixed successive periods of davs • they are separated (as some of the materials of several other secretions are) while yet living, and with a marvellous capacity of development, if only they be impregnated during the few days of life that remain to them after separation ; but if these days pass, and impregnation is not effected, they die. and are cast out, as impotent as the merest epithelium-cell." III. In striking contrast with the limited duration of such parts, is the condition of those tissues, whose function, instead of being transient, is t< « be indefinitely prolonged ; but such prolongation is seen only when the function, instead of being vital, is simply physical, — as in the case of parts that afford mechanical support, or resist tension, or supply elasticity. Thus we have seen that, among Plants, mechanical support is afforded hv the deposit of hard matter in the interior of the cells, tubes, Ac., of which the part may be composed, as in the ‘ heart-wood ’ of the axis, the ‘ stones ’ of fruit, &c. ; and that the tissues so consolidated are cut off from the general current of vital action. Among Animals, again, we meet with the same method of consolidation, by deposit within cells, in the shells of Mollusca (§ 197) and in the epidermic tissues (§ 179) of Vertebrata ; whilst the shells of Echinodermata (§ 195) and the bones of Vertebrata appear to be formed rather by the chemical union of calcareous matter with a fibrous basis. When this consolidation has been once effected, the hard tissues, if not subjected to disintegrating agencies external to them- selves,* may undergo little or no change for an almost indefinite period. Thus, the heart-wood of trees, the bones and shells of animals, and still more then - hair, hoofs, horns, &c., may remain unaltered through an un- limited series of years. Of some of these parts it can scarcely be said that they are less alive, when removed from the organism to which they belonged, than when included in it. In the heart- wood of a Plant, for example, no vital change takes place, from the time that the woody tubes and fibres have been consolidated by internal deposit ; it may decay, whilst still forming part of the stem, without the nutritive operations of the fabric being thereby interfered with ; and if we could possibly remove it entirely, without doing injury by the operation to the rest of the struc- ture, its absence would be productive of no other evil consequences, than such as would result from the withdrawal of the mechauical support afforded by it. The same may be said of the stony ‘ corals ’ formed in the soft bodies of Zoophytes and Bryozoa, and of the external skeletons of Echinodermata and Mollusca ; which remain unchanged, except lw addition, from the time of them first formation, not only during the whole life of the animal to which they belong, but for an indefinite period after its termination. It is probable that the same would hold good, also, of the Osseous skeleton of Vertebrata, if it were not for the necessity which exists for continually re-moulding this in accordance with the growth of the body, and for providing for its reparation when it has been injured after the attainment of its full growth (§ 211). — The same indisposition to spontaneous change shows itself in the simple Fibrous tissues, which, after them first formation, seem to require but little maintenance, their chemical * Thus the shells of many Mollusca are altered in form, not merely by the addition, but by the removal, of parts (§ 302); this, however, seems to be effected by some superficial rather than by interstitial action, like that by which many Mollusca bore into rocks. NUTRITION. GENERAL CONSIDERATIONS. 783 composition being such as indisposes them to spontaneous decay, and their functions in the economy being purely physical (§§ 161-165). Hence, when these tissues are formed by the transformation of cells (§ 166), it seems as if these cells, in becoming converted into fibres, had almost entirely parted with the distinguishing attributes of vitality, and had thus passed into a condition in which no necessary limitation is imposed on their continued existence. And when developed simply by the fibrillation of a blastema, or by the coalescence or extension of nuclear particles, it seems equally true that the vital attributes of the matter in which they originate, cease with the conversion of that matter into the form of an organised tissue. IV. The duration of the component tissues of the Nervo-Muscular apparatus seems to depend mainly upon the degree in which their vital energies are called forth. When they are left in a state of inaction, they appear to partake, with other soft tissues, in the general attribute of limited duration ; for we find that they undergo a gradual degeneration, and slowly waste away, the rate at which they do so being principally de- termined by the temperature. But when they are called into functional activity, it would seem as if the expenditure of their vital force put an immediate termination to their lives ; so that their elements are at once resolved into inorganic compounds, by the oxygenating power of the blood. Hence we see that no rule can be laid down, as to the duration of the muscular and nervous tissues ; since the exertion of their vital powers for no more than a few minutes, will occasion a lai’ger amount of disinte- gration, than would otherwise occur during many weeks in warm-blooded animals, or during as many months or even years in the prolonged tor- pidity of cold-blooded animals or even of hybernating Mammalia. 555. It may be stated, then, as a general rule, applicable to all the foregoing cases, that the duration of an organised structure is very closely related to the activity of its vital manifestations ; and that this, again, is related, on the one hand, to the character and attributes of the tissue, and on the other, to the conditions in which it is placed. Thus there are certain tissues (such as the Nervous substance of Animals), which would seem disposed to undergo very rapid change, in virtue alike of their che- mical composition and of their vital endowments ; whilst there are others (such as the leaf-cells of Plants) whose component elements have not the same inherent tendency to separation, and the discharge of whose func- tions does not seem to involve the same immediate loss of vital power. But in a cold-blooded Animal in a state of torpidity, the duration of the nervous tissue would seem to be protracted almost indefinitely by cold and inactivity ; whilst that of the leaf-cells of a Plant may be reduced by unusual intensity of heat and light to a much shorter period than usual ; and it has been ascertained by Mr. Paget, that “ if the general develop- ment of the Tadpole be retarded by keeping it in a cold dark place, and if hereby the function of the blood-corpuscles be slowly and im- perfectly discharged, they will maintain their embryonic state for even several weeks longer than usual, the development of the second set of corpuscles will be proportionally postponed, and the individual life of the first will be, by the same time, prolonged.”* We see, then, that * “ Ivirkes’ and Paget’s Hand-book of Physiology,” p. 284. 784 SPECIAL AND COMPARATIVE PHYSIOLOGY. external agencies have a most important influence upon the length of the life, either of the entire organism, or of its constituent parts. The greater the intensity of vital activity which they excite, the less is its duration : for, so soon as the series of vital operations, which each cell or other integral element is adapted to perform, has come to a close, the very same agencies that excited and maintained them, hasten its decomposition and decay. Of this we have a characteristic example in the case of annual Plants, whose term of life is inversely proportional to the amount of light, heat, &c., by which their vital activity has been called forth And we see it also still more remarkably in the case of then’ seeds, which may retain their vital endowments for an unlimited period, provided the vegetative actions are prevented by the deficiency of warmth, moisture, and oxygen ; but which, when called into germination, develope them- selves into structures whose duration is extremely transient. — It does not seem an unfair inference, then, from these and similar facts, that the Vital force takes the place, in an organised structure, of the Chemical Affinity, which holds together the component particles of Inorganic bodies ; and if, as formerly suggested (§ 54), this Vital force be nothing else than the modus operandi in Organised bodies, of the very same forces which are termed Physical when acting through Inorganic matter, the relation between the two agencies seems to justify such a conclusion. For, to take the case of the growing plant, so long as it is acted upon by heat and light, and supplied with nutritive materials, it continues to act upon these and to extend its own organism by the appropriation < >f them, giving origin to new tissue, and thus developing an additional amount of vital force ; during the period of its most active growth, it shows little or no tendency to decay ; but no sooner does the series of vital operations which it is fitted to perform, approach its termination, than the signs of degeneration show themselves, and the influence of Physical agencies is shown in promoting Chemical rather than Vital changes. 556. The operations in which the act of Nutrition appears essentially to consist, may be conveniently divided into two principal stages ; the first of which, Assimilation, comprehends the gradual preparation for organisation which the crude alimentary materials undergo, whilst they are yet in a fluid state ; the second, Formation, on the other hand, consists in the application of the assimilated or organisable particles to the genera- tion of new tissues, or to the replacement of those which have become effete. This distinction cannot be recognised in the lower forms of either Vegetable or Animal existence, in which eveiy component part executes both these operations alike, assimilating the nutritive matter, and either incorporating it with its own substance, or applying it to the production of new parts developed by its own vital endowments. It is in the higher Plants and Animals that we find distinct evidence, that the act of Assimilation may be performed by parts of the organism, which do net themselves apply the organisable material to the formation of tissue ; and that the act of Formation may take place in parts, which have had no participation in the previous preparative changes. Such a mod:- fication is quite in accordance with the general rule of ‘ division of labour ’ which has been so frequently alluded to as a characteristic of the higher organisms. — Both these operations are purely vital. They have NUTRITION. GENERAL CONSIDERATIONS. 785 nothing in common with physical or chemical changes ; and cannot be produced (so far as our present knowledge extends) except through the instrumentality of an organised structure. It would seem as if, in the act of Assimilation, the vital force is imparted by the solid tissues already formed, to the nutritive fluids in contact with them ; for there can be no doubt that the fluid becomes possessed of properties which are themselves purely vital, and that its manifestation of these is most complete when it is in immediate relation with the living solids. Thus, the coagulation of the ‘ plasma ’ of the blood, and the formation of a fibrous tissue which is the result of that act when it is most perfectly performed (§ II), can obviously be attributed only to the peculiar endowments of which the material has become possessed, subsequently to its introduction into the animal body ; it takes place most perfectly, when the plastic material is poured out upon the surface, or effused into the interstices, of living tissues ; and it may be altogether prevented by such a violent ‘ shock,’ as suddenly and completely destroys the vitality of the body. But we shall see reason to believe that there are certain organs, alike in the Animal and in the Vegetable, which are specially destined to effect the elaborat- ing process ; without superseding, however, the more general agency first alluded to. — In the act of Formation, a still higher measure of Vital Force seems to be communicated to the same elements ; since they are then enabled, not merely to assume a more perfectly-organised form, and to manifest endowments of a much higher character, but also to develope, in their turn, similar vital endowments in other nutritive materials. We may trace at least four distinct modes in which it takes place. — In the first place, the material may be applied solely to the interstitial augmenta- tion of the existing elements of the structure ; its component cells or fibres increasing in size without undergoing multiplication, and the whole organ being enlarged without the generation of any new integral part. This is probably the case with the leaves of Plants in general, which continue to enlarge after the number of cells they contain appears to have ceased increasing, and it may also happen in some other instances ; but in general, the enlargement of the cells, &c., already developed, takes place in conjunction with the next form of the nutritive process. — This, secondly , consists in the multiplication of the cells or other component parts, by the subdivision of those previously existing, or by outgrowth from them. Such appears to be the mode in which the most rapid increase of living structures takes place ; and it is seen alike in the Plant and in the Animal, in the most complex as well as in the simplest forms of both. The process has already been fully described as it takes place in the lower Cellular Plants (§§ 137 — 142) ; and this seems to be the type according to which it occurs even among the highest Animals. In the latter, however, it is especially during embryonic life, and in the develop- ment of entirely new parts at subsequent periods, that we meet with examples of it ; still there are certain tissues, as Cartilage (§ 157), in which it continues to take place throughout the whole of life. — In the third mode of Formation, the new tissues seem to arise from certain ‘ germinal centres,’ which appear to have the character of cell-nuclei ; 'sometimes, perhaps, not having been themselves fully developed into cells, and sometimes being residual components of cells which have under- gone transformation, but which leave to their nuclei the developmental 3 E 786 SPECIAL AND COMPARATIVE PHYSIOLOGY. power. Of this mode of formation, which seems peculiar to Animals, we have examples in the development of gdand-cells from the nucleus at the cte cal extremity of the follicle or tubule from which they are to be east forth (§ 185), and in the production of the component cells of muscular fibrillte from the nuclei contained within their » enclosing tube (§ 226). Whether the epidermic and epithelial tissues generally take then- origin in germinal centres contained in the ‘ basement membrane,’ as main- tained by Prof. Goodsir, or are developed in the fourth mode, has not been yet certainly determined. In the adult condition of Animals, in which no new parts are formed, but maintenance of those already existing is alone required, this mode of formation is undoubtedly more common than any other. For such ‘germinal centres’ or ‘ cytoblastfo are scattered through a large part of the fabric, then- relative abundance in different organs being nearly proportional to then - respective activity of growth, and then- disappearance being (as Mr. Paget has justly remarked) a sure accompaniment and sign of degeneration. — But. fourthly , new tissue may be formed at once by the coagulation of the plastic fluid, without the direct intermediation of pre-existing cells or germinal centres (§ 158) ; but it usually possesses a very low degree of vitality, consisting, iHit possess the fibrous type, of little else than an aggregation of similar particles into homogeneous fibres; whilst, if it lie developed into cells, these show a peculiar proneness to degeneration. This mode of formation is rare in Plants, and is most commonly seen in Animals in the reparation of injuries. 557. Under whichever of these methods the Formation of tissues may be performed, the same general conditions are required. — In the ini place, it is requisite that a due supply of the material be afforded, in a state in which it can be appropriated. The degree in which the f< nna- tion of each tissue is dependent upon the previous preparation of its nutritive materials, seems to vary greatly. Thus, as we have seen, among the lower tribes of Plants, every cell can effect the preparative operations, as well as appropriate the material so prepared ; whilst in the higher, a large paid of the organism is dependent for its pabulum upon the materials supplied elsewhere. But we shall presently see (§ 560), that out of the very same organic compounds, the different cells of the higher Plants can elaborate a great variety of new products, differing widely in their chemical character : so that they must retain a certain degree of converting power. Other circumstances being the same, the activity of growth is proportional to the abundance of the nutritive material, pro- vided that the formative power be not wanting. This is especially seen in Plants, in which there are fewer counteracting agencies ; and it is, in fact, the foundation of a large part of the art of Cultivation, which aims especially to accelerate the growth of the entire plant, or to augment the growth of some particular piart, by increasing the supply of the aliment which it may require. But we have also very characteristic examples of it in Animals ; such as the increase in the Adipose tissue, when the food has been abundant in quantity and rich in quality ; the increase in the size of glands, when the blood contains an unusual quantity ot the special product to be eliminated by then - agency : and (according t many eminent Pathologists), the development of abnormal growths (such as Cancer, Tubercle, Lepra, etc., Ac.), which remove particular morbid NUTRITION. GENERAL CONSIDERATIONS. 787 matters from the current of the circulation, with a rapidity proportional to the supply of the peculiar pabulum which their cells respectively require. It seems probable, indeed, that the Animal tissues have less converting power than those of Plants ; and that not only each tissue, but each part of the same tissue, selects some different material from the blood. For there are certain morbid matters, whose presence in the blood is manifested by the perversion of the nutritive process in certain spots only of the body, these spots being similar in size and situation on the two sides ; so that it would seem that the only parts of any tissue which are really identical in composition, are those which occupy sym- metrical positions on the opposite sides of the body.* Now in the healthy state, as in those diseased conditions which afford more striking exemplifications of this principle, every part of the body, by taking from the blood the peculiar substances which it needs for its own nutrition, thereby removes from it a certain part of its constituents, -which would interfere with the nutrition of the rest of the body if retained in the circulation. And this, as Mr. Paget has well remarked, seems to be the purpose answered by the development of many structures that perform no ostensibly useful part in the economy ; such as rudimental organs of various kinds, the lanugo or downy covering of the human foetus which falls off some time before birth, and the coat of hair which is formed in many of the lower Mammals during foetal life, and which gives place before birth to a more complete coat of a different colour. The deve- lopment of these and other structures, for which no other purpose can be assigned, may be fairly regarded, on the principle which has been now laid down and illustrated, as resulting from the presence of their peculiar materials in the blood, and as leaving it fitter by the removal of them, for the nutrition of other parts, or as adjusting the balance which might else be disturbed by the formation of some other part. “ Thus,” to use Mr. Paget’s own words, “ they minister to the self-interest of the indi- vidual, while, as if for the sake of wonder, beauty, and perfect order, they are conformed to the great law of the unity of organic types, and concur 1 with the universal plan observed in the construction of organised beings.” When the due supply of nutritive material is not afforded, imperfect or deficient formation must be the result ; and this is probably the ex- planation of the atrophy which normally occurs in the course of the life of higher animals, in many organs, which at earlier periods of life were of considerable size and importance, such as the thymus gland, the ductus arteriosus and venosus, the mammary glands, &c. 557 a. In the second place, the formative process is mainly dependent upon a due supply of heat. This agent has been commonly regarded as the mere stimulus to its activity (§ 68) ; but reasons have been already given for the belief, that Heat, acting through a substance previously organised, itself becomes the formative power (§ 54). This much, how- ever, is certain ; that the activity of growth bears a very close relation to the temperature of the fabric, so that the formative processes may be artificially retarded by cold or accelerated by warmth ; and it further appears that development requires a higher temperature (as it certainly * See Dr. W. Budd’s admirable paper on ‘ Symmetrical Diseases,’ in the “ Medico-Chirur- gical Transactions,” vol. xxv. ; and Mr. Paget’s ‘ Lectures on Nutrition, &c.’ in the “Medical Gazette” for 1847. 3 e 2 788 SPECIAL AND COMPARATIVE PHYSIOLOGY. seems to depend on a higher measure of vital force) than simple growth (§§ 96, 98). Hence we see that the very same agent which exerts sc remarkable an influence on the duration of the life of the tissues, exerts a corresponding influence on those processes by which their disintegration is compensated ; and the perfect balance between waste and repair is thus maintained. In Plants, indeed, we see this beautiful adjustment extending still further back ; for the heat which promotes the formative processes is accompanied (when not artificially supplied) by a correspond- ing amount of Light ; and these two agents, in proportion to the intensity of their operation, accelerate the absorption of fluid by the roots (§ 541), and also produce a corresponding increase in the supply of carbon fixed by the leaves. 2. Nutrition in Vegetables. 55 8. The phenomena of Nutrition in the simplest Cellular Plants having been already sufficiently considered (§§ 39 — 47), it is not requisite here to detail them again ; and it need only be mentioned, that the same facts hold good as to all Plants, in which the functions of Absorption and Aeration are not specialised by restriction to distinct organs. In the greater number of Vascular Plants, however, there is no doubt that the greatest proportion of the fluid imbibed into the system is derived from the soil surrounding the roots ; and that it holds in solution carbonic acid and ammonia, from the combination of whose elements are produced the proximate principles, gum, sugar, albumen, >f the matters which are sparingly dissolved in this liquid, the plant must absorb far more of it than it can apply to its own use. Hence the crude sap brought to the leaves undergoes a double change ; a large proportion of its water being got rid of, whilst a great addition is made to its carbon; * It is inferred by Prof. Schleiden and his followers, that these are generated by the tissues among which the liquid is diffused, which begin to exert an assimilating power upon the crude sap, as soon as it is brought within their reach. There are cogent objections, how- ever, to such an hypothesis ; and the fact may be as well explained in another way (i 44 7 V NUTRITION IN VEGETABLES. 789 and thus a great increase is presented in the proportion of organic com- pounds which the ‘proper juices’ of the leaves contain. These proper juices appear to furnish the chief part of the 'pabulum, at the expense of which the development of new parts takes place in every part of the organism ; for although it may not be distributed, as some have supposed, by any regular ‘descending current’ (§ 447), yet it is impossible to account for the large quantity of additional carbon taken in by the leaves, unless it is thus applied, since it is certain that these organs them- selves do not undergo any proportional increase, during the period when they are most actively performing the function of aeration. That a marked difference exists between the crude ascending sap whose materials are supplied by the roots, and the ‘ proper juices ’ whose formation thus seems chiefly to depend upon the leaves, is shown in a variety of ways. Thus, the inhabitants of the Canary islands tap the trunk of the Euphor- bia canariensis, and obtain a refreshing beverage from the ascending current, although the proper juice of. the plant is of a very acrid character. The phenomena of vegetable parasitism have a peculiar interest when viewed with reference to this subject. The Phanerogamic parasites may be arranged under two groups ; those provided with leaves, belonging for the most part to the order Loranthacece or mistletoe tribe ; and those which are destitute of leaves, such as the Cuscuta (dodder), Orobanche (broom-rape), and the Lathroea squamaria. Now the plants of the first group may be regarded as natural grafts, organically uniting themselves with their stock ; for the wood and bark of the Mistletoe grow in conti- nuity with the wood and bark of the tree to which it has attached itself, although the line of demarcation between the two may be seen when a vertical section is carried down through the part where they meet. It is obvious that it must receive its supply, like the branches of the stock itself, from the current of sap ascending in the latter ; and that this must be converted, by the elaborating action of its leaves, into the materials of its growth. On the other hand, the leafless parasites attach themselves to the bark alone, by means of suckers which apply themselves to its surface, or of fibres which penetrate into its substance ; and it seems obvious that, as they are not able to elaborate sap for themselves, they are dependent for their support, not upon the crude ascending current, but upon the ‘proper juices’ of the plants on which they live. And this view derives confirmation from the fact, that whilst the leafy parasites will grow almost indifferently upon a great variety of trees, their ascend- ing sap differing but little in quality, the leaf ess parasites are much more limited in their range, each kind being usually restricted to a few species whose ‘proper juices’ are suitable for its support, and those of other plants not being adapted to its nutriment. 559. The leaves of the higher Plants may be regarded, then, as the chief assimilating organs, by which the materials are prepared for the formative processes that take place in different parts of the fabric. That which seems to be the essential pabulum of the vegetable tissues, is the protoplasma (§ 143), containing saccharine, gummy, and albuminous matters in a peculiar state of combination, which is found in all rapidly- growing parts ; and if this be supplied, it is probable that each compo- nent cell of these tissues can elaborate for itself the peculiar compounds which it is destined to contain. And thus it happens that the growth of 790 SPECIAL AND COMPARATIVE PHYSIOLOGY. a ‘stock’ is not changed from its natural method, by the somewhat different quality of the proper juice that may be supplied by the leaves of a ‘ graft ’ ; nor does the mistletoe impart any of its peculiarities to the branch of the tree with which it has united itself. From the very same pabu- lum, the wood of the mistletoe and that ( e.g .) of the apple grow after their respective fashions ; just as in the different parts of the mistletoe itself, the wood grows after one pattern, the bark after a second, the leaves after a third, the flowers after a fourth, and the fruit after a fifth. Every growing part, in fact, turns the ‘ pabulum’ which it receives to its own account, and forms it into its own peculiar tissues. This ‘ pabulum ' appears to be especially diffused through all the cellular portion of the fabric ; and it is in this, as formerly explained, that all new formation originates. Thus we find new leaf-buds in Dicotyledonous stems (§ 280) developing themselves as extensions of the medullary rays ; whilst in the Monocotyledonous (§ 279) they spring from the general cellular mass of the axis ; and in the cells of the parts from which they spring, we usually find a much larger accumulation of starchy and other nutritious materials, than could be derived from the quantity of ascending sap attracted towards them, before exhalation is actively established by the expansion of the buds. — The addition to the central axis, again, is effected in Dicotyle- dons by the continual growth and transformation of the cambium-layer, which intervenes between the last-formed layers of wood and bark. The cambium was formerly supposed to be a mere glutinous sap, and various notions were entertained in regard to the mode of its conversion into the new wood ; but it is now quite ascertained that the cambium is real!} a mass of young cells turged with protoplasma, and that as the inner layers of these are converted into fibro-vascular bundles, they continue to increase by the multiplication of cells on them outer side ; and thus there is an absolute continuity of growth between the inner and outer parts of the axis, although, from the vegetative functions being periodically sus- pended in trees all whose leaves are cast off at once, lines of demarcate >n are more or less distinctly left between the portions formed at successive epochs. Thus are produced the so-called ‘ annual layers ’ in these stems ; which are not, however, to be regarded as by any means uniformly indi- cating the number of years during which a stem or branch has been growing. For there are many trees in tropical climates, whose leaves a:e thrown off and renewed twice or even thrice in eveiy year, or five times iu two years ; and as the whole series of nutritive operations then receives a check, it cannot be doubted that a new line of demarcation will be left by every exuviation. Even in temperate climates the same thing may 1 >e occasionally observed ; thus the author has known a long continuance of heat and dryness in the early part of the summer, to be followed by the entire exuviation of the leaves of trees growing in exposed situations : a new covering of leaves making its appearance within a few weeks after- wards. And temporary checks to the vegetative processes may arise from other sources ; thus the growth of the larch, which naturally thrives iu cold and moist situations, is stopped by heat and dryness ; and the author has been assured by a competent authority, that he has ascertained by actual observation that two thin layers of wood have been formed by larches, instead of one thick one, when the nature of the season had been such as to occasion a prolonged interruption. On the other hand, in NUTRITION IN VEGETABLES. 791 ‘evergreen’ trees, in which the leaves are not all cast off at once, the lines of demarcation are usually much less distinct, since the growth of the cambium-layer is not at any time completely stopped, except by intense cold. As the additional wood is always developed on the exterior of that previously formed, it is obvious that the term Exogenous may he appropriately applied to this mode of growth | it has been also designated as growth by indefinite fibro-vascular bundles. — On the other hand, in the stems of Monocotyledons, the fibro-vascular bundles once formed are not susceptible of increase, and they are therefore said to be closed. Hence the new bundles are not developed in continuity with the old, but take their origin in the midst of that p>art of the cellular mass of the stem which is in closest connection with the new leaves ; and instead of extending through the entire axis, so as to augment its diameter from the extremities of the branches to those of the roots, they p>ass towards its exterior at no great distance from the summit, and there terminate, so that the lower part of the stem undergoes no augmentation. To these stems, therefore, the term Endogenous cannot be justly applied; and it should be dropped altogether. 559 a. But besides ministering to the development of new tissue, the elaborated sap) of Plants supplies the materials for the production of that immense variety of organic compounds, in which the Vegetable World is so rich ; compounds whose use in the economy of the Plant is frequently by no means apparent, but whose value to the Animal Creation and to Man would frequently appear to be the sole end of their preparation. These compounds are frequently designated as Vegetable Secretions ; but this term cannot be applied to them in the sense in which it is used in Animal physiology. For all these products are contained and stored up in cells, which continue to form part of the organized fabric, instead of being cast forth from it (§ 573) ; and they might rather be likened to the fat of animals, which is in like manner separated from the blood by the development of adipose cells, that constitute permanent components of the organism. How of the immense variety of these products that presents itself in different plants, and even in different parts of the same plant, — the fixed and volatile oils, resins, gums, colouring matters, alkaloids, acids, &c., &c., — no other account can be given, than that each component cell generates its own peculiar products, at the expense of the nutrient mate- rials supplied to all alike, just as, among the simplest Cellular Plants, each species may form a distinct organic compound. Thus, to take a simple case, in the petal of a Heartsease or any similar parti-coloured flower, one cell forms purple colouring-matter, while another in close proximity with it forms red colouring-matter ; just as Protococci and Ilcematococci, growing under the same circumstances, and at the expense of the same inorganic compounds, generate green, and blood-red endochrome respectively. — To enter into the chemical characters of the Vegetable secretions, would be foreign to the purpose of this work ; but there is one which is too obvi- ously related to the vital operations of the economy, to be passed without notice. This is Starch, a substance very universally diffused through the Vegetable kingdom. When removed from the plant, Starch exists in the form of minute granules, presenting great diversities of figure and dimen- sion ; but having, for the most part, a limit of size, and a characteristic aspect, in each tribe of plants, by which its source may frequently be determined. Each granule, when examined by the microscope, is seen to 792 SPECIAL AND COMPARATIVE PHYSIOLOGY. be marked by numerous lines, having more or less of a concentric arrangement ; these, however, are not striations on the surface, as might be at first supposed, but seem to result from the existence, in each gra- nule, of a number of layers, one investing another. The composition of these layers appears to he uniformly the same ; but the outer ones con- tain less water than those which they inclose, and have even an almost membranous firmness. Several of these granules, of different sizes, are usually found in one cell ; and it seems obvious that they are formed by successive additions to their substance, though whether the outer layers are formed over the inner ones, or the latter are formed by imbibition through the former, has not yet been determined. When exposed to the heat of about 160°, the starch-grain bursts, and the inner layers are readily dissolved by water ; and this is the explanation of the fact, that Starch once dissolved in hot water can never be restored to its original form. In composition, Starch is very closely related to gum and cellu- lose ; and it may, in fact, be regarded, as a store of nutritive matter which has undergone such an alteration that it can be kept apart from the surrounding juices, and can thus be reserved for some special purpose. Thus, we find it stored up in the seeds of most species, either forming a separate albumen, as in the Grasses, or taken into the structure of the embryo and constituting the mass of the fleshy cotyledons, as in the Leguminosge, &c. (§ 278 a) ; in each of these cases it serves as a maga- zine of food for the nutrition of the embryo, previously to the development of those organs which enable it to maintain an independent existence. Similar reservoirs are occasionally formed by the enlargement of the stem into tubers, for the nutrition of the buds to be developed from them, — as in the Potato, Arrowroot plant, &c. ; or by the accumulation of the same material in fleshy roots, bulbs, &c., from which stems rapidly grow up. Starch is also found abundantly in the soft interior (improperly called pith) of the stem of the Sago Palm and other Monocotyledons, where it seems destined to assist the evolution of the young leaves ; and in the fleshy expansions of the flower-stalk (termed receptacles ), on which, in many orders, the flower is situated, and in which it seems to answer a corresponding purpose. — In all these cases, the immediate end of the accumulation of Fecula is, that it may be ready for the nutrition of the young germ before it is capable of obtaining food for itself ; and it may be observed that the deposit continues to increase as long as the plant is in active vegetation, — arrives at its maximum, — and then, remaining stationaiy during the winter, begins to decrease in the spring. The depo- sition of fecula fulfils, therefore, an obvious purpose in the Vegetable economy ; but we cannot doubt the wise and benevolent intention of the Creator, in thus providing a store of nutritious and palatable food for Man, in situations whence he can so easily obtain it ; and it is interesting to remark that, as it almost always exists in an insulated form, it may be obtained in a state of purity from many vegetables which would other- wise be very poisonous. Before it can be applied to the nutrition of the plant, however, its condition must be changed. Thus, in the germination of seeds, it is converted into sugar ; the same change takes place in the tuber of the potato ; and, from the researches of Dunal (§ 503), it seems probable that the Starch deposited in the receptacle is converted, during the period of flowering, into Sugar. This conversion is a process which NUTRITION IN ANIMALS. 793 the chemist can imitate ; for if the fecula be first heated, so that its vesi- cles are ruptured, and it he then treated with dilute sulphuric acid, it is converted into sugar ; and the salivary matter of animals possesses the same converting power in a remarkable degree (§ 410). This change is effected in the Vegetable economy by the operation of a secretion called diastase ; which seems to he formed for the express purpose, and which may be obtained in a separate state most readily from the neighbourhood of the eyes or buds of the potato. It is stored up in that situation, for the purpose of being conveyed, by the vessels connected with the bud, into the substance of the tuber, when the demand for nutrition is occa- sioned by the development of the shoot ; and, in the laboratory of the experimenter, it produces exactly the same effects as in the Vegetable economy. It is probable that the secretion of diastase takes place in every instance, in which fecula previously deposited is to be reabsorbed. 3. Nutrition in Animals. 560. In tracing the gradual incorporation of the alimentary materials ingested by Animals, into their organized fabric, it is desirable, as in the study of Plants, to give our chief attention to the cases in which the dif- ferent steps of the process are most distinctly separated from each other. For we see among the Protozoa nothing more than the fact, that each cell obtains from the fluids which it inhabits the materials of its growth, absorbs these into its own cavity, and applies them to its own enlarge- ment, or to its multiplication by subdivision. Whilst at the opposite extremity of the series, in which a Circulating system is interposed between the points at which nutriment is absorbed and those at which it is appropriated, we are able to trace also a gradual metamorphosis in the components of the nutritive fluids, whereby it is assimilated in nature to the tissues, whose formative operations are performed at its expense. This metamorphosis partly consists in the conversion of Albumen (the form to which fibrine, caseine, and other substances of like composition, as well as albumen itself, are reduced in the digestive process) into Fibrine ; a change which is not manifested by any appreciable difference in ulti- mate composition, but which consists in the acquirement of properties that are altogether different from those exhibited by any mere chemical compound, and are indicative of an incipient vitalization (§ 41). A fur- ther preparation, however, seems to be effected, by the intimate incorpo- ration of Fatty matter with albuminous substances ; which there is strong reason for regarding as one of the most important parts of the Assi- milating operation in Animals. For oily matter, as we have seen, is a constituent of the food of nearly all animals ; and where it does not exist as such in the food, it is generated in the system at the expense of farina- ceous compounds ; and since this takes place in cold- as well as warm- blooded animals, it is obvious that the oleaginous particles must have some special purpose in the system, altogether irrespective of the mainte- nance of the combustive process. Further, we shall see that one of the chief peculiarities of the chyle consists in the peculiar state in which these fatty particles are found ; and this state corresponds so closely with that in which we find them dispersed as ‘molecules’ through the solids and fluids of the body, that we can scarcely hesitate in attributing to it 794 SPECIAL AND COMPARATIVE PHYSIOLOGY. some peculiar relation to the nutritive operations. Of the nature of this relation we obtain some further indication from the fact, that oleaginous particles may be constantly distinguished in the nuclei or cytoblasts of growing cells or fibres, as well as in the nuclear bodies scattered through an ‘ organisable blastema so that it may be affirmed with much pro- bability, that the presence of fatty matter is not less essential to the formative operations, than is that of the albuminous compounds them- selves.* The successive stages of the assimilating process, so far as they are at present known, will be now described. 561. The nutritive materials prepared by the digestive process, are taken into the circulation of Vertebrated animals (as already shown, chap, xi.) through two distinct channels, the blood-vessels and the absorbents. Now all the veins which return the blood from the capilla- ries of the gastro-intestinal canal, converge into the portal trunk, which distributes the blood, thus charged with crude materials, to the secreting apparatus of the Liver. It was formerly supposed that the agency of that gland was limited to the elimination, from the blood subjected to its in- fluence, of the materials of the biliaiy secretion ; but there is now evidence that the blood itself is changed by its means, in a manner that indicates an assimilating as well as a depurating action. For it has been observed by Prof. E. H. Weber, that during the last three days of incubation of the chick, the liver is made bright yellow by the absorption of the yolk, which fills and clogs all the minute branches of the portal veins ; and that in time, the materials of the yolk disappear, part being developed into blood~corpuscles and other constituents of blood, which enter the cir- culation, and the rest forming bile, and being discharged into the intes- tines/! There is no evidence that blood-corpuscles are thus generated in the liver during later life ; but recent researches have shown that other materials of the blood here undergo an important assimilating action. F or the blood which comes to the liver during digestion, contains a large quantity of albuminous matter and very little fibrine ; whilst in that which has passed through the organ, there is a large increase in the amount of fibrine. Again, it appears that fatty matters are elaborated in the liver, from saccharine or some other constituents of the blood ; so that even when no fat can be detected in the blood of the vena portae, that of the hepatic vein contains a considerable amount of it. A portion of this fat may be destined to immediate elimination in the lungs ; but if the supply that should be introduced by the lacteals be deficient, it would doubtless be made subservient to the formative processes. So, again, it appears certain that the liver elaborates from some other constituents of the blood a saccharine compound (diabetic sugar), which is destined for immediate elimination by the lungs, and which, being much more readily * This view derives strong confirmation from the very remarkable effect of Cod-liver oil in improving the nutrition of Tubercular subjects ; and from the curious prophylactic influence of the oleaginous diet of the Icelanders, a people whose habits are such as would peculiarly favour the development of Scrofula, but who are most remarkably free from any form of it. The importance of oleaginous matter to the process of textural nutrition, and the probable rationale of the beneficial effects of Cod-liver oil (which may be regarded as by far the most important among the recent improvements in Medical practice), were first developed, the Author believes, by his friend Prof. J. H. Bennett, in his work on Cod- Liver Oil, published in 1841. T Hcnle and Pfeufer’s “ Zeitschrift j” 1846. NUTRITION IN ANIMALS. 795 carried off by the respiratory process than either grape-sugar or cane- sugar, may be regarded as its most appropriate pabulum.* — Thus we may look upon the liver as an assimilating organ of very comprehensive endowments ; being the seat (under certain circumstances at least) of the production of red-corpuscles, of the elaboration of fibrine, and of the generation of fatty matter, which are the three most important elements in the preparative stages of the nutrient process ; and being further capa- ble of preparing the blood most advantageously for the function next in importance, the generation of heat. 562. We have now to enquire whether the Chyle absorbed into the Lacteal system undergoes any corresponding changes, during its progress from the walls of the intestines to the thoracic duct. The fluid drawn from the lacteals that traverse the intestinal walls, has no power of spon- taneous coagulation ; whence we may infer that it contains little or no Fibrine. It contains Albumen in a state of complete solution, as we may ascertain by the influence of heat or acids in producing coagulation. And it includes a quantity of Fatty matter, which is not dissolved, but sus- pended in the form of globules of variable size. The quantity of this evidently varies with the character of the food ; it is more abundant, for instance, in the chyle of Man and of the Carnivora, than in that of the Herbivora. It was formerly supposed that the milky colour of the chyle is owing entirely to its oil-globules ; but Mr. Gulliver has pointed out that it is really due to an immense multitude of far more minute particles, which he has described under the name of the molecular base of the chyle. These molecules are most abundant in rich, milky, opaque chyle ; whilst in poorer chyle, which is semi-transparent, the particles float sepa- rately, and often exhibit the vivid motions common to the finely-divided molecules of various substances. Such is then’ minuteness, that, even with the best instruments, it is impossible to determine either then- form or their dimensions with exactness ; they seem, however, to be gene- rally spherical ; and their diameter may be estimated at between 1-36, 000th and 1-24, 000th of an inch. As they are readily soluble in ether, there would seem to be no doubt of their oleaginous nature ; but it appears probable that each of them is surrounded with that thin membranoid film, which, as first pointed out by Ascherson, is formed whenever oily and albuminous matters are brought into contact. — No other particles than these are observable in the chyle drawn from the lacteals near the villi ; but after the fluid has passed through the Mesenteric glands, it exhibits very marked changes. The presence of fibrine (which must have been formed at the expense of the albumen) now begins to declare itself, by the spontaneous coagulation of the fluid ; the quantity of molecules and larger oily particles diminishes, perhaps by their passage into the blood when the two fluids are brought into close relation in the mesenteric glands ; and the peculiar floating cells or < chyle-corpuscles’ formerly adverted to (§ 172) now for the first time make their appearance. The average diameter of these is about l-4600th of an inch ; but they vary from about l-7000th to l-2600th, — that is, from a diameter about half that of the human blood- corpuscles, to a size about a third larger. This variation probably depends in great part upon the period of their growth. They are usually minutely * See the Lectures of M. Cl. Bernard on the ‘ Functions of the Liver,’ delivered before the College de France, and published in “ L’Union Medicale ” for 1850. 796 SPECIAL AND COMPARATIVE PHYSIOLOGY. granulated on the surface, seldom exhibiting- any regular nuclei, even when treated with acetic acid ; but three or four central particles may sometimes be distinguished iu the larger ones. These corpuscles are par- ticularly abundant in the chyle obtained by puncturing the mesenteric glands themselves ; and it would seem not improbable, that they are iden- tical with the spherical nucleated particles, which are so copiously deve- loped within the dilated lacteal tubes in their course through those bodies (§429). The glandular character of these cells, and their continued pre- sence in the circulating fluid, seem to indicate that they have an impor- tant concern in the process of Assimilation. It is only in the Chyle which is drawn from the lacteals intervening between the mesenteric glands and the receptaculum chyli, that the spontaneous coagulability of the fluid is so complete, as to produce a perfect separation into dot and serum. The former is a consistent mass, which, when examined with the microscope, is found to include many of the chyle-corpuscles, each of them being surrounded with a delicate film of oil ; the latter bears a close resemblance to the serum of the blood, but has some of the chyle-corpus- cles suspended in it. Considerable differences present themselves, how- ever, both in the perfection of the coagulation, and in its duration. Sometimes the chyle sets into a jelly-like mass ; which, -without any sepa- ration into coagulum and serum, liquefies again at the end of half an hour, and remains in this state. The coagulation is usually most com- plete in the fluid drawn from the receptaculum chyli and thoracic duct ; and here the resemblance between the floating cells, and the white or colourless corpuscles of the blood, becomes veiy striking. 563. The Lymph, or fluid of the Lymphatics, sensibly differs from the Chyle in its comparative transparency : its want of the opacity or opa- lescence which is characteristic of the latter, being due to the absence, not merely of oil-globules, but also of the ‘ molecular base.’ It contains floating cells, which bear a close resemblance to those of the Chyle on the one hand, and to the colourless corpuscles of the Blood on the other; and these, as in the preceding case, are most numerous in the fluid which is drawn from the lymphatics that have passed through the glands, and in that obtained from the glands themselves. Lymph coagulates like chyle ; a colourless clot being formed, which encloses the greater part of the corpuscles. The chief chemical difference between the Chyle and the Lymph, consists in the much smaller proportion of solid matter in the latter, and in the almost entire absence of fat, which is an important constituent of the former. This is well shown in the following comparative analyses, performed by Dr. G. 0. Bees, of the fluids obtained from the lacteal and lymphatic vessels of a donkey, previously to their entrance into the tho- racic duct ; the animal having had a full meal seven hours before its death. Water ...... Chyle. 90-237 Lymph. 96-536 Albuminous matter (coagulable by heat) 3-516 1-200 Fibrinous matter (spontaneously coagulable) 0-370 0-120 Animal extractive matter, soluble in water and alcohol 0-332 0-240 Animal extractive matter, soluble in water only 1-233 1-319 Fatty matter ..... 3-601 a trace. Salts ; — Alkaline chloride, sulphate, and carbonate, with traces of alkaline phosphate, oxide of iron 0-711 0"585 100 000 100-000 NUTRITION IN ANIMALS. 797 The Lacteals may be regarded as the Lymphatics of the intestinal walls and mesentery ; for, during the intervals of digestion, they contain a fluid, which is in all respects conformable to the lymph of the lymphatic trunks. 564. Thus by the admixture of the aliment newly-introduced from without, with the matter which has been taken up in the various parts of the system, and by the elaboration which these undergo in their course towards the thoracic duct, a fluid is prepared, which bears a strong resem- blance to blood in every particular, save the presence of red corpuscles. Even these may sometimes be found in the contents of the thoracic duct, in sufficient amount to communicate to them a perceptible red tinge ; but there can be little doubt that they have found their way thither acci- dentally, — some of the lymphatic or lacteal trunks, which have been divided in the dissection necessary to expose the duct, having taken up blood by their open mouths, and rapidly transmitted it into the general receptacle. The fluid of the thoracic duct may be compared to the blood of Invertebrated animals ; from which the red corpuscles are almost or altogether absent, but which contains white or colourless corpuscles ; and whose coagulating power is comparatively slight, in consequence of its small proportion of fibrine. And we hence see, why these animals should require no special absorbent system ; since the blood-vessels convey a fluid, which is itself so analogous to the chyle and lymph to be absorbed, that the latter may be at once introduced into it, without injiu'ing its qualities. The elaboration of this fluid in the Vertebrata would seem most probably due to the assimilating agency of the cells contained within the absorbent glandulse, and of those which float in it during its passage through the vessels ; and we may, in fact, regard the -whole of the special Absorbent system in the light of an assimilating apparatus, fitted not merely to take up, but also to prepare for admission into the circu- lating fluid, such appropriate materials as may be presented to it in the requisite condition. 565. The same function may with great probability be assigned to certain bodies, known as 1 Vascular Glands’ or ‘ Glands without ducts,’ which are found in Vertebrated animals, in intimate relation with the lymphatic system. These are, the Spleen , the Thymus Gland , the Thyroid Gland, and the Supra-renal Capsules. Many points in the structure of these organs, especially of the one first-named, are still far from being satisfactorily made out ; and of their function it would be rash to speak too confidently. Nevertheless it may be said of them all, that when in most active operation they are largely supplied with blood, and that they are made up for the most part of a cellular parenchyma, which is con- tained either in isolated vesicles (as in the spleen, thyroid gland, and supra-renal capsules), or in one large branching cavity (as in the thymus). Thus, in every essential particular but the want of direct outlets, they correspond with the ordinary glands ; and it can scarcely be questioned that then- function is to separate certain materials from the blood, and, after submitting them to an elaborating or assimilating action, to return them again to the current of the circulation, either directly or through the medium of their lymphatics, which are commonly very large. It is a strong confirmation of this view, that the greatest activity of the Thymus and Thyroid Glands, and of the Supra-renal capsules, is during foetal life 798 SPECIAL AND COMPARATIVE PHYSIOLOGY. and early infancy, when the formative processes are being performed with extraordinary energy, and are making a large demand upon the assimi- lative powers ; and it has been farther shown by Prof. Goodsir, that these three organs may be regarded as involuted portions of the ‘ germinal membrane ’ which is the first assimilating organ possessed by the foetus (chap, xviii.), and are in absolute continuity with each other at an early period of foetal life. But there is no improbability that they may severally have some subsidiary or supplementary function to perform, varying accoi'ding to their respective structure, position, and connections. Thus it has been observed by Mr. Simon, that the Thymus of hybemating Mammalia, instead of dwindling away (as in other instances) when full growth has been attained, greatly enlarges and becomes laden with fat ; and that this accumulation takes place especially at the approach of winter ; so that the organ, after it has ceased to perform its original function, is then used as a sort of storehouse of combustive material. — The Spleen, which has a different origin from the other three, would appear to serve, through the extraordinary distensibility of its vessels, as a kind of diverticulum, to relieve the vessels of the digestive viscera, when they are compressed by undue accumulation of the contents of then- cavities, or when they are congested by obstruction to the flow of blood through the liver or the heart ; but no such mechanical arrangement can be fairly supposed to be the chief purpose of such a complicated organ in the economy. The spleen has been repeatedly removed, without any obviously injurious consequences ; whence it appears, either that its function is not of vital importance, or (which is more likely) that it is discharged by some other organ in its stead. In some of the instances in which animals have been allowed to survive longest after removal of the Spleen, the lymphatic glands, of- the neighbourhood have been found greatly enlarged and clustered together, so as nearly to equal the original spleen in volume ymd hencarir appears to be a fair inference, that the elaborat- ing function of tha^pleen corresponds closely to that of the lvmphatic glands.* W 566. Having thus traced the steps by which the Blood is elaborated and prepared for circulation through the body, and having in an earlier part of this treatise (chap, iv.) examined into the characters of its chief constituents, we have now to consider the fluid as a whole, to study the usual proportions of these constituents, and the properties which they impart to it. — The Blood, whilst circulating in the living vessels, may he seen to consist of a transparent, nearly colourless fluid, termed Liquor Sanguinis ; in which the Bed Corpuscles, to which the Blood of Yerte- brata owes its peculiar hue, as well as the White or Colourless corpuscles, are freely suspended and carried along by the current. — On the other hand, when the blood has been drawn from the body, and is allowed to remain at rest, a spontaneous coagulation takes place, separating it into clot and serum. The clot is composed of a network of Fibrine, in the meshes of which the Corpuscles, both red and colourless, are involve d : * In his article Spleen in the “ Cyclopmdia of Anatomy and Physiology,” Prof. Kblliker endeavours to show that one function of the Spleen is to dissolve and melt down the effete blood-corpuscles, in preparation for the secreting action of the liver. He has since, however, considerably modified this view, having met with similar collections of disintegrating blood- corpuscles elsewhere. See Kolliker's and Siebold’s “ Zeitschrift ” for 1849. NUTRITION IN ANIMALS. 799 and the serum is nothing else than the liquor sanguinis deprived of its Fibrine. When the Serum is heated, it coagulates, showing the presence of Albumen. And if it be exposed to a high temperature, sufficient to decompose the animal matter, a considerable amount of earthy and alka- line salts remains. — Thus we have four principal components in the Blood ; — namely, Fibrine, Albumen, Corpuscles, and Saline matter. In the circulating Blood they are thus combined :* — Fibrine Albumen > In solution, forming Liquor Sanguinis. Salts J Red Corpuscles,— Suspended in Liquor Sanguinis. But in coagulated blood they are thus combined : — Fibrine > Crassamentum or Clot Red Corpuscles > Albumen > -n ... ... r • o j, v Remaining in solution, forming Serum. The solid matter of the blood also contains various Fatty substances, which may be removed from it by ether. Some of these appear to cor- respond with the constituents of ordinary Fat (§ 189) ; whilst another contains phosphorus, and seems allied to the peculiar fatty acids of Neurine (§ 243) ; and another has some of the properties of Cholesterine, the fatty matter of the Bile (§ 590). — Besides these, there are certain substances known under the name of Extractive ; one group of which is soluble in water, and another in Alcohol. Of the precise nature of these, little is known. They have been aptly termed ‘ ill-defined ’ animal prin- ciples j and it is probable that they may include various substances in a state of change or disintegration, which are being eliminated from the Blood by the processes of Excretion. 