New York State College of Agriculture At Cornell University Ithaca, N. Y. Library .6/ - ^/yyA . 5 /. e^/cce-'. ■V V V Citc^^V^/u^- Cornell University UH""" QP 31.S84 A manual of physiology, with practical e 3 1924 003 108 499 A MANUAL OF PHYSIOLOGY Cornell University Library The original of tiiis book is in tine Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/details/cu31924003108499 Frag (OK the fiat) (side jiijju) Bird Plate I. Camel Blood o/ ituimiiiul 2. The colourless corpuscles of human blood, x 1000. a, eosinophile cells ; 6, finely granular oxyphlle cells ; c, hyaline cells ; d, lymphocyte ; e, polymorphonuclear neutrophile cells (Kanthack and Hardy). The magnification is much greater than in 1 . 3. Cover-glass preparation of spinal cord of ox, x 250. {Stained with methylene blue). Dendritic rproceases f Detached # axis-cylinder process Large -^. V % multipolar nerv6'Cell Axi8-cyliv4ir process Geo Waters ten A Soua ,L^ih, Edio Bipolar 7ierve-c$Us 4. Potassium in a, frog's erythrocyte (black) ; 6, nerve (black) ; c, striped muscle (black) ; d, cartilage cells (yellow) (Maoalluni). Capillary A MANUAL OF PHYSIOLOGY Mitb practical Eyercises G. N. STEWART, M.A., D.Sc, M.D.Edin., D.P.H.Camb. PROFESSOR OF EXPERIMENTAL MEDICINE IN WESTERN RESERVE UNIVERSITY; CLINICAL PHYSIOLOGIST TO LAKESIDE HOSPITAL, CLEVELAND; FORMERLY PROFESSOR OF PHYSIOLOGY IN THE UNIVERSITY OF CHICAGO ; PROFESSOR OF PHYSIOLOGY IN THE WESTERN RESERVE UNIVERSITY ; GEORGE HENRY LKWES STUDENT ; EXAMINER IN PHYSIOLOGY IN THE UNIVERSITY OF ABERDEEN ; SENIOR DEMONSTRATOR OF PHYSIOLOGY IN THE OWENS COLLEGE, VICTORIA UNIVERSITY, ETC. WITH COLOURED PLATE AND 467 OTHER ILLUSTRATIONS SEVENTH EDITION NEW YORK WILLIAM WOOD & COMPANY MDCCCCXIV First Edition, September, 1896 Second Edition, October, 1898 Third Edition, August, 1899 Fourth Edition, September, 1900 ■ Reprinted, September, igoi ; Juh, 1903 and July, 1904 Fifth Edition, November, 1905 Sixth Edition, September, 1910 Seventh Edition, September, 1914 PREFACE TO THE SEVENTH EDITION In the present edition the book has been extensively revised. The rapid progress of biochemistry has rendered it necessary to enlarge greatly and practically to rewrite the chapter on Metabolism. Many changes and additions have also been made in the chapters on Circulation, Respiration, Digestion, Absorption, and Internal Secretion. The blood-gases are considered in much greater detail than in the last edition, and more space is devoted to the general phenomena of the action of enzymes. The newer work on the relation of the heat production and the chemical changes in muscle to the contraction has been taken account of. The chapters on the Nervous System have been brought up to date. The arrange- ment of the book has been improved, it is hoped, by breaking the longer chapters up into sections, and increasing the number of chapters. Many new illustrations have been added, and many of the old ones redrawn. G. N. STEWART. Cleveland, August, 1914. EXTRACT FROM THE PREFACE TO THE FIRST Edition In this book an attempt has been made to interweave formal ex- position with practical work, according to a programme which I have followed for some time past in teaching Physiology to medical students on the other side of the Atlantic, and which has, it is believed, proved to be well adapted to their needs and opportunities. It ought, however, to be explained that, for various reasons, a somewhat wider range of experiment is open to the student in America than in this country. But as nobody will use this book except in a regular laboratory and under responsible guidance, it has not been thought necessary to mark in any special manner the parts of the exercises which the Enghsh student must do by proxy (that is, learn from demonstrations), and the parts he ought to perform for himself. An arrangement of the exercises with reference to the systematic course has this advantage — that by a little care it is possible to secure that practical work on a given subject shall actually be going on at the time it is being expounded in the lectures. Cross-refer- ence from lecture-room to laboratory, and from laboratory to lecture-room, from the detailed discussion of the relations of a phenomenon to the living fact itself, is thus rendered easy, natural, and fruitful. As some teachers may wish to know how a course such as that described in the Practical Exercises may be conducted for a fairly large class, a few words on the method, we have followed may not be out of place. It is obvious that many of the exercises require more than one person for their performance; and it may be said Viii EXTRACT FROM THE PREFACE TO THE FIRST EDITION that, except in the case of the simpler experiments and the chemical work as a whole, which each student does for himself, it has been found convenient to divide the class into groups of four, each group remaining together throughout the session. It is possible that some may find a group of four too large a unit, and it is certain that three, or perhaps even two, would be better; but in a large school so minute a subdivision is hardly possible, without entailing excessive labour on the teachers. The systematic portion of the book is so arranged that it can equally well be used independently of the practical work, and aims at being in itself a complete exposition of the subject, adapted to the requirements of the student of medicine. As to the matter of the text, it is hardly necessary to say that this book does not aspire to the dubious distinction of originality ; and it is literally impossible to acknowledge all the sources from which information has been derived. In many cases names have been quoted, but names no less worthy of mention have often been of necessity omitted. G. N. STEWART. Cambridge, September, 1895. CONTENTS CHAPTER I. INTRODUCTION. PAGE Chemical composition of living matter i Proteins ' i Carbo-hydrates; - 3 Fats - 3 Structure of living matter - 4 Functions of living matter - - 6 CHAPTER II. THE CIRCULATING LIQUIDS OF THE BODY. Section I. — Morphology of the Blood 14 Blood-corpuscles i «; Life-history of the corpuscles . 20 Section II. — General Physical and Chemical Properties of THE Blood 23 Viscosity of blood 23 Reaction of blood 24 Specific gravity of blood 26 Electrical conductivity of blood 26 Relative volume of corpuscles and plasma 27 Haemolysis 28 Agglutination - 30 Precipitins - 31 Anaphylaxis 32 Coagulation of blood t 33 Vaso-constrictor property of shed blood - 45 Section III. — The Chemical Composition of Blood - 47 Haemoglobin and its derivatives 50 Section IV. — Quantity and Distribution of the Blood 55 Quantity of blood - 55 Distribution of blood - - 56 X CONTENTS PAGE Section V. — Lymph and Chyle 57 Lymph - 57 Chyle ^ - - 58 Section VI. — Functions of Blood and Lymph 59 Phagocyiosis 59 Diapedesis • - 61 CHAPTER III. THE CIRCULATION OF THE BLOOD AND LYMPH. Section I. — Preliminary Anatomical and Physical Data 80 Physiological anatomy of the vascular system - 81 Flow of a liquid through tubes - 83 Section II. — ^The Beat of the Heart in its Physical or Mechanical Relations 85 Events in the cardiac cycle 85 The sounds of the heart - 88 The cardiac impulse 9° Endocardiac pressure 92 The ventricular pressure-curve 94 The auricular and venous pressure-curve - - 98 Section HI. — Physical or Mechanical Phenomena of the Circulation in the Bloodvessels - 10 1 The arterial pulse loi Arterial blood-pressure 109 Measurement of the blood -pressure in man 113 Velocity of the blood - 117 Measurement of velocity of blood 120 The volume-pulse 127 The circulation in the capillaries 129 The circulation in the veins ' 132 The circulation-time 135 Work and output of heart 139 Section IV.— The Heart-Beat in its Physiological Rela^ TIONS Intrinsic nerves of the heart - Cause of the heart-beat Conduction and co-ordination in heart 146 Auriculo-ventricular bundle 147 Fibrillary contractions 15 1 Chemical conditions of heart-beat - 152 Resuscitation of the heart 153 Refractory period of heart 155 140 141 141 CONTENTS xi PAGE Section V. — ^The' Nervous Regulation of the Heart (Ex- trinsic Venous Mechanism of the Heart) 156 Action of poisons on the heart 164 Normal excitation of cardiac nervous mechanism - i65 Section VI. — ^The Nervous Regulation of the Bloodvessels (Vaso-Motor Nerves) - 171 The chief vaso-motor nerves - 173 Vaso-dUator fibres 177 Course of the vaso-motor nerves - ■ 179 Vaso-motor centres 180 Vaso-motor reflexes 183 Influence of gravity on the circulation 188 Section VII. — The Lymphatic Circulation • rgo CHAPTER IV. RESPIRATION. Section I. — ^Preliminary Anatomical Data - 221 Physiological anatomy of the respiratory apparatus 222 Blood-supply of the lungs 222 Section II. — ^Mechanical Phenomena of External Respira- tion - - 224 T3rpes of respiration - 228 Artificial respiration 229 Respiratory sounds - 230 Frequency of respiration 233 Vital capacity - 235 Intrathoracic pressure 235 Respiratory pressure 237 Section III. — ^The Chemistry of External Respiration 238 Inspired and expired air 238 Respiratory quotient - 240 Ventilation 241 The quantity of carbon dioxide given off and of oxygen absorbed - - 242 Section IV. — The Gases of the Blood 244 Physical introduction 244 Quantity of the blood-gases 249 Distribution and condition of oxygen in the blood 250 Distribution and condition of carbon dioxide in the blood 253 The tension of the blood-gases 256 xii CONTENTS PAGE Section V. — Internal or Tissue Respiration 263 Seats of oxidation • - 263 Respiration of muscle - 265 Nature of the oxidative process 267 Section VI. — Relation of Respiration to the Nervous System 268 The respiratory centre and its connections 268 Regulation of respiration through the vagus - 270 Action of other afferent fibres on the respiration 274 The chemical regulation of the respiration 275 Apnoea 277 Automaticity of the respiratory centre 278 Special modifications of the respiratory movements - 281 Section VII. — ^The Influence of Respiration on the Blood- Pressure 283 Section VIII. — ^The Effects of breathing Condensed and Rarefied Air 289 Section IX. — Cutaneous Respiration 292 CHAPTER V. VOICE AND SPEECH. Voice 301 Speech - 306 CHAPTER VI. DIGESTION. Section I. — Preliminary Anatomical and Chemical Data 312 Anatomy of alimentary canal 313 Section II. — Mechanical Phenomena of Digestion - 315 Mastication 315 Deglutition 316 Movements of stomach ,320 Movements of intestines 322 Influence of central nervous system on gastro-intestinal movements - 325 Defaecation 326 Vomiting 328 Section III. — Chemistry of the Digestive Juices — Ferments 330 Saliva 338 Gastric juice - 343 Antiseptic functions of gastric juice - - 350 CONTENTS xiii PAGE Section III. (continued) — Pancreatic juice - 352 Bile 357 S11CCUS entericus - 364 Section IV. — Secretion of the Digestive Juices — ^Micro- scopical Changes in the Gland Cells- 368 Changes in pancreas and parotid during secretion 369 Changes in gastric glands during secretion 371 Changes in mucous glands during secretion 375 Mode of formation of the digestive juices 377 Why the tissues of digestion are not affected by the digestive fennents - 382 Section V. — Influence of the Nervous System on the Digestive Glands - 385 Influence of nervous system on salivary glands 385 Influence of nervous system on gastric glands 395 Influence of nervous system on the pancreas - 399 Secretin - 401 Influence of nervous system on secretion of bile 405 Influence of nervous system on the secretion of intestinal juice - 408 Secretion of the digestive juices (summary) 409 Section VI. — ^Survey of Digestion as a Whole 410 Reaction of intestinal contents 414 Bacterial digestion 416 Faeces - 418 CHAPTER VII. ABSORPTION. Section I. — Preliminary Physico-Chemical Data 420 Imbibition, diffusion, and osmosis ' 420 Electrolytes - 422 Surface tension - - - 423 Adsorption 424 Section II. — ^Mechanism of Absorption - 425 Theories of absorption - 427 Permeability of intestinal epithelium- - - 431 Absorption from the peritoneal cavity - 433 Section III, — Absorption of the Various Food Substances 435 Absorption of fat 435 Absorption of carbo-hydrates - - - 439 Absorption of water and salts - - 440 1: Absorption of proteins - 441 »v CONTENTS CHAPTER VIII. FORMATION OF LYMPH, PAGE Different kinds of lymph - - . - 460 Factors concerned in lymph formation - 461 The contribution of the tissue-cells to the Ijrmph - 466 CHAPTER IX. EXCRETION. Section I. — Excretion by the Kidneys — The Chemistry of Urine - - 470 The urine in disease - 480 Section II. — -The Secretion of the Urine 484 Bloodvessels and tubules of the kidney 484 Theories of renal secretion 487 Influence of the circulation on the secretion of urine 499 Section III. — Expulsion of the Urine - 503 Section IV.^Excretion by the Skin - 504 CHAPTER X. METABOLISM, NUTRITION AND DIETETICS. Section I. — ^Metabolism of Carbo-Hydrates — Glycogen 525 Glycogen-formers 528 Function and fate of the glycogen 531 Fate of the sugar — Glycolysis 532 Intermediary metabolism of carbo-hydrates - 534 ' The experimental glycosurias - - 538 Diabetes mellitus 543 Section II. — Metabolism of Fat 547 Formation of fat from carbo-hydrates - 551 Formation of fat from protein 553 Intermediary metabolism of fat 556 Non-nutritive functions of fat 559 Obesity - 559 Metabolism of sterins 561 Metabolism of phosphatides - 562 Section III. — ^Metabolism of Proteins - - 563 Living and dead proteins - - - 566 Formation of amino-acids from tissue-proteins - - 569 Formation of hippuric acid - . - - ^^i CONTENTS XV Section III. (contimted) — page Formation of urea 573 Formation of uric acid - 579 ; Metabolism of nucleic acids and purin bases - 582 Kreatin and kreatinin - : 587 Autolysis 588 Section IV. — Statistics of Nutrition — Income and Expendi- ture OF THE Body in Terms of Matter 590 The nitrogen balance-sheet — ^nitrogenous equilibrium 591 Relation of nitrogenous metabolism to muscular work 599 Relative value of different proteins in nutrition 601 The carbon balance-sheet 606 The oxygen deficit 608 Inorganic salts in nutrition G08 Section V. — Dietetics 5io Stimulants 617 Vitamines 619 CHAPTER XI. INTERNAL SECRETION. Of pancreas Of sexual organs Of thymus Of thyroid and parathyroid Of adrenals Of pituitary 622 626 629 631 637 644 CHAPTER XII. ANIMAL HEAT. Section I. — ^Thermometry and Calorimetry 651 Body-temperature 656 Section II. — -Income and Expenditure of the Body in Terms OF Energy 657 Heat-loss 657 Heat-production 659 ■ Seats of heat-production 662 Section HI. — Thermotaxis • 665 Relations between heat-production, surface and blood-flow- 671 The nervous system and thermotaxis - 674 Fever . 677 Section IV. — Temperature Topography 683 Normal variations in body temperaure 686 XVI CONTENTS CHAPTER XIII. THE PHYSIOLOGY OF THE CONTRACTILE TISSUES. PAGE Section I. — Preliminary Observations — Physical and Tech- nical Data 696 Cilia - 707 Section II. — Physical Properties and Stimulation of Muscle 709 - Elasticity of muscle 709 Stimulation of muscle 711 Direct excitability of muscle - 712 Section III. — Physical and Mechanical Phenomena of the Muscular Contraction 716 Optical phenomena — structure of muscle 716 Mechanical phenomena 719 Muscular fatigue 723 Electrical tetanus 730 Voluntary contraction . 734 Thermal phenomena and transformation of energy in mus- cular contraction - 736 Relation between mechanical energy and heat-production in active muscle 739 Section IV. — Chemical Phenomena of Muscular Contraction 741 Formation of lactic acid 743 The substances metabolized in muscular contraction 745 CHAPTER XIV. NERVE. Section I. — The Nerve-Impulse or Propagated Disturb- ance: its Initiation and Conduction 755 Stimulation of nerve 757 Excitability of nerve 758 Electrotonus - 759 Conduction in nerve - - 764 Velocity of the nerve-impulse 767 Section II. — Chemistry, Degeneration, and Regeneration OF Nerve 768 Chemistry of nerve 768 Degeneration of nerve - 769 Regeneration of nerve 772 Trophic nerves - - 778 Classification of nerves 780 CONTENTS- xvii CHAPTER XV. ELECTRO-PHYSIOLOGY. PAGE Currents of rest and action - - 796 Relation between action current and functional activity • - 797 Polarization of muscle and nerve - - 802 Electrotonic currents 803 Heart-currents 806 Human electro-cardiogram 808 Glandular currents 810 Eye-currents 811 Electric fishes 812 CHAPTER XVI. THE CENTRAL NERVOUS SYSTEM. Section I. — Structure — Histological Elements 819 Development 821 Histological elements 822 Nutrition of the neuron 830 Section II. — General Arrangement of the Grey and White Matter in the Central Nervous System 833 Section III. — Arrangement of Grey and White Matter in Spinal Cord - 835 Tracts of the, cord - 838 Section IV. — ^Arrangement of Grey and White Matter in THE Upper Portion of the Cerebro-Spinal Axis - 841 Section V. — Connections of the Long Paths of the Cord 842 Section VI. — Paths from and to the Cortex 850 Section VII. — Connections of Brain Stem with Cord- Connections of Cerebellum 856 Section VIII. — Functions of the Central Nervous System — THE Spinal Cord - 859 Decussation of the sensory paths - - 866 Reflex action - 869 Principle of the common path 871 Role of the receptor in reflex action - 872 Characteristic properties of the reflex arc 874 Irradiation of reflex action 877 Co-ordination of reflexes 880 Influence of the brain on spinal reflexes - - 882 Automatism, of the spinal cord " 888 xviii CONTENTS PAGE Section IX.— The Cranial Nerves - - 891 Section X. — Functions of the Brain - - 902 Functions of the cerebellum - - 904 Equilibration and orientation - - • 9^7 Forced movements - 912 Functions of the cerebral cortex 9^4 Motor areas 918 Histological differentiation of the cortex 924 Sensory areas - - 931 Aphasia - 934 Localization of function. in central nervous system 940 Reaction time - - 947 Section XI. — Fatigue and Sleep — Hypnosis - - 947 Section XII. — Size of Brain and Intelligence — Cerebral Circulation — Chemistry of Nervous Activity — Cere- bro-spinal Fluid - 952 CHAPTER XVII. THE AUTONOMIC NERVOUS SYSTEM. - 963 CHAPTER XVIII. THE SENSES. The Senses in General 966 Section I. — Vision 968 Physical introduction 968 Structure of the eye 974 Chemistry of the refractive media 976 Refraction in the eye - - 977 Accommodation - 980 Functions of the iris - 986 Defects of the eye - - 987 Ophthalmoscope . - . ggi Skiascopy - - ' 994 Diplopia - - - 997 Steroscopic vision . - . ggg Visual judgments and illusions - . - 1000 Purkinje's figures - ... 1002 Blind spot - 1004 Rods and cones in vision - - 1005 Talbot's law ... iqio Colour vision ... ion Contrast - - . . 1016 Perimetry - - - - -1018 CONTENTS XIX Section I. {continued) — page Colour-blindness 3^o^9 Movements of the eyes ^ '^"^^ Section II.— Hearing - - 1024 Section III. — Smell and Taste ' - 1035 Section IV. — Cutaneous and Internal Sensations - 1038 Tactile senses - 103° Sensations of temperature 1041 Pain - - 1043 Phenomena after section of cutaneous nerves 1045 Muscular sense - ' ^°52 Sensations of hunger and thirst 1Q54 CHAPTER XIX. REPRODUCTION. Regeneration of tissues 1074 Reproduction in the higher animals • io75 Menstruation - - ^°7^ Development of the ovum - 1078 Parthenogenesis 1080 Formation of the embryo - 1081 Development of the connections lO^S Exchange of materials in the placenta - 1085 Metabolism of the embryo - - '^"^^ Parturition - *. ^°93 Milk - - i°95 Cultivation of tissues outside of the body- - io97 Transplantation of tissues - - ^°9^ Parabiosis ^■'"■^ APPENDIX - "°3 INDEX - .1104 PRACTICAL EXERCISES. CHAPTER I. General reactions of proteins - - - - 7 Colour reactions of proteins - - ° Precipitation reactions of proteins ^ Special reactions of groups of proteins 9 Reactions of derivatives of proteins - 9 Carbo-hydrates - . - Fats ------ Scheme for testing for proteins and carbo-hydrates - 13 10 II CONTENTS CHAPTER II. 1. Reaction of blood - 62 2. Specific gravity of blood - 62 3. Coagulation of blood - ' 62 4. Preparation of fibrin-ferment 65 5; Preparation of extracts containing thrombokinasc 65 61 Serum 65 7. Action of serum on artery rings 66 8: Comparison of action of serum and epinephrin on artery rings 66 9. Comparison of action of serum and plasma on artery rings 66 10. Enumeration of the blood-corpuscles - 67 11. Haematocrite 68 12. Electrical conductivity of blood 68 13. Opacity of blood 70 14. Laking of blood 70 15. Haemolysis and agglutination - 71 16! Osmotic resistance of coloured corpuscles 73 17. Blood-pigment - 73 (i) Preparation of haemoglobin crystals 73 (2) Spectroscopic examination of haemoglobin and its derivatives 74 (3) Guaiacum test for blood 76 (4) Quantitative estimation of haemoglobin 76 (5) Haemin test for blood-pigment 78 CHAPTER III. 1. Microscopic examination of the circulating blood 191 2. Anatomy of the frog's heart - 191 3. Beat of the heart 192 4. Apex of the heart 192 5. Heart tracings - 192 6. Dissection of vagus and cardiac sympathetic in frog 194 7. Stimulation of the vagus in the frog - 196 8. Stimulation of the junction of the sinus and auricles 196 9. Action of muscarine and atropia on the heart 197 10. Stannius' experiment - . - . - igy 11. Stimulation of cardiac sympathetic in frof; 197 12. Action of inorganic salts on heart-muscle 198 13. Action of the mammalian heart 199 14. Perfusion of the isolated mammalian heart 203 15. Action of the valves of the heart -- - 204 i6. Sounds of the heart .. - -. - 205 17. Cardiogram . . - . . 205 CONTENTS xxi PAGE i8. Sphygmographic tracings - 206 19. Venous pulse tracing from jugular 207 20. 'Polygraph tracings - 207 21. Plethysmographic tracings - 208 22. Pulse-rate - - 208 23.. Blood-pressure tracing - - 208 24. Estimation of arterial pressure in man 211 25. Influence cf position of the body on blood-pressure' 211 26. Effects of haemorrhage and transfusion on blood-pressure 212 27. Influence of proteoses on blood -pressure 213 28. Effect of suprarenal extract on blood-pressure 214 29. Action of epinephrin on artery rings - - 214 30. Section and stimulation of cervical sympathetic in rabbit 215 31. Determination of the circulation-time 2I5 32. Measurement of the blood-flow in the hands - 218 33. Vaso-motor reflexes 220 CHAPTER IV. 1. Traicing of the respiratory movements in man 293 2. Production of apnoea and periodic breathing in man 294 3. Tracing of the respiratory movements in animals 294 4. Heat dyspnoea - - - - 296 5. Measurement of volume of air inspired and expired 297 6. Cardio-pneumatic movements 297 7. Auscultation of the lungs - 298 8. Measurement of the respiratory pressure 298 9. Estimation of carbon dioxide and water given off by an animal 299 10. Muscular contraction in the absence of free oxygen 300 11. Oxidizing ferments - 300 CHAPTERS VI. AND VII. 1. Contraction of isolated intestines in Ringer's solution 446 2. Effect of serum on the contractions of intestinal segments 447 3. Action of epinephrin on intestinal segments - - - 447 4. Quantitative estimation of ferment action - 447 5. Chemistry and digestive action of saliva ^148 6. Stimulation of the chorda tympani - - - 450 7. Effect of drugs on the secretion of saliva • - 451 8. Digestive action of gastric juice - - 152 9. To obtain chyme and gastric juice - 453 10. Digestive action of pancreatic juice - - 454 11. Chemistry of bile 456 12. Microscopical examination of faeces - - - 457 xxu CONTENTS PAGE 13. Absorption of fat 457 14. Time required for digestion, and absorption of food sub- stances - - 458 15. Quantity of cane-sugar inverted and absorbed in a given time - - 458 16. Auto-digestion of the stomach 459 CHAPTER IX. 1. Specific gravity of urine 508 2. Reaction of urine 508 3. , Chlorides in urine 508 4. Phosphates in urine - 509 5. Sulphates in urine 510 6. Indoxyl in urine 510 7. Urea - - - - 511 8. Ammonia in urine - - 513 9. Total nitrogen in urine - - - 514 10. Uric acid . - - 515 11. Kreatinin - 515 12. Hippuric acid 516 13. Proteins in urine 516 14. Sugar in urine - - - 517 15. Pentoses in urine - 520 16. Acetone in urine 521 17. Determination of the freezing-point of urine 521 18. Examination of urine - 523 19. Urinary sediments 523 CHAPTERS X., XL, AND XII. 1. Glycogen - - 689 2. Catheterism - 690 3. Experimental glycosuria - - 690 (i) Injection of sugar into the blood 690 (2) Phlorhizin glycosuria - 691 (3) Alimentary glycosuria - 691 4. Milk 691 5. Cheese - - - 692 6. Flour - - 692 ,7. Bread - 693 8. Excretion of urea (and total nitrogen) and proteins in food - 693 9. Action of epinephrin - - 6g3 10. Measurement of the heat given off in respiration 694 CONTENTS CHAPTERS XIII. AND XIV. 1. Difference of make and break induction shocks 780 2. Stimulation by the voltaic current 783 3. Ciliary raotion 784 4. Direct excitability of muscle — curara - 784 5. Graphic record of ' twitch ' - 784 6. Influence of temperature on the muscle-curve . 786 7. Influence of load on the muscle-curve - 786 8. Influence of fatigue on the muscle-curve 786 9. Seat of exhaustion in fatigue of the muscle-nerve preparation 786 10. Influence of veratrine on muscular contraction - 787 1 1 . Measurement of the latent period of muscular contraction 788 12. Summation of stimuli - - 789 13. Superposition of contractions - - 789 14. Composition of tetanus. 789 15. Contraction of smooth muscles - 790 16. Velocity of the nerve-impulse 791 17. Chemistry of muscle - - 792 18. Reaction of muscle in rest, activity, and rigor - 793 CHAPTER XV. 1. Galvani's experiment - - 814 2. Contraction without metals - - - 814 3. Secondary contraction - - 814 4. Demarcation and action currents with capillary electrometer 814 5. Action current of the heart - 816 6. Electrotonus - - 816 7. Paradoxical contraction - 816 8. Alterations in excitability and conductivity produced in nerve by a voltaic current - 816 9. Formula of contraction 817 10. Formula of contraction for (human) nerves in situ 818 11. Ritter's tetanus - 818 CHAPTER XVI. 1. Section and stimulation of nerve-roots - - 957 2. Reflex action in the ' spinal ' frog - 958 3. Reflex time - - 958 4'. Inhibition of the reflexes 959 5. Spinal cord and muscular tonus - - - 959 6. Spinal cord and tonus of the bloodvessels - - 959 7. Action of strychnine - 959 CONTENTS 8. Mammalian spinal preparation - - 959 9. Reflexes in man ■ - . . . 961 10. Excision of cerebral hemispheres in the frog - •■ 961 11. Excision of cerebral hemispheres in the pigeon 961 12. Stimulation of the motor areas in the dog - • 962 CHAPTER XVIII. 1. Dissection of the eye - - - - 1058 2. Formation of inverted image on the retina 1059 3. Helmholtz's Phakoscope 1059 4. Scheiner's experiment - 1060 5. Kiihne's artificial eye 1061 6. Astigmatism f ophthalmometer) 1062 7. Spherical aberration - 1063 8. Chromatic aberration 1063 9. Measurement of the field of vision - 1063 10. Mapping the blind spot - - 1064 11. The yellow spot - - 1064 12. Ophthalmoscope - - 1065 13. Retinoscopy ' 1066 14. PupUlo-dUator and constrictor fibres - 1067 15. Colour-mixing - - 1068 16. After-images - 1068 17. Retinal fatigue 1068 18. Visual acuity 1068 19. Colour-blindness 1069 20. Talbot's law - 1670 21. Purkinje's figures 1670 22. Relation of pitch and vibration frequency 1070 23. Beats 1070 24. Sympathetic vibration - 1070 25. Galton's whistle - - 1070 26. Cranial conduction of sound 1670 27. Taste - 1070 28. Smell 1071 29. Touch and pressure - 1071 30. Temperature sensations 1072 31. Pain 1073 CHAPTER XIX. Contractions of isolated uterine rings - - *■ - iioi A MANUAL OF PHYSIOLOGY CHAPTER I. INTRODUCTION Living tnatter, whether it is studied in plants or in animals, has certain peculiarities of chemical composition and structure, but especially certain peculiarities of action or function, which mark it off from the unorganized material of the dead world around it. Chemical Composition of Living Matter. — Although we cannot analyze the living substance as such, we can to a certain, but hmited, extent reconstruct it, so to speak, from its ruins. When subjected to analytical processes, which necessarily kill it, living matter invariably yields bodies of the class of proteins, exceedingly complex substances, which have approximately the following com- position: Carbon, 51-5 to 54-5 per cent.; oxygen, 20-9 to 23-5 per cent. ; nitrogen, 15-2 to 17 per cent. ; hydrogen, 6.-9 to 7-3 per cent., with small quantities of sulphur. Nucleo-proteins, which are com- pounds of ordinary proteins with nucleic acids, a series of sulphur- free organic acids rich in phosphorus, are constantly met with. Certain carbo-hydrates, composed of carbon, hydrogen, and oxygen (the last two in the proportions necessary to form water), of which glycogen (C5H,oOb)„ may be taken as a type, appear to be always present. Fats, which consist of carbon, hydrogen, and oxygen, and of whjch tristeaiin, a compound of stearic acid with glycerin, of the formula C3H5,3(Ci8H3502), may be given as an example, arc; often, and certain lipoids, e.g., lecithin (p. 4), are always, found. Finally, water and certain inorganic salts, such as the chlorides and phosphates of sodium, potassium, and calcium, are constantly present. The Proteins, — ^The constitution of the protein molecule is still un- known ; but when proteins are broken down by the action of ferments, such as exist in gastric and in pancreatic juice, or by chemical methods — ^for example, by boiling with dilute acids — ^the most important of the cleavage products are various amino-acids (p. 354). It has there- fore been suggested that proteins are built up by the linking together of amino-acids, the different proteins differing quantitatively or quali- tatively as regards the amino-acids present (E. Fischer). Thus serum- albumin and egg-albumin yield no glycin or glycocoll (amino^acetic acid, CHj.NHa.COQH;), while glycin is constantly found am,ong the cleavage products of serum-globulin. And while leucin ' (a-amino- 2 INTRODUCTION isobutylacetic acid) is present to the extent of about 20"5 per cent, in the cleavage products of (horse's) serum-albumin, (hen's) egg-albumin yields only y i per cent. On the other hand, egg-albumin yields 8'i per cent, of alanin (amino- propionic acid, CaH4.NHg.COOH), while serum-albumin yields only 2' 7 per cent. Of the aromatic amino-acids — ^that is, amino-acids united to the benzene ring — ^phenyl-alanin (amino-propionic acid in which one atom of H is replaced by phenyl, CgHg) is obtained to the extent of 4' 4 per cent, from egg-albumin,. 'and a little over 3 per cent, from serum- albumin. Tyrosin or oxyphenyl-alanin (amino-propionic acid in which a H atom is replaced by oxyphenyl, CgH^.OH) appears to the amovmt of I" 5 per cent, among the cleavage products of egg-albumin, and to the amount of 2'i per cent, among those of serum-albumin. It is an interesting point in this connection that gelatin, which yields i6"5 per cent, of glycin, yields no tyrosin at all; tryptophane, an aromatic amino-acid still more complex than tyrosin, is also absent. These facts afford an explanation of certain colour reactions of proteins long known empirically, but only recently understood (p. 8). The process by which the protein molecule is thus decomposed is called hydrolysis — ^that is, the molecule takes up water, and then splits into smaller molecules. The hydrolysis occurs in various stages, bodies like acid- or alkali- albumin (meta- or infra-proteins) being first formed, then proteoses, then, peptones. The peptones are further split into bodies containing a relatively small number of amino-acids linked together. These bodies are called pol3rpeptides, which finally are decomposed so as to yield the individual amino-acids, also called in this connection the peptides or monopeptides, the " building-stones " out of which the protein molecule is constructed. The inverse process can also be carried on to a certain extent, and Fischer has taken an important step towards the eventual synthesis of proteins by showing how polypeptides of increasing com- plexity can be built up by linking amino-acids together. When two amino-acids are so united, the resulting compound is called a dipeptide ; with three amino-acids we get tripeptides, etc. Still more complicated polypeptides may thus be formed in the laboratory, which give some of the characteristic reactions of peptones. The numerous substances included in the group of proteins may be classified as follows, beginning with the simplest: 1. Protamins, such as the bodies called salmin and sturin present in fish-sperm. 2. Histones, bodies separated from blood-corpuscles. Globin, the protein constituent of haemoglobin, is one of them. Unlike the other groups of proteins, they are precipitated by ammonia. ' 3. Albumins. 4. Globulins. 5. Sclero-proteins or albuminoids, such as gelatin and keratin. 6. Phospho-proteins, including such substances as vitellin, a body obtainable from egg-yolk, and caseinogen, the chief protein of milk. They are rich in phosphorus, but are to be distinguished from nucleo- proteins, which also contain a relatively large amount of phosphorus, by the fact that they do not yield tho purin bases, the characteristic products of the decomposition of nucleo-proteins. ■ 7. Conjugated proteins, substances in which the protein molecule is united to another constituent, usually spoken of as a ' prosthetic ' group. Thus the nucleo-proteins consist of protein united with nucleic acid, the chromo-proteins (e.g., haemoglobin) of protein united with a pig- ment, and the gluco-proteins (e.g., mucin) of protein united with a carbo-hydrate group. CHEMICAL COMPOSITION OF LIVING MATTER 3 Among the derivatives of proteins, the most important are those already mentioned as being produced in protein-hydrolysis, viz. : (a) Meta-proteins. (6) Proteoses, including albumose, the proteose derived from albu- min; globulose, that derived from globulin; gelatose, that derived from gelatin, etc. The proteoses may be further subdivided, according to the order in which they are formed in digestion into proto-proteoses, hetero-proteoses, and deutero-proteoses. (c) Peptones. \d) Polypeptides. The majority of these are artificial products, formed by the synthesis of amino-acids, although some can be obtained from proteins by hydrolysis. Only a few of those hitherto prepared give the biuret test. However formidable the above list may appear to the student, it gives an inadequate idea of the extreme complexity of the protein class and its richness in individuals. For, apart from the fact that the list has been purposely left incomplete, especially as regards the numerous vegetable proteins, there is the best evidence that proteins of the same name from different animal species have certain properties which dis- tinguish them from each o'ther. The serum-albumins can be crystal- lized much more easily in some animals than in others. The same is conspicuously true of the haemoglobins, which differ also in certain animals in the relative proportion of sulphur and iron in the molecule, as well as in the crystalline form. Even when no chemical or physical differences have as yet been made out, proteins of the same name from the blood or organs of different species show notable ' specific ' differ- ences when subjected to certain biological tests (see, e.g., the paragraph on Precipitins, p. 31 ; and that on Anaphylaxis, p. 32). Carbo-^Hydrates. — ^The most important carbo-hydrates in their physio- logical relations are dextrose, levulose, galactose, lactose, maltose, sucrose (cane-sugar), starch, and glycogen. As regards their chemical constitution, the simplest carbo-hydrates are aldehydes or ketones — that is, the first oxidation products of primary and secondary alcohols respectively. Thus dextrose is the aldehyde of sorbite, a hexatomic alcohol (an alcohol containing six OH groups), while levulose is the ketone of the isomeric alcohol called mannite, and galactose the alde- hyde of the isomeric alcohol called dulcite. The sugars containing six carbon atoms are termed hexoses. They include dextrose, levulose, and galactose. The empirical formula of these three simple sugars (or monosaccharides) is the same (C6Hi20g), but, owing to the different arrangement of the atoms or groups of atoms, they have each their characteristic properties by which they can be easily distinguished. For example, dextrose rotates the plane of polarization to the right, levulose to the left. By the union or ' condensation ' of two molecules of a monosaccharide, with loss of a molecule of water, a disaccharide is formed. Cane-sugar, maltose, and lactose, all with the same empirical formula, (Ci2H2aOii), are disaccharides. Cane-sugar yields on hydro- lysis a mixture of equal parts of dextrose and levulose ; lactose, a mix- ture of dextrose and galactose ; while maltose is converted into dextrose. By the condensation of more than two molecules of monosaccharide polysaccharides are formed, such as starch, dextrin, and glycogen. The exact molecular weights of these substances are unknown. Their general formula can be written (C6Hxo05)», where n represents the number of monosaccharide molecules condensed to form the poly- saccharide, in the case of starch probably some hundreds. Fats and Lipoids. — ^The fats are compounds of higher fatty acids with glycerin (glycerin esters). The ordinary body-fat consists of a 4 INTRODUCTION mixtura of three neutral fats (palmitin, stearin, and olein) which differ both chemically and physically from each other — e.g., in melting-point and in the so-called iodine value, the number which represeijits the amount of iodine taken up from a standard solution. Olein melts at -5° C, palmitin at 45° C, and stearin at a still higher temperature. It is, therefore, the presence of olein which keeps the body-fat liquid at the temperature of the body. . The fats are soluble in ether, in hot alcohol, and in many other liquids, but insoluble in water. ' Besides the ordinary fats, the tissues and liquids of the body contain phospha- tides, a group of compounds which stand in close relation to the fats, but differ in containing phosphoric acid and nitrogenous bases. The most important representative of this group is lecithin (C42Hg4NP09), a fat-like compound which yields on decomposition, in addition to glycerin and a fatty acid, phosphoric acid and a nitrogen-containing substance called cholin (p. 360). Lecithin, though found in all cells, is especially abundant in nervous tissues. It is associated with choles- terin and with other substances which, like lecithin and cholesterin, are soluble in ether and similar solvents of fat. For this reason these substances are often grouped together as lipoids, although some of them are chemically different from fat. Cholesterin, for instance, is an alcohol. Although usually present only in small amount, the lipoids play a very important part in the structure and in the economy of the cell. Structure of Living Matter — ^The Cell.*^ — Bioplasm is the name given to the living matter of cells. The portion of the bioplasm differentiated as the nucleus is distinguished by the term karyo- plasm, and the portion outside the nucleus by the term protoplasm or cytoplasm. Protoplasm, when examined in its most primitive undifferentiated condition in such cells as the amoeba or the white blood-corpuscles, appears on first view a homogeneous, structureless mass, except for certain granules embedded in it, and consisting either of products formed by its activity or of food materials. But even here more careful study reveals a certain complexity of struc- ture. At the very least, an external layer, or ectoplasm, can be dis- tinguished from the interior mass, or endoplasm. There is reason to believe that even where no histological demonstration of an ectoplasmic layer or a definite envelope is possible, the surface of the cell is physiologically different from its interior. In many cells the protoplasm presents the appearance of a honeycomb or net- work, with granules usually situated at the nodes, and holding in its vesicles or meshes a fluid, perhaps containing pabulum, from which the waste of the living framework is made good, or material upon which it works, and which it is its business to transform. Some observers, however, maintain that the network is an artificial appearance produced by the precipitation of the colloid constituents of the protoplasm by the fixing reagent, or even by the coagulative processes associated with the act of dying, and that the unaltered living substance is a homogeiieous fluid or jelly. It is known that * Space permits only the slightest sketch of this subject here. For de- tailed information the student is referred to textbooks of histology. STRUCTURE OF LIVING MATTER 5 changes of reaction occur when the living substance dies, and slight changes of reaction, i.e., changes in the relative concentration of hydrogen, ions (H + ) and hydroxyl ions (0H-), can bring about similar precipitates in colloid solutions. Nevertheless in some cells a certain differentiation in the structure of the protoplasm can be seen during life and before the addition of any reagent, and in such cases there can be no doubt that the structural details pre-exist and are not arte-facts. In certain respects protoplasm behaves like a liquid, and in others like a solid, a peculiarity which is un-- doubtedly associated with the fact that its chief constituents exist in the colloid state, as experiments with such substances as gelatin and agar have shown. In building up our typical ceU we start with a piece of protoplasm. Somewhere in the midst of this we find a body which, if not absolutely different in kind from the protoplasm of the rest of the cell or cytoplasm, is yet marked off from it by very definite morphological and chemical characters. This is the nucleus, generally of round or oval shape, and bounded by an envelope. Within the envelope lies a second network of fine threads, which do not themselves stain with nuclear dyes such as hsematoxylin. But in or on these ' achromatic ' filaments lie small, highly refractive particles, staining readily and deeply with dyes, and therefore described as consisting of chromatin. This chro- matin is either made up of nucleins (conjugated proteins particu- larly rich in, nucleic acid, and therefore in phosphorus), or yields nucleins by its decomposition; and it seems to owe its affinity for certain staining substances to the presence of nucleic acid. The meshes of the nuclear reticulum contain a semi-fluid material, which does not readily stain. The nucleus is distinguished from the cytoplasm, even as regards its inorganic constituents, by the absence of potassium.* Besides the nucleus, another much smaller structure, the centrosome, is differentiated from the protoplasm, of many cells. This is a minute dot staining deeply with such dyes as haematoxylin, and generally situated near the nucleus. Sur- rounding it is a clear area, the attraction sphere, in and beyond which fine fibrils radiate out into the cytoplasm. Both the attrac- tion sphere and the nucleus play an important part in division of the cell by the process known as karyokinesis, or mitosis, or in- direct division, which is by fat the most common mode. When the nucleus is about to divide, the chromatin granules arrange themselves into one or more coiled filaments or' skeins, which then break up into a number of separate portions called * This has been shown microchemically. The potassium is precipitated by a solution of hexanitrite of sodium and cobalt as orange-yellow crystals of the triple salt, hexanitrite of potassium, sodium, and cobalt. Where very minute traces of potassium are present, ammonium sulphide must be added, after washing out the excess of the cobalt reagent. Black cobalt sulphide is thus formed from the triple salt (Macallum, Frontispiece). 6 INTRODUCTION chromosomes. These undergo a remarkable series of transforma- tions, leading e-ventually to the segregation of the nuclear .chromatin in two separate daughter nuclei, each surrounded by a portion of the original cytoplasm. Apart from its role in the division, and therefore in the multiplication, of the cell, the nucleus is now known to exert an influence perhaps not less important upon those chemical changes in the cytof)lasm which are necessary for its normal nutri- tion and function.* It is doubtful whether any portion of proto- plasm can permanently survive the loss of its nuclear material. It must be remembered, however, that nuclear material may some- times be present in diffuse form in cells which do not show a nucleus in the histological sense. When we carry back the analysis of an organized body as far as we can, we find that every organ of it is made up of cells, which upon the whole conform to the type we have been describing, although there are many differences in details. Some organisms there are, low down in the scale, whose whole activity is confined within the narrow limits of a single cell. The amoeba sets up in life as a cell split off from its parent. It divides in its turn, and each half is a complete amoeba. When we come a little higher than the amoeba, we find organisms which consist of several cells, and ' specialization of function ' begins to appear. Thus the hydra, the ' common fresh-water polyp ' of our ponds and marshes, has an outer set of cells, the ectoderm, and an inner set, the endoderm. Through the superficial portions of the former it learns what is going on in the world; by the contraction of their deeply placed processes it shapes its life to its environment. As we mount in the animal scale, specialization of structure and of function are found con- tinually advancing, and the various kinds of cells are grouped together into colonies or organs. In some organs and tissues the bond of union is simple juxtaposition and similarity of function of the constituent cells. But in others the union is protoplasmic, pro- cesses of the cytoplasm actually passing from cell to cell. This is seen in certain epithelial tissues, and conspicuously in the cardiac muscle. The Functions of Living Matter.— The peculiar functions of living matter as exhibited in the animal body will form the subject of the main portion of this book; and we need only say here: (i) That in all living organisms certain chemical changes go on, the sum total of which constitutes the metabolism of the body. These may be divided into (a) integrative or anabolic changes, by which complex substances (including the living matter itself) are built up from * According to Hertwig, a precursor of chromatin, ' prochromatin,' a sub- stance without characteristic staining reaction, is formed in the cytoplasm, taken up by the nucleus, and there elaborated into chromatin. From the nucleus chromatin and its derivatives return to the cytoplasm to be used in its function. FUNCTIONS ;^0F LIVING MATTER 7 simpler materials; and (6) disintegrative or katdbolic changes, in which complex bodies (including the living substance) are broken down into comparatively simple products. In plants, upon the whole, it is integration which predominates; from substances so simple as the carbon dioxide of the air and the nitrates of the soil the plant builds up its carbo-hydrates and its proteins. In animals the main drift of the metabolic current is from the complex to the simple; no animal can construct its own protoplasm from the inorganic materials that lie around it; it must have ready-made protein in its food. But in all plants there is some disintegration; in all animals there is some synthesis. The progress of biochemistry in recent years has indeed shown that the synthetic powers of animal ceUs have been greatly underestimated. (2) The living sub- stance is excitable — that is, it responds to certain external im- pressions, or stimuli, by actions peculiar to each kind of cell. (3) The living substance reproduces itself. All the manifold activities included under these three heads have but one source, the trans- formation of the energy of the food. It is not, however, upon the whole, peculiarities in food, but in molecular structure, that underlie the peculiarities of function of different living cells. A locomotive is fed with coal; a steam-pump is fed with coal. The one carries the mail, and the other keeps a mine from being flooded. Wherein lies the difference of action ? Clearly in the build, the structure of the mechanism, which determines the manner in which energy shall be transformed within it, not in any difference in the source of the energy; So one animal cell, when it is stimulated, shortens or con- tracts; another, fed perhaps with the same food, selects certain constituents from the blood or lymph, and passes them through its substance, changing them, it may be, on the way ; and a third sets up impulses which, when transmitted to the other two, initiate the contraction or secretion. In the living body the cell is the machine ; the transformation of the energy of the food is the process which ' runs ' it. The structure and arrangement of cells and the steps by which energy is transformed within them sum up the whole of biology. PRACTICAL EXERCISES ON CHAPTER I. Reactions of Proteins. I. Gsneral Reactions of Proteins. — ^Egg-albumin may be taken as a typ3. Prepare a solution of it by adding water to white of egg, which coasists miinly of egg-albumin with a little globulin. In breaking the egg, take care that none of the yolk gets mixed with the white. Snip the white up with scissors in a large capsule, then add ten or fifteen times its volume of distilled water. The solution becomes turbid from the precipitation of traces of globulin, since globulins are insoluble in distilled water. Stir thoroughly, strain through several layers of muslin, and then filter through paper. INTRODUCTION Colour Reactions. (i) Add to a little of the solution in a test-tube a few drops of strong nitric acid. A precipitate is thrown down, which becomes yellow on boiling. Cool, and add strong ammonia; the colour changes to orange {xaniho-proteic reaction). The reaction depends upon the presence of aromatic groups in the protein (in phenylalanin, t}rrosin, tryptophane, oxytryptophane), which are conyerted into nitro-compounds. (2) To a third portion add a drop or two of very dilute cupric sulphate and excess of sodium or potassium hydroxide ; a violet colour appears {Piotrowski's test). Peptones and proteoses (albumoses) give a pink [biuret reaction) .* See p. 452. (3) To another portion add Millon's reagent ;t a white precipitate comes down, which is turned reddish on boiling. If only traces of protein are present, no precipitate is caused, but the liquid takes on a red tinge. The reaction is due to tyrosin. It is given by all aromatic substances which contain the group CgHg with at least one H replaced by OH, i.e., the hydroxyphenyl group C6H4OH. (4) Adamkiewicz' s Reaction [Hopkins's modification). — ^To a small quantity of the albumin solution add the same bulk of dilute glyoxylic acid. J Mix, and to the mixture add an equal volume of strong pure sulphuric acid. A purple colour is obtained. The substance in the protein molecule which gives the reaction is tryptophane (p. 354). (5) The Formaldehyde Reaction. — ^Add to the albumin solution a few drops of a very dilute solutioh of formaldehyde (i : 2,500), and then allow some strong (commercial) sulphuric acid to run from a pipette into the bottom of the test-tube. A purple ring appears at the surface of contact. This reaction depends on the presence of tryptophane in the protein. Precipitation Reactions. (6) Acidify another portion strongly with acetic acid, and add a few drops of a solution of potassium ferrocyanide. A white precipitate is obtained. Peptones do not give this reaction. (7) Heat a portion to 30° C. on a water-bath. Saturate with crystals of ammonium sulphate; the albumin is precipitated. Filter, and test the filtrate for proteins by (2). None, or only slight traces, will be found. The sodium hydroxide must be added in more than sufficient quantity to decompose all the ammonium sulphate. It will be best to add a piece of the solid hydroxide. Peptones are not precipitated by ammonium sulphate, but all other proteins are. (8) Add alcohol to a small quantity of the solution. The protein is * The reaction is also given, although more faintly, with the hydroxides of lithium, strontium, and barium. It is given by all substances containing at least two CONHg groups attached to one another (as in oxamide) , or to the nitrogen atom (as in biuret), or to the same carbon atom. •f Millon's reagent consists of a mixture of the nitrates of mercury with nitric acid in excess, and some nitrous acid. To make it, dissolve mercury in its own weight of strong nitric acid, and add to the solution thus obtained twice its volume of water. Let it stand for a short time, and then decant the clear liquid, which is the reagent. I A solution containing glyoxylic acid in the requisite strength can be prepared by treating half a litre of a saturated solution of oxalic acid with 40 grammes of 2 per cent, sodium amalgam in a tall cylinder. When all the hydrogen has been evolved, the solution is filtered, and diluted with twice its volume of water. Oxalic acid and sodium binoxalate are also present in the solution. PRACTICAL EXERCISES 9 precipitated. It can be redissolved at first, but rapidly becomes in- soluble. 2. Special Reactions of Certain Proteins — (i) Heat-Coagulable Pro- teins : (a) Albumins.^{o) Heat a little of the solution of egg-albumin in a test-tube; it coagulates. With another sample determine the tem- parature of coagulation, first very slightly acidulating with a 2 per cent, solution of acetic acid. To determine the Tem.perature of Coagulation. — Support a beaker by a ring which just grips it at the rim. Nearly fill the beaker with water, and slide the ring on the stand till the lower part of the beaker is im- mersed in a small water-bath (a tin can will do quite well) . In this bsaker place a test-tube, and in the test-tube a thermometer, both sup- ported by rings or clamps attached to the same stand. Put into the test-tube at least enough of the albumin solution to completely cover the bulb of the thermomster, and heat the bath, stirring the water in the bsaker occasionally with a feather or a splinter of wood, or a glass rod, the end of which is guarded with a piece of indiarubber tubing. Note the temperature at which the solution becomes turbid, and then the temperature at which a distinct coagulum or precipitate is formed. Repeat with the unacidulated alburnin solution. (j3) A similar experiment may he performed with serum-albumin obtained as on p. 65. (6) Globulins. — Use serum-globulin (p. 65), or myosinogen (p. 793). Fibrinogen is also a globulin, but cannot easily be obtained in quantity. Verify the following properties of globulins : (a) They coagulate on heating. (/3) They are insoluble in distilled water (p. 65). [y) They are precipitated by saturation with magnesium sulphate or sodium chloride (p. 65). They give the general protein tests (i) to (8). Both the heat-coagulated proteins and such proteins as the solid fibrin which is formed from fibrinogen in the clotting of blood give such of the general protein tests, (i), (2), (3) (p. 8), as with suitable modifica- tions can be instituted on solid substances. Thus, in performing (2), a flake of fibrin or a small piece of the boiled egg-white should be soaked for a few minutes in a dilute solution of cupric sulphate. Then the excess of the cupric sulphate should be poured off, and sodium hydroxide added, when the coagulated protein will become violet. Heat-coagu- Jated proteins are insoluble in water, weak acids and alkalies, and saline solutions; fibrin is slightly soluble in the latter. {2) G,elatin. — Add some pieces of gelatin to cold water in a test-tube. It does not dissolve. Immerse the tube in a boiling water-bath till the gelatin goes into solution. Then cool the test-tube under the tap; the solution sets into a jelly. On heating it redissolves. Try the general protein reactions (p. 8) on a dilute solution. In Piotrowski's test a violet colour is obtained. The tests which depend on the presence of tyrosin or tryptophane are not given by a 'solution of pure gelatin, since these amino-acids are absent from the gelatin molecule. Commercial gelatin may give a slight reaction due to traces of other proteins. 3. Reactions of Certain Derivatives of Native Proteins — (i) Meta- Proteins : (a) Acid-Albumin. — ^To a solution of egg-albumin add a little 04 per cent, hydrochloric acid, and heat to about body temperature — say 40° C. — ^for a few minutes. Acid-albumin is formed. It can be produced from all albumins and globijlins by the action of dilute acid. Make the following tests : (a) Add to a portion of the solution' in a test-tube a few drops of a 10 INTRODUCTION solution of litmus; the colour becomes red. Now add drop by drop sodium carbonate or dilute sodium hydroxide solution till the tint just begins to change to blue. A precipitate of acid-albumin is thrown down. Add a little more of the alkali, and the precipitate is redissolved. It can be again brought down by neutralizing with acid. (/3) Heat a portion of the solution to boiling; no precipitate is formed. (y) Add strong nitric acid ; a precipitate appears, which dissolves on heating, and the liquid becomes yellow. (b) Alkali-albumin. — To a solution of egg-albumin add a little sodium hydroxide, and heat gently for a few minutes. Alkali-albumin is produced. It can be derived by similar treatment from any albumin or globulin. (a) Neutralize, after colouring with litmus solution, by the addition of dilute hydrochloric or acetic acid. Alkali-albumin is precipitated when neutralization has been reached. It is redissolved in excess of the acid. (;8) To another portion of the solution of alkali-albumin add a few drops of sodium phosphate solution, then litmus, and then dilute acid till the alkali-albumin is precipitated. More of the dilute acid should now be required to precipitate the alkali-albumin, since the sodium phosphate must first be changed into acid sodium phosphate. (7) On heating the solution of alkali-albumin there is no coagulation. (2) Proteoses. — For preparation and reactions, see p. 452. They differ from albumins and globulins in not being coagulated by heat, and from meta-proteins in not being precipitated by neutralization. They are soluble (with the exception of hetero-albumose) in distilled water, and are not precipitated by saturation of their solutions with mag- nesium sulphate or sodium chloride. Saturation with ammonium sul- phate precipitates them. With a solution of ' commercial peptone,' which consists chiefly of albumoses, and contains only a little true peptone, perform the following tests: (a) Boil the slightly acidulated solution; there is no coagulation. (j3) Biuret reaction, p. 8. (■y) To a portion of the solution add its own volume of saturated ammonium sulphate solution. The primary albumoses (proto- and hetero-albumose) are precipitated. Filter. Add a drop of sulphuric acid to the filtrate and saturate it with ammonium sulphate crystals. The secondary or deutero-albumoses are precipitated. Filter. The filtrate still contains peptones. Use it for (3). (3) Peptones. — For preparation and tests, see p. 453. They differ from heat-coagulable proteins and meta-proteins in the same way as proteoses, and they differ from proteoses in not being precipitated by ammonium sulphate. On the filtrate from (2) perform the biuret test, as described in (7), p. 8; and note that the pink colour is the same as that given by proteoses. Carbo-Hydrates. I. Glucose or Dextrose. — Make a solution of dextrose in water, and apply to it Trommer 's test for reducing sugar. Put some of the desitrose solution in a test-tube, then a few drops of cupric sulphate, and then excess of sodium or potassium hydroxide. The blue precipitate of cupric hydroxide which is first thrown down is immediately dissolved in the presence of dextrose and many other organic substances. Now boil the blue liquid, and a yellow or red precipitate (cuprous hydroxide or oxide) is formed. r PRACTICAL EXERCISES ii 2. Cane-Sugar. — Perform Trommer's test with a sample of a solution. A blue liquid is obtained, which is not changed on boiling. Now put the rest of the solution in a flask. Add ^th of its bulk of strong hydro- chloric acid, and boil for a quarter of an hour. Again perform Trom- mer's test. Remember that excess of alkali must be present after the acid is neutralized. The test now shows much reducing sugar. The cane-sugar has been ' inverted ' — i.e., changed into a mixture of dextrose and levulose. 3. Starch. — (i) Cut a slice from a well-washed potato ; take a scraping from it with a knife, and examine with the microscope. Note the starch granules with their concentric markings, using a small diaphragm. Run a drop of dilute iodine solution under the cover-slip, and observe that the granules become bluish. Examine also with a polarization microscope. (2) Rub up a little starch in a mortar with cold water, then add boiling water and stir thoroughly. Decant into a capsule or beaker, and boil for a few minutes. After the liquid has cooled, perform the following experiments : {a) Add a few drops of iodine solution to a little of the thin starch mucilage in a test-tube. A blue colour is produced, which disappears on heating, returns on cooling, is bleached by the addition of a little Sodium hydroxide, and restored by dilute acid. (6) Test the starch solution for reducing sugar by Trommer's test. If none is found, boil some of the mucilage with a little dilute sulphuric acid in a flask for twenty minutes, and again perform Trommer's test. Abundance of reducing sugar will now be present. 4. Dextrin. — Dissolve some dextrin in boiling water. Cool. Add iodine solution to a portion; a reddish-brown (port-wine) colour results, which disappears on heating. As a control, the same amount of iodine should be added to an equal quantity of water in another test-tube. The colour returns on cooling. The colour is also bleached by alkali, restored by acid. Excess of iodine should be added for the bleaching experiment (i.e., more than enough to give the maximum d^pth of tint). If too little iodine has been added, there may be no restoration of the colour by the acid. The addition of a little more iodine to the acid solution will then cause the port-wine colour to return, and this may be again bleached by alkali, and will now be restored by acid. 5. Glycogen. — See p. 689. 6. Molisch's Test for Carbo-Hydrates. — ^This is a general test for carbo- hydrates. It is also given by proteins which contain, a carbo-hydrate group. Put a drop of dextrose solution in a test-tube. Add a drop of a 10 per cent, solution of a-naphthol in methyl alcohol, and then o'5 c.c. of water. Then cautiously allow i c.c. of pure concentrated sulphuric acid to run under the mixture, and shake gently. A violet or reddish colour appears. Fats. 1. Take a little lard or olive-oil, and observe that fat is soluble in ether or warm alcohol, but not in water. Put a drop of the ethereal solution of fat on a piece of paper, and note that it leaves a greasy stain. 2. Put a little alcohol in a test-tube, and then a drop of phenol- phthalein solution and a drop or two of dilute sodium hydroxide to give the solution a red colour. Add a few drops of an ethereal solution of the lard or olive-oil. If the red colour persists, the fat is neutral; if it disappears, the fat contains free fatty acids. 3. Saponification. — ^Melt some lard in a pdrcslain dish, and pour it 12 INTRODUCTION into an alcoholic solution of potassium hydroxide previously heated on ' a water-bath nearly to boiling. Mix well, and keep the mixture gently boiling on the bath till saponification is complete. This only takes a short time. Remove a little of the soap solution, and drop it into dis- tilled water in a test-tube. If unsaponified fat is present, it will rise to the top as drops of oil. In this case boiling should be continued. If all the fat has been saponified, the soap solution will mix with the water and no oil-drops will separate. 4. Fatty Acids. — Heat some 20 per cent, sulphuric acid in a small flask nearly to boiling, and drop into it some of the soap obtained in 3. The fatty acids separate out and rise to the top as an oily layer. Cool, skim off the fatty acid, and wash it with distilled water till the wash- water is no longer acid. (a) Dissolve' a little of the washed fatty acid in ether. Add a few drops of an alkaline solution of phenolphthalein to a few c.c. of water in a test-tube. Drop into this the etliereal solution of fatty acid. The red colour is discharged. (6) Put a small portion of the fatty acid on a glass slide resting on a piece of white paper. Place on it a drop or two of a i per cent, solution of osmic acid (osmium tetroxide). The osmic acid is reduced to a lower oxide (which is black) by the action of oleic acid present in the fatty acid mixture, which abstracts some of the oxygen. Any fat which contains olein or oleic acid, as body-fat does, is therefore blackened by osmic acid. (c) Add to a portion of the fatty acid some sodium hydroxide solution, and warm. Sodium soap is formed. Add warm water and shake up. A lather is produced. Keep the soap solution for 6. Keep a little of the fatty acid for 5 (6) and 6 (6) . 5. Glycerin. — (a) Add to a little glycerin in a dry test-tube a few crystals of potassium bisulphate (KHSO4), and heat over the free flame. Acrolein is given ofi, which is recognized by its pungent odour, and by blackening a piece of filter-paper moistened with ammoniacal silver nitrate solution, and held over the 'mouth of the test-tube. The paper is blackened owing to the reducing action of the vapour on the silver nitrate. , (6) Repeat this test with lard, and with a portion of the fatty acid from 4. Acrolein will be given ofi by the lard because glycerin is con- tained in neutral fat, but not by the fatty acid if it has been properly separated from the glycerin. 6. Emulsification. — (a) Take three test-tubes and label them A, B, and C. Put a few c.c. of water in A, a solution of soap in B, and a dilute solution of sodium carbonate or sodium hydroxide in C. To each add a few drops of fresh olive-oil and shake . An emulsion will be formed in B, but not in A. Probably there will be some emulsification in C also, owing to the presence in the oil of some fatty' acid, which forms soap with the alkali. But if the oil is free from fatty acid, no emulsion will be formed. (&) Repeat (a) with rancid olive-oil, which contains much fatty acid, or with fresh olive-oil to which some of the fatty acid obtained in 4 has been added. A good emulsion will be produced in C as well as in B. 7. Melting-Point of Fat. — Put into a very narrow test-tube or a short piece of narrow glass tubing some finely divided mutton fat, freed, as far as possible, from connective tissue. Fasten the test-tube on to the bulb of a thermometer with a rubber band, and immerse the ther- mometer and tube in a beaker filled with water and standing on a water- bath, which is gradually heated. Observe the temperature at which the fat melts. Repeat the experiment with hog's lard and dog's fat. PRACTICAL EXERCISES 13 SCHEME FOR TESTING A SOLUTION FOR THE MORE COMMON PROTEINS AND PROTEIN-DERIVATIVES. AND FOR CARBO- HYDRATES 1 . Note the reaction, and whether the liquid is coloured or colourless, clear or opalescent. A reddish colour suggests blood ; opalescence suggests glyco- gen or starch. Try one or more of the general protein tests (e.g., the xantho- proteic or biuret) . If the result is positive, proceed as in 2 ; if negative, pass to 3. 2. Test for Proteins. — (i) If the reaction is acid or alkaline,. neutralize with very dilute sodium carbonate or sulphuric acid. A precipitate = acid- or alkali-albumin, according as the original reaction is acid or alkaline. If the original reaction is neutral, no acid- or alkali-albumin can be present in solution. Filter ofE the precipitate, if any. (2) Boil some of the filtrate from (i) (or some of the original solution if it is neutral), acidulating slightly with dilute acetic acid. A precipitate = albumin or globulin. Filter, and keep the filtrate. (3) If a precipitate has been obtained in (2) , (a) saturate some of the original solution with magnesium sulphate, or half saturate it with ammonium sulphate (i.e., add to it an equal volume of saturated ammonium sulphate solution). If there is no precipitp,te, globulin is absent, and therefore the precipitate obtained in (2) must be albumin. A precipitate = globulin. But albumin may also be present in the solution. To see whether this is so, filter off the globulin and boil the filtrate after acidulation with acetic acid. A precipitate = albumin. (6) Half saturate the filtrate from (2) with ammonium sulphate (i.e., add its own volume of a saturated solution of the salt). A precipitate = primary proteoses. Filter. (c) Saturate the filtrate from (6) with ammonium sulphate crystals. A precipitate = secondary proteoses. Filter. {d) To the filtrate from (c) add excess of solid sodium hydroxide in small pieces at a time. Much ammonia is given ofi. Allow the test-tube to sta,nd fifteen minutes, shaking it at intervals. Then 'add dilute cupric sulphate, and if much of the sodium sulphate formed remains undissolved, add water to dissolve it. A well-marked rose colour = peptone. (4) If no precipitate has been obtained in (2), the solution contains neither albumin nor globulin. To test whether primary or secondary proteose or peptone is present, apply (3) (6), (c), and {d). 3. Test for Carbo-Sydrates. — Use the original solution, freed from coagu- lable proteins, if such have been found, by acidulation and boiling. (i) Add iodine. If the solution is alkaline neutralize it before adding the ■iodine. A blue co/om>-= starch. Confirm by boiling with dilute sulphuric acid and testing for reducing sugar. A reddish-brown colour with iodine — glycogen or dextrin. Glycogen gives an opalescent, dextrin a" clear, solution. Glycogen is pre- cipitated by basic lead acetate, dextrin is not (p. 689). Both are changed into reducing sugar by boiling with dilute acid. ^ (2) Add to some of the original solution cupric sulphate and excess of sodium hydroxide, and boil. Yellow or red precipitate = reducing sugar. (3) If (i) and (2) are negative, boil some of the liquid with one-twentieth of its volume of strong hydrochloric acid for fifteen minutes, and test as in (2) . A red or yellow precipitate indicates that a disaccharide like cane-sugar was originally present, and has been inverted. CHAPTER II THE CIRCULATING LIQUIDS OF THE BODY In the living cells of the animal bodyxhemical changes are con- stantly going on; energy, on the whole, is running down; complex substances are being broken up into siiripler combinations. So long as life lasts, food must be brought to the tissues, and waste products carried away from them. In lowly forms like the amoeba these functions are performed by interchange at the surface of the animal without any special mechanism; but in aU complex organ- isms they are the business of special liquids, which circulate in finely branching channels, and are brought into close relation at various parts of their course with absorbing organs, with eliminating organs, and with the tissue elements in general. In the higher animals three circulating liquids have been dis- tinguished: blood, lymph, and chyle. But it is to be remarked that chyle is only lymph derived from the walls of the alimentary canal, and therefore, during digestion, containing certain freshly- absorbed constituents of the food; while both ordinary lymph and chyle ultimately find their way into the blood, and are in their turn recruited from it. The blood contains at one time or another everything which is about to become part of the tissues, and every- thing which has ceased to belong to them. It is at once the scavenger and the food-provider of the cell. But no bloodvessel enters any cell;* and if we could unravel the complex mass of tissue elements which essentially constitute what we call an organ, we should see a sheet of cells, with capillaries in very close relation to them, but- everywhere separated from them by a thin layer of lymph. And to describe in a word the circulation of the food substances we may say that the hlood feeds the lymph, and the lymph feeds the cell. Section I. — Morphology of the Blood. The blood consists essentially of a liquid part, the plasma, in which are suspended cellular elements, the corpuscles. When the circulation in a frog's web or lung or in the tail of a tadpole is * Fine intracellular canaliculi, communicating with the blood-capillaries, and probably performing a nutritive function, since they seem to contain blood-plasma, have been described by Schafer and others in the liver cells. 14 THE BLOOD-CORPUSCLES 13 examined under the microscope, the bloodvessels are seen to be crowded with oval bodies— of a yellowish tinge in a thin layer, but in thick layers crimson — which move with varying velocity, now in single file, now jostling each other two or three abreast, as they are borne along in the axis of an apparently scanty stream of transparent liquid. Nearer the walls of the vessels, sometimes clinging to them for a little and then being washed away again, may be seen, especially as the blood-flow slackens, a few com- paratively small, roimd, colourless cells. The oval bodies are the red or coloured corpuscles, or erythrocytes; the colourless elements are the white blood-corpuscles, or leucocytes; the liquid in which they float is the plasma (Practical Exercises, p. 191). The Red Blood-Corpuscles, or Erythrocytes, differ in shape and size and in other respects in different animal groups. In amphib- ians, such as the frog and the newt, they are flattened ellipsoids containing a nucleus, and the same is true of nearly all the other ver- Elepiujit •009* tebrates, except mammals. In /^^^^^^\'" CaP oils mammals they are discs, hollowed //^^^^SC\.."a6ee;j -ooso out on both the flat surfaces, or (((0)"rn"T"M°'^ j 11°*' • 1 uvv\. ynn )„.J7osA-deer 002s biconcave, and possess no nucleus. But the red corpuscles of the llama and the camel, although non- „. , . „ , . , , J 1,. -J 1 • 1. Fig. I. — Diagram showing Relative nucleated, are elhpsoidal m shape, size of Red Corpuscles of Various like those of the lower vertebrates. Animals. As to size, the average diameter in man is between 7 and 8 /j,.* In the frog the long diameter is about 22 fj,, while in Proteus it is as much as 60 fx, and in Amphiuma, the corpuscles of which can be seen with the naked eye, nearly 80 fj, (Frontispiece) . As regards the structure of the red corpuscles, the most prob- able view is that they are solid bodies, with a spongy and elastic structureless framework, denser at the surface of the corpuscle than in its centre, but continuous throughout its whole mass (RoUett). The denser peripheral layer constitutes a physiological envelope which permits the passage of certain substances into or out of the corpuscles, and hinders the passage of others. In the large oval -corpuscles of Necturus (see Frontispiece) the envelope can be clearly demonstrated as a detachable membrane comparable to the mem- brane surrounding the nucleus. Envelope and spongework are sometimes spoken of as the stroma of the corpuscle, in contradistinction to its most important con- stituent, a highly complex pigment, the haemoglobin. This pigment is not in solution as such, for its solubility is not nearly great enough to permit this, but either in solution as a compound with * A micro- millimetre, represented by symbol /*, is x^^tt millimetre. 16 THE CIRCULATING LIQUIDS OF THE BODY some other unknown substance, or more probably bound in some solid or semi-solid combination to the stroma, and filling up the space within the envelope in the interstices of the spongework. Since there is good reason to believe that the haemoglobin as obtained artificially from the corpuscles is not quite the same sub- stance as the native blood-pigment within them, the latter is some- times distinguished by a separate name — hsemochrome. To the physical properties of the stroma it is usual to attribute the great elasticity of the corpuscles — that is, the power of recovering their original shape after distortion — for their elasticity is in no wise impaired by the removal of the hsemoglobin. Rouleaux Formation. — 'When blood .with disc-shaped corpuscles is shed, there is a great tendency for the corpuscles to run together into groups resembling rouleaux, or piles of coin. No satisfactory explana- tion of this curious fact has yet been given. Crenation of the corpuscles, a condition in which they become studded with fine projections, is caused by the addition of moderately strong salt solution, by the passage of shocks of electricity at high potential, as from a Leyden jar, or by simple exposure to the air. Con- centrated saline solutions, which abstract water from the corpuscles and cause them to shrink, make the colour of blood a brighter red, because more light is now reflected from the crumpled surfaces. On the other hand, the addition of water renders the corpuscles spherical; more of the light passes through them, less is reflected, and the colour becomes dark crimson [Frontispiece). The White Blood-Corpuscles, or Leucocytes. — The red corpuscles are peculiar to blood. The white corpuscles may be looked upon as peripatetic portions of the mesoderm (see Chap. XIX.), and some of them ought not in strictness to be called blood-corpuscles. They are more truly body corpuscles. Similar cells are found in many situations, and wan- der everywhere in the spaces of the connec- tive tissue. They pass into the bloodvessels with the lymph, and may pass out of them again in virtue of their amoeboid power. They consist of proto- , plasm, less differ- entiated than that of any other cells in the body, and under the microscope appear as granular, colour- less, transparent bodies, spherical in form when at rest, and containing a nucleus, often tri- or multi-lobed. Many of the leuco- cytes of frog's blood at the ordinary temperature, and of mam- malian blood when artificially heated on the warm stage, may be Fig. 2.— Amoeboid Movement. A, B, C, D, 'succes- sive changes in the form of an amcebSi THE BLOOD-CORPUSCLES 17 seen to undergo slow changes of form. Processes called pseudo- podia are pushed out at one portion of the surface, retracted at another, and thus the corpuscle gradually moves or ' flows ' from place to place, and envelopes or eats up substances, such as grains of carmine, which come in its way. This kind of motion was first observed in the amoeba, and is therefore called amoeboid. It is perhaps due to local alterations of surface tension; at any rate, similar phenomena can be thus produced artificially. The leuco- cytes of human blood are not all of the same size, and differ also in other respects. They may be classified according to the presence or absence of granules in their protoplasm, and the fineness or coarseness of the granules ; according to the chemical nature of the dyes with which the granules most readily stain, and according to the form of the nucleus. Five or six varieties of leucocytes may thus be distinguished in normal blood {Frontispiece) : I. Polymorphonuclear NeutrophUe Cells. — ^The . nucleus assumes a great variety of forms, often contorted or deeply lobed, the lobes being united by fine strands of chromatin. The cytoplasm contains numerous fine refractive granules, which stain best neither with simple acid dyes like eosin nor with simple basic dyes like methylene blue, but with mixtures which must be assumed to contain ' neutral ' stains, like Ehrlich's so-called triacid stain.* These cells make up 65 to 75 per cant, of the total number of leucocytes. Their diamster is 10 to 12 fi. 2: Eosinophils Cells (12 to 15 fi in diam.3ter), much less num.srous in normal blood than the neutrophiles (less than 5 per cent, of the whole), but found in considerable numbers in the serous cavities, the connec- tive tissue, and the bone-marrow. The granules in the cytoplasm are coarser than the neutrophile granules, and stain much more deeply with eosin. The nucleus may be simple, lobed, or even divided into fragments between which no connection can be traced. It is less rich in chromatin, and stains less easily with basic dyes, like methylene blue, than the nucleus of the first variety. 3. Large Mononuclear (also called Transitional) Leucocytes, with a diaraeter of 12 to 15 /i. They possess a large simple or slightly lobed nucleus, poor in chromatin, surrounded by a relatively great amount of cytoplasm, with faint neutrophile granules — i.e., granules which stain with neutral dyes. They constitute 3 to 5 per cent, of the total number of leucocytes. 4. Lymphocytes of Two Varieties — [a) Small Lymphocytes. — Smaller cells thg,n any of the preceding (diameter 6 fi), possessing a single large nucleus, surrounded by a comparatively small amount of non-granular cjrtoplasm; 20 to 25 per cent, of the leucocytes of the blood belong to this group. The lymphocytes are markedly deficient in the power of amoeboid motion in comparison with the other varieties of colourless corpuscles. (6) Large Lymphocytes. — ^The largest of all the white cells of the blood, and at least twice as large as the small lymphocytes. They possess a relatively great proportion of cytoplasm, which is devoid of granules. They constitute no more than i per cent, of the total number of the colourless corpuscles. * A mixture of orange G., acid fuchsin, and methyl green. 2 t8 THE CIRCULATING LIQUIDS OF THE BODY 5. ' Mast Cells,' or ' Basophiles,' the least numerous variety (o"5 per cent, of the total numjDer). Very few are to be found in the normal blood of adults, but more in children. They are somewhat smaller than the neutrophiles (aveTage diameter about 10 /i). The nucleus is irregularly trilobed. The protoplasm shows coarse granules, which do not glitter like the granules of the eosinophile cells, and are therefore less conspicuous in the unstained condition. Unlike the eosinophile granules, they stain with basic dyes, such as methylene blue. Blood-Plates, or Thrombocytes. — When blood is examined im- mediately after being shed, small colourless bodies (i to 3 ^m in diameter) of various shapes, but usually round or oval, may be seen. These are the blood-plates or platelets, also called thrombocytes, on account of their function in the coagulation of blood. If the blood is not at once subjected to some procedure which prevents clotting, the platelets swell and then break up. 1 here is reason to believe that in most of the methods of preventing coagulation the essential action is to hinder the break-up of the platelets (p. 37). They can be isolated by receiving a drop of blood from the finger upon a well-cleaned cover-slip, which is then laid, supported by two thin glass fibres, on a carefully cleaned slide. The plasma with the coloured corpuscles and leucocytes are washed away by irrigating the space between slide and slip with a suitable solution, e.g., a salt solution containing a certain proportion of manganese sulphate, which prevents disintegration of the platelets. The platelets stick to the cover-slip (Deetjen). They can then be fixed and stained. The blood-plates can even, like leucocytes, be kept alive on the warm stage in an appropriate medium (agar, to which certain salts have been added), and then show lively amoeboid movements (Deetjen). They have been described as nucleated cells, although the nucleus is not easy to stain, and with the ultra-microscope, a delicate means of testing whether such an object as a platelet is optically homogeneous, no evidence of the presence of a nucleus has been obtained. The origin of the platelets has been a matter of lively controversy. They are not produced by the breaking up of other elements of the shed blood, for they have been observed within the freshly excised, and therefore still living, capillaries — in the mesentery of the guinea-pig and rat (Osier). According to the best evidence, they are derivatives neither of the erythro- cytes nor of the leucocytes of the blood, but are developed from special elements (so-called megakaryocytes) of the blood-forming "organs (bone-marrow) (J. H. Wright). Enumeration of the Blood-Corpuscles. — 1 his is done by taking a measured quantity of ilood, diluting it to a known extent with a hquid which does not destroy the corpuscles, and counting the number in a given volume of the diluted blood (p. 67). The average number of red corpuscles in a cubic millimetre of blood is about 5,000,000 in a healthy man, and about 4,500,000 in THE BLOOD-CORPUSCLES 19 a healthy woman, but a variation of 1,000,000 up or dowii can hardly be considered abnprmal. In persons suffering from profound anaemia the number may sink to 1,000,000 per cubic millimetrt, or even less. In one case of pernicious anaemia, only i43',ooo corpuscles per cubic millimetre were present, the lowest number recorded. In new-born children the average is over 6,000,000, and in the inhabi- tants of high plateaus or mountains it may rise to 7,000,000, or even more. Fig. 3.— Curve showing the Number In the latter instance a residence of a of Red Corpuscles at Different Ages fortnight in the rarefied air is suffi- (after SSrensen's Estimations), cient to brinp about the inrrease and The figures along the horizontal ^^^"-^ ^" vj:uy^ aooui ine mcrease, ana axis are years of age, those along a Subsequent residence of a fortnight the vertical axis millions of cor- in the lowlands to annul it.* Over Woof ^'' ""''''' "^'™'*'' °* 13,000,000 erythrocytes to the cubic millimetre have been counted in a case of cyanosis (imperfect oxygenation of the blood, with blueness of the lips, etc.), due to congenital disease of the heart. The number of white blood-corpuscles is on the average about 10,000 per cubic millimetre of blood, ,.^^ or one leucocyte for every 500 red blood-corpuscles. But if the count is made when digestion is relatively inactive, four to five hours after a meal, it gives no more than 7,000 to the cubic millimetre. In new-born children the average number is over 18,000 per cubic millimetre. The total leucocyte count, and still more the so-called differential count, i.e., the determination of the relative number of the different kinds of leucocytes, is often resorted to in the study of pathological condi- tions. A distinct increase in the number is designated leucocytosis. In leukaemia the number of white corpuscles is enormously increased — on the average to about 300,000 600,000 per cubic millimetre- ' 7 'V r I . > \ / ' ' — A — i.-\.i — I K m Fig. of 4. — Curve showing Proportion White Cqrjmscles to Red at Different Times of the Day (after the Results of Hirt). At I the morning meal was taken; at II the midday meal; at III the even- ing meal. During active digestion the number of lymphocytes in the blood is greatly increased, both absolutely and relatively to the number of the other leucocytes. but in extreme cases to while at the same time the number * In 113 apparently healthy students (male) the average number of red corpuscles was 5,190,000 per cubic millimetre. In 104 of these, the number ranged from 4,000,000 to 6,400,000; in 71 (or 63 per cent, of the whole), from 4,400,000 to 5,500,000; in 3, from 3,500,000 to 3,900,000; in 5, from 6,500,000 to 7,000,000. In one observation the number reached 7,300,000. 20 THE CIRCULATING LIQUIDS OF THE BODY of the red corpuscles is diminished; and the ratio of white to red may approach 1:4. As the ansemia rapidly advances towards the fatal termination of an acute case, and the erythrocyte count falls to 1,000,000, or even less, the ratio may come still nearer to unity. An increase in the number of leucocytes has also been ob- served in certain infective diseases as part of the inflammatory reaction. There are also physiological variations, even within short periods of time; for example, the number of lymphocytes is in- creased when digestion is going on (digestive lymphocytosis). The normal number of blood-plates varies from a quarter to half a million to the cubic millimetre, but may be greater in disease and at high levels (Kemp). Life-History of the Corpuscles. — The corpuscles of the blood, like the body itself, fulfil the allotted round of life, and then die. They arise, perform their functions for a time, and disappear. But although the place and mode of their origin, the seat of their destruc- tion or decay, and the average length of their life, have been the subject of active research and still more active discussion for many years, much yet remains unsettled. Origin of the Erythrocytes. — In the embryo the red corpuscles, even of those forms (mammals) which have non-nucleated corpuscles in adult life, are at first possessed of nuclei, and approximately spherical in form. In the human foetus, at the fourth week all the red corpuscles are nucleated. Later on the nucleated corpuscles gradually diminish in number, and at birth they have almost or altogether disappeared, some of them, at least, having been converted by a shrivelling of the nucleus into the ordinary non-nucleated form. In the newly born rat, which corhes into the world in a comparatively immature state, many of the red corpuscles may be seen to be still nucleated. The first cor- puscles formed in embryonic life are developed outside of the embryo altogether. Even before the heart has as yet begun to beat, certain cells of the mesoderm (see Chapter XIX.) in a zone (' vascular area ') around the growing embryo begin to sprout into long, anastomosing processes, which afterwards become hollowed out to form capillary bloodvessels. At the same time clumps of nuclei, formed by division of the original nuclei of the cells, gather at the nodes of the network. Around each nucleus clings a little lump of protoplasm, which soon develops haemoglobin in its substance; and the new-made corpuscles float away within the new-made vessels, where they rapidly multiply by mitosis. In later embryonic life the nucleated corpuscles continue in part to be developed within the bloodvessels in the liver, allantois, spleen, and red bone-marrow, and in certain localities in the connective tissue, by mitotic division of previously existing nucleated corpuscles, in part to be formed endogenously within special cells in the liver and perhaps other organs. Still later the nucleated corpuscles give place in the blood of the mammal to non-nucleated erythrocytes. Many of these are doubtless derived from the nucleated corpuscles, but some appear to be produced in the interior of certain cells of the connective tissue, and are non-nucleated from the start. In the mammal in extra-uterine life the chief seat of formation of the red blood-corpuscles, or hsematopoiesis, is the red marrow of THE BLOOD-CORPUSCLES 21 the bones of the skull and trunk, and of the ends of the long bones of the limbs. Special nucleated cells in the marrow, originally colourless, multiply by karyokinesis, take up haemoglobin or, what is much more likely, form it within their protoplasm, and are transformed by various stages into the ordinary non-nucleated red corpuscles, which then pass into the blood-stream. These blood- forming cells have received the name of erythroblasts or hsemato- blasts. According to their size, erythroblasts have been distinguished as normoblasts, megaloblasts, and microblasts. The normo- blasts are most numerous, and have about the same diameter as the full-formed erythrocytes, into which they are believed to develop. The megaloblasts are larger, and the microblasts smaller, and they are thought to be the precursors of those aberrant forms of erythrocytes sometimes found in the blood in certain diseases. After haemorrhage rapid regeneration of the blood takes place, so that in a few weeks the loss of even as much as a third of the total blood is made good. The plasma is much sooner restored to its normal amount than the corpuscles. Microscopical examination shows in the red marrow the tokens of increased production of coloured corpuscles, and nucleated erythrocytes appear in the blood, the normoblasts being, as it were, hurried into the circula- tion before the transformation which normally results in the dis- appearance of the nucleus is complete. The same is true in severe pathological anaemias, e.g., pernicious anaemia. It is a matter of interest that other organs also, which in embryonic life perform a haematopoietic function, particularly the spleen, may, in such emergencies, again take on the office of forming blood-corpuscles. A constant destruction of red blood-corpuscles must go on, for the bile-pigment and the pigments of the urine are derived frorn blood-pigment. The bile-pigment is formed in the liver. It con- tains no iron; but the liver cells are rich in iron, and on treatment with hydrochloric acid and potassium ferrocyanide, a section of liver is coloured by Prussian blue. Iron must therefore be removed by the liver from the blood-pigment or from one of its derivatives; and there is other evidence that the liver is either one of the places in which red corpuscles are actually destroyed, or receives blood charged with the products of their destruction. Although it cannot be doubted that in all animals whose blood contains haemoglobin the iron found in the liver bears an important relation to the building up or breaking down of the blood-pigment, the injection of haemoglobin or haemin, indeed, increasing markedly tjie amount of iron in the liver, as well as in the spleen, bone-marrow and other tissues, this does not seem to be the only function of the hepatic iron, for the liver of the crayfish and the lobster, which have no haemoglobin in their blood, is rich in iron. Destruction of 22 THE CIRCULATING LIQUIDS OF THE BODY erythrocytes may also take place in the spleen and bone-marrow. Although the statement that free blood-pigment exists in demon- strable amount in the plasma of the splenic vein is incorrect, red corpuscles have been seen in various stages of decomposition within large amoeboid cells in the splenic pulp; and deposits containing iron have been found there and in the red bone-marrow in certain pathological conditions. But there is no good foundation for the statement sometimes rather fancifully made that the spleen is in any special sense the ' graveyard of the red corpuscles.' Some of the coloured corpuscles may break up in the blood itself, forming granules of pigment, which may then be taken up by the liver, spleen, and lymph glands. Indeed, it is probable that a large proportion of the worn-out erythrocytes are finally destroyed in the blood- stream. The portal circulation may be more than other vascular tracts a seat of this natural decay, perhaps in virtue of the presence of substances with a hsemolytic action (p. z8) absorbed from the alimentary canal. It has been argued that the erythrocytes must be short-lived, since they are devoid of nuclei (p. 6), and attempts have been made to calculate the average time for which they survive in the circulation from the amount of haemoglobin (or of its derivative, haematin) required to furnish the daily excretion of bile-pigment. The results arrived at, however, are not sufficiently trustworthy to warrant their citation. Origin and Fate of the Leucocytes. — ^There has been much dis- cussion as to the origin of the white blood-corpuscles. The numerous theories fall into two groups, which have been designated somewhat pompously the monistic and the dualistic. According to the first, all the colourless corpuscles arise from a single type of parent cell, namely, the Ijraphocyte type, in its small or large variety. According to the dualistic school, a fimdamental distinc- tion exists between the lymphocytes, or cells peculiar to lymphoid tissues, and to the blood on the one hand, and the remaining varieties of leucocytes on the other. The former are supposed to be derived from the lymphoblasts of lymphoid tissue, and the latter from the myeloblasts, the forerunners of the myelocj^es of bone-marrow. The question has recently been studied by Foot by a new method, namely, by cultivating chicken marrow outside of the body, and watching the transformation of certain of its cells. He concludes in favour of the development of the polymorphonuclear leucocyte from a lymphoid type of ceU existing in the marrow, a conclusion in harmony with the monistic view. As regards their immediate source, the small lymphocytes of the blood are undoubtedly derived from the lymph, and are identical with the lymph-corpuscles. That they are formed largely in the lymphatic glands is shown by the fact that the lymph coming to the glands is much poorer THE BLOOD-CORPUSCLES 23 in corpuscles than that which leaves them. The lymphatic glands, however, although the principal, are not the only seat of formation of lymphocytes, for lymph contains some corpuscles before it has passed through any gland; and although a certain number of these may have found their way by diapedesis from the blood, others are developed in the diffuse adenoid tissue, or in special collections of it, such as the thymus, the tonsils, the Peyer's patches and solitary follicles of the intestine, and the splenic corpuscles. To a very small extent white blood-corpuscles may multiply by karyokinesis or indirect division in the blood. The fate of the leucocytes is even less known than that of the red corpuscles, for they contain no characteristic substance, like the blood-pigment, by which their destruction may be traced. That they are constantly disappearing is certain, for they are constantly Doing produced. Not a few of them actually escape from the mucous membranes of the respiratory, digestive, and urinary tracts. The remnants of broken-down leucocytes have been found in the spleen and lymph glands. It must be assumed that many break up in the blood-plasma itself. Section II. — General Physical and Chemical Properties of THE Blood. Fresh blood varies in colour, from scarlet in the arteries to purple-red in the veins. It is a somewhat viscid liquid, with a saUne taste and a peculiar odour. Viscosity of Blood. — The viscosity, of normal dog's blood is about srx times greater than that of distilled water at body temperature. It can be determined by allowing the blood to flow through a capil- lary tube of known dimensions under a definite pressure, and measuring the amount which escapes in a given time. In general the viscosity and specific gravity of the blood vary in the same direction, although there is not an exact proportionality between them. Thus, sweating, which causes a diminution of the water of the blood, causes also an increase in its viscosity. With increasing temperature the viscosity of the blood diminishes, as is the case with other liquids (Burton-Opitz). In polycythemia, where the number of erythrocytes in propor- tion to plasma is greatly increased, the viscosity of the blood in- creases in an equal degree. In one case of polycythsemia, with a blood-count of 8,300,000, the viscosity was 9-4 times that of water; in a case of marked chlorosis it was only 2-14. But the importance of this factor in causing an abnormal blood-pressure by increasing 6r diminishing the resistance to the blood- flo;w has been exaggerated. Although it has been shown that in the living vessels, so long as 24 THE CIRCULATING LIQUIDS OF THE BODY their calibre remains constant, the flow is affected by changes in the viscosity of the blood, just as in glass tubes, compensation by adjustment of the vascular calibre is so ample and so easy that even the greatest alterations of viscosity produce little effect on the mean blood-pressure. Reaction of Blood.^ — In the sense in which the term is used in physical chemistry, the reaction of a solution depends on the pro- portion between its content of hydrogen (H-t-) and hydroxyl (OH — ) ions, an excess of hydrogen ions corresponding to an acid and an excess of hydroxyl ions to an alkaline reaction. It has been shown by a physical method (the determination of the electro- motive force of a cell containing blood or serum as one liquid) that hydroxyl ions are present only in small excess, and that blood is really but a little more alkaline than distilled water. Practically, it may be regarded as a neutral liquid. Under a great variety of conditions, physiological and pathological, its reaction remains almost unchanged. Yet it is known that acids (carbon dioxide, lactic, phosphoric, and sulphuric acids) are constantly being pro- duced in the normal metabolism of the tissues. The administra- tion of large quantities of acid or alkali causes a surprisingly small effect. In diabetes, even when it can be proved that an abnormal production of acid substances is taking place, the blood shows little, if any, diminution in the proportion of hydroxyl ions; it remains to all intents and purposes a neutral liquid. In diabetic coma, where the blood may in extreme cases turn blue litmus red, the true reaction is only slightly altered. The manner in which the reaction of the blood, the tissue liquids, and probably the protoplasm itself, is regulated within such narrow limits is a subject of great interest. For there is reason to believe that it is of the utmost moment that the equilibrium should be maintained not only in order that the functions of the tissues may be properly performed, but that danger to life may be averted. To be sure, the excretory organs, the lungs and the kidneys, provide the means by which the excess of acid (or of alkali) is finally, under normal circumstances, eliminated. Other regulative mechanisms- also exist. For example, it has been shown that when an excessive production of acids (acidosis) occurs in conditions of disordered metabolism, or when acids are purposely administered in large amount, a grmter quantity of ammonia, split off from the pro- teins, is mobilized to aid in neutralizing the acids. But very simple experiments on blood in vitro are sufficient to show that the blood itself has a great capacity, as compared with water, to resist a change in its reaction even when large amounts of acid or alkali are added to it. The secret of the reaction-regulating power lies, therefore, to a large extent in the blood itself. Two factors have been shown to be of importance: (i) The power of the proteins, GENERAL PHYSICAL AND CHEMICAL PROPERTIES 25 in virtue of their amphoteric character, to combine either with acids or with bases, so that, when excess of base is added to blood, the proteins act as acids, and neutrahze the base; when excess of acid is added, the proteins act as bases, and neutralize the acid. (2) The equilibrium of certain of the inorganic constituents of the blood (carbon dioxide, the carbonates, and the phosphates) is such that even great variations in the concentration of any of these, such as may normally occur, produce scarcely any effect upon the concentrations of the hydrogen and hydroxyl ions. Thus, when phosphoric acid and sodium hydroxide are added to water in certain proportions, and the solution placed under a certain tension of carbon dioxide (which is kept constant), we get a more or less accurate imitation of blood as regards the inorganic substances concerned in the regulation of its reaction, sodium bicarbonate (NaHCOj) and disodium phosphate (Na3HP04) being present in the solution as in blood. It is found that when the quantities are so chosen that the H+ concentration lies within the limits of variation of the normal blood reaction, relatively large quantities of alkalies can be added or withdrawn without causing much change in the H + concen- tration. It can be shown both theoretically and experimentally that precisely those weak acids present in blood (CO2, NaHaPOi) require the largest addition of alkali to alter the reaction to a given extent, and are therefore particularly suited to give stability to the reaction. Thus carbon dioxide requires twenty-four times, and monosodium phosphate thirty-three times, as much alkali as an equivalent solution of apetic acid to cause a given alteration of colour in rosolic acid (E. Henderson). The so-called ' titratable ' alkalinity of blood or serum, measured by the amount of standard acid which must be added before the colour of the indicator used changes from alkaline to acid, bears no necessary or fixed proportion to the actual alkalinity. When blood, for instance, is titrated with hydrochloric acid, with methyl orange as indicator, at the point where the red colour appears all the disodium phosphate and sodium bicarbonate will have been changed into monosodium phos- phate and carbon dioxide, all the alkali removed from combination with proteins, a certain amount of acid-protein compounds formed, and other minor reactions produced (Henderson). It is difficult to correjate the quantity deduced from such a titration with any physio- logical condition, although undoubtedly it bears some relation to the acid-neutralizing power of the blood, and some relation to its real reaction. Still, by titration information of value can be obtained which is not yielded by the physico-chemical method in regard to the potential ' acid capacity ' of the blood and its power of resistance against acid-poisoning. What is estimated here is the quantity of acid required to satisfy the proteins and to react with the carbonates and phosphates before that concentration of hydrogen and hydroxyl ions just necessary to cause the change of colour is established. This is not the same for different indicators, since there is a certain minimum ratio in the concentration of these ions at which each indicator turns in one or the other direction, none turning precisely at the neutral point. Thus serum appears to be acid When tested with phenolphthalein, and alkali must be added to the serum before the pink colour indicating alkalinity is produced. On the other hand, with litmus or methyl orange it gives the alkaline 26 THE CIRCULATING LIQUIDS OF THE BODY ' reaction, and a considerable amount of acid must be added befbre the colour of the indicator which denotes acidity appears. The true re- action of the serum is not, of course, at one and the same time both alkaline and acid ; but it is so near neutrality that it falls just below the degree of alkalinity necessary to give the pink colour with phenol- phthalein, and just below the degree of acidity which gives the pink colour corresponding to an acid reaction with methyl orange. Certain indicators — ^for example, rosolic acid — turn so as to give sharp colour reactions at about the concentration of hydrogen and hydroxyl ions in the blood, and these may possibly be of use in determining the changes in the true reaction for clinical purposes (Adler). More closely related to the true alkalinity of the blood than the titratable alkalinity is the carbon dioxide content. The estimation of the total carbon dioxide in a sample of blood throws light upon the capacity of the blood to perform one of its most important functions — the transportation of carbon dioxide — and to preserve one of its essential properties — an almost neutral reaction — in the presence of an excessive intake or production of acid substances. In herbivorous animals the carbon dioxide content of the blood is easily lessened by the administration of acids, but in carnivora and in man it is much more difficult to bring about such a decided effect, for the reason already mentioned, the acid being neutralized by ammonia. In many diseases, however, and particularly in those accompanied by fever, this protective mechanism breaks down. Specific Gravity of Blood. — The average specific gravity of blood is about 1066 at birth. It falls during infancy to about 1050 in the third year, then rises till puberty is reached to about 1058 in males (at the seventeenth year), and 1055 in females (at the four- teenth year). It remains at this level during middle life in males, but falls somewhat in females. In chlorotic anaemia of young women it may be as low as 1030 or 1035. It rises in starvation. Sleep and regular exercise inctease it (Lloyd Jones).* The specific gravity of the serum or plasma varies from 1026 to 1032. The Electrical Conductivity of Blood.— The liquid portion of the blood conducts the current entirely by means of the electrolytes dissolved in it, the most important of these being the inorganic salts; and the conductivity of the serum varies, in different speci- mens of blood, within a comparatively narrow range. The con- ductivity of entire (defibrinated) blood, on the contrary, varies within wide limits. For instance, in a case of pernicious anaemia the conductivity of the blood was found to be almost double that of normal human blood, while the conductivity of the serum was normal. The most influential factor which governs this variation * In 165 students (male) the average specific gravity of the blood, as deter- mined by Hammerschlag's method (p. 62) was 1054-4. In ^49 of these the variation was from 1050 to 1065; in 94 (or 57 per cent, of the whole), from 1054 to 1060; ill 4, from 1046 to 1049; in 9, from 1066 to 1070. In 3 the specific gravity was only 1040 to 1042. GENERAL PHYSICAL AND CHEMICAL PROPERTIES 27 is the relative volume of the corpuscles and serum. When the blood is relatively rich in corpuscles' and poor in serum, its con- ductivity is low; when it is poor in corpuscles and rich in serum, its conductivity is high. The explanation is that the corpuscle refuses passage to the ions of the dissociated molecules, which, in virtue of their electrical charges, render a liquid like blood a con- ductor (p. 422), or permits them only to pass very slowly, so that the intact red corpuscles have an electrical conductivity so many times less than that of serum, that they may, in comparison, be looked upon as non-conductors (Practical Exercises, p. 69) . The Relative Volume of Corpuscles and Plasma in Unclotted Blood, or, what can be converted into this by a small correction, the relative volume of corpuscles and serum in defiibrinated blood, can be easily determined, with approximate accuracy, by com- paring the electrical conductivity of entire blood with that of its serum.* Another method, more suitable for clinical work, though not so accurate, is the so-called hsematocrite method. A small quantity of blood is centrifugalized in a graduated glass tube of narrow bore until the corpuscles have been collected into a solid ' thread ' at the outer extremity of the tube. Their volume and that of the clear plasma which has been separated from them are then read off on the scale. The hsematocrite must rotate at such a high speed (10,000 turns a minute) that separation of the corpuscles from the plasma is accomplished before clotting has occurred. Dilution of the blood with liquids which prevent clotting is not permissible for exact work (Practical Exercises, p. 68). By these and other methods too elaborate for description here, it has been shown that the plasma or serum usually makes up rather less than two-thirds, and the corpuscles rather more than one-third, of the blood. But this proportion is, of course, liable to the same varia- tions as the number of corpuscles in a cubic millimetre of blood. It depends, further, the number of corpuscles being given, on the average volume of each corpuscle. For instance, when the mole- cular concentration, and therefore the osmotic pressure (p. 421), of the plasma is reduced, as by the addition of water or the abstrac- tion of salts, water passes into the corpuscles and they swell ; when the molecular concentration of the plasma is increased, by the abstraction of water or the addition of salts, water passes out of the corpuscles, and they shrink. In human seruiii the average * The formula p =jrj-r (174 - -^(6)), where p is the number of c.c. of serum in roo c.c. of blood; K{b), K{s), the specific conductivities respectively of the blood and serum (both measured at or reduced to 5° C, and, to obtain whole numbers, multiplied by 10*), may be used in the calculation. K is the specific conductivity of the liquid — i.e., the conductivity of a cube of the liquid of I centimetre side. The conductivity of a similar cube of mercury is 10,630. The number in brackets is the temperature at which the measurement was made. 28 THE CIRCULATING LIQUIDS OF THE BODY depression of the freezing-point below that of distilled water, which is a measure of the molecular concentration and of the osmotic pressure, is about 0-56° C. (Practical Exercises, p. 73). For clinical purposes, the determination of the relative volume of corpuscles and plasma is most useful in cases where the average size of the erythro- cytes departs from the normal, and where, accordingly, the enumera- tion of the corpuscles would give an erroneous idea of their total mass. Laking of Blood, or Haemolysis. — Even in thin layers blood is opaque, owing to reflection of the light by the red corpuscles. It becomes transparent or ' laky ' when by any means the pigment is brought out of the corpuscles and goes into true solution. Re- peated freezing and thawing of the blood, the addition of water, the passage of electrical currents, constant and induced,* putre- faction, heating the blood to 60° C, and many chemical agents (as bile-salts, ether, saponin), cause this change. Certain complex poisons of animal origin, such as snake-venoms, bee-poison, spider- poison or arachnolysin, and certain toxins produced by pathogenic bacteria — -for instance, tetanolysin, formed by the tetanus bacillus — also possess decided hsemoljrtic power. The blood-serum of certain animals acts on the coloured corpuscles of others, and sets free their pigment — for example, the serum of the dog or ox causes haemolysis of rabbit's corpuscles; the serum of the ox, goat, dog, or rabbit lakes guinea-pig's corpuscles. But rabbit's serum does not lake dog's corpuscles, and guinea-pig's serum is inactive towards the corpuscles both of the rabbit and the dog. It has been shown that in haemolysis by foreign serum two bodies are concerned: one, which is easily destroyed by heating to about 56° C, the so-called complement, and another,, the intermediary body or amboceptor , which is not affected by being heated to this temperature. Thus if dog's serum be heated to 56° C. for twenty minutes, no amount of it will lake rabbit's washed corpuscles — that is, rabbit's corpuscles freed from their own serum by repeated washing with salt solution and centrifugalization. If, however, serum which is not itself hsemolytic for rabbit's blood (e.g., rabbit's or guinea-pig's serum) be added to the washed rabbit's corpuscles, they will be laked by the heated dog's serum. Unheated dog's serum will lake rabbit's cotpuscles, whether they have been washed free from their own serum or not (Practical Exercises, p. 71). The hypothesis which best explains these facts and many similar ones is that dog's serum contains both of the bodies necessary for haemolysis of rabbit's corpuscles. When the complement has been rendered inactive by heatmg, the amboceptor cannot cause laking * The laking action of induced currents is due simply to the heating of the blood. Condenser discharges, which cause liberation of the hsemoglobin without raising the temperature of the blood as a whole to" the point at which heat-laking occurs, possibly act in the same way by causing local heating of the corpuscles owing to their high resistance. GENERAL PHYSICAL AND CHEMICAL PROPERTIES 29 by itself. Rabbit's serum contains complement, but not the specific amboceptor necessary for the laking of rabbit's corpuscles. Accordingly, the addition of fresh rabbit's serum to heated dog's serum restores complement to the latter, and thus it is again ren- dered active for rabbit's corpuscles. The amboceptor is supposed to unite on the one hand with certain groups in the corpuscle and on the other with the complement, which is thus enabled to develop its hsemoljrtic action upon the envelope or the stroma. The com- plement is incapable of acting, even in the presence of amboceptor, if the temperature is reduced to 0° C. Nevertheless, the corpuscles take up amboceptor at this temperature, and on this fact is based a method of freeing serum from amboceptor. For example, if dog's serum and excess of rabbit's washed corpuscles, both pre- viously cooled to 0° C, be mixed and placed at 0° C. for some hours, and the serum then removed, it will be found that it has lost the power of laking rabbit's corpuscles, washed or unwashed, at air or body temperature, although it will still do so on the addition of dog's serum in which the complement has been destroyed by heating it to 56° C. The real nature and mode of action of com- plements and amboceptors are not yet satisfactorily determined. The laws of chemical equivalents and definite proportions do not seem to be observed in the reactions into which they enter. It has therefore been suspected that the bodies in question belong to the group of ferments, or are closely related thereto, and there is some evidence that a fat-splitting enzyme, or lipase, is concerned in the complement action (Jobling). As to the manner in which haemolytic agents cause the liberation of the blood-pigment, the fact that in so many forms of laking the cor- puscles swell up before the haemoglobin escapes indicates that the entrance of water is an important step. The entrance of water is favoured by changes produced in the chemical and physical condition of certain constituents of the superficial layer (envelope) of the cor- puscle, as well as by changes in its interior. Saponin and ether, for example, are known to be solvents of cholesterin and lecithin, and cholesterin and lecithin are important constituents of the stroma and envelope of the erythrocyte. It is easy to understand that if a portion of one or both of these substances is dissolved, or altered without being actually dissolved, profound changes may be produced in the permea- bility of the corpuscle to water and to the salts dissolved in the liquid in which the erythrocytes are suspended. In addition to this change of permeability, many laking agents, perhaps all, exert also a more direct influence on the normal relations of tjie native blood-pigment to the stroma. Ether and saponin, for instance, seem to act in two ways — by disorganizing the envelope through solution of its lipoids, and thus increasing its permeability to water; and by helping to dissociate the blood-pigment-stroma complex by exerting a pull on the lipoids of the stroraa, while the water simultaneously exerts a pull on the pigment. The conclusion follows from this view of haemolysis, that the erythro- cytes, normally so perfectly adapted to the plasma in which they float, may, when the conditions on which their equilibrium with it depends 30 THE CIRCULATING LIQUIDS OF THE BODY are altered, be rapidly and inevitably destroyed by that very plasnia itself. It is, indeed, the very fact of the exquisite adaptation of liquid and cell for a strictly regulated exchange of material which constitutes the danger when the regulation is upset. A liquid like mercury, which is not adapted either to give anjrthing to erythrocytes in contact with ' it or to take anything from them, would not cause haemolysis, even if the permeability of the corpuscles for water or sodium chloride were increased to any extent. The continued survival of the erythrocytes in an aqueous solution of salts and proteins like the plasma — ^nay, more, the protection of the corpuscles up to a certain point by the plasma against the attack of extraneous hsemolytic agents — are facts we are , prone to take so much for granted as to forget that they depend entirely upon a most delicate adjustment of the permeability of the corpuscles for essential constituents of the plasma. Disturb these relations to a sufficient degree, and the plasma becomes a poison to the erythrocytes not much less deadly than distilled water. When we add to blood a haemolytic substance, and see that presently the blood-pigment has left the corpuscles, we are apt to attribute the whole effect to the foreign material added, and to say that the saponin, the ether, the alien serum, has laked the blood. In a certain sense this is true, but it is not the whole truth. In reality the haemolytic agent has acted in an essential degree, although not exclusively, by overthrow- ing the equilibrium between the corpuscles and the aqueous solution of certain substances in which they are suspended. To say that the foreign substance alone causes the haemolysis is no more accurate than it would be to say that a man swimming strongly in a rough sea, who sinks when, hit and stunned by a piece of wreckage, was drowned by the blow, and not by the sea. No doubt it is true that, but for the blow, he would have continued to S\niia ; yet, in reality, he loses his life because he is environed by a medium deadly to him as soon as his power of adjustment to it has been too much diminished. On land, the blow would have stunned, but would not have kiUed him. In like manner, to glance at one phase of the natural decay of the corpuscles within the boay, an erythrocyte may float secure in its watery environment through many rounds of the circulation. But its security is not static, like tnat of a log floating on the water. It is dynamic, a triumph of perfect physico-chemical poise, as the security of the swimmer, stiU more of the tight-rope dancer, is dynamic, a triumph of perfect neuro- muscular poise. The time, however, arrives when, either through changes in the corpuscle itseU (the changes of cellular senility, as we may call them), or through changes in the environing medium, or through a combination of the two, the adjustment is upset, and the erythrocyte is now destroyed by the plasma in which it has so long lived. In general haemolysis by foreign serum is preceded by agglutina- tion or aggregation of the corpuscles into groups. Agglutination may be obtained without haemolysis by heating the haemolytic serum to the temperature at which the complement is destroyed, since the agglutinating agents, or agglutinins, are relatively resistant to heat. Besides the amboceptors naturally present in the blood of certain animals, and capable, in conjunction with complement, of haemolyzihg the corpuscles of certain other animals, amboceptors may be produced in much greater strength by artificial means. GENERAL PHYSICAL AND CHEMICAL PROPERTIES 31 When the corpuscles of one animal are injected intraperitoneally or subcutaneouSly into an animal of a different kind, the serum of the latter acquires the property of agglutinating and laking the corpuscles of an animal of the same kind as that whose corpuscles have been injected. This is especially marked if the injection is several times repeated at intervals of a few days. If, for instance, dog's corpuscles are injected into a rabbit, the rabbit's serum after a time becomes strongly hsemolytic for dog's corpuscles. It also agglutinates them. Ihis is due to the appearance in the rabbit's serum of an amboceptor and an agglutinin which have a specific action on dog's corpuscles. Such a serum is often termed an immune serum, and the animal which has received the injections is spoken of as immunized in regard to this particular kind of corpuscles. For the reaction involved in the production of the aniboceptor and agglutinin is a particular case of the peculiar and specific response which the body makes to the presence of foreign juices or cells, including bacteria, and which constitutes an attempt to render itself ' immune ' to them. Many other animal cells besides the coloured blood'Corpuscles give rise, when injected, to similar specific substances (cytolysins), which cause destruction of cells of the same kind — e.g., leucocytes and spermatozoa.* The process of haemolysis is more easily followed than the cytolysis of ordinary cells. Yet in its main features it is essentially similar. In each case the specific antibody seems to be produced in response to the presence of some particular constituent of the foreign cell. The substances which on injection give rise to antibodies are spoken of as antigens. In the case of the erythroc5rtes there is evidence that the antigens (both the haemolysinogen, which causes the production of specific amboceptor, and the agglutininogen, the substance which gives rise to specific agglutinin) are lipoids, or are so closely associated with the lipoids of the corpuscles that they are extracted by the same solvents . Thus ethereal extracts of erythrocytes cause the production of haemol- ysin and agglutinin, just as the entire corpuscles do. The group of antibodies known as precipitins is of special interest. Precipitins. — When the serum of one animal is injected into another of a different group, the serum of the latter acquires the property of causing a precipitate in the normal serum of animals of the same group as that whose serum was injected, but not * Recent studies have tended to modify the view that the cytotoxins formed after the introduction of different foreign tissues into animals are quite spscific for each tissue. Thus Lambert, using tissues cultivated on media outside of the body for testing the toxic action, finds that the plasma of guinea-pigs which have received injections of either chick embryo heart or intestine becomes toxic for both of these tissues. In like manner the plasma of guinea-pigs treated by injection of rat sarcoma, a tumour which can be propagated by inoculation in rats, acquires a toxic action on cultures of both rat sarcoma and the skin of embryo rats. And the plasma of guinea-pigs treated with rat embryo skin is also toxic for cells of both types. 32 THE. CIRCULATING LIQUIDS OF THE BODY in the serum of any other kind of animal. Thus, if human blood or serum is repeatedly injected at short intervals into a rabbit, the serum of the rabbit will cause a precipitate in diluted human blood or serum, but not in the blood or serum of other animals, except that of monkeys, where a slight reaction may be obtained. The specific bodies which cause the precipitation are termed precipitins. The phenomenon has been made the basis of a method of dis- tinguishing human blood for forensic purposes. Other animal fluids and solutions containing tissue proteins likewise give rise to the production of precipitins. Thus, when cow's milk is injected into a rabbit, the rabbit's serum acquires the power of precipitating the caseinogen of cow's milk. Indeed, the iresponse of the animal body to the presence of foreign proteins is so catholic, and at the same time so approximately specific, that many artificially isolated proteins, even those of vegetable origin, after as careful purifica- tion as possible, occasion, when injected, the production of anti- bodies which will precipitate from a solution only the variety of protein injected, or sometimes also, though in slighter degree, pro- teins nearly related to it. Anaphylaxis. — Under certain conditions the injection of a toxin, a serum, or a protein solution, instead of eliciting an immunity reaction which tends to combat the effects of a subsequent injection of the same material, produces the opposite result — ^namely, a sensitization of the animal which renders the second dose far more harmful than the first. Thus, Richet found that animals into which eel serum, or the poison contained in the tentacles of Actinaria, was subcutaneously introduced became much more sensitive to the toxic action of a second injection. This phenomenon he designated anaphylaxis, as being the opposite of the prophylaxis or protection afforded by previous treatment with the toxins hitherto studied. Later on it was discovered that the sub- cutaneous injection of a great variety of proteins alien to the animal into which they are introduced causes anaphylaxis. Only very minute amounts are necessary for the first or sensitizing dose, and an interval considerably greater than that employed in the production of an immune serum (p. 31) is allowed to elapse before the second injection is made. The symptoms induced in the sensitized animal by a subse- quent dose of the same material used in sensitization differ somewhat in different animals, but may be designated in general terms as those of collapse or shock (anaphylactic shock). They have been especially studied in the rabbit and guinea-pig, the heart being particularly affected in the former, and the lungs in the latter. The symptoms are very severe, and manifest themselves within a very short time (a few minutes) of the injection. A large proportion of the animals die. If an animal recovers, it does so suddenly, and for sorne time afterwards it is in- sensitive to the particular protein. While the real nature of protein sensitization or anaphylaxis is not as yet understood, it affords a new and delicate test for the detection and discrimination of proteins, and has already been utilized in a number of practical applications. For instance, the sophistication of sausages with other than the orthodox ingredients — e.g., with horseflesh — can be thus exposed, since an animal sensitized by horseflesh will exhibit anaphylaxis to horseflesh, but not to beef or pork. In like manner, the anaphylactic reaction may be GENERAL PHYSICAL AND CHEMICAL PROPERTIES 33 used for the identification of human blood. It is probable that an- aphylaxis plays an important role in certain pathological reactions. It is well known, for example, that some persons are so susceptible to particular foods that the slightest indulgence in them brings on an attack of urticaria or nettlerash. It has been suggested that these persons have become sensitized to certain foreign proteins — such as those existing in eggs, veal, pork, strawberries, shellfish,, or whatever the peccant article of diet may be — possibly by absorption at some previous time, owing to gastro-intestinal disturbance, of small quan- tities of the proteins which have escaped complete digestion. It is- only when proteins are introduced parenterally {i.e., by some other route than the alimentary canal, such as the subcutaneous tissue, the blood, or the serous cavities) that the immunity reactions already described and the phenomenon of anaphylaxis can be experimentally produced. For in digestion the protein molecule is decomposed, and although, as will be seen later on, the decomposition products are not the same for each kind of protein, the factor on which the specificity , of the molecule depends does not survive the hydrolysis. Coagulation of the Blood. Since changes begin in the blood as soon as it is shed, having for their outcome clotting or coagulation, we have to gather from the composition of the stable factors of clotted blood, or of blood which has been artificially prevented from clotting, some notion of the composition of the unaltered fluid as it circulates within the vessels. The first step, therefore, in the study of the chemistry of blood is the study of coagulation. When blood is shed, its viscidity soon begins to increase, and after an interval, varying with the kind of blood, the temperature of the air, and other conditions, but in man seldom exceeding ten, or falling below three, minutes, it sets into a firm jelly. This jelly gradually shrinks and squeezes out a straw-coloured liquid, the serum. Under the microscope the serum is seen to contain few or no red corpuscles; these are nearly all in the clot, entangled in the meshes of a kind of network of fine fibrils composed of fibrin. In uncoaguiated blood no such fibrils are present ; they have accordingly been formed by a change in some constituent or constituents of the normal blood. Now, it has been shown that there exists in the plasma — the liquid portion of unclotted blood — a substance from which fibrin can be derived, while no such substance is present in the corpuscles. In various ways coagulation can be prevented or delayed, and the plasma separated from the corpuscles. For example, the blood of the horse clots very slowly, and a low tem- perature lessens the rapidity of coagulation of every kind of blood. If horse's blood is run into a vessel surrounded by ice and allowed to stand, the corpuscles, being of greater specific gravity than the plasma, gradually sink to the bottom, and the clear straw-yellow plasma can be pipetted off. Or the addition of neutral salts to 3 34 THE CIRCULATING LIQUIDS OF THE BODY blood may be used to delay coagulation, the blood being run direct from the animal into, say, a third of its volume of saturated mag- nesium sulphate solution. The plasma may then be conveniently separated from the corpuscles by means of a centrifugal machine. Again, two ligatures may be placed on a large bloodvessel, so that a portion of it can be excised full of blood and suspended vertically (the so-called experiment of the ' living test-tube ') ; coagulation is long delayed, and the corpuscles sink to the lower end. In these and many other ways plasma free. from corpuscles can be got; and it is found that when the conditions which restrain coagulation are removed — ^when, for instance, the temperature of the horse's plasma is allowed to rise or the magnesium sulphate plasma is diluted with several times its bulk of water — clotting takes place, with forma- tion of fibrin in all respects similar to that of ordinary blood-clot. The corpuscles themselves cannot form a clot.* From this we con- clude that the essential process in coagulation of the blood is the formation of fibrin from some constituent of the plasma, and that the presence of corpuscles in ordinary blood-clot is accidental. In accordance with this conclusion, we find that lymph entirely free from red corpuscles clots spontaneously, with formation of fibrin; and when fibrin is removed from newly shed blood by whip- ping it with a bundle of twigs or a piece of wood, it will no longer coagulate,. although all the corpuscles are still there. What, now, is the substance in the plasma which is changed into fibrin when blood coagulates ? If plasma, obtained in any of the ways described, be saturated with sodium chloride, a precipitate is thrown down. The filtrate separated from this precipitate does not coagulate on dilution with water; but the precipitate itself — the so-called plasmine of Denis — on being dissolved in a little water, does form a clot. Fibrin is therefore derived from something in this precipitate. Now, ' plasmine ' contains two protein bodies — '■ fibrinogen, which coagulates by heat at about 56° C, and serum- globulin, which coagulates at about 75° C, and it was at one time believed that both of these entered into the formation of fibrin (Schmidt). Hammersten, however, has shown that fibrinogen alone is a precursor of fibrin; pure serum-globulin neither helps nor hinders its formation. This observer isolated fibrinogen from blood- plasma by adding sodium chloride till about 13 per cent, was present. With this amount the fibrinogen is precipitated, while serum-globulin is not precipitated till 20 per cent, of salt is reached. After precipitation of the fibrinogen, the plasma no longer coagu- . lates; and a solution of pure fibrinogen can be made to clot and to form fibrin, while a solution of serum-globulin cannot. Blood- * Bird's corpuscles, however, washed free from plasma, will form a clot when laked in various ways, as by addition of water or by freezing and thi wing. COAGULATION 35 serum, too, which contains abundance of serum-globulin, Dut no fibrinogen, will not coagulate. So iar, then, we have reached the conclusion that fibrin is formed by a change in a substance, fibrinogen, which can be obtained by certain methods from blood-plasma. It may be added that there is evidence that fibrinogen exists as such in the circulating blood; for if unclotted blood be suddenly heated to about 56° C, the tem- perature of heat-coagulation of fibrinogen, the blood for ever loses its power of clotting. Ihe liver seems to be an important place of origin of fibrinogen, which may also be formed in the bone-marrow. 1 hat the liver is intimately concerned in the production of fibrinogen is indicated by a number of facts. In phosphorus poisoning, and notably in poisoning by chloroform, which causes necrosis, especially of the central portions of the hepatic lobules, the amount of fibrinogen in the blood is quickly diminished. The diminution is proportional to the extent of the injury to the liver, and the blood loses more or less completely its power of clotting. If the injury is repaired, the fibrinogen is, rapidly regenerated (Whipple). If the blood is allowed to circulate for a time in the head and thorax of an animal without passing into the rest of the animal's body, it becomes incoagulable, and the fibrinogen is found to be markedly deficient in amount. When the blood of an animal is defibririated by whipping, and reinjected, regeneration of the fibrinogen does not occur if the liver has been eliminated, whereas it takes place rapidly if the liver is intact (Meek) . Since fibrinogen is readily soluble in dilute saline solutions, and fibrin only soluble with great difficulty, we may say that in coagu- lation of the blood a substance soluble in the plasma passes into an insoluble form. How ,is this change determined when blood is shed ? We' have said that a solution of pure fibrinogen can be made to coagulate; but it does not coagulate of itself. 1 he addition of another substance in minute quantity is necessary. This sub- stance, to which the name thrombin has been applied, can be obtained in various ways, although not in a state of purity; for example, by precipitating blood-serum, or defibrinated blood, with fifteen to twenty times its bulk of alcohol, letting the whole stand for a month or more, and then extracting the precipitate with water. All the ordinary proteins of the blood having been ren- dered insoluble by the alcohol, the thrombin passes into solution in the water, and the addition of a trace of the extract to a solution of fibrinogen causes coagulation. When purified as well as possible, thrombin still gives protein reactions, but it is not known whether it is really a protein. The action of thrombin on fibrinogen helps to explain many experiments in coagulation. Thus, transudations like hydrocele fluid do not clot spontaneously, although they contain fibrinogen, 36 THE CIRCULATING LIQUIDS OF THE BODY which can be precipitated from them by a stream of carbon dioxide or by sodium chloride. But the addition of a little thrombin causes hydrocele fluid to coagulate. So does the addition of serum, not because of the serum-globulin which it contains, as was once believed, but because of the thrombin in it. The addition of blood- clot, either before or after the corpuscles have been washed away, or of serum-globulin obtained from serum, also causes coagulation of hydrocele fluid, and for a similar reason, the thrombin having a tendency to cling to everything derived from a liquid containing it. On the other hand, serum which, although thrombin is present in it, does not of itself clot, because the fibrinogen has all been changed into fibrin during coagulation of the blood, can be made to ' coagulate by the addition of hydrocele fluid, which contains fibrinogen-. We have thus arrived a step farther in our attempt to ex- plain^ the coagulation of the blood: it is essentially due to the formation 0/ ■fibrin from the fibrinogen of the plasma under the influence of thrombin. Up to this point there is agreement between physiologists. Some difference of opinion exists, however, as to the manner in which thrombin is formed or activated when blood is shed, and as to the nature of its action upon fibrinogen once it is fully formed. The Formation of Thrombin from its Precursors. — There is good reason to believe that thrombin is formed by the interaction of three factors: (i) A substance which, since it is a precurs'or of thrombin, is called thrombogen, or prothrombin. It is already present in the circulating plasma. (2) A substance liberated from the formed ele- ments of the shed blood, but which can be obtained also from the cells of all tissues. Since it has been supposed to act upon throm- bogen, changing it into fully formed thrombin, much in the same way as enterokinase (p. 366) acts upon trypsinogen, changing it into fully formed trypsin, it is called thrombokinase (Morawitz). (3) Calcium ions. The following experiments illustrate the role of these three factors: ' ■ The plasma obtained by drawing off bird's blood — e.g., the blood of a fowl or goose — through a perfectly clean cannula into a perfectly clean vessel, without contact with the tissues, and then rapidly centrifugal- izing off the formed elements, can be kept unelotted for days and even weeks. The addition of a small amount of tissue extract (procured by rubbing up blood-free liver, thymus, muscle, or other organs with sand, and extracting for several hours with salt solution) to the bird's plasma causes rapid coagulation. The plasma contains thrombogen and calcium salts, but is lacking in thrombokinase, which is supplied by the tissue extract. A solution of fibrinogen containing calcium will clot if serum, in which fibrin-ferment is always present, be added. It will not clot on addition of tissue -extract alone, nor on addition of bird's plasma alone (obtained as above), but will readily coagulate if both tissue extract and bird's plasma be added. Therefore, something in the. bird's plasma (thrombogen), plus something in the tissue extract (thrombokmase), produce in the presence of calcium the same effect as the thrombin of serum. It can be shown that calcium is only necessary COAGULATION 37 for the formation of the thrombin, but not for its action on fibrinogen. For instance, a calcium-free solution of fibrinogen can be made to clot ■ by serum from which the calcium has been removed. If a soluble oxalate (potassium or ammonium oxalate) is mixed with freshly drawn dog's blood to the amount of o' 2 or o' 3 per cent . , the blood remains unclotted. The plasma separated from this oxalated blood contains both thrombogen and thrombokinase, but it does not coagu- late, because the calcium has been precipitated out in the form of in- soluble calcium oxalate. In the absence of calcium the reaction of the thrombogen and thrombokinase which leads to the formation of thrombin, does not take place. All that is necessary to bring about coagulation is to add calcium chloride in somewhat greater quantity than is required to combine with any excess of oxalate present. If more than a certain amount of calcium be added, clotting is hindered instead of being helped, so that it is only within certain limits of concentration that calcium favours coagulation. From oxalate plasma a nucleo- protein or a mixture of nucleo-proteins can be separated which contains thrombogen and thrombokinase, but little or no calcium, and does not cause clotting, but which on treatment with a calcium salt acquires the properties of thrombin. ■ When sodium fluoride is added to freshly drawn blood to the amount of o'3 per cent., coagulation is also prevented. But thereis this differ- ence between oxalate- and fluoride plasma — ^that, although the calcium has been precipitated in both, the addition of calcium chloride to fluoride plasma is not sufiicient to induce clotting. Tissue extract containing thrombokinase must be supplied as well. In some way or other sodium fluoride interferes with the liberation of thrombokinase from the formed elements. of the blood, ia,lthQugh in the concentration mentioned it does not hinder the action of fully .formed thrombin, as is shown by the fact that fluoride plasma cosfgulates on the addition of a little serum, which supplies thrombin. The fluoride blood clots readily if it is diluted with water, and, ait the same time mixed with calcium chloride solution, for the water.damages the formed elements, and thus favours the liberation of thrombokinase. Sodium citrate solution prevents the coagulation of blood run into it, although there is no precipitation of the calcium. The addition of calcium chloride to citrate plasma induces clotting, and the action of the citrate is assumed to be due to the formation of a compound with the calcium of the blood, which does not dissociate so as to yield calcium ions. It ought to be remarked, however, that in all so-called decalci- fied plasmas, as ordinarily obtained, blood-platelets are present, and that platelets disintegrate under the influence of calcium salts. It has been shown, indeed, that many of the reagents and procedures which hinder the clotting of shed blood also prevent the breaking up of the platelets. Thus, the cooling of the blood, the addition of hirudin, sodium oxalate, sodium citrate, manganese salts, etc., which are cl5,ssical methods used in obtaining platelets for microscopical study, are also classical methods of hindering coagulation. These facts have not hitherto been sufficiently taken account of in interpreting experi- ments on decalcified blood. They indicate that the decalcifying agents may hinder clotting by interfering with the liberation of essential sub- stances from the platelets, and that this may be the decisive factor, and not merely the withdrawal of the calcium from the field where the already liberated thrombokinase and thrombogen would otherwise react to form thrombin. When proteoses (or peptones) are injected into the circulation of a dog or goose, the blood is deprived of the power of coagulation. The 38 THE CIRCULATING LIQUIDS OF THE BODY peptone plasma must be assumed to contain both thrombogen and thrombokinase, since it can be made to clot in various ways {e.g., by dilu- tion with water or by slight acidulation with acetic acid) without the addition of anything which could supply either of these factors. Yet a little tissue extract causes it to clot much more rapidly than simple dilution or acidulation, and more rapidly than the addition of serurn. So that either the thrombokinase already present in peptone plasma is present in an unavailable form, or in some way the formation of throm- bin from its precursors is hindered. But this is not the only cause of the incoagulability of peptone plasma. It may be shown to contain an antithrombin, a body which antagonizes the action of fully formed thrombin, and which does not seem to be a ferment, since it acts quan- titatively in proportion to the amount present. This is the reason why, although peptone plasma can always be made to clot by the addition of fibrin ferment, in serum, for instance, relatively large quantities of it must be supplied (Practical Exercises, pp. 64, 65). Fig, 5. — Fibriti Formation in Horse's Plasma (Ultrarnicroscope) (Stiibel). Several clumps of disintegrated platelets from which the hbrin filaments radiate. An extract of the head of the medicinal leech in salt solution prevents the clotting of blood both in the test-tube and when injected into the circulation. The plasma obtained differs from peptone plasma in refusing to coagulate unless tissue extract is added. It is therefore deficient in thrombokinase, or, rather, as has been shown, the kinase present is unable to act, because neutralized by antikinase present in the leech extract. Leech extract also contains an antithrombin, which can be neutralized by a sufficient amount of thrombin. In the small wound from which the leech sucks blood this sufficient amount is not present, and the blood remains unclotted, as it also does in the alimen- tary canal of the leech. The anticoagulant substance, hirudin, has been isolated, and gives the reactions of an albumose. Sources of Thrombogen and Thrombokinase. — It has already been stated that thrombogen exists in the circulating plasma. This is shown by the fact that fluoride plasma obtained from blood drawn directly through a wide cannula into sodium fluoride solution, with COAGULATION 39 all precautions to prevent alteration of the blood, and immediately separated from the formed elements by the centrifuge, will clot on the addition of tissue extract. The source of the thrombogen has been thought to be the blood-plates, but this has not been proved. Thrombokinase is not present in the circulating plasma. In shed and clotting blood which has not been allowed to come into contact with cut tissues, the only possible sources of thrombokinase, so far as we know, are the corpuscles and the blood-plates. The red corpuscles we may at once dismiss, for although the stromata, especially of nucleated corpuscles, contain thrombokinase, or can under artificial conditions be made to develop that action on coagulation by which we recognize its presence, not only do they remain intact under ordinary circumstances during coagulation, but there is strong evidence, as has already been pointed out, that they do not make any essen- tial contribution to the process. We have left over the leucocytes and the platelets, and it is highly probable that from the platelets thrombokinase is liberated in the first moments after blood is drawn, and, acting on the throm- bogen already present in the plas- ma, changes it into actual throm- bin. This surmise is strengthened by the fact that in freshly shed mammalian blood extensive de- struction of blood-plates takes place. Viewed with the ultra- microscope, the blood-platelets, in a drop of clotting plasma, which are at first homogeneous in appearance (optically empty), become granular. Then the platelets begin to agglutinate and swell up, and the agglutinated platelets are transformed into clumps of granules, from which needles of fibrin shoot out. Other needles and filaments of fibrin form in contact with the glass or free in the plasma, and soon the field is occupied by a felt-work of fibrin. The leucocytes have not been observed to be related to the process, at least, in the blood of mammals (Stiibel) . It is true that the white layer or ' buffy coat ' which tops the tardily formed clot of horse's blood, and consists of the lighter, and therefore more slowly sinking colourless cells, causes clotting in otherwise in- coagulable liquids like hydrocele fluid much more readily than the red portion of the clot, and yields far more thrombin on treatment with alcohol. It can also be easily verified that in mammalian Fig. 6. — Fibrin Formation in Plasma from a Case of Hsmophilia (Ultramicro- scope) (Stiibel). The needles of fibrin are slowly formed, and very large. 40 THE CIRCULATING LIQUIDS OF THE BODY blood collected in paraf&ned vessels, so as to delay clotting, and immediately centrifugalized, coagulation begins in and around the layer of white elements, and then spreads upwards- in the stratum of plasma and downwards in the stratum of erythrocytes. But in this white upper layer platelets are always intermingled with leuco- cytes. It has been shown, however, that the blood of the cray- fish, which coagulates with extreme rapidity, contains certain colourless corpuscles, which immediately it is withdrawn, break up with explosive , suddenness, and that substances which hinder the breaking up of these coipuscles restrain coagulation (Hardy). In the blood of another crustacean Limulus, the kingcrab, coagula- tion is preceded by an agglutination of the leucoc5;tes whidh exhibit amoeboid movements. They become entangled by the interlacing of the pseudopodia which they protrude (L. Loeb) . The disintegration of the platelets in shed blood has been attributed by Deetjen to an increase in the alkalinity of the blood, by escape of carbon dioxide, it may be. When blood is placed on a quartz slide and covered with a quartz cover-slip, the platelets, according to this observer, do not break up ; but if they are brought into con- tact with a medium whose OH — concentration is raised ten times or more above that of freshly drawn blood (still only a weak alka- line reaction) , disintegration ensues. He supposes that the contact of glass acts harmfully on account of the alkali in it. It is im- possible to say at present whether this observation has any bearing on normal coagulation. Thrombokinase has been shown to exist not only in the leuco- cytes, the platelets, and the stromata of the coloured corpuscles, but, as already stated, in all tissues hitherto examined. Under ordinary circumstances it appears that a larger amount of thrombogen is liberated or is already present in shed blood than can be changed into thrombin by the thrombokinase set free, since serum contains a surplus of thrombogen in addition to the fully formed ferment. This is shown by the fact that the activity of a given quantity of serum in causing the coagulation of a plasma not spontaneously coagulable or of a fibrinogen solution is increalsed by the addition of tissue extract (containing thrombokinase). The thrombin of any particular kind of vertebrate blood has no marked specific action — that is, will cause coagulation in solutions of fibrinogen or plasma of very different origin. For example, the sera of all vertebrates hitherto investigated induce clotting in goose's plasma. On the other hand, it appears that a greater degree of specificity exists in the case of the thrombokinase and throm- bogen, the specificity being absolute in some cases, relative in others. That is to say, the thrombokinase of one' animal may activate the thrombogen of an animal of another group, while it may fail to activate the thrombogen of an animal belonging to a third group. COAGULATION 41 But it will always activate the thrombogen of an animal of the same kind. To sum uf, we may say that when blood, is shed, thrombin is rapidly 'formed by the action of thrombokinase, liberated from the leucocytes, the blood-plates, and possibly to some extent from the erythrocytes, upon thrombogen, already present in the circulating plasma. Further — and this is of great practical importance — since no vessel is opened under ordinary circumstances except through a wound in the overly- ing structures, the cut tissues supply a store of thrombokinase at the point where it is required to aid in the stanching of the wound. Calcium is essential to the reaction by which thrombogen and thrombo- kinase form thrombin, but is not necessary for that action of thrombin on fibrinoges by which fibrin is produced (Practical Exercises, pp. 63-65). The Nature of the Action of Thrombin on Fibrinogen. — The usual view, first advanced by Schmidt many years ago, is that thrombin acts as an enzyme. Hence it is often spoken of as fibrin-ferment. In support of this theory it has been stated that the thrombin does not itself seem to be used up in the process, nor to enter bodih' into the fibrin formed; that a small quantity of it can cause an indefinitely large amount of fibrinogen to clot; and that its power is abolished by boiling (p. 331). There has been a disposition .among more recent observers to question this evidence. Accord- ing to Rettger, the quantity of fibrin formed when a small amount of thrombin is added to a fibrinogen solution tends to a fixed maxi- mum, which does not increase with the time of action.* Under certain conditions, also, it is said that thrombin is not destroyed at the temperature of boiling water. Whatever the precise nature of the reaction which leads to the precipitation of the fibrinogen in the form of fibrils, thrombin is very loosely combined if combined at all in the fibrin, since it is readily extracted by an 8 per cent, solution of sodium chloride. This, indeed, is one of the best ways of obtaining an active thrombin solution. The view which we have followed above, in accordance with Morawitz, that the substances in tissue extracts which favour coagulation do so by activating prothrombin to fully formed thrombin, has also been opposed by a number of the more recent workers. Some consider that they exert a direct action upon fibrinogen similar to, although not necessarily identical with, that of thrombin, and speak of them as coagulins (L. Loeb). Howell holds that these substances, which he prefers to term thromboplastic substances, since this makes no assumption as to their mode of action, play a quite different role, namely, that of neutralizing anti- thrombin. His observations have led him to the conclusion that * The inquiry is complicated by the fact that fibrin, once formed, tends to adscM-b the remaining thrombin and so to interfere with its further action. 42 THE CIRCULATING LIQUIDS OF THE BODY the effective thromboplastic substance in the tissues is a phos- phatide, probably kephalin, united with protein. Intravascular Coagulation Regulation of the Clotting Process, or Thrombotaxis. — So far we have been considering the problem of coagulation as if all the data for its solution could be obtained by a study of the blood itself. In other words, our main business up to this point has been the explanation of coagulation in the shed blood; it has been only incidentally, and with the object of casting light on the question of extravascular clotting, that we have touched on the coagulation of the blood within the living vessels. It is not possible here to adequately discuss, nor even to define, the differences between the two problems. All we can do is to warn the student, and to emphasize the warning by one or two illustrations, that valuable as is the knowledge derived from experiments on extravascular coagulation, it would be totally mis- leading if applied without modification to the circulating blood. For instance, we have recognized in the blood-plates an important source of the thrombokinase which plays so great a part in the clotting of shed blood; but we may be sure that blood-plates are constantly breaking down in the lymph and the blood, and we have to inquire how it is that coagulation does not occur, except in disease, within the vessels. Calcium is not wanting to the circu- lating plasma, fibrinogen is not wanting, and it has already been mentioned that thrombogen exists in perfectly fresh and, as we may say, still living blood. Why, then, does it not coagulate ? Some have said that coagulation is ' restrained ' by the contact of the living walls of the bloodvessels; but although it is certain that the contact of foreign matter — and all dead matter is foreign to living cells — does hasten the destruction of blood-plates or that alteration in them on which the liberation of the precursors of the ferment depends, it is evident that it is just this ' restraining ' in- fluence of the vessels, if it is due to anything more than the mere smoothness of their endothelial lining, which has to be explained. The best answer which can be given to the question is: First, that the quantity of thrombokinase free in the plasma at any given time must be small, since no evidence of its presence in fluoride plasma can be obtained. If thrombokinase is liberated in the circulating blood, we may assume that it is changed into some inactive sub- stance, or quickly eliminated. And it appears that, unlike the true ferments, thrombokinase acts quantitatively — i.e., in proportion to its amount — upon thrombogen. Second, an antithrombin exists in the circulating plasma, and even if fully formed fibrin-ferment were present, it could not cause coagulation until the antithrombin had been neutralized. This antithrombin is probably not manu- factured in the blood, or at least not exclusively in the blood, but in the tissues, and there is no reason to deny the vessels themselves COAGULATION 43 a share in its production, even if its presence has not hitherto been demonstrated in the internal coat (L. Loeb) . So that living blood within the living vessels may be said to be acted upon by two sets of influences, one tending to favour coagulation, the other to oppose it. In the clotting of extravascular plasma, free from corpuscles, we may indeed see the continuation, under modified conditions, of a normal process always going on within the bloodvessels. Under normal conditions, the processes that make for coagulation never obtain the upper hand. Indeed the margin of safety within which what may be called the thrombo-regulatiye mechanism works seems to be surprisingly wide, and the equilibrium in the circulating blood far more stable than observations on clotting outside of the body might lead us to suppose. Very considerable quantities of thrombin or of de- fibrinated blood or serum containing thrombin can be injected into the blood-stream without ill effect. According to Howell, the presence of the abnormally great amount of thrombin causes the formation of sufficient antithrombin to neutralize it, probably by a protective reflex secretion. In like manner the injection of tissue extracts or a solution of thrombo plastic substance (thrombokinase) prepared from them by precipitation does not necessarily induce coagulation in the vessels. On the contrary, when injected slowly or in small amount into the veins of an animal, it abolishes for a time the power of coagulation of the blood; and when this ' nega- tive phase,' as it is called, has been once established, even a very large and rapid injection produces no further effect, possibly because an antibody which neutralizes the action of thrombokinase has been produced. In both cases the limits of safety can be over- stepped, and intravascular clotting induced by the injection either of thrombin or of thrombokinase. When a considerable quantity of the active substance in tissue extract is introduced at the first injection, extensive coagulation in the vessels instantly ensues; the animal dies in a few minutes; and the right side of the heart, the venae cavae, the portal vein, and perhaps the pulmonary arteries, may be found choked with thrombi. Here the injected thrombo- kinase is responsible for the clotting, thrombogen and calcium being already present. Curiously enough, intravascular coagulation fails to be produced in a certain proportion of cases when albino animals are injected with material from pigmented animals, while there is no absolute failure of coagulation when albinos are injected with material from albinos, and no failure when pigmented animals are injected with material either from other pigmented animals or from albinos. Intravascular coagulation on injection of tissue extracts is especially striking in birds. To a certain extent the action of tissue extracts in coagulation can be imitated by other substances of animal origin, such as the 44 THE CIRCULATING LIQUIDS OF THE BODY venoms of some vipers (Maitin). It is not known whether these substances act on the blood-plates, leucocytes, or other cells, and thus cause an increased production or an increased liberation of one or more of the precursors of thrombin, or whether they take part directly in its formation. But there is some evidence that the venoms which favour coagulation do so in virtue of their con- taining a kinase. On the other hand, cobra-venom prevents coagula- tion by means of an antikinase — that is, a substance which antago- nizes the action of kinase^ and so hinders the formation of thrombin. It does not contain an antithrombin — that is, a body which will prevent the action of thrombin already formed (Mellanby). Relation of the Liver to Coagulation.— It is not known with any degree of certainty whether the thrombo-regulative processes are especially associated with any particular organ. But there are facts which suggest that the relations of the liver to the coagulation of the blood are peculiarly close. Not only, as previously shown, does it take an important share in the formation of fibrinogen, but there is some evidence that it is closely related to the formation of antithrombin. We have already mentioned that the injection of commercial peptone, which consists chiefly of proteoses, into the blood of dogs causes it to lose its coagulability. The effect gradually passes away, till after some hours the original power of coagulation is restored (p. 63). The liver is known to be intimately concerned in the production of this remarkable result, for if the circulation through it be interrupted, the injection of proteose is ineffective. Further, if a solution of proteose is artificially circulated through an excised liver, a substance (perhaps an anti- thrombin) is formed which is capable of suspending the coagula- tion of blood outside of the body, a property which proteoses them- selves do not possess, or possess only in slight degree. It is not believed that the proteose is actually changed into this anticoagu- lant substance, but rather that the liver cells produce it as a ' reaction ' to the presence of the foreign substance, being perhaps stimulated in some way by the circulating proteose. In part the abnormally great alkalinity of the peptone blood, due to the excess of alkali secreted by the liver, is responsible for itsislow coagulation. Under certain conditions, some of which are known and others not, the injection even of one or other of the purified proteoses causes not retardation, but hastening, of coagulation ; and if this has been the result of a first injection, a second is equally unsuccessful. It is possible that by an effort of the organism to restore the normal coagulability of the blood, on which its very existence depends, substances which favour coagulation are produced, and that the result of an injection of proteose is determined by the relative amount of coagulant and anticoagulant secreted in a given time. Protamins (products obtained from the ripe milt of certain fishes, COAGULATION. , 45 and believed to be the simplest proteins) exert, when injected intravenously, a retarding influence on coagulation, and lower the blood-pressure, just as albumoses do (Thompson). Even serum- albumin and serum-globulin possess this property in some degree. All these substances also cause a diminution in the number of leucocyte's in the blodd"^ owing, in the case of albumose at ainy rate, to their accumulation in the abdominal vessels, and not to any actual destruction of them. It has been lately announced that the adrenal glands have a relation to the coagulation of the blood. Stimulation of the splanchnic nerve, which supplies secretory fibres to the adrenal, greatly hastens coagula- tion, but has no such effect if the adrenal on the corresponding side has been previously removed (Cannon). It is possible that this effect is exerted through the liver, since it is known that one important function of the liver, the regulation of the sugar content of the blood, is intimately dependent upon the adrenal, and is affected by excitation of its splanchnic nerve-supply. In certain pathological conditions the normal balance of the factors that make for clotting and prevent it may beupset, and the scales may tip in either direction. In patients suffering from, the formation of spontaneous clots in the veins (thrombosis) it is stated that the anti- thrombin in the blood is diminished, the amount of prothrombin being normal. The mere slowing of the blood-stream in conditions where the circulatory mechanism is enfeebled may favour thrombosis. For anything which cripples the circulation, and consequently limits the free interchange between blood and tissues, interferes with the elimina- tion or neutralization of the precursors of thrombin, and with the entrance of the substances that render the fully formed thrombin in- active. This, as well as the injury caused by the ligature, which may favour the passage of thromboplastic substances into the lumen of the occluded vessel, is a possible factor in the formation of the clot on which the surgeon relies for the permanent sealing of ligated vessels. ■ , In haemophilia, a disease in which the coagulation of the blood is characteristically slow, and in which even slight wounds may occasion severe or fatal haemorrhage, the thrombogen (prothrombin) has been found deficient in amount, and the injection of normal serum or- the transfusion of normal blood has been used, with temporary advantage in the treatment of the condition. In certain cases of purpura, how- ever^. where haemorrhage also occurs with abnormal ease, no variation from the normal could be detected in the content of either antithrombin or prothrombin (Howell). Some have supposed that in such conditions ■t:he fault is an unnatural fragility of the small vessels rather than a. deficiency in the power of the blood to clot, but of this also no actual evidence has been adduced. Another factor on which the, promptness, and completeness of the sealing of wounded vessels may depend has been recently brought into notice, namely — The Vaso-Constrictor Property of Shed Blood.^It has been shown that when blood is shed and no precautions are taken to prevent clotting, it very quickly develops the power of causing marked constriction of bloodvessels. This can be demonstrated by allow ing the serum to act on rings cut from arteries (Practical Exercises, , 46 THE CIRCULATING LIQUIDS OF THE BODY p. 66), or by perfusing the hind-legs of frogs with a saline solu- tion containing serum. Plasma derived from blood in which the platelets have been prevented from breaking down, and which therefo c remains unclotted, has no such effect, or a much slighter Fig. 7. — Sheep Artery Rings. At 14 and 16 Ringer's solution was replaced by citrate plasma (two different specimens). At 15 and 17 the plasmas were replaced by the corresponding sera. At 58 and 1 9 the sera replaced Ringer's solution directly. Time-trace, half-minutes. Tracings reduced to J. effect. But when the platelets are separated from the plasma and then decomposed, the resulting extracts of platelets are rich in vaso-constrictor material. In the sealing of wounded vessels the platelets would therefore appear to play a double role, yielding Fig 8. — Frog Perfusion Experiment with Serum. The drops of liquid flowing through the preparation are recorded. At 11 citrate plasma was injected; at 13 the corresponding citrate serum. The tracing is to be read from left to right. Time is marked in half -minutes. a substance which causes constriction of the vessel in the neigh-,' bourhood of the wound while a plug of clot is being formed, thanks to other substances liberated from the platelets, which take an essential part in coagulation. The vaso-constriction may perhaps VASO-CONSTRICTOR PROPERTY OF SHED BLOOD 47 be looked upon as a form of ' first aid ' to diminish the haemor- rhage, and also to make it less eaSy for the beginning clot to be washed away. It is obvious that the two processes would be mutually advantageous in dealing with those injuries of the vascular system on the prompt repair of which the very existence of the '\ ^ -An ■n./- -r\ ^ v/ \ ^ "" V^ /cc. SeruM*- citrate ice Piasma. /cc. Plasma Ice Serum tcttmte Fig. 9. — Frog Perfusion Experiment with Serum. Curves showing the flow. The number o£ drops per half -minute is laid off along the vertical axis, and the time {in half-minutes) along the horizontal axis; 38 drops correspond to i cc. organism at all times depends, and it is not without interest to find that special formed elements in the blood, the platelets, are pre-eminently associated with both processes. Section III. — ^The Chemical CCimposition of Blood. The serum of coagulated blood represents the plasma minus fibrinogen ; the clot represents the corpuscles plus fibrin. Thus : Plasma - Fibrin(ogen) = Serum. Corpuscles + Fibrin = Clot. Plasma + Corpuscles = Serum + Clot = Blood. Bulky as the clot is, the quantity of fibrin is trifling (0-2 to 0-4 per cent, in human blood). The plasma contains about 10 per cent, of solids, the red corpuscles about 40 per cent., the entire blood about 20 per cent. Serum contains 7 to 8 per cent, of proteins, about o-8 per cent, of inorganic salts, and small quantities of neutral fats, soaps, cholesterin esters, lecithin, dextrose, urea, lactic acid, . glycuronic acid, amino - acids. Cor usffel iiiiii ll ^^^^l 1 1 1 ^"^ Other sub- Fig. lo.-Diagram showing Relative Quantity of Solids alhu'»fn and serum- and Water in Red Corpuscles and Plasma. gloouhn. In the rab- bit the former, in the horse the latter, is the more abundant ; in man they exist in not far from equal amount. A small quantity of nucleo-protein and of fibrino-globulin (which some consider a soluble product formed from 48 THE CIRCULATING LIQUIDS OF THt. aou x fibrinogen in dotting) is also present. Ferments which cause hydrolysis of proteins and carbohydrates, a ferment (lipase) which . acts upon fats, and certain oxidizing ferments (oxydases), have also been demonstrated. The chemical nature of the bodies which confer on serum or plasma its specific hasmolytic, agglutinating, precipitating, and bactericidal properties has not been definitely determined. ■ The quantitative composition of serum, especially as regards the inorganic salts,, is remarkably constant in animals of the same species, and even in animals of different species belonging to the same, or to not very widely separated, natural groups. In cold-blooded animals the serum-albumin is scantier than in mammals, the globulin relatively more plentiful. Serum-albumin belongs to the class of native albumins. It has been obtained in a crystalline form from the serum of horse's blood. It Fig. II. — Perspective View of Vivi-Diffusion Apparatus (Abel). This form of the ' app'aratus contains sixteen tubes. A, arterial cannula; B, venous cannula; C, side tube for introduction of hirudin; D, inflow tube; E, outlet tube for the blood ; F, G, supporting rod attached at H and K to branched V-tubes ; L, burette for hirudin; M, N, tube for filling and emptying liquid in outer jacket; O, air outlet; P, dichotomous branching-point of inflow tube; Q- and R, quadruple branching-points of the same ; S, S, wooden supports; T, thermometer. At each of the points H and K the blood is collected from four tubes into one, bending round to the back, and there redividing into four return flow tubes. Arrows show the direction of the flow. is soluble in distilled, water, and is not precipitated by saturating its solutions with certain neutral salts. Heated in neutral; or slightly acid solution, it coagulates first at 73°, then at 77°, then, at 84° C. Although this is not of itself sufficient proof, there is other evidence that it consists of a mixture of proteins. . Serum-globulin, also called paraglobulin, belongs to the globulin group of proteins. When heated, it coagulates at about 75° C. (p. 9). It is insoluble in distilled water, and is precipitated by saturation with such neutral salts as magnesium sulphate, or by half -saturation with ammonium sulphate. It appears that, as thus obtained, it is not a single substance, but a mixture of at least two proteins — eu-globulin, CHEMICAL COMPOSITION OF BLOOD 49 which can be precipitated from its saline solution by dialyzing off the salts, and pseudo-globulin, which cannot be so precipitated. In addition to the nitrogen. represented as protein, serum (or plasma) contains non-protein nitrogen, the amount of which varies with the nature of the food and the stage of digestion. Part of this fraction is attributable to urea and other metabolites on their way to be excreted, but another portion, and an important one, is due to amino-acids absorbed from the intestine during the digestion of proteins and on their way to be utilized in the tissues. Of the inorganic salts of serum, the most important are sodium chloride and sodium bicarbonate. Small amounts of potassium, calcium, and magnesium, united with phosphoric acid or chlorine, and a trace of fluoride, are also present. A portion of the salts is loosely combined with the proteins. Our knowledge of the chemistry of the circulating plasma is likely to be notably augmented by the method of vivi-diffusion recently intro- duced by Abel. An artery of an ansesthetized animal is connected by a cannula to a system of celloidin tubes immersed in a saline solution. Blood passes from the artery through the tubes, where it exchanges diffusible constituents with the solution, and is then returned to the animal's body by another cannula attached to a vein. Coagulation of the blood in the apparatus is prevented by hirudin, and under aseptic conditions the circulation may proceed through the tubes for a long time. The saline solution can then, be analyzed for substances which have entered it from the blood — amino-acids, for example (Fig. ii). The following tables give some details of the composition of blood : 1,000 Grammes of Pig's Blood (Corpuscles, 435'09; Serum, 564'9i) contained Corpuscles. Serum. Corpuscles. Serum, Water 272-20 518-36 P2O5 as nucleir I 0-0455 0-0123 Solids 162-89 46-54 NaaO * 2-401 Haemoglobin 142-20 K2O 2-157 0-152 Protein 8-35 38-26 FeaOg 0-696 Sugar 0-684 CaO 0-0689 Cholesterin 0-213 0-231 MgO 0-0656 0-0233 Lecithin 1-504 0-805 CI 0-642 2-048 Fat . . I-IO4 P2O5 0-895 O-III Fatty acids 0-027 0-448 Inorgai aic P2O5 0-719 0-296 Proteins of Plasma in 1,000 Grammes. Albumin. Globulin. Fibrinogen. Total. Man Dog Sheep Horse Pig - 40-1 31-7 38-3 28-0 44-2 28-3 22-6 30-0 47-9 29-8 4-2 6-0 4-6 4-5 6-5 72-6 60-3 72-9 80-4 80-5 * X'k^ «:„ The pig's erythrocytes are peculiar in that the sodium appears to be entirely confined to the plasma. 4 50 THE CIRCULATING LIQUIDS OF THE BODY The Coloured Corpuscles consist of rather less than 60 per cent, of water and rather more than 40 per cent, of solids. Of the solids the pigment haemoglobin makes up about 90 per cent. ; the proteins and nucleo-protein of the stroma about 7 per cent. ; lecithin and choles- terin 2 to 3 per cent. ; inorganic salts (which vary greatly in their relative proportions in different animals, but in man consist chiefly of phosphates and chloride of potassium, with a much smaller amount of sodium chloride) about i per cent. Potassium has been demonstrated microchemically in frog's erythrocytes (MacaUum) (Frontispiece). There is evidence that a portion of the salts is more firmly combined than the rest, so that, even after the action of the most energetic laking agents, this fraction remains attached to the stroma. The erythrocytes of some animals — e.g., the dog — contain dextrose. When dextrose is added to human blood it rapidly dis- tributes itself over corpuscles and plasma (Rona), although not exactly in proportion to their respective volumes (Masing). Hither- to the dextrose in blood has been reckoned as if it all belonged to the plasma. Haemoglobin. — Of all the solid constituents of the blood, haemoglobin is present in greatest amount, constituting as it does no less than 13 per cent., by weight, of that liquid. It is an exceedingly complex body, containing car- -6 bon, hydrogen, nitro- * Wa gen, and oxygen in 3 much the same pro- portions in which they exist in ordinary pro- teins (p. i). Iron is also present to the ex- tent of almost exactly Fig. 12. — Diagram of Spectroscope. A, source of light; one-thirdof I percent., B, layer of blood; C, collimator for rendering rays and there is also a little parallel; D, prism; E, telescope. sulphur, the amount of which stands in a very simple relation to the quantity of iron (i atom of iron to 3 of sulphur in dog's haemoglobin, and I atom of iron to 2 of sulphur in the haemo- globin of the horse, ox, and pig). Haemoglobin is made up of a protein element which contains all the sulphur and a pigment which contains aU the iron, the protein constituting by far the larger portion of the gigantic molecule, whose weight has been estimated at more than 16,000 times that of a molecule of hydrogen. Since its percentage composition is stUl undetermined with absolute precision, it is impossible to give an empirical formula that is more than approximately correct. For dog's haemoglobin Jaquet gives CjsgHiaosNigsSsFeOas, which would make the molecular weight 16,669. Direct determinations of the molecular weight gave 15,115 for oxyhaemoglobin of the horse, and 16,321 for that of the ox (Hiifner and Gansser). While these numbers need not be taken as more than a rough approximation, they at least show that the haemoglobin molecule is an exceedingly large one. The most remarkable property of haemoglobin is its power of combining loosely with oxygen when exposed to an atmosphere con- CHEMICAL COMPOSITION OF BLOOD D E b F SW 630 620 610 600 5^ 5S0 570 560 550 5¥) 5^ SW\ 510 500 4S0^ m I hmIii, J iiliiiiliiMLiiiliuiiMillmLlnlJlinlillllllillllllLllllmiLlllllllll 5t [•.•.'•■ 7 8 I I I r I I I ■ 1 ri I I I I I I I S 10 I ' i Fig. 13. — Table of Spectra of Hcemoglobin and its Derivatives (Zienika and Mliller)- I, Oxyhemoglobin; 2, reduced haemoglobin; 3,rnetha'moglobin; 4, acid ha-matin; 5. alkaline hematin; 6, htcmochromogen; 7, acid hiEinatoporphyrin; 8, alkaline hcematoporphyrin; 9, carbon monoxide ha;moglobin. 52 THE CIRCULATING LIQUIDS OF THE BODY taining it, and of again giving it up in the presence of oxidizable substances or in an atmosphere in which the partial pressure of oxygen (pp. 250-253) has been reduced below a certain limit. It is this property that enables hsemoglobin to perform the part of an oxygen-carrier to the tissues, a function of the first importance, which will be more minutely, considered when we come to deal with respiration. The bright red colour of blood drawn from an artery or of venous blood after free exposure to air is due to the fact that the haemo- globin is in the oxidized state — ^in the state of oxyhsemo- globin, as it is called. If the oxygen is removed by means of reducing agents, such as ammonium sulphide, or by ex- posure to the vacuum of an air-pump, the colour darkens, the blood-pigment being now in the form of reduced haemo- globin. In ordinary venous blood a large proportion of the pigment is in this condition, i but there is always oxyhaemo- globin present as well. In asphyxia (p. 276), however, nearly the whole of the oxy- haemoglobin may disappear. Crystallization of Hcsmoglohin. — In the circulating blood the haemoglobin is related in such a way to the stroma of the cor- puscles that, although the latter are suspended in a liquid readily capable of dissolving the pig- ment, it yet remains under ordinary circumstances strictly within them. In a few- inver- tebrates, however, it is nor- mally in solution in the cir- culating liquid. As a rare occurrence heemoglobin may form crystals inside the corpuscles (p. 71). When it is in any way brought into solu- tion outside the body, it shows in many animals, but not in the same degree in all, a tendency to crystallization ; and the ease with which crystallization can be induced is in inverse proportion to the solubility of the haemoglobin. Thus, it is far more difficult to obtain crystals of haemoglobin from human blood than from the blood of the rat, guiiiea- pig, or dog, whose blood-pigment is less soluble than that of man, and for a like reason the oxyhaemoglobin of the bird, the rabbit, or the frog crystallizes still less readily than that of human blood. As to the form of the crystals, in the vast majority of animals they Fig. 14.— Oxyhsemoglobin Crystals (Frey) ' a, b, from man ; c, from cat ; d, from guinea pig; e, from hamster;/, from squirrel. CHEMICAL COMPOSITION OF BLOOD 53 are rhombic prisms or needles, but in the guinea-pig they are tetrahedra belonging to the rhombic system, and in the squirrel six-sided plates of the hexagonal system (Fig. 14) . Careful study of the crystallography of haemoglobin from a large number of animals has established differences and resemblances so constant and so clear-cut that they may be used for the purposes of classification and for the identification of the source of a specimen of blood (Reichert and Brown) . Reduced hsemoglobin can also be caused to crystallize, though with more difficulty than oxyhsemoglobin, since it is more soluble. Crystals of reduced hemoglobin were first prepared from human blood by Htifner, who allowed it to putrefy in sealed tubes for several weeks. When a solution of oxyhaemoglobin of moderate strength is ex- amined with the spectroscope, two well-marked absorption bands are seen, one a little to the right of Fraunhofer's line t), and the other a little to the left of E. A third band exists in the extreme violet between G and H. It cannot be detected with an ordinary spectro- scope, but has been studied by the aid of a fluorescent eyepiece, by projecting the spectrum on a fluorescent screen, and by photograph- ing the spectrum. The addition of a reducing agent, such as ammonium sulphide, causes the bands in the visible spectrunj to disappear, and they are replaced by a less sharply defined band, of which the centre is about equidistant from D and E. This is the characteristic band of reduced haemoglobin. The spectrum of ordinary venous "blood shows the bands of oxyhsemoglobin. Carbonic oxide hcsmoglobin is a representative of a class of haemo- globin compounds analogous to oxyhaemoglobin, in which the loosely- combined oxygen has been replaced by other gases (carbon monoxide, nitric oxide) in firmer union. Its spectrum shows two bands very like those of oxyhaemoglobin, but a little nearer the violet end. Carbonic oxide haemoglobin is formed in poisoning with coal-gas. Owing to the great stability of the compound, the haemoglobin can no longer be oxidized in the lungs, and death may take place from asphjrxia. It is, however, gradually broken up, and therefore artificial respiration may be of use in such cases. Inhalation of oxygen and especially transfusion of blood are also of great value. Methcemoglobin is a derivative of- oxyhaemoglobin which can be formed from it in various ways, e.g., by the addition of ferricyanide of potassium or nitrite of amyl (Gamgee), by electrolysis (in the neigh- bourhood of the anode), or by the action of the oxidizing ferment ' echidnase ' in the poison of the viper (Phisalix) . It very often appears in an oxyhaemoglobin solution which is exposed to the air. It has been found in the urine in cases of haemoglobinuria, in the fluid of ovarian cysts, and in haematoceles. The strongest band in its spectrum is in the red, between C and D, but nearer C, nearly in the same position as the band of acid-haematin. Reducing agents, such as ammonium sulphide, change methsemoglobin first into oxyhaemoglobin and then into reduced haemoglobin. It has by some been regarded as a more highly -oxidized haemoglobin than oxyhaemoglobin . Rebutting evidence has, however, been offered to the effect' that the same quantity of oxygen is required to saturate both pigments, and this evidence appears to be sound. The difference lies rather in the manner in which the oxygen is united to the haemoglobin in the methaemoglobin molecule than in the quantity of oxygen which it contains. For methsemo- 54 THE CIRCULATING LIQUIDS OF THE BODY globin, unlike oxyhaemoglobin, parts with no oxygen to the vacuum, while, on the other hand, in the presence of reducing agents it yields up its oxygen even more readily than oxyhaemoglobin does (Haldane) (P- 249). ,. . ^ By the action of acids or alkalies oxyhaemoglobin is split into a pig- ment, haematib, and a protein, globin, belonging to the histon group. It is easily precipitated from solution by ammonia. On hydrolysis, it yields a large amount of histidin,to which its basic properties are chiefly (HHO^ pP cJWfe 3 *jr/yA/"- '^^^ OxyHb Carbonic OKI fie-Hh. I Tufo tfa^moch romoifen f bands Hacmatojiorithyrm (acidj\ Mcihaemo^lnbin. '\ flcidHaematin I ''««■ Alkaline Haematm \i/^^ Reduced Hb- J V. >Jk Fig. 15. — Diagram to siiow the Chief Characteristics by which Haemoglobin and some of its Derivatives may bs recognized Spectroscopically. The position of the middle of each band is indicated roughly by a vertical line. due. From 100 grammes of oxyhaemoglobin about 4 grammes of hae- matin are obtained. As to the pigment moiety, when haemoglobin is acted on by acids in the absence of oxygen, hcemochromogen is first formed, which then gradually loses its iron and is changed into haemato- porphyrin. If oxygen be present, haematin is the final product. Haematin may be considered as the compound which haemochromogen forms with oxygen. By the action of alkalies reduced haemoglobin yields haemochromogen, which is stable in alkaline solution, and gives a beautiful spectrum mmmmmmmmmm- mmim. Livtr 293 Ifoscles ig-2 fTTtaT vtst*ls,kMrTrJvngs ZZ'7 Sanes 8'2 Iitbisljnesr rnmhl organs S'3 ^hm il Kineys V6 Tferve cejiTfes Vi Spleen ">- Fig. 16. — Diagram to illustrate the Distribution of the Blood in the Various Organs of a Rabbit (after Ranke's Measurements). The numbers are percentages of the total blood. with two bands, bearing some resem- blance to those of oxyhaemoglobin, but placed nearer the violet end . The band next the red end is much sharper than the other (p. 76). Haemochromogen binds exactly the same amount of oxy- gen as the haemoglobin from which it is derived, and it is due to the haemochromogen in its molecule that the bood - pigment fulfils its function of takiag up and transporting oxygen. Hmmatin (C3aH3a04N4.FeOH), the most frequent result of the splitting up of haemoglobin, is generally obtained as an amorphous substance with a. bluish-black colour and a metallic lustre, insoluble in water, but soluble in dilute alkalies and acids, or in alcohol containing them. In addition to the iron of the haemoglobin, haematin contains the four chief elements of proteins — carbon, hydrogen, nitrogen, and oxygen (Practical Exercises, p. 75). QUANTITY AND DISTRIBUTION OF BLOOD 55 Hcsmatoporphyrin (CssHsgOgNJ, or iron-free hsBmatin, may be obtained from blood or haemoglobin by the action of strong sulphuric acid, from haematin or haemia by the action of hydrobromic acid. It is distinguished from these pigments by the fact that it contains no iron. When strong sulphuric acid is allowed to act on blood or haemo- globin solution, haematoporphyrin is also produced, as may be easUy shown by the spectroscope. Its spectrum in acid solution shows two bands, one just to the left of D, the other about midway between D and E. Like oxyhaemoglobin, reduced haemoglobin, carbonic oxide haemoglobin, methaemoglobin, and other derivatives of haemoglobin, it also has a band in the ultra-violet. Hcsmin (C32H3204N4.FeCl) is readily obtained from haematin and also from haemoglobin by heating with dilute hydrochloric acid, and also directly from blood, as described in the Practical Exercises, p. 78. It crystallizes in the form of small rhombic plates, of a brownish or brownish-black colour (Fig. 24, p. 78). They are insoluble in water, but readily soluble in dilute alkalies (Practical Exercises, p. 79). Chemistry of the White Blood- Corpuscles. — The composition of pus- cells and the leucocytes of lymphatic glands has alone been investigated. The chief constituents of the latter are a globulin coagulating by heat at 48° to 50° C. ; a nucleo-protein coagulating in 5 per cent, magnesium sulphate solution at 75° C., and causing coagulation of the blood on injection into the veins of rabbits; an albumin coagulating at 73° C. ; and a ferment with powers like the pepsin of the gastric juice. In pus- cells glycogen has been found, and it can be demonstrated micro- chehiically in the leucocytes of blood by the iodine reaction in various conditions. Fats, cholesterin, and lecithin are also present, as well as the so-called protagon. The ordinary inorganic constituents have been demonstrated — ^namely, potassium, sodium, calcium, magnesium, and iron, united with chlorine and phosphoric acid. The total solids amount to 11 to 12 per cent. Section IV. — Quantity and Distribution of the Blood. The Quantity of Blood. — ^The quantity of blood in an animal is most accurately determined by the method of Welcker. The animal is bled from the carotid into a weighed flask. When blood has ceased to flow the vessels are washed out with water or physiological saline solution, and the last traces of blood are removed by chopping up the bgdy, after the intestinal contents have been cleared away, and extracting it with water. The extract and washings are mixed and weighed; a given quantity- of the mixture is placed in a haema- tinometer (a glass trough with parallel sides, e.g.), and a weighed quantity of the unmixed blood diluted in a similar vessel till the tint is the same in both. From the amount of dilution required, the quantity of blood in the watery solution can be calculated. This is added to the amount of unmixed blood directly determined. Since haemorrhage is immediately followed by the entrance of liquid into the bloodvessels from the lymph and tissue fluids, somewhat too high a result will be obtained if the bleeding is at all prolonged. It is well, therefore, to take only a moderate amount of blood for direct estimation, and to compute the balance by the colorimetric method 56 THE CIRCULATING LIQUIDS OF THE BODY Many other methods have been devised on the principle of in- jecting a known quantity of some substance into the circulating blood, and then, after an interval has been allowed for mixture, determining the chsmge produced in a sample. Thus, the specific gravity of a drop of blood having been measured, a certain quantity of a solution of sodium chloride isotonic with the plasma may be injected into a vein, and the specific gravity again determined. Or the electrical resistance of a small sample of blood may be measured before and after injection of a given quantity of isotonic salt solution. The quantity of blood in the body was greatly overestimated by the ancient physicians. Avicenna put it at 251b., and many loose statements are on record of as much as 20 lb. being lost by a patient without causing death. By Welcker's method the proportion of blood to body- weight has been found to be in the dog i : 13, cat i : 14, horse i : 15, frog i : 17, rabbit i : 19, fowl i : 20. In new-born children the proportion was i : 19, in adult human beings (executed criminals) i : 13. The total mass of the blood in a living man has been estimated by causing the person to inhale a known volume of carbon monoxide mixed with oxygen or air, and then determining in a sample of blood taken from the finger the percentage amount to which the haemoglobin has become saturated with carbon monoxide. All that remains is to estimate the volume of carbon monoxide (or, what is precisely the same thing, the volimie of oxygen) which 100 c.c. of blood will take up. This latter quantity is called the percentage oxygen capacity. From these data the total volume of the blood can be calculated. If the volume is multiplied by the specific gravity the mass is obtained. Thus, if the haemoglobin was found to be 25 per cent, saturated with carbon monoxide after the person had absorbed 150 c.c. of that gas, the whole of the blood would require 600 c.c. of carbon monoxide to saturate it completely. If the percentage oxygen capacity was 20, 20 c.c. of oxygen or carbon monoxide would be needed to saturate 100 c.c. of blood. Therefore the total volume of the blood would be 600 X = 3,000 c.c. And the mass, if the specific gravity was l'055, would be 3,000 X fo55 = 3, 165 grammes. According to this method the blood on the average in man constitutes only 4-9 per cent., or 5J.5 of the body-weight (say, 3i kilogrammes in a 70 kilo man), varying in fourteen persons between ^ and ^. There is reason, however, for thinking that the method — at least, as hitherto employed — underesti- mates the quantity of blood. According to Dreyer, the blood volume is a function of the surface of the body, so that the smaller and lighter animals in. any given species have a relatively greater blood volume than the larger and heavier individuals. Accordingly, he considers that the practice of expressing the volume of blood as a percentage of the body-weight should be abandoned. Fig. 16 (p. 54) illustrates the distribution of the blood in the various organs of a rabbit. The liver and skeletal muscles each con- tain rather more than one-fourth; the heart, lungs, and great vessels LYMPH AND CHYLE 57 rather less than one- fourth; and the rest of the body about one-fifth, of the total blood. The kidney and spleen of the rabbit each contain one-eighth of their own weight of blood, the liver between one-third and one-fourth of its weight, the muscles only one-twentieth of their weight. Section V. — Lymph and Chyle. Lymph has been defined as blood without its red corpuscles (Johannes Miiller); it resembles, in fact, a dilute blood-plasma, containing leucocytes, some of which (lymphocytes) are common to lymph and blood, others (coarsely granular basophile cells, present only in small numbers) are absent from the blood. Lymph also contains thrombocytes. The reason of this similarity appears when it is recognized that the plasma of tissue-lymph (p. 460) is derived, in large part at any rate, from the plasma of blood hy a process of physiological filtration (or secretion) through the walls of the capillaries into the lymph-spaces that everywhere occupy the inter- stices of areolar tissue, while the lymph of the lymphatic vessels is in turn derived from the tissue fluid. But in addition to the con- stituents of the plasma, Ijmiph contains substances produced in the metabolism of the tissues which pass into it directly. As collected from one of the large lymphatic vessels of the limbs, or from the thoracic duct of a fasting animal, lymph is a colourless or some- times yellowish or slightly reddish liquid of alkaline reaction. Its specific gravity is much less than that of the blood (1015 to 1030). It coagulates spontaneously, but the clot is always less firm and less bulky than that of blood. The plasma contains fibrinogen, from which the fibrin of the clot is djerived. Serum-albumin and serum- globuhn are present in much the same relative proportion as in blood, although in smaller absolute amount. Neutral fats, urea, and sugar are also found in small quantities. The inorganic salts are the same as those of the blood-serum, and exist in about the same amount, sodium preponderating among the bases, as it does in serum. The following table shows the results of analyses of lymph from man and the horse (Munk) : Man. Horse. Water 95-0 per cent. 95 -8 per cent. Fibrin o-i ^ O'l "1 Other proteins 4-1 2-9 Solids - Fat - trace V 5-0 trace (■ 4'2 Extractives* 0-3 o-i I Salts 0-5 I'l , * The term ' extractives ' is somewhat loosely applied to organic substances which exist in so small an amount, or have such indefinite chemical characters, that they cannot be separately estimated, and are extracted together from the residue by various solvents. 58 THE CIRCULATING LIQUIDS OF THE BODY Chyle is merely the name given to the lymph coming from the ahmentary canal. The fat of the food is absorbed by the lym- phatics, and during digestion the chyle is crowded with fine fatty globules, which give it a milky appearance. There may also be in chyle a few red blood-corpuscles, carried into the thoracic duct by a back-flow from the veins into which it opens. Chyle clots like ordinary lymph, the size of the clot varjnng according to the quantity of fat present and enmeshed by the fibrin. Wounds of the thoracic duct or of lymphatics opening into it are occasionally produced in operations on the neck, and when these remain open chyle may be readily collected. In samples obtained from a patient only a week after the section of a branch of the diict during an operation for the removal of tubercular glands, water constituted 928-90 parts in 1,000, total solids yi-io, inorganic solids 6-04, organic solids 65-06, proteins 18-52, ether extract (fatty substances) 19-30 (SoUmann). The following is the composition of a sample analyzed by Paton, and obtained from a fistula of the thoracic duct in a man : Water Solids Inorganic - Organic Proteins Fats Cholesterin Lecithin 953-4 46-6 6-5 40-1 13-7 24-06 0-6 0-36 The quantity of chyle flowing from the fistula was estimated at as much as 3 to 4 kilos per twenty-four hours, or nearly as much as the whole of the blood. The flow has been calculated in various animals at one-eighteenth to one-seventh of the body-weight in the twenty- four hours. The quantity of lymph in the body is unknown, but it must be very great — ^perhaps two or three times that of the blood. Allied to tissue-lymph, but not identical with it, are the fluids present in health in very small amount in such serous cavities as the pericardium. The synovial fluid of the joints differs from lymph especially in containing a small amount of a mucin-like substance. The aqueous humour, and still more the cerebro-spinal fluid, are characterized by a marked deficiency in solids, especially protein. In the following table (from Spiro) the differences in the composition of lymph and allied fluids from different parts of the body are illus- trated. Man : Lymph from Fistula in Thigh. Horse : Lymph from Neck during Mastication. Aqueous Humour. Cerebro-Spinal Fluid. Water - Salts Fat Protein - 96-4 to 94-3 0-7 ,, 0-87 0-06 ,, 0-22\ 2-8 ,, 4-8 J 95 0-75 3-7 98-7 0-5 to 0-8 0-72 99 to 99-2 0-02 to 0-l6 FUNCTIONS OF BLOOD AND LYMPH 59 The gases of the blood and lymph will be treated of in Chapter IV., the formation of lymph in Chapter VIII., its circulation in Chapter III. Section VI. — ^The Functions of Blood and Lymph. We have already said that these liquids provide the tissues with the materials they require, and carry away from them materials which have served their turn and are done with. These materials are gaseous, liquid, and solid. Oxygen is brought to the tissues in the red corpuscles ; carbon dioxide is carried away from them partly in the erjrthrocytes, but chiefly in the plasma of the blood and lymph. The water and solids which the cells of the body take in and give out are also, at one time or another, constituents of the plasma. The heat produced in the tissues, too, is, to a large extent, conducted into the blood and distributed by it throughout the body. The leucocjrtes, as will be seen farther on, aid in some measure in the absorption of certain of the food substances from the intestine. It is not known whether, apart from this, they play any r61e in the normal nutrition of other cells, although it is probable that they exercise an influence on the plasma in which they live. But they have impor- tant functions of another kind, to which it is necessary to refer briefly here. Phagocytosis. — Certain of the amoeboid cells of blood and lymph, and the cells of the splenic pulp, are able to include or ' eat up ' foreign bodies with which they come in contact, in the same way as the amceba takes in its food. Such cells are called phagocjrtes ; and it is to be remarked that this term neither comprises all leucocytes nor excludes all other cells, for some fixed cells, such as those of the I endothelial lining of bloodvessels, are phagocjrtes in virtue of their power of sending out protoplasmic processes, while the small, relatively immobile lymphocjrte is not a phagocyte. Although it is not at present possible to assign a physiological value to all the phenomena of phagocytosis, either as regards the phagocytes themselves or as regards the organism of which they form a part, there seems little doubt that under certain circumstances the process is connected with the removal of structures which in the course of development have become obsolete, or with the neutral- ization or elimination of harmful substances introduced from with- out, or formed by the activity of bacteria within the tissues. During the metamorphosis of some larvae, groups of cilia and muscle-fibres may be absorbed and eaten up by the leucocytes. In the metamor- phosis of maggots, for example, the muscular fibres of the abdominal wall, which are used in creeping, and are therefore not required in the adult, degenerate, and are devoured by swarms of leucoc3rtes which migrate into them. In the human subject an example of 6o THE CIRCULATING LIQUIDS OF THE BODY absorption of tissue by the aid of leucocytes is the removal of the necrosed decidua reflexa, the fold of uterine mucous membrane which envelops the ovum (Minot). The behaviour of phagocytes towards pathogenic micro-organisms is of even greater interest and importance. Metchnikoff laid the foundation of our knowledge of this subject by his researches on Daphnia, a small crustacean with transparent tissues, which can be observed under the microscope. When this creature is fed with a fungus, Monospora, the spores of the latter find their way into the body-cavity. Here they are at once attacked by the leucocytes, ingested, and destroyed. But after a time so many spores get through that the leucocytes are unable to deal with them all ; some of them develop into the first or ' conidium ' stage of the fungus; the conidia poison the leucocytes, instead of being destroyed by them, and the animal generally dies. Occasionally, however, the leuco- cytes are able to destroy all the spores, and the life of the Daphnia is preserved. This battle, ending sometimes in victory, sometimes in defeat, is believed by Metchnikoff to be typical of the struggle which the phagocytes of higher animals and of man seem to engage in when the germs of disease are introduced into the organism. He supposes that the immunity to certain diseases possessed naturally by some animals, and which may be conferred on others by vaccina- tion with various protective substances, is, to a large extent, due to the early and complete success of the phagocytes in the fight with the bacteria; and that in rapidly- fatal diseases — such as chicken- cholera in birds and rabbits, and anthrax in mice — the absence of any effective phagocytosis is the factor which determines the result. Others have laid stress on the action of protective substances sup- posed to exist in the plasma itself. It is possible that such sub- stances are manufactured by the leucocytes, and either given off by them to the plasma by a process of ' excretion,' or liberated by their complete solution. The most recent investigations go to show that Metchnikoff' s phagocjrtic theory of immunity requires modification, at any rate in the case of the higher animals and man, although the brilliant biological observations on which it was originally built retain all their value. He supposed that in the immunizing process the leucocytes underwent certain changes, acquired, so to speak, a sort of ' education ' that enabled them to cope with bacteria against which they were previously powerless. It seems more probable that in the presence of the substances that confer immunity, not only the leucocytes, but other cells, are stimulated to produce bodies which cut short the life, or inhibit the growth, of the bacteria (alexins), or prepare them for being taken up by the phagocjrtes (opsonins). It has been shown that bacteria which have been in contact with serum containing the appropriate opsonins are taken FUNCTIONS OF BLOOD AND LYMPH 6i up readily by leucocytes washed free from serum constituents by physiological salt solution, whereas the washed leucocytes either do not ingest bacteria which have not been acted on by serum, or take them up in much smaller numbers. There is some evidence that in certain bacterial infections — for example, chronic furunculosis, a condition in which crops of boils continue to appear — the grip of the bacteria on the body is perpetuated by a deficiency in the amount or in the activity of opsonins capable of acting specifically upon the micro-organisms in question. A numerical expression, which in certain cases, perhaps, gives a measure of the patient's resistance to the infection, has been worked out by Wright under the name ' opsonic index.' This index is the ratio between thg average number of bacteria taken up, under certain fixed conditions, by each polymorphonuclear leucocyte in an emulsion made with the patient's serum, and the average number taken up by similar leucocytes in an emulsion made with normal serum. The significance of this index, and even the practicability of the methods used to ascertain it, are still the subject of discussion. Diapedesis. — The fact that leucocytes can pass out of the blood- vessels into the tissues has a very important bearing oil the subject of phagocytosis. The phenomenon is called diapedesis, and is best seen when a transparent part, such as the mesentery of the frog, is irritated. The first effect of irritation is an increase in the flow of blood through the affected region. If the irritation continues, or if it was originally severe, the current soon begins to slacken, the corpuscles stagnate in the vessels, and inflammatory stasis is pro- duced. The leucocytes adhere in large numbers to the walls of the capillaries, and particularly of the small veins, and then begin to pass slowly through them by amoeboid movements, the passage taking place at the junctions between, or it may be through the substance of, the endothelial cells. Plasma is also poured out into the tissues, the whole forming an inflammatory exudation. Even red blood- corpuscles may pass out of the vessels in small numbers. The exudation may be gradually reabsorbed, or destruction of tissue may ensue, and a collection of pus be formed. The cells of pus are emigrated leucocytes (Practical Exercises, Chap. III., p. 191). Their emigration is connected with the defence of the organism against the entrance of certain forms of bacteria at the seat of. injury, and with the repair of the injured tissue, but the nature of* the summons which gathers them there is not yet clearly under- stood. It is probably some sort of chemical attraction (chemio- taxis) between constituents of the bacteria or decomposition prod- ucts of the injured tissue on the one hand, and constituents of the leucocytes on the other. As for the blood-plates, it will suffice to say by way of summary that their important function in the sealing of wounded vessels (p. 46) 62 THE CIRCULATING LIQUIDS OF THE BODY is the sole office which at present can be attributed to them. And if it is permissible to consider the leucocytes as a patrol for the defence of the tissues in general against invading micro-organisms, it may perhaps not be too far-fetched an idea to look upon the blood- plates as essentially a patrol in the interests of the anatomical integrity of the vascular system itself. This does not exclude the possibihty that the clotting of extravasated plasma may furnish a more favourable medium for the processes of repair in all injured tissues. PRACTICAL EXERCISES ON CHAPTER II. N.B. — In the foUouring exercises all experiments on animals which would cause the slightest pain are to be done under complete anasthesia. 1. Reaction of Blood. — (i) Put a drop of fresh dog's or ox blood on a piece of glazed neutral litmus paper (the litmus paper can be glazed by dipping it into a neutral solution of gelatin and allowing it to dry) . Wash the blood off in lo to 30 seconds with distilled water. A bluish stain will be left, showing that fresh blood is alkaline. (2) Repeat with dog's or ox serum. It is not necessary to wash the serum off, as it does not obscure the change of Colour. (3) Repeat (i) with human blood. With a clean suture-needle or a good-sized sewing-needle which has been sterilized in the flame of a Bunsen burner, prick one of the fingers behind the nail. Bandaging the finger with a handkerchief from above downwards, so as to render its tip congested, will often facilitate the getting of a good-sized drop, but for quantitative experi- ments, like 2, 10, and 17 (4), this should not be done. 2. Specific Gravity of Blood — Hammerschlag's Method. — (i) Put a mixture of chloroform and benzol of specific gravity i -060 into a small glass cylinder. Put a drop of dog's or ox defibrinated blood into the mixture by means of a small pipette. If the drop sinks add chloroform, if it rises add benzol, till it just remains suspended when the liquid has been well stirred. Then with a small hydrometer measure the specific gravity of the mixture, which is now equal to that of the blood. Filter the liquid to free it from blood, and put it back into the stock-bottle. (2) Obtain a drop of human blood as in i, and repeat the measurement of the specific gravity. 3. Coagulation of Blood.* — (i) Take three tumblers or beakers, label them a, /3, and y, and measure into each 100 c.c. of water. Mark the level of the water by strips of gummed paper, and pour it out. (If a suf&cient number of graduated cylinders is available, they may of course be used, and this measurement avoided.) Into u put 25 c.c. of a saturated solution of magnesium sulphate, into ^ 25 c.c. of a i per cent, solution of potassium or ammonium oxalate in o-g per cent, solution of sodium chloride, and into y 25 c.c. of a i-z per cent, solution of sodium fluoride in c-g per cent, salt solution. If the dog provided is a large one, these quantities may be all doubled ; for a small dog they may be all halved. * This experiment requires two laboratory periods, the various blood mix- tures being obtained during the first and worked up during the second. PRACTICAL EXERCISES 63 (2) Insert a cannula into the central end of the carotid artery of a dog anesthetized with morphine* and ether, or A.C.E. mixture. f I'o put a Cannula into an Artery. — Select a glass cannula of suitable size, feel for the artery, make an incision in its course through the skin, then isolate about an inch of it with forceps or a blunt needle, carefully clearing away the fascia or connective tissue. Next pass a small pair of forceps under the artery, and draw two ligatures through below it. If the cannula is to be inserted into the central end of the artery, tie the ligature which is farthest from the heart, and cut one end short. Then between the heart and the other ligature compress the artery with a small clamp (often spoken of as ' bulldog ' forceps). Now lift the artery by the distal ligature, make a transverse slit in it with a pair of fine scissors, insert the cannula, and tie the ligature over its neck. Cut the ends of the ligature short. If the cannula is to be put into the distal end of the artery, both ligatures must be between the clamp and the heart, and the bulldog must be put on before the first ligature (the one nearest the heart) is tied, so that the piece of bloodvessel between it and the ligature may be full of blood, as this facilitates the opening of the artery. (3) Run into a, '/3, and 7 enough blood to fill them to the mark. Shake the vessels, or stir up once or twice with a glass rod, to mix the blood and solution. (4) Take a small thin copper or brass vessel, and place it in a freezing mixture of ice and salt. Run into it some of the blood from the artery. It soon freezes to a hard mass. Now take the vessel out of the freezing mixture and allow the blood to thaw. It will be seen that it remains liquid for a short time and then clots. (5) Run some of the blood into a porcelain capsule, stirring it vigorously with a glass rod. The fibrin collects on the rod ; the blood is defibrinated and will no longer clot. (6) Now let some blood run into a small beaker or jar. Notice that the blood begins to clot in a few minutes, and that soon the vessel can be tilted without spilling it. Note the time required for clotting to occur. Set the coagulated blood aside, and observe next day that clear yellow serum has separated from the clot. (7) Weigh out a quantity of Witte's ' peptone ' equivalent to 0-5 gramme for every kilo of body- weight of the dog. Dissolve the peptone in about twenty times its weight of o-g per cent, salt solution. Put a cannula into the central end of a crural vein (p. 212). Fill the cannula with the peptone solution and connect it with a burette. Put 15 drops of the peptone solution into a test-tube labelled ' Peptone A.' Put the rest into the burette, and see that the connecting tube is filled with the solution and free from air. Run into the test-tube about 5 c.c. of blood from the cannula in the carotid. Now let the peptone solution flow from the burette into the vein. Feel the pulse over the heart as the solution is running in. If the heart becomes very weak, stop the injection; otherwise the animal may die from the great lower- ing of blood-pressure (p. 214). As soon as the injection is finished, draw off a sample of 5 c.c. of blood into a test-tube labelled ' Pep- tone B,' and let it stand. In ten minutes collect five further samples of 5 c.c. (' Peptone C, D, E, F, G '), and a large one, H; in half an hour * One to 2 centigrammes of morphine hydrochlorate per kilogramme of body-weight should be injected subcutaneously about half an hour before the operation. Ten c.c. of a 2 per cent, solution is sufficient for a dog of good size. Note that diarrhoea and salivation are caused by such a dose. For directions for fastening the dog on the holder, see footnote on p. igg. t A mixture of i part of alcohol, 2 of ether, and 3 of chloroform. 64 THE CIRCULATING LIQUIDS OF THE BODY another set of five small samples, and at as long an interval as possible thereafter five more. Now letting the dog bleed to death, observe that the flow of blood is temporarily increased by pressure on the abdominal walls, which squeezes it towards the heart, by passive movements of the hind-legs, and also during the convulsions of asphyxia, which soon appear. Add to the peptone blood D 5 c.c. of serum, to E a little sodium, chloride extract of liver, to F a little extract of muscle, and to G 15 drops of a 2 per cent, solution of calcium chloride, and put C, D, E, F, and G into a water-bath at 40° C. Treat the other sets of small samples in the same way. Note how long each specimen takes to clot, and report your results.* (8) Observe that the blood in a, P, and 7 has not coagulated. Label four test-tubes ' Oxalate A, B, C, D,' and put into each about 5 c.c. of the oxalated blood. Add to A and B 5 or 6 drops of a 2 per cent, solution of calciurn chloride, to C 12 drops, and to D as much as there is of the blood. Leave A at the ordinary temperature, put the other test-tubes in a water-bath at 40° C, and note when clotting occurs. (9) To 10 c.c. of the fluoride blood add a little more CaClg than is re- quired to combine with the excess of fluoride present . Label four test- tubes ' Fluoride A, B, C, D,' and into each put about 2 c.c. of this ' recalcified ' fluoride blood. To B add I c.c. liver extract, to C 1 c.c. muscle extract, and to D 4 c.c. water. Label two more test-tubes ' Fluoride E and F.' Into each put 2 c.c. of the fluoride blood without CaClg. Add also to E i c.c. liver extract and to F I c.c. serum. Put all the tubes in a bath at about 40° C., and observe in which and in what time coagulation takes place. (10) By means of a centrifuge (Fig. 17) separate the plasma from the corpuscles in a, ^, and 7, and also from the peptone blood. With the oxalate plasma from (3, and the fluoride plasma from 7, repeat the observations in (8) and (9), using smaller quantities of the plasma, if necessary, in small test- tubes. With the plasma from a perform the following experiments: Put a small quantity of the plasma (i c.c.) into four test-tubes, labelling * Sometimes the injection of peptone hastens coagulation instead of hinder- ing it. It has been asserted that this is only the case when small doses are used (less than 0-02 gramme per kilo of body-weight). But in 2 dogs out of II a dose of o's gramme per kilo has been seen to hasten coagulation, and in I out of 12 to leave it unaffected; in the other 9 coagulation was markedly retarded 4 Fig. 17. — Centrifuge;_',(Jving). The four cylinders shown at the top of the figure are so swung that they become horizontal as soon as speed is up. PRACTICAL EXERCISES 65 them ' Magnesium Sulphate A, B, C, D.' Dilute B with four times, C with eight times, and D with twenty times as much distilled water as was taken of the plasma; Observe in which, if any, coagulation occurs, and the time of its occurrence, and report the result. (11) With peptone plasma from H and from the peptone blood obtained later repeat the experiments done in (7). In addition dilute I c.c. of the plasma with three volumes of water and i c.c. of it with ten volumes of water, and put in the bath at 40° C. Observe whether clotting occurs. Instead of dog's blood, the blood of an ox or pig may be obtained at the slaughter-house. 4. Preparation of Fibrin-Ferment. — Precipitate blood-serum with ten times its volume of alcohol. Let it stand for several weeks, then extract the precipitate with water. The water dissolves out the fibrin- ferment, but not the coagulated serum proteins. 5. Preparation of Tissue Extracts containing Thrombokinase. — In a dog or rabbit killed by bleeding insert a cannula into the lower end of the thoracic aorta. Fill the cannula with o-g per cent, salt solution, and connect it with a bottle also containing salt solution. Wash out the vessels of the lower portion of the body, making an opening in the inferior vena cava above the diaphragm to allow the liquid to escape. For the sake of cleanliness, a cannula armed with a piece of rubber tubing should be inserted for this purpose into the inferior vena cava. Continue the injection till the liquid issues colour- less. Then remove portions of liver and muscle. Mince each separately. Rub up with sand in a mortar. Add o-g per cent, sodium chloride solution and rub up again. Put into bottles and keep in the ice-chest. For use take off some of the liquid from the top with a pipette, or strain through cheese-cloth. 6. Serum. — Test the reaction, and determine, both by the hydrom- eter and the pycnometer, or specific gravity bottle, the specific gravity of the serum provided, or of the serum obtained in experi- ment 3. Serum Proteins. — (i) Saturate serum with magnesium sulphate crystals at 30° C. The serum-globulin is precipitated. Filter off. Wash the precipitate on the filter with a saturated solution of mag- nesium sulphate. Dissolve the precipitate by the addition of a little distilled water, and perform the following tests for 'globulins: (a) Satu- rate with magnesium sulphate. A precipitate is obtained. (6) Drop into a large quantity of water, and a flocculent precipitate falls down, (c) Heat. Coagulation occurs. Determine the temperature of coagu- lation (p. 9). (2) To a portion of the filtrate from (i) add sodium sulphate to saturation. The serum-albumin is precipitated. (Neither magnesium sulphate nor sodium sulphate precipitates serum-albumin alone, but the double sail; sodio-magnesium sulphate precipitates it, and this is formed when sodium sulphate is added to magnesium sulphate.) (3) Dilute another portion of the filtrate from (i) with its own bulk of water. Very slightly adidulate with dilute acetic acid, and de- termine the temperature of heat coagulation. (4) Precipitate the serum-globulin from another portion of serum by adding to it an equal volume of saturated solution of ammonium sulphate. Filter. Precipitate the serum-albumin from the filtrate by saturating with ammonium sulphate crystals. (5) Dilute serum with ten to twenty times its volume of distilled water, and pass through it a stream of carbon dioxide. The serum- globulin is partially precipitated. 5 66 THE CIRCULATING LIQUIDS OF THE BODY (6) Acidulate some serum with dilute acetic acid and boil. Filter off the coagulum, and to the filtrate add silver nitrate. A non-protein • precipitate insoluble in nitric acid, but soluble in ammonia, indicates the presence of chlorides. 7. Action of Serum on Artery Rings. — Cut a number of rings about ij millimetres wide from a fresh carotid artery of the sheep, obtained from the slaughter-house. Keep the rings in a dish in Ringer's solu- tion.* They should be as nearly as possible of uniform width. A small cylindrical glass vessel is supported on a stand in such a way that it can be easily lowered into a bath of water kept at a temperature of about 39° to 40° A stock of Ringer's solution is kept in a beaker or bottle immersed in the bath. A ring of the artery is put into the small cylinder, where it is held between two aluminium hooks, one fastened to the bottom of the cylinder, the other (the upper one) con- nected with the short arm of a lever, the long arm of which is arranged to write on a slowly revolving drum. A time-trace, say in half- minutes, is recorded below. The small cylinder is now filled with warm Ringer's solution and lowered into the bath. Oxygen is bubbled through the solution by means of a side-tube near the bottom of the cylindrical vessel. The artery ring is now stretched for five minutes by a weight of 10 grammes attached to the long arm of the lever at the same distance from the axis as that at which the ring is attached. After the stretching period the weight is removed, and a little time allowed to elapse till the writing-point traces a horizontal line on the drum. Then a bent pipette is filled with serum already heated to bath temperature in a vessel immersed in the bath. The pipette is intro- duced into the small cylinder so that its point is at the bottom, without disturbing the ring, and the serum is allowed to run in till the Ringer solution is displaced. The ring shortens under the influence of the con- strictor substance in the serum, and the tracing is continued till the shortening has reached its maximum and the trace is again horizontal (Fig. 3, p. 46). Various dilutions of the serum are now made with Rmger's solution, and the greatest dilution in which the serum will still cause a percep- tible constriction of the rings is determined. This affords a measure of comparison with other sera of the strength of the constrictor action. For each dilution of serum a separate ring must be used. It must be remembered that comparisons of this kind can only be made with arteries of the same sensitiveness, and difierent arteries vary much in this regard. 8. Comparison of the Action of Serum and Adrenalin (Epinephrin) on Artery Fiings. — Tracings showing the effect of various dilutions of adrenalin chloride on artery rings may now be taken for comparison with the serum effects. The adrenalin dilutions should be made just before use, as adrenalin is rapidly oxidized. Or a separate experiment on the action of adrenalin may be made under ' Circulation,' as on p; 214. 9. Cornparison of the Action of Serum and Plasma on Artery Rings. — Citrate plasma is obtained as follows : A cannula and attached rubber tube are boiled, oiled inside with fresh olive-oil, and filled with a citrate * This is the name given to a solution containing the most important of the inorganic constituents of blood-serum in approximately the normal pro- portions. The various ' Ringer's solutions ' used by different workers have varied slightly. That recommended by Locke (for perfusion of the isolated heart) contains NaCI, o'Q per cent.; KCl, 0-042 per cent.; CaCla, 0-024 per cent.; NaHCOs, o'Oi to 0-03 per cent.; with in addition o-i per cent, of dextrose, which can be omitted for such experiments as 7. PRACTICAL EXERCISES 67 solution made by dissolving sodium citrate in Ringer's solution to the extent of 2 per cent. The solution is prevented from escaping by a clip on the tube. The cannula is inserted into the carotid of a dog, the end of the rubber tube dipped below a quantity of citrate- Ringer solution in a beaker, and a volume of blood equal to that of the solution run in. Then the blood and solution are at once stirred gently, but sufficiently to insure proper admixture. Some blood is now run into another vessel, defibrinated, and measured. An equal volume of the citrate-Ringer solution is added to it while the mass of fibrin is still floating in the blood. After mixing, the fibrin is removed. Plasma is then separated by the centrifuge from the first specimen of blood, and serum from the second, and comparison experiments are made with each on artery rings. If the plasma has been properly obtained, it will have little constrictor effect on the rings in comparison with the serum. In making the comparison, arteries which give a decided effect with serum should be employed. The defibrinated blood and the unclotted citrate blood may also be used for the comparison. cfqif ;;::]:; 0.10 Oinm. "^^^ C. Zeiss Jena. Fig. 18. — Thoma-Zeiss Hsemocytometer. M, mouthpiece of tube G, by which blood is sucked into S; £, bead for mixing; a, view of slide from above; b, in section; c, squares in middle of B, as seen under microscope. 10. Enumeration of the Blood - Corpuscles. — Use the Thoma-Zeiss apparatus (Fig. 18). (i) Suck a drop of ox or dog's blood up into the capillary, tube S to the mark i. Wipe off any blood which may adhere to the end of the tube. Th^n fill it with 3 per cent, sodium chloride to the mark loi. This represents a dilution of 100 times. Mix the blood and solution thoroughly, then blow out a drop or two of the liquid to remove all the solution which remains in the capillary tube. Now fill the shallow cell B with the blood mixture. Put the cover-glass on, taking care that it does not float on the liquid, but that the cell is exactly filled. Put the slide under the microscope (say Leitz's oc. III., obj. 5), and count the number of red corpuscles in not less than ten to twenty squares. Sixteen squares is a good routine number. The greater the number of squares counted, the nearer will be the approximation to the truth. Now take the average number in a square. The depth of the cell is ^^ mm., the area of each square jjj^ sq. mm. The volume of the column of liquid standing upon a square is -^^ cub. mm. One cub. mm. of the diluted blood would therefore contain 4,000 times as many corpuscles as one square. But the blood has been diluted 100 times, therefore i cub. mm. of the 68 THE CIRCULATING LIQUIDS OF THE BODY undiluted blood would contain 400,000 times the number of corpuscles in one square. Suppose the average for a square is found to be 13. This would correspond to 5,200,000 corpuscles in i cub. mm. of blood. Compare your result with the true number supplied by the demon- strator. (2) Prick the finger to obtain a drop of blood, and repeat the count as in (i).* To Count the White Corpuscles. — Add to i part of blood 9 parts of J per cent, acetic acid, in order to lake the coloured corpuscles and render it easy to see the leucocytes. II. Relative Volume of Corpuscles and Plasma by Haematocrite. — (i) For practice, fill the two graduated glass tubes with the defibrinated blood of an animal. The rubber tube with mouthpiece supplied with the apparatus is to be attached to the glass tube, and the blood sucked up. Press the tip of the index-finger against the pointed end, and care- fully remove the rubber tube. Place the tube in the haematocrite frame, blunt end outwards — that is, farthest from the axis of rotation — and then slip the pointed end down into position against the spring. Instead of the rubber tube, a special suction pipette for automatically filling the graduated tubes may be employed (Daland) . Attach the haemato- crite frame to the centrifuge, and rotate till the volume of sediment 3 Fig. ig.^HEematocrite. A, hasmatocrite attachment with graduated tubes; B, auto- matic pipette for filling the tubes (Daland). (corpuscles) ceases to diminish. The graduations are best read with a hand lens. The leucocytes will be seen to form a thin whitish line proximal to the column of red corpuscles. (2) Prick the finger or the lobe of the ear, fill the tubes as in (i), and centrifugalize. Everything must be done as rapidly as possible, so that the blood may not clot till the separation of plasma and corpuscles is completed. The centrifuge must rotate very rapidly (about 10,000 revolutions a minute) for two or three minutes. For clinical purposes it is best to rotate the centrifuge always at the same speed for the same length of time rather than to aim at reaching a constant length of the column of corpuscles. In this way useful comparative results can be obtained. It is well, to avoid the risk of accident, to rotate the centrifuge under a guard. 12. Electrical Conductivity of Blood. — (i) FiU a small U-tube with blood up to a mark. In each limb insert a platinum electrodef con- * If the tube has not been properly filled, blow the blood out immediately On no account permit it to clot in the capillary tube. t If the platinum electrodes are of good size and the resistance of the tube of liquid considerable, it is not necessary to platinize them-^i.c, to cover them by electrolysis of a solution of.platinip chloride with a layer of platinum-black. PRACTICAL EXERCISES 69 nected with a holder, which insures that the electrode shall always dip to the same depth into the tube. Arrange the U-tube so that it is immersed at least to the mark in water of constant temperature. Water running freely from the cold-water tap into and out of a large vessel will have a sufficiently constant temperature for the' purpose. A ther- mometer must be fixed in the water with its bulb in contact with the U-tube. Connect the electrodes with a resistance-box in the Wheat- stone's bridge arrangement (Fig. 220, p. 699), so that the U-tube occupies the position of the unknown resistance CD. Instead of the battery F, connect the poles of the secondary of a small induction-coil, arranged for an iiiterrupted current, with A and C, and instead of the galvanometer G insert a telephone. The resistances AB and AD will be obtained by taking out two plugs from the appropriate part of the resistance-box. Whether AB and AD should be equal (say, 10 : 10, 100 : 100, or 1,000 : 1,000 ohms) or unequal (say, 10 : 100, or 100 : 1,000, or 10 : 1,000 ohms) will depend upon the resistance of the tube of liquid to be measured. Take out from the part of the box corresponding to BC a plug representing a resistance something like that which the tube of blood is expected to have. Close the primary circuit of the induction-coil, and apply the telephone to the ear. A buzzing sound will be heard, which will be louder the farther from the true resistance of the tube the resistance taken out of the box is. Go on altering the resistance in the box by taking out or putting in plugs till the sound disappears, or is reduced to a minimum. The tempera- ture of the water should now be read off. The resistance of the tube of blood for this temperature can easily be calculated from the formula on p. 699. It increases about 2 per cent, for each degree Centigrade of diminution of temperature. The conductivity is the reciprocal of the resistance. By determining once for all the resistance of the tube when filled with a standard solution of a salt whose conductivity is known, the specific conductivity of the blood can be expressed in definite units, but this is not necessary for the purposes of the student. Compare the resistances of defibrinated blood, serum, o-g per cent, sodium chloride solution, and a sediment of blood-corpuscles separated by centrifugalization. (2) Instead of the resistance-box a wire mounted on a scale may be used for the resistances AB, AD, the ends of the wire being connected at B and D. A , slider with an insulated handle moving along the graduated wire is joined by a flexible wire with one pole of the secondary coil, the other pole being connected at C. The resistance BC is consti- tuted by a rheostat from which a fixed resistance can be taken out. Instead of obtaining the minimum sound in the telephone by varying the resistance BC in the box, the measurement is made by varying the position of the slider; in other words, by changing the ratio AB: AD. (3) If no rheostat is available instructive comparative measurements may still be made with the graduated wire by substituting for the resistance BC a U-tube of another liquid. If the tubes are of the same dimensions, and the liquids with which they are filled are approximately at the same initial temperature, it is not necessary to immerse them in water at constant temperature. It is sufficient to place them side by side in the air. Perform the following experiments in this way : (a) Label the tubes A and B. Fill them both to the mark with 0-9 per cent. NaCl solution. Connect as in the figure, and move the slider along the wire till the sound is a minimum. Probably the two tubes are not exactly of the same dimensions, and therefore the slider will not be exactly in the middle of the wire. Suppose it is at 49-0 70 THE CIRCULATING LIQUIDS OF THE BODY the total length of the wire being loo. Then resistance of A : resistance of B: : 49-0 : si-o, i.e., resistance of A=— resistance of B. ^ -^ ' 51 (h) Fill A with defibrinated blood, keeping B filled with NaCl solu- tion, and repeat the measurement. The slider must now be moved much farther away from the zero of the scale. Suppose the minimum sound is obtained with the slider at 70-0. Then resistance of blood = a CI - X ^^ resistance of the NaCl solution. 7 49 (c) Compare in the same way the resistance of serum with that of the NaCl solution. It will be found much less than that of the blood. (d) Centrifugalize some of the blood for as long as is convenient, and compare the resistance of the blood from the top of the tubes and from the bottom of the tubes with that of the NaCl solution. The resistance of the blood from the bottom of the tubes will be found much greater than that of the blood from the top. 13. Opacity of Blood. — Smear a little fresh blood on a glass slide, and hold the slide above some printed matter. It will not be possible to read it, because the light is reflected from the corpuscles in all directions, and little of it passes through. 14. Laking of Blood by Chemical and Physical Agents. — (i) Put a little fresh blood into three test-tubes. A, B, and C. Dilute A with an equal volume, B with two volumes, and C with three volumes, of dis- tilled water, and repeat experiment 9. The print can now be read probably through a layer of A, but certainly through B and C, since the haemoglobin is dissolved out of the corpuscles by the water and goes into solution, the blood becoming transparent or laked. That the difference is not due merely to dilution can be shown by putting an equal quantity of blood in two test-tubes, and gradually diluting one with distilled water and the other with a o-g per cent, solution of sodium chloride, which does not dissolve out the haemoglobin. Print can be read through the first with a smaller degree of dilution than through the second. Examine the laked blood with the microscope for the ' ghosts,' or sha,dows of the red corpuscles. The addition of a drop or two of methylene blue will render them somewhat more distinct. (2) Heat a little dog's or ox blood in a test-tube immersed in a water- bath. Put a thermometer in the test-tube, taking care that there is enough blood to cover the bulb. Keep the temperature about 60° C. In a few minutes the blood becomes dark and laking occurs. (3) (a) Put a little blood into each of four test-tubes. To one add a little ether, to another a little chloroform, to the third dilute acetic acid in o-g per cent. NaCl, and to the fourth a dilute solution of bile salts {or of sodium taurocholate) in o-g per cent. NaCl solution. Laking occurs in all. (6) To 5 c.c. of blood add 0-5 c.c. of a 3 per cent, solution of saponin in o-g per cent. NaCl solution, and put the mixture at 40° C. Laking soon occurs. (c) Using a 10 per cent, dilution of blood (blood to which nine volumes of NaCl solution have been added) or a 5 per cent, suspension of washed corpuscles in NaCl solution (i.e., a suspension of corpuscles which have been washed free from serum by being repeatedly mixed with NaCl solution and centrifugalized), determine the minimum dose of 0-3 per cent, saponin solution which will just cause complete laking. Keep the tubes at about 40° C, and observe them from time to time. Now add to some of the 10 per cent, dilution or the 5 per cent, suspension of blood an equal volume of serum from the same kind of blood, and repeat the determination of the minimum dose of saponin necessary for laking. PRACTICAL EXERCISES 71 It will be found tl}.at more is now required. The cholesterin in the serum neutralizes the action of some of the saponin. (4) (a) Put I c.c. of blood into each of two test-tubes. To one add I c.c. of 2 per cent, aqueous solution of urea, and to the other 3 c.c. Laking will take place in the second, whether this has been the case ia the iirst or not. (6) Repeat the experiment with a 2 per cent, solution of urea ia o-g per cent. NaCl solution. Laking does not occur. This shows that the urea in the first experiment did not act as a hEemolytic agent. Laking occurred because urea penetrates the corpuscles easily, and therefore, although the freezing-point of the urea solution is not very different from that of the NaCl solution, its actual osmotic pressure, in relation to the envelopes of the corpuscles, is very much less, and the laking is really water-laking. (5) Put some blood into a flask or test-tube, cork up, and let it stand till it begins to putrefy. It becomes laked. The same occurs when the blood is collected aseptically in a sterile tube and sealed up, although it takes a longer time for the laking to become complete. (6) With blood containing nucleated corpuscles (necturus, frog or chicken) diluted with isotonic salt solution, perform the following experiments under the microscope : • (a) With a glass rod drawn to a fine point put a small drop of blood on a slide, and near it a drop of distilled water. Carefully lower the cover-slip and observe the interface with the microscope, first with the low and then with the high power. Then mix and see complete laking. Add a little methylene blue. Note that the nuclei still stain. (6) Place a small drop of a 3 per cent, solution of saponin in isotonic salt solution on a slide, and near it a small drop of blood. Observe as in (a). Repeat with a 2 per cent, solution of sodium taurocholate in salt solution. If necturus corpuscles, which are splendid objects for such experiments on account of their great size, have been used, intracorpuscular crystallization of the haemoglobin may be observed. (c) Repeat {a) and (6) with mammalian blood. Note that the cor- puscles swell before being laked by the saponin. If any of the corpuscles are crenated, it may be seen that before being laked by the saponin the crenations disappear, the corpuscles becoming round, while in the taurocholate solution they may remain crenated till laking has occurred. This indicates that the permeability of the envelopes is not affected in the same way by the two laking agents. 15. Hsmolysis and Agglutination by Foreign Serum. — (i) To a small quantity of rabbit's blood add an equal volume of dog's serum. Mix and let stand at 40° C. The colour of the blood is soon darker than before, and it can be seen to be laked. Examine microscopically. (2) Place a small drop of rabbit's blood and a somewhat larger drop of the dog's serum on a slide, near, but not quite in contact with, each other. Now put on a cover-slip, so that the drops just come together, and examine at once with the microscope with a moderately high power. Where the two drops mingle, the red corpuscles will be seen first to become agglutinated into groups, and then to fade out, leaving only their ' ghosts.' A few of the corpuscles which come into contact with the, as yet, undiluted serum may be entirely dissolved. (3) Heat some of the dog's serum to 60° C. for ten minutes, and repeat (i) and (2). No laking will now be produced in the rabbit's corpuscles, but agglutination may! be observed as before. {4) Repeat (i) and (2) with dog's blood and rabbit's serum. The blood will not be laked, although sometimes the dog's corpuscles may become crenated. There will be no agglutination. 72 THE CIRCULATING LIQUIDS OF THE BODY (5) With a 5 per cent, suspension of rabbit's washed corpuscles perform the following experiments:* Put into each of six small test-tubes i c.c. of the suspension. Label the tubes A, A', B. B', C, C (a) To A and A' add respectively o-i c.c. and 0-5 c.c. ox serum. (6) To B and B' add respectively o-i c.c. and 0-5 c.c. dog's serum, (c) To C arid C add respectively o-i c.c. and 0-5 c.c. of o-g per cent. sodium chloride solution. Put all the tubes in a bath at 40° C. Compare the amount of laking and agglutination in the various tubes at intervals of two minutes or less. Repeat (a), (6), and (c) with guinea-pig's washed corpuscles and serum of ox and dog. Determine which of these sera has the strongest haemolytic power.f (6) Heat i c.c. of ox and dog's serum respectively to 56° C, keeping it at that temperature, or not more than a couple of degrees above it, for tent minutes, and repeat experiment (5), labelling the tubes D, D', E, E', F, F'. Save the rest of the heated sera for (8). There is no laking in any of the tubes, but probably agglutination in D, D', and E, E'. (The complement is destroyed, but not the intermediary body or amboceptor, or the agglutinin— -p. 28.) (7) Put half of the contents of tubes D, D', E, E', into four separate test-tubes, and add to each 0-2 c.c. of rabbit's serum. If there is laking now it is because the rabbit's serum contains complement. Save the balance of D, D', E and E' for (8). (8) Allow 0'5 c.c. of ox serum to act at 0° C. on the rabbit's washed corpuscles contained in 5 c.c. of the 5 per cent, suspension after removal of the sodium chloride solution. The ox serum and rabbit's corpuscles are separately cooled to 0° C. before being mixed, and the mixture is then kept at 0° C. for one hour. Centrifugalize the serum ofi rapidly. Label it ' Serum S.' To 0-2 c.c. of the original 5 per cent, suspension of rabbit's washed corpuscles add o-i c.c. of this serum (labelling the tube G), and put at 40° C. with a control-tube containing the same amount of suspension plus salt solution instead of serum. Add the rest of the serum S, cooled to 0° C, to the same cooled rabbit's cor- puscles, and leave for a further period at 0° C. Then centrifugalize rapidly, and to 0-2 c.c. of the original suspension of washed rabbit's corpuscles add o-i c.c. of serum S (labelling the tube H), and put at 40° C. with a sodium chloride tube as control. There may be no * The material obtained from one medium-sized dog, two rabbits, and one guinea-pig is enough for fifty or sixty students, working together in sets of two, to perform experiments (5) to (8). In order to obtain a serum more ,strongly haemolytic for rabbit's corpuscles than normal dog's serum, a dog may be ' immunized ' by previous injection of all the washed corpuscles obtainable from a rabbit. The injection should be made under the skin or, better, into the peritoneal cavity — of course, with aseptic precautions. It should be repeated not less than twice, with an interval of ten days between the successive injections, and the dog's blood should be drawn ofE about ten days after the last injection. t To determine the amount of laking at any given moment, drop the small test-tubes into the metallic centrifuge cups after shaking them up, and centrif- ugalize. A very short time is sufficient to separate a clear supernatant liquid, from the tint of which the extent of the haemolysis can be deduced. Before replacing the tubes in the thermostat, they should, of course, be shaken up. Small test-tubes of about 8 mm. internal diameter and short enough to go conveniently into the centrifuge cups are the most serviceable. I For exact work a longer time is recommended. But for the student the time is made as short as possible, and it is only in exceptional cases that ten minutes is not enough. PRACTICAL EXERCISES 73 laking in either G or H, or if there is laking it may be greater in G than in H. The amboceptor has been removed from serum S by the rabbit's corpuscles. Add o-i c.c. of this ' inactivated ' serum to the balance of D, D', and E, E' (left from 6). Laking will occur because the serum S contains complement, and the heated serum added in (6) to these tubes contains amboceptor. Wash the rabbit's corpuscles which have been treated with ox serum at 0° C. with cooled sodium chloride solution. Add to them some of serum S (that from the top of tube H will do if no more is left), and put at 40° C. Laking will occur, showing that the amboceptor was fixed by the rabbit's corpuscles at 0° C. To a further portion of the washed rabbit's corpuscles which were treated with ox serum at 0° C. add normal rabbit's serum, and put at 40° C. If laking occurs it is because the rabbit's serum contains complement. Dog's serum may be used instead of ox serum for experiment (8) . 16. Osmotic Resistance of the Coloured Corpuscles. — Fill a burette with a I per cent, solution of sodium chloride and another with dis- tilled water. Take a series of ten test-tubes and run into the first 6 c.c. of the NaCl solution, into the second 5-8 c.c, into the third 5-6 c.c, and so on, always making a difference of 0-2 c.c. between successive test-tubes. From the other burette run in enough distilled water to make up 10 c.c. of solution in each tube — that is, 4 c.c. of dis- tilled water for the first tube, 4-2 c.c for the second, and so on. Shake up. The tubes now contain a series of solutions of salt differing in strength by 0-02 per cent, in successive tubes, the strongest being o-6 per cent., and the weakest 0-42 per cent. Number the tubes i to 10, beginning with the strongest solution. Put into each tube one drop of perfectly fresh blood. Shake moderately so as to mix the blood and salt solution, and allow the tubes to stand for ten to thirty minutes. Observe the colour of the clear liquid above the sediment of corpuscles. Determine in which tube the first. tinge of hsemoglobin appears. The next higher concentration of the salt solution is that in which all the corpuscles are just able to retain their hasmoglobin, and is a measure of the minimum osmotic resistance of the corpuscles, or the resistance of the weakest corpuscles. Repeat with blood which has stood at room temperature for twelve to twenty-four hours. For clinical purposes tubes, each containing 5 c.c of salt solution, may be used. A single drop of blood can then be distributed between the tubes with a fine pipette or a glass rod, beginning with the most concentrated solution, and passing down to the less concentrated. The blood must be dis- tributed rapidly before coagulation occurs. Only such concentrations of the salt solution as are known to correspond to the possible variations of the osmotic resistance for any particular disease or for any particular variety of blood need be employed. 17. Blood-Pigment — (i) Preparation of Haemoglobin Crystals. — {a) To a little dog's blood in a narrow test-tube add its own volume or twice its volume of chloroform. Invert the tube ten or twelve times so as to allow the chloroform to act on the blood, but avoid violent shaking. When the tube is now allowed to stand for a few minutes the laked blood all rises to the top. Remove a little of the lajTcr of blood without taking with it any of the chloroform layer, and examine the oxyhaemoglobin crystals with the microscope. They form long rhombic prisms and needles (Fig. 14, p. 52). (&) Add a little crude saponin to dog's blood in a test-tube. Shake up well, and allow it to stand till the colour becomes dark. Then shake vigorously, and a mass of haemoglobin crystals will be formed. (c) Put a small drop of guinea-pig's blood on a slide. Mix with a 74 THE CIRCULATING LIQUIDS OF THE BODY drop of Canada balsam and cover. Tetrahedral crystals of oxy- hasmoglobin will form after a time. The slide may be kept. (2) Spectroscopic Examination of Haemoglobin and its Derivatives. — (a) With a small, direct-vision spectroscope look at a bright part of the sky or a white cloud. Focus by pulling out or pushing in the eye- piece until the numerous fine dark lines (Fraimhofer's lines), runnmg vertically across the spectrum, are seen. Narrow the slit by moving the milled edge till the lines are as sharp as they can be made. Note especially the line D in the orange, the lines E and 6 in the green, and F in the blue. Always hold the spectroscope so that the red is at the left of the field. Now dip an iron or platinum wire with a loop on the end of it into water, and then into some common salt or sodium carbonate, and fasten or hold it in the flame of a fishtail burner. On examining the flame with the spectroscope, a bright yellow line will be seen occupying the position of the dark line D in the solar ■spectrum. This is a convenient line of reference in the spectrum, and in studying the spectra of haemoglobin and its derivatives, the position of the absorption bands with regard to the D line should always be noted. The dark lines in the solar spectrum are due to the absorption of light of a definite range of wave-lengths by metals in a state of vapour in the sun's atmosphere, and of course no dark lines are seen in the spectrum of a gas-flame. Put some defibrinated blood into a test-tube. B Fig. 20. — Direct Vision Spectroscope of Simple Type. A, slot in which a pin on the eyepiece C slides in focussing the spectrum; B, milled head, by the rotation of which the slit is narrowed or widened. Fasten it vertically in a clamp in front of the flame and examine it with the spectroscope, holding the latter in one hand with the slit close to the test-tube, and focussing the eyepiece with the other. Or arrange the spectroscope, test-tube and gas-flame on a stand as in Fig. 21. Nothing can be seen till the blood is diluted. Pour a little of the blood into another test-tube, and go on diluting till, on focussing, two bands of oxyhemoglobin are seen in the position indicated in Fig. 13, p. 51 . Draw the spectrum; then dilute still more, and observe which of the bands first disappears. Now put 5 c.c. of the blood into another test-tube, and dilute it with four times its volume of water. Take 5 c.c. of this dilution, and again add four times as much water, and so on till the solution is only faintly coloured. Note with what degree of dilution the bands disappear. Then examine each of the solutions with the spectroscope and draw its spectrum. (6) Make a solution of blood which shows the oxyhaemoglobin banas sharply. Add some ammonium sulphide solution to reduce the oxy- haemoglobin. Heat gently to about body temperature. A single, ill-defmed band now appears, occupying a position midway between the oxyhaemoglobin bands, and the latter disappear. This is the band of reduced hcemoglobin (Fig. 13). (c) Carbonic Oxide Hcemoglobin. — Pass coal-gas through blood for PRACTICAL EXERCISES 75 Test lube a considerable time. Examine some of the blood (after dilution) with the spectroscope. Two bands, almost in the position of the oxyhaemoglobin bands, are seen; but no change is caused by the addition of ammonium sulphide, since carbonic oxide haemoglobin is a more stable compound than oxyhaemoglobin. (d) Methcsmoglobin. — Put some blood into a test-tube, add a few drops of a solution of ferricyanide of potassium, and heat gently. On diluting a well-marked band will be seen in the red. On addition of ammonium sulphide this band disappears; the oxyhaemoglobin bands are seen for a moment, and then give place to the band of reduced hasmoglobin (Fig. 13, p. 51). (e) Acid Hcematin. — To a little diluted blood add strong acetic acid and heat gently. The colour becomes brownish. The spectrum shows a band in the red between C and D, not far from the position of the band of methaemoglobin. The addition of a drop or two of ammonium sulphide causes no change in the spectrum, and this is a means of distinguishing acid hsematin from methaemoglobin. If more ammonium sulphide be added, hasmatin will be precipitated when the acid solution has been rendered neutral, and a further addition of ammonium sulphide or sodium hydroxide will cause the haematin to be again dis- solved, a solution of alkaline haematin being formed. This in its turn may be reduced by an excess of ammonium sul- phide, and the spectrum of haemochromogen may be ob- tained (Fig. 13, p. 51). Since the watery solution of acid haematin obtained as above is usually somewhat tur- bid, a solution in acid ether is - sometimes employed for spec- troscopic examination. Add to a little undiluted defibrinated blood about half its volume of glacial acetic acid, and then not less than an equal volume of ether. Mix well, pour off the ethereal extract and examine it with the spectroscope, diluting, if necessary, with ether and glacial acetic acid. It shows a strong band in the red somewhat farther from the D line than the methaemoglobin band. On dilution, tliree additional fainter bands may be seen. (/) Alkaline Hcematin. — To diluted blood add strong acetic acid and warm gently for a few minutes. Then, when the spectroscopic ex- amination of a sample shows that acid haematin has been formed, neutralize with sodium hydroxide. A brownish precipitate of haematin is thrown down, which dissolves in an excess of sodium hydroxide, giving a solution of alkaline haematin (or alkali haematin). Or add sodium hydroxide to blood directly, and warm for a couple of minutes after the colour has changed decidedly to brownish-black. The spectrum of alkaline haematin is a broad but ill-defined band just overlapping the D line, and situated chiefly to the red side of it (Fig. 13). The solution should be shaken up with air before being examined, as Spectroscope Solulion ao Fig. 21. — Spectroscopic Examination of Blood-Pigment. 76 THE CIRCULATING LIQUIDS OF THE BODY some of the alkali hsematin is changed into haemochromogen by re- ducing substances formed by the action of the alkali on the blood. (§■) Hcemochromogen. — To a solution of alkaline haematin add a drop or two of ammonium sulphide. The band near D disappears, and two bands make their appearance in the green (Fig. 13, p. 51). [h) Htsmatoporphyrin. — ^Put some strong sulphuric acid into a test- tube. Add a few drops of blood, agitate the test-tube till the blood dissolves, and examine the purple liquid, diluting it, if necessary, with sulphuric acid. Its spectrum shows two well-marked bands, one just to the left of D, and the other midway between D and E (Fig. 13). (3) Guaiacum Test for Blood. — A test for blood — much used in hospitals, and, indeed, a delicate one, but quite untrustworthy unless certain precautions be taken — is the guaiacum test. A drop of freshly- prepared tincture of guaiacum is added to the liquid to be tested, and then peroxide of hydrogen. If blood be present, the guaiacum strikes a blue or greenish-blue colour. The decomposition of the peroxide by the blood is due mainly to the haemoglobin of the corpuscles. Any derivative of hsemoglobin which still contains the iron will act, and boUing does not abolish this power. On the other hand, oxydases or oxidizing ferments present, not only in the formed elements of blood, but elsewhere, e.g., in fresh vegetable protoplasm, milk, seminal fluid, and pus, will cause the same colour (p. 267), but not if they have been previously boiled.* The test has been considered chiefly of value as a negative test. When the blue colour is not obtained, we have good evidence that blood is absent. But, according to Buckmaster, if the precaution of first boiling the liquid suspected to contain blood be adopted, it is also a good positive test. It is, however, far inferior to the hasmin test (p. 78) where that can be obtained, and of course in- ferior to the identification of erythrocytes with the microscope, or to the spectroscopic identification of the blood-pigment where the material is suitable for this. (4) Quantitative Estimation of Haemoglobin — {a) By Haldane's Modi- fication of Gowers' Htsmoglohinometer. — Place in the graduated tube B (Fig. 22) an amount of water less than will ultimately be required to dilute the blood to the required tint. Puncture the finger or lobe of the ear with one of the small lancets in F, and fill the capillary pipette D to a little beyond the mark 20. Wipe the point of the pipette and dab it on a piece of filter-paper till the blood stands exactly at the mark. Blow the blood into the water in B, and rinse the pipette with the water. Attach the cap of tube G to a gas-burner. Introduce the rubber tube into B nearly to the level of the water, and allow gas to pass for a few seconds. Withdraw the tube while the gas is still passing. Immediately close the end of B with the finger, and move the tube so that the liquid passes from end to end of it. at least a dozen times, to saturate the hasmoglobin with carbonic oxide. While this is being done, the tube should be held in a cloth, otherwise it will become heated, and liquid will spurt out when the finger is removed. Water is now added * The formed elements of blood really contain no less than three ferments of interest in this connection: (i) A catalase which decomposes peroxide of hydrogen into water and molecular oxygen (i.e., oxygen not in the atomic or nascent state). This reaction is given by both blood and pus. (2) An oxydase (also spelled oxidase), which oxidizes guaiacum and similar substances without the presence of hydrogen peroxide. This reaction is obtainable even from aqueous extracts of leucocytes. (3) A peroxydase (also spelled peroxi- dase) which causes the oxidation of these substances only in the presence of hydrogen peroxide, a reaction also given by leucocytes. These ferments are all inactivated by boiling (Kastle). PRACTICAL EXERCISES 77 drop by drop with the pipette stopper of the bottle E, which is used for holding the water, the tube being inverted after each addition, till the tint in B is the same as that in A. In comparing the tubes, they should be held against the light from the sky or from an opal glass lamp-shade. It is necessary to transpose the tubes repeatedly. The level at which the tints are equal is read off on B half a minute after the addition of the last drop of water. Water is now again added by drops till the tint in B is just noticeably weaker than in A, and the mean of the two readings is taken. The result is the percentage actually present of the average proportion of haemoglobin in the blood of healthy adult males. Healthy women give an average of only 89 per cent., and healthy children an average of only 87 per cent., of the proportion in men. The liquid in A is a i per cent, solution of blood containing the average percentage of hasmoglobin found in the blood of healthy Fig. 32. — Haldane's Modification of Gowers' Haemoglobinometer. adult males, and having an oxygen capacity of i8'5 per cent. — i.e., 100 c.c. of the blood with which the standard was made would take up in combination from air 18-5 c.c. of oxygen. The solution in A has been saturated with carbonic oxide. This method is probably more accurate than any other used in clinical work, the error, in the hands of an experienced observer, not exceeding I per cent. (6) By FleiscM's Htsmometer (Fig. 23). — Fill with distilled water that compartment a' of the small cylinder {above the stage) which is over the tinted wedge. Put a little distilled water into the other compart- ment a. Now prick the finger and fill one of the small capillary tubes with blood. See that none of the blood is smeared on the outside of the tube. Then wash all the blood into the water in compartment a, and fill it to the brim with distilled water. By means of the milled 78 THE CIRCULATING LIQUIDS OF THE BODY head T move the tinted wedge K till the depth of colour is the same in the two compartments. The percentage of the normal quantity of haemoglobin is given by the graduated scale P. For example, if the reading is 90, the blood contains 90 per cent, of the normal amount; if 100, it contains the normal quantity. The observations should be made in a dark room, the white surface S, arranged below the compart- ments a and a', being illuminated by a lamp. Or the instrument may be placed in a small box, lighted by a candle . It is best that each result should be the mean of two readings, one just too large and the other just too small. In any case the instrument does not give readings accurate to less than 5 per cent. (c) Hoppe-Seyler" s Method. — Two parallel-sided glass troughs are used. In one is put a stajidard solution of oxyhaemoglobin of known strength, in the other a measured quantity of the blood to be tested. The latter is diluted a a' with water until its tint K appears the same as that of the standard solution, when the troughs are placed Fig. 23. — Fleischl's Hsemometer. Crystals of Hsemin (Frey). side by side on white paper. From the quantity of water added it is easy to calculate the proportion of hasmo- globin in the undiluted blood. Greater accuracy is obtained if the hasmoglobin in the standard solution and that of the blood are converted into carbonic oxide haemo- globin by passing a stream of coal-gas through them. {d) Tallquist's Method. — In this method the tint produced by a drop of blood on a piece of white filter-paper is compared with a scale representing 10 percentages of haemoglobin (from 10 to 100 per cent.). The standard filter-paper is supplied in the form of a book with the scale. To make an estimation, all that is necessary is to touch a drop of blood with a piece of the filter-paper, and allow the blood to diffuse slowly through the paper, so as to give an even stain. The position of the stain is then determined by the scale; e.g., it may be deeper than 90, but fainter than 100, in which case the percentage of haemo- globin lies between 90 and 100. The method is by no means a very accurate one, but more accurate than it appears at first sight. (5) Microscopic Test for Blood-Pigment. — Put a drop of blood on a slide. Allow the blood to dry, or heat it gently over a flame, so as to evaporate the water. Add a drop of glacial acetic acid ; put on a cover- PRACTICAL EXERCISES 79 glass, and again heat slowly till the liquid just begins to boil. Take the slide away from- the flame for a few seconds, then heat it again for a moment; and repeat this process two or three times. Now let the slide cool, and examine with the microscope (high power). The small black, or brownish-black, crystals of hsemin will be seen (Fig. 24, p. 78). This is an important test where only a minute trace of blood is to be examined, as in some medico-legal cases. If a blood-stain is old, a minute crystal of sodium chloride should be added along with the glacial acetic acid. Fresh blood contains enough sodium chloride. A blood-stain on a piece of cloth may first of all be soaked in a small quantity of distilled water, and the liquid examined with the spectro- scope or the micro-spectroscope (a microscope in which a small spectro- scope is substituted for the eyepiece). Then evaporate the liquid to dryness on a water-bath, and apply the hsemin test. Or perform the haemin test directly on the piece of cloth. In a fresh stain the blood- corpuscles might be recognized under the microscope. Very few liquids, however, are available for washing out the blood, as all ordinary solutions, and even serum itself, cause laking of dried corpuscles (Guthrie). Absolute alcohol, or 35 per cent, potassium hydroxide, may be used to soak and rub up the cloth in. CHAPTER III THE CIRCULATION OF THE BLOOD AND LYMPH . The blood can only fulfil its functions by continual movement. This movement implies a constant transformation of energy; and in i the animal body the transformation of energy into mechanical work is almost entirely allotted to a special form of tissue, muscle. In most animals there exist one or more rhythmically contractile muscular organs, or hearts, upon which the chief share of the work of keeping up the circulation falls. Section I. — ^Preliminary Anatomical and Physical Data. " Comparative. — ^In Echinus a contractile tube connects the two vascu- lar rings that surround the beginning and end of the alimentary canal, and plays the part of a heart. In the lower Crustacea and in insects the heart is simply the contractile and generally sacculated dorsal bloodvessel; in the higher Crustacea, such as the lobster, it is a well- defined muscular sac situated dorsally. A closed vascular system is the exception among invertebrates. In most of them the blood passes from the arteries into irregular spaces or lacunae in the tissues, and thence finds its way back to the heart. In the primitive vertebrate heart five parts can be distinguished as we proceed from the venous to the arterial end : (i) The sinus venosus, into which the great veins open ; (2) the auricular canal, from the dorsal wall of which is developed — (3) the auricle; (4) the ventricle; (5) the bulbus arteriosus, from which the chief artery starts (Fig. 25, p. 81). Amphioxus, the lowest verte- brate, has a primitive lacunar vascular system; a contractile dorsal bloodvessel serves as arterial or systemic heart, a contractile ventral vessel as venous or respiratory heart. From the latter, vessels go to the gills. Fishes possess only a respiratory heart, consisting of a venous sinus, auricle, ventricle, and bulbus arteriosus. This drives the blood to the gills, from which it is gathered into the aorta; it has thence to find its way without further propulsion through the systemic vessels. Amphibians have a venous sinus, two auricles, a single ventricle, and an arterial bulb; reptiles, two auricles and two incompletely-separated ventricles. In birds and mammals the respiratory and systemic hearts are completely separated. The former, consisting of the right auricle and ventricle, propels the blood through the lungs ; the latter, consisting of the left auricle and ventricle, receives it from the pul- monary veins, and sends it through the systemic vessels. The sinus venosus seems to be represented in the mammalian heart by certain small portions of tissue, especially the so-called sino-auricular node, a little knot of primitive fibres near the mouth of the superior 80 ANATOMICAL AND PHYSICAL DATA 8i vena cava. The auricular canal is probably represented by the auriculo-ventricular bundle (conveniently designated as the a.-v. bundle), which will again be referred to in relation to the conduction of the heart- beat from auricles to ventricles (p. 147). This bundle starts from a clump of primitive tissue, the auriculo-ventricular node (a.-v. node) at the base of the interauricular septum on the right side, below and to the right of the coronary sinus, and runs down the interventricular septum. The sino-auricular and the auriculo-ventricular nodes are connected by fibres which run in the interauricular septum, so that it may be considered that the primitive cardiac tube is still represented from' base to apex of the adult mammalian heart, although only by very slender threads of tissue, amidst the massive secondary developments of auricular and ventricular muscle (Keith and Flack). General View of the Circulation in Man. — The whole circuit of the blood is divided into two portions, very distinct from each other, both anatomically and function- ally — ^the respiratory or lesser circulation, and the systemic or greater circulation. Starting from the left ventricle, the blood passes along the systemic vessels — ar- teries, capillaries, veins — and, on returning to the heart, is poured into the right auricle, and thence into the right ventricle. From the latter it is driven through the pulmonary artery to the lungs, passes through the capillaries of these organs, and returns through the pulmonary veins to the left auricle and ventricle. The portal system, which gathers up the blood from the intestines, forms a kind of loop on the systemic circulation. The lymph-current is also in a sense a slow and stag- nant side-stream of the blood- circulation; for substances are constantly passing from the bloodvessels into the lymph-spaces, and returning, although after a com- paratively long interval, into the blood by the great lymphatic trunks. Physiological Anatomy of the Vascular System. — The heart is to be looked upon as a portion of a bloodvessel which has been modified to act as a pump for driving the blood in a definite direction. Morpho- logically it is a bloodvessel; and the physiological property of auto- matic rhythmical contraction which belongs to the heart in so eminent a degree is, as has been mentioned (p. 80), an endowment of blood- vessels in many animals that possess no localized heart. Even in some mammals contractile bloodvessels occur; the veins of the bat's wing, for example, beat with a regular rhythm, and perform the func- tion of accessory hearts. 6 Fig. 25.— Diagram, of Primitive Vertebrate Heart, combining Features found in tlie Eel, Dogfish, and Frog (Flack, after Keith), a, Sinus venosus; 6, auricular canal; c, auricle; d, ventricle; «, bulbus cordis; /, aorta; i-i, sino-auricular junc- tion and venous valves; 2-2, junction of canal and auricle; 3-3, annular part of auricle; 5, bulbo-ventricular junction. 82 THE CIRCULATION OF THE BLOOD AND LYMPH The whole vascular system is lined with a single layer of endothelial cells. In the capillaries nothing else is present; the endothelial layer forms the whole thickness of the wall. In young animals, at any rate, the endothelial cells of the capillaries are capable of contracting when stimulated; and changes in the calibre of these vessels can be brought about in this way. The walls of the arteries and veins are chiefly made up of two kinds of tissue, which render them distensible and elastic: non-striped muscular fibres and yellow elastic fibres. The muscular fibres are mainly arranged as a circular middle coat, which, especially in the smaller arteries, is relatively thick. One conspicuous layer of elastic fibres marks the boundary between the middle and inner coats. In the larger arteries elastic laminae are also scattered freely among the muscular fibres of the middle coat. The outer coat is composed chiefly of ordinary connective tissue. The veins differ , from the arteries in having thinner walls, with the layers less distinctly ; marked, and containing a smaller proportion of non-striped muscle and elastic tissue ; although in some veins, those of the pregnant uterus, for histance, and the cardiac ends of the large thoracic veins, there is a ^eater development of muscular tissue. Further, and this is of prime physiological importance, valves are present in many veins. These . are semUtmar folds of the internal coat projecting into the lumen in such a direction as to favour the flow of blood towards the heart, but to check its return. In some veins„as the venae cavae, the pulmonary veins, the veins of most internal organsy and of bone, there are no valves; in the portal system they are rudimentary in man and the great majority of mammals. The valves are especially well marked in the lower limbs, where the venous circulation is uphill. When a valve ceases to perform its function of supporting the column of blood between it and the valve next above, the foundation of varicose veins is laid; the valve .immediately below the incompetent: one, having to bear up too great a weight of blood, tends to yield in its turn, and so the condition spreads. The smallest veins, or venules, are very like the smallest arteries, or arterioles, but somewhat wider and less muscular. The transition from the capillaries to the arterioles and venules is not abrupt, but may be considered as marked by the appearance of the non-striped muscular fibres, at first scattered singly, but gradually becoming closer and more numerous as we pass away from the capillaries, until at length they form a complete layer. In the heart the muscular element is greatly developed and differ- entiated. Both histologically and physiologically the fibres seem to stand between the striated skeletal muscle and the smooth muscle. In the mammal the cardiac muscular fibres are generally described as made up of short oblong cells, devoid of a sarcolemma, often branched, and arranged in anastomosing rows, each cell having a single nucleus in the middle of it. But it has recently been shown that the muscle fibrils run right through the apparent cell boundaries, and form a con- tinuous sheet of tissue anastomosing' in every direction. The fibres are transversely striated, but the striae are not so distinct as in skeletal muscle. A sarcolemnia is not absent, althoiigh it is more delicate than in skeletal muscle, and perhaps of a difierent nature. Many fibres piss from one auricle to the other, and from one ventricle to the other. In the frog's heart the muscular fibres are spindle-shaped, like those of smooth muscle, but transversely striated, like those of skeletal muscle. From the sinus to the apex of the ventricle there is a con- tinuous sheet of muscular tissue. ANATOMICAL AND PHYSICAL DATA 83 The problems of the circulation are partly physical, partly vital. Some of the phenomena observed in the blood-stream of a living animal can be reproduced on an artificial model ; and they may justly be called the physical or mechanical phenomena of the circulation. Others are essentially bound up with the properties of living tissues ; and these may be classified as the vital or physiological phenomena of the circulation. The distinction, although by no meails sharp and absolute, is a convenient one — -at least, for purposes of description; and as such we shall use it. But it must not be forgotten that the physiological factors play into the sphere of the physical, and the physical factors modify the physiological. Considered in its physical relations, the circulation of the blood is the flow of a Uquid along a system of elastic tubes, the bloodvessels, under the influence of an intermittent pressure produced by the action of a central pump, the heart. But the branch of djmamics which treats of the movement of liquids, or hydrodynamics, is one of the most difficult parts of physics, and even in the physical portion of our subject we are forced to rely chiefly on empirical methods. It would, therefore, not be profitable to enter here into mathematical theory, but it may be well to recall to the mind of the reader one or two of the simplest data connected with the flow of liquids through tubes: Torricelli's Theorem. — Suppose a vessel filled with water, the level of which is kept constant; the velocity with which the water wUl escape from a hole in the side of the vessel at a vertical depth h below the surface will be v= sJ2gh, where g is the acceleration produced by gravity.* In other words, the velocity is that which the water would have acquired in falling in vacuo through the distance h. This formula was deduced experimentally by Torricelli, and holds only when the resistance to the outflow is so small as to be negligible. The reason of this restriction will be easily seen, if we consider that when a mass m of water has flowed out of the opening, and an equal mass m has flowed in at the top to maintain the old level, everything is the same as before, except that energy of position equal to that possessed by a mass w at a height h has disappeared. If this has all been changed into kinetic energy E, in the form of visible motion of the escaping water, then 'E=\mv^=mgh, i.e., v= ^2gh. If, however, there has been any sensible resistance to the outflow, any sensible friction, some of the potential energy (energy of position) will have been spent in over- coming this, and will have ultimately been transformed into the kinetic energy of molecular motion, or heat. Flow of a Liquid through Tubes. — Next let a horizontal tube of uni- form cross-section be fitted on to the orifice. The velocity of outflow wUl be diminished, for resistances now come into play. When the liquid flowing through a tube wets it, the layer next the waU of the tube is prevented by adhesion from moving on. The particles next this stationary layer rub on it, so to speak, and are retarded, although not stopped altogether. The next layer rubs on the comparatively ■ slowly moving particles outsideiit, and is also delayed, although not so much as that in contact with the immovable layer on the walls of * I.e., the amount added per second to the velocity of a, falling body (g = 32 feet). 84 THE CIRCULATION OF THE BLOOD AND LYMPH the tube. In this way it comes about that every particle of the liquid is hindered by its friction against others — ^those in the axis of the tube least, those near the periphery most — and part of the energy of position of the water in the reservoir is used up in overcoming this resistance, only the remainder being transformed into the visible kinetic energy of the liquid escaping from the open end of the tube. If vertical tubes be inserted at different points of the horizontal tube, it will be found that the water stands at continually decreasing heights as we pass away from the reservoir towards the open end of the tube. The height of the liquid in any of the vertical tubes indicates the lateral pressure at the point at which it is inserted ; in other words, the excess of potential energy, or energy of position, which at that point the liquid possesses as compared with the water at the free end, where the pressure is zero. If the centre of the cross-section of the free end of the tube be joined to the centres of all the menisci, it will be found that the line is a straight line. The lateral pressure at any point of the tube is therefore proportional to its distance from the free end. Since the same quantity of water must pass through each cross- section of the horizontal tube in a given time as flows out at the open end, the kinetic energy of the liquid at every cross-section must be constant and equal to \mv^, where v is the mean velocity (the quantity which escapes in unit of time divided by the cross-section) of the water at the free end. Just inside the orifice the total energy of a mass m oi water is mgh; just beyond it at the first vertical tube, mgh' + imv^, where h' is the lateral pressure. On the assumption that between the inside of the orifice and the first tube no energy has been transformed into heat (an assumption the more nearly correct the smaller the distance between it and the inside of the orifice is made), we have mgh=mgh' +\mv^, i.e., imv' = mg{h-h'). In other words, the portion of the energy of position of the water in the reservoir which is transformed into the kinetic energy of the water flowing along the horizontal tube is measured % by the difference between the height of the level of the reservoir and the lateral pressure at the beginning of the horizontal tube — that is, the height at which the straight line joining the menisci of the vertical tubes intersects the column of water in the reservoir. Let H represent the height corresponding to that part of the energy of position which is transformed into the kinetic energy of the flowing water. H is easily calculated when the mean velocity of efflux is known. For v= J2gH by Torricelli's theorem (since none of the energy corresponding to H is supposed to be used up in overcoming friction), or H = — At the 2g' second tube the lateral pressure is only h". The sum of the visible kinetic and potential energy here is therefore imv^ + mgh". A quantity of energy mg{h'-h") must have been transformed into heat owing to the resistance caused by fluid friction in the portion of the horizontal Fig. 26.