Ifiilij; New York State College of Agriculture At Cornell University ^ Ithaca, N. Y. Library Cornell University Library QP 34.B86 A text-book of human physiology.lncludin 3 1924 003 131 780 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/cu31 9240031 31 780 A TEXT-BOOK OF H€MAN PHYSIOLOGY. BRUBAKER. ■'air-eA TEXT-BOOK OF HUMAN PHYSIOLOGY INCLUDING A SECTION ON PHYSIOLOGIC APPARATUS. BY ALBERT P. BRUBAKER, A.M., M.D., PROFESSOR OP PHYSIOLOGY AND HYGIENE IN THE JEFFERSON MEDICAL COLLEGE; PROFESSOR OF PHYSIOLOGY IN THE PENNSYLVANIA COLLEGE OP DENTAL SDRGERY; LECTURER ON PHYSIOLOGY AND HYGIENE IN THE DREXEL INSTITUTE OF ART, SCIENCE, AND INDUSTRY. THIRD EDITION. REVISED AND ENLARGED. WITH COLORED PLATES AND 383 ILLUSTRATIONS. PHILADELPHIA: P. BLAKISTON'S SON & CO., 1012 WALNUT STREET. 1908. Copyright, 1908, by P. Blakiston's Son & Co. PHnted by The Maple Press York Pa. TO HENRY C. CHAPMAN, M.D., PROFESSOR OF INSTITUTES OF MEDICINE AND MEDICAL JURISPRUDENCE IN THE JEFFERSON MEDICAL COLLEGE. IN GRATEFUL RECOGNITION OF THE MANY KINDNESSES RECEIVED FROM HIM, DURING A PERIOD OF TWENTY-FIVE YEARS, THIS VOLUME IS RESPECTFULLY DEDICATED BY THE AUTHOR. PREFACE TO THIRD EDITION, The publishers having requested the preparation of a Third Edition of the Text-Book of Physiology, an attempt has been made to still further increase its value to those for whom it was primarily intended by subjecting the whole text to a careful revision, by the insertion of a number of new diagrams and by the incorporation of new material. In this attempt the author has been assisted by the suggestions con- tained in reviews and in letters received from teachers and students. Altogether some fifty additional pages have been added to the body of the text. This material will be found in the chapters on the chemistry of the proteids, the physiology of muscle tissue, absorption, the physiology of the heart and vascular apparatus, the nerve system and vision. Once again I wish to express my sincere thanks to those teachers and students who have used and recommended this book and for the generous praise they have bestowed on it verbally and by letter and trust that in its present form it will still further meet their approval. I am indebted to Dr. George Bachmann for the new diagrams in the text to which his name is appended. To Mr. I. A. Hagy my thanks are due for invaluable assistance in seeing the work through the press. To Messrs. P. Blakiston's Son & Co. I am greatly indebted for their encouragement and generosity in the promotion of every thing that pertains to the material value of this work. A. P. B. vii PREFACE. The object in view in the preparation of this volume was the selec- tion and presentation of the more important facts of physiology, in a form which it is believed will be helpful to students and to practi- tioners of medicine. Inasmuch as the majority of students in a medical college are preparing for the practical duties of professional life, such facts have been selected as will not only elucidate the normal functions of the tissues and organs of the body, but which will be of assistance in understanding their abnormal manifestations as they present themselves in hospital and private work. Both in the selection of facts and in the method of presentation the author has been guided by an experience gained during twenty years of active teaching. The description of physiologic apparatus and the methods of investigation, other than those having a clinical interest, have been largely excluded from the text, for the reason that both are more appropriately considered in works devoted to laboratory methods and laboratory instruction, and for the further reason that the student receives this information while engaged in the practical study of physiology in the laboratory, now an established feature in the curriculum of the majority of medical colleges. For those who have not had laboratory opportunities a brief account of some essential forms of apparatus and the purposes for which they are intended will be found in an appendix. I wish to acknowledge my indebtedness to Professor Colin C. Stewart for many valuable suggestions in the preparation of different sections of the volume; to Dr. Carl Weiland for assistance in the chapter on vision; to Dr. Joseph P. Bolton for excellent suggestions on questions. relating to physiologic chemistry. TABLE OF CONTENTS. CHAPTER I. PAGE Introduction i CHAPTER II. Chemic Composition op the Human Body 7 CHAPTER III. Physiology op the Cell 26 CHAPTER IV. Histology of the Epithelial and Connective Tissues 33 CHAPTER V. The Physiology op Movement 43 CHAPTER VI. The Physiology op thi^ Skeleton 48 CHAPTER VII. General Physiology of Muscle-tissue 53 CHAPTER VIII. The General Physiology of Nerve-tissue 96 CHAPTER IX. Foods 127 CHAPTER X. Digestion 145 CHAPTER XI. Absorption 213 CHAPTER XII. The Blood 236 CHAPTER XIII. The Circulation of the Blood 271 CHAPTER XIV. The Circulation of the Blood (Continued) 326 CHAPTER XV. Respiration 382 CHAPTER XVI. Animal Heat 436 xi xii TABLE OF CONTENTS. CHAPTER XVII. ^"^^ Secretion 44^ CHAPTER XVIII. Excretion 472 CHAPTER XIX. The Central Organs or the Nerve System and their Nerves 491 CHAPTER XX. The Medulla Oblongata; the Isthmcis oj the Encephalon; the Basal Ganglia S'P CHAPTER XXI. The Cerebrxjm 537 CHAPTER XXII. The Cerebellum 569 CHAPTER XXIII. The Cranial Nerves 577 CHAPTER XXIV. The Sympathetic Nerve System 616 CHAPTER XXV. Phonation; Articulate Speech 626 CHAPTER XXVI. The Special Senses 638 CHAPTER XXVII. The Sense of Sight 650 CHAPTER XXVIII. The Sense of Hearing 689 CHAPTER XXIX. Reproduction 701 APPENDIX. Physiologic Apparatus 721 Index. 745 TEXT-BOOK OF PHYSIOLOGY. CHAPTER I. INTRODUCTION. An animal organism in the living condition exhibits a series of phenomena which relate to growth, movement, mentality, and re- production. During the period preceding birth, as well as during the period included between birth and adult Ufe, the individual grows in size and complexity from the introduction and assimilation of material from without. Throughout its Hfe the animal exhibits a series of movements, in virtue of which it not only changes the relation of one part of its body to another, but also changes its position relatively to its environment. If, in the execution of these movements, the parts are directed to the overcoming of opposing forces, such as gravity, friction, cohesion, elasticity, etc., the animal may be said to be doing work. The result of normal growth is the attainment of a physical development that will enable the animal, and, more especially, man, to perform the work necessitated by the nature of its environment and ^ the character of its organization. In man, and probably in lower animals as well, mentality manifests itself as intellect, feeling, and volition.' At a definite period in the life of the animal it reproduces itself, in consequence of which the species to which it belongs is per- petuated. The study of the phenomena of growth, movement, mentahty, and reproduction constitutes the science of animal physiology. But as these general activities are the resultant of and dependent on the special activities of the individual structures of which an animal body is composed, physiology in its more restricted and generally accepted sense is the science which investigates the actions or functions of the individual organs and tissues of the body and the physical and chemic conditions which underlie and determine them. This may naturally be divided into: I. Special physiology, the object of which is a study of the vital phenomena or functions exhibited by the organs of any iildividual animal. 2 TEXT-BOOK OF PHYSIOLOGY. 2. Comparative physiology, the object of which is a comparison of the vital phenomena or functions exhibited by the organs of two or more animals of different species, with a view to unfold- ing their points of resemblance or dissimilarity. Human physiology is that department of physiologic science which has for its object the study of the functions of the organs of the human body in a state of health. Inasmuch as the study of function, or physiology, is associated with and dependent on a knowledge of structure, or anatomy, it is essential that the student should have a general acquaintance not only with the structure of man, but with that of typical forms of lower animal life as well. If the. body of any animal be dissected, it will be found to be com- posed of a number of well-defined structures, such as heart, lungs, stomach, brain, eye, etc., to which the term organ was originally ap- plied, for the reason that they were supposed to be instruments capable of performing some important act or function in the general activities of the body. Though the term organ is usually employed to designate the larger and more familiar structures just mentioned, it is equally applicable to a large number of other structures which, though possibly less obvious, are equally important in maintaining the life of the in- dividual — e. g., bones, muscles, nerves, skin, teeth, glands, blood- vessels, etc. Indeed, any complexly organized structure capable of performing a given function may be described as an organ. A descrip- tion of the various organs which make up the body of an animal, their external form, their internal arrangement, their relations to one another, constitutes the science of animal anatomy. , This may naturally be divided into : 1. Special anatomy, the object of which is the investigation of the construction, form, and arrangement of the organs of any in- dividual animal. 2. Comparative anatomy, the object of which is a comparison of the organs of two or more animals of different species, with a view to determining their points of resemblance or dissimilarity. If the organs, however, are subjected to a further analysis, they can be resolved into simple structures, apparently homogeneous, to which the name tissue has been given— e. g., epithelial, connective, muscle, and nerve tissue. When the tissues are subjected to a micro- scopic analysis, it is found that they are not homogeneous in structure, but composed of still simpler elements, termed cells and fibers. The investigation of the internal structure of the organs, the physical prop- erties and structure of the tissues, as well as the structure of their component elements, the cells and fibers, constitutes a department of anatomic science known as histology, or as it is prosecuted largely with the microscope, microscopic anatomy. INTRODUCTION. 3 Human anatomy is that department of anatomic science which has for its object the investigation of the construction of the human body. ^ GENERAL STRUCTURE OF THE ANIMAL BODY. The body of every animal, from fish to man, may be divided into — 1. An axial portion, consisting of the head, neck, and trunk; and — 2. An appendicular portion, consisting of- the anterior and posterior hmbs or extremities. The axial portion of all mammals, to which class man zoologic- ally belongs, as well as of all birds, reptiles, amphibians, and osseous fish, is characterized by the presence of a bony, segmented axis, which extends in a longitudinal direction from before backward, and which is known as the vertebral column or backbone. In virtue of the ex- istence of this column all the classes of animals just mentioned form oi>e great division of the animal kingdom, the Vertebrata. Each segment, or vertebra, of this axis consists of — 1. A solid portion, known as the body or centrum, and 2. A bony arch arising from the dorsal aspect and surmounted by a spine-hke process. At the anterior extremity of the body of the animal the vertebrae are variously modified and expanded, and, with the addition of new elements, form the skull; at the posterior extremity they rapidly di- minish in size, and terminate in man in a short, tail-Uke process. In many animals, however, the vertebral column extends for a consider- able distance beyond the trunk into the tail. The vertebral column may be regarded as the foundation element in the plan of organization ■of all the higher animals and the center around which the rest of the body is developed and arranged with a certain degree of conformity. In all vertebrate animals the bodies of the segments of the vertebral column form a partition which serves to divide the trunk of the body into two cavities — ^viz., the dorsal and the ventral. (See Fig. i.) The dorsal cavity is found not only in the trunk, but also in the head. Its walls are formed partly by the arches which arise from the posterior or dorsal surface of the vertebrae and partly by the bones of the skull. If a longitudinal section be made through the center of the vertebral column, and including the head, the dorsal cavity will be observed running through its entire extent. Though for the most part it is quite narrow, at the anterior extremity it is enlarged and forms the cavity of the skull. This cavity is Uned by a membranous canal, the neural canal, in which arc contained the brain and the neural •or spinal cord. Through openings in the sides of the dorsal cavity nerves pass out which connect the brain and spinal cord with all the structures of the body. The ventral cavity is confined mainly to the trunk of the body. Its walls are formed by muscles and skin, strengthened in most animals TEXT-BOOK OF PHYSIOLOGY. by bony arches, the ribs. Within the ventral cavity is contained a musculo-membranous tube or canal known as the alimentary or food canal, which begins at the mouth on the ventral side of the head, and, after passing through the neck and trunk, terminates at the posterior ex- tremity of the trunk at the anus. It may be divided into mouth, pharynx, esophagus, stomach, small and large intestines. In all mammals the ventral cavity is divided by a musculo-membranous partition into two smaller cavities, the thorax and abdomen. The former contains the lungs, heart and its great blood-vessels, and the anterior part of the alimentary canal, the gullet or esophagus; the latter contains the continuation of the alimentary canal — that is, the stomach and intestines — and the glands in connection with it, the liver and pancreas. In the posterior portion of the abdominal cavity are found the kidneys,- ureters, and bladder, and in the female the organs of reproduction. The thoracic and abdominal cavities are each hned by a thin serous membrane, known, respectively, as the pleural and perit- oneal membranes, which, in addition, are reflected over the surfaces of the organs contained within them. The alimentary canal and the various cav- ities connected with it are Hned throughout by a mucous membrane. The surface of the body is covered by the skin. This is com- posed of an inner portion, the derma, and an outer portion, the epidermis. The former consists of fibers, blood- vessels, nerves, etc. ; the latter of layers of scales or cells. Embedded within the skin are numbers of glands, which exude, in the different classes of ani- mals, sweat, oily matter, etc. Project- ing from the surface of the skin are Beneath the skin are found muscles. Fig. I. — Diagrammatic Longit- udinal Section of the Body. V, V. Bodies of the vertebrae which divide the body into the dorsal and ventral cavities, u., a'. The dorsal cavity. C, p'. The abdominal and thoracic divisions of the ventral cavity, separated from each other by a trans- verse muscular partition, the dia- phragm d. B. The brain. Sp.C. The spinal cord. e. The esophagus. S. The stomach, from which con- tinues the intestine to the opening at the posterior portion of the body. /. The liver, f. The pancreas, k. The kidney. ./. The bladder. /'.The lungs, h. The heart. hairs, bristles, feathers, claws, bones, blood-vessels, nerves, etc INTRODUCTION. S The appendicular portion of the body consists of two pairs of symmetric limbs, which project from the sides of the trunk, and which bear a determinate relation to the vertebral column. They consist fundamentally of bones, surrounded by muscles, blood-vessels, nerves and lymphatics. The limbs, though having a common plan of organization, are modified in form and adapted for prehension and locomotion in accordance with the needs of the animal. Anatomic Systems. — ^AU the organs of the body which have certain peculiarities of structure in common are classified by anato- mists into systems — e. g., the bones, collectively, constitute the bony or osseous system; the muscles, the nerves, the skin, constitute, respec- tively, the muscle, the nerve, and the tegumentary systems. Physiologic Apparatus. — ^More important from a physiologic point of view than a classification of organs based on similarities of structure is the natural association of two or more organs acting together for the accomplishment of some definite object, and to which the term physiologic apparatus has been appUed. . While in the community of organs which together constitute the animal body each one performs some definite function, and the harmonious cooperation of all is neces- sary to the life of the individual, everywhere it is found that two or more organs, though performing totally distinct functions, are cooperat- ing for the accompUshment of some larger or compound function in which their individual functions are blended — e. g., the mouth, stom.- ach, and intestines, with the glands connected with them, constitute the digestive apparatus, the object or function of which is the complete digestion of the food. The capillary blood-vessels and lymphatic vessels of the body, a.nd especially those in relation to the villi of the small intestine, constitute the absorptive apparatus, the function of which is the introduction of new material into the blood. The heart and blood-vessels constitute the circulatory apparatus, the function of which is the distribution of blood to all portions of the body. The lungs, and trachea, together with the diaphragm and the walls of the chest, constitute the respiratory apparatus, the function of which is the introduction of oxygen into the blood and the ehmination from it of carbon dioxid and other injurious products. The kidneys, the ureters, and the bladder constitute the urinary apparatus. The skin, with its sweat-glands, constitutes the perspiratory apparatus, the functions of both being the excretion of waste products from the body. The Uver, the pancreas, the mammary glands, as well as other glands, each form a secretory apparatus which elaborates some specific material necessary to the nutrition of the individual. The functions of these different physiologic apparatus — e. g., digestion, absorption of food, elaboration of blood, circulation of blood, respiration, production of heat, secretion, and excretion — are classified as nutritive junctions, and have for their final object the preservation of the individual. 6 TEXT-BOOK OF PHYSIOLOGY. The nerves and muscles constitute the nervo-muscle apparatus, the function of which is the production of motion. The eye, the ear, the nose, the tongue, and the skin, with their related structures, con- stitute, respectively, the visual, auditory, olfactory, gustatory, and tactile apparatus, the function of which, as a whole, is the reception of impressions and the transmission of nerve impulses to the brain, where they give rise to visual, auditory, olfactory, gustatory, and tactile sensations and volitional impulses. The brain, in association with the sense organs, forms an appa- ratus related to mental processes. The larynx and its accessory organs — the lungs, trachea, respiratory muscles, the mouth and resonant cavities of the face — form the vocal and articulating apparatus, by means of which voice and articulate speech are produced. The func- tions exhibited by the apparatus just mentioned — ^viz., motion, sensa- tion, language, mental and moral manifestations — ^are classified as functions of relation, as they serve to bring the individual into con- scious relationship with the external world. The ovaries and the testes are the essential reproductive organs, the former producing the germ-cell, the latter the sperm element. Together with their related structures — ^the fallopian tubes, uterus, and vagina in the female, and the urogenital canal in the male^ — they constitute the reproductive apparatus characteristic of the two sexes. Their cooperation results in the union of the germ-cell and sperm element and the consequent development of a new being. The function of reproduction serves to perpetuate the species to which the individual belongs. The animal body is therefore not a homogeneous organism, but one composed of a large number of widely dissimilar but related organs. As all vertebrate animals have the same general plan of organization, there is a marked similarity both in form and structure among corre- sponding parts of different animals. Hence it is that in the study of human anatomy a knowledge of the form, construction, and arrange- ment of the organs in different types of animal life is essential to its correct interpretation; it follows also that in the investigation and com- prehension of the complex problems of human physiology a knowledge of the functions of the organs as they manifest themselves in the differ- ent types of animal life is indispensable. As many of the functions of the human body are not only complex, but the organs exhibiting them are practically inaccessible to investigation, we must supplement our knowledge and judge of their functions by analogy, by attributing to them, within certain limits, the functions revealed by experimentation upon the corresponding organs of lower animals. This experimental knowledge, corrected by a study of the clinical phenomena of disease and the results of post-mortem investigations, forms the basis of modern human physiology. CHAPTER II. CHEMIC COMPOSITION OF THE HUMAN BODY. Since it has been demonstrated that every exhibition of functional activity is associated with changes of structure, it has been apparent that a knowledge of the chemic composition of the body, not only when in a state of rest, but to a far greater degree when in a state of activity, is necessary to a correct understanding of the intimate nature of physiologic processes. Though the analysis of the dead body is comparatively easy, the determination of the successive changes in com- position of the Uving body is attended with many difficulties. The Uving material, the bioplasm, is not only complex and unstable in com- position, but extremely sensitive to all physical and chemic influences. The methods, therefore, which are employed for analysis destroy its composition and vitality, and the products which are obtained are peculiar to dead rather than to hving material. Chemic analysis, therefore, may be directed — 1. To the determination of the composition of the dead body. 2. To the determination of the successive changes in composition which the Uving bioplasm undergoes during functional activity. A chemic analysis of the dead body, with a view to disclosing the substances of which it is composed, their properties, their intimate structure, their relationship to one another, constitutes what might be termed chemic anatomy. An investigation of the hving ma- terial and of the successive changes it undergoes in the performance of its functions constitutes what has been termed chemic physiology or physiologic chemistry. By chemic analysis the animal body can be reduced to a number of Uquid and sohd compounds which belong to both the inorganic and organic worlds. These compounds, resulting from a proximate analysis, have been termed proximate principles. That they may merit this term, however, they must be obtained in the form under which they exist in the hving condition. The organic compounds consist of representatives of the carbohydrate, fatty, and protein groups of organic bodies; the inorganic compounds consist of water, various acids, and inorganic salts. The compounds or proximate principles thus obtained can be further resolved by an ultimate analysis into a small number of chemic elements which are identical with elements found in many other organic as well as inorganic compounds. The different chemic elements which are thus obtained, and the percentages in which they exist in the body, are as follows — viz., oxygen, 72 percent. ; hydrogen, 9.10; nitrogen, 8 TEXT-BOOK OF PHYSIOLOGY. 2.5; carbon, 13.50; phosphorus, 1.15; calcium, 1.30; sulphur, 0.147; sodium, o.io; potassium, 0.026; chlorin, 0.085; fluorin, iron, silicon, magnesium, in small and variable amounts. THE CARBOHYDRATES. The carbohydrate compounds, which enter into the composition of the animal body, are mainly starches and sugar. In many respects they are closely related, and by appropriate means are readily con- verted into one another. In composition they consist of the elements carbon, hydrogen, and oxygen. As their name implies, the hydrogen and oxygen are present in these compounds in the proportion in which they exist in water, or as 2 : i. The molecule of the carbohydrates above mentioned consists of either six atoms of carbon or a multiple of six; in the latter case the quantity of hydrogen and oxygen taken up by the carbon is increased, though the ratio remains unchanged. The carbohydrates may be divided into three groups — ^viz. : (i) amylases, including starch, dextrin, glycogen, and cellulose; (2) dex- troses, including dextrose, levulose, galactose; (3) saccharoses, including saccharose, lactose, and maltose. According to the number of carbon atoms entering into the second group (six), they are frequently termed monosaccharids ; those of the third group, disaccharids — ^twice six; those of the first group, polysaccharids — ^multiples of six. Though but few of the members of the carbohydrate group are constituents of the human body, many are constituents of the foods; on account of their importance in this respect, and their relation to one another, the chemic features of the more generally consumed car- bohydrates will be stated in this connection. I. AMYLOSES, (CeHi„05)n. Starch is widely distributed in the vegetable world, being abundant in the seeds of the cereals, leguminous plants, and in the tubers and roots of many vegetables. It occurs in the form of microscopic granules which vary in size, shape, and appearance, according to the plant from which they are obtained. Each granule presents a nucleus, or hilum, around which is arranged a series of eccentric rings, alternately light and dark. . The granule consists of an envelope and stroma of cellulose, containing in its meshes the true starch material — granulosa. Starch is insoluble in cold water and alcohol. When heated with water up to 70° C, the granules swell, rupture, and liberate the granulose, which forms an apparent solution ; if present in sufficient quantity, it forms a gelatinous mass termed starch paste. On the addition of iodin, starch strikes a characteristic deep blue color; the compound formed — ^iodid of starch — ^is weak, the color disappearing on heating, but reappearing on cooling. Boihng starch with dilute sulphuric acid (twenty-five per cent.) converts it into , dextrose. In the presence of vegetable diastase or CHEMIC COMPOSITION OF THE HUMAN BODY. 9 animal ferments, starch is converted into maltose and dextrose, two forms of sugar. Dextrin is a substance formed as an intermediate product in the transformation of starch into sugar. There are at least two principal varieties — erythrodextrin, which strikes a red color with iodin, and achroodextrin, which is without color when treated -with this reagent. In the pure state dextrin is a yellow-white powder, soluble in water. In the presence of vegetable ferments erythrodextrin is converted into maltose. Glycogen is a constituent of the animal Hver, and, to a sUght ex- tent, of muscles and of tissues generally. In the tissues of the embryo it is especially abundant. When obtained in a pure state it is an amorphous, white powder. It is soluble in water, forming an opales- cent solution. With iodin it strikes a port- wine color. In some respects it resembles starch, in others dextrin. Like vegetable starch, glycogen or animal starch can be converted by dilute acids and ferments into sugar (dextrose). Cellulose is the basic material of the more or less solid framework of plants. It is soluble in ammoniacal solution of cupric oxid, from which it can be precipitated by acids. It is an amorphous powder; dilute acids can convert it into dextrose. 2. DEXTROSES, C^ifi^. Dextrose, glucose, or grape-sugar is found in grapes, most sweet fruits, and honey, and as a normal constituent of liver, blood, muscles, and other animal tissues. In the disease diabetes mellitus it is found also in the urine. When obtained from any source, it is soluble in water and in hot alcohol, from which it crystallizes in six-sided tables or prisms. As usually met with, it is in the form of irregular, warty masses. It is sweet to the taste. When examined with the polariscope, dextrose turns the plane of polarized Hght to the right. It is therefore termed dextro-rotatory. It has for a long time been known that when sugar, cupric hydroxid, and an alkaH — e. g., sodium or potassium — are present in solution, the sugar will abstract from the cupric hydroxid a portion of its oxygen, thus reducing it to a lower stage of oxidation giving rise to cuprous oxid. Sugar has a similar action on both silver and bismuth. On this property of sugar a standard solution of cupric hydroxid was suggested by Fehling which may be employed for both qualitative and quantitative tests for the presence of sugar in solution. Fehling's Test Solution.- — This is a solution of cupric hydroxid made alkaline by an excess of sodium or potassium hydroxid with the addition of sodium and potassium tartrate. It is made by dissolv- ing cupric sulphate 34.64 grams, potassium hydroxid 125 grams, so- dium and potassium tartrate 173 grams, in distilled water sufficient to make one liter. lo TEXT-BOOK OF PHYSIOLOGY. The reaction is expressed by the following equation : CUSO4 + 2KOH = Cu(OH)2 + K,S04. The object of the sodium and potassium tartrate is to dissolve the cupric hydroxid and hold it in solution. For quahtative analysis it is only necessary to boil a few cubic centi- meters of this solution in a test-tube; then add the suspected solution and again heat to the boiling-point. If sugar be present, the cupric hydroxid is reduced to the condition of a cuprous oxid, which shows itself as a red or orange-yellow precipitate. The color of the precipitate depends on the relative excess of either copper or sugar, being red with the former, orange or yellow with the latter. The dehcacy of this test is shown by the fact that a few minims of this solution will detect in i c.c. of water the -jij- of a milligram of sugar. For quantitative analysis, 10 c.c. of FehUng's solution, diluted with 40 c.c. of water, are heated in a porcelain capsule, to which the sus- pected solution is cautiously added from a buret until the blue color entirely disappears. The strength of this solution is such that i c.c. is decolorized by 5 milligrams of sugar (dextrose), from which the per- centage of sugar in any solution can be determined. All the sugars, with the exception of chemically pure saccharose, may be tested for with this solution. The Fermentation Test. — All the sugars with the exception of lactose undergo reduction to simpler compounds, mainly alcohol and carbon dioxid, under the action of the yeast plant, Saccharomyces cerevisia. The change with dextrose is expressed in the following equation : CaHiaOe = 2CjH60 -f- 2CO2. Dextrose = Alcohol + Carbon Dioxid. About 95 per cent, of the dextrose is so changed, the remaining 5 per cent, yielding secondary products — succinic acid, glycerin, etc. As a means of testing any solution for the presence of sugar this method may be adopted. It is generally very satisfactory. From the quantity of carbon dioxid and alcohol thus produced the quantity of sugar in the solution may be determined. Levulose, or fruit-sugar, is found in association with dextrose as a constituent of many fruits. It is sweeter than dextrose and more soluble in both water and dilute alcohol. From alcoholic solutions it crystallizes in fine, silky needles, though it usually occurs in the form of a syrup. Levulose is distinguished from dextrose by its property of turning the plane of polarized hght to the left; the extent to which it does so, however, varies vnth the temperature and concentration of the solution. For this reason it is turned levulo-rotatory. Under the influence of the yeast plant it slowly undergoes fermen- tation, yielding the same products as dextrose. It also has a reducing action on cupric hydroxid. CHEMIC COMPOSITION OF THE HUMAN BODY. ii Galactose is obtained by boiling milk-sugar (lactose) with dilute sulphuric acid. In many chemic relations it resembles dextrose. It is less soluble in water, however, crystallizes more easily, and has a greater dextro-rotatory power. It also undergoes fermentation with the yeast plant. 3. SACCHAROSES, CizHjPh. Saccharose, or cane-sugar, is widely distributed throughout the vegetable world, but is especially abundant in sugar-cane, sor- ghum cane, sugar-beet, Indian corn, etc. It crystalHzes in large mono- clinic prisms. It is soluble in water and in dilute alcohol. Saccharose has no reducing power on cupric hydroxid, and hence its presence can not be detected by Fehling's solution. It is dextro-rotatory. Boiled with dilute mineral, as well as with organic acids, saccharose combines with water and undergoes a change in virtue of which it rotates the plane of polarized Hght to the left, and hence the product was termed invert sugar. This latter has been shown to be a mixture of equal quantities of levulose and dextrose. This inversion of saccharose through hydration and decomposition is expressed in the following equation : Ci^H^Oii + HjO = CeHijOe + CeH^Oe Saccharose + Water = Levulose -f- Dextrose Invert Sugar. Saccharose is not directly fermentable by yeast, but through the specific action of a ferment, invertin or invertase, secreted by the yeast plant, or the inverting ferment of the small intestine, it undergoes in- version, as previously stated, after which it is readily fermented, yield- ing alcohol and carbon dioxid. Lactose is the form of sugar found exclusively in the milk of the mammaUa, from which it can be obtained in the form of hard, white, rhombic prisms united with one molecule of water. It is soluble in water, insoluble in alcohol and ether. It is dextro-rotatory. It reduces cupric hydroxid, but to a less extent than dextrose. Dilute acids decompose it into equal quantities of dextrose and galactose. Lactose is not fermentable with yeast, but in the presence of the lactic acid bacillus it is decomposed into lactic acid, and finally into butyric acid, as expressed in the following equation: C„H2,0„ -t- H.O = 4C3H6O3 Lactose -I- Water = Lactic Acid. 2C3H6O3 = C4H8O2 -f- 2CO2 + 2Hj Lactic Acid = Butyric Acid -r Carbon + Free Dioxid Hydrogen, Maltose is a transformation product of starch, and arises when- ever the latter is acted on by malt extract or the diastatic ferments in saliva and pancreatic jiuce. The change is expressed by the fol- lowing equation : 2C6H10O5 -\- H2O — C12H22O11 Starch. Water. Maltose. 12 TEXT-BOOK OF PHYSIOLOGY. Maltose crystallizes in the form of white needles, which are soluble in water and in dilute alcohol. It is dextro-rotatory. In the presence of ferments and dilute acids maltose undergoes hydration and decom- position, giving rise to two molecules of dextrose. It has a reducing action on cupric hydroxid. Fermentation is readily caused by yeast, but whether directly or indirectly by inversion is somewhat uncertain. Osazones. — ^All the sugars which possess the power of reducing cupric hydroxid are capable of combining with phenyl-hydfazin, with the formation of compounds termed osazones. The osazones so formed are crystalline in structure, but have different melting-points, varying degrees of solubihty and optic properties, all of which serve to detect the various sugars and to distinguish one from the other. Of the different osazones, phenyl-glucosazone is the most characteristic, and occurs in the form of long, yellow needles. It may be obtained from dextrose by the following method : To 50 c.c. of a dextrose solu- tion add 2 gm. of phenyl-hydrazin and 2 gm. of sodium acetate, and boil for an hour. On cooling, the osazone crystallizes in the form of long, yellow needles. THE FATS. The fats constitute a group of organic bodies found in the tissues of both vegetables and animals. In the vegetable world they are largely found in fruits, seeds, and nuts, where they probably originate from a transformation of the carbohydrates. In the animal body the fats are found largely in the subcutaneous tissue, in the marrow of bones, in and around various internal organs and in milk. In these situations fat is contained in small, round or polygon-shaped vesicles, which are united by areolar tissue and surroxmded by blood-vessels. At the temperature of the body the fat is Uquid, but after death it soon solidifies from the loss of heat. The fats are compounds consisting of carbon, hydrogen, and oxygen, of which the first is the chief ingredient, forming by weight about 75 per cent., while the last is present only in small quantity. The fat found in animals is a mixture, in varying proportions in different ani- mals, of three neutral fats — stearin, palmitin, and olein. Each fat is a derivative of glycerin and the particular acid indicated by its name — e. g., stearic acid, in the case of stearin, etc. The reaction which takes place in the combination of glycerin and the acid is expressed in the following equation: C.,H,(HO)3 -t- (HC,8H3,0,)3 = CjHsCCsHjsOs + 3H.O. Glycerin. Stearic Acid. Stearin. Water. Hence, strictly speaking, the fats are compound ethers, in which the hydrogen of the organic acid is replaced by the trivalent radicle, tritenyl, C3H5. Stearin, €3115(018113502)3, is the chief constituent of the more solid fats. It is soUd at ordinary temperatures, melting at 55° C, CHEMIC COMPOSITION OF THE HUMAN BODY. 13 then solidifying again as the temperature rises, until at 71° C. it melts permanently. It crystallizes in square tables. Palmitin, CjHsCCigHjiOj), is a semifluid fat, solid at 45° C. and melting at 62° C. It crystallizes in fine needles, and is soluble in ether. Olein, 03115(018113302)3, is a colorless, transparent fluid, liquid at ordinary temperatures, only solidifying at 0° C. It possesses marked solvent powers, and holds stearin and palmitin in solution at the tem- perature of the body. Saponification. — When subjected to the action of superheated steam, a neutral fat is saponified — i. e., decomposed into glycerin and the particular acid indicated by the name of the fat used : e. g., stearic, palmitic, or oleic. The reaction is expressed as follows: C3Hs(Ci8H330,)3 + 3H.O = C3H,=(HO)3 + sCCisHj^O,) Olein. Water. Glycerin. Oleic Acid. The fat acids thus obtained are characterized by certain chemic features, as follows: Stearic acid is a firm, white soHd, fusible at 69" 0. It is soluble in ether and alcohol, but not in water. Palmitic acid occurs in the form of white, glistening scales or needles, melting at 62° 0. Oleic acid is a clear, colorless hquid, tasteless and odorless when pure. It crystallizes in white needles at 0° 0. If this saponification takes place in the presence of an alkali — e. g., potassium hydroxid or sodium hydroxid — ^the acid produced combines at once with the alkah to form a salt known as a soap, while the glycerin remains in solution. The reaction is as follows: 3KHO + (C,8H340,)3 = 3(KCi8H330.) + 3H.0 Potassium. Oleic Acid. Potassium Oleate, Water. All soaps are, therefore, salts formed by the union of alkalies and * fat acids. The sodium soaps are generally hard, while the potas^ sium soaps are soft. Those made with stearin and palmitin are harder than those made with olein. If the soap is composed of lead, zinc, copper, etc., it is insoluble in water. Emulsification. — ^When a neutral oil is vigorously shaken with water or other fluid, it is broken up into minute globules that are more or less permanently suspended; the permanency depending on the nature of the liquid. The most permanent emulsions are those made with soap solutions. The process of emulsification and the part played by soap can be readily observed by placing on a few cubic centi- meters of a solution of sodium carbonate (0.25 per cent.) a small quant- ity of a perfectly neutral oil to which has been added 2 or 3 per cent, of a fat acid. The combination of the acid and the alkaU at once forms a soap. The energy set free by this combination rapidly divides the oil into extremely minute globules. A spontaneous emulsion is thus formed. 14 TEXT-BOOK OF PHYSIOLOGY. THE PROTEINS. The proteins constitute a group of organic bodies which are found in both vegetable and animal tissues. Though present in all animal tissues, they are especially abundant in muscles and bones, where they constitute 20 per cent, and 30 per cent, respectively. Though genetically related, and possessing many features in common, the different members of the protein group are distinguished by character- istic physical and chemic properties which serve not only for their identification, but for their classification into more or less well-defined groups. Chemic Composition. — ^A chemic analysis of proteins shows that they consist of carbon, hydrogen, oxygen, nitrogen and sulphur, though the percentage of each of these elements varies somewhat in the different proteins. A certain number of proteins contain phosphorus while almost all of them contain different inorganic salts in varying amounts. The average percentage composition of several proteins is shown in the following analyses: C. H. N. O. s. Egg-albumin, 52.9 7.2 15.6 23.9 0.4 (Wiirtz). Serum-albumin, 53.0 6.8 16.0 22.29 1.77 (Hammersten). Casein, 52.3 7.07 15.91 22.03 0.82 (Chittenden and Painter). Myosin, 52.82 7. 11 16.77 21.90 1.27 (Chittenden and Cummins). The molecular composition of the proteins is not definitely known and the formulse which have been suggested are therefore only approxi- mative. Leow assigns to albumin the formula C,2Hi„N,8022S, while Schiitzenberger raises the numbers to C24oH3p2N650„S3, either of which shows that the proteid molecule is extremely complex. Structure of the Protein Molecule.— From the large size of the protein molecule as indicated by its chemic composition it might be inferred that its structure was equally complex. This modern inves- tigation has shown to be the case. When any one of the typical proteins, found in animal or vegetable tissues, is hydrolysed by acids, alkaHes and animal ferments under appropriate conditions, it can be resolved through a series of descending stages into relatively simple nitrogen-holding bodies termed amino- acids and diamino-acids, of which somewhat more than twenty have been isolated and their properties determined. The principal amino- acids are as follows: GlycocoU, alanin, leucin, isoleucin, amino-iso- valerianic acid, serin, aspartic acid, glutamic acid, phenylalanin, tyrosin, prolin, tryptophan. The principal diamino-acids are as follows: Ornithin, lysin, histidin, arginin, cystin. The protein molecule is therefore structurally complex. The CHEMIC COMPOSITION OF THE HUMAN BODY. 15 or grouped in any given protein, is practically unknown. More or less successful attempts have been made at the reconstruction of the protein molecule by synthetic methods, by the union of two or more of the amino-acids. A number of such compounds have been formed by the union of from two to ten or more amino-acids, all of them exhibiting many of the protein reactions. Such bodies are termed polypeptids. Physical Properties. — ^As a class the proteins are characterized by the following properties : 1 . Indiffusibility. — ^None of the proteins normally assume the crystalline form, and hence they are not capable of diffusing through parchment or an animal membrane. Peptone, a product of the digestion of proteins, is an exception as regards its diffu- sibility. As met with in the body, all proteins are amorphous, but vary in consistence from the Hqmd to the soHd state. The colloid character of the proteins permits of their separation and purifi- cation from crystalloid diffusible compounds by the process of dialysis. 2. Solubility. — Some of the proteids are soluble in water, others in solutions of the neutral salts of varying degrees of concentration, in strong acids and alkalies. All are insoluble in alcohol and ether. 3. Coagulability. — ^Under the influence of heat and animal ferments, some of the proteins readily pass from the soluble liquid state to the insoluble solid state, attended by a permanent alteration in their chemic composition. To this change the term coagulation has been given. The various proteins, however, coagulate at different temperatures. Proteins are capable of precipitation without losing their solubiUty by ammonium sulphate, sodium chlorid, and magnesium sulphate. 4. Fermentability. — ^In the presence of specific microorganisms — bacteria — ^the proteins, owing to their complexity and instability, are prone to undergo disintegration and reduction to simpler compounds. This decomposition or putrefaction occurs most readily when the conditions most favorable to the growth of bacteria are present — ^viz., a temperature varying from 25° C. to 40° C, moisture, and oxygen. The intermediate as well as the terminal products of the decomposition of the proteins are numerous, and vary with the composition of the protein and the specific physiologic action of the bacteria. Among the intermediate products is a series of alkaloid bodies, some of which possess marked toxic properties, known as ptomains. The toxic symptoms which frequently follow the ingestion of foods in various stages of putrefaction are to be attributed to these com- pounds. The terminal products are represented by hydrogen sulphid, ammonia, carbon dioxid, fats, phosphates, nitrates, etc. i6 TEXT-BOOK OF PHYSIOLOGY. Classification. — ^The protein compounds by virtue of their struc- tural composition, their physical and chemic properties, permit of a provisional arrangement into groups as follows: PROTAMINS. These proteins are derived for the most part from the heads of the spermatozoa of fish. They take their names from the species of fish from which they are obtained, e. g., salmin (salmon), sturin (sturgeon), scombrom, (mackerel), etc. Inasmuch as they respond to Piotrowski's test in a characteristic way they are regarded as true proteins. When subjected to hydrolysis they can be resolved into the diamino bodies, lysin, arginin and histidin, of which they con- stitute about 90 per cent., and a small number of the mono-amino-acids. Because of the fact that the diamino bodies, lysin, histidin and arginin contain 6 atoms of carbon they are known as the hexone bases. Inasmuch as the protamins contain practically but these three bodies, they are regarded as the simplest of all the proteins. Since a typical protein always yields on hydrolysis the hexone bases, in addition to a variable number of mono-amino-adds, it is believed that the usual protein is composed of a nucleus of the hexone bases to which is attached a variable number of mono-amino-acids. The proportions in which the bases exist in the nucleus and the proportions in which the amino - acids are united to the nucleus, vary in different proteins. HISTONS. The proteins embraced in this class comprise a series of compounds which are somewhat more complex than the protamins and less com- plex than the typical proteins; for on hydrolysis they not only yield the hexone bases but in addition a certain number of amino-acids. They are, therefore, intermediate in structural composition between the protamins and the usual proteins. Their protein character is indicated by their reaction to Millon's reagent and to Piotrowski's test. The histons are usually found in combination with nucleic acid, in the spermatozoa of most animals and especially in fish, and in the coloring matter (the hemoglobin) of the red corpuscles. The proteins of the tissues usually contain from 25 to 30 per cent, of histons. ALBUMINS'. The members of this group are soluble in water, in dilute saline solutions, and in saturated solutions of sodium chlorid and mag- nesium sulphate. They are coagulated by heat, and when dried form an amber-colored mass. (a) Serum-albumin. — ^This most important protein is found in blood, lymph, chyle, and some tissue fluids. It is obtained readily by precipitation from blood-serum, after the other proteins have been removed, on the addition of ammonium sulphate. When freed from saUne constituents, it presents CHEMIC COMPOSITION OF THE HUMAN BODY. 17 strong nitric acid. It is coagulated at a temperature of 73° C, as well as by various acids — e. g., citric, picric, nitric, etc. It has a rotatory power of — 62.6°. (b) Egg-albumin. — ^Though not a constituent of the human body, egg-albumin resembles the foregoing in many respects. When obtained in the solid form from the white of the egg, it is a yellow mass without taste or odor. Though similar to serum-albumin, it differs from it in being precipitated by ether, in coagulating at 54° C, and in having a lower rotatory power, —35.5°. (c) Lact-albumin. — ^As its name imphes, this protein is found in milk. It can be precipitated from milk-plasma by sodium sulphate after the precipitation of the other proteids by half saturation with ammonium sulphate. It slowly coagulates at 77° C. (d) My -albumin. — ^This protein is found in muscle-plasma from which it subjects the plasma to fractional heat coagulation. At 73° C. myo-albumin coagulates. GLOBULINS. (a) Serum-globulin or Paraglobulin. — This protein, as its name implies, is found in blood-serum, though it is present in other animal fluids. When precipitated by magnesium sulphate or carbon dioxid, it presents itself as a fliocculent substance, insoluble in water, soluble in dilute acids and alkalies, and coagulating at 75° C. (b) Fibrinogen. — ^This protein is found in blood-plasma in asso- ciation with serum-globulin and serum-albumin. It is also present in lymph-tissue fluids and in pathologic transudates. It can be obtained from blood-plasma which has been pre- viously treated with magnesium sulphate on the addition of a saturated solution of sodium chlorid. It is soluble in dilute acids and alkahes, and coagulates at 56° C. (c) Para-myosinogen. — ^This protein is a constituent of the muscle-plasma from which it can be precipitated by a tem- perature of 47° C. (d) Myosinogen. — ^This protein is the chief constituent of the muscle-plasma and is of great nutritive value. During the living condition it is liqmd, but after death it readily undergoes a chemic change and contributes to the formation of an insoluble protein known as myosin. It is soluble in dilute hydrochloric acid and dilute alkalies. It coagulates at 56° C. (e) Crystallin or Globulin. — ^This is obtained by passing a stream of CO2 through a watery extract of the crystalline lens. SCLERO-PROTEINS (ALBUMINOIDS). The sclero-proteins constitute a group of substances similar to the i8 TEXT-BOOK OF PHYSIOLOGY. proteins in many respects, though differing from them in others. When obtained from the tissues, in which they form an organic basis, they are found to be amorphous, colloid, and when decomposed yield products similar to those of the true proteins. The principal members of this group are as follows: > (a) Collagen, Ossein. — ^These are two closely alhed, if not identical, substances, found respectively in the white fibrous connective tissue and in bone. When the tendons of muscles, the liga- ments, or decalcified bone are boiled for several hours, the collagen and ossein are converted into soluble gelatin, which, when the solution cools becomes solid. (b) Chondrigen. — ^This is supposed to be the organic basis of the more permanent cartilages. A\Tien the latter are boiled, they yield a substance which gelatinizes on cooHng, and to which the name chondrin has been given. Chondrin, however, is not a pure gelatin, but has associated with it a compound proteid known as chrondro-mucoid. (c) Elastin is the name given to the substance composing the fibers of the yellow, elastic connective tissue. (d) Keratin is the substance found in all homy and epidermic tissues, such as hairs, nails, scales, etc. It differs from most proteids in containing a high percentage of sulphur. PHOSPHO-PROTEINS. The two members of this group are distinguished by yielding on decomposition a protein which contains phosphorus. It was formerly regarded as a nuclein. V (a) Caseinogen.^ — ^This is the principal protein of milk, in which it exists in association with an alkaH, and hence was formerly regarded as an alkaU-albumin. It is precipitated by acetic acid and by magnesium sulphate. It is coagulated by rennet — that is, separated into an insoluble protein, casein or tyrein, and a soluble albumin. Calcium phosphate seems to be the natural alkaU necessary to this process, for if it be removed by dialysis, or precipitated by the addition of potassium oxalate, coagulation does not take place. (b) Vitellin. — ^VitelUn is a constituent of the viteUis or yolk of eggs. It differs from other proteins in the fact that it is semicrystalUne in character. Thdugh usually regarded as a nucleo-protein it is not definitely known whether or not it contains phosphorus in its composition. CONJUGATED OR COMBINED PROTEINS. The different members of this group are capable of being decom- posed by chemic methods into a protein and a non-protein substance ; e. g., a coloring matter, a carbohydrate, or .a nuclein. The chemic character of the non-protein substance furnishes the basis for the following classification : CHEMIC COMPOSITION OF THE HUMAN BODY. 19 CHROMO-PROTEINS. (a) Hemoglobin. — ^Hemoglobin is the coloring matter of the red corpuscles, of which it constitutes about 94 per cent. It possesses the power of absorbing oxygen as it passes through the lung capillaries and of yielding it up to the tissues as it passes through the tissue capillaries. In the arterial blood it is known as oxyhemoglobin, and in the venous blood as deoxy- or reduced-hemoglobin. When hydrolysed by acids or alkalies, hemoglobin undergoes a cleavage into a protein, globin, and a pigment hematin. (b) Myohematin. — ^Myohematin is a protein supposed to be present in muscle. It has never been isolated, hence its chemic features are unknown. Spectroscopic examination indicates that it is capable of absorbing and again yielding up oxygen. For this reason it is beUeved to be a derivative of hemoglobin. GLUCO-PROTEINS. (a) Mucin. — Mucin is the proteid which gives the mucus, secreted by the epitheUal cells of the mucous membranes and related glands, its viscid, tenacious character. It is also a constituent of the intercellular substance of the connective tissues. It is readily precipitated by acetic acid. When heated with dilute acids, mucin undergoes a cleavage into a simpler proteid and a carbohydrate termed mucose, which is capable of reducing FehHng's solution. (&) Mucoids. — ^The mucoids resemble the mucins though differ- ing from them in solubility and in not being precipitable from alkahne solutions by acetic acid. They are found in the vitreous humor, white of egg, cartilage, and in other situations. They differ shghtly one from the other in properties and chemic composition. They yield on decomposition a carbohydrate. NUCLEO-PROTEINS. The nucleo-proteins are obtained from the nuclei and- cell-sub- stance of tissue-cells. Chemically they are characterized by the presence of phosphorus in relatively large amounts. When hydrolysed, they separate into a protein and a nuclein. The nucleins derived from cell nuclei can be still further separated into a simpler protein and nucleic acid, which latter in turn yields phosphoric acid and the so-called purin bases, xanthin, hypoxanthin, adenin, and guanin. All nucleins which yield the purin bases are termed true nucleins. PROTEIN DERIVATIVES. The proteins of this group are derived from both albumins and globuHns by the gradual action of dilute acids and alkalies, and may be regarded as compounds of a proteid with an acid or an alkali. They have been designated. 20 TEXT-BOOK OF PHYSIOLOGY. INFRA-PROTEINS. (a) Acid-albumin. — This is formed when a native albumin is digested with dilute hydrochloric add (0.2 per cent.) or dilute sulphuric acid for some minutes. It is precipitated by neu- tralization with sodium hydroxid (o.i per cent, solution). After the precipitate is washed, it is found to be insoluble in distilled water and in neutral saline solutions. In acid solutions it is not coagulated by heat. (b) Alkali-albumin. — ^This is formed when a native albumin is treated with a dilute alkali — e. g., o.i per cent, of sodium hydroxid — for five or ten minutes. On careful neutrahzation with dilute hydrochloric acid, it is precipitated. It is also insoluble in distilled water and in saline solutions; it is not coagulable by heat. PROTEOSES AND PEPTONES. During the progress of the digestive process, as it takes place in the stomach and intestines, there is produced by the action of the gastric and pancreatic juices, out of the proteins of the food, a series of new proteins, known as proteoses and peptones. The chemic properties of these substances will be considered in connection with the process of digestion. COAGULATED PROTEINS. Although these proteins are not found as constituents of the animal organism, they possess much interest on account of their relation to prepared foods and to the digestive process. They are produced when solutions of egg-albumin, serum-albumin, or globulins are subjected to a temperature of 100° C. or to the prolonged action of alcohol. They are insoluble in water, in dilute acids, and in neutral sahne solutions. In this same group may be included also those coagulated pro- teins which are produced by the action of animal ferments on soluble proteins — e. g., fibrin, myosin, casein. V {a) Fibrin. — ^Fibrin is derived from a soluble protein — ^fibrinogen — ^by the action of a special ferment. It is not present under normal circumstances in the circulating blood, but makes its appearance after the blood is Withdrawn from the vessels and at the time of coagulation. It can also be obtained by whipping the blood with a bundle of twigs, on which it accu- mulates. When freed from blood by washing under water, it is seen to consist of bundles of white elastic fibers or threads. It is insoluble in water, in alcohol, and ether. In dilute acids it swells, becomes transparent, and finally is converted into acid-albumin. In dilute alkalies a similar change takes place, but the resulting product is an alkali-albumin. Fibrin pos- sesses the property of decomposing hydrogen dioxid, H,0, CHEMIC COMPOSITION OF THE HUMAN BODY. 21 — i. e., liberating oxygen, which accumulates in the form of bubbles on the fibrin. On incineration fibrin 3delds an ash which contains calcium phosphate and magnesium phosphate. (b) Myosin. — ^IVIyosin develops in muscles after death and is the cause of the stiffening of the muscles. It has been regarded as a derivative of the soluble protein myosinogen alone, but there is evidence that in its formation both paramyosinogen and myosinogen take part. It is not definitely known whether this is the result of the action of a special ferment or not. (c) Casein. — Casein is derived from the chief protein of milk — caseinogen — ^by the action of a special ferment known as rennin or chymosin. This ferment is a constituent of gas- tric juice. The Color Reactions of Proteins. — ^When proteins are present in solution, they may be detected by the following color reactions — viz. : - I. Xanthoproteic. The solution is boiled with nitric acid for several minutes, when the proteid assumes a light yellow color. After the solution has cooled, the addition of ammonia changes the color to an orange or amber-red due to the presence of phenyl- alanin and tyrosin. 2. The rose- red reaction. The solution is boiled with acid nitrate of mercury (Millon's reagent) for a few minutes, when the co- agulated proteid turns a purple-red color. This color is attributed to the presence of tyrosin. - - 3. The blue- violet reaction. A few drops of copper sulphate solution are first added to the protein solution, and then an excess of sodium hydroxid. A blue-violet color is produced, which deepens somewhat on heating, but no further change ensues. This is also known as Piotrowski's test : As this same color is developed with the substance biuret, it is also known as the biuret reaction. Biuret is formed by heating urea and driving off ammonia. Precipitation Tests. — ^Proteins in solution may be precipitated by nitric acid; acetic acid and potassium ferrocyanid, picric acid, copper sulphate, tannin, alcohol, etc. As stated in a foregoing paragraph, certain of the proteins, e. g., fibrinogen, caseinogen and myosinogen, will undergo, by the action of an animal ferment, a cleavage into a solid and a soluble portion. To this process the term jerment coagu- lation is apphed. The solidification of proteins by the action of heat, is designated heat coagulation. INORGANIC CONSTITUENTS. The inorganic compounds and mineral constituents obtained from the solids and fluids of the body are very numerous, and, in some instances, quite abundant. Though many of the compounds thus obtained are undoubtedly derivatives of the tissues and necessary to their physical and physiologic activity, others, in all probabiUty, 22 TEXT-BOOK OF PHYSIOLOGY. are decomposition products, or transitory constituents introduced with the food. Of the inorganic compounds, the following are the most important: WATER. ' Water is the most important of the inorganic constituents, as it is indispensable to hfe. It is present in aU the tissues and fluids without exception, varying from 99 per cent, in the saUva to 80 per cent, in the blood, 75 per cent, in the muscles to 2 per cent in the enamel of the teeth. The total quantity contained in a body weigh- ing 75 kilograms (165 pounds) is 52.5 kilograms (115 pounds). Much of the water exists in a free condition, and forms the chief part of the fluids, giving to them their characteristic degree of fluidity. Possess- ing the capabihty of holding in solution a large number of inorganic as well as some organic compounds, and being at the same time diffus- ible, it renders an interchange of materials between all portions of the body possible. It aids in the absorption of new material into the bl'ood and tissues, and at the same time it transfers waste products from the tissues to the blood, from which they are finally eliminated, along with the water in which they are dissolved. A portion of the water is chemically combined with other tissue constituents and gives to the tissues their characteristic physical properties. The consistency, elasticity, and phabiUty are, to a large extent, conditioned by the amount of water they contain. The total quantity of water eliminated by the kidneys, lungs, and skin amounts to about 3 kilograms (6| pounds). CALCIUM COMPOUNDS. Calcium phosphate, CajCPO^)^, has a very extensive distribu- tion throughout the body. , It exists largely in the bones, teeth, and to a shght extent in cartilage, blood, and other tissues. Milk con- tains 0.27 per cent. The soHdity of the bones and teeth is almost entirely due to the presence of this salt, and is, therefore, to be re- garded as necessary to their structure. It enters into chemic union with the organic matter, as shown by the fact that it can not be separated from it except by chemic means, such as hydrochloric acid. Though insoluble in water, it is held in solution in the blood and milk by the protein constituents, and in the urine by the acid phosphate of soda. The total quantity of calcium phosphate which enters into the forma- tion of the body has been estimated at 2.5 kilograms. The amount ehminated daily from the body has been estimated at 0.4 gm., a fact which indicates that nutritive changes do not take place with much rapidity in those tissues in which it is contained. ■ Calcium carbonate, CaCOj, is present in practically the same situations in the body as the phosphate, and plays essentially the same r61e. It is,, however, found in the crystalHne form, aggregated in small masses in the internal ear, forming the otoUths, or ear stones. CHEMIC COMPOSITION OF THE HUMAN BODY. 23 Though insoluble, it is held in solution by the carbonic acid diffused through the fluids. Calcium fluorid, CaF„ is found in bones and teeth. SODIUM COMPOUNDS. y Sodium chlorid, NaCl, is present in all the tissues and fluids of the body, but especially in the blood, 0.6 per cent., lymph, cr.5, and pancreatic juice, 0.25 per cent. The entire quantity in the body has been estimated at about 200 gm. Sodium chloiid is of much importance in the body as it determines and regulates to a large extent the phenomena of diffusion which are there constantly taking place. This is illustrated by the fact that a solution of albumin placed in the rectum without the addition of this salt will not be absorbed. When the salt is added, absorption takes place. The ingested water is absorbed into the blood largely in consequence of the percentage of this salt which it contains. The normal percentage of sodium chlorid in the blood-plasma assists in maintaining the shape and structure of the red blood-corpuscles by determining the amount of water entering into their composition. The same is true of other tissue elements. Sodium chlorid also influences the general nutritive process, in- creasing the disintegration of the proteins, as shown by the increased amount of urea excreted. During its existence in the body it under- goes chemic transformations or decompositions, yielding its chlorid to form the potassium chlorid of the blood-corpuscles and muscles and to form the hydrochloric acid of the gastric juice. Sodium phosphate, NajHP04, is found in all soUds and fluids of the body, to which, with but few exceptions, it imparts an alkaline reaction. This is especially true of blood, lymph, and tissue fluids generally. It is essential to physiologic action that all tissue elements should be bathed by an alkahne medium. v" Sodium carbonate, Na^COj, is generally found in association with the preceding salt. As it is an alkahne compound, it also assists in giving to the blood and lymph their characteristic alkalinity. In carnivorous animals the sodium phosphate is the more abundant, while in the herbivorous animals the sodium carbonate is the more abundant. Sodium sulphate, Na^SO^, is present in many of the tissues and fluids, especially in the urine. Though introduced in the food, it is also, in all probabiHty, formed in the body from the decomposition and oxidation of the protcids. POTASSIUM COMPOUNDS. ' Potassium chlorid, KCl, is met with in association with sodium chlorid in almost all situations in the body. It preponderates, how- ever, in the tissue elements, especially in the muscle tissue, nerve tissue, and red corpuscles. The plasma with which these structures 24 TEXT-BOOK OF PHYSIOLOGY. are bathed contains but a very small amount of this salt, but, as previously stated, a relatively large quantity of sodium chlorid. Though introduced to some extent in the food, it is very Ukely that it is also formed through the decomposition of the sodium chlorid. Potassium phosphate, K^HPO^, is found in association with sodium phosphate in all the fluids and sohds. As it has similar chemic properties, its functions are practically the same. Potassium carbonate, K^COj, is generally found with the pre- ceding salt. MAGNESIUM COMPOUNDS. Magnesium phosphate, MgjCPOJ^, is found in all tissues, in association with calcium phosphate, though in much smaller quantity. Magnesium carbonate, MgCOj, occurs only in traces in the blood. Both of these compounds have functions similar to the calcium compounds, and exist, in all probabiUty, under similar conditions. IRON COMPOUNDS. Iron is a constituent of the coloring-matter of the blood. Traces, however, are also found in lymph, bile, gastric juice, and in the pig- ment of the eyes, skin and hair. The amount of iron contained in a body weighing 75 kilograms is about 3 gm. It exists under various forms — e. g., ferric oxid, and in combination with organic compounds. Chemic analysis thus shows that the chemic elements into which the compounds may be resolved by an ultimate analysis do not exist in the body in a free state, but only in combination, and in char- acteristic proportions, to form compounds whose properties are the resultant of those of the elements. Of the four principal elements which riiake up 97 per cent, of the body, O, H, N are extremely mobile, elastic, and possessed of great atomic heat. C, H, N are distinguished for the narrow range of their affinities, and for their chemic inertia. C possesses the great atomic cohesion. O is noted for the number and intensity of its combinations. As the properties of the compounds formed by the union of ele- ments must be the resultants of the properties of the elements them- selves, it follows that the ternary compounds, starches, sugars, and fats must possess more or less inertia, and at the same time instabiUty; while in the more complex proteids, in which sulphur and phosphorus are frequently combined with the four principal elements, molecular instability attains its maximum. As all the foregoing compounds possess in varying degrees the properties of inertia and instability, it follows that living matter must possess corresponding properties, and the capabiUty of undergoing unceasingly a series of chemic changes, both of composition and decomposition, in response to the chemic and physical influences by which it is surrounded, and which underhe all the phenomena of Ufe. CHEMICAL COMPOSITION OF THE HUMAN BODY. 25 PRINCIPLES OF DISSIMILATION. In addition to the previously mentioned compounds — ^viz., carbo- hydrates, fats, proteids, and inorganic salts — ^there is obtained by chemic analysis from the tissues and fluids of the body: 1. A number of organic acids, such as acetic, lactic, oxalic, butyric, propionic, etc., in combination with alkaline and earthy bases. 2. Organic compounds, such as alcohol, glycerin, cholesterin. 3. Pigments, such as those found in bile and urine. 4. Crystalhzable nitrogenized bodies, such as urea, uric aci4, xanthin, Mppuric acid, creatin, creatinin, etc. While some few of these compounds may possibly be regarded as necessary to the physiologic integrity of the tissues and fluids, the majority of them are to be regarded as products of dissimilation of the tissues and foods in consequence of functional activity, and rep- resent stages in their reduction to simpler forms previous to being eUminated from the body. CHAPTER III. PHYSIOLOGY OF THE CELL. A microscopic analysis of the tissues shows that they can be re- solved into ultimate elements, termed cells, which may, therefore, be regarded as the primary units of structure. Though cells vary considerably in shape, size, and chemic composition in the different tissues of the adult body, they are, nevertheless, descendants from typical cells, known as embryonic or undifferentiated cells, examples of which are the leukocytes of the blood and lymph and the first offspring of the fertilized ovum. Ascending the line of embryonic development, it will be found that every organized body originates in a single cell — ^the ovum. As the cell is the elementary unit of all tissues, the function of each tissue must be referred to the function of the cell. Hence the cell may be defined as the primary anatomic and physiologic unit of the organic world, to which every exhibition of life, whether normal or abnormal, is to be referred. Structure of Cells. — ^Though cells vary in shape and size and internal structure in different portions of the body, a typical cell may be said to consist mainly of a gelatinous substance forming the body of the cell, termed protoplasm or bioplasm, in which is embedded a smaller spheric body, the nucleus. The shape of the adult cell varies according to the tissue in which it is foimd; when young and free to move in a fluid medium, the cell assumes a spheric form, but when subjected to pressure, may become cyUndric, fusiform, polygonal, or stellate. Cells vary in size within wide limits, ranging from 7.7 /i (swtr of an inch, the diameter of a red blood-corpuscle), to 135 ;« {tw^ of an inch, the diameter of the large cells in the gray matter of the spinal cord). (See Fig. 2.) The cell protoplasm consists of a soft, semifluid, gelatinous material, varying somewhat in appearance in different tissues. Though frequently homogeneous, it often exhibits a finely granular appearance under medium powers of the microscope. Young cells consist almost entirely of clear protoplasm. Mature cells contain, according to the tissue in which they are found, material of an en- tirely different character — e. g., small globules of fat, granules of glycogen, mucigen, pigments, digestive ferments, etc. Under high powers of the microscope the cell protoplasm is found to be pervaded by a network of fibers, termed spongio plasm, in the meshes of which is contained a clearer and more fluent substance, the hyaloplasm. The relative amount of these two constituents varies in different cells, the proportion of hyaloplasm being usually greater in young PHYSIOLOGY OF THE CELL. 27 cells. The arrangement of the fibers forming the spongioplasm also varies, the fibers having sometimes a radial direction, in others a concentric disposition, but most frequently being distributed evenly in all directions. In many cells the outer portion of the cell proto- plasm undergoes chemic changes and is transformed into a thin, transparent, homogeneous membrane — ^the cell membrane — ^which completely incloses the cell substance. The cell membrane is per- meable to water and watery solutions of various inorganic and organic substances. It is, however, not an essential part of the cell. The nucleus is a small vesicular body embedded in the proto- plasm near the center of the cell. In the resting condition of the cell it consists of a distinct membrane, composed of amphipyrenin, in- closing the nuclear contents. The latter consists of a homogeneous Nuclear membrane. Linin. Nuclear fluid (matrix) . - Nucleolus. Chromatin-cords (nuclear network). * Nodal enlargements ^''' of the chromatin. Cell membrane. '--" Exoplasm, '^ Spongioplasm. - Hyaloplasm. ?riy---^ — Foreign inclosures. Fig. 2. — Diagram of a Cell. Microsomes and spongioplasm are only partly drawn. —{Stohr.) amorphous substance — ^the nuclear matrix — ^in which is embedded the nuclear network. It can often be seen that a portion of one side of the nucleus, called the pole, is free from this network. The main cords of the network are arranged as V -shaped loops about it. These main cords send out secondary branches or twigs, which, uniting with one another, complete the network. The nuclear cords are composed of granules of chromatin — so called because of its affinity for certain staining materials — ^held together by an achromatin substance known as linin. Besides the nuclear network, there are embedded in the nuclear matrix one or more small bodies composed of pyrenin, known as nucleoli. At the pole of the nucleus, either within or just without in the protoplasm, is a small body, the centro- some, or pole corpuscle. Chemic Composition of the Cell. — The composition of living 28 TEXT-BOOK OF PHYSIOLOGY. bioplasm is difficult of determination, for the reason that all chemic and physical methods employed for its analysis destroy its vitality, and the products obtained are peculiar to dead rather than to living matter. Moreover, as bioplasm is the seat of extensive chemic changes, it is not easy to determine whether the products of analysis are crude food constituents or cleavage or disintegration products. Nevertheless, chemic investigations have shown that even in the Uving condition bioplasm is a highly complex compound — ^the resultant of the intimate union of many different substances. About 75 per cent, of bioplasm consists of water and 25 per cent, of sohds, of which the more important compounds are various nucleo-proteins (characterized by their large percentage of phosphorus), globuhiis, traces of lecithin, cholesterin, and possibly fat and carbohydrates. Inorganic salts, es- pecially the potassium, sodium, and calcium chlorids and phosphates, are almost invariable and essential constituents. MANIFESTATIONS OF CELL LIFE. Growth, Nutrition, and Reproduction. — ^AU cells exhibit the three fundamental properties of life — ^viz., growth, nutrition, repro- duction. Growth is an increase in size. When newly reproduced all cells are extremely small, but in consequence of their organization and the character of their surrounding medium, they gradually grow until they attain the size characteristic of the adult state. Nutrition is the maintenance of the physiologic condition of the cell and includes both growth and repair. So long as this is accom- plished, the cells and the tissues which are formed by them continue to exhibit their functions or their characteristic modes of activity. Both growth and nutrition are dependent on the power which Uving material possesses of not only absorbing nutritive material from the surrounding medium, the lymph, but of subsequently assimilating it, organizing it, transforming it into material hke itself and endowing it with its own physiologic properties. In the physiologic condition the living material of the cell, the bioplasm, is the seat of a series of chemic changes which vary in degree from moment to moment in accordance with the degree of functional activity, and on the continuance of which all Ufe phenomena depend. Some of these chemic changes are related to or connected with the molecules of the living material, while others are connected with the food material suppUed to them. Of the chemic changes occurring within the molecules some are destructive, dissimilative or disintegrative in character, whereby the molecule is in part eventually reduced through a series of descending chemic stages to simpler com- pounds which, apparently of no use in the cell, are eliminated from it. It is, therefore, said that the Uving material undergoes molecular disintegration as a result of functional activity. To these changes the term katabolism is also appUed. Other of these changes are con- PHYSIOLOGY OF THE CELL. 29 structive, assimilative or integrative in character, whereby a part at least of the food material is transformed through a series of ascending chemic stages into living material, and whereby it is repaired and its former physiologic condition restored. It is, therefore, said that the living material undergoes molecular integration as a preparation for functional activity. To these changes the term anaholism is also applied. The sum total of all chemic changes which go on in the cell, both anabolic and katabolic, is embraced in the general term metab- olism. During the course of its physiologic activities the cell bioplasm produces materials of an entirely different character which vary with the cell,' such as fat, glycogen, mucigen, pigments, ferments, etc., which are generally spoken of as metabolic products. The food materials, sugar, fat and proteins, which are constituents of the fluids circulating in the tissue spaces, as above stated, are also reduced to simpler forms and the energy which they contain liberated in the form of heat. It is, therefore, said that the food also undergoes metabolism. There is, however, much difference of opinion as to the extent to which the living material is metabolized and to the actual disposition of the food materials, and especially the proteins. Thus Voit contends that the tissue molecules are comparatively stable in composition and under ordinary conditions of nutrition do not undergo any material change during either rest or activity, and that metabolism is confined to the food materials occupying spaces in and around the living cell. The cause which initiates this metabolism is unknown, but is supposed to reside in the cell, if it is not a property of the cell itself. Because of the fact that but a very small amount, if any, of sugar or fat enters into the composition of bioplasm it is generally admitted that these foods are metabolized in the tissue spaces and in the manner just alluded to. The problem, however, is different in the case of the proteins. Voit contends, as previously stated, that the proteins of the tissue molecules, which he distinguishes as tissue proteins, do not metabolize and confines all protein metabolism to the food pro- teins circulating in the tissue spaces and which he distinguishes as circulating proteins. Even in starvation the tissue proteins, as such, do not metabolize until they have been dissolved in consequence of chemic changes and transformed into circulating protein. Pfliiger, however, asserts that the circulating proteins can not be metabolized but that they must first be built up into tissue proteins. The metabolism of protein is, therefore, confined, in this view, to the molecules of the living material. It is possible, however, that both views are correct and that in the physiologic condition the activity of the tissues is attended by a partial destruction of the tissue molecules which is followed in turn by construction during the subsequent rest, but that the greater part of the protein metabolism takes place out- side the cell, though in contact with it. Though the cell is, therefore, the seat of two opposing processes, 30 TEXT-BOOK OF PHYSIOLOGY. assimilation and dissimilation, it retains under normal conditions an average physiologic state, and so long as this is the case it is in a con- dition of nutritive equilibrium and capable of performing its various functions. Though the foregoing statements are applied to the individual cell they are equally apphcable to the body as a whole, inasmuch as the organs and tissues of which it consists are composed of cells. The body grows in size and maintains its nutrition, by the introduction of food materials which are utilized in part, for the repair of the tissues which have undergone molecular disintegration in consequence of activity, and in part for the hberation of energy. As a resiilt of the disintegration or the metaboHsm of tissue and food materials, products such as carbon dioxid, urea, etc., are formed which, apparently of no further use, are discharged from the body by eliminating organs as the kidney, lungs, skin, etc. Assimilation and dissimilation are constantly taking place. If the food assimilated and metabohzed exactly replaces the tissues dissimilated and the food metabohzed the body will retain a condition of nutritive equihbrium. Physiologic Properties of Bioplasm. — All Kving bioplasm possesses properties which serve to distinguish and characterize it — viz., irritability, conductivity, and motihty. Irritability, or the power of reacting in a definite manner to some form of external excitation, whether mechanic, chemic, or electric, is a fundamental property of all hving bioplasm. The character and extent of the reaction will vary, and will depend both on the nature of the bioplasm and the character and strength of the stimulus. If the bioplasm be muscle, the response will be a contraction ; if it be gland, the response will be a secretion; if it be nerve, the response will be a sensation or some other form of nerve activity. Conductivity, or the power of transmitting molecular disturbances arising at one point to all portions of the irritable material, is also a characteristic feature of all bioplasm. This power, however, is best developed in that form of bioplasm found in nerves, which serves to transmit, with extreme rapidity, molecular disturbances arising at the periphery to the brain, as well as from the brain to the periphery. Muscle bioplasm also possesses the same power in a high degree. Motility, or the power of executing apparently spontaneous move- ments, is exhibited by many forms of cell bioplasm. In addition to the molecular movements which take place in certain cells, other forms of movement are exhibited, more or less constantly, by many cells in the animal body — e. g., the waving of dha, the ameboid movements and migrations of white blood-corpuscles, the activities of spermato- zooids, the projection of pseudopodia, etc. These movements, arising without any recognizable cause, are frequently spoken of as spontaneous. Strictly speaking, however, all protoplasmic movement is the resultant of natural causes, the true nature of which is beyond the reach of present methods of investigation. PHYSIOLOGY OF THE CELL. 31 Reproduction. — Cells reproduce themselves in the higher ani- mals in two ways — ^by direct division and by indirect division, or karyokinesis. In the former the nucleus becomes constricted, and divides without any special grouping of the nuclear elements. It is probable that this occurs only in disintegrating cells, and never in a physiologic multiplication. In division by karyokinesis (Fig. 3) there is a progressive rearranging and definite grouping of the nucleus, the result of which changes is the division of the centrosome, the chromatin, and the rest of the nucleus into two equal portions, which Close Skein (viewed from the side). Polar field. Loose Skein (viewed from above — i, e., from the pole). Mother Stars (viewed from the side). Polar ' radia- tion. -" Spindle. Mother Star (viewed Daughter Star Beginning Completed from above). Division of the Protoplasm. Fig. 3. — Kaeyokinetic Figures Observed in the Epithelium of the Oral Cavity of a Salamander. The picture in the upper right-hand corner is from a section through a dividing egg of Siredon pisciformis. Neither the centrosomes nor the first stages of the development of the spindle can be seen by this magnification. X S^°- — IS/tohr.) form the nuclei. Following the division of the nuclei, the protoplasm divides. The process may be divided into three phases: I. Prophase.