567. The proportion of these components varies greatly in the different classes and orders of Animals. Thus in Man, in a state of health, we may reckon the whole solid matter of the blood at about 212 parts in 1000 ; the proportion of the several components averaging nearly as follows : — Fibrine ...... 2 Corpuscles . . . . . . 130 Solids f Albumen . . . . . .70 of ^ Extractive Matters and Salts ... 8 Serum (_ Fatty Matters ..... 2 In Carnivorous Mammalia, the average proportion of corpuscles is rather greater, being nearly 150 : whilst that of the solids of the serum is somewhat less, the fibrine remaining the same. On the other hand, in Herbivorous Mammalia, the proportion of corpuscles is considerably less, being reduced to about 95 ; that of the solids of the serum is about the same, but that of the fibrine seems to be above the human average. In Birds, the total amount of solid matter is greater than in the average of Mammals ; the excess being especially in the Red Corpuscles. In the cold-blooded Yertebrata, on the other hand, the proportion of solid matter, and especially of the Corpuscles, is greatly reduced ; and the blood is * The corpuscles of Frog’s blood may be separated from the liquor sanguinis by filtra- tion ; but this experiment cannot be performed with Human blood, because its corpuscles are small enough to pass through the pores of any filter that allows the liquor sanguinis to permeate it. 800 SPECIAL AND COMPARATIVE PHYSIOLOGY. manifestly paler and thinner. — In the blood of Man and the Mammalia in general, the Colourless Corpuscles usually bear a small proportion to the Red ; and being nearly of the same character, they have until recently attracted but little notice. In Reptiles and Fishes, however, they differ so much in form, size, and general appearance, that they force themselves on the attention, even whilst the blood is moving through the capillaries ; and they are the more easily watched, owing to the comparatively small number of Red corpuscles, to which they are found to bear an increasing proportion as we descend through the lower orders of the class of Fishes, until we come at last to the Arwpliioxus, in whose blood the red corpuscles are altogether wanting (§ 321). Among the Invertebrata, there are no decidedly coloured corpuscles, although, in their form and grade of devel- opment, some of the colomdess corpuscles which their blood contains, present an approximation to the early condition of the red corpuscle in the Oviparous Vertebrata. According to Mr. Wharton Jones,* who has attentively studied the characters of the Blood-corpuscles in different groups of animals, their progressive stages of development are as fol- lows : — 1. Coarse granule-cell. 2. Fine granule-cell. 3. Colomdess nucleated cell. 4. Coloured nucleated cell. 5. Free cellaeform nucleus. The first and second of these stages are seen in the blood of Inverte- brata generally, and in the lymph and blood of Vertebrata. The third presents itself in the blood of the higher Invertebrata ; the number of such cells being greater, the closer the approximation to the Vertebrated series; and it is seen also in the blood of the oviparous Vertebrata, as a transition-stage between the lymph-globule and the red corpuscle. The fourth is the highest form which the red corpuscle attains in the Ovipar- ous Vertebrata ; and this may be occasionally seen in the blood of adult Mammalia, as a transition-stage between the colourless nucleated cell and their peculiar red corpuscles. These transition-stages may be best seen, however, in the embryo. The non-nucleated red corpuscles of Mammalia are regarded by Mr. Wharton Jones as the escaped nuclei of the pre- ceding, which have undergone development into cells ; but the account of them already given (§ 175) seems much the more probable one. — Of the relative amount of Fibrine in the blood of different animals, no other estimate has yet been formed, than that which rests upon the coagulating power of the liquids. This coagulating power is much weaker in cold- blooded than in warm-blooded Vertebrata ; and it is still weaker in Inver- tebrata, in many of which the clot that separates itself from the scram is rather due to the aggregation of the corpuscles, than to the fibrillation of the plasma. — Of the relative proportions of the other constituents of the blood, little is certainly known. 568. It cannot be doubted that the due admixture of all the com- ponents of the Blood, is necessary to the regular performance of its ac- tions. But the vital properties of the fluid are still more immediately dependent upon the Fibrine it contains ; since, as we have seen reason to Philosophical Transactions,” 1846. NUTRITION IN ANIMALS. 801 believe, it is the material which is most completely prepared for organisa- tion, and which supjflies what is requisite for the nutrition of the larger proportion of the solid tissues of the body. It is, therefore, continually being withdrawn from the blood by the nutritive operations ; and the demand appears to be supplied, in part by the influx of that which has been prepared in the Absorbent system, and in part by the continued transformation of albumen which takes place during the circulation of the blood. — The Albumen of the Blood is the raw material, at the expense of which not only the Fibrine, but many other substances, are generated during the nutritive process. All the Albuminoid compounds of the Secretions, the Horny matter of the Epidermic tissues, the Gelatine of the simple Fibrous tissues, and the Hsematine of the Red Corpuscles, may be regarded as almost certainly produced by the transformation of the Albumen of the Blood ; and a continual supply of this from the food is therefore requisite, to preserve its due proportion in the circulating- fluid. — The Red Corpuscles in their fully developed state appear to be more connected with the function of Respiration than with that of N utrition (§176); and the stimulating action of Arterial blood, especially upon the Nervous and Muscular tissues, seems to depend upon their presence. It appears from the experiments of Dieffenbach on transfusion, that the Red Cor- puscles are more effectual as stimuli to the Heart’s action, than is any other constituent of the blood. The rapidity with which they may be decomposed and reconstituted, is made remarkably evident by the expe- riments of Magendie ; who found that, when the blood of one animal was injected into the veins of another having discs of very different size and form (care being taken to prevent the coagulation of the Fibrine during the operation), the Red particles so introduced soon disappeared, and were replaced by those characteristic of the species in whose veins the fluid was circulating. — The use of the Saline matter is evidently in part to supply the mineral materials, requisite for the generation of the tissues, and for the production of the various secretions. F urtlier, it is by the Saline and Albuminous matters in conjunction, that the specific gravity of the Liquor Sanguinis is kept up to the point, at which it is equivalent to that of the contents of the Red corpuscles ; and it is only in this condition, that the latter present their proper characters (§17 3). — The Fatty mat- ters of the Blood are evidently derived from the food, either directly, or by the transformation of its farinaceous ingredients ; and wdrile they are employed, as we have seen, in the ordinary operations of cell-formation, a special demand for them must be created in the nutrition of the Nerv- ous tissue ; and they are also continually drawn upon, in warm-blooded animals, for the maintenance of the combustive process. The quantity of them contained in the blood is subject to great variation ; for after a full meal including a considerable proportion of oleaginous matter (especially if preceded by a long fast), a great increase takes place, and the serum is often rendered opaque by the introduction of the 4 molecular base ’ of the chyle ; but after a few hours this excess is brought down again, through the elimination of fatty matters by the nutritive, secretory and combus- tive processes. 569. The new arrangement of the elements of the blood, which takes place when it is withdrawn from the living body and left to itself, or even within the body when its vitality is lost, consists chiefly in the passage of 3 F 802 SPECIAL AND COMPARATIVE PHYSIOLOGY. the Fibrine from the state of solution to that of a fibrous network, iu the meshes of which the Corpuscles are included. The rapidity of this change, and the completeness of its result, vary considerably ; and usually in an inverse ratio to each other. When the fibrine is imper- fectly elaborated, so that the coagulum is deficient in firmness, the process usually takes place rapidly, and is soon completed ; but when it has undergone a higher degree of assimilation, so that it transforms itself hit* * a definite fibrous tissue, the process is comparatively slow. The most perfect fibrillation is seen, not in blood itself, but in those effusions of modified liquor sanguinis, known as ‘ plastic ’ or £ organizable lymph,’ which are thrown out in the inflammatory process, or are effused in the first stage of the reparative operation consequent upon an injury. This may be due in part to the higher elaboration of the fibrine itself ; and in part to the influence of vital force imparted from the living parts around (§ 556). If the influence of this living surface be continued, and vessels shoot from it into the primitive tissue thus formed, it becomes part of the organised fabric ; and thus we see that coagulation is not an indication of the death of the plastic fluid, but is a stage iu its metamorphosis int > a living solid. The tissue thus formed, however, is of a low order, and veiy prone to degenerate. When the Blood has been withdrawn from the body, so that the coagulation takes place without any influence from a living surface, the fibrillated mass soon passes into decomposition ; so tha t the process may then be considered as the last act of the life of the vital fluid. Even under the most favourable circumstances, it does not seem that the plastic force of the blood, or of the plasma effused from it. is able to develope any form of tissue higher than cells or simple fibres ; for these are the sole kinds of organised structure, which ever directly result from the development of the ‘ nucleated blastema’ thrown out for the repair of injuries. And under less favourable circumstances, the same material resolves itself into a substance, pus, of far inferior character, in which no further organisation can ever take place, and which is only fir, therefore, to be cast out of the system. 570. In the foregoing account of the formation, composition, and properties of the Blood, it has been shown that the albuminous materials obtained from the aliment, or received back from the body itself, are gradually prepared for organisation, whilst yet remaining in the flu: i state, until they have reached the condition of Fibrine ; and that in this condition, they have been so far modified by the vitalising influences to which they are subjected, that they are capable of spontaneously pasung into a state of incipient organisation. The production of all the higher forms of organised tissue, however, is manifestly dependent, not upon the properties of the plasma alone, but upon the Formative powers of the tissues themselves ; each tissue, from the time when it first presents its characteristic form in the embryo, continuing to grow, develope, and main- tain itself, at the expense of the materials which it draws from the blood To the statements already made (Sect. 1) in regard to the general conditions under which this function is performed, it will be only requisite here to add certain facts which have reference solely to the Animal kingdom. It has been shown (§ 557) that in the Nutrition of the parts concerned iu the maintenance of Organic life alone, the quantity of new tissue produced (the requisite amount of formative power being supplied) depends chiefly NUTRITION IN ANIMALS. 803 upon the quantity of material specially adapted for its generation. But in the case of the tissues which minister to the Animal functions, a different rule seems to hold good ; for in these we find that the activity of reparation is dependent upon the degree of previous waste or disinte- gration, caused by the performance of their peculiar operations ; so that, if they be kept in complete inactivity, no nutritive changes can take place in them, notwithstanding the greatest abundance of their respective pabula, and degeneration immediately commences ; whilst, if their functions be performed with unusual energy, so that an increased amount of £ waste ’ takes place, this waste is more than repaired by the nutritive activity of the part (provided that a sufficient amount of duly prepared material be supplied), and a positive increase of its substance, with a like increase of functional power, is the result. It is a very remarkable circumstance that the nutrition of bones , at least in those situations in which they serve to afford fixed points of attachment to muscles, rather than to support and protect the softer parts, is closely related to the development of the muscles ; so that when this is aug- mented by continued activity, the bones become stronger, and their pro- minences and ridges more decided ; whilst if it be checked by disuse, the nutrition of the bone also is impaired, and its bulk speedily diminishes.* 570 a. Thus the condition of nutritive activity in the tissues which are the instruments of the Animal functions, is as strikingly opposed to that which prevails in the part of the organism appropriated to the Vegetative functions, as are the conditions under which those functions are re- spectively performed. For, as we have seen, Vegetative activity of every kind is entirely constructive, and its conditions are merely (1) the supply of organisable matter, and (2) the requisite organising force. On the other hand, Animal activity is in its essential nature destructive, and its conditions involve the re-conversion of organised tissue into the inorganic state. But, in some mode not yet understood, the performance of this destructive operation excites a corresponding exertion of the constructive ; and the tissues are renewed or even augmented. The explanation may perhaps lie in the fact, that the exercise of the endowments of the Ner- vous and Muscular tissues, which depends upon conditions external to themselves,! itself becomes a means of determining an increased afflux of * Thus Dr. John Reid found that the relative weights of the bones and muscles of the two hind-legs of a rabbit, from which a portion of the sciatic nerve on one side had been removed seven weeks before, so as entirely to paralyze the limb, were as follows. Sound limb. Paralyzed limb. Muscles . . 327 grains 170 grains Tibia and Fibula . 89 grains 81 grains. There was an obvious difference, also, in the ultimate structure of the muscles of the two limbs ; for the fibres of the paralyzed muscles were considerably smaller than those of the other side, exhibited the longitudinal and transverse striae much less distinctly, and had a shrivelled appearance. (“ Physiological, Pathological, and Anatomical Researches,” p. 10.) + The activity of Muscle must be called forth by the stimulus of innervation, or by other excitants immediately applied to itself. That of the afferent portion of the Nervous substance is entirely dependent upon the reception of impressions from without, so that if these be withheld it remains in a state of torpor, and its nutrition is proportionably affected, — as we see in the case of the eye, when the access of light to the retina is completely prevented by opacity of the cornea. In like manner, the activity of the central organs is dependent upon the excitement which they receive through the afferent nerves, or from mental operations ; 3 f 2 804 SPECIAL AND COMPARATIVE PHYSIOLOGY. blood to them on the principles already laid down (§ 481) ; and it is on the increase of vegetative activity which thus takes place, that the aug- mented nutrition seems to depend. On the other hand, when these tissues are not called into action, the circulation through them becomes extremely languid, and their nutrition suffers accordingly. •571. In addition to the foregoing peculiarities, it is to be remarked that the activity of nutrition among higher Animals varies greatly in proportion to the age of the individual. Among Plants, one part of the fabric may die, whilst another is vigorously growing and extending itself, the two having no relation of mutual dependence ; and the same may happen among Zoophytes. But among Animals, we find, as we ascend the series, that as the individual parts become more and more intimately dependent upon one another, the nutritive activity of each is more and more closely related to that of the remainder, and consequently to that of the organism at large. Moreover, we find that growth consists less in mere addition to the parts already formed (as it does for the most part in Plants), but more and more in interstitial enlargement ; and this involves a continual remodelling- of the entire fabric. Thus all the tissues, even those most consolidated, are undergoing continual changes in the young animal ; and it would seem as if the duration of then - com- ponent parts is much less than in more advanced life, so that disintegra- tion and renewal more rapidly succeed each other (§ 377). Here, as elsewhere, we find that duration and functional activity arc in the inverse ratio to each other. The dull perceptions, and slow and feeble move- ments of the aged individual, are in no less striking contrast with the acute sensibility, and the rapid and vigorous muscular actions, of the child, than is the sluggishness of the interstitial changes in the former, as compared with their energy and activity in the latter. Hence it may lie stated as a general fact, that the vital activity of the component parts of the organism diminishes with the prolongation of the general life of the whole. 571 a. That the function of Animal Nutrition is capable of being con- siderably modified by the influence of the Nervous system, cannot be doubted by any one who duly considers the facts which have been brought to light by experiment and observation. But still there appeal's to be no sufficient reason for regarding the ordinary Nutritive operations as in themselves necessarily dependent upon Nervous agency (§ 364). In fact. the results of injury to the nerves of a part seem to be exerted rather in a disturbance or perversion, than in an actual suspension of them. And seeing as we do, that active nutrition takes place in the embryo, long- before nerves have been developed, and that it goes on in parts (e. g. cartilage) into which it may be said with certainty that no nerves enter, we seem to have full confirmation of the inference, which rests upon the essential conformity between the processes of Animal and Vegetable Murn- and thus we find that a continuance of intellectual exertion, if not too severe, tends to aug- ment the nutrition of the brain. And the activity of the eft'erent portion of the nervous system is entirely dependent upon that of the central organs, so that the nutrition of any part of it ceases, as soon as it is separated from them. But it is a remarkable circumstance, first clearly ascertained by Dr. Waller, that if the connection of the sensory nerves with ih.’ central organs be interrupted, so that they cannot propagate onwards the impression thev have received, their nutrition is immediately impaired. (“Philosophical Transactions,’ 1850, Part II.) OF SECRETION. 805 tion, — that in the one case, as in the other, the operation is effected by the formative powers of the part itself, exerted upon the plastic material supplied to it ; but that these, in the Animal, are modified and directed by Nervous agency, in such a manner as to harmonize them with each other, and with the general requirements of its organism. CHAPTER XVI. OF SECRETION. 1. General Considerations. 572. The parallel between the Vegetable and Animal kingdoms, which has hitherto been so close, fails us when we arrive at this department of the enquiry ; for the conditions of Vegetable existence do not re- quire that provision for the maintenance of the purity of the nutritive fluid, by the removal of effete matters, which becomes the most constant and urgent necessity of Animal life. It wmuld seem as if all the nutri- tive material assimilated by the leaves of Plants, is applied to their growth and development, or is stored up as a provision for future operations of the same kind ; and the supply is so exactly proportioned to the demand (§ 557 a), that there is seldom any unappropriated residue of assimilated matter left to become injurious by its superfluity and by its tendency to decomposition. Moreover, the parts which have fulfilled their purposes in the system, and whose term of life is expired (such as the leaves and flowers), are cast off en masse ; their ultimate decay, in which they return to the state of simple binary compounds, takes place after their separa- tion ; and no such interstitial death and decomposition occur in the regular progress of Vegetable life, as we have seen to be necessary condi- tions of Animal activity. There is nothing, then, save Carbonic acid and Water, which needs to be cast out of the Vegetable organism ; and for the elimination of these we have found a special provision to be made. — In the Animal, on the other hand, there are numerous modes in which the Circulating fluid may be rendered impure. In the first place, a larger amount of nutriment may be introduced into the blood, than the Nutri- tive operations can appropriate ; for we have seen that the formative energy of those Nervo-muscular tissues, which make up by far the largest proportion of the bodies of the higher animals, has no relation whatever to the supply of food, but depends upon the exercise to which they have been subjected ; so that unless the amount of food ingested be propor- tionably reduced, there must remain a superfluity which soon becomes injurious by decomposition, its liighly-azotized nature rendering it pecu- liarly liable to undergo such a change, at the elevated temperature of warm-blooded animals. Further, it seems scarcely possible that the varied demands which are made upon the nutritive fluid of higher animals, for the supply of the requisite components of the several tissues, can be so accurately adjusted under all circumstances, as not to leave some residue incapable of further use, and therefore unsuitable to be retained. But the great and constant occasion for the excretory opera- 806 SPECIAL AND COMPARATIVE PHYSIOLOGY. tions, undoubtedly arises from the disintegration, to which, as so frequently mentioned, the whole apparatus of Animal life is subject as the very condition of its vital activity ; and therefore the demand for then per- formance will vary with the degree of that activity. All that has been already said on this point under another head, is here equally applicable (§§ 506, 507). So urgent is the necessity for this process, that, as Dr. M. Hall has justly remarked, the functions of egestion are more immedi- ately necessary to the maintenance of Animal life, than those of ingestion. This will more fully appear hereafter. 573. But besides that separation of effete matters from the hlood, for the purpose of maintaining its purity, which is usually distinguished as Excretion, we find that certain products are elaborated from it for special purposes in the economy ; and it is to the process by which this is accom- plished, that the term Secretion in its more restricted sense is applicable. But even this has scarcely any parallel in the Vegetable kingdom. For although there is a large class of substances which are commonly desig- nated as ‘ Vegetable Secretions,’ yet these are not poured out upon the surface nor into the cavities of the Plant, but are stored up in its consti- tuent cells, of which they form the characteristic contents ; and thus they bear the same relation to it, as the oleaginous contents of the fat-cells, or the calcareous deposit in the cells of shells, Ac., bear to the Animal organism (§ 559 a). We shall presently see, however, that the production of the true Secretions and even of the Excretions of Animals, is a process which is referable to the same general category as Nutrition ; the separa- tion of the several components of each, from the blood, being effected by the development of cells, to which they form the appropriate pabulum (§ 576). 2. Secretion in Vegetables. 5 74. It might seem almost superfluous to add anything under this head, to what has been just said, as to the absence of any tme secreting or excreting process in Plants. But there are certain doctrines current upon this subject, to which it seems necessary to make more particular allusion. — The nearest approach to a true Excretion seems to be presented by those cases, in which peculiar organic compounds are exuded from the surface ; as honey in flowers, wax upon leaves and fruits, gums and gum- resins upon bark, Ac. In these and other similar instances, however, the exudation appears chiefly to result from the superabundance of the product in the subjacent tissues, and is not to be considered in any other light than as relieving their distention. It may be subservient at the same time, however, to some secondary purpose ; thus the honey of the flowers is attractive to insects, whose services in the act of fertilization are frequently important and sometimes absolutely essential. And this, with the acrid fluids secreted in the so-called ‘glands,’ or agglomerations of cells, at the base of the hair of the Nettle, may be regarded as the nearest approximations exhibited by Plants to the true Secretions of Animals. 575. It has been imagined that Plants have the power of throwing oft from their roots, substances injurious to their economy ; and the matters thus separated have been regarded as real Excretions. Thus it was shown by Bonnet, that if a rapidly-growing plant have its roots divided into two bundles, and one of these be introduced into a solution of some saline SECRETION IN ANIMALS. 807 substance which it will absorb, whilst the other is immersed in pure water, the latter fluid will be found after a few days to have received a certain impregnation from the former ; showing that the acetate of lead must have been taken up through one set of roots, and cast out again through the other. Again, if a Leguminous plant be placed in distilled water, the fluid wall be found in a few days strongly impregnated wflth mucilaginous matter, which it has derived from the roots immersed in it. So the Cichoracece and Fapciveracece exude a large quantity of brownish hitter matter analogous to opium ; Eupliorbiaceee, an acrid gummy- resinous matter ; and so on, — the substances exuded being in all instances those which are characteristic of the proper juices of the plants which furnish them. Hence it seems fair to conclude, that this exudation is a simple act of exosmose , necessarily connected with the endosmose by which water is taken up into the organism (§ 423). The existence of such exudations, however, occasionally comes to be a matter of much practical importance. It has probably something to do with the fact, that the growth of particular species of plants favours the subsequent development of other kinds ; whilst there are other species, whose growth seems to leave the soil in a much worse condition than before. Now there can be little doubt that much of the latter result is to be attributed to the removal of particular ingredients from the soil, -which render it less fitting to sustain the continued growth of its own kind, whilst it may still be capable of affording the requisite support to such as need some different materials ; and it is probably by this principle that we are to explain the well known fact in forestry, that where a w T ood principally composed of one species of timber-trees has been cleared (by fire or other- wise), the trees wdiicli spring up spontaneously and supply the place of the former growth are for the most part of a different species ; and the success of the ‘ rotation of crops ’ in agriculture seems chiefly attributable to the same cause. But this is not all ; for it is well known that the soil is more benefited by the growth of a crop of certain Leguminous plants, such as Trefoil, than it would be by lying absolutely fallow ; and this can scarcely be due to anything else, than its impregnation wflth gummy exudations from the roots of these plants. On the other hand, there are exudations which are positively injurious ; this has been proved to be the case with tannin, which is given off by the roots of the oak, and which, even in very minute proportion, destroys the vitality of the growing tissues of the spongioles of other plants ; thus affording an explanation of the well-known fact, that trees transplanted into a soil in which oaks have previously grown, seldom flourish, and generally die. And it seems not unlikely, too, that Poppies and other ‘ rank ’ weeds, do more damage by communicating narcotic exudations to the soil, wdiich deaden the vital powers of other plants growing in it, than by any exhaustive influence exerted by them. 3. Secretion in Animals. 576. We have seen that, in the process of Animal Nutrition, the circu- lating current not only deposits the materials which are required for the renovation of the solid tissues ; but also takes back the substances which are produced by the decay of these, and which are destined to be thrown off from the body. We have also seen that it supplies the materials of 808 SPECIAL AND COMPARATIVE PHYSIOLOGY. certain fluids, which are separated from it to effect various purposes in the economy ; — such as the Salivary and Gastric fluids, which have for their office to assist in the reduction of the food. Now the process, by which the fluids of the latter kind are separated from the Blood, is precisely the same in character as that, by which the products of decay are elimi- nated from it ; and the structure of the organs concerned in the two is essentially the same. Hence both processes are commonly included under the general term Secretion ; which, considered in its most general point of view, may be applied to every act, by which substances of any kind are separated from the blood ; but which is usually restricted to those cases, in which the substances withdrawn are not destined either to be restored again to the circulating current (as in Assimilation), or to form part of the textures of the living fabric (as in Nutrition) ; and in which the sepa- rated product has a liquid or more rarely a solid form, and not a gaseous (as in Respiration). Viewed under this limitation, the essential character of the true Secreting operation seems to consist, — not in the nature of the action itself, for this is identical with those of Assimilation and Nutrition, being a process of cell-growth, — but in the position in which the cells are developed, and the mode in which the products of their action are after- wards disposed of. Thus the cells at the extremities of the Intestinal villi (if such there be, § 427), the cells of which the Adipose tissue is made up (§ 189), and the cells of which the greater part of the substance of the Liver is formed (§ 587), all have an attraction for fatty matter : and draw it from the neighbouring fluids, at the expense of which they are developed, to store it up in their own cavities. But the cells of the first kind, when they have come to maturity, set free then- contents, which are delivered to the absorbent vessels, to be introduced into the cir- culating current : — those of the second kind seem more permanent in their character, and retain their contents, so as to form part of the ordinary tis- sues of the body, imtil they are required to give them up : — whilst the cells of the third class cast forth their contents into the hepatic ducts, by which they are carried into the intestinal canal, whence a portion of them at least is directly conveyed out of the body. It is, then, in the position of the Secreting cells, — which causes the product of then' action to be deli- vered upon a free surface, communicating, more or less directly, with an external outlet, — that their distinctive character depends. All the proper Secretions are thus either poured out upon the exterior of the body, or into cavities provided with orifices that lead to it. Thus we have seen that a very large quantity of fluid, containing a considerable quantity of solid matter, of which it is desirable that the system should be freed, is carried off from the Cutaneous surface. Another most important secre- tion, containing a large quantity of solid matter, and serving also to regu- late the quantity of fluid in the body, — namely, the Urinary, — is set free by a channel expressly adapted to convey it directly out of the body. The same may be said of the Mammary secretion ; which is separated from the blood, not to preserve its purity, nor to answer any purpose in the economy of the individual, but to afford nutriment to another being. And of the matters secreted by the very numerous glandulse situated in the walls of the Intestinal canal, a .great part are obviously poured into it for no other purpose, than that they may be carried out of the body by the readiest channel. SECRETION IN ANIMALS. 809 577. The cells covering the simple membranes that form the free sur- faces of the body, whether external or internal, are all entitled to be regarded as secreting cells ; since they separate various products from the blood, which are not again to be returned to it. But the secreting action of some of these seems to have for its object the protection of the surface ; thus the Epidermic cells secrete a horny matter, by which den- sity and firmness are imparted to the cuticle (§ 177) ; whilst by the epithelial cells of the Mucous membranes of the alimentary canal and of other parts (§ 168), their protective mucus seems to be elaborated. But in general we find that special organs, termed Glands, are set apart for the production of the chief secretions ; and we have now to consider the essential structure of these organs, and the mode of their operation. — A true Gland may be said to consist of a closely-packed collection of fol- licles, all of which open into a common channel, by which the product of the glandular action is collected and delivered. The follicles contain the secreting cells in their cavities ; whilst their exterior is in contact with a network of blood-vessels, from which the cells draw the materials of their growth and development (Fig. 228). In any one of the higher animals, we may trace out a series of progressive stages of complexity, in the various glands included within their fabric ; and in following any one of the glands that attain the highest de- gree of development (such as the Liver or Kidney), through the ascending scale of the A nimal series, we shall be able to trace a very similar gradation from the simplest to the most complex form. Hence we see that there can be nothing Capillary network around the follicles of in the form or disposition of the com- Parotld GIand - ponents of the glandular structure, which can have any influence upon the character of the secretion it elaborates ; since the very same product, — i. e. the Bile, or the Urine, — is found to issue from nearly every variety of secreting structure, as we trace it through the different groups of the Animal kingdom. The peculiar power, by which one organ separates from the blood the elements of the Bile, and another the elements of the Urine, whilst a third merely seems to draw off a certain amount of its albuminous and saline constituents, is obviously the attribute of the ultimate secreting cells, which are the real agents in the secreting pro- cess. T Yhij one set of cells should secrete bile, another urea, and so on, we do not know ; but we are equally ignorant of. the reason, for which one set of cells converts itself into Bone, another into Muscle, and so on. Tiiis variety in the endowments of the cells, by whose aggregation and conversion the fabric of the higher Animals is made up, is a fact which we cannot explain, and which must be regarded (for the present, at least,) as one of the “ ultimate facts ” of Physiological Science. 578. Passing by the extended membranous surfaces, and the protective or secreting cells with which they are covered, we find that the simplest form of a secreting organ is composed of an inversion of that surface into follicles, which discharge their contents upon it by separate orifices. Of this we have a characteristic example in the gastric follicles, Fig. 228. 810 SPECIAL AND COMPARATIVE PHYSIOLOGY. even in the higher animals ; the apparatus for the secretion of the Gastric fluid never attaining any higher condition, than that of a series of distinct follicles, lodged in the walls of the stomach, and pouring then 1 products into its cavity by separate apertures. In Fig. 229, a. is Fig. 229. A b c a, Glandular follicles in Ventriculus Succenturiatus of Falcon : — b, Gastric gland from the middle of the Human Stomach ; — c, a more complicated form in the neighbourhood of the pylorus. represented a portion of the ‘ventriculus succenturiatus’ of a Falcon, in which the simplest form of such follicles is seen. A somewhat more com- plex condition is seen in some of the Gastric follicles of the Human sto- mach (b, c) ; the surface of each follicle being further extended by a sort of doubling upon itself, so as to form the commencement of secondary follicles, which open out of the cavity of the primary one. — Xow a con- dition of this kind is common to all glands, in the first stage of then- evolution ; and in this stage we meet with them, either by examining them in the lowest animals in which they present themselves, or by look- ing to an early period of their embryonic development in the highest. Thus, for example, the Liver consists, in certain Polypes and in the lowest Mollusca, of a series of isolated follicles, lodged in the walls of the stomach, and pouring their product into its cavity by separate orifices ; these follicles being recognized as constituting a biliary apparatus, by the colotu- of their secretion. And in the Chick, at an early period of incubation, the condition of the Liver is essentially the same with tue preceding ; for it consists of a cluster of isolated follicles, not lodged in walls of the intestine, but clustered round a sort of bud or diverticulum of the intestinal tube, which is the first condition of the hepatic duct, and into which they discharge them- selves (Fig. 239). So, again, the Mammary Gland presents an equally simple structure in that lowest type of the Mammalian class, the Ornithorhyncus ; for it consists of nothing else than a cluster of ere cal follicles, discharging their contents by separate orifices upon the surface of the mammary areola. And in like manner, the Pancreas makes its appearance in many Fishes in the condition ol a Fig. 230. Mammary Gland of Ornithorhyncus. SECRETION IN ANIMALS. 811 group of more prolonged csecal follicles, clustered round the commence- ment of the intestinal canal, and partly coalescing with each other before they enter it (Fig. 231). A still simpler condition of the pancreas is found in the Cephalopoda (§ 307 b), and in the Cyclostome Fishes. Thus, it may be stated as a general rule, that every gland presents itself under this most elementary state, alike in the lowest grade of the ani- mal series in which it is first traceable, and at its earliest appearance in the embryo of the higher. — The next grade of complexity is seen, where a cluster of the ultimate folli- cles open into one common duct, which discharges their product by a single outlet: a single gland often containing a number of such clusters, and having, therefore, several excretory ducts. A good example of such a condition, in which the clusters remain isolated from one another, is seen in the • Meibomian glands of the eyelid ; each of which consists of a double row of follicles, set upon a long straight duct, that receives the products of their secreting ^ 1 ° nc extremity of stomach; b < mtes ' action, and pours them out upon the edge of the eyelid. And of the more complex form, in which a number of such clusters are bound together in one glandular mass, we have an illustration in the accessory glands of the genital apparatus in several animals, which discharge their secretion into the urethra by numerous outlets ; or in the Mammary glands of Mammalia in general, the ultimate follicles of which are clustered upon ducts that coalesce to a considerable extent, though continuing to form several distinct trunks even to their termination. Such glands may be subdivided, therefore, into glandules or lobules, that remain entirely distinct from each other. — In the highest form of Gland, however, all the ducts unite, so as to form a single canal, which conveys away the products of the secreting action of the entire mass. This is the condition in which we find the Liver to exist in most of the higher ani- mals ; also the Pancreas, the Parotid Gland, and many others. In some of these cases, we may still separate the gland into numerous distinct lobules, which are clustered upon the excretory duct and its branches, like grapes upon a stalk (Fig. 232) ; in others, however, the branches of the excretory duct do not confine themselves to ramifying, but inoscu- late so as to form a network, which passes through the whole substance of the gland, and which con- nects together its different parts, so as to render the division into lobules less distinct. This seems to be the case in regard to the Liver of the higher Vertebrata. i 579. Whatever degree of complexity, however, prevails in the general arrangement of the elements of the Glands in higher animals, these ele- Fig. 232. Lobule of Parotid Gland of Human Infant , filled with mercury; magnified 50 diameters. Fig. *231. 812 SPECIAL AND COMPARATIVE PHYSIOLOGY. ments are themselves everywhere the same ; consisting of follicles or tubuli that enclose the real secreting cells (Figs. 233 and 234). Now from the history of the development of Glands in general, it appeal's that the follicles may be considered as parent-cells ; and that the secreting cells in their interior constitute a second generation, developed from the nuclei or germinal spots on the walls of the first. It has been pointed out by Prof. Goodsir,* that the continued development and decay of the glandular structure, in other words, the elaboration of its secretion, may take place in two different modes. — In one class of Glands, the parent-cell, bavin a begun to develope new cells in its interior, gives way at one point, an l bursts into the excretory duct, so as to become an open follicle, instead of a closed cell : its contained or secondary cells, in the progress of then own growth, draw into themselves the matter to be eliminated from the blood, and, having attained their full term of life, burst or liquefy, so as to discharge their contents into the cavity of the follicle, whence they pass by its open orifice into the excretory duct : and a continual new produc- tion of secondary cells takes place from the germinal spot, or nucleus, at the extremity of the follicle, which is here a permanent structure. In this form of gland, we may frequently observe the secreting cells existing in various stages of development within a single follicle ; then - size in- creasing, and the character of their contents becoming more distinct, iu proportion to their distance from the germinal spot (which is at the blind termination of the follicle), and their consequent proximity to the outlet (Fig. 234). In some varieties of such glands, however, — as in the greatly - prolonged follicles, or tubuli uriniferi, of the kidney, — the production of new cells does not take place from a single germinal spot at the extremity of the follicle, but from a number of points scattered through its entire length. — In the second type of Glandular structures, the parent-cell dues not remain as a permanent follicle ; but, having come to maturity and formed a connection with the excretory duct, it discharges its entire con- tents into the latter, and then shrivels up and disappears, to be replaced by newly-developed follicles. In each parent-cell of a gland foimed upon this type, we shall find all its secondary or secreting cells at nearly the same grade of development ; but the different parent-cells, of which the parenchyma of the gland is composed, are in very different stages of growth at any one period, some having discharged them contents and being in progress of disappearance, whilst others are just arriving at maturity and connecting themselves with the excretory duct ; others exhibiting an earlier degree of development of the secondary cell- ; others presenting the latter in their incipient condition ; whilst others are themselves just starting into existence, and as yet exhibit no traces of a second generation. — The former seems to be the usual type of the ordinary Glands ; the latter is chiefly, if not entirely, to be met with iu the Spermatic glands. 580. The purposes answered by the function of Secretion we have seen to be two-fold; namely, the separation of some material from the circu- lating fluid, which would be injurious to the welfare of the system il retained in it ; and the elaboration of a product which is destined to some other use iu the economy. Now it is probable that in almost every * “ Anatomical and Pathological Observations," chap. xv. SECRETION IN ANIMALS. 813 act of secretion, both these purposes are in some degree served ; the blood being freed from some ingredient whose accumulation would be superflu- ous, if not more positively injurious ; and the matter separated having some secondary purpose to answer. Thus, whilst biliary matter becomes a positive poison if it be retained in the blood, it contributes, when poured into the duodenum, to complete the digestive process, and to pre- pare the nutrient contents of the intestinal canal for absorption. So, again, the cutaneous exhalation not only removes the superfluous water of the blood, but is one of the chief means of regulating the temperature of the body. And even the urine, which seems to be eliminated merely for the removal of the products of the disintegration of the tissues from the circulating current, is sometimes made to serve an additional purpose ; its acridity, or its peculiarly offensive odour (increased under the influence of terror), frequently rendering it an effectual means of defence. On the other hand, the secretions which are separated from the blood for the purpose of discharging some important office in the economy, usually, if not always, contain some substances whose retention in the blood would be injurious, and which are therefore advantageously got rid of through this channel. Thus the Salivary, the Gastric, and the Pancreatic fluids, all contain an animal principle nearly allied to albumen ; but this prin- ciple seems to be in a state of change, or of incipient decomposition ; and it would not seem improbable, that whilst this very condition renders the albuminous matter useful in promoting the reduction of the aliment, it renders it unfit to be retained in the circulating current. So, again, the albuminous matter of Milk appears to have undergone a change, whilst yet within the sanguiferous system, which renders it unfit to be retained within the maternal blood ; for we find that when the Mammary secretion is checked, the peculiar substance termed Kiestine, which is nearly related to caseine, and which was eliminated by the kidneys during pregnancy, again passes off in the urinary excretion. — It is impossible, therefore, to divide the secreted products strictly, as some Physiologists have attempted to do, into the excrementitious and the recrementitious ; that is, into those which are purely excretory in their character, and those which are sub- servient to further uses in the economy ; since most, if not all of them, partake more or less of both characters. Still it is convenient to group them, for practical purposes, according to the predominance of one or other of the objects first mentioned ; those being regarded as excretions, in which the depuration of the blood is manifestly the chief end, any other purpose being rendered subservient to this ; and those being ranked as secretions, in which the ulterior purpose of the separated fluid would seem to be the principal occasion of its production. 581. Another classification has been proposed, of which the foundation is the degree of resemblance which the secreted products bear to the normal constituents of the blood : those being associated into the first group, in which the characteristic ingredients are altogether unlike those of the nutritive fluid ; whilst a second group is formed of those, whose elements seem nearly allied to the components of the blood. This classi- fication is, in fact, almost identical with the preceding ; for, as a general rule, all the cases in which the secreted products are very unlike the con- stituents of the blood, are those in which they are most directly and speedily removed from the body ; whilst those in which they serve some 814 SPECIAL AND COMPARATIVE PHYSIOLOGY. ulterior purpose, are for the most part also those, whose elements differ least from the components of the blood. — The products of the first class may be said to have their immediate origin in the disintegration of the tissues”; and we find the amount in which they are generated, to be in close rela- tion to the operation of the various causes, which tend to increase or diminish that disintegration (§§ 374-377). They act all of them a s poisons, if retained within the system and allowed to accumulate even for a short time ; and after their excretion, they speedily resolve themselves into simple combinations of inorganic elements. This is the case, for example, with urea, biliary matter, and the putrescent poi-tion of the faeces; all of which may be regarded as the products of the metamorphosis of the tissues, on their way towards their restoration to the inorganic universe (§ 260) ; and it would seem to be because they have undergone this com- plete metamorphosis, that their presence in the circulating current is not only useless but injurious. We may consider that portion of the carbonic acid thrown off by Respiration, which is the result of the disintegration of the tissues (§ 507), as the most important of these excretory products, being the one whose briefest accumulation gives rise to the most perni- cious results ; but the accumulation of biliary matter or of urea, caused I y the complete stoppage of the hepatic and renal excretions, usually proves fatal in from one to two days. Of the products of the first class it may be said, further, that many of them may be detected in the circulating blood, on account of then - marked dissimilarity in chemical proper- ties to its other constituents ; but their quantity is normally ex- tremely minute, since they are eliminated almost immediately that they are generated. But if any such secretory operation be checked, then the product which it is destined to remove speedily makes its presence apparent in the blood ; being alike detectible by chemical analysis, and recognisable by the symptoms of poisoning to which it gives rise. — On the other hand, the products of the second class seem to l»e supplied rather from the materials of the blood itself, than from the disintegration of the tissues ; and usually bear such a close relation to the normal constituents of the circulating fluid, as not to be dis- tinguishable from them. Such are the elements of the Salivary, Gas- tric, Pancreatic, Lachrymal, and Mammary secretions. Hence, when these secretions are checked, the consequences are more apparent in the suspension of the function to which they are especially subservient, than in that general disorder of the system which indicates a contamination of the blood. 582. As it would be quite foreign to the purpose of this Treatise, to enter into a comparative examination of all the Secreting structures which present themselves in the different parts of the Animal series, it will be advantageous to restrict ourselves for the most part to two sort i f organs, — the Biliary and Urinary — which are most generally distribute! and which present themselves under the greatest variety of forms and conditions. In the examination of these, we shall find ample illustration of the great principles of specialisation and concentration already so fre- quently referred to ; and shall also meet with some remarkable examples of that interchange of function, which occasionally takes place even in the most complex organisms. 583. The Liver ancl the secretion of Bile. — There are few animals possessed SECRETION IN ANIMALS. 815 of a distinct digestive cavity, in which some traces of a biliary apparatus (recognizable by the colour of the secretion) cannot be distinguished. Thus in th e Hydra, cells containing a yellowish-brown matter may occa- sionally be seen in the lining of the stomach, into the cavity of which they probably discharge their contents, by the rupture or solution of their own walls. Secreting cells of a similar kind are more distinctly seen in furrows, formed by duplicatures of the lining membrane of the stomach of the Actinia (Fig. 98). In the Laguncula and other Bryozoa (Fig. 114), very distinct spots may be seen in the parietes of the stomach, which seem to be composed of clusters of biliary cells contained within follicles ; and during digestion, the contents of the stomach are seen to be tinged with a rich yellow-brown hue, derived from the matter discharged from these follicles. In the Earthworm, again, the large annulated alimentary canal is completely encased in a flocculent external coating, which con- sists of a mass of minute flask-shaped follicles, filled with cells ; and several of these coalesce to discharge their contents by a common orifice into the digestive cavity. — Passing a little higher, as well in the Radiated, as in the Articulated and Molluscous groups, we find the hepatic cells clustering, not immediately around the digestive stomach, but around csecal prolongations of this, which thus present us with the first approxi- mation to the condition of the separate glandular mass, which we meet with iu the higher animals. Thus in the Asterias (Fig. 110), the radiat- ing extensions of the stomach have their walls dilated into numerous culs-de-sac ; and these are lined with large glandular cells, whose colour and aspect indicate their hepatic character. So in the Leech and many other Annelida, whose digestive cavity is more or less sacculated (Fig. 198), the walls of these sacculi are covered with biliary cells, as are also, although in a less degree, those of the central canal. In the Fycnogonidce, again, the only trace of hepatic organs is to be found in the biliary cells, which are dispersed through the walls of the caeca prolonged into their limbs (Fig. 158) ; and it is apparently in the same diffused condition, that the biliary apparatus exists in the A car idee (§319 d). The most re- markable example of this type of structure in the Molluscous series, is presented by certain of the Nudibranchiate Gasteropods (§ 304 c), in which, notwithstanding the high development which the liver attains in most other orders of the class, it is reduced to the condition of clusters of simple cells arranged round csecal prolongations of the stomach. In the Eolis and its allies, the stomach gives off on either side a number of branches, which usually re-divide, and then give off smaller tubes, which are continued into the numerous papillae that clothe the dorsal surface of these animals, and are subservient to the respiratory function. Each of these papillae contains a central canal, which is sometimes a mere dilated tube, but which in most species is more or less deeply sacculated ; and the inner surface of these simple or complex caeca is lined -with hepatic cells of irregular form. 584. In the greater number of Mollusca, however, as well as in most Myriapoda, Insectci, Crustacea, and Arachnida, we find the biliary organs considerably more detached from the digestive apparatus ; having the form either of prolonged tubes, or of clusters of shorter follicles, which coalesce with each other in such a manner, as to discharge their contents into the alimentary canal by a small number of orifices ; and being only 816 SPECIAL AND COMPARATIVE PHYSIOLOGT. Fig. 233 . connected with the alimentary canal, by the ducts through which their secretion is conveyed. The most simple type of this kind of structure is that which is met with in Insects ; in which class the biliary organs are usually regarded as consisting of a number of distinct filiform tubes (Fig. 199,/), lying in close apposition with the sides of the alimentary canal, but quite disconnected from it, except at the points where they enter it. Where their number is small, they are usually greatly elon- gated, being sometimes three or four times the length of the alimentary canal, and are tortuous and convoluted ; and they usually open separately into the intestinal tube. But when numerous, they are proportionally short ; and two or three of them frequently coalesce to form a common trunk, before entering the intestine. Their number varies considerably : in some Coleoptera, there are but two ; in Diptera, there are generally four • in Lepidoptera , six ; whilst in other orders, there are many more. In many larvae the biliary tubes are themselves furnished with lateral caeca ; but these almost always disappear as the insect approaches the imago state ; a less active biliary secretion being then apparently suffi- cient. In the Melolontha (Cockchaffer), however, which has but two biliary vessels, these are of great length, and retain their lateral caeca. When carefully examined, these tubes are found to consist of a delicate tube of transparent membrane, the inner surface of which is covered with secreting cells ; from the thinness of the tube, the cells often project, so as to give it a granulated appearance when viewed with the naked eye ; and generally at a distance from then’ outlets, the sides of the tubes are so irregular, that they appeal- as if folded upon the secreting cells to keep them together (Fig. 233). From the dif- ference in size and degree of deve- lopment between the cells of each tubulus, it seems probable that they originate at its upper extremity, that they are gradually being pushed on towards its outlet by the growth of new generations behind them ; and that, as they thus advance, they ac- quire an increase in size by their own inherent powers of development, at the same time drawing iuto them- selves the peculiar matters which they are destined to eliminate from the circulating fluids. The cells, haling attained then full growth, and completed their term of life, give up their contents by the rupture or deliquescence of their walls ; and these pass down the central cavity of the tube, to be discharged into the alimentary canal. It is by no means certain, however, that this constitutes the whole of the biliary apparatus of Insects. The hepatic tubuli, when traced as far as possible from their Structure of the biliary tubuli of Musca carnaria (flesh-fly): — a, portion of trunk and two branches, viewed by reflected light, and under a low magni- fying power : — b, portion of a biliary tube more highly magnified, exhibiting the arrangement of the secreting cells, and the mode of distribution of the tracheae. SECRETION IN ANIMALS. 817 intestinal connections, do not seem to terminate in free ciecal extremities but lose themselves in a mass which has been usually regarded as simply adipose, but which has been shown by Dr. T. Williams and Dr. C. H. Joues to be composed of cells in various stages of development, closely resembling those found withiu the hepatic tubuli ; these cells being usually inclosed in vesicles, which are sometimes free, but are sometimes connected with a network of tubuli. This has been termed by Dr. C. H. Jones the parenchymatous portion of the liver; and we shall hereafter see that it appears to combine the functions of the liver and of the adipose tissue of higher animals (§ 594). 