^ — Diagram to illustrate Flow of Water along a Horizontal Tube connected with a Reservoir, ANATOMICAL AND PHYSICAL DATA 85 tube between the first two vertical tubes. In general the energy of position represented by the lateral pressure at any point is equal to the energy used up in overcoming the resistance of the portion of the path beyond this point. Velocity of Outflow. — It has been found by experiment that v, the mean velocity of outflow, when the tube is not of very smaU calibre, varies directly as the diameter, and therefore the volume of outflow as the cube of the diameter. In fine capillary tubes the mean velocity is proportional to the square, and the volume of outflow to the fourth power of the diameter (Poiseuille) . If, for example, the linear velocity of the blood in a capillary of . 10 /i in diameter is i mm. per sec, it will be four times as great (or 2 mm. per sec.) in a capillary of 20 fi diameter, and one-fourth as great (or J mm. per sec.) in a capillary of 5 /i diameter, the pressure being supposed equal in all. The volume of outflow per second is obtained by multiplying the cross-section by the linear velocity. The cross-section of a circular capillary, 10 /i in diameter, is IT (sxyjsiijjjj^ =, say, ijIoct sq. mm. The outflow will be i^iviuxi = 25^(70 cub. mm. per sec. The outflow from the capillary of 20 /i diameter would be sixteen times as much, from the 5 /j. capillary only one-sixteenth as much. Some idea of the extremely minute scale on which the blood-flow through a single capillary takes place may be obtained if we consider that for the capillary of 10 jti diameter a flow of jsJinj cub. mm. per sec. would scarcely amount to i cub. mm. in six hours, or to i c.c. in 250 days. When the initial energy is obtained in any other way than by means of a ' head ' of water in a reservoir — say, by the descent of a piston which keeps up a constant pressure in a cylinder filled with liquid — the results are exactly the same. Even when the horizontal tube is distensible and elastic, there is no difference when once the tube has taken up its position of equilibrium for any given pressure, and that pressure does not vary. Flow with Intermittent Pressure. — When this acts on a rigid tube, everything is the same as before. When the pressure alters, the flow at once comes to correspond with the new pressure. Water thrown by a force-pump into a system of rigid tubes escapes at every stroke of the pump in exactly the quantity in which it enters, for water is practically incompressible, and the total quantity present at one time in the system cannot be sensibly altered. In the intervals between the strokes the flow ceases ; in other words, it is intermittent. It is very different with a system of distensible and elastic tubes. During each stroke the tubes expand, and make room for a portion of the extra liquid thrown into them, so that a smaller quantity flows out than passes in. In the intervals between the strokes the distended tubes, in virtue of their elasticity, tend to regain their original calibre. Pressure is thus exerted upon the liquid, and it continues to be forced out, so that when the strokes of the pump succeed each other with sufficient rapidity, the outflow becomes continuous. This is the state of affairs in the vascular system. The intermittent action of the heart is toned down in the elastic vessels to a continuous steady flow. Section II. — ^The Beat of the Heart in its Physical or Mechanical Relations. Events in the Cardiac Cycle. — In the frog's heart the contraction can be seen to begin about the mouths of the great veins which open into the sinus venosus. Thence it spreads in succession over the 86 THE CIRCULATION OF THE BLOOD AND j^Yivii-n sinus and auricles, hesitates for a moment at the auriculo-ventric- ular junction, and then with a certain suddenness invades the ventricle. In the mammahan heart the contraction likewise com- mences, so far as can be ascertained by inspection or the study of tracings, in the region near the mouths of the veins opening into the auricles. It will be seen, when the question of the origin of the rhythmical beat is being discussed (p. 141), that the actual starting- point is probably the sinus tissue of the right auricle (p. 142) near the opening of the superior vena cava, which is richly provided with muscular fibres akin to those of the heart. But the wave advances so rapidly that it is difficult to trace in its course a regular progress from base to apex, although the ventricular beat undoubtedly follows that of the auricle, and in a heart beating normally the electrical change associated with contraction of the ventricle begins at the base, then reaches the apex (p. 806), and finally passes towards the orifices of the great arteries. The most conspicuous events in the beat of the heart, in their normal sequence, are : (i) the auricular contraction or systole, (2) the ventricular contraction or systole, each followed by relaxation, (3) the pause. The auricles, into which, and beyond which into the ven- tricles, blood has been flowing during the pause from the great thoracic veins, contract sharply, the right, perhaps, a little before the left. The contraction begins in the muscular tissue that surrounds the orifices of the veins, so that these, destitute of valves as they are, are functionally, at least, if not anatomically, sealed up for an instant, and regurgitation of blood into them is to a great extent, if not entirely, prevented. Apparently, complete closure of the inferior cava is unnecessary, the pressure of the blood in it being sufficiently high to hinder any important back-flow. The action of the circular fibres of the veins in closing their orifices is reinforced by the contraction of a band of muscle (the tcenia ter- minalis) in the roof of the right auricle. This band compresses especially the mouth of the superior vena cava. The fiUing of the ventricles is thus completed. The actual amount of extra blood injected into the ventricles by the auricular contraction is not large. The ventricles are already nearly charged, but the auricles, so to speak, ram the charge home. The ventricular contraction follows hard on the relaxation of the auricles. The mitral and tricuspid valves, whose strong but delicate curtains have during the diastole been hanging down into the ventricles and swinging freely in the entering current of blood, are floated up as the intraventricular pressure begins to rise, so that, in the first momeiit of the sudden and powerful ventricular systole, the free edges of their segments come together, and the auriculo-ventricular orifices are completely closed (Fig. 98, p. 204). In the measure in which the pressure in the contracting ventricles increases, the contact of the valvular seg-j MECHANICS OF THE HEART-BEAT 87 ments becomes closer and more extensive; and their tendency to belly into the auricles is opposed by the puU of the chordae tendinese, whose slender cords, inserted into the valves from border to base, are kept taut, in spite of the shortening of the ventricles by the con- traction of the papillary muscles. The arrangement and connec- tions of the muscular fibres of the heart are such that during the auricular systole the auriculo-ventricular groove moves towards the base of the heart, while during the systole of the ventricles it moves towards the apex, which constitutes a relatively fixed point on account of the mutual action of the numerous fibres which converge here and constitute the ' whorl.' The line joining the apex and the origin of the aorta does not shorten when the ventricles contract, but all parts of the heart are drawn towards this line. The apex is, therefore, pushed forwards, while the rest of the ventricular surface is being drawn inwards. During the systole, the ventricles change their shape in such a way that their combined cross-section — which in the relaxed state is a rough ellipse with the major axis from right to left — -becomes approximately circular, and they then form a right circular cone. As soon as the pressure of the blood within the con- tracting ventricles exceeds that in the aorta and pulmonary artery respectively, the semilunar valves, which at the beginning of the ventricular systole are closed, yield to the pressure, and blood is driven from the ventricles into these arteries. The ventricles are more or less completely emptied during the contraction, which seems stiU to be maintained for a short time after the blood has ceased to pass out. The contraction is followed by sudden relaxation. The intraventricular pressure falls. The lunules of the semilunar valves slap together under the weight of the /blood as it attempts to regurgita;te, the corpora Arantii seal up the central chink, and the aorta and pulmonary artery are thus cut off from the heart. Then follows an interval during which the whole heart is at rest, namely, the interval between the end of the relaxa- tion of the ventricles and the beginning of the systole of the auricles. This constitutes the pause. The whole series of events is called a cardiac cycle or revolution (see Practical Exercises, p. 199). It will be easily understood that the time occupied by any one of the events of the cardiac cycle is not constant, for the rate of the heart is variable. If we take about 70 beats a minute as the average normal rate in a man, the ventricular systole will occupy about 0-3 second; the diastole,* including the ventricular relaxation, about * The term ' diastole ' is variously used, as meaning the pause, the pause plus the period during which relaxation is occurring, or the period of re- laxation alone. We shall employ it in the second sense. Henderson refers to the psriod during which the ventricular muscle is at rest, from the end of its relaxation to the onset of the auricular systole, as the ' diastasis ' and the period during which the relaxation is taking place as the ' diastole,' a termin- ology which seems worthy of general adoption. «8 THE CIRCULATION OF THE BLOOD AND LYMPH 0-5 second. The systole of the auricle is one-third as long as that of the ventricle. This rhythmical beat of the heart is the ground phenomenon of the circulation. It reveals itself by certain tokens — sounds, surface- movements or pulsations, alterations of the pressure and velocity of the blood, changes of volume in parts — all periodic phenomena, continually recurring with the same period as the heart -beat, and all fundamentally connected together. And if we hold fast the idea that when we take a pulse-tracing, or a blood-pressure curve, or a plethysmographic record, we are really investigating the same fact from different sides, we shall be able, by following the cardiac rhythm and its consequences as far as we can trace them, to hang upon a single thread many of the most important of the physical phenom- ena of the circulation. The Sounds of the Heart. — When the ear is applied to the chest, or to a stethoscope placed over the cardiac region, two sounds are heard with every beat of the heart. They follow each other closely, and are succeeded by a period of silence. The dull booming ' first sound ' is heard loudest in a region which we shall afterwards have to speak of as that of the ' cardiac impulse ' (p. 90) ; the short, sharp, ' second sound ' over the junction of the second right costal cartilage with the sternum. The heart-sounds can be registered by placing over the chest a microphone receiver connected with a string galvanometer. The magnified sounds are translated into electrical changes which cause movements of the fibre of the galvanometer, and the movements are photographed on a travelling plate (Einthoven). The record is called a cardiophonogram. When this is studied, a third sound can be detected, and it is probable that it is present in all persons, although it is as a rule inaudible to auscultation. It occurs early in diastole very shortly after the second sound. In those persons in whom it is audible it is most distinct over the region of cardiac impulse. It is described as softer and of lower pitch than the second sound (Thayer). There has been much discussion as to the cause of the first sound; That a sound corresponding with it in time is heard in an excised bloodless heart when it contracts is certain; and therefore the first sound cannot be exclusively due, as some have asserted, to vibra- tions of the auriculo-ventricular valves when they are suddenly rendered tense by the contraction of the ventricles, for in a bloodless heart the valves are not stretched. Part of the sound must accord- ingly be associated with the muscular contraction as such. Again, the fact that the first sound is heard during the whole, or nearly the whole, of the ventricular systole is against the idea that it is exclusively due to the vibrations of membranes like the valves, which would speedily be damped by the blood and rendered in- audible. But while there is good reason to believe that the vibra- tion of the suddenly-contracted ventricles is the fundamental factor, the shock sets up vibrations also in the blood, the chest-wall, and MECHANICS OF THE HEART-BEAT 89 perhaps the resonant tissue of the lungs. Further, as we shall see later on (p. 734), the sound caused by a contracting muscle readily calls forth sympathetic resonance in the ear, and the peculiar boom- ing character of the first sound may be due to the superposition of these various resonance tones upon the muscular note. But, in addition, the vibration of the auriculo-ventricular valves un- doubtedly contributes to the production of the sound, and some observers have been able to distinguish in the first sound the valvular and the muscular elements, the former being higher in pitch than the latter, but a minor third below the second sound. In the excised empty heart the deeper tone of the first sound is alone heard, while the higher note is elicited when in a dead heart the auriculo-ventric- ular valves are suddenly put under tension (Haycraft). When the mitral valve is prevented from closing by experimental division of the chordae tendinese, or by pathological lesions, the first sound of the heart is altered or replaced by a ' murmur.' This evidence is not only important as regards the physiological question, but of great practical interest from its bearing on the diagnosis of cardiac disease. It may be added that the point of the chest-wall at which the first sound is most easily recognized is also the point at which a changed sound or murmur connected with disease of the mitral valve is most distinctly heard. The sound is, therefore, best conducted from the mitral valve along the heart to the point at which it comes in contact with the wall of the chest. Changes in the first sound con- nected with disease of the tricuspid valve are heard best, in the com- paratively rare cases where they can be distinctly recognized, in the third to the fifth interspace, a little to the right of the sternum. The second sound is caused by the vibrations of the semilunar valves when suddenly closed, ' the recoiling blood forcing them back, as one unfurls an umbrella, and with an audible check as they tighten ' (Watson). The sharpness of its note is lost, and nothing but a rushing noise or bruit can be heard, when the valves are hooked back and prevented from closing. It is altered, or replaced by a murmur, yvhen the valves are diseased. As there is a mitral and a tricuspid factor in the first sound, so there is an aortic and a pul- monary factor in the second. The place where the second sound is best heard (over the junction of the second right costal cartilage and sternum) is that at which any change produced by disease of the aortic valves is most easily recognized. The sound is conducted up from the valves along the aorta, which comes nearest to the surface at this point. Changes connected with disease of the pulmonary valves are most readily detected over the second left intercostal space near the edge of the sternum, for here the pulmonary artery most nearly approaches the chest-wall. The first sound is ' systoHc ' — that is, it occurs during the ventricular systole; the second is ' diastohc,' beginning at the commencement of the diastole. 90 THE CIRCULATION OF THE BLOOD AND LYMPH Various explanations of the third sound have been given, but, as the authors who have studied it are not even agreed as to whether it is produced at the auriculo-ventricular orifices or at the aortic and pulmonary orifices, it would not be useful to discuss them at present. The Cardiac Impulse. — A surface-movement is seen, or an impulse felt, at every cardiac contraction in various situations where the heart or arteries approach the surface. The pulsation, or impulse, of the heart, often styled the apex-beat, is usually most distinct to sight and touch in a small area lying in the fifth left intercostal space, between the mammary and the parasternal Une,* and gener- ally, in an adult, about an inch and a half to the sternal side of the former. It is due to the systolic hardening of the ventricles, which are here in contact with the chest-waU, the contact being at the same time rendered closer by their change of shape, and by a slight movement of rotation of the heart from left to right during the contraction (Practical Exercises, p. 205). When the left ventricle is in contact with the chest at the position of the apex-beat, as is usually the case, an important element in the impulse is the actual forward thrust of the apex. When the apex-beat corresponds in position with the right ventricle, there is no actual forward movement, although the hardening of the ventricle may be felt as a thrust by the finger. Even in health the position of the impulse varies somewhat with the position of the body and the respiratory movements. In children it is usually situated in the fourth intercostal space. In disease its displacement is an important dragnostic sign, and may be very marked, especially in cases of effusion of fluid into the pleural cavity. It is sometimes, though not invariably, a Uttle lower in the standing than in the sitting position, and shifts an inch or two to the left or right when the person lies on the corresponding side. Various instruments, called cardiographs, have been devised for magnifying and recordiag the movements produced by the cardiac impulse. Marey's cardiograph (Fig. 27) consists essentially of a small * The mammary line is an imaginary vertical line supposed to be drawn on the chest through the middle point of the clavicle. It usually, but not necessarily, passes through the nipple. The parasternal line is the vertical line lying midway between the mammary line and the corresponding border of the sternum. Fig. 27. — Diagram of Marey's Cardiograph. MECHANICS OF THE HEART-BEAT 91 chamber, or tambour, filled with air, and closed at one end by a flexible membrane carrying a button, which can be adjusted to the wall of the chest. This receiving tambour is connected by a tube with a recording tambour, the flexible plate of which acts upon a lever writing on a travelling surface — a uniformly-rotating drum, for example — covered with smoked paper. Any movement communicated to the button forces in, the end of the tambour to which it is attached, and thus raises the pressure of the air in it and in the recording tambour; the flexible plate of the latter moves in response, and the lever transfers the movement to the paper. The tracing, or cardiogram, obtained in this way shows a small elevation corresponding to the auricular systole, succeeded by a large abrupt rise corresponding to the beginning of the first sound, and caused by the ventricular systole. This ventricular, elevation is the essential portion of" the curve; it is alone felt by the palpating hand, and the auricular elevation is often absent from the cardio§p:am in man. The rise is maintained, with small secondary oscillations, for about 0-3 of a second in a tracing from, a normal man,' then gives way to a sudden de- scent, that marks the relaxation of the ventricles, the beginning of the second sound, and the closure of the semilunar valves. An interval of about 0'5 second elapses before the curve begins again to rise at the next auricular contraction. Such was the interpretation which Chauveau and Marey put upon their tracings. Although neither their results noi'" their deductions from them' have ^'■S- 28.— Cardiogram taken with Marey s . escaped the criticism of sUcoeed- Cardiograph. A, auricular systole;, ing investigators, it is doubtful l: ventricular systole ; p, diastole whether anv ad emiate reason ^'^^ ^"°^ '■^°^'' ^^^ direction in which wnetner any adequate reason the tracing is to be read, has been brought forward for discarding them, and Chauveau has furnished further proofs of their accuracy. The difficulties that beset the subject are great, for the cardiogram is a record of a complex series of events. The very rapid variation of pressure within the ventricles, the change of volume and of shape of the heart, the slight change, of position of its apex, must all leave their mark upon the curve, which is besides distorted by the resistance of the elastic chest-wall, the inertia of the recording lever, and the compression of the air in the connecting tubes. It is only by comparing in anirnals the cardiographic record with the changes of ' blood-pressure in the heart and arteries that our present degree of knowledge of the human cardiogram has been attained. Could we register directly the fluctuations of pressure in the interior of the human heart, the cardiographic method would be rarely employed. For clinical purposes the receiving tambour can be advantageously replaced by a small glass funnel or a small metal cup, the open end of which is applied without a menlbrane over the cardiac impulse, the stem being connected with the recording tambour. In cases in which the right ventricle is in contact with the chest-wall at the position of the apex- beat the cardiogram is ' inverted ' — ^that is to say, the chest-wall is drawn in during systole and protruded during diastole of the ventricles. Inversion of the cardiogram is, therefore, not an infallible sign of the pathological condition known as adherent pericardium (Mackenzie). 92 THE CIRCULATION OF THE BLOOD AND LYMPH Endocardiac Pressure. — The function of the heart is to maintain an excess of pressure in the aorta and puhnonary artery sufficient to overcome the friction of the whole vascular channel, and to keep up the flow of blood. So long as the semilunar valves are closed, most of the work of the contracting ventricles is expended in raising the pressure of the blood within them. At the moment when blood begins to pass into the arteries, nearly all the energy of this blood is potential; it is the energy of a liquid under pressure. During a cardiac cycle the pressure in the cavities of the heart, or the endo- cardiac pressure, varies from moment to moment, and its variations afford important data for the study of the mechanics of the circulation. Manometers. — For the study of the endo- cardiac pressure, the ordinary mercurial manometer (p. no) is unsuitable, since, owing to the rela- tively great amount of work required to produce a given dis- placement of the mer- cury, it does not readily follow rapid changes of pressure, and the mercurial column, once dis- placed, continues for a time to execute vibrations of its own, which are compoun- ded with the true oscillations of blood-pressure. But by introducing in the connection between the manometer and the heart a valve so arranged as to oppose the passage of blood towards the heart, while it favours its passage towards the manometer, the maximum pressure attained in the cardiac cavities during the cycle may be measured with considerable accuracy. When the valve is reversed the apparatus becomes a minimum manometer. In this way it has been found that in large dogs the pressure in the left ventricle may rise as high as 230 to 240 mm. of mercury, and sink as low as - 30 to - 50 mm. ; while in the right ventricle it may be as much as 70 mm., and as little as — 25 mm. In the right auricle a maximum pressure of 20 mm. of nercury has been recorded, and a minimum pressure of — 10 mm. or 3 ven less. But these results were obtained under somewhat exceptional ;xperimental conditions, and the normal maximum pressures in the ieart cavities in man are probably not so high, especially in the right luricle and ventricle. Our knowledge of the maximum and minimum pressure attained n the cavities of the heart, even if it were far more precise than it ictually is, would only carry us a little way in the study of the endo- ;ardlac pressure-curve, for it would merely tell us how far above the Fig. 29. — Curves of Endocardiac Pressure taken witli Cardiac Sounds. Aur., auricular curve; Vent., ven- tricular curve; AS, period of auricular systole, in- cluding relaxation; VS, of ventricular systole, including relaxation; D, pause. MECHANICS OF THE HEART-BEAT 93 'T. ^ Fig. 30. — Diagram of Hurthle's Elastic-Mano- meter. T, small chamber covered by mem- brane; t, tube communicating with interior of heart ; L, compound lever to magnify the movements of the membrane. base-line of atmospheric pressure the curve ascends, and how far below the base-line it sinks. To exhaust the problem, we require to have tracings of the exact form of the curve for each of the cavities of the heart, and to know the time-relations of the curves so as to be able to compare them with each other, and with the pressure-curves of the great arteries and great veins. To obtain satisfactory tracings of the swiftly- changing endocardiac pressure is a task of the highest techni- cal difficulty, and it is only in very recent years that it has been accomplished, with any ap- proach to accuracy by the use of elastic manometers, in which the blood-pressure is counter- balanced, not by the weight of a column of liquid, as in the mercurial manometer, but by the resistance to compression of a small column of air or the tension of an elastic disc or of a spring. Modifications in the nature and dimensions of the elastic resistance of the recording apparatus and of the size of the cavity have produced successive improvements, as, e.g., in the manometers of Hiirthle (Fig. 30). The penetrating analysis of the principles of manometer construction by Frank has recently stimulated renewed investigation of the whole subject with the aid of instruments whose movements are optically re- Fig. 31. — Diagram of Optical Manometer (Wiggers). A is a vertical glass tube surmounted by a hollow- brass cylinder, B, which contains a stopcock, C, whose lumen comes info apposition with a plate, a, having a small opening in it. By opening the stopcockmore or less, the pulsations will be ' damped ' to a smaller or greater extent. Above a the cylinder ends in a segment capsule b (i.e., a capsule cut away at one side) 3 mm. in diameter, covered with rubber dam. Upon this a small piece of celluloid carrying a little mirror, c, is fastened, so that it pivots on the chord side of the capsule. Over the capsule and its recording mirror is moimted a support bearing an inclined reflecting mirror, E, adjustable about a horizontal axis by a screw, so that the image of the recording mirror appears within it. Upon this image a strong light is focussed. The incident rays are doubly reflected, as shown in the figure, and the movements of the capsule are thus greatly magnified. The beam of light is photographed on a moving plate. corded on a photographic plate, so as to eliminate all unnecessary fric- tion. Fig. 31 is a diagram of the manometer devised by Wiggers on this principle. Hurthle's spring manometer consists of a small drum covered with an indiarubber membrane, loosely arranged so as not to vibrate with a period of its own. The drum is connected with the heart or with a vessel, and the blood-pressure is transmitted to a steel spring by means of a light metal disc fastened on the membrane. The spring 94 THE CIRCULATION QF THE BLOOD AND LYMPH acts on a writing lever. The instrument is so constructed that for a given change of pressure the quantity of liquid displaced is as small as possible, and it is on this that its capacity to follow sudden varia- tions of pressure chiefly depends. The manometer is connected with the cavity of the heart by an appropriately curved cannula of metal or glass, which, after being filled with some liquid that prevents co- agulation (Practical Exercises, p. 209), is pushed through the jugfular vein into the right auricle or ventricle, or through the carotid artery and aorta into the left ventricle. Some observers fill only the cannula with fluid, and leave the capsule of the elastic manometer and as much of the connections as possible full of air. Others fill the whole system with liquid. And around the question of the relative merits of ' trans- mission 'by liquid and by air has raged a controversy which, however, now shows signs of coming to an end. For there is reason to suppose that the character of the curves obtained is modified among other circumstances by the manner in which the pressure is transrnitted, as it is certainly modified by the dimensions and rnass of the moving parts and the method of recording. As Wiggers has pointed out, the differ- ences in the records obtained by different observers, even with the latest methods of optical registration, are determined largely by the sensitive- ness and degree of damping of the manometer. The Ventricular Pressure-Curve. — Thus, the pressure-curve of the ventricle, according to most of those who have employed mano- meters with liquid transmission and small inertia of the moving parts (Fig. 33), remains after the first abrupt rise, which undoubtedly corresponds to the ventricular systole, weU above the abscissa line for a considerable time, and then descends somewhat less suddenly than it rose. This systolic ' plateau,' although usually broken by minor heights and hollows, which may be partly due to inertia oscilla- tions of the liquid or the recording apparatus, would indicate that the ventricular pressure, after its first swift rise, maintained itself at a considerable height throughout the greater part of the systole. The tracings yielded by most of the manometers with air trans- mission show the same suddenness in the first part of the upstroke and the last part of the descent — ^that is, the same abruptness in the beginning of the contraction and the end of the relaxa- tion. But they differ totally in the intermediate portion of the curve, which, climbing ever more gradually as it nears its apex, remains but a moment at the maximum, then immediately descend- ing forms a ' peak,' and not a plateau. It ought to be distinctly understood, however, that the use of the term ' plateau ' must not be taken to imply that the pressure remains constant and the curve parallel to the abscissa during this interval. Wiggers, using the optical method of recording the pressure- curve in the right ventricle (p. 93), finds, that when the auricular pressure and the pressure in the pulmonary artery are normal the curve of intraventricular pressure may be divided into (i) an auric- ular period; {3) a period of rising pressure while the ventricle is contracting and its cavity is closed by the auriculo-ventricular and semilunar valves; (3) an ejection period during which the pressure MECHANICS OF THE HEART-BEAT 95 still rises, reaches a summit, and then slowly falls; and (4) a relaxa- tion period (Fig. 32). Without entering further into a technical discussion, we may say CaT'otid Fig. 32. — Intraventricular Pressure Curves with Optical^Recording (Wiggers). Three types of normal curves are reproduced, taken with manometers of different degrees of sensitiveness. The second at the left-hand side was fallen with the least sensitive, a — b, auricular systolic; b — d. isometric period, during which the auriculo-ventricular and the semilunar valves are both closed; d—f, ejection period ; after /, diastole. the bulk of the evidence goes to show that the plateau is not, as the advocates of the peak have claimed, an artificial phenomenon, but does in reahty correspond to that continuation of the systole of the H ^H Fig- 33- — Simultaneous Record of Pressure in Left Ventricle (V) and Aorta (A). (Hiirthle.) The tracings werej taken with elastic mancmeters; o indicates a point just before the closure of the mitral valve; i. the opening of the semilunar valve; 2, beginning of the relaxation of the ventricle; 3, the closure of the semi- lunar valve; 4, the opening of the mitral valve. The ventricular curve shows a 'plateau.' ventricle, that dogged grip, if we may so phrase it, which it seems to maintain upon the blood after the greater portion of it has been expelled. 96 THE CIRCULATION OF THE BLOOD AND LYMPH This conclusion is essentially in accordance with the results of Chauveau and Marey, obtained long ago by means of their ' cardiac sound,' which was in principle an elastic manometer. It consisted of an ampulla of indiarubber, supported on a frame- work, and communicating with a long tube, which was connected with a recording tambour. The ampulla was introduced into the heart (of a horse) through the jugular vein or carotid artery in the way already described. Sometimes a double sound was employed, armed with two ampuUas, placed at such a distance from each other that when one was in the right ventricle the other was in the auricle of the same side. Each ampulla communicated by a separate tube in the common stem of the instrument with a recording tambour, and the writing points of the two tambours were arranged in the same vertical line. When any change in the blood-pressure takes place, the degree of compression of the ampullae is altered, and the change is transmitted along the air-tight connections to the recording tambours. On most of the endocardiac pressure tracings taken with modern manometers, whether the curves belong to the type of the peak or of the plateau, no sudden change of curvature, no nick, or crease, or undulation reveals the moment of opening or closure of any valve. This has been considered by some writers a striking tribute to the smooth working of the cardiac pump. There is reason to think, however, that the smoothness of the curve is still in some degree artificial, and on some of the records obtained by optical methods (Fig. 32) indications of changes of curvature, associated with the action of the valves, may be observed. But even in the absence of such indications, by experimentally graduat-irig-a''pair' of elastic manometers, and obtaining with them simultaneous records of the pressure in auricle and ventricle, or by using a ' differential ' mano- meter, in which the pressures in two cavities are opposed to each other, so that the movement of the membrane corresponds to their ; difference, we can calcula.te at what points of the ventricular curve the pressure is just greater than and just less than the pressure in the auricle. The first point, it is evident, will correspond to the instant at which the mitral or tricuspid valve, as the case may be, is closed, and the second to the instant at which it is opened. And in like manner, by comparing the pressure-curve of the aorta with that of the left ventricle, the moment of opening and closure of the semi- lunar valves may be determined (Figs. 33 and 34). According to the best observations, the closure of the semilunar valves takes place at a time corresponding to a point on the upper portion of the descend- ing limb of the intraventricular curve. On the blood-pressure curve of the aorta, simultaneously registered, the corresponding point is near the bottom of the so-called ' aortic ' notch (p. 105) which precedes the dicrotic elevation. For clinical purposes, in man the moment of closure of the semilunar valves (denoted by the abbreviation S.C. point) may be taken as 0-03 second before the bottom of the aortic notch in sphygmographic tracings from the carotid, this being approximately the average time occupied MECHANICS OF THE HEART-BEAT 97 by the pulse-wave in travelling from the aorta to the carotid. The S.C. point, the A.O. point, or moment of opening of the auriculo- ventricular valves, and the beginning of the ventricular systole, are three important points of reference in the measurement and inter- pretation of pulse-tracings in clinical work. The A.O. point in man may be taken as a point ' 0-03 second in advance of the summit of the dicrotic wave ' on the carotid pulse-tracing (Lewis). But this is the most dif&cult of the three standard points to determine clinically with anything like accuracy. The study of the curves of endocardiac pressure enables us to add precision in certain points to the description of the events of the cardiac cycle which we have already given, and, as regards the ventricles, to divide the cycle into four periods : (i) A period during which the pressure is lower in the ventricles than either in the auricles or the arteries, and the auricula-ventricular valves are consequently open, and the semilunar valves closed. This is the period of ' filling ' of the heart, or the pause. (2) A period, beginning with the ventricular systole, during which the pressure is increasing abruptly in the ventricles, while they are as yet completely cut off from the auricles on the one hand and the arteries on the other by the closure of both sets of valves. This is the period of ' rising pressure, ' during which the ventricles are, so to say, ' getting up steam.' The interval between the beginning of the ventricular systole and the opening of the semilunar valves is termed the ' presphygmic ' interval. (3) A period during which the pressure in the ventricles overtops that in the arteries, and the semilunar valves are open, while the auriculo- ventricular valves remain shut. This is the period of ' discharge ' or ' sphygmic ' period. (4) A period during which the pressure in the ventricles is again less than the arterial, while it still exceeds the auricular pressure, and both sets of valves are closed. This is the period of rapid relaxation. The interval between the closure of the semilunar and the opening of the auriculo-ventricular valves is sometimes called the ' post-sphygmic ' interval. Of the four periods, the second and fourth are exceedingly brief. The third is relatively long and constant, being but slightly depen- dent on either the pulse-rate or the pressure in the arteries. The duration of the first period varies inversely as the frequency of the heart ; with the ordinary pulse-rate it is the longest of all. From records taken in a person with a defect in the chest-wall which rendered the heart accessible the following results were obtained as to the duration of the various events of the cardiac cycle : First and fourth periods together, 0-445 ; third period, 0-254; second period (pre- sphygmic interval), 0-05 1 second, the pulse-rate being 80 a minute (Tigerstedt). In another case with a similar defect the first period lasted 0'32, the fourth period (post-sphygmic interval) o-o6, the second and third periods together 0-4, and the auricular systole o-i second, the pulse-rate being 66. 7 98 THE CIRCULATION OF THE BLOOD AND LYMPH The Auricular (and Venous) Pressure-Curve.— The fluctuations oi pressure in the auricles, although confined within narrower Umits than in the ventricles, are of equal interest. They have been studied in considerable detail both in animals and by indirect methods in man. No fewer than three distinct elevations or ' positive waves,' separated or followed by three depressions or ' negative waves,' have been described on the curve of intra- auricular pressure. The first elevation corresponds to the systole of the auricle. The second coincides with the onset of the ventricular systole, and is foa A.syst. V.jysCol.& J^aus^ 1 ^ t 1% 1 1 ^ 1 1 % ISO i 1^ .-'b \ m ] '6' \ £2 b 4 \ / ? >-—\ / \ r /-^ \ \ ,,,cl5i* II \J ary veins, and therefore upon the inflow into the right side of the heart from the systemic veins) varies widely, some of the mechanical effect of the contraction must be wasted when the quantity is less than the ventricle is capable of expelling. Output of the Heart. — If 4J kilos of blood pass through the heart in I minute with the average pulse-rate of 72 per minute, the quantity ejected by either ventricle with every systole will be — — = 62-5 grm., or a little less than 60 c.c. The output may be expressed in grammes or cubic centimetres per minute (the minute volume), or per second, or per beat. It has been measured in animals in several ways — e.g., by inserting a stromuhr (p. 121) on the course of the aorta, or by recording the variations in the volume of the heart, or, better, of the ventricles, by means of a plethysmograph (cardiometer of Henderson), in which the organ is enclosed. Another method, which does not entail the opening of the chest, is to allow a salt solution to run slowly, for a de- finite number of seconds, into the left ventricle through a tube passed into it from the carotid artery. A sample of the mixture of blood and salt solution is collected from a branch of the femoral artery, where its arrival is detected by the change of electrical resistance (p. 135). From the amount of salt solution which must be added to a normal saniple of blood drawn before the injection to make its conductivity the same as that of the sample taken during the passage of the mixture, the quantity of blood with which the solution was mixed in the ventricle during the injection can be approximately determined. By this method it has been shown in a series of experiments on more than twenty dogs, ranging in weight from 5 to nearly 35 kilos, that the output of the left ventricle per kilo of body-weight per second diminishes as the size of the animal increases ; and the relation between body-weight and out- put is such that in a man weighing 70 kilos we can hardly suppose that the ventricle discharges, during bodily rest, more than 105 grm. of blood per second, or 87 grm. (80 c.c.) per heart-beat with a pulse-rate of 72. Putting this result along with that deduced from the circulation- time, we can pretty safely conclude that the average amount of blood thrown out by each ventricle at each beat is not more than 70 or 80 c.c. Zuntz, from the quantity of oxygen absorbed by the blood in the lungs in a definite short time, and the difference between the oxygen content of samples of the arterial and venous blood, has estimated the output per beat at 60 c.c. But according to him this may be doubled during * Since the blood on expulsion is moving with a certain velocity, an addi- tion might be made for its kinetic energy. But this would only increase the total work by a small fraction (about i per cent.). I40 THE CIRCULATION OF THE BLOOD AND LYMPH severe muscular work, when, as a matter of fact, by the aid of the Rontgen-rays or by percussion of the chest, the volume of the heart may be shown to be considerably increased. Tigerstedt, on the basis of stromuhr measurements in animals, puts the ventricular output per beat in man at 50 to 100 c.c. ; Plesch, on the basis of gasometric ob- servations on man, at 59 c.c. Recently Krogh, using a gasometric method based on the absorption of nitrous oxide gas in the lungs, foimd that the minute volume during rest may vary between wide limits (2-8 to 8-7 Utres of blood per minute, corresponding, with a pulse-rate of 70, to 40 c.c. to 120 c.c. per beat). During muscular worJc there is a great and immediate increase, up to, it may be, 21-6 litres per minute. These great variations in the output of the ventricle depend primarily upon variations in the rate of return of the blood to the heart by the veins. According to Henderson, however, such great variations in the output per beat as are postulated by the majority of physiologists who have worked at the subject do not occur, and the fundamental variable is the rate of the beat. In healthy persons in whom the pulse-rate is permanently much below the normal (p. 107) the output of the ventricle per beat must, of course, be correspondingly increased. In a man with a pulse -rate always below 40 during rest in the sitting position, the flow in the hands was found to be normal in amount, and all the signs of a normal delivery of blood from the left ventricle were present. Here the output per beat must have been twice the usual amount during rest. Section IV.— The Heart-Beat in its Physiological Relations. So far we have been considering the circulation as a purely physical problem. We have spoken of the action of the heart as that of a force-pump, and perhaps to a small extent that of a suction- pump too. We have spoken of the bloodvessels as a system of more or less elastic tubes through which the blood is propelled. We have spoken of the resistance which the blood experiences and the pressure which it exerts in this system of tubes, and we have considered the causes of this resistance, the interpretation of this pressure, and the physical changes in the vascular system that may lead to variations of both. But so far we have not at all, or only incidentally and very briefly, dealt with the physiological mechanism through which the physical changes are brought about. We have now to see that, although the heart is a pump, it is a hving pump ; that, although the vascular system is an arrangement of tubes, these tubes are alive; and that both heart and vessels are kept constantly in the most delicate poise and balance by impulses passing from the central nervous system along the nerves. In many respects, and notably as regards the influence of nerves on it, we may look upon the heart as an expanded, thickened and rhythmically-contractile bloodvessel, so that an account of its innervation may fitly precede the description of vaso-motor action in general. THE HEART-BEAT IN ITS PHYSIOLOGICAL RELATIONS 141 The Relation of the Heart to the Nervous System. — A very simple experiment is sufficient to prove that the beat of the heart does not depend on its connection with the central nervous system, for an excised frog's heart may, under favourable conditions, of which the most important are a moderately low temperature, the presence of oxygen, and the prevention of evaporation, continue to beat for days. The mammalian heart also, after removal from the body, beats for a time, and indeed, if defibrinated blood be artificially circulated through the coronary vessels, for several or even many hours. But although this proves that the heart can beat when separated from the central nervous system, it does not prove that nervous influence is not essential to its action, for in the cardiac substance nervous elements, both cells and fibres, are to be found. The Intrinsic Nerves of the Heart. — In the heart of the frog numerous nerve-cells occur in the sinus venosus, especially near its junction with the right auricle (Remak's ganglion). A branch from each vagus, or rather from each vago-sympathetic nerve (for in the frog the vagus is joined a little below its exit from the skull by the sympathetic), enters the heart along the superior vena cava (pp. 157, 196). Running through the sinus, with whose ganglion-cells the true vagus fibres, or some of them, are believed to make physiological junction (p. 163), the nerves pursue their course to the auricular septum. Here they form an intricate plexus, studded with ganglion-cells. From the plexus nerve-fibres issue in two main bundles, which pass down the anterior and posterior borders of the septum to end in two clumps of nerve-cells (Bidder's ganglia), situated at the auriculo-ventricular groove. These ganglia in turn give off fine nerve-bundles to the ven- tricle, which form three plexuses — one under the pericardium, another under the endocardium, and a third in the muscular wall itself, or my6- cardium. From the last of these plexuses numerous non-meduUated fibres run in among the muscular fibres and end in close relation with them . Similar plexuses of nerve-fibres exist in the mammalian ventricle . But while scattered ganglion-cells are found in the upper part of the ventricular wall, most observers have been unable to demonstrate any either in the mammal or the frog in the apical half. In the rat's heart, according to the careful observations of Schwartz, true ganglion-cells are confined to an area on the posterior surface of the auricles, lying always under the visceral pericardium. Other writers, however, have stated that ganglion-cells do exist in the apex both of the cat's and of the frog's heart. In connection with the whole question it must be borne in mind that in other organs improved histological methods have brought typical nerve-cells to light in situations where they were not suspected or were denied to exist, and, further, that all investigators are not agreed upon the histological criteria by which ganglion-cells are to be distinguished. Cause of the Rhythmical Beat of the Heart. — Scarcely any physio- logical question has excited greater interest for many years than the mechanism of the heart-beat. Several properties of the cardiac tissue ought to be distinguished in discussing this question: (i) Its 142 THE CIRCULATION OF THE BLOOD AND LYMPH automatism — i.e., its power of beating in the absence of external stimuli; (2) its rhythmicity — i.e., its power of responding to con- tinuous stimulation by a series of rhythmically repeated contrac- tions; (3) its conductivity — i.e., its power of conducting the contrac- tion wave or the impulse to contraction once it has been set up ; and (4) the power of co-ordination, in virtue of which the various parts of the heart beat in a regular sequence. The excitability of the cardiac tissue — ^that is, its power of appro- priate response (namely, by contraction) to a suitable stimulus — does not particularly concern us here, since it is in no wise a property special to the heart. Only, as we shall see in the sequel, the time- relations of this excitability a,re of interest, for the existence of a refractory period — ^that is, an interval during which the cardiac muscle refuses to respond to excitation — ^throws light upon the rhythmicity of the heart-beat. The tonicity of the heart — i.e., its power of remaining contracted to a certain extent in the intervals between successive beats^ — is another property of great importance in certain aspects, but which only needs to be mentioned at present. Automatism of the Heart-Beat — ^Neurogenic and Myogenic Hypo- theses. — That the heart-beat is automatic is sufficiently shown by the fact that, as already mentioned, an excised and empty heart will go on beating for a time, for many hours or even for days in the case of cold-blooded animals. When blood, or even a suitable solution of such inorganic salts as exist in serum, is caused to circu- late through the coronary vessels of the excised heart of a warm- blooded animal, it also continues to contract for a long time. In trjdng to understand the real significance of the automatic beat of the heart, physiologists have endeavoured, first, to compare different portions of the heart as regards the degree in which they possess this property of automaticity ; and, second, to associate, if possible, one or other of the active tissues that compose the organ, muscle, and nervous tissue, with this characteristic property. It cannot be pretended that a final answer to this question is possible at present. Nor is the historical controversy which it has occasioned perhaps as important in itself as the space usually devoted to it in textbooks might imply. Yet it is probable that the series of fundamental facts in the physiology of the heart elicited in the long discussion can be best presented, even for the purposes of the elementary student, as they were originally brought forward in the form 6f pros and cons, of arguments for and against the neurogenic or the myogenic , hypothesis. There is good evidence that as in the amphibian ^ heart the contraction starts in the sinus venosus, so in the mam- malian heart it starts in the sinus tissue of the right auricle in the region of the sino-auricular node. Attempts have been made to demonstrate that the origination of the impulses which are after- wards conducted to all parts of the heart is normally confined to THE HEART-BEAT IN ITS PHYSIOLOGICAL RELATIONS 143 the node itself, and the sino-auricular node is by some authors denominated the pace-maker of the heart, the tissue which sets the pace for the rest of the organ and gives the time to auricles and ventricles alike. The experimental results, however, are by no means harmonious, some observers finding that destruction of the region of the node causes no change in the rate of the heart-beat, others that the beat is permanently slowed. But even were we in a position to sharply delimit a given region of the heart as the point at which the strong tendency to contraction inherent in the cardiac tissue as a whole first breaks into an actual beat, this would scarcely enable us to decide offhand where the cause of the automatism resides, in the muscular tissue or in the intrinsic nervous apparatus, because in nearly all animals hitherto investigated the muscular tissue, ganglion-cells, and nerve-fibres are inseparably intermingled. In Limulus, however, the horseshoe or king crab, the cardiac ganglion-cells are collected in a nerve-cord running longitudinally in the median fine along the dorsal surface of the segmented heart, and Fig. 63. — The Heart and the Heart Nerves of Limulus: Dorsal View (Carlson). (The heart is figured one-half the natural size of a large specimen.) aa, Anterior artery; la, lateral arteries; In, lateral nerves; mnc, median nerve-cord; os, ostia. sending off at intervals branches to two lateral cords, and also branches which enter the heart muscle directly (Fig. 63). When the median nerve-cord is removed, as can be done without injuring the muscle, the heart ceases for ever to beat spontaneously. It still contracts when directly stimulated, mechanically or electrically, but the contraction never outlasts the stimulation. The automatic power therefore resides in the nerve-cord alone, and not in the muscle. The same is true of the rhythmical power, for excitation of the nerves that pass from the median cord to the muscle produces, ' not a rhythmical series of beats in the resting, and an acceleration of the rhythm in the pulsating heart, but a tetanus closely resembling that produced in skeletal muscle on stimulation of a motor nerve ' (Carlson). Conduction and co-ordination are also effected in this heart through the nervous mechanism, and essentially through the median nerve-cord; for section of the longitudinal nerves in any segment of the heart abolishes the co-ordination of the two ends of the heart on either side of the lesion, while division of the muscle in any segment does not affect the co-ordination. It is not per- missible to transfer these results wholesale to higher hearts, and 144 THE CIRCULATION OF THE BLOOD AND LYMPH especially the conclusions as to rhythm, conduction, and co-ordina- tion. Nevertheless the Limulus heart affords one absolutely un- ambiguous example of a heart whose rh5^hmical beat is sustained by nervous imptfises. And in the case of the higher animals also facts may be adduced in favour of the neurogenic origin of the beat. The isolated auricular appendices of the mammaUan heart, in which no ganglion-cells have been found, refuse to beat spontaneously. If in the frog we divide the sinus, which is conspicuously rich in gangUon-cells, from the lower portion of the heart, it continues to pulsate. A fragment from the base of the ventricle will go on contracting if it includes Bidder's ganglion, but not otherwise. We cut off the lower two-thirds of the frog's ventricle, the so-called apex preparation, which either contains no ganglion-ceUs or is relatively poor in them, and it remains obstinately at rest. Further, if, without actually cutting off the apex, we dissever it physiologically from the heart by crushing a narrow zone of tissue midway between it and the auriculo- ventricular groove, we abolish for ever its power of spontaneous rhj'thmical contraction. The frog may hve for many weeks, but in general the apex remains in permanent diastole. It can be caused to contract by an artificial stimulus, but it neither takes part in the spontaneous contraction of the rest of the heart, nor does it start an independent beat of its own. What can be simpler than to assume that the sinus beats because it has numerous ganglion-cells in its walls, and that the apex refuses to beat because it has comparatively few or none ? Could we pick out the nerve-cells from the sinus, without injuring the muscular tissue, as easily as we can extirpate the median nerve-cord in Limulus, we may well suppose that it would lose its power of auto- matic contraction. And although, if we pursue our investigations a little farther, facts may emerge which seem to contradict the neurogenic hypothesis, the contradiction is usually only apparent. Let us inquire, for instance, what happens to the auricles and ven- tricle of the frog's heart when the sinus is cut off. The answer is that as a rule, while the sinus goes on beating, the rest of the heart comes to a standstill, in spite of the numerous ganglion-cells in the auricular septum and the auriculo-ventricular groove. Not only so, but if the ventricle be now severed from the auricles by a section carried through the groove, it is the former, poor in nerve-cells though it be, which Will usually first begin to beat. We shall again have to discuss this experiment (p. 165). It, at any rate, cannot be interpreted as proving that the automaticity of the heart does not depend upon the presence of ganglion-cells. For although a portion of the heart rich in ganglion-cells may, under the circumstances mentioned, refuse for a time to beat, there is good evidence that this is due either to a pecuhar condition called inhibition into which the muscular tissue or the nerve-cells of the lower portions of the THE HEART-BEAT IN ITS PHYSIOLOGICAL RELATIONS 14^ heart have been thrown by the first section, or more probably to the loss of the accustomed impulses from the sinus which normally give the signal for the auricular contraction. A stronger argument in favour of the myogenic theory is the. fact that the embryonic heart beats with a regular rhj^hm at a time when as yet no ganglion- cells have settled in its walls. But it may well be that this primitive automatic power of the cardiac muscle, absolutely necessary at first, since the early establishment of the circulation is essential for the development of the tissues in general and of the nervous system in particular, falls into abeyance when the intrinsic cardiac nervous mechanism is completed, or at least becomes subordinated to the latter. The advocates of the myogenic theory further state that the isolated bulbus aortse of the frog, and even tiny fragments of it, will pulsate spontaneously, and that the same is true of small portions of the great veins which open into the sinus. The rhyth- mical contraction of the veins of the bat's wing has also been con- sidered an argument in favour of myogenic automatism. In none of these cases, however, can the complete absence of ganglion-cells be considered satisfactorily demonstrated. The statement that a portion of the apex of the dog's ventricle continues for a considerable time to beat with a rhythm of its own when connected -with the rest of the heart by nothing but its bloodvessels and the narrow isthmus of visceral pericardium and connective tissue in which they lie has not been confirmed by all observers. But even if it be accepted, it can hardly be used as a decisive argument against the neurogenic theory so long as the absence of ganglion-cells from such a ventricular strip has not been demonstrated. The fact that under the influence of a constant stimulus portions of the heart can be made to beat rhythmically has been sometimes, though erroneously, brought forward as evidence of myogenic automatism. Thus the supposedly ganglion-free apex of the frog's heart, Ufeless as it seenns when left to itself, can be caused to execute a long and faultless series of pulsations when its cavity is distended with defibrinated blood or serum, or certain artificial nutritive fluids, or even physiological salt solution. The passage of a constant current through the preparation may also start a regular rhythm. But apart from the question whether nervous elements would not be subjected to the constant stimulus impartially with the muscular elements (and nerve-fibres, at any rate, are acknowledged to be present), the beat here produced ought not to be considered as an automatic beat, but as a contraction evoked by an external stimulus. Such experiments, in fact, throw no light- upon the automatism of the heart, but prove clearly its rhythmicity-^i.e., its power of responding to a continuous stimulus by regularly recusing con- tractions. While we are hardly at present in a position to dis- criminate sharply between the influence of constant stimulation 146 Tti^ ClRtHLATron OF THE BLOoD AIND LYMPH upon the nervous and upon the muscular elements of the heart, and certainly not in a position to deny to the nervous elements the power of responding to such stimulation by rhythmical discharges, it can hardly be doubted that the cardiac muscle itself possesses rhythmical power. This is a property which also belongs to the smooth muscle of such tubes as the ureter, whose rhjH:hmical con- traction is affected by distension much as that of the heart is, and in a smaller degree even to ordinary skeletal muscle, which can contract with a kind of rhythm under the stimulus of a certain tension and in certain saline solutions. But just as the primitive automatism of the cardiac muscle may have become subordinated in the course of development to the automatism of the nervous elements, so the primitive rhythmical power of the muscle may under ordinary conditions remain in abeyance and yet be capable of asserting itself in favourable circumstances, and when the normal rhythmical impulses from the nervous apparatus are withdrawn. In any case, in the normally beating heart the opportunity for the exercise of the rhythmical power of the muscle does not arise, at least in the case of the lower portions of the heart. For the impulses Which (in the frog's hea'-t), descending from the sinus, liberate the contraction of the auricles, and the impulses which, descending from the auricles, liberate the contraction of tfie ventricle, appear to be discrete, and not continuous; in other words, the lower portions of the heart do not receive from the upper portions a continuous stream of stimuli to which they respond by rhythmical contractions, but a series of rhythmically repeated impulses, each of which evokes a single contraction. One of the best proofs of this is that if the sinus is heated the ventricle beats much more rapidly in unison with the rapidly beating sinus and auricles, while if the ventricle itself is heated no change takes place in its rhythm. Now, if the ventricle responds to a constant stimulus by rhythmical beats, the condition of the ventricular tissue ought to affect the rate of its beat. In the mammalian heart, too, an alteration in the temperature of a definite area of the wall of the right auricle lying between the mouths of the venae cavae produces a change in the rate of the whole heart, while no effect is caused by altering the tempera- ture of any other portion of the heart. It has already been stated that the impulses from the nerve-cord which maintain the rhythm in the Limulus heart are also discontinuous. Conduction and Co-ordination. — The question of the conduction of the excitation over the heart and the co-ordination of its parts is in the same position as the question of the automatism and rhj^hmicity. In the horseshoe crab, as already remarked, the mechanism appears to be a nervous one. In higher hearts, on the other hand, facts have heen discovered which favour each of the rival hypotheses. In the frog's heart the probabihty that the con- THE HEART-BEAT IN ITS PHYSIOLOGICAL RELATIONS 147 traction wave is propagated from fibre to fibre of the muscle without the intervention of nerves has been much insisted upon, since the muscular tissue, although presenting certain variations in its character in the different divisions of the heart and at their junctions, forms a practically continuous sheet over the whole organ from base to apex. In support of this view has been brought forward the observation that the delay of the wave at the auriculo- ventricu- lar groove is much greater than it ought to be if the excitation were transmitted by nerves, since the velocity of the nerve-impulse is exceedingly great (p. 767) ; and the further observation that, when the ventricle is caused to contract by artificial stimulation of the auricle, this delay is appreciably greater when the stimulus is applied as far from the ventricle as possible than when it is applied as near to it as possible. The delay has been attributed to the ' embryonic ' character of the muscular tissue at the junction of the sinus with the auricles and of the auricles with the ventricles. But it has never been demonstrated that muscular fibres with the histological char- acters described do, as a matter of fact, conduct the contraction wave so much more slowly than the other cardiac muscular fibres. It is just as probable, and indeed more so, that, whether the con- traction travels in any particular division of the heart directly from muscle-fibre to muscle-fibre or not, the impulse to contraction is transferred from each division of the heart to the next by a nervous mechanism whose action is timed with the very object of securing a certain interval between the systoles of successive divisions. In any case, since we know that the velocity of the nerve-impulse is very different in different varieties of nerves, the question cannot be decided by general arguments of this kind. In Limulus, as a mEltter of fact, the velocity in the intrinsic heart nerves is only one-tenth as great as in the ordinary motor (limb) nerves of the animal (Carlson). In the mammalian heart the alleged absence of muscular con- nection between the auricles and ventricles was long the foundation of the general belief that the link_ was a nervous one. Certainly there is no dearth of nerves which might serve as such a bridge. But it has been shown (Kent, His, etc.) .that in the mammahan heart, too, a slender band of muscular fibres, arising at a definite point (the auriculo- ventricular node) near the coronary sinus on the right side of the interauricular septum below the fossa ovalis, passes forwards and downwards through the fibrous ring between the auricles and ventricles under the septal cusp of the tricuspid valve. It then divides into two branches, one for each ventricle, which run down the interventricular septum towards the apex, spreading out as the Purkinje fibres or their equivalent, to blend at last with the ordinary muscle of the ventricles, and particularly of the inter- ventricular septum. The fibres of the bundle are narrower than the other fibres of the auricles, very rich in nuclei, and only slightly 148 THE CIRCULATION OF THE BLOOD AND LYMPH differentiated into fibrillae. They seem to represent the remain's of the primitive cardiac tube, which by the development of certain pouches and twists becomes transformed into a multi-chambered heart. Their resemblance to embryonic fibres suggests that they may have retained the primitive capacity of the mesodermic tissue of the embryonic heart to conduct, and even to originate, the rhythmical contraction. But while there is no decisive evidence that they constitute an automatic cardio-motor centre, as some authors have supposed, they, or at least the narrow bridge of tissue in which they lie, do play an important part in the conduction of the contraction from the auricles to the ventricles. For compres- Fig. 64. — Right Auricle and Ventricle of Calf, to show Auriculo-Ventricular Band (Keith). I, central cartilage; 2, main auiiculo-ventricular bundle ; 3, auriculo- ventricular (A-V) node; 4, right septal division of the bundle; 5, moderator band; 6, medial or septal cusp of tricuspid valve; 8, coronary sinus. sion of the band produces a block, just as the pressure of a clamp in the auriculo- ventricular groove does in the frog's heart (Kent). With a certain degree of pressure the ventricle beats only once for two beats of the auricle, with greater pressure only once for three or more auricular beats. With a still greater pressure or after crushing or section of the bundle conduction is abolished, and the ventricle either remains at rest for a time, as in the frog's heart, or, what is much more common, immediately starts beating with an independent rhythm, which is slower than that of the auricles (Erlanger). It can be considered certain that in these observations nerves may have been involved in the block as well as the muscle of THE HEART-BEAT IN ITS PHYSIOLOGICAL RELATIONS 149 the auriculo-ventricular band, since this band is richly provided with nerve- fibres as well as ganglion- cells (Wilson). Yet it is unlikely that all the nerves capable of conducting the impulses to contraction should be .ga,thered into such a narrow compass, and therefore the experiment supports the view that the conduction is Fig. 65. — Jugular (Upper) and Carotid (Lower)(Pulse-Tracing from[^a Case of Arterio- .sclerosis, showing Partial Failure of Conduction in the Auriculo-Ventricular Bundle (Cushny and Grosh). The ventricle only beats once to two beats of the auricle. Time-trace, fifths of a second. carried out in the muscular tissue. And if the conduction of the excitation from auricles to ventricles is accomplished by a muscular conjUection, it is natural to suppose that the co-ordination of sym- metrical portions of the heart on either side of the longitudinal aixis. Fig. 66. — Tracing of Jugular (Upper) and Radial (Lower) Pulse from a Man with Heart-Block (Lewis and Macnalty). In the cycles marked 34, 35, and 36 the ventricular contraction, although less frequent than the auricular, was initiated from the auricle. In the last two cycles[(37 and 38)'and'the pause of 36 complete heart-block was present. On the jugular trace the a-c interval (representing the interval between the onset of the auricular and ventricular contractions) is given, and on the radial trace the duration of a cardiac cycle, both in fifths of a second. the co-ordination in virtue of which the two auricles contract together and the two ventricles together, is also achieved by the passage of impulses through the muscular tissue. In accordance with this, it has been shown that the ventricles in the dog and cat continue to beat in unison, after the attempt has been made to 150 THE CIRCULATION OF THE BLOOD AND LYMPH sever any nerves, connecting them by extensive zigzag incisions, so long as they are united by a narrow bridge of muscle (Porter). In disease, interference with the conduction of the stimulus from auricles to ventricles along the atrio-ventricular bundle is a not un- common phenomenon. According to the degree of interference, the ventricular contraction may be simply delayed, or only a certain pro- portion of the auricular contractions (every second, every third, or every fourth) may be conducted to the ventricle, or, finally, the block may be complete, and the ventricle then contracts quite independently of the auricle, the stimulus to contraction originating, perhaps, in the uninjured portion of the bundle below the seat of the block. These ccnditions are most easily recognized by comparing tracings simul- taneously obtained from the jugular vein and the radial artery or apex- beat (p. go). When the block is complete the rate of the ventricle is very slow (about 30 in the minute, or less), the time of the ventricular beat is clearly unrelated to that of the auricular, and the stability of the ventricular rhythm is abnormally great, such circumstances as usually cause a marked increase in the pulse-rate — mental excitement, for instance — affecting it little or not at all. This is the condition in Fig. 67. — Polygraph Tracing" from~a Case of True Bradycardia (Carter). The lower trace is the radial, the upper the jugular. Time-trace, fifths of a second. the so-called Stokes-Adams disease. In some of these cases pathological (syphilitic) changes in the A-V bundle have actually been discovered at necropsy. In others there 'is some reason to believe that abnormal excitation of the cardio-inhibitory nerves may be responsible even fcr long-continued block, especially when the conductivity of the bundle has been already permanently diminished. Cases of slow heart are also known in which there is no block in the conduction system, but the original rhythm of the auricle isfelow (so-called true bradycardia, Fig. 67). Kent has pointed out that the muscular connection between the auricles and ventricles is not single and confii.ed to the A-V bundle, but multiple, and that the co-ordinated action of the chambers of the heart is to some extent dependent upon the integrity of muscular connections other than that which exists in the A-V bundle. One of these he describes as the ' right lateral connection,' at the junction 01 the right auricle, the right ventricle, and the tricuspid valve, at the right-hand margin of the heart. The existence of this additional con- nection, the importance of which relatively to that of the A-V bundle need not be the same in every heart, may explain otherwise puzzling results'- both clinical and experimental — e.g., that sometimes co-ordina- THE HEART-BEAT IN ITS PHYSIOLOGICAL RELATIONS 151 tion between the ventricles and auricles has continued after destruction of the A-V bundle, while sometimes co-ordination has been upset by lesions not affecting the bundle. Fibrillary Contraction. — In the case of the warm-blooded heart a complete breakdown of co-ordination occurs under certain circum- stances, producing the phenomenon known as fibrillary contraction, or delirium cordis, a condition in which each minute portion, perhaps each fibre, of the whole heart, or of a portion of it, goes on contract- ing in a disorderly manner, quite independently of the rest. The condition is often seen in a heart that has been exposed for some time, particularly in the ventricle, and can be induced by stimulating it with strong induction shocks or by ligation of the coronary arteries. According to the best evidence, the condition is due to the fact that the conductivity of the fibrillating muscle is interfered with so that the contraction wave is prevented from running its usual course. The consequence of this ' blocking ' is that the normal co-ordinated action of the musculature gives place to the confused movement characteristic of fibrillation (Porter, Garrey). There is no reason to believe that fibrillary contraction is connected with the loss of impulses from any special co-ordinating centre, for it is not peculiar to the heart, but is typically seen in the tongue when the circulation after a long interruption is restored. The peculiar ' boiling ' movement is exactly similar to that observed in the heart, probably because the tongue also contains fibres running in several directions. The confused fibrillary contractions are quite ineffective for driving on the contents of the heart. Fibrillation of the ventricle is therefore incompatible with life. On the other hand, auricular fibrillation, far from being immediately fatal, is one of the most common of the chronic cardiac disorders in man. It is characterized by extreme irregularity of the pulse, due to the fact that the ventricles are played upon by an irregular stream of impulses from the fibrillating auricles to which they respond as they best can. The auricular wave (a, Figs. 65-67) is absent from the jugular pulse-tracing, and the P wave (Fig. 809), corresponding to the electrical change produced by the normally contracting auricles, is absent from the electrocardiogram. Auricular flutter is a condition which must be distinguished from auricular fibrillation. When a weak stimulus is applied to the auricle of a dog or cat, the auricular beats are greatly increased in frequency up to 300 or 400 a minute. Although the beats are so rapid, they are otherwise normal beats. When the strength of the stimulus is increased, this condition of flutter (MacWilliam) passes into fibrillation^^ Auricular flutter is also recog- nized clinically. In the majority of cases the ventricle does not respond to each beat of the auricle, and the arterial pulse is irregular; but each auricular contraction produces its appropriate effect upon the electro- cardiogram and often also upon the jugular tracing (Mackenzie). Without entering further into a discussion of the rival hypotheses, we may sum up by sajdng that for one heart {that of Limulus) the automatism and the rhythmical power have been clearly shown to reside 152 THE CIRCULATION OF THE BLOOD AND LYMPH in the local nervous apparatus ; for the hearts of other animals full and formal proof of the neurogenic theory, so far as those two properties of the cardiac tissue are concerned, has not been given. It is probable, but not proven. As regards the conduction and co-ordination of the contraction, the bulk of the evidence {leaving the Limulus heart out of account) points to the muscular tissue as the channel through which the effective impulses pass. The normal order or sequence in which the different parts of the heart contract depends upon the fact that the automatism of the upper portions is more pronounced than that of the lower, so that under strictly physiological conditions the contraction is only propagated, and not originated, by the lower parts of the heart. When, however, the signal to contraction normally given by the basal region is prevented from reaching the lower parts, an inde- pendent automatic rhythm of the latter may be developed, as in the case of the mammalian ventricle mentioned above. Here we may suppose that the automatic mechanism of the lower portions of the heart discharges itself as soon as a sufficient accumulation of energy has taken place in it, although it requires a longer time to reach the point of discharge than the automatic mechanism of higher parts, and therefore is normally discharged from above. Under certain conditions the normal sequence can be reversed. In the heart of the skate it is easy, by stimulating the bulbus arteriosus, to cause a contraction passing from bulbus to sinus. The power of pro- pagating the contraction may also be artificially altered. As already mentioned, it may be diminished or abolished by pressure. The same effect may be produced by fatigue or cold, while heating a portion of the heart in general increases its power of conducting the contraction. Chemical Conditions of the Beat. — When we have localized the essential mechanism of the rhythmical beat in the nervous or in the muscular elements, the question may still be asked what the chemical and physical conditions are which are necessary to its maintenance. While it is known that a supply of arterial blood at or near body-temperature, and under a sufficient pressure, is required for permanent cardiac contraction, much simpler solutions will suffice to maintain the activity even of the isolated mammalian heart for a considerable time. One of the best of these is a solution contain- ing sodium chloride, potassium chloride, calcium chloride, and sodium bicarbonate in the proportions in which they exist in blood-serum, with the addition of a small quantity of dextrose (Locke, p. 66). When this solution, properly oxygenated and warmed, is circulated through the coronary vessels of an excised rabbit's or cat's heart, strong and regular beats may be observed for many hours. Some investigators have claimed for sodium chloride, and even for sodium ions, others for calcium salts or calcium ions, a special r61e in the origination or maintenance of the rhythmical beat. There is no doubt that strips from the ventricle of the tortoise or turtle, which THE HEART-BEAT IN ITS PHYSIOLOGICAL RELATIONS 153 after isolation have ceased beating, and if left to themselves in a moist chamber do not develop rhythmical contractions, begin after a while to beat when immersed in or irrigated with a solution of sodium chloride or a solution of cane-sugar containing a little of that salt. They refuse to beat in any solution which does not contain sodium chloride (Lingle). The addition of calcium chloride to the sodium chloride solution, or preliminary treatment of the strip with a solution of a calcium salt before its immersion in the sodium chloride solution, hastens the onset of the contractions, and increases the length of time for which they are kept up (Erlanger). It is unquestionable that for the normal beat of the heart the presence of both salts is one of the necessary conditions, but there is at present no sufficient foundation for the view that either the one or the other acts as a special chemical excitant of the automatic contraction. Still less necessary is it to make this assumption for potassium. Certain potassium salts are, of course, beneficial to the heart as to other tissues. This might be assumed from their presence in blood and lymph, and it has been shown experi- mentally. But a terrapin's heart will continue beating, and beating well for a considerable time when irrigated with a solution con- taining sodium and calcium salts alone and free from potassium. That the reaction of the perfusive fluid is of great importance in connection with the origination of rh5H:hm is well established, and it is an interesting fact that the limits of H ion concentration within which the development of spontaneous beats is possible differs for the hearts of different kinds of animals (Mines), and even for the different portions of the frog's heart (Dale and Thacker). While these facts illustrate the importance of the inorganic com- position of the nutritive liquids for the action of the heart, they leave the old question of the existence and the nature of an inner stimulus to the rhythmic contraction very much where it was. Resuscitation of the Heart. — Not only can the beat of the freshly- excised mammalian heart be long maintained by artificial circulation, but many hours or even some days after somatic death pulsation may be restored by the perfusion of such a solution of inorganic salts as Locke's through the coronary vessels. Kuliabko in this way was able to restore a rabbit's heart which had been kept forty- four hours in the ice-chest. Even after an interval of three to five days from the death of the animal, in other experiments, pulsation returned in certain parts of the heart, while twenty hours after death from double pneumonia the heart of a boy three months old was restored, and went on beating for over an hour. He obtained also more or less complete restoration of the beat in the hearts of persons dead from bronchitis combined with peritonitis or menin- gitis, and from cholera infantum, but was unsuccessful in cases of diphtheria comphcated with septicaemia or erysipelas, and in cases 154 THE CIRCULATION OF THE BLOOD AND LYMPH of pleurisy with effusion. It is to be remarked, however, that although beats of a kind can be obtained a long time after death, they are either confined to the auricles or to portions of them, or, if they involve the ventricles too, thqy are only shallow and local contractions, especially seen in the neighbourhood of the larger coronary vessels, and are utterly inadequate to the maintenance of an efficient circulation. The heart can also be resuscit5.ted in situ for some time after complete stoppage without the injection of any solution by clamping the aorta in the thorax and practising direct cardiac massage, the lower end of the animal at the same time being elevated to a,llow blood to pass out of the engorged abdominal veins to the right auricle. The clamping of the aorta permits a sufficient pressure to be attained for the filling of the coronary arteries. The injection of adrenalin into the blood has also been recommended as •a means of raising the blood-pressure by constricting the small arteries, and stimulating the action of the cardiac muscle. The possibility of restoration of the mammalian heart many hours after somatic death has been considered by some a strong argument for the myogenic theory of cardiac automatism, since, they say, it is improbable that ganglion-cells, elsewhere such physiologically fragile structures, should in the heart retain their vitality for so long a time. But it is easy to overdo this argument, and we must not assume without proof that ganglion-cells in all parts of the body have an equal capacity of survival. Indeed, we know that there are great differences, the nervous mechanism concerned in respira- tion, e.g., being capable of restoration when the circulation is re- newed after total anaemia of the brain and cervical cord lasting for as much as an hour (in cats), while the nervous mechanism con- cerned in voluntary movements cannot be completely restored even after a much shorter interval. It is very probable that the cardiac ganglia, if the all-important automatic function of the heart depends upon them, are, like the cardiac muscle, endowed with exceptional powers of resistance to those changes which constitute death. The possibility also must not be overlooked that the contractions obtained after such long intervals are not truly automatic, but similar rather to the rhythmical beats developed under the influence of pressure in the frog's apex preparation or by immersion in salt solutions of tortoise ventricle strips. In addition to its marked power of rhythmical contraction, the cardiac muscle is distinguished from ordinary skeletal muscle by other pecuHarities. It used to be considered the most striking of these peculiarities that ' it is everything or nothing with the heart ' ; in other words, that the heart muscle, when it contracts, makes the best effort of which it is capable at the time; a weak stimulus, if it can just produce a beat, causing as great a contraction as a strong stimulus. Recent work, however, has indicated that this property THE HEART-BEAT IN ITS PHYSIOLOGICAL RELATIONS 155 is also possessed by the skeletal muscle- fibre. When a whole skeletal muscle is excited either directly or through its motor nerve, it is true that throughout a considerable range increase of stimulus is accompanied by an apparent increase in the strength of contraction. But there is reason to believe that, this is because a larger and larger number of fibres- become involved in the excitation as the stimulus is increased, and not because each fibre responds more and more strongly (Lucas). In skeletal muscle the fibres are completely isolated from each other, and the excitation does not spread from fibre to fibre, as happens in the Jieart. Refractory Period and Extra Contraction of Heart Muscle. — A more characteristic property of the cardiac muscle than the ' all oi: nothing ' law is that a true tetanus of the heart cannot be obtained at all, or only under very special conditions^' When the ventricle of a normally beating frog's heart is stimulated by a rapid series of induction shocks, its rate is generally increased, but there is no definite relation bet ween, the number of stimuli and the number of beats. Many of the stimuli are ineffective. In the same way a portion of the heart, such as the apex of the ventricle, when stimu- lated in the quiescent condition by an interrupted current, responds by a rhythmical series of beats, and not by a tetanus. It is evident that the cardiac muscle, like ordinary striped muscle, is for some time after excitation incapable of responding to a fresh stimulus — i.e., there is a refractory period. But this is immensely longer in cardiac than in skeletal muscle. When the phenomenon is analyzed, it is found that a stimulus falling into the heart muscle between the moment at which the contraction begins and the moment at which it reaches its maximum produces no effect^ — is, so to speak, ignored. When the stimulus is thrown in at any point between the maximum of the systole and the beginning of the next contraction, it causes what is called an extra contraction. The extra contraction is followed by a longer pause than usual — a so-called compensatory pause — wJiich just restores the rhythm, so that the succeeding systole falls in the curve where it would have fallen had there been no extra contraction (Fig. 68). In man, extra systoles followed by compensatory pauses may occur under pathological conditions, giving rise to an important group of cardiac irregularities. These extra systoles may be either auricular or ventricular, the auricle or the ventricle contracting prematurely without waiting for the signal of the sinus rhythm. The analysis of pulse- tracings showing these irregularities has led to results of great physio- logical and clinical interest (Cushny, Mackenzie, etc.), but cannot be dwelt on here. When every second beat is an extra systole, generally weaker than the preceding and the succeeding normal beat, the condi- tion is called pulsus higeminus. The weaker beat is always followed by a compensatory pause of greater duration than that preceding it. From the pulsus bigeminus must be distinguished that form of alterna- ting pulse termed pulsus alternans, in which every second beat is dimin- ished in size, but the intervals separating the beats are of uniform length. 156 TtiE CIRCULATION OF THE BLOOD AND LYMPH The refractory period is shorter for strong than for weak stimuU, and is markedly diminished by raising the temperature of the heart. So that stimulation of the heated Iieart with a series of strong induction shocks may cause,^a tetaniform condition, if not a typical tetanus. The con- traction of the normally beating heart is really a simple contrac- tion, and not a tetanus. The electrical changes correspond t( single contrac (p. 8o6);andw the nerve o: nerve-muscle ■ paration is on the heart,' the muscle responds to each beat by a simple twitch, and not by tet- anus (p. 201). That the cardiac Fig. 68. — Refractory Period,J]^and Compensatory jPauset (Marey). A frog's^heart was stimulated at a point [corre- sponding to tlie nick in the horizontal line below each curve. In i and 2 there was no response; in 3 and 4 there was an extra contraction, succeeded by a compensatory pause. muscle itself, apart from the intrinsic nervous mechanism, shows the phenomenon of ' refractory state ' has been shown in the Limulus heart after extirpation of the ganglion (Carlson). Like ordinary skeletal muscle, the cardiac muscle is at first bene- fited by contraction, perhaps by an ' augmenting ' action of fatigue- products such as carbon dioxide (Lee), so that when the apex is stimulated at regular intervals each contraction is somewhat stronger than the preceding one. To this phenomenon the name of the staircase or ' treppe ' has been given, from the appearance of the tracings (p. 723). Section V. — ^The Nervous Regulation of the Heart (Extrinsic Nervous Mechanism of the Heart). While, as we have seen, the essential cause of the rhythmical beat of the heart resides in the tissue of the heart itself, it is constantly affected by impulses that reach it from the central nervous system. These impulses are of two kinds, or, rather, produce two distinct effects : inhibition, shown by a diminution in the rate or force of the heart-beat, or in the ease with which the contraction is conducted over the heart-wall; and augmentation, or increase in the rate or THE NERVOUS HEGVLATION OF THE HEART 157 force of the beat or in the conductivity. Both the inhibitory and the augmentor impulses arise in the medulla oblongata, and perhap a narrow zone of the neighbouring portion of the cord; and they can be artificially excited by stimulation in this region. They pursue their course to the heart by fibres which may in certain animals be mingled together, but are anatomically distinct. We may, there- fore, divide the extrinsic or external nervous mechanism of the heart into a cardio-inhibitory centre with its efferent inhibitory nerve-fibres and a cardio-augmentor centre with its efferent acceler- ator or augmentor fibres. Both of those centres, as we shall see, have, in addition, extensive relations with afferent nerve-fibres from all parts of the body, including the heart itself. It was in thfe vagus of the frog that inhibitory nerves were first discovered by the brothers Weber seventy years ago, and even now our knowledge of the cardiac nervous mechanism is more com- plete in this animal than in any other. We shall, therefore, first describe the phenomena of inhibition and augmen- tation as we see them in the heart of the frog, and then pass on. to the mammal. In the frog the inhibitory fibres leave the medulla oblongata in the vagus nerve. The augmentor fibres come ofi from the upper part of the spinal cord by a branch from the third nerve to the third sympa- thetic ganglion, and thence find their way along the sympathetic cord to its junction with the vagus, in which they run, mingled with the inhibitory fibres, down to the heart. When the vago-sympathetic in the frog or toad is cut, and its peripheral end stimulated, the heart in the vast majority of cases is stopped or slowed, or its beat is distinctly weakened without slowing. In other words, the rate at working before the stimulation is greatly diminished, or reduced to zero. Such an effect, a diminution of the rate of working, we call Inhibiiion. What precise form the inhibition shall take, whether the stoppage shall be complete or partial, and if partial whether the beats shall be simply weakened without being slowed or both slowed and weakened, appears to depend partly upon the strength of the stimulus used and partly upon the state of the heart itself. Some hearts it may be impossible to stop with weak stimulation, although other signs of inhibition may be distinct; Fig. 69. — Diagram of Extrinsic Nerves of Frog's Heart (after Foster). Ill, 3rd spinal nerve; AV, annulus of Vieus- sens; X, roots of vagus; IX , glosso-pharyngeal nerve; VS, combined vagus and' sympa- thetic; I, .i, and 3, the is't, 2nd, and 3rd sympathetic ganglia. The dark line indi- cates the course of the sym- pathetic fibres. The arrows show the direction of the aug- mentor impulses. it may be, any marked which the heart was 158 THE CIRCULATION OF THE BLOOD AND LYMPH while they are readily stopped by stronger stimulation. In other cases the strongest stimulation may not produce complete standstill. Again, the inhibitory effect produced on a heated heart by a given strength of stimulation of the vagusmay be greater than that caused in a heart at the ordinary temperature or a cooled heart. This is especially evident on the auricular tracings when these are recorded separately from those of the ventricle. Even on the verge of heat standstill of the heart inhibition is easily obtained (Pig. 71). Some writers have assumed that the different inhibitory effects produced by the vagus are due to the existence in it of separate groups of fibres, some affecting only the rate of the contraction, others its strength, Fig. 70. — Tracing from Frog's Heart. A, auricular, V, ventricular tracing. Sinus stimulated (primary coil 70 mm. from secondary). Heart at temperature ii'2° C. Complete standstill. The time-tracing between the curves marks intervals of two seconds. others still the conductivity of the muscular tissue and its excita- bility. This theory has enriched the vocabulary of physiology with a number of sonor6us terms derived from the Greek, but has not otherwise established itself, although it has been useful in emphasizing the fact that the inhibitory nerves can influence the heart-beat in several distinct ways. But there are other points of importance to be noted in regard to this inhibition; (i) It does not begin for a little time after stimulation has begun. In other words, there is a distinct latent period; and the length of this latent period is related to the phase of the heart's TllE ISTERVOVS REGULATIOif OP THE HEART 159 contraction at which the stimulus is thrown in, and to the rate at which the heart is beating. As a general rule, the heart makes at least one beat before it stops. (2) The inhibition does not continue indefinitely, even if stimula- tion of the nerve is kept up. Sooner or later, and usually, in fact, after an interval of a few seconds, the heart begins again to beat if it has been completely stopped, or to quicken its beat if it has only been slowed, or to strengthen it if the inhibition has only weakened the contraction, and it soon regains its old rate of working. Not only so, but very often there follows a longer or shorter period during which the heart works at a greater rate than it did before the inhibi- tion, and this greater rate of working may be manifested by increased Fig. 71.— ^Activity of Vagus on Verge of Heat Standstill. Auricular and ventricular coutractiollS| 0i toad's heart recorded. Heart at 34-5° C v 50, stimulation of vagus {distaniie'of coils 50 mm.). The ventricle was already in heat standstill; j the auricle was at once inhibited. Then follows secondary augmentation (due ■'•to-the- sympathetic fibres), during which the ventricle also resumes beating. An interval of a minute elapsed between the first and second parts of the tracing, during which the heart remained at 34'5° C. The auricle was almost in standstill (contractions can still he seen on the curve with a lens), when the vagus was again stimulated at v 50 with the same distance between the coils. Complete inhibition followed by secondary augmentation. frequency of beat, or increased strength of beat, or by both. When the temperature of the heart is low, increased frequency; when it is high, increased strength, is generally seen during this period of secondary augmentation. * The cause of this secondary augmentation, and of the primary augmentation sometimes seen in fresh prepara- tions and often in hearts that have been long exposed (Fig. 73), excited much speculation before it was known that sympathetic fibres existed in the vagus. There is no longer any doubt that it is due to the stimulation of these accelerator or, as it is better to call * Augmentation, is termed ' secondary ' when it is preceded by inhibition, ' primary ' when it is not so preceded. l6o THE CIRCULATION OF THE BLOOD AND LYMPH them (since mere acceleration is not the only consequence of their stimulation), augmentor fibres in the mixed nerve. For (i) excita- tion of the roots of the vagus proper within the skull, and therefore above the junction of the sympathetic fibres, causes no secondary augmentation, or very little, and the inhibition lasts far longer than when the mixed trunk is stimulated. (2) Excitation of the upper or cephalic end of the sympathetic cord before it has joined the vagus causes, after a relatively long latent period, marked augmenta- tion. And if the contractions of the heart are registered, the tracing bears a close resemblance to the curve of secondary augmen- tation following excitation of the mixed nerve on the other side with an equally strong stimulus and for an equal time. (3) When the vago-sympathetic is stimulated weakly there is little or no Fig. 72. — Frog's Heart: Vagus stimulated. Temperature of heart 8° C; 78 mm. between the coils. Dimiaution in force of auricle and ventricle, but not com- plete standstill. Time-tracing shows two-second intervals. secondary augmentation. Now, it is known that the augmentor fibres require a comparatively strong stimulus to cause any effect when they are separately excited, whereas a weak stimulus will excite the inhibitory fibres. The question arises at this point, why it is that, when the inhibitory and augmentor fibres are stimulated together in the mixed nerve (and the same is true when the sympathetic on one side and the vagus on the other are stimulated at the same time), the inhibitory effect always comes first, when there is any inhibitory efiect, while the augmentation always has to follow. The answer has sometimes been given, that the latent period of the augmentor fibres is longer than that of the inhibitory fibres. But although this is certainly the case, the answer is insuffi- cient. For the period of postponement may be much greater than the THE NERVOUS REGULATION OF THE HEART i6i latent period of the sympathetic fibres when stimulated by themselves. The inhibition apparently runs its course without being affected by the simultaneous augmentor effect, which, lying latent until the end of the inhibition, then bursts out and completes its own curve. It is not like the passing of two waves through each other, but rather like the stopping of one wave until the other has passed by. It seems as if augmentation cannot develop itself in the presence of inhibition — ^at least, until the latter is nearly spent. Like a musical-box devised to play a series of melodies in a fixed order, and from which a particular tune cannot be obtained till those preceding it have been run through, the heart, in some way or other, is arranged, in the presence of competing impulses from its extrinsic nerves, to play the tune of inhibition before the tune Fig. 73- — Frog's Heart. A, auricular, V, ventricular tracing. Ventricle beating very feebly. Vagus stimulated (60 mm. between coils). Marked augmentation of ventricular beat. of augmentation. In the frog, at any rate, the two processes can hardly be considered as antagonistic, in the sense that a definite amount of augmentor excitation can overcome a definite amount of inhibitory excitation. Nor is it the case that, when the heart is played upon at the same time by impulses of both kinds, it pits them against each other and strikes the balance accurately between them. It is possible, how- ever, that when the inhibitory fibres are very weakly, and the augmentor 'fibres very strongly stimulated, the amount of inhibition may be some- what diminished. In mammals, on the other hand, a true antagonism seems to exist ; and stimulation of the inhibitory nerves is less effective when the augmentors are excited at the same time. The cardiac nerves affect not only the rate and force of the contraction, but also the con- 1 62 THE CIRCULATION OF THE BLOOD AND LYMPH ductivity of the heart. Thus in the frog's heart during stimulation of the vag^s, the contraction passes more slowly, and during stimulation of the sympathetic more quickly, from auricles to ventricle. In mammals (and in what follows we shall restrict ourselves chiefly to the dog, cat, and rabbit, as it is in these animals that the subject has been most carefully studied) the inhibitory fibres run down the vagus in the neck and reach the, heart by its cardiac branches. They are derived from the bulbar roots of the spinal. accessory, whose inner branch joins the vagus. The augmentor fibres leave the spinal cord in the anterior roots of the second and third thoracic nerves, and possibly to some extent by the fourth and fifth. Through the corresponding white rami communicantes they reach the sympa- thetic cord, and running up through the stellate ganglion (first thoracic), and the annulus of Vieussens, which surrounds the subclavian artery, to the inferior cervical ganglion, they pass off to the heart by separate ' accelerator ' branches, taking origin either from the annulus or from the inferior cervical ganglion. Some augmentor fibres are often, if not always, present in the dog's vago-sympathetic in the neck. It is especially easy to demon- strate their presence five or six days after section of the nerve, when the excitability of the inhibitory fibres has disappeared. In the dog the vagus and cervical sym- pathetic are, in the great majority of cases. Contained in a strong common sheath, and pass together through the inferior cervical ganglion. Upon opening this sheath they may with care be separ- ated, the fibres running in distinct strands, and not mixed together as in the vago- sympathetic of the frog. For some dis- tance below the superior cervical ganglion the cervical sympathetic is not connected with the vagus, and here the nerves may be separately stimulated without any artificial isolation . In the rabbit and some other mammals, including man, the vagus and sympathetic run a separate course in the neck. Fig.''74.' — Diagram of Cardiac Netves in the Dog (after Fbster). II, III, seootid and third dorsal nerves; SA, sub- clavian Artery ; AV, annulus of Vieussens; ICG, inferior cer- vical ganglion; CS, cervical sympathetic; i, first thoracic or stellate ganglion of the sympathetic; 2, second thora- cic ganglion; Ac, accelerator or augmentor fibres passing off towards the heart; X, roots of vagus; XI, roots of spinal accessory; JG, jugiilar gan- glion; GTV, ganglion trunci vagi; In., inhibitory fibres passing off towards the heart. The effects of stimulation of the vagus or vago - sympathetic in the mammal are very much the same as in the frog, except that secon- dary augmentation is in general less marked, though often present in some degree, and that in the mammal the inhibitory fibres have a smaller direct action on the ventricle. It indeed beats more slowly when the auricle is slowed, but this is only because in the normally beating heart the ventricle takes the time from the auricle. The strength of the ventricular contractions may be not at THE NERVOUS REGULATION OF THE HEART 163 all diminished, even when the auricle is beating very feebly during inhibition. When the auricle is completely stopped, which does not occur so readily as in the frog, the ventricle also stops for a short time, but soon begins to beat again with an independent rhythm of its own. In the frog the ventricle is directly affected by stimulation of the vagus, and the force of its beats is diminished independently of the inhibitory effects in the auricles (Practical Exercises, pp. 196, 201). The inhibitory fibres, then, influence the heart particularly through the auricles ; they are par excellence auricular nerves. On the other hand, the accelerantes in all mammals which have been investigated not only extend to the ventricles, bu,t are even mainly distributed to them. They are emphatically ventricular fibres, and in accordance with its greater mass the left ventricle receives more fibres than the right. Stimulation of the accelerator nerves in the dog causes an in- crease in the force of both the auricular and ventricular contraction, and as a rule, in addi- tion, some increase in the rate of the beat. As to the nature of the physiological link- age between the cardiac nerves and the mus- cular tissue of the heart we know but Uttle. Ganglion-cells lie on the course of the vagus fibres after they have entered the heart, and although the view has been advocated that they are simply stations where the inhibitory impulses pass from meduUated to non-meduUated fibres, . and where possibly other anatomical changes and rearrangements occur, they may be inter- mediate mechanisms which essentially modify the physiological impulses falling into them. It has been stated that in the dog the right vagus controls chiefly the rate of the heart, and the left vagus chiefly the conduction from auricles to ventricles, and the suggestion has been made that this is because the right vagus has a special relation to the sino-auricular node, in which impulses are supposed to arise, and the left vagus a special relation to the auriculo-ven- Fig. 75. — Blood- Pressure Tracings: Rabbit. Vagus stimulated at i. Stimulus stronger in B than in A (Hiirthle's spring manometer). 1 64 THE CIRCULATION OF THE SLOOD AND LYMPH tricular node, the upper end of the A-V bundle, the main conduction system (Cohn and Lewis). The nervi accelerantes are already non-meduUated before they reach the heart. The fact that the action of the accelerantes can be restored by perfusing the heart with a nutrient solution at a much longer interval after somatic death than the action of the vagus strengthens the suggestion that ganglion-cells are interposed on the inhibitory though not on the augmentor path, without, however, proving of itself that such a difference exists. In one experiment the heart of an anthropoid ape was revived when three successive periods — viz., four and a half, twenty-eight and a half, and fifty-three hours respectively — had elapsed after the death of Jihe animal, although during the last period the heart had been itwice frozen hard. The vagus was shown to be still capable of ;Causing some inhibition six hours after death, and the accelerans isome augmentation as late as fifty-three hours after death (Hering). I In the discussions that have arisen over the relation of the extrinsic to the intrinsic cardiac nervous apparatus appeal has frequently been made to the action of certain poisons on the heart. Thus, after nicotine has been injected subcutaneously, or painted directly on the heart of a frog, stimulation of the vago-sympathetic causes no inhibition; it may cause augmentation. But stimulation Of the junction of the sinus and auricle still causes inhibition, as in the i normal heart. ' Atropine and its allies, such as daturine, not only abolish the inhibi- tory efiect of stimulation of the vagus trunk, but also that of stimula- tion of the junction of sinus and auricle. I Muscarine, a poison contained in certain mushrooms (p. iqy), causes diastolic arrest of the heart, which, when the circulation is intact, be- comes swollen and engorged with blood. This action takes place in a heart already poisoned with nicotine or one of its congeners, but not in a heart under the influence of atropine or its allies. And a heart brought ' to a standstill by muscarine can be made to beat again by the applica- tion of atropine, although not by nicotine. These facts may be explained as follows : Nicotine paralyzes, not the very ends of the vagus, but the ganglia through which its fibres pass. Stimulation of the sinus, which is practically stimulation of the vagus fibres between the ganglion-cells and the muscular fibres, is therefore effective, although stimulation of the nerve-trunk is not (Langley). On the other hand, the atropine group paralyzes the nerve-endings themselves, or interferes with the reception of the inhibitory impulses by acting on a so-called receptive substance in the muscle (p. i8o), so that neither stimulation of the sinus nor of the nerve-trunk can cause inhibition. Muscarine, on the contrary, stimulates the vagus fibres between the nerve-cells and the muscle, or the actual nerve-endings, or . exerts an inhibitory action on the muscle itself through the appropriate receptive substance, and thus keeps the heart in a state of permanent inhibition, which is removed when atropine cuts out the nerve-endings, or combines with the receptive substance. It is' quite in accordance : with this that muscarine has no effect on a heart whose vagus nerves, as occasionally happens, have no inhibitory power. Pilocarpine has very much the same action as muscarine. The view that muscarine and atropine can directly affect the cardiac THE NEUVOUS REGULATION OF THE HEART 165 muscle gains a certain amount of support from tlie facts that these drugs act very much in the same way on the heart of the mammaHan embryo (rat, rabbit, etc.) before and after the development of its in- trinsic nervous system, and that the passage of an interrupted current through the heart of very young embryos causes distinct inhibition. But, as has already been pointed out, it is not legitimate to transfer without question to the muscle of the fully developed heart the proper- ties of the embryonic cardiac tissue. And, on the other hand, musca- rine fails to affect the heart in many invertebrate animals — ^for instance, in Daphnia (Pickering). Yet it is probable that, while the various tissues in the heart possess a different susceptibility to one and the same drug, if the dose is large enough it may affect them all. In the Limulus heart, where the question can be most easily tested, it has been found that the selective action of alkaloids, ancEsthetics, and various other substances on the three heart-tissues (gang- lion, motor nerve plexus, and muscle) is one of degree only (Meek). Stannius' Experiment. — Another series of pheno- mena, intimately related to our present subject, have excited, since they were first made known by Stannius, an enormous amount of discussion . The chief facts of this classical experiment we have, al- ready mentioned (p. 144), and they are also described in the Practical Exercises (p. 192). They are easy to verify, but difficult to interpret. The ^nost prob- able explanation of the standstill caused by the first ligature is that the lower portion of the heart, when cut, off from the sinus in which the beat normally originates, needs some time for the development of its automatic power to the point at which an indepen- dent rhythm can be maintained. The effects following the second Stannius ligature seem to depend upon the power of the ventricle to develop and maintain an independent rhythm, but the contractions are supposed by some to be started by stimulation of the muscular tissue in the auriculo-ventricular groove by the ligature. Nature of Inhibition and Augmentation. — So far we have been dis- cussing the phenomena of inhibition and augmentation as ultimate facts. We have not attempted to go behind them, nor to ask what it is that really happens when inhibitory impulses fall into a heart, which from the first days of embryonic life has gone on beating with a regular rhythm, and in the space of a second or two bring it to a standstill. The question cannot fail to press itself upon the mind of anyone who has ever witnessed this most beautiful of physiological experiments ; but as yet there is no answer except ingenious speculations. The rtiost j^lausible of these is the trophic theory of Gaskell, who sees in the vagus Fig. 76. — Frog's Heart. Synlpathetic stimulated (30 mm. between the coils). Temperature 12°. Marked increase in force. Only auricular tracing reproduced. Time-trace, two-second intervals. 1 66 THE CIRCULATION OF THE BLOOD AND LYMPH flTPWfiHI a nerve which so acts upon the chemical changes going on in the heart as to give them a trophic, or anabolic, or constructive turn, and thus to lessen for the time the. destructive changes underlying the muscular contraction. The augmentor nerves, on the other hand, are supposed to exert a katabolic influence, and to favour these destructive changes. And while, according to Gaskell, the natural consequence of inhibition is a stage of increased efficiency and working power when the inhibition has passed away, the natural complement of augmentation is a tem- porary exhaustion. But it must be remembered that this distinction is not as yet based upon any very solid foundation of actually observed and easily interpreted facts. Whatever the exact mechanism of augmentation may be, there is no basis for the statement that the cardio-augmentor nerves have an action on the heart so fundamentally different from the action of motor nerves on skeletal muscle that they cannot originate contractions in a heart entirely at rest. Excitation of the cardio - augmentor, nerves can cause rhyth- mical contractions in the perfectly quiescent heart of molluscs, and a sudden and prolonged outburst of beats of great force in the frog's heart, which has been brought to a standstill by cautiously heating it to 40° to 43° C. (Practical Exercises, p. 192) for a minute or two, or to a consider- ably lower tempera- ture, for a longer time (Fig. 77). A similar effect can be obtained on the quiescent mam- malian heart by stimu- lation of the nervi accelefantes. PS' 5 S30 1 1 1 M I M 1 1 1 I I I I 1 1 1 1 1 1 1 1 1 I T-rm 1 1 1 1 I 1 1 1 1 1 1 1 ITT rfj-rr^''-'^''^- Fig. 77. — Effect of Stimulation of Frog's Cardiac Sym- pathetic during Complete Standstill of the Heart at 28'5° C. Upper tracing, auricle; lower, ventricle. To be read from right to left. Time-trace, two- second intervals. The Normal Exci- tation of the Cardiac Nervous Mechanism. — We have now to in- quire how this elaborate nervous mechanism is normally set into action. And we may say at once that, striking as are the effects of experimental stimulation of the vagus trunk or the nervi accelerantes in their course, it is only under exceptional cir- cumstances that the efferent nerve-fibres, at any rate before they have entered the heart, can be directly excited in the intact body. In certain cases the pressure of a tumour or an aneurism on the nerve-trunks, or, in the case of the accelerators, the progress of a pathological change in the sympathetic gangha through which the THE NERVOUS REGULATION OF THE HEA^T 167 fibres pass, has been thought to bring about by direct stimulation a slowing or a quickening of the pulse. In some individuals the vagus has been excited by compressing it against a bony turnour in the neck; and by compressing the nerve against the vertebral column it is possible to cause inhibition in many normal persons* although it ought to be stated that the experiment is not free from danger. But it is from the cardio-inhibitory and cardio-augmentor centres in the medulla oblongata that the impulses which regulate the activity of the heart are normally discharged. Inhibitory im- pulses are constantly passing out from the medulla, for section of both vagi causes almost invariably an increase in the rate of the heart, at least in mammals, although the increase is less conspicuous in animals like the rabbit, whose normal pulse-rate is high, than in animals like the dog, whose pulse-rate is comparatively low. Section of one vagus usually causes only a comparatively slight increase, for the other is able of itself to control the heart. It is not certainly known whether the augmentor centre in like manner discharges a continuous stream of impulses, or is only roused to occasional activity by special stimuli. For the results of section of the nervi acceler- antes, or the extirpation of the inferior cervical and stellate ganglia, are dubious and conflicting. But if it does exert a tonic influence on the heart, this is feebler than the tone of the inhibitory centre. As to the nature of this inhibitory tone, and the manner in which it is maintained, we know but little. It may be that the chemical changes in the nerve-cells of the inhibitory centre lead of themselves to the discharge of impulses along the inhibitory nerves. But there is some evidence that, in the complete absence of stimulation from without, the activity of the centre would languish, and perhaps be , ultimately extinguished. For when the greater number of the afferent impulses have been cut off from the medulla oblongata by a transverse section carried through its lower border, division of the vagi produces little effect on the rate of the heart. Also, when the upper cervical cord and the brain are resuscitated after a period of ailaemia, the return of cardio-inhibitory tone is tardy in comparison with the return of the truly automatic function of respiration, and does not seem to precede the opening up of the afferent paths to the cardio-inhibitory centre. Indeed, reflex inhibition may be produced at a time when the inhibitory centre has regained none of its tone. The suggestion is that the normal tone of the centre is largely dependent upon reflex impulses. Be this as it may, we know that the activity of the inhibitory centre is profoundly influenced — and that both in the direction of an increase and. of a diminution — ^by impulses that fall into it through afferent nerves and by stimuli directly applied to it. And we may assume that the same is true of the augmentor centre. The common statement that stimulation of the central end of one vagus, the other being intact, produces i68 THE CIRCULATION OF THE BLOOD AND LYMPH distinct inhibition does not hold for all mammals. In dogs this is sometimes the case, but often (under anaesthesia, at any rate) there is little or no inhibition, or even augmentation. In etherized cats, on the other hand, some inhibition is always seen. Of all the afferent ^fibres of the vagus, the pulmonary fibres produce the most marked .reflex inhibition. The cardiac fibres are much less effective. These pulmonary nerves also influence the respiratory and vaso- motor centres. The respiration is temporarily arrested, and the .blood-pressure falls through the dilatation of the small arteries when they are excited. It is of interest in connection with the subject of death during the administration of anaesthetics, that the afferent vagus fibres coming from the alveoli of the lungs can be chemically stimulated when irritant vapours, such as chloroform, hydrochloric acid, ammonia, bromine, or formaldehyde are inhaled through a tracheal cannula, causing reflex arrest of the heart and of the respira- tory movements and a fall of blood-pressure through vaso-dilatation (Brodie). At a certain stage in chloroform anaesthesia, before it ■has become very deep, comparatively trifling causes may bring about great and sudden changes in the pulse-rate, owing to the abnormal mobility of the vagus centre (Mac William). The depressor nerve, a branch of the vagus, which is easily found in the rabbit as a slender nerve running close to the sympathetic in the neck, and a little to its inner side, but in the dog is usually blended with the vago-sympathetic, falls into the same category with the vagus itself as regards its reflex action on the heart, to which it bears an important relation. In all mammals some of its fibres end in the wall of the aorta, but some of them may run down over the heart to the ventricle. Stimulation of its peripheral end has no effect, for the fibres in it which influence the circulation are •afferent, not efferent. But excitation of its central end causes a marked fall of blood- pressure (p. 183), accompanied by, but not essentially due to, a distinct slowing of the heart. If the animal is not anaesthetized, there may be signs of pain, and for this reason the depressor has sometimes been spoken of, somewhat loosely, as the sensory nerve of the heart. The abdominal sympathetic (of the frog) also contains afferent fibres, through which reflex inhibition of the heart can be produced when they are excited mechanically by a rapid succession of light strokes on the abdomen with the handle of a scalpel. On the other hand, when the central end of an ordinary peripheral nerve like the sciatic or brachial is excited, the common effect is pure ■augmentation (Fig. 78), which sometimes develops itself with even ■ greater suddenness than when the accelerator nerves are directly stimulated. Occasionally, however, the augmentation is abruptly , followed by a t jrpical vagus action. Here the reflex inhibitory effect seems to break in upon and cut short the reflex augmentor effect. THE NERVOUS REGULATION OF THE HEART 169 These examples show that certain afferent nerves are especially related to the cardio-inhibitory, and others to the cardio-augmentor, centre, or at least that the central connections of some nerves are such that inhibition is the usual effect of their reflex excitation, while the opposite is the case with other nerves. But it is im- probable that the effect of a stream of afferent impulses reaching the cardiac centres by any given nerve is determined solely by anatomical relations. The intensity and the nature of the stimulus seem also to have something to do with the result. For when ordinary sensory nerves are weakly stimulated, augmentation is said to be more common than inhibition, and the opposite when they are strongly stimulated. And while a chemical stimulus, like the inhaled vapour of chloroform or ammonia, causes in the rabbit Fig. 78.— Myocardiographic Tracing of Cat's Ventricle. The signal line shows the point at which the central end of the brachial nerve was stimulated during resuscitation of the animal after a period of cerebral ana;mia. Some augmenta- tion of the ventricular beat is seen. The notches in the ventricular tracing are due to the artificial respiration. Time-trace, seconds. reflex inhibition of the heart through the fibres of the trigeminus that confer common sensation on the mucous membrane of the nose, the mechanical excitation of the sensory nerves of the pharynx and oesophagus when water is slowly sipped causes acceleration.* The stimulation of thte nerves of special sense is followed sometimes by the one effect and sometimes by the other. To complete the catalogue of the nervous channels by which impulses may reach the cardiac centres in the medulla, we may add that there must be an extensive connection between them and the cerebral cortex since every passing emotion leaves its trace upon the curve of cardiac action. The so-called ' reflex cardiac death,' which is an occasional consequence of intense psychical influences (anxiety, fright, etc.), wa'sin^rrlf^^Hf^""^ students the average pulse-rate (in the sitting position) was increased from 73 to 85 per minute by sipping water. I70 THE CIRCULATION OF THE BLOOD AND LYMPH may be due to the prolonged excitation of the cardio-inhibitory centre, as well as to the disturbance of other centres in the bulb by the cortical storm. It is a remarkable fact, too, and one that can only be explained by such a connection, that although in the vast majority of individuals the will has no influence whatever on the rate or force of the heart, except, perhaps, indirectly through the respiration, some persons have the power, by a voluntary effort, of markedly accelerating the pulse. In one case of this Idnd it was noticed that perspiration broke out on the hands and other parts of the body when the heart was voluntarily accelerated. A rise of blood-pressure due to constriction of the vessels has also been observed. The effort cannot be kept up for more than a short time, and the pulse-rate quickly goes back to normal. It has been recently shown that this peculiar power is more common than has been supposed, and that where it is present in rudiment it can be cultivated, although it is a dangerous acquisition. As an example of the direct action on a cardiac centre of a changed chemical composition of the blood, we may cite the inhibition produced by injection of bile into a vein and revealed in the.slow pulse of many cases of jaundice ; and as an instance of the direct action of a physical change, the slowing of the heart as the blood-pressure rises (p., i86) in asphyxia or on clamping the aorta. The variation in the pulse-rate associated with changes in the position of the body, to which we have already referred (p. 107), is brought about by direct stimulation of the in- hibitory centre by the increase of blood-pressure in the medulla oblongata when a person who has been standing assumes the supine, or even the sitting, posture. But it is also due in part to changes in the amount of muscular contraction, since muscular exercise causes acceleration of the heart either reflexly, through afferent muscular nerves, or by a direct effect of waste products of the metabolism of the muscles on the cardiac centres in the bulb or on the heart itself (P- 275)- Theoretically, quickening of the heart might be caused either by a diminution in the inhibitory tone or by an increase in the activity of the augmentor centre; and slowing of the heart might be due either to a diminution in the aUgmentor tone, if such exists, or to an increase in the activity of the inhibitory centre. So that it is not always easy to interpret such results as we have quoted above. But it would appear that under ordinary conditions the rate of the heart is mainly regulated by the inhibitory centre, which, within a considerable range, can produce variations in either direction. The augmentor mechanism is perhaps merely auxiliary to the inhibitory, being called into action only in emergencies. THE NERVOUS REGULATION OF THE BLOODVESSELS 171 Section VI. — ^The Nervous Regulation of the Bloodvessels (Vaso-Motor Nerves). Just as the muscular walls of the heart are governed by two sets of nerve-fibres, a set which keeps down the rate of working and a, set which may increase it, the muscular walls of the vessels are under the control of nerves which have the power of diminishing their calibre (vaso-constrictor) , and of nerves which have the power of increasing it {vaso-dilator). All nerves that effect the calibre of the vessels, whether vaso-constrictor or vaso-dilator, are included under the general name vaso-motor. These vaso-motor nerves, like the augmentor and inhibitory fibres of the heart, are connected with a centre or centres, which in turn are in relation with numerous afferent nerves. It is convenient to distinguish the afferent nerves which cause on the whole a vaso-cpnstriction and a consequent increase of arterial pressure as pressor nerves, and those which cause on the whole vaso- dilatation, with fall of pressure, as depressor nerves, reserving the terms vaso-constrictor and vaso-dilator for the efferent portions of the reflex arcs. It is through this reflex mechanism that the bloodvessels are mainly influenced, although the endings of the vaso-motor nerves in the smooth muscular fibres or the muscular fibres themselves are sometimes directly affected by sub- stances circulating in the blood. Proteoses, for instance, cause by peripheral action dilatation of the vessels and a fall of blood-pressure (p. 213); suprarenal extract, or its active principle, adrenalin, or epinephrin, constriction, with a rise of pressure (pp. 214, 638). Apo- codeine paralyzes the vaso-motor nerve-endings after a preliminary stimulation, and now adrenalin causes no constriction. Chryso- toxin, an active principle of ergot, causes a marked rise of blood- pressure by stimulating the sympathetic ganglion-cells or the pre- ganglionic fibres of the vaso-constrictor path. Vaso-motor nerves control chiefly the small arteries. They have no direct influence on the capillaries.* Nor has the existence of an effective vaso-motor regulation of the calibre of the veins, except in the portal system, been proved up to this time by any clear and unambiguous experi- ment, although there are grounds on which it has been surmised that the nervous system does influence the ' tone ' of the whole venous tract. These grounds will be mentioned in the proper place. * It is usually taught that the capillaries, being devoid of muscular fibres in their walls, are not supplied with vaso-motor fibres, and that the only kind of active contraction of which they are capable is due to a process analogous to the turgescence of vegetable cells, the thickness of the wall being increased at the expense of the lumen, while the total cross-section of the vessel remains unchanged. It has been asserted, however, that a true contraction, in which both the total section and the lumen are diminished, may be caused in the capillaries of the nictitating membrane of the frog either by direct stimulation or by excitation of vaso-motor fibres in thfe Sympathetic (Steinach and Kahn). 172 THE CIRCULATION OF THE BLOOD AND LYMPH Meanwhile, before describing the distribution of the best-known tracts of vaso-motor fibres and defining the position of the vaso- motor centres, we must glance at the metliods by which our know- ledge has been attained. (i) In translucent parts inspection is sufficient. Paling of the part indicates constriction; flushing, dilatation of tile small vessels. This method has been much used, sometimes in conjunction with (2), in such parts as the balls of the toes of dogs or cats, the ear of the rabbit, the conjunctiva, the mucous membrane of the mouth and gums, the web of the frog, the wing of the bat, the intestines, etc. (2) Observation of changes in the temperature of parts. This method has been chiefly employed in investigating the vaso-motor nerves of the limbs, the thermometer bulb being fixed between the toes. In such peripheral parts the temperature of the blood is normally less than that of the blood in the internal organs, because the opportunities of cooling are greater. The effect of a freer circulation of blood (dilatation of the arteries) is to raise the temperature ; of a more restricted circulation (constriction of the arteries), to lower it. (3) Measurement of the blood-pressure. If we measure the arterial blood-pressure at one point, and find that stimulation of certain nerves increases it without affecting the action of the heart, we can conclude that upon the whole the tone of the small vessel^ has been increased. But we cannot tell in what region or regions the increase has taken place ; nor can we tell whether it has not been 'acconlpanied by- diminution of tone in other tracts. But if we measure simultaneously the blood-pressure in the chief artery and chief vein of a part such as a limb, we can tell from the changes caused by section or stimulation of nerves whether, and in what sense, the tone of the small vessels within thisarea has been altered. For example, if we found that the lateral pressure in the artery was diminished, while at the same time it was increased in the vein, we should know that the ' resistance ' between artery and vein had been lessened, and that the blood now found its way more readily from the artery into the vein. If, on the other hand, the venous pressure was diminished, and the arterial pressure simultaneously increased, we should have to, conclude that the vascular resistance in the part was greater than before. If the pressure both in artery and vein was increased, we could not come to any conclusion as to local changes of resistance with- out knowing how the general blood-pressure had varied. (4) The measurement of the velocity of the blood in the vessels of the part. This may be done by the stromuhr or dromograph, or by allowing the blood to escape from a small vein and measuring the outflow in a given time, or, without opening the vessels, by estimating the circulation-time (p. 135). When changes in the general arterial pressure are eliminated, slowing of the bldod-stream through a part corresponds to increase of vascular resistance in it ; increase in the rate of flow implies diminished vascular resistance . Sometimes the red colour of the blood issuing from a cut vein, and the visible pulse in the stream, indicate with certainty that the vessels of the organ have been dilated. (5) Alterations in the volume of an organ or limb are often taken as indications of changes in the calibre of the small vessels in it. We have already seen how these alterations are recorded by means of a plethysmograph (p. 128). The brain is enclosed in the skull as in ?, natural plethysmograph, and changes in its volume may be registered by connecting a recording apparatus with a trephine hole. THE NERVOUS REGULATION OF THE BLOODVESSELS 173 ■ ■ '(6) For the separation of the effects' of stimulation of vaso-cdnstrictor and vaso-dilator fibres when they are mingled together, as is the case in many nerves, advantage is taken of certain differences between them. For example, the vaso-constrictorS lose their excitability sooner than ■the vaso-dilators when cut off from the nerve-cells to which they belong. So that if a nerve is divided, and some days allowed to elapse beforjs stimulation, only the dilators will be excited. . The vaso-dilators are more sensitive to weak stimuli repeated at long intervals than to strong and frequent stimuli, and the opposite is true of the constrictors. When a nerve containing both kinds of fibres is heated, the excitability of the vaso-Constrictors is increased in a greater degree than that of the dilators; when the nerve is cooled, the dilators preserve their excita- bility at a temperature at which the constrictors have ceased to respond to stimulation (Fig. 79). The Chief Vaso-Motor Nerves. — The first discovery of vaso-motor nerves was made in the cervical sympathetic. When this nerve is Fig- 79- — Plethysmograms : Hind-Limb of Cat (after Bowditch and Warren). To be read from right to left. On the left hand is shown thei effect of slow stimulation of the sciatic (i per second); on the right hand the effect of rapid stimulation (64 per second). In the first case the limb swelled owing to excitation of the vaso-dilators; in the second, it shrank through excitation of the vaso-constrictors. ■cut, the corresponding side of the head, and especially the ear, become greatly injected owing to the dilatation of the vessels. This experiment can be very readily performed on the rabbit, and the changes are most easily followed in an albino. . The ear on the side 'of the cut nerve is redder and hotter than the other;' the main arteries and veins are swollen with blood, and many vessels formerly invisible come into view. The slow rhythmical changes of caKbre, ' which in the' normal rabbit are very characteristically seen in the middle artery of the .ear, disappear for a time after section of the "Sympathetic^ although' they ultimately again become visible (Prac- tical Exercises, p. 215). Stimulation of the cephs^hc end of the cut sympathetic causes a inarked constriction of the vessels and a fall of temperature on the same side of the head. From these facts we know that the cervical 174 THE CIRCULATION OF THE BLOOD AND LYMPH sympathetic in mammals contains vaso-constrictor fibres for the side of the head and ear, and that these fibres are constantly in action. Certain parts of the eye, and the salivary glands, larynx, oesophagus, and thyroid gland, are also supplied with vaso-motor (constrictor) nerves from the cervical sympathetic. It has been asserted that the cervical sympathetic contains some of the vaso-constrictor fibres that supply the corresponding half of the brain and its membranes, but this has been disputed, and some ob- servers deny that the vessels of the brain have any vaso-motor nerves. Non-meduUated nerve-fibres, however, may be seen in and around the walls of the cerebral and spinal bloodvessels, and it is dificult to believe that these have not a vaso-motor function, although this has ^not as yet been clearly demonstrated by experimental methods. It has sometimes been argued that we ought not to expect the brain to be supplied with vaso-motors. For it is enclosed in a rigid box, and the quantity of blood in it can be increased or diminished only to the slight extent to which the cerebro-spinal liquid can be displaced into the Vertebral canal. Important changes in the cerebral blood-supply are therefore brought about, it is said, not by a widening or narrowing of the cerebral vessels, but by an alteration in the velocity of the blood in them as a result of a rise or fall of the systemic arterial pressure. This argument, however, leaves out of account the consideration that in general the brain does not function as a whole, but that certain parts of it may^gften, become active and relatively hyperaemic, while other parts are, inactive and relatively anaemic, and that important changes in the distribution of the blood in the encephalon may Ibe effected, although the total mass of blood in the organ undergoes little or no alteration. It is, of course, true that it is not the absolute quantity of blood in an organ which is a function of its activity, but the rate at which it is renewed. And it is theoretically possible t|jat an organ. at rest should contain as much blood as when it is active, or even more. But such cases, if they exist, are certainly rare. The fact that adrenalin generally constricts the vessels of a perfused brain (Wiggers) is in favour of the existence of vaso-motors. The retina, which from the stand- point of development is a portion of the brain, is undoubtedly suppUed with vaso-constrictor fibres which run in the cervical sympathetic. That the cervical sympathetic contains some dilator fibres is proved by the fact that stimulation of the cephahc end in the dog causes flushing of the mucous membrane of the mouth on the same side. Further, after division of the nerve on one side in the rabbit it may be observed that when the animal is excited the vessels of the ear whose nerve is intact may become still more dilated than those whose constrictor fibres have been paralyzed. The only explana- tion is that vaso-dilators are being excited from the central nervous system. In the cat the cervical sympathetic contains vaso-dilators for the submaxillary gland (p. 386). The vaso-motor fibres of the head run up in the cervical sympa- thetic, and then pass into various cerebral nerves, of which the fifth or trigeminus is the most important. The trigeminus nerve contains vaso-constrictor nerves for various parts of the eye (conjunctiva, sclerotic, iris), and for the mucous THE NERVOUS REGULATION OF THE BLOODVESSELS 175 membrane of the nose and gums, and section of it is followed by dilatation of the vessels of these regions. The lingual branch of the trigeminus supplies vaso-motor fibres to the tongue, and ap- parently both ,vaso-constrictor and vaso-dilator. In some animals — ^the rabbit, for instance — ^the ear derives part of its vaso-motor supply through the great auricular nerve, a branch of the third cervical nerve, which they reach as grey rami from the stellate ganghon. Another great vaso-motor tract, the most influential in the body, is contained in the splanchnic nerves, which govern the vessels of many of the abdominal organs. Section of these nerves causes an immediate and sharp fall of arterial pressure. The intestinal vessels are dilated and overfilled with blood. As a necessary consequence of their immense capacity, the rest of the vascular system is under- filled, and the blood-pressure falls accordingly. Stimulation of the- peripheral end of the splanchnic nerves causes a great rise of blood- pressure, owing to the constriction of vessels in the intestinal area. We therefore conclude that in the splanchnics there are vaso-motor fibres of the constrictor type, and that impulses are constantly passing down them to maintain the normal tone of the vascular tract which they command. When the splanchnic nerves are stimulated, the adrenal glands are so affected that adrenalin passes out by the veins into the blood-stream. It is clear that if the quan- tity thus hberated were sufficiently large and its hberation suffi- ciently prompt it might play a part in the rise of pressure (p.. 640) which follows stimulation of the nerves, whether they are excited directly or in the normal course of events reflexly. But it has not been demonstrated that this is an effective factor. Dilator fibres (for the intestines and the kidney., for example) have also been discovered in the splanchnic nerves, although the constrictors predominate, and special methods have to be employed for the detection of the dilators. The same is true of the nerves of the extremities, which certainly contain vaso-dilator fibres in addition to vaso-constrictors, although the difficulty of demonstrating the presence of the former is fully as great as it is in the splanchnics. For the investigation is com- plicated by the fact that such nerves as the sciatic supply with vaso-motor fibres two leading tissues — skin and muscle; and these are not necessarily affected in the same direction or to the same extent by stimulation of their vaso-motor fibres. The vaso-con- strictors under ordinary conditions preponderate, so that section of the sciatic or the brachial is generally followed by flushing of the balls of the toes and rise of temperature of the feet, stimulation by paling and fall of temperature. By taking advantage, however, of the unequal excitability of dilators and constrictors in a degenerating nerve, and of the differences between the two kinds of fibres in their 176 THE CIRCULATION OF THE BLOOD AND LYMPH reaction to electrical stimuli (p. 173), it has been shown that vaso- dilators are also present, and come to the front when the conditions are reqdered favourable for them and unfavourable for the constrictors. Vaso-motor fibres for the fore-limb (dog) issue from the cord in the anterior roots of the third to the eleventh dorsal nerves, and for the hind-limb in the anterior roots of the eleventh dorsal to the third lumbar. Stimulation of most of these roots causes constriction of the vessels, but stimulation of the eleventh dorsal may cause dilatation (Bayliss and Bradford). The Vaso-Moior Nerves of Muscle. — ^When the motor nerve of the thin mylo-hyoid muscle of the frog, which can be observed under the micro- scope, is cut, and the peripheral end stimulated, the vessels are seen to dilate distinctly, and this effect is not abolished when contraction of the m.uscle is prevented by a dose of curara insufficient to paralyze the vaso-motor nerves. This indicates the existence in the nerve of vaso- dilator fibres. But we must be cautious in transferring this result to ordinary skeletal muscle, for the mylo-hyoid is more closely allied to the muscles of the tongue than, for example, to the muscles of the limbs, aijd in the mammal the tongue muscles are known to be better supplied with vaso-dilator fibres than the limb muscles. The average flow of blood through a mammalian muscle is indeed increased during volun- tary contraction, and during rhythmically repeated artificial tetaniza- tion of its motor nerve. The outflow of blood from the main vein of the levator labii superioris, one of the muscles used in feeding in the horse, was found to be in one experiment nearly eight times, in anDther about seven times, and in a third three and a half times as great during voluntary work with it (in chewing) as in rest. But as no increase in the blood-flow through the skeletal muscles of a completely curarized mammal during excitation of their nerves has ever been satisfactorily demonstrated, we must conclude that they are very scantily provided with vaso-dilator fibres or not at all. It is uncertain whether they are supplied with vaso-constrictors. The undoubted increase in the blood- flow in contraction may therefore be connected in some way with the mechanical or chemical changes in the muscular fibres themselves. It has been suggested that the muscular vessels are widened by the direct action of the acid products of the active muscle, since very dilute acids (lactic acid, e.g.) cause general dilatation of the small vessels. A similar explanation has been extended to the dilatation of the vessels of the brain during cerebral activity by some of those who deny the existence of vaso-motor nerves for that organ, but here the evidence is by no means satisfactory. The vagus has been stated to contain vaso-constrictor, and the annulus of Vieussens vaso-dilator, fibres for the coronary arteries of the heart. But this question is far from being settled. Adrenalin causes dilatation and not constriction of the coronary vessels. There is some reason to believe that the metabolic products liberated in the heart-muscle, e.g., carbon dioxide, govern the changes in the calibre of the coronary arterioles. A close relationship exists between the output of carbon dioxide and the rate of flow through the coronary circulation. In asphyxia the flow through the coronary vessels is notably increased; indeed, it is at its maximum just before the heart fails altogether, as if an effort were being made to keep the heart going to the last by making up to it in the quantity of the blood supplied what it lacks in quality. As this increased flow is seen in the isolated heart-lung preparation, it has been concluded that metabolites produced in the cardiac muscle itself cause an increased coronary flow when increased demands are made on the heart, a local regulative THE NERVOUS REGULATION OF THE BLOODVESSELS 177 mechanism being thus constituted. There is some evidence that carbon dioxide is not the most potent of these substances. Vaso-Motor Nerves of the Lungs. — There has been much discussion as to the course, and even as to the existence, of vaso-motor fibres for the lungs. The problem is perhaps the most difficult in the whole range of vaso-motor topography, for the pulmonary circulation is so related to other vascular tracts, that changes produced in the vessels of distant organs by the stimulation or section of nerves may affect the quantity of blood received by the right side of the heart, and therefore the quantity propelled through the lungs and the pressure in the pulmonary artery. And changes in the systemic arterial pressure may- favour or hinder the discharge of the left ventricle, and therefore affect the pres- sure in the left auricle and the pulmonary veins. Nevertheless, evidence has been obtained from a number of sources that the lungs are supplied with vaso-constrictor fibres. Plumier, perfusing isolated, ' surviving ' lungs with blood under constant pressure and measuring the outflow, showed that adrenalin and also stimulation of the annulus of Vieussens caused great diminution in the flow — that is, constriction of the vessels. Wiggers also obtained constriction with adrenalin. Fiihner and Starling, working with a preparation including the heart as well as the lungs, found that adrenalin caused a rise of pressure in the pulmonary artery coupled with a fall of pressure in the left auricle, which could only be due to constriction of the vessels of the lungs. It is assumed that adrenalin produces vaso -constriction only in vessels supplied with vaso-constrictor nerves (p. 638), and that in organs where this substance does not cause vaso-constriction no such fibres are present. In mam- mals the vaso-constrictor fibres seem to pass out from the upper half of the dorsal spinal cord, and some of them can be detected nearer their destination in the annulus of Vieussens. The vago-sympathetic of the tortoise contains vaso-constrictors for the lung of the same side (Krogh) . Vaso-Dilator Fibres. — In most of the peripheral nerves these are mingled with vaso-constrictors; but in certain situations, for an anatomical reason that will be mentioned presently, nerves exist in which the only vaso-motor fibres are of the dilator type. Of these, the most conspicuous examples are the chorda tympani and the nervi erigentes or pelvic nerves; and, indeed, it was in the chorda that vaso-dilators were first discovered by Bernard. The chorda tympani contains vaso-dilator and secretory fibres for the sub- maxillary and subhngual salivary glands. With the secretory fibres we have at present nothing to do; and the whole subject will have to be returned to, and more fully discussed in Chapter VI. But a most marked vascular change is produced by stimulation of the peripheral end of the divided chorda tympani nerve. The glands flush red; more blood is evidently passing through their vessels. Allowed to escape from a divided vein, the blood is seen to be of a bright arterial colour and shows a distinct pulse. The small arteries have been dilated by the action of the vaso-motor fibres in the nerve. The resistance being thus reduced, the blood passes in a fuller and more rapid stream through the capillaries into the veins, and on the way there is not time for it to become completely venous. These vaso-dilator fibres are not in constant action, for section of the 178 THE CIRCULATION OF THE BLOOD AND LYMPH nerve, as a rule, produces little or no change. Vaso-constrictor fibres pass to the salivary glands from the cervical sympathetic along the arteries, and stimulation of that nerve causes narrowing of the vessels and diminution of the blood-flow, sometimes almost to complete stoppage. The nervi erigentes are the nerves through which erection of the penis is caused. When they are divided there is no effect, but stimulation of the peripheral end causes dilatation of the vessels of the erectile tissue of the organ, which becomes overfilled with blood. During stimulation of these nerves, the quantity of blood flowing from the cut dorsal vein of the penis may be fifteen times greater than in the absence of stimulation. It spurts out in a strong stream, and is brighter than ordinary venous blood (Eckhard). Stimulation of the peripheral end of the nervus pi4dendus causes constriction of the vessels of the penis, so that it contains vaso- constrictor fibres which are the antagonists of the nervi erigentes. Vaso-Motor Nerves of Veins. — Like arteries, veins have plexuses of nerve-fibres in their walls, and contract in response to various stimuli. In some cases — e.g., in the wing of the bat — ^rhythmical contractions of the veins are strikingly displayed, but they do not depend on the central nervous system, as they persist after section of the brachial nerves. The first clear proof of the existence of vaso-motor nerves for veins was furnished by Mall, who showed that vaso-constrictor fibres for the portal vein exist in the splanchnic nerves. When these were stimulated, after the disturbing effect of changes in the circulation through the intestines had been eliminated by compression of the aorta in the thorax, an actual shrinking of the vein could be observed. The fibres issue from the spinal cord by the anterior roots of the third to the eleventh dorsal nerves, but chiefly in the fifth to the ninth dorsal. When the liver is enclosed in a plethysmograph, and the central end of an ordinary sensory nerve, like the sciatic, excited, reflex vaso-constriction takes place in the portal area, the volume of the organ diminishes, and the blood-pressure rises in the portal vein (Franfois-Franck) . The vena portse and its branches are in the physiological sense arteries rather than veins, since they break up into capillaries, and it was to be expected that the regulation of the blood- flow in them would be carried out in the same way as in ordinary arteries, namely, by means of vaso-motor nerves. But we must not, without special proof, extend the results obtained in the portal system to ordinary veins. A certain amount of evidence, however, exists that even such veins as those of the extremities are supplied, though scantily, with vaso-constrictor (veno- motor) fibres. After ligation of the crural artery or aorta, stimulation of the peripheral end of the sciatic has been seen to cause contraction of short portions of the superficial veins of the leg. THE NERVOUS REGULATION OF THE BLOODVESSELS 179 Finally, adrenalin (epinephrin) causes constriction of rings of ' surviving ' veins just as of artery rings, although in correspondence with the smaller amount of muscular tissue in the former the con- traction is not so strong. As adrenalin is assumed to act only upon muscle supplied by sympathetic nerve-fibres (p. 638), this would seem to indicate the existence of such a supply for veins. The question is an important one in connection with the regulation of the filling, and therefore of the discharge, of the heart (Henderson), but the experimental data are as yet too meagre to justify further discussion of the matter here. Course of the Vaso-Motor Neryes. — In the dog the vaso-constrictors pass out as fitne meduUated fibres (i-8 to 3-6 /n in diameter) in the anterior roots of the second dorsal to about the second lumbar nerves. They proceed by the white rami communicantes to the lateral sym- pathetic gangha, where, or in more distal gangha such as the inferior mesenteric, they lose their medulla, and their axis-cylinder processes (p. 822) break up into fibrils that come into close relation with the nerve-cells of the ganglia. These ganglion-cells in their turn send off axis-cylinder processes, which, enveloped by a neurilemma, pass as non-medullated fibres by various routes to their final destination, the unstriped muscular fibres of the bloodvessels. Their course to the head has been already described. To iihe limbs they are distributed in the great nerves (brachial plexus, sciatic, etc.), which they reach from the sympathetic ganglia by the grey rami communicantes. The outflow of vaso-dilator fibres is not restricted to the same portion of the cord from which the outflow of constrictor fibres takes place. Their existence is indeed most easily demonstrated in nerves springing from those regions of the cerebro-spinal axis from which vaso-constrictor fibres do not arise, and where, therefore, we have not to contend with the difficulty of interpreting mixed effects. Vaso-dilators for the external generative organs and the mucous membrane of the lower end of the rectum pass out as small medullated fibres of the anterior roots of certain of the sacral nerves (mainly the second and third in the cat) into the pelvic nerve (nervus erigens). They end in relation with ganglion-cells in the neighbourhood of the organs which they supply. The seventh and ninth cranial nerves carry vaso-dilator fibres which are distributed by way of the lingual and other branches of the fifth to the salivary glands, the tongue, the mucous membrane of the floor of the mouth, and part of the soft palate. Those in the lingual, passing through the chorda tympani, end in ganglion-cells near the submaxillary and sublingual glands, and the axons of these cells continue the path to the vessels of the glands. It is supposed that the vaso-dilators dis- tributed in other branches of the fifth also have ganglion-cells on their course. In fact, there is good evidence that every eiferent vaso-motor fibre is interrupted by one, and only by one, ganglion-cell between the cord and the bloodvessels. The statement has been made that for certain regions of the body, especially the skin of the limbs, the vaso- dilator nerves are contained, not in the anterior, but in the posterior roots. And these, it is claimed, are not aberrant efferent fibres which have strayed in the course of development into the wrong roots, but true posterior root-fibres whose cells of origin lie in the spinal ganglia, and which conduct efferent vaso-dilator impulses in the wrong direction, so to speak, from the cord to the periphery — ' antidromic ' impulses (Bayliss). r i- j' f i8o THE CIRCULATION OF THE BLOOD AND LYMPH Effect of Nicotine on Nerve-Cells. — ^A method which has been found ' most fruitful in studying the relations of sympathetic ganglion-cells to the vaso-motor fibres, as well as to the pilo-motor* and secretory fibres which in certain situations are so intricately mingled with them, must here be mentioned. It depends upon the fact that when a suitable dose of nicotine (lo milligrammes in a cat) is injected into a vein, or a solution is painted on a ganglion with a brush, the passage of nerve- impulses through the ganglion is blocked for a time (Langley). The nerve-fibres peripheral to the ganglion are not afiected. The question whether eflerent fibres are connected with nerve-cells between a given point and their peripheral distribution can, therefore, be answered by observing whether any effect of stimulation is abolisihed by nicotine. If, for instance, the excitation of a nerve caused constriction of certain bloodvessels before, and has no efiect after, the application of nicotine to a ganglion, its vaso-constrictor fibres, or some of them, must be con- nected with nerve-cells in that ganglion. Langley has brought forward evidence that many of the bodies which are commonly supposed to act upon nerve-endings (as nicotine, curara, atropine, pilocarpine, adrenalin, etc.) really act upon ' receptive ' substances of the cells in connection with which the nerve-fibres end. These receptive substances are con- ceived to be capable of being specifically affected by chemical bodies and by nervous stimuli, and in their turn to be capable of influencing the metabolism of the main cell substance on which its function depends. The receptive substances thus form beyond the histological link of the nerve-ending a kind of chemical link between the nerve-fibre and the cell which it suppUes. We have thus traced the vaso-motor nerves from the cerebro- spinal axis to the bloodvessels which they control; it stiU remains to define the portion of the central nervous system to which these scattered threads are related, which holds them in its hand and acts upon them as the needs of the organism may require. Vaso-Motor Centres.— Now, experiment has shown that there is one very definite region of the spinal bulb which has a most intimate relation to the vaso-motor nerves. If while the blood-pressure in- the carotid is being registered, say, in a curarized rabbit, the central end of a peripheral nerve like the sciatic is stimulated, the pressure rises so long as the bulb is intact, this rise being largely due to the reflex constriction of the vessels in the splanchnic area. If a series of transverse sections be made through the brain, the rise of pressure caused by stimulation of the sciatic is not affected till the upper limit of the bulb is almost reached. If the slicing is still carried downwards, the blood-pressure sinks, and the rise following stimu- lation of the sciatic becomes less and less. When the medulla has been cut away to a certain level, only an insignificant rise or none at all can be obtained. The portion of the medulla the removal of which exerts an influence on the blood-pressure, and its increase by reflex stimulation, extends from a level 4 to 5 mm. above the point of the calamus scriptorius to within i to 2 mm. of the corpora quadrigemina. Stimulation of the medulla causes a rise, destruc- * Pilo-motor nerves supply the smooth arrectof pili muscles, whose contrac- tion causes the hair to ' stand on end.' THE NERVOUS REGULATION OF THE BLOODVESSELS i8i tion of this portion of it a severe fall, of general blood-pressure. There is evidently in this region a nervous ' centre ' so intimately related, if not to all the vaso-motor nerves, at least to such very important tracts as to deserve the name of a vaso-motor centre. Experiment has shown that this is much the most influential centre, and it is usually called the chief or general vaso-motor centre. Some writers prefer to speak of it as the vaso-constrictor centre, since it is undoubtedly connected with most or all o( the vaso-constrictor paths, and has not been shown to be similarly connected with the vaso-dilator paths. There is, indeed, not the same solid evidence for the existence of a general vaso-dilator centre in the bulb as for the existence of the general vaso-constrictor centre. Yet there are facts which indicate that the bulbar vaso-motor centre or centres, when reflexly stimulated, can, and often do, respond not merely by an increase or a remission of vaso-constrictor tone, but by a simul- taneous inhibition of vaso-constrictor fibres and excitation of vaso- dilators leading to a fall of pressure, or by a simultaneous inhibition of vaso-dilators and excitation of vaso-constrictors leading to a rise of pressure. ! The spinal cells of origin of the pre-ganglionic segments of the vaso-constrictor paths constitute subordinate centres which, eitlijer normally support a certain degree of vascular tone, or come to do ^so after the chief vaso-motor centre has been cut off. Thus, in the frog it is possible to go on destroying more and more of the cord from above downwards, and still to obtain reflex vaso- motor effects, as seen in the vessels of the web, by stimulating the central end of the sciatic nerve. Although these effects indeed diminish in amount as the destruction of the cord proceeds, yet a distinct change can be caused when only a small portion of the cord remains intact. Similarly, in the mammal evidence has been obtained of the existence of ' centres ' at various levels of the cord, capable of acting eventually, if not at once, as vaso-constrictor centres after the loss of the controlling influence of the bulb. The best example of a vaso-dilator centre is that situated in the lumbar cord, which controls the erection of the penis. After total section of the cord at the upper limit of the lumbar region, erection, which is known to be due to a reflex dilatation of the arteries of the organ through the nervi eri- gentes, can still be caused (in dogs) by mechanical stimulation of the glans penis, so long as the afferent fibres of the reflex arc con- tained in the ner vus pudendus are intact. Destruction of the lumbar cord abolishes the effect. It is impossible to avoid the conclusion that a vaso-dilator or erection centre, which is in relation on the one hand with the nervi erigentes, and on the other with the nervus pudendus, exists in the lower portion of the spinal cord. Vaso- motor centres for the hind-Hmbs have also been located in the 1 82 THE CIRCULATION OF THE BLOOD AND LYMPH same region. When the brain, the bulb, and the upper portion of the cord have been ehminated by ligation of all the arteries from which blood can possibly reach them, a sufficient vascular pressure persists to permit the circulation to go on in the lower portion of the body for hours. And while section — or freezing (Fig. 80) — of the cord in the lower cervical region causes a marked fall of pressure, this is not permanent if the animal is allowed to survive. Forty-one days after total section of the cord at the seventh cervical segment in a dog an arterial pressure of 130 mm. of mercury was found. A mechanism for the maintenance of vascular tone exists even beyond the limits of the central nervous system. For when the lower portion of the cord is completely destroyed, the dilatation of the vessels of the hind-limbs, which is at first so conspicuous, passes away after a time, the functions of vaso-motor centres having perhaps been assumed by the sympathetic ganglia (Goltz and Ewald). When the lumbo-sacral sympathetic chain is extirpated, Fig. 80. — Effect on Blood-Pressure of Freezing Spinal Cord (Pike). At i the first or second dorsal segment of a dog's cord was frozen with liquid air; at 2 and 3 central end of sciatic stimulated without effect on pressure (respectively one and a half and three minutes after freezing of cord). (Four -fifths of original size.) there is a further loss of vascular tone in the affected region. But even this is not irremediable. After a time recovery again occurs, although it may be more partial and tardy than before. This may take place either through the intervention of still more peripheral ganglia, or through the development of a certain tonus by the muscular fibres of the vessels when abandoned to themselves. As to the nature of the tone of the general vaso-motor centre, the same question may be asked which has been already discussed for the cardio-inhibitory centre. Is it reflex, or does it depend upon direct excitation of the centre by some constituent of the blood or lymph, or some substance produced in the centre itself ? The best answer which can at present be made is that a constant central excitation by the carbon dioxide formed in the centre or circulating in the blood is a not unimportant factor in the maintenance of the vaso-motor tone. A marked diminution in the carbon dioxide tension of the blood, a condition which is termed ' acapnia,' may indeed contribute to the severe fall of blood-pressure associated with THE NERVOUS REGULATION OF THE BLOODVESSELS 183 surgical shock (Henderson). In addition to the direct influence of carbon dioxide, and possibly of other substances, the arrival of affereQt impulses at the centre seems to play a part in maintaining that continual discharge of efferent impulses along the vaso-motor nerves which constitutes its tone. In this regard, the vaso-motor centre occupies an intermediate position between the respiratory centre, the most purely automatic, and the cardio-inhibitory centre, the most purely reflex of the three great bulbar mechanisms. Of the' anatomical relations of the nerve-cells that make up the bulbar and -spinal vaso-motor centres, little more is known than may he deduced from the physiological facts we have been reciting. It has been surmised on histologicar groimds that certain cells of small size scattered up and down the ttoracic and upper lumbar regions of the cord in the. lateral horn (intermedio-lateral tract), and perhaps cropping out also in the bulb, are vaso-motor cells. There is good evidence that the pre-ganglionic sympathetic fibres, including the vaso-motor fibres which we have already discovered emerging from the cord in the spinal roots, are connected with these cells. And, indeed, there is reason to believe that the connection is made without the intervention of any other nerve-cells, and that the axis-cylinders of these vaso-motor fibres are the axis-cylinder processes of the vaso-motor cells. So that the simplest efferent path along which vaso-motor impulses can pass may be considered as built up of two neurons, one with its cell-body in the cord, and the other in a sympathetic ganglion. Less is known of the elements which constitute the bulbar centre and of their connections. But since it would appear that the spinal vaso-motor centres are under the control of the chief centre in the bulb, it is necessary to suppose that the axis-cylinder processes of some of the cells of the bulbar centre come into relation with the spinal vaso-motor cells, and that inapulses passing, let us say, from the bulb to the vessels of the leg, would have to traverse three neurons (p. 823). Vaso-Motor Reflexes. — We have already seen that the cardiac centres are constantly influenced by afferent impulses, and that in the direction either of augmentation or inhibition. The vaso-motor centre in the bulb is equally sensitive to such impulses. They reach it for the most part along the same nerves, and by increasing or diminishing its tone cause sometimes constriction and sometimes dilatation of the vessels, the result depending partly upon the ana- tomical connection of the afferent fibres, but apparently in part also upon the state of the centre. Of the afferent nerves that cause vaso-dilatation, the most im- portant is the depressor, whose reflex inhibitory action on the heart has been already described. The fall in the arterial pressure is due chiefly, not to the inhibition of the heart, but to inhibition of the vaso-constrictor tone of the bulbar vaso-motor centre, combined with stimulation of vaso-dilat&r nerves, and consequent general dila- tation of the arterioles throughout the body. That the depressor action involves excitation of vaso-dilators follows from the fact that vaso-dilatation occurs in the hmbs on stimulation of the depressor after their vaso-constrictor nerves have been cut. Stimulation of 184 THE CIRCULATION OF THE BLOOD AND LYMPH the depressor produces its usual result after section of the vagi. It has been suggested that the function of the nerve is to act as an automatic check upon the blood-pressure in the interest both of the heart and the vessels, its terminations in the aorta or the ventricular wall being mechanically stimulated when the pressure tends to rise towards the danger limit. In rare cases, efferent inhibitory fibres for the heart have been found in the depressor of the rabbit. Many of the peripheral nerves contain fibres whose stimulation is followed by dilatation of the bloodvessels in special regions. Fig. 81. — Diagram of De- pressor Nerve in Rabbit. X, vagus; SL, superior laryngeal branch of vagus ; D, depressor fibres. The arrows show the course of Fig. 82. — Blood -Pressure Tracing: Rabbit. Central the impulses that affect end of depressor stimulated/at i; stimulation the blood-pressure. stopped at 2. Time-trace, seconds: usually the areas to which they are themselves distributed, accom- panied by constriction of distant and, it may be, more extensive vascular tracts. Thus, the usual local effect of stimulating the afferent fibres of the lowest three thoracic nerves, in whose anterior roots run the vaso-motor fibres for the kidney, is a dilatation of the renal vessels (Bradford), and the usual local effect of stimulating the infra-orbital or supra-orbital nerve a dilatation of the external maxillary artery. But the general effect in both cases is vaso- constriction in other regions of the body, which more than com- pensates the local dilatation, so that the arterial blood-pressure rises. It is not difficult to see that both of these changes render it easier for the part to obtain an increased supply of blood. Sometimes the reciprocal relation between vase -dilatation in one part of the bodjr and vaso-constriction in another is only apparent. For exarnple, stimulation of the cut end of the sciatic causes, as we have already seen, extensive vaso-constriction and a notable rise in the blood- pressure. The constriction certainly involves the splanchnic area; but THE NERVOUS REGULATION OF THE BLOODVESSELS 185 superficial parts, as the lips, may be seen to be flushed with blood. In asphyxia, when the vaso-motor centres are directly stimulated by the venous blood, this apparent antagonism is still better marked : the cutaneous vessels are widely dilated and engorged, the face is livid, but the abdominal organs are pale and bloodless (Heidenhain). The blood-pressure rises rapidly, reaches a maximum, and then gradually falls as the vaso-motor centre becomes paralyzed (Figs. 84 and 85). It has been shown that in both cases vaso-constriction of the skin is really produced as well as vaso-constriction of the internal organs, but the increased blood-pressure mechanically overcomes the constriction of the cutaneous vessels. The kind of stimulus seems to -have something to do with the direction of the reflex vaso-motor change. For while electrical stimulation of every muscular nerve, even of the very finest twigs that can be isolated and laid on electrodes, provokes always, whether the shocks follow each other rajpidly or slowly, a rise of general Fig. 83. — Pressor Effect of Stimulation of Central End of Vagus in a Cat during Resuscitation after Cerebral Ansemia. The depressions in the signal line ABC indicate the duration of three successive excitations of equal strength, sixty-five, seventy-three, and seventy-nine minutes respectively after restoration of the circulation. The pressor effect increases as resuscitation proceeds. Later on the original depressor effect was again obtained. The upper tracing is that of the artificial respiration. (Two-thirds original size.) blood-pressure, mechanical stimulation of a muscle, as by kneading or massage, causes a fall. The condition of the afferent fibres also exerts an influence. For example, excitation of the central end of a sciatic nerve that has been cooled is followed by vaso- dilatation and fall of pressure, the opposite of the ordinary result. These and similar facts have led to the idea that most afferent nerves contain two kinds of fibres, whose stimulation can affect the activity of the vaso-motor centres — ' reflex vaso-constrictor,' or ' pressor ' fibres, and ' reflex vaso-dilator,' or ' depressor ' fibres. The branch of the vagus, however, to which the name ' depressor ' has been specially given is usually described as the only peripheral nerve the excitation of which is in all circumstances followed by a general diminution of arterial pressure. But this is not strictly correct, for at an early period in the resuscitation of the brain after anaemia excitation of t86 THE CIRCULATION OF THE BLOOD AND LYMPH the rabbit's ' depressor ' causes a slight rise of pressure not followed by any fall. This, perhaps, indicates the presence in the ' depressor ' of a small number of pressor fibres, which are resuscitated sooner than the depressor fibres proper. The same phenomenon, only more marked, may be seen when the central end of the cat's vagus, containing the depressor fibres, is excited at intervals during resus- citation (Fig. 83). Or the result may depend upon a change in the response of the altered vaso-motor centres to impulses reaching them along the depressor fibres. If specific ' depressor ' fibres exist in other nerves, they are so mingled with ' pressor ' fibres that their action is masked when both are stimulated together. The state of the vaso-motor centre is unquestionably a factor which has some importance in determining the result of reflex vaso-motor stimula- tion. For instance, in an animal deeply anaesthetized with chloro- form or chloral, excitation of pressor fibres (in an ordinary sensory Fig. 84.- — Rise of Blood-Pressure in Aspliyxia : Rabbit. Respiration stopped at i. ' Interval between 2 and 3j{not reproduced) 44 seconds, during which the blood- pressure steadily rose. At 4, respiration resumed. Time -trace, seconds. nerve) causes, not a rise, but a fall of blood-pressure ; while in an animal fully under the influence of strychnine stimulation of the depressor nerve causes not a fall, but a rise. The vaso-motor reflexes in man can be conveniently studied by the calorimetric method described on p. 220. One of the most important of the vaso-motor reactions is that by which the vessels of the skin respond to the temperature of the environment so as to regulate the loss of heat from the body (p. 674). When one hand, e.g. the left, is immersed in cold water (say at about 8° C), the blood-flow in the right is at once reduced owing to reflex vaso- constriction. Other parts of the body are also affected, but not so readily as the contra-lateral hand, since the segments of the cord into which the afferent fibres from a given skin area run are at the same time the segments from which the efferent vaso-motor fibres for the symmetrically-placed area on the opposite side of the body arise. The reflex diminution in the flow persists for a time which THE NERVOUS REGULATION OF THE BLOODVESSELS 187 varies with the individual, the external temperature, and other circumstances, and then as a rule rather suddenly the vaso-con- striction gives way and the flow begins .again to increase, even while the left hand is still kept in the cold water. When the left hand is transferred from the cold to warm water (at 43° or 44° C), the first effect is a transient diminution in the blood-flow in the right hand. This soon gives place to an increase (vaso-dilatation). As an ex- ample, the following table gives the condensed results of three experiments on two young men. Experiments II. and III. were on the same man at an interval of three days. Experi- ment. Temperature of — '*-■"• M0"d'.' ^^''"^'■^■ II. III. 36-6 36-0 36-5 Duration of Observation in Minutes. 16 4 6 II 13 13 2 3 7 15 5 5 3 7 Flow in Grms. per 100 c.c. of Right Hand per Minute. 12-2 6-9 9-9 II-6 lO-I 5-0 3-4 8-4 15-0 12-4 5-9 10-7 7-9 17-6 Left Hand in- Cold water. Cold water still. Warm water. Cold water. Warm water. Warm water still. Warm water still. Cold water. Cold water still. Warm water. Warm water still. Such facts enable us to some extent to understand the manner in which the distribution of the blood is adjusted to the requirements of the different parts of the body, so that to a certain degree of approximation no organ has too much, and none too little. The blood-supply of the organs is always shifting with the caUs upon them. Now, it is the actively- digesting stomach and the actively- secreting glands of the alimentary tract which must be fed with a full stream of blood, to supply waste and to carry away absorbed nutriment. Again, it is the working muscles of the legs or of the arms that need the chief blood-supply. But wherever the call may be, the vaso-motor mechanism is able, in health, to answer it by bringing about a widening of the small arteries of the part which needs more blood, and a compensatory narrowing of the vessels of other parts whose needs are not so great. It is also through the vaso-motor system, and especially by the action of that portion of it which governs the abdominal vessels, and i88 THE CIRCULATION OF THE BLOOD AND LYMPH of the nerves that regulate the work of the heart, that in animals to which the upright position is normal (monkey) and in man the influence of changes of posture on the circulation is almost com- pletely compensa1:ed.* The pressure in the upper part of the human brachial artery has been measured with a sphygmoman- ometer, first in the horizontal and then immediately afterwards in the standing posture, and in health it has been found to remain practically unchanged (Hill). But if the person was overworked or out of sorts, the compensation was less complete. It is well known that in debilitated persons, especially if long confined to bed, the sudden assumption of the upright position may cause vertigo, and even syncope, the normal compensatory mechanism being deranged. In such animals as the .rabbit this compensation is totally inefficient. When a domesticated rabbit, which has been kept in a hutch, is suspended vertically with the feet down, the blood drains into the abdominal vessels, S5mcope speedily ensues, and in a period that ranges from less than a quarter to three-quarters of an hour the animal dies in the convulsions of acute cerebral anaemia (Salathe, Hill). The head-down position has no ill-effects. In wild rabbits, whose abdominal wall is more tense and elastic, these fatal symp- toms are not easily produced, and the same is true of cats and dogs. But in all animals, when the compensation is destroyed, as in paralysis of the vaso-motor centre by chloroform, the circulation may be profoundly influenced by the position of the body: elevation of the head may lead to cerebral anaemia, syncope, and even death; elevation of the legs, and particularly the abdomen, may restore the sinking pulse by filling the heart and the vessels of the brain. If a chloralized dog be fastened on a board which can be rotated about a horizontal axis passing under the neck, the blood-pressure in the carotid artery falls greatly when the animal is made to assume the vertical position with the head up, and either rises a little or remains practically unchanged when the head is made to hang down. So great may the fall of pressure be in the former position that death may occur if it be long maintained (Practical Exercises, p. 212). * Two factors may be distinguished in the blood-pressure, the hydrostatic and the hydrodjoiamic elements. The hydrostatic portion of the pressure is due to the weight of the column of blood acting on the vessel; the hydro- djoiamic portion of the pressure is due to the work of the heart. If a dog be securely fastened to a holder arranged in such a way that the animal can be placed vertically, with the head up or down, and the mean blood-pressure in the crural artery be measured in the two positions, there will be a considerable difference. For when the legs are uppermost the heart has to overcome the weight of the column of blood rising above it to the crural artery; when the head is uppermost the action of the heart is reinforced by the weight of the blood. And if no change were produced in the action of the heart, or in the general resistance of the vascular path, by the change of position, this differ- ence would be equal to the pressure of a column of blood twice as high as the straight-line distance between the cannula and the point of the arterial system at which the pressure is the same with head up as with head down (indifierent ■ point) . THE NERVOUS REGULATION OF THE BLOODVESSELS 189 Finally, it is in virtue of the amazing power of accommodation possessed by the vascular system, as controlled by the vaso-motor and cardiac nerves, that so long as these are not disabled the total quantity of blood may be greatly diminished or greatly increased, without endangering life, or even causing more than a transient alteration in the arterial pressure. It is not until at least a quarter of the blood has been withdrawn that there is any notable effect on the pressure, for the loss is quickly compensated by an increase in the activity of the heart and a constriction of the small arteries. An animal may recover after losing considerably more than half its blood.* Conversely, the volume of the circulating liquid may be doubled by the injection of blood or physiological salt solution without causing death, and increased by 50 per cent, without any marked increase in the pressure. The excess is promptly stowed <^-«S ' I ■ ■ ' I ' I ' ' ■ ■ I 1 I ■ 11 I I I I ' I I ti 1 1 I I I I r I ■! II I ilihi 1 1 1 II Seamdi Fig. 85. — Blood-Pressure Tracing from][a Dog poisoned with Alcoliol. Tlie respiratory centre being paralyzed, respiration stopped, and the typical rise of blood-pressure in asphyxia took place. The pressure had again fallen, and total paralysis of the vaso-motor centre was near at hand, when at A the animal made a single respiratory movement. The quantity of oxygen thus taken in was enough to restore the vaso-motor centre, and the blood-pressure again rose. This was repeated five or six times. (Three-fourths original size.) away in the dilated vessels, especially those of the splanchnic area ; the water passes rapidly into the lymph, and is then more gradually eliminated by the kidneys. From these facts we can deduce the practical lesson, that blood- letting, unless fairly copious, is useless as a means of lowering the general arterial pressure, while it need not be feared that transfusion of a considerable quantity of blood, or of salt solution, in cases of severe haemorrhage will dangerously increase the pressure. And from the physiological point of view the term 'haemorrhage ' includes more than it does in its ordinary sense. For as dirt to the sani- tarian is ' matter in the wrong place,' haemorrhage .to the physiolo- gist is blood in the wrong place. Not a drop of blood may be lost * It is not usually possible to obtain quite two-thirds of the total blood by bleeding a dog from a large artery. In seven dogs bled from the carotid, the ratio of the weight of the blood obtained to the body-weight was I : 24'7, 1 : 217, i : 20-7, i : 20-5, i : i8-6, i : 16, i : 13-5 In the last case, the blood clotted with abnormal slowness, and the animal died in a few minutes. tgo THE CIRCULATION OF THE BLOOD AND LYMPH from the body, and yet death may occur from haemorrhage into the pleural or the abdominal cavity, into the stomach or intestines. Not only so, but a man may bleed to death into his own blood- vessels ; in surgical shock, it would appear that the blood which ought to be circulating through the brain, heart and lungs may stagnate in the dilated veins. ■■ }. Section VII. — -The Lymphatic Circulation. As has already been stated, some of the constituents of the blood, instead of passing back to the heart from the capillaries along the veins, find their way by a much more tedious route along the lymphatics. The blood capillaries are everywhere in very intimate relation with lyinph capillaries, which, completely lined with epithelioid cells, lie in irregular spaces in the connective-tissue that everywhere accompanies arid supports the bloodvessels. The constituents of the blood-plasma are filtered through, or secreted by the capillary walls into these lymph spaces, and mingling there with waste products discharged by the cells of the tissues, form the liquid known as tissue liquid or tissue lymph. From the tissue liquid the lymph capillaries take up the constituents of the ' lymphatic ' lymph, which then passes into larger lymphatic vessels, with lymphatic glands at intervals on their course. These fall into still larger trunks, and finally the greater part of the lymph reaches the blood again by the thoracic duct, which opens into the venous system at the junction of the left subclavian and internal jugular veins. The lymph from the right side of the head and neck, the right extremity, and the right side of the thorax, with its viscera, is collected by the right lymphatic duct, which opens at the junction of the right sub- clavian and internal jugular veins. The openings of both ducts are guarded by semilunar valves, which prevent the reflux of blood from the veins. Serous cavities like the pleural sacs, although differing from ordinary lymph spaces, are connected through small openings, called stomata, with lymphatic vessels. The rate of flow of the lymph in the thoracic duct is very small com- pared with that of the blood in the arteries — only about 4 mm. per second, according to one observer. Nevertheless, a substance injected into the blood can be detected in the lymph of the duct in four to seven minutes (Tschirwinsky). The factors which contribute to the main- tehance of the lymph flow are : (i) The pressure under which it passes from the blood capillaries into the lyinph spaces and from the lymph spaces into the lymph capillaries. The pressure in the thoracic duct of a horse may be as high as 12 mm. of mercury; in the dog it may be less than i mm. The difierence is probably due, in part at least, to a difierence in the experimental con- ditions, dogs being usually anaesthetized for such measurements, horses not. The pressure in the lymph capillaries must, of course, be higher than in the thoracic duct — how much higher we do not know. (2) The contraction of muscles increases the pressure of the lymph by compressing the channels in which it is contained, and the valves, with which the lymphatics are even more richly provided than the veins, hinder a backward and favour an onward flow. The contractions of the intestines/ and especially of the villi, aid the movement of the chyle. By the contraction of the diaphragm, substances may be sucked from the peritoneal cavity into the lymphatics of its central tendon, through the stomata in the serous layer with which THE LYMPHATIC CIRCULATION 191 its lower surface is clad. It is even possible by passive movements of the diaphragm in a dead rabbit to inject its lymphatics with a coloured liquid placed on its peritoneal surface. Passive movements of the limbs and massage of the muscles are also known to hasten the sluggish current of the lynaph, and are sometimes employed with this object in the treatment of disease. {3) The movements of respiration aid the flow. At every inspiration the pressure in the great veins near the heart becomes negative, and lymph is sucked into them (p. 225). (4) In some animals rhythmically - contracting muscular sacs or hearts exist on the course of the lymphatic circulation. The frog has two pairs, an anterior and a posterior, of these lymph hearts, which pulsate, although not with any great regularity, at an average rate of sixty to seventy beats a minute, and are governed by motor and inhibi- tory centres situated in the spinal cord. The beat is not directly ini- tiated from the cord, but the tonic influence of the cord is necessary in order that the lymph hearts may continue to beat (Tschermak). Such hearts are also found in reptiles. It is possible that in animals without localized lymph hearts the sraooth muscle, which is so conspicuous an element in the walls of the lymphatic vessels, may aid the flow by rh)rthmical contractions. PRACTICAL EXERCISES ON CHAPTER III. 1. Microscopic Examination of the Circulating Blood. — (i) Take a tadpole and lay it on a g;lass slide. Cover the tail with a large cover- slip, and examine it with the low power (Leitz, oc. III., obj. 3). Generally the tail will stick so closely to the slide, and the animal will move so little, that a sufficiently good view of the circulation can be obtained. If there is any trouble, destroy the brain with a needle. Observe the current of the blood in the arteries, capillaries and veins. An artery may be easily distinguished from a vein by looking for a place at which the vessel bifurcates. In veins the blood flows in the two branches of the fork towards the point of bifurcation, in arteries away from it. Sketch a part of a field. To Pith a Frog. — ^Wrap the animal in a towel, bend the head forwards with the index-finger of one hand, feel with the other for the depression at the junction of the head and backbone, and push a narrow-bladed knife right down in the middle line. The spinal cord will thus be divided with little bleeding. Now push into the cavity of the skull a piece of pointed lucifer match . The brain will thus be destroyed. The spinal cord can be destroyed by passing a blunt needle down inside the vertebral canal. (2) Take a frog and pith its brain only, inserting a match to prevent bleeding. Pin the frog on a plate of cork into one end of which a glass slide has been fastened with sealing-wax. Lay the web of one of the hind-legs on the glass and gently separate two of the toes, if necessary by threads attached to them and secured to the cork plate. Put the plate on the microscope-stage and fasten by the clips (see pp. 15, 118). (3) After the normal circulation has been studied thoroughly put a very small drop of tincture of cantharidcs on- the portion of the web which is in the field of the microscope, using a fine pipette. Observe the process of inflammation, including stasis and diapedesis (p. 61). 2. Anatomy of the Frog's Heart. — Expose the heart of a pithed frog by pinching up the skin over the abdomen in the middle line, dividing it with scissors up to the lower jaw, and then cutting through the 192 THE CIRCULATION OF THE BLOOD AND LYMPH abdominal muscles and the bony pectoral girdle. The external ab- dominal vein, which will be observed on reflecting the skin, can be easily avoided. The heart wUl now be seen enclosed in a thin mem- brane, the pericardium, which should be grasped with fine-pointed forceps and freely divided. Connecting the posterior surface of the heart and the pericardium is a slender band of connective tissue, the frsenum. A silk ligature may be passed around this with a threaded curved needle, or curved fine-pointed forceps, and tied, and then the frsenum may be divided posterior to the ligature. The anatomical arrangement of the various parts of the heart should now be studied. Note the single ventricle with the bulbus arteriosus, the two auricles, and the sinus venosus, turning the heart over to see the latter by means of the ligature. Observe the whitish crescent at the junction of the sinus venosus and the right auricle (Fig. 86). |t„3- The Beat of the Heart. — Note that the auricles beat first, and then the ventricle. The ventricle becomes smaller and paler during its systole, and blushes red during diastole. Count the number of beats of the heart in a minute. Now excise the heart, lifting it by means of the ligature, and tak- ing care to cut wide of the sinus venosus. Place the heart in a small por- celain capsule on a little blotting - paper moist- ened with physiological salt solution.* Observe that it goes on beating. Put a little ice or snow in contact with the heart, and count the number of beats in a minute. The rate is greatly dimin- ished. Now remove the ice and blotting-paper, cover the heart with the salt solution, and heat, noting the temperature with a thermometer. Observe that the heart beats faster and faster as the temperature rises. At 40° to 43° C. it stops beating in diastole (heat standstill). Now at once pour off the heated liquid, and run in some cold salt solution. The heart will begin to beat again. 4. Cut off the apex of the ventricle a' little below the auriculo-. ventricular groove. The auricles, with the attached portions of the ventricle, go on beating. The apex does not contract spontaneously, but can be made to beat by stimulating it mechanically (by pricking it with a needle) or electrically. Divide the still contracting portion of the heart by a longitudinal incision. The two halves go on beating. 5. Heart Tracings. — (i) Fasten a myograph-plate (Fig. 87) on a stand. Take a long light lever consisting of a straw or a piece of thin chip, armed at one end with a writing-point of parchment-paper, supported near the other end by a horizontal axis, and pierced not far from the axis by a needle carrying on its point a small piece of cork or a ball of seahng-wax. * For frog's tissues this should be 0-7 to 0-75 per cent, sodium chloride solution, for mammalian tissues a little stronger (about o-g per cent.). Fig. 86. — Frog's Heart with Stannius' Ligatures in Position (Cyon). Anterior surface of heart shown on the left, posterior surface on the right, a, right auricle; 6, left auricle; c, ventricle; d, bulbus arte- riosus; e,f, aortas; g, sinus venosus. PRACTICAL EXERCISES 193 Fig. 87. — Arrangement for obtaining a Heart Tracing from a Frog. A counterpoise is adjusted on the short arm of the lever in the form of a small leaden weight. Cover a drum with glazed paper and smoke it. The paper must be put on so tightly that it will not slip. To smoke the drum, hold it by the spindle in both hands over a fish-tail burner, depress the drum in the flame, and rotate rapidly. Avoid putting on a heavy coating of smoke, as a more delicate tracing is obtained when the paper is lightly smokgd- The speed of the drum can be varied by putting in or taking out a small vane. Arrange an electro -magnetic time- marker for writing seconds (Fig. 88). Pith a frog (brain only), expose the heart, and put under it a cover-slip to give it support. Pin the frog on the myograph- plate, and adjust the foot of the lever so that it rests on the ven- tricle or the auriculo-ventricular junction. Bring the writing-point of the lever and that of the time- marker vertically under each other on the surface of the drum. Set off the drum at the slow speed (say, a centimetre a second). When the lever rests on the auriculo-ventricular junction, the part of the tracing corresponding to the contraction of the heart will be broken into two portions, representing the systole of the auri- cles and ventricle re- spectively. Cut the paper off the drum with a knife (keeping the back of the knife to the drum to avoid scoring it) and carry it to the vamishing- trough, holding the tracing by the ends witli both handSj smoked side up. Im- merse the middle of it in the varnish, draw first one end and then the other through the varnish, let it drip for a minute into the trough, and fasten it up with a pin to dry. (2) Heart Traciiig, with Simultaneous Re- la) For this purpose two Fig. 88. — Electro - Magnetic Time - Marker connected with Metronome. The pendulum of the metro- nome carries a wire which closes the circuit when it dips into either of the mercury cups, Hg. cord of Auricular and Ventricular Contractions. levers may be arranged, one resting on the auricle, the other on the ven tricle, the writing-points being placed in the same vertical straight line on the drum. A convenient form of apparatus is shown in Fig. 89. (6) Gashell's Method [a modification of). — Attach a silk ligature to the very apex of the ventricle. Divide the frsenum, cut the aorta 13 194 THE CIRCULATION OF THE BLOOD AND LYMPH across close to the bulbus, pinch up a tiny portion of the auricle and ligature it. Remove the intestines, liver, lungs, etc., care being taken in cutting away the liver not to injure the sinus. Then remove the lower jaw, and cut away the whole of the body except the head, part of the oesophagus, and the tissue connecting it with the heart. Fix the head in a clamp sliding oh an ordinary stand. The heart is held at the auriculo-ventricular junction in a (^skell's clamp supported on a separate stand. The thread connected with the ventricle is brought round a pulley and attached to a lever above the heart. The auricle is connected with another lever. The writing-points of the two levers are arranged in a vertical line on the drum. The small pulley must be oiled from time to time to lessen the friction (Fig. 90). If tortoises or turtles are available, the much larger, heart of these animals may be used for Experiment? 5 (2) (a) and (b). The animal having beeh killed by cutting off its head, the ventral portion of the carapace is detached by the saw. The pericardium can now be slit open, and the pads of the levers arranged on auricles and ventricle Aurtcului' le-ncr , Pad io rest Ofi Auricle . Pad to rest on Ventricle -Apparatus for obtaining a Siinultaneous Tracing of Auricular and Ventricular Contractions. , respectively, as in Experiment 3 (2) (a), without further disturbing the heart. Or the heart may be removed, together with the upper portion of the body, the pericardium opened, and the liver cut away. The aortic trunk is then divided, and the portion of it attached to the heart grasped by a small forceps clamp. Fine silk ligatures are attached to the apex of the ventricle and the top of the right auricle. The vagus nerves are exposed in the neck, Ugated, and divided. The upper portion of the body is supported on a stand. The forceps grasp- ing the aorta is fixed in an ordinary holder, and the threads are attached to the levers, as in Experiment 5 (2) (6). With the vagi, Experiment 7 may be performed. It must be remem- bered that the activity of the two vagi is unequal in the tortoise, the right being the more active. 6. Dissection of the Vagus and Cardiac Sympathetic Nerves in the Frog. — (i) Put the tissues in the region of the neck on the stretch by passing into the gullet a narrow test-tube or a thick glass rod moistened with water, and by pinning apart the anterior limbs. Expose the heart PRACTICAL EXERCISES 195 tM by cutting through the pectoral girdle in the way described in 2 (p. 192). On clearing away a little connective tissue and muscle with a seeker, three large nerves will come into view. The upper is the glosso- pharyngeal, the lower the hypoglossal; the vagus crosses diagonally between them (Fig. 91). Above the vagus trunk, running parallel to it, and separated from it by a thin muscle and a bloodvessel (the carotid artery), lies its laryngeal branch. The vagus should be traced up to the gang- lion situated on it near its exit from the skull. (2) Then cut away tlio lower jaw, dividing and reflecting the membrane covering the roof of the mouth. At the junction of the skull and the back- bone will be seen on each side the levator anguli scapulae muscle (Fig. 92). Remove this muscle care- fully with fine forceps. Clear away a little con- nective tissue lying just over the upper cervical vertebrae, and the sym- pathetic chain, with its ganglia, will be seen. Pass a fine silk thread beneath the sympathetic about the level of the large brachial nerve, by means of a sewing-needle which has been slightly bent in a flame and fastened in a handle. Tie the ligature, divide the sympathetic be- low it, and isolate it care- fully with fine scissors up to its junction with the , vagus ganglion. Batteries — To set up a Daniell Cell. — Fill the por- ous pot (Fig. 219, p. 697), previously well soaked in water, with dilute sulph- uric acid (i part of com- mercial acid to 10 or 15 parts of water) to within Fig. 90. — Arrangement for recording Auricular and Ventricular Contractions (and studying the Influence of Temperature of the Heart). C, clamp holding the heart at the auriculo-ven- tricular groove ; P, pulley round which a thread attached to the apex of the ventricle passes to the lever L'; L, lever connected with auricle. (The rest of the arrangement is for studying the influence of temperature on the heart and its nerves, G teing a vessel filled with physiological salt solution in which the heart is immersed ; R, an inflow tube from a reservoir containing salt solution at the temperature required; O', an out- flow tube by which G may be emptied into the beaker B'; O, a tube passing to the beaker B to prevent overflow from G; T, a thermometer.) I J inches of the brim, and place in it the piece of amalgamated zinc. If the zinc is not properly amalgamated, leave it in thepot for a minute or two to clean its surface. Then lift it out, pour over it a little mercury, and rub the mercury thoroughly over it with a cloth. Put the pot into the outer vessel, which contains the copper plate, and is filled with a saturated solution of sulphate of copper, with some undissolved 196 THE CIRCULATION OF THE BLOOD AND LYMPH crystals to keep it saturated. After using the Daniell, it must always be taken down. The outer pot is left with the copper plate and the sulphate solution in it. The zinc is washed and brushed bright. The sulphuric acid is poured into the stock bottle, and the porous pot put into a large jar of water to soak. The Bichromate Cell contains only one liquid — a mixture of i part of sulphuric acid with 4 parts of a 10 per cent, solution of potassium bichromate. In this is placed one, or in some forms two, carbon plates and a plate of amalgamated zinc. After using the battery, take the zinc out of the liquid. The Leclanche battery consists of a porous pot filled with a mixture of manganese dioxide and carbon packed around a carbon plate, which ioims the positive pole. The pot stands in an outer jar of glass filled with a saturated solution of ammonium chloride, into which dips an amalgamated zinc rod, which constitutes the negative pole. Various forms of dry batteries can be conveniently used for running induction- coils or time-markers, but are not ; Gictss rod Closso- pharyn^tal Lqiyn^eaL branch of Vagus adapted for yielding constant cur- rents of long duration. 7. Stimulation of the Vagus in the Frog. — ^Make the same arrange- ments as in 5 (i) (p. 192), but in addition set up an induction machine arranged for an inter- rupted current (Fig. 93), with a Daniell, a bichromate, a Leclanche, or a dry cell in the primary circuit, which should also include a simple key. Insert a short-circuiting key in the secondary circuit. Attach the electrodes to the short-circuit- ing key, push the secondary coil up towards the primary untU the shocks are distinctly felt on the tongue when the Neef 's hammer is set going and the short-circuiting key opened. Pith the brain of a frog, expose the heart, dissect out the vagus on one side, Ugature it as high up as possible, and divide above the ligature. Fasten the electrodes on the cork plate by means of an indiarubber band, and lay the vagus on them. Set the drum off (at slow speed). After a dozen heart -beats have been recorded, stimulate the ^vagus for two or three seconds by opening the short- circuiting key. If the nerve is active, the heart will be slowed, weakened, or stopped. In the last case the lever will trace an unbroken straight line ; but even if the stimulation is continued the beats will again begin. 8. Stimulation of the Junction of the Sinus and Auricles. — After a ■ sufficient number of the observations described in 7 have been taken with varying time and strength of stimulation, take the writing-points off the drum, apply the electrodes directly to the crescent at the junc- tion of the sinus venosus with the right auricle, and stimulate. The heart will be affected very much in the same way as by stimulation of the vagus, except that during the actual stimulation its beats may be quickened and the inhibition may only begin after the electrodes have been removed (Fig. 70, p. 158). Fig. 91. — The Relations of the Vagus in the Frog. PRACTICAL EXERCISES 197 ZAS g. Effect of Muscarine (or Pilocarpine) and Atropine. — Paint on the sinus venosus with a small camel's-hair brush a very dilute solution of muscarine (or of pilocarpine). The heart will soon be seen to beat more slowly, and will ultimately stop in diastole. Now apply a dilute solution of sulphate of atropine to the sinus. The heart will again begin to beat. Stimulation of the vagus will now cause no inhibition of the heart, because its endings have been paralyzed by atropine. (Muscarine or pilocarpine has also been applied to the heart, but it could be shown by a separate experiment that atropine by itself|has the same effect on the vagus endings — p. 164.) 10. Stannius' EKperiment. — ^Pith a frog. Expose the heart in the way described under 2 (p. 191). Ligature the fraenum with' a fine silk thread, and use the thread to manipulate the heart. With a curved needle pass a moistened silk thread between the aorta and the superior vena cava, and tie it round the junction of the sinus and right auricle (Fig. 86). The auricles and ventricle stop beating as soon as the ligature is tightened. The sinus venosus goes on beat- ing. Now separate the ven- tricle from the rest of the heart by an incision through the auriculo-ventricular groove, or tie a second ligature in the groove. The ventricle begins to beat again, the auricle re- maining quiescent in diastole (p. 165). Occasionally both auricle and ventricle, or only the auricle; may begin to beat. 11. Stimulation of Cardiac Sympathetic Fibres in the Frog — (i) In the vagosympathetic after the inhibitory fibres ' have been cut out by atropine. — Arrange everything as in 7 (p. 196). Assure yourself, by stimulating the vagus, that it inhibits the heart, and take a tracing during stimulation. Then paint a dilute solution of atropine on the sinus. Stimulation of the vagus, which is really the vago-sympathetic (see Fig. 92), will now cause, not inhibition, but augmentation (increase in rate or force, or both), since the endings of the inhibitory fibres have been paralyzed by atropine. The strength of the stimulating: current required to bring out a typical augmentor effect is greater than that needed to stimulate the inhibitory fibres. ■ Take a tracing to show augmentation produced by stimulating the nerve. (2) By direct stimulation of the cervical sympathetic. — ^Make the same arrangements as in 11 (1), but, instead of isolating the vagus, dissect out the sympathetic on one side in the manner described in 6 (2) (p. 195) , and do not apply atropine to the heart. Lay the upper (cephalic) end of the sympathetic on very fine and well-insulated electrodes, and stimulate (Fig. 76, p. 165). (To insulate electrodes the points may be covered with nlelted paraffin. When the paraffin has cooled, a narrow Fig. 92. — Relation of the Sympathetic to the Vagus in the Frog (after Gaskell). Sym, sympathetic chain ; G, ganglion of the vagus; VS, vago-sympathetic; GP, glosso -pharyngeal nerve; LAS, levator anguli scapulee muscle ; SA, subclavian artery; A, descending aorta; V,^ vertebra] column; OC, occipital part of skull; 1-4, spinal nerves. 1^8 THE CIRCULATION OF THE BLOOD AND LYMPH groove, just sufficient to lay bare the wires on the upper side, is made in it, and the nerve is laid in this groove.) Experiments 7, 11 (i) and 11 (2) will be rendered more exact by connecting a second electro-magnetic signal with a Pohl's commutator without cross-wires (Fig. 94), in such a way that the circuit is inter- rupted at the instant when stimulation begins. iz. The" Action of Inorganic Salts on Heart-Muscle. — Expose and remove the heart of a tortoise or turtle (p. 194). Cut off the apical two- thirds of the ventricle by an incision parallel to the auriculo-ventricular groove. By a second parallel cut remove a ring of tissue 2 or 3 milli- metres wide from the upper end of this portion of the ventricle. Divide the ring at opposite ends of a diameter, so as to form two strips. Tie a fine silk thread to each end of one strip. Attach one of the threads to the ^hort limb of a glass rod bent at right angles, so that it can be immersed at will in a beaker. The other end of the rod is fixed in a holder ^sliding on a stand. Attach the second thread to the short arm Fig. 93. — Arrangement of Induction Machine for Tetanus. B, battery; K, simple key; P, primary coil; S, secondary coil; A, C, binding screws to be connected with battery for single shocks; F, G, binding screws for tetanizing current; N, Neef's hammer; D, short-circuiting key in secondary; E, electrodes. D and E are drawn to a much larger scale than the rest of the figure. of a counterpoised lever arranged to write on a slowly-moving drum. If the strip is still beating, wait till the contractions have ceased ; then (i) Immersef the strip in a beaker filled with 07 per cent, solution of sodium chloride. After a time it begins to beat rhythmically. The contractions become rapidly stronger, and then after a while diminish, and gradually cease. The tone or tonus of the strip is diminished by the solution. (2) Arrange the other strip in the same way, and immerse it in a solution of calcium chloride (about i per cent.) isotonic with the sodium chloride solution used in (i). If the strip is contracting, the contrac- tions will cease. Rhythmical contractions will not appear as in the sodium chloride solution. The tone of the strip may be increased. (3) Remove most of the calcium chloride solution from the beaker, and fill it up with o'7 per cent, sodium chloride solution. The rhj^thmi- cal contractions will appear after a longer or shorter latent period, and will be stronger and last for a longer time than in the sodium chloride solution alone. (4) Immerse a fresh strip in a solution containing sodium chloride PRACTICAL EXERCISES I99 (o'7 per cent.), calcium chloride (o'o25 per cent.), and potassium chlonde (0-03 per cent.) (a modified Ringer's solution). A longer series of rhythmical contractions will be obtained than in either (l) or (3). That this is not due to the pptassium chloride acting alone can be shown by immersing a strip in a solution of potassium chloride (about 09 per cent.) isotonic with the sodium chloride solution used in (i). No contractions will be caused. 13. The Action of the Mammalian Heart. — Inject under the skin of a dog (preferably a small one) i c.c. of a 2 per cent, solution of morphine hydrochlorate for every kilo of body-weight. As soon as the morphine' has taken effect (in 15 to 30 minutes, but better after an hour), fasten the animal back down on a holder (as in Fig. 135, p. 295), pushing the mouth-pin behind the canine teeth and screwing the nut home.* In the meantime select a tracheal cannulaf of suitable size, arid get ready instruments for dissection — one or two pairs of artery-forceps, a pair of artery-clamps (bulldog pattern), two or three glass cannulsc of mnnMm •& Fig. 94. — ^Arrangement for recording tlie Beginning and End of Stimulation. C, Pohl's commutator witliout cross-wires ; B, battery in circuit of primary coil P ; B', battery in circuit of electro-magnetic signal T;K, simple key in primary circuit; S, secondary coil. When the bridge of the commutator is tilted into the position shown in the figure, the primary circuit is closed and the circuit of the signal broken. various sizes for bloodvessels, ten strong waxed ligatures, sponges, hot water, a towel or two, and a pair of bellows to be connected with the tracheal cannula when the chest is opened. Arrange an induction- * A simple but ef&cient and convenient holder for a dog may be easily constructed as follows: Take a board of the length required (2^ to 5 feet, according to the size of the dog) . At one end fasten two short upright wooden pins, with a clear space of 4 to 6 inches between them. These are pierced from side to side with four or five holes at different heights. An iron pin passes behind the canine teeth of the animal through two corresponding holes in the uprights, and the muzzle is tied over this by a cord which secures the head. For a large dog an upper pair of holes is used, for a small dog a lower pair. The feet are fastened by cords to staples inserted into the sides of the board, the fore-legs being drawn tailwards for all operations on the neck or head, head wards for operations on the thorax. A rabbit-holder can be made in exactly the same way. t A tracheal cannula is easily made by heating a piece of glass tubing, about 6 inches long, a short distance from one end, and drawing it out slightly so as to form a ' neck.' The tubing is then bent about its middle to an obtuse angle, and the end next the neck is ground obliquely on a stone. The diameter of the cannula should be about the same as that of the trachea, into which it is to be inserted by its oblique end. 200 THE CIRCULATION OF THE BLOOD AND LYMPH coil and electrodes for a tetanizing cuTrent (Fig. 93, p. 198). With scissors curved on the flat cUp away the hair from the front of the neck. Put the hair carefully away, and remove all the loose hairs with a wet sponge so that they may not get into the wounds. Give ether, or pour into the stomach by a tube 5 c.c. of a o'5 per cent, solution of chloroform in 10 per cent, alcohol per kilo of body-weight, diluted before administration with 3 or 4 volumes of water (Grehant's method). To put a Cannula in the Trachea. — The hair having been clipped in the middle line of the neck and the skin shaved, a mesial incision is to be made, beginning a little below the cricoid cartilage, which can be felt with the finger. The trachea is then cleared from its attach- ments by forceps or a blunt needle, and two strong ligatures are passed beneath it. A single loop is placed on each of these, but is not drawn tight. Raising the trachea by means of the upper ligature, the student makes a longitudinal incision through two or three of the cartilaginous rings, inserts the cannula, and ties the lower ligature firmly around its neck. The upper ligature can now be withdrawn. Clip off the hair on each side of the sternum. Make an incision on each side through the skin and down to the costal cartilages about 2 inches from the edge of the breast-bone, and long enough to expose about four costal cartilages (say, 3rd to 6th). With a curved needle pass waxed ligatures round the cartilages, and tie firmly to compress the intercostal vessels. The bellows should now, or earher if any symptoms of impeded respiration have appeared, be connected with one end of the horizontal limb of a glass T-piece, the other end of which is similarly connected with the tracheal cannula. The stem of the T-piece is provided with a short piece of rubber tubing, which, when artificial respiration is being carried on, is to be alternately closed and opened — closed during inflation of the lungs, and opened when the air is to be allowed to escape from them. Or a screw-clamp may be adjusted on the piece of tubing so that the opening is sufficiently narrow to permit the lungs to be properly inflated when the bellows are compressed, and yet sufficiently wide to permit easy escape of the air and collapse of the lungs at the end of each inflation. Ether may, when necessary, be administered, by inserting between the T-piece and the tube from the bellows an ether bottle with two tubes passing through the cork to within an inch or two of the ether. If the cannula has a side-opening, as is usually the case with metal cannulae, the T-piece may be dispensed with. One student should take sole charge of the artificial respiration, which ought to be begun as soon as the chest has been opened, and continued at the rate of about twenty inflations per minute. The costal cartilages are rapidly cut through with strong scissors just on the sternal side of the ligatures, the artificial respira- tion being suspended for an instant, as each cut is made, to avoid wounding the lungs. The sternum is divided at its lower end and turned up. If there is much bleeding a ligature ^ould be tied round its upper end. With a curved needle a ligature ife passed below the internal mammary arteries as they approach the sternum. That bone may now be removed, and the heart, enclosed in the pericardium, comes into view. A thread is passed with a suture-needle through each side of the pericardium, which is then stitched to the chest-wall and opened. (a) Note the various portions of the heart, right and left ventricles, right and left auricles, with the auricular app:ndices. Feel the heart with the hand, and observe that the right ventricle is softer and has thinner walls than the left, and that the. auricles are softer than the ventricles. Note how all the parts of the heart harden in the hand during systole and soften during diastole (pp. 86, 90). PRACTICAL EXERCISES Arl (6) Dissect out the vago-sympathetic on one side in the neck of the dog. The guide to the nerve is the carotid artery. These two struc- tures and the internal jugu- lar vein lie side by side in a common sheath. Feel for the artery a little ex- ternal to the trachea, cut down on it, open the sheath, isolate the vago - sympa- thetic for about an inch, pass two ligatures under it, tie them, and divide be- tween the ligatures. The peripheral and central ends of the nerve may now be successively stimulated. Stimulation of the peri- pheral end causes slowing of the heart, or stoppage in diastole. Feel that it softens when it stops. It soon begins to beat again. Stimulation of the- central end of the vago-sympa- thetic may or may not cause inhibition. If it does, expose the other vago- S)anpathetic, divide it, and repeat the stimulation of the central end . There will now be no inhibition of the heart. Incidentally it inay be seen that stimulation of the central end of the vago - sympathetic causes strong, though, of course, with opened chest , abortive , respiratory movements. (c) Pith a frog (brain and cord), dissect out the sciatic nerve on one side up to the sacral plexus. Cut ofE the whole leg. Drop the cut end of the nerve on the heart, and hold the prep- aration so that the ngrve touches the heart also by its longitudinal surface. At each cardiac beat the nerve is stimulated by the action current (p. 807), and the muscles of the leg contract. (d) Raise the board so that the head of the animal is down and the hind-feet up, and note whether there is any efiect on the action B \y^ M Fig. 95. — Myocardiograph of Adami and Roy (modified by Cushny and Matthews). AB, a perpendicular rod descending from a universal joint, which is not shown in the figure; CD, a brass sheath, moving easily on the rod, and bearing on its upper end an ivory pulley, and at its lower end a horizontal bar, which is inter- rupted by a plate of hard rubber, I. The per- pendicular rod EF moves on the horizontal bar by the hinge-joint, J . EF is hooked at one end for attachment to the heart, and bored at the other for a thread which, passing over the pulley at C, passes through the universal joint and moves a writing lever not shown in the figure. CD is prevented from moving up AB by a ring of brass, G, which is screwed to AB, but is not attached to CD ; the hook F can therefore move to and from AB, and can rotate round it, while it cannot move up or down. The hooks F and B are insulated from each other by the hard rubber, I. H is a binding post through which, and ■ through another connected with A, induction shocks may be sent at will into the portion of the heart lying between the hooks. 202 THE CIRCULATION OF THE BLOOD AND:''LYMPH and filling of the heart, feet down. Repeat the observation with head up and (e) Compress the aorta with the fingers, and observe the effect on the degree of dilatation of the Various cavities of the heart. Repeal the experiment with the inferior vena cava, and compare the results. "" (/) Smoke a drum. Insert the hooks of the myocardiograph (Fig. 95 ] into the ventricle, taking care not to : penetrate deeply into the wall. Arrange the lever to write on the drum. While a tracing is being taken stimulate the peripheral end of the vagus. Unhook the cardiograph. (g) Stop the artificial respiration, and observe the changes which take place in the auricles and ventricles, comparing particularly the right side of the heart with the left. Before the heart has stopped beating, re- commence the artificial respiration. (h) Connect a cylinder of oxygen with a good-sized rubber catheter. D ,c Fig. 96. — Arrangement to illustrate Action of Cardiac Valves in the Heart of an Ox (Gaa.). C, glass window in left auricle; D, window in aorta; E, tube inserted through apex of heart into left ventricle and connected with pump P; A, side tube on E, through which wires are connected with a tiny incandescent lamp in the ventricle; W, water in bottle B; T, T', tubes. and pass the catheter down the tracheal cannula or through a separate opening in the trachea. Allow a small stream of oxygen to flow into the lungs. Artificial respiration is now unnecessary. The lungs remain at rest, yet the blood is sufficiently oxygenated, and the heart goes on beating. The myocardiographic tracing thus goes on undis- turbed by respiratory movements. («') Stop the oxygen, and resume the artificial respiration. Make a PRACTICAL EXERCISES 203 -a -d small penetrating wound with a scalpel in the left ventricle. Observe the course of the haemorrhage, and note especially the difference in systole and diastole. {;') Lay the electrodes on the heart, and stimulate it with a strong interrupted current. The character of the contraction soon becomes profoundly altered. Shallow, irregular contractions flicker over the surface, with a kind of simmering movement sugges- tive of a boiling pot (delirium cordis, fibrillar contraction). Now kill the ani- mal by stopping the artificial respiration . Observe how long the heart continues to beat, and which of its divisions stops last. (k) Make a dissection of the cervical sympathetic up to the superior cervical ganglion, and down through the inferior cervical ganglion to the stellate or first thoracic ganghon. Make out the annulus of Vieussens and the cardiac sympa- thetic (accelerator) branches going off from the annulus or the inferior cervical ganglion to the cardiac plexus (Fig. 74, p. 162). 14. Perfusion of the Isolated Mam- malian Heart. — ^The heart of a dog em- ployed for some other experiment may be used. Or a rabbit may be killed by a blow on the back of the head, and the heart at once removed. The aorta should not be cut off too short. Tie a cannula into the aorta and detach it to a T-piece connected by rublaer tubes, which must be perfectly clean, with two bottles, one containing Ringer's solution (pp. 66, 199), preferably that made with dextrose, Ihe other containing defibrin- ated blood diluted with Ringer's solu- tion. The defibrinated blood should be strained so as to remove any small pieces of fibrin. The bottles are supported on a high stand, so that the level of the bottles above the he^irt can be altered, and the pressure of the perfusion liquid thus varied. Perfusion may be begun with Ringer, to wash out any remaining blood and obviate the possible formation of clots in the small vessels. Oxygen is allowed to bubble through the Ringer's solution, but this is not necessary for the blood, since, if shaken up, it will retain far more oxygen than the Ringer's solu- tion. The temperature of the liquids should be at about 40° C. when nearing the heart. This can be most easily insured by interposing a worm immersed in a heated bath or other heating arrangement between the cannula and the T-tube, and for the study of its movements by inspection the heart itself can be placed in a glass vessel immersed in the bath. When records of the contractions Fig. 97. — Mammalian Heart Per- fusion Apparatus (Gunn). u, Liebig condenser, . cut off as shown ; b, inlet for the warm Water ; d, thermometer almost filling up the lumen of the thin glass tube c ; e, cork ; /, cannula for aorta fitted with a collar of rubber tubing, g, in the end of the tube c ; h, Y-tube connected with two\reservoirs, one contain- ing Ringer's solution, the other any other liquid which is to be perfused. 204 THE CIRCULATION OF THE BLOOD AND LYMPH are to be obtained, threads are attached to the auricle and to the apex of the ventricle. The heart is suspended by fastening the cannula in a holder on a stand, and the threads, after passing over pulleys to give them a convenient direction, are attached to writing-levers. As the heart cannot now be easily kept immersed in the bath, it is suspended in the air, and can be kept warm by the following simple arrangement : A copper pipe about 4 inches long is slit on one side, and on the opposite side is screwed or riveted to a copper rod, under which is hung a spirit-lamp. The lamp is adjusted at such a point on the rod that when the coppsrtube is placed around the heart the heat conducted along the rod keeps the air around the heart at about body-temperature. The perfusion liquid before it enters the heart may be heated thus : A Liebig's condenser is cut through the middle, and the large end closed by a paraffined cork. A glass tube is run down from the top through this cork, and the aorta is attached directly to this, so that the heart is very near the condenser. This tube is mostly filled up by a thermometer, so that the perfusion liquid passes through it in a thin stream which is easily heated by the water in the condenser, which contains a second ther- mometer. This water is kept constantly flowing through the condenser from a heated bath. The T-piece connecting with the perfusion bottles is attached to the upper end of the glass tube to which the heart is attached (Gunn and Cushny). 15. Action of the Valves of the Heart. — (i) Study the action of the valves of the ox- heart, connected with the pump P and bottle B in the artificial scheme, as shown in Fig. 96. The cavity of the heart is illuminated by means of a small electric lamp, the wires of which pass in at A. When the piston of the pump is pushed down, water is forced through the aorta D along the tube T into the bottle, and flows back again into the left auricle by the tube T' During each stroke of the pump the auriculo-ventricular valve is seen through the glass disc inserted into C to close, and the semilunar valve is seen through the glass in D to open. When the piston is raised, the semilunar valve is seen to be closed and the auriculo-ventricular valve to be opened. For comparison, a human heart with a valvular lesion might be used. (2) With the sheep's or dog's heart provided, perform the following experiments : [a) Open the pericardium and notice how it is reflected around the great vessels at the base of the heart. Distinguish the pulmonary artery, the aorta, the superior and inferior venae cavje, and the pul- monary veins. The trachea and portions of the lungs may also be attached. If so, remove them carefully without injuring the heart. Fig. 98. — Diagram of Valves of the Heart. The valves are supposed to be viewed from above, the auricles having been partially removed. A, aorta with semilunar valve; B, pulmonary artery and valve; C, tricuspid, and D, mitral valve; E, right, and F, left coronary artery ; G, wall of right, and H, of left auricle; I, wall of right, and J, of left ventricle. PRACTICAL EXERCISES 205 (6) Take two wide glass tubes, drawn slightly into a neck at one end. One of the tubes should be about 10 cm. long, and the other about 50 cm. Tie the short tube A firmly by its neck into the superior vena cava, the long tube B into the pulmonary artery. Ligature the inferior vena cava. Connect A by a small piece of rubber tubing with a funnel supported in a ring on a stand. Pour water into the funnel till the right side of the heart is full. It will escape from the left azygos vein, which must be tied. Put on any additional ligatures that may be needed to render the heart water-tight. Support B in the vertical position by a clamp. Fill the funnel with water, and it will rise in B to the same level as in the funnel. Now compress the right ventricle with the hand, and the water will rise higher in B. Relax the pressure and notice that the water remains at the higher level in B, being pre- vented by the semilunar valves from flowing back into the ventricle. By alternately compressing the ventricle and allowing it to relax, water can be pumped into B till it escapes from its upper end, and if this is so curved that the water falls into the funnel, a ' circulation ' which imitates that of the blood can be established. Note that during the pumping the sinuses of Valsalva, behind the semilunar valves at the origin of the pulmonary artery, become prominent. (c) Take out B and tear out one of the segments of the semilunar valve. Replace B, and notice that, while compression of the ventricle has the same effect as before, the water no longer keeps its level on relaxation, but regurgitates into the ventricle. This illustrates the condition known as insufficiency or incompetence of the valves. But if the injury is not too extensive, it is still possible, by more vigorously and more rapidly compressing the heart, to pump water into the funnel. This illustrates the establishment of compensation in cases of valvular lesion. {d) Now remove both tubes. Tie the pulmonary artery. Cut away the greater part of the right auricle. Pour water into the aurjculo- ventricular orifice, and notice that the segments of the tricuspid valve are floated up so as to close the orifice. Invert the heart, and the ventricle will remain full of water. Open the right ventricle carefully, and study the papillary muscles and the chordae tendineae, noting thjit the latter are inserted into the lower surface of the segments of the tricuspid valve, as Well as into their free edges. (e) Repeat (6), (c), and [d) on the left side of the heart, tying tube B into the aorta as far from the heart as possible, and A into the left auricle. (/) Separate the aorta from the left ventricle, cutting wide of its origin so as not to injure the semilunar valves, and tie a short wide tube into its distal end. Fill the tube with water, and notice that the valves support it. Cut open the aorta just between two adjacent segments of the valve, and notice the pockets behind the segments, andhowthey are related to each other, and connected to the wall of the vessel. 16. Sounds of the Heart. — (a) In a fellow-student notice the position of the cardiac impulse, the chest being well exposed. Use both a binaural and a single-tube stethoscope. Place the chest-piece of the stethoscope over the impulse, and make out the two sounds and the ■pause. (6) With the hand over the radial or brachial artery, try to determine whether the beat of the pulse is felt in the period of the sounds or of the pause, (c) Listen with the stethoscope over the junction of the second right costal cartilage with the sternum, and cornpare the relative intensity of the two sounds as heard here with their relative intensity as heard over the cardiac impulse. 17. Cardiogram. — Smoke a drum, and arrange a recording tambour and a time-marker beating half or quarter seconds to write on it (Fig. 88, 206 THE CIRCULATION OF THE BLOOD AND LYMPH p. 193). Apply the button of a cardiograph (Fig. 27, p. 90) over your own cardiac impulse, and fasten it round the body by the bands attached to the instrument. Connect the cardiograph by an indiarubber tube with a recording tambour (Fig. 99). Set the drum off at a fast speed, take a tracing, and varnish it. Compare with Fig. 28 (p. 91), and if the tracing is sufficiently typical, as is often not the case with human cardiograms, measure out the time-value of the various events in the cardiac revolution. Fig. 99. — Marey's Tambour. For the cardiograph, a small glass funnel, or thistle-tube, the stem of which is connected with the recording tambour, may be substituted, the broad end of the funnel being pressed over the apex-beat. 18. Sphygmographic Tracings. — ^Attach a Marey's sphygmograph (Fig. 37, p. 165) to the arm. Fasten a smoked paper on the plate D. Apply the pad C of the sphygmograph to the wrist over the point where the pulse of the radial artery can be most distinctly felt. Adjust the pressure by moving the screw G. The writing-point of the lever E will rise and fall with every pulse-beat. When everything is satisfactorily arranged, set off the clockwork which Fig. 100. — Dudgeon's Sphygmograph. moves the plate D, and a pulse tracing will be obtained. Study the changes which can be produced in the pulse curve — (a) by altering the position of the body (sitting, standing, and lying down) ; (6) by exercise (Fig. loi) ; (c) by inhalation of 2 drops of amyl nitrite poured on a hand- kerchief by the demonstrator (Fig. 102); {d) by raising the arm above the head and letting it hang at the side ; (e) by compression of the brachial artery at the bend of the elbow; (/) by altering the pressure of the pad. Varnish the tracings after marking on them the conditions under which they were obtained. A Dudgeon's sphygmograph (Fig. 100) may also be employed. In this the clockwork carries the strip of blacl^ened paper along beneath PRACTICAL EXERCISES 207 Fig. loi. — Effect of Exercise on the 'IPulse (Marey). Upper tracing, normal; lower, after running. the needle which records the movements of the artery. Or a small glass funnel or thistle-tube connected with a recording tambour may be pressed over the carotid artery. The lever of the tambour writes on a drum, on which at the same time half or quarter seconds are marked by an electro-magnetic signal. 19. Venous Pulse Tracing from the Jugular Vein. — ^Arrange a recording tambour to write on a drum. Con- nect the tambour with the stem of a small glass thistle-tube or funnel (or with a small metal cup) by a piece of narrow rubber tubing, and apply the cup-shaped end of the thistle-tube over the right jugular bulb of a fellow-student. This lies about 1 inch external to the right stemo-clavicular articulation, and a little above it. The receiver may have to be moved about a little until the best pulsation is obtained. The ' patient ' should be lying down, the shoulders slightly raised, the head on a pillow and turned slightly to the right, in order to relax the right stemo-mastoid"muscle (Mackenzie). 20. Polygraph Tracings.-rr-Arrangs the polygraph over the radial artery, as with an ordinary sphygmograph, so that the lever will record the radial pulse when the strip of paper is set moving. If the instru- ment has only one tambour, con- nect the tambour to a receiver or thistle-tube over the jugular bulb. The writing-point of the tambour is arranged so as to be immediately below the writing- point connected with the radial. If the polygraph is provided with clockwork to -record time, set off the time-marker writing fifths of a second. When it is seen that the writing-points are marking properly, start the clockwork which moves the strip of smoked paper. Repeat the observation with the tam- bour connected with the apex- beat. Letter the curves as far as possible as in Figs. 65 and 66 (p. 149) without at present attempting their exact analysis. , . ,^ if the polygraph has two tam- bours, simultaneous tracmg of the radial pulse, the jugular pulse, and the cardiac impulse, or of the carotid pulse, the jugular pulse, and the apex^ peat, may be taken, and other combinations as well. If no polygraph is available, a drum may be employed, the tracings being all taken with tmstle-tubes connected with recording tambours. The levers of the tambours must be arranged to write on the drum in the same vertical straight line, or, without making the adjustment quite exact, vertical hnes 01 reference may be drawn through each curve, with the drum at rest maicatmg the relative positions of the writing-points. Fig. 102.— Effect of Amyl Nitrite on tlie Pulse (Marey). Upper tracing, normal ; lower, after inhalation of amyl nitrite. 2o8 THE CIRCULATION OF THE BLOOD AND LYMPH 21. Plethysmographic Tracings. — Connect the vessel D (Fig. 36, p. 12%), directly with a recording tambour by the tube F, omitting for simplicity the recording arrangement in the figure. Place the arm in the plethysmograph, and adjust the indiarubber band to make a watertight connection. Support D so that the arm rests easily within it, and fill it with water at body temperature. No water must get into the tambour, and it is well to insert a piece of glass tubing in the connection between it and the plethysmograph, so that it may be seen when the water is rising too high. A T -piece with a short piece of rubber tubing on the stem should be inserted in the course of the tube leading to the tambour. All adjustments are made with the T-piece open, and when a tracing is to be taken the short rubber tube is closed by .a clip. Arrange a time-marker to write half or quarter seconds (Fig. 88, p. 193). Adjust the writing-point to write on a drum, and close the upper tubulure C with a cork. The quantity of blood in the arm is increased with every systole of the left ventricle, diminished in diastble. The lever will therefore rise when thfe ventricle contracts, and sink when it relaxes. (i) Take tracings with the arm (a) horizontal, (b) hanging down. I2) With the arm horizontal, take tracings to show the effect (a) of closing and opening the fist inside the plethysmograph;* (6) of apply- ing a tight bandage round the arm a little way above the indiarubber band; (c) of inhaling 2 drops of amyl nitrite. Instead of the arm plethysmograph, a small plethysmograph to hold a finger may be employed. It consists of a glass tube drawn out at one end. The wide end is provided with a rubber collar. The narrow .end is, connected by a small rubber tube with a very small and sensitive recording tambour, a T-piece being inserted on the connection as before'. With the T-piece closed fill the tube w^h water. Then, holding up the ■vvide end of the tube, the tip of the finger is put in so as just to close the tube. The T-piece is then raised arid opened, and the finger pushed in as far as it will go. The collar must fit the finger so as to form a watertight joint. Now get the propeir pressure in the tambour by blowing into the T-piece, and close the clamp. A time-tracing can be taken as before. 22. Pulse-Rate. — (i) Count the radial pulse for a minute in the sitting, supine, and standing positions. Use a stop-watch, setting it ofP on a pulse-beat and counting the next beat as one. Make three ob.servations in each position. (2) Count the pulse in a person sitting at rest, and then again in the sitting position immediately after active muscular exertion. Note how long it takes before the pulse-rate comes back to normal. (3) Count the pulse in a person sitting at rest. Repeat the observa- tion while water is being slowly sipped, and note any change. (4) With one hand over the thorax of a rabbit, count its pulse. Then nbtice the effect (a) of suddenly closing its nostrils, (6) of bringing a small piece of cotton-wool sprinkled with ammonia or chloroform in front of the nose [reflex inhibition of the heart). 23. Blood-Pressure Tracing. — (a) Put a dog under morphine (p. 63). Set up an induction machine arranged for an interrupted current (Fig. 93, p. 198). Fill the U-shaped manometer tube (if this has hot already been done) with clean mercury to the height of 10 to 12 cm. in each limb. If the float tends to stick, half an inch of oil may be put above the mercury in the distal (straight) limb before putting in the float. But where the mercury is clean and dry, and the * Closing the fist causes a fall in the curve — i.e., a diminution in the volume of the arm. On opening the hand, the curVe regains its level, Practical exerCisbs io§ size of the float properly adjusted to that of the tube, this is not neces- sary, and is to be avoided. Then, tilting the tube carefully, fill the proximal limb [i.e., the limb which is to be connected with the blood- vessel) with a saturated solution of sodium carbonate or a half -saturated solution of magnesium sulphate, or, what is better for most purposes, a 2 per cent, solution of sodium citrate. This is easily done by means of a pipette furnished with a long point. Now attach a strong rubber tube to the proximal end of the manometer, and fill it also with the solution. All air must be got out of the manometer and its connecting- tube. Raise th: end of the rubber tube and blow into it, so as to cause a difference of about lo cm. in the height of the mercurj' in the two limbs of the manometer, and, without releasing the pressured clamp the tube with a pinchcock or screw clamp (Fig. 41, p. no). Now smoke a drum, and arrange the writing-point of the manometer- float so that it will write on it. Suspend a small weight by a piece of silk thread from a support attached to the stand of the drum, so that it hangs' down outside of the writing-point of the manometer-float and always keeps it in contact with the smoked surface without undue friction. Or a piece of glass rod drawn out to a fine thread in the blowpipe flame answers very well. Below the writing-point of the float, and in the same vertical line with it, adjust the writing-point of a time-marker beating seconds (Fig. 88, p. 193). Next fasten the animal on a holder, back down. Give ether and insert a tracheal cannula (p.. 200). (The tracheal cannula is not abso- lutely required for the experiment, but it is convenient, as the animal is more under control, and artificial respiration can be begun at any moment, should this be necessary.) Insert a glass cannula, armed with a short piece of rubber tubing, into the central (cardiac) end of the carotid artery (p. 63). Leaving the bulldog forceps on the artery, fill the cannula and tube with the sodium citrate or one of the other solutions. Slip the rubber tube over a short glass connecting-tube. Fill this also with the solution, and connect it with the manometer-tube, seeing that both are quite full of liquid, so that no air may be enclosed. Where a permanent working place is provided for blood-pressure experiments it is convenient to connect the cannula and manometer with a pressure-bottle containing the sodium citrate solution, and to use a three-way cannula for the bloodvessels (Fig. 103). The cannula has a bulbous enlargement, which hinders clotting. The end of the cannula is connected with the tube from the pressure-bottle, which is closed by a clip, and the side-tube is connected with one limb, E, of the manometer shown in Fig. 104. E is itself provided with a side- tube, F, armed with a short piece of rubber tubing. The cannula does not require to be filled with liquid before being inserted into the artery. By opening F and releasing the clip on the tube from the pressure- bottle the cannula and the tube connecting it with the manometer can be fiUed, and any blood-clots can be easily washed out in the course of ail experiment. Before the bulldog forceps is taken off the artery to obtain a blood-pressure tracing, F must be closed, and the clip on the tube from the pressure -bottle opened. The bottle is attached to a strong cord passing over a pulley, by which it is raised to a height sufiicient to balance approximately the pressure in the artery. The tube to the pressure-bottle is then clipped. If no manometer with side-tube is available, a T-piece can be inserted in the connection between the cannula and the manometer, and the cannula can be washed out through this. Now take the btilldog forceps off the artery, and allow the drum to revolve at slow speed. The writing-point of the manometer-float will 14 tSe circulation of the blood and lymph trace a curve showing an elevation for each heart-beat, and waves due to the movements of respiration. (6) Isolate the vago-sympathetic nerve in the neck. Ligature doubly, and cut between the ligatures. Stimulate the peri- pheral (lower) end ; the heart will be slowed or stopped, and the blood-pressure will fall. Stimulate the central (upper) end; there may be inhibition of the heart or accelera- tion', and the pressure may fall or rise (p. 168). (c) Expose and divide the other vago- sympathetic while a tracing is being taken. Again stimulate the central end of the nerve and observe whether there is any effect. (i^) Expose the sciatic nerve in one leg, as follows: The leg having been loosened from the holder, the foot is seized by one hand and lifted straight up, so as to put longer Fig. 103. — Three-way Cannula. the skin of the thigh on the stretch. An incision is now made in the middle line on the posterior aspect of the thigh, through the skin and subcutaneous tissue. The muscles are separated in the line of the incision with the fingers, and the sciatic nerve comes into view lying deeply be- tween thein. Place a double ligature on it, and divide between the ligatiires. Stimu- late the upper (central end); the blood- pressure probably rises, and the heart may be accelerated. Stimulate the peripheral end of the nerve ; there is little change in the blood-pressure and none in the rate of the heart. Fig. 104. — Manometer with Side-tube (Guthrie). A, float j B, collar through which the wire C of the float moves ; D, vertical wire fixed to mano- meter-holder, which keeps the writing^point on the drum; E, limb of manometer con- nected with cannula, with its side-piece, F. («) Note, incidentally, that stimulation of the central end of the sciatic or the upper (cephkhc) end of the vago-sympathetic may cause increase in the rate and depth of the respiratory movements. Dilatation of the pupil is also caused by stimulation of the upper end of PRACTICAL EXERCISES 211 the Vago-sympathetic through the sympathetic (pupillo -dilator) fibres that supply the iris. (/) Again stimulate the peripheral end of one vagus, or of both at the same time; while a tracing is being taken, and see how long it is possible to keep the heart from beating. Sometimes, but rarely in the dog, inhibition can be kept up so long that the animal dies. {g) Close the tracheal cannula so that air can no longer enter the lungs. In a very short time the blood-pressure curve begins to rise (rise of asphyxia). After some minutes the pressure falls, and finally, when the circulation has stopped completely and the pressure has become,' equalized throughout the whole vascular system, a residual pressure of only a few mm. (usually about 10 mm. Hg) is indicated. In order to get the true zero pressure, disconnect the arterial cannula Fig.'jioj. — Bbod- Pressure Tracing from a Dog: Stimulation of Central and Peripheral Ends of Vagus. Tlie other vagus was intact. Stimulation of the peripheral end caused stoppage of the heart and a marked fall of pressure. Stimulation of the central end produced a great rise of pressure, with, perhaps, a slight acceleration of the heart. from the manometer, and allow the writing-point to trace a horizonta straight line (line of zero pressure) on the drum (Figs. 84 and 85). 24. Estimation^of the Arterial Blood-Pressure in Man. — Use the Riva- Rocci apparatus, as described on p. 113. Begin with the subject in the sitting position. The observer's left hand may be used for palpating the pulse, and the right for working the bulb. Employ the ausculta- tory method as well as palpation, and determine the systolic and dias- toUc pressures. Repeat the observations with the person standing up and lying down. Investigate the effect of muscular exercise on the blood-pressure. 25. The Influence of the Position of the Body on the Blood-Pressure. —Inject into the rectum of a dog 3 to 4 grm. of chloral hydrate dis- solved in a little water. See chat it does not run out again immediately 212 THE CIRCULATION OF THE BLOOD AND LYMPH after injection. In ten minutes anaesthetize the animal fully with a mlkture jbl equal parts of alcohol, chloroform, and ether (one of the so-called A.C.E. mixtures), or with chloroform, and tie it very securely, back downward, on a board, which can be rotated around a horizontal axis, corresponding in position to the point at which the cannula is to be inserted.* Set up a drum and manometer as in 23 (p. 208), but with a rubber connecting-tube of such length as will allow free rotation of the board. Put a cannula in the trachea. Insert a cannula into the central end of the carotid artery at a point immediately above the axis of rotation of the board, and connect it with the manometer.. (a) Take a blood-pressure tracing with the board horizontal. (6) Whilst the tracing is being taken, rotate the board so that the position of the animal becomes vertical, with the feet down. Mark on the tracing the moment when the change of position takes place. •The pressure falls. Replace the dog in the horizontal position. The manometer regains its former level. Now rotate the board, till the animal is again vertical, but with feet up and head down, and observe the efiect on the blood-pressure. The respiratory variations in the pressure are usiiaUy greater with feet down than with head down. Notice in both cases whether there is any change in the rate of the hear^. (c) Take the board o£E the stands, lay it on a table, expose the femoral artery, and insert a cannula into it. Shift the axis so that it now lies below this cannula. Replace the board on the stands, and repeat (a) and (6). The fall of pressure will now take place in the head-down position. f In the feet-down position (with the cannula in the femoral artery) a rise of pressure in general takes place. But sometimes this is very small, and lasts only a few seconds, being succeeded by a fall, during which the heart-beats on the tracing are much weaker than before, since enough blood is not reaching the heart to enable it to maintain the pressure. In the feet-down position see whether the corneal reflex can be got. If not, as is likely, turn the animal into the head-down position. The reflex may now soon be obtained, and it 'may again disappear on putting the animal in the feet-down position. If the chloroform anaesthesia is light the reflex may not be aujolished in the feet-down position, although strong respiratory movements may occur, owing to anaemia of the medulla oblongata. \,26. Effecte of Haemorrhage and Transfusion on the Blood-Pressure. ^^^aesjBdetizea ^o^ with morphm and^ether, and .insert a c?,nnu}» into the trachea. Put a cannula into the central end of the carotid artery and another into the central end of the femoral artery. Then insert a cannula, which should have a piece of indiarubber tubing 2 to 3 inches in length on its wide end, into the central end of the femoral vein on the opposite side. In doing this more care is necessary than * A simple arrangement for this purpose is a board with a number of staples fastened in pairs into its lower surface, so that an iron rod can be pushed through any pair, and form a horizontal axis at right angles to the length of the board. The dog having been tied down, the rod is pushed through the pair of staples corresponding to the position of the cannula in the artery that IS to be connected with the manometer. The projecting ends of the rod rest in two ordinary, clamp-holders, fastened at a convenient height on two strong stands, Whose bases are clamped to the end of a table. The other end of the board is supported by a piece of wood that rests on the floor, and can be re- moved when the board is to be rotated. f In 16 dogs the fall of pressure in the carotid in the feet-down position varied from 12 to 100 mm. of mercury; average fall, 44-4 mm. In 12 out o£ the 16 animals the rise of pressure in the head-down position varied from 2 to 36 mm.; in i there was no change; in 3 there was a fall of 5 to 24 mm. PRACTICAL EXERCISES 213 in putting a cannula into an artery. Feel for the femoral ajrtery, cut down over it, and with forceps or a blunt needle separate the femoral vein from it for about an inch. Pass two ligatures under the vein, and tie a loose loop on each. Put a pair of bulldog forceps on the vein between the ligatures and the heart. Now tie the lower (distal) liga- ture, and cut one end short. The piece of vein between it and the bulldog forceps is thus distended with blood, and this facilitates the next step. With fine-pointed scissors make a snip in the wall of the vein. The cannula is now pushed through the slit in the vein, and the upper ligature tied firmly round its neck. By the aid of a pipette, made by drawing a piece of glass tubing out to a long point, the cannula and rubber tube are then completely filled with 09 per cent, salt solution. Be sure to pass the point of the pipette right down to the point of the cannula, so as to dislodge any bubble of air that may tend to cling there. Then, holding up the open end of the rubber tube, close it, without allowing any air to enter, by means of a screw clamp or bulldog forceps, or a small piece of glass rod. Connect the cannula in the carotid with a manometer, arranged to write on a drum as in~ experiment 23 (p. 208). Take the bulldog off the carotid, and measure the difference in the level of the mercury in the two limbs of the man- ometer with a millimetre scale. (i) (a) While a tracing is bfeing taken, draw off about 10 c.c. of blood from the femoral artery, and observe whether there is any effect 011 the tracing. Mark on the tracing the moment when the removal of the blood begins and ends. (b) Repeat (a), but run off about lod c.c* of blood, and let this be immediately defibrinated. Theri draw off portions of 100 c.c* at short intervals until a distinct fall of blood-pressure has been produced. All the samples of blood should be defibrinated and strained through cheese-cloth. , ■ (2) (a) Now, while a tracing is being taken, inject the whole of th^ defibrinated blood slowly through the cannula in the femoral vein; by means of a funnel supported by a stand at such a height that the blood runs in easily. A pinchcock should be put on the tube connecting the funnel and the cannula, and this should be closed before the funnel is quite empty, so as to obviate any risk of air getting into the vein. Of course, the cannula and connecting-tubes must all be freed from air before injection is begun. Again measure the difference in the level of the mercury and compare the pressure with that observed before the first haemorrhage. (6) Inject into the vein, while a tracing is being obtained, about 100 c.c* of o'9 per cent, salt solution heated to 40° C, and go on injecting portions of 100 c.c. until a distinct rise of pressure has taken place, keeping a record of the total amount injected, and marking the time of each injection on the curve. (c) After an interval of thirty minutes, again measure the height of the mercury in the manometer. Then bleed the dog to death while a tracing is being recorded. 27. The Influence of Proteoses (and Peptones) on the Blood-Pressure. — Set up the apparatus for taking a blood-pressure tracing as in experi- ment 23 (p. 208), but omit the induction-coil. Weigh a dog. Weigh out a quantity of Witte's peptone equivalent to o'5 grm. for every kilo of body-weight. Dissolve the peptone in about ten times its weight of o'9 per cent, salt solution. Anaesthetize the dog with morphine and ether or A.C.E. mixture. Insert a cannula into the trachea. Put cannula; into the central end of one carotid and of one femoral vein * 200 c.c. for a large dog. 214 THE CIRCULATION OF THE BLOOD AND LYMPH (p. 212). Connect the carotid with the manometer, and the femoral vein with a burette or large syringe containing the peptone solution. Take care that the connecting-tube and cannula are free from air. Now commence to take a blood-pressure tracing, and while it is going on inject the peptone solution. The pressure falls owing largely to a dilatation of the small arteries through the direct action of the pep- tone on their muscular tissue or on the endings of the vaso-motor nerves.* 28. Effect of Suprarenal Extract on the Blood-Pressure. — Make the arrangements for a blood-pressure tracing from a dog as in 23 (p. 208). Put a cannula in the carotid and another in the femoral vein or one of its branches (p. 212). Expose both vagi in the neck, and pass threads loosely under them. Connect the carotid with the manometer and take a tracing. Then, while the tracing is continued, inject slowly into the femoral vein an amount of watery extract corresponding to aboiit o'2 grm. of suprarenal, or, what is more convenient, a few c.c. of a solution of adrenalin chloride of the strength of i to 50,000 in o'g per cent.f' sodium' chloride solution, the dose depending, of course, on the size of the animal. The blood-pressure risest owing to con- Fig. 106. — Effect of Injection of Peptone on the Blood-Pressure in a Dog. (To be read from right to left.) striction of the arterioles by direct excitation of the junction between their vaso-constrictor nerves and their muscular tissue. The heart is slowed, but its beat is strengthened. At once cut both vagi while a tracing is being taken; the blood-pressure rises still more (p. 638). The rise of pressure is sometimes so great that to prevent the mercury from being forced out of the manometer the tube must be clipped. The rise is not long maintained, but a second injection causes a renewed increase of pressure. 29. Action of Epinephrin (Adrenalin) on Artery Rings. — The experi- ment (8) described on p. 66 in connection with the constrictor action of serum may equally well be performed here. * In 12 dogs the blood-pressure always fell, the amount of the fall varying from 81 to 21 mm. of mercury (average, 60 mm.). It sometimes returned to normal in twenty to thirty minutes, but usually required a longer time. In some dogs, after the injection of the whole of this amount of peptone, death occurs before there has been any considerable recovery of the pressure. f The amount of the initial rise of pressure is very variable, since the slow- ing of the heart tends to diminish the pressure, while the constriction of the arterioles tends to increase it. Thus, in one experiment the increase of pres- sure on injection of the extract was only 6 mm. of mercury, while in another it was 56 mm. On section of the vagi in this second experiment, there was an additional rise of 64 mm., and after a second injection a further rise of 70 mm., making an increase of 190 mm. in all above the original pressure. PRACTICAL EXERCISES 215 30. Section and Stimulation of the Cervical Sympathetic in the Rabbit. — Set up an induction-coil arranged for an interrupted current (Fig. 93, p. 198), and connect it through a short-circuiting key with electrodes. The preparations necessary for an operation with antiseptic precautions are supposed to have been previously made — the instruments, sponges, and ligatures boiled in water; the instruments then immersed in a 5 per cent, solution of carbolic acid, the sponges and ligatures in cor- rosive sublimate solution (o'l per cent.). Instead of sponges swabs of sterile gauze or cotton may be employed, and until the observations on the nerve have been made it is better to use sterile o'g per cent, salt solution for such slight sponging as the wound may require rather than the antiseptic solutions. The hands are to be thoroughly washed, with diligent use of the nail-brush, in soap and water before the cutting operation begins, and then soaked successively in alcohol and in the corrosive sublimate solution. Fasten the rabbit on a holder, back downwards, as in Fig. 61. Keep the animal warm by covering it with a cloth, and do not handle or wet its ears. Clip off the hair on the anterior surface of the neck. Remove loose hairs with a wet sponge, shave the neck, and wash it thoroughly, first with soap and water, and then with corrosive sublimate. Give ether. Make a longitudinal incision in the middle line over the trachea, beginning a little below the thyroid cartilage and extending downwards for an inch and a half. Feel for the carotid artery, expose, and raise it up. Two nerves will now be seen coursing beside the artery. The larger is the vagus, the smaller the sympathe;tic. A third and much finer nerve (the depressor, or superior cardiac, branch of the vagus) may also be seen in the same position, but the Student should neglect this for the present. Pass a ligature under the sympathetic, and tie it, the ear being held up to the light while this is being done, so that its vessels may be clearly seen. A transient constriction of the arteries may be seen at the moment when the nerve is ligatured. This is due to stimulation of the vaso-constrictor fibres. Then follows a marked dilatation of the bloodvessels, due to paralysis, of these fibres. The ear is flushed and hot. Note also that the pupil is probably narrower on the side on which the nerve has been tied. On stimulation of the upper (cephalic) end of the syrnpathetic with the electrodes, the vessels are markedly constricted, the ear becomes pale and cold, and the pupil dilates. Cut the nerve above and below the ligature, and take out the ligature. Wash the wound thoroughly with corrosive sublimate, then with sterile (boiled) water, and close it, the muscles being first brought together by- a row of interrupted sutures and then the skin by another row. Since it is difficult to thoroughly disinfect the hair-follicles, and a suture passed through a septic follicle is apt to give rise to suppura- tion, subcutaneous stitches — i.e., stitches passed by a curved needle through the deep layer of the skin without coming through to the surface — ^may be employed. The wound is to be protected by a coating of collodion. No other dressing is required. The animal is now removed from the holder and put back to its hutch. The student must examine it at least once a day for the next week, and study the differ- ences between the two ears (p. 173) and the two pupils. 31. Determination of the Circulation-Time. — {a) Begin with an arti- ficial scheme (Fig. 107). Fill the syringe with a o'2 per cent, solution of methylene blue. Allow the water to flow from the bottle by loosen- ing the clamp. Inject a definite quantity of the methylene-blue solu- tion, and with a stop-watch observe how long it takes to pass from the point of injection to the end of the glass tube filled with beads. Make ten readings of this kind, and take the mean. Then raise the 2i6 THE CIRCULATION OF THE BLOOD AND LYMPH bottle so as to increase the rate of flow of the water, and repeat the observations. The ' circulation-time ' will be found to be diminished. This corresponds to an increase of blood-pressure due to increased activity of the heart, without change in the calibre of the bloodvessels. Next, leaving the bottle in its present position, diminish the outflow by tightening the clamp; the circulation-time will be increased. This corresponds to an increase of blood-pressure due to diminution in the calibre of the small arteries. (6) Fill the «yringe* with methylene-blue solution (o-2 per cent, in 0-9 per cent, salt solution), as in (a). Keep the solution warmed to 40 C. by immersing the small beaker containing it in a water-bath, or Fig. 107. — Artificial Scheme to illustrate a Method of measuring the Circulation- Time. B, bottle containing water, the rate of outflow of which is regulated by screw-clamp a ; S, syringe filled with methylene-blue solution, connected with T-piece A; M, beaker containing methylene-blue solution; b, c, screw-clamps; C, T-piece, inseirted in the course of the fiexible tube E, and connected with the glass tube T, which is filled with beads; F, outflow tube. The clamp c having been closed and b opened, the syringe is filled with the methylene-blue solution; 6 is then closed, c opened, and a definite quantity of the solution injected into the system. The time from the beginning of injection till the appearance of the blue at G is measured with the stop-watch. heating it over a bunsen with a small flame. Weigh a rabbit or cat. In the case of the rabbit, inject J grm. chloral hydrate into the rectum, and later on give ether if necessary. If a cat, give ether alone. Fasten it on a holder, back downwards (Fig. 61, p. 136). Cover it with a towel to keep it warm. Clip off the hair on the front of the neck, and make an incision i\ inches long in the middle line, beginning a little * A burette, sloped so as to make a small angle with the horizontal, may be substituted for the syringe. The burette is supported on a stand at such a height (say 10-15 cm. above the level of the cannula) that the methylene- blue solution runs without great force into the jugular. The danger of pro- ducing an abnormal result by suddenly raising the pressure in the right side of the heart is thus avoided. PRACTICAL EXERCISES ZI7 way below the cricoid cartilage. Reflect the skin and isolate the external' jugular vein, which is quite superficial. Carefully separate about f inch of the vein from the surrounding tissue, and pass two ligatures under it. but do not tie them. Compress the vein with a pair of bulldog forceps between the heart and the ligatures. Now tie the uppermost of the two ligatures (that next the head), but only put a single loose loop on the other. The piece of vein between the upper ligature and the bulldog is now distended with blood. With fine- pointed scissors make a small slit' in the vein, taking great care not to divide it completely, insert the cannula, and tie the loose Jigature firmly over its neck. Fill the cannula and the small piece of rubber tubing attached to it with o'g par cent, salt solution by means of a pipette with a long point. Expose the carotid on the other side, isolate it lor f inch, clear it carefully from its sheath, slip under it a strip of thin sheet indiarubbsr, and between this and the artery a little piece of white glazed paper. Connect the cannula in the ju^lar with the T-piece attached to the syringe. Care must be taken that no air remains in the cannula or its connecting-tube, as a rabbit not unfrequently dies instantaneously when a bubble of air is injected into the right heart, although a considerable quantity of air can generally be injected into the jugular of a dog without killing it. Now take off the bulldog from the vein, and make a series of observa- tions on the pulmonary circulation -time. The animal must be so placed that a good light falls on the carotid. If necessary, the light of a gas-flame may be concentrated on it by a lens. The student holds the stop-watch in one hand, and injects a measured quantity of the methylene-blue solution with the other. Uniformity in the quantity injected is secured by fastening on the piston of the syringe a screw- clamp, which stops the piston at the desired point. The observation consists in setting off the watch at the moment when injection begins and stopping it when the blue appears in the carotid. After each injection the screw-clamp or pinchcock on the tube connected with the cannula must be tightened, the other opened, and the syringe refilled. Great care must be taken never to open the two clamps at the same time, as in that case blood may regurgitate through the jugujEtr^ and fill the syringe, or methylene blue ipay be sucked into the circulation. As many observations as possible should be taken, and the mean determined. The circulation-time observed is approximately that of the lesser circulation, the time taken by the blood to pass from the left ventricle to the carotid being negligible for the purposes of the student. * The specific gravity of the blood may also be tested at the beginning and end of the experiment by Hammerschlag's method (p. 62). If a large number of injections have been made in quick succession, the specific gravity will be less than normal; but if a considerable interval has been allowed to elapse after the last injection, little or no differ- ence may be found, as the surplus liquid readily passes out of the bloodvessels. Necropsy. — Observe particularly the state of the lungs, whether the bladder is distended or not, and whether any of the serous cavities or the intestines contain much liquid ; so as to determine, if possible, by what channel the water injected into the blood may have been elimin- ated. Study the distribution of the methylene blue in such organs as the kidneys and the muscles immediately after death, and notice that the blue colour becomes more pronounced after exposure for a time to the air. Make a longitudinal section through a kidney, and observe that the pigment is found especially in the cortex and around the 2i8 THE CIRCULATION OF THE BLOOD AND LYMPH pelvis at the apices of the pyramids, or it may be only in the cortex. The urine is greenish. If some methylene blue has been injected after the heart ceased to beat, the bloodvessels, particularly in the mesentery, may be beautifully mapped out by the pigment. This is not the case if the last injection took place before death, since the methylene blue is rapidly reduced by living tissues to a colourless substance, leuco-methylene blue. 32. Measurement of the Blood-Flow in the Hands. — Arrange the calorimeters as in Fig. ic8. The thermometers in the calorimeters should be graduated in tenths of a degree, so that ]Dy means of the small lenses or ' readers ' which slide on the stems hundredths of a degree can be estimated. Where it is desirable that a number of students should make observations in as short a time as possible, one calorimeter can be allotted to each subject, the other hand being kept in the pocket or covered with a glove if the room is cool, so as to avoid reflex vaso- motor interference. A felt collar is chosen which fits the wrist closely. A horizontal pencil-mark is made at the lower edge of the styloid process of the ulna, and another parallel mark at a distance above this slightly greater than the thickness of the collar. When this second mark is just kept in view above the collar with the hand in the calorimeter, the first (lower) mark will be just below the level of the lid. A large bath holding 20 or 30 litres or more (a clean ' garbage ' or ' offal ' can is suitable) is filled with water at about 32° C. The exact temperature is not important, but it should be about the same in all measurements which are to be compared. An ordinary ther- mometer graduated in degrees is all that is necessary for reading the tempfiiature of the bath. The calorimeters are now filled from the bath. They are conveniently made of such a size that 3 litres of water and fhe hand can be contained in them without any slopping over when, the water is stirred. Time is saved by having a metal flask which just holds the quantity of water that goes into each calorimeter. The orifices of the calorimeters are closed by felt discs. The subject, sitting in a high chair placed between the calorimeters, now immerses his hands in the bath to a point between the two marks. The fingers are kept spread. The bath is occasionally stirred. An ordinary ther- mometer suspended at the back of the chair gives the room tempera- ture. After ten minutes the hands are withdrawn from the bath, the wrists rapidly dried with a towel, the hands at once introduced into the calorimeters," and the felt collars adjusted round the wrists. The sub- ject leans back comfortably in the chair, allowing the arms to hang down without effort. The fingers are kept slightly spread. The ob- server sits on a low seat behind the subject, and reads the thermometers from time to time, always after stirring the water well with goose- feathers passing through the stirring-holes in the lid. The readings can be made at intervals of a minute, two minutes, or any interval which is convenient. At the end the hands are quickly withdrawn, the felt discs put over the orifices, and the water vigorously stirred for ten or fifteen seconds before the thermometers are read. In this way any errors due to imperfect stirring or to accidental contact of the hands with the thermometers are eliminated. The volume of each hand is now measured by immersing it exactly to the lower mark in water contained in a glass douehe-can connected by a short rubber tube with a pipette furnished with a side-tube at its lower end. The lowest graduation on the burette (50 on a 50 c.c. burette) is brought level with the water before the hand is immersed. While the hand is being held steadily and vertically in the water by an assistant, the level of the water in the burette_is read off. All that PRACTICAL EXFRCTSES 21 9 is necessary to get the volume of the hand is to pour water into the can from a graduated measure after withdrawal of the hand until the same level is reached. Or the value of a division of the burette can be determined once for all. The burette is simply used as a transparent scale. When the two hands are successively measured, the small amount of water removed by the first is automatically restored by dippmg the second into a separate vessel of water, and putting it wet into the douche-can. The rectal temperature should now be obtained. The temperature of the arterial blood entering the hand is taken as o'5° C. below that of the rectum. If only the mouth temperature can be got, the thermometer should be put in a second time without shaking Fig. -Calorimetric Method of measuring Blood-Flow in Hands. down to see if it rises any more. The mouth temperature is taken as equal to the arterial blood temperature. After thorough stirring, the calorimeter temperatures can now be read again. The two being noted, the amount of cooling of the calor- imeters can be determined. This has to be added to the actually observed rise of the thermometers during immersion of the hands. Suppose an experiment jdelded the following data: Rise of ther- mometer in a calorimeter in twenty minutes during immersion of a hand in it, i'o° C. ; temperature of calorimeter at beginning of the twenty minutes, 3i'o° C. ; at end of twenty minutes, 320° C. ; cooling ]of calor- imeter in twenty minutes, 01° C. ; water in calorimeter, 3,000 c.c; 220 THE CIRCULATION OF THE BLOOD AND LYMPH volume of hand, 450 ex.; rectal temperature, 370° C; water equivalent of caloiimeter, 100 c.c. The water equivalent of the hand is 450 x o'8*= 360 c.c. The water equivalent of the calorimeter is 100 c.c. Water - - 3,000 c.c. Total 3.460 c.c. 3,460x1-1 = 3,806 small calories given off by the hand in twenty- minutes. Temperature of arterial blood (36-5°) minus temperature of venous blood (31-5°, the mean temperature of the calorimeter) = 50. Flow per minute through hand=^^^ x ' =42-3 grm. "Flow per 100 c.c. of hand per minute=9-4 grm. The readings of the calorimeter thermometers for the first one or two minutes may not be usable, owing to disturbance caused by the intro- duction of the hands. As soon as they begin to rise steadily and uniformly, the readings can be utilized for the calculation of the flow. 33. Vasomotor Reflexes. — Begin as in 32. Then, after the hands have been in the calorimeters for a sufficient period (say ten minutes) to allow satisfactory readings for the determination of the blood-flow to be obtained, rapidly transfer one hand to cold water (at about 8° C), while the other remains in the calorimeter. Continue reading the calorimeter thermometer. Its rise will be checked by reflex vaso- constriction. If the hand is kept for a few minutes in the calorimeter, the reflex vaso-constriction of the hand in the calorimeter will probably disappear, and the thermometer -will rise faster. When a sufficient number of readings have been obtained for calculating the alteration in the flow, which will usually be the case in eight or ten minutes, transfer the hand from the cold-water to warm water (at about 43° C), and continue reading the calorimeter thermometer. There is usually a reflex vaso-constriction followed by vaiso-dilatation. * This factor is the product of the specific gravity and the specific heat of the hand. The volupie multiplied by the specific gravity gives the mass of the hand, which multiplied by the specific heat, gives the water equivalent of the hand. t The reciprocal of the specific heat of blood (see formula, p. 122). CHAPTER IV RESPIRATION Respiration in its widest sense is the sum total of the processes by which the ultimate elements of the body gain the oxygen they require, and get rid of the carbon dioxide they produce. Section I. — Preliminary Anatomical Data. Comparative. — In a unicellular organism no special mechanism of respiration is needed; the oxygen diffuses in, and the carbon dioxide diffuses out, through the general surface. The simple wants of such multicellular animals as the coelenterates, the group to which the sea- anemone belongs, are also supplied by diffusion through the ectoderrh from and into the surrounding water, and through the endoderm from and into the contents of the body-cavity and its ramifications. But in animals of more complex structure special arrangements become necessary, and respiration is divided into two stages: (i) Ex- ternal respiration, an interchange between the air or water and a cir- culating medium or blood as it passes through richly vascular skin, gills, tracheae, or lungs; and (2) internal respiration, an interchange between the blood, or lymph, and the cells. In the lower kinds of worms respiration goes on solely through the skin, under which plexuses of bloodvessels often exist, but in some higher worms there are special vascular appendages that play the part of gills. The Crustacea also possess gills, while in the other arthropoda respiration is carried on either by the general surface of the body (in some low forms), or more commonly by means of tracheae, or branched tubes surrounded by blood spaces and communicating externally with the air and internally by their finest twigs with the individual cells. Most of the mollusca breathe by gills, but a few only by the skin. Among vertebrates the fishes and larval amphibians breathe by gills, but most adult amphibians have lungs. The skin, too, in such animals as the frog has a very important respiratory function, more of the gaseous exchange taking place through it in some conditions than through the lungs. One small group of fishes, the dipnoi, has the peculiarity of possessing both gills and a kind of lungs, the swim-bladder being surrounded with a plexus of bloodvessels and taking on a respiratory function. In all the higher vertebrates the respiration is carried on by lungs; the trifling amount of gaseous interchange which can possibly take place through tlje skin is not worth taking into account. The lungs are to be regarded as developed from outgrowths of the alimentary canal, beginning near the mouth. 