- — ^The centrosome, at first small and lying within the nucleus, increases in size and moves into the protoplasm, where it lies near the nucleus, surrounded by a clear zone, from which delicate threads radiate through an area known as the attraction sphere. The nucleus enlarges and becomes richer in chromatin. The lateral twigs of the chromatin cords are drawn in, while the main cords become much contorted. These cords have a general direction transverse to the long axis of the cell, and parallel to the plane of future cleavage. They are seen as V- shaped segments or loops, chromosomes, having their closed ends directed toward a common center, the polar field, while the other 32 TEXT-BOOK OF PHYSIOLOGY. ends interdigitate on the opposite side of the nucleus — the anti- pole. The polar field corresponds to the area occupied by the cen- trosome. This arrangement is known as the close skein; but as the process goes on, the chromosomes become thicker, shorter and less contorted, producing a much looser arrangement, known as the loose skein. During the formation of the loose skein, the centrosome divides into two portions, which move apart to posi- tions at the opposite ends of the long axis of the nucleus. At the same time delicate achromatin fibers make their appearance, arranged in the form of a double cone, the apices of which corre- spond in position to the centrosomes. This is known as the nuclear spindle. During the prophase the nuclear membrane and the nucleoU disappear. 2. The Metaphase. — ^The two centrosomes are at opposite ends of the long axis of the nucleus, each surrounded by an attraction sphere, now called the folar radiation. The chromosomes become yet shorter and thicker, and move toward the equator of the nucleus, where they lie with their closed ends toward the axis, presenting the appearance, when seen from the poles, of a star — -the so-called mother star, or monaster. While moving toward the equator of the nucleus, and often earlier, each chromo- some undergoes longitudinal cleavage, the sister loops remaining together for a time. Upon the completion of the monaster, one loop of each pair passes to each pole of the nucleus, guided, and per- haps drawn by the threads of the nuclear spindle. The separation of the sister segments be^ns at their apices, and as the open ends are drawn apart they remain connected by delicate achromatin filaments drawn out from the chromosomes. This separation of the daughter chromosomes, and their movement toward the daughter centrosomes, is called metakinesis. As they approach their des- tination, we have the appearance of two stars in the nucleus — ^the daughter stars, or diasters. 3. Anaphase. — ^The daughter stars undergo, in reverse order, much the same changes that the mother star passed through. The chro- mosomes become much convoluted, and perhaps united to one another, the lateral twigs appear, and the chromatin resumes the appearance of the resting nucleus. The nuclear spindle, vnth most of the polar radiation, disappears, and the nucleoU and the nuclear membrane reappear, thus forming two complete daughter nuclei. Meanwhile the protoplasm becomes constricted midway between the young nuclei. This constriction gradually deepens until the original cell is divided, with the formation of two complete cells. CHAPTER IV. HISTOLOGY OF THE EPITHELIAL AND CONNECTIVE TISSUES. I. EPITHELIAL TISSUE. The epithelial tissue consists of one or more layers of cells resting on a homogeneous membrane, the other side of which is abundantly- supplied with blood-vessels and nerves. The form of the epithelial cell varies in different situations, and may be flattened, cuboid, sphe- roid, or columnar. (See Figs. 4, 5, and 6.) The form of the cell in all instances is related to some specific function. When arranged in layers or strata, the cells are cemented together by an intercellular substance. Fig. 4. — Epithelial Cells or Rabbit, Isolated. X S^° i- Squamous cells (mucous membrane of mouth). ^. Columnar cells (corneal epithelium). 3. Colum- nar cells, with cuticular border, j (intestinal epithelium). 4. Ciliated cells; h, cilia (bronchial epithelium) . — {Stohr.) The epithelial tissue forms a continuous covering for the surfaces of the body. The external investment (the skin) and the internal investment (the mucous membrane, which lines the entire alimentary canal as well as associated body cavities) are both formed, in all situations, by the homogeneous basement membrane, covered with one or more layers of cells. The glands of the skin, the lungs and the glands in connection with the aUmentary canal and the uro-genital apparatus are formed of the same elemental structures. All materials, therefore, whether nutritive, secretory, or excretory, must pass through epithehal cells before they can enter into the formation. of the blood or be eliminated from it. The nutrition of the epithelial tissue is maintained by the nutritive material derived from the blood diffusing itself into and through the basement membrane. Chemically, the epithehal cells of the epidermis — ^hair, nails, etc. — are- composed of 3 33 34 TEXT-BOOK OF PHYSIOLOGY. an albuminoid material (keratin), a small quantity of water, and inorganic salts. In other situations, especially on the mucous mem- , branes, the cells consist largely of mucin, in association with other proteins. The consistency of epithelium varies in accordance with external influences, such as the presence or absence of moisture, pres- sure, friction, etc. This is well seen in the skin of the palms of the hands and the soles of the feet — situations where it acquires its greatest density. In the alimentary canal, in the lungs, and in other cavities, where the reverse conditions prevail, the epithelium is extremely soft. Epithelial tissues also possess varying degrees of cohesion and elasticity —physical properties which enable them to resist considerable pressure and distention without having their physiologic integrity destroyed. Inasmuch as these tissues are poor conductors of heat, they assist in preventing too rapid radiation of heat from the body, and cooperate with other mechanisms in main- taining the normal temperature. Epithelium (Larynx or Man). X240- I. Columnar cells. 2. Prickle- cells. 3. Squamous cells.^{Stdhr.) Fig. 6. — Stratified Ciliated Epithelium. X 560. From the res- piratory nasal mucous membrane of man. i. Oval cells. 2. Spindle- shaped cells. 3. Columnar cells.— (Slohr.) The physiologic activity of all epithelial tissue depends on a due supply of nutritive material derived from the blood, which not only maintains its nutrition, but affords those materials out of which are formed the secretions of the glands, whether of the skin or mucous membrane. The cells lining the blood-vessels, the lymph- vessels, the peritoneal, pleural, pericardial, and other closed cavities are usually termed endothelial cells. These cells are flat, irregular in shape; with borders more or less wavy or sinuous in outline. Functions of Epithelial Tissue. ^In succeeding chapters the form, chemic composition, and functions of epithelial cells will be considered in connection with the functions of the organs of which they constitute a part. In this connection it may be stated in a general way that the functions of the epithelial tissues are : I . To serve on the surface of the body as a protective covering to the THE CONNECTIVE TISSUES. 35 underlying structures which collectively form the true skin, thus protecting them from the injurious influences of moisture, air, dust, microorganisms, etc., which would otherwise impair their vitality. Wherever continuous pressure is applied to the skin, as on the palms of the hands and soles of the feet, the epithelium increases in thickness and density, and thus prevents undue pressure on the nerves of the true skin. The density of the epidermis enables it to resist, within Umits, the injurious influence of acids, alkalies, and poisons. 2. To promote absorption. Inasmuch as the skin and mucous mem- branes cover the surfaces of the body, it is obvious that all nutritive material entering the body must first traverse the 'epithelial tissue. Owing to their density, however, the epitheUal cells covering the skin play but a feeble r61e as absorbing agents in man and the higher animals. The epithelium of the mucous mem- brane of the alimentary canal, particularly that of the small intestine, is especially adapted, from its situation, consistency, and properties, to play the chief r61e in the absorption of new materials from the canal. The epithelium Uning the air- vesicles of the lungs is engaged in promoting the absorption of oxygen and the exhalation of carbon dioxid. 3. To form secretions and excretions. Each secretory gland con- nected with the surfaces of the body is lined by epithelial cells, which are actively concerned in the formation of the secretion peculiar to the gland. Each excretory organ is similarly provided with epithelial cells, which are engaged either in the production of the constituents of the excretion or in their removal from the blood. 2. THE CONNECTIVE TISSUES. The connective tissues, in their collective capacity, constitute a framework which pervades the body in all directions, and, as the name imphes, serve as a bond of connection between the individual parts, at the same time affording a basis of support for the muscle, nerve, and gland tissues. The connective-tissue group includes a number of varieties, among which may be mentioned the areolar, adipose, retiform, white fibrous, yellow elastic, cartilaginous and osseous. Notwithstanding their apparent diversity, they possess many points of similarity. They have a common origin, developing from the same embryonic material; they have much the same struc- ture, passing imperceptibly into one another, and perform practically the same functions. Areolar Tissue. — ^This variety is found widely distributed through- out the body. It serves to unite the skin and mucous membrane to the structures on which they rest; to form sheaths for the support of blood-vessels, nerves, and lymphatics ; to unite into compact masses the muscular tissue of the body, etc. Examined with the naked eye, 36 TEXT-BOOK OF PHYSIOLOGY. it presents the appearance of being composed of bundles of fine fibers interlacing in every direction. In the embryonic state the elements of this form of connective tissue are united by a ground substance, gelatinous in character. In the adult state this substance shrinks and largely disappears, leaving intercommunicating spaces of vailing size and shape, from which the tissue takes its name. When subjected to the action of various reagents, and examined microscopically, the bundles can be shown to consist of extremely delicate, colorless, transparent, wavy fibers, which are cemented together by a ground substance composed largely of mucin. Other fibers are also observed, which are distingmshed by a straight course, a sharp, well-defined outline, a tendency to branch and unite with adjoining fibers, and to layers. Fig. 7. — Adipose Tissue. — {Stohr.) Fig. 8. — Fat-cells from the Axilla op Man. i. The equator of the cell in focus. 2. The ob- jective somewhat devated. 3, 4. Forms changed by pressure, p. Traces of protoplasm in the vicinity of the flat nucleus k. — {Stohr.) curl up at their extremities when torn. From their color and elasticity they are known as yellow elastic fibers. Distributed throughout the meshes of the areolar tissue are found flattened, irregularly branched, or stellate corpuscles, connective-tissue corpuscles, plasma cells, and granule cells. Adipose Tissue. — ^This tissue, which exists very generally through- out the body, though found most abundantly beneath the skin, around the kidneys, and in the bones, is practically but a modification of areolar tissue. In these situations it presents itself in small masses or lobules of varying size and shape, surrounded and penetrated by the fibers of connective tissue. (See Fig. 7.) Microscopic examination shows that these masses consist of small vesicles or cells, round, elUp- tical or polyhedral in shape, depending somewhat on pressure. (See Fig. 8.) Each vesicle consists of a thin, colorless, protoplasmic membrane, thickened at one point, in which a nucleus can usually be detected. This membrane incloses a globule of fat, which during fife is in the liquid state. It is composed of olein, stearin, and palmitin. The origin of the fat is to be referred to a retrograde change in the proto- THE CONNECTIVE TISSUES. 37 plasmic material of the connective-tissue cells. When this protoplasm becomes rich in carbon and hydrogen, it is speedily converted into fat, which makes its appearance in the form of minute drops in different portions of the cell. As the drops accumulate, at the expense of the cell protoplasm, they gradually coalesce, until there remains but a thin stratum of the protoplasm, which forms the wall of the vesicle. Adi- pose tissue may, therefore, be regarded as areolar tissue, in which, and at the expense of some of its elements, fat is stored for the future needs of the organism. A diminution of food, especially of fat and carbohydrates, is promptly followed by an absorption of fat by the blood-vessels and by its transference to the tissues, where it is either utilized for tissue construction or for oxidation purposes. In the situations in which adipose tissue is found it serves, by its chemic and physical properties, to assist in the prevention of a too rapid radiation of heat from the body, to give form and roundness, and to diminish angularities, etc. Retiform and adenoid tissue are also modifications of areolar tissue. The meshes of the former contain but little ground substance, its place being taken by fluids ; the meshes of the latter contain large numbers of lymph corpuscles. Fibrous Tissue.— This variety of connective tissue is widely distributed throughout the body. It constitutes Fig. 9.— Connective-tissue almost entirely the ligaments around the Bundles of Various Thick- ...... ^ , r ii 1 ,1 NESSES OF THE InIEEMUSCUXAE joints, the tendons of the muscles, the connective Tissue op Man. membranes covering organs such as the X 240.— (Stshr.) heart, Uver, nerve system, bones, etc. All fibrous tissue, wherever found, can be resolved into elementary bundles, which on microscopic examination are seen to consist of delicate, wavy, transparent, homogeneous fibers, which pursue an independent course, neither branching nor uniting with adjoining fibers. (See Fig. 9.) A small amount of ground substance serves to hold them together. Fibrous tissue is tough and inextensible, and in consequence is admirably adapted to fulfil various mechanical func- tions in the body. It is, however, quite pliant, bending easily in all directions. When boiled, fibrous tissue yields gelatin, a derivative of collagen. Elastic Tissue. — ^The fibers of elastic tissue are usually associated in varying proportions with the white fibrous tissue; but in some structures — ^as the Hgamentum nuchas, the ligamenta subflava, the middle coat of the larger blood-vessels — ^the elastic fibers are almost the only elements present, and give to these structures a distinctly yellow appearance. The fibers throughout their course give off 38 TEXT-BOOK OF PHYSIOLOGY. many branches, which unite with adjoining branches to form a more or less close network. As the name implies, these fibers are highly elastic, and are capable of being extended as much as 60 per cent, before breaking. (See Fig. 10.) Cartilaginous Tissue. — ^This form of connective tissue differs from the preceding varieties chiefly in its density. As a rule, it is firm in consistency, though somewhat elastic. It is opaque, bluish- white in color, though in thin sections translucent. All cartilaginous tissues consist of connective-tissue cells embedded in a solid ground ?a^%a^ mm Fig. 10. — Elastic Fibers. X S^°- -^^ ^^^^ elastic fibers, /, from intermuscular connective tissue of man; 6, connective-tissue bundles swelled by treatment with acetic acid- B. Very thick elastic fibers, /, from ligamentum nuchae of ox; b, connective-tissue bundles. C From a cross-section of the ligamentum nuchae of ox; /, elastic fibers; b, connective-tissue bundles. — (Stohr.) substance. According to the amount and texture of the ground substance, three principal varieties may be distinguished : I . Hyaline cartilage, in which the cells, relatively few in number, are embedded in an abundant quantity of ground substance (Fig. ii). The body of the cells is in many instances distinctly marked off from the surrounding substance by concentric hnes or fibers, which form a capsule for the cell. Repeated division of the cell substance takes place, until the whole capsule is completely occupied by daughter cells. The ground substance is pervaded by minute channels, which communicate on one hand with the spaces around the cells, and on the other with lymph-spaces in the connective tissue surrounding the cartilage. By means of these channels, nutritive fluid can permeate the entire structure. Hyaline cartilage is found on the ends of the long bones, where it enters into the formation of the joints; between the ribs and sternum, forming the costal cartilage, as well as in the nose and larynx. THE CONNECTIVE TISSUES. 39 White fibro-cartilage, the ground substance of which is pervaded by white fibers, arranged in bundles or layers, between which are scattered the usual encapsulated cells. (See Fig. 12.) White fibro-cartilage is tough, resistant, but flexible, and is found in joints where strength and fixedness are required. Hence it is present between the vertebrae, forming the intervertebral discs, between the condyle of the lower jaw and the glenoid fossa, in the knee-joint, around the margins of the joint cavities, etc. In these situations it assists in maintaining the apposition of the Fig. II. — Hyaline Cartilage. X 240- ^- Surface view of the ensiform process of frog, fresh; p, protoplasm of cartilage-cell, which entirely fills the lacuna; k, nucleus; g, hyaline matrix. B. Portion of cross-section of human rib-cartilage several days after death; examined in water: the protoplasm, z, of the cartilage-cells has withdrawn from the walls of the lacunae, /s; the nuclei are invisible, i. Two cells within one capsule, k; X, a developing partition. 2. Five cartilage-cells within one capsule; the lowest cell has fallen out, and here only the empty space is seen. 3. Capsule cut obliquely, and apparently thicker on one side. 4. Capsule not cut, but showing the cell within, g. Hyaline matrix transformed into rigid fibers, /. — (Stohr.) bones, in giving a certain degree of mobility to the joints, and in diminishing the effects of shock and pressure imparted to the bones. 3. Yellow fibro-cartilage, the ground substance of which is pervaded by opaque, yellow elastic fibers, which form, by the interlacing of their branches, a complicated network, in the meshes of which are to be found the usual corpuscles. (See Fig. 13.) As these fibers are elastic, they impart to the cartilage a very considerable degree of elasticity. Yellow fibro-cartilage is well adapted, there- fore, for entering into the formation of the external ear, epiglottis, Eustachian tube, etc. — structures which require for their func- tional activity a certain degree of flexibility and elasticity. Osseous Tissue. — Osseous tissue, as distinguished from bone, is a member of the connective-tissue group, the ground substance of 40 TEXT-BOOK OF PHYSIOLOGY. which is permeated with insoluble lime salts, of which the phosphate and carbonate are the most abundant. Immersed in dilute solutions of hydrochloric acid, they can be converted into soluble salts and dis- solved out. The osseous matrix left behind is soft and pliable. When boiled, it yields gelatin. A thin, transverse section of a decalcified bone, when examined microscopically, reveals a number of small, round, or oval openings, which represent transverse sections of canals which run through the bone, for the most part in a longitudinal direction, though frequently anastomosing with one another. These so-caUed Haver- sian canals in the Uving state contain blood-vessels and lymphatics. (See Fig. 14.) Around each Haversian canal is a series of concentric laminte, composed of white fibers. Between every two laminae are found small cavities (lacunae), from which radiate in all directions small canals (canalicuh), which communicate freely with one another. The Haversian canals, with their associated lacunee and canalicuh, form a system of inter- communicating passages, which circulate lymph destined for the Fig. 12. — From a Horizontal Section of the Intervertebral Disc oe Man. g. Fibrillar connec- tive tissue, z. Cartilage-cell (nucleus invisible), k. Capsule surrounded by calcareous granules. X 240. — (Stohr.) «u#*'«- lan^ «- Fig. 13. — Elastic Cartilage. X 240- i- Portion of section of vocal process (ante- rior angle) of arytenoid cartilage of a woman thirty years old; the elastic substance in the form of granules. 2 and 3. Portions of sections of epiglottis of a woman sixty years old; a fine network of elastic fibers in 2, a coarser network in 3. z. CartUage-cell, nucleus not visible; k, capsule. — (Stohr.) nourishment of bone. Each lacuna contains the bone corpuscle, which bears a close resemblance to the usual branched connective-tissue cor- puscle, and whose function appears to be the maintenance of the nutrition of the bone. THE CONNECTIVE TISSUES. 41 The surface of every bone in the living state is invested with a fibrous membrane, the periosteum, except where it is covered with cartilage. The inner surface of this membrane is loose in texture, and supports a fine plexus of capillary blood-vessels and numerous protoplasmic cells— the osteoblasts. As this layer is directly con- cerned in the formation of bone, it is spoken of as the osteogenetic layer. A section of a bone shows that it is composed of two kinds of tissue — compact and cancellated. The compact is dense, resembling ivory, and is found on the outer portion of the bone; the cancellated Periosteum. Outer ground lamellae. Haversian canals. Haversian lamellas. Interstitial lamellas. Inner ground lamellee. Marrow. Fig. 14. — From a Cross-section of a Metacaep of Man. X 50. The Haver- sian canals contain a little marrow (fat-cells). Resorption line at h. — {Stdhr.) is spongy, and appears to be made up of thin, bony plates, which intersect one another in all directions, and is found in greatest abun- dance in the interior of the bones. The shaft of a long bone is hollow. This central cavity, which extends from one end of the bone to the other, as well as the interstices of the cancellated tissue, is filled in the living state with marrow. The marrow or medulla is composed of a connective-tissue framework supporting blood-vessels. In its meshes are to be found characteristic bone cells or osteoblasts, the function of which is supposed to be the formation of bone. In the long bones the marrow is yellow, from the presence in the connective-tissue corpuscle of fat globules, which arise through the transformation of the cell protoplasm. In the cancellated tissue, near the extremities of the long bones, this fatty transformation does not take place to the same extent, and the marrow appears red. The cells of the red marrow are believed to give birth indirectly to the red blood-cor- puscles. Physical and Physiologic Properties of Connective Tissues.— Among the physical properties may be mentioned consistency, cohesion, and elasticity. Their consistency varies from the semiliquid to the solid state, and depends on the quantity of water which enters into 42 TEXT-BOOK OF PHYSIOLOGY. their composition. Their cohesion, except in the softer varieties, is very considerable, and offers great resistance to traction, pressure, torsion, etc. In all the movements of the body, in the contraction of muscles, in the performance of work, the consistence and cohesion of these tissues play most important r61es. Wherever the various forms of connective tissue are found, their chemic composition and structure are in relation to their functions. If traction be the preponderating force, the structure becomes fibrous, as in Ugaments and tendons, and the cohesion greatest in the longitudinal direction. If pressure be exerted in all directions, as upon membranes, the fibers interlace and offer a uniform resistance. When pressure is exerted in a definite direction, as on the extremities of the long bones, the tissue becomes expanded and cancellated. The lamellae of the cancellated tissue arrange themselves in curves which correspond to the direction of the greatest pressure or traction. Extensibility is not a characteristic feature, except in those forms containing an abundance of yellow elastic fibers. The elasticity is an essential factor in many physiologic actions. It not only opposes and Hmits forces of traction, pressure, torsion, etc., but on their cessation returns the tissues or organs to their original condition. Elasticity thus assists in maintaining the natural form and position of the organs by counterbalancing and opposing temporarily acting forces. The Skeleton. — ^The connective tissues in their entirety con- stitute a framework which presents itself under two aspects: (i) As a solid, bony skeleton, situated in the trunk and limbs, affording attachment for muscles and viscera; (2) as a fine, fibrous skeleton, found everywhere throughout the body, connecting the various viscera and affording support for the epithelial, muscle, and nerve tissues. CHAPTER V. THE PHYSIOLOGY OF MOVEMENT. The Animal Body as a Machine for Doing Work.— Of the four phenomena presented by an animal, that which more immediately interests the physiologist is movement, for the reason that it is not only its most characteristic form of activity, and that which serves to distinguish it in the main from forms of vegetable Ufe, but its solution affords an explanation of many physiologic processes occurring within the human body. It is also for this reason that movement constitutes for the most part the subject matter of physiologic experimentation. The movements may be general as when the animal changes its position relatively to its environment as in the various acts of loco- motion, or special as in the changes of relation of one part of the body with reference to another. In the execution of these movements the animal, of necessity, meets with various forms of resistance, viz. : gravity, cohesion, friction, etc. When its different parts are appUed or directed, either volitionally and in a determinate manner, or non-volitionally and in an indeter- minate or reflex manner to the overcoming of these opposing forces in the environment, the animal may be said to be doing work. In addition to these obvious external movements, a series of less obvious though no less characteristic internal movements are being exhibited by the various organs of the animal, e. g., heart, lungs, stomach, intestines, bladder, etc.; and when these organs are applied to the overcoming of opposing forces or resistances, as they are from moment to moment in the performance of their functions, it may be said that they also are doing work. The cooperation of both external and internal organs is necessary, not only for the maintenance of the Ufe of the animal but also for the accompUshment of work. In the con- ception of the animal body as a machine for doing work, the skeletal, the muscle and nerve tissues constitute a primary mechanism in which each bears a certain definite relation to the other. The internal organs collectively constitute a secondary mechanism in which each bears not only a definite relation to the other, but to the primary mechanism as well. The relation of skeletal, muscle and nerve tissue, are shown in Fig. 15. General Considerations. — The skeleton is the passive framework of the body, the axial portions of which (the vertebral column, ribs, sternum and skull) impart more or less fixity and rigidity to the mechanism, while the appendicular portions (the bones of the arms and legs) impart extreme mobihty. The bones of the arms and legs, 43 44 TEXT-BOOK OF PHYSIOLOGY. CSC Fig. 15. — Diagram {Showing the Relaton of Skeletal, Muscle and Nerve Tissues. (G. Bachman.) f.a. Bones of the forearm representing the skeletal tissue; e.j. the elbow joint, the fulcrum of the lever formed by the bones of the forearm; W. a weight acting in a downward direction and representing the passive force of gravity; sk.m. a skeletal muscle acting in an upward direction and the source of the active power to be ap- plied to the lever; sp.c. transection of the spinal cord showing the relation of the white and the gray matter: m.c. a motor cell in the anterior horn of the gray matter; ef.n. an effer- ent nerve-fiber connecting the motor cell from which it arises with the skeletal muscle and contained in the ventral roots of the spinal nerves ; af.n. an afferent nerve-fiber arising from the ganglion cell along its course and connecting the skin, s., on the one hand with the spinal cord on the other hand and contained in the dorsal roots of the spinal nerves; c.s.c. coronal section of the cerebrum showing the relation of the gray to the white matter; v.c. a volitional or motor cell; d.a. a descending axon or nerve-fib?r connecting the volitional cell from which it arises vpith the motor cell in the spinal cord; s.c. a sensor cell; a.a. an ascending axon or nerve-fiber connecting a receptive cell from which it arises (not shown in the diagram) with the sensor cell in the gray matter of the cerebrum. The nerve-fibers which pass outv/ard from the spinal cord to the glands, blood-vessels, and the muscle walls of tie viscera, have for the sake of simplicity been omitted from the diagram. THE PHYSIOLOGY OF MOVEMENT. 45 more especially, may be looked upon as constituting a system of levers, the fulcra of which, the points around which they move, he in the joints. That a lever may be effective as an instrument for the accomplish- ment of work, it must not only be capable of moving around its ful- crum, but it must, at the same time, be acted on by two opposing forces, one passive, the other active. In the movements of the bony levers of the animal body, the passive forces to be overcome are largely those connected with the environment, e. g., gravity, cohesion, friction, elasticity, etc. The active forces by which these are opposed and overcome through the mediation of the bony levers are found in the muscles attached to them. The muscle tissue may therefore be regarded as the seat of those energies that impart movement to the levers. This tissue is arranged in masses of irregular shape and size, termed muscles. The majority of the muscles of the body are connected with the bones of the body in such a manner that by an alteration in their form, they can change not only the position of the bones with reference to one another, but also the individual's relation to surrounding objects. When the lever is appKed to the overcoming of an opposing force, the muscle is said to be doing work. Though consisting of a highly active tissue, muscles in themselves do not possess spontaneity of action, but require for the manifestation of their energy the stimulating influence of nerve energy or nerve impulses developed in, and transmitted by, nerve tissue to the muscles. The nerve tissue is the seat of origin of those energies or impulses necessary to the physiologic excitation of the muscles. This tissue is partly arranged in masses contained in the cavities of the head and spinal column (the brain and spinal cord), forming the central organs of the nerve system, and partly in the form of cords or nerves, forming the peripheral organs of the nerve system; the latter connects the for- mer, not only with the muscles, but with glands, blood-vessels, skin, mucous membranes, etc., as well. A transection of the spinal cord shows that it is composed externally of white matter and internally of grey matter. The grey matter is arranged in the form of two crescents united in the median line by a transverse band or commissure forming a figure resembhng the letter H. Though varying in shape in different regions of the cord, the grey matter in all situations presents on either side an anterior or ventral and a posterior or dorsal horn. In the ventral horns of the grey matter are located nerve cells which not only generate, but under appropriate circumstances discharge, a form of energy termed a nerve impulse, which is transmitted by efferent nerve fibers arising from the cells, and by way of the ventral roots of the spinal nerves, to the muscles, glands, blood-vessels, and walls of the viscera with which they are directly or indirectly connected. 46 TEXT-BOOK OF PHYSIOLOGY. The arrival of the nerve impulse at once calls forth the form of activity characteristic of the structure excited. Thus the muscle, for example, passes from the passive to the active state, that is, the muscle becomes shorter and thicker, and the bone to which it is attached is moved. This is at once followed by a return of the muscle to the passive state; that is, it lengthens, be- comes narrower, and resumes its original form; the bone at the same time returns to its former position. Coincident with this change of shape there is a hberation of heat and electricity. The nerve impulse which occasions this transforma- tion of potential into kinetic energy is the normal or the physiologic stimulus. The muscle responding to the stimulus is said to be irrit- able or to possess irritability. The glands in response to the nerve impulse pour out a secretion. The blood-vessels and viscera change their caliber. These tissues, too, responding to the nerve impulse, are said to be irritable. The nerve cells of the spinal cordy however, do not possess spontaneity of action but require for their excitation the arrival of other nerve impulses. These may come, (i) from the per- iphery through afferent nerve fibers by way of the dorsal roots of the spinal nerves; and (2) from the brain as a result of an act of volition through descending axons or nerve fibers. In the first instance, the resulting movement taking place independently of volition, and in re- sponse to a peripheral or surface excitation, is termed a reflex move^ ment ; in the second instance, the resulting movement taking place in response to an act of volition is termed conventionally a volitional or voluntary movement. In the case of reflex movements, the nerve impulses are primarily developed in speciahzed organs located in the skin or mucous membranes and as a result of the impact of various external agents, which for this reason are termed stimuli. The nerve impulses thus developed are transmitted by the afferent nerves to the nerve cells which afe in turn excited to activity. In the case of volitional muscle movements, the nerve impulses which cause the movement are discharged from certain motor or voHtional nerve cells in the grey matter of the cerebrum and trans- mitted by descending axons or nerve fibers direct to the nerve cells in the spinal cord, by which they in turn are excited to activity. The volitional movements are however the immediate or the more or less remote effects of sensations which have been evoked in the sense areas of the brain, by the arrival of nerve impulses coming through ascending axons or nerve fibers from peripheral sense organs, e. g., skin, eye, ear, nose, tongue, and which have been developed by the impact of objects in the external world. The nerve cells and their related nerve fibers, responding by the development and conduction of nerve impulses are also said to be irritable. The transformation of energy, however, manifests itself mainly as electricity and molecular motion. Thp animal body in its THE PHYSIOLOGY OF MOVEMENT. 47 entirety may therefore be regarded as a machine for the transformation of potential energy into kinetic energy, viz. : heat and electricity, move- ments of muscles and bony levers, secretion, sensation and other forms of nerve activity. When muscles and bones are appUed to the over- coming of opposing forces, mechanic work is accomplished. In the following chapters some of the problems connected with the activities of the skeletal, muscle and nerve tissues will be considered. CHAPTER VI. THE PHYSIOLOGY OF THE SKELETON. The skeleton in its entirety determines the plan of organization of the animal body. Its axial portion is the foundation element and the center around which the appendicular portions are developed and arranged with a certain degree of conformity. The character and the arrangement of the bones of the axial portion endow the animal mechanism with a certain degree of fixity, combined with slight mo- bility, while the character and arrangement of the bones of the append- icular portions endow it with extreme mobility. The bones collectively constitute a system of levers, the fulcra of which lie in the points of union of the bones, and with which the animal is enabled to execute a variety of movements, to change its position relatively to its environ- ment and overcome opposing forces. The structure and the chemic composition of the bones, consisting as they do of inorganic matter 67 per cent, and of organic matter 33 per cent, endow them with both rigidity and elasticity, physical properties which admirably adapt them to the character of the work necessitated by the environment and the organization of the animal. The rigidity of bone is considerable as compared with other hard and rigid materials. The breaking limit, in terms of the weight in kilos required to tear across a rod one square millimeter in cross-section of various materials is as follows : Cast iron 13; bone 12; oak 6.5; granite 1.9. The elasticity is about one-sixth that of wrought iron and twice that of oak parallel to the grain (MacAUster). In youth bones are quite elastic; in old age they are fragile because of a diminution of tissue and an increased porosity, and, therefore, at both periods less capable of functionating as effectively as in the middle period of life. The skeleton also serves for the attachment of muscles and affords support and protection to viscera. For the manifestation of the activities of the animal it is essential that the relation of the various portions of the bony skeleton to one another shall be such as to permit of movement while yet retaining close apposition. This is accomplished by the mechanical conditions which have been evolved at the points of union of bones, and which are technically known as articulations or joints. A consideration of the body movements involves an account of (i) the static conditions, or those states of equilibrium in which the body is at rest — e. g., standing, sitting; (2) the dynamic conditions, or those states of activity characterized by movement- — e. g., walking, running, etc. In this connection, however, only those physical and physiologic pecuharities of the skeleton, especially in its relation to 48 THE PHYSIOLOGY OF THE SKELETON. 49 joints, will be referred to, which underUe and determine both the static and dynamic states of the body. Structure of Joints.— The structures entering into the formation of joints are : 1. Bones, the articulating surfaces of which are often more or less expanded, especially in the case of long bones, and at the same time variously modified and adapted to one another in accordance with the character and extent of the movements which there take place. 2. Hyaline cartilage, which is closely applied to the articulating end of each bone. The smoothness of this form of cartilage faciU- tates the movements of the opposing surfaces, while its elasticity dinunishes the force of shocks and jars imparted to the bones during various muscular acts. In a number of joints, plates or discs of white fibro-cartilage are inserted between the surfaces of the bones. 3. A synovial membrane, which is attached to the edge of the hyaline cartilage, entirely inclosing the cavity of the joint. This mem- brane is composed largely of connective tissue, the inner surface of which is hned by endothehal cells, which secrete a clear, color- less, viscid fluid — the synovia. This fluid not only fills up the joint-cavity, but, flowing over the articulating surfaces, diminishes or prevents friction. 4. Ligaments — ^tough, inelastic bands, composed of white fibrous tissue — which pass from bone to bone in various directions on the different aspects of the joint. As white fibrous tissue is in- extensible but phant, ligaments assist in keeping the bones in apposition, and prevent displacement while yet permitting of free and easy movements. Classification of Joints. — All joints may be divided, according to the extent and kind of movements permitted by them, into (i) diarthroses; (2) amphiarthroses ; (3) synarthroses. I. Diarthroses. — ^In this division of the joints are included all those which permit of free movement. In the majority of instances the articulating surfaces are mutually adapted to each other. If the articulating surface of one bone is convex, the opposing but corresponding surface is concave. Each surface, therefore, represents a section of a sphere or a cyhnder, which latter arises by rotation of a hne around an axis in space. According to the number of axes around which the movements take place all diarthrodial joints may be divided into: I. Uniaxial Joints. — In this group the convex articulating surface is a segment of a cyhnder or cone, to which the opposing surface more or less completely corresponds. In such a joint the single axis of rotation, though nearly, is not exactly at right angles to the long axis of the bone, and hence the movements — ^flexion and extension — ^which take place are not confined to one plane. so TEXT-BOOK OF PHYSIOLOGY. Joints of this character — e. g., the elbow, knee, ankle, the pha- langeal joints of the fingers and toes — are, therefore, termed ginglymi, or hinge-joints. Owing to the obliquity of their ar- ticulating surfaces, the elbow and ankle are cochleoid or screw- ginglymi. Inasmuch as the axes of these joints on the opposite sides of the body are not coincident, the right elbow and left ankle are right-handed screws; the left elbow and right ankle, left-handed screws. In the knee-joint the form and arrangement of the articulating surfaces are such as to produce that modifica- tion of a simple hinge known as a spiral hinge, or helicoid. As the articulating surfaces of the condyles of the femur increase in convexity from before backward, and as the inner condyle is longer than the outer, and, therefore, represents a spiral surface, the Une of translation or the movement of the leg is also a spiral movement. During flexion of the leg there is a simultaneous inward rotation around a vertical axis passing through the outer condyle of the femur; during extension a reverse movement takes place. Moreover, the slightly concave articulating surfaces of the tibia do not revolve around a single fixed transverse axis, as in the elbow-joint, for during flexion they slide backward, during extension forward, around a shifting axis, which varies in posi- tion with the point of contact. In some few instances the axis of rotation of the articulating surface is parallel with rather than transverse to the long axis of the bone, and as the movement then takes place around a more or less conic surface, the joint is termed a trochoid or pulley — e. g., the odonto-atlantal and the radio-ulnar. In the former the collar formed by the atlas and its transverse ligament rotates around the vertical odontoid process of the axis. In the latter the head of the radius revolves around its own long axis upon the ulna, giving rise to the movements of pronation and supination of the hand. The axis around which these two movements take place is continued through the head of the radius to the styloid process of the ulna. 2. Biaxial Joints. — ^In this group the articulating surfaces are un- equally curved, though intersecting each other. When the sur- faces lie in the same direction, the joint is termed an ovoid joint — e. g., the radio-carpal and the atlanto-ocdpital. As the axes of these surfaces are vertical to each other, the movements per- mitted by the former joint are flexion, extension, adduction, and abduction, combined with a slight amount of circumduction; the latter joint permits of flexion and extension of the head, with inclination to either side. When the surfaces do not take the same direction, the joint, from its resemblance to the surfaces of a saddle, is termed a saddle-joint — e. g., the trapezio-metacarpal. The m.ovements permitted by this joint are also flexion, exten- sion, adduction, abduction, and circumduction. THE PHYSIOLOGY OF THE SKELETON. 51 3. Polyaxial Joints. — In. this group the convex articulating surface is a segment of a sphere, which is received by a socket formed by the opposing articulating surface. In such a joint, termed an enarthrodial or ball-and-socket joint — e. g., the shoulder-joint, hip-joint — ^the distal bone revolves around an indefinite number of axes, all of which intersect one another at the center of rotation. For simphcity, however, the movement may be described as taking place around axes in the three ordinal planes — ^viz., a .transverse, a sagittal, and a vertical axis. The movements around the transverse axis are termed flexion and extension; around the sagittal axis, adduction and abduction; around the vertical axis, rotation. AVhen the bone revolves around the surface of an imaginary cone, the apex of which is the center of rotation and the base the curve described by the hand, the movement is termed circumduction. 2. Amphiarihroses. — ^In this division are included all those joints which permit of but slight movement — e. g., the intervertebral, the interpubic, and the sacro-iUac joints. The surfaces of the opposing bones are united and held in position largely by the intervention of a firm, elastic disc of fibro-cartilage. Each joint is also strengthened by ligaments. 3. Synarthroses. — In this division are included all those joints in which the opposing surfaces of the bones are immovably united, and hence do not permit of any movement — e. g., the joints between the bones of the skull. The Vertebral Column. — ^In all static and dynamic states of the body the vertebral column plays a most essential r61e. Situated in the middle of the back of the trunk, it forms the foundation of the entire skeleton. It is composed of a series of superimposed bones, termed vertebras, which increase in size from above downward as far as the brim of the pelvic cavity. Superiorly, it supports the skull ; laterally, it affords attachment for the ribs, which in turn support the wfeight of the upper extremities ; below, it rests upon the pelvic bones, which transmit the weight of the body to the inferior extremities. The bodies of the vertebrae are united one to another by tough elastic discs of fibro-cartilage, which, collectively, constitute about one- quarter of the length of the vertebral column. The vertebrae are held together by Ugaments situated on the anterior and posterior surfaces of their bodies, and by short, elastic Hgaments between the neural arches and processes. These structures combine to render the verte- bral column elastic and flexible, and enable it to resist and diminish the force of shocks communicated to it. The amphiarthrodial character of the intervertebral joints endows the entire column with certain fornls of movement which are neces- sary to the performance of many body activities. While the range of movement between any two vertebras is shght, the sum total of move- ment of the entire series of vertebrae is considerable. In different 52 TEXT-BOOK OF PHYSIOLOGY. regions of the column the character, as well as the range of move- ment, varies in accordance with the form of the vertebrae and the incUnation of their articular processes. In the cervical and lumbar regions extension and flexion are freely permitted, though the former is greater in the cervical, the latter in the lumbar region, especially between the fourth and fifth vertebras. Lateral flexion takes place in all portions of the column, but is particularly marked in the cer- vical region. A rotatory movement of the column as a whole takes place through an angle of about twenty-eight degrees. This is most evident in the lower cervical and dorsal regions. CHAPTER VII. GENERAL PHYSIOLOGY OF MUSCLE-TISSUE. The muscle-tissue, which closely invests the bones of the body and which is famihar to all as the flesh of animals, is the immediate cause of the active movements of the body. This tissue is grouped in masses of varying size and shape, which are technically known as muscles. The majority of the muscles of the body are connected with the bones of the skeleton in such a manner that, by an alteration in their form, they can change not only the position of the bones with reference to one another, but can also change the individual's relation to surrounding objects. They are, therefore, the active organs of both motion and locomotion, in contradistinction to the bones and joints, which are but passive agents in the performance of the corre- sponding movements. In addition to the muscle masses which are attached to the skeleton, there are also other collections of muscle- tissue surrounding cavities such as the stomach, intestine, blood- vessels, etc., which impart to their walls motiUty, and so influence the passage of material through them. Muscles produce movement of the structures to which they are attached by the property with which they are endowed of changing their shape, shortening or contracting under the influence of a stimulus transmitted to them from the nervous system. Muscles are divided into: 1. Voluntary muscles, comprising those the activity of which is called forth by an act or effort of volition. 2. Involuntary muscles, comprising those the activity of wJiich is entirely independent of the volition. The voluntary muscles are also known from their attachment to the skeleton as skeletal, and from their microscopic appearance as striped or striated muscles. Though for the most part these muscles are red, there are certain muscles in man and other animals which are pale in color and in many muscles pale fibers are extensively dis- tributed among the red fibers. The involuntary muscles, from their relation to the viscera of the body, are known also as visceral, and from their microscopic appearance as plain, smooth, or non-striated muscles. THE VOLUNTARY OR SKELETAL MUSCLE. All skeletal muscles consist of a central fleshy portion, the body or belly, provided at either extremity with a tendon in the form of a cord or membrane. The body is the active, contractile region, the 53 S4 TEXT-BOOK OF PHYSIOLOGY. source of the movement; the tendon is the inactive region, the passive transmitter of the movement to the bones. A skeletal muscle is a complex organ consisting of a framework of connective tissue, supporting muscle-fibers, blood-vessels, nerves, and lymphatics. The general body of the muscle is covered by a dense layer of connective tissue, the epi-mysium, which blends with and partly forms the tendon. From the under surface of this covering, septa of connective tissue pass inward, dividing and grouping the fibers into larger and smaller bundles, termed f ascicuU. The fascicuU, invested by a special sheath, the peri-mysium, are prismatic in shape and on cross-section pre- sent an irregular outUne. The muscle-fibers composing the fas- cicuU are separated one from another and supported by a very deUcate connective tissue, the endo-mysium. The connective tissue thus surrounding and penetrating the muscle binds the fibers into a distinct organ and affords support to all remaining structures (Fig. i6). Histology of the Skeletal Muscle -fiber. — ^The muscle- fiber is the ultimate anatomic units of the muscle system. The fibers for the most part are ar- ranged parallel one to another and in a direction corresponding to the long axis of the muscle. They vary in length from 30 to 40 milUmeters and in breadth from 20 to 30 micromilHmeters. There are exceptional fibers, however, which have a much greater length. As the fibers have but a Umited length in the vast majority of muscles, each end, more or less pointed or beveled, is united to adjoining fibers by cement. In this way a muscle is increased in length. When examined with the microscope, the muscle-fiber is seen to be cyhndric or prismatic in shape and to consist of a thin transparent membrane, the sarcolemma, in which is contained the true muscle or sarcous substance. The sarcolemma is elastic and adapts itself to all changes of form the sarcous substance undergoes. Beneath the sarcolemma there are several nuclei surrounded by granular material. Each fiber also presents a series of transverse bands alter- nately dim and bright which give to it a striated appearance. If the Fig. 16. — From a Cross-section of the Adductor Muscle of a Rabbit. P. Peri- mysium, containing two blood-vessels, at g; m, muscle-fibers; many are shrunken and be- tween them the endomysium, p, can be seen; at X the section of muscle-fiber has fallen out. X 60.— (Stokr.) GENERAL PHYSIOLOGY OF MUSCLE-TISSUE. 55 Fig. 17. — Muscle-fiber OF A RABBir. a. Daxk band. b. Light band. c. In- termediate line. «. Nucleus. — {Landois and Stirling. — Ranvier.) bright bands are examined with high magnifying powers, each one is seen to be crossed by a fine dark line which at the time of its discovery was regarded as the optic expression of a mem- brane attached laterally to the sarcolemma. It has since been resolved into a row of gran- ules (Fig. 17). The muscle-fiber also presents a longit- udinal striation which indicates that it is composed of finer elements placed side by side, termed fibrillae. The fibrillae extend through- out the entire length of the fiber, though they are not of uniform thickness (Fig. 18). That portion of the fibril corresponding in position to the dim band is thick, prismatic, or rod-like in shape, and termed a sarcostyle; that por- tion corresponding in position to the bright band is extremely thin and narrow and pre- sents at its middle a slight enlargement or granule. The fibrillas are embedded in a clear transparent fluid which, from its supposed nutritive character, is termed sarcoplasm. The diminution in caliber of the fibrillae at different levels permits of the accumulation and storage of a larger amount of nutritive material than could otherwise be the case. It is for this reason that the fiber at these points presents a brighter appearance. When the muscle-fiber is examined under crossed Nichol prisms, the dim band appears bright and the bright band appears dim against a dark background, indicating that the former is doubly refracting or anisotropic, the latter singly refracting or isotropic. The pale muscle fiber is histologically similar to the red. It is, however, somewhat larger, paler in color and does not contain so much muscle material. The transverse striation is closer and the nuclei are not so abundant. The Blood-supply. — Muscles in the physiologic condition require for the main- tenance of their activity a large amount of nutritive material. This is obtained directly from the lymph and indirectly from the blood furnished by the blood-vessels. The vascular supply to the muscles is very great and the disposition of the capillary vessels mXh. refer- ence to the muscle-fiber is very characteristic. The arterial vessels, after entering the muscle, are supported by the peri-mysium; in this Fig. 18 — A. Diagram of arrangement of the contrac- tile substance according to the view of RoUett; the granular figures represent the contractile elements, the intervening light areas the sarcoplasm. B. Small muscle-fiber of man, the corresponding parts in the two figures are indicated; t, i, I, respectively the trans- verse, the intermediate, and lateral discs, n. Muscle nuclei. — (Piersol.) 56 TEXT-BOOK OF PHYSIOLOGY. LYMPH SPACE MUSCLE . FIBER ■CAPILLARY BLOOD VESSEL situation they give off short, transverse branches, which immediately break up into a capillary network of rectangular shape within which the muscle-fibers are contained. The relation of the capillary vessel to the niuscle-fiber is shown in Fig. 19. The muscle-fiber, in intimate relation with the capillary, is bathed with lymph derived from it. Its contractile substance, however, is separated from the lymph by its own investing membrane, through which all interchange of nutritive and waste material must take place. The nutritive material passes through the capillary wall into the lymph-space, then through the sarcolemma into the interior of the fiber, where it comes into re- lation with the Uving muscle material. The waste prod- ucts arising in the muscle as a result of nutritive changes pass in the reverse direction first into the lymph and then into the blood, by which they are carried away to ehminat- ing organs. Lymphatics are present in muscle, but confined to the connective tissue, in the spaces of which they take their origin. The Nerve-supply. — ^The nerves which carry the stimuli to a muscle enter near its geometric center. Many of the fibers pass directly to the muscle-fibers with -^phich they are connected ; others ar^ dis- tributed to blood-vessels. Every muscle-fiber is suppUed with a special nerve-fiber except in those instances where the nerve-trunks entering a muscle do not con- tain as many fibers as the muscle. In such cases the nerve-fibers divide near their termination until the number of branches equals the number of muscle-fibers. The individual muscle-fiber is penetrated near its center by the nerve where it terminates; the ends being practically free from nerve influence. The stimulus that comes to the muscle- fiber acts primarily upon its center, the effect of which then travels in both directions to the ends. The manner in which the nerve-fibers terminate in muscle will be more fully described in connection with the histology of the nerve tissue. CHEMIC COMPOSITION OF MUSCLE. The chemic composition of living muscle is but imperfectly under- stood owing to the fact that shortly after death some of its constituents Fig. 19. — Relation of the Blood-ves- sel TO THE MtJSCLE-riBER. GENERAL PHYSIOLOGY OF MUSCLE-TISSUE. 57 undergo a spontaneous coagulation and for the reason that the methods employed for analysis also tend to alter its composition. To human muscle, the following average percentage composition has been given : Water, ^j c Proteins, including those of sarcolemma, connective- tissue, pigments, 18.02 Gelatin, i.pg Fat, 2.27 Extractives, 0.22 Inorganic salts, 3.12. (Halliburton.) (The composition of muscles of different animals, consumed as foods, will be found in the chapter on Foods.) When fresh muscle is freed from fat and connective tissue, frozen, rubbed up in a mortar, and expressed through linen, a sHghtly yellow syrupy alkahne or neutral liquid is obtained which has b^en termed muscle-plasma. This fluid at normal temperatures'" coagulates spontaneously, the phenomena resembling in many respects those observed in the coagulation of blood-plasma. The coagulum subse- quently contracts and squeezes out an acid muscle-serum. The coagulated protein is known as myosin and belongs to the class of globuhns. Inasmuch as it is not present in Uving muscle and only makes its appearance under conditions not strictly physiologic, it is regarded as a derivative of a pre-existing protein which has been termed myosinogen. According to HalUburton, the proteins of Uving muscle are four in number, distinguished by their varying solubilities in different salts and by the varying temperatures at which they coagulate. From muscle-plasma may then be obtained: (i) Para- myosinogen and (2) myosinogen, the former coagulating at 47° C, the latter at 56° C. It is myosinogen which is converted into myosin under the influence of some special ferment, though both enter into the formation of the muscle-clot. From the muscle-serum may also be obtained at 68° C. a globulin body termed myoglobuUn and a small quantity of myoalbumin. Among the proteids may be men- tioned hemoglobin, which gives the color to the muscles. Spectro- scopic investigation reveals the presence of a special pigment, myo- hematin, which is supposed to have a respiratory function, inasmuch as its absorption bands change by oxidation and reduction. Among the extractives containing nitrogen may be mentioned creatin, creatinin, xanthin, carnin, urea, uric acid, carnic acid, etc. Among the extractives free of nitrogen, glycogen, dextrose, inosite, lactic acid, fat, are the most important. Inorganic salts are relatively abundant, of which potassium is the most abundant among the bases, and phosphoric acid among the acids. THE PHYSICAL AND PHYSIOLOGIC PROPERTIES OF MUSCLE- TISSUE. Consistency. — The consistency of muscle-tissue during life varies considerably in accordance with different states of the muscle. 58 TEXT-BOOK OF PHYSIOLOGY. Fig. 20. — Extension Curve of Muscle. — {Gad.) In a state of tension it is hard and resistant ; in the absence of tension it is soft and. fluctuating to the sense of touch. Tension alone gives rise to hardness. Cohesion. — The cohesion of a muscle is largely dependent on the quantity of connective tissue it contains. A band of fresh human muscle one square centimeter in cross-section was able to resist a weight of 14 kilograms without rupture (MacAlister). Cohesion resists the forces of traction and pressure. Elasticity.— Muscle, in common with many other organic as well as inorganic substances, is capable of being extended beyond the normal length through the action of external forces and of resuming the normal length when these forces cease to act. All such bodies are said to be elastic; and the greater the variations between the natural and acquired lengths, the greater is their elasticity said to be. Muscle, therefore, possesses extensibility and elasticity.* If the muscle of a frog, preferably the sartorius, the fibers of which are arranged in a practically parallel manner, be fastened at one extremity by a clamp, and then extended by a series of successive weights which differ by a common increment, it will be found that the extensibility of muscle does not follow the law of elasticity as deter- mined for inorganic bodies; i. e., directly proportional to the weight and to the length of the body extended; but that while in- creasing in length with each successive weight, the increase is always in a diminish- ing ratio. Thus, for example, as shown in Fig. 20: The extension produced by 5 grams is 5 millimeters, that produced by 10 grams is only 4 millimeters more, and so on with additional weights until the increase in passing from 25 to 30 grams is only i milli- meter. The extensibility is thus shown to be proportionately greater with small than with larger weights. It is, however, actually greater with the larger weights. The extension curve A B formed by joining the ends of the muscle approximates that of a parabola. The muscle in returning to its original length also shows a variation from the behavior of inorganic bodies. With the successive removal of the weights, the elasticity of the * By this latter term is here meant the power by virtue of which the muscle returns to its original length and is used synonymously with perfect retractibility. Fig. 21. — Curve of Elas- ticity Produced by Continu- ous Extension and Recoil OF a Frog's Muscle. x. Ab- scissa before; x', after exten- sion. — {Landois and Stirling.') GENERAL PHYSIOLOGY OF MUSCLE-TISSUE. 59 muscle asserts itself with gradually increasing energy until its normal length is nearly, if not entirely, regained (Fig. 21). Though it is usually stated that the elasticity of muscle is incomplete, it must be borne in mind that the experiments have usually been made on muscles removed from the body, deprived of blood and nerve influences, and hence under abnormal conditions. It is highly probable that in the living body muscles possess perfect elasticity which enables them to completely return to their normal length after extension. The extension and elastic recoil of muscle depends on the maintenance of physiologic conditions. If the nutrition is impaired by fatigue, deficient blood- supply, or any pathologic condition, the elasticity is at once impaired. Tonicity. — ^This is a property possessed by all muscles in the body in consequence of being stretched to a shght extent beyond their normal length. This may be due to the action of antagonistic muscles or to their mode of growth, the muscles growing somewhat more slowly than the bones to which they are attached. That muscles are so stretched is shown by the shortening which at once takes place when their tendons are divided. This muscle tonus or tension is closely connected with the elasticity and plays an important r61e in muscle contraction; being always on the stretch, the muscle loses no time in acquiring that degree of tension necessary to immediate action on the bone to which it is attached. The working power of a muscle is also increased by the presence, within limits, of some resistance to the act of contraction. According to Marey, the amount of work is considerably increased when the muscle energy is transmitted by an elastic body to the mass to be moved, while at the same time the shock of the contraction is lessened. The position of a passive limb is the resultant also of the elastic tension of antagonistic groups of muscles. Another explanation for the tonicity of muscle is found in the fact that the skeletal muscles of the body receive continuously nerve im- pulses from the nerve cells of the spinal cord as a consequence of the arrival of nerve impulses reflected through afferent nerves from the tendons and muscles themselves. The stimulus here being the sHght degree of extension and variations in extension to which the muscles are being subjected from moment to moment. That this is a con- siderable factor in the production of the tonus is shown by the effects which follow division of the afferent or dorsal roots of those spinal nerves coming from any given muscle group. With the division of the nerves the muscles relax and lose their usual tone. As a result of this shght but constant stimulation from the spinal cord, the metaboUc changes of the muscle material are maintained at a certain level with a corresponding liberation of heat. The chief function of the tonicity would thus be the production of heat, other functions which the tone subserves being merely secondary. Irritability, Contractility.— These are terms employed to denote that property of muscle-tissue by virtue of which it responds by a change of form, becoming shorter and thicker on the application of 6o TEXT-BOOK OF PHYSIOLOGY. a stimulus. On the withdrawal of the stiinulus the muscle again undergoes a reverse change of form, becoming longer and narrower, and returning to its original condition. All muscles which possess this capabiUty are said to be irritable and contractile ; and all agents which call forth this response of the muscle are termed stimuli. The rapid change of form which a highly irritable muscle undergoes in response to the action of a stimulus of short duration is usually termed a twitch or pulsation. With appropriate apparatus it can be shown that the muscle at the time of the twitch becomes warmer and exhibits electric phenomena. The muscle is therefore an apparatus for the conversion of potential into kinetic energy: viz., heat, electricity, and mechanic motion. Though usually associated with fhe activity of the nerve system, and to some extent dependent on it, irritabiUty is nevertheless an independent endowment of the muscle and persists for a longer or shorter period, as shown by many experiments, after all nerve con- nections have been destroyed. Among the proofs which may be presented in support of this view are the following : The introduction of the drug, curara, into the body of an animal produces in a short time complete paralysis. Experiment has shown that curara sus- pends the conductivity of the intramuscular terminations of the nerve- fiber and thus separates the muscle entirely from the nerve. Though tlje animal is incapable of executing a single movement, its muscles respond promptly on the application of a stimulus. Moreover, portions of muscles exhibit irritability in which there is no trace of nerve structure. This is the case with the ends of the sartorius muscle of the frog and the anterior end of the retractor muscle of the eyeball of the cat. These and other facts demonstrate the independence of muscle irritability. In the living body irritabihty and nutritive activity, with which it is closely associated, are maintained by a due supply of oxygen, of nutritive material, the removal of waste products, and a normal temperature. The muscles of the cold-blooded animals, and especially the frog, retain their irritability for a much longer period after death than the muscles of the warm-blooded animals. This is the case also with the individual muscles after removal from the body of the animal. The reason for this is found in all probability in the difference in the rate of their nutritive activities and in the quantity of nutritive material stored up in their cells. The duration of the irritability of isolated muscles can be considerably prolonged by keeping them in a moist atmosphere. Muscle Stimuli. — ^Though consisting of a highly irritable tissue, muscles do not possess spontaneity of action. They require for the manifestation of their characteristic activity the application of a stimulus. In the living body all contractions, at least of the skeletal muscles, occurring under normal or physiologic conditions are caused by the action of "nerve impulses" transmitted by the nerves from the GENERAL PHYSIOLOGY OF MUSCLE-TISSUE. 6i central nerve system to the muscles. The nerve impulse is the normal or physiologic stimulus. After removal from the body and freed from nerve connections muscles can be excited to action by various agents of a mechanic, chemic, thermic, or electric nature. These are artificial or non-physiologic stimuli. 1. Mechanic Stimuli. — Cutting, pinching, sharply tapping the muscle will cause it to contract, providing the stimulus has sufi&cient intensity. With each stimulation a short, fleeting contraction ensues. If repeated with sufficient rapidity, a series of con- tinuous but irregular pulsations are produced. 2. Chemic Stimuli. — ^Various chemic substances in solution will excite single or continuous pulsations if the strength of the solu- tion is not such as to destroy at once the irritabihty. They owe their efficiency as stimuli to the rapidity with which they alter the composition of the muscle-substance. Among these may be mentioned solutions of potassium and sodium, weak solutions of the mineral and organic acids, ammonium vapor, distilled water, glycerin, and sugar. 3. Thermic Stimuli. — ^The application of a heated object, such as a hot wire, causes the muscle to rapidly contract. 4. Electric Stimuli. — ^The most efficient stimulus and the one least injurious to the tissue is the electric current. Either the con- stant or the induced current may be used.* The Constant Current. — ^If the ends of the wires in connection with an electric cell be provided with non-polarizable electrodes and the latter placed on opposite ends of a freshly prepared sartorius muscle of a frog which has been previously curarized, it will be found on closing or making the circuit that the muscle will exhibit a short quick pulsation. During the actual passage of the current, especially if it is weak, there may be no apparent change in the muscle. If the current is strong, the muscle may, on the contrary, remain in a state of continuous contraction. With the opening or breaking of the current the muscle at once relaxes, or perhaps again contracts and then relaxes. The extent of the contraction depends mainly on the strength of the current, being greater with strong, less with weak currents. When the current is sufficiently strong to elicit both making and breaking contractions, it is found that the contraction occurring on the make or closure of the circuit is always greater than that occur- ring on the break or opening of the circuit. Moreover, it has been shown in many ways that the contraction occurring on the closure of the circuit has its origin at the point where the current is leaving the muscle — i. e., in the immediate neighborhood of the negative pole or * Since the study of the physiologic properties of both muscle-tissue and nerve-tissue involves the employment of electricity as a stimulus, it becomes necessary for the student to familiarize himself with certain forms of apparatus by which it is generated, controlled, and applied. For the purpose of not interrupting the continuity of the text this inform- ation is embodied in an appendix. The facts therein contained should be mastered at this time by the student. 62 TEXT-BOOK OF PHYSIOLOGY. cathode — and propagates itself to the opposite extremity; while the contraction occurring on the opening of the circuit has its origin at the point where the current is entering the muscle, i. e., in the neighborhood of the positive pole or anode. These facts can be readily demonstrated by destroying the ir- ritability and contractility of one extremity of a muscle with parallel fibers such as the sartorius. On applying non-pplarizable electrodes to the muscle as in Fig. 22, A, it will be found that when the circuit is made a contraction occurs which must, of course, have developed at the irritable cathodic region, for on the break of the circuit the muscle remains at rest. When the electrodes are applied as in Fig. 22, B, and the circuit made the muscle remains at rest, but on the break of the circuit a contraction occurs which must have developed at the irritable anodic region. The Induced Current. — If the primary spiral of the inductorium be connected with an electric cell and the secondary spiral be con- nected with a muscle, it will be found that the current induced in the A. B. Fig. 22. — Diagram to Show the ErFEcr of Local Injury on the Irritability of A Muscle (after Starling). C Z an electric cell from which wires pass to non-polarizable electrodes, anode and kathode, in contact with a muscle, the injured end of which is more deeply shaded. The arrows indicate the direction of the current. secondary circuit, both on the make and break of the primary, will also cause the muscle to sharply and rapidly pulsate if the two spirals are sufficiently near each other. Observation, however, makes it evident that the pulsation occurring with the break of the primary circuit. is more energetic than that occurring with the make, a result the opposite of that obtained with the constant current. This is not due to any difference in the electricity, however, but to peculiarities in the construction of the inductorium. When the primary circuit is interrupted with sufficient frequency, as it can be by throwing into the circuit Neef's hammer or some other form of interrupter, the con- tractions excited by the induced currents may be made to succeed one another so rapidly that they become fused together, producing a spasm or tetanus of the muscle. The rapidity with which the induced current appears and disappears, its brief duration, the ease with which its strength can be regulated, combine to render it a most efficient stimulus for either muscle or nerve. Conductivity. — ^All muscle protoplasm possesses conductivity. GENERAL PHYSIOLOGY OF MUSCLE-TISSUE. 63 The change excited in a muscle-fiber by the arrival of a nerve impulse is at once conducted with great rapidity in opposite directions to the end of the fibers; the advance of the excitation process is im- mediately succeeded by the contraction process, the change of form which constitutes the contraction. With the disappearance of .the former, the latter also disappears and the muscle resumes its pre- vious passive condition. There is no evidence, however, that the excitation process travels transversely — ^that is, into adjoining fibers — ^being prevented from doing so by the presence of the Smiting membranes, the sarcolemmata. The fact that each muscle-fiber receives its own, or at least a branch of a nerve-fiber, and hence its own nerve impulse or stimulus, would also indicate that the excitation process can not be conducted longitudinally into adjoining fibers, or at least with sufl&cient rapidity for the purposes of ordinary muscle actions. Nevertheless if a long muscle, such as the sartorius, from a curarized frog be stimulated at one end with an induced electric cur- rent, the excitation and the contraction processes will be conducted with extreme rapidity to the opposite end of the muscle. The rapidity of conduction in human muscles has been estimated at from 10 to 13 meters per secpnd, and in frog's muscle at from 3 to 4.5 meters per second. The contraction process, the thickening of the muscle, is termed the contraction wave. As it is the result of the excitation process and immediately succeeds it, its rate of conduction must be the same as that given above. With appropriate apparatus the du- ration of the wave at any given point has been shown to be, in the frog's muscle, one-tenth of a second and its length three-tenths of a meter. PHENOMENA ATTENDING A MUSCLE CONTRACTION. PHYSICAL PHENOMENA. The most obvious change in a muscle during the contraction is that relating to its form. The muscle not only becomes shorter, but at the same time thicker. The extent to which it may shorten when unopposed may amount to 30 per cent, or more of its original length. The increase in thickness practically compensates for the diminution in length, for there is no observable diminution in volume. The change in form of the entire muscle results from a corresponding change of form of its individual fibers as determined by microscopic examination, each of which becomes shorter and thicker. The successive changes in both the muscle and the individual fibers are represented in Fig. 23. , When the contraction begins, the dim band increases and the bright band diminishes in width. This Engelmann attributes to the passage of fluid material from the bright into the dim band. At the time of relaxation there is a return of this material and the bands assume their original shape and volume. As the contraction wave 64 TEXT-BOOK OF PHYSIOLOGY. reaches its maximum the optic properties of the bright and dim bands change. The former now becomes darker and less transparent until at the crest of the wave it assumes the appearance of a distinct dark band; the latter now becomes clear and bright in comparison. This change in the appearance of the fiber is due to an increase in refrangibility of the bright, and a decrease in the refrangibility of the dim band, coincident with the passage of the fluid from the former into the latter. There is at the height of the contraction a complete reversal in the positions of the striations. At a certain stage between the beginning and the crest of the wave the striae almost entirely dis- appear, giving to the fiber an appearance of homogeneity. There is, Fig. 23. — Showing the Changes in a Muscle and Muscle-fiber during Contraction. however, no change in refractive power as shown by the polarizing apparatus. When the contraction wave has reached the stage of greatest intensity, there is a reversal of the above phenomena as the fiber returns to its former condition, that of relaxation. Elasticity. — ^During the contraction of a muscle there is a greater or less alteration in its elasticity, as shown by the fact that it is ex- tended to a greater degree by the same weight in the active than in the passive condition. The degree to which the extensibihty is in- creased and the elasticity decreased is dependent on the amount of the resisting force. These facts, as determined experimentally, are represented in Fig. 24. Let A B and A b represent the length of the normal unweighted muscle, passive and active states respectively; the Hne B B', the extension curve of the passive muscle produced by successive weights, 5, 10, 15, 20, 25, 30 grams, differing by a com- mon increment; the Hne b B', the extension curve of the active con- tracted muscle when weighted with the same weights ; A' B' the length of the muscle when the weight is sufficiently great to prevent shorten- ing. It will be observed from these facts that while the muscle is extended in both the passive and active states by corresponding GENEIifAL PHYSIOLOGY OF MUSCLE-TISSUE. 6S weights, the extension during the latter is progressively greater, until with a given weight the length of the muscle is the same. Under such circumstances, there being no shortening of -the muscle, the energy of its contraction manifests itself physically merely as tension. In the successive actions of the muscle represented in the same figure there is to be observed also a combination of a change of length and a change of tension, the ratio of the one to the other being deter- FiG. 24. — Extension Curves: B B', of the resting; b B', of the contracting muscle. mined by the amount of the supported weights. When the weight is slight in amount, the shortening of the muscle reaches a maximum and the tension a minimum; when the weight is large in amount, the reverse conditions obtain. THE CONTRACTION PROCESS. METHODS OF INVESTIGATION. The contraction of a muscle as it takes place in the hving body and under normal physiologic conditions is a complex act persisting for a variable length of time in accordance with the number of stimuU transmitted to it in a given unit of time, and as determined experi- mentally is the resultant of the fusion of a greater or less number of separate and individual contractions or pulsations. To this enduring contraction the term tetanus has been given. With the aid of ap- propriate apparatus it has become possible to obtain and record single muscle contractions, to analyze and decompose them into their constituent elements, or to combine them in such a manner as to pro- duce practically a normal physiologic tetanus. As in the experi- mental study of the phenomena of a muscle contraction it frequently becomes necessary to remove the muscle from the body of the animal, the muscles of warm-blooded animals are not well adapted for this purpose, owing to the rapid alteration in composition they undergo, with a consequent loss of irritability, when deprived of their normal blood-supply. The excised muscles of cold-blooded animals, par- ticularly of the frog — in which, owing to the relatively slow rate of s 66 TEXT-BOOK OF PHYSIOLOGY. the nutritive activities, the irritability and contractility endure for a relatively long period of time, even though deprived of blood — are particularly valuable for experimental studies. The muscles generally employed are the gastrocnemius, the sartorius, and the hyoglossus. If kept at 'a normal temperature and moistened with 0.6 per cent, solution of sodium chlorid, such a'muscle -mil contract for a long period of time on the application of any form of stimulus, but especially the electric. Graphic Record of a Muscle Contraction. — ^Inasmuch as the changes in the form of a muscle during a single contraction take place with extreme rapidity, their succession, peculiarities, and time re- lations cannot be determined with any degree of accuracy by the unaided eye. This difficulty can largely be overcome by the employ- ment of the graphic method, the principle of which; consists in record- FiG. 25. — Myograph. K. Recording cylinder. M. ^oist chamber. L. Recording lever. W. Weight. I. Induction coil. ing the movements by means of a pen on some appropriate moving arid receiving surface. To accompUsh this object the muscle is at- tached at one extremity by a clamp to a firm support, and at the other extremity to a weighted lever, which is, however, sufficiently hght to enable it to take up, reproduce, and magnify its movements. The end of the lever provided with a pen is applied to a smooth surface, such as glazed paper on a cyUnder or plate, and covered with lamp- black. If the surface is stationary, the contraction is recorded as a vertical line; if jt is placed in movement at a uniform rate by clock- wprk, the contraction is recorded in the form of a curve, the width of the arms of which will depend on the rate of movement. The time relations of the phases of the contraction can be obtained by placing beneath the lever a pen attached to an electro-magnet thrown into GENERAL PHYSIOLOGY OF MUSCLE-TISSUE. 67 action by a tuning-fork vibrating in hundredths of a second. In order to determine the rapidity with which the contraction follows the stimulation, it is essential that the movement of the latter be also recorded. This is accomplished by an automatic key, the opening or closing of which develops the stimulus which excites the muscle. A combination of these different apphances constitutes a myograph and the curve of contraction a myogram. (See Fig. 25.) The Isotonic Myogram. — ^With the object of obtaining a curve of successive changes in the length of a muscle during a single con- traction and at the same time avoiding changes in tension, the weight attached to the lever should be appUed close to its axis, a mechanic Fig. 26. — The Isotonic Myogeam. condition which practically maintains a uniform tension throughout the contraction. To this method the term "isotonic" has been given and the curve so obtained an isotonic myogram.* The Character of an Isotonic Myogram. — With the muscle arranged as previously described and stimulated directly with a single induction shock, the contraction will be recorded in the form of a curve similar to that represented in Fig. 26, in which the horizontal line represents the abscissa of time; a, the moment of stimulation; and bed, the degree of shortening at each successive moment. The undulating line shows the time relations, the distance from crest to crest represent- ing hundredths of a second. The curve may be divided into three portions: I. A short but measurable portion between the point of stimulation and the first evidence of the shortening, a b, known as the "latent period." The duration of this period for the skeletal muscle of the frog was originally determined to be o.oi second, but with the employment of more accurate apparatus it has been reduced to 0.0025 to 0.004 second. During this period it is supposed that certain chemic changes are taking place preparatory to the *In the ordinary method of recording a muscular movement, i. e., with the weight attached to the lever immediately beneath the muscle and known as the "loaded method," a certain momentum is imparted to the weight, which continues after the muscle has ceased to act, both when shortening and relaxing, and so imparts to the recording lever additional movements which vitiate the true character of the curve. 