585. The biliary apparatus of the Myriapoda closely resembles that of Insects ; but in that of the Crustacea we have a much closer approxima- tion to the Molluscan type, in which the biliary cells are inclosed within a multitude of follicles with distinct ciecal terminations (Fig. 234), aggregated together into a lobulated glandular mass. On a careful ex - amination of these follicles, and a comparison of the size and contents of the cells at the Fig. 23-1. bottom and towards the outlet, it becomes r . j evident that they originate in the former situa- • tion, and gradually increase in size as they advance towards the latter. It is also to be observed, that the cells which lie deepest in f the csecum (a, 6) contain for the most part the yellow granular matter, which may be regarded as the proper biliary secretion : but as they ©) — increase in size, they also increase in the '** quantity of oil-globules which they contain (c) ; ; : until beyond the middle of the tube, where ...J they are found full of oil, so as to have the . ^ appearance of ordinary fat-cells (d, e). From this circumstance it happens, that when a c|^ j : caecum is examined with the microscope, its *** : lower half appears filled with a finely granu- : lar matter, intermingled with nucleated par- tides ; and the upper half with a mass of fat cells, whose nuclei are obscured by the oily | §r particles. These follicles are clustered into ^ lobules (Fig. 235), and discharge the products of then’ secreting action into a cavity in the ® *3*?’ centre of each ; and from these it is collected One of the hepatic cseca of Asta- by ducts that coalesce with each other to form fod.lhowing 1 ^ a small number on either side, by which the lopment of the secreting ceils from i ; O ,1 -l -i /T -,. ~t r~ p* j? /*\ • the blind extremity to the mouth of prOClUCt OI the whole mass (big. ldo^y, J j IS the follicle; specimens of these, in discharged into the alimentary canal. Here, separateij^atT 6 * ^ d 6S e are shown as in other Articulata, the hepatic organs pre- sent a perfect bi-lateral symmetry. In some of the Entomostracous Crus- tacea, however, as the common Daphnia pulex, the biliary apparatus is - reduced to that simple condition, which it has been already described as presenting in the Bryozoa and lower Annelida ; namely, a set of cells lodged in the walls of the intestine itself. — The structure of the biliary ■organs of the Arachnida has not been clearly made out ; but they would 3 G 818 SPECIAL AND COMPARATIVE PHYSIOLOGY. Lobule of Liver of Squilln Mantis ; seem to partake rather of the character of those of the Crustacea, than of those of Insects. From the short, straight, alimentary canal, there pass off' on either side, in the abdominal region of the A raneidce, but in the thoracic region of the Scorpionidae, Fig. 235. several pairs of caeca (Fig. A B 162, e), which subdivide, and then lose themselves in a fatty mass of considerable size ; this fatty mass has been usually regarded as nothing else than adipose tissue, but its hepatic cha- racter appears from recent observations to he unques- tionable ; and this circum- stance adds weight to the opinion of those, who con- sider the great adipose mass , exterior ; b, cut open, in the body of Insects as biliary in its nature.* 586. Throughout the Molluscous series, with the exceptions already named, the liver presents the same type of structure as in the Crustacea ; and the chief advance which we see in ascending the series, consists in the increasing compactness of the glandular mass, and its isolation from the walls of the digestive canal. For whilst, in the higher Tunicata and Conchifera, its lobules cluster round the pyloric portion of the stomach and the commencement of the intestine, and discharge their secretion by a multitude of separate orifices, these cluster in the higher Gasteropoda around the branches of a single or multiple common duct (Fig. 127, /. /, /). very much as in the Crustacea ; and in Cephalopoda, the lobules are so connected together as to form a compact mass, nearly resembling the liver of Fishes, and (like it) removed to a distance from the intestinal canal, into which it pours its secretion by a single long duct. 587. The intimate structure of the Liver in Yertebrata has not yet been satisfactorily made out. The organ presents, as we ascend the series, a more and more solid parenchymatous texture, which strikingly contn sts with its loosely-lobulated appearance in all but the highest Invertebrata ; and there is not the least difficulty in de- monstrating that this parenchyma is composed of cells. „ which are obviously the instruments by which the biliary ,_g matter is eliminated. These biliary cells, in the liver of Man and of higher Yertebrata, are of a flattened spheroi- dal form (Fig. 236), their diameter being usually from l-1500th to l-2000th of an inch ; and they commonly lie in piles, their faces adhering to one another. Each of them presents a distinct nucleus ; and the cavity of the cell is occupied by yellow amorphous biliary matter, usually having one or two large adipose globides, or five or six small ones, intermingled with it. It is interesting to remark that, in the Amphioxus (§ 321), the liver * See, on this part of the enquiry, the paper of Dr. C. H. Jones on the Structure and Development of the Liver, in the “ Philosophical Transactions' 5 for 1849. Fig. 236. Glandular cells of Liver : — a, nucleus ; b , nucleolus; c, adi- pose particles. SECRETION IN ANIMALS. 819 reverts to the type which, it presents in the lower Articulata ; for it is nothing else than a csecal prolongation from the alimentary canal (Fig. 172, 1,1), surrounded by hepatic cells, which are found also in the walls of the intestinal canal itself, as in the Earth-worm. But in the higher Fishes, as in the classes above them, it has been hitherto found impossible to obtain an entirely satisfactory demonstration of the relation of the cells of the hepatic parenchyma to the biliary ducts, by which the products of their secreting action are carried off. — The following are the general results of the observations which have been made upon the struc- ture of the Liver in Man and the Mammalia, on which attention has been chiefly bestowed. The entire mass is made up of a vast number of minute lobules of irregular form, but about the average size of a millet seed ; and each of them contains the elements of which the entire organ is composed, namely, a plexus of biliary ducts connected with their main trunks, and a mass of biliary cells ; and each is connected, in like manner, with the three blood-vessels which minister to the circulation in this organ, namely, the hepatic artery which brings blood for its nutrition, the vena portae which brings blood for the secreting ope- ration, and the hepatic vein which carries back the blood received from both sources. On endeavouring to trace the main excretory canal, or £ hepatic duct,’ from its ter- mination in the intestine to its sources in the liver, it is easy to make out that it subdivides and ramifies among the lobules of the whole mass ; and in general it would appear that the ducts connected with neighbouring lobules inosculate with each other, so as to form a plexus that is contin- uous through the entire organ. This plexiform arrangement was inferred by Mr. Kiernan to be continued into the interior of the lobules (Fig. 237) ; and from various recent observations, it seems probable that this is really the case, and that the cells of the parenchyma are actually included within a limitary membrane, which is a continuation of that lining the biliary plexus, so that they discharge their contents immediately into the ducts. 588. The blood which is brought to the liver for the elimination of the biliary secretion, is distributed by the branches of the vena portse to a similar inter-lobular plexus ; and from these, a set of converging twigs oroceed towards the centre of each lobule (Fig. 238), which supply a papillary network in its substance. In the centre of the lobule a radicle |*f the hepatic vein takes its origin, collecting the blood from the capillary letwork, and uniting with other radicles to form the trunk by which it is landed back to the vena cava. Owing to the peculiar position of the Fig. 237 . Horizontal section of two superficial lobules of the Human Liver , showing the probable arrangement of the biliary ducts ; a , a, intralobular branches of the hepatic veins ; b, b, trunks of biliary ducts, proceeding from the plexus which is supposed to traverse the lobules ; c, interlobular tissue ; d, parenchyma of the lobules. 820 SPECIAL AND COMPARATIVE PHYSIOLOGY. branches of the hepatic vein in the centre of each lobule, the lobules are appended (as it were) to these branches, like fruits upon a stalk. The branches of the hepatic artery are principally distributed upon the walls of the hepatic ducts, and upon the tranks and branches of the portal and hepatic veins, sup- plying them with then’ vam vasorum ; and also upon their fibrous sheaths (formed by an inversion of the investing mem- brane of the gland, and teimed ‘ Glisson’s capsule’), from which prolongations extend into the substance of the liver, to hold together its elements. The pre- cise relation of the capillaries of the hepatic artery to those of the portal and venous sys- tems, has not yet been ascer- tained beyond all doubt; but there seems reason to believe, with Mr. Kiernan, that the Horizontal section of three superficial lobules, showing , • --i-j • -j • 1 the two principal systems of blood-vessels; a, a , intra- STtGlltli CQ.piilcU.16S (llSCilQlgc lobular veins, proceeding from the Hepatic veins ; b. b, inter- themselves into the ultimate lobular plexus, formed by branches of the Portal reins. . . . ramifications of the portal vein ; and that thus the blood of the former, having become venous by trans- mission through the nutritive capillaries of the liver, mingles with the other venous blood collected by the vena portae, to supply the ma- terials of the secretory function, which are eliminated from it during its passage into the hepatic vein. Thus the parenchyma of each 1< bale will exercise its secretory action upon the blood, during the passage of the fluid from its periphery towards its centre ; and the bile thus eliminated will be discharged, by the rapture or dissolution of the hepatic cells, into the plexus of ducts by whose limitary membrane they are invested. — It is only in the warm-blooded Vertebrata, that the cavity of these cells is chiefly occupied by biliary matter. In Fishes and Reptiles, the paren- chyma is as fatty as in the inferior classes ; and the oily matter fills the cells almost as completely as in the Crustacea (Fig. 234'. so that the livers of the Cod and other large Fishes yield a considerable supply of it. In Birds, on the other hand, the hepatic cells are even more free from fat globules than are those of Mammalia ; and they are almost entirely filled with amorphous biliary particles. These differences will be seen to have an important signification, when taken in connection with the probable action of the organ in these different groups (§ 504).* * Much valuable information has lately been contributed by microscopic examination ot the minute structure of the Liver in different classes of animals. The Author has especially availed himself, in the foregoing account, of the “ Anatomical and Pathological Observations of Prof. Goodsir (1845); Dr. T. Williams’s Essay in the “Guy’s Hospital Reports ” for 1846; Dr. Leidy's valuable communication in the “American Journal of the Medical Sciences” for Jan. 1848 ; a Report on the researches of Prof. Retzius in “ Muller's Arcbiv.. 1849; and Dr. C. Handheld Jones’s paper in the “ Philosophical Transactions” for 1849. Mr. Kiernan's paper in the “ Phil. Trans.” for 1833 will always remain of standard value. Fig. 238. I SECRETION IN ANIMALS. 821 589. The history of the development of the Liver in the higher animals, presents many points of most interesting analogy to the permanent condi- tions which the organ has been thus shown to possess in the lower. The first rudiment of the gland is formed by the thickening of the cells in the wall of the alimentary canal, at the spot in which the hepatic duct is sub- sequently to discharge itself. This thickening increases, so as to form a projection upon the exterior of the canal ; and soon afterwards the lining membrane of the intestine dips down into it, so that a kind of caecum is formed, surrounded by a mass of cells, as shown in Fig. 239. Now here we have the obvious representa- tion of the hepatic organs of the Fl<5, 239, lower Invertebrata ; which, in their simplest forms, are nothing else than isolated cells, situated in the walls of the alimentary canal it- self ; whilst in their next grade, they are clustered round a csecal diverticulum from its cavity. The condition of the Liver in Amphi- oxus is almost precisely repre- sented by that of the embryo Fowl at the fourth or fifth day. The increase of the organ seems to take place by a continual new budding-fortli of cells from its peri- pheral portion ; and a considerable mass is thus formed, before the caecum in its interior undergoes any extension by ramifications into it. Gradually, however, the cells of the exterior become metamorphosed into fibrous tissue for the investment of the organ ; those of the interior break down into ducts which are developed in continuity with the intestinal caecum, and which are lined by muscular and fibrous tissues developed from the primitive cellular blastema ; whilst those which occupy the intervening space, and which form the bulk of the gland, give origin to the proper secreting cells, which are now to come into active operation. As this is going on, the hepatic mass is gradually removed to a distance from the wall of the alimentary canal ; and the caecum is narrowed and lengthened, so as to become a mere connecting pedicle, forming, in fact, the main trunk of the hepatic duct. — Such is believed to be the general history of the process ; and it applies equally to the other glands developed from the parietes of the alimentary canal, such as the salivary glands and the pancreas ; but many of the minuter questions which it involves, still remain for elucidation. Origin of the Liver from the intestinal wall, in the embryo of the Fowl , on the fourth day of incubation : — a, heart; b, intestine; c , everted portion giving origin to liver; d, liver; e, portion of yolk-bag. 590. We have now to enquire into the characters of the Bile , and the ’ purposes to which it ministers in the Animal economy. — Of the solid matter, which forms nearly a tenth part of the whole secretion, about an , eighth part is composed of alkaline and earthy salts, corresponding with those found in the blood, but not bearing the same proportions to each , other ] soda being present (apparently in a state of actual combination j with biliary matter) in such amount as to be peculiarly characteristic of this product. The organic constituents, which make up the remainder of the solid matter of the bile, are acted upon by reagents with peculiar facility, and are thus liable to be changed by the processes which are 822 SPECIAL AND COMPARATIVE PHYSIOLOGY. merely intended to separate them ; and hence the accounts of them given by different chemists are far from being conformable with each other. For our present purpose it will be sufficient to state that the proper Biliary matter , or Biline (of Berzelius), whether a single or a composite substance, is allied to resinous and fatty matters in its composition and properties, and appears to unite (like the fatty acids) with soda or other bases ; and its ultimate composition is represented by Prof. Liebig to be 76 Carbon, 66 Hydrogen, 22 Oxygen, 2 Nitrogen, with a considerable proportion of sulphur. — The fatty matter of the Bile of Mammalia (which has been the principal subject of chemical research) for the most part consists of the peculiar spermaceti-like substance termed Cholesterine ; the proportion of this, however, in healthy bile, is extremely small ; but in many disordered states it accumulates in large amount, and it usually forms the principal if not the sole ingredient in biliary concretions. It is a white crystallisable fatty matter, somewhat resembling spermaceti, free from taste and odour, and composed almost entirely of carbon and hydro- gen, its formula being 36 Carbon, 32 Hydrogen, and 1 Oxygen. The ordinary fatty acids (§ 189) may occasionally be detected in the bile, but seldom in large proportion. — The peculiar Colouring Matter of bile is distinct from both the preceding substances ; its chemical characters have not yet been clearly made out ; but there is much probability that it is related to the colouring-matter of the blood. For the addition of a mineral acid to the colouring-matter of bile produces remarkable changes in its hue, converting the yellowish matter successively into green, blue, violet, red, and brown ; in fact, precisely into those colours which are exhibited by the colouring-matter of the blood, as left in an ‘ ecchyinosis ' in process of departure. 591. There is every probability that these substances exist in the blood, in a condition not very dissimilar to that in which they are found in the product secreted from it. Thus Cholesterine may be obtained from blood- serum by an analytical process of no great complexity ; and its presence there is manifested by its occasional deposit, as a result of diseased actiou. in other parts of the body, especially in the fluids of local dropsies. The Colouring-matter appears to be derived, directly or indirectly, from the hsematine of the blood. Of the Biline, however, it can only be said that its indefinite nature and reactions have hitherto prevented its detection in the blood by chemical tests ; and much yet remains to be learned regard- ing its history. From the considerations to be presently adduced, it would seem probable that biliary matter does not exist as such in the blood, pre- viously to the formation of the secretion ; but that its elements, derived from the disintegration of the tissues, are present in the circulating fluid under some more pernicious form, and are transformed into biline, by the agency of the liver, in order that they may be re-absorbed in a less noxious form, to be finally eliminated by the respiratory process. It is certain that the effects of the re-absorption of bile into the blood, as seen in ordinary cases of jaundice dependent upon obstruction of the biliary ducts, are not nearly so injurious as are those of the retention of the elements of the secretion, consequent upon deficiency of the secreting power of the liver : for whilst in the latter case, death speedily supiervenes, if no other outlet be found for the excrementitious matter, in the former, no severe injury necessarily arises from the accumulation of biliary matter, to even such an SECRETION IN ANIMALS. 823 extent that the tissues in general are tinged by it. And, as will presently appear, there is strong reason to believe that the re-absorption of biliary matter into the circulating current, is the means by which it is finally carried out of the system. 592. The special purposes answered by the secretion of Bile are still involved in considerable obscurity ; but more definite and probably more correct ideas in regard to them are gradually being evolved ; and the following may, perhaps, be regarded as tolerably well established truths. In the first place, the bile must be considered as an excrementitious sub- stance, derived from the disintegration of the tissues, or from the decom- position of the elements of the blood ; and it serves especially to remove from the blood the hydro-carbonaceous portion of the effete matters, the nitrogenous being eliminated in the urine. It has been pointed out by Prof. Liebig, that if we add to half the formula representing the rdtimate composition of bile, the formula of urate of ammonia (which is the charac- teristic element of the urine of all animals save Mammalia), the sum gives the proportionals of the ultimate components of dried blood or of flesh, with the addition of 1 eq. of oxygen and 1 eq. of water. For ; — Half an equivalent of Biliary matter One equivalent of Urate of Ammonia The sum of which And, in like manner ; — Formula of Blood 1 Eq. of Water + 1 Eq. of Oxygen = 38 C, 33 H, N, 1 1 0 = IOC, 7 H, 5 N, 6 0 = 48 C, 40 H, G N, 17 0 = 48 C, 39 H, 6 N, 15 0 = H 20 48 C, 40 H, 6 N, 17 0 Now although it must be admitted that, by a dextrous management of formulas, almost any kind of transformation may be effected on paper, yet this coincidence, without any management at all, is so close that it cannot be regarded as accidental ; and we seem fairly entitled to look upon the principal materials of the animal fabi’ic as partly resolving themselves, in their disintegration, into the characteristic components of the two principal excretions, the urinary and the biliary. Of course this resolution only expresses a part of the metamorphoses which the tissues undergo ; for the kreatine and kreatinine of the urine, and the faecal matters of the alimentary canal, must be regarded in the same light ; and there are doubtless other excrementitious matters (included in the general term ‘ extractive ’), of which we know still less, that must be attributed to the same origin. But it seems satisfactorily to account for the com- ponents of the biliary matter ; since they form the complement of the urinary ; so that the formation of each excretion seems to involve that of the other. — Now, that the bile is in its essence an excrementitious product, and that the assistance it may afford in the digestive process is not the principal purpose of its secretion, appears further from this ; that it is eliminated diming the latter part of embryonic life, and that it then accu- mulates in the alimentary canal, forming a large part of the meconium which is discharged soon after birth. But from the time that respiration commences, and during the whole subsequent life, it appears from che- 824 SPECIAL AND COMPARATIVE PHYSIOLOGY. mical analysis of the faeces, that not much of the bile, save its colouring matter, is evacuated in the state of health ; so that a large part of that which is poured into the alimentary canal, must be re-absorbed through the blood-vessels and lymphatics of its walls. And there can be little doubt that, when thus re-absorbed, and taken into the current of the circulation in a less noxious form than that in which its elements previously existed, the biliary matter is chiefly eliminated by the respiratory process ; its sulphur, however, being oxidised, that it may be carried off by the urine, and its soda being eliminated by the same channel. In this point of view, the secretion of bile may he regarded as the intermediate stage between the disintegration or ‘ waste’ of the tissues, and the final elimination of the combustible products of that waste by the instrumentality of the lungs. 593. But that the bile performs some subsidiary function in the Digestive process, would also seem beyond doubt. It is not, however, a sufficient indication of this, that we should find the outlet of the hepatic- organs, through the entire Animal Kingdom, to be in the part where digestion is most actively going on ; for it may be, that the discharge of the bile into the upper part of the alimentary canal, has reference merely to its re-absorption. But it appears from the results of numerous experi- ments, that when the bile-duct is divided, and its extremity is drawn out of the body, in such a manner that it can freely discharge the hepatic secretion, but this is prevented from passing into the alimentary canal, the animals, although they do not at first appear to suffer much in health, gradually become emaciated, and at last die of inanition, unless they are allowed to receive back the bile by licking the wound, in which case they survive. And it seems proved, by the recent investigations of Lehmann and Frerichs, that although the pancreatic fluid certainly pos- esses considerable power of £ emulsifying ’ the oleaginous matters in the alimentary canal, yet that it answers this purpose much more effectually when mingled with bile, which possesses an emulsifying power of its own. It also appears from the experiments of H. Meckel, that bile has the power of effecting the transformation of sugar - into oleaginous matter ; thus effecting that for a portion of the contents of the intestine, which the passage of the portal blood through the liver effects for a portion of the saccharine matter which it contains (§ 561). And lastly, from the expe- riments of Gorup-Besanez it appears that bile has a special solvent action upon caseine. — Hence it may be stated that the function of the bile iu digestion is chiefly the preparation of the combustive material for the respiratory process ; since by its means the saccharine matters are partly transformed into fat, and the absorption of the fatty matter's is assisted by its admixture with the pancreatic fluid. And thus we see why animals should die of inanition when the bile is wholly discharged out of the body ; since they thus lose, not only the combustive materials supplied by the bile itself, but also the greater part of those contained in the food, which they are prevented from assimilating. 594. Thus, then, if we bring together all the facts at present in our possession, with reference to the offices of the Hepatic organs, they seem to lead to these conclusions. — 1. That they are essentially organs of excretion, designed to remove from the circulating fluid that portion of the products of disintegration, of which the principal component of the SECRETION IN ANIMALS. 825 urinary excretion is the ‘complement.’ — 2. That in doing this, they con- vert the greater part of the excrementitious matters into Biline ; a substance which can be reabsorbed with less injury, and which, after per- forming its part in the digestive process, is taken back into the blood, to be eliminated by oxidation through the lungs.* — 3. That the temporary presence of bile in the alimentary canal is subservient to the digestion and absorption of the non-azotized compounds, and perhaps also in some degree to that of the albuminous. — 4. That not only by the separation of biliary matters from the blood, and by the operation of this upon the alimentary substances, but also by the change which it effects in the constituents of the blood itself, the liver aids in preparing oleaginous materials for the combustive process. — 5. It also aids (if we are to rely upon the statements of M. Bernard, § 561) in the assimilation of albumen, taking a share in effecting its metamorphosis into fibrine. — Now if we are to regard the production and separation of fatty matter as one of the great purposes answered by the Liver, it need not surprise us to find that this organ should frequently serve, not only to prepare this, but also to store it up ; and this we might expect to be the fact especially in those classes of animals, in which its final elimination by the respiratory actions is slow. Such we have seen to be the case ; the great bulk of the liver in the Crustacea, Mollusca, and cold-blooded Yertebrata, having reference apparently, not to a large production of bile, but to an accumulation of fat : whilst in Mammalia the fat is in very small amount, unless the respiration be impeded ; and in Birds, whose respiration is pre-eminently active, scarcely any traces of fat are to be found. The fat thus stored up in the liver will probably be taken into the current of the circulation, as it is wanted, by the blood which passes through the organ ; and may never be discharged as an excretion into the alimentary canal, t 595. Of the Kidneys and, the Urinary Excretion. — The Kidneys are to be regarded as purely excreting organs ; their sole function being to sepa- rate from the blood certain matters which would be injurious to it if retained ; and these matters being destined to immediate and complete removal from the system. These glands almost always present a tubular structure ; the required extent of surface being given, not by the multi- plication of separate follicles, but by a great prolongation of the individual cseca. — No distinct traces of urinary organs can be made out among the lower Invertebrata. In Insects and Aracknida, however, they present themselves as a group of decal tubnli, discharging themselves into the cloacal termination of the alimentary canal (Fig. 162, h, h ) • and similar tubuli are found in some of the higher Crustacea.— In the Molluscous series, however, the urinary organs, where they exist, possess rather a follicular character. It is probable that a glandular mass found in some Conchifera near the termination of the intestine (which has been regarded by some as a calcifying gland) is really a rudimentary kidney ; and this * When an unusually large quantity of bile is poured into the intestinal canal, as after the action of a mercurial purgative, or in ‘ bilious diarrhoea,’ the greater portion of it escapes reabsorption, and is ejected per anum. *f The doctrine that the agency of the Liver is preparatory to the Respirator}' function, was first propounded by Prof. Liebig (“Animal Chemistry,” 1842); and although it has had to contend against many important objections, it has gradually made its way into Physiological Science:— in the form in which it is above stated, the Author believes that it will be found consistent with all the facts at present known. 826 SPECIAL ANI) COMPARATIVE PHYSIOLOGY. may be said with more certainty of a conglomerate mass of small follicles, with a single efferent duct or ureter which opens externally near the anus, in many Gasteropoda, this body having been found to contain uric acid. It is remarkable that in animals so highly organised as the Cephalopoda , the presence of a proper renal organ is as yet undetermined. Some have attributed this character to the £ ink-bag ; ’ but this only exists in a part of the class ; and it is thought more probable by Prof. Mayer, that the masses of follicles connected with the great veins (Fig. 135, r) are to be regarded as a urinary apparatus. 596. When we pass to the Vertebrated series, however, we usually find the renal organs attaining a large size in the class of Fishes ; but their type of conformation is low. They very commonly extend through the whole length of the abdomen ; and consist of tufts of uniform-sized tubules, which shoot forth transversely at intervals from the long ureter, and are con- nected together by a loose web of areolar tissue, that supports the network of vessels distributed upon then- walls. This condition of the urinary apparatus is very analogous to that of the Corpora Wolffiana, or tempo- rary renal organs of the embryo of the higher Yertebrata, which are afterwards superseded by the permanent kidneys (§ 599). In the lowest members of the class, however, the structure is yet simpler ; for in the Cyclostomi, each long duct sends off at intervals, instead of a bundle of tubuli, a short wide tube which commimicates with a single caecum ; and at the bottom of this is a small £ vaso-ganglion,’ or convoluted plexus of blood-vessels, which reminds us of the Corpora Malpighiana in the kidneys of higher Yertebrata. In the Amphioxus, the only rudiment of a kidney is the slightly opaque, slender, elongated glandidar-looking body (Fig. 172, li), which is considered by Prof. Owen to possess this character, though its structure has not yet been fully elucidated. 597. The size of the Kidneys is usually considerable in Reptiles ; but their form differs greatly in the several orders of the class, being narrow and much elongated in Batra- chia and Ophidia, but broader and shorter in Sarnia and Che- lonia. Their essential struc- ture, however, is nearly the same throughout ; for the ure- ter gives off a large number of transverse caeca, which are short and nearly straight in the lower Reptiles and in the early condition of the higher (Fig. 240), but which in the more developed conditions of the organ become long and con- voluted (Fig. 241), each group forming a lobule, which receives branches from the portal trunk that supplies the organ v\ itli blood. In the Crocodile the distinction between the cortical and the me- dullary portion of the kidney becomes evident ; the former being the part in which the blood-vessels are most copiously distributed, and in which the Fig. 240. Kidney of fetal Boa .-—the urinary tubes as yet short and straight. Fig. 241. Portion of Kidney from Coluber : — a, a , vascular trunk: b, b, ureter ; c, c, converging fasciculi of tubuli uriniferi. SECRETION IN ANIMALS. 827 Fig. 242. Pyramidal fasciculi of tubuli urini- Fig. 243. tubuli have the most convoluted arrangement ; whilst the tubuli in the latter are straighter, and converge more directly to the points at which they discharge themselves into the ureter. The Corpora Malpighiana (§ 598), where they exist in this class, are scattered through the entire substance of the kidney ; not being re- stricted, as in the higher animals, to its corti- cal portion. — In Birds, too, the kidneys are of large size ; and they present also a greater compactness of texture. The lobules of which they are composed, can often not be distin- guished externally ; but they are connected with separate branches of the ureter, and each consists of a converging bundle of tubuli uriniferi, forming its ‘ medullary’ portion (Fig. 242), and of the dichotomous ramifications of these in the outer part of the lobule, of which feri of Bird, terminating in one of the . , . -i . , • tj • branches of the ureter. they constitute the 4 cortical portion. — It is in Mammalia, however, as in Man, that the organ becomes most com- pact, and the distinction between its cortical and medullary portions most clearly marked. The separate clusters of tubes do not now open into distinct branches of the ureter ; but discharge their contents into a capacious cavity formed by the dilata- tion of the ureter in the interior of the kidney (Fig. 243), which looks (in section) as if it were doubled together around it. The lobules composed of these separate clusters are usu- ally blended together so closely, that they cannot be distinguished externally, and can- not be separated from each other anatomi- cally. In the Human foetus, however, when the kidney is first being developed, its lobular character is very apparent ; and this type is permanently retained in many of the lower Mammalia. 598. The proper secreting apparatus con- sists of the epithelial cells lining the tubuli uriniferi (Fig. 244), which draw the peculiar elements of the urinary excretion from the vascular plexus which surrounds the exterior of the tubes, and then deliver them up to be made u p "of Turin c t "lobes 7 T." The supra-renal capsule. 2 . The vascular portion of the kidney. 3, 3. Its tu- bular portion, consisting of cones. 4, 4. Two of the papillae projecting into their corresponding calices. 5, 5, 5. The three infundibula; the middle 5 is situated in the mouth of a calyx. 6 . The pelvis. 7 . The ureter. A section of the Kidney, surmount- ed by the supra-renal capsule ; the swellings upon the surface mark the original constitution of the organ, as carried off through the interior of these, to their terminations in the ureter. It does not ap- peal’ that this process necessarily involves the continual exuviation and renewal of the se- creting cells ; for it would seem as if they could give up, by their free surface, the mat- ters which they have taken in, when these are in a state of such perfect solution as to be able to transude the cell-wall, and thus to escape with- out rupturing it. For the liberation of the constituents of the almost SPECIAL AND COMPARATIVE PHYSIOLOGY. Fig. 244. solid urine of Reptiles and Birds, however, it is probable that the exuvia- tion of the secreting cells is necessary.— But the Kidney contains another apparatus, of a very peculiar description, which appears specially destined for the elimination of the superfluous fluid of the blood, by a process of simple transudation. When a section of the cortical substance is slightly magnified, the cut surface is seen to be studded with a number of little dark points, each one of which, when ex- amined under a higher magnifying power, is found to consist of a knot of minute blood- vessels, formed by the convolutions of thin-walled capillaries (Fig. 245, m). These bodies, termed Corpora Malpighiana after their first discoverer, are included in little flask-shaped capsules, the necks of which are continuous with the tubuli uriniferi ; so that these may be considered as sending off little diverticula to invest the Corpora Malpighiana, which appear to lie freely within them.* Each of these little vascular knots is directly supplied by a branch of the renal artery (Fig. 245, a, b), which, when it reaches the capsule, subdivides into a group of capillaries ; and these, after forming the convoluted tuft (?«), coalesce into a single efferent trunk {(f), which may be considered as representing a branch of the vena portae. For the efferent trunks of the Malpighian bodies discharge their blood into the capillary plexus, which surrounds the tubuli uriniferi, and from which the solid matter of the urinary secretion is elaborated ; just as the vena portae supplies the capillary plexus, from which the biliary secretion is elaborated in the liver. Thus the whole Malpighian system of vessels may be considered (as Mr. Bowman has pointed out) in the light of a portal system within the kidney ; the efferent vessels of the Corpora Malpighiana being collectively the representatives of the Vena Portae, since they convey the blood from the first (or systemic) to the second (or secreting) set of capillaries. In Reptiles (in which, as in Fishes, the Kidney is partly supplied from the Hepatic Portal Portion of tubulus uriniferus, with its tesselated epithelium. Fig. 245. Distribution of the Reual ves- sels ; from Kidney of Horse : — a, branch of Renal artery; uf, afferent vessel ; m, in, Malpighian tufts ; ef, ef, efferent vessels ; p, vascular plexus surrounding the tubes ; st, straight tube ; ct, con- voluted tube. Magnified about 30 diameters. * By Mr. Bowman, the first discoverer of this curious relation, it was supposed that the flask-shaped dilatation was formed by the expansion of the extremity only of the tubulus uriniferus ; and that the membranous wall of the capsule is perforated by the afferent and efferent vessels, so that the capillary knot is really in its inside, and has no investment of its own. (“ Phil. Trans.” 1842). Subsequent investigations, however, have proved that the Corpora Malpighiana are connected with the sides, as well as with the extremities of the tubuli, so that each tubulus is in relation with several of them ; and have rendered it probable that the flask-shaped capsule does not enclose the Corpus Malpighianum, but is reflected over it like the pericardium over the heart, so that the capillary knot really lies on its exterior, although embraced by it in such a manner that fluid transuding from the vessels at once enters the cavity of the capsule. SECRETION IN ANIMALS. 829 system, § 472), the efferent vessels of the Malpighian bodies, which receive their blood (as elsewhere) from the renal artery, unite with the branches of the Portal vein to form the secreting plexus around the tubuli urini- feri ; so that this plexus, like the secreting plexus of the Liver (§ 588), has a double source, the vessels which supply it receiving their blood in part from the capillaries of the organ itself, and in part from those of other viscera. In Mammalia, how T ever, the secreting plexus is entirely supplied by the efferent vessels of the Corpora Malpighiana ; though in Birds the oviparous type of distribution seems still to prevail to a certain extent (§ 475). 599. In the embryological development of the Kidneys of the higher Vertebrated animals, we have a very interesting example of the evolu- tion of an organ, which is to serve only a temporary purpose in them, but which remains as the permanent instrument of the function in the lowest class, although superseded in the higher by an organ formed upon a more elaborate type. The first appearance of anything like a Urinary apparatus in the Chick, is seen on the second half of the third day ; and the form then presented by it is that of a long canal, extending on each side of the spinal column, from the region of the heart towards the allan- tois : and the sides of this present a series of elevations and depressions, indicative of the incipient development of caeca. On the fourth day, the Corpora Wolffiana, as they are then termed, may be distinctly recognised as composed of a series of caecal appendages, which are attached along the whole course of the first-mentioned canal, opening into its outer side ; and thus closely corresponding with the condition of the (so called) kidneys of Fishes (§ 596). On the fifth day, these appendages are con- voluted, and the body which they form acquires thickness ; they evidently then possess a secreting function, and the fluid which they separate is conveyed by the duct of each side into the allan- tois, a sac which, though employed as a temporary respiratory organ (chap, xviii.), is also used as a urinary bladder. Vestiges of Corpora Malpigh- iana may even be detected in connection with the secreting cieca. — The development of the true Kidneys commences in the Chick at about the fifth day. They are seen on the sixth as lobu- lated grayish masses, which sprout from the outer edges of the Wolffian bodies ; and they gradually increase in size, the temporary organs diminishing in the same proportion. The sexual organs also originate as offsets from the Wolffian bodies ; and it seems to be in connection with them, that the last traces of these temporary organs are found. The Kidneys, in the Human embryo, soon after their first development in the manner just de- scribed, consist of seven or eight lobes, the future pyramids ; their excretory ducts still terminate in the same canal, as that which receives those of the Wolffian bodies and of the sexual organs ; and this opens, with the rectum, into a cloaca, analogous to that which re- mains permanent in the Oviparous Vertebrata. Thelobulated appearance increased breadth and Fig. 24C. Chick: — kidney; b, b, ureters; c, corpus Wolffianum ; d, its ex- cretory duct ; e, e, testes. 830 SPECIAL AND COMPARATIVE PHYSIOLOGY. of the kidney gradually disappears, partly in consequence of the con- densation of the areolar tissue which connects the different parts, and partly through the development of additional tubuli in the interstices ; it is persistent, however, in the kidney of many of the lower Mammalia, as the Bear. — Thus we have, in the development of the Urinary apparatus, the same kind of progress from the more general to the more special type, as we have seen in the Respiratory • and it is not a little curious that the more general form of both should be retained in the same class, namely that of Fishes. There is this difference, however, between the two cases ; that whilst the branchial arches of higher animals are not ever deve- loped so far as to be instrumental in the respiratory function, their Corpora Wolffiana appear to be true temporary kidneys, eliminating a real urinary product (§ 601). 600. The Urine of Man and of Mammalia generally, is characterised by the large proportion of ivater which it contains, in comparison with its solid constituents; the latter being seldom above 5 parts in 100, and being very commonly less. The two, in fact, bear no constant relation to each other ; for the amount of liquid in the secretion depends mainly on the degree of fulness of the blood-vessels ; while that of the solid matter is governed by that of the previous ‘ waste ’ of the tissues. It would seem as if a much larger quantity of water is habitually taken-in, than is needed in the system ; in order to provide for the reduction of the tem- perature of the body by cutaneous Exhalation, when it might otherwise be unduly elevated (§ 547) ; but if the usual quantity of water be not thus drawn off, in consequence of the depression of the external tempera- ture, or the saturation of the atmosphere with dampness, or if an unduly large amount have been absorbed, the Kidneys afford the channel for its elimination. This appears to be the special function of the Malpighian bodies ; whose thin-walled capillaries allow the transudation of water to take place, under a certain pressm-e, into the tubuli uriniferi ; and thus act the part of regulating valves, permitting the passage of whatever is superfluous, while they retain the liquid that is needed in the system. In Birds, on the other hand, it would seem as if there is much less occa- sion for any provision to reduce the temperature, which is habitually kept up at a standard higher than that of any other animal ; and they accord- ingly drink very little, so that the proportion of water in their urine i- only sufficient to give it a semifluid consistence. The urinary excretion of Reptiles appears to be in general yet more solid ; for these animals usually ingest but little water, and a part of this is given off by cutaneous exhalation when the external temperature is high. The condition of that of Fishes and Invertebrata appears to be generally the same ; but from this statement the ‘bombardier' beetles must be excepted, which emit their urine, as a means of defence, in little puffs of vapour, having a very acrid character, and believed to contain nitric acid. — The solid matter of the Urine partly consists of organic compounds formed within the body, as the result of the disintegration of the tissues, or of the decomposition of sub- stances taken in as food ; and partly of inorganic salts, such as normally exist in the serum of the blood, then- proportion being liable to an increase, however, under circumstances to be presently alluded to. 601. The Organic Compounds are not the same in all animals : but yet they are nearly related to each other, and agree in the very large SECRETION IN ANIMALS. 831 proportion of nitrogen which they contain. The most characteristic of them, when completely isolated, present a crystalline form, which seems to he completely incompatible with the possession of plastic or organisable properties, and marks their affinity to inorganic substances. In the Urine of Man, the most characteristic ingredient is Urea, a neutral sub- stance, isomeric with cyanate of ammonia (§ 29), which is very soluble in water, and may be crystallised-out in transparent, colourless, four-sided prisms. When pure, it has very little tendency to decomposition ; but when associated with other substances which act as ‘ ferments,’ it takes to itself the oxygen and hydrogen of 2 equiv. of water, and resolves itself int*o carbonate of ammonia. The uses of this arrangement in the economy of Nature have already been remarked upon (§ 260 note). The Urine of Man also contains a small quantity of Uric Acid, a substance which is not readily soluble in pure water, but which is more easily dissolved by water holding phosphate of soda in solution, especially when warm ; and this seems to be its condition in human urine. It is also more readily dissolved, when in combination with ammonia ; and in this condition it forms a large part of the almost solid urine of Serpents, which also con- tains, however, like the urine of Birds, a large quantity of undissolved m’ic acid. When separated and purified, uric acid forms a glistening- snow-white powder, apparently amorphous, but shown by the microscope to consist of minute but regular crystals. It is tasteless and inodorous ; and its acid reaction is very feeble. Uric Acid is replaced in the Urine of Herbivorous Mammals by Hippuric Acid ; which is much more soluble in cold water, and dissolves readily in warm. When pure, it crystallizes in long transparent four-sided prisms, and has a strong acid reaction, with a bitterish taste. When dissolved in a liquid containing putrescent albu- minous compounds, hippuric acid is converted into benzoic acid, ammonia being at the same time given off. In the fluid of the Allantois of the foetal Calf (and probably also of other animals), which may be regarded as a temporary urinary bladder, receiving the product of the secreting action of the Corpora Wolffiana or temporary kidneys, is found another substance, termed Allantoin, which may be artificially obtained from uric acid by boiling it with peroxide of lead. This is a neutral substance, forming small but most brilliant prismatic crystals, which are destitute of taste, and moderately soluble in cold water ; when decomposed by strong acids it is resolved into ammonia, carbonic acid, and carbonic oxide ; and when acted on by alkalies, it is resolved into ammonia and oxalic acid. Two other substances, Kreatine and Kreatinine, have recently been dis- covered in the urine of Man and the Mammalia, which seem intermediate in character between the albuminous compounds and the characteristic components of the urinary excretion. Kreatine, which may be obtained from the juice of raw flesh, is a neutral substance, having the form of colourless, prismatic crystals, sparingly soluble in cold water, but dis- solving readily in warm. By the action of strong acids, kreatine is con- verted into kreatinine, which only differs from it in composition by containing two proportionals less of the elements of water, but is a substance of very different chemical relations, having a strong alkaline reaction, and serving as a powerful organic base to acids. It pre-exists in the juice of flesh to a small extent ; and is found, in conjunction with kreatine, in the urine. When long boiled with caustic baryta, kreatinine 832 SPECIAL AND COMPARATIVE PHYSIOLOGY. is gradually resolved into urea. — The composition of these substances, in relation to each other, and to that of Albuminous compounds, is shown in the following table ; which gives for each the number of combining equivalents of its individual components, and their respective atomic weights, so as to show the proportion which the nitrogen bears to the whole. Carbon. Hydrogen. Nitrogen. Oxygen. Total. . „ ( Liebig .Albumen ( MuW " r 49 (294) 36 (36) 6(84) 14(112) 526 40 (240) 31 (31) 5(70) 12 (.96) 437 Urea 2 (12) 4 (4) 2 (28) 2 (16) 60 Uric Acid . 10 (60) 4 (4) 4 (56) 6 (48) 168 Hippuric Acid 18 (108) 8 (8) 1(14) 5 (40) 170 Allantoin . 4 (24) 3 (3) 3 (42) 3 (24) 93 Kreatine 8 (48) 9 (9) 3(42) 4 (32) 131 Kreatinine 8 (48) l 0) 3(42) 2 (16) 113 Thus we see that the proportion of Nitrogen in the whole, by weight, is in albumen as 1 : 6 -2 4 ; in urea as 1 : 214 ; in allantoin as 1 : 2-21 ; in kreatinine as 1 : 2-69 ; in uric acid as 1 : 3 - 00 ; in kreatine as 1 : 3T2 : and in hippuric acid as 1 : 12 T 4. Hence the proportion of nitrogen in the compounds of urine ranges from double to triple that which exists in the albuminous constituents of the living fabric ; the only exception being in the case of hippuric acid, whose proportion of nitrogen is only half of that which exists in albumen.*- — Besides the foregoing substances, the Urine contains others whose nature has not yet been clearly determined ; these are at present included under the general designation Extractive matters, and appear to consist in part of non-azotized compounds in a state of change. — The Inorganic Compounds which are found in the mine, partly consist of salts which are taken-in as such in the food ■ and partly of salts which are formed in the economy, the acids being furnished by the oxygenation of bases contained in the aliment, and an ammoniacal base being supplied by the decomposition of the albuminous compounds. To the former class belongs chloride of sodium (common salt), of which the mine always contains a large amount, obviously derived directly from the serum of the blood ; and also the phosphates of lime and magnesia, the proportion of which in the mine appears entirely to depend upon the amount ingested in the food. To the latter class belong the alkaline sulphates amMphosphates ; whose acids appear to be chiefly formed by the oxygenation of the sulphur and phosphorus which are constituents of all the albuminous compounds used as food ; while their alkaline bases, when not ammoniacal, are supplied by the potass and soda that wen ingested in combination with citric, tartaric, oxalic, and other organic acids, these acids being decomposed in the system, and carried off by the respiratory process. Such weakly-combiued bases abound in the food of Herbivorous animals, but they are for the most pail wanting in that i f the purely Carnivorous ; and the fixed alkalies are there replaced in greater proportion by ammonia. 602. Hence we may say, that the Urinary excretion is specially des- * It would seem fair to surmise, therefore, that as Hippuric acid is restricted (so far is known) to the urine of animals of whose food non-azotized substances form a large part, it must have some other source than the metamorphosis of the organised tissues, and must he formed by the union of the products of this operation with some of the farinaceous or other superfluous components of the food. SECRETION IN ANIMALS. 832 tined (1), for the elimination of those products of the disintegration of the tissues, and of the metamorphoses taking place in the living body, which are of a highly azotized nature, or which, being in the condition of soluble salts, readily find their way by transudation through membranous cell- walls that hold back the albuminous element of the serum of the blood (§ -598) ; it is also obvious (2) that the kidneys are destined to remove, in the same form, whatever components of the food are superfluous, and are undergoing decomposition from not being applied to the purposes of nutrition ; and it is further their office (3) to draw off any soluble saline matters taken into the system, which are either useless or injurious to it. — Although the relations of the amount of the organic compounds in the urine, to food, exercise, &c., have been as yet studied almost entirely in the Human subject, there can be no reasonable doubt that the same general rides will be found to hold good elsewhere. The proportion of urea which is voided in a given time, is proportional, ceteris paribus, to the amount of muscular exertion that has been put forth ; showing that its presence depends in part upon disintegration of the Muscular tissue. But this is not its sole source. For it is greatly augmented, also, by an excess of azotized compounds in the food ; these compounds, as already shown (§ 570), not being applied to the nutrition of the muscular sub- stance, unless a demand for augmented formation has been created by previous functional activity. Thus, the average proportion of Urea in the Human urine, under ordinary circumstances as to food and exercise, appears to be from about 20 to 35 parts in 1000 ; but it may be raised to 45 parts by violent exercise, and to 53 parts by an exclusively animal diet ; whilst it may fall as low as to 15 or even 12 parts, when the diet is deficient in azotized matter. The average daily amount excreted by adult males, is about 430 grains ; by adult females, about 300 grains ; in children of eight years old, it is neai’ly half what it is in adults ; whilst in very old persons, the quantity sinks to one-tliird or even less ; showing that the proportion is greatly influenced by the rapidity of interstitial change at different periods of life (§ 571). There can be no doubt that hreatine and kreatinine have the same origin and character, since they are actually found in the juice of flesh, as well as in the urine. — So the pro- portion of the alkaline phosphates in the urine is found to bear such a close relation to the previous energy of the nervous system, that there can be little doubt that, ceteris paribus, their amount may be taken as a mea- sure of its disintegration by functional activity. It has been pointed out that, for the maintenance of this activity, a constant supply of arterialised blood is a necessary condition ; and whilst the other elements of the nervous tissue (whose composition is almost entirely adipose, § 243) will be carried off by oxygenation in the form of carbonic acid and water, the phosphorus which largely enters into it will be oxygenated, and taken back into the blood in the form of phosphoric acid, uniting there with alkaline oases, as already pointed out. — The proportion of extractive matters appears ihiefly to depend upon the nature of the food ; being greatly augmented py an exclusively vegetable regimen, and greatly diminished by an exclu- sively animal diet. — The importance of the urinary excretion in removing Superfluous or injurious saline compounds from the system (the introduc- ion of which into it has taken place by endosmotic action, § 435), is further shown by the increase in the secretion which most of these sub- 3 ii 834 SPECIAL AND COMPARATIVE PHYSIOLOGY. stances produce, this increase being the result of an augmented determina- tion of blood to the kidneys, and a consequently increased transudation < f their various actions, which have the power of generating Heat when it is not sufficiently imparted to them from external sources, and of resisting its influence when it is excessive. Having already considered the means by which the temperature of the living body is kept clown to its proper standard (chap, xiv.), we have now to enquire into the sources of that generation of Heat, within the living body, which keeps up its temperature in ‘warm-blooded’ animals to a certain fixed point, and which assists in enabling even the £ cold-blooded’ to resist the effects of extreme depres- * “ An Account of several cases of the Evolution of Light in the Living Human Subject by Sir Henry Marsh, M.D., M.R.I.A., &c. j- Such facts appear to give support to the idea, that a preternatural combustibility may sometimes exist in the body, owing to the retention of phosphorus-compounds, which shouid normally be excreted from it by the urine after undergoing oxidation. It has been observed that the breath of drunkards has sometimes exhibited luminosity, as if it contained the vapour of phosphorus or of some of its compounds ; and it has been found by experiments upon docs, that if phosphorus be mixed with oil and injected into the blood-vessels, it escapes unburned from the lungs. (“ Casper’s Wochenschrift,” 1849, No. IS.) — The Author has seen are- markable case, drawn up under the hand of the subject of it (a highly respectable clergyman), and shown him by his friend Dr. M. Barry, in which a troublesome sore, occasioned by the combustion of phosphorus on the hand, twice at distant intervals emitted a flame which burned the surrounding parts. It was particularly stated that ignition could not have been effected by any neighbouring flame, and that the combustion could not be due to any particles of phosphorus remaining in the wound ; and it does not seem improbable that, in the peculiar condition of the sore, an unusual amount of phosphorus-compounds had been deposited in it, so as even to become spontaneously inflammable on the contact of oxygen. EVOLUTION OF HEAT. 845 sion of the external temperature. — It is well known that almost all Chemical changes are attended with some disturbance of the temperature of the agents concerned ; and it may not unreasonably be surmised that, of those which are so constantly occurring in the living system, some may be connected with the disengagement of the Heat peculiar to it. Much uncertainty still prevails on this subject ; but there can be little doubt that a large proportion of the caloric liberated by organised beings, is generated by the combination of atmospheric oxygen with the carbon and hydrogen furnished by them, to form the carbonic acid and water which they are constantly excreting ; since we find these two changes every- where bearing a close relation with each other. Several other changes of composition are going on, however, in the living body, to which a part of the effect must be attributed ; and there are some residual phenomena, which seem to indicate that Heat may occasionally be a direct product of the metamorphosis of the Nervous Force. 615. Evolution of Heat in Vegetables . — Much dispute has occurred at different periods, as to wdiether Plants could be considered as having a proper heat or not ; and this has resulted from the limited view -which has been taken of the processes of the Vegetable Economy. Although the excretion of carbonic acid is constantly going on, under the conditions formerly described (§ 499), it usually takes place so slowly, and from a surface so openly exposed to the atmosphere, that it could scarcely be expected that there should be any sensible elevation of the temperature of the part from this source, — especially when it is considered that a constant loss of heat is taking place by evaporation : and as there is no provision for the conveyance of caloric set free in one part to distant portions of the system, a general maintenance of vital warmth would be still less antici- pated. In plants of small or moderate size, accordingly, the temperature is found to vary with that of the atmosphere ; but the interior of large trunks seems to maintain a more uniform degree, being colder than the at- mosphere in summer, and warmer in winter. This fact may be accounted for on two different grounds. The slow conducting powder of the -wood, which is much less transversely to the direction of its fibre, than with it, would prevent the interior of a large trunk from being rapidly affected by changes in the heat of the external air ; and accordingly, it is found that, the larger the trunk on which the observation is made, the greater is the difference. Again, some motion of the sap takes place even in winter ; and as the earth, at a few feet below the surface, preserves a very uniform temperature, it is not improbable that the transmission of fluid derived from it through the stem, may have an influence on the state of the latter ; a supposition which is countenanced by the fact, that the temperature of the interior of a large trunk, and that of the soil four feet below the surface (which may be regarded as the medium depth of roots), bear a very close correspondence. It is reasonable to suppose that both these causes may be in operation. — By experiments, however, made with in- struments of great susceptibility to changes of temperature, Dutrochet ascertained that Plants do possess some power of generating heat, in the parts in which the most active changes are taking place. In order to obtain unexceptionable evidence to this effect, it was necessary to exclude the influence of evaporation in depressing the temperature. This was effected by making the comparison, not between the temperature of the 846 SPECIAL AND COMPARATIVE PHYSIOLOGY. plant and that of the surrounding air, but between similar parts in a livim- plant, and in one recently killed by immersion in hot water, which would be (after cooling) equally susceptible with the former of the diminution of temperature which evaporation causes. In some instances, this source of error was still farther guarded against, by the immersion of both plants in an atmosphere saturated with aqueous vapour. The temperature < f the leaves and young shoots was ascertained, in preference to that of the stem ; both in order to avoid the source of fallacy already mentioned, and because in them the greatest proper heat might be expected. With these precautions, the result was constantly the same. An elevation of temperature, sometimes to the amount of nearly a degree (Fahr.), was observed in the herbaceous parts of actively-growing plants ; differing with the species, the energy of vegetation, and the time of the day. The highest temperature is observed about uoon ; it increases previously, and afterwards diminishes. This diurnal change is partly influenced by that of the light to which the plant is exposed.