222 RESPIRATION The object of all special respiratory arrangements being, in the first instance, to facilitate the gaseous exchange between the surrounding medium (air or water) and the blood, a prime necessity of a respiratory organ, be it skin, gill, trachea, or lung, is a free supply of blood, in vessels so fine and thin that diffusion readily takes place into them and out of them. But a free supply of blood would be of no avail if the medium to which the blood gave up its carbon dioxide and from which it drew its oxygen was not being constantly and sufficiently renewed. Sometimes the natural currents of the water or the air are of them- selves sufficient to secure this renewal ; in other cases, artificial currents are set up by cilia, or special bailing organs, like the scaphognathites of the lobster. In all the higher animals, active movements by which air or water is brought into contact with the respiratory surfaces, are necessary; and it is possible that such movements take place even in the tracheae of insects and other air-breathing arthropoda. Fishes, by rhythmical swallowing movements, take in water through the mouth and pass it over the gills and out by the giU-sIits, while the frog distends its lungs by swallowing air. Physiological Anatomy of the Respiratory Apparatus. — In man the respiratory apparatus consists of a tube (the trachea) widened at its upper part into the lar3mx, which contains the special mechanism of voice, and communicates through the nose or mouth with the external air. Below, the trachea divides dendritically into innumerable branches, the ultimate divisions of which are called bronchioles. Each bronchiole breaks up into several wider passages, or infundibula, the walls of which are everywhere pitted with recesses or alcoves, called alveoli. The infundibula constitute the essential distensible elements of the lung, by the alternate stretching and relaxation of which the respiratory changes in the volume of the organ are mainly brought about. The trachea and larger bronchi are strengthened by hyaline cartilage in the form of incomplete rings, connected behind by non- striped muscular fibres, which also exist in the intervals between the rings. The middle-sized bronchi within the lungs have the cartilage in the form of detached pieces in the outer portion of the wall, while nearer the lumen lies a complete ring of non-striped muscle. In the bronchioles, no cartilage is present, but the circularly-arranged muscular fibres still persist, and also form a thin layer in the infundi- bula. In the air-cells, or alveoli, however, there are no muscular fibres. Their walls consist essentially of a netwprk of elastic fibres, continuous with a similar layer in the infundibula and bronchioles, and covered on the side next the lumen by a sin-gle layer of large, clear epithelial scales, with here and there a few smaller and more granular polyhedral cells. From the larjmx to the bronchioles the mucous membrane is ciliated on'ifcs free surface, the cilia lashing upwards so as to move the secre- tion towards the larynx and mouth. In the infundibula the ciUated epithelium begins to disappear, and is absent from the alveoli. Part of the nasal cavity and the upper part of the pharynx are also lined with ciliated epithelium. Mucous glands are present in abundance in the upper portions of the respiratbry passages, but disappear in the smaller bronchi. Blood-Supply of the Lungs. — ^^fhe quantity of blood traversing the lungs bears no proportion to the amount required for their actual nourishment. Small, however, as this latter quantity is, it cannot apparently be derived from the vitiated blood of the right ventricle, but is obtained directly from the aortic system by the bronchial arteries. These are distributed with the bronchi, which they supply as well as the connective tissue of the interlobular septa running through the PRELIMINARY ANATOMICAL DATA 223 substance of the lung, the pleura lining it and the walls of the large bloodvessels. Most of the blood from the bronchial arteries is returned by the bronchial veins into the systemic venous system, but some of it finds its way by anastomoses into the pulmonary veins'. The branches of the pulmonary artery are also distributed with the bronchi, and break up into a dense capillary network around the alveoli. From the capillaries veins arise which, gradually imiting, form the large pulmonary veins that pour their blood into the left auricle. The same quantity of blood must, on the whole, pass per unit of time through the lesser as through the greater circulation, otherwise equilibrium could not exist, and blood would accumulate either in the lungs or in the systemic vessels. But it does -not follow that at each heart-beat the output of the two ventricles is exactly equal. If, indeed, the capacity of the lesser circulation were constant, the quantity driven out at one systole by the right ventricle would be the same as that ejected at the next by the left ventricle. But it is known that the capacity of the pulmonary vessels is altered by the movements of respiration and probably in other ways, so that it is only on the average of a number of beats that the output of the two ventricles can be supposed equa,l. The time required by a given small portion of blood — e.g., by a single corpuscle — ^to complete the round Of the lesser circulation, is, as we have seen (p. 137), much less than the average time needed to complete the systemic circulation. In man the ratio is probably about 1:5. Since all the blood in a vascular tract must pass out of it in a period equal to the circulation time, the average quantity of blood in the lungs and right heart of a man would thus be about one-fifth of that in the systemic vessels. That is to say, not less than 700 grm. out of the 4^ kilos* of blood in a 70-kilo man would be contained in the lesser cir- culation, and about 3|- kilos in the greater. This corresponds sufficiently well with calculations from other data. For example, the average weight of the lungs in three persons exe- cuted by beheading, was 457 grm. (Gluge). The average weight of the lungs in a great number of persons who had died a natural death was 1,024 gi'™- (Juncker). The weight of the pulmonary tissue alone in the first set of cases must be less than 457 grm., for the lungs of a person who has bled to death are never bloodless. In a dog kiUed by bleeding from the carotid, one-quarter of the weight of the lungs con- sisted of blood. Assuming the same proportion for the decapitated individua,Js, we get 343 grm. as the net weight of the blood-free lungs. Deducting this from 1,024 grm., we arrive at 681 grm. as the average quantity of blood in the limgs. Adding to this the quantity in the right side of the heart (p. 140), we get, in round numbers, 750 grm. as the amount in the lesser circulation. It is true that in the living body the conditions are not the same as after death; but it is probable that in a large number of cases taken at random the differences would be approximately equalized. It has been further calculated that the total area of the alveolar surface of the lungs of a man is about 100 square metres (sixty times greater than the area of the skin), of which, perhaps, 75 square metres are occupied by capillaries. The average thickness of this immense sheet of blood has been reckoned to be equal to the diameter of a red blood-corpuscle, or, say, 8 /i. This would give 600 c.c. (630 grm.) as the quantity of blood in the lungs, which is probably somewhat too low an estimate. * See footnote on p. 139 224 RESPIRATION If we take the pulmonary circulation-time as 13 seconds (p. 137), and the quantity of blood in the lungs as 700 grm., then — ==194 kilos of blood will pass through the lungs in an hour, or 4,656 kilos (say, 4,400 litres) in twenty-four hours. This would fill a cubical tank in which the man could almost stand upright with the lid closed. Section. II. — Mechanical Phenomena of External Respiration. The lungs are enclosed in an air-tight box, the thorax ; or it may be said with equal truth that they form part of the wall of the thoracic cavity, and the part which has by far the greatest capacity of adjustment. The alveolar surface of the lungs is in contact with the air. The pleura, which covers their internal surface, is reflected over the chest-walls and diaphragm, so as to form two lateral sacs, the pleural cavities. In health -these are almost obliterated, and the visceral and parietal pleurae, separated and lubricated by a few drops of lymph, glide on each other with every movement of respiration. But in disease the pleural cavities may be filled and their walls widely separated by exudation, as in pleurisy, or by blood, as in rupture of an aneurism, or by air in the condition known as pneumo-thorax. Between the two pleural sacs lies a mesial space, the mediastinum, commonly divided into an anterior medias- tinum in front of the heart, and a posterior mediastinum behind it. The pleural and pericardial sacs and the mediastinum constitute together the thoracic cavity. The external surface of the chest- wall and the alveolar surface of the lungs are subjected to the pressure of the atmosphere, to which the pressure in the thoracic cavity (intra-thoracic pressure) would be exactly equal if its bound- aries were perfectly yielding. But in reality the intra-thoracic pressure is always normally something less than this. For even the lungs, the least rigid part of the boundary, oppose a certain resistance to distension, and so hold off, as it were, from the thoracic cavity a portion of the alveolar pressure ; and in any given position of the chest the intra-thoracic pressure is equal to the atmospheric pressure minus this elastic tensioniof the lungs. The object of the respiratory movements is the renewal of the air in contact with the alveolar membrane — ^in other words, the ventila- tion of the lungs. Two main methods are followed by sanitary engineers in the ventilation of buildings: they force air in, or they draw it in. In both cases the movement of the air depends on the establishment of a slope of pressure from the inlet to the interior. In the first method, this is done by increasing the pressure at the inlet ; in the second, by diminishing the pressure at the outlet. In certain, animals Nature, in solving its problem of ventilation, has made use of the first principle. Thus, the frog forces air into its MECHANICAL PHENOMENA OF EXTERNAL RESPIRATION 225 lungs by a swallowing movement. In artificial respiration, as practised in physiological experiments, the same method is usually employed : air is driven into the lungs under pressure. But in the vast majority of air-breathing animals, including man, the opposite principle has been adopted; and the 'indraught ' of air from nose and pharynx to alveoli is not set up by increasing the pressure in the former, but by diminishing it in the latter. This ' indraught,' or inspiration, is brought about by certain movements of the chest- wall, which increase the capacity of the thoracic cage and lower the pressure in the thoracic cavity. The expansion of the highly-distensible lungs keeps pace with the diminution of pressure in the pleural sacs, and they follow at every point the retreating chest - wall and diaphragm, although they do not expand equally in all directions. The dorsal surface in con- tact with the vertebral column, the mediastinal surface in contact with the pericardium and the contents of the mediastinum, and the surface of the apex, move but httle. The surfaces in contact with the diaphragm, ribs, and sternum have the greatest range of movement. Intermediate portions of the parenchyma of the lungs expand in a degree determined by their distance from the relatively stationary and mobile surfaces. The pres- sure of the air in the alveoli during the rapid expansion of the lungs necessarily sinks below that of the atmosphere, and air rushes in through the trachea and bronchi till the difference is equalized. Then commences the movement of ex- piration. The expanded chest falls back to its original Umits; the pressure in the thoracic cavity increases; the distended lungs, in virtue of their elasticity, shrink to their former volume; the pressure of the air in the alveoli rises above that of the atmo- sphere, and with this reversal of the slope of pressure air streams out of the bronchi and trachea. In inspiration the chest dilates in all its diameters. Its vertical diameter is increased by the contraction of the diaphragm, which, cornposed of a central tendon, a peripheral ring of muscular tissue, and the two muscular crura, bulges up into the thorax in the form of two flattened domes, one on each side, and thus closes its lower 15 Fig. 109. — Scheme to illus- trate the Movements of the Lungs in the Chest. T is a bottle from which the bottom has been removed ; D, a flexible and elastic membrane tied on the bottle, and capable of being pulled out by the string S so as to increase the ca- pacity of the bottle. L is a thin elastic bag repre- senting the lungs. It com- municateswith the external air by a glass tube fitted airtight through a cork in the neck of the bottle. When D is drawn down, the pressure of the external air causes L to expand. When the string is let go, L con- tracts again, in virtue of its elasticity. 226 RESPIRATION aperture. When the diaphragm contracts, even in ordinary quiet breathing, the central tendon descends distinctly (about half an inch) after the manner of a piston. The acute angle which the muscular ring makes during relaxation with the thoracic wall opens out around its whole circumference, so as to form a groove of trian- gular section. But the most peripheral portion of the ring is always kept in close apposition to the chest- wall by the negative intra- thoracic pressure. The lungs follow the descending diaphragm,, their lower borders keeping accurately in contact with it. The descent of the diaphragm is not directly downwards, but downwards and forwards. For it is compounded of two movements, the spinal segment of the muscle (the crura) causing a vertical elongation of the thorax, while the sterno-costal part (the muscular ring) pushes the abdominal viscera downwards and forwards (Keith). Since the diaphragm is attached to the lower ribs, there is a tendency during its contraction for these to be drawn inwards and upwards; but this is opposed by the pressure of the abdominal viscera, and by the action of the quadratus lumhorum, which fixes the twelfth rib, and of the serratus posticus inferior, which draws the lower four ribs backward. When these and the other inspiratory muscles that act especially upon the ribs are paralyzed by injury to the spinal cord, and respiration is carried on by the diaphragm alone, the line of its attachment to the ribs is distinctly marked during inspiration by a shallow circular groove. The thorax is also enlarged by the action of certain muscles that act upon the ribs. Among the elevators of the ribs, as their name indicates, are usually reckoned, although erroneously, the levatores costarum— twelve in number on each side. They arise from the transverse processes of the last cervical and first eleven dorsal vertebrae, and passing obliquely downwards and outwards, are in- serted between the tubercle and the angle into the first or second rib below their origin. They do not elevate the ribs, but take part in lateral movements of the spinal column. The scalene muscles, which may in a lean person be felt to be tense during inspiration, fix the first and second ribs (scalenus anticus aiid medius, the first; scalenus posticus, the second rib), and so afford a fixed line for the intercostal muscles to work from on the lower ribs. The most important elevators of the ribs are the external inter- costals. The intercartilaginous portions of the internal inter- costals (the intercartilaginei muscles, as they are sometimes called) also contract simultaneously with the diaphragm, and may there- fore be included in the list of inspiratory muscles; but instead of elevating the ribs they depress the costal cartilages, and thus help to widen the angles between them and the ribs. In addition to increasing the capacity of the chest, the contraction of the external intercostals and the intercartilaginous muscles aids in inspiration MECHANICAL PHENOMENA OF EXTERNAL RESPIRATION 227 by augmenting the rigidity of the intercostal spaces, and so pre- venting them from being drawn in as easily as would otherwise be the case when the thorax is expanded by the action of the dia- phragm and the other inspiratory muscles. Leaving out of account the floating ribs, which functionally form a part of the abdominal wall, the ribs in relation to their respiratory functions may be divided into the following groups: (i) The first rib, which, moving itself very little, provides a fixed line towards which the next set of ribs may be raised. (2) An upper costal series consisting of the ribs from the second to the fifth. These are raised in inspiration towards the fixed first rib by the contraction of the intercostal muscles. The movement of these ribs is, mainly at any rate, a rotation around a transverse axis, the axes on which they move corresponding to their necks. The manner in which they are articulated to the vertebrae prevents any sensible rotation around an antero-posterior axis or ' bucket- handle ' movement. Since these ribs slant downwards and forwards to their sternal attachments, the sternum is raised when they are elevated; or, rather, since the manubrium is practically immovable in ordinary breathing, the body of that bone is bent on the manu- brium at the manubrio-sternal joint. This causes an increase in the antero-posterior diameter of the thorax. Further, since the arches formed by the ribs widen in regular progression from above downwards in the upper portion of the thoracic cage, so that the second rib is a segment of a larger circle than the first, and the third than the second, it is clear that a general elevation of the chest will tend to increase the transverse diameter at any given level. Such an increase is also favoured by the opening out of the angles between the bony ribs and the costal cartilages under the influence of the couple (or pair of oppositely directed forces) that acts on them — viz., the upward pull of the external intercostals exerted on the ribs, and the downward pull of the intercartilaginei and the resist- ance of the sternum to further displacement exerted on the carti- lages. The whole arrangement is perfectly adapted to permit the expansion of the roughly conical upper lobes of the lungs. (3) The lower costal series, consisting of the ribs from the sixth to the tenth. These ribs, with their muscles, form a mechanism which normally acts along with the diaphragm (Keith). They are so arranged that in inspiration the lateral and anterior part of each moves outwards to a greater extent than the one above it. There is not only a rotation around a transverse axis, by which the lower end of the sternum, connected to these ribs by the combined cartilages of the sixth to the ninth, is elevated, but also a rotation around an antero-posterior axis. The movement of the lower ribs results, therefore, in increasing both the back-to-front diameter and the transverse diameter of the lower portion of the thorax. The 228 RESPIRATION widening of the thorax from side to side may also be in a slight degree ascribed to a twisting movement of the ribs, which tends to evert their lower borders. With the diaphragm, these lower ribs arranged in a vertical series of not very different curvature con- stitute a mechanism for the inspiratory expansion of the roughly cylindrical lower lobes of the lungs. Expiration in perfectly tranquil breathing is brought about with less aid from active muscular contraction. The sense of effort disappears as soon as the chest ceases to expand. The diaphragm and the elevators of the ribs relax. The structures that have been stretched or twisted recoil into their original positions; the struc- tures that have been raised against the force of gravity fall back by their weight, and in the measure in which the pressure increases in the thoracic cavity the elasticity of the lungs causes them to shrink. The pressure in the alveoli, which at the end of inspiration was just equal to that of the atmosphere, is thus increased, and the air expelled. It is probable that, even in man and in quiet respira- tion, the interosseous portions of the internal intercostals help by their contraction in depressing the ribs, and that a slight contrac- tion of the abdominal muscles hastens the return of the diaphragm to its position of rest. In reptiles and birds, expiration is normally effected by an active muscular contraction. This is also true in some mammals — the rabbit, for instance, in which the external oblique muscles of the abdominal wall take an important share in the expiratory act. Types of Re^iration. — -Differences exist also, not only between different groups of animals, but even between women and men, in the relative importance in inspiration of the diaphragm and the muscles that raise the lower ribs on the one hand, and the muscles that elevate the upper ribs on the other. When the movements of the diaphragm predominate, the respiration is said to be of the abdominal or diaphragmatic type ; when the movements of the upper ribs and sternum are most conspicuous, of the costal or thoracic type. In abdominal respiration, the inspiratory movement commences at . the diaphragm, and then involves the lower ribs and the tip of the sternum. In costal respiration, the upper ribs initiate the move- ment, and are followed by the abdomen. In the rabbit, during quiet breathing, the respiration is purely diaphragmatic, the ribs remain motionless; arid herbivorous animals in general conform more or less closely to this type. In the carnivora, on the contrary, the costal type prevails. Man alhes himself as regards his respira- tion with the rabbit and the sheep ; he uses his diaphragm more than his upper ribs. Civilized woman falls into the class of the wolf and the tiger; she uses her upper ribs more than her diaphragm. The cause of the difference between men and women has been much discussed. It is not a primitive sexual difference, for it is far from MECHANICAL PHENOMENA OF EXTERNAL RESPIRATION 229 being universal; in the uncivilized and semi-civilized races that have been investigated, the women breathe like the men. It is therefore probable that the predominance of the costal type among women of European race is a peculiarity developed by a mode of dressing which hampers the movements of the diaphragm while permitting the elevation of the ribs. This conclusion is strengthened by the fact that in children no difference exists ; both boys and girls show the abdominal type of respiration. All this refers to ordinary breathing. In forced respiration, when the need for air becomes urgent, costal breathing always becomes prominent alike in men, in women, and in animals, for by elevation of the ribs the capacity of the chest can be increased to a greater degree than by any contraction of the diaphragm. In forced inspiration, indeed, all the muscles that can elevate the ribs may be thrown into contraction, as well as other muscles which give these fixed points to act from. During a paroxysm of asthma, for example, the patient may grasp the jback of a chair with his hands, so as to fix the arms and shoulders and allow the pectorals and serratus magnus to raise the ribs. Similarly in forced expiration all the muscles are used which can depress the ribs, or increase the intra-abdominal pressure and push up the diaphragm. Artificial Respiration.— An efficient pulmonary ventilation can be obtained by various methods when the natural breathing is in abey- ance. In animals the method most commonly employed for ex- perimental purposes is the rhythmical inflation of the lungs by a pump or bellows, or by a stream of compressed air which is regularly interrupted, the chest being allowed to coUapse after each inflation. When the animal is to be kept alive after the experiment the inflation is produced through a tube introduced through the glottis. If the animal is not to be kept alive, the apparatus is generally connected with a cannula in the trachea. In man the exchange of air between the atmosphere and the lungs may be most readily accomplished by strong rhythmical compression of the lower part of the chest. This forces out some of the air from the lungs; on relaxing the pressure the chest expands again and air is drawn in. Schafer has shown that this is the most efficient method of respiration in re- suscitation of the apparently drowned. ' The patient is placed face downwards on the ground, with a folded coat under the lower part of the chest. The operator puts himself athwart or at the side of the patient, facing his head and kneeUng upon one or both knees (Fig. no), and places his hands on each side over the lower part of the back (lowest ribs). He then slowly throws the weight of his body forward to bear upon his own arms, and thus presses upon the thorax and forces air out of the lungs. He then gradually relaxes the pressure by bringing his own body up again to a more erect position, but without moving the hands.' Air is thus 230 RESPIRATION drawn into the lungs. The process is repeated twelve to fifteen times a minute. Certain accessory phenomena (movements and sounds) are asso- ciated with the proper movements of respiration. The larjmx rises in expiration, and sinks in inspiration. The glottis (and particu- larly its posterior portion, the glottis respiratoria) is widened during deep inspiration and narrowed during deep expiration. The same is the case with the nostrils, and, indeed, in some persons the alse nasi move even in ordinary breathing. It has long been known that in deep respiration changes in the calibre of the bronchi syn- chronous with the respiratory movements may occur. In young parsons it may be directly observed with the bronchoscope, an instrument used by laryngologists for exploring the larger bronchi, that these dilate in inspiration and constrict in expiration (In- galls). In part at least these movements are passively pro- duced by the changes of intra- thoracic pres- sure, but it has not been defi- nitely deter- mined whether they are not in part caused by alternate contraction and relaxation of the circular bronchial muscles. To these muscles has sometimes been attributed the function of regelating the flow of air into and out of the infundib- ula, as the muscle of the arterioles regulates the distribution of the blood in the organs. As regards the respiratory sounds, all that is necessary to be said here is that when we listen over the greater portion of the lungs with the ear, or, much better, with a stethoscope, a soft breezy murmur, that has been compared to the rustling of the wind through distant trees, is heard. This has been called the vesicular murmur. It is only heard in health during inspiration and the very beginning of expira- tion, and is louder in children than in adults. Around the larger bronchi and the trachea a blowing sound is heard, which certainly originates at the glottis, and is strengthened by the resonance of the air-tubes. In health this is not recognized over the greater portion of the lung. But in certain diseases in which the alveoli are devoid of air, whether from compression or because they are filkd up with exudation, and in other conditions, this bronchial or tubmar breathing Fig. no. -Artificial Respiration in Cases of Drowning (after Schafer). MECHANICAL PHENOMENA OF EXTERNAL RESPIRATION 231 may be heard over the affected area. The bronchi themselves, how- ever, must still be patent and contain air. The most commonly accepted explanation is that the laryngeal sound is better conducted through the smaller bronchi towards the surface of the lungs when their walls have been rendered more rigid by the solidification of the parenchyma, in spite of the fact that the consolidated tissue as such does not conduct the sound so well as the air-containing alveoli* It seems probable that, in addition, the columns of air in the bronchi, which are encased in solid tissue, may actually increase the intensity of the transmitted laryngeal murmur by resonance. It has been much debated whether the vesicular murmur also arises at the glottis, and is modified by transmission through the pulmonary tissue, or whether it arises somewhere in the terminal bronchi, the infundibula or the alveoli. Both views may be supported by certain arguments, and to both some objections may be raised. The fact appears to be that there are two elements in the inspiratory murmur— a true vesicular sound, produced about the place where the terminal bronchioles give off the infundibula, and a resonance sound set up in the trachea and bronchi by the glottic murmur. This resonance sound as heard over portions of the lung containing only small bronchi has a different character from that heard over large bronchi, inasmuch as the fundamental note, and to a still greater extent the overtones (p. 304), are much weakened in those small and easily-distensible tubes. The true vesicular element is heard all over the lungs, but the resonant laryngeal element in large animals, like the horse and ox, dies out as an audible murmur before it reaches the remotest lobules, and can only be distinguished over a portion of the pulntonary area. When the glottic sound is eliminated by causing an animal to breathe through a tracheal fistula, the vesicular murmur is still heard, and in the horse is even somewhat sharper than normal, although in the dog it is softer and weaker. The expiratory murmur does not seem to contain a true vesicular element, but is exclusively due to the resonance of the expiratory glottic sound (Marek). It is generally admitted, and this is of great importance in practical medicine, that when the normal vesicular sound is heard over any portion of the lung tissue, it may be inferred that this portion is being properly distended, and that air is freely entering its alveoli. Up to this point we have contented ourselves with a purely quahtative description of the mechanical phenomena of respiration. We have now to consider their quantitative relations, and the methods by which these have been studied. The expansion of the lungs in inspiration may be easily demonstrated in man, and even a rough estimate of its amount obtained, by the clinical method of percussion. For example, the resonant note that is elicited when a finger laid on the chest at a part where it overlies the right lung is smartly struck can be followed down until it is lost in the ' liver dulness.' If the lower limit of the resonant area be marked on ihe chest-wall first in fuU inspiration, and then in full expiration, the mark will be lower in the former than in the latter, and the difference will represent the difference in the vertical length of the shrunken and dis- tended lung. A similar enlargement in the transverse direction may be demonstrated in the same way, the inner borders of the lungs coming nearer to the middle line in inspiration, and receding from it in expira- tion. The examination of the chest by the Rontgen rays has also 232 RESPIRATION yielded results of importance in the study of normal respiratory con- ditions, and still more important results in pulmonary disease. For most physiological purposes, however, a faithful graphic record of the respiratory movements is indispensable. This may be obtained — (i) By registering the movements of a single point, or the variations in a single circumference, of the bound- ary of the thoracic cavity. In man changes in the circumference of the thorax at any level can be recorded by means of a tambour adjusted to the chest (Figs, iii and 134), and in communication with another, which is provided with a writing lever (Figs. 99 and 137). Or an elastic tube, with a spiral spring in its lumen, rtiay be fastened around the thorax or abdomen and connected with a piston-recorder (a small cylin- der in which works a piston carrying a writing-point) (Fitz). (2) By recording the changes of pressure produced in the air-passages by the respiratory movements. This can be done by connecting a cannula in the trachea of an animal with a recording tambour in the manner described in the Practical Exercises (p. 295). The variations of pressure may be measured by connecting a manometer with the trachea, or in man with the nostril. (3) By writing off the changes of pressure which occur in the thoracic cavity during respiration. For this purpose a trocar (Fig. 1 13) is intro- duced through an intercostal space into one of the pleural sacs, without the admission of air, or into the pericardium, and then connected with a manometer or other recording apparatus. Or a tubs, similar in con- struction to a car- diac sound (p. 96), may be pushed down the oesopha- gus. The varia- tions in the intra- thoracic pressure are transmitted to the air in the elas- tic bag, and thence to a tambour. (4) In the rabbit the part of the dia- phragm attached to the ensiform cartilage may be isolated from the rest and its contractions recorded by a lever (Head). For some purposes this is the best method. When the respiratory movements are studied in any of these ways, it is found that there is practically no pause between the end of Fig. III. — Scheme of Tambour for recording Respiratory Movements. C, a metal capsule connected airtight with B, A, two caoutchouc mem- branes, the chamber formed by which can be inflated by means of the tube and stopcock E. The tube D con- nects the space H with a registering tambour provided with a lever. The membrane A is applied to the chest, round which the inextensible strings Fare tied. At every expansion of the chest the pressure in H is increased, and the increase of pressure is trans- mitted to the registering tambour. Fig. 112. — Respiratory Tracing from Man (Marey). stroke, inspiration; up stroke, expiration. Down MECHANICAL PHENOMENA OF EXTERNAL RESPIRATION 233 d )-f^ Fig. 113. inspiration and the beginning of expiration. Nor, although the chest collapses more graduaiUy than it expands, is there any distinct interval in ordinary breathing between the end of expiration and the beginning of the succeeding inspiration. When, however, the respiration is unusually slow, an actual pause (expiratory pause) may occur at this point. Expiration takes somewhat longer time than inspiration, the ratio varying from 7 : 6 to 3 : 2, according to age, sex, and other circumstances. The frequency of respiration is by no means constant even in health. All kinds of in- fluences affect it. It is difficult even to direct the attention to the respiratory act without bringing about a modification in its rhjrthm. In the adult 15 to 20 respirations per minute may be taken as about the normal. In young children the frequency may be twice as great (new-born child, 50 to 70; child from i to 5 years old, 20 to 30 per minute). It is greater in a female than in a male of the same age. A rise of temperature increases it; 150 respira- tions per minute have been seen in a dog with a high temperature. Sudden coohng of the skin, exercise, and various emotional states, increase the rate, and sleep diminishes it. The will can alter the frequency and depth of respiration for a time, and even stop it altogether, but in less than a minute, in ordinary individuals, the desire to breathe becomes imperative. Cato's assertion that he could kill himself at any time ' merely by holding his breath ' is only a proof that he was a better philosopher than physi- ologist. After a period of forced respiration the breath can be held for a much longer time. This is due to the ' washing out ' of the carbon dioxide, the normal stimulus to the respiratory centre (p. 276). After six minutes of forced breathing the interval of voluntary inhibition can be extended be- yond four minutes. A professional diver has remained under water in a tank for about four and three-quarter minutes. When oxygen is inhaled instead of air duririg the last few breaths of the forced respiration, the interval during which the breath can be held may be much increased (up to nine or ten minutes). In animals the rate of respiration can be greatly affected by drugs and by the section and stimulation of certain nerves; but to this we shall return when we come to consider the nervous mechanism of respiration. Simple Pleural Cannula. B, a line of small spurs which, after the can- nula C has been pushed without ad- mission of air through an intercostal space into the pleural cavity, stick in the parietal pleura and securely fasten the cannula. Traction being made on the cannula, a ligature is tied at L around the protruding tissue for greater security. S, side- tube by which the cannula is connected with a manometer or tambour. 234 RESPIRATION It cannot fail to be observed that to a great extent the rate of respiration is affected by the same circumstances as the frequency of the heart (p. 107), and in the same direction. And, indeed, in health, these two physiological quantities, amid all their absolute variations, maintain to each other a fairly constant ratio (i to 4 or I to 5 in man) . Even in many diseases this proportion remains tolerably stable, although in others it is dis- turbed. The total quantity of air expired, or, what comes to the same thing, the alteration in the capacity of the chest during expiration, can be measured by means of a gas-meter or of a spiro- meter (Fig. 114), which consists of an inverted graduated glass cyUnder dip- ping by its open mouth into water and balanced by weights. The vessel is sunk till it is full of water, the air being allowed to escape by a cock. The expired air is now permitted to enter it through a tube, and displaces some of the water. The spirometer is adjusted so that the level of the water inside and outside is the same, and then the volume of air contained in it is read off. This gives the volume of the expired air at atmospheric pressure. Similarly, by breathing air from the spirometer the amount inspired can be measured (p. 297). From 400 to 500 c.c. of air* are taken in and given out at each respiration in quiet breathing. This is called tidal air. It amounts to 35 pounds by weight Ktal ^ Capacitu'" Fig. 114. — Diagram of Spirometer. A, vessel filled with water. B, glass cylinder with scale C, swung on pulleys and counter- poised by weights W. D, tube for breathing through. Comphnjeotal air lidal ai> SupfiejQenfal aif Residual air Fig. 115. — Diagram to illustrate the Relative Amount of Complemental, Tidal, Supplemental, and Residual Air. in twenty-four hours, or enough to fill, at atmospheric pressure, a cubical box with a side of 8 feet. With the deepest possible in- spiration room can be made for 2,000 c.c. more ; this is called complemental air. By a forced expiration 1,500 c.c. can be expelled besides the tidal air ; and to this * The average for 81 healthy students, with an average body- weight of 66 kUos, was 460 c.c, or 7 c.c. per kilo. In 4 new-born children the tidal air varied from 20 to 30 c.c, and from 7'6 to 73 c.c. per kilo, which is not very difEerent from the amount in the adult. The pulmonary ventilation must therefore be far more rapid in the child, since its respiratory frequency is so much greater. MECHANICAL PHENOMENA OF EXTERNAL RESPIRATION 235 quantity the name of supplemental or reserve air has been given. After the deepest expiration there alvs^ays remains 1,000 to 1,200 c.c. of air in the lungs (Durig), and this is called the residual air. After a normal expiration following a normal inspiration the lungs still contain stationary air to the amount of about 2,500 c.c. The term vital or respiratory capacity is appUed to the quantity of air which can be expelled by the deepest expiration following the deepest inspiration, and amounts in an adult of average height to 3,500 or 4,000 c.c. The maximum quantity of air which the lungs can contain is evidently equal to vital capacity plus residual air. At one time the vital capacity was thought to be capable of affording valuable information in the diagnosis of chest diseases; but little stress is now laid upon it, as it varies from so many causes. For instance, it can be increased by practice with the spirometer. It is greater in mountaineers than in the inhabitants of lowland plains. It is clear from the figures we have given that in ordinary breath- ing only a small proportion of the air in the lungs comes in direct at each inspiration from the atmosphere, and only a small proportion escapes into the atmosphere at each expiration. The greater part of the air in the lungs is simply moved a little farther from the upper respiratory passages, or a little nearer them; and fresh oxygen reaches the alveoli, as carbon dioxide leaves them, mainly by diffu- sion, aided by convection currents due to inequalities of temperature, and to the churning which the alternate expansion and shrinking of the lungs, and the pulsations of their arteries, must produce. But that some of the tidal air strikes right down to the alveoli is evident enough. For the respiratory ' dead space ' — that is, the capacity of the upper air-passages and the bronchial tree down to the infundibula — is only 140 c.c, or one-third of the amount of the tidal air (Zuntz, Loewy). There is no direct way of determining whether any respiratory exchange goes on through the walls of the upper air-passages. But by indirect methods it has been estimated that about 30 per cent, of the volume of the tidal air is pure air (Haldane and Priestley). This, of course, corresponds to the ' effec- tive ' dead space. Taking the average tidal air at 460 c.c. (p. 234), it is clear that the effective corresponds very closely with the ana- tomical dead space — ^that is to say, the respiratory function of the air-passages above the point where the infundibula are given off is negligible. Although such calculations can only be approximately correct, the agreement is of interest. The immense extent of the pulmonary surface, and the extreme thinness of the layer of blood in the capillaries of the lungs and of the alveolar walls, facilitate the interchange between the gases of the blood and the gases of the alveoli. The Anaount and Variations of the Intrathoracic Pressure. — In the deepest expiration the lungs are never completely collapsed; their 236 RESPIRATION elastic fibres are still stretched ; and the tension of these acts in the opposite direction to the external atmospheric pressure, and dimin- ishes by its amount the pressure inside the thoracic cavity. In the dead body Bonders measured the value of this tension, and there- fore of the negative pressure of the thorax, by tjdng a manometer into the trachea, and then causing the lungs to collapse by opening the chest. It varied from 7-5 mm. of mercury in the expiratory position to 9 mm. in the inspiratory. So far as can be judged from observations made on persons suffering from various diseases of the Fig. 116. — Variations of Intrathoracic Pressure. Upper curve, carotid blood-pres- sure (dog); lower curve, intrapleural pressure. At 42 the trachea was closed; the blood-pressure curve shows the rise of asphyxia, and the intrapleural curve, greatly exaggerated pressure variations due to the strong and slow but abortive respirations. respiratory organs, the alterations during ordinary breathing do not amount to more than 3 or 4 mm. of mercury. But when an attempt is made in the dead body to imitate a deep inspiration by making traction on the chest-walls so as to expand the lungs, the intra- thoracic pressure may fall to —30 mm. of mercury; and in a living rabbit, during a deep natural inspiration, a pressure of —20 mm. has been seen. The reason why the lungs collapse when the chest is opened is that the pressure is now equal on the pleural and alveolar surfaces, being in both cases that of the atmosphere. There is therefore MECHANICAL PHENOMENA OF EXTERNAL RESPIRATION 237 nothing to oppose the elasticity of the lungs, which tends to con- tract them. So long as the chest is unopened, the pressure on the pleural surface of the lungs is less than that on the alveolar surface, and the elastic tension can only cause them to shrink until it just balances this difference. In intra-uterine life, and in stillborn children who have never breathed, the lungs are completely collapsed (atelectatic), and there is no negative intrathoracic pressure. They are kept in this con- dition by adhesion of the walls of the bronchioles and alveoli. If the lungs have been once inflated, this adhesion ceases to act, and they never completely collapse again. Amount and Variations of the Respiratory or Intrapulmonary Pressure. — As we have already remarked, the pressure in the alveoli and air-passages is less than that of the atmosphere while the in- spiratory movement is going on, greater than that of the atmosphere during the expiratory movement, and equal to that of the atmo- sphere when the chest -walls are at rest. When the external air- passages are closed — e.g., by connecting a manometer with the mouth and pinching the nostrils — ^the greatest possible variations of pressure are produced. In the deepest inspiration under these conditions a negative pressure of about 75 mm. of mercury {i.e., a pressure less than that of the atmosphere by this amount) has been found, and in deep expiration a somewhat greater positive pressure* (Practical Exercises, p. 298). 1 But with ordinary breathing, the variations of pressure as measured by this method do not exceed 5 to 10 mm. of mercury above or below the pressure of the atmosphere. When the external openings are not obstructed, as, for example, when the lateral pressure is taken in the trachea of an animal by means of a cannula with a side-tube connected with a manometer, stiU smaller, and doubtless truer, values have been found (2-3 mm, of mercury as the positive expiratory pressure and i mm. as the negative inspiratory pressure in dogs). But since the respiratory passages are abruptly narrowed at the glottis, the variations of pressure must be greater below than above it, and in general they must increase with the distance from that orifice, being greater, for instance, in the alveoli than in the bronchi. The mechanical phenomena of respiration having been described, it might seem logical to consider next the nervous mechanism by which the respiratory movements are controlled ; but the regulation of these movements through the nervous system is in so important a degree a chemical regulation that it cannot be properly understood * The maximum negative pressure in deepest inspiration averaged for 49 students -73 mm, (highest observation - 137 mm.) of mercury ; the maxi- mum positive pressure in deepest expiration, + 80 mm. (highest observation + 140 mm.). 238 RESPIRATION without some knowledge of the chemical changes in the blood associated with external and internal respiration. We therefore pass to the consideration of — Section III. — ^The Chemistry of External Respiration. Our knowledge of this subject has been entirely acquired in the last 200 years, and chiefly in the last century. Boyle showed by means of the air-pump that animals die in a vacuum, and Bernouilli that fish cannot Uve in water from which the air has been driven out by boiling. Mayow, of Oxford, seems to a considerable extent to have antici- pated Black, who in 1757 demonstrated the presence of carbonic acid (carbon dioxide) in expired air by the turbidity which it causes in lime-water. A fundamental step was the discovery of oxygen by Priestley in 1771, and his proof that the venous blood could be made crimson, like arterial, by being shaken up with oxygen. Lavoisier discovered the composition of carbonic acid, and applied his discovery to the explanation of the respiratory processes in animals, the heat of which he showed to be generated, hke that of a candle, by the union of carbon and oxygen. He made many further important experiments on respiration, publishing some of his results in 1789, when the French Revolution, in which he was to be one of the most distinguished victims, was breaking out. He made the mistake, however, of supposing that the oxidation of the carbon takes place in the blood as it passes through the lesser circulation. That some carbon dioxide is formed in the lungs there is no reason to doubt, and the quantity may even be considerable. But that they are not the chief seat of oxidation was sufficiently proved as soon as it was known that the blood which comes to them from the right heart is rich in carbon dioxide, while the blood which leaves them through the pulmonary veins is comparatively poor. There are two main lines on which research has gone in trying to solve the chemical problems of respiration: (i) The analysis and comparison of the inspired and expired air, or, in general, the in- vestigation of the gaseous exchange between the blood and the air in the lungs. (2) The analysis and comparison of the gases of arterial and venous blood, of the other liquids, and of the solid tissues of the body, with a view to the determination of the gaseous exchange between the tissues and the blood. We shall take these up as far as possible in their order. The methods which have been used for comparing the composi- tion of inspired and expired air and estimating the respiratory ex- change are very various. THE CHEMISTRY OF EXTERNAL RESPIRATION 239 (i) Breathing into one spirometer and out of another, the inspired and expired air being directed by valves. The contents of the spiro- meters are analyzed at the end of the experiment (Speck). In the arrangement of Zuntz and Geppert, instead of the whole of the expired air, a sample is collected for analysis during the entire duration of the experiment, while the total volume expired is measured by a gas-meter. This is a very convenient method for observations on man, especially in disease, but each experiment can only be carried on at most for fifteen to twenty minutes. (2) A small apparatus, much on the same principle, was used for rabbits by Pfliiger and his pupils. A cannula in the trachea was con- nected with a balanced and self-adjusting spirometer containing oxygen, and the inspired and expired air separated by potassium hydroxide valves, which absorbed the carbon dioxide. The amount of oxygen used could be read oflf on the spirometer, and the amount of carbon dioxide produced estimated in the liquid of the valves. (3) Elaborate arrangements, such as Pettenkofer's great respiration apparatus, and the still larger and more efficient modifications of it constructed since his time, in which a man, or even several men, can remain for an indefinite period, working, eating, and sleeping. Air is drawn out of the chamber by an engine, its volume being measured by a gas-meter. But as it would be far too troublesome to analyze the whole of the air, a sample stream of it is constantly drawn off, which also passes through a gas-meter, through drying-tubes containing sulphuric acid, and through tubes filled with baryta water. The baryta solution is titrated to determine the quantity of carbon dioxide; the increase in weight of the drying tubes gives the quantity of aqueous vapour. A similar sample stream of the air before it passes into the chamber is treated exactly in the same way, and from the data thus got the quantity of carbon dioxide and aqueous vapour given off can readily be ascertained. The oxygen can be calculated, as the difference be- tween the final body-weight and the original body-weight plus the weight of the carbon dioxide and water eliminated, but may also be directly estimated by special methods. (4) Haldane and Pembrey have elaborated a gravimetric method, which is very suitable for small animals. It depends upon the absorp- tion of carbon dioxide by soda lime. (See Practical Exercises, p. 299.) In Atwater's so-called respiration calorimeter, which will be referred to again under ' Animal Heat,' and by which, not only the gaseous metab- olisrn, but the heat production can be measured in man, the carbon dioxide is estimated in the same way. Inspired and Expired Air. — The expired air is at or near the body temperature, and is saturated with watery vapour. In ordinary breathing it contains about 4 per cent, of carbon dioxide, while the inspired air only contains a trace. The expired air contains 16 or 17 per cent, of oxygen, the inspired air about 21 per cent. The percentage of carbon dioxide in the alveolar air is, of course, greater than in the ordinary expired air, since the relatively pure air of the dead space constitutes a substantial fraction of the tidal air. The carbon dioxide percentage in the alveolar air at the end of expira- tion, with the body at rest, is remarkably constant in one and the sanie individual at constant atmospheric pressure (p. 261). There aie m addition in expired air small quantities of hydrogen and marsh- 240 RESPIRATION gas derived from the alimentary canal, either directly from eructa- tion or after absorption into the blood. Sometimes a trace of ammonia can be detected in the air of expiration, but this is due to decomposition of proteins taking place in the mouth, especially in carious teeth, or in the air-passages and lungs in disease of these organs. It has indeed been shown that the lungs are practically impermeable for ammonia. Expired air is entirely free from float- ing matter (dust), which is always present in the inspired air. The volume of the expired air, owing to its higher temperature and ex- cess of watery vapour, is somewhat greater than that of the inspired air, but if it be measured at the temperature and degree of satura- tion of the latter, the volume is somewhat less. Since the oxygen of a given quantity of carbon dioxide would have exactly the same volume as the carbon dioxide itself at a given temperature and pressure, it is clear that the deficiency is due to the fact that all the oxygen which is taken up in the lungs is not given off as carbon dioxide. Some of it, going to oxidize hydrogen, reappears as water. A small amount of it unites with the sulphur of the proteins (p. 478). Respiratory Quotient. — ^The quotient of the volume of oxygen given out as carbon dioxide by the volume of oxygen taken in is the respiratory quotient. It shows what proportion of the oxygen is used to oxidize carbon. It may approach unity on a carbo-hydrate diet which contains enough oxygen to oxidize all its own hydrogen to water. With a diet rich in fat it is least of all; with a diet of lean meat it is intermediate in amount. For ordinary fat contains no more than one-sixth, and proteins not one-half, of the oxygen needed to oxidize their hydrogen (p. 608). In man on a mixed diet the respiratory quotient may be taken as o-8 or o-g. So long as the type of respiration is not changed, the respiratory quotient may remain constant for a wide range of metabolism. In hibernating animals, however, the respiratory quotient may become very small during winter sleep (as low as 0-25), both the output of carbon dioxide and the consumption of oxygen falling enormously, but the former in general more than the latter. This has been explained on the assumption that oxygen is stored away in winter sleep in the form of incompletely oxidized substances. On the other hand, in dyspnoea accompanying muscular exertion the respiratory quotient has been found as high as 1-2. It must be remembered that even a voluntary increase in the respiratory movements causes an imme- diate temporary increase in the respiratory quotient, owing to the ' washing out ' of carbon dioxide from the blood and tissues. This change has no metabolic significance. Indeed, the determination of the respiratory quotient for short periods has only a hmited value, and such observations must be interpreted with great care. In starvation the respiratory quotient diminishes, the production of carbon dioxide falling off at a greater rate than the consumption THE CHEMISTRY OF EXTERNAL RESPIRATION 241 of oxygen, for the starving organism lives on its own fat and pro- teins, and has 'only a trifling carbo-hydrate stock to draw upon. In a diabetic patient, fed on a diet of fat and protein alone, the respiratory quotient was only o-6 to 07, just as in a starving man. Total Respiratory Exchange. — The amount of oxygen absorbed in a man at rest has been determined under certain conditions as about • 0-29 gramme per hour, and the discharge of carbon dioxide as about 0'33 gramme per hour per kilogramme of body-weight. In an average man weighing 70 kilos the mean production of carbon dioxide is about 800 grammes (400 litres) in twenty- four hours, and the mean consumption of oxygen about 700 grammes (490 litres). But there are very great variations depending upon the state of the body as regards rest or muscular activity and on other circum- stances. In hard work the production of carbon dioxide was found to rise to nearly 1,300 grammes, and in rest to sink to less than 700 grammes, the consumption of oxygen in the same circumstances increasing to nearly 1,100 grammes and diminishing to 600 grammes. In rest, in moderate exertion, and in hard work, the production of carbon dioxide was found to be nearly proportionate to the numbers 2, 3, and 6 respectively. When unaccustomed work is performed, the increase in the carbon dioxide output (and oxygen intake) may be much greater. With training it diminishes. In a case of diabetes the consumption of oxygen was 50 per cent, greater than in a healthy man, corresponding to the higher heat-equivalent of the food of the diabetic patient. Ventilation. — ^Taking 400 litres per twenty-four hours, or 17 litres per hour, as the mean production of carbon dioxide by an average male adult at rest or doing only light work, we can calculate the quantity of fresh air which must be supplied to a room in order to keep it properly ventilated. It has been f oimd that when the carbon dioxide given off in respiration amounts to no more than 2 parts in 10,000 in the air of an ordinary room, the air remains sweet. When the carbon dioxide given off reaches 4 parts in 10,000, the room feels distinctly, and at 6 in 10,000 disagree- ably, close, while at 9 parts in 10,000 it is oppressive and almoist in- tolerable. This is not due to the carbon dioxide as such,, for pure carbon dioxide added alone in similar proportions to the air of a room has not the same bad effect, and the amount of this gas is only taken as an index of the extent to which the air has been vitiated by some other products or processes connected with the occupation of the room. Very often the mere rise of temperature in a crowded and ill- ventilated space is suf&cient to induce disagreeable symptoms, especially as it is inevitably associated with an increase in the humidity of the air, which reduces the capacity of the body to cool itself by increasing the secretion of sweat. Thus it has been found that persons in a respiratory chamber feel quite comfortable with only moderate ventila- tion when the carbon dioxide has risen to i per cent., if care is taken that the temperature and ■*he proportion of watery vapour do not rise too high. In addition, however, it has been supposed by some that a volatile poison exhaled from the luiigs is peculiarly responsible for the 16 242 RESPIRATION evil effects. Certain observers, indeed, alleged that the condensed vapour of the breath, when injected into rabbits, produced fatal symp- toms. But this has been shown to be erroneous; and the most careful experiments have failed to detect in the air expired by healthy persons any trace of such a poison. It has therefore been suggested that the odour and some of the other ill-effects of a close room are due to sub- stances given off in the sweat and the sebum, and allowed by persons of uncleanly habits to accumulate on the skin, and also to the products of slow putrefactive processes constantly going on, under favourable conditions, on the walls, floor, or furniture, but only becoming per- ceptible to the sense of smell when ventilation is insufficient. In a small, newly-painted chamber, presumably free from such impurities, it was not until the carbon dioxide reached 3 to 4 per cent., an immensely greater proportion than occurs even in very badly ventilated rooms, that marked discomfort, with dyspnoea, began to be felt. No close odour could be detected. Nevertheless, experience has shown that it is a good working rule for ventilation to take the limit of permissible respiratory impurity at 2 parts of carbon dioxide per 10,000; and the 17 litres of carbon dioxide given off in the hour will require 85,000 litres (or 3,000 cubic feet) of air to dilute it to this extent. This is the average quantity required for the male adult per hour. For men engaged in active labour, as in factories or mines, twice this amount may not be too much. For women and children less is required than for men. If a room smells close, it needs ventilation, whatever be the proportion of carbon dioxide in the air. It must be remembered that in permanently occupied rooms mere increase in the size will not compensate for incomplete renewal of the air, although it niay be easier to ventilate a large room than a small one without causing draughts and other inconveniences. But as few apartments are occupied during the whole twenty-four hours, a large room which can be thoroughly ventilated in the absence of its inmates has a distinct advantage over a small one in its great initial stock of fresh air. The &ubic space per head in an ordinary dwelling- house should be not less than 28 cubic metres or 1,000 cubic feet. The quantity of carbon dioxide given off (and of oxygen consumed) is not only affected by muscular work, but also by everything which influences the general metabolism. In males it is greater on the average than in females (in the latter there is a temporary increase during pregnancy), but for the same body- weight and under similar external conditions there is no difference between the sexes. The gaseous exchange is greater in proportion to the body-weight in the child than in the adult. This depends largely on the fact that, other things being equal, the metabolism is relatively to the body- >• weight more active in a small than in a large organism, since the surface (and therefore the heat loss) is relatively greater in the former. But it has been shown that even in proportion to the surface the metabolism is greater in youth than in adult life, and greater in the vigorous adult than in the old man. So that the age •of the organism has an influence apart from the extent of surface. The taking of food increases the gaseous exchange, partly from the increased mechanical and chemical work performed by the ah- mentary canal and the digestive glands. But that this is not the THE CHEMISTRY OF EXTERNAL RESPIRATION M3 sole cause of the increase is shown by the fact that it varies with different kinds of food to a greater extent than can be explained by differences in the ease with which they are digested. For in- stance, maize produces a much greater increase than oats when given in equal amount, and a protein diet a greater increase than a diet of carbo-hydrate or fat. Sleep diminishes the production of carbon dioxide partly because the muscles are at rest, but also to some extent because the external stimuli that in waking life excite the nerves of special sense are absent or ineffective. Even a bright light is said to cause an increase in the amount of carbon dioxidi produced and of oxygen consumed ; but probably only by increasing muscular movements, including the movements of respiration. The external temperature also has an influence. In poikilothermal animals (such as the frog), the temperature of which varies with that of the surrounding medium, the production of carbon dioxide, on the whole, diminishes as the external temperature falls, and increases as it rises. In homoiothermal animals, that is, animals with constant blood temperature, external cold increases the pro- duction of carbon dioxide and the consumption of oxygen. But if the connection of the nervous system with the striated muscles has been cut out by curara, the warm-blooded animal behaves like the cold-blooded (Pfliiger and his pupils in guinea-pig and rabbit). These interesting facts will be returned to under ' Animal Heat.' Cold-blooded animals produce far less carbon dioxide, and con- sume far less oxygen, per kilo of body- weight than warm-blooded. The following table shows the relation between the body-weight and the excretion of carbon dioxide in man : Age. Weight in Kilos. "CO2 excreted per Kilo per Hour. Male r.58 44 35 28 16 ^ 9-6 84-6 76-5 6.5 82 577 22 0-41 gramme 0-48 0-51 „ 0-49 0-59 0-92 /■66 Female- f 66-9 53-9 557 23 0-39 ,, 0-54 ,, 0-45 ., 0-83 The next table illustrates the difference in the intensity of metab- olism in different kinds of animals, a difference, however, largely dependent upon relative size; 244 RESPIRATION ^^ r-i- : ] Oxygen absorbed per Carbon Dioxide given off Kilo per Hour. per Kilo per Hour. Respiratory Quotient Animal. CO2 O2 (in con) In Grms. In C.C. In Grms. In C.C. 0,°' O2 • Greenfinch - 13-000 9091 13-590 6909 0-76 Hen 1-058 740 1-327 675 0-91 Dog 1-303 911 1-325 674 0-74 Rabbit 0-987 • 690 1-244 632 0-91 ''Sheep 0-490 343 0-671 341 0-99 Boar 0-391 273 0-443 225 0-82 Freg - 0-105 73-4 0-113 57-7 0-78 Crayfish 0-054 3S 0-064 32-7 0-86 Forced respiration, although it will temporarily increase the quantity of carbon dioxide given off by the lungs, and thus raise for a short time the respiratory quotient, does not sensibly affect the production; it is only the store of already formed carbon dioxide in the body which is drawn upon. The amount of oxygen taken up is little altered by changes in the movements of respiration. Within wide hmits the oxygen consumption of the organism is in- dependent of the supply of oxygen offered to it. How it is that the depth of the respiration may affect the rate at which carbon dioxide is eliminated, we can only understand when we have examined the process by which the gaseous interchange between the blood and the air of the alveoh is accomplished; and before doing this it is necessary to consider the condition of the oxygen and carbon dioxide in the blood. Section IV. — ^The Gases of the Blood. Physical Introduction. — ^Matter may be assumed to be made up of molecules beyond which it cannot be divided without aJtering its essen- tial character. A molecule may consist of two or more particles of matter (atoms) bound to each other by cheinical links. The kinetic theory of matter supposes the molecules of a substance to be in constant motion, frequently colliding with each other, and thus having the direc- tion of their motion changed. In a gas the mean free path, that is, the average distance which a molecule travels without striking another, is comparatively long, and far more time is passed by any molecule without an encoimter than is taken up with collisions. Although the average. velocity of the mole- cules is very great, these coUisions will produce all sorts of difierences in the actual velocity of different molecule^ at any given time. Some will be moving at a greater, some at a slower rate, than the average; while some may be for a moment at rest. If the gas is in a closed vessel, the molecules will be constantly striking its sides and rebounding from them. If a very small opening is made in the vessel, some mole- cules will occasionally hit on the opening and escape altogether. If the opening is made larger, or the experiment continued for a longer time THE GASES OF THE BLOOD 245 with the small opening, all the molecules will in course of time have passed out of the vessel into the air, while molecules of the oxygen, nitrogen, and argon of the air will have passed in. In a gas, then, not enclosed by impenetrable boundaries, there is no restriction on the path which a molecule may take, no tendency for it to keep within any limits. When two chemically indifferent gases are placed in contact with each other, diffusion will go on till they are uniformly mixed. The diffusion of gases may be illustrated thus. Suppose we have a perfectly level and in every way uniform field divided into two equal parts by a visible but intangible line, the well-known whitewash line, for instance. On one side of the line place 500 blind, men in green, and on the other 500 blind men in red. At a given signal let them begin to move about in the field. Some of the men in green wiU pass over the line to the ' red ' side; some of the men in red will wander to the ' green ' side. Some of the men may pass over the line and again come back to the side they started from. But, upon the whole, after a given interval has elapsed, as many green coats will be seen on the red side as red coats on the green. And if the interval is long enough there will be at length about 250 men in red and 250 in green on each side of the boundary- line. When this state of equilibrium has once been reached, it will henceforth be maintained, for, upon the whole, as many red uniforms will pass across the line in one direction, as will recross it in the other. In a liquid it is very different; the molecule has no free path. In the depth of the liquid no molecule ever gets out of the reach of other molecules, although after an encotmter there is no tendency to return on the old path rather than to choose any other; so that any molecule may wander through the whole liquid. Although the average velocity of the molecules is much less in the liquid state than it would be for the same substance in the state of gas or vapour (gas in presence of its liquid), some of them may have velocities much above the average. If any of these happen to be moving near the surface and towards it, they may overcome the attraction of the neighbouring molecules and escape as vapour. But if in their further wanderings they strike the liquid again, they may again become bound down as liquid molecules. And so a constant interchange may take place between a liquid and its vapour, or between a liquid and any other gas, until the state of equi- librium is reached, in which on the average as many molecules leave the liquid to become vapour as are restored by the vapour to the liquid, or as many molecules of the dissolved gas escape from solution as enter into it. For the sake of a simple illustration, let us take the case of a shallow vessel of water originally gas-free, standing exposed to the air. It will be found after a time that the water contains the atmospheric gases in certain proportions — in round numbers, about jj^ of its volume of oxygen and -^g of its volume of nitrogen (measured at'760 mm. mercury and 0° C). Now, let a similar vessel of gas-free water be placed in a large airtight box filled with air at atmospheric pressure, and let the oxygen be all absorbed before the water is exposed to the atmosphere of the box. The latter now consists practically only of the nitrogen of the air, and its pressure will be only about four-fifths that of the external atmo- sphere. Nevertheless, the quantity of nitrogen absorbed by the water will be exactly the same as was absorbed from the air. If the box was completely exhausted, and then a quantity of oxygen, equal to that m it at first, introduced before the water was exposed to it, the pressure would be found to be only about one-fifth that of the external atmo- sphere; but the quantity of ojcygen taken up by the water would be exactly equal to that taken up in the first experiment. 246 RESPIRATION Two well-known physical laws are illustrated by our supposed ex- periments: (i) In a mixture of gases which do not act chemically on each other the pressure exerted by each gas {called the partial pressure of the gas) is the same as it would exert if the others were absent. (2) The quan- tity {mass) of a gas absorbed by a liquid which does not act chemically upon it is proportional to the partial pressure of the gas. It also depends upon the nature of the gas and of the liquid, and on the temperature, increase of temperature in general diminishing the quantity of gas absorbed. It is to be noted that when the volume of the absorbed gas is measured at a pressure equal to the partial pressure under which it was absorbed, the same volume of gas is taken up at every pressure. The volume of a gas (reduced to 0° C. and 760 mm. pressure) physi- cally absorbed or dissolved in i c.c. of a liquid exposed to the gas at 760 mm. pressure is called the absorption coefficient of the gas in that liquid. The following table from Bohr shows the absorption coefficients of the three gases of physiological interest — oxygen, nitrogen, and carbon dioxide in water, blood-plasma, whole blood and blood-corpuscles at the body temperature (38° C.) : Oxygen. Nitrogen. Carbon Dioxide. Water Blood-plasma Blood - Blood-cells 6-0237 0-023 0-022 o-oig 0-0I22 0-0I2 O-OIl O-OIO o'555 0-541 0-511 0-450 Suppose, now, that a vessel of water, saturated with oxygen and nitrogen for the partial pressures under which these gases exist in the air, is placed in a box filled with pure nitrogen at full atmospheric pres- sure. As we have seen, there is a constant interchange going on between a liquid which contains gas in solution and the atmosphere to which it is exposed. Oxygen and nitrogen molecules will therefore continue to leave the water; but if the box is large, few oxygen molecules will find their way back to the water, and ultimately little oxygen will remain in it. In other words, the quantity of oxygen absorbed by the water will become again proportional to the partial pressure of oxygen, which is not now much above zero. On the other hand, molecules of nitrogen will at first enter the water in larger number than they escape from it, for the pressure of the nitrogen is now that of the external atmosphere, of which its partial pressure was formerly only four-fifths. In unit volume of the gas above the water there will be 5 molecules of nitrogen for every 4 molecules in the same volume of atmospheric air. There- foire, on the average 5 nitrogen molecules will in a given time get en- tangled by liquid molecules for every 4 which came within their sphere of attraction before. On the whole, then, the water will lose oxygen and gain nitrogen, while the atmosphere of the airtight box will gain oxygen and lose nitrogen. In the case of water, in which oxygen and nitrogen are absorbed solely in solution, the partial pressures of these gases under which the water was originally saturated could, of course, be easily calculated from the amount dissolved and the coefficient of absorption. But supposing that these partial pressures were unknown, it is evident that by exposing it to an atmosphere of known composition, and afterwards determining the changes produced m the composition of that atmo- THE GASES OF THE BLOOD 247 sphere by loss to, or gain from, the gases of the water, we could find out something about the original partial pressures. If, for example, the quantity of oxygen in the atmosphere or the chamber was increased, we could conclude that the partial pressure of oxygen under which the water had been saturated was greater than that in the chamber at the beginning of the experiment. And if we found that with a certain partial pressure of oxygen in the atmo- sphere of the chamber there was neither gain nor loss of this gas, we might be sure that the partial pressure (the temperature being sup- posed not to vary) was the same when the water was saturated. We shall see later on how this principle has been applied to deter- mine the partial pressure of oxygen or carbon dioxide which just suffices to prevent blood, or any other of the liquids of the body, from losing or gaining these gases when they are not merely dissolved, but also combined in the form of dissociable compounds. This pressure is evidently equal to that exerted by the gases of the liquid at its surface, which is sometimes called their ' tension ' ; for if it were greater, gas would, upon the whole, pass into the blood ; and if it were less, gas would escape from the blood. Thus, the tension of a gas in solution in a liquid is equal to the partial pressure of that gas in an atmosphere to which the liquid is ex- posed, which is just sufficient to prevetit gain or loss of the gas by the liquid (p. 256). The following imaginary experiment may further illustrate the meaning of the term ' ten- sion ' of a gas in a liquid in this connection. Suppose a cylinder filled with a liquid con- taining a gas in solution, and closed above by a piston moving airtight and without friction, in contact with the surface of the liquid (Fig. 117). Let the weight of the piston be balanced by a counterpoise. The pressure at the sur- face of the liquid is evidently that of the atmosphere. Now, let the whole be put into the receiver of an air-pump, and the air gradually exhausted. Let exhaustion proceed until gas begins to escape from the liquid and lies in a thin layer between its surface and the piston, the quantity of gas which has become free being very small in proportion to that still in solution. At this point the piston is acted upon by two forces which balance each other, the pressure of the air in the receiver acting downwards, and the pressure of the gas escaping from the liquid acting upwards. If Fig. 117. — Imaginary Ex- periment to illustrate ' Tension ' of a Gas in a Liquid. P, frictionless piston; L, liquid in cy- linder; G, gas beginning to escape from liquid. P is exactly counter- poised. In addition to the manner described in the text, the experiment may be supposed to be performed thus: Let the weight, W, be deter- mined which, when the receiver is completely exhausted, suffices just to keep the piston in contact with the liquid. The pressure of the gas is then just counter- balanced by W; and if S is the area of the cross- section of the piston, the pressure of the gas per W unit of area is ^. Or, if the piston is hollow, and mercury is poured into it so as just to keep it in contact with the liquid, the height of the column of mercury required is also equal to the pressure' or tension of the %3S. the pressure in the receiver is now slightly increased, the gas is again absorbed. The pressure at which this just happens, and against which the piston is still supported by the impacts of gaseous molecules flying out of the liquid, while no pressure is as yet 248 RESPIRATION exerted directly between the liquid and the piston, is obviously equal to the pressure or tension of the gas in the liquid. From the above principles it follows that a gas held in solution may be extracted by exposure to an atmosphere in which the partial pressure of the gas is made as small as possible. Thus, oxygen can be obtained from liquids in which it is simply dissolved by putting them in an atmosphere of hydrogen or nitrogen, in which the partial pressure of oxygen is zero, or in the vacuum of an air-pump, in which it is extremely small. Heat also aids the expulsion of dissolved gases. Some gases held in weak chemical union, like the loosely-combined oxygen of oxyhaemoglobin, can be obtained by dissociation of their compounds Fig. Ii8.- — Scheme of Gas-Pump. A, the blood bulb; B, the froth chamber; C, the drying tube; D, fixed mercury bulb ; E, movable mercury bulb comiected by a flexible tube with D; F, eudiometer; G, a narrow delivery tube; i, 2, 3, 4, taps, 4 being a three-way tap. A is filled with blood by connecting the tap i by means of a tube with a bloodvessel. Taps i and 2 are then closed. The rest of the apparatus from B to D is now exhausted by raising E, with tap 4 turned so as to place D only in communication with G, till the mercury fills D. Tap 4 is now turned so as to connect C with D, and cut off G from D, and E is lowered. The mercxn-y passes out of D, and air passes into it from B and C. Tap 4 is again turned so as to cut off C from D and connect G and D. E is raised and the mercury passes into D and forces the air out through G, the end of which has not hitherto been placed under F. This alternate raising and lowering of E is continued till a man- ometer connected between C and 4 indicates that the pressure has been sufficiently reduced. The tap 2 is now opened; the gases of the blood bubble up into the froth chamber, pass through the drying-tube C, which is filled with pumice-stone and sulphuric acid, and enter D. The end of G is placed under the eudiometer F, and by raising E, with tap 4 turned so as to cut off C, the gases are forced out through G and collected in F. The movements required for exhaustion can be repeated several times till no more gas comes off. The escape of gas from the blood is facilitated by immersing the bulb A in water at 40° to 50° C. when the partial pressure is reduced. More stable combinations may require to be broken up by chemical agents — carbonates, fcr instance, by acids. Extraction of the Blood-Gases. — ^This is best accomplished by ex- posing blood to a nearly perfect vacuum. The gas-pumps which have been most largely used in blood analysis are constructed on the principle of the Torricellian vacuum. A diagram of a simple form of Pfluger's gas-pump is given in Fig. 118, The gases obtained are ultimately dried and collected in a eudiometer, which is a graduated glass tube with its mouth dipping into mercury. The carbon dioxide is estimated by introducing a little potassium hydroxide to absorb it. The diminution in the volume of the gas contained in the eudiometer gives the volume of the carbon dioxide. The oxygen may be estimated by putting into the eudiometer more than enough hydrogen to unite with all the oxygen so as to form water, and then, after reading off the volume, exploding the mixture by means of an electric spark passed through two platinum wires fused into the glass. One-third of the diminution of volume represents the quantity of oxygen present. It can also be estima ted THE GASES OF THE BLOOD 249 by absorption with a solution of pyrogallic acid and potassium hydrox- ide, or an alkaline solution of sodium hydrosulphite, which is more cleanly. The remainder of the original mixture of blood-gases, after deduction of the carbon dioxide and oxygen, is put down as nitrogen (with, no doubt, a small proportion of argon). For the sake of easy comparison, the observed volume of gas is always stated in terms of its equivalent at a standard pressure and temperature {760 mm., or some- times on the Continent i metre of mercury, and 0° C). It is also possible in various ways to estimate the amount of oxygen in blood without the use of the pump. Thus, since a definite volume of oxygen (l"338 c.c. at 0° C. and 760 mm. pressure) combines with a gramme of haemoglobin, we can calculate the total volume of oxygen present if we know how much of the blood-pigment is in the form of oxyhaemoglobin ; and this can be determined by means of the spectro- photometer. Or potassium ferricyanide may be added to the blood. This expels the oxygen from its combination with the haemoglobin, which then unites with an exactly equal amount of oxygen obtained from the ferricyanide to form methaemoglobin (Haldane) (p. 75). The Quantity of the Blood-Gases. — In arterial and in venous blood oxygen, carbon dioxide, nitrogen, and argon are constantly found. Both the oxygen and the carbon dioxide vary considerably in amount in the arterial blood, even of individuals of the same animal group, and, of course, much more in the venous blood, as might naturally be expected, since even to the eye it varies greatly accord- ing to the vein it is obtained from, the rapidity of the circulation, and the activity of the tissues which it has just left. In one observation on blood obtained directly from a human artery, 21-6 c.c. of oxygen, 40-3 c.c. of carbon dioxide, and i-6 c.c. of nitrogen were found in 100 c.c. of blood. The quantity of oxygen taken up outside of the body by specimens of human blood drawn from six normal persons, when shaken up with atmospheric air, varied from 17-6 c.c. to 22-5 c.c. per 100 c.c. of blood, the variations depending mainly on the haemoglobin content. The arterial blood as it actually left the lungs of those persons must have contained somewhat less oxygen (about I c.c. less per 100 c.c. of blood), since the partial pressure of " oxygen in "the alveolar air is decidedly below that in atmospheric air. In dogs the amount of carbon dioxide in arterial blood has been found to vary from 35 to 45 c.c. per 100 c.c. of blood, the differences being due to variations in the extent of the pulmonary ventilation and to other factors. In a series of observations on the venous blood of dogs the oxygen content ranged from 5-5 to i6-6 c.c. (averageii-g c.c), and the carbon dioxide content from 38-8 c.c. to 47-5 (average 44-3 cc.) per 100 c.c. of blood (Schoffer). It will be sufficiently accurate to assume that on the average, voiun.es of; 02. CO2. N2. loo volumes of arterial blood yield - 20 46 1-2 mixed venous blood (from right heart) yield 10-12 45-50 1-2 (reduced to 0° C. and 760 mm. of mercury). 250 RESPIRATION Average venqus blood contains 7 or 8 per cent, by volume less oxygen, and 7 or 8 per cent, more carbon dioxide, than arterial blood. Thus, in the lungs the blood gains about twice as many volumes of oxygen per cent, as the air loses, and the air gains about half as many volumes of carbon dioxide per cent, as the blood loses. It is easy to see that this must be so, for the volume of air inspired in a given time is about twice as great as that of the blood which passes through the pulmonary circulation (pp. 223, 234). Even arterial blood is not quite saturated with oxygen ; it can still take up a variable small amount. The percentage saturation with oxygen of the arterial blood of a normal woman from whom blood was being transfused into a patient was directly determined. The blood proved to be 94 per cent, saturated — i.e., it could still have taken up about one-sixteenth of the quantity contained in it. Nor is venous blood nearly saturated with carbon dioxide; when shaken with the gas it can take up about 150 volumes per cent. When the gases are not removed from blood immediately after it is drawn, it yields more carbon dioxide and less oxygen than if it is evacuated at once (Pfliiger). From this it is concluded that oxidation goes on in the blood for some time after it is shed. The oxidizable substances are, however, confined to the corpuscles, which suggests that ordinary metabolism simply continues for some time in the formed elements of the shed blood, and that the disappearance of oxygen is not due to the oxidation of substances which have reached the blood from the tissues. The Distribution and Condition of the Oxygen in the Blood. — ^The oxygen is nearly all contained in the corpuscles. A little oxygen can be pumped out of serum (0-2 or 0-3 per cent, by volume), but this follows the Henry-Dalton law of pressures — that is, it comes off in proportion to the reduction of the partial pressure of the oxygen in the pump, and is simply in solution. When blood at body temperature is shaken up with air at the ordinary pressure, corresponding to a partial pressure of oxygen of a little over one- fifth of an atmosphere (in round numbers 160 mm. of mercury), the blood-pigment becomes saturated with oxygen or nearly so. When the blood is now pumped out, very little oxygen comes off till the pressure has been reduced to about half an atmo- sphere, corresponding to a pressure of oxygen of about 80 mm. At about 70 mm. partial pressure the dissociation is somewhat greater. At a third to a quarter of an atmosphere (50 to 40 mm.) the amount of oxygen liberated is markedly increased, and the dissociation becomes more and more rapid as the pressure falls towards zero. This behaviour shows that the oxygen is not simply absorbed, but is united, as a dissociable compound, to some con- stituent of the blood- The same thing is, of course, seen when THE GASES OF THE BLOOD 251 defibrinated blood is saturated at body temperature with oxygen at different pressures. As the partial pressure of the gas is increased from zero the first increments of pressure correspond to a much greater absorption of oxygen than further equal increments. Thus, as is seen in Fig. 120, with an oxygen pressure of 10 mm. 100 c.c. of blood took up 6 c.c. of oxygen, or 30 per cent, of the amount required to saturate it. When the pressure of oxygen was 30 mm. over 16 c.c. of oxygen was absorbed, the blood being 80 per cent, saturated. A further increase of the oxygen pressure to 40 mm. Fig. 119. — Curve of Dissociation of Oxyhsemoglobin at 35° C. (after Hilfner's Re- sults). Along the horizontal axis are plotted the partial pressures (numbers' below the curve) of oxygen in air, to which a solution of hsemoglobin was exposed. The corresponding percentages of oxygen are given above the curve. Along the vertical axis is plotted the percentage saturation of the heemoglobin with oxygen. Thus, on exposure to an atmosphere in which oxygen existed to the extent of I per cent., corresponding to a partial pressure of y6 mm. of mercury, the haemo- globin took up about 75 per cent, of the amount of oxygen required to saturate it. When the oxygen was present in the atmosphere to the amount of about 10 per cent., corresponding to a partial pressure of 76 ram. of mercury, the quantity taken up by the haemoglobin was about 96 per cent. 'of that required for satu- ration. increased the quantity of the gas taken up by only 2 c.c. (to 90 per cent, saturation). The next increment of 10 mm. in the oxygen pressure only produced an additional absorption 01 1 c.c, and above this increasing the pressure had very little effect. We may suppose that at the ordinary temperature and pressure some oxygen is continually escaping from the bonds by vsrhich it is tied to the haemoglobin; but, on the whole, an equal number of free mole- cules of oxygen, coming within the range of the haemoglobin molecules, are entangled by them, and thus equilibrium is kept up. If now the 252 RESPIRATION atmospheric pressure, and therefore the partial pressure of oxygen, is "^educed, the tendency of the oxygen to break off from the haemoglobin will be unchanged, and as many molecules on the whole will escape as before ; but even after a considerable reduction of pressure the haemo- globm, such is its avidity for oxygen, will still be able to seize as much oxygen as it loses. The more, however, the partial pressure of the oxygen is diminished^-that is to say, the fewer oxygen molecules there are in a given space above the haemoglobin— the smaller will be the chance of the loss being made up by accidental captures. At a certain pressure the escapes will become conspicuously more numerous than the captures ; and the gas-pump will give evidence of this. The higher the temperature of the haemoglobin is, the greater will be the average velocity of the molecules, and the greater the chance of escape of mole- cules of oxygen. It is easily proved that the substance in the corpuscles which unites with oxygen is the blood-pigment. Although a solution of 20 M. 18 ac 14 CC 12 CC lOCC see. SCJC 4CC. 2C.C. 01 cc. 10 20 30 40 so eo 70 aO so lOO no 120 IJO 140 ISO Fig. 120. — Curves of Dissociation of Oxygen for Horse's Blood (B) and Dog's Hsemo- globin solution (H) at 38° C. (Bohr). The figures along the base-line are the partial pressures of oxygen to which the blood and haemoglobin solution were exposed. Those along the vertical axis on the left are the percentage saturations with oxygen. The figures along the vertical at the right give the actual number of CO. of oxygen chemically combined by 100 c.c. of the blood for each pressure of oxygen. The interrupted line P indicates the amount of oxygen dissolved in the plasma of the blood at each partial pressure on the assumption that the plasma is two-thirds of the volume of the blood. Thus, at 150 mm. oxygen pressure the plasma of 100 c.c. of blood took up 03 c.c. oxygen. oxyhaemoglobin crystals behaves towards oxygen somewhat differ- ently from blood containing the same proportion of the native pig- ment, the maximum amount of oxygen taken up is the same for each. The differences in the results of the various investigators who have worked out the curves of dissociation for haemoglobin and for blood (Figs. 119, 120, 121) are paitly explained by the fact that, as is the case with similar dissociable compounds, the dissociation tension varies with the temperature and the concentration of the pigment, Another factor which was overlooked in the earlier 100 90 SO 70 B, -" ==" ■^^ »- "■" ^■~ ■■M / > --' / / / f / VI / 40 SO 20 10 f / 11 /.. _. _ __ _ __ i. __ _, THE GASES OF THE BLOOD 253 observations is the influence of the carbon dioxide of the blood on the binding power of hcemoglobin for oxygen. It has been shown that the presence of carbon dioxide increases the dissociation ten- sion of oxyhsemoglobin, or, what is a different way of expressing the same thing, diminishes the quantity of oxygen taken up with a given oxygen partial pressure (Bohr, Barcroft, Fig. 121). The influence of salts is also considerable. The form of curve obtained by Hiifner (an equilateral hyperbola. Fig. 119) is only found when the haemoglobin solution is thoroughly freed from salts. But even when allowance is made for all these factors, the discrepancies seem still sufficiently definite to warrant the conclusion, which is also sup- ported by other facts, that the substance in blood with which the oxygen is loosely united, although, of course, intimately irelated to the haemoglobin which can be artificially prepared from it, is yet not absolutely identical with the crystalline product. Some writers for this reason prefer to give the special name haemo- chrome to the native blood-pig- ment as it exists within the unaltered corpuscles, reseirving the term haemoglobin for the more or less artificial though, perhaps, only slightly altered product. The Distribution and Condition of the Carbon Dioxide in the Blood. — ^The question is much more complicated than for the oxygen, which is practically con- fined to one of the morphologi- cal elements of the blood (the erythroc3rtes), and exists in the form of a single compound. Carbon dioxide is distributed over the entire blood in important amounts, and is present is several forms. The serum yields a larger per- centage of carbon dioxide than the clot, but this percentage is not great enough to allow us to assume that the whole of the carbon dioxide is contained in the plasma. Somewhat more than a third of it belongs to the corpuscles. As regards the condition of the carbon dioxide, it is known that some of it is simply dissolved in the plasma and corpuscles; but although this fraction, on account of the relatively high coefficient of absorption of the gas (p. 246) , is much greater than the corresponding oxygen fraction, it is insignificant in comparison with the quantity chemically combined. Carbon dioxide is united in dissociable 90 ^ ■^1^ — 1 ^^ 80 V / / y 10 / 'f^v / / 60 1 1 / I 1 / / J 50 ■ // / 10 II 1 / 1 / 1 / 30 ■ //// 1 ■ 20 10 III / w • 10 20 30 40 50 60 70 80 90 100 Fig. 121. — Dissociation Curves of Blood, with Different Tensions of CO2 (o, 3, 20, 40, and 90 mm.). Ordinates = percentage saturatibn. Abscissae = oxygen pressure. (After Barcroft.) 254 RESPIRATION combinations with a number of the constituents of the blood, both inorganic and organic, and our knowledge of these combinations, especially of the compounds formed with organic substances, is far from complete. The inquiry is complicated by the circumstance that the proportion of the total combined carbon dioxide united with a given constituent or bound by plasma and corpuscles respec- tively is not constant, but varies with the varying tension of the gas, while the total amount of carbon dioxide is itself dependent upon the varjdng ' titratable alkalinity ' (p. 25). There is no doubt that some of the carbon dioxide in blood is combined with alkali, but the amount of alkali available is not nearly sufficient to unite with all the carbon dioxide even in the form of bicarbonate. Some of the dissociable carbon dioxide must therefore be combined with organic substances. The relations, even of that portion which exists as bicarbonate, are peculiar. This is sufficiently indicated by the fact that from defibrinated blood the whole of the carbon dioxide can in time be pumped out without the addition of an acid to displace it from the bases with which it is united. On the other hand, from a bicarbonate solution whose concentration corresponds to that of the blood, not much more than half of the loosely bound carbon dioxide (that is, the carbon dioxide which comes off accord- ing to the equation zHNaCOg = NajCOg + COg + H2O) can be ob- tained even when the evacuation is kept up for days. This is only about one-fourth of the total carbon dioxide in the bicarbonate; yet, when sodium bicarbonate is added to blood, even in consider- able amount, all the carbon dioxide in it can be obtained by the pump. From serum a great deal, but not the whole, of the carbon dioxide can be likewise pumped out, and the liberation of the gas does not stop, as in the case of the bicarbonate solution, when all the bicarbonate has been changed into carbonate. The residue (from 10 to 18 per cent, of the whole) is set free on the addition of an acid — e.g., phosphoric acid. The most satisfactory explanation is that in the serum there exist substances which can act as weak acids in gradually driving out the carbon dioxide, when its escape is rendered easier by the vacuum. The quantity of these, however, is not so large but that a portion of the carbon dioxide remains in the serum. The proteins of the serum, such as serum-globulin, behave in certain respects like weak acids, and may contribute to the driving out of the carbon dioxide. When defibrinated blood is pumped out, the whole of the carbon dioxide can be removed, apparently because substances of acid nature pass from the corpuscles into the serum and help to break up the carbonates. The haemoglobin in the corpuscles acts as a weak acid, and as some corpuscles are hsemolyzed during the evacuation, the hsemoglobin may exert this action in the serum as well as in the interior of the corpuscles. The quantity of carbon dioxide combined with alkali (as bicar- bonate) has not been exactly determined. Bohr estimated it by shaking blood with atmospheric air, which was supposed to leave THE GASES OF THE BLOOD 255 the bicarbonate intact, while removing nearly all the rest of the carbon dioxide, the compounds of the gas with organic constituents of the blood being more easily dissociated than the bicarbonate. The best known of these compounds is that which carbon dioxide seems to form with haemoglobin. A solution of haemoglobin absorbs more of the gas than water, and the quantity taken up is not pro- portional to the pressure. There is also evidence of the existence of dissociable combinations between carbon dioxide and the proteins of the plasma, by which considerable amounts of the gas can be bound at such carbon dioxide tensions as normally exist in blood. In the red corpuscles a portion of the carbon dioxide is in com- bination with alkalies. We know that the corpuscles contain more alkali than the serum, and the titratable alkalinity of ' laked ' blood (pp. 25, 28) is greater than that of unlaked blood, unless a long time is allowed in the case of the latter for the alkalies of the corpuscles to reach the acid used in titration. The haemoglobin of the corpuscles holds a portion of the carbon dioxide in weak com- bination. Although the student is warned not to give too much weight to the actual numbers, the present position of our knowledge in regard to the distribution and condition of the carbon dioxide of the blood may be summed up by quoting the calculation of Loewy, that in 100 c.c. of arterial blood containing (with a carbon dioxide tension of 30 mm. of mercury) 40 c.c. of carbon dioxide, there are — In Plasma. In Corpuscles. In Blood. Physically absorbed Combined as bicarbonate In organic combinations 1-2 C.C. I2-0 ,, II-8 ,, 07 C.C. 6-8 „ 7-5 .. i-g c.c. i8-8 ,, 19-3 ,. When blood is saturated with carbon dioxide and then separated into serum and clot, the serum is found to yield more gas than the clot'; but if the serum and clot are separately saturated, the latter takes up more carbon dioxide than the former. From this it is argued that a substance combined with carbon dioxide must in blood saturated with the gas pass out of the corpuscles into the serum. The corpuscles at the same time gain water and becom? larger. The molecular concentration (p. 420) of the serum of defibrinated blood, as measured by the lowering of the freezing-point, increases when it is saturated with ca,rbon dioxide. On the other hand, when blood is saturated with oxygen, the corpuscles lose water and shrink in volum2, while the molecular concentration of ths serum is diminished. Hamburger has extended these observations to ths circulating blood, and has shown that the plasma of venous blood has a higher percentage of alkali, protein, sugar, and fat than the plasma, of arterial blood, and that the corpuscles have a greater volume, though not a greater diameter. He therefore supposes that in the pulmonary capillaries, under the influence of oxygen, water passes mto the plasma from the corpuscles. In the systemic capillaries the blood becomes loaded with carbon dioxide, and therefore the corpuscles 456 RESPIRATION take up water from the plasma, which accordingly has a more concen- trated supply of food-substances to offer to the tissues than the plasma of arterial blood itself. Some writers see in this interchange an auto- matic arrangement by which oxidation is favoured. Whatever may be thought of this view — and objections to it are hot wanting — the current theory, that the corpuscles are simply passive carriers of oxygen, and exercise no further influence on the plasma, breaks down in face of the facts. We must admit that an active and many-sided commerce exists between them and the liquid in which they float. The nitrogen of the blood is simply absorbed.* The Tension of the Blood-Gases. — If the gases of the blood existed in simple solution, their tension or partial pressure could be deduced from the amount dissolved and the coefficient of absorption. We have seen that they are mainly combined, and it is characteristic of dissociable compounds of this kind that the relation between the partial pressure of the gas in contact with the liquid and the quantity of gas taken up is much more complicated than in the case of pure physical absorption. It is therefore necessary to determine the tension directly. This can be done by means of arrangements called aerotonometers. There are various forms of aerotonometer, but the object of aU is to bring the blood into contact with an atmosphere or gaseous mixture into which the gases of the blood can difiuse and from which gases can enter the blood. When the composition of the mixture has ceased to change, the gases in it are imder the same partial pressure as the corre- sponding gases in the blood, and all that is necessary in order to arrive at the tension of the blood-gases is to determine the final composition of the gaseous mixture by analysis. The speed with which equilibrium is attained depends essentially upon the magnitude of the surface of contact between the blood and the gas mixture in proportion to the volume of the gas space. In the earlier observations with the aerotono- meter it was foxmd to be very difficult to get complete equilibrium, £ind therefore the gas space was filled at the beginning with a mixture whose gases had partial pressures as nearly equal as possible to those expected in the blood. In one form of the apparatus the blood is made to pass directly from the vessel to glass tubes, which it traverses at the same time, the stream being divided between them; it then passes out again. The tubes are warmed by means of a water-jacket to the body-tem- perature. Some of them are filled with gaseous mixtures having a greater, and the others with mixtures having a smaller, partial pressure, say of carbon dioxide, than is expected to be foimd in the blood. As the latter runs in a thin sheet over the waUs of the tubes, it loses carbon dioxide to some of them and takes up carbon dioxide from others. From the alteration in the proportion of the carbon dioxide in the tubes, the partial pressure of that gas in the blood is calculated — ^that is, the partial pressure which would be necessary in the tubes in order that the blood might pass through thein without losing or gaining carbon dioxide (p. 247). Bohr's aerotonometer, constructed and worked much in the same way as a stromuhr (p. 121), permits the blood after passing through the gas space to return to the circulation. A stream of blood can thus be * But according to Buckmaster and Gardner, the volume of nitrogen in blood does not follow the ordinary physical laws of absorption with varying nitrogen pressures in the alveolar air. THE GASES OF THE BLOOD 257 kept in contact with the gas for a quarter of an hour or longer, so as to insure equilibrium. Finally, in Krogh's microtonomelev the gas space is reduced to the smallest possible dimensions, being composed merely of an air-bubble 2 mm. in diameter, which is exposed to the contact of a stream of blood from an artery or vein. Equilibrium is estabUshed so quickly that it is indifferent whether a bubble of air or of pure nitrogen is employed. The bubble is analyzed at the end of the obser-. vation, and its composibion gives the tension of the blood gases. Suppose that the gaseous mixture which is in equilibrium wi.th the blood contains 10 per cent, of oxygen and 5 per cent, of carbon dioxide, Fig. 133. — Krogh's Microto- nometer. The apparatus, which is filled with salt solu- tion, consists of a graduated capillary tube, 3, the lower part of which, with its ex- panded lower end, is shown on a much enlarged scale in A. The rest of the capillary tube, surrounded by a water- jacket to control the tempera- ture, is shown on a smaller scale in B. 2 is a gas-bubble, against which blood flowing from the very narrow mouth of the tube i plays, i is con- nected by a rubber tube with a cannula in a bloodvessel. The blood forces its way up above the gas-bubble, which is pressed a little down by the current, and kept oscillating rapidly. The blood flows off through the tube 5, and is collected drop by drop and measured. By means of the screw 4, shown in B, which moves in mercury, the gas- bubble can be drawn into the capillary for measurement. The upper end of the capil- lary tube also expands into a funnel-shaped cavity, which is closed by a stopper, and is only used for cleaning the apparatus. the tension of oxygen in the blood would be one-tenth of an atmosphere [\.e., oi 760 mm. of mercury), or 76 mm., and the tension of the carbon dioxide in the blood one-twentieth of an atmosphere, or 38 mm. of mercury. Another method by which the tension of the gases in the venous blood passing from the right heart through the lungs has been estimated depends upon the use of the pulmonary oatheier. This consists of two tubes, one within the other. The iniier tube, which is a fine elastic catheter, projects free from the' other for a little distance at its lower end. The outer tube terminates in a thin india-rubber balloon, through which the inner tube passes without communicating with the balloon. 17 258 RESPIRATION The balloon can be inflated so as to block the bronchus into which it is passed, and cut off the corresponding portion of the lung from com- munication with the outer air. A sample of the air below the block can be drawn off through the inner tube, which opens free in the bronchus. This method has been applied both to animals and to man. In observations on man the cathete'r was passed into the right bronchus so as to occlude at will any one of the!' lobes of the right lung. On the assumption that the gaseous exchange in the lungs depends essentially on the physical process of diffusion, the occluded alveoli will correspond to the gas space of an aerotonometer. . When the occlusion has lasted long enough for the gases in the alveoli and the blood gases to come completely into equilibrium — say half an hour — all that is necessary is to draw off the air, and from its composition to deduce the tensions in the blood. Since the respiratory function of the occluded lobe is in abeyance, the blood circulating in it is all unaltered venous blood, as it comes from the right ventricle, so that the gas tensions found can be considered those of the mixed venous blood. For estimating the oxygen tension in the arterial blood of rnan the following method 'was" introduced by Haldane and Smith: The subject of the experiment breathes air containing a definitely known very small percentage of carbon monoxide until the haemoglobin has united with as much of that gas as it will take up for the given concentration of it in the air. Then the percentage amount to which the haemoglobin has become saturated with carbon monoxide is determined in a sample of blood taken, say, from the finger. Now, the final saturation with carbon monoxide of a haemoglobin solution brought into contact with a gaseous mixture containing carbon monoxide and oxygen, depends on the relative tensions of the two gaseS i^ the liquid. But the tension of carbon monoxide in the blood leaving the lungs will (after absorption has ceased) be the same as that in the inspired air. Knowing this tension and the degree of saturation of the haemoglobin with carbon monoxide, the oxygen tension in the blood leaving the lungs — i.e., in the arterial blood — is known. Before proceeding to the consideration of the results obtained by these diverse methods, it may be well to point out that when a gas is stated to be under such and such a tension in the blood, no direct information is given as to the quantity of gas present. For instance, the oxygen tension in blood exposed to atmospheric air will be the same foi;' the erjrthrocytes as for the serum — namely, about i6o mm. of mercury; but lOO c.c. of serum will scarcely contain | ex. of oxygen, while lOO c.c. of corpuscles will have absorbed about 60 c.c. of the gas. When we now turn to the actual blood-gas tensions obtained by different observers and by different methods, these, as displayed in such a table as appears on p. 259, seem to present, at first sight, nothing but a welter of widely diverging and contradictory figures. As regards the venous blood, we have aheady learnt that very considerable variations in the content of oxygen and of carbon dioxide are associated with the varying functional activity of the tissues from which the blood comes. This factor, of course, is also not without influence upon the gas tensions of the venous blood. The carbon dio^dde tension of arterial blood is affected by variations THE GASES OF THE BLOOD 259 in the amount of the pulmonary ventilation, which affect the partial pressure of the carbon dioxide in the alveolar air and thus alter the steepness of the slope of pressure between the two sides of the pul- monary membrane. Arterial Blood : Venous Blood : Tension in Mm. Hg. Ten.sion in Mm. Hg. Observer and Method. Oxygen. Carbon Dioxide. Oxygen. Carbon Dioxide. Strassburg 21-43- 16-29 10-35 38-49 1 (29-6)* (20) (20-6) (41) a Herter - 39-79 17-31 — a- Bohr - 101-144 20-32 — — $ Bohr (later series) — 9-27 — 26-43 S Fredericq 91-105 17-19 < Falloise — 17-37 32-54 ^u (42-5) g S rWolffberg — (26) 18-37 u _ Nussbaum a -H Loewy and Schrotter "j q ^ Haldane and Smith j '"^'^ — — '• (27) 24-33 [200 + — (37-7) 34-59 (45) It is chiefly the enormous differences in the recorded oxygen tensions of the arterial blood which excite surprise. To some ex- tent, indeed, these also may depend upon differences in the partial pressure of the oxygen in the alveoli, and it has been shown experi- mentally (by the aerotonometer) that with increasing oxygen tension of the inspired air the* oxygen tension of the arteriaL blood increases (Fredericq). Still, the differences which can possibly have existed in the partial pressure of the oxygen in the alveoli in the various seiries of observations can only to a small extent account for the differences in the results. The main reason for the great range of values Ues unquestionably in the different experimental procedures by which they were obtained. There is no doubt that in the earlier observations with the aerotonometer (Strassburg) the oxygen of the blood could not have come into equihbrium with the mixture in the gas space, in which the oxygen pressure was at the beginning much lower than that in the blood; the results are therefore too low. The same is true for the oxygen tension of the venous blood, but as this is in any case considerably smaller than that of the arterial blood, the proportional error is not so great. The later experiments (of Herter), given in the second line of the table, yield much higher values, owing to improved technique, but the findings are still to be regarded as minimal and not average results. At the other end of * The numbers in brackets are averages. 26o RESPIRATION the scale stand the results of Haldane and Smith, who found in n an oxygen tension in the arterial blood of over 200 mm. of merci equal to more than 26 per cent, of an atmosphere. This exce( the partial pressure of oxygen in the external air, and is about tw as great as that of the air of the alveoli. In the bird they fou an oxygen tension of between 300 and 400 mm., equal to 45 ] cent, of an atmosphere. These results, however, differ so vas from those of all other observers that for the present it is best leave them out of account. The method by which they were ( tained, although perhaps correct enough in principle, seems to exposed to several somrces of error in practice, and has not escaj criticism as to its details (Osborne, etc.). We are left, then, wit] series of values for the oxygen tension of arterial blood which always below the partial pressure of the gas in atmospheric air, a usually do not much exceed or fall much below 100 mm. of mercu corresponding to about 13 per cent, of oxygen. The average tension of carbon dioxide in the venous blood passi through the lungs, as determined by the pulmonary catheter, man was 45 mm., corresponding to 6 per cent, of an atmosphe "this agrees fairly well with most of the observations made with t aferotonometer. The lower results (of Wolffberg and of Nussbau: with the lung catheter are probably due to the fact that in the dc used, which breathed through tracheal cannulse, the catheter caus greater interference with the respiration than in man and indue dyspnoea, with the consequent washing out of carbon dioxide (p. 24 The chief interest of this discussion of the blood-gas tensions. 1 in their fundamental importance in the problem of the gasec exchange in the lungs, on the one hand, and between the blood a tissues on the other. We are now in a position to consider the form Calculations made on the basis of such anatomical and physi( data as are available (total surface of the lungs, thickness of t membrane which separates the air of the alveoli and the blood the capillaries, velocity of diffusion of oxygen and carbon dioxid indicate that even with differences of oxygen tension between t blood and the alveolar air, which would lie within the limits error of our present methods of measurement, enough oxyg could diffuse across the pulmonary membrane to cover the whi normal intake, The speed of diffusion of carbon dioxide acre such a membrane being much greater than that of oxygen, si smaller differences of tension would suffice to permit the wh( normal output of that gas to be eliminated by diffusion. Accoi ingly, the problem in its present phase reduces itself to this, wheth as a matter of fact, the slope of the oxygen pressure is always frc the alveolar air to the blood passing through the lungs, and t slope of the carbon dioxide pressure always from the blood to t alveolar air ? THE GASES OF THE BLOOD 261 In order to answer this question it is necessary to know the partial pressures of oxygen and carbon dioxide in the alveoU. The per- centage of oxygen or carbon dioxide in expired air cannot tell us the pressure of the gas in the alveoli, for the air in the upper part of the respiratory tract is necessarily expelled along with the alveolar air, and alters the proportions. But the mean of the oxygen or carbon dioxide percentages in samples taken from the last por- tions of the air of two deep expirations, one follpwing an ordinary inspiration and the other following an ordinary expiration, is the mean percentage in the alveoli. The average percentage of oxygen may be taken as ■14-5, corresponding to 109 mm. of mercury. The percentage of carbon dioxide in the alveolar air while, as already remarked (p. 239), very constant in a given individual, varies in different men from 4-6 to 6'2 (mean 5-5) per cent, of the dry alveolar air. In women and in children of both sexes it is less than in men. From this we conclude that in men the partial pressure of carbon dioxide in the alveoli may be at least one-eighteenth of an atmo- sphere; or 42 mm. of mercury (Fitzgerald and Haldane). If we take the average oxygen tension in the alveolar air as 100 mm., it is clear that the slope of pressure is very decidedly from the alveoli to the venous blood coming to the lungs, the average oxygen tension in the observations with the pulmonary catheter being only 377 mm. It must be clearly pointed out, however, that in the lungs the air is in relation with arterialized as well as with venous blood ; and if the partial pressure of oxygen in the alveoli, while exceed- ing that in the venous blood, is inferior to that in the arterial blood, the only conclusion which could be drawn would be that some of the oxygen might pass into the blood by diffusion, but that the whole of it could not do so. For as soon as the oxygen tension in the blood, as it became better and better oxygenated in its circuit through the lungs, reached the level of the alveolar partial pressure, diffusion would, of course, come to an end. According to the majority gf observers, however, the diffusion hypothesis surmounts this test also, since the oxygen tension in the alveoli is invariably at least as great as that in the arterial blood. Bohr, however, found that in the majority of his observations on dogs the oxygen tension was distinctly greater in the arterial blood than in the pul- monary air. Even if we accept Bohr's results, and they have been severely criticized, the conclusion that the alveolar oxygen tension far exceeds that in the blood of the right heart is in no way affected, and this establishes the possibility of a large absorption of oxygen by the venous blood in the lungs through diffusion alone. It must be carefully remembered that even if it be admitted that diffusion can account for the absorption of the whole of the oxygen, this is not of itself a proof that it is by diffusion that the thing is actually done; it is only a reason for refusing to call in the aid of a more 262 RESPIRATION recondite hypothesis, until the necessity for doing so is clearly demonstrated. It is unfortunate that complete unanimity has not been attained on this question in regard to the oxygen absorption, for the avail- able differences of partial pressure between air and blood are much greater than in the case of the carbon dioxide, and were it ^definitely shown that the process is a physical one for oxygen, there would be little chance that it could be anything else for carbon dioxide. A glance at the table on p. 259 shows that ^^hile the carbon dioxide tension of venous blood may sometimes, perhaps generally, exceed that of the alveolar air, the difference is quite small. The average for the observations on man with the pulmonary catheter was 45 mm., which compares with an average alveolar tension of 42 mm. If this excess of 3 mm. in favour of the blood be taken to show, as it certainly could be, if the difference were a constant one, that carbon dioxide can diffuse from the venous blood, as it enters the pulmonary capillaries, into the air of the alveoli, the marked de- ficiency in the carbon dioxide tension of arterial blood ought to be interpreted as meaning that diffusion is not the only way in which the blood gets rid of its carbon dioxide in making the round of the pulmonary circulation. In Bohr's experiments, in some of which the animals were made to breathe air containing carbon dioxide in various proportions, the tension of that gas in the alveolar air was often greater than in the arterial and even than in the venous blood, and yet carbon dioxide was given off by the blood to the lungs. It does not seem improbable in itself that the physical process of diffusion, which is generally considered to play a great part, is aided by some other process, which may provisionally be termed secre- tion, and which can move the gases even against the slope of pres- sure. It is possible, too, that when the conditions are especially unfavourable to diffusion — ^when, for instance, the partial pressure of carbon dioxide is artificially increased in the alveoli — ^the cells which line them are stimulated to increased activity, just as Bohr has supposed that under the influence of the carbon monoxide used in the observations of Haldane and Smith the absorption of oxygen was greatly stimulated. Additional evidence in favour of the view that there is, besides diffusion, an element of selective secretion in the exchange of gases through the pulmonary membrane has been found by some writers in the results of a study of the gases of the swim-bladder in fishes; and to the extent that this study has demonstrated the existence of animal^cells which actually secrete gases, it removes a presump- tion against, if it does not establish a presumption in favour of, the secretion theory of external respiration. These gases consist of oxygen, nitrogen, and usually a small quantity of carbon dioxide, THE GASES OF THE BLOOD 263 but in very different proportions from those in which they exist in the air or the water. Thus, as much as 87 per cent, of oxygen has been found in the bladder of fishes taken at a considerable depth, but a smaller amount in those captured near the surface. When the gas is withdrawn by puncturing the bladder with a trocar, the organ rapidly refills, and the percentage of oxygen increases. Further, this process of gaseous secretion is under the influence of nerves, for gas ceases to accumulate in the organ when the branches of the vagi that supply it are cut. In the tortoise stimulation of the peripheral end of the vagus causes a fall of gaseous exchange in the corresponding lung, with an accompanying rise in the other lung. That this is not the consequence of an alteration in the pul- monary circulation is indicated by the fact that the change is greater in the intake of oxygen than in the output of carbon dioxide. In the mammal, however, no such effect has been clearly demon- strated, and the decisive proof that the lungs are gas-secreting glands which would be afforded by the discovery of secretory nerves is still wanting. We have now completed the description of the phenomena of external respiration, with the discussion of its central fact, the exchange of gases between the blood and the air at the surface of the lungs. It remains to trace the fate of the absorbed oxygen, and to determine where and how the carbon dioxide arises. Section V. — ^Internal or Tissue Respiration. Seats of Oxidation. — The suggestion which lies nearest at hand, and which, as a matter of fact, was first put forward, is that the oxygen does not leave the blood at all, but that it meets with oxidizable substances in it, and unites with their carbon to form carbon dioxide. While there is a certain amount of truth in this view, oxygen, as already mentioned, being to some extent taken up by freshly shed blood, and also by blood under other conditions, to oxidize bodies, other than haemoglobin, either naturally contained in it or artificially added, there is no doubt that the cells of the body are the busiest seats of oxidation. This is shown by the presence of carbon dioxide in large amount in lymph and other liquids which are, or have been, in intimate relation with tissue elements; by its presence, also in considerable amount, in the tissues themselves — in muscle, for instance ; by its continued and scarcely lessened pro- duction not only in a frog whose blood has been replaced by physio- logical salt solution, and which continues to live in an atmosphere of pure oxygen, but in excised muscles ; and by the remarkable con- nection between the amount of this production and the functional state of those tissues. In insects the finest twigs of the tracheae, through which oxygen passes to the tissues, actually end in the cells; 264 RESPIRA TION and in luminous insects, like the glow-worm, it has been noticed that the phosphorescence, which is certainly dependent on oxida- tion, begins and is most brilliant in those parts of the cells of the light-producing organ that surround the ends of the tracheae. Micro- scopic evidence has been obtained that the nucleus plays a predomi- nant part in intracellular oxidation — e.g., in the indophenpl (p. 268) and similar reactions the coloured oxidation products are deposited chiefly in and around the nuclei of such cells as liver and kidney cells and frog's red corpuscles (Lillie). The fact observed by Bohr, and already alluded to (p. 252), that an increase in the carbon dioxide tension of blood diminishes its combining power for oxygen, and therefore favours the giving up of oxygen to the lymph and tissues, may have an important influence on internal irespiration. The effect is much more marked where the oxygen tension is low than where it is high, so that in the lungs the taking up of oxygen is scarcely interfered with even by a high carbon dioxide tension. Lymph, bile, urine, and the serous fluids contain very little oxygen, but so much carbon dioxide that the pressure of that gas in all of them is greater than in arterial blood, while in lymph alone (taken from the large thoracic duct) has it been found less than that of venous blood. And it is probable that lymph gathered nearer the primary seats of its production (the spaces of areolar tissue) would show a higher proportion of carbon dioxide, Strassburg found that with a pressure of carbon dioxide in the arterial blood of 21 mm. of mercury, the pressure in bile was 50 mm., in peritoneal fluid 58 mm., in urine 68 mm., in the surface of the empty intestine 58 mm. Saliva, pancreatic juice, and milk, also contain much carbon dioxide, and only a little, if any, oxygen. From muscle no free oxygen at all can be pumped out, but as much as 15 volumes per 100 of carbon dioxide, some of which is free — that is. is given up to the vacuum alone^ — ^while some of it is fixed, and only -^omes off after the addition of an acid. ' Muscle may be safely taken as a type of the other tissues in regard to the problems of internal respiration. It is instructive, therefore, to observe that the great scarcity of oxygen in the parenchymatous liquidswhich bathe the tissues, here in the tissues themselves, deepens into actual famine. The inference is plain. The active tissues are greedy of oxygen; as soon as it enters the muscle it is seized and ' fixed ' in some way or other. The traces of oxygen in the lymph cannot therefore be journe5nng away from the tissue elements; they must have come from another source, and this can only be the blood. Could we gather tissue lymph for analysis directly from the thin sheets that lie between the blood capillaries and the tissues, we might find more oxygen present as well as more carbon dioxide. But if we did find more oxygen, it would still be oxygen in transit from the capillaries towards places where the partial pressure of INTERNAL OR TISSUE RESPIRATION 265 oxygen is less. In the lymph, the pressure is kept low by the avidity of the tissues with which it is in contact, and possibly by the exis- tence in it of oxidizable substances which have come from the tissues. In the tissues there is no partial pressure at all, because the oxygen that reaches them is at once stowed away in some compound in' which it has lost the properties of free oxygen. Assuming, then, that at least a great part of the oxidation and consequent production of carbon dioxide goes on in the. tissues, let us follow the steps of the process, as far as we can, in the light of our knowledge of the respiration of muscle. Respiration of Muscle. — It is a remarkable fact that an excised frog's muscle is capable of going on yielding carbon dioxide for a long time, in the entire absence of oxygen, in a chamber, for instance, filled with nitrogen or other indifferent gas. Not only so, but it Fig. 123. — Fatigue of a Pair of Sartorius Muscles (Fletcher). A, in an atmosphere of oxygen; B, in an atmosphere of nitrogen. A is partially restored by a rest of five minutes, can be made to contract many times in this oxygen-free atmosphere, although it loses its power of contraction sooner than in oxygen, and does not show the same capacity for recuperation during an interval of rest. In mammals the muscles can also be made to contract repeatedly when the dissociable oxygen has, as far as pos- sible, been got rid of from the blood by asphyxiating the animal, and to give off a correspondingly large quantity of carbon dioxide, although they lose their coritractibility much more rapidly than the muscles of the frog. This has usually been interpreted as meaning that the carbon dioxide does not arise, so to speak, on the spot, from the immediate union of carbon and oxygen, but that a stock of it is taken up by the muscle, and stored in some compound or compounds, which are broken down during contraction, and more slowly during rest, carbon dioxide in both cases being one of the end-products. In a normal muscle with intact circulation. 266 RESPIRATION while carbon dioxide is given off, certain of the other decomposition products are supposed, in conjunction with oxygen and some sub- stance rich in carbon, like sugar, to be regenerated into the material which breaks down in contraction. When oxygen is not available, as in an atmosphere of nitrogen, carbon dioxide is still given off, but the other decomposition products are supposed not to be regenerated to contractile substance, but to accumulate in the muscle, producing the phenomena of fatigue, and eventually of rigor. When muscle goes into rigor (p. 751) — and this is most strik- ingly seen when the rigor is caused by raising the temperature of frog's muscle to about 40° or 41° C. — ^there is a sudden increase in the quantity of carbon dioxide given off. Moreover, in an isolated muscle the total quantity of carbon dioxide obtainable during rigor is markedly less if the muscle has been previously tetanized. From this it has been argued that the hypothetical substance (" inogen "), the decomposition of which yields carbon dioxide in contraction, is also the substance which decomposes so rapidly in rigor ; that a given amount of it exists in the muscle at the time it is removed from the influence of the blood; and that this can all explode either in con- traction or in rigor, or partly in the one and partly in the other. Recent work, however, has tended to show that this famous inogen theory has very little foundation. According to Fletcher, there is no increase in the amount of carbon dioxide given off during tetanus by an excised frog's muscle unless the stimulation is so severe and prolonged as to hasten the onset of rigor. He therefore supposes that in the contraction the decomposition does not proceed quite to the formation of carbon dioxide, which in the intact body is after- wards liberated from some more complex carbon-containing waste- product. He considers that the carbon dioxide yielded by excised muscles is really preformed carbon dioxide, already existing in a state of loose combination, from which it is displaced by the lactic acid formed after excision. There is no reason to suppose that any independent new formation of carbon dioxide occurs within the isolated muscle in the absence of a good supply of oxygen. The respiration of muscles in situ can be studied by collecting samples of the blood coming to and leaving them and analyzing the gases. The mere difference of colour between the venous and arterial blood of a muscle, or other active organ, is sufficient to show that oxygen is taken up and carbon dioxide given out by it to the blood. This is the case in muscles at rest, and even in muscles with artificial circulation after they have become inexcitable. In active muscles more oxygen is used up and more carbon dioxide produced than in the resting state. Chauveau and Kaufmann, in their experiments on the levator labii superioris muscle of the horse in feeding, foUnd that the consumption of oxygen and the production of carbon dioxide might be many times as great in activity as in rest. INTERNAL OR TISSUE RESPIRATION 267 Thus in one experiment the amount of oxygen taken in, expressed in c.c. per gramme of muscle per minute, was o-oo8 during rest, and 0-14 during work; the corresponding quantities for the carbon dioxide given off were o-oo6 and o-i8. The respiratory quotient rose to 1-3 in two experiments, and even to 17 in a third, showing that the increase in the production of carbon dioxide was relatively greater than the increase in the intake of oxygen. These experi- ments were performed under conditions so normal that the animal continued to eat its hay with s^emyig unconcern throughout the obser- vations, although these involved the exposure of the main blood- vessels of the muscle, and the collection of samples of blood from them. For skeletal muscle at rest, Barcroft gives 0-004 cc. per gramme per minute as the oxygen consumption; during maximal activity twenty times as much (o-o8 gramme). In the heart of a small dog through which blood was pumped by a largei; dog the oxygen intake when the heart was beating feebly was, on the average, about o-oi c.c. per gramme of heart-muscle per minute. When the heart was caused to beat very strongly under the influence of adrenaUn, the oxygen intake rose in one case to o-o8, and in two others to 0-04. In the resting pancreas the oxygen intake has been found to be 0-03 to 0-05 c.c. per gramme per minute; in the active pancreas, o-i c.c. The corresponding number for the submaxillary gland at rest is 0-03, and in activity 0-09; for the kidney, 0-03 at rest or during scanty secretion, and 0-07 or even o-og during active secretion. Nature of the Oxidative Process. — When we have recognized the cells as the seat of oxidation, the question immediately presents itself. How do they accomplish the feat of burning such masses of food substances as can only be rapidly oxidized in the laboratory at the temperature of the body by the most energetic chemical reagents ? The researches of late years have furnished a key to the solution of this long-standing puzzle by demonstrating the existence in the tissues of oxidizing ferments or oxydases. Of these, one of the most widely distributed is a ferment which splits off oxygen from hydrogen peroxide. Since any oxidation produced is only secondary to this decomposition, ferments which decompose hydrogen peroxide are often spoken of as catalases, to distinguish them from the oxydases proper. A catalase is found in practically all the tissues of the body, as well as in vegetable cells, and we have already mentioned instances of its action in connection with the oxidation of the guaiaconic acid in tincture of guaiacum in the presence of the peroxide (p. 76). As regards the activity of this ferment, blood comes first; then follow spleen, liver, pancreas, thymus, brain, muscle, and ovary. It is present in the blood-free organs as well as in the W^od. Some tissues, both animal and vegetable, contain a ferment, an oxydase, which causes the oxida- tion of guaiaconic acid in the presence of atmospheric oxygen, and 268 RESPIRATION these do not need peroxide of hydrogen in order to render guaiacum blue. An allied ferment which also induces the blue colour in tincture of guaiacum is the so-called laccase found in the most active form in the latex of the tree from which Japanese lacquer is ob- tained, but also in many other plants. Many fungi contain a fer- pient, tyrosinase, which oxidizes tyrosin, and in certain animals tyrosinases have also been demonstrated. Another well-known oxidizing ferment in fresh s^nimal tissues is characterized by the property of forming indophenol by oxidation in an alkaline solution of paraphenylenediamin and a-naphthol, and may therefore be termed indophenyloxydase. The colourless solution becomes reddish or violet. This ferment is contained in pancreas, salivary glands, spleen, thymus, and bone-marrow, but has not been de- tected in muscle, lungs, brain, kidneys, and other organs. It is to be expected that pther oxydases capable of favouring oxida- tion of specific kinds of food substances or their decomposition products will be discovered, but it ought to be remarked that those at present known are only capable of attacking relatively simple organic substances, and it would be rash to conclude that this is the only way in which living protoplasm can bring about the rapid, but at the same time the regulated, oxidation which is so characteristic a feature of its activity. Yet the capacity of the cell to regulate the intensity and the extent of the intra- cellular oxidations would seem to find a simple explanation if we assign an important role to oxidizing ferments formed by the cell itself in accordance with its needs. In this connection we may mention a ferment, aldehydase, which was formerly included among the oxydases, but is now known to be a hydrolytic enzyme. It splits aldehydes so as to jneld the corresponding acid — e.g., salicylic aldehyde is split into salicylic acid and saligenin. Evidence of its presence in most organs has been obtained. Section VI. — ^Relation of Respiration to the Nervous Syst6m. The Respiratory Centre and its Connections. — Unlike the beat of the heart, the respiratory movements are entirely dependent on the central nervous system. The ' centre ' which presides over them is situated in the spinal bulb. It is a bilateral centre — that is, it has two functionally symmetrical halves, one on each side of the middle line. Each of these halves has to do more particularly with the respiratory muscles of its own side, for destruction of one-half of the spinal bulb causes paralysis of respiration only on that side. Anatomically the respiratory centre has not been sharply localized, but it lies lower than the vaso-motor centre, not far from the point of the calamus scriptorius. Stimulation of this region during apnoea (p. 277) is stated to cause co-ordinated inspiratory movements and RELATION OF RESPIRATION TO THE NERVOUS SYSTEM 269 widening of the opening of the glottis through abduction of the vocal cords. The centre is brought into relation with the muscles of respiration by efferent nerves. The phrenic nerves to the dia- phragm, and the intercostal nerves to the muscles which elevate the ribs, are the most important of those concerned in ordinary breathing. The respiratory centre is further related to afferent nerves, of which the most influential are those which supply the respiratory tract itself, particularly the pulmonary fibres and superior laryngeal branch of the vagus. But almost any afferent nerve may powerfully affect the centre ; and it is also influenced by fibres pass- ing to it from the higher parts of the central nervous system. Section of the spinal cord in animals above the origin of the phrenic nerves causes complete paralysis of respiration, and con- sequent death. The phrenics arise from the third and fourth cervical nerves, and are joined by a branch from the fifth; and in man fracture of any of the four upper cervical vertebrae is as a rule instantly fatal. But in one case respiration was carried on, and life maintained for thirty minutes, merely by the contraction of the muscles of the neck and shoulders in a man entirely paralyzed below this level (Bell). Section of the cord just below the origin of the phrenics leaves the diaphragm working, although the other respiratory muscles are paralyzed. A case has been recorded of a man in whom, from disease of the spine in the lower cervical region, all the ribs became completely immovable. He was able to lead an active life, and to carry on his business, although he breathed entirely by his diaphragm and abdominal muscles. Section of one phrenic is followed by paralysis of the correspond- ing half of the diaphragm, section of both phreAics by complete paralysis of that muscle, and although respiration still goes on by means of the muscles which act upon the ribs, it is usually inadequate to the prolonged maintenance of life. In the horse, however, not only has survival been seen after this operation, but the animal, after the first temporary increase in the frequency of the breathing had disappeared, could be driven in a light vehicle without any marked dyspnoea. The phrenic nuclei in the two halves of the cord are connected across the middle line. For when a semisection of t];ie cord is made between this level and the respiratory centre in the medulla, respiratory impulses are still able to reach both phrenic nerves. In some animals both halves of the diaphragm go on con- tracting. But when, as usually happens, this is not the case, and the diaphragm on the side of the semisection has ceased to act, it at once begins to contract again when the opposite phrenic nerve is cut, and the respiratory impulse, descending from the bulb, is blocked out from the direct, and forced to follow the crossed path. It has been shown that the crossing takes place at the level of the phrenic nuclei, and nowhere else (Porter), 270 RESPIRATION The Regulation of the Respiration through the Afferent Vagus t''ibres. — When one vagus is divided, there is little or no change in the' respiratory movements. Half an inch of one vagus nerve has been excised in removing a tumour, and the patient showed no symptoms whatever. But section of both vagi in such animals as the dog, cat and rabbit causes respiration to become much deeper and slower, the one change for a time compensating the other, so that the total amount of air taken in and given out, the amount of carbon dioxide eliminated, and the partial pressure of that gas in the pulmonary alveoh are not greatly altered. The relative dura- tion of the two respiratory phases is completely changed, inspira- tion being much more prolonged than expiration. It has been shown that the effect is really due to the loss of impulses that nor- mally ascend the vagi, not to any irritation of the cut ends. For a nerve can be frozen without exciting it ; and when a portion of each vagus is frozen, the respiration is affected in precisely the same way as when the nerves are divided. After section of both vagi certain fibres coming from the brain above the respiratory centre appear to take a share in the regulation of the respiratory movements. The bloodvessels supplying these fibres, or the centres from which they come, can be blocked by injection of paraffin wax into the common or internal carotid, or the bulb can be severed with the knife above the level of the re- spiratory centre, without any effect being produced upon the breath- ing, except that the rate is as a rule somewhat lessened. But when both the vagi and these uppei paths are cut the character of the respiration is changed, exceedingly prolonged inspiratory spasms alternating with long periods of complete relaxation of the diaphragm till the animal dies. From these facts it appears that the periodic automatic discharges ol the respiratory centre are being continually controlled and modi- fied by impulses passing up the vagus, and that in the absence of these impulses a certain degree of control is exercised by the higher paths, which, however, do not appear to be normally in action, at any rate to the full measure of their capacity. When the vagi are severed, the control of the higher paths comes into play, and is sufficient still to keep the breathing regular, although it is slowed. When the higher paths are cut off, the vagus of itself is able to regu- late the discharge. But when both are gone, the respiratory centre, fr6ed from nervous control, passes into a condition of alternate spasm and exhaustion. Of the central connections of these upper paths but little is surely known. The corpora quadrigemina, how- ever, seem to contain centres which can affect the respiration. Certain areas on the cerebral cortex have also been described, the excitation of which modifies the respiratory movements. There is no question that the cortex is connected, and extensively connected, RELATION OF RESPIRATION TO THE NERVOUS SYSTEM 271 with the respiratory centre, since the rate and depth of the co- ordinated respiratory movements, which are universally acknow- ledged to involve the activity of the centre, can be altered not only by the will, but by the most varied psychical events. The rhythmical excitation of the regulating vagus fibres must be brought about by either mechanical stimulation of the nerve- endings in the lungs, due to the alternate stretching and shrinking, or by chemical stimulation of these endings depending on the changes that occur with each respiration in the content of oxygen and carbon dioxide in the alveolar air, and therefore in their pressure (p. 260) in the blood. Both views have found advocates, but whatever influence the chemical changes in the blood may exert, there is no doubt that the mechanical factors are the more important. That the vagus is really excited is shown by the fact that a negative varia- tion (p. 801) is set up in the nerve when the lungs are inflated. An electrical change is also observed when air is sucked out of the lungs (Alcock and Seemann, Einthoven). When the normal excitation of the vagus fibres by expansion of the lungs is exaggerated by closing the trachea at the e'nd of in- spiration, the diaphragm immediately relaxes, and a long expira- tory pause ensues, broken at last by a series of inspirations much deeper and more prolonged than those which were taking place before occlusion. When the trache^ is occluded at the end of expiration, a series of deep and long-drawn inspirations occurs, the first of which begins at the moment when the next normal inspira- tion ought to have taken place had the windpipe been left free. The most obvious explanation of these results is that the expansion of the lungs sets up impulses in the vagi which cut short the in- spiratory activity of the respiratory centre (inspiration-inhilsiting fibres), while in collapse impulses are set up which excite it to re- newed inspiratory discharge (inspiration-exciting fibres). Since ordinary expiration is in the main not associated with active muscular contraction, the inspiration-inhibiting fibres would be at the same time expiration -exciting. Clearly this would constitute a so-called ' self-steering ' arrangement, each inspiration leading inevitably to the succeeding expiration, and each expiration providing the neces- sary stimulus for the succeeding inspiration. On this hypothesis section of the vagi must necessarily be followed by slowing of the respiratory movements, and we have seen that this is the case. A rival hypothesis is that the automatic activity of the respira- tory centre leads normally to the discharge of motor impulses to the inspiratory muscles, which are cut short at each expansion of the lungs by the inhibitory action of the vagus, the nerve not being excited during pulmonary collapse, and therefore carrying no in- spiratory impulses to the centre. On this assumption, we may think of the centre as being ' wound up ' like a clock, the periodic 272 RESPIRATION arrival of regulating impulses acting like an escapement movement, and allowing a certain amount of discharge. When the vagi are cut, the inspirations are greatly prolonged and deepened, because the check on the discharge of the centre has been removed. Attempts have been made by experimental stimulation of the vagus trunk to determine whether, as a matter of fact, it contains both inspiratory and expiratory fibres. But the results are neither so clear nor so constant that we can confidently appeal to them in making a decision, and even some of the investigators who main- tain the existence of but one anatomical set of fibres beUeve that these are affected differently by different kinds of stimulation- momentary stimuli, for example, setting up in them impulses which we may call inspiratory, and long-lasting stimuh impulses which we may call expiratory. Excitation of the central end of the cut vagus below the origin of its superior laryngeal branch, with induction shocks of moderate Fig. 124. — Respiratory Tracings: Dog. A, normal; B, eiiect of stimulation of the central end of vagus; C, eflect of section of both vagi. (Tracing taken as in Fig. 135. p. 295.) Time-tracing, seconds. strength, certainly causes quickening of respiration. If the excita- tion be strong, there is arrest in the inspiratory phase. A brief mechanical stimulus, or a series of such, has a similar effect. But chemical stimulation (e.g., with a strong solution of potassium chloride) or long-continued mechanical excitation like that produced by Stretching or compression of the nerve, or certain kinds of elec- trical stimulation — for instance, the very weakest induction shocks, or the closure of an ascending voltaic current* — cause slowing of the respiratory movements or expiratory standstill. This is also the usual, though not the invariable result of stimulating the superior laryngeal, even when weak induction shocks are employed. With stronger stimulation energetic contractions of the, expiratory muscles may occur. These facts undoubtedly suggest the existence in the vagus of two kinds of afferent nerve-fibres that affect the * I.e., a current passing towards the head in the nerve. RELATION OF RESPIRATION TO THE NERVOUS SYSTEM 273 respiratory centre in opposite ways — inspiratory fibres, which stimulate it to greater activity of discharge, and expiratory fibres, which inhibit its action. The latter variety we may suppose to be more numerous in the superior laryngeal, the former in the pul- monary branches of the vagus. And there is nothing forced in the hypothesis that certain kinds of stimuh act particularly on the one set of fibres, and certain kinds on the other, for we have already seen an instance of this in studying the differences between the vaso- constrictor and the vaso-dilator nerves (p. 173). The most probable conclusion, and the one which best reconciles the conflicting hypotheses, is that two sets of fibres are present : (i) Fibres which inhihit inspiration {and cause expiration), and are excited in ordinary inspiration by the expansion of the lungs. (2) Fibres which Fig.'i25,^-Effect of Stimulation of Central End of Vagus in a Cat. Upper Trace, Respiration; Lower Trace, Blood- Pressure. At the top are the time-trace (seconds), and below it the signal line, the depression in which indicates the duration of the excitation. Practically no effect was produced on the respira- tion, but a fall of blood-pressure with slowing of the heart. *' cause inspiration [and inhibit expiration), and are excited in st/ong expiration, as in dyspnoea, by the collapse of the lungs, but are not active in ordinary expiration. However this may be, the facts we have been discussing have an importance of their own, apart from any hypothetical explanations of them. Some of them have been more than once unintentionally illustrated on man. In one case the left vagus trunk was included in a ligature with the common carotid. The respiratory move- ments immediately stopped,, the pulse was slowed, and death occurred in thirty minutes (Rouse). The superior laryngeal fibres, unlike those of the vagus proper, are not constantly in action, as section of both nerves has no effect on respiration. Any source of irritation in the larynx may stimulate these fibres and produce a 18 274 RESPIRATION cough, which may also be caused by irritation, of the pulmonary fibres of the vagus. Action of Other Afferent Fibres on the Respiration. — ^The cutaneous nerves, and especially those of the face (fifth nerve), abdomen and chest, have a marked influence on respiration. They can be easily excited in the intact body by thermal and mechanical stimulation. A cold bath, for instance, usually causes acceleration and deepening of the respiratory movements ; and the efRcacy of mechanical stimu- lation of sensory nerves in stirring up a sluggish respiratory centre is well known to midwives, who sometimes slap the buttocks of a new- Fig. 126. — Effect of Stimulation of Central End of Brachial Nerve on Respiration (Upper Tracing) and Blood-Pressure (Lower Tracing) in the Cat. At the top of the figure Eire the time-trace (seconds) and the signal line, showing beginning and end of stimulation. born child to start its breathing. The reflex expiratory standstill caused in rabbits by inhalation of such sharp-smelling substances as ammonia, acetic acid, and tobacco-smoke is due to afferent impulses passing up the trigeminus fibres from the mucous membrane of the nose, and is still obtained after section of the olfactory nerves. Another set of afferent nerves which have been supposed by some to bear an important relation to the respiratory centre are those which supply the muscles. We have already noticed that the frequency of respiration is greatly augmented by muscular exercise. The simplest explanation would seem to be that afferent muscular nerves are stimulated either by mechanical compression of their RELATION OF RESPIRATION TO THE NERVOUS SYSTEM 275 terminal ' spindles,' or by the chemical action on them of certain waste products produced in contraction. It is quite likely that this is one way in which the adjustment is achieved. But this is not the only, and perhaps not the most important, way. For an in- crease in the respiratory movements is caused by tetanizing ; the muscles of a limb whose nerves have been completely severed, and which is indeed connected with the rest of the body by no other structures than its bloodvessels. This can only be due to two things : a direct action on the respiratory centre by the blood that has passed through, and been altered in, the contracting muscles, or an action exerted by the blood indirectly on the centre through .the excitation of afferent respiratory nerves whose connection with it is still intact — for example, the other muscular nerves or the |)ul- monary branches of the vagus. That the action is direct is shown by the fact that after section of the vagi, the sympathetic, and the spinal cord below .the origin of the phrenics, an increase in the respiratory movements is still produced by tetanizing a limb. The Chemical Regulation of the Respiration. — However important the regulation of respiration by afferent nervous impulses may be, the normal discharge of the respiratory centre is intimately associ- ated with the gases of the blood. It is generally acknowledged that the centre may be excited both by blood that is rich in carbon dioxide and by blood that is poor in oxygen. Stimulation by deficiency of oxygen has to some minds presented a metaphysical difficulty — namely, that it is not easy to see how the absence of a thing could cause stimulation. The diffi- culty does not exist, but none the less there is some evidence that when oxygen is lacking the respiratory centre can be excited by substances like lactic acid, which are easily oxidizable and rapidly disappear from properly oxygenated blood. On the other hand, it it stated that, when the oxidative processes of the medullary centres are decreased by the administration of carbon monoxide or sodium cyanide, the latent period which precedes the excitation of the respiratory (and other) centres is so short that the stimulation cannot be attributed to the accumulation of acid products, and that the mere oxygen want is of itself a stimulus for these cei^tres (Rosenthal, Gasser and Loevenhart). Be that as it may, it has been the subject of long-continued dis- cussion whether excess of carbon dioxide or deficiency of oxygen is the more potent stimulus for the respiratory centre. The best evi- dence points to the conclusion that comparatively small alterations in the amount of carbon dioxide in the inspired air cause a relatively great increase in the respiration, while in the case of the oxygen the departure from the normal proportion must be much more decided to bring about any notable effect. Nor is it at all out of harmony with this that, when very large quantities of carbon dioxide (30 per 276 RESPIRATION cent, and upwards in rabbits) are inhaled, a condition of narcosis comes on without any previous respiratory distress. For many substances act differently in large and in small doses. Haldane has pointed out how exquisitely sensitive the respiratory centre is to even small changes in the partial pressure of carbon dioxide in the alveolar air, and therefore in the blood and the centre itself, and has demonstrated that this is the way in which the amount of the pulmonary ventilation (the volume of air breathed per unit of time) is chiefly regulated in ordinary breathirl^. For instance, an increase of as little as o-2 per cent, of carbpn dioxide in the alveolar air, corresponding to an increase of 1-4 rnm. ■ of mercury in the partial pressure (p. 246) of the gas, caused an increase in the pulmonary ventilation of 100 per cent. The alveolar oxygen pressure had to be diminished to 13 per cent, of an atmo- sphere before any decided increase in the respiration occurredv During moderate muscular work the percentage of carbon dioxide in the alveolar air, and therefore in the blood, increases slightly, causing an increase in the ventilation, and this is one of the ways in which the hyperpnoea associated with muscular exercise is brought about. In severe work lack of oxygen, with accumulation of lactic , acid and other metabolic products, which stimulate the respiratory centre or render it excitable by smaller pressures of carbon dioxide, also plays a part. To sum up, the regulation of normal breathing is twofold- — a chemical regulation {through the carbon dioxide) of the amount of air moved into ■ and out of the lungs per unit of time ; and a nervous regulation [chiefly through the vagi) of the rate and depth of the movements necessary to effect the given amount of ventilation. When the vagi have been divided, an increase in the carbon dioxide pressure within certain limits is responded to by an increase in the total ventilation, just as in the normal animal, but the form of the response is different. Whereas in the normal animal both the rate and the depth of respiration are increased, in the vagoto- mized animal there is a marked increase in depth, with little or no increase in rate (Scott). When the gaseous exchange in the lungs from any cause becomes insufficient, the respiratory movements are exaggerated, and ulti- mately every muscle which can directly or indirectly act upon the chest -wall is called into play in the struggle to pass more air into and out of fhe lungs. To a lesser and greater degree of this exag- geration of breathing the terms Hyperpnoea and Dyspnoea have been respectively applied. If the gaseous interchange remains insuffi- cient, or is altogether prevented, asphyxia sets in. Sometimes in man impending asphyxia from loss. of function.;by a part of the lungs (with crippling of the lesser circulation), as in pneumonia, may be warded off by inhalations of oxygei^. Increase iri the temperature RELATION OF RESPIRATION TO ^HE NERVOUS SYSTEM 277 of the blood circulating through the spinal bulb, as when the carotid arteries of a dog are laid on metal boxes through which hot water is kept flowing, also causes dyspnoea, (heat-dyspnosa,) (p. 296). But if the temperature be too high, the respiratory movements may be slowed, perhaps by a partial paralysis or inhibition of the respiratory centre. When the blood is cooled the respiration becomes deeper and slower, but if the temperature is greatly and suddenly lowered, the centre may be stimulated and the breathing quickened. In man the increased temperature of the blood in fever is a cause, though not the only one, of the increase in the rale of respiration. Apnoea. — The physiological opposite of dyspnoea is apndx. This condition may be produced in an animal by rapid or prolonged artificial respiration. It is especially easy to obtain in an animal in which the circulation through the brain and bulb is interrupted for a time and then restored, while artificial respiration is being kept up. Spontaneous respiration returns after a longer or shorter interval, but if the artificial respiration be still maintained, it again ceases. In a successful experiment the animal remains without breathing for many seconds after the artificial respiration is stopped. In apnoea the chest remains at rest in the expiratory phase if the lungs have been inflated by the artificial respiration and then allowed to collapse of themselves (expiratory apnoea), but in the inspiratory phase if they have been emptied by suction and then permitted of themselves to expand (inspiratory apnoea). The apnoea is not pro- duced, as some have thought, by the accumulation of an excess of oxygen in the blood, for rapid and repeated inflation of the lungs with hydrogen may cause the condition. Indeed, towards the end of the apnceic period the venous blood may be very distinctly poorer in oxygen than normal venous blood. Apnoea is easily caused in man by a period of deep and rapid breathing and in other ways. The essential thing in this chemical or true apnoea (apnasa vera) is the lowering of the partial pressure of carbon dioxide in the alveolar air, and therefore in the arterial blood and the respiratory centre. The carbon dioxide is washed out of the body, so to say, by the excessive pulmonary ventilation. In addition to chemical apnoea, which is obtainable whether the vagi are intact or not, a so-called mechanical apnoea, or apncea vagi, exists — ^that is to say, a stoppage of the respiration due to an inhibitory effect produced through the vagi on the respiratory centre when the vagus endings in the lungs are excited mechanically by inflation. Some observers state that this vagus apnoea does not outlast the inflation. Others beUeve that the results of successive inflations can be ' summated ' in the centre, giving rise to an apnoea which persists after stoppage of the artificial respiration. That a ' memory ' of a prolonged rhythmical inflation of the lungs can impress itself in some way on the respiratory centre is shown by 278 RESPIRATION the curious phenomenon that in resuscitation of the bulb after a period of anaemia the natural respiration, when it returns, may have for a short time exactly" the same rhythm as the artificial respiration which has just been stopped. That the blood when the gaseous exchange in the lungs is inter- fered with produces dyspnoea by acting on some portion of the brain may be shown in an interesting manner by establishing what is called a cross-circulation in two rabbits or dogs. The vertebral arteries and one carotid are tied in both animals; the remaining carotids are divided and connected crosswise by glass tubes, or, what is better, as it avoids the risk of clotting, they are crossed by suturing the cut ends, so that the brain of each is supplied by blood from the other. When the respiration is artificially hindered or stopped in one of the animals, it shows no dyspnoea; it is in the other, whose brain is being fed with improperly ventilated blood, that the respiratory movements become exaggerated. The point of attack of the ' venous ' blood has been further locaUzed in the spinal bulb by the observation that when the brain has been cut away above it, the cord severed below the origin of the phrenics, and all other nerves connected with the region between the two planes of section divided, any interference with the gaseous ex- change in the lungs is at once followed by dyspnoea.* Autotnaticity of the Respiratory Centre. — The question has been raised whether, in the absence of this ' natural ' stimulation by the blood, and of the impulses that constantly reach the centre along its afferent nerves, it would continue to discharge itself, or whether it would sink into inaction. We have already discussed a similar question in regard to the cardiac and vaso-motor centres, and the subject must again present itself when we come to examine the functions of the central nervous system. In the meantime it is only necessary to say that there is evidence that it is not the mere presence of carbon dioxide (or other substances) in the blood circu- lating through the respiratory centre which determines the constant excitation of the centre, but rather the accumulation of carbon dioxide in the centre itself when the partial pressure of that gas in the blood is raised. The idea that the continuous excitation of the centre is ' autochthonous ' — in other words, that it is due to an internal stimulating substance or substances manufactured in the centre itself, as well as carried to it in the blood — ^renders it easy to imderstand that the discharge of the respiratory centre, although modified by the quality of the blood which circulates in it, is not essentially dependent on it. Indeed, in cold-blooded animals whose blood has been replaced by physiological salt solution, and (in frogs) * The conclusion is doubtless correct, but this experiment is not decisive. For the phrenic nerves themselves contain afferent fibres, the stimulation of which can influence the respiration after section of the vagi. RELATION OF RESPIRATION TO THE NERVOUS SYSTEM 279 even after the circulation has been stopped altogether by excision of the heart, quiet, regular breathing may be seen for a considerable time. Of course, blood is essential for the continued nutrition of the centte and its connections, and it eventually breaks down and ceases to discharge. The respiratory discharge is still less dependent for its initiation upon the arrival of afferent impulses. For after section of the bulb above the centre, of the cord below the origin of the phrenics, of the vagi and of the posterior roots of all the upper cer- vical nerves, the spasmodic respiration which we have already described as occurring when the vagi and the higher paths have been severed continues without essential modification. It has also been observed that during resuscitation of the bulb and upper cervical cord after a period of anaemia, stimulation of afferent nerves, in- cluding the vagi, is entirely without influence on the respiratory movements for some time after respiration has returned, presumably because the synapses (p. 824) on the afferent paths lying within the previously anaemic area are as yet unable to conduct the nerve impulses. Nevertheless, the respiratory centre continues steadily to discharge itself along the efferent paths, whose synapses are situated beyond the anjemic region. Section of the bulb above the level of the respiratory centre, and of the cord below the origin of the phrenic nerves, in addition to the anaemia, makes no essential difference in the result. The initial rate of discharge of the centre thus isolated from afferent impulses is approximately constant in different experiments (about four a minute in cats). Spinal Respiratory Centres. — Although the chief respiratory centre lies in the medulla oblongata, under certain conditions impulses to the respiratory muscles may originate in the spinal cord. Thus, in young mammals (kittens, puppies), especially when the excitability of the cord has been increased by strychnine, in birds and in alli- gators, movements, apparently respiratory, have been seen after destruction of the brain and spinal bulb. In adult cats, when the functions of the brain, medulla, and cervical cord have been abolished by occlusion of their vessels, similar movements of the thoracic and abdominal muscles may be seen, but they are not suffi- cient for effective respiration. No proof has ever been given that in the intact organism the spinal cord below the level of the bulb takes any other part in respiration than that of a mere conductor of nerve impulses; and it is not justifiable to assume the existence of automatic spinal respiratory centres on the strength of such experi- ments as these. , Death after Double Vagotomy. — ^Alterations in the rhythm of respira- tion are not the only effects that follow division of both vagi (or vago- sympathetics) in the neck. In certain animals, at least, this operation IS incompatible with life. In the rabbit, as a rule, death takes place in twenty-four hours. A- sheep ma5'- live three days, and a horse five or SIX. Dogs often live a week, occasionally a month or even two, and in 28o RESPIRATION rare instances they survive indefinitely. The most prominent S5anp- toms (in the dog), in addition to the marked and permanent slowing of respiration, quickening of the pulse and contraction of the pupils, are difficult deglutition, accompanied by frequent vomiting and pro- gressive emaciation. The appetite is sometimes ravenous, but no sooner is the fopd swallowed than it is rejected ; and this is particularly true of water or liquid food. Sometimes the rejected food is simply regurgitated after having reached the lower end of the oesophagus, without entering the stomach. The fatal result is usually caused, or at least preceded, by changes of a pneumonic nature in the lungs. The precise significance of the pulmonary lesion is obscure. But it would seem that paralysis of the laryngeal and oesophageal muscles, with the consequent entrance of saliva, food, or foreign bodies, carrying bacteria into the lungs, is responsible to a great extent. And when only a partial palsy of the glottis is produced, by dividing the right vagus' below the origin of the recurrent laryngeal, and the left as usual in the neck, pneu- monia either does not occur or is longdelayed. It may be that the tissue of the lungs is rendered particularly susceptible to such insults in conse- quence; of trophic or vascular changes induced by section of the pul- monary and cardiac fibres in the vagi. It may be quite clearly demon- strated, however, in animals which live for some weeks, that, not- withstanding the paralysis of the glottis associated with aphonia, no pulmonary symptoms may be present till a day or two before death. The picture presented in these cases is that of an animal suffering, above all, from alimentary disturbances. The respiration is, to be sure, very different from the normal in frequency, depth, and type, but there is nothing to suggest that the lungs are the seat of any pathological process. Suddenly the picture changes. Pulmonary symptoms ob- trude themselves. The physical signs of consolidation of the lungs may be detected, and in a short time the animal is inevitably dead. Occasionally the determining cause of the pulmonary lesion seems to be some external circumstance, as a sudden fall of the air temperature. The idea is exceedingly apt to present itself to the observer that the pneumonia is an accident, an acute intercurrent affection breaking the course of a chronic malnutrition, which in any case must have ended in death. Of course, the vagotomized animal is predisposed to this accident, but there is no definite time after section of the nerves at which it must take place. The vomiting is certainly connected with the paralysis and consequent dilatation of the oesophagus ; and by previously making an artificial opening into the stomach or by a surgical prophy- laxis still more heroic, the establishment of a double gastric and oesophageal fistula (p. 395), death may be prevented for many months. Elimination of all the pulmonary fibres of the vagi, by extirpation of one lung, followed after an interval by section of the opposite vagi:s in the neck, is not fatal in rabbits. This is also in favour of the view that in double vagotomy the stress falls mainly on the digestive system. Innervation of the Bronchial Muscles. — Both constrictor and dilator fibres for the bronchi are contained in the vagus. They are not constantly in action, but can be reflexly excited, most easily (in the dog and cat) by stimulating the nasal mucous membrane, and particularly a small area well back upon the nasal septum. Cauterization of the corresponding area in man is said to give per- manent relief in certain cases of spasmodic asthma, a condition in which the recurrent attacks of dyspnoea seem, according to the most RELATION OF RESPIRATION TO THE NERVOUS SYSTEM 281 generally accepted view, to be associated with spasnl of the bronchial muscles. Special Modifications of the Respiratory Movements'. — Gheyne- Stokes Respiration is the name given to a peculiar type of breathing, marked by pauses of many seconds alternating with groups of respirations. In each group the movements gradually increase to a maximum amplitude, and then become gradually shallower again, till they cease for the next pause. The phenomenon often occurs in certain diseases of the brain and of the circulation, and pressure on the spinal bulb may produce it. In cats in which the circulation in the brain and medulla oblongata has been interrupted for a time and then restored it is often noticed at a certain stage of resuscita- tion of the respiratory centre. In frogs, Cheyne- Stokes breathing has been observed as the result of interference with the circulation in the spinal bulb, ' drowning,' or ligature of the aorta, and also as a consequence of removal of the brain, or parts of it (hemispheres and optic thalami) . But it is not peculiar to pathological conditions, being also seen, more or less perfectly, in normal sleep, especially in children, in healthy men at high altitudes, in hibernating animals, and in morphine and chloral poisoning. Well-marked Cheyne-Stokes breathing can be obtained experi- mentally in normal persons in a variety of ways. If, for exarhple, the subject is caused to breathe deeply and frequently for about two minutes, so as to produce a prolonged apnoea, the respiration, when it is resumed spontaneously, is of the Cheyne-Stokes type (Haldane). The explanation given by Haldane is that the fall in the partial pressure of the oxygen in the pulmonary alveoli (p. 277) during the primary apnoea, with the consequent fall of oxygen pressure in the arterial blood and the respiratory centre, leads to the production of lactic acid in the respiratory centre and elsewhere, which stimu- lates the centre in the same way as carbon dioxide, and thus permits it to be excited by a srrialler partial pressure of carbon dioxide than that normally necessary. As soon as the pressure of carbon dioxide, which is increasing during the period of apnoea, has reached the exciting value breathing is resumed. The respirations, beginning as very feeble movements, rapidly increase in strength till the breathing becomes quite deep or actually dyspnoeic. The store of oxygen is replenished by this thorough ventilation of the lungs, the changes in the excitability of the respiratory centre due to lack of oxygen disappear (perhaps by oxidation of the lactic acid), and the centre relapses into a period of repose. During this period of apnoea the oxygen pressure sinks once more to the point at which the change in the excitability of the respiratory centre by carbon dioxide occurs, and the breathing again starts. In pathological cases the want of oxygen may be associated either with deficient circulation through the bulb-centre or with deficient intake by the lungs. The adminis- 282 RESPIRATION tration of oxygen through a mask has been shown in such cases to abolish the periodicity in the respiration, and to render it more normal. Peculiarly modified, but more or less normal, respiratory acts are coughing, sneezing, yawning, sighing, and hiccup. A cough is an abrupt expiration with open mouth, which forces open the previously closed glottis. It. may be excited reflexly from the mucous membrane of the respiratory tract or stomach through the afferent fibres of the vagus, from the back of the tongue or mouth, and (by cold) from the skin. Sneezing is a violent expiration in which the air is chiefly expelled through the nose. It is usually excited reflexly from the nasal mucous membrane through the branch of the fifth nerve which supphes it. Pressure on the course of the nasal nerve will often stop a sneeze. A bright light sometimes causes a sneeze, and so in some individuals does pressure on the supra-orbital nerve, when the skin over it is slightly inflamed. Yawning is a prolonged and very deep inspiration, sometimes accompanied with stretching of the arms and the whole body. It is a sign of mental or physical weariness. A sigh is a long-drawn inspiration, followed by a deep expiration. Hiccup, or hiccough, is due to a spasmodic contraction of the dia- phragm, which causes a sudden inspiration. The abrupt closure of the glottis cuts this short and gives rise to the characteristic sound. The following readings of the intervals between successive spasms were obtained in one attack: 13 sees., 12 sees., 15 sees., 9 sees., 14 sees., etc. — i.e., one-fourth or one-fifth of the frequency of the ordinary respiratory movements. The mere fixing of the attention on the observations soon stopped the hiccup. Hiccup is generally considered to be a reflex movement, brought about through the respiratory centre by afferent impulses originating in the stomach. The irritation may be merely due to some slight digestive disturbance set up by overfilling of the stomach, perhape. This is exceedingly common in infants. But persistent hiccup may also be a distressing symptom of very formidable diseases — for example, carcinoma of the pylorus. Experimentally, reflex con- tractions of the diaphragm can sometimes be elicited by stimulation of the central end of the vagus at a time when no spontaneous respiratory movements are going on. This has been observed, for instance, in cats during resuscitation of the brain after a period of anaemia. In man also, in a case of Cheyne-Stokes respiration accom- panied by hiccup, it was seen that the hiccup persisted during the periods of apnoea. If the respiratory centre is the centre for the hiccup reflex, it can therefore be excited by afferent nervous im- pulses at a time when it is not excited by the normal chemical stimulus (MacKenzie and Cushny). INFLUENCE OF RESPIRATION ON THE BJLOOD-PRESSURE 283 Section VII. — The Influence of Respiration on the Blood- Pressure. We have already stated, in treating of arterial blood-pressure (p. Ill), that a normal tracing shows a series of waves corresponding with the respiratory movements. The relationship between the respiratory phases and the rise and fall of the blood-pressure is not by any means a simple and invariable one. It depends upon a number of factors, which need not be equally influential under different conditions or in different animals (Lewis). Something depends upon the rate, something upon the relative preponderance of costal and abdominal respiration, and something probably upon the size ol the animal. For instance, an inspiratory rise of blood-pressure occurs in man with pure dia- Fig. 127. — Respiratory Waves in tKe Blood-Pressure: Simultaneous Tracings of Movements of Respiration and of Radial Pulse in Human Subject (Lewis). In A the respiration was diaphragmatic; in B, costal. In A the respiratory tracing was taken from the abdominal wall; in B, from the chest. phragihatic, and a fall with pure thoracic, breathing (Fig. 127). In cats with fairly fast and not very deep respiration the blood-pressure rises in expiration and sinks in inspiration. With deep and slow respiration the opposite effect may, upon the whole, be seen. In dogs, according to Einbrodt, although the mean bload-pressure is falling for a short time at the beginning of inspiration, it soon reaches its minimum, then begins to rise, and continues rising during the rest of this period. At the commencement of expiration it is still mounting, but soon reaches its maximum, begins to fall, and con- tinues falling through the remainder of the expiratory phase. A partial explanation is afforded by a consideration of the mechan- ical changes produced in the thorax by the respiratory movements. Of these, the influence of variations in the intrathoracic pressure on the filling of the heart is of special importance. With deep abdominal breathing the changes of intra-abdominal pressure also 284 RESPIRATION affect the filling of the heart, an increase of pressure (in inspiration) tending to cause more blood to be squeezed from the abdominal veins towards the chest. The changes of vascular resistance in the lungs, due to the alteration in the calibre of the pulmonary vessels, may also contribute, but, for such variations of intrathoracic pressure as normally occur, only in a minor degree. The changes in the vascular capacity of the lungs — ^that is, in the amount of blood contained in the pulmonary vessels — are of importance espe- cially in delaying or accelerating the alterations of blood-pressure in the systemic arteries due to the other factors. The intrathoracic pressure, which, as we have seen, is always less than that of the atmosphere, unless during a forced expiration when the free escape of air from the lungs is obstructed, diminishes in inspiration and increases in expiration. The great veins outside the chest, the jugular veins in the neck, for example, are under the atmospheric pressure, which is readily transmitted through their thin walls, while the heart and thoracic veins are under a smaller pressure. The venoUs blood both in inspiration and ex- piration will, accordingly, tend to be drawn into the right auricle. In inspiration the ven- ous flow will be increased, since the pressure in the thorax, and therefore in the pericardial cavity, is diminished ; and upon the whole more venous blood will pass into the right heart during inspiration than during expiration. Now, the right ventricle is not in general working as hard as it can work. Hence, the excess of blood which reaches it during an inspiration is at once sent into the Itings, although not even the first of it can have passed through to the left side of the heart at the end of the inspiration, since the pulmonary circulation-time (four to five seconds in a small dog, two to three seconds in a rabbit) is longer than the time of a com- plete inspiration at any ordinary rate. The increase in the quantity of blood pumped into the pulmonary artery will, if not counteracted by other circumstances, tend to raise the blood-pressure in the artery and its branches, and therefore at once to accelerate the out- flow through the pulmonary veins. This will be aided if at the same time the vascular resistance in the lungs is reduced, as is generally stated to be the case. The left ventricle, like the right, is capable of discharging more blood than it ordinarily receives. The excess of blood coming to it is easily and promptly ejected. The systemic arteries are better filled and the arterial pressure rises. Fig. 128. — The. upper tracing sJiows the respiratory movements in a rabbit with rather deep and slow diaphragmatic breathing; the lower tracing is the blood-pressure curve; /, inspiration; E, expiration, including the pause. INFLUENCE OF RESPIRATION ON THE BLOOD-PRESSURE 285 In expiration the contrary will happen. The return of blood to the thorax will be checked. This is well shown by the swelling of the veins at the root of the neck in expiration, their shrinking in inspiration, the so-called respiratory venous pulse. Less blood being drawn into the right heart, less will be pumped into the pul- monary artery, in which the pressure will, of course, fall. The out- flow into the left auricle will thus be diminished — all the more if in the expiratory phase the vascular resistance in the lungs is increased — and the systemic arterial pressure will be lowered. In both cases, however, the change seen in the blood-pressure curve will be belated, and will not coincide exactly with the commencement of the inspira- tion or the expiration. If it is delayed for a period about equal to the length of an inspiration or an expiration, the blood-pressure will be seen to sink in inspiration and to rise in expiration. If the period of delay is less than this, the pressure will be mounting during a part of. each respiratory phase and falling during the rest. As to the explanation of the delay, several factors may be concerned. The negative pressure of the thorax acts on the aorta, as well as on the thoracic veins, although, on account of the greater thickness of its walls, to a smaller extent than on the veins. The diminution of pressure in inspiration tends to expand the thoracic aorta, and to draw blood back out of the systemic arteries, while expiration has the opposite effect. And although the hindrance caused in this way to the flow of blood into the arteries during inspiration, and the acceleration of the flow during expiration may not be great, they will, of course, be better marked in small animals with compara- tively yielding arteries than in large animals. Yet, whether gireat or small, the tendency will be to diminish the pressure in the one phase and increase it in the other. As soon as the changes of pres- sure produced by alterations in the flow of venous blood into the chest and through the lungs are thoroughly established, the arterid.1 effect will be overborne; but before this happens — ^that is, at the beginning of inspiration and expiration — it will be in evidence, and will help to delay the main change. Another factor in this delay is found in the changes of vascular capacity which take place in the lungs when they pass from the expanded to the collapsed condition. The expansion of the lungs in natural respiration causes a widening of the pulmonary capillaries, with a consequent increase of their capacity and diminution of their resistance. When the vessels at the base of the heart are ligatured either at the height of inspiration or the end of expiration, so as to obtain the whole of the blood in the lungs, it is found that they invariably contain more blood in inspiration than in expiration. During inspiration, as we have seen, the right ventricle is sending an increased supply of blood into the pulmonary artery ; but before 286 RESPIRATION any increase in the outflow through the pulmonary veins can take place, the vessels of the lung must be filled to their new capacity. The first effect, then, of the lessened vascular resistance of the lungs in inspiration is a temporary faUing off in the outflow through the aorta, and therefore a fall of arterial pressure. As soon as a more copious stream begins to flow through the lungs, this is succeeded by a rise. In like manner the first effect of expiration, which in- creases the resistance and diminishes the capacity of the pulmonary vessels, is to force out of the lungs into the left auricle the blood for which there is no room. This causes a rise of arterial blood- pressure, succeeded by a fall as soon as the lessened blood-flow through the lungs is established. The changes in the diastolic capacity of the chambers of the heart itself, with the changes of pericardial pressure, must also act in the Fig. 129. — Effect on Blood-Pressure of Inflation of tlie Lungs: Rabbit. Artificial respiration stopped in inflation at i. Interval between z and 3 (not reproduced) 51 seconds, during which the curve was almost a straight line. Time tracing shows seconds. same direction. It is obvious, then, how greatly the rate and depth of respiration in relation to the size of the animal and the other cir- cumstances already mentioned may influence the time relations of the respiratory oscillations in the arterial pressure curve, so that we ought not to expect them to be absolutely constant. In artificial respiration oscillations of blood-pressure, synchronous with the movements of the lungs, are also seen. During inflation (inspiration) the arterial pressure rises; during deflation (expiration) it falls. When artificial respiration is stopped at the height of inflation and the lungs kept inflated (Fig. 129), the arterial blood-pressure falls rapidly, and continues low until the rise of asphyxia begins. In the fall of pressure the increased intrathoracic pressure due to the inflation is an important factor. When the respiration is stopped in collapse, instead of a fall a steady rise of pressure occurs (as in Fig. 84, p. 186). This ultimately merges in the elevation due to asphyxia, which shows itself sooner than in inflation. When the tracheal cannula is closed in natural respiration, no initial fall of pressure takes place (Fig. 130). INFLUENCE OF RESPIRATION ON THE BLOOD-PRESSURE 287 Besides the mechanical effects of the respiratory movements on the circulation, it may be influenced by changes in the cardio- inhibitory and vaso-motor centres synchronous with the rhythm of the respiratory centre. In many animals (the dog, for instance) and in man, it can be very easily made out that the rate of the heart is greater during inspiration,' especially towards its end, than in expiration. The phenomenon is especially distinct in deep and slow respiration. ■ It is caused by a rhythmical rise and fall in the activity of the cardio-inhibitory centre, synchronous with the respiratory movements, for the difference disappears after division of both vagi. The normal respiratory oscillations of blood-pressure are not due in any great degree to such changes in the rate of the heart, for they persist after section of the vagi, and they are seen in animals like the rabbit, in which p ordinary breathing little or no variation in the ra,t« of the heart is connected with the phases of respiration. The "most probable explanation of the respiratory Fig, 130. — Blood-Pressure Tracing; Rabbit, under Chloral. Natural respiration stopped at I in inspiration, at E in expiration. The mean blood-pressure is scarcely altered; but the respiratory waves become much larger owing to the abortive efforts at breathing. Time tracing shows seconds. variations in the pulse-rate is that the respiratory centre, when it is discharging itself in inspiration, sends out impulses as a sort of over- flow along, fibres connecting it with the cardio-inhibitory centre. These increase the tone of that centre, but, owing to the long latent period of the cardio-inhibitory apparatus, the inhibition does not reveal itself till the succeeding expiration. It may be, however, that the impulses discharged from the respiratory centre in inspiration diminish the tone of the cardio-inhibitory centre, and thus lead to acceleration of the heart towards the end of the inspiratory phase. In certain pathological conditions the influence of the respiration on the pulse-rate is exaggerated (so-called ' respiratory arh57thmia '), Traube-Hering Curves. — Rhythmical changes in the activity of the vaso-motor centre, also associated with periodic discharges from the respiratory centre, may be observed under certain conditions — e.g., when in an animal paralyzed by curara, and therefore unable to breathe spontaneously, the artificial respiration is stopped for a time. 288 RESPIRATION If such a dose of cyrara be given as will still permit slight spontaneous respiration to go on, and both vagi be cut, it can be seen on stopping the 'artificial respiration that the waves on the blood-pressure curve are exactly synchronous with the slow respiratory movements. The Traube-Hering waves sink in inspiration and rise in expiration. The fact that they have invariably a longer period than the natural respiratory movements indicates that they are not concerned in the production of the normal respiratory oscillations of arterial pressure. Probably the reason why the Traube waves appear after section of the vagi is the increased vigour of the slow respiratory discharges, coupled with a hyperexcitability of the vaso-motor centre, due to the long pauses in the aeration of the blood. In the asphyxial rise of pressure in a curarized dog they are constantly seen, and are often observed when the circulation in the medulla %#«%, *^W^ti?^**?>^; Fig. 131.- -Traube-Hering Waves as the Blood-Pressure is falling during Occlusion of the Cerebral Arteries in a Cat. oblongata is in any way interfered with (Fig. 131). In addition to the true Traube-Hering waves, other and much longer periodic variations in the blood-pressure are sometimes noticed. If spon- taneous respiration is going on, their long sweeping curves then show the ordinary respiratory waves superposed on them. The normal respiratory oscillations in the veins, as might be expected, run precisely in the opposite direction to those in the arteries, and so do the Traube-Hering curves. The increased flow from the veins to the thorax during inspiration lowers the pressure in the jugular vein, while it increases the pressure in the carotid. The constriction of the small bloodvessels to which the Traube- Hering curves are due increases the blood-pressure in the arteries, because it increases the peripheral resistance to the blood-flow; in the veins it lowers the pressure, because less blood gets through to them. Accordingly, when the Traube-Hering cyrye is ascending in. the carotid, it is descending in the jugular. EFFECTS OF BREATHING CONDENSED AND RAREFIED AIR 289 The respiratory variations in the volume of the brain, which are so striking a phenomenon when a trephine hole is made in the skull, but which can also take place, thanks to the displacement of cerebro-spinal fluid (p. 174), when the cranium is intact, have by some been attributed to interference with the venous outflow from the cranial cavity during expiration, and by others to those changes in the arterial pressure whose causes we have just been discussing. The truth is that neither factor is exclusively concerned. The ques- tion turns largely upon the time-relations of the movements. The swelling of the brain is sometimes synchronous with expiration, and the shrinking with inspiration. Here the damming back of the blood in the sinuses when the outflow is checked by the expiratory rise of pressure in the thoracic veins either conspires with ari expira- tory rise of arterial pressure or is more than enough to counter- balance an expiratory fall of pressure in the cerebral arteries if the respiratory conditions are such as to lead to an expiratory fall. But sometimes the dura mater bulges into the trephine hole in inspira- tion and sinks down in expiration. Here the increase in the volume of the brain produced by the increased pressure in the arteries and capillaries in inspiration is more than sufficient to counterbalance the quickened escape of blood from the cerebral veins. Section VHI. — ^The Effects of breathing Condensed and Rarefied Air. These are — (i) mechanical, shown chiefly by changes in the cir- culation, in the blood-pressure, for instance ; (2) chemical. The mechanical effects differ according to whether the whole body, MX only the respiratory tract, is exposed to the altered pressure. When the trachea of an animal is connected with a chamber in which the pressure can be raised or lowered, it is found that at first the arterial blood-pressure rises as the pressure of the air of respira- tion is increased above that of the atmosphere. But a maximum is soon reached; and when respiration begins to be impeded, the pressure falls in the arteries and increases in the veins. When the pressure of the air in the chamber is diminished a little below that of the atmosphere, there is a slight sinking of the arterial blood- pressure, which rises if the air-pressure is further diminished. It is clear that any change of the air-pressure which tends to diminish the intrathoracic pressure will favour the venous return to the heart, and therefore, if the exit of blood from the thorax is not proportionally impeded, the filling of the arteries. An increase in the intra-alveolar pressure must tend on the whole to increase, and a diminution in it to lessen,- the pressure inside the thorax, which always remains equal to the mtra-alveolar pressure, minus the elastic tension of the lungs. Breathing compressed air should, therefore, under the conditions aescnbed, be upon the whole unfavourable to the venous return to the 19 290 RESPIRATION heart and to the filling of the arteries, and the arterial pressure should fall; while breathing rarefied air should have the opposite effect. But a very great diminution of the intrathoracic pressure is not necessarily favourable to the circulation, sinfce the auricles are then unable to con- tract perfectly. Certain chest diseases have been treated by the use of apparatus by which the patient is made to breathe. either compressed or rarefied air; or to inspire air at one pressure and to expire into air at another pressure. And it has, upon the whole, been found, in agreement with theory, that condensed air cannot help the circulation however it is applied, but always hinders it ; while rarefied air aids the circulation both in inspira- tion and in expiration. But the increased work of the in- spiratory muscles may coun- terbalance the advantage. Valsalva's experiment. which is performed by closing the mouth and nostrils after a previous inspiration, and then forcibly trying to expire , is an imitation of breathing into compressed air. The intrathoracic pressure is raised, it may be, to considerably more than that of the atmosphere; the venous return to the heart is impeded, and may be stopped ; and the pulse curve is altered in such a way as to indicate first an increase and then a decrease of the arterial blood- pressure succeeded by a second rise (Fig. 132). Milller's experiment, which should be bracketed with Valsalva's, consists in making, after a previous expiration, a strong inspiratory effort with mouth and nostrils closed. Here the intrathoracic pressure is greatly diminished, more blood is drawn into the chest, and upon the whole effects opposite to those of Valsalva's experinient are produced (Fig. 133). Neither experiment is quite free from danger. In both the dicrotism of the pulse becomes more marked. Fig. 132.- -Pulse Tracing in Valsalva's Experi- ment (Rollett). Fig. 133. — Pulsejj Tracing in Muller's Experiment (Rollett), When the whole body is sub- jected to the changed pressure, as in a balloon or on a mountain, in a diving-bell or a caisson used in building the piers of a bridge, the'conditions are very different. For the blood-pressure, the intrathoracic pressure, and the intra- alveolar pressure, all fall together when the pressure of the atmo- sphere is diminished, and all rise together when it is increased. It is possible not only to live, but to do hard manual labour, at very different atmospheric pressures. As regards the chemical effects of condensed and rarefied air, Loewy found that the quantity of oxygen absorbed by a man breath- ing air in the pneumatic cabinet remained constant at all pressures between about two atmospheres and half an atmosphere. At 440 mm. of mercury dyspnoea became evident ; but if the person was now made to work, the dyspnoea passed away, and did not again manifest itself EFFECTS OF BREATHING CONDENSED AND RAREFIED AIR 291 till the pressure was reduced to 410 mm. There are towns on the high tablelands of the Andes, and in the Himalayas, where the barometric pressure is not more than 16 to 20 inches, yet the in- habitants feel no ill effects. And in the caissons of the Forth Bridge the workmen were engaged in severe toil under a maximum pressure of over three atmospheres, while in the caissons of the St. Louis Bridge in America a maximum pressure of over four atmospheres (i.e., more than three atmospheres in addition to the ordinary air- pressure) was reached. Inside the caissons the men sometimes suffer from pain and noise in the ears, due to excessive pressure on the external surface of the tym- panic membrane. If the pressure in the tympanum is raised by a swallowing movement, which opens the Eustachian tube and permits air to enter it, the symptoms generally disappear. The suddenness of the change of pressure has much to do with its effects, and it is found that the men are most liable to dangerous symptoms while passing through the air-lock from the caissons to the external air. It may be concluded, from experiments on animals, that some of the most serious of these — ^the localized paralysis usually affecting the legs (paraplegia) and the circulatory disturbances — are due to the formation of gaseous emboli, by the liberation of nitrogen in the blood and other body- fluids when the pressure is abruptly reduced. And, indeed, it is found that the symptoms can often be caused to disappear, both in animals and men, by promptly subjecting them again to compressed air. To avoid gas embolism on decompression, the shift should be so short that the body-fluids do not become fully saturated with nitrogen, and the decompression should be slow. Even with a rate of decompression of twenty minutes for each atmosphere of excess pressure, the equilibrium between the dissolved and the atmospheric nitrogen is not entirely established fifteen minutes after decompression. But that the action of air under a high pressure is not merely mechan- ical follows from the singular fact that in pure oxygen at a pressure of 4 to 5 atmospheres, which ctorresponds to air at 20 to 25 atmospheres, convulsions are often produced in vertebrate animals, while exposure to 6 to 25 atmospheres of oxygen causes dyspnoea and coma, usually without convulsions. All animals, so far as investigated, are instantly convulsed and killed under a pressure of 50 atmospheres of oxygen (Hill and Macleod). Even seeds and vegetable organisms in general are killed in a short time in oxygen at 3 to 5 atmospheres; and an atmosphere of pure oxygen, equal to 5 atmospheres of air, hinders the development of eggs. Lorrain Smith has shown that in small birds and mice exposure for many hours to a pressure of between I and 2 atmospheres of pure oxygen causes pneumonia. He confirms Bert's observations on the acute toxic effects produced by higher pressures, and supposes that in the production of caisson disease the special action of the oxygen at high pressure may play a part as well as the rapid decompression. When the air-pressure is diminished below a certain limit, death takes place from asphyxia, more or less gradual according to the rate at which the pressure is reduced. The hsemoglobin cannot get or retain enough oxygen to enable it to perform its respiratory func- tion; its dissociation tension is no longer balanced by an equal or 292 RESPIRATION greater partial pressure of oxygen in the air. The tension of carbon dioxide in the blood is also lessened, owing to the dyspnoea and the consequent increase of pulmonary ventilation. To such changes, as well as to the cold, some of the deaths in high balloon ascents must^ be attributed. Messrs. Glaisher and Coxwell supposed that they reached the height of 37,000 feet ; the former became unconscious at 29,000 feet (8,800 metres), at which height the amount of oxygen in the arterial blood would probably not exceed 10 volumes per cent., but recovered during the descent. The symptoms of the ' mountain sickness ' so familiar to Alpine cUmbers (nausea, headache, and marked depression), the undue hyperpnoea produced by muscular exertion, and the sleep disturbed by irregular breathing, are also mainly due to deficiency of oxygen in the blood. The most rational prophy- laxis is to leave the high peaks severely alone. But for the enthusiasts who cannot do this a portable apparatus for generating oxygen has been devised. Experiments in the pneumatic cabinet indicate that the hyperpnoea is due to the indirect action of want of oxygen already referred to in discussing the normal regulation of respiration (p. 275) —that is, to the formation, in consequence of the insuflfiicient oxygen supply, of lactic acid or other substances which have the same influence as carbon dioxide on the respiratory centre — so that less carbon dioxide is required to excite the centre. Although the hyperpnoea leads to a diminution in the partial pressure of carbon dioxide in the pulmonary alveoli, there is no evidence that lack of carbon dioxide (' acapnia ') is the primary cause of mountain sickness (Haldane) . It must be remem- bered, however, that here the influence of the low barometric pressure is complicated by other conditions. For example, while in the pneu- matic cabinet, as already stated, diminution of the pressure does not afiect the oxygen consumption, it is relatively much greater on the high mountain levels both during rest and during work than on the plains. This is not the case in balloon ascents. And evidence has been brought forward that changes in the mechanics as well as in the chem- istry of respiration are concerned (the breathing, for instance, taking on a periodic character, with some approach to the Cheyne-Stokes type — p. 282), and that there is something not connected with the want of oxygen which diminishes the capacity for muscular work. This ' some- thing ' is perhaps a peculiar excitation of the nervous system in the fierce light of those high levels, which acts not only on the retina, but on the skin, and may even .affect the distribution of the blood. It is said that a so-called light bath, as used in the treatment of certain diseases, may increase the quantity of blood in rabbits by 25 per cent, in four hours. The shorter wave-lengths which are relatively more intense in the mountain light are most effective. Section IX. — Cutaneous Respiration. It has already been remarked that a frog survives the loss of its lungs for some time, respiration going on through the skin. Indeed, it has been calculated that in the intact frog, under ordinary conditions, as much as three-quarters of the total gaseous exchange may be cutaneous. Two frogs were seen to live thirty-three days, and one even forty days, after excision of the lungs. The effect of exclusion of the pulmonary respiration on the gaseous exchange depends on the previous intensity of the metabolism. If this is high the gaseous exchange sinks markedly; if it is low there is scarcely any alteration. At their maximum efficiency CUTANEOUS RESPIRATION 293 the frog's lungs are capable of sustaining a much greater exchange than the skin. Besides this quantitative, there is a qualitative differ- ence, the carbon dioxide passing more easily through the skin than the oxygen, so that the respiratory quotient is increased by elimination of the lungs. In mammals the structure of the skin is different, and respiration can only go on through it to a very slight extent. The amount of carbon dioxide excreted in man, although only about 4 grm. or 2 litres in twenty-four hours, is much greater than corresponds to the quantity of oxygen absorbed through the skin. It has been as- serted, and no doubt with justice, that some at least of the carbon dioxide given off is due to putrefactive processes taking place on the surface of the body. Such processes, as has already been pointed out, seem alsQ responsible in part for the heavy odour of a ' close ' room. For no harmful products appear to be exhaled from the skin when it is properly cleansed. In spite of the romantic statements to the con- trary in ancient and modem books (for instance, the story of the child that was gilded to play the part of an angel at the coronation of a medieval pope, but died before the ceremony began), the whole of the human skin may be coated with an impermeable varnish without any ill effects. The entire surface of the body of a patient with cutaneous disease was covered with tar, and kept covered for ten days. There was not the least disturbance of any normal function. The serious effects of varnishing the skin. in animals are due, not to retention of poisonous substances, but to increased heat loss. Varnishing is not so rapidly harmful in larg; animals like dogs as in rabbits, which have a relatively great surface and a delicate skin. The danger of widespread superficial burns is well known. But it is not due to diminished excretion by the skin, for death occurs when large cutaneous areas remain uninjured. The patient nearly always dies when a quarter of the whole skin is burnt; yet the remaining three-quarters may surely be considered capable, from all analogy, of making up the loss by increased activity. One kidney is enough to eliminate the products of the nitrogenous metabolism of the whole body. It is difficult to see why the excretion of the trifling amount of solid matter in the perspiration should be interfered with by the loss of 25 per cent, of the sweat-glands. The real explanation of the serious effects of extensive superficial bums is perhaps the excessive irritation of the sensory nerves, which may lead to changes in the nervous centres. Or reflexly in other organs, or the chemical changes in the damaged tissue, for example, in the blood- corpuscles, or the transudation of lymph at the injured part, and con- sequent increase in the concentration of the blood. PRACTICAL EXERCISES ON CHAPTER IV. I. Tracing of the Respiratory Movements in Man. — Pass a tape through the rings B of the stethograph shown in Fig. 134, and then around the neck or over the shoulders, so as to support the instrument on the chest at a convenient height. Fasten tapes to the hooks and tie them by a slip-knot round the chest. The tube E is •^o^ected to a recording tambour, writing on a drum. Or use the belt stethograph or spirograph of Fitz (p. 232), fastening the elastic tube ™™d the chest with the chain, and connecting it with a tambour or the bellows recorder shown in Fig. 137. Compare the extent of the excursion when the tube is adjusted at different levels over the thorax and abdomen. 294 RESPIRATION 2.* Production of Apnoea and Periodic Breathing in Man. — Arrange for taking tracings of ■tiie respiratory movements from a fellow-student as in I. Let the subject of the experiment recline in a -perfectly easy position in an armchair. Let him then breathe deeply and frequently for about two minutes, so as to produce a prolonged apnoea of about two minutes' duration. Whenever any desire to breathe returns, the breathing is to be allowed to take its own course. It may be expected at first to be of the pariodic (Cheyne-Stokes) type. 3. Tracing of the Respiratory Movements in Animals.^(a) Set up the arrangement shown in Fig. 135, and test whether it is air-tight. Have also in readiness an induction machine and electrodes arranged for an interrupted current. Anaesthetize a rabbit with chloral or ether (p. 216), or a small dogf with morphine and ether, or A.C.E. mixture. Insert a cannula into the trachea (p. 199), and connect it with the large bottle by a tube. Connect the bottle with a recording Fig. 134. — Stethograph. tambour adjusted to write on a drum, and regulate the amount of the excursion of the lever by slackening or tightening the screw-clamp. Set the drum off at slow speed, and take a tracing. (6) Then disconnect the cannula from its tube. Dissect out the vagus in the lower part of the neck, pass a ligature under it, but do not tie it. Connect the cannula again with the bottle, and while a tracing is being taken ligature the vagus. Cut below the ligature and stimulate its central end with weak shocks, marking the time of stimulation on the drum. Repeat the stimulation with strong shocks, and observe the results. (c) Apply a strong solution of potassium chloride with a camel's- hair brush to the central end of the vagus while a tracing is being taken, and observe the effect. * This experiment is only to be attempted under the direct supervision of the demonstrator. f If a large dog is used the bottle should be omitted, the tracheal cannula being connected with the stem of a T-tube. One end of the horizontal limb of the T-tube is connected with the tambour; the other is provided with a rubber tube, which can be partially closed by a screw-clamp to regulate the excursion. Ether may be given when required by connecting the horizontal limb of the T-tube with a bottle with two glass tubes in the cork (p. 199). PRACTICAL EXERCISES 295 [d] Isolate the sciatic nerve (p. 210), ligature it, and cut below the ligature. Stimulate its central end while a tracing is being taken. The respiratory movements will be increased. («) Disconnect the cannula, and isolate the vagus on the other side. While a tracing is being taken, divide it. The respiratory movements will probably at once become deeper and less frequent. (j) Again disconnect the cannula. Isolate the superior laryngeal branch of the vagus. This will be found entering the larjrnx at the point where the laryngeal horn of the hyoid bone is connected with the thyroid cartilage. If the finger is passed back along the upper border ^rachtral Cann, Fig- 135- — ^Arrangement for Respiratory Tracing. Two glass tubes are inserted through a cork in the mouth of the large bottle. One of them has a small piece of indiarubber tubing on it, which is closed or opened, as may be required, by a screw-clamp. The other is^connected by a rubber tube with a recording tambour. The tubulure at the bottom of the bottle is closed by a cork, through which passes a glass tube, connected by a rubber tube with the tracheal cannula. If no bottle with tubulure is available, it is only necessary to pass tlirough the cork, down to the bottom of the bottle, a third glass tube, which' is connected with the tracheal cannula. While a tracing is being taken the animal breathes the air contained in the bottle. When this becomes vitiated the respiratory movements are exaggerated and a normal tracing is no longer obtained. For this reason the tracheal cannula must be connected with the bottle only at the moment when a tracing is to be taken. The arrangement is most suitable for a small animal. of the thyroid cartilage, this point will easily be felt. Ligature the nerve, and divide it between the larynx and the ligature. Reconnect the cannula. Take a tracing first with weak, and then with strong stimulation of the central end of the superior laryngeal. (?) Make an incision through the abdominal wall in the linea alba, and study the movements of the diaphragm. Find the nerves from which the phrenics take origin in the neck. In the dog they arise from the fifth, sixth, and seventh cervical nerves. Divide the phrenic fibres on one side, and observe that the diaphragm on the corresponding side IS now paralyzed. 296 RESPIRATION (h) Insert a cannula into the carotid artery. While a respiratory tracing is being taken, allow blood to flow from the artery. Dyspnoea and exaggeration of the respiratory movements will be seen when a coiisiderable quantity of blood has been lost. Mark and varnish the tracings. In the whole of this experiment the tracheal cannula is to be dis- connected, except when the lever is actually writing on the drum, in order that the period during which the animal must breathe into the confined space of the bottle may be diminished as much as possible. Instead of the method described, the stethograph shown in Fig. 136 may be used to obtain respiratory tracings from animals, a broad canvas band being put round the animal's chest. To each end of this band is clamped with suf&cient tension a strong thread (F), fastened to a small metal disc on the in.side of the rubber dam closing the obliquely- cut ends of the metal cylinder D. The tube G is connected with a, tambour or with a bellows recorder (Fig. 137). 4. The Effect of Temperature on the Respiratory Centre— Heat Dysp- noea. — Set up an arrangement for Fig. 136. — Stethograph (Crile). Fig. 137. — Bellows Recorder. B, a lead tube connected with the small bellows A, which consists of a light wooden base and top, to which is cemented very flexible (organ key) leather, properly creased for expansion and con- traction; C, writing lever. taking a respiratory tracing as in 2 (footnote, p. 294). Anaesthetize a dog, and fasten it, back downward, on a holder. Make an incision in the middle line of the neck, com- mencing a little below the cricoid cartilage, and extending down for 4 or 5 inches. Insert a cannula into the trachea. Isolate both carotid arteries for as great a distance as possible, and arrange them on the brass tubes shown in Fig. 138. Connect two adjacent ends of the tubes by a short rubber tube. Connect one of the remaining ends to a funnel, supported on a stand, and the other to a rubber tube hanging over the table above a large jar. Slip two or three folds of paper between the tubes and the vagus nerves. Heat two or three litres of water to about 65° C. (a) Now connect the tracheal cannula with the tambour. As soon as the tracing is under way, let the hot water run through the funnel and tubes into the jar. Mark on the tracing the point at which the flow of the hot water was begun, and go on passing it until it has produced an efiect. Then stop the drum, and circulate water at the ordinary temperature till the breathing is again normal. Then, while a tracing is being taken, pass ice-cold water through the tubes, and again notice the effect. PRACTICAL EXERCISES 297 (6) Expose the sciatic. Pass ice-water through the tubes, and while a respiratory tracing is being taken stimulate its central end with in- duction shocks so weak as just to cause ail effect. Pass water at air temperature through the tubes, and repeat the stimulation with the coils at the same distance. Do the same while hot water is being passed through the tubes, and compare the results. Always allow the water to pass for a time before making an observation. 5. Measurement of Volume of Air inspired or expired — ^Vital Capacity. — A spirometer (Fig. 114, p. 234) of sufficient accuracy for this experi- ment can be made by removing the bottom of a large bottle with a capacity of not less than 4 litres. A good cork, through which passes a glass tube connected with a rubber tube, is fitted into the neck. The bottle is fixed vertically, mouth downwards, the glass tube being closed for the time, and graduated, by pouring in measured quantities of water, say 100 c.c. at a time, and marking the level. The divisions are then etched in. If the cork does not fit air-tight, it is covered with wax. The bottle is swung on two pulleys, counterpoised and immersed, bottom down, in a large glass jar or a small cask nearly full of water. A smaller bottle may be used for the determination of the tidal air, so as to reduce the error of reading. , (i) Submerge the bottle -A- to the stopper, after opening Fig. 138. — Arrangement for Heating or Cooling the pinchcock on the rub- ber tube. Breathe into the bottle, close the cock, ad- just the bottle so that the level of the water is the same inside and outside, and then read off the level. De- termine the volume of air expired in — (a) A normal expiration after a normal inspiration (tidal air) ; the Blood in the Carotid Arteries. A, cylin- drical portion of tube; B, flattened portion in the groove, between which and A the artery lies ; C, cross-section, showing the lumen extend- ing into B; D, rubber tube attached to a brass tube soldered into A. The other end of A h^s a similar brass tube soldered into it {not shown in the figure). This is connected by a rubber tube with a similar apparatus, on which the other carotid lies. D is connected with a funnel containing hot or cold water or with the outflow tube, as the case may be. expiration after a normal inspiration after the greatest possible (6) The greatest possible (supplemental air plus tidal air) (c) The greatest possible expiration inspiration (vital capacity). (2) Open the cock and raise the bottle till it is nearly full of air. Determine the volume of air inspired in — (a) A normal inspiration after a normal expiration (tidal air) ; {b) The greatest possible inspiration after a normal expiration (complemental air plus tidal air) ; (c) The greatest possible inspiration after the greatest possible expiration (vital capacity). Make several observations of each quantity, and take the mean. (3) Count the rate of respiration for three minutes, keeping the breathing as nearly normal as possible; repeat the observation; and from the mean result and the amount of the tidal air calculate the quantity of air taken into the lungs in twenty-four hours (pulmonary ventilation). 6. Cardio-Pneumatic Movements. — Fill a U-tube with tobacco- smoke. One end of the tube is placed in the nostril of a fellow-student. 298 RESPIRATION and made tight with a Uttle cotton-wool. The other nostril and the mouth are closed, and respiration suspended. The column of smoke moves in and out at each beat of the heart. By feeling the apex-beat, try to verify the fact that during systole the cardio-pneumatic movement is inspiratory, and in diastole expiratory. 7. Auscultation of the Lungs. — ^This is taught in the course of physical diagnosis, but in connection with the subject the student may perform the following experiment on a dog used for some other purpose: Open the trachea as described on p. 199. Insert into it the cross-piece of a glass T-tnbe of as large a bore as possible, t3nmig the trachea over it on each side of the stem. The stem projecting from the wound is armed with a short piece of rubber tubing, which can be closed at will with a clip. When the tube is thus closed the animal breathes through the glottis in the ordinary way. When the tube is open, and the mouth and nose covered tightly with a cloth, no air goes through the glottis. The tube being closed, Usten with the stethoscope or the ear alone over a part of the chest where the vesicular murmur is well heard. If the Fig. 139. — Haldane's Apparatus for measuring the Quantity of COa and Aqueous Vapour given off by an Animal. A, chamber into which the animal is put; I and 4, Woulff's bottles filled with soda-lime to absorb carbon dioxide; 2, 3. and 5, Woulff's bottles filled with pumice-stone soaked in sulphuric acid to absorb watery vapour; B, glass bell-jar suspended in water, by means of which the negative pressure is known ; P, water-pump which sucks air through the appar- atus; I and 2 are simply for absorbing the carbon dioxide and water of the ingoing air. rubbing of the hairs below the stethoscope causes disturbing sounds, shave a portion of the skin. Continue hstening while an assistant closes the tube and covers up the animal's muzzle. Determine whether any change takes place in the vesicular sound. Repeat the obssrvation while hstening over the lower part of the trachea, and determine whether any change takes place in the bronchial breathing sound. 8. Respiratory Pressure. — Connect a strong rubber tube with a glass bulb, and the bulb with a mercurial manometer provided with a scale, (i) Fasten the tube with a little cotton-wool in one nostril, breathe through the other with closed mouth, and observe the amount by which the level of the mercury is altered in ordinary inspiration and ex- piratonv (2) Repeat the observation with forced breathing, pinching the tube at the height of inspiration and expiration, and reading ofi the maximum inspiratory and expiratory pressure. PRACTICAL EXERCISES 299 (3) Repeat (i) with the tube connected to the mouth by a glass tubs held between the lips, and the nostrils opsn. (4) Repeat (2) with the tube in the mouth and the nostrils closed. 9. Estimation of the Quantity of Water and of Carbon Dioxide given off by an Animal (Haldane's Method). — (i) Connect the apparatus shown in Fig. 139 with the water-pump. Allow a negative pressure of 5 or 6 inches of water to bs established in it, as shown by the rise of water in the bell-jar B. Then close the open tube of carbon dioxide bottle I, and clamp the tube between the water-pump and the bell-jar. If the negative pressure is maintained, the arrangement is air-tight. Now weigh bottle 3 and bottles 4 and 5, the last two together. Place a cat in the respiratory chamber A, connect the chamber directly with the water-pump, and test whether it is tight. Then take the stopper out of bottle I, and adjust the rate at which air is drawn through the apparatus. Let the ventilation go on for a few minutes, then insert bottles 3, 4, and 5 again. Note the time exactly at this point, and after an hour dis- connect 3, 4, and 5, and again weigh. The difference of the two weighings of 3 shows the quantity of water given off by the animal in an hour ; the dif- ference in the combined weight of 4 and 5, the quantity of carbon dioxide . Weigh the cat , and calculate the amount of water and of carbon dioxide given ofi per kilo per hour. (2) For the student it is more convenient to use smaller animals. The mouse may be taken as an example of a warm-blooded animal, and the frog of a cold-blooded. Insteaid of the WoulfE's bottles use wide test - tubes connected as in Fig. 140, and for the animal chamber a small beaker, closed with a ^ery carefully fitted cork which has been boiled in paraffin. The inlet and outlet tubes of the chamber are to be introduced through this cork. The holes for these are to be bored with the greatest care, and the tubes to be put in while the cork is still hot from boiling in paraflin. Also insert a thermom- eter about 6 inches long registering from 0° C. to 45° C. Modeller's wax is to be used finally to render all the junctions air-tight. Add to the series of tubes described in the apparatus a single tube containing baryta-water. This is placed to the left of tube 5, and so arranged that the air-current bubbles through the water. As long as the absorption of carbon dioxide is complete, the baryta-water remains clear. Beyond this a water-bottle should be placed to act as a valve and to indicate the negative pressure in the apparatus. It can be most simply constructed by using a cylinder of stout glass tubing in a wide-mouthed bottle containing some water, the inlet and outlet tubes passing through a paraffined cork which seals the upper end of the cylinder. Fig. 140. — Absorption Tubes for CO^ and Moisture. A, soda -lime tube; B, jSul- phuric acid tube; C, wooden frame, in which A and B are supported by wires d ; b, wire hook, which grips the glass tube firmly, and by means of -ivhich the tubes are lifted out of the frame in order to be weighed; u, short piece of glass tubing, by taking out which the absorption tubes are disconnected from the rest of the apparatus ; e, glass tube going off to animal chamber. 30° RESPIRATION Before making an observation, test whether the apparatus is air- tight, as explained above, after introducing the animal into the cham- ber, seaUng the latter with wax, and connecting it with the absorption- tubes. But a negative pressure of 2 or 3 inches of water is a sufficient test for the small apparatus. To make an observation, set the air-current going at the desired rate. Allow it to run for a few minutes till the carbon dioxide, which has accumulated during the testing, has been swept out. At a time which has been decided on and noted, stop the current by disconnecting the water-pump. Disconnect and stopper up the animal chamber, and weigh it as quickly as possible. Connect up again, using only recently- weighed absorption-tubes, and finally connect with the water-pump and allow the current to pass for a definite period, say an hour. The soda-lime should not be too dry, or absorption is not sufficiently rapid. The following facts are made out: (a) Loss of weight by the animal chamber (chiefly loss by the animal) ; (6) gain of the sulphuric acid tube in water; (c) gain of the soda-lime tubes in carbon dioxide. The total loss and total gain do not correspond, the gain being always greater than the loss. The surplus can only be oxygen absorbed by the animal and added to the hydrogen and carbon of its substance to form water and carbon dioxide. Calculate the respiratory quotient (p. 240). 10. Muscular Contraction in the Absence of Free Oxygen (see p. 265). — Pith a frog (brain and .cord). Cut off one hind-leg at the middle of the thigli, and strif the skin from it. Pass a thread under the tendo Achillis, tie it, and divide the tendon below it. Free the tendon and the gastrocnemiud muscle from the loose connective tissue lying between them and the bones of the leg, and divide the latter just below the knee: Remove superfluous thigh muscles, and fasten the gastrocnemius in a moist chamber by means of the femur. Attach the thread on the tendon to a lever. Connect the poles of the secondary coil of an induc- tion machine by fine copper wires to the femur and the tendon. Put a battery and simple key in the primary, and arrange it for single shocks. Stimulate the muscle and observe the height of the contraction. Now pass into the chamber a current of washed hydrogen gas from a bottle containing granulated zinc, upon which a little dilute sulphuric acid is poured from time to time. The air in the moist chamber will soon be entirely displaced by the hydrogen, but the muscle will contract on being stimulated, and the stimulation can be repeated many times. 11. Oxidizing Ferments. — ^Wash out the bloodvessels of a dog or rabbit (Practical Exercises, p. 65). Chop up finely portions of pancreas, spleen, muscle, lungs, and ladney, keeping each separate, and avoiding any contamination of one by another. Grind up half of each portion with sand in a small mortar, and extract with a small quantity of water, keeping all the extracts separate. Into each of eleven test-tubes put 10 c.c. of a colourless dilute alkaline solution of paraphenylenediamin and d-naphthol (freshly made by mixing solutions of the two sub- stances in equimolecular proportions* and adding a little sodium carbonate). To five of the tubes add the chopped organs, to five the watery extracts of the organs, and enough water to make the volume equal in all the tubes. To the remaining tube add the same amount of water. Observe in which tube a change of colour takes place (p. 268). * I.e., the weight of each of the two substances in the mixture should bs proportional to its molecular weight. A convenient solution contains 01 44 per cent, of a-naphthol and o'loS per cent, of paraphenylenediamin. These quantities are one-hundredth-molecular. Sodium carbonate is added to the amount of 025 per cent. The a-naphthol can be kept as a i per cent, solu- tion in 50 per cent, alcohol. CHAPTER V VOICE AND SPEECH Voice. — Sounds of various kinds are frequently produced by the movements of animals as a whole, or of individual organs. The muscular sound, the sounds of the heart and of respiration, we have already had to speak of. Such sounds may be considered as purely accidental as the footfall of a man or the buzzing of a fly. The wings 6f an insect beat the air, not to cause sound, but to produce motion; the respiratory murmur is a mere indication that air is finding its way into the lungs, it is in no way related to the oxidation of the blood in the pulmonary capillaries. But in many of the higher animals mechanisms exist which are specially devoted to the utterance of sounds as their prime and proper end. In man the voice-producing mechanism consists of a triple series of tubes and chambers: (i) The trachea, through which a blast of air is blown; (2) the lar5mx, with the vocal cords, by the vibrations of which sound-waves are set up ; and (3) the upper resonance chambers, the pharynx, mouth, and nasal cavities, in which the sounds produced in the larynx are modified and intensified, and in which independent notes and noises arise. The larynx is a cartilaginous box, across which are stretched, from front to back, two thin and sharp-edged membranes, the (true) vocal cords. In front the cords are attached to the thyroid carti- lage, one a little to each side of the middle line; behind they are connected to the vocal or anterior processes of the pyramidal arytenoid cartilages. The thyroid and the two arytenoids are mounted upon a cartilaginous ring, the cricoid. The arytenoids can rotate on the cricoid about a vertical axis, while the cricoid can rotate on the thyroid cartilage around a transverse horizontal axis. The cricoid can thus be raised by the contraction of the crico- thyi-oid muscle, and the vocal cords stretched. By the pull of the posterior crico-arytenoid muscles, attached to the external or mus- cular processes of the arytenoid cartilages, the vocal processes are rotated outwards, the cords separated from each other or abducted, and the chink between them, the rima glottidis, widened. When the vocal processes are approximated by contraction of the lateral 301 302 VOICE AND SPEECH crico-arytenoid muscles and the consequent forward movement of the muscular processes, the- vocal cords are brought close together, or adduded, and the rima is narrowed. The transverse or posterior arytenoid muscle, which connects the two arjrtenoid cartilages behind, also helps, by its contraction, to narrow the glottis by shift- ing the cartilages on their articular surfaces somewhat nearer the middle line. Running in each vocal cord, and, in fact, incorporated with its elastic tissue, is a muscle, the thjnro-arytenoid, the external portion of which may to some extent cause inward rotation of the vocal processes and adduction of the cords; but the main function, at least of its inner part, is to alter the tension of the cords. The diagrams in Figs. 141 and 142 illustrate the action of the abductors and adductors of the vocal cords. The crico-thyroid muscle and the deflectors of the epiglottis are supplied by the superior laryngeal branch of the vagus, which also Fig. 141. — Diagrammatic Hori- zontal Section of Larynx to show the Direction of Pull of the Posterior Crico-Arytenoid Muscles, which abduct the Vocal Cords. Dotted lines show position in abduction. Fig. 142. — Direction of Pull of the Lateral Crico-Arytenoids, which adduct the Vocal Cords. Dotted lines show position in adduction. contains the sensory fibres for the mucous membrane of the larynx above the vocal cords. In the dog and rabbit motor fibres also reach the crico-thyroid by the so-called middle laryngeal nerve which arises from the superior pharyngeal branch of the vagus. All the other intrinsic muscles are supplied by the recurrent laryngeal branch of the vagus. It receives these motor fibres from the spinal accessory, and supplies sensory fibres to the mucous membrane of the larynx below the vocal cords and to the trachea. The voice is produced, like the sounds of a reed instrument, by the rhythmical interruption of an expiratory blast of air by the ■vibrating vocal cords. When a bell is struck, vibrations are set up in the metal, which are communicated to the air. It is not the same with the vibrations of the vocal cords; if they were plucked or struck, they would only produce a feeble note. The air in the mouth, pharynx, larynx, trachea, and lungs is the real sounding VOICE 303 • body; a pulse of alternate rarefaction and condensation is set up in it by the interference, at regular intervals, of the vocal cords with the expiratory blast. Forced abruptly from their position of equi- hbrium as the blast begins, they almost immediately regain and pass below it, in virtue of their elasticity, and continue to vibrate as long as the stream of air continues to issue in sufftcient strength. Not only do they vibrate up and down, but also towards and away from the middle line, so that, at least in the chest voice, they come into contact with each other at each swing. The sound-waves thus set up spread out on every side, impinge on the tympanic membrane, set it quivering in response, and give rise to the sensation of sound. We may say, in a word, that the whole exquisite mechanism of cartilages, ligaments, and muscles, has for its object the production of a sufficient pressure in the blast of air driven through the wind- pipe by an expiratory act, and of a suitable tension in the vibrating cords. An approximation of the cords, a narrowing of the glottis, is essential to the production of voice; with a widely-opened glottis the air escapes too easily, and the necessary pressure cannot be attained. The pressure in the windpipe was found in a woman with a tracheal fistula to be about 12 mm. of mercury for a note of medium height, about 15 mm. for a high note, and about 72 mm. for the highest possible note. The period of vibration of structures like the vocal cords depends on their length, thickness, density and tension ; the shorter, thinner, more dense and less tense a stretched string is, the greater is the vibration frequency, the higher the note. In the child the cords are short (6 to 8 mm.), in woman longer (10 to 12 mm. when slack, 13 to 15 mm. when stretched), in man longest of all (14 to 18 mm. in the relaxed, and 18 to 22 mm. in the stretched position) ; and the lower Hmit of the voice is fixed by the maximum length of the relaxed cords. A boy or a woman cannot utter a deep bass note, because their vocal cords are relatively short, and do not vibrate with sufficient slowness. It is true that by the action of the crico-thyroid muscle the cords can be length- ened, and that the maximum length in a woman approaches or exceeds the minimum length in a man. But the lengthening of the vocal cords in one and the same individual is always accompanied by other changes — increase of tension, decrease of breadth and thickness — which tell upon the vibration frequency in the opposite way, and more than compensate the effect of the increase of length, so that for high notes the cords are longer than for low. The con- traction of the thyro-arytenoid muscle is a more influential factor in altering the tension of the cords than the contraction of the crico- thyroid. It is probable that, when the highest notes are uttered, only the anterior portions of the cords are free to vibrate, their posterior portions being damped by the approximation of the vocal processes of the arytenoid cartilages by the contraction of the 304 VOICE AND SPEECH lateral crico-arytenoid and transverse arytenoid muscles. The range of an ordinary voice is 2 octaves; by training 2 J octaves can be reached; but in exceptional cases a range of 3, and even 3 J, octaves (as in the celebrated singer Cat9,lini) has been known. The development of the voice in children is of great interest. At the age of six years the boy's voice has a rather narrower range than the girl's in both directions. The boy's voice reaches its full height in the twelfth and its full depth in the thirteenth year, when the range is almost 3 octaves, its upper limit being a semitone higher than the girl's, but its lower limit a whole tone deeper. When the voice ' breaks ' in boys at the age of puberty it falls about an octave. The control of the vocal organs becomes so incomplete that only in one-fourth of the cases can notes of sufficient steadiness to be used in music be produced. The vocal cords, as may be seen with the, laryngoscope, are frequently, though not always, congested. The pitch of a note, while it depends chiefly, as has been said, on the tension of the vocal cords, rises and falls somewhat with the strength of the expiratory blast ; the highest notes are only reached with a strong expiratory effort. The intensity of all vocal sounds is determined by the strength of the blast, for the ampUtude of vibration of the cords is proportional to this. Besides pitch and intensity, the ear can still distinguish the quality or timbre of sounds; and the explanation is as follows: Two simple tones of the same pitch and intensity, that is, the sounds caused by two series of air- waves of the same period and amplitude — of the same frequency and height, to use less technical terms — ^would appear absolutely identical to the sense of hearing; just as the aerial disturbances on which they depend would be absolutely alike to any physical test that could be applied. But no musical instrument ever produces sound-waves of one definite period, and one only; and the same is true of the voice. When a stretched string is displaced in any way from its position of rest, it is set into vibration ; and not only does the string vibrate as a whole, but portions of it vibrate independently and give out separate tones. The tone corresponding to the vibra- tion period of the whole string is the lowest of all. It is also the loudest, for it is more difficult to set up quick than slow vibrations. The ear therefore picks it out from all the rest ; and the pitch of the compound note is taken to be the pitch of this, its fundamental tone. The others are called partial or overtones, or harmonics of the fundamental tone, their vibration frequency being twice, three times, four times, etc., that of the latter. Now, the fundamental tone of a compound note or clang produced by two musical instru- ments may be the same, while the number, period, and intensity of the harmonics are different ; and this difference the ear recognizes as a difference of timbre or quality. The timbre of the voice de- pends for the most part on partial tones produced or intensified in the upper resonance chambers. / voicn 305 A great deal of our knowledge as to the mode and mechanism of the production of voice has been acquired by means of the laryngo- scope (Fig. 143). This consists of a small plane mirror mounted on a handle, which is held at the back of the mouth in such a position that a beam of light, reflected from a larger concave mirror fastened on the forehead of the observer, is thrown into the larynx of the patient. The observer looks through a hole in the centre of the large mirror; and an image of the interior of the larynx is seen in the small mirror, in which the parts that are anterior appear as posterior, the arytenoid cartilages in front, the thyroid behind, and the vocal cords stretching between. The small mirror is warmed to body-temperature before being introduced, so as to prevent the condensation of moisture on it. The tendency to retch, which is Lamp Concave M/rror JLaryoxi Fig. 143. — Diagram of Laryngoscope. caused by contact of the instrument with the soft palate, may be removed or lessened by the application of a solution of cocaine. Examined with the laryngoscope during quiet respiration, the glottis is seen to be moderately, though not widefly, open, and the vocal cords almost motionless. Although the portion between the arytenoid cartilages has received the name of glottis respiratoria, in contradistinction to the glottis vocalis between the vocal cords, the rlma in its whole extent from front to back is really concerned in ther respiratory act. In deep expiration the vocal cords come nearer to the middle Une, and the glottis is narrowed; in deep inspiration they are widely separated, and the rings of the trachea, and even its bifurcation, raiy be disclosed to view. When a sound is produced —a note sung, for example— the cords are approximated (Figs. 144 and 145) ; and with a high note more than with a low. 306 VOICE AND SPEECH The essential difierence between the production of notes in the lower register, or chest voice, and in the higher register, or falsetto, has been much debated. The lowest notes which can be uttered by any given voice are chest notes, the highest are falsetto notes; but there is a de- batable land common to both registers, and medium notes can be simg either from the chest or from the head. Chest notes impart a vibration or fremitus to the thoracic walls, from the resonance of the lower air- chambers, the trachea and bronchi; and this can be distinctly felt by the hand. In head notes or falsetto the resonance is chiefly in the upper cavities, the pharynx, mouth, and nose. As to the mechanical conditions in the larynx, there is a pretty general agreement that during the production of falsetto notes the vocal cords are less closely approxi- mated than in the sounding of chest notes. The escape of air is conse- quently more rapid in the head voice, and a falsetto note cannot be niaintained so long as a note sung from the chest. But it is only the anterior part of the rima glottidis that is wider in the falsetto voice; the whole of the glottis respiratoria, and even the posterior portion of the glottis vocalis, are closed during the emission of falsetto notes. Fig. 144. — Position of tlie Glottis preliminary to the Utterance of Sound, rs, false vocal cord; ri, true vocal cord; ar, arytenoid cartilage; 6, pad of the epiglottis. Fig. 145. — Position of Open Glottis. I, tongue; e, epiglottis; ae, ary- epiglottidean fold; c, cartilage of Wrisbeirg; ar, arytenoid cartilage; 0, glottis; V, ventricle of Mor- gagni; ti, true vocal cord; ts, false vocal cord. Oertel has stated, and the statement has been confirmed by others, that the free edge of the vocal cord alone vibrates in the falsetto voice, one or more nodes or motionless lines parallel to the edge being formed by the contraction of the internal part of the thyro-arytenoid muscle, which thus acts like a stop upon the cord. Approximation of the vocal cords may take place in certain acts unconnected with the production of voice. Thus, a cough, as has already been mentioned, is initiated by closure of the glottis. During a strong muscular effort, too, the chink of the glottis is obliterated, and respiration and phonation both arrested. The object of this is to fix the thorax, and so afford points of support for the action of the muscles of the limbs and abdomen. But con- siderable efforts can be made even by persons with a tracheal fistula. Speech. — Ordinary speech is articulated voice — voice shaped and fashioned by the resonance of the upper air-cavities, and jointed SPEECH 307 together by the sounds or noises to which the varying form of these cavities gives rise. Here we come upon the fundamental distinction between vowels and consonants. Vowels are musical sounds; con- sonants are not musical sounds, but noises— that is to say, they are due to irregular vibrations, not to regularly recurring waves, the frequency of which the ear can appreciate as a definite pitch. This difference of character corresponds to a difference of origin: the vowels are produced by the vibrations of the vocal cords ; the con- sonants are due to the rushing of the expiratory blast through certain constricted portions of the buccal chamber, where a kind of temporary glottis is established by the approximation of its walls. One of these ' positions of articulation ' is the orifice of the lips; the consonants formed there, such as p and b, are called labials. A second articulation position is between the anterior part of the tongue and the teeth and hard palate. Here are formed the dentals, t, d, etc. The ordinary English r, and the r of the Berwickshire and East Prussian ' burr,' also arise in this position through a vibratory motion of the point of the tongue. The third position of articula- tion is the narrow strait formed between the posterior portion of the arched tongue and the soft palate. To the consonants arising here the name of gutturals has been given. They include k, g, the Scottish ch, and the uvular German r. The latter is produced by a vibration of the uvula. The aspirated h is a, noise set up by the air rushing through k moderately wide glottis, apd some have there- fore included the glottis as a fourth articulation position for con- sonants. Certain sounds like n, m, and ng, when final (as in pen, dam, ring), although produced at the glottis, are intensified by the resonance of the air in the nose and pharynx, and are sometimes spoken of as nasal consonants. As we have said, the vowels are produced by vibrations of the vocal cords, but to what they owe their special timbre or quaUty has been much discussed. According to the view with which Helm- holtz's name is particularly connected this is due to the reinforce- ment of certain overtones by the resonating cavities, the shape and fundamental tone of which are different for each vowel. When a vowel is whispered, the mouth assumes a characteristic shape, and emits the fundamental tone proper to the form and size of the particular ' vowel-cavity,' not as a reinforcement of a tone set up by the vibrations of the vocal cords, but in response to the rush of air through the cavity; just as a bottle of given shape and size gives out a definite note when the air which it contains is set in vibration, by blowing across its mouth. A whisper, in fact, is speech without voice; the larynx takes scarcely any part in the production of the sound ; the vocal cords remain apart and comparatively slack; and the expiratory blast rushes through without setting. them in vibration. The fundamental tone of the ' vowel-cavity ' may be found for each, vowel by placing the mouth in the position necessary for uttering it, then bringing tuning-forks of different period in front of it, and noting 308 VOICE AND SPEECH which of them sets up sympathetic resonance in the air of the mouth, and so causes its sound to be intensified. The fundamental tone is lowest for m (as in lute). Next comes o ; then a (as in path) ; then a (as in fane) ; then i ; while e is highest of all. A simple illustration of this may be found in the fact that when the vowels are whispered in the order given, the pitch rises. When « or o is sounded, the buccal cavity has the form of a wide-belUed flask, with a short and narrow neck for u, a still shorter but wider neck for o. For e the tongue is raised and almost in contact with the palate, and the cavity of the mouth is shaped like a flask with a long narrow neck and a very short belly. For i the shape is similar, but the neck is not so narrow. For a (as in path) the vowel-cavity is intermediate in form between that of u and e, being roughly funnel-shaped, and the mouth is rather widely opened. For u (oo) the resonating cavity is made as long as possible, the larynx being depressed and the lips protruded; for e the resonating cavitj is at its shortest, the larynx being raised as much as possible and the lips retracted (Figs. 146 to 148). According to Helmholtz, all that the resonating cavity does is to strengthen certain of the partials or overtones of the laryngeal note. Fig. 146. Fig. 147. Fig. 148. If this is true, the partials which give a vowel-sound the timbre by which we recognize it as difierent from other vowel-sounds cannot preserve the same numerical relation to the fundamental tone when the pitch of the latter is altered. Suppose, for example, that a given vowel is sounded with a pitch corresponding to 100 vibrations a second, and that the partial which is particularly strengthened by the resonance of the mouth cavity is the fifth overtone, corresponding to 600 vibra- tions. Then when the same vowel is sounded with a pitch of 200 vibra- tions, the reinforced partial which will now give the quality to the sound will still correspond to 600 vibrations a second, since this is the rate which most easily elicits the resonance, but it will not now' be the fifth but the second overtone. Universally accepted for a time, the Helmholtz theory has been in recent years assailed, especially by Hermann, who bases his criticism on microscopic examination of curves obtained by the Edison phono- graph, and on reproductions of such records obtained by photographing on a moving drum covered with sensitive paper a beam of light re- flected from a small mirror attached to a system of levers whose move- ments follow the curves faithfully and greatly magnify them. Hermann SPEECH 309 has come to the conclusipn that the mouth does not act as a mere resonator, but that for each vowel, in addition to the fundamental note due to the vibration of the vocal cords, the pitch of which is, of course, variable, one or, it may be, two other notes (formants, as he calls them), not necessarily harmonics of the lar5mgeal note, but separ- ated from it by a constant or nearly constant musical interval, are directly produced by the passage of the regularly interrupted expiratory blast through the mouth, the air contained in that cavity being for an instant set into vibration at each interruption. On this view it is the musical effect produced by the oscillation or continual recurrence, in short series, of these vibrations which gives the vowels their quality. The fact that it is by no means difficult to sing (with the larynx) and whistle (with the mouth) at the same time, shows the possibility of Hermann's view, that a fixed tone can be generated in the mouth by the intermittent stream of air issuing from between the vibrating vocal cords, just as a tone is generated in a pipe by blowing into or over it, and his records do show continually recurring groups of vibrations as his theory requires. McKendrick takes up a middle position, believing that both theories are partially true, and this seems to be the best conclusion which can at present be arrived at. It seems clear, at any rate, that more than one factor is concerned in the timbre of the vowel sounds. When the vowels are being uttered, the soft palate cloSes the entrance to the nasal chambers completely, as may be shown 'by holding a candle in front of the nose, or tr3nng to inject water through the nares. If the cavities of the nose are not completely blocked off, the voice assumes a nasal character in pronouncing certain of the vowels; and in some languages this is the ordinary and correct pronunciation. Many animals have the power of emitting articulated sounds; a few have risen, like man, to the dignity of sentences, but these only by imitation of the human voice. Both vowels and consonants can be distinguished in the notes of birds, the vocal powers of which are in general higher than those of mammalian animals. The latter, as a rule, produce only vowels, though some are able to form con- sonants too. The nervous mechanism of voice and speech will have to be again considered when we come to study the physiology of the brain and spinal cord. But the curious physiological antithesis between the functions of abduction and of adduction of the vocal cords may be mentioned here. The abductor muscles are not employed in the production of voice; they are associated with the less specialized, the less skilled and purposive function of respiration. The adductor muscles are not brought into action in respiration; they are asso- ciated with the highly specialized function of speech. Correspond- ing to this difference of function, we find that adduction is pre- ponderatingly represented in the cortex of the brain, abduction in the medulla oblongata. Stimulation of an area in the lower part of the ascending frontal convolution, near the fissure of Rolando, in the macaque monkey, causes adduction of the vocal cords, never 3 CO VOICE AND SPEECH abduction. In the cat, however, abduction of the cords may also be obtained by stimulation of the cortex. The same is true of the dog, but only when the peripheral adductor nerves have been divided. Stimulation of the medulla oblongata (accessory nucleus) causes abduction, never adduction. The skilled adductor function is, therefore, placed under control of the cortex. The vitally im- portant, but more mechanical, abductor function is governed by the medulla. The abductor movements are more hkely to be affected by organic disease, the adductor movements by functional changes. But the distinction between the two groups of muscles is not entirely due to a difference of central connections, since by altering the strength of the stimulus and the external conditions the one or the other may be separately excited through the inferior laryngeal nerve. Thus, strong stimulation of the inferior laryngeal causes closure of the glottis, for although it suppHes both abductors Fig. 149. — Diagram of Vocal Cords in Paralyses of the Larynx, a. Paralysis of both l^inferior laryngeal nerves. The vocal cords have taken up the ' mean ' position. b. Paralysis of right inferior laryngeal nerve. An attempt is being made to narrow the glottis for the utterance of sound. The right cord remains in its ' mean ' position, c. Paralysis of the abductor muscles only, on both sides. The cords are, approximated ■ beyond the ' mean ' position by the action of the adductors. and addilctors,- the 'latter, as the stronger muscles, prevail. With weak stimiilation, and^in young animals, the abductors, owing to the greater excitability of the neuro-muscular apparatus, carry off the victory, and the glottis is opened (Russell). When the nerve is cooled the abductors give way before the adductors. The same is true when it is allowed to become dry. And after death in a cholera patient it was observed that the pos- terior crico-arytenoid, an abductor muscle, was the first of the intrinsic laryngeal muscles to lose its excitability. Lesions of the medulla oblongata are often accompanied by marked changes in the character of the voice and the power of articulation. Section or paralysis of the superior laryngeal nerve causes the voice to become hoarse, and renders the sounding of high notes an impossibility, owing to the want of power to make the vocal cords tense. Stimulation of the vagus within the skull causes contraction SPEECH ^ 311 of the crico-thyroid muscle and increased tension of the cords. Sec- tion or paralysis of the inferior laryngeal nerves leads to loss of voice or aphonia, and dyspnoea (Fig. 149). Both adductor and abductor muscles are paralyzed ; the vocal cords assume their mean position — the position they have in the dead body — and the glottis can neither be narrowed to allow of the production of a note, nor widened during inspiration. It is said, however, that young animals, in which the structures around the glottis are more jdelding than in adults, can still utter shrill cries after section of the inferior laryngeals, the contraction of the crico-thyroid muscle alone being able, while in- creasing the tension of the cords, to draw them together. Interference with the connections on one side between the higher cerebral centres and the medulla oblongata, as by rupture of an artery and effusion of blood into the posterior portion of the internal capsule (giving rise to hemiplegia, or paralysis of the opposite side of the body), is not followed by loss of voice; the laryngeal muscles on both sides are still able to act. CHAPTER VI DIGESTION In the last chapter we have described the manner in which. the interchange of gases between the tissues and the air is carried out. We have now to consider the digestion and absorption of the soUd and Uquid food, its further fate in relation to the chemical changes or metabolism of the tissues, and finally the excretion of the waste products by other channels than the lung. Logically, we ought to take metabohsm after absorption and before excretion, tracing the food through all its vicissitudes from the moment when it enters the blood or lymph till it is cast out as useless matter by the various excretory organs. Unfortunately, however, many of the intermediate steps of the process are as yet hidden from us ; we know best the beginning and the end. We can follow the food from the time it enters the alimentary canal till it is taken up by the tissues of absorption; and we have really a fair knowledge of this part of its course. We can collect the end-products as they escape in the urine, or in the breath, or in the sweat ; and our knowledge of them and of the manner in which they are excreted is considerable. But of the wonderful pathway by which the dead molecules of the food mount up into life, and then descend again into death, we catch only a glimpse here and there. Only the introduction and the conclusion of the story of metabolism are at present in our possession in fairly continuous and legible form. We will read these before we try to decipher the handful of torn leaves which represents the rest. Section I. — Preliminary Anatomical and Chemical Data. Comparative. — In the lowest kinds of animals, such as the amceba, there is neither mouth, nor alimentary canal, nor anus: the food, wrapped round by pseudopodia, is taken in at any part of the animal with which it happens to come in contact. A vacuole is formed around it. Acid is secreted into the vacuole,' the food is digested within the cell-substance, and the part of it which is useless for nutrition is cast out again at any part of the surface. Coming a little higher, we find in the Coelenterates a mouth and alimentary tube, which opens into the body-cavity, where a certain 312 PRELIMINARY ANATOMICAL AND CHEMICAL DATA 313 amount of digestion seems to take place, and from which the food is absorbed either through the cells of the endoderm, or, as in Medusa, by means of fine canals, which radiate from the body-cavity into its walls, and form part of the so-called gastro-vascular system. In the Echinodermata we have a further development, a complete alimentary canal with mouth and anus, and entirely shut off from the body-cavity. In many Arthropods it is possible already to distinguish parts corre- sponding to the stomach, and the small and large intestines of higher forms, the digestive glands being represented by organs which in some groups seem to be homologous with the liver, and in others with the salivary glands of the higher Vertebrates. A few Molluscs seem in addition to possess a pancreas. Among Vertebrates fishes have the simplest, and birds and mammals the most complicated, alimentary system. In the lowest fishes the stomach is only indicated by a slight widening of the anterior part of the digestive tube. In water-living Vertebrates there are no salivary glands. In birds the oesophagus is generally dilated to form a crop, from which the food passes into a stomach consisting of two parts, one pre-eminently glandular (proventriculus), the other pre-eminently muscular (ventriculus). Among mammals a twofold division of the stomach is distinctly indicated in rodents and cetaceae, but this organ reaches its greatest complexity in ruminants, which possess no fewer than four gastric pouches. The differentiation of the intestine into small and large intestine and rectum is more distinct,, both anatomically and functionally, in mammals than in lower forms ; but there are marked differenqes between the various mammalian groups both in the relative size of the several parts of the digestive tube, and in the proportion between the total length of the alimentary canal and the length of the body. In general, the canal is longest in herbivora, shortest in carni- yora. Thus, the ratio between length of body and length of intestine is in the cat I : 4, dog i : 6, man 1 : 5 or 6, horse i : 12, cow i : 20, sheep I ; 27. The relative capacity of the stomach, small intestine, and large intestine, is in the dog 6 :"2 : 1-5, in the horse i : 3-5 : 7, in the cow 7:2:1. The area of the mucous surface of the alimentary canal is very considerable, in the dog more than half that of the skin, the surface of the small intestine being three times that of the stomach and four times that of the large intestine. In the horse the mucous surface has twice the area of the skin. Anatomy of the Alimentary Canal in Man. — The alimentary canal is a, muscular tube, which, beginning at the mouth, runs under the various names of pharynx, oesophagus, stomach, small intestine, large intestine, and rectum, till it ends at the anus. Its walls are largely composed of muscular fibres; its lumen is clad with epithelium, and into it open the ducts of glands, which, morphologically speaking, are involutions or diverticula formed in its course. In virtue of its muscular fibres it is a contractile tube ; in virtue ot its epitheUal hning and its • special glands it is a secreting tube ; in virtue of both it is fitted to per- form those mechanical and chemical actions upon the food which are necessary for digestion. Its inner surface is in most parts richly supplied with bloodvessels, and in special regions beset with peculiarly- arranged lymphatics; by both of these channels the alimentary tube performs its function of absorption. From the beginning of the oeso- phagus to the end of the rectum the muscular wall consists, broadly speaking, of an outer coat of longitudinally-arranged fibres, and a +h + V, ^^^^' '^°^^ °^ fibres running circularly or transversely around the tube. Between the layers lies a plexus of non-meduUated nerves and nerve-cells (Auerbach's plexus). In the stomach the longitudinal 314 DIGESTION fibres are found only on the two curvatures, and a third incomplete coat of oblique fibres makes its appearance internal to the circular layer. In the large intestine, again, the longitudinal fibres are chiefly collected into three isolated strands. In the pharynx the typical arrangement is departed from, inasmuch as there is no regular longi- tudinal layer; but the three constrictor muscles represent to a certain extent the great circular coat. The muscles of the mouth and of the pharjnax are of the striped variety. So is the muscle of the upper half of the oesophagus in man and the cat, and of the whole oesophagus in the dog and the rabbit. In the rest of the alimentary canal the muscle is smooth, except at the very end, where the external sphincter of the anus is striped. In certain situations the circular coat is de- veloped into a regular anatomical sphincter, a definite muscular ring, whose function it is to shut one part of the tube off from another (sphincter pylori, ileo-colic sphincter), or to help to close the external opening of the tube (internal sphincter _ of anus). Elsewhere a tonic contraction of a portion of the circula'r coat, not anatomically de- veloped beyond the rest, creates a functional sphincter (cardiac sphincter of stomach). Throughout the greater part of the digestive tract the peritoneum forms a thin serous layer, external to the muscular coat. Internally the muscular coat is separated from the mucous membrane, the lining of the canal, by some loose areolar tissue containing bloodvessels, lymphatics, and nerves (Meissner's plexus), and called the submucov? coat. Between the mucous and submucous layers, but belonging to the former, in the whole canal below the beginning of the oesophagus, is a thin coat of smooth muscular fibres, the muscularis mucosae, con- sisting in some parts, e.g., in the stomach, of two, or even three, layers. Between this and the lumen of the canal lie the ducts and alveoli of glands, surrounded by bloodvessels and embedded in adenoid or lymphoid tissue, which in particular regions is collected into well- defined masses (solitary follicles, Peyer's patches, tonsils), extending, it may be, into the submucous tissue. In the mouth, phar5m,x, and oesophagus, the glands lie in the submucosa, as do the glands of Brunner in the duodenum; everywhere else they are confined to the mucous membrane proper. Between the openings of the glands the mucous membrane is lined with a single layer of columnar epithelial cells, some- times (in the small intestine) arranged along the sides of tiny projec- tions or villi. When the intestine is contracted the villi are long and cylindrical in shape, when it is relaxed or distended they are flat and conical. At the ends of the alimentary canal, viz., in the mouth, pharynx, and oesophagus, and at the anus, the epithelium is stratified squamous, and not columnar. The purpose of food is to supply the waste of the tissues, to replenish the stores of material from the oxidation of which the energy required for the running of the bodily machine is derived, and thus to maintain the normal composition of the body. In the body we find a multitude of substances marked off from each other, some by the sharpest chemical differences, others by characters much less distinct, but falling upon the whole into the few fairly definite groups already described (p. i). Now, although it is by no means necessary that a substance in the body belonging to one of these great groups should be derived from a substance of the same group in the food, it has been found PRELIMINARY ANATOMICAL AND CHEMICAL DATA 315 that upon the whole no diet is sufficient for man unless it contains representatives of all; a proper diet must include proteins, carbo- hydrates, fats, inorganic salts, and water. These proximate prin- ciples have to be obtained from the raw material of the foodstuffs — that is, as regards the first three groups, which can alone yield energy in the body, from the tissues and juices of other living things, plants or animals; it is the business of digestion to sift them out and to prepare them for absorption. This preparation is partly mechan- ical, partly chemical. The water and salts and some carbo-hydrates, such as dextrose, are ready for absorption without change. Fats are split into glycerin and fatty acids before absorption. Indiffusible colloidal carbo-hydrates, like starch and dextrin, are changed into diffusible and readily soluble sugars, and the natural proteins into diffusible peptones, and eventually into much simpler decomposition products. These changes are obviously favourable to absorption. But this is not their whole significance. For disaccharides, such as cane-sugar, maltose, or lactose, although easily soluble in the contents of the gut, and in themselves perfectly capable of being absorbed without change, are, unless present in unusually large amount, all converted into monosaccharides, such as dextrose, levulose, or galactose, either in the lumen or in the wall of the alimentary tube. The reason is that the disaccharides are unsuitable as pabulum for the cells. ■ Digestion is not only a preparation of the food for absorption by the gut, but for assimilation by the tissues after absorption. An equally important instance of this double function is seen in the digestion of proteins. The complete shattering of the protein mole- cule into amino-acids and the other groups yielded by its decom- position (p. 354) is required, in the case of that portion of the protein which goes to build up the tissues, because of the high degree of specificity of the tissue proteins. The myosinogen of beef cannot be cobbled into the myosinogen of human muscle, still less we may suppose into the serum-albumin of human blood. It is necessary that the food protein should be completely ' wrecked ' in digestion so that protein which is to take its place in protoplasm may be built exactly to order from the bricks. A satisfactory ' fit ' cannot be obtained with ready-made protein. Mechanical division of the food is an important aid to the chemical action of the digestive juices. We shall see that this mechanical division forms a great part of the work of the stomach, but it is normally begun in the mouth, and it is of consequence that this preliminary stage should be properly performed. Section II.— The Mechanical Phenomena of Digestion. Mastication. — It is among the mammaUa that regular mastication of the food first makes its appearance as an important aid to diges- tion, The amphibian bolts its fly, the bird its grain, and the fish 31 6 DIGESTION its brother, without the ceremony of chewing. In ruminating mammals we see mastication carried to its highest point ; the teeth work all day long, and most of them are specially adapted for grinding the food. The carnivora spend but a short time in masti- cation; their teeth are in general adapted rather for tearing and cutting than for grinding. Where the diet is partly animal and partly vegetable, as in man, the teeth are fitted for all kinds of work ; and the process of mastication is in general neither so long as in the purely vegetable feeders, nor so short as in the carnivora. In man there are two sets of teeth : the temporary or milk teeth, and the permanent teeth. The milk teeth are twenty in number, and consist on each side of four incisors or cutting-teeth, two canines or tearing-teeth, and four molars or grinding-teeth. The central incisors emerge at the seventh month from birth, the other incisors at the ninth month, the canines at the eighteenth, and the mola.rs at the twelfth and twenty-fourth month respectively. Each tooth in the lower jaw appears a little before the corresponding one in the upper jaw. Each of the milk teeth is in course of time replaced by a permanent tooth, and in addition the vacant portion of the gums behind the milk set is now fiUed up by twelve teeth, six on each side, three above and three below. These twelve are the permanent molars; they raise the number of the permanent teeth to thirty-two. The permanent teeth which occupy the position of the milk molars now receive the name of premolars. The first tooth of the permanent set (the first true molar) appears at the age of 6J years; the last molar, or wisdom-tooth, does not emerge till the seventeenth to the twenty-fifth year. In mastication the lower jaw is moved up and down, so as to alternately separate and approximate the two rows of teeth. It has also a certain amount of movement from side to side, and from front to back. The masseter, temporal and internal pterygoid muscles raise, and the digastric, with the assistance of the mylo- and genio- hyoid, depresses, the lower jaw, but its downward movement is mainly a passive one. The external pterygoids pull it forward when both contract, forward and to one side when only one con- tracts. The lower fibres of the temporal muscle retract the jaw. The buccinator and orbicularis oris muscles prevent the food from passing between the teeth and the cheeks and lips. The tongue keeps the food in motion, works it up with the saliva, and finally gathers it into a bolus ready for deglutition. Deglutition. — -This act consists of a voluntary and an involun- tary stage. Just before the beginning of the voluntary stage mastication is suspended, and a slight contraction of the dia- phragm generally takes place. The anterior part of the tongue is suddenly elevated and pressed against the hard palate, and the elevation travels back from the tip towards the root, as the mylo- THE MECHANICAL PHENOMENA OF DIGESTION 317 hyoid muscles in the floor of the mouth contract sharply so as to thrust the bolus through the isthmus of the fauces. As soon as this has happened, and the food has reached the posterior portion of the tongue, it has passed beyond the control of the will, and the second or involuntary stage of the process begins. This stage may be divided into two parts: (i) Pharyngeal, (2) oesophageal — both being reflex acts. During the first the food has to pass through the pharynx, the upper portion of which forms a part of the respiratory tract, and is in free communication with the larynx during ordinary breathing. It is therefore necessary that respiration should be interrupted and the larynx closed while the food is being moved through the pharynx. But that the inter- ruption may be short, the food must be rapidly passed over this perilous portion of its descent. The main propelling force under which the bolus is shot through the back of the pharynx is derived from the contraction of the mylo-hyoid muscles already mentioned, assisted to some extent by the stylo- and palato-glossi ; and that none of the purchase may be lost, the pharyngeal cavity is cut off from the nose and mouth as soon as the bolus has entered it. The soft palate is raised by the levator palati and palato-pharyngei muscles; at the same time the upper part of the phar5mx, narrowed by the contraction of the superior constrictor, comes forward to meet the soft palate, closes in upon it, and so prevents the food from passing into the nasal cavities. The pharynx is cut off from the mouth by the closure of the fauces through the contraction of the palato-phar5mgeal muscles which lie in their posterior pillars. The upper free end of the epiglottis (the so-called pharjmgeal part) aids the back of the tongue in completing a movable partition across the pharynx, which keeps close to the bolus as it passes down between the posterior surface of the epiglottis and the posterior wall of the pharynx. Almost immediately after the contraction, of the mylo-hyoids the larynx is pulled upwards and forwards by the contraction of the thyro-hyoid muscle, and the elevation of the hyoid bone by the muscles which connect it to the lower jaw. The base of the tongue is simultaneously drawn backwards by the stylo- and palato-glossus. The lower or laryngeal portion of the epiglottis is thus caused to come into contact with the upper orifice of the larynx, occluding it completely, but the pharjmgeal portion projects beyond the larynx, and takes no share in its closure (Eykman). The glottis is closed by the approximation of the vocal cords and the arytenoid cartilages. The epiglottis, however, is not absolutely indispensable for closing the lar3mx, since swallowing proceeds in the ordinary way when it is absent. The morsel of food, grasped by the middle and lower constrictors as it leaves the back of the tongue, passes rapidly and safely over the closed larynx, the process being accelerated by the puUing up of the lower portion 3i8 DIGESTION of the phar3mx over the bolus by the action of the palato- and stylo- pharyngei. The second or oesophageal portion of the involuntary stage is a more leisurely perforlnance. The bolus is carried along by a peculiar ' peristaltic ' contraction of the muscular wall of the oesophagus, which travels down as a wave, constricting the tube and pushing the food before it. In front of the constricting wave moves a wave of inhibition, so that the part of the oesophagus into which the bolus is about to pass is always relaxed, while the part behind it is contracted. This exact co-ordination of inhibition and contraction is the essential thing in peristalsis. When the food reaches the lower end of the gullet the tonic contraction of that part of the tube is for a moment relaxed by reflex inhibition, and the morsel passes into the stomach. Beaumont saw, in the case of St. Martin, that the oesophageal orifice of the stomach contracted firmly after each morsel was swallowed, and so did the gastric walls in the neighbourhood of the fistula when food was introduced by this opening. In the dog the whole process of swallowing from mouth to stomach has been shown to occupy four to five seconds, but the time is by no means constant. In man the peristaltic wave requires about five to six seconds to travel from the level of the glottis to the cardiac orifice. The rate of movement is greater in the upper than in the lower portion of the oesophagus. Such is the mechanism of deglutition when the bolus is of such consistence and size that it actually distends the oesophagus. But it has been shown that liquid food is swallowed in a different way. The food Is^ng on the dorsum of the tongue, suddenly put under pressure by the sharp contraction of the mylo-hyoid muscles, is shot rapidly down to the lower part of the lax oesophagus, or, occa- sionally, some of it even into the stomach. So far the process has only occupied one-tenth of a second. After several seconds, the food, or the portion which still remains in the oesophagus, is forced through the cardiac sphincter into the stomach by the arrival of the tardy peristaltic contraction of the oesophageal waU (Kronecker and Meltzer). Two sounds may be heard in man on listening in the region of the stomach or oesophagus during deglutition of Uquids, especially when, as generally happens, they are mixed with air. The first sound occurs at once, and is due to the sudden squirt of the liquid along the gxillet ; the second, which is heard after a distinct interval (about six seconds), is caused by the forcing of the fluid through the cardiac orifice of the stomach by the contraction of the oesophagus. There are certain peculiarities which distinguish this peristaltic movement of the oesophagus from that of other parts of the alimen- tary canal. It is far more closely related to the central nervous system, and, unlike the peristaltic contraction of the intestine, can THE MECHANICAL PHENOMENA OF DIGESTION 319 pass over any muscular block caused by ligature, section, or crush- ing, so long as the nervous connections are intact. But division of the oesophageal nerves causes, as a rule, stoppage of oesophageal movements; although an excised portion of the tube retains its vitality for a long time, and may, under certain circumstances, go on contracting in the characteristic way after removal from the body (p. 790) . Stimulation of the mucous membrane of the pharynx will cause reflex movements of the oesophagus, while stimulation of its own mucous membrane is ineffective. From these facts we learn that although the oesophageal wall may possess a feeble power of spontaneous peristaltic contraction, yet this is usually in abeyance, or at least overmastered by central nervous control ; so that impulses discharged as a ' fusillade ' from successive portions of the vagus centre, and travelling down the oesophageal nerves, excite the muscular fibres in regular order from the upper to the lower end of the tube. Nervous Mechanism of Deglutition. — The centre for the whole involuntary stage (both pharyngeal and oesophageal) lies in the upper part of the meduUa oblongata. When the brain is sliced away above the medulla, deglutition is not affected ; but if the upper part of the- medulla is removed, the power of swallowing is abolished. In man, disease of the spinal bulb interferes far more with deglutition than disease of the brain proper. Normally, the afferent impulses to the centre are set up by the contact of food or saliva with the mucous membrane of the posterior part of the tongue, the soft palate and the fauces, the nerve- channels being the superior laryngeal, the pharyngeal branches of the vagus, and the palatal branches of the fifth nerve.* A feather has sometimes been swallowed involuntarily by a reflex movement of deglutition set up while the soft palate or pharynx was being tickled to produce vomiting. Artificial stimulation of the central end of the superior laryngeal will cause the movements of deglutition independently of the presence of food or liquid; but if the central end of the glosso- pharyngeal nerve be stimulated at the same time, the movements do not occur. The glosso-pharyngeal is therefore able to inhibit tlie deglutition centre, and it is owing to the action of this nerve that in a series of efforts at swallowing, repeated within less than a certain short interval (about a second), only the last is successful. It is also through the glosso-pharyngeal nerve that the respiratory movements are inhibited during deglutition. When the central end of this nerve is stimulated, respiration is stopped * It appears that the most influential reflex paths may differ in difierent animals. In the rabbit, e.g., the reflex is set up by excitation of the trigeminal fibres which supply the mucous membrane anterior to the tonsils, in the dog and cat by excitation of the glosso-pharyngeal fibres in the posterior wall of the pharynx, and in monkeys by excitation of the trigeminal branches dis- tributed to the mucous membrane over the tonsils (Kahn) . 