68 TEXT-BOOK OF PHYSIOLOGY. exhibition of the movement. The duration of the latent period is influenced by a variety of conditions, e. g., temperature, fatigue, strength of stimulus, etc. 2. An ascending portion, b c, the contraction or period of increasing energy. The contraction as shown by the character of the curve begins slowly, then proceeds rapidly, and again slowly as the shortening reaches its maximum. The contraction may be said to end when the tangent to the curve becomes parallel with the abscissa. 3. A descending portion, c d, the relaxation or period of decreasing energy. The relaxation as shown by the character of the curve begins slowly, then proceeds rapidly, and again slowly as the muscle attains its original length. The termination of the re- laxation is at the point where the curve cuts the abscissa. The curve beyond this point may be compUcated by the presence of one or more residual or after-vibrations, which are probably due to. the inertia of the lever as well as to changes in the muscle elasticity. The duration of the period of shortening is about 0.04 second, and of the period of relaxation 0.05 second. A single pulsation of the isolated muscle of the frog therefore occupies, from the moment of stimulation to termination, the tenth of a second. Muscles of many other animals have a contraction period the duration of which varies considerably from this. Thus, in man the time of a single contrac- tion is one-twentieth of a second, in some insects one three-hundredth of a second, and in the turtle one second. Pale muscles have a shorter period than the red. Influences Modifying the Contraction Process. — ^The con- traction process in its entirety as well as in its individual parts is considerably modified by both external and internal conditions, among which may be mentioned the following: I. Stimulus. — ^As the contraction is the response of the muscle to a stimulus, the vigor of the former is proportional, within Hmits, to the strength of the latter. Thus using as a stimulus the single induced current, it has been found that if the strength of the current is progressively increased, the height of the contraction will correspondingly increase until^a certain maximum height is attained (Fig. 27, A); then notwithstanding a continued increase in the strength of the stimulus, this height will not be exceeded for some time. But if the strength of the stimulus be yet further increased, there comes a moment when the contractions again increase in vigor and a second maximum height is attained (Fig. 27, B). Beyond this no further increase in height is observed. The second maximum has been attributed to the presence in the muscle of two different substances differently affected by changes in temperature, by fatigue and by various drugs. The rate at which the muscle is stimulated with a given stimulus GENERAL PHYSIOLOGY OF MUSCLE-TISSUE. 69 of uniform strength will also.influence the character of the contrac- tion process. If the intervals between the successive stimulations be such as permit the muscle to recover from the effects of the contraction, it may contract as many as a thousand times without showing any particular variation from the normal form; but if the intervals are shorter than that just stated it is found that froni the beginning of the stimulation each succeeding contraction sUghtly exceeds in height the preceding contraction, until a cer- tain maximum is reached and maintained, indicating that for some reason the irritability and the energy of the contraction have been increased. This gradual increase in the height of the con- traction has been termed the stair case effect, or the treppe. In A. B. Fig. 27. — Tracing Showing the Effects of a Gradual Increase in the Strength OF THE Stimulus on the Height of the Contraction, a. Minimal contraction; a b. progressive increase in the height; b c. first maximum (a number of contractions have been omitted for economy of space); de. second maximum. the beginning of the period of stimulation there is sometimes ob- served a decrease in the height of the contraction following , several stimulations before the stair case effect develops, indi- cating a temporary decrease in the irritability. These stair case contractions have been observed in the muscle of both warm- blooded and cold-blooded animals. The cause for this increase in irritability upon which the effect depends is attributed to the presence of certain chemic substances in the muscle arising as a result of its kataboUsm, such as carbon dioxid, mono-potassium phosphate, and paralactic acid. These compounds when pres- ent in small amounts or in larger amounts for a short time, aug- ment the action of the muscle and give rise to the treppe effect. (I^ee.) In time, however, if the stimulation be continued, the irritability dechnes, the height of the contraction diminishes and finally the muscle ceases to respond to any stimulus. 2. Temperature. — ^The temperature at which all phases of the con- traction process, as represented by the myogram, attain their physiologic maximum value is about 30° C. If the tempera- 7° TEXT-BOOK OF PHYSIOLOGY. ture of the muscle falls to 20° C. there is a corresponding decline in activity, as shown by an increase of the latent period, a de- crease in the height of curve — i. e., in the shortening of the mus- cle — an increase both in the contraction and relaxation periods. As the temperature approaches 0° C. the height of the curve again suddenly increases, indicating, for some unknown reason, an increase in the irritabihty. This is, however, scarcely a physio- FiG. 28. — Single Contractions of the Gastrocnemius Muscle at Different Temperatures. Time tracing 200 per second. — (Brodie.) logic condition. At a temperature of 40° C. to 50" C. the muscle suddenly contracts and passes into the condition of heat rigor. The proteid constituents of the muscle are coagulated and the irritability destroyed. (Fig. 28.) The Load. — ^The extent to which a muscle is loaded or weighted wiU not only determine the height of the contraction, but also the time relations of all its phases. This is apparent from an ex- FiG. 29. — Contractions of a Gastrocnemius Muscle WITH Different Loads. — {Brodie.) amination of Fig. 29, in which it is shown that with an increase in Joad there is a decrease in the height of the contraction, an increase in the latent period, and a general increase in the dura- tion of both the periods of rising and falling energy. Continuous Stimulation. — -Prolonged or excessive activity of our own muscles is accompanied by a feehng of stiffness or soreness and lassitude. There is at the same time a diminution in the speed GENERAL PHYSIOLOGY OF MUSCLE-TISSUE. 71 and vigor of the contractions and the power of doing work. To this condition the term fatigue has been given. The cause of the fatigue is attributed to a diminution in the amount of the energy- yielding compounds as well as to the production and accumulation of waste products resulting from katabolic activity. Among the waste products mono-potassium phosphate, paralactic acid, and carbon dioxid are the most important. These substances, when present in small amounts or in larger amounts for a short time, increase the irritabiUty of \he muscle, but when they accumulate more rapidly than they are removed, as is the case during excessive activity, they exert a depressive influence on the irritabihty of the muscle and thus diminish its contractile power and its capacity for doing work. The more rapidly they are removed, the sooner is a fatigued muscle restored to its normal condition. The condi- tion of fatigue with its attendant phenomena is shown by stimulat- FiG. 30. — Fatioue Curves. Every Twentieth Contraction Recorded. ing through its nerve an excised frog muscle with induced electric currents at intervals of one second. In a variable period of time the muscle shows an increase in the duration of the latent period, a diminution of the height of the contraction, in the power of doing work, and an increase in the time required for relaxation. (Fig. 30.) If the stimulation is continued the contractions gradually decUne as the muscle becomes exhausted. When a muscle will no longer re- spond to stimulation through its related nerve, it can be made to respond to direct stimulation with the electric current. This taken in connection with the fact that stimulation of a nerve-trunk even for several hours does not fatigue it, leads to the inference that the cause of the cessation of contraction does not wholly he in the muscle but partly in the nerve endings in the muscle. These structures it is beheved fatigue more readily than the muscle structures, and hence fail to conduct the nerve impulse to the muscle. By this means it is protected from absolute exhaustion. Nutrition. — ^The irritabihty of a muscle which conditions the con- traction process is dependent on the maintenance of its nutrition; hence a continuous and sufficient supply of nutritive material and a rapid removal of waste products are essential conditions for the exhibition of normal contractions. A diminution of blood supply or an accumulation of waste products sooner or later im- pairs the irritabihty and diminishes the vigor and extent of the 72 TEXT-BOOK OF PHYSIOLOGY. contraction. Various drugs — e. g., veratrin, barium, etc. — ^in- troduced into the circulation and finding their way into the muscle modify the contraction process,- as shown by a very great increase in the duration of the relaxation period. The Isometric Myogram. — ^With the object of obtaining a curve of the increase and decrease in the tension of a muscle during a single contraction, with the exclusion as far as possible of a change in length, the muscle may be made to contract against a strong spring or similar resistance sufficient to practically though not absolutely prevent short- ening. To this method the term isometric has been given, and the curve so obtained an isometric myogram or a tpnogram. The record- ing portion of the lever is prolonged some distance so that the very shght upward movement at its axis, close to which the muscle is at- tached, will be considerably magnified.' That the ordinate value of anisometric curve itnay be known, the apparatus must be graduated by subjecting the spring to a series of weights playing over a pulley supported by the muscle clamp. The curve of the variation in tension obtained by the iso- metric method is shown in Fig. 31, b, in which the two curves are contrasted. The form of Fig. 31.— a. Diagram of Isotonic; b, the curve indicates that the mus- op Isometric Muscle Curves.— (lawdoij cle attains its maximum of ten- andsming.) ^^^^ ^^^^ rapidly than its max- imum of shortening; that the tension endures for a certain period of time unchanged; that the fall in tension takes place more rapidly than the muscle relaxes. The Myogram Due to the Make and the Break of a Galvanic Current. ^The contraction of the muscle which has heretofore been recorded has been caused by the momentary action of an induced current. The contraction of the muscle which is caused by the action of a constant or galvanic current presents features which are some- what different and as it serves to illustrate the difference in the effects of a constant or galvanic and an induced or interrupted current, a myogram of a contraction due to the make and break of a galvanic current is introduced at this place. The effects which are observed in a muscle during the passage of both feeble and strong currents have been alluded to in a previous section. (See page 61.) In Figure 32 these effects are graphically represented. It will be observed that on the closure of the circuit at c the muscle at once contracted and so long as the current was flowing, the muscle remained in a more or less con- tracted state known as galvanotonus; on opening the circuit at o the muscle again contracted, after which it gradually relaxed and returned to its original condition. The record shows also that during the GENERAL PHYSIOLOGY OF MUSCLE-TISSUE. 73 actual passage of the current the muscle substance was being stimu- lated by it. The Work Accomplished by a Muscle during the Time of a Single Contraction. — By work is meant the overcoming of opposing forces. In the physiologic activities of the body the muscles at each contraction not only overcome the resistances of antagonistic muscles, Fig. 32. — A Myogram Due to the Action op a Galvanic Ciirkent, Applied Di- rectly TO A Muscle, when the Circuit was Closed (c) and when it was Opened ('o): the weight of the Umbs, the friction of joints, etc., but in addition over- come various external resistances connected with the environment — e. g., gravity, cohesion, friction, elasticity,, etc. The muscles may therefore be regarded as machines for the accomphshment of work. Experimentally the work done by an isolated muscle may be calcu- lated when first the height of the contraction is ob- tained and then multiply- ing it by the weight raised. The influence of the weight on the height of the contraction is shown in Fig. 33. From this tracing it will be observed that the extent to which a muscle will shorten in response to a maximal stimulus is greatest when it is unweighted; but as weights differing by a common increment are added, the height of the contraction diminishes until with a given weight it is nil. A careful study of the results of this experiment will show that the work done gradually increased as the load was increased from o to 70 grams, when it amounted to 210 milligrammeters; but that after this, even though the weight lifted was greater, the height to which it was lifted was less, and hence the work done gradually decreased, until it amounted to nothing. Fig. 33. — Tracing Showing the Gradual Diminution in the Height of the Contrac- tion as the Weight was Increased by a Com- mon Increment op 10 Grams prom o to 180 Grams. Magnipication of the Lever, 4. 74 TEXT-BOOK OF PHYSIOLOGY. The following table will also show the work done by a frog's muscle according to Rosenthal. Weight. Height. Work DONE. o grams 14 mm. gram- millimeters S° " 9 :: 450 (( lOO " 7 " 700 ti ISO " s " 75° It 200 2 " 400 ti ti 250 From the preceding figures it is evident that the mechanical work of a muscle increases with increasing weights up to a certain maximum, and then declines to zero. Equally when the muscle contracts to its maximum without being weighted, and when it does not contract at all from being overweighted, no work is done. Between these two extremes the muscle performs varying amounts of work. The maximum amount of force which a muscle puts forth during a contraction is naturally measured by the amount of work done; but as this varies with the degree to which the muscle is weighted, another rheasure has been adopted, to which the term absolute muscle force ,or static force has been given. The absolute force is measured by the weight which is sufiicient to prevent the muscle from shortening. This is best determined by the method of after-loading in which the muscle is not extended by the weight previous to the contraction. It has been found that the absolute force of a muscle is directly de- pendent on the number and not the length of the fibers it contains and proportional to the physioto^c transverse section of the muscle. The transverse section of a muscle is obtained by dividing its volume (obtained by dividing its actual weight by the specific weight of mus- cle-tissue, 1 .058) by the average length of the fibers. Assuming that the muscle weighs 622 grams, its volume would be 576 c.c; and if it be further assumed that the fibers have an average length of 4 centimeters the transverse section would contain 144 sq. centimeters each of which would have a length of 4 centimeters. For purposes of comparison it is customary to refer the absolute force to these units of diameter — ^viz., one square centimeter. Rosen- thal estimates the force for the square centimeter of the muscle of the frog at from 2 to 8 kilograms ; for the muscles of man at 6 to 8 kilograms ; Koster at about 10 kilograms for the muscles of the leg and 7 or 8 kilograms for the muscles of the arm. Summation Effects. — If a series of successive stimuli be applied to a muscle, the effect will vary according to the rapidity with which they follow one another. As previously stated, if the interval preced- ing each stimulus be sufficiently long to enable the muscle to recover from the effects of the previous contraction, there will be no change in the form or the character of the contraction for a long time except a sHght increase, in the early period, of the irritability as shown by the in- creased height of the curve or shortening of the muscle. If, however, a second stimulus be applied to a muscle during the period of relaxation, GENERAL PHYSIOLOGY OF MUSCLE-TISSUE. 75 a second contraction immediately follows which is added to or super- posed on the first; the effect produced will be greater than that pro- duced by either stimulus separately. See Fig. 34. A third stimulus appUed during the relaxation of the second con- traction produces a third contraction which adds itself to the second, and so on. The increment of increase in the extent of the success- ive contractions gradually di- minishes, however, until the muscle reaches a maximum of contraction. The superposition of the second contraction on the first, the third on the second, and so on, is termed summation of contractions or effects. Experi- ment has shown that the greatest effect of a second stimulus — that is, the highest contraction— is produced when the stimulus is applied during the last third of the period of rising energy, when the sum of the two contractions is almost twice as great as the first contraction (Fig. 34). The effects following both maximal and submaximal stimuli indicate that the muscle cannot attain its maximum of shortening except through a summation of several stimuli. If a second maximal stimulus enter a muscle during the latent period following the first, the effect produced will be no greater than that produced by a single stimulus. The muscle • during this period is said to be refractory or non-responsive to a second stimulus. If, however, the stimuli are submaximal they add themselves together, and though the effect is but a single contraction, it is larger than either would have produced separately.. This is termed the summation of stimuli. Still further, if a series of subminimal stimuH, each of which is alone insufiicient to produce a contraction of the muscle, be applied in rapid succession,, a contraction frequently results. This is termed the summation of subminimal stimuli. Tetanus. — ^Tetanus may be defined as a more or less continuous contraction of a muscle which arises when the time intervals between Fig. 34. — Tracing Showing the Er- FECTS OF Two Successive Stimuli, a. a' WITH Gradually Diminishing Inter- val ON A Muscle Contraction. To be read from below upward. 76 TEXT-BOOK OF PHYSIOLOGY. the stimuli are shorter than the time of the contraction process. Tetanus will be incomplete or complete according to the number of stimuli that fall into the muscle in a pecond of time. When a muscle is stimulated directly or, better, indirectly through its related nerve by a series of induced currents at the rate of four or six per second, it undergoes a corresponding number of contractions of about equal ex- tent. If the rate of stimulation is increased up to the point when the interval between each stimulus is less than the duration of the entire coijtraction process, the muscle does not have time to coinpletely relax before the arrival of the succeeding stimulus, and hence remains in a more or less contracted state, during which it exhibits a series of alternate partial contractions and relaxations. To this condition of muscle activity the term incomplete tetanus or clonus is applied. A graphic record of an incomplete tetanus is given in Fig. 35. Fig. 35. — Curves, Showing the Analysis of Tetanus of a Frog's Muscle (Gastrocnemius). The numbers under the curves indicate the number of shocks per second applied to the'^uscle. There is almost complete tetanus with twenty-five per second, and it is a little lower than the previous one because the muscle was slightly fatigued. — {Sthrling.) ~ In such a tracing it is observed that the second stimulation, occurring before the muscle had time to relax, gave rise to a second contraction, which was superposed on the first; the same result followed the third stimulus, the fourth, the fifth, and so on. Owing largely to this sum- mation of the contractions there is a gradual rise in the height of the con- traction curve. This condition of the muscle, viz., continued contrac- tion, combined with diminished power of relaxation, is termed con- tracture. The tracing also shows that as the stimulus continues, the base line, that connecting the lowest points of the contractions, grad- ually rises and takes the form of a curve which increases in height with the stimulation. The apex line, that connecting the highest points of the contractions, also rises at the same tiijie, indicating a continuous increase in the height of the contractions. The length of time a muscle will exhibit incomplete tetanus depends on a variety of circumstances, e. g., character of muscle, rate and strength of stimulation, etc., but mainly on the rapidity with which the muscle becomes fatigued. With the oncoming of fatigue the muscle begins to relax, and ultimately returns to its normal condition, notwithstanding the continued stimu- GENERAL PHYSIOLOGY OF MUSCLE-TISSUE. 77 lation. If the stimulation be withdrawn, the muscle does not at once return to its original length but remains more or less contracted for a variable time. This contraction after stimulation is known as the con- traction-remainder. If the stimulation be still further increased in frequency, the in- dividual contractions become fused together and the curve described by the lever becomes a continuous line. (See Fig. 35.) Notwith- standing the fact that the individual contractions are no longer visible, it can be shown by other methods that the muscle is undergoing a series of slight alternate contractions and relaxations or vibrations at least. After a varying length of time the muscle becomes fatigued, relaxes, and returns to its natural condition even though the stimu- lation be continued. The number of stimuli per second necessary to Fig. 36. — Development op Fatigue and Contraction. Muscle stimulated once a second by a strong induced current. l yvC&LA. t-'j->x' develop complete tetanus will depend under normal circumstances on the period of duration of the individiial contractions. The longer this period, the less the number of stimuli required, and the reverse. Hence the number of stimuli will vary for different classes of animals and for different muscles in the same animal, e. g., 2 or 3 for the muscles of the tortoise, 10 for the muscles of the rabbit, 15 to 20 for the frog, 70 to 80 for birds, 330 to 340 for insects. An effect which follows frequent stimulation of a muscle, e. g., 50 to 60 times per minute, and especially when the muscle is somewhat fatigued or cold is shown in Fig. 36. It is evidently a combination of contracture and fatigue. It will be observed that at the beginning of the stimulation there is a stair case effect, a-b, combined with dim- inished relaxation. This in turn is followed by a decline in the height of the contractions, b-c, and a fall of the base line which may be attributed to fatigue conditions. After a short time there is a second rise of the base line, d, and a rapid development of contracture. The muscle at this period is in a condition of incomplete tetanus which gradually passes into complete tetanus attended by fatigue. 78 TEXT-BOOK OF PHYSIOLOGY. The tetani of muscles may be classified in accordance with their causes as follows : — ■ni • 1 • VoUtional. 1. Physiologic [ Reflex. 2. Experimental. 3. Pharmacologic. T, .1 1 • Bacterial. 4. Pathologic [ Reflex. 1. Physiologic Tetanus. — i. VoUtional. — Because of the fact that during the continuance of a volitional movement the muscle is in a state of continuous contraction, it may be accepted that volitional con- tractions are states of tetanus, more or less complete ; for the shortest possible volitional contraction, however quickly it takes place, has al- ways a longer duration than a single contraction caused by an induced electric current. As the volitional contraction is similar to that observed when a muscle or its related nerve is stimulated by rapidly repeated induced currents, it is assumed that the nerve-cells in the spinal cord are discharging in a rhythmic manner a certain number of nerve im- pulses per second in consequence of the arrival of nerve impulses com- ing from the cerebral cortex, the result of volitional acts. In other words the volitional tetanus is the result of a discontinuous stimulation. The number of stimuli transmitted to a muscle during a volitional tetanus has been estimated by the employment of the graphic method at from 8 to 13 per second, 10 being about the average. When a voli- tional contraction is recorded the myogram not infrequently exhibits a series of small wave-like elevations which indicate that the muscle is not in a state of complete tetanus but is undergoing slight alternate contractions and relaxations. Unless the contraction process in human muscle differs from that of frogs it is difi&cult to see how 10 or even 20 stimuli per second can give rise to even an incomplete tetanus when the single contraction is Yiy of ^ second in duration. 2. Reflex. — ^A tetanus of muscle, physiologic in character, arises during the performance of many muscle movements in consequence of peripherally acting causes and may therefore be termed a reflex tetanus. The duration of a tetanus thus induced, like the duration of a volitional tetanus, will vary with the duration of the exciting cause. Reflex tetani are presented by the muscles of the lower jaw during mastication, by the intercostal muscles during breathing, by the muscles of the limbs during walking, etc. In these and other in- stances there are reasons for believing that for a variable period of time the muscles are in a state of continuous contraction from the dis- charge of nerve impulses from the nerve cells in the spinal cord as the result of the arrival of nerve impulses coming from a peripheral sur- face. GENERAL PHYSIOLOGY OF MUSCLE-TISSUE. 79 2. Experimental Tetanus. — The tetanus of muscle developed in accordance with the method described in foregoing paragraphs, i. e., by the employment of instrumental procedures, may be termed ex- perimental tetanus. Its mode of development serves to illustrate and explain the method by which individual contractions are summated and continuous contractions made possible for the performance of volitional acts. 3. Pharmacologic Tetanus. — The administration of certain drugs, e. g., strychnin, in sufficient amounts, is followed in a short time by a series of intermittent spasms in which all the muscles of the body are involved. At the beginning of the spasms the muscles are thrown into tonic or complete tetanus, during the continuance of which the muscles are hard and firm. In a short time this tonic state begins to subside, giving way to tremors or a series of irregular contractions resembling incomplete tetanus or clonus. A tetanus of this char- acter may be termed pharmacologic. Though the onset of the tetanus is occasioned largely by peripheral stimulation, the seat of action of strychnin is central and for the most part focalized in the spinal cord. The exact seat of its action is not definitely determined but there are reasons for believing that it is on the end-tufts of afferent nerves in the spinal cord or on the intercalated neuron between them and the nerve-cells in the anterior horns of the gray matter, the irrita- bility of which is raised and the resistance to the transmission of nerve impulses coming from the periphery diminished. As a result the nerve impulses are transmitted to the nerve-cells more readily, not only in a horizontal but also in a longitudinal direction, and the effects they pro- duce enormously increased. 4. Pathologic Tetanus. — i. Bacterial. — The introduction of a specific bacillus into a wound in any region of the body is followed after a period of incubation of from three or four days to a week by a tetanus in which nearly all the muscles of the body are involved, char- acterized by a tonic contraction and clonic exacerbations. A tetanus of this character may be termed pathologic. The persistent tonic contraction is the result of a more or less continuous discharge of nerve impulses from the nerve- cells of the spinal cord which have been rendered a,bnormally irritable by the action of a toxin, produced by the bacilli, and which has a selective action on these structures. The clonic exacerbations are evoked from time to time by various forms of peripheral stimulation. 2. Rejlex — A tetanus of individual muscles more or less continu- ous in character is occasionally the result of peripheral irritations of a pathologic character. A tonic contraction of the masseter muscles, for example, firmly closing the jaws for weeks and months at a time is caused in some instances by an impacted wisdom tooth or an ulcera- tive condition of the mouth. Since the removal of the cause is followed by a relaxation of the muscle, this form of tetanus, known as trismus, may be regarded as pathologic in character and reflex in origin. 8o TEXT-BOOK OF PHYSIOLOGY. The Muscle Sound. — If a stethoscope or a myophone with tele- phone connections be placed on a muscle while in a condition of voli- tional tetanus and at the same time kept in a certain degree of tension, there will be developed in the observer a sensation of sound or tone which is spoken of as a muscle sound or tone. It is also readily heard in the masseter muscle when the side of the face is placed on a receiv- ing body such as a pillow, and the masseter muscles made to con- tract volitionally. This tone is attributed to a vibration or an alternate contraction or relaxation of the muscle or to an intermittent rhythmic variation in tension, the result of the rate of stimulation. This tone corresponds to a vibration frequency of from 1 8 to 20 per second and is accepted as one of the proofs that the physiologic volitional tetanus is not continuous but discontinuous in character. If a muscle is tetanized with induced currents, the tone increases in pitch for a limited time as the frequency of the current per second increases up to a certain maximum, which for frogs is about 200 and for mammals about 1000. CHEMIC PHENOMENA. The chemic . changes which underlie the transformation of energy in the living muscle even when in a state of rest are active and com- plex, though but little is known as to their exact character. As shown by an analysis of the blood flowing to and from the resting muscle, it has, while flowing through the capillaries, lost oxygen and gained carbon dioxid. The amount of oxygen absorbed by the muscle (9 per cent.) is greater than the amount of carbon dioxid (6.7 per cent.) given off. Notwithstanding the relation of the oxygen absorbed to the carbon dioxid produced, there is no parallelism between these two processes, as the carbon dioxid will be given off in the absence of free oxygen or in an atmosphere of nitrogen. In the active or contracting muscle all the chemic changes are increased, as shown both by an increased absorption of oxygen and an increased production of carbon dioxid, though the ratio existing between them differs considerably from that of the resting muscle. Thus, according to Ludwig, an active muscle absorbs 12.26 per ceiit. of oxygen and gives off 10.8 per cent, carbon dioxid. During the activity of a muscle its tissue changes from a neutral to an acid reac- tion, from the development of sarcolactic acid and possibly phos- phoric acid. The degree of the acidity depends to some extent on the duration of the contraction periods. Chemic analysis of a tetanized muscle shows that it contains less glycogen than a resting muscle, and that it contains a larger amount of water. Coincident with muscular contraction, the blood-vessels become widely dilated, leading to a large increase in the blood-supply and a rapid removal of the products of decomposition. Rigor Mortis. — A short time after death the muscles pass into a condition of extreme rigidity or contraction known as death stiffening GENERAL PHYSIOLOGY OF MUSCLE-TISSUE. 8i or rigor mortis, which lasts from one to five days. In this state they offer great resistance to extension. At the same time their tonicity dis- appears, their cohesion diminishes, and their irritability ceases. The time of the appearance of this post-mortem rigidity varies from a quarter of an hour to seven hours. Its onset and duration are influenced by the condition of the muscle irritability at the time of death. When the ir- ritability is impaired from any cause, such as chronic disease or defec- tive blood-supply, the rigidity appears promptly but is of short duration. After death from acute diseases it is apt to be delayed, but will continue for a longer period. The rigidity first appears in the muscles of the lower jaw and neck; next in the muscles of the abdomen and upper extrem- ities; finally in the trunk and lower extremities. It disappears in prac- tically the same order. Chemic changes of a marked character ac- company this process. The muscle becomes acid in reaction from the development of sarcolactic acid and there is a large increase in the amount of carbon dioxid given off. The. immediate cause of the rigidity appears to be coagulation of the myosinogen within the sar co- lemma with the formation of an insoluble protein, myosin. In the early stages of the coagulation restitution is possible by. the circula- tion of arterial blood through the vessels. The final disappearance of this post-mortem rigidity is due probably to the action of acids which render the myosin soluble, and possibly to the action of various micro- organisms which give rise to putrefactive changes. Source of the Muscle Energy. — Notwithstanding many in- vestigations, the nature of the materials which are the immediate source of the muscle energy is not known. The absence of any noticeable increase in the quantity of urea or other nitrogen-holding compounds excreted renders it probable that the energy does not come from the metabolism of proteid materials. The marked production of carbon dioxid and sarcolactic acid points to the decomposition of some un- stable compound, of a carbohydrate character, rich in carbon and oxygen. It has been suggested that glycogen furnishes the energy, inasmuch as this substance, generally present in muscle, disappears during activity. A muscle which has been tetanized contains less glycogen than the corresponding muscle at rest. A muscle which has been separated from the nervous system by division of its nerves and thus prevented from contracting accumulates glycogen. Bunge is of the opinion that though the carbohydrates are the main, they are not the only sources of muscle energy. If there is a deficiency or ab- sence of carbohydrate food, the muscle will utilize fat and proteid, for experiment has shown that the available glycogen is entirely con- sumed the second or third day. The mechanism by which the energy is liberated, whether by decomposition or direct oxidation, is unknown. The fact that muscle will contract in an atmosphere free of oxygen, that no free oxygen can be obtained from muscle, would support the idea that the mechanism is one of decomposition. Hermann suggests that the energy of a contraction is liberated by the splitting and subsequent 6 82 TEXT-BOOK OF PHYSIOLOGS?^. re-formation of a complex body belonging neither to the carbohydrates nor fats, but to the proteids — to this hypothetic body the term inogen is given. This complex molecule, the product of the nutritive activity of the muscle-cell in undergoing decomposition, would yield carbon dioxid, sarcolactic acid, and a proteid residue resembling myosin. On the cessation of the contraction the muscle-cell recombines the proteid residue with oxygen, carbohydrates, and fats, and again forms the energy-holding compound, inogen. The phenomena of rigor mortis support this view. At the moment of this contraction the muscle gives ofiF COj in large amount, develops sarcolactic acid and myosin. There is thus a close analogy between the two processes; in other words, a contraction is a partial death of the muscle. If this view is correct, then the oxygen is required mainly for heat production through oxida- tion processes. THERMIC PHENOMENA. The potential energy liberated during a contraction is transformed into kinetic energy — ^viz., heat and mechanic motion. Though heat production is taking place even during the passive condition, prob- ably through oxidation processes, it is largely increased by muscle activity. The skeletal muscle of the frog, the gastrocnemius, shows after tetanization an increase in temperature from 0.14° C. to 0.18° C, and after a single contraction from 0.001° C. to 0.005° C. The amount of heat thus produced will vary with a variety of conditions, as strength of stimulus, tension, work done, etc. Stimulus. — It has been experimentally determined that an in- crease in the strength of the stimulus from a minimal to a maximal value increases the amount of heat liberated. This is the direct result of increased chemic change naturally following increased stimulation. Tension. — The greater the tension of a muscle, the greater, other conditions being the same, is the amount of heat liberated. If the muscle is securely fastened at both extremities so that shortening is practically impossible during the stimulation, the maximum of heat production is reached. In the tetanic state the great increase in tem- perature is due to the tension of antagonistic and strongly contracted muscles. In both instances, mechanic motion being prevented, the liberated energy is transformed into heat. Mechanic Work. — If the muscle is permitted to shorten and raise a weight, some of the energy liberated takes the form of mechanic motion. If the weight is removed at the height of the contraction, external work is accomplished. The greater the weight raised, within limits, the greater is the percentage of energy which takes the direction of mechanic motion. The percentage of the total energy liberated which is thus utilized, has been estimated at from 25 to 40 per cent. In accordance with the law of the conservation of energy, the heat produced, stated in calories, plus the energy required in the raising of the weight, Missing Page Missing Page Missing Page Missing Page Missing Page Missing Page Missing Page Missing Page Missing Page Missing Page Missing Page Missing Page Missing Page Missing Page Missing Page Missing Page GENERAL PHYSIOLOGY OF NERVE-TISSUE. 99 The neurons in their totality constitute the neuron or nerve tissue. From the fact that they are arranged both serially and collaterally into a regular and connected whole, they collectively constitute a system known as the neuron or nerve system. Neurons, moreover, are grouped into more or less completely organized masses, termed organs, which in accordance with their locations may for convenience be divided into central and peripheral organs. The central organs of the nerve system consist of the en- cephalon or brain and the spinal cord ; the peripheral organs consist of the cranial nerves, the spinal nerves, the sympathetic gangha and their branches. Nerve-fibers. — ^The nerve-fibers which constitute by far the larger part of both the peripheral and central organs of the nerve system, are simply the axonic processes of neurons with their secon- dary investments, the myeUn and neurilemma; according as they possess or do not possess the medullary sheath, they may be divided into two groups — viz., meduUated and non-meduUated fibers. Medullated Nerve-fibers. — ^These consist for the most part of three distinct structures: 1 . An external investing sheath, tubular in shape, termed the neuril- emma. 2. An intermediate semifluid substance — ^the medulla or myehn. 3. An internal dark thread — the axis-cylinder. The neurilemma is a thin, transparent, homogeneous membrane closely adherent to the medulla. Owing to its colorless appearance, it can be seen only with difficulty in fresh tissue. When treated with various reagents, it becomes distinct. Physically, it is quite resistant and elastic. Its function is doubtless that of a protective agent to the structures within. The medulla, myelin, or white substance of Schwann completely fills the neurilemma and closely invests the axis-cylinder or axon. In fresh tissue the medulla is clear, homogeneous, semifluid, and highly refracting. When the nerve is treated with various reagents which alter its composition, the medulla becomes opaque and imparts a white, ghstening appearance. The function of the medulla is quite unknown. At intervals of about seventy-five times its diameter the medul- lated nerve-fiber undergoes a remarkable diminution in size, due to an interruption of the medullary substance, so that' the neurilemma hes directly on the axis-cyhnder. These constrictions, or nodes of Ranvier, taking their name from their discoverer, occur at regular intervals along the course of the nerve, separating it into a series of segments. The portion between the nodes is termed the internodal segment. It has been suggested that in consequence of the absence of the myehn at these nodes, a free exchange of nutritive material and decomposition products can take place between tbe axis-cylinder and the surrounding plasma. Beneath the neurilemma in each inter- loo TEXT-BOOK OF PHYSIOLOGY. nodal segment there is a large nucleus surrounded by a small amount of granular protoplasm. The axis-cylinder, or axon, the direct outgrowth of the nerve-cell, is the most essential element of the nerve-fiber, as it alone is uni- formly continuous throughout. In the natural condition it is trans- parent and invisible; but when treated with proper reagents, it presents itself as a pale, granular, flattened band, more or less solid and some- what elastic. It is albuminous in composition. With high magnifi- cation the axis presents a longitudinal striation, indicating a fibrillar structure. The fibrillae appear to be embedded in an intervening semifluid substance, the neuroplasm. Non-MeduUated Nerve-fibers,— These consist, for the most part, only of the axis-cylinder, though in some portions of the nerve system a neurilemma is also present. Though much less abundant than the former variety, they are distributed largely throughout the nerve system, but are particularly abundant in the sympathetic. Owing to the absence of a medulla, they present a rather pale or grayish appearance. Sympathetic Ganglia. — A sympathetic ganglion consists essen- tially of a connective-tissue capsule with an interior framework. The meshes of this framework contain nerve-cells provided with dendrites and branching axons. The majority of the axons are non- medullated. In all instances, with the exception of the ganglion cells of the heart, the axons are distributed to non-striated muscle tissue and to the epithelium of glands. The nerve-cells of the ganglia are also in histologic connection with the terminal branches of fine meduUated nerve-fibers which leave the spinal cord by way of the anterior roots of the spinal nerves. These nerve-fibers are designated autonomic or pre- ganglionic fibers, while those emerging from the cells are designated post-ganglionic fibers. (See Sympathetic System.) The Peripheral Organs of the Nerve System. — These consist of the cranial and spinal nerves and the sympathetic ganglia. Each nerve consists of a variable number of nerve-fibers united into firm bundles by connective tissue which supports blood-vessels and lym- phatics. The bundles are technically known as nerve-trunks or nerves. The nerve-trunks connect the brain and cord with all the re- maining structures of the body. Each nerve is invested by a thick layer of lamellated connective tissue, known as the epineuriunt. A transverse section of a nerve shows (see Fig. 47) that it is made up of a number of small bundles of fibers, each of which possesses a separate investment of connective tissue — the perineurium. With- in this membrane the nerve-fibers are supported by a fine stroma — the endoneurium. After pursuing a longer or shorter course, the nerve-trunk gives off branches, which interlace very freely with neigh- boring branches, forming plexuses, the fibers of which are distributed to associated organs and regions of the body. From their origin to GENERAL PHYSIOLOGY OF NERVE-TISSUE. lOI their termination, however, nerve-fibers retain their individuality, and never become blended with adjoining fibers. As nerves pass from their origin to their peripheral terminations, they give off a number of branches, each of which becomes invested Tsdth a lamellated sheath — an offshoot from that investing the parent trunk. This division of nerve-bundles and sheath continues through- out all the branchings down to the ultimate nerve-fibers, each of which is surrounded by a sheath of its own, consisting of a single layer of endotheUal cells. This deUcate transparent membrane, the sheath of Henle, is separated from the nerve-fiber by a considerable Fig. 47. — Transverse Section of a Nerve (Median), ep. Epineurium. pe. Perineurium, ed. Endoneurium. — {Landois and Stirling.) space, in which is contained lymph destined for the nutrition of the fiber. Near their ultimate terminations the nerve-fibers themselves undergo division, so that a single fiber may give origin to a number of branches, each of which contains a portion of the parent axis- cylinder and myelin. Blood-supply. — ^Nerves being parts of living cells require for the maintenance of their nutrition a certain amount of blood. This is furnished by the blood-vessels ramifying in and supported by the connective-tissue framework. Here as elsewhere there is a constant exchange, through the capillary wall and the neurilemma, of nutritive material to the nerve proper and of waste materials to the blood. The Chemic Composition and Metabolism. — Chemic analysis of nerve-tissue has shown the presence of water, proteins (two glob- ulins and a nucleo-protein), neurokeratin and nuclein, two phos- phorized bodies (protagon and lecithin), several cerebrosides (nitro- I02 TEXT-BOOK OF PHYSIOLOGY. gen-holding bodies of a glucoside character, as shown by their yielding the reducing carbohydrate galactose), inorganic salts, and a series of nitrogen-holding bodies such as creatin, xanthin, urea, leucin, etc. As to the metabohsm that is taking place in nerve-cells and fibers, practic- ally nothing definite is known. That such changes, however, are taking place would be indicated first by the blood-supply, and second by the fact that withdrawal of the blood-supply is followed by a loss of irritability. The metabolism of the central organs of the nerve sys- tem is more active and extensive. In this situation any withdrawal of blood from compression or occlusion of blood-vessels is followed by impairment of nutrition and loss of function. THE RELATION OF THE PERIPHERAL ORGANS OF THE NERVE SYSTEM TO THE CENTRAL ORGANS. Spinal Nerves. — ^The nerves in connection with the spinal cord are thirty-one in number on each side. If traced toward the spinal column, it will be found that the nerve-trunk passes through an in- tervertebral foramen. Near the outer limits of the foramina each nerve-trunk divides into two branches, generally termed roots, one of which, curving slightly forward and upward, enters the spinal cord on its anterior or ventral surface, while the other, curving back- ward and upward, enters the spinal cord on its posterior or dorsal surface. The former is termed the anterior or ventral root ; the latter, the posterior or dorsal root. Each dorsal root presents near its union with the ventral root a small ovoid grayish enlargement known as a ganglion. Both roots previous to entering the cord subdivide into from four to six fasciculi. A microscopic examination of a cross-section of the spinal cord shows that the fibers of the ventral roots can be traced directly into the body of the nerve-cells in the ventral horns of the gray matter. The fibers of the dorsal roots are not so easily traced, for they diverge in several directions shortly after entering the cord. In their course they give off collateral branches which, in common with the main fiber, end in tufts which become associated with nerve-cells in both the ventral and dorsal horns of the gray matter. Cranial Nerves. — ^The nerves in connection with the base of the brain are known as cranial nerves; some of these nerves present a similar ganglionic enlargement, and therefore may be regarded as dorsal nerves, while others may be regarded as ventral nerves. Their relations within the medulla oblongata are similar to those within the spinal cord. Efferent and Afferent Nerves. — ^Nerves are channels of com- munication between the brain and spinal cord, on the one hand, and the muscles, glands, blood-vessels, skin, mucous membrane, viscera, etc., on the other. Some of the nerve-fibers serve for the transmission of nerve energy from the brain and spinal cord to certain peripheral GENERAL PHYSIOLOGY OF NERVE-TISSUE. 103 organs, and so increase or retard their activities; others serve for the transmission of nerve energy from certain peripheral organs to the brain and spinal cord which gives rise to sensation or other modes of nerve activity. The former are termed efferent or centrifugal, the latter afferent or centripetal nerves. Experimentally it has been de- termined that the anterior or ventral roots contain all the efferent fibers, the posterior or dorsal roots all the afferent fibers. , THE PERIPHERAL ENDINGS OF NERVES. The efferent nerves as they approach their ultimate terminations lose both the neurilemma and myelin sheath. The axon or axis- cylindter then divides into a number of branches which become directly Motor plate Medullated nerve-fibers. Nerve-fiber y^ bundle. Fig. 48. — Motor Nerve-endings of Intercostal Muscle-fibers of a Rabbit. X ISO-— {Stokr.) and intimately associated with tissue-cells. The particular mode of termination varies in different situations. These terminations are generally spoken of as end-organs, terminal organs, or end-tufts.. In the skeletal muscle the nerve-fiber loses both neurilemma and myelin sheath at the point where it comes in contact with the muscle- fiber. After penetrating the sarcolemma, the axon or axis-cyUnder divides into a number of small branches which appear to be embedded in a relatively large mass of sarcoplasm and nuclei, the whole form- ing the so-called "motor plate." Each muscle-fiber possesses one such plate or end-organ in mammalia, several in the frog. (Fig. 48.) In the visceral muscle the terminal nerve-fibers derived from sympathetic or peripheral neurons are primarily non-medullated. The axons divide and subdivide and form plexuses which surround the muscle-cell bundles. Fine fibers from the plexuses are given off I04 . TEXT-BOOK OF PHYSIOLOGY. which ultimately come into relation with each individual cell, on the surface of which they terminate in the form of one or more granular masses. In the glands, taking as an illustration the parotid and mammary glands, the nerve-fibers, also derived from sympathetic or peripheral neurons, pass into the body of the gland and ultimately reach the acini, on the outer surface of which they ramify and form a plexus. From this plexus fine fibers penetrate the acinus wall and end on the gland-cell. The fibers present a varicose appearance (Fig. 49)- The afferent nerves as they approach their ultimate terminations undergo similar changes. The end-tufts become associated, in some situations, with speciaUzed end-organs which are extremely complex; e. g., the retina in the eye, the organ of Corti in the ear, the taste- beakers in the tongue, the olfactory cells in the nose. In the skin and mueous mem- branes the mode of termination Fig. 49.— Terminations of Nerve- varies considerably. The foUow- HBERs IN THE Gland-cells. A. Cell jug ^j-c some of the principal of the parotid gland of a rabbit. B. A . Cells of the mammary gland of a cat in moaes . gestatibn.— (Uoyojt and Moral.) i. Free endings in the epithelium. 2. Tactile cells of Merkel. 3. Tactile corpuscles in the papillae of the true skin. 4. Pacinian corpuscles found attached to the nerves of the hand and feet, to the intercostal nerves, and to nerves in other situations. ■ 5. End-bulbs of Krause in the conjunctiva, clitoris, penis, etc. (A consideration of these end-organs will be found in the chapters devoted to the organs of which they form a part.) In the skeletal muscles afferent fibers become associated with small spindle-shaped structures known as muscle-spindles or neuromuscle end-organs. These spindles vary in length from i mm. to 4 mm. They consist of a capsule of fibrous tissue containing from five to twenty muscle-fibers. After penetrating the several layers of the capsule, the nerve-fibers lose the neurilemma and myeUn sheaths. The axons or axis-cylinders then divide into several long narrow branches which wind themselves in a spiral manner around the con- tained muscle-fiber and terminate in small oval-shaped discs. Similar endings have been observed in the tendons of muscles. Development and Nutrition of Nerves. — ^The efferent nerve- fibers," which constitute some of the cranial nerves and all the ventral roots of the spinal nerves, have their origin in cells located in the gray matter beneath the aqueduct of Sylvius, beneath the floor of the fourth ventricle, and in the anterior horns of the gray matter of the spinal cord. These cells are the modified descendants of independent, oval, pear-shaped cells — ^the neuroblasts — which migrate from the medullary tube. As they approach the surface of the cord their axons are GENERAL PHYSIOLOGY OF NERVE-TISSUE. loS Poihenor Xoot directed toward the ventral surface, which eventually they pierce. Emerging from the cord, the axons continue to grow, and become invested with the myelin sheath and neurilemma, thus constituting the ventral roots. The afferent nerve-fibers, which constitute some of the cranial nerves and all the dorsal roots of the spinal nerves, develop outside of the central nervous system and only subsequently become con- nected with it. (See Fig. 50.) At the time of the closure of the medullary tube a band or ridge of epithelial tissue develops near the dorsal surface, which, becoming segmented, moves outward and forms the rudimentary spinal gang- lia. The cells in this situa- tion develop two axons, one from each end of the cell, which pass in opposite direc- tions, one toward the spinal cord, the other toward the periphery. In the adult con- dition the two axons shift their position, unite, and form a T shaped process, after which a division into two branches again takes place. In the ganglia of all the sensori-cranial and sensori- spinal nerves the cells have this histologic peculiarity. The efferent fibers are therefore to be regarded as outgrowths from the nerve- cells in the ventral horns of the gray matter, and serve to bring the cells into anatomic and physiologic relationship directly with the skeletal muscles and indirectly, through the intermediation of ganglia (see sympathetic nervous system), with visceral muscles and glands. The afferent fibers are to be regarded as outgrowths from the cells of the dorsal nerve ganglia, and serve to bring the skin, mucous membrane, and certain visceral structures into relation with special- ized centers in the central nerve system. Nerve Degeneration. — If any one of the cranial or spinal nerves be divided in any portion of its course, the part in connection with the periphery in a short time exhibits certain structural changes, to which the term degeneration is applied. The portion in connection with the brain or cord retains its normal condition with the exception of a few millimeters at its peripheral end. The degenerative process begins simultaneously throughout the entire course of the nerve, and consists in a disintegration and reduction of the myelin and axis- cylinder into nuclei, drops of myelin, and fat, which in time disappear JSooi Fig. 50. — Diagram Showing the Mode OF Origin of the Ventral and Dorsal Roots. — {Edinger, after His.) io6 TEXT-BOOK OF PHYSIOLOGY. through absorption, leaving the neurilemma intact. Coincident with these structural changes there is a progressive alteration and diminu- tion in the excitability of the nerve. Inasmuch as the central portion of the nerve, which retains its' connection with the nerve-cell, remains histologically normal, it has been assumed that the nerve-cells exert over the entire course of the nerve-fibers a nutritive or a trophic in- fluence. This idea has been greatly strengthened since the discovery that the axis-cylinder, or the axon, has its origin in and is a direct outgrowth of the cell. When separated from the parentcell, the fiber appears to be incapable in itself of maintaining its nutrition. The relation of the nerve-cells to the nerve-fibers, in reference to their nutrition, is demonstrated by the results which follow section of the ventral and dorsal roots of the spinal nerves. If the anterior Fig. 51. — Degeneration of Spinal Nerves and Nerve-roots after Section. A. Section of nerve-trunk beyond the ganglion. B. Section of anterior root. C. Sec- tion of posterior root. D. Excision of ganglion, a. Anterior root. p. Posterior root. g. Ganglion. — (JDalton.) root alone be divided, the degenerative process is confined to the peripheral portion, the central portion remaining normal. If the posterior root be divided on the peripheral side of the ganglion, de- generation takes place only in the peripheral portion of the nerve. (See Fig. 51.) If the root be divided between the ganglion and the cord, degeneration takes place only in the central portion of the root. From these facts it is evident that the trophic centers for the ventral and dorsal roots lie in the spinal cord and spinal nerve ganglia, re- spectively, or, in other words, in the cells of which they are an integrai part. The structural changes which nerves undergo after separation from their centers are degenerative in character, and the process is usually spoken of, after its discoverer, as the Wallerian degeneration. When the nerve-cells from which the nerve-fibers arise, whether efferent or afferent, undergo degeneration from any cause whatever, the nerve-fiber becomes involved in the degenerative process and when it is completed the structures to which they are distributed, especially the muscles, undergo an atrophic or fatty degeneration, with a change or loss of their irritability. This is, apparently, not to be attributed merely to inactivity, but rather to a loss of nerve influences, inasmuch as inactivity merely leads to atrophy and not to degeneration. GENERAL PHYSIOLOGY OF NERVE-TISSUE. 107 Reunion and Regeneration. — When a nerve-trunk is divided there is a loss of function of the parts to which it is distributed, and usually involves both motion and sensation. This, however, is not necessarily permanent, for after a variable period of time it not in- frequently happens, that the functions are restored because of a reunion of the separated ends and a regeneration of the peripheral portion. A histologic study of the nerve-fibers after separation from the nerve- cells shows that coincidently with the degenerative process there occurs a regenerative process, consisting in a multiplication of the nuclei lying just beneath the neurilemma and an accumulation around them of a granular protoplasm which in due time completely fill the neuril- emma. At this stage the fiber is known as a band-fiber. If now the physical conditions are such as to permit of a reunion of the nerve, this takes place, and under the nutritive influence of the cell the axis- cylinder grows into the band-fiber and the protoplasm becomes trans- formed into myelin as in the original fiber. The axis-cylinder con- tinues to grow and extend itself forward until it reaches its ultimate termination. CLASSIFICATION OF NERVES. The efferent nerves may be classified, in accordance with the characteristic form of activity to which they give rise, into several groups, as follows: 1. Skeletal-muscle or motor nerves, those which convey nerve energy or nerve impulses to skeletal-muscles and excite them to activity. 2. Gland or secretor nerves, those which convey nerve impulses to glands, and cause the formation and discharge of the secretion peculiar to the gland. 3. Vascular or vaso-motor nerves, those which convey nerve impulses to the muscle-fibers of the blood-vessels and change in one direction or the other the degree of their natural contraction. Those which 'increase the contraction are known as vaso-constrictors or vaso- augmentors; those which decrease the contraction are known as vaso-dilatators or vaso-inhibitors. The nerves which pass to that specialized part of the vascular apparatus, the heart, transmit nerve impulses which on the one hand accelerate its rate or augment its force, and on the other hand inhibit or retard its rate and diminish its force. For this reason they are termed cardio-motor nerves, one set of which is known as accelerator and augmentor, the other as inhibitor nerves. 4. Visceral or viscero-motor nerves, those which transmit nerve im- pulses to the muscle walls of the viscera and change in one direc- tion or another the degree of their contraction. Those which increase or augment the contraction are known as viscero-aug- mentor, while those which decrease or inhibit the contraction, are known as viscero-inhibitor nerves. io8 TEXT-BOOK OF PHYSIOLOGY. 5. Hair bulb or pilo-moior nerves, those which transmit nerve im- pulses to the muscle-fibers which cause an erection of the hairs. Of the foregoing nerves the skeletal-muscle or motor nerves^ alone pass directly to the muscle. The gland, the vascular and the visceral nerves, all terminate at a variable distance from the peripheral organ around a local sympathetic ganglion, which in turn is connected with the peripheral organ. The former are termed pre-ganglionic. The latter post-ganglionic fibers. (See Fig. 61.) The afferent nerves may also be classified, in accordance with their distribution and the character of the sensations or other modes of nerve activity to which they give rise, into several groups, as fol- lows: I. Tegumentary nerves, comprising those distributed to skin, mucous membranes and sense organs and which transmit nerve impulses from the periphery to the nerve centers. They may be divided into sensorifacient and reflex nerves. A. Sensorifacient nerves, those which transmit nerve impulses to the brain where they give rise to conscious sensations. They may be subdivided into: 1. Nerves of special sense — e. g., olfactory, optic, auditory, gustatory, tactile, thermal, pain, pressure — ^which give rise to correspondingly named sensations. 2. Nerves of general sense — e. g., the visceral afferent nerves — ^those which give rise normally to vague and scarcely perceptible sensations, such as the general sen- sations of well-being or discomfort, hunger, thirst, fatigue, sex, want of air, etc. B. Reflex nerves, those which transmit nerve impulses to the spinal cord and medulla oblongata, where they give rise to different modes of nerve activity. They may be divided into : 1. Reflex excitator nerves, which transmit nerve impulses which cause an excitation of nerve centers and in conse- quence increased activity of peripheral organs, e. g., skeletal muscles, glands, blood-vessels and viscera. 2. Reflex inhibitor nerves, which transmit nerve impulses which cause an inhibition of nerve centers and in conse- quence, decreased activity of the peripheral organs. It is quite probable that one and the same nerve may sub- serve both sensation and reflex action, owing to the col- lateral branches which are given off from the afferent roots as they ascend the posterior column of the cord. Muscle nerves, comprising those distributed to muscles and tendons and which transmit nerve impulses from muscle and tendons to the brain where they give rise to the so-called muscle sensations, e. g., the direction and the duration of a movement, the resistance offered and the posture of the body or of its indi- vidual parts. GENERAL PHYSIOLOGY OF NERVE-TISSUE. 109 PHYSIOLOGIC PROPERTIES OF NERVES. Nerve Irritability or Excitability and Conductivity. — These terms are employed to express that condition of a nerve which enables it to develop and to conduct nerve impulses from the center to the periphery, or from the periphery to the center, in response to the action of stimuli. A nerve is said to be excitable or irritable so long as it possesses these capabilities or properties. For the manifestation of these properties the nerve must retain a state of physical and chemic integrity; it must undergo no change in structure or chemic composi- tion. The irritability of an efferent nerve is demonstrated by the contraction of a muscle, by the secretion of a gland, or by a change in the caliber of a blood-vessel, whenever a corresponding nerve is stimulated. The irritability of an afferent nerve is demonstrated by the production of a sensation or a reflex action whenever it is stimu- lated. The irritability of nerves continues for a certain period of time after separation from the nerve-centers and even after the death of the animal, the time varying in different classes of animals. In the warm-blooded animals, in which the nutritive changes take place with great rapidity, the irritability soon disappears — a result due to disintegrative changes in the nerve, caused by the withdrawal of the blood-supply and other non-physiologic conditions. In cold-blooded animals, on the contrary, in which the nutritive changes take place relatively slowly, the irritability lasts, under favorable conditions, for a considerable time. Other tissues besides nerves possess irritability, that is, the property of responding to the action of stimuli — e. g., glands and muscles, which respond by the production of a secretion or a contraction. Independence of Tissue Irritability. — The irritability of nerves is distinct and independent of the irritability of muscles and glands, as shown by the fact that it persists in each a variable length of time after their histologic connections have been impaired or destroyed by the introduction of various chemic agents into the circulation. Curara, for example, induces a state of complete paralysis by modifying or depressing the conductivity of the end-organs of the nerves just where they come in contact with the muscles, without impairing the irrita- bility of either nerve-trunks or muscles. Atropin induces complete suspension of gland activity by impairing the terminal organs of the secretor nerves just where they come into relation with the gland- cells, without destroying the irritability of either gland-cell or nerve. Nerve Stimuli. — -Nerves do not possess the power of spontaneously generating and propagating nerve impulses; they can be aroused to activity only by the action of an external stimulus. In the physiologic condition the stimuli capable of throwing the nerve into an active condition act for the most part on either the central or peripheral end of the nerve. In the case of motor nerves the stimulus to the excitation, originating in some molecular disturbance in the nerve- no TEXT-BOOK OF PHYSIOLOGY. cells, acts upon the nerve-fibers in connection with them. In the case of sensor or afferent nerves the stimuli act upon the peculiar end-organs with which the sensor nerves are in connection, which in turn excite the nerve-fibers. Experimentally, it can be demonstrated that nerves can be excited by a sufiiciently powerful stimulus applied in any part of their extent. Nerves respond to stimulation according to their habitual function; thus, stimulation of a sensor nerve, if sufficiently strong, results in the' sensation of pain; of the optic nerve, in the sensation of light; of a motor nerve, in contraction of the muscle to which it is distrib- uted; of a secretor nerve, in the activity of the related gland, etc. It is, therefore, evident that peculiarity of nerve function depends neither upon any special construction or activity of the nerve itself nor upon the nature of the stimulus, but entirely upon the pecul- iarities of its central and peripheral end-organs. Nerve stimuli may be divided into — 1. General stimuli, comprising those agents which are capable of exciting a nerve in any part of its course. 2. Special stimuli, comprising those agents which act upon nerves only through the intermediation of the end-organs. The end-organs are specialized highly irritable structures placed between the nerve-fibers and the surface. They are especially adapted for the reception of special stimuli and for the liberation of energy, which in turn excites the nerve-fiber to activity. General stimuli: 1. Mechanic: Sharp taps, sudden pressure, cutting, etc. 2. Thermic: Sudden application of heated object. 3. Qhemic: Contact of various substances which alter their chemic composition quickly, e. g., strong acids or alkalies, sol. sodium chlorid 15 per cent., sugar, urea, etc. 4. Electric: Either the constant or induced current. Special stimuli: For afferent nerves — 1. Light or ethereal vibrations acting upon the end-organs of the optic nerve in the retina. 2. Sound or atmospheric undulations acting upon the end-organs of the auditory nerve. 3. Heat or vibrations of the air acting upon the end-organs in the skin. 4. Chemic agencies acting upon the end-organs of the olfactory and gustatory nerves. For efferent nerves — A molecular disturbance in the central nerve-cells from which they arise, the nature of which is unknown. Nature of the Nerve Impulse. — As to the nature of the nerve impulse generated by any of the foregoing stimuli, either general or special, but little is known. It has been supposed to partake of the nature of a molecular disturbance, a combination of physical and GENERAL PHYSIOLOGY OF NERVE-TISSUE. iii chemic processes attended by the liberation of energy, which propag- ates itself from molecule to molecule. The passage of the nerve impulse is accompanied by changes of electric tension, the extent of which is an indication of the intensity of the molecular disturbance. Judging from the deflections of the galvanometer needle it is probable that when the nerve impulse makes its appearance at any given point it is at first feeble, but soon reaches a maximum development, after which it speedily declines and disappears. It may, therefore, be graphically represented as a wave-like movement with a definite length and time duration. Under strictly physiologic conditions the nerve impulse passes in one direction only; in efferent nerves from the center to the periphery, in afferent nerves from the periphery to the center. Experimentally, however, it can be demonstrated that when a nerve impulse is aroused in the course of a nerve by an ade- quate stimulus it travels equally well in both directions from the point of stimulation. When once started, the impulse is confined to the single fiber and does not diffuse itself to fibers adjacent to it in the same nerve-trunk. Rapidity of Conduction of the Nerve Impulse. — The passage of a nerve impulse, either from the brain to the periphery or in the reverse direction, requires an appreciable period of time. The velocity with which the impulse travels in human sensory nerves has been estimated at about 50 meters a second, and for motor nerves at from 28 to 33 meters a second. The rate of movement is, however, somewhat modified by temperature, cold lessening and heat increasing the rapidity; it is also modified by electric conditions, by the action of drugs, the strength of the stimulus, etc. The rate of transmission through the spinal cord is considerably slower than in nerves, the average velocity for voluntary motor impulses being only 11 meters a second, for sensory impulses 12 meters, and for tactile impulses 40 meters a second. Nerve Fatigue. — Inasmuch as nerves are parts of living cells, the seat of nutritive changes, it might be supposed that the passage of nerve impulses would be attended by the disruption of energy-holding compounds, the production of waste products, the liberation of heat, and in time by the phenomena of fatigue. Though it is probable that changes of this character occur, yet no reliable experimental data have been obtained which afford a clue as to the nature or extent of any such changes. Stimulation of motor nerves with the induced electric current for four hours appears to be without influence either on the intensity of the nerve impulse or the rate of its conduction. Identity of Efferent and Afferent Nerves and Nerve Impulses. — Notwithstanding the classification of nerve-fibers based on differ- ences of physiologic actions, there are no characters, either histologic or chemic, which serve to distinguish them from one another. More- over, as the nerve impulse is conducted through a nerve-fiber equally well in both directions, as determined by experiments, it is probable TEXT-BOOK OF PHYSIOLOGY. that it does not differ in character in the two classes of nerves. That the efferent fibers conduct the nerve impulses from the nerve-centers to the periphery, and the afferent nerves from the periphery to the centers, is because of the fact that they receive their stimulus physio- logically only in the centers or at the periphery. The fundamental reason for difference of effects produced by stimulation of different nerves is the character of the organ to which the nerve impulse is conducted. A nerve is merely the transmitter of the nerve impulse, which if conducted to a muscle excites contraction; to a gland, secre- tion; to a blood-vessel, variation in caliber; to special areas in the brain, sensations of light, sound, pain, etc. Electric Excitation of Nerves. — ^For the pur- pose of studying the physiologic activities of nerves it has been found convenient to employ the nerve- muscle preparatioil (the gastrocnemius muscle and sciatic nerve) and to use as a stimulus the induced electric current. (See Fig. 52.) When kept moist, this preparation is extremely sensitive to either the galvanic or the induced current. Though the development and conduction of a nerve impulse may be demonstrated by the deflec- tion of the galvanometer needle or the movement of the mercury in the capillary electrometer, it is more conveniently demonstrated by the contraction of a muscle, the vigor of which, within limits, may be taken as a measure of the intensity of the im- pulse. The preparation should be enclosed in a moist chamber and the nerve connected with the inductorium through the intervention of non-polarizable electrodes. The muscle may be attached to the muscle-lever and its contractions recorded. A single shock of an induced current develops, it is believed, a single nerve impulse followed by a single muscle contraction. A minimal contraction following a minimal electric stimulus presupposes the development of a nerve impulse of low intensity. Within certain limits a maximal contraction following a maximal electric stimulus presupposes the development of a nerve impulse of high intensity. Intermediate contractions indicate nerve impulses of corresponding intensity. Tetanization of a muscle indicates that the nerve impulses arrive at the muscle with a frequency so great that the muscle does not succeed in relaxing from the effect of one stimulus before the next arrives. Complete as well as incomplete tetanus may be developed by gradually increasing the frequency of the stimulus. The character of the contraction caused by indirect stimulation — i. e., through the nerve — does not differ in any essential respect from that due to direct stimulation. Fig. 52. — Nerve- muscle Prepaila.- TiON OE A Frog. F. Femur. S. Sciatic nerve. I. T e n d o Achillis. — (Landois and Stirling.') GENERAL PHYSIOLOGY OF NERVE-TISSUE. 113 ELECTRIC PHENOMENA OF NERVES. Electric Currents from Injured Nerves. — It was discovered by du Bois-Reymond that electric currents can be obtained from nerves as well as from muscles, and that the electric properties of the former correspond in most respects to those of the latter. The laws governing the development and mode of action of the currents derived from muscles are equally applicable to the currents derived from nerves. A nerve-cylinder obtained by making two transverse sections of any given nerve presents, as in the case of muscles, a natural and two artificial transverse surfaces. A line drawn around the cylinder at a point lying midway between the two end sur- faces constitutes the equator. From such a cylinder strong currents are ob- tained when the natural longitudinal surface and the transverse surface are connected with the electrodes of the galvanometer circuit. The strength of the current thus obtained will diminish or increase according as the electrode on the longitudinal surface is removed from or brought near to the equator. If two symmetric points on the longitudinal sur- face equidistant from the equator are united, no current is obtainable. When asymmetric points on the longitudinal surface are connected, weak currents are obtained, in which case the point lying nearer the equator becomes positive to the point more distant, which becomes negative. From these facts it is evident that all points on the longitudinal surface are electrically positive to the transverse surface and that the point of greatest positive tension is situated near the equator (Fig. 53). The electromotive force of the nerve current varies in strength with the length and thickness of the nerve. The strongest current obtained from the nerve of the frog is equal to the 0.002 of a Daniell cell; that obtained from the nerve of the rabbit, 0.026 of a Daniell. The existence of the nerve current, its strength, duration, etc., depend largely on the maintenance of physiologic conditions. All influences which impair the nutrition of the nerve diminish the current. With the death of the nerve all electric phenomena disappear. Negative Variation of the Nerve Current. — During the pas- FiG. 53. — Diagram to Illus- trate THE Currents in Nerves. The arrowheads indicate the direc- tion; the thickness of the lines in- dicates the strength of the currents. — {Landois and Stirling.') 114 TEXT-BOOK OF PHYSIOLOGY. sage of the nerve impulse the resting nerve current, or the demarca- tion current, diminishes more or less completely in intensity, undergoes a negative variation, as shown by the return of the galvanometer needle, due to a change in its electromotive condition or to a diminu- tion of the difference in potential between the positive longitudinal and negative transverse sections. This negative variation of the de- marcation current is observed equally well from either the central or peripheral end of the nerve. If the two ends of the nerve are con- nected with galvanometers and the nerve stimulated in the middle, the demarcation currents simultaneously undergo a negative variation. This may be taken as a proof that the excitation process propagates itself equally well in both directions. The negative variation is inti- mately connected with changes in the molecular condition of the nerve and is not due to any extraneous electric or other influence. And du Bois-Reymond was also enabled to obtain a negative variation of the current in the nerves of a living frog which were yet in connection with the spinal cord. In this experiment the sciatic nerve was divided at the knee and freed from its connections up to the spinal column; the transverse and longitudinal surfaces were then placed in connection with the electrodes of the galvanometer wires and the current per- mitted to influence the needle. The animal was then subjected to the action of strychnin. "Upon the appearance of the muscle spasms the needle was observed to swing backward toward the zero point to the extent of from i to 4 degrees, and upon the cessation of the spasms to return to its previous position. In an experiment of this nature it is obvious that the negative variation was the result of a physiologic stimulation of the nerve arising within the spinal cord. The question also here arises as to whether the negative variation is due to a steady, continuous decrease of the natural current, or whether it is due to successive and rapidly following variations in its intensity ,^ similar to that observed in muscles. Though this can not be demonstrated with the physiologic rheoscope, as was the case with the muscle, there can be no doubt, both from experimentation and analogy, that the latter supposition is the correct one. It has been shown that when non-polarizable electrodes connected with Siemen's telephone are placed in connection with the longitudinal and trans- verse sections of a nerve, low, sonorous vibrations are perceived during tetanic stimulation — a proof that the active state of the nerve is connected with the production of discontinuous electric currents. The oscillations of the mercurial column of the capillary electrometer also reveal similar electric changes. It was also demonstrated by Bernstein with a specially devised apparatus, the repeating rheotome, that the negative variation is composed of a large number of single variations which succeed each other in rapid succession and sum- marize themselves in their effect on the needle. Electric Currents from Uninjured Nerves. — The pre-existence of electric currents in living and wholly uninjured nerves while at GENERAL PHYSIOLOGY OF NERVE-TISSUE. 115 rest has also been denied by Hermann, who regards all portions of the nerve as isoelectric, any difference of potential being the result of some injury to its surface. Action Currents. — ^For reasons to be stated below, it is very diffi- cult to determine the presence of diphasic action currents during the passage of an excitatory impulse through the nerve-fiber. The so- called negative variation of the resting nerve current — the demarca- tion current — ^which is occasioned by tetanic stimulation, Hermann regards as the expression of an action current which flows in the nerve in a direction opposite to the demarcation current. The origin of this action current is to be sought for in the continuous negativity of that portion of the longitudinal surface of the nerve in contact with the diverting electrode, while the dying substance of the transverse surface takes no part in the excitation. This tetanic action current, or nega- tive variation, was discovered by du Bois-Reymond, and Bernstein later succeeded in obtaining this action current during the passage of a single excitation process. That the return of the galvanometer needle toward the zero point is not due to an annulment of the demarc- ation current itself, but to the appearance of an action current, is shown by the fact that if the former be compensated by a battery current until the needle rests on the zero point the appearance of the latter current will cause the needle to swing in a direction the opposite of that caused by the demarcation current. The negative variation and action current may therefore be regarded as one and the same thing. It is the expression of the change the nerve is undergoing during the passage of the nerve impulse. The rapidity with which the negative variation or action current travels, the variation in its intensity from moment to moment, the time required for it to pass a given point, would express the change in the nerve to which the term nerve impulse is given. From experiments made with the differential rheotome, Bernstein calculated that the speed of the negative variation is about 28 meters a second; that it is at first feeble, soon rises to a maximum, and then declines; that it requires 0.0006 to 0.0008 of a second to pass a given point. From these data it is evident that the negative variation or action current has a space value of 0.0006 of 28 meters or about 18 mm. Transferring these statements to the nerve impulse, it may be said that it is a molecular disturbance, traveling at the rate of about 28 meters a second, is wave-like in char- acter, the wave being 18 millimeters in length, and occupying from 0.0006 to 0.0008 of a second in passing any given point. Absence of Diphasic Action Currents. — When any two points on the longitudinal surface which do not exhibit a current are connected with the galvanometer and a single wave of excitation passes beneath the electrodes, it might be expected that, as in the case of the muscle, a diphasic action current would be observed, from the fact that the portions of the nerve beneath the electrodes become alternately neg- ative with reference to all the rest of the nerve. This, however, is ii6 TEXT-BOOK OF PHYSIOLOGY. not the case, the absence of the two opposing phases of _ the action current being explained on the supposition that the negativity of the two led-off points is of equal amount, and that, owing to the great rapidity with which the excitation wave travels, the two phases fall together too closely in time to alternately influence the galvanometer needle. During stimulation of the nerve, when two currentless or iso- electric points are connected, there is also an absence of the action current, as was observed first by du Bois-Reymond, and which is^ to be explained on similar grounds. It is true that an apparent action current is sometimes seen when the stimulating current is very power- ful or the seat of stimulation too near the diverting electrodes. This, however, must be attributed to an electrotonic state of the nerve. The Effects of a Galvanic Current on a Nerve.— When a con- stant galvanic current of medium strength is made to pass through a portion of a nerve, several distinct effects are produced: /^^ + 't POLAR I Z I N G I CURRENT ; ANELECTROTONIC CURRENTS KATELECTROTONIC CURRENTS Fig. 54. — Electrotonic Currents. 