* 616. It is, however, when the processes of vegetation give rise to an extraordinary liberation of Carbonic acid (§ 503), that the evolution of heat becomes manifest. This is the case diming Germination, when the elevation of temperature, scarcely manifested by a single seed, becomes evident if a number are brought together, as in the process of malting, in which the thermometer has been seen to rise to 110°. — The same may he said of the other period of vegetable growth, in which the function of respiration is carried on to a remarkable extent ; that of Flowering From the large surface exposed, it is evident that, in by far the greater number of instances, the heat will be earned off by the atmosphere the instant it is developed ; nevertheless the flowers of a Cistus showed a temperature of 79° whilst the air was at 76°, and those of a Geranium 87 c when the ah' was at 81°. It is in plants of the Arum tribe, however, where flowers are collected in great numbers within cases which act as non-conductors, that the elevation of temperature becomes most appre- ciable ; and it bears a definite relation with the quantity of oxygen con- verted into carbonic acid. Thus, a thermometer placed in the centre of five spadixes of the Arum Cordifolium has been seen to rise to 111 0 , and in the centre of twelve to 121°, while the temperature of the external air was only 66° ; but the production of heat was wholly checked, by preventing the spadix from coming in contact with the air. — The truth of statements of this sort, which has been questioned by many physiologists, has been placed beyond all doubt by the observations of Adolphe Brong mart. He found that, at the first opening of the spathe of Colombia odora, the temperature of the spadix was 8T° above that of the surround- ing air ; that this increased during the next day to 18° ; and, during the emission of the pollen on the three succeeding days, to 20° ; after which it began to diminish with the fading of the flower. More recently these observations have been confirmed by MM. Vrolik and Vriese ; who have added to them some important facts. The rise of the temperature was found to be more rapid and considerable, in a spadix placed in oxygen, than in one at a corresponding stage surrounded by common air: and a larger proportion of carbonic acid gas was evolved. On the other * “ Ann. des Sci. Nat.,” Deuxieme Serie, tom. xii. EVOLUTION OF HEAT. 847 hand, when a spadix, of which the flowers had already begun to expand, and the temperature to rise, was placed in nitrogen, the temperature sank, and exhibited no elevation during the emission of the pollen ; nor was any carbonic acid evolved.* 617. Evolution of heat in Animals . — Although we find many instances in the Animal kingdom, in which the capability of maintaining an elevated and uniform temperature is exhibited in a degree to which nothing comparable exists in plants, yet this is by no means a constant function of animal any more than of vegetable organisms. Among the lower tribes of Animals, in which the power of locomotion is but feeble, and the supply of the wants of the system not immediately dependent upon it, very little more heat is generated than in Plants. But wherever a high degree of muscular energy is required, in connection with a general acti- vity of the functions of the nervous system, the evolution of caloric to a remarkable extent is provided for in the nutritive processes. We may regard it, therefore, as in its degree essentially connected with the deve- lopment of the Animal powers relatively to the system of Organic life ; although really dependent, as it would appear, upon the changes occur- ring in the latter. — It is worthy of notice that, although the temperature of the various parts of the Animal body is usually much more uniform than that of the different organs in Vegetables (owing to the comparative rapidity with which the general circulation of the former diffuses the heat evolved in any one part, and thus tends to equalise the whole), wherever processes are going on which call the nutritive functions into extraor- dinary activity, there a corresponding elevation of temperature occurs. Thus, a slightly increased evolution of heat from the stomach has been observed during the determination of blood to its capillaries, which takes place during digestion ; the same is observable in the reproductive organs of those animals, in -which the aptitude for the function is periodic only; and the temperature of a muscle (as ascertained by MM. Becquerel and Breschet) rises a degree or more during its contraction. 618. Our knowledge of the heat evolved by the lower Invertebrata is very limited. The Infusoria have been observed to possess a certain degree of power of resisting cold. When the water containing them is frozen, they are not at once destroyed ; but each lives for a time in a small uncongealed space, where the fluid seems to be kept from freezing by the caloric liberated from the animalcule. What is known in regard to other classes is principally derived from the experiments of John Hunter. He found that a thermometer, introduced in the midst of several Earthworms, stood at 58^°, when the temperature of the external air was 57° ; and in another instance, when the atmosphere was at 55°, the worms were at 57°. The amount of heat manifested by Leeches appeared to be nearly the same, viz. from one to two degrees above that of the atmosphere. Of the Mollusca, nearly the same may be said. Hunter found that the black slug ( Limax ater) exhibited a temperature of 55^°, when that of the atmosphere was 54° ; and the garden snail ( Helix pomatia) has been observed by others to evolve about the same amount of heat. Further experiments, however, are desirable, for the purpose of ascertaining; whether the power of generating caloric varies in such Op. cit. tom. xi. 848 SPECIAL AND COMPARATIVE PHYSIOLOGY. animals with different degrees of external temperature ; or whether the heat of their bodies always bears the same close relation with that of the medium in whicli they exist. The experiments of Hunter furnish the only information on this subject which we possess. He put several leeches into a bottle which was immersed in a freezing mixture, and, the ball of the thermometer being placed in the midst of them, the quicksilver sunk to 31°; by continuing the immersion for a sufficient length of time to destroy life, the quicksilver rose to 32°, and then the leeches froze. A similar result was obtained with a snail. It would appear, therefore, that these animals have the power of resisting, for a time, the physical effects of cold ; but how far this resistance is due to the power of generating heat, or to the causes arising from their structure (as in Vegetables § 615), cannot be determined without further inquiries. The simple maintenance of a temperature equal to that of the atmosphere, by an animal whose body has a soft surface exposed to the air, implies a certain degree of power of generating caloric ; since that surface (as in plants) is constantly being cooled by the evaporation of its moisture. 619. In many Vertebrated animals, the heat of the body is almost equally dependent upon that of the surrounding medium. Thus, Fishes in general do not seem capable of maintaining a temperature more than two or three degrees higher than that of the water in which they live. There are, however, some remarkable exceptions ; for Dr. J. Davy found that certain marine fishes, as the Bonito and Thunuy, whose gills are supplied with nerves of unusual magnitude, and which have also a very powerful heart, and a quantity of red blood sufficient to give the muscles a dark red colour, maintain a temperature much higher than that of the white fishes of fresh water on winch Hunter experimented. Thus, Dr. D. observed in the bonito a temperature of 99°, whilst that of the sea was but 80-|°. Although the conditions of existence in Yertc- brata, in which the animal powers are developed to then- greatest extent, might have seemed to require a greater power of generating heat than Fishes usually possess, it is to be remembered that this class is less liable to suffer from alternations of temperature connected with the seasons, than those which inhabit the air. In climates subject to great atmo- spheric changes, the heat of the sea is comparatively uniform through the year, and that of deep lakes and rivers is but little altered. 51 any have the power of migrating from situations where they might otherwise suffer from cold, into deep waters ; and it is an unquestionable fact, that the species which are confined to shallow lakes and ponds, and which are thus liable to be frozen during the winter, are frequently endowed with tena- city of life, sufficient to enable them to recover after a process which is fatal to animals much lower in the scale. Fishes are occasionally found imbedded in the ice of Arctic Seas ; and some of these have been known to revive when thawed. (See § 103.) 620. In Reptiles the power of generating caloric is somewhat greater. In all cases, however, the temperature of their bodies is greatly dependent upon that of the medium which they inhabit ; but in proportion to the depression of the latter, do they seem endowed with the power of main- taining their own above it. Thus, when the air was at 68°, a Proteus manifested the same degree of heat; but when the air was lowered to 55°, the temperature of the animal was 65°. In the same manner, it appeared EVOLUTION OF HEAT. 849 that the edible frog [liana esculenta ) possessed a temperature of 72^°, when examined in an atmosphere of 68° ; and that in ice of 21°, the animal maintained a heat of 37|°. The Chelonia do not seem endowed with the power of evolving heat to the same degree with the Saurian and Ophidian reptiles. In some of the more agile of the Lizard tribes, the high temperature of 86° has been noticed, when that of the external air was but 71°. — In all experiments on the influence of change of tempera- ture on such animals, it is necessary to guard against the fallacy arising from the slowness (resulting from their non-conducting power) with which then' bodies acquire the altered heat of the medium, whether it be increased or diminished. By attending to this precaution, it has been shown that many of the statements which have been made regarding their power of modifying their temperature are liable to exception ; but it cannot be questioned that Reptiles have some capability of generating heat, which is called into action in resisting the depressing influence of cold. This is unequivocally proved by the fact, that frogs will remain alive in water which is frozen around them (even when the thermometer has fallen to 9°), the water in contact with the body remaining fluid, and the temperature of the body being 33°. 621. The classes of animals, which are especially endowed with the power of producing and maintaining heat, are Insects, Birds and Mam- malia. The temperature of Insects has been very ably investigated by Mr. Newport.* — In the Larva condition, the temperature of the animal corresponds much more closely with that of the atmosphere, than in the perfect state ; thus, the larva of the higher species of Hymenoptera (Humble-bees, &c.) is usually from 2° to 4° above the surrounding me- dium, whilst the perfect Insect has a range of from 3° to 10°, or even more ; and the Caterpillar of the Lepidoptera is seldom more than from |° to 2° warmer than the atmosphere (the amount varying in close rela- tion with the activity of the individual) ; whilst the perfect insect is, when much excited, 5° or 9° above it. It is probable that in those tribes, in which no complete metamorphosis exists, but in which the difference between the development of the larva and that of the perfect insect is but trifling, there is not the same variation with regard to the production of heat. — The Pttp>a state being, in all Insects which undergo a complete metamorphosis, a condition of absolute rest, the temperature of the indi- vidual is in general lower than at any previous or subsequent period of its existence ; and it is only equal to, or at most very little above, that of the surrounding medium. But in those species, which, not undergoing a complete metamorphosis, continue active during the whole of life, this diminution of the power of maintaining heat probably does not occur. Within a short period after the first change, however, the Pupa often retains some of the characteristics of the larva state, and exhibits a tempe- rature somewhat elevated ; and if it be at any time excited to motion, a slight degree of heat is manifested. The pupa appears to follow varia- tions in atmospheric temperature more rapidly than the larva ; and as an elevation of temperature becomes necessary towards the epoch when the final metamorphosis is to take place, means are provided for it. In the Lepidoptera, the Chrysalis has itself the power of generating heat, at the Philosophical Transactions,” 1837. 850 SPECIAL AND COMPARATIVE PHYSIOLOGY. period when its energies are aroused, and it is about to buret forth from its silky envelope ; whilst in the Hymenoptera, it is most curious to observe an artificial warmth communicated to the pupae, by an increased evolution of heat from the bodies of the perfect insects which crowd over their cells (§ 623). 622. The increase in the power of generating heat which is character- istic of the Imago or perfect Insect, is not manifested immediately on its emersion from the pupa state ; in fact at that period, when the body is soft and delicate, and the unexpanded wings hang uselessly from its sides, it parts with its heat with great rapidity. It is not until its active respi- ratory movements have commenced, and the whole system has been stimulated by the exercise of its locomotive powers, that the evolution ot heat takes place to any remarkal >le extent ; and whether these processes be delayed or hastened by the influence of external circumstances, the elevation of the temperature of the individual is still proportional to them. Thus, a specimen of the Sphinx ligustri which had only left the pupa state about an hour and a quarter, had a temperature of but 4° above the atmosphere ; whilst, at the expiration of two horn's and a quarter, when it had become strong and had just taken its first flight, it had a temperature of 5-2° ; and another specimen, which had been longer exerting itself in rapid flight, was as much as 9° warmer than the sur- rounding air. In the states of abstinence, inactivity, sleep, and hyber- nation, the evolution of heat is checked ; and the temperatm-e of the perfect insect may fall veiy nearly to that of the atmosphere. By inor- dinate excitement, on the other hand, a very rapid evolution of heat may be produced. Thus, a single individual of Bombus terrestris (Humble-bee; enclosed in a phial of the capacity of three cubic inches, had its tempera- ture gradually raised, by violent excitement, from that of rest (2° or 3° above that of the atmosphere) to 9° above that of the external air, and had communicated to the air within the phial as much as 4° of heat within five minutes. In an experiment upon another species, Bombus Jonella, the temperature of the ah’ within the phial was raised by the motion of the insect, during six or eight minutes, as much as 5'S° above that of the atmosphere ; but when the bulb was held near enough to the insect to touch the tips of its wings, the mercury sunk- 2 '2°. This obser- vation, which was repeated several times with the same results, shows that the vibration of the wings tends to cool the body of the insect dining its flight. — In regard to the relative amount of heat evolved by different tribes of perfect Insects, Mr. Newport has ascertained that the volant insects in their perfect state have the highest temperature, while those species which have the lowest temperatm-e are located on the earth. Among the volant insects, those Hymenopterous and Lepidopterous species have the highest temperatm-e, which pass nearly the whole of the day- time on the wing • of these the Hive-bee, with its long train of near and distant affinities, and the elegant and sportive Butterflies, have the highest. Next to these are probably their predatory enemies the Hornets ani Wasps, and others of the same order ; and lastly, the Ants, the tempera- ture of whose dwelling has been found to be considerably above that of the atmosphere. Next below the Diurnal insects, are the Crepuscular, the highest of which are the Sphinges and Moths, and almost equal with them are the Chaffers. In some of the Coleoptera (Beetle tribe) the EVOLUTION OF HEAT. 851 amount of heat is found to approach very nearly to that in Hymenoptera ; in both of these tribes the organs of respiration are of large extent, and the quantity and activity of aeration considerable. On the other hand, the inferior temperature of crepuscular Insects to that of diurnal species of the same orders is associated with a lower degree of respiration. Nearly all the Hymenoptera are diurnal, and bear the privation of atmospheric air with greater difficulty than many other tribes. Further it would appear that some of the volant Coleoptera have, even in a quiescent state, a higher temperature than some of the terrestrial Coleoptera in a state of moderate activity, the difference being much increased in the active con- dition of the former. 62 3. It is among the Insects which live in societies, however (nearly all of them belonging to the order of Hymenoptera), that the greatest evolution of heat is manifested. Mr. Newport’s observations were made principally upon the JBombus terrestris (Humble-bee) and Apis mellifica (Hive-bee). — A single individual of the former species has frequently, when moderately excited, a temperature 9° above that of the atmosphere ; but that of the nest, examined in its natural situation, was from 14° to 1 6° above that of the atmosphere, and from 1 7° to 1 9° above that of the chalk bank in which it was formed. But the generation of heat is in- creased to a most extraordinary degree, at the period when the last change is about to take place in the inclosed pupae, which require an elevated temperature for the completion of their developmental processes (§ 96). This is furnished by the individuals denominated by Huber Nurse-bees, of which Mr. Newport gives the following interesting account : — “ These individuals are chiefly young female bees ; and, at the period of hatching of nymphs, they seem to be occupied almost solely in in- creasing the heat of the nest, and communicating warmth to the cells by crowding upon them and clinging to them very closely, during which time they respire very rapidly, and evidently are much excited. These bees begin to crowd upon the cells of the nymphs, about ten or twelve hours before the nymph makes its appearance as a perfect bee. The incubation during this period is very assiduously persevered in by the nurse-bee, who scarcely leaves the cell for a single minute ; when one bee has left, another in general takes its place : previously to this period the incubation on the cell is performed only occasionally, but becomes more constantly attended to nearer the hour of the development. The manner in which the nurse-bee performs its office, is by fixing itself upon the cell of the nymph, and beginning to respire very gradually ; in a short time its respiration becomes more and more frequent, until it sometimes respires at the rate of 130 or 140 per minute.” In one instance, the thermometer introduced among seven nursing-bees stood at 92^°, whilst the temperature of the external air was but 7 0°. The greatest amount of heat is generated by the nurse-bees just before the young bees are liberated from the combs, at which period they require the highest tem- perature. It is just after its emersion that the young insect is most susceptible of cold ; it is then exceedingly sleek, soft, and covered with moisture ; it perspires profusely, and is highly sensitive of the slightest I current of air. It crowds eagerly among the combs and among the other bees, and everywhere that warmth is to be obtained. It is not until after some hours, that it becomes independent of external warmth. It is 3 i 2 852 SPECIAL AND COMPARATIVE PHYSIOLOGY. interesting to remark that these bees do not incubate on cells that contain only larvcB ; the temperature of the atmosphere of the nest being suffi- ciently high for the young in that condition, as veil as to perfect their change into the pupa state. — Similar observations have been made by Mr. Newport upon the temperature of the Hive-bees ; and he has shown that the fallacy of the statements of other experimenters, as to the degree of heat maintained by them during the winter, is caused by the rapidity with which, ivhen aroused, they can generate caloric. The temperature of individual bees in a state of moderate excitement, is usually from 10 ° to 15° above that of the atmosphere 3 but it is greatly increased about the swarming season, when incubation of the pupae is going on, and also when clusters are formed round the entrance of the hive. At such timet Mr. N. has seen the thermometer raised as high as 96° or 98°, when the range of atmospheric temperature was only between 56° and 58°. The mean temperature of a hive dining May was 90°, that of the atmosphere being 60° 3 whilst, in September, the mean of the atmosphere being also 60°, that of the hive was only 66 |°. During the winter, it appears that bees, like other insects, exist in a state of hybernation 3 though then- torpidity is never so profound, as to prevent their being aroused by moderate excitement. The temperature of the hive is usually from 5° to 20 ° above that of the atmosphere 3 but it is sometimes depressed even below the freezing point. It is when artificially excited in a low tem- perature, that their power of generating caloric becomes most evident. Mr. N. mentions one instance in which the temperature of a hive, of which the inmates were aroused by tapping on its exterior, was raised to 102° 3 whilst a thermometer in the air stood at 34^°, and the temperature of a similar hive which had not been disturbed was only 48 1°. 624. In regard to the degree of Heat which Insects are capable of generating, therefore, it appears that they may be ranked between 'col el- and ‘ warm-blooded ’ animals. Like the former, they are much influenced by external temperature 3 although the higher species are, when in a state of moderate exercise, relatively warmer than the least cold-blooded among the Reptiles. The degree of heat they are occasionally capable of evolving, is nearly equal to that generated by Mammalia 3 but this is only required for the pei-formance of particular functions, and, if con- stantly maintained in Insects, would have occasioned an unnecessary activity in the processes on which it is immediately dependent, and, by consequence, in the whole of the nutritive system. In Birds and Mam- malia, however, — where, from the high development of the animal powers, the constant maintenance of an elevated temperature is necessary, — all the functions are adapted to its support 3 and in them we no longer find any dependence upon the state of the external medium, the calorific and frigorific processes being so delicately adjusted, as to render the heat of the system extremely uniform. 625. The temperature of Birds is, almost without exception, higher than that of the Mammalia, varying from 100° to 111^°. The first is that of the Gull, the last that of the Swallow. In general, the same statement may be applied to Birds, as has been made with respect to Insects, — that the temperature is greater in the species of most rapid and powerful flight, and less in those which principally inhabit the earth, as the Fowl tribe 3 but we find the lowest temperature of the class in the EVOLUTION OF HEAT. 853 aquatic birds (which most closely approximate Reptiles in their general organisation), notwithstanding that they possess, in the thick and soft down with which they are clothed, and which is rendered impervious to fluid by the oily secretion applied with the bill, a special provision for retaining that heat within their bodies, which would otherwise be too rapidly conducted away. It is to be remembered that, from the compa- ratively small size of most of the members of this class, and the larger surface which they consequently expose in proportion to then bulk, the cooling action of the surrounding medium will have a greater relative effect upon them, than upon larger animals ; and the amount of heat they must generate to maintain the same internal temperature, will be greater. — The embryo of Birds requires for its development a heat nearly equal to that of the body of the parent ; and this is afforded by the process of incubation. The contents of the egg, when lying under the body of its parent, are so situated, that the germ-spot (§ 685) is brought into closest proximity with the source of warmth. Eggs may, however, be artificially incubated, — a practice which is carried to a great extent in Egypt ; and in tropical climates the heat of the sun is in some instances sufficient. Thus, the Ostrich is said to leave her eggs to be hatched by the sun’s rays alone, when she breeds in the neighbourhood of the Equator ; and to sit upon them, if inhabiting a more variable climate. It was observed by Mr. Knight, that a Fly-catcher, which built for several successive years in one of his stoves, quitted its eggs whenever the thermometer was above 71° or 72°, and resumed her place upon the nest when the thermometer sunk again. The young of many kinds of Birds are deficient, for some time after their emergence from the egg, in the power of maintaining an independent temperature. Thus, Dr. Edwards found that young Sparrows, a week after they are hatched, have, while in the nest, a temperature of from 95° to 97° ; but when they are taken from the nest, their temperature falls in one hour to 66|°, the temperature of the atmosphere being at the same time 62 1°; and this rapid cooling was shown by parallel experiments not to be owing to the want of feathers. This fact, however, is not com- mon to all Birds (§ 325 l). 626. The temperature of Mammalia seems usually to range from about 96° to 104°; but more accurate observations are still required for the sake of comparison. It is remarkable that a still higher point should be attained by the animals inhabiting the coldest regions ; the Arctic Fox having been found by Capt. Lyon to possess a temperature of nearly 107°, when that of the atmosphere was 14°. That of the Cetacea (Whale tribe) does not seem to be inferior to that of other orders ; and, to retain it within the body, the skin is enormously thickened and penetrated with oil (so as to form the substance known as blubber), by which the conduct- ing power of the medium they inhabit is prevented from operating too energetically and injuriously. As far as is yet known, the temperature of the Cheiroptera (Bat tribe) seems more variable than that of any other order; for it has been found by Mr. Paget* that the amount of heat evolved by a Noctule under his observation, varied (like that of Insects) with its degree of activity, its body being but a few degrees warmer than the atmosphere after a period of prolonged repose, but its temperature rising ‘ Lectures on Inflammation,’ in “ Medical Gazette,” June 7, I860. 854 SPECIAL AND COMPARATIVE PHYSIOLOGY. to 99° when it had been making active exertions. The heat of different parts of the body varies a good deal according to the degree of surface exposed ; and it seems greater among the viscera, than in any situation ever exposed to the air. Thus, the temperature of the Human body is usually stated at 98° or 99°, from the height of thermometers placed in the mouth, axilla, &c. ; but that of the stomach, according to Dr. Beau- mont, is generally 100°; and that of the blood from 100^° to 101|°. — In the young Mammalia also, there is usually a considerable deficiency of calorifying power ; but the degree of this values considerably in the different orders of the class (§ 326 bb). 627. The phenomena of Hybernation, a state in which certain warm- blooded animals are reduced to the condition of those of least independent temperature, having been already described (§ 94), it need only here be added, that, like many Insects, hybernating Mammals still preserve the capability of evolving heat, when a stimulus of any kind arouses the animal functions, and gives a temporary excitement to those of organic life. This effect may be produced by mechanical irritation, which arouses the animal from its torpidity, and accelerates its respiratory movements. Extreme cold will produce the same effect ; but it does not last long ; and a more profound torpidity then comes on, which speedily ends in death, if the cold continue. — Hybernating animals, however, are by no means the only ones which exhibit a periodical change in the power of generating heat. It appears probable that all species of animals inhabiting climates in which the seasons are subject to much variation, vary in this respect at different parts of the year. Their power of evolving heat is greatest in the winter ; so that, if exposed to severe cold during summer, their bodies are speedily cooled. The change in the colour of the fur, from dark to white, which many animals exhibit at the approach of winter, has the evi- dent purpose (besides other objects) of diminishing the radiation of heat from its surface. That this change is occasioned by the cold of the air around, has been proved by an experiment of Capt. Ross upon a Lemming, which he had kept in his cabin until Feb. 1, and which retained its s umm er fur. It was then exposed on the deck to a temperature of 30° below zero ; and on the following day the change of the fur to white commenced on the cheeks and shoulders, from which it gradually extended itself over the body. The Common Stoat of this country turns completely white in Scotland and the North of Continental Eimope (whence it is obtained as the Ermine) ; in the North of England this change is occasional only ; and in the midland and southern counties it is of rare occurrence. It is interesting to perceive the depressing cause thus counteracting itself, by means of that provision in the structure of the animal, which occasions the change of the colour of its fur. 628. We have now to enquire what are the conditions of the evolution of Heat in the animal economy. That many of the nutritive processes are subservient to it, can scarcely be doubted ; but it seems peculiarly to depend upon those changes in which the function of Respiration is con- cerned — viz. the union of oxygen derived from the atmosphere, with com- pounds of hydrogen and carbon existing in the living system. M herever the aeration of the blood is extensively and actively carried on, there is a proportionate elevation of temperature. And, on the other hand, wherever the respiration is naturally feeble, or the aeration of the blood EVOLUTION OF HEAT. 855 is checked by disease or accidental obstruction, the temperature of the body falls. Thus, in spasmodic Asthma, the temperature of the Human body during a paroxysm has been found as low as 82° ; in the Asiatic Cholera, a thermometer placed in the mouth has indicated but 77° ; and in Cyanosis (or ‘ blue disease,’ arising from malformation of the heart im- peding perfect arterialisation, § 49 1 ), the same low temperature has been observed. Again, whenever the temperature of an animal is, by any extraordinary stimulus, quickly raised above that which it Was previously maintaining, it is always in connection with increased activity of the respi- ratory movements, and increased consumption of oxygen. Thus, during the incubation of Bees, the insect, by accelerating its respiration, causes the evolution of heat and the consumption of oxygen to take place at least twenty times as rapidly as when in a state of repose. — Now it has been seen that arterial blood contains a larger proportion of oxygen, than exists in venous blood ; whilst, on the other hand, the latter contains a larger proportion of carbonic acid than exists in the former ; and it seems obvious, therefore, that during the passage of blood through the capillaries, a part of its oxygen has been exchanged for carbonic acid. The source of this product is obviously the union of atmospheric oxygen with Carbon, furnished by disintegration of the tissues, or (more directly) by the ele- ments of the food (§ 392) ; and by this union, caloric must be generated, precisely as by the more rapid union of the same materials in the ordi- nary process of combustion. Further, since these materials yield Hy- drogen as well as carbon, and since more oxygen almost always disappears from the air which has been respired, than is contained in its carbonic acid (§ 534), it seems probable (although this cannot be demonstrated) that this element also is subjected to the combustive process, with a fur- ther generation of caloric ; and that, of the water which is exhaled from the lungs, a part has been thus produced. Further, there can be no doubt that other combustive processes take place in the system, although these may be the chief ; for example, of the sulphur and phosphorus which the albuminous elements of the food contain, the greater proportion is thrown off by the urinary excretion (§ 601) in the condition of sulphuric and phosphoric acids, having been combined with oxygen in the system. When all these actions are taken into account, — when the amount of heat that should be generated by the production of the quantity of carbonic acid found in the air expired during a given time, is carefully estimated, and the oxygen which has disappeared is considered as having been simi- larly employed in other combustive processes, especially in the formation of water, — it is found that the total so closely corresponds with the amount of heat actually generated by the animal during the same time, that it can scarcely be doubted that this process is the main source of calorification.* 629. Notwithstanding, however, that the Chemical theory of Animal Heat may be considered as accounting for the ordinary maintenance of a fixed temperature in the body at large, yet there are some ‘ residual phe- * From the experiments of Dulong and Despretz it appeared that the whole amount of caloric generated by an animal in a given time could not thus be accounted for ; but it has been more recently shown by Prof. Liebig, that the discrepancy nearly vanishes, when the calculation is based on the more correct data since determined, as to the amount of caloric generated in the combustion of carbon and hydrogen. 856 SPECIAL AND COMPARATIVE PHYSIOLOGY. nomena ’ to which it scarcely appears applicable. Of this kind are the sudden elevation of temperature that occurs under the influence of nervous excitement, which may be either general or local ; the equally sudden diminution which marks the influence of the depressing passions ; and the rapid cooling of bodies in which the nervous centres have been de- stroyed, notwithstanding that the respiration is artificially maintained, and the circulation continues. So, again, when the spinal cord of a warm- blooded animal is divided, there is a temporary elevation of temperature in the lower extremities ; but this soon subsides, and the temperature of the paralysed parts then remains permanently below the natural standard. This last fact, which corresponds with the constant deficiency of warmth observable in paralysed limbs in the Human subject, may be explained by attributing the imperfect calorification of the part to the torpor of the nutrient operations, consequent upon the deficiency of nervous energy and the want of functional activity. But this will scarcely apply to the cases first cited ; and it seems not unreasonable to regard them as indications of that direct conversion of Nervous Power into Heat, of which we have strong evidence in regard to Light and Electricity. (See §§ 54 and 639). 4. Evolution of Electricity. 630. Electricity is a force which may be made to act through any form of matter, and which may be produced by any other of the physical forces, acting under certain conditions. Thus Motion will generate Elec- tricity, as in the ordinary mode of obtaining it by the friction of two dissimilar substances ; and it is scarcely possible for such friction to occur, without some degree of electric disturbance. So, Heat will generate Electricity, when applied to two dissimilar metals in contact ; and the electric equilibrium is disturbed, even when two parts of the same bar arc unequally heated. Magnetism, again, may be made to develope Elec- tricity ; as in the action of the ordinary Magneto-Electric machine. But the most frequent and powerful source of Electric disturbance, is Chemical Action; there being probably no instance of chemical union or decom- position, in which the electric condition of the bodies is not altered, although it may not be always easy to demonstrate the change. And, by means of the chemical actions which it produces, Light also may become the generator of the Electric force. — Now as many of these forces are operating in the living body, under circumstances which would appear to be peculiarly favourable to the excitement of Electricity, there can be no difficulty in accounting for its production in the organic processes of Nutrition and Secretion ; and the wonder perhaps is, that the manifesta- tions of it should be ordinarily so trivial. Thus, when the change of form of a body from solid to liquid, or from liquid to gaseous, is attended with the least chemical decomposition (as when water containing a small quantity of saline matter in solution is caused to evaporate and to leave it behind), a very decided electric disturbance is produced ; and if it were not for the provisions which everywhere exist for the neutralization of the effects of such actions, by the conducting power of the parts in winch they occur and of the bodies around them, such electric disturbances could scarcely fail to exert an important influence on the organic func- tions. (See § 108). EVOLUTION OP ELECTRICITY. 857 631. Electricity in Vegetables . — That the ordinary processes of Vege- table growth are attended with a disturbance of electric equilibrium, which is manifested when the bodies in which it takes place are effectually insulated, seems to have been proved by the experiments of Pouillet. Several pots filled with earth, and containing different seeds, were placed on an insulated stand in a chamber, the air of which was kept dry by quick-lime ; and the stand was placed in connection with a condensing elec- trometer. During germination, no electric disturbance was manifested : but the seeds had scarcely sprouted, when signs of it were evident ; and when the young plants were in a complete state of growth, they separated the gold leaves of the electrometer half an inch from each other. It was calculated by him that a vegetating surface of 1 00 metres square in extent produces in a day more electricity than would be sufficient to charge the strongest battery ; and he not unreasonably considers that the growth of plants may be one of the most constant and powerful sources of atmo- spheric electricity. The disengagement of vapour from the surface of the leaves would alone be sufficient to produce such a disturbance, as the fluid from which it is given off is always charged with saline and other ingre- dients ; and the gaseous changes which are effected by the leaves, upon the oxygen and carbonic acid of the atmosphere, may be regarded as addi- tional sources of its development. During the various processes of decom- position and recomposition which take place in the assimilation of the Vegetable juices, we should expect that Electric equilibrium would be sometimes disturbed, sometimes restored. Of this, the following facts, amongst others, appear to be sufficient evidence. If a wire be placed in apposition with the bark of a growing plant, and another be passed into the pith, contrary electrical states are indicated, when they are applied to an electrometer. If platinum wires be passed into the two extremities of a fruit, they also will be found to present opposite conditions. In some fruits, as the apple or pear, the stalk is negative, the eye positive ; whilst in such as the peach or apricot, a contrary state exists. If a prune be divided equatorially, and the juice be squeezed from its two halves into separate vessels, its portions will in like manner indicate opposite elec- trical states, although no difference can be perceived in then chemical qualities.* 632. Electricity in Animals — All that has been said of the effects of vegetation in producing a disturbance of Electric equilibrium, will mani- festly apply to the nutritive processes of Auimals also ; and there is no deficiency of indications that such is the case. Thus, Donne found that the skin and most of the internal membranes are in opposite electrical states ; and Matteucci has seen a deviation of the needle amounting to 15° or 20°, when the liver and stomach of a rabbit were connected with the platinum ends of the wires of a delicate galvanometer. It may be questioned whether or not the differences in the secretions of these parts were the causes or the effects of their electric conditions. According to Matteucci, it could not be by their chemical action on the wires that the manifestation w 7 as produced, since it became very feeble or entirely ceased on the death of the animal. The more recent experiments of Mr. Baxter,! * “ Annales de Chimie,” tom. lvii. t “ Philosophical Transactions,” 1848, p. 243. 858 SPECIAL AND COMPARATIVE PHYSIOLOGY. which were directed to the determination of the relative electrical con- dition of secreting surfaces, and of the blood in the veins returning from them, seem to confirm the belief that an electric disturbance takes place in the very act of secretion. He found that when one of the electrodes was placed on the intestinal surface, and the other inserted into tie branch of the mesenteric vein proceeding from it, a deflection of the needle to the extent of 4° or 5° was produced, indicating a positive condition of the blood ; no effect, on the other hand, was produced, when the second electrode was inserted into the artery of the part. These effects cease soon after the death of the animal, which is not the case with those which proceed from simple chemical differences between the blood ar.d the secreted product. — The foregoing experiments may seem confirmatory of Dr. Wollaston’s theory respecting Secretion. Observing the connection between electricity and chemical action, he was led to think that all the secretions in the body are the effect of electrical agency acting in various modes, and that the qualities of each secretion point out what species of electricity preponderated in the organ which forms it : thus, the existence of free acid in the urine and gastric juice, and of free alkali in the bile and saliva, mark the prevalence of positive electricity in the kidneys and stomach, whilst an excess of negative electricity is indicated in the liver and salivary glands. But it is more likely that the disturbance of Elec- tricity is the result of chemical changes, whose source is the Vital force, than that it is itself the cause of those changes. 633. From experiments on the Human subject, it would appear that the living body would be never in perfect equilibrium with those around it, were this not constantly maintained by free contact with them ; thus, if two persons, both insulated, join hands, sufficient electricity is developed to affect the electrometer. Some electric disturbance is manifested by almost every individual, if it be carefully sought for. In men it is most frequently positive ; and irritable men of sanguine temperament have more free electricity than those of phlegmatic character ; whilst the elec- tricity of women is more frequently negative than that of men. Some individuals exhibit these phenomena much more frequently and power- fully than others. There are persons, for instance, who scarcely ever pull off articles of dress which have been worn next the skin, without sparks and a crackling noise being produced, especially in dry weather; this may, however, be partly due to the friction of these materials on the surface and with each other, as it has been proved to be greatly influenced by their nature. The most remarkable case of the generation of elect ii- city in the human subject at present on record, is one related a few years since in America.* The subject of it, a lady, was for many months in an electric state so different from that of surrounding bodies, that, whenever she was but slightly insulated by a carpet or other feebly-conducting medium, sparks passed between her person and any object which she approached. From the pain which accompanied the passage of the sparks, her condition was a source of much discomfort to her ; when most favourably circumstanced, four sparks per minute would pass from her finger to the brass ball of the stove at a distance of 1 J inch. The cir- cumstances which appeared most favourable to the generation of elec- * “American Journal of Medical Science,” January, 1838. EVOLUTION OF ELECTRICITY. 859 tricity, were an atmosphere of about 80°, tranquillity of mind, and social enjoyment ; while a low temperature and depressing emotions diminished it in a corresponding degree. The phenomenon was first noticed during the occurrence of a vivid Aurora Borealis ; and though its first appearance was sudden, its departure was gradual. Various experiments were made, with the view of ascertaining if the electricity was generated by the friction of articles of dress j but no change in these seemed to modify its intensity. 634. Electricity may be generated, however, not merely by the Organic functions, but by changes more peculiarly connected with the exercise of the Animal powers ; and this through both the Muscles and the Nerves. — It has been shown by Matteucci,* that if an incision be made into a muscle of a living animal, and the nerve of a ‘ galvanoscopic frog’t be introduced into the wound, in such a manner that its extremity is applied to the deepest part, and another portion to the lips, or to the surface of the muscle, the leg of the frog is thrown into contraction ; thus showing that a difference exists in the electric condition of the deeper and more super- ficial parts of the animal. This phenomenon is exhibited even when the muscle is separated from the body ; but when the experiment is many times repeated, the contractions are observed to become feebler and feebler, and at last cease altogether, their duration being greater when a muscle of a cold-blooded animal, than when that of a warm-blooded animal is employed. A galvanic pile may even be formed of pieces of fresh muscle, provided they be so arranged that the internal substance of each is in contact with the external surface of the next ; and thus it may be shown, by the galvanometer, that a current is continually proceeding from the interior to the surface of every muscle. This current exists when the muscle is in a completely passive state ; and its intensity seems then to depend upon the activity of the nutrient changes taking place in it. But its intensity is greatly increased when the muscle is thrown, by a stimulus applied to its own nerve, into a state of energetic contraction. There can be little doubt that the source of this electric disturbance lies in the molecular changes taking place in the muscle, either during the ordinary process of nutrition, or in the alterations connected with its contractile action ; and the explanation of its constant direction from the interior to the exterior of the muscle, appears to lie in the difference in the rate of these changes in these two situations ; since they will go on more ener- getically in its interior, than they do nearer its surface, where the proper muscular fibres are mingled with a larger proportion of areolar and ten- dinous substance. — The existence of an Electrical current in the Frog, passing during its whole life from its extremities towards its head, has been known since the time of Galvani ; but it has been usually supposed to be peculiar to this animal. Even Matteucci, for some time after his * “ Lectures upon the Physical Phenomena of Living Beings Lect. ix. + The ‘ galvanoscopic frog,’ which has been continually employed by Prof. Matteucci to test minute electric disturbances which are scarcely appreciable by a galvanometer, is simply the leg of a recently-killed frog, with the crural nerve, dissected out of the body, remaining in connection with it ; the leg being enclosed in a glass tube covered with an insulating varnish, and the nerve being allowed to hang freely from its open end, when two points of the nerve are brought in contact with any two substances in a different electrical state, the muscles which it supplies are thrown into contraction. 860 SPECIAL AND COMPARATIVE PHYSIOLOGY. discovery of the 4 muscular current,’ regarded the ‘ proper current of the frog’ as something essentially different. More recently, however, he has shown that the latter is probably but a special case of the former. For he has found that in every living muscle, of which the tendinous extremi- ties are not equally disposed (one, for example, forming a cord, whilst the other is broad and ribbon-like), a current circulates in the muscle, from the tendinous extremity to the muscular surface ; so that a galvanic pile may be formed by a succession of such muscles, the tendinous extremity of one being placed in contact with the middle of the surface of the ne.\t. And the arrangement of the muscles and tendons in the limbs and bodv of the frog, is such as peculiarly to favour the development of such a current, in the direction in which it is actually discovered. The com- munity of origin of the ‘ muscular current’ and of the ‘proper current of the frog,’ is further indicated by the circumstance that they are both modified in the same manner by agents which affect the vitality of the muscle ; and it is particularly remarkable that poisoning with sulphuretted hydrogen should almost immediately put an end to both, although nar- cotic poisons have very little influence. The ‘ proper current of the frog’ bears this curious analogy to the electric discharges of Fishes presently to be described ; — that it is not manifested if the connection be made between corresponding points of the opposite sides ; but that it shows itself when the communication is made between points higher or lower than the body, whether on the same or opposite sides. 635. There are certain animals which possess the power of accumulating Electric force within their bodies, and of discharging it at will in a violent form ; and with the exception of some Insects and Mollusca, which have been said (though this is doubtful) to communicate sensible shocks, these animals are all included in the class of Fishes. About seven species of this class, belonging to five genera, are known to possess electric proper- ties ; and it is curious that these genera belong to tribes very dissimilar from one another, and that, though each has a limited geographical range, one species or other is found in almost every part of the world. Thus, the three species of Torpedo, belonging to the Ray tribe, are found on most of the coasts of the Atlantic and Mediterranean, and sometimes so abundantly as to be a staple article of . food. The Gi/mnotus, or electric Eel, is con- fined to the rivers of South America. The Silurus (more correctly tire Malapterurus), which approaches more nearly to the Salmon tribe, occurs in the Niger, the Senegal, and the Nile. The Trichi urns* or Indian Sword-fish, is an inhabitant of the Indian Seas ; and the Tetraodon (one of a genus allied to the Diodon or globe-fish) has only been met with on the coral banks of Johanna, one of the Comoro Islands. — These Fishes have not all been examined with the same degree of attention ; but it seems probable that the phenomena which they exhibit, and the structural peculiarities with which these are connected, are essentially the same through- out. The peculiar characteristic of all is the power of giving, to any living body which touches them, a shock resembling in its effects that produced by the discharge of a Leyden jar. This is of very variable intensity in different species and individuals, and at different times. The Gymnotus * There is some doubt, however, as to the real character of the fish to which this name has been given. EVOLUTION OP ELECTRICITY. 861 will attack and paralyse horses, as well as kill small animals ; and the discharges of large fish (which are 20 feet long) sometimes prove sufficient to deprive men of sense and motion. The effects of the contact of the Torpedo are less severe, and soon pass off ; but the shock is attended with considerable pain when the fish is vigorous. The electrical organs appear to be charged and discharged to a certain extent at the will of the animals. Their power is generally exerted by the approach of some other animal, or by some external irritation ; but it is not always possible to call it into action, even in vigorous individuals. It usually diminishes with the general feebleness of the system, though sometimes a dying fish exerts considerable power. All electrical fishes have their energy exhausted by a continued series of discharges ; hence it is a common practice with convoys in South America, to collect a number of wild horses and drive them into the rivers, in order to save themselves, when they pass, from being injured by the fish. If excessively exhausted, the animals may even die • but they usually recover their electrical energy after a few hours’ rest. 636. The Torpedo, from its proximity to European shores, has been most frequently made the subject of observation and experiment ; and the following are the most important results of the investigations which have been made upon it by various enquirers. — That the shock perceived by the organs of sensation in Man is really the result of an Electric discharge, has now been fully established. Although no one has ever seen a spark emitted from the body of one of the fish, it may be easily manifested, by causing the Torpedo or Gymnotus to send its discharge through a slightly interrupted circuit. The galvanometer is influenced by the discharge of the Torpedo, and chemical decomposition may be effected by it, as well as magnetic properties communicated to needles. It seems essential to the proper reception of the shock, that two parts of the body should be touched at the same time, and that these two should be in different electrical states. The most energetic discharge is procured from the Torpedo, by touching the back and belly simultaneously, the electricity of the dorsal surface being positive, and that of the ventral negative ; and by this means the galvanometer may be strongly affected, every part of the back being posi- tive with respect to every part of the opposite surface. When the two wires of the galvanometer are applied to the corresponding parts of the two sides of the same surface, no influence is manifested ; but, if the two points do not correspond in situation, whether they be both on the back or both on the belly, the index of the galvanometer is made to deviate. The degree of proximity to the electric organ appears to be the source of the difference in the relative state of different parts of the body ; those which are near to it being always positive in respect to those more distant. Dr. Davy found that, however much Torpedos were irritated through a single point, no discharge took place ; and he states that, when one < surface only is touched and irritated, the fish themselves appear to make an effort to bring the border of the other surface, by muscular contraction, into contact with the offending body ; and that this is even done by foetal fish. If a fish be placed between two plates of metal, the edges of which are in contact, no shock is perceived by the hands placed upon them, since the metal is a better conductor than the human body ; but, if the plates be separated, and, while still in contact with the opposite sides of 862 SPECIAL AND COMPARATIVE PHYSIOLOGY. the body, the hands be applied to them, the discharge is at once ren- dered perceptible, and it may be passed through a line formed by the moistened hands of two or more persons, the extremities being brought into relation with the opposite plates. — The electrical phenomena of the Gymnotus are essentially the same with those of the Torpedo ; but the opposite electrical states are found to exist, not between the dorsal and ventral surfaces, but between the head and tail ; so that the shock is most powerful, when the connection is formed between these two extreme points. 637. It has been ascertained by experiment, that the manifestation of this peculiar power depends upon the integrity of the connection between the Nervous centres and certain organs peculiar to Electrical fishes. In the Torpedo the Electric Organs are of flattened shape (Fig. 247, d), and occupy the front and sides of the body, forming two large masses, which extend backwards and out- wards from each side of the head. They are composed of two layers of membrane, between which is a whitish soft pulp, divided into columns by processes of the mem- brane sent off so as to form parti- tions like the cells of a honey- comb ; the ends of these colunms being directed towards the two surfaces of the body. The co- lumns are again subdivided hori- zontally by more delicate parti- tions, which form each into a number of distinct cells ; the partitions are extremely vascular, and are profusely supplied with nerves, of which the fibres seem to break up into minuter fibrillae to form plexuses upon these mem- branes. The fluid contained in the electrical organs forms so large a proportion of them, that the specific gravity of the mass is only 1026, whilst that of the body in general is about 1060 ; and, from a chemical examination of its con- stituents, it seems to be little else than water, holding one-tenth part of albumen in solution, with a little chloride of sodium. — The electrical organs of the Gymnotus are essentially the same in structure, though differing in shape in accordance with the conformation of the animal : they occupy one-third of its whole bulk, and run along nearly its entire length ; there are, however, two distinct pairs, one much larger than the other. The prisms are here less numerous, but are much longer ; for they run in the direction of the length of the body, a difference which is Electrical Organs of the Torpedo : — a, brain ; b. cu- taneous glandulse ; c, eye : d y electrical apparatus ; e , branchiae ; /, spinal nerves supplying the pectoral fins; g, spinal cord: h , lateral nerves to the body; i, pneumogastric nerves to the electric organ. EVOLUTION OF ELECTRICITY. 8G3 productive of a considerable modification of the character of the discharge. — In the Silurus there is not any electrical organ so definite as those just described ; but the thick layer of dense cellular tissue, which completely surrounds the body, appears to be subservient to this function ; it is com- posed of tendinous fibres interwoven together, and of an albuminous sub- stance contained in their interstices, so as to bear a close analogy with the cellular partitions in the special organs of the Torpedo and Gymnotus. — The organs of the other known electrical fishes have not yet come under the notice of any anatomist. 