320 DIGESTION for four or five seconds, and this cessation is distinguished from that produced by any other afferent nerve by the circumstance that it occurs not in expiration exclusively or in inspiration ex- clusively, but with the respiratory muscles in the precise degree of contraction in which they happened to be at the moment of stimu- lation. The efferent nerves of the reflex act of deglutition are the hypoglossal to the tongue and the thyro-hyoid and other muscles concerned in raising the lar37nx; the glosso-pharyngeal, vagus, facial and fifth to the muscles of the palate, fauces, and pharynx; the fifth to the mylo-hyoid; and the vagus to the larynx and oesophagus. Section of the vagus interferes with the passage of food along the oesophagus; stimulation of its peripheral end causes oesophageal movements. Movements of the Stomach. — The whole of the stomach does not take part equally in the movements associated with digestion. We may divide the organ, both anatomically and functionally, into two portions — a pyloric portion, or antrum pylori, comprising about a fifth of the stomach, and a larger cardiac portion, or fundus* At the junction of the antrum and the fundus the circular muscular coat is slightly thickened into a ring called the ' transverse band,' or ' sphincter of the antrum.' In the living stomach the region of the transverse band is usually contracted so strongly and con- tinuously that a distinct groove is seen to separate the tubular antrum from the bag-like cardiac end. The suggestion of a massive constricting ring of muscle is belied by an examination of the dead viscus. The transverse band is really little more than a physio- logical sphincter. The empty stomach is contracted and at rest. A few minutes after food is taken contractions begin in the antrum, and run on in constricting undulations (in the cat at the rate of six in the minute) towards the pyloric sphincter. Each wave takes about twenty seconds (in the cat) to pass from the middle of the stomach to the pylorus. Feeble at first, they become stronger and stronger as digestion proceeds, and gradually come to involve the portion of the fundus next the sphincter of the antrum, but their direction is always towards the pylorus, never, in normal diges- tion, away from it. The food is thus subjected to energetic churn- ing movements in the pyloric end of the stomach, and worked up thoroughly with the gastric juice. Kept in constant circulation, it gradually becomes reduced to a semi-liquid mass, the chyme, which is at intervals driven against the pylorus by strong and regular peristaltic contractions of the lower end of the stomach, * Here ' fundus ' is used in the sense in which it is generally employed in speaking of the stomach of the dog or cat as signifying the whole of the organ with the exception of the antrum pylori. By the fund us of the human stomach most writers mean only the cul-de-sac at the cardiac end; the portion inter- vening between it and the antrum pylori is often termed the body of the stomach. THE MECHANICAL PHENOMENA OF DIGESTION 321 the sphincter relaxing from time to time by a reflex inhibition to admit the better-digested portions into the duodenum, but tighten- ing more stubbornly at the impact of a hard and undigested morsel. The nature, as well as the consistence of the food, influences the length of its sojourn in the stomach. Carbo-hydrate food passes more rapidly through the pylorus than fatty food, and fat more rapidly than protein. The reason is that the acidity of the gastric juice varies with the different kinds of food, hydro- chloric acid being secreted in abundance in the presence of proteins, and to a much smaller extent in the presence of fats and carbo-hydrates. Now, dilute hydrochloric acid when introduced into the stomadji remains there for a much longer time than water. This depends upon the fact that such portions of the . acid as getinto the duodenum stimulate *af£erent flbires in its mucous membrane, and' so caufee reflex spasm of the pyloric sphincter. , When the acid chyme be- comes neutralized to a certain point by the bile and pancf'^atic juice, inhibitory impulses pass up from the duodenum and cause the sphincter to relax. The cardiac division of the stomach, with the exception of the portion that borders the transverse band, takes no share in. the peristaltic movements. And, indeed, it is far more difficult to cause such con- tractions by artificial stimulation in the fund.us than in the pylorus. The two portions of the stomach are partially, or in certain animals from time to time completely, cut off from each other by the contraction of the sphincter of the antrum. The fundus, so far as its mechanical functions are concerned, acts chiefly as a reservoir for the food, which, like a hopper, it gradually passes into the pyloric mill as digestion goes on by a tonic contraction of its walls. The existence of this reservoir enables larger quantities of food to be taken at one meal, which can then be digested gradually. These facts have been mainly ascertained by observations on animals, such as the dog and the cat, either by direct inspection after opening the abdomen (Rossbach), or in the intact body, under Fig. 150. — Cat's Stomach seen by Rontgen Rays (Cannon). The outlines of the stomach containing food mixed with bismuth subnitrate ' were drawn at intervals from II a.m. to 4.30 p.m. 322 DIGESTION absolutely physiological conditions, by means of the Rontgen rays (Cannon). In the latter method the food is mixed with subnitrate of bismuth, which is opaque to these rays, so that when the animal is looked at through a fluorescent screen the stomach appears as a dark shadow in the field (Fig. 150). This method has even been applied with success to the study of the passage of the food along the human alimentary canal from deglutition to defaecation (Hertz). It has been shown in this way that in the living body in the erect position the long axis of the stomach is much more nearly vertical than had been supposed. When food is taken it sinks into the lower (pyloric) end, and at the upper end gas collects. When the person lies down the lower end of the Stomach passes more towards the left, so that the long axis lies more transversely. Other methods have thrown light on the gastric movements — e.g., direct inspection through a fistula of the stomach, and the study Fig. 151. — Human Stomach studied by Rontgen Rays, a. Empty stomach in ver- tical positiop ; b, shortly after a meal (peristaltic contractions are .occurring the' THE MECHANICAL PHENOMENA OF DIGESTION 323 )endulum movements, but running indifferently in both direc- ions, can be set up by local stimulation. The function of these jendulum movements seems to be the thorough mixing of the food ffith the digestive juices in the intestine. When an animal is fed with food containing bismuth subnitrate and observed with the Rontgen rays, it is seen that the food in a coil is often divided into small segments, which then join together to form longer masses, these being in turn again divided. This segmentation is rhyth- mically repeated (in the cat at the rate of thirty times a minute). Although of itself it insures only thd mixing of the contents of the gut, and not their onward progress, it is usually accompanied by peristalsis, so that while the food is undergoing segmentation it is also slowly passing down the intestine. Often, however, a column of food remains for a considerable time, dividing, uniting, and divid- ing again, without sensibly shifting its position. In addition to the relatively rapid pendulum movements, much slower periodic varia- tions of tone of the whole musculature may be normally observed. (2) True peristaltic movements, in which a ring of constriction, obliterating the lumen, moves slowly doWn the tube, with a speed, it may be, no greater than i mm. per second. The portion of the intestine immediately below the advancing constriction is relaxed and motionless, so that we may say that a wave of inhibition pre- cedes the wave of contraction. The peristaltic movements of the small intestine, the most typical of their kind, are most easily excited by mechanical stimulation of the mucous membrane, as by the contact of a morsel of food or an artificial bolus of cotton- wool. TravelUng, under normal conditions, always downwards, the constriction squeezes the contents of the tube before it, and the wave usually ends at the ileo-caecal valve, which separates the small intestine from the large. The cause of the definite direction of the peristaltic wave is grounded in the anatomical relations of the intestinal wall. For when a portion of the intestine is resected, turned round in its place and sutured, so that what was before its upper is now its lower end, the contraction wave is unable to pass, and the obstruction to the onward flow of the intestinal contents causes marked dilatation of the gut, and sometimes serious disturb- ance of nutrition. The most probable explanation is that the peri- stalsis is governed by a local reflex nervous mechanism (Auerbach's plexus), the stimulation of which by the contact of the foo^i with the mucous membrane or by the distension of the gut causes excitation of the circular muscular fibres above the point of stimula- tion, and inhibition of them below it. The automatic pendulum movements, and also the slow, rhythmical variations of tone, have a different relation to the local nervous mechanism, for they behave differently to poisons like cocaine and nicotine, which act on that mechanism. The pendulum movements are, if anything, increased 324 DIGESTION in intensity and made more regular. But the peristaltic waves, although they can be locally excited by direct stimulation of the 'muscular fibres, are no longer propagated, and a bolus introduced into the intestine remains at rest where it is placed. Some have interpreted these facts as indicating that the pendulum movements are myogenic in origin. But evidence has lately been obtained that, although they are not reflex movements elicited by afferent impulses from the mucous membrane, since they continue in unaltered in- tensity, in isolated loops of intestine immersed in Ringer's (or Locke's) solution (p. 66) after removal of both mucosa and sub- mucosa, they are nevertheless dependent upon Auerbach's plexus. JFor when the circular muscular coat is separated from this plexus, the automatic movements of this coat are abolished, although the excitability of the musculature to direct stimulation is not affected. The longitudinal coat, which is still in connection with Auerbach's plexus, goes on contracting spon- taneously (Magnus). Under certain conditions a movement of food or secretions in the reverse of the normal direction can be set up in the small intestine in the intact body — e.g., in the case of. obstruc- tion of the intestine leading to vomiting of its contents. But this does not necessarily indicate a re- versal of the normal direction of the peristalsis. Such a reversal, if it occurs at all, is not easy to realize by artificial stimulation, and even when an antiperistaltic wave is apparently started, it travels up the intestine only for a short distance and then dies out., A thiyd variety of intestinal movement has sometimes been described, the so-called 'peristaltic rush' (Melt^er, etc.). It consists ',of a rapidly moving peristaltic contraction, preceded by relaxatibft of .3. long portion of the tube. Such a contraction may even sweep .down without pause from the duodenum to the end of the ileum. .The Movemeints of the Large Intestine differ from those of the small mainly in the great frequency of antiperistalsis. This, indeed, seems to be the usual movement of the transverse and ascending' .colon. The antiperistalsis. recurs in periods about every fifteen minutes, and each period generally lasts about five minutes. The ■constrictions, running towards the caecum, thoroughly churn and mix the remnants of the food, a considerable absorption of which may take place in the upper part of the large intestine. Regurgita- ;tion into the ileum in man is prevented pai'tly by the oblique entry Fig. 152. — Intestine "Segment beating in Ringer's Solution. At 6 the oxy- gen stream was increased. To be read from left to right. Time trace, half -minutes. (Reduced to one -half.) THE MECHANICAL PHENOIHENA OF DIGESTION 3-25 of the ileum through the wall of the colon (so-called ileo-c'sec41' valve), but essentially by the tonic contraction of the ileo-coli'c' sphincter. The sphincter usually permits the passage of material- only in the direction from small to large intestine. But as an occasional phenomenon, a reverse movement may occur. Thus food may actually pass back through the ileo-colic sphincter into the small intestine under the action of a long-continued and vigorous antiperistalsis, and in this way a considerable portion of a bulky enema may be eventually disposed of (Cannon). This so-called antiperistalsis is not precisely the same kind of movement, leaving out of account its direction, as the peiristalsis already described in the small intestine, since it is not preceded by a wave of inhibition. True peristaltic contractions preceded by relaxation of the gut may also be observed to start in the caecum, and to travel down the large intestine. They are not very frequent in comparison with those of the small intestine, and they die away before reaching the end of the colon, allowing the food to be driven back again towards the caecum by the antiperistalsis. A true downward peristalsis is more commonly seen in the descending colon, and is here asso- ciated with the propulsion and collection of the faeces, which are mainly stored in the sigmoid flexure. These peristaltic contractions do not normally reach the rectum, which, except during defaecation, remains at rest. Influence of the Central Nervous System on the Gastro- Intestinal Movements.— As we have already said, these movements are much less closely dependent on the central nervous system than are those of the oesophagus. They can not only go on, but are in general better marked when the extrinsic nervous connections are cut ; they cannot spread when the continuity of the tube is destroyed, and the mere presence of food will excite them when other than local reflex action has been excluded by section of the nerves. Never- theless, the central nervous system does exercise some influence in the way of regulation and control, if not in the way of direct initiation of the movements, and the swallowing or even the smeU of food has been observed to strengthen the contractions of a loop of intestine severed from the rest, but with its nerves still intact. The vagus is the efferent channel of this reflex action : stimulation of its peripheral end may cause movements of all parts of the aUmentary canal from oesophagus to large intestine, and may strengthen movements already going on ; but section of it does not stop them, nor hinder the food from causing peristalsis vvherever it comes. The vagus also contains inhibitory fibres for the lower end of the oesophagus and the whole of the stomach. Stimulation of it is followed first by inhibition, and then, after an interval, by an increase of tone and augmentation of the contraction of the whole stomach, including the cardiac and pyloric sphincters. The 3Z&' DIGESTION splanchnic nerves contain fibres by which the intestinal movements can be inhibited, and they appear to be always in action, for after section of these nerves the movements are strengthened. On the other hand, stimulation of the peripheral end of the cut splanchnic causes arrest of the movements. Occasionally, however, it has the opposite effect. Contractions of the small intestine are more easily caused by excitation of the vagus after the inhibitory splanch- nic nerves have been cut. The splanchnics also contain inhibitory fibres for the stomach, and it is only when these are intact that complete reflex inhibition of the organ can be obtained in the rabbit (Auer). The gastric movements are not permanently affected by section of these nerves alone, or even by simultaneous section of the splanchnics and the gastric branches of the vagi. But if the vagi are cut while the splanchnics remain intact, the, peristalsis of the stomach is weakened, its onset delayed, and the proper emptying of the viscus through the pylorus interfered with. In all probability these results are due to the uncontrolled action of the inhibitory fibres. The splanchnics have a special relation to the ileo-cohc sphincter, which closes when they are stimulated, and becomes in- sufficient when they are cut. The vagus does not affect it. The lower part of the large intestine is influenced by the sacral nerves (second, third, and fourth sacral in the rabbit), and by certain lumbar nerves, in the same way as the higher parts of the alimentary canal, and particularly the small intestine, are influenced by the vagus and the splanchnics. Stimulation of these sacral nerves within the spinal canal, or of the pelvic nerves (nervi erigentes) into which they pass, causes contraction of the parts of the large intestine concerned in defaecation — that is, in the dog, of the whole colon, with the exception of the caecum; in the cat, of the distal two-thirds of the colon. The colon first undergoes rapid shortening due to the contraction of the longitudinal fibres and the recto-coccygeus muscle. After a few steconds this is followed by contraction of the circular fibres, beginning at the lower limit of the region in which antiperistalsis can occur, and spreading downwards, so as to empty the portion of the bowel involved in the contraction. This is a very close imitation of what occurs, in natural defaecation. In man the parts involved in these movements ate probably the sigmoid flexure and rectum. In addition to these characteristic motor effects on the lower part of the large intestine, stimulation of the pelvic nerves causes an increase in the antiperistalsis of its upper portions. Stimulation of the lumbar nerves or of the por- tions of the sympathetic into which their visceral fibres pass (lumbar sympathetic chain from second to sixth ganglia, or the rami from it to the inferior mesenteric ganglia) causes inhibition of the movements of the caecum and the whole colon, including the antiperistaltic move- ments. Excitation of the sacral nerves initiates or increases the con- traction of both coats of the portions of the large intestine on which they act, excitation of the lumbar nerves inhibits both. And in the small intestine the same law holds good ; the two coats are contracted together by the action of the vagus or inhibited together by that of the splanchnics . Defaecation is partly a voluntary and partly a reflex act. But in the infant the voluntary control has not yet been developed; THE MECHANICAL PHENOMENA OF DIGESTION 327 in the adult it may be lost by disease; in an animal it may be abolished by operation, and in each case the action becomes wholly reflex. Owing to the tonic contraction of the rectum and the acute angle formed at the pelvi-rectal flexure, the fseces are arrested at this point. In consequence the pelvic colon becomes filled with faeces from below upwards, and the rectum remains empty till immediately before defaeoation. This has been verified in man by observations with the Rontgen rays (Hertz). In persons whose bowels are opened regularly after breakfast, the passage of faeces into the' rettum gives rise to the characteristic sensation which may be termed the ' call to defsecation. ' It is the distension of the rectum, and of the rectum alone, which is associated with this sensation, for in persons from whom the entire rectum has been removed for ' malignant disease the sensation is absent, and it may be elicited by artificially distending the rectum, though not any other part of the alimentary canal. The minimum pressure required to eUcit the sensation is smaller the greater the length of the gut exposed to it, varying in one individual from 32 to 48 mm. of mercury, according to the length of a balloon introduced into the rectum. The passage of the fseces from the pelvic colon into the rectum isdue to the discharge of that reflex contraction of the lower portion of the bowel already described (p. 325), of which the pelvic nerves constitute the efferent path. This reflex peristalsis is elicited by various causes, among which one of the most important is the taking of food at breakfast into the empty stomach, and another the muscular activity associated with getting up and dressing. The desire to defsecate may for a time be resisted by the will, or it may be yielded to. In the latter case the abdominal muscles, and, according to Hertz, the diaphragm also, are forcibly contracted; and the glottis being closed, the whole effect of their contraction is expended in raising the pressure within the abdomen and pelvis, and so aiding the muscular wall of the bowel itself in driving the fsces fronj the sigmoid flexure to the rectum. The two sphincters which close the anus — the internal sphincter of smooth muscle, and the external of striated — are now relaxed by the inhibition of a centre in the lumbar portion of the spinal cord, through the activity of which the tonic contraction of the sphincters is normally maintained. This relaxation is partly voluntary, the impulses that come from the brain acting probably through the medium of the lumbar centre. But in the dog, after section of the cord in the dorsal region, the whole act of defaecation, including contraction of 'the abdominal muscles and relaxation of the sphincters, still takes place, and here the process must be purely reflex. Even after complete destruction of the lumbar and sacral portions of the spinal cord the tone of the sphincters returns after a time, and defaecation IS carried on as in a normal animal, the control of the sphincters 328 DIGESTION being due either to a property of the muscular tissue itself or to local ganglia. The contraction of the levatores ani helps to resist overdistension of the pelvic floor and to pull the anus up over the faeces as they escape. The nervi erigentes carry efferent constrictor fibres, and the hypogastrics, as a rule, efferent dilator fibres, to the sphincters. While the internal sphincter is by itself capable of maintaining a tonus of considerable strength, the external sphincter contributes an important share (30 to 60 per cent.) to the closure of the rectum. If the call to defaecation is neglected, the desire passes away. This is not due to the faeces being carried back into the pelvic colon by antiperistalsis, as has generally been stated. The faeces which have passed into the rectum remain there, as can be shown by examination with the finger after the desire to empty the bowels has disappeared. The reason for the disappearance of the sensation is the relaxation of tone which occurs in the muscular coat of the rectum after a period of distension. It is not till it has been again distended by the entrance of a further portion of faeces that the call to defaecation is again experienced. When the call is repeatedly neglected, the sensibility of the rectum to dis- tension becomes blunted, and this is a common cause of constipation. The time of passage of substances through the alimentary canal has been studied by administering cpUodion capsules filled with subnitrate of bismuth to human beings, and observing their pro- gress by taking shadow pictures of them at intervals with the Rontgen rays. During' the first twenty'minutes tv^o such capsules swallowed at the same time by a healthy young man were clearly seen in the greater curvature of the stomach, but in the interval between the first half-hour and the seventh or eighth hour no further trace of them was detected. About the eighth hour they re- appeared in the caecum, where they remained with little or no onward movement till the fourteenth hour. From the fourteenth to the sixteenth hour they travelled along the ascending colon, and tarried a long time at the left angle of the colon. From the nine- teenth to the twenty-second or twenty-fourth hour they slowly passed downward in the descending colon, and stopped at the sig- moid flexure till their expulsion in defaecation. In some subjects the entire passage of the capsules was complete in sixteen hours, in others not until after thirty hours. A one cent piecfe swallowed by a healthy child four years old was recovered in the faeces 52 hours, later, and a button, slightly larger, swallowed by the same child, appeared after almost exactly the same interval. Vomiting. — We have seen that under normal conditions the movements of the alimentary canal always tend to carry the food in one definite direction, along the tube from the mouth to the rectum. The peristaltic waves generally run only in this direction, and, further, regurgitation is prevented at three points by the THE MECHANICAL PHENOMENA OF DIGESTION 329 cardiac and pyloric sphincters of the stomach and the ileo-cblic sphincter and valve. But in certain circumstances the peristalsis may be reversed, one or more of the guarded orifices forced, and the onward stream of the intestinal contents turned back. In obstruc- tion of the bowel, the fsecal contents of the large intestine may pass up beyond the ileo-csecal valve, and, reaching the stomach, be driven by an act of vomiting through the cardiac orifice ; in what is called a ' bilious attack,' the contents of the duodenum may pass back through the pylorus and be ejected in a similar way; or, what is by far the most common case, the contents of the stomach alone may be expelled. Vomiting is usually preceded by a feeling of nausea and a rapid secretion of saliva, which perhaps serves, by means of the air carried down with it when swallowed, to dilate the cardiac orifice of the stomach, but may be a mere by-play of the reflex stimula- tion bringing about the act. The diaphragm is now forced down upon the abdominal viscera, first with open and then with closed glottis. The thoracic portion of the oesophagus is thus placed under diminished pressure, and therefore widened, while sahva and air are aspirated into it out of the mouth. The abdominal muscles strongly contract. At the same time the stomach itself, and par- ticularly the antrum pylori, contracts, the cardiap orifice relaxes, and the gastric contents are shot up into the lax oesophagus, and through it into the pharynx, and issue by the mouth or nose. The movements of the stomach during vomiting induced by apomorphine have been studied in the cat by the Rontgen ray method. There is first observed extreme relaxation of the cardiac end; then a deep constriction appears a little below the cardiac orifice, and runs towards the pylorus, increasing in depth as it goes. When the transverse band is reached, this contracts firmly and remains con- tracted, and the constriction passes on over the antrum pylori. Ten or twelve similar waves follow, at the end of which time the constriction in the region of the transverse band divides the stomach into the firmly-contracted antrum and the relaxed fundus. Now follows a sudden contraction of the diaphragm and abdominal muscles accompanied by the opening of the cardiac orifice. Either the diaphragm and abdominal muscles alone, without the stomach, or the diaphragm and stomach together, without the abdominal muscles, can carry out the act of vomiting. For an animal whose stomach has been replaced by a bladder filled with water can be made to vomit by the administration of an emetic (Magendie) ; and Hilton saw that a man who lived fourteen years after an injury to the spinal cord at the height of the sixth cervical nerve, which caused complete paralysis below that level, could vomit, though with great difficulty. In a young child in which very slight causes will induce vomiting, the stomach alone contracts during the act. 330 DIGESTION But in the adult such a contraction is ineffectual, and the same is the case in animals, for a dog under the influence of a moderate dose of curara, which paralyzes the voluntary muscles but not the stomach, cannot vomit. The nerve-centre is in the medulla oblongata. It may be excited by many afferent channels: the sensory nerves of the fauces or pharynx, of the stomach or intestines (as in strangulated hernia), of the liver or kidney (as in cases of gall-stone or renal calculi), of the uterus or ovary, and of the brain (as in cerebral tumour), are all capable, when irritated, of causiiig vomiting by impulses passing along them to the vomiting centre. The vagus nerve in man certainly' contains afferent fibres by the stimulation of which this centre can be excited, for it has been noticed that when the vagus was exposed in the neck in the course of an operation, the patient vomited whenever the nerve was touched (Boinet, quoted by Gowers). In meningitis, vomiting is often a prominent symptom, and is sometimes due to irritation of the vagus nerve by the inflammatory process. Some drugs act as emetics by irritating surfaces in which efficient afferent impulses may be set up, the gastric mucous membrane, for example; sulphate of zinc and sulphate' of copper act mainly in this way. Apomorphine, on the other hand, stimulates the centre directly, and this-is also the mode in which vomiting is pro- duced in certain diseases of the medulla oblongata. The efferent nerves for the diaphragm are the phrenics, for the abdominal muscles the intercostals. The impulses which cause contraction of the stomach pass along the vagi. Dilatation of the cardiac ori-fice is brought about by the inhibitory fibres in the vagus already mentioned. Section III. — The Chemistry of the Digestive Juices. Ferments.^The chemical changes wrought in the food as it passes along the alimentary canal are due to the secretions of various glands which line its cavities or pour their juices into it through special ducts. These secretions owe their power for the most part to substances present in them in very small amount, but which, nevertheless, act with extraordinary energy upon the various constituents of the food, causing profound changes with- out, upon the whole, being themselves used up, or their digestive power affected. The active agents are the enzymes, sometimes speken of as unformed or unorganized ferments — unorganized because their action does not depend upon the growth of living cells, which was long supposed to be the case for some other fer- ments, such as yeast. Since it has been shown that specific enzymes can be separated from cells which were formerly believed to act THE CHEMISTRY OF THE DIGESTIVE JUICES 331 by their mere growth, the distinction between formed and unformed ferments has lost its significance, and has to a great extent been superseded by the distinction between intra- and extra-cellular enzymes (also called endo- and exo-enzymes) — i.e., between ferments which normally act in the interior of the cells where they are produced and ferment's which act outside of the cells that secrete them. From yeast cultures, for instance, by crushing the cells, a ferment (zymase*) can be obtained which in the complete absence of living yeast-cells, and, indeed, of any living micro-organism, forms alcohol and carbon dioxide from sugar, just as living yeast does. There is every reason to believe that it is by the intracellular action of this endoenzyme that the yeast-cell normally causes alcoholic fermentation. The digestive ferments are typical extracellular enzymesj Their chemical nature has not been exactly made out; some of them at least do not appear to be proteins, or to contain a protein group. Many of them apparently exist in the colloidal condition, although this has not been shown for all. In certain cases the more or less stable union of a definite inorganic substance with the ferment, or its actual inclusion in the ferment molecule, seems to be a condition of its action. Thus there is reason to believe that in gastric digestion hydrochloric acid is loosely combined with the pepsin. In the plant oxydase, laccase (p. 268), manganese is present. And the fact that manganese salts oxidize certain substances as laccase does suggests that it is the manganese in combination with some protein or other organic compound in the ferment molecule which confers upon laccase its oxidizing power. A similar relation between iron and some animal oxydases is possible, though not definitely proved. But none of the ferments of the digestive juices has as yet been satisfactorily isolated, and at present it is only by their effects that we recognize them. The difficulty of isolating them is increased by the fact that, like other colloids, they readily adhere to surfaces, and are carried down by the most diverse precipitates of substances to which they are chemically indifferent. On the other hand, this very property is taken advan- tage of to procure more concentrated, although still impure, solutions of them than exist in the natural secretions. Thus in the prepara- tion of many ferments the first step is to produce an inert pre- cipitate, such as calcium phosphate, in the juice or extract. Some of the ferments act best in an alkaline, some in an acid medium. They all agree in having an ' optimum ' temperature, which is more favourable to their action than any other; a low temperature sus- pends their activity, and boiling abolishes it for ever. The optimum temperatures of the majority of enzymes lie between * Ferments are usually designated by names with the termination ' ase,' and indicating the kind of substances on which they act, or sometimes their source. Thus proteases are ferments acting on proteins, amylases ferments, acting on starch, etc. 332 DIGESTION 37° and 33° C. ; the ' killing ' temperatures between 60" and 75° C. when they are heated in solutions, but considerably higher when they are heated dry. The action of the digestive enzymes is hydrolyiic — i.e., it is accompanied with the taking up of the elements of vfa.ter by the substance acted upon. The accumulation of the products of the action first checks and then arrests it. In many cases this seems to be due to combination of the ferment with one' or other of the end products, and the consequent segregation of the ferment from the reaction mixture. The enzyme is not affected indiscriminately by any of the end products. On the contrary, their action is curiously selective. Thus the hydrolysis of lactose by lactase is retarded by galactose, but not by the other end product dextrose. The hydrolysis of cane-sugar by invertase is retarded by levulose, but not by dextrose. The splitting of the dipeptid (p. 2) glycyl-Z-tyrosin by a ferment in the expressed juice of yeast-cells is greatly delayed by one of the products (Wyrosin), but not by the other (glycocoU) . Combination of the ferment with an end product is not, however, the only way in which the reaction may stop before the whole of the substrate, as the substance acted on by the ferment is termed, has been changed. It has been demonstrated in some cases that this is due to the action of the enzymes being reversible. For example, lipase (p. 357) not only decomposes the esters ethyl butyrate or glycerin butjnrate, but also builds them up again from the decomposition products — ethyl but37rate from ethyl alcohol and butyric acid, glycerin butyrate from glycerin and butjn-ic acid (Kastle and Loevenhart, Hanriot). Thus: C3H,COOC2H6 + HaO^rzZ^-CgHfCOOH + C2H5OH. Ethyl butyrate. Water. Butyric acid. Etliyl alcohol. The action of the enzyme is merely to accelerate the estabhsh- ment of the proportions in which the four bodies entering into the reaction are in equilibrium, and the point of equilibrium is the same whether we start from one or the other side of the equation repre- senting the reaction. Such reversible reactions in the presence of enzymes seem to afford the key to the explanation of many of the S5mtheses which are known to occur in the body. Sometimes the action is not strictly reversible in the sense that precisely the original material is reconstructed, but from the products of the hydrolysis substances are sjmthesized or condensed, which are then incapable of being split by the ferment. When a concentrated solution of dextrose is acted on for a long time by yeast maltase, a ferment obtained from yeast which changes maltose into dextrose, some of the dextrose is reconverted into isomaltose and dextrin-hke bodies. Isomaltose is not again hydrolysed by maltase. The ferment emulsin contained in almonds behaves in the converse way. It hydrolyses isomaltose so as to form dextrose, and then condenses dextrose to maltose (Armstrong). THE CHEMISTRY OF THE DIGESTIVE JUICES 333 Many of the ordinary substances of the laboratory will accelerate a reaction which goes on slowly in their absence. , These are called catalysers. Some writers also speak of catajysers ' which retard a reaction progressing quickly in their absence. The process by which the reaction is accelerated (or retarded) is termed catalysis. A typical catalyser can exert its action when it is present in ex- ceedingly, small amount in comparison with the substance acted lUpon. However it may enter into the reaction, it does not take part in the formation of the final products nor contribute to the energy changes, and for this reason is often apparently unaltered at the end of the process. The catalysers have therefore been compared to the lubricants used for machinery as contrasted with the coal or other source of energy. If it be remembered that the expression is a purely metaphorical one, we may say that the catalyst oils the reaction so that it slips on smoothly and swiftly to an end-point which would, however, have been reached just the jsame in time. A classical instance of catalysis is the inversion of cane-sugar by weak acids, i.e., the change of the cane-sugar into a mixture of equal quantities of dextrose and levulose — a reaction which may be represented by the equation Ci2H2aOn + H20= CgHiaOg -I- CgHjaOn. Cane-sugar. Water. Dextrose, Levulose. ■ This is a reaction which occurs also when the sugar is simply dis- solved, in water, but with extreme slowness at the ordinary tempera- ture, although more rapidly at 100° C. The effect of the acid is to catalyse the reaction, to markedly accelerate it. The hydrogen ions of the free acid are responsible for the catalysis, and they are not used up in the process, for the reaction at the end is unaltered. The same action upon cane-sugar is exerted by an enzyme, invertase, found in intestinal juice, although the laws governiiig the reaction are somewhat different. Reversibility of the reaction can be even more clearly denionstrated for catalysers than for enzymes. For example, the condensation of acetone to diacetone-alcohol, which is accelerated by hydroxyl ions (as by the addition of sodium hydroxide, ammonia, etc.), only proceeds to a certain point, at which equilibrium is establishied between the proportions of acetone and , the condensation product. Henceforth as much of the latter is decomposed as is condensed. Thus: 2CH3.CO.CH3«=^CH3.CO.CH2.C(CH3)aOH. Acetone. . Acetone alcohol. On the other hand, the final equilibrium point need not be the same for a catalyser and an enzyme. For example, amyl butyrate is formed and decomposed according to the equation CgHuOH -HCgH,COOH<— -^ C3H,COO.C6Hu -j-HgO. Amyl alcohol. Butyric acid. Amyl butyrate. WateV. 334 DIGESTION The reaction can be accelerated either by a catalyst — e.g., H ions — as by addition of free hydrochloric or picric acid, or by pancreas lipase. When the concentrations of the reacting substances are appropriately chosen, the same equilibrium point will be reached from either side of the equation — i.e., the same percentage of the butyric acid will be converted into the ester if we start with the alcohol and acid, as will remain combined as ester if we start with the amyl butyrate. But the proportion will not be the same when the reaction is acclerated by H + as when it is accelerated by the enzyme. And although it is probable that there is no fundamental difference between the action of the digestive enzymes and that of the inorganic catalysers, it is much too early to dogmatize. Not even the markedly specific action of the digestive ferments can be considered an essential distinction. It is true that invertase will act upon dextrose, and not at all upon maltose or lactase. But there are other sugars, e.g., rafifinose, a trisaccharide with the formula C^gHagOj^, obtained from beet-sugar residues, which it will hydrolyse. Raffinose is made up of one molecule each of dextrose, levulose, and galactose. On heating with dilute acids, it is decom- posed into these substances. Invertase, however, only splits off the levulose molecule, leaving a disaccharide isomeric, but not identical with lactose. Similarly lactase, which is without action upon cane-sugar or maltose, will hydrolyse the jS-galactosides, and maltase, inert as regards cane-sugar or lactose, will hydrolyse the a-glucosides. On the other hand, emulsin decomposes the iS-gluco- sides, to which group most of the natural glucosides belong, as well as the ^-galactosides and lactose. From rafiinose emulsin splits off galactose, leaving cane-sugar. Since the u and /3 compounds are isomeric, and differ not in their composition but in their struc- ture, it has been concluded that the structure of the molecule of a substance must be related to the structure of the enzyme which can act on it, in some such way as a lock is related to its proper key. Thus the key lactase fits in the lock lactose, but not in the lock dextrose or the lock maltose. Although the same specificity is not to be observed in the action of catalysers as in the action of enzymes, it is not difficult to find many instances in which inorganic substances show a marked limitation of their cataljd:ic effects to particular reactions. Thus hydriodic acid is slowly oxidized in • presence of hydrogen peroxide, with formation of iodine and water. This reaction is accelerated by the addition of many substances, e.g., tungstic acid. But tungstic acid has no catalytic effect on the oxidation of hydrogen peroxide by bromic acid. The existence of an optimum temperature for ferment action, above which it rapidly decreases, and eventually comes to a com- plete stop, is also in all probability only a superficial distinction between enzymes and catalysers. For enzymes are easily altered, THE CHEMISTRY OF THE DIGESTIVE JUICES 335 or even destroyed, at temperatures which very likely would- favour their action were they as thermostable as the majority of catalysing agents. And inorganic catalysts are known which also show the phenomenon of an optimum temperature depending on changes produced in their physical condition when the temperature is raised above this point. Thus a colloidal solution (or ' sol,' as it is called) of platinum, prepared by passing electric sparks between two platinum electrodes immersed in distilled water, and containing the metal in the form of ultra-microscopic particles, acts as a cata- lyser of a number of reactions. As the temperature is increased up to a certain ' optimum,' the velocity of the catalysed reaction is increased. But beyond this, as the boiling-point is approached, the colloidal platinum is precipitated, and ceases to influence the reaction. As to the manner in which an enzyme increases the velocity of its appropriate reaction, it is not easy to make any very positive statement. Several possibilities are recognized, of which two have been especially discussed, (i) The existence of the enzyme in colloidal solution may be important. It is characteristic of colloidal solutions, in which the dissolved substance is present in the form of extremely fine particles, that the total surface of the particles is very great in proportion to ths mass of the substance in solution. Thus, a sphere of about the same volume as the eyeball, with a diameter of, say, 2 centimetres, would have a surface of i2-5 square centimetres. If this material were subdivided into spheres of about the same volume as a leucocyte, with a diameter of, say, 10 jj., it would form eight thousand million of these spheres, with a total surface of over 2^ square metres. If the small spheres were further subdivided into spherical particles, with a diameter only the thousandth part of that of a leucocyte, say -!—, each would form a thousand million of these particles, and the total surface of all the particles would be about 2,500 square metres. Now, it is known that the intensity of action of some of the inorganic catalysers is proportional to the' surface exposed. For example, hydrogen peroxide, if left to itself, is slowly decomposed into water and oxygen. The addition of finely divided platinum, in the form of platinum black, greatly hastens the decomposition, and the oxygen bubbles off. The colloidal platinum sol is still more effective. The nature of the surface effect is not entirely clear. One factor has been thought to be an increase in the concentration of dissolved substances - or condensation of gases at the surface, and the better opportunity for mutual action thus afforded to the ferment and the substrate. The great extension of the surface cannot be the only factor in the catalysis; otherwise any fine powder or suspension would have a cata- lytic action. But kaolin, or fine sdnd, or colloidal solutions of ordinary proteins or gelatin, have little, if any, effect on the decomposition of hydrogen peroxide. (2) Enzymes may produce their effects by contributing to the for- mation of bodies intermediate between the substrate and the end- products. If the time required for the formation of a given quantity of the intennediate compound and the time required for the decom- position of this compound into the final products of the ferment action are in sum less than the time requireid for the direct change of the 336 DIGBSTWN substrate into the end-products, the enzyme will clearly act as a cata- lyser of the reaction. It has been shown that in the case of certain inorganic catalysers this does occur. Thus, in the oxidation of hydriodic acid by hydrogen peroxide, which has been already referred to, molybdic acid has the power of acting as a catalyser. It has been proved that the reaction occurs in two stages, permolybdic acid being first formed by the action of the peroxide on molybdic acid. The permol- ybdic acid then acts on hydriodic acid, producing iodine and water, and being itself reduced again to molybdic acid, which therefore comes out at the end of the reaction unchanged. The velocity of the double reaction is much greater than that of the direct oxidation of hydriodic acid by hydrogen peroxide. There is evidence that the ferment actually combines with the sub- strate, the combination then breaking up to form the end-products. For instance, it has been shown that the amount of lactose hydrolysed by lactase in a given time, when the ferment is present in very small quantity in comparison with the substrate, is proportional to the con- centration of the ferment, . and independent of the concentration of the lactose. Also with a given small concentration of ferment the amount of lactose hydrolysed is at first the same for successive equal intervals of time. These facts can only be explained by the assump- tion that the ferment first combines with a portion of the substrate, the rest of which remains inactive as regards the reaction, and that this combination then takes ujp water and decomposes into the end- products, in this case dextrose and galactose, setting free the ferment to combine with another portion of the substrate. ' The Quantitative Estimation of Ferment Action. — Since we have as yet no certain method of freeing the digestive ferments from impurities, our only quantitative test is their digestive activity. And since a very small quantity of ferment can act upon a practically indefinite amount bf material if allowed snfiicient time,. we can only make comparisons When the time of digestion and all other conditions are the same. If we find that a given quantity of one gastric extract, acting on a given weight of fibrin, dissolves it in half the time required by an equal amount of another gastric extract, or dissolves twice as much of it in a given time, we conclude that the digestive activity of the pepsin is twice as great in the first extract as in the second. But this does not permit us to say that the one contains twice as much pepsin as the other. For it has been found that the amount of digestion in a given time is not directly proportional to the quantity of ferment present, but to the square root of the quantity of ferment (Schiitz's law). This law was deduced by Schiitz for pepsin, but is said to hold also for trypsin, steapsin, and ptyalin (Pawlow, Vernon). To determine the amount of ■ proteolysis the nitrogen of the protein which has gone into solution may be estimated (p. 514). The following table shows the results of one experiment : Pepsin Solution used in C.C. Digested Nitrogen in Grammes. Found. Calculated.- , I • 4. 9 16 0-0230 0-0427 0-0686 0-0889 0-0223 0-0446 . 00669 0-0892 THE CHEMISTRY OP THE DIGESTIVE JUICES 337 Or a piece of a glass capillary-tube filled with heat-coagulated egg-white may be cut off and placed in the digestive mixture (Mett's tubes). At the end of the period of digestion the length of the piece of tube and that of the undigested remnant of the column of coagulated protein are measured with a millimetre scale under a low-power microscope. The difierence gives the length of the column digested. If i c.c. of gastric juice caused in a given time digestion of 2 mm, of the egg-white, 4 c.c. of the same juice would digest in the same time and under identical conditions about 4 mm., and 9 c.c. about 5 mm. As a test of the activity of a diastatic ferment, we take the amount of sugar formed in a given time in a given quantity of a standard starch solution. To determine the activity of a liquid, say, the pancreatic juice, as regards fat-splitting ferment, the acidity of the einulsion formed by the juice and fat after standing for a definite time at a given temperature (with occasional shaking) can be estimated by titration with baryta solution. In addition to the ferments of the digestive juices which act extra- cellularly in the lumen of the alimentary canal, and those which do their work intracellularly in its walls, micro-organisms are present in the gut, and even in normal digestion contribute to the changes brought about in the food ; while under abnormal conditions they may awaken into troublesome, and even dangerous, activity. It is now known that these act by producing intracellular enzymes. It may be noted here, although the subject must be again referred to (p. 384), that specific substances capable of inhibiting the action of ferments exist. Some of these antiferments are normally present in the body — -an antitrypsin, for instance, in normal blood-serum. Numerous antiferments may be artificially obtained by immunizing animals with the original ferments. Thus an antilipase is found in the serum of rabbits after injection of pancreatic lipase, and an antiemulsin after injection of emulsin. Injection of rennin causes the formation of antirennin, which can be deinonstrated in the blood-serum and milk of the immunized animal. Besides the anti- ferments, bodies, sometimes, spoken of as ' co-enzymes,' are known which aid the action, of certain enzymes, not in the general way in which, for instance, increase of temperature up to the optimum does, but in some quite special manner. Thus, as we shall see, bile salts greatly facilitate the fat-splitting action of lipase. This co-operation is not to be confounded with the activation of the proferment or zymogen which in many cases represents the inactive form of the enzyme, while it is still within the secreting cells. For, once activated, the fully formed enzyme cannot be made to revert to the zymogen stage. For example, the active trj^sin of the pancreatic juice cannot be changed into inactive trypsinogen, whereas substances which simply co-operate or co-act with enzymes leave thclatter unaltered when they are removed. Thus lipase does not preserve the increased activity conferred upon it by bile salts when the bile salts are again separated from the digestive mixture. It is now .necessary to consider in detail the nature of the various 338 DIGESTION juices yielded by the digestive glands, and the inechanism of their secretion. Since it is along the digestive tract that glandular action is seen on the greatest scale, this discussion will practically embrace the nature of secretion in general. And here it may be well to say that, although in describing digestion it is necessary to break it up into sections, a true view is only got when we look upon it as a single, though complex, process, one part of which fits into the other from beginning to end. It is, indeed, the business of the physiologist, wherever it is possible to insert a cannula into a duct and to draip off an unmixed secretion, to investigate the properties of each juice upon its own basis; but it must not be forgotten that in the body digestion is the' joint result of the chemical work of five or six secretions, the greater number of which are actually mixed together in the alimentary canal, and of the mechanical work of the gastro- intestinal walls. ' Saliva. — The sahva of the mouth is a mixture of the secretions of three large glands on each side, and of many small ones. The large glands are the parotid, which opens by Stenson's duct opposite the second upper molar tooth ; the submaxillary, which opens by Wharton's duct under the tongue; and the subhngual, opening by a number of ducts near and into Wharton's. The small glands are scattered over the sides, floor, and roof of the mouth, and over the tongue. Two types of salivary glands, the serous or albuminous and the mucous, are distinguished by structmral characters and by the nature of their secretion; and the distinction has been extended to other glands. The parotid of many, if not all, mammals is a purely serous gland; it secretes a watery juice with a general re- semblance in composition to dilute blood-serum. The submaxillary of the dog and cat is a typical mucous gland; its secretion is viscid, and contains mucin. The submaxillary gland of man is a mixed gland; mucous and serous alveoli, and even mucous and serous cells, are intermingled in it. The submaxillary of the rabbit is purely serous. The sublingual is, in general, a mixed gland, but with far more mucous than serous alveoli. Some of the small glands are serous, others mucous in type. The mixed saliva of man is a somewhat viscous, colourless liquid of low specific gravity (1002 to 1008, average about 1005), alkaline to litmus, acid to phenolphthalein, but when tested by the electrical method (p. 24) almost neutral. Besides water and salts, it contains mucin (entirely from the submaxillary, the sublingual and the small mucous glands of the mouth), to which its viscidity is due, traces of serum-albumin and serum-globulin (chiefly from the parotid), and a ferment, which hydrolyses starch, and therefore belongs to the group of amlyases or diastases. It differs somewhat from the amylase of pancreatic juice. But the small differences THE CHEMISTRY OF THE DIGESTIVE JUICES 339 o m w w H tn W N I I H en W fq w w H O H !/3 O o Cm S o •US gs ^^ -2° ~ -g o o ^ C3 60 cnCM ■^•a 03 Q a lu '-J o g a s 2 C3 0) (3 "O o g <« "= P S .. A .a 5 cl III .g|gs.s||is^i||i ■?t3ss^ III Irfgllils^lla-s .2 o wOCMm o- o ■•-• > V cl. (U - ^1 4J M ^8 SDnl 2 +■ 3 CO ' © o H »03 ta fl a.gsi5 3ia3 ^'^-a is ■« (§ "fcl ts >, m ft ft-H »,-,-„ d ft>,ar y Srj Q, O-Tl J3 i' d d 0<(!lz; CHCH2CH(NH2)COOH. ■iNorleucin (^a-amino-caproic acid), CH3.CH2.CH2.CH2.CH(NH2).COOH. Isoleucin (a-amino-/3-methyl-/3-etliyl-propionic acid), ^^3\cH.CH{NH2).COOH. Cystein (a - amino-;8 - thiopropionic acid), CH2 (SH).CH(NH2)COOH, which is unstable, two molecules of it easily yielding cystin di-(o-amino-/3-thiopropionic acid) , COOH.CH(NH2).CHa.S.S.CH2.CH{NH2).COOH. /Aspartic or aminosuccinic acid, CH(NH2).COOH. :2 ol CH2COOH. Glutamic or glutaminic acid, CHg/^g^^^alj^COOH o ,0 p g -y .£ I Tyrosin (para-oxyphenylaminopropionic acid) , N g "S -[ _ C^H40H .CH2 .CH (NH2) .COOH . PP g.S ^ Phenylalanin (phenylaminopropionic acid), CgH5CH2CH(Ntl2)C00H. tlT).> 1^ «,§ Prolin (pyrrolidin carboxylic acid). Oxyprolin (oxypyrrolidin carboxylic acid). Tryptophane (a-amino-/3-indol-propionic acid), /NH— CH HC<^ II ^N — C— CH2.CH(NH2).COOH. Histidin (/(5-imidazol-a-aminopropionic acid), CgHgNjOa- DiAMlNO-AciDS AND THEIR COMPOUNDS. Lysin {a-amino-E-amino-caproic acid, CH2NH2(CH2)3CH(NH2)COOH. Arginin (guanidinaminovalerianic acid), c/NH2 \NHCH2(CH2)2CH (NH2)COOH. HN = Ammonia (representing the so-called " amide-nitrogen,' and liberated from the products of acid hydrolysis of proteins by heating the mixture after addition of alkali). It has not been shown that ammonia is itself one of the ' Bansteine ' of the proteins. THE CHEMISTRY OF THE DIGESTIVE JUICES 355 In the artificial compounds of two or more amino-acids which have been synthesized by Fischer and named by him polypeptides (p. 2), the carboxyl group of one amino-acid is linked with the amino group of another. For example, a molecule of alanin and a molecule of glycin form, with loss of a molecule of water, a molecule of alanyl-glycin, according to the equation NHa.CHa.COIOH +HiNH.CH.CH3— H20= NHo.CHo.CO.NH.CH.CHo. " I I COOH COOH Glycin. Alanin. Glycyl-alanin. Two or more molecules of the same amino-acid can be linked in the same way; e.g., two molecules of glycin yield a molecule of glycyl-glycin, and so on. It has been proved that polypeptides identical with, some of these S5aithetic bodies are present in the peptone mixtures derived from the native proteins, so that it must be assumed that one of the ways at least in which amino-acids are linked in the protein molecule is that described. It has been suggested that the early appearance of some of these amino-acids in pancreatic digestion is not really due to trypsin, but to other ferments, peptases, which act upon the peptones formed by the trypsin. There is, however, no clear evidence of the existence of a separate peptone-splitting enzyme in pancreatic juice, like the erepsin of intestinal juice, and it is therefore most natural to suppose that under the influence of trypsin the protein molecule breaks at different points from those at which it ruptures under the influence of pepsin. After the most prolonged artificial digestion with trypsin, a residue of the protein remains unconverted into these relatively simple substances. But even this small portion of the original protein has undergone a great change, for it no longer gives the biuret reaction. It can be split into amino-acids, etc., by heating with acid, and also by the action of the erepsin of the intestinal juice, and then yields mainly prolin and phenylalanin, substances which are gefterally not to be detected among the decomposition products of protein after digestion with pancreatic juice. This illustrates the important fact that some of the ' building stones ' of the protein molecule can be separated from it with far greater ease than others. Tyrosin, tryptophane, and cystin appear very early in the digestive fluid, and tyrosin, as shown in the following example from Abder- halden, may be completely liberated at a time when glutaminic acid is scarcely more than beginning to appear. The plant protein edestin, obtained from cottonseed, was digested with pancreatic juice or an extract containing trypsin. The quan- tities of tyrosin and glutaminic acid liberated at different periods of the experiment are expressed as percentages of the total amounts of these substances contained in the edestin. 336 DIGESTION Duration of Digestion : I Day. 2 Days. 3 Days. 7 Days. 16 Days. Tyrosin Glutaminic acid 78-4 4' 3 97-6 976 io'9 100 3i'i 100 6o'2 When trypsin acts upon protein already digested by pepsin, this partially hydrolysed residue is smaller than when the trypsin acts alone, no matter for how long a time. Also the decomposition of a given quantity of protein by trj^sin is accomplished in a notably shorter time if it has been previously subjected to the action of pepsin. This illustrates the co-operative ' relation of these two ferments — a relation still more clearly implied in the fact that, although trypsin readily forms albumoses and peptones from native pjTotein when such is offered to it, yet in natural digestion the great albumose- and peptone-forming ferment is pepsin. In the lumen of the intestine the trypsin is confronted mainly with protein already hydrolysed to the albumose and peptone stage in the stomach. In other words, instead of the very large molecules of the original protein food with a weight of perhaps 5,000 to 7^000, the trypsin begins its action on a much larger number of much smaller molecules of only one-twentieth the initial weight or less. The statement is sometimes made that trypsin is a stronger proteolytic ferment than pepsin. This may be true in the sense that trypsin carries the de- composition down to bodies of smaller molecular weight than pepsin. But within the range of its hydrolytic action pepsin decomposes certain proteins and allied bodies more readily than trypsin— e.^., the serum proteins, and especially elastin and the constituents of connective tissue. In all that we have hitherto said regarding tryptic digestion we have supposed that putrefaction has been entirely prevented — e.g. , by the addition of toluol. If no antiseptic is added to a tryptic digest, it rapidly becomes filled with micro-organisms, and emits a very disagreeable faecal odour ; and now various bodies which are not found in the absence of putrefaction make their appearance, such as indol, skatol, and other substances, to which the faecal odour is due. They are not true products of tryptic digestion, but are formed by the putrefactive micro-organisms, which can themselves split off from proteins numerous decomposition products, including tyrosin, and change tyrosin into indol. Amylase or pancreatic amylase, the diastatic or sugar- forming ferment of pancreatic juice, changes starch into dextrin and maltose, just as the ptyalin of saliva does. The two ferments are possibly identical, but under the conditions of action of the pancreatic juice" its diastatic power is greater than that of saliva, and it readily acts THE CHEMISTRY OF THE DIGESTIVE JUICES 357 on raw starch as well as boiled. Pancreatic amylase is mainly, perhaps entirely, present in the juice in the form of active ferment. If a zymogen stage exists, the mother-substance is less stable or less easily extracted from the gland than is trypsinogen. In this respect amylase also resembles ptyalin. A small amount of maltase is contained in pancreatic juice, and further hydrolyses to dextrose a portion of the maltose formed by the amylase. Steapsin or pancreatic lipase splits up neutral fats into glycerin and the corresponding fatty acids. The latter unite with the alkalies of the pancreatic juice and the bile to form soaps. In this important process bile acts as the helpmate of pancreatic juice; together they effect much more than either or both can accomplish by separate action. Many tissues contain fat-splitting ferments or lipases, some of which are perhaps identical with the pancreatic lipase. The lipase exists as active ferment in the pancreatic juice, but there is reason to believe that a portion of it may be present as a zymogen in the gland, and probably in the secretion as well. It is changed into active ferment by the bile salts. Active lipase can also be extracted from the pancreas by glycerin or water. It is to be noted that it is only the proteolytic enzyme which is totally inactive till it reaches the intestine. The significance of this will be discussed later on. Bile. — Bile is a liquid the colour of which varies in different groups of animals, and even in the same species is not constant, depending on the length of time the fluid has remained in the gall-bladder and •other circumstances. When it is recognized that the colour is due to a series of pigments, which are by no means stable, and of which one can be caused to pass into another by oxidation or reduction, this want of uniformity will be easily intelligible. The fresh bile of carnivora is golden-red. The bile of herbivorous animals is in general of a green tint, but, when it has been retained long in the gall-bladder, may incline to reddish-brown. Fresh human bile, as it flows from a fistula just established, is of a reddish-brown, golden- yellow or yellow colour. Beaumont speaks of the yellowish bile which he could press into the stomach of St. Martin by manipulating the abdomen. In a case observed by the writer, it was seen that when the bile flowing from a fistula was allowed to spread out in a dressing, it became greenish, because of oxidation of a part of the bilirubin to biliverdin, although as it actually escaped from the fistula it was yellow. The bile of a monkey taken from the gall-bladder immediately after death is dark green, but if left a few hours in the gaU-bladder it is brown, the green pigment having been reduced. It should be remembered that human bile from the post-mortem room may alter its colour in the interval which must elapse before it can usually be procured after death. Bile, as obtained from fistulae in otherwise healthy persons, has a specific gravity of about 1008 to loio. In the gall-bladder water is absorbed from the bile and 358 DIGESTION mucin added to it, so that the specific gravity of bladder bile is as high as 1030 to 1040. The reaction is feebly alkaline to litmus. . The composition of two specimens of human bile — one from a fistula, the other from the gall-bladder — is shown in the following table : 1 Bladder Bile. Fistula Bile. Water 8g8-i 977-4 Solids loi-g 22-6 Mucin and other substances insoluble in alcohol I4-5 2-3 Sodium taurocholate and sodium glycocholate 56-5 lO-I Inorganic salts 6-3 8-5 Fat ] Lecithin J- 30-9 0-05 CholesterinJ 0-56 The substance which renders bladder bile viscid, but which is present in much smaller amount in bile from a fistula, and is probably entirely absent from the fluid as it is secreted by the Uver-cells, is commonly termed 'mucin.' It has been shown, however, that in many animals — for example, the ox, dog, sheep, etc. — ^the substance is not a true mucin. It does not yield, like mucin, on boiling with dilute acid, a carbo- hydrate group (viz., glucosamine, CgHnOgNHj, corresponding to dextros; in which OH is replaced by NH2) . It is relatively rich in phos- phorus, and consists — ^mainly, at any rate — of a phospho-protein (p. 2). The mucilaginous substance of human bile consists largely of true mucin. Mucin is scarcely to be looked upon as an essential constituent of bile ; it is not formed by the actual bile-secreting cells, but by mucous glands in the walls and goblet-cells in the epithelial lining of the larger bile-ducts, and especially of the gall-bladder. Bile-Pigments. — rit has been said that these form a series, but only two of the pigments of that series are present in normal bile, bilirubin, and biliverdin. In human bile, the former, in herbivorous bile and that of some cold-blooded animals, such as the frog, the latter is the chief pigment. But bilirubin can be extracted in large amount from the gall-stones of cattle; while the placenta of the bitch contains bili- verdin in quantity, although, as in all camivora, it is either absent from the bile or exists in it in comparatively small amount. These facts show that the two pigments are readily interchangeable, but there is no question that bilirubin is the pigment which is formed by the liver-cells. Bilirubin (C32H36N4O6) can be prepared from powdered red gall- stones by dissolving the chalk with hydrochloric acid, and extracting the residue with chloroform, which takes up the pigment. From thii solution, on evaporation, or from hot dimethyl anilin, beautiful rhombii tables or prisms of bilirubin separate out. Biliverdin (C32H38N4O8) can be obtained from the placenta of thi bitch by extraction with alcohol. It is insoluble in chloroform, and h] means of this property it may be separated from bilirubin when the twi happen to be present together in bile. Biliverdin can also be formei from bilirubin by oxidation. By the aid of active oxidizing agents THE CHEMISTRY OF THE DIGESTIVE JUICES 359 such as yellow nitric acid (which contains some nitrous acid'), a whole series of oxidation products of bilirubin is obtained, beginning with biliverdin, and passing through bilicyanin, a blue pigment, and other intermediate bodies, to choletelin, a yellow substance. It is question- able whether these are all definite compounds. This is the foundation of Gmelin's test for bile-pigments (see Practical Exercises, p. 456). The same colours are produced, and in the same order, when a solution of bilirubin in chloroform is treated with a dilute alcoholic solution of iodine. The positive pole of a galvanic current causes the same oxidative changes, the same play of colours, while the reducing action of the negative pole reverses the effect, if the action of the positive electrode has not gone too far. These reactions can also be used for the detection of bile -pigments. By the reducing action of sodium amalgam on bilirubin, hemi- bilirubin (C33H44N40g) is obtained. It gives a beautiful red colour with ^-dimethylaminobenzaldehyde (Ehrlich's reaction). Hemibilirubin is identical with the urobilinogen of urine from which urobilin is derived. Urobilinogen and urobilin (often called in this connection stercobilin) are also found in the faeces from birth onwards, although not in the meconium (p. 418). Urobilinogen is derived from the normal bile- pigment by reduction in the intestine itself, where reducing substances due to the action of micro-organisms are never absent in extra-uterine life. The bile of most animals shows no characteristic absorption spectrum. But the fresh bile of certain animals, the ox, for instance, does show bands. These, however, are not due to the normal bile-pigment, and they are not essentially changed when this is oxidized or reduced by electrolysis. MacMunn attributes the spectrum of the bile of the ox and sheep to a body which he calls cholohaematin, and which does not belong to the bile-pigments proper. The Bile-Salts. — ^These are the sodium salts of certain acids, of which glycocholic and taurocholic are the chief. In the bile of omnivora, including man, both are in general present, and in various proportions ; in human bile there is more glycocholic than taurocholic acid; some- times taurocholic acid is entirely absent. In the bile of many camivora — e.g., the dog and cat — only taurocholic acid is found; in that of the camivora generally it is by far the more important of the two acids. In the bile of most herbivora there is much more glycocholic than taurocholic acid. The bile acids are paired acids: glycocholic acid (better named cholyl-glycin) formed by the union of glycin and cholic acid, and taurocholic acid (or cholyl-taurin), consisting of cholic acid united with taurin. The decomposition of the bile-acids into these substances is effected by boiling them with dilute acid or alkali, a molecule of water being taken up; thus — C28H43NO8 + HaO = CHa (NH^) .COOH + C24H40O5 ; (glycocholic acid. Glycin. Cholic acid. C26H45NSO, 4-H20=CHa(NH2).CH2.S02.0H -FCa4H4o05. Taurocholic acid. Taurin. Cholic acid. A notable difference between glycocholic and taurocholic acid is that the latter contains sulphur. The whole of this belongs to the taurin. Both glycin and taurin are derived from the disintegration of proteins. We have already seen that among the products of protein hydrolysis a sulphur-containing body, cystein, which is readily changed into cystin, is found, and there is good evidence that taurin is derived from cystein 36o DIGESTION or cystin. In certain patliological conditions cystin appears in the urine (cystinuria). The source of the chohc acid which goes to form the bile acids is unknown, but it has been surmised that it may be derived from cholesterin. Thus, CH2.SH + 3O CHo.SOa.OH CH2.SO2.OH I I i CH.NH2 = CH.NHo = CH.NH2 I I COOH COOH— COa Cystein. Cysteinic acid. Tauriii. Traces of cholic acid, formed by hydrolysis from the bile-acids by the action of putrefactive bacteria, are found in the intestines, especially in the lower portion. Pettenkofer's test for bile-acids (Practical Exjrcises, p. 456), acciden- tally discovered in examining the action of bile upon sugar, depends upon three facts: (i) That cholic acid and furfurald.;hyde give a purple colour when brought together; {z) that the bile-salts yield cholic acid when acted upon by sulphuric acid ; (3) that when cane-sugar is decomposed by strong sulphuric acid, furfuraldehyde is formed. Since a similar colour is given when the same reagents are added to a solution containing albumin, it is necessary to remove this, if present, from any Uquid which is to be tested for bile-acids. Lecithin and cholesterin, or cholesterol, are by no means peculiar to bile (p. 4). They are very widely distributed in the body. Lecithin (C44H90NPO9) belongs to the group of phosphatides, fat-like phosphorus- containing substances presenlt in all cells. It is a compound of glycerin with two molecules of fatty acid and one of phosphoric acid. The phosphoric acid is at the same time united with a base cholin (CgHjgNOa). The fatty acid (stearic, palmitic, oleic, etc.) varies in different varieties of lecithin . Heated with baryta-water, lecithin yields the corresponding fatty acid in the form of a soap, along with cholin and glyceryl-phos- phoric acid. Glyceryl-phosphoric acid can be further spUt so as to 3rield a molecule of glycerin and one of phosphoric acid. Cholesterin is a substance with the empirical formula C27H46O. It contains an alcohol group in virtue of which fatty acids can be linked to it, forming esters. It is best obtained from white gaU-stones, of which it is the chief, and sometimes almost the sole constituent (see Practical Exercises, p. 457). All the compounds related to cholesterin are grouped together under the name of sterins. The sterins are very vndely distributed both in animals (zoosterins) and in plants (phytosterins) . Every cell seems to contain sterins and sterin esters (compounds of the same nature as the compounds of fatty acids with the alcohol glycerin which constitute the neutral fats) . In the vertebrates cholesterin and its products constitute the chief, perhaps the only sterins, but in invertebrate animals and plants there is a much greater variety/ of these substances. The chief inorganic salts of bile are sodium chloride, sodium carbonate, and alkaline sodium phosphate. The phosphoric acid of the ash comes partly from the phosphorus of organic compounds (lecithin and bile- mucin), the sulphuric acid from the sulphur of taurocholic acid, the sodium largely from the bile-salts. Iron is a notable inorganic con- stituent of bile, although it exists only in traces, in the form of phosphate of iron. Manganese is also present in minute amount. 100 c.c. of fresh bile yields 50 to 100 c.c. of carbon dioxide, part of which is in solution and part combined with alkalies. THE CHEMISTRY OF THE DIGESTIVE JUICES 361 The quantity of bile secreted in twenty- four hours in an average man is probably from 750 c.c. to a litre. In nine cases of fistula of the gall-bladder in patients operated on for gall-stones or echino- coccus the daily quantity varied from 500 to 1,100 c.c. (Brand). Digestive Functions of Bile. — The great action of the bile in digestion is undoubtedly the preparation of the fats for absorption. In this preparation four processes are important: two chemical actions, hydrolysis of neutral fats to glycerin and fatty acids, and saponification, or the formation of soaps by the union of fatty acids with bases, especially sodium ; and two physical processes, emulsifi- cation, or the formation of a mechanical suspension of such fine globules of unaltered neutral fat as exist in milk, and solution of soaps and fatty acids. While there has been much discussion as to the relative share taken by these processes, and especially by saponi- fication and emulsification in the absorption of fat (p. 435), there is no doubt that they are all concerned in the digestion of fat or the preparation of it for absorption and assimilation. In this, indeed, the processes are complementary to each other, for an essential pre- liminary to emulsification in the intestine seems to be the formation of a certain amount of soaps, soluble in the intestinal contents, while the formation of an emulsion enormously increases the surface of contact between the unaltered fat and the digestive juices, and so favours more rapid hydrolysis, saponification, and solution. In the whole series of changes the bile plays a part, though not an indepen- dent one; it acts always in conjunction with the pancreatic juice. ' While no complete explanation has been given of the precise nature of this partnership, it is certain that the fat-splitting ferment of the pancreatic juice on the one hand, and the bile-salts on the other, contribute largely to the total action. An alkaline solution, a solution of sodium carbonate, e.g., is unable of itself to emulsify a perfectly neutral oil ; but if some free fatty acid be added, emulsifi- cation is rapid and complete (p. 12). Now, there is no doubt that here a soap is formed by the action of the alkali on the fatty acid, and there is equally little doubt that the formation of the soap is an essential part of the emulsification. But it is not clear in what manner the soap acts, whether by forming a coating round the oil- globules, or by so altering the surface-tension, or other physical properties of the solution in which it is dissolved, that they no longer tend to run together. However this may be, in pancreatic juice we have the two factors present which this simple experiment shows to be necessary and sufficient for emulsification ; we have a ferment which can split up neutral fats and set free fatty acids, and an alkali which can combine with those acids to form soaps. ' Accordingly, pancreatic juice is able of itself to form emulsions with perfectly neutral oils. It is possible that the protein constituents of pancreatic juice may have a share in emulsification, since the addition of protein 362 DIGESTION — e.g., egg-white — to a soap solution increases the stability of the emulsions formed by the soap. In bile, on the contrary, although the alkaU is present, there is no fat-splitting ferment, and, according to the best experiments, bile alone has no emulsif57ing power on perfectly neutral fat. But we now come to a remarkable fact: this inert bile when added to pancreatic juice greatly intensifies its emulsifying action, and a solution of bile-salts has much the same effect as bile itself. The fact is undoubted, but the explanation is obscure. What it is that the bile or bile-salts can add to the pancreatic juice which so increases its power of emulsification, we do not know. It has been surmised that a characteristic physical property of bile, the diminution of the surface-tension of watery liquids to which it is added, may play an important part, perhaps, in enabling the fat-splitting ferments or the emulsifying soaps to get into closer contact with the unaltered fat. It is also true that bile, presumably in virtue of the chemical action of its alkaline salts, can, in presence of a free fatty acid, rapidly form an emulsion. But the pancreatic juice itself contains so considerable a quantity of sodium carbonate that it would scarcely seem to require the rela- tively feeble reinforcement of the alkaline salts of the bile. An important part of the effect of the bile is certainly due to its favouring the fat-splitting action of the pancreatic juice. By the addition of bile, the quantity of fat split up by a definite amount of dog's pancreatic juice may be increased two to threefold. It has been shown that this is an action of the bile-salts. The sodium salts of synthetically-obtained glycocholic and taurocholic acids produce the same effect. It is in virtue of this action that the bile- salts are sometimes spoken of as the co-ferment of the lipase. As already pointed out, this action is exerted in presence of the fully formed enzyme, and should not be confounded with the effect of the bile-salts in activating the lipase zymogen. The capacity of dis- solving soaps, which is a property of the bile-salts, is also of great importance in supplementing the solvent power of the intestinal hquids for the products formed by the pancreatic juice. The solution of soaps in the bile-salts has the power in its turn of dis- solving free fatty acids. The significance of this in fat absorption will be referred to again. A further illustration of the mutual adaptation of the various digestive juices, of the remarkably precise manner in which the action of each dovetails into the action of others, is afforded by the facts already mentioned in connection with the lipase of the stomach. It is highly probable that the fatty acids formed by the gastric lipase, even if formed only in small amount, may exertan important influence in emulsifying the fat as soon as it enters the intestine. The intestinal juice itself also unquestionably takes a share in the digestion of fat along with the pancreatic secretion and the bile. There exists also, as will be seen later on, a THE CHEMISTRY OF THE DIGESTIVE JUICES 363 certain adaptation between the food and the digestive secretions. Not the best illustration of this, but one which suits the present topic, is the fact that the food itself probably always contains some free fatty acids when it contains fat at all. Although our knowledge of the mutual action of the pancreatic juice and the bile on the digestion of fats is still incomplete, there is no doubt that they are equally necessary. For in some diseases of the pancreas fat or fatty acid often appears in the stools, and this token of imperfect digestion of the fatty food may be confirmed by the wasting of the patient. The same may occur when the bile is prevented by obstruction of the duct or by a biliary fistula from entering the intestine. Yet in some cases of fistula, where there is every reason to believe that all the bile is escaping externally, the nutrition of the patient — at any rate, on a diet not abnormally rich in fat — is unaffected. The mere deficiency of bile in the intestine is, of course, complicated in obstructive jaundice by the harmful effects of the biliary constituents circulating in the blood. The white stools of jaundice owe their colour, not merely to the absence of bile-pigment, but also to the presence of fat. Their highly offensive odour used to be adduced as evidence that bile is the ' natural antiseptic ' of the intestine. It seems rather to be due to the coating of the particles of food with undigested fat, which shields the proteins from the action of the digestive juices, while permitting the putrefactive bacteria to revel in them unchecked. As a matter of fact, the bile itself has little, if any, power of hindering the growth of micro-organisms, although the free bile-acids are tolerably active antiseptics. In suckling children it is not uncommon to see the faeces white with fat. This is a less serious symptom than in adults, and perhaps betokens merely that the milk in the feeding-bottle is undiluted cow's milk, which is richer in fat than human milk, and ought to be mixed with water. Bidder and Schmidt found that the chyle in the thoracic duct of a normal dog contained 3-2 per cent, of fat. In a dog with the bile-duct ligatured the proportion fell to 0-2 per cent. It is an instance of the extraordinarily exact adaptation of the digestive juices to the nature of the food, the mechanism of which will present itself for discussion later on, that the reinforcing action of the bile upon the fat-splitting ferment of the pancreatic juice is said to be greater when the food is rich in fat (p. 408). Bile has been credited with a physical power of aiding the passage of fat through membranes moistened with it by diminishing the surface tension, and it has been inferred that this has an important bearing on the absorption of fat from the intestine. But the inference does not follow from the stateiflent, and the statement has been itself denied. There is at present no evidence that the digestive function of the bile extends beyond the preparation of the food for absorption to the preparation of the mucosa for absorbing it. On proteins bile has either no digestive action, or only a feeble one. Fibrin is slightly digested by the bile of the dog and of man.. But the addition of it to fresh pancreatic juice considerably increases the proteolytic power of that secretion (Rachford), although not so decidedly as in the case of the fat-splitting action. The amylolytic 364 DIGESTION action of the pancreatic juice is also favoured by the bile, and in about the same degree as its proteol5H:ic effect. Although bile some- times exerts by itself a feebly amylolytic action, this is not to be included among its specific powers, for a diastatic ferment in small quantities is widely diffused in the body. The addition of bile or bile-salts to a gastric digest causes the precipitation of any unaltered native protein, acid-albumin, albu- mose, and pepsin. The precipitate, which is a salt -like compound of protein with taurocholic acid, is redissolved when the liquid is rendered alkahne, and therefore in excess of bile, or of a solution of bile-salts, but the pepsin has no longer any power of digesting, proteins. Pait of the bile-acids and bile-mucin is also thrown down by the acid of the digest. It has been suggested that by thus precipitating the constituents of the chyme which have not been carried to the peptone stage bile prepares them for the action of the pancreatic juice. But it is difficult to see how the precipitation of a substance can prepare the way for its digestion, and it is more probable that if any physiological value is to be given to this reaction, it has the function of preventing the absorption of proteins which have not been sufficiently split up. There is little doubt, however, that the rendering of the pepsin inactive has physiological signifi- cance, for pepsin exerts an injurious influence upon the ferments of the pancreatic juice. In digestion, then, the bile has a twofold func- tion, favouring greatly the activity of the pancreatic ferments, especially the fat-splitting ferment, and aiding in establishing the conditions necessary for the transition of gastric into intestinal digestion. Succus Entericus. — ^This is the name given to the special secretion of the small intestine, which is supposed to be a product of the Lieberkiihn's crypts or intestinal glands. In order to obtain it pure, it is of course necessary to prevent admixture with the bile, the pan- creatic juice, and the food. This can be done by dividing a loop of intestine from the rest by two transverse cuts, the abdomen having been opened in the linea alba. The continuity of the digestive tube is restored by stitching the portion below the isolated loop to the part above it. One end of the loop is sewed into the lips of the wound in the linea alba, and the other being closed by sutures, the whole forms a sort of test-tube opening externally (Thiry's fistula). Or both ends are made to open through the abdominal wound (Vella's fistula). Another method is to make a single opening in the intes- tine, and by means of two indiarubber balls, one of which is pushed down, and the other up through the opening, and which are after- wards inflated, to block off a piece of gut from communication with .the rest. Or several openings may be made at different levels in the intestine, each being allowed to heal into a wound in the abdominal wall. When pure juice is required it is collected from the lower fistulse, while the upper fistulas are opened to permit the escape of the THE CHEMISTRY OF THE DIGESTIVE JUICES 365 secretions which enter the higher portions of the alimentary canal (gastric juice, pancreatic juice, and bile). The intestinal juice so obtained is a thin yellowish liquid of alkaline reaction, generally somewhat turbid from the presence of a certain number of leucocytes and epithelial cells. Its specific gravity is about loio, the total solids about 1-5 per cent. It contains a small amount of proteins, including serum albumin and serum globulin, and about the same proportion of inorganic salts as most of the liquids and solids of the body, namely, 07 or o-8 per cent., chiefly sodiiim carbonate and sodium chloride; but, like the other digestive liquids, it is adapted to the nature of the food, and therefore its composition is not quite constant. Like bile, intestinal juice acts but feebly on the food substances by itself, and if we contented ourselves with examining the pure and isolated secretion, we should greatly, underestimate its importance. The sodium carbonate, in which it is relatively rich, will, to be sure, form soaps with fatty acids produced by the action of the pancreatic juice or of the fat-sphtting bacteria in which the intestine abounds, and thus aid in the digestion of fats. A lipase, feebler than that of the pancreatic juice, or present in smaller con- centration, is also a constituent of the succus entericus. That a great deal of fat may be split up in the aUmentary canal in the absence both of bile and pancreatic juice is well ascertained. The alkali of the succus entericus must at the same time aid in neutraliz- ing the original acidity of the chyme, and in preserving the proper reaction of the intestinal contents. A ferment called invertase, or sucrase — which is not introduced with the food or formed by bacterial action as has been suggested, since it occurs in the aseptic intestine of the new-born child — will invert cane-sugar. The ferments maltase and lactase will cause a corresponding change in maltose and lactose (see footnote, p. 350). It is worthy of remark that these inverting enzymes are present in the intestinal mucosa as well as in the intestinal juice, and extracts of the mucosa are usually distinctly more active than the juice itself. So that there is reason to believe that hydrolysis of the disaccharides may take place both in the lumen of the gut before absorption and in the wall of the gut during absorption. Inverting enzymes appear in the .intestine early in embryonic life. Maltase is the most generally distributed of all these enzymes, and it is found along with lactase in the intestine of the embryo pig, while invertase is missing till after birth (Mendel). On native proteins and starch the isolated succus entericus has little or no action. But it contains a ferment, erepsin, which, although it does not affect native proteins like serum- and egg-albumin (fibrin and caseinogen may be slightly digested), exerts a powerful action on the first products of protein hydrolysis, albumoses, and peptones, breaking them up into bodies which no longer give the biuret re- action (ammonia, mono-amino acids, hexone bases, etc.). It 366 DIGESTION destroys the diphtheria toxin, which is also rendered innocuous by trypsin. Erepsin, however, is not specific to the secreted intestinal juice, for it occurs also, not only in the mucous membrane of the intestine, which, indeed, contains a greater quantity of it than the succus entericus, but in all animal tissues hitherto investigated. It is said even to be sometimes present in pancreatic juice, since in- activated pancreatic juice, which does not digest other proteins, will sometimes digest casein. But the matter is far from being settled, and the presence of erepsin in the pancreatic tissue is a complicating circumstance. For under abnormal conditions, most glands pro- vided with artificial fistulae have an increased liability to injuries of various kinds, which might permit constituents not normally present in the secretion to pass into it from the ceUs. The kidney in mammals is even richer in erepsin than the intestinal mucous membrane. Next to these come the pancreas, spleen, and liver, then at a long interval the heart muscle, while skeletal muscle and brain-tissue are poorest of all in the ferment. The intestinal mucosa varies in its erepsin content at different levels and on different diets. In cats on a meat or a mixed diet the duodenum is about five times richer in the ferment than the stomach. The ileum is about half as rich as the duodenum, and the jejunum occupies an intermediate position between the duodenum and ileum (Vernon). The secretion of Brunner's glands in the duodenum, which resemble in structure the pyloric glands of the stomach, digests coagulated albumin, although its proteolytic powers are feebler than those of the pan- creatic juice. Enterokinase. — The most characteristic constituent of succus entericus is a ferment, enterokinase, which differs from all the fer- ments we have hitherto described in acting not directly upon the foodstuffs, but upon the trypsinogen of the pancreatic juice, chang- ing it into the active enzyme trypsin. It may therefore be spoken of as a ferment of ferments. It has been previously stated that freshly secreted pancreatic juice is without action upon proteins. The addition of succus entericus immediately confers upon it a high degree of proteolytic power. In one experiment pancreatic juice, obtained by a temporary fistula, required four to six hours to dissolve fibrin, and did not attack coagulated albumin even in ten hours. On addition of succus entericus, the same pancreatic juice dissolved fibrin in three to seven minutes, and rapidly digested coagulated albumin (Pawlow). In like manner a glycerin extract of a fresh pancreas has hardly any effect on proteins; a similar extract of a stale pancreas is active. The fresh pancreas contains trypsinogen, which is soluble in glycerin, for the inert extract becomes active when it is treated with dilute acetic acid, or even when it is diluted with water and kept at the body-temperature. If the fresh pancreas be first treated with dilute acetic acid, and then with glycerin, the THE CHEMISTRY OF THE DIGESTIVE JUICES 367 extract is at once active. The trypsinogen can therefore be activated within the pancreatic cells, gradually when the pancreas is simply allowed to stand after excision, more rapidly in presence of the dilute acid. The ordinary tests for ferment action (destruction by boiling, activity in very small amounts, etc.) have shown that this property of the intestinal juice is due to a ferment, although it differs in certain respects from most ferments — -for instance, in requiring a relatively high temperature to inactivate it. The smallest trace of enterokinase will convert a large quantity of trypsinogen into trypsin if time be given. At the same time, although to a much smaller extent, the fat-splitting and starch-digesting activity of the pancreatic juice is increased. The secretion of the duodenum causes a greater increase in the proteolytic power than that of the other portions of the small intestine, while no such difference has been made out in the case of the amylolytic and Hpolytic functions. It is probable that the enterokinase, which is secreted mainly in the upper two-sevenths of the small intestine, and solely by the intestinal epithelium, acts only on the trypsinogen, and that the amylopsin and steapsin are aided in some other way. Enterokinase is only found in the intestinal juice when pancreatic juice is present in the gut. It is therefore secreted in response to the presence of tryp- sinogen or of some other constituent of the pancreatic juice. Delezenne has attempted to explain the interaction of enterokinase and trypsinogen as an adaptive phenomenon of the same kind as the formation of antitoxins and haemolysins (p. 31). According to him, enterokinase acts like a complement in haemolysis, while trypsinogen plays the part of an intermediary body or amboceptor which enables the enterokinase to attack the protein molecule. He asserts that enterokinase, or a substance which produces a similar effect on tryp- sinogen, is contained not only in the mucous membrane of the intestine, but also in leucocytes, in fibrin (one of whose properties it is to pick out ferments from liquids containing them), in lymph-glands, in snake venom, and even in certain anaerobic bacteria. On this view trypsin would not be a definite substance produced by the interaction of enterqjiinase and trypsinogen, but only an expression for these two bodies acting together. Strong evidence against this view, and in favour of the independent existence of trypsin, had been brought forward by Bayliss and Starling, and it does ijot seem to merit, further con- sideration. According to Mellanby, enterokinase is really a proteolytic ferment, and trypsinogen contains a protein moiety with which trypsin is firmly combined. The conversion of trypsinogen into trypsin depends on the digestion of this protein moiety, and the consequent liberation of trypsin. Vernon has put forward the view that, while enterokiiiase starts the activation of trypsinogen in the intestine, and can no doubt in time complete it, the trypsin as it is formed aids in the activation of more trypsinogen to trypsin, and so on by a process of so-called auto-catalysis of the trypsinogen. This idea can be har- monized with Mellanby's conception by assuming that the trypsin fornied from trypsinogen can itself digest the protein moiety of a further portion of trypsinogen. 368 DIGESTION According to Pawlow, the reason why the trj^sin is not secreted in the active form is that active trypsin readily destroys the amylo- lytlc and lipolytic ferments. In the intestine, where trypsin is rendered active by enterokinase, these ferments are protected from its attack by the proteins of the food and by the bile. Enterokinase is itself immediately destroyed in the presence of free acid (centi- normal hydrochloric acid). Having now finished our review of the chemistry of the digestive juices, our next tisk is to describe what is known as to their secre- tion — ^the nature of th ; cells by which it is effected and their histo- logical appearance in activity and repose, and the manner in which it is called forth and controlled. Section IV.— The Secretion of the Digestive Juices — Microscopical Changes in the Qland Cells. The digestive glands are formed originally from involutions of the mucous membrane of the alimentary canal, the salivary glands from the ectoderm, the others from the endodenn (Chap. XIX.). Some are simple unbranched tubes, in which there is either no distinction into body and duct, as in Lieberkiilm's crypts in the intestines, or in which one or more of the tubes open into a duct, as in the glands of the fundus of the stomach. Some are branched tubes, several of which may end in a common duct ; such are the glands of the pyloric end of the stomach and the Brunner's glands in the duodenum. In others the main duct ramifies into a more or less complex system of small channels, into each of the liltimate branches of which one or more (usually several) of the secreting tubules or alveoli open. The salivary glands and the pancreas belong to this class of compound tubular or" racemose glands, and so does the liver of such animals as the frog. But in the latter organ the typical arrangement is obscured in the higher vertebrates by the pre- dominance of the portal bloodvessels over the system of bile-channels as a groundwork for the grouping of -the cells. In every secreting gland there is a vascular plexus outside the cells of the gland-tubes, and a system of collecting channels on their inner surface ; and in a certain sense the cells of every gland are arranged with reference to the bloodvessels on the one hand, and the ducts on the other. But in the ordinary racemose gland the blood-supply is mainly required to feed the secretion ; the cells of the alveoh have either no other function than to secrete, or if they have other functions, they are not such as to entail a great disproportion between the size of the cells and the lumen of the channels into which they pour their products.- For both reasons the relation of the grouping of the cells to the duct- system is very obvious, tc the blood-system very obscure. In the liver, the conditions are precisely reversed. We cannot suppose that the manufacture of a quantity of bile less in volume than the secretion of the salivary glands, though doubtless containing far more solids, requires an immense organ like the liver, and a tide of blood like that which passes through the portal vein. And, as we shall see, the liver has other functions, some of them certainly of at least equal importance with the secretion of bile, and one of them evidently requiring from its very nature a bulky organ. Accordingly, both the richness of the blood-supply and the size of the secreting cells are out of proportion to THE SECRETION OF THE DIGESTIVE JUICES 3^9 the calibre of the ultimate channels that carry the secretion away. The so-called bile-capillaries, which represent the lumen of the secreting tubules, are mere grooves in the surface of adjoining cells; and the architectural lines on which the liver lobule is built are : (i) the inter- lobular veins which carry blood to it; (2) the rich capillary network which separates its cells and feeds them; and (3) the central intra- lobular vein which drains it. Thus a network of cells lying in the meshes of a network of blood-capillaries takes the place of a regular dendritic arrangement of ducts and tubules; and in accordance with this the bile-capillaries, instead of opening separately into the ducts, form a plexus with each other within the hepatic lobule (see also foot- note, p. 14). The ducts and secreting tubules of all glands are lined by cells of columnar epithelial type, but the type is most closely preserved in the ducts. In none of the digestive glands is there more than a single complete layer of secreting cells. But the alveoli of the mucous salivary glands show here and there a crescent-shaped group of small deeply-staining cells (crescents of Gianuzzi) outside the columnar layer (Fig. 158, A", B"), and between it and the basement membrane, while the gland-tubes of the fundus of the stomach have in the same situation a discontinuous layer of large ovoid cells, termed parietal from their position, 0X5mtic (or acid-secreting) from their supposed function (Figs. 155-157). Access to the lumen of the glands is provided for these deeply-placed parietal cells and for the cells of the crescents by fine branching channels which enter and surround the cells. The serous salivary glands, the pyloric glands of the stomach, and the Lieberkiihn's crypts, have but a single layer of epithelium; and since there is no hepatic cell which is not in contact with at least one bile capillary, the liver may be regarded as having no more. The same is true of the pancreatic alveoli, except that in the centre of many of the acini a few spindle-shaped cells (centro-acinar cells), apparently con- tinued from the lining of the smallest ducts, may be seen. Remarkable histological changes, evidently connected with changes in functional activity, have been noticed in most of the digestive glands. In dis- cussing these, it will be best to omit for the present any detailed reference to the liver, since, although there are histological marks of secretive activity in this gland as well as in others, and of the same general character, they are accompanied, and to some extent overlaid, by the microscopic evidences of other functions (p. 526). The serous salivary glands and the pancreas can be taken together ; so can the glands of the various regions of the stomach; the mucous salivary glands must be considered separately. Changes in the Pancreas and Parotid during Secretion. — The cells of the alveoU of the pancreas or parotid during rest, as can be seen by examining thin lobules of the former between the folds of the mesentery in the hving rabbit, or fresh teased preparations of the latter, are filled with fine granules to such an extent as to obscure the nucleus. In the parotid the whole cell is granular, in the pancreas there is still a narrow clear zone at the outer edge of the cell which contains few granules or none; in both, the divisions between the cells are very indistinct, and the lumen of the alveolus cannot be made out. During activity the granules seem to be carried from the outer portion of the cell towards the lumen, and; 24 37° DIGESTION there discharged. The clear outer zone of the pancreatic cell grows broader and broader at the expense of the inner granular zone, until at last the granular zone may in its turn be reduced to a narrow contour Una around the lumen (Fig. 153) • In the uniformly clouded parotid cell a similar change takes place; a transparent outer zone arises; and, after prolonged secretion, only a thin edging of granules may remain at the inner portion of the cell (Fig. 154). In both glands the outlines of the cells be- come more clearly indicated, and a distinct lumen can now be recognized. The cells are smaller than they are during rest. The disappearance of granules from without in- wards during activity sug- gests that these are manu- factured products eliminated in the secretion, and they are generally spoken of as zymogen granules. Bensley, who has made a careful study of the pancreas in the guinea-pig, has been able to distinguish, even in fresh preparations examined in the animal's own serum, but better after staining with such, a dye as neutral red, another kind of granules, which he regards A. B Fig. .153. — A, alveolus of rabbit's pancreas, ' loaded ' (resting) ; B, ' discharged ' (active), observed in the living animal (Kiihne an,d Lea). Fig. 154. — Alveoli of Parotid Gland; A, at Rest; B, after a Short Period of Activity; C, after a Prolonged Period of Activity (Fresh Preparations) (Langley). In A and B the nuclei are obscured by the granules of zymogen. as zymogen granules in the course of formation, and therefore designates prozymogen granules. The resting acini show a clear basal zone which is unstained, and a zone next the lumen containing coarse zymogen granules which are ' faintly stained. In the active gland — e.g., after a meal or after the injection of secretin (p. 401) — prozymogen granules which stain much more intensely than the zymogen granules with neutral red make their appearance between the zymogen granules, now much reduced in number and size, and THE SECRETION OF THE DIGESTIVE JUICES 371 the clear outer zone. After prolonged secretion the zymogen granules may be entirely absent from the cells, and only a narrow rim of prozymogen granules can be seen around the lumen. In one respect the pancreas differs remarkably from the salivary glands — ^namely, in the presence of the islets of Langerhans — characteristic groups of small polygonal cells, richly sup- plied with bloodvessels, but not arranged in the form of alveoli. Some observers state that they are remarkably in- creased in size, and even in number when the pancreas is caused to secrete actively by repeated injections of secretin, and also in starvation. But it has been shown that this conclusion was based upon faulty methods of counting the islets, and even of identi- fying the islet cells. There appears to be no foundation for the view that they are derived from the ordinary secreting cells, and that they can, in turn, give rise to new alveoli by a process of pro- liferation. It is far more pfobable that they are inde- pendent structures, with a different function from the pancreatic alveoli (p. 624). Changes in the Glands of the Stomach during Secre- tion. — The mucous membrane of the stomach is covered with a single layer of colum- nar epithelium, largely con- sisting of mucigenous goblet- cells. It is studded with minute pits, into which open the ducts of the peptic and pyloric glands, the ducts being Uned with cells just Uke those of the general gastric surface. Three varieties of gastric glands have been distinguished: (i) The glands of the cardia. In man these occupy a small portion of the mucous membrane at the cardiac end, near the orifice of Fig. Fig. 155- Fig. 156. 155. — A Fundus Gland of Simple Form from the Bat's Stomach (Osmic Acid Pre- paration) (Langley). c. Columnar epithe- lium of the surface; », neck of the gland with chief or central and parietal cells; /, base, occupied only by chief cells, which show the granules accumulated towards the lumen of the gland. Fig. 156. — A Fundus Gland prepared by Golgi's Method, showing the Mode of Com- munication of the Parietal Cells with the Gland-Lumen (Schafer, after E. Muller). 372 DIGESTION the oesophagus. Some of the glands are single tubules, but others have two or more tubules opening into a common duct. Both are lined by a single layer of short columnar epitheUum, which contains granules. {2) The glands of the pyloric canal or antrum. These consist of short, branched tubules, which ojien by twos and threes into long ducts. (3) The glands of the fundus or ox5mtic glands, occupjnng the intermediate and greater portion of the organ. The gland tubules are long and seldom branched, and the ducts, into each of which open from one to three tubules, are relatively, short. The secreting parts of both kinds of glands are lined by short columnar granular cells ; and in the pyloric tubules no others are present. But, as we have said, in the glands of the fundus there are besides Idrge ovoid cells scattered at intervals like beads between the basement membrane and the lining or chief cells. The cells of the pyloric glands have a general resemblance to the chief cells of the fundus glands, but they are not quite the same. For example, the granules are less distinct in the pyloric glands. In the human stomach it is only quite near the pylorus that the parietal cells disappear altogether. The parietal cells also contain granules, but they are smaller and less numerous than those of the chief cells, so that the deeper portions of the fundus glands are much darker in appearance than the more superficial portions, since the oxyntic or parietal cells are more numerous in the neighbourhood of the ducts (Bensley). The histological changes connected with gastric secretion do not differ essentially from those described in the pancreas and the parotid, but there is much greater difficulty in making observations on the living, or at least but slightly altered, cells. For the mammal the best method is to use animals with a permanent gastric fistula, and to remove from time to time small portions of the mucous membrane for examination in the fresh condition. During digestion the granules disappear from the outer part of the chief cells of the fundus glands, leaving a clear zone, the lumen being bordered by a granular layer. Or, more rarely, there may be a uniform decre;ase in the number of granules throughout the cell. The total volume of the cell is less than in the fasting condition. The parietal cells, which are small in the fasting animal, swell up, so as to bulge out the membrana propria. Thej' reach their maximum size (in. the dog) very late in digestion (the thirteenth to the fifteenth hour). No such definite changes in their contents as those observed in the other cells have been made out. The granules in the ovoid cells during and after activity seem to be as large and as numerous as in the resting cell, or even larger. After sham feeding in dogs the histo- logical changes in the gastric glands are very shght, even when-con- siderable amounts of gastric juice have been secreted (Noll and Sokoloff). . , THE SECRETION OF THE DIGESTIVE JUICES a?.-? The chief cells of the oxyntic, and the similar if not identical cells of the pyloric glands, are believed to manufacture the pepsin-form- ing substance. The ovoid cells of the former are supposed to secrete the hydrochloric acid. The evidence on which this belief is based is as follows: The glands of the antrum pylori in the dog, in which in most situations no ovoid cells are to be seen, secrete pepsin, but no acid. The pyloric end of the stomach or a portion of it' has been isolated, Fig. 157. — The Gastric Glands (Ebstein). On the left, oxjTitic; right, pyloric the continuity of the alimentary canal restored by sutures, and the secretion of the pyloric pocket collected. It was found to be alka- line, and contained pepsin. The glands of the frog's oesophagus, which contain only chief cells, secrete pepsin, but no acid. It seems fair to conclude that the chief cells of the fundus glands in the mammal secrete none of the free hydrochloric acid, but certainly some of pepsin. But it does not follow that all the pepsin is formed by these cells, although it would seem that all the hydrochloric acid 374 DIGESTION must be secreted by the only other glandular elements present, the parietal or ' border ' cells. And, indeed, the glands in the fundus of the frog's stomach, which are composed only of ovoid cells, whilst secreting much acid, also form some pepsin, although far less than the oesophageal glands. During winter sleep (in the marmot) the production of hydrochloric acid in the parietal cells stops altogether, while the chief cells continue to accumulate granules of pepsinogen. That some pepsin is secreted by the pyloric end of the stomach is not difficult to prove. The secretion collected from the isolated pyloric portion is, indeed, like the secretion of the Brunner's glands in the duodenum, quite unable to digest protein until dilute hydro- chloric acid is added. But this is because in both cases the juice as it flows from the glands is slightly alkaline, and, as we have already seen, pepsin only acts in the presence of an acid. The milk-cmrdliijg action of these two juices also unfolds itself only when the secretions are first acidulated, and later on again neutralized; in other words, the ferments must be activated by the addition of an acid. In normal digestion the pepsin of the (in itself) alkaline secretion of the pyloric end of the stomach becomes a constituent of the acid gastric juice; and it may perhaps be considered a morphological accident, so to speak, that the oxjmtic cells of the fundjis should mingle their acid products with the (presumedly) alkaline secretion of the chief cells in the lumen of each gland-tube, instead of being massed as a separate organ with a special duct. We are not without other examples of digestive juices fitted or destined to act in a medium with an opposite reaction to their own. The ' saliva ' of the cephalopod Octopus macropus, strongly acid though it is, contains a proteolytic ferment which in vitro acts,- like trypsin, better in an alkaline than in an acid solution. And trypsin, whose precursor is a constituent of the alkaline pancreatic juice, while the enterokinase which activates it is a constituent of the alkaline succus entericus, performs a part at least of its work in an accid medium. Attempts made to demonstrate an acid reaction in the border cells have hitherto failed. Harvey and Bensley on the basis of experiments with dyes (cyanimin and neutral red), which give different colours according to whether the reaction is acid, alkaline, or neutral, have concluded that free acid exists only on the internal surface of the stomach, or at most also in the necks of the glands. The parietal cells they find alkaline. They suggest that these cells form in some way, of course ultimately from the chlorides of the blood, a chloride of an organic base which does not decompose so as to yield free hydrochloric acid until it reaches the mouth of the gland. The nature of this decom- position, if it occurs, is unknown. It may be mentioned, although only as a matter of historical interest, that some observers have denied that the acid is secreted in the depths of any cell from the chlorides of the blood, and have asserted that it is formed at the surface of contact of the stomach-wall with the gastric contents from the sodium chloride of the food by an exchange of sodium ions (p. 422) for hydrogen ions from THE SECRETION OF THE DIGESTIVE JUICES 375 the blood or lymph. It was pomted out in favour of this view that when, instead of sodium chloride, sodium bromide is given in the food, the hydrochloric acid in the stomach is to a large extent replaced by hydrobromic acid. And it was argued that this cannot be due to the decomposition of the bromide by hydrochloric acid, since it occurs in animals deprived for a considerable time of salts, and in ' salt-hunger ' the stomach contains no acid (Koeppe). It may be, however, that evsn m ' salt-hunger ' the presence of sodium bromide in the stomach stimulates the secretion of hydrochloric acid, which then decomposes the bromide, with the formation of hydrobromic acid. The sodium chloride formed in the double decomposition might be re-absorbed, and the stock of chlorides in the blood remain undiminished. It is in any case a decisive objection to this now defunct theory that a copious secretion of gastric juice, containing hydrochloric acid in abundance, can be obtained, without the introduction of any food into the stomach, either by the process of sham feeding (p. 395) or by psychical stimulation of the gastric glands when food is shown to an animal. Changes in Mucous Glands during Secretion. — In the mucous salivary and other mucous glands similar, but apparently more complex, changes occur. During rest the cells which line the lumen may be seen in fresh, teased preparations to be filled -vyith granules or ' spherules.' After active secretion there is a greall diminution in the number of the granules. Those that remain are chiefly collected around the lumen, although some may also be seen in the peripheral portion of the cell; and there is no -very distinct differentiation into two zones. That a discharge of material takes place from these cells is shown by their smaller size in the active gland. That the material thus discharged is not protoplasmic is indicated by the behaviour of the cells to proto- plasmic stains such as carmine. The resting cells around the lumen stain but feebly, in contrast to the deep stain of the demilunes, while the discharged cells take on the carmine stain much more readily. Further, when a resting gland is treated with various reagents (water, dilute acids, or alkalies), the granules swell up into a transparent sub- stance identical with mucin, which fills the meshes of a fine protoplasmic network. In ordinary alcohol-carmine preparations only the network and nucleus are stained ; the nucleus, small and shrivelled, is situated close to the outer border of the cell. When a discharged gland is treated in the same way there is proportionally.more ' protoplasm ' (or ' bioplasm ') and less of the clear material, what remains of the latter being chiefly in the inner portion of the cell, while the nucleus is now large and spherical, and not so near the basement membrane (Fig. 158). Everjrthing, therefore, points to the granules in what we may now call the mucin-forming cells as being in some way or other precursors of the fully- formed mucin ; manufactured during ' rest ' by the proto- plasm and partly at its expense, moved towards the lumen in activity, discharged as mucin in the secretion. It has been asserted that not only is the protoplasm lessened in the loaded cell and re- newed after activity, but that many of the mucigenous cells may be altogether broken down and discharged, their place being supplied by proliferation of the small cells of the demilunes. This conclusion, however, is not supported by sufficient evidence. The cells of the crescents contain fine granules, but none which can be changed into 376 DIGESTION mucin. They are of serous and not of mucous type. But the fact on which we would specially insist is that the granules of the resting mucigenous cell may be looked upon as a mother-substance from which the mucin: of the secretion is derived ; they are not actual, but potential, mucin. So in the pancreas, the serous or albuminous salivary glands, and the glands of the stomach, there is every reason to believe that the granules which appear in the intervals of rest, and are moved towards the lumen and discharged during activity, are the pre- cursors, the mother-substances, of important constituents of the secretion. These granules are sharply marked off from the proto- plasm in which they lie and by which they are built up. By every mark, by their reaction to stains, for instance, they are non-living substance, formed in the bosom of the hving cell from the raw material which it culls from the blood, or, what is more likely, fprmed from its own protoplasm, then shed out in granular form and Fig. 158. — Mucous Cells {from Submaxillary of Dog) in Rest and Activity (Langley). A, B, fresh; A', B', after treatment with dilute acetic acid; A', B', alveoli hard- ened in alcohol and stained with carmine. A, A', and A" represent the loaded; B, B', and B', the discharged condition. secluded from further change. The granules in the ferment-forming glands are not in general composed of the actual ferments, and, indeed, in several instances it has been shown that the actual fer- ments are not present in the secreting cells at all. We have already seen that the pancreas and even the fresh pan- creatic juice are devoid of active trypsin. Similarly, a glycerin extract of a fresh gastric mucous membrane is inert as regards proteins, or nearly so. But if the mucous membrane has been pre- viously treated with dilute hydrochloric acid, the glycerin extract is active, as is an extract made with acidulated glycerin. Here we must assume the existence in the gastric glands of a mother-sub- stance, pepsinogen, from which pepsin is formed. The rennin of the gastric juice, which is formed in the chief cells, also has a precursor, which, if the ferment is identical with pepsin (p. 347), must be pepsinogen. The proteolytic power of an extract of the pancreas. THE SECRETION OF THE! DIGESTIVE JUICES 377 when the trypsinogen has been activated into trypsin, or of the gastric mucous membrane, when the pepsinogen has been changed into pepsin, seems to be, roughly speaking, in proportion to the quantity of granules present in the cells. Therefore it is concluded that the granules represent mother-substances of the ferments or zymogens. Some observers believe they have obtained evidence of stages in the elaboration of the ferments still further back than the mother-substances, grandmother-substances so to speak, or pro- zymogens. Bensley, e.g., concludes that the nuclei of the chief cells in the fundus glands of the stomach take part in the formation of a prozymogen, the precursor of the zymogen or pepsinogen, as pepsino- gen is the precursor of the enzyme pepsin. A glycerin or watery extract of the salivary glands always con- tainsactive amylolytic ferment, if the natural secretion is active. So that if ptyalin is preceded by a zymogen in the cells, it must be very easily changed into the actual ferment. But we should greatly deceive ourselves if we supposed that granules of this nature in gland-cells are necessarily related to the production of ferments. The miicigenous granules have no such significance. Most digestive secretions contain- protein constituents, with which the granules may have to do as well as with ferments. And bile, a secretion which contains no mucin, no proteins, and either no ferments or mere traces, as essential and original constituents, is formed in cells with granules so disposed and so affected by the activity of the gland as to suggest some relation between them and the process of secretion. In the liver-cells of the frog, in addition to glycogen, and oil-globules small granules may be seen, especially near the lumen of the gland tubules ; they diminish in number during digestion, when the secretion of bile is active and increase when food is withheld and secretion slow. And in fasting dogs the secreting cells of Brimner's glands, the pyloric glands and the pancreas, as well as the lining epithelium of the bile-ducts, have been found to contain many fatty granules. Possibly some of these represent the fat which is known to be excreted into the alimentary canal (pp. 437, 438). The Nature of the Process by which the Digestive Secretions are Formed.-^We have spoken more than once of the gland-cells as manufacturing their secretions. It is an idea that rises naturally in the mind as we follow with the microscope the traces of their functional activity. And when we compare the composition of the digestive juices with that of the blood-plasma and lymph, the suggestion that the glands which produce them are not merely passive filters, but living laboratories, acquires additional strength. It is evident that everything in the secretion must, in some form or other,iiexist in the blood which comes to the gland, and in the lymph which bathes'its cells. No glandular cell, if we except the leucocytes, which in some respects are to be coilsidered as unicellular glands, dips directly into the blood ; everything a gland-cell receives must pass through the wdlls of the bloodvessels. (But see footnote 378 DIGESTION on p. 14). So that anything which we find in the secretion and do not find in the blood must have been elaborated by the gland epithelium (or by the capillary endothelium) from raw material brought to it by the blood. Take, for example, the saliva or gastric juice. These liquids both contain certain things that also exist in the blood, but in addition they contain certain things specific to themselves: mucin in sahva, hydrochloric acid in gastric juice, ferments in both. It is true that a trace of pepsin and a trace of a diastatic ferment may be dis- covered in blood; but there is no reason whatever to believe that this is the source of the pepsin of the gastric juice, or the ptyalin of the salivary glands, except, perhaps, in animals like the cat, whose saliva contains a diastase in still smaller concentration than -the serum (Carlson). On the contrary, it is possible that the fer- ments of the blood may be in part absorbed from the digestive glands, the rest being formed by the leucocytes and liberated when they break down. Formation of Bile. — ^The liver affords an even better example of this ' manufacturing ' activity of gland-cells, and many facts may be brought forward to prove that the characteristic constituents of the bile, the bile-pigments and bile-acids, are formed in the liver, and not merely separated from the blood. Bile-pigment has indeed been recognized in the normal serum of the horse, and bile-acids in the chyle of the dog, but only in such minute traces as are easily accounted for by absorption from the intestine. Frogs live for some time after excision of the liver, but no bile-acids are found in the blood or tissues. But if the bile-duct be ligatured, bile-acids and pigments accumulate in the body, being absorbed by the lymphatics of the liver (Ludwig and Fleischl). If the thoracic duct and the bile-duct are both ligatured in the dog, no bile-acids or pigments appear in the blood or tissues. Wertheimer and Lepage state that bile or bilirubin injected into a bile-duct appears sooner in the urine than in the lymph of the thoracic duct, and therefore conclude that the bloodvessels are the most important channel of absorption. This conclusion, however, cannot be accepted until it is shown that in these experiments the injection did not cause rupture of some of the hepatic capillaries and direct entrance of the bile-pigment into the blood. It is not improbable that the pressure attained by the bile in the bile-capillaries is a factor in determining the path by which it is absorbed, and that when the pressure rises beyond a certain limit it may pass both into the bloodvessels and into the lymphatics. In mammals life cannot be maintained for any length of time after Ugature of the portal vein, since this throws the whole intestinal tract out of gear. But after an artificial communication has been made between the portal and the left renal vein or the inferior cava, the portal may be tied and the animal live for months THE SECRETION OF THE DIGESTIVE JUICES 379 (Eck). The liver can now be completely removed, but death follows in a few hours. A good method of establishing an Eck's fistula is to make a longitudinal incision in the inferior vena cava and the portal or superior mesenteric vein, and to suture the edges of the two openings together with a very fine sewing-needle and thread (Carrel and Guthrie). In birds there exists a communicating brajich between the portal vein and a vein (the renal-portal) which passes from the posterior portion of- the body to the kidney, and there breaks up into capillaries; and not only may the portal be tied, but. the liver maybe completely destroyed without immedi- ately killing the animal. In the hours of life that still remain to it no accumulation of biliary substances (acids or pigments) takes place in the blood or tissues. A further indication that bile-pig- ment is produced -in the liver is the fact that the liver contains iron in relative abundance in its cells (p. 21), and eliminates small quantities of iron in its secretion. Now, bile-pigment, which con- tains no iron, is certainly formed from blood-pigment, which is rich in iron. For hsematin, when injected under the skin, has been found to appear almost quantitatively in the form of bile-pigment in the bile, and haematoidin (Fig. 159), a crystalline derivative of haemoglobin found in old ex- travasations of blood, especially in the brain and in the corpus luteum, is identical with biHrubin. The fact that one of the derivatives of hgematin, haematoporphyrin (C33H3gN40g), contains no iron, and is prolDably nearly related to bihrubin (C32H3gN406), suggests that haematoporphyrin may be an inter- pig. isg^H^ematoidin. mediate step in the formation of bile-pigment from blood-pigment. In any case, the seat of formation of bile- pigment might be expected to be an organ peculiarly rich in iron. The existence of haematoidin, however, shows that bile-pigment may, under certain conditions, be formed outside of the hepatic cells. The occurrence of biliverdin in the placenta of the bitch points in the same direction. But the pathological evidence in favour of the pre-formation of the biliary constituents tends rather to shrink than to increase. For many cases of what used to be considered ' idiopathic ' or ' haematogenic ' jaundice, i.e., an accumu- lation of bile-pigments and bile-acids in the tissues, due to defective elimination by the liver, are now known to be cfosed by obstruction of the bile-ducts and consequent re-absorption of bile (' obstructive ' or ' hepatogenic ' jaundice). But if substances such as the ferments, mucin, hydrochloric acid, the bile-saks and bile- pigments, are undoubtedly manufactured in the gland-cells, it is different with the water and inorganic salts which form so large a part of every secretion. No tissue lacks them ; no 38o DIGESTION physiological process goes on without them; they are not high and special products. As we breathe nitfogen which we do not need because it is mixed with the oxygen we require, the secreting cell passes through its substance water and salts as a sort of by-play or adjunct to its specific work. But this is not the whole truth. The gland-cell is not a mere filter through which water and salts pass in the same proportions in which they exist in the liquids that the cell draws them from. When, e.g., the salivary glands secrete ag^ainst the resistance of an abnormally high pressure in the ducts, the percentage of salts in the saliva increases. The secretidms of different glands differ in the nature, and especially' in the relative proportions, of their inorganic constituents. They differ also in their osmotic pressure and electrical conductivity, which depend so largely upon those constituents, notvvithstanding the fact that the osmotic pressure and conductivity of the blood-serum (p. 26) vary only within narrow limits. Even the secretion of one and the same gland is by no means constant in this respect, as we shall have to note more especially when we come to deal with the in- fluence of the nervous system on secretion (p. 389). The following tables illustrate this point ; Dog. Blood-Serum.* Filtrate of Gastric Contents. • At KtCsoCjxio*. A KKs^C-jXio*. - I. II. III. 0-643° . 0-628° 0-602° 92-0 87-6, 87-7 ' 0-585° 0-585° 0-642° ■ 312-5 179-4 351-7 Vomit of man with complete intes- tinal obstriiction ' 0-433° 84-7 Pancreatic Juice of Dog {Pincussohn) . Diet. A Milk Cauliflower Horseflesh o-57°-^-63'' 0-58°— 0-63° 0-62°— 0-63° * The blood and gastric contents were obtained from the animals in the writer's laboratory twenty-four hours after the last meal. f The depression of the freezing-point below that of distilled water. { See footnote on p. 27. ' THE SECRETION OF THE DIGESTIVE JUICES 381 Gastric Juice from Miniature Stomach in a Dog in Different Experiments (Bickel) . Milk Diet. Meat Diet. A K(2S°C.)Xio*. A K(25»C.)Xio'. 0-52' 0-65" 0-64° 0-69° 0-81° 195-9 402-6 436-5 ., 104-2 , 1 436-5 0-60° 0-71" 1-21° ,,. 0-79° 0-70° 310-3 473-5 483-3 514-1 514-1 A of Blood and Saliva Compared (Jappelli). A of Blood. A of Submaxillary Saliva pf Dog. 0-570° 0-410° 0-610° 0-350° , o-6oo° 0-430° 0-590° 0-410° 0-580° 0-450° 0-605° 0-425° 0-650° 0-380° 0-610° 0-475° A of Human Fistula Bile. A of Human Bladder Bile. 0-56° 0-547" 0-615° 0-60° 0-545° 0-65° 0-865° 0-78° 0-92° A of Dog's Submaxillary Saliva. Chorda stimulated : Left submaxillary Both glands Spontaneous secretion : Right submaxillary A of dog's serum 0-293 0-408° 0-195° 0-590° The protein substances, such as serum-albumin and globulin, common to blood and to some of the digestive secretions, take a middle place between the constituents that are undoubtedly manu- 382 DIGESTION factured in the cell and those which seem by a less special and laborious, though a selective, process to be passed through it from the blood. Their practical absence from bile, and, as we shall see, from urine, their relative abundance in pancreatic and scantiness in gastric juice, point to a closer dependence upon the special activity of the gland-cell than we can suppose necessary in the case of the salts. Although it is in the cells of the digestive glands that the power of forming ferments is most conspicuous, it is by no means confined to them. It seems to be a primitive, a native power of protoplasm. Lowly animals, like the amoeba, lowly plants, like bacteria, form ferments within the single cell which serves for all the purposes of their life. The ferment-secreting gland-cells of higher forms are perhaps only lop- sided amoebse, not so much endowed with new properties as dispro- portionately developed in one direction. The contractility has been lost or lessened, the digestive power has been retained or increased; just as in muscle the power of contraction hcis been developed, and that of digestion has fallen behind. The muscle-cell and the cartilage- cell are parasites, if we look to the function of digestion alone. They live on food already more or less prepared by the labours of other cells; and it is a universal law that in the measure in which a power becomes useless it disappears. But the presence of pepsin in the white blood- corpuscles, the parasites as well as the scavengers of the blood, and of amyloljrtic, proteol5rtic and lipolytic ferments in many tissues, should warn us not to conclude that the power of forming ferments belongs exclusively to any class of cells. There is good and growing evidence that food-substances absorbed from the blood are further decomposed and, in turn, elaborated by ferment action within the tissues them- selves; while many facts show that the power of contraction is widely diffused among structures whose special function is very different, and a few point to its possession in some degree even by glandular epithelium. On the other hand, it must be remembered that none of the digestive glands absorb food directly from the alimentary canal to be then digested within their own cell-substance; the ferments which they form do their work outside of them ; their cells feed also upon the blood. Why are the Tissues of Digestion not affected by the Digestive Ferments ? — This is the place to mention a point which has been very much debated. Why is it that the stomach or the small intes- tine does not digest itself ? This is really a part of a wider question : Why is it that U ving tissues resist all kinds of influences, which attack dead tissues with success ? And we have to inquire whether the immunity of the aUmentary canal to the digestive juices is an example of a general resistance of all living tissues to destructive agencies, or a specific resistance of certain tissues to certain in- fluences. That all living tissues cannot withstand the action of the gastric juice has been shown by putting the leg of a living frog inside the stomach of a dog; the leg is gradually eaten away (Bernard). It is true that it has first been killed and then digested, but the question is, why the stomach-wall is not first killed and then THE SECRETION OF THE DIGESTIVE JUICES 383 digested ? When the wall has been injured by caustics or by an embolus, the gastric juice acts on it. But the living epithelium that covers it is able to resist the action of the acid and pepsin, which destroys the tissues of the frog's leg. The explanation is not to be found in the alkalinity of the blood, for the frog's blood is also alkaline, and the cells that line the intestine are preserved from the pancreatic juice, which is intensely active in an alkaline medium, while the living frog's leg is not harmed by a weakly alkaline pan- creatic extract, which does not digest the epithelium because it cannot kill it. A certain amount of protection may be afforded to the walls of the stomach by the thin layer of mucus which covers the whole cavity, for mucin is not affected by peptic digestion. And a mucous secretion seems in some other cases to act as a protective covering to the walls of hollow viscera, whose contents are such as would certainly be harmful to more delicate membranes, e.g., in the urinary bladder, large intestine, and gall-bladder. Still, how- ever important such a mechanical protection may be, it does not explain the whole matter, and it is necessary to suppose that the gastrid epithelium has some special power of resisting the gastric juice, either by turning any of the ferment which may invade it into an inert substance and neutraUzing any intrusive acid, or by opposing their entrance as the epithelium of the bladder opposes the absorption of urea. There is reason to believe that, as a matter of fact, free hydrochloric acid' cannot penetrate the living cells, and it is' to be noted that both active pepsin and free acid must be present at the same point within the cells before digestion of them can take place. In the gland-cells of the pancreas the protoplasm is, no doubt, shielded from digestion by the existence of the ferment in an inert form as zymogen ; and it is possible that this is one of the reasons for the existence of the mother- substance. But no such explanation is, of course, available for the intestinal epithelium. Trypsin when injected below the skin causes the tissue to break down ^nd ulcerate. And while an active solution of trypsin can be allowed to remain a long time in an isolated loop of small intes ■ tine without producing any ill effect, damage is soon caused not only to the intestinal wall, but also to the liver, when the mucous membrane of the loop has been injured before the introduction of the trypsin. We must suppose, then, that the normal mucous membrane of the intestine prevents the absorption of trypsin, or, if it absorbs any of it, renders it harmless. On the other hand, the intestinal mucosa is injured by the natural gastric juice when intro- duced directly into it unless the animal takes food simultaneously or a Uttle earlier. But for reasons already given (p. 364) injury to the intestine cannot be produced in this way in normal digestion. It is impossible to escape the conclusion that each membrane becomes accustomed, and, so to speak, ' immune, ' to the secretion normally 384 ., c DIGESTION in contact with it, altlioi:igh not necessarily to other secretions. It is easy to multiply illustrations of this principle. , The mucosa of the dog's urinary bladder is digested by the natural activated pancreatic juice of the dog, and still more readily by the natural gastric juice. Yet few tissues but the lining of the urinary tract or of the large intestine could bear the constant contact of urine or faeces. When urine is extravasated under the skin, or the contents of the alimentary canal burst into the peritoneal cavity, they come into contact with tissues which, although aUve, are much less fitted to resist them than the surfaces by which they are normally enclosed; and the consequences, which are not wholly due to infection, are often disastrous. Leucoc5rtes thrive in the blood, but perish in urine. Blood does not harm the endo- thelial cells of the vessels, but kills a muscle whose cross-section is dipped into it. The defensive or, in some cases, offensive liquids secreted by many animals are harmless to the tissues which produce and enclose them. A caterpillar investigated by Poulton secretes a liquid so rich in formic acid that the mere contact of it would kill most cells. The so-called saliva of Octopus macropus contains a substance fatal to the crabs and other animals on which it preys. The blood of the viper contains an a-ctive principle similar to that secreted by its poison-glands, but its tissues are not affected by this substance, so deadly to other animals. A step in the solution of our problem has been taken by Wein- land. Starting with the idea that if special protective mechan- isms against the digestive juices were anywhere to be found, it would be in the intestinal parasites whose whole existence is passed among them, he has made the important discovery that in these parasitic worms specific antiferments exist — i.e., substances which inhibit the action either of pepsin or of trypsin or of both. These substances can be precipitated from the expressed juice of the worms by alcohol, without completely losing their activity. Fibrin can be impreg- nated with them, and it is then, just like the ' living, tissue,' rendered for a longer or shorter time unassailable by the proteolytic ferments. These facts are full of suggestion for future work, although the sup- posed proof that similar antiferments are contained in the cells of the mucous membrane of the stomach and intestines of the higher animals appears to have broken down. Substances can indeed be obtained by Weinland's method from the gastric and intestinal mucosa which, when added to a digestive mixture, strongly inhibit the digestion of proteins. But there is no clear proof that these sub- stances are specific antiferments. They are probably merely some of the split products of protein (Langenskjold). There is, however, some evidence of the existence of an antipepsin in many tissues including the mucous membrane of the stomach. As already men- tioned, it is known that an antitrypsin exists in the blood, with the INFLUENCE OF NERVOUS SYSTEM ON DIGESTIVE GLANDS 385 same properties as the antitrypsin in the intestinal worms (Hamill). This explains the resistance of blood-serum to the digestive action of trypsin. In addition to this body, which hinders the action of fully-formed trypsin, and has no effect upon enterokinase, the serum of some animals contains an antikinase — i.e., a substance which hinders the action, not of trypsin, but of enterokinase, pre- venting it from activating the trypsinogen into trypsin. Section V. — The Influence of the Nervous System ON THE Digestive Glands. The Influence of Nerves on the Salivary Glands. — All the salivary glands have a double nerve-supply, from the medulla oblongata through some of the cranial nerves, and from the spinal cord through the cervical sympathetic (Fig. 160). In the dog the chorda tympani branch of the facial nerve carries the cranial supply of the sublingual and submaxillary glands. It joins the lingual branch of the fifth nerve, runs in company with it for a little way, and then, breaking off, after giving some fibres to the lingual, passes, as the chorda tympani proper, along Wharton's duct to the submaxillary gland. In the hilus of this gland most of its fibres break up into fibrils around nerve-cells situated there, and lose their medulla in doing so. A few fibres terminate in a similar manner before entering the hilus, and a few deeper in the gland. The nervous path is continued by the axis-cylinder processes (p. 824) of these nerve-cells, which, passing in as non-medullated fibres, end in a plexus on the basement membrane of the alveoli. From the plexus fibrils run in among the gland-cells, but do not seem to penetrate them. The lingual, the chorda tympani proper, and Wharton's duct form the sides of what is called the chordo-lingual triangle. Within this triangle are situated many ganglion cells, a special accumulation of which has received the name of the submaxillary ganglion. This, however, should rather be called the sublingual ganglion,- since its cells, as well as the others in the chordo-lingual triangle, are the cells of origin of axons which proceed as non-medullated fibres to the sublingual gland. The sublingual gland receives its cerebral fibres partly from branches given off from the lingual in the chordo-lingual triangle after the chorda tympani proper has separated from it, and ending around the nerve-cells within that triangle, partly from the chorda itself in the terminal portion of its course. These statements rest on anatomical and physiological evi- dence. The latter we shall return to. The cerebral fibres for the parotid (in the dog) pass from the tympanic branch of the glosso-pharyngeal (Jacobson's nerve) through connecting filaments to the small superficial petrosal branch of the facial, with this nerve to the otic ganglion, and thence by the auriculo-temporal branch of the fifth to the gland. The sympathetic fibres for all the salivary glands appear to arise from nerve-cells in the upper dorsal portion .of the spinal cord. Issuing from the cord in the anterior roots of the upper thoracic nerves (first to fifth, but mainly second thoracic for the submaxillary), they enter the sympathetic chain, in which they run up to the superior cervical ganglion. Here they break up into terminal twigs, and thus come into 25 386 DIGESTION relation with ganglion cells, whose axons pass out as non-meduUated fibres, and, surrounding the external carotid, reach the salivary glands along its branches. Langley has shown, by means of nicotine (p. i8o), that the sympathetic fibres for the submaxillary and sublingual, and, indeed, for the head in general in the dog and cat, are connected with nerve-cells in this ganglion, but not between it and their termination, or between it and their origin from the spinal cord. Stimulation of the Cranial Fibres. — When in a dog a cannula is placed in Wharton's duct, and the saUva collected (p. 450), it is found that stimulation of the peripheral end of the divided chorda causes a brisk flow of watery saliva, and at the same time a dila- tation of the vessels of the gland, which we have already described in dealing with vaso- motor nerves (p. 177). Notwithstanding the vaso - dilatation, the volume of the gland is in general dimin- ished, owing to the rapid passage of water into the duct (Bunch). The blood has been shown to lose water in making the circuit of the submaxillary gland during excita- tion of the chorda, but doubtless some of the water of the saliva comes directly from the cells or from the lymph. That the increased secretion is not due merely to the greater blood-supply, and the consequent increase of capillary pres- sure, is shown by the injection of atropine, after which stimulation of the nerve, although it still causes dilatation of the vessels, is not followed by a flow of saliva. Mere increase of pressure could not in any case of itself account for the secretion, since it has been Fig. 160. — Nerves of the Salivary Glands. SM and SL, submaxillary and sublingual glands; P, parotid; V, fifth nerve; VII, facial; GP, glosso-pharyngeal; L, lingual; CT, chorda tympani; CL, chordo-lingual ; D, submaxillary (Wharton's) duct; C, ganglion cell of so-called submaxillary ganglion in the chordo- lingual triangle, connected with a nerve fibre going to sublingual gland; C ganglion cell in hilus of sub- maxillary gland; SSP, small superficial petrosal branch of the facial; OG, otic ganglion; IM, inferior maxillary division of fifth nerve; AT, auriculo- temporal branch of fifth; JN, Jacobson's nerve; C ganglion cells in superior cervical ganglion (SG) connected with sympathetic fibres going to parotid, submaxillary and sublingual glands. The figure is schematic. INFL UENCE OF NER VO US S YSTEM ON DIGEST I VE GLA NDS 387 found that the maximum pressure in the saUvary duct when the outflow of saUva from the duct is prevented may, during stimula- tion of the choirda, much exceed the arterial blood-pressure (Ludwig). In one experiment, for example, the pressure in the carotid of a dog was 125 mm. , in Wharton's duct 195 mm. of mercury. Even in the head of a decapitated animal a certain amount of saliva may be caused to flow by stimulation of the chofda, but too much may easily be made of this. And since the blood is the ultimate source of the secretion, we could not expect a permanent or copious flow in the absence of the circulation, even if the gland-cells could continue to live. In fact, when the circulation is almost stopped by strong stimulation of the sympathetic, the flow of saliva caused by excitation of the chorda is at the same time greatly lessened or arrested, even though the sympathetic itself possesses secretory fibres. So that, while there is no doubt that the chorda tympani contains fibres whose function is to increase the activity of the gland-ceUs, its vaso-dilator action is, under normal conditions, closely connected with, and, indeed, auxiliary to, its secretory action, although the dilation of the vessels does not directly produce the secretion. This is only a particular case of a physiological law of wide application, that an organ in action in general receives more blood than the same organ in repose, or, in other words, that the tissues are fed according to their needs. The contracting muscle, the secreting gland, is flushed with blood, not because an increased blood- flow can of itself cause contraction or secretion, but because these high efforts require for their continuance a rich supply of what blood brings to an organ, and a ready removal of what it takes away. The quantity of blood passing through the parotid of a horse when it is actively secreting during mastication may be quadrupled (Chauveau). The parallel between the muscle and the gland is drawn closer when it is stated that electrical changes accompany secretion (p. 810), and that the rate of production of carbon dioxide and consumption of oxygen (in the submaxillary gland) is three or four times greater during activity than during rest. The temperature of the saliva flowing from the dog's submaxillary during stimulation of the chorda has been found to be as much as 1-5° C. above that of the blood of the carotid, although with the gland at rest no con- stant difference could be detected between the arterial blood and the interior of Wharton's duct. But such measurements are open to many fallacies; and while there is no doubt that more heat is produced in the active than in the passive gland, it will not be surprising, when the vastly-increased blood-flow is remembered, that no difference of temperature between the incoming and out- going blood has been satisfactorily demonstrated. It has already been mentioned that most of the fibres of the chorda tjonpani proper become connected with ganglion-cells, and lose their 388 DIGESTION medulla inside the submaxillary gland, only a few having already lost it by a similar connection with ganglion-cells in the chordo-lingual triangle. These facts have been made out by means of the nicotine method previously described (p. 180). Thus, it is found that, after the injection of nicotine (5 to 10 mg. in a rabbit or cat, 40 or 50 mg. in a dog), stimulation of the chorda tjrmpani proper or of the chordo- lingual nerve causes no secretion from the submaxillary gland; but stimulation of the hilus of the gland is followed by a copious secretion — as much, if the stimulation is fairly strong, as was caused by excitation of the nerve before injection of nicotine. That this is due neither to any direct action on the gland-cells, nor to stimulation of the sympa- thetic plexus on the submaxillary artery, but to stimulation of chorda fibres beyond the hilus, is shown by the fact that after atropine has been injected in sufficient amount to paralyze the nerve endings of the chorda, but not of the sympathetic, stimulation of the hilus causes httle or no flow of saliva. The application of nicotine solution to the chordo- lingual triangle does not afiect the submaxillary secretion caused by stimulation of the chordo-lingual nerve, even m cases where a few secretory fibres for the submaxillary do not leave the chordo-lingual nerve in the chorda tympani proper, but are given ofi to the chordo- lingual triangle. This shows that none of the ganglion-cells in the triangle are connected with the secretory fibres of the submaxillary gland. By observations of the same kind they are known to be con- nected with fibres going to the sublingual. In a similar way, by observ- ing the effects of stimulation of the chorda on the bloodvessels before and after the application of nicotine, it has been found that the vaso- dilator fibres are connected with ganglion-cells in the same positions as the secretory fibres (Langley). Stimulation of the Sympathetic Fibres. — ^The sjnnpathetic, as has been already indicated, contains both vaso-constrictor and secretory fibres for the salivary glands. If the cervical sympathetic in the dog is divided, and the cephalic end moderately stimulated, a few drops of a thick, viscid and scanty saliva flow from the submaxillary and sublingual ducts, while the current of blood through the glands is diminished. As a rule, no visible secretion escapes from the parotid, but microscopic examination shows that many of the ductules are filled with fluid, which is apparently so thick as to plug them up (Langley) ; while the cells show signs of ' activity ' (p. 370). Simultaneous Stimulation of Cranial and S3nmpathetic Fibres. — When the chorda and sympathetic are stimulated together, the former prevails so far, with moderate stimulation of the latter, that the submaxillary saliva is secreted in considerable quantity, and is not particularly viscid. It is, however, richer in organic matter than is the chorda saliva itself. When the chorda is weakly, and the sympathetic strongly, excited, the scanty secretion (if there is any) is of sympathetic type, thick and rich in organic matter. With strong stimulation of both nerves, the secretion, 'at first plentiful and watery, soon diminishes, even below the amount obtained by stimulation of the chorda alone, because of the diminution in the blood-flow, and therefore in the oxygen- supply, produced by the vaso-constrictors of the sympathetic (Heidenhain). With stimula- INFLUENCE OF NERVOUS SYSTEM ON DIGESTIVE GLANDS 389 tion just strong enough to cause secretion when apphed separately to either nerve, there is no secretion when it is applied simul- taneously to both. All this refers to the dog. In this animal, then, there seems to be a certain amount of physiological antagonism between the secretory action of the two nerves. But it differs in one respect from the antagonism between their vaso-motor fibres ; for with strong stimu- lation the constrictors of the sympathetic always swamp the dilators of the chorda, while the secretory fibres of the chorda appear upon the whole to prevail over those of the sympathetic. And in all probability this apparent secretory antagonism is very superficial, and is due largely to the difference in the vaso-motor effects of the two nerves. For it has been shown that, when the blood- flow through the submaxillary gland is artificially diminished by gradu- ated compression of its artery, stimulation of the chorda gives rise to a thick, viscid and scanty saliva, relatively rich in organic soHds (Heidenhain). When the amount of blood passing through the gland is made approximately the same as during stimulation of the sympathetic, the chorda saliva becomes practically identical in composition with the sympathetic saliva. This is one reason, perhaps the chief one, why the sympathetic, when both nerves are stimulated together, without artificial interference with the blood- supply, always appears to add something to the common secretion when there is a secretion at all, this something being represented by an increase in the percentage of organic matter. The observation that the sympathetic effect persists after stimulation has been stopped, and that excitation of the chorda after previous stimula- tion of the sympathetic causes a flow of saliva richer in organic matter than would have been the case if the sympathetic had not been stimulated, has long been considered a proof that the secretory fibres of the two nerves are widely different in function. To explain this result, Heidenhain assumed the existence in the sympathetic of a preponderance of fibres concerned in the building up in the cells of the organic constituents of the saliva (so-called ' trophic,' or, better, since the word ' trophic ' is usually associated with the building up of the bioplasm itself, ' trophic-secretory ' fibres). It would seem, however, that the increase in organic constituents is only reahzed when a sufficient time has not been allowed, after stimulation of the sympathetic, for the normal circulation to become re-established in the gland. The saliva obtained by stimulation of the chorda immediately after a period of artificially diminished blood-flow, without any previous excitation of the sympathetic, also contains a surplus of organic matter (Carlson). Indeed, the distinction between chorda and sympathetic saliva, which, by taking account of the parotid as well as the submaxillary and sublingual glands, has been generalized into a distinction 390 DIGESTION between cerebral and sympathetic saliva, and which, when the vaso-motor conditions are left out of account, appears to hold good . in the dog and the rabbit, breaks down before a wider induction. For in the cat the sympathetic saliva of the submaxillary gland, although much more scanty, is more watery than the chorda saliva (Langley), which, however, is by no means viscid; and the two secretions differ far less than in the dog. The discovery of Carlson that the cat's cervical sympathetic contains so many vaso-dilator fibres for the submaxillary gland that the usual effect of its stimu- lation with a weak interrupted current is a marked augmentation in the blood-flow affords an explanation. In accordance with this functional similarity, there is a much smaller difference in the action of atropine on the two sects of fibres in the cat than in the dog, although even in the cat the sympathetic is less readily paralyzed than the chorda. In their secretory action there is not even an apparent antagonism in the cat, with minimal stimulation of both nerves, which causes as much secretion as would be produced if both were separately excited. Fiuiher, even in the dog, after prolonged stimulation of the sympathetic, the submaxillary saliva is no longer viscid, but watery, the proportion of solids, and especially of organic solids, being much lessened, as it is also in chorda saliva after long excita- tion. When the cerebral nerve of the resting gland is strongly excited, it is found that up to a certain hmit the percentage of organic matter in a small sample of saliva subsequently collected during a brief weak excitation increases with the strength of the previous stimulation ; this is also true of the inorganic solids. But there is a striking difference when the experiment is made on a gland after a long period of activity; here increase of stimulation causes no increase in the percentage of organic material, while the inorganic solids are still increased. In both cases the absolute quantity of water, and therefore the rate of flow of the secretion, is augmented. All this points to the same conclusion as the microscopic appear- ances in the gland-cells, that the cells during rest manufacture the organic constituents of the secretion, or some of them, and store them up, to be discharged during activity. The water and the inorganic salts, on the other hand, seem rather to be secreted on the spur of the moment, so to speak, and not to require such elaborate preparation. And it has been stated that when the chorda tympani is stimulated with currents of var5dng strength, the quantity of organic substances in small samples of saliva collected from a fresh gland is more nearly proportional to the rate of secretion than is the quantity of water and salts, which varies also with the blood-supply. Lest the apparently insignificant result of artificial stimulation of the sympathetic in such animals as the dog should cause its INFLUENCE OF NERVOUS SYSTEM ON DIGESTIVE GLANDS 391 secretory action to be appraised at too low a value, it should be remembered that in the intact body the sympathetic secretory fibres, when they are excited, are, it may be assumed, excited independently of the vaso-constrictors, and even in conjunction with the vaso- dilators of the salivary glands. It is conceivable that such differences between chorda and sympathetic saliva as are not accounted for by the differences in the blood-flow during their stimulation are due, not to the nerve fibres, but to the end organs with which they are connected; that is, the two nerves may supply, not the same, but different gland- cells. And it is well known that even after prolonged stimulation of the chorda or chordo-lingual alone, some alveoli of the dog's submaxillary gland remain in the ' resting ' state ; after stimulation of the sympathetic alone, the number of unaffected alveoli is much greater; while after stimulation of both nerves, few alveoli seem to have escaped change. If there is no essential difference between the cranial and sympathetic secretory fibres, it is easy to understand that they will be distributed to different secreting elements. The supposed proof that there must be some overlapping in the nerve- supply — i.e., that some cells must be supplied from both sources, since excitation of the sympathetic influences the amount of organic material in the saliva obtained by subsequent stimulation of the chorda — -is, as we have just seen, by no means so cogent as has been assumed. And, indeed, we know nothing of a division of labour between the cells of a gland,' except when there are obvious anatom- ical distinctions. Thus, the submaxillary gland in man contains both serous and mucous acini, and mucin- making cells are scattered over the ducts of most glands, and, indeed, on nearly every surface which is clad with columnar epithelium. In these cases we cannot doubt that one constituent — -mucin — of the entire secretion is manu- factured by a portion only of the cells. In the cardiac glands of the stomach, too, the ovoid cells, in all probability, yield the whole of the acid of the gastric juice. But, so far as we know, every hepatic cell is a liver in little. Every cell secretes fully-formed bile ; every cell stores up, or may store up, glycogen. So it is with the secretory alveoli of the pancreas, if we consider the islands of Langerhans as having no connection with the alveoli; one cell is just like another; all apparently perform the same work; each is a unicellular pan- creas. (See p. 624.) Paraljrtic Secretion. — When the chorda tympani is divided, a slow ■ paralytic ' secretion from the submaxillary gland begins in a few hours, and continues for a long time accompanied by atrophy of the gland. There is also a secretion of the same kind from the submaxillarjr on the opposite side, but it is less copious. This is called the ' antilytic ' secretion, which is most pronounced in the first few days after the opsration, and seems to be a transient phenomenon. It can be at once abolished by section both of the chorda and the sympathetic on the 392 DIGESTION corresponding side, and is therefore due to impulses arising in the central nervous system. The cause of the paralytic secretion has not been fully made out. If within two or three days of division of the chorda the sympathetic on the same side is cut, the secretion is greatly diminished or stops altogether; and it is concluded that up to this time it is maintained by impulses passing along the sym.pathetic to the gland from the salivary centre, the excitability of which has been in some way increased by division of the chorda, possibly by some such degenerative process in the cells as the changes seen in cerebro-spinal motor cells whose axons have been divided (p. 830). This may also account for the antilytic secretion. But if section of the sjrmpathetic is not performed for several days, it has no effect on the paralytic secretion, which at this stage seems to depend on local changes in or near the gland itself, leading to a mild continuous excitation of those nerve-cells on the course of the fibres of the chorda to which reference has already been made. Section of the sympathetic alone causes neither secretion nor atrophy, nor does removal of the superior cervical ganglion. The histological characters of the -gland-cells during paralytic secretion are those of ' rest.' Reflex Secretion of Saliva. — The reflex mechanism of salivary secretion is very mobile, and easily set in action by physical and mental influences. It is excited normally by impulses which arise in the mouth, especially by the contact of food with the buccal mucous membrane and the gustatory nerve-endings. The mere mechanical movement of the jaws, even when there is nothing between the teeth, or only a bit of a non-sapid substance like india- rubber, causes some secretion. The vapour of ether gives rise to a rush of saliva, as does garghng the mouth with distilled water. The smell, sight, or thought of food, and even the thought of saliva itself, may act on the salivary centre through its connections with the cerebrum, and make ' the teeth water.' A copious flow of saliva, reflexly excited through the gastric branches of the vagus, is a common precursor of vomiting. The introduction of food into the stomach also excites salivary secretion. The researches of Pawlow and his pupils have shown that the salivary glands are not excited indifferently by everjrthing which comes into contact with the buccal mucous membrane. A remark- able adaptation exists between the properties of food or foreign bodies introduced into the mouth and their effects upon the secre- tion of saliva. When solid dry food is given to a dog saliva is copiously poured out; much less is secreted when the food is moist. Acids or salts induce an abundant flow, in order that they may be neutralized, diluted or washed out of the mouth. In this case a watery liquid, poor in mucin, flows from the mucous glands. Mucin is a lubricant to facilitate the swallowing of solid food, and here it could be of no use. When clean pebbles are put in the dog's mouth the animal may try to chew them, but eventually ejects them. Either no saliva or very little is secreted, since it could not aid in their expulsion. If, however, the very same stones are INFLUENCE OF NERVOUS SYSTEM ON DIGESTIVE GLANDS 393 reduced to sand and again introduced into the animal's mouth, saliva is plentifully secreted to wash it out. The serous and mucous sahvary glands are not necessarily excited by the same food materials, and here again we can trace an astonish- ingly exact adaptation. A permanent parotid or submaxillary fistula can easily be made in a dog by freeing Stenson's or Wharton's duct from the surrounding mucous membrane for a little distance, bringing the natural orifice of the duct out through a small wound in the cheek, and stitching it in position there. When it is desired to collect saliva, the wide end of a funnel-shaped tube, whose stem is bent so as to hang vertically, can be attached by a little shellac of low melting-point to the skin around the orifice of the duct and at some distance from it, and on the narrow end can be hung a small graduated tube, into which the saliva drops. When fresh meat is given to the animal little or no parotid saliva is secreted, while a copious flow takes place from the submaxillary gland, mucin being required to lubricate it for deglutition, while water is not speciaEy needed. But if the meat is in the form of a dry powder the parotid pours out a plentiful secretion, while the submaxillary also secretes a fluid relatively rich in mucin. The same difference is seen between fresh moist bread and dry bread. The afferent nerve-endings from which impulses are carried to the reflex centres (or the portions of the salivary centre) which preside over the various salivary glands must possess the power of very delicate selection as regards the kinds of stimulation by which they are affected. The mere relish of the animal for the different kinds of food plays but a small part. Most dogs display a much livelier interest in a piece of meat than in a piece of dry biscuit^ yet it is the biscuit which excites the parotid to activity. The sight of dry food causes an abundant flow of watery saliva from the parotid, and a flow of fluid rich in mucin from the sub- maxillary. Various uneatable substances, including substances which in contact with the mucous membrane of the mouth produce strong and disagreeable. stimulation of it, and excite disgust, cause also, when viewed from a distance, secretion by all the salivary glands; but the submaxillary saliva, as ought to be the case for substances unfit for food, and therefore not destined to be swallowed, is poor in mucin. When the animal is shown pebbles and sand the phenomena are qualitatively the same as when they are put into its mouth — the glands remaining inactive in presence of the pebbles, but secreting plentifully at sight of the sand. In short, the same adaptation is observed in the case of the so-called psychical secretion as when the stimulating substances act directly upon the endings of the afferent salivary nerves in the buccal raucous membrane. It is further worthy of note that when the animal is hungry the psychical secretion is most copious and most 394 DIGESTION easily obtained. After a full meal it cannot be excited at all. When food (or other exciting substance) is repeatedly shown to a fasting animal the reaction becomes each time weaker, and finally the glands cease to respond. All that is then necessary to restore the reaction is to put into the animal's mouth a little of the food (or other object). When it is now shown it at a distance the ordinary effect follows promptly. This indicates that the condition of the salivary centre exercises an important influence upon the psychical secretion, its excitability to the weaker stimulus set up by the sight of the object being increased by the stronger reflex stimulation coming directly from the mouth. In the condition of satiety the inexcitability of the centre may be due to the action of food- products in the blood. In most animals and in man the activity of the large saUvary glands is strictly intermittent. But the smaller glands that stud the mucous membrane of the mouth never entirely cease to secrete, and the same is the case with the parotid in ruminant animals. The centre is situated in the medulla oblongata, stimulation of which causes a flow of saliva. The chief afferent paths to the salivary centre are the lingual branch of the fifth and the glosso- pharyngeal ; but stimulation of many other nerves may cause reflex secretion of saliva. In experimental reflex stimulation, the sole efferent channel seems to be the cerebral nerve-supply of the glands. After section of the chorda, no reflex secretion by the submaxillary gland can be caused, although the sympathetic remains intact. It was alleged by Bernard that, after division of the chordo- lingual, a reflex secretion could be obtained from the submaxillary gland by stimulating the central end of the cut lingual nerve between the so-called submaxillary ganglion and the tongue, the ganglion being supposed to act as ' centre. ' It has been shown, however, that this is not a true reflex effect, but is due to the excitation of certain (recurrent) secretory fibres of the chorda that run for some distance in the lingual, then bend back on their course and pass.to the gland. It may be in part a pseudo- or axon-refiex (p. 885), elicited by excitation of efferent fibres, which send branches to some of the ganglion-cells. The salivary centre can also be inhibited, especially by emotions of a painful kind — for instance, the nervousness which often dries up the saliva, as well as the eloquence, of a beginner in public speaking, and the fear which sometimes made the medieval ordeal of the consecrated bread pick out the guilty. In rare cases the reflex nervous mechanism that governs the salivary glands appears to completely break down ; and then two opposite conditions may be seen — xerostomia, or ' dry mouth,' in which no saliva at all is secreted, and chronic ptyalism, or hydro- stomia, where, in the absence of any discoverable cause, the amount INFLUENCE OF NERVOUS SYSTEM ON DIGESTIVE GLANDS 395 of secretion is permanently increased. Both conditions are said to be more common in women than in men. The Influence of Nerves on the Gastric Glands. — Like sahva, gastric juice is not secreted continuously, except in animals such as the rabbit, whose stomachs j,re never empty. The normal and most efficient stimulus is the eating of food and its presence in the stomach. Mechanical stimulation of the gastric mucous membrane with a non-digestible substance, such as a feather or a glass rod, causes secretion of mucus, but not of gastric juice. But the observations mentioned above on the difference of response of the salivary glands to different substances suggest that the local mechan- ical stimulation of the food on the gastric glands may be more effective. There is also at first thought much to indicate that the gastric glands are stimulated chemically in a more direct manner than the salivary glands by the local action of food substances reaching the cells by a short-cut from the cavity of the stomach, or in a more roundabout way by the blood. And it might be very plausibly argued that the gastric glands are favourably situated for direct stimulation, while the large salivary glands are not; and that the great function of saliva being to aid deglutition, an almost momentary, and at the same time a perilous act, it is necessary to provide by a nervous mechanism for an immediate rush of secre- tion at any instant, while it is not important whether the gastric juice is poured out a little sooner or a little later, and therefore it is left to be called forth by the more tardy and haphazard method of local action. Nevertheless, on looking a little closer, we find that this does not exhaust the subject, and that the gastric secretion can be influenced by events taking place in distant parts of the body, just as the salivary secretion can. In a boy whose oesophagus was completely closed by a cicatrix, the result of swallowing a strong alkali, and who had to be fed by a gastric fistula, it was found that the presence of food in the mouth, and even the sight or smell of food, caused secretion of gastric juice (Richet). Here there must have been some nervous mechanism at work. The secretion cannot have been excited by the direct action of absorbed food-products circulating in the blood — -an explanation which might be given, though an insufficient one, of the secretion seen in an isolated portion of the cardiac end of the stomach during the digestion of food in the rest. The efferent nervous channels through which these effects are produced have been defined by Pawlow's experiments on dogs. He first made a gastric fistula, then a few days afterwards divided the oesophagus through a wound in the neck, and stitched the two cut ends to the edges of the wound. After the animals had recovered, it was observed that when meat was given to them by the mouth, a copious secretion of gastric juice followed in five or six minutes, notwithstanding the 396 DIGESTION posterior vagi. fact that in this ' sham feeding ' the food immediately escaped from the opening in the upper portion of the divided oesophagus. Much the same result was seen when the food was simply shown to the animal. Indeed, when a hungry animal is tempted with the sight of meat, the flow of gastric juice, always occurring after a latent period of five or six minutes, may be even greater than with sham feeding. Division of the splanchnic nerves had no effect on this reflex secretion, while it could not be obtained after division of both vagi below the origin of their cardiac and pulmonary branches, by which disturbance of the| heart and respiration are avoided. Further, stimulation of the peripheral end of the vagus in the neck* caused secretion. These experiments show that secretory fibres for the gastric glands run in the vagi. It is probable that the vagi also contain efferent fibres ^^ • \\ > , which inhibit Py/crus ^„>; / —foITophsgus ^^^ gastric ^'""-.ff^.fri':" secretion. The excitation of the secretory fibres is not produced re- flexly by the processes of mastication and deglutition as such. Di- lute acid is the most powerful chemical stim- ulus for the buccal mucous membrane, and when it is introduced into the mouth of a dog with a double oesophageal and gastric fistula, an abundant secretion of saliva at once ensues. But no matter how long the animal continues to swallow the mixture of saliva and acid, no gastric juice is formed. The same is the case in sham feeding with salt, pepper, mustard, smooth stones, and even extract of meat. It is the desire for food ■ — -the appetite, as we call it — and the feeling of satisfaction associa- ated with eating food that the animal relishes, which is the efficient cause of the gastric secretion in sham feeding. The more eagerly the dog eats, the greater is the flow of gastric juice. * The nerve was not stimulated till a few days after the section, so as to allow the cardio-inhibitory fibres to degenerate. Otherwise the heart would have been stopped by the stimulation. Fig. i6i. — Pawlow's Stomach Pouch. AB, line of incision; C, flap for forming the stomach pouch. At the base of the flap the serous and muscular coats are preserved, and only the mucous membrane divided, so that the branches of the vagus going to the pouch are not severed. INFLUENCE OF NERVOUS SYSTEM ON DIGESTIVE GLANDS 397 Mas cu/aris Pawlow also performed the converse experiment. In dogs in which a pouch had been isolated from the stomach and made to open to the exterior by the surgical procedure illustrated in Figs. 161 and 162, he introduced into the large stomach, without the animal's knowledge, food of various kinds. This is best done in a sleeping dog. The secretion of gastric juice, both in the main stomach and in the pouch or miniature stomach, which is known in a great variety of conditions to present an exact picture of the process of secretion in the large, is markedly delayed and scanty when it does appear. Bread and coagulated egg-white did not yield a single drop during the first hour or more. Raw flesh excited a secretion, but after an inter- val of fifteen to forty - five minutes, in- stead of five or six to ten, as in sham feeding. It was very scanty during the first hour (only o n e - third the nor- mal amount), and possessed a very low di- gestive power. The impor- tance of the psychical ele- ment is shown by the fact that in one dog, which, after a weighed amount of meat had been introduced into its stomach (without its knowledge), received a sham meal of meat, the amount of protein digested after one and a half hours was five times greater than in another animal treated exactly in the same way, except that the sham meal was omitted. But even after division of the vagi, gastric secre- tion is still caused by the introduction of various substances into the stomach, especially water and meat extract. The active substances in the meat extract are, for the most part, insoluble in alcohol. Kreatin is inactive. It is in virtue of these substances that raw meat placed directly in the stomach causes some secretion after a Fig. 162. — Pawlow's Stomach Pouch. S, the completed pouch; V, cavity of stomach. 398 DIGESTION time. Milk and gelatin solution are also direct excitants of gastric secretion apart from the water in them. Starch, fat, and egg-white are totally inert. After section of both vagi in dogs, no marked qualitative or quantitative changes have been observed in the gastric juice. The secretion caused by the presence of food in the stomach is still obtained when, in addition to the vagi, all other nerves which can possibly connect the central nervous system with the organ have been severed and the sjmipathetic abdominal plexuses have been destroyed (Popielski). We must therefore sug- pose that the gastric glands, while normally under the control of a nervous mechanism in the upper portion of the cerebro-spinal axis whose efferent fibres run in the vagi, are also capable of being locally stimulated through the peripheral gangUa in the stomach walls or the chemical action of the products of digestion absorbed into the blood. Edkins showed that the injection of food substances or the products of their digestion (broth, dextrin, peptone) or of acid into the blood caused no secretion of gastric juice, while the injection of an extract of the pyloric mucous membrane, made by boiling it with water, acid, or peptone, excited a certain amount of secretion. He therefore" concluded that the secondary secretion of gastric juice is determined, not by local stimulation of a reflex mechanism in the gastric waU, but by the production in the mucous membrane of the pyloric end of a chemical substance, the gastric secretin or gastric hormone,* which is absorbed by the blood, and acts as an excitant to all the gastric glands. The cardiac mucosa was foimd incapable of forming this substance. It is not to be imagined that the ' psychical ' secretion and the secretion called forth by the direct action of the food or food- products in the stomach perform independent of&ces. They can, in various instances, be shown to supplement each other. For example, not more than one-half or one-third of the gastric juice secreted during the digestion of bread or boiled egg-albumin can be ascribed to the psychic effect. Yet these substances, when introduced directly into the stomach, cause practically no secretion. We must suppose that during the digestion of the bread and albu- min by the psychically secreted juice certain products analogous to those in the meat extract are formed, which act as chemical excitants of the local secretory apparatus. The psychic juice is indispensable in this case to start the process, ' to set the stove ablaze,' as Pawlow puts it. In the case of meat it is not indispensable, since the meat can chemically excite the gastric glands; but it greatly hastens the process of digestion. These facts emphasize the importance of * ' Hormone ' (from opuau, I arouse or excite) is the name given to a sub- stance which, carried by the blood from the place where it is formed, acts as a chemical messenger in exciting the activity of some more or less distant organ. The classical example is the pancreatic secretin which, manufactured in the intestinal mucosa, excites the secretion of the pancreatic juice. INFLUENCE OF NERVOUS SYSTEM ON DIGESTIVE GLANDS 399 appetite in digestion, a truism in treatment which thus receives for the first time a rational explanation. The influence of good- humour upon nutrition, which experience has crystallized into the proverb ' L?i.ugh and grow fat,' has also been shown to depend — ■ in great part, at least — upon a beneficial action on the digestive functions, both motor and chemical. The movements of a cat's stomach and intestines have been observed to cease when the animal became angry or excited by unpleasant emotions; and in a dog whose gastric glands were pouring out a copious psychical secretion in response to a sham meal, secretion stopped abruptly when the animal's wrath was awakened by what is probably to the normal dog the most specifically ' adequate ' stimulus for the emotion of anger — the sight of a cat which he was restrained from chasing. By means of experiments with the miniature stomach it has been further shown that each kind of food has its own characteristic curve of gastric secretion. With flesh diet the maximum rate of secretion occurs during the first or second hour, and in each of the first two hours the quantity of juice furnished is approximately the same. With bread diet we have always a sharply-indicated maximum in the first hour, and with milk a similar one during the second or the third hour (Fig. 163). The juice secreted on different diets also differs in digestive power — i.e., in the amount of protein which a given quantity of it will digest in a given time. ' Bread juice ' is much stronger in ferment than ' meat juice,' and ' meat, juice ' somewhat stronger than ' milk juice ' (Fig. 164). But ' meat juice ' has a higher acidity than ' bread juice,' ' milk- juice ' being intermediate. These differences do not necessarily indicate that the gastric mucous membrane responds in a specific way to each kind of food substance, as suggested by Pawlow. They may depend on several circumstances, and particularly on this — ^that the quantity, though not the quality, of the psychical or ' appetite ' juice is related to the relish with which the ariimal eats the food. The products formed in the digestion of the different foods by the psychical juice may therefore be different in nature and amount, and thus the quantity of the gastric hormone which determines the secondary secretion may vary with the food. The young mammal, like the adult, secretes gastric juice before the food reaches the stomach. In puppies from one to eighteen days old sham feeding (sucking the teats of the mother after an CEsophageal fistula has been made in the younger animals and a double oesophageal and gastric fistula in the older) causes a liquid with the properties of gastric juice to gather in the stomach. This power, then, is a congenital one. The individual does not gain it by experience; 'it comes into the world with him (Cohnheim). The Influence of Nerves on the Pancreas. — Like the stomach, the pancreas receives secretory fibres through the vagus. These are 400 DIGESTION 3 rs 1 2 3 ♦ 5 6 78 1 2 3 4 5 6 7 a 9 10 1 2 3 4 S 6 12 _- in \ \ « \ / \ ^ / 1 fi \ / \ ^ / \ 4 1 \ . \ j \ . ) / 7 I N \ l^ ^ Y \ 1 \ \ / • \ 1 ■^ ' . Flesh, Bread, 2oo grm. Milk, 6oo cc. Fig. 163.- -Rate of Secretion of Gastric Juice with Diets of Meat, Bread, and Milk (Pawlow). probably connected with a reflex centre in the medulla oblongata. It has long been known that when the medulla is stimulated a flow of pancreatic juice is occasionally set up, or is increased if already going on. The same is true when the vagus is stimulated in the ordinary way in the neck. But the experiment often failed, for the pancreas is peculiarly susceptible to circulatory disturbances, and stimula- tion of the bulb or the vagus may interfere with the blood- flow through the gland by exciting its vaso-con- strictor fibres or causing inhibition of the heart. These disturbing influences may be avoided, as Pawlow has shown, by stimulating the vagus, three or four days after dividing it, with slowly-recurring stimuli (induction shocks or light blows from a small hammer worked by an electro - mag- net at the rate of about one in the second). The secretory fibres are still susceptible of excitation, while the car- dio-inhibitory fibres, which degenerate more rapidly, are almost or altogether in- excitable, and the vaso-constrictors are but little affected by these slow rhythmical stimuli, which excite the secretory nerves (p. 173)- A pancreatic fistula has previously been established by excising a small portion of the duodenal wall containing the open- ing of the pancreatic duct, closing the intestine by sutures, and Hours 1 2 : ) << ^ ! > c i • r i i : } ; ) < i ; > e i ■ ' ( ■ ) i ) ;< S K I i 6 ^ An » / ■s. P': 60 / N i:6 ^■° \ ■~- / t5 ^ Q 40 1 f 5*a \\ixn\z r\f fh#i PRACTICAL EXERCISES ' 447 head. The small intestine is immediately removed. It may be cut between double ligatures into several pieces for this purpose. The contents are rapidly washed out by a stream of warm Ringer, and the pieces placed in (6) , through which oxygen is kept bubbling. The ■pieces are conveniently supported in the liquid by threads Sxed by the cork of the bottle. There is a hole in the cork for the escape of the oxygen. The movements of the intestines in (6) can be studied very well by inspection. Or a separate length of intestine maybe kept for this purpose, the contents not beiag removed, but prevented from escaping by ligatures at each end. Thi§ can be most easily ob.served in a shallow dish of warm Ringer. Or a separate experiment can be made in which the whole alimentary canal of a rabbit is carefully removed, and examined in oxygenated Ringer's solution. ^ A segment of intestine about 2 or 2^ cm. in length is now cut off one of the pieces. A small ring of platinum or aluminium is tied to a point on the circumference of one end of the preparation by a silk thread passed through the wall. The other end is caught by a serre- fine at a point exactly corresponding to the attachment of the ring,, so that the pull of the contracting lohgitudin,al muscle should be in the straight line joining these two points. The serre-fine has attached to it a thread with a hook on the other end. In preparirig the intestinal segment it lies on a plate of glass above a vessel of iSvarm water. The small cylinder (c) is now partially filled with w^rm (Ringer's solution! The ring is grasped by fine forceps, and made to engage with the hook at the bottom of the cylinder, care being taken not to injure the prepara- tion in the process. The cylinder is then fastened on its stand anc} lowered into the bath. The thread is connected by its attached hook to the lever, and oxygen allowed to bubble slowly aiid regularly through the cylinder. Very soon rhythmical contractions begin (Fig. 173), and continue for a long time. The effect on. these contractions of abolish- ing, reducing, or increasing the oxygen-supply may first be studied. 2. Effect of Blood-Serum on the Contractions of Intestinal Segments. — While a tracing is being taken as in'i, fill a small bent pipette with serum already warmed in the bath, pass-the point-of the pipette down to the bottom of the cylinder without interfering with the preparation, and allow the serum to flow in till the Ringer's solution is displaced. Almost at once the lever will begin to rise, indicating strong tonic contraction. The increase of tone lasts for some time, but can soon be removed on washing the preparation with Ringer. This is most easily done, while the drum is stopped, by introducing pipetteful after pipetteful of Ringer's solution into the cylinder in the way described, allowing the liquid to overflow into the bath. The subsequent addition of serum causes a renewed increase of tone, and this may be many times repeated. Determine the greatest dilution of the serum which still produces a distinct effect upon the intestinal segment. 3. Action of Epinephrin (Adrenalin) on Intestinal Segments. — Pro- ceed as in 2, but use various dilutions of adrenalin chloride instead of serum. They must be freshly prepared. Instead of increase of tone, inhibition of the movements and decrease of tone will be obtained (Fig. 173). This experiment may be performed at another stage in the course (P- 693)- 4. Quantitative Estimation of Ferment Action. — For pepsin an easy method, although not a very accurate one, of estimating the rate at which the fibrin disappears is to use fibrin stained with carmine. As solution goes on, the dye colours the liquid more and more deeply, and by comparing the depth of colour at any time with standard solutions nf ^r^^,^: — — 4- 4-1, „ .^,....«4-;+,, ^f J-*.. ^^4- f^^^ .,v,j 4.v.^^^f^ — ^i cu_:« 448 DIGESTION AND ABSORPTION digested. This method cannot be used for trypsin. A much better method is that of Mett (p. 336). Fluid egg-white is sucked up into fine glass tubes (of i to 2 mm. bore). The tubes are then heated in a bath at 95° C. For use short pieces (i or 2 cm. long) are cut ofi and placed in I or 2 c.c. of the liquid to be tested, the whole being kept at 38° to 40° C. For amylolytic ferments where rapid work is required, glass tubes filled with tinted starch paste may be used in the same way as the Mett's tubes. A more accurate method, and yet a rapid and convenient one, is based upon the time which is necessary in order that the iodine reaction with starch may just disappear when a standard starch solution is digested with a dilution of the ferment solution at 40° C. 5. Saliva — Collection atidMicroscopic Examina- tion of Saliva. — Chew a piece of paraffin-wax, or inhale ether or the vapour of strong acetic acid. The flow of saliva is increased. Collect it in a porcelain capsule. Examine a drop under the microscope. It may contain a few flat epithelial scales from the mouth and a few round granular bodies, the sali- vary corpuscles, the gran- ules in which often show a lively, dancing move- ment (Brownian motion) . Filter the saliva to free it from air-bubbles, and per- form the following ex- periments : (a) Test the reaction with litmus paper. It is usually alkaline. An acid reaction may indicate that bacterial processes are abnormally active in the mouth. (b) Add dilute acetic acid. A precipitate indicates the presence of mucin {p. 338). Filter. (c) Add a drop or two of silver nitrate solution to the filtrate from (6). A precipitate insoluble in nitric acid, soluble in ammonia, proves that chlorides are present. (d) Add to another portion a few drops of dilute ferric chloride slightly acidulated with dilute hydrochloric acid, and the same quantity to as much distilled water in a control test-tube. A reddish coloration is obtained, due to the presence of sulphocyanic acid, which is com- bined with potassium and other bases in the saliva. The colour is dis- charged by mercuric chloride. Or, a drop of saliva may be allowed to fall from the mouth on a test-paper (prepared by soaking filter-paper with a dilute starch solution, containing a little iodic acid, and allowing it to dry in the air). The paper is coloured blue by the union of the Fig. 173. — Efiect of Serum' arid Adrenalin on Con- tractions of a Segment of Intestine. Rabbit's intestine contracting in Ringer's solution. At 55 the Ringer's solution was replaced by dog's seriun, and this at 56 by adrenalin (i : 5,000,000) in serum. At 57 this weak adrenalin solution was replaced by a stronger one (i: 500,000) in serum. Time-trace, half-minutes; (Reduced to half.) PRACTICAL EXERCISES 449 starch with iodine set free from the iodic acid by the action of the sulplio- cyanic acid. (e) Take some boiled starch mucilage, and test it for reducing sugar by Trommer's test (p. lo). If no sugar is found, take three test- tubes, label them A, B, C, and nearly half fill each with the boiled starch. To A add a little saliva,* to B some saliva which has been boiled, to C a little saliva which has been neutralized, and as much 0-4 per cent, hydrochloric acid as has been taken of the mucilage, so as to make the strength of the acid in the mixture 0-2 per cent., a propor- tion well below that of the gastric juice . Put the test-tubes into a water- bath at 40° C. In a few minutes test the contents for reducing sugar. Abundance will be found in A, none in B or C. In B the ferment ptyalin has been destroyed by boiling; in C its action has been inhibited by the acid. If the test-tubes have been left long enough in the bath, no blue colour will be given by A on the addition of iodine, but a strong blue colour by B and C— i.e., the starch will have completely disappeared from A. (/) Put some starch in a test-tube, add a little saliva, and hold in the hand or place in a bath at 40° C. On a porcelain slab place several separate drops of dilute iodine solution. With a glass rod add a drop of the mixture in the test-tube to one of the drops of iodine at intervals as digestion goes on. At first only the blue colour given by starch will be seen; a little later a violet colour, due to the presence of erythro- dextrin in addition to some unaltered starch. A little later the colour will be reddish, the starch having disappeared and the erythrodextrin reaction being no longer obscured. Later still no colour reaction will be obtained, the erythrodextrin having undergone further changes, and only sugar (maltose, isomaltose, and perhaps a trace of dextrose^ and achroodextrin — a kind of dextrin which gives no colour with iodiiie — • being present. (g) Put two pieces of glass tube filled with tinted starch paste (p. 448) into separate test-tubes. Cover one with 3 c.c. and the other, with 6 c.c. of saliva. The saliva must all be taken from the same stock, and must hot be collected separately. Put in a bath at 38° C, and when a fair amount of digestion has taken place in each, measure the length of the column digested, and determine the relation between the amount digested in the two tubes (p. 336). (h) Dilute 2 c.c. of saliva with distilled water up to 20 c.c, and filter. Take six test-tubes of the same width, and label them A, B, C; etc. Measure into A 3 c.c. of the diluted saliva, into B 2 c.c, into C 1-3 c.c, into Do'9 c.c, into E 06 c.c, and into F 0-4 c.c. Thus a series is obtained in which each tube contains (approximately) two-thirds as much ferment as the one it follows. Add distilled water to tubes B to F, sufficient to make up the volume in each to 3 c.c. Place the tubes in a beaker of iced water; add to each 10 c.c. of a i per cent, solution of boiled starch previously cooled in iced water, and shake so as to mix the contents. Each tube now contains starch in uniform concentration, and ferment in varying concentration. The low temperature prevents digestion till all the tubes are ready. Now put the tubes simultaneously into a water- bath at 40° C. for half an hour, and then back again into iced water to prevent further digestion. Move them about in the iced water to cool rapidly. Fill up the tubes with distilled water nearly to the top, add a drop or two of iodine solution to each, and mix uniformly. The tubes to which the smallest amounts of saliva were added will probably * As it filters slowly, unfiltered saliva may be used for Experiments (e), (/), and (i). 29 450 DIGESTION AND ABSORPTION still show a distinct blue colour, while those at the other end of the series will be brown or yeUow, and the intermediate tubes bluish-violet. Suppose D is the last tube still showing a bluish tint, then in the next higher tube, C, all the starch has been hydrolysed at least to dextrin — that is, 1-3 c.c. of the ten-times diluted saliva, or 0-13 of the original saUva, has been sufficient to change all the starch in 10 c.c. of the i per cent, solution. With another specimen of saliva the same result might be reached in tube E, containing an amount of ferment equal to that in 0'o6 c.c. of the original saliva. We could then conclude that the diastatic power of the second saliva was about twice as great as that of the first. A closer approximation can now be made by setting up two fresh tubes (C and E respectively for the two salivas) and deter- mining the time required for the blue reaction, with iodine to disappear, taking out a drop from time to time and testing on a porcelain slab. (j) Put a little distilled water into a porcelain capsule, and bring the water to the boil. Now put into the mouth some boiled starch paste, and move it about as La mastication. After half a minute spit the starch out into the boiliag water. Divide the water into two portions. Test one for sugar, and the other for starch. Repeat the experiment, but keep the starch two minutes in the mouth. Report the result. (7) Starch solution to which saliva has been added is placed in a dialyser tube of parchment-paper for twenty-four hours. At the end of that time the dialysate (the surrounding water) should be tested for sugar and for starch. Sugar will probably be foimd, but no starch. If no reaction for sugar is obtained, the dialysate should be concen- trated on the water-bath, and again tested. 6. Stimulation of the Chorda Tympani. — (i) Having previously studied the anatomy of the mouth and submaxillary region in the dog by dissecting a dead animal (Fig. 174), put a good-sized dog under morphine. Set up an induction-machine for a tetanizing current (p. 198), and connect it with fine electrodes. Fasten the dog on the holder, give ether if necessary, and insert a cannula in the trachea (p. 199). Then make an incision 3 or 4 inches long through skin and plat3rsma muscle, along the inner border of the lower jaw, beginning about the angle of the mouth, and continuing backwards towards the angle of the jaw. Such branches of the jugular vein as come in the way may be generally pushed aside, but if necessary they may be doubly ligated and divided. Feel for the facial artery, so as to be able to avoid it. Divide the digastric muscle about its anterior third, and clear it carefully from its attachments ; or, without dividing it, puU it outwards with a hook. The broad, thin mylo-hyoid muscle will now be seen with its motor nerve lying on it. Divide the muscle about its middle at right angles to its fibres, and raise it carefully. The lingual nerve will be seen emerging from under the ramus of the jaw. It runs transversely towards the middle line, and then, bending on itself, passes forwards parallel to the larger hypoglossal nerve. In its transverse course the lingual wiU be seen to cross the ducts of the submaxillary and sublingual glands. These structures having been identified, the lingual nerve is to be ligatured before it enters the tongue and cut peripherally to the ligature. Then a glass cannula of suitable size is to b3 inserted into the submaxillary duct (the larger of the two), just as if it were a bloodvessel (p. 63). A short piece of narrow rubber tubing is carefully slipped on the end of the cannula. The lingual is now to be lifted by means of the ligature, and traced back towards the jaw till its chorda tympani branch is seen coming ofi and running backwards along the duct. The chordo-lingual nerve (Fig. 160, p. 386) is then to be cut centrally to the origin of the chorda tympani, which can'now be PRACTICAL EXERCISES 451 easily laid on electrodes by means of the ligature on the lingual. On stimulating the chorda, the flow of saliva through the cannula will be increased. The current need not be very strong. If the flow stops after a short time, it can be again caused by renewed stimulation after a brief rest. A quantity of saliva may thus be collected, and the experi- ments already made with human saliva repeated. (2) Expose the vago-sympathetic nerve in the neck on the same side ; ligature it ; divide below the ligature ; and note the efiect pro- duced by stimulation of the upper end on the flow of saliva. I"* (3) Set up another induction-machine, and connect it with electrodes. Stimulate the chorda, and note the rate of flow of the saliva. Then, while the chorda is still being excited, stimulate the vago-sympathetic, and observe the efiect. If the experiment is successful, finish by stimulating the chorda for a long time. Then harden both sub- Digastric Muscle (cut). Hypoglossal Nerve. Mylo-liyoid Lingual Wharton's Muscle (cut). Nerve. Duct. Fig. 174. — Dissection for Stimulation of Chorda Tympani (after Bernard). maxillary glands in absolute alcohol, make sections, stain with carmine, and compare them. 7. Effect of Drugs on the Secretion of Saliva. — (i) Proceed as in 6 (i), but, in addition, insert a cannula into the femoral vein (p. 217). On the cannula- put a short piece of rubber tubing, filled with o-g per cent, salt solution and closed by a small clamp, or a small piece of glass rod, or a pair of bulldog forceps. While the chorda is being stimulated inject into the vein 10 to 15 milligrammes of sulphate of atropine by pushing the needle of a hypodermic syringe through the rubber tube. This will stop the flow of saliva, and abolish the efiect of stimulation of the chorda. See whether the sympathetic is also inactive, and report the result. (2) Now empty the cannula in the submaxillary duct by means of a feather, and fill it with a 2 per cent, solution of pilocarpine nitrate by means of a fine pipette. Fill also the short rubber tube attached to the cannula, and close it again. Compress the tube, and so force into 452 DIGESTION AND ABSORPTION the duct a small quantity of the solution. Open the tube. Secretion of saliva will again begin, and stimulation of the chorda will again cause an increase in the flow. But after a few minutes the action of the atropine will reassert itself, and the flow wiU stop. Renewed secretion may be caused by a fresh injection of pilocarpine. 8. Gastric Juice — (a) Preparation of Artificial Gastric Juice. — Take a portion of the pig's stomach provided, strip ofi the mucous membrane (except that of the pyloric end, which is relatively poor in pepsin), cut it into smaU pieces with scissors, and put it in a bottle with loo times its weight of 0-4 per cent, hydrochloric acid. Label and put in a bath at 40° C. for three hours, and then in the cold for twelve hours. Then filter. (b) Take another portion of the mucous membrane, cut it into pieces, and rub up with clean sand in a mortar. Then put it in.a small bottle, cover it with glycerin, label, and set aside for two or three da3rs. The glycerin extracts the pepsin. (c) Take five test-tubes. A, B, C, D, E, and in each put a little washed and boiled fibrin or a small cube of coagulated egg-white. To A add a few drops of glycerin extract of pig's stomach, and fill up the test-tube with 0-4 per cent, hydrochloric acid. To B add glycerin extract and distilled water; to C glycerin extract and i per cent, sodium carbonate; to D 0'4 per cent, hydrochloric acid alone ; to E glycerin extract which has been boiled, and 0-4 per cent, hydrochloric acid. Put up another set of five test-tubes in the same way, except that a few drops of a watery solution of a commercial pepsin are substituted for the glycerin extract. Label the test-tubes A', B', C, D', E'. Into another test-tube put a little fibrin (or an egg-white cube), and fill up with the filtered acid extract from (a). Label it F. Place all the test-tubes in a tumbler, and set them in a water-bath at 40° C. Put a piece of a Mett's tube (p. 336) into each of two test-tubes, and add 15 c.c. of 0-4 per cent, hydrochloric acid. To one tube add 5 drops and to the other 10 drops of the same filtered glycerin extract of gastnc mucous membrane. Put the tubes in the bath, and when digestion is distinct at the ends of both tubes measure the length of the column digested in each. What is the relation between the two (p. 336) ? The experiment can be repeated with the hydrochloric acid extract of the mucous membrane. After a time the fibrin {or egg-white) will have almost completely dis- appeared in A, A', and F, but not in the other test-tubes. Filter the contents of A, A , and F into one dish. (d) Test the filtrate for the products of gastric digestion : (a) Neutralize a portion carefully with dilute sodium hydrox- ide. A precipitate of acid-albumin may be thrown down. Filter. 0) To a portion of the filtrate from (a) add excess of sodium hydroxide and a drop or two of very dilute copper sulphate. A rose colour indicates the presence of proteoses or peptones. The cupric sulphate must be very cautiously added, because an excess gives a violet colour, and thus obscures the rose reaction. If still more cupric sulphate be added, blue cupric hydroxide is thrown down, and nothing can be inferred as to the presence or the nature of proteins in the liquid. (y) Heat another portion of the filtrate from (a) to 30° C, and add crystals of ammonium sulphate to saturation. A precipitate of proteoses (albumoses) may be ob- tained. Filter off. PRACTICAL EXERCISES 453 (8) Add to the filtrate from (7) a trace of cupric sulphate and excess of sodium hydroxide. A rose colour indicates that peptones are present. More sodium hydroxide must be added than is sufficient to break up all the ammonium sulphate, for the biuret reaction requires the presence of free fixed alkali. A strong solution of the sodium hydroxide should therefore be used, or the stick caustic soda. The addition of ammonium sul- phate will cause the red colour to disappear; so will the addition of an acid. Sodium hydroxide will bring it back. Ammonia does not affect the colour, (e) To some milk in a test-tube add a drop or two of rennet extract,, and place in a bath at 40° C. In a short time the milk is curdled by the rennin. (See p. 347.) 9. (i) To obtain Normal Chyme. — Inject subcutaneously into a dog, one and a half hours after a meal of minced meat and water, 2 mg. of apomorphine. Half of one of the ordinary tabloids is enough. Collect the vomit. (2) To obtain Pure Gastric Juice. — If the laboratory possesses a dog with Pawlow's double oesophageal and gastric fistula, the juice may be obtained in large amount by sham feeding with meat (p. 395). If not, proceed as follows: Put a fasting dog under ether, and fasten on the holder. Clip the hair and shave the skin in the middle line below the sternum. Make a longitudinal incision ,.^^_ through the skin and subcutaneous tissue from the xiphoid cartilage downwards for 3 or 4 inches. The hnea alba will now be seen as a white mesial streak. Open the abdomen by an incision through it. Pull over the stomach towards the right, and stitch it to the abdominal wall, open it, and insert a stomach cannula (Fig. 175). Make an incision Fig. 175.— Stomach through the serosa and muscularis. Doubly Cannula, ligate and divide any vessels exposed in the submucosa. Then make an opening in the mucosa of sufficient size to just admit the gastric cannula. This will go into a smaller opening if it is provided with a nick in the flange which enters the stomach. Be careful to prevent blood from getting into the stomach. Immediately stitch the wound in the stomach over the flange of the cannula, but do not pass the stitches through to the internal surface of the mucosa. Suture the muscles and skin separately. Then stitch up the wound in the abdomen. Wash out any stomach contents with warm water. Put a cork in the cannula, and cover the animal with a cloth. The follow- ing experiments may now be performed: Expose both vagi in the neck. Connect two pairs of electrodes with the secondary coil of an induc- torium arrahged for single shocks. By means of a key in the primary stimulate the nerves with slow rhythmical induction shocks at the rate of about one a second. Continue the stimulation for fifteen minutes, collect any juice that may have been secreted, and apply the tests in (3). If secretion is slow, a little distilled water may be put into the stomach, and the vagus stimulation repeated. Mechanical stimulation of the mucous membrane with a feather causes no secretion of acid gastric juice, but may cause a secretion of alkaline mucus. (3) (a) Test the reaction to litmus of the chyme obtained in (i), and of the pure juice obtained in (2). (6) Test their proteolytic powers by putting in a bath at 40° C. for two hours two test-tubes containing respectively filtered chyme and 454 DIGESTION AND ABSORPTION fibrin, and gastric juicfe and fibrin. The fibrin will be digested in both. Estimate the proteol3rtic power quantitatively by Mett's tubes (p. 448). (c) Add a few drops of the chyme and gastric juice to milk in two test-tubes, and place them in a bath at 40° C. Repeat (c) after neutral- izing the liquids. (d) Examine a drop of the unfiltered chyme under the microscope. Partially digested fragments of the food wiU be seen. — ^muscular fibres or fat cells. Filter, and proceed as in 8 {d) (p. 453). (4) Test the filtrate from the chyme and the gastric juice for lactic acid by UfEelmann's test or Hopkins's test (p. 794), and for hydrochloric acid by Gunzburg's reagent. Uffelmann's Test for Lactic Acid. — ^The reagent is a dilute solution 'of carbolic acid to which dilute ferric chloride has been added till the colour is bluish (say a drop of a i per cent, ferric chloride solution to 5 c.c. of a I per cent, carbolic acid solution). The blue colour of the mixture is turned yellow by lactic acid, but not by dilute hydrochloric acid. Since Ufielmann's test is given also by phosphates, alcohol, and sugar, which may sometimes be present in the stomach contents, it is best to shake the gastric contents with ether, dissolve the ethereal extract in water, and then make the test on the watery solution. Giinzburg's Reagent for Free Hydrochloric Acid in Gastric Juice is made by dissolving 2 parts of phloroglucinol and i part of vanillin in 30 parts by weight of absolute alcohol. A few drops of the reagent are added to a few drops of the filtered gastric juice in a small porcelain capsule, and the whole evaporated to dryness over a small bunsen flame. If free hydrochloric acid is present, a carmine-red residue is left. If all the hydrochloric acid is united to proteins in the stomach contents, the reaction does not succeed. It is also hindered by the presence of leucin. 10. Pancreatic Juice. — (a) Take a piece of the pancreas of an ox or dog which has been kept twenty-four hours at the temperature of the laboratory, and make a glycerin extract in the same way as in the case of the pig's stomach in 8 (6). Put in a small bottle, and set aside for a day or two. (6) Put a little boiled fibrin into each of six test-tubes. A, B, C, D, E, F. To A add a few drops of glycerin extract of pancreas, and fiU up with a I per cent, sodium carbonate solution ; to B add glycerin extract and distilled water; to C glycerin extract and excess of o'05 per cent, hydrochloric acid; to D i per cent, sodium carbonate alone; to E i per cent, sodium carbonate in which a few drops of glycerin extract of pancreas have been previously boiled ; to F glycerin extract and excess of 0-2 per cent, hydrochloric acid.* Set up six test-tubes. A', B', C, D', E*, F', in the same way, but substitute a few drops of a solution of commercial pancreatin for the glycerin extract. Set up two test-tubes as in experiment 8 (p. 452) with Mett's tubes. Put all the test-tubes in a tumbler, and place in a bath at 40° C. The fibrin wiU be gradually eaten away in A and A*^, by the action of the trypsin, but will not swell up or become clear before disappearing, as it does in dilute hydrochloric acid with glycerin * With hydrochloric acid of different strengths the rapidity of digestion of boiled fibrin by glycerin extract of dog's pancreas (i volume of extract to 25 of acid) was found about the same for o"3 and o'ly per cent, acid; much less for o-o8 per cent., while in 0-04 per cent, acid there was practically no digestion at all. In 0-4 per cent, acid digestion took place more rapidly than in o'o8 per cent., but much less rapidly than in o'i7 per cent. luacid of all strengths digestion was markedly slower than in i per cent, sodium car- bonate. PRACTICAL EXERCISES 453 extract of stomach. Filter the contents of these test-tubes. Neutralize the filtrate with dilute acid; a precipitate will consist of alkali-albumin. If such a precipitate is obtained, filter it off and test the filtrate for proteoses and peptones as in 8 (i) (p. 452) . Some digestion, and perhaps a considerable amount, may also have taken place in F and F'; less or none at all in C and C ; and none in the other „est-tubes (pp. 353, 415). (c) Add a few drops of the glycerin extract to a test-tube containing starch mucilage, which has been previously found free from reducing sugar. Put in a bath at 40° C. After a short time abundance of reducing sugar will be found, owing to the action of the ferment, amylopsin, or pancreatic amylas3. {d) Mince thoroughly a good-sized piece of fresh pancreas, and shake up well with three or four times its bulk of water. Put 5 c.c. of fresh cream into a test-tube, then 10 c.c. of the extract, a few drops of chloro- form to prevent the growth of bacteria, a few drops of litmus solution, and if necessary enough of very dilute sodium hydroxide to just render the colour distinctly blue. Shake up, and divide the mixture into two portions, A and B. Boil one portion (B), and place the test-tubes at 40° C. Examine from time to time. The blue colour will disappear in A, owing to the formation of fatty acids from the neutral fats, and sodium hydroxide must be added to it to restore the colour. In B the fat-splitting ferment has been destroyed by boiling, and fat-splitting will not occur. Probably a distinct result will not be obtained for several hours, and it will be best to leave the tubes in the incubator overnight. («) If the laboratory possesses an animal with a pancreatic fistula, the following experiment may be done by a limited number of students with fresh pancreatic juice* collected from the fistula. Take five test- tubes. A, B, C, D, E. Add 5 c.c. of pancreatic juice to each tube. Boil E, and then cool it. Put into A and B small pieces of heat-coagulated egg-white, into C a little starch mucilage, and into D and E 5 c.c. of fresh cream. Add further to B a scraping of the mucous membrane of the upper part of the small intestine which has first been washed free of contents. To D and E add a drop or two of litmus solution, and, if necessary, enough of dilute sodium hydroxide to just establish a blue colour. Then put the test-tubes at 40° C, and examine after a time. No digestion will have taken place in A, because the pancreatic juice, as secreted, does not contain active trypsin. In B digestion may take place, because the enterokinase in the intestinal mucous membrane will activate the trypsinogen to trypsin. In C and D there will be evidence of the production of reducing sugar and fatty acids respec- tively, since the pancreatic juice, as secreted, contains active amylase and steapsin. E will be unchanged unless by bacterial action. if) Leucin and Tyrosin. — As examples of amino-acids formed when pancreatic digestion of proteins (fibrin or casein, e.g.) is allowed to go on for some days,| leucin and tryosin maybe isolated. Add bromine- water by drops to 5 c.c. of the digest; a pink colour indicates trypto- phane. If the ' digest ' be neutralized, then filtered, and the filtrate concentrated and allowed to stand, a crop of tyrosin crystals will separate out, since tyrosin is only slightly soluble in watery solutions of neutral salts. These crystals having been filtered off, the proteoses (albumoses) and peptones can be precipitated together by alcohol, and * A considerable flow of pancreatic juice can be obtained from a dog with a pancreatic fistula by injecting intravenously an extract of intestinal mucous membrane containing secretin (p. 401). t A little chloroform is added to prevent bacterial growth. 456 DIGESTION AND ABSORPTION afterwards separated, if that is desired, by redissolving the precipitate in water and throwing down the proteoses by saturation with am- monium sulphate. The alcoholic filtrate will contain any leucin that may be present, since that body is moderately soluble in alcohol, as well as traces of tyxosin, which, however, is much less soluble in this medium. On concentration, crystals of both substances will be ob- tained. Tyrosin crystallizes characteristically from animal liquids in beautiful silky needles united into sheaves, leucin in the form of in- distinct fatty-looking balls, often marked with radial striae and coloured with pigment (Figs. i86 and 187, p. 483). Tests for Tyrosin by Morner's Test. — Put a small quantity of tyrosin into a test-tube. Add about 3 c.c. of the reagent,* and heat gradually and gently to the boiling-point. A green colour is obtained. II. Bile. — (a) Test the reaction of ox bile. It is alkaline to litmus. (6) Add dilute acetic acid. A precipitate of bile-mucin (really nucleo-albumin) falls down. Some of the bile-pigment is also pre- cipitated. Filter. (Pig's bile contains more of the mucin-like sub- stance than ox bile.) (c) Put a little of the filtrate from (6) or of the original bile into a porcelain capsule, add a drop or two of a dilute solution of cane-sugar, and mix with the bile. Then add a few drops of strong sulphuric acid, and stir; then a few drops more of the sulphuric acid, stirring all the time. A purple colour appearing at once, or after gentle heating, shows the presence of bile-acids (Pettenkofer's reaction). The bile may be diluted before the addition of the sulphuric acid. In this case a, greater amount of the acid must be added. Examine the purple liquid in a test-tube with a spectroscope (p. 74). Dilute the liquid with water, adding some sulphuric acid to partially clear up the precipitate caused by the water. Two absorption bands are seen, one to the red side of D, and the other, a stronger and broader band, over and to the right of E. When only a very small amount of bile-salts is present, the reaction is made more sensitive if a solution of furfuraldehyde (i to 1,000) is used instead of cane-sugar. {d) Hay's Sulphur Test. — Sprinkle a little sulphur (in the form of the fine powder known as flowers of sulphur) on the surface of some bile in a small beaker or deep watch-glass. The sulphur will soon sink to the bottom. Repeat with water; the sulphur will float. The rjaction is due to the diminution of the surface tension produced by the bile-acids, and succeeds also in a solution of bile-salts. The test is very sensitive. But in stomach contents, vomit, or stools, it rarely gives good results, since alcohol or acetic acid is often present in the gastric liquid, and phenol and its derivatives in intestinal contents, and all of these cause such an alteration in the surface tension that the sulphur sinks. Ether, chloroform, turpentine, benzine and its deriva- tives, anilin and soaps, also vitiate the test in the same way. (e) Add yellow nitric acid (containing nitrous acid) to a little bile on a white porcelain slab. A play of colours, beginning with green and running through blue to yellow and yellowish-brown, indicates the presence of bile-pigment (Gmelin's reaction). The reaction may also be obtained by putting some yellow nitric acid into a test-tube, and then running a little bile from a pipette on to the surface of the acid. The play of colours is seen at the surface of contact. Where the bile- pigment is present only in traces, some of the liquid may be filtered * The reagent for this test is prepared by mixing thoroughly i volume of formalin, 45 volumes of distilled water, and 55 volumes of concentrated sulphuric acid. PRACTICAL EXERCISES 457 through white filter-paper, and the test applied by putting a drop of the nitric acid on the paper. if) Cholesterin or Cholesterol (Fig. 176) — Preparation. — Extract a powdered gall-stone (preferably a white one) with hot alcohol and ether in a test-tube. Heat the test-tube in warm water, not in the free flame. Put a drop of the extract on a slide. Flat crystals of cholesterin, often chipped at the comers, separate out. (a) Carefully allow a drop of strong sulphuric acid and a drop of dilute iodine to run under the cover- glass. A play of colours — ^violet, blue, green, red — ^is seen. (/3) Evaporate a drop of the "solution of cholesterin. in a small porce- lain capsule, add a drop of strong nitric acid, and heat gently over a flame. A yellow stain is left, which becomes red when a drop of am- monia is poured on it while it is still warm. (7) Dissolve a little cholesterin in chloroform. Add an equal bulk of strong sulphuric acid, and shake gently. The solution turns red and the subjacent acid shows a green fluorescence. (S) Put a drop or two of water in a watch-glass, and add a drop of an ethereal solution of cholesterin. The cholesterin is precipitated, and will not dissolve in the water even on heating. Repeat the observation with bile instead of water. The cholesterin dissolves in the bile. (g) To a little of the filtrate from a peptic digest {e.g., fibrin which has been digested for twenty-four hours with pepsin and hydrochloric acid) add some bile. A precipitate is thrown down, which is redissolved in excess of the bile (p. 364). (h) Shake up a little bile with some rancid olive-oil in a test-tube . An emul- sion is formed. Repeat the experiment with the same quantities of bile and oil, but use perfectly fresh oil. Compare the stability of the two emulsions, allowing the tubes to stand together for a while. Fig. 176. — Crystals of Cholesterin (i) To some starch mucilage, shown (Frey). to be free from sugar, add a little bile, and place in a bath at 40° C. After a time test for reducing sugar. Report the result. Bile often has a slight diastatic power. Q) To demonstrate the Presence of Iron in the Liver Cells. — Steep sec- tions of liver in a solution of potassium ferrocyanide, and then in dilute hydrochloric acid . Or a i -5 per cent . solution of potassium ferrocyanide in 0-5 per cent, hydrochloric acid may be used. (The iron may pre- viously be fixed in the tissue by hardening it in a mixture of alcohol and ammonium sulphide.) The sections become bluish from the formation of Prussian blue. A fine-pointed glass rod or a platinum lifter should be used in manipulating them. A steel needle cannot be employed. Mount in glycerin or Farrant's solution, or (after dehy- drating with alcohol and clearing in xylol) in xylol-balsam. Blue granules may be seen under the microscope in some of the hepatic cells. Sections of spleen may also be examined for this reaction. 12. Microscopical Examination of Faeces. — Examine under the micro- scope the slides provided. Draw, and as far as possible determine the nature of, the objects seen (p. 418). 13. AbsorpticAi of Fat.— (a) Feed a rat or frog with fatty food; kill the rat in three or four hours, the frog in two or three days. Imme- diately after killing the rat open the abdomen, carefully draw out a loop of intestine, and look through the thin mesentery. The white 458 DIGESTION AND ABSORPTION lacteals will probably be seen ramifying in the mesentery. They appear white on account of the presence of globules of fat in the chyle with which they are filled. Strip ofi tiny pieces of the mucous mem- brarie of the small intestine, and steep them in J per cent, solution of osmic acid for forty-eight hours. Then tease fragments of the mucous membrane in glycerin and examine under the microscope. To preserve the specimens take off the glycerin with blotting-paper and mount in Farrant's medium, which is a preservative glycerin mixture. Other portions of the mucous membrane may be hardened for a fortnight in a mixture of 2 parts of MiiUer's fluid and i part of a i per cent, solution of osmic acid. Sections are then maae with a freezing microtome after embedding in gum. No process must be used by which the fat would be dissolved out (Schafer). (See Fig. 172, p. 435.) (6) Feed a cat with 30 grammes of butter stained a deep red with the dye Sudan III. After five hours anaesthetize the animal with ether, insert a cannula in the carotid artery, and obtain a sample of blood. Defibrinate the blood, and separate the serum by the centrifuge. If digestion and absorption of the fat have proceeded normally, the serum will contain numerous fat droplets, and will be tinged pink by the dye, which can be dissolved out of it by shaking up with ether. On opening the abdomen it will be seen that the mucous membrane of the small intestine, as far down as the fat has reached, is stained pink, and that the lacteals in the mesentery are also pink. Observe whether any of the pigment has passed into the urine. 14.* Time required for Digestion and Absorption of Various Food Substances. — Feed three dogs. A, B, and C, which have previously fasted for twenty-four hours, with a meal containing starch (proved to be free from sugar), lard, and meat. (i) After fifteen minutes inject subcutaneously into A 2 c.c. of a O'l per cent, solution of apomorphine. Note the time which elapses before the animal vomits. Collect the vomit. (a) Examine a little of it under the microscope, and make out fat globules, muscular fibres and starch granules. The latter can be recog- nized by their being coloured blue by a drop or two of iodine solution. (6) Filter the chyme, mixing it, if necessary, with a little water, and test it as in 8 {d) (p. 452) for the products of digestion of proteins. In addition, test for starch, dextrin, and reducing sugar. (2) One and a quarter hours after the meal inject apomorphine into dog B, and proceed as in (i). (3) Two and a half hours after the meal inject apomorphine into dog C, and proceed as in (i). Compare the results from the three specimens of chyme. 15.* Qucintity of Cane-Sugar inverted and absorbed in a Given Time. — Take three dogs, A, B, and C, which have fasted for twenty-four hours. The animals should be about the same size, ^eed A and B with 100 c.c. of a standard solution of cane-sugar (about a 20 per cent, solu- tion), or as much more as they will take. If the dogs have been kept without water for a day they will more readily take the sugar solution. Or it may be given through a tube passed into the stomach, and in this case a larger quantity of sugar can be given. A gag consisting of a. piece of wood with a hole in the middle of it, through which the tube is passed, must first be secured in the dog's mouth. Feed C with * Experiments 14 and 15 are conveniently done in a class by assigning each of the three animals to a separate set of students. The contents of the stomach and intestine are divided into three portions, so that each set has a sample from each dog. PRACTICAL EXERCISES 459 50 grammes of powdered cane-sugar mixed with lard, the mixture being rolled into little balls. (l) After a quarter of an hour put A under chloroform or the A.C.E. mixture, and fasten it on a holder. Kill the animal with chloroform, open the abdomen, tie the oesophagus, place double ligatures on the pyloric end of the. stomach and the lower end of the small intestine, and divide between them. Cut out the stomach and intestine ; wash their contents into separate vessels, and test the reaction with litmus paper. Add water and rub up thoroughly. Filter. Wash the residue re- peatedly with small quantities of water, and pass all the washings through the filter. Make up each of the two filtrates to 200 c.c. (a) Test the filtrates from the contents of the stomach and intes- tines qualitatively for dextrose by Trommer's (p. 10) or Fehling's (p. 517) and the phenyl-hydrazine test (p. 517). (6) If no reducing sugar is present, add to 20 c.c. of each filtrate i c.c. of hydrochloric acid, boil for half an hour, and again test for reducing sugar. If it is now found, some cane-sugar is present. (c) If reducing sugar is found, estimate its amount as dextrose by Fehling's solution (p. ,^5 18) in a measured quantity of the original filtrate of the gastric or intestinal contents before and after boiling with one-twentieth of its volume of hydrochloric acid. {d) Estimate in the satne way the amount (as dextrose) of the invert sugar in the standard solution of cane-sugar after inversion, and before inversion if it gives the qualitative test for reducing sugar before it has been "boiled with acid. From the data obtained (and taking 95 parts of cane-sugar as equal to 100 parts of dextrose) calculate the amount of cane-sugar absorbed, left unchanged, and inverted, though not absorbed. (2) One and a half hours after the meal anesthetize B, and proceed as in (i). (3) Two hours after the meal proceed in the same way with C. But in addition observe the lacteals in the mesentery, by gently lifting up a loop of intestine immediately after opening the,!abdomen. If the absorption of the fat has begun, they will be easily visible, as a network of fine milk-white Vessels. Also examine the gastric and' intestinal contents with the microscope for fat globules. Compare your results on the amount of sugar obtained from the three animals. Probably much more unabsorbed sugar will be found in C than in B, as the lard hinders it from being dissolved. 16. Auto-Digestion of the Stomach. — In some of the previous experi- ments the stomach of an animal killed during digestion should be removed from the body after double ligation of oesophagus and duo- denum, and placed in a water-bath at 40° C. After several hours the contents should be washed out and the mucous membrane examined. It may be entirely eaten away in parts. CHAPTER VIII FORMATION OF LYMPH Different Kinds of Lymph. — ^We ought to distinguish the lymph as we collect it from the great lymphatic trunks, not only from the liquids of the serous cavities, but stUl more sharply from the Uquid which fiUs the multitudinous clefts and spaces of the tissues. It is now pretty definitely established that the tissue spaces do not com- municate by actual passages with the lymphatic vessels, but that the latter form ever3rwhere a closed system like the blood- vascular system, the lymph capillaries merely" lying in the tissue spaces (Sabin, etc.). This conception entails a radical change in the current views of lymph production. If the lymphatics form a closed system, the lymph cannot be actual tissue fluid, but only tissue fluid modified by its passage through the walls of the lymph capillaries, just as tissue fluid is not actual blood-plasma, but plasma modified by its passage through the walls of the blood capillaries as well as by exchange with the tissue elements. Although it is customary to speak of the lymph obtained from the lymphatic vessels as if it were perfectly homogeneous, there is no experimental ground for supposing that the Ijrmph from different tracts, or the tissue liquid in contact with the cells of different organs, or even the tissue liquid in contact with one and the same cell at different parts of its periphery, has a uniform composition, or even a uniform molecular concentration. There are, indeed, certain general considerations which show that this cannot be so. StUl less can it be assumed that the serous cavities, although they come into relation with lymphatics and bloodvessels in their walls, are analogous to colossal tissue spaces or even to expansions of the closed lymphatic system, or that the liquids contained in them, normally in scant amount, are simply tissue, or if not tissue, then simply lymphatic lymph. The cerebrospinal fluid, which bathes the external surface of -the central nervous system and fiUs its cavities, and the special liquids of the eyeball — the aqueous humour and the liquid of the vitreous humour — although no doubt, in addition to their other functions, they may in some degree minister to the nutrition of the tissues with which they are in contact, are Ada FACTORS CONCERNED IN LYMPH FORMATION 461 as regards their composition and mode of formation scarcely more closely allied to lymph than sweat is. They are almost free from protein, and are secreted by special structures— the choroid plexus and the uveal epithelium — quite different from any that can be concerned in the formation of ordinary lymph. It is very true these liquids are not blood, but that is scarcely a sufficient reason for calling them lymph, else we might classify sweat or even milk as lymph also. If a term is desirable to indicate that they have certain relations with lymph, they might perhaps be spoken of as lymphoid secretions. It may be that the essential difference in the chemical composition of these lymphoid secretions and lymph — ^the practical absence of protein — ^is related to the difference in the manner of their formation. ' The uveal and choroidal epithelial cells, interposing the depth of their columns or cubes between the blood and the free surface at which the liquid escapes, may well be suited to hinder the passage of the protein molecules which find their way with greater ease through the thin endothelium of the capillary wall into the tissue spaces, and from these into the lumen of the closed lymphatics (see p. 433). Nevertheless, we shall recognize later on in the glomeruli of the kidney an instance of blood capillaries but little pervious to proteins, and there are several other facts which show that the capillaries may differ considerably in different organs in the readiness with which they permit the various constituents of the plasma to pass through their walls. Further, in discussing the mechanism by which lymph is formed, we shall see reason to doubt whether mechanical filtration, due to differences of hydrostatic pressure on the two surfaces of the capillary endothelium, has much, if anything, to do with the passage either of protein or of the other constituents of the lymph from the lumen of the capillaries into the tissue spaces. At first glance, indeed, such a process would seem to be admirably fitted to explain the fact that, while lymph differs but little from blood- plasma in the proportions of its other constituents, it is at most no more than half as rich in protein. For there are many filters which allow substances in ordinary solution and their solvent to pass through without alteration in their relative proportions, while substances like proteins in colloid solution are kept back to a greater or less extent. Factors concerned in Lymph Formation. — The teaching of Ludwig, that lymph is formed by the filtration, and in a minor degree by the diffusion, of the constituents of blood-plasma through the walls of the capillaries into the tissue spaces, was based on such facts as the increase in the tissue liquid of a limb or organ which occurs when the exit of blood from it by the veins is hindered, or when the quantity of the circulating liquid is increased by the injection of blood or salt solution. It was first seriously called in question by 4^2 FORMATION OF LYMPH Heidenhain, who advanced the theory that lymph is secreted by the endothelium of the blood capillaries. One of Heidenhain's strongest arguments in favour of his secretion theory was the existence of substances which, when injected into the blood, in- creased the flow of lymph from the thoracic duct of the dog without affecting appreciably the arterial pressure. He divided these so- called lymphagogues into two classes: (i) Substances like peptone, extracts of the head and liver of the leech, extract of crayfish muscle, egg-albumin, etc., which cause not only an increase in the rate of flow, but an increase in the specific gravity and total solids of the lymph; (2) crystalloid substances, like sugar, salt, etc., which cause an increased flow of lymph more watery than normal. Starling has shown that, although the Ijnnphagogues of the second class do not raise the arterial pressure, they do, by attracting water from the tissues and thus causing hydraemic plethora (an excess of blood of low specific gravity), bring about a marked rise of venous, and therefore, what is the important thing for lymph filtration, of capillary pressure. But it can be demonstrated that vaso-dilata- tion with increase of capillary pressure is not in itself suf&cient to increase the formation of lymph. We have seen, e.g. (p. 177), that when the chorda tympani nerve is stimulated in the dog the arteri- oles of the submaxillary gland are dilated, and no doubt the pres- sure in the capillaries is increased. No increased flow of lymph, however, takes place from the submaxillary lymphatics during even prolonged excitation of the chorda, nor do the lymph spaces of the gland become distended (Heidenhain). In the horse also the spon- taneous flow of lymph from the quiescent parotid is not appreciably altered by excitation of the secretory nerves of the gland or by pilocarpine (Carlson) . There is every reason to believe that during active secretion of saliva tissue liquid is really formed from the blood in increased amount, and that it is from the tissue spaces that the gland-cells directly obtain the increased supply of water and other substances necessary to sustain the increased secretion. But a balance is maintained between the production of tissue liquid and its removal by the gland-ceUs. When the gland is quiescent, the small amount of tissue liquid normally formed from the blood capillaries for the nutrition of the cells is balanced by, upon the whole, an equal amount of lymph secreted from the tissue spaces into the lymph capillaries. We may say, indeed, that the closed lymphatic system has for its great function the regulation of the quantity and quality of the tissue liquid. In glands with an external secretion increased irriga- tion of the tissue spaces from the blood does not as a rule lead to increased flow of lymph, because the surplus fluid is required to form the secretion. In other organs, however, such as the muscles and the ductless glands, it is probable that the augmented irriga- FACTORS CONCERNED IN LYMPH FORMATION 463 tion rendered necessary by functional activity is always associated with an accelerated flow of lymph, which carries off the surplus liquid, including a portion of the waste products. It is probable that an important factor in the production of cedema may be the derangement of the mechanism, whatever it is, through which the adjustment of the rate of formation of tissue hquid to that of lymphatic lymph is achieved. But it must be remembered that in all the organs the blood capillaries not only supply materials to the tissue spaces, but take up materials from them. Indeed, there are facts which indicate that in general water and dissolved substances pass more easily and in greater amount back from the tissues into the blood than into the lymphatics. So that, while the lymphatics constitute an accessory drainage system, the bloodvessels irrigate the tissues and drain them as well. "A consequence of this, as well as of the great difference in the capacity of the different tissues for storing water, is that the amount of tissue lymph formed can never be estimated from the amount of lymphatic lymph leading an organ. Thus the flow of lymph from the limbs at rest is very small in com- parison with the flow from the abdominal viscera, which constitutes the great bulk of the lymph passing along the thoracic duct. This, however, does not prove that very little liquid passes out of the limb capillaries, for the chief tissue of the limbs, the muscles, pos- sesses a far greater storage capacity for water than the intestines and digestive glands. In the one case we have a field whose soil takes up a great deal of water and is not easily saturated; in the other a field whose soil soon becomes water-logged and refuses to take up any more. With the same supply from the irrigating ditch, little or no water may drain off at the foot of the first field, a great deal at the foot of the second. Where the main lymphatics are themselves blocked by mechanical pressure or by inclusion in a ligature, the balance is, of course, grossly upset by the failure of the outflow of lymph to keep pace with its formation. Where an obstruction on the course of the veins is responsible for the oedema, the lymphatic outflow, to be sure, is not directly interfered' with. But the nutrition and the respiration of the vascular walls themselves, including the endo- thelium of the capillaries, necessarily suffers from the insufficiency of the blood-flow, and the crippled capillaries may very well become abnormally permeable for water, salts, and the other constituents of lymph. And while ordinary mechanical filtration may not be a factor, or a very unimportant one, in the passage of liquid through the normal capillary wall, it may become far more effective when it acts on a- damaged wall. The tissue cells also suffer from lack of oxygen and nutritive material, and from the accumulation of waste products, including acid substances, which cause them to take up and to hold more water than normal. Even in the absence 464 FORMATION OF LYMPH of changes in the mechanical conditions of the blood and lymph circulations, alterations in the tissues must be recognized as among the causes of oedema (Fischer). Thus it is clear that the interpre- tation of such an apparently simple experiment as the production of oedema by the ligation of a vein needs great care. Whatever it proves, it may be said with confidence that it does not prove that the increased capillary pressure is the direct cause of the oedema. A mere increase in the capillary blood-pressure does not of itself accelerate the formation of tissue liquid from the blood any more than that of lymph from the tissue liquid, as is shown by the fact that, when the chorda tympani is stimulated after injection of a dose of atropine sufficient to prevent all salivary secretion, there is neither oedema of the gland nor increase in the flow of lymph from, it, although the arterioles are as widely dilated as before. When the blood-pressure is greatly increased in the anterior portion of an animal by clamping the aorta, or in the whole animal by continued stimulation of the cut medulla oblongata or the splanchnic nerves, the blood does not alter in concentration in the least, showing that no sensible increase in the passage of water into the lymph has occurred. After division or embolism of the medulla oblon- gata, and consequent paralysis of the vaso-motor centre and general vascular dilatation, it is stated that the injection of sodium chloride produces an increase in the lyniph-fiow as great and as durable as in the normal animal, and which can continue even after death (Pugliese). The action of the first class of lymphagogues, which cannot be explained as the consequence of an increase of capillary pressure, because the pressure in the capillaries is not consistently increased, and may even in the case of some of these lymphagogues be diminished, is attributed by Starling to an injiu-ious effect on the capillary endothelium (and especially on the endo- thelium of the capillaries of the liver, since nearly the whole of the increased lymph- flow comes from that organ), which increases its permeability. But it is not easy to distinguish an increase of per- meability produced by lymphagogues from an increase of secretory activity of the endothelial cells. Hamburger, too, has brought forward results which it is difficult to reconcile with a theory of filtration even for the second class of lymphagogues. Further, Heidenhain has shown that some time after injection of a crystalloid substance, like sugar, into the blood, a greater percentage of the substance may be found in the lymph than in the blood. Now, when a mixture of crystalloids and col- loids is filtered through a thin membrane, the percentage of crystal- loids in the filtrate is never, at most, greater than in the original liquid. And although Cohnstein states that, if time enough be allowed, the maximum concentration of sodium chloride in the lymph, after intravenous injection, becomes approximately the FACTORS CONCERNED IN LYMPH FORMATION 465 same as the maximum in the blood, this fact loses its weight as an argument in favour of the filtration hypothesis when we re- member that, according to Asher, all the solids of the lymph are markedly increased when even small quantities of crystalloids are injected into the veins. Upon the whole, then, it may be con- cluded that up to the present it has not been shown that filtration due to the excess of pressure in the capillaries over that in the lymph spaces is an effective factor in the formation of lymph. Nor is it at all easier to explain lymph formation as a matter of pure osmosis or diffusion. Lazarus-Barlow found, for example, that in the great majority of his experiments the injection of a concen- trated solution of sodium chloride, dextrose or urea into a vein was followed, not by an initial diminution in the outflow of lymph (as might have been expected if the exchange of water between the blood and the tissue spaces, and between the tissue spaces and the lymph capillaries, was regulated solely by differences in osmotic pressure), but by an immediate increase. And Carlson has shown that the osmotic pressure of lymph coming froni the active salivary glands, as measured by the freezing-point method, may, under chloroform or ether anaesthesia, be distinctly less than that of the blood-serum. Water must therefore be passing from a liquid of higher to one of lower osmotic concentration. Nevertheless, it would be erroneous to assume that because osmosis and diffusion have not been shown to satisfactorily account for all the phenomena of lymph formation, they exert no influence upon it. It is probable, indeed, that their action is fully as im- portant as in absorption from the alimentary canal, although, as in absorption, it is often overlaid and always modified by the specific permeability of the blood-capillary walls, the lymph-capillary walls, and the tissue cells in general, in virtue of which they exert an action upon the quantity and composition of the lymph analogous to the action exerted in a higher degree by the cells of the digestive glands upon the quantity and composition of the liquids passing into their ducts. It is not difficult to illustrate the fact that phenomena of osmosis and difiusion emerge, although not of course in such purity as in physical experiments, when we study the interchange between the blood and the tissue liquids. If, for example, a hypertonic solution of sodium chloride is injected into the blood, water rapidly passes from the tissues to the blood as it would through a semipermeable membrane, and the blood becomes diluted. At the same time sodium chloride leaves the blood and passes into the tissues, as it would do by diffusion from a place of higher to a place of lower concentration. But after this has gone on for some time, and the concentration of the blood in salt has sunk, it may be, to that in the lymph, salt still continues to pass out of the blood, and the excess of water also leaves the bloodvessels till the osmotic pressure of the blood has again become norjnal. When isotonic and even hypotonic solutions of sodiumi chloride are injected, salt cLlSo 30 4^6 FORMATION OF LYMPH leaves the blood and enters the lymph, although it ought not to do so by diffusion, while water, which might pass from the h)rpotonic blood into the lymph by osmosis, moves in the same direction from the blood to which the isotonic salt solution has been added. Regulative mechan- isms, in short, exist which tend, with, but also without, the co-opera- tion of diffusion and osmosis, and even, so to say, in their teeth, to bring back the quantitative and qualitative composition of the blood to the normal. Exactly similar phenomena are witnessed when the equilibrium is upset from the other side by the injection of salt solutions into the subcutaneous tissue or the intramuscular connective tissue. Hypotonic sodium chloride solution injected into the sub- conjunctival connective tissue quickly loses water and gains sodium chloride, as it ought to do if under the influence of osmosis and diffusion, and hypertonic salt solutions gain water. But eventually hypertonic, hypotonic, and isotonic solutions, and even serum itself, are completely absorbed, which could not occur in the presence of diffusion and osmosis alone. Sometimes in dropsy it appears that the oedema liquid is absorbed when the patient is put on a diet free as far as possible from salts. The suggestion is that the regulative mechanism which tends to keep the molecular concentration of the blood and lymph approximately constant provides that as the salt content of the body falls, which it does through continued excretion of salts in the urine, water is eliminated in corresponding amount. The Contribution of the Tissue-Cells to the Lymph. — So far we have considered the passage of the lymph constituents, on the one hand through the endothehum of the blood capillaries into the tissue spaces; on the other, from the tissue spaces through the endo- thelium of the lymph capillaries. But it is not to be supposed that the liquid lying in clefts, partly bounded by blood capillaries, partly by lymph capillaries, partly by tissue-cells, should be affected solely by the first two. The third anatomical element must contribute something to, or withdraw something from, the tissue liquid, and may thus play a part in the formation of lymph from the latter. The recent researches of Asher and his pupils have raised the ques- tion of the relation between the physiological activity of the organs, and especially of the glands, and the formation of the lymph. They conclude that the common doctrine that lymph is simply a diluted blood-plasma is erroneous. Lymph, they say, far from being a mere filtrate, or even a secretion from the blood, is formed by the activity of the organs, and may actually be absorbed by the blood from the tissue spaces. In fact, according to their view, the in- travenous iiljection of lymphagogues, both crystalloid and colloid, only causes an increased flow of lymph in so far as it leads to in- creased glandular secretion. But this generalization has had only a. short-lived vogue,' and one by one some of the main results which sisemed to support it have been dispfovfed'or shown to be capable ctf^iliterpretation in a d,if£erent sense. For example, it was stated 'fiiat .secretin causes a flow of lymph from the lymphatics of the pancreasj as well as a flow of pancreatic juice. But it has been Shown that the increased production of lymph is not due to the INFLUENCE OF NERVES ON LYMPH FORMATION 467 secretin at all, but to lymphagogue substances, including proteose, extracted with the secretin from the intestinal mucous membrane. A solution of secretin can be prepared which causes a considerable increase in the secretion of pancreatic juice and bile, but no augmen- tation whatever in the flow of lymph from the thoracic duct. Again, it was asserted that peptone, a noted lymphagogue, produces a great increase in the biliary secretion. It has been demonstrated, however, thait the action of the peptone is merely to cause expul- sion of the contents of the gall-bladder by the mechanical effect of the swelling of the liver, and not at all to stimulate the liver-cells to form more bile. For it produces no effect on, the flow of bile if the gall-bladder be emptied or the cystic duct tied before the injection (EUinger). That active salivary secretion is not accom- panied by increased lymph-flow from the lymphatics of the salivary glands has been mentioned above. Nevertheless, we may safely assume that the activity of the organs does make a contribution to the lymph^ — to its solids, if not in any important degree to its water- Content, although to say that they alone are concerned in its forrnaT tion, to the exclusion of the capillaries, is altogether an over-statCr ment. The waste-products of the tissues pass into the lymph, and possibly, as Koranyi suggests, may, by increasing its molecular concentration, cause the passage of some water into it from the blood. Or the decomposition of the large protein molecules, which in tissue metabolism are breaking down into numerous smaller molecules, may entail an increase of osmotic pressure in the cells themselves, which in turn may lead to withdrawal of water by the cells from the tissue liquid. The osmotic pressure of the liquid may thus rise, and water may pass into the tissue spaces from the blood. The molecular concentration of lymph (except in ansesthe? tized animals as mentioned above) is in general somewhat greater than that of blood-serum — e.g., in one observation A of serum was 0-605° C., and of lymph 0610° C. For the solid tissues, the freezing-point of which, however, cannot be as satisfactorily deter- mined as that of liquids, the following values of A were obtained : Brain, 0-65°; muscle, 068°; kidney, 0-94°; liver, 097°; while for blood it was 0-57° (Sabbatani). To sum up, we may say that while the physical processes of .filfra- tion, osmosis and diffusion may. play a part in the. passage of water and solids through the walls of the blood capillaries, as well as from the tissue-cells into the tissue spaces, and from these spaces into the lymph capillaries, there' is much which they leave unexplained, and, which at present, for the want of a more precise term, we must attribute to secretory activity. Influence of Nerves on Lymph Formation. — 'In one instance it appears to have been shown that lymph may be formed under the influence of secretory nerves. In the males of certain 468 FORMATION OF LYMPH aquatic birds erection is due to the filling of the corpus cavernosum, •not with blood, but with lymph. The lymph is secreted rapidly by the so-called bodies of Tannenberg when certain sympathetic nerve- fibres are experimentally stimulated, and passes into the corpus cavernosum, which swells up. If a small incision is made in the corpus, a large quantity of clear lymph, which clots slowly on stand- ing, escapes. There is a simultaneous vasodilatation. After erection, the lymph is rapidly and completely reabsorbed (Eckhard, Miiller). Although no definite lymph-secretory nerve-fibres have as yet been discovered in mammals and for ordinary tissues, it is possible that they exist (Sihler). As already pointed out, the same volume of liquid as escapes into the ducts of the active submaxillary gland must, upon the whole, pass out of the blood capillaries. On what principle shall we distinguish one only of these processes as physio- logical secretion ? They begin together when the chorda tympani is stimulated. A drug which paralyzes secretory nerve-endings abolishes both effects. The simplest explanation is that the chorda contains secretory fibres which influence the formation both of saliva and of the tissue liquid from which it is recruited; and, so far as this consideration goes, it is just as logical to consider the increase in the supply of tissue liquid as the cause of the, increase in the flow of saliva as to consider the increased salivajry secretion as the cause of the increased flow of liquid into the tissue spaces. The increased flow of liquid may be brought about either by an action of the nerve on the gland-cells, causing them to produce a hormone, which then effects the blood capillaries (Carlson), or by a direct action on the capillary endothelium. The advantage to cells engaged in the active secretion of saliva of being immersed in an abundant bath of tissue liquid is obvious. The post-mortem flow of lymph, which may continue in some cases long after complete cessation of the circulation — for an hour after injection of dextrose to produce hydraemic plethora; for as much as four hours after injection of extract of the strawberry, which is a lymphagogue of Heidenhain's first group (Mendel and Hooker) — is a phenomenon whose relation to normal lymph forma- tion has not been definitely settled. It ought to be remembered in this whole discussion that the epithelium of ordinary glands derives its supplies of material from the tissue lymph. The vicissitudes of blood-pressure affect it only in a secondary and indirect manner. On the other hand, the endo- thelial cells of the capillaries are in direct contact with the blood. And it is interesting to observe that in this respect the glomeruli of the kidney and the alveoli of the lungs (if the endothelial lining of Bowman's capsule and the alveolar membrane are assumed to -be complete) take a middle place between the glands proper and the quasi-glandular capillaries. CHAPTER IX EXCRETION We have now followed the ingoing tide of gaseous, liquid, and solid substances within the physiological surface of the body. There we leave them for the present, and turn to the consideration of the channels of outflow, and the waste products which pass along them. In a body which is neither increasing nor diminishing in mass the outflow must exactly balance the inflow; all that enters the body must sooner or later, in however changed a form, escape from it again. In the expired air, the urine, the secretions of the skin, and the faeces, by far the greater part of the waste products is elimin- ated. Thus the carbon of the absorbed solids of the food is chiefly given off as carbon dioxide by the lungs; the hydrogen, as water by the kidneys, lungs and skin, along with the unchanged water of the food; the nitrogen, as urea by the kidneys. The faeces in part represent unabsorbed portions of the food. A small and variable contribution to the total excretion is the expectorated matter, and the secretions of the nasal mucous membrane and lachrymal glands. Still smaller and still more variable is the loss in the form of dead epidermic scales, hairs, and nails. The dis- charges from the generative organs are to be considered as excre- tions with reference to the parent organism, and so is the milk, and even the fcetus itself, with respect to the mother. Excretion by the lungs and in the faeces has been already dealt with. All that is necessary to be said of the expectoration and the nasal and lachrymal discharges is that the first two generally contain a good deal of mucin, and are produced in small mucous and serous glands, the cells of which are of the same general type as those of the mucous and serous salivary glands. The lachrymal glands are serous like the parotid; and the tears secreted by them contain some albumin and salts, but little or no mucin. The sexual secretions and milk will be best considered under reproduction (Chap. XIX.), so that there remain only the urine and the secre- tions of the skin to be treated here. 469 47° EXCRETION Section I. — Excretion by the Kidneys — ^The Chemistry of THE Urine. Normal urine is a clear yellow liquid acid to litmus and similar indicators, but nearly neutral or very weakly acid in the physico- chemical sense (p. 24). The average specific gravity is about 1020, the usual limits being 1015 and 1025, although when water is taken in large quantities, or long withheld, the specific gravity may fall to 1005, or even less, or rise to 1035, or even more. The quantity passed in twenty-four hours is very variable, and is especially .dependent on the activity of the sweat-glands, being, as a rule, smaller in summer when the skin sweats much, than in winter when it sweats little. The average quantity for an adult male is 1,200 to 1,600 c.c. (say, 40 to 50 oz.).* Composition of Urine. — This is very closely related to the quantity and quality of the food. Hence it is impossible to speak of a definite normal composition of the urine. It is essentially a solu- tion of urea and inorganic salts, the proportion of the latter being generally about 1-5 per cent., or double the usual amount of physio- logical liquids. Besides urea, there are other nitrogenous bodies in much smaller quantity, such as ammonia, uric acid, and the allied purin bases, hippuric acid, and kreatinin. Some of these at least are products of the metabolism of proteins within the tissues. And besides the inorganic salts there are certain organic bodies — indoxyl, phenyl, pyrokatechin, skatoxyl — united with, sulphuric acid, which are primarily derived from the products of the putre- faction of proteins within the digestive tube. Fplin has published analyses of ' normal ' urines from six persons, weighing from 56-6 to 70-9 kilos (average 63-4 kilos), who were kept for seven days on one standard uniform diet. The diet consisted of 500 c.c. of milk, 300 c.c. of cream (containing 18 to 22 per cent, of fat), 450 grammes of v^ggs, 200 grammes of Horlick's malted milk, 20 grammes of sugar, 6 grammes of sodium chloride, water enough to make the whole up to two litres, and 900 c.c. of additional water. The ingredients contained 119 grammes of protein, about 148 grammes of fat, and 225 grammes of carbo-hydrates. The average results of all the deter- minations are given in the following table : * The average quantity of urine varies not only with the season, but also with the habits of the person, especially as regards the amount of Uquid taken. The average for seventeen healthy (American) students, whose urine was collected for six to eight successive days in winter, was 1,166 c.c. The highest average in any one individual for the observation period was 1,487 c.c. (for seven days), and the lowest 743 c.c. (for eight days). The greatest quan- tity passed in any one period of twenty-four hours was 2,286 c.c. (by the in- dividual whose average was the highest). The smallest quantity passed in twenty-four hours was 650 c.c. (by the individual whose average was the lowest. EXCRETION BY THE KIDNEYS 471 Grammes. Containing Nitrogen (Grammes). Percentage of Total Nitrogen. Urea - Ammonia Kreatinin Uric acid Nitrogen in other compounds 29-8 1-55 0-37 13-9 o-yo 0-58 0-12 o-6o 87-5 4-3 0-8 3-75 T9tal nitrogen — 16-00 — ' Inorganic SO3 ■ Ethereal SO3 - ■ Neutral ' SO3 Total sulphur as SO3 Total phosphates as PjOj Chlorine 2-92 0'22 o-iy 3-31 3-87 6-1 Percentage of Total Sulphur. 87-8 6-8 5-1 Titratable acidity in c.c. of decinormal acid ^ndican (Fehling's solution=ioo*) /Volume of urine (, iraintTAl, 304. ',\ organic, 313. 77 1430 c.c. ' The great influence of diet on the composition of the urine is illus- trated in the following table. Urine I. was obtained from a man weigh- ing 87 kilos on the, standard protein-rich diet described above. Urine II. was obtained from the same person on a diet very poor in protein (400 grammes of starch and 300 c.c. of cream), containing only about r gramme of nitrogen, as against 19 grammes in the first diet. I. II. Volume of urine 1 170 c.c. 385 c.c. Grammes. Per Cent. Grammes. Per Cent. Total nitrogen i6-8 3-60 Urea-nitrogen 14-70 = 87-5 2-20 = 6l'7 Ammonia-nitro gen 0-49 = 3-0 0-42 = II-3 Uric acid-nitrogen 0-18 = i-i 0-09 = 2-5 Kreatinin-nitrogen 0-58 = 3-6 0-60 = 17-2 , Nitrogen in other compounds 0-85 = 4-9 0-27 = 7-3 Total SO3 3-64 Inorganic SO3 - 3-27 = 90-0 0-46 = 60-5 Ethereal SO3 - 0-19 = 5-2 o-io = 13-2 Neutral SO3 o-i8 = 4-8 0-20 = 26-3 Total phosphates as P2O5 4-1 I-O Chlorine 6-1 1-6 mineral, 398 /mineral, 123. organic, 120 407-" ^(^ organic, 20T N ( Titratable acidity in c.c. — acid - 805-^ Indican (Fehling's solution = 100) * The indican is given in arbitrary units, the Indigo blue being obtainetl Torn the urine and then estimated colorimetrically, using Fehling's solu 472 EXCRETION The titrable acidity of urine (see p. 25) is chiefly due to the acid (mono- basic) phosphates, such as acid sodium phosphate (NaH2P04), but in an important degree also to organic acids. According to Folin, indeed, the organic acidity may be more than half the total acidity. Normally the acidity diminishes distinctly, or even gives place to alkalinity, during digestion, when the acid of the gastric juice is being secreted. This is sometimes fancifully denominated the alkaline tide. After a fast, as before breakfast, the opposite condition, the acid tide, occurs. The acidity varies with the quantity of vegetable food in the diet. The urine of herbivora and vegetarians is alkaline, and is either turbid when passed, or on standing soon becomes turbid from precipitated carbonates and phosphates of earthy bases, while that of camivora and of fasting herbivora, which are living on their own tissues, is strongly acid and clear. Normal human urine may deposit urates soon after discharge, as they are more soluble in warm than in cold water. They carry down some of the pigment, and thsrefore usually appear as a pink or brick-red sediment. When urine is allowed to stand after being voided, what is generally described as ' acid fermentation ' occurs. The acidity gradually increases; acid sodium urate is produced from the neutral urate, and comes down in the form of amorphous granules, while the liberated uric acid is deposited often in ' whetstone ' crjrstals, coloured yellow by the pigment (Fig. 177). Calcium oxalate may also Fig. 177. — Uric Acid. Fig. 178. — Calcium Oxalate be thrown down as ' envelope,' a, b, or less frequently, ' sand-glass ' crystals, c (Fig. 178). If the urine is allowed to stand for a few" days, especially in a warm place, or in a place where urine is decomposing, the reaction becomes ultimately strongly alkaline, owing to the forma- tion of ammonium carbonate from urea by the action of micro-organ- isms (Micrococcus urecB, Bacterium urea, and others) which reach it from the air, and produce a soluble ferment, in whose presence the urea is split up with assumption of water. Thus : -/ NH» /O.NH4 C|=0 -I-2H2O = C^O \NH2 \O.NH4. Urea. Ammonium carbonate. This is a reaction of considerable interest, for the reverse reaction occurs when blood containing ammonium carbonate is circulated through the liver, the ammonium carbonate being converted into urea with loss of water. tion as a standard. Fehling's solution is employed because it is a blue liquid of a definite depth of tint already prepared in every physiological laboratory. EXCRETION BY THE KIDNEYS 473 The substances insoluble in alkaline urine are thrown down, the deposit containing ammonio-magnesic or triple phosphate, formed by the union of ammonia with the magnesium phosphate present in fresh urine, and precipitated as c&ar crystals of ' knife-rest ' or ' coffin-lid ' shap3 (Fig. 179), along with amorphous earthy phosphates, and often acid ammonium urate in the form of dark balls occasionally covered with spines (Fig. 182). Calcium phosphate (CaHP04) is another phos- phate found in sediments deposited from alkaline or faintly acid urine. It is usually amorphous, but sometitnes in the form of long prismatic crystals arranged in star fashion, and hence spoken of as stellar phos- phate (Fig. r8i). It is not pigmented. It is only in pathological conditions that the alkaline fermentation takes place within the bladder. The reaction of the urine can readily o Fig. 179.— Triple Phosphate. Fig. 180.— Cystin. Fig. 181.— Stellar Phos- phate Crystals. be made alkaline by the administration of alkalies, alkaline carbonates, or the salts of vegetable acids like malic, citric, and tartaric acid, which are broken up in the body and form alkaline carbonates with the alkalies of the blood and lymph. It is not so easy to increase the acidity of the urine, although mineral acids do so up to a certain limit. If the admin- istration of acid be j)ushed farther, ammonia is split ofi from the pro- teins, and is excreted in the urine as the ammonium salt of the acid. Determination of the Acidity. — ^A titration method is described in the Practical Exercises (p. 508). In speaking of the reaction of blood, it has already been mentioned (p. 25) that we can- not determine by titration the true acidity or alka- linity of a liquid in the physico-chemical sense — i.e., the concentration of the dissociated hydrogen and hydroxyl ions respectively. E.g., when we titrate equal quantities of decinormal* acetic acid and deci- normal hydrochloric acid with decinormal potassium hydroxide, using, say, phenolphthalein as the indi- cator, nearly the same volume of the potassium hydroxide solution will be needed to neutralize each acid. Yet it can be shown by physico-chemical methods that the acetic acid in the strength used is only dissociated to the extent of a little more than I per cent., while about 80 per cent, of the hydrochloric acid is dissociated. The concen- tration of the hydrogen ions is therefore eighty times as great in the hydrochloric as in the acetic acid solution. What we determine by the titration is not the true acidity, but the total amount of hydrogen which can be replaced by metal. The concentration of the hydrogen ions in normal urine is very small, on the average only about 0-003 milli- * A normal solution of a substance contains in a litre a number of grammes of the substance equal to the number which expresses its equivalent weight — a decinormal (usually written ^) solution one-tenth of this amount, a centinormal one-hundredth, etc. Thus, a normal solution of potassium hydroxide contains 56 grammes of KOH, and a decinormal solution 5' 6 grammes in 1,000 c.c. Fig. 182. — Ammo, nium Urate (after Milroy). 474 EXCRETION gramme in the litre, or about thirty times as much as is present in the purest distilled water. Urine departs about as much from neutrality in the one direction as blood does in the other. Urea, CO(NH2)g, is the form in which by far the greater part of the nitrogen is under ordinary conditions discharged from the body. Its amount is as important a measure of protein metabolism as the quantity of carbon dioxide given out by the lungs is of the oxidation of carbon- aceous material. Yet a glance at the table on P.-471 shows that, when the total protein metabolism is greatly reduced by diminishing the protein in the food, the relative as well as the absolute amount of nitrogen eliminated as urea suffers a great diminution. The relative amount of the other nitrogenous urinary constituents, especially of the kreatinin, is markedly increased. The significance of this difierence is alluded to in speaking of the kreatinin content of urine, and will have to be again considered under Protein Metabolism. Urea is soluble in water and in alcohol, and crystallizes from its solutions in the form of long colourless needles, or four-sided prisms with pyramidal ends. It can be easily prepared from urine . Urea can also be obtained artificially by heating its isomer, ammonium cyanate (NH4 — O— CIM), to 100° C. This reaction is of great historical interest, as it forms the final step in Wohler's famous synthesis of urea, the first example of a complex product of the activity of living matter being formed from the ordinary- materials of the laboratory. Heated in watery solution in a sealed tube to 180° C-., urea is entirely split up into carbon dioxide and am- monia, a change which can also be brought about, as already mentioned, by the action oi micro-organisms. Nitrous acid, h3rpochlorous acid, and salts of hypotiromous acid carry the decomposition still farther, carbon dioxide, nitrogen, and water being the products of their oxidizing action on urea. Thus: C0.2(NH2) -l-3NaBrO=3NaBr-|-2H20-hC02-l-N2. This reaction is the basis of the hypobromite method of estimating the quantity of urea in urine (Practical Exercises, p. 512). Ammonia. — The ammonia in urine is united with acids in the form of salts. Its formation from proteins is determined, as we shall see later on, by the necessity of neutralizing certain acids produced in metabolism — e.g., those derived from the sulphur and phosphorus of the proteins, or acids administered experimentally. According to some observers, the percentage amount of the total nitrogen in the urine in the forna of ammonia remains the same whether the food be rich or poor in protein (Schittenhelm, etc.), but others state that when the protein is reduced there is a relative increase in the ammonia-nitrogen (see table on p. 471) (Folin). Uric acid (C6H4N4O3) exists in large amount in the urine of birds. The excrement of serpents consists almost entirely. of uric acid. In both cases it is mainly in the form of acid ammonium urate. In con- trast to urea, uric acid is very insoluble, requiring 1,900 parts of hot •and 15,000 parts of cold water to dissolve it. In man and mammals the quantity is comparatively small in health, but is increased after a meal containing material [e.g., thjrmus gland) rich in nucleins, from the nucleic acid of which purin' bodies are derived, or sub- stances containing purin bases in the free state — e.g., hypoxanthin -in .meat. In mammals the amount of uric acid excreted depends little, if at all, upon the quantity of protein in the food, but a great deal upon the quantity of purin bodies,, whether free or combined. When nitrogenous food is omitted altogether, the absolute quantity of uric acid is diminished,, but the proportion of the total nitrogen of the urme 'eliminated as uric acid is increased, since the 'endogenous' uric acid (p. 585) still continues to be formed and excreted. EXCRETION B Y THE KIDNE YS 475 The purin bases (sometimes called the nuclein bases, the alloxuric bases, or the xanthin bases) are a group of substances allied to uric acid, and including, besides xanthin itself, hypoxanthin, guanin, adenin, and other bodies. They exist in very small amount in urine, but, like uric acid, are increased in amount by the ingestion of nuclein-coritain- ing substances. The greater part of the purin bases produced in the body is transformed into uric acid ; it is only the untransformed residue which appears in the urine. An interesting fraction of the purin bases in the urine which is not related to the nuclein metabolism is composed of the so-called heteroxanthin, derived from caffeine in the coffee and tea, Z-methylxanthin, derived from theobromine in the cocoa, and para- xanthin, derived from theophyllin in the tea, consumed as beverages. Hippuric acid (C9H9NO3) occurs in considerable quantity in the urine of herbivora (Practical Exercises, p. 516) ; in the urine of camivora and of man only in traces ; in that of birds not at all. Its amount is much more dependent on the presence] of particular substances in the food than that of the other organic constituents of urine; Anything which contains benzoic acid, or substances which can be readily changed into it (such as cinnamic and quinic acids), causes an increase of the hippuric acid in urine. In fact, one of the best ways of obtaining the latter is from the urine of a person to whom benzoic acid is given by the mouth; the sweat Fig. 183.^ — Kreatin. Fig. 184. — Kreatinin -Zinc-Chloride. may also in this case contain a trace of hippuric acid. Chemically it is a conjugated acid formed by the union of benzoic acid and glycin. Amino-Acids. — ^The only amino-acid hitherto detected with certainty in normal urine is glycin. Oxalic acid is always present, although in very small amount. Some of it comes from the oxalates of the food, but a portion of it arises in the metabolism of the tissues, probably from the decomposition of uric acid. It is known that outside of the body uric acid maybe made to yield oxalic acid. Ca cium oxalate crystals are often seen in urinary sediments. Kreatinin (C4H7N3O) . — Kreatinin is the anhydride of kreatin (Fig. 183). Its formula differs from that of kreatin only in possessing the elements of one molecule of water less; and kreatinin can be obtained by boiling kreatin with dilute sulphuric acid. From its alcoholic solution it crystallizes in colourless prisms. Kreatinin forms crystalline comppunds .with various acids and salts. One of the best known of these is kreatinin-zinc-chloride, formed on the addition of zinc chloride to an alcoholic or watery solution of kreatinin, often in the shape of beautiful thick-set rosettes of needles (Fig. 184). A pofr 476 EXCRETION tion of the urinary kreatinin is derived from the kreatin of the meat taken as food. But this is not its only source, for on a meat-free diet and in starvation kreatinin is still excreted. The absolute quantity in the urine on a meat-free diet is constant for one and the same individual, although diflEerent in different persons, and independent of the total amount of nitrogen eliminated. Hence on a diet poor in protein the percentage of the total nitrogen excreted as kreatinin is much greater than on a protein-rich diet, as shown in the table on p. 471 . So constant is the quantity that a determination of the kreatinin may be used as a check upon the complete collection of the urine. Carbo-hydrates are normally present in human urine, but only in very small amounts. Three are known with certainty — dextrose, isomaltose, and the so-called animal gum or urine dextrin. Glycuronic acid (C6H10O7), a body which can be derived from dextrose, is con- stantly present in small amount as a conjugated acid, paired with phenol or indoxyl. It gives Fehling's test, and thus may easily be mistaken for sugar. Glycuronic acid becomes coupled very easily with a large variety of substances, including many drugs, and care must be taken after the administration of camphor, chloral hydrate, chloro- form, nitrobenzol, etc., not to confound the largely increased excretion of conjugate glycuronates in the urine with glycosuria. The yeast test will turn out negative if the reduction is due to glycuronic acid, and the polarimeter will show rotation to the right if it is due to dextrose. The total quantity of carbo-hydrates, including glycuronic acid, excreted in the urine of the twenty-four hours has been estimated at 2 'to 3 grammes. The quantity of dextrose in normal human urine is about 0-02 per cent., or about one-fifth of the proportion in blood. Proteins, mainly serum-albumin, are also found in normal urine in minute quantities, on the average about 0-0036 per cent. (Momer). Pigments of Urine. — ^The pigments of urine have not hitherto been exhaustively studied ; but we already know that normal urine contains several, and pathological urines probably additional, pigmentary sub- stances. The best-lmown pigments in normal urine are urochrome, the yellow substance which gives' the liquid its ordinary colour; uyocrythrin, the pink pigment which often colours the deposits of urates that separate even from healthy urine ; and urobilin, which, as has been already stated, is identical with the faecal pigment stercobilin, and occurs not only in many febrile conditions, but also in cases with no fever, such as functional derangements of the liver, dyspepsia, chronic bronchitis, and valvular diseases of the heart. The urobilin of urine represents, mainly at least, the portion of the stercobilin which is not excreted with the fasces, but absorbed from the intestine into the blood. The urobilin in normal urine only exists in small amount in the fully-formed con- dition, most of it being present as a chromogen or mother-substance (urobilinogen), which by oxidation, as on standing exposed to the air, is converted into urobilin. On the addition of ammonia and zinc chloride to a solution of urobilin a beautiful green fluorescence is obtained, and the solution now shows an absorption band between 6 and F. Urobilin and urochrome are related substances, but the exact nature of the relation has not been settled. There is some evidence that a portion of the urobilin of urine is not derived from the intestine, but manufactured probably in the liver. In hunger urobilin is still excreted in the urine, although in greatly reduced ainount. During menstruation it is markedly increased, both in fasting and in normally fed individuals. Urorosein is a red pigment which is produced from its chromogen by the action of mineral acids — e.g., strong hydrochloric acid — in the presence oif an oxidizing agent, especially nitrites. EXCRETION BY THE KIDNEYS 477 The pigments of the blood and bile and some of their derivatives are of common occurrence in the urine in disease. Hmmatoporphyrin has not only been found in some pathological conditions, but is constantly- present in minute traces in normal urine. Certain drugs — e.g., sulphonal — cause an increase in its amount. In paroxysmal haemoglobinuria,- methesmoglobin, mixed with some oxyhsemoglobin, is found in the urine in large amount ; and it is worthy of note that it is not formed in the urine after secretion, but is already present as such when it reaches the bladder. In the rare condition termed alkaptonuria, a body, alkapton, now known to be identical with homogeiltisinic acid, CbH3.