1. The development of a nerve impulse at the moment the current enters and at the. moment the current leaves the nerve, i. e., at the moment the circuit is made and at the moment it is broken. The development of the nerve impulse is made evident by the contraction of the muscle if the nerve-muscle preparation be used. If the current be either very weak, or very strong, the muscle contraction may not always take place. 2. The development of electric currents on each side of the positive pole or anode, and the negative pole or kathode (see Fig. 54), which can be led off by means of wires into a galvanometer circuit from either the artificial transverse and longitudinal surfaces, or from any two points on the longitudinal surface as shown by the deflection of the galvanometer needle. The direction of these electric cur- rents in the nerve coincides with that of the galvanic or "polarizing current." The "natural nerve currents," the currents of injury or demarcation currents, as they are variously termed, are at the same time increased and decreased at opposite extremities of the nerve according to the direction of the polarizing current. To this changed condition of the electromotive forces in a nerve GENERAL PHYSIOLOGY OF NERVE-TISSUE. 117 the term electrotonus was given (du Bois-Reymond). The currents themselves are known as electrotonic currents; from their relation to the anode and kathode, they are termed anelectrotonic and kat- electrotonic currents. The condition of the nerve around the poles both in the intra-polar and extra-polar regions is known as anelectro- tonus and katelectrotonus. The electrotonic currents vary considerably in strength and ex- tent, according to the intensity of the polarizing current, increasing steadily with the intensity of the latter up to the point at which the polarizing current begins to destroy the physical and chemic integrity •of the nerve. The electrotonic currents are strongest in the imme- diate neighborhood of the electrodes, but gradually diminish in strength as the distance between the polarized and led-off portions is increased. The distance to which the electrotonic currents extend along the nerve will depend very largely upon the strength of the polarizing current, though it is conditioned by the physical state of the nerve; for if it be ligated or injured beyond the polarized portion, the electrotonic cur- rents are abolished. The electrotonic currents have no necessary connection with the natural nerve currents, nor are they to be regarded as branchings of the galvanic current. They are in all probability of artificial origin, due to an inner positive and negative polarization of the nerve which extends for a variable distance on each side of the poles, and due to the action of the polarizing or the galvanic current. 3. An alteration in the excitability and conductivity of the nerve in the neighborhood of the poles, whereby the results of nerve stimu- lation — that is, muscle contraction, sensation, and inhibition — are increased or decreased according to the strength and direction of the current. To this condition the term electrotonus was also given (Pfliiger). This word has thus been employed to express two distinct series of effects exhibited by a nerve through a portion of which a con- stant galvanic current is passing. It appears desirable, for the sake of clearness, to limit the term electrotonus to the electric or electrotonic currents which can be led off from either extremity of the nerve, and to apply to the modifications of irritability which accompany electro- tonus the expression, electrotonic alteration of excitability and con- ductivity. During the passage of the current the excitability of the intra- polar as well as the extra-polar regions undergoes a change which, as shown on examination, is found to be diminished in the neigh- borhood of the anode or positive pole and increased in the neighbor- hood of the kathode or negative pole. These alterations in the excita- bility are most marked in the immediate vicinity of the electrodes, though they extend for some distance into both the ex:tra-polar and intra-polar regions, though with gradually diminishing intensity, until they finally disappear. Between the electrodes there is a point where the excitability is unchanged and known as the neutral or indifferent point (Fig. 55). The extent to which the excitability is ii8 TEXT-BOOK OF PHYSIOLOGY. modified as well as the position of the neutral point will depend largely on the strength of the polarizing or galvanic current. The electrotonic alterations of excitability and conductivity can be experimentally demonstrated on the muscle-nerve preparation in the following manner: I. With a descending current of medium strength. Previous to the N f " Fig. ss- — Scheme or the Electrotonic Excitability. — (^Landois and Stirling.) closure of the polarizing current, the nerve is stimulated first in the extra-polar anodic region and the extra-polar kathodic region with an induction shock of medium intensity and the height of the contraction recorded. On repeating the stimulation ajter closure of the polarizing current the contraction resulting SECONDARY COIL Fig. 56. — Diagram Showing the Region of Increased Excitability Caused BY THE Passage of a Galvanic Current, Stimulation of which Gives Rise to In- creased Contraction. from stimulation of the anodic region will be enfeebled or may be entirely wanting, while the contraction from stimulation of the kathodic region will be decidedly increased. (See Fig. 56.) "With an ascending current of the same strength. After prelim- inary testing of the excitability and the subsequent closure of GENERAL PHYSIOLOGY OF NERVE-TISSUE. iig the polarizing current, it will be found that stimulation of the extra-polar anodic region will provoke a much .less energetic contraction or perhaps none at all. Stimulation of the extra- kathodic region, though of increased excitability, as shown by the previous experiment; may also fail to provoke a contraction, owing to the diminished conductivity of the region in the neighbor- hood of the anode. The impulse on reaching this region is blocked in its passage. A similar if not more; marked decrease in the conductivity may be developed in the region of the kathode if the current strength be very great. (See Fig. 57.) The Law of Contraction; Polar Stimulation-. — It was stated in a previous paragraph that when a galvanic current of medium strength is made to enter a nerve, and when it is withdrawn from the nerve, there is a contraction of its related muscle. These are generally REGION OF DECREASED EXCITABILITY •A Fig. 57. — Diagram Showing the Region of Decreased Excitability Caused by THE Passage of a Galvanic Current, Stimulation of which Gives Rise to De- creased Contraction. known as the make and break effects. During the actual passage of the current no effect is observed so long as its strength remains uniform. Any sudden variation in the strength of the current at once arouses the nerve to activity, as shown by a muscle contraction. The muscle response to the make and break of the constant current is more or less variable unless the direction of the current as well as its strength be taken into consideration. If the current is made to flow from the central toward the peripheral end of the nerve it is termed a direct, descending, or centrifugal current; if it is made to flow in the reverse direction, it is termed an indirect, ascending, or centripetal current. The strength of the current is determined and regulated by means of a rheocord. The make and break of currents of different but known strengths and directions give rise to contractions which occur with more or less regularity. The order in which they occur under these varying 120 TEXT-BOOK OF PHYSIOLOGY. conditions of experimentation has been determined and tabulated as follows by Pfliiger, and is termed the law of contraction: Ascending Current. Descending Current. Current Intensity. Make. Break. Make. Break. Weak, Crurraction. CcM.action. Res' Rest. Contraction. Contraction. Contraction. Contraction. Contraction. Rest. Contraction. Strong, Rest or weak contraction. The results as above tabulated are sometimes complicated on the opening of the circuit by a series of irregular pulsations of the muscle, an apparent .tetanus, and long known as the opening tetanus of Ritter, which is attributed to rapid changes in the irritability of the nerve, in the region of the anode. A similar tetanic contraction of the muscle is sometimes observed on the closure of the circuit due to continued excitation in the region of the kathode. This is known as the closing tetanus of Wundt or of Pfliiger. All the phenomena of the law of contraction were explained by Pfliiger on the assumption that the current stimulates the nerve only at the one electrode, at the kathode on closing, and at the anode on opening; or, in other words, by the appearance of katelectrotonus or by the disappearance of anelectro- tonus, both conditions being attended by a rise of excitability— not, however, by the opposite changes. It is further assumed that the appearance of katelectrotonus is more effective as a stimulus than the disappearance of anelectrotonus. For these reasons the term polar stimulation is generally employed in discussing the make and break effects of the galvanic current. The law of contraction may then be explained as follows: Very feeble currents, either ascending or de- scending, produce contraction only upon the closure of the circuit, the sudden increase of the excitability in the katelectrotonic area being alone sufficient to generate . an impulse. The contraction which follows the closing of the weak ascending current depends upon the fact that the decrease of excitability and conductivity at the anode is insufficient to interfere with the conduction of the kathodal stimulus. Medium currents, either ascending or descending, produce contrac- tion both on closing and opening the circuit. The appearance of katelectrotonus and the disappearance of anelectrotonus are both sufficiently powerful to generate an impulse without, however, seri- ously impa:iring the conductivity of the nerve. ,Very strong currents produce contraction only upon the opening of the ascending and. closure qf the descending currents, or upon the passage of the excitability in the former from the marked anelectro- tonic decrease to the normal conditim,, and in the latter from the normal to that of katelectrotonic increase. The absence of contraction upon. the closure, of the ascending current is dependent . upon the With Descending Current. K. C. C. A. O. Ct K. C. C. A. O. C. A. O. C. K. C. C. A. O. C.(?) GENERAL PHYSIOLOGY OF NERVE-TISSUE. 121 blocking of the kathodal stimulus by the decrease of the excitability and conductivity at the anode. With the opening of the descending current the disappearance of anelectrotonus should also be followed by contraction, which would indeed be the case if the stimulus so generated was not blocked by the decrease of the conductivity at the kathode in consequence of the fall of a high state of katelectrotonus to the normal condition. The order in which the contractions occur may be tabulated as follows : With Ascending Current. Weak, I. K. C. C* Medium, i,. K. C. C. Strong, 3. Polar Stimulation of Human Nerves. — The preceding state- ments as to changes in the excitability caused by the passage of a constant current, as well as to the law of contraction, are based en- tirely on experiments made with the isolated nerve of the frog. It is probable, however, that the same phenomena would have been observed had the nerve of a mammal been used and its excitability been maintained. If the electrodes connected with the wires of a sufficiently strong galvanic battery be applied to the skin over the course of a superficially lying nerve, e. g., the brachial, it will be found that there occurs on the closure of the circuit an increase in the excitability in the extra- polar anelectrotonic region and a decrease in the excitability in the extra-polar katelectrotonic region, as shown by stimiilating the nerve in the extra-polar regions with the induced current — results which are in apparent contradiction to those obtained with the isolated nerve. This want of accordance in the results of the two classes of experi- ments arises from a failure to recognize the fact that the physiologic anode and kathode do not coincide with the physical anode and kathode. It has been experimentally demonstrated that owing to the large amount of readily conducting tissue by which the nerve is surrounded, the current density, though great immediately under the electrode, quickly decreases at a short distance from it, so that for the nerve it becomes almost nil. The current, therefore, shortly after eritering, again leaves the nerve at various points which become physiologic kathodes. Stimulation of this physiologic kathode with the induced current gives rise, therefore, to the phenomenon of increased excita- bility in the region of the anode. If, however, the galvanic and stimulating current be combined in one circuit and both be applied to the same tract of nerve, results will be obtained which harmonize with those obtained with the frog's nerve. The changes in the excitability of a nerve of a living man and the *K. C. C, kathodal closing contraction. f A. O. C, anodal opening contraction. 122 TEXT-BOOK OF PHYSIOLOGY. contractions which follow the closing and opening of the constant current have been thoroughly studied by Waller and de Watteyille. These observers employed a method similar to that of Erb, conjoin- ing in one circuit the testing and polarizing currents. By the graphic method they recorded first the contraction produced by an induc- tion shock alone; and, secondly, the contraction produced by the same stimulus under the influence of the polarizing current. As a result of many experiments, they also demonstrated an increase of the excitability in the polar region when it is cathodic, and a decrease when it is anodic. Following the suggestion of Helmholtz, that the current density quickly decreases with the distance from the elec- trodes, they recognize, at the point of entrance and exit of the current from the nerve, two regions — a polar, having the same sign as the electrode, and a peripolar, having the opposite sign (Figs. 58 and 59). Fig. 58. — Anode of Battery. Polar region of nerve is anodic. Peri- polar region of nerve is cathodic. Fig. 59. — Cathode of Battery. Polar region of nerve is cathodic. Peri- polar region of nerve is anodic. — (Waller.) The peripolar regions also experience similar alterations of excita- bility, though less in degree, according as they are kathodic or anodic. As it is impossible to confine the current to the trunk of the nerve when surrounded by living tissues, as is easily the case when experi- menting with the frog's nerves, it is incorrect to speak of either as- cending or descending currents. Waller,* who has thoroughly studied the electrotonic effects of the galvanic current from this point of view, sums up his conclusions in the following words: "We must apply one electrode only to the nerve and attend to its effects alone, completing the circuit through a second electrode, which is applied according to convenience to some other part of the body. "Confining our attention to the first electrode, let us see what will happen according as it is anode or kathode of a galvanic current (Figs. 58 and 59). If this electrode be the anode of a current, the latter enters the nerve by a series of points and leaves it by a second series of points; the former, or proximal series of points, collectively constitutes the polar zone or region; the latter, or distal series of points, collectively constitutes the peripolar zone or region. In such case the polar region is the seat of entrance of current into the nerve — *" Human Physiology," p. 363, 1891. GENERAL PHYSIOLOGY OF NERVE-TISSUE. 123 i. e., is anodic; the peripolar region is the seat of exit of current from the nerve — i. e., is kathodic. If, on the contrary, the electrode under observation be the kathode of a current, the latter enters the nerve by a series of points which collectively constitute a 'peripolar' region, and it leaves the nerve by a series of points which collectively con- stitute a 'polar' region. The current, at its entrance into the body, diffuses widely, and at its exit it concentrates; its 'density' is greatest close to the electrode, and, the greater the distance of any point from the electrode, the less the current density at that point; hence it is obvious that the current density is greater in the polar than in the peripolar region. These conditions having been recognized, we may apply to them the principles learned by study of frogs' nerves under simpler conditions. " Seeing that, with either pole of the battery, whether anode or kath- ode, the nerve has in each case points of entrance (constituting a collective anode) and points of exit to the current (constituting a col- lective kathode), and admitting as proved that make excitation is kathodic, break excitation anodic, we may, with a sufficiently strong current, expect to obtain a contraction at make and at break with either anode or kathode applied to the nerve; and we do so, in fact. When the kathode is applied, and the current is made and broken, we obtain a kathodic make contraction and a kathodic break contraction; when the anode is applied, and the current is made and broken, we obtain an anodic make contraction and an anodic break contraction. These four contractions are, however, of very different strengths; the kathodic make contraction is by far the strongest; the kathodic break contraction is by far the weakest; the kathodic make contraction is stronger than the anodic make contraction; the anodic break con- traction is stronger than the kathodic break contraction. Or, other- wise regarded, if, instead of comparing the contractions obtained with a sufficiently strong current, we observe the order of their appearance with currents gradually increased from weak to strong, we shall find that the kathodic make contraction appears first, that the kathodic break contraction appears last, and the formula of contraction for man reads as follows: "Weak current, . . . K. C. C. Medium current, . . K. C. C. A. C. C. A. O. C. Strong current, . . . K. C. C. A. C. C. A. O. C. K. O. C." The constant or the galvanic current is frequently used for thera- peutic and diagnostic purposes. In accordance with the statements above quoted, one electrode should be applied to the part to be in- vestigated, the other to some indifferent region. The electrode con- veying the current to or from this part should be of a size sufficient to localize the current and to increase its density. It was discovered by Duchenne that there are certain points all over the body stimula- tion of which is more quickly followed by muscle contraction than others. It was subsequently discovered by Remak that these points 124 TEXT-BOOK OF PHYSIOLOGY. coincide with the entrance of the nerve into the muscle. It is to these motor points that the one electrode should be applied.^ The position of some of these points on the forearm is shown in Fig. 60. Reactions of Degeneration.— In consequence of the degen- eration and changes in irritability which occur in nerves when separ- ated from their centers and in muscles when separated from their related nerves, either experimentally or as the result of disease, the response of these structures to the induced, and the make and break of the constant current, differs from that observed in the physiologic condition. The facts observed under the application of these two ' M. biceps brachii. M. brach. anticus. N. medianus. M. pronator teres. M. flex, digitor. commun. profund. M. flex, carpi radialis. M. flex, digitor. sublim. M. flex. dig. subl. (dig. ind. et min.) \M. flex. poll, longus. N. ulnaris. M. flexor carpi ulnaris. N. ulnaris. Fig. 60. — Motor Points op the Median and Ulnar Nerves, with the Muscles Supplied by Them. — {Landois and SHrlmg.) forms of electricity are of importance in the diagnosis and thera- peutics, of the precedent lesions. The principal difference of behavior is observed in the muscles, which exhibit diminished or abolished excitability to the induced current, while at the same time manifesting an increased excitability to the constant current; so much so is this the case that a closing contraction is just as likely to occur at the positive as at the negative pole. This peculiarity of the muscle response is termed the reaction oj degeneration. The synchronous diminished excitability of the nerves is the same for either current. The term "partial reaction of degeneration" is used when there is a normal reaction of the nerves, with the degenerative reaction of the muscles. This condition is observed in progressive muscular atrophy. Reflex Action. — Inasmuch as many of the muscle movements of the body, as well as the formation and discharge of secretions from glands, variations in the caliber of blood-vessels, inhibition and ac- GENERAL PHYSIOLOGY OF NERVE-TISSUE. "5 celeration in the activity of various organs, are the resuh of stimu- lations of the terminal organs of afferent nerves, they are termed, for convenience, reflex actions, and, as they take place for the most part through the spinal cord and medulla oblongata and independently of the brain or of volitional influences, they are also termed involun- tary actions. A reflex action of skeletal muscles, glands, or non- striated muscles of blood-vessels or of viscera, therefore, may be defined as an action which takes place independent of volition and in response to peripheral stimulation. As many of the processes to be described in spc Fig. 6i. — Diagram Showing the Structures Involved in the Production OP Reflex Actions. (G. Bachman.) r.s. Receptive surface; af.n. afferent nerve; e.c. emissive or motor cells in the anterior horn of the gray matter of the spinal cord, sp.c; ej. n. efferent nerves distributed to responsive organs, e.g., directly to skeletal muscles, sk.n., and indirectly through the intermediation of sympathetic ganglia, syjw.^., to blood-vessels, b.v., and to glands, g. The nerves distributed to viscera are not represented. succeeding chapters are of this character, requiring for their per- formance the cooperation of several organs and tissues associated through the intermediation of the nerve system, it seems advisable to consider briefly, in this connection, the parts involved in a reflex action, as well as their mode of action. As shown in Fig. 6i, the necessary structures are as follows : 1. A receptive surface, skin, mucous membrane, sense-organ, etc. 2. An afferent nerve-fiber and cell. 3. An emissive cell, from which arises — 4. An efferent nerve, distributed to a responsive organ, as 5. Skeletal muscle, gland, blood-vessel, etc. Such a combination of structures constitutes a reflex mechanism or arc, the nerve portion of which, in the case of skeletal muscles, is composed of but two neurons — an afferent and an efferent. In the 126 TEXT-BOOK OF PHYSIOLOGY. case of glands and non-striated muscles, whether of blood-vessels or viscera, the efferent neuron instead of passing direct to the responsive organ, arborizes around the nerve-cells of a peripheral sympathetic ganglion. The reflex arc is then continued by the propesses of the ganglion cells. An arc of this simplicity would of necessity sub- serve but a simple movement. The majority of reflex activities, however, are extremely complex, and involve the cooperation and coordination of a number of nerve centers situated at dififerent levels of the spinal cord on the same and opposite side, and of re- sponsive organs frequently situated at distances more or less remote from one another. This implies that a number of neurons are as- sociated in function. The transference of nerve impulses coming from a localized area of a sentient surface to emissive cells situated at different levels is accom- plished by the intercalation of a third neuron situated in the gray matter which is in connection, on the one hand, with the central terminals of the afferent neuron, and, on the other hand, through its collateral branches with the dendrites of the efferent neurons situated at differ- ent levels of the cord. (Fig. 62.) For the excitation of a reflex action it is essential that the stimulus applied to the sentient surface be of an intensity sufflcient to develop in the terminals of the afferent nerve a series of nerve im- pulses, which, traveling inward, will be distributed to and received by the den- drites of the emissive or motor cell. With the reception of these impulses there is apparently a disturbance of the equilib- rium of its molecules, a liberation of. energy, and, in consequence, a transmis- sion outward of impulses through the efferent nerve to muscle, gland, or blood-vessel, separately or collectively, with the production of muscle contraction, a secretion, vascular dilatation or contraction, etc. The reflex actions take place, for the most part, through the spinal cord and medulla oblongata, which, by virtue of their contained centers, coordinate the various organs and tissues concerned in the performance of the organic functions. The movements of mastication; the secretion of saliva; the muscle, gland, and vascular phenomena of gastric and intestinal digestion; the vascular and respiratory movements; the mechanism of micturition, etc., are illustrations of reflex activity. Fig. 62. — Diagram Showing the] Relation of the Third Neuron a, to the Aiteeent Netjeon b, and to the Efferent Neurons c, c, c. — {After KoUiker.) CHAPTER IX. FOODS. The functional activity of every organ and tissue of the body is accompanied by a more or less active disintegration of the living material, the bioplasm, of which it is composed, as well as of the food materials circulating in its interstices. The complex molecules of the living material and of the non-living food materials are continually undergoing disruption and falling into less complex and more stable compounds; these, through oxidative processes, are eventually re- duced through a series of descending chemic stages to a small number of simpler compounds which, being of no further value to the organ- ism, are eliminated by the various eliminating or excretory organs, the lungs, skin, kidney, and liver. Among these excreted compounds derived from tissue and from food metabolism the more important are urea, uric acid, and carbon dioxid. Many other compounds, organic as well as inorganic, are also eliminated from the body in the various excretions, though they are present in but small amounts. Coincident with this metabolic process there is a transformation of potential into kinetic energy, which manifests itself for the most part as heat and mechanic motion. In order that the organs and tissues may continue in the per- formance of their functions, it is essential that they be supplied with nutritive materials similar to those which enter into their own com- position: viz., proteids, fat, carbohydrates, water, and inorganic salts. These compounds, though originally derived from the food, are immediately derived from the blood as it flows through the capil- lary blood-vessels. The blood is therefore to be regarded as a reser- voir of nutritive material in a condition to be absorbed and trans- formed into utilizable and living material. Inasmuch as the materials lost to the body daily, through disintegration and oxidation, though considerable, are supplied by the blood, it is evident that this fluid would diminish rapidly in volume, with a corresponding decline in functional activity, were it not restored by the introduction into the body of new material in the food. With the diminution of the volume of the blood and an insufhcient supply to the tissues, there arise the sensations of hunger and thirst, which lead to the consump- tion of food and the subsequent restoration of the physiologic condi- tion of the tissues. These two sensations are also partially dependent on the empty condition of the stomach and the dryness of the mucous membrane of the mouth and throat. 127 128 TEXT-BOOK OF PHYSIOLOGY. The foods which are consumed daily in response to the sensations of hunger and thirst are complex in composition and contain, though in varying amounts, proteins, fats, carbohydrates, water and inor- ganic salts, which, in contradistinction to foods, are termed food principles or nutritive principles. In these compounds is also to be found the potential energy necessary to maintain the dynamic equi- librium of the body and which will become manifest as heat and mechanic motion in the transformations of the material underlying the nutritive processes. The animal body may be therefore regarded as a machine capable each day of performing a certain amount of work by the expendi- ture of a definite amount of energy. In the performance of its work, whether it be the raising of weights against gravity, the overcoming of friction, cohesion, or elasticity, the machine suffers disintegration and metabolizes a portion of the food materials and loses a portion of its available energy. Unlike other machines, however, it possesses the power, within limits, of self-renewal, self-adjustment, when supplied with foods in proper quantity and quality. QUANTITIES OF FOOD PRINCIPLES REQUIRED DAILY. In order that the body may continue in the performance of its work and yet retain a given weight, it is essential that the loss to the body daily shall be exactly compensated by the introduction and assimilation of a corresponding amount of food principles. If this condition is realized, the body neither gains nor loses, but remains in a condition of nutritive equilibrium. The determination of the exact quantities of the different food principles required daily and their ratio one to another is made from an examination of the quantity and composition of the daily excretions. Since the proteins disintegrated are represented in the excretions by urea and similar nitrogen-holding compounds and the fats and carbohydrates by carbon dioxid, it becomes possible to determine from them the quantities required to restore equilibrium under any given condition. But as the activity of the nutritive changes will vary in accordance with climatic condi- tions, work done, etc., and as the excreted products will vary in the same ratio, it is obvious that the required amounts of food will vary in accordance with these varying conditions, if equilibrium is to be maintained. Various estimates have been made by different investigators as to the amounts of the excreted products and the food principles re- quired daily, which, though differing to some extent, have, neverthe- less, an average nutritive and energy-producing value. The follow- ing table shows the diet scale of Vierordt and the excretions to which it would give rise. As the income and outgo practically balance, there would be no change in the weight. FOODS. 129 COMPARISON OF THE INCOME AND OUTGO. Income. Protein, Fat Carbohydrates, Salts, Water Oxygen, Grams. Ounces. 120 4.25 90 33° 3-17 11.64 32 2818 756 I-I3 99-30 26.66 4146 146.13 Outgo. Water, Urea, ...._. Feces, dry, Salts, Carbon dioxid, Water formed in body Grams. 2818 40 38 32 922 296 4146 Ounces. 99-3° 1.40 1. 60 I-I3 32-37 10-33 146-15 Other estimates as to the amounts of the organic food principles required daily are as follows: Ranke. Voit. Moleschott. Atwater. Hultgren. Grams. Grams. Grams. Grams. Grams. Protein 100 118 130 125 134 Fat, 100 56 84 125 79 Starch, 250 500 550 400 522 In arranging tables showing the relation between the income and the outgo, it is generally customary to state merely the amounts by weight of the nitrogen and carbon each contains. This method furnishes suflSciently accurate information regarding the metabolism of the body, for the reason that the nitrogen represents the protein, and the carbon, with the exception of that contained in the protein, the fat and carbohydrates which have undergone disintegration or metabolism. The following balance table, as given by Ranke, shows the rela- tion of the nitrogen to the carbon in the average mixed diet and in the excretions of a man weighing 70 kilograms, in a condition of nutritive equilibrium : Income. Grams. N. c. 100 100 230 iS-5 53-° 79-0 93-0 Fat,... Carbohydrates iS-5 225.0 Outgo. Grams. N. C. Urea, 3i-S\ o-SJ 14.4 I.I 6.16 10.84 CO2, : ^■5 225.00 130 TEXT-BOOK OF PHYSIOLOGY. From the above it will be observed that the daily discharge for each kilogram of body-weight is 0.21 gram nitrogen and 3.03 grams of carbon; the relation of the two being ^--14.$. On a diet in which there is an excess of either proteid or carbohydrates this ratio necessarily changes. CLASSIFICATION OF FOOD PRINCIPLES. Though the food principles are grouped as proteids, fats, carbo- hydrates, etc., the members of each group differ somewhat in chemic composition, digestibility, and nutritive value. These groups are as follows : I. Proteins. Principle. Where found. Myosin, Flesh of animals. Albumin, vitellin, White of egg, yolk of egg. Caseinogen, Milk. Serum-albumin, fibrin, Blood contained in meat. Glutin, Grain of wheat and some other cereals. Vegetable albumin, Soft-growing vegetables. Legumin, Peas, beans, lentils, etc. 2. Fats. Animal fats, In adipose tissue of animals. Vegetable oils, In seeds, grains, nuts, fruits, and other I vegetable tissues. 3. Carbohydrates. Dextrose or grape-sugar, 1 j^ ^^^. Levulose or fruit-sugar, J Lactose or milk-sugar, Milk. Saccharose or cane-sugar, Sugar-cane, beet roots. Maltose, Malt and malted foods. Starch \ ^^^^^^' tuberous roots, and legumin- ' J ous plants. Glycogen, Liver, muscles. < 4. Inorganic. Water, Sodium and potassium chlorid, Sodium, potassium, and calcium phosphates and carbonates, Iron, In nearly all animal and vegetable foods. 5. Vegetable Acids. Citric, tartaric, acetic, malic, In fruit and vegetables. i 6. Accessory Foods. Coffee, Tea, Cocoa, Alcohol. Disposition of Food. — The protein principles of the food, after undergoing all those changes which are embraced under the term di- FOODS. 131 gestion, are absorbed from the intestine. During the act of absorption they are transformed into the forms of protein characteristic of blood. After being distributed by the blood-stream to the tissues, they are brought into relation with the living cells. The disposition made of the protein material by the bioplasm of the cell has not been definitely determined. According to Voit, of the protein thus brought into con- tact with the living tissues, only a small percentage is utilized and assim- ilated for tissue repair. This he terms tissue or organ protein. The remaining large percentage circulating in the interstices of the tissues, though not forming an integral part of them, is acted on directly by them, merely in virtue of contact — split up, oxidized, and reduced to simpler comp9unds. This he terms circulating protein. According to Pfliiger and others, this view is not tenable. Pfliiger asserts that, as material changes or metabolism can only take place within living cells, all the protein must first be assimilated and organ- ized by the cells before it can undergo metabolic changes. Metab- olism by contact action is denied, and the division of proteins into organ and circulating protein is not justifiable. In the process of metabolism the protein suffers disintegration, giv- ing rise through oxidation to some carbon-holding compound, possibly fat, and to some nitrdgen-holding compounds, which eventually give rise to urea. The intermediate stages, however, are not definitely known; the immediate antecedents of urea are probably carbamate and carbonate of ammonia. The disintegration of the protein's is attended by the disengagement of heat, thus contributing to the general store of the energy of the body. The fai principles, after digestion, are absorbed by the lymphatic vessels and discharged by the thoracic duct into the blood, from which they rapidly disappear. Though it is possible that a portion of the fat enters directly into the formation of the living material, it is gener- ally believed that it is at once oxidized and reduced to carbon dioxid and water with the liberation of energy. The natural supposition that a portion of the ingested fat was directly stored up in the cells of the areolar connective tissue, thus giving rise to adipose tissue, has been a subject of much controversy, though modern experimentation renders this very probable. The body-fat, under physiologic con- ditions, is also a product of the metabolic activity of connective-tissue ■cells and is a derivative of both proteids and carbohydrates. The carbohydrate principles, after digestion, are absorbed into the blood as dextrose. This compound is then stored up in the liver and muscles as glycogen. The intermediate stages which glycogen passes through before it is reduced to carbon dioxid and water are only imperfectly known. Though a large part of the carbohydrate material is at once oxidized, it is now well established that another portion contributes to the formation of, if it is not directly converted into, fat. As the carbohydrates form a large portion of the food, they •contribute materially to the production of energy. 132 TEXT-BOOK OF PHYSIOLOGY. The inorganic principles, though not playing apparently as active a part in the metabolism of the body as the organic, are nevertheless essential to its physiologic activity. Water is promptly absorbed after ingestion and becomes a part of the circulating fluids— blood and lymph. In the digestive appa- ratus it favors the occurrence of those chemic changes in the food necessary for their absorption, it promotes absorption of the food, holds various constituents of the blood and other fluids in solution, hastens the general metabolism of the body, holds in solution various products of metabolic activity, and, leaving the body through the excretory organs, promotes their elimination. Sodium chlorid is absorbed into the blood and, unless taken in excess, is utilized in replacing that which is lost to the organism daily. The exact rdle which sodium chlorid plays in the nutritive process is unknown; but, as it is present as a necessary constituent in all the fluids and solids of the body, and as it is instinctively employed as a condiment, it may be assumed to have a more or less important function. When taken as a condiment, it imparts sapidity to the food and excites the flow of the digestive fluids; it ultimately furnishes the chlorin for the hydrochloric acid of the gastric juice. Judging from the impairment of the nutrition as observed in animals after depriva- tion of salt for a long period of time, if favorably influences the growth and functional activity of all tissues. It is well known that herbivorous animals, races of men as well as individuals who live largely on vegetable foods, require a larger addi- tional amount of sodium chlorid than carnivorous animals, or human beings who live largely on animal foods, even though the two classes of foods contain relatively the same amounts. The explanation is that the vegetable foods contain potassium salts which, meeting in the blood with sodium chlorid, undergo decomposition into potassium chlorid and sodium carbonate or phosphate, all of which, when in excess, are at once eliminated by the kidneys. The blood, therefore, becomes poorer in sodium chlorid, one of its necessary constituents. Potassium phosphate and carbonate are also essential to the nor- mal composition of the solids and fluids. They impart a certain degree of alkalinity to the blood and lymph, one of the conditions necessary to the life and activity of the tissue-cells bathed by them. When administered in small doses, they increase the force of the heart, raise the arterial pressure, and increase the activity of the circulation. Calcium phosphate and carbonate are partly utilized in maintain- ing the solidity of the bones and teeth, replacing the amount metab- olized daily. Inasmuch as the metabolism of these two tissues is slight, there is not much need in the adult for lime as an article of food. In young animals lime is essential to the solidification and development of bone. When deprived of it, the skeleton undergoes FOODS. 133 a defective development similar to the pathologic condition known as rickets. Lime is present in milk to the extent of 0.15 per cent., as well as in eggs and peas in relatively large quantities. Iron is contained in both animal and vegetable foods, not, how- ever, in the form of inorganic iron, nor in the form of an organic salt, but as a compound with nuclein, thus forming an integral part of the proteid molecule. After absorption the iron is utilized in the forma- tion of the coloring-matter of the blood-corpuscles — hemoglobin. The organic compounds of iron and the nucleins have been termed hematogens. The amount of iron ingested has been estimated at from 10 to 90 milligrams, the larger part of which is eliminated in the feces. The relatively small part eliminated by the kidneys and liver is usually taken as the amount metabolized, though it is probable that this is not wholly true, as there is evidence that iron can be re- tained in the body and utilized again in the formation of new hemo- globin. Contrary to what might be expected, milk contains but a very small quantity of iron, not more than 3 or 4 milligrams in 1000 grams (human milk) — an amount insufficient for the development of the necessary hemoglobin. This is compensated for, however, by the accumulation of iron in the liver during intrauterine life. Ac- cording to Bunge, the liver of a newly born rabbit contains as much as 18.2 milligrams per 100 grams of body-weight, while at the end of twenty-four days it only contains 3.2 milligrams per 100 grams of body-weight. Vegetable acids increase the secretions of the alimentary canal, and are apt, in large amounts, to produce flatulence and diarrhea. After entering into combination with bases to form salts, they stimulate the action of the kidneys and promote a greater elimination of all the urinary constituents. In some unknown way they influence nutrition; when deprived of these acids, the individual becomes scorbutic. The accessory foods — coffee, tea, and cocoa — when taken in moderation have a stimulating influence on the nervous system, as shown by the removal of both mental and physical fatigue, by an increased capacity for sustained mental work, by the persistent wake- fulness among those unaccustomed to their use. Coffee more especially increases the frequency and force of the heart-beat, raises the arterial pressure, and hastens the general blood-flow. It has no influence either in the way of increasing or decreasing proteid metabolism. Tea frequently acts as an astringent on the alimentary canal on account of the tannin which passes into the water when the infusion is made. Inasmuch as tannin also coagulates peptones, the excessive use of tea as a beverage is apt to derange the digestive organs and the general process of digestion. Cocoa is more nutritive than either coffee or tcci, on account of the large amount of fat and proteid it contains. It is, however, less stimulating. The active principles in coffee, tea, and cocoa, and to which their 134 ' TEXT-BOOK OF PHYSIOLOGY. effects are to be attributed, are caffein, thein, and theobromin respec- tively. These alkaloids are chemically closely related one to the other and to the compound xanthin. They are present in the coffee seeds, the tea leaves, and the cocoa bean to the extent of 1.7 per cent., 1.4 per cent., and 1.6 per cent, respectively. When prepared as a beverage, however, there is three times as much caffein in coffee as thein in tea. Alcohol when taken in small quantities stimulates the digestive glands to increased activity and thus promotes digestive power. Its absorption into the blood is followed by increased action of the heart, dilatation of the cutaneous blood-vessels, a sensation of warmth, and an excitation of the brain. In large quantities it acts as a paraly- zant, depressing more especially the vaso-constrictor nerve-centers and certain areas of the brain, as shown by an impairment in the power of sustained attention, clearness of judgment, and muscle coordination. Alcohol is undoubtedly oxidized in the body, as only about 2 per cent, can be obtained from the urine and expired air. It thus contributes to the store of the body-energy. As to whether for this reason it can be regarded as a food — that is, whether it can be sub- stituted in part at least for fat or carbohydrate material without im- pairing the proteid metabolism — is at present a subject of experimen- tation and discussion. According to some investigators, alcohol does not retard proteid metabolism, for when it is introduced into the body in amounts equivalent to the carbohydrates withdrawn from the food there is at once a rise in the amount of nitrogen excreted. Hence it cannot be regarded as a food. According to other investigators, alcohol retards or protects proteid metabolism just as effectually as an equivalent amount of starch or sugar. Many more experiments are required to decide this question. When taken habitually in large quantities, alcohol deranges the activities of the digestive organs, lowers the body temperature, impairs muscle power, lessens the resistance to depressing external conditions, diminishes the capacity for sustained mental work, and leads to the development of structural changes in the connective tissues of the brain, spinal cord, and other organs. In zymotic diseases and in cases of depression of the vital powers it is most useful as a restorative agent. THE ENERGY OR HEAT VALUE OF FOOD PRINCIPLES. The food consumed not only restores the material metabolized and discharged from the body, but also the energy which has been expended as heat and mechanic motion. The food principles are products of the constructive processes taking place in the vegetable world during the period of growth and activity. At the time of their formation there is an absorption and storing of the sun's energy which then exists in a potential condition. During the metabolism of the animal body these compounds are reduced through oxidation to rela- FOODS. I3S tively simple bodies, such as carbon dioxid, water, urea, etc., with the liberation of their contained energy. All of the energy of the body, whatever its manifestations may be, can be traced to chemic changes going on in the tissues, and more particularly to those changes in- volved in the oxidation of the food principles. The amount of heat or energy which any given food principle will yield can be determined by burning a definite amount (e. g., i gram) to carbon dioxid and water and ascertaining the extent to which the heat thus liberated will raise the temperature of a given amount of water (e. g., i kilogram). The amount of heat may be expressed in gram or kilogram degrees or calories, a gram calorie or kilogram 'calorie being the amount of heat required to raise the temperature of a gram or a kilogram (looo grams) of water i° C. The apparatus employed for this purpose is termed a calorimeter, and consists es- sentially of a closed chamber in which the oxidation takes place, surrounded by a water jacket, the rise in temperature of the water indicating the amount of heat produced. The results obtained by investigators employing different calorim- eters and different food principles of the same group vary, though within certain limits: e. g., i gram of casein yields 5.867 kilogram calories; i gram of lean beef, 5.656 calories; i gram of fat yields 9.353, 9.423, 9.686 calories; i gram of carbohydrate, 4.182, 4.479, etc., calories. These numbers represent the physical heat values of these food principles. In the human body as determined by calorimetric methods the oxidation of the food principles yields practically the same amount of heat they yield when oxidized outside the body, with the excep- tion of the proteins, which are oxidized only to the stage of urea. As this compound is capable of further reduction in the calorimeter to carbon dioxid and water with the liberation of heat, the quantity of heat it contains must therefore be deducted from the calorimetric heat value of the protein. According to Rubner, i gram of urea will yield 2.523 kilogram calories. As the urea which results from the oxidation of i gram of protein is about -J of a gram, the amount of heat to be deducted from the heat value of the protein is ^ of 2.523, or 0.841 calories. It has also been shown that some of the ingested protein escapes in the feces, the heat value of which must also be determined and deducted. This having been done, the physiologic heat value becomes 4.124 calories. The following estimates give approximately the number of kilo- gram calories produced when the food is. burned to carbon dioxid, water, and urea in the body : I gram of protein yields, 4-124 calories. I " "fat " 9-3S3 " I " " carbohydrate yields, 4.116 " The total number of kilogram calories or kilogram degrees of heat yielded by any of the previously given diet scales can be readily deter- 136 TEXT-BOOK OF PHYSIOLOGY. mined by multiplying the quantities of food principles consumed by the above-mentioned factors. The diet scale of Vierordt, for example, yields the following: I20 grams of protein yields, • • 494-88 calories. 90 " "fat " 841.77 330 " " starch " 1358-28 " 2694.93 The total calories obtained from other diet scales would be as follows : Ranke, 2335; Voit, 3387; Moleschott, 2984; Atwater, 3331; Hultgren, 3436- Starvation.— The relation of the different food principles to the" general nutritive process'becomes more apparent from an examination of the excretions from the body during the process of starvation com- bined with an examination of the organs and tissues after death. If an animal be deprived entirely of food, a decline in body-weight at once sets in, which continues until about 40 per cent, of the weight has been lost, when death generally ensues. This results from the fact that the active tissue cells consume, for the purpose of maintain- ing the normal temperature of the body, not only their own reserve food material, but that of the less active or storage tissues as well; and, in consequence, there is a progressive diminution in weight. The phenomena which characterize this non-physiologic con- dition are as follows: hunger, intense thirst, gastric and intestinal uneasiness and pain, diminished pulse-rate, and respiration, muscular weakness and emaciation, a lessening in the amount of urine and its constituents, diminished exhalation of carbon dioxid, an exhalation of a fetid odor from the body, vertigo, stupor, delirium, at times con- vulsions, a sudden fall in body-temperature, and finally death. The duration of life after complete deprivation of food varies from eight to thirteen days or more, though this period can be prolonged if the ani- mal be supplied with water, this being more essential under the cir- cumstances than the organic materials which can be supplied by the organism itself. The duration of the starvation period will vary in accordance with the previous condition of the animal and the amount of reserved food the body contains. The excretion of urea declines very rapidly during the first two days — a fact which has been attrib- uted to a rapid consumation of the surplus proteid food. After this period, when the tissues begin to metabolize their own proteid, the excretion remains fairly constant until toward the close, when the amount eliminated falls very rapidly. As proteids contain about 16 per cent, of nitrogen, i part of nitrogen equals 6.25 parts of protein. Hence, for every i gram of nitrogen or 2.14 grams urea excreted, it may be assumed that 6.25 grams of protein or, according to Voit, 30 grams of flesh have been metabolized. The daily excretion of urea, therefore, indicates the extent of the protein metabolism. It has been observed also that there is a steady diminution in the FOODS. 137 cretion of carbon dioxid, though this is greatest in the last few days, i fat contains about 76 per cent, of carbon, 1 part of carbon equals 1.3 1 irts of fat. Hence, for every i gram of carbon or 3.66 grams carbon oxid excreted it may be assumed that 1.3 1 grams of fat have been etabolized. The daily excretion of carbon, therefore, indicates the tent of fat metabolism. The carbohydrates are here left out of con- ieration, as they constitute only about i per cent, of the body-weight. must be borne in mind, however, that in the metabolism of protein a rtain quantity of fat is produced which also undergoes oxidation, he amount of the carbon or the fat that the protein would give rise to, previously determined, must therefore be subtracted from that clim- ated by the lungs, etc., in order to determine the amount of body- t metabolized. Observations of human beings in the fasting con- tion show that for a period of ten days there is a daily excretion of )out 21 grams of urea, equivalent to about 70 grams of protein. This nount, however, may be reduced to from 50 to 60 per cent, if the in- vidual has a surplus of body-fat. Human beings under similar rcumstances may lose during the first few days 200 grams of fat lily. The following table shows the excretion of nitrogen and carbon and le calculated amounts of protein and fat metabolized from an experi- ent made by Ranke on himself during a fast of twenty-hours, be- nning twenty-four hours after the last meal: Disintegration op Tissue. (Calculated.) Expenditure. Nitrogen. Carbon. Nitrogen. Carbon. 7.8 0.0 26.5 157-5 7.2 0.0 •otein, 50 gm., It. 100.6 ffm . . Uric acid, 0.2 gm., ... J Carbon dioxid 3-4 180.6 7.8 184.0 7.2 184.0 Coincidently with these losses to the body there is also a gradual ss of inorganic salts, and toward the termination of the period a idden fall in temperature of several degrees centigrade, in conse- j.ence of the final consumption of all available foods, when death isues, in all probability, from a cessation in the action of the heart. Post-mortem Appearances. — It has been experimentally determined lat animals die when the body-weight has declined to about 40 per ;nt. Post-mortem examination shows that the loss of material, lOugh very generally distributed throughout the body, is greatest L organs and tissues least essential to life. The results of an analysis of the organs and tissues of a cat after thirteen-day period of starvation, during which the animal lost 1017 138 TEXT-BOOK OF PHYSIOLOGY. grams in weight, are given in the following table, based on data fur- nished by Voit: Organ. Adipose tissue, . Spleen, .' Liver, Testes, Muscles, Blood Kidneys, Skin and hair, , Lungs Intestines, Pancreas, Bones, Heart, Nervous system. Percentage. ACTUAL l>OSS OF Tissue. Grams. 97 267 67 6 54 49 40 I 31 429 27 37 26 7 21 89 18 3 18 21 17 I 14 SS 3 3 I It will be observed from this table that the adipose tissue suffers the greatest loss, the entire amount disappearing with the exception of a small portion in the posterior part of the orbital cavity and around the kidneys. The muscles, though only losing 31 per cent, of their weight, yet furnish 429 grams of presumably proteid material, for nutritive purposes. The heart and nervous system experience but slight loss. Mixed Diet.— The chemic composition of the tissues, taken in connection with their metabolism during starvation, implies that no one article of food is sufficient for tissue repair and heat production; but that all classes of foods — in other words, a mixed diet — are essential to the maintenance of a normal nutrition. Experimental investiga- tion has also conclusively established this fact. Moreover, the amounts of nitrogen and carbon eliminated daily, and the ratio existing between them, indicate the amounts of proteid, fat, and carbohydrate which are required to cover the loss. Metabolism on a Purely Protein Diet. — Notwithstanding the chemic composition of the proteins and the possibility of their giving rise to both fat and a carbohydrate during their metabolism it has been found extremely difficult to maintain the normal nutrition for any length of time on a pure protein or fat-free flesh diet. This, however, has been accomplished with dogs. It was found, however, that, in order to maintain the equilibrium, it was necessary to increase the proteins from two to three times the usual amount. Thus, a dog weighing 30 to 35 kilograms required from 1500 to 1800 grams of flesh daily in order to get the requisite amount of carbon to prevent consumption of its own adipose tissue. Under similar circumstances, a human being weighing 70 kilograms would require more than 2000 grams of lean beef — an amount which, from the nature of the digestive FOODS. 139 pparatus, it would be practically impossible to digest and assimilate Dr any length of time. Even the slight habitual excess beyond the mount normally required is imperfectly assimilated and gives rise D the production of nitrogen-holding compounds which, on account f the difficulty with which they are eliminated by the kidneys, ac- umulate within the body and develop the gouty diathesis, with all :s protean manifestations. Metabolism on a Fat and Carbohydrate Diet. — As nitrogen i an indispensable constituent of the tissues, it is evident that neither it nor carbohydrates can maintain nutritive equilibrium except for ery short periods. On such a diet the tissues consume their own roteids, as shown by the continuous excretion of urea, though the mount is less than during starvation. An excess of fat retards the letabolism of proteids. The same holds true for the carbohydrates. Thus, in any well-arranged dietary there should be a combina- on of proteins, fats, and carbohydrates in amounts sufficient to m^in- lin nutritive equilibrium; in other words, to repair the loss of tissue nd to furnish the requisite amount of heat in accordance with work one, as well as with climatic and seasonal variations. COMPOSITION OF FOODS. The food principles essential to the maintenance of the nutrition f the body are contained in varying proportions in compound sub- ;ances termed foods; e. g., meat, milk, wheat, potatoes, etc. Their utritive value depends partly on the amounts of their contained food rinciples and partly on their digestibility. The dietary of civilized lan embraces foods derived from both the animal and vegetable orlds. Composition of Animal Foods. — The following table shows the i^erage percentage composition of various kinds of meats, cow's ilk, and eggs: In 100 Parts 'ater, rotein It, '.. irbohydrates,, ilts, Beef. Veal. Mut- ton. Pork. Fowl. Fish. Cow's Milk. 76.25 77.82 75-59 72-57 70.80 79-3° 86.87 20.24 19.86 17.11 19-31 22.70 18.30 4-7S 1.68 0.82 5-47 S.82 4.10 0.70 3-5° 0.50 0.80 0.60 0.60 T.20 o.go 4.00 1.38 0.70 1.23 1.70 1.20 0.80 0.17 Eggs. 73-67 I2-5S 12. II 0-55 I-13 Meats. — It will be observed from these analyses that the meats )ntain from 18 to 20 per cent, of a protein which belongs in virtue ' its chemic relations to the group of globulins. In the living con- ition this body, known as myosinogen, is in a semifluid condition. Lit shortly after death undergoes coagulation, giving rise to solid lyosin and a soluble albumin. There are also present in meat small 140 TEXT-BOOK OF PHYSIOLOGY. percentages of other forms of protein; e. g., myoalbumin, myoglob- ulin, paramyosinogen, etc. After being subjected to the cooking process, meats contain the albuminoid body gelatin, a product of the transformation of the proteids of the connective tissue. The percentage of fat, contained within the meat substance, is very small except in mutton and pork, where it rises to 5.4 per cent, and 5.8 per cent, respectively. The fat-globules in these meats are packed closely between the muscle-fibers, and prevent the easy entrance of the digestive fluids, and hence they are more difficult of digestion than beef. The carbohydrates vary from 0.5 to i per cent., and are represented by glycogen. The principal inorganic salts are potassium phosphate . and sodium chlorid. Cooking, when properly done, not only makes the meat more palatable and appetizing from the development of agreeable flavors, but converts the connective tissue, which, in old animals especially, is tough and resisting, into gelatin, thus rendering it more easy of mastication and digestion. At the same time parasitic organisms, such as the embyronic forms of tenia or tapeworm, trichina spiralis, as well as bacterial growths, which frequently infest the bodies of an- imals, are destroyed and made harmless. Milk is the natural food of the young of all mammals, and is usu- ally regarded as typical on account of the ratio existing among its nu- tritive principles. The analysis given above is that of cow's milk. Examined microscopically, milk is seen to consist of a clear fluid, the milk plasma, holding in suspension an enormous number of small, highly refractive oil-globules, which measure on the average about j-^^^-^ of an inch in diameter. Each globule is supposed by some observers to be surrounded by a thin albuminous envelope, which enables it to maintain the discrete form. Others deny the existence of such a mem- brane. The chief protein constituent of milk, caseinogen, is held in solution by the presence of phosphate of lime. On the addition of acetic acid or sodium chlorid up to the point of saturation the casein- ogen is precipitated as such and may be collected by appropriate chemic methods. When taken into the stomach, caseinogen is coagulated; that is, it is separated into casein or tyrein and a small quantity of a new soluble proteid. This change is brought about by the presence in the gastrir juice of a special ferment known as rennin or pexin. The fat of milk is more or less solid at ordinary temperatures. It is a combination of olein, palmitin, and stearin, with a small quan- tity of butyrin and caproin. When milk is allowed to stand for some time, the fat-globules rise to the surface and form a thick layer known as cream. When subjected to the churning process, fat-globules run together and form a coherent mass— butter. Lactose is the particular form of sugar found in milk. In the presence of Bacillus acidi lactici, the lactose is decomposed into lactic acid and carbon dioxid, the former of which not only imparts a sour FOODS. 141 ste to the milk, but causes a precipitation of the caseinogen. The lief salt found in milk is phosphate of lime, and this is the chief source this agent in the formation of bones. Eggs are also to be regarded as complete natural foods, inasmuch 1 they contain all the necessary food principles. The analysis given the above table represents the composition of the entire egg. The hite of the egg contains 12 per cent, of proteid and 2 per cent, of fat. he yolk, however, contains 15 per cent, of proteid and 30 per cent. ■ fat. Composition of Cereal Foods. — The average composition of le principal cereals is shown in the following table : In 100 Parts. ater, ■otein, It, irbohydrate. illulose, . . . dts, Wheat, Rye. Barley. Oats. Corn. Rice. 13-56 12.65 13-77 12.37 13.10 ^Hl 12-35 12-55 II. 14 10.41 9-«5 7.88 1-75 1.97 2.16 5-23 4-57 0.85 67.90 67-95 64-93 57-78 68.42 76-55 2.63 3.00 S-31 II. 19 2-5° o-SS 1.81 1.88 2.69 3.02 1-S6 i-°S Buck- wheat. 12.62 10.02 2.24 64-43 8.67 That the cereals are most important and useful articles of diet is ^ident from their composition, consisting, as they do, of proteins id carbohydrates in large proportion. Owing to the cellulose or oody fiber which envelops and penetrates the grain, they are some- hat difficult of digestion. A section of a grain of wheat shows the eternal cellulose envelope, the husk, beneath which is a layer of large ills containing the chief protein — the gluten. The interior of the rain consists of small cavities, the walls of which are formed of cellu- ise and which contain the granules of starch, fat, small quantities of roteid, and inorganic salts. All other cereals have a similar structure. In "the preparation of white flour from wheat it is customary to ;move the husk, a process which involves the removal also of a por- on, if not all, of the gluten cells, so that such flour contains less nitrog- lized material than the original grain. It is possible, however, in le milling of wheat, to remove only the husk and retain the gluten in le flo.ur, as in the preparation of whole wheat flour.' Bread is an artificially prepared food made either of wheat or 't. Owing to the fact that the proteids of the other cereals do not assess the same adhesive properties when kneaded with water, they m not be used for bread-making purposes. In the making of bread, le flour is kneaded with water until a glutinous mass — dough— is irmed. During this process, salt, sugar, and yeast are added. It then placed in a temperature of about 100° F. In the presence of sat and moisture the natural ferment of the flour — diastase — con- ;rts a portion of the starch into sugar, which in turn is split up into 142 TEXT-BOOK OF PHYSIOLOGY. carbon. dioxid and alcohol by the yeast plant. The bubbles of carbon dioxid, becoming entangled in the dough, cause it to swell or rise and subsequently give the porous or spongy character to the bread. When baked at a temperature of 400° F., the alcohol is driven off; yeast cells and other organisms are destroyed; the starch, particularly that on the surface, is dextrinized. Thus prepared, white bread consists of water, 32 per cent.; protein, 8.8 per cent.; fat, 1.7 per cent.; carbohydrate, 56.3 per cent.; salts, 0.9 per cent. The principal salts are potassium and magnesium phosphate. Whole wheat bread consists of water, 40 per cent.; protein, 12.2 per cent.; fat, 1.2 per cent.; carbohydrate, 43.5 per cent.; salts, 1.3 per cent.; cellulose, 1.8 per cent. Composition of Vegetable Foods.— The average composition of some of the principal vegetables is shown in the following table : In Parts. Pota- TUR- TOMA- ASPA- Beans. Peas. Beets. toes. 13-74 14.99 75-47 82.20 89.42 96.30 93-75 23.21 22.85 1-95 1.80 '■35 0.90 1-79 2.14 1.79 o.is 0.30 0.18 0.50 0.25 53-67 52-36 20.69 13.00 7-36 2.80 2.63 3-69 5-43 0.76 0.94 1.04 3-SS 2.58 0.98 1.60 0-75 0.40 0-54 Cab- bage. Water, Protein, Fat, Carbohydrates Cellulose, .... Salts, 89.97 1.89 0.20 4.87 1.84 1.23 The vegetable foods, as a class, vary considerably in nutritive value and digestibility, the latter depending on the amount of cellu- lose they contain. A section of a vegetable shows not only the pres- ence of an external cellulose envelope, but also an inner framework which penetrates its substance in all directions. The nutritive prin- ciples are contained in small cavities, the walls of which are formed by the framework. Nearly all vegetables require cooking before being eaten. When subjected to heat and moisture, not only is the texture of the vegetable softened and disintegrated, but the starch grains are hydrated and partially prepared for conversion into dextrin and sugar. At the same time various savory substances are set free, which make the food more palatable. Beans and peas contain large quantities of a proteid, legumin, and starch, and hence are especially valuable as nutritive foods. The presence of the cellulose envelope, especially in ripe beans and peas, combined with rather a dense texture, renders them somewhat difficult of digestion. Potatoes, though largely employed as food, are ex- tremely poor in protein, 2 per cent., and carbohydrates, 20 per cent. When sufficiently cooked they are easily digested, owing to the small amount of cellulose they contain. Green vegetables, — e. g., lettuce, spinach, tomatoes, asparagus, onions, etc., though containing food principles in small amounts, are, nevertheless, valuable adjuncts to the dietary, for the reason that they contain inorganic as well as organic salts, which appear to be necessary FOODS. 143 > the maintenance of the normal nutrition. The want of green veget- Dles has been supposed to be the cause of scurvy. Ripe fruits, grapes, cherries, apples, pears, peaches, strawberries, mons, oranges, etc., though consumed largely, possess but little nutri- ve value. They consist largely of water, 75 to 85 per cent., proteins trace, sugar from 5 to 13 per cent., organic acids (citric, malic, tar- iric), pectose, and various inorganic salts. Relative Value of Animal and Vegetable Foods.— Though oth animal and vegetable foods contain the different classes of food rinciples, it is not a matter of entire indifference as to which are con- amed. It has been found by experiment that animal proteids are lore easily and completely digested and absorbed than vegetable roteids; that cellulose is not only highly indigestible, but by its pres- ace in large quantities retards the digestive process and impairs the ctivity of the entire digestive mechanism, though in moderate quan- ty it undoubtedly aids digeston indirectly by mechanically promot- ig peristalsis. The following table shows the relative digestibilit}' f the two classes of foods : Vegetable. Animal. Digested. Undigested. Digested, Undigested. f 100 parts of solids, f 100 parts of protein, .- f 100 parts of fats or carbohy- drates Hi 90-3 24-5 53-4 9-7 89.9 81.2 96.9 II. I 18.8 3-1 Construction of Dietaries. — Inasmuch as neither animal nor egetable foods contain the food principles in proper quantities and roportions, the instinctive choice of mankind has led to a combina- on of the two classes of foods. From the analyses tabulated above becomes comparatively easy to construct a suitable dietary, com- osed of different articles of food, in which the food principles shall ear the proper ratio one to the other — a ratio based on the total quan- ty of nitrogen (15 to 20 grams) and carbon (225 to 300 grams) elim- lated from the body daily. It is only necessary, therefore, to combine two or more foods, the Dmposition of which is known, in quantities sufficient to furnish le requisite amount of nitrogen and carbon, or their equivalents in roteid, fat, and carbohydrates. As illustrations of such combina- ons the following examples are given: Foods. [eat, 250 gm., 8.8 oz. read, 400 gm., 14.2 oz. It 100 gm., 3.5 oz. igar, 70 gm., 2.5 oz. :;} Pood Principles. N. 0. Protein, loo gm. 15 50 }Fat, ICO gm. . . 75 Carbohydrates, 250 gm. . . 100 IS 225 144 TEXT-BOOK OF PHYSIOLOGY. Foods. N. C. Meat, 22$ g™ Bread, 450 gm. Fats, 113 gm. Potatoes, 450 gm. Milk, 225 gm. Eggs, 113 gm i lb. 7.5 gm. 34 gm. I lb. 5.5 gm. 117 gm. i lb. ... 84 gm. I lb. 1.3 gm. 45 gm. J pint 1.7 gm. 20 gm. J lb. 2.0 gm. IS gm. Cheese, 56 gm., j lb. 3.0 gm. 20 gm 21.0 Waller. DAILY RATION OF THE UNITED STATES SOLDIER. Fresh beef, 20 oz. or pork, 12 oz. or bacon, 12 oz. Flour, 18 oz. or soft bread, 18 oz. or hard bread, 16 oz. Potatoes, 16 oz. or potatoes ii-J and tomatoes 43- 16 oz. Beans or peas, 2.4 oz. Rice or hominy, 1.6 oz. Coffee, 1.6 oz. Sugar, 2.00 oz. Vinegar, u.32 gill Salt, 0.60 gUl CHAPTER X. DIGESTION. Foods are heterogeneous compounds consisting of organic and lorganic nutritive principles associated with a varying amount of on-nutritive material, such as the dense parts of the connective tissue [ the animal foods and the woody fiber or cellulose of the vegetable )ods. Before the nutritive principles can be utilized they must be issociated from the non-nutritive material. Even when consumed I the free state, the food principles are seldom in a condition to be bsorbed into the blood and assimilated by the tissues. When foods re consumed in their natural state or after they have been subjected ) the cooking process, they are subjected while in the food canal to the )lvent action of various fluids by which they are disintegrated and :duced to the liquid condition. The nutritive principles freed from leir combinations are changed in chemic composition and transformed ito substances capable of absorption. To all the physical and chemic langes which foods undergo in the food canal the term digestion has sen given. The digestive apparatus comprises the entire alimentary or food mal and its various appendages: the teeth, the tongue, the mouth, le gastric and intestinal glands, the pancreas, and the liver (Fig. 63) . The canal itself is a musculo-membranous tube about thirty-two :et in length, and extends from the mouth to the anus. It may be ibdivided into several distinct portions j as mouth, pharynx, esoph- ^us, stomach, small and large intestines. The mouth is provided :) with teeth, by which the food is divided, (2) with the tongue, and 1) with glands, by which a solvent fluid, the saliva, is secreted. The ands, though situated for the most part outside the mouth, are con- ;cted with it by means of ducts. Posteriorly the mouth opens into le pharynx or throat, .a somewhat pyramidal-shaped structure about i^e inches in length, which in turn is followed by the esophagus or jllet, a tube about nine inches in length. As the esophagus passes irough the diaphragm it expands into the stomach, a curved pyri- irm organ, which serves as a reservoir for the reception and retention ': the food for a varying length of time. The small intestine is that Drtion of the alimentary canal extending from the end of the stomach I the beginning of the large intestine; owing to its length, about twenty- mo feet, it presents a very convoluted appearance in the abdominal ivity. Embedded in its walls are the intestinal glands which open on 3 surface and secrete the intestinal fluid. In the upper portion of 10 145 146 TEXT-BOOK OF PHYSIOLOGY. the small intestine, within five inches of the stomach, there are generally two orifices, the terminations of the ducts of the liver and pancreas, organs which secrete the bile and pancreatic juice respectively. The large intestine is from five to six feet in length and extends from the end of the small intestine to the anus. Its walls contain a large num- ber of glands. ) '%- /""' ^ / ,''/Voaa ^,f:^%/m' Salivary eland ^^^§ f- --->'-4--| V^ Liver— -^^^L ^e.//e. On/I r^ ^^5^^ ^H ^BHrV^,/^^^ .-^W ^B IBIif f/jj^^^^^^^^^^'^'^^^^^^^^^^^ ^m iiif i&^^^P^^^^ Duodenum Large- Infeahhe -U — Smafl IrjtesNne) Fio. 63.— Diagram of the Alimentary CA.-Sk-L.— {Modified from Landois.) The general process of digestion is largely accomplished by the chemic action of the digestive fluids : the saliva, the gastric, intestinal, and pancreatic juices, and the bile. Though taking place through- out a large portion of the food canal, the process may be subdivided into several stages: viz., mouth digestion, gastric digestion, and intesti- nal digestion. DIGESTION. 147 As a result of the action of these fluids the nutritive principles are epared for absorption into the blood; the non-nutritive principles, jng with certain waste products, pass into the large intestine to be lally extruded from the body. MOUTH DIGESTION. The digestion of the food as it takes place in the mouth comprises series of physical and chemic changes, the result of the action of e teeth, the tongue, and the saliva. The mechanic division of the od and the incorporation of the saliva with it are termed respectively astication and insalivation. MASTICATION. Mastication is the mechanic division of the food, and is accom- ished by the teeth and the movements of the lower jaw under the fluence of muscle contractions. Complete mechanic disintegra- >n of the food is essential to its subsequent solution and chemic msformation; for when finely divided it presents a larger surface the action of the digestive fluids and thus enables them to exert eir respective actions more effectively and in a shorter period of ne. The Teeth. — In man passing from childhood to adult life two ts of teeth make their appearance. The first set constitute the tem- irary, deciduous, or milk teeth; the second set constitute the per- ment teeth, which should last with proper care through life or to an vanced age. The temporary teeth, twenty in number, ten in each jaw, though laller than the permanent teeth, have the same general conforma- in. They are divided into four incisors, two cuspids or canines, .d four molars for each jaw. The permanent teeth, thirty-two in number, sixteen in each jaw, s divided into four incisors, two cuspids or canines, four bicuspids or emolars, and six molars for each jaw. Each tooth may be said to consist of three portions: (i) the crown, that portion which projects above the gums; (2) the root or fang, it portion embedded in the alveolar socket; (3) the constricted por- m or neck, which is surrounded by the free margin of the gum. The ;th are firmly secured in their sockets by a fibrous membrane, the ridental membrane, which is attached, on the one hand, to the alveo- ■ process, and, on the other, to the cementum. A vertical section of a tooth shows that it consists of three distinct id structures, the enamel, the dentine, and the cementum, which ve the anatomic relationship as represented in Fig. 64. In the titer of the dentine there is a cavity the general shape of which varies 148 TEXT-BOOK OF PHYSIOLOGY. ^X^^' in different teeth, and which is occupied during the living condition by the tooth pulp. Microscopic examination of the tooth reveals the presence of ir- regular stellate spaces, the interglobular spaces, between the dentine and the cementum, which are occupied by connective-tissue cells. Clefts of varying size are also observed at the junction of the dentine and the enamel, and which extend for some distance into the latter. The enamel is composed of dense hard cylinders which, on account of their small size and close relationship, appear to be hexagonal in shape. These cylinders are held together by cement substance. The free border of the enamel is covered, in early life at least, by a thin mem- brane known as the cuticle or mem- brane of Nasmyth. The dentine is somewhat less dense than the enamel. It is composed of connective-tissue fibers embedded in a ground-substance, both of which have undergone calcification in the course of development. The dentine is pen- etrated by a series of fine canals, the dentine canals or tubules, which begin by open mouths on the pulp side. From this point the tubules pass out- ward to the cementum and enamel, where their terminal branches com- municate with and terminate in the interglobular spaces and clefts. In their course the tubules give off a series of branches which communicate freely with one another. The dentine bordering the tubule is somewhat more dense than the intertubular por- tion and constitutes what is known as the dentinal sheath or Neumann's sheath. The cementum resembles bone be- . cause it contains both lacunae and canaliculi, though it is, as a rule, devoid of Haversian canals. The pulp consists of a framework of connective tissue which affords support to blood-vessels and nerves, both of which enter the pulp chamber through a small foramen at the apex of the root. The outer surface of the pulp is covered with a layer of large spheric cells, the odontoblasts. Each cell presents on its inner surface short processes which pass into the pulp; on its outer surface it presents a long process which enters a dentine tubule and extends as far as its ultimate ter- minations. Collectively these processes are known as the dentine Fig. 64. — Vertical Section of Tooth in Jaw. E. Enamel. D. Dentine. P. M. Periodontal mem- brane. P.C. Pulp cavity. C. Ce- ment. B. Bone of lower jaw. V. Vein. a. Artery. N. Nerve. — (Stir- ling.) DIGESTION. 149 fibers. Inasmuch as the fibers do not completely occupy the lumen of the tubule, it is probable that there is a free circulation of lymph from the pulp chamber through the dentine tubules into the enamel clefts, into the interglobular spaces, and possibly into the lacunae of the cementum. The peridental membrane is composed of connective-tissue fibers abundantly supplied with blood-vessels, and nerves. Movements of the Lower Jaw. — The lower jaw is capable of a downward and upward, an antero-posterior, and a lateral movement, all dependent on the peculiar construction of the — Temporo-maxillary Articulation. — This articulation is formed by the anterior portion of the glenoid cavity, the eminentia articularis and the condyle of the inferior maxilla, all of which are united by means of ligaments. Situated between the glenoid cavity and the condyle is a plate of fibro-cartilage oval in shape and biconcave. This ■cartilage divides the joint into two cavities — one above, the other below — each of which is provided with a synovial membrane. The function of the cartilage is to present constantly an articulating surface to the condyle in the various movements of the lower jaw, which it is enabled to do by virtue of its mobility. In the downward movement of the lower jaw each condyle glides forward, carrying with it the interarticular fibro-cartilage the upper concave surface of which is applied to the convex surface of the emin- entia articularis. In the upward movement of the jaw both the con- dyles and the cartilages pass backward and resume their normal posi- tion. The movements of depression and elevation are made possible by the transverse direction of the condyle. In the carnivorous ani- mals, whose food requires considerable cutting, these movements are especially well developed. In the antero-posterior movement the jaw moves in a horizontal direction and the condyles and the articular car- tilages glide forward and backward in the glenoid fossa;. In the rodent animals the long axis of the condyle runs in the antero-posterior direc- tion, which allows of a considerable gliding movement. When the jaw performs a lateral movement, the condyle and cartilage of one side remain in their normal position, while the opposite condyle and cartil- age glide forward in the glenoid fossa, directing the symphysis of the jaw to the opposite side of the median line. The lateral movements are well exhibited by the herbivorous animals, in which they are quite extensive, and made possible by the small size of the condyle and the large extent of articulating surface. In man the structure of the joint is such as to admit of all these possibilities, and the lower jaw acquires in consequence great freedom of movement. The Functions of the Muscles of Mastication.— The move- ments of the lower jaw are caused by the action of numerous muscles, which, having a fixed point of origin, are attached to various points on its surface. The muscles concerned in the movements of mastication are presented in the following table : ISO TEXT-BOOK OF PHYSIOLOGY. Anterior belly of digastic Mylohyoid Geniohyoid Temporal Internal portion of masseter Internal pterygoid External pterygoids External portion of masseter Anterior fibers of temporal Posterior fibers of temporal Internal portion of masseter Digastric, mylohyoid, and genio- hyoid Internal pterygoids External pterygoids Pterygoids, external and internal Temporal Masseter Depress the lower jaw and open the mouth. Elevate the lower jaw and close the mouth. Draw the lower jaw forward and cause the lower teeth to project beyond the upper. Draw the lower jaw back to its nor mal position. Contracting alternately, draw the jaw to the opposite side. Produce grinding movements of the lower jaw. The action of the depressor muscles becomes apparent when their points of origin and insertion are considered. The anterior belly of the digastric, the mylohyoid, the geniohyoid muscles, agree in having a similarity of origin — the hyoid bone — and a common area of inser- tion, the anterior portion of the inferior maxillary. Their anatomic relation is such that their combined action will depress the lower jaw and open the mouth. The elevator muscles arise from various points on the side of the head, and are inserted into the coronoid process, ramus, and internal surface of the angle of the lower jaw. When the mouth has been opened, the simultaneous contraction of these muscles elevates the jaw and closes the mouth with considerable force. The power of these muscles is very great, and depends on the shortness and thickness of the muscle-bundles. The action of the rotator muscles, those which give rise to the lateral movements of the jaw, depends in like manner on their origin and in- sertion. Arising from the superior maxillary and sphenoid bones, -they are inserted into the neck of the condyle and angle of the lower jaw respectively. When they contract, the condyle on the correspond- ing side is drawn forward, while the opposite condyle remains stationary. As a result, the symphysis of the jaw is directed to the opposite side. The grinding movements of the jaw are produced by the coordinated action of all the groups of muscles acting more or less successively. For the proper mastication of the food it is essential that it be kept between the opposing surfaces of the teeth. This is accomplished by the contraction of the orbicularis oris and buccinator muscles from without and the tongue muscles from within. The Nerve Mechanism of Mastication.* — The movements of mastication, though originating in efforts of the will and under its control, are for the most part of an automatic or reflex character; for * By this term is meant a combination of nerves, afferent and efferent, and nerve centers which when stimulated coordinates the actions of the organs with which it is associated. DIGESTION. 151 when once initiated by a voluntary effort they continue for an indef- inite period — so long, in fact, as the impressions which the food makes upon the afferent nerves are received by the nerve-centers which regulate and control them. That the masticatory movements are of this reflex nature is shown by the fact that they will be maintained even though the voluntary effort which called them forth has sub- sided and the attention has been directed to some entirely different subject. It would appear that all that is necessary under such con- ditions is the exciting action of the food upon the periphery of the afferent nerves distributed to the tongue and mouth. The nerves involved in this reflex are shown in the following table : Afferent Nerves. Efferent Nerves, 1 . Lingual branch of fifth nerve. i . Inferior maxillary division of fifth nerve, n. Glossopharyngeal. u. Hypoglossal or sublingual. 