638. In all these instances, the Electrical Organs are supplied with Nerves of very great size, larger than any others in the same animals, and larger than any nerve in other animals of like bulk. They all arise in the Torpedo from a ganglionic mass situated behind the Cerebellum, and connected with the Medulla Oblongata, to which the name of ‘ electric lobe’ has been given ; the first two of them issue from the cranium in close proximity with the 5th pair, and have been regarded as belonging to it, although their real origin is different ; whilst, from the distribution of the third electrical nerve to the stomach, after sending its principal portion to the electrical organ, it would seem analogous to the 8th pair or pneumogastric. — The electrical nerves in the Gymnotus are believed to i arise from the spinal marrow alone ; and those of the Silurus are partly intercostals and partly belong to the 5th pair. — The integrity of the nerves is essential to the full action of the electrical organs. If all the trunks be cut on one side, the power of that organ will be destroyed, but that of the other may remain uninjured. If the nerves be partially destroyed on either or both sides, the power is retained by the portion of the organs still in connection with the centres. The same effects are produced by tying the nerves, as by cutting them. Even slices of the organ entirely separated from the body, except by a nervous fibre, may exhibit electrical properties. Discharges may be excited by irritation of the brain when the nerves are entire, or of the part of the divided trunk distributed on the organ ; but on destroying the £ electric lobe’ of the brain, the electric power of the animal ceases entirely, although all the other ganglionic centres may be removed without impairing it. It is remarkable however that, after the section of the electrical nerves, Torpedos appear more lively than before the operation, and actually live longer than others not so injured, which are excited to discharge frequently.— Poisons which act violently on the nervous system, have a striking effect upon the electrical manifestations of these fish ; thus, two grains of muriate of Morphia were found by Matteucci to produce death after about ten minutes, during which time the discharges were very numerous and power- ful ; and Strychnia also excited powerful discharges at first, succeeded by weaker ones, the animals dying in violent convulsions. When the animals were under the influence of strychnia, it was observed that the slightest rritation occasioned discharges ; a blow given to the table on which the mimal was placed, being sufficient to produce this effect. If the spinal ;ord were divided, however, no irritation of the parts situated below the lection called forth a shock. — It has also been ascertained by Matteucci, hat the electric power is suspended when the Torpedo is plunged into water at 32°, and is recovered again when it is immersed in water of a emperature from 58° to 68° ; and that this alternation may be repeated 864 SPECIAL AND COMPARATIVE PHYSIOLOGY. several times upon the same fish. But if the temperature be raised to 86°, the Torpedo soon ceases to live, and dies while giving a great number of violent discharges.* 639. From all these facts it seems an almost unavoidable inference, that the Electric force is developed in the electric organ, by a disturbance of its equilibrium consequent upon Nervous Agency. Such a disturbance may be conceived to take place in every one of those minute cells, into which the prism is divided by transverse partitions ; the two electricities (to use the current phraseology) being separated by nervous agency, as they are in the tourmaline by heat. Now by the multiplication of such cells in each prism, a ‘pile’ would be produced, at the two extremes of which the greatest differences in electric condition would be found ; and the intensity of the discharge would depend upon the number of elements in the pile, while its quantity would be proportional to the multiplication of the separate prisms. Now this is precisely what holds good in nature ; for the electric discharge of the Gymnotus is far more intense than that of the Torpedo, as might be expected from the multiplication of its cells ; so that, according to Prof. Faraday, a single medium discharge from this animal gives a shock equal to that of a battery of fifteen Leyden jars, containing 3500 square inches, charged to its highest degree. — Further evidence that the force which enables the Electric Fishes to give sensible manifestations of Electricity, is the same as that which excites Contrac- tion when transmitted to the Muscles, is derived from the remarkable conformity between the conditions under which the two phenomena respectively occur. The connection of the organs specially appropriated to each of these actions with the nervous system, the dependence of their functions upon the integrity of this connection and upon the will of the animal, the influence of stimulation applied to the nervous centres or trunks, the effect of ligature or section of the nerve, and the results of poisonous agents, are all so remarkably analogous in the two cases, that it seems scarcely possible to refuse assent to the proposition, that the Nervous power is the agent which is instrumental in producing both sets of phenomena. Still, however, no proof whatever can be derived from this source, of the identity of nervous influence with any form of Electricity ; since all that can be inferred from it is, that, as by the influence of the Nervous system on one class of organs, sensible Contraction is produced, so by its influence on another class of organs, Electricity is generated. On the other hand, all the experiments of Prof. Matteucci confirm the con- clusion formerly stated (§ 246), that the two forces are not identical, but that they are ‘correlated,’ each being able to generate the other. t * See Matteucci’s “ Lectures,” p. 216. + A remarkable experiment confirmatory of this view, is recorded in Prof. Matteucci's most recent paper (“ Philosophical Transactions,” 1850, p. 295). "When an electric current is transmitted through the muscles of the thighs of a living animal, the positive pole being placed above and the negative pole below, so that the current passes in the direction of the efferent or motor nerves, there is not only a strong contraction of the muscle traversed, but also of the muscles of the leg below ; indicating that an influence is transmitted through the nerves which proceed to them. On the other hand, when the current is transmitted iu the inverse direction, the positive pole being below, so that it is directed along the course of ’he afferent nerves, or towards the nervous centres, pain is produced, without any muscular contraction save in the muscle traversed ; showing that the influence is now transmitted towards the nervous centres. Now it has been clearly established by other experiments. EVOLUTION OF ELECTRICITY. 865 640. Regarding the uses of the Electrical organs to the animals pos- sessing them, no very certain information can be given. It is doubtful to what extent their power is subservient to the prehension of food, which was once supposed to have been their principal object ; since it is known that the Gymnotus eats veiy few of the fishes which it kills by its dis- charge ; and young Torpedos kept by Dr. Davy for five months ate nothing, though supplied with small fishes both dead and alive, but nevertheless increased in strength and in electrical energy. The electric power of young Torpedos is much less exhaustible than that of the adults ; this is readily accounted for by the fact of the greater energy of the vital processes in young than in old animals. Dr. D. experienced shocks from foetal fish, which he was removing from the abdomen of the parent. He believes that the electric action may assist the fimction of respiration, by decomposing the water in the neighbourhood of the gills, when the animal, being buried in sand or mud, might be unable to obtain the requisite supply of oxygen in the ordinary way ; but its chief use he considers to be to guard the fish from its enemies. Another function, however, may not improbably be influenced by it,— that of digestion ; and this in two ways. It is well known that the vital properties of living- tissues are so completely destroyed by a violent electric discharge, that they are disposed to pass more readily than in other cases into decom- position, the incipient stage of which is favourable to digestion ; the shortness of the intestinal canal of the Torpedo would seem to render some assistance of this kind peculiarly necessary. The process of diges- tion may also be aided by the continued action of electricity through the nerves of the stomach, which have been mentioned to be peculiai’ly connected in the Torpedo with those of the Electrical organs ; a suppo- sition 'which derives some confirmation from the fact mentioned by Dr. Davy, that digestion appeared to be arrested in an individual which had been exhausted by frequent excitement to discharge itself : it should be kept in view, however, that the general depression of the system may have been the cause of the check put to the process. It is not unlikely, as Dr. Roget has suggested, that the electrical organs may communicate to the fish perceptions of electrical states and changes in the surrounding- bodies (very different from any that we can feel), in the same way as other organs of sense convey perceptions with regard to light and sound. Such perceptions may be conceived to be very useful and pleasurable to animals living in the dark abysses of the waters. that an electric current traversing a muscle, never quits its substance to travel along the fila- ments of the nerves distributed through it ; so that it is obvious that the influence which in the one case excites muscular contraction, and in the other case occasions pain, is not Electri- city, but Nervous Force generated by the passage of the electric current through the muscle, in the nerves which are distributed to it. 866 SPECIAL AND COMPARATIVE PHYSIOLOGY. CHAPTER XVIII. REPRODUCTION OF ORGANISED BEINGS. 1. General Considerations. 641. If the changes which living beings undergo during the period of their existence, and the termination of that existence by the separation of their elements at a period more or less remote from their first combina- tion, be regarded as distinguishing them in a stiiking and evident manner from the masses of inert matter which surround them, still more is their difference manifested in the series of processes, which constitute the function of Reproduction. A very unnecessary degree of mystery has been spread around the exercise of this function, not only by general enquirers, but by scientific physiologists. It has been regarded as a process never to be comprehended by man, of which the nature and the laws are alike inscrutable. A fair comparison of it, however, with other functions, will show that it is not in reality less comprehensible or more recondite than any one of them ; — that our acquaintance with each depends upon the facility with which it may be submitted to investiga- tion ; — and that, if properly enquired into by an extensive survey of the animated world, the real character of the process, its conditions, and its mode of operation, may be understood as completely as those of any other vital phenomenon. 642. It may be considered as a fundamental truth of Physiological Science, that every living organism has had its origin in a pre-existing organism. The doctrine of ‘ spontaneous generation,’ or the supposed origination of organised structures de novo out of assemblages of inorganic particles, although at different times sustained with a considerable show of argument, based on a specious array of facts, cannot now be said to have any claim whatever to be received as even a possible hypothesis ; all the facts on which it claimed to rest having either been themselves dis- proved, or having been found satisfactorily explicable on the general principle omne vivum ex ovo. Thus, the appearance of Animalcules in infusions of decaying organic matter, the springing-up of Fungi in spots to which it woidd not have been supposed that their genus could have been conveyed, the occurrence of Entozoa in the bodies of various animals into which it seemed almost beyond possibility that them eggs could have been introduced, with other facts of a like nature, may now lac- accounted for, without any violation of probability, by our increased knowledge of the mode in which these organisms are propagated. Thus it is now well ascertained that the germs of Fungi and of many kinds of Animalcules are diffused through the atmosphere, and are conveyed by its movements in every direction ; and that if to decomposing substances, of a kind that would otherwise have been most abundantly peopled by these organisms, such air only be allowed to have access, as has been deprived of its organic germs by filtration (so to speak) through a red-hot GENERAL CONSIDERATIONS. 867 tube or strong sulphuric acid, no living organisms will make their appear- ance in them ; whilst, in a few hours after the exposure of the very same substances to ordinary atmospheric air, it has been found to be crowded with life.* And when it is borne in mind in the case of the Entozoa, that the members of this class are remarkable for the immense number of eggs which most of them produce, for the metamorphoses which many of them are known to undergo, and for the varieties of form under which there is reason to suspect that the same germs may develope themselves (§ 309), it becomes obvious that no adequate proof has yet been afforded that they have been, in any particular case, otherwise than the products of a pre-existing living organism. — This, again, is the conclusion to which all the most general doctrines of Physiology necessarily conduct us. For it is most certain that we know nothing of Vital Force, save as manifested through Organised structures ; whilst, on the other hand, the combina- tion of Inorganic matter into Organised structures is one of the most characteristic operations of Vital Force ; hence it is scarcely conceivable that any operation of Physical Forces upon Inorganic matter should evolve a living Organism. Nor is such a conception more feasible, if it be admitted that Vital Force stands in such a relation to the Physical Forces, that we may regard the former as a manifestation of the latter, when acting through Organised structures (§ 54) ; since no Vital force can be manifested (according to this view), and no Organisation can take place, except through a pre-existing organism. 643. It may be further considered as an established Physiological truth, that when placed under circumstances favourable to its complete evolution, every germ will develope itself into the likeness of its parent ; drawing into itself, and appropriating by its own assimilative and forma- tive operations, the nutrient materials supplied to it ; and repeating the entire series of phases through which its parent may have passed, how- ever multiform these may be.t Now the germs of all tribes of Plants and Animals whatever, bear an extremely close relation to each other in their earliest condition ; so that there is no appreciable distinction amongst them, which would enable it to be determined whether a particular mole- cule is the germ of a Conferva or of an Oak, of a Zoophyte or of a Man. But let each be placed in the conditions it requires ; and a gradual evolu- tion of the germ into a complex fabric will take place, the more general characters of the new organism preceding the more special, as already ex- plained (chap. viii.). These conditions are not different in kind from those which are essential to the process of Nutrition in the adult ; for they consist, on the one hand, in a due supply of aliment in the condi- tion in which it can be appropriated ; and, on the other hand, in the ope- ration of certain external agencies, especially Heat, which seems to supply the force requisite for the developmental process (§ 55 7). Now although we may not be able to discern any such ostensible differences in the germs of different orders of living beings, as can enable us to discriminate them from each other, yet, seeing so marked a diversity in their operations under circumstances essentially the same, we cannot do otherwise than * See the experiments of Schulze, in the “ Edinb. New Phil. Journ.” 1837, p. 165. t The apparent exceptions to thi3 rule, which have been brought together under the col- lective term ‘ Alternation of Generations,’ will be presently considered, and will be shown to be only exceptional when misinterpreted (§ 649). 3 k 2 868 SPECIAL AND COMPARATIVE PHYSIOLOGY. attribute to them distinct properties ; and it will be convenient to adopt the phrase germinal capacity as a comprehensive expression of that peculiar endowment, in virtue of which each germ developes itself into a structure of its own specific type, when the requisite forces are brought to bear upon it, aud the requisite materials are supplied to it.* Thus, then, every act of Development may be considered as due to the force supplied by Heat or some other physical agency, which, operating through the organic germ, exerts itself as formative power ; whilst the mode in which it takes effect is dependent upon the properties or endowments of the substances through which it acts, namely, the germ on the one hand, the alimentary materials on the other, — just as an Electric current trans- mitted through the different nerves of sense, produces the sensory im- pressions which are characteristic of each respectively ; or, as the same current, transmitted through one form of Inorganic matter, produces Light and Heat, through another, Chemical change, or through another, Magnetism. 644. In the development of any living being, therefore, from its pri- mordial germ, we have three sets of conditions to study namely, first, the physical forces which are in operation ; second, the properties of the germ, which these forces call into activity ; and third, the properties of the alimentary materials which are incorporated in the organism during its development. There is evidence that each of these may have a consi- derable influence on the result ; but in the higher organisms it would seem that the second is more dominant than it is in the lower. For among many of the lower tribes, both of Plants and Animals, there is reason to believe that the range of departure from the characters of its parent, which the organism may present, is considerably greater than that of the higher ; and that this is chiefly due to the external conditions under which it has been developed. The forms of a number of species of the lower Fungi, for example, appear to be in great part dependent on the nature of their aliment (§27 2) ; so among the Entozoa, there seems strong reason to believe that those of the Cystic order are only Cestoidea, that are prevented by the circumstances under which they exist from attaining their full development (§ 309 b) ; and the production of a fertile ‘ queen ’ or of an imperfect ‘ worker,’ among the Hive-bees, appeal's to be entirely determined by the food with which the larva is supplied (§ 60). N o such variations have been observed among the higher classes ; in which it would seem as if the form attained by each germ is more rigidly deter- mined by its own endowments ; a modification in the other conditions, which in the lower tribes would considerably affect the result, being iu them unproductive of any corresponding change. For if such modifica- tion be considerable, the organism is unable to adapt itself to it, and conse- quently either perishes or is imperfectly developed ; whilst, if it be less potent, it produces no obvious effect. Thus, a deficiency of Food in the * This term is preferred to that of ‘ germ-power’ suggested by Mr. Paget, because the latter seems to imply that the force- of development exists in the germ itself. Xow if tins were true, not only must the whole formative power of the adult have been possessed by its first cell-germ, but the whole formative power of all the beings simultaneously belonging to any one race, must have been concentrated in the first cell-germ of their original progenitor. This seems a reductio ad absurdum of any such doctrine ; and we are driven back on the assumption (which all observation confirms), that the force of development is derived from external agencies. GENERAL CONSIDERATIONS. 869 growing state of the higher Animal, will necessarily prevent the attain- ment of the full size ; but it will not exert that influence on the relative development of different parts, that it does among Plants, in which it favours the production of flowers and fruit in place of leaves, or that it seems to exercise in several parallel cases among Animals (§ 359). So, again, a deficiency of Heat may slightly retard the development of the Chick ; but if the egg be allowed to remain long -without the requisite warmth, the embryo dies, instead of passing into a state of inactivity like that of Reptiles or Insects. — The extent, indeed, to which these external conditions may affect the development of the inferior organisms, must not be in the least judged of by that to which their operation is restricted in the higher ; and it is probable that we have yet much to learn, on the subject. At present, it may be stated as a pro- blem for determination, whether, from a being of superior organization, lower forms of living structure, capable of maintaining an independent existence, and of propagating their kind, can ever originate, by an imper- fect action of its formative powers (§ 7U9). Various morbid growths, such as Cancer-cells, to which the higher organisms are liable, have been looked upon in this light : these have certainly a powerful vitality of their own, which enables them to increase and multiply at the expense of the or- ganism which they infest ; and they have also an energetic reproductive power, by which they can propagate their kind, so as to transmit the disease to other organisms, or to remote parts of the same organism : but such growths are not independent ; they cannot maintain their own existence, when detached from the organism in which they are developed ; and they have not, therefore, the attribute of a separate individuality. Various phenomena hereafter to be detailed, however, respecting the ‘ gemmiparous ’ production of living beings, when taken in connection with that just cited, seem to render it by no means impossible that the individualization may be more complete in other cases, so that inde- pendent beings of a lower type may possibly originate in a perverted condi- tion of the formative operations in the higher. But no satisfactory evidence has ever been afforded by experience, that such ‘ equivocal generation ’ has actually taken place ; and its possibility is here alluded to only as a contingency which it is right to keep in view. That no higher type has ever originated through an advance in developmental power, may be safely asserted ; for, although various instances have been brought for- ward to justify the assertion that such is possible, yet these instances en- tirely fail to establish the analogy that is sought to be drawn from them.* * Thus the Author of the “ Vestiges of the Natural History of Creation” refers to the various modifications which have taken place in our cultivated Plants and Domesticated Animals, in proof that such elevation is possible ; quite overlooking the fact that these exter- nal influences merely modify the development, without elevating it, and that these races, if left to themselves, speedily revert to their common specific type. And he adduces the phe- nomena of metamorphosis — the transformation of the worm-like larva into an Insect, and of a fish-like tadpole into a Frog, — as giving some analogical sanction to the same doctrine ; totally overlooking the fact, that these transformations are only part of the ordinary develop- mental process, by which the complete form of the species is evolved, instead of being transi- tions from the perfected type of one class to the perfected type of one above it. So, again, ’ie quotes the transformation of the worker-grub of the Hive- Bee, into the fertile Queen, as m example of a similar advance ; without regarding the circumstance that the worker is psychical ly higher (according to human ideas, at least,) than the queen, whose instincts appear united to the performance of her sexual functions ; and that the utmost which the fact is 870 SPECIAL AND COMPARATIVE PHYSIOLOGY. 645. The developmental power which each germ possesses, under the conditions just now detailed, is manifested, not merely in the first evolu- tion of the germ into its complete specific type, but also in the mainte- nance of its perfect form, and, within certain limits, by the reproduc- tion of parts that have been destroyed by injury or disease. This repro- duction, as Mr. Paget has pointed out,* differs from the ordinary process of nutrition in this, — that “ in grave injuries and diseases, the parts that might serve as models for the new materials to be assimilated to, or as tissue-germs to develope new structures, are lost or spoiled ; and yet the effects of injury and disease are recovered from, and the right specific form and composition are retained — and, again, “ that the reproduce'! parts are formed, not according to any present model, but according to the appropriate specific form, and often with a more strikingly evident design towards that form, as an end or purpose, than we can discern in the natural construction of the body.” In the reproduction of the leg of a full-grown Salamander after amputation, which was observed to take place by Spallanzani, it is clear that whilst the process was from the first of a nature essentially similar to that by which its original develop- ment took place, it tended to produce, not the leg of a larva, but that of an adult animal. Hence, it is obvious that, through the whole of life, the formative processes are so directed, as to maintain the perfection of the organism, by keeping it up, so far as possible, to the model or arche- type that is proper to the epoch of its life which it has attained. — The amount of this regenerating power, however, varies greatly in different classes of organized beings, and at different stages of the existence of the same being • and, as Mr. Paget has pointed out,+ it seems to bear au inverse ratio to the degree of development which has previously taken place in each case. Thus in the Hydra and other Zoophytes, it would appear (as in Plants) to be almost unlimited : for the developmental pro- cess in them is checked at such an early period, that both the form of the organism and the structure of its tissues retain the most simple type (§ 289) ; and by the subdivision of one individual, no fewer than fifty were produced by Trembley. In this, as probably in all the cases in which new individuals have been obtained by artificial subdivision, there is some natural tendency to then 1 production hy the vegetative process of gemmation (§ 647) ; but this does not always manifest itself. It is a curious fact, that the first attempt at regeneration, in some of these cases, is not always complete ; but that successive efforts are made, each of which approximates more and more closely to the perfect type. This was well seen in one of Sir J. G. Daly ell’s experiments ; for he observed that, having cloven the stem of a Tubularia (a Hydroid Zoophyte), after capable of proving is, that the same germ may be developed into two different forms, accord- ing to the circumstances of its early growth. It must always be borne in mind that the character of a species, to be complete, should include all its forms, perfect and imperleet, modified and unmodified; since in this mode alone, can that ‘capacity for variation' be determined, which is so remarkable a feature in many cases, and is that which specially distinguishes the races of plants and animals that have been subjected to human influence. In no instance has this variation tended to confuse the limits of well-ascertained species ; it has merely increased our acquaintance with the number of diversified forms into which the same germ may develope itself. * “ Lectures on Reproduction and Repair.” f Loc. cit. GENERAL CONSIDERATIONS. 871 the natural fall of its head, an imperfect head was at first produced, which soon fell off and was succeeded by another more fully formed ; this in its turn was succeeded by another ; and so on, until the fifth head was produced, which was as complete as the original. 646. As a general statement of the amount of this regenerating power which exists in most of the different classes of animals, has been already given (chap, vii.), it is unnecessary here to do more than allude to some of those facts which most strongly bear out the doctrine just laid down. — Next to Zoophytes, there are no animals in which the regenera- tive power is known to be so strong, as it is in the lower Articulata (as the Cestoid Entozoa, and the inferior Annelida) and in the Planaria (§ 309 d), which may perhaps be regarded as rather approximating to the Molluscous type ; and here, again, we see that a low grade of general development is favourable to its exercise, and that the spontaneous mul- tiplication which occasionally takes place in these animals by fission or gemmation, is only another form of the same process. In the higher forms of both these sub-kingdoms, as we no longer meet with multiplica- tion by gemmation, so do we find that the reparative power is much more limited ; the only manifestation of it among the fully-formed Aracltnida and Crustacea being the reproduction of limbs, and the power of effecting even this being usually deficient in perfect Insects. The enquiries of Mr. Newport, however, upon the reproductive powers of Myriapods and Insects, in different stages of their development,* confirm the general principle already stated ; for he has ascertained that in their larval condi- tion, Insects can usually reproduce limbs or antennse ; and that Myria- pods, whose highest development scarcely carries them beyond the larvae of perfect Insects, can regenerate limbs or antennae, up to the time of their last moult, when, their normal development being completed, the regenerative power seems entirely expended. The Phasmidce and some other Insects of the order Orthopterci retain a similar degree of this power in their perfect state ; but these are remarkable for the similarity of their larval and imago states, the latter being attained, as in Arach- nida, by a direct course of development, without anything that can be called a ‘ metamorphosis.’ — Little is known of the regenerative power in the higher Mollusca ; but it has been affirmed that the head of the Snail may be reproduced after being cut off, provided the cephalic ganglion be not injured, and an adequate amount of heat be supplied (§ 98). — In Verte- brata, again, it is observable that the greatest reparative power is found among Batrachian Reptiles, whose development is altogether lower, and whose life is altogether more vegetative, than that of probably any other group in this sub-kingdom. In Fishes it has been found that portions of the fins which have been lost by disease or accident, are the only parts that are reproduced. But in the Salamander, entire new legs, with per- fect bones, nerves, muscles, &c., are reproduced after loss or severe injury of the original members ; and in the Triton a perfect eye has been formed to replace one which had been removed. In the true Lizards, an imperfect reproduction of the tail takes place, when a part of it has been broken off ; but the newly-developed portion contains no perfect vertebrie, its centre being occupied by a cartilaginous column, like that of the lowest * “ Philosophical Transactions,” 1844. 872 SPECIAL AND COMPARATIVE PHYSIOLOGY. Fishes. In the warm-blooded Yertebrata generally, as in Man, the power of true reproduction after loss or injury seems limited, as Mr. Paget has pointed out * to three classes of parts : — namely, (1). “ Those which are formed entirely by nutritive repetition, like the blood and epethelia, their germs being continually generated de novo in the ordinary condition of the body ; (2). Those which are of lowest organization, and (which seems of more importance) of lowest chemical character, as the gelatinous tissues, the areolar and tendinous, and the bones ; (3). Those which are inserted in other tissues, not as essential to their structure, but as accessories, as connecting or incorporating them with the other struc- tures of vegetative or animal life, such as nerve-fibres and blood-vessels. With these exceptions, injuries or losses are capable of no more than re- pair, in its more limited sense ; i. e., in the place of what is lost, some lowly organized tissue is formed, which fills up the breach, and suffices for the maintenance of a less perfect life.’’ — Yet, restricted as this power is, its operations are frequently most remarkable ; and are in no instance, perhaps, more strikingly displayed, than in the re-formation of a whole bone, when the original one has been destroyed by disease. The new bony matter is thrown out, sometimes within, and sometimes around, the dead shaft ; and when the latter has been removed, the new structure gradually assumes the regular form, and all the attachments of muscles, ligaments, &c., become as complete as before. A much greater variety and complexity of actions are involved in this process, than in the repro- duction of whole organs in the simpler animals ; though its effects do not appear so striking. It would seem that in some individuals this regene- rating power is retained to a greater degree than it is by the class at large ;+ and here again we find, that in the early period of development the power is more strongly-exerted than in the adult condition. The most remarkable proof of its persistence even in Man, has been collected by Prof. Simpson ; who lias brought together numerous cases, in which, after “ spontaneous amputation of the limbs of a foetus in utero,” occur- ring at an early period of gestation, there has obviously been an imper- fect effort at the re-formation of the amputated part from the stump. | * “ Lectures on Reproduction and Repair.” + One of the most curious and well-authenticated instances of this kind is related by Mr. White in his work on the “ Regeneration of Animal and Vegetable Substances,” 1785, p. IP. “ Some years ago, I delivered a lady of rank of a fine boy, who had two thumbs upon one hand, or rather, a thumb double from the first joint, the outer one less than the other, each part having a perfect nail. When he was about three years old, I was desired to take o;f the lesser one, which I did ; but to my great astonishment it grew again, and along with it, the nail. The family afterwards went to reside in London, where his father showed it to that excellent operator, William Bromfield, Esq., surgeon to the Queen’s household ; who said, he supposed Mr. White, being afraid of damaging the joint, had not taken it wholly out, but he would dissect it out entirely, and then it would not return. He accordingly executed the plan he had described, with great dexterity, and turned the ball fairly out of the socket; notwithstanding this, it grew again, and a fresh nail was formed, and the thumb remained in this state.” — The Author has been himself assured by a most intelligent Surgeon, that he was cognizant of a case in which the whole of one ramus of the lower jaw had been lost by disease in a young girl, yet the jaw had been completely regenerated, and teeth were developed and occupied their normal situations in it. J These cases were brought by Prof. Simpson, before the Physiological Section of the British Association, at its Meeting in Edinburgh, Aug., 1850. The Author, having had the opportunity of examining Prof. Simpson's preparations, as well as two living examples, is perfectly satisfied as to the fact. GENERAL CONSIDERATIONS. 873 By the knowledge of these facts and principles, we seem justified in the surmise, that the occurrence of supernumerary or multiple parts is not always due (as usually supposed) to the ‘ fusion ’ of two germs, but that it may result from the subdivision of one ; for if it be supposed that this subdivision has taken place when the developmental process has advanced no further than in a Hydra or a Planaria, it seems by no means impos- sible that each part might, as in those creatures, advance in its develop- ment up to the attainment of its complete form. 647. There are many tribes, both of Plants and Animals, in which multiplication is effected not only artificially but spontaneously, by the separation of parts, which, though developed from the same germ in perfect continuity with each other, are capable of maintaining an inde- pendent existence, and which, when thus separated, take rank as distinct individuals. This process, which is obviously to be regarded, no less than the preceding, as a peculiar manifestation of the ordinary operations of Nutrition, may take place in either of four different modes — 1. In the lowest Cellular Plants, and the simplest Protozoa, every component cell of the aggregate mass that springs from a single germ, being capable of existing independently of the rest, may be regarded as a distinct indi- vidual ; and thus every act of growth which consists in the multiplication of cells (§ 556), makes a corresponding augmentation in the number of individuals. — 2. In many organisms of a somewhat higher type, in which the fabric of each complete individual is made up of several component parts, we find the new growths to be complete repetitions of that from which they are put forth ; and thus the composite organism presents the semblance of a collection of individuals united together, so that nothing is needed but the severance of the connection, to resolve it into a number of separate individuals, each perfect in itself. The most characteristic example of this is presented by the Hydra (§ 289), which is continually multiplying itself after this fashion ; for the buds or ‘ gemmse ’ which it throws off, are not merely structurally but functionally complete (being capable of seizing and digesting their own prey), previously to their detachment from the parent. — 3. In by far the larger proportion of cases, on the other hand, the ‘ gemma ’ does not possess the complete structure of the parent, at the time of its detachment, but is endowed with the capacity for developing whatever may be deficient. Thus, the bud of a Phanerogamic Plant possesses no roots, and its capacity for independent existence depends upon its power of evolving those organs. On the other hand, the ‘zoospore’ of an Ulva or a Conferva (§ 267 a) is nothing else than a young cell, from which the entire organism is to be evolved after it has been set free ; and even in the ‘ bulbels’ of the Marchantia (§ 275), the advance is very little greater. The ‘ bulbels ’ of certain Phanerogamic plants, however, bear more resemblance to ordinary buds. — 4. In the preceding cases, the organism which is developed by this process, re- sembles that from which it has been put forth ; but there are many cases in which the offset differs in a marked degree from the stock, and evolves itself into such a different form, that the two would not be supposed to have any mutual relation, if their affinity were not proved by a knowledge of their history. Sometimes we find that the new individual thus budded off is in every respect as complete as that from which it proceeded, though developed upon a different type ; but in other instances it is made up of 874 SPECIAL AND COMPARATIVE PHYSIOLOGY. little else than a generative apparatus, provided with locomotive instru- ments to carry it to a distance, its nutritive apparatus being veiy imper- fect. Of the first, we have an example in the development of Medusae from the Hydroid Polypes (§ 290) ; and of the second in the peculiar subdivision of certain Annelida, hereafter to be described (§ 715). — Now it is obvious that in this process, do agency is brought into play, that differs in any essential mode from that which is concerned in the ordinary nutritive operation. The multiplication of individuals is performed exactly after the same fashion as the extension of the parent organism ; and the very same parts may be regarded as organs belonging to it, or as new individuals, according to their stage of development, and the relation of dependence which they still hold to it. The essence of this operation is the multiplication of cells by continual subdivision (§ 556). 648. We have now, on the other hand, to enquire into the nature of the true Generative process, by which the original germ is endowed with its developmental capacity ; and this we shall find to be of a character precisely the opposite of the preceding. For, under whatever circum- stances the generative process is performed, it appears essentially to consist in the reunion of the contents of two cells,* of which the genii, which is the real commencement of a ‘ new generation,’ is the result. This process is performed under the three following conditions. — 1. All the cells of the entire aggregate produced by the previous subdivision, may be capable of thus uniting with each other indiscriminately ; there being no indication of any sexual distinction. This is what we see in the simplest Cellular plants (§ 652). — 2. All the component cells of each organism may. in like manner, pair with other cells, to produce fertile germs ; but there are differences in the shares which they respectively take in the process, which indicate that their endowments are not precisely similar, and that a sexual distinction exists between them, notwithstanding that this is not indicated by any obvious structural character. This condition is seen in the Zygnema and its allies (§ 654). — 3. The generative power is restricted to certain cells, which are set apart from the rest of the fabric, and destined to this purpose alone ; and the endowments of the two sets are so far different, that the one furnishes the germ, whilst the other supplies the fertilizing influence ; whence the one set have been appropriately designated ‘ germ-cells ’ and the other ‘ sperm-cells.’ Such is the case in all the higher Plants among which a true generative apparatus has beeu discovered ; and also throughout the Animal kingdom. 649. Thus, then, in the entire process in which a new being originates, possessing like structure and endowments with its parent, two distinct classes of actions participate, — namely, the act of Generation, by which the germ is produced, — and the act of Development, by which that germ is evolved into the complete organism. The former is an operation alto- gether sui generis ; the latter is only a peculiar modification of the Nutri- tive function ; yet it may give origin, as we have seen, to new individuals, by the separation (natural or artificial) of the parts which are capable of existing as such. Now between these two operations there would seem to be a kind of antagonism. Whilst every act of Development tends to * In very rare instances, it is the reunion of the two parts of the contents of the same cell, which had previously tended to separate from each other, as if in the process of subdivision (§ 653 ). REPRODUCTION IN PLANTS. 875 diminish the ‘germinal capacity,’ the act of Generation renews it; and thus the tree which has continued to extend itself by budding until its vital energy is well-nigh spent, may develope flowers and mature seeds from which a vigorous progeny shall spring up. But the multiplication of individuals does not directly depend upon the act of generation alone ; it may be accomplished by the detachment of gemmae, w T hose production is a simple act of development ; and the individuals thus produced are sometimes similar, sometimes dissimilar, to the beings from which they sprang. When they are dissimilar, however, the original type is always reproduced by an intervening act of generation ; and the immediate pro- ducts of the true generative act always resemble one another. Hence the phrase “ alternation of generations ” can only be legitimately employed when the term generation is used to designate a succession of individuals, by whatever process they have oi’iginated ; an application of it which cannot but lead to a complete obliteration of the essential distinction which the attempt has been here made to draw, between the generative act and the act of gemmation. For when it is said that “generation a produces generation b, which is dissimilar to itself, whilst generation b produces generation c, which is dissimilar to itself but which returns to the form of generation a,” * it is entirely left out of consideration that generation A produces (the so-called) generation b by a process of gemmation ; whilst the process by which generation b produces generation c, is one of true gene- ration. So generation c developes d by gemmation, which resembles b ; and d, by a true generative act, produces E, which resembles a and c. This distinction, although it may at first sight appear merely verbal, will yet be found of fundamental importance in the appreciation of the true relations of these processes, and of their resulting products. So, in the Author’s opinion, the application of the term ‘ generation ’ to the entire product of the development of any germ originating in a generative act, whether that product consist of a single individual, or of a succession, will be found much more appropriate, and more conducive to the end in view, than the indiscriminate application of it to each succession, whether pro- duced by gemmation or by sexual reunion. It is of great importance to the due comprehension of certain phenomena of Reproduction, which will come under consideration in the Animal kingdom, that the relations of the products of these two processes should be rightly appreciated ; and this appreciation of them will, it is believed, be best gained by a careful enquiry into the phenomena of Reproduction in the Vegetable kingdom. 2. Reproduction in Plants. 650. Our knowledge of the reproductive process in the Vegetable Kingdom has of late been greatly extended by Microscopic research ; and although it is not possible at present to give a complete sketch, much less * Such is the doctrine propounded in the highly original and ingenious work of Prof. Steenstrup on the “ Alternation of Generations,” a translation of which has been published by the Ray Society. The Author feels satisfied, from careful and repeated consideration of the phenomena on which this doctrine is founded, that it cannot be accepted in its original form, and that it requires to be modified in the manner indicated in the text. His reasons for this modification, and likewise for his dissent from the hypothesis put forth by his friend Prof. Owen in his treatise on “ Parthenogenesis,” will be found in the “ Brit, and For. Med. Chir. Rev.” vol. i. p. 183, and vel. iv. p. 43G. 876 SPECIAL AND COMPARATIVE PHYSIOLOGY. a full account, of the mode in which it is performed in all the principal groups of Plants, still enough has been ascertained in some of the most important, to make it probable that the general doctrines founded upon the phenomena which they present, may be applied to those whose history has been less completely studied. It may be stated as a certainty that the true Generative act is not confined, as was long supposed by most Physiologists, to the Phanerogamic or Flowering Plants ; but that it is performed by many of the Cryptogamia ; and as it has been discovered to take place at both extremities of the Cryptogamic series (namely, among the simplest Cellular plants, and in the Ferns), further research may be confidently expected to show, that the intermediate groups form no exception to the general law. So far as our present knowledge ex- tends, this generative act may be performed in three distinct modes, each of which is peculiar to a certain assemblage of Plants. — The first of these is the ‘ conjugation’ of two apparently similar cells, and the a dmix ture of their contents, whereby a new body, the sporangium, is produced ; this, so far as we at present know, is peculiar to the lower Algae ; hut it may he surmised that a similar process takes place in the lower Fungi and Lichens. — In the second case, the generative act is effected by the agency of cells which are manifestly dissimilar in their endowments ; for the one set develope within themselves certain moving filaments, that convey their influence to the cells of the other set, within which last the germ origi- nates ; so that the distinction between ‘sperm-cells’ and ‘germ-cells’ is here characteristically shown. This is the mode in which the generative act seems to he performed in the higher Algae and in Ferns; and pro- bably in all the intermediate tribes of Cryptogamia. In the inferior groups, the fertilized germ appears to be at once thrown upon the world, and is dependent for its supply of food upon its own absorbing and assi- milating powers ; hut in the superior, it is supplied for a time with nutriment prepared and imparted to it by its parent. — In the third form of the generative process, which is peculiar to Phauerogamia, there is the same distinction between ‘ sperm-cells ’ and ‘ germ-cells,’ hut the mode in which the action of the former upon the latter is brought about, is dif- ferent ; for the ‘ sperm-cell ’ (pollen-grain) does not evolve self-moving filaments, hut puts forth long tubes, which extend themselves until they reach the ‘ germ-cell,’ and thus convey to it the fertilizing influence. Moreover, the germ is supplied with nutriment by its parent, not merely whilst it remains in connection with the organism which evolved it, but for some time subsequently : its continued growth being provided for by the store laid up around it in the seed ; and it being at the expense of the materials which it thence obtains, that the germ is enabled to develope and put forth the first of those organs, which are subsequently to act as its own instruments of absorption and assimilation.- — Each of these pro- cesses will now be described in more detail. 651. It is among the lowest tribes of Plants, that we see the closest relation between the functions of Nutrition and Reproduction. For we do not here find, as in the higher Plants, that the development of distinct organs is requisite, to enable the descendants of the original germ-cell to exist independently of each other ; since this power is possessed by every one of the cells that are produced by its fission (§ 139), so that they have as much title to be accounted distinct ‘ individuals,’ as have the leaf-buds of REPRODUCTION IN PLANTS. 877 Fig. 248. i Multiplication of Coccocliloris cystifera by sub- division of cells ; a, b, c, d, e, various stages of development. the Flowering-plant. Thus, a frond of the Coccochloris cystifera, one of the simple Algae belonging to the family Palmellece, presents us with a mass of independent cells, held together by a common mucous investment (Fig. 248 ) ; and in such a mass we usually find cells in various stages of development. At a is seen a simple globular cell, surrounded by a well- defined mucous envelope ; at b are cells which present an elongated form, and are evidently about to undergo subdivision ; at c we observe a pair of cells, whose separation has apparently been recent, as they are still surrounded by the same mucous envelope ; at d we have a group of three cells, each having its own mucous envelope, but all of them still held together by the original investment; the two smaller of these cells are obviously produced by a secondary subdivision of one of the first pair : whilst the other shows by its elongated form that it is about to subdivide ; and at e is a group of small cells formed by a continuance of the same subdividing pro- cess, each of them subsequently increasing in size, acquiring a mucous envelope of its own (as at a), and in its turn going through the same series of changes. And as this will take place equally, whether the cell be detached from the general mass, or remain as a component part of it, we cannot refuse to it the designation of a distinct individual. It will be found convenient, however, to distinguish the individuals which are thus related as the products of a single generative act, by the appella- tion of co-individuals. — To what extent this method of multiplication may proceed, has not been ascertained. There is reason to think that there is a limit to its performance, since we find it giving place, at a certain period of the year, to the converse process ; namely, the true Generative act. But upon the principle already laid down, we should expect that it may be carried on to a vast extent, under favourable circum- stances ; since there is here no advance upon the very simplest condition of an organised structure, and consequently but little exhaustion of the ‘ germinal capacity’ (§ 645). 652. As amongst the independent cells of which the simplest Algse are composed, each seems capable of going through the whole series of Nutritive operations for and by itself, so there is no perceptible difference in their endowments as regards the Generative act ; the process of 1 con- jugation’ appearing to take place indifferently amongst any pair of them. As examples of this process, two instances will be first selected from the families of Desmidece and Diatomacece ; in which its occurrence is a fact of peculiar interest, as tending to show their close alliance to the simpler Algse, from which they have been separated by Prof. Ehrenberg and other naturalists, who have considered them deserving of a place in the Animal kingdom. — The Desmidece are characterised by the peculiar structure of their cells : for the -wall of each consists of two distinct and 878 SPECIAL AND COMPARATIVE PHYSIOLOGY. Fig. 249. symmetrical valves, united to each other by a transverse suture ; and the endochrome shows a tendency to separation into two equal halves at the same part. In some instances there is a constriction at the suture, so that the two portions of the cell are connected only by a narrow neck, as in Euastrum (Fig. 249, a) ; but in other cases, as in Closterium, the median portion traversed by the suture is the broadest paid of the cell. The plants of this family are generally met with in the condition of detached cells or ‘fronds;’ but in some instances, these are associated into confervoid filaments ; the difference simply arising from the continu- ance of the connection, in the latter case, between the cells which have been multiplied by subdivision, and from the separation of these cells in the former. The first stage in the conjugating process is the approxima- tion of two cells, in whose contained granules an active movement is often seen at the same time. Each of the two cells then gives way at the plane of its transverse division (b), and the four halves discharge their contained ‘ endochromes’ into the central space, so that those of the two cells are thus completely incorporated. At first the new mass is merely surrounded by the mucous envelope, and has no definite shape ; but it soon becomes invested by a distinct cell-wall, which presents a regular form, usually globular or spheroidal ; and this cell-wall is often furnished with curious prolongations which give it a hispid surface (as seen at b), these being sometimes very long, and furnished with hooks at their extremities. The new body thus formed is termed the sporangium. The mode in which it gives origin tr the ordinary frond, has not yet been clearly ascertained ; but it seems pro- bable that from each sporangium, several new fronds may be produced. Every one of the new cells thus generated may multiply itself by subdivi- sion ; and thus each sporangium may develope itself into a large mass of such cells, all having similar forms and endowments. It has been remarked that the sporangia of the Desmidcse (which are for the most part inhabi- tants of small collections of fresh water, that are liable to be dried up by the heat of summer,) are most abundant in spring j so that it may be surmised that they are not killed by desiccation, like the grow- ing fronds, but are enabled to regenerate the race after the period of drought has passed away. — The mode in which the conjugation and development of the sporangia take place in the entire family of Desmidece , is essentially the same ; and even where the cells that have been multi- plied by subdivision remain in continuity with each other, so as to form a filament, they dissociate themselves before the conjugation takes place. Euastrum oiblongum : — a, single frond: — b, two fronds in conjugation, showing the sporangium in the centre between the four emptied valves. REPRODUCTION IN PLANTS. 879 Sometimes it is observable that instead of the complete separation of the two halves of each cell in the act of conjugation, they only partially open for the discharge of the endochrome; and in some of the higher forms of this family, we find the mixture to take place in one of the conjugating cells, within which the sporangium is de- veloped. This hap- pens especially in the genus Desmi- dium, which ap- proaches closely to the conjugating Confervse.* 653. In the fa- mily Diatomacece, the membranous envelope of each cell is covered by a siliceous incrus- tation, which usu- ally presents very beautiful and re- gular markings ; and th is hard ‘ shell’ is composed of two ‘ valves,’ which are divided longitudi- nally, and are ca- pable of being se- parated from each other. Like the plants of the pre- ceding family, the cells may occur either singly (Fig. 250, a, b), or in connection with each other • in the first case they are known as ‘ frus- tules in the se- cond, they form bands or filaments composed of several cells adherent by their long sides, which cells have all been produced by subdivision from a single Conjugation of Eunotia turgida : — a, single frustule ; b, side view of the same ; c, two frustules with their concave surfaces in close apposition ; n, front view of the same, showing the separation of each frustule longitudinally into two halves ; e and f, side and front views after the formation of the sporangia; g, the sporangia more advanced, exceeding the parent frustules in size ; h, completion of the sporangia. * See Mr. Ralfs's admirable Monograph on the “ British Desmideae.” 880 SPECIAL AND COMPARATIVE PHYSIOLOGY. one, and have not become detached from each other. When two frus- tules (as in Eunotia turgida ) are about to enter into conjugation, they move towards each other, and come into close approximation by their concave edges (c) ; the valves of each frustule separate from each other (d), and the inner membrane forms two protuberances, which meet two similar ones of the other frustule. These protuberances soon give way, and the endochromes of the two frustules discharge themselves and mingle, not, however, into one mass, but into two ; these masses are at first irre- gular in shape ; but they shortly become covered with a smooth membrane, and present a symmetrical elongated form (e, f), which presents some resemblance to that of the parent frustule. These ‘ sporangia! frustules’ continue to increase in length (g), and at last they acquire the transverse markings of the parent frustules, which they greatly exceed in dimensions (h). These large sporangial frustules seem to give origin to a new brood by the ordinary fissiparous multiplication ; and it may be conjectured that this process cannot take place indefinitely, but that the race would die out, were it not for the renewal of the powers of growth by the act of conjugation. — In the Meloseirece, which constitute a filamentous tube of Diatomaeeie, there is a curious departure from the ordinary type ; for no conjugation can be here seen to take place between two distinct frustules ; but a disturbance of a somewhat equivalent nature takes place in the endochrome of a single frustule, its particles moving from the extremities towards the centre, rapidly increasing in quantity, and aggregating into a mass, round which a new envelope is developed, so that it becomes a sporangium resembling that of other plants of its kind (Fig. 251). At first sight, this might seem an exception to the general rule of the reunion of the con- tents of two cells ; but it is more so in appearance than in reality. For in some of the group of Conjugates, whose conjugation will be presently described (§654), the union of endochromes takes place, not between Development of sporangia in Aulacoseira crenulata:— 1, simple filament; xi -n., if distinct 2, filament developing sporangia; a , b,c, successive stages in tne formation c 1 25 of sporangia; — 3, sporangial frustules, a,b,c, in successive stages of multi- filfHHGlltS, blit by the evolution of a stolon or creeping stem from its base (resembling that of many Zoophytes), from which gemmae sprout forth, these generally be- coming subsequently detached. Thus from a single individual, a large colony (c) may be produced in no very long time, the detached gemmae most commonly remaining in proximity with each other ; and the rate of multiplication, as in the Hydra, is chiefly influenced by the temperature, and by the supply of food. Further, it has been ascertained by Sir J. G. Dalyell, that this creature has the same power of regenerating parts of the body that have been removed, or of reproducing the complete body and tentacula from a portion of it, as that which is possessed by the common Hydra ; and that both portions of a bisected Strobila could not merely oe developed into new and perfect individuals, but could multiply the stock by gemmation, as before their fission. There appears no definite limit to its continuance in this state ; it has been kept by the last-named excellent * It lias recently been asserted by M. Derbes (Ann. des Sci. Nat.” Juin, 1050). that the Cvaruea chrysaora is hermaphrodite ; but it docs not appear certain that he has not mistaken the ' thread-cells ' or urticating organs for sperm-cells. REPRODUCTION IN ANIMALS. 