(OH)2CH2.COOH, a dioxyphenylacetic acid, is present. The urine becomes dark brown on the addition of an alkali, or simply on exposure to air. It gives Fehling's test for sugar. The substance has relations to the aromatic amino-acids tyrosin and phenyl-alanin, and when either of these is given to a person sufiering from alkaptonuria, the amount of alkapton excreted is increased. We may suppose, therefore, that in this con- dition the normal decomposition of these products of proteolysis is interfered with. Ferments. — The urine contains traces of proteolytic and amylolytic ferments (Fig. 185). These may be easily separated from it by putting a little fibrin, which has the power of fixing (adsorbing) enzymes, into the urine. Of the inorganic constituents of urine the most important and most easily estimated are the chlorine, phosphoric acid, and sul- phuric acid. 185. — Pepsin in Urine. Diastatic Ferment in Urine. At Different Times of the Day (Hoffmann). Chlorine. — Much the greater part «)f the chl»rine is united with sodium, a smaller amount with potassium. The chlorides of the urine are undoubtedly to a great extent derived directly from the chlorides of the food, and have not the same metabolic significance as the organic, and even as some of the other inorganic constituents. But it is note- worthy that in certain diseased conditions the chlorine may disappear entirely from the urine, or be greatly diminished — e.g., in pneumonia, and in general in cases in which much material tends to pass out from •the blood in the form of effusions (p. 508). Phosphoric Acid. — The phosphoric acid of the urine is chiefly derived from the phosphates of the food, but must partly come from the waste products of tissues rich, in phosphorus-containing substances, such as lecithin and nuclein. The phosphoric acid is united partly with alkalies, especially as acid sodium phosphate,, and partly with earthy bases, as phosphates of calcium and magnesium. The eari'hy phosphates are precipitated by the addition of an alkali to urine.t or m the alkaline 478 EXCRETION fermentation. In some pathological urines they come down when the carbon dioxide is driven ofi by heating; a precipitate of this sort differs from heat-coagulated albumin in being readily, soluble in acids (Practical Exercises, p. 516^. A small amount of phosphorus may appear in the urine in a less oxidized form than phosphoric acid. Sulphuric Acid. — This is only to a sUght extent derived from ready- formed sulphates in the food. The greater part of it is formed by oxidation of the sulphur of proteins. About nine-tenths of the sulphur in normal urine is present as inorganic sulphates, mainly those of potassium and sodium. Of the other tenth, a portion is represented by ethereal sulphates, and the remainder by the so-called ' neutral ' sulphur, including the sulphur associated with the pigment urochrome. A small amount of sulphur occurs in less oxidized forms than sul- phates in such compounds as the sulphocyanide, which is probably, in part but not entirely, derived from that of the saliva, and ethyl ■sulphide, a substance -with a penetrating odour, which appeg-rs to be a constant constituent of dog's urine (Abel). Thiosulphuric acid (HgSgOs) occurs alinost constantly in cat's urine, often in dog's. It is not free, but combined with bases. The ethereal sulphates are compounds in which the sulphuric acid is united with aromatic bodies (indol, phenol, etc.). Such are potassium- phenyl-sulphate (CJH5KSO4), potassium-kresyl-sulphate (C7.H7KSO4), potassium-indoxyl-sulphate (C8HgNKS04), potassiium-skatoxyl-sulphate (CgHgNKSO^), and two double sulphates of potassium and pyroca.techin. The formation. of potassium indovxl sulphate may be thus represented: Indol, €5114^ •^ttt' on absorption from the intestine is changed into indoxyl, C6H4<'-]^tx ' ' which + SOjC^qj^ (potassium hydrogen sulphate) yields ^^zCryii ' (potassium indoxyl sulphate) -I- HgO. The ' pairing ' of these aromatic bodies with sulphuric acid renders them ifl'nocuous to the organism. Most of the compounds are present in greater amount in the urine of the horse than in the normal urine of man. But in disease the quantity of indican in the latter may be much increased ; and to a csrtain extent it must be looked upon as an index of the intensity of putrefactive processes in the intestine and of absorp- tion from it. Munk made the observation that in the urine of a starving dog the phenol-forming substances are absent, while in the urine of a starving man they are present in abnormally large amount. The indigo-forming substances (indican), on the other hajid, are in hunger excreted in considerable quantity by the dog, and hot at all by man (Practical Exercises, p. 5 10) . According to Folin, the indoxyl potassium sulphate' or indican of the urine is not to any appreciable ejctent related to protein metabolism, but for the most part to the putrefaction of protein in the intestine . The indoxyl-potassium sulphate taken by itself may therefore afford a rough index ■ of the intensity of the intestinal putrefactive processes. On the other hand, the total ethereal sulphuric aCid cannot be taken as an index of the ejctent of the putrefaction, for, although absplutely diminished, it is increased relatively to the total excretion of sulphur on a diet poor in protein, or even protein-free (sefe tables on p. 471). Phenbl and kresol can easily be obtained from horse's urine by mixing it with. strong) hydrochloric acid and distiUing. These aromatic bodies pass over in the distillate. Pyrocatechin remains behind, and can be extracted by ether. It gives a green colour with ferric chloride. Which becomes violet on the addition .of sodium carbonate. EXCRETION BY THE KIDNEYS 479 The sulphur of the inorganic sulphates is the fraction of the total sulphur which fluctuates in proportion to the total protein metabolism. In this regard it follows the variations in the urea. It represents ' exogenous ' metabolism. The neutral sulphur occupies a position analogous to that of the kreatinin: the smaller the amount of protein' in the food, and the smaller therefore the total protein decomposed, the larger is the fraction which the neutral sulphur forms of the total sulphur. The neutral sulphur accordingly represents endogenous metabolism. The ethereal sulphur takes an intermediate position in this regard, but upon the whole it also becomes a more prominent fraction of the total sulphur when the food contains little or no protein. The ethereal sulphates are therefore not entirely derived from the putrefaction of protein. Carbonates of sodium, ammonium, calcium, and magnesium occur in alkaline urine. ' Their source is the carbonates and the vegetable organic acids of the food. In acid urine a certain amount of carbon dioxide is present, although not firmly united with bases, so that most of it can be pumped out. Physico- Chemical Analysis of Urine. — The freezing-point of urine is often determined to obtain a measure of the molecular concentration, which with the total quantity of urine secreted in a given time is an index of the work of the kidney. The greater the volume of urine secreted per unit of time, and the greater the number of molecules dissolved in unit volume of it, the greater is the work of the secretory apparatus in separating it from the blood (p. 497). Normally, A has a higher value for urine than for blood — i.e-., the molecular concentration of the urine is higher than that of the serum. But when large draughts of water are taken A may be lower for urine than for blood,, and in general' it varies within 'far wider limits (from o-ii5° to 2-546° C, according to Koppe). The following table from Kovesi and Roth- Schulz shows the changes in A imder the influence of water : Time. Urine in C.C. , A i 10 to 2 240 .i-8o 2, to 6 255 1-72 6 to 10 ,161 1-93 , 10 to 2 , 131 2-l8 2 to. 6 160 2,-23 6 to 10 120 I -91 II to 12 I -S litres ' Salvator ' water taken 12 to 12.30 500 0;I2 12. 30 to I 444 o-ii I to 1.30, 442 . o-io 1,30 to, 2 46 1 0-78 . .i2tQ 2.30 - ■ . 45 • I -30 ■ 1 If the electrical conductivity is determined, we obtain- an approxi- mate measure Of the number of. dissociated ions in unit volume, mainly the inorganic salts.; Deducting this from the total number of molecules per unit volmne (tiieaisjred by A), we arrive at the concentration of the urine in .non-dissoigj&ted. .inoleoules, mainly urea and other orgahib constituents. , PteciatBiVuis. addtd. to such calculations by estimating also in the ordinary way te titration, e^g.) one or more of the. inorganic 480 EXCRETION constituents, especially the chlorine, since sodium chloride is quanti- tatively the most important of the salts. Various formulae have been deduced from such determinations connecting the freezing-point and conductivity with other physical constants of the urine. E.g., —r^ = K=75, where s is the specific gravity and K a constant with the value 75. The quotient -^ _-., representing the ratio of the total concen- tration to the sodium chloride concentration, varies within relatively narrow limits in health, according to Koranyi, the diet exercising no influence upon it whatever. Thus, in a large number of heedthy individuals ,, p. fluctuated only between 1-23 and i-6g, while A varied from i'26° to 2-35°, ajid the percentage of sodium chloride from 0-85 to 1154. This is illustrated in the table : Urine in C.C. in Percentage of A Twenty-four Hours. NaCl. NaCl' I.-365 1-43° I -08 1-32 1.745 I -60° 1-24 1-29 1,680 1-68' 1-28 I-3I 1. 015 I-84*' I-I5 I -60 865 i-8i° 1-26 1-44 1,360 1-62° I'og 1-49 840 2-26'' 1-50 I-5I 1,600 1-46° 1-14 1-28 2,080 i-as" 0-85 1-68 1 The Urine in Disease. — Although, strictly speaking, a truly patho- logical urine has no place in physiology, the line which separates the urine of health from that of disease is often narrow, sometimes invisible ; while the study of abnormal constituents is not only of great importance in practical medicine, but throws light upon the physiological processes taking place in the kidney, and upon the general problems of metaboUsm. Even in health the quantity of the urine, its specific gravity, its acidity, may vary within wide limits. A hot day may increase the secretion of sweat, and correspondingly diminish the secretion of urine, and the de'ficiency of water may lead to a deposit of brick-red urates. A meal richin fruit or vegetables may render the urine alkaline, and its alkalinity may determine a precipitate of earthy phosphates. But neither the scanty acid urine with its sediment of urates, nor the alkaline urine with its sediment of phosphates, comes into the category of pathological urines; the deviation from the normal does not amoimt to disease. The maximum deviation from the line of health is the total suppression of the urine. If this lasts long, a train of S3rmptoms, of which con- vulsions may be one of the most prominent, and which are grouped imder the name of uraemia, appears. At length the patient becomes comatose, and death closes the scene. Suppression of urine may be the consequence of many pathological conditions, but there is one case on record in the human subject which, in effect, though not in intention, belongs to experimental physiology. A surgeon diagnosed a floating kidney in a woman. With a natural impatience of loose odds and ends of this sort, he ofiered to remove it, and in an evil hour the patient consented. The surgeon, a perfectly skilful man, who acted for the EXCRETION BY THE KIDNEYS 481 best, and to whom no blame whatever attached, carried the kidney to a well-known pathologist for examination. The latter, to the horror of the operator, suggested, from the appearance of the organ, that it was the only kidney the woman possessed. This turned out to be the fact. Not a drop of urine was passed. Apart from this ominous symptom, all went well for seven or eight days; but then uraemic troubles came on, and the patient died on the eleventh or thirteenth day after the operation. The necropsy showed that her only kidney had been taken away. In disease the urine may contain abnormal constituents, or ordinary constituents in abnormal amounts. Of the normal constituents which may be altered in quantity, the most important are the water, the inor- ganic salts, the urea, the uric acid, and the aromatic substances. Water. — A marked and persistent diminution in the quantity of urine^that is to say, practically in the water, with or without an increase in the specific gravity — -is suggestive of disorganization of the renal epithelium. In some infective diseases the kidney is liable to be secondarily involved, its secreting cells being perhaps crippled in the attempt to eliminate the bacterial poisons. In the form of paren- chymatous or tubal nephritis which so frequently complicates scarlet fever, the quantity of urine has in some cases fallen to 50 or 60 c.c. in the twenty-four hours. In chronic interstitial nephritis (' granular kidney '), on the other hand, where the structural changes in the tubules are, for a long time at least, comparatively circumscribed, the quantity of urine is often increased and of low specific gravity. In these cases the increase in, the blood-pressure, associated with hypertrophy of the heart, may be. a factor in the exaggerated renal secretion. In diabetes mellitus the quantity of urine is greatly increased, perhaps in some cases because more urea is excreted than normal, and urea acts as a diuretic, perhaps also because the elimination of sugar draws with it an increased excretion of water to hold it in solution. Although a specific gravity as low as J002 has been seen in healthy persons (after copious potations), the persistence of a density below loio should suggest hydruria. Watson mentions the case of a boy with diabetes insipidus, who voided in twenty-four hours 9 or 10 pints (5 to 6 litres) of urine with a specific gravity of 1002. On the other hand, while the specific gravity has been occasionally observed to mount in health to at least 1036, its persistence at 1025 or 1030 or anything above this, especially if the urine is pale and apparently dilute, should suggest diabetes mellitus. Inorganic Salts. — ^The changes in the quantity of the inorganic con- stituents of the urine in disease are not, in the present state of our knowledge, of as much importance as the changes in the organic con- stituents. The chlorides are diminished in most acute febrile diseases and may even totally disappear from the urine, and their reappearance 3,fter the crisis is, so far as it goes, a favourable symptom. In most cases in which the quantity of the urine is markedly lessened, all the inorganic substances are diminished in amount. Urea. — The quantity of urea is, as a rule, increased in fever, either absolutely or in proportion to the amount of nitrogen in the food. In the interstitial varieties of kidney diseg,se the urea is usually not diminished, but when the stress of the change falls on the tubules (parenchymatous nephritis), it is distinctly decreased — it may be even to one-twentieth of the normal. Uric acid is diminished in the urine in gout (perhaps to one-ninth of the normal), not only during the paroxysms, but in the intervals. It accumulates in the blood and tissues, and, as sodium urate, may form 31 482 EXCRETION concretions in the joints, the cartilage of the ear, and other situations. Watson relates the case of a gentleman who used to avail himself of his resources in this respect by scoring the points at cards on the table with his chalky knuckles. In leukaemia the quantity of uric acid and purin bases in the urine is greatly increased, not only absolutely, but also in proportion to the urea. As much as 4|- grammes of free uric acid, in addition to about ij grammes of ammonium urate, has been found in a urinary sediment in a case of leukaemia. The aromatic bodies, of which indoxyl may be taken as the type, are increased when the conditions of disease favour the growth of bacteria in the intestine — e.g., in cholera, acute peritonitis, and carci- noma of the stomach. A marked increase' in the amount of the indican in the urine may, as far as it goes, be taken as an indication that the bacteria are gaining the upper hand in the intestinal tract ; a marked diminution is usually a sign that the battle has begun to turn in favour of the organism (Practical Exercises, p. 510). Tryptophane, a sub- stance which we have already recognized among the products of the tryptic digestion of proteins, has been shown to be a precursor of indol, which is formed from it under the influence of bacteria. When trypto- phane is injected into the caecum of rabbits, the indican in the urine is markedly increased. Putrefactive processes in other parts of the body than the intestine may also increase the indican in the urine — e.g., a collection of putrid pus in the pleural cavity. Abnormal Substances in Urine. — Sugar, proteins, the pigments of bile and blood, or their derivatives, are the most important abnormal sub- stances found in solution in the urine. Normal urine, as has been stated, contains a trace of dextrose, but so little that it cannot be detected by ordinary tests, and for practical purposes it may be con- sidered absent. Dextrose is the sugar found in the urine in diabetes. In the urine of nursing mothers lactose may be present. Pentoses, sugars with five carbon atoms in the molecule (instead of six, as in the hexoses, of which group dextrose is a member), miay also occasionally occur in urine. Pentoses give the ordinary reduction tests for sugar, and yield osazones, but do not ferment with yeast. Various plants contain pentoses, and when these are eaten the pentoses are excreted in the urine, but in cases of true pentosuria they originate in the body, possibly from nucleo-proteins. The condition has not the same sinister significance as diabetes. Specific toxic substances produced by bac- terial action have been demonstrated in the urine in certain diseases. Red blood-corpuscles and leucocjrtes (pus corpuscles, white blood- corpuscles, mucous corpuscles) are the chief organized deposits ; but spermatozoa may occasionally be found, as well as pathogenic bacteria- — e.g., the typhoid bacillus; and in disease of the kiiiey casts of the renal tubules are not uncommon. These tube-casts may be composed chiefly of red blood -corpuscles, or of leucocytes, or of the epithelium of the tubules, sometimes fattily degenerated, or of structureless protein, or of amyloid substance. Abnormal crystalline substances, such as the amino-acids, leucin (Fig. 186), tyrosin (Fig. 187), and cjrstin (Fig. 180), may be on rare occasions found in urinary sediments; but generally the unorganized deposits of pathological urine consist of bodies actually contained in, or obtainable from, the normal secretion, but present in excess, «ither absolutely, or relatively to the solvent power of the urine. Cystin is of interest because of its relations to the sulphur of the protein molecule (p. 354). It is not found in the normal organism. It very occasionally forms calculi in the bladder. There are individuals who constantly pass as much as one-fourth of all the sulphur in the form of cystin, without any other symptoms. EXCRETION BY THE KIDNEYS 483 Various amino-acids are present in solution in the urine in many- pathological conditions. Of these the least soluble are leucin and tyrosin, and this is the reason why they are most easily detected. A general reaction for amino-acids is their precipitation as sparingly soluble compounds (^-naphthalinsulphones) by /3-naphthalinsulpho- chloride in the presence of an alkali (sodium hydroxide). In acute yellow atrophy of the liver leucin and tyrosin have been found in large amounts in the liver itself, as well as in the urine. In phosphorus poisoning these amino-acids, as well as glycocoU, have been detected in the urine, and there is no doubt that other amino-acids, arising from the decomposition of proteins, are also present in such conditions. Sugar. — In diabetes mellitus, although the quantity of urine is usually much increased, its specific gravity is above the normal; and this is due chiefly to the presence of sugar (dextrose), which generally amounts to 1 to 5 per cent., but may in extreme cases reach 10 or even 15 per cent., more than half a kilogramme being sometimes given ofi in twenty- four hours. The name of the tests for dextrose is legion. They are mostly founded on its reducing action in alkaline solution. Hydrated oxide of bismuth (Boettcher), salts of gold, platinum and silver, indigo (Mulder), and a host of other substances, are reduced by dextrose, and may be used to show its presence. The reduction of cupric salts (Trommer), Fig. 186. — Leucin Crystals. Fig. 187. — Tyrosin Crystals. fermentation by yeast, and the formation of crystals of phenyl-gluco- sazone are the best established tests. (See Practical Exercises, p. 517.) Proteins. — Serum-albumin and serum-globulin are the proteins most commonly found in pathological urine. Both are coagulated by heating the urine, slightly acidulated if it is not already acid, or by the addition of strong nitric acid in the cold. Proteoses (albumoses) are also occa- sionally present, e.g., in the disease called ' osteomalacia ' and in con- ditions associated with the formation and especially with the decom- position of pus. They may be recognized by the tests given in the Practical Exercises (p. 517). It is doubtful whether the presence of true peptone has as yet been satisfactorily made out. The presence of bile-salts may be shown by Hay's test or Petten- kofer's test (p. 456). The pigments of blood and bile may be detected by the characters described in treating of these substances; the spectrum of oxyhaemo- globin, ormethaemoglobin, or any of the other[derivatives of haemoglobin, with the formation of haemin crystals, would afiord proof of the presence of the former, and Gmelin's test of the latter. The red blood-corpuscles, seen with the microscope, are the most decisive evidence of the presence of blood, as leucocjrtes in abundance are of the presence of pus. It should be remembered that pus in the urine of women has sometimes no signifi- cance except as showing that there has been an admixture of leucorrheal discharge from the vagina. (See Practical Exercises, pp. 74, 523.) 484 EXCRETION Section II. — The Secretion of the Urine. We have now to consider the mechanism by which the urine is formed in the kidney from the materieJs brought to it by the blood. And here the same questions arise as have already been discussed in the case of the salivary and other digestive glands: (i) Are the urinary constituents, or any of them, present as such in the blood ? {2) If they do exist in the blood, can they be shown to be separated from it by processes mainly physical or mainly ' vital ' — in other words, by ordinary filtration, diffusion and osmosis, or by the selec- tive action of living cells ? In the case of the digestive juices it has been seen that some constituents are already present in the blood, but that physical laws alone, so far as we at present under- stand them, cannot explain the proportions in which they occur in the secretions, or the conditions under which they are separated; while other constituents — and these the more specific and important — are manufactured in the gland-cells. In the kidneys the conditions seem at first sight favourable to physical separation, as opposed to physiological secretion. Urine has been described as essentially a solution of urea and salts, and both are ready formed in the blood. The arrangement of the blood- vessels, too, suggests an apparatus for filtering under pressm-e. Bloodvessels and Secreting Tubules of Kidney. — The renal artery splits up at the hilus into several branches, which pass in between the Mal- pighian p3n:cimids, and form at the boundary of the cortex and medulla vascular arches, from which spring, on the one side, interlobular arteries running up into the cortex between the pyramids of Ferrein, and, on the other, vasa recta running down into the boimdary layer of the medulla (Fig. 188). The interlobular arteries give ofi at intervals, afferent vessels. Each of these soon breaks up into a glomerulus or tuft of vascular loops, which gather themselves up again into a single efferent vessel of somewhat smaller calibre than the afferent. The glomerulus is fitted into a cup or capsule (of Bowman), which is closely reflected over it, except where the afferent and efferent vessels pass through, and forms the beginning of a urinary tubule. If we suppose the tuft pushed into the blind end of the tubule so as to indent it, it will be easily understood that the single layer of flattened epithelium reflected on the glomerulus is continuous with that lining the capsule, which in its turn is continuous with the epithelial layer of the rest of the urinary tubule. This has been divided by histologists into a number of parts which it is unnecessary to enumerate here, further than to say that the urinary tubule proper begins in the cortex in Bowman's capsule and the proximal convoluted tubule (with its continuation, the spiral tubule), and ends in the cortex with the distal convoluted tubule, the connection between the two being made by a long loop (Henle's) with a descending and an ascending Umb (Fig. 189). Between the ascending limb and the distal convoluted tube is interposed the zigzag tubule. The tubule throughout its length is bounded by a basement membrane lined by a single layer of epithelium, which differs in its character in different parts of the tubule THE SECRETION OF THE URINE 485 • Fig. 188. — Diagram of Blood- vessels of Kidney (Klein, after Ludwig). ai, interlobular ar- tery; vi, interlobular vein; g, glomerulus, to which an afferent artery is seen coming from the interlobular artery, and from which an efferent artery proceeds to break up into a capillary network sur- rounding the renal tubules; vs, vena stellata; ar, arteriae rectae ; vh, leash of venae rectae ; vp, vascular network round ducts at apex of a papilla. Fig. 189. — Diagram of Renal Tubule (Klein). A, cortex ; a, layer of cortex immediately under capsule containing no Malpighian corpuscles; a', inner layer of cortex devoid of Malpighian cor- puscles; B, boundary layer; C, papillary zone of medulla; i. Bowman's capsule; 2, neck of cap- sule; 3, proximal convoluted tubule; 4, spiral tubule; 5, descending part of Henle's loop- t-abule; 6, the loop; 7, 8, and 9, ascending limb of loop-tubule; 10, irregular tubule; 11, distal convoluted tubule; 12, junctional tubule; 13, collecting tubule in a medullary ray or pyra- mid of Ferrein; 14, collecting tubule in the boundary layer; 15, large collecting tubule ending in a duct of Bellini. 486 EXCRETION The distal convoluted tube joins by means of the short connecting tubule one of the straight tubules which form the pyramids of Ferrein or medullary rays in the cortex, and which run down into the medulla, alwasrs uniting into larger and larger tubes as they go, until at length they open as ducts of Bellini on the apex of a papilla. The two convo- luted tubules (with the spiral and zigzag tubules) are lined by similar epithelial cells with granular contents, and the tendency of the granules to be arranged in rows perpendicular to the basement membrane gives them a striated or ' rodded ' appearance (Fig. 190). The granules are eosinophile (p. 17), which is also a character of the granules of other secreting cells. . Towards the lumen the cells may show a brush of pro- Fig. 190. — From a Vertical Section of Dog's Kidney to show the Structure of Different Portions of the Renal Tubule (Klein), a. Bowman's capsule enclosing glomerulus, the capillaries of which are arranged in lobules separated by a little connective tissue. The capsule and glomerulus together constitute a Malpighian body or corpuscle; », neck of capsule; c, c, convoluted tubules, cut in various directions; 6, irregular or zigzag tubule ; d, e, and/ are straight tubules, which take part in the formation of a medullary ray or pyramid of Ferrein ; d, collecting tubule ; e, e, spiral tubule; /, narrow part of ascending limb of Henle's loop-tubule; 6, c, and e are lined with rodded epithelium. cesses, looking like cilia, but in mammals these are not motile. The ascending part of Henle's loop also has cells of the same general char- acter, with numerous granules, although the ' rodding ' may not be so distinct. We shall see directly that the morphological resemblance is the index of a functional likeness. The blood-supply of the tubules, especially of the convoluted portions, is exceedingly rich, the efferent vessels of the glomeruli breaking up around them into a close-meshed network of capillaries, from which the blood is collected into inter- lobular veins running parallel to the interlobular arteries between the pyramids of Ferrein. The straight tubules of the medulla are also surrounded by capUlanes given off from straight arteries (arteriae THE SECRETION OF THE URINE 487 rectas) running down into it partly from the arterial arches and partly from efferent vessels of the glomeruli nearest the boundary layer, the blood passing away by straight veins (venae rectte) which join the larger veins accompanying the arterial arches. The greater part of the blood going through the kidney has to pass through two sets of capil- laries, one in the glomeruli, the other around the tubules. Even the portion of it which does not go through the glomeruli has for the most part a long route to traverse in narrow arterioles and venules to and from its capillary distribution. And the mean circulation-time through the Mdney has been found to be longer than that through most other organs (p. 137). Theories of Renal Secretion. — ^To come back to our problem of the nature of renal secretion, the anatomical structure of the kidney might be expected to throw light upon the question. And, indeed, it was on a purely histological foundation that Bowman established his famous ' vital ' theory of renal secretion. Impressed with the resemblance between the renal epithelium and the epithelial cells of other glands, and with the distribution of the bloodvessels in the kidney, he came to the conclusion that the characteristic con- stituents of urine, including urea, were secreted from the blood by the tubules. To the Malpighian bodies he assigned what he doubt- less considered the humbler office of separating water from the blood for the solution of the all-important solids. To Ludwig, on the other hand, with his whole attention fastened on the mechanical factors by which the flow of urine could be influenced, the tubules seemed of secondary importance, while the glomeruli appeared a complete apparatus for filtering urine from the blood into Bow- man's capsule. He saw that the efferent vessel was smaller than the afferent ; that it was therefore easier for blood to come to the glomerulus than to get away from it, and that the pressure in the capillaries of the tuft must be higher than in ordinary capillaries, because the resistance beyond them in the comparatively narrow efferent vessel, and especially in the second plexus, is greater than the resistance beyond a single capillary network. And experi- mental investigation soon showed him that the rate at which urine was formed could be greatly influenced by changes in the blood- pressure. On such considerations, Ludwig founded the ' mechanical ' theory of urinary secretion, which, although in a much modified form, still divides with the ' vital ' theory the allegiance of physiologists. It is impossible here to enter in detail into a controversy that has extended over more than half a century and produced an extensive literature. The result of the discussion has been, in our opinion, to establish in its essential principles the ' vital ' theory of Bowman, or at least to show that no purely physico-chemical theory as yet constructed will account for all the facts. Ludwig supposed that the urine, qualitatively complete in all its constituents, was simply filtered through the glomeruli, the work 488 EXCRETION done in this filtration being performed entirely at the expense of the energy of the heart-beat represented as lateral pressure in the vessels of the tufts. But as the proportion of salts, and especially of urea, is very far from being the same in urine as in blood, it had further to be assumed that the liquid which passes into Bowman's capsule is exceedingly dilute, and that absorption of water, and perhaps of other constituents, takes place in its passage along the renal tubules. This process of reabsorption he pictured as a purely physical diffusion between the dilute urine in contact with the free ends of the epithelial cells lining the tubules and the much more concentrated lymph with which their deep ends are bathed. The great length of these tubules, as compared with those of most other glands, might indeed seem to indicate a long sojourn of the xuine in them, and the probability of important changes being caused in its passage along them. But if we consider the immense length (60 to 70 cm.) of the seminal tubules and of their gigantic ducts (epididymis 6 metres), where, of course, absorption of water on a large scale is out of the question, it will be granted that little can be built upon the mere length of the renal tubules. On the other hand, the saUvary glands, where there are no glomeruli, secrete as much water as the kidneys are supposed to filter; and the pancreas, whose capillaries form the first of a double set, and therefore in this respect correspond to the renal glomeruli, secretes less water than the liver, whose capillaries correspond to the low-pressure plexus around the convoluted tubules of the kidney. So that deductions drawn from the anatomical relations of the bloodvessels are not in this case of much value, imless supported by physiological results. It is somewhat unfortunate that systematic writers have fallen into the habit of discussing the mechanism of urinary secretion as if the Ludwig theory and the Bowman theory presented an exact antithesis, as if the one offered a complete ' mechanical ' explana- tion of a process, which the other viewed as entirely ' vital,' and therefore withdrawn from physical explanation. We need not concern ourselves here with the historical develop- ment of this discussion. Three main questions require oiu: attention : 1. Is there any evidence that reabsorption actually occurs in the tubules ? If reabsorption on an important scale does take place, it follows at once that there must be a difference of function between the two parts of the renal apparatus, through which urinary con- stituents pass in opposite directions. 2. But if there is no reabsorption, or none of importance, it may still be asked whether, the direction of movement of the urinary constituents through the glomeruli and the tubular epithelium being the same, some quantitative or qualitative difference in their activity may not exist, certain constituents, e.g., passing mainly or exclusively through the one or the other. THE SECRETION OF THE URINE 489 3. When these questions have been settled, we are in a position to consider the nature of the process by which the urinary con- stituents find their way from the blood into the lumen of the capsules and the tubules, or, if there is reabsorption, out of the tubules into the lymph and blood again, and to see whether or no it can be entirely explained on mechanical and physico-chemical principles. The Question of Reabsorption from the Tubules. — That some absorption can take place from the kidney when the pressure in the ureter is abnormally raised need not be doubted, and when substances like potassium iodide or strychnine are introduced into the ureter or the pelvis of the kidney under these circumstances, they can speedily be detected in the blood. When the ureter pressure (in dogs) is only slightly increased, instead of evidence of reabsorption, we obtain evidence of increased secretion. The volume of urine, the total quantity of sulphate in the urine when sodium sulphate is injected into the blood as a diuretic, and the total amount of reducing sugar when phlorhizin is injected, are all greater on the obstructed than on the normal side. These facts are quite opposed to the idea that filtration and reabsorption are im- portant factors in the preparation of normal urine (Brodie and CuUis) . Changes in the blood- flow through the kidney have nothing to do with the results, since the small increase in pressure in the ureter was shown not to affect the rate of flow of the blood. The attempt has been made to decide whether absorption normally occurs by removing as much of the tubules as possible, and seeing whether the character of the urine is altered. In rabbits the whole or a large portion of the medulla has been excised from one kidney and the other then extirpated. From the mutilated kidney two or three times as much urine was said to flow as was secreted by a control rabbit operated on in the same way, except for the removal of the renal medulla (Ribbert). The conclusion was drawn that the greater quantity of urine escalping was due to the smaller opportunity for reabsorption of the water. But experiments men- tioned on p. 649 suggest a different interpretation of these observa- tions. And Boyd, who repeated Ribbert's work, obtained quite different results after partial removal of the meduUa. He found it impossible to remove the whole. So that hitherto the direct method of eliminating the tubules has left the matter where it was. Some light has been thrown on this question, by taking advantage of the anatomical fact that the kidney of batrachians, and, indeed, that of fishes arid ophidia as well, has a double blood-supply. The renal artery gives off afferent vessels to the glomeruli; the vena advehens, or renal portal vein, breaks up, like the portal vein in the liver, into a plexus of capillaries surrounding the tubules, and there seems to be no communication between the two vascular systems. 490 EXCRETION By t3nng all the arteries going to the kidneys in frogs the circula- tion through the glomeruli can be completely cut off, while ligation of the renal portal vein does not affect the blood-supply of the glomeruli, though markedly interfering with that of the tubules. Gurwitsch has found that, after ligation of the renal portal vein of one kidney in (male) frogs, the flow of urine from that kidney is much diminished as compared with the other. He argues that if reabsorption of dilute urine filtered through the glomeruli takes place in the tubules, the opposite result ought to be obtained, since the glomeruli are not affected, while any absorptive power of the tubules must be crippled or abolished. Experiments on the Excretion of Pigments by the Kidney. — In connection with the second question, and also incidentally with the first, the results of experiments on the distribution of pigments in the kidney after their injection into the blood have often been appealed to. Heidenhain injected indigo - carmine into the blood of rabbits, and after a variable time killed them, cut out the kidneys, and flushed them with alcohol, in which the pigment is insoluble. His results were as follows: (i) When the ^*?inn^'f"^-^^^f™ ^^°'""?*"" spinal cord was cut before the injec- tion of Pigment in Kidney after ," . , , , j.-l 1.1 j Injection into Blood. The cor- tion m order to reduce the blood- tex between a and b and be- pressure, the blue granules were found tween c and d was cauterized j ^^ < rodded ' epithelium of the before the injection. In the 1 ,,,,,, ii j- blank wedge-shaped portions, i, Convoluted tubules and the ascending there was no pigment. In the limb of Henle's loop, and in the lumen zones shaded like 2 there was (,f ^^e tubules, but nowhere else. some pigment, but not so much _, , , j. • j as in the areas shaded like 3. Bowman s capsules contained no pig- ment. The renal cortex was coloured blue. (2) When the spinal cord was not cut, the pigment was found in the medulla and pelvis of the kidney, as well as in the cortex, but always in the lumen of the tubules, and not in the epithelium, except in the situations mentioned. (3) If a portion of the cortex of the kidney had been cauterized with nitrate of silver before in- jection of the pigment, the spinal cord being left intact, a wedge of the renal substance, corresponding to this area, remained coloured only in the cortex, although the rest was blue in the medulla also. The ' rodded ' epithelium was filled with blue granules as before (Fig. 191). (i) shows that the epithelium is capable of excreting some sub- stances at least. (2) appears to show that when the blood-pressure is normal water is poured out from some part of the tubule, and washes the pigment separated by the ' rodded ' epithelium down towards the papillte. (3) suggests that it is through the glomeruli THE SECRETION OF THE URINE 491 that most of the water passes. For cauterization has not destroyed the power of the epithehum to excrete pigment, and therefore, presumably, would not have destroyed its power to excrete water if it possessed this power in any great degree; and the glomeruli and their capsules are the only other part of the renal mechanism which can have been affected. The fact that in birds and serpents, whose urine is solid or semi-solid, the glomeruh are smaller than in mammals is corroborative evidence that the glomeruli have to do with the excretion of water. When pigments are injected into the dorsal lymph-sac of a frog without interference with the renal circulation, they are found plentifully in the lumen of the convoluted tubules, and also in the epithelial cells lining them. The suggestion has been made that the pigments have been absorbed by the cells from the lumen, and not excreted by them into it. And certainly pigments soluble in the cytoplasm or in the substances that form the envelopes of cells, and therefore capable, like methylene blue, of staining them during life, might be taken up by the renal epithelium if excreted into the tubules by the glomeruli, and might cause staining .of them, par- ticularly, of course, of the free ends of the cells next the lumen. But this suggestion is inadmissible, since, on injection of the same pigments after ligation of the renal portal vein, the convoluted tubules contain little or no pigment in their lumen. And when the urinary flow is stopped on one side in mammals by temporary com- pression of the renal artery, the corresponding kidney takes up fully as much carmine as its fellow (Carter). There is no doubt that not only pigments capable of ' vital staining,' like methylene blue, but also pigments which do not stain living cells, are taken up from the blood (or lymph) by the epithelial cells, and, lying in vacuoles in their cytoplasm, are transported towards the lumen, and there extruded. It is not the solubility of the pigments in lipoids, and therefore their solubility in the supposed lipoid envelope of the cells, which determines whether they shall be excreted. The degree in which they are capable of being presented to the cells in non-colloid solution appears to some extent to be a determining factor. The pigments not taken up are highly colloidal (Gurwitsch, Hober). Shafer has recently confirmed Heidenhain's statements as to the place of excretion of indigo-carmine. When leuco-indigo-carmine (a colourless reduction-product of indigo-carmine) was injected, the blue oxidized substance was found in the lumen of the convoluted tubules and in the collecting tubules, but not at all in the Bow- man's capsule. The cells of the convoluted tubules were colour- less, because they kept the pigment in its reduced condition, and it only became oxidized in the lumina of those parts of the tubules whose contents, according to Dreser, show an acid reaction. On oxi- dation by peroxide of hydrogen the cells of the convoluted tubules 492 EXCRETION became faintly green, but the Bowman's capsule remained colourless. This can only be explained on the assumption that the leuco-product of the pigment was excreted by the cells of the convoluted tubules. But these cells are far from taking up all pigments indifferently. Some pigments are extruded mainly by one part, others mainly by another part, of the renal tubule, and some even by the glomeruli, as shown long ago for ammonium carminate. The glomeruli, how- ever, are in general far less active in this regard than the epithelial cells, and the fact that the latter pick out from the blood such sub- stances as these foreign pigments, which pass through the Mal- pighian tufts xmchallenged, renders it likely that the tubules also exercise a special function in the secretion of the normal con- stituents of urine. More direct evidence of this is not wanting, for Bowman saw crystals of mdc acid in the epithelium of the convoluted tubules of birds. Heidenhain found that urate of soda injected into the blood of a rabbit is excreted by the epithelium of the convoluted tubules and the ascending part of Henle's loop, just as is the case with indigo-carmine. And Nussbaum's experi- ments, although not quite conclusive, have made it probable that in the frog urea is actually separated by the epithelium of the tubules. They were founded on the anatomical peculiarity in the renal circulation of the frog already mentioned. By t37ing the renal arteries in that animal, he thought he could at will stop the circula- tion in the glomeruli, and he found that after this was done there was no further spontaneous secretion of urine. But when urea was injected intravenously the secretion of urine again began, urea being eliminated by the kidneys, and water along with it. Sugar, peptone, and egg-albumin, injected into the blood, no longer passed into the urine, even when the secretion was excited by simultaneous injection of urea, although they readily did so when the arteries were not tied. He concluded that the Malpighian corpuscles have the power of excreting water, sugar, peptone, and albumin, while the epithelium of the tubules excretes urea as well as water. Beddard has confirmed Nussbaum's statement that when aU the arteries going to the kidney are tied the glomeruli are completely and permanently deprived of blood. The spontaneous secretion of urine is totally stopped, as Nussbaum found, but only in three experiments out of eighteen was it possible to start the secretion by injection of urea. The epithelium of the tubules degenerated and desquamated after complete ligation of all the renal arteries, showing that it requires some arterial blood as well as the venous blood from the renal portal to maintain its vitality. The degenera- tion of the epithelium can be prevented by keeping the frogs in an atmosphere of oxygen after ligation of the arteries. In six such frogs, in which the complete elimination of the glomeruli was con- trolled by subsequent injection, secretion of urine followed the THE SECRETION OF THE URINE 493 injection of urea, alone or in combination with dextrose, phlorhizin, or di-sodium hydrogen phosphate (Na2HP04). In all the cases the urine contained urea, chlorides, and sulphates, and was acid to phenolphthalein. In one case after injection of urea and dextrose, and in another after nrea and phlorhizin, the urine reduced Fehling's solution, and therefore presumably contained dextrose (Beddard and Bainbridge). When the frog's kidney is perfused in situ with oxygenated salt solution a certain flow of urine takes place. Sub- stitution of non-oxygenated saline markedly slows the flow (CuUis). Apparently, then, the tubules have the capacity to secrete prac- tically all the constituents of urine, and when the flow of urine is small, probably most of it comes from the tubules. When, as in the diuresis produced by salt solutions, large quantities of water and salts have to be rapidly excreted, the bulk of the liquid comes from the glomeruli, but also by a process of secretion. Lindemann has endeavoured to exclude the glomeruli in mam- mals by injecting oil through the renal artery. After a short time, according to him, the oil emboli clear away from practically all parts of the kidney except the glomeruli, which remain plugged. If indigo-carmine be subsequently injected into the blood, it is not only taken up from it by the embolized kidney as well as by a normal one, but is excreted. The quantity of urine is much diminished, and its specific gravity increased, but its composition is not essen- tially altered. He infers that the tubules are in a high degree independent of the glomeruli as an apparatus for the secretion of urine. More conclusive observations have lately been reported in which the tubules were eliminated by producing an artificial nephritis in rabbits by the subcutaneous injection of sodium tartrate. Tar- trates act almost specifically upon the tubules, causing no noticeable effect upon the glomeruli. After the intravenous infusion of a solution containing sodium chloride and urea during pronounced tartrate nephritis, all the chlorine appears in the urine within forty- eight hours, but little, if any, of the urea. In the light of the histological findings, this is interpreted to mean that under normal conditions chlorides and water are passed through the glomerular mechanism, and urea through the convoluted tubules (Underbill, Wells, and Goldschmidt). These results constitute a direct and striking confirmation of the Bowman hypothesis. As regards our first two questions, we may conclude that there is no good evidence that reabsorption of water or other constituents of the urine in the renal tubules plays an important part in the preparation of that secretion. Many facts favour the conclusion that the glomeruli and the renal epithelium act as distinct, although, of course, mutually supplementary mechanisms, the glomeruli separating the larger portion of the water and salts, the epithelium the larger portion, if not the whole, of the characteristic organic constituents. 494 . EXCRETION As regards the third question, it is now generally admitted, even by those who uphold a modified ' mechanical ' theory, that if the urine is originally separated firom the blood by filtration at the expense of the energy of the heart -beat represented by the pressure of the blood in the glomeruli, the reabsorption in the tubules cannot be attributed to simple diffusion, but must be a selective process analogous to absorption in the intestine and entailing the expendi- ture of a large amount of work at the expense of the food materials or the protoplasm of the epithelial cells. Every attempt at a strictly mechanical explanation breaks down for the kidney, as for other glands. The practical absence from urine of the proteins and sugar of the blood under normal circumstances, and the elimination by the kidney of egg-albumin, peptone, and other bodies when injected into the veins, show a selective permeability inexplicable by refer- ence to any known structural or physico-chemical property of the renal epithelium or the glomeruli, but precisely the kind of thing which the physiologist has, without being hitherto able to explain it, learnt to associate with the activity of living cells. Urea and dextrose, both highly diffusible substances, circulate side by side in the bloodvessels of the kidney. The one is taken and the other left. The urea is a waste-product of no further use in the economy. The sugar is a valuable food-substance. The kidney selects with unerring certainty the urea, of which only 4 parts in 10,000 are present in the blood, but rejects the sugar, of which there is three times as much. The theory that the dextrose of the blood or a part of it is combined with substances in the colloid state, and not in ordinary solution, has been advanced from time to time as an explanation of the practical impermeability of the kidney for this sugar under normal conditions. But no proof of the truth of this h3rpothesis has ever been given. On the contrary, there is good evidence that all the dextrose which is estimated in blood by analytical methods is in the free condition. For instance, dextrose easily escapes from blood circulating in the vivi-diffusion apparatus previously described (p. 48). And when the plasma of shed blood is placed in a dialyser tube of animal membrane surrounded by a liquid in which dextrose is dissolved in exactly the same concentration as that determined in the plasma by the ordinary chemical methods, the contents of the dialyser neither lose nor gain dextrose. Now, the plasma ought to gain sugar by diffusion if a portion of the dextrose in it exists in a combination which prevents its diffusion, just as it does gain dextrose when the liquid outside the dialyser contains sugar in greater concentration than the plasma (Michaelis and Rona). Egg-albumin injected into the blood passes through the renal circulation side by side with the serum-albumin of the plasma. THE SECRETION OF THE URINE 495 Both are indiffusible through membranes, and to the physical chemist the differences between them may appear superficial and minute. But the kidney does not hesitate for an instant. A large part of the egg-albumin is promptly excreted as a foreign substance ; the serum-albumin passes on untouched. Not only does the kidney exercise a power of qualitative selec- tion; it also takes cognizance of the quantitative composition of the blood. So long as there is less sugar in the plasma than about 15 to 2 parts in 1,000, it is refused passage into the renal tubules. But when this limit is passed, and the proportion of sugar in the blood becomes excessive, the kidney begins to excrete sugar, and continues to do so till the balance is redressed. The advocates of the theory of filtration through the glomeruli under the influence of the difference of hydrostatic pressure in the capillaries and in the lumen of the capsules have made their firmest stand on the excretion of the inorganic constituents of the urine. They have laid stress particularly on the fact that the hydrsemic plethora caused by intravenous injection of salts is accompanied by diuresis. It is true that the direct introduction of water into the blood, or its attraction from the lymph-spaces when the osmotic pressure of the blood is increased by the injection of substances like urea, sugar, and sodium chloride, may cause a condition of hydrcemic plethora, and that this plethora may sometimes be associated with an increase of pressure in the capillaries in general, and therefore in the vessels of the Malpighian tuft. It may also be admitted that such an increase of pressure might be accompanied by an increased filtration of water and salts into Bowman's capsule. Even in the excised kidney, after the vital activity of its cells may be presumed to have ceased, filtration of the most varied solutions occurs when the organ is perfused with them through the renal artery. The liquid which escapes from the ureter always has the same composi- tion as the perfusion fluid (SoUmann). It would certainly appear unlikely that the glomerular epithelium should make no use what- ever for the furtherance of its task of the difference of hydrostatic pressure on its two surfaces. It is in taking advantage of such circumstances for the promotion of its specific work up to the point at which they cease to favour it that a great part of the true secretory activity of cells may be supposed to consist. When we see a barge passing through a lock, and being gradually lifted to the proper level by the inrush of water, we never dream of saying that the whole thing is an affair of the laws of hydrostatics. We know that the part played by the lock-keeper, the opening and closing of the gates and sluices at the proper time, is all-important, although he does not lighten by one ounce the Weight which the water must lift. He uses the head of water for a specific purpose — ^namely, to lift the barge. In like manner it is to be expected 496 EXCRETION that the glomerular epithelium, when the difference of pressure on its two surfaces is increased by. hydrsemic plethora, will use the increased facility of filtration to rapidly excrete a portion of the water. But who will believe that the addition of a tumbler of water, absorbed from the ahmentary canal, to 4 or 5 litres of blood circulating in a system of vessels whose capacity can and does vary within wide limits, should cause in the capillaries of the kidney an increase of pressure exactly proportional to the increase in the elimination of water in the urine, lasting for the same time and disappearing at the moment when the normal composition of the blood is restored ? Nor is it easier to explain on any mechanical hypothesis how it is that in a starving animal the quantity of inorganic substances eliminated in the urine drops almost to zero, while the proportional amount in the blood and tissues is little, if at all, affected. In a rabbit rendered poor in sodium chloride by feeding it with salt-free food, the injection of a solution of sodium chloride isotonic with the blood produces no diuresis for a con- siderable time, but, on the contrary, a diminished flow of urine, while a similar solution injected into the veins of a rabbit previously fed with salted food causes an imrnediate and considerable diuresis. When small quantities of isotonic solutions of various salts are injected, those not normally present in the blood produce a greater diuresis than normal constitiients. Sodium chloride, which is present in normal plasma in greater amount than any other salt, causes the smallest diuresis of all (Haake and Spiro). Such facts suggest that the secreting cells of the kidney are stimu- lated or inhibited by the contact of blood or lymph in which the normal constituents are present in too great or in too small amount, and that the intensity of the action is proportional to the degree of deficiency or excess. The greater the velocity of the circulation in the kidney, the more effective will be the stimulation produced by any given substance present in excess, and therefore the greater the total amount of it eliminated in a given time. For in making the round of the renal circulation the concentration of the sub- stance in any given portion of blood will fall less, and therefore the average stimulation exerted by it during the round will be greater the faster the blood flows. It is quite in agreement with this that when plethora is occasioned by transfusion of blood there is little or no diuresis, although the increase of arterial, capillary, and venous pressure, and the dilatation of the kidney, are evident. For the rapid passage of liquid out of the vessels would lead to a great increase in the relative proportion of corpuscles to plasma — that is to say, to an abnormal condition of the blood. On the other hand, when plethora is produced by injection of serum diuresis occurs (Cushny). This, again, is what we should expect, since the elimination of the superfluous liquid will restore the normal^ pro- THE SECRETION OF THE URINE 497 portion. The diminished viscosity of the blood (p. 23) produced by the excess of serum will aid the flow through the kidney and therefore increase the diuresis, while in the case of the plethora produced by injection of blood the elimination of liquid will at once increase the viscosity, diminish the velocity of the renal flow, and tend to lessen diuresis. There is, then, little more reason to assume that the copious flow of urine which follows the absorption of a large quantity of water is due to a mere process of filtration than there is to believe that filtration, and not selective secretion, is the cause of the gush of saliva which precedes vomiting, or the sudden outburst of sweat on sudden and severe exertion. In addition, there are the positive proofs already mentioned that the ' rodded ' epithelium of the tubules, which no one supposes to be abandoned more to mere physical influences than the epithelium of the salivary glands, plays a part in the secretion of some of the urinary constituents. As to the nature of the mechanism set in motion, and the series of events that take place as the constituents of the urine journey from the interior of the bloodvessels to the lumen of the tubules, we know no more than in the case of other glands. This alone is clear, that the separation of the urine from the blood implies the performance of a large amount of work by the kidney. For the osmotic pressure of urine is several times as great as that of the plasma of the blood. Blood- plasma freezes at —0-55° to —0-65° C. (on the average, say, — o-6° C). The osmotic pressure correspond- ing to -0-6° C. is 5,662 millimetres of mercury (p. 422), or, in round •numbers, 75 metres of water. Human urine has been found to freeze at —1-38° to — 2-ii° C. (say, on the average, — 1-8° C), and for highly concentrated urines the depression of the freezing-point may be considerably greater. The osmotic pressure corresponding to — 1-8° C. is 16,986 millimetres of mercury or 225 metres of water. This exceeds the osmotic pressure of the plasma by 150 metres of water. In separating a kilogramme of urine from the blood the kidney accordingly does work approximately equivalent to raising a weight of a kilogramme to the height of 150 metres — i.e., 150 kilo- gramme-metres. It is evident that the excess of the blood-pressure in the glomeruli over the pressure of the urine in the tubules, which, even if we neglect the latter altogether — since there is only slight resistance to the flow of urine towards the bladder — cannot at most be greater than 100 millimetres of merciiry, or i -35 metres of water, will account for only an insignificant part of this work. The rest must be done at the expense of the energy of the food materials taken up by, and transformed in, the cells concerned with the secre- tion of the urine. But we do not know in what way these cells, by applying this energy, perform the remarkable feat of perma- nently maintaining a difference of fifteen atmospheres in the osmotic 498 EXCRETION pressure of the liquids in contact with their attached and free sur- faces. A token of the intensity of the metabolic effort required is the marked increase in the absorption of oxygen which occurs during diuresis, although it is not in proportion to the amount of the diuresis. In one experiment the oxygen absorbed by a dog's kidneys was ii per cent, of what would have been used up by the entire animal under normal conditions. There is no definite relation between the oxygen taken in and the carbon dioxide given out at any moment. What is the significance of the peculiar arrangement of the glomerular bloodvessels, if the epithelium of the capsules has secretive powers like that of ordinary glands ? It is difl&cult to believe that these unique vascular tufts have not a near and important relation to the renal function ; but it is by no means clear what that relation is . The secretion of water, and even its rapid secretion, is not at all bound up with any set arrangement of bloodvessels. Gland-cells all over the body secrete water under the most varied conditions of blood -pressure, although a comparatively high pressure is upon the whole favourable to a copious outflow. But the kidney has other functions than mere excretion (p. 649), And it may >be that the simplest part of the latter process, the eUmination of water and salts, is largely thrown upon the. Malpighian corpuscles, as a physiologically cheaper machine than the epithehum of the tubules, which is left free for more complex labours. These may include not only the separation of nitrogenous metaboUtes, but also synthetic processes possibly concerned in the regulation of protein metabolism. One characteristic sjrnthesis, the union of benzoic acid a,nd glycin to hippuric acid, has already been referred to. As will be shown later (p. 571), it takes place mainly, in some animals perhaps exclusively, in the kidney. The epithelium of the glomerulus, being a less highly organized and less delicately selective mechanism than that of the convoluted tubules, may more easily respond to increase of blood- pressure by increased secretion. At the same time, placed as it is at' the last flood-gate of the circulation, where the escape of anything valuable means its total loss, the glomerular epithelium may be endowed with a general power of resistance to transudation, which renders a comparatively high blood-pressure a necessary condition of its acting at all. And as a matter of fact water ceases to be secreted by the kidney long before the blood-pressure in the glomeruli can have fallen below that which suf&ces for the highest activity of the liver. Perhaps, however, the high minimum pressure required (30 to 40 mm. of mercury in the dog) is merely the necessary consequence of the long and difi&cult path which most of the blood going through the kidney has to take, and, that a sufficient blood-flow caimot be kept up with less. It may be, too, that the comparatively small surface of the glomeruli, restricted in order to leave room for the more highly organized parts of the renal mechemism, entails the more intense and concentrated activity which the high blood-pressure renders possible, and the simplicity of work and organization renders harmless. An obvious result, and perhaps an important one, of the peculiar arrangement of the bloodvessels of the kidney is that the renal tubules proper are shielded from an excessive blood-pressure by the inter- position of the glomeruli as a block. This may be either because the epithelium of the tubules would not perform its work so well under a THE SECRETION OF THE URINE 499 high, blood-pressure, or because there would be a danger of substances which ought to be retained being cast out into the urine. In this con- nection it is interesting to note that the specific constituents of urine are separated by epithelium surrounded by capillaries of the second order, and therefore with a smaller blood-pressure than exists in the capillaries of most glands, while the same is true of bile, another (practically) protein-free secretion. The maximum secretory pressure in the kidney, as shown by a manometer tied into the divided ureter, is about 60 mm. of mercury in the dog, or less than half that of saliva. If the escape of the urine is opposed by a greater pressure than this, or if the ureter is tied, the kidney becomes oedematous. Whether the oedema is due to reabsorption of urine or to the pouring out of lymph owing to the pressure of the dilated tubules on the veins has not been de- finitely settled. It has been already pointed out that there is no necessary relation between the blood-pressure in the capillaries of a gland and its secretory pressure ; and, so far as this goes, water might just as well be secreted at a pressure of 60 mm. of mercury from the low-pressure blood of the second set of renal capillaries as from the high-pressure blood of the glomeruli. By obstruction the molecular concentration of the urine is diminished to half or three-quarters of the normal. The Influence of the Circulation on the Secretion of Urine. — Although the activity of no organ in the body is governed more by the indirect effects of nervous action than that of the kidney, no proof has been given of the existence of secretory fibres for it comparable to those of the salivary glands. All the changes in the rate of renal secretion which follow the section or stimulation of nerves can be explained as the consequences of the rise or fall of local or general blood-pressure, and of the corresponding variations in the velocity of the blood in the renal vessels. The best way to study variations in the calibre of the renal vessels is the plethysmographic method, and the oncometer of Roy is a plethysmo- graph adapted to the kidney (Fig. 192) . It consists of a metal capsule lined with loose membrane, between which and the metal there is a spac3 filled with oil. The two halves of the capsule open and shut on a hinge; and the kidney, when introduced into it, is surrounded on all sides by the membrane, the vessels and ureter passing out through an opening. The oil-space is connected with a cylinder also filled with oil, above which a piston, attached to a lever, moves. The lever registers on a drum the changes in the volume of the kidney — i.e., practically the changes in the quantity of blood in it, and therefore in the calibre of its vessels. A still better oncometer is that of Schafer, in which air is employed instead of oil. Nerves of the Kidney. — Both vaso -constrictor and vaso-dilator fibres for the renal vessels, but most clearly the former, have been shown to leave the cord (in the dog) by the anterior roots of the sixth thoracic to second lumbar nerves, and especially of the last three thoracic. They run in the splanchnics, and then through the renal plexus — around the renal artery — into the kidney. The vaso-constrictors predominate, 500 EXCRETION so that the general effect of stimulation of the nerve-roots, the splanch- nics, or the renal nerves is shrinking of the kidney, with diminution or cessation of the secretion of urine. But slow rhythmical stimulation of the roots causes increase of volume, the scanty dilators being by this method excited in preference to the constrictors. The renal nerves, entering at the hilum, branch repeatedly, so as to form a wide-meshed plexus around the arteries, and accompany them even to their finest ramifications in the cortex. Coming off from the nerves surrounding the arteries are fine fibres which are distributed to the convoluted tubules. Some of them terminate in globular ends, others in fine threads that pass through the membrana propria (Berkeley). Section of the renal nerves is followed by relaxation of the smaU arteries in the kidney, and consequent swdling of the organ. The flow of urine is greatly increased, and sometimes albumin appears in it, the excessive pressure in the capillaries (particularly in those of the glomeruli) being supposed to favoiu" the escape of substances to which a passage is refused under normal conditions. An experiment which is sometimes quoted as a de- cisive test of the relative importance of changes in the rate of flow, and in the pressmre of the blood within the glomeruli, is that of tying the renal vein. This undoubtedly does not lower the intra- glomerular pressure — on the contrary, it must in- crease it — but the secretion of urine stops. If the venous outflow from the kidney is only partially interfered with, the flow of urine is immediately diminished, but the administration of a diuretic like potassium nitrate causes an increase. It is more than likely that in these experiments the secretion stops or slackens not because a high blood- pressure, but because an active circulation is its necessary condition. When the blood stagnates in the kidney the natural stimulus to the renal apparatus speedily disappears owing to the elimination of the urinary constituents to the neutral or indifferent point (p. 496). The experiment, however, is. not perfectly conclu- sive. For few glands can go on performing their function after the circulation has ceased. The kidney must be able to feed itself in order to continue its work. Above all, it needs oxygen; and it might be urged that if the blood in the glomeruli could be kept at the normal standard of arterial blood, secretion might still go on after ligation of the renal vein. Fig. 192. — Diagram of Organ-Plethysmograph or Oncometer. B, metal box in two halves open- ing on the hinge H ; M, thin membrane ; A, space filled with oil; O, organ enclosed in oncometer ; V, vessels of organ ; t, tube for filling instrument with oil; T, tube connected with D, which opens into cylinder C; C is also filled with oU; P, pis- ton attached by E to a writing lever. THE SECRETION OF THE URINE 501 According to Ludwig, indeed, the flow of urine stops, in spite of continued filtration through the glomeruli, because the swelling of the veins in the boundary layer compresses the tubules, and may even obliterate their lumen. There is no conclusive experimental evidence, however, and no a priori probability, that the obstruction so produced is sufficiently sudden or sufficiently complete to cause instant and total cessation of the flow. It is even less justifiable to conclude from the experiment that the liquid part of the urine is, at any rate, not separated by the epithelium of the tubules, since the blood-pressure in the capillaries around the tubules must rise very greatly after ligature of the vein, and yet secretion is stopped. It might equally well be argued that the renal epithelium normally secretes water under a low blood-pressure, but is disorganized under the excessive and entirely unaccustomed pressure which follows the closure of the vein. It is not only through nerves directly governing the calibre of the vessels of the kidney that the rate of urinary secretion can be affected. Any change in the general blood-pressure, if not counter- acted by, still more if conspiring with, simultaneous local changes in the renal vessels, rnay be followed by an increased or diminished flow of urine ; and the law which explains all such variations, or at least serves to sum them up, is that in general an increase in the rate of the blood- flow through the kidney is followed by an increase in the rate of secretion. It will be remarked that this is the converse of the great law, of which we have already seen so many illustra- tions, that functional activity increases blood-flow. It is probable that this law holds for the kidney as well as for other organs, but that the influence of activity on blood-supply is subordinated to that of blood-supply on activity, while in most tissues, as in the muscles, the opposite is the case. It is evident that an increase in the blood-flow would favour the secretory activity of the renal cells, since the average concentration of the blood presented to them as regajds those constituents which they select would remain relatively high in its circuit through the kidney. The ' stimulus ' to secretion would, therefore, be relatively intense. Destruction of the medulla oblongata {i.e., of the vaso-motor centre), or section of the cord below it, diminishes the secretion of urine, because the arterial pressure is lowered so much as to over- compensate the dilatation of the renal vessels, which the operation also brings about. If the blood-pressure falls below 40 mm. of mercury, the secretion is abolished. Stimulation of the medulla or cord also lessens the flow of urine by constricting the arterioles of the kidney so much as to over-compensate the rise of general blood- pressure, caused by the constriction of srnall vessels throughout the body. If the renal nerves have been cut, stimulation of the medulla 502 EXCRETION oblongata increases the urinary secretion, because now the rise of general blood-pressure is no longer counterbeilanced by constriction of the renal vessels. An increase in the urinary flow can be pro- duced in the rabbit by a lesion in a part of the funiculi teretes, which can be reached in the floor of the fourth ventricle (Eckhard), perhaps by destroying the portion of the vaso-motor centre govern- ing the renal nerves, while the rest remains uninjiu-ed, or is even stimulated, and thus keeps up or even increases the general blood-pressure. There is either no glycosuria, or it is very slight. Section of the splanchnic nerves causes a faU of arterial pressure, which is, however (in animals like the dog, in which compensation soon takes place), more than balanced by the simultaneous dilata- tion of the renal vessels, and therefore for some time the flow of urine is increased, but not so much as when the renal nerves alone are cut. In the rabbit there is no increase. On the other hand, stimulation of the splanchnics stops the urinary secretion, because the general rise of pressure is not enough to make up for the con- striction of the renal vessels. Diuretics are substances that increase the flow of urine. Some of them act mainly on the circulation, as by increasing the general blood- pressure, others mainly by a direct influence on the secreting mechanism. Digitalis is a representative of the first class ; urea and caffein belong to the second. The action of digitalis is to strengthen the beat of the heart, which is at the same time somewhat slowed, and to constrict the arterioles. Both eSects contribute to the increase of pressure. But the accompanying diuresis is due to the cardiac factor, the vaso-con- striction which involves the renal vessels also, being over-compensated. The diuretic efiect of digitaUs is much greater in cardiac disease. with dropsical effusions than in health. Caffein, when injected into the blood, affects the pressiu-e but little. It causes dilatation of the renal vessels after a passing constriction, and an increase in the flow of urine after a temporary diminution. The vascular dilatation is not the chief reason for the diuretic effect, for the latter is still obtained when the vaso-motor mechanism has been paralyzed by chloral hydrate, and even after the secretion of urine has been stopped by the fall of pressure consequent on section of the spinal cord. Caffein, therefore, acts directly on the renal epithelium. The action of urea, potassium nitrate, and the saline diuretics is probably also a direct action on the secreting structures, although some haye supposed that their primary effect is to cause vaso-dilatation in the kidney, and a consequent local increase in the capillary pressure. Summary. — Our knowledge of renal secretion may be thus summed up: The water and salts of the urine are chiefly separated by the glomeruli ; the process is not a mere physical filtration, hut a true secretion. Substances like sugar, peptone, egg-albumin, and heemoglobin, when injected into the blood, are probably excreted mainly by the glomeruli ; and so is the sugar of diabetes. Urea, uric acid, and presumably the other organic constituents of normal urine, with EXPULSION OF THE URINE 503 a portion of the water and salts, are excreted by the physiological activity of the ' rodded ' epithelium of the renal tubules. The rate of secretion of urine rises and falls with the pressure, and still more with the velocity, of the blood in the renal vessels. No secretory nerves for the kidney have been found ; the effects of section or stimulation of nerves on the secretion can all be explained by the changes produced in the renal blood-flow. Some diuretics act by increasing the blood-flow, others directly on the epithelium of the tubules or the glomeruli. Section III. — Expulsion of the Urine. Micturition. — ^The urine, like the bile, is being constantly formed ; although secretion varies in its rate from time to time, it never ceases. Trickling along the collecting tubules, the urine reaches the pelvis pf the kidney, from which it is propelled along the ureters by peristaltic contractions of their walls, and drops from their valve- like orifices into the bladder. When this becomes distended, rhyth- mical peristaltic contractions are set up in it, and notice is given of its condition by a characteristic sensation, which is perhaps aided by the squeezing of a few drops of urine past the tonically con- tracted circular fibres that form a sphincter round the neck of the bladder, and into the first part of the urethra. The desire to empty the bladder can be resisted for a time, as can the desire to empty the bowel. If it is yielded to, the smooth muscular fibres in the wall of the viscus are thrown into contraction. This is aided by an expulsive effort of the abdominal muscles. The sphincter vesicae is relaxed; and the urine is forced along the urethra, its passage being facilitated by discontinuous contractions of the ejaculator urinse muscle, which also serve to squeeze the last drops of urine from the urethral canal at the completion of the act. Regurgitation into the ureters is to a great extent prevented by their compression between the mucous and muscular coats of the bladder, where they run for more than half an inch before opening at the posterior angle of the trigone. But it has been shown that a certain amount of back flow can take place. Small bodies like diatoms suspended in water and pigments dissolved in it have been found in the pelvis of the kidney, the renal tubules, and even the circulation after being injected into the bladder. The pressure in the bladder of a man may be made as high as 10 cm. of mercury during the act of micturition; about half this amount is due to the contraction of the vesical walls alone, the rest to the contraction of the abdominal muscles. A pressure of 16 to 26 mm. of mercury is required to open the sphincter of a rabbit's bladder in life. Although the whole performance seems to us to be completely voluntary, there are facts which show that it is at bottom a reflex 504 EXCRETION series of co-ordinated movements, that can be started by impulses passing to a centre in the spinal cord from above or from below — from the brain or from the bladder. In dogs, with the spinal cord divided at the upper level of the lumbar region, micturition takes place regularly when the bladder is full, and can be excited by such slight stimuli as sponging of the skin around the anus (Goltz). Here, of course, the act is entirely reflex; and the centre is situated at the level of the fifth lumbar nerves. The efferent nerves of the bladder, like those of the rectum; come partly from the cord directly through the sacral nerves, and partly through the lumbar sympa- thetic chain (second to sixth ganglia). The sacral fibres are con- nected with nerve cells in the hypogastric plexus, and the sympa- thetic, partly at least, in the inferior mesenteric ganglia. This anatomical coincidence acquires interest in view of the striking physiological similarity between the processes of micturition and defaecation, a similarity which is emphasized by the fact that the latter is almost invariably accompanied by the former. An im- portant difference, however, is that the wiU can far more readily set in motion the machinery of micturition than that of defaecation; a man can generally empty his bladder when he likes, but he cannot empty his bowels when he likes. Sometimes in disease, and especially in disease of the spinal cord, the mechanism of micturition breaks down ; the bladder is no longer emptied; it remains distended with urine, which dribbles away through the urethra as fast as it escapes from the ureters. To this condition the term incontinence of urine is properly applied. Reflex emptying of the bladder, without an act of will or during unconsciousness, is not true incontinence. The involuntary mic- turition of children during sleep, for example, is a perfectly normal reflex act, although more easily excited and less easily controlled than in adults. Section either of both nervi erigentes, or of both hypogastrics, is never followed by more than quite temporary dis- turbance of function of the bladder in dogs, both male and female. In a few days the urine is normally passed. In bitches the same is true when both pairs of nerves are divided. But in male dogs true incontinence of urine follows section of the four nerves, as well as intense tenesmus due to paralysis of the lower part of the large intestine. Section IV. — Excretion by the Skin. Besides permitting of the trifling gaseous interchange already referred to (p. 292), the skin plays an important part in the elimina- tion of water by the sweat-glands. Sweat is a clear colourless liquid of low specific gravity (1003 to 1006), consisting chiefly of water with small quantities of salts. EXCRETION BY THE SKIN 505 neutral fats, volatile fatty acids, and the merest traces of proteins and urea. It is acid to litmus except in profuse sweating, when it may become neutral or even alkaline. It is secreted by simple gland-tubes, which form coils lined with a single layer of columnar epithelium, in the subcutaneous tissue, with long ducts running up to the surface through the true skin and epidermis. Unless col- lected from the parts of the skin on which there are no hairs, such as the palm, it is apt to be r. .xed with sebum, a secretion formed by the breaking down of the cells of the sebaceous glands, which open into the hair follicles, and consisting chiefly of glycerin and cholesterin fats, soaps, and salts. Sebum is probably of consider- able importance for maintaining the normal condition of the hair and skin. Although it is only occasionally that sweat collects in visible amount on the skin, water is always being given off in the form of vapour. This invisible perspiration leaves behind it on the skin, or in the glands, the whole of the non- volatile constituents, which may be to some extent reabsorbed ; and since even the visible per- spiration is in large part evaporated from the very mouths of the glands in which it is formed, the sweat can hardly be considered a vehicle of solid excretion, even to the small extent indicated by its chemical composition. The total quantity of water excreted by the skin, and the relative proportions of visible and invisible perspiration, vary greatly. A dry and warm atmosphere increases, and a moist and cold atmo- sphere diminishes the total, and, within limits, the invisible per- spiration. Visibld sweat — given the condition of rapid heat-produc- tion in the body as in muscular labour — ^is more readily deposited on freely exposed surfaces when the air is moist than when it is dry. The air in contact with surfaces covered by clothing is never far from being saturated with watery vapour. Here, accordingly, a comparatively slight increase in the activity of the sweat-glands suffices to produce more water than can be at once evaporated; and the excess appears as sweat on the skin, to be absorbed by .the clothing without evaporation, or to be evaporated slowly, as the pressure of the aqueous vapour gradually diminishes in con- sequence of diffusion. The power of imbibition (p. 420) of water by the various layers of the skin diminishes as we pass outwards, and the cells of the epidermis are characterized by the rapidity with which they return from a condition of excessive imbibition to their normal state. This constitutes a protective mechanism against excessive loss of water. When the skin is thoroughly moistened, its degree of imbibition is three times the normal. The quantity of sweat given off by a man in twenty-four hours varies so much that it would not be profitable to quote here the' numerical results obtained under different conditions of tempera- 506 EXCRETION ture and humidity of the air (but see p. 666). It is enough to say that the excretion of water from the skin is of the same order of magnitude as that from the kidneys: a man loses upon the whole as much water in sweat as in urine. But it is to be carefully noted that these two channels of outflow are complementary to each other; when the loss of water by the skin is increased, the loss by the kidneys is diminished, and vice versa. The Influence of Nerves on the Secretion of Sweat. — ^The sweat- glands are governed directly by the nervous system; and though an actively perspiring skin is, in health, a flushed skin, the vascular dilatation is a condition, and not the chief cause of the secretion. Stimulation of the peripheral end of the sciatic nerve causes a copious secretion of sweat on the pad and toes of the corresponding foot of a young cat, and this although the vessels are generally constricted by excitation of the vasomotor nerves. Not only so, but when the circulation in the foot is entirely cut off by compres- sion of the crural artery or by amputation of the limb, stimulation of the sciatic still calls forth some secretion. As in the case of the salivary glands, injection of atropine abolishes the secretory power of the sciatic, while leaving its vaso- motor influence untouched ; and pilocarpine increases the flow of sweat by direct stimulation of the endings of the secretory nerves in the glands. That the sweating caused by a high external temperature is normally brought about by nervous influence, and not by direct action on the secreting cells, is shown by the following experiments. One sciatic nerve is divided in a cat, and the animal put into a hot- air chamber. No sweat appears on the foot whose nerve has been cut, but the other feet are bathed in perspiration. Similarly, a venous condition of the blood (in asphjrxia) causes sweating in the feet whose nerves have not been divided, but none in the other foot ; and stimulation of the central end of the cut sciatic has the same effect. All this points to the existence of a reflex mechanism ; and it is certain that asphyxia acts by direct stimulation of the centre or centres. The vaso-motor centre is at the same time stimulated, and the bloodvessels constricted, as in the cold sweat of the death agony. Fear may also cause a cold sweat, impulses passing from the cerebral cortex to the vaso-motor and sweat centres. It is probable that a general sweat - centre exists in the medulla oblongata, but its position has not been exactly determined nor even its existence definitely proved. On the other hand, it is known that in the cat there are at least two spinal centres, one for the fore-limbs in the lower part of the cervical cord, and another for the hind-limbs where the dorsal portion of the cord passes into the lumbar. That this latter centre does not exist or is Comparatively inactive in man is indicated by the following case : A man fell from a window and fractured his backbone at the fifth dorsal vertebra. The lower half of the body was paralyzed for a time, but recovered. Ultimately, however, the EXCRETION BY THE SKIN 507 paralysis returned ; and shortly before his death (twenty-one years after the accident) it was noticed that a copious perspiration broke out several times on the upper part of the body, while the lower portion remained perfectly dry. If there is any functional spinal centre in man, it appears to lie above the fifth spinal segment. For it was seen in a professional diver who fractured his neck at that level, and lived three months after the accident, that sweat frequently appeared on parts of the body above the lesion, but never below. At the autopsy the whole thickness of the cord, except perhaps a small portion of the anterior columns, was found destroyed. Of course, it may be that in man the spinal centres, although normally active, lose their function for a long time after such severe injuries to the cord, owing to the condition known as shock. The secretory fibres for the fore-limbs (in the cat) leave the cord in the anterior roots of the fourth to ninth thoracic nerves. They pass by white rami communicantes to the sympathetic chain, in which they reach the ganglion stellatum, where they are all connected with nerve- cells. Then, as non-meduUated fibres, they gain the brachial plexus by the grey rami, and run in the median and ulnar to the pads of the feet. The fibres for the hind-limbs leave the cord in the anterior roots of the twelfth thoracic to the third or fourth lumbar nerves ; pass by the white rami to the sympathetic ganglia-, in which they form connections with ganglion cells ; then, as non-meduUated fibres, run along the grey rami, and are distributed to the foot in the sciatic. The evidence of the direct secretory action of nerves on the sweat- glands is singularly striking and complete, in contrast to what we know of the kidney. In the latter, blood-flow is the important factor; increased blood- flow entails increased secretion. In the former, the nervous impulse to secretion is the spring which sets the machinery in motion; vascular dilatation aids secretion, but does not generally cause it. It would, however, be easy to lay too much stress on this distinction, for in the horse the mere dilatation ' of the bloodvessels of the head after section of the cervical sympa- thetic has been found to be accompanied by increased secretion of sweat, and urinary secretion can certainly be affected by the direct action of various substances on the secretory mechanism, indepen- dently of vascular changes. But the broad difference stands out clearly enough, and the reason of it lies in the essentially different purpose of the two secretions. The water of the urine is in the main a vehicle for the removal of its solids; the solids of the sweat are accidental impurities, so to speak, in the water. The kidney eliminates substances which it is vital to the organism to get rid of; the sweat-glands pour out water, not because it is in itself hurtful, not because it cannot pass out by other channels, but because the evaporation of water is one of the most important means by which the temperature of the body is controlled. In short, urine is a true excretion, sweat a heat-regulating secretion. No hurtful effects are produced when elimination by the skin is entirely prevented by varnishing it, provided that the increased loss of heat is compensated. A rabbit with a varnished skin dies 5o8 EXCRETION of cold, as a rabbit with a closely-clipped or shaven skin does; suppression of the secretory function of the skin has nothing to do with death in the first case any more than in the second (p. 293). PRACTICAL EXERCISES ON CHAPTER IX. Urine. For most of the experiments human urine is employed — ^in the quantitative work the mixed urine of the twenty-four hours. Urine may also be obtained from animals. In rabbits pressure on the abdomen will usually empty the bladder. Dogs may be taught to micturate at a set time or place, or kept in a cage arranged for the collection of urine. Or a catheter may be used (p. 690). I. Specific Gravity. — Pour the urine into a glass cylinder, and remove froth, if necessary, with filter-paper. Place a ufinometer (Fig. 193) in the urine, and see that it does not come in contact with the side of the vessel. Read off on the graduated stem the division which corresponds with the bottom of the meniscus. This gives the specific gravity. 2. Reaction. — (a) Test with litmus-paper. Generally the litmus is reddened, but occasionally in health the urine first passed in the morning is alkaline. Some- times urine has an amphicroic reaction — i.e., affects both red and blue litmus-paper. This is the case when there is such a relation between the bases and acids that both acid and ' neutral ' (dibasic) phosphates are present in certain proportions. The acid phosphate reddens blue litmus, and the ' neutral ' phosphate turns red litmus blue. (6) Titratable Acidity. — To 25 c.c. of urine add 15 to 20 grammes of powdered potassium oxalate, and one or two drops of a 1 per cent, solution of phenol- phthalein. Shake the mixture rapidly for a minute or two, and then titrate with decinormal sodium hy- droxide at once (while still cold from the solution of the oxalate) till a faint pink colour remains permanent on shaking. The potassium oxalate is added to counteract the tendency of the calcium present in urine to form basic phosphates, which would be precipitated', and the acidity of the urine thus increased (Folin). 3. Chlorides — (a) Qualitative Test. — ^Add a drop of nitric acid and a drop or two of silver nitrate solution. The nitric acid is added to prevent precipitation of silver phosphate. A white precipitate soluble in ammonia shows the presence of chlorides. The precipitate appears to be incompletely soluble in ammonia, since the ammonia brings down a small precipitate of earthy phosphates. (&) Quantitative Estimation. — ^The quantitative estimation of the chlorine in urine without previous evaporation and incineration is best made by one of the modifications of Volhard's method. It depends upon the complete precipitation of the chlorine combined with the alkaline metals, and also of sulphocyanic acid, by silver from a solution con- taining nitric acid in excess ; and avoids the error introduced into simpler methods, like Mohr's, by the union of some of the silver with other substances than chlorine. A given quantity of a standard solution of Fig. 193. — Urin- ometer. PRACTICAL EXERCISES 509 silver nitrate (more than sufficient to combine with all the chlorine) is added to a given volume of urine. The excess of silver is now estimated by means of a standard solution of ammonium sulphocyanide, which precipitates the silver as insoluble silver sulphocyanide. A fairly strong solution of the double sulphate of iron and ammonium (known as iron- ammonia-alum) is taken as the indicator, since a ferric salt does not give the usual red colour with a sulphocyanide so long as any silver in the solution is uncombined with sulphocyanic acid. The iron-ammonia- aluEQ forms the red salt, ferric sulphocyanide, when any excess of ammonium sulphocyanide is present, but it does not react with silver sulphocyanide. The standard solution of silver nitrate can be naade by dissolving 29-065 grammes of pure f use^ silver nitrate in distilled water and making up the volume of the solution accurately to i litre. The solution should be kept in the dark. One c.c. of this solution corresponds to O'Oi gramme NaCl or 0-00607 gramme CI. The standard solution of ammonium sulphocyanide is prepared as follows: Dissolve 13 grammes of pure ammonium sulphocyanide (NH4CNS) in a litre of distilled water. Measure with a pipette into a beaker 20 c.c. of the standard silver nitrate solution, and add 5 c.c. of the iron alum solution and 4 c.c, of pure nitric acid (specific gravity 1-2). Fill a burette with the sulphocyanide solution, and run it into the silver nitrate solution until a faint permanent red tinge is obtained. Note the number of c.c. of the sulphocyanide solution required, and then dilute the solution till 2 c.c. of the sulphocyanide solution corre- spond exactly to i c.c. of the silver solution, so as just to allow of the end reaction with the iron solution being seen, and no more. To carry but the method, put 10 c.c. of urine, which must be free from albumin, in a stoppered flask, with a mark corresponding to 100 c.c. or a graduated cylinder. Add 50 c.c. of water, 4 c.c. of pure nitric acid (specific gravity 1-2), and 15 c.c. of the sta'ndard silver solution; shake well, fill with water to the mark, and again shake. After the precipitate has settled, filter it off. Take 50 c.c. of the filtrate, add 5 c.c. of the solution of iron-ammonia-alum, and run in from a burette the standard solution of ammonium sulphocyanide until a weak but permanent red coloration appears. Suppose X c.c. of the sulphocyanide solution are required, then the chlorine in 10 c.c. of urine evidently corresponds to (15 — x), o'oi gramme NaCl. 4. Phosphates — (i) Qualitative Tests. — [a) Render the urine alkaline with ammonia. A precipitate of earthy phosphates (calcium and mag- nesium phosphates) falls down. Filter. The filtrate contains the alkaline phosphates. To the filtrate add magnesia mixture.* The alkaline phosphates (sodium, potassium, or ammonium phosphates) are precipitated as ammonio-magnesic or triple phosphate. (6) Add to uriue half its volume of nitric acid and a little molybdate of ammonium, and heat. A yellow precipitate of ammonium phospho-molybdate shows that phosphates are present. This test is given both by alkaline and earthy phosphates. (2) Quantitative Estimation. — The quantitative estimation of phos- phoric acid in urine is best done volumetrically, by titration with a standard solution of uranium nitrate, using ferrocyanide of potassium as the indicator. Uranium nitrate gives with phosphates, in a solution containing free acetic acid, a precipitate with a constant proportion of * Magnesium chloride no grammes, ammonium chloride 140 grammes, ammonia (specific gravity o'gi) 250 c.c, and water 1,750 c.c. 510 EXCRETION phosphoric acid. As soon as there is more uraniuta in the solution than is required to combine with all the phosphoric acid, a brown colour is given with potassium ferrocyanide, due to the formation of uranium ferrocyanide. In carrying out the method, 5 c.c. of a mixture of acetic acid and sodium acetate (there are 10 grammes of sodium acetate and 10 grammes of glacial acetic acid in 100 c.c. of the mixture) are added to 50 c.c. of urine, which is then heated in a beaker on the water-bath almost to boiling. The standard uranium solution (which contains 35'5 grammes of uranium nitrate in the litre, and i c.c. of which corre- sponds to 0-005 gramme PaOs) is now run in from a burette, until a drop of the urine gives, with a drop of potassium ferrocyanide solution, on a porcelain slab, a brown colour. Uranium acetate may be used instead of uranium nitrate, but the latter keeps best. When uranium acetate is employed it is not necessary to add the sodium acetate mixture. 5. Sulphates — (i) Qualitative Test. — ^Add to urine a drop of hydro- chloric acid and then a few drops of barium chloride. A white pre- cipitate comes down, showing that inorganic sulphates are present. The hydrochloric acid prevents precipitation of the phosphates. (2) Quantitative Estimation of the Sulphates [Inorganic and Ethereal). — ^Add to 50 c.c. of albumin-free urine in a 200-c.c. Erlenmeyer flask 5 c.c. of a 4 per cent, potassium chlorate solution and 5 c.c. of strong hydrochloric acid, and boil the mixture to break up the ethereal sul- phates. In five to ten minutes it becomes perfectly colourless. While it continues to boil, 25 c.c. of a 10 per cent, solution of barium chloride are added by drops, at such a rate that it takes about five minutes to add this quantity. The flask is now put on the water-bath for one-half to one hour, till the precipitate has settled. Then filter through an ash-free filter. Wash the precipitate on the filter for haU an hour with hot water. During the first twenty minutes of the washing, at intervals of a few minutes, substitute hot 5 per cent, ammonium chloride solution for the water. At the end of the half-hour's washing, as soon as the water has run through the filter, fold up the latter and press it gently between dry filter-papers to remove a portion of the water. Then place the filter in a weighed porcelain crucible. Pour into the crucible 3 or 4 c.c. of alcohol, and ignite it, to dry and partially bum the filter-paper. Then incinerate till all the carbon is burned ofi, cool, and weigh. From the weight of the barium sulphate, the sulphuric acid in 50 c.c. of urine is easily calculated (SO4 in i gramme of barium sulphate, 0'4ii87 gramme) (Folin). (3) Quantitative Estimation of the Sulphuric Acid united with Aromatic Bodies {Aromatic or Ethereal Sulphates). — Put 200 c.c. of the same urine as used in (2) into a beaker. Add 100 c.c. of 10 per cent, barium chloride solution in the cold. Let stand for twenty-four hours. Then decant ofi the clear supernatant liquid, and filter it. Measure 150 c.c. of the clear filtrate, corresponding to 100 c.c. of the urine, into a 400-c.c. Erlenmeyer flask. Add 10 or 15 c.c. of concentrated hydrochloric acid and 10 to 15 c.c. of 4 per cent, potassium chlorate. Heat the mixture to boiUng, and proceed as in (2). From the weight of the barium sul- phate, the ethereal sulphuric acid in 100 c.c. of urine can be calculated. Deducting this from the quantity per 100 c.c. of urine obtained in (2), we get the amount of inorganic sulphuric acid per 100 c.c. (Folin). 6. Indoxyl (contained in the urine as indican, the potassium salt of indoxylrsulphuric acid) can be oxidized into indigo, and so detected and estimated. A qualitative test is the following: Ten c.c. of horse's urine is mixed with 10 c.c. of Obermayer's reagent (pure concentrated hydrochloric acid containing 2 to 4 parts of ferric chloride in 1,000), and shaken well PRACTICAL EXERCISES 511; for a minute or two; a bluish colour appears if, as is generally the case, indoxyl is present, indigo (Cj^6HioN202) being formed by the oxidizing action of the ferric chloride on the indoxyl, the compound of which with sulphuric acid has been broken up by the hydrochloric acid. The urine must be free from albumin. In performing the test in human urine, which contains a smaller quantity of the indigo-forming sub- stance, the faint blue liquid should be shaken up with a few drops of chloroform. The latter takes up the colour, which is thus rendered more evident. If there is difficulty in obtaining the reaction, the urine may first be decolorized by precipitating it with acetate of lead, avoiding excess. The precipitate is filtered off, and the test then applied to the clear filtrate. The skatoxyl of urine can alsij) be oxidized to indigo, but it is present in far smaller amount. The average quantity of indigo obtained from a litre of horse's urine is about 150 milligrammes ; from a litre of human urine, not a twentieth of that amount. For comparative quantitative determinations the method of Folin may be used. One-hundredth of the twenty-four hours' urine is taken. In this the indigo is developed by the addition of an equal volume of Obermayer's reagent (p. 510), and the indigo-blue dissolved by means of 5 c.c. of chloroform. The chloroform solution is then compared colorimetrically with Fehling's solution. This can be done by putting the indigo solution and 5 c.c. of the Fehling's solution respectively into small test-tubes of equal calibre, and comparing the depth of tint. If the Fehling's solution is stronger than the indigo solution, run water into the former from a pipstte, graduated in tenths of a c.c, shaking up after each addition, till equality of tint has been reached. If the indigo solution has a stronger blue colour than the Fehling's solution, dilute a measured amount of it first of all with such a quantity of chloroform (say an equal volume) as will make its tint distinctly weaker than that of the Fehling's solution. Then dilute the Fehling's solution with water, as before, till the tint is the same. From the amount of dilution the quantity of indigo can be expressed in arbitrary units, taking Fehling's solution as 100. Thus, if I c.c. of water must be added to the 5 c.c. of Fehling's solution, the indican can be expressed as = 5^ = 83. The comparison can be made more accurately by 5 a colorimeter, if one is available. To determme the absolute amount of indigo obtained, comparison must be made with a standard solution of indigo. 7. Urea — (i) Decomposition of Urea. — ^Heated dry in a test-tube, it gives ofi ammonia. The residue contains biuret, which, when dissolved in water, gives a rose colour with a trace of cupric sulphate and excess of sodium hydroxide (or of the hydroxides of certain other metals of the alkalies and alkaline earths (p. 8) . Some proteins — peptones and albumoses — in the presence of the same reagents, give a similar colour, the so-called biuret reaction. (2) Quantitative Estimation — Folin s Method. — -Put 3 c.c. of urine, 20 grammes of magnesium chloride, and 2 c.c. of concentrated hydro- chloric acid into an Erlenmeyer flask of 200 c.c. capacity fitted with a short backflow tube (200 mm. long and 10 mm. in diameter). Add a small piece of paraf&n to prevent foaming. Boil briskly, and then continue boiling moderately for forty-five to sixty minutes. Now cautiously dilute the mixture with water and wash it into a litre flask. Add about 7 c.c. of a 20 per cent, solution of sodium hydroxide, and distil ofi into decinormal acid. Usually about 350 c.c. of water should 512 EXCRETION be distilled off, which takes about sixty miautes. Then titrate the acid with decinormal alkali (sodium hydroxide). Deduct from the number of c.c. of acid taken the number of c.c. of the decinormal alkali needed to neutrahze it. The difference gives the number of c.c. of decinormal ammonia which passed into the acid. Each c.c. of deci- normal ammonia contained in the distillate corresponds to 3 mg., or o-i per cent, of urea. Corrections for the ammonia content of the mag- nesium chloride used, as well as for preformed ammonia in the urine, are made separately. A less exact method which is very rapid, and is therefore much used in clinical determinations, is the Hypobromiie Method. The urea is split up by sodium hypobromite {p. 474), and the carbon dioxide being absorbed by the excess of sodium hydroxide used in preparing the hypobromite, the nitrogen is collected over water in an inverted burette. It is easy to cal- culate the weight of urea corresponding to a given volume of nitrogen measured at a given temperature and pressure. The nitrogen of urea is f§, or ^ of the whole molecular weight. Now, i c.c. of N weighs, at 760 millimetres of mercury and 0° C, o-ooi25 gramme. Therefore, i c.c. of N corresponds to 0-00125 ^ ^= 0-00268 gramme urea. Suppose, now, that i c.c. of urine was found to yield 10 c.c. of N measured at 17° C. and 750 millimetres barometric pressure. Since a gas expands ^5 part of its volume at 0° for every degree above 0°, we must correct the apparent volume of nitrogen by multiplying by f|^. Since the volume of a gas is inversely propor- tional to the pressure, we must further multiply by |fg. Thus we get 10 x f^ ^flS = ^W^=9'29 c.c. as the volume of the nitrogen reduced to 0° C. and 760 mUhmetres of mercury. Multiplying this by 0-00268, we get 0-0249 gramme urea for i c.c. urine, which for a daily yield of 1,200 c.c. would correspond to 29-88 grammes urea. As a matter of fact, however, it has been found that there is always a de- ficiency of nitrogen — that is, a given quantity of urea yields less than the estimated amount of gas. A gramme of urea in urine, instead of giving ofE 373 c.c. of nitrogen, gives only 354 c.c. at 0° C. and 760 millimetres pressure. We must therefore take i c.c. of N as correspond- ing to 0-00282 gramme, instead of 0-00268 gramme urea. But it is affectation to make this correction if, as is seldom done in hospitals, the temperature is not taken into account. A convenient apparatus is shown in Fig. 194. In B place 10 c.c. of a solution made by adding bromine to ten times its volume of 40 per cent, sodium hydroxide solution. Mix 5 c.c. of urine with 5 c.c. of water. Put 5 c.c. of the mixture into the thimble A, which is then set in the small bottle B. The cork is now carefully fixed in B, and the tube F being open, the level of the water in the burette is read off. Fig. 194. — Hypobromite Method of estimating Urea. A, glass thimble; B, bottle, through the rubber cork of which pass two short glass tubes, one connected by the rubber tube C with a burette D, and the other armed with a short piece of rubber tube F. F is provided with a pinch- cock. The burette is supported on a stand, and immersed in water contained in the glass cylinder E. PRACTICAL EXERCISES 513 The pinchcock having been closed, the bottle B is now tilted so that the urine in the thimble is gradually mixed with the hypobromite solution, and the nitrogen given off is added to the air in the burette and its connections. The level of the water in the burette is therefore depressed. When gas ceases to be given off, and a short time has been allowed for the whole to cool, the tube is raised till the level of the water is once more the same inside and out. The level is again read off; the difference of the two readings gives the volume of nitrogen at the temperature of the air and the barometric pressure. In order that the temperature of the water may be the same as that of the air, the cylinder should be filled a considerable time before the observations are begun. For most clinical purposes sufficiently accurate results may be very easily obtained with the so-called ureometer of Doremus (Fig. 195). A little urine is poured into the side-tube A, the stopcock C being closed. The stopcock is then opened for an instant, so as to fill its bore, and then closed again. Any urine which has passed into the tube B is washed out with water, and B is then filled with hypobromite solution. A is now filled up with urine to the top of the graduation. By opening the stopcock, i c.c. of urine (or less if the urine is concentrated) is per- mitted to pass into B and to mix with the hypo- bromite solution. The nitrogen collects in B, and when it has ceased to come off, the meniscus of the liquid is read off. The corresponding degree on the scale gives the amount of urea in grammes contained in the quantity of urine employed. 8. Estimation of the Ammonia in Urine (Folin's Method). — Ammonia is liberated by addition of a weak alkali (sodium carbonate). Then the am- monia is driven out at ordinary temperature by a strong current of air and taken up in decinormal acid, which is then titrated with decinormal alkali. The apparatus employed consists of — (i) A cylinder of about 45 cm. height and 5 cm. diam- eter, with a rubber stopper through which pass two glass tubes. One of the tubes goes nearly to the bottom of the cylinder, and the other end is connected, through a U-tube filled with cotton, with a tube containing sulphuric acid. The s.cond tube is cut off short below the rubber cork, and its other end is connected, through a U-tube containing cotton, with a sulphuric acid tube (or with two in series). (2) A water-pump to draw or force air through th3 apparatus (600 to 700 litres in an hour) . Put into the first sulphuric acid tube 25 c.c, into the second 10 c.c. decinormal acid and some water; into the cylinder 25 c.c. of filtered urine, 8 to 10 grammes sodium chloride, 5 to 10 c.c. of petroleum or toluol to prevent foaming, and last of all i gramme dried sodium carbonate. At once close the cylinder and allow a strong stream of air to pass through the apparatus. At a temperature of 20° to 25° (room temperature), and using 600 to 700 litres of air an hour, all the ammonia is in the sulphuric acid in one to one and a half hours. The contents of the sulphuric acid tubes are put into a beaker and titrated with decinormal alkali, using lacmoid (litmoid) or rosolic acid as indicator. Deduct the number of c.c. of alkali used from the number of c.c. of the decinormal acid originally taken, and multiply the remainder by i"7034 to get the quantity of ammonia in milligrammes. The method can be employed also for albuminous urine. Fig. 195. — Doremus Ureometer. 514 EXCRETION g. Estimation of the Total Nitrogen. — It is sometimes more important to determine the total nitrogen of the urine than the urea alone. This is conveniently done by Kjeldahl's method (or some modification of it), which can also be applied to the estimation of the nitrogen in the faeces, or in any of the solids or liquids of the body. It depends on the oxidation of the nitrogenous matter (or, rather, in the case of urine, mainly its hydrolysis) in such a way that the nitrogen is all represented as ammonia.". The ammonia is then distilled over, collected and esti- mated, and from its amount the nitrogen is easily calculated. In urine the method can be carried out by adding to a measured quantity of it (say 5 c.c.) four times its volume of strong sulphuric acid, and boiling in a long-necked flask (capacity 200 c.c), after the addition of a globule of mercury '(about o-i C.c), which hastens oxidation and obviates bumping. A pj.rt of the mercuric sulphate formed remains in solution ; the rest forms a crystalline deposit. The heating should continue for haU an hour, or until the liquid is decolourized. It should be kept gently boiling. This completes the process of "oxidation ; and the next step is to liberate the ammonia from the substances with which it is united in the solution, and to distil it over. Dilute the liquid with water, after cooling, up to about 150 c.c, and pour into a larger long- necked flask. Add enough of a solution of sodium hydroxide (specific gravity about i-is) to render the liquid alkaline, avoiding excess, as this favours bumping. The proper quantity can be found by deter- mining beforehand how much of uhe alkali is needed to neutralize the acid used for oxidation, and a little more than this amount should be added. Twenty c.c of strong sulphuric acid needs about 75 c.c. of 40 per cent, sodium hydroxide to neutralize it. Bumping may further be prevented by the addition of a little granulated zinc Shake the flask two or three times. Add also about 12 cc of a concentrated solution of potasEfium sulphide (i part to ij parts water), which favours the setting tree of the ammonia from the amino-compounds of mercury that have been formed during oxidation . Commercial ' liver of sulphur ' will do quite well. Immediately connect the distilling-flask with a worm or Liebig's condenser, and distil the ammonia over into 50 cc. of standard (decinormal) sulphuric acid (see footnote, p. 473) con- tained in a flask into which a glass tube connected with the lower end of the worm dips. Heat the distilling-flask at first gently, then strongly, and boil for three-quarters of an hour, or until about two-thirds of the liquid has paissed over. Then lift the tube out of the standard acid, and continue the distillation for two or three minutes longer. The ammonia is now all united with the standard acid, a certain amount of which is left over. By determining this amount we arrive at the quantity com- bined with ammonia, and therefore at the quantity of ammonia. Fill a burette with a decinormal solution of potassium or sodium hydroxide. Add a little methyl-orange splution to the standard sulphuric acid, to serve as indicator. Then run in the potassium or sodium hydroxide till the pink tinge gives place to a permanent but just recognizable yellow. Let x he the number of c.c. run in. Since i c.c. of any deci- normal solution is equivalent to i c.c. of any other, x represents also the number of cc. of the standard sulphuric acid left uncombined with ammonia; and 50— x, the quantity combined with ammonia. Then, I c.c. of decinormal sodium or potassium hydroxide being equivalent to 1 c.c. of decinormal ammonium hydroxide, and i c.c of decinormal ammonium hydroxide' containing 0'00i4 gramme nitrogen, we get (50 — Ar)x 0-0014 as the quantity of nitrogen in 5 c.c. of urine. Instead of mercury, potassium sulphate and copper sulphate may be added to the sulphuric acid in order to aid the decomposition in the PRACTICAL EXERCISES 515 first stage of the estimation. About 3 grammes of potassium sulphate and 1 gramme of copper sulphate are added to 5 c.c. of urine, and then 5 c.c. of sulphuric acid. The liquid is gently boiled for an hour, or until it is quite clear. The neutralization and distillation are conducted as before, the proper quantity of sodium hydroxide being determined in advance. No potassium sulphide is added, but a small quantity of talc may be put in to prevent bumping. Instead of methyl orange, ' alizarin red,' which is bright red in the presence of the slightest trace of alkali, may be used. 10. Uric Acid^— (i) Qualitative Test for Uric Acid — Murexide Ted. — A small quantity of uric acid or one of its salts is heated with a little dilute nitric acid. The colour of the residue left by evaporation becomes yellow, and then red, and on the addition of ammonia changes to deep purple-red. Potassium or sodium hydroxide changes the yellow to violet. In the reaction alloxantin is formed by oxidation of the uric acid. When ammonia acts on alloxantin it is changed into purpuric acid, and this into its ammonium purpurate, the purple-red substance called murexide. Thus: CgHeN^Og -t- NH3 =C8H6N60e+ 2HaO. Alloxantin. Furpu')ric Acid. The reaction is also given by theobromine (diingthylxanthin), an alkaloid in cocoa, and theine or caffeine (trimethylxanthin), an alkaloid in tea and coffee, which are also purin derivatives (p. 475). (2) Quantitative Estimation — Folin's Modification of Hopkins's Method. — ^The chief reagent is a solution of 500 grammes ammonium sulphate, 5 grammes uranium acetate, and 60 c.c. 10 per cent, acetic acid, in 650 c.c. of water. One hundred and fifty c.c. of urine is measured into a tall, narrow beaker or a cylinder, and 37^ c.c. of the reagent added. If enough urine is available, 200 c.c. of urine and 50 c.c. of reagent are to be used. Allow the mixture to stand without stirring for about half an hour. The uranium precipitate has then settled, and the clear supernatant liquid is removed by siphoning or decantation. One hundred and twenty-five c.c. of this liquid is measured into another beaker, 5 c.c. of strong ammonia added, and the mixture set aside till next day. The precipitate is then filtered off, and washed with 10 per cent, ammonium sulphate solution until the filtrate is quite or nearly free from chlorides. The filter is then removed from the ftmnel, opened, and the precipitate rinsed back into the beaker. Enough water to make about 100 c.c. is added, and the precipitate is then dissolved by means of 15 c.c. con- centrated sulphuric acid, and at once titrated with f^ (one-twentieth normal) potassium permanganate solution (made by dissolving 1-581 grammes of the permanganate in a litre of water), each c.c. of which corresponds to 3-75 milligrammes of uric acid. The very first pink coloration, extending through the entire liquid on the addi- tion of two drops of permanganate solution, marks the end point. A correction of 3 milligrammes, owing to the solubility of ammonium urate, is added to the result. II. Kreatinin. — Qualitatively, kreatinin may be recognized in very small amounts by Weyl's test. A few drops of a dilute solution of sodium nitro-prusside are added to urine, and then dilute sodium hydroxide drop by drop. A ruby-red colour appears, which soon.tums yellow. If the urine is now strongly acidified with acetic acid and heated, it becomes first greenish and then blue. Enough acid must be added to more than neutralize the alkali. Another test which has been made the basis of a quantitative method 5i6 EXCRETION by Folin is Jaffa's test. A little urine (say 5 c.c.) is put in a test-tube, and then a solution of picric acid in water. The mixture is rendered alkaline by the addition of potassium or sodium hydroxide solution, and a reddish colour is produced, which turns yellow on the addition of acid. A similar red colour is given by dextrose, but not unless the solution is heated. Quantitative Estimation of Kreatinin by Folin' s Method. — It depends upon the comparison of the colour which kreatinin gives with picric acid in an alkaline solution with that of a standard solution of potassium bichromate. Ten c.c. of urine is measured into a 500 c.c. measuring- flask; 15 c.c. of a saturated picric acid solution (containing about 12 grammes per Utre) and 5 c.c. of a 10 per cent, solution of sodium hydroxide are added. The mixture is allowed to stand for five minutes. Then water is added up to the 500 c.c. mark, and the flask shaken to mix imiformly. Samples of the liquid are then at once compared colorimetricaUy with a half-normal solution of potassium bichromate containing 24-55 granunes per litre. The colour of the urine does not introduce a sensible error on account of the great dilution. For exact work the comparison must be made with a good colorimeter. It has been found experimentally that, when 10 miUigrammes of kreatinin are present in 500 c.c. of a solution made as described, a layer of the solution 8-1 millimetres in thickness has the same depth of tint as 8 milli- metres of the bichromate solution. Suppose it takes 9 millimetres of the urine-picrate solution to equal 8 millimetres of the bichromate, 8*1 then the 10 c.c. of urine contains 10 x — =9-0 milligrammes of kreatinin. 12. Hippuric Acid. — ^From horse's or cow's urine hippuric acid is prepared by evaporating to a small bulk, and adding strong hydrochloric acid. The crystalline precipitate is washed with cold water, then dissolved in hot water, and filtered hot. Hippuric acid separates out from the filtrate in the cold in the form of long four-sided prisms with P5rramidal ends. Heated dry in a test-tube, the crystals melt, and benzoic acid and oily drops of benzonitrile, a substance with a smell like that of oil of bitter almonds, are formed. ABNORMAL SUBSTANCES IN URINE. 13. Proteins — (i) Qualitative Tests. — (a) Boil and add a few drops of nitric acid. A precipitate on boiling, increased or not afiected by the acid, shows the presence of coagulable proteins (serum-albumin or globulin). A precipitate of earthy phosphates sometimes forms on boiling. It is distinguished from a precipitate of proteins by dissolving on the addition of acid. (6) Heller's Test. — Put some nitric acid in a test-tube. Pour care- fully on to the surface of the acid a Uttle urine. A white ring at the junction of the liquids indicates the presence of albumin or globulin. If much albumose is present, a white precipitate, which disappears on heating, may be formed. When this test is performed with undiluted urine, uric acid may be precipitated and cause a brown colour at the junction. A similar ring may be found in the absence of proteins when the test is made on the urine of a patient who has been taking copaiba. In very concentrated uiine a white ring of nitrate of urea may be formed. A coloured ring is frequently seen, owing to the oxidation of certain chromogens of urine. (c) Filter some urine, and add to the filtrate its own volume of acetic acid. A precipitate may indicate mucin or nucleo-albimiin. If any is PRACTICAL EXERCISES 517 formed, filter it ofi, and add to the filtrate a few drops of potassium ferrocyanide. A white precipitate shows the presence of proteins. {d} Test for Globulin in Urine. — Serum-globulin probably never occurs in urine apart from serum-albumin. It may be detected thus: Make the urine alkaline with ammonia, let it stand for an hour, and filter. Half saturate the filtrate with ammonium sulphate — i.e., add to it an equal volume of a saturated solution of ammonium sulphate. Ssrum-globulin is precipitated, serum-albuniin is not. (e) Test for Albumose in Urine (^ /fiMmosMna). ^-Coagulable proteins are removed by boiling the urine (acidulated if necessary), and filtering off the precipitate if any. The filtrate is neutralized. If a further precipitate falls down it is filtered off, the clear filtrate is heated in a bsaker placed in a boiling water-bath, and there saturated with crystals of ammonium sulphate. A precipitate indicates that albumoses (proteoses) are present. A slight precipitate might possibly be due to the formation of ammonium urate. A further test may be performed on the original urine if it is free from coagulable proteins, or on the filtrate after their removal. Add a drop or two of pure nitric, acid. If albumoses are present, a precipitate is thrown down which disappears on heating, and reappears on cooling the test-tube at the cold-water tap. (2) Quantitative Estimation of Coagulable Proteins [Serum-Albumin and Globulin) — (a) Gravimetric Method. — ^Heat 50 to 100 c.c. of the; urine to boiling, adding a dilute solution (2 per cent.) of acetic acid by drops as long as the precipitate seems to be increased. Filter through a weighed filter. Wash the precipitate on the filter with hot water, then with hot alcohol, and finally with ether. Dry in an air-bath at 110° C, and weigh between watch-glasses of known weight. (6) Esbach's Method. — Esbach's reagent is made by dissolving 10 grammes of picric acid and 20 grammes of citric acid in boiling water (800 or 900 c.c), and then making up the volume to a litre. The so-called albuminimeter is simply a strong glass tube graduated and marked in a certain way. Fill the tube up to the mark U with the urine. Then add the reagent up to the mark R. Close the tube with the rubber cork, and invert it a dozen times without shaking. Set the tube aside for twenty-four hours, then read off the graduation on the tube which corresponds with the top of the precipitate. The figures indicate the number of grammes of dry protein in a litre of the urine. Suppose the top of the sediment is at 4, this will indicate 4 grammes per litre, or 0-4 per cent. The method is of some clinical importance, owing to its simplicity, although it is, of course, not very accurate. 14. -'Sugar — (i) Qualitative Tests — (a) Trommer's Test (p. 10). — It is to be remarked that some substances present in small amount in normal urine reduce cupric sulphate — e.g., uric acid (present as urates) and kreatinin — -but although a normal urine may thus decolourize the copper solution, it rarely causes so much reduction that a yellow or red precipitate is formed, as is the case in diabetic urine. Glycuronic acid (p. 476) also reduces cupric salts, as does alcapton or homogentisinic acid, a substance found in rare cases in disease (p. 477). (6) Fehling's Test. — Fehling's solution (p. 518) is brought to the boil in a test-tube, a little of the urine then added, and the change of colour noted. Benedict's modification of Fehling's solution may also be used. It has the advantage that it keeps indefinitely, and therefore is always ready for use, and is also said to be more delicate. (c) Phenyl-Hydrazine Test. — ^This test depends upon the fact that phenyl-hydrazine forms with sugars such as glucose (dextrose), maltose, isomaltose, etc., but not with cane-sugar, characteristic crjrstalline substances (phenyl-glucosazone, phenyl-maltosazone, etc.) which can 5i8 EXCRETION be recognized under the microscope, and are distinguished from each other by melting at difiererit temperatures. Phenyl-glucosazone (CjgH2aN404) melts at 205° C. To perform the test for dextrose in the urine, proceed thus: Put 5 c.c. of unne in a test-tube, add i decigrarame of hydrochlorate of phenyl-hydrazine and 2 decigrammes of sodium acetate. It is sufficiently accurate to add as much phenyl-hydrazine as will lie on a sixpence (or a dime) and twice as much sodium acetate. Heat the test-tube in a boihng water-bath for half an hour. Then cool at the tap and examine the deposit under the microscope for the yellow phenyl-glucosazone crystals (Fig. 196). Sometimes the osazone pre- cipitate is amorphous. If this should be the case, the precipitate, if no crystals can be seen, must be dissolved in hot alcohol. The solution is then diluted with water and the alcohol boiled off, when the osazone, if any be present, will crystallize out. Very minute traces of sugar can be detected in this way (as little as o-i per cent, in urine) . Often in normal urine yellow crystals are deposited during the first fifteen minutes' heating. They must not be mistaken for gluco- sazone. They probably consist of a compound of glycuronic acid and phenyl-hydrazine. They are changed as the heating goes on into an amor- phous brownish - yellow precipitate (Abel). (d) The Yeast Test is an important confirmatory test for distinguishing the fermentable sugars from other re- ducing substances, but it is not very delicate, and will with difficulty detect sugar when less than 0-5 per cent, is present. It can be performed thus: A little yeast (the tablets of com- pressed yeast do very well) is added to a test-tube half filled with uriae. The test-tube is then filled up with mercury, closed with the thumb, and inverted over a dish containing mer- cury. The dish may be placed on the top of a water-bath whose temperature is about 40° C. After twenty-four hours the sugar will have been broken up into alcohol and carbon dioxide. The latter will have collected above the mercury in the test-tube, and the former will be present ia the urine. The tests for sugar will either be negative or will be less distinct than before. A control test-tube containing water and yeast should also be set up, as impurities in the yeast sometimes yield a small amount of carbon dioxide. Specially-constructed tubes are also often used for performing the test. (2) Quantitative Estimation of Sugar in Urine. — (a) V olumetrically , the sugar can be estimated by titration with Fehling's solution. As this does not keep well, two solutions containing its ingredients should be kept separately and mixed when required. Solution I.: Dissolve 34-64 grammes pure cupric sulphate in distilled water, and make up the volume to 500 c.c. Solution II.: Dissolve 173 grammes Rochelle salt in 400 c.c. of water, add to this 5i'6 grammes sodium hydroxide, and Fig. 196. — Phenyl-Gluoosazone and Phenyl-Maltosazone Crystals (Mac- leod). The phenyl - glucosazone crystals are in the upper part of the figure, the phenyl-maltosazone in the lower. PRACTICAL EXERCISES 5i9 make up the volume with water to 500 c.c. Keep in well-stoppered bottles m the dark. For use, mix together equal volumes of the two solutions. Ten c.c. of this mixture is reduced by 0-05 gramme dextrose. To estimate the sugar in urine, put 10 c.c. of the mixture into a porcelain capsule or glass flask, and dilute it four or five times with distilled water. Dilute some of the urine, say ten or twenty times, according to the quantity of sugar indicated by a rough determination. Run the diluted urine from a burette into the Fehling's solution, bringing it to the boil each time urine is added, until, on allowing thfe precipitate to settle, the blue colour is seen to have entirely disappeared from the supernatant liquid. The observation of the colour must be made while the liquid is still hot. Benedict's modification of Fehling's solution* may also be employed. Suppose that 10 c.c. of Fehling's solution is decolourized by 20 c.c. of the ten-times diluted urine. Then 2 c.c. of the original urine contains 0-5 gramme dextrose. If the urine of the twenty-four hours (from which this sample is assumed to have been taken) amounts to 4,000 c.c, the patient will have passed 0-05x2,000= 100 grammes sugar, in twenty-four hours. (6) The polarimeter affords a rapid and, with practicfe, a delicate means of estimating the quantity of sugar in pure and colourless solu- tions, but diabetic urine must in general be first decolourized by adding lead acetate and filtering off the precipitate. What is measured is the amount by which the plane of polarization of a ray of polarized light of given wave-length (say sodium light) is rotated when it passes through a layer of the urine or other optically active solution of known thickness. Let a be the observed angle of rotation, / the length in decimetres of the tube containing the solution, w the number of grammes of the optically active substance per c.c. of solution, and {a)r, the specific rotation of the substance-for light of the wave-length of the part of the spectrum corresponding to the D line {i.e., the amount of rotation expressed in degrees which is produced by a layer of the substance I decimetre thick, when the solution contains i gramme of it per c.c). Then {a)i, = '±--.. In this equation a and / are known from direct measurement; {a)^ has been determined once for all for most of the important active substances, and therefore w is easily calculated. For dextrose (a)o may be taken as 52-6°. It varies somewhat with the concentration, but for most investigations on the urine these variations may be neglected. It is net possible to describe here the numerous forms of polarimeter that are in use. Those constructed on what is called the ' half-shadow ' system (Fig. 197) give sufficiently satisfactory results. A half-shadow polarimeter consists, like other polarimeters, of a fixed Nicol's prism (the polarizer), and a nicol capable of rotation (the analyzer). In addition, there is an arrangement which rotates by a definite angle the plane of polarization in one-half of the field, but not in the other — e.g., a small nicol occupying only half of the field. In the zero position of the analyzer, both halves of the field are equally dark. The solution to be investigated is placed in a tube of known length, the ends of which * It contains I7'3 grammes of cupric sulphate, lys'o grammes of sodium citrate, lod'o grammes of anhydrous sodium carbonate made up with distilled water exactly to one litre. In making the solution the citrate and carbonate are dissolved with the aid of heat in aUout 600 c.c. of water, and then made up to about 800 c.c. The cupric sulphate is dissolved in about 100 or 150 c.c. of water and added to the other solution, the whole being then made up to a litre. 520 EXCRETION are closed by glass discs secured by brass screw caps. The glass_ discs must be slid on, so as to exclude all air. The tube having been intro- duced between the polarizer and analyzer, the sharp vertical line which indicates the division between the two half-fields is focussed with the eye-piece, and then the analyzer is rotated till the two halves are again equally shadowed. The angle of rotation, a, is read off on the graduated arc, which is provided with a vernier. Pentoses reduce Fehling's solution, but do not give the yeast test. They give the following characteristic tests, which may be performed with gum arable, a substance containing arabinose, one of the pentoses: (i) Phloroglucin Reaction. — Warm in a test-tube some pure concen- trated hydrochloric acid to which an equal volume of distilled water has been added. Add phloroglucin until a little remains undissolved. Fig. 197. — Mitscherlich's Polarimeter. (Half-shadow instrument.) (Simple form ) ' Add a small quantity of gum arable, and keep the test-tube in a water- bath at 100° C. The solution becomes cherry-reS, and a precipitate gradually separates, which may be dissolved in amyl alcohol. The solution shows with the spectroscope a band between D and E. (2) Orcin Reaction. — Use orcin instead of phloroglucin in (i). The solution becomes reddish-blue on warming, and shows a band between C and D, near D. The colour quickly changes from violet to blue, red, and finally green. A bluish-green precipitate separates, which is soluble in amyl alcohol. Glycuronic acid gives all the above reactions of pentoses. Bile-Salts (Hay's Test). — Put a little finely-divided sulphur, in the form of flowers of sulphur, on the top of a glass of urine. If bile-salts are present the sulphur will sink to the bottom. If there are no bile- PRACTICAL EXERCISES Szi salts it will float on the top. The difference is due to an alteratiop in the surface tension of the urine produced by the bile-salts. We must exclude the presence of acetic acid, alcohol, ether, chloroform, turpen- tine, benzine and its derivatives, phenol and its derivatives, anilin arid soaps, all of which also cauSe such an alteration in the surfat* teiision of urine that the sulphur sinks to the bottom. The urine should be fresh, and if it has to be kept it should be preserved from decomposition by cyanide of mercury, which does not alter the surface tension. The reaction has the great advantage over other tests of being easily carried out at the bedside. Acetone — (i) Legal's Test (Rothera's modification). — To 5 to 10 c.c. of the acetone-containing urine add enough ammonium sulphate crystals to form a layer at the bottom of the test-tube, then 2 or 3 drops cf a fresh 5 per cent, solution of sodium nitro-prusside and i to 2 c.c; of strong ammonia. The development of a colour like that of perman- ganate of potassium, often in the form of a ring a little above the undissolved salt, indicates the presence of acetone. The reaction must not be declared negative till half an hour has elapsed. The colour slowly fades. (2) Where there is doubt as to the presence of acetone, it is best first to distil it over. Put 250 to 500 c.c. of the urine suspected to contain acetone into a litre flask.. Add a few c.c. of phosphoric acid; connect the flask with a worm, and distil over the urine int& a small flask. For qualitative tests it is best to collect only the flfSt 20 to 30 c.c, as most of the acetone is contained in this. Test the distillate for acetone by (i) or by Lieben's Test. — To a few c.c. of the distillate in a test-tube add a few drops of solution of iodine in potassium iodide, and then sodium or potassium hydroxide. A precipitate of yellow iodoform crystals (six- sided tables) is thrown down if acetone be present. Examine them under the microscope. On heating, the odour of iodoform may be recognized. If the precipitate is amorphous it may be dissolved in ether (free from alcohol), which is allowed to evaporate on a slide, when crystals may be obtained. Determination of the Freezing-Point of Urine. — Study Beckmann's apparatus shown in Eig. 171, p. 421. Note the large thermometer D graduated in hundredths of a degree centigrade. It is inserted through a rubber cork into the inner thick test-tube A. A platinum wire F, bent at the lower end into a circle or a spiral, which passes easily up and down between the bulb of the thermometer and the tube, serves to stir the urine. The thermometer must be so supported by the rubber cork that the bulb is in the axis of the tube and a centimetre or two from the bottom of it. The side-piece E on the tube A is not absolutely necessary, but it is convenient for ' inoculating ' the urine with a crystal of ice at the proper time. A passes through a rubber cork into a shorter and wider outer glass tube B. The space between A and B serves as a badly conducting mantle, which prevents too rapid cooling of the contents of A. B passes through a hole in the metal or wooden cover of a strong glass jar C, which contains the freezing mixture. B should fit the hole so tightly that it does not bob up out of the mixture when A is removed. In C is a stirrer, G, of strong copper wire, the end of which passes through the lid. This serves to stir up the freezing mixture from time to time. Pulverize some ice by pounding it in a strong wooden box with a heavy piece of wood. Take the inner tube with the thermometer out of the apparatus. It is convenient to take the thermometer out of the tubs, and to hang it up carefully on a stand by means of a fine flexible 522 EXCRETION copper wire passing through the eye. The rubber cork can be taken out with the thermometer, and the platinum wire also, the bent free end of the latter supporting it in the cork, or it may be fastened tempor- arily to the thermometer stem by a small rubber band, which is slid up over the cork when the therniometer is reinserted. Tube A can be set temporarily in a specially heavy .test-tube rack. Remove the lid of C, and with it tube B. Now put ice and salt alternately into C until it is nearly full, mixing them up well. Add some cold water from the tap till the stirrer G can move freely up and down in the mixture. For very exact work the temperature of the freezing mixture must not be more than a few degrees below the free^g-point of the liquid which is being examined. Put on the lid, and immerse tube B. Into A, which must be perfectly clean, put enough pure distilled water to fully cover the bulb of the thermometer, and introduce the latter. For ordinary purposes distilled water previously boiled to expel the carbon dioxide, and then cooled in a stoppered flask, is sufficiently pure. Immerse A directly in the freezing mixture through the hole by which G comes out, or through a separate hole (not shown in the figure) tiU some ice has formed in the water. Take A out of the mixture, wipe it with a cloth, and hold the lower part of it in the hand till nearly the whole of the ice has melted. If there is a cake of ice at the bottom, see that it is displaced by the platinum stirrer. A trace of ice being still left floating in the water, place A in B, and allow the temperature to fall to a few tenths of a degree below the freezing-point you expect to get, as deter- mined by a previous rough experiment. The freezing mixture is stirred up occasionally. The meniscus of the thermometer is to be carefully followed, as it goes on falUng, by means of a weak hand lens. Now stir the water in A iMskly. Suddenly it will be seen that the mercury begins to rise. Keep stirring witii the platinum wire, and read ofi the maximum height of the mercury, at which it is stationary for some time. The temperature can be estimated between the gradu- ations to thousandths of a degree. Take out A, and observe the fine ice crystals in the water. Heat A in the, hand as before till nearly all the ice has disappeared; then replace A in iB, and make another freezing- point determination. A third one may also be made, and the mean of the three readings taken. Take, out the thermometer, and dry it and the platinum wire with clean filter-paper, or dip them in some of the urine, which is then thrown away. Dry A or rinse it with urine. Then make a determiaation of the freezing-point of the urine in the same way as was done with the water. The freezing-point of the urine will lie much lower on the scale. Instead of freezing the liquid first and then leaving a Uttle ice in it when A is placed in B, A may be put into B before any ice has formed. Cooling is then allowed to go on with gentle stirring to a few tenths of a degree below the anticipated freezing-point. A small crystal of clean dry ice is then introduced through the side-piece on a clean splinter of wood or the loop of a cooled platinxim wire, the end of which passes through a piece of cork, by which it is held to prevent conduction of heat. The platinum stirrer can be raised to receive the crystal. The liquid is now vigorously stirred ; freezing occurs, and the observation is made as before. Instead of the above method, the liquid may first be cooled directly in the freezing mixture, but not so much that ice forms. A is then put in B, and cooUng allowed to go on while it is being stirred. When it has been undercooled to a certain extent— i.e., cooled below its freezing- point — ^the vigour of the stirring is increased. Ice forms suddenly, as before,- and the temperature rises to the freezing-point. With urine PRACTICAL EXERCISES 523 this method is sufficiently satisfactory, but it is not usually easy to get freezing of the distilled water till the undercooling is considerable, and it has been shown that this introduces some error. Suppose the freezing-point of the distilled water on the scale of the thermometer was 5-245° and that of the urine 3-625°, the value of A for the urine is 1-620°. Since for most purposes it is sufficient to fix the second decimal point, much smaller and less expensive thermometers than the ordinary Beckmann may be employed. In the same way the freezing-point of blood-serum (or blood), bile, and other jDhysiological liquids can be determined. Systematic Examination of Urine. — In examining urine, it is con- venient to adopt a regular plan, so as to avoid the risk of overlooking anything of importance. The following simple scheme may serve as an example; but no routine should be slavishly followed, the object being to get at the important facts^with the minimum of labour. More extensive information must be sought in the treatises on examination of the urine for clinical purposes. 1. Anything peculiar in colour or smell ? If the colour suggests blood, examine with spectroscope, haemin test, guaiacum test (pp. 76, 267) ; if it suggests bile, test for bile-pigments by Gmelin's test (p. 456), and for bile-salts by iPettenkofer's test (p. 456) and by Hay's test (pp. 456, 520). 2. Reaction. 3. Sediment or not ? .Sediment may be procured by letting the urine stand in a conical glass, or in a few minutes by the centrifuge. If the appearance of the sediment suggests anything more than a little mucus, examine with the microscope. The sediment may contain organized or unorganized deposits. Organized Sediments. — (a) Red blood-corpuscles (considerably altered if they have come from the upper part of the urinary tract). (6) Leucocjrtes. A few are present in health. A large number indicates pus. When pus is present the sediment may be white to the naked eye. (c) Epithelium from the bladder, ureters, pelvis of the kidney Or the renal tubules. A few squamous epithelial cells from the urethra are always present in normal urine. (d) Tube casts. (e) Spermatozoa (occasional). if) Bacteria. (§•) Parasites (rare). (h) Portions of tumours (rare) . Unorganized Sediments. IN ACID URINE. Uric Acid. — Crystals coloured brownish - yellow with urinary pigment. Various shapes, espe- cially oval ' whetstones,' rhom- bic tables, and elongated crystals, often in bundles (Fig. 177). Urates. — Usually amorphous, in the form of fine granules, often tinged with urinary pigment, sometimes brick-red. Soluble on heating. On addition of acids (including acetic acid) they dis- IN ALKALINE URINE. Triple Phosphate. — Clear, col- ourless, coffin - lid or knife - rest crystals. Also deposited in the form of feathery stars (Fig. 179). Calcium Hydrogen Phosphate (■ stellar ' phosphate), CaHPOi.— Crystals often wedge-shaped and arranged in rosettes. May also occur in a dumb-bell form. (A phosphate of calcium is also occa- sionally seen in weakly acid urine.) (Fig. 181, p. 473.) 524 EXCRETION Unorganized Sediments {continued) — IN ACID URINE. solve and urib. acid. pystals -appear in their pla'efe. Acid .urate of sodium and df airitnohium occa- sionally found in the crystalline form (rosettes of needles). Calcium Oxalate. — Octahedral, ■ envelope ' crystals, not coloured. Insoluble in acetic acid. Soluble in hydrochloric acid (Fig. 178, p. 472). Cystin. — Hexagonal plates. Rare (Fig. 180, p. 473). Leucin and Tyrosin (Figs. 186, 187, p. 483). — Rare. Also found in alkaline urine, but rarely. .'' Triple Phosphate. — Sometimes found in weakly acid urine. IN ALKALINE URINE. Calcium Phosphate, Ca3(P04)2. — Amorphous. Magnesium Phosphate. — Long rhombic tablets, which are dis- solved at the edges by ammonium carbonate solution, unlike triple phosphate. All the above are soluble in acetic acid without effervescence. Calcium Carbonate. — Small spherical or dumb - bell - shaped "bodies soluble in acetic acid with effervescence. Ammonium Urate. — Dark balls, often covered with spines. Soluble in acetic or hydrochloric acid, with formation of uric acid crys- tals (Fig. 182, p. 473). 4. Specific gravity. 5. Quantity of urine in twenty-four hours. If the quantity is abnormally large and the specific gravity high, test for sugar. 6. Inorganic constituents not generally of clinical importance, but in special diseases they should be examined — e.g., chlorides in pneu- monia. 7. Normal organic , constituents. Sometimes quantitative estima- tion of urea or total nitrogen in fever, and in diabetes and Bright's disease. 8. Chemical examination for abnormal organic constituents, especi- ally albumin and sugar. Albumin. — (i) Heat to boiling some of the urine in a test-tube. A precipitate insoluble on addition of a few drops of acetic acid consists of coagulable protein. A precipitate soluble in acetic acid consists of earthy phosphates. (2) Heller's test. Put some strong nitric acid in a test-tube and run on to it some urine. A white ring indicates protein. A quantitative estimation may be made by the method of Roberts and Stolnikow or Esbach (p. 517). Sugar. — (i) Trommer's test. (Fehling's solution may be used.) If the result is indecisive — (2) Phenyl-hydrazine test (p. 518). (3) In case of doubt confirm by yeast test. A quantitative estimation may be made with Fehling',s solution or the polarimeter. CHAPTER X METABOLISM, NUTRITION AND DIETETICS We return now to the products of digestion as they are absorbed from the alimentary canal, and, still assuming a typical diet con- taining carbo-hydrates, fats, and proteins, we have to ask, What is the fate of each of these classes of proximate principles in the body ? what does each contribute to the ensemble of vital activity ? It will be best, first of all, to give to these questions what roughly qualitative answer is possible, then to look at metabolism in its quantitative relations, and lastly to focus our information upon some of the practical problems of dietetics. Section I. — Metabolism of Carbo-Hydrates — Glygogen. The carbo-hydrates of the food, passing into the blood of the portal vein in the form of dextrose, are in part arrested in the liver, and stored up as glycogen in the hepatic cells, to be gradually given out again as sugar in the intervals of digestion. The proof of this statement is as follows : Sugar is arrested in the liver, for during digestion, especially of a meal rich in carbo-hydrates, the blood 9f the portal contains more sugar than that of the hepatic vein. Popielski, on the basis of experiments in which he fed with known quantities of sugar dogs whose inferior vena cava and portal vein had been united by an Eck's fistula, and determined the amount of sugar which passed into the urine, estimates the quantity of sugar kept back by the liver at from 12 to 20 per cent, of the whole. In the liver there exists a store of sugar-producing material from which sugar is gradually given off to the blood, for in the intervals of digestion the blood of the hepatic vfeins contains more dextrose (2 parts per 1,000) than the mixed blood of the body or than that of the portal vein (about I part per 1,000). When the circulation through the liver is cut off in the goose, the blood rapidly becomes free, or nearly free, from sugar (Minkowski). And a similar result follows such inter- ference with the hepatic circulation as is caused by the ligation of the three chief arteries of the intestine in the dog, even when the 525 326 METABOLISM, NUTRITION AND DIETETICS animal has been previously made diabetic by excision of the pancreas (p. 622). The nature of the sugar-forming substance is made clear by the following experiments: (i) A rabbit after a large carbo-hydrate meal, of carrots for instance, is killed and its liver rapidly excised, cut into small pieces, and thrown into acidulated boiling water. After being boiled for a few minutes, the pieces of liver are rubbed up in a mortar and again boiled in the same water. The opalescent aqueous extract is filtered off from the coagulated proteins. No sugar, or only traces of it, are found in this extract; but another carbo-hydrate, glycogen, a polysaccharide giving a port-wine colour with iodine and capable of ready conversion into sugar by amylolytic ferments, is present in large amount. (See Practical Exercises, p. 689.) (2) The liver after the death of the animal is left for a time in situ, or, if excised, is kept at a temperature of 35° to 40° C, or for a longer period at a lower temperature; it is then treated exactly as before, but no glycogen, or comparatively little, can now be obtained from it, although sugar (dextrose) is abundant. The inference plainly is that after death the hepatic glycogen is con- verted into dextrose by some influence which is restrained or de- stroyed by boiling. This transformation might theoretically be due to an imformed ferment or to the direct action of the liver-cells, for both unformed ferments and living tissue elements are destroyed at the temperatiure of boiling water. It has been clearly shown that the action is brought about by a diastatic enzyme, which some writers call glycogenase, for it readily occurs when the minced liver is mixed with chloroform water, and chloroform kills all hving tissues. Although blood contains a diastase in small amount, the change does not depend essentially upon this, since the glycogen also undergoes hydrolysis (glycogenolysis) to dextrose when aU the blood has been washed out of the organ. Lymph also contains a diastase, but there is evidence that the post-mortem glycogenolysis is chiefly due to an enzyme contained in the hepatic cells (an endo- enzyme) (Macleod). The diastases in the blood and lymph seem to be ' discards ' of the tissues which are on the way to destruction or elimination (Carlson) . The post-mortem change is to be regarded as an index of a similar action which goes on during life: sugar in the intact body is changed into glycogen; glycogen is constantly being changed into sugar. There is no reason to doubt that here, too, the hydrolysis is effected by the endo-enzyme. It might be supposed, indeed, that the adjustment of the two processes glyco-" genesis and glycogenolysis is simply a matter of the alteration of the equilibrium in a reversible reaction (p. 332), according to whether the dextrose content of the blood tends to rise or fall. If the concentration of dextrose in the blood is increased, more dex- METABOLISM OF CARBO-HYDRATES— GlYCOGEN 527 trose might be expected to ' diffuse ' into the hepatic cells, whose content of dextrose in proportion to glycogen would increase till the equilibrium was restored by the conversion of the excess of sugar into glycogen. Contrariwise, a diminution in the dextrose content of the blood might be expected to lead to diffusion of dextrose out of the liver-cells, and a consequent acceleration of the hydrolysis of the glycogen. We have already learnt, however, that in physiology — above all, perhaps, in the physiology of the glands — ' simple ' explanations are usually suspect. And when we come to study those conditions in which, as a consequence of the derange- ment of the mechanisms which regulate the carbo-hydrate metabo- lism, sugar appears in the urine, it will be seen that the matter is more complicated. For one thing, the nervous system seems to take a hand in the regulation, and where the nervous system takes a hand things are generally doing which the experienced physiologist does not expect to be simple. We may be certain, as in the case of the intracellular proteolytic ferments, that the vital action of the hepatic cells is a most important factor in controlling the rate of production of the ferment, and therefore its concentration in rela- tion to that of the substrate and the rate at which it works. (3) With the microscope, glycogen, or at least a substance which is very nearly akin to it, which very readily yields it, and which gives the characteristic port-wine colour with iodine, can be actually seen in the liver-cells. The liver of a rabbit or dog which has been fed on a diet containing much carbo-hydrate is large, soft, and very easily torn. Its large size is due to the Ipading of the cells with a hyaline material, which gives the iodine reaction of glycogen, and is dissolved out by water, leaving empty spaces in a network of cell- substance. If the animal, after a period of starvation, has been fed on protein alone, less glycogen is found in the shrunken liver- cells; if the diet has been wholly fatty, little or no glycogen at all may be found. Glycogen can even be formed by an excised liver when blood containing dextrose is circulated through it. Formation of Glycogen from Protein. — In the liver-cells of the frog in winter-time a great deal of this hyaline material — ^this glycogen, or perhaps loose glycogen compound — ^is present; in summer, much less. The difference is remarkable if we con- sider that in winter frogs have no food for months, while summer is their feeding-time; and at first it seems inconsistent with the doctrine that the hepatic glycogen is a store laid up from surplus sugar, which might otherwise be swept into the general circulation and excreted by the kidneys. It has been found, however, that the quantity of glycogen is greatest in autumn at the beginning of the winter-sleep, and slowly diminishes as the winter passes on, to fall abruptly with the renewal of the activity of the animal in the spring. The glycogen present at any moment is, therefore. 528 METABOLISM, NUTRITION AND DIETETICS believed to be a residue, which represents the excess of glycogen formed over glycogen used up ; and the amount is larger in winter, not because more is manufactured than in summer, but because less is consumed. It is possible, indeed, to produce the ' summer ' condition of the hepatic cells merely by raising the temperature of the. air in which a winter frog lives ; at 20° or 25° C. glycogen dis- appears from its liver. Conversely, if a summer frog is artificially cooled, a certain amount of glycogen accumulates in the liver. The meaning of this seems to be that at a low temperature, when the wheels of life are clogged and metabolism is slow, some substance, probably dextrose, is produced in the body from proteins in greater amount than can be used up, and that the surplus is stored as glycogen ; just as in plants starch is put by as a reserve which can be drawn upon — ^which can be converted into sugar — ^when the need arises. That carbo-hydrates may be formed from proteins (or their constituent amino-acids) has been shown in various ways — ■ for example, by feeding dogs with almost pure protein (casein) after the production of permanent glycosuria by removal of the pancreas (p. 622). To induce the animal to take the casein it had to be mixed with a certain amount of butter, or serum, or meat extract. The amount of sugar excreted was much more than could possibly have come from the glycogen originally present in the animal's body, computing, it on the most generous scale (41 grammes per kilo- gramme of body-weight, according to. Pfliiger), or from free carbo- hydrate present in traces in the food, or as prosthetic groups (p. 2) in the ingested protein. That the source of the sugar was protein and not fat was indicated by the fact that when the amount of protein food was increased, the dextrose and the nitrogen excreted increased proportionally (see also p. 530). Glycogen^Fomiers. — ^As true glycogen-formers in the higher anima,ls — ^that is, compounds whose elements (particularly the carbon) actually enter into the composition of the glycogen mole- cule — may be mentioned such substances as proteins (including gelatin), the fermentable sugars, and glycerin. In the case of proteins it is, of course, not the entire molecule which is transformed bodily into glycogen, but amino-acids yielded by them, or dextrose derived from the amino-acids. The liver is of itself capable of dealing only with the dextrose, and not with the amino-acids. At least, when the isolated liver (of the tortoise) is perfused with blood containing amino-acids no increase in the glycogen of the liver occurs. When glycerin is added to the blood the glycogen content of the liver is very distinctly increased. Glycerin is a tri- valent alcohol (CgHgOs) whose aldehyde, obtained from it by gentle oxidation, is glycerose (CgHgOs), a substance with the typical properties of a sugar. In the laboratory it has been shown that two molecules of glycerose can be combined to form one molecule of METABOLISM OF CARBO-HYDRATES— GLYCOGEN 529 sugar of the hexose type with six carbon atoms (CgH^gOg) . A similar transformation is accomplished in the liver, and then a number of the monosaccharide molecules (CgHigOg) are condensed with loss of water to form glycogen. Thus, n(C^H^2'^g)~niisP=(CgB.-^fi5)n. Since glycerin is a normal product of the hydrolysis of fats, the possibility that the fats of the food may contribute through their glycerin component to glycogen formation must be admitted. The monosaccharides dextrose, levulose, and galactose gave a similar result, while the disaccharides cane-sugar and lactose caused no increase in the glycogen of the perfused liver, since the liver contains no ferment capable of splitting them into monosaccharides. And although the first step in the linking of the monosaccharide mole- cules would seem to be the formation of a disaccharide such as maltose, the glycogen molecule must apparently be built up from single ' bricks,' the monosaccharides, and cannot be constructed from bricks which are already coupled in pairs, the disaccharides. Of course, since the disaccharides are hydrolysed in the digestive tube to simple sugars, they are to be reckoned with the true glycogen- formers, for in the intact body they are presented to the "hepatic cells in the form of monosaccharides. It is probable that levulose and galactose are first changed into dextrose. By the action of alkalies such structurally related sugars can 'easily be transformed into each other. Thus dextrose is an aldehyde of an alcohol with six carbon atoms, and levulose the corresponding keto- hexose. By oxidizing the alcohol we get an aldehyde or a ketone, according to whether a primary alcohol group (CH2.OH) or a secondary group (CH.OH) is oxidized, with the loss of t\iiro atoms of hydrogen. The aldehyde is characterized by the presence of the group C^tt, the ketone by the group CO. Both the aldehyde and the ketone are sugars, and since each contains six carbon atoms, they are both sugars of the group known as ' hexoses.' Dextrose, being not only a hexose but an aldehyde, may be called an ' aldohexose,' and levulose, being not only a hexose but a ketone, a ' ketohexose.' CHg.OH c/2 CHg.OH I 1^ 1 CH.OH CH.OH CO I I I CH.OH CH.OH CH.OH CH.OH CH.OH CH.OH I I I CH.OH CH.OH CH.OH I 1 I CH2.OH CH2.OH CHg.OH 6-valent alcohol. Aldehyde. Ketone. That levulose can be changed into dextrose in the body is indi- cated by the observation that after extirpation of the pancreas in 34 53Q METABOLISM, NUTRITION AND DIETETICS dogs the- administration of levulose is followed by an increase in the excretion of dextrose nearly equal to the amount of levulose ingested. It is also stated that, when the surviving liver of a normal dog is perfused with a suspension of washed blood-corpuscles to which levulose has been added, dextrose accumulates in the; blood and levulose disappears from it. ,.v' It has not hitherto been proved that the fatty acid component of the food fats can be converted into glycogen. But a fatty acid, propionic acid, is capable of complete transformation into dextrose when given either by the mouth or subcutaneously to dogs under the influence of phlorhizin (Ringer). Many other bodies are known to influence the formation of glycogen by ' sparing ' Sfubstances which are true glycogen-producers, but their carbon does not actu- ally take its place in the glycogen molecule. It has been shown that proteins can directly form glycogen or sugar apart from carbo- hydrate groups contained in the protein molecule. For the proteins of meat, gelatin, and casein are capable of forming 60 per cent, of their own weight of dextrose in diabetic metabolism, and even the end products of pancreatic digestion of meat jdeld so much sugar that the greater part of.it must have come from the amino-bodies, ,and not from a sugar-group in the protein. ..When given to dogs with total phlorhizin glycosuria (p. 542), glycin and alanin are connpletely, glutamic and aspartic acids in great part (corresponding to piacent°a ""containing ^^°^^ ^^^ee carbon atoms of their respective Glycogen. molecules) , converted into dextrose (Lusk and Ringer), and there is no reason to doubt that when such substances are produced by hydrolysis of protein in the normal body, and are not all utilized in rebuilding the bio- plasm, a portion of the surplus, after deamidization, can be trans- formed into glycogen. Extra-Hepatic Glycogen. — While the liver in the adult (containing as it does from 2 to 10 per cent, of glycogen, or even, with a diet rich in sugar or starch, more than 18 per cent.) may be looked upon as the main storehouse of surplus carbo-hydrate, depots of glycogen are formed, both in adult and foetal life, in other situations where the strain of function or of growth is exceptionally heavy — in the muscles of the adult (03 to 0-5 per cent, of the moist skeletal muscle, or on a carbo-hydrate regimen 0.7 to 3.7 per cent.), in the placenta, in many developing organs in the embryo (muscles, lungs, epithehum of the trachea, oesophagus, intestine, ureter, pelvis of. kidney, and renal tubules). The foetus, however, is not, compared with the adult, especially rich in glycogen. In the adult under favourable circumstances the absolute amount of glycogen in the METABOLISM OF CARBO-HYDRATES— GLYCOGEN 531 muscles may be several times greater than that in the liver, and usually the hepatic glycogen makes up considerably less than half the total glycogen of the body. That the muscles do not derive their glycogen by the migration of hepatic glycogen, but can them- selves form it from dextrose, has been shown by injecting that sugar subcutaneously into frogs after excision of the liver. The muscle glycogen was found to be increased. Function and Fate of the Glycogen. — ^The glycogen store of the liver fulfils a different function from that of the muscles. This is indicated by the fact that when dogs, after being put on a given diet for two or three days, are starved for a time, and then put again on the original diet, the hepatic and the muscular glycogen behave differently at first during the period of re-alimentation. While glycogen accumulates in the liver in greater quantity than under normal conditions of nutrition, in the muscles it at first accumulates much less rapidly than normally. This is entirely in accordance with the view that the hepatic glycogen store has for its great func- tion the regulation of the sugar content of the blood in the interest of all the tissues, while the glycogen store of the muscles and other _ tissues is mainly in the interest of their own nutrition and a source of energy for their own work. This does not imply that, when sugar is being absorbed in quantities too great for the liver to deal with after the current needs of the tissues have been satisfied, they do not add to their glycogen reserves. There is every reason to suppose that they do so, and thus act as a subsidiary regulating mechanism, although a less elastic one than that supplied by the liver. A third way in which a portion of the surplus sugar can be stored is in the form of fat. When a fasting dog is made to do severe muscular work, the greater part of the glycogen soon disappears from its liver. When a dog is starved, but allowed to remain at rest, the glycogen still markedly diminishes, although it takes a longer time; and at a period when there is still plenty of fat in the body, there may be only a trace of hepatic glycogen left. The glycogen which is usually contained in the skeletal muscles also diminishes very rapidly in the first days of hunger, but the heart contains the normal amount of glycogen at a time when the proportion in the skeletal muscles has sunk to J5- to -^ of the normal. These facts have been taken to indicate that glycogen and the sugar formed from it are the readiest resources of the starving and working organism, for the transformation of chemical energy into heat a^id mechanical work. To borrow a financial simile, the fat of the body has sometimes been compared to a good, but rather inactive security, which can only be gradually realized; its organ-proteins to long-date bills, which will be discounted sparingly and almost with a grudge; its glycogen, its carbo-hydrate reserves, to consols, which can be turned into money at an hour's warning. Glycogen, on this view, is especially 532 METABOLISM. NUTRITION AND DIETETICS drawn upon for a sudden demand, fat for a steady drain, tissue-protein for a life-and-death struggle. While there may be some such diflerence in the tenacity with which the different kinds of reserve material are held back from consumption when the floating supplies are wearing low, modem investigation tends to the conclusion that the interchange- ability of the various groups of nutritive substances is greater than had been supposed, and that in the long-run the cellS^ — in normal cir- c\imstances at least — ^never work without dextrose, even after the glycogen store has been practically all consumed, but secure it from other sources. Pavy has put forward the heterodox view that the glycogen formed in the liver from the sugar of the portal blood is never reconverted into sugar under normal conditions, but is changed into some other substance or substances, and he denies that the post-mortem formation of sugar in the hepatic tissue is a true picture of what takes place during life. But in spite of the brilliant manner in which he has defended this thesis, both by argument and by experiment, it must be said that the older doctrine of Bernard, which in. the main we have followed above, is attested by such a cloud of modem witnesses that it seems to be firmly and finally established. Fate of the Sugar — Glycolysis. — ^What, now, is the fate of the sugar which either passes right through the portal circulation from the intestine without undergoing any change in the hver, or is gradually produced from the hepatic glycogen ? When the pro- " portion of sugar in the blood rises above a certain low limit (about 1-5 or 2 parts per 1,000), some of it is excreted by the kidneys (Practical Exercises, p. 691). A large meal of carbo-hydrates is frequently followed by a temporary glycosuria, but much depends upon the form in which the sugar-forming material is taken. We have seen that poly- saccharides are quite incapable of absorption as such, and that they must be very completely hydrol;^sed in the lumen of the alimentary canal before their constituent sugars have any chance of reaching the blood. It is therefore not to be expected that the rapid absorption of such considerable quantities of sugar as would lead to its excretion should easily occur when the carbo-hydrate is in this form. Miura for example, after an enormous meal of rice (equivalent to 6-4 grammes of ash- and water- free starch per kilo of body- weight) , which, as he mentions, tasked even his Japanese powers of digestion for such food to dispose of, found not a trace of sugar in the urine. Dextrose, cane-sugar and lactose, on the other hand, when taken in large amount, were in part excreted by the kidneys, as was also the case with levulose and maltose in a dog (Practical Exercises, p. 690).* The amount of any carbo-hydrate which can be eaten * Twenty-four healthy students, whose urine had previously been shown to be free from sugar, ate quantities of cane-sugar varying from 250 grammes to 750 grammes. The urine was collected in separate portions for twelve to twenty-four hours after the meal. In only three cases was reducing sugar found in the urine (by Fehling's and the phenyl-hydrazine test), and then merely in traces. In eight cases cane-sugar was found, and estimated by the polarimeter, and, after boiling with hydrochloiric acid, by Fehling's solution. METABOLISM OF CARBO-HYDRATES , 333 without the appearance of sugar in the urine is sometimes called the assimilation limit for that carbohydrate. The attempt has been made to fix the limit of tolerance of dextrose for normal persons and persons suffering from incipient diabetes, with the object of aiding in early diagnosis of that condition. But the limit varies with so many conditions, only some of which can be controlled, that such observations are not easily interpreted. Except as an occasional phenomenon, glycosuria other than ali- mentary is inconsistent with health; and therefore in the normal body the sugar of the blood must be either destroyed or transformed into some more or less permanent constituent of the tissues. The transformation of sugar into fat we have already mentioned, and shall have again to discuss; it only takes place under certain con- ditions of diet, and no more than a small proportion of the sugar which disappears from the body in twenty-four hours can ever, in the most favourable circumstances, be converted into fat. The dextrose which is taken up from the blood by the tissues and there condensed to glycogen suffers sooner or later the converse change, in all probability under the influence of diastases or glycogenases produced in the cells, and reappears as dextrose to take its place in the cellular katabolism and begin the series of cleavages and oxidations by which its chemical energy is set free. Accordingly, it is the destruction of sugar which concerns us here, and there is every reason to believe that this takes place, not in any particular organ, but in all active tissues, especially in the muscles, and to a less extent in glands. It has been asserted that the blood which leaves even a resting muscle, or an inactive salivary gland, is poorer in sugar than that coming to it ; and the conclusion has been drawn that in the metabo- lism of resting muscle and gland sugar is oxidized, the carbon passing off as carbon dioxide in the venous blood. This is indeed extremely likely, for we know that, when the skeletal muscles of a rabbit or guinea-pig are cut off from the central nervous system by curara, the production of carbon dioxide falls much below that of an intact animal at rest ; and the carbon given off by such an animal on its ordinary vegetable diet can be shown, by a comparison of the chemical composition of the food and the excreta, to come largely from carbo-hydrates. But, considering the relatively feeble metabolism of muscles and glands when not functionally excited, the large volume of blood which passes through them, the difficulty of determining small differences in the proportion of sugar in such a liquid, the possibility that even in the blood itself sugar may be < The greatest quantity of cane-sugar recovered from the urine was 8 grammes (7.92 grammes by Fehling's method arid 8-29 grammes by the polarimeter) ; the highest proportion of the quantity taken which appeared in the urine was 2-5 per cent. When dextrose was found, cane-sugar was always present as well. 534 METABOLISM, NUTRITION AND DIETETICS destroyed, or that it may pass from the blood, without being oxi- dized, into the lymph, too much weight may be easily given to the results of direct analysis of the in-coming and out-going blood. And although the results of Chauveau and Kaufmann, obtained in this way, fit in fairly well with what we have already learnt by less direct, but more trustworthy, methods, such as the study of the respiratory exchange, they cannot be accepted as yielding exact quantitative information. They found that in one of the muscles of the upper lip of the horse the quantity of dextrose used up during activity (feeding movements) was 3-5 times as much as in the same muscle at rest, and this corresponded with the deficit of oxygen in the blood entering the muscle, and with the excess of carbon dioxide in the blood leaving it. More dextrose was also destroyed in the active than in the passive parotid gland of the horse, but the excess per unit of weight of the organ was far less than in muscle. In dogs whose abdominal viscera have been removed, so that they constitute practically preparations composed of skeletal muscles it has been found that the amount of dextrose which disappears from 100 grammes of blood per minute varies from 0-47 to i-8 milligrammes, the irregu- larities probably depending largely upon the irregular consumption by the muscles of the glycogen stored in them (Macleod and Pearce). Intermediary Metabolism of Carbo- Hydrates. — Concerning the pro- cesses and the stages by which dextrose is destroyed in the tissues, we have no very exact information, and it cannot be definitely stated at present what share is taken by cleavage and what by oxidation, or rather through what intermediate products, formed, it may be, now by simple cleavage, now by oxidation, again by a combination of cleavage and oxidation, the dextrose molecule is finally resolved into carbon dioxide and water. It must be remembered that the S5mthetic powers of animal cells are now known to be very extensive. They build carbo-hydrates, fats, phosphatides, and proteins, as well as destroy them, and at any of the earlier stages at any rate the degradation products of dextrose, or some of them, may be utilized in the construction of new compounds — for example, of fat — either in the cells where they arise or elsewhere in the body. In like manner the decomposition of a molecule of dextrose begun in one cell or in one tissue may be consummated in another to which intermediate products are conveyed in the blood. In such ways it is obvious that the katabolic processes may be finely regulated both qualitatively and quantitatively in accordance with the specific wants of different organs and the intensity of their func- tional activity from time to time. It must be said, however, that at present there are few definitely ascertained facts to guide us in tr5ring to form a scheme of the actual changes which occur in the intermediate katabolism of carbo-hydrates, and the sequence which METABOLISM OF CARBO-HYDRATES 535 they normally follow. Glycuronic acid has been previously inen- tioiied as a substance occurring even in normal urine in small amount. It is very closely related to dextrose, as a comparison of their constitutional formulae shows: COH COOH COH I 1,1 H— C— OH H— C— OH H— CO— H I I I OH^C— H OH— C— H OH— C— H ' I I I : H— C— OH H— C— OH H— C— OH I I I H— C— OH H— C— OH H— C— OH I I I CHgOH CHjOH COOH (^.dextrose. d-glyconic acid. (f-glycuronic 'acid. Glycuronic acid agrees with dextrose in containing the character- istic aldehyde group Cf^Tj, but differs in that by oxidation two atoms of hydrogen in the primary alcohol group CHgOH have been replaced by one atom of oxygen. There is reason to believe that in the tissues glycuronic acid can be formed from dextrose in the same way, possibly through the mediation of an enzyme, and it may therefore represent a stage in the katabolism of sugar. But it is not known whether this is a normal transformation through which the whole or the greater part of the dextrose passes,! or only a transformation involving a small part of the sugar under normal conditions. The appearance in the urine of large quantities of glycuronic acid, paired as already explained with various com- pounds, in certain pathological states or after the administration of certain drugs (p. 476), might be explained either as the result of an increased production of that substance through a deflection of the normal trend of carbo-hydrate degradation, or as the result of a failure on the part of the cells to further transform the glycuronic acid in the quantities normally produced. Lactic acid is the one intermediate stage in the decomposition of dextrose in the tissues whose importance seems to be definitely ascertained. The muscles and the liver have been proved to possess the power of producing lactic acid from dextrose obtained directly from the blood or from the hydrolysis of their own store of glycogen, and there is little doubt that this power is shared by many, perhaps by all, of the other organs. There is also good evidence that the lactic acid thus formed can be, and under normal conditions is, in large part oxidized so as eventually to yield carbon dioxide and water, although there is reason to believe that a portion of it may be utilized for the synthesis of more complex bodies. The chemistry of the change or series of changes by which lactic acid is produced from dextrose and the end-products, carbon dioxide 536 METABOUSM. NUTRITION AND DIETETICS and water, from lactic acid has given rise to nrach discussion, and is not yet cleariy known. The following scheme, based on the researches of Embden and others, and quoted from Abderhalden, illustrates one suggestion as to the course of the series of transformations, although it must be taken only as a diagram of the sequence of some of the possible stages. A molecule of dextrose is represented as giving rise to two molecules of glyceric aldehyde, each of which then yields a mole- cule of lactic acid. Each molecule of lactic acid, losing two atoms of hydrogen, becomes converted into a molecule of pyruvic acid, which by the loss of the elements constituting a molecule of carbon dioxide becomes acetaldehyde or acetic aldehyde, and this by Oxidation acetic acid, which is then oxidized to carbon dioxide and water. Thus — ?<:: H— C— OH I HO— C— H I H— G— OH I H— C— OH I CH20H (/•dextrose. COOH c/o |\H H— C— OH I ^ CHaOH |\H H— O-OH I CHgOH 2 molecules glyceric aldehyde. \ y" dAactic acid. ^H COOH CO -CO, — > + CH3 Pyru\'ic acid. CH3 Acetaldehyde. + 40 CH3 Acetic acid. zCOa 2H2O Carbon dioxide and water. It has been shown 'ttiat acetaldehyde and carbon dioxide are formed from pyruvic acid by the action of a ferment contained in yeast, and there is some evidence' that a similar transformation may occur in the liver. It is to be particularly remarked that according to this scheme the whole of the dextrose molecule is still represented in the lactic acid formed from it. Up to this stage no part of the molecule has been burnt. Nearly the whole of the chemical energy — i.e., all but about 3 per cent, of it — is still available. For a gramme of lactic acid 5^elds 3,661, and a gramme of dextrose 3,762, small calories on complete combustion. The intermediate products of the decom- position may therefore be transported from the place of origin and utilized elsewhere with scarcely any loss of energy. Further, it is indicated in the scheme that the degradation process is not merely a series of cleavages and oxidations, but that these may be inter- spersed with stages of reduction. It is also clearly suggested that at certain points the metabolism may become recessive and syn- METABOLISM OF CARBO-HYDRATES 537 theses be started, which may go far to retrace the steps of the pre- ceding katabolism in respect to a portion of the dextrose. Thus, lactic acid can be retransformed into dextrose, and this, of course, into glycogen. The formation of fat from sugar may also start from some of the stages displayed in the scheme, for it is only a short step to obtain by the reduction of glyceric aldehyde its alcohol glycerin. And from acetaldehyde fatty acids can be derived. Not only does lactic acid afford a point of contact between the metaboUsm of carbo-hydrates and that of fats — a junction, so to speak, where these two great metabolic currents cross each other, and where material originating in the one may be shunted into the other — but it also affords a point of junction and interchange with the current of protein metabolism. For example, certain of the amino-acids, such as alanin, yield as a decomposition product a compound called methylglyoxal (CH3.CO.CHO), which by the assumption of a molecule of water can be changed into lactic acid. It may also be one of the intermediate stages in the decomposition of dextrose as a precursor of lactic acid, and one of the ways in which the conversion of amino-acids into dextrose is accomplished may be through this link. Pyruvic acid is another possible link. As has just been mentioned, it probably forms a stage in the decomposition of dextrose, and has, in addition, chemical relations on the one hand to certain of the amino- acids, especially to alanin, and on the other to glycerin and even to fatty acids. Thus — CHo CHq I I CH.NHo-fO = CO -I-NH3 I • I COOH COOH Alanin (a-amino-pro- Pyruvic acid, pionic acid. CH2.OH CH3 CH.OH + 2O = CO+2H2O CHg.OH Glycerin. COOH Pyruvic acid. The more completely the various steps in the metabolism of the three great groups of food substances are unveiled, the more clearly does it appear that, far from -being independent circuits, the three currents are constantly exchanging materials with each other. It is to be supposed that in many of these transformations enzymes are concerned, although comparatively little is definitely known as to this. Normal blood itself has been credited with a ferment which has the power of destro3nng sugar (glycolysis). But with rigid aseptic precautions the loss of sugar, even in several hours, is small, and it is doubtful whether such a ferment exists. On the other hand, Cohnheim stated that while no glycolytic ferment can be demonstrated in the pancreas, and only an exceedingly weak glycolytic action in muscular tissue (Brunton), by combining ex- 338 METABOLISM, NUTRITION AND DIETETICS tracts of pancreas and extracts of muscles, distinct glycolysis, due to a ferment action, could be produced. He suggested that the glycoljrtic ferment is activated by another substance, as trypsinogen is activated by enterokinase (p. 366). This announcement aroused great interest, since it is known that the pancreas is intimately concerned in the metabolism of sugar. That sugar disappears under the conditions of Cohnheim's experiments has been confirmed by a number of observers. But his interpretation of his results has not been generally accepted. According to Levene and Meyer, the dextrose, far from being burnt, seems to be condensed to a poly- saccharide, and can be recovered by hydrolysing this compound when the mixture is acted on by dilute acid. The action of the pancreas-muscle mixture is, therefore, not a true glycolysis. In- deed, of all the tissues investigated by I.evene, leucocytes alone can be credited with a real glycoljrtic action. Excision of the pancreas in dogs causes permanent glycosiuria (pancreatic diabetes) (v. Mering and Minkowski), which is prevented if a portion of the pancreas be left (p. 622). Diabetes in man is known to be frequently associated with pancreatic lesions. Although much stUl remains obscure, the study of this pathological form of glycosuria and of the experimental glycosurias has thrown light upon the normal metabolism of carbo^ hydrates and upon those regulative mechanisms whose breakdown is responsible for the excretion of sugar. It will be best to discuss the experimental glycosurias first, and to begin with the form which probably is better understood than any other, the so-called punc- ture glycosuria. Puncture Glycosuria- — Sugar-Regulating Mechanism. — ^An arti- ficial and temporary glycosuria, in which the sugar in the urine un- doubtedly arises from the hepatic glycogen, can be caused by punc- turing the medulla oblongata in a rabbit — for example, at a level between the origins of the auditory nerves and the vagi. It is stated that a puncture of the thalamencephalon, or 'tween-brain (p. 822), produces the same effect-. If the animal has been previously fed with a diet rich in carbo-hydrates — ^that is, if it has been put under con- ditions in which the liver contains much glycogen — ^the quantity of sugar excreted by the kidneys will be large. The immediate cause of the glycosuria is an increase in the sugar content of the blood (hyperglycaemia), an increase which is most pronounced in the blood of the hepatic vein. If, on the other hand, the animal has been starved before the operation, so that the liver is free, or almost free, from glycogen, the puncture will cause little or no sugar to appear in the urine, and the proportion of sugar in the blood wiU remain normal. That nervous influences are in some way involved in the mobilization of the glycogen reserve of the liver is shown by the absence of glycosuria if the splanchnic nerves, or the spinal cord above the third or fourth dorsal vertebra, be cut before the puncture METABOLISM OF CARBO-HYDRATES— GLYCOSURIAS 539 is made. But sometimes these operations are themselves followed by temporary glycosuria, due, it is believed, to irritation of the same efferent nervous path whose elimination when the splanchllics are divided prevents the glycosuria. The simplest explanation of the phenomena is that a ' sugar centre ' — ^that is to say, a centre which has the important office of regulating the sugar content of the blood by governing the rate at which glycogen is built up and de- composed in the liver, as the salivary centre regulates the rate at which the constituents of saliva are formed and discharged — has been injured or irritated by the puncture. If a nervous centre does in fact preside over this internal secretion of the liver, it will, of course, be connected with efferent and afferent nerves. The former, as defined by the experiments alluded to, seem to be confined to the splanchnic nerves; the latter are believed to run especially, though not exclusively^ in the vagus. Section of the vagi has no effect either in causing glycosuria of itself or in preventing the ' puncture ' glycosuria, but stimulation of the central ends of these and of other afferent nerves may cause sugar to appear in the urine, although not, it is said, if precautions are taken to prevent any degree of asphyxia. Asphyxia produces an increase in the sugar content of the blood, an increase in the flow of urine and glycosuria. It has usually been assumed that this action of asphj^a is due to the effect upon the centre of blood over-rich in carbon dioxide (and other metabolic products) or impoverished as regards oxygen. But there is some evidence that the altered blood may also affect the liver-cells directly, or, what comes to the same thing in the long-run, that interference with the internal respiration of the hepatic tissue, operating, it may be, through an increase in the concentration of the hydrogen ions, upsets the equilibrium of those intracellular reactions by which glycogen is formed from dextrose and dextrose from glycogen. In like manner it may be supposed that under normal conditions the rate of transformation of the hepatic glycogen into dextrose is adjusted to the dextrose content of the blood, not only by reflex nervous impulses passing through the sugar-regulating centre, but also by the direct influence of the dextrose itself circu- lating in the blood, upon whose concentration the reaction of the centre on the one hand and of the liver-cells on the other may in part depend. So that when the proportion of sugar in the blood tends to sink we may perhaps picture the centre as sending impulses to the liver which increase the rate at which the glycogen is hydro- lysed; and when the proportion tends to rise, we may think of it as sending impulses which inhibit the hydrolysis, both effects being accentuated by the direct influence of the changes of concen- tration on the hepatic cells. Whatever the mechanism may be through which the puncture hastens the transformation of glycogen 54° METABOLISM, NUTRITION AND DIETETICS into dextrose in the liver, there is evidence that the amoiint of the enzyme which hydrolyses the glycogen is increased. Whether the action of the enzyme is favoured in some other way — e.g., by the production of a co-ferment or by some change in the condition of the glycogen which renders it more open to attack — ^is unknown. Certain facts have recently been brought forward which go to show that the action of the splanchnic fibres on the liver is not a direct action, but that in some way or other the concomitant activity of the adrenal glands is essential. For if the adrenals have been previously extirpated, the puncture does not cause glycosuria. It was at first thought that the reason for this was that the removal of the adrenals is itself followed by the disappearance of glycogen from the liver, and, as has been pointed out, the presence of glycogen in the liver is essential to the success of the puncture experiment. The matter, however, is not so simple. For although in certain animals — e.g., the dog — ^the liver does lose all its glycogen when thei adrenals have been taken away, this is not the case in the rabbit, and yet in the rabbit also the urine remains free from sugar after puncture in the absence of the adrenal glands. In some way or other, then, the adrenals do intervene in the production of puncture glycosuria. The observation, which is easily confirmed, that the injection of adrenalin (or epinephrin) (p. 541) under the skin or into the blood, or into one of the serous sacs, does cause a pronounced increase in the sugar content of the blood, and the appearance of dextrose in the urine, seemed at first to supply the missing link in the chain of evidence. What could be simpler than the assumption that the splanchnic fibres stimulated in the puncture experiment were fibres going not to the liver, but to the adrenals, which occa- sioned an outpouring of adrenalin into the blood, and that puncture glycosuria was therefore merely a particular case of adrenalin glyco- suria ? It is known that excitation of the splanchnic nerves causes the passage of adrenalin into the blood of the adrenal veins (p. 640). It is known that puncture of the medulla oblongata diminishes the epinephrin content of the adrenal glands. The argument seemed straightforward, and the adrenal hypothesis of puncture glycosuria triumphant. As soon, however, as the matter was put to the test of quantitative experiments, the hj^othesis began to crumble. It was shown, for example, that during a stimulation of the splanchnic nerves sufficient to cause a decided increase in the dextrose content of the blood, a quantity of adrenalin was given off to the adrenal veins, which, when mingled with the rest of the blood on its way to the liver, could not possibly amount to more than one in a hundred million parts of blood, a concentration in which adrenalin, when introduced artificially into the blood-stream, produces no glycosuria whatever, nor, indeed, any recognizable physiological effect. Still more significant is the fact that, after destroying the hepatic jlexus. METABOLISM OF CARBO-HYDRATES—GLYCOSVRIAS 541 stimulation of the splanchnic nerves causes no increase in the blood- sugar in spite of the increased output of adrenahn by the way of the adrenal veins. On the other hand, excitation of the hepatic plexus causes hj^jerglycaemia (Macleod and Pearce). It is not, then, a direct action on the liver of epinephrin secreted in response to stimulation of splanchnic fibres supplying the adrenal glands which is responsible for the increase in the dextrose content of the blood. The adrenals, however, play some part. For in their absence stimulation of the hepatic plexus is not followed by hyperglycaemia. But whether this is due to general derangement of the normal carbo-hydrate metabolism in their absence, or to the loss of some special influence on the liver, without which stimulation of the hepatic plexus is ineffective, is unknown. Although several of the operations which lead to temporary glycosuria undoubtedly bring about changes in the hepatic circula- tion, it is as yet inrpossible to say whether vaso-motor effects con- tribute essentially^ the result, or whether it is due entirely to nervous stimulation of the liver-cells, or to withdrawal of such stimulation or control (see also p. 502). There is some evidence that excitation of the uncut great splanchnic nerve (on the left side) in dogs may cause hyperglycemia, diuresis, and glycosuria, even under conditions in which as far as possible circulatory effects are eliminated. Contrariwise, when in the puncture experiment on an unnarcotized animal the small instrument does not wound the medulla oblongata in the right place, a rise of blood-pressure due to excitation of the vaso-motor centre may occur without any glycosuria. But absolute proof of the, existence of glycogenolytic nerve fibres going to the liver — that is, fibres whose stimulation accelerates the hydrolysis of glycogen into dextrose (Macleod) — has not yet been brought forward. Adrenalin Glycosuria. — In adrenalin glycosuria the sugar-content of the blood is increased. Subcutaneous injection of adrenalin chloride causes a mild, intravenous injection a greater glycosuria, and intraperitoneal injection the greatest glycosuria of all (Herter). The best evidence is that the glycosuria is produced by some action on the liver, possibly through the excitation of sympathetic fibres controlling the production of dextrose from glycogen (Underbill and Closson), or by a direct effect on the hepatic cells, which hastens the normal transformation of glycogen into dextrose, or hinders the normal transformation of dextrose into glycogen. It has been stated that in the isolated surviving liver of the frog adrenalin causes the glycogen to be rapidly converted into dextrose. While this confirms the view that experimental adrenalin glycosuria is due to an action on the liver which increases the sugar-content of the blood, it does not necessarily show that the action is exerted directly on the hepatic cells without the intervention of nerve fibres. For the 542 METABOLISM, NUTRITION AND DIETETICS sympathetic nerve-endings may survive a considerable time. The theory that epinephrin causes glycosuria by inhibiting the internal Secretion of the pancreas, and that the condition is therefore a par- ticular variety of pancreatic diabetes, is erroneous. Adrenalin glycosiuria does not seem to be in any way related to true diabetes. The complete metabolism of dextrose is not interfered with. Indeed, a much larger proportion of the total heat produced comes from the destruction of sugar after the subcutaneous injection of epinephrin into dogs than in the normal animals (Lusk and Riche). If in spite of this glycosuria ensues, it is only because the carbo- hydrate reserve of the body is mobilized so rapidly that it cannot possibly be all consumed. Nor does epinephrin cause any increased production of sugar from protein or from fat. For in dogs rendered diabetic by phlorhizin and freed from glycogen by shivering, injec- tion of epinephrin is not followed by an increase of either sugar or nitrogen in the urine (Ringer). After repeated injections of adren- alin, a tolerajice for it is established, and glycosuria is no longer caused. Phlorhizin Glycosuria, produced by subcutaneous injection of the glucoside phlorhizin, agrees with pancreatic, but differs from punc- ture diabetes in this, that it can be produced in an animal free from gly<;ogen, and is accompanied by extensive destruction of proteins. It differs from other forms of diabetes in being associated, not with an increase, but with a diminution, in the sugar of the blood. This is best explained by supposing that the phlorhizin acts on the kidney in such a way as to increase the permeability of the glomeru- lar epithelium for sugar, or (in terms of the secretion theory of urine formation) in such a way as to increase its sensitiveness to the stimulus of sugar circulating in the blood. The sugar is therefore rapidly swept out of the circulation, and this leads secondarily to an increased production of sugar to make good the loss. In addi- tion, within certain limits there is a total inability on the part of the body to consume dextrose. After the preliminary sweeping out of the sugar already in the body, a definite ratio is established between the dextrose and the nitrogen eliminated in the urine (dextrose : nitrogen : : 3-6or 37 : i). The sugar at this stage is produced entirely from proteins, and not at all from fat. It is a fact of considerable interest that, if small quantities of dextrose are now given, the amount of protein de- stroyed is reduced to some extent, although all of the dextrose is excreted, and none of it is burnt (Ringer). This supports the hypothesis of Landergren that in starvation some of the protein is metabohzed for the formation of the indispensable dextrose, and that this fraction can be ' spared ' by carbohydrate, though not by fat. The protein metabolized is so much increased under the influence of phlorhizin that it exceeds the starvation requirement METABOLISM OF CARBO-HYDRATES— GLYCOSURIAS 543 by a greater amount than in pancreatic diabetes, perhaps because the diminished content of sugar in the blood constitutes a more insistent call upon the proteins to produce sugar. In pancreatic diabetes, where hyperglycsemia exists, there can at least be no reason for the formation of sugar from protein for the maintenance of the normal sugar-content of the blood, and it is interesting that i*i this condition the giving of dextrose does not seem to spare any protein (p. 595). The degree of intolerance for carbo-hydrates in pathological diabetes may be arrived at by putting the patient on a diet of protein and fat (rich cream, meat, butter, and eggs), and determining the ratio of dextrose to nitrogen excreted. If it is 3.6 or 37: I, intolerance is complete, none of the dextrose produced from protein being burned, and there will probably be a quickly fatal issue (Lusk and Mandel). Glycosuria can be caused in many other ways than those already mentioned. Sometimes the action seems to be a direct one on the sugar-regulating centre — e.g., in concussion of the brain, occlusioii and subsequent release of the arteries supplying the brain and cervical cord, and acute haemorrhage. Carbon monoxide has a, similar action owing to the deficiency of oxygen occasioned by it. Many drugs also cause glycosuria, including curara, morphine, strychnine, phosphorus, chloroform, ether, and other substances, some of which may act on the ' sugar centre,' although others — e.g., phosphorus and chloroform — are poisons which can affect the liver directly. Injection of water or physiological salt solution into the bile-ducts, or into the mesenteric veins, or of salt solution in consider- able amount into the general circulation, is followed by glycosuria (Fischer, etc.). Diabetes Mellitus. — In the natural diabetes of man, as in all the forms of glycosuria mentioned, with the exception of that produced by phlorhizin, the immediate cause of the glycosuria is the increase; of sugar in the blood. Instead of the i part per 1,000, or a little more or less, which constitutes the normal proportion in a healthy man, in diabetes 3 or 4 parts, and in exceptional cases even 7 to 10 parts per 1,000 may be present. The riddle of diabetes is the explanation of this persistent hyperglycsemia. Innumerable hypotheses have been framed to account for this, but on the whole three possibilities have been emphasized: (i) That the ppwer of temporarily storing carbohydrates is deranged; (2) that the power of the tissues to utilize carbo-hydrates [i.e., eventually dextrose) is diminished or abolished; (3) that too much sugar is produced in the body. In addition, some writers have postulated a fourth factor to explain certain cases (of so-called ' renal diabetes ') — to wit, an increase in the permeability of the kidneys for sugar, as in phlorhizin glycosuria. Lest the student should be bewildered amongst all these theories, he should take note that there is every 544 METABOLISM: NUTRITION AND DIETETICS reason to believe that diabetes mellitus is not a single pathological condition, but comprises a group of such conditions. Some cases present a picture conforming closely to one or to another of the experimental glycosurias, but many cases a picture compounded of features characteristic of two or of several of these experimental conditions. It is possible that in some cases the sugar coming from the ali- mentary canal passes entirely or in too large amount through the liver, owing to a deficiency in its power of forming glycogen. But although in certain cases of diabetes specimens of the hepatic cells, obtained by plunging a trocar into the liver, have been found free from glycogen, in others glycogen has been present. The muscles also are usually stated to be much poorer in glycogen than normal . muscles, but this might just as well be the case because glycogen was being transformed into sugar with abnormal ease as because there was interference with glycogen formation. Indeed, it is said that in the heart muscle of depancreatized dogs there is more glyco- gen than in normal heart muscle. It must be carefully remembered that the amount of glycogen present in a tissue gives no information as to the rate at which it is being formed or decomposed. And if the cause of the supposed defect in glycogen-forming power be the absence of a glycogen-forming ferment, or its production in too small an amount, the same circumstance may occasion a too tardy trans- formation into sugar of whatever glycogen happens to be present. In this case the sugar-regulating function of the glycogen store would be equally lost, whether the storehouses were permanently filled with long-formed glycogen or only half-filled or empty. In addition to an interference with the due and regulated storage of the surplus sugar as glycogen, it has usually been thought neces- sary for a rational explanation of the facts of diabetes, or at least of some forms of it, to assume that from some change in the tissues sugar has ceased to be a food for them, or is used up in smaller amount than in the healthy body. Why the tissues cannot decompose and utilize dextrose as they normally do, if it be really the case that they fail in this regard, is a question of great interest, but as yet no satisfactory answer can be given. It appears probable that the failure occurs at one or more of the earliest stages in the intermediate metabolism of carbo- hydrates (p. 534) or in the preliminary processes, whatever they may be, which, without profoundly altering the dextrose molecule, prepare it for the series of decompositions, in the course of which it eventually parts with aU its chemical energy. For it has been shown that many of the products of the cleavage or oxidation of sugar, even those in which the decomposition has proceeded but a little way — e.g., glyconic and glycuronic acids (p. 535) — are com- pletely utilized by the tissues of diabetics and of depancreatized METABOLISM OF CARBO-HYDRATES— GLYCOSURIAS 545 dogs. And the derangement in the normal sequence of events, of whatever nature it may be, is not so deep-reaching as to prevent the retracing of the chemical steps by which sugar i& synthesized from such derivatives of the proteins as amino-acids or their further degradation products. As to the actual cause of the alleged in- capacity of the tissues to consume dextrose, the change has by some been supposed to be the loss or diminution of a glycolytic ferment or a substance necessary for the activation of such a ferment. And although the sugar-destroying power of blood from diabetic patients, or from animals in which glycosuria has been caused by phlorhizin, is not at all inferior to that of healthy blood, it has been maintained that the intracellular glycolytic ferments, if such really exist, are much less active, especially in the more severe forms of the disease, which conform so closely in their clinical manifestations to the pic- ture presented by the depancreatized animal. Nevertheless, up to the present all attempts to satisfactorily demonstrate for isolated tissues a loss or even a diminution in the capacity to utilize dextrose have broken down. In eviscerated dogs, for example — that is, in preparations consisting mainly of skeletal muscle — it has been found impossible to make out any deficiency as compared with normal animals in the amount of dextrose disappearing in a given time from the blood, even when the animals have been deprived of the pancreas as long as a week before the experiment, and therefore exhibit the condition of pancreatic diabetes in full intensity (Macleod and Pearce). This conclusion has been confirmed for the isolated heart -lung preparation. As regards the hypothesis that an increased production of sugar from proteins, or it may be from fat, is the essential proximate cause of the hyperglycaemia and the glycosuria, there is no good evidence that this factor, acting by itself in the absence of a derangement of the regulative influence of the glycogen store, and in the absence of a derangement of the normal katabolism of dextrose, is ever respon- sible for pathological diabetes. But a secondary overproduction of sugar unquestionably occurs in many cases. The tissues, bathed as they are in liquids rich in dextrose, are nevertheless starving for sugar, if they cannot use what is offered to them, and the body labours to avert the famine by increasing its production of sugar, the sugar-forming tissues being stimulated to their task either through nervous influences or by chemical messengers circulating in the blood. In depancreatized dogs, and in dogs under the influence of phlorhizin, glycerin, given by the mouth, causes an increase in the excretion of sugar up to two or three times the original amount. The giving of fat does not increase the amount of sugar excreted, which, however, is increased by such substances as egg-yolk, which contain lecithin. These should accordingly be avoided in cases in 35 546 METABOLISM, NUTRITION AND DIETETICS which a strictly antidiabetic diet is desired. It is much more im- portant to exclude carbo-hydrates largely or entirely from the food, although oatmeal and potatoes are said to occupy an exceptional position, and have even been recommended as beneficial. Calcium chloride has been stated to diminish the sugar excretion in diabetes (Boigey), and it has a similar effect in certain of the artificial glyco- surias (Brown, Fischer). In many cases, even when carbo-hydrates are completely, or almost completely, omitted from the food, sugar, derived from the breaking-down of proteins, and possibly to some extent from fats, still continues to be excreted, although in smaller quantity. Other prod- ucts formed or imperfectly transformed in the deranged metabolism, especially of fats, such as acetone, aceto-acetic acid, and oxybutyric acid (the so-called acetone bodies), may also appear in the urine, or, accumulating in the blood, may, by uniting with its alkalies, seriously diminish the quantity of carbon dioxide which that liquid is capable of carrying, and thus lead to the condition known as diabetic coma. The small amount of carbon dioxide in the venous blood may also be partly due to the hyperpncea, marked by increased depth of the respiratory movements produced by stimulation of the respiratory centre by other substances than carbon dioxide. The increased ventilation causes a fall in the carbon dioxide pressure in the alveolar air, and therefore an increased elimination of that gas from the blood. This form of coma appears to be really in part an acid-poisoning comparable to the condition produced in animals by doses of mineral acids too large to be neutralized by the ammonia split ofi from the proteins. The administration of very large doses of alkahes (sodium bicarbonate, for instance, to the amount even of hundreds of grammes) has been recommended for the treatment of this serious complication, and in many cases it is successful in staving it off for a time. Often, however, in spite of a prolonged course of treatment, during which the urine has continued distinctly alkaline, fatal coma eventually occurs. The coma then is not merely a symptom of acidosis, but is also due to the specific toxic effects of the acids even when neutralized. Other toxic products may also be formed in the deranged metabolism. The appearance of the acetone bodies in diabetes presents a problem which cannot be said to have been as yet completely solved. Oxybutyric acid, from which aceto-acetic acid and acetone are easily derived (p. 558), seems to be one of the inter- mediate steps in the normal metabolism of fats. But whereas imder ordinary circumstances it is readily oxidized in the body, in diabetes the power of the tissues to burn oxybutyric acid seems to suffer just as does the power to utiUze dextrose. The suggestion that in diabetes the abnormally gieat consumption of fat entailed by the loss of avail- ability on the part of the carbo-hydrates causes the intermediary metabolism of fats to be scamped, as it were, is not satisfactory. THE METABOLISM OF FAT 547 For many animals and some races of men dwelling in cold climates consume with impunity much greater quantities of fat than any diabetic organism. Section II. — The Metabolism of Fat. Chemistry of Fats. — The fats are compounds (esters) of an alcohol with fatty acids, and can be split, with assumption of water, into these constituents by the action of acids or alkalies or of enzymes (lipases). In the majority of the ordinary fats, and in all those which are. of physiological importance (the triglycerides), the alcohol is glycerin. The fatty acid components which may be united with the glycerin are very numerous, and the physical properties of the different fats — e.g., their melting-points and solubilities — are closely related to the physical properties of the corresponding fatty acids. Thus palmitic and stearic acids are solid at ordinary temperatures, and so are palmitin and stearin, the glycerin esters or fats forriied with these acids. Oleic acid, on the contrary, is fluid at the ordinary temperature, and the corresponding fat, olein, is a liquid fat or oil. On the chemical side the fatty acids can be distinguished as saturated and unsaturated. The fatty acids, of the series CnHgK+i.COOH are saturated acids. Where w is o we have formic acid, H.COOH; where n is i, acetic acid, .CH3.COOH; where n is 2, propionic acid, CHg.CHg.COOH ; where n is 3, butyric acid, CH3.CHg.CH2.COOH, and so on, each acid in the series differing from the one immediately preceding it in possessing an additional CHg group. In the case of the higher members of the series these carbon chains become very long. In palmitic acid, for instance, CH3.(CH2)x4.COOH, there are fourteen CHg groups, and in stearic acid, CH3.(CHa)ie.COOH, sixteen. Oleic acid, ^8§"%C=c/Sjj > COOH is a representative of a series of unsaturated fatty acids whose general formula is CmH^n-iCOOH. As the formula of oleic acid shows, the unsaturated fatty acids contain' in their molecule two carbon atoms united by a double link, and one of these valencies can be occupied by halogens {e.g., chlorine) or by oxygen. Erucic acid, a fatty acid occur- ring in certain vegetable oils — for example, in rape oil— also belongs to this series, and linolic acid, found in linseed oil, to another series of unsaturated fatty acids. Then there are the so-called oxyfatty acids, which in their turn comprise saturated and unsaturated acids. They differ from the ordinary fatty acids in containing one or more OH groups. Thus a dioxystearic acid, Cj7H33(OH)g.COOH, in which two of the H atoms in stearic acid are replaced by OH, is found in castor-oil. It is clear, from the great variety of the fatty acids, that by their union with glycerin (with loss of water) a very large number of different fats can be formed. Thus, when all the OH groups in the trivalent alcohol are replaced by palmitic acid we have tripalmitin ; when they are replaced by stearic acid, tristearin ; when they are replaced by oleic acid, triolein ; and so on. As a group such fats may be termed homo-acid fats, since all the OH groups are replaced by the same fatty acid. I* hus — CHg.OH C17H36.COOH CHa.O.OC.Ci^Has CH.OH + Ci^Hgs.COOH = CH.O.OC.CuHgs-fsHaO CHg.OH CijHgg.COOH CH2.O.OC.C17H36 Glycerin. 3 molecules stearic acid. Tristearin. Water. 544 METABOLISM. NUTRITION AND DIETETICS But it is not necessary that each OH group in the alcohol should be replaced by the same fatty acid, and when this does not occur we have hetero-acid fats. For instance, one can be replaced by stearic acid, and the remaining two by palmitic acid, yielding a fat called ' stearo- dipalmitin.' Conversely, one OH may be replaced by palmitic and two by stearic acid, forming palmito-distearin. Similarly, a dioleo-stearin (glycerin combined with two riiolecules of oleic and one of stearic acid), and an oleo-distearin (glycerin combined with two molecules of stearic and one of oleic acid) are known. Such compounds have been isolated from the fat of animals, and also formed S3mthetically. Again, each of the OH groups in the alcohol can be replaced by a different fatty acid. It is obvious, then — and this is the point to which these chemical detaUs are intended to lead up — that the number of different fats which the animal organism has at its disposal for concocting those varied mixtures designated as body fat is very great, and that there is room for a considerable degree of specificity in the fat stores of different animals, and it may be in the fat contained in different organs of the same animal, even if this specificity is not as marked as in the case of the proteins. It may be added, in connection with the composition of the body fat, that small quantities of free fatty acids and of glyperin may be present ; but there is reason to believe that these are simply the surplus of raw materials which is about to be sjmthetized to neutral fat, or the surplus of decomposition products of the ^neutral fat which have not yet left the fat depots to take their place in the metabolism of the tissues. The discussion of the metabolism of fat involves a study — (i) of the transformations and migrations of the food fat before it begins to be utilized ; (2) of the possible production of fat from other con- stituents of the food ; (3) of the processes and the stages by which fat, whatever its origin, undergoes katabolism to its end products. The fat of the food, passing along the thoracic duct into the blood- stream, is soon removed from the circulation, for normal blood contains only traces, except during digestion. Where does it go ? What is its fate ? Transformation and Migration of the Food Fat. — ^The presence of adipose tissue in the body might suggest a ready answer to these questions. The fat-cells of adipose tissue are ordinary fixed connective-tissue cells which have become filled with fat, the protoplasm being reduced to a narrow ring, in which the nucleus is set like a stone. It would, at first thought, seem natural to suppose that the fat of the food is rapidly separated by these cells from the blood, and slowly given up again as the needs of the organism require, just as carbo-hydrate is stored in the liver for gradual use. And it has been found that a lean dog, fed with a diet containing much fat and little protein, puts on more fat, as estimated by direct analysis, or keeps back more carbon, as esti- mated by measurements of the respiratory exchange, than can be accounted for on the supposition that even the whole of the carbon of the broken-down protein corresponding to the excreted nitrogen THE METABOLISM OF FAT 549 has been laid up in the form of fat. Even with a diet of pure fat — and with such a diet digestion and absorption are carried on tmder unfavourable conditions — more carbon is retained than can have come from the metabolism of the proteins of the body, as measured by the nitrogen given off in the urine and faeces: the fat passes rapidly from the blood into the organs, and especially into the liver (Hofmann, Pettenkofer and Voit). It is thus certain that some of the absorbed fat may be stored up as fat in the body. This is borne out by the careful experiments of Munk and Lebe- deff, who found that, when dogs are fed with excess of foreign fat (linseed oil, rape oil, mutton fat), a fat is laid down which is quite different from dog's fat, and has the greatest resemblance to the fat of the food. Thus, when rape oil, which contains a fatty acid, erucic acid, not found in animal fat, was given, erucic acid could be detected in the fat laid on. When the dogs were fed with mutton fat, whose melting-point is much higher than that of dog's fat, the fat laid on did not melt till it was heated to 40° C. or more. When they were fed with linseed oil, the body-fat was found liquid even at 0° C. We have already referred (p. 438) to the fact that neutral fat can be built up in the wall of the intestine from fatty acids given in the food. Munk has shown that fat formed in this way can also be laid down as body-fat. But besides the fat and fatty acids of the food, the fat of the body has other sources, and some of it is produced by more complex processes. The fat of a dog consists of a mixture of palmitin, olein, and stearin. When a starved dog was fed on lean meat and a fat con- taining palmitin and olein, but no stearin, the fat put on contained all three, and did not sensibly differ in its composition from the normal fat of the dog (Subbotin). Stearin must, therefore, have been formed in some way or other in the body. If it was produced from the olein and palmitin of the food, the portion of these deposited in the cells of the adipose tissue must have undergone changes before reaching this comparatively fixed position. But there is conclusive evidence that fat may be derived from other sources, certainly from carbo-hydrates, and probably from proteins; and the stearin may have been formed from the carbo-hydrates or proteins of the food or tissues, and not directly from fat. And if the stearin was produced from proteins or carbo-hydrates, it is evident that the olein and palmitin might have been formed in this way too, the portion of the carbo-hydrate or protein devoted to this purpose l;eing sheltered from oxidation by the combustion of the fats of the food. It is well known that not only neutral fats, but also fatty icids, exert such a ' protein-sparing ' action. It is possible also that :he fat which is normally excreted into the intestine (p. 438), and Afhich is perhaps derived from broken-down proteins, may be re- ibsorbed, and take its place among the fat ' put on. ' 550 METABOLISM. NUTRITION AND DIETETICS At this point in the discussion it is necessary to remark that a distinction ought to be established between that store of svirplus fat laid down in the connective tissue which, in order to avoid com- plicating the matter unduly, has hitherto been referred to as if it constituted the whole of the body-fat, and the fat which is contained in greater or less amount in all the tissue cells. The fat con- tained in the tissue elements — e.g., in the liver cells — ^in the visible form of droplets, and which can be easily extracted from them by solvents such as chloroform, should also be distinguished from the fat which is so intimately incorporated or combined with the cell substance that it can only be extracted after this has been digested by the aid of proteoljrtic ferments or acids. The latter fraction of the body-fat is probably an integral and indispensable constituent of the protoplasm. Now, it is in the great fat depots of the sub- cutaneous tissue and the mesentery and omentum that variations in the proportions of the various fatty adds corresponding to varia- tions in the natiire of the food-fat are most easUy produced, or, at least, most easily observed. These depots are laid down, not in the interest of the fat cells themselves, but to serve the purpose of a reserve of fat which may be drawn upon for the nutrition of the body as a whole, just as the glycogen store of the liver forms a general carbo-hydrate reserve. The free fat in the cells of the organs is superficially analogous to the glycogen reserves of such tissues as muscles and glands, and certain facts are known which might be interpreted as indicating that this fraction of the body-fat, like the fat of the connective tissue, is not a definite and specific mixture of fats with an unvarying composition for each kind of animal, but a mixture whose composition can be made to vary by altering the nature of the fats in the food. On the other hand, the fat combined in the tissues appears to preserve a certain specificity which is inde- pendent of the fats supplied in the food. Thus, when dogs were fed with rape oil, and had accumulated considerable quantities of this fat of low melting-point in the subcutaneous and other fat depots, the fat combined in the organs remained in all respects the same as normal dog's fat. This was also the case with animals fed on fat of highmelting-point.suchassheep'stallow (Abderhalden) . Although the liver appears to have a special relation to the metabo- lism of fat, it is not known whether any particular organ is more than the rest responsible for the manufacture of this specific mix- ture of fats. It appears more probable that each cell has the power of forming for itself the characteristic fats from the crude materials represented by the food-fat directly absorbed from the tissue lyriiph, or the fat of the depots after it has been mobilized and has found its way again into the blood, or, finally, from other materials than fats, such as dextrose or some of its decomposition products. Even in the case of the subcutaneous and similar collections of THE METABOLISM OF PAT / 5^f fat, it must be noted that upon the whole, under normal conditions, it is their specificity of composition rather than their dependence upon the composition of the fat mixture in the food which is the striking fact, and undue weight can easily be given to the results of feeding experiments where great quantities of quite foreign fats are administered. When small quantities of fats very far removed in their properties from the normal fat of an animal are given in the food, they are either completely utilized before reaching the fat depots, or transformed into normal body-fat, since no change whatever can be detected in the latter. If they have been utilized, then it may be that a corresponding amount of fat, formed, say, from dextrose, has been laid down in the fat stores. If this fat is formed from dextrose, it will, of course, be the kind of fat which the particular animal is accustomed to form from dextrose — ^that is, the fat characteristic of the animal. If the foreign fat is itself transformed into body-fat when given in small amount, this same feat can without doubt be gradually accomplished in the case of the surplus of foreign fat laid down in the depots when a large quantity of it is given in the food. Formation of Fat from Other Sources than the Fat of the Food — (i) From Carbo- Hydrates. — It has been found that the addition of protein to a diet of fat, and especially to a diet of carbo-hydrate, in larger arhount than is just necessary for nitrogenous equilibrium (p. 594)' leads to a more rapid increase in the carbon deficit — -that is, in the fat put on — -than if the minimum quantity of protein required for nitrogenous equilibrium had been given. From this it is inferred that the carbonaceous residue of the broken-down protein is shielded from oxidation by the fat, and to a still greater extent by the carbo-hydrates, and so retained in the body as fat. And there is little doubt that the high repute of carbo-hydrates as fattening agents is in part due to their taking the place of proteins and fats in ordinary ' current ' metabolism, and so allowing body-fat to be laid down from these. Voit, indeed, has gone so far as to assert that this is the only sense in which carbo-hydrates can be said to form fat, and that, in carnivorous animals at least, a direct con- version never occurs. But the experiments of Rubner have shown that in a dog fed with a diet rich in carbo-hydrates, and containing but little fat and no proteins at all, the carbon deficit was greater than could be accounted for by the proteins being broken down in the body and the fat of the food. In the pig and goose, too, the direct formation of fat from carbo-hydrates has been demonstrated. For example, in an experiment by Tscherwinsky two young pigs of the same litter were taken. They weighed respectively 7,300 grammes and 7,290 grammes. One was killed, and the amount of fat and nitrogen in its body directly estimated. From the nitrogen the maximum quantity of protein which could be present was calculated. The other pig was fed for four months with barley, which was analyzed. The excreta were also analyzed to determine the amount of unabsorbed 552 METABOLISM, NUTRITION AND DIETETICS fat and protein. At the end of the four months the pig was killed. It now weighed 24 kilogrammes, and contained 2-52 kilogrammes protein and 9-25 kilogrammes fat. Subtracting the protein (0-96 kilo- gramme) and fat (o-eg kilogramme) originally present, 1-56 kilogrammes of protein and 8-56 kilogrammes of fat must have been put on. The amount of protein taken in the food was 7-49 kilogrammes, and of fat 0-66 kilogramme. Therefore, 5-93 kilogrammes of protein must have been used up, and 7-90 kilogrammes of fat laid on. At least 5 kilo- grammes of this fat must have come from the carbo-hydrate of the food. Only a small amount of the fat put on could possibly have come from the protein. The production of wax by beeSi which used to be given as a proof of the formation of fat from sugar, is not decisive, for in raw honey proteins are present ; and even when bees fed on pure honey or sugar manufacture wax, it may be derived from the broken-down proteins of their own bodies. It is probable that in the formation of fats the carbo-hydrates are first split up to some extent, and that the fats are then con- structed from their decomposition products, oxygen being lost in the process, since fat is much poorer in oxygen than carbo-hydrate. But the chemistry of the transformation as it takes place in the body is still imperfectly known, and all that can be done here is to indicate one or two of the ways in which chemists conceive that it may occur. The formation of the glycerin component of the neutral fats from carbo-hydrates would appear to present little difficulty. In dis- cussing the formation of glycogen from glycerin (p. 528), it was stated that two molecules of glycerose (glycerin aldehyde), a triose or sugar with three carbon atoms, can be condensed to form a hexose or sugar with six carbon atoms like dextrose, from the condensation or union of a number of molecules of which, with abstraction of water, glycogen is built up. The reaction can be worked equally well in the reverse direction — that is, from the hexose dextrose two molecules of glycerin aldehyde can be formed, and then from each molecule of the alde- hyde, by reduction, a molecule of the alcohol glycerin. As a matter of fact, it has been demonstrated that glycerin is produced when the cor- responding aldehyde is brought into contact with minced liver. As regards the fatty acid components of the fats, it will be seen from .the schematic representation of the katabolism of dextrose on p. 536 that acetic acid, a fatty acid, is represented at one of the stages as being formed by the oxidation of a molecule of acetaldehyde. Lactic acid is represented in the same scheme as a previous stage in the decom- position of dextrose, and lactic acid can be converted into acetaldehyde and formic acid, the lowest of the same series of fatty acids of which acetic acid is the next highest member. Thus : Q^sNcH.COOH = CHg.c/^ + H.COOH Lactic acid. Acetaldehyde. Formic acid. Aldehydes (as well as ketones) have a great capacity for entering into reactions with other substances, and their molecules show also a marked tendency to combine with one another, forming new compounds by their condensation. Thus, from two molecules of acetaldehyde one THE METABOLISM OF FAT 553 molecule ol aldol is formed, which by transposition of certain groups, becomes butyric acid, the fourth member of the fatty acid series of which acetic acid is the second member, and palmitic and stearic acids, which form such important constituents of the ordinary body -fats, the sixteenth and eighteenth members respectively. By oxidation aldol becomes -/3-oxybutyric acid, which by further oxidation yields aceto- acetic acid, compounds already referred to in connection with diabetes (p.- 546). The following equations illustrate these reactions: CHg.C^g + CHg.c/^ = CH3.CH (OH) .CHg.c/g Ac^taldehyde. Acetaldehyde. Aldol. CH3.CH(OH).CHg.CHO =CH3.CH2.CHa.COOH ' Aldol. Butyric acid. CH3.CH (OH) .CH2.CHO + O =CH3.CH{OH) .CH2.COOH Aldol. j3-oxybutyric acid. CHj.CH (OH) .CHa.COOH + O = (CH3.CO) .CH3.COOH + HaQ )3-oxybutyric acid. Oxygen, Aceto-acetic acid. Water. By reduction aceto-acetic acid is reconverted into ^-oxybutyria acid. Other aldehydes can react in similar ways, and thus many of the other fatty acids can be formed. It may be added that acetone (another of the so-called acetone bodies which appear in the urine in diabetes mellitus) is easily obtained from aceto-acetic acid by the splitting off of carbon dioxide. Thus : (CH3.CO).CHa.COOH =CH3.CO.CH3 4- CO2 Aceto-acetic acid. Acetone. Carbon dioxide. Formation of Fat— (2) From Protein. — Dry protein contains on the average 16 per, cent, of nitrogen and 50 per cent, of carbon, and urea contains 46 per cent, of nitrogen and 20 per cent, of carbon. Urea is therefore three times as rich in nitrogen as the protein from which it is derived, but two and a half times poorer in carbon ; and less than one-seventh of the carbon of protein will be eliminated in a quantity of urea sufficient to carry off all the nitrogen. It is probable that a portion of the remaining carbon may, after passing through various stages, take its place as the carbon of fat. We have seen that certain amino-acids derived from proteins can be converted into dextrose, and that dextrose can be converted into fat. So that the mere question whether carbon atoms or carbon chains originally present in protein molecules are ever capable of appearing in fat molecules, can be straightway answered in the affirmative. But it is still in doubt whether amino-acids can be transformed into glycerin or into fatty acidsy or into both, by processes which do not involve the production of dextrose from them. Aild>;in any case proof is required that the' extent of the transformation, let the steps be what they may, is great enough to be satisfactorily demonstrated. In regard to this point it must be said that absolutely flawless experiments to prove the direct production of fat from protein seem still to be wanting. 554 METABOLISM, NUTRITION AND DIETETICS Phosphorus Poisoning and Migration of Fat. — In the experiments of Bauer, the amount of oxygen consumed and of carbon dioxide and nitrogen excreted was determined in starving dogs. Phosphorus, which, as is well known, causes extensive fatty changes in the organs, was then administered in small doses for several days. The excretion of nitrogen was doubled, the excretion of carbon dioxide and the consumption of oxygen diminished to one-half. When the animals died, in a few days, the organs were all found loaded with fat. In one case 42-4 per cent, of the solids of the muscles and 30 per cent, of the solids of the liver consisted of fat. This is much more than the normal amount. It was assumed that the fat could not have been simply transferred from the adipose tissue, since the dog had been starved for twelve days before the phosphorus was given, and died on the twentieth day of starvation. Now, after such a period of hunger the amount of fat in the adipose tissue is greatly reduced. It was there- fore concluded that the source of the fat could only have been the broken-down protein. Since the nitrogen excretion was increased, while the carbon excretion was diminished, it was supposed that a residue rich in carbon must have been split off from the proteins, and, remaining unburnt in the body, must have been converted into fat. Experiments of this kind are open to criticism on several.grounds, but especially on this : that unless the fat-content of the whole body before the adminis- tration of the poison is known, it is impossible to be sure that the fat in a particular tissue has not been increased simply by the transportation of fat from some other tissue. It has been conclusively shown that migration of preformed fat does occur, and on an extensive scale, in phosphorus poisoning. For example, a dog was fed for a time with sheep's tallow, and fat was laid down in its adipose tissue with the physical and chemical characters, not of dog's, but of sheep's fat. The animal was then poisoned with phosphorus, and the fat which accumu- lated in the liver examined. It also resembled sheep's fat, as it should have done had it migrated from the adipose tissue, and not dog's fat, as it might have been expected to do had it been formed in the hepatic Cells from protein . The ease with which connective-tissue fat — i.e., food fat — emigrates to the liver suggests, with other facts, that the liver has a special relation to the transformation of this fat into the fat of the organs. This ' organized ' intracellular fat differs in various ways from the fats of adipose tissue. Its ' iodine value ' (p. 4) is higher (Leathes), and a large proportion of it consists of phosphatide lipoids (p. 562). The most convincing evidence that fat is not produced in increased amount under the influence of phosphorus has been obtained by deter- mining by actual analysis the total fat in animals, then poisoning similar animals with phosphorus and again estimating the total fat. Far from being increased, the fat may even be decreased in the poisoned animals (Taylor, etc.). There is no ground, then, for the assumption that phosphorus and other substances, like arsenic, antimony, etc., which bring about so-called ' fatty degeneration ' of the organs, act by causing or accelerating the transformation of protein into fat. Yet there is good evidence that they do accelerate the decomposition of protein, or at least interfere with its normal metabolism, for after phosphorus poison- ing amino-acids (leucin, tyrosin, glycin) appear in the urine. The observations of Lusk and his pupils indicate that phosphorus does not directly increase the amount of protein broken down, but does so indirectly, by favouring the conversion of the carbohydrate-like radicle of the protein molecule into leucin, tyrosin, and perhaps fat, and thereby necessitating an increased consumption of protein. A celebrated experiment, performed nearly forty years ago, was long THE METABOLISM OF FAT 555 supposed to furnish an absolute proof of the formation of fat from protein, under strictly physiological conditions, although in a humble form of animal life. Maggots were allowed to develop from the egg on blood containing a known amount of fat. The quantity of fat in the eggs was also taiown. After the maggots had grown, ten times as much fat was foilnd in them as had been contained in the blood and eggs together. The trifling quantity of sugar in the blood was utterly inadequate to account for the fat, which, it was concluded, must there- fore have come from the proteins of the blood (Hofmann). It can be objected to this experiment that no precautions were taken to prevent the growth of micro-organisms on the blood, and fat might have been formed by them from the proteins. Further, the fat estimations would scarcely pass muster according to the present standards. The experiments of Pettenkofer and Voit, which were supposed to have demonstrated that in the higher animals also fat is formed from proteins under normal conditions, are in the same position. According to them, a dog fed for a time on a liberal diet of lean meat may go on excreting a quantity of nitrogen equal to that in the food, while there is a deficiency in the carbon given off. Or if the dog is not in nitrog- enous equilibrium (p. 591), but putting on nitrogen in the form of ' flesh,' the deficiency in the carbon given off may be too great in pro- portion to the nitrogen deficit to warrant the assumption that all the retained carbon has been put on in the form of protein. In either case, carbon in large amount can only come from the proteins of the food, and can only be' stored up in the body in the form of fat. For lean meat contains but a trifling quantity of carbon in any other proximate principle than protein, and the non-protein carbon of the animal body is only to a very small extent contained in carbo-hydrates or other substances than fat. Pfliiger has criticized these experiments, and has shown that lean meat contains more fat than was supposed, and this is now generally admitted. He has endeavoured to show that the fat and glycogen in the meat given to the animals fully accounts for the carbon retained. Pfliiger, indeed, takes up the position that the fat of the body comes exclusively from the carbo-hydrates and fats of the food, and not at all from the proteins. But there is little doubt that in this he has gone too far, although his criticism has rendered it impossible any longer to appeal to Pettenkofer and Volt's results as good evidence on the other side. If none of the supposed quantitative proofs of the conversion of proteins into fat which have hitherto been brought forward are free from flaw, the same is true of the alleged qualitative indications of its possibility and of its actual occturence. The accumulation of fat between the hepatic cells caused by phlorhizin is, at the best, no better evidence than the accumulation within the cells in phos- phorus poisoning. The formation of adipocere (a cheesy substance, rich in fatty acids united with calcium or ammonium), sometimes seen in dead bodies which have remained a long time under water . or in moist graveyards, is largely, if not entirely, due to the fat already present in the parts which have undergone the change, or to fat removed by the water from other parts of the body. If any portion of the adipocere represents fat formed from protein, this transformation may well be credited to the numerous micro- organisms present, and throws no light upon the question of fat formation in the normal organism. The fat in the cells of the 556 METABOLISM, NUTRITION AND DIETETICS sebaceous glands, and of the mammary glands, may be produced from protein by a transformation of the cell-substance. But abso- lutely convincing proof is wanting. The old idea that the cells of these glands underwent a physiological process of transformation into fat analogous to the fatty degeneration of pathology, and then broke down bodily into the secretion, has been long since disproved for milk formation, and is probably erroneous also as regards the secretion of sebum. The rule which experience has taught, that a woman during lactation requires an excess of proteins in her food corresponding not only to the proteins, but also to the fat given off in the milk, suggests such an origin for the milk-fat, but does not prove it. Other fat-containing secretions are the ear-wax formed by glands in the wall of the extemcil auditory meatus, and the smegma formed by the glands of the prepuce, but nothing is known of the sources from which the fatty substances are derived. The Intermediary Metabolism of Fat. — ^The mechanism and the stages of the transformation, including the migration, of fats is not well understood — ^indeied, not, as well as that of the carbo- hydrates. Many of the tissues contain intracellular, soluble, fat-splitting ferments called lipases, especially the liver, the active mammary gland, and the intestinal mucosa. We have already seen that there is evidence that these lipases, like some other enzymes, have a reversible action. They are either fat- splitting or fat-forming ferments, according to the conditions (Kastle and Loevenhart). It is stated that the perfectly aseptic blood does not split ordinary neutral fats, although it contains a ferment . which splits up monobutyrin (glycerin butyrate) into glycerin and butyric acid. The question how the fat, after absorption from the intestine, passes from the blood into the cells, and how it is enabled again to pass out of the fat-ceUs when the needs of the tissues call for its mobilization, cannot ' at present be definitely answered. It is possible that just as fat is split in the lumen of the intestine before being absorbed, and then rebuilt in the epithelium, so it is split in the blood or in the lymph before being taken up by the fat-ceUs> The lipase in these cells would then be capable of synthetizing the glycerin and fatty acids to fat in their interior. When the fat is about to pass out of the cells in response to the call, of whatever nature it is, of the tissues for fat, it may again be split, res5mthetized in the blood, and again hydrolysed for entrance into the tissue cells. Or it may be carried to the cells in the forrn of glycerin and fatty acids, or soaps, in such small concentration as to be harmless, and there built up again into the original fat, or transformed into other fats characteristic of the particular tissues, including the fatty acid components of the phosphatides, or utilized without S5mthesis into fat. An alternative hypothesis avoids this series of decomposi- THE METABOLISM OF FAT 557 tions and syntheses by assuming 1;hat the fat passes in the form of very line droplets through the walls of the cells and of the capil- laries. The reader will observe that we seem to be discussing again, and almost in the same terms, the question of the absorption of fat from the intestine. It is indeed at bottom the same question, and it might be argued that by analogy it should receive the same solu- tion. Analogy, however, is a dangerous guide in such matters, and it is even more difficult to secure an unambiguous experimental test of the manner in which the internal migration of fat is accomplished than to secure the like for its absorption from the digestive tube. As to the ultimate fate of the fat, from whatever source it may be derived, our knowledge may be compressed into very few sen- tences : Sooner or later it is split and oxidized to carbon dioxide and water, its energy being converted into heat or, directly or indirectly, into mechanical or other functional work ; some of the fat absorbed from the intestine rapidly undergoes this change without entering the fat-cells of the adipose tissue. A portion of the fat may be changed into carbo- hydrates. This has been proved for the glycerin component ; its possi- bility must be admitted for the fatty acids, but proof has not yet been given. Of the intermediate stages by which the fatty acids are degraded into the simple end products but little is surely known. Included among these stages must be the compounds with which the forma- tion of the acetone bodies (p. 553) starts, if and in so far as their formation is a normal event which is merely unveiled by the dis- turbance of the ordinary course of the metabolism in diabetes. Among these intermediate stages must also be included, it is to be supposed, the compounds, whatever they may be, which act as connecting links between the currents of fatty acid and of carbo- hydrate metabolism, and with which the transformation of fatty acids into carbo-hydrates commences, if this occurs at all. According to the observations of Knoop, the saturated as well as some of the other series of fatty acids when oxidized decompose in a very characteristic way. As already remarked, these acids are made up of a larger or smaller number of CHg groups forming a chain which at one end terminates with a carboxyl (COOH) group, and at the other with a CH3 group. The carbon atoms in the chain are designated by Greek letters, a, 0, etc., the a position being that next the carboxyl group, the /3 position one remove from the carboxyl group, and so on. Accord- ing to Knoop, the oxidation of the fatty acid chain takes place in such a way that the chain is shortened by the cutting off from the carboxyl end the a CHg group along with the carboxyl group, while in place of the ^CHj group there is left a carboxyl group, an operation which might be fancifully compared to the naval manoeuvre of breaking the enemy's V -^u . -A CH3.CH2.CH2.CHa. I CHg.COOH, ,„^ Ime. Thus from caproic acid eSyBla get by oxidation butyric acid, CH3.CH2.CH2.COOH, ^^^^^^^^ dioxide and water. It appears that the oxidation proceeds in two stages, the hydrogen of the /3 group being first oxidized with formation of an 558 METABOLISM, NUTRITION AND DIETETICS oxyacid oxycaproic acid, CHj.CHg.CHa.CHOH.CHg.COOH, which is then by further oxidation converted, with loss of two carbon atoms, into butyric acid. The oxidation process may then start afresh on the j3 group of butyric acid. On the long carbon chains of the higher fatty acids this operation may be repeated again and again, the chain losing two atoms of carbon at each attack. If this represents what occurs in the normal metabolism, the groups cut o£E may then and there undergo the fate of the ships isolated by a successful application of^the manoeuvre alluded to, complete destruction — that is to say, oxidation to the end products carbon dioxide and water, a portion of the energy of the fatty acid being thus liberated at each oxidation of the /3 group. Eventually a fatty acid or acids with very few carbon atoms will be left. There is some reason to think that acetic acid (and perhaps similar simple acids) may be one of the normal stages in the decom- position. Thus, butyric acid may first yield by oxidation of the jS group the oxyacid ;8-oxybutyric acid, ^'q '^ *' which by further oxidation of the ;8 group and the cutting off of the a and carboxyl groups would give CH3.COOH, or acetic acid. If this is the general course of the oxidation of the fatty acids in the body, it is to be assumed that numerous intermediate stages unrepresented in such a simple scheme may exist. Thus it is known, as has been mentioned more than once in other connections (p. 553), that ;8-oxybutyric acid by oxidation yields aceto-acetic acid, by losing from the /3 group two atoms of hydrogen which unite with oxygen to form water. A molecule of aceto-acetic acid contains ■the elements of two molecules of acetic acid minus the elements of one molecule of water. It is therefore possible that aceto-acetic acid, if it is a normal stage in the katabolism of fatty acids, yields by its hydrolysis as a further step acetic acid, according to the equation CH3.CO.CH2.COOH -I- HgO =2 (CH3.COOH) . Aceto-acetic acid. Acetic acid. It is worth while, perhaps, to point out once more that even the relatively simple products now arrived at are not necessarily at once completely oxidized to their end products. That, it is to be assumed, will depend upon the needs of the organism. Acetic acid, for example, when added to blood and perfused through the sur- viving liver, can be transformed into aceto-acetic acid, and may thus become the starting-point of new syntheses. The Liver and Fats. — The liver seems to play an important part in the metabolism of fat, as it does in the metabolism of carbo-hydrates and of proteins. It contains an oxidizing ferment, j3-oxybutyrase (or /3-hydroxybutjn:ase), which transforms /3-oxybutyric acid into aceto-acetic acid (DaMn). This oxidation appears to occur in the normal as well as in the diabetic organism. The liver seems also to possess the power of transforming aceto-acetic acid into acetone, a reaction which does not involve an oxidation, and this may also be accomplished by means of an enzyme. But it is not at all likely THE METABOLISM OF FAT 559 that acetone forms a stage in the normal katabolism of the fatty acids or of the /3-oxyacids derived from them. The importance of the liver in the metabolism of fats is further indicated by the extent of the migration of fat to that organ when the fat stores are mobilized in unusual amoiint (p. 554). The reason for this migration seems to be that the fats undergo preparatory changes which facilitate their utilization by the tissues. For example, there is evidence that saturated fatty acids are changed in the liver into unsaturated acids, which are then carried to the organs to be metabolized. The desaturation may serve the purpose of facilitating the rupture of the long carbon chains, or their capacity for entering into reac- tions with other substances, at the points where double links exist between carbon atoms (p. 547). ' Non-Nutritive Functions of Fat. — In connection with the metabo- lism of fat, it ought to be noted that, in addition to their value as reserve material for the nutrition of the body, the deposits of fat under the skin and in other situations perform important functions in protecting delicate structures from mechanical injury, in facili- tating their movements upon each other, and in hindering the loss of heat. It would doubtless be a gross exaggeration to say that the mechanical and physical properties of 'the fat depots are as im- portant in comparison to their chemical relations as is the case for the bones and ligaments, but it would be an error not less gross to consider them as of little account. It will even, perhaps, be thought not unworthy of mention, from the point of view of the propagation of the race, that in the human species, at least, the amount and distribution of the cutaneous fat plays a part of some consequence in the aggregate of qualities which determine the physical attractiveness of the individual, especially of the female, although the standard in this regard varies widely in different communities. It is perhaps partly because the fat depots have important mechanical functions that the fat reserve is far less mobile than the glycogen reserve. The s^mi-solid panniculus adiposus, the fatty tissue around the great nerve trunks, between the muscles, around •the eyeball, on the soles of the feet, etc., possesses as a protective packing the good qualities of a water cushion -with none of its dis- advantages. But if the fat-cells were subject to sudden depletion, as the hepatic cells are — ^nay, in still greater degree, since they contain hardly any protoplasm — ^they would never serve for such a function. Of course, in the emergency of starvation, when even the glands and the muscles themselves are wasting, the fat reserves are neces- sarily mobilized, let their mechanical functions suffer as they may. Obesity.^— The proportion of the total mass of the body which is made up of fat varies greatly in different individuals, and often in the same individual at difierent stages in life. When the accumulation of fat 56o METABOLISM. NUTRITION AND DIETETICS passes beyond a certain point it causes obvious changes in the contours of the body, and often some embarrassment in its movements. This condition is termed obesity. It is extremely difficult to say when the condition oversteps the physiological boundary and becomes actually pathological. Some individuals who are notoriously stout are noted also for their intellectual activity, and may not fall below the average even in the ordinary kinds of physical effort. It would be an exaggera- tion to speak of such persons as suffering from a disease . In other cases the pathological stamp is clearly imprinted upon the metabolic anomaly which leads to the overfilling of the fat depots. This is perhaps best illustrated in those cases of extreme obesity in children where, in spite of the intense metabolism associated with growth, with the restless muscular activity characteristic of that age, and with the relatively great surface through which heat is lost, great quantities of fat continue to be put on. Muscular activity by itself is no certain antidote to or prophylactic against obesity, and it is a mistake to suppose that the condition is exceedingly rare among manual workers sufficiently well paid to be able to gratify their tastes in the quality and quantity of their food. Statistics or rough estimates covering the whole of the hand-workers of a country throw no light on such a question, for few indeed are the lands where the masses of the people have such weU-filled purses that they are able to nourish themselves according to their wishes. While it is true that the great majority of normal individuals (although not all, since even in the fattening of stock for market some animals are rejected as bad feeders) can be compelled to lay on fat when overfed with fat and especially with carbo-hydrates, and prevented from taking riiuch exercise or from losing heat freely, the most important factor in the excessive storing of fat by human beings leading a free life seems to be an anomaly in the metabolism which permits the machine to be run on less than the usual amount of fuel. From the point of view of thermo- dynamics the fat man, in very many instances at least, grows fat and fatter because his body is a machine whose ' efficiency ' is greater than the normal — ^that is to say, a machine which is capable of doing a given amount of work and of keeping itself in repair with a food intake of smaller heat value than is usually needed. Whether this anomaly is to be considered a metabolic fault or a metabolic virtue depends largely upon the ease with which the intake is adjusted to the actual require- ment of the body. If the adjustment is rendered accurate, the man with the anomalous tendency to put on fat, the adiposophil, as he might be called, is in all probability just as well off in every physiological sense on a smaller diet than a so-called normal individual of the same age, weight, and daily routine, on a larger quantity of food, and on this smaller diet he does not become fat. In this connection it may be recalled that, in speaking of the blood-flow in the hands and feet (p. 127), which are in this relation to be regarded as essentially an ' outcrop ' of the cutaneous circulation, it was pointed out that some healthy persons have habitually small flows and a habitually cool skin which perspires little, in comparison with others living practically the same life. It was suggested that this difference in the blood-flow through the skin, ivhich of course would correspond with a difference in the rate of heat loss, and therefore in the rate of heat production, may be correlated with a difference in the intensity of the metabolism and the intake of food. The difficulty of adjusting the appetite to the actual physiological requirement is perhaps the real anomaly in adiposophilia. Several factors seem to be involved in the group of sensations comprised under appetite and hunger (Chapter XVIII), and the onset and intensity of these sensations are unquestionably influenced by habit. The real THE METABOLISM OF FAT 561 question in many cases of obesity may be not why, the metabolism is managed so parsimoniously — that is, in the physiological sense, so thriftily — but why the fat man or the man tending to become fat still experiences so strong a desire for food after he has eaten what in pro- portion to his metabolic wants is enough, whereas the man with no tendency to obesity is no longer hungry after he has eaten an amount of food sufficient for the requirements of his tissues. Is there here perhaps an anomaly in the nervous mechanism in virtue of which, for instance, the gastric hunger contractions are more readily initiated and less easily stilled than in the normal person ? It is recognized that in the usually much more serious anomaly of the carbo-hydrate metabolism, diabetes mellitus, the nervous element may be important. The influence of the loss of certain of the internal secretions on .the deposit of fat will be alluded to in the next chapter. In the treatment of obesity the factor of appetite and hunger control has to be specially kept in mind. Bulky but comparatively innu- tritions food, such as green vegetables, e.g., in the form of salads, should form an important constituent of the dietary, since the mere distension .of the stomach staves off hunger. The total heat value of the food must be reduced gradually. Carbo-hydrates must be largely excluded, and also fats, although a certain amount of fat, say in the form of butter, is permissible and even beneficial as aiding in the passage of the food along the digestive tube. Alcoholic beverages are in general contra-indicated, because alcohol, as an easily oxidizable substance, protects the carbo-hydrates and fats from oxidation, and perhaps also because the normal oxidative power of the tissues may be depressed by its habitual use. On the other hand, tobacco smoking, which has some power of inhibiting the gastric hunger contractions, may be permitted. Muscular exercise, cold baths, light clothing both during the day. and at night, and a cool environment, are favourable to the reduction of fat by increasing the consumption of material and the loss of heat, just as a sedentary life in an overheated house in a person predisposed to obesity, and eating too much for his requirements, favours the putting on of fat. But if the appetite of the patient is allowed to govern the intake of food, the increased decomposition brought about by exercise, etc., is very likely to be balanced by an increased ingestion, and no progress will be made. Metabolism of Sterins or Sterols. — It has been previously stated that cholesterin appears to be the only representative of the sterins in the higher animals. Its source and function haVe been much discussed of late years. As to its source, there seems to be no reason to beheve that any part of the cholesterin of the tissues is formed from decomposition products of ordinary fats, carbo- hydrates, or proteins. It is probably entirely derived from the cholesterins of animal, and the phytosterins of vegetable food. On this assumption, its metabolism, unlike that of the great groups of food substances, is carried on in a closed circuit. Evidence that it can be synthesized from other substances in the body is lacking. No increase in the cholesterin has been observed during the develop- ment of eggs, and the cholesterin content of growing chickens appears to correspond to the sterins taken in the food (Gardner). The portion of the cholesterin which is ingested in the form of esters 36 562 METABOLISM. NUTRITION AND DIETETICS is probably split, with liberation of the fatty acid, in the course of digestion. But if this be so, cholesterin esters are again formed in the tissues, for the cells and the blood contain both cholesterin esters and free cholesterin. While some cholesterin is excreted in the faeces (p. 419), there is evidence that a portion of the cholesterin of the bile may be reabsorbed, a ' circulation ' of cholesterin taking place analogous to the circulation of bile-salts. The appearance of cholesterin in the bile has been connected by some writers with the destruction of erythrocytes in the liver, or the conveyance of the products of their decomposition to that organ (p. 21), but there are no means of distinguishing between the cholesterin set free from blood-corpuscles and that liberated from other cells. Since it is contained in all cells, every cell may be supposed to contribute something from time to time to the cholesterin excretion. As to the office of the tissue-cholesterin, it can only be suggested that a substance so ubiquitous must be important. There is some evidence that cholesterin, free or combined, plays a part in con- ferring on the- cells those peculiarities in their permeability upon which their functions, and indeed their integrity, depend. Free cholesterin, for instance, hinders the haemolytic action of the saponins (p. 28), apparently by forming compounds with them. Whether it or its esters are actually concentrated at the surface of the cell, and contribute to the formation there of the so-called ' lipoid ' envelope, is not definitely known, although there are facts in favour of this idea. Metabolism of Phosphatides. — The lecithins, which are the best- known members of this class of compounds, have been already described (p. 360). They are built up of glycerin, fatty acids, phosphoric acid in the form of glyceryl-phosphoric acid, and a nitrogenous base cholin. There is some reason to think that the lecithins of the tissues are, in part at least, not free, but combined with proteins or with carbo-hydrates. Other bodies belonging to the phosphatide group are kephalin, a constituent of nervous tissue and of yolk of egg, cuorin found in heart muscle, etc. It is probable, as stated in the chapter on Digestion, that the phosphatides of the food are hydrolysed in the alimentary canal with liberation of the glycerin, fatty acids, and the other com- ponents. It is not known whether they are resynthesized in the intestinal wall, but it is more probable that they pass directly to the tissues, where they can be utilized for building up the phos- phatides of the cells. Cholin is found in small quantities free in the tissues, and also, it is said, in the • blood-plasma. Glyceryl-phos- phoric acid has also been obtained in small amount from various tissues. The other components of lecithin are, of course, never wanting, and there can be no doubt that the cells possess the power of reconstructing phosphatides from such materials. They can do METABOLISM OF PROTEINS 563 more than this: they can prepare the ' building-stones ' themselves. For even when the ingestion of phosphatides in the food is excluded, or the intake is so small as to be negligible, the formation of phos- phatides in the body goes on apparently without check. An instance of this will be given on a future page in discussing experiments on the relative value of different proteins for nutrition and growth. A very striking observation has been recorded by McCpUom, who fed three hens on a diet almost free from fat. In about three and a half months they laid, fifty-seven eggs, containing over 9 per cent, of ) phosphatides. Calculation showed that here, first of all, fats or their components must have been constructed from carbo-hydrates. Then the nitrogenous component of the phosphatides (chohn in the case of lecithin, at least) must have been obtained from some source, possibly from an amino-acid by the addition of methyl groups /CH3 (CH3), of which cholin, OH.HjC.HgC-NcT^^s (trimethyl-oxyethyl- OH ammonium hydroxide) contains three. Section III. — Metabolism of Proteins. Blood-Proteins.— The two chief proteins of the plasma, serum- globulin and serum-albumin,* must, as has been already pointed out, be recruited from proteins absorbed from the intestine and for the most part, at any rate, profoundly altered in its lumen and in their passage through the epithelium which lines it. Even when proteins are being actively absorbed, the plasma, after the blood-proteins have been separated, contains no substances which give the biuret reaction (p. 444). So that the peptones, which can be demonstrated in the intestinal contents, suffer great changes before or diiring their absorption. The physiological reasons for this alteration are in a measure known, and have already been alluded to in connection with the digestion of proteins. No doubt the far-reaching decom- position of the protein molecule may to some extent facUitate the absorption of protein food. No doubt also it is imperative that such comparatively slightly hydrolysed products as peptone, and particularly proteose, should not appear in quantity in the blood, for when injected they cause profound changes in that liquid, one expression of which is the loss of its power of coagulation, and are rapidly excreted by the kidneys, or separated out into the lymph. But the passage of the food from the stomach is so gradual an affair, the quantity of digesting protein present at one time in any loop of intestine is so small, and the rush of blood which irrigates the * It is probable that plasma contains a mixture of different albumins and globulins. 