3. Facial or portio dura. The nerve-center coordinating the movements of mastication is situated in the medulla oblongata. The afferent or excitor nerves which receive the impressions of the food are distributed largely to the mucous membrane of the tongue. AVhen these impressions are received by the center in the medulla oblongata, it discharges nerve impulses, which, passing outward through motor nerves, excite con- traction in the masticatory muscles. The motor nerves innervating the muscles are : (i) The small root of the fifth nerve, which, after emerg- ing from the cavity of the cranium through the foramen ovale, joins the inferior maxillary division of the large sensor root. It then is dis- tributed to the masseter, temporal, internal, and external pterygoids, anterior belly of the digastric, and mylohyoid muscles, and controls their movements. (2) The hypoglossal nerve, which, after emerging through the anterior condyloid foramen, passes downward and forward to be distributed to the extrinsic and intrinsic muscles of the tongue. (3) The facial or portio dura, which, after emerging from the stylo- mastoid foramen, is distributed to the muscles of the face. Irritation of any one of these nerves produces convulsive movements in the mus- cles to which it is distributed, while their division is followed by paral- ysis of these muscles. The medulla not only generates the impulses which are directly responsible for the movements of mastication, but also coordinates them in such a manner that the movements of mastica- tion may be directed toward the accomplishment of a definite purpose. INSALIVATION. Insalivation is the incorporation of the saliva with the food, and takes place for the most part during mastication. The saliva ordi- narily present in the mouth is a complex fluid composed of the various secretions of the parotid, submaxillary, and sublingual glands and the muciparous follicles of the mouth, which collectively constitute the salivary apparatus (Fig. 65). 152 TEXT-BOOK OF PHYSIOLOGY. The parotid gland is situated in front of and partly below the external ear, where it is held in position by the fascia and skin. From the anterior border of the gland there emerges a duct (Stensen's), which, after crossing the masseter muscle to its anterior border, turns inward, pierces the buccinator, and opens on the surface of the cheek opposite the second upper molar tooth. The submaxillary gland is situated below the jaw in the anterior part of the submaxillary triangle. From the gland there emerges a duct (Wharton's) which opens into the mouth by a minute orifice on the surface of a papilla by the side of the tongue. The sublingual gland is situated just beneath the mu- cous membrane in the anterior part of the mouth, where it forms a projection between the gums and tongue. The posterior part of the gland gives origin to a duct (the duct of Rivinus, described also by Bartholin) which opens into the mouth with or very near to the duct of Wharton. The anterior part of the gland gives origin to a varying num- ber of ducts (Walthers) which open separately along the edge of the sublingual plica of the mucous membrane. Histologic Structure of the Salivary Glands. — In their ultimate structure the salivary glands bear a close resemblance to one another. They are compound tubulo-alveolar glands composed of small irregularly shaped lobules united by areolar tissue and connected by branches of the salivary ducts. Each lobule is made up of a number of srriall alveoli or acini more or less tubular in shape which are the terminal expansions of the smallest ducts. (See Fig. 66.) The wall of the acinus is formed by a reticulated basement mem- brane, surrounded externally by blood-vessels, the spaces between which constitute lymph-spaces or channels. The inner surface of the acinus membrane supports a single layer of irregular spheric or polygenic epithelial cells. The cells do not entirely fill up the cavity of the acinus, but leave an intercellular space, the lumen. Fig. 65. — Salivary Glands. 1, 2. Parotid. 3. DuctofSteno. 4. Submaxillary. 5. Sublin- gual. 6. Mylohyoid muscle. 7. Lingual branch of the fifth nerve. 8. Duct of Wharton. 9. Di- gastric muscle. 10. Sternomastoid muscle. 11. External jugular vein. 12. Facial vein. 13. Temporal vein. 14, 15. Internal jugular vein. 16. Branch of the cervical plexus. 17. Sublin- gual nerve. — {Le Bon.) j DIGESTION. 153 which constitutes the beginning of the duct for the transmission of the secretion to the mouth. From each acinus there passes a narrow intercalary duct lined by a layer of flattened cells. The common excretory duct— formed by the union of the intralobular and inter- lobular ducts — consists also of a basement membrane, lined, however, by tall columnar epithelial cells. The salivary glands are abundantly supplied with blood-vessels and nerves which are closely related to their activity. Based partly on the character of their secretions and partly on the microscopic appearance of their secreting cells, the salivary glands have been divided by Heidenhain into two classes: viz., serous or al- buminous, and mucous glands. To the first class belong the parotid, a portion of the submaxillary, and a portion of the glands of the tongue. To the second class belong a portion of the submaxillary gland, the sub- lingual, a portion of the glands of the tongue, the glands of the cheeks, lips, palate, and pharynx. It is possible that a single alveolus of any gland may contain both albuminous and mucous cells. In the serous glands the cells are more or less spheric in shape, nu- cleated, and almost completely filled with dark granular material (Fig. 67). In the mucous glands the cells are large, clear in appearance, and loaded with a highly refracting material re- sembling mucin (Fig. 68). Between the basement membrane and the clear cells are to be found in the acini of the submaxillary glands small crescentic shaped cells filled with granular material which stains deeply with various coloring-matters. These are known as the demilunes of Heidenhain. At one time it was supposed that they were young cells destined to take the place of the clear cells which were believed to be exhausted and to have undergone disintegration. At the present time they are regarded as albuminous or serous cells which exhibit changes similar to the cells of the parotid gland. The glands embedded in the mucous membrane covering the tongue, lips, cheek, palate, and pharynx are for the most part lined with epithelial cells which contain a highly refracting matter similar to, if not identical with, that found in the cells of the submaxillary and sublingual glands. Nerve-supply. — Histologic investigation has demonstrated that the cells and blood-vessels of the salivary glands are supplied with Secretory tubule. Intercalated pieces. Acini. Fig. 66. — .Scheme of the Htjman Submaxillary Gland. — {Stshr.) 154 TEXT-BOOK OF PHYSIOLOGY. nerve-fibers directly from ganglion cells situated in their immediate neighborhood. Thus the cells and blood-vessels of the submaxillary and sublingual glands receive nerve-fibers from the submaxillary, sublingual and superior cervical ganglia, while the cells apd blood- vessels of the parotid gland receive nerve-fibers from the otic and the superior cervical ganglia. From their ultimate distribution it may be inferred that some of the ganglion cells and fibers influence the pro- duction of the secretions (secretor nerves), while others influence the caliber of the blood-vessels causing either constriction or dilatation (vaso-constrictor and vaso-dilatator nerves). (Fig. 69.) The secretor fibers penetrate the basement membrane enclosing the gland acinus and finally terminate between and on the surface of the secretory cells. The vaso-motor fibers terminate between and on the muscle cells in the walls of the blood-vessels. The local ganglion cells, however, are in anatomic relation with nerve-trunks coming directly from the medulla oblongata and the spinal Fig. 67. — Section op Human Par- otid Gland Including Several Acini. d. Cut intralobular duct. — {Pier sol.) Fig. 68. — Section of Human Sub- lingual Gland. Among the clear cells lining the mucous acini are nests {g, g) of granular elements which constitute the demilunes of Heidenhain. — {Piersol.) cord. As they approach the ganglia, their terminal branches arborize around and closely invest the cells of the ganglia and come into in- timate histologic and physiplo^c connection with them. The nerve- fibers coming from the central nerve system are known as pre-ganglionic fibers, while those coming from the ganglia are known as post-gangli- onic fibers. Through the intermediation, therefore, of the ganglion cells, the secretor cells of the salivary glands and the blood-vessels surrounding them, are brought into relation with the central organs of the nerve system and become susceptible of being influenced by them. The Parotid Saliva. — ^The parotid sahva, as it flows from the orifice of Stensen's duct, is clear, Umpid, free from viscidity, distinctly alkaUne in reaction, with a specific gravity of 1.003. Chemic analysis shows that it consists of water, a small quantity of proteid matter, a trace of a sulphocyanogen compound, and inorganic salts. The secre- tion is increased during mastication, and especially on the side engaged in mastication. Dry food causes a larger flow than moist food. The situation of the orifice of the parotid duct is such that as the secretion DIGESTION. I5S is poured into the mouth it is at once incorporated with the food by the movements of the lower jaw, and thus fulfils the physical function of softening and moistening it. The Submaxillary Saliva. — ^The submaxillary saliva is clear, slightly viscid, alkaline in reaction, and has a specific gravity of 1.002. It consists of water, proteid matter (mucin), and inorganic salts. The Sublingual Saliva. — ^The sublingual saliva is clear, extremely viscid, and strongly alkaline in reaction. It consists of water, proteid matter (chiefly mucin), and inorganic salts. Glesso-Piar^n^eal Otic Ganyl.ion.^ Parotid Gland. Jacoisen's Jveriie Ou.l-Marilla.ri) GanqlLon, l"^ SuhMaxilla.ri/ Gtam Cnorda.JympaniJlterve! Oup.CeriticaZ uanolion Sympathetic Mines Fig. 69. — Scheme of the Nerves Involved in the Secretion oe Saliva. The small racemose glands embedded in the mucous membrane on the inner surface of the cheeks and lips, on the hard and soft palate, on the tongue and pharynx, secrete a fluid which is grayish in color, extremely viscid and ropy. It contains a large amount of mucin. Mixed Saliva.^The saliva of the mouth is a complex fluid com- posed of the secretions of all the salivary glands. As obtained from the mouth it is frothy, opalescent, slightly turbid, and somewhat viscid. The specific gravity is low, ranging from 1.003 to 1.006. The reac- tion is usually distinctly ,alkaline. It may, however, be neutral or even acid in reaction if there is any fermentation of food particles in the mouth or certain disorders of the alimentary canal. When ex- iS6 TEXT-BOOK OF PHYSIOLOGY. amined with the microscope, the saliva is seen to contain epithelial cells, salivary corpuscles resembling leukocytes, particles of food, various microorganisms, and especially Leptothrix huccalis. The chemic composition of the saliva is shown in the following table : COMPOSITION OF HUMAN SALIVA. Water, 995-i6 994-20 Epithelium, 1-62 2.20 Soluble organic matter, 1.34 i-4o Potassium sulphocyanid, 0.06 0.04 Inorganic salts, 1-82 2-20 1000.00 1000.04 (Jacubowitsch.) (Hammerbacher.) Water constitutes the main ingredient, amounting to 99.5 per cent. The soluble organic matter is proteid in character and is a mixture of mucin, globulin, and serum-albumin. The potassium sulpho- cyanid is mainly derived from the parotid gland. Its presence can be demonstrated by the addition of a few minims of a dilute solution of slightly acidulated ferric chlorid, when a characteristic red color is developed. The inorganic constituents comprise the sodium, calcium and magnesium phosphates, sodium carbonate, sodium and potassium chlorids. The relative amounts of the different constituents of the saliva will depend on the relative degree of activity of the different glands, and this in turn will be determined by the character of the food. When the food is dry, there will be an excess of the parotid secretion; when it partakes of the consistence of meat, there will be a larger secretion of the submaxillary saliva. The glands apparently adapt their activity to the character of the food. Quantity of Saliva. — The estimation of the total quantity of mixed saliva secreted in twenty-four hours is exceedingly difficult, and the results obtained must be only approximative. It is, of course, subject to considerable variation, depending upon habit, the nature of the food, etc. The experiments of Professor Dalton and the results obtained by him are eminently trustworthy, and in all probability represent as nearly as possible the exact amount secreted. He found that without any artificial stimulus he was enabled to collect from the mouth about 36 grams (540 grains) of saliva per hour, but that upon the introduction of any stimulating substance into the mouth the quantity could be greatly increased. During mastication the saliva was poured out in greater abundance, the amount depending upon the relative dryness of the food. He found that wheaten bread ab- sorbed 55 per cent, of its weight, and fresh cooked meat 48 per cent. If, therefore, the average quantity of bread and meat required daily by a man of ordinary physical development and activity be assumed to be 540 grams (19 oz.) of the former and 450 grams (16 oz.) of the DIGESTION. 157 latter, these two substances would absorb respectively 297 grams (4573.8 grains) and 216 grams (3326.4 grains), making a total of 513 grams (7900 grains). If, therefore, the amount secreted and mixed with the food during an estimated two hours of mastication be added to the amount secreted during the remaining twenty-two hours, sup- posing that it continues at the rate of 36 grams per hour, we have a total amount of 513 -=- 792 grams, or 1305 grams (19,780 grains), or about 2.8 pounds. Histologic Changes in the Salivary Glands during Secretion. — During and after secretion very remarkable changes take place in the cells lining the acini, which are in some way connected with the production of the essential constituents of the salivary fluids. In the case of the parotid gland, which may be regarded as the type of a serous or albuminous gland, the following changes have been observed by Langley (Fig. 70). During the period of rest and just previous to secretory activity, the epithelial cells are enlarged and swollen, and ( I Fig. 70. — Cells or the Alveoli of a Serous or Watery Salivary Gland. A. After rest. B. After a short period of activity. C. After a prolonged period of activity. — {Yeo's "Text-Book of Physiology.") encroach on the lumen of the acinus. The protoplasm of the cells is so completely filled with dark fine granules as not only to obscure the nucleus, but to almost obliterate the line of union of the cells. As soon as secretion becomes active, however, the granules begin to disappear from the outer region of the cell and move toward the inner border and into the lumen of the acinus. From these observations it might be inferred that during rest the protoplasm of the cells gives rise to granular material, and that during and after secretory activity there is an absorption of new material from the lymph and a recon- struction of the granular material. In the submaxillary gland, a portion of which may be taken as a type of a mucous gland, similar changes have been observed (Fig. 71). During rest the epithelial cells are large, clear in appearance, highly refractive, and loaded with small globules resembling mucin. The nucleus, surrounded by a small quantity of protoplasm, lies near the margin of the cell. That the granules are not protoplasmic in character is shown by the fact that they do not stain on the addition of carmine. When treated with water or dilute acids, the globules swell up, coalesce, and form a uniform mass. The chemic relations iS8 TEXT-BOOK OF PHYSIOLOGY. of this substance indicate that it is the precursor of mucin— nainely, mucigen. During secretory activity the cells discharge these mucigen granules into the lumen of the acinus where they are transformed into mucin. Though the appearance of the gland-cell appears to in- dicate it, there is no evidence for the view that the cell itself undergoes disintegration in the process. The Physiologic Actions of Saliva.— The constant presence of salivary glands in the different classes of animals and the large amount of secretion they pour daily into the alimentary canal point to the conclusion that this mixed fluid plays an important role in the general digestive process. Experiment has demonstrated that it has a two- fold action, physical and chemical. ^Physically, saliva softens and moistens the food, unites its par- ticles ijito a consistent mass by means of its contained mucin, and thus facilitates swallowing. Chemically it converts starch into sugar. This action is more marked with boiled than with raw starch, a fact which depends on the physical structure of the starch grain. In the natiu-al condition each starch grain con- sists of a cellulose envelope or stroma in the meshes of which is contained the true starch ma- terial, the granulose. When boiled for some minutes, the starch grain absorbs water, the granulose swells and ruptures the cellulose envelope, after which it passes into an imperfect opalescent solution more or less viscid, depending on the relative amounts of water and starch. This is the change largely brought about by the process of cooking. If a portion of this hydrated starch be kept in the mouth for a few minutes it will be converted into sugar, a fact made evident by the sense of taste. The chemic action of saliva in converting starch into sugar, as well as the intermediate stages, can be experimentally shown in the following manner: To 5 volumes of a thin starch solution in a test- tube add two volumes of filtered saliva. Place the mixture in a water-bath at a temperature of 35° C. In a few minutes the starch passes into a soluble condition and the fluid becomes clear and trans- parent. On testing the solution from time to time with iodin the characteristic blue reaction will be found to gradually disappear, the color passing from blue to violet, to red, to yellow. If now the solu- tion be boiled with a solution of cupric hydroxid (Fehling's solution) a copious red or yellow precipitate of cuprous oxid is formed, which indicates the presence of sugar. The polariscope shows that this sugar is maltose, C^Hj^Oi,. During the conversion of the starch Fig. 71. — The Appeakance Presented BY the Cells of the Submaxillary Gland or the Dog atter Prolonged Secretion. — {Modified from Landois and Stirling.') DIGESTION. IS9 intermediate substances are formed to which the term dextrin is applied. After the starch has been rendered soluble it undergoes a cleavage into maltose and a dextrin, which, as it gives rise to a red color with iodin, is termed erythrodextrin. At a later stage this erythrodextrin also undergoes a cleavage into maltose and a second variety of dextrin, which, as it does not give rise to any color with iodin, is termed achroodextrin. It is claimed by some investigators that this form can also in time be transformed into sugar. It is pos- sible that a small quantity of dextrose is also formed. The successive stages of the conversion of starch into sugar may be represented by the following schema: r T^ ii_ J 1 • f Achroodextrin. Starch = Soluble Starch = | mTw '^ ''■ Maltose. This change consists in the assumption by the starch of a molecule of water, and for this reason the process is termed hydrolysis. The nature of the chemic change is shown in the following formula: 3(C6HioOs) + H,0 = C„H,,0,i + CeHioO; Starch + Water ■= Maltose + Dextrin. The amylolytic or starch-changing action of saliva depends on the presence of an unorganized ferment or enzyme known as ptyalin. This enzyme is present in the secretion of each of the salivary glands. The chemic character of ptyalin is unknown, though there are reasons for believing that it partakes of the nature of a proteid. It is a prod- uct in all probability of the katabolic activity of the secretory cells. According to Latimer and Warren, ptyalin is a derivative of the zymogen, ptyalogen. This latter compound has been shown to be present in the glands of the dog, cat, and sheep. Ptyalin effects the transformation of starch merely by its presence, and undergoes no perceptible consumption in the process. The activity of this enzyme is very great, and unless interfered with by an excess of sugar and dextrin, it acts practically indefinitely. The activity of ptyalin is modified by various external conditions, among which may be mentioned the chemic reaction of the medium in which it is placed. It is most active when the medium is moder- ately alkaline. Its activity is arrested either by strong alkalies or acids, though the presence of a small percentage of an acid does not appear to have any effect in either hastening or retarding the process. This fact has a bearing upon the question as to whether the action of the saliva is interfered with in the stomach by the presence of the gastric juice. At present it is a disputed matter, but the weight of authority is in favor of the view that the transforming action may continue for almost half an hour during the early stages of gastric digestion. The temperature also influences the rapidity with which the transformation of the starch is effected. At a temperature of 95° to io6° F. the ptyalin acts most energetically, while its activity i6o TEXT-BOOK OF PHYSIOLOGY. is entirely arrested by reducing the temperature to the freezing-point or raising it to the boiling-point. The Nerve Mechanism of the Secretion of Saliva.— The secretion of the saliva is a complex act and involves the cooperation of gland-cells, blood-vessels, and nerves. During the intervals of mastication the glands are practically at rest as far as the discharge of saliva is concerned. The cells, however, are actively engaged in absorbing from the surrounding lymph-spaces materials derived from the blood from which they construct their characteristic con- tents. The blood-vessels possess that degree of dilatation necessary for nutritive purposes. With the beginning of mastication the blood-vessels suddenly dilate, the blood-supply is increased, and the gland-cells begin to dis- charge water, inorganic salts, and their organic constituents into the lumen of the acinus. This continues until mastication ceases, when all the structures return to their former condition of relative inactivity. The entire process is reflex in character and takes place through the medulla oblongata. It requires the usual mechanism necessary for all reflex acts — viz., a sentient surface, afferent nerves, emissive cells, efferent nerves, and the responsive organs. With the introduction of food into the mouth impressions are made on the terminal branches of the afferent nerves distributed in the mucous membrane. Nerve impulses developed by the mechanic and chemic action of the food are then transmitted to the medulla oblongata and received by emissive cells. These in turn discharge nerve impulses which are transmitted through efferent nerves to the structures, producing the vascular and secretory effects already stated. The nerves and nerve-centers which constitute the reflex mechan- ism for the secretion of saliva are shown in the following table: Afferent Nerves. Nerve-centers. Efferent Nerves. 1. Lingual branch of fifth Medulla oblongata. Chorda tympani for the submax- nerve. illary and sublingual glands, auriculo-temporal branch of the 2. Taste fibers in the chorda fifth nerve for the parotid gland. tympani. 3. Glossopharyngeal. Sympathetic nerve for all glands. That the secretion of the saliva is regulated by the above mechan- ism, and that the lingual branches of the fifth nerves and the glosso- pharyngeal are the afferent nerves, can be demonstrated by exposing the glands and their nerve connections and subjecting them to ex- periment. Under such circumstances, if a cannula be placed in the duct of the submaxillary gland, and the lingual nerve stimulated by an induced electric current of moderate strength, a copious flow of saliva at once takes place. If now the glossopharyngeal nerve be stimulated in a similar manner, the effect on the secretion will be the same. Division of these two nerves in an animal, in such a wavas to DIGESTION. i6i prevent the nerve impulses from reaching the medulla oblongata, is followed by a marked diminution in the amount of saliva secreted. The reflex centers, however, may receive impulses and be excited to activity by impulses coming through other nerves — e. g., the pneumo- gastric, when the mucous membrane of the stomach is stimulated; the sciatic, when after division its central end is stimulated. The central mechanism which causes through its efferent nerves the discharge of saliva is also capable of being excited to activity by psychic influences. It is well known that ideas and emotional states developed by the sight and the odor of foods, especially after long abstinence, cause a discharge of saliva into the mouth. The watering of the mouth under such circumstances is a demonstration of the influence of such psychic states. This fact was experimentally demonstrated on dogs by Pawlow. This investigator caused the ducts of the glands to be brought to the surface in such a manner that they healed into the edges of the skin wounds. By means of suitable receivers applied over the orifices of the ducts, the saliva could be readily collected. When the dog, under such circumstances, was tempted by the sight of food there was at once a free discharge of saliva. Whenever the central mechanism is stimulated, either by nerve impulses coming through afferent nerves, from the periphery or from the brain, impulses are generated which pass outward through effereni nerves — the chorda tympani nerve to the submaxillary and sublingual glands, the auriculo-temporal nerve to the parotid gland, and the sympathetic nerve to all three. The Chorda Tympani. — The chorda tympani nerve is a branch of the facial, the trunk of which it leaves in the aqueduct of Fallopius. It then crosses the tympanic cavity, emerges through the Glaserian fissure, and joins the lingual branch of the inferior maxillary division of the fifth nerve. After passing forward as far as the sublingual gland, nearly all of the original fibers leave the lingual nerve by four or five strands to become connected by terminal branches with nerve- ganglion cells in relation with the submaxillary and sublingual glands. (See Fig. 69.) The effects on the secretion and flow of saliva from the submaxil- lary gland which follow division and stimulation of the chorda tym- pani nerve are shown in the following way: a cannula is inserted into Wharton's duct and the rate of flow estimated; the nerve is then divided, after which the flow ceases. The peripheral end of the nerve is then stimulated with the induced electric current, when a copious secretion of a thin saliva takes place, accompanied by a marked dilatation of the blood-vessels of the gland. The quantity of blood passing through the vessels is so great as to give to the venous blood an arterial hue and to the small veins a distinct pulsation. It would appear from these effects that the chorda contains two sets of fibers, one of which inhibits the action of a local vaso-motor mechan- ism permitting the blood-vessels to dilate (vaso-dilatator fibers), the i62 TEXT-BOOK OF PHYSIOLOGY. other of which stimulates the secretor cells to activity, either directly or through the intermediation of local ganglia. That local ganglia ai;e involved is shown by the effects which follow -the injection of nicotin into the circulation. After a sufficient dose— lo milligrams for the cat— stimulation of the chorda has no effect. Stimulation of the nerve-plexus beyond the ganglion, however, is at once followed by vascular dilatation and secretion. It might be inferred that the increase in the flow of saliva is due to filtration, the result of the increased blood-supply to the gland, and not to the influence of any true secretor fibers stimulating the activities of the secretor cells. That this is not the case, however, can be demonstrated in several ways: First, the pressure in the duct of the submaxillary gland, as shown by the mercurial manometer, rises, when the gland is secreting, considerably above the pressure in the carotid artery, which could not be the case if it were due to a mere filtration; for if pressure alone were the cause, the flow of saliva would cease as soon as the pressure in the tube equaled the pressure in the blood-vessels. Second, even in the absence of blood the gland can be made to yield a secretion, as shown by stimulating the nerve in a recently killed animal. Third, after the injection of atropin into the circulation the secretion is abolished, but the local vaso-motor mechan- ism is unimpaired, for stimulation of the nerve, as in the previous instance, gives rise to a dilatation of the vessels and an increased blood-supply. There is thus abundant proof that the chorda tym- pani contains two sets of fibers — one regulating the blood-supply to the gland, the other stimulating the secretor cells. The Auriculo-temporal Nerve. — The nerve-fibers which conduct nerve impulses outward from the medulla to the parotid gland are believed to pass through the glossopharyngeal nerve, through the tympanic branch or nerve of Jacobson, to the otic ganglion, with which they become connected. From this ganglion new nerve-fibers arise which pass into the fifth nerve and reach the secretor cells of the parotid gland through the auriculo-temporal nerve. The influence of the auriculo-temporal branch of the fifth nerve on the parotid gland is similar to the action of the chorda tympani on the submaxillary gland. The active fibers of this nerve are prob- ably derived from the ninth nerve or glossopharyngeal. If the nerve be stimulated by the induced electric current, there follows a dilata- tion of the blood-vessels and an abundant discharge of a thin saliva, rich in water and salts, but containing a small amount of organic matter. Division of the nerve, extirpation of the otic ganglion,, or division of Jacobson's nerve, is followed by a loss of reflex secretion. Stimulation of Jacobson's nerve, as shown by Heidenhain, gives rise to the secretion. The Sympathetic Nerves. — The sympathetic fibers which in- fluence the salivary secretion emerge from the spinal cord mainly through the second, third, and fourth thoracic nerves. After passing DIGESTION. 163 into the sympathetic chain they ascend to the superior cervical ganglion, with the cells of which they become connected through the interme- diation of fine terminal branches. From this point non-medullated nerve-fibers follow the branches of the external carotid artery to the different glands. There is no evidence that these fibers have any connection, anatomic or physiologic, with local ganglia at or near the submaxillary, sublingual or parotid glands. If the sympathetic nerve in the neck, especially in the dog, be divided and the peripheral end stimulated with the induced electric current, there is at once a contraction of the smaller blood-vessels of the submaxillary and sub- lingual glands and a diminution of the blood-supply, a result showing the presence of vaso-constrictor fibers. Nevertheless both the sub- maxillary and sublingual glands pour out a saliva which is different from that poured out when the chorda tympani is stimulated. The ' quantity is less, it is more viscid, richer in organic matter, of a higher specific gravity, and more active in the transformation of starch into sugar. Stimulation of the sympathetic fibers passing to the parotid gland is followed by contraction of the vessels and an abolition of the secre- tion; but at the same time there is an increased activity of the secretor cells, for subsequent stimulation of the auriculo-temporal nerve not only causes an increase in the amount of water and inorganic salts, but an increase also in the amount of organic matter far beyond thai produced when the auriculo-temporal has alone been stimulated. Histologic examination shows that the small ducts of the gland are filled with thick organic matter after stimulation of the cervical sym pathetic. The foregoing facts led Heidenhain to the conclusion that there are two physiologically distinct efferent nerve-fibers distributed to the glands, viz., trophic nerves, derived from the sympathetic which stimulate the cells to the production of organic constituents; and secretor nerves, derived from the cranial nerves, which stimulate the cells to the production of water and inorganic salts. This view has however, been controverted by Langley who regards the secretor fibers to the glands as essentially the same, and considers the differ- ences in the character of the secretion to be dependent on differences in the quantity of the blood-supply induced by the simultaneous stimulation of the vaso-motor nerves. DEGLUTITION. Deglutition is that part of the digestive process which is concerned in the transference of the food from the mouth through the pharynx and esophagus into the stomach. This is an extremely complex act and- involves the action of a large number of structures, all of which are made to act in proper sequence under the coordinating influence of the nervous system. The deglutitory canal consists of the' mouth. 164 TEXT-BOOK OF PHYSIOLOGY. pharynx, and esophagus, each of which presents certain anatomic features on which its physiologic action depends. The cavity of the mouth communicates posteriorly with the pharynx by a narrow orifice, the isthmus of the fauces. This orifice is bounded above by the soft palate, laterally by the anterior and posterior half arches, and below by the tongue. The pharynx is an oval-shaped cavity extending from the base of Fig. 72. — Vertical Section of the Nasal Fossa and Mouth, i. Left nares. 2. Lateral cartilage of the nose. 3. Portion of the internal alar cartilage forming the skeleton of the lower part. 4. Superior meatus. 5. Middle meatus. 6. Inferior meatus. 7. Sphenoidal sinuses. 8. External boundary of the posterior nares. 9. Internal elliptical opening of the Eustachian tube. 10. Soft paJate. it. Vestibule, of the mouth. 12. Vault of palate. 13. Genioglossus muscle. 14. Geniohyoid muscle. 15. Cut margin of the mylohyoid muscle. 16. Anterior pillar of the palate (anterior half-arch), presenting a triangular figure with the base inferiorly, covering partly the tonsil. 17. Posterior pillar (posterior half-arch) of the palate. 18. Tonsil. 19. Follicular (mucous) glands at the base of the tongue. 20. Cavity of the larynx. 21. Ventricle of the larynx. 22. Epiglottis. 23. Cut os hyoides, 24. Cut thyroid cartilage. 25. Thyrohyoid membrane. 26. Section of posterior portion of the cricoid cartilage. 27. Section of the anterior portion of the same cartilage. 28. Crico-thyroid membrane. — {Sappey.) the skull to the lower border of the cricoid cartilage, a distance of about 12 centimeters. (See Fig. 72.) Its walls are formed mainly by three pairs of muscles — the superior, middle, and inferior constrictors — each consisting of red, striated muscle-fibers, and hence capable of rapid and energetic contractions. Superiorly the pharynx is attached to and supported by the basilar process of the occipital bone; inferiorly it becomes continuous with the esophagus. The anterior wall of the pharynx is imperfect and presents openings which DIGESTION. i6s communicate with the nasal chambers, the mouth, and the larynx. The lateral wall on either side presents the opening of the Eustachian tube which leads directly into the cavity of the middle ear. The interior of the pharynx is lined by mucous membrane. The pharynx is partially separated from the mouth by the velum pendulum palati, a muscular structure attached above to the hard palate; its lower edge or border is directed downward and backward and presents in the middle line a conical process, the uvula. On either side the palate presents two curved arches, the anterior and posterior, formed re- spectively by the palato-glossei and palato-pharyngei muscles. The laryngeal orifice or glottis is placed just beneath the base of the tongue. It is triangular in shape, wide in front, narrow behind, and directed downward and backward. It is bounded above by a thin plate of cartilage, the epiglottis, placed just behind the tongue and so arranged that it can easily be depressed and elevated. The esophagus, the continuation of the deglutitory canal, extends downward from the lower border of the cricoid cartilage for a dis- tance of from 22 to 25 centimeters, to a point opposite the ninth thoracic vertebra, where it expands into the stomach. Its walls are composed of an internal or mucous and an external or muscular coat, united by areolar tissue. The muscular coat consists of an external layer of longitudinal fibers arranged in three bands and of an internal layer composed of fibers arranged circularly in the upper part and obliquely in the lower part of the esophagus. In the upper third the fibers are striated; in the middle third they are a mixture of both striated and non-striated; in the lower third they are entirely non- striated. The muscle fibers surrounding the esophago-gastric orifice are arranged in the form of and play the part of a sphincter muscle, and for this reason may be termed the sphincter cardim muscle. By its action it prevents a return under normal conditions of food into the esophagus. The deglutitive act may be for convenience divided into three stages, viz.: 1. The passage of the food from the mouth into the pharynx. 2. The passage of the food through the pharynx into the esophagus. 3. The passage of the food through the esophagus into the stomach. In the first stage the bolus of food is placed on the superior surface of the tongue. The mouth is then closed and respiration is momen- tarily suspended. The tip of the tongue is placed against the pos- terior surfaces of the teeth. The tongue, because of its intrinsic musculature, then arches from before backward against the roof of the mouth and pushes the bolus of food through the isthmus of the fauces into the pharynx. This completes the first stage. It is a voluntary effort and accomplished partly by the tongue, though, as shown by Meltzer, mainly by the mylohyoid muscles. The second and third stages, or the passage of the food through i66 TEXT-BOOK OF PHYSIOLOGY. the pharynx and esophagus into the stomach, have been attributed until quite recently entirely to peristaltic movements of their muscu- lature.* It has been stated that with the passage of the food through the isthmus of the fauces the posterior wall of the pharynx advances and seizes the food, and in consequence of a rapid peristaltic move- ment running through its constrictor muscles from above downward is transferred to the esophagus; that with the entrance of the food into the esophagus a similar peristalsis, varying in rapidity in different sections in consequence of a change in the character of its muscula- ture, gradually transfers the food into the stomach. There can be but slight doubt that by this method the bolus of food, especially if it is of firm consistence and of a size sufficient to distend the esoph- agus, is transferred into the stomach, but that it is the exceptional rather than the usual method has been demonstrated by Kronecker, Falk, and Meltzer. In 1880 the first of these experimenters made the observation that the sensation in the stomach following the swallowing of a mouthful of cold water occurred too quickly to be explained by the prevalent belief that its transference was caused by ordinary peristalsis, the rate of progression of which was known to be slow. Falk then discovered the fact, by introducing through the mouth into the pharynx a tube connected externally with a water manometer, that during the act of swallowing there is a sudden rise of pressure equal to about twenty centimeters of water. These experiments demonstrated that at the beginning of degluti- tion there is a sudden rise of pressure, the result of a quickly acting force resident in the mouth or pharynx, in consequence of which the food is rapidly thrown down into the stomach, peristalsis playing no part in the process. The proof, however, of these statements was furnished by Meltzer. This observer introduced into the pharynx and esophagus rubber tubes, the ends of which were provided with thin-walled rubber balloons which could be distended with air. The outer ends of the tubes were connected with Marey's recording tam- bours. Any compression of the balloon would be followed by the passage of the air into the tambour and an elevation of the lever. With one balloon in the pharynx and the other in the esophagus at varying depths, and the recording levers of the tambours applied against the surface of a revolving cylinder, it became possible, with the addition of a chronogram, to obtain a graphic representation of the time relations of simultaneous and successive compressions of the two balloons. It was found as the result of many experiments that no matter * Peristalsis may be defined as a progressive wave-like movement which passes over different portions of the walls of the alimentary canal. Its effect physiologically is the propulsion of its solid and semisolid contents. It is characterized by a contraction of the muscle-fibers behind the object and an inhibition or relaxation of the muscle-fibers in front of it. (Bayliss and Starling.) DIGESTION. 167 how deep the position of the esophageal balloon, it was compressed simultaneously with the pharyngeal- balloon, as shown by the rise of the levers on swallowing a mouthful of water. The interval of time between the rise of the two levers did not amount to more than the tenth of a second. The inference was that the water was projected or shot down the pharynx and esophagus in this period of time, and in its passage compressed both balloons practically at the same in- stant. The same was found to be true when small masses of more consistent food were swallowed. The curves of the entire deglutitive act recorded by the two levers are, however, different in form. (See Fig. 73.) The pharyngeal curve, I, presents two crests, the first, A, being due to the compression caused by the passage of the bolus, the second, B, due to the compression Tig. 73. — Tracing OF the Act or Deglutition. 1. A indicates the compression of the elastic bag caused by the bolus projected by the contraction of the mylohyoid muscles. B. Contraction of the pharynx. 2. Line marking seconds. 3. Tracing of the bag in the esophagus 12 cm. from the teeth. C. Compression of the bag by the bolus corre- sponding to A. D. Compression by the residues of the bolus carried on by the contraction of the pharynx, B. E. Contraction of the esophagus. — (Landois and Stirling.) exerted by the contraction of the pharyngeal muscles. The interval of time between these two crests amounts to not more than 0.3 second. In the esophageal curve, 3, the elevation, C, corresponds to the eleva- tion, A, and is likewise due to the compression exerted by the bolus. The interval of time between the beginning of the first and second curves was not more than o.i second, regardless of the depth to which the esophageal balloon was plunged. At a later period a second rise of the lever was recorded; the time of its appearance, height, duration, etc., were found to increase with the depth of the balloon. These facts demonstrate that deglutition consists of two phases: (i) a rapid rise of pressure in the pharynx, as a result of which the bolus is suddenly shot down to the lower end of the esophagus; (2) a peristaltic contraction of the musculature of the canal, which, acting as a supiplementary force, carries onward any particles of food in the canal and forces the bolus through the closed sphincter cardies at the end of the esophagus. i68 TEXT-BOOK OF PHYSIOLOGY. The immediate cause of the sudden rise of pressure was shown by Meltzer to be the contraction of the mylohyoid muscles. When the nerves going to these muscles were divided in a dog, deglutition was practically abolished. These muscles are probably assisted in their action by the contraction of the hyoglossus muscles as well as the tongue itself. It was also demonstrated in these experiments that the contrac- tion of the esophagus did not partake of the character of ordinary peristalsis. It was found that the esophagus contracted in three distinct segments, corresponding in all probability to the difference in the character of their muscular fibers. The first segment, about six centimeters in length, was found to begin to contract about 1.2 seconds after the beginning of the first curve and lasting 2 seconds ; the second segment, about twelve centimeters in length, beginning to contract about 1.8 seconds or 3 seconds after the beginning of the first section, and lasting for from 5 to 7 seconds; the third segment, six centimeters in length, contracting from 6 to 7 seconds. The beginning and the end of the contraction for each segment occurred simultaneously throughout its entire extent. If, however, a series of deglutitory acts follow each other in quick succession, there is an inhibition of the peristaltic contractions until after the final swallow. An examination of the action of the esophagus during degluti- tion, made by Cannon and Moser with x-rays and the fluoroscope, disclosed the fact that the method of food transmission varied in different animals. In the cat and dog the transmission was effected by peristalsis alone. The time required for the food to reach the stomach varied in the cat from nine to twelve seconds and in the dog from four to five seconds. The descent of the bolus was more rapid in the upper than in the lower part of the esophagus. In man, liquids descended rapidly, at the rate of several feet a second, in con- sequence of the rapid and energetic contraction of the mylohyoid mus- cles. A peristaltic contraction, passing over the entire esophagus, was necessary to the passage of solid and semisolid food through it, Closure of the Posterior Nares and Larynx. — Notwithstand- ing the rise of pressure in the pharynx during the act of swallowing, it is seldom under normal circumstances that any portion of the bolus ever finds its way either into the larynx or nasal chambers, for the reason that .the openings of these cavities are fully closed by appropriate means. At the moment the food passes into the pharynx the posterior nasal openings are closed against the entrance of the food by a septum formed by the pendulous veil of the palate and the posterior half arches. The palate is drawn upward and backward until it meets the posterior wall of the pharynx, and at the same time is made tense, by the action of the levator palati and tensor palati muscles respec- tively (Fig. 74). This septum is completed by the advance . toward the middle line of the posterior half arches caused by the contraction DIGESTION. 169 of the muscles which compose them — the palato-pharyngei. When these structures are impaired in their functional activity, as in diph- theritic paralysis and ulcerations, there is not infrequently a regurgita- tion of food, especially liquids, into the nose. The larynx is equally protected against the entrance of food during deglutition under normal circumstances. That this accident occasion- ally happens, giving rise to severe spasmodic coughing, and even in extreme cases to suffocation, is abundantly shown by the records of clinical medicine. Usually it does not occur, for the following rea- sons : Just preceding and during the act of deglutition there is a com- plete suspension of the act of inspiration by which particles of food might otherwise be drawn into the larynx; at the same time the larynx is always drawn well up under the base of the tongue and its entrance closed by the downward and backward move- ment of the epiglottis. The action here attributed to the epiglottis has been denied by Stuart and McCormick. These observers had the oppor- tunity of looking into a naso- pharynx which had been laid open by a surgical operation for the removal of a morbid growth. In this patient, the epiglottis, at the time of deglutition, was always more or less erect and closely applied to the base of the tongue. So complete was this that the food passed over its posterior or inferior surface for a certain distance it ever observed to fold backward like a lid. Because of the possibility that this position of the epiglottis was due to pathologic causes, Kanthack and Anderson instituted a new series of experiments with a view of determining the action of the epiglottis. As a result of many experiments on animals and of ob- servations on themselves, these observers reaffirm the generally accepted view, that under normal conditions, the entrance of the larynx is always closed by the epiglottis after the manner of a lid. In addition to the downward and backward movement of the epi- glottis and the ascent of the larynx under the base of the tongue, it is also certain from the observations of Meltzer that the larynx is protected from the entrance of food, in the rabbit at least, by the closure of the glottis itself. This experimenter noticed, while observing the interior Fig. 74. — Diagram Showing the Man- ner OF Closure of the Posterior Nares AND Larynx during Deglutition. — {Lan- dois and Stirling.) In no instance was I70 TEXT-BOOK OF PHYSIOLOGY. of the larynx, both from above, through an opening in the hyothyreoid membrane, and from below, through an opening in the trachea that when an act of deglutition was excited by touching the soft palate with, a sound, there was simultaneously with the contraction of the mylo- hyoid muscles, a firm closure of the glottis. This was accomplished by an approximation of the true vocal bands, a close approximation and a downward and forward movement of the arytenoid cartilages, until they almost touched the anterior wall of the thyreoid cartilage. This movement preceded the ascent of the larynx. When the larynx was separated from all surrounding structures with the exception of the laryngeal nerves, a touch of the palate excited the same phenomena. Under such circumstances the closure of the glottis must have been due to the contraction of its own intrinsic muscles and in consequence of a reflex action through the inferior laryngeal nerves. The Nerve Mechanism of Deglutition. — Deglutition is almost exclusively a reflex act throughout its entire extent, and requires for its inauguration merely a stimulus to some portion of the mucous membrane of the deglutitory canal. The first stage is primarily voluntary, but from inattention to the process may become secon- darily reflex. The origin and course of the afferent nerves, stimu- lation of which excite reflexly the movements of the pharynx and esophagus, however, are practically unknown. In the rabbit deg- lutition can be excited by stimulating the anterior central part of the soft palate; in man it has not yet been possible to locate an area stimulation of which will give rise to a reflex deglutitory act. Though electric stimulation of the superior laryngeal nerve will cause reflex deglutitory movements, it is obvious that the terminals of this nerve can not be the source of the natural afferent impulses. Stimulation of the glossopharyngeal nerve causes an inhibition of the movements. The center from which emanate nerve impulses- which excite the various .muscles to action has been located experimentally in the medulla oblongata just above the alae cinerese. The efferent nerves comprise branches of the facial, hypoglossal, motor filaments of the third division of the fifth nerve, motor filaments of the glossopharyn- geal and vagus derived in all probability directly from the medulla oblongata. Inasmuch as the different mechanisms of this reflex, act not only in a coordinate but sequential manner, it would appear as if the deglutition center sent out, in response to the nerve impulses com- ing from a single peripheral area, a series of nerve impulses successively to succeeding portions of the canal, through the groups of nerve-ceUs corresponding to the origins of the efferent nerves. That this orderly and progressive peristalsis usually observed is due to a sequence of changes in the central nerve system is shown by the fact, that if the esophagus is divided or a ring of it excised, the extremity in connection with the stomach will exhibit a well-marked peristalsis after a short interval, when an act of deglutition is excited in the customary manner. The efferent nerve fibers, which stimulate the esophageal muscles to DIGESTION. 171 action are contained in the trunk of the vagi nerves for after their division the peristalsis is abolished. In addition to this primary reflex mechanism, the esophagus appears to possess a secondary reflex mechanism consisting of a series of reflex arcs, vsfhose afferent and efferent paths are found in the trunk of the vagus and both connected with successive portions of the esophagus. The first mechanism is temporarily suspended during deep anesthesia while the second persists. (Meltzer.) Though the peristalsis of the esophagus is excited by nerve impulses coming through the vagus nerves and is abolished by their division, Cannon has shown by means of the Rontgen rays that this effect for the lower portion of the esophagus, at least in the cat and monkey, is of a temporary duration only, lasting from one to several days, after which a peristalsis again develops and of sufficient vigor to force food through the cardiac orifice into the stomach. The muscle coat of this portion of the esophagus is composed of non-striated muscle-fibers, is supplied with a myenteric nerve plexus and resembles lower portions of the alimentary canal. It is capable of developing a peristalsis, solely in response to the pressure of food within and independent of extrinsic nerves. GASTRIC DIGESTION. After the food has passed through the esophagus it is received by the stomach, where it is retained for a variable length of time, during which important changes are induced in its physical and chemic com- position. The disintegration of the food inaugurated by mastication and insalivation is still further carried on in the stomach by the sol- vent action of the acid fluid there present, until the entire mass is reduced to a liquid or semiliquid condition. The stomach is dilated and highly specialized portion of the alimentary canal intervening between the esophagus and small intes- tine. When moderately distended with food, it is somewhat conical or pyriform in shape and slightly curved on itself. It is situated obliquely and in some individuals almost vertically in the upper part of the abdominal cavity, extending from the left hypochondrium to the right of the epigastrium. The dimensions and capacity of the stomach undergo considerable periodic variation according to the extent to which it is distended by food. In the average condition it measures in its long diameter from 25 to 35 centimeters, in its vertical diameter at the cardia 15 centimeters, in its antero-posterior diameter from II to 12 centimeters. The capacity of the stomach varies from 1500 to 1700 c.c. In the empty condition its walls are con- tracted and partly in contact, and the entire organ is dravm up into the upper part of the abdominal cavity. The opening through which the food passes into the stomach is known as the esoph- ago-gastric orifice or the cardia. The opening through which it 172 TEXT-BOOK OF PHYSIOLOGY. passes into the intestine is known as the pylorus, the pyloric or gastro- duodenal orifice. Between these two orifices the stomach along its upper border presents a curve and along its lower border a much larger curve, known as the lesser and greater curvatures respectively. The left end of the stomach is termed the fundus or cardiac end; the right, the pyloric end. Passing from the fundus toward the pylorus, the stomach gradually narrows, and at a point situated about 5 cm. from the pyloric opening it frequently presents a constriction which divides the general cavity into two portions: viz., the fundus and the antrum of the pylorus. The walls of the stomach are formed by four distinct coats united Fig. 75- — Fibeks Seen with, the Stomach Everted. 1,1. Esophagus. 2. Circular fibers at the esophageal opening. 3, 3. Circular fibers at the lesser curvature. 4, 4. Circular fibers at the pylorus. 5, 5, 6, 7, 8. Oblique fibers. 9, 10. Fibers of this layer covering the greater pouch. 11. Portion of the stomach from which these fibers have been removed to show the subjacent circular fibers. — (Sappey.) by areolar tissue and named, from without inward, as the serous, muscle-, submucous, and mucous. The external or serous coat is thin and transparent and formed by a reduplication of the general peritoneal membrane. The middle or muscle-coat consists of three layers of non-striated muscle-fibers, named from their direction the longitudinal, circular, and oblique (Fig. 75). The longitudinal fibers are most abundant along the lesser curvature and are a continuation of those of the esophagus; over the remainder of the stomach they are thinly scat- tered, but toward the pyloric orifice they are more numerous and form a tolerably thick layer which becomes continuous with the fibers of the small intestine. The circular fibers form a complete DIGESTION. 173 ' Epithelium. Mucosa. layer encircling the entire organ, with the exception, perhaps, of a portion of the fundus. The fibers of this coat cross the longitudinal fibers at right angles. At the lower end of the esophagus and sur- rounding the cardia the circular muscle fibers form a true sphincter which is known as sphincter cardicB. At the junction of the fundus with the pyloric antrum the circular fibers are arranged in a well-de- fined bundle termed the sphincter antri pylorici. In the pyloric region the circular fibers are more closely arranged, forming thick well- defined rings. At the pyloric opening the circular fibers are again crowded together and form a distinct muscle band — ^the sphincter pylori — which projects for some distance into the interior of the stomach. It has been stated by Riidinger that the inner fibers of the lon- gitudinal coat become con- nected with this circular band and constitute a dis- tinct muscle, the dilatator pylori. The oblique fibers are most distinct over the cardiac portion of the stomach, but extend from left to right as far as the junction of the middle and last thirds of the stomach. They are continuations of the circular fibers of the esophagus. The submucous coat con- sists of loose areolar tissue carrying blood-vessels, nerves, and lymphatics. It serves to unite the muscle to the mucous coat. Its inner surface bears a thin layer of muscular tissue, the muscularis mucosa, which supports the mucous membranes. The internal or mucous coat is loosely attached to the muscular coat. In the empty and contracted state of the stomach it is thrown into longitudinal folds, or rugae, which are, however, obliterated when the organ is distended with food. The mucous membrane in adult life is smooth and velvety in appearance, gray in color, and covered with a layer of mucus. Its average thickness is about one millimeter. The surface of the membrane is covered with a layer of columnar epithelial cells, each of which possesses a nucleus and nucleolus. At the pylorus there is a circular involution of the mucous membrane Muscu- laris. Fig. 76. — ^Transverse Section of the Wall OF A Human Stomach. _X iS- The tunica pro- pria contains glands standing so close together that its tissue is visible only at the base of the glands toward the muscularis mucosae. — {Stohr.) 174 TEXT-BOOK OF PHYSIOLOGY. which is known as the pyloric valve. This is strengthened by fibrous tissue and embraced by the sphincter muscle previously described. Gastric Glands. — The surface of the mucous membrane when examined with a low magnifying power presents throughout innumer- able depressions polygonal in shape and separated by slightly elevated ridges. At the bottom of these spaces are to be seen small orifices, which are the mouths of the glands embedded in the mucous mem- brane. A vertical section of the gastric walls (Fig. 76) shows not only the position and appearance of the glands, but the relation of the various tissues which enter into the formation of these walls. An ex- amination of the mucous membrane in different regions of the stomach reveals two distinct types of glands, cardiac or fundic, and pyloric, which d-i Fig. 77. — Peptic Gland from Stomach of Dog. a. Wide mouth and duct which receive the terminal divisions of the gland, b, c. Neck and fundus of the tubes, e. Central or chief cells, i. Parietal or acid cells. — {AJter Piersol.) ^ Lumen. Secretory capillaries. Fig. 78. — Section of Fundus Gland OF Mouse. Left upper half drawn after an alcohol preparation, right upper half after a Golgi preparation. The entire lower portion is a diagrammatic combi- nation of both preparations. — (Stohr.) differ not only in histologic structure, but also in function. Both types extend through the entire thickness of the mucosa. The cardiac or fundic glands are formed by an involution of the basement membrane of the mucosa and lined by epithelial cells. Each gland may be said to consist of a short duct, or neck, and a body, or fundus (Fig. 77). The latter portion is wavy or tortuous and fre- quently subdivided into as many as 8 to 10 distinct and separate tubules. The duct is lined by columnar epithelial cells similar to DIGESTION. 175 those covering the surface of the mucosa. The lumen of the fundus is bordered by epithelial cells, cuboid in shape, and consisting of a granular protoplasm containing a distinct spherical nucleus. These cells are generally spoken of as the chief or central cells. In addition to the chief cells, the fundus contains a second variety of cell, which is of a larger size, of a triangular or oval shape, and consisting of a finely granular protoplasm. From their situation in and just beneath the gland wall they have been termed parietal or border cells. Each parie- tal cell appears to be surrounded and penetrated by a system of pas- sages which open into the lumen of the gland by means of a delicate cleft or canaliculus (Fig. 78). Glands with these histologic features are most abundant in the middle zone of the stomach. The pyloric glands are also formed by an involution of the mucous membrane and lined by epithelial cells (Fig. 79). The ducts are much longer than the ducts of the fundic glands. At its ex- tremity each duct becomes branched, giving rise to a number, from 2 to 16, of short tubes, each of which has a large lumen and communicates with the duct by a narrow short neck. The ducts are lined throughout by columnar epithelium. According to Mall, the total number of openings on the surface of the mucous membrane of the dog's stomach is some- what over 1,000,000, and the total num- ber of blind tubes opposite the muscularis mucosa exceeds 16,500,000. According to Sappey, the surface of the mucous membrane of the human stomach pre- sents over 5,000,000 orifices of gastric glands. Blood-vessels, Nerves, and Lymphatics. — The blood-vessels of the stomach after entering the mucosa break up into a number of branches which are distributed to the muscular and mucous coats. The branches to the latter soon form a capillary network with oblong meshes which not only -surround the tubules but form a network just beneath the surface of the mucosa. Veins gradually arise from the capillaries which empty into the larger veins of the mucosa. The glands are also supported by processes of smooth muscle-fibers passing up from the muscularis mucosa. The nerve-fibers distributed to the stomach are derived from the vagus and the sympathetic branches of the solar plexus. After pierc- ing the serous coat the fibers form or unite with a plexus of fibers sit- uated between the circular and longitudinal layers of the muscle-coat. At the nodal points of this plexus large nerve-ganglion cells are to be Fig. 79. — Section of Pylo- ric Glands from Human Stom- ach, u. Mouth of gland leading into long, wide duct (i), into which open the terminal divisions. c. Connective tissue of the mu- cosa. — {After Piersol.) 176 TEXT-BOOK OF PHYSIOLOGY. found, the whole forming the mechanism known as Auerbach's plexus. A similar plexus of cells and fibers in more or less intimate anatomic connection with the foregoing is found between the muscle and submucous coats, and is known as Meissner's plexus. From this plexus fine nerve filaments are distributed to muscle-fibers, blood-vessels, and glands. In the latter structure terminal arbori- zations have been detected in close contact with the secreting cells themselves. The lymphatics, which are quite numerous, originate in the meshes of the mucosa. The larger trunks enter lymph-glands lying along the greater and lesser curvatures of the stomach. Gastric Fistulas. — The general process of digestion, as it takes place in the stomach, has been studied in human beings and animals with a fistula in the walls of the stomach and abdomen, the result either of accident or of necessary surgical or experimental procedures. The earliest observations on gastric digestion were made by Dr. Beaumont on Alexis St. Martin, who, as the result of a gunshot wound, was left with a perAianent fistulous opening into the fundus of the stomach. This opening two years after the accident was about two and a half inches in circumference and usually closed from within by a fold of mucous membrane which prevented the escape of the food. This valve could be readily displaced by the finger and the interior of the stomach exposed to view. After the complete recovery of St. Martin, Dr. Beaumont during the years between 1825 and 1831 at intervals made numerous experiments on the nature of gastric diges- tion. As the result of an admirable series of investigations it was established that the digestion of the food is largely a chemic act, due to the presence of an acid fluid secreted by the mucous membrane; that this fluid is secreted most abundantly after the introduction of food into the stomach; that different articles of food possess varying degrees of digestibility; that the duration of digestion varies according to the nature of the food, exercise, mental states, etc., and that the process is aided by continuous movements of the muscular walls. Since Dr. Beaumont's time the establishing of a gastric fistula in human beings has been necessitated by pathologic conditions of the esophagus. After recovery these cases offered fair facilities for the study of the process when the food was introduced through the opening. Similar fistulae have been established in both carnivorous and her- bivorous animals with a view of studying the process as it takes place in them. The results obtained in these instances in many respects corroborate those obtained by Dr. Beaumont, though many new facts, unobserved by him, have been brought to light. Much additional information as to the mode of secretion and the characteristics of the gastric juice has been obtained, since the intro- duction of two new procedures by Pawlow. The first consists in establishing a gastric fistula and subsequently dividing the esophagus in the neck, and then so adjusting the divided ends that they heal DIGESTION. 177 separately into an angle of the $kin incision. The second procedure consists in forming a diverticulum or pouch out of the cardiac end of the stomach which opens on the surface of the abdomen but is sepa- rated from the rest of the stomach by a thin septum formed of two layers of mucous membrane. (Fig. 80.) The serous and muscle-coats of this pouch are in direct continuity with the large stomach and all possess the same vascular and nerve connections. Because of this fact this miniature stomach, about one-tenth the size of the natural stomach, exhibits the same phenomena, so far as the secretion of the gastric juice is concerned, that the large stomach does. The phenomena which are observed in it may be taken as an indication as to the phenomena which are taking place in the natural stomach. By the first procedure it is possible to feed an animal with different kinds of food and to observe the effects of psychic states on the secretion of gas- tric juice. As the swallowed food is discharged from the lower end of the divided esophagus the appetite con- tinues, and hence the animal will eat for several hours. By the second procedure it is possible to collect gas- tric juice from the miniature stomach and to study the effects on its quantity and quality produced by psychic states, mastication, difiFerent articles of food, and by the process of digestion itself as it goes on in the large stomach. In both instances the juice is obtained free from admixture with saliva or food. Gastric Juice. — The gastric juice obtained from the human stomach free from mucus and other impurities is a clear, colorless fluid with a constant acid reaction, a slightly saline and acid taste, and a specific gravity varying from 1.002 to 1.005. The juice ob- tained from the dog's stomach possesses essentially the same char- acteristics, though its acidity as well as its specific gravity are slightly greater. When kept from atmospheric influences, it resists putre- factive change for a long period of time, undergoes no apparent change in composition, and loses none of its digestive power. It will also prevent and even arrest putrefactive change in organic matter. The chemic composition of the gastric juice has never been satisfactorily determined, owing to the fact that the secretion as obtained from fistulous openings has not been absolutely normal. The following analyses represent the composition of a sample obtained by Schmidt from the stomach of a woman who had a fistula, but who Fig. 80. — Diagram Showing THE Relation of the Nattjrai- Stomach to the Miniature Stomach or Pouch made Ac- cording to the Procedure De- vised BY Pawlow. V. The nat- ural stomach. S. The minature stomach, e, e. The septum formed by the mucous membrane. A, A. The abdominal walls. 178 TEXT-BOOK OF PHYSIOLOGY. was nevertheless in good health; also the composition of the juice from a dog: COMPOSITION OF GASTRIC JUICE. Human. Dog. "Water, 994-40 973-°6 Organic matter, 3-i9 i7-i3 Hydrochloric acid, . , .• o-20 ? 3.34 Calcium chlorid, 0.06 0.26 Sodium chlorid, 1.46 2.50 Potassiuni chlorid, 0.55 1.12 Calcium phosphate 1 i-73 Magnesium " \ 0.12 0.23 Ferric " J o-o8 Ammonium chlorid, 0.47 The organic matter present in gastric juice is a mixture of mucin and a proteid, products of the metabolic activity of the epithelial cells on the surface of the mucous membrane and of the chief or central cells of the gastric glands respectively. Associated with the proteid material are two ferment or enzyme bodies, termed pepsin and rennin. As is the case with other enzymes, their true chemic nature is practically unknown. Pepsin, though present in gastric juice, is not present as such in the chief cells of the glands, but is derived from a zymogen, propepsin or pepsinogen, when the latter is treated with hydrochloric acid. This antecedent compound is related to the granules observed in and produced by the cell protoplasm during the period of rest. Though pepsin is largely produced by the central cells of the fundic glands, it is also produced, though in less amount, by the cells of the pyloric glands. Pepsin is the chief proteolytic agent of the gastric juice and exerts its influence most" energetically in the presence of hydrochloric acid and at a temperature of about 40° C. Other acids — e. g., phos- phoric, nitric, lactic, etc. — are also capable of exciting it to activity, though with less intensity. Rennin or pexin is present in the gastric juice not only of man and all the mammalia, but also of birds and even fish. In its origin from a zymogen substance, in its relation to an acid medium and an optimum temperature, it bears a close resemblance to pepsin. Its specific action is the coagulation of milk, a condition due to the cleavage of caseinogen into a solid flaky body, casein, and a soluble, albumin. Hydrochloric acid is the agent which gives to the gastric juice its normal acidity. Though the juice frequently contains lactic, acetic, and even phosphoric acids, it is generally believed that they are the result of fermentation changes occurring in the food, the result of bacterial action. The percentage of hydrochloric acid has been the subject of much discussion. The analysis of human gastric juice made by Schmidt shows a percentage of 0.02, while that of the dog is 0.34. It is probable, however, that the low percentage of HCl in human gastric juice was due to the admixture with saliva. At present DIGESTION. 179 it is believed from analyses made for clinical purposes that the acid is present to the extent of at least 0.2 per cent. This degree of acidity is not constant during the entire process of digestion. In the earlier as well as in the later stages it is much less. The immediate origin of the hydrochloric acid is difficult of ex- planation. That it is derived, however, primarily from the chlorids of the food and secondarily from the blood-plasma has been established by direct experiment. If all the chlorids be removed from the food and all chlorids be withdrawn from the animal tissue by the adminis- tration of various diuretics^ — e. g., potassium nitrate — there will be a total disappearance of hydrochloric acid from the stomach. On the addition of sodium or potassium chlorids to the food, there is at once a reappearance of the acid. As to the nature of the process by which the acid is formed, noth- ing definite is known. Various theories of a chemic and physical character have been offered, all of which are more or less unsatis- factory. As no hydrochloric acid is found either in the blood or lymph, the most plausible view as to its origin is that which regards it as one of the products of the metabolism of the gland-cells, and more particularly of the parietal or border cells, and which for this reason have been termed acid-producing or oxyntic cells. From the chlorids furnished by the blood the chlorin is derived, which, uniting with hydrogen, forms the HCl. The base set free returns to the blood, which in part accounts for its increased alkalinity during digestion as well as the diminished acidity of the urine. The acid thus formed passes through the canaliculi, which penetrate and surround the cells, into the lumen of the gland. Hydrochloric acid exerts its influence in a variety of ways. It is the main agent in the derivation of pepsin and rennin or pexin from their antecedent zymogen compounds, pepsinogen and pexinogen (Warren) ; it imparts activity to these ferments; it prevents and even arrests fer- mentative and putrefactive changes in the food by destroying micro- organisms; it softens connective tissue, it dissolves proteids and acid- ifies the proteids, thus making possible the subsequent action of pepsin. The inorganic salts of the gastric juice are probably only inci- dental and play no part in the digestive process. Mode of Secretion. — The observations of Dr. Beaumont and the experiments of many physiologists have made it certain that the secretion of the gastric juice is intermittent and not continuous, that it is only on the introduction and digestion of the food that the normal amount is poured out. During the intervals of digestive activity the stomach is practically free from all traces of the juice. The mucous membrane is pale and covered with a layer of mucus having an alka- line or neutral reaction. The introduction, however, of small por- tions of food or irritation with a glass rod causes a change in the ap- pearance of the mucous membrane. At the points of irritation the membrane becomes red and va?cular and in a few minutes small i8o TEXT-BOOK OF PHYSIOLOGY. drops of a secretion make their appearance; these coalesce and run down the sides of the stomach. The statements of Beaumont and many subsequent investigators that the secretion thus obtained is gastric juice have been apparently disproved by Pawlow, who asserts that it is only an alkaline mucous the function of which is protective in character. Mechanic stimula- tion is incapable of exciting the secretion. The primary stimulus to gastric secretion, according to Pawlow, is a psychic state induced, on the one hand, by the sight or the odor of food especially if the animal is hungry and the food appetizing; and on the other hand by the mastication of food which is agreeable to the animal. Thus when a dog was tempted by the sight of food, the secretion made its appearance at the end of six minutes and during the time of the experiment, which lasted for an hour and a half, 80 cubic centimeters of the juice were obtained. This is known as psychic or appetite juice. The character of a psychic state, however, greatly influences the amount of the juice secreted. Agreeable emo- tions increase, depressing emotions inhibit it. Again when a dog with a divided esophagus and a gastric fistula was subjected to sham feeding, mastication continued for five or six hours during which time 700 cubic centimeters of juice were obtained from the stomach. Similar results have been obtained in human beings with an occluded esophagus and a gastric fistula. It is evident from these facts that the secretion of gastric juice is favorably influenced by the sight and odor of appetizing food, by exhilarating emotional states and thorough mastication. Though the secretion of the gastric juice can be initiated by these means, the amount secreted is but small compared with the quantity secreted after digestion has begun. Then it is that the blood-vessels dilate to their full capacity and furnish for several hours the requisite materials for the production of the juice on a relatively large scale. That some factor is active and keeping up the secretion in the large stomach, is apparent from the increase in the quantity and the change in the quality of the juice secreted by the miniature stomach. This secondary stimulus to the gastric secretion is in all probability chemic in character and developed in the stomach or in its walls during digf s- tive activity, inasmuch as the secretion takes place independent of nerve influences and after division of all afferent and efferent nerves that pass from and to the stomach. On the assumption that this factor might be developed in the walls of the stomach itself , Edkins conducted a series of experiments, the re- sults of which lead to the inference that there is developed in the pyloric mucous membrane, by the action of the first products of digestive activity a chemic agent which, absorbed by the blood, is carried to the glands throughout the stomach and which stimulates their cells in a specific manner. For this reason it has been called the gastric hor- mone or the gastric secretin. Whatever the agent or the mechanism DIGESTION. i8i may be, there is not only an increase in the quantity but a change in the quality of the juice in accordance with the character of the food; in other words, there is an adaptation of the juice to the kind of food to be digested. Thus the protein of bread causes a secretion of five times more pepsin than the same amount of the protein of milk, while the protein of meat causes secretion of 25 per cent, more pepsin than milk. Meat extract and bouillon have a very stimulating effect on the quan- tity of juice produced, while alkalies have an inhibitor effect. Histologic Changes in the Gastric Cells during Secretion.— During the periods of rest and secretor activity the cells of the gas- ...-.pr Fig. 81. — Sections of Deep Ends of Fundus Glands of the Cat in Different ■Secretive Phases. X iooo- — (Bensiey.) 4. From a fasting stomach. The chief cells are filled with large zymogen granules; nuclei near the outer ends of cells. Gentian- violet preparation, b b b. Border cells. B. Six hours after an abundant meal of raw flesh. The chief cells exhibit two zones, the inner occupied by large zymogen granules, the outer by a deeply staining, obscurely fibrillar element, prozymogen; the nuclei lie at the junction of the two zones, bbb. Border cells, pr. Prozymogen. c. Mucin-secreting ■cell, similar to those found in the neck of the gland. Gentian-violet preparation. — {Hemmeter after Bensiey.) trie glands undergo changes in histologic structure which are believed to be connected with the production of the enzymes, pepsin and rennin, and the acid. In the resting period the protoplasm of the chief or central cells of the fundus glands becomes crowded with large and well- defined granules, which during the period of secretory activity largely disappear, so much so, that only the luminal border of the cell is occupied by them, the outer border being clear and hyaline in appear- ance. The parietal cells during rest are large and finely granular, but after secretion they are smaller in size though still granular. (See Fig. 81, ^ and B.) The cells of the pyloric glands, though containing granules, do not show any marked difference between the resting and active condition. According to some observers, they contain pepsinogen; according to others, mucin. The epithelial cells lining the ducts of the pylorus i82 TEXT-BOOK OF PHYSIOLOGY. and fundus glands, if not identical with the epithelial cells on the sur- face of the mucous membrane, pass by transitional forms into them. Among these cells are found many goblet cells which secrete a portion of the mucus found in the stomach and gastric juice. In the period of rest the protoplasm of the epithelial cells absorbs and assimilates from the surrounding lymph-spaces material which eventually makes its reappearance as a product of metabolism in the form of granules and hydrochloric acid. With the onset of digestive activity there is a dilatation of the blood-vessels, an increase in the blood-supply, a stimulation through the nerve-supply of the cells, and an output of a fluid to which the name gastric juice is given. Influence of the Nerve System. — The primary secretion of gas- tric juice is a reflex act and under the control and influence of the central nerve system. Experimental investigations render it probable that the central mechanism is located in the medulla oblongata and that the efferent path lies in the trunk of the vagus nerve. Though this nerve has been the subject of much experimentation, the results which have been obtained have not been uniform. The investigations of Pawlow seem to be the most reliable. He found that after division of the nerve, secretion was arrested, and that stimulation of the per- ipheral ends with induced electric currents at the rate of one or two per second, after a latent period of several minutes' duration caused a flow of gastric juice. This center is excited to activity by nerve im- pulses descending from the cerebrum as shown by the effects which follow the development of psychic states caused by the sight or the odor of food, as well as by the mastication of agreeable food; for with their development there is a dilatation of blood-vessels and a copious discharge of the juice in a few minutes, showing the cooperation of both vaso-motor and secretor nerves. It is uncertain if the medullary center can be influenced by nerve impulses reflected from the stomach. The Physiologic Action of Gastric Juice.— In the study of the physiology of gastric digestion as it takes place under normal con- ditions it is important to bear in mind that the foods introduced into the stomach are heterogeneous compounds consisting of both nutritive and non-nutritive materials, and that before the former can be digested and utilized for nutritive purposes they must be freed from their combinations with the latter. This is accomplished by the solvent action of the gastric juice, which in virtue of the chemic activity of its constituents on proteids, gradually disintegrates the food and reduces it to the liquid or semiliquid condition. The nature of this change and the respective influence which the acid and pepsin exert can be studied with almost any form of proteid. The most suitable form, however, is coagulated fibrin obtained from blood by whipping and thoroughly freed from blood by washing under a stream of water. The chemic features of proteins, as well as the typical forms contained in the different articles of food, have been considered in connection with the chemic composition of the body DIGESTION. 183 and the composition of foods (see pages 16 and 139). For purposes of experimentation artificial gastric juice may be employed. This is as effective as the normal secretion and in no essential respect differs from it. A glycerin extract of the mucous membrane acidulated with 0.2 per cent, hydrochloric acid is probably the best. If the small pieces of fibrin be suspended in clear gastric juice and kept at a temperature of 104° F. (40° C.) for an hour or two, they will be dissolved and will entirely disappear, giving rise to a slightly opales- cent mixture. In the early stages of the process the fibrin becomes swollen and transparent and partly dissolved. If at this time the solution be carefully neutralized, the dissolved portion can be regained in the form of acid-albumin or syntonin — a fact which indicates that the first effect of the gastric juice is the acidification of the proteids. This having been accomplished, the pepsin becomes operative, and in a varying length of time transforms the acid-albumin into a new form of proteid, termed peptone. This form of proteid differs from all other forms of proteid in being soluble in both' acids- and alkalies and non-coagulable by heat. In the transformation of acid-albumin into peptone it is possible to isolate by the addition of magnesium sulphate and ammonium sulphate intermediate bodies to which the term albumoses or proteoses has been given, and which differ somewhat in their solubility. The proteoses are termed, from the order in which they make their appearance, primary and secondary. The primary proteoses are precipitated by magnesium sulphate, the secondary by ammonium sulphate. As some of the primary proteoses are soluble in water while others require in addition sodium chlorid for their solu- tion, they have been divided into two groups — viz. : proto- and hetero- albumoses. The secondary proteoses or deutero-albumoses are solu- ble in water. Though in the subjoined scheme two forms of deutero- albumose are represented and two forms of peptone developed out of them, the results of chemic investigation would indicate that there is but one form of deutero-albumose and hence but one form of peptone. This supposed change produced by gastric juice is represented by the following scheme: Albumin Acid-albumin Proto-albumose = (p . ) = Hetero-albumose Deutero-albumose( p,f,teo^ \\/ ^-^ --- colorless protoplasmic KZ/ O? stroma and a coloring- a. b. matter which difiuses into the surrounding ^~^ ^-TK ^ (^ liquid. The presence of ^^ ^^ e~\ the former can be dem- (^ ^1 ^ ^ onstrated by the addi- Oj '• ^^ tion of iodin, which im- ^ ^^ parts to it a faint yellow <^- ?