927 observer for several years ; and from the entire conformity of its general structure and habits to that of the Hydra, and the absence of any indica- tion of its larval nature, it was considered by him as a complete being, “entitled to a distinct position in the Systema Naturae — A minute examination of its structure, how- ever, shows that it is distinguish- able from the ordinary Hydraform polype. The part surrounding the mouth is capable of being pro- truded as a sort of proboscis ; and the aperture of the mouth is then seen to have a quadrangular form. The inner surface of the lips and mouth, and the external surface of the tentacula and body, are covered with cilia ; so that cur- rents of water, unless when the mouth is shut, are continually passing in and out from the mouth and along the tentacula. No pro- per Hydroid polype has its arms or body ciliated. The wall of the stomach consists of two layers, of which the inner one is folded in- wards, so as to form four longi- tudinal projections into the cavity of the stomach, a sort of pouch or blind canal being left between tbe folds of each plait. These four short canals terminate at the upper end in another canal, that passes between the outer margin of the mouth and the peripherv of the group of five, the gemmae remaining: in connection with ■j. , • x xi • • f the parent; c, a colony consisting of numerous indivi- CllSK j and into tnis circular canal, duals proceeding from a common stock, but most of the tubular cavities of the tenta- them detached from 1{ - cula also open.* This arrangement is obviously indicative of the Medusan affinities of the Strobila; but it does not render it the less a Polype. 693. The Strobila does not always remain, however, in its original con- dition. Under certain conditions not yet ascertained, it ceases to multiply by ordinary gemmation, although this process may have gone on with regularity and activity for months and even years previously; and enters upon an entirely different series of operations, of which the first is the assumption of a more elongated cylindrical form than it had previously possessed. A constriction or indentation is seen around this cylinder, just below the ring which surrounds the mouth and gives origin to the tenta- cula ; and similar constrictions are soon repeated around the lower parts of the cylinder, so as to give to the whole body somewhat the appearance of a rouleau of coins (Fig. 271). Still, however, a sort of fleshy bulb, * This description is derived from the excellent “ Observations on the Development of the Medusas,” by the late Dr. John Reid, contained in the “Annals of Natural History ” for Jan. 1848, and in his “Physiological, Pathological, and Anatomical Researches.” Fig 270. Strobila (or polypoid state of Medusa) propagating by gemmation : — a, single individual, attached by its base, its tentacula extended in search of food ; b, a 928 SPECIAL AND COMPARATIVE PHYSIOLOGY. somewhat of the form of the original polype, is left at the base or attached extremity (Fig. 27 2 , a). The number of circles is indefinite ; and all are Fig. 271 . Group of Strobiles , or Medusan Laruee , attached to a shell of Pecten. not formed at once, new constrictions appearing below, after the upper portions have been detached ; as many as 30 or even 40 have thus been produced in one specimen. The constrictions then gradually deepen, so Development of Medusa-buds from Strobila: — a, Strobila enlarged, with incipient production of disc-like gemmae between the body a and the circle of tentacular,- — b. more advanced development of the Medusa-buds, the constriction between them being deepened, and their margins becoming lobed ; — c, a specimen still further advanced, the original tentacular circle being now detached, and a new set of tentacula c developed at the base of the pile of discs, of which those nearest the extremity b are most advanced, having acquired the proboscis d. and the general form ot the Medusa; — d, a Strobila returning to its original polypoid state, several ot the Medusa-buds having been detached, the new tentacula c being fully developed ; and a polype-bud e being in process of formation. REPRODUCTION IN ANIMALS. 929 as almost to divide the cylinder into a pile of saucer-like bodies ; the division being most complete above, and the upper discs usually present- ing some increase in their diameter ; and whilst this is going on, the edges of the discs become lobed, and the lobes soon present the clefts and ocelli characteristic of the detached Medusse (b). Up to this period, the tenta- cula of the original polype surmount the highest of the discs ; and a general contraction and relaxation of the whole cylinder, causing the in- tervals between the discs to be diminished or increased, may he occasion- ally seen to take place. But before the detachment of the topmost disc, the circle of tentacula around the original mouth disappears ; and a new circle is developed upon the summit of the bulb, which remains at the base of the pile of discs (c). At last, the topmost and largest disc begins to exhibit a sort of convulsive action ; it becomes detached, and swims freely away ; and the same series of changes takes place from above downwards, until the whole pile of discs is detached and converted into free-swimming Medusae. But the original polypoid body still remains ; and may return to its polype-like and original mode of gemmation (d). becoming the pro- genitor of a new colony of Strobilce, every one of which may develope in its turn a pile of medusa-discs.* The bodies thus detached have all the essential characters of Medusae. Each consists of an umbrella-like disc, divided at its edge into a variable num- , FlG ’ . , ber of lobes, eight being apparently the normal type ; and of a sto- mach, which occupies a considerable propor- tion of the disc, and projects downwards in the form of a pro- boscis, in the centre of which is the quad- rangular mouth (Fig. 273, a, b). At first there are no indica- tions of genital organs; but these subsequent- ly appear. Each of the lobes of the mar- gin is bifid ; and at the bottom of the cleft is a little body which has been sup- posed to be a rudi- mentary eye (c) ; but it is very doubtful if it be really entitled to this designation. As the animal advances towards maturity, the segments or lobes of the border of the disc increase comparatively little in size, whilst the intervals between them gra- * This last fact, which is of fundamental importance in the philosophical interpretation of this wonderful process, was first brought to light by Sir J. G. Dalyell. (See his “ Rare and Remarkable Animals of Scotland,” vol. i.) 3 o Development of Medusa from detached portions of Strobila ; — a, individual viewed sideways, and enlarged, showing a proboscis, and b bifid lobes ; b, individual seen from above, showing the bifid lobes of the margin, and the quadrilateral mouth ; c, one of the bifid lobes still more enlarged, showing the ocellus at the bottom of the cleft ; d, group of young Medusae, as seen swimming in the water, of the natural size. 930 SPECIAL AND COMPARATIVE PHYSIOLOGY. Fig 274. dually fill up ; tubular prolongations of the stomach extend themselves over the disc ; and its borders become furnished with long, pendent, prehensile cirrhi. The mouth, which even in the youngest detached animal ad- mits of being greatly extended and protruded, is quadrangular, and pre- sents four extensible angles. These angles grow more rapidly than the four-sided oral tube or proboscis ; so that, in the more advanced animals,, the mouth appears during the growth to have divided or split into four lobes ; and the minute serratures which appear on the edges of these, are the commencement of the lobes and fringes that are observed on the ten- tacula of the adult animal (Fig. 106, b). — The Medium which are as yet known to be developed after this mode, belong to the ‘ hooded-eved ’ group of Pulmograda ; whilst those which are budded-off from the com- posite Hy droid Zoophytes belong to the ‘ naked-eyed ’ subdivision. Even after the complete Medusan form has been attained, propagation occasion- ally takes place by gemma- tion ; but only, so far as yet observed, in the infe- rior or ‘ naked-eyed ’ group. This curious fact was first observed by Sars in Cytceis; from the peduncular sto- mach of which he observed medusan genuine to arise, these being symmetrically arranged around its four sides, but in different stages of advancement (Fig. 274). He also observed in Thau- mantias a similar gemma- tion from the ovaries. Both these observations have been verified by Prof. E. Forbes ; who has also seen in a species of Sarsia an evolution of gemma; around the whole length of the peduncle, these being in different stages of deve- lopment, and having an indistinct spiral arrangement; whilst in another Sarsia he has found clusters of gemmae hanging like bunches of grapes from the tubercles at the base of the four marginal cirrhi.'' 694. Now if the history of the development and propagation of the Medusae be compared with that of the corresponding processes in the Hydroid Zoophytes, it will be seen that the two series of phenomena ire of precisely the same order. The immediate product of the ovum, in each case, is a Hydraform polype. This lives for an indefinite time iu the polypoid condition, and multiplies itself by gemmation ; the gemmae either detaching themselves, so as to form distinct zooids ; or remaining in con- Gemmiparous multiplication of Cytceis octopunctata ; a,b , c, d , buds in different stages of development, sprouting from the wall of the stomach h ; at e is seen one of the four radiating canals ; at ///the marginal cirrhi, and at h the proboscis. * “ Monograph of the British Naked-Eyed Medusae, ” published by the Bay Society, 1848, p. 17. REPRODUCTION IN ANIMALS. 931 tinuity with the original stock. But to whatever extent this process is carried on, it merely consists in a multiplication of the original Hydra ; and all the zooids thus produced have the same homological relation to each other, as have the several parts of any single body among the higher Animals. But, under some change either of internal or external condi- tions, which cannot yet be distinctly stated, the polypoid gemmation gives place to the medusan,— the multiplication of the products of the original generative act, to the preparation for a repetition of that act. Now although the medusa-buds of the Strobila are formed in a different situa- tion from those of the Coryne or Campanularia, being developed between the body and tentacular circle of the original polype in the one case, and from the sides of the polype-body or from the connecting stem in the other, yet there is otherwise no essential difference between the two oper- ations. In the Strobila, as in the Coryne and Campanularia, the original polype-body remains ; so that, after having given off a large number of medusa-buds, it continues its ordinary polypoid-existence, and may repeat the same process at a future time.* It is in the Medusa-buds only, that generative organs are evolved ; these are subservient, in the one case as in the other, to the performance of the true generative process ; and it is from the ova thus produced, that the polypoid larvae arise. — Now that the whole of this series of processes is the continuation of one and the same developmental operation, and that the polypoid and medusan states should he included in the idea of one generation, instead of being regarded as distinct generations, will appear from this consideration ; that they do, in fact, hold the same relation to each other, both homologically and func- tionally, as do the nutritive and generative organs in higher animals. The polype is no more a complete organism, than is the larva of an Insect or the tadpole of a Frog ; for, although possessed of the capacity to main- tain its own life, it has no generative apparatus for the continuance of the species. — So, again, we do not consider a Plant as having attained its com- plete development, however great may he its extension by leaf-buds, until it has developed flowers and produced fruit. The Medusan zooid is obvi- ously the flower-bud or generative apparatus of the polype ; it originates in a process of continuous development, precisely resembling that which evolves the generative organs, both in the annual Plant and in the Animal, after the nutritive apparatus has attained nearly its full development ; and its special endowments obviously have reference to its peculiar func- tion, that of propagating the race in localities remote from the original stock. In some of the Cam pan ular i die, as we have seen, the Medusa-buds never become completely detached from the Zoophytic structure, but seem to remain dependent upon it for their nutriment ; and here the embryoes seem to have an unusual motor power, which serves for their dispersion. * The production of Medusas from the Strobila was represented by Sars as the result of fission; the fact that the polypoid body remains behind, forms a new set of tentacula, and may again give off polypoid and medusan gemmae, not being known to him. This view is taken up by Steenstrup, who goes so far as to maintain that the original polypoid animal is really a medusa of the female sex, which acts as a sort of nurse to a brood of young ones. How these young ones are developed, whether from ova or gemmae, he does not clearly explain ; but he considers them as a new generation, alternating with the polypoid form, — a mode of viewing the subject which tends to confuse the essential distinction between the product of gemmation and that of the true generative act. 3 o 2 932 SPECIAL AND COMPARATIVE PHYSIOLOGY. In most other cases, however, the Medusa-buds are not only detached, hut, being separated when as yet they have attained but a small size and a low grade of development, they are provided with a nutrient apparatus of their own, whereby they can obtain and prepare the needful materials. This is perfectly conformable to what has been shown to take place in the Fern (§ 664). Of the detachment of a self-moving body containing the gene- rative products, we shall meet with several curious examples, among the higher Animals. 695. The Reproductive process among other tribes usually assembled together under the class of A calephae* is not less remarkable. In all of them, as it would appear, is there a double method of multiplication, namely, by gemmation, and by true generation ; but the results of the former process differ very considerably in the different groups. — In Beroe, as probably in other Giliograda (§ 294 a), gemma; are occasionally pro- duced, which seem to evolve themselves into the likeness of their stock ; but the ordinary method of reproduction is by time generation, tbeir progeny being apparently developed at once into the parental form. If this prove to be the case (which is not yet certain), it constitutes an im- portant point of agreement betweeen the Ciliograda and the Helianthoil Zoophytes. — The Oirrhigrada (§ 294 b) are, in Mr. Huxley’s view, com- posite polypoid animals, analogous to the compoimd Hydroida, although their parts have a very different arrangement. The multiplication of the organs (every one of the so-called tubular cirrhi being according to Mr. H. a distinct polype) takes place by a process of gemmation : but the generative act is provided for in a different mode. The bodies developed from the bases of the cirrhi, and formerly termed ovarian capsules (Fig. 108, d, e), are in his Mew rather medusiform buds. They become detached in some instances, and swim freely through the water ■ and in due time produce either spermatozoa or ova. From each of the latter, when fertilized, a single polypoid animal originates ; and this gradually evolves the composite structure by gemmation, the central proboscis-like organ being the stock, and the surrounding tubular cirrhi the buds. The cavities of all these have been found by Mr. Huxley to communicate with each other and with the general reservoir, thus forming a polygas trie digestive apparatus. — The order Physograda includes a great number of forms, whose true nature was long misunderstood, hut which are now shown to be, like the preceding, composite organisms, the multiplication of whose parts is effected by gemmatiou. This is the case with the Pfiys- sophoridce and Diphydce , two groups whose relation to the Medusa; seems to consist in this, — that the contractile natatorial organ, which is deve- loped in the latter around the stomach, is developed separately and on •one side of it in the former. In each of these, long chains may be formed by the multiplication of similar organs by gemmation ; but there is always a separate provision for the generative process, consisting of a * Since the outline view of this class contained in the earlier portion of this work (ct 2:1o, 204) was in print, much has been added to our knowledge of it, chiefly by the researches t f Mr. Huxley, the Assistant-Surgeon of HALS. Rattlesnake, recently engaged in a Surveying Voyage in the Eastern Archipelago. A portion of his results, which will probably tend to modify very greatly the existing arrangements of the classes Acaleph® and Polypifera, are contained in his Paper on ‘ The Anatomy and Affinities of the Medusas,' in the " Philoso- phical Transactions” for 184.0, Part ii. For further information on some points not there treated of, the Author is indebted to communications made to him by Mr. Huxley himself. REPRODUCTION IN ANIMALS. 933 gemma of a peculiar kind, which contains within it either ovisacs or sperm-cells. Both sets of these are formed in each individual of the Physsophoridse, as in monoecious plants ; but only one set in each indi- vidual of the Diphydie, as in dioecious plants. This gemma, in some instances, is not developed into any special form, but is a mere protuber- ance containing the essential organs within it ; but in other cases it has a natatorial disc developed around it ; and some of those which are pro- vided with this organ become detached, and swim about after the manner of Medusae. — The Physalia, in Mr. Huxley’s view, is simply one of the Physograda, in which the air-vesicle that all possess has undergone an unusual development ; and its cirrhi are so many polype-mouths, or por- tions of a polygastric digestive apparatus. Its reproduction is effected after the same manner ; the ova being developed within a medusiform gemma, that possesses a power of independent movement, and the sperma- tozoa within a gemma of less specialized character ; whilst, from the ova which are thus generated and fertilized, a polypoid animal would probably spring, that would gradually evolve itself by gemmation into the likeness of its parent. — Much labour and research will undoubtedly be required to complete our knowledge of this subject ; but when it shall have been thoroughly investigated, it will unquestionably be found to be one of the most remarkable pages in the whole volume of Natural History, abound- ing, though this does, in wonders and beauties throughout. 696. In the class Echinodermata , in which the Radiated type is carried to its highest degree of development, the structure of each organism being more complex, and its parts more specialized and mutually depen- dent, the power of multiplication by gemmation seems to be altogether wanting. Still w T e find a very extraordinary power of regeneration of lost parts to be retained. The Ophiurida can reproduce the arms, and the Asteriada can replace the rays, which they may have lost ; and although neither arms nor rays can reproduce the body, when separated from it, yet if a small part of the disc (in some species at least) remain attached to the separated arm, the whole may in time be developed from this.* This regenerative power seems to extend, in the Holothuricula, to the reproduc- tion of the entire visceral apparatus ; and instances of spontaneous fission are said to occur in the same group, especially in those aberrant forms of it which lead towards the Articulated series (§ 296 tce, which exhibit this curious property, there is also another departure from the usual type ; the two sets of sexual organs being combined in the same organism, instead of being, as in all other Echinodermata, allotted to dis- tinct individuals. The position of these organs in the different orders has been already noticed in the general account of the class (§§ 296-296 e ) ; but it must be remarked that the so-called “ ovaries ” are as frequently “ testes ” or spermatic organs ; the male and female generative apparatus being scarcely distinguishable from one another in this class, either by their external aspect or their internal structure, except when their respec- tive products are nearly mature.— We have now to trace the history of * The Author once met with a single ray of a Star-fish in a state of activity, as was evidenced by the energetic movements of its tubular suckers ; although it was obvious, from the appearance of the wound, that it must have been detached from the body for some days or even weeks. Might this have continued to live, and have re-formed the body ? 934 SPECIAL AND COMPARATIVE PHYSIOLOGY. embryonic development in this class, so far as it is known ; and we shall find it to present some very curious features. 697. Of the orders Crinoidea and Cystidea, nothing can be said under this head. The fossil remains of the latter group do not afford any clue to the history of their growth from the egg ; and little is known in regard to the formei’, save the fact of the change from the pedunculate to the free state, of which mention has already been made (§ 296). — The accounts of the development of the Asteriada given by different trust- worthy observers, exhibit so much discrepancy, that there can be little doubt that the process takes place after at least two very diverse plans. The first aud simplest of these has been witnessed by Sars * in the Eclii- naster riibens ; and the observations of Agassiz t are on the whole in accordance with those of that industrious naturalist: In the early stages, the segmentation of the yolk takes place as in other animals (Fig. 275, c, d. e, f) ; and the embryo comes forth from the egg soon after it has Fig. 275. Development of the ovum of Echinaster rubens ; — a, ovarian caecum, containing ova in various stages of development, the most mature being seen at a; b, ovarian ovum highly mag- nified, showing the germinal vesicle and spot ; c, an ovum after its deposition, the natural size being also shown ; d, e, f, successive stages of the segmentation of the yolk ; g, embryo soon after its escape from the ovum, showing at a, b, two of the rudimentary organs of attachment ; h, end view of the same, showing all three {a, a, b) of these organs; i, more advanced embryo, attached by four organs, two at o, and two at b, with a papillary projection c between the n : k, embryo more advanced, free and furnished with cirrhi e. attained the state of the ‘ midberry mass,’ and swims freely about, by means of the cilia with which it is covered, in a sort of marsupial chamber which is formed by the drawing-together of the rays of the parent around its mouth. Soon after its emersion, the embryo begins to put forth an organ of attachment, resembling the stem of a Crinoid ; this at first pos- sesses two tubercles, then three (g, h), then four, with a fifth smaller one between them (i).j. At the same time, the principal mass gradually be- * “ Fauna littoralis Norvegiae,” 1846. •f “Lectures on Comparative Embryology,” New York, 1849. J From the figures and descriptions of Sars, it would appear as if the organ of adhesion was developed from one side of the disc ; but it is aflirmed by Agassiz that it hangs down as a pedicle from the centre of the under side of the body, and that its apparent lateral projection REPRODUCTION IN ANIMALS. 935 comes flattened, and shapes itself into five lobes surrounding a central disc ; thus sketching out the body and rays. When in this state, it attaches itself to fixed objects by its organ of adhesion ; but if detached, it swims through the water by the action of the cilia with which the body and arms are clothed ; so that it bears a strong analogy to the Pentacri- nus in process of conversion into a free-moving Comatula (§ 296). At the same time, five double rows of small tubercles may be perceived radiating from the centre of what is to become the ventral surface of the body; these gradually elongate themselves, and become cirrhi (k, e), each furnished with a sucker at its extremity. A peculiar tubercle is also seen at the edge of each of the five lobes of the body ; and this is the rudiment of the ocellus, which is afterwards found at the extremity of each ray. As development proceeds, the primitive organ of adhesion gradually decreases in size, and the animal creeps by means of its cirrhi ; and at last the pedicle is drawn (as it were) into the body, the lobes of the body lengthen into rays, the animal loses its ciliograde progression, and the ordinary characters of the Star-fish become apparent. — The progress of the internal organization is thus described by Agassiz. The earliest deposit of calcareous matter takes place around the prominent tubercles of the lower surface ; at first in the condition of little isolated crystals, which are formed as nuclei in the cells ; and then as a network formed by the coalescence of several of these. Of these networks there are at first ten, symmetrically disposed on the ventral surface, in a manner corresponding to the arrangement of the solid plates in Crinoids ; but they gradually increase in number, and more distinctly mark out the rays ; new ones being interposed in pairs between those already existing, and small spines projecting from the older ones. The calcareous deposit in the dorsal surface, on the other hand, seems to proceed from a central nucleus above the yolk-mass. The progress of development is obviously from without inwards ; the cells on the surface of the yolk-mass being the first to undergo metamorphosis into the permanent structure. Those occupying the central part of the body and pedicle; undergo liquefaction ; and a kind of circulation is seen in the latter. Gradually what remains of the yolk-mass is more distinctly circumscribed in the interior of the animal, and forms a central cavity with prolongations extending into the rays ; but it is not until the pedicle has contracted itself into a mere vesicle, that the mouth is formed, by the thinning-away of the envelope of the yolk-mass on the lower surface, a little to one side of the base of the pedicle ; and it is not until after the formation of the mouth, that the nervous ring can be traced, with its prolongations extending to the ocelli at the extremities of the rays. 698. The second plan of development seems much more conformable to what will be presently described as taking place in the Ophiurida and Echinida ; for. the body first developed from the embryonic mass is a larva of which little remains in the permanent structure, and the Star- fish is budded-off, as it were, from the anterior extremity of this. This larva, which has received the name of Bipinnaria from the symmetrical wing-like arrangement of its natatory organs, presents much more re- semblance to an Articulated, or to a Vertebrated, than to a Radiated is simply due to the flexion it undergoes, when the embryo is taken out of the water in which it floats, and is laid upon a plane surface. 936 SPECIAL AND COMPARATIVE PHYSIOLOGY. animal (Fig. 276). Its body is elongated, and carries at its anterior extre- mity the portion of the yolk-mass not yet metamorphosed, from which the Star-fish is afterwards to be det r eloped ; and into the cavity of this a pas- sage is formed, through what may be termed the mouth of the larva (a), which opens in the middle of a transverse furrow ; whilst another tube passing forth from it ( b ) seems to answer to an intestine. On either side of the anterior portion of the body are six, or more, narrow fin-like appen- dages, which are fringed with cilia ; and the posterior part of the body is prolonged into a sort of pedicle, bilobed towards its extremity, which also tegument has taken place, and its true mouth has been formed, but long before it has attained the adult condition; and as its ulterior de- velopment has not hitherto been observed in any instance, it is not yet known what are the species in which this mode of evolution prevails. The larva continues active for several days after its detachment ; but there is no reason to believe that its existence is prolonged for any considerable time ; and as the Star-fish is not formed by gemmation from it, but from a portion of the yolk-mass which remained unconsolidated after its completion, it is obvious that the larva does not stand in the same relation to the Star-fish, as the Hydraform polype to its medusa-bud. We shall hereafter see how close is the analogy borne by this temporary apparatus, to the first product of embryonic development in the higher animals.* 699. For our knowledge of the history of embryonic development Fig. 276. is covered with cilia. The or- ganisation of this larva seems completed, and its movements through the water are very active, before the mass at its anterior extremity presents any- thing of the aspect of the Star- fish ; in this respect corre- sponding with the movements of the ‘ pluteus ’ of the Echi- nida. The temporary mouth of the larva does not remain as the permanent mouth of the Star-fish ; for it is on what is to become the dorsal side of the body ; and the true mouth is subsequently formed by the thinning-away of the integu- ment (which has completely enclosed the yolk-mass) on the ventral surface. The young ventral surface. The young Star-fish is separated from the careous consolidation of its in- * See the Observations of Koren and Daniellsen (of Bergen) in the “ Zoologiske Bidrag." Bergen, 1047 (translated in the “Ann. des Sci. Nat.,’’ Juin, 1847); and the Memoir of REPRODUCTION IN ANIMALS. 937 in the orders Ophiurida and Echinida, we are principally indebted to the researches of Prof. Muller.* The embryo issues forth from the ovum as soon as it has attained, by the repeated segmentation of the yolk, the condition of the £ mulberry- mass and the superficial Fig. 277. cells of this are covered with cilia, by whose agency it swims freely through the water. So rapid are the early processes of develop- ment, that no more than from twelve to twenty-four hours intervene between fe- cundation and the emersion of the embryo ; the division into two, four, or even eight segments taking place within three hours after impregna- tion. Within a few hours after its emersion, the embryo changes from the spherical into a sub-pyramidal form with a flattened base ; and in the centre of this base is a depression, which gradu- ally deepens, so as to form a mouth that communicates with a cavity in the interior of the mass, which is sur- rounded by a portion of the yolk-mass that has returned to the liquid granular state. The pyramid is at first tri- angular, but it afterwards becomes quadrangular; and the angles are greatly pro- longed round the mouth (or base), whilst the apes of the pyramid is sometimes greatly prolonged in the op- posite direction, but is some- times rounded off into a kind of dome (Fig. 277, a). All parts of this curious body, and especially its most projecting por- tions, are strengthened by a frame-work of thread-like calcareous rods (e). In this condition, the embryo swims freely through the water, being Embryonic development of Echinus; — a, Pluteus-laroa at the time of the first appearance of the disc; a, mouth in the midst of the four-pronged proboscis ; b, granular yolk-mass ; c , echinoid disc; d,d,d,d, four arms of the pluteus-body ; e, calcareous framework; f, ciliated lobes ; g, g,g> g, ciliated pro- cesses of the proboscis: — b, disc, with the first indication of the cirrhi : — c, disc, with the origin of the spines between the cirrhi : — d, more advanced disc, with the cirrhi and spines pro- jecting considerably from the surface. (N.B. In Figs, b, c, and d, the pluteus is not represented, its parts having undergone no change, save in becoming relatively smaller.) Prof. Muller ‘ Uber die Larven uud die Metamorphose der Echinodermen,’ in “ Abhaldlungen der Koniglichen Akademie der Wissenschaften zu Berlin,” 1848. * ‘ Ueber die Larven und die Metamorphose der Ophiuren und Seeigel,’ in “Abhaldlungen der Koniglichen Akademie der Wissenschaften zu Berlin,” 1846. See also, for the earlier stages, a Memoir by M. Derbes, in the “ Ann. des Sci. Nat.,” Aout, 1847. 938 SPECIAL AND COMPARATIVE PHYSIOLOGY. propelled by the action of cilia, which clothe the four angles of the pyramid and its projecting arms, and which are sometimes thickly set upon two or four projecting lobes (/) ; and it has received the designa- tion of Pluteus. The ‘ pluteus ’ of the Ophiura and that of the Echin us at first differ very little in their general form and structure ; but the ‘ plutei ’ of different species vary considerably in the number of their arms, some having as many as thirteen. The mouth is usually surrounded by a sort of proboscis, the angles of which are prolonged into four slen- der processes ( which they attacli themselves by their gelatinous envelope. The Gobius niger constructs a regular nest among sea-weeds, in which it deposits its ova ; and these it watches with maternal care until they are hatched. A very elaborate nest is constructed by the male of the fresh-water Stickle- back; and it is carefully watched by him during the period of embryonic development, in order to guard the eggs from the voracity of the female?, who are continually endeavouring to make a prey of its contents. 725. In Rept ties, we find the generative apparatus exhibiting a manifest * In these animals, the temporary branchial filaments are extremely long, and appear to serve as special absorbing organs. In Dr. Davy’s opinion, their function is particularly snl>- servient to the development of the electrical apparatus. See his “ Anatomical and Physiolo- gical Researches,” vol. i. p. G7. REPRODUCTION IN ANIMALS. 965 advance, in degree of organisation, from the highest form in which it exists in Fishes (§ 324 u) ■ and the provision for the fertilization of the ova is such as more effectually to secure it. In the Batracliia, as in most Fishes, the eggs are fertilized after they are extruded from the genital orifice of the female : but that of the male is in close contact with it at this pei'iod, so that the seminal fluid is most advantageously applied to them. In the higher Reptiles, however, there is an actual introduction of the male fluid into the oviducts of the female ; this is accomplished in Ophiclia by a temporary eversion of the orifices of the spermatic ducts ; but in the higher Sauria and Chelonia, there is a regular intromittent organ, which is, however, sometimes completely bifid, and always grooved. The groove is completed, by the swelling of the organ at the time of ex- citement, into a canal, which receives the spermatic ducts. The ovaria are constructed upon the same general plan as in the higher Fishes ; but are usually more compact in their form. The ova escaping from them are received into the funnel of the oviducts, and are conveyed by them into the cloaca , which receives both their orifices, as well as those of the ureters and rectum. Of the Batracliia, one species, the Land Salamander, is viviparous ; and each oviduct is dilated, near its termination, into a sort of uterine cavity, in which the young are developed. Among the higher Reptiles of this country, moreover, the Viper, Slow-worm, and common Lizard, frequently produce their young alive ; the egg being- retained until the embryo has attained its full development, and the enveloping membrane being usually burst (as Mr. Bell* considers) in the act of parturition. With these animals, as indeed with all Reptiles, a high degree of heat is necessary for the maturation of the eggs ; a portion of this heat seems to be developed within the body of the parent herself, by some peculiar excitement of the calorifying power which these animals possess (§ 620) ; but the gravid female may often be seen basking in the sun, acquiring from its rays the caloric which she is not herself capable of generating. In the oviparous Reptiles, it is not common to find the parent affording any protection to the eggs when once deposited, or to the young produced from them. The Pipa Americana (Surinam Toad), how- ever, is an exception ; in this species, the female has on her back a num- ber of cells hollowed iu the integument ; in these the eggs are placed by the male, when she has deposited them ; and here they remain for about 80 days, during which time the embryoes undergo their metamorphosis, so as to come forth as perfect Frogs. It has been satisfactorily determined that the gelatinous mass in which the ova of Frogs are imbedded, serves for the first nutriment of the young Tadpoles on their emergence from the egg ; and the same is probably the case in many other instances. 726. In Birds, the generative apparatus does not manifest any great advance beyond the form it presents in Reptiles (§ 325 l) ; but there is always a provision for the fertilization of the ova, by sexual congress, soon after they have been discharged from the ovaria. The ovarium, which is usually developed on one side only, consists of a thin and condensed layer of cellular parenchyma, constituting a stroma, in which the ovules are evolved ; its size is much smaller in proportion to the whole body, except at the period of the development of the ova, than in most of the History of British Reptiles,” pp. 35, 63. 966 SPECIAL AND COMPARATIVE PHYSIOLOGY. classes we have yet considered. It is covered by an envelope of its own, and then by the peritoneum ; its surface is usually smootli ; but, when the ova are being developed, their size causes them to project more or less, so as to form a number of convexities upon it (Fig. 188). As the ovisacs enlarge, they gradually project from the ovarium, carrying before them its envelopes ; and at last, the ovarium presents almost the appearance of a bunch of grapes, — the ovisacs hanging from it only by a short peduncle, which contains vessels. Each of these projecting bodies, therefore, con- tains an ovum, which is enveloped in its ovisac ; around this is a thin layer of the stroma ; this, again, is enclosed in the proper envelope of the ovarium, consisting of two layers, of which the inner one is vascular ; and the peritoneum envelopes the whole. At last the ovum escapes by the rupture of its coverings ; and these remain as a sort of cup, which is termed the calyx, and which subsequently disappears by absorption. — The oviduct commences by a wide slit, which receives the ovum at its escape from the ovary ; at its lower part it dilates into a thick glandular sac, which secretes the shell. The ovum, during its passage along the oviduct, receives, as in most of the higher animals, an additional layer of albumen : and this is surrounded by a membrane, which has been commonly re- garded as composed simply of the hardened external layer of the albumen, but which has in reality a regular fibrous texture (§ 160) : and is properly analogous to the chorion of Mammalia (§§ 325 l, 727). This membrane is composed of two layers, which separate at the blunt end of the egg, to include a bubble of air ; its outer surface is somewhat shaggy, and sends little processes into the shell, which is formed by the calcification of a similar membrane. The outer paid of the albumen is extremely watery ; but the inner part is viscid, and clings closely to the yolk. The yolk-bag is held in its place within the egg, by two twisted cords, termed chalazce, which appear composed of coagulated albumen ; these arise, by a funnel- shaped expansion, from the sides of the yolk-bag opposite the poles of the egg, towards which they pass. In this manner, the yolk-bag is prevented from quitting the central part of the egg ; but it is permitted to rise towards the side ; and, as it is rather lighter than the albumen, it will always approach the shell, whichever side the egg may be resting on. Further, it is permitted to turn on its own axis : and in this manner, the cicatricula or germ-spot (§ 728), being specifically lighter than the res':, will always be turned uppermost. By this very simple contrivance, two necessary conditions are provided for ; the embryo is placed, during its development, in the most advantageous position for receiving the influence of the maternal heat • and is also brought into the nearest possible relation with the external air, by which the aeration of its fluids is effected. The former condition is indispensable throughout the class, except in a few instances, in which the solar heat is sufficiently intense, or in which arti- ficial heat is designedly substituted (§ 625). The latter, here as elsewhere, is absolutely necessary to the development of the embryo : and the shell, being porous, does not interpose any obstacle to the aeration of the fluids it contains.* Of the various provisions so remarkable in this class, as in * An attempt was made some time since, to show that no respiratory process takes place in the egg through the medium of the shell-membrane ; and that the development of the embryo takes place with equal perfection, when the air is completely excluded from the interior. The result, however, was completely fallacious, in consequence of the imperlcct REPRODUCTION IN ANIMALS. 967 that of Insects, for the protection and maintenance of the young, some notice has already been taken (§ 325 l). In general it is to be remarked, that the attention which the young receive after they break the shell, is prolonged in proportion as the plumage, and especially the feathers of flight, are to be of a more perfect character ; so that, in this comparatively trifling variation, we have an illustration of the general law, — that the higher the grade of development which the being is ultimately to attain, the more is it assisted in the early stages by its parent. 727. This law is remarkably exemplified in the class Mammalia, which unquestionably ranks at the head of the Animal kingdom, in respect to degree of intelligence and general elevation of structure. It is the uni- versal and most prominent characteristic of this class, that the young are retained within the body of the parent, until they have made considera- ble progress in their development ; and that they are afterwards nourished by a secretion from the blood of the female parent. In regard to the degree of development attained by the young, however, at the period of parturition, there is much variation in the different orders ; and the lower tribes. of the class are so truly intermediate, as to the mode in which this part of the generative function is performed, between the oviparous Birds and Reptiles, and the true viviparous Mammalia, as to be appropriately separated as a distinct sub-class, that of Ovo-vimparous, or more properly Implacental, Mammalia (§ 326). And, as already pointed out, this in- ferior grade manifests itself also in the general development of these animals ; especially in their osteology, which presents many oviparous characters, and in the low type of conformation of their brain. At the other extremity in the scale of development, we find Man ; in whom the dependence of the offspring on the parent is extended through a much longer period than in any other animal ; and this has an obvious relation to the high amount of intelligence which he is ultimately to attain.- — The transition from the generative apparatus of Birds, to that of the lowest Mammalia, is by no means abrupt ; and the passage from the latter to the highest forms which the class presents, is very gradual (§ 326 s). — The ovum itself corresponds, in all essential characters, with that of Ovi- parous animals ; — chiefly differing in its minute size, in proportion to that of the ovisac (§ 683). Its external envelope is thicker than the ordinary yolk-bag of oviparous animals ; and has received the name of Zona pel- lucida, from its appearing under the microscope (when the ovum is flattened by compression) as a broad transparent ring* (Fig. 261 m v). In the immature ovisac, the space between its inner layer and the ovum is for the most part filled up with cells ; these, however, gradually dissolve away, especially on the side nearest the surface of the ovary, whilst an albuminous fluid is effused from the deeper part of the ovisac, which pushes before it the residual layer that immediately surrounds the ovum, forming the discus proligerus, and thus carries it against the opposite wall. At this period, the vascular tunic enveloping the ovisac, which is nature of the means of exclusion employed ; and the very accurate experiments of Schwann leave no doubt, that the access of oxygen is as necessary to the embryo, in all but the very earliest period of development, as it is to the adult. See “Brit, and For. Med. Rev.” vol. x. p. 229. * This Zona pellucida has been described by some writers under the name of chorion ; but the true chorion is afterwards formed around it. 968 SPECIAL AND COMPARATIVE PHYSIOLOGY. derived (as in Birds) from the parenchyma of the ovary, is acquiring great thickness and consistency ; and the two membranes together form the structure known as the Graafian Follicle, which was formerly regarded as peculiar to the Mammalia, but which is really analogous to the inner part of the calyx of Birds and other Ovipara. When the ovum has at- tained its full development, it is carried to the side of the Graafian follicle nearest the surface of the ovary, by the effusion of fluid on the side more deeply imbedded ; and there it remains, until the rupture of the envelopes allows of its exit from the cavity. This rapture is preceded bv a gradual thinning of these membranes at that point ; but on the opposite side, the outer or vascular tunic becomes much thickened ; and a fleshy substance of a reddish-yellow colour (entirely composed of cells) sprouts from the interior of the ovisac, which it gradually fills after the discharge of the ovum, forming the corpus luteum . — When set free from the ovarium, the ovum is received into the Fallopian tube ; and its fertilization appeal's to take place almost immediately on its entrance into that canal, if it have not been pi-eviously accomplished. During its passage towards the uterus, it acquires an additional envelope, the true chorion, which has subsequently to perform veiy important functions in the nutrition of the embryo. This is at first seen as a layer of cells in contact with the outer surface of the Zona pellucida ; it usually, however, imbibes fluid, which separates it from the Zona pellucida ; and a change takes place in its own structure, by the alteration in the form of its cells, which extend themselves and interlace in various directions, so as to give the fabric a fibrous texture, and to produce asperities on its outer surface. In its situation and mode of production, the chorion is evidently homologous with the membrana testae of Birds ; and the fluid which it absorbs is comparable to the albumen of the egg of the latter. The connection between the two is clearly established by the ovum of the Monotremata, in which there is a distinct albumen around the yolk-bag, contained in a membrane, which, — as these ova are not to be incubated, but are destined to receive their early development within the uterus, — must subsequently become the chorion. In all Mammiferous animals, the oviduct, instead of imme- diately conveying the ovum out of the body, deposits it in the receptacle provided for its further development ; within which it forms a new connection with the parent, and is supplied with nutriment from the fluids of the latter, until it has arrived at a state of completeness usually corresponding with that which the Chick presents, when it emerges from its shelly covering. 728. It now remains for us to consider the peculiarities which dis- tinguish the development of the embryo of the Vertebrata, from the process already described in the other Sub-Kingdoms. These peculiarities chiefly consist in this ; — that the permanent structure is developed from a very limited portion only of the £ nndberry mass ’ (§ 685) : and that the remainder of this, with that extension which it forms over the large mass of ‘ food-yolk ’ that forms the bulk of the vitelline body in Birds and Fishes, and probably in the higher Reptiles, has but a very sub- ordinate and temporary office, — being destined (like the cotyledonous expansion of the higher Plants) solely for the imbibition and assimilation of nutriment, during the early period of evolution. In the ova of Batrachia and Mammalia, there is no ‘ food-yolk ’ in addition to the REPRODUCTION IN ANIMALS. 969 ‘ germ-yolk / and as the entire vitellns consists of the latter, the whole of it is involved in the process of segmentation. This arrangement, which is common to them with Invertebrata generally (save the highest of the Articulated and Molluscous series), appears to be related to the fact, that in neither of these two cases is the development of the embryo carried on far at the expense of the vitellus ; for that of the Frog proceeds no further within the ovum, than to the production of a fish-like larva ; and the continued development of the Mammal is very early provided for by a different arrangement. — The evolution of the fractious of the vitellus (Fig. 286, a) into true cells, by the formation of a cell-wall around them (§ 685), takes place soonest, as well as most completely, at its peripheral portion ; and its cells arrange themselves, in the act of formation, into a kind of membrane lining the yolk-bag, at the same Fig. 286. A B Latter stage in the segmentation of the yolk of the Mammalian Ovum; at a is shown the “ mulberry mass ” formed by the minute subdivision of the vitelline spheres; at b, a further increase has brought its surface into contact with the vitelline membrane, against which the spherules are flattened. time assuming a pentagonal or hexagonal shape from mutual pressure, so as to resemble pavement-epithelium (b). Of the globular masses of the interior, those nearest the surface seem to be developed into true cells, and to increase the thickness of the membrane already formed by the more superficial layer ; but the spherules of the interior appear for the most part to licpiefy again ; so that the embryonic mass is now in the condition of a cellular stratum, known as the ‘ germinal membrane,’ inclosing a liquid yolk,- — a condition which is closely paralleled by the embryo of Zoophytes, previously to the formation of the oral aperture. — In the ova of Fishes and Birds, however, and probably in those of true Reptiles, there is a large food-yolk, in which the segmentation does not take place ; and the mode in which this becomes enclosed by the ‘ ger- minal membrane,’ is by the extension of the peripheral portion of the cicatricula , wdiicli seems to be formed by the segmentation of the ‘ germ- yolk.’ — The ‘ germinal membrane ’ is sometimes termed the blastoderma ; and the new envelope which is formed by it, between the vitellus and its original sac, has been termed by Bischoff the blastodermic vesicle. This vesicle, very soon after its formation, presents at one point an opaque, roundish spot, which is produced by an accumulation of cells and nuclei of less transparency than elsewhere ; within this, which is termed the area germinativa, all the structures of the permanent organism originate. The 970 SPECIAL AND COMPARATIVE PHYSIOLOGY. germinal membrane increases in extent and thickness, by the formation of new cells (whose mode of production has not been clearly made out) ; and it subdivides into two layers, which, although both at first composed of cells, soon present distinctive characters, and are concerned in very different ulterior operations. The outer one of these is commonly known as the serous layer ; but being the one in whose substance the foundation is laid for the vertebral column and the nervous system, it is sometimes called the animal layer. The inner one is usually known as the mucous layer ; and being the one chiefly concerned in the formation of the nutritive apparatus, it is sometimes called the vegetative layer. This division is at first most evident in the neighbourhood of the area germinativa ; but it soon extends from this point, and implicates nearly the whole of the germinal membrane. Fig. 287. 729. The ‘area germinativa’ (Fig. 287 ag ), at its first appearance, has a rounded form ; but it soon loses this, becoming first oval, and then pear-shaped. While this change is taking place in it, there gradually appears in its centre a clear space, termed the area pell ucida ; and this is bounded externally by a more opaque circle (whose opacity is due to the greater accumulation of cells aud nuclei in that part than in the area pellucida), which subsequently be- comes the area vascidosa. In the formation of these two spaces, both the serous and mucous layers of the germinal membrane seem to take their share ; but the foundation of the embryonic structure, known as the primitive trace, is laid in the se- rous lamina only. This consists in a shallow groove, lying between two oval masses (y), known as the laminae dor sales. The form of these changes with that of the area pellucida ; at first they are oval, then pyriform, and at last become of a guitar shape. At the same time, they rise more aud more from the surface of the area pellucida, so as to form two ridges of higher elevation, with a deeper groove between them ; aud the summits of these ridges tend to approach each other, and gradually unite, so as to convert the groove into a tube. At the same time, the anterior portion of the groove dilates into three recesses or vesicles, which indicate the position of the three principal divisions of the Encephalon, afterwards to be developed as the prosencephalon, the mesencephalon, and the epence- phalon (Fig. 290 b, x, y, z). The most internal parts of these lamime, bounding the bottom and sides of the groove, appear to furnish the rudiments of the nervous centres which this cranio-vertebral canal is to contain ; wdiilst the outer parts are developed into the rudiments of the vertebral column and cranium. Even before the laminse dorsales have closed over the primitive groove, a few square-shaped, at first indistinct, plates, which are the rudiments of vertebne (Fig. 288, a, q, q), begin to appear at about the middle of each. The position of the bodies of the The germ, and surrounding parts, from a more advanced Uterine Ovum ; b, blastoderma, or germi- nal membrane ; ag, area germinativa; c, cephalic extremity of the germ ; v, first indications of verte- brae ; q, caudal extremity. REPRODUCTION IN ANIMALS. 971 vertebrae is indicated at this period, by a distinct cylindrical rod of nucleated cells, termed the chorda dorsalis ; and this retains its embryonic type in the Myxinoid Fishes (§ 322). While this is going on, an accu- mulation of cells takes place between the two lamina; of the germinal membrane at the ‘ area vasculosa ;’ and these cells speedily form thern- Fm. 288. A B Ovum of Coregonus pul a a , eleven days after fecundation ; a, as seen in front through the transparent ovum; b, as seen sideways; — a, shell-membrane; b, yolk; e e, oil globules; f albumen; g q, vitelline membrane; h, yolk- vesicle; k, trunk of the embryo; /, tail ; o o, optic lobes ; p , chorda dorsalis ; q q, vertebral divisions ; u, curve of the trunk. selves into a distinct layer, the vascular lamina, in which the first blood- vessels of the embryo are developed, as will be presently described (§ 730). From the dorsal laminae on either side, a prolongation passes outwards and then downwards, forming what is known as the ventral lamina ; in this are developed the ribs and the tranverse processes of the vertebrae ; and the two have the same tendency to meet on the median line, and thus to close-in the abdominal cavity, which the dorsal laminae have to inclose the spinal cord. At the same time, the layers of the ‘ germinal membrane,’ which lie beyond the extremities of the embryo, are folded in, so as to make a depression on the yolk ; and their folded margins gradually approach one another under the abdomen. The first rudiment of the intestinal canal presents itself as a channel along the under surface of the embryonic mass, formed by the rising-up of the inner layer of the germinal membrane into a ridge on either side. The two ridges gradually arch over and meet, so as to form a tube, which is thus (so to speak) pinched off the general vitelline sac; but it remains in connection with this by means of an unclosed portion, which constitutes the ‘ vitelline duct.’ In oviparous animals generally, the yolk-bag, as it is emptied of its contents, is gradually drawn into the abdominal cavity ; but in Mammalia, and also in the higher Cartilaginous Fishes, it is cut off from the intestine by the obliteration of the vitelline duct, and the complete closure of the abdominal parietes around the peduncle. The minute yolk-hag of Mammalia is known under the name of the ‘ um- bilical vesicle.’ 730. Whilst these new structures are being produced, a very remarkable change is taking place in that part of the serous lamina which surrounds 972 SPECIAL AND COMPARATIVE PHYSIOLOGY. Fig. 289. Diagram of Ovum at the commence- ment of the formation of the Amnion; — a, chorion; b, yolk-sac; c, embryo; d f and e, folds of the seroirs layer rising up to form the amnion. the ‘ area pellucida. This rises up on either side in two folds ; and these gradually approach one another, at last meeting in the space between the general envelope and the embryo, and thus forming an additional invest- ment to the latter. As each fold contains two layers of membrane, a double envelope is thus formed • of this, the outer lamina adheres to the general envelope ; whilst the inner remains as a distinct sac, to which the name of Amnion is given. (See Figs. 289 and 292.) This takes place during the third day in the Chick ; and at about the same period, a very important provision for the future support of the embryo begins to be made, by the deve- lopment of blood-vessels and the formation ol blood. Hitherto, the embryonic struc- ture has been nourished by direct absorp- tion of the alimentary materials supplied to it by the yolk ; in the same manner as the simplest Cellular Plant is developed at the expense of the carbonic acid, moisture, &c., which it obtains for itself from the sur- rounding elements. But its increasing size, and the necessity for a more free com- munication between its parts than any structure consisting of cells alone can permit, call for the development of vessels, through which the nutri- tious fluid may be conveyed. These vessels are first seen in that part of the Vascular lamina of the germinal membrane, which immediately sur- rounds the embryo ; and they form a network, bounded by a circular channel, which is known under the name of the Vascular A rea (§ 484, Fig. 214). This gradually extends itself, until the vessels spread over the whole of the membrane containing the yolk. The first blood-discs appear to be formed from the nuclei of the cells, whose cavities have become con- tinuous with each other to form the vessels (§ 175) ; and from these, the subsequent blood-discs of the first series are probably generated. This network of blood-vessels serves the purpose of absorbing the nutritious matter of the Yolk, and of conveying it towards the embryonic structures, which are now in process of rapid development. The first movement of the fluid is towards the embryo ; and this can be witnessed before any distinct heart is evolved. The same process of absorption from the yolk, and of conversion into blood, probably continues as long as there is any alimentary material left in the sac. The mode in which the heart is formed, and the subsequent phases of its development, as well as that of the principal blood-vessels, have been already described (§§ 485-489) ; it is ouly necessary here to add, that the mass of cells in which it originates may be considered as a thickened portion of the vascular layer. The trunks which connect the circulating system of the embiyo with that of the ‘vascular area,’ are called Omphalo-Mesenteric, Meseraic, or Vitelline vessels. It was formerly believed, that the nutrient matter of the yolk passes directly through the vitelline duct, into the (future) digestive cavity of the embryo, and is from it absorbed into its structure ; but there can now be little doubt, that the vitelline vessels are the real agents of its absorption, and that they convey it to the tissues in process of formation. REPRODUCTION IN ANIMALS. 