564 METABOLISM, NUTRITION AND DIETETICS active mucosa is so large, that the concentration of peptone or proteose necessary to produce injurious effects coiild hardly in any case be realized. Again, there is no evidence that the simpler decomposition products of further hydrolysis are not in equal con- centration as" poisonous as proteose and peptone. ' Apart from any influence which it may have in favouring absorp- tion, the complete shattering of the protein molecule has a double significance. In the first place, as already pointed out, the food- proteins cannot be used directly in the upbuilding and repair of the protoplasm (p. 442), since the tissue-proteins differ from them and from each other in the amount and nature of the amino-acids and other groups in their molecule (p. 2). Secondly, under ordinary dietetic conditions a surplus of nitrogen in the protein food has to be got rid of by being converted into urea without being built up into the tissue substance. Here we come upon the fundamental fact that the protein katabolism is not a single uniform process. Two forms may be distinguished which are essentially independent in course and character. One kind varies extremely in its quantita- tive relations, according to the amount of protein in the food. Its chief end-products are urea, representing the nitrogen, and inorganic sulphates, representing the sulphur of the proteins. Since this form of katabolism, as we shall see directly, is not essentially connected with the life and nutrition of the living substance, it is termed •exogenous. The other variety is practically constant in amount for one and the same individual, and independent of the quantity of protein in the food. Its characteristic end-products are kreatinin and neutral sulphur. This form of protein katabolism is essentially an expression of the waste of the living substance itself, and is therefore spoken of as endogenous. Some have stipposed that the intestinal mucosa has as one of its special functions the resjmthesis of a. great part of the digestive decomposition products into the proteins of the blood-plasma. If this is the case, these proteins must be again decomposed in the cells of the various tissues in order that the ' building-stones ' may be recombined to form the tissue-proteins. For the proteins of the organs are not the same as those .of the blood, and the proteins of different ofgans differ characteristically from each other. The •significance of. the synthetic function of the intestinal wall would then lie in this: tha;t from the varying mixture of amino-acids, etc., derived from the food- proteins an always uniform and suitable protein mixture (the blood-proteins) is fabricated for the feeding - of the tissues. Experiments intended to test this hj^othesis have 'hitherto jdelded a negative result. No accumulation of protein in the wall either of the intestine in situ or of the isolated surviving intestine has been detected during absorption of the decomposition products of protein. An alternative assumption, and superficially METABOLISM OF PROTEINS 565 at least a simpler one, is that no more extensive synthesis of proteins occurs in the wall of the alimentary canal than is necessary for the needs of the tissues composing it, and, perhaps in addition, for the maintenance of the normal composition, of the plasma, and that the decomposition products of the proteins are mainly absorbed as such, and pass in the blood to the tissues for which they are destined. If this is the case, the blood-proteins can no longer be looked upon as representing the main current of protein supply for the organs, but rathpr the store of protein material proper to the circulating tissue blood itself, and which confers on it certain chemical and physico- chemical properties {e.g.; the due degree of viscosity) necessary for its function. Slowly accumulated, under ordinary conditions, and slowly consumed, this protein store may, of course, be at the dis- posal of the organs in an emergency — for instance, in starvation — • or may be rapidly recruited from the organ-proteins, as after haemorrhage, just as in prolonged hunger the proteins of skeletal muscle may be utilized to feed the heart. That the blood-proteins can serve as nutritive material for the cells without undergoing digestion in the alimentary canal is well shown by the observations of Carrel and Burrows on the growth of isolated tissues in a medium composed of clotted blood-plasma. But, as previously pointed out in another connection (p. 444), a portion, and probably a large portion, of the digested protein is absorbed from the intestine by the blood in the form of amino-acids. Considerable quantities of these compounds can be separated by dialysis from blood drawn off during the absorption of proteins or by the process of vivi- diffusion (p. 48) (Abel)> Among these amino-acids, glycocoU, alanin, glutaminic acid, and leucin, have been identified. While the quantity of amino-acids in the blood, which is very srnall in the fasting animal, is decidedly increased during protein digestion, it is probable that even in starvation amino-acids derived from the decomposition of the body-proteins are not entirely lacking. It has been surmised that they constitute the form in which proteins are transported from tissue to tissue, as well as the form in which proteins are normally utilized by the cells. Although this cannot be regarded as yet established, there is reason to believe that the amino-acids play a great part in protein metabolism, perhaps as great a part as the dextrose does in the metabolism of the carbo- hydrates. There is some evidence that serum-albumin is more directly related to the proteins of the food than serum-globulin. And it is said that during starvation the albumin is relatively diminished, and the globulin relatively increased. It is, of course, not at all improbable that the plasma-proteins have a double source — organ-proteins on the one hand, food-proteins on the other. In any case, it is certain that serum-albumin and serum-globulin cannot be interchangeable without far-reaching decomposition, for 566 METABOLISM, NUTRITION AND DIETETICS their composition is very different. The globulin, e.g., yields glyco- coll, but the albumin does not. That the plasma-protein mixture maintains a very constant composition in the face of wide variations in the composition of the food-protein is indicated by the following experiment : A horse fed mainly on hay and oats was bled to the amount of 6 litres, and in the total protein of the serum the content of tyrosin and glutaminic acid was determined. In order to eliminate the influence of remains of the food in the digestive canal, nothing was given to the animal for a week. Then 6 litres of blood were agaia removed, and the tyrosin and glutaminic acid in the serum-protein again estimated. The horse was now fed with gliadin (one of the prolamins or alcohol- soluble proteins obtained from flour) , a substance which contains 36'5 per cent, glutaminic acid and 2-37 per cent, tyrosin — ^that is, about the same amount of tjrrosin as the serum-protein, but about four times as much glutaminic acid. The serum-protein was again analyzed for the two amino-acids after this diet. The results of one experiment are shown in the table : Normal. After 8 Days' Hanger. After Feeding with 1,500 Grammes Gliadin. After Feeding again with 1,500 Grammes Gliadin. Tyrosin - Glutaminic acid 2-43 8-85 2 -60 8-20 2-24 7-88 2-52 8-25 No increase in the glutaminic acid content of the serum-protein occurred, although, owing to the loss of blood, much new serum-proteia must have been formed. If the amino-acids of the gliadin were used without change to build up the new serum-protein, three-quarters of the glutaminic acid must have been superfluous, and the nitrogen of this portion may have been straightway changed into urea and excreted. But the possibility that the glutaminic acid, or a portion of it, may have been changed into other amino-acids in the body cannot be excluded. In the case of some of the amino-acids it has been shown that such a transformation occurs (p. 602) (Abderhalden). The high degree of independence of the food and body proteins is still more clearly exhibited in the table from Abderhalden on p. 567, in which the proteins of milk are compared with some of the proteins which must be formed from them in the body of the suckling. The numbers represent percentages of the weight of each protein. Living and Dead Proteins. — Carried to the tissues, the decomposition products of the food-proteins, or the regenerated proteins of the plasma, which ia ordinary language are stUl to be regarded as dead material, are built up into the liviug protoplasm, at any rate to the extent neces- sary to make good its waste. In this form they sojourn for a time within the cells, and then they become dead material again. The nature of this tremendous transformation has, of course, been the subject of speculation, but the truth is that we do not understand wherein the difierence between a living and a dead cell, between a living and a dead particle in one and the same cell, really consists. All we METABOLISM OF PROTEINS 567 know is that now and again a protein molecule or an aggregate of such molecules incorporated in the colloid mass which constitutes the proto- plasm of a muscle-fibre, or a gland-cell, or a nerve-cell, must fall to pieces. , Now and again a molecule of protein, hitherto dead (or perhaps, to speak more correctly, hitherto not a constituent of living protoplasm, since protoplasm is certainly more than protein), or a molecule of a particular amino-acid, or perhaps a polypeptide group intermediate in complexity between amino-acid and protein, coming within the grasp of the molecular forces or chemical affinities of the living substance, is caught up by it, takes on its peculiar motions, acquires its special powers, and is, as we phrase it, made alive. Each cell has the power of selecting and, if necessary, further decomposing or further synthesizing the protein materials offered to it ; so that a particle of serum-albumin, or a mixture of amino-acids may chance to take its place in a liver-cell and help to form bile, while an exactly similar particle or mixture may furnish constituents to an endothelial scale of a capillary and assist in forming lymph, or to a muscular fibre of the heart and help to drive on the blood, or to a spermatozoon and aid in transferring the peculiarities of the father to the offspring. And just as a tomb and a lighthouse, a palace and a church, may be, and have been, built with the same kind of material, or even in succession with the very same stones, so every organ builds up its own characteristic structure from the common quarry of the blood. i "5 «) "5 J 1 1! .2>, Kg i Glycocoll 3-5 3-0 0-5 26-0 4-7 Alania 0-9 2-5 27 2-2 4-2 3-6 3-5 6-6 1-5 Valin i-o 0-9 some some i-o — i-o 0-9 Leucin 10-5 19-5 20-0 18-7 29-0 15-0 II-8 21-4 7-1 Serin 0-2 0-6 0:6 some — Cystin 0-07 — 2-3 07 0-3 — — — 0-6 Asparaginic acid I -2 I-O 3'i 2-5 4-4 2-0 — — 1 0-0 Glutaminic acid 1 1-0 lO-O 77 8-5 17 8-0 0-5 0-8 3-7 Lysin 5-8 — 4-3 — 6-9 ^- Arginin 4-8 — — — 5-4 — 15-5 0-3 — Phenylalanin - 3-2 2-4 3-1 3-8 4-2 2-0 2-2 3*9 — Tyrosin 4-5 1-0 2-1 2 "5 1-5 .3-5 5-2 0-34 3-2 Prolm 3-1 4-0 I-O 2-8 2-3 2-5 1-5 17 3-4 Histidin 2-6 — II-O — 1-5 I It is not any difference in the kind of protein offered them which determines the difference in structure and action between one organ and another. In this quarry alongside of the plasma proteins the tissue cells find what is probably more important for their individual nutrition, the building-stones of the shattered food-protein molecules. They are only under exceptional circumstances confronted with intact molecules of food -proteins. ' The body cells do not know what the kind of food was ' (Abderhalden). In the case of the more highly developed tissues, at least, no mere change of food will radically alter the structure of the cells, nor even, as we have seen, the composition of the tissue proteins. A cell may be fed with different kinds of food, it may be overfed, it may 568 METABOLISM, NUTRITION AND DIETETICS be ill-fed, it may be starved; but its essential peculiarities remain as long as it continues to live. What may be called its organization, per- haps at bottom a more or less metaphorical expression for its essential physico-chemical make-up, dominates its nutrition and function. ' We must assume that many of the enigmatical properties of living matter depend upon the activity of intact protein molecules. We can obtain some idea of the possible variety in the combinations of the '' building-stones " of the proteins by recalling the fact that they are as numerous as the letters of the alphabet, which are capable of expressing an infinite number of thoughts. Every peculiarity of species and every occurrence affecting the individual may be indicated by special com- binations of the "building-stones" — that is to say, by specific proteins. Consequently we may readily understand how peculiarity of species may find expression in the chemical nature of the proteins constituting living matter, and how they may be transmitted through the material contained in the generative cells ' (Kossel). Add to the great variety of compounds rendered possible by the enormous number of permuta- tions aind combinations of the protein ' building-stones, ' * the still greater variety rendered possible by the fact that the quantitative relations of given amino-acids may vary greatly in different proteins, and it will be seen what a practically infinite power of functional adjustment and reaction, correlated with a practically infinite variety of chemical changes in the midst of which the cell still preserves its specificity through and through, may be conferred upon the living substance by its content of protein. Some have supposed that the protein of the living substance is es- sentially different from dead protein, especially in possessing a character- istic instability, a prodigious power of dissociation and reconstruction. All the older theories which attempted to explain this alleged difference require revision in accordance with the newer chemistry of proteins, and speculations on the subject are probably in any case premature till. the constitution of the proteins is thoroughly understood. In the meantime it is enough to say that the velocity of the reactions into which the proteins of liviiig protoplasm or their constituent amino- acids may enter must depend upon intracellular conditions, which may vary rapidly and within wide limits. For example, enzymes may be present in greater or smaller concentration, or be activated and aided more or less powerfully by other substances, or by a more or less favour- able chemical reaction of the medium. The protein itself, too, or such part of it as is ready for decomposition, maj' exist in a physical con- dition now more and again less favourable to the attack of the enzymes. It may not be superfluous at this point to again warn the reader that protoplasm and tissue-proteins are by no means synonymous. The physical, physico-chemical, and chemical changes involved in the katabolism of the colloid aggregates, including water, salts, phosphatides, sterins, and probably fats and dextrose as well as proteins, to which the term protoplasm is applied, may be many and complex before the individual proteins known to the chemist come face to face in the interior of the cells with the ferments which decompose them. On the other hand, it has not been proved that in the katabolic processes of the living substance isolated proteins ever form a stage. It may well be that without the complete decomposition of the protein molecules, or * Twenty different amino-acids, each used only once, but in a different order, would be capable of forming about 2,000,000,000,000,000,000 (two thousand million times a thousand millions) of different polypeptides, all containing the various amino-acids in the same proportions (Abderhalden). METABOLISM OF PROTEINS 569 even without their complete detachment from the protoplasm, indi- vidual amino-acids or mixtures of amino-acids, or polypeptide groups, are cut out of the protoplasmic mass. It is now necessary to follow, as far as is possible, the steps in the degradation of the body-proteins. Since there is reason to believe that these, like the food-proteins, are first split up into the amino-acids from which they were originally synthesized before undergoing further decomposition, a study of protein metabolism is to a great extent a study of the metabolism of amino-acids. In this study it is for most purposes impracticable, even if it were desirable,, to distinguish between amino-acids directly derived from the food, and which have not yet been, and may never be, built up into tissue-proteins, and those derived from the tissue proteins. There is nothing to indicate that the fate of a given amino-acid, once it has reached the blood, depends in the least upon its source. It may be said at once that the katabolism of the amino-acids is not a single and uniform process, one step in which inevitably follows another till the final end-products are reached. On the contrary, certain of the stages may become the starting-points of syntheses, which may lead back to the original or to another protein, or it may be to sugar or to fat. The extent of such sjmthesis, and even in some degree the stage from which it starts, may be assumed to depend upon the needs of the tissues and the relative abundance of protein and of other foods. Formation of Amino-Acids from Tissue-Proteins. — ^That amino- acids are formed in the metabolism of the cells and by the action of intracellular enzymes is indicated by the fact that proteolytic en- zymes (proteases) are invariably present in the tissues, and can be obtained from them by appropriate methods — e.g., by subjecting the organ in a finely divided state to a high pressure and collecting the expressed juice. Not only do unicellular organisms, like leuco- cytes, yeast cells, and bacteria, which must naturally depend upon themselves alone for all enzymatic reactions, yield ferments which have the power of splitting proteins, peptones, and polypeptides into amino-acids, but their existence has been demonstrated in practically all- the organs of the higher animals and man. When a piece of liver, e.g., is removed with aseptic precautions and kept at body-temperature, extensive auto-digestion occurs, and ammonia and other basic substances, glycin, and tryptophane, appear among the products. Tyrosin appears so early that it is scarcely possible to doubt that it must be a product of protein decomposition in the Uver-cells under normal conditions — a decomposition which could be observed also in the organ in situ were the circumstances as favourable. The circumstances are less favourable in an organ whose circulation is going on, because the amino-acids are removed by the blood as they are formed. Further, it is to be assumed that 57° METABOLISM, NUTRITION AND DIETETICS the regulation of the ferment action, which is a characteristic property of the normal cell, becomes feebler the longer it is with- drawn from normal conditions. Similar antolytic processes have been observed in the spleen, muscle, lymph-glands, kidneys, lungs, stomach wall (independently of pepsin), thymus, and placenta; also in pathological new growths like carcinoma, in the breaking down of which and in the removal of such exudations as occur in the alveoli in pneumonia, these proteolytic ferments seem to play a part. It is to be assumed that the S3mtheses of the proteins or their products, which are scarcely less characteristic of the tissue cells than the decompositions effected by them, are also due to the action of separate intracellular ferments or upon the reversed activity of the proteoljrtic ferments. More direct proofs of the production of amino-acids in the tissues are not lacking. A rare condition known as cystinuria has been alluded to on a previous page (p. 482). Here there is a continuous excretion of the sulphur-containing amino-acid cystin in the mine. Sometimes the cystin is accompanied by other amino-acids, as leucin and tj^rosin, a condition which might be called amino- aciduria. Cystinuria, while of course resulting from a gross anomaly in metabolism, is of little clinical importance unless the sparingly soluble cystin should form calculi somewhere in the urinary tract. The most plausible explanation of the condition is that in the normal course of the metabolism each of the ' building-stones ' of the proteins is sooner or later further decomposed by special fer- ments, and that for some reason the ferment which acts on cystin is absent, or if it is stiU produced, the conditions are more or less unfavourable to its action. Unable to take its place in the meta- bolic current, except in so far as it can be utilized to form taurin, and therefore taurocholic acid — for this property it has not lost — cystin becomes a chemical outcast in the body of the cystinuric individual, and is got rid of by the kidneys as an ' unertiployable. ' By comparing the amount of cystin excreted with the amount ingested in the food-proteins and with the (undiminished) amount contained in the tissues (especially the hair and the nails, since keratin is exceptionally rich in cystin), it has been shown that a portion of the cystin in the mine must have come from the tissues (Abderhalden). Observations on animals in prolonged starvation afford additional evidence. The hair and nails continue to grow and to maintain the high cystin content, and taurocholic acid con- tinues to be excreted. In like manner glycin continues to be pro- duced and to unite with cholic acid to form the glycocholic acid of the bile. There are other and more striking proofs that glycin can be formed in the body in large amount. For example, as already stated (p. 475), when benzoic acid is ingested it is not excreted as METABOLISM OF PROTEINS 571 such in the urine, but coupled with glycocoll as hippuric acid. Thus — CgHs.COOH + CHalNHa) .COOH - H2O =CgH5.CO.NH.CHg.COOH Benzoic acid. Glycocoll. Hippuric acid. Benzoic acid, therefore, meets glycin in the body, and combines with it, as fatty acids meet glycerin and combine with it. Even starving animals fed with benzoic acid excrete large quantities of hippuric acid. Yet their tissues, as shown by analysis after death, 3deld as much glycocoll as starving animals which have received no benzoic acid, and excreted little or no hippuric acid. Many other acids which are totally foreign to the body are, when ingested, paired in the same way with glycocoll and excreted in the urine. Even substances whose chemical nature does not permit of a direct union with the glycin are often altered by oxidation or reduction till they can unite with it, and then the coupling takes place, and the conjugated acid is eliminated by the kidneys. The paired or aromatic sulphuric acid which we have already recognized as a normal constituent of the urine affords another instance of this coupling. Cystein among the derivatives of proteins, and glycu- ronic acid (p. 476) among the derivatives of carbo-hydrates, can also unite in the same way with numerous compounds. There is some evidence that the physiological significance of this process is that the toxicity of the foreign substances, or, as in the case of the aromatic sulphuric acid of the urine, of substances formed by bacteria in the intestine, or even produced in the metabolism of the tissues, is diminished by the pairing. The place and manner of formation of hippuric acid have been investigated with the following result : If an excised kidney is per- fused with blood containing benzoic acid, or, better, benzoic acid and glycin, hippuric acid is formed. Oxygen is required, for if the blood is saturated with carbon monoxide, or if serum is employed for perfusion, the synthesis does not take place. The kidney cells must be intact, for if a mixture of blood, glycin, and benzoic acid be added to a minced kidney immediately after its removal from the body, hippuric acid is produced, but not if the kidney has been crushed in a mortar. Nevertheless, there is some evidence that a ferment is concerned, and the known mechanism of similar reactions in the body scarcely permits the physiologist to acquiesce in any other explanation. It must not be forgotten that the urinary constituents which must come into contact with the ferment when the kidney is crushed may injure or inhibit the enzyme. In herbivora hippuric acid cannot normally be detected in the blood; it is present in large quantities in the urine; it must therefore be manufactured in the kidney, not merely separated by it. In certain animals, as the dog) the kidney is the sole seat of the production 572 METABOLISM, NUTRITION AND DIETETICS of hippuric acid. But in the rabbit and the frog some of it must also be formed in other tissues, for after extirpation of the kidneys the administration of benzoic acid causes hippuric acid to appear in the blood. It has, indeed, been recently shown that when the rabbit's liver is perfused with blood containing benzoic acid, hip- puric acid is produced. The benzoic acid required for the normal excretion of hippuric acid comes mainly from substances of the aromatic group contained in vegetable food, but a small amount is produced in the body, since hippuric acid does not entirely dis- appear from the urine in starvation. The differences which may exist in the metabolism of different groups of animals is well illustrated by the fact that in birds, when benzoic acid is given in the food, it unites not with glycin, but with omithin, a derivative of arginin, forming not hippuric acid, but ornithuric acid (dibenzoyl-ornithin). A much more important instance of such a difference will be seen when we come to consider the formation of urea and mric acid. The method by which the presence and the production of glyco- coll in the body are demonstrated by coupling it with benzoic add, and so saving it from decomposition and bringing it to excretion, can also be applied to other amino-acids. If instead of fishing with the bait benzoic acid we fish with a bait called brom-benzol (CgHg.Br) (or bromo-benzene), a substance derived from benzol by the substitution of an atom of bromine for an atom of hydrogen), we capture the amino-acid cystein in the form of a compound called mercapturic acid, produced by the union of brom-benzol with cystein and acetic acid, with oxidation and loss of water. In other words, when this substance is administered, mercapturic acid is excreted in the urine, the cystein, which is very unstable and readily changes into cystin (p. 354), being thus preserved from decom- position. Another instance in which amino-acids (tjnrosin and phenylalanin) , which would normally be decomposed and so escape detection, come to the surface by being excreted in the urine, has already been alluded to in connection with alkaptonuria (p. 477). In this condition, which seems to have no serious significance so far as the well-being of the patient is concerned, not only does the taking of food containing the aforesaid amino-acids lead to an increased excretion of homogentisinic acid, but even in starvation this substance still continues to appear in the urine. Since homo- gentisinic acid is undoubtedly formed from tyrosin and from phenyl- alanin, this observation constitutes a convincing proof that these amino-acids are produced from tissue-proteins. Fate of Amino-Acids in the Body. — The problem of the katabol- ism of proteins is thus reduced to the question. What becomes of the amino-acids ? Where, how, by what stages, and to what end- products are they decomposed ? As to the end-products, the answer METABOLISM OF PROTEINS 573 is easy. The amino-acids, whatever intermediate stages they may pass through, whatever cleavages, oxidations, or reductions they may undergo, jdeld eventually carbon dioxide, water, and comparatively simple nitrogen-containing substances, which after further changes appear in the urine principally as urea, and in birds and reptiles as uric acid. When amino-acids are fed to mammals or introduced parenterally, a very large proportion of the nitrogen appears in the urine as urea. The same is true when, instead of simple amino- acids, polypeptides, like glycyl-glycin, alanyl-alanin, or leucyl-leucin, are given. When amino-acids are administered to birds, the great bulk of the nitrogen is excreted in the form of uric acid. Whether in mammals, and if so to what extent, uric acid is also one of the nitrogenous end-products of the decomposition of ordinary proteins or of the amino-acids which they jdeld, are moot questions. In any case, the most important and characteristic source of the uric acid in mammals and the other groups of animals whose chief nitrogenous end-product is urea, is not the ordinary proteins, but the nucleins which form constituents of the nucleo-proteins. We have no definite information as to the production of water from the hydrogen of the tissues, except what can be theoretically deduced from the statistics of nutrition (p. 608). A few words will be said a little farther on about the production of carbon dioxide from proteins ; we have now to consider the seat and manner of formation of the nitrogenous metabolites. And since in man and the other mammals urea contains, under ordinary conditions, by far the greater part of the excreted nitrogen, it will be well to take it first. Formation of Urea.^-The starting-point of all inquiries as to the place of formation of urea is the fact that it occurs in the blood in small amount (4 to 6 parts per 10,000 in man; 3 to 15 parts per 10,000 in the dog), the largest quantity being found when the food contains most protein and at the height of digestion, the smallest quantity in hunger (Schondorff) . Evidently, then, some, at least, of the urea excreted in the urine may be simply separated by the kidney from the blood; and analysis shows that this is actually the case, for the blood of the renal vein is poorer in urea than that of the renal artery, containing only one-third to one-half as much. If we knew the exact quantity of blood passing through the kidneys of an animal in twenty-four hours, and the average difference in the percentage of urea in the blood coming to and leaving them, we should at once be able to decide whether the whole of the urea in the urine reaches the kidneys ready made, or whether a portion of it is formed by the renal tissue. Although data of this kind are as yet inexact and incomplete, it is not difficult to see that all, or most of, the urea may be simply separated by the kidney. If we take the weight of the kidneys of a dog of 35 kilos at 160 grammes (j^jth of the body-weight is the mean result of a great number o| 574 METABOLISM, NUTRITION AND DIETETICS observations in man), and the average quantity of blood in them at rather less than one-fourth of their weight, or 35 grammes, and con- sider that this quantity of blood passes through them in the average time required to complete the circulation from renal artery to renal vein, or, say, ten seconds, we get about 300 kilos of blood as the flow through the kidneys in twenty-four hours. Even at 0-3 per 1,000, the urea in 300 kilos of blood would amount to 90 grammes. Now, Voit found that a dog of 35 kilos body-weight, on the minimum protein diet (450 to 500 grammes of lean meat per day) which sufficed to maintain its weight, excreted 35 to 40 grammes of urea in the twenty-four hours. If, then, the renal epithelium separated somewhat less than half of the 90 grammes urea ofiered to it in the circulating blood, the whole excre- tion in the urine could be accounted for, and the blood of the renal vein would still contain more than half as much urea as that of the renal artery. So that the whole of the urea in the urine may be simply separated by the kidney from the ready-made urea of the blood. Another line. of evidence leads to the same conclusion: that the kidney is, at all events, not an important seat of urea-formation. When both renal arteries are tied, or both kidneys extirpated, in a dog, urea accumulates in the blood and tissues ; and, upon the whole, as rriuch urea is formed during the first twenty-four hours of the short period of life which remains to the animal as would under normal circumstances have been excreted in the urine. Where, then, is urea chiefly formed ? The answer to this question is that, while some urea is probably produced from amino-acids in all the tissues, one organ is particularly associated with this function — ^namely, the liver. There is no reason to suppose that the hepatic cells, so far as the repair of their own protoplasm or the supply of energy for their own special work is concerned, require to metabolize particularly large quantities of amino-acids as compared, for instance, with the muscles. Glycin, however, they must have for the manufacture of glycocholic, and cystin for the manufacture of the taurin of taurocholic acid. In addition, the liver is known to possess the power of utilizing amino-acids for the formation of dextrose and eventually of glyco- gen, and a portion of the surplus amino-acids of the food may be withdrawn from the blood of the portal vein for this purpose, just as the surplus of dextrose is withdrawn. The liver contains a relatively large amount of urea, and there is strong evidence that it is the manufactory in which a great part of the nitrogenous relics of broken-down proteins reach the final stage of urea. This evidence may be summed up as follows: (i) An excised ' surviving ' liver forms urea from ammonium carbonate mixed with the blood passed through its vessels, while no urea is formed when blood containing ammonium carbonate is sent through the kidney or through muscles. Other salts of am- monium, such as the lactate, the formate, and the carbamate, imder- go a like transformation in the liver. It is difficult, in the light of this experiment, to resist the conclusion that the increase in the METABOLISM OF PROTEINS 575 excretion of urea in man, when salts of ammonia are -taken by the mouth, is due to a similar action of the hepatic cells. (2) If blood from a dog killed during digestion is perfused through an excised liver, some urea is formed, which cannot be simply washed out of the liver-cells, because when the blood of a fasting animal is treated in the same way there is no apparent formation of urea (v. Schroeder). This suggests that during digestion certain substances which the liver is capable of changing into urea enter the blood in such amount that a surplus remains for a time unaltered. These substances may come directly from the intestine; or they may be products of general metabolism, which is increased while •digestion is going on; or they may arise both in the intestine and in the tissues. Leucin — -which, as we have seen, is constantly, or, at least, very frequently, present in the intestine during digestion — can certainly be changed into urea in the body. So can other amino-acids of the fatty series, like glycocoU or glycin, and aspartic acid, and it has been shown by perfusion experiments that this change can take place in the liver. Further, the blood of the portal vein during digestion contains several times as much ammonia as the arterial blood, and the excess disappears in the liver. (3) Uric acid — which in birds is the chief end-product of protein metabolism, as urea is in mammals — is formed in the goose largely, and almost exclusively, in the liver. This has been most clearly shown by the experiments of Minkowski, who took advantage of the communication between the portal and renal-portal veins (p. 379) to extirpate the liver in geese. When the portal is ligated the blood from the alimentary canal can still pass by the round- about road of the kidney to the inferior cava, and the animals survive for six to twenty hours. While in the normal goose 50 to 60 per cent, of the total nitrogen is eliminated as uric acid in the urine, and only 9 to 18 per cent, as ammonia, in the operated goose uric acid represents only 3 to 6 per cent, of the total nitrogen, and ammonia 50 to 60 per cent. A quantity of lactic acid equivalent to the ammonia appears in the urine of the operated animal, none at all in the urine of the normal bird. The small amount of urea in the normal urine of the goose is not affected by extirpation of the liver. And while urea, when injected into the blood, is in the normal goose excreted as uric acid, it is in the animal that has lost its liver eliminated in the urine unchanged. (4) After removal of the liver in frogs, or in dogs which have survived the previous connection of the portal vein with the inferior vena cava by an Eck's fistula (p. 379), the quantity of urea excreted is markedly diminished, and the ammonium salts in the urine are increased. When the Eck's fistula is established and the portal vein tied, without any further interference with the hepatic circula- tion, the amount of urea in the urine is not lessened to nearly the 576 METABOLISM, NUTRITION AND DIETETICS same extent,* evidently because the substances from which urea is formed still, for the most part, gain access to the liver through the hepatic artery and by means of the back-flow which is known to take place through the hepatic vein. Yet while in normal dogs the proportion of ammonia to urea in the urine is only i: 22 to i: 73, in dogs with Eck's fistula it rises to i: 8 to i: 33. If the animals are kept on a diet poor in proteins, no symptoms may develop for a considerable time. But if much protein is given, characteristic symptoms, including convulsions, always appear. These may be produced by the saturation of the organism with ammonia com- pounds, which are formed from the proteins as in the normal animal, but which the liver, with its circulation crippled, is unable to cope with, and to completely change into urea, although the statement has been made that when ammonia or ammonium salts are injected into the blood larger quantities must be present to produce these symptoms than are found in animals with the Eck's fistula. Although the portal vein carries to the liver a much greater supply of blood than the hepatic artery, ligation of the latter causes a greater diminution in the ratio of the amount of urea to the total nitrogen in the urine than ligation of the former. This indicates that a good supply of oxygen is an important factor in the formation of urea in the liver (Doyon and Duf ourt) . But this is no proof that the process by which it is formed is an oxidation. The work of the liver, like that of other tissues, is no doubt deranged by lack of oxygen. (5) In acute yellow atrophy, and in extensive fatty degeneration of the liver, urea may almost disappear from the urine, and leucin, t5n-osin, and other amino-acids may appear in it along with a much larger amount of ammonia than normal. Ifere it may be supposed that the amino-acids and ammonia formed in the intestine in the digestion and absorption of proteins, perhaps also amino- groups formed in the tissues which would normally be culled from the blood by the hepatic cells for the manufacture of urea, pass unchanged through the degenerated liver, and are excreted by the kidney. It would, however, be very easy to overdo this argument; for it is sometimes observed that in pathological and experimental condi- tions in which the liver has suffered severely considerable quantities of urea continue to be excreted. Urea does not entirely cease to be produced even when the liver is removed; and it must again be pointed out that there is reason to believe that the formation of urea is not a function peculiar to the liver, but one shared prob- ably with all tissues. The liver certainly does not arrest the whole of the amino-acids coming from the alimentary canal ; for the non- protein nitrogen in the muscles is distinctly increased during the absorption of amino-acids, and the muscular tissue, even when freed from blood, contains some urea, which in all probability is formed METABOLISM OF PROTEINS 577 there in the decomposition of amino-acids. Some writers, indeed, take the view that the muscles, containing as they do three-fourths of the proteins of the body, and utiUzing as they appear to do a large proportion of the amino-acids of the food-protein, are more important seats of urea formation than the liver. Yet the fact that it is far easier to demonstrate this power — -e.g., by perfusion experi- ments — for the liver than for such tissues as the muscles, renders.it difficult to avoid the conclusion that in the preparation of an end- product so important as that in which the great bulk of the nitrogen leaves the body, a certain degree of specialization has been developed, and that this preparation has been largely entrusted to a special organ. And, while it may be true that larger amounts of amino-acids are taken up and utilized by the muscles than by the liver under certain conditions, this does not show that amino-grdups removed from the amino-acids in the muscles may not be largely transferred to the liver before being changed into urea. Further, the transforma- tion of amino-acids into dextrose (and glycogen) may be assumed to entail a considerable absorption of amino-acids by the hepatic cells. Processes by which Urea is formed. — In the case of only one of the amino-acids derived from proteins can urea be obtained by a simple process of hydrolytic cleavage. This is arginin (a-amino-S-guanidin- «-valerianic acid) — that is to say, normal valerianic acid, CH3.CH2.CH3.CH2.COOH, S y ii a in which an amino group is attached to the a carbon atom, while guanidin ^^u'^'^C.NH is attached to the S carbon atom (p. 557). When arginin is hydrolysed by barium hydroxide it yields urea and ornithin (diamino-valerianic acid), half of the nitrogen of the arginin appearing in each. Thus, NHa ^^a^C.NH.CHa.CHa.CHa.CH.COOH + HgO = Avginin. NHa NH*/*^ =0-H NH2.CH2.CHa.CH3.CH.COOH Uroa. Ornithin. The amount of arginin, and therefore the amount of urea which can be artificially obtained in this way, varies extremely with the different proteins. Thus, salmin, a protamin (p. 2), prepared from the milt of salmon, yields 84-3 per cent, of its weight of arginin, while the casein of cow's milk yields only 4-8 per cent., and gluten-fibrin, one of the proteins of wheat, only 3 per cent. In the body the hydrolysis of arginin to urea and ornithin is accomplished by the ferment arginase (Kossel and Dakin). This ferment is found in the liver, and also in many other organs. The urea formed in this way appears very rapidly in the urine. The ornithin itself is then more slowly transformed into urea. Since the ordinary food-proteins are poor in arginin, the 37 578 METABOLISM, NUTRITION AND DIETETICS amount of urea which can possibly be formed in mammalian metabolism by this process cannot be large, even if most of the arginin, as is the case when it is fed to an animal, is transformed into urea. There is no reason to suppose that urea can be directly split off from the other amino-acids with which we are concerned. A comparison of their constitutional formulae with that of urea (or with that of uric acid) shows that a more far-reaching decomposition must take place before products are obtained from which urea (or uric acid) can be formed. Urea has been artificially obtained from protein by oxidation with an ammoniacal solution of permanganate at body -temperature. When the protein is- first split into its cleavage products, and these are then oxidized, a very large amount of urea is produced — e.g., as much as 3 grammes of urea from lo grammes of glycin. While these facts suggest possible ways of formation of urea in the body, we cannot assume that what happens in the test-tube must happen in the tissues. The best evidence is to the effect that in the body the removal of the amino-group (NHg) in the form of ammonia from the amino-acids is the essential step in the formation of at least a great part of the urea, which is then sjmthesized from ammonia and carbonic acid. The possibility exists that this deamination (or deamidi- zation) of the amino-bodies is the result of hydrolysis, or of oxidation, or of reduction, or of a combination of these processes. In any case it is to such ammonium compounds as have been already mentioned as being transformed into urea when circulated through an excised liver (p. 574) that we have to look for the source of a portion, and probably a large portion, of the urea. Ammonia in the form of carbonate or carbamate is constantly found in the blood (p. 575). The excess of ammonia in the portal blood, which, however, is not admitted by all observers to be very large or very constant, has been interpreted as indicating that a considerable decomposition of amino-acids with libera- tion of the amino-groups occurs in the intestinal lumen or the intestinal wall. It is not established beyond doubt that ammonia is itself present in the protein molecule, or that its liberation in the hydrolysis of proteins can take place except at the expense of further decomposition of amino-bodies. It has been shown, however, that a great part of the ammonia in the blood is produced in the decomposition of protein in the digestive tube by putrefactive bacteria (Folin and Denis) . This is a necessary part of the reaction by which phenol and indol arc formed in the intestine. It has been generally taught that the deamidization of the surplus of amino-bodies takes place chiefly in the liver, the extra nitrogen being thus ' shunted ' out of the blood-stream before it has had a chance to reach the tissues. It would seem more advantageous in the light of our present knowledge that a large and, so to say, a miscellaneous assortment of amino-bodies should be placed at the disposal of the tissues to facilitate the selection of those which are indispensable. We have seen that tissues such as muscle can and do take up amino- acids when protein is digested in the intestine, and it is very probable that they take up not merely the relatively small amount necessary to replace their wear and tear, but also a portion of the surplus, which after deamidization in the ceils takes its place as a source of energy to drive the machine. The nitrogen in the form of ammonia may pass back into the blood, and may thtis be carried to the liver for conversion into urea. It is not necessary, however, to suppose that aU of the nitrogen must perforce make this journey before being changed into urea. There is evidence that all the tissues share to some extent with the liver the power of forming urea just as they share with the liver the METABOLISM OF PROTEINS 579 power of splitting off NH2 from the amino-bodies. It may be that the liver surpasses other tissues in its deamidizing power just as it seems to surpass other tissues in its power of transforming ammonium com- pounds iato urea. But this does not prevent at least a considerable proportion of the amino-substances absorbed from the intestine from passing into the general circulation. It is of importance to remark that such hydrolytic cleavages as are associated with the splitting of protein into amuio-acids, etc., only slightly reduce the available energy of the compounds. If, as is most probable, the liberation of the nitrogen from the amino-acids is also accomplished by hydrolytic cleavage (sup- posedly by a ferment desaminase), the residue, relatively rich in carbon, will still be available for yielding to the body by its oxidation an amount of energy not much less than could be obtained from the original protein. The combination of ammonia with carbon dioxide and the conversion of the carbonate into urea, perhaps through the intermediate stage of ammonium carbamate, does not require any oxidation. Thus, /OH /O.NH, /O.NH4 /NH, C^O +2NH3=C=0 ; -H20=C=-0 ; -H30=Ci^0 \OH \O.NH4 \NH2 \NH2 Carbonic acid. Ammonium Ammonium Urea, carbonate. carbamate. Another way in which some of the urea may be produced is by the direct formation of ammonium carbamate in the katabolism of amino- acids without the preliminary liberation of ammonia. By the loss of a molecule of water the carbamate would then become urea. But if, as there is every reason to believe, a part of the carbonaceous residue is converted into carbo-hydrate, a certain amount of oxidation must occur in the transformation. Such compounds as guanin, sarkin or hypoxanthin, xanthin, uric acid, and kreatin, used to be cited as among the possible intermediate substances between protein and urea. But while there is now complete evidence that the first three bodies can be and are converted into uric acid, there is nothing at all to indicate that they are stages on the way to urea. Uric acid is, indeed, very closely related to urea, and can be made to yield it by oxidation outside the body. Not only so, but it is, in part at least, excreted as urea when given to a mammal by the mouth and it replaces urea as the great end-product of nitro- genous metabolism almost wholly in the urine of birds and reptiles. But none of these things can be admitted as evidence that in the normal metabolism of mammals uric acid lies on the direct line from protein to urea. Kreatin exists in the body in greater amount than any of these, muscle containing from 0-2 to 0-4 per cent, of it; and the total quantity of nitrogen present at any given time as kreatin is not only greater than that of the nitrogen present in urea, but greater than the whole excretion of nitrogen in twenty-four hours. But although there are facts which indicate that kreatin is an important derivative of the decomposed tissue proteins (p. 586) there is no evidence that it is related to urea formation. Formation of Uric Acid. — Uric acid, like urea, is separated from the blood by the kidneys, not to any appreciable extent formed in them. In birds, and often in man, it can be detected in normal blood. It is present in increased amount in the blood and transuda- tions of gouty patients, in whose joints and ear-cartilages it often 58o METABOLISM, NUTRITION AND DIETETICS forms concretions. ' Chalk-stones ' may contain more than half their weight of sodium urate. As to the place and manner of formation of uric acid, it has already been stated that in birds, after extirpation of the liver, the uric acid excretion is greatly diminished, and that ammonium lactate appears instead in the urine. The simplest interpretation of this result is, that ammonia and lactic acid pass into the urine because they can no longer be utilized for the sjmthesis of uric acid. Chemical schemata can indeed be constructed, which show more or less plausibly how lactic acid, pyuvic acid (p. 537), and other substances reacting with ammonia or with the urea derived from it (and birds form some urea) might jdeld uric acid. It has been further stated that when blood containing ammonium lactate is circtdated through the surviving liver of the goose, an increase in the uric acid content of the blood, occurs. As demonstrated by control experiments, this inciease is too great to be due merely to the sweeping out of pre- viously formed uric acid from the hepatic cells; also the feeding of lactic acid, pyuvic acid, and other organic acids leads to an increased output of uric acid. The story seems fairly complete, although criticisms have not been lacking. It has been suggested, for instance, that for some reason the loss of the liver leads to acidosis, an in- creased production of acids, especially lactic acid, in the organism; that ammonia, which would otherwise be employed in the formation of uric acid, is needed to neutralize these acids, and that the appear- ance of this ammonia in the urine is only a secondary consequence of the elimination of the liver. The deficiency in the uric acid excreted, it is said, is therefore due, not to inability on the part of the remaining tissues to form uric acid, but to the absence of the ammonia which they require for its formation. This criticism, if it were admitted as against the ciurent interpretation of such ob- servations on the bird's liver, could scarcely be denied some validity as against the current interpretation of similar observations on the results of interference with the mammalian liver. It is therefore important to point out that there is still the same deficiency of uric acid when alkali is administered to neutralize the acids, although ammonia ought now to be available. There can be no question, then, that the liver in birds is the seat of an extensive synthesis of uric acid, and theie is little doubt that ammonia compounds are essentially concerned in the process, whatever the role of the lactic or other acids may be. A similar synthetic formation of uric acid from ammonia and a derivative of lactic acid may take place in mammals, and probably exclusively in the liver, but it is of much less importance. Another way in which uric acid arises both in mammals and in birds is by the spUtting and oxidation of nucleins This is by far the most important mode of formation in mammals, as synthesis is the chief mode of formation in birds. In both groups METABOLISM OF PROTEINS 581 of animals the oxidative production of uric acid takes place, not in any particular organ, but in the tissues in general, including the liver. It has been shown that when air is blown through a mixture of splenic pulp and blood, uric acid is formed from purin bodies already present in the spleen. When the quantity of these is increased by the decomposition of nucleins induced, by slight putrefaction, the yield of uric acid is also increased. Uric acid is also formed by the perfectly fresh surviving spleen, liver, and thymus in the presence of oxygen, and the quantity is increased when purin bodies are artificially added. Sources of the Uric Acid. — It is well established that in the bird it arises both from amino-acids derived from the hydrolysis of protein and from nuclein compounds and their derivatives in the food and tissues. The amino-acids constitute by far the greatest source of uric acid in these animals and in the reptiles, and it is practically certain that the course of the decomposition of the amino-acids and the form in which nitrogen is liberated from them in its transformation into this end-product are not essentially differ- ent from what obtains in the formation of urea in the mammal and the amphibian. This is sufficiently illustrated by the role played by ammonia and ammonia compounds in the production of uric acid in the birds and their congeners. In the mammal, the taking of food rich in nucleated cells, and therefore in nucleo-proteins and nucleins, the characteristic conjugated proteins of nuclei (thymus gland, pig's pancreas, and herring roe), or of food rich in purin bases (Liebig's meat extract), increases the quantity of uric acid in the urine. The increase is mainly due to the production of uric acid from the nuclein substances of the food. But this is not the only source of the uric acid, since extracts of the thymus gland containing only traces of nucleins or nucleic acid cause, when in- jected, a characteristic increase in the uric acid excretion, just as the entire gland does when taken by the mouth. And during the period of increased nitrogen excretion occasioned by a meal contain- ing protein, the increase in the uric acid occurs particularly in the hours immediately following the ingestion of the food, and does not last so long as the increase in the urea. Now, the nucleins of the food are comparatively little affected during the earlier stages of digestion (Hopkins and Hope). Whether in mammals any portion of the uric acid comes from amino-acids is still in doubt, but there are facts which indicate that a fraction of it may do so. We may conclude, therefore, that in the mammal, as well as in the bird, a portion of the uric acid, although certainly a far smaller, portion in the mammal, is derived from bodies other than the nuclein substances of the food — that is Lo say, from the nuclein substances of the tissues contained particularly in the cell-nuclei and probably from the ordinary proteins of both food and tissues. The portion derived 582 METABOLISM, NUTRITION AND DIETETICS from the proteins may be assumed to be that small fraction v.hich has already been spoken of as synthetically formed. Metabolism of the Nucleic Acids and Purin Bases. — Our know- ledge of the metabolism of the nucleo-proteins and nucleins has been greatly augmented in recent years. When nucleo-protein is digested by gastric juice, a certain amount of protein is easily split off and hydrolysed to peptone and the other ordinary products of proteolysis. An insoluble residue of nuclein remains. This is acted upon with difficulty by gastric juice, although eventually an active juice will split it up also. By the action of pancreatic juice, or by heating with dilute acids, it is more easily hydrolysed, yidding a further quantity of protein along with nucleic acid. This second fraction of protein, which is split off with so much more difficulty than the first, undergoes proteolysis in the usual way. The result- ing amino-acids no doubt take their place in the general metabolism precisely like the amino-acids derived from ordinary proteins, and yield the same end-products. As regards the nucleic acid (or rather acids, since different nucleo-proteins contain different nucleic acids), pancreatic juice is practically inert, although succus entericus can effect a partial hydrolysis. For their complete decomposition more drastic treatment is required — ^namely, heating with hydrochloric acid in a sealed tube. Thus treated, nucleic acids yield a number of components, out of which they may be assumed to be built up, as the proteins are built up out of amino-acids, etc. The charac- teristic components are purin bases (adenin, C5HJN4.NH2; guanin, C5H3N4O.NH2; hypoxanthin, CBH4N4O; and xanthin, C5H4N4O2); pyrimidin bases (TU-acil, C4H4Ng02; cytosin, C4H3NgO.NH2; thymin, C4H3N2O2.CH3) ; phosphoric acid and a carbo-hydrate group. Some of the nucleic acids contain all these components; they are sometimes spoken of as the true nucleic acids. In others certain of the components are absent, and to these nucleic acids the name nucleotids has been applied. The purin bases are always present. The carbo- hydrate group varies in different nucleic acids, being in some a hexose (p. 529), in others a pentose (p. 482). The pentose d-Tihose is especially often met with. It is probable that the nucleotids are merely simpler decomposition products of the true nucleic acids. Thus, inosinic acid, a nucleotid first isolated from meat extract, yields phosphoric acid, (i-ribose, and the purin base hypoxanthin. The nucleotid guanylic acid found in the pancreas yields phosphoric acid, CH N— C— N Purin. HC C— NH >CH N— C— N Adenin. NH— CO NH- -co N = =C.OH NH— CO NH2.C C— NH CO C— NH HC C— NH CO C— NH II II /™ I ! >o II 1 >^ I II /' N — C— N NH— C— NH N— C— N NH— C— N Guanin. Uric acid. Hypoxanthin. Xanthin. CH Besides the purin bases combined in the nuclein substances, purin bases and uric acid are widely spread in the tissues in the free state, although in very small amounts. A portion of the intake of purin bodies is therefore ready formed, especially in the animal constituents of the food, and does not require the decomposition of nucleic acid for its liberation. The nuclei of vegetable cells contain nucleo-proteins, and accordingly can contribute to the purin intake. The most interesting contribution of vegetable origin has been previously alluded to (p. 475) — ^namely, the methyl purins forming the active principles of tea, coffee, and cocoa, caffein, or I, 3, 7-trimethylxanthin (CgH]^oN402), tbeobromin, or 3, 7-dimethyl- xanthin (C7H8N4O2), and theophyllin, or i, 3 - dimethylxanthin (CgHgN^Oa). CH3.N— CO GO C— N.CHg CO C— N.CHg CO C— NH j,ZH I I ^Ci^ I ^CH CH3.N — C^ CHg.N- C— N CafTein. Theobromin. Theophyllin. Nucleic acid, as stated, can be partially decomposed by the succus entericus, by means of a ferment called nuclease or, more accurately, nucleic-acidase. The groups into which it is split are nucleotids (see above). By another ferment, nucleotidase, a portion, at any rate, of the nucleotids is further decomposed to yield nucleosides, bodies of the glucoside class containing a com- pound of a purin base with the carbo-hydrate group of the nucleic acid, to which phosphoric acid is also coupled. Beyond this stage the hydrolysis of nucleic acid does not proceed in the intestine. 584 METABOLISM, NUTRITION AND DIETETICi, The resultant products, probably along with unchanged nucleic acid, are absorbed, mainly at least, by way of the bloodvessels. It will be well, however, to remember that our knowledge of the digestion of the nuclein bodies is still incomplete, and the natural tendency of the mind to think in diagrams is apt to give it greater precision than is justified by the facts; for example, it is known that even gastric juice is capable of liberating some of the phosphoric acid from nucleo-proteins. In the tissues the absorbed products of the digestion of nucleic acids may be partially utilized without further decomposition for the synthesis of nucleo-proteins, to take the place of those which are destroyed in the metabolism of the cells; or they may be split com- pletely into their components, and these resynthesized. Finally, and this fate is probably not long delayed in the case of the surplus of purin compounds contained in ordinary dietaries, both the purins of the food and the purins arising from the waste of the tissues are for the most part converted into uric acid and excreted in the urine. Small quantities of purins leave the body in the faeces (p. 419). The phosphoric acid can be utilized not only for the building of nucleo-proteins, but for the synthesis of phosphatides. Eventually it is eliminated as phosphates in the urine. The carbo-hydrate groups, so far as they are not utilized in the synthesis of nucleic acids, may be supposed to undergo metabolism like other carbo- hydrates. The metabolic history of the pyrimidin bases has not been made clear. Steps in Formation of Uric Acid. — As to the manner in which uric acid arises from the nuclein substances, we may picture the process as taking place by the following steps : Certain organs have been shown to contain ferments which split up nucleo-proteins into protein and nucleic acid. This nucleic acid, or nucleic acid arising in other ways in the metabolism of nuclein, and also any nucleic acid absorbed as such from the alimentary canal in the digestion of nuclein-containing substances, are then decomposed by another ferment, similar to or identical with the nuclease or nucleic-acidase previously encountered in the intestine. The resulting nucleotids are split up by a special ferment (nucleotidase) so as to yield nucleosides. These, are in turn decomposed by appropriate enzjones (nucleo- sidases), so that we finally arrive at the individual ' building-stones,' the nucleic acid molecule, phosphoric acid, the carbo-hydrate group, pyrimidin and purin bases, especially adenin and guanin. Then follows the action of ferments (adenase and guanase), which remove the amino-group from these purin bases, transforming adenin into hypoxanthin, and guanin into xanthin (Jones). The deaminiza- tion is associated with hydrolysis. Thus : CgHgNs -I- H2O =C5H4N40 -I- NH3 ; CgHgNsO + H2O =C6H4N402 + NHj. Adenin. Hypoxanthin. Guanin, Xanihin. METABOLISM OF PROTEINS 585 By oxidation hypoxanthin is changed into xanthin and xanthin into uric acid, and the oxidation seems to be accomphshed by a separate oxidizing ferment, xanthin oxidase, whose action may be thus represented : C5H4N4O+ O =CgH5N402; CBH6N4O2+ O =C6H4N403. Hypoxanthin. Xanthin. Xanthin. Uric Acid. Evidence of the existence of these ferments, and of their wide dis- tribution, has been obtained by making experiments on the various substances mentioned with extracts of different tissues. The portion of the uric acid which comes from the food (mainly from the purin bodies in it) is sometimes denominated the exogenous portion, while that which arises from the tissues is called the endog- enous portion. The latter moiety, which generally amoimts to about 0-6 gramme in the twenty-four hours, can be estimated by restricting the diet to articles of food free from purin bodies, such as bread, milk, cheese, eggs, and butter. It is stated that the endog- enous uric acid remains practically constant in the same individual under constant conditions, and is unaffected by changes in the diet. The total excretion of uric acid (and the other purin bodies) is by no means identical with the sum of the uric acid taken in as purin bases in the food and that produced in the body. A con- siderable destruction of uric acid (and other purin bodies) goes on in the body, and mainly in the liver. The quantity of endogenous uric acid excreted by the kidneys bears a certain ratio to the total amount which has entered the circulation. This ratio varies much in different mammalian species. In man a fuU half is said to be excreted and about a half destroyed, being mainly changed into urea. Some of the exogenous moiety is also broken down. When uric acid is heated in a sealed tube with strong hydrochloric acid, it is broken up into glycin, carbon dioxide, and ammonia. There are grounds for believing that a similar decomposition takes place in the body, and that the products are then transformed to urea in the liver. The process of uricolysis, or destruction of uric acid, is usually attributed to a ferment called the uricolytic ferfnent, and it has been supposed that one of the factors in the production of gout may be a diminution in the amount or activity of this ferment. In some cases it is said to be entirely absent. It is doubtful, however, whether in man and the anthropoid apes the oxidizing enzyme, uricase or uricoxydase, which oxidizes uric acid to allantoin (C4HgN403), exists. In all other mammals hitherto investigated it has been found in some of the tissues. In accordance with this, only a trace of allantoin is present in human urine and in the urine of the higher apes, while in the other mammals— for example, in the dog — a large proportion of the purin excretion assumes this form. It is probable that there is more than one way in which uric acid may be decomposed in the body, and, if so, that there is 586 METABOLISM, NUTRITION AND DIETETICS more than one ferment concerned in its transformation. It would be well, therefore, not to speak of uricolysis as if it were synonymous with the well-ascertained process by which allantoin is formed from uric acid, and not to identify all enzymes which may take part in uricolysis with uricoxydase. It is worthy of remark in this connection, as a further illustration of the differences which may exist in the piirin metabolism in different kinds of animals, that in man and the anthropoid apes the quantity of purin bases in the urine is small in proportion to the quantity of uric acid. In the pig, which is included among the animals that form allantoin from uric acid, the purin bases exceed the uric acid in amount, whereas in the dog, which likewise excretes allantoin, the purin bases exist in very small amount compared with the uric acid. In concluding our consideration of the metabolism of the nucleic acids, the question may be raised whether it is related to the metabo- lism of the other substances — carbo-hydrates, fats, and proteins— in such a way that derivatives of nucleic acid can contribute to the formation of any of these, or derivatives of carbo-hydrates, fats, or proteins contribute to the formation of any of the components of nucleic acid. It has been already mentioned that the phosphoric acid can aid in the synthesis of phosphatides, and that the carbo- hydrate groups probably take their place in the ordinary carbo- hydrate metabolism. There is no evidence that the purin bases can ■ take part or can yield products capable of taking part in the forma- tion of any of the other substances. The ptirin metabolism, so far as is known, moves in a closed circuit. Of the fate of the pyrimidin bases nothing is surely known. Without doubt nucleic acid can be formed in the body when none is contained in the food. More than one source of the phosphoric acid and the carbo-hydrate are known and have been already pointed out, but how and from what materials the purin and pjnrimidin bases are formed cannot yet be stated. Recently the S5mthesis of nucleosides has been accomplished in the laboratory (Fischer). It only needs the introduction of phosphoric acid in the appropriate way into the molecule to give nucleic acid. The Significance of Kreatin and Kreatinin in Protein Metabolism. — A glance at the tables of composition of the urine (p. 471) will show that kreatinin, as regards the quantity excreted, is a much more important product of nitrogenous metabolism than uric acid, stand- ing, indeed, with the ammonia compounds, next in order to urea; but our information as to its source and significance is very scanty. Kreatin is a-methylguanidiii-acetic acid, and kreatinin is derived from it by loss of the elements of water : /NHa /NHg /NHg /NH— CO C^NH C^NH CH3.COOH C^NH C^NH NNHg XNH-CHgu \N{CH3).CH2.C00H ^-N(CH3).CH2 Guanidin. Methylguanidin. Acetic acid. Kreatin. Kreatinin. METABOLISM OF PROTEINS 587 On heating with baryta-water kreatin is decomposed, yielding urea, methylglycocoll or sarcosin, and other substances. It can be prepared synthetically from sarcosin and cyanamid. Thus: /NHg H.N{CH3) /NHo Cz£.-N + I = C^NH CHg.COOH \N(CH3).CH2.C00H Cyanamid. Methylglycocoll. Kreatin. Kreatin is found in considerable amount (p. 741) in muscular tissue, and in traces in other tissues and in blood-plasma. Kreatinin can be so readily obtained from kreatin outside the body that it is tempting to suppose that the portion of the kreatinin of the urine which is not formed from the kreatin in the food is derived from the kreatin of the muscles and other tissues, and many theories have been evolved to connect the kreatinin of urine with the kreatin of the muscles. But it is doubtful whether there is any direct connection. The alleged absence of kreatinin from muscle seemed to be opposed to the idea that the kreatin store of the muscular tissue was an important source of urinary kreatinin ; for if a constant transformation of this kind was going on, traces of kreatinin not yet absorbed by the blood might have been expected to be present in the muscles. Recently, however, it has been reported that small quantities of kreatinin do exist in fresh muscle (4 to 8 milligrammes in 100 grammes of tissue), and that when the muscle is allowed to undergo autolysis the kreatinin increases at a very uniform rate at the expense of the kreatin. Added kreatin experiences the same fate as the kreatin originally present, while added kreatinin inhibits the reaction, or even reverses it (Myers and Fine) . A parallelism with the conversion of glycogen into dextrose in the liver easily suggests itself, and it is possible that we are here in the presence of a normal reaction which may account for at least a portion of the kreatinin excretion. It is probable that both kreatin and kreatinin can undergo changes in the body, especially in the liver, and it is possible that the products may be further utilized in metabolism. If this were so, the kreatin store of the muscles would acquire new significance as a reserve of useful material with perhaps a long and varied metabolic career before it, and would not constitute merely a temporary depot of waste material whose metabolic history was ended, and which was waiting to be excreted. However this may be, the constancy of the kreatinin elimination on a meat-free diet (p. 475), and its complete independence of the changes in the total nitrogen excretion, show that it has a different significance in protein metabolism from the urea. Evidence is accumulating that it is especially in the metabolism of the organized or tissue protein that the product eventually excreted as kreatinin arises; in other words, that it represents especially the nitrogenous waste connected with the wear and tear of the bodily machinery, while urea represents also, and under ordinary conditions of diet 588 METABOLISM. NUTRITION AND DIETETICS chiefly, the nitrogen of the surplus amino-acids which are not utilized in the building of new or the repair of old tissue elements. The fact that the amount of kreatinin excreted by different persons seems to be related to the weight of active tissue in the body, exclud- ing fat, is in favour of this suggestion, and there is other evidence pointing in the same direction; for example, in ordinary circum- stances kreatin is either absent firom the urine or present in very small amount, except in young children. When, however, the de- composition of tissue-protein is abnormally increased, as in starva- tion, in fevers, in women after delivery, while involution of the uterus and the associated destruction of a considerable mass of smooth muscle is taking place, kreatin appears in larger quantities in the urine, perhaps because it can no longer be all converted into kreatinin. Now, the increased excretion of kreatin in starvation can be prevented by giving carbo-hydrate food, which is known (p. 595) to lead to sparing of tissue-protein (Mendel and Rose). The statement that the content of the urine in kreatinin is in- creased by muscular work may indicate that the muscular machine wears out faster during activity than during rest, or perhaps only that already-formed kreatin leaves the muscles in greater amount when the blood- flow is increased; but recent observations tend to show that this statement may require revision. As to the manner in which kreatin is changed into kreatinin in the body, a highly suggestive fact is the presence of ferments in various organs which possess this power. Ferments also exist which can decompose both kreatin and kreatinin. The existence of such enzymes is presumptive evidence that the changes which they are capable of producing actually occur in the organism; but the seat of the changes if they do take place, and their metabolic significance, are unknown. Kreatin when given by the mouth or injected into the blood does not cause any increase Hn the urinary kreatinin, nor when administered in moderate quantities does it seem to be excreted as kreatin. Kreatinin, on the other hand, when added to the food, causes an increase in the kreatinin of the urine. Intracellular Ferments — Autolysis.— As to the agencies by which the decomposition of the proteins is carried out in the cells, we have already spoken of the oxidizing cell ferments, or oxydases (p. 267). Re- ducing ferments, or reductases, are also known, and can be extracted from most organs, if not all. Like oxydases, they act in a weakly alkaline medium, causing in the presence of hydrogen such reduc- tions as the formation of nitrites from nitrates. There is some evidence that one and the same ferment may act as an oxydase or a reductase according to the conditions. Recent researches have brought to light in addition hydrolytic intracellular ferments, which split up proteins very much in the same way as the proteolytic ferments of the digestive 'juices. METABOLISM OF PROTEINS 589 The significance of these autolytic enzymes in the normal metabo- Usm of proteins has been already discussed (p. 569) ; indeed, so many of the chemical reactions of the body have been found to depend upon enzymes that modern physiology may at first thought seem almost to have reverted to the position of van Helmont and his school in the seventeenth century, who resolved all difficulties by murmuring the magic word ' ferment.' No fewer than eleven fer- ments have been stated to be present and active in the liver alone — -viz., a proteolytic and a nuclein-splitting ferment, a ferment which splits off ammonia from amino-acids, a milk-curdling ferment, a fibrin ferment, a bactericidal ferment, an oxydase, a lipase, a maltase, a ferment called glycogenase, which changes glycogen into dextrose, and an autolytic ferment. In the presence of such an array of enzymes the organs might seem to be little more than incubators in which the' ferments do their work. It must not be supposed, however, that the intracellular ferments, whether they cause decomposition or synthesis, oxidation or reduction, work in- dependently of what, for want of a better name, we must call the organization of the cell. We may be sure they are the servants and not the masters of the protoplasm, and that a drop of an extract containing intracellular ferments has very different powers from a living cell. ' It is not in the existence of the ferments, but in their combined action at the proper time and in the proper intensity, that the riddle of metabolism lies ' (Hober). Summary. — At this point let us sum up what we have learnt as to the relation between the proximate principles of the tissues and the proximate principles of the food. Inside the body we recognize representatives of the three groups of organic food-substances in a typical diet — proteins, carbo-hydrates, and fats. But we should greatly err if we were to imagine that the three streams of food- materials have flowed from the intestines into the tissues each in its separate channel, neither giving to nor taking from the others. The fats of the body may, indeed, in part he composed of molecules which were present as fat in the food ; but they may also be formed from carbo-hydrates, and probably from proteins. The carbo-hydrates of the body—the glycogen of the liver and muscles, the sugar of the blood — may undoubtedly be derived from carbo-hydrates in the food, but they may also he derived from proteins and from fats {certainly from their glycerin constituent, perhaps from the fatty acids as well). The pro- teins of the body come mainly, if not solely, from the proteins of the food. Although, of course, neither fats nor carbo-hydrates can by themselves form protein, being devoid of nitrogen, it is possible that products arising in the intermediary metabolism of either may, by combining with nitrogenous groups, be transformed into amino-hodies, which can then take part in the synthesis of proteins. In any case there is no doubt that both carbo-hydrates and fats can economize proteins and shield them from an overhasty metabolism. 590 METABOLISM, NUTRITION AND DIETETICS Section IV. — -Statistics of Nutrition — ^The Income and Expenditure of the Body in Terms of Matter* Preliminary Data. — ^The office of the food is to maintain the con- stituents of the body upon the whole in their normal proportions. A knowledge of the chemical composition of the body is, therefore, an important datum in the consideration of the statistics of its metabolism. The body of a man analyzefl by Volkmann had the following composition : Inorganic substances i ^^^^"^ ' ^5 '^ P^'^ '^®'^*- inorganic su Dstances | ^i^g ja.1 matter 4-4 (Carbon i8'4 per cent.~| Ntaojfr 2-6 ;: [29-7 - Oxygen 6-o ,, J The muscles, the adipose tissue, and the skeleton form nearly four- fifths of the total body-weight in the adult. . The following table shows the percentage amount of each of these tissues in a man, a woman, and a child (Bischoff) : I Man. Woman. Child™ Voluntary muscles ! 41 -8 Adipose tissue 18-2 Skeleton I5'9 Rest of body 24'i 35-8 i 23-5 28-2 13-5 I5-I 15-7 20-9 I 47-3 The nitrogen is contained chiefly in the muscles, glands, and nervous system, and in the constituents of the connective tissues, which yield gelatin, various mucoids, and elastin. The ordinary proteins make up about 9 per cent, of the weight of the body, or 22 per cent, of its solids ; the albuminoids or sclero-proteins (gelatin-yielding material, etc.) (p. 2) about 6 per cent, of the body-weight. Nitrogen exists in proteins to the extent of 16 per cent., so that the 6-5 kilos of protein of a 70-kilo body contain about i kilo of nitrogen. The carbon is contained chiefly in the fat, which forms a very large proportion of the water-free substance of the body, and in the proteins. A small amount is present as calcium carbonate in the bones. In the body of a strong young man weighing 68-5 kilos, Voit found the following quantities of dry fat in the various tissues : Adipose tissue - 8809-4 grammes. Skeleton - 261 7-2 Muscles - 636-8 Brain and spinal cord - 226-9 Other organs - - 73-2 ,, Total - - - - 12363-5 equivalent to 18 per cent, of the whole body-weight, or 44 per cent, of the solids. In dry fat rather more than 75 per cent, of carbon is present, and in protein about 50 to 55 per cent. ; so that while the fat of the body analyzed by Voit contained more than 9 kilos of carbon, only about a third of this amount would be found in the proteins. * The income and expenditure of the body in terms of energy are considered in Chapter XII. STATISTICS OF NUTRITION 591 In the fat there is, roughly speaking, 12 per cent, of hydrogen, in proteins only 7 per cent. ; so that from three to four times as much hydrogen is contained in the fat of the body as in its proteins. Oxygen forms about 12 per cent, of fat, and 20 to 24 per cent, of proteins ; the protein constituents of the body, therefore, contain about as much of its oxygen as the fat. Of the inorganic salts, calcium phosphate, Ca3(P04)2, is much the most abundant, owing to the large amount of it in bone, in the ash of which it is found to the extent of 83 per cent., along with 13 per cent. of calcium carbonate. Income and Expenditure of Nitrogen — -The Nitrogen Balance-Sheet. Nitrogenous Equilibrium. — It is a matter of common experi- ence that the weight of the body of an adult may remain approxi- mately constant for many months or years, even when the diet varies greatly in nature and amount. And not only may the weight reijiain constant, but the relative proportions of the various tissues of the body, so far as can be judged, may remain constant too. Here it is evident that the expenditure of the body must precisely balance its income : it must lose as much nitrogen as it takes in, otherwise it would put on flesh; it must lose as much carbon as it takes in, otherwise it would put on fat. Or, again, the body may be losing or gaining fat, giving off more or less carbon than it receives, while its ' flesh ' (its protein constituents) remains constant in amount, the expenditure of nitrogen being exactly equal to the income.* In both cases we say that the body is in nitrogenous equilibrium. A starving animal or a fever patient, on the other hand, is living upon capital, the former entirely, the latter in part ; the expenditure of nitrogen is greater than the income. A growing child is living below its income, is increasing its capital of flesh. In neither case is nitrogenous equilibrium present. The starving animal, as long as life lasts, excretes urea, kreatinin, and other nitrogenous substances, and gives off carbon dioxide; but its expenditure, and especially its expenditure of nitrogen, is pitched upon the lowest scale. It lives penuriously, it spins out its resources; its glycogen goes, its fat goes, a certain part of its protein goes, and when its weight has fallen from 25 to 50 per cent, it dies. At death the heart and central nervous system are found to have scarcely lost in weight ; the other organs have been sacrificed to feed them. Fig. 199 shows the percentage loss of weight and the proportion of the total loss which falls upon each of the organs of a cat in starvation (Voit). * For long experiments extending over many days the nitrogen balance may be considered as practically the same as the protein balance, but this is not necessarily true of short periods of time, since the stock of nitrogen present in the body in other forms than proteins, although relatively small, is subject to variations. 592 METABOLISM, NUTRITION AND DIETETICS For the first day of starvation the excretion of urea in a dog or cat is not diminished ; it takes about twenty-four hours for all the nitrogen corresponding to the proteins of the last meal to be elimin- ated. On the second day the quantity of urea sinks abruptly; then begins the true starvation period, during which the daily output of urea remains constant or diminishes very slowly until a short time before death, when it rapidly falls, and soon ceases altogether. An increase in the excretion may precede the final abrupt decline (pre- mortal increase). This seems to indicate the time at which all the available fat has been used up, and after which protein is no longer ' spared ' by the fat.* If the animal has little fat in its body to begin with, the rise in the urea excretion takes place even after the first few days. So long as the fat lasts the rate at which it is Fig. 199. — Diagram showing Loss of Weight pf the Organs in Starvation. The numbers under I. are the percentages of the total loss of body-weight borne by the various organs and tissues. The numbers under II. give the percentage loss of weight of each organ calculated on its original weight as indicated by com- parison with the organs of a similar animal killed in good condition. destroyed — as estimated from the amount of carbon given off minus the carbon corresponding to the broken-down proteins — ^remains very nearly constant after the first day. The fat to a certain extent economizes the proteins of the starving body, but however much fat may be present, a steady waste of the tissue-proteins goes on. If non-nitrogenous food in the form of sugar is supplied to an other- wise starving animal, the premortal rise in the nitrogen excretion does not occur. By giving a sufficient quantity of sugar, or of sugar and fat, but practically no protein (so-called nitrogen starva- tion), the excretion of nitrogen may be reduced to one-third of its amount when no food at all is given. This is true both in animals * If the animal has been for some time on a diet containing an abundance of proteins, several days may elapse before the constant excretion of urea is reached; if the previous diet has been poor in protein, the constant star- vation output may be at once established. STATISTICS OF NUTRITION 593 and man. In this way the daily excretion of nitrogen in a man has been reduced to 4 grammes. It is a remarkable fact that while a mixture of carbo-hydrate and fat will act just as well as carbo- hydrate alone in bringing about this reduction in the nitrogen output, fat without carbo-hydrate is much less effective. The h5^othesis suggested by Landergren to explain this is alluded to on another page (p. 542). The results obtained on fasting men differ in some respects from those obtained on starving animals. In ten days of hunger, Cetti, a professional ' fasting man ' of meagre habit, excreted 112 grammes nitrogen, or an average of 11 grammes a day. The excretion was least on the eighth, ninth, and tenth days — ^namely, about 9 grammes a day. On the third day it was higher than on the second, and almost as, high on is grams the fourth as on the third. A similar rise in the nitrogen lograros excretion on the second day has been observed in other "5 grdms fasting men, but is either rare or absent in fasting dogs. The explanation appar- ently is that in the ordinary food of man there is a greater abundance of carbo - hydrates and fats, the pro- tein - sparing action Ai5 A30 /\U5 A60 B/4 66 6// E^I6 62/ Fig. 200. — Excretion of Urea in Starvation, A is a curve representing the quantity of urea excreted daily by a fat dog in a starvation period of sixty days. B is the curve of urea excretion in a lean young dog in a starvation period of twenty -four days. Both are con- structed from Falck's numbers, but in A only every third day is put in, in order to save space. The num- bers along the vertical axis represent grammes of urea; those along the horizontal axis days from the beginning of starvation. of which is most pronounced at the very beginning of the starvation period. The quantity of chlorine and alkalies in the urine was also diminished, while the phenol was increased. The respiratory quotient sank to 0-66 to 0-69 — even less than ,the quotient corresponding to oxida- tion of fats alone. The meaning of this, in all probability, is that some of the carbon of the broken-down proteins was laid up in the body as glycogen (Zuntz). In another professional fasting man (Sued) with a considerable amount of body-fat, the excretion of nitrogen was found to diminish continuously during a fast of thirty days, being less than 7 grammes on the tenth day. In another fast of twenty-one days by the same person it was a little less than 3 grammes on the last day. The surprisingly small nitrogenous waste in this case is perhaps to be accounted for by the protein- sparing action of the abundant body-fat. The nitrogenous metabo- 38 594 METABOLISM, NUTRITION AND DIETETICS lism has also been investigated during long-continued hypnotic sleep (Hoover and Sollmann). The results were very much the same as in an ordinary starvation experiment. It might be supposed that if an animal was given as much nitrogen in the food in the form of proteins as corresponded to its daily loss of nitrogen diuring starvation, this loss would be entirely prevented and nitrogenous equilibrium restored. The supposition would be very far from the reality. If a dog of 30 kilos weight, which on the tenth day of starvation excreted 11-4 grammes urea, had then received a daily quantity of protein equivalent to this amount — that is to say, about 34 grammes of dry protein, or 175 grammes of lean meat — ^the excretion of nitrogen would at once have leaped up to nearly double its starvation value. If the quantity of protein in the diet was progressively increased, the output of urea would increase along with it, but at an ever- slackening rate; and at length a condition would be reached in which the income of nitrogen exactly balanced the expenditure, and the animal neither lost nor gained flesh. In an experiment of Voit's, for instance, the calculated loss of flesh in a dog with no food at all was 190 grammes a day. The animal was now fed on a gradually increasing diet of lean meat, with the following result : Flesh in the Flesh used up in Net Loss of Food. the Body. Body-flesh. 190 190 250 341 91 350 411 61 400 454 54 450 471 21 480 492 12 The loss of nitrogen in the urine and faeces is what was measured. Knowing the average composition of ' body-flesh ' (muscles, glands, etc.), it is possible to translate results stated in terms of nitrogen into results stated in terms of ' flesh.' Muscle contains approximately 3-4 per cent, of nitrogen. Here, with a diet of 480 grammes of meat, the dog was still losing a little flesh; it would probably have required from 500 to 600 grammes for equilibrium. The results are graphically represented in Fig. 201, p. 596. The quantity of protein food necessary for nitrogenous equili- brium varies with the condition of the organism ; an emaciated body requires less than a muscular and well-nourished body. The least quantity which would suffice to maintain in nitrogenous equilibrium the famous 35 kilo dog of Voit, even in very meagre condition, was 480 grammes of lean meat, corresponding to 16 grammes of nitrogen, or 35 grammes of urea — that is, about three times the daily loss STATISTICS OF NUTRITION 595 during starvation. From this lower limit up to 2,500 grammes of meat a day nitrogenous equilibrium could always be attained, the animal putting on some flesh at each increase of diet, until at length the whole 2,500 grammes was regularly used up in the twenty-four hours. A fxurther increase was only checked by digestive troubles. A man, or at least a civilized man, can consume a much smaller amount both absolutely and in proportion to the body- weight. Rubner, with a body- weight of 72 kilos, was able to digest and absorb over 1,400 grammes of lean meat; Ranke, with about the same body-weight, could only use up 1,300 grammes on the first day of his experiment, and less than 1,000 grammes on the third. But whether the surplus of protein food above the necessary minimum is great or small, nitrogen equihbrium is eventually attained, and thereafter all the nitrogen of the food regularly appears in the excreta; the explanation of this fact will be considered a little later (p. 598). So much for a puiely protein diet. When fat is given in addition to protein, nitrogenous equilibrium is attained with a smaller quantity of the latter. A dog which, with protein food alone, is putting on flesh, will put on more of it before nitrogenous equilibrium is reached if a considerable.quantity of fat be added to its diet. Fat, therefore, economizes protein to a certain extent, as we have already recog- nized in the case of the starving animal. On the other hand, when protein is given in large quantities to a fat animal, the consumption of fat is increased ; and if the food contains little or none, the body- fat will diminish, while at the same time ' flesh ' may be put on. The Banting cure for corpulence consists in putting the patient upon a diet containing much protein, but little fat or carbo-hydrate ; and the fact just mentioned throws light upon its action. All that we have here said of fat is true of carbo-hydrates. To a great extent these two kinds of food substances are complementary. Carbo-hydrates economize proteins as fat does, but to. a greater extent, so that with an abundant supply of carbo-hydrate .in the food the minimum protein requirement can be forced down much below what is possible on a diet of protein and fat alone. Carbo- hydrates also economize fat, so that when a sufficient quantity of starch or sugar is given to an otherwise starving animal, all loss of carbon from the body, except that which goes off in the urea, krea- tinin, etc., still excreted, can be prevented. Of course, the animal ultimately dies, because the continuous, though diminished, loss of protein cannot be made good. The fact that carbo-hydrates econo- mize proteins so much more efficiently than fat indicates that sugar is essential in the bodily metabolism, so that when carbo-hydrates are absent from the food some of the protein must be broken down so as to yield eventually the compounds necessary for the formation of carbo-hydrate. It is probable, dndeed, that purified proteins. 596 METABOLISM, NUTRITION AND DIETETICS Granjs SOO 4.00 300 200 100 \ 1 \ ^ i V + 100 ip absolutely free from admixture with carbo-hydrates, which, of course, is not the case with the natural protein foods, will not per- manently suffice for nutrition, but that the protein must be supple- mented by a certain amount of carbo- hydrate in some form available for the tissues. It would appear, indeed, that fats are not absolutely indispensable either for maintenance or for growth. White rats have been seen to grow nor- mally over long periods with dietaries devoid of fat ; for example, mixtures of the purified protein edestin (from hemp seed) or casein with starch, sugar, and ' protein- free milk ' freed from , fat by extraction with ether (Osborne and Mendel). While in these experiments the food might not have been free from the so-called ' lipoids,' it has been demonstrated that an impor- tant group of substances of this class, the phosphatides, can be sjmthesized in the body, the necessary phosphorus being ob- tainable even from inorganic phosphates (McCollom). Relation between Nitrogen excreted and the Quantity of Protein Food. — At this point we may consider a little more closely a phenomenon already alluded to, and to which much discussion used to be devoted by writers on metabolism. It has been stated that within the limits of nitrogenous equilibrium, which is the nor- mal state of the healthy adult, the body lives up to its income of nitrogen; it lays by nothing for the futiore. In the actual pinch of starvation the organism, when its behaviour is tested by a comparison of the intake and excretion of nitrogen, appears to have become suddenly econo- mical. When a plentiful supply of protein is presented to the starving body, it seems, judged by the same criterion, to pass at once from extreme frugality to luxury. Some fiesh may be put on for a short time, some nitrogen may be stored up ; but the excretion of nitrogen is soon adjusted to the new scale of supply, and the protein income is apparently spent as freely as it is received. These facts were usually summed up in the Fig. 201. — Curves constructed to illustrate Nitrogenous Equilibrium (from an Ex- periment of Voit's), The loss of flesh in grammes is laid ofi along the horizontal axis. The income and expenditure correspondiQg to a given loss are laid off (in grammes of ' flesh ') along the vertical axis. The continuous curve is the curve of income ; the dotted curve, of expenditure. With no income at all the expen- diture is jjjo grammes; with an income of 480 grammes the expenditure is 492 and the loss 12 grammes. Nitrogenous equilibrium is represented as being reached with an income of , about 530 grammes; here the two curves cut one another. STATISTICS OF NUTRITION 597 dictum, often dignified as a ' law ' of nitrogenous metabolism that : Consumption of protein is largely determined by supply (Practical Exercises, p. 693). To explain this many hypotheses were invented. The famous theory of Voit assumed that the food-protein after absorption (the so-called ' circulating-protein ') is carried to the tissues and taken up by the cells, where the greater part of it, without being incorporated with the protoplasm, is nevertheless acted upon, rendered unstable, shaken to pieces, as it were, by the whirl of life (by the intracellular enzymes we might now say less dramatically) in the organized framework, the interstices of which it fills. Pfliiger, on the other hand, maintained that we have no right to draw a distinction between the consumption of organ- and circulating- protein; that the whole of the latter ultimately rises to the height of organ- or tissue-protein, and passes on to the downward stage of metabolism only through the topmost step of organization. An increase in the supply of nitrogenous material in the blood must, on this view, be accompanied with an increased tendency to the break-up, the dis- sociation, as Pfliiger put it, of the living substance. The actual organ- ized elements, however, the existing cells, were not supposed to be destroyed ; the building remained, for although stones were constantly crumbling in its walls, others were being constantly built in. A much less plausible view was that the tissue elements themselves are short-lived ; that the old cells disappear bodily and are replaced by new cells ; and that the whole of the proteins of the food take part in this process of total ruin and reconstruction. Histological evidence, as soon as the methods of examining tissues with the microscope became sufficiently refined, told strongly against this idea. Although the cells of certain glands, such as the mammary, perhaps the mucous glands, and especially the sebaceous glands (p. 556), exhibit changes which, hastily interpreted, might seem to indicate that they break down bodily, as an incident of functional activity, no proof could be obtained of the production of new cells on the immense scale which this theory would require. The relatively small and constant amount of the endogenous metabolism indicates that the actual protoplasmic sub- stance, the living framework of the cell, is comparatively stable; that it does not break down rapidly; and that only a small and fairly constant amount of food- or circulating-protein, or of the decomposition products of protein, is required to supply the waste of the organ-protein. We have referred to these theories because there could scarcely be a more instructive instance of the way in which theories become obsolete with the advance of knowledge and of the way in which, with the advance of knowledge, a phenomenon which appears an absolute riddle to one generation may become fairly intelligible to the next, perhaps childishly simple to a third. The student will not derive much benefit from the perusal of this page should he fail to recognize that the hypotheses of the twentieth century are mortal too, and bound for the same bourne as those of the nineteenth. It is apparent in the first place from our study of the metabolism )f the proteins that the conclusion, ' consumption of protein is pro- portional to supply,' cannot be drawn from the equality of nitrogen ntake and nitrogen output. The amino-acids derived from proteins, ;xcept that relatively small fraction employed in repairing the 598 METABOLISM, NUTRITION AND DIETETICS waste of the tissues, which in nitrogen equilibrium is exactly com- pensated for by a corresponding release of amino-acids or their equivalent from the cell-proteins, are indeed speedily deaminated and the nitrogen of the amino-groups excreted as urea (with ammonia compounds and kreatinin). But, as we have seen, only a small proportion of the chemical energy of the amino-acids and only a small fraction of their carbon are liberated in this process. The carbon-containing residues are katabolized only to the extent re- quired by the momentary needs of the tissues, any balance being stored as part of the reserve of carbo-hydrate or of fat. The body does not possess the means of storing surplus amino-acids as such or even in the form of proteins, except to the small extent corre- sponding to any increase which may occur in the body-protein when the food-protein is increased beyond the minimum required for nitrogen equilibrium. Why the organism has not devdoped the capacity to store large quantities of protein is, of course, an interesting question, but it need scarcely be discussed here. One obvious reason is that protein is not a suitable source, nor are amino- acids apparently a suitable source of energy for the tissues until they have been deaminated and have probably undergone further decomposition and transformation. Therefore they are decomposed at once and their available residue stored, if it is in any case to be stored, in the more available form of carbo - hydrate (or fat). Where the food-proteins differ greatly from the body-proteins in the proportions of the various amino-acids, there would be no object in storing a great surplus of those which are most plentiful in the food, if they were at the same time the scarcest in the tissues, or, in the case of gland-cells, the scarcest in the proteins which they manufacture for their secretions. At any moment the magnitude of this non-utilizable surplus will depend upon the quantity of that one of the indispensable amino- acids which is present in the smallest amount. For the proper proportion must be preserved between the different ' stones ' out of which the molecule is built. When a single amino-acid is intro- duced into the body, it is at once changed into urea and excreted, since it cannot be utilized by itself for building up protein. When the cells have once culled from the mixture circulating in the blood the amino-acids, a full supply of which they have most difficulty in obtaining, a residue, large or small, according to the quantity and quality of the protein intake, will be left, and this can only be utilized to supply energy or to add to the fat and carbo- hydrate stores. For these uses removal of the amino-group is an essential preliminary. The question whether the deamination of a large part of the amino-acids coming from the intestine takes place in the liver, so that the surplus nitrogen is shunted out of the main metabolic current at its very source, has been already touched upon STATISTICS OF NUTRITION 599 (P- 578)- Som& writers conceive that in such a short-cut from pro- tein to urea we have a kind of physiological safety-valve to protect the tissues from the burden of an excessive metabolism. And if by this is meant that it is advantageous to the tissues that a special mechanism should exist to eliminate a surplus of nitrogen which they do not require, and which they cannot store, and to present them with a residue which they can utilize, the conception is certainly correct. But there is no good evidence that in the presence of an over-abundant supply of amino-acids the endogenous protein meta- bohsm would be essentially modified. Relation of Nitrogenous Metabolism to Muscular Work. — ^This is another of those classical physiological problems which it is difficult to present properly apart from its historical setting. The general result of much experimental work and long-continued discussion is that when the work does not transgress what may be called ' normal limits,' the excretion of nitrogen is nearly independent of mus- cular work — that is to say, the quantity of nitrogen excreted by a man on a given diet is practically the same whether he rests or works. Before this was known it was maintained by Liebig that proteins alone could supply the energy of muscular contraction — ^that, in fact, proteins were solely used up in the nutrition and functional activity of the nitrogenous tissues, while the non-protein food yielded heat by its oxidation. As exact experiments multiplied, it was found that muscular work, the production of which is the function of by far the greatest mass of protein-containing tissue, had little or no effect upon the excretion of urea in the urine. More than this, it was shown that a certain amount of work accomplished (by Fick and Wislicenus in climbing a mountain) on a non-nitrog- enous diet had double the heat equivalent of the whole of the pro- tein consumed in the body, as estimated by the urea excreted during, and for a given time after, the work. On the assumption that all the urea corresponding to the protein broken down was eliminated during the time of this experiment, a part at least of the work must have been derived from the energy of non-nitrogenous material. And other experiments in which account was taken of the increase in the carbon dioxide given off (as conspicuous an accompaniment of muscular work as the constancy of the urea excretion), showed that during muscular exertion carbonaceous substances other than proteins — ^that is to say, fats and carbo-hydrates — are oxidized in greater amount than during rest. So the pendulum of physiological orthodoxy came full-swing to the other side. Liebig and his school had taught that proteins alone were consumed in functional activity; the majority of later physiologists following Voit denied to the proteins any share whatever in the energy which appears as muscular contraction. The proteins, they said, ' repair the slow waste of the framework of the muscular machine, replace a loose rivet, a worn-out belt, as occasion may require ; the 6oo METABOLISM. NUTRITION AND DIETETICS carbo-hydrates and fats are the fuel which feeds the furnaces of hfe, the material which, dead itself, is oxidized in the interstices of the living substance, and yields the energy for its work.' Now, it is a singular coincidence, and full of instruction for the ingenuous student of science, that the facts which were supposed absolutely to disprove the older theory, and absolutely to establish its more modem rival, are now seen to do neither the one thing nor the other. The fact — and it is a fact — that the excretion of nitrogen is but little affected by muscular contraction, does not prove that none of the energy of muscular work comes from proteins; the fact that, under certain conditions, some of the muscular energy must apparently come from non-nitrogenous materials, does not prove that these are the normal source of it all. The distinction had again been made too absolute. The pendulum must again swing back a little; and the experiments of Pfliiger and his pupils were soon to set it moving. In the first place, it is not perfectly correct to say that work causes no increase in the excretion of nitrogen; excessive work in man, and work, severe but not excessive, in a flesh-fed dog (Pfliiger), do cause somewhat more nitrogen to be given off. On the first day of work the increase is always much less than on the second and third ; and on the first and second rest days, following work, the elimina- tion of nitrogen is still increased. After excessive exercise in man not only is the urea increased, but also the ammonia, kreatinin, and if the subject is in poor training, the uric acid and purin bases (Paton, Stockman, etc.). Moderate exercise causes no increase on the first day, but a slight increase on the second. The meaning of these facts seems to be that during muscular work the intensity of which does not exceed certain limits, the protein waste of the muscular substance itself is no greater than during rest. When, however, the machine is ' speeded up ' beyond a certain point the wear and tear is sensibly increased and an excess of tissue-protein is katabolized. There is no reason to suppose that the tissue-protein thus broken down will not yield energy for the muscular work by the oxidation of its non-nitrogenous residue, just as well as the surplus amino- bodies derived from food-protein. The muscular machine has the peculiarity that it is constructed of combustible material ; even the dust and the splinters, if we may so express it, which represent the wear and tear of the machine can be burnt in the furnace which keeps it going. In the second place, even if the excretion of nitrogen were entirely unaffected by work, this would not prove that none of the energy of the work comes from proteins. For, as we have seen, it is after the nitrogen has been split off and converted into urea that the energy of a great part of the food-protein is developed by oxidation. Further, since the animal body is a beautifuUy-balanced mechanism which constantly adapts itself to its conditions, it is conceivable that it may, when called upon to labour, save proteins from lower uses to devote them to muscular contraction. In this STATISTICS OF NUTRITION 6ot case the excretion of nitrogen would not necessarily be altered ; the proteins which, in the absence of work, would have been oxidized within the muscular substance or elsewhere, their energy appearing entirely as heat, may, when the call for protein to take the place of that broken down in muscular contraction arises, be diverted to this purpose. In any case, there is no doubt that a dog fed on lean meat may go on for a long time performing far more work than can be yielded by the energy of fat and carbo-hydrates occurring in traces in the food, or taken from the stock in the animal's body at the beginning of the period at work. A large portion, and perhaps the whole, of the work, must in this case be derived from the energy of the pro- teins (Pfliiger). On the other ha.nd, it is well estabUshed that when fats and carbo-hydrates are present in sufficient quantity in the tissues or the food, they constitute the main source of the energy of muscular contraction (p. 746), and there is some evidence that of the two classes of food materials carbo-hydrates in the form of dextrose (or glycogen) is the material oi election. The outcome, then, of this famous controversy is essentially a compromise. Everybody now admits that the muscular machine can and does utilize predominantly any one of the great groups of food substances, be it carbo-hydrate, fat, or protein, when the dietetic conditions are such that only one of these is offered to it in large amount, the others being either absent or offered in small amount. To be sure, amino-acids are not the first choice, but if it must do so the muscle can make shift with them, and can indeed make them serve excellently well. When all the food substances are present in abundance, carbo-hydrate is favoured above fat, and fat above protein. Experience has shown that the minimum quantity of nitrogen required in the food of a man whose daily work involves hard physical toil is higher than the minimum required by a person lead- ing an easy, sedentary life. This is evidently in accordance with the view that protein is actually used up in muscular contraction ; but it is not inconsistent with the opposite view. For the body of a man fit for continuous hard labour has a greater mass of muscle to feed than the body of a man who is only fit to handle a composing- stick, or drive a quill, or ply a needle ; and the greater the muscular mass, the greater the muscular waste. But if an animal just in nitrogenous equilibrium on a diet of lean meat when doing no work is made to labour day after day, it will lose flesh unless the diet be increased. This must mean that some of the protein is being diverted to muscular work, and that the balance is not sufficient to keep up the original mass of ' flesh ' (see p. 615). Relative Value of Different Proteins in Nutrition — Synthesis of Amino-Acids.— The fact that the various proteins differ quantita- 6o2 METABOLISM. NUTRITION AND DIETETICS ti vely and qualitatively in respect to their amino-acids raises the ques- tion of the relative value of different proteins in nutrition. In this is involved the further question, whether the body can itself S57nthesize from other materials any of the amino-acids which may be deficient, or change one amino-acid into another. That the ' peptones ' derived from a protein which is itself capable of permanently supplying the whole nitrogenous intake of the organism can be substituted for the protein scarcely needs demonstration, since it is known that the protein is converted into peptones in digestion. Nevertheless, this has been proved conclusively by feeding experiments with peptones. It was to be expected, too, leaving out of account all consideration of the means of overcoming the repugnance of animals to accepting such unnatural food substances, that the further products of protein hydrolysis, the amino-acids, etc., could be substituted for the original proteins when these were themselves adequate. For it is in the form of amino-acids or at most of such relatively simple polypeptide groups as may still hang together after complete digestion and absorption, that the nitrogenous food substances are normally offered to the tissues. Experimental demonstration of the feasibility of this substitution has also been obtained. The split products of meat, for example, will keep an animal in nitrogen equilibrium as well, as the meat from which they are derived. But what happens when one or more of the amino- acids found in the proteins of the body are missing from the protein of the food ? That the components of an amino-acid like arginin (ornithin and urea), into which it can be split not only by the possibly crude and violent methods of the chemical laboratory, but also by the delicate and precisely-adapted ' biological ' action of a special enzyme (arginase), should be able to replace the original amino-acid is a fact which does not greatly help towards an answer. For when these components, or ornithin alone, since urea is always present in the body, are given instead of arginin, the reversal of the enzyme reaction by which arginin is decomposed is all that is neces- sary for its synthesis, and the reversal of such a reaction is doubtless a very commonplace affair in tissue metabolism. The formation of one amino-acid from another, or from materials which do not originate exclusively from protein, is a different thing, and the answer to the question raised, so far as it can yet be given, is that the way in which the body deals with a deficiency in the protein ' building stones ' depends upon the nature of the missing amino- acids. Thus, the phospho-protein casein does not jrield glycin on hydrolysis; yet it has been shown that casein is a perfectly adequate or complete protein food capable of covering the whole nitrogen requirement of the body over long periods. The same is true of the cleavage products of casein which has been subjected to pancreatic digestion. In an animal fed on no other protein than casein, with STATISTICS OF NUTRITION 603 suitable quantities of carbo-hydrate and fat in addition, the glycin contained in certain of the body proteins must therefore have been produced in the body itself. We have already seen (p. 571) that for the S3mthesis of hippuric acid after the administration of benzoic acid, glycin is necessary, and the quantity of hippuric acid which can be thus produced is so great that it is impossible to suppose that it all comes from glycin preformed in the body or from glycin in the food substances. It may accordingly be taken as proved that the tissues have the power of synthesizing at least this one of the amino-acids (amino-acetic acid), reckoned among the protein ' building stones. ' It is said that if the casein has been hydrolysed by acid, the products will not preserve nitrogen equilibrium, per- haps because the acid has broken up all the polypeptides (p. 2), some of which the cells may need as the starting-point of protein synthesis. This, however, is uncertain. Lysin also appears to be capable of being synthesized in the body, and protein foods free from lysin, or containing only a trace of it, may yet be adequate for nutrition and growth. . Prolin, too, is not indispensable, and this is of special interest, for the amino-acids hitherto mentioned as capable of being built up in the tissues are all more or less directly related to each other, being derivatives of the series of saturated fatty acids. The task ofj[changing one of these into another in which the food is deficient may, therefore, be considered a comparatively easy one. But prolin has no obvious relation to most of the other amino-acids. It is a-pyrrolidin carboxylic acid, CHg — CHg CHg — CH2 CH- CH.COOH— ?.e., pyrrolidin, CHa CHg \ / \ / NH NH in which H in one of the CHg groups is replaced by carboxyl (COOH). It has been suggested that prolin may be formed in the body from glutaminic (amino-glutaric) acid, which by loss of a molecule of water can be made to yield o-pyrrolidon carboxylic acid. Thus, CH2.CH2.CH.COOH CH2— CH2 COOH NH- -HoO = CO CH.COOH \ / NH Glutaminic acid. a-pyrrolidon carboxylic acid. By reduction the latter compound might be changed into prolin. With proteins deficient in certain other amino-acids a totally different result has been obtained. Gelatin, for example, contains most of the amino-acids and other groups which compose the body proteins, but tyrosin, cystin, and tryptophane are lacking in the 6o4 METABOLISM, NUTRITION AND DIETETICS gelatin molecule. Zein, an alcohol- soluble protein or prolamin* derived from maize, yields no tryptophane, glycin, or lysin. Now, it is found that neither gelatin nor zein can replace the whole of the ordinary proteins in the food. When only enough protein is taken to prevent loss of nitrogen from the body, one-fifth of the necessary nitrogen can be supplied in the form of gelatin. When the food is much richer than this in ordinary protein, a correspond- ingly greater proportion of the protein can be replaced by gelatin. The surplus is not used in the endogenous metabolism of the cells (p. 564), but supplies energy to the body after the elimination of its nitrogen as urea, just as the surplus protein would do. Thus gelatin economizes protein in the same way that fat and carbo- hydrates do, but also to some extent in a different way by supplying ' building stones ' for the protoplasm. It is therefore an interesting question whether gelatin can fully replace protein when the missing substances are given in addition. Kauffmann has stated that his own nitrogen requirement (15-2 grammes) was almost completely covered by a mixture containing 93 per cent, of the nitrogen in the form of gelatin, 4 per cent, as t5n:osin, 2 per cent, as cystin, and I per cent, as tryptophane, in addition to the same amounts of carbo-hydrate and fatty food as in the comparison diet, in which the nitrogen was supplied in the form of plasmon, a commercial preparation of casein. Similar results have been reported in experiments on animals in which attempts have been made to ' complete ' such inadequate proteins by addition of the missing amino-bodies, with fair but, according to Osborne and Mendel, not entirely satisfactory results. The converse experiment, in which an amino-acid such as trypto- phane has been purposely eliminated from the food mixture, has also been tried, with the result that rapid deterioration in the condition of the animal ensued. It would seem, indeed, that whatever capacity the animal body may have for synthesizing certain of the amino-acids, this power does not extend to the cyclic compounds tr3rptophane, tyrosin, phenylalanin, and histidin, which must be supplied in the food. It has been suggested by Osborne that in this regard an essential difference exists between the animal and the plant, the latter alone being endowed with the function of ' cyclopoifisis,' or formation of substances of the cyclic type. It is not clearly understood as yet on what this difference really hinges, whether, as some have supposed, on the inability of the animal organism to form the appropriate fatty acid radicals, or on some * The prolamins are so called because on hydrolysis they yield exceptionally large amounts of prolin (p. 354) and ammonia. They are insoluble in water and absolute alcohol, but soluble in 70 to 80 per cent, alcohol and in dilute acids and alkalies. Besides zein they include gliadin (from wheat and rye), hordein (from barley), and bynin (from malt). They are extraordinarily rich in glutaminic acid, hordein yielding more than any protein hitherto investi- gated (over 41 per cent). STATISTICS OF NUTRITION 605 other limitation of its chemical powers. While the cyclic (and hetero- cyclic) compounds cannot be replaced by other ' Bausteine ' of the proteins, they may to some extent replace each other. Thus it would seem that tyrosin can be replaced by phenylalanin (Abderhalden). While some of the food proteins like casein are sufficient by themselves to supply all the amino-bodies necessary not only for the maintenance, but also for the growth of the body, and can accordingly be termed adequate or complete protein food sub- stances, others, Uke gelatin, are insufficient bythemselves to supply the protein required for mere maintenance, still less for growth, and may be spoken of as inadequate or incomplete proteins. There is a third intermediate group, comprising proteins which suffice when given as the sole protein food to maintain the body for an indefinitely long period, and to repair the tissue waste without per- mitting growth of the animal to take place. Gliadin and hordein (see footnote, p. 604) are representatives of this group. The ex- periments of Osborne and Mendel with ghadin are of special interest, since this substance is very differently constituted from the ordinary food proteins, as well as from the tissue prdteins of the animal body. While, as already stated, it yields very large quantities of glutaminic acid, prolin, and ammonia, it either contains no lysin and no glycin, or yields too little to be detected with certainty. It also yields comparatively little histidin and arginin. Now, it has been found that a dietary containing carbo-hydrate, fats, and inorganic salts, but no protein except gliadin, suffices to maintain adult rats in good condition for very long periods (up to 290 days), and also to maintain young rats in a stationary condition as regards growth, but in perfect health. The youthful appearance of the rats whose growth was thus inhibited was very striking, and corresponded with their size rather than with their age. The capacity for growth on a normal diet was apparently not in the least diminished; the growth processes simply remained in abeyance. ' In one rat, after a continuous suppression of growth lasting 277 days, when the animal was 314 days old — an age at which normally little or no growth takes place — satisfactory growth was resumed on a suitable diet.' A still more remarkable experiment was the following: ' A male rat, kept for 154 days with gliadin as the sole protein in the food, was paired with a female also on the gliadin diet. At the end of 178 days on the gliadin diet she gave birth to four young, which were satisfactorily nourished by the mother, still on gliadin, during the first month of their existence. After a month three of the young rats were removed from the mother and put on diets of casein food {i.e., casein plus suitable proportions of carbo-hydrate, fat, and inorganic materials), edeotin food and milk food respectively. The fourth was left with the mother. The fourth rat began to evince a failure to grow at about the period (thirty days) when young rats are wont to depend upon extraneous food. The meaning of this last observation can only be that the young animal, when obliged to depend upon its share of the gliadin food 6o6 METABOLISM. NUTRITION AND DIETETICS left with the mother in place of the milk formed by the mother from this same gUadin food mixture, showed the typical failure to grow on a diet inadequate as regards the power of producing growth in respect to the protein contained in it. On the other hand, in the body of the mother this inadequate diet had been so transformed that not only had she maintained her body-weight and repaired her tissue waste completely, but she had produced from it every- thing necessary for the development of the embryo rats up to full term, and after that- everything necessary (in the form of milk) for their normal growth dviring the period of suckling. On the whole, a very large amount of body tissue in proportion to the original weight of the mother must have been formed or renewed in the 200 days or more during which the experiment continued, and during which gliadin was being steadily transmuted into tissue protein, and latterly into the proteins of milk as well, by what might almost be called a feat of chemical legerdemain. There must have occurred a sjmthesis ' not only of the Bausteine (the " building- stones ") deficient in the protein intake) but likewise of' tissue and milk components like the nucleic acids (with their content of purins, pyrimidins, and organically combined phosphorus) and phospho proteins like casein, etc., which were completely missing ' in the food. It has been suggested that the bacteria of the alimentary canal, which, of course, are plant cells, may have and may exercise on a large scale the power of building up new amino-acids from a variety of materials in the intestinal contents, and that they raay thus be synthe- sizing agents, thanks to which inadequate proteins inay be reshaped to proteins adequate to the needs oi the body. It is precisely, however, in the case of incomplete proteiiis like gelatin deficient in cyclical compounds, that the body fails to effect the necessary tiansformation in spite of the fact that plant cells are supposed to be specially capable of forming these compounds. In any case bacterial action would not explain why proteins like gliadin and hordein are only adequate for the renewal of tissue, and not for its growth. This points rather to the possibility that the processes by which the nitrogenous compounds degraded in cellular metabolism are replaced are not of the same char- acter as the processes by which new nitrogenous complexes are built up into growing protoplasm. If, for instance, the protein molecule is not completely disrupted in ordinary metabolism, it will not need to be completely reconstructed, while in the formation of new tissue complete protein molecules will have to be S3mthesized. Incomplete proteins like gliadin may furnish building-stones adequate for repairing the house, but inadequate for building it from the foimdations. Income and Expenditure of Carbon — ^The Carbon Balance-Sheet. — This division of the subject has been necessarily referred to in treating of the nitrogen balance-sheet, and may now be formally completed. Carbon Equilibrium. — A body in nitrogenous equilibrium may or may not be in carbon equihbrium. It has been repeatedly pointed out that the continued loss or gain of carbon by an organism in STATISTICS OF NUTRITION 607 nitrogenous equilibrium means the loss or gain of fat; and, since the quantity of fat in the body may vary within wide limits without harm, carbon equilibrium is less important than nitrogen equili- brium. It is also less easily attained when the carbon of the food is increased, for the consumption of fat is not necessarily increased with the supply of fat or fat-producing food, and there is by no means the same prompt adjustment of expenditure to income in the case of carbon as in the case of nitrogen. Carbon equilibrium can be obtained in a flesh-eating animal, like a dog, with an exclusively protein diet ; but a far higher minimum is required than for nitrogenous equilibrium alone. Voit's dog required at least 1,500 grammes of meat in the twenty-four hours to prevent his body from losing carbon. For a man weighing 70 kilos, the daily excretion of carbon on an ordinary diet is 250 to 300 grammes. About 2,000 grammes of lean meat would be re- quired to yield this quantity of carbon; and, even if such a mass could be digested and absorbed, more than three times the necessary nitrogen would have to undergo preliminary cleavage and excretion as urea or be thrown upon the tissues. Not only may carbon equilibrium be maintained for a short time in a dog on a diet containing fat only, or fat and carbo-hydrates, but the expenditure of carbon may i)e less than the income, and fat may be stored up. But, of course, if this diet is continued, the animal ultimately dies of nitrogen starvation. So far we have spoken only of the income and expenditure of carbon and nitrogen; and from these data alone it is possible to deduce many important facts in metabolism, since, knowing the elementary composition of proteins, fats, and carbo-hydrates, we can, on certain assumptions, translate into terms of proteins or fat the gain or loss of an organism in nitrogen and carbon, or in carbon alone. But the hydrogen and oxygen contained in the solids and water of the food, and the oxygen taken in by the lungs, are just as important as the carbon and nitrogen ; it is just as necessary to take account of them in drawing up a complete and accurate balance- sheet of nutrition. Fortunately, however, it is permissible to devote much less tirpe to them here, for when we have determined the quantitative relations of the absorption and excretion of the carbon and nitrogen, we have also to a large extent determined those of the oxygen and hydrogen. Income and Expenditure of Oxygen and Hydrogen. — The oxygen absorbed as gas and in the solids of the food is given off chiefly as carbon dioxide by the lungs; to a small extent as water by the lungs, kidneys, and skin; and as urea and other substances in the urine dnd faeces. The hydrogen of the solids of the food is excreted in part as urea, but in far larger amount as water. The hydrogen and oxygen of the ingested water pass off as water, without, so far as 6o8 METABOLISM, NUTRITION AND DIETETICS we know, undergoing any chemical change, or existing in any other form within the body. But it is important to recognize that although none of the water taken in as such is broken up, some water is manufactured in the tissues by the oxidation of hydrogen. We have already considered (p. 240) the gaseous exchange in the lungs, and we have seen that all the oxygen taken in does not reappear as carbon dioxide. It was stated there that the missing oxygen goes to oxidize other elements than carbon, and especially to oxidize hydrogen. We have now to explain more ftilly the cause of this oxygen deficit. The Oxygen Deficit. — The carbo-hydrates contain in themselves enough oxygen to form water with all their hydrogen ; they account for a part of the water-formation in the body, but for none of the oxygen deficit. The fats are very difierent ; their hydrogen can be nothing like com- pletely oxidized by their oxygen. A gramme of hydrogen is contained in 8'5 grammes of dry fat, and needs 8 grammes of oxygen for its com- plete combustion. Only i gramme of oxygen is yielded by the fat itself; so that if a man uses 100 grammes of fat in twenty -four hours, rather more than 80 grammes of the oxygen taken in must go to oxidize the hydrogen of the fat. The proteins also contribute to the deficit. In 100 grammes of dry proteins there are 15 grammes of nitrogen, 7 grammes of hydrogen, and 21 grammes of oxygen. The carbon does not concern us at present. The 33 grammes of urea, corresponding to roo grammes of protein, contains 15 grammes of nitrogen, a little more than 2 grammes of hydrogen, and a little less than 9 grammes of oxygen. There remain 5 grammes of hydrogen and 12 grammes of oxygen. But 5 grammes of hydrogen needs for complete combustion 40 grammes of oxygen ; there- fore 28 grammes of the oxygen taken in must go to oxidize the hydrogen of 100 grammes of protein. Taking 140 grammes of protein as the amount in a liberal diet for a man, we get 39 grammes as the required quantity of oxygen. This, added to the 80 grammes needed for the hydrogen of the fat, makes a total of, say, 120 grammes, equivalent to about 85 litres of oxygen. A small amount of oxygen also goes to oxidize the sulphur of proteins. With a diet containing less fat and protein and more carbo-hydrate, the oxygen deficit would of course be less. The Production of Water in the Body. — One gramme of hydrogen corresponds to 9 grammes of water. In 140 grammes of proteins and 100 grammes of fat there are, in round numbers, ;22 grammes of hydS'o- gen; in 350 grammes of starch, 21-5 grammes. With this diet, 43 "5 grammes of hydrogen is oxidized to water within the body in twenty-four hours, corresponding to a water production of 39r grammes, or 15 to 20 per cent, of the whole excretion of water. It has been observed that during starvation the tissues sometimes become richer in water, even when none is drunk. The only explanation is that the elimination of water does not keep pace with the rate at which it is produced from the hydrogen of the broken-down tissue-substances, or set free from the solids with which it is (physically ?) united. Inorganic Salts. — ^The inorganic salts of the excreta, Uke the water, are for the most part derived from the salts of the food, STATISTICS OF NUTRITION ,609 which do not in general undergo decomposition in the body. A portion of the chlorides, however, is broken up to jdeld the hydro- chloric acid of the gastric juice. Within the body some of the salts are more or less intimately united to the proteins of the tissues and juices, some simply dissolved in the latter. The clilorides, phos- phates and carbonates are the most important ; the potassium salts belong especially to the organized tissue elements, the sodium salts to the liquids of the body; calcium phosphate and carbonate pre- dominate in the bones. The amount and composition of the ash of each organ only change within narrow limits. In hunger the organism clings to its inorganic materials, as it clings to its tissue- proteins; the former are just as essential to life as the latter. In a starving animal chlorine almost disappears from the urine at a time when there is still much chlorine in the body; only the inorganic salts which have been united to the used-up proteins are excreted, so that a starving animal never dies for want of salts. When sodium chloride is omitted as an addition to the food of man, the decomposition of protein seems to be slightly accelerated, but for a time, at least, there are no serious symptoms (Belli). It is a general rule that purely carnivorous animals do not desire salt, and the same is true of human beings living on a purely animal •diet, while vegetable feeders eagerly seek it. On the other hand, when an animal, even a carnivore, is fed with a diet as far as possible artificially freed from salts, but otherwise sufficient, it dies of sali- hunger. The blood first loses inorganic material, then the organs. The total loss is very small in proportion to the quantity still retained in the body; but it is sufficient to cause the death of a pigeon in three weeks, and of a dog in six, with marked symptoms of muscular and nervous weakness. A deficiency of lime salts causes changes particularly in the skeleton, although the nutrition of the rest of the body is also interfered with. These changes are of course most marked in young animals, in which the bones are growing rapidly. In pigeons on a diet containing very little calcium the bones of the skull and sternum become e^ttremely thin and riddled with holes, while the bones concerned in movement scarcely suffer at all (E. Voit). It is not indifierent in what form the calcium, is taken, nor can it be replaced to any great extent by other earthy bases, as magnesium or strontium. Weiske fed five young rabbits of the same litter on oats, a food relatively poor in calcium. One of the rabbits received in addition calcium carbonate, another calcium sulphate, a third mag- nesium carbonate, and a fourth strontium carbonate. At the end of a certain time it was found that the skeleton of the rabbit fed with calcium carbonate was the heaviest and strongest of all, and contained the greatest proportion of mineral matter. Then came the rabbit fed with calcium sulphate. The animal which received only oats had the wOrst- developed skeleton; the condition of the animals fed with magnesium and strontium carbonates was but little better. 39 6io METABOLISM. NUTRITION AND DIETETICS Milk as a Food. — Milk is a food rich in calcium and also in phos- phorus, a circumstance evidently related to the rapid development of the skeleton in the young child. As in the other natural foods, the calcium and phosphorus are partly in the form of organic com- pounds, united with the proteins, the calcium especially with caseinogen, and partly in the form of inorganic salts. Both of these elements are more easily assimilated by the body in the organic than in the inorganic form. The same is true of iron, which exists in organic combination in the bran of wheat, in the haemoglobin of the hlood and of muscular fibres, in the nuclei of most cells, vegetable and animal, and conspicuously in the nuclein compounds of the yolk of the egg. Attempts have been made to increase the amount of iron in hen's eggs by giving them food mixed with preparations of iron — e.g., iron citrate. An increase takes place, but only after a long time. Thus in one experiment loo grammes of egg- substance contained 4-4 milligrammes of Fe203 before the administration of the iron was begun; after feeding with iron for three and a half weeks the amount was 4-5 milligrammes, after more than two months 7-4 milligrammes; and after a year only 7-3 milligrammes. Although, as we have seen, inorganic iron can be absorbed, it is certainly the case that under ordinary conditions all the iron that the body receives or needs is taken in the form of organic com- pounds, since there is no inorganic iron in the natural food sub- stances. Stockman, from careful estimations of the quantity of iron in a number of actual dietaries, finds that it only amounts to about 8 to 10 milligrammes a day. He concludes that the greater part of it must be retained in the body and used over and over again. Milk is poor in iron, but this does not hinder the development of the young child, except when it is weaned too late, when it is apt to become ansemic unless the milk is supplemented ^with a food rich in iron, such as yolk of egg. The explanation is that the foetus, especially in the last three months of intra-uterine life, accumulates a store of iron in the liver and other organs; so that, in proportion to its body-weigKt, it is at birth several times richer in iron than the adult. This iron, of course, all comes from the mother, and the loss is not exactly balanced by the excess of iron in her food ; certain of her organs, the spleen, for instance, though not apparently the liver, are impoverished as regards their content of iron. Section V. — ^Dietetics. There are two ways in which we can arrive at a knowledge of the amount of the various food substances necessary for an average man : {a) By considering the diet of large numbers of people doing fairly definite work, and sufficiently, but not extravagantly, fed — e.g., soldiers, gangs of navvies, or plantation labourers; (6) by making special experiments on one or more individuals. DIETETICS ■ 6ii- Voit, bringing together a large number of observations, concluded that an ' average workman,' weighing 70 to 75 kilos, and working ten hours a day, required in the t\^enty-four hours 118 grammes protein, 56 grammes fat, and 500 grammes carbo-hydrate, corre- sponding to about 18 -8 grammes* nitrogen; and at least 338 grammes carbon. Ranke found the following a sufficient diet for himself, with a body-weight of 74 kilos: Proteins 100 grammes. Fat 100 Carbo-hydrates . - 240 This corresponds to only 16 grammes nitrogen and, say, 230 grammes carbon. A German soldier in the field receives on the average : Proteins 151 grammes. Fat - 46 Carbo-hydrates 522 representing about 24 grammes nitrogen and 340 grammes carbon. The average ration for four British regiments in peace-time con- tained 133 grammes protein, 115 grammes fat, and 424 grammes carbo-hydrate ( = 3,400 calories). But in addition the soldiers constantly obtained at their own expense a supper, generally com- prising meat (Pembrey). The Russian army war ration in the Manchurian campaign is said to have comprised 187 grammes protein and 775 grammes carbo-hydrate, but only 27 grammes fat ( = 4,900 calories). The diet of certain miners (Steinheil) and lum- berers (Liebig) contained respectively 133 and 112 grammes protein, 113 and 309 grammes fat, and 634 and 691 grammes carbo-hydrates. The diet of a Japanese jinricksha man with a body-weight of 62 kilos contained 158 grammes protein, and its total healt value was 5,050 calories. The work of these men in running long dis- tances with passengers is very laborious. They consume large amounts of fish, eggs, beef, and pork during their periods of rest, and large quantities of rice during their working periods (McCay). The diet of prize-fighters and of athletes in training is richer in protein than any of these. The members of two college football teams are stated to have consumed on the average 225 grammes protein, 334 grammes fat, and 633 grammes carbo-hydrates ( = 6,800 calories). Caspari, from a study of the phenomena of training, concluded that continuous bodily work at a rate above the ordinary requires a large amount of protein (2 to 3 grammes a day per kilo of body- weight) . But there seems to be a considerable difierence between different individuals. So that a definite and tj^Tpical diet for severe labour does not exist. And although perhaps the hardest physical work ever done in the world is to break athletic * 'taking the percentage of nitrogen in protein at i5. 6i2 METABOLISM, NUTRITION AND DIETETICS records, to cut and handle timber, to mine coal, and to make war, the diet on which these things are accomplished is very variable. Recent observations tend to reduce the amount of protein con- sidered necessary for a person under ordinary conditions. Siven remained in nitrogen equilibrium, for a time at least, with an intake of only o-oy to o-o8 gramme of nitrogen (0-4 to 0-5 gramme of protein) per kilo of body-weight, or not much more than one-third of the amount in Ranke's diet. It is obvious that the endogenous protein katabolisni sets the Hmit below which it must be impossible permanently to reduce the allowance of protein. But it would be very hazardous to assume that this theoretical minimum limit corresponds with the permissible physiological limit. From ex- periments on men of various callings extending over many months, Chittenden has concluded that the average man eats at least twice as much protein as he really requires. We have already seen that the amount of nitrogen required to repair the actual waste of the tissues is comparatively small, and that with the ordinary amount of protein in the food a very large fraction of the total nitrogen is rapidly excreted as urea. There is no doubt, also, that many persons consume too much protein, at any rate in the form of animal food, and would feel better, work better, and probably live longer, if they restricted themselves in this regard. But there is no evidence that the digestion of such quantities of protein as the average healthy man eats, or the elaboration and excretion of the corresponding amounts of urea, ' strain ' in the least the digestive apparatus, the liver, or the kidneys. And it may just as well be argued that it is advantageous that much more than the minimum protein requirement shoiJd be offered to the tissues, so that the appropriate amino-acids, even the scarcest of them, may be sure to be present in sufficient amount, rather than that the organs should be subjected to the unnecessary ' strain ' of reconstructing some of the amino-acids themselves, supposing that they possess this power. In a question of this sort the immemorial experience and instinct of mankind cannot be lightly waved aside. McCay points out that while Bengalis in Lower Bengal subsist on food containing only about one-third the amount of protein in such a ' standard ' diet as Voit's (6 to 7 grammes of nitrogen a day), and may therefore be supposed to be immune from the dangers of an excessive protein metabolism, the large intake of carbo-hydrate rendered necessary by the poverty of the foocj in protein is associated with perhaps greater evils, among them a marked predisposition to diabetes and renal troubles. Their weight, chest measurement, and muscular development are inferior to those of other Asiatics living in the same climate, but with dietetic habits or economic conditions which ensure them a larger supply of protein. Thus the natives of Behar, with a larger intake of nitrogen, derived from wheat, and the patives of Eastern Bengal with a larger intake of nitrogen, derived DIETETICS . 613 from wheat and fish, are physically much superior to the rice-eating Bengalis of Lower Bengal, although all belong to the same race. If we decide the matter merely on physiological grounds, we may say that for a man of 70 kilos, doing fairly hard, but not excessive, work, 15 grammes, nitrogen and 250 grammes carbon are a sufficient allowance. The 15 grammes nitrogen will be contained in 95 grammes dry protein, which will also yield 50 grammes of the required carbon. The balance of 200 grammes carbon could theoretically be supplied either in. 450 grammes starch or in 260 grammes fat. But it has been found by experiment and by experience (which is indeed a very complex and proverbially expen- sive form of experiment) that for civilized man a mixture of these is necessary for health, although the nomads of the Asian steppes, and the herdsmen of the Pampas, are said to subsist for long periods on flesh alone, and a dog can live very well on proteins* and fat. The proportion of fat and carbo-hydrates in a diet may, however, be varied within wide limits. Probably no ' work ' diet should contain much less than 40 grammes of fat, but twice this amount would be better; 80 grammes fat give about 60 grammes carbon, so that from proteins and fat we have now got no grammes of the necessary 250, leaving 140 grammes carbon to be taken in about 310 grammes starch, or an equivalent amount of cane-sugar or dextrose. Adding 30 grammes inorganic salts, we can put down as the solid portion of a normal diet sufficient from the physiological point of view for a man of 70 kilos: 95 grammes proteins - =y^ of body-weight. 80 ,. fat =^ 310 ,, carbo-hydrates =^5 30 ,, salts. 525 ,, solid food - =i4(, Now, knowing the composition of the various food-stuffs, we can easily construct a diet containing the proper quantities of nitrogen and carbon, by using a table such as appears on p. 614. Econqmic and social influences — prices and habits — and not purely physiological rules, fix the diet of populations. The Chinese labourer in a rice district, for example, is apt to live on a diet which no physiologist would commend. In order to obtain 15 grammes nitrogen or 95 grammes protein, he must consume more than 1,500 grammes rice, which will yield 700 grammes carbon, or twice as much as is required. But if many of the Chinese labourers could not live on rice, or often on grains cheaper than rice, they could not five at all. The Irish peasant, in the days when the potato was his staple, was even in worse case; he would have been obhged to consume nearly 4 kilos of potatoes to obtain his 15 grammes nitrogen, while little more than half this amount would have furnished the * A little glycogen is, of course, supplied in the meat. 6i4 METABOLISM,. NUTRITION AND DIETETICS necessary 230 grammes carbon. Of course a diet consisting, week in week out, entirely of potatoes or rice, would represent an extreme case, and no doubt the total nitrogen ingested would be considerably below the usual proportion. A certain amount of the necessary nitrogen is obtained even by the poorest populations, in the form of fish, milk, eggs, or bacon. A' man attempting to live on flesh alone would be well fed as regards nitrogen with 500 grammes of meat, but neajly four times as much would be required to yield 250 grammes of carbon. Oatmeal and wheat-flour contain nitrogen and carbon in nearly the right proportions (i N: 15 C), oatmeal being rather the better of the two in this respect; and the best-fed labour- ing populations of Europe still live largely on wheaten bread, whUe, one hundred years ago, the Scotch peasant stiU cultivated the soil, as the Scotch Reviewer the Muses, ' on a Uttle oatmeal.' But although bread may, and does, as a rule, form the great staple of diet, it is not of itself sufficient. Quantity Quantity Carbo- liydrate required required Nin Cin Protein Fat in Water to yield 15 Grms. N. to yield 100 100 in 100 too in loa 250 Grms. C. Grms. Grms. Grms. Grms. in 100 Grms, Grms. Cheese* (Gruydre) - 300 640 5 39 31 31 — 34 Peas (dried) 430 700 3-5 35-7 22 2 55 15 Lean meat - 440 i860 3-4 13-5 21 3-5 74 Wheat-flour 650 625 2-3 39-8 12 2 70 15 Oatmeal - - 580 620 2-6 40-3 13 5-5 65 15 Eggs- 790 1700 1-9 14-7 II-5 12 75 Maize 810 610 1-85 40-9 10-5 7 65 15 Wheat- bread 1200 II20 1-25 22'4 8 1-5 49 40 Rice - 1530 685 0-9 36-6 5 I 83 10 Milk 2380 3540 0-6 7 4 4 5 85 Potatoes 3750 2380 0-4 10-5 2 0-I5 21 75 Good butter 1 0000 360 0-I5 69 I 90 — 8 It is necessary to recognize that habit has much to do with the quantity as well as the quality of the food used by an individual or a community. Some concession may be made to custom in what is after all, not a purely physiological question, and in this country it is probable that 20 grammes of nitrogen and 300 grammes of carbon, while a liberal is not an excessive allowance, although it is certain that a man can maintain a normal body-weight and perform a normal amount of work on considerably less, in some cases even with advantage to his health. We may take 500 grammes of bread and 250 grammes of lean meat as a fair quantity for a man fit for hard work. Adding ♦ A cheese manufactured from whole milk, curdled before the cream has had time to rise, and therefore rich in fat. DIETETICS 615 500 grammes milk, 75 grammes oatmeal (as porridge), 30 grammes butter, 30 grammes fat (with the meat, or in other ways), and 450 grammes potatoes, we get' approximately 20 grammes nitrogen and 300 grammes carbon contained in 135 grammes protein, rather less than 100 grammes fat, and somewhat over 400 grammes carbo- hydrates. Thus — N. c. Proteins. Fat. Carbo- hydrates. 245 25 95 48 Salts. {9 oz.) 250 grms. lean meat - (18 oz.) 500 grms. bread (i pint) 500 grms. milk (i oz.) 30 grms. butter (i oz.) 30 grms. fat (16 oz.) 450 grms. potatoes - (3 oz.) 75 grms. oatmeal 8 6 3 1-5 1-7 33 112 35 20 22 47 30 55 40 20 Id 10 8-5 7-5 20 27 30 4 4 6-5 3-5 0-5 4-5 2 20-2 299 135 97 413 21 This would be a fair ' hard work ' diet for a well-nourished labourer. But the great elasticity of dietetic formulae is shown in the following tables, which give the ration of the German soldier in peace and war and the minimum allowance per ' statute adult ' prescribed in the British regulations concerning passenger-ships from Great Britain to America. Ration of the German Soldier. Peace. War. Bread 750 grammes. Bread - 750 grammes Meat 150 Biscuit 500 Rice 50 Meat 375 or barley groats 120 Smoked meat 250 Legumes - 230 or fat 170 Potatoes 1.500 Rice 125 or barley groats 125 Legumes 250 Minimum Ration foi ' Passenger Ships. Bitead or biscuit 227 grammes. Sugar - 65 grammes Wheaten flour 65 or treacle - 97 or bread 81 Tea 8 Oatmeal 97 or coffee or cocoa 14 Rice 97 Salt 8 Peas 97 Mustard 2 Potatoes 130 Pepper I Beef 81 Vinegar or pickles 20 c.c. Pork or preserved meat 65 In prisons the object is to give the minimum amount of the plainest food which will suffice to maintain the prisoners in health. A ' hard work ' prison diet in Munich was found to contain 104 grammes proteins, 38 grammes fat, and 521 grammes carbo-hydrates; a 'no-work' diet, only 87 grammes proteins, 22 grammes fat, and 305 grammes carbo- hydrates. Here we recognize the influence' of price; carbon can be much more cheaply obtained in vegetable carbo-hydrates than in 6l6 METABOLISM, NUTRITION AND DIETETICS animal fats; the cheapest possible diet contains a minimum of animal fat and proteins. Many poor persons live on a diet which would not maintain a strong man, for an emaciated body has a smaller mass of flesh to keep up, and therefore needs less protein; it can do- little work, and therefore needs less food of all kinds. A London needlewoman, according to Playfair, subsists, or did subsist thirty years ago, on 54 grammes protein, 29 grammes fat, and 292 grammes carbo-hydrates. But this is the irreducible minimum of the deepest poverty, not so much in the protein content, perhaps, as in the very low heat equivalent (1,600 calories); and a woman, with a smaller mass of flesh and leading a less active life than a man, requires less food of all sorts. Even the Trappist monk, who has reduced asceticism to a science, and, instead of eating in order to live, lives in-order not to eat, consumes, accoriding to Voit, 68 grammes protein, 11 .grammes fat, and 469 grammes carbo-hydrates; but manual labour is a part of the discipline of the brotherhood, and this must be still above the lowest subsistence diet. The question whether it is best to derive the proteins (and fats) of the food mainly from plants or mainly from animals is one which is never left to physiology alone to decide. But it has been definitely proved that vegetable proteins and vegetable fats are (when properly prepared) digested and absorbed as completely as those of animal origin, and play the same part in the metabolism of the body. A growing child needs far more food than its weight alone would indicate ; for, in the first place, its income must exceed its expendi- ture so that it may grow ; and, in the second place, the expenditure of an organism is pretty nearly proportional, not to its mass, but to its surface. Now, speaking roughly, the cube of the surface of an animal varies as the square of the mass; when the weight is doubled, the surface only becomes \/4, or one and a half times as great. The surface of- a boy of six to nine years, with a body-weight of 18 to 24 kilos, is two-fifths to one-half that of a man of 70 kilos; and he should Jiave about half as much food as the man. A child of four months,- weighing 5-3 kilos, consumed per diem food con- taining 0-6 gramme nitrogen per kilo of body-weight, or 3-18 grammes nitrogen altogether, as against a daily consumption of only 0-275 gramme nitrogen per kilo in a man of 71 kilos (Voit). An infant for the first seven months should have nothing except milk. Up to this age vegetable food is unsuited to it ; it is a purely- carnivorous animal. By careful observations on the amount of carbon dioxide and nitrogen excreted by a child nine weeks old, fed exclusively on its mother's milk, it has been shown that the ab- sorption and assimilation of milk in the infant is very complete, over gi per cent, of the total energy being utilized ; while an adult, taking as much milk as is necessary for the maintenance of nitrog- enous equiUbrium, does not utilize at most more than 84 per cent. Human milk contains about 2 per cent, of protein (mainly caseino- gen), 3 per cent, of fat, 5 or 6 per cent, of carbo-hydrate (lactose or milk-sugar), and from 0-2 to 0-3 per cent, of salts. Cow's milk contains about 4 per cent, of protein, 4 to 6 per cent, of fat, 4 per cent, of lactose, and 07 per cent, of salts. When given to infants it DIETETICS 617 should, as a general rule, be diluted with water, and some sugar should be added to it. Ass's milk has about the same amount of protein, lactose, and salts as human milk, but less than half as much fat. It is very well borne and very completely absorbed. As to the place of water and inorganic salts in diet, it is neither necessary nor practicable to lay down precise rules. In most well- settled countries they cost little or nothing; very different quantities' can be taken and excreted without harm ; and both economics and physiology may well leave every man to his taste in the matter. Salt is indeed for the most part used,not as a special article of diet, but as a condiment to give a relish to the food, just as a great deal more water than is actually needed is often drunk in the form of beverages. It is certain that the quantity of salt required, jji addition to the salts of the food, to keep the inorganic constituents of the body at their normal amount, is very small. When the food is entirely animal, no additional gait is necessary. A 30-kilo dog obtains in his diet of 500 grammes of lean meat only o -6 grapame sodium chloride, and needs no more. An infant in a litre of its mother's milk, which is a sufficient diet for it at six to nine months, gets only o 8 gramme sodium chloride. The Hereros in Damaraland, who are physically one of the finest races in Africa, do not use salt (Reclus). In this they resemble other tribes in different parts of the world who eat no vegetable food, for example the Kirghiz,, who, live on meat and milk, and the Todas, a pastoral tribe in Southern India, who are ignorant of the use of vegetable foods and know nothing of salt (McCay). Bunge has explained the difference between the flesh and the vegetable feeder by showing that the proportion of potassium and sodium salts in the food is a factor in determining the quantity of sodium chloride required. A dojible decomposition takes place in the body between potassium phQs;^ate and sodium chloride, potassium chloride and sodium phosphate being formed and excreted ; and the loss of sodium and chlorine in this way depends on the relative proportions of potassium and sodium in the food. In most vegetables the proportion of potassium to sodium is much greater than in- animal food, so that vegetable- feeding animals and men as a rule desire and need relatively great quantities of sodium chloride. But it is stated that the inhabitants of a portion of the Soudan use potassium chloride instead of sodium chloride, obtaining the potassium salt by burning certain plants which leave an ash poor in carbonates, and then extracting the residue with water and evaporating (Dybowski). A beef-eating English soldier in India consumes about 7 grammes (J 6z.),, a vegetarian Sepoy about 18 grammes (| oz), of common salt per day. Stimulants. — ^Wine, beer, tea, coffee, cocoa, etc., belong to the important class of stimulants. Some of them contain small quanti- ties pf food substances, but these are of secondary interest. In beer, for example, there are not inconsiderable amounts of proteins, 6i8 METABOLISM. NUTRITION AND DIETETICS dextrin, and sugar. But 14 litres of beer would be required to yield 15 grammes nitrogen, and 10 litres to give 250 grammes carbon; and nobody, except a German corps student, could consume such quantities. The minimum nitrogen requirement, however, as well as the necessary heat value, could theoretically be covered by 6 or 7 litres of good German beer. In some cocoas there is as much as 50 per cent, of fat, 4 per cent, of starch, and 13 per cent, of proteins; and in the cheaper cocoas much starch is added. Still, a large quantity of the ordinary infusion would be needed for a satisfying meal. Frederick the Great, indeed, in some of his famous marches dined off a cup of chocolate, and beat combined Europe on it ; but his ordinary menu was much more varied and substantial. Alcohol. — ^The great social and hygienic evils connected with the abuse of alcohol, as well as its applications in therapeutics, render it necessary, or at least permissible, to state a little more fully, though only in the form of summary, some of the chief conclusions that may be drawn as to its action and uses. (i) In small quantities alcohol is oxidized in the body, a little of it, however, being excreted unchanged in the breath and urine. A certain amount of protein is saved from decomposition when alcohol is taken, just as when fat or sugar is taken. For example, the addition of 130 grammes of sugar to the daily food of an individual caused a 'sparing' of 0-3 gramme nitrogen. The substitution of 72 giammes alcohol for the sugar caused 0-2 gramme nitrogen to be spared (Atwater and Benedict). Alcohol is therefore to some extent a food substance, although it is not, under ordinary circumstances, taken for the sake of the energy its oxidation can supply, but as a stimulant. (2) There is no reason to suppose that this energy cannot be utUized as a source of work in the body. Indeed, a certain amount of alcohol may "be normally formed in the tissues as one of the intermediate products in the oxidation of sugar. Heat can certainly be produced from it, but this is far more than counterbalanced by the increase in the heat loss which the dilatation of the cutaneous vessels caused by alcohol brings about. (3) It is a valuable drug, when judiciously employed, in certain diseases — e.g., pneumonia and puerperal insanity (Clouston). (4) Alcohol is occasionally of use in disorders not amoimting to serious disease — e.g., in some celscs of slow and difficult digestion. In these cases it may act by increasing the flow of certain of the digestive secretions, as saliva and gastric juice. This effect seems to more than counterbalance the retarding influence which, except when well diluted, it exerts on the chemical processes of digestion. The action of alcohol on the secretion of gastric juice has been studied in a dog with a double gastric and oesophageal fistula. Before or during a sham meal of meat, alcohol diluted with water was given as an enema. After the enema the quantity of hydrochloric acid secreted increased in about the same proportion as the quantity of juice, but the pepsin was diminished, reaching a minimum after three-quarters to one and a quarter hours. The increase in the total quantity of the juice and in the acid over-compensated the moderate diminution in the digestive power, so that the net result was beneficial (Pekelharing). But it must be remembered that strong alcoholic beverages, when mixe4 DIETETICS 619 with the gastric juice, and therefore when taken by the mouth, retard the proteolytic action, so that any favourable effect on the secretion of the juice may easily be lost in the subsequent digestion, unless the alcohol is dilute (Chittenden and Mendel). The action of alcohol intro- duced into the rectum on the gastric secretion is both reflex and direct, (5) Alcohol is of no use for healthy men. (6) Alcohol in strictly moderate doses*, properly diluted and especi- ally when taken with the food, is not harmful to healthy men, living and working under ordinary conditions. (7) Modern experience goes to show that in severe and continuous exertion, coupled with exposure to all weathers, as in war and in exploring expeditions, alcohol is injurious, and it is well known that it must be avoided in mountain climbing. Alcohol in small doses, when given by the stomach or (in animals) injected into the blood, causes stimulation of the respiratory centre and increase in the pulmonary ventilation. In man, this increase usually amounts to 8 to 15 per cent., but is occasionally much greater. But the ' limit which separates the favourable action of the small dose from the hurtful action of the large, is easily overstepped. When this is done, and the dose is continually increased, the activity of the respiratory centre is first diminished and finally abolished. In dogs, for instance, after the injection of considerable quantities of alcohol into the stomach, death takes place from respiratory failure, and the breathing stops while the heart is still unweakened (Fig. 85, p. 189). This is the final outcome of a progressive impairment in the activity of the centre, of which the slow and heavy breathing of the drunken man represents an earlier stage. Tea, cofiee, and cocoa are more suitable stimulants for healthy persons, because they are less dangerous than alcohol, and they leave no unpleasant effects behind them. But it should be remem- bered that there is no stimulant which is not liable to be abused. It has been shown by ergographic experiments (p. 726) that, like alcohol, tea, coffee, mat6, and cola-nut, which all contain the alka- loid theine or caffeine, restore the power of performing muscular work after exhaustion, but only if food has been recently or is simultaneously taken. Vitamines. — Certain substances, although neither in the ordinary sense foods nor condiments, seem to be necessary for the main- tenence of health, for in circumstances in which these cannot be obtained for long periods so-called ' deficiency diseases,' such as scurvy, are liable to occur. Scurvy used to be the scourge of the sailing-ship in the days when fresh meat, and particularly fresh vegetables and fruits, were unobtainable on long voyages. It has long been known that it is prevented by the use of lime or lemon- juice, in which citric and a trace of malic acid are contained, and it used to be thought that it was the organic vegetable acids which were the important thing. Recent researches have shown, how- ever, that scurvy is only one of a group of diseases, including beri- beri, and probably pellagra, rickets, and others which are induced * Not more than i J oz. of absolute alcohol, corresponding to about 4 oz. of whisky, or 2 to 3 wineglasses of sherry or port, or a pint of claret, or a couple , of pints of light beer in 24 hours. 620 METABOLISM, NUTRITION AND DIETETICS by deficiency in the food of certain substances minute in amount but essential to proper nutrition. These substances are some- times termed ' vitamines.' But their chemical nature is imper- fectly known, and there is no certainty that the bodies which exert the beneficial influence belong to the same chemical group.* The best investigated representative of the important food constituents in question is a basic substance separated by Funk from the polish- ings of rice, and named by him ' vitamine.' Polished rice is rice deprived of the outer coats by modern milling processes, and the polishings are the coats which have been removed. It is a general rule that the vitamines in the cereals, including wheat, maize, oats, and barley, are contained exclusively in the outer coats. Since the introduction of steel rollers instead of the primitive millstones which used to crush the whole grain, beri-beri, a disease characterized by inflammatory and degenerative changes in the peripheral nerves (peripheral neuritis) and consequent paralysis, has greatly increased among the rice-eating Japanese. In Bengal, although much rice is eaten, there is practically no beri-beri, as country rice and not the highly polished variety is consumed. When birds — e.g., pigeons, —are fed on polished rice, pol3meuritis similar to that seen in human beri-beri is produced, and both in man and in birds the condition is quickly cured by reverting to rice prepared according to the old- fashioned methods, or by adding the polishings or an alcoholic extract of them containing the essential substance, or the isolated base itself. The addition of various legumes to the diet, or alcoholic extracts of these, will produce the same beneficial effect (McCay). Potatoes, carrots, fresh vegetables, lime and other fruit juices, also certain animal foods, such as fresh milk, fresh meat, and yolk of egg, are all valuable, in addition to their ordinary nutritive constituents, for their content of vitamines. Yeast contains them in exceptionally large amount, and it is possible, though not proved, that such fermented liquors as beer, or some varieties of it, may derive some part of their value from these substances liberated both from the yeast and the barley and not destroyed in the process of brewing. Since vitamines exert so great an effect on nutrition and growth, it might be expected that their absence would tell on those glands of internal secretion which appear to be concerned in the metabolism of growth. As a matter of fact, it has been found that in pigeons suffering from the typical deficiency disease beri-beri, certain of these glands show marked changes. The thymus gland, normally very large and persistent in these birds, can be caused to atrophy com- pletely by a diet of polished rice. Changes also occur in the patuitary, and decided atrophy in the testes and ovaries (Funk and Douglas). * It might be better in the present state of our knowledge to avoid giving those bodies a name which may easily mislead. They might possibly be pro- iVisionally spoken of as " vitines," a term involving no assumption as to their chemical nature, and implying only their importance in the nutritional pro cesses associated with the life (and growth) of the tissues. CHAPTER XI INTERNAL SECRETION It is long since Caspar Friedrich Wolfl[ expressed the idea that ' each single part of the body, in respect of its nutrition, stands to the whole body in the relation of an excreting organ,' and thus emphasized the importance of substances produced by the activity of one kind of cell for the normal metabolism of another. But it is only in recent years that it has become possible to illustrate this mutual relation by any large number of experimental facts. Certain of the substances taken in from the blood by the liver find their way, after undergoing various changes, inl o the biliary capillaries, and are excreted as bile; certain other substances, such as sugar and the precursors of urea, are taken up by the hepatic cells, transformed and sometimes stored for a time within them, and then given out again to the blood. Bile we may call the external secretion of the liver, glycogen and urea constituents of its internal secretion. In one sense it is evident that all tissues, whether glands in the morphological sense or not, may be considered as manufac- turing an internal secretion. For everything that an organ absorbs from the blood and lymph it gives out to them again in some form or other except in so far as it forms or separates a secretion that passes away by special ducts. But it is usual to employ the term only in relation to organs of glandular build, whether provided with ducts or not. For convenience the action of extracts of some other tissues, such as nervous tissue, will also be considered here, although there is no reason to suppose that they form any specific internal secretion. The capacity of manufacturing internal secretions of high im- portance can neither be attributed to all glands with ducts nor denied to all other organs. For the salivary, mammary, and gastric glands may be completely removed without causing any serious effects, while death follows excision of the, so far as mere bulk is concerned, apparently insignificant masses of tissue in the ductless thyroid, parathyroid, suprarenal or pituitary bodies. ■, It is known that in the case of the liver the internal secretion is more important than the external, for an animal cannot sur\dve 621 622 INTERNAL SECRETION without its liver, while it may be but little affected by the con- tinuous escape of the bile through a fistulous opening. Pancreas. — The internal secretion of the pancreas is also indis- peiisable. For when the pancreas is excised death follows in many species of animals, and especially in carnivorous animals; and in man severe and ultimately fatal diabetes is often associated with pancreatic disease, while the mere loss of the pancreatic juice through a fistula does not necessarily shorten Ufe, although the absorption of fat is seriously interfered with. The ultimate cause of death seems to be a profound disturbance of metaboUsm, of which the most significant token is the increased proportion of sugar in the blood, and its speedy appearance in the urine — in dogs always within twenty-four hours following total removal of the organ. Associated with the glycosuria is an increase- in the quantity of the urine (polyuria), excessive thirst (polydipsia), and a ravenous appetite (polyphagia accompanied by intense hunger contractions of the stomach — Luckhardt), in spite of which the animal becomes more and more emaciated — in short, the classical symptoms of a severe type of pathological diabetes in man, but, of coiurse, far more acute in their onset, and far more rapid in their progress towards the inevitable end. Dogs rarely survive more than two or three weeks, the immediate cause of the rapidly fatal result being perhaps the extensive suppuration which is apt to ensue on slight and practically unavoidable superficial injuries. The resistance of the tissues to bacterial invasion and their tendency to spontaneous healing are reduced by the overloading of the blood and tissue liquids with sugar. Even when carbo-hydrates are ex- cluded from the food, or when no food at all is given, sugar continues to be excreted in large amounts. The destruction of proteins is increased. It is a significant fact that glycosuria does not appear or is only transient when the pancreas is partially removed, so long as a comparatively small fraction of the gland (one-quarter or one- fifth) is left. Even when such a remnant is transplanted from its original position, care being taken not to interfere with its circula- tion, and grafted in the peritoneal cavity oi', indeed, under the skin, the animal remains in good health. In the dog this operation can be practised on the lowest part of the descending division of the pancreas, which is not united with the duodenum, but lies free in the mesentery. Removal of the fragment of pancreas is followed by the whole train of symptoms associated with total extirpation of the organ. Although as yet we are ignorant of the precise manner in which the pancreas influences the metabolism of the body, it is impossible to doubt, in view of the facts we have mentioned, that, like the liver, in addition to carrying on the exchanges necessary for the prepara- tion of the ordinary or external secretion, the gland has other PANCREAS 623 important relations with the circulating fluids, giving to them or taking from them substances on the manufacture or destruction of which the normal metabolic processes depend. It has been sug- gested that the pancreas neutralizes or renders harmless some toxic substance formed elsewhere in the body, the action of which produces glycosuria. But no evidence of the existence of any such substance has been obtained, and the transfusion into a normal dog of blood from a depancreatized animal, which ought to be laden with the hypothetical toxic material, does not cause glycosuria. It is much more probable that the hyperglycsemia on which the glycosuria depends is caused by the absence of something normally produced by the pancreas, and which is indispensable for the due regulation of the sugar-content of the blood. This something, as already pointed out in discussing pathological diabetes, may be necessary to regulate the transformation of sugar into glycogen, or eventually, it may be, into fat, so that too great a surplus of sugar does not remain unchanged; or to regulate the transformation of glycogen into dextrose, and prevent too hasty and too extensive action by the glycogenase; or to regulate the production of sugar from sources other than the carbo-hydrates; or, finally, to regulate and to aid in the normal utilization of the sugar in the organs (P- 357)- While the liver contains less than the normal content of glycogen, its power to form glycogen is certainly not abolished. On the con- trary, there is some reason to think that a great deal of this reserve carbo-hydrate may be synthesized in the diabetic organism, and that the comparative poverty of the hepatic cells in glycogen may be due to rapid glycogenolysis, despite the hyperglycaemia, in response to the insistent demand for sugar on the part of the tissues, which in the midst of plenty are hungry for dextrose on account of their inability to utilize it, or some of its decomposition products, in the normal way. It has indeed been shown by numerous ex- periments that interference with the formation or with the hydrol- ysis of glycogen, although it may be a factor, is not of itself sufficient to explain pancreatic glycosuria. Failure in the katabolism of dextrose, as already mentioned (p. 545), has been asserted by some observers and denied by others. A great production of sugar from proteins {i.e., from amino-acids) has been demonstrated, but it is quite possible that just as much is produced from this source in the norrnal organism, although here its formation is masked by a corresponding utilization. The clearest evidence that the pancreas produces something of high importance in carbo-hydrate metabolism has been obtained by experiments in which animals were united in such a way that substances could pass from one to the other (parabiosis, see Chap. XIX.). When two young dogs were so united and the pancreas 624 INTERNAL SECRETION then removed from one, no glycosuria followed. The internal secretion of the remaining pancreas was sufficient for both (Forsch- bach). In like manner the removal of the pancreas from pregnant bitches not far from full term caused no glycosuria or very little till the pups were born, when the usual train of events associated . with pancreatic diabetes ensued. Obviously the pancreatic tissue of the embryos in the uterus supplied the mother with the indis- pensable secretion (Carlson and Drennan). The question has often been raised why it should not be possible to supply animals or human beings suffering from pancreatic defi- ciency with the missing material by administering pancreas or pancreatic extracts. Hitherto, however, little if any success has attended attempts of this kind, perhaps because the active substance or substances are very easily destroyed. This has been all the more disappointing, as in the case of the internal secretion of the thyroid the so-called ' substitution therapy ' has been brilliantly successful (P- 634)- The seat of the internal secretion of the pancreas seems to be the very vascular epithelioid tissue which is peculiar to this gland, and occurs in islands between or imbedded in the alveoU (islands or islets of Langerhans) (Schafer). For animals survive the complete atrophy of the ordinajy secreting epithelium caused by the injec- .tion of paraffin into the ducts, and no sugar appears in the urine. The islets remain intact. When a portion of the pancreas is sepcirated from the rest, and its duct hgated, it undergoes extensive atrophy, a tissue remaining which is apparently composed of en- larged islands of Langerhans and remains of pancreatic ducts. If the rest of the gland is now removed, no glycosuria occurs, even when considerable quantities of dextrose are injected. But when the atrophied remnant is also removed, typical pancreatic glycosuria at once ensues (W. G. MacCallum). As further evidence that the islets have a different function from the pancreatic alveoli may be cited the statement that in teleostean fishes, in which the islands are so large that they can be separated from the rest of the tissue, the cells of the islets, instead of containing an amylolytic ferment like the alveolax cells, contain a glycolytic .ferment, or at least possess the power of destroying sugar. Yet the question of the significance of the islets can hardly be considered settled, although, as previously mentioned, the supposed experi- mental basis of the theory that they do not differ essentially from the alveolar tissue, but are formed by certain changes in the arrange- ment and properties of the alveolar elements, appears to have collapsed under the criticism of Bensley and others. Far from being interchangeable with the cells of the acini, the islet cells present definite and permanent criteria by which they can be sharply distinguished from the alveolar epithelium. At least two types of PANCREAS 625. islet cells may be identified by their staining reactions, the so-called A and B cells (Lane). The B cells are the most abundant in all the islets, and many of the small islets are composed of them. In the guinea-pig, on account of the great size of some of the islets, and because many of them are situated in the interstitial tissue (between the acini), it is not difficult to pick out an islet, isolate it from the surrounding tissue, and examine it in serum or salt solu- tion. The cells are crowded with very fine granules exhibiting Brownian movement. In the fresh preparation the granules of the A cells cannot be distinguished from those of the B cells. Both varieties stain intensely with neutral red and other dyes, and the islet tissue can in this way be easily differentiated from the tissue of the acini. By differences in their staining reactions and certain properties of their nuclei, which need not be gone into here, the two varieties of islet cells can be identified. The important point for our purpose is that by an appropriate histological tech- nique the islet tissue can be studied in all the functional vicissitudes of the gland. When this is properly done, it is not found that there is any close connection between the secretory activity of the cells of the acini and the islets. Nor is there any evidence that the amount of islet tissue in the pancreas is ever affected by the forma- tion of new islets out of acini. On the other hand, it seems that the islets, or the great majority of them, consist of epithehal cells which are in direct continuity with the pancreatic ducts, and that after removal of a portion of the pancreas, establishing an insufficiency of islet tissue, new islets can be developed from the duct epithelium, in addition to the increase by interstitial growth in the size of islets already existing (Bensley, etc.). If the islets are connected with the ducts, the possibility may be admitted that they yield some- thing to the external secretion of the pancreas as well as to its internal secretion. But if this be so, there is no improbability in the idea that the alveolar epithelium, which is undoubtedly mainly concerned in the preparation of the pancreatic juice, may also contribute something to the internal secretion of the gland. While, then, the importance of the pancreas in carbo-hydrate metabolism is certain, and the dependence of this function upon an internal secretion is highly probable, it is not yet definitely settled whether this secretion is formed in the organ as a whole, or only in the islets. That lesions of the pancreas may be concerned in pathological diabetes is well established, and it is of interest in connection with the question we have just been discussing that in a certain number of cases the changes observed have been in the islands (Opie). And in diabetes accompanying cirrhosis of the liver, which has usually been considered to depend upon the hepatic changes, it has been shown that in many, if not all, of the cases the pancreas is also affected by a growth of connective tissue outside the acini (Stein- 40 626 INTERNAL SECRETION haus). Some authors, indeed, have gone so far as to say that in all cases of diabetes mellitus there is disease of the pancreas, but of this there is no evidence. Ligation, or the establishment of a fistula, of the thoracic duct, causes glycosuria in dogs. It is possible that this is really a mild form of pancreatic diabetes, due to interference with the supply of the internal secretion of the pancreas, or of that part of it which reaches the blood by the lymph-stream (Tuckett). Pfliiger has brought forward evidence that it is not the removal of the pancreas, as such, but the section of certain nerves running into or through it from the duodenum, which is the cause of the glycosuria. For when these nerves are divided or the duodenum removed while the pancreas remains untouched, the result is the same as if the pancreas itself had been excised. He imagines that these nerves are ' anti- diabetic ' — ^that is, in some way oppose the production of sugar — while nerves coming from the so-called ' sugar centre ' in the bulb (the centre assumed to be affected in the puncture experiment) favour sugar pro- duction. Between these the normal balance is struck in health; it is the upsetting of this balance by the crippling of the duodenal fibres which is at the bottom of ' pancreatic ' diabetes. It is too early to appraise the value of this conception, especially as the facts upon which it is founded have only been clearly established for frogs, and it is doubtful whether they can be extended to mammals. But if these nerves end in the pancreas, and do not simply run through it, say, to the liver, it is possible that they act on the sugar metabolism by regulating the internal secretion of the pancreas. Sexual Organs. — ^The influence of castration in preventing the development of the sexual characters, and especially the physical and psychical changes that normally occur at puberty, is also due to the loss of the internal secretion of the generative glands, and does not appear to depend at all upon the loss of nervous impulses arising in these organs. In Herdwick sheep an outstanding sexual difference is the presence of horns in the males, their absence in the females. Removal of the testes from ram lambs arrests further growth of horns forthwith and at any stage of development. The retention of the epididymes, provided that the testes proper are removed, does not alter the result of castration in the least. The removal of one testicle slows horn growth without arresting if (Marshall and Hammond). In partially castrated cocks it has been seen that, so long as a portion of one testicle remains, the male characters are preserved, but after removal of this residue the comb and wattles wither in a few weeks (Hanau). At the breeding- time the muscles of the forearm of the brown land frog [Rana fusca) become hypertrophied in the male, so that it can more tightly hold the female. At the same time the balls of the toes increase in size, and become covered with a peculiar black growth. After the breeding season these secondary sexual characters disappear. If the male frog is castrated, the periodic return of these phenomena does not occur, but the presence of one testicle suffices for their SEXUAL ORGANS 627 development on both sides. When pieces of testicle frorii normal, frogs are introduced, under the skin of the castrated firogs, the phenomena occur, just as if the animals had not been castrated (M. Nussbaum). The remarkable observations of Steinach indicate thclt the internal secretion is not furnished by the proper reproductive elements (those which form the spermatozoa), but by the interstitial cells of Leydig, which are distributed in groups througho.Ut the substance of the testes between the seminal tubules. When the testes of a young rat or guinea-pig are transplanted to another part pf its body (the peritoneal cavity -or subcutaneous tissue), the animal develops all the secondary sexual characters at the proper time. The penis grows" to. the normal size. The seminal Vesicles and prostate develop in -the ordinary way, and yield a plentiful secretion. Sexual ;desire and potency lappear. in due season, and in normal or, in not a few cases, indeed, increased intensity. Yet histological examination shows that not a single spermatocyte or spermatid' (Chapter XIX.) has developed, while outside the seminal tubules the' interstitial cells form large masses which much surpass in size the interstitial islands of the normal testis. Similar changes are observed, though with less certainty and after a longer interval, when the.vas deferens is ligated, a method often recom- mended and occasionally practised for the sterilization of the human male. .On account of thei influence, thus demonstrated, of the inter- stitial cells in producing the sexual development observed at puberty, Steinach desigfiates.these cells collectively as the ' puberty gland.' When the ovaries of a young female rat or guinea-pig are transplanted into the peritoneal cavity or under the skin of a pre- viously castrated male animal of the. same kind (preferably, to facilitate accurate comparison, a male of the same litter), the graft takes (in about half the cases), and. the implanted .ovaries grow and mature in the male body. There is this differehce in the fate of the ovary and the testis when transplanted, that the generative elements of the latter, the Graafian f oUicles, with the ova contained in them, generally develbp as well as the large interstitial cells rich in protoplasm lying in the stroma, which cells appear to constitute the female puberty gland. The strict isolation of the female puberty gland is only realized in those cases in which by some accident of healing the^stroma of the transplanted ovary maiijtains itself while the generative elements disappear. In these cases the influence of the ovary on the development of the sexual characters is the same as when; the reproductive elements proper persist and grow. Male animals into which successful implantation of ovaries has been accompUshed, instead of developing sexually in the way observed even in castrated inales, become feminized. The grov^th of the exterijal generative organs is inhibited, the mammary glands 628 INTERNAL SECRETION and the nipples develop to the form and size seen in the normal female. The dimensions and shape of the body conform more and more to the feminine type. The hair becomes smooth and silky, in contrast to the coarse hair of the normal or even of the castrated male. The psychical characteristics also become feminized, and certain reflexes peculiar to the female make their appearance. The feminized male is sought by normal males as if it were a true female. A more general influence of the sexual organs on metabohsm seems also to be well established. The exact experiments of Loewy and Richter on the metabohsm of bitches before and after cas- tration throw hght upon the changes which follow that operation, and afford decisive proof that they are connected with the absence of substances specific to the ovary. They conclude that in the castrated animal the oxidative energy of the cells is lessened. The oxygen consumption sinks, even although protein is laid on and the total amount of active tissue thus increased. Under certain cir- cumstances this specific diminution of metabohsm may be balanced by conditions which cause an increase in the metabohsm. The lessening of the oxidative power is due to the loss of ovarian sub- stance, for the administrarion of an extract of the ovary (oophorin) not only neutraUzes it, but actucdly causes an increase in the gaseous metabohsm to far above the original amount, while it has no effect on the metabolism of the uncastrated animal. It is not the de- composition of proteins, but of non-nitrogenous substances, which is accelerated. Oophorin also brings about a notable increase in metabolism in the castrated male dog, while, curiously enough, extract of testicle causes only a small increase, due to a basic sub- stance, spermin (C5Hj4N2), which can be isolated from the testicle. But the orchitic extract is not without influence in other ways. It certainly increases the capacity for muscular work (Zoth and Pregl), as tested by the ergograph (p. 726), and this distinct physio- logical action is sufficient to encourage the hope that it may possess some therapeutic value, although far from what has been daimed for it by its more enthusiastic advocates. The only constituent of extracts of the testicle made with salt solution which causes any pronounced effect on the blood-pressure when injected into the circulation is a nucleo-protein, the most plentiful of the protein substances. The pressure faUs, mainly owing to inhibition of the heart, but partly through vaso-dilatation in the splanchnic area (Dixon). The testicles also influence the growth of the bones. In eunuchs and in young men with atrophy of the testicles a tendency has been observed for the long bones to go on growing far beyond the usual period. This has been shown by the Rontgen rays to be due to delay in the ossification of the epiphyses. The same has been observed in animals, and is supposed to be caused by the loss of THYMUS 629 some substance normally formed in the testicle which influences the metabolism of the bones and the deposition of the bone salts. A temporary diminution in the haemoglobin and in the number of the erythrocytes has been observed in castrated bitches, an observation which, so far aS it goes, is in favour of the view that an insufficient internal secretion of the ovaries is the cause of the form of anaemia known as chlorosis. , While these effects on general metabolism and nutrition, as well as the influence on the development of the sexual characters, are probably to be ascribed to changes in the internal secretion of the interstitial cells, there are facts Which indicate that other elements may be concerned. For example, evidence has been brought ' forward that the corpus luteum is a gland with an internal Secretion, whose function is connected with menstruation and with the im- plantation of the ovum and the subsequent growth of both ovum and uterus in pregnancy (Born, Fraenkel) (Chap. XIX.). Thymus. — -Our knowledge of the function of the thymus is very incomplete. Even its histological structure, and especially the source and nature of its cellular elements, have long been, and still are, the subject of controversy. It is developed as a pair of diver- ticula from the ventral part of the third and fourth, perhaps also to some extent from the second, branchial cleft. These pouches grow downwards into the thorax. At this stage the organ is a purely epithelial structure. Soon connective tissue and blood- vessels begin to grow into it, the two halves coalesce in the middle line, and the thymus becomes transformed by degrees into a struc- ture with a general resemblance to a big lymph gland, and con- sisting mainly of small cells like lymphocytes. Some observers believe that, these cells are true lymphocytes, derived from the mesoderm, which have migrated into and displaced the earlier epithelial tissue. Others maintain that the resemblance is merely superficial, and that they are simply epithelial cells diminished in size and altered in shape, but derived from the original epithelium by repeated division, and remaining epithelial to the end. Any theory of the function of the thymus must needs depend lairgely upon the view adopted as to its structure. For if it is in its fully developed state merely a large collection of lymphocytes, it would appear quite unlikely that it should possess functions very different from those of other collections of lymphoc5rtes. On the other hand, if the essential elements in the organ are epithelial, they may well,, like the epithelial elements of the thyroid or of other glands with an internal secretion, be concerned in the elaboration of sub- stances which exercise an important influence upon nutrition and growth. On the whole, the best histological evidence seems to favour the view that the thymus cells are different from the cells of lymph glands. Chemical differences also exist. For example. 630 INTERNA L SECRETION nuclein- substances characteristic of the iraclear fraraework of the true glailds are much more abundant ifi the thjmius than in lymph glands; After a period of further development, which varies in duration in different animals, the organ undergoes involution. In mammals (includihg man) the thymus does not completely disappear in the adult. Islands of thymus tissue are found at all ages among the fat by which the bulk of the organ is replaced. It is usually stated that in man the thymus begins to diminish in size about the end of the second year, but the careful observations of Hammar indicate that this is incorrect. According to him, the organ continues to grow till puberty is reached, weighing on the average 13 grammes at birth, 37 grammes at eleven to fifteen years, 25 grammes ai. sixteen to twenty years, and only 6 grammes at sixty-six to seventy- five years. Besides this involution with age, great changes in the size of the thymus may occur at any time under the influence of toxic substances or of deficient nutrition. In starvation, even in the first three days of hunger, the weight of the^ thymus in rabbits has been observed to shrink to one- half , and during prolonged underfeeding even. to one-thirtieth, of the normal (Jonson). The opposite effect, namely, cessation of the involution process, or even new formation of thymus tissue, may also occur, leading to the presence of an unusually large so-called persistent thymus in the adult. The point most clearly established in the physiology of the thymus seems to be its relation to the sexual glands. It is well known that in castrated animals the thymus is larger and persists longer than in entire animals. In bulls and unspayed heifers the normal atrophy of the thymus, which begins after the period of puberty, is greatly accelerated when the bulls have been used for breeding, and when the heifers have been pregnant for several months. There is a reciprocal influence of the thymus on the testicles, and removal of the thymus before the time at which it naturally atrophies is followed by a more rapid growth of the testes (in guinea-pigs) (Paton). The relation of the thymus to the growth of bones is less well established, but according to some observers extirpation of the gland retards their calcification. In young mammals the loss of the thymus is said to cause transient disturbances of nutrition, a temporary decrease in the number of all varieties of leucocytes, and a diminished resistance to the pus- forming micrococci, probably connected with the relatively feeble leucoc5rtosis (or increase in the number of leucocjrtes) by which the animals react to the infection. In the frog the thymus persists throughout hfe. Yet the removal of it is not fatal if precautions against infection be taken. The chief effect of intravenous injection of extract of human THYROIDS AND PARATHYROIDS 631 or ox thymus is a lowering of blood-pressure ; but there is nothing specific in this, a similar effect being given by th3n:oid extract and the extracts of many other tissues. The heart may be at the same time accelerated. Thyroids and Parathyroids. — ^The thyroid consists of two lobes connected by an isthmus across the middle line in man and some animals, but often separate. In the neighbourhood of the thj^oid, or embedded in its tissue, are certain bodies called parathyroids, consisting of solid columns of epithelial cells. The number and situation of the parathyroids are not constant. . As a rule, there are four in mammals, two on each side, but this number is subject to variations in different individuals of the same species. The varia- biUty in their anatomical relations to the thyroid is of greater significance. For much of the uncertainty in which the whole question of the symptoms following ex- tirpation of the thyroids was until lately involved arose from ignorance or insufiicient recognition of this variability. In most animals the inferior, anterior, or external pair of parathyroids is more or less distinctly separated from the thyroid. The separation is especially evident in the her- bivora, in the monkey, and in inan, and this pair of para- thyroids is much larger than the other. In carnivorous animals, as the dog and cat, the anterior pair of parathyroids is closely adherent to the thyroid capsule. The superior, posterior, or internal pair, both in herbivora and carnivora, is always very closely associ- ated with the capsule of the thyroid, and frequently embedded in the substance of the gland. The consequence of this arrangement is that in the older experiments the chief masses of parathyroid tissue were. much more hkely to escape removal with the thyroid in the case of herbivorous than in the case of carnivorous animals. But even in one and the same species considerable variations may exist. It is easy to see, then, that in removing the thyroid the parathyroids would sometimes be completely removed as well, Fig. 202. — Parathyroid (Vincent and Jolly). A small portion of parathyroid of cat em- bedded in thyroid tissue. It consists for the most part of solid columns of epithelial cells (3, 5, 8) with strands of vascular con- nective tissue (6). A thyroid vesicle (11) and portions of two others (i, 10) are seen in the lower part of the figure, separated from the parathyroid by a fibrous capsule (2). 4, 7, bloodvessels; 9, lower boundary of the parathyroid tissue. ( x 500.) 632 INTERNAL SECRETION while at other times all or some of the parathyroid tissue would be spared. Add to this that sporadic masses of thyroid tissue (acces- sory thyroids), often existing as far down as the root of the aorta (always, indeed, in certain animals — e.g., the dog), must necessarily be spared in the most complete thyroidectomy, and it will cease to excite surprise that the sjmiptoms and pathological changes de- scribed after that operation should have been so various and so contradictory. We know now that the parathyroids are perfectly distinct organs from the thyroid in histological structure, in func- tion, and in the consequences of their removed. The parathyroids, for instance, contain no iodine, while iodine is a characteristic constituent of the thyroid. Nor do the parathyroids show any compensatory hypertrophy when the thyroid alone is excised, or any changes which would indicate a definite relation to, still less an active participation in, the pathological processes occurring in the thyroid in goitre. This does not mean, however, that there are no points of contact between the functions of the two glands. The more the matter is probed, the more clearly does it appear that none of the organs is quite independent of the rest, and the reciprocal relations of the ductless glands are probably of exceptional im- portance. But the premature attempts which have been made, in the absence of a sufficiency of exact data, to represent their mutual influence by crude schemata, have retarded rather than advanced our knowledge, and need not be referred to here. Parathyroidectomy. — ^Total extirpation of the parathyroids is followed by a train of acute symptoms, ending fatedly, as a rule, in from one to ten days. The t jrpical nervous symptoms following the operation have been described as those of ' tetany,' and the tetany which used to be included among the consequences of removal of the thyroid is now known to be due to the simultaneous excision of the parathyroids (Kocher). A cat, after the combined operation, is perfectly well on the first day. On the second day a curious shaking of the paws is seen, tremors of central origin soon appear, and increase in severity, imtil at length they culminate in general spasmodic attacks. Even when the animal is at rest the fore-legs tend to be flexed, while the hind-legs are extended, and this attitude is exaggerated in the convulsions. In the later stages unconscious- ness is associated with the onset of the convulsions. Similar results follow e.xcision of the parathyroids alone in dogs. Although the tetany is the most striking symptom, it is only one token of a pro- found general disturbance of nutrition. The pulse-rate and the rate of respiration are markedly increased. There is fever and pro- fuse salivation, with dilatation of the stomach and duodenum, due to the loss of muscular tonicity. In the intervals between attacks the tonus returns to the normal. The secretion of the gastric juice, pancreatic juice, and bile are interfered with (Carlson, etc.). THYROIDS AND PARATHYROIDS 633 The excitability of the vaso-constrictor mechanism is said to be increased. The exact significance of these symptoms is unknpwn. It has been suggested th^t the loss of the parathyroid function is in some way associated with an augmentation of the irritability of the whole sympathetic system (Hoskins). The administration of calcium completely relieves the symptoms, and by its use death may be long or perhaps indefinitely postponed (W. G. MacCallum). The mode of action of the calcium has not been made clear as yet. It does not seem to be so efficacious in rabbits as in dogs (Arthus). Thyroidectomy. — -The symptoms that follow removal of the th3Toid alone are perfectly different. The metabolic disturbance is eventually, in most animals, not less far-reaching than that which ensues when the jparathyroids are alone excised. But it is far more chronic, reveals itself by totally distinct changes, is not amenable to calcium, and is completely corrected by the administration of thyroid substance. While no animals which have been examined survive the total removal of the parathyroids, certain species — e.g., the goat — are but ,shghtly affected by thyroidectomy, and survive indefinitely. In man, before the consequences of th3^oid- ectomy were known, the whole gland was not infrequently excised for goitre. If the parathyroids happened also to be comjiletely involved in the operation, death quickly followed. But where only the thyroid itself, or the thyroid plus the smair internal pair of parathyroids, was extirpated, the condition called cachexia strumipriva was observed to supervene. The symptoms resemble those of the disease known as myxcedema, in which the. charac- teristic anatomical change is an increase (a h3^erplasia) of the connective tissue in and under the true skin. Newly-formed connec- tive tissue always contains an excess of mucoids, and for this reason in the early stages of myxoedema there is somewhat more than the usual amount of these substances in the subcutaneous tissue. The skin is dry, and the hair falls off. The features are swollen and heavy, the movements clumsy and trembling. As the disease progresses the mental powers deteriorate too; the patient becomes stupid and slow, and perhaps, at last, imbecile. When the gland is so affected in early life that extensive atrophy of the true secreting tissue occurs, a peculiar condition of idiocy (cretinism) results. In animals there is a great difference in the results of total ex- cision of the thyroids, both between different groups and between different individuals of the same group. In young animals the symptoms come on more rapidly and are more severe than in old. Monkeys develop symptoms resembling those of myxcedema. The older descriptions of the very acute onset of the symptoms and the quickly fatal result in carnivorous animals were vitiated by the circumstance that, for the anatomical reason already alluded to, the parathyroids were also involved in the operation. Never- ^34 INTERNAL SECRETION tbeless, the consequences of complete removal of the thyroid proper are in general more serious in the carnivora than in the herbivora. Muscular weakness soon becomes marked; the tissues waste, the temperature becomes subnormal, and this is associated with changes in the heat regulation (p. 673). If a portion of the thyroid be left, or a graft be made of some thyroid tissue from an animal of the same species, these effects are entirely obviated so long as the graft survives. It has not been estabUshed that a hetero-thyroid graft— I.e., a graft of thyroid tissue from an animal of a different rkind — even temporarily succeeds. The alien thyroid cells are destroyed by cytolysins (p. 31) in the serum and tissue Uquids of the animal. When a small part of a thyroid is left, it may undergo great h57pertrophy, and the same is true of the accessory thjnroids. The administration of extracts of the thyroid glands or the glands themselves by the mouth brings about a cure, permanent so long as the thyroid treatment is continued, in cases of myxoedema in man, and prevents the development of the symptoms in animals or removes them when they have appeared. The same is true of a compound, rich in iodine, the- so-called thyroiodin, which has been extracted from the organ. Under this treatment the total metab- ohsm, which in myxoedema is below the normal, is markedly in- creased. This is partly due to an increase in the metabohsm of ■protein. An increase in the destruction of protein is also caused in normal persons and in normal animals by feeding with thyroid or with thyroid preparations. The excretion of nitrogen, carbon dioxide, and phosphoric acid, and the intake of oxygen, are aug- mented. But in spite of increased appetite the body-weight falls off, arid diarrhoea is often caused. For these reasons the use of thyroid preparations to reduce weight in cases of obesity, without evidence of thyroid insufficiency, is a dangerous remedy. For while a fat man can very well spare a great deal of his fat, he cannot spare much of his tissue-protein. That the gland exerts in some way an important influence on the metabolism of proteins is also indicated by other facts. The question whether the thyroid or parath3nroid is, in addition, concerned in the carbo-hydrate metab- olism is at present the subject of discussion, but the data are so contradictory that it would not be advisable to enter into the matter here. The ready response of the thyroid by hyperplasia or involution to changes in the nutritive conditions is one of its most striking characteristics, and further illustrates the significant role which it plays in the chemical activities of the body. The thyroid swells and shrinks almost as easily and under almost as great a variety of conditions as the spleen. One of the most interesting of the physiological changes is the hyperplasia of the gland which is a normal accompaniment of pregnancy. A pathological change of THYROIDS AND PARATHYROIDS 635 great -interest, because of the careful manner in which it has been; stiidied, is the endemic goitre (sometimes erroneously tef med ' car- cinoma ') of brook trout kept under artificial conditions in hatcheries. Marine has shown that this depends upon overfeeding with unsuitable food (such as livers of cattle, pigs, or sheep), over- crowding, and insufficiency of water-supply, and that the goitre can be readily cured or prevented by changing the conditions -in these respects. Similar results have been obtained in mammals fed exclusively with meat. Thus, hon cubs at the Zoological Gardens in London on a diet consisting only of raw meat developed rickets and goitre, as did puppies fed with meat, lungs, hver, or heart, and nothing else; whereas When milk; bread, and bone were added to meat the puppies grew nor- mally (Marine) . A meat .diet caused hyperplasia of the thyroid in rats (Chalmers Watson) . The relation of the disease known as exophthal- mic goitre to the thyroid has been much debated'. The best evidence is against the hypothesis that the Symp- toms are due to increased activity of the thyroid func- tion (so-called hyperthyroid- ism). All attempts to pro- duce anything resembling the pathological condition by the administration of large amounts of thyroid or of thyroid products have failed. Nor has it ever been shown that the changes in the gland are the primary cause of the sjmdrome. Indeed, no specific anatomical or chemical changes have as yet 'been demonstrated in the thjn-oid in this condition. The thyroid gland of exophthalmic goitre has the same action on animals and on patients suffering from exophthalmic goitre as any other thyroid gland with like iodine content (Marine). The relations of iodine to the gland itself, and the modifications in its structure and function determined by the giving or withholding of iodine, recently studied by Marine, are of great interest. In all animals, so far as examined, the normal thyroid contain^ iodine. The amount is variable, but the minimum percentage of iodine necessary, if the normal histological structure is to be maintained, is quite constant for a givenspedes. So also the highest percentage Fig, 203. — •Microphotograph of Active Thyroid Hyperplasia from a Case of Exophthalmic Goitre (Marine). The characteristic changes in the hyperplastic gland — ^the infoldings and plications of .the alveolar epithelium, the great reduction in the colloid, and the increase'in the stroma — are shown. 636 INTERNAL SECRETION Fig. 204. — Microphotograph of a Colloid Gland (Goitre), (Marine). The effect of ad- ministration of iodine is shown in the return towards the normal structure from a pre- ceding active hyperplasia, such as is shown in Fig. Z03. of iodine associated with any degree of active hyperplasia (develop- ing goitre) is always belOw the normal minimum, as shown by Marine in the dog, sheep, man, and other mammals. As active hyperplasia of the thyroid (goitre) (Fig. 203) develops, the iodine content of the gland, both relative and absolute, decreases, until in extreme degrees of the condition there may be no demonstrable iodine present at all. Since the iodine is contained in the colloid as an iodine-protein compound, the generahzation may be made that in the thyroid the iodine varies directly with the amount of coUoid, and inversely with the degree of hyperplasia. The administra- tion of any iodine-containing substance to animals with actively hyperplastic thjToids (goitres) quickly (in two to three weeks in dogs) induces a histological change, the end stage of which is the so-called colloid goitre (Fig. 204). This is a reversion to the normal histological struc- ture (Fig. 205), so far as this is possible in a gland which has once undergone hyper- plasia. The physiological influence of iodine on the thyroid may be summed up as follows : Iodine is abso- lutely essential for the normal activity of the gland. It prevents spontaneous hyper- plasia (goitre), and also the compensatory hyperplasia which follows partial re- moval of the thyroid. It exercises a curative effect on active hyperplasias, The physiological and therapeu- tical activity of thyroid substance vaiies directly with the amount of iodine in it in organic combination (thyroiodin). Fig. 205. — Microphotograph of Normal Himian Thyroid (Marine). ADRENALS 637 As in the case of other glands forming an internal secretion, it has been debated whether the function of the thyroid is to destroy toxic bodies or to form substances indispensable or advantageous to the organism. While the precise role played by tlie organ in the economy remains obscure, it is evident that in most animals and in man its secretion is of great importance, whether it be solely the quasi-external secretion of ' colloid,' containing the th5rroiodin, that collects in its alveoli and slowly passes out of them by the lym- phatics, or perhaps, in addition, some other substance, which, likg the glycogen of the liver, never finds its way into the lumen of the gland-tubes at all.' It may also be admitted that, by aiding in the maintenance of the normal level of general nutrition, particularly that of the central nervous system, the ability of the organism to cope with toxic substances introduced from the outside or manu- factured in the body is favoured. There is, however, no evidence . that an actual destruction or neutralization of toxic substances occurs in the gland itself. It is probable that the secretion of the thyroid is influenced by nerves. Section of the superior and inferior thyroid nerves going to the gland is followed by degenerative changes in it. It has been stated that stimulation in the dog of the nerves entering one thyroid lobe on the bloodvessels, or of the cephalic end of the vago-sympa- thetic nerve below the superior cervical ganglion, causes a diminu- tion in the iodine content of that lobe as compared with the other (Fawcett and Beebe). This result has been interpreted as due to the excitation of fibres which accelerate the passage of the active substance out of the gland. It has long been known that vaso- motor fibres for the dog's thyroid run up in the cervical sympathetic to the superior cervical ganglion, and thence to the lobe of the same side. These were first discovered by the effect produced by their stimulation on the th5n:oid circulation time (p. 135). Further evidence of the existence of secretory fibres has been brought for- ward by Asher and Flack. They compared the excitabihty of the depressor nerve, and also the effect on the blood-pressure of the intravenous injection of adrenalin, before and during stimulation of the th3n:oid nerves. They conclude that, when all the other conditions remain unchanged, both the effect of excitation of the depressor and the effect of adrenalin are greater during stimulation of the thyroid nerves than shortly before it without such stimula- tion. The difference is really connected with the internal secretion of the thyroid, since it is not obtained if the thyroids are previously extirpated, and injection of thyroid extracts influences the result exactly in the same way as stimulation of the thyroid nerves. Adrenal Bodies. — It had been observed by Addison that the malady which now bears his name, and in which certain vascular changes, with muscular weakness, anaemia, and pigmentation or 638 INTERNAL SECRETION \ ■bronzing ' of the skin, are prominent symptoms, was associated with disease, usually tuberculous, of the adrenal bodies, commonly called in human anatomy the ' suprarenal capsules.' This clinical result was soon supplemented by the discovery that extirpation of the adrenals in animals is incompatible with life (Brown-Sequard), Our knowledge of the functions of these hitherto enigmatic organs was extended by the experiments of Oliver and Schafer, who in- vestigated the action of extracts of the adrenals (of calf, sheep, dog, guinea-pig, and man) when injected into the veins of animals. The arteries are. greatly contracted, and this mainly through direct action on the v^so-mbtor nerve-endings or some structure inter- mediate between them and the smooth muscle of the vessels, but partly through the vaso-motor centre. The blood-pressure rises rapidly, although the heart may be inhibited through the vagus centre. The heart is at the same time directly stimulated, so that, .although it beats slowly, the beats are stronger than before. When the vagi are cut the action of the heart is markedly augmented, and the arterial pressure rises enormously (it may be to four or five times its original .amount). Stimulation of the depressor is of no avail in combating this increase of blood-pressure. The generaliza- tion may be made that suprarenal extract or adrenalin — also called '. epinephrin ' and ' suprarenin '^ts active principle, acts upon all plain muscle and . gland-cells that are supplied. with syrhpathetic nerve-fibres, and the result of the action, whether augmentation >or inhibition, is the same as would be produced by stimulation of the sympathetic fibres going to the muscle or gland in question. Yet it is not through excitation of these fibres that the adrenalin acts, for its effect is even more pronounced when the nerve-fibres have been caused to degenerate, in the case of the pupillo- dilator fibres, e.g., by excision of the superior cervical ganglion. Nor is the effect a direct one on the muscular fibres. For smooth muscle, which is not, arid never has been, in functional union with sympathetic nerve- fibres is indifferent to adrenalin (ElUott). It seems, then, to act on some structure intermediate between the nerve and the muscle, but so related, to the latter that it continues to live so long as it is in connection with the muscle-fibre. Instead of a definite histological structure, the seat of the action may be a special ' re- ceptive '■ substance at the myoneural junction. Thus adrenahn causes marked diminution of tone in the small intestine, with dis- appearance of the peristalsis and pendulum movements. The same effect is produced on an isolated loop of intestine immersed in Locke's solution, and the action is therefore local. The drug is effective in a dilution of i : 1,000,600, or even in much greater dilution. A similar effect has been observed on the stomach. The vessels of the conjunctiva are constricted by. local action when an extract of the capsules is' dropped into the eye, a fact which has proved of ADRENALS 639 value in ophthalmological practice. Inhibition of the contraction of the stomach, intestine, urinary bladder, and gall-bladder; con- traction of the uterus, vas deferens, and seminal vesicles; dilatation of the pupil and retraction of the nictitating membrane ; stimulation of the salivary and lachrymal secretions, are among its actions (Langley). The curve of contraction of the skeletal muscles is lengthened as in veratrine poisoning (p. 729), though to a less extent, Meltzer has shown that the dilatation of the pupil caused by the intravenous injection of adrenalin is distinct, though fleeting, in cats, less marked in rabbits. Subcutaneous injection has no effect. Instillation of the drug into the conjunctival sac is without effect on the pupil inthe normal rabbit's eye, but causes dilatation if the superior cervical ganglion has been removed. The influence of adrenalin in increasing the sugar content of the blood, and thus causing glycosuria, has been previously discussed (p. 541). A new and interesting action has recently been added by Cannon to the already long list of the effects of adrenalin, by the discovery that small doses (o-ooi milligramme per kilo of body- weight) and larger doses injected subcutaneously into cats shorten the coagulation time of the blood to one-half or one-third of its previous duration, probably by stimulating the liver to greater activity in discharging sonie substance or substances concerned in clotting. • ' Methods for the detection and the assay of adrenalin in the small quantities in whichit can only be supposed to be present in physio- logical liquids have been based upon certain of these actions. Such; for example, is the extraordinary power of this active principle that a dose of one-millionth of a gramme per kilo of bddy-weight is sufficient to cause a distinct effect upon the heart and bloodvessels (a rise of pressure of 14 milhmetres Hg) when it is injected into the veins of a mammal. The reaction is rendered more constant, although less delicate, when the brain is previously destroyed and the animal used as a spinal preparation. In pithed cats the assay can be accurately performed to o-oi milligramme. Another delicate, and for certain purposes a convenient, reaction for the detection and the physiological assay of adrenalin is the perfusi.on test on the legs of frogs already alluded to (p. 46). The dilatation of the pupil in the excised^ eyeball of the frog, the contraction of stretched airtery rings (p. 66), the increase in the tone of isolated segments of the uterus of rabbits or guinea-pigs, and the diminution in the tone of isolated segments of intestine (Practical Exercises, p. 447), have also been employed as physiological tests. A dilute solution of adrenalin chloride is used in medicine as a styptic, and for reducing congestion in accessible parts. The intense local anaemia which it causes when given subcutaneously or by the mouth is one reason, perhaps the most itoportfant. f&r the 640 INTERNAL SECRETION slow absorption on which depends the absence of its general effects, including that on the blood-pressure, when it is administered in thts way. Function of Epinephrin (or Adrenalin). — ^The striking effects produced by adrenalin have naturally led to the assurhption that its function in the body must be important. It has been con- clusively proved that under certain conditions it is given off to the blood, but only in such quantities as, when diluted by the general mass of the blood, lie far below the concentration necessary for detec- tion by any of the biological methods mentioned above. In fact, no proof has ever been given that in blood withdrawn from an artery, either in health or disease, adrenalin exists at all. When the adrenal gland of a dog is directly massaged by the fingers, the blood coming from the adrenal vein has been shown to contain a small but detectable amount of adrenalin. The same is true of the blood coming from the adrenal during stimulation of the splanchnic nerves on the corresponding side (Fig. 206). The existence of secretory fibres for the adrenal glands in the splanchnic nerves was first indicated by the experiments of Dreyer, who found that the amount of active substance in the blood of the suprarenal vein, as tested by its physiological effect when injected into an animal, was increased by stimulation of those nerves. Under the influence of strong emotions, painful stimulation of sensory nerves, and other conditions, the quantity of adrenalin which can be extracted from the adrenals is markedly diminished, but not if the splanchnic fibres have been previously cut. -That adrenalin is actually given off to the blood during stimula- tion of a nervous mechanism of which the efferent fibres run in the splanchnics is further indicated by facts like the following: Similar cjianges in the clotting time of blood are produced by splanchnic stitiaulatiori as by injection of adrenalin, and these changes do not occur if the adrenal on the side of the stimulated nerve has been previously excised. Excitation of afferent nerves under light anaesthesia and emotional excitement also shorten the coagulation time, but this effect is not obtained after section of the splanchnic nerves. Temporary improvement in the response to stimulation of a faHgued muscle, still in connection with the circulation, is observed on excitation of the splanchnic nerve if the corresponding adrenal is intact, and a similar reaction is obtained on injection of epinephrin. The existence of a reflex nervous mechanism through which the gland can be stimulated to secrete adrenalin into the blood can therefore be considered as definitely established. But the function of the adrenalin, once it has entered the circulation, is involved in doubt. The common view is that it exerts an important physiological action upon the sympathetic system, contributing especially to the ADRENALS 54 [ « *? O in cii i*" O «*" rt 3 S ^ -a '=^ • * I S .3 P ^ S en >^ "■§ e g " g:^ rt .S ^ 5 rt 9 " OS 3 ja a '-■'ti a Scum! x)^ 5) ■ ■§5 " " § .a IS •« ° ci i^<. S " ffl rt ■ w rt 3 1 « &-a= « ?i ffS "I t3 " 3 2 S n tJ (/) aqn CO , '■ '^ aj tu l*^ -d .S B' 73 ^ ^Tj3.a5" 3 15 HH O '^ .9 ^ o d o t:} ft to o a ftS O) J B ■5 H .3 -i^ S .3 o > pj which expresses it, is called Ohm's Law. It states that the current varies directly as the electromotive force, and inversely as the resist- ance. For the measurement of electrical quantities a system of units is necessary. The common unit of resistance is the ohm, of current the ampire, of electromotive force the volt. The electromotive force of a Daniell's cell is about a volt. An electromotive force of a volt, acting through a resistance of an ohm, yields a current of one ampere. But the current produced by a Daniell's cell, with its poles connected by a wire of i ohm resistance, would be less than an ampere, because the internal resistance of the cell itself — ^that is, the resistance of the .liquids between the zinc and the copper — must be added to the external resistance in order to get the total resistance, which is the quantity represented by R in Ohm's Law. Measurement of Resistance. — To find the resistance of a conductor, we compare it with known resistances, as a grocer finds the weight of a packet of tea by comparing it with known weights. The Wheatstone's bridge method of measuring resistance depends on the fact that if four resistances, AB, AD, BC, CD, are connected, as in Fig. 220, with each PRELIMINARY DATA 699 other, and with a galvanometer, G, and a battery, F, no current will flow through the galvanometer when xts= t^- In making the measurement, a resistance box, containing a large number of coils of wire of different resistances, is used (Fig. 221). The resistances corresponding to AB and AD may be made equal, or may stand to each other in a ratio of i : 10, i : 100, etc. Then, the un- known resistance being CD, BC is adjusted by taking plugs out of the box Fig. 220. — Wheat- stone's Bridge. Fig. 221. — -Diagram of Resistance^Box. till, on closing the current, there is either no deflection, or the deflection is as small as it is possible to make it with the given arrangement.^ Galvanometers. — A galvanometer is an instrument used to detect a current, to determine its direction, and to measure its intensity. Since, by Ohm's law, electromotive force, resistance, and current strength are connected together, any one of them may be measured by the gal- vanometer. A galvanometer of the kind ordinarily used in physiology- consists essentially of a small magnet suspended in the axis of a coil Fig. 222. — Scheme of Wiedemann's Galvanometer (with Telescope Reading). T, telescope; S, scale; M, mirror; m, ring magnet suspended between the two gal- vanometer coils G, the distance of which from m can be varied; F, fibre suspend- ing mirror and magnet. of wire, and free to rotate under the influence of a current passing through the coil. The most sensitive instruments possess a small mirror, to which the magnet is rigidly attached. A ray of light is allowed to fall on the mirror, from which it is reflected on to a scale; and the rotation of the mirror is magnified and measured by the ex- cursion of the spot of light on the scale. In the Thomson galvanometers the magnet is very light — e.g., a strip or two of magnetized watch spring. The magnet is ' damped ' — ^that is, |its tendency, when once displaced, to go on oscillating about its new position of equilibrium is 700 THE PHYSIOLOGY OF THE CONTRACTILE TISSUES overcome by enclosing it. in a narrow air space. In the Wiedemann instrument the magnet is heavier (Fig. 222). It swings in a chamber with copper walls. Every movement of the magnet ' induces ' cur- rents in the copper; these tend to oppose the movement, and so ' damping ' is obtained. It is usual to read the deflections of the Wiedemann galvanometer by means of a telescope. An inverted scale is placed over the telescope at a distance of, say, a metre from the mirror ; an upright image of the scale is formed in the telescope after reflection from the mirror, and with every movement of the latter the scale divisions appear to move correspondingly. The method of reading by a telescope can be applied to any mirror galvanometer, and is often extremely conveiiieht iii physiological work. Sometimes a small scale is fastened on the tairror itself, and observed di- rectly through a low- power microscope. Fig. 223. — Astatic Pair of Magnets. SN and NS are the magnets, fixed to the vertical piece P. M is a mir- ror. The arrow-heads show the direction of a current which deflects both mag- nets in the same direction. Fig. 224. — Diagram of String Galvanometer. The string or fibre CC is stretched between the poles of a powerful electromagnet. When a current passes down the string it is deflected in the direction of the large arrow a — i.e., at right angles to the magnetic field NS. When the current is reversed, the string moves in the opposite direction. The movements of the string can be observed by a microscope. A (objective E), passing through a hole bored through the centre of the magnet poles. For obtaining records a source of light is placed at B and concen- trated on the fibre by a condenser, F, and the move- ments of the shadow are recorded by photography. In many galvanometers the magnets attached to the mirror form an ' astatic ' pair (Fig. 223). Two small magnets of nearly equal strength are connected to a light slip of horn or an aluminium wire, with their poles in opposite directions. The earth's magnetism affects them op- positely, so that the resultant action is nearly zero. Either one or both magnets may be surrounded by the galvanometer coils. If both are so surrounded, each must be within a separate coil, and the current must pass in opposite directions in the two coils, otherwise they would neutralize each other. In the d' Arsonval galvanometer the current passes through a small coil of fine wire suspended in the field of a strong magnet. When the current passes the coil is deflected, carrying with it a small mirror attached to the suspending filament. A great advantage of this galvano- meter in many situations is that it is unaffected by neighbouring currents . PRELlMtNARV DATA 701 The string galvanometer of Einthoven has peculiar merits for certain physiological purposes. It consists of a silvered quartz- or glass-fibre stretched in a very strong magnetic field. When traversed by a current the fibre is deflected, and by means of a beam of light the deflection is greiitly magnified (Fig. 224). A rheocord is an instrument by means of which a current may be divided, and a definite portion of it sent through a tissue (Fig. 225). A compensator is simply a rheo- cord from which a branch of a current is led off, to balance or ' compen- sate ' any electrical difference in a tissue, like that which gives rise to the current of rest of a muscle, for example (Fig. 226). An electrometer is an instrument for measuring electromotive force — Fig. 225. — Diagram of Rheocord (after Du Bois-Reymond's Model). Fig. 226. — Compensator. Description of Fig., 225 : 1 to VII are pieces of brass connected with the wires a to / in such a way that, by taking out any of the brass plugs i to 5, a greater or less resistance may be interposed, between the binding-screws A and B. The two wires a are connected by a slider s, filled with mercury or otherwise making contact between the wires. The current from the battery B divides at A and B, part of it passing through the rheocord, part through N, the nerve, muscle, or other conductor which forms the alternative circuit. When a sufficient resistance R is interposed in the chief circuit to make the total strength of the current independent of changes in the resistance of the rheocord, the strength of the current passing through N will vary inversely as the resistance of the rheocord. When all the plugs are in, and the slider close up to A, there is practically no resistance in the rheocord, and all the current passes across the brass pieces and plugs to B, and thence back to the battery. As s is moved father away from A, the resistance of the rheocord is increased more and more, and the intensity of the current passing through N becomes greater and greater. The scale S shows the length of wire interposed for any position of s, and this gives a rough measure of the fraction of the current passing through N. When plug r or 2 is taken out, a resistance equal to that of the two wires a is interposed; plug 3, twice that of a ; plug 4, five times; plug 5, ten times. Description of Fig. 226; W is a wire stretched alongside a scale S. A battery B is connected to the binding-screws at the ends of the wire. A pair of unpolarizable electrodes are connected, one with a slider moving on a wire, the other through a galvanometer with one of the terminal binding-screws. In the figure a nerve is shown on the electrodes, one of which is in contact with an uninjured portion, the other with an injured part. The slider is moved until the twig of the compensating current just balances the demarcation current of the nerve and the galvanometer Shows no deflection. 702 THE PHYSIOLOGY OF THE CONTRACTILE TISSUES that is, differences of electric potential. Lippmann's capillary elec- trometer has been much employed in physiology. A convenient form of it is shown in Fig. 227. A simple form, suitable for students work- ing in a class where a considerable number of copies of the instrument is needed, can be conveniently made as follows : A glass tube is drawn out to a capillary at one end and filled with mercury. The tube is inserted into a small glass bottle,* and fastened in its neck by a cork or a plug of sealing-wax which does not quite fill the opening, so that the interior of the bottle is still in communication with the external air. The upper end of the tube is connected by a short piece of rubber tubing with a glass T-tube as in Fig. 228. The bottle is partially filled with 5 to 10 per cent, sulphuric acid, under which the capUlary dips. By means of a small reservoir made from a piece of glass tubing filled with mercury, and connected with the stem of the T-tube, a little mercury is forced through the capillary so as to expel the air in it. When the Fig. 227. — Capillary Electro- meter (after Frey), as arranged for mounting on the Microscope Stage. The electrometer consists (i) of a small table carrying a parallel-sided glass vessel contain- ing mercury and sulphuric acid. (2) The capillary tube, which can be moved in two directions at right angles to each other, and so adjusted in the field of the micro- scope. (3) A pressure-vessel, and a manometer connected with it for measuring the pressure. (4) Two binding-screws connected by wires to the mercury in the capillary tube and in the parallel, sided vessel. The binding-screws can be short-circuited by closing the friction-key shown at the right side of the figure, thus pre- venting any difference of elec- tromotive force between two points connected with the screws from affecting the electrometer. pressure is lowered again, sulphuric acid is drawn up, and now lies in the capillary in contact with the meniscus of the mercury. A platinum wire fused through the tube, or simply inserted through its upper end, dips into the mercury. Another, passing through the cork, or, better, fused through the bottom of the bottle, makes contact with the sul- phuric acid through some mercury. The bottle is fastened on the stage of a microscope, the capillary brought into focus, and the meniscus adjusted by raising or lowering the reservoir. When the platinum wires are connected with points at different potential, a current begins * A parallel-sided bottle is best, as it gives the clearest image of the menis- cus. But it is easiest to make a cylindrical bottle from a piece of wide glass tubing, and to insert a platinum wire into it before closing it at the bottom in the blow-pipe flame. The tube can then be firmly fastened with sealing-wax in a depression in a piece of wood, the wire being brought out through a hole in the wood. Once the instrument is arranged, there is little chance of the capillary getting broken, Emd there is very little evaporation of the acid. Preliminary data 703 to pass through the instrument, and the meniscus of the mercury in the capillary tube, where the current density is the greatest, becomes polarized by the ions separated from the sulphuric acid at the surface of contact between the acid and the mercury, so that the meniscus is no longer in equilibrium in the tube. The surface tension (p. 423) is diminished when the direction of the current is from mercury to acid (mercury at a higher potential than acid), and is no longer able to coun- terbalance the hydrostatic pressure of the mercury. The meniscus there- fore moves down in the tube. With the opposite direction of current (mercury at a lower potential than acid) the surface tension is increased, and the meniscus moves up. The polarization develops itself almost in- stantaneously, and thus an electromotiveJJ force is at once established in the opposite direction to that between " r a the points connected with the electro- ^^y' meter, and equal to it so long as the '""^ external electromotive force is not sufficiently great to cause continuous electrolysis of the acid — ^that is, so long Fig. 228. — A Simple Capillary Electro- meter. B, bottle containing sulphuric acid; Hg,'mercury ; £, £', platinum wires. . E dips into the mercury in the M'ertical tube, and E' is fused 'through the bottom of B, so as to make contact with the mercury in B, the other end of it passing out through a small" hole in the Kooden platform F, on which B rests. F is fastened to the stage of the microscope, S, by a pin, G, passing through" one of the clip-holes, and to the wooden upright, D, by the pin, H. D fits tightly over the microscope stage, but can be moved laterally a little so as to bring the capil- lary into the middle of the field. /, stem of glass T-tube passing through a hole in D. L, rubber tube connecting the capil- lary point with the vertical portion of the T-tube. v4 is a reservoir containing mercury connected by the rubber tube iW to /. A can be raised or lowered by sliding it in the clips K. C, magnified portibn of the capillary tube showing the meniscus, as it is below about 2 volts. The external current is therefore at once compensated, and after the first moment no current passes through the instrument, which is accordingly not a measurer of current, but of electromotive force. Induced Currents. — When a coil of wire in which a current is flowing is brought up suddenly to another coil, a momentary current is developed in the stationary coil in the opposite direction to that in the moving coil. Similarly, if instead of one of the coils being moved a current is sent through it, while the other coil remains at rest in its neighbour- hood, a transient oppositely-directed current is set up in the latter. When the current in the first coil is broken, a current in the same direction is induced in the other coil. Du Bois-Reymond's Sledge Inductorium (Fig. 229). — This consists of two coils, the primarj'- and the secondary, the former having a com- 704 THE PHYSIOLOGY OF THE CONTRACTILE TISSUES paratively small number of turns of fairly thick copper wire, the latter a large number of turns of thin wire. The object of this is that the resistance of the primary, which is connected with one or more voltaic cells, may not cut down the current too much; while the currents induced in the secondary, having a high electromotive force, can readily pass through a high resistance, and are directly proportional in intensity to the number of turns of the wire. By means of various binding-screws and the electro-magnetic inter- rupter or Neef's hammer, shown in the figure and explained below it, the current can be made once in the primary or broken once, or a con- stant alternation of make and break can be kept up. We can thus get a single make or break shock in the secondary, or a series of shocks, sometimes called an interrupted or faradic current. Such a series of stimuli can also be got by making and breaking a voltaic current at any given rate. A ' self -induced ' current can also be obtained from a single coil; for instance, from the primary coil alone of the induction apparatus. The Fig. 229. — Du Bois-Reymond's Inductorium. B, primary, B'. secondary, coil; H, guides in whicli B' slides, witli scale; D, electro-magnet; E, vibrating spring; i, wire connecting wire of D to end of primary; v, screw with platinum point, connected with other end of primary ; A, A', binding-screws, to which are attached the wires from battery. A' is connected with the wire of the electro-magnet D, and through it and i with the primary. reason of this is, that when a current begins to flow through any turn of a coil of wire it induces in all the other turns a current in the opposite direction, and, when it ceases to flow, a current in the same direction as itself. The former current, ' the make extra shock,' being in the opposite direction to the inducing current, is retarded in its develop- ment, and reaches its maximum more slowly than ' the break extra shock.' But, as we shall see, the suddenness with which an electrical change is brought about is one of the 'post important factors in elec- trical stimulation, and therefore the bi'eak extra shock is a much more powerful stimulus than the make. Owing to these self -induced currents, the stimulating power of a voltaic stream may be much increased by putting into the circuit a coil of wire of not too great resistance. The self-induction of the primary also affects the stimulating power of the currents induced in the secondary; the shock induced in the secondary by break of the primary current is a stronger stimulus than that caused at make of the primary. The reason is that with a given distance of primary and secondary, and a given intensity of the voltaic PRELIM I NA RY DATA 705 current in the primary, the abruptness with which the induced current in the secondary is developed depends upon the rapidity with which the primary current reaches its maximum at closing, or its minimum (zero) at opening. Now, the make extra current retards the development of the primary current, while in the opened circuit of the primar^ coil the current intensity falls at once to zero. The inequality between the make and break shocks of the secondary coil can be greatly reduced by means of Helmholtz's wire. Connect one P°!? A°,\.^ battery with v (Fig. 229), and the other with A'. Join A and A' by a short, thick wire. With this arrangement the primary cir- cuit is never opened, but the current is alternately allowed to flow tiirough the_ primary, and short-circuited when the spring touches v. The make now corresponds to the sudden increase of intensity of the current m the primary when the short-circuit is removed, and the break to its sudden decrease when the short-circuit is established. In both cases self-induced currents are developed, and therefore both shocks are weakened. But the opening stimulus is now slightly the weaker of the two. because the opening extra shock has to pass through a smaller resistance (the short-circuit) than the closing extra shock (which passes by the battery), and therefore opposes the decline of current intensity on short-circuiting more than the closing shock opposes the increase of current intensity on long- circuiting through the primary. By means of wires con- nected with the terminals of the secondary coil, and leading to electrodes, a nerve or muscle may pig. 230. — Unpolarizable Electrodes. A, hook- be stimulated. It is usual shaped; B, U-tubes; C, straight; D, clay in to connect the wires to contact with tissue; S, saturated zinc sulphate a short-circuiting key solution; Z, amalgamated zinc wire. (Fig. 232), by opening which the induced current is thrown into the tissue to be stimu- lated. For some purposes the electrodes may be of platinum; but all metals in contact with moist tissues become polarized when currents pass through them — ^that is, have decomposition products of the electrolysis of the tissues deposited on them. And as any slight chemical difference, or even perhaps a difference of physical state, be- tween the two electrodes will cause them and the tissues to form a battery evolving a continuous current, it is often desirable to use un- polarizable electrodes. Unpolarizable Electrodes. — Some convenient forms of these are represented in Fig. 230. A piece of amalgamated zinc wire dips into saturated zinc sulphate solution contained in the upper part of a glass tube. The lower end of the tube may be straight, but drawn out so as to terminate in a not very large opening, or it may be bent into a hook, in the bend of which a hole is made. Before the tube is filled with the zinc sulphate solution, the lower part of it is plugged with china clay made up with physiological salt solution. The clay just projects through the opening, and thus comes in contact with the tissue. When these electrodes are properly set up, there is very little polarization for several hours, but for long experiments, U-shaped tubes, filled with saturated zinc sulphate solution, are better. The amalgamated zinc dips into one limb, and a small glass tube filled with clay, on which the tissue is laid, into the other. 45 7o6 THE PHYSIOLOGY OF THE CONTRACTILE TISSUES Pohl's Commutator or Reverser (Fig. 231) consists of a block of paraf- fin or wdod with six mercury cups, each in connection with a binding- screw (not shown in the figure) . Cups i and 6 and 2 and 5 are connected by copper wires, which cross each other without touching. The bridge consists of a glass or vulcanite cross-piece a, to which are attached two wires bent into semicircles, each connected with a straight wire dip- ping into the cups 3 and 4 respectively. With the bridge in the posi- tion shown in the figure, a current coming in at 4 would pass out by the wire connected with i, and back again by that connected with 2, in the direction shown by the arrows. When the bridge is rocked' to the other side so that the bent wires dip into 5 and 6, the direction of the current is reversed. The cross-wires may be taken out altogether, and the commutator used to send a current at will through either of two circuits, one connected with i and 2, and the other with 5 and 6. Du Bois -Reymond's Short- circuiting Key.— A cheap and convenient form is shown in Fig. 232. Time - Markers — Electric Sig- nal. — It is of importance to know the time relations of many physiological phenomena which are graphically recorded ; for Fig. 231. — Pohl's Commutator. Fig. 232. — Short-Circuiting Key. example, the contraction of a skeletal muscle or the beat of a heart. For this purpose a tracing showing the speed of the travelling sur- face in a given time is often taken simultaneously with the record of the movement under investigation. For a slowly-moving surface it is sufficient to mark intervals of one or two seconds, and this is very readily done by connecting an electro-magnetic marker (such as the electric signal of Deprez) with a circuit which is closed and broken by the seconds pendulum of an ordinary clock (Fig. 233) or a metronome (Fig. 88, p. 193). Special clocks have also been constructed which permit pf the time intervals being varied. For shorter intervals a tuning-fork is used, which makes and breaks a circuit including an electromagnetic marker, or writes on the drum directly by means of a writing-point attached to one of the prongs. Amoeboid movement (p. 16) is the most primitive, the least elaborated form of contraction. The maximum velocity of the movement has been reckoned at o-oo8 millimetre a second. Stimu- lation with the constant current or induction shocks causes the whole of the pseudopodia to be drawn in. This illustrates a CILIA 707 universal property of protoplasm, excitability, or the power of re- sponding to certain influences, or stimuli, by manifestations of the peculiar kind which we distinguish as vital or physiological. Many unicellular organisms and the chief varieties of the white blood- corpuscles possess the power of amoeboid movement ; and we have already dwelt upon some of the important functions fulfilled by such movement in the higher animals and in man. A great dis- tinction between this kind of contraction and that of a muscular fibre is that it takes place in any direction. Cilia. — Ciha possess a higher and more speciahzed grade of contractility. They are very widely distributed in the animal kingdom; and analogous structures are also found in many low plants, such as the motile bacteria. In the human subject ciUated epithelium usually consists of several layers of cells, the most superficial of which are pear-shaped, the broad end being next the sur- face, and covered with extremely fine processes, or cilia, about 8 /j. in length, which are planted on a clear band. It lines the respi- ratory passages, the middle ear and Eustachian tube, the Fallopian tubes, the uterus above the middle of the cervix, the epididymis, where the cilia are extremely long, and the central cavity of the brain and spinal cord. Ciliary motion can be readily studied by placing a scraping from the palate of a frog or a small portion of the gill of a fresh-water mussel under the microscope in a drop of physiological salt solution. The motion of the cilia is at first so rapid that it is impossible to make out much, except that a stream of liquid, recognized by the solid particles in it, is seen to be driven by them in a constant direc- tion along the ciliated edge. When the motion has become less quick, which it soon does if the tissue is deprived of oxygen, it is seen to consist in a swift bending of the cilia in the direction of the stream, followed by a slower recoil to the original position, which is not at right angles to the. surface, but sloping streamwards. All the cilia on a tract of cells do not move at the same time; the motion spreads from cell to cell in a regular wave. The energy of ciliary motion may be considerable, although far inferior to that of mus- cular contraction. The work which cilia are capable of performing Fig. 233. — Time-Marker. Arrange- ment for marking 2 .second inter- vals. D, seconds peudulmn, with platinum point E soldered on; A, mercury trough, into which E dips at end of its swing ; B , D aniell cell ; C, electro - magnet which draws, down writing - lever F when the current is closed by E dipping into A; G, spring (or piece of india- rubber), which raises F as soon as current is broken. 7o8 THE PHYSIOLOGY OF THE CONTRACTILE TISSUES can be calculated by removing the membrane, fixing it on a plate of glass, cilia outwards, putting weights on the glass plate, and allowing the cilia, like an immense number of feet, to carry it up an inclined plane. Bowditch found in this way that the cilia on a square centimetre of mucous membrane did nearly 7 gramme- millimetres of work per minute (equal to the raising of 7 grammes to a height of a millimetre). Since the cilia in the respiratory tract all lash upwards, they must play an important part in carr57ing up foreign particles taken in with the air, and the mucus in which they are entangled, as well as patho- logical products. Engelmann found that the energy of cihary motion in- creases as the temperature is raised up to about 40° C, after which it Fig. 234.— Ciliated Cell (M. Heidenhain). From a ' liver duct ' of the garden snail X 2,500. Fig. 235. — Ciliated Cell (Schneider). From a flatworm (Planocera folium). I, space between two adjoining ciliated cells; 2, basal bodies; 4, inner granule ; 5, ' cilia roots ' ; 6, boundary layer. diminishes quickly. Over-heating causes cilia to come to rest, but if the temperature has not been too high, and has not acted too long, they recover on cooling, thus exhibiting the phenomena of heat standstill which we have already studied in the heart. It is not well understood in what way the contraction of the cilia depends upon their connection with the body of the ciliated cell. Very few cases occur in which cilia have the power of independent motion when severed from, the sell-body. It has been observed in certain low forms of animals that cilia which have been broken off from the cell are still able to contract when a small portion of the substance of the PHYSICAL PROPERTIES OF MUSCLE 709 cell-body at the point wliere the cilium is attached to the cell, the so-called basal piece, or basal body (Fig. 235), has come off along with them. In other forms isolated cilia can contract in the absence of anything corresponding to the basal piece. It cannot, therefore, be said that continuity with the basal piece is absolutely necessary. Nor is it known what significance for the ciliary movements is possessed by the long fibrillse, called the ' roots of the cilia,' which in some animals run down through the cell from the basaf bodies (Figs. 234, 235). In some worms and molluscs ciliated cells are supplied with nerve-fibres, but this has not been demonstrated for the higher animals. Section II. — Physical Properties and Stimulation of Muscle. Since most of our knowledge of the general physiology of muscle has been gained from striped muscle, in what follows we always refer to ordinary skeletal muscle, unless it is otherwise stated. The sartorius and the gastrocnemius are the classical objects for experiments on striated muscle. For smooth muscle the adductor muscle of Anodon, the fresh-water mussel, a'ring cut from the middle portion of the frog's stomach, the rabbit's ureter and uterus, and the cat's bladder, have been most used. Physical Properties of Muscle — Elasticity. — All bodies may have their shape or volume altered by the application of force. Some require a large force, others a small force, to produce a sensible amount of dis- tortion. The elasticity of a body is the property in virtue of which it tends to recover its original form or bulk when these have been altered. Liquids and gases have only elasticity of volume; solids have also elasticity of form. Most solids recover perfectly, or almost perfectly, from a slight deformation. The limits of distortion within which this occurs are called the limits of elasticity, and they vary greatly for different substances. Living muscle has very wide limits of elasticity; it may be deformed — stretched, for example — to a very considerable extent, and yet recover its original length when the stretching force ceases to act. The extensibility of a body is measured by the ratio of the increase of length, produced by unit stretching force per unit of area of the cross-section, to the original length of a uniform rod of the substance. If e is the extensibility, «=T-p. where / is the increase of length, L the original length, 5 the cross-section, and F the stretching force. Suppose we wish to compare the extensibility of two substances. Let A and B be strips or rods of the substances, the length of A being 506 mm., that of B 1,000 mm.; the cross-section of A, loo sq. mm., of B, 200 sq. mm. Let the elongation produced by a weight of i kilo be 10 mm. in each, then the extensibility of A is y i ~ ^ ' ^^"^ ^^** of B is ^°^^°° =2; that is, the substances are equally extensible. 1,000 X I Young's modulus of elasticity, or the coefficient of elasticity, is the quotient of the deforming force acting on unit area of the given body by the deformation produced (within the limits of elasticity). In the above example it is — h-=-, that is, -r-, the reciprocal of the extensi- 7IO THE PHYSIOLOGY OF THE CONTRACTILE TISSUES bility e. For steel the coefficient of elasticity is very large, for muscle small. Or, as we may otherwise express it, living muscle within its limits of elasticity is very extensible ; a small force per unit area of cross-section of a prism of it will produce a comparatively great elonga- tion. The extensibility, however, diminishes continually with the elongation, so that equal increments of stretching force produce always less and less extension. If, for instance, the sartorius or semi-mem- branosus of a frog be connected with a lever writing on a blackened surface, and weights increasing by equal amounts be successively attached to it, the recording surface being allowed to move the same distance after the addition of each weight, a series of vertical lines, representing the amount of each elongation, will be traced. When the lower ends of all the vertical lines are joined, a smooth curve with the concavity upwards is obtained (Fig. 236). This is a property common to living and dead muscle and to other animal structures, such as arteries. Marey's method, in which the weight is continuously in- creased from zero and then continuously decreased to zero again by the flow of mercury into and out of a vessel attached to the muscle, gives directly the curve of extensibility. The elongation of a steel rod or othet inorganic solid is proportional within limits to the extending force per unit of cross-section ; and a curve plotted with the weights for abscissae and the amounts of elongation for ordinates would be a straight line. But this is not a fundamental dis- tinction between animal tissues, and the materials of unorganized nature, as some writers seem to suppose. For when the slow after-elongation which follows the first rapid increase in length in the loaded, Fig. 236.— Curves of Extensi- excised muscle is waited for, the curve of bility. M, of muscle; S, of an extensibility comes out a straight line ordinary inorganic solid. (Wundt), and within limits this is also the case for human muscles in the intact body. And although a steel rod much more quickly reaches its maximum elongation for a given weight when loaded, and its original length when the weight is removed, than does a muscle, time is required in both cases, and the difference is one of degree rather than of kind. When muscle (striated or smooth) is not stretched beyond the limit of physiological relaxation, the amount of stretching is proportional to the weight, and the same is true of all the simple tissues of the body (Haycraft) . Dead muscle is less extensible than living, and its limits of elasticity are much narrower. In the state of contraction the extensibility is increased in excised frog's muscle. When fatigue comes on after many excitations, the after-elongation becomes more pronounced, but the return after unloading is very incomplete. Donders and Van Mans- veldt have found that contraction causes little difference in the muscles of a living man, although fatigue increases the extensibility. The great extensibility and elasticity of muscle must play a con- siderable part in determining the calibre of the vessels, and in lessening the shocks and strains which the heart and the vascular system in general are called upon to bear, and must contribute much to the smoothness with which the movements of the skeleton are carried out, and immensely reduce the risk of injury to the bones as well as to the muscles themselves, the tendons and the other soft tissues. And not only is smoothness gained, but economy also ; for a portion of the STIMULATION OF MUSCLE 711 energy of a sudden contraction, which, if the muscles were less ex- tensible and elastic, might be wasted as heat in the jarring of bone against bone at the joints, is stored up in the stretched muscle and agam given out in its elastic recoil. The skeletal muscles, too, are even at rest kept slightly on the stretch, braced up, as it were, and ready to act at a moment's notice without taking in slack. This is shown by the fact that a transverse wound in a muscle ' gapes,' the fibres being retracted, in virtue of their elasticity, towards the fixed points of origin and insertion. Smooth muscle, as we meet it in the hollow viscera, is highly distensible and elastic, as is suited to organs whose capacity is continually varying within wide limits (Fig. 237). In the further study of muscle it is necessary first of all to consider the means we have of calling forth a contraction — in other words, the various kinds of stimuli. Stimulation of Muscle.— A muscle may be excited or stimulated either directly or through its motor nerve. It is usual to classify stimuli as electrical, mechanical, chemical, or thermal. Electrical stimuU are by far the most commonly employed, and will be dis- cussed in detail. A prick, a cut, or a blow are examples of mechani- Fig. 237. — Extensibility of Smooth Muscle (Grtitzner). The upper group of four cells (I to 4) is from a hollow organ, whose walls are contracted, and its lumen almost abolished ; the under group represents the same fibres when the organ is full. The fibres are longer and somewhat darker. They are also displaced somewhat along each other. cal stimuli. The action of a fairly strong solution of common salt or of a dilute solution of a mineral acid is usually described as chemical stimulation. But in considering the excitation of nerve (P- 757) we shall see that physical changes are often mixed up with so-called chemical stimulation. The contraction caused is not A single brief twitch, as is the case with a not too severe mechanical excitation, but a sustained contraction or a tetanus. Sudden cooling or heating acts as a stimulus for muscle, but thermal stimulation is somewhat uncertain. It is not quite settled whether the contrac- tion which can be obtained from a muscle when it is subjected to brief local heating — to a ' thermic shock,' as some writers prefer to say {e.g., by the momentary glow of a platinum wire below but not touching it) — is an ordinary muscular contraction, or a physical, although transient, contracture analogous to that caused by certain drugs (Waller). Smooth, like striped, muscle is susceptible to electrical, mechanical, thermal, and chemical stimulation. In addition, in certain situations it can be excited by light (photic stimulation), as in the case of the excised iris of fish and amphibia. 712 THE PHYSIOLOGY OF THE CONTRACTILE TISSUES In all artificial stimulation there is an element of sudden or abrupt change, of shock, in other words; but we cannot tell in what the ' natural ' or ' physiological ' stimulus to muscular contraction in the intact body really consists, nor how it differs from artificial stimuU. All we know is that there must be a wide difference, and that our methods of excitation must be very crude and inexact imitations of the natural process. Direct Excitability of Muscle.— The famous controversy on the existence of independent ' muscular irritability ' has long been forgotten, and has no further interest except for the antiquaries of science, if such exist. The direct excitability of muscle in the modern sense is not quite the same as the ' muscular irritability,' the dis- cussion of which occupied Haller and his contemporaries. What the modern physiologists have been called upon to decide is whether muscular fibres can be caused to contract except by an excitation that reaches them through their nerves. In this sense there can exist no doubt that muscle is directly excitable, and some of the proofs are as follows : (i) The ends of the frog's sartorius contain no nerves, yet they respond to direct stimulation. (2) Certain chemical stimuli — ammonia, for instance — excite muscle but not nerve. (3) When the motor nerves of a limb are cut they degenerate, and after a certain time stimulation of the nerve-trunk causes no muscular 'contraction, while the muscles, although atrophied, can be made to contract by direct stimulation. (4) Finally, there is the. cele- brated curara experiment of Claude Bernard, which is described in a somewhat modified form in the Practical Exercises, p. 784. A hgature is tied firmly round one thigh of a frog, omitting the sciatic nerve; then curara is injected, and in a short time the skeletal muscles are paralyzed. That the seat of the paralysis is not the contractile substance of the muscles itself is shown by their vigorous response to direct stimulation. The ' block ' is not in the nerve- trunk, nor above it in the central nervous system, for the ligated leg is often drawn up — ^that is, its muscles are contracted — although the poison has circulated freely in the sacral plexus and the spinal cord. Further, if the nerve of the ligated leg be prepared as high up above the ligature as possible, where the curara must undoubtedly have reached it (just above the ligature the nerve has been isolated and the circulation in it more or less interrupted), stimulation of it will cause contraction of the muscles of the limb ; while excita- tion of the other sciatic is ineffective. It can be also shown, by means of the negative variation or current of action (p. 797), that a nerve-trunk on which curara has acted remains excitable, and capable of conducting the nerve- impulse. The conclusion, therefore, is that the curara paralyzes neither nerve-fibre nor the contractile substance of the muscular STIMULATION OF MUSCLE 713 fibre, but some link between the two. If the assumption be made that the efferent medullated nerve-fibres within the muscle, since they are anatomically similar to those in the nerve-trunk till near their terminations, are similarly affected by curara — and it is a justifiable assumption — the seat of the curara paralysis must either be the nerve-ending or some mechanism, physiological if not anatomical, interposed between the nerve-ending and the con- tractile substance. Now, Langley has shown that the contractions caused by the local application of dilute nicotine solution to points of the skeletal muscles of the frog, both in normal muscles and in muscles whose motor nerves and nerve-endings have degenerated after section of the nerves, are prevented by curara. He there- Fig. 238. — Frog's Motor Nerve-Ending (Wilson). A, B, C, three muscle-fibres. The medullated nerve a loses its medullary sheath and breaks up on B at i. It gives off at 2 a large non-meduUated branch, which also breaks up on B. The nerve- endings send ultraterminal flbrillEe to A, B, and C, some of which were seen to end in small knobs. A separate non-meduUated nerve, », is shown, which forms a small plexus on B, one fibre of which penetrates to a lower plane than the other, and ends by forming a knob under the sarcolemma. fore concludes that, since nicotine produces its effects by a direct action on muscle, and not by an action on nerve-endings or on any special structure (such as the protoplasmic mass or ' sole ' at the nerve-ending in many animals) interposed between the nerve and the muscle, no such special structure existing in the frog (Fig. 238), curara must also act directly on the muscle. But obviously curara does not paralyze the general contractile substance of the muscle, else the curarized muscle would not contract on direct stimulation. Langley accordingly assumes that, in addition to the contractile or ' general ' substance, ' receptive ' substances exist in the fibre, through which the excitation is transferred to the con- tractile substance when the motor nerve is stimulated. He pictures these receptive substances as ' side-chains ' of the contractile mole- cule, in accordance with Ehrlich's theory of immunity (p. 31), 714 THE PHYSIOLOGY OF THE CONTRACTILE TISSUES and distinguishes those in the neighbourhood of the nerve-ending from those present throughout the muscle fibre. Both the slow local tonic contraction and the quick, brief conducted contractions or twitches set up in a muscle fibre by nicotine, but especially the latter, are^much more easily elicited in that part of it which lies under the nerve-ending than elsewhere. Indeed, the position of the nerve-endings in the superficial fibres of a muscle can be ascer- tained by observing the points which respond most readily to nico- tine. Nicotine and curara, etc., are supposed to combine with the receptive substance, which is then in both cases rendered incapable of being affected by nerve impulses. In the case of nicotine an additional action results from the combination with the receptive substance — viz., the change in the contractile substance which leads to contraction. Curara paralyzes the transmission; of the excitation from the motor nerves to smooth muscle — the muscles of the Fig. 239. — ^Tonic Contraction of Muscle during Passage of Constant Current. Two sartorius muscles of frog connected by pelvic attachments. Current from 12 small Daniell cells in series passed through their whole length. Current closed at m, opened at 6. Time trace, two-second intervals. bronchi, for instance— with much greater difficulty than to ordinary skeletal muscle, and the same is true of the inhibitory nerves of the heart. The action of curara gives us the means of stimulating muscle directly; when electrical currents are sent through a non-curarized muscle, there is in general a mixture of direct and indirect stimula- tion, for the nerve-fibres within the muscle are also excited. Induced currents stimulate nerve more readily than muscle. Voltaic currents may excite a muscle whose nerves have degenerated, while induced currents are entirely without effect. For direct stimulation, a curarized frog's sartorius or semi-mem- branosus is generally used on account of their long parallel fibres. For indirect excitation, a muscle-nerve preparation, composed of a frog's gastrocnemius with the sciatic nerve attached to it, is commonly em- ployed, as it is easy to isolate the muscle without hurting its nerve. STIMULATION OF MUSCLE 715 Stimulation by the Voltaic Current. — ^While the current continues to pass through a nerve without any sudden or great change in its in- tensity, there is no stimulation, and the muscle connected with the nerve remains at rest. The same is true of striated muscle when a weak current is passed directly through it. But in muscle the con- stancy of the rule is more and more frequently broken by exceptional results as the current is strengthened, a state of permanent contrac- tion being very apt to show itself during the whole time of flow (Wundt) (Fig. 239). Above a certain intensity of current a greater ot less degree of permanent contraction is invariably produced. This is some- times called the ' closing tetanus.' It is, however, not a true tetanus, but a tonic contraction, which is strongest in the neighbourhood of the kathode, and does not spread far from it. A similar condition, the so-called galvanofonus , is normally seen in human muscles when they or their motor nerves are traversed by a stream of considerable inten- sity. Under certain con- ditions, too — e.g., when a strong current is allowed to flow for a comparatively long time through a muscle — ^the muscle remains (Contrac- ted after the opening of the current (so-called ' opening or Ritter's tet- anus '). Srriooth muscle is excited to contraction even when a voltaic cur- rent is very gradually passed into it and slow- ly increased, and again when it is caused very gradually to disappear. But striped muscle is not stimulated under these conditions. For nerve, and with these qualifications for muscle, too, the law holds that the voltaic current stimulates at make and at break, hut not during its passage. Or, generalizing this a little, since it has been shown that a sudden increase or decrease in the strength of a current already flowing also acts as a stimulus, we may say that the voltaic current stimulates only when its intensity is suddenly and sufficiently increased or diminished, but not while it remains constant.* 1 When a strong current is closed through a muscle there is an im- mediate sharp contraction (initial contraction). The muscle then promptly relaxes, but incompletely. When the current is opened, there is another contraction (Fig. 240). The force of the initial cori- traction, as measured by the resistance necessary to prevent it, is greater than that of the tonic contraction which follows it. A second laVv of great theoretical importance is that of polar stimula- tion. At make the stimulation occurs only at the kathode ; at break only at the anode. This is true both for muscle and nerve, but it is most * This law of Du Bois-Reymond has been questioned by Hoorweg and others. It seems to need modification, but the subject cannot be discussed here. Fig. 240.— Tonic Contraction during and after Flow of Voltaic Current. Curve from frog's gastroc- nemius. At M constant current closed, at B broken. Contracture continues after opening Of current. Time trace, two-second intervals. 7i6 THE PHYSIOLOGY OF THE CONTRACTILE TISSUES directly and simply demonstrated on muscle. A long parallel-fibred curarized muscle is supported about its middle; the two ends, which hang down, are connected with levers writing on a revolving drum, and a current is sent longitudinally through the muscle. It is not dif&cult to see from the tracings that at make the lever attached to the kathodic end moves first, and that the other lever only moves when the contrac- tion started at the kathode has had time to reach it in its progress along the muscle. Similarly, at break the lever connected with the anodic end moves first. The law of polar excitation holds both for striated and for smooth muscle. Not only is there no excitation of unstriped muscle at the anode on closure of the current, but a previ- ously existing contraction disappears. For skeletal muscle the make is stronger than the break contraction. It has not been proved that this is the case for smooth muscle. Section III. — Physical and Mechanical Phenomena of the Muscular Contraction. When a muscle contracts, its two points of attachment, or, if it be isolated, its two ends, come nearer to each other; and in exact proportion to this shortening is the increase in the average cross- section. The contraction is essentially a change of form, not a change of volume. The most delicate observations fail to detect the smallest alteration in bulk (Ewald). Living fibres kept con- tracted by successive stimuli can be examined under the microscope ; or fibres may be ' fixed ' by reagents like osmic acid, and sometimes a very good opportunity of studying the microscopic changes in contraction is given by a group of fibres in which the ' fixing ' reagent has caught a wave of contraction, and, so to speak, pinned it down. It is then seen that the process of contraction in the fibre is a miniature of that in the anatomical muscle. The individual fibres shorten and thicken, and the sum-total of this shortening and thickening is the muscular contraction which we see with the naked eye. The phenomena of the muscular contraction may be classified thus: (i) Optical, (2) Mechanical, (3) Thermal, (4) Chemical, (5) Sonorous, (6) Electrical. (5) will be treated under ' Voluntary Contraction ' ; (6) in Chapter XV. (i) Optical Phenomena — Microscopic Structure of Striped Muscle. — The structure of striped muscle has long been the enigm.a of histology; and the labours of many distinguished men have not sufficed to make it clear. On the contrary, as investigations have multiplied, new theories, new interpretations of what is to be seen, have multiplied in proportion, and a resolute brevity has become the chief duty of a writer on elementary physiology in regard to the whole question. The muscle-fibre, the unit out of which the anatomical muscle is built up, is surrounded by a structureless membrane, the sarcolemma. The length and breadth of a fibre vary greatly in different situations. The maximum length is about 4 cm. ; the breadth may be as much as 70 [J, and as little as 10 ix. When we come to analyze the muscle- fibre and to determine out of what units it is built up, the difficulty begins. The fibre shows alternate dim and clear transverse stripes, and OPTICAL PHENOMENA OF MUSCULAR CONTRACTION 717 can actually be split up into discs by certain reagents. It also shows a longitudinal striation, and can be separated into fibrils. Some have supposed that the discs are the real structural units which, piled end to end, make up the fibre. The fibrils they consider artificial. This view is erroneous. It seems certain that the fibres are built up from fibrils ranged side by side, and that the discs are artificial. The con- tents of the muscle-fibre appear to consist of two functionally different substances, a contractile substance, and an interstitial, perhaps nutri- tive, non-contractile material of more fluid nature. The contractile substance is arranged as longitudinal fibrils embedded in interfibrillar matter (sarcoplasm) . In a muscle impregnated with chloride of gold the interfibrillar matter appears as a network. Schafer has described the contractile elements of the muscle-fibre (Figs. 241, 242) as fine columns (sarcostyles), divided into segments (sarcomeres) by thin transverse discs (Krause's membranes), occu- 4-limSSiuiuiiuS!!fBIBBHBltejiiuuitui, Pyi^g the position of the middle of each light stripe. Each sarcomere contains a sarcous element (a por- tion of the dark stripe) with a clear substance at its ends, filling up the Fig. 241. — Living Muscle of Water- Beetle (highly magnified) (Schafer). s, sarcolemma ; a, dim stripe ; 6, bright stripe; c, row of dots in bright stripe, which appear to be the enlarged ends of rod-shaped particles, d, but in reality represent expansions of the interstitial sub- stance (sarcoplasm). Fig. 242. — Portion of Leg Muscle of Insect, treated with Dilute Acetic Acid (Schafer). S, sarco- lemma; D, dot-like en- largement of sarcoplasm; K, Krause's membrane. The sarcous elements have been swollen and dissolved by the acid. space between the sarcous element and Krause's membrane, and con- stituting a portion of the light stripe. The sarcous element is itself double, and if the fibre be stretched, the two portions separate at a line which runs transversely across the middle of the dim stripe (Hensen s line) . Schafer considers that the appearance of longitudmal fibrillation in the sarcous elements is due to the presence in them of fine longi- tudinal canals or pores. The Krause's membrane of the individual fibrils is scarcely ever visible in an intact mammalian fibre, and the apparent line m the clear stripe of an intact fibre is an optical appearance due to interference of light. Kuhne, who was fortunate enough to find one day a small nematode worm moving in the interior of a fibre, saw it pass along the fibre with perfect freedom, ignoring Krause's membrane. Possibly, however, it was moving in the sarcoplasm, the fibrils being simply pushed aside. 7i8 THE PHYSIOLOGY OF THE CONTRACTILE TISSUES Changes during Contraction — Theories of Contraction. — In contractions, according to Schafer, the clear substance between Krause's membrane and the sarcous element passes into the canals, which are open towards Krause's membrane, but closed towards Hensen's line. The sarcous element therefore swells up, and the sarcomere is shortened. In the extended muscle the clear substance leaves the pores of the sarcous element, and accumulates in the space between it and Krause's mem- brane. The sarcomere is thus lengthened and narrowed. While the existence of Schafer's pores is not admitted by all observers, there is a pretty general agreement that the sarcomere, like the cytoplasm of an amoeboid cell, does consist of two substances, one of which (the hyalo- plasm of the cell, the clear material of the sarcomere) interpenetrates the other (spongioplasm of the cell, substance of the sarcous element) ; and that in relaxation the clear fluid passes from the sarcous element to the ends of the sarcomeres, whereas in contraction it passes in the reverse direction into the sarcous elements. Whether the fluid passes into and out of the meshes of an actual network, or along actual physical pores in the sarcous element, or whether it is transferred by some process like molecular imbibition (p. 420), need not be discussed here, since it is not definitely known. The fundamental question by what process the transference is determined when the muscle is excited also remains unsettled. So far as is known at present, it is probable that the mechanical energy of the contracting muscle must be derived from the transformation of chemical energy into one of three forms : energy associated with osmotic processes, energy associated with imbibition, and energy associated with changes oft surface tension. It is not diffi- cult to see that a sudden increase in the osmotic concentration in the sarcous element, due to the breaking up of large molecules or colloid aggregates into small molecules, or the liberation of electrolytes from the colloids, might lead to the rapid passage of water into it from the bright bands. A sudden change of permeability of the sarcous elements for dissolved substances in the clear fluid would have a similar efiect. The same is true of a change in their power of imbibi- tion. But, according to Bernstein, it is scarcely to be supposed that the extraordinarily rapid movement of water molecules which must occur in contraction can be accounted for either by osmosis or by im- bibition. tA more plausible theory is that the surface tension — say between the substance of the sarcous elernent and the clear fluid — is altered. That the shortening of the muscle in fatigue (p. 723) and rigor (p. 748), as well as its shortening in normal contraction, is due in some way to the liberation of metabolic products, especially lactic acid, is a theory of some standing, and fresh evidence in its favour has been recently supplied. Thus it has been pointed out that the course of heat production in the active muscle, and its relation to the time of the mechanical response, and the development and time relations of the electrical change which precedes that response, can be very naturally explained on the supposition that the liberation of lactic acid on or near some surface in the contractile substance is an essential factor in the contraction (Mines, etc.). It is known that in the presence of acid on the surface of certain colloid structures shortening occurs (Fischer and Strietman). The substance of the sarcous element which forms the dark stripe is doubly refracting, and therefore rotates the plane of polarization, but the clear substance of the light stripe is singly refracting. When an uncontracted fibre is viewed with crossed nicols, the dim stripe accordingly appears bright in the otherwise dark field. In the con- tracted fibre the doubly refractive material remains in the stripe which MECHANICAL PHENOMENA OF MUSCULAR CONTRACTION 719 is dim in. ordinary light. There is no transference of it, but, according to most writers, the bands which are dim in ordinary light increase in size by the transference of liquid from the isotropous band. Diffraction Spectrum of Muscle. — When a beam of white light passes through a striped muscle, it is broken up into its constituent colours, and a series of diffraction spectra are produced, just as happens when ' the light passes through a diffraction grating (a piece of glass on which are ruled a number of fine parallel equidistant lines). The nearer the lines are to each other, the greater is the displacement of a ray of light of any given wave-length. It has accordingly been found that when a muscular fibre contracts, the amount of displacement of the diffraction spectra increases. At the same time the whole fibre becomes more transparent. (2). Mechanical Phenomena. — ^The muscular contraction may be graphically recorded by connecting a muscle with a lever which is moved either by its shortening or by its thickening. The lever writes on a blackened surface, which must travel at a uniform rate if the form and time-relations of the muscle curve are to be studied, but may be at rest if only the height of the con- traction is to be recorded. The whole arrangement for taking a muscle- tracing is called a myograph (Fig. 278, p. 785). The duration of a ' twitch ' or single contraction (in- cluding the relaxation) of a frog's muscle is usually given as about one-tenth of a second, but it may vary considerably with temperature, fatigue, and other circumstances. It is measured by the vibrations of a tuning-fork written immediately below or above the muscle curve. When the muscle is only slightly weighted, it but very gradually reaches its original length after con- traction, a period of rapid relaxation being followed by a period of ' resi- dual contraction,' during which the descent of the lever towards the base-line becomes slower and slower, or stops altogether some distance above it. The duration of the con- traction of smooth muscle evoked by a single momentary stimulus is much greater than that of striped muscle (two to seven seconds for the rabbit's ureter; five to fifteen seconds for the cat's nictitating mem- brane; one to two minutes for the frog's stomach). Latent Period. — If the time of stimulation is marked on the tracing, it is found that the contraction does not begin simultaneously with it, but only after a certain interval, which is called the latent period. This can be measured by means of the spring myograph (Fig. 224) or of the pendulum myograph, a pendulum which in its swing carries a smoked plate against the writing-point of a lever connected with a muscle- The carrier of the recording- plate opens, at a definite point in its passage, a key in the primary coil of an induction machine, and so causes a shock to be sent through the muscle or nerve, which is con- nected with the secondary- The precise point at which the stimulus is thrown in can be marked- on -the tracing by carefully bringing the Fig. 243. — Living Muscular Fibre (from Geotru/pes stercorarius). i, in or- dinary; 2, in polarized light. (Van Gehuchten.) In living muscle (at least in fibres which are not extended) in contrast to dead muscle after treat- ment with reagents, the doubly re- fracting or anisotropous substsince is present in the greater part of the fibre ; and with crossed nicols the position of the singly refracting or isotropous material is indicated only by narrow transverse black lines or rows of dark dots. 720 THE PHYSIOLOGY OF THE CONTRACTILE TISSUES plate to the position in which the key is just opened, and allowing the lever to trace here a vertical line (or, rather, an arc of a circle). The portion of the time-tracing between this line and a parallel line drawn through the point at which the contraction begins gives the latent period. Helmholtz measured the length of the latent period by means of the principle of Pouillet, that the deflection of a magnet by a current of given strength and of very short duration is proportional to the time during which the current acts on the magnet. He arranged that at the moment of stimulation of the muscle a current should be sent through a galvanometer, and should be broken by the contraction of the muscle the moment it began. In this way he obtained the value °^ tJs second for the latent period of frog's muscle. The tendency of Fig. 244. — Spring Myograpli. A , B, iron uprights, between which are stretched the guide-wires on which the travelling plate a runs; ft, pieces of cork on the guides to gradually check the plate at the end of its excursion, and prevent jarring; b, spring, the release of which shoots the plate along; h, trigger -key, which is opened by the pin d on the frame of the plate. later observations has been to make the latent period shorter. Burdon Sanderson found that the change of form begins in unweighted or very slightly weighted muscle with direct stimulation in yg^ second after, and the electrical change (p. 797) simultaneously with, the excitation. It is known that the apparent latent period depends upon the resistance which the muscle has to overcome in beginning its contraction. The maximum rhortening, or ' height of the lift,' depends upon the length of the muscle, the direction of the fibres, the strength of the stimulus, the excitability of the tissue, and the load it has to raise. In a long muscle, other things being equal, the absolute shortening, and therefore the maximum height of the curve, will be greater than in a short muscle; in a muscle with fibres parallel to its length — ^the sartorius, for instance — it will be greater than in a muscle like the gastrocnemius, with the fibres directed at various angles to the long axis. For stimuli less than maximal, the absolute contraction increases MECHANICAL PHENOMENA OF MUSCULAR CONTRACTION 721 with the strength of stimulation, and a given stimulus will cause a greater contraction in a muscle with a given excitability than in a muscle which is less excitable. Under ordinary experimental condi- tions at least, weak stimuli cause a smaller contraction than strong, not only because each stimulated fibre contracts less, but because a smaller number of fibres are excited (p. 155). The objects used for the study of muscular contraction contain many fibres, and it is not in Fig. 245. — Curve of a Single Muscular Contraction or Twitcli talcen on Smoked Glass with Spring Myograph and photographed. Vertical line A marks the pomt at which the muscle was stimulated; time tracing shows ^^ of a second (reduced). general possible to distribute the stimulus equally to all. This is true for smooth muscle as well as for striped. Finally, increase of the load per unit of cross-section of the muscle diminishes above a certain limit the ' height of the lift." Influences which affect the Time- Relations of the Muscular Contrac- tion. — Many circumstances affect the form of the muscle curve and its time-relations. [a] Influence of the Load — Isotonic and Isometric Contraction.— Ihs first effect of contraction is to suddenly' stretch the muscle, and- ^He more the muscle is loaded the greater will this elongation be. So that at the beginning of the actual shortening part of the energy of contraction is already expended without visible effect, and has to be recovered from the elastic reaction during the ascent of the lever. The contraction of a muscle loaded by a weight which is not increased or diminished during the contraction is said to be isotonic, for here the tension of the muscle is the same throughout, and its length alters . When the muscle is attached very near the fulcrum of the lever, so that it acts upon a short arm, while the long arm carrying the writing-point is prevented from moving much by a spring, the muscle can only shorten itself very slightly; but the changes of tension in it will be related to those in the springs, and therefore to the curve traced by the writing-point. Such a curve is called isometric, since the length of the muscle remains almost unaltered. In the body muscles usually contract under conditions more nearly allied to those of the isometric than to those of the isotonic contraction. The work done by a muscle in raising a weight is equal to the product of the weight by the height to which it is raised. Beginning with no load at all, it is found that the weight can be increased up to a certain limit without diminishing the height of the contrg,ction ; perhaps the height may even increase. Up to this limit, then, the work evidently increases with the load. If the weight is made still greater, the con- 46 Fig. 246. — Contrac- tions of Smooth Mus- cle: Cat's Bladder (C. C. Stewart). Stimulated with pro- gressively stronger induction shocks. The lowest line is the time trace (lo-second intervals). Immedi- ately below the mus. cular contractions are marked the points at which the stimuli were thrown in. 722 THE PHYSIOLOGY OF THE CONTRACTILE TISSUES traction becomes less and less, but up to another limit the increase of weight more than compensates for the diminution of ' lift,' and the work still increases. Beyond this, further increase of weight can no Fig. 247. — Influence of Load on the Form of the Muscle Curve, i, curve taken with unloaded lever; 2, 3, 4, weight successively increased; 5, abscissa line; time trace, T^ second (reduced). longer make up for the lessening of the lift, and the work falls off till ultimately the muscle is unable to raise the weight at all. The ' absolute contractile force ' of an active muscle may be measured by determining the weight which, brought to bear upon the muscle at Fig. 248.— Influence of Temperature on the Striated Muscle Curve. 