* 'J . 3 m ^jK'ifei ^"•.^' ' '%^' :A ••■sf. "•loV »*• ••ro-'i.V -f '.*. fe &■-•.•• •■■■-# \, ,/ \ / l& ^ v'^-i '»(U»:*' '■%. The Leukocytes. (2-16, TriaciJ Slain: 17-36. £osm ami Mctliyh-HC-bhie.) (E. F. Faber, iVc.) (From DaCosta's "Clinical Hcmaiology.") THE BLOOD. 257 at the opposite end of the tube which renders the divergent rays of light parallel. These parallel rays subsequently fall on the prism, by which they are dispersed and directed into the tube, A, which is nothing more than a small telescope. On looking into it at the ocular end the spectral colors are seen. If the light has been derived from the sun, the spectrum will present vertical dark lines, the so-called Fraunhofer's lines. They are given from A to F in Fig. 117. If a colored medium be held in front of the slit so that the light has to pass Fig. 116. — The Specteoscope. A. Telescope. B. Tube for the admission of light and carrying the collimator. C. Tube containing a scsJe, the image of which when illuminated is reflected above the spectrum. D. The fluid examined.^(ioKdoii and Stirling.) through it first, certain dark bands will appear in the spectrum, owing to the absorption of certain rays. Dilute solutions of arterial blood show two absorption bands between the Fraunhofer lines, D and E, in the green and yellow portion of the spectrum. (See Fig. 117.) The band nearest D frequently designated as alpha is dark in the center and sharply defined. The band which lies toward E is broader and less sharply defined. As the amount of light absorbed varies with the concentration of the solution as well as its thickness, and gives rise to absorption bands of different widths and intensities, it becomes necessary, in order to obtain the characteristic bands, to employ only dilute solutions. The absorption spectra, as seen with different strengths of solution one centimeter thick, are shown graphically in Fig. 118. It will be observed that solutions varying in strength from o.i per cent, to 0.6 17 2s8 TEXT-BOOK OF PHYSIOLOGY. per cent, give rise to the two characteristic bands, but with gradually increasing breadths. With a percentage greater than 0.65 per cent, the light between D and E, the yellow-green, becomes extinguished and the two bands fuse together, forming a single band overlapping slightly the lines D and E. At the same time there is a progressive darkening of the violet end of the spectrum. At 0.85 per cent., all the light is absorbed with the exception of a small amount of the red. Yellow, Cyan Blue, Oxy hemoglobin 0.8%. Fig. 117. — Spectra of Hemoglobin and Some of its Compounds. — (Landois and Stirling.) Solutions less than o.oi per cent, to 0.003 P^"^ cent, show but a single absorption band — that nearest D. A solution of venous blood or of reduced hemoglobin shows but a single absorption band (see Fig. 117), frequently designated as gamma, broader and less marked between the lines D and E, but extending slightly beyond D. Fig. 117 shows in the same graphic manner the increasing breadth of the absorption band with increasing strengths of solution, as well as the simultaneous absorption of light at both the red and violet ends of the spectrum. Compounds of Hemoglobin. — The coloring-matter of the blood is characterized by the property of combining with and of again yielding up oxygen. The union is a chemic one, taking place under certain pressure conditions. It therefore may exist in two states of oxidation, distinguished by a difference in color and their absorption spectra. If hemoglobin either in blood or in solution be shaken with air, it at once combines with oxygen and is converted into oxyhemo- globin, which imparts to the blood or solution a bright red or scarlet color. If the blood or solution be now deprived of oxygen, the oxy- THE BLOOD. 2S9 hemoglobin is converted into reduced hemoglobin, which imparts to the blood or solution a dark bluish or purple color. The quantity of oxygen absorbed by i gram of hemoglobin is estimated at 1.56 c.c. measured at 0° C. and 760 mm. of mercury. The compound formed by the union of oxygen and hemoglobin is a very feeble one; for when the pressure is lowered the union becomes less stable, and as the zero point is approached, as in the Torricellian vacuum, a rapid dissociation of the oxygen takes place. This, how- ever, is not due entirely to a fall of pressure but partly to the dis- sociating force of heat, which increases in power as the pressure falls. Fig. 118. — Graphic Representa- tion OP THE Absorption of LigSt in A Spectrum by Solutions of Oxy- hemoglobin of Different Strengths. The shading indicates the amount of absorption of the spectrum, and the numbers at the side the strength of the solution. aCB b Bb F Fig. 119. — Graphic Representation of the Absorption op Light in a Spectrum by Solutions of Hemoglobin op Different Strengths. The shad- ing indicates the amount of absorption of the spectrum, and the numbers at the side the strength of the solution. The same dissociation of oxygen can be brought about by passing through blood indifferent gases, such as hydrogen, nitrogen, carbon dioxid, which lower oxygen pressure, or by the addition of reducing agents, such as ammonium sulphid or Stokes' fluid. These experimental determinations of the relation of oxygen to hemoglobin partly explain the oxidation and deoxidation of the hemoglobin in the lungs and tissues.. As the blood passes through the lungs and is subjected to the oxygen pressure there, the hemoglo- bin combines with a definite quantity of oxygen, and on emerging from the lungs exhibits a bright red or scarlet color; as the blood passes through the systemic capillaries where the oxygen pressure in the surrounding tissues is low, the oxyhemoglobin yields up a por- tion of its oxygen, becoming deoxidized or reduced, and the blood on emerging from the tissues exhibits a dark bluish color. The portion of oxygen given up to the tissues is termed respiratory oxygen. In 100 parts of arterial blood the coloring-matter presents itself almost exclusively in the form of oxyhemoglobin. In passing through the 26o TEXT-BOOK OF PHYSIOLOGY. capillaries about 5 per cent, only gives up its oxygen and becomes reduced, so that both kinds are present in venous blood. In asphyx- iated blood only reduced hemoglobin is present. It is this capa- bility of combining with and of again yielding up oxygen, that enables hemoglobin to become the carrier of oxygen from the lungs to the tissues. Carbon Monozid Hemoglobin. — Carbon monoxid is a con- stituent of both coal-gas and water-gas. From either source it is likely to accumulate in the air, and if inspired for any length of time produces a series of effects which may eventuate in death. If blood be brought into contact with this gas, it assumes a bright cherry-red color, which is quite persistent and due to the displacement of the loosely combined oxygen and the union of the carbon monoxid with the hemoglobin. The compound thus formed is very stable and resists the action of various reducing agents. The passage of air or of some neutral gas through the blood for a long period of time will gradually displace the carbon monoxid and enable the hemoglobin to again absorb oxygen. It is for this reason that partial poisoning with the gas is not fatal. It is to its power of displacing oxygen and form- ing a stable compound with hemoglobin and thus interfering with its respiratory function that carbon monoxid owes its poisonous properties. Examined spectroscopically, solutions of carbon monoxid hemoglobin exhibit two absorption bands closely resembling in position and extent those of oxyhemoglobin; but. careful examination shows that they are slightly nearer the violet end of the spectrum and closer together. (See Fig. 117.) A useful test for CO blood is the addition of caustic soda, which produces a cinnabar red precipitate. Methemoglobin. — This is a pigment, closely related to oxy- hemoglobin, found in the blood after the administration of various drugs, in cysts and in the urine in hematuria and hemoglobinuria. It is also produced when a solution of hemoglobin is exposed to the air and becomes acid in reaction and brown in color. The spectrum shows two absorption bands similar to oxyhemoglobin, but in addition a new and quite distinct band near the line C, in the red. If the acid solution be rendered alkaline by the addition of ammonia, this band disappears and another makes its appearance near the line D. The addition of ammonium sulphid develops reduced hemoglobin, which, on the absorption of oxygen, produces again oxyhemoglobin, as shown by the spectroscope. Hematin. — Boiling hemoglobin or adding to it acids or alka- lies decomposes it and develops one or more proteid bodies to which the general term globulin has been given, and an iron-holding pig- ment termed hematin. This is regarded as aii oxidation product of hemoglobin and constitutes about 4 per cent, of its composition. When obtained in a pure state, it is a non-crystallizable blue-black powder with a metallic luster. According as it is treated with acids or alkalies, two forms of hematin can be obtained (acid and alkaline), THE BLOOD. 261 each of which has special properties, giving rise to different absorp- tion bands. Hemochromogen. — This pigment is derived from hemoglobin, of which it constitutes about 96 per cent., during decomposition in the absence of oxygen. In solution it produces a purple color, but soon absorbs oxygen and is converted into hematin. Hemin. — This pigment is a derivative of hematin, presenting itself in the form of microscopic rhombic plates or rods (Teichmann's crystals), which are so characteristic as to serve as tests for blood- stains in medicolegal inquiries. These crystals are readily obtained by adding to a small quantity of dried blood on a glass slide a few drops of glacial acetic acid and a crystal of sodium chlorid; after heating gently for a few minutes over a spirit lamp and then allowing the mixture to cool, crystallization of the hemin soon takes place. Hematoidin. — This term has been applied to a pigment which occurs in the form of yellow crystals in old blood-clots or in blood which has been extravasated into the tissues. In its chemic com- position and in its reactions it closely resembles bilirubin, the pigment of the bile, exhibiting the same characteristic play of colors on the addition of nitric acid. The Stroma. — The stroma of the red corpuscles obtained by the methods which dissolve out the hemoglobin has been shown by analysis to consist of from 60 to 70 per cent, of water and 40 to 30 per cent, of solid material, containing a proteid resembling cell-globulin, lecithin, cholesterin, and inorganic salts, among which potassium phosphate is especially abundant. HISTOLOGY OF THE WHITE CORPUSCLES OR LEUKOCYTES. The presence of white corpuscles in the blood can be readily observed under the same conditions as the red corpuscles are observed. Thus when the mesentery of the frog or the guinea-pig is examined with the microscope the white corpuscles are seen adhering to the walls of the blood-vessels; in a drop of freshly drawn blood they are found in the spaces between red' corpuscles (Fig. 104). Shape and Size. — In the resting condition, whether seen in the vessel or on the stage of the microscope, the white corpuscle, as its name implies, is grayish in color, round or globular in form, though often presenting a more or less irregular surface. Its diameter varies from 0.0004 to 0.0013 mm., though the average is about o.ooii mm. or about ^s^oo inch. Structure. — A typical white corpuscle consists of a ground- substance uniformly transparent and apparently homogeneous, in \yhich are embedded a number of granules of varying size, some of which are very fine, while others are larger. By various reagents it has been demonstrated that the granules are fatty, proteid, and carbohydrate (glycogen) in character. In the fresh cells the existence 262 TEXT-BOOK OF PHYSIOLOGY. of a nucleus is difficult of detection, though its presence can be demon- strated by the addition of acetic acid, which renders the perinuclear cytoplasm more transparent and makes the nucleus conspicuous and sharply defined. From its structure it is apparent that the white corpuscle belongs to the group of undifferentiated tissues and re- sembles the cells of the embryo in its earliest stages as well as the unicellular organism, the amoeba. Chemic Composition. — The chemic composition of the white corpuscles has been inferred from an analysis of pus-corpuscles, with which they are practically identical, and of lymph-corpuscles from the lymph-glands. Of the corpuscle about 90 per cent, is water and the remainder solid matter consisting mainly of proteids, of which nuclein, nucleo-albumin, and cell globulin are the most abundant. The two former are characterized by the presence of a considerable quantity of phosphorus, amounting to as much as 10 per cent. Lecithin, fat, glycogen, and earthy tod alkaline phosphates are also present. Number of White Corpuscles. — The number of white cor- puscles per cubic millimeter of blood is much less than the number of red corpuscles, the ratio being in the neighborhood of i white to 700 red. This ratio, however, varies within wide limits in different portions of the body and under normal variations in physiologic conditions. In the blood of the splenic artery there is but i white to 2260 red, while in the splenic vein there is i white to every 60 red; or about thirty-eight times as many as in the artery. In the portal vein there is i white to 740 red, while in the hepatic vein there is i white to 170 red. The total number of white corpuscles per cubic millimeter has been estimated at from 5000 to 10,000, though the average is about 7500. The number, however, is influenced by a variety of physio- logic conditions. The ingestion of food rich in proteid material raises the count from 30 to 40 per cent., as compared with the count before the meal. Fasting for a few days always lowers the count, and in a case of total abstinence of food for a week, reported by Luciani, the count fell to 861 per cubic millimeter, after which it rose to 1530, where it practically remained for the succeeding three weeks of the fasting period. In the new-born the number is greater than in adults — 17,000 to 20,000 per cubic millimeter. Cabot states that 30,000 is never a high count after a meal in infants under two years. In the later months of pregnancy, especially in primiparas, the number in- creases to 16,000 to 18,000. Many pathologic conditions of the body also influence the count very considerably. The method for counting the white corpuscles is similar to that used in counting the red. The given volume of blood should, however, be diluted with 10 or 20 volumes of a one per cent, solution of acetic acid, which disintegrates the red corpuscles and thus facilitates the counting of the white. The pipette should have a larger bore' than THE BLOOD. 263 that used for the red, and a much greater number of squares in the counting chamber should be counted, so as to diminish the percent- age of error. Physiologic Properties. — The white corpuscles possess the characteristic property of exhibiting movements similar to those observed in the amoeba, and are therefore termed amoeboid. These movements consist in alternate protrusions and retractions of portions of the cell body, as a result of which they exhibit a great variety of forms. (See Fig. 120.) The protruded process can also attach itself to some point of the surface, on which it rests, and then draw the body of the corpuscle after it. By a repetition of this process the corpuscle can slowly creep about and change its position in space. In virtue of these ameboid movements the corpuscle can appropri- ate small particles of pig- ment, such as indigo or carmine, and after a short time eliminate them from various parts of the surface. It is also capable of thrusting a process into and through the wall of the capillary vessel, after which the remainder of the corpuscle follows (Fig. 121). This continues until the corpuscle is outside the vessel and in the lymph-space, where it resumes its original shape and movement. This process is best observed in inflammatory conditions, when the blood. has come to rest and the vessels are occluded with both red and white corpuscles. To this passage of the white blood-corpuscles' through the capillary wall the term diapedesis is given. The movements of the white corpuscles are increased by a rise in temperature up to 40° C, beyond which they cease, owing to the coagulation of the cell-substance. A low temperature also arrests the movements. Induced electric cur- rents also cause contraction and death of the cell. Moisture and oxygen are necessary to their activity. From their similarity to lower organisms the white corpuscles may be regarded as independent organisms living in the animal fluids, just as the amoeba lives in its natural liquid medium. Varieties of Leukocytes. — A detailed study of the blood with the aid of the triacid staining fluid of Ehrlich or any of the various eosin and methylene-blue stains, reveals the presence of five distinct varieties of leukocytes and transitional forms which may be classified as follows : Fig. 126- OF Shape Bachman.) -Human Leukocytes Showing Changes Due to Amceboid Movements. (G. 264 TEXT-BOOK OF PHYSIOLOGY. I. 2. Small lymphocytes, so called from their resemblance to the cor- puscles of the lymph-glands, consisting of a deeply staining and relatively large round nucleus, encircled by a narrow rim of cytoplasm. Found in from 20 to 25 per cent, of all leukocytes. Large lymphocytes or hyaline cells, which are believed by some to represent the preceding type at a later stage of development, by others to have an independent origin, are distinguished by a round or ovoid nucleus staining faintly and surrounded by a relatively larger layer of cytoplasm than is seen in the small lymphocyte. The large lymphocyte is present to the extent of from 4 to 8 per cent. The transitional forms are very much like the large lymphocyte in appear- ance and size, with the exception, how- ever, that they possess a crescentic or in- dented nucleus having a somewhat greater affinity for basic dyes. They are usually counted with the large lymphocytes. Both varieties of lymphocytes are char- acterized by a cytoplasm which is devoid of granules. Rarely, basophilic granules may be present. Polymorphonuclear leukocytes or neutro- philes. The nucleus of this cell is irreg- ular and assumes a great variety of shapes in different cells, a feature which has suggested the name given to the cell. The perinuclear cytiDplasm contains a large number of fine granules which are neutrophilic or faintly acidophilic in their staining reaction. They make up about 60 to 70 per cent, of the whole number of the white blood-cells. Eosinophile cells. The nucleus resembles in many respects that of the preceding variety; it is, however, less apt to stain so deeply. It is also very irregular in shape and many cells possess several apparently distinct nuclei. The cytoplasm is ill- defined but its presence is easily revealed through the large, in- tensely acidophilic granules which it possesses. It is present to the extent of 0.5 to 2 per cent. Basophile cells, the nucleus of which is round or slightly irregular. The granules which may be large or small are basophilic and stain more deeply than the nucleus, though they have the same color. It is rare for this cell to be present above 0.5 per cent, of all leukocytes. Fig. 121. — Small Ves- sel SHOWING Various Stages in the Diapedesis OF Leukocytes. {G. Bach- man.) THE BLOOD. 265 Origin of the White Corpuscles. — Of the various theories ad- vanced to explain the origin of leukocytes, that formulated by Ehrlich has found the most credence. According to this theory the leukocytes may genetically be classed into two groups. In the first group are the large and small lymphocytes which take their origin entirely from the lymph-adenoid tissues of the body, e.^., the lymph-glands, solitary and agminated follicles of the intestines, etc. As the lymph flows through these structures the lymph-corpuscles, as the future lymph- ocytes of the blood are called in these situations, are washed out and carried by way of the lymph-stream into the general circulation. In the second group are the transitional forms, the polymorpho- nuclear, eosinophile and basophile leukocytes which originate from the bone-marrow only. The immediate ancestors of these cells are known as myelocytes and are normally found in the red bone-marrow. These cells, through transitional stages, assume the characteristics of the leukocytes just mentioned and pass directly into the capillaries of the marrow whence they are distributed throughout the body. Several attempts have been made by different investigators to trace all varieties of leukocytes to a common mother cell. While this is believed to take place during embryonal life, the proofs of such an origin of leukocytes in the normal adult are insufficient and uncon- vincing. After an unknown period of life the leukocytes undergo dissolution and disappear. Functions. — The functions of the white corpuscles are but im- perfectly known, and at present no positive statements can be made. It has been suggested that wherever found in the body, whether in blood or tissues, they are engaged in the removal of more or less in- soluble particles of disintegrated tissues, in attacking and destroying more or less effectively various forms of invading bacteria and thus protecting the body against their deleterious activity. This they do by surrounding, enveloping, and incorporating either the tissue par- ticle or bacterium and digesting it. On account of this swallowing action these cells were termed by Metchnikoff phagocytes and the process phagocytosis. The cells engaged in this process are the polymorphonuclear leukocytes and the large and the small lymphocytes. He regards them as the general scavengers of the body. It has been suggested that they are also engaged in the absorption of fat from the lymphoid tissue of the intestine. In their dissolution they con- tribute to the blood-plasma certain proteid materials which assist under favorable circumstances in the coagulation of the blood. HISTOLOGY OF THE BLOOD-PLATES. The blood-plates or plaques are small histologic elements circu- lating in the blood-plasma. They were discovered by Hayem, who applied to them the term hematoblasts, on the supposition that they 266 TEXT-BOOK OF PHYSIOLOGY. were the early stages in the development of the red corpuscles. This is now known to be erroneous. On account of their specific, distinct characters, and their constant presence in the blood of living animals (guinea-pig and bat), they are now regarded as normal constituents of the blood and designated as the third corpuscle. When blood is freshly drawn from the body, the plaques rapidly undergo disintegra- tion and disappear; but by treating the blood with osmic acid, the form and structure of the plaque may be retained. The blood-plaque may be defined as a colorless, grayish-white, homogeneous or finely granular protoplasmic disk, varying in diam- eter from 1.5 to 3.5 micro- millimeters. The edges are rounded and well defined, but it is not certain whether they are only flattened or are slightly biconcave. There is, however, no nucleus. The ratio of the plaques to the red corpuscles is i to 18 or 20, and the total number per cubic millimeter has been estimated to be 250,000 to 300,000. When blood is shed they tend to adhere to each other and form irregular masses known as Schultze's granular masses. If threads are suspended in blood, the plaques accumulate in enormous numbers upon them and appear to form a center from which fibrin filaments radiate as coagulation proceeds. The white thrombi which form in blood-vessels in consequence of diseased states — e. g., endocarditis, atheromatous ulceration, etc.^-are composed very largely of blood- plaques and fibrin threads. The function of the blood-plaques is unknown, but it has been surmised that in some way they are, like the leukocytes, concerned in the coagulation of the blood. When- ever they are diminished in number, as in purpura and hemophilia, coagulation takes place very slowly. The blood-plaques can be seen with high powers of the micro- scope in the blood-vessels of the omentum of the guinea-pig and rat, especially when the blood-stream begins to slow. They are also readily seen in the blood-vessels of subcutaneous connective tissue of various animals, and especially in that of the new-born rat. A small quantity of this tissue moistened with normal saline and exam- ined microscopically with suitable powers will show large numbers of plaques within the blood-vessels. THE TOTAL QUANTITY OF THE BLOOD; ITS GENERAL COMPOSITION. The determination of the total quantity of the blood in an animal is best made by the chromometric method, somewhat modified at present, of Welcker. This consists, first, in bleeding an animal, collecting all the blood it yields, and weighing it; second, in washing out the vessels with a normal saline solution until the fluid comes from the veins clear and free from blood; third, in mincing the tissues of the body, after removal of the contents of the alimentary canal, soaking them in water for twenty-four hours, and then expressing them. All the washings are collected and weighed. A given volume THE BLOOD. 267 of the normal defibrinated blood, treated with carbon monoxid so as to give it uniform color, is then diluted with water until its tint is identical with that of the washings similarly treated with carbon monoxid. From the quantity of water necessary to dilute the blood the quantity of blood in the washings is readily determined. The animal having been previously weighed and the weight of the contents of the alimentary canal deducted, the ratio of the total weight of the blood to the weight of the body at once becomes apparent. By this method it has been shown that the ratio of blood to body-weight in a human adult is 1:13; in an infant, 1:19; in a dog, 1:13; in a cat, 1:21. Thus an adult man of 75 kilos weight would have 5769 grams of blood. The amount of blood in the different organs has been determined by ligating the blood-vessels in the living animal, removing the organ, and after allowing the blood to escape subjecting the tissues to the chromometric methods described above. According to Ranke, the volume of the blood is distributed as follows: Heart, lungs, arteries, and veins, ^; liver, |; muscles, J; other organs, |. General Composition. — The results of the analyses of the blood will vary with the animal and the methods employed. The following table, taken from Gad, shows the average composition, expressed in whole numbers, of horse's blood. In essential respects the ratio of the constituents in human blood would not be materially different. One thousand parts of blood contain: Water, 200 200 Cells, 328 i Solids, 128 Hemoglobin, 116 Other organic matter, 10 Salts, 2 { Water, 604 604 Fibrin, 7 Albumin, 52 Fat, I Other organic matter, 3 Potassium and sodium salts, 4 Calcium and magnesium salts, i Plasma, 672 i l- Solids, 68 ■ CHEMISTRY OF COAGULATION. The changes which eventuate in the formation of fibrin, and hence all the subsequent phenomena of coagulation, are chemic in character; but as these changes take place in organic compounds the composition of which is but imperfectly known, the intimate nature of the process is quite obscure. All the theories which have been advanced in explanation, though approximating the truth, are more or less incomplete and in some respects contradictory. Since the coagulation is coincident with the appearance of the fibrin, the ante- cedents of this substance, the physical and chemic conditions which condition its development, and the succession of chemic changes in- volved must be determined, before any consistent theory can be established. 268 TEXT-BOOK OF PHYSIOLOGY. Extra-vascular Coagulation.— At present it is generally be- lieved that the immediate factors concerned in extra-vascular coagu- lation are jSbrinogen, a calcium salt, and a ferment-body. As to the manner in which these three bodies react one with another there is a diversity of opinion. At least five different theories are current at the present time, all of which have some features in common, though presenting points of difference. Alexander Schmidt long contended that fibrin was the result of a union of fibrinogen and paraglobulin; that the union was brought about by a ferment-body; that the presence of the neutral salts of the plasma was necessary to the activity of the ferment. Previous to his death in 1893 Schmidt rnodified his view as follows: The insoluble fibrin is developed out of a soluble fibrin derived from paraglobulin, which in turn is a product of general cell disintegration; the conver- sion of the fibrinogen into fibrin is due to the activity of a ferment, thrombin, a derivative of pro-thrombin, a product of the disintegra- tion of leukocytes, lymph-cells, etc.; that the production of thrombin is conditioned by the presence of the neutral salts of the plasma in normal percentages; that no one of these salts, calcium included, acts in a specific manner; finally, that fibrin is not a compound of a pro- teid and calcium. Hammersten, as a result of many years of investigation, believes that paraglobulin is not necessary to the process, fibrinogen alone being transformed into fibrin under the influence of the ferment, in the presence of a neutral salt, especially calcium, which acts specifically in a manner different from the sodium salts. Inasmuch as the quan- tity of fibrin produced is always less than the quantity of fibrinogen previously present, Hammersten concludes that the latter substance, under the influence of the ferment, undergoes a cleavage into two unequal portions, one of which remains in solution, the other solidify- ing as fibrin. While admitting that the calcium salts act specifically, he believes that they are concerned rather with the production of the ferment than the fibrin, for if the ferment is present in sufficient quantity coagulation takes place in a typical manner even in the total absence of calcium. Arthus and Pages conclude that for the transformation of fibrin- ogen into fibrin the calcium salts are absolutely essential and act in a specific manner; that the ferment causes a cleavage of fibrinogen into two substances, one of which remains in solution, the other com- bines with calcium to form fibrin. They offer in support of this view the fact that if a i per cent, solution of potassium oxalate be added to blood in quantity sufficient to precipitate the calcium, coagu- lation will not take place; but if calcium is restored coagulation pro- ceeds in the usual manner. They transfer the sphere of influence of calcium to the formation of the fibrin rather than to the formation of the ferment. Pekelharing's researches led him to the conclusion that there THE BLOOD. 269 arises from the disintegration of the leukocytes a nucleo-proteid, pro-thrombin, which combining with the calcium salt forms the ferment thrombin. This compound then transfers the calcium to the fibrinogen, which in turn becomes fibrin; the latter is therefore a proteid-calcium compound. Lilienfeld asserts that fibrin formation is a cleavage process by which fibrinogen is separated into two bodies, one an albumose which remains in solution, the other a proteid to which he has given the name thromhosin. This cleavage is attributed to the action of the usual ferment, a product of the disintegration of leukocytes. Throm- hosin combines, according to Lilienfeld, with calcium to form fibrin. In a critical examination of these different theories Hammersten denies that fibrin is a compound of a proteid and calcium; for chemic analysis of both fibrinogen and fibrin shows that the former contains as much calcium as the latter, and that therefore the view of coagu- lation according to which fibrinogen unites with calcium to form fibrin is without foundation. On the contrary, he maintains that the specific influence of the calcium is directed toward the production of the ferment, for if this be present in sufficient quantity coagulation takes place in a typical manner, no matter whether the blood has been decalcified by potassium oxalate or not. Intra-vascular Coagulation. — So long as the relations of the blood and the vascular system remain physiologic no coagulation occurs in the vessels. The reason assigned for this is that the fer- ment, though continually being produced, is as rapidly being de- stroyed, and hence never accumulates in amount sufl&cient to develop fibrin. This view is supported by the fact that if a solution of cell- protoplasm, leukocytes, lymph-corpuscles, etc., presumably contain- ing a large amount of the ferment, be injected into the blood-vessels, extensive intra-vascular coagulation promptly follows. It is also believed that the lining of the blood-vessel in some unknown way restrains the coagulation process even though the circulation has come to rest. Under pathologic conditions of the circulatory apparatus, espe- cially of the internal lining, intra-vascular coagulation frequently arises, though the process can not be considered as identical with extra-vascular coagulation. Many pathologists assert that in its origin, mode of formation, and structure the intra-vascular coagulum or thrombus is not a true coagulum as ordinarily understood, but rather a conglutination of blood-plaques and leukocytes. Whenever the integrity of the internal wall of the vessel is impaired by disease or by the introduction of foreign bodies, there is primarily a deposition and accumulation of blood-plaques at the injured area or on the foreign body which constitutes to a large extent the mass of the throm- bus which at once forms. The thrombi which form on the surface of atheromatous ulcers, on the valves of the heart, and- in the veins in consequence of diseased states, on threads or needles passed, through 270 TEXT-BOOK OF PHYSIOLOGY. the vessels, at the orifices of torn blood-vessels, consist largely of blood-plaques. A thrombus so formed may contain a number of delicate fibrin threads, which, however, present a different appearance from the fibrin of the extra-vascular clot. In the thrombi which form around foreign bodies there is always a larger quantity of fibrin than in those originating from causes wholly within the vessel. CHAPTER XIII. THE CIRCULATION OF THE BLOOD. Each organ and tissue of the body is the seat of a more or less active metabolism, the maintenance of which is essential to its physio- logic activity. This metabolism is characterized by the assimilation of food materials and the production of waste products; that it may be maintained it is imperative that there shall be a continuous supply of the former and a continuous removal of the latter. Both condi- tions are subserved by the blood. In order, however, that this fluid may fulfil these functions it must be kept in continuous movement, must flow into and out of the tissues in volumes varying with their activity, under a given pressure and with a certain velocity. The apparatus by which these results are attained is termed the circulatory apparatus. This consists of a central organ, the heart; a series of branching diverging tubes, the arteries; a network of minute passageways with extremely delicate walls, the capillaries; a series of converging tubes, the veins. These structures are so arranged as to form a closed system of vessels within which the blood is kept in continuous movement mainly by the pressure produced by the pumping action of the heart, though aided by other forces. (See Fig. 122.) In this system a particle of blood which passes any given point will eventually return to the same point, no matter how intricate or tortuous the route may be through which it in the meanwhile travels; for this reason the blood is said to move in a circle, and the movement itself is termed the circulation. In order to understand the reasons for the movement of the blood in one direction orily, as well as for many other phenomena connected with the circulation, a knowledge of the structure of the heart and its internal mechanism is of primary importance. THE PHYSIOLOGIC ANATOMY OF THE HEART. The heart is a cone or pyramid-shaped hollow muscular organ situated in the thorax just behind the sternum. The base is directed upward and to the right side; the apex downward and to the left side, extending as far as the space between the cartilages of the fifth and sixth ribs. In this situation the heart is enclosed and suspended in a fibroserous sac, the pericardium, attached to the great vessels at its base. The heart is a hollow, double organ, consisting of a right and a 271 272 TEXT-BOOK OF PHYSIOLOGY. left half, separated by a musculo-membranous septum. The general cavity of each side is subdivided by an incomplete transverse fibrous septum into two smaller cavities, an upper and a lower, known respectively as the auricle and the ventricle. The heart may therefore be said to consist of four cavities, the walls of which are composed of muscle-tissue. Of these four cavities, the right auricle and the right ventricle constitute the venous heart; the left auricle and the left ventricle, the arterial heart. The right auricle is quadrangular in shape and presents on its posterior aspect two large openings, the terminations of the two final trunks o'f the venous system, the superior and inferior vena cava (Fig. 123). Below, the. auricle communicates with the ventricle by a large opening which, from its position, is termed the auriculo-ventricular opening. The walls of the auricle are extremely thin, not measuring more than two millimeters in thickness. The right ventricle, as shown on cross-section, is crescentic in shape owing to the projection of the ventricular sep- tum. It presents at its upper left angle a cone-shaped prolongation, the conus arteriosus. From this prolongation, and continuous with it, arises the pulmonary artery. The wall of the ventricle meas- ures in the middle about four millimeters in thickness. The inner surfaces of the ventricle show: (i) a complicated system of muscle ridges and bands, the columna carnea (fleshy columns), and (2) a set of muscle projections, the musculi papillares (papillary muscles), which arise by a broad base from the walls of the ventricle and project upward toward the auriculo- ventricular opening. From the apex of each papillary muscle there are given off fine tendinous cords, the chorda tendinea, which become attached above to the under surface of the auriculo-ventricular valve. The left auricle, similar in general shape to the right, presents Fig. 122. — Diagram of Cir- culation. I. Heart. ^. Lungs. 3. Head and upper extremities. 4. Spleen. 5. Intestine. 6. Kid- ney. 7. Lower extremities. 8. Liver. — (Dalton.) THE CIRCULATION OF THE BLOOD. 285 while Landois places it as occurring at d, Edgren places it at e, while Marey places it at a point midway between d and e. The Cardiac Cycle. — The entire period of the heart's pulsation may be divided into three phases, viz.: 1. The auricular contraction. 2. The ventricular contraction. 3. The pause or period of repose, during which both auricles and ventricles are at rest. These three phases collectively constitute a cardiac cycle or a cardiac revolution. The duration of a cycle, as well as the duration of each of its three phases, varies in different animals in accordance with the number of cycles which recur in a unit of time. In human beings in adult life there are about 72 cycles to the minute; the average duration therefore is 0.83 second. From this it follows that the time occupied by any one of the three phases must be extremely short and difficult of determination. From observations made on human beings and from experiments on animals the following estimates have been made and accepted as approxi- mately correct: ._/ 01 Q2 qa a-» OS oa qy oBfatti oa r~ 5 >U 4D r IS S3Uf D The Phases of the Heart's Pulsation. The auricular systole, 0.16 soi second. 2. The ventricular systole, 0.32 second. Fig. 133.- 3. The period of rest for both auric- les and ventricles, 0.32 second. The relations of these three phases to one another may be illus- trated by the accompanying diagram (Fig. 133), in which the space 1-2 is the duration of a cardiac cycle divided into eight equal spaces, each of which represents one-tenth of a second. The line A represents the auricular, the line V the ventricular phase. The rise in the line A represents the contraction; the fall and subsequent continuation, the relaxation and pause. The rise in the line V and its continuation represent the contraction; the fall and subsequent continuation, the relaxation and the pause. From this it is apparent that the auric- ular contraction or systole has a brief duration, 0.16 second, while the relaxation or diastole has a long duration, 0.64 second; that the ventricular contraction immediately following the auricular has a duration of 0.32 second, while the relaxation and diastole have a duration of 0.48 second; that the pause of the entire heart, that is, the period between the termination of the ventricular systole and the be- ginning of the next auricular systole, is only 0.32 second. The frequency of the heart-beat varies with a variety of con ditions: e. g., age, sex, posture, exercise, etc. 286 TEXT-BOOK OF PHYSIOLOGY. Age. — The most important normal condition which modifies the activity of the heart is age. Thus: Before birth, the number of beats a minute averages 140 During the first year it diminishes to 128 During the third year it diminishes to 95 From the eighth to the fourteenth year it averages 84 In adult life it averages 72 Sex. — The heart-beat is more rapid in females than in males. Thus while the average beat in males is 72, in females it is usually 8 or 10 beats more. Posture. — Independent of muscle efforts the rate of the beat is influenced by posture. It has been found that when the body is changed from the lying to the sitting and to the standing position, the heart will vary as follows — from 66 to 71 to 81 on the average. Exercise and digestion also temporarily increase the number of beats. A rise in blood-pressure from any cause whatever is usually attended by a decrease, while a fall in blood-pressure is attended by an increase in the rate. The Action of the Valves. — As previously stated, the forward movement of the blood is permitted and regurgitation prevented by the alternate action of the auriculo-ventricular and the semilunar valves. As a point of departure for a consideration of the action of the valves and their relation to the systole and diastole of the heart, the close of the ventricular systole may be conveniently selected. At this moment, if the blood is not to be returned to the ventricles, the semilunar valves must be instantly and completely closed. This is accomplished in the following manner: During the outflow of blood from the ventricles the valves are pushed outward toward the walls of the vessels, though not coming into contact. with them, for behind them are the pouches of Valsalva, containing blood, continuous with and under the same pressure as that in the vessels themselves. With the cessation of the outflow and the beginning of the relaxation the pressure of the blood behind the valves suddenly forces them inward until their free edges, including the lunulse, come into complete appo- sition. By this means the orifices of the pulmonary artery and aorta are securely closed and a return flow prevented. Reversal of the valves is prevented by their mode of attachment to the fibrous rings of the orifices. During the ventricular systole the relaxed auricles have been filling with blood. With the ventricular relaxation this volume, or its equivalent, flows readily into the empty and easily distensible ventricles, its place being taken by an additional volume of blood flowing from the venae cavae and pulmonary veins. .Whether the ventricles exert a suction power at the moment of their relaxation is an undecided question. A steady stream of blood into the auricles and ventricles continues throughout the entire period of rest until both cavities are filled. The tricuspid and bicuspid valves which THE CIRCULATION OF THE BLOOD. 287 hang down into the ventricular cavities are now floated up by cur- rents of blood welling up behind them until they are nearly closed. The auricles now contract, forcing their contained volumes, or at least the larger portions of them, into the ventricles, which become fully distended. With the cessation of the auricular systole the ventricular systole begins. If the blood is not to be returned to the auricles at this moment, the tricuspid and mitral valves must be suddenly and com- pletely closed. This is readily accomplished by reason of the position S.a.-D.v D.a.-S.v. Fig. 134. — Diagrammatic Representation of the Auricular Systole, S.a., WITH the Ventricular Diastole, D.v., and of the Auricular Diastole, D.a., with the Ventricular Systole, S.v. C.s. and C.i. Superior and inferior cavas; A.d. (atrium dex- trum) right auricle; A.s. (atrium sinistrum) left auricle; V.d. (ventriculus dexter) right ven- tricle; V.s. (ventriculus sinister) left ventricle; P. pulmonary artery; A. aorta; P.P. papillary muscles. — {Landois.) of the valves, which have been floated up and placed almost in apposi- tion by the blood itself. With the beginning of the ventricular pressure the blood is forced upward against the valves until their free edges are brought together and the orifices closed. Reversal of these valves is prevented by the contraction and shortening of the papil- lary muscles, which in consequence exert a traction on their under surfaces. The blood now confined in the ventricle between the closed auriculo-ventricular and semilunar valves is subjected to pressure from all sides. As the pressure rises proportionately to the vigor of the contraction, there comes a moment when the intra-ven- tricular pressure exceeds that in the aorta and pulmonary artery. 288 TEXT-BOOK OF PHYSIOLOGY. Immediately the semilunar valves of both vessels are thrown open and the blood discharged. Both contraction and outflow continue until the ventricles are practically empty, after which ventricular relaxa- tion sets in, attended by a rapid fall of pressure. Under the influence of the positive pressiure of the blood in the sinuses of Valsalva the semilimar valves are again closed, the column of blood supported, and regurgitation is prevented. With the accumulation of blood in the auricles the cardiac cycle is completed. The changes in the shape of the heart, the variations in the size of its cavities and in that of the blood-vessels arising from them, the relative position of the valves during systole and diastole are shown in Fig. 134. Relative Functions of Auricles and Ventricles. — ^Though both auricles and ventricles are essential to the continuous movement of blood, they possess imequal values in this respect. The passage of the blood through the pulmonary and systemic vessels is accom- plished by the driving power of the right and left ventricles respec- tively, aided, however, by minor extra-cardiac forces. They may be regarded therefore as force-pumps. If the heart consisted of ventricles only, the flow of blood from the venae cavae and pulmonary veins would be temporarily arrested during their systole and their subsequent refilling delayed. This is obviated, however, by the addition of the auricles; for during the ventricular systole the blood continues to flow into the auricles, in which it is temporarily stored imtil the ventricular relaxation sets in. With this event the accumulated blood passes into the ventricles, which are thus practically filled before the auricular systole occurs by which the filling is completed. By this means there is no delay in .the filling of the ventricles, and hence their effective working as force-pumps is more readily secured. The aiuricles may therefore be regarded as feed-pumps. For this reason it is probable, notwithstanding the contraction of the circular muscle-fibers at the terminations of the venous system, the flow of blood into the aiu"icles is never entirely arrested. Regurgitation in these vessels does not occur for the reason that the pressure in the auricles is not higher than, if as high as, in the great veins. Synchronism of the Two Sides of the Heart. — If the balance of the circulation is to be maintained, the two sides of the heart must act synchronously. That they do so can be shown by attaching levers to their walls, and thus recording their activities. The syn- chronism is so perfect that until recently it was generally believed to be dependent on nerve connections; but Porter has shown that if the ventricles are cut away from the auricles, in which the nerve mechan- ism seemed to lie, the synchronism of the former is not interfered with; that the apical halves of the ventricles will beat synchronously if perfused with blood through an artery; that a very small bridge of muscle-tissue will carry the wave of excitation from one part to neigh- THE CIRCULATION OF THE BLOOD. 289 max valve to manometer mm valva boring parts of the ventricle. It is therefore probable that the syn- chronism is accomplished through muscle connections only. The left ventricle, in keeping with the greater work it has to do, has a greater development than the right, and therefore contracts more energetically. The ratio between the energy of the left and right sides is approximately 3 to i. Intra-cardiac Pressure. ^ — It has been stated that during the pause of the heart when its cavities are filling with blood the semilunar valves are kept closed by the pressure of the blood in the pulmonary artery and aorta, a pressure due to the resistance, as will be explained later, offered to the flow of the blood mainly by the smaller arteries and capillaries; that they are opened only when the pressure of the blood within the ventricle exceeds that in the arteries. It becomes, therefore, a matter of importance to determine the extent of this pressure as well as its variations during the course of a cardiac cycle. This can be done by inserting a long catheter into either the right or left ventricle, through the jugular vein or the carotid artery re- spectively, and connecting its free ex- tremity with a mercurial manometer. By the interposition of a double valve such as represented in Fig. 135, it becomes possible, according to the direction the blood is permitted to flow, to obtain either the maximal or the minimal pressure that occurs in the heart during a series of cycles. Thus Goltz found in the left ventricle of the dog a maximal pressure of 114 to 135 mm.; in the right ventricle, a pressure of 35 to 62 mm. Minimal pressures of —23 to —52 mm. for the left ventricle have also been obtained. The maximal pressure in the ventricles during the systole, though always higher than that in the arteries, is not a fixed or an invariable pressure, as it rises and falls with the latter from moment to moment. Within limits the cardiac power, and therefore the intra-cardiac pressure, is capable of considerable increase. The function of the heart is to drive the blood through the vessels with a given velocity. This is only possible by first overcoming the resistance to the flow offered by the vessels, as indicated by the arterial pressure. As this is a variable factor, rising and falling very considerably at times, the heart must meet and exceed each rise, if the circulation is to be main- tained. The power put forth by the heart is proportional to the work it has to perform. If the arterial pressure continues higher 19 to heart Fig. 135. — V. Frank's Valve. This is placed in the course of the tube between heart and manometer, so that the latter may be used as a maximum, minimum, or ordinary manometer according to the tap which is left open. — (Starling.) 290 TEXT-BOOK OF PHYSIOLOGY. A •H\JW 0/2 wwwwwvwwv/v wvlvwwww J* s than the average for any length of time, the heart meets the condition by an hypertrophy of its walls. The Intra-ventricular Pressure Curve.— An accurate interpre- tation of the play of the heart mechanism necessitates the obtaining of a graphic record of the course of the intra-ventricular pressure, its varia- tions and time relations. With such a record may be compared the records of the pressures in the venae cavas, on the one hand, and in the aorta, on the other hand, and their relations one to another accurately defined. The intra-ventricular pressure has been obtained by specially de- vised manometers or tonometers or tonographs, as they are variously termed, the construction of which is such as to enable them to respond instantly to the very rapid variations of the pressure which occur during the brief cardiac cycle. One of the best is that of Hiirthle. This consists of a small metallic tam- bour s or 6 millimeters in diameter, covered by a thin rubber membrane. A small button resting on the mem- brane plays against an elastic steel spring, by the tension of which the pressure of the blood is counterbal- anced. The movements of the mem- brane are taken up, magnified, and recorded by a suitable lever. A long cannula is inserted into the right ventricle through the jugular vein or into the left ventricle through the carotid artery. Both cannula and tambour are filled with an alkaline solution to prevent coagulation of the blood, and then joined air-tight. The pressure of the blood in the ventricle is thus transmitted by a liquid column to the tambour and to its attached lever. With such a manometer a curve is registered similar to that shown in Fig. 136. To obtain the absolute value of this curve in millimeters of mercury it is necessary to previously graduate the instrument. An examination of the curve shows that previous to the ventricular contraction there is a very slight rise of pressure above that of the atmosphere, repre- sented by the line a — h. This may be due to the inflow of blood from the auricle during the diastole. At o the pressure suddenly rises, passes quickly to its maximum value;, (2), which is maintained with slight variations for -some time, and then suddenly (3) begins to fall, and rapidly reaches the line of atmospheric pressure, or even passes below it, becoming negative in fact for a short period. The curve may also be taken as a record of the ventricular contraction, for there are reasons to believe that the two closely coincide through- out their entire course. A characteristic feature of this curve is the Fig. 136. — V. Curve of the pres- sure in the ventricle of the dog. — (Hiirthle.) A. Curve of the pressure in the aorta. The curves were taken simultaneously, i. Tuning-fork vi- brations, 100 per second. THE CIRCULATION OF THE BLOOD. 291 more or less horizontal portion comprised between the points 2 and 3, marked by several elevations and depressions, which has been termed the systolic plateau. With other forms of elastic manometers, especially those in which the transmission of the intra-ventricular pressure is effected by air or by a combination of air and liquid, this portion of the curve is represented by a single peak, which is taken as an indication that the maximum pressure once reached is not maintained, but immediately begins to fall to its original level, notwithstanding the continued contraction of the ventricle. Those who adhere to this view attribute the plateau to the closure of the orifice of the catheter by the contracting and. ap- proximating walls of the ventricle. There are reasons for believing, however, that the former curve is the more correct representation of the course of the intra-ventricular pressure. Bayliss and Starling photo- graphed on a moving surface the oscillations of a fluid, a solution of sodium sulphate, in a capillary glass tube one end of which was closed, the other end placed in connection with an intra-cardiac catheter, the oscillations representing the variations in pressure. The photogram thus obtained resembles the curve obtained by Hiirthle's membrane manometer. The Relation of the Intra-ventricular Pressure Curve to the Intra-cardiac Mechanisms. — ^By itself the curve of the intra-ventric- ular pressure affords no indication as to events occurring within the heart :_ i. e., as to the times during the systole, of the closure of the auricolo-ventricular valves and the opening of the semilunar valves, or the times during the diastole, of the closure of the semilunar valves and the opening of the auriculo-ventricular valves. By registering the curve of pressure in the aorta simultaneously with the pressure in the left ventricle (Fig. 136), and by comparing these with the curve of the successive differences of pressure in these two cavities as determined by the "differential manometer," it be- comes possible to mark on the ventricular pressure curve the points at which the foregoing events take place. During the systolic plateau the blood is passing from the ventricle into the aorta. Independent of the slight elevations and depressions there is an absolute fall of pressure between the beginning and the end of the plateau. There is also a corresponding fall in the aortic pressure, corresponding to these two points. The curve of the dif- ference of pressure shows, however, that the ventricular pressure is slightly higher than the aortic. This fall in both ventricular and aortic pressures is due to the escape of blood from the arterial into and through the capillary system. At 3, however, whether completely emptied or not, the ventricle suddenly relaxes, and its pressure soon falls below that in the aorta. As soon as this takes place the semi- lunar valves must close, if regurgitation into the ventricular cavity is to be prevented. A comparison of the aortic pressure curve shows a slight notch, the "dicrotic notch," just preceding a slight elevation, 292 TEXT-BOOK OF PHYSIOLOGY. the "dicrotic" wave. This notch is taken as the moment when the semilunar valves close. The corresponding point on the ventricular pressure curve has been placed just where the ordinate 4 cuts the de- scending portion. As yet, however, the pressure is higher in the ven- tricle than in the auricle, and so continues until near the line of atmos- pheric pressure. At this point the pressure in the auricle, due to the accumulation of blood during the ventricular systole, now forces open the mitral valve and the blood flows into the ventricle. The opening of the mitral valve occurs about the point where the ordinate 5 cuts the curve. The ventricular pressure curve affords- but slight, if any, indication of the auricular systole. It apparently does not give rise to any noticeable increase in the ventricular pressure. The slight rise in the pressure curve, which just precedes the abrupt rise due to the ventricular systole, may be taken as an indication of an increasing pressure due to the inflow of blood from the auricle. As soon as the pressure in the ventricle exceeds that in the auricle the mitral valve closes. This is marked on the cinrve where the ordinate cuts it, at o. Coincident with this, the ventricular systole begins, and as the blood is contained within a closed cavity the pressure abruptly rises. A comparison of the aortic curve shows that for a short time during the ventricular systole, the pressure is falling, but at one point it turns at a sharp angle and rapidly rises. This is an indication that the semi- lunar valves are suddenly thrown open and the blood begins to pass into the aorta. This event occurs at a moment marked on the ventric- ular curve by the ordinate i. Beyond this point the pressure continues to rise, for the aortic pressure must not only be exceeded, but a certain velocity must be imparted to the blood. Between the ordinates i and 4, the semilunar valves remain open and the blood passes into the aorta. In accordance with the foregoing The ventricular systole may be subdivided into two periods: 1. The period of rising tension, from the beginning of the systole to the opening of the semilunar valves, occupying from 0.02 to 0.04 second. 2. The period of ejection, from the opening of the semilunar valves to the end of the systole, occupying about 0.2 second. The ventricular diastole may also be divided into two periods: 1. The period of falling tension or relaxation, from the end of the systole to the time of lowest pressure in the ventricle, occupying about 0.05 second. 2. The period of filling, from the opening of the mitral valve to the beginning of the systole. Negative Pressure. — As shown by the ventricular pressure curve there is a moment when the pressure falls below atmospheric pres- sure, becoming negative to it. The extent to which this takes place, its duration and frequency, have never been satisfactorily determined. THE CIRCULATION OF THE BLOOD. 293 The cause of the negative pressure, its influence on the opening of the auriculo-ventricular valves, and on the entrance of blood into the ventricles are equally unknown. The most probable cause is an expansion of the base of the ventricles due to the enlargement of the aorta and pulmonary artery. That it is not due to the expansion of the thorax is evident from the fact that it occurs when the thorax is open and the heart exposed. Heart-sounds. — Two sounds accompany each pulsation of the heart, both of which may be heard by applying the ear or the stetho- scope to the chest-walls, especially over the region of the heart. One of these sounds is low in pitch, dull and prolonged; the other is high in pitch, clear and short. These sounds can be approximately repro- duced by pronouncing the syllables lubb-dupp, lubb-dupp. The long dull sound occurs with the systole, the first phase of a new cardiac cycle, and is therefore termed the -first sound; the short clear sound occurs at the beginning of the diastole, with the second phase of the cardiac cycle, and is therefore termed the second sound. The first sound is the systolic, the second the diastolic, sound. With the ear it can readily be determined that there is a brief pause between the first and second sounds, and a longer pause between the second and the first sounds. The duration of the first sound is almost equal to the diu-ation of the systole — viz., 0.3 second; the duration of the second sound is not more than o.i second. The systoHc sound is heard most distinctly over the body of the heart; the diastolic sound is heard most distinctly in the neighborhood of the third rib to the right of the sternum. The causes of the heart-sounds have enlisted the attention of clinicians and physiologists for years, and many factors have been assigned for their production. At present it is generally believed that the first sound is the product of at least two, possibly three, factors : viz., the contraction of the muscular walls of the ventricles, the simul- taneous closure and subsequent vibration of the tricuspid and mitral valves, and the sudden increase of pressure of the apex of the heart against the chest-wall. That the contraction of the ventricular muscle gives rise to a sound is certain from the fact that it is perceptible in an excised heart when the cavities are free from blood and when the valves are prevented from closing. The explanation of this sound is extremely difficult, as the contraction, though prolonged, is not of the nature of a tetanus and therefore not characterized by rapid variations of tension. The apex element may be eliminated by placing the individual in the recumbent position. The second sound is the product of the simultaneous closure and subsequent vibration of the aortic and pulmonary valves which occurs at the beginning of the ventricular diastole as the blood surges back against the closed valves. This has been definitely proved by the fact that the sound disappears when the valves are destroyed or held back 294 TEXT-BOOK OF PHYSIOLOGY. by hooks introduced into the aorta and pulmonary artery. It is also possible that the vibration of the column of blood produces an addi- tional tone which adds itself to that produced by the valves. The relation of the sounds to the systole and diastole of the heart is represented in Figs. 133 and 137. The Blood-supply to the Heart.^— The nutrition of the heart, its contractility, the force and frequency of the beat, are dependent on and maintained by the introduction of arterialized blood into and the removal of waste products from its tissue. This is accomplished by the coronary arteries, on the one hand, and the coronary veins, on the other. The arteries, two in number, the right and left, arise from the aorta in the pouches of Valsalva just above the right and left semilunar valves. Turning in opposite direc- tions, they ultimately anastomose, forming a circle around the base of the ventricles. From both the right and left artery branches are given ofE which run over the walls of both auricles and ventricles, the most important of which in man are the anterior and posterior inter-ventricular. These main vessels lie in grooves on the surface of the heart beneath Fig. 137. — Scheme of the visceral pericardium, surrounded by con- fnne^r''''d^cre Shows' what nective tissuc and fat. Smallbranches pene- events. occur in the heart, trate the heart-muscle in which they divide and the outer, the relation j^to Capillaries. From the capillary areas of the sounds and silences n • • i • i . i i i to these events. Small veins arise which, passmg backward, converge to form the coronary veins. These follow the course of ihe arteries and finally terminate in the coronary sinus, located in the auriculo-ventricular groove on the posterior surface of the heart. This sinus opens into the right auricle between the opening of the inferior vena cava and the auriculo-ventricular opening. Its orifice is guarded by a valve, which is usually single, though sometimes double. While by far the larger portion of the blood is returned by the coronary veins, it is also certain that some of it is returned by small veins which open into little pits or depressions on the inner surface of the heart-walls, known as the foramina Thebesii. It has, however, been shown by Pratt that these foramina are present not only in the auricular wall, as generally stated, but in the walls of all the cavities. These foramina communicate through a capillary plexus with both arteries and veins, and by special large passages with the veins alone. The period of time in the cardiac cycle during which the coronary (the extra-muj-al) arteries are filled with blood, whether during the systole or the diastole, has been a subject of much discussion. At present, however, as the result of many experiments it is generally believed that they are filled at the time of the systole. A comparison of the tracings of the pulse-wave taken simultaneously in the carotid THE CIRCULATION OF THE BLOOD. 295 and coronary arteries shows that the pressure rises and falls simul- taneously in both vessels; that there is a complete agreement between the two tracings, and as a corollary both vessels are filled during the systole. But because of the pressure which the heart-muscle must exert upon the smaller arteries and veins within its own substance during systole, it is probable that there is a freer circulation in the coronary (the extra-mural) vessels, during the period of diastolic repose. During the systole the intra-mural vessels are compressed and the blood driven out of the capillaries into the veins; during the diastole, these vessels again dilate and permit the blood to re-enter freely, from the extra-mural arteries. The greater the force and frequency of the beat, the greater the volume of blood passing through the coronary system. As stated in a foregoing paragraph the nutrition of the heart- muscle, its irritability and contractility, depend on the blood-supply derived from the coronary vessels. This is shown by the effects which follow its withdrawal. Ligation of both coronary arteries in the dog is followed by a diminution in the force and frequency of the heart-beat, and in a few minutes by complete cessation. Ligation of even a single branch of a coronary artery of the dog heart provided it supply a sufficiently large territory — e. g., the arteria circumflexa — is sufficient to cause arrest in at least 80 per cent, of animals (Porter). With the ligation of this vessel there occurs a gradual diminution in the force and frequency of the systole. As the power of coordinate contraction ceases the heart-muscle frequently exhibits a series of independent contraction of individual fibers and cells known as fibrillary contraction. All the results which follow ligation are to be attributed in the light of experi- ment to the sudden anemia which is thus established. The removal of the ligature and the return of the blood will restore the nutrition and re-establish coordinate contractions. The excised heart of the mammal which has passed into the condition of fibrillary contraction may be again made to beat rhythmically and vigorously, by first cooling it with normal saline, and then perfusing it with warm defibrinated blood through the coronary vessels under a suitable pressure. The same result can be brought about by first perfusing it with a i per cent, solution of potassium chlorid until the heart comes to rest and then perfusing it with Ringer's solution. In frogs and allied animals the heart is nourished by blood flow- ing, during the diastole, from the interior of the heart into a system of irregular channels which penetrate the walls in all directions. With the systole the blood is returned to the cavities. The excised heart of the mammal — e. g., the cat — may be partially nourished in a similar manner through the foramina Thebesii. If the warm defibrinated blood of the same animal be introduced into the ventricle under a pressure of about 75 mm. of blood, the heart will recommence and continue to beat for a period varying from one to several hours. 296 TEXT-BOOK OF PHYSIOLOGY. The Beat of the Excised Heart.— The beat of the heart, its frequency and regularity, its continuance from the early stages of fetal development till death, has long been an interesting subject for physiologic investigation. Though related to the functional activities of the body at large, the activity of the heart is in a sense independent of them, for it will continue for a variable length of time after they have ceased. The heart of the frog and the turtle will continue to beat under appropriate conditions for hours after separa- tion of all its anatomic connections and removal from the body. The heart of the dog or cat will, however, beat but for a few minutes. The human heart would in all probability act in the same way. Never- theless there are good reasons for believing that though the spontaneous beat has ceased, the irritability yet endures though perhaps in lessened degree. For if, after the heart has ceased to beat for some time, warm defibrinated and oxygenated blood be passed through the coronary vessels the beat will reappear and continue at its usual rate for some hours. The reason for the longer continuance of the beat of the excised heart of the cold-blooded animal beyond that of the warm-blooded animal lies probably in the difference in the rate of their respective metabolisms. There is reason to believe that each cell of the heart- muscle, in common with other tissue-cells, during life stores up and holds in reserve a larger quantity of nutritive material than is necessary for its immediate needs. When separated from the general blood- supply, the cells begin to utilize this reserved material. With its consumption the irritability declines and after a variable period of time disappears. As the metabolism is far more rapid in the warm-blooded than in the cold-blooded animal, it is probable that the reserved nutritive material is utilized more quickly in the former than in the latter other conditions being equal. So long as it lasts in either class, the irritability and contractility persist. Whatever the immediate or exciting cause of the heart contraction may be, the fundamental condition for its manifestation is the main- tenance of the irritability. So long as this persists at a sufficiently high level the heart-muscle will contract in response to the appropriate stimulus. PROPERTIES OF THE HEART-MUSCLE. The heart-muscle is characterized by the following properties, viz., irritability, conductivity, rhythmicity, tonicity, and automaticity. I. Irritability. — The heart-muscle in common with other muscles possesses irritability by virtue of which it responds by a change of form to the action of a stimulus. Whatever the stimulus, here, as elsewhere, there is a conversion of potential into kinetic energy — heat, electricity and mechanic motion. The normal physiologic stimulus has not been positively, determined. In THE CIRCULATION OF THE BLOOD. 297 common with other forms of muscle-tissue, the heart-muscle may be made to contract by artificial stimuli — e. g., mechanic, thermic, chemic, and electric. For the demonstration of this fact it is necessary to eliminate the action of the physiologic stimulus and to bring the heart to rest in the condition of diastole. This can be done with the frog heart, by ligating the tissues between the sinu-auricular junction, a procedure which prevents the passage of the contraction wave which originates in the sinus, over the auricles and ventricles (a fact that will be more fully alluded to later). With the heart thus prepared and while still in situ, the apex may be connected with a recording lever and its provoked contractions registered on a recording surface. In this condition it will respond by a contraction to any form of an adequate stimulus and especially to the induced electric current. Experiment has shown that the irritability is most marked in the neighborhood of the venae cavae terminations. It is least marked in the ventricles. In its irritability, contractility, and manner of response to stimuli, the heart of the mammal corresponds in all essential respects to the heart of the frog or turtle. The irritability of the heart-muscle depends primarily on the blood-supply and secondarily on the maintenance of a normal temperature, and so long as- both conditions are maintained the muscle will respond by a contraction to any adequate stimulus, physiologic or artificial. a. The Blood-supply. — The supply of blood to the mammalian heart is derived from the coronary arteries which, though filled during the systole, deliver the blood to the intra-mural arterioles and capillaries during the diastole. The facts relating to the blood-supply have been presented fully in a foregoing paragraph. b. The Influence of Temperature. — ^For the manifestation of the irritability and contractility it is essential that the heart-muscle be kept at a sufficiently high temperature in order that the physio- logic or a given artificial stimulus may provoke a maximal con- traction. This is accomplished by immersing the suspended heart in a bath of Ringer's solution the temperature of which can be readily decreased or increased by appropriate means. The optimum temperature for the frog heart is about 25° C. As the temperature is lowered both rate and force decrease until at about from 4° C. to 0° C. both cease. Beyond 35° C. it also ceases to contract, because of a coagulation of the muscle sub- stance. The mammalian heart attains its maximum activity at a temperature of 37° C. It ceases to beat at about 47° C. on the one hand and at about 17° C. on the other hand. Conductivity. — The heart-muscle possesses conductivity. The excitation process and the subsequent contraction wave, both of which take their rise under physiologic conditions near the venae 29& TEXT-BOOK OF PHYSIOLOGY. cavae terminations, are conducted over the auricles, thence to the ventricles from base to apex. It is now generally believed that the propagation of both processes is accomplished by muscle- tissue alone, independently of the nerve system. The conduc- tivity, however, is not equally well developed in every part of the heart. This is especially true of the tissue at both the sinu- auricular and the auriculo-ventricular junctions. At these points in the frog heart the contraction wave is delayed for an appreciable period, a condition attributed to the embryonic character of the muscle-tissue. In the frog's heart the excitation process begins in the sinus venosus, from which it passes to the auricles, thence to the ventricles. In Fig. 138, which is a graphic record of the heart-beat, the two elevations of the level on the up-stroke, a and b, repre- sent the contraction of the sinus and the auricle respectively, while the two depressions, c and d, indi- cate the delay in the transmission of the contraction wave at the two junctions. There is here an ana- tomic obstacle to the conduction of the contraction wave. This may CoNx^ACT^lN-r °T^H. °FKoo's be artificially increased by com- Heart. pressmg the heart between the au- ricles and ventricles with a clamp. By carefully regulating the pressure it is possible to so block the wave that three or four auricular contractions may occur before the excitation process forces the block and excites a ventricular contraction. (Fig. 139.) If the block is complete, rather than partial, the ventricle will come to rest and so remain. From the foregoing facts it is evident that the physiologic stimulus exerts its action in the sinus venosus and that the auricular and ventricular beats are in turn dependent on it. In the mammalian heart the contraction wave arising at the terminations of, or at a point between the terminations of, the venae cavae, is likewise conducted over the auricles and thence to the ventricles; between the end of the auricular and the beginning of the ventricular contraction there is also a perceptible interval similar to that observed in the frog heart. For a long time this was attributed to an interference with the passage of the contrac- tion wave across the auriculo-ventricular junction because of the extreme scarcity of the muscle-fibers in this region or to their embryonic character. In recent years, however, this view has been abandoned because the real bond of union between the auricular and ventricular tissues, across which the contraction wave passes, has been found, as stated on page 280, in the system of muscle-fibers, described in part by His, Retzer and Braunig THE CIRCULATION OF THE BLOOD. 299 and completed by Tawara and termed by him the conduction system of the heart. This system it is now believed constitutes the anatomic and physiologic path across which the contraction wave passes from auricles to ventricles. The supposition that this was the case was demonstrated by Hering and others who succeeded in dividing the muscle-bundle in the excised hearts of rabbits and dogs, kept actively beating by perfusion with Ringers' solution. On division of the bundle both auricles and ventricles continued to beat though with different rates and in- dependently of each other. These and experiments of a similar character have demonstrated beyond question that the auriculo- ventricular bundle with its widespread ramifications is the true Fig. 139. — Record of the Aueicular and Ventricular Contractions before AND AFTER THE CLOSURE OF THE ClAMP AT ffl. conducting system between auricles and ventricles. The cause assigned by Tawara, for the interval between the auricular and ventricular contraction is not so much the embryonic character of the fibers of the system, as it is the length of the system as a whole, which he estimates at from 4 to 6 centimeters. Because of this fact, the excitation process requires time to pass from the beginning to the ends of the system and to all parts of the ventricles, which time is represented by the inter-systolic period or interval. With the mammalian heart as with the frog heart it is possible to increase the length of the interval between the auricular and the ventricular contraction, the inter-systolic period, by com- pression of the tissues between auricles and ventricles including presumably the central part of the conducting system, the muscle band of His. This has been accomplished in the dog by Erlanger by means of a specially devised hook clamp. When the com- pression is brought about suddenly and completely the ventricles at once cease beating though the auricles continue to beat with their customary rate and regularity. After a variable period of time, varying from a few seconds to 70 seconds, during which the ventricles are relaxed and gradually filling with blood from the Ven. rate per minute. Aur. rate per minute. Max. 69.8 Min. 34.8 Ave. 54.1 216 117.8 162. 1 300 ■ TEXT-BOOK OF PHYSIOLOGY. auricles, the ventricular beat returns, at first slowly but with a gradually increasing frequency until a definite but a comparatively slow rate is attained. In experiments on the dog heart performed by Erlanger the following results were obtained when the auriculoTventricular bundle was completely crushed. A v 3-7 2.4 3-oS The reason assigned for the cessation of the ventricular contrac- tion is the non-arrival of the excitation process at the ventricular end of the conducting system, because of the blocking or com- pression. Under physiologic conditions the ventricular beat is directly dependent on the arrival of the excitation process from the auricles and if it fails to arrive the ventricle does not con- tract for some seconds. The retuirn of the beat during complete blocking is attributed to the development of a hitherto dormant inherent rhythmicity. When this is established both auricles and ventricles continue to beat though with a totally different rhythm. The effects which follow gradual compression of the muscle- bundle are somewhat different from those which follow sudden compression. If the clamp is accurately adjusted and the com- pression gradually applied, the first perceptible effect is a length- ening of the normal pause between the auricular and the ventric- ular contraction. With an increase in the compression there will come a moment when one of the auricular contraction waves fails to reach' the ventricle, or if it does, it is so enfeebled that it is incapable of exciting the ventricle, which in consequence fails to contract. This dropping out of a ventricular contraction may occur once in every 10, 9, 8, 7, 6, etc., auricular beats, in accordance with the degree of compression. With a further tightening of the clamp, the blocking of the excitation process may be still further increased so that only every second, third or fourth auricular beat will be followed by a ventricular beat, es- tablishing what has been termed the 2:1, 3:1, 4:1, rhythms re- spectively; and finally when the blocking is complete no excitation process can reach the ventricle. Owing to the capability of the mammalian ventricle to develop an independent rhythm when not stimulated by the auricles for a few seconds or more, it is not always possible to state at what particular moment in the successive stages of compression the independent ventricular rhythm becomes manifest. Usually when the rhythm is of the 3 : i type, i. e., when the third auricular contraction fails to reach the ventricle, it will begin to beat of itself. Under such circumstances the auricles and ventricles become dissociated even though the block is not quite complete. 1. rate per minute. Aur. rate per minute. 22.4 31.0 79.6 84.6 THE CIRCULATION OF THE BLOOD. 301 These experimental facts have afforded an explanation of the altered rhythm between auricles and ventricles in that pathologic condition known as Stokes-Adams disease. In this disease the rhythm may be any one of the rhythms stated in the foregoing paragraph. In two instances the following ratio of the ventricle to the auricle was observed by Erlanger. A V 3-55 2-73 In a few cases of death from this disease a postmortem ex- amination showed a pathologic lesion of the auriculo-ventricular bundle. 3. Rhythmicity. — The beat of the heart is a uniform movement, occurring at regular intervals. Each phase of each beat occupies a regular measure of time. The beat is therefore rhythmic in char- acter. The heart-muscle as a whole varies in rhythmic power in its different parts. It is best developed in the frog and tortoise, in the sinus venosus, less so in the auricles, least in the ventricles. This may be shown by division of the tissue between sinus and auricles in situ. At once the auriculo-ventricular portion ceases to beat, while the sinus continues contracting as usual. In a short time the auricles and ventricles begin to beat, though less rapidly than formerly. Separation of the auricle from the ven- tricle is again followed by rest. In due time the auricle begins to beat, while the ventricle remains quiescent. If the ventricle be now stimulated in a rhythmic manner, it may resume rhyth- mic activity. These facts are taken as an indication that the rhythmic power is developed in unequal degree in the three divisions of the heart. In the warm-blooded animal, e. g., dog, cat, rabbit, there is also a difference in the rhythmicity between the auricles and ventricles. This is shown by the effects which follow division of the auriculo-ventricular bundle, or sudden and complete com- pression of the heart at the auriculo-ventricular groove. In either case the ventricle for a short time remains at rest, though the auricles continue to beat at their usual rate. After a variable number of seconds the ventricle develops a rhythm of its own, though it never attains that of the auricle. 