973 They do, in fact, correspond to the Mesenteric veins of Invertebrated ani- mals, which are the sole agents in the absorption of nutriment from their digestive cavity (§ 426) ; and the blastodermic vesicle may be regarded as the temporary stomach of the embryo, — remaining as the permanent stomach in the Radiated tribes. Previously to the ninth day of incubation (in the Fowl’s egg), a series of folds are formed by the lining membrane of the vesicle, which project into its cavity ; these become gradually deeper and more crowded, as the bag diminishes in size by the absorption of its contents. The vitelline vessels that ramify upon the vesicle, send into these folds (or valvulse conniventes) a series of inosculating loops, which immensely increase the extent of this absorbing apparatus. But these minute vessels are not in immediate contact with the yolk ; for there in- tervenes between them a layer of nucleated cells, which is easily washed away. It was from the colour of these, communicated to the vessels be- neath, that Haller termed the latter vasa lutea ; when the cellular layer is removed, the vessels present their usual aspect. There seems good reason to believe that these cells are the real agents in the process of absorbing and assimilating the nutritive matter of the yolk ; and that they deliver this up to the vessels, by themselves undergoing rupture or dissolution, being replaced by newly-formed layers. 731. The development of the embryo of Fishes takes place without any further change of plan ; the nutrient material supplied by the yolk being gradually taken into the circulation, through the medium of the germinal membrane, and being applied to the nutrition of the various tissues and Fiu. 200. A B Ovum of Corerronus palcea, sixteen clays after impregnation; — A, seen in front; — b, seen sideways;— «, shell-membrane; 0, yolk; c , crystalline lens; (l, choroidal system, e e, oil- globules; /, albumen; g, vitelline membrane; h, yolk-vesicle; i, keel of the encephalon; A;, ear; /.tail; q, vertebral divisions ; s, cephalic bend ; /, nuchal bend; v, epidermoidal stra- tum ; .r, prosencephalon; y, mesencephalon ; z, tpencephalon. organs which successively make their appearance. Up to the time of the emersion of the foetal fish from the egg, the aeration of the blood is pro- vided for in no other way, than by its distribution upon the surface of tire yolk-bag ; and even after it has come forth, whilst the yolk-bag is still far from being emptied, and hangs down from its abdomen, and the gills have 974 SPECIAL AND COMPARATIVE PHYSIOLOGY. not yet come into play, the dispersion of the blood over the vascular area still serves to aerate it, as well as to enable it to absorb the nutritive con- tents yet remaining to be appropriated. In most of the Osseous Fishes, the blood-vessels which ramify upon the yolk-bag may be regarded as part of the portal system ; for they branch off from the intestinal veins, and return into the vena cava. As the yolk diminishes in size, and the permanent respiration is established, the blood is transmitted more directly through the liver to the heart ; by the enlargement of vessels that were at first capillary, into regular trunks. In the higher Cartilaginous fishes, however, the blood sent to the respiratory surface is derived from an arterial trunk, as in all the higher Vertebrata. — The general course of the develop- ment of the principal organs in Fish, may be understood from the aecom- Fig. 291 . Embryo of Coregonus palaea, thirty three days after fecundation ; — b. yolk; e, oil- globules ; m, pectoral fin; n, ear; p, chorda dorsalis; r, sheath of the dorsal cord: x, prosencephalon: y, mesencephalon ; z , epencephalon : 1, pineal gland; 2, buccal intestine; 3, ureter: 4, anus; 5, ventral intestine; 6, kidneys; 7, liver; 8, mouth; n and in, second and third branchial fissures. panying figures ; the history of the evolution of each principal organ has been already given under its own head ; with the exception of that of the Nervous system, which will be found in its pi'oper place (§§ 782-786). 732. In all the higher Vertebrata, we find that at an early period of embryonic development, provision is made for a more effectual per- formance of the respiratory function. This consists in the development of a peculiar membrane, the Allantois ; which sprouts forth from the caudal extremity of the embryo, at first as a little mass of cells, whose interior is soon observed to be hollow ; and which continues to enlarge, until it surrounds the entire embryo (Figs. 292-295). In Reptiles and Birds, it extends itself around the yolk-sac, intervening between it and the mem- brane of the shell ; and the porosity of the latter allows the air to have free access to the blood, which is copiously distributed upon it. It is a remarkable circumstance that this membrane should not merely serve REPRODUCTION IN ANIMALS. 975 as a respiratory organ, but that it should also stand to the Corpora Wolffi- ana, or temporary kidneys, in the light of a receptacle for their secretion, Fig. 292.* Fig. 293.fi .* Diagram of Human Ovum , with the Amnion in process of formation: — a , the chorion; b , the yolk-bag, surrounded by the serous and vascular laminae ; c, the embryo ; d, e, and /, external and internal folds of the serous layer, forming the amnion; g , incipient allantois. t Diagram representing a Human Ovum in second month : — a 1, smooth portion of chorion; a 2, villous portion of chorion; k, k, elongated villi, beginning to collect into Placenta; b , yolk-sac or umbilical vesicle ; c, embryo; /, amnion (inner layer) ; g , allantois ; h, outer layer of amnion, coalescing with chorion. or urinary bladder. The greater portion of this allantois is afterwards cast off by the contraction of its pedicle ; but a part of its root is usually retained, to be converted into the permanent urinary bladder. — In Mammalia, however, the chief office of the allantois is to convey the vessels of the embryo to the internal surface of the Chorion (Fig. 294) ; the tufts of which are rendered vascular by the penetration of these vessels into them (Fig. 295) ; and in this mode the embryo is nourished for a time, the absorbent tufts drawing their supplies by endosmose from the very vascular lining of the uterus. 733. Whilst the foregoing changes have been taking place in the ovum, the mucous membrane lining the uterus has undergone an im- portant change ; for it has become extremely thick and vascular, and its follicles are greatly enlarged, and appear to secrete a nutritive material from the blood. When the tufts of the chorion come into apposition with it, they insinuate themselves into these glandular follicles ; and ab- sorb the material which has been elaborated by them for the support of the embryo. Each villus of the chorion contains a capillary loop ; this is inclosed in a layer of cells, and this again in a lamina of basement- membrane ; the whole constituting the foetal tuft. On the other hand, this tuft is in apposition with a layer of cells covering the orifices of the uterine glandulae, and these lie upon the basement-membrane of its mucous lining ; forming the maternal portion of the apparatus. Such is the highest form which it assumes in the Implacental Mammalia (§ 326) ; the ovum being ejected from the uterus, before any further changes have taken place. In the higher orders, however, we find such a concentration of this arrangement in one or more spots, as is adapted to bring the foetal and maternal blood-vessels into closer approximation. This is accom- plished, in the Ruminants, by an enlargement and multiplication of the 976 SPECIAL AND COMPARATIVE PHYSIOLOGY. vascular tufts at several points of the surface of the chorion, so as to form what are termed the foetal cotyledons ; and these come into relation with a corresponding series of maternal cotyledons , formed by the enlargement Fig. 294.* Fig. 295. + * Diagram of a Human Ovum in a more advanced stage ; — a n\ the allantois developing itself around the embryo ; cc', the cephalic and caudal prolongations of the amniotic sac, nearly united; e, serous layer of the germinal membrane; e\ vertebral structure of the foetus; ?, mucous layer of the germinal membrane, partly converted into the rudimentary intestine i', and partly forming the umbilical vesicle o ; m, portion of the serous layer of the germinal membrane reflected to form the amnion ; n, rudimentary portion of the intestine, representing the rectum ; p, pedicle of the umbilical vesicle ; r, pedicle of the allantois; t y limit of the area vasculosa ; v, vitelline membrane. f Diagram of an Ovum still more advanced, the two prolongations of the allantois having now met and united at a , and the amniotic sac being now complete ; the external layer of the germinal membrane, e, is now disappearing ; vascular villosities, 7 7 , are being developed from the allan- tois ; and the intestine i' is forming its first loop.— The other references as in the preceding figure. and multiplication of the blood-vessels of the lining membrane of the uterus. No inosculation takes place between these two sets of vessels; but the loopings of each are received into the interspaces between those of the other ; and each tuft is covered with its own layer of base- ment-membrane and of cells. In Man, however, and in the Quadru- mana, Carnivora, and other orders, a still more concentrated arrangement is found. The vascular tufts are enormously developed from one por- tion only of the chorion ; and they dip down, as it were, into a cavity, whose inner wall is formed by an extension of the lining membrane of the large veins or sinuses of the uterus. The former, which are the ultimate ramifications of the umbilical arteries, constitute the foetal portion of the Placenta (as this compound organ is termed) ; whilst the latter, which constitutes its maternal portion, is irregularly subdivided by partitions, formed by reflexions of its wall over the fcetal tufts ; and each of these reflexions carries with it an extension of the cel- lular layer that covers the mucous surface, so that the extremity of every loop of foetal vessels forming a placental villus is invested first by a foetal, and then by a maternal, layer of cells, a space being usually observed to intervene between them (Fig. 296). The blood is conveyed into the pla- REPRODUCTION IN ANIMALS. 977 Extremity of a Placental villus : — a, external membrane of the villus, continuous with the lining: membrane of the vascular system of the mother ; I), external cells of the villus, belong- ing to the placental decidua ; c, c, germinal centres of the external cells ; d, the space between the maternal cental cavity by the 1 curling arteries ’ of the uterus ] and after it has been brought into relation with that of the foetus, it is returned to the uterus by large apertures communicating with its sinuses. Thus the placental tufts, in which the foetal blood is circulating, are bathed (as it were) in the maternal blood, just as the branchiae of aquatic animals are bathed in the medium they inhabit ; and an interchange can freely take place by endosmose between the two fluids, al- though no more direct communication exists between the two systems of vessels. This inter- change is subservient to two most important purposes. The foetal tufts draw from the ma- ternal blood the materials which are required for the nutrition of the embryo, these materials having been first elaborated by the two sets of intervening cells ; and in this character, the foetal tufts resemble the villi of the intestinal surface, which dip down into the fluids of the alimentary canal, and absorb the nutritive ma- terials which they furnish. But the placenta serves also as the instrument for the depura- tion of the blood of the foetus, especially by the respiratory process : for the aerated blood of the tinuous with the external membrane mother will necessarily impart oxygen to that the villus, belonging to the chorion . of the foetus, and will carry off its carbonic acid ; o, the loop of umbilical vessels, so that in this respect the foetal tufts may be likened functionally, as well as structurally, to the gills of aquatic animals. But it is probable that during the early stages of embryonic life, before the proper depurating organs of the foetus have come into full activity, other excretory matter may be got rid of through the same channel ; and there even seems a strong probability, that the foetal blood may so react upon the maternal, as to communicate to it some of the properties which it lias derived from its male parent. Such, at least, seems the most feasible explanation of tire fact, which has now been observed to take place in a great number of instances, that if a mare, cow, bitch, or other mammiferous female (not even excepting the human species), be impregnated in the first instance by a male possessing some marked peculiarity, and on subsequent occasions by a different male, the offspring of the second parentage will bear in greater or less degree the characters of the first ; these peculia- rities becoming less and less obvious on each successive occasion.* 734. Notwithstanding the peculiarity of the provision which is made in Mammalia for the nutrition of the embryo, the earlier part of its course of development presents little to distinguish it from that of the inferior classes. It may, in fact, be said, that all the most important parts of the apparatus of organic life, and even the fundamental portions of that of animal life, are developed upon the same general plan in all Vertebrata ; and that the special peculiarities of each class only gradually evolve them- selves. The conditions under which the alimentary canal (§§ 409, 729), the heart and blood-vessels (§§ 485, 730), the liver (§ 589), the corpora * See the very ingenious papers on this subject by Dr. Harvey, in the “ Edinb. Monthly Journal” for 1849 and 1851 ; and his pamphlet “ On a remarkable effect of Cross-breeding,” Edinburgh, 1851. 3 R 978 SPECIAL AND COMPARATIVE PHYSIOLOGY. wolffiana (§ 599), the vertebral column (§ 729), the nervous centres (§ 782), and the eye and ear (§§ 826, 830), first present themselves, exhibit no essential differences in the FlG ' Fish, Reptile, Bird, or Mam- Emhryo of Dog , twenty-five days after the last copulation, enlarged three times ; — a , prosencephalon ; 6, deutencephalon ; c, mesencephalon ; d, epencephalon ; e, eye ; /, auditory vesicle ; g, h, i, visceral arches; k, right auricle; l, right ventricle ; m, left ventricle; n, bulbus aorticus ; o, pericardium; p, liver; q, loop of intestine, communicating through r, the ductus om- phalo-mesentericus, with s, s, the umbilical vesicle ; f, allantois; u, amnion ; v, anterior extremity ; x, posterior extremity ; z, nose. mal; and the first consider- able divergence from the common type is shown in that alteration in the struc- ture of the Heart, and in the arrangement of the great vessels, which distinguishes the air-breathing Vertebrata from the Fish ; whilst in the latter, the original conform- ation is retained, and is made subservient to the distribution of the blood in the branchial filaments. So, again, at a later period, the Corpora Wolffiana give place in the air-breathing Verte- brata to the permanent Kid- neys, but remain persistent in the Fish. In like man- Fig. 298. ner, the course of develop- ment of the vertebral ele- The same Embryo, straightened, and seen in front; — a, u, nostrils ; 6, b, eyes ; c, c , first visceral arch, forming the lower jaw ; d, d , second visceral arches ; e, right auricle ; /, left auricle ; g, right ventricle ; h, left ventricle ; i, aortic bulb ; k , k , liver, between the two lobes of which is seen the divided orifice of the omphalo-mesenteric vein ; /, stomach ; m, intestine, communi- cating with the umbilical vesicle n, n; o, o, corpora Wolffiana ; p, allantois; q , q, anterior extremities; r, r, posterior ex- tremities. however, it is retained in that cavity, ments very early shows those peculiarities, which are cha- racteristic of the type of the Fish ; whilst it is not until a subsequent period, that in these or any other parts (save in the provisions already de- scribed for the nutrition ofthe foetus and the aeration of its blood) is there anything that can be accounted distinctive of either of the higher classes of Vertebrata. — The accom- panying figures exhibit an early phase of the evolution of the Mammalian embryo, in which most of the organs are in a veiy rudimentary condition. It is in a state not much further advanced than this, that the embryo of Monotremata and Mar- supialia is expelled from the uterus ; in the higher order's, . and continues to derive its support REPRODUCTION IN ANIMALS. 979 through the placenta, until nearly all the principal organs in the body, save the generative, are prepared for the active performance of their respective functions. Still there are considerable differences in the de- gree of indejiendence manifested by the young Mammalia of different orders at the epoch of their birth ; and to the most important of these allusion has been already made (§ 326 bb). 735. Although nothing is certainly known in regard to the causes which influence the Sex of the offspring, some facts which have been ob- served on this curious subject are worthy of being here adverted to, with the purpose of stimulating further enquiry. It is stated by Mr. Knight, that several kinds of monoecious Plants can be made to produce solely- male or solely-female flowers, by regulating the quantity of light and heat under which they are grown. If the heat be excessive, compared with the quantity of light which the plant receives, male flowers only appear ; — but if light be in excess, female flowers alone wall be produced. In this case it is evident that both sets of sexual organs exist in a rudimentary state ; and that one or the other kind is developed according to external circumstances. To the same industrious experimenter we ow T e some inter- esting facts in regard to Animals, to which this explanation is not appli- cable. He remarks that, in flocks or herds of domesticated quadrupeds, it is no uncommon thing to meet with females, whose offspring is almost invariably of the same sex, although it may have resulted from intercourse with several different males ; whilst, on the other hand, he has never met with males that exhibited any such uniformity in the sex of their offspring with different females. Hence he concludes that the female parent exer- cises the chief influence in determining the sex. An experiment upon the fecundation of Birds, which he states to have been frequently repeated, gave the following curious result. When the female was kept without intercourse with the male, up to nearly the time of laying, so that the eggs had advanced very far in their development at the time of fertiliza- tion, the proportion of males among the offspring was very large, — com- monly about six out of seven.* Some observations which have been made on the Human species tend to show, that there is a probability of a ma- jority in the number of one sex over the other, according to the relative ages of the parents ; — the male children predominating in those families in which the father’s age is considerably above the mother’s, and vice versa. + 736. In no tribe of animals save Mammalia, do we find that the young- are nourished exclusively, for some time after their birth, by a fluid secreted from the blood of the maternal parent; the nearest approach to this is presented by the Pigeon (§ 325 l). Of the variety in the condition which the Mammary glands present, in the different orders of the class, some account has been already given (§§ 326 aa, 578); and it is 'only requisite here to describe the most important characters of the fluid they secrete. The Milk of most Mammalia consists of Water, holding in solu- tion a peculiar albuminous substance termed Caseine, and various Saline ingredients, together with (in most instances) a certain form of Sugar ; and having Oleaginous globules suspended in it. — The most important difference between Caseine and Albumen consists in the fact, that the * “ Selection from Mr. Knight’s Physiological Papers,” pp. 347, 357. + Quetelet “ Sur L’Homme,” tom. i. pp. 52, 53. 3 r 2 980 SPECIAL AND COMPARATIVE PHYSIOLOGY. former is not coagulated by heat, which precipitates the latter ; and that it is coagulated by acetic acid, which has no effect upon the latter. Caseine is very remarkable for the facility with which it may be coagu- lated by the contact of certain animal membranes ; thus a piece of £ ren- net,’ which is nothing else than the dried stomach of a calf, will coagulate the caseine of 1800 times its weight of milk. Further, Caseine appears to surpass albumen in its power of combining with the phosphates of lime and magnesia, and rendering them soluble ; and it seems to be in this mode, that the earthy phosphates, which are so important for the consoli- dation of the bones of the suckling animal, are introduced into its system. — Save in the larger proportion of these substances, the Saline matter of Milk is nearly the same as that of blood. — The Sugar of Milk is peculiar as containing nearly 12 per cent, of water ; so that it may really be consi- dered as a hydrate of sugar. It is nearly identical in its composition with starch ; and it appears to be directly formed at the expense of the farina- ceous elements of the food. Sugar of milk is chiefly remarkable for its proneness to metamorphosis into lactic acid, under the influence of a de- composing animal membrane which acts as a ‘ feiment and it is thought by some, that the agency of such a membrane in occasioning the coagu- lation of caseine, is first exerted in producing lactic acid, which in its turn acts upon the caseine. — The Oleaginous globules, which consist of the substance called butyrine, are surrounded by a thin pellicle, that keeps them from coalescing whilst the milk is at rest ; but when it is agitated, the envelopes of the oil-globules are broken, and they coalesce, so as to form butter. It has been ascertained that butyric acid is one of the pro- ducts of the change in sugar produced by the contact of putrescent animal membranes ; and it can scarcely be doubted that it is directly producible by the transformation of that substance in the living body. — Tlius ordi- nary Milk contains the three classes of organic principles, which form the chief part of the food of animals, namely, the albuminous, the saccharine, and the oleaginous ; together with those mineral compounds, which are required for the development and consolidation of the fabric of the infant. It would appear - , however, that the combination of all these is not neces- sary • but rather has reference to the composition of the food, on whicli the animal is habitually supported. Thus, in the Carnivora, so long as they live upon a purely animal diet, the milk contains no sugar, which principle is altogether wanting in the food of the adult ; but if they be fed upon a mixed diet, sugar soon appears in their milk. Amongst the differ- ent species of Herbivorous animals, the proportion of the several ingre- dients varies considerably ; and it is also liable to variation in the same individual, according to the nature of the food, the amount of exercise taken by the animal, and other circumstances. Thus in the milk of the Cow, Goat, and Sheep, the average proportions of Caseine, Butter, and Sugar are nearly the same one with another, each amounting to from 3 to 5 per cent. In the milk of the Ass and Mare, on the other hand, the proportion of Caseine is under 2 per cent,, the oleaginous constituents are scarcely traceable, whilst the sugar and allied substances rise to nearly 9 per cent. In the Human Female, the saccharine and oleaginous elements are both present in large amount ; whilst the Caseine forms a moderate proportion. — The proportion of the saccharine and oleaginous elements appears to be specially affected by the amount in which these are pre- LAWS OP REPRODUCTION. 981 sent in the food, and by the degree in which the quantity ingested is consumed by the respiratory process. Thus, a low external temperature, and out-door exercise, by increasing the production of carbonic acid from the lungs, occasion a consumption of the oleaginous and saccharine matters which might otherwise pass into the milk, and thus diminish the amount of cream. On the other hand, exercise favours the secretion of caseine ; which would seem to show, that this . ingredient is derived from the disintegration of the azotised tissues. Thus in Switzerland, the cattle which pasture in exposed situations, and which are obliged to use a great deal of muscular exertion, yield a very small quantity of butter, but an unusually large proportion of cheese ; yet the same cattle, when stall-fed, give a large quantity of butter, and very little cheese. 4. On the Laws of the Exercise of the Reproductive Function. 737. When we contemplate the immense number of diversified forms, which the study of the Organised creation brings under our notice, and witness these distinct forms perpetuated, as it would seem, by the process of Reproduction, so as to constitute separate races, the question naturally arises, whether all these had a different origin ; or whether the characters of any of them have been so modified in the course of time, as to lead to the belief in a diversity of origin among those, which were at first really identical. When it can be shown that two races have had a separate origin, they are regarded as of different sjoecies; and, in the absence of proof, this is inferred, when we see some peculiarity of organisation, cha- racteristic of each, so constantly transmitted from parent to offspring, that the one cannot be supposed to have lost, or the other to have acquired it, through any known operation of physical causes. It cannot be regarded as an unimportant question to the Naturalist, to ascertain what these con- stant distinctions are ; whilst it is an investigation of high interest, in a Physiological point of view, to trace the modifying influence of external circumstances upon the structure and functions of living beings, and to enquire how far the results of such influences may be transmitted heredi- tarily, so that the differences produced by them may be perpetuated. Where races which have originally sprung from a common stock present marked differences, they are spoken of as varieties ; and the variety may be transient, from its peculiarity manifesting a tendency to disappear, or permanent, where it continues to be transmitted without change. The uncertainty of the limits of species is daily becoming more and more evident ; and every Naturalist is aware, that a very large number of races are usually considered as having a distinct origin, when they are nothing more than permanent varieties of a common stock. Whilst the exertions of the enterprising discoverer are adding to our already enormous list of species, from the unexplored resources of foreign lands, the skill of the horticulturist and of the breeder is exerted to produce new varieties of species already in our catalogues ; and it has unfortunately too often hap- pened, that a new specific name has been invented for the latter as well as for the former ; and that a mere hybrid or transient variety has thus taken the rank of a species, to the confusion of all true principles of arrange- ment. The philosophic naturalist, on the other hand, aims to reduce the number of species, by investigating the degree of variation which each is 982 SPECIAL AND COMPARATIVE PHYSIOLOGY. liable to undergo, the forms it assumes at different periods of its existence, the permanent characters by which it may he distinguished during its whole life, the habits which are natural to it, the degree in which these may be changed by the influence of circumstances ; and, in fine, he endeavours to become acquainted with the whole natural history of a reputed species, before separating it from another to which it may be closely allied. 738. Many examples maybe given, of the success with which this mode of investigation is now being prosecuted. The belief which is gaining ground, that many diversified forms of the simpler Cryptogamia may arise from similar germs developed under different circumstances, has already been noticed (§ 272) ; and among the higher Plants, the experiments of Mr. Herbert on the primrose, cowslip, oxslip, and polyanthus (which he proves to be all varieties of one species), are sufficient evidence of the im- portant results which would probably accrue from a similar investigation in other quarters. The uncertainty of all principles of arrangement founded upon arbitrary characters, has been demonstrated by the fact recently published,* that the flowers and pseudo-bulbs of three reputed genera of Orchideous plants have been produced by the same individual, t — In Zoology, again, it is only necessary to recal the phenomena which have been arranged under the head of ‘ alternation of generations,’ to see how diverse may be the forms presented by the very same being at differ- ent epochs of its existence. The great influence of external circumstances in modifying the foi'm of shells, has been pointed out by Mr. J. E. Gray ; % who has shown, among other instances, that what have been regarded as six distinct species of Murex are in reality but different states of one ; and Mr. S. Stutchbury has been equally successful in reducing the number of species of Patella, Cyprcea, and Oliva, by attending to the changes of form which each individual undergoes in the progress . of its development. Many instances might be related, in proof of the uncertainty of reputed specific distinctions among higher classes ; thus, Insects have been seen presenting the characters of different species on the two sides of the body ; and it is now certain that an erroneous multiplication of species among Birds, especially in the migrating tribes, has been occasioned by their change of plumage at different seasons. 739. The Naturalist endeavours to simplify the pursuit of his science, by the adoption of easily -recognised external characters, as the basis of his classification of the multitudinous forms which he brings together ; but such can only be safely employed, when indicative of peculiarities in internal structure, which are found to be little subject to variation, and which are not liable to be affected by the influence of physical causes. The colour of flowers, for example, is liable to so much alteration from the influence of soil and climate, that it is seldom regarded as of itself any test of the unity or diversity of species. In certain moths and butterflies, on the other hand, the uniform appearance of particular spots on the wings is held sufficient to constitute a specific character, because it is never * “ Linn. Trans.” vol. xvii. t This fact has also come under the Author’s own notice in the Durdham Down Nursery, near Bristol, two of the genera being the same as in the instance just quoted, but the third a different one, so that four may thus be regarded as of the same species. J “ Philos. Transactions,” 1833. LAWS OF REPRODUCTION. 983 known to vary in those kinds ; and it would probably be found associated, if the examination were pushed far enough, with some unequivocal differ- ences in the configuration of internal organs : in other cases, however, there is considerable tendency to change ; but, as in the colouring of plants, there are usually certain limits within which the varieties of shade are restrained. Sometimes one sex varies extremely little, while the form and colour of the other present much diversity. Amidst all these diffi- culties attending the discrimination of species from structural characters alone, it is not unreasonable to enquire, if there be any other means of effecting the object with greater certainty. This subject has been fully considered by Dr. Prichard in his elaborate work on the Physical History of Man ; all that can be here considered, are the laws according to which the intermixture of species, and the transmission of hereditary or acquired peculiarities from parent to offspring, appear to take place. — The conclusion which has now been attained on the first of these points, and which (if stated in a sufficiently general form) is equally applicable to both the Animal and Vegetable kingdoms, may be regarded as one of the most valuable tests which the naturalist possesses. In Plants, the stigma of the flower of one species may be fertilized with the pollen of an allied species ; and, from the seeds produced, plants of an intermediate cha- racter may be raised. But these hybrid plants will not perpetuate the race ; for, although they may ripen their seed for one or two generations, they will not continue to reproduce themselves beyond the third or fourth. But, if the intervention of one of the parent species be used, its stigma being fertilized by the pollen of the hybrid, or vice versa, a mixed race may be kept up for some time longer • but it will then have a manifest tendency to return to the form of the parent whose intervention has been employed. Where, on the other hand, the parents were themselves only varieties, the hybrid is only another variety, and its powers of reproduction are rather increased than diminished ; so that it may continue to propa- gate its own race, or may be used for the production of other varieties, almost ad infinitum. In this way many beautiful new varieties of garden flowers have been obtained, especially among such species as have a natural tendency to change their aspect.* Amongst animals, the limits of hybri- dity are more narrow, since the hybrid is totally unable to continue its race with one of its own kind ; and although it may be fertile with one of its parent species, the progeny will of course be nearer in character to the pure blood, and the race will ultimately merge into it.t In Animals, as among Plants, the mixed offsprings originating from different races within the limits of the same species, generally exceed in vigom - , and in the ten- * There are many instances in which foreign plants, that have been introduced into this country under different specific names, have been found capable of producing fertile hybrids ; in these cases a more accurate examination of the original locality has gene- rally shown, that the parents were nothing more than permanent varieties, or even hybrids naturally occurring between other varieties. This is particularly the case with many of the South American genera, such as that elegant garden flower, the Calceolaria ; and this is probably the explanation of the almost indefinite number of splendid varieties, well known to horticulturists, which may be obtained from the South American Amaryllis. + One or two instances have been mentioned, in which a mule has, from union with a simi- lar animal, produced offspring ; but this is certainly the extreme limit, since no one has ever maintained that the race can be continued further than one generation, without admixture with one of the parent species. 984 SPECIAL AND COMPARATIVE PHYSIOLOGY. dency to multiply, the parent races from which they are produced, so as often to gain ground upon the older varieties, and gradually to supersede them. Thus, the mixture of the European raees with the Hindoo and South American, has produced tribes of such superior characters of body, and of such rapid tendency to multiplication, that there is reason to believe that they will ultimately become the dominant powers in the com- munity.* The general principle, then, is that beings of distinct species, or descendants from stocks originally different, cannot produce a mixed race which shall possess the capability of perpetuating itself ; whilst the union of varieties has a tendency to produce a race superior in energy and fertility to its parents. 740. In examining into the characters of the different species of Plants and Animals with which different regions on the earth’s surface are peo- pled, the Naturalist soon becomes aware, that there are many kinds which are restricted to particular localities, whilst others are diffused extensively or even universally over the globe ; — that there are some spots (especially insular ones), of which the aboriginal inhabitants are almost entirely dif- ferent from those elsewhere found ; — and yet that amongst these, there will always be found species holding the same rank with regard to the remainder, and thus representing each other in different countries. + Thus, the species of Plants and Animals, originally inhabiting the eastern and western hemispheres, were probably almost entirely different, until the agency of man changed their geographical distribution ; and almost the same may be said of the species north and south of the equator. On the other hand, Man, and his constant attendants the dog and the house-fly, exist in every quarter of the globe. Again, we find in New Holland no quadrupeds which do not belong to the order Marmpialia or J lonotre- mata, with the exception of a dog which is believed to have been introduced by man, and to have run wild ; and none of these species are found else- where. The greater part of the plants also belong to distinct genera ; and those included in the genera occurring elsewhere constitute distinct species, — with scarcely any exception but among the Cryptogamia, the distribution of which seems more extended than that of Flowering-plants. The Flora of insular situations, if at a great distance from land, contains very few species which occur elsewhere. Thus, among the Flowering-plants of St Helena, which is so far removed even from the western shores of Africa, there have been found, out of 61 native species, only two or three which exist in any other part of the globe. From these and many similar facts it appears fair to conclude, that every species of plant and animal had originally a distinct locality, from which it has been dispersed, according to the capabilities possessed by its structure of adapting itself to changes in its external conditions, its own locomotive powers, and the degree in which it is subject to external agencies. “ What is a rare plant,” says Decandolle, “ but one which is so organised that it can only live in a * Several additional instances of this kind are related in Dr. Prichard’s work, vol. i. p. 147, and in Mr. G. Combe's “ Constitution of Man,” chapter v. ■f “ We see in two distant countries a similar relation between the Plants and Insects of the same families, though the species of both are different. When Man is the agent in introducing into a country a new species, this relation is often broken ; as one instance of this I may mention that the leaves of the cabbages and lettuces, which in England afford food to such a multitude of slugs and caterpillars, in the gardens near Rio are untouched.” Darwin, “ Journal of the Voyage of the Beagle.” LAWS OF REPRODUCTION. 985 particular locality, and which perishes in all others ; such a plant is incapable of assuming different forms. What, on the other hand, is a common plant 1 It is one robust enough to exist in very different locali- ties, and under very different circumstances, and which will therefore put on many different forms.” Plants, then, are liable to run into varieties, in proportion as they are more robust, more common, or more cultivated ; and some native species are, from this cause, domesticated with greater difficulty than many exotics. Precisely the same may be said of Animals ; those which have the power of adaptation to differences of temperature, food, &e. are most universally diffused ; while those that can only exist within narrower limits of variation, are restricted to the neighbourhood of their original locality.* 741. It becomes a’ most interesting question, then, to determine what are the changes which may be produced by the influence of external cir- cumstances, and how far these are hereditarily transmissible. On this subject, a few facts may be stated, which will give an insight into the nature of the enquiry ; but it is one which deserves more attention than it has yet received, since it is not only essential to the correctness of all Natural-history classifications, but is connected with some of the highest questions in Physiological science.— One of the most obvious distinctions, where it is well marked, is that of size ; and yet, as we have already seen (§§ 378, 379), it is one peculiarly open to fallacy. Not only the size of the entire organism, but the relative development of individual parts, may be greatly modified by the supply of food ; this is especially the case in Plants, whose tissues are simple, and whose different organs closely resemble each other in elementary constitution. Thus, cidtivation often converts a single flower into a double one, by the metamorphosis of its stamens into petals, or by the development of a row of petals previously abortive, or by the change of the small tubular florets of a ‘composite’ flower (like those composing the ‘ disc ’ or ‘ eye ’ of the Dahlia) into the flat expanded florets which constitute the ‘ray.’ Cultivation has a similar effect in obliterating the spines, prickles, and thorns, from the surface of many plants ; a change which was fancifully, but not improperly, termed by Linnaeus “ the taming of wild fruits.” The instances of such altera- tions effected by external agency in the Vegetable kingdom, are almost innumerable ; and they are not confined to structure, being observed in habit also. Thus, many plants, which are annuals in a cold climate, become 'perennial if transported to the torrid zone ; and plants which are usually biennial, forming their organs of vegetation one year, and those of fructification in the second, and then perishing, may be converted into annuals by heat, or into triennials by cold. It is very difficult, however, to say how far the varieties thus created may become permanent by their hereditary transmission. The usual principle is, that propagation by seeds will only reproduce the species, the race not being continued with any cer- tainty. In most Plants which have been much altered by cultivation — such as the Apple, the Cabbage, or the Dahlia — the seeds, if dropped on a poor soil, will produce offspring which approximates to the original type of * The geographical distribution of organised beings is a question of tile highest interest to the Physiologist as well as to the Naturalist ; and it is one of those which requires the utmost elucidation it can obtain from the combined researches of both. It is a department of enquiry which is at present being most successfully prosecuted by Prof. E. Forbes. 98 G SPECIAL AND COMPARATIVE PHYSIOLOGY. the species ; whilst from the seeds of the Cerealia (com-grains), which are believed to have been originally grasses of some very different aspect, no other forms are ever produced, which might assist in the solution of the curious problem of their origin. It is not improbable that, as among animals, varieties which arise from some peculiarity in the constitution of the being itself, are more liable to be reproduced in the offspring, than those which are simply the result of external agencies. It is evident, at least, that here also the capability of undergoing such modifications, is that which renders the species most truly valuable to man. 712. Amongst Animals, the various breeds of domestic cattle, of the horse, dog, &c. afford abundant evidence of the modifying influence of external conditions ; since there is little doubt that they have respectively originated from single stocks, and that their peculiarities have been engrafted, as it were, upon their specific characters. Between the Shetland pony and the Arabian racer, for example, or between the Newfoundland dog and the Italian greyhound, there would seem much greater difference than between the Lion and Tiger (the skulls of which are so much alike that even Cuvier was not always able to distinguish them), or between various other species of the Feline tribe, which, from the incapability of domestication, have not been exposed to such influences. But that these domesticated races, however different their external characters, have a common origin, is indicated by the perfect freedom with which they breed together ; and by the fact that, whenever they return to a state of nature, — as is the case with the dogs introduced by the Spaniards into Cuba, and the horses and wild cattle which now overspread the plains of South America, — the differences of breed disappear, and a common form is pos- sessed by all the individuals. It is not a little curious, too, that instincts which must have remained dormant for many generations during the domesticated condition of the race, should re-appear when this change takes place in its habits ; thus, among the wild horses of South America, there is the same tendency to associate in herds, under the protection of a leader, as among those of Asia, whose ancestors are not known to have been ever reduced to subjection. “ It seems reasonable to conclude/’ as Sir C. Lyell has justly remarked, “ that the power bestowed on the horse, the dog, the ox, the sheep, the cat, and many species of domestic fowls, of supporting almost every climate, was given expressly to enable them to follow man throughout all parts of the globe, in order that he may obtain their services, and they our protection.” “ L T nless some animals had manifested in a wild state an aptitude to second the efforts of man, their domestication woidd never have been attempted. If they had all resem- bled the wolf, the fox, and the hyaena, the patience of the experimentalist would have been exhausted by innumerable failures, before he at last succeeded in obtaining some imperfect results ; so, if the first advantages derived from the cultivation of plants, had been elicited by as tedious and costly a process as that by which we now make some slight additional improvement in certain races, we should have remained to this day in ignorance of the greater number of their useful qualities.” 7 43. How all the varieties have been produced, which are now so nu- merous and striking, is a question much more easily asked than replied to satisfactorily. That very important changes may be induced by the direct influence of climate, food, and habits of life, upon successive gene- LAWS OF REPRODUCTION. 987 rations, cannot be doubted ; since experience shows that particular races of animals, placed under new conditions, gradually undergo modifications which adapt them to those conditions. Thus, of the number of new varieties which have sprung up amongst the domesticated races first introduced into South America by the Spaniards, there are many which are clearly traceable to climatic influences ; and even within a recent period, numerous examples of a similar modification have occurred. For example, Sir C. Lyell mentions that some Englishmen engaged in con- ducting the operations of the Keal del Monte Company in Mexico, carried out with them some greyhounds of the best breed, to hunt the hares which abound in that country. The great platform which is the scene of sport, is at an elevation of about 9000 feet above the level of the sea, and the mercury in the barometer stands habitually at the height of about 1 9 inches. It was found that the greyhounds could not support the fatigues of a long chase in this attenuated atmosphere; and before they could come up with their prey, they lay down gasping for breath ; but these same animals have produced whelps, which have grown up, and are not in the least degree incommoded by the want of density in the air, but run down the hares with as much ease as do the fleetest of their race in this country. — But peculiarities sometimes arise, to all appearance de novo, which cannot be attributed with the same degree of probability to external agencies ; thus, it is by no means uncommon to find individuals of the human species with six fingers and six toes ; and such peculiarities are more likely to be continued hereditarily, than are those which have been acquired. Sometimes advantage has been taken by man, of spontaneous variations of this kind, for some purpose useful to him ; and he has exerted his skill to perpetuate them. The following example is of comparatively recent occurrence. In the year 1791, one of the ewes on the farm of Seth Wright in the state of Massachusets, produced a male lamb, which, from the singular length of its body and the shortness of its legs, received the name of the otter breed. This physical conformation, incapacitating the animal from leaping fences, appeared to the farmers around so desir- able, that they wished it continued. Wright determined on breeding from this ram, and the first year obtained only two with the same pecu- liarities ; in the following years, he obtained greater numbers ; and, when the offspring became capable of breeding with one another, a new and strongly-mai’ked variety, before unknown to the world, was established.* — This history shows the influence which the circumstance of a scanty population may have formerly had on the production of varieties, both in the human and other species. At the present time, any peculiarity which may occasionally arise, speedily merges by intermixture, and returns to the common standard ; but it may be surmised that, in the older ages of the world, some individuals in which a peculiarity existed, may have been so far separated from the rest, as to necessitate frequent union among themselves, so that the character would be rendered still more marked, instead of disappearing ; and, being propagated for a few generations, would be rendered permanent. 744. Acquired peculiarities, on the other hand, are seldom reproduced in the offspring, unless they have a relation with the natural habits and * “Philosophical Transactions,” 1813. 988 SPECIAL AND COMPARATIVE PHVSIOLOGT. physical wants of the species ; but, when this relation exists, they may be transmitted as regularly as the specific character. Thus, in dogs, the relative perfection of the organs of sight and smell, perhaps also of hearing, varies much in different breeds, and their mode of hunting their prey undergoes a corresponding change ; but in these cases, no new instinct is developed, the difference merely consisting in the relative proportion of those already existing ; and the new peculiarities have an intimate relation to the habits of the animal in a wild state. For example, in a mongrel race of dogs employed by the inhabitants of the banks of the Magdalena almost exclusively in hunting the white- lipped Pecari, a peculiar instinct appears to have become hereditary, like that of the pointers and other dogs of this country. The address of these dogs consists in restraining then’ ardour, and attaching them- selves to no animal in particular, but keeping the whole herd in check. Now among these dogs some are found, which, the veiy first time they are taken to the woods, are acquainted with this mode of attack ; whereas, a dog of another breed starts forward at once, is surrounded by the Pecari, and, whatever may be its strength, is destroyed in a moment.'"' It is impossible not to recognise in many acquired habits, however, some- thing more than a relation to the instincts necessary for the preservation of the species ; they evidently arise, in part at least, from the connection of the race with Man. This is more particularly exemplified in the instance of the breed of Shepherds’ Dogs, which often display an extra- ordinary hereditary sagacity respecting their peculiar vocation ; as well as in cases which have been frequently mentioned, where the descendants of dogs to which peculiar tricks have been taught, have displayed an unusual aptitude for learning the same. It may then be considered, that the capa- bility of undergoing such modifications is a part of the psychical as well as structural character of the Dog, even in a wild state ; and that his relation to man may have as important an influence on h is hereditary propensities, as the supply of their physical wants has on animals of other species. The same may, perhaps, be said of the Horse, in the races of which we find peculiar habits transmitted from parent to offspring, which are the pure results of human instruction. ■ — It is from the want of this relation towards either the natural habits of the species or their subser- viency to man, that habits acquired by other animals do not become hereditary. Thus, Pigs have been taught to hunt and point game with great activity and steadiness, and other learned individuals of the same species have been taught to spell ; but these acquirements have in no instance been transmitted to the offspring, not being the result of the development or modification of any instinctive propensity naturally exist- ing. In like manner, however artificially the forms of domesticated animals may have been altered in all the individuals of successive genera- tions, the usual character of the species and variety is maintained in each one of the offspring ; unless, as sometimes happens, this alteration happens to coincide with natural varieties of the species. Thus instances are on record, in which Dogs, that have been deprived of their tails by accident or design, have produced puppies with a similar deficiency 3 but as breeds * Some curious instances of a similar propagation of acquired peculiarities connected with the natural habits of the race, are given by Mr. Knight, “ Phil. Trans.” 1837 ; the most remarkable, perhaps, are the facts related of the Retriever. SENSIBLE MOTIONS OF LIVING BEINGS. 989 of tail-less dogs have spontaneously arisen, there would be a stronger tendency to the perpetuation of this acquired peculiarity, than when no such peculiarity naturally occurred. There can be no doubt that much has yet to be learned, of the influence of the state of the parent upon the development of the offspring ; and that, though credulity and the love of the marvellous have been the occasion of many strange fictions being- transmitted to us, we are by no means justified in rejecting the doctrine without further enquiry.* And when it is borne in mind, that the races of animals among which the so-called spontaneous variations are most apt to spring up, are also those which are most susceptible of the modifying- influence of external conditions, it seems highly probable that these spon- taneous variations in the offspring are really attributable to the influence of external agencies in modifying the constitution of the parent. CHAPTER XIX. SENSIBLE MOTIONS OF LIVING BEINGS. 745. Although we are ordinarily accustomed to think of Animals as self-moving, and to regard Plants as altogether destitute of the power of spontaneous motion, yet, as already pointed out on several occasions, this distinction between the two kingdoms is one of which the validity cannot be sustained. For the ‘zoospores’ of many of the inferior Algae are as active in their movements, as are the Animalcules which they so much resemble ; and are there some Protophytes, which seem to be in a state of more or less active motion during the whole period of their lives, at least until the occurrence of ‘ conjugation.’ Thus the Oscillatorice, which are long filamentous cells, have a movement of alternate flexion and extension, writhing like worms in pain ; sometimes they appear to twist spirally, and then to project themselves forwards by straightening again. These move- ments are greatly influenced by temperature and light, being more active in heat and sunshine than at a low temperature and in shade ; and they are checked by any strong chemical agents, which also put a stop to the motions of Animalcules inhabiting the same water. And most of the Diatomacece move slowly through the water, as if by the action of cilia; although these organs are not distinctly discernible, t Among the higher Algse, and probably in the whole Cryptogamic series, the ‘ phytozoaires ’ possess an activity in no degree inferior to that of the spermatozoa of Animals. And although the different mode in which the reproductive function is performed in the Phanerogamia, renders it unnecessary that the contents of the sperm-cells should possess such a power of spontaneous movement, and the entire organisms are firmly rooted to the ground * Montgomery, “ On the Signs of Pregnancy,” p. 1 6. See also Walker “ On Intermar- riage,” pp. 275 — 8, and Dr. Harvey “ On a remarkable Case of Cross-breeding,” for several examples of the influence of the mental condition of the mother at the time of conception, upon the offspring, in various domesticated animals. f The Bacitlaria parudoxa has a very remarkable movement; for the long staff-like seg- ments of which the plant is composed, slide edgeways over one another until they become almost completely detached, and then return and slide in the opposite direction; repeating this movement with a rhythmical regularity. 990 SPECIAL AND COMPARATIVE PHYSIOLOGY. during the whole of life, yet we shall hereafter find that they exhibit movements of one part upon another, which are scarcely inferior in cha- racter to those of Zoophytes, these also being fixed during the greater paid of their existence, although its earliest jieriod has been passed by them in a condition resembling that of the zoospores of the inferior Algse. 746. Now with regard to ciliary movements, it maybe asserted without hesitation that they do not in the least indicate consciousness or self-deter- mining power on the part of the beings which exhibit them ; and there is no reason whatever, why the fact of their performance by any organism should be regarded as entitling it to a place in the Animal kingdom. And it cannot but induce us to look at those movements of Animalcules, which are due to ciliary action, as of a purely automatic character, when we see that movements of a precisely similar nature are performed by beings whose place is indubitably in the Vegetable kingdom. In fact, as already pointed out (chap, v.), there is no other line of demarcation to be drawn between the Protophyta and Protozoa, than that which is based upon the nature of their respective aliments, and the mode in which these are obtained and appropriated. — And the same may be said of those rhyth- mical motions which are exhibited by Oscillatoriae, and by ‘ phytozoaires ’ and ‘ spermatozoa their very uniformity and constancy being indications that they do not proceed from a self-determining power, but are entirely automatic. Such movements, as already pointed out, appeal' to result from the expenditure of a certain amount of Vital force, which becomes re-converted into the Physical, through the peculiarity of the material sub- stratum which is their instrument (§§ 50-54). 747. The sensible motions of the higher Plants, however, which arc usually performed under the influence of stimuli applied to their tissues, or to certain parts of them, appear referable to a somewhat different cate- gory ; being, in fact, manifestations of the same property of contractility, as that which exists in certain Animal tissues, but especially in the muscu- lar. This property of contractility on the application of a stimulus, may be readily distinguished from the elasticity which is simply due to the mechanical relation of the particles composing the tissue ; the latter being retained as long as there is no evident decomposition, whilst the former is an essentially vital endowment. An elastic ligament, when stretched, tends to contract only in virtue of the distiu'bance of its molecrdar arrange- ment, that has been produced by the force applied to it ; but a muscle which contracts powerfully upon the stimulus of a simple touch, or upon one of a still less mechanical nature, can do so only by a property of its own, which is connected with its attributes as a living being. In the lowest and simplest Animals, whatever degree of contractility is possessed, appears to be almost equally diffused through the system (§§ 283 a, 289) ; and we can neither discover in them any structure specially endowed with this property, nor anything resembling a nervous system fitted to call it into exercise. In proportion as we ascend the scale, however, we find a distinct muscular structure evolved, in which the general con- tractility of the body becomes, as it were, concentrated ; and, in propor- tion to its development and complexity, it supersedes the corresponding but more feeble powers of the remainder of the tissues. It is now almost entirely subjected to the nervous system ; and all those parts of it, which are not connected with the functions of organic life merely, are rendered SENSIBLE MOTIONS OF LIVING BEINGS. 