2, air tempera- ture; I, 25° — 30° C. ; 3, 7° — 10° C. ; 4, ice in contact with muscle. The fifth curve was taken at a little above air temperature. the instant of contraction, is just able to prevent shortening without stretching the muscle. It, of course, depends, among other things, on the cross-section of the muscle. During the contraction the absolute; force diminishes continually, so that a smaller and smaller weight is MECHANICAL PHENOMENA OF MUSCULAR CONTRACTION 723 suf&cient to stop any further contraction, the more the muscle has already shortened before it is applied. At the maximum of the co;i- traction the absolute force is zero. Hence a muscle works under the most favourable conditions when the weight decreases as it is raised, and this is the case with many of the muscles of the body. During flexure of the forearm on the elbow, with the upper arm horizontal, a weight in the hand is felt less and less as it is raised, since its motoent, which is proportional to its distance from a vertical line drawn thrpugh the lower end of the humerus, continually diminishes. (6) Influence of Temperature on' the Muscular Contraction. — Increase of temperature of the muscle up to a certain limit diminishes the latent period and the length of the curve, and increases the height of the contraction, but beyond this limit the contractions are lessened in height (Fig. 248). Marked diminution of teinperature causes, in general, an increase in the latent Jieriod and length, and a decrease in the height of the contraction. In the heart the effect of cold in strengthening the beat is often very marked. Temperature affects the contraction curve of smooth muscle much ip. the same way as that of striated muscle (Fig. 249). (c) Influence of Previous Stimulation — Fatigue. -If. a muscle is stimulated by a series of equal shocks thrown in at regular intervals. ■Fig. 249.— Influence of Temperature on the Smooth Muscle Curve : Cat's Bladder (C C. Stewart). Contractions at different temperatures with the same strength of stimulus. The temperatures (Centigrade) are marked on the curves. and the contractions recorded, it is seen that at first each curve overtops its predecessor by a small amount. This phenomenon, which is regularly observed in fresh skeletal muscle (Fig. 253), although it was at one time supposed to be peculiarly a property of the muscle of the heart (Fig. 254), is called the ' staircase," and seems to indicate that within hmits the muscle is benefited by contraction and its excitabiUty increased for a new stimulus. Soon, however, in an isolated preparation, the contractions begin to dechne in height, till the muscle is at length utterly exhausted, and reacts no longer to even the strongest stimulation (Figs. 251, 252, 279). A conspicuous feature of the contraction-curves of fatigued muscle is the progressive lengthening, which is much more marked in the descending than in the ascending periods; in other words. 724 THE PHYSIOLOGY OF THE CONTRACTILE TISSUES relaxation becomes more and more difficult and imperfect (Fig. 279, p. 787) . In smooth muscle (cat's bladder or ring from frog's stomach) fatigue can be very easily demon- strated in the same way, and the curves present similar features, with the exception that, instead of be- coming longer in fatigue, the suc- cessive contractions become shorter. It is by no means so easy to fatigue a muscle still in connection with the circulation as an isolated muscle. But even the latter, if left to itself, will to some extent re- cover, and be again able to con- tract, although exhaustion is now more readily induced than at first. Fig. 2SI. — Fatigue Curve of Muscle: Frog's Gastrocnemius. The arrange- ment with which the curve figured was obtained was a so-called auto- matic muscle interruptor (Fig. 250). A wire on the lever is made to close and open the primary circuit of an inductorium, the muscle or nerve being connected with the secondary. Every time the needle touches the mercury the muscle is stimulated automatically. > Fig. 25o.-^Automatic Muscle Interruptor. K, battery; P, primary; S, secondary coil; A, axis of lever; N, needle; Hg, mercury cup. In man, muscular fatigue can be studied by means of an arrange- ment called an ergograph (Fig. 255). A record of successive con- Fig. 252. — Fatigue Curve taken on a Slowly-moving Drum (reduced to Half): Frog's Gastrocnemius. Excited through the sciatic nerve by maximal shocks once in six seconds. MECHANICAL PHENOMENA OF MUSCULAR CONT'RACTION 723 tractions, say, of one of the flexor muscles of a finger, in raising a weight (isotonic method) or in deforming a spring (isiDmetric method) is taken on a drum. When the contractions are repeated every second, or every half-second, distinct evidence of fatigue is seen on the tracing after a longer or shorter period, according to the conditions. What is the cause of muscular fatigue ? An exact answer is not possible in the present state of our knowledge, but we may fairly con- clude that in an isolated preparation it is twofold: (i) Waste products, among which some are so directly related to the onset of fatigue as to deserve the name of ' fatigue sub- stances,' are formed by the active muscle faster than they can be re- moved, oxidized or otherwise decom- posed. (2) The material necessary for ^'f; 253.-' Staircase; in. Skeletal . ,- ■ , y , , Muscle ! Frog. Stimulation by contraction is used up more quickly an automatic arrangement. than it can be reproduced or brought to the place where it is required. That the accumulation of fatigue products has something to do with the exhaustion is shown by the fact that the muscles of a frog, exhausted in spite of the con- tinuance of the circulation, can be restored by bleeding the animal, or washing out the vessels with physiological salt solution, while injection of a watery extract of exhausted muscle into the blood- Fig. 254. — 'Stairca^' in Cardiac Muscle. Contractions recorded on a much more quickly moving drum than in Fig. 253. The contractions were caused by stimu- lating a heart reduced to standstill by the first Stannius' ligature (p. 197). The contractions gradually increase in height. vessels of a curarized muscle renders it less excitable (Ranke). This observer supposed that it was specially the removal of the acid products of contraction which restored the muscle. Such acid products as carbon dioxide and lactic acid, or the lactates which it may form with bases in the blood, lymph or tissues, when they act on muscle in more than a certain concentration, produce the 726 THE PHYSIOLOGY OF THE CONTRACTILE TISSUES same effects on its power of contraction as are produced by fatigue' and there is some reason to suppose that lactic acid is the most influential of the fatigue substances. In smaller concentration, on the contrary, they increase the excitability of the muscle, and, according to Lee, the phenomenon of the ' staircase ' is due to the augmenting action of these, and perhaps other fatigue substances, before they have accumulated sufficiently to cause fatigue. The lack of oxygen holds a conspicuous place among the con- ditions which permit an excessive accumulation of fatigue substances, and may contribute also to the failure of the processes normally going on in the muscle which replenish the store of materials needed for contraction. An isolated muscle is necessarily an asphyxiated muscle, and the favourable action of an atmosphere of oxygen on restoration- of its contractile power after exhaustion Fig. 255. — Ergograph (MosSo's, modified by Lombard). (Fig. 123, p. 265) shows that asphyxia is itself an important factor in the onset of fatigue. Injection of arterial blood, or even of an oxidizing agent like potassium permanganate, into the vessels of an exhausted muscle causes restoration (Kronecker). The depletion of the available store of carbo-hydrate in the form of glycogen (and dextrose) seems to be another factor in fatigue, although not the chief direct cause of the phenomena associated with that condition. Seat of Exhaustion in Fatigue. — When a fatigued muscle responds no longer to indirect stimulation, it can still be directly excited. The seat of exhaustion must therefore be either the nerve-trunk or the nerve-endings. It is not the nerve-trunk which is first fatigued, for this still shows the negative variation (p. 797) on being excited: And if the two sciatic nerves of a frog or rabbit be stimu- MECHANICAL PHENOMENA OF MUSCULAR CONTRACTION 727 lated continuously with interrupted currents of equal strength, while the excitation is prevented from reaching the muscles of one Umb till those of the other cease to contract, it will be found that when the ' block ' is removed the corresponding muscles contract vigorously on stimulation of their nerve. The passage of a constant current through a portion of the nerve or the application of ether between the point of stimulation and the muscles niay be used to prevent the excitation from passing down (p. 786). Or a dose of curara just sufficient to paralyze the motor innervation may be given to a rabbit, and the animal kept alive by artificial respiration. The sciatic is now stimulated for many hours. As soon as the influence of the curara begins to wear off, the muscles of the leg contract. The possible seats of fatigue caused by voluntary muscular con- traction are (i) the muscle; (2) the nerve-endings (or the receptive substances in the muscles, p. 713); (3) the nerve-trunk; and (4) the path of the voluntary motor impulses in the central nervous system, which includes the pyramidal cells in the motor region of the cerebral cortex (p. 847), the fibres of the pyramidal tract, and the motor cells in the anterior horn of the spinal cord. The two weak links in this chain appear to be the motor nerve- endings and the muscles. The nerve-fibres, whether peripheral or central, are certainly the strongest link. Ergographic experi- ments have hitherto yielded results too discordant to justify any very definite statement as to the point at which the chain snaps in complete fatigue, if, indeed, it always necessarily breaks at the same point. The muscles and motor endings appear to be always affected. The position of the nerve centres, including the synapses (p. 824), is in doubt. That the synapses easily lose their power of con- ducting nerve impulses under the influence of repeated excitations is indicated by the experiments of Sherrington on fatigue of reflex mechanisms in which two or more afferent paths can cause discharge along a common efferent path (p. 874). When excitation of one of the afferent paths has ceased to be effective, the reflex contrac- tions can still be obtained on exciting the other. In this case the motor neuron from cell-body to nerve-ending and the muscle are eliminated as the seats of the fatigue block. Whether the tem- porary loss of conduction in this case is comparable to the fatigue of muscle, or iS a perfectly different phenomenon (' pseudo-fatigue ' of Lee), scarcely bears on our present question. For if ' pseudo- fatigue ' of afferent synapses can cause a reflex to miss fire, this at least shows that the conductivity of the S3mapse is very easily affected by repeated excitation, just as it is known to be very easily affected by ansemia. The fact that a muscle, completely fatigued by direct electrical stimulation, can still be voluntarily contracted, has been supposed to indicate that the voluntary excitation is more effective 728 THE PHYSIOLOGY OF THE CONTRACTILE TISSUES than any artificial stimulus. But the alternative explanation that the electrical stimuli cannot be applied to a muscle in situ, so as to cause uniform excitation, and therefore uniform fatigue, of all the fibres of the muscle, is more probable (Hough). It has been shown that the injection of the blood of an animal exhausted by running or other muscular effort into the circulation of a normal animal produces in the latter all the symptoms of fatigue. Here the fatigue-producing substances will have the opportunity of acting on both the central and the peripheral mechanisms. There are reasons for believing that the fatigue process is fundamentally the same in different tissues. The fatigue substances produced in Fig. 256. — Influence of Mental Fatigue on Muscular Contraction, i, series of con- tractions of flexors of middle finger before, and 2, series of contractions imme- diately after, a period of three; and a half hours' hard mental work. In both cases the muscles were stimulated directly every two seconds by an electrical current, and caused to raise a certain weight till temporary exhaustion occurred. In the first series fifty-three contractions were found possible, in the second only twelve (Maggiora). 1 muscle, and not immediately ehminated or transformed during active muscular exertion, may therefore very well be a factor in inducing fatigue of the central nervous mechanisms in addition to the formation of fatigue products, and the using up of necessary material in these mechanisms themselves. Conversely, active and long-continued mental exertion may occasion muscular fatigue (Fig. 256). The sensation of fatigue is alluded to in Chapter XVIII, (d) The Influence of Drugs on the Contraction of Muscle. — ^The total work which a muscle can perform, its excitability and the absolute force of the contraction, may all be altered either in the plus or the minus sense by drugs. But in connection with our present subject those drugs which conspicuously alter the form and time-relations of the muscle-curve have most interest. Of these veratrine is especially MECHANICAL PHENOMENA OF MUSCULAR CONTRACTION 729 important. When a small quantity of this substance is injected below the skin of a frog, spasms of the voluntary muscles, well marked in the limbs, come on in a few minutes. These are attended with great stiffness of movement, for while the animal can contract the extensor muscles of its legs so as to make a spring, they relax very slowly, arid some time elapses before it can spring again. If it be killed before the reflexes are completely gone, the peculiar alterations in the form of the muscle-curve caused by veratrine will be most marked. The poisoned 1 .2 Fig. 257. — Veratrine Curve compared with Normal: Frog's Gastrocnemius. The tuning-fork marks hundredths of a second. Between i and 2 a portion of the tracing corresponding to one aad a half seconds has been out out, and between 2 and 3 a portion corresponding to one second. The veratrine curve does not show a peak. At 3 it has not yet fallen to the base-line. muscle, stimulated directly or through its nerve, contracts as rapidly as a normal muscle; while the height of the curve is about the same, but the relaxation is enormously prolonged (Fig. 257). This effect seems to be to a considerable degree dependent on temperature, atid it may temporarily disappear when the muscle is made to contract several times without pause. Barium salts, and, in a less degree, those of strontium and calcium, have an action on muscle similar to that of veratrine. Sometimes the curve shows a peak (Fig. 258), due to a rapid descent of the lever for a certain distance. This is followed by a slow relaxation. The p3ak appears to be analo- gous to the initial con- traction when a strong voltaic current is passed through a muscle, and the rest of the curve to the tonic contraction. (e) The individuality of the muscle itself has an influence on the muscle- curve. Not only do the muscles of different animals vary in the rapidity of contraction, but there are also differences between the skeletal muscles of the same animal. In the rabbit there are two kinds of striped muscle, the red and the pale (the semitendinosus is a red, and the adductor magnus a pale muscle), and the contraction of the former is markedly slower than that of the latter. In many fishes and birds, and in some insects, a similar difference of colour and structure is present. Even where there is no distinct histological difference, there may be great variations in the length of contraction. In the frog, for instance, the hyoglossus muscle contracts much more slowly than the gastroc- Fig. 258. — Veratrine Curve: Frog's Gastrocnemius. The curve shows a peak, the lever falling a little before the sustained contraction begins. ^30 THE PHYSIOLOGY OF THE CONTRACTILE TISSUES nemius. The wave of contraction, which in frogs' striped muscle lasts only about o-oy second at any point, may last a second in the forceps muscle of the crayfish, though only half as long in the muscles of the tail. In the muscles of the tortoise the contraction is also very slow. The muscles of the arm of man contract more quickly than those of the leg. Summation of Stimuli and Superposition of Contractions. — Hitherto we have considered a single muscular contraction as arising from a single stimulus, and we have assumed that the muscle has completed its curve and come back to its original length before the next stimulus was thrown in. We have now to inquire what happens when a second stim.ulus acts upon the muscle during the contraction caused by a first stimulus, or during the latent period before the contraction has actually begun; and what happens when a whole series of rapidly-succeeding stimuli are thrown into the muscle. First let us take two stimuli separated by a smaller interval than the latent period (p. 719). If they are both maximal — i.e., if each by itself would produce the greatest amount of contraction of which the muscle is capable when excited by a single stimulus — -the second has no effect whatever ; the contraction is precisely the same as if it had never acted. But if they are less than maximal, the contraction, although it is a single contraction, is greater than would have been due to the first stimulus alone ; in other words, the stimuli have been sjmmed or added to each other during the latent period, so as to produce a single result. Next let us consider the case of two stimuli separated by a greater interval than the latent period, so Fig. 259.— Superposition of Contractions, that the second falls into the I is the curve when only one stimulus muscle during the contraction pro- is thrown in; 2, when a second stimulus duced by the first. The result here acts at the time when curve i has nearly j different : traces of two con- reached its maximum height. j. j.--' xi. „ 1 tractions appear upon the muscle- curve, the second curve being that which the second stimulus would have caused alone, but rising from the point which the first had reached at the moment of the second shock (Fig. 259). Although the first curve is cut short in this manner, the total height of the contraction is greater than it would have been had only the first stimulus acted ; and this is true even when both stimuli are maximal. Under favourable circum- stances, when the second curve rises from the apex of the first, the total height may be twice as great as that of the contraction which one stimulus would have caused (p. 789). It is worthy of note that striated muscle has no power of summation of subminimal stimuli each of which is just too weak to cause contraction. No matter how rapidly they are thrown in, the muscle remains at rest. It is otherwise with smooth muscle. Stimuli which are singly ineffective cause contraction when repeated. Tetanus. — Not only may we have superposition or fusion of two contractions, but of an indefinite number; and a series of rapidly following stimuli causes complete tetanus of the muscle, which remains contracted during the stimulation, or till it is exhausted (Fig. 260). MECHANICAL PHENOMENA OF MUSCULAR CONTRACTION 731 The meaning of a complete tetanus is readily grasped if, beginning with a series of shocks of such rapidity that the muscle can just completely relax in the intervals between successive stimuli, we gradually increase the frequency (p. 789). As this is done, the ripples on the curve become smaller and smaller, and at last fade . out altogether. The maximum height of the contraction is greater than that produced by the strongest single stimulus ; and even after complete fusion has been attained, a further increase of the fre- quency of stimulation may cause the curve still to rise. Fig. 260. — Analysis of Electrical Tetanus (reduced to |). Four curves showing the effect of increasing frequency of stimulation of the frog's. gastrocnemius through its nerve. In the lowest curve the frequency is such that the muscle relaxes almost completely between the successive contractions. In the uppermost curve, with a frequency more than three times greater, the contractions are almost completely fused. In all the curves the fusion becomes more nearly con5|)lete as stimulation goes on, owing to the slower relaxation of the fatigued muscle. It is evident from what has been said that the frequency of stimulation necessary for complete tetanus will depend upon the rapidity with which the muscle relaxes; and everything which diminishes this rapidity will lessen the necessary frequency of stimulation. A fatigued muscle may be tetanized by a smaller number of stimuli per second than a fresh muscle, and a cooled by a smaller number than a heated muscle. The striped muscles of insects, which can contract a million times in an hour, require 300 stimuh per second for complete tetanus, those of birds 100, of man 40, the torpid muscles of the tortoise only 3. The pale muscles of the ;rabbit need 20 to 40 excitations a second, the red 732 THE PHYSIOLOGY OF THE CONTRACTILE TISSUES muscles only lo to 20 ; the tail muscles of the crayfish 40, but the muscles of the claw only 6 in winter and 20 in summer. The gastrocnemius of the frog requires 30 stimuli a second, the hyo- glossus muscle only half that number (Richet). The frequency of stimulation necessary for complete tetanus of unstriped muscle is much less than for striped muscle. Smooth tetanus of a band of muscle from the frog's stomach was obtained with strong opening induction shocks at the rate of i in 5 seconds. We see, then, that there is a lower limit of frequency of stimulation below which a given muscle cannot be completely tetanized. There appears also to be an upper limit beyond which a series of stimuli becomes too rapid to produce complete tetanus, and at which an inter- rupted current acts like a constant current, causing a single twitch at its commencement or at its end, but no contraction during its passage. This limit undoubtedly does not depend upon the frequency of stimula- tion alone ; the intensity of the individual excitations, the temperature of the muscle, and probably other factors, affect it. For Bernstein found that with moderate strength of stimulus tetanus failed at about 250 per second, and was replaced by an initial contraction; with strong stimuli at more than 1,700 per second, tetanus could still be obtained. Krpnecker and Stirling, stimulating the muscle by induced currents set up in a coil by the longitudinal vibrations of a magnetized bar of iron, saw tetanus even with the utmost frequency attainable, 4,000 shocks a secpud, according to Roth; while v. Kries in a cooled muscle found tetanus replaced by the simple initial twitch at 100 stimuli per second, although in a muscle at 38° C. stimulation of ten times this frequency still caused tetanus. Recently Einthoven, exciting the nerve of a frog's nerve-muscle preparation with extremely frequent oscillatory condenser discharges, observed tetanus up to even a million vibrations a second, if the current intensity was at the same time very greatly increased (to more than i6;ooo times the intensity needed with a constant current). These results are not really so discordant as they appear; for it is known that with electrical stimulation the number of excitations is not necessarily the same as the nominal number of shocks. By applying a telephone to a muscle excited through its motor nerve, it has been shown that the pitch of the note produced by the tetanized muscle corresponds exactly to the rate of excitation up to a certain frequency. This frequency is about 200 per second for frog's and about 1,000 per second for mammalian muscle under the best condi- tions. If the rate of excitation is still further increased, there is no corresponding increase in the pitch. Therefore, some of the stimuli are now producing no effect — ' falling flat,' so to speak (Wedensky). Out reason for this is that even very brief currents leave alterations of conductivity and excitability behind them (Sewall), which we shall have to discuss in another chapter (p. 759). (See also p. 761.) It is only while the actual shortening is taking place that a tetanized muscle can do external work. But, although during the maintenance of the contraction no work is done, energy is nevertheless being ex- pended, for the metabolism of a muscle during tetanus is greater than during rest, and, among other changes, lactic acid is produced. There are great differences in the ease with which different muscles can be exhausted by tetanus. For example, the muscles which close the forceps of the crayfish or lobster have, as everyone knows, the power of most obstinate contraction. Richet tetanized one for over seventy MECHANICAL PHENOMENA OF MUSCULAft CONTRACTION 733 minutes, and another for an hour and a half, before exhaustion came on, while a tetanus of a single minute exhausted the muscles of the crayfish's tail. The gastrocnemius of a summer frog kept up for twelve minutes, and a tortoise muscle for forty minutes. Continuous stimulation is not always necessary for the production of continuous contraction ; in some conditions a single stimulus is suffi- cient. A blow with a hard instrument may cause a dying or exhausted, and in thin persons even a fairly normal, muscle to pass into long- continued contraction. This so-called ' idio-muscular ' contraction seems to depend, in part at least, on the great intensity of the stimulus. It can sometimes be obtained in the frog's gastrocnemius, particularly in spring after the winter fast. It is not a tetanus and is not propa- gated along the muscular fibres, as an electrical tetanus is, but remairs localized at the spot where it arises. Similar non-tetanic contractions have already been mentioned, such as the tonic contraction during the passage of a strong voltaic current and the sustained veratrine contrac- tion. Ammonia causes also a long but non-tetanic contraction, and this, too, does not spread when the substance has acted only on a portion of the muscle. The contraction force of all these tonic contractions, as measured by the resistance necessary to overcome or prevent them, is less than the contraction force in electrical tetanus (Schenck). The rate at which the wave of muscular contraction travels may be measured by stimulating the muscle at one end, and recording, by means of levers, the movements of two points of its surface as far apart from each other as possible. Time is marked on the tracing by means of a tuning-fork, and the distance between the points at which the two curves begin to rise from the base-line divided by the time gives the velocity of the wave. Another method is founded upon the measure- ment of the rate at which the negative variation (p. 797) passes over the muscle, this being the same as the velocity of the contraction-wave. In frog's muscle it is about three metres a second, or six miles an hour. Rise of temperature increases, fall of temperature lessens it. When a rduscle is excited through its nerve, the contraction springs up first of all about the middle of each muscular fibre where the nerve-fibre enters it, and then sweeps out in both directions towards the ends. But so long is the wave, that all parts of the fibre are at the same time involved in some phase or other of the contraction. The wave of contraction in unstriped muscle lasts a relatively long time at any given point, and in tubes like the intestines and ureters, the walls of which are largely composed of smooth muscle arranged in rings, the wave shows itself as a gradually-advancing constriction travelling from end to end of the organ. There is no evidence that the contraction of smooth muscular fibres is discontinuous — ^that is, composed of summated contractions like a tetanus; it appears to be a greatly-prolonged simple contraction. An artificial stimulus, mechani- cal or electrical, causes, after a long latent period, a very definitely- localized contraction in a rabbit's ureter, which slowly spreads in a peristaltic wave in one or both directions along the muscular tube» Here, as in the cardiac muscle, the excitation passes from fibre to fibre, while in striped skeletal muscle only the fibres excited directly or through their nerves contract. That the rhy-thmical contraction of the heart is not a tetanus has already been seen. It is a simple con- traction, intermediate in its duration and other characters between the twitch of voluntary muscle and the contraction of smooth muscle. The contraction both of unstriped and of cardiac muscle is lengthened and made stronger by distension of the viscera in whose walls they occur, just as a skeletal muscle contracts more powerfully against resistance. 734 THE PHYSIOLOGY OF THE CONTRACTILE TISSUES Voluntary Contraction. — There is evidence that the voluntary contraction is a tetanus. One of the strongest buttresses of the theory of natural tetanus has been the muscle-sound, a low rumbling note which can be heard by listening with a stethoscope over the contracting biceps, or, when all is still, by stopping the ears with the fingers and strongly contracting the masseter and the other muscles concerned in closing the jaws.* Discovered about ninety years ago, first by Wollaston and then by Erman, half a century passed away before it was investigated more fully by Helmholtz. The latter observer, confirming the results of his predecessors, put down the pitch of the sound at 36 to 40 vibrations per second. He found, however, that little vibrating reeds with a rate of oscillation of about 19-5 per second were more affected when attached to muscle thrown into voluntary contraction, than those that vibrated at a smaller or a greater rate. He therefore concluded that the fundamental tone of the muscle corresponded to this frequency, although, since such a low note is not easily appreciated, the sound actually heard was really its octave or first harmonic (p, 304). The objection has been brought forward that the resonance tone of the ear also corresponds to a vibration frequency of 36 to 40 a second. In other words, this is the natural rate of swing of the elastic struc- tures in the middle ear, the rate they will most easily fall into if set moving by an irregular mixture of faint, low-pitched tones and noises, and not compelled to vibrate at some other rate by a distinct sound of definite pitch. Now, this resonance tone might be elicited by a quivering muscle if, among many diverse rates of oscillation of different portions of its substance, the rate of 36 to 40 a second anywhere ap- peared, and the note corresponding to the real rate of vibration of the muscle as a whole might be overpowered. Or, even if there were no regular rate of vibration of the whole muscle, but, instead, a series of irregular tremors or pulls due to irregularities in the contraction, con- nected with a want of co-ordination of all the fibres (Haycraft), the ear might from time to time pick out c f the turmoil of feeble aerial waves those corresponding to its resonance tone, just as a tuning-fork or a piaiio-string attuned to a particular note would cttch it up amid a thousand other sounds and strengthen it. But while this renders it highly probable that the resonance of the ear contributes to the production of the muscle-sound, and shows that we cannot from the pitch of the muscle-sound alone deduce the rate at which the muscle-substance is vibrating, it does not invalidate Helmholtz's objective observations with the oscillating reeds. And several observers (Schafer, Horsley, v. Kries) have noticed periodic oscillations, at the rate of 10 or 12 per second, in the curves taken from muscles (Fig. 261), contracted voluntarily against a small resistance. When the resistance is greater, the rate may be as much as 18 or 20 a second, and in quick and rapidly * In order that a muscular sound may be produced there must be a certain abruptness in the contraction. Thus, the slowly-contracting smooth muscles do not produce a sound, nor the slowly-contracting heart-muscle of cold- blooded animals. MECHANICAL PHENOMENA OF MUSCULAR CONTRACTION 735 repeated movements of the fingers even 40 a second. In habitual movements, such as those employed by a mai^ in his trade, the tremors are much less coarse than in unaccustomed movements For this reason the tremors of the left hand are greater than those of the right in executing a movement usually made with the latter (Eshner). In disease these tremors are often increased — e.g., in the clonic convulsions of epilepsy — but the frequency is the same. W#^#Wlf Fig. 261.- -Vibrations of Contracted Arm Muscles (Griffiths). Ttie arm was stretched out, holding a weight of about 6 kilos. Similar vibrations, and at about the same rate, are seen in curves traced by muscles excited through stimulation of the motor areas of the surface of the brain. Since this rate remains the same whether the motor cortex, the corona radiata, or the spinal cord is excited, and, unlike the rate of response to excitation of peripheral nerves, is independent of the frequency of stimulation (so long as the rate of stimulation is greater than 10 or 12 a second), it has been supposed to represent the rhythm with which impulses are dis- charged from the motor cells of the cord (Fig. 262). It is prob- able that the cortical centres discharge at about the same rate, for not only is it im- possible to articulate more rapidly than eleven .syllables per second, but it is impossible to reproduce the act of articulation in thought at a greater rate than this (Richet). But while this rate of 10 or 12 a second does seem to represent a fundamental rhythm of the central discharge, there are facts which indicate that upon this relatively slow rhythm a quicker rhythm is superposed. In other words, each of these discharges is itself discontinuous, and made up of a number of^separate impulses. Fig. 2C2. -Contractions caused by Stimulation of the Spinal Cord. 736 THE PHYSIOLOGY OF THE CONTRACTILE TISSUES Thus, according to Piper, the total number of simple discharges, each associated with an electrical change in the muscle, as recorded by the string galvanometer, is 47 to 50- a second. The rhythm of strych- nine tetanus in the frog is about 8 to 12 per second. By means of the capillary electrometer (p. 702) large electrical oscillations at this rate can be demonstrated, each of which represents a short tetanic spasm, as is shown by the fact that a number of smaller electrical oscillations are superposed upon the large ones (Sanderson). The electrical changes suggest that each discharge causes a, simple contraction much more prolonged than the twitch of a ditectly stimulated muscle. This removes the difficulty of understanding how such a small number as 10 contractions per second could be smoothly fused, and indicates that even the shortest possible voluntary movement, which can be executed in -^ to aV oi a second, is not caused by a single impulse, but is a tetanus, tor these brief movements the frequency of oscillation, as shown by the action currents, is the same as for sustained contractions. The electrical changes in the voluntarily contracted muscle seem to difier in amplitude or abruptness from those produced in experimental tetanus. For secondary tetanus (p. 806) is not caused by muscle in voluntary contraction. But this is also the case with the other pro- longed contractions caused by continuous artificial stimulation — e.g., Ritter's tetanus (p. 715) and the contraction produced by sodium chloride or ammonia. We need not hesitate to conclude, then, that the voluntary contraction is discontinuous, in the sense that it is not a perfectly smooth and uniform tonic contraction, although we still lack a decisive proof that it is maintained by a strictly intermittent outflow of nervous energy, and not by a continuous outflow causing a sustained contraction, which, it niay be, remits and is reinforced at intervals. The apparent discrepancies as to the rate of discharge in the results obtained by difierent observers, and by different methods, far from exciting distrust of them all, really lend support to the idea of a fundamental and fairly constant rhythm in the outflow as soon as it is recognized that the higher rates are approximately multiples of the lower. Thus, the number deduced by Helmholtz from the ex- periment of the springs is twice the lowest rate calculated from graphic records of the contraction. The rates corresponding to the muscle- sound and to the frequency of the electrical oscillations are about four times this number. Now, in a vibrating elastic body like a contracting muscle, a simple mathematical relation of this sort might be expected to appear when determinations of the rate of oscillation and of accom- panying periodic changes are made by methods varying in principle and in delicacy. For instance, an arrangement suited to record and to count coarse vibrations could not be expected to give the same result as an arrangement suited to record and count fine vibrations. But if both the coarse and the fine vibrations were related to a fundamental rhythm, a simple proportion might be expected to exist between the two sets of results. (3) Thermal Phenomena and Transformation of Energy in the Muscular Contraction. — ^When a muscle contracts, its temperature rises; the production of heat in it is increased. This is most dis- tinct when the muscle is tetanized, but has also been proved for single contractions. The change of temperature can be detected by a delicate mercury or air thermometer; and, indeed, a ther- mometer thrust among the thigh-muscles of a dog may rise as much THERMAL PHENOMENA OF MUSCULAR CONTRACTION 737 as i" to 2° C. when, the muscles are thrown into tetanus. In 'the isolated muscles of cold-blooded animals the increase of tempera- ture is much less ; and thermo-electrical methods, which are the most delicate at present known, have generally been used for its detection and measurement. They depend upon the fundamental fact of thermo-electricity, that in a circuit composed of two metals a current is set up if the junctions of tha metals are at different tempera- tures. Where no very fine differences of temperature are to be measured, a single thermo-j unction of German silver and iron, or copper and iron, is inserted into a muscle or between two muscles. But the electromotive force, and therefore the strength of the thermo-electric cur- rent, is proportional for any given pair of metals to the number of junctions, and for delicate measurements it may be necessary to use several connected together ■ in series. A thermopile of antimony - bismuth junctions gives a stronger current for a given difference of temperature than the same number of German silver-iron couples, but from its brittle nature is otherwise less convenient . The direction of the current in the cir- cuit is sUch that it passes through the heated junction from bismuth to anti- mony and from copper or German silver to iron. Knowing this direction, we are aware of the changes of temperature which take place from the movements of the mirror of the galvanometer with which the pile is connected. In the thermopiles employed in the recent ex- tensive investigations of Hill the alloy constantan is coupled with iron, the electromotive force of this combination being exceptionally great. The muscle which is to be excited is brought into close contact with one junction or set of junctions, the other set being kept at constant temperature. The image will now come to rest on the scale; and excitation of the muscle will cause a movement indicating an increase of temperature in it, the amount of which can be calculated from the deflection. In one form (Fig. 263) the thermopile con- stitutes a hollow cone, in which a muscle can be arranged so as to eliminate largely the errors due to differences of temperature of the muscle, or to the " slip " of the contracting muscle over the junctions. In this way Helmholtz observed a rise of temperature of 0-14° to '18° C. in excisedfrogs' muscles when tet'anized for a couple of minutes. 47 Fig. 263. — Conical Thermopile containing Gastrocnemius Mus- cle Reversed. C, copper leads tb galvanometer; S, stimulating wire. The straight lines indicate iron, the crossed lines constan- tan, the external junctions em- bedded in the ebonite frame being] at a, the internal junc- tions,' 6, in contact jwith the muscle. 738 THE PHYSIOLOGY OF THE CONTRACTILE TISSUES ^idenhain, with a very delicate pile, found a rise of o-ooi° to -005° C. for a single contraction of a frog's muscle. On the assump- tion that the pile had time to take on the temperature of the muscle before there was any appreciable loss of heat, this would be equal to the production by every gramme of muscle of a thou- sandth to five-thousandths of a gramme-calorie (p. 653) of heat. From pick's observations we may take about three-thousandths of a gramme-calorie as the maximum production of a gramme of frog's muscle in a single contraction. Hill has shown that in the case of the single contraction or twitch the evolution of heat may be so rapid as to be practically instan- taneous, indicating that it depends upon some sudden ' explosive ' chemical reaction; or, on the other hand, under certain conditions it may last as long as two seconds — that is, from four to ten times as long as the contraction itself. In the absence of oxygen, when the muscle is left, for in- stance, in an atmosphere of hydrogen, the heat produc- tion becomes markedly pro- longed. When abundant oxygen is supplied, the dura- tion of the discharge of heat is decreased. In a tetanus the evolution of heat lags behind the excitation, and the discharge associated with each stimulus is not complete till 0-5 to 2-5 seconds after the stimulus. In a prolonged complete tetanus the heat production corresponding to the first tenth of a second of excitation is far greater than that corresponding to the second tenth of a second, and so on, until eventually a uniform dis- charge of heat, at a rate much smaller than the initial rate, is reached. When frogs' muscles are rapidly stimulated indirectly (through the nerves) till fatigue has occurred, the maximum value of the heat evolved approximates to 0-9 gramme - calorie per gramme of muscle, about 70 or 80 per cent, being liberated in the first two minutes (Peters). A fact of great significance in regard to the relation of the reaction upon which the heat production depends and the mechanical conditions in the active muscle is that the production of heat is determined by the length of muscular fibres existing at the time when the heat is being evolved. From this it has been assumed that the production of heat in active muscle is a surface effect, and Fig. 264. — A, a single copper-iron thermo- electric couple ; B, two pairs, one inserted into the tissue b, the other dipping into water in a beaker a. The temperature of the jwater may be adjusted so that the galvanometer shows no deflection. The temperature of the tissue is then the same as that of the water. THERMAL PHENOMENA OF MUSCULAR CONTRACTION 739 not an effect taking place uniformly throughout the muscle substance and related accordingly to the volume or mass of the muscular substance (Bhx). Much evidence has been accumulated in favour of this hypothesis. For example, a muscle contracting isometrically (p. 721) produces more heat the greater is the initial tension (the more it is stretched at the begining of the excitation) — ^that is, the greater its length during contraction (Heidenhain). When a muscle is allowed to shorten in a tetanus, th6 heat production as compared with that of an isometric contraction of the same dura- tion, and evoked by the same strength of stimulus, is diminished by as much as 40 per cent. Relation between the Development of Mechanical Energy and Heat Production in Active Muscle. — ^There is no simple relation between the external work done in a muscle twitch and the heat set free. The efficiency of the muscular machine, as estimated by the proportion of the work done to the total energy degraded, varies with a number of factors — e.g., the load, the number of fibres excited, and the intensity of the excitation of each fibre, the two latt,er factors depending upon the strength of the stimulus. The greater the resistance, so long as the muscle can overcome it so as to do its utmost amount of external work,* the larger is the proportion of energy which appears as work, the smaller the proportion which appears as heat. For every muscle, under given conditions, there is a certain load which can be raised more advan- tageously than any other; but even in the most favourable case, an excised frog's muscle never does work equal to more than J of the heat given off. Generally the ratio is much less, and may sink as low as -^j. In the intact mammalian body the muscles work somewhat more economically than the excised frog's muscles at their best; for both experiment and calculation show (p. 662) that in a normal man under the most favourable conditions as much as ^ of the energy is converted into work. According to Zuntz and Katzenstein, 35 per cent, of the total energy appeared as muscular work in chmbing a mountain, and in bicycling only 25 per cent. Movements which have been much practised are more econSmicilly performed than unaccustomed ones, and this explains the superior efficiency of the muscles concerned in climbing, for no movements can possibly be more familiar than those con- cerned in locomotion. So far as this indication goes, it would seem that in the treatment of obesity unfamiliar, and therefore physio- logicaiUy expensive, forms of exercise should be recommended, in so far, of course, as they do not injuriously react upon the general condition, especially upon the circulation. * This statement, based on experiments with excised frog's muscles, is not, of course, inconsistent with the fact mentioned on p. 662, that in the intact body the fraction of the energy transformed into heat is greater in hard than in moderate work. 740 THE PHYSIOLOGY OF THE CONTRACTILE TISSUES When a muscle, excited by maximal stimuli, is made to lift con- tinuously increasing weights, both the woik done and the heat given out increase up to a certain limit. The muscle, as it were, burns the candle at both ends. The heat-production reaches its maximum some- what sooner than the work. It is certain that when work is done by a muscle an equivalent amount is subtracted from its sum-total of energy, and under proper conditions this can be actually demonstrated by the deficiency in the heat-production. This is done by means of a contrivance called a work-adder. It consists of a wheel, the rotition of which raises a weight attached to a cord wound round its axle. The muscle acts on the periphery of the wheel, and by rotating it raises the weight a little at each contraction. At the end of the contraction the wheel is pre- vented from moving back by a catch. The work done in a series of contractions is calculated from the total height to which the weight has been raised. Suppose a frog's gastrocnemius is made to contract a certain number of times while attached to the work-adder, and that simultaneously the heat-production is measured by means of a thermo- pile. Let H represent the heat actually produced, and h the heat equivalent of the work done. Now let the muscle be disconnected from the adder and made to raise the same weight, directly attached to it, by a series of contractions elicited in precisely the same way as the previous ones, except that the weight is allowed to fall with the muscle when it relaxes after each contraction. Here heat corresponding to the external work disappears from the muscle during the contraction just as in the first experiment, but this heat is returned to the muscle during the relaxation, since on the whole no external work is done. The heat produced in the second experiment is found, as a matter of fact, allowing for unavoidable errors, to be equal to H+h. According to Hill, the true ' efficiency ' of the muscle is not the ratio WjH. where W is the external work and H the total heat liberated, but TjH where T is the maximum increase of tension set up during the twitch whan the muscle is contracting isometrically. This fraction TjH is constant whatever be the initial tension, the number of fibres excited, or the strength of excitation of each fibre. For the theory of the muscular contraction the tension auring an isometric muscle twitch, which represents the potential energy suddenly developed in conse- quence of the excitation, is accordingly much more important than the height of the contraction, which is related to the work actually done. The essential thing in muscular contraction may be the abrupt develop- ment of this tension through a chemical reaction which liberates certain substances at some membrane or surface in the muscle. The potential energy once in being may or may not be transformed into work, and if so transformed the change may be accomplished economically or waste- fully, according to the conditions of the contraction. The ratio TjH decreases in fatigue, and with the time during which the muscle has been deprived of its blood-supply. Hill has calculated the absolute value of the heat -production in tetanus of a sartorius or semimem- branosus muscle of the frog. This quantity, reckoned per centimetre of length of the muscle, per gramme weight of the tension developed and per second of maintenance of the tension, is relatively constant at about 0'00ooi5 gramme-calorie. Including the recovery processes of oxidation following the contraction the total heat-production would amount to about 0-000025 calorie. The potential energy possessed by a muscle of 'a length of a centimetre when maintaining a tension of a gramme is about 0-000004 calorie. So that to maintain this state of potential energy six or seven times as much energy must, be liberated CHEMICAL PHENOMENA OF MUSCULAR CONTRACTION 741 per second. The maintenance of prolonged tension is, therefore, from the point of view of tlie mechanical result, an exceedingly wasteful process, with a very low effi.ciency in comparison with the high efficiency in a rapid twitch. This enables uo to see how important a part in heat- production, and therefore in temperature regulation, the tonus of muscle and the prolonged contractions of shivering may possess (p. 670). We are as yet in the dark as to the precise relation of the energy which appears as heat and of that which is converted into work. The ultimate source of both is, of course, the oxidation (and cleavage) of the food substances. It was at one time a favourite theory that in a muscle, as in a heat-engine, the chemical energy is first converted into heat, and part of the heat then transformed into work. There is no evidence that this is the case. It is, indeed, impossible that such differences of temperature can exist as would be compatible with the known efficiency of the muscular machine. Hypotheses based on the assumption that the chemical energy is immediately changed into work, perhaps through the pro- duction of surface effects, have met with increasing favour, but data are as yet too few for the formulation of any really satisfactory theory. The close relation between the heat-production and the formation of lactic acid in contraction which have been shown to- exist, is a suggestive fact whose full significance will only be revealed by further investigation. The restitution processes by which the original state of the muscle is restored after contraction are, of course, intimately related to those concerned in the actual shorten- ing; but unless we know how, and in consequence of what chemical or physical changes, the equilibrium of the resting muscle has been disturbed, we cannot know how, or in consequence of what chemical or physical changes, it is restored. Section IV. — Chemical Phenomena of the Muscular Contraction. The composition of dead mammalian muscle of the striped variety may be stated, in round numbers, as follows, but there are considerable variations, even within the same species : Water 75 per cent. Proteins - - 20 ,, Fats, lecithin, and cholesterin 2 Nitrogenous extractives, kreatin, carnosin, phos- ' pho-carnic acid, inosinic acid, purin bodies, such as uric acid, hypoxanthin, xanthin, etc. - Carbo-hydrates (glycogen, dextrose, maltose) - Non-nitrogenous organic substances (lactic acid, inosit) - - - - - - Pigment (myoheEmatin or myochrome, a haemoglobin not precisely identical with that of blood) . Inorganic substances less than i per cent, (chlorides, carbonates, phosphates, and sulphates of potassium, sodium, iron, calcium, magnesium) . Potassium is absent from the nuclei (Frontispiece) . 742 THE PHYSIOLOGY OF THE CONTRAQTILE TISSUES Of the nitrogenous extractives, kreatin (p. 587) and carnosin are present in greatest quantity, muscle containing 0-2 to 0-4 per cent, of kreatin. Carnosin (C9H14N4O3) is a substance with basic properties, and can be split up into histidin and /3-amino-propionic acid, an amino-acid not identical with alanin (or a-amino-propionic acid), but having the NHj coupled to the |8 instead of the a carbon atom (p. 557). There is more water in the muscles of young than of old animals (v. Bibra), and more in tetanized than in rested muscle (Ranke). The fats are variable in amount, and belong to a small extent to the actual muscle-fibres. For even when the visible fat is separated with the utmost care, nearly i per cent, of fat still remains (Steil). The glycogen content varies extremely in different muscles and in the same muscle under difierent nutritive and functional conditions. Thus, in one and the same dog the biceps brachii contained 0-17 and the quadriceps femoris 0-53 per cent. In dogs on a diet rich in carbo- hydrate and protein the percentage in the whole skeletal musculature ranged from 0-7 to 3-7, and in the heart from o-i to i-a. The average for human muscles has been given as 0-4 per cent. In lean horse-flesh Pfiiiger found 0-35 per cent, of glycogen, but no sugar. The total nitrogen was 3-21 per cent, of the moist tissue. The lactic acid of muscle and other tissues is the (i-lactic acid, which rotates, the plane of polarization to the right. By the action of certain bacteria on cane- sugar Z-lactic acid is obtained, which is left rotatory. The optically inactive fermentation lactic acid is obtained by the fermentation of lactose. Smooth muscle is somewhat richer in water than the striated variety from the same species, because skeletal muscle is richer in fat. Glycogen is either absent or present only in traces in the smooth muscle (of the stomach and bladder). Lactic acid, kreatin, and kreatinin are also found in much smaller amount than in striped muscle (Mendel and Saiki). As in striated muscle, hypoxanthin is the conspicuous purin base occurring in the free form — i.e., obtainable in muscle extracts. The most remarkable difference in the quantitative relations of the inorganic constituents is that in striated muscle potassium preponderates over sodium and magnesium over calcium, whereas in the smooth variety this relation is reversed. It would be natural to expect that the proteins, which bulk so largely among the solids of the dead muscle, and which are so obvi- ously important in the living muscle, should be affected by contrac- tion. But up to the present time no quantitative difference in the proteins of resting and exhausted muscle has ever been made out. The quantity of kreatin (and kreatinin) is said by some authorities to be increased. The following chemical changes have been defi- nitely established. In an active muscle — • («) More carbon dioxide is produced. (6) More oxygen is consumed, (c) Lactic acid is formed, {d) Glycogen is used up. (e) The substances soluble in water diminish in amount; those soluble in alcohol increase. Production of Carbon Dioxide and Consumption of Qxygen during Contraction. — This subject has already been dealt with in part in connection with tissue respiration (p. 265). The fact that muscular exercise increases the carbon dioxide output and the oxygen absorp- tion at 'the pulmonary surface, shows that oxidation processes involving ultimately the combustion of carbon-containing substances CHEMICAL PHENOMENA OF MUSCULAR CONTRACTION 743 are associated with the activity of the muscular tissue, but does not of itself prove that the final steps of the oxidation occur in the muscles themselves. This has been demonstrated, however, by observations on isolated muscles. When well supphed with oxygen, these, in addition to the stock of carbon dioxide in solution, in the form of carbonates and in other combinations, which they possess at the moment of isolation, continue to produce carbon dioxide, and this production is markedly increased by stimulation; The best evidence is to the effect that only preformed carbon dioxide is given off by isolated muscles in the absence of oxygen. They can go on contracting indeed, as previously stated, in an atmosphere of hydrogen or nitrogen, and may seem to be producing carbon dioxide, but the increased output appears to be due simply to an accelerated decomposition of already existing carbonates, or perhaps other combinations in which carbonic acid is loosely held, brought about by lactic acid, which in the absence of oxygen, is not trans- formed as it is under normal conditions, and accumulates in the muscular substance, uniting with bases, and thus displacing carbonic acid. Formation of Lactic Acid — Reaction of Muscle. — ^To litmus-paper fresh muscle is amphicroic — that is, it turns red litmus blue and blue litmus red. This is due, partly at least, to the phosphates. Mono- phosphate (tribasic phosphoric acid, H3PO4, in which one hydrogen atom is replaced, say, by sodium or pofassium) reddens blue litmus, while diphosphate (where two hydrogen atoms are replaced) turns red litmus blue. Litmoid (lacmoid) differs from litmus in not being affected by monophosphates. Diphosphates turn red litmoid blue, and so does fresh muscle, which has no effect on blue litmoid. A cross-section of fresh muscle is about neutral (sometimes faintly acid) to turmeric paper, which is turned yellow by monophosphates. A muscle which has entered into rigor or has been fatigued by prolonged stimulation is distinctly acid to blue litmus and to brown turmeric, reddening the former and turning the latter yellow, but does not affect blue litmoid. Perfectly fresh resting muscle excised with avoidance of all un- necessary manipulation contains very little lactic acid (as .little as 0-02 per cent, expressed as zinc lactate). Mechanical injury, heating, and chemical irritation cause a marked increase in the amount. Under anaerobic conditions — in an atmosphere of hydrogen, for instance — lactic acid is spontaneously developed in the resting muscle so long as irritability persists, but not longer. In air, which for even small excised muscles corresponds to a partial asphyxia, there is a small increase in the lactic acid, but its pro- duction is very slow in comparison with that in the hydrogen atmosphere. In pure oxygen not only is there no accumulation of lactic acid for a long time after excision, but a portion of the amount originally present in the resting excised muscle disappears. The same is true of the lactic acid formed in a muscle fatigued by stimulation'when it is afterwards placed in an atmosphere of pure 744 THE PHYSIOLOGY OF THE CONTRACTILE TISSUES oxygen. There is no doubt that the production of lactic acid in functional activity and its transformation into other substances are processes that go on also in the muscles of the intact body. The formation of the acid in the excised muscle, far from being a sign of death, is an index of the ' survival ' of a process by which it is normally formed, as the accumulation of it is an index of the crippling, in the absence of oxygen, of a mechanism by which it is normally transformed. The lactic acid which accumulates in the excised muscle in rigor and activity does not remain free, since blue litmoid paper is not reddened as it would be by free lactic acid. It causes a repartition of the bases at the expense of the sodium carbonate and disodium phosphate, the latter being changed into monophosphate, which, in part at least, accounts for the acid reaction to turmeric (Roh- mann). It is of great interest that this oxidative transformation of lactic acid only occurs in muscle whose structure is so far pre- served that its irritability is not lost. In minced or triturated muscle it does not take place. The relations between the heat production, the formation of carbon dioxide, and the production of lactic acid indicate that liberation of lactic acid from some precursor is an essential stage in the sudden, ' explosive ' reaction or series of reactions which precedes and induces the mechanical response to stimulation. This stage takes place whether oxygen be present or absent, and it seems to be accompanied by a considerable liberation of energy, at the expense of which alone the anaerobically contracting muscle works. It is most probable that the liberation of lactic acid follows the same course in the muscle abundantly supplied with oxygen, although it has not been shown that oxidative processes, resulting in the forma- tion of carbon dioxide, do not contribute also at this stage to the energy which is transformed into the mechanical effect. But while in the absence of oxygen the reaction stops at the formation of lactic acid, when oxygen is available the cycle is completed by restitution processes which lead to the disappearance of the lactic acid either by restoration to its original position in the precursor from which it is derived, or perhaps in the case of a portion of the lactic acid to its combustion to carbon dioxide and water. For these restitution processes oxygen is essential, and it is to be supposed that the energy required for the rebuilding of the lactic acid precursor, or, to speak more generally, for the restoration of the muscle to its original state in readiness for a fresh contraction, is derived largely from oxidations in which carbon dioxide makes its appearance. The Precursor of Lactic Acid.^ — ^What material is the lactic acid formed from ? There are reasons for thinking that lactic acid is an intermediate substance which in metabolism serves as a link between CHEMICAL PHENOMENA OF MUSCULAR CONTRACTION 745 the products of protein decomposition and carbo-hydrates, and between carbo-hydrates and fat. From what we know of the production of lactic acid both outside the body and in the intestine from carbo-hydrates, it might seem a most plausible suggestion that in the active muscle it comes from glycogen. Glycogen is the one solid constituent of muscle which has been definitely proved to diminish during activity. It accumulates in a resting muscle, especially in a muscle whose motor nerve has been cut ; but rapidly disappears from the muscles of an animal made to do work while food is withheld ; or from the muscles of an animal poisoned by strychnine,which causes violent muscular contractions. But the best evidence points the other way — e.g., in rigor mortis lactic acid is produced just as in muscular contraction. Nay, more, the amount of lactic acid (as much as 0-5 per cent, expressed as zinc lactate) produced in full heat rigor (at 40° to 45° C.) is con- stant for similar excised muscles. This " acid-maximum ' is the same when fresh muscle is at once put into rigor; or when fatigue is first induced, with formation of lactic acid, before rigor; or, finally, when the lactic acid of the fatigued muscle is caused to disappear under the influence of oxygen, and heat rigor is then brought about in the muscle (Fletcher and Hopkins). Yet in rigor mortis the quantity of glycogen is unaltered (Boehm). Further, under certain conditions an excised muscle is capable of producing a quantity of lactic acid much greater than could be derived from the glycogen contained in it. An indirect argument against the view that the lactic acid pre- cursor is glycogen has been based by Hill on the results of his studies on the heat production of surviving muscle. From the amount of heat evolved, he calculates that the precursor of lactic acid must have a heat value 10 per cent, greater than that of lactic acid. Now, the heat of combustion of dextrose is only about 3 per cent, more than that of lactic acid. He concludes that the precursor which yields lactic acid is a body of greater energy than dextrose. This, of course, does not preclude the possibility that the complex, whatever it is, from which lactic acid is liberated, contains a carbo- hydrate group. But it would not be profitable to pursue these speculations at present. The facts just mentioned suggest that it is the same precursor which yields the lactic acid developed with the onset of rigor. Further evidence of the close relations between the chemical changes occurring in contraction and those occurring in rigor will be developed in considering the latter phenomenon. The Substances metabolized in Muscular Contraction.— If the liberation of lactic acid were assumed. to be the immediate cause of the mechanical changes in muscular contraction, if the nature of the body which yields lactic acid were known, and if it were proved, which is far from being the case, that the whole of the energy con- 746 THE PHYSIOLOGY OF THE CONTRACTILE TISSUES cerned in initiating and carrying out the mechanical effect is derived from the decomposition of this precursor, the question would still confront us, What are the materials at the expense of the energy of which the muscle is restored to its original condition ready for another contraction ? If the lactic acid is used over and over again, it is indeed the metaboUsm of these substances which will be chiefly represented in the waste products given off by the muscle ; the lactic acid complex will merely represent a chemical machine through which the energy of these other substances is transformed into mechanical energy, and they will constitute the ultimate source of energy of the muscular contraction. In this sense the muscular glycogen, whether it yields lactic acid or not, is almost certainly one source of energy for the active muscle, being converted into dextrose, of course, before utilization. Dextrins and maltose, the intermediate products of this decomposition, have been detected in muscle, more maltose, indeed, than dextrose being present (Osborne), since the dextrose is rapidly oxidized. Glycogen cannot be the only source of muscular energy, for its amount is too small. For example, the heart of an average man, which weighs 280 grammes, contains about 60 grammes of solids, and among these certainly not more than i gramme of glycogen. In twenty-four hours it produces 120 calories of heat (pp. 138, 663), equivalent to the complete com- bustion of a little less than 30 grammes of glycogen. To supply this amount, the whole store of glycogen in the heart would have to be used and replaced every fifty minutes. But the accumulation of glycogen is immensely slower in the muscles of a rabbit made glycogen-free by strychnine, and therefore we have to look around for some other source of energy to supplement the glycogen. We have already brought forward evidence (p. 599) that, under ordinary circumstances, not a great deal, at any rate, of the energy of muscular contraction comes from the proteins. Of carbo-hydrates, the only one except the glycogen of the heart muscle which is at all adequate to the task of supplying so much energy is the dextrose of the blood. The quantity of blood passing through the coronary circulation has been estimated at 30 c.c. per 100 grammes of cardiac muscle per minute (Bohr and Henriques), which would be equivalent for an average man to about 120 litres in twenty-four hours. This quantity of blood will contain at least 120 grammes of dextrose, and about 32 grammes will sufi&ce to supply all the heat produced by the heart. There is no reason to suppose that this dextrose must first be changed into muscular glycogen, which only represents a certain amount of reserve carbo-hydrate. Of proteins a little less than 30 grammes would be needed, of fat a little more than 12 grammes. We see, therefore, how intense must be the metabolism that goes on in an actively contracting muscle. On any probable assumption as to the source of muscular energy, a quantity of material equal to half of its solids must be used up by the heart in twenty-four hours. Or, to put it in another way, the heart requires not less than two-fifths of its weight of ordinary solid food in a day. The body as a whole requires ^ to ^i^ of its weight. The genera] conclusions to which physiologists have been led as to the relative importance of the different food substances for CHEMICAL PHENOMENA OF MUSCULAR CONTRACTION 747 muscular work have been previously given (p. 601), and need not be repeated here. It may be added that the various food substances yield muscular energy in isodynamic relation. In other words, a given amount of muscular work requires the expenditure of approxi- mately the same quantity of chemical energy, whether it comes almost entirely from protein, or chiefly from carbo-hydrates, or chiefly from fat. Some observers have stated that the taking of even a comparatively small quantity of sugar vastly increases the capacity for muscular work as measured by the ergograph (p. 726). But although it is not to be doubted that sugar is under normal circumstances one of the most important substances used up in muscular contraction, the claim that sugar is, par excellence, the food for muscular exertion has not yet been made out. Physico-Chemical Conditions of Muscular Contraction. — For excised fresh muscle A (p. 421) has been estimated at o-68° C. But this is probably higher than in the living body, for after excision waste products, with their relatively small and numerous molecules, are still for a time produced, and are no longer removed by the blood. In salt solutions isotonic with the muscle substance — e.g., for the frog's gastrocnemius at room-temperature a 0-75 per cent, solution of sodium chloride — the resting muscle neither gains nor loses water for some hours. The active muscle behaves quite differently. When a muscle immersed in isotonic salt solution is tetanized, water enters it, leading to an increase in weight and a diminution in specific gravity (Ranke, Loeb, Barlow). The same occurs even when blood is circulated through active muscles, the blood becoming poorer in water (Ranke). This may be explained by the increase of osmotic pressure in the muscle substance which must accompany the decomposition of large molecules into small. As fatigue progresses a movement of water in the reverse direction occurs, and the muscle rapidly loses water. Exposure of the fatigued muscle for a sufficient time to an atmosphere of oxygen restores the osmotic proper- ties of the resting muscle. Striking differences have also been demon- strated in the behaviour of resting and fatigued muscle to hypotonic solutions or water. Hales observed long ago that, on injecting large quantities of water into the bloodvessels of a dog, so as to replace the blood, marked swelling of the muscles occurred. This physiological fact is well known to the pork-butchers in China, who have given it a practical, if not a very praiseworthy, application in sophisticating their product by increasing its weight (MacGowan). So long as the muscular fibres are uninjured they are permeable or impermeable lor exactly the same compounds as other animal and vegetable cells. All substances easily soluble in media like ether or olive oil readily penetrate them (Overton). To most salts they are relatively impermeable, as is shown by the fibres retaining their oiiginal volume in isotonic solutions of them. In particular, they cannot easily take up or retain the salts of the blood-plasma, otherwise the observed qualitative differences — e.g., the preponderance of potassium in the muscle and sodium in the plasma — could not be maintained. There are facts which indicate that temporary changes in the permeability to ions, not only of muscular fibres, but also of nerve fibres and other excitable structures, are concerned in their stimulation. Potassium salts after a time seem to produce an effect upon frog's muscle, which alters its permeability so that it takes up water from hypertonic solutions. 748 THE PHYSIOLOGY OF THE CONTRACTILE TISSUES Calcium salts have the opposite effect (Loeb). Sodium (and in a minor degree lithium) salts have a peculiar relation to the contraction of skeletal muscle, for which they appear to be indispensable. Yet sodium chloride produces a paralyzing action on the frog's motor nerve-endings, so that after perfusion with a solution of that salt stimulation of the motor nerve causes no contraction, or with a slighter degree of paralysis contraction only after a long interval. The effect can be counteracted by solutions containing calcium salts (Locke, Gushing). Rigor Mortis. — ^When a muscle is dying, its excitability, after perhaps a temporary rise at the beginning, diminishes more and more until it ultimately responds to no stimulus, however strong. The loss of excitability is not in itself a sure mark of death, for, as we have seen, an inexcitable muscle may be partially or com- pletely restored; but it is followed, or, where the death of the muscle takes place very rapidly, perhaps accompanied, by a more decisive event, the appearance of rigor. The muscle, which was before soft and at the same time elastic to the touch, becomes firm; but its elasticity is gone. The fibres are no longer translucent, but opaque and turbid. If shortening of the muscle has not been opposed, it may be somewhat contracted, although the absolute force of this contraction is small compared with that of a living muscle, and a slight resistance is enough to prevent it. The reaction is now distinctly acid to litmus. This is rigor mortis, the death-stiffening of muscle. An insight into the real meaning of this singular and sometimes sudden change was first given by the- experiments of Kiihne. He took living frog's muscle, freed from blood, froze it, and miaced it in the frozen state. The pieces were then rubbed up in a mortar with snow containing i per cent, of common salt, and a thick neutral or alkaline liquid, the ' muscle-plasma,' was obtained by filtration. This clotted into a jelly when the temperature was allowed to rise, but at 0° C. remained fluid. The clotting was accompanied by a change of reaction, the liquid becoming acid. An equally good, or better, method is to use pressure for the extraction of the plasma from the frozen fragments of muscle. A low temperature is essential, otherwise the plasma will coagulate rapidly within the injured muscle. A similar plasma can be expressed from the skeletal muscles of warm-blooded animals (Halliburton), and with greater difficulty from the heart. When the muscle, after exhaustion with water, is covered with a solution of a neutral salt, a 5 per cent, solution of magnesium sulphate or 10 per cent, solution of ammonium chloride being the best, certain proteins are extracted which clot or are precipitated much in the same way as the muscle-plasma obtained by cold and pressure; and the process is hastened by keeping them at a temperature of 40° C. In the extracts of mammalian muscle three chief proteins are present : paramyosinogen (v. Fiirth's myosin), coagulating by heat at 47° to 50° C. ; myosinogen (v. Furth's myogen), coagulating at 55° to 60° C, usually about 56°) ; and serum-albumin, coagulating about 73° The CHEMICAL PHENOMENA OF MUSCULAR CONTRACTION 749 serum-albumin belongs to the blood and lymph, and is not a constituent of the muscle-fibre. The most recent work on the subject is that of Botazzi, who obtained muscle juice without the addition of water or salt solutions, by rubbing muscles up with sand, and then subjecting the triturated material to a pressure of many atmospheres. He finds that, leaving out of account the serum-albumin, muscle juice contains only one protein in solution, and this corresponds upon the whole in its properties to myosinogen. A second protein, and only these two have been proved to exist in muscle juice, is not in solution, but in the form of very fine granules revealed by the ultramicroscope. This corresponds in a general way to paramyosinogen. Botazzi supposes that it repre- sents the substance of the muscular fibrils. The granules show a ten- dency even at the ordinary temperature to agglutinate and to be pre- cipitated. The process is hastened by dilution with water, removal of the salts by dialysis, addition of acids, and the agglutination and pre- cipitation are accomplished very rapidly at 45° to 55° C, giving rise to ' heat coagulation.' The protein in solution (myosinogen) is in- soluble in distilled water when thoroughly freed from salts, and is pre- cipitated by dialysis, but not so easily as paramyosinogen. The total proteins in the juice obtained by pressure varied from 5-3 to 9-5 per cent., a great deal of the muscle protein being, of course, left in the residue. The granules (paramyosinogen) constituted from a third to two-thirds of the protein in different experiments, and the true pro- portion must have been considerably higher, since on account of tneir small size the loss in separating them by filtration was great. The ' myosin ' precipitate, which rapidly forms in muscle-plasma at body temperature, is sometimes called the muscle-clot, and the liquid which is left the muscle-serum, but it would probably be better to avoid these terms, as they suggest an analogy with the coagulation of blood-plasma, which is apt to be misleading. A similar precipitate or clot seems to be formed in the interior of the muscular fibres in natural rigor and in the rapid rigor produced by heating a muscle to a little above the body- temperature. But in natural rigor the whole of the paramyosinogen and myosinogen do not undergo the change, since a certain amount of these substances can as a rule be extracted from dead muscle by saline solutions. Thus, in rabbit's muscles, before the onset of rigor mortis, 87-3 per cent, of the total protein was found to be soluble in 10 per cent, ammonium choride solution, and only 12-7 per cent, coagulated; while after rigor had occurred, 7i-5 per cent, was coagulated, and only 28-5 per cent, remained soluble (Saxl). It is not known whether in the living muscle paramyosinogen and myosinogen exist as such. It has, indeed, been stated that, if a tracing is taken from a muscle which is gradually heated, it first shortens at the temperature of coagulation of para- myosinogen, and then again at that of myosinogen, and that in frog's muscle there is an additional shortening at 40°, the temperature at which in.extracts an additional heat precipitate occurs. The conclusion has been drawn that these substances must be present as such in the living fibres, and that the successive shortenings are mechanical phe- nomena due to their heat coagulation. Similar shortenings have been described in nerve and liver tissue at about the temperatures at which the proteins in extracts of these tissues are coagulated by heat. But Meigs has shown that the supposed correspondence is far from being exact, and that muscles whose proteins have been already coagulated in a mixture of alcohol and salt solution still show the typical shortening on being heated. The heat shortening is, therefore, dependent on some other process than aggregation of the particles of coagulable protein. 750 THE PHYSIOLOGY OF THE CONTRACTILE TISSUES Certain analogies between rigor and muscular contraction were early pointed out. In both there is (i) shortening; (2) heat-pro- duction; (3) formation of lactic acid; (4) discharge of carbon dioxide ; (5) electrical changes. As regards the production of lactic acid, there is reason to believe that the process is fundamentally the same as in contraction, and the study of rigor, especially of certain of the artificially induced forms — e.g., heat rigor — ^in relation to the liberation of lactic acid, carbon dioxide, and heat, has thrown light upon the changes normally occurring in muscle. Another analogy might be forced into the list by anyone who was deter- mined to see only rigor in contraction: the rigor passes oft as the contraction passes off, although the ' resolution ' of a rigid muscle takes days, the relaxation of an active muscle a fraction of a second. The disappearance of rigor is not dependent on putrefaction; it takes place when growth of bacteria is prevented (Hermann). Possibly it is connected with autolytic processes due to intracellular ferments (p. 588). Why does coagulation of myosin occur at the death of the muscle ? To this question no clear answer can be given. Some have looked on the process as analogous to the clotting of blood when it is shed, and it has even been suggested that just as a fibrin ferment is developed when the leucocytes and blood-plates begin to die, a myosin ferment, which aids coagulation, is developed in dead or dying muscle. But no proof has been given of the existence of such a ferment. And it is easy to make too much of the apparent analogy between the clotting of muscle and the clotting of blood, for there are differences as well as resemblances. For instance, the addition of potassium oxalate does not prevent coagulation of muscle extracts, as it does of blood and blood-plasma. If the development of lactic acid in the muscle is not the primary cause of the coagulation which constitutes the essential feature of rigor mortis, it seems to be closely related to it. For when excised muscles are abundantly supplied with oxygen, no lactic acid accumulates in them, and the final loss of excitability of the muscle is not followed by rigor. In any case, direct precipitation of hitherto unclotted muscle proteins may be induced by the acid, or the acid salts formed in its presence. Deficiency of oxygen is associated with the occurrence of rigor mortis, as it is with the accumulation of lactic acid, and a developing rigor can be abolished by oxygen, and its onset long or indefinitely delayed. When strict aseptic technique is observed an excised sartorius muscle of the frog may remain irritable in sterile Ringer's solution, even without oxygenation, for as long as three weeks (Mines). Various influences affect the onset of rigor. Fatigue hastens it; heat has a similar effect; the contact of caffeine, chloroform, and other drugs causes most pronounced and immediate rigor. CHEMICAL PHENOMENA OF MUSCULAR CONTRACTION 751 Blood applied to the cross-section of a muscle first stimulates the fibres with which it is in contact, and then renders them rigid. But it is to be remembered that normally the blood does not come into direct contact even with the sarcolemma, much less with its contents. The effect of heat is of special interest. A skeletal muscle of a frog, like the gastrocnemius, if dipped into physiological saline solution at 40° or 41° C. goes into rigor at once; the frog's heart requires a temperature 3° or 4° higher; the distended bulbus aortas can withstand even a temperature of 48° for a short time. An excised mammahan muscle passes into immediate rigor at 45° to 50° In heat rigor the reaction of the muscle becomes strongly acid owing to the formation of lactic acid, and the evolution of carbon dioxide is also increased. Tlie total discharge of carbon dioxide in heat rigor induced at 40° amounts to 35 to 40 c.c. per 100 grammes of muscle. An additional 15 to 20 per cent, is obtained on heating to 75° C. to completely coagu- late the proteins, and a further 15 to 20 per cent, on heating to about 100° C. When a muscle is scalded by being suddenly immersed in boiling salt solution, lactic acid is not formed, but carbon dioxide to the amount of 60 to 70 per cent, is discharged. An excised muscle kept in oxygen for many hours, during which it has discharged several times as much carbon dioxide as is ever Uberated by heating, still yields the normal discharge on heating whether to 40° C. or to 100° C. On the other hand, previous survival in an anaerobic atmosphere (of nitrogen) reduces greatly or abolishes the yield of carbon dioxide at 40° C, although not that at 100° C, the sum of the carbon dioxide given off to the atmosphere of nitrogen and that given off on heating to 100° C. being about equal to the total amount which would have been dis- charged by a freshly-excised muscle on heating first to 40° C. and then to 100° C. If acid is added to a fresh muscle at about 0° C. even more carbon dioxide is liberated than in heat rigor, while the yield of lactic acid is, even after many hours, very little increased above the normal amount for fresh resting muscle. When the acidified muscle after the discharge of the carbon dioxide is now heated to 40° C, the yield of lactic acid is increased, but only traces of carbon dioxide are given off. From these and similar observations, Fletcher concludes that the carbon dioxide discharged during heat rigor at 40° C. is pre-existent carbon dioxide set free from carbonates or other compounds by the lactic acid known to be produced in heat rigor. The carbon dioxide discharged at 75° and 100° C. he regards as held by muscle colloids or in combination with amino-acid groups. These results render untenable the ' inogen ' theory (p. 266), with its assumption that ' intramolecular oxygen ' is stored away in the muscle, which was largely based upon erroneous observations on the discharge of carbon dioxide from heated muscles. According to this theory, carbon dioxide and lactic acid were supposed to arise from a common precursor into which oxygen had been previously introduced. 752 THE PHYSIOLOGY OF THE CONTRACTILE TISSUES The production of heat in heat rigor is also of great interest. Hill has shown that it amounts to from o-6 to i-o gramme calorie per gramme of muscle. Of this no more than 0-05 calorie can be due to the heat of neutralization of lactic acid by the sodium bicarbonate in the muscle, with which it reacts as soon as it is liberated. The rest of the heat is associated with the chemical reaction by which lactic acid is formed from its precursor, a reaction which, there is every reason to believe, is the same as that which occurs in muscular contraction. The heat production can only be due in very slight degree to the physical alteration (clotting or precipitation) of the muscle proteins. The so-called rigor caused by water, which is not a true rigor, causes no increase in the carbon dioxide given off. Chloroform, on the other hand, produces a marked increase in the carbon dioxide production, and this is evidently related to its action in hastening the onset of rigor. Rigor mortis is to some extent in- fluenced by the nervous system, for section of its nerves retards the onset of rigor in the muscles of a limb. Ante-mortem stimula- tion of the peripheral ends of the vagi, even, with currents too weak to cause a perceptible effect upon the heart-beats, prolongs the period of spontaneous contraction and the irritability of the ven- tricles after death, and retards the onset of rigor (Joseph and Meltzer). Cold rigor is obtained when frog's muscles are cooled to —15° C. The muscles remain perfectly translucent. They do not recover their irritability on thawing, but if cooled only to — 7° C. they recover (Folin). In a human body rigor generally appears not earlier than an hour, and not iater than four or five hours, after death. In ex- ceptional cases, however, it may come on at once, and the annals of war and crime contain instances where a man has been found after death still holding with a firm grip the weapon with which he had fought, or which had been thrust into his hand by his murderer (so-called cataleptic rigor). It is related that after one of the battles of the American Revolutionary War some of the dead were found with one eye open and the other closed as in the act of taking aim. A high temperature favours a rapid onset; a body wrapped up in bed will, other things being equal, become rigid sooner than a body lying stripped in a field. Muscular exhaustion, as we have said, is another favouring condition: hunted animals and the victims of wasting diseases go quickly into rigor. It is a rule, but not an invariable one, that rigor, when it comes on quickly, is short, and lasts longer when it comes on late. All the muscles of the body do not stiffen at the same time; the order is usually from above downwards, beginning at the jaws and neck, then reaching the arms, and finally the legs. After two or three days the rigor disappears in the same order. The position of the CHEMICAL PHENOMENA OF MUSCULAR CONTRACTION 753 limbs in rigor is the same as at death; the muscles stiffen without any marked contraction. This can be strikingly shown on a newly- killed animal by cutting the tendons of the extensors of one foot and the flexors of the other ; when natural rigor comes on, the feet remain just as they were. If heat rigor, however, is caused, the one foot becomes rigid in flexion and the other in extension; and the contraction-force is considerable, although not so great as that of an electrical tetanus in a living muscle. The Possibility of Recovery of Muscles after Rigor. — ^When the circu- lation in the hind legs of rabbits is interrupted by compression or ligation of the abdominal aorta (Stenson's experiment), the muscles lose their excitability, but speedily recover, if they have not been deprived of arterial blood for too long a time, when the blood is again allowed to reach them. A longer interruption of the circulation leads not only to total inability to respond to stimulation, but also to rigor, and most observers are agreed that, as regards the skeletal muscles at least, this is the irrevocable end of excitability. Brown-Sequard, indeed, stated that after the full development of rigor in the rabbit's muscles (Stenson's experiment), and also in the hand of an executed criminal through which an artificial circulation was established, recovery ensued. But probably the rigor was incomplete or did not involve all the fibres. In heart muscle the conditions appear to be somewhat different, and Heubel has alleged that rhythmical contractions of the frog's heart can be restored by filling its cavity with blood, after rigor has been caused by heat and in other ways, and we have already seen that the same is true of the mammalian heart after the onset of rigor. Excised frog's muscles which have undergone rigor mortis become less stiff when exposed to an atmosphere of oxygen. 48 CHAPTER XIV NERVE The voluntary movements are originated by efferent or outgoing impulses from the brain, which reach the muscles along their motor nerves. The involuntary movements and the secretions are in many cases able to go on in the absence of central connec- tions, but are normally under central control. Afferent impulses are continually ascending to the cord and brain from the skin, joints, bones, muscles, and organs of special sense like the eye and the ear. Everywhere the connection between the nervous centres and the peripheral organs, and between different parts of the central nervous system, is made by nerve-fibres. Those which run outside the brain and cord are called peripheral nerve-fibres to distinguish them from the intracentral fibres of the central nervous system itself. In this chapter we propose to consider certain of the general properties of nerve-fibres. Most of our knowledge of these proper- ties has been derived from experiments on the peripheral, and particularly the peripheral motor nerves ; but there is every reason to believe that the main results are true of all nerve-fibres, afferent and efferent, peripheral and central. What we call nerve-fibres were known and named, and many im- portant facts in their physiology discovered, long before their true morphological significance was recognized. The researches of recent years have shown that every nerve-fibre is, as regards its essential con- stituent the axis-cylinder, a process of a nerve-cell. The nerve-cells, each of which, including all its processes, may be conveniently termed a neuron, are the essential elements of the nervous system. The cell- bodies of most of the neurons are situated in, or in close relation to, the spinal cord and the brain, and therefore the detailed description of them will be reserved till we come to treat of the central nervous system (see p. 822 and Figs. 318 to 330). It is enough to say here that in general a nerve-cell gives off two kinds of processes: (i) one or more dendrites or protoplasmic processes, which repeatedly bifurcate like the branches of a tree into thinner and thinner twigs, and extend only for a relatively short distance from the cell-body; (2) an axis-cylinder process or axon, which as a rule runs for a considerable distance withoiit altering its calibre, and either gives off no branches (as in the peripheral nerves) or only a comparatively small number of lateral twigs (col- 754 THE NERVE-IMPULSE OR PROPAGATED DISTURBANCE 755 laterals). Ultimately the axis-cylinder process and its collaterals, if it has any, end by breaking up into a brush, a plexus or a feltwork or basketwork of fibrils. The axons of different nerve-cells vary greatly in length. Some terminate within the grey matter of the brain or spinal cord not far from their origin; others run in the white tracts of the central nervous system or in the peripheral nerves for half the height of a man. AH except the shortest axis-cylinder processes become clothed at a little distance from the cell-body with a protective covering, which continues to invest them (and their collaterals) throughout the rest of their course, disappearing only when they begin to break up at their terminations. An axis-cylinder process (spoken of simply as the axis-cylinder, when considered apart from the nerve-cell) constitutes, with its covering, a nerve-fibre. The axis-cylinder is the essential conducting part of the fibre, for it is present in every nerve-fibre, running from end to end of it without break, and towards the periphery it is alone present. It is made up of fine longitudinal fibrils embedded in interstitial substance (Fig. 319, p. 823). Such a fibrillar structure is best shown after treatment of the nerve- fibres with certain reagents, although it is certain that it exists pre- formed in the living fibres. Section I. — The Nerve-Impulse or Propagated Disturbance: ITS Initiation and Conduction. So far as we know, the only function of nerve- fibres is to conduct impulses from nerve-centres to peripheral organs, or from peripheral organs to nerve-centres, or from one nerve-centre to another. In the normal body these impulses never, or only very rarely, originate in the course of the nerve-fibres; they are set up either at their peripheral or at their central endings. By artificial stimu- lation, however, a nerve-impulse may be started at any part of a fibre, just as a telegram may be dispatched by tapping any part of a telegraph wire, although it is usually sent from one fixed station to another. Nature of the Nerve- Impulse. — ^What the nerve-impulse actually consists in we do not know. All we know is that a change or dis • turbance of some kind, of which the most evident token is an electrical change, passes over the nerve with a measurable velocity, and gives tidings of itself, if it is travelling along efferent fibres — that is, out from the central nervous system — by the contraction or inhibition of muscle or by secretion; if it is travelling along afferent fibres — that is, up to the central nervous system — by sensa- tion, or by reflex muscular or glandular effects. Whether the wave which passes along the nerve is a wave of chemical change (such, to take a very crude example, as runs along a train of gunpowder when it is fired at one end), or a wave of mechanical change, a peculiar and most delicate molecular shiver, if we may so phrase it, or a shear in a definite direction along the colloidal substance of the axis-cylinder (Sutherland), there is no absolutely definite experimental evidence to decide. An 756 NERVE electrical change accompanies the nerve-impulse travelling at the same rate, and although this is to be distinguished from the impulse itself, there is little doubt that the latter is essentially connected with a disturbance of the electrical equilibrium of the nerve- substance. An attempt has been made to settle the question by determining the temperature coefficient of the velocity of conduction of the impulse — i.e., the quantity which measures the change of velocity for a given change of temperature. For most physical processes the quotient velocity at Tot -I- 10 , _ . . , j. ■ 4. j. ^— ^rr tt; , where Tw xs any given temperature, is not greater than I -2, while for frog's sciatic nerve the temperature coefficient for the most part lies between 2 and 3 (Snyder). The mean value of a large number of observations is i'79, with Tn= 8° to 9° C. (Lucas). For the pedal nerve of the giant slug the mean value of the temperature coefficient is i-yS (Maxwell). In other words, while for the majority of physical processes an increase of 10° C. increases the velocity of the process by at most one-fifth, the same increase of temperature increases the velocity of conduction of the nerve-impulse by four-fifths, or even more. While it is true that it may not be entirely safe to apply such a criterion to a biological process which need not be either entirely chemical cr entirely physical, and very likely is a complex one, the suggestion, £0 far as it goes, is undoubtedly in favour of the chemical hypothesis. That chemical changes go on in living nerve we need not hesitate to assume; and, indeed, if the circulation through a limb of a warm- blooded animal be stopped for a short time, the nerves lose their excitability. Even the nerves of cold-blooded animals gradually become inexcitable and incapable of conduction when placed in an oxygen-free medium, as the oxygen already contained in the tissue is exhausted. The excitability and conductivity of the nerve are restored by oxygen. It is clear, then, that even a resting nerve requires oxygen, and it can be shown that the loss of function is acceleiated by stimulation in the absence of oxygen. But the metabolism is very slight compared with that in muscle or gland. Until recently even in active nerve no measurable production of carbon dioxide had ever been observed, nor, in fact, had any chemical difference between the excited and the resting state ever been unequivocally made out. However, it has been announced that by the aid of an extremely delicate method of estimating small quantities of carbon dioxide, a measurable production of carbon dioxide can be detected even in resting frogs' nerves, and that this pro- duction is increased two to three times on stimulation (Tashiro). This result is somewhat puzzling in view of the fact that neither in cold- blooded nor in mammalian nerves is there any sensible rise of temper- ture during stimulation. With the apparatus shown in Fig. 265 (an electrical resistance thermometer or bolometer whose use depends upon the fact that the electrical resistance of a metallic conductor varies with its temperature) an increase even of 0-0003° C. in the temperature of the sciatic nerves of dogs could not be detected during tetanization. RoUeston failed to find evidence of a rise of even 0-0002° C. in frog's nerves during, stimulation. And according to the latest investigation with a more suitable and much more sensitive thermo-electric arrange- ment, the passage of a single nerve impulse along a frog's nerve cannot be associated with an increase of temperature in the nerve of even the hundredth million of a degree (A. V. Hill). The difficulty of inducing fatigue in nerves under ordinary conditions has been considered a strong THE NERVE-IMPULSE OR PROPAGATED DISTURBANCE 757 support of the physical nature of the conduction process. Neverthe- less, it is possible to show by special methods that nerve can be tempor- arily fatigued, although it recovers very rapidly. When a meduUat^d nerve is stimulated, a brief period ensues during which it refuses to respond to a second stimulus. This refractory period is normally very short — ^not more than 0-002 second for the frog's sciatic. But it can be greatly prolonged by cold, asphyxia, or anaesthesia, especially by the alkaloid yohimbine (Tait and Gunn), and when the refractory period is thus prolonged, fatigue phenomena are readily induced by stimulation. And while the nerves of warm-blooded animals at body temperature and those of cold-blooded animals at about 32° C. can hardly be shown to undergo fatigue when tetanized in atmospheric air, fatigue phe- nomena are easily elicited when the temperature is lowered even although air is supplied (Thorner). Stimulation of Nerve. — With some differences, the same stimuli are effective for nerve as for muscle (p. 711) ; but chemical stimula- tion IS not in general so easily obtained. The so-called thermal Fig. 265. — Electrical - Resistance Thermometer (Natural Size) as used for investigating heat-pro- duction in mammalian nerves in situ. A, a piece of hard rubber in the hook-shaped part of which the fine platinum wire P is fixed, and covered with insulating varnish; c, c, thick copper wires connected with P, fastened in grooves, and covered with paraffin. Above they end in contact with the small binding posts, Bj, Bg. B is a hard rubber sliding piece, with a slot s. When B is in position the screw, u, projects througli the slot. By a nut on this screw B is fixed on A when the nerve has been arranged in the groove. B_ B ( ifa B ED stimulation is not a real stimulation due to the sudden change of temperature. The irregular contractions of the muscle caused by the locdl application of heat to the nerve are dependent on desicca- tion of the nerve. I Chemical Stimulation. — When hyper- or hypotonic solutions are em- ployed, the withdrawal or entrance of water may be an important factor. For salts which penetrate the fibres with equal difficulty this factor can be eliminated by applying them as isotonic solutions. There is evidence that chemical stimulation proper, as distinguished from the stimulation produced by changes in the water content of the fibres by osmosis, is connected with the electrical charges on the dissociated ions of the salts (p. 422). Electrical stimulation, indeed, may only be a variety of chemical stimulation (Loeb, Mathews, etc.). Mechanical Stimulation may be applied to a nerve by allowing a small weight to fall on it from a definite height or by permitting mercury to drop upon it from a vessel with a fine outflow tube. A regular tetanus may thus be obtained. Tigerstedt found that the smallest amount of work spent on a frog's nerve which would sufiice to excite it was a little less 758 NERVE than a gramme-millimetre — ^that is, the work done by a gramme falling through a distance of a millimetre, or (taking an erg as equivalent to tbWo gramme-centimetre) about loo ergs. No doubt a great part of this is wasted, as a much smaller quantity of work done by a beam of light on the retina or by an electrical current on an isolated nerve, both of which may be supposed to act more directly on the excitable con- stituents, suffices to cause stimulation. Thus, the work done by the minimal, natural or specific, stimulus for the retina in the form of green light may be as little as — j erg (S. P. Langley), or only one-ten-thousand- millioneth part of the minimum work necessary for raechanical stimula- tion. Again, with electrical stimulation (closure of a voltaic cuirent, or condenser discharges) it has been shown that an amount of work equal to — ^ erg may be enough to cause excitation of a frog's nerve. This is ten thousand times as great as the minimal luminous stimulus, but a million times less than the minimal mechanical stimulus. The laws of electrical stimulation for nerve are essentially the same as those we have already discussed for muscle (p. 715). The voltaic current stimulates a nerve, as it does a muscle, at closure and opening. During the flow of the current, so long as its intensity remains constant, there is as a rule no excitation, or at least none which is propagated along the nerve, so that the muscles supplied by it remain uncontracted. But under certain conditions — ^for example, when the nerve is .more excitable than usual (as is the case with nerves taken from frogs which have been long kept in the cold) — a closing tetanus may be seen while the current continues to pass through the nerve, and an opening tetanus after it has ceased to flow, just as when the current is led directly through the muscle. Sensory nerve-fibres, too, are stimulated by a voltaic current during the whole time of flow. Induction shocks are relatively more powerful stimuli for nerve than the make or break of a voltaic current. The opposite, as we have seen, is true of muscle; and, ■upon the whole, we may say that muscle is more sluggish in its response to stimuli, and is excited less easily by very brief currents, than nerve is. An apparent illustration of this difierence is the fact that the nervous excitation has no measurable latent period, while muscular excitation has. But it is quite possible that, if the conditions of experi- ment were as favourable in nerve as in muscle, a sensible latent period might be found here too. In nerve as in muscle, strength of stimulus and intensity of response correspond within a fairly wide range, when we take the height of the muscular contraction or the amount of the negative variation (p. 797) as the measure of the nervous excitation. Summation of stimuli, super- position of contractions, and complete tetanus, are caused by stimulating the muscle through its nerve, just as by stimulating the muscle itself (P- 730). Excitability of Nerve. — It has usually been stated that the ex- citability of frog's nerve, as measured by the muscular response to stimulation, is increased by rise of temperature, and diminished by fall of temperature. It has, however, been shown that this increase of excitability is only apparent, and due to the strengthening of the current by diminution of the resistance, since the resistance of all animal tissues, like that of electrolytic conductors in general, diminishes as the temperature rises (Gotch). When precautions THE N^RVE-IMPULSE OR PROPAGATED DISTURBANCE 759 are taken to keep the current intensity the same at the various temperatures compared, it is found that cooling of a (frog's) nerve, even to 5° C, increases the excitability for currents of long duration (several hundredths of a second). It has, indeed, been shown both for muscle and for nerve that the cooler tissue requires a smaller current strength for its excitation when the current is of long dura- tion. With brief currents this effect is masked, either partially or completely, by the greater increase of current strength needed in the case of the cooler tissue to compensate f ora given decrease m duration (p. 763) (Lucas and Mines). This is the reason that for induction shocks or voltaic currents of short duration, the excitabihty of the nerve seems to be increased by a rise of temperature (up to about 30° C. in the case of frog's nerve), and diminished by cooling. Drying of a nerve at first increases its excitability ; and the same is true of separation of a nerve from its centre. In the latter case the increase of irritability begins at the proximal end of the nerve, and travels towards the periphery. As time goes on, the excita- bility diminishes, and ultimately disappears in the same order (Ritter-Valli Law). At a certain stage it may be found that a given stimulus causes a smaller and smaller contraction the farther down the nerve — that is, the nearer to the muscle — ^it is applied. On this was based the now abandoned ' avalanche theory,' according to which the impulse continually unlocked new energy as it passed along the nerve, and so gathered strength in its course like an avalanche. It is now known that no material change takes place in the intensity of the excitation while it is being propagated along a normal uninjured nerve. For instance, experiments on the phrenic nerve, in its natural position, and with all its connections intact, have shown that with a given strength of stimulus the amoimt of contraction of the diaphragm is the same whether the nerve be excited in the upper, middle, or lower portion of its course. In the above experiment on the isolated, and therefore injured, nerve, the contraction varies in height with the distance of the point of stimulation from the muscle, not because the excitation grows as it travels, but because it is already greater at the moment when it sets out from a point near the central end of the nerve than at the moment when it sets out from a point near the muscle. Electrotonus. — Although the constant current does not, unless it is very strong or the nerve very irritable, cause stimulation during its passage, it modifies profoundly the excitability and conductivity of the nerve. In the neighbourhood of the kathode the excitability is increased (condition of katelectrotonus), while around the anode it is diminished (anelectrotonus). Immediately after the opening of the current these relations are for a brief time reversed, the excitability of the post-kathodic area (area which was at the kathode during, the flow) being diminished, and that of the post-anodic 760 NERVE increased. In the intrapolar area there is one point the excita- bility of which is not altered. This indifferent point, as it is called, shifts its position when the intensity of the current is varied, moving towards the kathode when the current is increased, towards the anode when it is diminished. It is only under certain definite conditions that these phenomena, first d ascribed by Pfliiger, appear in their purity and uncomplicated by other changes. The nerve should be quite fresh, the current a weak or at most a moderately strong one, and the stretch of nerve employed should be as far as possible from the cross section, and from the cross sections of branches. The middle region of the frog's sciatic nerve is the best. When all these conditions are fulfilled, the whole stretch of nerve in katelectrotonus — i.e., the part on both sides of the kathode and at the kathode itself shows an increased stimulation effect, the more pronounced the nearer to the kathode the point of stimulation. This condition, however, only lasts an instant. Then the excitability begins to sink sharply first at the kathode, then on both sides of it, till it ultimately becomes decidedly less than the initial excitability. This secondary depression of excitability, always most marked at the very kathode, is just as con- Fig. '266. — Katelectrotonus. — Weak tetanus of muscle (the right-hand elevation), greatly intensified in katelectrotonus of the motor nerve (the left-hand elevation). Fig. 267. — Anelectrotonus. Strong tetanus of muscle (left-hand ele- vation), lessened in strength by anelectrotonic condition of the mo- tor nerve (right-hand elevation). stant a phenomenon as tlie preliminary increase. The stronger the cur- rent the more profound is the depression, the more quickly it is de- veloped, and the greater is the distance to which it spreads along the nerve. With a certain strength of current the depression appears so rapidly that the preliminary increase of excitability may be completely missed. When the current is opened the excitability quickly increases again, but with strong currents it may remain depressed for a while. At the anode changes in the reverse direction may be observed, although they are less pronounced than at the kathode. Thus at the anode during the passage of the current the initial depression of the excitability tends to give place to an increase (Werigo). These statements have been made on the strength of experiments in which the height of the muscular contraction was taken as the index of the excitability of the nerve at any given point. It is difficult, however, to disentangle the effects of alterations in the excitability from the effects of alterations of conductivity — i.e., of the power of a portion of the nerve to conduct an impulse set up elsewhere. Whether these two properties are distinct or not is a question which will be considered a little lEiter on. But it is perfectly clear that in deducing conclusions THE NERVE-IMPULSE OR PROPAGATED DISTURBANCE ybi as to the effect produced on a nerve by excitation at a given point from the resultant effect on the muscle to which the nerve is attached (or on a galvanometer or electrometer if we are following the effect by means of the electrical changes), we must know whether the change set up in the nerve at the point of excitation can pass freely along the nerve to the muscle or to the point at which it is led off to the galvanometer. Now, changes of conductivity are certainly produced in a nerve by th^ constant current, which even oatlast its flow. For all currents above a certain strength the conductivity at the kathode and in its neigh- bourhood is eventually dim- inished, and with currents still only moderately strong the block deepens into im- pissability. The conduc- tivity at the anode is, during this stage, higher than at the kathode, so that at the time of full, kathodic block the nerve - impulse still passes through the region around , the positive pole . With still s.ronger currents the con- ductivity here, too, dimin- ishes, until the anode as well as the whole intrapolar re- gion is blocked. After the opening of the current, the relation between kathodic and anodi: conductivity is reversed, for now the post- kathodic region conducts the nerve-impulse relatively bet- ter than the post-anodic. It will be seen that these changes of. conductivity up- . on the whole run parallel to, the (secondary) changes of excitability, depression of excitability corresponding to depression of conductivity, and vice versa. With the relatively strong currents re- Fig. 268. — Diagram of Changes of Excitability and Conductivity produced in a Nerve by a Voltaic Current. E, changes of excitability during the flow of the current, according to Pfliiger. These are seen most typically witli the .weaker currents. In particular the in- creased excitability at and around the kathode when the current is strong very quickly gives place to depression. The ordinates drawn from the abscissa axis to cut the curve repre- sent the amount of the change. C(i), changes of conductivity found shortly after the closure and during the flow of a moderately strong current. Conductivity greatly reduced around kathode; little affected at anode. C(2), changes of conductivity during flow of a very strong current. Conductivity reduced both in anodic and kathodic regions, but less in the former. C^, changes of conductivity just after opening a moderately strong current. Con- ductivity greatly reduced in region which was formerly anodic; little affected in region for- merly kathodic. ' quired to produce decided effects on the conductivity, any preliminary change in the same sense as the (so-called primary) effects on the excitability (increase at kathode, decrease at anode) might be expected to be fleeting, and theiefore less easy to detect. The above facts serve to explain the manner in which the effects of stimulation of a nerve with the constant current vary with the strength 762 NERVE and direction of the stream. These effects, so far as the contraction of the muscles supplied by the nerve is concerned, have been formulated in what has been somewhat loosely termed the law of contraction. In this fomiula the direction of the current in the nerve is commonly dis- tinguished by a thoroughly bad but now ingrained phraseology, as ascending when the anode is next the muscle, and descending when the kathode is next the muscle. Current. Ascending. Descending. M. B. M. B. Weak Medium - Strong c c c c c c c c Here M means traction follows.' make,' B, ' break,' of the current; C means ' con- The explanation generally given is as follows : Wherever there is an increase of excitability sufficiently rapid and sufficiently large, stimula- tion is supposed to take place; where there is a fall of excitability, stimulation does not occur. Accordingly, at closure the kathode stimu- lates — the anode does not; while at opening, the anode, at which the depressed excitability jumps up to normal or more, is the stimulating pole; the kathode, at which it declines to normal or under it, is inactive. With a weak current, (i) contraction only occurs at make, and (2) the direction of the current is indifferent. The explanation of the first fact is that the make is a stronger stimulus than the break, and when the current is weak enough the break is less than a minimal stimulus. No sensible change of conductivity is caused by weak currents, which suffices to explain (2). With a ' medium ' current, contraction occurs at make and break with both directions. Here the break excitation is effective as well as the make. With anode next the muscle (ascending current), there is, of course, nothing to prevent the opening excitation, which starts at the anode, from passing down the nerve and causing contraction ; and since there is no block around the anode or in the intrapolar region with ' medium ' currents, there is nothing to keep the closing (kathodic) excitation from reaching the muscle too. With the kathode next the inuscle (descending current), the closing excitation, which starts from the kathode, has no reigon of diminished conductivity to pass through, nor has the opening (anodic) excitation, for the kathodic block, caused by moderately strong currents, is removed as soon as the current is broken. With ' strong ' currents there are only two cases of contraction out of the four, just as with ' weak,' but for very different reasons. There is a break-contraction with ascending, and a make-contraction with descending current. With ascending current the anode is next the muscle, and the break-excitation starting there has nothing to hinder its course. The make-excitation, although as strong or stronger, has to pass through the whole intrapolar region and over the anode, and here the conductivity is depressed and the nerve-impulse blocked. With descending current the kathode is next the muscle, and there is no hindrance to the passage of the make-excitation. The break-excitation, however, has to traverse the whole intrapolar region, and this does not THE NERVE-IMPULSE OR PROPAGATED DISTURBANCE 763 at once, after a strong current, become passable. The break-excitation, accordingly, cannot get through to the muscle. A formula similar to the law of contraction has been shown to hold for the inhibitory fibres of the vagus (Donders), ' inhibition ' being substituted for ' contraction.' There is also some evidence that a similar law obtains for sensory nerves. It is not difficult to see that with currents of brief duration the break follows so quickly on the make that interference of their opposed effects may occur. This is the reason — or, at least, one reason — ^why, above a certain frequency, a muscle or nerve ceases to respond to all of a series of rapidly recurring electrical stimuli (p. 732). It is also the reason why, with single very brief stimuli, a greater current intensity must be employed in order to cause excitation than when the duration of the stimulating current is greater (Woodworth, Lucas). The Law of Contraction for Nerves ' in Situ.' — ^When a nerve is stimu- lated without previous isolation — in the human body, for instance, through electrodes laid on the skin — ^the current will not enter and leave it through definite small portions of its sheath, nor will it be possible to make the lines of flow nearly parallel to each other and to the long axis of the nerve, as is the case in a slender strip of tissue when there is a considerable dis- tance between the electrodes. On the contrary, when, as is usual in electro-therapeutical treatment, a single electrode — say, the positive — is placed over the position of the nerve, and the other at a distance on some convenient part of the body, the current will enter the nerve by a broad fan of stream-lines cutting it more or less obliquely, and pass out again into the surrounding tissues; so that both an anode (surface of en- trance) and a kathode (still larger surface of exit) will correspond to the single positive pole. Similarly, the single nega- tive electrode will correspond to an anodic surface where the now narrowing sheaf of lines of flow enters the nerve, and a smaller kathodic surface, where they emerge. Even if the two elec- trodes were on the course of the nerve, the stream-lines would still cut it in such a way that each electrode would correspond both to anode and kathode (Fig. 269). It is impossible under these circumstances to take account of the direction of a current in a nerve, or to connect direction with any specific effect. When we place one of the electrodes over the nerve and the other at a distance, the law of contraction only appears in a disguised form; for since a kathode and an anode exist at each pole, there is, with a current of sufficient strength (' strong current '), excitation at each, both at make and break. The negative make contraction is, however, stronger than the positive, for the excitation corresponding to the latter arises at the secondary kathodic surface, where the sheaf of current-lines spreading from the positive electrode passes out of the nerve. Now, this is much larger than the primary kathodic surface, through which the narrow wedge of stream-lines passes to reach the nega,tive electrode, and the current density at the latter is accordingly muchgreater. The Fig. 269. — Diagram of Lines of Flow of a Current passing through a Nerve. A, an isolated nerve;B,anervemx»i«. Secon- dary anodes ( + ) are formed where the current re-enters the nerve below the negative elec- trode after passing through the tissues in which it is embedded and secondary kathodes ( - ) where the current passes out of the nerve into the surrounding tissues below the positive elec- trode. 764 NERVE positive break-contraction is, for a similar reason, stronger than the negative. With a ' weak ' current, the only contraction is a closing one at the kathode; with a ' medium ' current there are both opening and closing contractions at the positive pole, and a closing but no opening con- traction at the negative (Practical Exercises, p. 818). Conductivity of Nerve. — The disturbance which is called the nerve-impulse, once set up, is propagated along the fibres. Are the changes in the nervous substance involved in the initiation of the disturbance at a given point identical with those involved in its transmission from one point to the next, or are they different ? This is a question which has been much discussed, and many attempts have been made to prove that the two processes can be dissociated by acting on nerves with substances like carbon dioxide, ether, and alcohol, which gradually suspend their functions, by cutting nerves off from the circulation and allowing them to die gradually, by depriving them of oxygen and in other ways. Many of the results obtained from such experiments seem at first sight to be favourable to the view that the local change is different from the propagated disturbance. Nevertheless, careful examination of the results on which such statements are based indicates that none of them supplies a crucial test of the question at issue. For example, when a stretch of frog's sciatic nerve is treated with ether or another of the narcotics which act on nerve, and the strength of stimulus determined which is necessary to elicit a contraction when applied to an untreated portion more remote from the muscle than the narcotized area, this strength is found, for some time after the application of the narcotic, to be just the same as it was previous to the application. ' The conductivity '' of the narcotized stretch appears to be unaltered. On the other hand, the stimulus, when . applied within the narcotized region, must be strengthened, and the narcotic appears to have diminished the ' excitability ' of the nerve. When the narcotic has acted for a longer time, the reverse effect appears. No stimulus, however strong, applied to the central non-narcotized stretch will cause a contraction, the ' conductivity ' having been apparently totally abolished by the narcotic, whereas a strong stimulus applied in the narcotized region will still cause a contraction, showing that ' excitability ' still remains. As to the facts there is general agreement; it is their interpretation which is in doubt. Now, it has been shown that in passing along a narcotized nerve the propagated disturbance diminishes in pro- portion to the length of nerve which it has to traverse. Accordingly in the second stage of narcosis the failure of the stimulus applied to the upper part of the nerve to elicit a contraction is explained most naturally as due to the extinction of the disturbance, which must pass through the whole narcotized region, whereas the dis- turbance set up by stimulation in that region succeeds in reaching THE NERVE-IMPULSE OR PROPAGATED DISTVRBANCE 765 the muscle, since it has a shorter stretch .of narcotized nerve to traverse. This experiment, then, in reahty affords no proof that excitability and conductivity can vary independently. Facts are also known, to which allusion need not be made here, but which greatly modify the ordinary interpretation of the experimental results obtained in the first stage of narcosis, and upon the whole it may be said that these direct methods of determining the question- have failed to yield a satisfactory answer. Indirect evidence exists, however, that the local process initiated by stimulation is not quite the same as the process involved in the propagation of the disturbance (Lucas). Thus, a brief current too weak to set up a propagated disturbance, nevertheless causes some change at the point of stimulation, since a second cixrrent, also too weak to be effective by itself, will, when thrown in a short time after the first, cause a disturbance which is propagated along the nerve. There is good reason to believe that the change produced by the first current is not the same in kind as that produced by the second, only weaker, but that it is inherently different in quality. Above all, it is a local change incapable of being itself propagated, but constituting the necessary prelude to the starting of the propagated disturbance. Lucas has called this preliminary local effect the ' local excitatory process. ' There are many facts which indicate that the capacity of different functional or anatomical groups of nerve-fibres for responding to stimu- lation and for conducting the nerve-impulse can be differently affected by one and the same influence. For example, piessure abolishes the conductivity of sensory fibres sooner than that ot motor fibres. Cocaine locally applied to a nerve diminishes or abolishes its con- ductivity, according to the dose. It exercises a selective action as regards nerve-fibres of different kinds, picking out and paralyzing sensory fibres before motor; vagus fibres conducting upwards before those conducting downwards, vaso-constrictors before vaso-dilators, and broncho-constrictors before broncho-dilators (Dixon). The conduction or propagation of a definite disturbance or impulse is a pheijpmenon not confined to nervous tissue. It is also characteristic- ally seen in muscle, although there the mechanical effect which con- stitutes the normal response to the arrival of the propagated disturbance obtrudes itself and tends to divert attention from the latter. It is unlikely that the conduction process in muscle should be essentially different from that in nerve, and in muscle, as in nerve, there is evidence that it is associated with only a small, perhaps not even a detectable, liberation of heat. The main heat-production in muscle is essentially a feature not of conduction, but of contraction. Con- duction in muscle can be completely dissociated from the contraction process in various ways. For example, if a portion of a muscle is immersed for a time in distilled water, so-called water rigor ensues, and the altered muscle has lost the power of contraction. It will never- theless conduct the impulse which on reaching the unaltered part of the muscle causes it to contract normally. Double Conduction. — When a nerve (or muscle) is stimulated artificially, the excitation runs along it in both directions from the 766 NERVE point of stimulation; so that nerve-fibres which in the intact body are afferent can conduct impulses towards the periphery and efferent fibres can conduct impulses away from the periphery. In the normal state, however, double conduction must seldom occur, for efferent fibres are connected centrally, and afferent fibres peripherally, with the structiores in which their natural stimuli krise. In general, too, an impulse, if it did pass centrifugally along an afferent fibre, would not give any token of its existence, for the peripheral organ would not be able to respond to it ; and there is no ground for assuming that the central mechanisms connected with afferent fibres are better fitted to answer such foreign and un- accustomed calls as impulses reaching them along normally efferent nerves. There is good evidence that muscular excitation is not carried over to the motor nerve- fibres; in other words, the wave of action flows from the nerve to the muscle, but cannot be got to flow backwards. Excitation of the central end of an efferent (anterior) spinal root is not transferred to the corresponding afferent (posterior) root, the connection between the efferent and afferent neurons presenting the character of a physiological ' valve,' which permits impulses to pass only in one direction. We have seen that vaso-dilator impulses possibly pass out to the limbs over fibres which, morphologically speaking, are afferent fibres (p. 179). And we shall see that a nutritive influence is exerted over the afferent fibres of the spinal nerves by the ganglion cells of the posterior root ganglia (p. 770), an influence which must spread along these fibres in the opposite direction to that of the normal excitation. The best proofs of double conduction in nerves, with artificial stimu- lation, are: (i) The propagation of the negative variation or action current in both, directions. This holds for sensory as well as for motor fibres, as du Bois-Reymond showed on the posterior roots of the spinal nerves of the frog and the optic nerves of fishes. (2) Stimulation of the posterior free end of the electrical nerve of Malapterurus (p. 813) causes discharge of the electric organ, although the nerve-impulse travels nor- mally in the opposite direction. (3) If the lower end of the frog's sartorius is split into two, gentle stimulation of one of the tongues causes contraction of individual fibres in the other. This is supposed to be due to conduction of the nerve-impulse up a twig of a nerve-fibre distributed to the one tongue, and down another twig of the same fibre going to the other tongue. A similar experiment can be done on the gracilis of the frog. This muscle is divided by a tendinous inscription into two parts, each supplied by a branch of a nerve which divides after entering the muscle. Stimulation of either twig is followed by contraction of both parts of the muscle (Kiihne). Bert's much-quoted experiment on the rat is' valueless as a proof of double conduction. He caused union of the point of the tail with the tissues of the back, then divided the tail at the root, and found that stimulation of what was now the distal end caused pain. From this he concluded that the sensory fibres of the ' transposed ' tail conducted in the direction from root to tip. But the conclusion is not warranted, for sensation disappeared in the tail after the section, and did not THE NERVE-IMPULSE OR PROPAGATED DISTURBANCE 767 return till some months later, when the nerve-fibres, after degenerating, would have been replaced by new sensory fibres growing down from the dorsal nerves (Ranvier). For a similar reason the so-called union of the peripheral end of the cut hypoglossal nerve (motor) with the central end of the cut lingual (sensory) proves nothing as to double conduction, nor as to the possibility of motor nerves taking on a sensory function. For while sensation is after a time restored in the affectedr portion of the tongue, this is due to the growth of sensory fibres from the central stump of the lingual down through the degenerated hypoglossal , and not to the conduction upwards of sensory impulses by the motor, fibres of the latter. Every fibre of a nerve is physiologically isolated from the rest,, so that an impulse set up in a fibre runs its course within it, and does not pass laterally into others (law of isolated conduction). In connection with this physiological fact there is the anatomical fact that nerveTfibres do not normally branch in the trunk of a peripheral nerve. (But see p. 776.) It has, however, been shown that bifurca- tion of nerve-fibres may occur in the spinal cord (Sherrington), The axis-cylinder of a peripheral nerve-fibre only begins to branch where complete isolation of function is no longer required, as within a muscle. The exjferiment of Kiihne on double conduction, men- tioned above, shows that an excitation set up in one twig or one fibril of an axis-cylinder which has branched can spread to the rest. Velocity of the Nerve- Impulse. — ^We have said that the nerve- impulse travels with a measurable velocity. It is now time to describe how this has been ascertained (p. 791). For motor fibres the simplest method is to stimulate a nerve successively at two points, one near its muscle, the pther as far away from it as possible, and to record the contractions on a rapidly-moving surface (pendu- lum or spring myograph) (p. 720). The apparent latent period of the curve corresponding to the nearer point will be less than that of the curve corresponding to the point which is more remote, by the time which the impulse takes to pass between the two points. The distance between these points being measured, the velocity is known, Helmholtz found the velocity for frog's nerves at the ordinary temperature of the air to be a little under, and for human nerves, cooled so as to approximate to the ordinary temperature, a little over 30 metres per second. For observations on man the contraction cmrves of the flexors of one of the fingers or of the thumb may be recorded, first with stimulation of the brachial plexus at the axilla, and then with stimulation of the median or ulnar nerve at the elbow. Probably at the same temperattnre there is little difference in the rate of transmission in the nerves of warm- blooded and cold-blooded animals, but temperature has a con- siderable influence (p. 756). By cooling a frog's nerve Helmholtz reduced the rate to ^g of its value at the ordinary temperature. In the human arm' he found a variation from 30 to 90 metres per second, according to the temperature, 50 metres 768 NERVE being about the normal rate. This is greater than the speed of the fastest train in the world. According to Piper's recent measurements the velocity in human medullated nerve is even greater than Helmholtz concluded, about 120 metres a second under ordinary conditions. The rate is independent of the intensity of the excitation. The velocity with which the negative variation is propagated (p. 800) is the same as that of the nerve-impulse. In sensory nerves there is no reason to believe that the velocity of the nerve-impulse differs from that in motor nerves, but experiments on man really free from objection are as yet wanting. The usual method is to stimulate the skin first at a point distant from the brain, and then at a much nearer point. The person experimented on, as soon as he feels the stimulation, makes a signal, say, by closing or opening with the hand a current connected with an electric time- marker, writing on a moving surface. There is, of course, a measurable interval between the excitation and the signal, and this being in general longer the more remote the point of stimulation is from the brain, it is assumed that the excess represents the time taken by the nerve-impulse to pass over a length of sensory nerve equal to the difference in the length of the path. But there is this difficulty, that the propagation of the impulse from the point of stimulation to the brain is only one link in the chain of events of which the signal marks the end. The impulse has first to be transformed into a sensation, and then the will has to be called into action, and an impulse sent down the motor nerves to the hand. And while the time taken by the excitation in travelling up and down the peripheral nerve-fibres is probably fairly constant, the time spent in the intermediate psychical processes is very variable. Section II. — Chemistry, Degeneration, and Regeneration of Nerve. Chemistry of Nerve. — Our knowledge of this subject is still scanty ; and most of what we do know has been obtained from analyses, not of the peripheral nerves, but of the white matter of the central nervous system Proteins are present, especially in the axis-cylinder. The proteins of nervous tissue include two globulins, one coagulated by heat at 47° C, the other at 70° to 75° C, and a nucleo-protein coagulating at 56° to 60° C. Very important constituents are certain substances soluble in organic solvents, like benzol and ether, and , comprising cholesterin, certain phosphatides (kephalin and lecithin), and certain cerebrins or cerebrosides. The cerebrins are glucosides containing nitrogen, but no phosphorus, and they yield a reducing sugar (galactose) on hydrolysis. In the nervous tissue there is also present, according to some authorities, a compound called protagon. Others consider it a mere mixture of phos- phatides and cerebrosides. The lipoids of nerve-fibres belong largely to the medullary sheath, but they are not confined to it, since non-medul- lated nerves also yield a considerable quantity of lipoids (11 "5 per cent, of the solids as against 46-6 per cent, for medullated nerves). Non- meduUated nerves (splenic nerves of the ox) are distinguished from medullated nerves (human sciatic) by the high proportion of their total lipoids constituted by the phosphatides (kephalin and lecithin) and cholesterin. Thus, in non-medullated fibres 47 per cent, of the lipoid CHEMISTRY OF NERVE 769 extract consisted of cholesterin, and 23-7 per cent, of kephalin; while in the meduUated fibres cholesterin made up only 25 per cent, of the extract, and kephalin 12-4 per cent. On the other hand, the cere- brosides are present, both relatively and absolutely, in much larger quantity in medullated than in non-medullated nerves. In both varieties of fibres kephalin, and not lecithin, is the chief phosphorus- containing body (Falk). The medullary sheath further contains a kind of network of a peculiar resistant substance, neurokeratin. The neurilemma consists of substances insoluble in dilute sodium hydroxide. Gelatin is obtained from the connective tissue which binds the nerve- fibres together. There may also be ordinary fat in the meshes of the epineurium connecting the bundles. Small quantities of xanthin, hypoxanthin, and other extractives, can also be obtained from nerve. According to Halliburton's analyses, the water in sciatic nerves amounts to 65-1 per cent., and the solids to 34-9 per cent. The proteins make up 29 per cent, of the solids. For an analysis of the white matter of the brain, see Chapter XVI. Nerve-cells contain no potassium, according to Macallum; and this is true both of the dendrites and the axons. In medullated nerves, how- ever, potassium compounds are present external to the axons, chiefly at the nodes of Ranvier (Frontispiece) and in the neurokeratin framework of the sheath. The only chemical difference between living and dead nervous tissue which has been made out with any degree of certainty is that the former is neutral or faintly alkaline, and the latter acid, in reaction to such indicators as litmus. This is especially true of the grey matter of the central nervous system, although the white matter also is often found acid. The change of reaction is due to the accumulation of lactic acid. Such a change has not hitherto been clearly demonstrated in peripheral nerves, either after death or after prolonged stimulation. The (non- medullated) splenic nerves of the dog, even after stimulation for six hours, never became acid (Halliburton and Brodie). Degeneration of Nerve. — Nerve-fibres are ' bound in the bundle of life ' with the nerve-cells from which their axis-cylinders arise; the connection between cell and axon once severed, the nerve- fibre dies inevitably. This is an illustration of a general law that no portion of a cell can live once it is separated from the nucleus. We shall see later on that changes also occur in the nerve-cell whose axon has been divided from it, although they are of a different nature (rather a slow atrophy than an acute degeneration), and do not necessarily lead to the destruction of the cell. We must regard the neuron not only as a morphological unit, a single cell from nucleus to remotest end-brush, but also as a functional and nutritive unit, the fortune of any portion of which is not in- different to the rest. Thus, when a man's arm is amputated the arm fares worse than the man, for the arm dies. But the man is not unaffected. He lives, but he suffers much temporary disturb- ance and some permanent loss. What is left of him is not quite the same as it was. The acute changes that occur in severed nerve- fibres are most conveniently studied in the peripheral nerves, although essentially similar phenomena take place also in the fibres of the central nervous system. 49 77° NERVE A spinal nerve is composed of efferent fibres whose cells of origin are in the grey matter of the anterior horn, and afferent fibres whose cells of origin are in the posterior root ganglion. When such a nerve is cut below the junction of its roots, muscular paralysis and impairment of sensation at once follow in the region supplied by the nerve; but for a time the nerve remains excitable to direct stimulation. The excitability gradually diminishes, and in a few days is completely gone. If portions of the nerve distal to the lesion are examined at different periods after section, a remarkable process of degenera- tion (commonly spoken of as Wallerian degeneration) is seen to be going on. In the medul- lated fibres this begins on the second or third day with a swelling of the axis-cyUnder, which breaks up into detached pieces (fragmentation), and as- sumes a granular appearance. The medullary sheath also under- goes fragmentation at the lines of Lantermann, and a little later separates into clumps and drop- lets of myelin. The nuclei under the neurilemma increase in size, proliferate by mitosis, and in- sinuate themselves between the fragments of the medullary sheath and axis-cylinder,, which ultimately disappear, leaving the nerve- fibre represented only by a kind of mummy of connective tissue, in which the neurilemma with its abnormally numerous nuclei can still be recognized. The fragmentation of the myelin sheath is not dependent upon the proliferation of the nuclei, since it occurs also in nerves removed from the body and kept under con- ditions in which the nuclei do not proliferate (Feiss and Cramer). The protoplasm around the nuclei of the neurilemma also increases in amount , and undergoes other changes, which will be more particularly referred to in describing the regeneration of nerve. The degenerative process begins near the cut end, and extends gradually to the peri- Fig. 270. — Degeneration of Nerve-Fibres after Section (Barker, after Thoma). I, normal fibre; II, degenerating fibre; III, further stage of degeneration; S, neurilemma; m, medullary sheath; A, axis-cylinder; L, Lantermann's line or cleft ; R, node ; mt, drops of myelin ; a, remains of axis-cylinder ; w, prolifera- ting cells of neurilemma. DEGENERATION OF NERVE 771 phery, and more rapidly in warm- than in cold-blooded animals. At any rate, that is the interpretation generally given to the fact that at a given period after section the changes— especially the breaking- up of the myehn— are more pronounced near the proximal end of the peripheral stump. In a mammal degeneration is far advanced in a fortnight, although the last remnants of the myehn may not be absorbed for months. In the degenerated nerve (cat's sciatic) the percentage of phosphorus undergoes a diminution from about the third day. About the eighth day the loss of phosphorus— i.e., of the phosphatides (lecithin, kephaHn)— is markedly accelerated, coinciding with the appearance • of a strong Marchi* staining re- action. By the twenty-ninth day the degenerated nerve is prac- tically devoid of phosphorus. A progressive increase in the water and a diminution in the total solids also culminate about the same time (Mott and Halli- burton). In the portion of the nerve-fibre still connected with the nerve-cell the degeneration only extends as far back as the next node of Ranvier, and seems to be due to the direct effect of the injury. In non-meduUated fibres, such as the fibres arising from the cells of the superior cervical ganglion (Tuckett), the degeneration is confined to the axis-cylinders. It begins in about twenty-four hovis after section, and the loss of excitability and conductivity is complete by the fortieth hoiu:. It follows from what has been said as to the position of the cells of origin of the root fibres of the spinal nerves that section of the anterior root causes degeneration on the peripheral, but not on the central side of the lesion, -f Only the anterior root fibres in the mixed nerve degenerate. Section of the posterior root above the ganglion causes degeneration of the central stump, but not of the portion still connected with the ganglion, nor of the posterior root fibres below the ganglion or in the mixed nerve. Section of the posterior root below the ganglion causes degeneration of the fibres of the root below the section and in the mixed nerve, but not above it. * The chief constituents of Marchi's solution are potassium bichromate and osmic acid. It stains medullated nerve-fibres black in the earlier stages of degeneration. t A few fibres in the peripheral stump of the anterior root do not degenerate, and a few fibres in the central stump do. These are the 'recurrent fibres,' whose course is described on p. 864. Fig. 271. — Degeneration of Spinal Nerves and their Roots after Section. The shading shows the degenerated portions. 772 NERVE Regeneration of Nerve.— Degeneration of nerve is followed, if its divided ends are not kept artificially apart, by a process of re- generation, already distinct under favourable conditions in from three to four weeks after the section, and indeed in some cases commencing as early as the second week. This consists in the outgrowth of new axis-cylinders, in the form of fine fibres, from the ends of the divided axis-cylinders of the central stump of the nerve. These push their way into and along the degenerated fibres, ultimately acquire a medullary sheath, and develop into complete nerve-fibres, restoring first sensation, and later on volun- tary motion, to the paralyzed part. The process needs several months for its completion, even in warm-blooded animals. It takes place under the influence of the nucleated portion of the neuron (the cell-body), and is never completed if the peripheral and central portions of the nerve are permanently separated by a substance through which the new axis-cylinders cannot grow or by a gap too wide for them to bridge over. When the cut ends of the nerve are carefully sutured together, the conditions for com- plete and speedy regeneration are rendered more favourable — a fact which finds its application in the surgical treatment of injured nerves. The cycle of chemical changes described in the degenerating nerve is retraced in the reverse order. In the cat's sciatic the first sign of the return of the phosphorus was seen with the beginning of the normal myelin reaction about the sixtieth day after section. At the one-hunchredth day the phosphorus content was almost as great as that of the normal nerve (a little under i per cent, of the solids for the regenerated, as compared with a little over i per cent, for the normal nerve). It is not as yet well understood how the regenerating fibres are directed in their growth, so that they join their centres to the appro- priate end-organs without mistake. That they have a high capacity for finding their way is indicated by the results of cross-suturing such nerves as the median and ulnar — i.e., of uniting the central end of the one with the peripheral end of the other. Howell and Huber found that after this operation in the dog, both co-ordinated volun- tary motion and sensation returned in large measure in the parts supplied by the nerves. Here the motor fibres of the median nerve must, of course, have made connection with muscles previously supplied by the ulnar, being guided to them along the nerve-sheaths of the latter. Doubtless the old nerve-sheaths serve to some extent as mechanical guides by offering to the new axons a path of least resistance. And when a nerve-trunk containing motor and sensory fibres is simply crushed so as to destroy all physiological continuity, but is not cut, no distortion of the motor and sensory ' patterns ' of the nerve — in other words, no ' straying ' of the fibres from their old paths — can be detected on regeneration^ When the REGENERATION OF NERVE 773 Fig. ■272.- — Regenerating Fibres crossing in the Scar after Ligation of a Dog's Sciatic Nerve 165 Days previously. Weigert- Pal stain. Drawn under oil- immersion (Feiss). nerve is cut and then sutured, a certain amount of distortion of the pattern is inevitable. The mechanical apposition of central and peripheral stumps is, of course, much more nearly perfect in the crushed nerve than in the cut nerve, however exact the suturing may be (Osborne and Kilvington) Yet, even after crushing or liga- tion of nerves, or after section and suturing, the regenerating fibres do not pass straight through the scar tissue from the central to the peripheral stump, but cross and mingle, ap- parently in the most inextricable confusion (Feiss) (Fig. 273). This is due to the prolifera- tion of cells in the scar which run in all direc- tions, and show no signs of following the parallel arrangement of the nerve-sheaths in the central or the distal segment. These being formed before the regenerating nerve-fibres, the latter must necessarily grow also in all direc- , , „ ,, , tions in the scar. Int. Popliteal ^^ T-1.- I, This, however, is a local phenomenon. Beyond the scar the arrangement of the regenerating axis- cylinders recovers its regularity, and the amount of dis- tortion of the nerve pattern, as indicated by histological ex- amination and functional tests, is by no means so great as the com- plete effacement of the pattern in the scar might appear to promise. That the degenerated peripheral stump directs the growth of the axons from the central stump in some other than a merely mechanical way is evident from the experiments of Langley on Fig. 273. — Semidiagrammatic Representation of Longi- tudinal Section through Neuroma or Scar produced by ligating the Sciatic Nerve with Catgut, and crushing it with a haemostat just above its Division into the Ex- ternal and Internal Popliteal Nerves. Weigert-Pal preparation (Feiss). 774 NERVE regeneration of the cervical sympathetic in the cat after section below the superior cervical ganglion. The nerve contains fibres of various functions which reach it from the upper thoracic nerves. The anterior roots of the first and third thoracic nerves supply the cervical sympathetic mainly with fibres which end in the ganglion around cells that give off dilator fibres for the pupil. The fibres connected with the cells in the ganglion which send vaso-motor fibres to the vessels of the ear are for the most part contained in the anterior roots of the second and fifth thoracic nerves; and the fibres connected witii the cells that give origin to the pilo-motor fibres for the hairs of the face and neck in the anterior roots of the fourth to the seventh. Stimulation of any one of the upper thoracic roots accordingly causes a specific effect, which, according to Langley, is in general the same after regeneration as before section of the cervical sympathetic. We must assume, therefore, that each regenerating fibre seeks out either the ganglion cell with which it was originally connected, or one belonging to the same class. No mere mechanical guidance of the growing axons by the old neuri- lemmas will suffice to explain this selective growth. It is necessary to postulate, in addition, an attraction of a chemical or physico- chemical nature (chemiotaxis), dependent upon a specific relation between the new axons and the scaffolding of the peripheral stump or the ganglion cells. But it is not possible at present to form any very precise conception of the properties on which the chemiotactic phenomena depend. And the specificity is not an absolute one. Under certain conditions these pre-ganglionic nerve-fibres (that is to say, nerve-fibres running from the spinal cord to end around the sympathetic ganglion cells) can form connections with nerve-cells of a different class — e.g., pupillo-dilators with cells whose axons end in the erector muscles of the hairs. Further, after section of the sympathetic above the superior cervical gangUon, the post- ganglionic nerve-fibres {i.e., the fibres coming off from the cells of the ganglion) may also, if the opportunity be favourable during regeneration, exchange their old end-organs for new ones; pilo- motor fibres, for instance, finding their way into the iris and becoming pupillo-dilators. After excision of the superior cervical ganglion, the cervical sympathetic does not recover its function. Accordingly the pre-ganglionic fibres cannot form direct functional connection with the post-ganglionic fibres, but can become connected with them only indirectly through the ganglion cells. Nor can efferent post-ganglionic fibres achieve regenerative union with a cerebro- spinal (somatic) motor nerve, although they can themselves re- generate, as has been shown, e.g., in the case of the vaso-constrictors of the limbs. On the other hand, union easily takes place between pre-ganglionic fibres and efferent somatic fibres, and vice versa. For example, the cervical sympathetic can unite with the phrenic REGENERATION OF NERVE 775 nerve, and cause contraction of the diaphragm, or with the recurrent laryngeal nerve, and cause movement of the vocal cords, or with the spinal accessory, and cause contraction of the sterno-mastoid muscle. Conversely, the phrenic nerve, when united with the cer- vical sympathetic, can, when stimulated, produce the usual effects observed on exciting the latter nerve (Langley and Anderson). Central and Autogenetic Theories of Regeneration.— Although the establishment of connection with the central end of the cut nerve is necessary for complete regeneration, it must not be supposed that no share whatever is taken in the process by the peripheral stump. Even while it remains completely isolated from the central nervous system, changes occur which are often described as the third or final stage of degeneration, but which are more correctly interpreted as forming a stage in the regenerative cycle. Spindle-shaped cells or fibres with elongated nuclei make their appearance, produced by the proliferation of the nuclei of the primitive sheath already described, and the increase of the proto- plasm in which these nuclei are embedded. These so-called axial strand fibres or this fibrillated protoplasm may appear long before the remains of the degenerated axis-cylinder and myelin sheath have been completely removed. It is generally acknowledged that in the adult they do not develop beyond this, so long as the peri- pheral portion of the nerve remains completely isolated, but neither do they disappear even after a very long interval. When strict precautions against union with other nerve-trunks were taken, the radial nerve of an adult cat was found in this resting-stage nearly a year and a half after division, and the same was true after two years and a half in a nerve divided in a human being. The fibres are incapable of being excited or of conducting nerve impulses. The precise relation between these axial strand fibres of the peri- pheral stump and the myelinated fibres found there after regenera- tion has been much debated. All are agreed that nerve-fibrils sprout from the central stump, and the weight of evidence is in favour of the long-accepted view that it is by the growth of these fibrils along the peripheral stump that the new axons are formed, and that all the changes in the distal portion of the nerve, however important for directing and perhaps sustaining the growth of the central fibrils, are subsidiary to this. But some maintaiii that the outgrowing central fibrils meet and unite with corresponding fibrils sprouting from the peripheral stump, and that the new axis- cylinders arise from the fibrils of the axial strand. It is said that very shortly after being brought into connection with the central portion of the same or of another nerve by careful suturing the spindle cells begin to lengthen, and form non-medullated fibres, like those of the sympathetic. Four weeks after union the afferent fibres, although still non-medullated, are capable of being stimulated 776 NERVE mechanically and electrically, and of conducting impulses towards the centre. In about eight weeks they become medullated, but at first are of small calibre (Head and Ham) . Bethe, the most strenuous defender of the inherent regenerative power of the isolated peri- pheral stump (autogenetic theory), has even stated that complete regeneration occurs in young animals in nerves entirely separated from their centres. There is no doubt that this result is due to some error of technique or of interpretation. The controversy turns largely upon the precautions judged necessary to prevent the ingrowth of central fibres. And while it is comparatively easy to make sure, by removing a large part of it, that the central end of the nerve under observation shall remain completely vmconnected with the peripheral end, it is often a matter of the greatest difficulty to prevent the union of the distal stump with central fibres from other sources — e.g., from the nerves cut in the wound. Many of the results which seemed to favour the autogenetic theory were cer- tainly due to this cause. The most conclusive evidence in favour of central and against autogenetic regeneration, because the most direct and uncompli- cated, has been afforded by the demonstration that the development of axis-cylinders occur in vitro in a suitable plasmatic medium, in the absence of any other elements than the nerve-cells from which they arise (Harrison). This observer, working with the medullary plates of tadpoles, in which the nerve-cells originate in the embryo, showed further that peripheral nerves do not develop when the nerve-centres are removed, and that the sheath-cells of Schwann are not essential to the growth of axis-cylinders, since in their ab- sence the latter continue to grow and reach their normal length. It has also been proved that nerve-fibres grow out from pieces of the cerebellum and spinal ganglia of young mammals when cultivated on clotted plasma outside of the body (Fig. 327, p. 829). Many fibres sprouting out from the spinal ganglia attain a length of more than half a millimetre in forty-eight hours, and their growth need not be accompanied by either neuroglia or connective tissue (Ingebrigtsen). A fact of great physiological interest, and also of practical impor- tance, in connection with the anastomosis of nerves for the relief of certain forms of paralysis, is the bifurcation of axons in regeneration, when the conditions are such that the axons of the central stump are offered more than one path along which to regenerate. If, for instance, a limb nerve-trunk containing motor fibres is cut, and its central end sutured both to its own distal end and to the distal end of an adjacent nerve-trunk, the sum of the nerve-fibres in the two distal trunks after regeneration has occurred is greater than the number of fibres in the central stump (Kilvington). That this is due to splitting of axons is shown by the fact that an axon reflex (p. 885) can be elicited on dividing one of the distal trunks and stimulating its central end after complete separation of the proximal or parent stem from the central nervous system. Even when the second path offered to the regenerating motor axon is a sensory path, bifurcation of the axon occurs, one branch DEGENERA TION OP M VSCLE 777 passing down along the previous motor path to its proper muscular termination, and the other passing down the sensory path. Although there is no evidence that efferent fibres can unite with afferent fibres, a degenerated afferent path can therefore serve as a cheniiotactic scaffold- ing or guide for the growth of regenerating motor axons, though not such an efficient one as a degenerated motor path. Sensory fibres, however, cannot regenerate along motor paths or make functional union with the receptive substance of skeletal muscle. Regeneration of the fibres of the central nervous system either does not in general occur, or is exceedingly difficult to realize. This lends support to the doctrine of the importance of the neurilemma in regeneration, since its elements are scantily developed in the fibres of the brain and cord (p. 832) . Regeneration of the fibres which proceed from the cells of the spinal ganglia along the posterior roots into the cord may take place after the roots have been cut, so that the normal reflexes through the res- piratory, cardiac, and vaso-motor centres may be once more obtained. Degeneration of Muscle. — Experimental section or, in man, traumatic division or compression of a nerve leads not only to its degeneration, but ultimately, if regeneration of the nerve does not take place, to degeneration of the muscles supplied by it as well. The muscle- fibres dwindle to a quarter of their normal diameter; the stripes disappear ; the longitudinal fibrillation fades out ; and at length only hyaline moulds of the fibres are left, filled, and separated by fatty granules and globules and surrounded by engorged capil- laries. Amidst the general decay, the muscular fibres of the tei-minal ' spindles ' with which the afferent nerves of muscles are. coimected alone remain unchanged (Sherrington). Certain dis- eases of the cord which interfere with the cells of the anterior horn cause degeneration of motor nerves, and ultimately of muscles. The motor nerve-endings degenerate sooner than the sensory. Both may, under suitable conditions, regenerate (Huber). Reaction of Degeneration. — ^Muscles whose motor nerves have been separated from their trophic centres show, when a certain stage in degeneration has been reached, a peculiar behaviour to electrical stimulation, called the ' reaction of degeneration.' To the constant current the muscles are more excitable, and the contraction slower and more prolonged than normal. When a current, either constant or induced, is passed through a normal muscle, the muscular fibres may be stimulated either directly, or indirectly through the intramuscular nerves. Under ordinary conditions the nerves respond more readily than the muscular fibres, especially to momentary stimuli like induction shocks, and therefore the so-called direct stimulation of uncurarized muscle is as a rule an indirect stimulation. When the muscle is curarized and the nerves thus eliminated, the excitability to induced currents is found to be diminished. The same is the case in a muscle which exhibits the reaction of degeneration after section of its motor , nerve, only the loss of excitability to induced currents is greater, and may even be complete. The common statement that the closing anodic contraction is stronger than the closing kathodic — the opposite of the ordinary law — is subject to so many exceptions that it has no diagnostic value. The nerves are inexcitable either to constant or induced currents . The reaction of degeneration is only obtained from paralyzed muscles when the paralyzing lesion is situated in the cells of the anterior 778 NERVE horn Irom which the motor nerves take origin, or below that level, Accordingly, it is sometimes of use in localizing the position of a lesion. For instance, a group of muscles might be paralyzed by a lesion in the grey matter of the brain or in the nerve-fibres connecting this with the grey matter of the anterior horn of the cord, or in the grey matter of the anterior horn itself, or in the peripheral nerve-fibres leading from this to the muscles. In the first two cases the reaction of degeneration would be absent, although the muscles, if the lesion was of long standing, would be atrophied to some extent ; in the last two there would be acute atrophy of the muscles, and the reaction of degeneration would be obtained. Trophic Nerves. — There is no question that nerves exert a very important influence upon the nutrition of the parts supplied by them, in influencing the specific function of those parts. So that in this sense all nerves are trophic nerves. The fact that the proper nutrition of nerve-fibres and striated muscular fibres is dependent on their connection with nerve-cells has been by some writers generalized into the doctrine that all tissues are provided with ' trophic ' nerves, which, apart from any influence of functional activity, regulate the nutrition of the organs they supply. But the evidence for this view, when weighed in the balance, is found wanting; and it may be said that up to the present no unequivocal proof, experimental or clinical, has ever been given of the existence of specific trophic fibres, anatomically distinct from other efferent or afferent nerves. It is true that in various diseases and injuries of the nervous system nutritive changes in the skin, and sometimes in the bones and joints, are apt to appear. But it is very difficult in such cases to disentangle the effects produced by accidental injuries acting on structures whose normal sensibility is lost or lessened, or whose circulation is deranged, from true trophic changes. The most that can be said is that there is some evidence that the power of the skin to resist injury, and the capacity of recovering froia it, are diminished by interference with its nerve-supply, so that a large sore may result from a trifling lesion, and healing may be slow and difficult. Experimentally it has been found that division of the trigeminus nerve within the skull is sometimes followed by cloudiness of the cornea, going on to ulceration, and ulti- mately inflammation and destruction of the eyeball. Ulcers also form on the lips and on the mucous membrane of the mouth and gums ; and the nasal mucous membrane on the side corresponding to the divided nerve becomes inflamed . But in this case the sensibility of the eye is lost, and reflex closure of the eyelids ceases to prevent the entrance of foreign bodies. The animal is no longer aware of the contact of particles of dust or bits of straw or accumulated secretion with the conjunctiva, and makes no effort to remove them. The lips, being also without _ sensation, are hurt bythe teeth, particularly as the muscles of mastica- " tion on the side of the divided nerve are paralyzed, and decomposed food, collecting in the mouth, and inhaled dust in the nose, will tend still further to irritate the mucous membranes. There is thus no more need to assume the loss of unknown trophic influences in order to explain the occurrence of the ulcerative changes than there is to explain the production of ordinary bed-sores, bunions or corns on parts peculiarly liable to pressure. And, as a matter of fact, if the eye be TROPHIC NERVES Tjg artificially protected, after section of the trigeminal nerve, the ophthalmia either does not occur or is much delayed. In man, too, a case has been recorded in which both the fifth and the third nerves were paralyzed. The eye was still shielded by the contraction of the orbicularis oculi supplied by che seventh nerve, as well as b}^ the drooping of the upper eyelid that accompanies paralysis of the third. It remained perfectly sound for many months, till at length the tumour at the base of the brain which had affected the other nerves involved the seventh too. The eye was now no longer com- pletely closed; inflammation came on, and vision was soon permanently lost (Shaw). In another case a patient lived for seven years with complete paralysis of the fifth nerve, yet the eye remained free from disease and sight was unimpaired (Gowers). The so-called ' trophic ' effects following division of both vagi we have already discussed (p. 280) so far as they are concerned with the respiratory system. The degenerative changes sometimes seen in the heart are perhaps due to its being overworked in the absence of nervous restraint on its functional activity. The nutritive alterations in muscles and salivary glands after section of motor and secretory nerves seem to depend in part on functional and vaso-motor changes. In the paralyzed muscles nutrition is not only interfered with in consequence of their inactivity, as would be the case even if the paralysis were due to a lesion above the level of the anterior comual cells, but the already poorly nourished fibres are continually pressed upon by the capillaries, which are dilated owing to the division of the vaso-motor nerves. The degeneration must also be in part ascribed to the loss of a tonic influence exerted on the muscles by the motor cells of the spinal cord, through the ordinary motor nerves (p. 889). When all allowance has been made for these factors, the rapid and characteristic degeneration of the striated muscles, after their connection with the central nervous system is severed, is still inexplicable, except on the assumption that their nutrition is specially related to the integrity of their efferent nerves. In other words, it is necessary to suppose, not, indeed, that distinct trophic nerves exist for the muscles, but that an influence or impulses, which can be termed trophic or nutritive, do normally pass out to them from the spinal cord along their motor nerves. Section of the cervical sympathetic in young rabbits and dogs increases the growth of the ear and of the hair on the same side. But it is impossible to separate these consequences from the vaso-motor paral- ysis ; and the same is true of the hypertrophy following section of the vaso-motor nerves of the cock's comb and of the nerves of the bones. After section of the superior laryngeal the vocal cord on the side of the section is at once rendered motionless, and remains so, but the muscles, notwithstanding their inaction, do not degenerate. And Mott and Sherrington have found that, although section of the posterior roots in monkeys is followed after a time (three weeks to three months) by ulceration over certain portions of the foot, no corresponding lesions occur in the hand. They believe, therefore, that the lesions are not due to the withdrawal of a reflex trophic tone, but are accidental injuries in positions specially exposed to mechanical or microbic insults. One of the best examples of interference with the proper nutrition of a part produced by a lesion in the nerves supplying it is an eruption (herpes zoster), limited to the skin supplied by the nerve-fibres coming from one or more spinal ganglia, and dependiiig on an (infectious) inflammatory change in the ganglia. It has been suggested that the vesicles are formed either because the passage of afferent impulses normally concerned in the nutrition of the skin is interfered with or 780 NERVE Centripetal or afferent fibres. 2. Nerves of general sensation because the skin is bombarded by antidromic (p. 179) impulses dis- charged from the inflamed ganglia. But an alternative hypothesis is that a toxine spreads out along the nerves from the ganglia, just as in traumatic tetanus the toxine is known to pass in the opposite direc- tion along the nerves from the seat of injury to the central nervous systeim. Classification of Nerves. — Omitting the group of ' trophic ' nerves, and the even more problematical ' thermogenic ' fibres (which some have supposed to preside over the production of heat, and therefore to assist in the regulation of the temperature of the body, but of whose existence as distinct and specific nerve-fibres with no other function there is not the slightest proof), peripheral nerves may be classified as follows: f Smell. . Nerves of special sensation -j xs-^^:' i Sight. ' Touch (hght touch). Pressure (perhaps in- cluding the nerves of muscular sense). ( Warmth— Cold. Pain. Calibre of small arteries (pressor, depressor). Action of heart. Respiratory movements. Visceral movements. Glandular secretion. Ordinary skeletal muscles. ' Skeletal muscles Visceral f Vaso-constrictor I Cardio-augmentor. Erector muscles of hairs (pilo-motor (^ fibres). ( Visceral muscles 2 . Inhibitory nerves for < ( Vaso-dilator. t Vascular , , \ Cardio-inhibi- 3. Secretory nerves \ tory. * It is not known whether the afferent portion of a reflex arc is always com- posed of fibres included in the first two categories, although undoubtedly in some cases it is. y. '■ Possibly nerves other than those included under i and 2, concerned in reflex changes in Centrifugal or efferent fibres. I. Motor nerves for Vascular PRACTICAL EXERCISES ON CHAPTERS XIII. AND XIV. 1. Difference of Make and Break Shocks from an Induction Machine. — Connect a Daniell or. other cell B (p. 697) with the two upper binding- screws of the primary coil P, and interpose a spring key K in the circuit. Connect a pair of electrodes with the binding-screws of the secondary coil (Fig. 274). PRACTICAL EXERCISES 781 Electrodes can be very simply made by pushing copper wires through two glass tubes, filling the ends of the tubes with sealing-wax and bmding them together with waxed thread. The projecting points may be filed, and the nerve laid directly on them, or they may be tipped with small pieces of platinum wire soldered on. (a) Push the secondary away from the primary, until no shock can be felt on the tongue when the current from the battery is made or broken with the key. Then bring the secondary gradually up towards the primary, testing at every new position whether the shock is per- ceptible. It will be felt first at break. If the secondary is pushed still further up, a shock will be felt both at make and at break. From this we learn that for sensory nerves the break shock is stronger than the make. The same can easily be demonstrated for motor nerves and foi muscle. (6) Smoke a drum and arrange a myograph, as shown in Fig. 278. But omit the brass piece F, and do not connect the primary through the drum, as there shown, but connect it as in Fig. 274. Pith a frog (brain and cord), and make a muscle-nerve preparation. To make a Muscle-Nerve Preparation. — Hold the frog by the hind legs back upwards; the front part of the body will hang down, making an angle with the posterior portion. With strong scissors divide the back- bone anterior to this angle, and cut away all the front portion of the body, which will fall down of its own weight. Make a circular incision at the level of the tendo Achillis, and another at the lower end of the femur, through the skin. The sciatic nerve must now be dissected out, as follows: Remove the skin from the thigh, and, pig. 274._Arrangemeiit of Coil for Single Shocks. holding the leg in the left hand, slit up the fascia which connects the external and internal groups of muscles on the back of the thigh. Complete the separa- tion with the two thumbs. Cut through the iliac bone, taking care that the blade of the scissors is well pressed against the bone, otherwise there is danger of severing the sciatic plexus. Now divide in the middle line the part of the spinal column which remains above the urostyle. A piece of bone is thus obtained by means of which the nerve can be manipulated without injury. Seize this piece of bone with the forceps, and carefully free the sciatic plexus and nerve from their attachments right down to the gastrocnemius muscle, taking care not to drag upon the nerve. The muscles of the thigh will contract, as the branches going to them are cut. This is an instance of mechanical stimulation. Now pass a thread under the tendo Achillis, tie it, and divide the tendon below it. Strip up the tube of skin that covers the gastrocnemius, as if the finger of a glove were being taken off. Tear through the loose connective tissue between the muscle and the bones of the leg, and divide the latter with scissors just below the knee. Cut across the thigh at its middle. Fix the preparation on the cork plate of the myograph by a pin passed 782 MUSCLE AND NERVE through the cartilaginous lower end of the femur, and attach the thread to the upright arm of the lever by one of the holes in it. Hang not far from the axis by means of a hook a small leaden weight (5 to 10 grammes) on the arm of the lever which carries the writing-point, and move the myograph plate or the muscle-nerve preparation until this arm is just horizontal. Fasten the electrodes from the secondary coil on the cork plate with an indiarubber band ; lay the nerve on them ; and cover both muscle and nerve with an arch of blotting-paper moistened with physiological salt solution, taking care that the blotting- paper does not touch the thread. Or put the preparation in a moist chamber* (Fig. 312, p. 815). Muscle troughs of various kinds may also !Fig. 275. — Lucas's Muscle Trough. A, trough made of hard rubber ; B, a hard rubber ' boss with a hole drilled in it to receive the pin which fastens the gastrocnemius preparation; H, H, electrodes cased in hard rubber except at the ends, which I in the trough carry platinmn wires; C, a brass plate mounted on one side of the trough, carrying a lever with a vertical arm F ending in a hook, which is attached •' by a loop of thread to the tendon of th^ preparation; G, the writing arm of the I lever; K, M, holes in G for lo.ading the muscle. C can be slid horizontally by means of the slots in it, ahd'clamped by the screw E. I, tube for running off the solution. be used, which permit immersion of a muscle (or nerve) in Ringer's solution. A convenient form is shown in Fig. 275, but a trough suffi- cient for the purposes of the student can be easily improvised in any laboratory. Adjust the writing-point to the drum. Begin with such a distance between the coils that a break contraction is just obtained on opening the key in the primary circuit, but no make contraction. The lever will trace a vertical line on the stationary drum. Read off on the scale of the induction machine the distance between the coils, and mark this on the drum. Now allow the drum to move a little, still keeping the writing-point in contact with it ; then push up the secondary coil I centimetre nearer the primary, and close the key. If there is a * Porter's moist chamber is found in many laboratories, and is very con- venient. It consists of a porcelain plate around which runs a groove. A bell- shaped glass cover, which can be lifted ofi at will, rests in the groove. The femur of the muscle-nerve preparation is fixed in a small clamp, composed of a split screw on which moves a nut. By means of the nut the clamp is tightened on the femur. The gastrocnemius hangs vertically down, the thread on the tendo Achillis passing through a hole in the porcelain plate to a lever separately supported on the same stand as the moist chamber. A piece of wet blotting-paper fixed inside the cover keeps the air in the chamber saturated. PRACTICAL EXERCISES 783 contraction, let the drum move a little before opening the key again, so that the lines corresponding to make and break may be separated from each other. If there is still no contraction at make, go on moving the secondary up, a centimetre (or less) at a time, till a make con- traction appears. When the coils are still further approximated, the make may become equal in height to the break contraction, both being maximal — i.e., as great as the muscle can give with any single shock (Fig. 276). (c) Attach a thin insulated copper wire to each terminal of the secondary. Loop the bared end of one of the wires through the tendo Fig. 276. — Contractions caused by Make, and Break Shocks from an Induction Machine. M, make, B, break, contractions. The numbers give the distance between the primary and secondary coils in centimetres. AchiUis, and coil the other round the pin in the femur, so that the shocks will pass through the whole length of the muscle. Repeat the experi- ment of (6), with direct stimulation of the muscle. 2. Stimulation of Nerve and Muscle by the Voltaic Current. — (a) Con- nect a Daniell cell through a key with a pair of electrodes on which the nerve of a muscle-nerve preparation lies. Observe that the muscle con- tracts when the current is closed or broken, but not during its passage. Fig. 277. Simple .Rheocordi arranged to send a Twig of a Current through a Muscle or Nerve. B, battery ;"R, rheocord wire (German silver); S, slider formed of a short piece of thick indiarubber tubing filled with mercury; K, spring key; W, W, wires connected with electrodes. Connect the cell with a simple rheocofd, as shown in JPig. 277, so that a twig of the current of any desired strength may te sent through the nerve. As the strength of the current is decreased by moving the slider S, it will be found that it first becomes impossible to obtain a contraction at break. The current must be still further reduced before the make contraction disappears, for the closing of a galvanic stream is a stronger stimulus than the breaking of it. The break or make con- 784 MUSCLE AND NERVE traction obtained by stimulating a neive with an induction machine must not be confused with the break or make contraction caused by the voltaic current. In the case of the induction machine, the break or make applies merely to what is done in the primary circuit, not to what happens to the current actually passing through the nerve. The current induced in the secondary at make of the primary circuit is, of course, both made and broken in the nerve — ^made when it begins to flow, broken when the flow is over; the shock induced at break of the primary is also made and broken in the nerve. .And although make and break of the actual stimulating current come very close together, the real make, here, too, is a stronger stimulus than the real break. (b) Repeat (a) with the muscle directly connected to the cell by thin copper wires, or, better, unpolarizable electrodes (p. 705). 3. Ciliary Motion. — Cut away the lower jaw of the same frog, and place a small piece of cork moistened with physiological salt solution (0-75 per cent.) on the ciliated surface of the mucous membrane covering the roof of the mouth. It will be moved by the cilia down towards the gullet. Lay a small rule, divided into millimetres, over the mucous membrane, and measure with a stop-watch the time the piece of cork takes to travel over 10 millimetres. Then pour salt solution heated to 30° C. on the ciliary surface, rapidly swab with blotting-paper, and repeat the observation. The piece of cork will now be moved more quickly than before, unless the salt solution has been so hot as to injure the cilia. 4. Direct Excitability of Muscle — Jetton of Curara. — Pith the brain of a frog, and prevent bleeding by inserting a piece of match. Expose the sciatic nerve in the thigh on one side. Carefully separate it, for a length of half an inch, from the tissues in which it lies. Pass a strong thread under the nerve, and tie it tightly round the limb, excluding the nerve. Now inject into the dorsal or ventral lymph-sac a few drops of a I per cent, curara solution. As soon as paralysis is complete, make two muscle-nerve preparations, isolating the sciatic nerves right up to the vertebral column. Lay their upper ends on electrodes and stimu- late; the muscle of the ligatured limb will contract. This proves that the nerve-trunks are not paralyzed by curara, since the poison has been circulating in them above the ligature. The muscle of the leg which was not ligatured will contract if it be stimulated directly, although stimulation of its nerve has no effect. The ordinary contaactile sub- stance of the muscular fibres, accordingly, is not paralyzed. The seat of paralysis must therefore be some structure or substance physiologic- ally intermediate between the nerve-trunk and the general contractile substance of the muscular fibres (p. 712). 5. Graphic Record of a Single Muscular Contraction or Twitch. — Pith a frog (brain and cord), make a muscle-nerve preparation, and arrange it on the myograph plate, as in i (6). Lay the nerve on electrodes connected with the secondary coil of an induction machine arranged for single shocks. Introduce a short-circuiting key (Fig. 232, p. 706) between the electrodes and the secondary coil, and a spring key in the primary circuit. Close the short-circuiting key, and then press down the spring key with the finger. Let the drum off (fast speed); the writing-point will trace a horizontal abscissa line. Open the short- circuiting key, and then remove the finger from the spring key. The nerve receives an opening shock, and the muscle traces a curve. Now adjust the writing-point of an electrical tuning-fork (Fig. 278), vibrating, say, 100 times a second, to the drum, and take a time-tracing below the muscle-curve. Stop the drum, or take ofi the writing-point, the moment the time-tracing has completed one circumference of th? drum, PRACTICAL EXERCISES 785 so that the trace may not run over on itself. Cut off the drum-paper, write on it a brief description of the experiment, with the time-value of s'*s--s?g.s- a H > « o C'Ss o T3 a, t^ « « C I 5- rt "fit* §^-1^5.^ Q, O r.*^ "'2 O c C , w O « " S " ^ "13 c -« -•^S « c-d tS &" (iu-SJS.SdS fe n-§So.sSgg"S i ft*""'- V.M = o s' •2 £ 8 Sj^ M-°s „ 0. S 9 R S (S ci ^ c ■s 4 g o..i2^ « "„-o .H36--J-3S^g'-S2 g V u^ ffl S_ 2 S = Qiog-g -gQ-Ss -i^lifi |§ 1l^'^ 1 |h 1 "M en > s E . 5M § bO |X4 each vibration of the fork, the date, and the name Of the maker of the tracing, and then varnish it. An exactly similar tracing can be obtained by directly stimulating the muscle (curarized or not) . 50 786 MUSCLE AND NERVE 6. Influence of Temperature on the Muscle-Curve. — Pith a frog (brain and cord), make a muscle-nerve preparation, and arrange it on a myograph. Lay the nerve on electrodes connected through a short- circuiting key Vvith the secondary coil of an induction machine, or connect the muscle directly with the key by thin copper wires. Take a Daniell cell, connect one pole through a simple key with one of the upper binding-screws of the primary coil, and the other pole with the metal of the drum. A wire, insulated from the drum, but clamped on the verticalpart of its support, and with its bare end projecting so as to make contact with a strip of brass fastened on the spindle, is con- nected with the other upper terminal of the primary (Fig. 278). At each revolution of the drum the primary circuit- is made and broken once as the strip of brass brushes the.projectiiig end of the wire. The object of this arrangement is to ensure that when the writing-point of the myograph lever has been once adjusted to the drum, successive stiinuli will cause co tractions, the curves of which all rise from the same point. Close the key in the primary, set the drum, Off (fast speed), open the short-circuiting key, and as soon as the rtiuscle'has contracted once, close it again. Now stop the drum, mark with a pencil the position of the feet of the stand carrying the myograph plate, take the writing-point off the drum, and surround the muscle with pounded ice or snow. After a couple of minutes brush away any ice which could hinder the movement of the muscle, rapidly replace; the stand in exactly its original position, with the writing-point on' the drum, and take another tracing. Again take ofr the writing-point, and remove all unmelted ice or snow. With a fine-pointed pipptte irrigate the muscle with physiological salt solution at "30° C, and quickly take another tracing. Then put on a time-tracing with the electrical tuning-fork. Fig. 248,. p. 722, shows a Series of curves obtained in this way. 7. Influence of Load on the Muscle-Curve.— Arrange everything as in 6. Take a tracing first with the lever alone, then with a weight of 10 gra,nimes, then with 50, 100, 200, and 500 grammes (Fig. 247, p. 722). 8. Influence of Fatigue on the Muscle-Curve. — Arrange as in 7, but leave on the same weight (say 10 grammes) all the time. Place the nerve on the electrodes. Leave the short-circuiting key open. The nerve will be stimulated at each revolution of the drum, and the writing- point will trace a series of curves, which become lower, and especially longer, as the preparation' is* fatigued. - Two or four curves can be taken at the same time, if both ends of one or of two brass slips be arranged so as to make contact with the projecting wire at an interval of a semicireumference or quadrant of the drum (Fig. 278). (For specimen curve, see Fig. 279, p. 787.) 9. Seat of Exhaustion in Fatigue of the Muscle-Nerve Preparation for Indirect Stimulation. — ^Whenjthe nerve of a muscle-nerve preparation has' been stimillated until contraction no longer occurs, the muscle can, under ordinary conditions, be madfe to contract by direct stimulation. The seat; of exhaustion is, therefore, not the general contractile sub- stance of the muscular fibres thenlselves. Tfi»determine whether it is the nerve-fibres or some structure or substance intermediate between therii an^' the. ordinary contractile substance of the muscle, perform the following experiments : (a) Pith a frog; make two muscle-nerve preparations; arrange them bo'th on a myograph plate, which has two levers connected with it. Attach each of the muscles 'to a lever in the usual way, and lay both, nerves side by side on the saihe pair of electrodes.. Cover with moist blotting-paper. The electrodes are coniifected with the secondary of an induction machine' arranged for tctknuS. ' With a Cam'el's hair brush PttACTICAL E^EkCISES lH moisten one of the nerves between the electrodes and the muscle with a mixture of equal parts of ether and alcohol, diluted with twice its volume of water, to abolish the conductivity. Or put the mixture in a small bottle, in which dips a piece of filter-paper. The projecting end of the filter-paper is pointed, and the nerve is laid on the point. As soon as it is possible to stimulate the nerves without obtaining contraction in this muscle, proceed to tetanize both nerves till the contracting muscle is exhausted. If the other musile begins to twitch during the stimu- lation, more of the ether mixture must be painted on the nerve. As soon as the stimulation ceases to cause contraction in the non-etherized preparation, wash off the mixture from the other nerve with physio- logical salt solution, and soon contraction may be seen to take place in Fig. 279.— Fatigue Curve of Skeletal Muscle: Gastrocnemius of Frog. Indirect stimulation; taken witti arrangement shown in Fig. 378 (p. 785). , Time-tracing, ■^ of a second. the muscle of this preparation. This shows that the nerve-trunk is still excitable. Now, both nerves have been equally stimulated, and there- fore the exhaustion in the non-etherized preparation was not due to fatigue of the nerve-fibres, but of something between them and the contractile substance of the muscle. 10. Influeiice of Veratrine on Muscular Contraction. — Arrange a drum as in Fig. 278. Pith a frog (brain only), expose the sciatic nerve in one thigh, and isolate it for ^ inch from the surrounding tissues. Pass under it a strong thread, and ligature everything e^fcept the nerve. Now inject into the dorsal or ventral lymph-sac a few drops of o-i per cent, solution of sulphate of veratrine. In a few minutes make two muscle-nerve preparations from the posterior limbs, First put the preparation from the unligatured limb on the myograph plate. Lay 788 MUSCLE AND NERVE the nerve on electrodes connected tlirough a short-circuiting key with the secondary of an induction machine arranged as in Fig. 278. Put the writing-point on the drum and set it off (fast speed). Open the short-circuiting key till the nerve has been once stimulated, then close it again. The curve obtained differs from a normal curve, in that the period of descent (relaxation) is exceedingly prolonged. Now connect the preparation from the ligatured limb with the lever, and take a tracing of a single contraction. Put on a time-tracing with the electrical tuning-fork (see Figs. 257, 258, p. 729). II. Measurement of the Latent Period of Muscular Contraction. — (i) For this the drum must travel at a faster speed than usual. It is most convenient to use a drum rotated very rapidly by a cord attached to a falling weight or by the recoil of a stretched rubber band or spring. The arrangement for automatic stimulation described in Experiment 6 (p. 786) may be employed. Or an electro-magnetic signal may be connected in the primary circuit of the induction coil so that when the primary is closed or opened the writing-point of the signal moves. Arrange the writing-point of the signal on the drum in the same vertical line as the writing-point of the muscle lever, and in the same line place the writing-point of a vibrating electric tuning-fork. The coil is adjusted for single opening shocks as in Experiment 5 (p. 784). Pith a frog, and make a muscle-nerve preparation. Arrange it on the myograph plate. The muscle, or the nerve very near the muscle, is to be excited by a single opening shock while the drum is moving. When the curve has been traced, the latent period is got by drawing a vertical line through the point at which the curve just begins to rise from the abscissa line, and another through the signal mark. The number of vibrations of the tuning-fork included between these two verticals gives the latent period. Or (2) use fhe spring myograph (Fig. 244, p. 720), raising it on blocks of wood. Smoke the glass plate over a paraffin flame, or cover it with paper, and smoke the paper. Connect the knock-over key of the myo- graph with the primary circuit of an induction coil. Arrange a muscle- nerve preparation on the myograph plate. Place electrodes below the nerve as near the muscle as possible, and connect by a short-circuiting keyuWith the secondary. Bring the writing-point in contact with the smoked surface of the spring myograph, so as to get the proper pressure. Seel. that the writing-point of the tuning-fork is in the right position for tracing, time. Then push up the plate so as to compress xhe spring, till Ihe rod connected with the frame which carries the plate is held by the catch. With the short-circuiting key closed, press the release and allow an abscissa line to be traced. Again shove back the frame till it is caught. Push home the rod by means of which the prongs of the tuning-fork are separated, and rotate it through 90°. Close the knock-over key, open the short-circuiting key, shoot the plate again, and a muscle-curve and time-tracing will be recorded. Again close the short-circuiting key, withdraw the writing-point of the tuning-fork, push back the plate, close the trigger key, then open the short-circuiting key, and, holding the travelling frame with the hand, allow it just to open the knock- over and stimulate the nerve. The writing-point now records a vertical line (or, rather, an arc of a circle), which marks on the tracing the moment of stimulation. The latent period is obtained by drawing a parallel line (or arc) through the point of the muscle-curve where it just begins to diverge irom the abscissa line. The value of the portion of the time-tracing between these two lines can be readily determined, and is the latent period. PRACTICAL EXERCISES 789 12. Summation of Stimuli. — Arrange two knock-over keys on the spring myograph at such a distance from each other that the plate travels from one to the other in a time less than the latent period. Connect each key with the primary circuit of a separate induction coil having a couple of Daniells in it. Join two of the binding-screws of the secondaries together; connect the other two through a short- circuiting key with electrodes, on which the nerve of a muscle-nerve preparation is arranged. Push up the secondaries till the break shocks obtained on opening the two knock-over keys are maximal. Then shoot the plate as described in 11, first with one trigger key closed, and then with both. The curves obtained should be of the same height in the two cases, as a second maximal stimulus falling within the latent period is ignored by the nerve or muscle. Repeat the experiment with submaximal stimuli — i.e., with such a distance of the coils that opening of either trigger key does not cause as strong a contraction as is caused when the coils are closer. The curve will now be higher when the two shocks are thrown in successively than when the nerve is only once stimulated. This shows that (submaximal) stimuli can be summed in the nerve. The same could be demonstrated for muscle (p. 730). 13. Superposition of Contractions. — Smoke a drum arranged for auto- matic stimulation as in Fig. 278. Adjust the brass points with a distance of, say, i centimetre between them, so that a second stimulus may be thrown into the nerve at an interval greater than the latent period of muscle. Put two Daniells in the primary circuit. Lay the nerve of a muscle-nerve preparation on electrodes connected through a short-circuiting key with the secondary. Allow the drum to revolve (fast speed) ; open the short-circuiting key till both brass points have passed the projecting wire, then close it. Now bend back the second brass point, and take a tracing in which the first curve is allowed to complete itself. This will not rise as high as the second curve obtained when the two stimuli were thrown in. Repeat the experiment with varying intervals between the brass points — that is, between the two successive stimuli. Put on a time-tracing with the electrical tuning- fork. (For specimen curve, see Fig. 259, p. 730.) 14. Composition of Tetanus. — (a) Adjust a muscle-nerve preparation on a myograph plate, the nerve being laid on electrodes connected through a short-circuiting key with the secondary of an induction machine, the primary circuit of which contains a Daniell cell and is arranged for an interrupted current (Fig. 93, p. 198). The lever should be shorter than that used for the previous experiments, or the thread should be tied in a hole farther from the axis of rotation, so as to give less magnification of the contraction. Set the Neef's hammer going, let the drum revolve (slow speed), and open the key in the secondary. The writing-point at once rises, and traces a horizontal or perhaps slightly-ascending line. Close the short-circuiting key, and the lever sinks down again to the abscissa line. If it does not quite return, it should be loaded with a small weight. This is an example of complete tetanus. (6) Connect the spring shown in Fig. 280 with one of the upper terminals of the primary coil, and the mercury cup with the other. Fasten the end of the spring in one of the notches in the upright piece of wood by means of a wedge, so that its whole length can be made to vibrate. Let the drum off, set the spring vibrating by depressing it with the finger, then open the key in the secondary. The muscle is thrown into incomplete tetanus, and the writing-point traces a wavy curve at a higher level than the abscissa line. Close the short-circuiting key, and the lever falls to the horizontal. Repeat the experiment with 79° MUSCLE AND NERVE ig. 280. — Arrangement for Tetanus. A, upright with notches, in which the spring S is fastened (shown in section); C, horizontal board to which A is attached, and in a groove in which the mercury-cup E slides. The primary coil P is connected with E, and through a simple key, K, with, the battery B, the other pole of which is connected with the end of the spring. The wires from the secondary coil, P', go to a short- circuiting key, K', from which the wires F go off to the electrodes. the spring fastened, so that only f , f, J, J of its length is free to vibra,te. The rate of interruption of the primary circuit increases in proportion to the shortening vof the spring, and the tetanus becomes more and more complete, till ultimately the writing -point marks an unbroken straight line. Put on a time- tracing by means of an electro - magnetic marker connected with a metronome beating seconds or half-seconds (Fig. 88, p. 193). (For speci- men curves, see Fig. 260, p. 730.) 15. Contraction of Smooth Muscles — (i) Spontaneous Rhythmical Contrac- tions. — Immerse in oxygenated Ringer's solution a ring of oesophagus obtained Immediately after death from a cat, or, still better, from a chicken. Or a. segment of rabbit's intestine may be employed as described on p. 446 Use the arrangement lescribed on p. 446. In the case of the cat's oeso- phagus the ring should 3e taken from the lower lalf of the oesophagus, since the upper portion lontains purely striated muscle. Obtain tracings Df the rhythmical con tractions on a slowh moving drum (Fig. 28] ^ (2) Fix one 'end "t 1 piece of cat's oesi phagus, 2 to 5 cen1 1 metres long, to a musclc- :lamp in a moist ;hamber, and the other snd to "a lever writing an a drum. Connect thin copper wires from the secondary coil of an inductorium with the two ends of the piece jf oesophagus. Take tracings to show [a) the curve of a single contraction caused by a single make or break shock, with estimation of the latent period, as in Experi- ment II, p. 788; (6) summation, as in Experiment 12, p. 789; (c),genesig Fig. z8i. — Rhythmical Contractions of CEsophagus of Chicken (Botazzi), PRACTICAL EXERCISES 791 .of tetanus, as in Experiment 14, p. 789; [d) the relations between strength of stimulus and amount of contraction. For this last experi- ment the drum should be stationary while the contraction is being recorded, and should be allowed to move a little between successive contractions. Begin with the secondary at such a distance from the primary that a contraction is just caused by a break shock. Then gradually increase the strength of the stimulus (always using the break) till maximum contraction is obtained. The gradual increase in the response is very clearly seen with the oesophageal preparation (Waller). For further experiments on the contraction of smooth muscle, see pp. 66 and 447. 16. Velocity of the Nerve-Impulse. — Use the spring myograph (Fig. 244, p. 720) or a very rapidly rotating drum. Make a. muscle- nerve preparation from a large frog (preferably a bull-frog), so that the sciatic nerve may be as long as possible. Connect the knock-over key with the primary circuit of an induction machine, which should contain Fig. 282. — Arrangement for Measuring the Velocity of the Nerve-Impulse. A, travel- ling plate of spring myograph ; M, muscle lying on a myograph^plate ; N, nerve lying on two pairs- of electrodes, E and E'; C, Pohl's commutator without cross- wires; K, knock-over key of spring myograph (only the binding-screws shown); K', simple key in primary circuit; B, battery; P, primary coil; S, secondary coil. a single Daniell cell. Arrange two pairs of fine electrodes under the nerve on the myograph plate, one near the muscle, the other at the central end. Connect the electrodes with a Pohl's commutator (with- out cross-wires), the side-cups of which are joined to the terminals of the secondary coil, as shown in Fig. 382. By tilting the bridge of the commutator the nerve may be stimulated at either point. Great care must be taken to keep the nerve in a moist atmosphere by means of wet blotting-paper or a moist chamber; but at the same time it must not lie in a pool of sa,lt solution, as twigs o,f the stimulating current would in this case spread down the nerve; and we could never be sure that the apparent was always the real point of stimulation. The writing- points of the lever and tuning-fork having been adjusted to the ^moked plate, as in 11 (p. 788), the bridge of the Pohl's commutator is arranged for stimulation oC the distal point of the nerve, the, plate is shot witji the short-circuiting key in the secondary closed, and an abscissa line and time-curve traced. Then the .writing-point of the fork is removed and'^^the plate again shot with the key in the secondary open, and a 792 MUSCLE AND NERVE muscle-curve is obtained. The commutator is now arranged for stimu- lation of the central end of the nerve, and another muscle-curve taken. Vertical lines are drawn through the points where the two curves just begin to separate out from the abscissa line. The interval between these lines corresponds to the time taken by the nerve-impulse to travel along the nerve from the central to the distal pair of electrodes. Its value in time is given by the tracing of the tuning-fork. The length of the nerve between the two pairs of electrodes is now carefuUy measured with a scale divided in millimetres, and the velocity calculated (p. 767). 17. Chemistry of Muscle. — Mince up some muscle from the hind-legs of a dog or rabbit (used in some of the other experiments), of which the bloodvessels have been washed out by injecting o'g per cent, salt solution through a cannula tied into the abdominal aorta until the washings are no longer tinged with blood. To some of the minced muscle add twenty times its bulk of distilled water, to another portion ten times its bulk of a 5 per cent, solution of magnesium sulphate. Let stand, with frequent stirring, for twenty-four hours. Then strain through several folds of linen, press out the residue, and filter through paper, (i) With the filtrate of the watery extract make the following observations : (a) Reaction. — To litmus-paper acid. (6) Determine the temperatures at which coagulation of the various proteins in the extract takes place, according to the method described on p. 9.* Put some of the watery extract in the test-tube, and heat the bath, stirring the water in the beakers occasionally with a fpather. Note at what temperature a coagulum first forms. It will be about 47° C. Filter this off, and again heat ; another coagulum will form at 56° to 58°- Filter, and heat the filtrate; a third slight coagulum may be formed at 60° to 65° C, but this represents merely a residue of the myosinogen which was left in solution at the previous heating. A fourth precipitate (of serum-albumin) will come down at 70° to 73°. Saturate some of the watery extract with magnesium sulphate ; a large precipitate will be formed, showing the presence of a considerable amount of globulin. Filter oS the precipitate and heat the filtrate; coagulation will again occur at very much the same temperatures as before, although the total amount of precipitate will be less. Note in particular that there is still some precipitate at 47° to 50°- Paramyo- sinogen possesses some of the characters of both globulins and albumins, for it is partially but not entirely precipitated by saturation with magnesium sulphate, and is not precipitated by sodium chloride. (2) (a) Test the reaction of the magnesium sulphate extract. It will usually be faintly acid to litmus. (6) Heat some of it. Precipitates will be obtained at the same tem- peratures as in (i) (6), but those at 47° to 50° and 56° to 58° will be more abundant. Of the two, that at 47* to 50° will usually be the larger when time is given for it to come down and the heating is gradual. (c) Dilute some of the magnesium sulphate extract with three times, another portion with four times, and another with five times, its volume of water in a test-tube, and put in a bath at 40° C. Coagulation or * It should be remembered that the temperature of heat-coagulation of any substance is by no means an absolute constant. It depends on the reaction, the proportion and kind of neutral salts present, perhaps on the strength of the protein solution and the manner of heating. A solution of egg-albumin, e.g., can be coagulated at a temperature much below 70° when it is heated for a week. Small differences in the temperature of heat-coagula- tion, unless supported by well-marked- chemical reactions, are not enough to characterize protein substances as chemical individuals.* PRACTICAL EXERCISES 793 precipitation will occur in one or all of these test-tubes. To another test-tube of the extract diluted in the proportion which has given the best ' muscle-clot ' add a few drops of a dilute solution of potassium oxalate, and place in a bath at 40°. Coagulation occurs as before. Filter off the clot from all the test-tubes. The filtrate is the ' muscle- serum,' and yields a precipitate of serum-albumin at 70* to 73" C. (3) Myosinogen, like other globulins, is insoluble in distilled water, but soluble in weak saline solutions. Saturation with neutral salts like sodium chloride and magnesium sulphate precipitates myosinogen, but not albumin, from its solutions; saturation with ammonium sulphate precipitates both. Verify the following reactions of myosinogen, using the original magnesium sulphate extract of the muscle : (a) Dropped into water, it is precipitated in flakes, which can be redissolved by a weak solution of a neutral salt (say 5 per cent, mag- nesium sulphate). (6) When a solution of myosinogen is dialyzed, it is after a time pre- cipitated on the inside of the dialyzer as the salts pass out. (c) If a piece of rock-salt is suspended in a solution, the myosin gradually gathers upon it, diffusion, of the salt out through the precipi- tated myosin always keeping a saturated layer around it. {d) Saturate a solution containing myosinogen with crystals of magnesium sulphate, stirring or shaking at frequent intervals. The myosinogen is precipitated. (e) Without adding any salt, simply shake a myosinogen solution vigorously; a certain amount of the myosinogen will be precipitated and the solution will become turbid. This reaction can also be ob- tained with solutions of other proteins, such as albumins (Ramsden). Extracts qualitatively similar to those obtained from the muscles of a freshly-killed animal can be got from muscles that have entered into rigor, but the quantity of the various proteins going into solution is less. 18. Reaction of Muscle in Rest, Activity, and Rigor Mortis. — (a) Take a frog's muscle, cut it across, and press a piece of red litmus-paper on the cut end; it is turned blue. Yellow turmeric paper is not affected. (6) Immerse another muscle in physiological salt solution (0-75 per cent, for frog's tissues)at 40° to 42° C. It becomes rigid. The reaction becomes acid to litmus-paper, and also turns brown turmeric paper yellow. (c) Plunge another muscle into boiling physiological salt solution. It becomes harder than in (6). (d) Stimulate another nruscle with an interrupted current from an induction machine (Fig. 93, p. 198), till it no longer contracts. The reaction is now acid to litmus-f)aper. Brown turmeric paper may also be turned yellow. (e) To demonstrate the formation of lactic acid in muscle in heat rigor or fatigue, perform the following experiment: Pith a frog, and afterwards leave it for half an hour at rest, so that the lactic acid pro- duced in the movements connected with the pithing operation may disappear from the muscles. See that the circulation in the hind-limbs is not interfered with by pressure or flexion. Then remove both hind- limbs. Carefully, but rapidly, remove the muscles of one from the bones with as little manipulation as possible. Immediately place them in a small mortar cooled in ice, and containing some sand and 20 or 30 c.c. of ice-eold 95 per cent, alcohol, and quickly grind them up. Produce heat rigor (p. 751) of the muscles of the other hind-limb, or fatigue them with induction-shocks, and then grind them up under alcohol in the same way. Filter the alcoholic extracts, and then •794 MUSCLE AND NERVE evaporate them to dryness on the water-bath. Rub up the residues with a few c.c. of hot water. Add to each aqueous extract a small quantity (say a decigramme) of finely powdered charcoal. Then heat each extract to boiling in a test-tube, and filter. Evaporate the filtrates to dryness, and apply Hopkins's Reaction for Lactic Acid. — ^The reagents required are (i) a very dilute alcoholic solution of thijophene (C4H4S) (10 to 20 drops in 100 c.c); (2) a saturated solution of copper sulphate; and (3) ordinary strong sulphuric acid. Have ready a glass beaker containing water briskly boiling. Place about 5 c.c. of strong sulphuric acid in a test-tube, with i drop of the copper sulphate solution.* Add to the mixture a few drops of the solution to be tested, and shake welLf (In the case of the muscle extracts the dry residues are dissolved in the 5 c.c. of strong sulphuric acid, the acid transferred to test-tubes, and the test proceeded with by the addition of the copper sulphate solution, etc.) Now place the test-tube in the boiling water for one to two minutes. Then cool it well under the cold-water tap, and add 2 or 3 drops of the thiophene solution from a pipette. Replace the tube in the boiling water, and immediately observe the colour. If lactic acid is present, the liquid rapidly takes on a bright cherry-red colour, which is only permanent if the test-tube be cooled immediately after its appearance. The tube should al-ways be cooled as described, before addition of the thiophene, as the gradual appearance of the colour on re-warming makes the test more delicate. (The extract of the resting limb generally gives a negative, that of the other a strongly positive, reaction.) * The copper sulphate is added to hasten the oxidation that follows. t For practice use a i per cent, alcoholic solution of lactic acid. The test cannot be applied directly to material which chars with the strong sulphuric acid used. In this case preliminary extraction of 'the lactic acid is necessary. Alcohol should be used as the solvent, or if ether is employed it must first be well washed to remove aldehyde-yielding products, since the colour-change is due to an aldehyde reaction with thiophene. CHAPTER XV ELECTRO-PHYSIOLOGY A LITTLE more than a hundred years ago the foundation both of electro- physiology and of the vast science of voltaic electricity was laid by a chance observation of a professor of anatomy in an Italian garden. It is indeed true that long before this electrical fishes were not only popularly known, but the shock of the torpedo had been to a certain extent scientifically studied. But it was with the discovery of Galvani o'f Bologna that the epoch of fruitful wOrk in electro-physiology began. Engaged in experiments on the effect 'of static and atmospheric elec- tricity in stimulating animal tissues, he happened one day to notice that some frogs' legs, suspended by copper hooks on an iron railing, twitched whenever the wind brought them into contact with one of the bars (p. 814), He concluded that electrical charges were developed in the animal tissues themselves, and discharged when the circuit was completed. Volta, professor of physics at Pa via, fixing his attention on the fact that in Galvarii's experiment the metallic part of the circuit was composed of two metals, maintained that the contact of these was the real origin of the current, and that the tissues served merely as moist conductors to complete the circuit ; and after a controversy lasting for more than a decade, he finally clinched his argument by constructing the voltaic pile, a series of copper and zinc discs, every two pairs of which were separated by a disc of wet cloth, or paper moistened with salt solution. The pile yielded a continuous current of electricity. ' So,' said Volta, ' it is clear that the tissue in Galvani's experiment only acts the part of the cloth." Galvani, however, had shown in the meantime that contraction without metals could be obtained by dropping the nerve of a preparation on to the muscle (p. 814) ; and itsoon began to be recog- nized that both Galvani and Volta were in part right, that the tissues produce electricity, and that the contact of different metals does so too. Although it is curious to note how completely the growth of that science of which Volta's discovery was the germ has overshadowed the parent tree planted by the hand of Galvani, yet animal electricity has been deeply studied by a large number of observers, and many interesting and important facts have been brought to light. Since it is in muscle and nerve that the phenomena of electro- physiology are seen in their simplest expression, and have been chiefly studied, we shall develop the fundamental laws with reference to muscle and nerve alone, and afterwards apply them to other excitable tissues. T. All points of an uninjured resting muscle or nerve are approxi- mately at the same potential {or iso-electric). In other words, if any 795 796 ELECTRO-PH YSIOLOG Y two points are connected with a galvanometer by means of un- polarizable electrodes, little or no current is indicated. (Although it is scarcely possible to isolate a muscle without its showing some current, the more carefully the isolation is performed, the feebler is the current ; and between two points of the inactive, uninjured ventricle of the frog's heart no electrical difference has been found. Frogs' nerves kept ten to twenty hours after excision in physiological salt solution to which a little calcium salt and frog's blood have been added, are absolutely iso-electric.) 2. Any uninjured point of a resting muscle or nerve is at a different potential from any injured point. The difference of potential is such that a current will pass through the galyanometer from uninjured to injured point and through the tissue from injured to uriinjured point (current of rest, or demarcation current, or injury response) (Fig. 283). 3. Any unexcited point of a muscle or nerve is at a different potential from any excited point, and any less excited point is 'at a different Fig. 283. — A, uninjured, B, injured, portion of nerve; G, galvanometer. The large arrows show direction of demarcation current or current of rest, the small arrows direction of negative variation or action current. Fig. 284. — Diagram of Currents ifof Rest in a Regular Muscle, or Muscle Cylinder. E, equator. The dotted lines join points at the same po- tential, between which there is' no current. potential from any more excited point. The difference of potential is such that a ciirrent will pass through the galvanometer to the excited from the unexcited or less excited point (action current, or negative variation, or excitatory electrical response). It has been customary in physiological writings to speak of the electrical change in injured or active tissue as a negative one, because when the tissue is led off to a galvanometer the current passes from the galvanometer to the injured or excited portion of the tissue. It may be called with greater precision ' galvanometrically negative; ' It is in this sense that we sljair employ the term. The best object for experiments on the demarcation current is a straight-fibred muscle like the frog's sartorius. If this muscle be taken, and the ends cut off perpendicularly to the surface, a muscle-prism or muscle-cylinder is obtained (Fig. 284). The strongest current is got when one electrode is placed on the middle of either cross-section, and the other on the ' equator ' — ^that.is, on a line passing round the longi- tudinal surface midway between the ends. The direction of this current is from the cross-section towards, the equator in. the muscle. If the electrodes are placed on symmetrical points on each side of the equator, there is no current. NEGATIVE VARIATION 797 Current of Action, or Negative Variation. — ^When a muscle or nerve is excited, an electrical change sweeps over it in the form of a wave. Suppose two points, A and B (Fig. 285), on the longi- tudinal surface of a muscle to be connected with a capillary electro- meter (p. 702), the movements of the mercury being photographed on a travelling surface — ^for example, a pendulum carrying a sensitive plate. Let the muscle be excited at the end, so that the wave of excitation will be propagated in the direction of the arrow. The wave will reach A first, and while it has not yet reached B, A will Eig. 285. — Diagram to illustrate Propagation of the Electrical Change along an Active Muscle or Nerve. Suj)pose AB to be a horizontal bar representing the muscle or nerve. Let C be a curved piece of wood representing the curve of the electrical change at any point. Let W, W be two glass cylinders connected by a flexible tube, the whole being filled with water. Suppose the rims of the cylinders originally to touch AB at the points A and B, and let them be movable only in the vertical direction. The level of the water being the same in both, there is no tendency for it to flow from one to the other. This represents the resting state of the tissue when A and B are symmetrical points. Now let C be moved along the bar at a uniform rate. The cylinder W, being free to move down, but not horizontally, will be displaced by C, and, if it is kept always in contact with its curved margin, will, after describing the curve of the electrical variation, come again to rest in its old position at A. B will do the same when C reaches it. But since C reaches A before B, the level of the water in B will at first be higher than that in A, and water will flow from B to A as the current flows through the galvanometer. This will correspond to the time during which the point of the tissue represented by A would be galvanometrically negative to a point represented by B. Later on, when C has reached the position shown by the dotted lines, the level of the water in A will be higher than that in B, and a flow will take place in the opposite direction to the first flow. This corresponds to a second phase of the electrical variation. become negative to B; If there is a resting difference of poteritial between A and B, this will be altered, the new and transitory differ- ence adding itself algebraically to the old. When the wave reaches B, it may already have passed over A altogether, and B now be- coming, negative to A, there will be a movement of the meniscus of the electrometer in the opposite direction. This is called the 798 Etncrno-PHystoLOGV diphasic current of action. If the wave has not passed over A before it reaches B, as would in general be the case in an actual experiment, there will be first a period during which A is relatively negative to B (first phase); this will end as soon as B has become iso-electric with A, and will be succeeded by a period during which B is rela- tively negative to A (second phase). Since the wave takes time to reach its maximum, it is evident that a well-marked first phase will be favoured when the interval between its arrival at A and at B is Ipng, for in this case A will have a chance of becoming strongly negative while B is still normal. Similarly, if A has again become norma;l, or nearly normal, before the maximum negative change has passed over B, a strong second phase will be favoured. The heart- muscle, accordingly, wherethe wave of contraction, and its accom- pan3ang electrical change, move with comparative slowness, is better suited for showing a well-marked diphasic variation than skeletal muscle, and 'still better suited than nerve. In the gastroc- Fig. 2&6. — Photographic Electrometer Curves from Sartorius Muscle (Sanderson). The darkly-shaded curve represents the ■ diphasic veiriation of ' the' uninjured muscle; the lightly-shaded curve the monophasic variation of the muscle after injury of one end. The toothed curve at the top is the tinie-tracing registered by photographing the prong of a tuning-fork vibrating five hundred times a second. nemius muscle of the frog, when excited through its nerve, the elec- trical response begins about ixrW second, and the change of form of the muscle about -x-i^sis second, after the stimulation. It is believed that in a muscle directly excited the electrical change begins in less than YiTbTT second, and the mechanical change in yw^ts second (Burdon Sanderson, Figs. 286-291). When one electrode is placed on an injured part, the wave of action and of electrical change diminishes as it reaches the injured tissue; and if the tissue is killed at this part, it diminishes to zero; so that here the second phase may be greatly weakened or may disappear altogether, and we then have what is called a monophasic variation. In this case the current of action can be deanonstrated, even for a single excitation, but still better for a tetanus, with an ordinary galvan- ometer, which in general is not quick enough to analyze a diphasic variation with equal phases, and gives, therefore, only their algebraic sum — that is, zero. When the muscle or nerve is tetanized, the action current appears, while stimulation is kept up, as a permanent deflection representing the ' sum ' of the separate effects. It is in the opposite direction to the current of rest, since the injured tissue, being less NEGATIVE VARIATION 799 affected by the excitation, and therefore undergoing a smaller negative change than the uninjured, becomes relatively to the latter less nega- tive. Appearing as a diminution or reversal of- the current of rest, it was called the negative variation. The term negative is not used here iri ' its electrical, but in its algebraic, sense, and merely as indicating the direction of the current with reference to that of the demarcation current. It is in this sense that ' negative variation ' and the converse term, 'positive variation,' are used (pp. 8io, 8ii )in speaking of the electrical changes produced in glands and in the retina by stimulation. i^jxnixr A. B. Fig. 287. — ' Spike ' (Dipliasic Variation) of Uninjured Gastrocnemius (Sanderson). A photograplied on slow, B on fast-moving, plate. Fig. 288. — Variation of 1 jured Gastrocnemius (Se derson). A 'spike' f lowed by a ' hump.' Fig. 289. — Variation of Injured Gas- trocnemius (Sanderson). The plate was moving ten times faster than [in Fig. 288. Fig'. 290. — Variation of Uninjured Muscle excited Eighty- Four Times a Second (Sanderson). Fig. 291.— Curve of an Injured Muscle' excited Sixty Times a Second (Sanderson). 8oo ELECTRO-PHYSIOLOGY When the current of rest is compensated by a branch of an external current just sufficient to balance it and bring the galvanometer image back to zero (Fig. 226, p. 701), the action current appears alone in un- diminished strength. This shows that the latter is not due to a change of electrical resistance during excitation, since such a change would equally affect current of rest and compensating current, and they would still balance each other. The action current is really due to a change of potential, which can be measured by determining what electro- motive force is just required to balance it, and which may actually exceed that of the current of rest. Thus, Sanderson and Gotch obtained an average of 0"o8 of a Daniell cell (the electromotive force of the Daniell would be about a volt) as the electromotive force of the action current due to a single indirect excitation of a vigorous frog's gastroc- nemius, and about 0-04 Daniell as that of the cijrrent of rest. The electromotive force of the current of rest in the rabbit's nerve was found by du Bois-Reymond to be 0-026; Gotch and Horsley fouhd the average for the cat o-oi, and for the monkey only 0'005. That the fusion of the successive variations of a tetanized muscle, as seen with an ordinary galvanometer, is only apparent has been shown by means 6f tl^e capillary electrometer_or the string galvanometer. Even with a frequency of stimulation far beyond what is necessary for complete tetanus, each stimulus is answered by a movement of the meniscus (Figs. 290, 291). In nerve, also, each of two successive stimuli causes its appropriate electrical change when they are separated by an interval longer than a certain small fraction of a second. The precise interval at which the second stimulus ceases to be efiective depends on the temperature of the nerve, being markedly inrceased by cold (Gotch and Burch). The rate of propagation of the electrical change in muscle is the same as that of^the mechanical change, and in nerve the same as that of the nervous impulse. The velocity of propagation of the diphasic variation along a fresh sartorius at 14° C. was in one experiment , 2-8 metres, in another at 18° C., 3-5 metres (Sanderson). (See p. 733.) ' Lucas has pointed out that in strict accuracy what is observed is merely ' that the time interval separating contraction at one point of the muscle \ from contraction at another is equal to the time interval, separating the electrical changes which occur at the same points. The facts observed do not formally prove that either the contraction or the elec- trical disturbance. is propagated at all. So far as they go, some other perfectly distinct change may be propagated, which at all points of the fibre at which it arrives sets up both the contraction and the electrical change. Subh direct evidence, howeye^', as we possess goes to show that it is the electrical disturbance which is the propagated one, and that this evokes the contractile disturbance. There is ample evidence that- the excitatory electrical response is a normal physiological phenomenon. In human skeletal muscles the current of action has been demonstrated by connecting a gal- vanometer with ring electrodes passing round the forearm, and throwing the muscles into contraction. A diphasic variation is thus obtained; and the electrical change travels with a velocity of as much as twelve metres per second> which is greater than the velocity in frogs' mjiscles. Electroniotive changes are likewise associated with the beat of the heart. Action currents Jiave also been detected in the phrenic nerves of living animals accompanying the respiratory THEORIES OF DEMARCATION AND ACTION CURRENTS 8oi ddschaxge (Reid and Macdonald), in the vagi accompanying the movements of the lungs, in the oesophagus during swallowing, in the cutaneous sensory nerves in response to the ' adequate ' stiniulus of pressure (Steinach), in the retina in response to the adequate stimulus of light, in glands during secretion, in the central nervous system during the passage of impulses along its conducting paths. Some of these will be further considered a little later on. As to the interpretation of the facts we have been describing, and which are summed up in the three propositions on p. 796, two chief doctrines long divided the physiological world: (i) the theory of du Bois-Reymond, the pioneer of electro-physiology, and (2) the theory of Hermann. It was believed by du Bois-Reymond that the current of rest seen in injured tissues is of deep physiological import, and that the electrical difference which gives rise to it is not developed by the lesion as such, but only unmasked when the electrical balance is upset by injury. He looked upon the muscle or nerve as built up of elec- tromotive particles, with definite positive and negative surfaces ar- ranged in a regular manner in a sort of ground-substance which is elec- trically indifferent. The ' negative variation' he supposed to depend on an actual diminution of previously existing electromotive forces ; and from this conception arose its his- toric name . Hermann and his school assumed that the uninjured muscle or nerve is ' streamless,' not because equal and opposite electromotive forces exactly balance each other in the substance of the tissue, but be- cause electromotive forces are absent until they are called into existence (by chemical changes) at the boun- dary, or plane of demarcation, be- tween sound and injured tissue. For this reason du Bois-Re5miond's current of rest is called in the terminology of Hermann the ' demarca- tion ' current. The newer theories, such as Macdonald 's, have sought to take account of the recent developments of physical chemistry, and it is unquestion- able that it is here the real explanation is to be found. There is little doubt that the electrical phenomena of the tissues are connected with the existence in them of membranes, envelopes, or sheaths, physiological if not always anatomical, which are relatively impermeable to certain ions. When such a sheath is injured, these ions, carrying with, them their electrical charges, may be supposed to migrate with abnormal freedom through the injured part. A new distribution of electricity is thus established in the tissue, and differences of potential depending upon differences in the concentration of the ions at different points are set-up. Bernstein and Tschermak, from an investigation of the thermo- d3mamic relations of bio-electrical currents, have come to the conclusion that they are analogous to the currents produced by so-called concentra- 51 Fig. 292. — ^Upper Curve, Record of the Electrical Changes in the Vagus Nerve (so-called ' Electrovagogram '), taken with the String Galvanometer. The small waves on it are synchronous with the heart-beats, while the large waves are synchronous with the respi- ratory movements, the mechanical record of which constitutes the second curve (ascent, inspiration). The lowest curve is a mechanical record of the pulse (Einthoven), ' 8o2 ELECTRO-PHYSIOLOGY tion cells — i.e., arrangements of solutions of electrol5rtes of difierent concentration in contact with each other. Since the development of the new electrical condition depends upon the fundamental structure of the tissue, these modern views lead us back to du Bois-Reymond's doctrine of a pre-existing electrical equilibrium connected with the essential physiological properties of muscle or nerve. But instead of his electromotive elements and their definite arrangement, we have the ions and their definite relation to the semi-permeable membranes. Relation between the Action Current and Functional Activity. — Although the negative variation is so general an accompaniment of excitation, and is even within tolerably wide limits, in muscle and nerve at least, pretty nearly proportional to the strength of the stimulus, it is at present impossible to say definitely what the chemical or physical changes are which underlie it. Unquestionably the electrical changes are closely related to the excitatory process and to the functional activity of the tissues. In the case of nerve some writers, indeed, assume that the redistribution of potential associated with the excited state is identical with the nervous impulse, but the common view is that the negative variation is an accompaniment of some other change which constitutes the propagated disturbance. There is at present no clear experimental evidence sufiicient to decide the question. From time to time attempts have been made to show that the two processes can be dissociated, but none of the experiments so far reported are really crucial. Like the demarcation current, the action current and the excitation which accompanies it may be due to changes in the permeability of membranes or changes in the concentration of certain ions. Although the electromotive changes caused by excitation are much more transient than those caused by injury, everything suggests that there must be some deep analogy between the two conditions. Some have supposed that what may be called a subdued and more or less permanent excitation exists in the neighbourhood of the injured tissue, an excitation which, like some other forms, does not spread, and that this explains the similarity of electrical condition in activity and injury. It is, of course, clear that energy must be transformed to produce an electromotive force capable of doing work. It may be assumed that this energy is ultimately derived from the stock of chemical energy in the tissue-substance. But whether in the final transformation the eleotrical phenomena are the expression of chemical changes or of physical (osmotic) changes, or of both, we do not know. In the case of muscle it is possible that the liberation of lactic acid, which there are several reasons for regarding as essentially concerned in the initiation of the mechanical change, is associated in some way with the appearance of the negative variation. It is known that the latter, although it begins before the contraction, and very rapidly reaches its maximum, declines more gradually, so that it overlaps the mechanical change of form. This is particularly well seen in veratrinized muscles (p. 729), in which the electrical variation, like the contraction, is greatly prolonged (Garten). Polarization of Muscle and Nerve.— We have already spoken of electrical excitation and of the changes of excitability caused by the passage of a constant current (p. 759). We are now to see that these physiological effects are accompanied by, and indeed very ■closely related to, more physical changes which the galvanometer or electrometer reveals to us. Since these throw light on the ELECTROTONIC CURRENTS 803 physical, and therefore ultimately on the physiological, structure of the tissues, they have been deeply studied, especially in nerve. There is no question that they depend upon the presence in the tissues of membranes presenting a relatively great resistance to the passage of ions. When a current is passed by means of unpol arizable electrodes (Fig. 230, p. 705) through a muscle or nerve for several seconds, and the tissue connected to the galvanometer immediately after this polarizing current is opened, a deflection is seen indicating a current (negative polarization current) in the opposite direction. This (negative) polarization, like the poJarization of the electrodes seen after passage of a current through any ordinary electrolytic con- ductor, dilute sulphuric acid, e..g., depends on the liberation of ions (p. 423) at the kathode and anode. It is seen not only in muscle, nerve, and other animal tissues, but also in vegetable structures, and indeed, to a certain extent, in unorganized porous bodies soaked with electro- lytes. In muscle and nerve, however, it is particularly well marked; and although it is not bound up with the life of the tissue, and may be obtained when this has become quite inexcitable, it is nevertheless dependent on the preservation of the normal structure, for a boiled muscle shows but little negative polarization. When the polarizing current is strong, and its time of closure short, we obtain, on connecting the tissue with the galvanometer after opening the current, not a negative, but a positive deflection, indicating a current in the same direction as that of the polarizing stream. This is really an action stream, due to the opening excitation set up at the anode (p. 715). It is only obtained when the tissue is living, and is far more strongly marked in the anodic than in the kathodic region. Suppose that the nerve in Fig. 293 is stimulated by the opening of the battery B, and that, immediately after, the nerve is connected with the galvanometer G by the electrodes E, Ej. Suppose, furthter, that the shaded region near the anode remains more excited for a short time than the rest of the nerve, and we have seen (p. 761) that after the opening of a strong current there is a defect of conductivity, especially in the neighbourhood of the anode, which would tend to localize excita- tion. An action current will pass through the galvanometer from E^ to E, and through the nerve in the same direction as the original stimu- lating stream. Under certain conditions a state of continuous excita- tion in the anodic region of a nerve is shown by a tetanus of its muscle {RitUr's tetanus, p. 715, and Fig. 294). Electrotonic Currents. — If a current be passed from the battery through a meduUated nerve (Fig. 295) in the direction indicated by the arrows, while a galvanometer is connected with either of the extrapolar areas, as shown in the figure, a current will pass through the galvanometer, in the same direction in the nerve as the polar- izing current, so long as the latter continues to flow. These currents are called electrotonic (in the kathodic region katelectro- tonic ; in the anodic, anelectrotonic) . The exact mode of their produc- tion is obscure. Similar currents can be detected in artificial models consisting of a good conducting core and a badly conducting envelope ; for example, a platinum wire in a glass tube filled with saturated zinc sulphate solution, or a zinc wire covered with cotton-wool soaked in salt solution. In such models it appears to be essential that there ELECTRO-PH YSIOLOG Y should be polarization (separation of ions) at tte boundary between the core and the sheath — i.e., between the wire~and the liquid, where • the current passes, from the one to the other. ' A current led into the sheath tries, so to speak, to pass inostly by the good conducting wire. If this is not polarizable — if it is, e.g., a Fig. 293.— Diagram, to show Dis- tribution of ' Positive Polar- zation' after openiag Polar- izing Current. B, battery; G, galvanometer. The dark shading signifies that, the ex- 'pitation to which the current ' Causing the positive deflection after the opening of the polar- izing currfent is due is greatest in the immediate neighbour- hood of the anode, and fades away in the intrapolar region. + indicates the anode, and - the kathode of the polariz- ing current. Fig. 294. — Ritter's Tetanus. A strong voltaic current was passed for some time through the nerve of a muscle - nerve preparation. On opening the circuit, the muscle gave one strong con- traction, and then entered into irregular tetanus, which continued for four minutes. (Only the first part of the ^ tracing is reproduced.) zinc wire surrounded by saturated zinc sulphate solution — ^there is little or no spreading of the current outside the electrodes : it passes at once into the core, and so on to the other electrode. If, however, there is polarization when the current passes from the liquid into the wire, as is the case in the platinum-zinc sulphate .or the zinc-sodium chloride .combinations, the stream }spreads longitudinally in the sheath, since the polarization introduces a virtual resistance at the surface, of the wire, in comparison with which the difference in resis- tance of an oblique and a direct transverse path through the liquid becomes small. It has been supposed that in meduUated nerve a similar polarization takes place at the boundary between some part of the nerve-fibre which may be called a core, and another part which may be called a sheath — ^for instance, between the axis-cylinder and the medullary sheath, or between the latter and the neurilemma. It is known that the electrical resistance of nerve in the Fig. 295. — Diagram showing Direction of the Extra- polar Electrotonic Currents. + is the anode . and - the kathode of the polarizing current. ELECTROTONIC CURRENTS ,805 transverse direction is much greater (five to seven times)* than the longitudinal resistance., Since a rapidly-established polarization would, by the ordinary methods of measurement, appear as a resistance, this has been adduced as evidence of the great capacity of nerve for polar- ization by a current passing across the fibres. It is, however, probable, from what we know of the high electrical resistance of the physiological envelopes of such cells as the red blood-corpuscles (p: 26),- that the great transverse resistance of nerve, and indeed the electrotonic currents, are due in part, if not wholly, to the true resistance of one or more of its envelopes (perhaps the medullary sheath). Examples of such differences of resistance even in the fiuid constituents of one and the same animal structure are not wanting. For instance, the specific resistance of the yolk of a hen's egg may be three times greater than that of the white. The electrotonic currents cannot spread beyond a ligature ; they are stopped by anything which destroys the structure of the tissue; they are affected by various reagents. But this does not prove that they are other than physical in origin, for what destroys the structure of the tissue or modifies its molecular condition may destroy or diminish its capacity for polarization, or alter its electrical resistance. There are, however, certain facts which indicate that physiological factors, as well as physical, are concerned. While the currents obtained from core-models show a general resemblance to the electrotonic currents of meduUated nerve, there is one significant difference : in the former the katelectrotonic and anelectrotonic currents are of equal intensity; in the latter the anelectrotonic preponderates. The most probable ex- planation is that the anelectrotonic current of medullated nerve is made up of two distinct electrical effects, one physiological in nature, the other dependent merely on the structure and physical properties of the fibres, while the katelectrotonic current is wholly physical. It is in favour of this hypothesis that under the influence of ether, which abolishes the physiological functions of nerve, the anelectrotonic curirent diminishes till it becomes equal to the kateletrotonic. Non-meduUated nerves, in which the conditions for physical electrotonus, if present at all, are only feebly developed, and which exhibit no katelectrotonic current, or only a very weak one, show an anelectrotonic current, which is abolished by ether, and seems to represent the physiological portion of the anelectro- tonic current of medullated nerve. A nerve may be stimulated by an electrotonic current produced in nerve-fibres Ijring in contact with it. A well-known illustration of this is the experiment known as the paradoxical contraction (Practical Exercises, p. 816). The current of action of a nerve can also stimulate another nerve when the excitability of both is greater than normal, as is the case in the nerves of frogs kept in the cold. This comes under the head •of secondary contraction. But the best-known form of secondary contraction is where a nerve, placed on a muscle so as to touch it in two points (Fig. 296), is . stimulated by the action-current of the muscle, and, causes its own muscle to contract. A secondary tetanus * Since a part of the current is conducted by the connective tissue and other structures lying between the nerve-fibres, and the longitudinal and transverse resistance of these tissues may be supposed equal, the disproportion between the longitudinal and transverse resistance of , the nerve fibres them- selves is probably much greater than this. 8o6 ELECTRO-PHYSIOLOGY can be obtained in this way by dropping a nerve on an artificially tetanized muscle. The beat of the heart causes usually only a single secondary contraction when the sciatic nerve of a frog is allowed to fall on it (p. 201). But when the diphasic variation is well marked, as it is in an uninjured heart, there may be a secondary contraction for each phase — i.e., two for each heart-beat. Excitation of one muscle may in the same way cause secondary contraction of another with which it is in close contact. Fig. 296. — Secondary Contrac- tion. The nerve of muscle M touches muscle M' at a and 6. Stimulation of the nerve of M' at S causes contraction of M. Fig. 297. — Electrometer Record from Rab- bit's Heart (Gotch). The heart was ex- posed and beating in situ. Contacts, one on base of right ventricle, the other on right apex. The commencement of the beat is on the left-hand edge of the dark line V. The length of the dark line shows the duration of the beat. Upward move- ment signifies relative negativity (activity) of the part at or near the base contact. Time-trace at top, one-fifth second. The electromotive phenomena of the heart and of the central ner- vous system are naturally included under those of muscle and nerve. Heart. — Records of the electrical changes obtained with the capillary electrometer or string galvanometer from the exposed ventricles vary in certain details with the position of the two contacts. When one contact is on the base of the ventricles (in the rabbit) near the auriculo-ventricular groove, and the other on the apex (Fig. 297), for each beat of the ventricle the electrometer record shows (i) a sharp rise, indicating relative negativity (activity) of the base; (2) an equally sharp fall, indicating relative negativity at the apex ; (3) a slower but marked rise, indicating an increase or a fresh development of relative negativity at the base; (4) a more rapid fall, which returns first slowly, then quickly, until (i) follows again (Gotch). The time between the beginning and the top of rise (i) is believed to correspond to the time of transmission of the ELECTRO-CARDIOGRAM 807 active state from the base to the apex. The rate of propagation on the rabbit's ventricle varies from i to 3 metres a second, accord- ing to the rate of the heart-beat! Such observations have been inter- preted as indicating that the excitation, with its accompanying electrical change, begins at the base, then develops in the region of the apex, and finally involves the poition of the ventricles near the aorta and pulmonary artery, possibly extending even into the roots of these vessels. The full explanation of this seemingly erratic course of the excitation wave is doubtless dependent upon a full knowledge of the course and connections of the conducting system. Fig. 298.— Electrometer Record from Tortoise Heart (Gotch). One contact upon the sinus, the other on the apex of the ventricle. One complete beat shown. Upward movement signifies relative negativity of the sinus contact. The dark line A shows the auricular effect, and the dark line V the ventricular effect. Time-trace at top, one-fifth second. and this we do not possess as yet. The fact that the ventricle is originally developed from a tube with a venous and an arterial end, and that this tube later on becomes bent upon itself so that the two ends (the auricular or venous and the aortic or arterial) lie together at the base of the ventricle, probably affords the clue. It should be mentioned, however, that this explanation of the second rise of the ventricular curve (the T-wave, according to Einthoven's nomen- clature. Fig. 304) is by no means universally accepted. Einthoven, who has worked with the string galvanometer, believes that be- tween the first (R) and the second (T) ventricular waves the whole of the ventricle is in contraction, and that there is no difference of 8o8 ELECTRO-PHYSIOLOG Y Fig. 299. — Electro-Cardiograms from Man (Capillary Electrometer) (Einthoven and Lint). Obtained from the same individual at rest (upper curve), and immediately after vigorous muscular exercise (lower curve). The elevations A, C, D, indicate negativity of base to apex ; the notches B and Cj^, nega- tivity ;of apex to base. potential at this time between its various parts. The T-wave he considers to be produced, when it is present, merely because the excitated state does not dis- appear simultaneously over the whole ventricle. In the ventricle of the frog and tortoise the same order of development of the negative change is seen, the base first becoming rela- tively negative, then the apex, and then the neigh- bourhood of the origin of the aorta (Fig. 298). Under certain conditions the action current of the heart may stimulate the phrenic nerves, causing the dia- phragm to contract synchro- nously virith the heart. The Human Electro-Cardi- ogram. — ^An electrical change accompanies each beat of the human heart. Waller first showed how this may be demonstrated by means of the capillary electrometer. Einthoven and Lint then investi- gated the pheno- menon on a large number of persons. From the photo- graphic records of the movements of the meniscus they constructed the true electro - car- diographic curves* (Fig. 300), which express the actual changes in the po- tential difference between the two ■points led ofif. They distinguished in every one of these constructed electro-cardio- grams five points or cusps, three of which indicate re- lative negativity Fig. 300. — Constructed Elec- tro-Cardiograms from Man (Einthoven and Lint). Time is laid ofl along the hori- zontal, and electromotive force along the vertical axis, the same space being allot- ted to ten millivolts {i.e., rhi volt) as to one second.. Fig. 3or. — Illustrating the Position of Favourable and Unfavourable Leads for the Human Electro- Cardiogram (Waller). * In all accurate work with the capillary electrometer such curves must be obtained by constructionirom the direct photographic records, which do not themselves give an absolutely true picture of the variations. ELECTRO-CARDIOGRAM 809 of the base of the heart to the apex, and two negativity of the apex to the base. The capillary electrometer has now been superseded by the string galvanometer (p. 700) for the investigation of the human electro- cardiogram (Figs. 302-305). But a sample of the records obtained by the former method (Fig. 299), with the corresponding constructed 1 1 1 i I ITT -'---'r-'tr-'l'^ ^ V¥\ ' ilc I" X I Fig, 302. — Human Electro-Cardiogram (String Galvanometer) (Einthoven). Led off from the two hands, i mm. of the abscissa corresponds to o-oi second. Fig. 303. — Human Electro-Cardio- gram (String Galvanometer) (Einthoven). Led off from right hand and left foot. o,iSec. Fig. 304. — Schematic Representation of Electro-Cardiogram (String Galvanometer) (Einthoven). Five points arellettered at which the curve changes sign. P cor- responds to the auricular contraction; the other four are included in the ven- tricular cycle. Fig- 305. — Electro-Cardiogram from Man (String Galvanometer) (Lewis). From a case of paroxysmar tachycardia. The heart-rate was 200 a minute. The upper notohed'line is the time-trace in one-fifth seconds. electro -cardiograms, (Fig. 300) is reproduced for their historical interest. In their main features it is obvious that they agree with the records obtained by the string galvanometer. The electro-cardiograms are distinctly affected by exercise and by the position of the body, and very markedly in disease. The galvanometer may be connected with the two 8io ELECTRO-PHYSIOLOGY hands, or, better, with the right hand and the left foot. The two feet are the most unfavourable combination. The reason is obvious from the direction of the long axis of the heart, which determines the direction ofthe lines of flow of currents due to differences of potential between base and apex (Fig. 301). Central Nervous System. — ^It was discovered by du Bois-Reymond that the spinal cord, like a nerve, shows a current of rest between longi- tudinal surface and cross-section, and that a current of action is caused by excitation. Setschenow stated that when the medulla oblongata of the frog was connected with a galvanometer, spontaneous variations occurred which he supposed due to periodic functional changes in its grey matter. Gotch and Horsley have made experiments on the spinal cords of cats and monkeys. Leading off from an isolated portion of the dorsal cord to the capillary electrometer, and stimulating the motor ' region of the cortex cerebri, they obtained a persistent nega- tive variation followed by a series of intermittent variations. This agrees remarkably with the muscular contractions in an epileptiform convulsion started by a similar excitation of the cortex, which consist of a tonic spasm followed by clonic or phasic (interrupted) contractions. By means of the galvanometer, the same observers have made in- vestigations on the paths by which impulses set up at different points travel along the cord. To these we shall have to refer again (p. 867). Electrical Phenomena of Glands. — These have been studied with any care chiefly in the submaxillary gland and in the skin, although the liver, kidney, spleen, and other organs also show currents when injured. In the sub- maxillary gland the hilus is galvanometrically positive to any point on the external surface of the gland; a current passes from hilus to surface through the galvanometer, and from surface to hilus through the gland (Fig. 306). Fig. 306. — Current of Sub- When the chorda tympani is stimulated with . maxillary Gland. rapidly - succeeding shocks of moderate strength, there is a positive variation — i.e., the hilus becomes still more positive to the surface. This variation can be abolished by a small dose of atropine. Skin Currente. — So far as has been investigated, the integument of all animals shows a permanent current passing in the skin from the external surface inwards. This is feebler in skin which possesses no glands. In skin containing glands the current is chiefly, but not altogether, secre- tory. As such, it is affected by influences which affect secretion, a positive variation being caused by excitation of secretory nerves — e.g., in the pad of the cat's foot by stirtiulation of the sciatic. The deflection obtained when a finger of each hand is led off to the galvanometer, which was at one time looked upon as a proof of the existence of currents of rest in intact muscles, is due to a secretion current. Of more doubtful origin is the current of ciliated mucous membrane, which has the same direction as that of the skin of the frog and the mucous membrane of the stomach of the frog and rabbit — viz., from ciliated to under surface through the tissue, or from ciliated surface to cross-section, if that is the way in which it is led off. The current is strengthened by induction shocks, by heating, and in general by influ- ences which increase the activity of the cilia. Some circumstances point to the goblet-cells in the membrane as the source of the current; but, on the whole, the balance of evidence is in favour of the cilia being the chief factor (Engelmann), although the mucin-secreting cells may be concerned, too. Electrical changes associated with secretion have ELECfROMOTIVE PHENOMENA OF THE EYE 8ii been observed in the frog's tongue on excitation of the glosso-phaiyngeal nerve. Eye-Currents. — If two unpolarizable electrodes connected with a galvanometer are placed on the excised eye of a frog or rabbit, one on the cornea and the other on the cut optic nerve, or on the posterior surface of the eyeball, it is found that a current passes in the eye from optic nerve to cornea, the fundus of the eye being therefore negative as regards the cornea (Fig. 307). The current has the same direction if the anterior electrode is placed on the an- terior surface of the retina itself, the front of the eyeball being cut away, or if one electrode is in contact with the anterior and the other with the posterior surface of the isolated retina. There is nothing of special interest in this ; but the important point is that if light be now allowed to fall upon the eye, or upon the isolated retina, characteristic electrical changes are caused. These are spoken of as the photo- electric reaction, and are best studied by means of the string galvanometer. The features of the curve representing the photo-electric reaction vary with the duration and intensity of the illumination and with the previous condition of the eye as regards illumination. A careful analysis of the curves obtained under different conditions supports the hypothesis that there occur in the eye three separate processes, which may for convenience be con- sidered to depend upon the existence in the retina of three separate photo-chemical substances. When light of moderate intensity is allowed to act upon an eye which has not shortly before been exposed Fig. 307. — Eye-Current. Fig. 308. — Photo-Electric Reaction of Frog's Eye (Einthoven and Jolly). The duration of the flash (of green light) was o-oi second. The eye had been pre- viously in the dark, i millimetre of the abscissa corresponds to 0-5 second, I millimetre of the ordinate to 10 microvolts. Curve to be read from left to right. to strong light, a form of curve is obtained which seems to represent the combined reaction of the three substances (Einthoven and Jolly) (Fig. 308). After a latent period a small preliminary negative deflec- tion A is observed (downward movement of the string). This is at once followed by a much larger upward movement (positive variation) in the same direction as the resting effect, the fundus becoming relatively more negative to the cornea than before. After the peak B has been reached, the curve sinks first rapidly, then more gradually, but soon mounts again, and reaches a second maximum C, which is higher than B 8l2 ELBCTRO-PH YSIOLOG Y (second positive variation). Finally, the ciirve descends to its original level.* The photo-electric reaction is substantially the same in all vertebrate eyes hitherto investigated. In the cephalopod retina, too, the only important electrical change on illumination is in the same direction as the resting effect. The reaction depends upon the retina alone, and does not occur when it is removed. Bleaching of the visual purple does not much affect it, so that it is not connected with chemical changes in this substance. Its seat must be the layer of rods and cones, since in the Fig. 309. — Diagram showing Direction of Shook in Gymnotus. cephalopods the structure called the retina contains only this layer, the other layers of the vertebrate retina being represented in the optic nerve and ganglion (Beck). Of .the spectral colours, yellow light causes the largest variation ; blue, the least ; but white light is more powerful than either (Dewar and McKendrick). (For ■ visual purple,' see Chap. XVIII.) Electric Fishes. — Except lightning, the shocks of these fishes were probably the first manifestations of electricity observed by man. The Torpedo, or electrical ray, of the coasts of Europe was Imown to the Greeks and Romans. It is mentioned in the writings of Aristotle and Pliny, and had the honour of being described i?! verse 1,500 years before Faraday made the first really exact investigation of the shock of the Gymnotus, or electric eel, of South America. Another of the electric fishes, Malapierurus eleciricus, al- though found in many of the African rivers, the Nile in par- ticular, and known forages, was scarcely investigated till fifty years ago. In all these fishes there is a special bUateral organ immediately under the skin, called the electrical organ. It is in this that the shock is developed . It consists of a series of plates arranged parallel to each other. To one side of each plate a branch of the electrical nerve supplying each lateral half of the organ is distributed, so that each half of the organ represents a battery of many cells arranged in series. In Gymnotus the plates are vertical, and at right angles to the long axis of the fish, and the nerves are distributed to their posterior surface; * In the figure the last portion of the curve while it is still slowly descending has not been reproduced. 310. — Diagram showing Direction of Shock in Malapterurus. ELECTRIC FISHES 8,13 the shock passes in the animal from tail to head. In Malapterurus, although the direction of the plates is the same, and the nerve-supply is also to the posterior, surface, the shock passes from head to tail. In Torpedo, the plates or septa dividing the vertical hexagonal prisms of which each lateral half of the organ consists are horizontal ; the nerve- supply is to the lower or ventral surface ; and the shock passes from belly to back through the organ. In all electric fishes the discharge is dis- continuous ; an active fish may give as many as 200 shocks per second. The electrical nerve of Malapterurus is peculiar. It consists of a single gigantic nerve-fibre on each side, arising from a giant nerve-cell. The fibre has an enormously thick sheath, the axis-cylinder forming a relatively small part of the whole ; and the branches which supply the plates of the organ are divisions of this single axis-cylinder. The electromotive force of the shock of the Gymnotus may be very considerable; and even Torpedo and Malapterurus are quite able to kill other fish,, their enemies or their prey. Indeed, Gotch has esti- mated the electromotive force of i cm. of the organ of Torpedo at 5 volts. Schonlein finds that the electromotive force of the whole organ may be equal to that of 31 Daniell cells, or 0-08 volt for each plate, and it is one of the most interesting questions in/rthe ;whole of electro-physiology, how they are pro- tected from their own currents. There is no doubt that the current density inside the fish must be at least as great as in any part of the water sur- rounding it, and . probably much greater. The central nervous system and the great nerves must be struck by strong shocks, yet the fish itself is Fig. 311. — Diagram stiowing Direc- not injured; nay, more, the young in tion of Shock in Torpedo, the uterus of the viviparous Torpedo are unharmed. The only explanation seems to be that the tissues of electric fishes are far less excitable to electrical stimuli than the tissues of other animals; and this is found to be the case when their muscles or nerves are tested with galvanic or induction currents. It requires ex- tremely strong currents to stimulate them; and the electrical nerves are more easily excited mechanically, as by ligaturing or pinching, than elec- trically. In general, too, the shock is more readily called forth by reflex mechanical stimulation of the skin than by electrical stimulation. But that the organ itself is excitable by electricity has been shown by Gotch. He proijed that in Torpedo a current passed in the norme^l direction of the shock is strengthened, and a current passed in the opposite direction weakened, by the development of an action current in the direction of the shock. And , indeed, a single excitation of the electrical nerve is followed by a series of electrical oscillations in the organ, which gradually die away. The latent period of a single shock is about 5^5 second. The skate must be included in the list of electric fishes. Although its organ is relatively small, and its electromotive force rela- tively feeble, yet it is in all respects a complete electrical organ. It is situated on either side of the vertebral column in the tail. The plates or discs are placed transversely and in vertical planes. The nerves enter their anterior surfaces ; the shock passes in the organ from anterior to posterior end. Gotch and Sanderson have estimated the maximum electromotive force of a length of i cm. of the electrical organ of the skate at about half a volt. < Whether the electrical organ is the homologue of muscle or of nerve- ending, or whether it is related to either, has been much discussed. Our surest euide in a Question of this sort is the study of development; 8i4 ELECTRO-PHYSIOLOGY and researches along this line have shown that there are two kinds of electrical organ, one being modified muscle (as in Gymnotus, Torpedo, and the skate); the other transformed skin-glands (as in Malapterurus). The scanty blood-supply of the electrical organs in comparison with that of muscle is noteworthy. In no case do bloodvessels enter the sub- stance of the plates. PRACTICAL EXERCISES ON CHAPTER XV. 1 . Galvani's Experiment. — Pith a frog (brain and cord). Cut through the backbone above the urostyle, and clear away the anterior portion of the body and the viscera. Pass a copper hook beneath the two sciatic plexuses, and hang the legs by the hook on an iron tripod. If the tripod has been painted, the paint must be scraped away where the hook is in contact with it. Now tilt the tripod so that the legs come in contact with one of the iron feet. Whenever this happens, the circuit for the current set up by the contact of the copper and iron is completed, the nerves are stimulated, and the muscles contract (p. 795). 2. Make a muscle-nerve preparation from the same frog. Crush the muscle near the tendo Achillis, so as to cause a strong demarcation current. Cut off the end of the sciatic nerve. Then lift the nerve with a small brush or thin glass rod, and let its cross-section fall on or near the injured part of the muscle. Every time the nerve touches the muscle a part of the demarcation current passes through it, stimulates the nerve, and causes contraction of the muscle (p. 795). 3. Secondary Contraction. — Make two muscle-nerve preparations. Lay the cross-section of one of the sciatic nerves on the muscle of the other preparation (Fig. 296, p. 806). Place under the nerve near its cut end a small piece of glazed paper or of glass rod, and let the longi- tudinal surface of the nerve come in contact with the muscle beyond this. Lay the nerve of the other preparation on electrodes connected with an induction machine arranged for single shocks, with a Daniell cell and a spring key in the primary circuit (Fig. 274, p. 781). On closing or opening the key both muscles contract. Arrange the induc- tion machine for an interrupted current. When it is thrown into one nerve, both muscles are tetanized; the nerve lying on the muscle whose nerve is directly stimulated is excited by the action current of the muscle. 4. Demarcation Current and Current of Action with Capillary Elec- trometer. — (a) Study the construction of the capillary electrometer (Fig. 227, p. 702). Raise the glass reservoir by the rack and pinion screw, so as to bring the meniscus of the mercury into the field. Place two moistened fingers on the binding-screws of the electrometer, open the small key connecting them, and notice that the mercury moves, a difference of potential between the two binding-screws being caused by the moistened fingers. (6) Demarcation Current. — Set up a pair of unpolarizable electrodes (Fig. 230, p. 705). Fill the glass tubes about one-third full of kaolin mixed with physiological salt solution tiU it can be easily moulded. To do this, make a piece of the clay into a little roll, which will slip down the tube. Then with a match push it down until it forms a fixm plug. Next put some saturated zinc sulphate solution in the tubes, above the clay, with a fine-pointed pipette. Fasten the tubes in the holder fixed in the moist chamber (Fig. 312). Now amalgamate the small pieces of zinc wire (p. 195) which are to be connected with the binding-screws of the chamber. (Or use Porter's ' boot ' electrodes. These are made of unglazed potter's clay. In use the leg of the boot is half-fiUed with saturated zinc sulphate solution, into which dips a thick amalgamated PRACTICAL EXERCISES 815 zinc wire. In the foot of the boot is a hollow (or well) which is filled with physiological salt solution and serves to keep the feet well moist- ened with the salt solution. The nerve is laid on the feet of the boots. When not in use the boots should be kept in physiological salt solution.) The zincs are now placed in the tubes, dipping into the zinc sulphate. A piece of clay or blotting-paper moistened with physiological salt solu- tion is laid across the electrodes to complete the circuit between their points, and they are connected with the electrometer to test whether they have been properly set up. There ought to be little, if any, move- ment of the mercury on opening the side-key of the electrometer. If the movement is large, the electrodes are 'polarized,' and must be set up again. The second pair of bind- ing - screws in the chamber are con- nected with a pair of platinuip-pointed electrodes on the one side, and on the other, through a short-cir- cuiting key, with the secondary coil of an induction machine ar- ranged for tetanus. Next pith a frog (cord and brain), and make a muscle-nerve preparation. Injure the muscle near the tendo Achillis. Lay the injured part over one unpolarizable electrode, and an un- injured part over the other. Put a wet sponge in the chamber to keep the air moist, and place the glass lid on it. Focus the meniscus of the mercury, and open the ■ key of the electrometer; the mercury will move, perhaps right out of the field. Note the direction of movement, and, remembering that the real direction is the opposite of the apparent direction, and that when the mercury in the capillary tube is connected with a part of the muscle which is relatively positive to that connected with the sulphuric acid, the movement is from capillary to acid, determine which is the galvano- metrically positive and which the negative portion of themuscle (p. 796). (c) Action Current. — Now, without disturbing its position on the electrodes, fasten the muscle to the cork or paraffin plate in the moist chamber by pins thrust through the lower end of the femur and the tendo Achillis. Lay the nerve on the platinum electrodes. Open the key of the electrometer, and let the meniscus come to rest. This happens very quickly, as the capillary electrometer has but little inertia. If the meniscus has shot out of the field, it must be brought back by raising or lowering the reservoir. Stimulate the nerve by opening the key in the secondary circuit ; the meniscus moves in the direction oppo- site to its former movement. (d) Repeat (6) and (c) with the nerve alone, laying an injured part (crushed, cut, or overheated) on one electrode, and an uninjured part on the other.' Of course, the nerve does not need to be pinned. Fig. 312. — Moist Chamber. E, unpolarizable electrodes supported in the cork C; M, muscle stretched over the electrodes and kept in position by the pins A, B, stuck in the cork plate P; B, binding-screws connected with galvanometer or capillary electrometer. The othei pair of binding-screws serves to connect a pair of stimulating electrodes inside the chamber with the secondary coil of an induction machine. 8i6. ELECTRO-PHYSIOLOGY Clean the unpolarizable electrodes, and be sure to lower the reservoir of the electrometer; otherwise the mercury may reach the point of the capillary tube and run out. In 4 a galvanometer (p. 699) may be used with advantage by students, if one is available, instead of the electrometer, the unpolarizable elec- trodes being connected to it through a short-circuiting key. The spot of light is brought to the middle of the scale by moving the control- magnet; or if a telescope-reading (Fig. 222, p. 699) is being used, the zero of the scale is brought by the same means to coincide with the vertical hair-line of the telescope. The short-circuiting key is then opened. 5. Action Current of Heart. — Pith a frog (brain and cord). Excise the heart, and lay the base on one unpolarizable electrode, and the' apex on the other, having a sufficiently large pad of clay on the tips of the electrodes to insure contact during the movements of the heart, or having little cups hollowed in the clay and filled with physiological salt solution, into which the organ dips. Connect the electrodes with the capillary electrometer and open its key. At each beat of the heart the mercury will move (p. 806). 6. Electrotonus.— 5et up two pairs of unpolarizable electrodes in the moist chamber. Connect two of them with a capillary electrometer (or galvanometer), and two with a battery of three or four small Daniell cells, as in Fig. 295. Lay a frog's nerve on the electrodes. When the key in the battery circuit is closed, the mercury (or the needle of the galvanometer) moves in such a direction as to indicate that in the extra- polar regions parts of the nerve nearer to the anode are relatively positive to parts more re- mote, and parts nearer to the kathode are rela- tively negative to parts more remote. The direction of movement of the mercury (or gal- vanometer needle) must be made out first for one direction of the polarizing current. Then the latter must be reversed, and the movement of the mercury (or needle) on closing it again noted (p. 803). 7. Paradoxical Contraction. — Pith a frog (brain and cord). Dissect out the sciatic nerve down to the point where it splits into two divisions, one for the gastrocnemius b, and the other for the peroneal muscles a. Divide the peroneal branch as low down as possible, and make a muscle-nerve preparation in the usual way. Lay the central 313. — Paradoxical end of the peroneal nerve on electrodes con- Contraction, nected through a simple key with a battery of two Daniell cells . When the peroneal nerve is stimu- lated, the gastrocnemius muscle contracts. This result is not due to the current of action, for it is not obtained with mechanical stimulation of the nerve. But it is not the result of an escape of current, for if the peroneal nerve be ligatured between the point of stimulation and the bifurcation, no contraction is obtained. The contraction is really due to a part of the electrotonic current set up in the peroneal nerve passing through the fibres for the gastrocnemius, where they lie side by side in the trunk of the sciatic. 8. Alterations in Excitability (and Conductivity) produced in Nerve by the Passage of a Voltaic Current through it, — Set up two pairs of unpolarizable electrodes in the moist chamber. Connect a battery, of two or three Daniell cells, arranged in series through' a simple key PRACTICAL EXERCISES 817 with the side-cups of a Pohl's commutator with cross-wires in. Con- nect the commutator to one pair of the unpolarizable electrodes (' the polarizing electrodes '), as in Fig. 314. The other pair of unpolarizable electrodes (' the stimulating electrodes ') are to be connected through a short-circuiting key with the secondary of an induction machine arranged for tetanus. A single Daniell is put in the primary coil. Pith a frog (brain and cord), make a muscle-nerve preparation, pin the lower end of the femur to the cork plate in the moist chamber, attach the thread on the tendo Achillis to the lever connected with the chamber through the hole in the glass provided for this purpose, and arrange the nerve on the electrodes so that the stimulating pair is between the muscle and the polarizing pair. By moving the secondary, seek out such a strength of stimulus as just suffices to cause a weak tetanus when the polarizing current is not closed. Set the drum off (slow speed), and take a tracing of the contraction. Then close the polarizing current with a Pohl's commutator so arranged that the anode is next the stimulating electrodes — i.e., the current ascending in the nerve. Again open the short-circuiting key in the secondary; the contraction will now be weaker than before, or no contraction at all may be obtained. Allow the preparation two minutes to recover, Fig. 314. — Arrangement for showing Changes of Excitability produced by the Voltaic Current. Ji, muscle; N, nerve; E-^, Eg, electrodes connected with secondary ooilS; E3, E4, unpolarizable electrodes connected with Pohl's commutator (with cross-wires) C; B', 'polarizing' battery; B, ,' stimulating ' battery in primary circuit P; K, K", simple keys ; K', short-circuiting key. then'J^stimulate again, as a control, without closing the polarizing current. If the contraction is of the same height as at first, close the polarizing current with the bridge of the commutator reversed, so that the kathode is now next the stimulating electrodes. On stimu- lating, the contraction will now be increased in height. (See Figs. 266, 267, p. 760.) 9. PflUger's Formula of Contraction (p. 762). — To demonstrate this, connect two unpolarizable electrodes, through a spring key and a commutator, with a simple rheocord (Fig. 277, p. 783), so as to lead off a twig of a current from a, Daniell cell. The unpolarizable elec- trodes are placed in a moist chamber. A muscle-nerve preparation is arranged with the nerve on the electrodes and the muscle attached to a lever. The effects of make and break of a weak current, ascending and descending, can be worked out with the simple rheocord. The effects of a medium current will probably be obtained with a single Daniell connected directly with the electrodes through a key. The effects of a strong current will be got when three or four Daniells are connected with the electrodes. Care must be taken to keep the prepara- tion in a moist atmosphere, and more than one preparation may be needed to^verify the whole formula. 8i8 ELECTRO-PHYSIOLOGY 10. Formula of Contraction for (Human) Nerves in Situ, — Connect eight or ten dry cells in series.