4. Tonicity. — The heart-muscle, like the vascular muscle, main- tains continuously a certain degree of contraction, termed tone, upon which the efficiency of the heart as a pumping organ is largely dependent. In the physiologic condition the ventricular muscle neither contracts nor relaxes to its fullest possible extent, but maintains an intermediate position between the two extremes and for this reason is said to possess tonicity. This tone may, however, be increased or decreased by the action of various ex- 302 TEXT-BOOK OF PHYSIOLOGY. ternal agents. Thus the passage of dilute solutions of various drugs — e. g., alkalies, digitalis— through the cavities of the excised heart wiU so increase the tone, or the contractile power, that complete relaxation is prevented, until finally the heart comes to a stand- still in the condition of systole. The passage of dilute solutions of lactic acid, muscarine, etc., through the heart will, on the con- trary, so decrease the tone or the contractile power that the nor- mal contraction is not attained. The relaxation therefore gradu- ally increases until the heart finally comes to a standstill in the condition of diastole. In the first instance the tonicity is said to be increased; in the second instance, decreased. 5. Automaticity. — Inasipuch as the heart continues to contract in a perfectly rhythmic manner after removal from the body and apparently without the aid of an external stimulus, it is said that the heart-muscle is automatic or spontaneous in action. Strictly speaking, however, this is not the case, for the reason that all movement, that of the heart included, is the resultant of the action of natural causes though their true nature may be beyond the reach of present methods of investigation. The Nature of the Stimulus. — As the heart continues to beat after removal from the body, it is evident that the stimulus does not originate in the central nerve system but in the heart itself. Two views have been held as to its origin and nature: 1. That it originates in the nerve-cells found in various parts of the heart-muscle; that it is a nerve impulse rhythmically and auto- matically discharged by these cells and transmitted by their axons to the heart-muscle cells. 2. That it originates in the muscle-cells themselves; that it is chemic in character and due to a reaction between the chemic constituents, organic and inorganic, of the muscle-cells and those in the lymph by which they are surrounded. According to the first view the stimulus is neurogenic, according to the second view myogenic, in origin. The presence of nerve-cells; their relation to the muscle-cells; the pronounced rhythmic activity of the sinus and auricles in which the nerve-cells are abundant; the feeble activity of the apex, in which they are wanting — these and other facts lend support to the view that the stimulus originates in the nerve-cells. To them have been attributed the power of automatic activity. The absence of nerve-cells in portions of the heart-muscle, which nevertheless exhibit rhythmic contractions for quite a long period of time; the rhythmic beat of the embryonic heart before the migration of nerve-cells to its walls shows, that the stimulus does not necessarily originate in nerve-cells. Moreover, Porter has conclusively shown that the apex of the dog's heart, which is generally believed to be totally devoid of nerve-cells, can be made to beat for hours by feeding it through its nutrient artery with warm defibrinated blood. Unless THE CIRCULATION OF THE BLOOD. 303 it be assumed that the heart-muscle contracts automatically, without a cause, it is a fair assumption that the exciting cause of the contraction arises within the muscle-cells themselves, and that it is in all proba- bility the outcome of a reaction between the chemic constituents and more especially the inorganic constituents of the blood or lymph on the one hand, and the chemic constituents of the muscle-cells on the other. The discovery that some of the inorganic salts of the blood have a specific physiologic action on the heart-muscle was made in 1882 by Ringer. Since then, many attempts have been made to isolate these constituents, to determine not only their individual, but also their collective action, when combined in proportions approxi- mating those in which they exist in the blood. The Action of Inorganic Salts. — i. On the Frog and Terrapin Heart. — The inorganic salts which are most directly concerned in ex- citing and sustaining the heart-beat are sodium chlorid, calcium phos- phate or chlorid, and potassium chlorid. A combination of these salts in the proportions in which they exist in the blood was first sug- gested by Ringer and is made by saturating a 0.65 per cent, solution of sodium chlorid with calcium phosphate, and then adding to each 100 c.c, 2 c.c. of a I per cent, solution of potassium chlorid. A frog's heart immersed in this solution will continue to beat for some hours. A combination of the chlorids of sodium, calcium, and potassium in amounts which will vary for different animals is equally efficient in maintaining the heart-beat. The collective as well as the individual actions of these salts have been strikingly brought out by the experiments of Profs. Howell and Greene, from whose published results the following statements are derived. Instead of employing the entire heart, they used for various reasons strips from the terminations of the vense cavse and from the ventricle of the terrapin heart. The proportion of the inorganic salts most favorable for the contraction of the vena cava strips is the follow- ing: viz., sodium chlorid, 0.7 per cent.; calcium chlorid, 0.026 per cent.; potassium chlorid, 0.03 per cent. When vena cava strips are immersed in this solution, they begin in a short time to exhibit rhythmic contractions which may continue for several days. In the same strength of solution the ventricular strips remain inactive but if the percentage of the calcium chlorid be raised from 0.026 per cent, to 0.04, or 0.05 per cent., spontaneous contractions soon develop and continue for several days or more. In the foregoing solution when the calcium chlorid is present only to the extent of 0.026 per cent., though the ventricular strip does not contract, it is kept in good condition- for contraction, for even after many hours the raising of the percentage of calcium chlorid to 0.04 or 0.05 per cent, will call forth after a brief latent period, rapid and energetic contractions. From this fact it is inferred that the venae cavse region is more sensitive to the combined action of the salts than is the ventricle. The action of the individual salts is also best shown with ventricular 304 TEXT-BOOK OF PHYSIOLOGY. strips. In a 0.7 per cent, sodium chlorid solution the strip beats rhythmically and energetically, but for a short period and with gradually diminishing force, until it entirely ceases to beat. A reason assigned for this is the removal of other salts necessary to the excitation of the contraction. In a calcium chlorid solution — 0.9 per ceiit. — i. e., isotonic with the sodium chlorid — the heart strip is thrown into strong tone, but does not rhythmically contract. If, however, the strip is placed in normal saline, and calcium chlorid added in amounts equal to that present in the blood, it will after a very short latent period begin to contract rapidly and energetically and for a longer time than when in sodium chlorid solution alone. The contractions not infre- quently occur before relaxation is completed, so that the strip passes into the condition of contracture. In potassium chlorid solutions isotonic — 0.9 per cent. — with sodium chlorid solution the heart strip also fails to contract. This is the case also when the potassium is added to the sodium chlorid in amount practically equal to that found in the blood. 2. On the Mammalian Heart. — The collective action of the in- organic salts on the isolated heart of all members of this class of animals, which have been made the subject of experimentation, is as marked, if not more so, than it is on the heart of the frog or terrapin especially when the coronary blood-vessels are perfused with Ringer's solution or the modification of it suggested by Locke, as follows: NaCl 0.90 per cent.; CaCl^ 0.024 P^r cent.; KCl 0.042 per cent.; NaHCOj 0.02 per cent., dextrose o.i per cent. The reviving and sustaining power of this solution is extraordinary. Locke and Rosenheim were able to revive the isolated heart of a rabbit and to excite it to active contrac- tion, for several hours at a time, on four consecutive days by perfusing it with this solution saturated with oxygen and at a temperature of 35° C. No special precautions were observed other than keeping it cool (10° C.) and moist during the intervals of experimentation. The duration of the irritability and contractility extended over a period of 95 hours. Kuliabko revived the heart of a rabbit for an hour nearly three days after removal from the body of the animal. It was then placed on ice, and after four days it was again revived by per- fusing it with Ringer's solution. Altogether this heart retained its irritability for seven days. Hering revived the heart of a monkey on three different occasions, the first, 4^ hours, the second, 28 hours, and the third 54 hours after the death of the animal. In the intervening periods the heart was also kept on ice. In this animal it was even possible to increase and decrease the activity of the heart by stimula- tion of the nerves which normally control the rate of the beat. Kuliabko was also able to revive the isolated heart of a child 20 hours after death from a double pneumonia. It was made to beat rhythmically at a rate varying from 70 to 80 per minute when the solution had a tem- perature of 39° C, and at a rate of 98 to 102 per minute when it had a temperature of 41° C, though at this temperature the beat became THE CIRCULATION QF THE BLOOD. 305 arrhythmic. All these instances demonstrate the extreme longevity of the irritability of the heart-muscle under appropriate conditions. The action of individual salts has been shown experimentally on the hearts of rabbits, cats, dogs, monkeys, by Gross, Howell and others. Thus it has been found that when an isolated heart is rhythmically beating in response to the perfusion of Ringer's or Locke's solution, the addition of potassium chlorid in small amounts is followed by a decrease in the rate and force of the contraction, and in larger amounts by a complete cessation of the contraction and a standstill in diastole. On the withdrawal of the potassium, the former frequency and vigor are regained. Potassium exerts a depressor or an inhibitor influence on the irritability and contractility of the heart-muscle. Under the same conditions, the addition of calcium chlorid in sufficient amounts is followed by an increase in the rate and in the vigor of the contractions; on its withdrawal both rate and force return to the previous condition. Calcium exerts an accelerator and an aug- mentor influence on the irritability and contractility of the heart. The Cause of the Heart-beat.— From the foregoing facts it seems probable that the heart-beat is connected with and dependent on the presence and interaction of the inorganic salts present in the lymph, though as to the manner in which they interact to initiate the beat, there is some obscurity. A very plausible theory as to the part played by the inorganic salts in initiating the contraction and one in accordance with the facts has been presented by Howell as follows: The heart-muscle, it is assumed, contains a stable organic energy- yielding compound of which potassium is one of its constituents and on which its stability depends. This compound must be present in relatively large amounts as the heart will continue to contract and expend energy for many hours after the blood-supply has been with- drawn. During the diastole a reaction takes place between this compound and the calcium or the calcium and the sodium salts, whereby a portion of the organic compound is freed from potassium and is then com- bined with calcium, or with calcium and sodium. In consequence, this portion of the organic compound in combination with the calcium acquires and gradually increases in instability, reaching its maximum at the end of the diastole, when it undergoes a dissociation giving rise to a chain of events that culminate in a contraction. The initial step, therefore, is a dissociation of a complex unstable molecule followed by an oxidation of the dissociated products. That an active dissocia- tion of some character takes place is evident from the consumption of oxygen, the production of carbon dioxid, the liberation of heat, electricity and mechanic motion. Inasmuch as the contraction is always maximal and as the heart is refractory to a stimulus during the systole, the probabilities^ are that all of the energy-yielding unstable compound is dissociated with each contraction. With the relaxation there is a renewal of the unstable 3o6 TEXT-BOO^ OF PHYSIOLOGY. molecules along the lines stated above, which increase in number and instability until the maximum is again attained when another dissociation occurs followed by another contraction. The rhyth- micity of the heart's action, the appearance of a refractory condition during the systole and its gradual disappearance during the diastole, as well as other phenomena, are readily explained by the foregoing hypothesis. The cause of the dissociation of the energy-yielding material is, however, a subject of discussion. According to Howell it is not necessary to assume the presence of any cause other than the extreme instability of the organic compound in question. According to Engelman, Langendorff and others, the dissociation is not spontaneous but is the result of the action of a specific stimulus, an "inner stimu- lus," arising within the muscle elements themselves through metabolic processes; and so long as these processes are chemically arid physically conditioned by blood or tissue fluids containing the inorganic salts, so long will this stimulus be produced. As to the nature of this stimulus, whether chemic, electric or enzymic, nothing definite can be stated at present. The Response of the Heart to the Action of a Stimulus. — The heart of the frog as well as of some other animals may be brought to a standstill by the ligation of the tissues between the sinus veno- sus and the auricle, a procedure first introduced by Stannius and now known as the first Stannius ligature. Under such circumstances the heart may be made to contract by stimulating it with the single induced current. With each passage of the current the heart con- tracts. Contrary to what is observed in other muscles, the heart- muscle, if it contracts at all, at once reaches its maximal value. Any increase in the strength of the stimulus above the threshold value has no greater effeet on the extent or force of the contraction than the minimal stimulus. A conclusion which may be drawn from this fact, according to Engelman, is as follows: By reason of the fact that the heart contracts at its maximum value to the action of any strength of stimulus, under given conditions, there is always en- sured a complete emptying of the ventricular contents and a uni- form discharge of blood into the arteries, which would not be the case if the extent of the contraction varied with the strength of the stimulus; and there are reasons for believing that the normal stimu- lus for the contraction varies within wide limits above the threshold value both in normal and abnormal conditions of the heart. The changes in the extent or force of the contraction are the result, not of changes in the intensity of the stimulus, but of changes in the heart- muscle, caused by variations in mechanical resistances. The periodicity of the heart's action or its rhythm may also be elucidated by the foregoing fact. There are reasons for believing that at the time of the contraction practically all of the available energy-yielding material is completely utilized, after which the heart THE CIRCULATION OF THE BLOOD. 307 relaxes and remains at rest in the diastolic condition for a given period; and before a second excitation wave can be developed and pass from the sinus over the heart there must be a re-accumulation of energy- yielding material, and a restoration of the irritability. This is accom- plished during the diastole. By virtue of this fact the heart can not act otherwise than in a periodic or rhythmic manner. Inasmuch as there is a conversion of all of the potential energy into kinetic energy during the systole, there is of necessity, a lowering of the irritability, and to so great an extent is this the case that the heart will not respond to the action of a second stimulus either physiologic or artificial during the systolic period. This non-responsiveness of the heart may be shown by throwing into it a second stimulus at any moment during the systole. Whatever the moment or whatever the strength of the stimulus may be the extent of the contraction remains the same. During the systolic period the heart is said, therefore, to be refractory to a second stimulus. If, however, a second stimulus of average strength be thrown into the ventricle at any moment during the relaxation, a , second contraction or extra systole will be developed. The extent of this contraction cont^ction IJ.^^.^.^Cou- will be proportional to the time at which pensatoky Pause. The break the stimulus is thrown into the ventricle as ™ ^^^ horizontal hne indicates • ■ c ii- -L • • i ii J r the moment the electric cur- It passes from the beginning to the end of j,ent passes through the heart. its relaxation. Whatever the extent of the extra contraction it superposes itself on the first, though its height is no greater than the first. For this reason it is believed a tetanic con- traction can not be developed. If the stimulus be thrown into the heart just as the relaxation is completed, the extra contraction attains the same height as the preceding contraction. In passing from the beginning to the end of the relaxation and into the diastolic or resting period, it has been found that the extra contraction can be provoked by a stimulus which is steadily decreased in intensity. It is evident from this fact that the restoration of the energy-yielding material and the return of the irritability gradually increases from the beginning of the relaxation to the end of the diastole. For this reason weak stimuli are more effective in the later than in the earlier period of the relaxa- tion and the diastole. After the development and disappearance of the extra contraction a considerable pause in the heart's action occurs to which the term compensatory pause has been given (Fig. 140), on the assumption that it was necessary on the part of the heart to compensate for the dis- turbance of the rhythm by remaining at rest until the time of the next beat and thus restore the rhythm. This was thought to be a special property of the heart-muscle. This view, however, is no longer enter- tained. For if an isolated ventricle of a frog heart be employed and made to contract rhythmically by an artificial stimulus, or if a sponta- 3o8 TEXT-BOOK OF PHYSIOLOGY. neously beating portion of the dog's heart be employed for experi- mentation instead of the whole heart, the results of the same methods of stimulation are different. Though an extra contraction is called forth as usual, there is no compensatory pause; indeed, if anything the pause is shorter than the regular pause. The theory of a necessary compensation is unnecessary. The explanation assigned and generally accepted at present for the production of a compensatory pause is as follows: In a sponta- neously beating heart the ventricular contraction is provoked by the arrival of an excitation process coming from the auricles. When the extra contraction is induced by an artificial stimulus, the next succeed- ing excitation from the auricle falls into the refractory period and hence the ventricle is not stimulated. It, therefore, simply waits for the arrival of the third succeeding excitation, when it responds and takes up the regular rhythm. If a series of successive stimuli be thrown into the heart-muscle the effect will vary in accordance with their time intervals. Should this be less than about three seconds there will be a gradual increase in the height for some half dozen contractions, a result to which the term "staircase" or "treppe" has been given. This increase in the height of the contraction is attributed to an increase in the irritability and con- tractility of the muscle the result of the primary stimulating action of fatigue products. THE NERVE MECHANISM OF THE HEART. By this term is meant a combination of nerves and nerve-centers which cooperate to increase or decrease either the rate or force — or both — of the heart's contraction in accordance with the needs of the system. That the heart is normally influenced by the central organs of the nerve system in response to the action of nerve impulses re- flected to them from many organs of the body is a matter of personal experience; that it is abnormally influenced by the same or other organs in response to nerve impulses reflected to them in consequence of pathologic and traumatic processes occurring in different regions of the body, and that both heart and nerves are modified in different ways by the action of drugs introduced into the body, are matters of daily clinical experience. The nerves comprising this mechanism and the relation they bear one to another are represented in Fig. 141. It was stated in a previous paragraph, page 296, that the con- traction of the heart-muscle is independent of its connection with the central organs of the nerve system, and that it will continue to contract in a rhythmic manner for a variable length of time even after its removal from the body of the animal, the length of time varying with the animal and the conditions to which it is subjected; that the stimulus is myogenic in origin and chemic in character, the THE CIRCULATION OF THE BLOOD. 309 result of a reaction between the chemic constituents, organic and inorganic, of the muscle-cells and those in the lymph by which they are surrounded. It has also been further shown that even in the living animal the heart will continue to beat and fulfil its functions after Emotional Centers Ethitarating (ewe) Jiepnsa'mg (hm) Cantio-Inklbitor Center. Ganglion Stellatum Intra-CardiarMroe Caniio-Accelerator Center UiffusNeroe "^""'yiikiHtor (BLUE) Mmnt uahibltor (/icd) Sympathetic Mroes Accelerator iAujmentor Fig. 141. — DiAOEAM op the Nerve Mechanism oe the Heart. — (G. Bachman.) division of all nerves in connection with it. A dog thus experimented on lived for eleven months, and beyond the fact of becoming fatigued more readily upon exertion than formerly, exhibited no striking dis- turbance of his functions. Nevertheless groups of nerve-cells are 3IO TEXT-BOOK OF PHYSIOLOGY. present in certain portions of the heart in all classes of vertebrate animals, which bear an anatomic and physiologic relation to the heart- cells on the one hand, and to the nerves connecting them with the central organs of the nerve system on the other hand. Intra-cardiac Nerve-cellfe. — In the frog heart a group of nerve- cells is found in the sinus at its junction with the auricle, known as the crescent or ganglion of Remak; a second group is found at the base of the ventricle on its anterior aspect, and known as the gan- glion of Bidder; a third group is found in the auricular septum, known as the septal ganglion, or v. Bezold's or Ludwig's. The majority of the cells are situated on the surface of the heart just beneath the pericardium. From the cell-body fine non-meduUated fibers pass into the substance of the heart, to become histologically and physiologically related with the muscle-fiber. In the dog heart and in the mammaUan heart generally, though nerve-cells are present, they are not arranged in such definite groups, but are more widely distributed in the terminations of the venae cavse, pulmonary veins, the walls of the auricles, and in the neighborhood of the base of the ventricles. Extra-cardiac Nerves. — The extra-cardiac nerves which connect the heart with the central nerve system and through which the activities of the heart are influenced are two: viz., the sympathetic and the vagus or pneumogastric. Experimental investigation has established the fact that the sympathetic is the motor nerve to the heart, the nerve which accelerates the rate and augments the force of the iiatural beat; while the vagus is the inhibitor nerve, the nerve which inhibits or con- trols the rate and the force of the beat in accordance with the necessi- ties of blood distribution. For this reason these two nerves will be considered in the following order. The course of the fibers compos- ing these nerves, from their origin to their termination, and the relation they bear to one another and to neighboring structures, vary some- what in difiEerent animals. The Origin and Distribution of the Sympathetic Nerves in Mammals. — The sympathetic nerve-fibers which influence the action of the heart, are connected on the one hand with the heart-muscle itself and on the other hand with nerve-fibers coming from the central nerve system. The former are non-medullated and post-ganglionic, the latter meduUated and pre-ganglionic. The pre-ganglionic fibers have their origin in the medulla oblongata and very probably from nerve-cells in the gray matter beneath the floor of the fourth ventricle. From this origin they descend the spinal cord as far as the level of the second and third thoracic nerves. At this level they emerge from the cord in company with the nerve-fibers composing the anterior roots of the second and third thoracic nerves. After a short course, they enter the white rami communicantes, enter the sympathetic chain and pass upward to the ganglion stellatum, and by way of the annulus of Vieussens to the inferior cervical ganglion THE CIRCULATION OF THE BLOOD. 311 as well around the nerve-cells of which their terminal branches arborize. From the nerve-cells of both the stellate and inferior cervical ganglia, the sympathetic nerves proper arise which after emerging from the ganglia pass towards the heart and become associated with the fibers of the vagus and assist in the formation, of the cardiac plexuses. On reaching the heart they may terminate directly in the muscle-cell or indirectly through the intermediation of intra-cardiac nerve-cells. The former mode of termination is the more probable.' Experiment has shown that both the pre- and post-ganglionic fibers are efferent in function. The Origin and Distribution of the Vagus Nerve in Mammals. — The vagus nerve-fibers which influence the heart are connected on the one hand with the heart through the intermediation of the intra- cardiac cells, and on the other hand with the central nerve system. Histologic investigation has shown that the vagus nerve-trunk of man and mammals generally, contains medullated fibers of large and small size. Experiment has shown that the large fibers are afferent, the small fibers efferent in function. The large afferent fibers arise in the ganglia situated on the trunk of the nerve. From their contained nerve-cells a short axon process proceeds which soon divides into a central and a peripheral branch. The central branch passes toward and into the gray matter beneath the floor of the fourth ventricle where its end-tufts arborize around nerve-cells; the peripheral branch passes toward the general periphery to be distributed to the mucous membrane of the lungs, stomach, intestine, etc. The small efferent fibers are the peripherally coursing axons of nerve-cells situated in the gray matter beneath the floor of the fourth ventricle at the tip of the calamus scriptorius. The exact course of these fibers from their origin into the trunk of the vagus is not positively known. According to some investigators, they leave • the medulla by way of the spinal accessory nerve and enter the trunk of the vagus through its internal or anastomotic branch; according to recent investigations made by Schaternikoff and Friedenthal, they leave the medulla along the path by which the afferent fibers enter and never become associated with the spinal accessory nerve at its origin. In the neighborhood of the inferior or recurrent laryngeal nerves, branches containing efferent fibers are given off, which pass to the heart by way of the cardiac plexus. The terminal branches of these fibers are not distributed directly to the heart-muscle, but to the intra- cardiac nerve-cells, around the bodies of which they end in basket- like formations. The fibers in the vagus are pre-ganglionic; those of the nerve-cells post-ganglionic. (See Fig. 142). In the frog and allied animals the relation of these two sets of nerve-fibers, viz., the efferent sympathetic fibers and the efferent vagus fibers, is somewhat different; and because of the fact that these nerves in this animal are largely employed for determining experi- 312 TEXT-BOOK OF PHYSIOLOGY. ■Sympathetic Neuron mentally their respective actions on the heart, this relation should be clearly understood. The sympathetic nerve-fibers in this animal are also in connection with the heart on the one hand and with nerve-fibers coming from the central nerve system on. the other hand. The pre-ganglionic fibers take their origin very probably in nerve-cells in the medulla 'oblongata. From this origin they descend and emerge from the spinal cord in, the anterior roots of the third spinal nerve, then pass through the white rami communicantes to the third sympathetic ganglion around the nerve-cells of which their terminal fibers arborize. From the nerve-cells of this ganglion, the sympathetic nerves proper, the post-ganglionic, non-medullated fibers arise. Froin this origin they ascend, passing successively through the second sympathetic ganglion, the annulus of Vieussens, the first sympa- thetic ganglion, to the ganglion on the trunk of the vagus, at which point they enter the sheath of the vagus fibers and in company with them pass to the heart. For this reason the common trunk is generally spoken of as the vagosympa- thetic nerve. The vagus nerve is connected with the medulla oblongata by a series of from six to eight roots. A short distance from the medulla, the nerve trunk passes through a large opening in the cranium beyond which it presents an enlargement, termed the vagus ganglion. The peripheral end of this ganglion gives off two trunks, one the glossopharyngeal, the other the vagus proper. The vagus nerve proper in the frog also consists of both afferent - and efferent fibers which have practically the same origin, distribution and termination as the corresponding fibers in the mammal. After the union of the sympathetic fibers with the vagus fibers, the common trunk passes forward to the angle of the jaw, winds around the pharynx just beneath the border of the petro-hyoid muscle and in close relation with the carotid artery. As the nerve approaches the heart it divides into two branches, the pulmonary and the cardiac. At the sinus venosus some of the fibers become related, histologically and physiologically, with the ganglion cells, while others plunge into the heart, course along the auricular septum on the left side and finally terminate at or near the ganglion cells of the base of the ventricle. The mode of termination of both the vagus and sympathetic fibers is similar to that observed in the mammals. The Physiologic Actions of the Sympathetic Nerves in the Frog. — ^The information now possessed regarding the influence which the central nerve system exerts on the heart through these Fig. 142. — Diagram showing THE. Relation of the Vagus to THE Heart Muscle-cell. THE CIRCULATION OF THE BLOOD. 313 nerves, has been derived largely from experiments made on the nerves of the frog, toad, and turtle. Inasmuch as the sympathetic and vagus nerves in the frog and related animals are bound up in a common sheath, it is necessary in order to demonstrate their respective functions to first divide the nerves, before their union at the vagus ganglion, and ^irr'r^'f'll^'l, !i 'M I ''' ' ' ' I i ^ 'f :Mj! Fig. 143. — Tracings Showing the Effects on the Heart-beat of the Frog from Stimulation of the Sympathetic Nerves Prior to Their Union with the Vagus Nerve. The upper tracing shows an increase in the rate which before stimulation was 15 per minute and during stimulation 30 per minute. Before stimulation the height of the ventricular beat was 9 mm. and during the stimulation it was 12 mm. The lower tracing shows a similar series of effects, the differences being only of degree. — {Brodie.) then stimulate their peripheral ends. The heart should be exposed and attached to a recording lever so that its movements may be taken up and recorded on a moving recording surface. Stimulation of the sympathetic fibers with induced electric currents, prior to their union with the vagus, is followed by an increase in the rate or an augmentation in the force of the heart-beat or both at the same time. The effects of such a stimulation with induced currents of moderate intensity are graphically shown in Fig. 143. The upper tracing shows that the heart was first accelerated, the beats increasing from 15 per minute before stimulation, to 30 per minute during stim- ulation. On the cessation of the stimulation, the heart slowly returned to its former rate. Coincidently with this acceleration of the rate there was an augmentation of the force of the ventricular contraction as shown by an increase in the height of the ventricular contraction which before stimulation was 9 mm., but during stimulation 12 mm. In addition to the foregoing changes in the heart-beat there is an alteration in the sequence of the beat. The natural delay in the con- duction of the excitation process from the auricles to the ventricle is 314 TEXT-BOOK OF PHYSIOLOGY. increased, in consequence of which the auricle completely relaxes before the ventricular contraction begins. Moreover, the auricular contrac- tion again occurs before the ventricle has completely relaxed. After the effect of the stimulation passes away, the acceleration diminishes, the augmentation declines and a reverse change in the sequence occurs. The lower tracing shows a similar series of effects. If the stimulus be applied to the pre-ganglionic sympathetic nerves, an acceleration or augmentation of the heart follows, similar in all respects to that which follows stimulation of the post-ganglionic or sympathetic fibers proper; and the inference may be drawn that if the stimulus could be applied directly to the nerve- cells in the medulla oblongata from which the fibers take their origin, the same acceleration or augmentation would follow; for this reason this collection of nerve-cells is known as the cardio- accelerator or augmentor center. Since stimulation of the nerve in any part of its course, which in all probability exaggerates its normal func- tion, is followed by an acceleration or an augmentation, the sympa- thetic is said to have an accelerator or an augmentor influence on the heart-beat; with the cessation of the stimulation, and very fre- quently before, the heart returns to its normal condition. The Physiologic Action of the Vagus Nerve in the Frog. — Stimu- lation of the intra-cranial roots of the vagus with very weak induced elec- tric currents is followed by a gradual diminution in the rate and a dim- FlG. 144. — Te AGING SHOWING THE EFFECT ON THE HeART-BEAT OF THE TOAD OF LONG Stimulation of the Intra-ceanial Roots of the Vagus with Moderately STRONG Electeic CURRENTS^ — (Gaskell.) inution in the force of the heart-beat. If the induced currents are moderate in strength, the heart will at once come to a standstill in diastole. (Fig. 144.) If the stimulus be applied to the trunk or the peripheral portion of the vagus, for example close to the sinu-auricular junction, an inhibition occurs similar in all respects to that which follows stimulation of the intra-cranial roots, and judging from what is known regarding the action of nerve-cells, the inference may be drawn that if the stimulus could be applied directly to the group of nerve-cells from which the efferent fibers arise, the same inhibition would follow; for this reason this collection of nerve-cells is known as the cardio-inhibitor center. Since stimulation of the nerve, either at its center, in its course, or at its periphery, and which in all probability exaggerates its normal function, is followed by a period of rest or inactivity, the vagus is said to have a retarding or an inhibitor in- fluence on the beat of the heart. During the continuance of the inhibition, the heart-muscle is THE CIRCULATION OF THE BLOOD. 315 relaxed, its cavities dilated and filled with blood. The dilatation usually exceeds that observed prior to the vagus stimulation. After cessation of the stimulation, the heart resumes its activity. At first the beat usually is slow and feeble, but with each succeeding beat both rate and force increase, until they attain or exceed that observed prior to the stimulation. In some cases, however, the heart begins to beat with as much and even more vigor than it did prior to the stimulation. The duration of the inhibitor effect varies with the di;ration of the stimulation. Thus during and after a stimulation of thirty-eight seconds the heart of the toad remained at rest for 292 seconds (Gaskell) ; the heart of a snake for from one-half to one hour "■'•vmwwwvvrvY Fig. 145- — Tracing showing the Diminution in the Rate of the Heart-beat FOLLOWING Weak Tetanization of the Vagus Trunk. (Meyer) ; the heart of a turtle for four and a half hours (Mills) . The period of inhibition will depend on the strength of the electric current employed, the nerve stimulated, the season of the year, etc. The effects on the heart-beat which will follow stimulation of the vago-sympathetic in its course vary, however, because of the antagon- istic action of the inhibitor and accelerator nerve impulses. Thus stimulation of the peripheral end of the divided trunk of the vagus in Fig. 146. — Tracing showing Complete Inhibition following Strong Tetan- ization OF THE Vagus Trunk. the frog or the toad with weak tetanizing induced electric currents is followed by an increase in the rate of the heart-beat because of the stimulation of the accelerator fibers which apparently respond before the inhibitor fibers; stimulation with somewhat stronger currents is followed by a diminution in the rate of the beat because of the greater effect on the inhibitor nerve-fibers (Fig. 145). Stimulation with strong tetanizing currents is followed by complete inhibition (Fig. 146). The foregoing facts lead to the inference that the cardio-accelerator 3i6 TEXT-BOOK OF PHYSIOLOGY. and the cardio-inhibitor centers have as their function the discharge of nerve impulses which are conducted by their related nerves, the efferent sympathetic and vagal fibers, to the heart, and which, in an as yet unexplained manner, accelerate or augment or inhibit, the action of the heart. The relation which these two centers bear one tothe other and the manner in which they are influenced in their activities both directly and reflexly and thus regulate the action of the heart from moment to moment will be considered in a subsequent paragraph. Changes in the Conductivity of the Heart.— In addition to the changes in the rate and force of the heart caused by stimulation of the inhibitor and the augmentor nerves, it is stated by Gaskell that there is also during the inhibition a decrease in the conductivity of the heart at both the sinu-auricular and auriculo-ventricular junc- tions, and an increase in the conductivity during acceleration of the beat. The decrease in conductivity may be so pronounced that only every second or third contraction of the auricle will be follo;wed by a contraction of the ventricle. In other instances both auricles and ventricles remain at rest while the sinus maintains its uSual rate. The increase in conductivity is shown by first artificially , blocking the contraction wave at the auriculo-ventricular junction with the clamp, until only every second or third auricular contraction is con- ducted to the ventricle, and then stimulating the sympathetic. At once the auricular contraction forces the block, and passes to the ventricle, calling forth a normal contraction. The Physiologic Actions of the Sympathetic Nerves in Mam- mals. — ^In the mammal, stimulation of the sympathetic nerves in any part of their course, either through the rami communicantes, the ven- tral portion of the annulus of Vieussens, or after their emergence from the stellate or inferior cervical ganglia is followed by effects similar to those observed in the frog: viz., an acceleration or augmentation, or both, of the heart-beat. The percentage increase in the acceleration varies in different animals. In some instances the increase varies from 58 per cent, to 100 per cent. (Hunt). If the heart is beating slowly before stimulation, the acceleration is more marked than if it is beating rapidly. The effect of the accelerator impulses is apparently a change in the inner mechanism of the heart-muscle itself and not to a change in the peripheral portion of the inhibitor apparatus. This is indicated by the fact that acceleration occurs after the full physiologic action of atropin which acts upon, and impairs the conductivity of, the intra- cardiac nerve- cell terminals. A peculiarity of the sympathetic nerve is that it does not respond to stimulation as rapidly as do many nerves, so that a rather iong latent period intervenes between the moment of stimulation and the appearance of the acceleration as shown in Fig. 147. A further pe- culiarity is that the acceleration sometimes continues after the stim- ulus is withdrawn, and sometimes ceases before it is withdrawn. THE CIRCULATION OF THE BLOOD. 317 Though an increase in both the rate and force frequently occur simultaneously, there is no necessary relation or connection between the two as they can and do occur independently of each other. For this reason it is generally assumed that the sympathetic nerves contain two groups of fibers, viz., accelerators and augmentors, the functions of which are to respectively accelerate the rate and augment the force of the heart-beat. From the fact that both auricles and ventricles exhibit these changes it is assumed that the nerve impulses stimulate both chambers. This is rendered probable also from the experiments of Erlanger, who found that after complete heart block, stimulation of the sympathetic caused independent acceleration of both auricles and ventricles. fA^J-J"'''v\j\f\fJA^^N\N^^/^^\^J"J"^"^AN^^ Fig. 147. — Acceleration of the Heakt following Stimulation op the Cardiac Branches which come from the Anndhjs of Vieussens. The Physiologic Action of the Vagus Nerve in Mam- mals. — In the mammal the same or similar effects result from stimulation of the vagus as in the frog. If the thorax of the dog is opened and artificial respiration maintained the heart will continue to beat in a practically normal manner for a long time. Under such conditions if the vagus nerve on one side be divided and its peripheral end stimulated with induced electric currents of moderate strength the heart will be seen to come to a standstill almost immediately in the condition of diastole, and may be so kept for a variable period, from fifteen to thirty seconds or more, during which its walls are relaxed and its cavities filled with blood. On cessation of the stimulation the contractions return and in a very short time the fornier rate and force of the beat are regained. If the electric currents are of feeble strength, the heart will come to rest gradually through a gradual diminution in the rate and force of the contraction. During the period of the inhibition the heart presents an appearance similar to that presented by the heart of the cold-blooded animal. When the heart of an animal is thus exposed, the auricle and the ventricle of one side may be attached by threads to writing levers and their contrac- tions registered on a moving recording surface. The effects on both auricles and ventricles which follow vagus stimulation will then become more apparent. Fig. 148 is a tracing thus obtained. The animal employed was a rabbit. The inhibitor effect of the vagus varies in degree and duration in different animals. In the dog the effect of vagus stimulation is usually pronounced, lasting from 15 to 30 seconds; in the rabbit it is perhaps equally well pronounced but somewhat less iii duration; in the cat it is almost wanting. In this latter animal a complete standstill, 3i8 TEXT-BOOK OF PHYSIOLOGY. even for a few seconds, is very rarely seen; usually there is produced merely a slight diminution in the rate of the beat even though the stim- ulus employed is quite strong. In all these animals, however, after a very short time the nerve impulses lose their inhibitor influence on Fig. 148. — Result of the Stimulation of the Peripheral End of the Divided Left Vagus in the Rabbit. — {Brodie.) the heart-muscle, and notwithstanding continued stimulation of the vagus, the heart returns to its former rate and vigor. This result is in marked contrast to that observed during stimulation of the vagus in the cold-blooded animals, in which the heart may be kept at rest for relatively very long periods of time. No satisfactory explanation THE CIRCULATION OF THE BLOOD. 319 for this loss of vagus control or escape of the heart from the vagus control has as yet been offered. Seat of Action of the Vagus Impulses.— In a foregoing experi- ment of which Fig. 148 is a graphic result, stimulation of the left vagus with a fairly strong current was followed by a diminution in both the rate and force of the contraction of both auricles and ventricles, though the effect was most marked in the auricles. From this and similar facts it has come to be the general belief that the inhibitor nerve im- pulses exert their influence mainly, if not exclusively, on the auricle, and that the cessation of ventricular action is a secondary effect due to the non-arrival across the conducting apparatus of the normal excitation process from the auricle. This is the case undoubtedly in the cold-blooded animals, and the experiments of Erlanger on the heart of the dog indicate that the same holds true for the mammals. This investigator has found that when the auriculo-ventricular tissues are suddenly clamped, including presumably the muscle band of His, there is for a time a complete cessation of ventricular activity, but after a variable period of time, fifty seconds or more, the ventricle develops an independent rhythm which gradually increases in frequency, but seldom , if ever, attains that of the auricles. Under such circumstances tetanic stimulation of the auriculo-ventricular tissues by means of the clamp now transformed into stimulating electrodes, failed to bring about a stoppage of the ventricles. Moreover, if during the time the clamp is apphed and after the ventricle has developed a rhythm of its own, the vagus is stimulated, the auricles will cease to beat as usual, but the ventricles will continue to beat at their usual rate. These and similar facts lead to the conclusion that vagal inhibitor action is lim- ited to the auricles. From the foregoing facts it is apparent that the accelerator and augmentor effects of the sympathetic nerve impulses, and the inhibitor effects of the vagus nerve impulses, closely resemble on the one hand the accelerator and augmentor effects of increasing amounts of diffusible calcium salts, and on the other hand the inhibitor effects of increasing amounts of diffusible potassium salts in the blood or other circulating fluid; and so closely do these two sets of phenomena resemble each other, that they are by some observers regarded as identical. Some additional facts in this connection have been presented by Howell, viz., that an increase (within limits) and a decrease in the percentage of diffusible calcium salts in a circulating fluid passing through the cavities of the mammalian (cat) heart, increases on the one hand, and decreases on the other hand, the sensitiveness of the heart to sympathetic acceleration and augmentation. From this the inference is deduced that the acceleration and augmentation of the heart-beat which follow stimulation of the sympathetic nerves are due to the presence in the heart tissue of a certain percentage of diffusible calcium salts, which have been freed from combination with organic matter by the action of the sympathetic nerve impulses. 320 TEXT-BOOK OF PHYSIOLOGY. Again, that an increase (within Hmit) and a gradual decrease in the percentage of diffusible potassium salts in a circulating fluid passing through the cavities of the frog and the cat heart, increases on the one hand and decreases and finally abolishes on the other hand the sensi- tiveness of the heart to vagus inhibition. From this the inference is deduced that the inhibition of the heart-beat which follows stimulation of the vagus nerve is due to the presence in the heart tissue of a certain percentage of diffusible potassium salts, which have been freed from combination with organic matter by the action of the vagus nerve impulses. The Cardio-accelerator Center.— The collection of nerve-cells from which the pre-ganglionic fibers of the sympathetic system arise is known as the cardio-accelerator or augmentor center. The exact location of this center in the central nerve system has not been as yet accurately determined. It is probably located in the medulla oblongata. From experiments which have been made on the sympathetic nerve apparatus in its entirety, it is believed that the function of this center is the discharge of nerve impulses which, conducted to the heart by the pre-ganglionic and post-ganglionic sympathetic fibers, cause an acceleration in the rate or an augmentation in the force, or both, of the heart-beat. It is also generally believed since the publication of Hunt's investigations that this center is in a state of tonic activity. This is shown by the fact that after the division of the vagus nerves and the removal of all inhibitor influences, division of the sympa- thetic nerves or extirpation of the stellate or inferior cervical ganglion, is yet followed by a decrease in the rate of the heart-beat. After division of the sympathetic nerves and the removal of accelerator influences it is also easier to bring about inhibition through vagus stimulation. The Factors which Determine the Activity of the Cardio-ac- celerator Center. — The question has been raised as to whether the tonic activity of this center is maintained by central or peripheral stimuli, i. e., whether it is maintained by causes within itself, the result of an interaction between the constituents of the cell substance and those of the surrounding lymph, or whether it is maintained by nerve impulses reflected to it through various afferent or sensor nerves. Inasmuch as there is no way of determining whether the causes are central, except by dividing all afferent nerves, it is impossible to state how much influence is to be attributed to this fkctor. On the contrary, though it is readily demonstrable that stimulation of many afferent nerves will cause an acceleration of the heart it can not be stated positively that this is the result of a reflex stimulation of the accelerator center. Though earlier investigators believed this to be the correct interpretation, the more recent experiments of Hunt apparently dis- prove it; for this investigator has shown that if the vagus nerves are divided it is impossible to produce reflex acceleration of the heart. THE CIRCULATION OF THE BLOOD. 321 His conclusion, conj&rming that of others, is that cardiac acceleration is the result of an inhibition of the cardio-inhibitor center. A freer play to the tonic activity of the accelerator center would thus be made pos- sible. The Cardio-inhibitor Center. — The collection of nerve-cells from which the small efferent fibers of the vagus nerve arise is known as the cardio-inhibitor center. It is situated in the medulla oblongata and more especially in the gray matter beneath the floor of the fourth ventricle near the tip of the calamus scriptorius. It is in all prob- ability, a part of the nucleus ambiguus. From the experiments which have been made on the vagus inhibitor apparatus in its entirety it is believed that the function of this center is the discharge of nerve impulses, which conducted to the 'heart by the vagus fibers cause an inhibition of its beat of greater or less extent. In the dog, and probably in many other mammals, this center exerts a more or less constant inhibitor or restraining influence on the heart's activity. This is indicated by the fact that the rate of the beat is very much increased by simultaneous division of both vagi. The degree of the inhibition which this center exerts varies greatly, however, in different animals. In the cat and in the rabbit the inhibitor control is normally so slight that there is but a relatively slight increase in the rate of the beat after division of the vagi. The tone of the vagus in these animals is, therefore, said to be slight or feeble. In human beings the tone of the inhibitor apparatus is poorly developed in early childhood, as shown by the fact that the administration of atropin, which removes temporarily inhibitor control is not followed by an increase in the rate of the beat. It develops steadily and reaches a maximum at from the twenty-fifth to the thirtieth year. In ad- vanced years the tone again declines. For these and other reasons it is believed that this center is in a state of tonic activity in many if not all mammals, discharging nerve impulses which exert a regulative influence on the cardiac mechanism in accordance with its needs and especially in reference to the variable resistances oSered to the flow of blood which the heart must overcome. The Factors which Determine the Activity of the Cardio- inhibitor Center. — The question has also been raised as to whether the tonic activity of this center is maintained by central or peripheral stimuli, i. e., whether it is maintained by causes within itself the result of an interaction between the constituents of the cell substance and those of the surrounding lymph, or whether it is maintained by nerve impulses reflected to it through various afferent or sensor nerves. Though both factors play an important part in the maintenance of its activity, the trend of evidence points to the conclusion that the reflected impulses are by far the more important of the two. This latter sup- position is supported by the results of direct experimentation upon sensor nerves in almost any region of the body. Thus stimulation of the dorsal roots of the spinal nerves, the trunks of the cranial sensor 322 TEXT-BOOK OF PHYSIOLOGY. nerves, the splanchnic nerves, the pulmonary branches of the vagus, etc., gives rise to a more or less pronounced inhibition of the heart. As a rule, stimulation of the peripheral terminations of these nerves is more effective than stimulation of their trunks, hence an explanation is at hand for the cardiac inhibition which results from sudden disten- tion of the stomach and intestines, or operative procedures in the nose, mouth, and larynx. Reflex inhibition of the heart, even to the stage of absolute and permanent standstill, eventuating in the death of the individual is a not infrequent result of peripherally acting causes of a pathologic or operative character. From the results of experimental procedures the inference is drawn that normally, nerve impulses, developed by the action of physiologic causes, are reflected continuously from many peripheral regions of the body, which falling into this center gently stimulate and maintain it in a condition of necessary tonicity or activ- ity. The Causes of the Variations in the Heart-beat. — ^It has been stated elsewhere in the text (page 286), that the rate of the heart-beat is influenced by age, muscle activity, the position of the body, meals, variations in blood pressure, etc. In addition to these factors there is abundant evidence that other factors, e.^., the action of peripheral stimuli of a physiologic or pathologic character in various regions of the body, can and do cause reflexly at one time or in one individual an acceleration of a marked character, and at another time or in another or the same individual an inhibition which may be so pro- nounced as to lead to a complete standstill in diastole. The records of clinical medicine contain many instances which show that gastric, intestinal, uterine and other organic disorders as well as various opera- tive procedures in different regions of the body cause now an accelera- tion, now an inhibition of the heart. The first explanation, that acceleration of the heart, the result of a peripherally acting stimulus, is due to a stimulation of the cardio- accelerator center by the arrival of nerve impulses coming through afferent nerves, having been made questionable and improbable by the results of Hunt's experiments, the alternate explanation must be, that the acceleration is due to an inhibition of the normal activity of the cardio-inhibitor center, and that inhibition is due to an excitation of the normal activity of the cardio-inhibitor center, and hence there fol- lows the corollary that afferent nerves contain two sets of nerve-fibers which are in physiologic relation with the cardio-inhibitor center, one of which when stimulated peripherally inhibits its activity, the other of which when stimulated excites or augriients its activity. The extent to which both sets of fibers are present in any one afferent nerve is unknown. In the trigeminus it is believed the ex- citator fibers preponderate for the reason that peripheral stimulation of this nerve is followed by inhibition of the heart; in the sciatic, it is believed the inhibitor nerves preponderate, for the reason that stimu- . THE CIRCULATION OF THE BLOOD. 323 lation of the central end of the divided nerve is follovired generally by acceleration of the heart. It is probable from the effects which follow gastro-intestinal dis- orders, that the vagus nerve contains both classes of fibers as repre- sented in Fig. 141, inasmuch as stimuli of a pathologic character in one individual may reflexly excite or increase the activity of the cardio- inhibitor center, to be followed by an inhibition of the heart ; and in another individual, may reflexly inhibit the activity of the same center and to such an extent that the cardio-accelerator center may be enabled to increase either the rate or the force or both, of the heart move- ments. Palpitation of the heart from gastric irritation might thus be explained. The Influence of Psychic States.— The cardio-inhibitor and the cardio-accelerator centers may be increased in activity also by nerve impulses descending from the cerebrum, the result of emo- tional states; thus depressing emotions according to their intensity may so increase the activity of the cardio-inhibitor center, that the heart's action may not only be retarded but even completely inhibited; joyous emotions, on the contrary, may so increase the activity of the cardio-accelerator center or what is more probable inhibit the activity of the cardio-inhibitor center that the heart's action will be increased in both its rate and force. From the results of stimulation of the sympathetic (accelerator) and vagus (inhibitor) nerves under a great variety of conditions it has been established that their respective centers are mutually antagonistic; that the activity of the accelerator center at one moment limits the ac- tivity of the inhibitor and at another moment is limited in turn by it; that the rate of the heart-beat at each moment is the resultant of the relative degree of activity of the two centers. The Depressor Nerve. — The vagus trunk also contains afferent fibers stimulation of which not only brings about a reflex inhibition of the heart, but also a dilatation of the peripheral arteries and a fall of blood-pressure through a depressive influence on the vaso-motor centers. To this nerve the term depressor has been given. A cdn- sideraJ;ion of the physiologic action of this nerve will be found in the section devoted to the nerve mechanisms concerned in the mainte- nance of the blood-pressure. Modifications of the Nerve Mechanism of the Heart due to the Physiologic Action of Drugs. — ^The functions of different parts of the nerve mechanism of the heart may be demonstrated by an analysis of the effects which follow the administration of slightly toxic doses of the alkaloids of various drugs. The effects can be shown to be due to a stimulation or to a depression of the normal activity of one or more portions of the mechanism. The alkaloid may exert its specific action on the central portions in the medulla, or on the peripheral portions in the heart, or on both simultaneously. The heart- mus- cle may at the same time be stimulated or depressed in its action 324 TEXT-BOOK OF PHYSIOLOGY. either in the same or in the opposite direction to that of the nerve mech- anism. As a result the heart-beat may be increased or decreased both in rate and force. The following examples will illustrate the action of alkaloids in general. Atropin. — After the administration of atropin in sufficient amounts the heart-beat increases in frequency in all animals in which the cardio- inhibitor centers exert a steady inhibitor influence over the heart. This is especially true in man and the dog. In animals in which the inhibitor control is slight, as the rabbit and frog, the increase in fre- quency is not very marked. In all animals thus far experimented on after the administration of atropin, neither stimulation of the trunk of the vagus nor stimulation of the intra-cardiac ganglia will arrest or even retard the heart-beat. The inference, therefore, is that the alkaloid exerts its action upon the ganglion cells and their ter- minal branches, impairing their chemic integrity and abolishing their normal function, that of conducting nerve impulses from the vagus nerve proper to the heart- muscle. In consequence of this, the influ- ence of the cardio-inhibitor center is cut off and the cardio-accelerator being unopposed in its activity, the rate of the beat is increased. After a variable period the heart returns to its normal rate. Stimu- lation of the vagus is again followed by the usual inhibition. As atropin is partly oxidized, and partly excreted, it is assumed that the nerve terminals have been restored by nutritive forces to their normal condition and their conductivity regained. This having been accom- plished the vagus nerve impulses can again reach the heart- muscle and the cardio-inhibitor center is therefore enabled to re-establish inhibitor control and antagonize the activity of the cardio-accelerator center. Nicotin. — After the administration of nicotin in sufficient amounts the heart-beat is primarily decreased in frequency even to the point of standstill in diastole for a few seconds, and secondarily increased both in frequency and force beyond the normal. If the vagus nerve's be first divided this primary decrease is not so marked and the inference is that the alkaloid primarily stimulates the cardio-inhibitor center and increases its normal function and perhaps the terminal branches of the vagus fibers, the pre-ganglionic, as well. After the secondary in- crease in the rate is established stimulation of the vagus trunk fails to inhibit the heart, though stimulation of the intra-cardiac ganglia is at once followed by the usual inhibitor phenomenon, arrest of the heart in diastole. For this reason it is believed that nicotin acts on the periph- eral terminations of the pre-ganglionic fibers of the vagus as they arborize around the intra-cardiac ganglia, depressing them and sus- pending their normal function, that of conducting nerve impulses from the vagus to the ganglion cells. Since stimulation of the pre-ganglionic fibers of the accelerator apparatus fails to accelerate the rate of the heart-beat, though stimulation of the post-ganglionic fibers has the usual accelerating effect, the inference is that nicotin acts upon and THE CIRCULATION OF THE BLOOD. 325 suspends the conductivity of their terminal branches in the ganglia. The acceleration of the heart must therefore be attributed either to a stimulation of the post-ganglionic fibers or of the cardiac muscle itself (Cushney). Pilocarpin and Muscarin. — These alkaloids, whether adminis- tered internally or applied locally to the heart, diminish the frequency and the force of the beat to such an extent that it very shortly comes to rest in diastole. For the reason that the internal administration or the local application of atropin in proper doses, which has a depressive action on the intra-cardiac cell terminations, removes the inhibition and restores the normal rhythm, the inference is drawn that both these alkaloids either increase the irritability of the nerve-cells or heighten the conductivity of their terminal fibers. The return of the heart-beat is attributed to a decline in irritability to the normal level in conse- quence of the antagonistic action of the atropin. Digitalin. — The administration of digitalin gives rise to effects, the character and extent of which vary in different animals. In the frog, as a rule, the only effect produced is a gradual increase in the duration and force of the ventricular systole, with a corresponding decrease in the duration of the diastole, until the heart comes to rest in the systohc state. As this effect is observed after division of the vagus trunk and also after the suspension of the activity of the intra-cardiac cell- fibers by atropin, it is evidently due to a direct stimulation of the heart- muscle. In some instances, however, the opposite effect is produced, viz., a gradual increase in the length of the diastole, .a decrease in the duration of the systole, until the heart comes to rest in the diastolic state. As this effect only arises when the vagus nerve is intact it is very probably due to a stimulation of the cardio-inhibitor center and a consequent increase of its functional activity. Though either effect may be produced in the frog the predominant effect is the increase in the contraction of the heart-muscle rather than an inhibition of the beat. In mammals both effects are observed, viz., a diminution in the rate of the beat, a lengthening of the diastole and an increase in the vigor of the systole, which are evidently due to a simultaneous stim- ulation of the cardio-inhibitor center and of the cardiac muscle. Digitalin thus expends itself on two opposing mechanisms; as to which gains the ascendency will depend on the dosage and the character of the animal. CHAPTER XIV. THE CIRCULATION OF THE BLOOD (Continued). THE VASCULAR APPARATUS: ITS STRUCTURE AND FUNCTIONS. The systemic vascular apparatus consists of a closed system of vessels extending from the left ventricle to the right auricle, and in- cludes the arteries, capillaries, and veins. Though serving as a whole to transmit blood from the one side of the heart to the other, each one of these three divisions has separate but related functions, which are dependent partly on differences in structure and physiologic properties, and partly on their relation to the heart and its physiologic activities. The Structure, Properties and Functions of the Arteries. — The arteries serve to transmit the blood ejected from the heart to the capillaries; that this may be accomplished they divide and subdivide and ultimately penetrate each and every area of the body. Their repeated division is attended by a diminution in size, a decrease in the thickness and a change in the structure of their walls. A typical artery consists of three coats: an internal, the tunica intima; a middle, the tunica media; an external, the tunica adventiiia. The internal coat consists of a structureless elastic basement mem- brane, on the inner surface of which rests a layer of elongated spindle- shaped endothelial cells. The middle coat consists of several layers of circularly disposed, non-striated muscle-fibers, between which are networks of elastic fibers. The external coat consists of bundles of connective tissue of the white fibrous and yellow elastic varieties. Between the external and middle coats there is an additional elastic membrane. In the small arteries there is but a single layer of muscle- fibers. In the large arteries the elastic tissue is very abundant, ex- ceeding largely in amount the muscle-tissue. It is also more closely and compactly arranged. The external coat is well developed in the large arteries (Figs. 149 and 150). In virtue of the presence in their walls of both elastic and con- tractile elements, the arteries possess the two properties of elasticity and contractility. The elasticity is especially well developed in the large arteries, which are capable, therefore, of both distention and elongation, and, when the distending force is withdrawn, of returning to their previous condition. The elasticity permits of a wide variation in the amount of blood the arterial system can hold between its minimum and maximum disten- tion. Thus the capacity of the aorta and carotid artery of the rabbit can be increased four times and six times respectively by raising the in- 326 THE CIRCULATION OF THE BLOOD. 327 tra-arterial pressure from o to 200 mm. of mercury. The elasticity also converts the intermittent movement of the blood imparted to it by the heart as it is ejected from the ventricle, into a remittent move- ment in the arteries and finally into the continuous and equable movement observed in the capillaries. This is accomplished in the following manner: With each contraction of the left ventricle more blood is ejected into the aorta than the arteries can discharge into the capillaries and veins during the time of the contraction. The portion not so discharged exerts a lateral pressure against the walls of the arteries which at once dilate until a condition of equilibrium is established between the pressure from within and the elastic re- action of the arterial walls from without. With the cessation of the contraction the elastic walls recoil and propel the blood toward the capillaries. The intermittent action of the heart is thus succeeded by the continuously reacting arterial wall. As the blood advances toward the periphery of the arterial system and larger amounts pass into the capillaries, both the distention and the elastic recoil di- minish, and by the time the blood reaches the capillaries its intermittency of move- ment has been so far obliterated by the elastic recoil, that as it enters the capil- laries the movement becomes equable and continuous. The elasticity thus serves the purpose of equalizing the movement of the blood throughout the arterial system. In youth the arterial walls are highly distensible and elastic; in advanced years they are frequently 'relatively rigid and inelastic; and in consequence the flow of blood toward and into the capillaries approximates in its characteristics the flow of a fluid through a rigid tube under the intermittent action of a pump; that is, the inter- mittent movement imparted by the heart is not so completely converted into a continuous movement, and hence the blood flows through the capillaries during the systole with greater velocity, and during the dias- tole with less velocity, than is the case when the vessel is normally elastic. For these and other reasons the tissues are not so well nour- ished and hence their nutrition and functional activities decline. The contractility permits of a variation in the amount of blood passing into a given capillary area in a unit of time. Normally each artery has a certain average caliber dup to a given contraction of the Fig. 149.' — Coats of a Small Artery, a. Endoth elium. b. Internal elastic lamina, c. Cir- cular musciJar fibers of the middle coat. d. The outer coat. — (^Lan- dois and Stirling.') 328 TEXT-BOOK OF PHYSIOLOGY: '_»iS|^ muscle coat. Beyond this average condition the artery can pass in one direction or the other by either a relaxation or increased contraction of the muscle coat. During the functional activity of any organ or tissue there is need for an increase in the amount of blood beyond that supplied during' functional inactivity or rest. This is accomplished by a relaxation of the muscle-fibers. With the cessation of activity the muscle-fibers again contract and reduce the amount of blood to that required for nutritive purposes only. An increased contraction of the muscle-fibers beyond the average, diminishes the outflow of blood, and if sufficiently great may give rise to anemia and pallor. The contractile elements at, the periphery of the arterial system, in the "' so-called arteriole re- gion, therefore regu- late the supply of blood to the tissues in accordance with their functional needs. Moreover, as will be stated in subse- quent paragraphs the degree of contraction of the arteriole mus- cle influences very markedly the degree of friction which the blood has to overcome in passing from the arteries into the capillaries. If the muscle contracts vigorously the caliber of the arteriole is diminished and the friction increases; if the muscle relaxes, the caliber of the arteriole is augmented and the friction decreases. By virtue of its tonic activity, the arteriole muscle at the periphery of the arterial system offers considerable resistance to the outflow of the blood and which is therefore spoken of generally as the peripheral resistance, though there is included under this term the resistance, offered by the small caliber of the capillary blood-vessel as well. This latter factor is constant, the former variable. The Structure, Properties and Functions of the Capillaries.— The capillaries are small vessels that connect the arteries with the veins. Though different in structure from a small artery or vein, there is no sharp boundary between them, as their structures pass imperceptibly one into the other. A true capillary, however, is of uniform size in any given tissue and does not undergo any noticeable decrease in size from repeated branchings. The diameter varies in different tissues from 0.0045 mm. to 0.0075 mm., just sufficiently large to permit the easy passage of a single red corpuscle. The length varies from 0.5 mm. to I mm. The wall of the capillary (Fig. 151) is composed of a single layer of nucleated endothelial cells with serrated edges united by a Fig. 150. — Transverse Section of Part of the Wall of the Posterior Tibial Artery (Man). ■ — {Schafer.) a. Endothelium lining the vessel, ap- pearing thicker than natural from the contraction of the outer coats. 6. The elastic layer of the intima. c. Middle coat composed of muscle-fibers and elastic tissue, d. Outer coat consisting chiefly of white fibrous tissue. — {From Yeo's "Physiology.") THE CIRCULATION OF THE BLOOD. 329 cement material. Though extremely short, the capillaries divide and subdivide a number of times, forming meshes or networks, the close- ness and general arrangement of which very in different localities. As the endothelial cells are living structures and characterized by irritability, contractility and tonicity, it may be assumed that the cap- illary wall as a whole is characterized by the same properties. Upon the possession of these properties, the functions of the capillary depend. The function of the capillary wall is to permit of a passage of the nutritive materials of the blood into the surrounding tissue spaces and of waste products from the tissue spaces into the blood. The structure of the capillary wall is well adapted for this purpose. Composed as it is of but a single layer of endothelial cells, the thickness of which defies accurate, measure- ment, it readily permits, under certain conditions, of the necessary exchange of materials between the blood and the tissues. The forces which are concerned in the passage of materials across the capillary wall are embraced under the terms diffusion, osmosis, and filtra- tion. As a result of the in- terchange of materials the tissues are provided with nourishment and relieved of the presence of waste products. The blood at the same time changes to a variable extent in chemic composition; because of the loss of oxygen and the gain of carbon dioxid it also changes in color from red to bluish-red. In order that the nutritive materials may pass through the cap- illary wall in amounts sufficient to maintain the necessary supply of lymph in the lymph or tissue spaces, it is essential that the blood shall flow into and out of the capillary vessels constantly and equably, in volumes varying with the activities of the tissues, under a given pres- sure and with a definite velocity. These conditions are made possible . by the cooperation of the physical properties and physiologic functions of the heart and vascular apparatus, the nature of which will be ex- plained in subsequent pages. The Structure, Properties and Functions of the Veins.— The veins serve to collect the blood from the capillary areas and return it to the right side of the heart. As they emerge from the capillary areas the veins, which in these regions are termed venules, are quite Fig. 151. — Capillaries. The Outlines of THE Nucleated Endothelial Cells with the Cement Blackened by the Action or Silver Nitrate. — {Landois and Stirling.') 33° TEXT-BOOK OF PHYSIOLOGY. small. By their convergence and union the veins gradually increase in size in passing from the periphery toward the heart. Their walls at the same time correspondingly increase in thickness. The veins from the lower extremities, the trunk, and abdominal organs finally terminate in the inferior vena cava. The veins from the head and upper extremities terminate in the superior vena cava. Both venae cavas empty into the right auricle. A typical vein consists of the same three coats as the artery: viz., the tunica intima, the tunica media, and the tunica adventitia. The media, however, does not possess as much of either the elastic or muscle tissue as the artery, but a larger amount of the fibrous tissue. Hence they readily collapse when empty. In virtue of their structure the veins also possess both elasticity and contractility, though in a far less degree than the arteries. These properties come into play and are of value in further- ing the movement of the blood toward the heart, es- pecially after a temporary obstruction. Pjg J Veins are distinguished by the presence of valves Valves of a throughout their course. These are arranged in pairs Vein. •;;. Semi- and formed by a reduplication of the internal coat, hanar valve. J. strengthened by fibrous tissue. They are always valve. — {David- directed toward the heart and in close relation to the son.) walls of the veins, so long as the blood is flowing for- ward (Fig. 152). An obstruction to the flow causes the valves to turn backward until they meet in the middle line, when they act as a barrier to regurgitation. Under these circumstances the elastic tissue permits the veins to distend and accommodate the blood. With the removal of the obstruction the recoil of the elastic tissue, and perhaps the contraction of the muscle-tissue, forces the blood quickly onward. HYDRODYNAMIC CONSIDERATIONS. The blood flows through the arteries, capillaries and veins in ac- cordance with definite laws. During its transit certain phenomena are presented by each of these three divisions of the vascular appara- tus. Since these phenomena, as well as the laws which govern them are similar to, though moi^e complex than the phenomena presented by relatively simple tubes with rigid or elastic walls, while liquids are flowing through them under a steadily acting or an intermittently acting pressure, it will be conducive to clearness of conception of the mechanics of the vascular apparatus, if there be considered : 1. The flow of a Hquid through a horizontal tube with rigid walls and of uniform or variable diameter under a steadily acting pressure, and 2. The flow of a liquid through a tube with elastic walls under an intermittently acting pressure. THE CIRCULATION OF THE BLOOD. 331 THE FLOW OF A LIQUID THROUGH A HORIZONTAL TUBE WITH RIGID WALLS. The phenomena and the laws which govern them, that attend the flow of a liquid through a rigid tube of uniform diameter under a steadily acting pressure may be readily observed in an apparatus similar to that represented in Fig. 153, wTiich consists of a reservoir or pressure vessel, P, provided with a horizontal tube into which is inserted at equal distances a series of vertical tubes. If the reservoir be filled with a liquid, water for example, the latter under certain conditions will exert a downward pressure and act as a pro- pelling or driving power, the degree of which will depend on the height of the column and may be represented by H. If the stopcock at O be opened the column of water, which has heretofore been exerting an equal pressure in all directions, will now exert a downward pressure only, and Fig. 153. — A Pressure Vessel, P, with an outflow horizontal tube, 0-n, into which vertical tubes or manometers are inserted. in consequence it will be driven into and through the horizontal tube and discharged from its free extremity with a definite velocity. The velocity can be determined by measuring the quantity, q, discharged in a given time, t, (i sec.) and dividing it by the area of the tube^ tp^; e.g., v=— Moreover in a tube of uniform diameter the velocity through each cross- section will be the same. As the water flows through the horizontal tube it meets with resistance, namely, the cohesion and friction of its molecules, and which must be overcome if the flow is to continue. Because of the fact that water will moisten most surfaces with which it comes in contact there will be an adhe- sion between the walls of the tube and the outer layer of the column of water, in consequence of which it will become more or less stationary. Between the outer stationary layer and the axis of the stream, there is an infinite number of layers of molecules, the cohesion of which one for the other steadily diminishes in passing from the periphery to the center of the stream. The extent of this cohesion will increase as the tube is lengthened, 332 TEXT-BOOK OF PHYSIOLOGY. and decrease as it is widened. In tubes of large diameter the axial cohe- sion is slight and hence the friction of the nu)lecules, the resistance to the flow, is readily overcome; in the tubes of small diameter the axial cohesion is relatively great and the resistance less easily overcome. As a result of the resistance the forward movement of the water under the pressure in P is somewhat retarded, and as a consequence it will exert a lateral or radial pressure against the walls of the tube, as shown by the rise of the water in the vertical tubes. The amount of the lateral pressure at any -given point is indicated and measured by the height to which the water rises. For this reason these tubes are termed pressure tubes or piezometers. Since the resistance in a tube of uniform diameter is proportional to its length the lateral pressure will gradually but progressively decrease from the reservoir to the outlet. Therefore the pressure at any given point is proportional to the resistance yet to be overcome and conversely the resist- ance to be overcome is indicated by the height of the pressure. (In the conduct of an experiment the propelling power should be kept constant by permitting fluid to flow into the reservoir as rapidly as it flows out of the horizontal tube.) The power or force which overcomes the resistance in the horizontal tube and imparts velocity to the fluid is the downward pressure of the water in the reservoir, represented by H. The amount of this power util- ized in overcoming the resistance is approximately indicated by the height of the fluid, y, at which poipt the line uniting the upper limits of the water in the vertical tubes intersects it. The height of the fluid at this point is a measure, therefore, not only of the resistance but also an indication of the relative amount of the pressure used in overcoming it and is therefore known as the pressure height. The amount of the pressure consumed in imparting the observed ve- locity is determined by ascertaining the height from which a particle must fall in empty space to acquire this velocity. This is obtained by dividing the square of the velocity by twice the accelerating force of gravity as ex- pressed in the formula, jg ; the quotient is the height and is known as the velocity height. Conversely if the moving fluid were discharged into empty space through an opening in the tube at n, it would ascend an equal dis- tance. If now this height is represented by F, and a line be drawn from it, parallel to the line of pressure until it meets the reservoir at x, it will be seen what percentage, x y, of the primary propelling power is consumed in imparting the observed velocity. Of the total pressure a small portion is left over which is utilized in forcing into, and overcoming the resistance offered by, the orifice of the horizontal tube. The initial pressure in P therefore divides itself into two portions; one, the larger by far, is utilized in overcoming the resistance to the flow of the water; the other, the smaller, in imparting velocity. Thus the two phenomena presented by the flow of a liquid through a tube with rigid walls and of uniform diameter are velocity and pressure, of which the former is the same for each cross-section, and the latter at any point directly proportional to the resistance to be overcome. If, instead of a horizontal tube of uniform diameter, there be substi- tuted a tube, the middle third of which is enlarged, the conditions will be the same as in the previous case until the fluid flows into the enlarged por- tion, when the velocity will diminish and will become inversely proportional THE CIRCULATION OF THE BLOOD. 