991 subservient to the will, and thus become the instruments of its operation upon the place and condition of the body. 748. Of the evident movements observable in the higher Plants, there are some which take place as a part of the regular series of phenomena of growth and reproduction, and which must be regarded as the ordinary operations of the vital forces which they have derived from the light, heat, &c. that have sustained their organic activity ; but there are others which are performed in respondence to excitement of a mechanical kind. The immediate connection of these movements with the organic functions, in the first class of instances, and the indication they would seem to give of consciousness and sensibility in the second, have led many persons to seek for an explanation of them in the hypothesis of the existence of a nervous system in these beings. But it will be seen, if the question be fairly investigated, that, whilst no evidence of its presence is furnished by the minutest anatomical research, no argument for its operation can be deduced from the phenomena observed. In the simplest and most intelli- gible instances of sensible motions in Plants, the change is the result of the contraction of the part to which the stimulus is applied ; this con- traction being consequent upon the change of form of its cells, which must be regarded as a manifestation of their peculiar vital endowments (§ 138). Thus, the leaf of the wild Lettuce exudes, when the plant is in flower, the milky juice contained in its vesicles, if these be irritated by the touch ; and the contraction of the poison-gland of the Nettle, when the tubular hair which surmounts it is pressed, forces out its secretion, and produces urtication. So, again, if the base of the filament of the Berberry be touched with the point of a pin, the stamen immediately bends over and touches the style. In this case, the movement is produced by the peculiar contractility of the tissue on the interior side of the filament, which, when called into operation by the application of a stimulus, necessarily occasions the flexion of the stalk. This peculiar irritability has a relation with the functions of the flower ; since, when called into play (as it frequently is) by the contact of insects, the fertilization of the stigma will be assisted. Many similar instances might be adduced, in which a corresponding operation is connected with the process of reproduction in Plants. — There are cases of more com- plexity, however, in which an irritation of one part produces motion in a distant and apparently-unconnected organ. Thus, in the Dioncea masci- pula (Venus’ fly-trap), the contact of any subtance with one of the three prickles which stand upon each lobe of the leaf, will occasion the closure of the lobes together, by a change taking place in their leaf-stalk. And in the Mimosa pudica (Sensitive Plant), any irritation applied to one of the leaflets will occasion, not only its own movement towards its fellow, but the depression of the rib from which it springs ; and, if the plant be healthy, a similar depression will be produced in the principal leaf-stalk, and even in the petioles of other leaves. This propagation of the effect of the stimulus, from one part to another more or less remote, is usually accomplished in Animals through the nervous system ; but there is little doubt that in Plants the transmission is made through an entirely different channel. The experiments of Dutrochet upon the Mimosa have shown it to be through the vascular system, that an irritation in one part is made to produce movement in a distant organ ; and there can be little 992 SPECIAL AND COMPARATIVE PHYSIOLOGY. doubt that the same is time of the Dionoea also. Where each leaflet of the Mimosa is implanted upon its rib, there is a little swelling or intumes- cence ; this is more evident where the lateral ribs join the central one : and it is of considerable size at the base of the petiole, where it is articu- lated with the stem. The experiments which have been made upon its properties, have been performed, therefore, in the latter situation ; but the description of their results will apply equally well to the rest. The intumescence consists of a succulent tissue, which, on the upper side, appears very distensible, and on the lower very irritable. In the usual position of the leaf or leaflet, the distension of the two sides seems equally balanced ; but anything which causes an increase of fluid on the upper side, or a contraction of the vesicles on the lower, will obviously give lise to flexion of the stalk. The latter effect may be readily produced by touch- ing that part of the intumescence itself ; and then the leaf or leaflet will be depressed by the contraction of the part immediately irritated, just as in the case of the stamen of the Berberry. The same result follows the stimulation of this part by an electric spark, by the concentration of the sun’s rays upon it with a burning glass, or by chemical agents : and if, instead of applying a temporary stimulus, whose effect is speedily recovered from, a notch be made in the lower side of the intumescence, the balance between its resistance and the expansive tendency of the upper side is then permanently destroyed, and the stalk remains depressed. Now, supposing the lower side to be in its usual condition, flexion of the stalk may result also from whatever distends the vesicles of the upper part of the intumescence ; and this is the mode in which the movement is usually effected. For a stimulus applied to any part of the leaf, will cause a con- traction of its vesicles ; and the fluid expelled from them is carried by the circulating system to the distensible portion of the intumescence belonging to each leaflet, and to that of the petiole itself. — It appeal's, then, that these evident motions are readily explicable on the supposition, that con- tractility is a property of various tissues of Plants, and that this may be excited by stimuli of a physical nature. To suppose more, would be impliilosophical because unnecessary. 749. There are other movements, however, arising from causes which originate in the system itself, of which some notice should be taken. Such are, the folding of the flowers and drooping of the leaves, known as the sleep of plants. These phenomena seem due to a diminution in the activity of those vital processes, by which the turgescence of the soft parts of the structure is maintained ; and this diminution appears partly to result from the withdrawal of the usual stimuli, especially light, and to be in part of a periodical character. For it is found that artificial light and warmth will cause many flowers and leaves to erect themselves for a time ; and that, by proper management, the usual periods may be completely reversed. But the phenomenon cannot be altogether explained on this principle, since there are many plants of which the flowers only expand in the night, and which must be kept in darkness to prevent them from closing ; and in the present state of our knowledge, we can only consider this periodicity, like that periodical cessation or augmentation of particular functions which are common to nearly all organised beings, as a part of that regular train of operations, which is characteristic of each living being, and which proceeds from the original endowments of its organism, called SENSIBLE MOTIONS OF LIVING BEINGS. 993 into exercise by forces external to itself. — One other spontaneous Vegetable motion may be instanced, as of a very inexplicable character ; that of the Hedysarum gyrans, a Bengalese plant, each of whose petioles supports three leaflets, of which the central one is large and broad, and the two lateral ones, which are situated opposite to each other, small and narrow. The position of the central leaflet appears peculiarly influenced by light : for in the day-time it is usually horizontal ; by the action of strong solar light it is raised towards the stalk, whilst in the evening it bends down- wards ; and it is manifestly depressed, if placed in the shade for a few minutes only. The small lateral leaves are in incessant motion ; they describe an arch forwards towards the middle leaflet, and then another backwards towards the footstalk ; and this by revolving on their articula- tion with the petiole. They pass over the space in 30 or 40 seconds, and then remain quiet for nearly a minute ; the leaflets do not move together, but in opposite directions, one usually rising while the other is sinking ; the inflexion downwards is generally performed more rapidly and uni- formly than that upwards, which occasionally takes place by starts. These movements continue night and day ; being slower, however, in cold nights, and more rapid in warm and moist weather. They seem less affected by mechanical or chemical stimuli, than do those of any other plant ; and continue for a longer time in separated parts. 750. One class of spontaneous Vegetable movements has been shown by Dutrochet to be due to the action of Endosmose (§ 415) in the organs which execute them. This is particularly the case in various seed-vessels, which burst when ripe, in such a manner as to eject their contents with force, — as in the instance of the Momordica elaterium (common Squirting- Cucumber). His experiments upon the capsule of the Balsam termed Impatiens noli-me-tangere are particularly interesting. The valves of this capsule, when the fruit is ripe, suddenly spring from each other and curl inwards, scattering the seeds to some distance. Now an examination of the tissue of the valves shows, that the outer part consists of much larger vesicles than the inner ; and that the fluids contained in it are the densest. According to the law of Endosmose, the fluids contained in the tissue of the interior will have a tendency to pass into the vesicles of the exterior ; and it will distend them in such a manner, as to produce a disposition in that side to expand, when permitted to do so, whilst the inner side has an equal disposition to contract. This at last occurs from the separation of their edges consequent upon their ripening ; and then each valve rolls inwards. If, however, the valves be placed in a fluid more dense than that contained in the exterior vesicles, such as syrup or gum-water, these will be emptied on the same principle, and the valves will become straight, or even curl outwards. — A very curious movement, which probably depends upon a similar cause, may be observed in a little Fungus, which is not uncommon in some parts of Britain, named Carpobolus, from its peculiar manner of scattering its fruit. The sporules are collected into one mass, and inclosed in a globular bag, which is called a sporangium. This lies in a cavity, of which the inner wall is capable of separating itself from the outer, and of suddenly everting itself, so as to project in a globular form from the mouth of the cavity which it previously lined. This sudden eversion ejects the sporangium (with a degree of violence which for so minute a plant is very remarkable) from the cavity in which it was formed ; 3 s 994 SPECIAL AND COMPARATIVE PHYSIOLOGY. the mouth of this, which was at first nearly closed, spontaneously dilating itself as the sporules are mature. The eversion of the membrane is probably due to a change having taken place in the relative distension of the cells forming its inner and outer layer, which would operate much as in the capsule of the Balsam. 751. Among the simplest Protozoa, it seems as if the change of form of the single cell of which each individual is composed, were the sole means of movement which it possesses (§ 283 a ) ; and this change of form seems rather to be due to operations taking place within the cell, than to occur in respondence to mechanical irritation applied to its exterior, — a circum- stance which throws some light upon the phenomena just now described as occurring in Plants (§ 749). In these movements it is impossible to imagine with any probability that consciousness can participate ; nor can it be supposed that a nervous system can be then’ instrument. Now although the movements of the Hydra (§ 289), and of other Zoophytes of its class, may appear to indicate consciousness and self-determining power, yet it is very doubtful whether such endowments can justly be assigned to these animals. For their contractile tissue is of the simplest possible character, resembling that which is found in a very early state of newly-forming parts of higher Animals ; and the most careful scrutiny has failed to discover the faintest vestiges of a Nervous System in them, not- withstanding that the extreme transparency of their bodies renders them peculiarly favourable subjects for the most minute examination. And further, when their movements are fairly compared with those of higher animals, it becomes obvious that they resemble those by which food is immediately introduced into the stomach, the tentacula of the Hydra being homologous with the oesophageal muscles of Man ; and as we know that the latter act without our will, and even without our consciousness, it would be inconsistent with sound philosophy to regard them as depen- dent upon such springs of action in the Hydra, without some very cogent evidence that they are so. — Again, the rhythmical movements of the disc of the Pulmograde Acalephce (§ 294), whereby they are propelled through the water, bear a much closer resemblance to the rhythmical contractions of the heart of higher animals, than they do to any other of then- actions ; and as the latter are pei’formed without any exercise of will, and even without the guidance of consciousness, it seems reasonable to suppose that the former are so likewise. And such an interpretation is confirmed by what is known of the nervous system of these animals ; for its extent of diffusion is so limited, that it is impossible to imagine the whole contrac- tile tissue of the disc to be influenced by it ; and, moreover, portions of the disc entirely separated from the l’est, and not containing any portion of the nervous centres, will continue their alternating contractions and relaxations, just like the heart of a cold-blooded animal taken out of its body. — In the young of the Compound Ascidians (§ 703), again, we find an active movement of the embryonic mass through the water, effected by the lateral undulations of the tadpole-like tail ; and this at a time when this organ consists of nothing but cells, and when it is certain that the formation of the nervous system has not even commenced. Such movements are strictly comparable, on the one hand, with the rhythmical movements of the Oscillatorise or the Hedysarum among Plants, and on the other, with the rhythmical contractions of the heart in the embryo of SENSIBLE MOTIONS OF LIVING BEINGS. 995 the higher Animals, when as yet its walls consist of nothing but cells (§ 485). They cannot be attributed to any external stimulation, and must be regarded as a part of the regular - series of vital operations of the cells which exhibit them, as the ciliary vibrations are of the ciliated epithelium- cells, or as the acts of secretion or reproduction are of the cells of glands or ovaries (§ 51). 752. There are many movements in higher Animals, which, though performed by the instrumentality of the Muscular tissue, are perfectly involuntary, take place without exciting any consciousness of their occurrence, and appear - to be essentially independent of Nervous agency, although capable of being affected by it. All the actions of this class are immediately subservient to the maintenance of the Organic frmctions ; thus we find the propulsion of food along the alimentary canal effected by the peristaltic contractions, alternating with relaxations, of its proper muscular coat ; and the circulation of the blood in great measure dependent upon the alternating contractions and relaxations of the muscular walls of the heart. Between these two sets of actions, however, there is this important difference ; — that whilst the former, like the closure of the fly- trap of the Dionsea, or the bending-down of the filaments of the Ber- berry, are chiefly dependent upon the application of a stimulus, which calls into activity the contractile power of the tissues, — the latter, like the lhyth- rflical movements of Plants previously referred to, take place without any external stimulation, the regular alternation of contraction and relaxation being apparently their peculiar manifestation of vital activity. Thus we find that the peristaltic movements of the alimentary canal are for the most part excited by the contact of solid or liquid substances with its lining membrane • and that, when they are not otherwise taking place, they may be called into activity by a slight mechanical irritation applied to its external surface, or, in a warm-blooded animal at least, by the admission of cold ah into the abdominal cavity. On the other hand, the alternate contractions and relaxations of the heard will continue with extraordinary regularity (in a cold-blooded animal), after the organ has been removed from the body, and has been completely drained of its blood ;* and it seems impossible to refer them to the agency of any stimulation derived from an external source. But, on the other hand, the peristaltic move- ments of the alimentary canal may be often seen to take place without any ostensible stimulation ■ and the alternating movements of the heart, after tliey have ceased, may often be re-excited by a slight mechanical irritation. Hence it is obvious, that both kinds of action are referable to the same endowment, namely, a motility which is inherent in the tissues, and is a part of then - vital endowments, and which may manifest itself either spontaneously, or in respondence to a stimulus : but that the former method is the rule in the case of the heart, the exception in that of the alimentary canal ; whilst the latter method is the rule in the case of the alimentary canal, the exception in that of the heart. — The endowment in question, however, is not limited in the higher animals to the heart and alimentary canal ; for it pervades, in a greater or less degree, the walls of their blood- vessels and absorbents, those of the principal gland-ducts, those of the * It has been stated by Dr. Mitchell of Philadelphia, that the heart of a Sturgeon which had been cut out and hung up to dry, continued its rhythmical movements, until its tissues had lost so much of their fluid, that a crackling sound was heard with each contraction, 3 s 2 996 SPECIAL AND COMPARATIVE PHYSIOLOGY. urinary bladder, uterus, etc., all of which parts are furnished with non- striated muscular fibres (§ 225), or with those elongated contractile cells, whichhave been recently shown by Prof. Kiilliker to take a considerable share in the movements of the contractile tissues of even the highest animals.* 752 a. By far the larger proportion of the sensible movements of higher Animals, however, are called forth by the instrumentality of the Nervous System ; through which the contractility of the Muscular apparatus is called into exercise. This is the case, not only with all those actions that are designated as voluntary, being the result of an individual self-deter- mining power, but also with many that are as involuntary or automatic as those which we have been just considering; these last being connected, more or less directly, with the maintenance of the organic functions, or, in some mode or other, with the conservation of the bodily fabric. We shall find that these are performed with no more dependence on conscious- ness, than the rhythmical contractions of the heart, or the peristaltic movements of the alimentary canal ; and that the nervous system is made to participate in them simply as a conductor of stimuli, from the parts which receive the impressions, to those which are to respond to them by contraction ; the conditions of Animal existence requiring that this conduction shall be performed with greater rapidity and energy, than w r e see to take place in the parallel cases that present themselves among Plants (§ 759).— It is very interesting, however, to observe, that the form of Muscular tissue which most readily obeys the stimulus of innervation, and which is the instrument of the most pm-ely Animal functions, is that which most perfectly exhibits that cellular type of structure, which seems common to all the parts most actively engaged in the vital operations ; and that the act of muscular contraction in the highest Animal is due to the same kind of change in the form of the cells of the ultimate fibrillse, as that which produces the sensible motions of Plants (§ 224). The only essential difference, then, between the contractile tissues of Plants and those of the higher Animals, consists in this ; that the latter are capable of being called into activity by a kind of stimulus — the Nerve-force — which does not operate in the former. But this speciality of endowment does not supersede the more general susceptibility which exists among the inferior grades ; for we find that the ‘ striated muscular fibre,’ although called into action more readily and powerfully by the stimulus of Innervation than by any other, is yet induced to change its state, like the contractile tissues of Plants, by electrical, chemical, or mechanical irritation; whilst the ‘ noil-striated fibre’ responds so much more readily to this kind of irritation, than it does to the stimulus of Innervation, that it is even difficult to demonstrate the action of the latter upon it (§ 231). — A general account of the Movements which are characteristic of the different classes of Animals has already been given (chap, vm.) ; and to enter into any detail as to the various mechanical adaptations, by which the force generated by Muscular contraction is applied to their production, would carry us far beyond the limits of the present Treatise. * See Siebold and Kblliker’s “Zeitschrift fur Physiologie,” Band i., 1849. 997 CHAPTER XX. FUNCTIONS OF THE NERVOUS SYSTEM. 1. General Considerations. 753. We have now completed our survey of the entire series of operations which are common to the Vegetable and Animal kingdoms ; and it only remains for us to consider those which are peculiar to the latter. Such are the various actions of the Nervous System,- — an appara- tus to which we find among Plants nothing in the least analogous ; and the possession of which, therefore, must be considered as the peculiar and characteristic endowment of the Animal. Still, as already pointed out on several occasions, we cannot justly regard it as universally present throughout those classes, which possess on other grounds a title to he ranked in the Animal kingdom ; for there are many such beings in which we seem entitled to affirm that it is absent ; and there are many others, in which, if present at all, it exists in a condition quite rudimentary, and can take little share in the actions of the organism. But the life of all such beings is chiefly vegetative in its nature ; their movements are not dissimilar in kind to those which we see in Plants ; and their title to a place in the Animal kingdom chiefly rests upon the nature of their food, the mode in which they appropriate it, and their resemblance to the embiyonic conditions of beings undoubtedly belonging to that group. In proportion, however, as we ascend the Animal series, do we find the Nervous System presenting more and more of structural complication, and obviously acquiring more and more of functional importance ; so that in Vertebrated animals, and more especially in Man, it obviously becomes that part of the organism, to whose welfare everything else is rendered subordinate (§ 320). And we observe that this is the case, not merely in virtue of its direct instrumentality as the organ of Mind ; but also as the medium through which the conditions are provided for carrying on many of the Organic functions, which we find to be brought into more and more intimate dependence upon it, iu proportion as we rise in the scale. Thus the Ingestion of food, aud the Aeration of the circulating fluid, which are provided for in many of the lower tribes of animals by ciliary action, in which Nervous agency has no participation whatever, are made to depend in the higher upon the Nervo-nmscular apparatus, which, without interfering with those organic processes that take place (so to speak) in the -penetralia of the system, guards the portals for entrance and exit. But we find that, when thus brought into intimate relation with the Organic functions, the Nervous system is employed in its very simplest mode of operation ; that which does not involve Intelligence, Will, or even Instinct (iu the proper sense of that term), but which may take place independently of all Consciousness, by the simple reflexion of an impression, conveyed to a ganglionic centre by one set of fibres pro- ceeding towards it from the periphery, along another set which passes from it to the muscles, and calls them into operation (§ 244). This 998 SPECIAL AND COMPARATIVE PHYSIOLOGY. ‘reflex function,’ therefore, by which ‘automatic’ movement is effected without the necessary participation even of consciousness, is the simplest application of the Nervous System in the Animal body. We shall presently see reason to believe, that a very large proportion of the move- ments of many of the lower animals are of this reflex character ; and that they are not necessarily accompanied by sensation, although this may usually be aroused by the same cause which produces them. As we rise, however, in the scale of Animal existence, we find the reflex movements forming a smaller and smaller proportion of the whole ; until, in Man, they constitute so limited a part of that entire series of movements of which the Nervous system is the agent, that their very existence has been overlooked. 754. But the main purpose of the Nervous System is to serve as the instrument of those Psychical* powers, which are altogether peculiar to the Animal. Now when we attempt to analyse these attributes, we may resolve them, like the properties of the material body, into different groups. We find that the first excitement of all mental changes, whether these involve the action of the feelings or of the reason, depends upon sensations ; which are produced by impressions made upon the nerves of certain parts of the body, and are conveyed by these to a particular ganglionic centre, which is termed the sensorium, being the part in which Sensation, or the capability of feeling external impressions, especially resides. Now there are numerous actions, especially among the lower Animals, which seem to be as far removed from the influence of the Will, as little directed by Intelligence, and hence as truly ‘ automatic’ in their nature, as the ‘reflex’ movements themselves; but which, nevertheless, seem to depend upon sensation for then excitement. The movement seems, in some instances, to be immediately consequent upon the produc- tion of the sensation ; of this kind are the act of sneezing, and the sudden start produced by a loud and unexpected sound, in ourselves. But in other instances it appears rather to proceed from an internal feeling excited by the sensation, more akin to the Emotions of Man ; which, without any calculation of consequences, any intentional adaptation of means to ends, any exertion of the reason, or any employment of a discriminating Will, may produce an action, or train of actions, as directly and obviously adapted to the well-being of the individual, as we have seen those of the reflex character to be. It is impossible to say, in regard to many of the actions of the lower animals, to what extent they involve feelings or emotions at all analogous to those which tve expe- rience ; and it would seem better to apply the generic term Consensual to those, in which the Sensation excites the motor action, either immediately, or through the agency of an indiscriminating impidse excited by the sensation. This class will include all the purely ‘ instinctive’ actions of the lower Animals ; which make up, with the reflex, nearly the whole of the Animal functions in many tribes, and which are peculiarly elaborate in their character, and wonderful in their results, in the class of Insects. It is obvious that such actions, not being the result of any intelligent choice or voluntary direction on the part of the animals which perform * This term is used to designate the sensorial and mental endowments of Animals, in the most comprehensive acceptation of those terms. FUNCTIONS OF THE NERVOUS SYSTEM. 999 them, are not less truly ‘automatic’ than those belonging to the ‘ reflex’ group ; the only essential difference being, that the latter are performed without the necessary participation of sensation, the excitement of which appears to be essential to the performance of the latter. — The Automatic movements are found to be gradually brought under the control of the Intelligence and Will, as we rise towards Man, in whom those faculties are most strongly developed, so as to keep the consensual as well as the reflex actions in subordination to the more elevated purposes of his existence. 7 55. There are many sensations, however, which do not thus imme- diately give rise to muscular movements ; their operation being rather that of stimulating to action the Intellectual powers. There can be little doubt that all Mental processes are dependent, in the first instance, upon Sensations ; which serve to the Mind the same kind of purpose, that food and air fulfil in the economy of the body. If we could imagine a being to come into the world with its mental faculties fully prepared for action, but destitute of any power of receiving sensations, these faculties would never be aroused from the condition in which they are in profound sleep ; and the being must remain in a state of complete unconsciousness, because there is nothing of which it can be made sensible, no kind of Idea which can be aroused within it. But after the mind has once been in active operation, the destruction of all future power of receiving sensations would not reduce it again to the inactive condition ; for sensations and ideas are so stored up in the mind, by the power of Memory, that it can feed (as it were) upon the past. Now the ideas which are excited by sensa- tions, if associated with feelings of pleasure or pain, constitute those Emotions, or £ moving powers of the mind,’ which are, either directly or indirectly, the springs of the greater part of our actions. When strongly excited, the emotions may produce movements which are purely invo- luntary, and which the Will may not be able to restrain ; but in their normal exercise, they act in subordination to the reasoning processes, and their operation upon the movements of the body is through the medium of volition. It is, indeed, the peculiar character of voluntary movement, that it is the result of the exercise of the Reasoning powers, being the expression of a definite purpose , of a designed adaptation of means to ends, on the part of the individual performing it ; instead of proceeding from a mere blind indiscriminating impulse, such as that which is the main- spring of the instinctive and emotional actions. It is in Man, that we find the highest development of the reasoning faculties ; but it is quite absurd to limit them to him, as some have done ; since no impartial observer can doubt, that many of the lower animals can execute reasoning- processes, as complete in their way as those of Man, though much more limited in their scope. 756. Thus, then, we have to consider the Nervous system under three heads ; — first, as the instrument of the Reflex actions ; — second, as the instrument of the Consensual and Instinctive actions ; — third, as the instrument of the Intellectual processes, and of Voluntary movements. Now there is good reason to believe, that to each of these groups of actions a particular portion of the Nervous Centres, with its afferent and efferent nerves, may be assigned ; one ganglion, or series of ganglia, being- limited in its function to the reception of simple impressions, and to the 1000 SPECIAL AND COMPARATIVE PHYSIOLOGY. origination of reflex movements consequent thereon ; whilst another gan- glion, or series of ganglia, is set apart for the reception of impressions of which the mind becomes conscious as sensations, and for the origination of the consensual movements which these excite ; whilst a third, which is superadded to the foregoing in Vertebrated animals, receiving from the proper sensorium the sensations which have been there called up, is the instrument through which these sensations call ideas into existence, by which these ideas are employed in the various processes of intellection, and by which the voluntary determination is brought to bear upon the muscular apparatus. But we shall find reason to believe, that just as this Cerebral ganglion is called into action through the sensoiy portion of the automatic apparatus, so does it react through the motor portion of the same appara- tus ; and that it has no more direct connection with the muscular system, than it has with the organs of sense. 757. What has been hitherto said refers exclusively to that division of the Nervous system, which is concerned in the reception of impressions, the production of sensations, and the stimulation of muscles to contrac- tion ; and as these are all purely animal functions, it has been called the nervous system of animal life. There is another set of ganglia and nerves, however, which constitute what is termed the Sympathetic or visceral system ; this is distributed to the various parts of the nutritive and secretory apparatus, its fibres forming a plexus upon the walls of the blood-vessels, and being distributed with them to all parts of the body ; and hence it has been designated the nervous system of organic life. We have seen reason to believe, however, that the functions of Nutrition and Secretion are not themselves dependent upon nervous agency (§ 3G4) ; and the relation of the Sympathetic system to them, therefore, can scarcely be so intimate as that designation would seem to imply. Probably it is the vehicle for the involuntary action of the emotions upon those func- tions ; and it may perhaps serve also to harmonize them with each other. — We shall now take a brief survey of the comparative structure of the Nervous system in the principal groups of Animals, and enquire what actions may be justly attributed to its several parts in each instance ; commencing with those in which the structure is the simplest, and the variety of actions the smallest ; and passing on gradually to those, in which the structure is increased in complexity by the addition of new and distinct parts, and in which the actions present a corresponding variety. 2. Comparative View of the Structure and Actions of the Nervous System, in the principal groups of the Animal Kingdom. 758. In the beings which have been associated under the designation of Protozoa, no definite traces of a Nervous System can be discovered : and it is maintained by many Physiologists, that, in these animals, the nervous matter is present in a “ diffused form,” — that is to say, incorpo- rated with the tissues ; but it would be difficult to assign a valid reason for a supposition so gratuitous. An arrangement of this kind cannot be required to confer on the individual parts of the organism then- vital pro- perties ; since these exist, to as great an extent, in beings which are allowed to be entirely destitute of it, namely the entire Vegetable king- STRUCTURE AND ACTIONS OF THE NERVOUS SYSTEM. 1001 clom. The simplest office of a nervous system is, as we have seen (§ 752), to establish a communication between parts specially modified to receive impressions, and others particularly adapted to respond to them. Where every portion of the body has similar endowments, there can be no object in such a communication 3 just as, where every part of the surface is. equally capable of absorption, and every part of the tissue equally perme- ated by nutrient fluid, there is no necessity for a circulating system. The motions exhibited by Animals of these lowest classes, would seem to be scarcely less directly dependent upon external stimuli, than are those of Plants 3 being, in fact, the result of the general diffusion of that exalted degree of irritability, which is restricted in most plants to particular parts of the structure (§ 751). 759. No valid ground can be shown for assigning a higher character to the movements even of Zoophytes , such as the Hydra and Actinia, or their composite allies 3 since all those actions which are concerned in the pre- hension and ingestion of food, are, as already remarked, really analogous rather to the peristaltic movements of the oesophageal muscles and stomach of higher animals, than to the motions of their limbs 3 and the few which they exhibit, that are not referable to this category, appear to be the result of the direct stimulation of light or other physical agencies, which may operate by producing organic changes in their tissues, as in those of Plants. At any rate it may safely be affirmed, that if these Zoophytes do possess any rudiment of a Nervous System, and if any of their actions are to be attributed to its instrumentality, its participation in the general course of their vital actions must be very trifling. We have at present, it must be acknowledged, no certain means of appreciating the degree of sensibility possessed by the lowest members of the Animal kingdom. The motions which follow the impressions of external agents, are our only means of judging of its possession by a particular being 3 and in the interpretation of these phenomena, the greatest care is requisite to avoid being misled by false analogies. Much error has probably arisen, from comparing the manifestations of life exhibited by creatures of this doubtful character, with those of the highest Animals 3 and thence infer- ring that, because motions are witnessed in the former which bear some analogy to those of the latter, they must be equally dependent on a nervous system. But, when it is considered how completely vegetative is the life of such beings, and how closely all their motions are connected with the performance of their organic functions, it would seem obvious that the general comparison should be made with Plants, rather than with Animals 3 and that we should seek the assistance of principles of a higher character, only when those we already possess are insufficient to explain the phenomena. A nervous system would seem to be required only in a being possessed of a number of distinct organs, whose actions are of such a character that they cannot be brought into mutual relation, without a more immediate and direct communication than that afforded by the circulating system, which, as we have seen, is the only bond of union pos- sessed by Plants between distant parts. In the lowest and simplest Animals, whatever degree of contractility exists, appears to be almost equally diffused through the system 3 and we neither find any special sensory organs, adapting one part more than another to the reception of impres- sions, nor do we observe any portion of the structure peculiarly endowed 1002 SPECIAL AND COMPARATIVE PHYSIOLOGY. with the power of motion ; neither can we discover anything like a nervous system fitted to receive such impressions, and to excite respon- dence to them in distant parts. To use the forcible expression of Sir Gilbert Blane — “ Mr. Hunter, by a happy turn of expression, calls the function of the nervous system internuncial. It is evident that some such principle must exist in the complicated system of the superior Animals, in order to establish that connection which constitutes each individual a whole.” But where all the parts act for themselves, there is, as we have seen, no necessity for such an internuncial communication ; and consequently, although, when united, their functions all tend towards the maintenance of the system to which they belong, they are capable of being separated from it and from each other, without these functions being necessarily abolished. 760. It is in the higher Ecidiata, that we find the first definite indica- tions of the existence of a connected Nervous system. It is probable that such exists in all the Acalephae, although the softness of their tissues renders it difficult of detection. According to Ehrenberg, two nervous circles may be detected in the Medusa ; — one running along the margin of the mantle, and furnished with eight ganglia, from which filaments proceed to the eight red spots which he supposes to be eyes, — whilst the other is disposed around the entrance to the stomach, and is furnished with four ganglia, from which filaments proceed to the tentacula. A nervous ring has also been detected by Prof. Agassiz in Sarsia, one of the ‘ naked-eyed ’ Puimogrades ; and he states that it is entirely composed of ganglionic cells.* In Beroe, it is affirmed by Dr. Grant that a nervous ring exists round the mouth, furnished with eight ganglia, from each of which a filament passes towards the other extremity of the body, while others are sent to the lips and tentacula, — In the Echinodermata, how- ever, its manifestations are much less equivocal. In the Asterias, for instance, a ring of nervous matter surrounds the mouth, and sends three filaments to each of the rays ; of these, one seems to traverse its length, and the two others to be distributed on the c fecal prolongations of the stomach. In the species examined and figured by Tiedemann, no gan- glionic enlargements of this ring seem to exist, and it seems not impro- bable that, as in the Medusse, the entire ring is composed of vesicular nervous matter ; but this element is usually collected into distinct ganglia, which are found at those points of the ring whence the branches diverge, the number of these ganglia being always equal to that of the rays. In those species 'which possess ocelli at the extremities of the rays, the nervous cord proceeding towards each swells into a minute ganglion in its neigh- bourhood. — In the Echinus, the arrangement of the nervous system fol- lows the same general plan : the filaments which diverge from the oral ring being distributed (in the absence of rays) to the complicated dental apparatus, whilst others pass along the course of the vessels to the diges- tive organs. The transition between the Radiata and Articulata, pre- sented by the Holothuria and Sipunculus, is peculiarly well marked in the * “ Contributions to tlie Natural History of the Acaleph® of North America,” Part i., p. 232.' — It is considered by Prof. Agassiz that the movements of these Acalephte are volun- tary and exhibit a purpose ; but his statements on this subject exhibit a very strange confu- sion of ideas ; and the Author is assured by Mr. Huxley that he has never seen the slightest evidence of anything beyond automatic action in these animals. STRUCTURE AND ACTIONS OF THE NERVOUS SYSTEM. 1003 nervous system of these animals ; for the ring which encircles the mouth is here comparatively small, but two filaments traverse the length of their prolonged bodies, running near the abdomiual surface, which is their situation in the Articulated classes. — When we compare the character of the nervous system of these Radiated classes with that of higher Animals of more heterogeneous structure, we find that every segment of the body which is similar to the rest, is connected with a ganglionic centre, that seems subservient to the functions of its own division alone, and to have little communication with or dependence upon the remainder ; these centres being all apparently similar to each other in their endowments. This is the case, indeed, not merely with the tribes we are now consider- ing, but with the lower Articulata ; the chief difference being, that the individual portions are here disposed in a radiate manner round a com- mon centre, whilst in the latter they are longitudinally arranged. But when the different organs are so far specialized as to be confined to dis- tinct portions of the system, and each part consequently becomes possessed of a different structure, and is appropriated to a separate function, this repetition of parts in the nervous system no longer exists ; its individual portions assume special and distinct offices ; and they are brought into much closer relation to one another, by means of the commissures or con- necting fibres, which form a large part of the nervous masses in the higher Animals. It is evident that, between the most simple and the most complex forms of this system, there must be a number of inter- mediate gradations,— each of them having a relation with the general form of the body, its structure and economy, and the specialization of its distinct functions. This will be found, on careful examination, to be strictly the case ; and yet, with a diversity of its parts, as great as exists in the conformation of any other organs, its essential character will be found to be the same throughout. 761. Among the Molluscous classes, there is no repetition of parts like that just described ; and the nervous system partakes of that general want of symmetry in the body, which seems so characteristic of the pre- dominance of the vegetative organs in these animals (§ 297). Its gan- glionic centres are for the most part disposed near the mouth, then - actions being apparently destined to little else than the supply of the digestive organs ; and their size is usually proportional to the development of the organs of special sensation which are connected with them, and to the energy of the masticatory movements required for the reduction of the food. Where, however, unusually active powers of locomotion are pos- sessed, we commonly find ganglia situated in the neighbourhood of the organs destined to seive this purpose ■ but the most constant of all the ganglia in the Mollusca is that which is specially connected with the gills, and is hence termed the branchial ganglion. This, indeed, is the only one which distinctly presents itself in the lowest animals of this type ; those, namely, belonging to the groups of Bryozoa and Tunicata. In many of the former group, but most distinctly in the fresli-water species, a small body may be seen at the base of the tentacular circle (Fig. 114, a, a), between the oral and anal orifices ; which, from its appearance, and its correspondence in position with the undoubted ganglion of the Tunicata, may be regarded almost with certainty as a nervous centre. No branches, however, have been seen to proceed from it, either to the tentacula, or to 1004 SPECIAL AND COMPARATIVE PHYSIOLOGY. the muscles ; but the activity of the movements of these animals, with the perfection of their muscular structure (the fibres being striated in many species), would seem to indicate that nervous instrumentality is concerned in them. — In the Tunicata there is no difficulty in discovering the nervous ganglion, which is always situated between the oral and anal orifices (Fig. 117, b,/, Fig. 119,7, Fig- 121,y), being sometimes single, and sometimes bilobed or even double ; this ganglion is connected by branches with the sensory tentacula which guard the oral orifice, and branches extend from it also over the muscular coat of the branchial sac, filaments being spe- cially transmitted to the oral and anal sphincters. Other ganglia of smaller size have been described by Meckel as situated between the stomach and the branchial sac ; but it is doubtful whether the bodies in question belong to the nervous system at all. — No beings possessed of a complex internal structure, a distinct stomach and alimentary tube, a pulsating heart and ramifying vascular apparatus, with branchial appen- dages for aerating the blood, and highly-developed secretory and repro- ductive organs, can be imagined to spend the period of their existence in a mode more completely vegetative than these. The continuous and equable current of water which enters the cavity of the mantle, and which serves at the same time to convey food to the stomach and to aerate the blood, is maintained, without any nervous agency, by the vibrations of the cilia with which the walls of the cavity are clothed ; and it is only when substances come in contact with the sensitive tentacula, which ought not to enter the orifice, or when they have been introduced and are to be expelled, or when some external irritation is applied, that we see anything like a definite muscular movement, indicating the agency of a nervous system. The ganglion appeal's to control the circular sphincters which surround the orifices of the mantle, as well as the muscular fibres which are spread in a network over the whole sac ; by the contraction of the former, the ingress of improper substance is prevented ; and by that of the latter, the fluid contained in the cavity is violently ejected, as occurs when the ani- mal is alarmed by any external contact, or is excited to the same action by any internal irritation. This ganglion, then, must be regarded as chiefly branchial ; and, from the analogy of higher animals, we should be led to regard its actions as altogether ‘automatic,’ the movemeuts just described being analogous to those of coughing and sneezing in Man. Rudimentary organs of special sense, however, are foimd in some Tunicata ; and we must regard this ganglion as the seat of whatever consciousness these animals may possess. 762. In the Conchifera, there is a greater separation of the nervous centres, in accordance with the higher development of the general organ- isation ; but the branchial ganglion is still the principal one, and this is found near the posterior extremity of the body. At the sides of the oesophagus, however, there are seen two small ganglia (Fig. 299, a, a), united by a band which arches over it, and usually, also, by another pair of filaments which meet beneath it ; in some of the higher Conchifera (such as the Mactra ) these two ganglia nearly coalesce into a single mass above the oesophagus, and are then seen to be the evident analogues of the cephalic ganglia of the Articulata and Yertebrata. These ganglia are con- nected by nervous filaments (probably of an afferent character) with the sensory tentacula that guard the oral orifice ; and they send backwards a STRUCTURE AND ACTIONS OF THE NERVOUS SYSTEM. 1005 pair of trunks, e, which enter the branchial ganglion, whose branches are extensively distributed to all parts of the respiratory apparatus, siphons, &c., and also to the adductor muscle. In the Solen, and other Conchifera which possess a foot, a third principal nervous centre, the pedal ganglion (b), is found at its base, and seems peculiarly connected with its actions ; but where the foot is absent, as in the Oyster tribe, this does not exist. The pedal ganglion has always an immediate connection with the cephalic ; and in general, this connection is quite distinct from that of the branchial ganglion, so that the pedal and branchial ganglia have not even an apparent connection with each other. Some- times, however, the pedal ganglion has no such obviously distinct communication with the cephalic, being situated on the nervous cords which pass backwards from the cephalic to the branchial ; and the pedal and branchial ganglia thus seem to be connected toge- ther. The normal relations of the parts, however, are not altered by such an arrangement ; for the trunks which connect the bran- chial ganglia with the cephalic pass over, not through, the former, which has its own band of union as structurally distinct as in the Solen. — Besides these principal centres, we meet with numerous smaller ones (/,/,) upon the nerv- ous cords which proceed from them to the different parts of the general muscular envelope. 763. A good deal of variety exists in this class, in regard to the actions to which the nervous system is subservient. Many of those which have no foot (such h ’ anus ' as the Oyster) are attached for life to the place where they originally fix themselves ; and no evident motion is exhibited by these, save the opening and closure of the shell, which corresponds with the dilata- tion and contraction of the sac of the mantle in the Tunicata. This is principally accomplished by the posterior ganglion, which supplies the Fig. 299. Nervous system of Solen; — a , a, cephalic ganglia, connected by a transverse band arching over the oeso- phagus; b, pedal ganglion, the branches of which are distributed to the powerful muscular foot; c , branchial ganglion, the branches of which proceed to the gills g, the siphons, i, i, and other parts ; e, trunks connect- ing the cephalic and branchial ganglia ; /, /, f f, minute ganglia on the branches distributed to the 1006 SPECIAL AND COMPARATIVE PHYSIOLOGY. adductor muscle. Other species, being unattached, are enabled to swim by the flapping of the valves in the water. Among those which have a foot, this organ is employed for many different purposes, such as burrow- ing, leaping, &c. ; which are generally executed in such a manner, as to imply that they are in some degree under the direction of consciousness. And it is in the most free and active of these Bivalves, that we have the most distinct indications of the existence of organs of special sensation ; these, however, where they exist, are not restricted to any particular part of the body, but are disposed along the free margins of the lobes of the mantle. For reasons which will be given hereafter, it appeal's probable that, whilst the general movements of the foot are directed by the cephalic ganglia, under the guidance of sensations, the particular actions by which it fixes itself on a given surface, and adapts its disc to the inequalities which it encounters, may be produced simply by impressions conveyed to this ganglion through its afferent nerves, and reflected through its motor fibres, in which sensations are not necessarily concerned. The same may be inferred respecting the actions of the branchial ganglion ; which, so far as they participate in the respiratory function, may probably be considered as simply reflex ; but when the adductor muscle is caused by it to draw together the valves, either for protection from threatened injury, or for the propulsion of the body through the water, the movement being prompted or directed by consciousness, we must regard the cephalic ganglia as its original source. — The whole course of the lives of these animals shows them to be so little elevated in the scale of psychical endowment, that we can scarcely regal'd the motions executed by them as possessing a voluntary character. The greater part of them are concerned in protecting the animal from danger, and in the prehension of its food ; and may be compared in the higher animals to the closing of the glottis against irritating matters, and to the contraction of the pharynx in swallowing. 764. As the head is not otherwise indicated, in the two preceding classes of Mollusca, than by the position of the mouth, and as the organs of special sensation are but very imperfectly evolved, it is not to be wondered at that the cephalic ganglia should be inferior in size to those more connected with powerful and active muscles. But in the higher classes it is very different ; and in proportion as we meet with evidence of the possession of the senses of sight, hearing, &c., and find these organs no longer scattered over the periphery of the body, but collected upon a £ head,’ do we observe a greater concentration of the ganglionic system towards their neighbourhood. — In the lower species of Gasteropoda, the general distribution of the nervous system is not very dissimilar to that which has been last described. Thus, in Car inaria (Fig. 129), the cephalic ganglia are still found separate, lying at the sides of the oesophagus, and connected by a transverse band ; and these receive the optic nerves and tentacular filaments. Besides other branches trans- mitted to the neighbouring organs, two principal trunks are sent back- wards, which unite in a large ganglion situated among the viscera ; and from this, nerves proceed to the foot, respiratory organs, and posterior part of the trunk ; so that it may be regarded as combining the functions of the pedal and branchial ganglia, as is further indicated by its quadrilo- bate form. Generally, however, the cephalic ganglia are much larger in STRUCTURE AND ACTIONS OF THE NERVOUS SYSTEM. 1007 proportion to the rest, than they are in Conchifera ; and they show a tendency to approximate each other, and to gain a position above the oesophagus, as we see in Paludina (Fig. 128, u, u) ; or even to meet upon it in a single mass, as in Aplysia (Fig. 127, d). The branchial ganglion is constantly to be met with, but its position is extremely variable, like that of the organs to whose action it is related ; it is generally found in the immediate neighbourhood of the respiratory organs ; and, as in the pre- ceding group, it is always connected with the cephalic centres by a distinct commissural trunk. It is not unfrequently conjoined into one mass with the pedal or palleal centres ; but its presence may always be recognised by the distribution of its nerves, as well as by its separate con- nection with the cephalic ganglia. The position of the pedal ganglion, which is constantly found in the Gasteropoda, and which is generally double though the foot is single, also varies, but in a less degree ; since it is usually near the ventral surface, at no great distance behind the head. In addition to the foregoing centres, which are common to the Gasteropoda with the higher Conchifera, we find that the muscular mantle usually has a pair of ganglia of its own, which are termed the palleal ; these are sometimes united with the branchial, sometimes with the pedal, according, perhaps, as the movements of the mantle most participate in the respira- tory acts, or in the general locomotion of the body ; but they are recog- nized by the distribution of their nerves, and by their separate connection with the cephalic ganglia. Thus in Aplysia, the cord of communication which unites the single supra-cesophageal ganglion (Fig. 127, d ) with the two infra-cesophageal ganglia (onding also, like the buccal gan- glion of Mollusca, with the third division of the Fifth pah in Vertebrata, by which the muscles of mastication are especially supplied, and with the glosso-pharyngeal, which is especially concerned in the act of deglutition. The union between these nerves in Fishes, near then - origin, is extremely close ; and they may almost be considered as proceding from the same ganglionic centre. On the other hand, the lateral ganglia seem more analogous to the centres of the Sympathetic system in Vertebrata ; espe- cially in the connection of their branches with all the other systems of nerves, and in the share which they have in the formation of the coeliac ganglia. And this view of the relative functions of the two divisions included under the general term of ‘ stomato-gastric system,’ is strengthened by the fact, that the connection between the Sympathetic system and the Par Vagum is peculiarly intimate in Fishes, so that the two sets of nerves can scarcely be isolated from each other. 774. Although the cephalic ganglia are usually larger than those of the ventral trunk, yet their relative size varies considerably. They receive nerve-trunks from the eyes, antennae, and other sensory organs ; and the history of their development shows that they are to be regarded as com- posed of several distinct pairs, which are fused (as it were) into one mass on either side. According to Mr. Newport, the cephalic ganglia of the Centipede are formed by the coalescence of the ganglia of the four segments of which the anterior portion of the head is composed ; whilst the first sub-oesophageal ganglion is in like manner composed of those of four segments which have coalesced to form the posterior part of the head. The relative development of the cephalic ganglia, however, is so closely accordant with that of the visual organs, — as we see, not only in comparing different species with each other, but in comparing the larva and imago states of the same Insects, — that there can be no doubt that they are to lie regarded as chiefly optic ganglia, corresponding with the STRUCTURE AND ACTIONS OF THE NERVOUS SYSTEM. 1017 optic lobes of Fishes. There is not the least trace, in Articulata generally, of anything that can be fairly considered homologous with the cerebrum and cere- bellum of Yertebrata ; the first suboesophageal ganglion, which has been likened to the latter, being really homologous (as the distribution of its nerves abun- dantly proves) with the Medulla Oblongata. 775. We shall now briefly notice the principal varie- ties of this general plan, which are presented to us in the principal subdivisions of the Articulated series. — In the Entozoa, no other than cephalic ganglia (Fig. 141,/) have yet been detected; these give off a band which encircles the oesophagus ; and send back a pair of longitudinal filaments that run towards the caudal extremity of -the body, diverging from one another more or less widely, and giving off branches at inter- vals, that encircle the body. No ganglionic enlarge- ments can be detected in these ; but it does not seem improbable that they may contain nerve-cells. The arrangement of the supposed nervous system in the Rotifera, has been already described (§ 310 «). — In the Annelida we are led to that condition of the nervous system, which has been spoken of as typical of the group of Articulata ; for, whilst the soft-skinned species, in which there are neither organs of special sensation, nor distinct members for propulsion, have scarcely any ganglionic enlargements on the nervous cord, the higher tribes, in which the division into segments be- comes distinct, and in which the animal relies for locomotion more upon the action of its members than upon that of its trunk, have ganglia regularly disposed at intervals corresponding with the division into seg- ments. These ganglia, however, as in the inferior Myriapoda, in which the segments of the body are very numerous, are often so small and so closely set- together, that they seem almost to form one continuous tract (Fig. 306). The cephalic ganglia are usually of small bulk in the Annelida, in accordance with the very imperfect development of their organs of special sense ; but as we pass from them to the lower Myria- poda, and thence through the higher forms of that class to the Insects which are most distinguished for their visual powers, we find a very remarkable increase in the relative size of these organs. — In harmony with the increased development of the sensori-motor portion of the nervous system, do we find the respiratory, stomato-gastric, and visceral centres becoming more distinct, and obviously rising in functional importance. 776. The nervous system of Insects, like the rest of their organs, presents very different aspects at the dif- ferent stages of their metamorphosis; and these have a Fig. 306 . Nervous System of lulus terrestris ; — a, antennal nerves ; b, ce- phalic ganglia; cl, optic nerves ; h, m , lateral ganglia ; l , recurrent nerve; 1, 2, 3, 4, gan- glia of the ventral cord. 1018 SPECIAL AND COMPARATIVE PHYSIOLOGY. peculiarly interesting relation with the general characters and habits of the animals. The Larva, as formerly stated (§ 315