* Connect one terminal of the battery to a large plate electrode, and the other to a small electrode, both covered with cotton, flannel, or sponge, moistened with salt solution. Include in the circuit a simple key for making or breaking the current, and a commutator for changing its direction at will. Leave the key open. Place the large electrode behind the shoulder (or on the back of the neck), and the small electrode over the ulnar nerve at the elbow, between the internal condyle and the olecranon. Arrange the com- mutator so that the small electrode shall be the kathode. Close, and then open the key. If no contraction occurs at closing, the battery is too weak, and more cells must be added. If contraction occurs at closing, but not at opening, reverse the commutator, making the small electrode the anode, and observe whether contraction now occurs at closing, at opening, or at both. Note also the relative strength of the various contractions. If the current is ' weak,' the only contraction will be a closing one when the kathode is over the nerve. If the current is of ' medium ' strength, a closing kathodic contraction and both, opening and closing anodic contractions will be obtained. With ' strong ' currents contractions will occur at closing and at opening, whether the kathode or the anode is over the nerve. The contractions will vary in strength, as described on p. 763. To work out the different cases of the formula summarized in the table, the number of cells must be increased or diminished. Weak Currents, M ed ium" Currents . Strong Currents, KCC KCC ACC AOC KCC ACC AOC KOC The abbreviations KCC, ACC, are used respectively for kathodic closing contraction and anodic closing contraction; KOC, AOC, for kathodic opening contraction and anodic .opening contraction. KCC is stronger than KOC, and ACC than AOC. KCC is stronger than ACC, and AOC than KOC. Therefore, as the strength of the current is increased, in the case of normal tissues, KCC is first obtained, then ACC, then AOC, and finally KOC. II. Ritter's Tetanus. — Lay the nerve of a muscle-nerve preparation on a pair of unpolarizable electrodes connected through a simple key with a battery of three or four small Daniells. Connect the muscle with a lever. Pass an ascending current (anode next the muscle) for a few minutes through the nerve, and let the writing-point trace on a slowly-moving drum. When the current is closed there may be a smgle momentary twitch, or the muscle may remain somewhat con- tracted (galvanotonus) as long as the current is allowed to pass, or it may continue to contract spasmodically (' closing tetanus '). When the current is opened the muscle will contract once, and then immedi- ately relax, or there may be a more or less continued tetanus (Ritter's or ' opening tetanus '). If opening tetanus is obtained, divide the nerve between the electrodes: the tetanus continues. Divide it be- tween the anode and the muscle : the tetanus at once disappears. This shows that the seat of the excitation which causes the tetanus is in the neighbourhood of the anode (p. 804). * If the laboratory possesses a battery (with rheostat), such as is used by neurologists, the experiment is more conveniently performed with this. CHAPTER XVI THE CENTRAL NERVOUS SYSTEM In other divisions of our subject we have been able to follow to a greater or less extent the processes which take place in the organs described. The chemistry and the physics of these processes have bulked more largely in our pages than the anatomy and histology of the tissues themselves. In deahng with the central nervous system, we must adopt a method the very reverse of this. Its ana- tomical arrangement is excessively intricate. The events which take place in that tangle of fibre, cell, and fibril are, on the other hand, almost unknown. So that in the description of the physiology of the central nervous system we can as yet do little more than trace the paths by which impulses may pass between one portion of the system and another, and from the anatomical connections deduce, with more or less probability, the nature of the physiological nexus which its parts form with each other and the rest of the body. And here it may be well to remark that, although for convenience of treatment we have considered the general properties of nerves in a separate chapter, there is not only no fundamental distinction between the central nervous system and the outrunners which connect it with the periphery, but obviously a central nervous system would be meaningless and useless without afferent nerves to carry information to it from the outside, and efferent nerves along which its commands may be conducted to the peripheral organs. Section I. — Structure of the Central Nervous System — Histological Elements. In uiuravelling the complex structure of the central nervous system, we avail ourselves of information derived (i) from its gross anatomy ; (2) from its microscopical anatomy ; (3) from its develop- ment ; (4) from what we may call, although the term is open to the criticism of cross-division, its physiological and pathological anatomy. Certain tracts of white or grey matter are differentiated from each other by the size of their fibres or cells. For example, the postero- median column of the spinal cord has small fibres, the direct cere- 819 820 THE CENTRAL NERVOUS SYSTEM bellar tract large fibres; the large pyramidal cells (giant cells or cells of Betz), in what we shall afterwards have to distinguish as the ' motor area ' (p. 918) of the cerebral cortex, are the cells of origin of fibres of the pyramidal tract subserving the volitional movements of the limbs and trunk. The pyramidal cells of the ' face area ' are comparatively small. In general, an efferent or motor nerve-cell is larger the longer its axon is — e.g., the largest of all the pyramidal cells in the ' motor ' region are found in the portion known as the ' leg area,' from which the pyramidal fibres have to pass all the way down the cord to the segments from which the spinal nerves going to the lower limbs arise. The recent work of Brodman and of Campbell has shown that the cerebral cortex may be histologically diflerentiated into regions which correspond to a great extent to the various functional regions mapped out by physiological methods (p. 924). The study of development enables us not only to determine the homology, the morphological rank, of the various parts of the brain and cord, but also, by comparison of animals of different grades of organization, sometimes to decide the probable function and physio- logical importance of a strand of nerve-fibres or a column of nerve- cells. It is of special value in helping us to differentiate the various areas of grey matter on the surface of the brain, and to trace the various tracts or paths into which the white matter of the central nervous system may be divided. For the medullary sheath is not developed at the same time in all the tracts, and a strand of nerve-fibres in which it is wanting — e.g., the pyramidal tract (p. 850), which is the last of the spinal tracts to become myelinated — is readily distinguished under the microscope. Then, again — and this is what we propose to include under the fourth head — experimental physiology and clinical and pathological observation throw light not only on the functions, but also on the structure, of the central nervous system. For instance, complete or partial section, or destruction by disease, of the white fibres of the cord or brain, or of the nerve-roots, or removal of portions of the grey matter, is followed by degeneration in definite tracts. And since, as we have already seen, degeneration of a nerve-fibre is caused when it is cut off from the cell of which it is a process, the amount and dis- tribution of such degeneration teaches us the extent and position of the central connections of the given tract. Conversely, the cells in which a tract of nerve-fibres arises may sometimes be identified by the alterations in the chromatin (p. 831) and other changes which occur in them after section of their axons. Particularly in young animals, removal of a peripheral organ — an eye or a limb — or section of its nerves, may be followed by atrophy of portions of the central nervous system immediately related to it. ' Softening ' of a definite portion of the white or grey matter may also in certain cases be caused by depriving it of its blood-supply by the injection of artificial emboli, and the resulting degenerations may then be studied. For instance, fine particles like lycopodium spores are injected into the abdominal aorta between the origins of the renal and inferior mesenteric arteries. They are prevented by clamps from entering these vessels, and, passing through the lumbar arteries, stick in the branches of the anterior spinal artery, and cause softening mainly of the grey matter of the lumbar portion of the cord. When the abdominal aorta of a rabbit is temporarily compressed (for about an hour) below the origin of the renal arteries, the grey matter of the corresponding portion of the cord is so seriously injured that it and the fibres that arise from it degenerate, while the fibres whose cells of DEVELOPMENT 821 Fig- 315- — Formation of the Neural Canal at an Early Stage (Beard). origin are not situated in this part of the grey matter are not affected, or at least completely recover. Certain tracts may also be marked out by means of the electrical variation, which gives token of the passage of nervous inipulses along them when portions of the central nervous system or peri- pheral nerves are stimulated (Horsley and Gotch) . Development of the Central Nervous System. — Very early in development (Fig. 315) the keel of the vertebrate embryo is laid down as a groove or gutter in the ectoderm of the blastodermic area (Chap. XIX.). The walls of this ' medullary ' or ' neural ' groove grow inwards, and at length there is formed, by their coalescence, the " neural canal ' (Fig. 316), which expands at its anterior end to form four cerebral vesicles (Fig. 317). Thus there is a continuous tunnel from end to end of the primary cerebro -spinal axis; and this persists as the central canal of the spinal cord and the ven- tricles of the brain, whose ciliated epithelium represents the ectodermic lining of the primitive neural canal. In the adult portions of the canal may - become obliterated from an overgrowth of the lining cells, and the cilia are, if present at all, less distinct than in the child, and far less distinct than in the lower animals. From the wall of this canal is formed the cerebro-spinal axis, in which developing nerve-cells or neuro- Fig. 317- — Diagram to illustrate the Formation of the Cerebral Vesicles. A. i indicates the cavity of the secondary fore- brain, which eventually becomes the lateral ventricles. In B the secondary fore-brain has grown backwards so as to overlap the other vesicles. I, first cerebral vesicle (primary fore-braiu or 'tween brain); II, second cerebral vesicle (mid-brain); III, third cerebral vesicle (hind-brain) ; IV, fourth cerebral vesicle (after- brain). Fig. 316. — Neural Canal at a Later Stage (Beard). C, neural canal; G, posterior spinal gangUon. blasts soon become differentiated from the supporting cells or spongio- blasts, and wander outwards from the neighbourhood of the central canal (Fig. 328) till their further progress is checked by the barrier of the mar- 822 THE CENTRAL NERVOUS SYSTEM ginal veil, a closely-woven network or thicket, into which the processes of the spongioblasts break up at the outside of the primitive cerebro-spinal axis. Although the neuroblasts themselves are unable to penetrate the marginal veil, the axis-cylinder processes of some of them do so, and form the motor roots of the spinal nerves. The neuroblasts from which the fibres of the white columns of the cord are developed are apparently unable to send their axons through the marginal veil. They are accordingly forced to assume a longitudinal direction, and in this way the central grey matter becomes covered with a sheath of longitudinal white fibres. For a time only motor nerve-cells and the fibres connected with them are developed in the cerebro-spinal axis. The ganglia on the posterior roots arise from a series of ectodermic thickenings or sprouts from the neural crest which runs along the dorsal aspect of the neural canal. These sprouts contain the neuroblasts which develop into the spinal ganglion cells with the posterior root- fibres. From each pole of each neuroblast a process grows out, one towards the periphery, which forms a peripheral nerve-fibre, the other centrally to connect the cell with the cord. From the after-brain (or myelencephalon) is developed the meduUa oblongata or spinal bulb, from the hind-brain (or metencephalon) the cerebellum and pons, from the mid-brain (or mesencephalon) the corpora quadrigemina and crura cerebri. The fore-brain, or primary fore-brain (thalamencepha- lon), gives rise of itself only to the third ventricle and optic thalamus; but a secondary fore-brain (telencephalon) buds off from it and soon divides into two chambers, from the roof of which the cerebral hemi- spheres, and from the floor the corpora striata, are derived. Their cavities persist as the lateral ventricles, which communicate with the third ventricle by the foramen of Monro. The olfactory tracts are formed as buds from the secondary fore-brain. To complete the story of the development of the brain, it may be added that the retina is really an expansion of its nervous substance, A hollow process, the optic vesicle, buds out on each side from the primary fore-brain. A button of ectoderm, which afterwards becomes the lens, grows against the vesicle and indents it so that it becomes cup-shaped, the inner concave surface of the cup representing the retina proper, the outer convex surface the choroidal epithelium. The stalk becomes the optic nerve. Histological Elements of the Central Nervous 'System. — The central nervous system is built up (i) of true nervous elements, (2) of sup- porting tissue. The nervous elements have usually been described as consisting of nerve-fibres and nerve-cells, but the antithesis of a time- honoured distinction must not lead us to forget that the essential part of a nerve-fibre, the axis-cylinder, is a process of a nerve-cell, and the medullary sheath a structure whose integrity is intimately related to that of the axis-cylinder.* In strictness, the term ' nerve- cell ' ought to include not only the cell-body, but all its processes, out to their last ramifications. But the habit of speaking of the position of the cell-bodyj- as that of the nerve-cell is so ingrained, that it seems better to continue the use of the latter term in its old signification, and to speak of the cell and branches together as a neuron (also spelled neurone). * While the medullary sheath, like the axis-cylinder, seems to be as regards its nutrition under the control of the nerve-cell, and must therefore be looked upon as an integral portion of the neuron, although not essential for its de- velopment, the neurilemma in respect both of its nutrition and its develop- ment appears to be an independent structure. ■f Foster and Sherrington call the cell-body the perikaryon. mstoLO&ICAL ELEMENTS 823 The Neurons.— A typical nerve-cell (Figs. 318, s2o ^22^ is a knot of granular protoplasm, containing a large nucleus, inside of which lies a highly refractive nucleolus. A centrosome and attraction sphere (p. 5) have also been found in some nerve-cells though not ^^^rZ demonstrated in all. Pign.ent may also be present esfeciaHy In Id age. By certain methods of stammg it may be shown that fibrils (neuro-fibrils) run through the protoplasm of the cell, forming a felt- work m It, and entering the dendrites on the one hand and the axis- cylinder process on the other Figs. 318, 321, 32s) In the axit cylmders of nerve-fibres the fibrils (Fig:'3i9)\ppeir^io preserve tSir identity down to the distribution of the fibre. In the ground substance between the fibrils lie round, an- gular, or spindle-shaped bodies (Nissl's bodies) which stain with basic dyes (Fig. 330) . * These bodies vary in appearance in different kinds of nerve-cells, and in the sarne nerve-cell under different con- ditions. According to Macallum, they contain organically combined iron. In a multipolar cell, like those Fig. 318. — Anterior Horn Cell from Man showing Fibrils (Bethe). Fig. 319. — MeduUated Nerve- Fibre showing Fibrils of Axis- Cylinder (Bethe). The fibrils are seen passing, without interrup- tion, across a node of Ranvier. in the anterior horn of the spinal cord, several processes — ^it may be five or six, or even more — pass off from the cell-body (Frontispiece). The most complete pictures of them are given by preparations impregnated according to the method of Golgif (Figs. 320, 323). One of the pro- * In Nissl's method the sections are stained in a solution of methylene blue, and decolourized in anilin-alcohol. t The method depends upon the deposition of mercury, or silver, in or around the cell-bodies and their processes in tissues which have been hardened in bichromate of potassium and then soaked in a solution of mercuric chloride or silver nitrate. In Pal's improvement of Golgi's method a solution of sodic sulphide follows the mercuric chloride. 824 IH^ CEkTML NERVOUS SYSTEM cesses of most nerve-cells is distinguished from the rest by the fact that it maintains its original diameter for a comparatively great distance from the cell, and gives off comparatively few branches. This process, which in favourable preparations can be traced on till it becomes the axis-cylinder of a nerve-fibre, is called the axis-cylinder process, or more shortly the axon. The few slender brai ches that come off from it, usually at right angles, are called collaterals. The collaterals consist essentially of one or more fibrils of the axon. Both the main thread of the axon and the collaterals end by breaking up into an arborescent system of fibrils or telodendrion. The telodendrions vary greatly in appearance from simple end-brushes to far-branching tliickets, or such special end-organs as motor plates (Fig. 325) or muscular spindles. The rest of the processes of the cell, wluch are termed dendrites or protoplasmic processes, very rapidly diminish in diameter, as they pass away from the cell, by breaking up into Fig. 320.^-Multipolar Nerve-Cell: Golgi Preparation (Barker, after Kolliker). «, axon; c, collaterals. fibrils like the branches of a tree. The Nissl bodies extend for some distance into the dendrites, but not into the axon. The dendrites of some cells, especially the pyramidal cells of the cerebral, and the Purkinje's cells of the cerebellar cortex, have small sweUings, the so-called lateral buds or gemmules, on their course. Their signifi- cance is unknown. The dendrites terminate at a little distance from the cell, where they come into relation with the end-arborizations of the axons of other neurons. In this way two or more neurons are linked together to form a nervous path. According to the view most commonly held (neuron hypothesis), the relation is not one of actual anatortiical continuity, but the processes come so close together that nerve impulses are able to pass across from the terminal brush of the axon of one nervous element to the dendrites or cell-body of another. This kind of junction is called a synapse. It has been suggested that the contact may be rendered more or less close through amoeboid movements of the dendrites, and that in this HISTOLOGICAL ELEMENTS 825 way the nervous impulse may be switohed, like a railway-train from one path to another. But there is no experimental basis for this some- what crude, if fascinating, hypothesis. Sherrington has suggested that the presence of a ' membrane ' at the synapse may liAiit ,the con- duction and determine its direction. ' Some membranes; suc^ as frog's skin, are known to possess a So-called irreciprocal permeability for certain substance*, permitting them to pass more easily lit one direc- tion than the other, and it is conceivable that' a meipbrane at the synapse might have a similar action jn respect to4he movement of ions concerned in the propagation of the nervous excitation. Whatever Fig. 321.— Nerve-Cells of Hirudo (ScMfer, after Apdthy). A, unipolar motor cell; a, network of neuro-fibrils near the sur- face of the cell; b, near the nucleus n ; c, afferent, d. efferent neuro-fibril. B, bi- polar sensory cell g, with its nucleus n ; cu, cuticle; ep, epidermis c^ls between which a neuro-fibril passes up from its branched ending near the surface of the sliin to the nerve-cell, where it forms a network, which gives off a fibril passing towards the central nervous system. Fig, 322. — Large Pyramidal Cell [of Cerebral Cortex (Barker, after Bech- 'terew). a, axon; 6, dendrite. the nature of the relation between two superposed neurons may be, it does not permit the conduction of nerve-impulses indiscriminately in both directions. For instance, stimulation of the central end of the posterior root of a spinal nerve causes an electrical response (p. 797) in the anterior root of the same segment, while no electrical change is produced in the posterior root by stimulation of the anterior. We shall see later on (p. 8i|,4) that soriie of the fibres of the posterior root and their collaterals end by arborizing around the dendrites of the cells of the anterior horn. ;The excitation is,' therefore, able to pass from the 826 THE CENTRAL NERVOUS SYSTEM telodendrions of the posterior root-fibres through the dendrites of the anterior horn cells towards their cell-bodies, butinot in the opposite Fig. 323. — a — e shows the development of the pyramidal nerve-cells of the cerebral ^ cortex in a typical mammal; a, neviroblast with commencing axon; 6, dendrites appearing; d, commencing collaterals. A — D shows the different degree of com- plexity in the fully-developed pyramidal cells in different vertebrates: A, frog; B, lizard; C, rat; D, mah (Donaldson, after Ram6u y Cajal). direction, and in general the direction of conduction is from the den- drites towards the cell-body. fj Some investigators believe that the fibrils already spoken of as forming a felt-work in the protoplasm of the nerve-cell may run right Fig. 324. — Cells from the Gasserian Ganglion of a Developing Guinea-Pig. The originally bipolar cfeUs are seen changing into cells apparently unipolar. The same process occurs in the cells of the spinal ganglia (Van Gehuchten). through from one cell to another, thus constituting an actual anatomical connection between the neurons, and that Such a connection may be established also by fibrils which do not enter the cells at all, but run in the intercellular substance of the grey matter. Such a continuity of HISTOLOGICAL ELEMENTS 827 fibrils from cell to cell has been demonstrated in some of the invertebrates — e.g., in annelids' (Fig. 321) — where previously the best examples of strictly iso- lated neurons were sup- posed to be found (Apathy) . The supporters of the theory of continuity look upon the cell-body as merely necessary for the nutrition of the nerve-net, but deny that it is neces- sary for the conduction of nerve-impulses. If this is the case, it is obvious that the neurons can no longer be considered as functional units in which the law of isolated conduction of nerve-impulses (p. 767) holds good. Nor is it by any means so easy to un- derstand as on the neuron hypothesis such facts as the strict limitation of Wal- lerian degeneration to the boundaries of the neurons directly affected, or the strict limitation of the silver reduction in Golgi preparations to single neu- rons. It is, of course, true that the simplicity and order introduced by the neuron hypothesis into our conceptions of the nervous conduction paths by no means prove its accuracy. Yet they are reasons for not lightly abandoning it, and it has recently been corroborated by important new evidence on the growth of nerve-cells on artificial media outside of the body (p. 776; Fig. 327, p. 829). Varieties of Neurons. — Nearly all the nerve-cells of the cerebro-spinal axis agree with the cells of the anterior horn in the posses- sion of an axon and one or more dendrites, although sometimes the dendrites are scanty in number and Fig. 325. — Scheme of Lower Motor Neuron (Barker), a, h, axon-hillock (the portion of the cell from which the axon comes off), containing no Nissl bodies, and showing fibrillation; ax, axis-cylinder or axon; m, medullary sheath, outside of which is the neurilemma; c, cell- substance (cytoplasm), showing Nissl bodies in a lighter ground substance; d, protoplasmic processes or dendrites containing Nissl bodies; n, nucleus; »', nucleolus; n, R, node of Ranvier; s , /, side fibril ; » of », nucleus of the neurilemma ; tel., motor end-plate; m', striped muscle-fibre; s, I., incisure. 828 THE CENTRAL NERVOUS SYSTEM insignificant ' in size. In the cerebral cortex the typical cells are of pyramidal shape. From the base comes off the axon, and from the angles dendritic processes, a particularly massive dendrite proceeding from the apex of the pyramid towards the surface of the. brain. Sometimes an axon, instead of ending in an arborization which comes into relation with the dendrites of another nerve-cell, or, as is more frequently the case, with the dendrites of more than one cell, breaks up into a sort of basket-work of fibrils surrounding the cell- body. The cells of Purkinje, for instance, in the cerebellum are sur- rounded by such pericellular baskets (Fig. 326). Thfe cells of the spinal ganglia have two axons, which in the embryo arise one from each end of the bipolar cell, but in the adult, in all vertebrates except some fishes, are connected to the cell by a single process (Fig. 324). It has been commonly held that the unipolar cell with a single T-shaped process is developed from a bipolar cell, which grows towards one side, so that the two processes come together and fuse. Such observations as that of Harrison on the bifurcation of the growing end of the main process of isolated nerve-cells culti- vated in vitro suggest an alternative and a simpler explanation — viz., that the T-shaped process is derived from the splitting of a single chief process. If this be the case, one of the original processes at the poles probably undergoes a retarded development or disappears, since the great majority of the spinal ganglion cells with the T-shaped process appear to have no dendrites. Another kind of cell which seems undoubtedly to be of nervous nature is the ' granule-cell.' Granule-cells are much smaller than the nerve-cells we have been describing. Their processes are much less easily followed, but all appear to give off an axon and several dendrites. They contain a relatively large nucleus (5 to 8 /* in diameter), with only a mere fringe of ceU-substance. The nucleus, unlike that of a large nerve-cell, stains deeply with haematoxylin. Some parts of the grey matter are crowded with these granule-cells — e.g., the nuclear layer of the cerebellum and the substantia gelatinosa, or substance of Rolando, which caps the posterior horn in the cord. In other parts they are more thinly scattered, but probably they are as widely diffused as the large nerve-cells proper, and no extensive area of the grey matter is wholly without them. Although there are several varieties of granules (Hill), they all agtee in this, that their axons run a comparatively short course, and never, or rarely, pass beyond the grey matter. Another kind of neuron which is also confined to the grey matter, and is typically seen in the cortex of the cerebrum and cerebellum, presents the peculiarity of an axon which branches into an intricate network immediately after coming off from the cell (cell of Golgi's second type). Unlike the long axon of the typical large nerve-cell, the axis-cylinder process of this Golgi cell remains vnmeduUated. The sympathetic ganglion cells are developed from immature neuro- Fig. 326. — Pericellular Baskets (Schafer, after Cajal). Two cells of Purkinje from the cerebellum are seen sur- rounded by end ramifications forming a basket-work, b, de- rived from the branching of axons of small nerve-cells in the molecular layer; a, axon. HISTOLOGICAL ELEMENTS 829 blasts that migrate, in the course of development, from the rudiments of the spinal ganglia, and gathering in clumps form the ganglia of the sympathetic chain (His). They agree in general with the cells of the cerebro-spinal axis in possessing an axon and one or more, commonly several, dendrites, although a few of them are devoid of dendrites. The great majority of the axons remain unmeduUated, but a few acquire a very fine medullary sheath. The epitheUum lining the centrarl canal of the cord and the ventricles of the brain has also been considered by some as of nervous nature. The fact that the deep ends of the cells are continued into processes which pierce far into the grey substance has been supposed to lend weight to this opinion, but there is no good ground for it. Growth of Neurons. — ^The growth of a neuron from origin to com- pletion is a comparatively slow process in the higher animals. Early in foetal life (about the third or fourth week in man) certain round germinal cells make their appearance amid the columnar ectodermic cells surrounding the neural canal. From their division are formed, in the first months of embryonic life, the primitive nerve-cells or neuroblasts. These soon elongate and push out processes, first the Fig. 327. — Isolated nerve-cells"from the spinal cord of a tadpole growing in clotted lymph. A, B, CJare cells in different stages of growth. The lower view of C was drawn under the microscope 4I hours later than the upper (Harrison). axon or axons, and then the dendrites (Fig. 323). The formation of the axons from the nerve-cell is most clearly followed in isolated cultures (Fig. 327). As development goes on, the cell-body grows larger, and the processes longer and more richly branched. The axon and its collaterals, when it has any, in the case of the great majority of the nervous elements of the brain and cord, ultimately acquire a medullary sheath, although, as we have said, the time at which medul- lation is completed varies in different groups of elements, and in some nervous tracts it is even wanting at birth. At birth, too, the branches of many of the cells are less numerous, and the connections between different nervous elements therefore less intimate than they will after- wards become. For many years the processes, and particularly the axons, continue not only to grow longer, but to grow thicker as well. The cell-body also enlarges, and the quantity of material in it that stains with basic dyes increases. In the growing (lumbar) spinal ganglia of the white rat the increase in volume of the largest cell- bodies is very closely correlated with the increase in area of the cross- section of the nerve-fibres growing out of them. The cross-section of the axis-cylinder is, and remains, almost exactly equal to the area 830 THE CENTRAL NERVOUS SYSTEM of tHe medullary sheath (Donaldson). Even after puberty is reached the anatomical organization of the nervous system may still continue to advance, although at an ever-slackening rate, and the finishing touches may only be given to its architecture in adult life. In old age the nervous elements decay as the body does. The cell- body diminishes in size; the stainable material lessens in amount; .vacuoles form in the protoplasm and pigment accumulates ; the nucleus shrinks; the nucleolus is obscured or may disappear altogether. At the same time the processes of the cell, and especially the dendrites, tend to atrophy (Fig. 329). Nutrition of the Neuron. — ^We have already seen that when an axon is cut off from its cell-body, it and its medullary sheath, when it possesses one, undergo a rapid degeneration. It was long supposed Fig. 328. — -Se'c'tion through Half of Neural Tube (Barker, after His). The pear- shaped neuroblasts are seen migrating out- wards. The axons of some of them are seen pushing their way out through the marginal veil as the anterior root of a spinal nerve. Fig. 329. — I, spinal ganglion cells of a still-born male child; 2, of a man ninety-two years old ( X 250) — N, nuclei; 3, nerve-cells from the antennary ganglion of a honey-bee just emerged in the per- fect form ; 4, of an old honey-bee. The nucleus is black in the figure. In 3 it is very large, in 4 it is shrunken and the cell-substance contains vacuoles (Hodge). that no change took place in the nerve-cell. The researches of recent years have shown that not only does loss of the specific function and trophic influence of the cell-body affect the nutrition of the axon, but loss of function of the axon reacts on the cell-body.* In many cases at least, when a nerve-fibre is divided from its cell, characteristic changes are produced in the latter and in its dendritic processes, and they are scarcely less rapid, although usually less profound, and far more transient than the degeneration in the peripheral portion of the nerve-fibre. The cell-body and the nucleus swell. Many of the Nissl bodies (Fig. 330) disintegrate, and are reduced to a finely granular condition. After a time much of the disintegrated chromatic sub- stance disappears altogether. The nucleus may be displaced to one HISTOLOGICAL ELEMENTS 831 side of the cell. Certain changes in the neurofibrils of the cell may accompany the changes in the chromatin. In rabbits after division of the facial nerve the alterations in its nucleus of origin have been fpund to reach a maximum in about three weeks, after which there is a tendency to recovery on the part of the majority of the cells, even when regeneration of the nerve has been prevented by cutting out a portion of it. Some of the cells may completely atrophy and disappear. Similar changes have been found by Warrington in the motor cells of the anterior horn after section of the posterior (dorsal) spinal roots. Since in this case no anatomical injury has been inflicted on the motor neurons, it has been surmised that the cause of the alterations is the loss of impulses which normally reach them along their dendrites. In short, we may say, with Marinesco, that the functional and anatomical integrity of the neuron depends on the integrity of all its constituent parts, and of the neurons which carry to it functional excitations — i.e., excitations connected with its proper physiological work. The neuron, in fact, lives by its function, or, in common language, by doing its work. Yet the anatomical tokens of mere disuse, as in the motor cells Fig. 330. Cells from the Nuclei of the Oculo-Motor Nerves of the Cat Thirteen Days after Division of the Root-Fibres on one Side : Nissl's Stain (Barker, after Flatau). a, normal cell from side on which the roots were not cut ; &, cell from side operated upon. Only a few Nissl bodies are present in 6, and the nucleus is displaced to one side of the cell. of the anterior horn after division of the cord at a higher level, are less distinct than those which follow section of the axon. Therefore it must be concluded that the latter, although not indispensable for the nutrition of the cell as the cell is for the axon, exerts an influence upon it. Similar changes in the chromatin may also be produced in nerve- cells by a period of anaemia, in extensive superficial burns, in tetanus caused by the injection of bacterial cultures, in acute alcoholic poisoning, in fatigue, and in other ways. According to Wright, the inhalation of ether or chloroform (in dogs) so alters the chromatic substance that it loses its affinity for aniline dyes. In long-continued anaesthesia the nucleus is also affected, while the nucleolus is the last part of the cell to suffer. A greater alteration occurs in the cells in the three hours between the sixth and ninth hours of anaesthesia than in the five hours between the first and sixth. Although the changes are transitory, the cells, after a narcosis of nine hours, being practically normal in forty- eight hours, they indicate that the duration of safe surgical anaesthesia has a limit measured by hours, it is probable that the alterations in the chromatic substance should 832 THE CENTRAL NERVOUS SYSTEM not be looked upon as the token of any specific lesion; they are the common structural response of the cell to injurious influences of the most varied nature. Grey and White Matter. — Nerve-cells are the most distinctive his- tological feature of the grey nervous substance. Sown thickly in the cerebral cortex, the basal ganglia, the floor of the fourth ventricle, and the cervical and lumbar enlargements of the cord, they are scattered more sparingly wherever the grey matter extends. They also occur in the spinal ganglia, and their cerebral homologues (such as the Gas- serian ganglion), in the ganglia of the S3rmpathetic system, and the sporadic ganglia in general. But wide as is their distribution, and great as is the size of the individual cells, some of which have a diameter of 140 /* , or even more , they yet make up but a. small portion of the whole of the central nervous substance, the total weight of the g,ooo millions of nerve-cell bodies in the human brain being less than 27 grammes (Donaldson). And although it is not to be wondered at that objects so notable when viewed under the microscope should have struck the imagination of physiologists, it is probable that the very high powers which it is so common to attributfe exclusively to them are, in part at leastj shared with the network or feltwork formed by their processes. The grey matter, in addition to this ex- ceedingly 'deli6ate f eltworkpf .npnt-meduUated fibres' and filaments reprfese^ting the den- drites arid such axons and, collaterals as *' terminate within itself, contains also, as may , bepeen in preparations stain^ by Weigert's J method, * great numbers of exceedingly fine ' medullated fibres, many fs of the Long Paths of the Cord Coming back now to our" question as to the connections of the long tracts. of the cord, let us consider, first of all. The, Gonnection,s of the Postero-Median and Postero- External Coluirins. — ^When a single posterior root is divided, say, in the dorsal region, between the cord and the ganglion, its fibres, as we have already seen (p. 771), degenerate above the section. Since the cell- bodies of these neurons lie in the ganglion, if a series of microscopic sections of the spinal cord be made, well-marked degeneration will be found at the level of entrance of the root on the same side of the cord, while below that level there will be only a few degenerated fibres in the comma tract. Immediately above the plane of the divided root the degeneration will be confined to Burdach's column and to its external border. Higher up it will be found in the internal portion of Burdach's and the external rim of GoU's column. Still higher up the degenerated fibres will be confined to the postero- median column ; the postero-external will be free from degeneration. When a number of consecutive posterior roots are cut, the whole of the postero-external column in the sections immediately above the highest of the divided roots will be found occupied by degene- rated fibres, while GoU's column may be free from degeneration, or CQNNECTIONS OF THE LONG PATHS OF THE CORD 843 degenerated only at its outer border. Higher up degeneration will be found to have involved the whole of the postero-median column, and to have cleared away altogether from the postero-external. The degeneration in the column of GoU may be traced along the whole length of the cord to the medulla, although the number of degenerated fibres diminishes as we pass upward. The explanation of these appearances is as follows: It may be seen in preparations of the cord impregnated by Golgi's method that the fibres of the posterior roots soon after their ■ entrance into the cord divide into two processes, one of which runs up and the other down in the posterior column, or in the adjoining portion of the posterior horn. From both of these collaterals are given oft at intervals to the grey matter. The descending branches run downwards only fof a short dis- tance, and the degeneration in the comma tract seen after section of the cord is due to the division of theise branches. Many of the as- cending branches pass up for a short distance in the postero-external column, sweeping obliquely through it to gain the tract of GoU. In this tract some of them run right on to the meduUa ob- longata, to end by arborizing among the cells of the nucleus gracilis. Other fibres, both of GoU's and of Burdach's tract, end at various levels in the cord, their collaterals, and ultimately the coming into relation with nerve-cells in the grey matter. When the cervical posterior roots are cut, many of the degenerated fibres remain in Burdach's column up to the medulla, where they terminate Fig- 338. — Diagrams of Degeneration at Different Levels in the Cord after Sec- tion of a Number of Posterior Roots of Nerves forming the Lumbo-Sacral Plex- us (Mott). Fig- 339- — Branching of Posterior Root-Fibres in Cord (Donald- son, after Cajal). Collaterals, Col, are seen coming off from the two main branches of the root-fibres, DR, and ending in arborizations. CC, cells in the grey matter of the cord, whose axons also give off collaterals. main branches themselves. 844 THE CENTRAL NERVOUS SYSTEM in the nucleus cuneatus. In the posterior column, then, the numerous fibres of the posterior roots which do not end in the spinal cord are arranged in layers, the fibres from the lower roots being nearest the median fissure (in the postero-median column), and those from the higlier roots farthest away from it (in the postero-extemal column. Th^s, in a sectiop through the upper cervical region Goll's column is almost entirely composed of fibres from the posterior limb, while the column of Buftiach consists of fibres from the anterior limb. Othei- collaterals from the posterior root-fibres, and many of the main root-fibres themselves, run into the anterior horn and terminate in arborizations around its cells; some pass into the posterior horn, and doubtless come into relation with its scattered cells and, in the dorsal region, with the cells of Clarke's column. Some of the posterior root-fibres and their collaterals also form synapses with the cells of the intermedio-lateral tract. Other collaterals and probably some axons cross the middle line in the anterior and posterior commissures and end in the grey matter of the opposite side. Connections of the Direct or Dorsal Cerebellar Tract. — Since the dorsal or direct cerebellar tract does not degenerate after section of the posterior nerve-roots, but does degenerate above the level of the lesion after section of the spinal cord, the nerve-ceUs from which its axons arise must be situated somewhere or other in the cord. Now, it has been observed that the vesicular column of Clarke first becomes prominent in the lower dorsal region, and that in this same region the direct cerebellar tract begins. Atrophy of the cells of Clarke's column has sometimes in disease been shown to accompany de- generation of the direct cerebellar fibres. After an experimental lesion of these fibres in animals, some of the cells of the vesicular column show the changes in the Nissl bodies and the other changes which we have already described as occurring in nerve-cells whose axons have been cut. After two or three months these cells may be found almost completely atrophied (Schafer). Finally, axis- cylinder processes have been seen sweeping out from Clarke's column into the direct cerebellar tract (Mott). The evidence, then, is complete that the cells of origin of this tract are in Clarke's column. Clarke's cells are surrounded by arborizations, some of which, as previously stated, represent the terminations of posterior root-fibres and of their collaterals. The neurons whose axons run in the dorsal cerebellar tract are therefore the second link in an afferent path. The direct cerebellar tract runs right up to the cerebellum through the restiform body, without crossing and without being further interrupted by nerve-cells. The restiform body ends partly in the dentate nucleus of the cerebellum, partly in the vermis, and among the fibres which end in the vermis are those of the direct cerebellar tract. In the dorsal cerebellar tract there is a definite stratification CONNECTIONS OF THE LONG PATHS OF THE CORD 845 of the fibres: the fibres from the lowest segments of the cord lie outermost ; beneath these come fibres from the lowest thoracic seg- ments, then fibres from the higher thoracic segments ; and, internal to all, fibres from the topmost thoracic and lowest cervical segments^ Connections of the Antero-Lateral Ascending Tract. — According to Schafer, the axons of this tract are probably connected with cells situated in the middle and posterior parts of the grey crescent, mainly .p.mf. ff- 77 fc. ^, "/ y n.c "i^' .'■ n.t ig. 340. — Transverse Section of Medulla Oblongata at the Level of the Decussa- tion of the Fillet (Halliburton, after Schwalbe). a.m.f, anterior, and p.m.f, posterior, median fissure; f.a and f.a^, external arcuate fibres; f.a', internal arcuate fibres becoming external; n.a.r, nuclei of arcuate fibres; py, pyramid; 0,0', lower end of nucleus of olive ; f.r, formatio reticularis; n.l, lateral nucleus; ».g, nucleus gracilis; /.g, funiculus gra- cilis; n.c, nucleus cuneatns; n.c', external cuneate nucleus; /.c, funiculus cuneatus; g, substance of Rolando ; c.c, central canal surroundedby grey matter ; ». XI, nucleus of spinal accessory; n.XII, of hypoglos- sal; a.V, ascending root of fifth nerve; s.d, the decussation of the fillet, or superior decussation. Fig. 341. — Transverse Section of MeduU; Oblongata at about the Middle of th Olive (Schwalbe). f.l.a, anterior mediai fissuire; n.a.r, arcuate nucleus; p., pyra mid; n.XII, hypoglossal nucleus; XII root bundle of hypoglossal nerve comin, off from the surface ; at 6 it runs betweei the pyramid and the dentate nucleus o the olive, 0; f.a.e, external arcuate fibres n.l, lateral nucleus; a, arcuate fibres goini to restiform body c.r, partly through thi substantia gelatinosa |, partly superficia to the ascending root of the fifth nervi a.V; X, root-bundle of vagus; n.X, n.X' two portions of vagus nucleus; f.r, for matio reticularis; «.g, nucleus gracilis n.c, nucleus cuneatus; n.t, nucleus of thi funiculus teres; n.am, nucleus ambiguus r, raphe ; 0', 0", accessory olivary nucleus p.o.l, peduncle of the olive. on the opposite side of the cord, although also on the same side. None of the fibres of the tract can come directly from the posterior nerve-roots, since no degeneration is seen in it on section of the roots alone. The antero-lateral ascending tract passes up through the medulla, where some of its fibres perhaps form synapses with the cells of the 846 THE CENTRAL NERVOUS SYSTEM lateral nucleus, a collection of grey matter in the lateral portion of the spinal bulb. But its main strand runs on unbroken through the medulla, in front of the restiform body, and behind the olive, and after reaching the upper part of the pons bends back over and in company with the superior peduncle as the ventral spino-cerebellar bundle, to end in the worm of the cerebellum (Fig. 353). A few fibres of Gowers' tract niay pass by the middle peduncle to the opposite cerebellar hemisphere. Some of its fibres do not go to the cerebellum at aU. One group can be followed to the corpora quadri- gemina (spino-tectal fibres), and another by way of the tegmentum of the cms cerebri to the optic thalamus {spino-thalamic fibres). Through the relay of the gracile and cuneate nuclei, the postero- internal and postero-extemal columns of the cord are further con- nected on the one hand with the cerebrum, and on the other with the cerebellum. The cells of the nuclei give off fibres (internal arcuate fibres) which, sweeping in wide arches across the mesial raphe to the opposite side, take up a position behind the pyramid in the tract of the fillet, a bundle of fibres which becomes more compact, and therefore more distinct, as it passes brainwards. Receiving fibres from other sources on its way, and also giving off fibres, it rims up- wards through the dor- sal or tegmental portion of the pons. In the mid- brain it divides into two portions, the lateral fillet, also called the lower fillet or fiUd of Reil, and the intermediate, also called the upper fillet. The lateral fillet contains mainly fibres arising in the cochlear nucleus of the auditory never, and ends in grey matter of the pos- terior corpus quadrigeminum, and partly in the mesial genictdate body. It appears to be a path for the conduction of auditory impulses. The intermediate fillet contains chiefly the fibres that come off from the gracile and cuneate nuclei, but is enlarged by the accession of fibres from the sensory nuclei of the cranial nerves. It terminates in the lateral nucleus of the optic thalamus by forming synapses with nerve-ceUs, whose axons, passing through the posterior limb of the internal capsule and the corona radiata, continue the afferent path to the cerebral cortex. Not all of the axons from the cells of the cranial sensory nuclei run Fig.^34S. — Diagrammatic Transverse Section of Crura Cerebri and Aqueduct of Sylvius, a, an- terior corpora quadrigemina; b, aqueduct; c, red nucleus; d, fillet; e, substantia nigra;/, pyramidal tract in the crusta of the crura cerebri; g, fibres from frontal lobe of cerebrum; h, fibres from tem- poro-occipital lobe ; », posterior longitudinal bundle. CONNECTIONS OF THE LONG PATHS OF THE CORD 847 in the fillet . Many of them occupy a position in the reticular forma- tion of the tegmentum dorsal to the fillet as they pass through the pons and mid-brain to end in the thalamus and the region below it (sub-thalamic region). From the sensory nucleus of the fifth nerve a separate bundle of fibres ascends to the thalamus, in the tegmen- tum of the mid-brain lateral to the posterior longitudinal bundle. Connections of the Pyramidal Tracts. — ^When the cortex in and in front of the fissure of Rolando is destroyed by disease in man, or removed by operation in animals, it is found that in a short time degeneration has taken place in the fibres of the corona radiata which pass off from this area. The degeneration can be followed down through the genu and the anterior two-thirds of the posterior limb of the internal capsule r^ (Fig. 343) and the crusta of the cerebral peduncle of the corre- sponding side into the medulla oblongata. Below the decussation of the pyramids it is found that the degene- ration has involved the two p5Tamidal tracts, and only these — the , crossed pyramidal tract on the side oppo- site the cortical lesion, the direct pyramidal tract on the same side — and that the cross- section of the two de- generated tracts goes on continually dimin- ishing as we pass down the cord. (We overlook for the moment, in the interest of simphcity of statement, the fact that some degenerated fibres are found in the crossed pyramidal tract on the same side as the lesion. ) This is proof positive that the cell-bodies of the neurons whose axons run in these tracts are situated in the cerebral cortex. They have indeed been identified with certain of the large pyramidal cells (the so-called giant cells or cells of Betz) in the cortex of the ' motor ' region in front of the Rolandic fissure (p. gtS). For after division of the motor pyramidal fibres in the upper cervical region of the cord (in monkeys) changes in the chromatin (so-called chromatolysis) and atrophy of these large cells occur. The same has been found to be true in man in cases where the cord was injured by fracture of the spine in such a way as to interrupt the tract (as well as other tracts) CORD MID. BRAIN Fig. 343. — Pyramidal Path (after Gowers). Degenera- tion after destruction of the ' motor ' area of the right cerebral hemisphere. The degenerated areas are indicated by the shading. 848 THE CENTRAL NERVOUS SYSTEM completely and permanently, without entailing death for a consider- able time (Holmes and May) . The fact that after destruction of the cortex or the path in its course the degeneration below the lesion does not spread to the anterior roots shows that at least one relay of nerve-cells intervenes between the pyramidal fibres and the root- fibres. The results both of normal and morbid histology enable us to identify the cells of the anterior horn as the cells of origin of the axons of the anterior root-fibres. For (i) Axis-cylinder processes have been actually observed passing out from certain of the so-called motor cells of the anterior horn to become the axis-cylinders of the anterior root. (2) In the pathological condition known as anterior poliomyelitis, the cells of the anterior horn degenerate, and so do the anterior roots of the affected region, the motor fibres of the spinal nerves, and the muscles supplied by them. (3) As already mentioned (p. 830), comparatively transient but decided changes occur in the anterior horn cells on section of the corre- sponding anterior roots. (4) An enumeration* has been made in a small animal (frog) of the cells of the anterior horn and of the anterior rcot-fibres, and it has been found that the numbers agree in a remarkable manner. From all this it cannot be doubted that most, at any rate, of the cells of the anterior horn are connected with fibres of the anterior root. But since the number of fibres in the pyramidal tracts (about 80,000 in each half of the human cord) falls far short of the number of fibres in the anterior roots (not less than 200,000 in man on each side), it is necessary to suppose either that one pyramidal fibre may be connected with several cells or that all the anterior root-fibres are not in functional connection with the pyramidal tract. Thfere is no reason to assume any such connection in the case of the fine meduUated root-fibres arising in the lateral horn and going to the visceral and vascular muscles. . While there is no doubt that anterior root-fibres and pyramidal fibres of the brain and cord form segments of the same nervous path, the connection between the pyramidal fibres and the cells of the anterior horn has not yet been anatomically demonstrated. Many of the pyramidal fibres pass into the grey matter between the anterior and posterior horns or near the base of the posterior horn. The anterior horn cells are surrounded by arborizations. Some of these are probably the terminations of axons whose cell-bodies are situated in the posterior horn, others the terminations of posterior root-fibres or their collaterals. Many of them very likely represent the end arborizations of pjn-amidal fibres or their collaterals. Some ob- servers, however, suppose that the pyramidal fibres do not come into immediate relation with the anterior horn cells, but that another neuron is intercalated between them and the cells. The pjnramidal fibres are unquestionably paths for voluntary motor impulses passing down from the. cortex to the cord. But * Such enumerations can be made with great accuracy from photographs of sections of the nerves (Hardesty, Dale). (See Fig. 331. p. 832.) CONNECTIONS OF. THE WNG, PATBS OP THE CORD 849, they are not the only cortico-spinal efferent paths, and in niany, animals they are not even the mosj: important paths for voluntary movements. It is the more skilled and dehcate movements which the pyramidal tract subserves in man, and it is these movements which are permanently lost when the tract is destroyed. The size of the path is proportioned to the degree of development of the brain. Thus, it is larger in the monkey than in the dog, larger in the anthropoid apes than in the lower monkeys, and larger in man than Fig. 344. — Paths from Cortex in Corona Radiata (Starr). A, tract from frontal con- volutions to nuclei of pons and sq to cerebellum; B, motor pyramidal tract; C, afferent tract for tactile sensations (represented in the diagram as separated from B by an interval for the sake of clearness); D, visual tract; E, auditory tract; F, G, H, superior, middle, and inferior cerebellar peduncles; J, fibres from the auditory nucleus to the posterior corpus quadrigeminum ; K, decussa- tion of the pyramids in the bulb; FV, fourth ventricle. The roman numerals indicate the cranial nerves. in even the highest of the apes. In the lower mammals it is exceed- ingly small. . While in man the pj^amidal tracts constitute nearly 12 per cent. I of the total cross-section of the cord, they ma,ke up little more than i per cent, in the mouse, 3 per cent, in the guinea-pig, 5 per cent, in the rabbit, and nearly 8 per cent, in the cat. In some mammals, as the rat, mouse, guinea-pig, and squirrel, the pyramidal tracts lie, not in the antero-lateral, but in the posterior columns. In, vertebrates below the mammals the pyramidal system does not exist as a collection of neurons which send their axons with-, 850 THE CENTRAL NERVOUS SYSTEM out interruption down from the cortex to the cord. In birds, e.g., after the removal of a hemisphere, the degeneration does not extend below the mid-brain (Boyce). Section VI. — ^Paths from and to the Cortex. Thus far we have been able to map out two great paths from the cerebral cortex to the periphery — one efferent, the other afferent, (i) The great efferent or motor pyramidal path, which, starting in the cortex in front of the fissure of Rolando, where its axons give off numerous collaterals to the grey matter soon after emerging from the cells, and sweeping down the broad fan of the corona radiata, passes through the narrow isthmus of the internal cap- sule into the crusta of the crus cerebri, and thence into the pons (Figs. 344, 345). At this level, the fibres destined to make connection with the motor nuclei of the cranial nerves in the grey matter underlying the aqueduct of Sylvius and the fourth ven- tricle terminate. Most of these fibres decussate to make physiological connection with nuclei on the opposite side, but some join nuclei on the same side. The question whether they arborize di- rectly around the cells of the motor nuclei or make junc- tion with them through another intercalated neuron is precisely in the same position as the corresponding question for the spinal pyra- midal path (p. 848). On their way through the pons they send off collaterals to the nuclei pontis, as they do higher up to the grey matter of the basal gangUa of the cerebrum and the substantia nigra, and the path may be continued to the motor nuclei by axons arising here. There is no proof, however, that this is the case. The rest of the pyramidal fibres run on into the P5n-amid of the bulb, where the greater part (usually about 90 per cent.) of the fibres decussate, appearing in the cervical cord as the massive crossed Fig. 345. — Motor Pyrsimidal Tracts (Diagram- matic) (Halliburton, after Gowers). The convolutions are supposed to be cut in vertical transverse section, the internal capsule, I, C, and the crus in horizontal section. O, TH, optic thalamus; CN, cau- date nucleus; L2 and L3, middle and ex- ternal portions of lenticular nucleus; /, a, /, fibres from the face, arm, and leg areas of the cortex respectively; E, S, Sylvian fis- sure. The genu or knee of the internal capsule is indicated by the asterisk. PATHS FROM AND TO THE CORTEX 851 pyramidal tract of the opposite side. A few (usually about 10 per cent.) remain on the same side as the slender direct pyramidal tract. The size of this tract varies much in different individuals, and it is occasionally absent. Its breadth constantly diminishes as it proceeds down the cord, and it disappears before the middle of the thoracic region is reached, its fibres continually decussating across the anterior white commissure and plunging into the opposite anterior horn. They either end among its cells, or, passing through it, reinforce the crossed pyramidal tract. The fibres of this crossedtract are, in their turn, con- tinually passing oft into the grey matter to make connection (p. 848) with the cells of the anterior horn, whose axis-cylin- der processes enter the anterior roots of the ■ spinal nerves. The losses which it suffers as it descends the cord may be in some slight degree compensated by the bi- furcation of some of its fibres (geminal fibres), but ultimately the whole tract forms synapses with cells in the grey matter, and dwindles awav aino-Cer£bellAr|of the nostrils. Each olfactory cell gives off two processes, a short one, representing a dendrite, which runs out to the surface of the mucous membrane, and a longer but more slender process, representing an axon. THE CRANIAL NERVES 893 which as a fibre ol the olfactory nerve pierces the cribriform plate of the ethmoid bone, and plunges into the olfactory bulb. In the olfactory bulh at least four layers can be distinguished — (i) on the surface, beneath the pia mater, the layer of entering olfactory nerve-fibres ; (2) the layer of olfactory glomeruli, peculiar structures, each of which is made up of an intricate basket-like arborization formed by an olfactory nerve-fibre, or, it may be, more than one, and a brush-like arborization belonging to a dendrite of one of the mitral cells of the next layer; (3) the molecular or mitral layer, which contains a number of large nerve-cells called, from their most common shape, mitral cells, along with sm.aller nerve-cells (' granules ') and neuroglia ; (4) the nuclear layer, containing numerous small nerve-cells or ' granule? ' intermingled with white fibres. The mitral cells give ofi axons, which pass through the fourth layer, and then as fibres of the olfactory tract to the grey matter of the hippocampal region of the brain. The course of the impulses from the olfactory mucous membrane to the brain is shown in Fig- 359- The olfactory tract, as it runs back, divides into portions Fig. 359. — Scheme of the Olfactory Nervous Apparatus (Cajal). A, olfactory cells; B, glomeruli; C, mitral cells; t>, olfactory granule cell; E, lateral root of olfactory tract; F, cortex of brain in the region of the uncinate gyrus; a. small cell of mitral layer; 6, brush of dendrite of a mitral cell ending in a glomerulus; c. thorns or spines on the processes of an olfactory granule; e, collateral coming ofi from the axon of a mitral cell; /, collaterals ending in the molecular layer of the uncinate gyrus; g, pyramidal cells of the cortex; h, supporting epithelial cells of the olfactory mucous membrane. called its ' roots.' Of these the lateral is the most important, and it terminates in the hippocampal and uncinate gyri of the same side. Fibres of'the olfactory tract are also connected either directly or through the relay of another neuron with the opposite side of the bram, especially the opposite uncinate gyrus. The anterior commissure contains numerous fibres, which connect the hippocampal regions of the two sides. Other central connections of the olfactory tract exist, but some are imperfectly known. The name ' rhinencephalon ' is given to the portions of the brain concerned with the sense of smell. Disturbances of smell sensation may be caused by lesions in any part of the rhinen- cephalon, and also by changes in the olfactory mucous membrane and olfactory fibres; but the symptoms do not obtrude themselves, and are doubtless often overlooked. Excessive stimulation of the olfactory nerve by exposure to a strong odour has been said to cause complete and permanent loss of smell. The second or optic nerve contains mainly afferent fibres, which arise from the ganglion cells of the retina, and terminate by forming synapses with nerve-cells in the lateral or external geniculate body, the 894 THE CENTRAL NERVOUS SYSTEM pulvinar (or posterior portion) of the optic thalamus, and the anterior corpus quadrigeminum. In young animals all these structures undergo atrophy after extirpation of the eyeball. The visual path is continued from the pulvinar and the external corpus geniculatum by the axons of these nerve-cells, which proceed in the optic radiation (p. 855) to the occipital cortex. The fibres which pass from the retina to the anterior corpus quadrigeminum are distinguished by their small size, and probably constitute the path of the impulses which cause contraction of the pupil when light falls on the retina. The reflex arc is schematic- ally shown in Fig. 360, where optic nerve-fibres are represented as forming synapses with cells in the anterior corpus quadrigeminum whose axons pass to the nucleus of the third nerve and arborize around some of its cells (Figs. 344, 346, and 360). At the chiasma the fibres of the optic nerve de- cussate, partially in man and some mammals, as the rabbit, dog, cat, and monkey, com- pletely in animals whose visual field is entirely independent for the two eyes, as in fishes and birds. In rnan the fibres for the nasal halves of both retina; cross the middle line at the chiasma, those for the temporal halves do not. This does not mean, however, that exactly half of the optic nerve-fibres deciissate. The number of un- crossed fibres is smaller than that of crossed. The chiasma also contains fibres in its pos- terior portion, which extend from one optic tract to the other, but are not connected with the retinae or the optic nerves. They are commissural fibres which connect the two mesial geniculate bodies across the middle line, and are called Gudden's commissure. A suffi- ciently extensive lesion involv- ing the occipital cortex on one side, or the posterior portion of the optic thalamus, or the optic tract, causes hemianopia* or defect of the visual field on the side opposite to the lesion, with blindness of the corresponding halves of the two retinae. Thus, a lesion equivalent to complete section of the right optic tract would cause blindness of the nasal half of the left, and of the temporal half of the right eye, and the left half of the field of vision would be blotted out — ^the patient would be unable, with his eyes directed forwards, to see an object at his left. Such a complete hemianopia is much rarer in disease of the cortex than in disease of the * The terms ' hemiopia,' ' hemianopia,' ' hemianopsia,' are used with refer- ence sometimes to the blind side of the retinae, but ordinarily to the half of the visual field which is deficient We shall always use the word ' hemianopia ' in the latter sense. Ill Nerve Fig. 360. — Scheme of the Visual Path (after Schafer). THE CRANIAL NERVES 895 optic tract. A lesion — e.g.. a tumour of the pituitary body — involving the whole of the optic nerve in front of the chiasma, would cause com- plete blindness in the corresponding eye. Sometimes in disease of the optic nerve vision is not totally destroyed in the eye to which it belongs, but the field is narrowed by a circumference of blindness. In this case the pathological change involves the circumferential fibres of the nerve. When the chiasma is affected by disease, a very frequent symptom is bitemporal hemianopia, blindness of the nasal halves of the retinae, with loss of the outer or temporal half of each field of vision. The optic nerve and tract contain a few efferent fibres for the retina, whose cell-bodies have not yet been certainly located. The third nerve, or oculo-motor, arises from an elongated nucleus, or a series of nuclei, containing large nerve-cells in the floor of the Sylvian aqueduct below the anterior corpora quadrigemina. The root- bundles coming off from the most anterior of the nuclei carry fibres that innervate the ciliary muscle, and thus have to do with the mechanism of accommodation, and also fibres that innervate the sphincter muscle of the iris, and thus cause contraction of the pupil when light falls on the retina. Both groups of fibres terminate by arborescing around sympa- thetic cells in the ciliary ganglion, from which the path to the (unstriated) ciliary and sphincter muscles is continued by post-ganglionic fibres. Further back in the oculo-motor nucleus arise the motor fibres for four of the extrinsic muscles of the eyeball and the elevator of the upper eyelid. In the dog these fibres come ofi in the following order, from before backwards; internal rectus, superior rectus, levator palpebrre superioris, inferior rectus, inferior oblique. Most of the fibres of the third nerve arise from nerve-cells on their own side of the middle line, but a certain number decussate to enter the nerve of the opposite side. Complete paralysis of the third nerve causes loss of the power of accommodation of the corresponding eye, dilatation of the pupil by the unopposed action of the sympathetic fibres, diminution of the power of moving the eyeball, ptosis, or drooping of the upper lid, external squint, and consequent diplopia, or double vision. The fourth or trochlear nerve arises from the posterior part of the same tract of grey matter which gives origin to the third nerve. It supplies the superior oblique muscle. Paralysis of the nerve causes internal squint when an object below the horizontal plane is looked at, owing to the unopposed action of the inferior rectus. There is also, diplopia on looking down. Unlike the other cranial nerves, the two trochlear nerves decussate completely after they emerge from their nuclei of origin. The fifth or trigeminus nerve appears on the surface of the pons as a large sensory root and a smaller motor root. Its deep origin is more extensive than that of any of the other cerebral nerves, stretching as it does from the level of the anterior corpus quadrigeminum above to the upper part of the spinal cord below. Its sensory root, in fact, seems to include the sensory divisions of several motor cranial nerves. The motor root arises partly from a nucleus [principal motor nucleus) in the floor of the fourth ventricle below the pons, partly from large round nerve-cells lying at the side of the grey matter bounding the aqueduct of Sylvius all the way from the anterior quadrigeminate body to the point at which the motor root is given off (accessory or superior motor nucleus). The fibres of the sensory root have their cells of origin in the Gasserian ganglion, whence they pass into the pons. Here they bifurcate into ascending and descending branches. The ascending branches end in the principal sensory niwleus, a collection of grey matter at the side of 896 THE CENTRAL NERVOUS SYSTEM the principal motor nucleus. The descending branches, turnmg down- wards into the medulla oblongata, terminate in a long tract of scattered cells, constituting with the fibres the so-called spinal root, and extending from the level of the second cervical nerve through the medulla oblon- gata and the pons, where it is continued into the principal sensory nucleus. The afferent path is continued by the axons of cells of the sensory nuclei (or nuclei of reception) of the nerve, many of which cross the middle line and enter the intermediate fillet of the opposite side, and also the special ascending bundle going to the thala- mus. Some of the axons do not decussate, but ascend in the fillet of the same side. Tr. sp. n. V. . 361. — Scheme of Motor and Sensory Neurons of Trigeminus (Gehuchten). G. a. G., Gasseriau ganglion; Nu. m. m. ». V., nucleus of the descending root; Nu. m. pr. n. V., chief motor nucleus of the fifth nerve; Rod. desc. mes. n. V., accessory motor nucleus, sometimes called the descending root; Tr. sp. n. V., tractus spinalis, or spinal root of the fifth. The motor fibres of the fifth nerve supply the muscles of mastication and the tensor tympani. The sensory fibres confer common sensation on the face, conjunctiva, the mucous membranes of the mouth and nose, and the structures contained in them, and, according to Gowers, special sensation, through branches given o3 to the facial and glosso-phaiyngeal nerves, on the organs of taste.* Complete paralysis of the nerve causes * It should be stated that some physiologists behave that the glosso-pharyn- geal is the nerve of taste, and that none of the taste fibres go to the sensory nuclei of the fifth nerve. The majority hold that the glosso-pharyngeal supplies the posterior third, and the chorda tympani and lingual llie anterior two-thirds of the tongue with gustatory fibres. The removal of the Gasserian ganghon and the adjacent portion of the fifth nerve for severe and persistent neuralgia, has afforded opportunities to test this question. But, unfortunately the results described by various observers do not agree, some finding that taste is unimpaired, others that it is aboUshed. Gowers states that the gus- tatory sensations may persist for some time after the operation, although ultimately (in two or three weeks) they disappear. It may be, however, that this disappearance is due to secondary changes produced in the end-organs of the true taste fibres, the taste buds, by degeneration of the supporting cells consequent on section of the trigeminus, or to degeneration and swelUng of the trigeminal fibres in the lingual nerve and consequent interference with the conductivity of the intermingled chorda tympani fibres. Gushing believes THE CRANIAL NERVES 897 loss of movement in the muscles of mastication, sometimes 'impaired hearing, and loss of common sensation in the area supplied by it. Loss or impairment of taste in the corresponding half of the tongue is also often seen in disease involving the sensory root, although not in affections of the trunk of the nerve, since the taste fibres leave it near its origin (Gowers) . Both taste and touch are lost in the monkey in the anterior two-thirds of the tongue after intracranial section of the trigeminus (Sherrington). Vaso-motor changes are occasionally, and ' trophic ' changes fre- quently, observed in disease of the fifth nerve. The trophic disturbance is most conspicuous in the eyeball (ulceration of the cornea, going on, it may be, to complete disorganization of the eye). These effects are partly due to the loss of sensation in the eye, and the consequent risk of damage from without, and the unregarded presence of foreign bodies and accumulation of secretion within the lids (p. 778). The sixth or abducens nerve takes origin from a nucleus in the floor of the fourth ventricle at the level of the posterior portion of the pons. It is a purely efferent nerve, and supplies the external rectus muscle of the eyeball. Paralysis of it causes internal squint. The motor fibres of the seventh or facial nerve arise from a nucleus in the reticular formation of the medulla oblongata, and running up some distance into the pons. They supply the muscles of the face; and when these are greatly developed, as in the trunk of the elephant, the nerve reaches very large proportions. Since the fibres which connect the cerebral cortex with the nucleus decussate about the middle of the pons, a lesion above this level which causes hemiplegia paralyzes the face on the same side as the rest of the body— i.e., on the side opposite the lesion. But the paralysis is confined to the muscles of the lower portion of the face, and affects especially the muscles about the mouth. Sometimes the pyramidal tract and the facial nerve, or nucleus, are involved in a common lesion. In this case paralysis of the face is on the side of the lesion, and is total, while the rest of the body is para- lyzed on the opposite side. Paralysis of the seventh nerve is more common than that of any other nerve in the body. It is often caused by an inflammatory process in the nerve itself (neuritis). The symp- toms of complete facial palsy are very characteristic. The face and forehead on the paralyzed side are smooth, motionless, and devoid of expression. The eye remains open even in sleep, owing to paralysis of the orbicularis palpebrarum. A smile becomes a grimace. An attempt to wink with both eyes results in a grotesque contortion. The mouth appears like a diagonal slit in the face, its angle being drawn up on the sound side, and the patient cannot bring the lips sufficiently close together to be able to blow out a candle or to whistle. Liquids escape from the mouth, and food collects between the paralyzed buc- cinator and the teeth. The labial consonants are not properly pro- nounced. Taste may be lost in the anterior two-thirds of the tongue when the nerve is injured above the exit of the gustatory fibres in the chorda tympani, but not when the lesion is in the nucleus of origin, or anywhere above it. Hearing is sometimes impaired because the auditory and facial nerves, lying close together for part of their course, are apt to suffer together, but perhaps also because the stapedius muscle is supplied by the seventh. that the fifth nerve suppUes no taste fibres, but that the taste fibres for the anterior two-thirds of the tongue have their cells of origin in the geniculate ganglion of the pars intermedia of the seventh nerve, and those for the pos- terior third in the ganglion petrosum of the ninth nerve. 57 898 THE CENTRAL NERVOUS SYSTEM The seventh nerve is not purely motor. From the cells of a ganglion on it corresponding to a spinal ganglion (the geniculate ganglion) afferent fibres arise, which pass in the pars intermedia or nerve of Wrisberg into the pons between the seventh and eighth nerves, and there bifurcate into ascending and descending branches, like other afferent fibres originating in ganglia of the spinal type. The descend- ing branches enter the fasciculus solitarius, and end by arborizing around nerve-ceUs in the upper part of that bundle. The peripheral axons of the nerve-cells in the geniculate ganglion enter the large super- ficial petrosal nerve and the chorda tympani, in which they, or some of them, perhaps represent taste fibres. The eighth or auditory nerve enters the medulla oblongata by two roots (a dorsal and a ventral), one of which passes in on each side of vin Fig. 362. — Scheme of Path of Auditory Impulses (Lewandowsky). Sp, ganglion spirale; G, accessory nucleus; T, acoustic tubercle; Tr, trapezium; H, Held's fibres; St, striae acusticae; tr, trapezoid nucleus; Os, upper olive; LI, lateral fillet, with its nucleus, nL; P, commissure of the lateral fillets; Qp, posterior corpora quadrigemina, with Cq, their commissure, and Bq, the bracbia; Gm, mesial or internal geniculate body; R, cerebral cortex. the restiform body. The cells of origin, both of the dorsal and of the ventral root, are situated in the internal ear, the former in the ganglion spirale, or ganglion of Corti, which is embedded in the bony spiral of the cochlea, the latter in the ganglion vestibulare, or ganglion of Scarpa, which lies in the vestibvile. These cells correspond to the ganglion cells on the posterior root of a spinal nerve, but, unlike them, they remain, even in mammals, bipolar throughout life. Their central processes form the axons of the eighth nerve. Their peripheral pro- cesses are distributed in the case of the dorsal root to the organ of Corti, in the case of the ventral root to the semicircular canals and the vestibule. For this reason the dorsal root is often called the cochlear division, and the ventral root the vestibular division of the auditory THE CRANIAL NERVES 899 nerve. And the cochlear and vestibular roots are physiologically as well as anatomically distinct. For the cochlea subserves the function of hearing, the semicircular canals and vestibule the function of equilibration. As they enter the medulla oblongata, the fibres of the dorsal root bifurcate. Of the two branches, one is considerably thicker than the. other. Many of the thicker branches terminate by arborizing around the cells of the accessory auditory nucleus, whose position is indicated by a swelling on the ventral surface of the resti- form body at the junction of the dorsal and ventral roots; but some pass over the restiform body to end in another nucleus (lateral nucleus), also indicated by a swelling (tuberculum acusticum) lying over the restiform body. The nerve-cells of the accessory nucleus and the acoustic tubercle, therefore, constitute nuclei of reception for the dorsal root-fibres. The more slender branches of the cochlear root- fibres run downwards for some distance before breaking up into fibrils. The path to the higher parts of the brain is continued by the axons of nerve-cells in the accessory nucleus and the acoustic tubercle. The fibres from the accessory nucleus pass into the trapezium, a mass of transverse fibres lying in the pons behind the pyramidal fibres. In their course through the trapezium some of the fibres terminate around the cells of the nucleus of the trapezium, others run into the superior olive of the same side, and end there; but most of them cross the middle line, and enter the trapezoid nucleus and superior olive of the opposite side, where many of them terminate. Others, however, run through those nuclei and pass into the lateral fillet, to end in its nucleus or in the posterior corpora quadrigemina. The path of the fibres which terminate in the nuclei of the trapezium, superior olive, and lateral fillet, is continued by another relay of fibres, which link them also to the posterior corpora quadrigemina. The axons of the cells of the acoustic tubercle enter for the most part the strics acusticce, a series of prominent strands that run transversely across the floor of the fourth ventricle. Passing across the raphe, they join the fibres from the accessory nucleus on their way to the superior olive, and accompany them into the lateral fillet, which terminates in the grey matter of the posterior corpus quadrigeminum. We must assume, from clinical and experimental data, that the dorsal root is ultimately connected with the first or first and second tempore -sphenoidal convolutions on the opposite side. From the posterior corpora qiiadrigemina the auditory path to the convolutions seems to run in the brachium to the internal or mesial geniculate body, whence it is continued in the posterior extremity of the internal capsule. The fibres of the ventral root of the eighth nerve, better termed the vestibular nerve, after entering the medulla oblongata, pass to a nucleus called the principal nucleus of the vestibular division. Here each bifurcates into a descending and an ascending branch. The descending branches running down in the medulla terminate at dif- ferent levels around cells in the principal nucleus, and the grey matter continued down from it (descending vestibular nucleus). The ascending branches run up on the inner side of the restiform body towards the nucleus of the roof {nucleus tecti) in the cerebellar worm. On their course they enter into relation through their collaterals with the nuclei of Deiters and Bechterew. The nucleus of Deiters, as already stated, sends fibres into the posterior longitudinal bundle. Through ascend- ing branches of these fibres a communication is established with the nuclei of the third and sixth nerves, and through descending branches that pass into the antero-lateral descending tract of the cord with the anterior horn cells. It is obvious that through these connections 900 THE CENTRAL NERVOUS SYSTEM which link the vestibule with the cerebellum, the nuclei of the motor nerves of the eyeball and the motor cells of the cord, the nucleus of Deiters has an important relation to the co-ordination of those move- ments mainly concerned in equilibration. Nothing is known of the connections of the vestibular nerve with the cerebrum. Two promi- nent symptoms may be associated with disease of the auditory nerve — {a) disturbance or loss of hearing; (b) loss or impairment of equilibration. The ninth or glosso-pharyngeal nerve comprises both sensory and motor fibres — sensory for the posterior third of the tongue and the mucous membrane of the back of the mouth, motor for the middle constrictor of the pharynx and the stylo-pharyngeus. It also contains the netves of taste for the posterior third of the tongue. The efferent fibres arise from a nucleus (motor nucleus of the glosso-pharyngeal) a little posterior to the facial nucleus. The afferent fibres take origin from unipolar cells in ganglia of spinal type connected with the nerve (ganglion petrosum and ganglion superius). Entering the medulla oblongata, the central processes of these cells bifurcate into ascending and descending branches. Their peripheral processes pursue their course as the axons of sensory fibres to the structures to which the nerve is distributed. The ascending branches terminate in a nucleus (principal nucleus of the glosso - pharyngeal) beneath the floor of the fourth ventricle. The descending branches, as well as similar branches from the pars intermedia of the seventh nerve and from the afierent fibres of the vagus, form a bundle called the fasciculus solitarius (some- times termed the descending root of the facial, vagus, and glosso-pharyn- geal). It can be traced to the lower boundary of the spinal bulb. Along the mesial border of the fasciculus sohtarius are strung out the somewhat scattered nerve-cells (descending nucleus of facial, vagus, and glosso-pharyngeal), around which the descending branches arborize: At its upper end the grey matter of the fasciculus solitarius is con- tinuous with the principal nuclei of the glosso-pharyngeal and vagus.' The tenth nerve, or vagus, "also contains both motor and sensory fibres. The efferent fibres arise partly from the nucleus ambiguus or ventral nucleus of the vagus, a collection of large nerve-cells situated in the reticular formation, and extending from a point a little below the facial nucleus to a point a little above the lower limit of the medulla oblongata, where it becomes continuous with the column of cells from which the spinal fibres of the eleventh nerve take origin. A second nucleus of origin for efferent vagus fibres is constituted by the upper part of the dorsal accessory-vagus nucleus, a collection of rather small cells extending from a little below the lower margin of the pons to nearly the level of the first cervical nerve. The afferent fibres of the vagus arise from unipolar cells in ganglia connected with the nerve (ganglion jugulare, ganglion nodosum). In the medulla oblongata they bifurcate, like other fibres coming off from the cells of ganglia of spinal type. The ascending branches, which are short, terminate in the upper sensory or principal nucleus, and the descending branches, which are long, in the cells of the fasciculus soli- tarius, just as in the case of the glosso-pharyngeus. The motor fibres of the vagus are partly derived from the accessory, whose internal branch joins the vagus not far from its origin. The distribution of the nerve is more extensive than that of any other in the body. The oesophagus receives both motor and sensory branches from the oesophageal plexus. The pharyngeal branch of the vagus is the chief motor nerve of the pharynx and soft palate (including the tensor palati) . The superior laryngeal branch is the nerve of common sensation for the larynx above the vocal cords, and the motor nerve THE CRANIAL NERVES 901 oi the crico-thyroid muscle. The inferior or recurrent laryngeal sup- plies the rest of the laryngeal muscles, and the sensory fibres for the mucous membrane of the trachea and the larynx below the glottis. The superior laryngeal contains afferent fibres, stimulation of which gives rise to coughing, slows respiration, or stops it in expiration. Reflex movements of deglutition are also caused. The vagus supplies the lungs both with motor and sensory filaments through the pulmonary plexus. The motor fibres when stimulated cause constriction of the bronchi; excitation of the afferent fibres causes reflex changes in the rate or depth of respiration. The cardiac branches contain inhibitory fibres probably derived from the spinal accessory, and depressor fibres which pass up in the vagus trunk (dog), or as a separate nerve to join the vagus or its superior laryngeal branch or both (rabbit) . The gastric and intestinal branches contain both motor and sensory nerves for the stomach and intestines. The sensory are probably large medullated fibres (7 /* to 9 /*). The afferent vagus fibres from the stomach carry up impulses which excite the action of vomiting. Lesions of the vagus, its nuclei of origin, or its branches, are associated with many interest- ing forms of paralysis and other symptoms. Paralysis of the pharynx is generally caused by disease of the nucleus in the medulla. From its anatomical relation to the nuclei of the glosso-pharyngeal and hypo- glossal, it will be easily understood that these nerves are often involved in localized central lesions along with the vagus. But the fact that in progressive bulbar palsy (glosso-labio-laryngeal paralysis) — a condition characterized by progressive paralysis and atrophy of the muscles of the tongue, lips, larynx, and pharynx — ^the orbicularis oris and other muscles of the mouth and chin are paralyzed, while the rest of the muscles supplied by the facial remain intact, might seem to indicate that in system diseases it is not so much anatomical groups of nerve- cells which are liable to simultaneous degeneration and failure, as physiological groups normally associated in particular functions. Such functional groups of cells, occupied with the same kinds of labour at the same times and under the same conditions, might be supposed to take on a similar bias or tendertcy to degeneration — a tendency not indicated, it may be, by any structural peculiarity, but traced deep in the molecular activity of the cells. There is no foundation for the view that the lips are involved in progressive bulbar palsy because i;he fibres of the facial which supply them arise from the hypoglossal nucleus, any more than for the idea that the upper part of the face escapes because its motor fibres, while reaching it in the seventh nerve, really arise from the oculo-motor nucleus (Bruce). Difficulty in swal- lowing is the chief symptom of pharyngeal paralysis. The symptoms of laryngeal paralysis have been already described under ' Voice ' (p. 310). Tachycardia, or a permanent increase in the rate of the heart, has been stated to occur in certain cases of paralysis of the vagus, caused by disease or accidental interference; and a persistent slowing of the respiration has been occasionally attributed to the same cause. But it is difficult to reconcile many of these cases with experi- mental results, for in most of them the lesion only involved one vagus ; and in animals section of one vagus has no permanent effect on the rate of the heart or of the respiratory movements. Destruction of the nerve near its origin has been sometimes found associated with disappearance of the food-appetites, hunger and thirst, and it has been assumed that this was due to loss of afferent impulses from the stomach. But clinical testimony is by no means unanimous on this point, and experiments on animals show that other factors are involved in these sensations (see Chapter XVIII. ). 902 THE CENTRAL NERVOVS SYSTEM The eleventh or spinal-accessory nerve contains only efferent fibres. The cells of origin of its spinal portion lie in the lateral horn of the cord, from about the level of the first to the fifth or sixth cervical nerves. The bulbar portion, sometimes called the bulbar accessory, arises from the lower two-thirds of the dorsal accessory-vagus nucleus, from about the level of the first cervical nerve up to the level of the tip of the calamus scriptorius. The accessory portion of the nucleus lies behind and to the side of — i.e., dorso-lateral to — the central canal; the upper or vagus portion is more laterally placed in the floor of the fourth ventricle. Soon after the junction of its bulbar and spinal portions, the nerve divides into two branches, an internal and an external. The external branch, containing the spinal fibres, passes out to supply the trapezius and sterno-mastoid muscles with motor fibres. The internal branch, containing the bulbar fibres, passes bodily into the vagus. The twelfth or hypoglossal nerve is exclusively an efferent nerve. Its nucleus of origin is an elongated collection of large nerve-cells ex- tending throughout approximately the lower two-thirds of the bulb close to the median line and parallel to it. It contains the motor supply of the intrinsic and extrinsic muscles of the tongue and of the thyro- and genio-hyoid. Paralysis of it causes deficient movement of the corresponding half of the tongue. When the tongue is put out, it deviates towards the paralyzed side, being pushed over by the un- paralyzed genio-hyoglossus of the opposite side, which is thrown into action in protruding the tongue. Section X.— Functions of the Central Nervous System — (2) The Brain. The paths by which the various parts of the central nervous system are connected with each other and with the periphery have been already described, and we have completed the examination of the functions of the spinal cord and medulla oblongata. The events that take place in the upper part of the central nervous stem and in the cortex ]of the jcerebellum and cerebrum now claim our attention. From very early times the brain has been popularly believed to be the seat of all that we mean by consciousness — sensation, ideation, emotion, volition. And he who loves to trace the roots of things back into the past may see, if he choose, running through the whole texture of the older speculations a belief that the brain does not act as a whole, but is divided jnto mechanisms, each with its special work — a fore- shadowing, often in grotesque outlines, of the doctrine of localization so widely held to-day. But until comparatively recent times, cerebral physiology remained a kind of scientific terra incognita ; and no notable additions were made for a thousand years to the doctrines of Galen. Even to-day the utmost limit of our knowledge is reached when in certain cases we have connected a particular movement or sensation with, a more or less sharply-defined anatomical area. How the cere- bral processes that lead to sensations and movements, to emotions and intellectual acts, arise and die out; what molecular changes are asso- ciated with them; above all, how the molecular changes are translated into consciousness — ^how, for example, it is that a series of nerve- impulses from the optic radiation flickering across the labyrinth of the FUNCTIONS OF THE BR4IN 903 occipital 'cortex should light ,up there a visual sensation— these are questions to which we can as yet give no answer, and the answers' to some of which must for ever remain hidden from us. Functions of the Upper Part of the Central Stem and Basal Ganglia. — The function of the pons is sufficiently indicated by its name. The grey matter so plentifully scattered, especially in its ventral portion, may exercise a not unimportant influence on the impulses that traverse it. But on the whole its main office is to provide a bridge along which impulses may travel between other portions of the nervous system. We have already seen that many of its transverse fibres arising from the cells of the pontine grey matter, and then crossing the middle line to the opposite middle, peduncle, are the cerebellar segments of com- missural arcs connecting the cerebral with the opposite cerebellar hemispheres. The cerebral segments of these arcs are the cortico- pontine fibres originating in the prefrontal, temporal, and occipital portions of the cerebral cortex, and passing through the corona radiata, internal capsule, and crura cerebri, to end in the nuclei pontis. Many fibres and collaterals of the pyramidal tract also terminate here. On the dorsal aspect of the pons in the floor of the fourth. ventricle are the nuclei of origin (or reception) of the fifth, sixth, and seventh cranial nerves. Various reflex centres are situated in this region — e.g., that for the closure of the eyelids, when the conjunctiva is stimulated. The posterior corpora quadrigemina and internal geniculate bodies are connected with the cochlear division of the auditory nerves, and form important stations on the auditory path to the cortex. The anterior corpora quadrigemina and the lateral corpora geniculata are connected with the optic tracts. Their development is arrested after extirpation of the eyeball in young animals, and they may there- fore be assumed to be concerned in vision, although the size of their homologues, the optic lobes or corpora bigemina, in animals below the rank of mammals (birds, reptiles, amphibians), does not seem to be related to the development of the organs of sight. Proteus and the Hag-fish, e.g., have large optic lobes, rudimentary eyes and optic tracts. The optic nerve, the anterior corpus quadrigeminum, the nucleus of the oculo-motor nerve in the wall of the Sylvian aqueduct, and the fibres which it carries to the iris, form a reflex arc foi: the contraction of the pupil to light, as represented in Fig. 360, p. 894. The ftmaions of the optic thalami have not been fully defined either by experiment or pathological observation, except in so far as they can be deduced from their connections. Tying as they do in the isthmus of the brain, begirt by the great motor and sensory paths, it is to be expected that lesions of the thalami should affect also the internal capsule, and give rise to the symptoms of motor and sensory paralysis. But it is questionable whether any definite defect of motor power or common sensation has ever been unequivocally associated with a lesion restricted to the thalami. The most constant features of the so-called thalamic syndrome (or symptom-complex) are partial loss of sensibility, ■ especially to tactile impressions, and of the muscular sense on the opposite side, with some degree of inco-ordination and disorder, though little, if any, actual paralysis of voluntary movements. These phe- nomena are accounted for by the extensive connections of the thalami. Each of the thalamic nuclei is linked with a definite cortical region in such a way that destruction of the cortical area in young animals or human beings leads to degeneration of the corresponding nucleus. Some of the fibres connecting the cortex (and the corpus striatum) with the thalamus end in the thalamic grey matter, and are therefore efierent with respect to the cortex (corticofugal). It is, however, the 904 THE CENTRAL NERVOUS SYSTEM afferent paths to the cortex with which the thalami are specially related as centres of relay. The fibres of the upper fillet carrying afferent im- pulses up from the opposite posterior column of the cord to the cere- brum end in the grey matter of the thalamus, as does the central path of the afferent febres of the opposite fifth nerve. The posterior portion of the thalamus, or pulvinar, forms part of the central visual apparatus ; for (a) it is found to be undeveloped in animals from which the eyeballs have been removed soon after birth ; (6) a portion of the optic tract is certainly connected with it; (c) in some cases of atrophy of the occipital cortex, which, as we shall see, is undoubtedly a central area for visual sensations,' atrophy of the pulvinar has also been noticed ; (d) a lesion of the pulvinar may give rise to hemianopia (p. 894) . Hfcmorrhage into the caudate cr lenticular nucleus of the corpus striatum often causes hemiplegia, but this is frequently due to implica- tion of the internal capsule. It is said, however, that lesions presumably confined to the lenticular nucleus cause paralysis or paresis of the limbs or face, which is less severe than that produced by lesions in the internal capsule. Experimental lesions in dogs and rabbits are stated to be followed by disturbances of the heat-regulating mechanism and rise of temperature. Certain structures belonging to the primary fore-brain which have now lost some or all of their functional importance may nevertheless be mentioned as milestones in the march of development. The pineal body is made up of the vestiges of the unpaired mesial eye of such animals as the ancient labyrinthodonts, which resembled the eye of invertebrates in having the retinal rods directed towards the cavity instead of towards the circumference of the eyeball. . In many living forms, especially in certain lizards, this pineal or parietal eye is found in a more perfect condition, though covered by a thin membrane. The ganglia habenules, two small collections of nerve-cells, one of which is situated at the posterior part of each thalamus, are supposed by some authorities to represent the optic ganglia of this Cyclopean eye. They are less prominent in -man than in many of the lower animals. The infundibulum is probably what remains of the gullet of the ancestors of the vertebrates. The pituitary body is in a different category. It is now known that, far from being a useless vestigial remnant, it has a highly important function (p. 644). It consists of two portions, the anterior lobe, or hypophysis, derived from the buccal cavity, the pos- terior lobe, or infundibular body, from the primary fore-brain. Functions of the Cerebellum. — ^The elaborate pattern of the arbor vitse, the appearance given by the branched laminae in a section of the cerebellum, excited the speculation of the old anatomists. A structure so marvellous must be matched, they thought, with functions as unique. At a time when the discoveries of Galvani and Volta were fresh, and the world ran mad on electricity, the hypothesis of Rolando, that ' nerve-force ' was generated by the lamellae of the cerebellum as electrical energy is generated by the plates of the voltaic pile, ridiculous as it now appears, was not unnatural. The speculation of Gall, who connected the cerebellum with the development of sexual emotions and the action of the generative mechanisms, was based on no fact. It has been definitely disproved by the observations of Luciani, who found that a bitch deprived of its cerebellum showed all the phenomena of heat or FUtfCTIONS OF THE BRAIN 905 ■ rut,' was impregnated, whelped at full term in an entirely normal manner, and manifested the maternal instincts in their full intensity. Flourens put forward the doctrine that the cerebellum is an organ concerned in the co-ordination of movements, and especially the maintenance of equilibrium, supporting his conclusions by an elaborate series of experiments. Notwithstanding the very large amount of experimental and clinical study which has been devoted to the cerebellum since the time of Flourens, our actual knowledge Fig. 363.— Cerebellar Cortex: Section in Direc- tion of Lamina (Cajal). u, Purkinje's cell; b, granule cell in inner layer; c, dendrite of a granule cell; d, axon of a granule passing into the molecular layer, where it bifur- cates into two fine longitudinal branches (Golgi's method). Fig. 364. — Cerebellar Cortex : Section across a Lamina (Cajal). u, Purkinje's cell; the numerous dots in the molecular layer represent cross-sections of the bifur- cated axons of the granule cells (Golgi's method). of its functions has not greatly advanced beyond the point then reached. Some of the more modern authorities restrict its influence entirely to the actions on which equihbration depends ; others extend it to all voUtional movements. Luciatii looks upon it as ' an organ which by processes that dp not awaken consciousness exerts a con- tinual strengthening (reinforcing) action upon the activity of all other nerve-centres.' Sherrington conceives of the cerebellum as the head gangUon of the proprio-ceptive system— i.e., of the system of neurons whose receptors lie not on the surface, but in the deeper 906 THE CENTRAL NERVOUS SYSTEM parts of the body (labyrinth of ear, muscles, tendons, joints, viscera, etc.) (p. 886). After removal of the whole cerebellum (in the dog or monkey), there is at first rigidity and tonic spasm of certain muscles, which contribute to the difficulty of co-ordinating their movements. When this stage has passed, the muscles all over the body, but especially those of the loins and hind-limbs, and those which fix the head, are weaker than normal, are deficient in tone, and contract with a peculiar want of steadiness (Luciani). When one lateral half of the cerebellum is removed, the symptoms affect especially the muscles on the same side. In extensive lesions of the cerebellum in man what has been noticed is a marked inability to maintain the upright posture, giddiness, a staggering gait, twitching movements of the eyes (nystagmus), tremor accompany- ing voluntary movements — ^in a word, a general breakdown of the co-ordinating machinery, and especially of the part of it concerned in the movements necessary for locomotion, and for the maintenance of the equilibrium of the body — the so-called cerebellar ataxia. There is no sensory paralysis and none of voluntary movement such as lesions of the cerebral cortex produce, nor is there any psychical disturbance. In cases of congenital defect of the' cere- bellum, the power of walking, and even of standing, may be late in being acquired, and imperfect. But it is remarkable what great deficiencies in the cerebellar substance are often compensated for when established early in life, so that even cases of marked atrophy or lack of development have sometimes been recognised for the first time at the necropsy. The connections of the cerebellum with other parts of the central nervous system and with the periphery corroborate the direct results of experiment. For, in addition to the visual impressions, the most important afferent impulses concerned in equilibration are those from the semicircular canals and vestibule of the internal ear, the muscles, tendons, joints, etc., and certain portions of the skin, such as that of the soles of the feet. And the cerebellum, as we have seen (p. 857), is linked with all of these, and has besides an extensive crossed connection through the middle and superior peduncles with the opposite cerebral hemisphere. The importance and extent of this crossed connection with the great brain is illustrated by the facts that in disease atrophy or deficient development of one cerebellar hemisphere is associated with a similar condition of the opposite cerebral hemisphere, and that a lesion in one-half of the cerebellum affects chiefly the co-ordination of the movements of the same side of the body — ^that is to say, of the side connected with the opposite cerebral hemisphere. We do not as yet know the full significance of this extraordinarily free communication of the grey matter of the cerebellum with every part of the central nervous system. But it is evident that by the broad FUNCTIONS OF TITE BRAIN 907 highway of the restiform body, or the cross-country routes from cere- bral cortex to cerebellum, impulses may reach it from every quarter; while impulses passing out from it along its peduncles may influence the motor discharge either indirectly through the Rolandic cortex and the pyramidal tract, or more directly through the antero-lateral de- scending spinal path that brings it into relation with the nuclei of origin of the motor nerves. It is an organ so connected that is suited to take cognizance of the multitudes of afferent impressions concerned in the co-ordination of movements and the maintenance of equilibrium, and to regulate the outflow of efferent impulses in correspondence with the inflow of afferent. Sherrington points out that all the modern theories of cerebellar function harmonize with his conception of the cerebellum as the head ganglion of the proprio-ceptive system (p. 905). The most influential of the proprio-ceptive organs being the labyrinth, the central organ of the whole proprio-ceptive mechanism is built up over the central con- nections of the labyrinth. Thither converge connecting (internuncial) paths from the central endings of proprio-ceptive neurons in aU seg- ments of the body (from joints, muscles, tendons, ligaments, viscera, etc.). Thus a central organ is developed, which varies in size and complexity in different kinds of animals accord- ing to the com- plexity of their habitual m o v e - ments. Afferent Im- pulses concerned in Equilibration and Orientation. — ^This is a con- venient place to consider a little more in detail the nature and peripheral sources of some of the most important afferent impressions concerned in equilibration and orientation. (1) Afferent Impulses from the Semicircular Canals. — The semi- circular canals are three in number, and lie nearly in three mutually rectangular planes: the external canal in the horizontal plane, the superior canal in a vertical longitudinal plane, and the posterior canal . in a vertical transverse plane. Each canal bulges out at one end into a swelling, or ampulla, which opens into the utricular division of the vestibule (Figs. 365, 436). The other extremities of the superior and posterior canals join together, and have a common aperture into the utricle, but the undilated end of the external or horizontal canal opens separately. The utricle and the semicircular canals are thus connected by five distinct orifices. The greater part of the internal surface of the membranous canals, utricle and saccule, is lined by a single layer of flattened epithelium. But at one part of each ampulla projects a transverse ridge, the crista acustica, covered not with squamous, but with long columnar epithelium. Hair-like processes (auditory hairs) Fig. 365. — The Semicircular Canals (Diagrammatic) (after Ewald). H, horizontal or external; S, superior; P, pos- terior. The two horizontal canals lie in the same plane. The plane of the superior vertical canal of one side is parallel to the plane of the posterior vertical canal of the opposite side. 9o8 THE CENTRAL NERVOUS SYSTEM are borne by so'me of the columnar cells, between which lie more elongated fibre-like supporting cells. The hairs project into a mucus- like mass, sometimes containing otoconia, or crystals of calcium car- bonate," The ampullae, like the rest of the membranous labyrinth, is filled with a watery fluid called endolymph. The utricle and saccule have each a somewhat similar biit broader elevation, the macula acustica, covered with epithelium and hair-cells of the same character, and the hairs project into a similar mass in which otoconia are con- stantly present. In some animals, as fishes, the calcareous matter in the utricle and saccule forms masses of considerable size {otoliths). Fibres of the auditory nerve end in arborizations around the bodies of the hair-cells of the maculae and cristas acusticae. We have already seen that it is the ventral or vestibular division of the nerve which is especially related to the vestibule (p. 898). There is very strong evidence that the semicircular canals are con- cerned, not in hearing, but in equilibration. A pigeon from which the membranous canals have been removed stiU hears perfectly well so long as the cochlea is intact, but exhibits the most profound disturbance of equilibrium. If the horizontal canal is destroyed or divided, the pigeon moves its head continually from side to side around a vertical axis; if the superior canal is divided, the head moves up and down around a horizontal axis. The power of co- ordination of movements is diminished, but not to the same extent in all kinds of animals. Thrown into the air, the pigeon is helpless; it cannot fiy; but a goose with divided semicircular canals can still swim. The condition is only temporary, even when the injury involves the three canals on one side ; but if the canals on both sides are destroyed, recovery is tardy, and often incomplete. In mam- mals the loss of co-ordination is much less than in birds; and move- ments of the eyes, the direction of which depends on the canal destroyed, take to a large extent the place of movements of the head. The effects of destructive lesions have their counterpart in the phenomena caused by stimulation; excitation of a posterior canal, for example, in the pigeon causes movements of the head from side to side. Lee's results in fishes are, on the whole, of similar tenor. Mechan- ical stimulation of the ampullae in the dogfish, by pressing on them with a blunt needle, calls forth characteristic movements of the eyes and fins, and electrical stimulation of the auditory nerve causes movements compounded of the separate movements obtained by stimulation of the ampullae one by one. Lee concludes that the semicircular canals are the sense-organs for dynamical equilibrium (i.e., equilibrium of an animal in motion), and the utricle and saccule for statical equilibrium (i.e., equilibrium of an animal at rest). The evidence from all sources points strongly to the conclusion that afferent impulses are actually set up in the fibres of the auditory nerve, through the hair-cells, by alterations of pressure or by stream- ing movements of the endolymph when the position of the head is FUNCTIONS OF THE BRAIN 909 changed. Rotation of the head to the right may be supposed to cause the endolymph in the right external canal, in virtue of its inertia, to lag behind the movement, and to press upon the anterior surface of the ampulla. The disorders of movement after lesions of the canals may be explained as the result of the withdrawal of certain of these afferent impulses, and the consequent overthrow of that equipoise of excitation necessary for the maintenance of equi- librium. An experiment of Kreidl on a crustacean (palaemon) has made it probable that the otoliths by their weight may mechani- cally affect the hair-cells, and so increase their sensitiveness to changes of position. This animal has the peculiarity that in moult- ing the inner Hning of the otocysts, in which the otoHths lie and which open to the exterior, are shed along with the otoliths. When moulting is over, the animal by means of its claws conveys fine sand grains into the otocysts, where they function as otoliths. Kreidl placed the animal after moulting upon finely powdered iron, some of which was conveyed into the otocyst instead of sand. It was now found possible to obtain definite reactions from the animal in the presence of a magnet, which, of course, tended to attract the ferruginous otoliths, and so to alter their position with reference to the hairs. The way in which the animal changed its position in response to the magnet could be satisfactorily accounted for on the hypothesis that normally the contact of the otoliths with the hairs is altered under the influence of gravity when such changes of posi- tion occur. Even in man there is evidence of the existence of some mechanism not depending on the muscular sense or on impressions passing up the channels of ordinary or special sensation, by which orientation (the determination of the position of the body in space) is rendered possible. For a man lying perfectly still, with eyes shut, on a horizontal table which is made to rotate uniformly, can not only judge whether, but also in what direction, and approximately through what angle, he is moved. The phenomena of pathology afford weighty additional testimony in favour of the equilibratory function of the semicircular canals. For many cases of vertigo are associated with changes in the internal ear (Meniere's disease). And while nearly every normal individual becomes dizzy when rapidly rotated, 35 per cent, of deaf-mutes are entirely unaffected (James), and the proportion seems to be much higher among con- genital deaf-mutes. Kreidl and Bruck, too, have found that ab- normalities of locomotion and equilibration are much more common in deaf-and-dumb children than in others. Now, in these cases the defect is usually in the internal ear. We must conclude, then, that the co-ordination of muscular movements necessary for equili- brium is achieved in some centre, to which afferent impulses pass from the internal ear by the vestibular branch of the auditory nerve, and from which efferent impulses pass out to the muscles. If, as gro THE CENTRAL NERVOUS SYSTEM there is strong reason to believe, this centre is situated in the cere- bellum, the efferent path is, as already suggested (p. 906), partly an indirect one (perhaps by commissural fibres to the Rolandic area, and then out along the pjnramidal tract), or more probably to lower centres, perhaps in the posterior portion of the optic thalamus, which control such massive co-ordinated movements as those con- cerned in walking and the maintenance of the normal attitude, and thence out along certain tracts that connect the thalamus to the spinal cord (p. 858). Ewald has mad« an observation which illustrates the peculiar relation of the semicircular canals to the muscular system — -namely, that the labyrinth (in rabbits) influences the course of rigor mortis in the striped muscles. Rigor does not come on so soon on the side from which the labyrinth has been removed. He attributes to the lab5a:inth, as one of its functions, the maintenance of a certain tonus in the entire skeletal musculature. (2) Afferent Impressions from the Muscles. — Muscles are richly supplied with afferent fibres, for about half of the fibres in the nerves of skeletal muscles degenerate after section of the posterior roots beyond the gangha (Sherrington). Various kinds of impressions may pass up these nerves: (a) Impressions giving rise to pain, as in muscular cramp and in experimental excitation of even the finest muscular nerve-filament ; (b) impulses causing a rise of blood-pres- sure ; (c) impulses which are not associated with a distinct impres- sion in consciousness, but which- enable us to localize the position of the limbs, head, eyes, and other parts of the body; {d) impulses which inform us as to the extent and force of muscular contraction, and seem to underlie the so-called muscular sense. It is the last two kinds — if, indeed, they are distinct — -which must be concerned in equilibration. In locomotor ataxia such impressions are blocked by degeneration in a part of the afferent path (p. 887), and disorders of equilibrium are the result. (3) Afferent Impressions from the Skin. — Of the various kinds of impulses that arise in the nerve-endings of the skin, only those of touch and pressure seem to be concerned in the maintenance of equilibrium. When the soles of the feet are rendered insensitive by local anaesthesia or by cold, and the person is directed to close his eyes, he staggers and sways from side to side. The disturbance of equilibrium in locomotor ataxia must be partly attributed to the loss of these tactile sensations, for numbness of the feet is a frequent symptom, and the patient asserts that he does not feel the ground. An interesting illustration of the importance of afferent impulses from the skin in the maintenance of equilibrium is afforded by the behaviour of a frog deprived of its cerebral hemispheres. Such a frog will balance itself on the edge of a board like a normal animal, but if the skin be removed from the hind-legs, it will fall like a log. Leuia FUNCTIONS OF THE BRAIN 911 In birds and lower vertebrates the cerebellum is only representeti by the worm. Yet m many of these animals the same characteristic disturbances follow its removal as in the higher animals where the cerebellar hemispheres have become so prominent. Indeed, it was mamly on the pigeon that Flourens made his classical experiments. At first the pigeon can neither fly nor feed itself. When it attempts to walk, extensor spasms of the legs come on, and it faUs, wildly struggling and apparently panic-stricken, to the ground. The power of flight IS soon regained, but for a long time the animal is unable to perch, the legs and talons stiffening in rigid extension as it attempts to alight. In the higher animals stimulation of certain parts of the worm and lateral lobe causes conjugate movements of the eyes towards the same side, both eyes being turned to the right— e.g., when the cerebellum is stimulated to the right of the middle line. Inhibition of movement can also be elicited from the organ. Excitation of the cerebellar cortex for some distance outwards from the line of junction of the superior worm with the lateral lobe in animals which exhibit tonic con- traction af extensor muscles after ex- cision of the cere- bral hemispheres (decerebrate rigid- ity or acerebral tonus, a.6 it is called) causes immediate relaxation of the rigid muscles of the neck, tail, and especially the an- terior limb, particu- larly on the same side. The relaxation of the extensors may be accom- panied by contrac- tion of the antago- nistic flexors — ^for example, relaxation of the triceps and contraction of the biceps (Horsley and Lowenthal) . Bu t this can scarcely be considered a reaction specific to the cerebellum. For Sherrington, who finds that the tonus or spasm is largely due to centripetal impulses coming from the rigid limb, has been able to inhibit it by stimulation of various other regions, including the portion of the cerebral cortex in front of the fissure of Rolando (p. 921). Localization of Function in the Cerebellum. — The confusion which so long reigned in regard to this matter has in great measure been cleared up by recent physiological work following on a more accurate anatomical mapping of the lobes and lobules of the cerebellum in accordance with their genetic relations (Bolk) (Fig. 366). Following this scheme, van Rynberk has obtained satisfactory evidence of localization of function. Thus the lobulus simplex con- stitutes a centre for the neck muscles, and the elimination of its influ- Fig. 366. — Scheme of Dog's Cerebellum (Dorsal View), ac- cording to the Anatomical Division of Bolk (after vaii Rynberk). La, lobus anterior, which is separated from the larger posterior lobe by the deep primary fissure (sulcus primarius), Spr; Ls, lobulus simplex; Si, sulcus intercruralis; O-, crus primum; C^, crus secundum; L.ans, lobulus ansiformis; Lp, lobulus paramedianus; Lmp, lobulus medianus posterior; Fv, formatio vermi- cularis (pars tonsillaris); Sp, sulcus paramedianus. 912 THE CENTRAL NERVOUS SYSTEM ence by excision leads to movements of the head (so-called head nystagmus) . The anterior extremity is represented by a centre in the crus primum, and. the posterior extremity by a centre in the cnis secundum, of the ansiform lobule of its own side, and injury in the region of these, centres is associated with abnormal movements of the corresponding fore and hind foot respectively. Extirpation of a lobulus paramedianus causes rolling movements of the body around its long axis or bending of the body to one side, and this centre is connected with the muscles of the trunk. It is still an open question whether in the function of these centres only the cortex of the lobules is concerned, or in addition the corresponding portions of the central nuclei of the cerebellum. These observations are supported by other facts. For example, microscopical studies have shown that definite regions of the cerebellar cortex are especially connected with definite levels of the spinal cord. Further, the lobulation of the cerebellum in mammals keeps pace with the increase in complexity of the voluntary motor apparatus of the whole body, and the variations in the degree of development of definite lobules are related to the variations in the anatomical and physiological development of the corresponding groups of muscles. All this fits in well with the idea that the cerebellum is a great reflex mechanism standing in intimate relation on the one hand to numerous afferent paths (skin, muscles, labyrinth, etc.), and on the other to the voluntary muscles. It is the precise nature of the influ- ence exerted by it upon the latter which is in doubt, whether an augr menting sthenic influence, as Luciani supposes, or a co-ordinating in? fluence, as Flourens assumed, or a combination of these. Forced Movements. — ^We have incidentally mentioned that in fishes injuries to the semicircular canals may give rise to movements which seem to be beyond the control of the animal, and which have conse- quently received the name of ' forced movements.' It may be added that when the internal ear of a Necturus (one of the tailed amphibia) is destroyed on one side, rapid movements of rotation around a longi- tudinal axis are observed. The animal spins round and round ap- parently without voluntary control, purpose, or fatigue. The direction of rotation is towards the side of the lesion, the observer being sup- posed to look down upon the animal as it lies in its normal position. After a time it becomes quiescent; but the forced movements can be again produced by pinching or exciting it in other ways. In man, too, during the passage of a galvanic current through the head by electrodes applied just behind the ears, a tendency to move the head towards the anode is experienced. The person may resist the tendency, but if the current be strong enough his resistance will be overcome ; he will exe- cute a forced movement. When the head turns towards the anode the eyes move in the same direction, and then undergo jerking move- ments towards the kathode. There is at the same time a feeling of vertigo. Complex as such an experiment is, involving as it does stimu- lation of so many structures within the cranium, there is reason to believe that it is the excitation of the semicircular canals, or their cerebellar connections, that is responsible for these forced movements. For when the experiment is. performed on a pigeon, forced movements are caused so long as the membranous canals are intact, but not after they have been destroyed (Ewald). The observation of Rawitz, that the peculiar rotatory movements of the so-called Japanese dancing mice are associated with marked anatomical peculiarities in the laby- rinth, is another fact in favour of the connection of the canals with the maintenance of equilibrium and the sense of rotation. So is the relation between the degree of development of the canals in different FUNCTIONS OF THE BRAIN 913 species of birds and the degree of agility in the co-ordination of bheir movements (Laudenbach). But forced movements may also follow injuries {especially unilateral) to many portions of the brain — e.g., the pons, crus cerebri, posterior corpora quadrigemina, corpus striatum, even the cerebral cortex, and above all the cerebellum. The movements are of the most various kinds. The animal may run round and round in a circle (circus move- ment) ; or, with the tip of its tail as centre and the length of its body as radius, it may describe a circle with its head, as the hand of a clock does (clock-hand movement) ; or it may rush forward, turning end- less somersaults as it goes. Intervals of rest alternate with paroxysms of excitement, and the latter may be brought on by stimulation. In man forced movements associated with vertigo have been sometimes seen in cases of tumour of the cerebellum — e.g., involuntary rotation of the body in tumour of the middle peduncle. No entirely satisfac- tory explanation of these forced movements has been given. They are evidently connected with disturbance of the mechanism of co-ordina- tion, leading to a loss of proportion in the amount of the motor dis- charge to muscles or groups of muscles accustomed to act together in executing definite movements. For instance, in circus movements the muscles of the outer side of the body contract more powerfully than those of the inner side, and the animal is therefore constrained to trace a circle instead of a straight line, the excess of contraction on the outer side being analogous to the acceleration along the radius in the case of a point moving in a circle. In connection with the consideration of the mechanism of equilibra- tion, a short account of the miiscular actions concerned in the main- tenance of the erect posture so characteristic of man, and of those concerned in locomotion, is subjoined here: Standing. — In the upright posture the body is supported chiefly by non-muscular structures, the bones and ligaments. But muscles also play an essential part, for it is only peculiarly-gifted individuals, like some of the fishermen of the North Sea, who can go to sleep on their feet, and a dead body cannot be made to stand erect. The condition of equilibrium is that the perpendicular dropped from the centre of gravity to the ground should fall within the base of support — that is, within the area enclosed by the outer borders of the feet and lines joining the toes and heels respectively. The centre of gravity alters its position with the position of the body, which tends to faU whenever the perpendicular cuts the ground beyond the base of support. In the comfortable and natural erect position the centre of gravity of the head is a little in front of the vertical plane passing through the occipital condyles, and as much as 4 centimetres in front of the vertical plane passing through the ankle-joints. A certain degree of contra.c- tion of the muscles of the nape of the neck is required to balance it. When these muscles are relaxed, as in sleep, the head must fall forward, and this is the reason why Homer or any lesser individual nods. In animals which go upon all-fours none of the weight of the head bears directly upon the occipito-atloid articulation; its support by muscular action alone would be an intolerable fatigue, and the ligamentum nuchae is specially strengthened to hold it up. The vertebral column is kept erect by the ligaments and muscles of the back. The centre of gravity of the trunk lies almost vertically over the horizontal line joining the two acetabula, but the centre of gravity of the whole body is about the level of the third sacral vertebra, and a little more than 4 centimetres in front of the vertical plane passing through the ankle-joints. Equilibrium is maintained by con- 58 9X4 THE CENTRAL NERVOUS SYSTEM traction of the muscles of the back and of the legs. By means of the muscular sense, and the tactile sensations set up by the pressure of the soles on the ground, alterations in the position of the centre of gravity, and consequent deviations of the perpendicular passing through it, are detected, and adjustment of the amount of contraction of this or the other muscular group is promptly made. In standing at ' attention ' the heels are close together, the legs and back straightened to the utmost, and the head erect; the weight falls equally upon both legs, but the advantage may be more than counter- balanced by the muscular exertion associated with this more orna- mental than useful position. In ' standing at ease,' practically the whole weight is supported by one leg, the perpendicular from the centre of gravity passing through the knee and ankle-joint. The centre of gravity is brought over the supporting leg by flexure of the body to the corresponding side, and comparatively little muscular effort is required. The other foot rests lightly on the ground, the weight of the leg itself being almost balanced by the atmospheric pressure acting upon the air-tight and air-free cavity of the hip-joint. The light touch of this foot varies slightly from time to time, so as to maintain equilibrium. When the head or arms are moved, or the body swayed, the centre of gravity is correspondingly displaced, and it is by such movements that tight-rope dancers continue to keep the perpendicular passing through it always within the narrow base of support. In sitting, the base of support is larger than in standing, and the equihbrium therefore more stable. The easiest posture in sitting without support to the back or feet is that in which the perpendicular from the centre of gravity passes through the horizontal line joining the two tubera ischii. Locomotion. — In walking, the legs are alternately swung forward and rested on the ground. With most persons the swinging foot first strikes the ground by the heel; then the sole comes down, the heel rises, the leg is extended, and, with a parting push from the toe, the leg again swings free. By this manoeuvre the body is raised vertically, tilted to the opposite side, and also pushed in advance. The forward swing of the leg is only slightly, if at aU, due to mus- cular action; it is more like the oscillation of a pendulum displaced behind its position of equilibrium, and swinging through that position, and in front ot it, under the influence of gravity. For this reason the natural pace of a tall man is longer and slower than that of a short man; but it may be modified by voluntary effort, as when a rank of soldiers of difierent height keeps step. The lateral swing of the body is illustrated by the everyday experience that two persons knock against each other when they try to walk close together without keeping step. In step both swing their bodies to the same side at the same moment, and there is no jarring. Even in the fastest walk- ing on level ground there is a short time during which both feet touch the ground together, the one leg not beginning its swing until the other foot has begun to be set down. In running, on the other hand, there is an interval during which the body is completely in the air, while in walking uphiU or in carrying a load the one foot is not raised until the other has been firmly planted. Functions of the Cerebral Cortex.— When an animal, like a frog, is deprived of its cerebral hemispheres, the power of automatic voluntary movement appears to be definitively and entirely lost. FUNCTIONS OF THE BRAIN 915 The animal, as soon as the effects of the anaesthetic and the shock of the operation have passed away, draws up its legs, erects its head, and assumes the characteristic position of the normal frog at rest. So close maybe the resemblance, that if all external signs of the opera- tion have been concealed, it may not be possible for a casual ob- server to tell merely by inspection which is the intact and which the ' brainless ' frog. The latter will jump if it be touched or otherwise stimulated. It will croak if its flanks be stroked or gently squeezed together. It wiU swim if thrown into water. If placed on its back, it will promptly recover its normal position. But it will do all these things as a machine would do them, without purpose, without regard to its environment, with a kind of ' fatal ' regularity. Every time it is stimulated it will jump, every time its flanks are squeezed it will croak, and, in the absence of all stimulation, it will sit still till it withers to a mummy, even by the side of the water that might for a while preserve it. A Necturus, without its cerebral hemispheres, will, like the frog, refuse to lie on its back. On stimulation it moves its feet or tail, or its whole body; but if not interfered with, it lies for an indefinite time in the same position. Its gills are seen to execute rhythmic movements, which never stop, and rarely slacken, except for an instant, when some part of the skin, particularly in the region of the head, is mechanically or electrically stimulated. The normal Necturus, on the other hand, lies for long periods with its gills at perfect rest, and when stimulated, moves for a considerable distance. After a time — two months or more — ^it is true the brainless frog, if it be kept alive, as may be done by careful attention, will recover a certain portion of the powers which it has lost by removal of the cerebral hemispheres; and, indeed, the longer it lives, the nearer it approximates to the condition of a normal frog. A brainless frog has been seen to catch flies and to bury itself as winter drew on. A fish even three days after the destruction of its cerebrum has been seen to dart upon a worm, seize it before it had time to sink to the bottom of the aquarium, and swallow it. Even in the pigeon the loss of the hemispheres, which at first induces a state of profound and seemingly permanent lethargy, is to a great extent compensated for, as time passes on, by the unfolding in the lower centres of capabilities previously dormant or suppressed. A brainless pigeon has been known to come at the whistle of the attendant and follow him through the whole house. In the mammal the removal of the whole or the greater part of the cerebral hemispheres at a single operation is uniformly and speedily fatal ; even rabbits or rats, which bear the operation best, survive but a few hours. During those hours they manifest phenomena similar to those observed in the bird and the frog. In the dog the entire cortex has been removed piecemeal by successive 9i6 THE CENTRAL NERVOUS SYSTEM operations. In this case, of course, the change in the condition of the animal . is more gradually produced, and an opportunity is afforded for a certain recovery of function in the intervals between the operations. On the whole, however, as might be expected from its greater intellectual development, recovery is more imperfect in the dog than in the bird, much more imperfect than in the frog. But even in the dog wonderful resources lie hiddein in the grey matter of the central neural axis, and are called forth by degrees to replace the lost powers of the cerebral cortex. It is true that a brainless dog is a less efficient animal than a brainless fish, or even than a brainless frog; but in favourable cases, even in the dog, the movements of walking may stiU be carried out with tolerable pre- cision in the absence of the cerebral hemispheres. The animal can swallow food pushed well back into the mouth, although it cannot feed itself. Stupid and listless as it is compared with the normal dog, it seems to be by no means devoid of the power of experiencing sensations as the result of impressions from without, or of carrying on mental operations of a low intellectual grade. Goltz had a dog which lived more than a year and a half practically without its cerebral hemispheres, and another which hved thirteen weeks. He believes that they had lost understanding,refiection, and memory, but not sensation, special or general, nor emotions and voluntary power. Their condition may be best described as one of general imbecility. Hunger and thirst are present. They experience satis- faction when fed, become angry when attacked, see a very bright light, avoid obstacles, hear loud sounds, such as those produced by a fog-horn, and can be awakened by them. They are not com- pletely deprived of sensations of taste and touch. But it ought to be remembered that the interpretation of the objective signs of sensation in animals is beset with difficulties; and although every- body admits the accuracy of Goltz's description of what is to be seen, his interpretation of the facts has been severely criticized, particularly by H. Munk. To the monkey there can be no doubt that the loss of the cerebral hemispheres would be a still heavier and more irremediable blow than to the dog. But nobody has yet succeeded in keeping a monkey alive after complete removalof even one hemisphere. In man the destruction of considerable masses of brain-substance, particularly if gradual, is not necessarily fatal. How great a loss is compatible with life cannot be exactly stated. It depends to a large extent on the position of the lesion. But it is possible that one cerebral hemisphere may be rendered functionally useless without immediately putting a term to existence. In the foetus, however, no portion of the great brain is absolutely indispensable for life and movement. An anencephalous foetus (in which the brain has remained undeveloped) may be born alive, and live for a short time. FUNCTIONS OF THE BRAIN 917 We see, then, that homologous organs are not necessarily, nor indeed usually, of the same physiological value in different kinds of animals. A loss which perhaps hardly narrows the range of the psychical, and certainly restricts only to a slight extent the physical powers of a fish, impairs in a marked degree the voluntary move- ments of a dog, in addition to cutting off from it a great part of its intellectual life, and is in man incompatible with life altogether. The results of the removal of the entire cerebral hemispheres help us to fix their position as a whole in the physiological hierarchy. A more minute analysis shows us that the cerebral cortex itself is not homogeneous in function, that certain regions of it have been set aside for special labours. Our knowledge of this localization of function in the cerebral cortex has been derived partly from clinical, coupled with pathological observations on man, and partly from the results of the removal or stimulation of definite areas in animals. In addition, the study of the development of the myelin sheath, and especi- ally in recent years the minute study of the hist- ology of the various regions, have aided materially in mapping out the cortex. It is a fact which might appear strange and almost inexplicable did the history of science not constantly present us with the Uke, that fifty years ago the universal opinion among physiologists, pathologists, and physicians j, . , i.- r it, 4. was that the cerebral cortex is inexcitable to artificial stimuli, that no visible response can be obtained from it. The great names of Flourens and Magendie stood sponsors for this error, and repressed research. In 1870, however, Hitzig and Fritsch showed that no* only was it possible to elicit muscular contractions by stimulation of the cortex of the brain in the dog with voltaic currents, but that the excitable area occupied a definite region in the neighbourhood of the crucial sulcus or sulcus centralis, which runs put over the convexity of the hemispheres nearly at right angles to the longitudinal fissure. In this region they were further able to isolate several distinct areas, stimulation of which was followed by movements respectively ot the head, face, neck, hind-leg, and fore-leg (Fig. 367). This was the starting-point of a long series of researches by Ferrier. Munk, Horsley, Fig. 367. — Motor Areas of Dog's Brain, n, neck ; f.L, fore-limb; h.l., hind-limb; t, tail;/, face; C.S., crucial sulcus; e.m., eye movements; p, dilatation of the pupil in both eyes, but espe- cially in the opposite eye. All the areas are marked in the figure only on the left side except the eye areas, whose position, to avoid confusion, is indicated on the right hemi- sphere. 9i8 THE CENTRAL NERVOUS SYSTEM Schafer, Heidenhain, and many others, on the brains of monkeys as well as dogs — researches which have formed the basis of an exact cortical localization in the brain of man, and have enriched surgery with a new province. In these later experiments the interrupted cur- rent from an induction machine has been found the most suitable form of stimulus (see Practical Exercises, p. 962), especially when one elec- trode only is placed on the cortex and the other on some indifferent part of the body — e.g., in the rectum (unipolar stimulation), a pro- cedure which permits of finer localization than when both electrodes are applied to the brain (bipolar stimulation). ' Motor ' Areas.* — ^These have been localized with great care (both by stimulation and by removal of portions of the cortex) in the brains of the higher , apes (gorilla, orang, and chimpanzee) by Sherrington and Griinbaum, and there can be no doubt that the results, in their general outlines at least, can be applied to the human brain. These observers employed the so-called 'unipolar method of stimulation. The ' motor ' region in- cludes the whole length of and the whole of the free width of the precentral or ascending frontal con- volution, and dips down to the bottom of the central sulcus (fissure of Rolando in man), but does not extend behind the sulcus. It extends also into the depth of all the fissures, so that the hidden part of the excitable area probably equals, perhaps exceeds, the part which is free on the surface of the hemisphere. The anterior limit of the ' motor ' field is not quite * Since the so-called ' motor ,' area, as is now well known, is really sensori- motor, and a region having to do purely with the discharge of motor impulses does not exist, it would be better to call it the sensori-motor, or, following Bastian's suggestion, the kinaesthetic area. Probably, however, the altera- tion of a term so long sanctioned by custom in physiological writings would lead to confusion. Accordingly, in what follows the word ' motor ' will be retained, but to show that it is used in a special sense it will be enclosed in quotation marks. Fig. 368. — Dog's Brain with Lesion. A portion of the cortex indicated by the shaded area was destroyed by cauterization. The symp- toms were complete blindness of the opposite eye (in this case the right); weakness of the muscles of the limbs and of the neck on the right side ; slight weakness of the limbs on the left side. When the animal walked, there was a tendency to turn to the left in a circle. In eating or drinking, the head was turned to the left, so that the mouth was oblique, and the right angle of the mouth was lower than the left. The tail movements were normal, and there was no deviation of the tail to one side. FUNCTIONS OF THE BRAIN 919 sharp, but shades off somewhat gradually into inexcitable cortex. The sulci in this region cannot be considered to represent physio- logical boundaries, and they vary so much in these higher brains, that they can easily prove fallacious landmarks. On the mesial surface of the hemisphere the ' motor ' area does not extend quite to the caUoso-marginal fissure. Within this area are localized movements of the leg and arm and their various joints, of the head, face, mouth, tongue, ear, nostril, and vocal cords, of the neck, chest, and abdominal wall, of the pelvic floor, and the anal and vaginal orifices. jtmjs !$• va.gina. Abdomen Cheat finders i thumbs Ear-'' y Eyeiid / Nose CLdaure ' Opening cfjaw. Voca.L cords. Sulcils eenbralva. Ma^ticaCion "^ *'■ Fig. 369.- -' Motor ' Area of Cortex of Chimpanzee (Griinbaum and Sherrington). Lateral aspect of the hemisphere. The arrangement of the various regions follows very closely the order of the cranio-spinal nerves, which supply them, but the organs whose nerves come off lowest down are represented highest up in the ' motor ' area. Figs. 369, 370 will make this clear. In the frontal region, isolated from the ' motor ' area by a strait of inexcitable cortex, lies an area the stimulation of which causes conjugate devia- tion of the eyes. But the reaction differs from that obtained on excitation of the ' motor ' area proper in front of the Rolandic fissure. It is to be particularly noted (i) that within the larger areas, such as those of the arm and leg, smaller foci can be mapped off which are related to movements of the separate joints — thus, in the leg area, the hip, knee, and ankle-joints, and the great toe, are repre- 920 THE CENTRAL NERVOUS SYSTEM sented by separate and special centres ; (2) that stimulation of any one of these areas leads, not to contraction of individual muscles, but to contraction of muscular groups which have to do with the execution of definite movements. Sulccalloso rrutrg.^ Sulc.parieto oocip. SuLcCenCral. Anus & \/<^gin^ ■^ Sulc.preceiUrmarg. SvdiccaloarUi GSS. rfei. Fig. 370. — ' Motor ' Area on Mesial Surface of Hemispliete: Brain of a Chimpanzee {Troglodytes Niger) (Griinbaum and Sherrington). Left hemisphere: mesial sur- face. The extent of the ' motor ' area on the free surface of the hemisphere is indicated by the black stippling. On the stippled area ' LEG ' indicates that the movements of the lower limb are represented in all the regions of the ' motor ' area visible from this aspect. The minuter subdivisions in this area overlap each other so much that no attempt is made to distinguish them in the diagram. ' Anus and vagina ' indicates the position from which perineal movements can be primarily elicited. Sulc. central. = central fissure ; Sulc. calcarin. = calcarine fissure; Sulo. parieto occ4^.=parieto-occipitalflssure; Sulc. calloso warg.—calloso- marginal fissure ; Sulo. precentr. marg. = precentral marginal fissure. The single italic letters mark spots whence, occasionally and irregularly, movements of the foot and leg (//), of the shoulder and chest (s), and of the thimib and fingers (h), have been evoked by strong faradization. The shaded area marked ' EYES ' indicates a field of free surface of cortex which, under faradization, yields con- jugate movements of the eyeballs. The conditions under which these reactions are obtained separates them from those characterizing the ' motor ' area. The Stability of the Reactions obtained by Stimulating Cortical Points. — ^The question whether stimulation of a ' motor ' area, or point invariably causes the same movements, when it causes any movements at all, has been recently investigated by Graham Brown and Sherrington. They observed the contractions of two isolated antagonistic muscles acting on the elbow-joint (in monkeys) after elimination of all the other muscles of the arm and shoulder by section of their ' motor ' nerves, when a point on the area, of the cortex in which the movements of the elbow are represented was excited by the unipolar method. They find that a cortical point which has given flexion at the elbow, sometimes on FUNCTIONS OF THE BRAIN 921 investigation the next day, may give the opposite result of extension of the elbow. Even within short intervals reversal of the reaction elicited from one and the same point may be seen. They do not question at all the general regularity of the results which such cortical points give when investigated by suitable methods after sufficient intervals of rest, and on which the current statements as to the reactions elicited from the various 'motor' areas are based. But they see in the influence of transient excitation either of the point itself or of other more distant points in modifying or reversing the reaction an indication that one of the functions of the cortex may be the carrying out of such phe- nomena .of reversal, a function which may play some part in the co-ordination of voluntary movements. Inhibition from the Cortex. — Contraction is not the only efiect on the muscles which can be elicited by stimulating the cortex. Cor- tical inhibition of tonus and of active contraction is just as char- acteristic, though not so obvious a result. There is abundant evidence of reciprocal innervation of volitional movements from the cortex. When, e.g., the part of the arm area which presides over extension of the elbow is stimulated (in the monkey), it can be shown that the biceps relaxes as the triceps contracts, In like manner, stimulation of the appropriate part of the leg area will cause along with contrac- tion of the extensors of the hip relaxation of such flexors as the psoas- iliacus and the tensor fascim femoris. Such observations are most easily made when, in a certain stage of narcosis, the limbs, instead of hanging limp, assume a position of tonic flexion, especially at the elbow and hip. Under other conditions the position of tonic exten- sion of a joint may be assumed, and then it can be shown that excita- tion of the appropriate focus for flexion of that joint will cause simultaneous contraction of the flexors and relaxation of the extensors. The observer cannot fail to be struck with the general resem- blance between these cortical reactions and their co-ordination and the co-ordinated bulbo-spinal reflex movements previously studied. There are, however, certain differences which place the cortical reactions upon a higher level. One of the most important is the part played by visual, auditory, and pure ' touch ' stimuli in eliciting cortical motor responses— e.g'., ' the closure of the hand, pricking of the ear, opening of the eyes, and turning of the head in the direction of the gaze ' (Sherrington). The facility of response to stimuli acting from a distance through the distance-receptors, such as those of the retina and labyrinth, is one of the great characteristics of the cerebrum as an organ concerned in movements, and helps to place the ' motor ' cortex at the helm, since these distance- receptors control more than others the skeletal musculature as a whole. Spinal reflex movements are mainly such as are elicited by harmful (nocuous) stimuli (protective reflexes), or through the sexual skin nerves, or from the visceral afferent fibres, or such as are concerned in the chief movements of locomotion. 922 THE CENTRAL NERVOUS SYSTEM Decerebrate Rigidity is a phenomenon closely related to the in- hibitory function of the cerebral cortex. It is a condition of pro- onged spasm of certain groups of skeletal muscles (especially the retractor muscles of the head and neck, the elevators of the jaw and tail, and the extensors of the elbow, knee, shoulder, and hip), supervening on removal of the cerebral hemispheres by transection anywhere in the mid-brain or in the posterior part of the thalamus, and favoured by suspending the animal in the vertical posture. If the afferent roots belonging to one of the rigid limbs are-severed, it at once becomes flaccid, while the other limbs remain rigid. The tonus is therefore reflex through the local afferent nerves, and, to be more precise, through those that supply the deep structures (joints, muscles, etc.). The centre must be situated somewhere between cerebrum and spinal bulb, since section of the bulb abolishes the rigidity. It is not apparently in the cerebellum. It is noteworthy that the muscles mainly involved in decerebrate rigidity are those which are much more easily inhibited than excited from the ' motor ' cortex, and also in the local spinal reflexes. After removal of the cerebrum, the mechanism which maintains their tonic contraction has free play. Sherrington points out that this mechanism sustains the steady muscular tension necessary to pre- serve against the force of gravity the attitude or posture of the body. When the transient spinal reflex or the transient cortical effect breaks in upon this tonic contraction^ — e.g., in locomotion — ^inhibi- tion of the contracted extensors accompanies contraction of the flexors (see also p. gii). Removal of a single ' motor ' region leads to paralysis of the corresponding limb, or part of a limb, on the opposite side. For example, after extirpation of the hand area the hand is for a few days practically useless and apparently powerless. In a few weeks, however, it recovers remarkably, so that it is once more used in climbing or in conveying food to the mouth. It is an important question in what way this recovery is brought about. If the whole of the corresponding area in the opposite hemisphere is now removed, a similar paralysis occurs in the other hand, but the hand whose ' motor ' area was first extirpated remains entirely unaffected by the second lesion. On the contrary, the first hand is used more freely and more adroitly than before the second operation, probably be- cause the animal needs to use it more. The second hand recovers eventually, like the first. If when this has taken place the remain- ing part of the arm area from which the hand area was first excised be removed, neither hand is apparently affected, although there is severe paralysis of the shoulder and slighter paralysis of the elbow on the side opposite to the lesion, which is again largely recovered from. The recovery of the hand movement cannot therefore be attributed to the taking on of the function of the corresponding FUNCTIONS OF THE BRAIN 923 ' motor ' area either by the opposite hand area or by the adjacent ' motor ' cortex of the same hemisphere. According to some authorities, the recovery is due to the representation of the upper Hmb in the post-central gyrus (ascending parietal convolution in man) acting through fibres that descend from this gyrus to the optic thalamus, and thence through the rubro-spinal tract, which runs to the spinal cord (p. 839). Removal of the whole of the ' motor ' cortex of one hemisphere, in such animals as this operation has been performed on, causes paralysis of movement on the opposite side of the body. The paralysis is less marked in the case of bilateral muscles that habitu- ally act together than in the case of those which ordinarily act alone. Thus the muscles of respiration and the muscles of the trunk in Fig. 371. — Cerebral Cortex Man (seen from Above). The frctat of the brain is towards the left. The dotted line shows the position of the fissure of Rolando, as fixed by Thane's rule (p. 929). general are, although perhaps weakened, never completely para- lyzed. This is an indication that each member of such functional pairs of muscles is innervated from both hemispheres; and this physiological deduction is supported by the anatomical fact already referred to, that after removal of the ' motor ' cortex, or injury to the pyramidal tracts in the internal capsule or crus, some degener- ated fibres (homolateral fibres) are found in the crossed pyramidal tract on the side of the lesion (p. 847). In the dog afte'r a time the paralysis may more or less completely disappear. In the monkey restoration is less complete. Some interesting observations have been made on a monkey, which was carefully watched for eleven years after the removal by two operations of the cortex of the greater portion of the frontal 924 THE CENTRAL NERVOUS SYSTEM and parietal lobes on the left side. The character of the animal, which had been studied for months before the operations, was en- tirely unaffected. All its traits remained unaltered. There was no loss of memory or intelligence. On the other hand, disturbances of movement on the right side were very noticeable up till its death. It learned again to use the right limbs in locomotion ; but, although they were not markedly weaker than those of the left side, their movements had a certain clumsiness, which was associated with a permanent diminution in the sensibility of the skin of these limbs. Muscular sensibility was also lessened. In acts requiring the use only of one hand, the right was never willingly employed, and it evidently cost the animal a great effort to use it in such movements, but by special training it learnt again to give the right hand when asked for it, and to make use of it for other purposes. The movements with which the ' motor ' areas are concerned are essentially skilled movements, and we may suppose that it is more difficult for a monkey to educate again a centre for such complex and elaborate manoeuvres as are performed by its hand than for a dog to regain normal control of the comparatively simple movements of its paw. In man in cases of hemiplegia, when the patient lives for some time, a certain amount of recovery usually takes place, especially in young persons, in the paralyzed leg, but much less in the paralyzed arm. In the lower monkeys the ' motor ' area was formerly stated to extend behind the sulcus centralis into what in man would be called the ascending parietal convolution (post-central gyrus), and also to be more extensively represented on the mesial surface of the hemi- sphere than in the higher apes. Such observations, however, require to be reinterpreted in view of the results of Sherrington and Griin- bauin, especially as they were carried out by the bipolar method of stimulation, with both electrodes on the cortex. This method does not admit of such strict localization of the stimulus as the unipolar method. The most recent work with the unipolar method has indicated that in the lower apes also excitation of the gyrus post- centralis does not cause movements (C. and O. Vogt). It is in the light of the results obtained in monkeys, and by the aid of histological, embryological, clinical, and pathological ob- servations, that the ' motor ' areas in man have to a great extent been mapped out. The histological differentiation of the various cortical regions recently demonstrated by Brodmann and by Campbell are of especial interest (Figs. 372-376). It has long been customary to divide the cortex into layers, although the number and the boundaries of these layers are somewhat arbitrarily fixed. Brodmann distinguishes six layers : (i) A zonal or peripheral layer, containing many nerve-fibres and neuroglia cells, but few nerve-cells; (2) a layer containing ' granules ' and small pyramidal cells (external granular layer) ; (3) a layer of medium and large pyramidal cells {pyramidal layer) ; (4) a layer of small irregular FUNCTIONS OF THE BRAIN 925 cells {internal granular or stellate layer) ; (5) a ' gang- lionic ' layer, containing the largest pyramidal cells (deep large pyramids) ; (6) a layer (lamina multiformis) of spindle-shaped or polymor- phous cells. These layers vary in their structural de- tails, and especially in their relative development in animals of different rank in the mammalian scale, in one and the same animal at different periods in its em- bryonic and extra - uterine growth, and also in different parts of the cortex in an adult animal of given species. The region in front of the central sulcus (fissure of Rolando), e.g., is characterized by the presence of the giant pyra- Fig. 372.— Cell-Lamination of Gyrus Postcen- mids of Betz, which give origin to the pyramidal fibres going to the trunk and limbs (Fig. 373). trails (Campbell). A, just behind upper end of fissure ofJRclEindo; B, from the posterior edge of the gyrus (intermediate postcentral area of Campbell). r.vV •,», 1, '<■^ A /■■*■ ^ .i/ :*■.■'■ Fig. 373. — Cell-Lamination of Gyrus Pre- centralis (Campbell). From the portion of the gyrus immediately in front of the central sulcus (Campbell's precentral aJea in Figs. 375, 376). Although the results are less definite, the work of Flechsig on the time of development of the medullary sheath of the fibres in the various cerebral convolutions has also contributed to our knowledge of localiza- Fig. .374. — -Cell-Lamination of Gyrus Precentralis (Campbell). From an- ■ terior part of the gyrus (Campbell's intermediate precentral area in Figs. 375. 376). 925 THE CENTRAL NERVOUS SYSTEM Visuo-pMyetiio T»inf° Fig. 375. — Structurally Difierentiated Cortical Areas (Campbell). External surface of hemisphere (human brain). ./ isntory .^y f'l'rntdiate Fig. 376. — Structurally Differentiated Cortical Areas (Campbell). Mesial surface of hemisphere (human birain). FUNCTIONS OF THE BRAIN 927 tion in the cortex. In the development of a neuron four stages can be distinguished: (i) Cells without processes; (2) the appearance of pro- cesses, first the axon and then the dendrites ; (3) the formation of col- Pii- 377- — Flechsig's Developmental Zones (after Fieohsig). Outer surface of human cerebral hemisphere. Primary zones (i-io), darkly shaded; intermediate zones (11-31), less deeply shaded; terminal zoass (32-36), unshaded. laterals; (4) myelination or the formition of the medullary sheath (Fig. 323, p. 826). Myelination occurs in the cerebral convolutions in a regular order. In some areas the fibres miy bs meduUated three months before birth. Fig. 378.- -Flechsig's Developmental Zones (after Flechsig). cerebral hemisphere. Inner surface of human in others not till six months later. For instance, the Rolandic and olfactory regions, the calcarine portion of the occipital lobe associated with vision, and the portion of the temporal lobe associated with hearing, are plentifully provided with medullated fibres a short time after birth, at any rate before the first month, whereas the remaining .regions of the cortex are completely, or almost completely, free from 928 THE CENTRAL NERVOUS SYSTEM such fibres. In this way Flechsig has distinguished thirty-six cortical fields (Figs. 377, 378), which he divides according to the time of myelination.into three groups: 1 . Primary fields, ten in number, which are well provided with mye- linated fibres at birth. They include the cortical centres for the various sensations and also the ' motor ' area. They are connected especially with the so-called projection fibres. Thus, the cutaneous and muscular sense is assumed to be represented in field i, the sense of smell in field 2, of vision in 4, and of hearing in 5. From field i arise the fibres of the pyramidal tract, chiefly from the ascending frontal convolution, while the sensory fibres from the skin and muscles end mainly in the ascending parietal. This is an illustration of what Flechsig considers a general rule for these primary fields — viz., that each primordial sensory region is connected both with an afferent (cortici-petal) and with an efferent (cortici-fugal) tract. From the visual area (4), e.g., arises a tract which proceeds mainly to the anterior corpus quadrigeminum. 2. Terminal fields (32 to 36 in the figures) which become myelinated late, the process not beginning until at least a month after birth. 3. Intermediate fields (11 to 31) which become myelinated earlier than the terminal, but later than the primary. They and the terminal fields constitute par excellence association centres, which furnish fibres (association fibres) connecting the centres represented in the primary fields — e.g., such fibres as must be continually conveying impressions from the visual centre to the ' motor ' cortex when the hand is sketching a landscape. It may also be considered a function of these association centres to store up the memories of previous sense impressions. Flech- sig divides the association centres represented in the terminal fields into — (i) The great anterior association centre in the frontal lobe in front of the ' motor ' area ; (2) the great posterior association centre in the parieto-temporal region; (3) the smaller middle or insular associa- tion centre, which coincides with the island of Reil, an area which, according to Sherrington and Griinbaum, is totally ' inexcitable ' as regards the production of movement in the anthropoid apes. These association centres are foci, from which issue and to which come the long association paths. The reader must bear in mind that Flechsig's conclusions as to the functions of his very numerous areas are in many cases hypothetical, and can only be accepted when corroborated by other methods. We are far from being able at present to subdivide the functions of the cortex so minutely as is suggested by his map. Clinical and Pathological Observations in man agree, upon the whole, with wonderful pi'ecision with the results of experiments on animals ; and, indeed, before any experimental proof of the minute and elaborate subdivision of the cortex had been obtained, Broca had already, from the phenomena of the sick-bed and the post- mortem room, located a centre for speech in the left inferior frontal convolution (but see p. 936), and Hughlings Jackson had associated pathological lesions of the Rolandic area with certain cases of epi- leptiform convulsions. An extensive haemorrhage involving the Rolandic area of the cerebral cortex, or an embolus blocking the middle cerebral artery, causes paralysis of the opposite side of the body. An embolus of a branch of the middle cerebral artery causes paralysis of the muscles, FUNCTIONS OF THE BRAIN 929 or rather movements, represented in the area supplied by it. A tumour causes symptoms of irritation, motor or sensory— convul- sions beginning in, or sensations referred to, the parts represented in the regions on which it presses. In connection with the localiza- tion of lesions in the ' motor ' area of the cortex, and operative interference for their cure, the cortex has been frequently stimulated in man. There is no doubt that the ' motor ' region corresponds closely in position to that of the higher apes. It does not include the postcentral gyrus, for stimulation of this convolution with such strengths of current as are permissible evokes no movements, while movements are readily elicited from the precentral gyrus (Horsley, etc.). In exposing the ' motor ' region, or any particular part of it, the exact position of the fissure of Rolando becomes important ; and Thane has given the following simple method for fixing it: The point midway between the point of the nose and the occipital pro- tuberance is fixed by measuring the distance with a tape. The upper end of the fissure of Rolando lies half an inch behind this middle point. The fissure makes an angle of 67° with the longi- tudinal fissure (Fig. 371). The minor fissures are so inconstant as to afford no safe guidance in the locahzation of a given area. This must be delimited by stimulation. Sensory Functions of the Rolandic Area.— There are many proofs that the ' motor ' region is not a purely motor, but a sensori-motor, or kincBsthetic, area. Histological and embryological studies on the course of the sensory paths, as already pointed out, support this conclusion. It has also been mentioned that, according to Goltz's observations (p. 916), removal of the Rolandic cortex causes defects of sensation as well as of movement. In man, in connection with operations on the brain, still better evidence has been obtained. In two cases Gushing was able to elicit tactile sensations by electrical ' stimulation of the gyrus postcentralis (ascending parietal convolu- tion), and the sense of muscular movement by electrical stimulation of the gyrus precentralis. In a very careful study of a case in which he removed the upper limb area of the right hemisphere in a boy for violeiit convulsive movements of the whole of the left arm, Horsley came to the conclusion that the precentral gyrus in man is the seat of representation of (i) slight tactile sensation (after the operation appreciation of the lightest tactile stimuli was lost); (2) topognosis — i.e., appreciation of the localization in space of the point touched ; (3) muscular sense ; (4) stereognosis, or the power of recognizing the form of objects touched and handled; (5) pain — eg., that caused by a pin-pridk; (6) volitional movement. The postcentral gyrus in man appears to be the seat of a similar sensory representation, but as its relation to the efferent impulses concerned in volitional movements is less decided than that of the precentral . gyrus, so its relation to afferent impulses, both from the skin and the 59 930 THE CENTRAL NERVOUS SYSTEM deeper structures, is better marked. From the field of experiment further evidence of the sensori-motor nature of the ' motor ' region is forthcoming. (i) It has been found that if the posterior roots of the nerves suppljdng one of the limbs be cut in a monkey, all the most delicate and skilled movements of the limb are either greatly impaired or totally abolished (Mott and Sherrington). The limb is not used for progression or for climbing, but hangs limp, and apparently help- less, by the side of the animal. That this condition is not due to any loss of functional power by the peripheral portion of the motor path may be assumed, since the anterior roots remain intact. That it is not due to any want of capacity on the part of the ' motor ' centres to discharge impulses when stimulated may be shown by exciting the cortical area of the limb — either electrically or by inducing epileptic convulsions by intravenous injection of absinthe ' — ^when movements of the affected limb take place just as readily as movements of the sound limb. The cause of the impairment of voluntary motion, then, can only be the loss of the afferent impulses which normally pass up to the brain, and presumably to the ' motor ' cortex. When only one sensory nerve-root is cut, no defect of move- ment can be seen ; and this is evidently in accordance with the fact previously mentioned (p. 863), that complete anaesthesia of even the smallest patch of skin is never caused by section of a single posterior root. And that it is the loss of impulses from the skin which plays the chief part is shown by the fact that after division of the posterior roots supplying the muscles of the hand or foot, which only partially interferes with the sensory supply of the skin, joints, sheaths of tendons, etc., movement is unimpaired; while section of the nerve- roots supplying the skin, those of the muscles being left intact, causes extreme loss of motor power. (2) If a strength of stimulus be sought which will just fail to cause contraction of the muscular group related to a given motor area, and a sensory nerve, or, better, a sensory surface (best of all, the skin over the corresponding muscles), be now stimulated, con- traction may occur — that is to say, the excitability of the motor centres may be increased. This shows that the ' motor ' region is en rapport not only with efferent, but also with afferent fibres, that it receives impulses as well as discharges them. The same experiment is a proof that the results of excitation of the motor cortex are due to stimulation of the grey matter, and not, as might be objected, of the white fibres of the corona radiata. It is undoubtedly possible to excite these fibres by electrodes directly applied to the motor cortex, but in the latter case the current has to be made stronger than is sufficient to excite the grey matter alone. Further evidence is afforded by the following facts : (a) The ' period of delay ' — ^that is, the period which elapses between stimulation and contraction — ^is greater by nearly 50 per cent, when the cortex is stimu- FUNCTIONS OF THE BRAIN 931 lated than when the white fibres are directly excited. (6) Morphine greatly increases the period of delay for stimluation of the cortex, and at the same time renders the resulting contractions more prolonged than normal, while the results of direct stimulation 01 the white fibres are much less, if at all, affected, (c) Stimulation of the grey matter, when separated from the subjacent white matter by the knife, but left m position, IS without effect unless the strength of stimulus be increased, although twigs of the current ought, of course, to pass into the corona radiata as easily as before. Perfectly definite movements JJp RF LF can, however, be excited or ' ' ' inhibited by stimulating de- finite spots in the corona radi- ata, and even in the internal capsule. This simply means that in these positions the fibres representing these move- ments are not yet intermingled with fibres representing other movements. SensoryAreas— Visual Cen- tres.— In the occpital lobe in animals an area of consider- able extent has been found, destruction of which causes hemianopia (p. 894). Thus, if the right occipital cortex is destroyed, the right halves of the two retinae are para- lyzed, and the left half of the field of vision is a blank. There is conjugate deviation of the head and eyes to the same side as the lesion — in other words, the animal turns its head and eyes to the right. Destruction of this region on both sides causes complete blindness. When the same region is stimulated, the eyes and head are turned to the left — that is, there is conju- gate deviation to the opposite side. In the higher monkeys the eye movements can be elicited only from the extreme posterior apex of the occipital lobe and from its calcarine region, and then not easily. The movements differ from those produced by stimulation of the area for eye movements in the frontal lobe. They are not so certain, their latent period is longer, and a stronger stimulus is required to evoke them. It cannot be Fig. 379. — Diagram of Relations of Occipital Cortex to the Retinse. RO, LO, right and left occipital cortex ; RE, LE, right and left retina ; C, optic chiasma ; RF, LF, right and left visual fields. The continuous lines passing back from the retinse to the occi- pital cortex- represent the crossed, the broken lines the uncrossed, fibres of the optic nerves and tracts. For the sake of simplicity the intermediate stations on the visual path in the anterior corpora quadri- gemina, lateral geniculate bodies, and pul- vinar are not represented in the diagram. For these connections, see Fig. 360, p. 894. 932 THE CENTRAL NERVOUS SYSTEM doubted that the occipital region is concerned in vision, and it is a very natural suggestion that the movements are the result of visual sensations in the excited occipital cortex. The right occipital lobe is concerned with vision in the right halves of the two retinse (Figs. 360 and 379). Now, under normal conditions, a visual image would be cast on the two right retinal halves by an object placed towards the left of the field. The movements of the head and eyes to the left may therefore be plausibly explained as an attempt to look at, and a rotation towards, the supposed object. The pathological evidence is very Clear that disease of the occipital Icbe, especially of the cuneus, a triangular area on its mesial surface, causes hemianopia in man. A limited lesion may even be associated with an incomplete hemianopia, and cases have been recorded in which colour hemianopia (blindness of the corresponding halves of the two retinae for coloured objects) co-existed with normal vision for white light. The precise limits of the occipital visual area are still disputed. It probably occupies, in addition to the cuneus, the lingual lobule and a portion of the external aspect of the occipital lobe. The question of the projection of the retina upon the visual cortex — i.e., the question whether each retinal area is represented in a definite cortical area — has given rise to much debate. The representation of the fovea cen- tralis, the area of most distinct vision, has aroused especial interest. It has been asserted that a circumscribed area in the region of the cal- carine fissure is the centre for the fovea (Henschen). But it is totally opposed to this view that extensive lesions of the occipital cortex, even on both sides, do not, except in rare cases, cause total blindness in the foveal region, although peripheral vision is destroyed. On the other hand, in no case has a purely cortical lesion been found associated with blindness confined to the fovea (Monakow). The fibres of the optic radiation which are on the path from the fovea are accordingly dis- tributed diffusely to the visual cortex. Sometimes dimness of vision in the whole of the opposite eye (crossed amblyopia), and not hemianopia, is caused by a lesion of the occipital cortex. It seems impossible to explain this and other facts without postulating the existence of more than one visual centre; and it has been supposed that in the angular gyrus and the neighbouring region a higher visual centre exists which is connected with the lower occipital centres for the two halves of the opposite eye. Thus, the right angular gyrus would be in connection with the part of the right occipital cortex which has to do with vision in the nasal half of the left eye, and with the part of the left occipital cortex which has to do with vision in the temporal half of that eye. This higher centre, which perhaps functions as a storehouse of visual memories, probably corresponds to the structurally differentiated area (x'isuo-psychic area of Campbell), as the lower centre corresponds to his structurally differentiated visuo-sensory area (Figs. 376, 377). Auditory Centre. — On the outer surface of the temporo-sphenoidal lobe, mainly in the first temporal convolution, lies an area asso- ciated with the sense of hearing. Stimulation in the region of the first temporal convolution may cause the animal to prick up its ears on the opposite side. Destruction of this area on both sides is followed by complete and irremediable loss of hearing. If it is destroyed only on one side, there is partial deafness of the opposite FUNCTIONS OF THE BRAIN 933 ear, and also to some extent of the ear on the same side. This is gradually recovered from. If it is destroyed on the left side there is also the peculiar condition called ' word-deafness,' which will be referred to directly (p. 937). In deaf-mutes the first temporal convolution may be atrophied. There is evidence that the posterior corpora quadrigemina and the mesial geniculate body form an in- ferior relay on the route between the fibres of the auditory nerve and the temporal cortex. There are indications that within the auditory area so-called ' musical centres ' exist — that is, an orderly Fig. 380.- SYLVIAW FISSURE -Lateral View of Left Hemisphere with Sensory Areas : Man. of the brain is towards the left. The froi.t arrangement of the cell-bodies of the neurons that have to do with the perception of pitch, so that a hmited lesion may cause deafness to notes of a particular pitch when it is situated on one part of the area, and deafness to notes of a different pitch when it is situated elsewhere (Larionow). Centre for Smell.— As to the position of the centre for smell, direct experiment on animals cannot teach us much, for if the outward tokens of visual and auditory sensations are dubious and fluctuating, still more is this the case with the signs of sensations of smell. A 934 THE CENTRAL NERVOUS SYSTEM further source of fallacy is the fact that other sensations than those of smell are caused by stimulation of the mucous membrane of the nose. Substances like ammonia, for example, affect entirely the endings of the trigeminus, which is the nerve of common sensation for the nostrils. Pathological and clinical evidence would be of great value, but it is as yet scanty, and of itself indecisive. Some cases of epilepsy have been reported in which the attack was heralded by smells for which there was no objective cause. At necropsy the un- cinate gyrus was found diseased. So far as it goes, such evidence supports the view derived from the anatomical connections of the olfactory tracts, that the centre for smell is situated in the uncinate gyrus on the mesial aspect of the temporal lobe, for the olfactory track may be traced into this region. In animals with a very acute sense of smeU, this gjnrus is inagnified into a veritable lobe, called from its shape the pj^iform lobe; from its supposed function, the Fig. 381. — Sensory Areas of Mesial Surface of Human Brain. The front of the brain is towards the right. rhinencephalon. The centre for taste is supposed to be situated in the same region as the centre for smell (in the hippocampal convolu- tion posterior to the uncinate gyrus). Ordinary and Tactile Sensations, including the muscular sense, have been located in the Rolandic area (p. 929) ; and there are good grounds for believing that afferent fibres from the joints, the muscles and their accessory structures and the skin terminate here in arboriza- tions which come into contact either with the motor pyramidal cells, or with intermediate cells which link them to the pjn-amidal cells. Aphasia. — ^Words are, at bottom, arbitrary signs by which certain ideas are expressed. The power of intelligent communication by spoken or written language may be lost: (i) by paralysis of the muscles of articulation or the muscles which guide the pen; (2) by inability to hear or see the spoken or written word — i.e., by deafness or blindness; (3) by inability to comprehend the meaning of spoken or written lan- guage, although sensations of hearing and sight may not be abolished FUNCTIONS OF THE BRAIN 935 — ^that is to say, by inability to interpret the auditory or visual symbols by which ideas are conveyed ; (4) by inability to clothe ideas in words, although the words may be present in the patient's consciousness, and the ideas conveyed by speech or writing may be comprehended. Neither (i) nor (2) is considered to constitute the condition of aphasia; (3) represents what is called amnesia, or sensory aphasia ; (4) is aphasia in the ordinary restricted sense, or motor aphasia. Motor aphasia may be divided into two varieties — subcortical or pure motor aphasia, and cortical, or Broca's aphasia. In the subcortical type the patient understands speech and writing perfectly, and is able to wiite normally; but he cannot speak spontaneously o"r read aloud, or repeat words when requested to do so. He may know quite well what to reply in answer to a question, but the words necessary to express his meaning do not come to him. In Broca's type of aphasia, which is the most common, the patient may understand spoken and written words — often imperfectly, it is true — but he is unable to speak spontaneously, to repeat words spoken to him, and to read aloud. Unlike the person suffering from the subcortical type of motor aphasia, he has difficulty in reading by the eye without articulation, and in writing spontaneously or to dictation. There is often or always some intellectual deficiency. The gradations in the loss of the expressive factor in speech may be infinite. A patient may sometimes sing a song without a single slip in words or measure, and yet be unable to speak or write it. In a case recorded by Larionow an aphasic could speak only one syllable, ' tan,' but could sing the ' Marseillaise.' In certain cases the change is confined to los^ of the power of spontaneous speech, and the patient may be able to read intelligently. Sometimes he can express his ideas in speech, but not in writing (agraphia) . Sometimes the loss is restricted to certain sets of ideas. For example, a boy was injured by falling on his head. Typical symptoms of motor aphasia developed, but the power of dealing with ideas of number was not interfered with, and the boy continued to learn arithmetic as if nothing had happened. Proper names and nouns are more easily lost than adjectives and verbs. Motor aphasia is generally accompanied by paralysis, frequently transient, of voluntary niovement on the right side, sometimes amount- ing to complete hemiplegia, but more often involving the right arm alone. This association is generally explained by the proximity of the inferior frontal convolution to the motor area of the arm, and their common blood-supply. It has already been stated that since Broca it has been generally assumed that in most persons the inferior frontal convolution on the left side is concerned in the expression of ideas in spoken or written language. It is even said that oratorical powers have been found associated with marked development of this convolution (as in the case of Gambetta, the French statesman). It is the cortical or Broca's type of motor aphasia which has been supposed to be associated with a lesion in the left inferior frontal convolution. The portion of the con- volution concerned is the posterior extremity, where it borders on the fissure of Sylvius, and it either completely coincides with or largely overlaps the centre for the movements of the tongue, lips, and larynx concerned in articulation. The failure, however, does not lie in the articulatory mechanism. The patient uses the same muscles of articu- lation, without any marked impairment of function, for chewing and swallowing his food. It is only when the corresponding area in the right inferior frontal convolution, or the path from it to the internal capsule, is also destroyed, that articulation is greatly and permanently interfered with.'^ The question obviously presents itself why it is that motor aphasia is 936 THE CENTRAL NERVOUS SYSTEM commonly due to a lesion in the left hemisphere alone. The answer to this question is supposed to be partly supplied by the important and curious observation that in left-handed individuals damage to the right inferior frontal convolution may cause aphasia. In the right-handed man the motor areas of the left hemisphere may be supposed to be more highly educated than those of the right hemisphere. The movements of the right side which they initiate or control are stronger and more delicate and precise than those of the left side. It is only necessary to assume that this processs of specialization, of selective training, has been carried on to a still greater extent in the left frontal convolution, that in most men the speech-centre there has taken upon itself the whole, or the greater part, of the labour of clothing ideas in words, leaving to the right centre only its primitive but undeveloped powers. In left- handed persons the speech-centre on the right side may be supposed to share in the general functional development of the right hemisphere. That great capabilities are lying dormant in the right speech-centre of the ordinary right-handed individual is indicated by the fact that after complete destruction of the left inferior frontal convolution the power of speech may be to a considerable extent, though slowly and laboriously regained; and it is said that this second accumulation may be swept away, and without remedy, by a second lesion in the right inferior frontal convolution. But frail is the tenure of life in a person who has twice suffered from such a lesion ; and we do not know whether recovery might not take place to some extent even after destruction of both inferior frontal convolutions, if the patient only lived long enough. Recently Marie has reopened the whole question of the relation of aphasia to lesions of the inferior frontal convolution. He believes that the ^o-caUed Broca's area has nothing to do with aphasia in the proper sense of the term — i.e., it is not a cortical area concerned in ' internal ' speech processes, or in which motor or kinaesthetic ' speech memories ' are stored — but simply a ' motor ' area for the movements of articula- tion. He maintains that there is but'one form of true apheisia — ^the aphasia of Wernicke — ^which has for its basis a lesion of the so-called zone of Wernicke (the supramarginal and angular gyri, and the posterior portions of the first and second temporal convolutions) . This, according to him, is the true speech-centre. The symptom-complex known as Broca's aphasia, which everybody admits to exist as a distinctly charac- terized clinical condition, is due, he says, to a double lesion. One lesion causes aphemia (loss of the power of co-ordinating the movements needed in the articulation of words without actual paralysis of the muscles), and the other the disturbance of internal speech, and the difficulty of reading and of writing, which constitute the true aphasia . According to Marie, the lesion which causes the aphemia is not even situated in Broca's convolution, but somewhere in a rather badly de- fined region, which he denominates the lenticular zone, since it includes the lenticular as well as the caudate nucleus, in addition to the external and internal capsules and the cortex of the island of Reil. It would be out of place to enter more minutely here upon such controversial matters. The conclusion which emerges most definitely from the dis- cussion" is that Broca's localization was based upon a very narrow foundation, and must probably be modified. It is generally recognized that in almost all cases of aphasia in which the brain has been studied after death, some lesion of association fibres has been present, and not merely a cortical lesion. Interference with the association fibres causes confusion in the processes of association which are so important in mental activity, and defects of intelligence are there- fore commonly observed in aphasia. FUNCTIONS OF THE BRAIN 937 A so-called temporary aphasia may occur without any structural change in the speech-centre — for example, during an attack of migraine. In children it may even be caused by some comparatively slight irrita- tion in the digestive tract, such as that due to the presence of a tape- worm. In the anthropoid apes no evidence of the existence of any " speech- centre,' even distantly foreshadowing the human, has been obtained by stimulating the inferior frontal convolution on either side. No move- ments, and particularly no movements connected with vocalization, are elicited. Sensory Aphasia. — In typical motor aphasia spoken and written words convey to the patient their ordinary meaning. They call up in his mind the usual sequence of ideas, but the chain is broken at the speech-centre, and the outgoing ideas cannot be clothed in words. The expressive factor in speech is deranged. In sensory aphasia the percep- tive factor in speech is deranged. In ordinary' sensory aphasia (Wer- nicke's, or cortical sensory) aphasia) the patient cannot understand spoken or written language, but, far from being unable to speak, he often babbles incessantly. He may string together a series of words, each correctly articulated, but having no meaning, or may utter a jargon not composed of known words at all. Instead of the words which he desires to use to express his meaning, he may use others having a similar sound [paraphasia). Damage to two regions of the brain has been found associated with this condition: (i) the middle part of the first and second temporal convolutions, (2) inferior parietal convolu- tions and the angular gyrus in the neighbourhood of the occipital visual centre. When the temporal region is alone afiected, it is the spoken word that is missed, the written that is understood {word-deafness). When, as occasionally happens, the lesion is confined to the occipitsl region, spoken language is perfectly understood, written language not at all {word-blindness). It is the left hemisphere which is affected in right-handed persons, the right hemisphere in left-handed person?. Sensory, like motor aphasia, may exist in any degree of completeness, from absolute word-deafness or word -blindness, in which no spoken or printed word calls up any mental image, to a condition not amounting to much more than a marked absence of mind or unusual obtusenesf . Motor and sensory aphasia may be present together. In well-markec cortical word-deafness speech is always interfered with to some extent. In so-called pure word -deafness (subcortical sensory aphasia) the patient may be perfectly capable of rational speech. He may talk to himsel' or on a set topic with fluency and sense, may write intelligently, and understand what he reads; but he may be unable to understand a single, word spoken to him, or to repeat words when asked to do so. Cortical Epilepsy. — ^Disturbed action of the motor centres may tale the form either of depression or of increased excitability. The former will be associated with partial or complete paralysis of the movemenls represented in the area, the latter by abnormally intense or prolonged discharge leading to the condition called cortical epilepsy — ^that if, epileptic attacks associated with cortical lesions. Among these are tl e cases of so-called Jacksonian epilepsy — a condition characterized by the fact that the seizure does not begin by general, but by local, convulsions. They may remain confined to a single limb, or to one side of the face, or to one side of the body. So long as the convulsions are not general, consciousness need not be lost. Or a seizure beginning as Jacksonian may spread so as to involve the whole body, in which case the sympton: s become identical with those of ordinary epilepsy, including the loss of consciousness. It has been found possible in some cases to localize the 93S THE CENTRAL NERVOUS SYSTEM position of the lesion from the part of the body in which the fit, or the aura (the sensation or group of sensations peculiar to each case, which precedes and announces the attack) begins. For example, if the con- vulsions commence with a twitching of the right thumb and extend over the arm, or if the aura consists of sensations beginning in the thumb, there is a strong presumption that the seat of the lesion is the part of the arm-area known as the ' thumb-centte ' in the left cerebral hemisphere. It is the seat of the convulsion at its commencement, not the regions to which it may afterwards spread, that is important in diagnosing the position of the lesion. For just as strong or long-continued electrical stimulation of a given ' centre ' of the ' motor ' cortex may give rise to contractions of muscles associated with other " centres,' so the excita- tion set up by localized disease may spread far and wide from its original focus, involving area after area of the ' motor ' region first in the one hemisphere and then in the other. The part of the tody to which a sensory aura is referred is as significant an indication of the seat of the discharging lesion as is the part of the body which first begins to twitch. This is one of the proofs that the ' motor ' region is not a purely motor area. Disturbed action of the sensory areas on the cortex may, as in the case of the motor regions, take the foim either of de- ficiency or of excitation. Excitation expresses itself by hallucinations, the person having the impression of a sight, a sound, a smell or taste, or one or other of the cutaneous sensations in the absence of the related objects. Seat of Intellectual Processes — ^Association Areas. — ^When we have deducted from the cortex of the hemisphere the whole Rolandic region and the sensory centres, there still remains a large territory unaccounted for. Considerable portions of the occipital, parietal, and temporal lobes, nearly the whole of the island of Reil and the greater part of the frontal lobe anterior to the ascending frontal convolution are ' silent areas,' and respond to stimulation by neither motor nor sensory sign. They correspond to the association centres previously referred to. They are connected with the sensory and motor areas and with each other, but are not directly connected by projection fibres with the lower parts of the central nervous system, as the motor area, for example, is by the pyramidal path. By a process of exclusion it has been supposed that, in addi- tion to, or partly in virtue of, their associative function, they are the seat of intellectual and psychical operations. It is supposed that the sensations aroused in the various sensory areas by the impulses received from the sense organs are linked in the assccia- tion areas into complex perceptions. For instance, when an orange is taken into the hand, the visual sensation of a yellow body, the tactile sensation of a smooth round body, and perhaps the olfactory sensation characteristic of an orange, are collected from the sensory areas, connected and combined, or synthesized in an association area to the concept of Jan orange. Somewhere in the association areas it is to be supposed is stored the memory of past experiences. The intellectual function has been more particularly assigned to the frontal lobes, and with great probabiHty, although FUNCTIONS OF THE BRAIN 939 we have little real knowledge to guide us to a decision. Extensive destruction and loss of substance of the prefrontal region may sometimes occur without any marked symptoms. But usually there is restriction of mental power, or it may be loss of moral restraint. Thus in the famous ' American crowbar case,' an iron bar completely transfixed the left firontal lobe of a man engaged in blasting. Although stunned for the moment, he was able in an hour to climb a long flight of stairs, and to answer the inquiKes of the surgeon. Finally, he recovered, and hved for nearly thirteen years without either sensory or motor deficiency, except that he suffered occasionally from epileptic convulsions. But his intellect was impaired; he became fitful and vacillating, profane in his language and inefficient in his work, although previously decent in conversation and a diligent and capable workman. Flechsig supposes that his great anterior association centre in the frontal lobe is concerned in the retention of the memory of all conscious bodily experiences, especially those connected with voluntary acts. The great posterior association centre he imagines to be engaged in the formation and collection of ideas of external objects and of the ' word pictures ' which represent them, and with the preparation of speech in respect of the thoughts to be expressed and the form of expression, the office of the Broca's area (but see p. 936) being to execute the mechanical part of the process by transforming these thoughts into actual spoken words. This posterior association centre may be looked upon as the seat of intellect in the narrower sense, as the anterior is of will and feeling. The experiments of Franz on the relation of the cerebral association areas, and especially the frontal area, to certain acquired habits are of interest. Cats were allowed to acquire certain habits involving simple mental processes, and then it was seen how these were affected by cortical lesions. After bilateral extirpation of the frontal lobes (the area anterior to the crucial sulcus) newly-formed, but not long-standing, habits are lost. This cannot be due to shock, since other brain lesions are not followed by loss of the habits. Extirpation of one frontal area usually causes a partial loss of newly-acquired habits, or, rather, a slow- ing of the association process leading to unusual delay in the execution of the movements connected with the habit. Habits once lost after removal of the frontal lobes may be releamed. The influence of psychical events upon bodily functions is well known, and has been more than once illustrated in preceding pages. The con- verse question of the influence of bodily states upon psychical events has also been raised, especially in connection with the genesis of emotion. Some psychologists assume that the bodily changes associated with such emotions as grief, fear, rage, or love, are not evoked as a consequence of the emotions, but that the bodily changes follow directly the perception of the exciting fact — e.g., a spectacle which causes fear or rage, 'and that our feeling of the same changes as they occur is the emotion ' (James). Sherrington, however, has shown that in dogs in which, by transection of the vagi and the spinal cord, all sensation of viscera, skin, and muscles behind the level of the shoulder was eliminated, no obvious emotional defect was caused. Notwithstanding the immense abridg- ment of the field of sensation, anger, joy, fear, disgust (as on being offered dog's flesh, which most dogs refuse to eat), were as marked as ever, and were evoked by the same objects as before the operation. 940 THE CENTRAL NERVOUS SYSTEM When the afferent field is still more restricted, as in the head of a dog grafted on the circulation of another dog by anastomosis of the blood- vessels, with precautions to avoid interruption of the bleed-flow, not only does the respiratory centre continue to discharge itself with a regular rhythm, but cortical volitional movements persist (Guthrie, Pike, and Stewart), and, so far as can be judged, sense perception, emotional, and even intellectual, processes continue. In one case the picture presented by the engrafted head was essentially the same as that presented by the head of the ' host ' for over two hours. In a trans- planted head from a younger dog in which the circulation had been interrupted for twenty-nine minutes, a remarkable return of cerebral function was observed (Guthrie). Localization of Function in the Central Nervous System. — Let us now consider a little more closely the real meaning of this localization of function. Scattered all over the grey matter of the primitive neural axis, and, as we have seen, over the grey mantle of the brain as well, are numerous ' centres ' which seem to he related in a special way to special mechanisms, sensory, secretory, or motor. The question may fitly be asked whether those centres are really distinct from each other in quality of structure or action, or whether they owe their peculiar properties solely to differences in situation and anatomical connection. It is clear at the outset that the nature of the work in which a centre is engaged must be largely determined by its connections. The kind of activity which goes on in the vaso-motor centre in the bulb, for example, may in no essential respect differ from that which goes on in the respira- tory centre. The calibre of the bloodvessels will alter in response to a change of activity in the one because it is anatomically con- nected with the muscular coat of the bloodvessels. The rate or depth of the respiratory movements will alter in response to a change of activity in the other, because it is connected with muscles which can act upon the chest-walls. Experiments on the anastomosis of nerves afford a very interesting illustration of the determining influence of their peripheral con- nections on the function of nerve-fibres. It has, in fact, been shown that the central end of any efferent somatic fibre — i.e., any fibre running from the central nervous system and ending in striated muscle — can make functional connection with the periph- eral end of any other efferent fibre of the same class, whatever te the normal actions produced by the two fibres. Advantage has been taken of this in surgery. For instance, in a case of severe facial (motor) tic the facial nerve was divided, and its peripheral end united with a portion of the fibres of the spinal accessory. The voluntary movements of the face, after regeneration had occurred, were normally carried out through impulses descending the spinal accessory. In cases of local paralysis, due to destruction of anteric r horn-cells (anterior poliomyelitis), restoration of movement has also been obtained by connecting the motor nerve of the paralyzed FUNCTIONS OF THE BRAIN 941 muscles to a portion of a nerve coming off from an uninjured region of the cord. By such operations it has been possible to transpose motor areas on the cerebral cortex associated with the flexion and extension of a particular joint, so that the part of the cortex which originally caused flexion after the nerve anastomosis causes extension, and vice versa. When the nerves supplying a group of muscles of the dog's fore-limb are eliminated, the nerves of the antagonistic group may be used to supply both groups, and co-ordinated movements may be restored, although this does not occur so rapidly as when the nerves supplying the two groups are simply cut and cross- sutured (Kennedy). However, the limitations of this method ought to be recognized. Before any anastomosis of nerves can be made, good fibres must first be destroyed. Under favourable cir- cumstances these may all regenerate and find their way to the struc- tures they are intended to innervate. When regeneration is com- plete, the number of fibres capable of functioning will at best be the same as before the operation, and may easily be considerably less. The benefit, whatever it is, will be associated solely with the re- distribution of the fibres. There is reason to think that the closer to the cell of origin a nerve is injured or divided, the less is the chance of restoration, and Feiss has found that after lesions in the cord or the spinal roots neither the anatomical pattern of the affected nerves nor their functional power is much affected by subsequent nerve anastomosis. The central end of any efferent somatic fibre can also make functional union with the peripheral end of any of the efferent fibres which run from the central nervous system and. end in ganghon cells (pre-ganglionic fibres), and the central end of any pre-gan- glionic fibre can do the same with the peripheral end of any efferent somatic fibre (Langley and Anderson). For instance, Langley divided (in cats) the vagus nerve and the cervical sympathetic. The peripheral end of the former degenerated, of course, below the section, and the peripheral (cephalic) end of the latter degenerated above the section, up to the terminations of its axons in the superior cervical ganglion. The central end of the cut vagus was subsequently sutured to the peripheral end of the cut sympathetic. After a time the vagus-fibres grew along the course of the degener- ated sympathetic up to the ganglion, where some of them formed arborizations around the ganghon cells. It was now found that stimulation of the vagus produced the effects usually caused by stimulation of the cervical sympathetic— for example, dilatation of the pupil and constriction of the bloodvessels of the head and neck. From these experiments it follows that the functions of the various groups of fibres in the cervical sympathetic do not depend on anything pecuhar to the fibres; any fibre which can make con- 942 THE CENTRAL NERVOUS SYSTEM nection with one of the ganglion cells that send axons to the dilator muscle of the iris will, when stimulated, act as a pupillo- dilator fibre, just as well as a cervical sympathetic fibre. Other instances of the same law have already been given in connection with the regeneration of nerves (p. 774). Functional union does not take place between efferent somatic fibres (or pre-ganglionic fibres) and post-ganglionic fibres — i.e., fibres arising in peripheral ganglia, and ending in smooth muscle and glandular tissue; e:g., the cervical sympathetic after excision of the superior cervical ganglion does not unite with the fibres leaving the anterior end of the ganglion in such a way that stimula- tion of it can produce any of the effects normally produced through these fibres. No proof has been given that afferent fibres can unite with efferent fibres or efferent with afferent. Afferent fibres of one nerve can unite with afferent fibres of another nerve, but there is not sufficient evidence to show whether fibres concerned in one sensation can unite with fibres concerned in another. The localization of function in the cerebral cortex has been likened to the localization of industries in the multiplex commercial life of the modern world. The barbarian household in which cloth is woven and worked into garments; sandals, or moccasins cobbled together; rough Dottery baked in the kitchen fire, and all the rude furniture of the lodge f ishioned by the hands which built it, and which rest beneath its roof at night — ^this state of things where centralization has not yet begun, it has been said, is a picture of what goes on in the undeveloped brains of the frog, the pigeon, and the rabbit. The ' diffusion ' of industries which is characteristic of a primitive state has given place among the most highly civilized men to extreme centralization and concentra- tion. ISfenchester spins cotton and Liverpool ships it. Chicago handles wheat and pork that have been produced on the prairies of Minnesota and Illinois. Amsterdam cuts diamonds. Munich brews beer. Lyons weaves silk. New York and London are centres of finance. This, it is said, is the picture of the highly specialized brain of a monkey or a man. But ingenious and alluring though such analogies are, they do not rest upon a sufficient basis of fact. Indeed, the more deeply the structure and function of the central nervous system are studied, the more clearly does its essential solidarity appear, the more clearly does it emerge as an organized co-ordinated system, not an aggregate of separate mechanisms jumbled together for convenience of storage in the protected cranio- spinal cavity. It has never been shown — ^nor is it likely that the proof will soon be forthcoming — that there is any difierence whatever in the physical, chemical, or psychical processes which go on in the various centres of the ' motor ' cortex. It may be supposed, indeed, that the so-called sensory areas of the cortex differ more widely in their internal activity from the ' motor ' areas than the latter do among themselves, and that the activity of the anterior portion of the brain, the portion which has been credited par excellence with pyschical functions, differs in kind, not merely in degree, from that of all the rest. But, as we have just seen, even the ' motor ' areas have sensory functions. A cast-iron physiology may explain this by the assumption of ' sensory ' as well as ' motor ' cells in the Rolandic area", and may find support for such an assumption FUNCTIONS OF THE BRAIN 943 in the well-known fact that the large pyramidal cells whose axons form the pyramidal tract make up but a small proportion of the total number of pyramidal cells in this region, which, besides, contains numerous cells of .Golgi's second type (p. 828). And although it may be true that the tactile sensations constituting the so-called body-sense are represented mainly not in the motor region itself, but in the adjacent gyrus post- centralis posterior, to the Rolandic fissure (p. 918), there is nothing to contradict the supposition that the discharge of energy from the most circumscribed motor area or element may be accompanied with con- sciousness. And, indeed, some writers have supposed that such a consciousness of, or even conscious measurement of, the discharge from the ' motor ' areas is the basis of the muscular sense (Bain, Wundt). So far, at least, as the ' motor ' region and the grey matter imme- diately around the neural canal are concerned, the analogy of an electrical switch-board connected with machines of various kinds might be more correct. Touch one key or another, and an engine is set in motion to grind corn, or to saw wood, or to light a town. The difference in result lies not in any difference of material or workmanship in the switches, but solely in the difference in their connections. Grey matter in the upper part of the precentral convolution is excited, and the muscles of the leg contract. Grey matter on the lower part of the convolution is excited, and there are movements of the face and mouth. Grey matter in the medulla oblongata is excited, and the salivary glands pour forth a thin, watery fluid, poor in proteins, and containing an amylolytic ferment. Another portion of grey (?) matter in the mqduUa is thrown into activity, and the pancreatic ducts become flushed with a thicker secretion, relatively rich in proteins and in ferments which act on proteins, starch, and fat. Here, too, there is a variety in result according as one or another nervous switch is closed ; here, too, the variety is due, not to essential difierences in the structure of the activity or the nervous centres, but to their connection, by nervous paths, with peripheral organs of different kinds. There is, indeed, a specialization, a localization, of function, but the localization is at the periphery, the specialization is in the peripheral organ?. It may be asked whether, if this is the case for the peripheral organs of efferent nerves, the converse does not hold true for the afferent nerves — in other words, whether the localization here is not at the centre. And that there is in some degree a central localization of sensation may be considered proved by the well-known clinical fact, already referred to, that sensations of various kinds may be produced by pathological changes in the cortex. For example, a tumour involving the upper part of the temporal lobe may give rise to epileptiform convulsions preceded by an auditory aura, a sound, it majr be, resembling the ringing of bells; a tumour involving the occipital region may cause a visual aura, and so on. Central sensory localization is the fundamental idea of the old doctrine of the specific energy of nerves, which, in modem phraseology, expresses the fact that excitation of the central end of a sensory nerve by various kinds of stimuli causes always — or at least very often — the particular kind of sensation appropriate to the nerve. The observation so frequently made in surgery before the days of ansesthetics, that when the optic nerve was cut in removing the eyeball the patient experienced the sensation of a flash of light,* was long looked upon as the strongest prop of the law of specfic energy, and well illustrates the meaning of the term. Here a mechanical excitation of the optic fibres in their course gives rise to the same sensation as excitation of the retina by the * It is said that this is not always the case. 944 THE CENTRAL NERVOUS SYSTEM natural or homologous or adequate stimulus of light. Since a similar mechanical stimulus applied to the auditory nerve gives rise to a sensa- tion of sound, and, applied to the trigeminal nerve, to a sensation of pain, many physiologists have assumed that the impulses set up in the auditory nerve when sound impinges on the tympanic membrane do not differ essentially from those set up in the optic nerve when a ray of light falls upon the retina, or from those set up in the fifth nerve by the irri- tation of a carious tooth, or from those set up in certain fibres of the cutaneous nerves when a warm body comes in contact with the skin. Since the results in consciousness are very different, this assumption has necessitated the further conclusion that somewhere or other in the central nervous system there exist organs that are differently afiected by the same kinds of afferent impulses — ^in other words, that sensory localization is at the centre. On this view, the viscual areas in the cortex respond to all kinds of stimuli by visual sensations ; the auditory areas by sensations of sound, and so on. But while it cannot be doubted that special sensory regions exist in the grey matter of the brain, where the afferent paths concerned in the different kinds of sensation end, it has not been proved that the nerve- impulses which travel up the various paths are absolutely simUar until they have reached the centres, and there suddenly become, or produce, sensations absolutely different. There is, indeed, evidence of a certain amount of sensory specialization at the periphery. For example, when an ordinary nerve-trunk is touched, the resultant sensation is not one of touch. If there is any sensation at all, it is one of pain. Heating or cooling a naked nerve-trunk gives rise to no sensations of tempera- ture. When the ulnar nerve is artificially cooled at the elbow, the first effect is severe pain in the parts of the hand supplied by the nerve. The pain disappears somewhat abruptly as cooling goes on, and is succeeded by gradual loss of all sensation in the ulnar area of the hand ; but the cooling of the nerve-trunk does not give rise to any sensation of cold ( Weir Mitchell) . Stimulation of the receptors or end -organs is normally essential in order that sensations of touch and temperature should be experienced. Although as previously stated, one great function of the receptor is to lower the threshold of the adequate stimulus, and thus to render the afferent neuron more easily excited by an adequate stimulus than by any other, it may also serve to impress a particular rhythm or other character upon the nerve impulse, so that the afferent impulses may be to some extent differentiated before they reach their centres. One reason, then, why excitation of the temporal cortex by impulses falling into it along the auditory nerve-fibres causes a sensa- tion different from that caused by impulses reaching the occipital cortex through the fibres of the optic nerve may be a difference in the nature of the impulses. If this were the only reason, it would follow that were it possible to physiologically connect the fibres of the optic radiation with the temporal cortex, and those of the temporal radiation with the occipital cortex, sights and sounds would still be perceived and dis- criminated in a normal manner, although now the integrity of the occipital lobe would be bound up with the perception of sound, the integrity of the temporal lobe with visual sensation. This state of affairs would correspond to complete specialization for sensation in the peripheral organs, complete absence of specialization in the centres. On the other hand, it is conceivable that, after such an ideal experiment, sound-waves falling on the auditory apparatus might cause visual sensations, and luminous impressions falling on the retina sensations of sound. This would correspond to complete specialization of sensation in the centres, complete absence of speciaUzation at the periphery. A FUNCTIONS OF THE BRAIN 945 third possibility would be that the • transposed ' centres, responding at first feebly or not at all to the new impulses, might, by slow degrees, become more and more excitable to them. This would correspond to a peripheral specialization, combined with a tendency to development 01 central specialization. And, indeed, it is not easy to conceive in what way, except as the result of differences in the nature of impulses coming from the periphery, specialization of sensory areas in the central nervous system could have at first arisen. Degree of Localization in Different Animals. — Before leaving this subject, two points ought to be made clear: (i) The degree of localization of function in the cortex goes hand in hand with the general development of the brain. In man and the monkey, the motor localization is more elaborate than in the dog — that is to say, a greater number of movements can be associated with definite cortical areas. In the rabbit, whose ' motor ' centres have been particularly studied in recent years by Mann and Mills, localization is still less advanced than in the dog. Towards the bottom of the mammahan group certain ' motor ' areas can still be demonstrated, though they are rather ill-defined, for instance in the hedgehog (Mann), opossum (Cunningham), and ornithorhynchus (Martin). In general the movements of the anterior limb are easier to obtain than those of the posterior. In birds Mills found no evidence of the existence of any ' motor ' centres. (2) Areas of the same name (homologous areas) in different groups of animals do not necessarily have the same function — that is, in the case of the ' motor ' areas, are not necessarily associated with the same movements. Taking the position of the centre for the orbicularis oculi as a test, Ziehen has come to the conclusion that in the anthropoid apes and in man this centre has been pushed forward by the encroachment of the centres behind it, and especially of the visual centre, the arm centre, and the speech centre, which have undergone a great functional development. Acquisition of Co-ordination of Voluntary Movements. — ^The co-ordina- tion of movements has already been alluded to in connection with the spinal reflexes. No fundamental distinction can be drawn between the co-ordination of reflex and of voluntary movements, but the conscious and often long-continued efforts necessary to acquire mastery over the latter lends to their co-ordination a special interest. The new-bom child brings with it into the world a certain endowment of co-ordinative powers; it has inherited, for example, from a long line of mammalian ancestors the power of performing those movements of the cheeks, lips, and tongue, on which sucking depends; perhaps from a long, though soniewhat shadowy, race of arboreal ancestors the power of clinging with hands and feet, and thus suspending itself in the air. Many move- ments, such as walking and the co-ordinated muscular contractions involved in standing, and even in sitting, which, once acquired, appear so natural and spontaneous, have to be learnt by painful effort in the hard school of (infantile) experience, and- this despite the fact that in these movements the voluntary co-ordination mechanism makes use to 60 94^ THE CENTRAL NERVOUS SYSTEM a great extent of a motor machinery already existing in the cord and capable of discharging well co-ordinated reflexes. In addition to such fundamental movements, most people consciously learn, and are willing to confess that they have learnt, to execute a considerable number of co-ordinated movements with the arms, and especially with the fingers. Some part even of the extreme dexterity of jaws, tongue, and teeth displayed by a hungry school-boy, in a minor degree, perhaps, by a hungry mouse, is the result of the much practice, entailing at first some conscious effort, which maketh perfect. The exquisite co-ordina- tion of the muscles of the eyeball, which we shall afterwards have to speak of, and the no less wonderful balance of efiort and resistance, of power put forth and work to be done, of which we have already had glimpses in studying the mechanism of voice and speech, become to a great extent the common propenty of all fully-developed persons. But the technique of the finished singer or musician, of the swordsman or acrobat, and even the operative skill of the surgeon, are in large part the outcome of a special and acquired agility of mind or body, in virtue of which highly-complicated co-ordinated movements are promptly deter- mined on and immediately executed. ,With such special and elaborate movements it is impossible to occupy ourselves in a book like this. Their number may be almost indefinitely extended, and their nature almost infinitely varied, by the needs and training of special trades and professions. It will be sufiicient for our purpose to sketch in a few words the mechanism of one or two of the most common and fundamental co-ordinations of muscular efiort, passing over the rest with the general statement that the more refined and complex movements are in general brought about, not by the abrupt contraction of crude anatomical groups of muscles, but by the contrac- tion of portions of muscles, perhaps even single fibres or small bundles of fibres, while the rest remain relaxed. The excitation may gradually wax and wane as the difierent stages of the movement require. Antago- nistic muscles may be called into play to balance and tone down a con- traction which might otherwise be too abrupt. Many interesting illustrations of this process of ' give and take ' between opposing muscles have been reported, especially by Sherring- ton. Some have been already alluded to in discussing reflex move- ments (p. 875). One or two additional observations may be given here. In the cortex cerebri, as we shall see (pp. 919, 931), there is an area in the frontal region, and another in the occipital region, stimulation of which gives rise to conjugate deviation of the eyes — that is, rotation of both eyes— to the opposite side. Sherrington divided the third and fourth cranial nerves in monkeys — say on the left side. The external rectus, which is supplied by the sixth nerve, caused now by its unopposed contraction external squiflt of the left eye. When either of the cortical areas referred to, or even the subjacent portion of the corona radiata, was stimulated on the left side, both eyes moved towards the right, the left eye, however, only reaching the middle line — ^that is, the position in which it looked straight forward. The same thing was observed when the animal, after complete recovery from the operation, was caused to voluntarily turn its eyes to the right by the sight of food. Here an inhibitory influence must have descended the fibres of the abducens, the only nervous path connected with the extrinsic muscles of the left eye, and the relaxation of the left external rectus must have kept accurate step with the contraction of the right internal rectus. Hering has made an exhaustive analysis of the co-ordinated movements concerned in opening and closing the hand in monkeys. These movements can be produced by stimulation of the cortex or the internal capsule, but nol FUNCTIONS OF THE BRAIN 947 by stimulation of the anterior spinal roots. When the hand is opened the muscles that open it are excited, and those which close it are in- hibited from the cortex. Reaction Time. — Just as in a reflex act a certain measureable time {reflex time), is taken up by the changes that occur in the lower nervous centres,^ so we may assume that in all psychical processes the element of time is involved. And, indeed, when the interval that elapses between the application of a stimulus and the signal which announces that it has been felt {reaction time) is measured, it is found that for the cerebral processes associated with the per- ception of the simplest sensation and the production of the simplest ' voluntary contraction it is longer than the time which the spinal centres require for the elaboration of even complex and co-ordinated reflex movements. Suppose, e.g., that the stimulus is an induction shock applied to a given point of the skin, and that the signal is the closing of the circuit of an electro-magnet, then, if both events are automatically recorded on a revolving drum, the interval can be readily determined. It is evident that this includes, not only the time actually consumed in the central processes, but also the time required for the afferent impulse to reach the brain, and the efferent impulse the hand, along with the latent period of the muscles. The time taken up in these three events can be approximately calculated, and when it is subtracted, the remainder represents the reduced or corrected reaction time — that is, the interval actually spent in the centres themselves. This is by no means a constant. It is in- fluenced not only by the degree of complexity of the psychical acts involved, and the mental attitude of the person (whether he expects the stimulus or is taken by surprise, whether he has to choose between several possible kinds of stimuli and respond to only one, etc.), but it varies also for different kinds of sensation, for the same sensation at different times, and, as is recognized in the personal equation of astronomers, in different individuals. For sensations of touch and pain it may be taken as one-ninth to one-fifth, for hearing one-eighth to one-sixth, and for sight one-eighth to one- fifth of a second. So that the proverbial quickness of thought is by no means great, even in comparison with that of such a gross process as the contraction of a muscle (one-tenth of a second). Nor is it the case that the man ' of quick apprehension ' has always a short reaction time, or the dullard always a long one, although in all kinds of persons practice will reduce it. Section XI. — Fatigue and Sleep — Hypnosis. Sleep and Fatigue. — Certain gland-cells, certain muscular fibres, and the epithehal cells of ciliated membranes, never rest, and perhaps hardly ever even slacken their activity. But in most 948 THE CENTRAL NERVOUS SYSTEM organs periods of action alternate at more or less frequent intervals with periods of relative repose. In all the higher animals the ceiitral nervous system enters once at least in the twenty-four hours into the condition of rest which we call sleep. What the cause of this rfegular periodicity is we do not know. It is accompanied by changes in the microscopical appearance of the nerve-cells. Thus, Hodge found differences between the cells of certain portions of the cerebral cortex in birds, and of certain ganglia in the honey-bee after a long day of work and after a night's rest. Mann, Lugaro, and other observers, found similar differences in the cells of the cerebral cortex and the anterior horh, and D&lley in the Purkinje's cells of the cerebellum in dogs fatigued by muscular exercise as compared with rested dogs (Fig. 382); According to DoUey, there is, as a result of continued activity, at first a steady increase of the basic chromatic material. This increase affects first the extra- nuclear chromatin, the Nissl sub- stance, which, according to the most modem view, is really nuclear substance distributed through the cytoplasm, and func- tions as such (Goldschmidt). The size and number of the gramiles are increased, and some of the chromatic material is diffused Fig. 382.-Effect of Fatigue on Nerve-Cells throughout the cytoplasm, as in- (Barker, after Mann): Two motor cells dicated by diffuse stammg. Tlien from lumbar cord of dog fixed in sublimate ^he intranuclear chromatm also and stained with toluidin blue, a, from undergoes an increase, and the rested dog; i, pale nucleus; 2, dark Nissl size of the cell is increased too. spindles ; 3, bundles of nerve fibrils. In moderate activity the change 6, from the fatigued dog; 4, dark shrivelled goes no farther: At this stage nucleus; 5, pale spindles. .^jie cell is hyperchromatic — i.e., as compared .with a normal resting cell it contains an excess of chromatin. The production of chromatin having reached the maximum of which the nucleus is capable, and functional activity, which entails the using up of the extranuclear chromatin, still continuing, the total chromatin content begins to diminish, first in the nucleus, through the passage of its chromatin into the cytoplasm to recruit the Nissl substance, then in the C5rtoplasm as well. Accompanying the disappearance of the chromatic material there is diminution in the size bf both cell and nucleus, but especially of the nucleus, so that tlie normal proportion between volume of cell and volume of. nucleus (nucleus-plasma relation of Hertwig) is disturbed in favour of the cytoplasm. Both cell and FATIGUE AND SLEEP— HYPNOSIS 949 nucleus become irregular in outline or crenated. Later on, and, it would seem, rather abruptly, swelling of the nucleus and, after some time, of the cytoplasm occurs. This is due to oedema, and may be taken to indicate an upset of their normal osmotic relations. The earlier occur- rence of oedema in the nucleus leads to another change in the nucleus- plasma relation, which is now disturbed in favour of the nucleus. In the measure in which fatigue progresses the extranuclear chromatic material . contmues to be used up, and, in spite of its replenishment from the nucleus, it almost or entirely vanishes from the cytoplasm. Then follows what is perhaps a ' last effort ' on the part of the nucleus to supply the cytoplasm, in the form of a discharge of chromatic substance, which first masses itself around the outside of the nuclear membrane, and thence gradually diffuses into the cytoplasm. With the using up of this supply all the basic chromatic material of the cell, except that in the karyosome (nucleolus), is exhausted. Finally, this too is yielded up to the cytoplasm, and with its consumption there remains a totally exhausted cell, devoid of basic chromatin and incapable of recuperation. According to Pugnet, even in extreme fatigue, as when dogs were caused to run forty to nearly sixty miles in a special apparatus, the changes varied greatly in degree in different cortical cells, from mere diminution of the chromatic substance to complete disappearance of it, and such disintegration pi the cell as must have precluded its recovery, had the animal been allowed to live. Many, and indeed most, of the cortical cells were quite unaffected. Histological alterations may also be caused in sympathetic ganglion cells by prolonged artificial stimula- tion of the nerves connected with the ganglia. Experiments on fatigue changes in the cells of the spinal ganglia after electrical excitation of the posterior root-fibres are less decisive, some observers having obtained positive, others negative, results (p. 885). Theories of the Causation of Sleep. — (i) Some have suggested that sleep is iriduced by the using up of substances necessary for the functional activity of the neurons — e.g., the stored-up or intramolecular oxygen — or by the action of the waste products of the tissues, and especially lactic acid, when they accumulate beyond a certain amount in the blood, or in the nervous elements themselves. (2) Others have looked for an explanation to vascular changes in the brain, but so far are the possible causes of such changes from being understood, that it is even yet a question whether in sleep the brain is congested or anaemic. Certain writers have settled this question by the summary statement that when the brain rests the quantity of blood in it must be supposed to be diminished, as in other resting organs. But this is a fallacious argument. For when the whole body rests, as it does in sleep, it has as much blood in it as when it works ; in sleep, therefore, if some resting organs have less blood than in waking life, other resting organs must have more ; and it is the province of experiment to decide ' which are congested and which anjemic. In coma, a pathological con- dition which in some respects has analogies to profound and long- continued sleep, the brain is congested, and the proper elements of the nervous tissue presumably compressed. And artificial pressure (applied by means of a distensible bag introduced through a trephine hole into the cranial cavity) may cause not only unconsciousness, but absolute anaesthesia. But it is possible that this artificial increase of intra- cranial pressure may produce its effects by rendering the brain anaemic, and it has been actually observed that the retinal vessels, as seen with the ophthalmoscope, and the vessels of the pia mater exposed to direct observation in man by disease of the bones of the skull, or in animals by operation, shrink during sleep. Statements to the contrary may be due 950 . THE CENTRAL NERVOUS SYSTEM to neglecting the influence of difference of position in the sleeping and waking states. Iii sleeping children the fontanelle sinks in, an indica- tion that the intracranial pressure is rednced. Observations with the plethysmograph have shown that the arm swells in sleep, and shrinks when the sleeper awakes, or even when he is subjected to sensory stimuli not sufficient to arouse him — e.g., a tune played by a musical-box (Howell). The tone of the vaso-motor centre is therefore diminished, and the arterial pressure falls during sleep. But a fall of general arterial pressure is usually accompanied by a diminution of the quantity . of blood passing through the brain. So that the balance of evidence is in favour of the view that sleep is associated with a certain degree of cerebral ancsmia. As to the nature of the relation between the two conditions, it has been suggested that the anaemia is produced by fatigue of the vaso- motor centre, which causes it to relax its grip upon the peripheral blood- vessels, and that the condition of the cortical nerve-cells which we call sleep is directly produced by the lack of blood. But there does not appear to be any good reason for believing that the vaso-motor centre is more susceptible of fatigue than the higher cerebral centres. On the contrary, it is probable that the bulbar centres are less delicately- organized and more resistant than the higher centres. In any case, if the cerebral nerve-cells ' go to sleep ' because their blood-supply is diminished, ought we not to look for a similar cause for diminished activity of the vaso-motor centre ? Or if the answer is made that the activity of the vaso-motor cells is directly lessened by fatigue, or by the cessation of external stimuli, why should not this be the case also for the cortical cells ? It can be shown by means of the sphygmomanometer (p. 114) that the fall of arterial pressure is not essentially connected with sleep, but is produced by the bodily rest and' warmth which accompany it. Further, even a great diminution in the supply of blood going to the brain is not necessarily followed bysleep. For example, both carotids and both vertebral arteries may frequently be tied in dogs at the same time without producing any symptoms, the anastomosis of the superior intercostal arteries with 'the anterior spinal artery providing a sufficient channel for the blood absolutely required by the brain. Monkeys after ligation of both carotids may be most alert and active. To produce sopor in animals the cortical circulation must be reduced almost to the vanishing-point, and to a far greater degree than ever occurs in sleep (Hill). We must, therefore, conclude that although sleep is normally a,ssociated with some ancsmia of the brain, it is not directly caused by it. The cortical centres go to sleep because they are ' tired,' or because the stimuli which usually excite them have ceased, arid not because their blood-supply is diminished. (3) The idea that thedendrites are contractile, and by pulling them- selves in, and thus breaking certain nervous chains, cause sleep, is a mere theory, unsupported by any real evidence. The same is true of the notion that the fibrils of the neuroglia insinuate themselves into the 'joints,' by which one neuron comes into contact with another, and, acting as insulating material, block the nerve-impulses. In general, the depth of sleep, as measured by the intensity of sound needed to awaken the sleeper, increases rapidly in the first hour, falls abruptly in the second, and then slowly creeps down to its minimum, which it reaches just before the person awakens. As to the amount of sleep required, no precise rules can be laid down. It varies with age, occupation, and perhaps climate. An infant, whose main business is to grow, spends, or ought to spend, if mothers were wise and feeding-bottles clean, the greater part of its time in sleep. The man, whdse main FATIGUE AND SLEEP—HYPNOSIS 95! business it is to work with his hands or brain, requires his full tale of eight hours' sleep, but not usually more. The dry and exhilarating air of some of the inland portions of North America, and perhaps the plains of Victoria and New South Wales, incites, and possibly enables a new- comer to live for a considerable period with less than his ordinary amount of sleep. Idiosyncrasy, and perhaps to a still greater extent habit, have also a marked influence. The great Napoleon, in his heyday, never slept more than four or five hours in the twenty-four. Five or six hours or less was the usual allowance of Frederick of Prussia throughout the greater part of his long and active life. Hypnosis is a condition in some respects allied to natural slumber; but instead of the activity of the whole brain — or perhaps we should rather say, the whole activity of the brain — ^being in abeyance, the susceptibility to external impressions remains as great as in waking life, or may be even increased, while the critical faculty, which normally sits in judgment on them, is lulled to sleep. The condition can be induced in many ways — by asking the subject to look fixedly at a bright object, by closing his eyes, by occupying his attention, by a sudden loud sound or a flash of light, etc. The essential condition is that the person should have the idea of going to sleep, and that he should surrender his . will to the operator. In the hypnotic condition the subject is extremely open to suggestions made by the operator with whom he is en rapport. He adopts and acts upon them without criticism. If, for example, the hypnotizer raises the subject's arm above his head, and suggests that he cannot bring it down again, it stays fixed in that position for a long time without any appearance of fatigue; or the whole body may be thrown, on a mere hint, into some unnatural pose, in which it remains rigid as a statue. Suggested hemiplegia or hemianaesthesia, or paralysis of motion and sensation together or apart in limited areas, can also be realized; and surgical operations have been actually performed on hypnotized persons without any appearance of suffering. If, on the other hand, the operator suggests that the subject is undergoing intense pain, he will instantly take his cue, writhing his body, pressing his hands upon his head or breast, and in all respects behaving as if the suggestion were in accord with the facts. If he is told that he is blind or deaf, he will act as if this were the case. If it is suggested that a person actually present is in Timbuctoo, the subject will entirely ignore him, will leave him out if told to count the persons in the room, or try to walk through him if asked to move in that direction. "What is even more curious is that the organic functions of the body are also liable to be influenced by suggestion. A postage-stamp was placed on the skin of a hypnotized person, and it was suggested that it would raise a blister. Next day a blister was actually found beneath it. The letter K, embroidered on a piece of cloth, was suggested to be red-hot. The left shoulder was then ' branded ' with it, and on the right shoulder appeared a facsimile of the K as if burnt with a hot iron. The secretions can be increased or diminished, subcutaneous haemorrhages, veritable stigmata,* can be caused, and many of the ' miracles ' of Lourdes and other shrines, ancient and modem, repeated or surpassed by the aid of hypnotic suggestion. Hypnotism has also been practically employed in the treatment of various diseases, and particularly in functional derangements of the * I.e., bleeding spots on the skin generally corresponding to the wounds of Christ. In the well-known case of Louise Latour, which excited great interest in France in 1868, blisters first appeared; they burst, and then there was bleeding from the true skin. The probable explanation is that she con- centrated her attention on these parts of her body and so influenced them, perhaps by causing congestion through the vaso-motor centre. 952 THE CENTRAL NERVOUS SYSTEM nervous system. But care and judgment are necessary on the part of the operator, and although as a rule there is no difficulty in putting an end to the condition by a suitable suggestion, it is said that in rare instances grave mischances have occurred. There seems to be no ground for the opinion that women are more easily hypnotized than men. Out of more than a thousand persons, Liebault found only seventeen abso- lutely refractory. Section XII. — Size of Brain and Intelligence — Circulation IN AND Resuscitation of Central Nervous System after Anemia — Chemistry of Nervous Activity — Cerebro-spinal Fluid. Relation of Size of Brain to Intelligence. — ^While it is the case that some men of great ability have had remarkably heavy and richly convoluted brains, it would seem that in general neither great size nor any other obvious anatomical peculiarity of the cerebrum is constantly associated with exceptional intellectual power. In the animal kingdom, as a whole, there is undoubtedly some relation between the status of a group and the average brain development within, the group. But that this is a relation which is complicated by other circumstances than the mere degree of intelligence is sufficiently shown by the fact that a mouse has more brain, in pro- portion to its size, than a man, and thirteen times more than a horse ; while both in the rabbit and sheep the ratio of brain-weight to body- weight is nearly twice as great as in the horse, in the dog only half as great as in the cat, and not very much more than in the donkey. The following tables, too, which illustrate the weight of the brain in man at different ages, show that, although we might give ' the infant phenomenon ' an anatomical basis, we should greatly over- rate the intellectual acuteness of the average baby if wje were to measure it by the ratio of brain to body- weight alone. Age. Brain-weight. Age. Brain-weight. I year 885 grui. 8 years 1,045 g""- 2 years 909 ,, 10 1,315 ., 3 1,071 ,, II I,l6» ,, 4 1,099 „ 12 1,286 ,, 5 1,033 „ ! 13 1,505 .■ 6 1,147 ,, 14 1,336 ,, 7 1,201 ,, i 15 1,414 ■• — BiscHorr. Brain-weight- Brain-weight — Brain-we ght- Brain-weight— Age. Men. Women. Age. Men Women. 10-19 1,411 grm . . 1,219 grin. 50 -59 ■ • 1,389 i ;rm . . . 1,239 grm. 20-29 1,419 ., . . 1,260 ,, 60 -69 ■ ■ 1,292 , , . . 1,219 ,, 30-39 1,424 ,, . . 1,272 ,, 70 -79 •• 1,254 ,, .. 1,129 ,, 40-49 1,406 ,, ■ • 1,272 ,, 80- -90 • ■ 1.303 " . . 898 „ HuSCHKE. THE CEREBRAL CIRCULATION 953 In some small birds the ratio is as high as i : 12, in large birds as low as i : 1,200 ; in certain fishes a gramme of brain has to serve for over 5 kilos of body. As a rule, especially within a given species, the brain is proportionally of greater size in small than in large animals. It is to be supposed that quality as well as quantity of brain substance is a potent factor in determining the degree of mental capacity. The Cerebral Circulation. — ^The arrangement of the cerebral blood- vessels has certain peculiarities which it is of importance to remember in connection with the study of the diseases of the brain, many of which are caused by lesions in the vascular system — haemorrhage or embolism. Four great arterial trunks carry blood to the brain.two internal carotids and two vertebrals. The vertebrals unite at the base of the skull to form the single mesial basilar artery, which, running forward in a groove in the occipital bone, splits into the two posterior cerebral arteries. Each carotid, passing in through the ^rotid foramen, divides into a middle and an anterior cerebral arteryj the latter runs forward in the great longitudinal fissure, the former lies in the fissure of Sylvius. A communicating branch joins the middle and posterior cerebrals on each side, and a short loop connects the two anterior cerebrals in front. In this way a hexagon is formed at the base of the brain, the so-called circle of Willis. While the anastomosis between the large arteries is thus very free, the opposite is true of their branches. All the arteries in the substance of the brain and cord are ' end-arteries ' — ^that is to say, each terminates within its area of distribution without sending communicating branches to make junction with its neighbours. The - consequence of these two anatomical facts is: (i) that interference with the blood-supply of the brain between the heart and the circle of Willis does not readily produce symptoms of cerebral ansmia; (2) that the blocking of any of the arteries which arise from the circle or any of their branches leads to destruction of the area supplied by it. Nearly all dogs recover after ligation in one operation of both carotids and both vertebral arteries. In monkeys both carotids may, as a rule, be safely tied, and one carotid in man. If, in addition to the two carotids, one vertebral be ligated at the same time in the monkey, sopor results, and this is generally followed by extensor rigidity, coma, and death in twenty-four hours. In one case a monkey survived this triple ligation, but became demented. The motor paralysis and rigidity were much greater than in the dog. In the condition of partial anaemia the cortex is more excitable than normal, but the excitability disappears at once when the anaemia is rendered complete (Hill). The basal ganglia are fed by twigs from the circle of WiUis and the beginning of the posterior, middle, and anterior cerebral arteries. Of these the most important are the lenticulo-striate and lenticulo-optic branches of the middle cerebral, which are given off near its origin, and run through the lenticular nucleus into the internal capsule, and thence to the caudate nucleus and optic thalamus respectively. The chief part of the blood from the circle of Willis is carried by the three great cerebral arteries over the cortex of the brain. The white matter, with the exception of that in the immediate neighbourhood of the basal ganglia, is nourished by straight arteries which penetrate the cortex. The middle cerebral supplies the whole of the parietal as well as that portion of the frontal lobe which lies immediately in front of the fissure of Rolando and the upper part of the temporal lobe. The rest of the 954 THE CENTRAL NERi^OUS SYSTEM frontal lobe is supplied by the anterior cerebral, and the occipital lobe, with the lower part of the temporal lobe, by the posterior cerebral. The medulla oblongata, cerebellum, and pons are -fed from the verte- brals and the basilar artery before the circle of Willis has been formed. Resuscitation of the Central Nervoias System after Total Anaemia. — Complete temporary anaemia of the brain and upper cervical cord can be produced in most cats by passing temporary ligatures around the innominate artery and left subclavian proximal to the origin of the vertebral artery. Artificial respiration is main- tained through a tube passed through the glottis. The eye reflexes disappear very quickly, and a period of high blood-pressure imme- diately follows the occlusion. A fall of pressure succeeds, due to vagus inhibition of the heart, and this is followed by a second rise after the vagus centre succumbs to the ansemia. Respiration stops temporarily (in twenty to sixty seconds) after occlusion; then follows a series of strong gasps, and finally cessation of all respiratory movements. The blood-pressure slowly falls to a level which is then maintained approximately constant for the remainder of the occlu- sion period. The anterior part of the cord and the encephalon lose all function ; no reflexes can be elicited from this part of the central nervous system. The intra-ocular tension is much reduced, and the cornea is characteristically wrinkled. When the cerebral circulation is restored by releasing the vessels, the general arterial pressure soon begins to rise if the period of occlusion has not overstepped the limit of successful cardio-vascular resuscitation. The respiration returns suddenly, the time of return depending on the length of the occlusion and on other factors. The respiratory rate, at first slow, soon becomes normal, and then more rapid than normal. The eye-reflexes reappear more gradu- ally; the intra-ocular tension increases, and the shrunken cornea becomes smooth and hard. The anterior part of the cord recovers its functions gradually; the reflexes connected with it return, first the homonymous, then the crossed. A short period of quiet follows ; then spasms of the skeletal muscles appear, gradually increase in severity and extent, and terminate in (a) death, (b) partial, or (c) complete recovery. In partial recovery, disturbances of loco- motion, such as walking in a circle, paralysis, apparent dementia or loss of intelligence, and loss of sight or hearing, may be observed. Voluntary movements of the head, neck, shoulders, and fore-limbs have been seen eight minutes after release from an occlusion of six minutes. Blindness has been observed without loss of the pupillary light reflex. In this case the visual cortex would seem to have suffered more than the lower centres, an illustration of a general rule. Complete recovery is rare after total anaemia lasting as much as fifteen minutes, and has not been observed after an anaemia of twenty minutes. Ten to fifteen minutes of total anaemia represent CHEMISTRY OF NERVOUS ACTIVITY 955 the limit beyond which recovery of the brain, and therefore successful resuscitation of the animal, cannot be expected. Chemistry of Nervous Activity. — Of this we are practically ignorant. The percentage composition of the solids and the percentage of water in the brains of three persons of different ages are ex- hibited in the following table (W. Koch) : Child 6 Wfeeks (Brain 64a Grms.). Child 2 Years (Brain 1,100 Grms.). Adult 19 Years (Brain 1,670 Grms.), Whole Brain, Grey, White. Whole Brain,* Grey. White. Whole Brain, t Proteins Extractives . . Ash . . Lecithins and kephalins . . Cerebrins Lipoid S as SO4 Cholesterint 46-6 I2'0 8-3 24-2 69 O'l 1-9 48-4 lO-O 5-8 24-7 8-6 O-I 2-4 31-9 5-9 3-2 26-3 17-2 0-5 15-0 40-1 8-0 4-5 25-5 12-9 0-3 8-7 47-1 9-5 5-9 23-7 8-8 O-I . 4-9 27-1 3-9 2-4 31-0 16-6 0-5 i8-5 37-1 6-7 4-1 27-3 12-7 0-3 II-7 76-42 Water . . 88-78 84-49 76-45 80-47 83-17 69-67 The next table shows the variations in the content of water, solids, and protein in different parts of the nervous system (Halli- burton) : Water. Solids. Percentage of Pro- teins in Solids. Cerebral grey matter Cerebral white matter Cerebellum Spinal cord as a whole . Cervical cord Dorsal cord Lumbar cord Sciatic nerves 83-5 69-9 79-8 71-6 72-5 69-8 72-6 65.r 16-5 30-I 20-2 28-4 27-5 30-2 27-4 34-9 51 33 42 31 31 28 33 29 The grey matter of the cerebrum in the adult contains 8i to 86 per cent, of water, the white matter 68 to 72 per cent., the brain as a whole 81 per cent., the spinal cord 68 to 76 per cent,, and the peripheral nerves 57 to 64 per cent. In the foetus more water is present (92 per cent, in the grey and 87 per cent, in the white matter). The superior richness of the grey matter in proteins and the preponderance of water in it are the chief chemical peculiarities which distinguish it from the white matter. That it should have * Calculated. f Calculated by difference. 956 THE CENTRAL NERVOUS SYSTEM a high protein content is easily understood, for the protoplasmic structures, the nerve-cells, are situated in the grey matter. But that the most important functions should have their seat in a tissue containing only 14 to 19 per cent, of solids is surprising, and should warn us that the water is no less significant a constituent of living matter than the solids, and that it is not the mass of the sohd substances in a tissue which is the essential thing, but the whole colloid complex, which cannot be constituted without the water. Fresh nervous tissues are alkaline to litmus, but become acid soon after death. No change of reaction has been detected during activity. That oxygen is used up during cerebral activity is certain, and when the brain is coloured with methylene blue, by injecting it into the circulation, any spot of it which is stimulated loses the blue colour, the pigment being reduced. But if the animal is so deeply narcotized that it does not respond to stimulation, the change of colour does not occur. Gholin (p. 360), a substance which can be derived from lecithin, is believed to represent one of the waste products of nervous activity. Exceedingly small traces of it are present in normal cerebro-spinal fluid, and in certain diseased conditions of the brain, as in general paralysis, the quantity is said to be notably increased, indicating an increased decomposition of lecithin. The fatty acid constituent of lecithin is liberated in degenerating nerve, giving rise to the reaction with osmic acid (p. 771). Some writers assert that this "increase in the cholin can be used as a test to distinguish organic nervous disease from that which is purely functional. But the matter is in dispute. Cerebro-spinal Fluid.^ — ^The cerebro-spinal fluid, which fills the ventricles of the brain and the central canal of the cord, is con- tinuous with that contained in the subarachnoid space through the foramen of Magendie, an opening in the piece of pia mater that helps to roof in the fourth ventricle. It is secreted in part by the cubical cells covering the choroid plexus, a fold of pia mater which projects into each lateral ventricle. Extracts of choroid plexus, when in- jected intravenously, increase the rate of secretion. This action is dependent upon the presence of some substance in the choroid plexus, which, however, is not a specific product of the activity of the plexus, since extracts of the brain produce the same effect. It may therefore be some product of the metabolism of the brain which passes to the choroid plexus and stimulates secretion by the epithelium. The substance is removed from the fluid by filtration through a Chamberland filter, and is therefore probably of high molecular weight. It is probable that variations in the rate of secretion of the cerebro-spinal'fluid by the choroid plexus are more influential in governing the intracranial pressure than variations in PRACTICAL EXERCISES 957 the arterial and venous pressures. The idea that the cranial contents constitute a fixed quantity, without the power of contraction or expansion, can no longer be maintained (Dixon and Halliburton); Cerebro-spinal fluid can easily be obtained in man by lumbar puncture with a hypodermic needle sufficiently long to enter the subarachnoid space in the spinal canal. The point usually selected for the puncture is between the fourth and fifth lumbar vertebrae. The normal pressure of the fluid is such that it trickles out by dropsj but in disease it is sometimes so high that it spurts out in a steady stream. An examination of the fluid, especially for leucocytes or bacteria, is of great diagnostic value in certain conditions. Nor- mally it is a thin, clear, watery fluid; faintly alkaline in reaction .to litmus, and with a specific gravity of about 1O04 to 1007. It contains the ordinary salts, but more potassium than sodium, unlike other body fluids; a very small amount of protein (globulin) — usually about o-i per cent.^ — ^and a little dextrose (Nawratzki). Its composition is evidently different from that of ordinary lymph: Only a few lymphocytes are present in health, but in some diseases (as in general paralysis of the insane, tabes, and cerebro-spinal sj^hilis) a marked increase occurs. In acute cerebro-spinal menin- gitis numerous polymorphonuclear leucocytes are found, which are absent from the normal fluid. The depression of the freezing-point (A) usually lies between -o-eo" and 0-65° C. In a case of hydrocephalus it was -0-65° C Normally, cerebro-spinal fluid is somewhat hypertonic to the blood- serum. In injury of the cribriform plate of the ethmoid bone and also in some cases where there is no traumatic injury, the fluid escapes from the nose; and the rate of its formation can thus be ascertained. In one case it was found to be as much as 2 c.c. to nearly 4 c.c. in ten minutes. PRACTICAL EXERCISES ON CHAPTER XVI. I. Section and Stimulation of the Spinal Nerve-Roots in the Frog.— (a) Select a large frog (a bull-frog, if possible). Pith the brain. Fasten the frog, belly down, on a plate of cork. Make an incision m the middle line over the spinous processes of the lowest three or four vertebrae; separate the muscles from the vertebral arches, and with strong scissors open the spinal canal, taking care not to injure the cord by passing the blade of the scissors too deeply. Extend the opening upwards till two or three posterior roots come into view. Pass fine silk hgatures under two of them, tie, and divide one- root central to the ligature, the other peripheral to it. Stimulate the central end, and reflex movements will occur. Stimulate the peripheral end: no effect is produced. Now cut away the exposed posterior roots and isolate and ligature two of the anterior roots, which are smaller than the posterior. Stimulate the central end of one : there is no effect; Stimulation of the peripheral end of the other causes contractions of the corresponding muscles. ._ (6) Stimulation of the roots may be repeated on the mammal; usmg 958 THE CENTRAL NERVOUS SYSTEM the dog employed for the experiment on the motor areas (p. 962). Place the animal, belly down, and insert a good-sized block of wood between it and the board at the level of the lumbar vertebrae of the spine. Divide the skin and muscles on either side of this region till the laminae of the vertebrae are exposed. Snip through them with strong forceps, and open the spinal canal, exposing a length of cord correspond- ing to three or four vertebrae. Ligate and stimulate the roots as in (a); 2. Reflex Action in the ' Spinal ' Frog. — Pith the brain of a frog, destroying it down to the posterior third of the medulla oblongata, (i) Note the position of the limbs immediately after the operation, and again thirty to forty minutes later. Its hind-legs possess tone, and are drawn up against the flanks. The animal can still execute certain co-ordinated movements — e.g., pulling away its leg if a toe is pinched. The power of maintaining equilibrium is lost. If placed on its back, it lies there. When thrown into water it sinks usually without any attempt at swimming. Verify the following facts, using mechanical stimulation (pinching the toes or skin of the leg) : (a) If the stimulus provokes muscular movements only on one side of the body, this is usually on the same sid^ as the stimulated point. (6) When the stimulus causes reflex movements on both sides of the body, the stronger con- tractions are on the side to which the stimulus was applied. Determine whether it is easier to obtain movement of a portion of the body innervated from a region of the cord above the level of the stimu- lated nerves or below that level. (2) With electrical stimuli (using a coil arranged for single shocks de- termine if reflex movements are elicited by a single induced shock ap- plied to the skin. Verify the fact that a series of shocks is more ef&cient, the effects of the separate stimuli being summated in the reflex centres. (3) To test the effect of thermal stimuli, dip the leg into a beaker of warm water. Vary the temperature of the water, using a series of beakers with water at 10° C, 15° C, 20° C, etc., above the temperature of the room. Place the leg for a moment in each, and determine which is the most ef&cient stimulus. Immediately on withdrawing the leg from each of the hot-water beakers immerse it in a beaker of water at room tem- perature. Finally, dip the leg into a beaker of cold water, and heat it gradually to a temperature at which a reflex was previously obtained. Probably it will not be elicited by the gradual warming. (4) ' Purposive ' Movements. — Touch the skin of one thigh with blot- ting-paper soaked in strong acetic acid. The leg is drawn up, and the foot moved as if to get rid of the irritant. If the leg is held, the other is brought into action. Immerse the frog in water to wash away the acid. (5) Spread {Irradiation) of Reflexes. — Gently stimulate a toe or a small spot on the flank with weak induction shocks or weak mechanical stimuli, and note the reflex effect obtained. Then go on gradually increasing the strength of stimulation without increasing the area of the field stimulated, and observe the extent and order of spread of the reflex movements. 3. Reflex Time. — Pass a hook through the jaws. Holding the frog by the hook, dip one leg into a dilute solution of sulphuric acid (o'2 to 0'5 per cent.), and note with the stop-watch the interval which elapses bsfore the frog draws up its leg (Tiirck's method of determining the reflex time). Wash the acid off with water. Determine how the reflex time varies with the strength of the stimulus. This can be done by using various strengths of acid. The reflex time will be shorter the stronger the stimulus up to a certain point. Compare the reflex time of movements on the same side of the body as the point of application of the stimulus and on the opposite side. PRACTICAL EXERCISES 959 4. Inhibition of the Reflexes.— (i) Destroy the cerebrum of a frog. IJip one leg into dUute sulphuric acid as in 3, and estimate the reflex time Then apply a crystal of common salt to the upper part of the spinal cord . If the opening made for pithing the frog is not large enough to enable the cord to be clearly seen, enlarge it. Again dip the leg in the dilute acid . It will either not be drawn up at all, or the interval will be distinctly longer than before. ' | (2) Expose the viscera, including the heart, taking care not to injure the cardiac nerves. Tap the intestines sharply with the handle of a scalpel many times m succession. The heart is inhibited. (3) Tie strings tightly around both fore-lsgs of a normal frog. Place the animal on its back; it does not turn over. The hind-legs may be pulled about in various ways without the frog turning over into its normal position. The reactions concerned in the maintenance of equilibrium are inhibited. Remove the strings. The animal cannot be made to lie on its back except by force. 5. Spinal Cord and Muscular Tonus. — Destroy the brain of a frog. Isolate the gastrocnemius, and cut away the bone below the knee. Isolate the sciatic nerve without injuring it. Remove the muscles from the femur, cut the bone and fix it in a clamp for graphic recording. Connect the tendon with a lever, weighted with 5 to 10 grammes. Take a base line. Destroy the spinal cord, or cut the sciatic and again take a base line. The length of the muscle is slightly altered. 6. Spinal Cord and Tonus of the Bloodvessels. — Destroy the brain of a frog. Arrange the web of the foot on the stage of a microscope, and note the calibre of the bloodvessels in the field. Destroy the cord, and observe the change in their calibre. They will dilate. 7. Action of Strychnine. — Pith a frog (brain only). Inject into one of the lymph-sacs three or four drops of a o'l per cent, solution of strych- nine. In a few minutes general spasms come on, which have inter- missions, but are excited by the slightest stimulus. The extensor muscles of the trunk and limbs overcome the flexors. Destroy the spinal cord; the spasms at once cease, and cannot again be excited. 8. Mammalian Spinal Preparation (Sherrington).* — Deeply anses- thetize a cat with ether. Insert a cannula into the trachea (p. 200), and continue the anassthesia through this. Expose and ligate both common carotids. Make a transverse incision through the skin over the occiput, and extend it laterally behind the ears. Pull back the skin so as to expose the neck muscles at the level of the axis vertebra. Feel for the ends of the transverse processes of the atlas, and divide the muscles down to the bone just behind these processes. Now start artificial respiration (p. 200), or sooner if necessary. Notch the spinous process of the axis with bone forceps. Pass a strong thick ligature by a sharp- ended aneurism needle close under the body of the axis, and tie it tightly in the groove left by the incision behind the transverse processes of the atlas and the notch made in the spinous process of the axis. This com- presses the vertebral arteries where they pass from transverse process of axis to transverse process of atlas. Pass a second strong ligature * A similar preparation can be used for certain experiments on the circu- lation (Crile, Guthrie). For these, as well as for the study of many reflexes, a good preparation is obtained by occlusion of the cerebral blood-supply in cats (without decapitation) . Even a human ' spinal preparation ' is capable of executing reflex movements. The Turkomans are stated to have decapitated their prisoners and immediately placed on the neck a hot metal plate, whxh sealed up the cut vessels. The (reflex) movements, which are described as very lively, were then watched with an interest, it is to be supposed, not wholly scientific 96o THE CENTRAL NERVOUS SYSTEM under, the trachea at the level of the cricoid cartilage and include in it the whole neck, except the trachea, but at present only tie a single loop on it. Now decapitate the animal with a large knife (an amputating knife) passed from the ventral aspect of the neck through the occipito- atlantal space, severing the cord just behind its junction with the bulb. At the moment of decapitation tighten the ligature round the neck and complete the knot. Destroy the head. If there is oozing of blood from the vertebral canal; arrest it by raising the neck somewhat above the level of the body. The carcass must be kept warm by placing it on a metal box or table containing hot water, and the air used for artificial respiration must also be warmed, as by passing it through a coil of rubber tubing immersed in a water-bath which is kept hot. Stitch the skin-flaps together so as to cover the cut end of the spinal cord and the other structures cut in decapitation. By this procedure the spinal cord is usually severed about 4 millimetres behind the point of the calamus scriptorius. Although the blood-pressure remains low, reflexes employ- ing the skeletal muscles can be fairly well elicited for hours. Study on the preparation the reflexes described in the text (pp. 873, 876) — e.g., the flexion reflex of the hind and fore limb, as elicited from the skin, or one of the afierent nerves of the limb— the crossed extension reflex of hind and fore limb, the scratch reflex. (i) Scratch Reflex. — (a) Evoke the reflex by rubbing the skin of the neck behind the pinna. The scratching movements are in the hind-leg of the same side. Record them on a drum, on which is also written a time-tracing in seconds. The record can be obtained by tying a piece of tape, not too tightly, round the foot, leg, or thigh, and connecting this by a thread with a lever. The thread is passed over a pulley below the lever, so that its pull may be exerted at right angles to the axis of rotation of the lever. The lever is attached to a light spring or a rubber band, which is stretched when it moves in one direction, and in recoiling brings it back again to its position of rest at the end of the contraction. If the reflex is not easily evoked, it can be facilitated by producing a slight degree of asphyxia by temporarily clamping the respiration tube. Some time must elapse after the decapitation before a fair scratch reflex can be expected. It is usually sufficiently well marked within an hour. (6) While the reflex is occurring, stimulate with an interrupted current the central stump of the popliteal nerve of the opposite hind-limb. The scratch reflex may be cut short by inhibition. Also, during the stimulation of this nerve the reflex may be incapable of being elicited till the excitation of the inhibitory afferent nerve is stopped. , (2) Flexion Reflex. — (a) Stimulate with a weak interrupted current the skin of some part of the hind-limb— rsay one of the toes. The flexion reflex of the hind-limb on the same side may be evoked — i.e., a flexion niovement at the knee, hip, and ankle. Record the movements of one af the joints or of flexor muscles after severing them from their insertion. (6) Stimulate with a weak interrupted (faradic) current the central stump of one of the nerves of a hind-limb — say the peroneal nerve. The flexion reflex of the same limb may be elicited. Record the move- ments. Now produce temporary asphyxia by clamping the respiration bube, and repeat the stimulation at half -minute intervals. The reflex ivill be increased by the asphyxia. Do not interrupt the respiration for nore than two or three minutes, and immediately start it if the heart, vhich can be felt through the chest, begins to weaken. (3) Elicit the knee-jerk, as described in the text (p. 876). It is generally exaggerated. (4) By the unipolar method (p. 918) stimulate with a point electrode )ne lateral half of the cross-section of the cervical cord exposed in PRACTICAL EXERCISES 961 decapitation. The large electrode is placed on a shaved part of a fore- arm. Various effects may be elicited according to the point of the cross- section stimulated — e.g., stepping and scratch movements of the hind- hmbs. Other facts mentioned in the text in regard to spinal reflexes can be verified on this preparation. 9. Reflexes in Man. — Study systematically on a fellow-student and on yourself the chief reflexes described in the text (p. 886), especially The Knee-jerk. — (i) Elicit the jerk in the usual way by striking the ligamentum patellse and observe its height. Then cause the patient to make a strong voluntary movement (squeezing the hands together or clenching the jaws) at the moment when the tendon is struck, and note whether the height is increased by ' reinforcement.' (2) Attach a suitable recording apparatus to the foot of a person sitting with his legs over the edge of a table, and record the jerks elicited by taps made as uniform in strength as possible. A small hammer worked by an electro-magnet or a spring might be employed for this purpose. Com- pare the records obtained when the jerk is elicited while the person is squeezing his hands together with those previously obtained. The influence of mental activity, especially of excitement or irri- tation (opportunities of studying such psychical states occasionally offer themselves in physio- logical laboratories) in increasing the height of the knee-jerk may also be verified (Lombard). 10. Excision of Cerebral Hemispheres in the Frog (Fig. 383). — Ansesthetize a frog lightly by putting it under a bell-jar or tumbler with a small piece of cotton-wool soaked in ether. Put very little ether on the cotton, and leave the frog only a very short time under the bell-jar. Then, holding it in a cloth, make an incision through the skin over the skull in the mesial line. With scissors open the cranium about the position of a line drawn at a tangent to the posterior borders of the two tympanic membranes. Remove the roof of the skull in front of this line with forceps, scoop out the cerebral hemispheres, and sew up the wound. As soon as the animal has recovered from the ether, the phenomena described at p. 915 should be verified. The frog will swim when thrown into water, will refuse to lie on its back, and will not fall if the board on which it lies be gradually slanted. Let the frog live for a day, keeping it in a moist atmosphere; then expose the brain again, determine the reflex time by Turck's method ; apply a crystal of common salt to the optic lobes, and repeat the observa- tion. The reflex movements will be completely inhibited or delayed. Remove the salt, wash with physiological salt solution, excise the optic lobes, and see whether the frog will now swim. 11. Excision of the Cerebral Hemispheres in a Pigeon. — Feed a pigeon for two or three days on dry food, etherize it by holding a piece of cotton-wool sprinkled with ether over its beak, or inject into the rectum I gramme chloral hydrate. The pigeon being wrapped up in a cloth, and the head held steady by an assistant, the feathers are clipped off the 61 Fig. 383. — Brain of Frog (after Steiner). a, cere- bral hemispheres; 6, position of optic thala- mi; c optic lobes; d, cerebellum ; e, medulla oblongata ; A , upper end of spinal cord. 962 THE CENTRAL NERVOUS SYSTEM head, an incision made in the middle line through the skin, and the flaps reflected so as to expose the skull. Cut through the bones with scissors, and make a sufiiciently large opening to bring the cerebral hemispheres into view. They are now rapidly divided from the corpora bigemina and lifted out with the handle of a scalpel. The bleeding is very free, but may be partially controlled by stufBng the cavity with the vegetable fibre known as Pengavar Djambi, which should be removed in a few minutes, the wound cleansed with iodoform gauze wrung out of physio- logical salt solution at 50° C, and sewed up. Study the phenomena described on p. 915. 12. Stimulation of the Motor Areas in the Dog. — (a) Study a hardened brain of a dog, noting especially the crucial sulcus (Fig. 367, p. 917), the convolutions in relation to it, and the areas mapped out around it by Hitzig and Fritsch and others. (6) Inject morphine under the skin of a dog. Set up an induction-coil arranged for tetanus, with a single Daniell in the primary circuit. Connect a pair of fine but not sharp- pointed electrodes through a short-circuiting key with the secondary. Fasten the dog on the holder, belly down, and put a large pad under the neck to support the head. Clip the hair over the scalp. Feel for the condyles of the lower jaw, and join them by a string across the top of the head. , Connect the otiter canthi of the eyes by another thread. The crucial sulcus lies a little behind the mid-point between these two lines. Now give the dog ether, make a mesial incision through the skin down to the bone, and reflect the flaps on either side. Detach as much of the temporal muscle from- the bone as is necessary to get room for two trephine holes, the internal borders of which must be not less than ^ inch from the middle line, so as to avoid wounding the longitudinal sinus. Carefully work the trephine through the skull, taking care not to press heavily on it at the last. Raise up the two pieces of bone with forceps, connect the holes with bone forceps, and enlarge the opening as much as may be necessary to reach all the ' motor ' areas. At this stage only enough ether should be given to prevent suffering. Now unbind the hind- and fore-limbs on the side opposite to that on which the brain has been exposed, apply blunt elec-brodes successively to the areas for the fore- and hind-limbs, and stimulate.* The 'unipolar' method of stimulation (p. 918) may also be employed. Contraction of the corresponding groups of muscles will be seen if the narcosis is not too deep. Movements of the head, neck, and eyelids may also be called forth by stimulating the ' motor ' areas for these regions. Stimulation in front of the crucial sulcus may also cause great dilatation of the pupil, the iris almost disappearing. The dilatation takes place most promptly and is greatest on the opposite side, but the pupil on the same side is also widened. Even after section of both vago-sympathetic nerves in the neck, a slow and slight dilatation, greatest perhaps on the same side, may be caused by cortical stimulation. Repeat the whole experiment on the opposite side of the brain. In the course. of his observations the student will perhaps have the opportunity of seeing general epileptiform convulsions set up by a localized excitation. They begin in the group of muscles represented in the portion of the cortex directly stimulated. After the convulsions have been sufficiently studied, they should be again induced, and the stimulated ' motor ' area rapidly excised during their course. In some cases this will be followed by immediate cessation of the spasms, (c) The same animal can be used for stimulation of the spinal nerve-roots, as described in Experiment i (p. 957). * It is not necessary to remove the dura mater. CHAPTER XVII THE AUTONOMIC NERVOUS SYSTEM (THE SYMPATHETIC AND ALLIED NERVES) The efferent fibres of the body can be divided into two classes: (i) Those which supply multinuclear striated muscle (skeletal muscle) ; (2) those which supply other structures (smooth muscle, heart muscle, glands). The second group is called ' autonomic,' to indicate that it possesses a certain independence of the central nervous system, although this independence is far from absolute. The autonomic fibres arise from four regions of the central nervous system : (i) The mid-brain; (2) the bulb; (3) the thoracic and upper lumbar cord; (4) the sacral portion of the cord. All autonomic fibres after issuing from the central nervous system end sooner or later by forming synapses around nerve-cells of sympathetic type, by whose axons the path is continued to the peripheral distribution. The autonomic path accordingly comprises two neurons, the fibre which arises from the brain or cord being termed the ' pregang- lionic,' and that which arises from the sympatlietic ganglion the * postganglionic ' fibre. The autonomic fibres originating in the mid^brain emerge in the oculo-motor nerve, and form synapses with cells in the ciliary ganglion, which in turn send fibres to the cihary muscle and the constrictor muscle of the iris (pp. 894, 984). The bulbar autonomic fibres emerge in the seventh, ninth, and tenth cranial nerves. Those in the vagus include inhibitory fibres for the heart muscle, motor and inhibitory fibres for the smooth muscle of the alimentary canal from the oesophagus to the descending colon, and for the muscles of the trachea and lungs, and secretory fibres for the gastric glands and the pancreas. The sympathetic ganglion cells with which these preganglionic fibres form synapses have not always been definitely located, but lie near or in the tissue supplied (p. 179). The auto- nomic fibres in the seventh and ninth nerves supply the mucous membranes of the mouth and nose with vaso-dilator and secretory fibres. The preganglionic portion ,of the path terminates in such ganglia as the submaxillary and sublingual (p. 385) and the spheno- palatine and otic ganglia. 963 964 THE AUTONOMIC NERVOUS SYSTEM \3u/b The part of the autonomic system which originates in the middle region of the spinal cord (in the cat from the first thoracic to the fourth or fifth lumbar nerves) is the sympathetic proper. The course of the fibres has already been described in connection with the vaso-motor nerves (p. 179). Among the fibres may be men- tioned the dilators of the pupil, the augmentors of the heart, motor (viscero-motor) and inhibi- tory fibres for the smooth muscle of the alimen- tary canaj, sweat-secretory, pilo-motor and vaso- constrictor fibres. The preganglionic fibres issue from the cord in the anterior roots, and leave the corresponding, spinal nerve in the white ramus communicans, which connects it with the corresponding ganglion of the lateral syihpa- thetic chain. A fibre may either end in this ganglion by forming a S3mapse, or it rnay run up or down in the chain for some distance before terminating. Some of the preganglionic fibres, particularly the vaso-constrictors for the ab- dominal and pelvic viscera, do not end in the lateral chain at aU, but, issuing from^ it still as meduUated fibres, terminate in one of the pre- vertebral ganglia — e.g., coeliac ganglion, inferior mesenteric ganglion — from which postganglionic fibres proceed to the viscera, as previously described (p. 326). The postganglionic fibres arising from cells of the lateral ganglia return as non-medullated fibres in grey rami com- municating to the spinal nerves, and are dis- tributed with them to the head, limbs, and the superficial parts of the trunk. The autonomic fibres arising from the sacral region of the cord emerge as preganglionic fibres in the anterior roots of the second to the fourth sacral nerves, from which they pass to the pelvic nerve (nervus erigens) (pp. 179, 326)^ They comprise vaso-dilator fibres for the rectum, anus, and external genitals, motor (viscero-motdr) fibres for the smooth muscle of the descending colon, rectufn, and anus, inhibitory fibres for the smooth muscle of the anus, and the ■muscles of the external genitals, motor fibres for the bladder, etc: The preganglionic fibres terminate by forming, synapses with sympathetic ganglion cells in the pelvic plexus, or in the neighbour- hood of the organs which they supply. From these ganglion cells the postganglionic fibres arise. . ...;.' 'Fig. 384. — Diagram showing the Cen- ' ■ tral Origin of the Autonomic Fibres