333 to the area of the cross-section. The resistance will be also diminished and as a result the pressure"" rises or at least less of the initial pressure is utilized' in this than in the first section of the tube. When the liquid flows into the narrow or third section, the primary velocity returns. Though the resistance again increases the amount to be overcome is small, and hence there is a rapid and steady fall of pressure. On the contrary, if a tube be substituted, the middle third of which is narrowed, the conditions will be the same as in the previous cases until the liquid flows into the narrowed section, when at once the velocity increases and becomes inversely proportional to the area of the cross-section; the re- sistance being increased at the same time, there will be a rapid consump- tion and a steep fall of pressure. On flowing into the third section, the velocity again diminishes and the pressure falls though less slowly to the end of the tube. THE FLOW OF A LIQUID THROUGH A SERIES OF BRANCHING AND AGAIN RE-UNITING TUBES WITH RIGID WALLS. In a system of this character, such as represented in Fig. 154, there may be assumed as a result of the repeated branchings, a progressive increase in the total sectional area of the collective tubes coincident with a progres- sive decrease in the sectional area of individual tubes in the section B c, Fig. 154. — A Semes op Branching and again Re-uniting Tubes. while in the section c d, there is a progressive decrease in the total sectional area of the collective tubes coincident with a progressive increase in the sectional area of individual tubes. Assuming the system to be connected with a pressure vessel, as in the preceding instance, but which has been omitted from the figure, and the stop cock to be suddenly opened, the column of water will now exert a downward pressure, and in consequence the water will be driven into and through the system with a definite velocity and pressure. The velocity of a fluid through such a system should theoretically be decreased from b to c in a ratio inversely proportional to the total area of each cross-section. This, however, will not be the case because of the presence of the angles formed by the repeated branchings which, as de- termined experimentally, simultaneously increase the velocity. The extent to which the initial velocity will be changed will therefore be proportional 334 TEXT-BOOK OF PHYSIOLOGY. to the ratio between these two factors. If they balance each other there will be no change; according as the one or the other preponderates, there will be a corresponding decrease or increase in velocity. In flowing from c to d the velocity will be changed in the opposite di- rection and for the reverse reasons and in e it will have regained the value it had in A, if the entrance and exit tubes are of the same diameter. The lateral pressure should theoretically be increased from b to c be- cause of the increase in the total sectional area and the consequent diminu- tion of friction, but because of the increase in resistance due to the decrease in sectional area of individual tubes as well as by the angles formed by the branching of the tubes it will be simultaneously decreased. The extent to which the pressure will be changed will also be proportional to the ratio between these two factors. If they balance each other, there will be no change; according as one or the other preponderates will there be an in- crease or decrease in pressure. In passing from c to d the pressure will be changed in the same direction but for the reverse reasons. In a general way it may be said that in a system in which the succes- sive branchings are accompanied by a progressive decrease in dianleter, the influence of an increase in the total sectional area on velocity will prepon- derate over that due to the production of angles,* and hence the initial velocity will be decreased from B to c, and increased from c to D, for the reverse reasons; and if the tubes in the center of the system are capillary in character, the resistance offered by them and hence the consumption of the propelling power will preponderate over the decrease in resistance due to the widening of the stream bed, and hence as the liquid flows from b to c there will be a fall in pressure, and in flowing from c to d there will be an additional fall of pressure, owing to the narrowing of the stream bed and the increase in resistance, which is however slight in amount. The pressure throughout this system is the result of the resistance to the flow of the water and the extent of the pressure in any one section will be proportional to the resistance yet to be overcome. It will naturally be high in the section a-b and low in the section d-e. The value of these two pressures in these sections and their relation to each other may be varied temporarily or permanently by the introduction along the course of the tubes between b and c of a series of stopcocks. If the lumen of each stopcock has a certain average value, so as to permit of a certain outflow of water, the pressure will have a certain value in both A-B and D-E. But if the lumen of each stopcock is decreased, there will be an increase in the resistance and hence a rise of pressure in a-b and a fall of pressure in d-e. If, on the contrary, the lumen of each stopcock is increased, there will be a decrease in the resistance and hence a fall of pressure in a-b and a rise of pressure in d-e. The stopcocks may be spoken of as a variable peripheral resistance. In the foregoing exposition it has been assumed that in all instances the pressure in the pressure vessel was steadily acting. If, however, the pres- sure be made to act intermittently as it can be by alternately opening and closing the stopcock, both the velocity and the pressure will be alternately increased and decreased. The outflow of the fluid during the moment the pressure is acting will be rapid, and during the moment the pressure is not acting the outflow will cease. It becomes therefore intermittent. Coinci- dently there is an alternate temporary increase and decrease of the lateral pressure. THE CIRCULATION OF THE BLOOD. 335 THE FLOW OF A FLUID THROUGH A TUBE WITH ELASTIC WALLS. When a tube with elastic walls is connected with a pressure vessel, the conditions which are established on opening the stopcock and the conse- quent flow of water, will soon approximate those observed in a tube with rigid walls. As the water moves forward, it encounters friction, exerts a lateral pressure and causes a distention of the tube. This latter effect con- tinues until the elastic recoil of the walls of the tube exactly counterbal- ances the pressure of the water from within. When this condition is es- tablished the tube becomes practically a tube with rigid walls, and hence so long as the primary pressure is uniform, the velocity and lateral pressure will obey the laws which hold true for rigid tubes. If, however, the primary pressure be intermittently applied or alter- nately increased or decreased, and the water forced into the tube, previously filled with water but under no particular pressure, it will be forced out of the peripheral end of the tube more or less rapidly during the period of the increase of pressure and less rapidly during the period of the decrease of pressure or it may cease entirely. The extent to which the outflow be- comes merely remittent, or entirely intermittent, will depend on the amount of resistance, whether this be due to length of tube or a narrowed outlet, and the degree of elasticity. When these factors have a negative value the outflow will be intermit- tent. But if they are made to gradually change, and this is especially the case with the resistance, from a negative to a positive value, the outflow gradually changes from an intermittent to a remittent and finally to a con- tinuous outflow and for the following reasons: With a given resistance and elasticity, the fluid which is driven into the tube by the action of the primary pressure exerts more or less lateral pres- sure, gives rise to a distention of the tube and acquires a certain velocity of outflow. In consequence of the distention, a portion of the fluid accumu- lates. With the cessation in the action of the primary pressure, the elastic walls recoil and force the accumulated fluid forward and so maintain more or less effectively the same velocity of outflow until there is a return of the pressure. If the resistance and elasticity have a negative value this is im- possible and the outflow will be entirely intermittent. But if they are made to increase in value, the proportionate amount of the fluid which ac- cumulates during the action of the primary pressure will also increase in amount and hence there will be an increase in the distention of the tube. The elastic recoil will therefore be greater in amount and longer in dura^ tion, and hence the outflow will change to a remittent and finally to a con- tinuous outflow. Coincident with the action and cessation of action of the primary pres- sure there is a corresponding increase and decrease of the lateral pressure and when the intermittency in their action is suflSciently rapid, the excess of fluid entering the tube over that discharged becomes sufficiently great to maintain a certain average or mean pressure, which, however, undergoes an alternate increase and decrease with each variation in the primary pressure. The temporary increase and decrease of the pressure and the consequent expansion and recoil of the tube in the neighborhood of the pressure vessel, gives rise to a wave on the surface of the fluid which is propagated with 336 TEXT-BOOK OF PHYSIOLOGY. more or less rapidity — though with decreasing size from the beginning to the end of the tube and causing in each section a corresponding expansion and recoil, and known as the expansion wave. THE APPLICATION OF THE FOREGOING FACTS TO THE VASCULAR APPARATUS. The systemic vascular apparatus may be conceived of as a system of tubes which have symmetrically divided and subdivided and after- wards again united and reunited in a corresponding manner. The arteries, arterioles, capillaries, venules and veins may therefore be sche- matically arranged (Fig. 155) in a manner identical with the schematic arrangement of tubes represented on page 333. The heart, with which they are in connection, when filled with blood may, be compared with the reservoir filled with water, and the intra-ventricular pressure devel- CAPILL ARIES ARTERIES VEINS Fig. 155. — Schematic Arrangement of the Vascui-ar Apparatus. oped during the contraction, to the downward pressure of the water when the stopcock at O is opened (See Fig. 153). The Stream-bed. — The stream-bed, the path along which the blood flows, varies widely in its total sectional area in different parts of its course, being least in the aorta and venae cavse, and greatest in the capillaries. In passing from the base of the aorta toward the capil- laries the sectional area of individual arteries, in consequence of re- peated branching, diminishes, though their total sectional area in- creases and in direct proportion to their distance from the heart. In the capillary system the sectional area of an individual capillary attains its minimal value, though the total sectional area attains its maximal value. Comparing one with the other, it has been estimated that the total sectional area of the aortic bed is to the total sectional area of the capillary bed as i is to 600 or 800. In passing from the capillary into the venous system the sectional area of individual veins increases, though the total sectional area decreases and in direct pro- portion to their distance from the capillaries. The stream-bed in the aorta is relatively narrow, but widens grad- THE CIRCULATION OF THE BLOOD. 337 ually as it approaches the capillaries, where it attains its maximum width; it again narrows gradually as it passes into the veins, until in the venae cavas it becomes almost as narrow as in the aorta. As the combined sectional areas of the venae cavae are greater than the sectional area of the aorta, the stream-bed of the former never becomes as narrow as that of the latter. The gradual increase in the width of the stream-bed from the beginning of the aorta to the middle of the capillary system, and the gradual decrease in the width of the stream-bed from the middle of the capillary system to the terminations of the venae cavae, which Fig. 156- — Diagram Designed to Give an Idea op the Aggregate Sectional Area of the Different Parts' of the Vascular System. A. Aorta. C. Capillaries. V. Veins. The transverse measurement of the shaded part may be taken as the width of the various kinds of vessels, supposing them fused together. — ( Yeo.) results from the repeated branching and subsequent reuniting, as well as its relative width in the arteries, capillaries and veins, is shown graphically in Fig. 156. When the heart contracts and when the intra-ventricular pressure rises above the pressure in the aorta, the aortic valves are forced open and the blood is driven into and through the arteries, capillaries and veins and empties into the right side of the heart with a definite velocity and pressure. Because of the fact that the stream-bed progressively increases in width from the aorta to the middle of the capillary system the velocity through each cross-section should theoretically be inversely proportional to its total sectional area. This, however, strictly speak- ing, is not the case, owing to the accelerating influence of the angles formed by the repeated branchings. But for the reason that the in- fluence of the former factor so largely preponderates over the influence 338 TEXT-BOOK OF PHYSIOLOGY. of the latter factor, it may be accepted that the initial velocity of the blood in the aorta gradually decreases inversely as the sectional area increases, until it attains its minimal value in the capillary system. The actual velocity is the resultant of these two opposing forces. For the reverse reason, viz., the narrowing of the stream-bed and the retarding influence of the angles, the velocity of the blood gradually increases from the middle of the capillary system to the end of the venas cavae though it will never attain its initial velocity in these vessels because their combined sectional areas are greater than that of the aorta. The accelerating effect of the narrowing of the stream-bed preponderates over the retarding effects of the angles and the velocity will be the resultant again of these two opposing forces. The same facts of course hold true for the pulmonic vascular apparatus. In its passage through the vessels the blood meets with resistance, viz., the cohesion and friction of its molecules which must be over- come if the flow or the movement of the blood is to continue. The statements made in previous paragraphs regarding the causes and the results of friction in tubes with either rigid or elastic walls hold true for the most part also for the blood-vessels. As a result of friction the forward movement of the blood is somewhat retarded and in consequence will exert a lateral or radial pressure against the walls of the vessels, and from the facts previously stated, this pressure should diminish in a more or less progressive manner from the origin of the aorta to the ends of the venae cavse. That this is the case can be demonstrated for the arteries and veins at least by the insertion of pres- sure tubes and into which the blood can ascend. The height of the pressure at any given point in the system will be proportional to the resistance yet to be overcome. Inasmuch as the stream-bed is not uniform in diameter the fall in pressure will not be uniform. As it enlarges in its middle portion the resistance lessens and theoretically there should be a rise of pressure in passing towards the capillary system, but as the repeated branching of the vessels is attended by an enormous diminution in the diameter of individual vessels, the resistance is proportionately increased and for this reason there should be a great consumption of the propelling power and a marked and rapid fall of pressure at the periphery of the arterial and throughout the capillary systems. That this is the case, though not to the extent it might be, is because the effect of the increase in resistance so largely preponderates over the effect of the increase in the width of the stream-bed. The actual fall of pressure is therefore the resultant of these two opposing factors. For the reverse reasons, however, viz., the narrowing of the stream-bed and the increase in the sectional area of individual tubes, there will be a further consump- tion of the propelling power and a further fall of pressure to the ends of the venae cavse. The effect of the increase in resistance of the rapidly narrowing stream-bed preponderates over the effect of the enlarge- ment of diameter of individual vessels. The fall of pressure from THE CIRCULATION OF THE BLOOD. 339 the middle of the capillary system to the ends of the venae cavffi will be again the resultant of these two factors. The pressure throughout the vascular apparatus is, of course, the result of the resistance offered to the flow of the blood and therefore it will be high in the aorta and its main branches and low in the large veins. The amount and the relation of these two pressures in these two sections of the vascular apparatus can be temporarily or permanently changed in one direction or another by an increase or decrease in the resistance offered to the flow of blood from the arteries through the capillaries into the veins. This variation in the resistance is brought about by an increase or a decrease in the degree of the contraction of the arteriole muscles. Thus, if the muscle contraction increases, the resistance is increased and the pressure in the arteries rises; if, on the contrary, the muscle contraction decreases, the resistance diminishes and the pressure falls in the arteries and rises in the veins. The contraction of the arteriole muscle is spoken of as the peripheral resistance. The Distribution of the Intra-ventricular Pressure. — The pressure developed during the ventricular contraction is thus expended in imparting velocity to the blood and overcoming the cohesion and friction of its molecules. The percentage of the pressure utilized in overcoming the resistance could be approximately determined from the pressure in the aorta if this were accurately known; the percent- age of the pressure utilized in imparting velocity could be determined with the formula — , if the actual velocity of the blood in the aorta could be experimentally determined. On account of the difhculty in obtaining this latter factor at least, the results must only be approxi- in-ative. An id£a of the ratio between the velocity pressure and the resistance pressure, hoWever, may be obtained from the distribution of the aortic pressure in the dog in reference to the carotid artery. Thus, if it be assumed that the average velocity of the blood is 35 cm., the velocity pressure is equal to ^^ or 0.62 centimeters of blood or 0.046 centi- meters of mercury, and if the average aortic pressure is 150 mm. of mercury, the ratio of the velocity pressure to the resistance pressure is as I to 326. The phenomena which for the most part characterize the flow of blood through the blood-vessels are velocity and pressure, combined with an alternate expansion and recoil of the arterial vessels due to the intermittent character of the heart-beat. For special reasons it is convenient to consider the pressure first. BLOOD-PRESSURE. From theoretic considerations alone it may be inferred that the blood, as it flows through the vascular apparatus, exerts a pressure against the walls of the vessels, and that this pressure is greatest at the 340 TEXT-BOOK OF PHYSIOLOGY. beginning of the aorta, and least at the ends of the venae cavse. The fact that the blood flows from the aorta to the venae cavae indicates that there is a higher pressure in the former than in the latter. The same holds true for the pulmonary artery and veins. So long as this is the case, the blood must flow from the point of high to the point of low pressure. To this pressure the term blood-pressure is given, and may be defined as the pressure exerted radially or laterally by the moving blood-stream against the sides of the vessels. That there is such a pressure within the arteries, capillaries, and veins, different in amount in each of these three divisions of the vascular apparatus, is evident from the results which follow division of an artery or a vein of corre- sponding size. When an artery is divided, the Wood spurts from the opening for a considerable distance and with a certain velocity. The reason for this lies in the fact that the vessel has been distended by the pressure from within and its walls thrown ipto a condition of elastic tension, so that at the moment there is an outlet, the vessel suddenly recoils and forces the blood out with a velocity proportional to the distention. When a vein is divided, the blood as a rule merely wells out of the opening with but slight momentum, and for the reason that the vessel has been but slightly, if at all distended by the pres- sure. These results indicate that the blood in the arteries stands under a pressure considerably higher than that of the atmosphere, while that in the veins stands under a pressiu"e perhaps but slightly above that of the atmosphere. Especially true is this of the larger veins. The same facts may be demonstrated in another and more striking way. A dog or cat is anesthetized and securely fastened in an appro- priate holder. The carotid artery on the right side and the jugular vein on the left side are freely exposed and clamped. Into the artery there is inserted on the distal side of the clamp and in the direction of the heart a cannula to which is connected a tall glass tube, 200 cm. high and of about 4 mm. internal diameter. Into the vein there is passed on the proximal side of the clamp and in the direction of the capillaries a second cannula, to which is connected a similar tube, though of less height. If the two clamps are removed at the same time, the blood will mount in both tubes simultaneously. In the arterial tube the blood will ascend by leaps corresponding to the heart- beats until a certain height is reached, when the column becomes relatively stationary, being kept in equilibrium by the blood-pressure within the vessel and the atmospheric pressure without. Though stationary in a general sense, nevertheless, the blood-column oscillates, rising and falling with each contraction and relaxation of the heart. Not infrequently larger excursions of the column are seen which corre- spond in a general way to the respiratory movements. This experi- ment was originally performed on the horse, by the Rev. Stephen Hales (1732). THE CIRCULATION OF THE BLOOD. 341 In the venous tube the blood also rises to a certain height, after which it remains quite stationary, as the effect of the cardiac con- traction is not propagated under normal conditions beyond the arterial system. The height to which it rises is but slight as compared with that in the arterial tube. The pressure in both vessels is thus recorded in millimeters of blood. Strictly speaking the pressure thus obtained does not represent the lateral pressure in the carotid artery but in the vessel from which it arises. The central end of the carotid is, under the circumstances, but a continuation of the cannula and the pressure thus obtained is the lateral pressure of either the innominate artery ^M^ Fig. 157. — Diagram to show the Relation of the Mercurial Manometer to THE Artery, on One Hand, and to the Recording Cylinder, on the Other Hand, WHEN Arranged for Recording Blood-pressure. , or the aorta as the case may be. In order to obtain the lateral pressure in the carotid or any other artery it is only necessary to take the end pressure of any one of its branches or what amounts to the same thing, to divide the vessel and insert the horizontal portion of a T-shaped tube into the central and distal ends and through which the blood can continue to flow, and to connect the vertical portion with a vertical pressure tube or with a mercurial manometer. The absolute pressure on any given unit of vessel surface— e. g., 1 sq. mm.— is obtained by multiplying the height of the column, expressed in millimeters, by the unit of surface, and then determining the weight of this mass of blood. Thus if the height of the column of blood in the carotid artery tube is 2000 mm., then the pressure on i sq. mm. is 2000 mm. of blood. The weight of 2000 c.mm. of blood is equal to 2.1 grams. 342 TEXT-BOOK OF PHYSIOLOGY. The Arterial Blood-pressure. — ^For accurate and long-continued observation the arterial blood-pressure is more conveniently studied by means of a U-shaped tube (a manometer) partially filled with mercury. One limb of the manometer is connected by means of a tube and a cannula with an artery (Fig. 157). For the purpose of retarding coagulation of the blood and for preventing the escape of a large volume of blood from the vessels, the system is filled with a solu- tion of carbonate of soda of sp. gr. 1060, 5.58 grams per 1000 c.c, and under a pressure approximately equal to that in the vessel of the Fig. 158. — A Portion of a Blood-presstjke Tracing Obtained from the C.«iotid Artery of the Rabbit with a MERCtntiAi, Manometer. The small oscillations are due to the heart-beat, the large osciUations are due to the respiratory movements. animal as determined in previous experiments. When commun- ication is established between the vessel and the cannula, the mercurial column adjusts itself to the pressure in the artery and at once exhibits the, same cardiac oscillations and respiratory undulations as did the column of blood in the previous experiment. The height of the mercurial column kept in equilibrium by the pressure of the blood within, and the pressure of air without the vessel is that between the lower level of the mercury in the proximal, and the higher level in the distal limb of the manometer, both of which can be read off on a scale placed between the two limbs. The height of the mercury as well as its oscillations in the distal limb may be recorded by placing on the top of the mercury a light THE CIRCULATION OF THE BLOOD. 343 max valve to manometer mm valve float, the upper end of which carries a writing point. When the latter is placed in contact with the moving blackened surface of a recording cylinder or kymograph, the height and the oscillations are recorded in the form of a tracing similar to that shown in Figs. 157 and 158, in which the smaller oscillations represent the changes in pressure due to the systole and diastole of the heart and the larger oscillations to variations in the average pressure due to the respiratory movements. The height of the mercurial column kept in equilibrium at any particular moment is determined by measuring the distance between a base-line or abscissa, which represents the position of the mercury at atmospheric pressure, and any given point on the trace above, and multiplying it by 2, for the reason that the mercury sinks in the proximal limb as high as it rises in the distal limb of the manometer and hence the column of mercury sup- ported is that observed between the upper and lower level of the mercury in the distal and proximal limbs of the manometer. The blood-pressure as revealed by the tracing may be resolved into two components: viz., (i) a constant ele- ment and represented by the pressure in the arteries during the period of the cardiac diastole, which is termed the diastolic or minimum pressure; and (2) a variable element and represented by that additional pressure occurring at^the time of the cardiac systole, which is termed the systolic or maximum pressure. The diastolic pressure is represented by the distance between the base-line and the points of the curve corresponding to the diastolic rest; the systolic pressure, by the distance between the base-line and the apices of the curves following the cardiac systole. The relation of these two components varies in different animals and in the same animal at different times. If the diastolic pressure is low, the systolic increase may be considerable; if the former is high, the latter may be slight The relation, however, of these components is not so accurately shown by the mercurial manometer, owing to the inertia of the mercury, as by one of the various forms of quickly responsive spring man- ometers used in determining the rapid variations of intra-cardiac pressure These instruments show a much larger rise of pressure during the systole, often amounting to as much as one-third or one- fourth of the diastolic pressure. For the purpose of obtaining the maximum systolic and the mini- mum diastolic pressures, it is best, however, to insert between the Fig. 159. — V. to heart Frank's Valve. This is placed in the course of the tube between heart and manometer, so that the latter may be used as a maximum, minimum, or ordinary manometer according to the tap which is left open. — (Starling.) 344 TEXT-BOOK OF PHYSIOLOGY. cannula and the manometer a maximum and a minimum valve similar in principle to that shown in Fig. 159. By permitting the blood to exert its pressure first through the maximum valve and then per- mitting the mercurial column to exert its pressure through the mini- mum valve in the reverse direction for a certain length of time, or by permitting each to exert its pressure alternately with each heart-beat, the maximum systolic and the minimum diastolic pressures will be re- corded. By this method Dawson found an average maximum pres- sure in the carotid artery of the dog of 162, and a minimum pressure of 103 mm. of mercury, a difference of 59 mm. Hg. The difference between these two pressures is known as the pulse pressure. / (A dia- gram showing the relation of these different pressures one to another will be found on page 346.) In a series of experiments it will be found that the blood-pressure in the arteries, though rising and falling a certain number of milli- meters, yet retains a fairly constant general average, the result of an adjustment between the number of heart-beats per minute and the amount of the resistance offered to the escape of blood into the capil- laries and veins. Between the two extremes there ,is, of course, a certain average or mean pressure which represents the power driving the blood- through the vessels. It is frequently stated that in a tracing in which the respiratory undulations are absent, that the mean pressure is th« ar-ithmetic mean of the systolic and diastolic pressures. This is, however, not strictly correct, as can be demonstrated experi- mentally. Thus, if at some one point between the artery and the manometer, the lumen of the connecting tube be largely obliterated by a constriction, the variations in the pressure following the systole and diastole of the heart will be largely, if not entirely excluded, and the mercury, instead of rising rapidly in the manometer and fluctuating with each heart-beat, will rise slowly to a certain level and then remain at rest. The number of millimeters of mercury thus supported represents the mean or absolute pressure.. The same result can be obtained by employing the compensatory manometer of Marey which presents a constriction of this character. From many experiments made by Dawson it has been learned that the mean pressure lies nearer to the diastolic than to the systolic pressure and may be ex- pressed nunierically by the statement that it is equal in millimeters of mercury to the diastolic pressure plus one-third of the pulse pres- sure. In a tracing in which the respiratory undulations are present the mean pressure can be calculated. The method by which this is done, however, is rather complicated a'nd need not be detailed here. In a general way the mean pressure in such a tracing may be repre- sented by a line drawn horizontally across the tracing midway between the apex and trough of the undulation. Estimates of the Mean Arterial Pressure. — Because of the difficulty in obtaining the pressure in small arteries, the experimental determinations have for the most part been confined to large arteries THE CIRCULATION OF THE BLOOD. 345 such as the carotid, brachial and femoral, and hence the results which have been obtained have reference to the lateral pressure in the aorta or in the large vessels which immediately arise from it. The pressure obtained in the usual way at the central end of a divided carotid is generally known as the "end pressure" and represents the mean lateral pressure in the aorta or in the innominate artery. Among the results, thus obtained in different experiments from the carotid artery of different animals are the following: In the horse, from 122 to 214 mm. Hg.; in the dog, from 140 to 160 mm.; in the cat, 150 mm.; in the rabbit, from 90 to 100 mm.; in the sheep, 170 mm.; inthe calf, from 133 to 165 mm. In two observations made on human beings previous to the amputation of a limb, the pressure was found in the brachial artery of one patient to vary from no mm. to 120 mm. Hg., and in the anterior tibial artery of the other patient from no mm. to 160 mm. Hg. The investigations made in different parts of the arterial system indicate that the mean pressure is remarkably constant and uniform and does not show any noticeable falling off until near the arteriole region where the resistance suddenly and rapidly increases. Thus Volkman found simultaneously in the carotid artery and in the meta- tarsal artery of the sheep a mean pressure of 165 and 146 mm. Hg. respectively and this for the reason that the resistance throughout the arterial system does not markedly increase, until the arteriole region is reached. The careful investigations of Dawson show that in the large blood-vessels of the dog the diastolic pressure is as con- stant as the mean pressure though it undergoes slight variations in different regions; but that the systolic pressure, as shown by taking the end pressure in the thyroid and similar sized arteries in different parts of the arterial tree, undergoes a considerable falling off, though it, too, remains high in large arteries. The numerical expressions of these various pressures in different parts of the arterial system are shown in the following table abstracted from the more extensive tables of Dawson. The results were obtained from experiments made on dogs. The figures represent in milli- meters of mercury certain average end pressures in the arteries named. Artery. Systolic. Mean. Diastolic. Pulse Pressure. Brachio-cephalic 163 160 160 168 160 i6S 152 140 121 118 123 123 118 123 118 118 103 no lOI lOS no 103 102 97 60 s° Left carotid S9 Left subclavian T^pft brachial 63 5° Left renal 62 5° Thyroid 43 The Capillary Pressure.— The small size of the capillaries pre- cludes an investigation of their pressure by manometric methods. It may be stated, however, to be approximately equal to the pressure 346 TEXT-BOOK OF PHYSIOLOGY. required to obliterate their lumen and to whiten the skin. The apparatus of v. Kries is based on this theory. A small glass plate, from 2.5 to 5 sq. mm., is fastened to the under surface of a support of suitable size carrying a small scale pan. The glass plate is placed on the skin near the root of a finger-nail and the scale pan gradually weighted until the vessels are obliterated, as shown by the blanching of the skin. From results obtained with this apparatus v. Kries L ine of SYSTOLIC PRESS UQE Line of /^eAf^ PRESSU/fE PULSE P/f£SSU/fE^ TJje cfifference Seiiveen SYSTOLIC ancf O/ASTOLIC PPESSt/ftE c Fig. 160. — A Diagram Designed to Show the Amount and the Relation of THE BlOOD-PEESSTTRE IN THE THREE DIVISIONS OF THE VaSCCTLAR APPARATUS, AS WELL AS THE Relation of the Diastolic. The mean and the systolic pressures in the arterial system. Based on experiments made on dogs. H. Heart. A. Arteries. C. Capillaries. V. Large veins. O, O, being the zero line (= atmospheric pressure), the pressure is indicated by the height of the curve. The numbers on the left give the pressure (approxi- mately) in millimeters of mercury, h. Pressure in heart, a. Arteriole region showing sudden fall of pressure, c. The fall of pressure in the capillaries, v. The negative pres- sure in the large veins. estimated the pressure in the capillaries of the hand at 37 mm. Hg. and in the ear at 20 mm. The Venous Pressure. — ^In passing from the capillaries to the heart the pressure continues to fall. The increasing size of the veins permits again of manometric observations in different regions. In the crural vein the pressure has been found to be equal to 14 mm. Hg., and in the brachial vein 9 mm. of Hg. In the jugular and subclavian and other vessels near the heart it is zero or even negative; that is, less than atmospheric pressure to the extent of from i to 10 mm. of mercury. The amount and relation of the different pressures in the three divisions of the systemic vascular apparatus are approximately shown in Fig. 160. THE CIRCULATION OF THE BLOOD. 347 RESUME OF THE FACTS OF THE BLOOD-PRESSURE AND OF THE FACTORS WHICH CAUSE IT. From a consideration of the foregoing facts and statements the following resum^ may be made: i. The blood during its flow exerts a pressure against the sides of the blood-vessels. 2 . This pressure is the resultant on the one hand of the intra-ventricular pressure developed at the time of the contraction, and on the other hand of the resistance to the forward movement of the blood. 3. The resistance is to be sought for in the cohesion and friction of the molecules of the blood. 4. The resistance is proportional to the diameter of the vessel and is therefore least in the large arteries and veins and great- est in the arterioles and capillaries. 5. The pressure is highest in the aorta where it may amount in man to 150 mm. of mercury above that of the atmosphere, and lowest at the ends of the venae cavas where it may be no greater than that of the atmosphere or may be even 10 mm. Hg. below it. 6. The pressure falls from the beginning to the end of the vascular apparatus, though not progressively, for throughout the large vessels of the arterial system it continues relatively high. 7. The high pressure in the aorta is due to the total resistance of the vascular apparatus and the pressure at any given point- of the appa- ratus represents the resistance yet to be overcome. 8. The high pressure in the arterial system and its marked fall at its periphery is more especially the result of the very great resistance at this point, known as the peripheral resistance, the result of a rapid diminution in the diameter of the arterioles and the capillary vessels, modified by the tonic contraction of the arteriole muscles. 9. The pressure in the arterial system undergoes considerable variation both above and below the mean pressure during the systole and diastole of the heart. The Heart. — The primary factor in the production of the pressure is the pumping action of the heart. Should there be any cessation in its activity, the elastic walls of the arteries would recoil and force the blood into the veins. There would be coincidently a fall of pres- sure equal to that of the atmosphere. Even under normal circum- stances this condition is approximated during the diastole. The recoil of the arterial wall by which the forward movement of the blood is maintained is attended by a fall in pressure. But before this reaches any considerable extent, the heart again contracts and forces its con- tained volume of blood into the arteries. That this may be accomplished it is essential that the cardiac energy be sufficient not only to drive a portion of the blood through the capillaries into the veins, but to oppose the recoiling arteries, and to distend them to their previous extent, so that the incoming volume of blood may be accommodated. This at once reestablishes the pressure at its former level. During the contraction of the heart the kinetic energy is trans- 348 TEXT-BOOK OF PHYSIOLOGY. formed into potential energy, represented by the tense distended walls of the arteries. With the relaxation of the heart and the closure of the semilunar valves the potential energy of the arteries is again trans- formed into kinetic energy, represented by the moving blood. The artery thus continues the work of the heart during its period of in- activity. The rapidity with which the cardiac contractions succeed each other prevents the pressure from sinking below a certain average level. The Resistance. — The secondary factor is the resistance to the flow of blood through the vessels, the nature of which has been pre- viously stated. So long as the resistance, and especially that variable element of it at the periphery of the arterial system-, maintains a certain average value, so long will the pressure in each division of the vascular apparatus maintain an average or a physiologic value. Should the resistance at the periphery of the arterial system vary in either direction, the result of an increase or a decrease in the degree of the contraction of the arteriole muscle, there will arise a change in the relative degree of pressure in each of the three divisions of the vascular apparatus. The Elasticity of the Vessel Walls. — A tertiary factor is the elasticity of the arterial wall. While it can hardly be said that the elasticity is a cause of the pressure, there can be attributed to it the capability of modifying and assisting in the maintenance of the pressure at a more or less constant level; for were it not for this property of the vessel wall the variations in pressure during and after the systole would be far more extensive than they are, and would approximate the variations observed in tubes with rigid walls. The elasticity, moreover, assists in the equalization of the blood-stream, converting the intermittent and remittent flow characteristic of the large arteries into the continuous equable stream characteristic of the capillaries. It also permits of wide variations in the amount of blood the arteries can contain between their minimum and maximum distention. VARIATIONS IN THE BLOOD-PRESSURE. A. In the Arterial Pressure. — It is evident from the preceding statements that the arterial blood-pressure as a whole may be increased by: 1. An increase in the rate or force of the heart's contraction. 2. An increase in the peripheral resistance. 3. An increase in both the force of the heart and the peripheral resistance. And that it may be decreased by : 1. A decrease in the rate and force of the heart's contraction. 2. A decrease in the peripheral resistance. 3. A decrease in both the force of the heart and the peripheral resistance. THE CIRCULATION OF THE BLOOD. 349 If when the arterial pressure is in a condition of equilibrium the heart ejects into the arteries in a given period of time an increased quantity of blood as a result of an increased rate of contraction, there will be an accumulation of blood temporarily in the arteries and a rise of pressure (the peripheral resistance remaining the same), for the reason that the pressure is only sufficient to force into the capillaries a given volume, in the same period of time. As the pressure rises the velocity and the outflow will be increased until equilibrium is restored though at a somewhat higher level. A rise of pressure from an increase in the rate of the beat alone has been questioned, for it has apparently been demonstrated that there is a definite relation between the normal rate and the volume discharged from the ventricle, and that when the rate is increased, the volume discharged diminishes Fig. 161. — A Tracing showing an Increase in the Blood-pressure in the Carotid Artery of a Rabbit dde to an Increase in the Peripheral Resistance FROM A Contraction of the Arterioj.es Caused by Reflex Stimulation of the Vaso-motor Center. The nerve stimulated was the sciatic. Stimulation began at s. The rate of the heart-beat is unchanged. With the cessation of the stimulation the blood- pressure falls for the reverse reasons. and hence the pressure remains normal or even falls below the normal. An increase in the pressure is readily brought about by an increase in the force or power of the contraction, the frequency remaining the same. An increase in the volume of blood ejected at each contraction will necessarily lead to an accumulation. With the accumulation there goes an increased distention of the artery and a corresponding increase of pressure. In a short time, therefore, the increased pressure will force out of the arteries at a higher rate of speed this excess of blood until the outflow again equals the inflow. This restores the equilibrium but establishes the mean pressure at a higher level. If the peripheral resistance is increased by a contraction of the muscle walls of the arterioles, the frequency and force of the heart remaining the same, there will also be 'an accumulation'of blood in the arteries, an increased distention and consequent rise of pressure (Fig. 161). The outflow of blood will at the same time be diminished. Arise of pressure from this cause much beyond the normal is to a large extent prevented by a simultaneous decrease in the rate and force of the heart-beat. This is due to a stimulation of the peripheral ends of the 3SO TEXT-BOOK OF PHYSIOLOGY. depressor nerve, and a consequent reflex stimulation of the cardio- inhibitor center, and not to a direct action on the heart-muscle, inas- much as the effect is not observed after division of the vagi. When both the force of the heart and the peripheral resistance are simul- taneously increased there is a rapid increase in pressure; the former factor tends to increase, the latter factor, to decrease, the velocity of the outflow. According as the one or the other preponderates, will there be an increase or decrease in velocity. If they balance each other, there will be no change. A rise of pressure from a combi- nation of these factors is rather L pathologic than a physiologic ;ondition and is observed in cer- ain diseases of- the vascular ap- paratus. The converse of these state- ments also holds true. If when the general arterial pressure is in a condition of equilibrium the heart ejects into the arteries in a given period of time a less- ened quantity of blood, either as a result of a decrease in the rate or force, there will soon be a diminution of the arterial disten- tion and a consequent fall in pressure (Fig. 162). The velocity at the same- time diminishes. This continues until the outflow no longer exceeds the inflow. Equilibrium will again be estab- lished, but the pressure will be at a lower level. If the peripheral resistance is diminished by a dilatation of the arterioles, the heart's con- tractions remaining the same, the existing pressure soon diminishes. The outflow of blood at once increases in velocity. As a rule a diminution in peripheral resistance is attended by an increase in the rate or force of the heart, and this is especially the case if the pressure has been above the normal. When both the force of the heart and the peripheral resistance are simultaneously diminished, there will be a rapid fall in pressure. The former factor tends to decrease, the latter factor to increase the velocity of outflow. According as the one or the other preponderates will there be a decrease or an increase in velocity. If they balance each other there will be no change. This condition is also a pathologic Fig. 162. — A Tracing of the Blood- pressure IN THE Carotjp Artery of a Rabbit, showing a sudden decrease in the pressure due to an arrest in the rate and force of the heart-beat the result of stimulating the vagus nerve from " on " to "off." With the cessation of the stimu- lation the pressure began to rise as the rate and the force of the heart-beat re- turned. (The abscissa should be 20 mm. lower.) THE CIRCULATION OF THE BLOOD. 351 rather than a physiologic condition and observed in states of profound depression due to serious injuries. Local Variations in the Arterial Blood-supply.— The varia- tions in pressure and velocity from variations either in the activity of the heart or in the peripheral resistance recorded in preceding para- graphs, have reference to the arterial system in its entirety; but it is evident from many facts that similar variations take place in special re- gions or organs of the body. Thus, it is a well-known fact that for the exhibition of the functional activity of every organ there must be an in- crease in the volume of blood supplied to it in each unit of time. This is accomplished by an active dilatation of the arterioles of the artery of supply, and unless the area or organ supplied is large, as the splanchnic area for example, there will be no necessary diminution in either the general blood-pressure or the average velocity. With the cessation of functional activity, there is no longer any need for so large a blood-supply and hence the arterioles contract, diminish the outflow, and raise the pressure. If, on the other hand, the area to be supplied be large, as the splanchnic area, the dilatation of the intestinal arteries will be attended by such a large inflow of blood that not only will there be a fall of pressure in these vessels, but a fall of pressure in other arteries as well, combined with a diminution in velocity through them. With the contraction of the intestinal arteries the reverse conditions at once arise. By constant variations in the peripheral resistance of individual arteries in each and every region of the body, and in association with variations in the rate or force of the heart, the blood is shunted now into this, now into that organ in ac- cordance with its functional needs. All variations in peripheral re- sistance are largely brought about reflexly by the vaso-motor nerves, the origin, distribution and mode of action of which will be considered in subsequent paragraphs. B. In Capillary Pressure. — The pressure in the capillaries, though for the most part possessing a permanent value, is subject to variations in accordance with variations in the pressure in either the arterial or venous systems or both. The marked difference in the pressure in the large arteries and the capillaries is partly due to the resistance offered by the narrow arterioles. If the latter dilate in any given area, the capillary pressure increases because of the propaga- tion into them of the arterial pressure. The reverse condition would decrease the pressure. On the other hand, any interference with the outflow from any given area, due to venous compression, would likewise increase the pressure; any factor which would, on the con- trary, favor the outflow would decrease the pressure. Independent of any change in the arteriole resistance, it is evident that a rise in arterial pressure alone would increase the capillary pressure. If both arterial and venous pressures rise, the capillary pressure increases; if both fall, it decreases. C. In Venous Pressure.— Independent of any change m the 352 TEXT-BOOK OF PHYSIOLOGY. venous pressure in a given area from local or temporarily acting causes — e. g., aspiration of the thorax or heart, muscle contractions, change of position, etc. — the general venous pressure will be increased by a decrease in the value of those factors which produce the difference of pressure between the arteries and veins. An increase in the value of these factors would necessarily decrease the pressure. Variations in the Relation of the Arterial and Venous Pres- sures. — So long as the heart maintains a given rate and force and the resistance at the periphery of the arterial system (due to the contraction of the arteriole muscle) a given value, will the usual physiologic difference between the pressure in the arteries and veins remain un- changed. If, however, either factor changes in one direction or another, there will arise a change in the relative degree of pressure in the dif- ferent divisions of the vascular apparatus. Thus if the heart force increases and a larger volume of blood is discharged into the arteries in a unit of time, the amount of blood in the venous system diminishes, and the result is a rise of the arterial and a fall of the venous pressures. If, on the contrary, the heart force decreases or the mitral valve permits of a regurgitation, a smaller volume of blood is ejected into the arteries in a unit of time, the amount of blood in the venous system increases, and the result is a fall of the arterial and a rise of the venous pressure. Again if the arteriole muscle relaxes and a larger volume of blood flows from the arteries into the veins in a unit of time, the result will be a fall of arterial and a rise of venous pressure. If, on the contrary, the arterial muscle contracts and a smaller volume of blood flows into the veins, the reverse change of pressure obtains. The Determination of the Arterial Blood-pressure in Man.^ Inasmuch as the blood-pressure undergoes considerable variation in both physiologic and pathologic conditions as well as in response to the action of drugs, it seemed desirable to possess some means by which an accurate knowledge of the pressure under a variety of conditions could be obtained both for diagnostic and therapeutic purposes. The foregoing method of obtaining the blood-pressure not being of general application to human beings for obvious reasons, special instruments have been devised by which the pressures may be deter- mined at least approximately without resorting to any surgical proce- dure. These instruments are turned sphygmomanometers. Some of the many forms of this instrument are adapted for obtaining the systolic pressure only, while others are adapted for obtaining either the systolic or the diastolic pressure, or both. The principle involved in the first group is the application of an elastic pressure to an artery, e. g., the temporal, radial, etc., until the lumen is completely obliterated as indicated by the disappearance of the pulse beyond the point of compression, and at the same time reg- ister the pressure applied, by means of a mercurial or spring man- ometer. The pressure just sufficient to obliterate the pulse or to allow it to reappear after obliteration, is taken as the systolic pressure. THE CIRCULATION OF THE BLOOD. 353 The principle involved in the second group is based on a suggestion of Marey, that the maximum pulsation of the artery or the maximum distention and recoil following a heart-beat would be most likely to take place when an elastic pressure applied to the outside of an artery is just sufficient to equalize the diastolic pressure within. Inasmuch as these pulsations can be transmitted to, taken up and reproduced by a mercurial column in connection with the pressure appliances, it becomes possible, when the maximum oscillation of the mercurial column is attained, to read off the diastolic pressure. With either form of apparatus it became necessary to devise a suitable elastic sac or tube enclosed by non-elastic or rigid walls and Fig. 163. — The Sphygmomanometer of Mosso. which could be made to encircle a finger or an arm, and which could in turn be connected with a pressure apparatus, and with a manometer by which any given pressure could be registered. One of the best known of the sphygmomanometers in that of Mosso represented in Fig. 163. It consists essentially of rubber capsules, contained within metallic tubes and into which two fingers of each hand can be inserted. This system is connected, on the one hand, with a pressure apparatus, and, on the other, with a manometer pro- vided with a scale. A float and writing-pen record the movements of the mercurial column on a moving blackened surface. In using this apparatus the pressure is adjusted to the point at which the mer- curial column exhibits the greatest oscillations. Mosso' s interpretation of the results obtained with this apparatus was that when the greatest oscillations of the mercurial column were taking place, the external pressure was just equal to the mean arterial pressure, the latter being the mean between the maximum pressure 23 354 TEXT-BOOK OF PHYSIOLOGY. during the systole and the minimum pressure during the diastole of the heart. It was only necessary, therefore, to take the readings cor- responding to the excursions of the mercurial column and to determine from them the mean arterial pressure. It has been experimentally demonstrated, however, by Howell and Brush that this interpretation, either for this or any similar form of apparatus, is not correct, but that the maximum oscillations take place when the pressure applied to the exterior of the artery is just equal to the pressure within the artery at the end of the cardiac diastole; or in other words, the pressure in the manometer from which the greatest oscillation takes place indicates diastolic pressure. These experimenters connected the right carotid artery of a dog with a mercur- ial manometer, interposing along the coiu-se of the connecting tube a maximum and a minimum valve. The left carotid artery was sur- rounded by a plethysmograph which was connected, on the one hand, with both a mercurial and a spring manometer, the former for the purpose of indicating the pressure necessary to obtain the greatest oscillation, the latter for the purpose of magnifying and recording the pulsation. When the observations were simultaneously made it was found that the diastolic pressure in the right carotid measured by the minimum manometer was almost exactly equal to the pressure measured by the manometer in connection with the sphygmomanometer surrounding the left carotid artery, when it was exhibiting its maxi- mum excursions. The difference in the results of the two sides scarcely exceeded more than one or two millimeters of mercury. It was, therefore, established that the greatest oscillations record diastolic pressure. It was also shown by the same investigators that ^NIosso's apparatus is not adapted for obtaining systolic pressure. Among the many forms of sphygmomanometers adapted for clinic purposes and with which both systolic and diastolic pressures may be obtained is that devised by Stanton* (Fig. 164). The pressure is ap- plied to the arm by the rubber armlet h, which is 3 J inches wide. This is the widest armlet that can be adjusted to the average-sized arm and presents distinct advantages over the narrow armlet hitherto employed. This armlet is prevented from expanding outward by a cuff, F, of double thick canvas with inserted strips of tin, which is held in place by two straps which completely encircle the cuff. On the rigidity of this depends to a large extent the transmission of pulsation. The rubber armlet is connected by glass with a stiff-walled rubber tube, G, which in turn connects with the manometer. The manometer is perhaps the most important part of the apparatus. It is constructed entirely of metal except for the glass tube containing the mercury column. The chamber c communicates by means of a metal tube with the glass column d, which is connected by. a screw- thread at 3, the caliber of c being approximately 100 times that of d. ♦ The following description of this apparatus is abstracted from the Univ. of Pa. Medical Bulletin, Feb., 1903. THE CIRCULATION OF THE BLOOD. 363 of the aorta and its branches expand in quick succession, and by the time the systole is completed the expansion has traveled over the entire arterial system as far as the capillaries. With the cessation of the systole and perhaps even before, the recoil of the arterial walls at once occurs, beginning at the root of the aorta and rapidly passing over the arteries to the capillaries. This expansion movement which thus passes from the beginning to the end of the arterial system in the form of a wave is known as the pulse-wave or pulse. Preceding and causing the expansion and recoil of the arterial system there is an alternate increase and decrease of the general blood-pressure, as shown by the small curves on a blood-pressure tracing, and for this reason the pressure which causes the expansion and recoil is termed the pulse pressure. It is defined as the rhythmic change in pressure at any given point of the arterial system; and in amount, is the difference between the diastolic and the systolic pres- sures, at the corresponding points. The volume of blood ejected from the ventricle is frequently termed the pulse volume. The pulse-wave which thus spreads itself over the entire arterial system with each systole of the heart can be perceived in certain locali- ties by the eye, by the sense of touch, and investigated with various forms of apparatus or instrumental means. The pulse-wave, or at least the elevation of the soft tissues overlying it, can be seen in the radial artery, where it passes across the wrist-joint, in the carotid artery, in the temporal artery, in the arteries of the retina under certain conditions, with the ophthalmoscope. If the ends of the fingers are firmly placed over the radial artery, not only the increase and decrease of pressure, but also many of the peculiarities of the pulse-wave, may be perceived. Without much difficulty it may be perceived that the ex- pansion takes place quickly, the recoil relatively slowly; that the waves succeed one another with a certain frequency, corresponding to the htart-beat; that the pulsations are rhythmic in character, etc. Inas- much as the individuality of the pulse-wave varies at different periods of life and under different physiologic and pathologic conditions, vari- ous terms more or less expressive, have been suggested for its varying peculiarities. Thus the pulse is said to be frequent or infrequent accord- ing as it exceeds or falls short of a certain average number — 72 per minute; quick or slow, according to the suddenness with which the ex- pansion takes place or strikes the fingers; hard or soft, tense or easily compressible, according to the resistance which the vessel offers to its compression by the fingers; large, full, or small, according to the vol- ume of blood ejected into the aorta, or, in other words, the degree of fullness of the arterial system. Frequency of the Pulse. — As the pulse or the arterial expansion and recoil is the direct result of the heart's action, its frequency must, under physiologic conditions, coincide with that of the heart. All conditions which modify the rate of the heart will modify at the same time the rate of the pulse. 364 TEXT-BOOK OF PHYSIOLOGY. The Velocity of Propagation of the Pulse-wave. — The propag- ation of the pulse-wave from its origin at the root of the aorta to any given point of the arterial system occupies an appreciable period of time. The difference in time between the systole and the appearance of the pulse-wave at the dorsal artery of the foot can be appreciated by the sense of touch. The absolute time occupied by the wave in reaching this point was determined by Czermak to be 0.193 second. The rate at which the wave is propagated over the vessels of the lower extremity has been estimated by the same observer at 11. 16 meters per second, and for the upper extremities at but 6.7 meters per second. Other,, experimenters have obtained for the lower extremities some- FiG. 170. — Von Feey's Sphygmograph. G. S. Metal framework. P. Button at tached to spring. F. Vertical rod. U. Clock-work which turns the recording cylinder. VI. Time marker. what different results, varying from 6.5 to 11 meters per second. Weber's original estimate was from 7.92 to 9.24 meters per second. The slower rate of movement in the vessels of the upper extremities has been attributed to a greater distensibility of their walls, a condi- tion unfavorable to rapid propagation. For this reason a low arterial tension will occasion a delay in the appearance of the pulse-wave in any portion of the body; a high arterial tension will of course have the opposite effect. The difference in the speed of the pulse-wave and the blood-current shows that they are not identical and must not be confounded with each other. The Sphygmograph. — The sphygmograph is an apparatus de- signed to take up, reproduce, and record the alternate expansion and recoil of an artery caused by the temporary increase and decrease of pressure following each heart-beat. The tracing or record obtained with it is termed the pulse-curve or the sphygmogram. Different forms of this apparatus have been devised by Marey, Dudgeon, v. Frey, and THE CIRCULATION OF THE BLOOD. 365 many others. The instrument of v. Frey is shown in Fig. 170. This consists first of a metal framework by which the apparatus is fastened to the arm and support given to the lever, recording surface, etc. The essential part is the spring carrying a button which is placed over the artery, usually the radial, before it crosses the wrist-joint. A vertical rod transmits the movement of the spring to the recording lever; the movements of the latter are recorded on a small cylinder inclined slightly so that the upstroke may be vertical. A small electro-magnet serves to record the time relations of the changes in the blood-pressure. An average tracing taken from the radial artery is shown in Fig. 171. This, however, is not a tracing of the pulse-wave, but rather a record of the changes in pressure, their succession and time relations, which follow each beat of the heart. The artery usually selected for obtaining a sphygmogram is the radial. This artery lies quite superficially, covered only by connective tissue and skin and supported by the flat surface of the radial bone, conditions most favorable to technical investigation. Pj^, 171.— The Pulse-curve or The sphygmogram or pulse-curve Sphygmogram. may be divided into two portions: viz., a line of ascent from a to b, and a line of descent from b to d (Fig. 171). In normal tracings the former is almost vertical and caused by the sudden expansion of the artery immediately following the ventric- ular contraction; the latter is in general oblique, due to the recoil of the arterial walls, occupies a longer period of time, and is marked by several elevations and depressions, both of which indicate that the restoration to equilibrium is neither immediate nor uncomplicated. One of these elevations is quite constant and known as the dicrotic wave, c; the depression or notch just preceding it is known as the dicrotic notch. Pre- and post-dicrotic waves are not infrequently present. The summit is generally sharp and pointed. The vertical direction of the line of ascent is taken as an indica- tion that the arterial walls expand readily, that the blood is discharged quickly, and that the ventricular action is not impeded. An oblique direction of the line of ascent is an indication that the reverse condi- tions obtain. The height varies inversely as the arterial pressure, other things being equal; being high with a low pressure, and low with a high pressure. The dicrotic elevation shows that a second expansion wave is de- veloped which interrupts temporarily the recoil of the arterial walls. The origin of this second expansion has been the subject of much investigation, and at present it may be said that the question is not fully decided. It is asserted by some investigators that it is central in origin, beginning at the base of the aorta and passing to the per- iphery; by others, that it is peripheral in origin, beginning near the 366 TEXT-BOOK OF PHYSIOLOGY. capillary region and reflected to the heart. The former view is the one more generally accepted. According to it, the expansion is the result of the sudden closure of the aortic valves, and a backward surge of the blood column against them. The sudden arrest of the blood and its accumulations again expands the aorta. The dicrotic notch is therefore taken as the moment at which the ventricular systole ceases and the aortic valves close. From this fact it is evident that immediately after the first expansion the pressure begins to fall, even though the ventricular systole continues, owing to the discharge of blood from the arterial into the capillary and venous systems. The height of the dicrotic wave or the depth of the dicrotic notch is favored by low arterial tension and highly elastic arteries. Both features are diminished by the reverse conditions. The apex is sometimes rounded and even flat, indicative of a great diminution in arterial elasticity. The sphygmogram not infrequently varies con- FiG. 172. — Mosso's Plethysmograph. Flask for varying the water-pressure in G. Stirling.) G. Glass vessel for holding a limb. F. T. Recording apparatus. — (Landois and siderably from the normal type in different pathologic conditions of the circulatory apparatus. A consideration of these variations does not fall within the scope of this work. The Volume Pulse.— If an individual artery expands with each systole and recoils with each diastole of the heart, the same is true of all arteries, and as a tesult the volume of any organ or part of the body must undergo similar changes. To such alternate changes in volume the term volume pulse is given. The extent to which an organ will increase in volume will depend to some extent on its elasticity. The reason for the increase in volume is the resistance offered to the flow of blood into and through the capillaries; the decrease in volume to the overcoming of the resistance through the arterial recoil. The variations in volume may be recorded by enclosing the organ in a rigid glass or metal vessel, which at one point is in communication with a recording apparatus, e. g., a tambour with a lever or mer- curial manometer with float and pen. The space between the organ and vessel is filled with normal saline, air, or oil. Such an apparatus is known as a plethysmograph. A well-known form of plethysmograph THE CIRCULATION OF THE BLOOD. 367 is that of Mosso (Fig. 172). Many forms of this apparatus have been devised in accordance with the character of the organ — spleen, kidney, etc. — to be investigated, though the principle underlying them is es- sentially the same. In addition to changes in volume due to the heart's action, most organs undergo additional changes in volume from vaso- motor and respiratory causes. Indeed the plethysmographic is the most generally employed method to show the action of vaso-motor nerves in changing the resistance of the arterioles and hence the outflow of blood. Thus when an organ is enclosed in a plethys- mograph and the arterial pres- sure raised by either a direct stimulation of vaso-motor nerves or a reflex stimulation of the vaso-constrictor center, there is always a decrease in the volume of the organ under observation; and on the contrary, when the vaso-dilatator nerves or centers are stimulated and the vessels dilated, there will be a fall of pressure, an increased outflow of blood and an increase in the volume of the organ. From this it is learned that the functional activity of an organ which is attended and conditioned by an increased blood-supply is always associated with an increase in volume. On plethysmographic records large undulations are frequently observed which are regarded as of respiratory origin. Fig. 173. — The Vessels of the Frog's Web. a. Trunk of vein, and (6, 6) its tributaries passing across the capillary net- work. The dark|spots are pigment cells. — {Yeo's "Physiology.") THE CAPILLARY CIRCULATION. In certain regions of the body of many animals it is possible, on account of the delicacy and transparency of the tissues, to observe not only the flow of blood through the smaller arteries, capillaries, and veins, but many of the. phenomena connected with it, to which reference has already been made. The structures usually selected for the observation of these phenomena are the interdigital membranes (Fig. 173), the tongue, the lung, the bladder, and the mesentery of the frog. Though any one of these structures will afford an admirable view of the blood-flow, the mesentery for many reasons is the most satisfactory. For a comparison of the phenomena observed in the cold- blooded animals with those in the warm-blooded animals the omentum 368 TEXT-BOOK OF PHYSIOLOGY. of the guinea-pig may be employed. If the frog is the subject of ex- periment, it should be slightly curarized and the brain destroyed by pithing. The animal is then placed on a small board capable of ad- justment to the stage of the microscope. The abdomen is then opened along the side and a loop of intestine withdrawn and placed around a cork ring which surrounds an opening in the side of the frog board. The loop of the intestine should be so placed that it will lie between the observer and the body of the frog. The mesentery thus exposed must be kept moist with normal saline solution. When examined with low powers of the microscope, arteries, veins, and capillaries will be found occupying the field of vision. Their general arrangement, their size and connections, can be readily deter- mined. After a few preliminary adjustments a region will be found in which the blood is flowing in opposite directions. The vessel apparently carrying blood away from the observer is an artery; the vessel apparently carrying blood toward the observer is a vein; the smallest vessels are capillaries. The blood in the artery is of a brighter color than the blood in the vein; the blood in the capillaries is almost colorless. The arterial blood-stream not infrequently shows remit- tancy, an alternate acceleration and retardation, corresponding to each heart-beat; the capillary and venous streams are uniform and con- tinuous. The relative velocities in the three sets of vessels are indicated by the movement of the red corpuscles. In the arteries they pass before the eye so rapidly that they can not be distinguished; in the capillaries they pass so slowly that both form and structure may be determined; in the veins, though again moving rapidly, they can often be distinguished. The relative positions of the red and white corpuscles in the blood-stream are also apparent; the former occupy the central, the latter the peripheral portion, at the same time adhering to the sides of the vessel. Between the axial portion of the stream occupied by the red curpuscles and the wall of the vessel there is a clear still layer of plasma, the result of an adhesion of the plasma to the wall. It is this feature which gives rise to the friction between successive layers of the blood-stream, the resistance to the blood-flow, and the devel- opment of blood-pressure. The relative breadth of the still layer and amount of friction are greater in small than in large vessels. The volume of blood passing into any given capillary area is de- termined by the degree of contraction of the arterioles. Thus on the application of warm saline solution, which relaxes the arterioles, there is a large increase in the inflow of blood; vessels previously invisible suddenly come into view as the blood with its corpuscles passes into them. On the application of cold water, which contracts the arterioles and diminishes the inflow, many of the smaller vessels entirely disappear from view. The alternate contraction and relaxation of the arterioles will therefore determine the quantity of blood flowing into and through the capillary system. THE CIRCULATION OF THE BLOOD. 369 Migration of the White Corpuscles.— A phenomenon fre- quently observed in the capillary vessels of the mesentery or of the bladder of the frog is the passage of the white corpuscles through the walls into the surrounding lymph-spaces. To this process the term migration or diapedesis is given. After the tissues have been exposed to the air for some time or subjected to an irritant, the vessels dilate and become distended with blood. In a short time the blood-stream slows, and finally comes to rest. The condi- tion of stasis is then established. During the development of this condition the white cor- puscles accumulate in large numbers along the inner surface of the vessels and soon begin to pass through the vessel-walls. This they do by protruding a portion of their substance and inserting it into and through the vessel-wall. This once accomplished, the remainder of the cell in due time follows until it has entirely passed out into the tissue-space. The opening in the cell-wall now closes. The successive Steps in this process are shown in Fig. 174. As this migration occurs -mainly after the cir- culation has ceased or when the tissues present the phenomena of approaching inflammation, it is difficult to state in how far it is strictly a physiologic process. The Venous Circulation. — The blood, having passed through the capillary vessels, is gathered up by the veins and conveyed to the right side of the heart. As the veins converge and unite to form larger and larger trunks the sectional area gradually diminishes, and hence the velocity of the blood-flow increases, though it never attains the velocity, even in the ven^ cavse, that it had in the aorta, for the reason that the sectional area of the venae cavee is considerably larger than that of the aorta. The pressure also is very low in the larger veins because the friction still to be overcome is relatively very slight. The capacity of the venous system is considerably greater than that of the arterial system, as there are usually two and even three veins accompanying each artery. This, taken in connection with its greater distensibility, makes of the venous system a reservoir in which blood can be stored. On this reservoir the arterial system can call for that amount of blood necessary for the maintenance of its normal volume and pressure, and into it any excess can be discharged. The relative amounts contained in the two systems are regulated by the nervous system. The movement of the blood through the veins is Fig. 174. — Diagram to SHOW Various Stages in THE diapedesis OR MI- GRATION OE White Cor- puscles. 24 37° TEXT-BOOK OF PHYSIOLOGY. accomplished by the cooperation of several forces, reference to which will be made in a following paragraph. THE PULMONIC VASCULAR APPARATUS. The pulmonic vascular apparatus consists of a closed system of vessels extending from the right ventricle to the left auricle, and includes the pulmonary artery, capillaries, and pulmonary veins. In its anatomic structure and physiologic properties it closely resembles, if it is not identical with, the systemic apparatus. The stream-bed widens from the beginning of the pulmonary artery to the middle of the capillary system; it again narrows from this point to the terminations of the pulmonary veins. The movement of the blood from the beginning to the end of the system is due to a difference of pressure between these two points, the result of the friction between the blood and the vascular walls. From the difference in the extent of the pulmonic and systemic systems it is evident that, other things being equal, the friction is less, and therefore also the pressure is less in the former than in the latter. This view is supported by the difference in the thickness of the walls of the right and left sides of the heart. The pressure in the pulmonary artery of the dog was shown by Beutner to be about one-third that in the aorta; by Bradford and Dean to be one-fifth. The- velocity of the blood-stream in each of the three divisions of the system can not well be determined. The time occupied by a particle of blood in passing from the right to the left ventricle has been estimated at one-fourth the time required to pass from the left to the right ven- tricle. Assuming the latter to be thirty seconds, the former would be seven and one-half seconds. The capillary vessels are spread out in a very elaborate manner just beneath the inner surface of the pulmonary air-cells, and form, by their close relation to it, a mechanism for the excretion of carbon dioxid and the absorption of oxygen. The extent of the capillary surface is very great. It has been estimated at 200 square meters. The amount of blood flowing through this system hourly and exposed to the respiratory surface is about 800 liters. The reason for the ex- istence of the pulmonary circulation is the renewal of the oxygen volume in the blood and the elimination of the carbon dioxid; for the accomplishment of both objects ample provision is here made. The flow of blood through the cardio-pulmonary vessels is subject to varia- tion during both inspiration and expiration in consequence of their relation to the respiratory apparatus. The mechanism by which these variations are produced will be considered in the chapter devoted to Respiration. FORCES CONCERNED IN THE CIRCULATION OF THE BLOOD. I. The Contraction of the Heart. — The primary forces which keep the blood flowing from the beginning of the aorta to the right THE CIRCULATION OF THE BLOOD. 371 side of the heart and from the beginning of tlie pulmonary artery to the left side are the contractions of the left and right ventricles respectively. This is evident from the fact that each ventricle at each contraction not only overcomes the pressure in the aorta and pulmonary artery, the sum of all resistances, but imparts a given velocity to the blood. Since the pressure continuously falls from the beginning to the end of each system, it follows that the blood must flow from the point of high to the point of "low pressure. During the interval of the heart's activity the walls of the arteries, to which the heart's energy was largely transferred, now take up and continue the work of the heart, and by recoiling drive the blood forward and into the venous system. Though the heart's energy is probably sufficient to drive the blood into the opposite side of the heart, it is supplemented by other forces — e. g.: 2. Muscle Contraction. — As a result of the relation which the veins bear to the muscles in all parts of the body it is clear that with each contraction and relaxation of the muscles there will be exerted an intermittent pressure on the veins. With each contraction, the blood on the proximal side will at once be driven forward with increased velocity, while that on the distal side will be re- tarded, will accumulate and distend the veins, owing to the closure of the valves; with the relaxation of the muscle the elastic and contractile tissues in the walls of the veins will come into play and force the blood forward. 3. Thoracic Aspiration. — The inspiratory movement aids the flow of blood through the venae cava and their tributaries. With each inspiration the pressure within the thorax but outside the lungs undergoes a diminution more or less pronounced in ac- cordance with the extent of the movement. As a result, the blood in the large veins outside of the thorax, being now subjected to a pressure greater than that in the thorax, flows more rapidly toward the heart. With each expiration the reverse obtains. 4. Action of the Valves. ^ — It is quite probable that gravity opposes to some extent the flow of blood through the veins below the level of the heart. This opposition to the upward flow is largely pre- vented by the valves, for each retardation is immediately checked by their closure and support given to the column of blood. The influence of gravity is shown when the relation of the arm to the heart is changed. Thus, if the arm be allowed to hang pas- sively by the side of the body, the veins, especially on the back of the hand, will become distended with blood. If now the arm be raised, the blood will flow rapidly toward the heart, as shown by the rapid emptying of the veins. Work Done by the Heart. — The work which the left ventricle performs at each contraction when it discharges its contained volume of blood into the aorta is: 372 TEXT-BOOK OF PHYSIOLOGY. 1. To overcome the total resistance of the systemic vascular appa- ratus expressed in terms of aortic pressure; and — 2. To impart velocity to the blood. The pressure in the aorta is not absolutely determined, though for many reasons it may be assumed to be about 150 mm. Hg., orits equivalent, a column of blood 1.93 meters in height. If the volume of blood which the heart discharges is assumed to be 188 grams, the work done may be calculated by multiplying the weight by the height : viz., 0.188 X 1.93 = 0.3628 kilogrammeter. The velocity of the blood in the aorta has been approximately estimated at 0.5 meter per second. The work done in imparting this velocity to 188 grams is estimated by squaring the velocity and dividing by the accelerating force of gravity (75^^37) and multiplying the quotient by 0.188. The quotient of the first two values represents the distance a body would have to fall to acquire this velocity: viz., 0.0127 meter. The work done is therefore 0.188 X 0.0127, °^ 0.0023 kilogrammeter. The entire work of the left ventricle is the sum of these two amounts, or 0.3651 kilogrammeter. Assuming that the heart beats 72 times per minute, the work done daily would be 0.604 X 72 X 60 X 24, or 37,857.6 kilogrammeters. The right ventricle approximately per- forms about one-third of this amount of work in overcoming the resistance offered by the p^ulmonary system and in imparting velocity to its contained volume of blood. The work of the entire heart would therefore be for the twenty-four hours about 50,476 kilogrammeters. THE NERVE MECHANISM OF THE VASCULAR APPARATUS. The middle coat of the arteries, and especially of those in the peripheral region of the arterial system, consists of well-defined layers of non-striated muscle-fibers arranged in a circular direction or at right angles to the long axis of the vessel. In the physiologic con- dition these fibers are in a state of continuous contraction, more or less pronounced, and give to the arteries a certain average caliber which permits a definite volume of blood to flow through them in a given unit of time. The cause of this tonic contraction is not definitely known. It has been attributed to the action of local nerve-ganglia, to the pres- sure of blood from within, to the influence of organic substances in the blood, the products of gland activity: e. g., adrenalin or epinephrin. This tonic contraction of the vascular muscle is subject to increase or decrease in accordance with the action of various agents. In- creased contraction will result in a decrease of the caliber and a reduction in the outflow of blood. Decrease of the contraction or relaxation will result in an increase both of the caliber and outflow of blood. The small arteries thus determine the volume of blood THE CIRCULATION OF THE BLOOD. 373 passing to any given area or organ in accordance with its functional activities. The Vaso-motor Nerves. — The activities of the vascular muscle are regulated by the central nerve system through the intermedia- tion of nerve-fibers, termed vaso-motor nerves. Of these there are two kinds, one which increases or augments the contraction, the vaso-constrictors or vaso-augmeniors; another which decreases or inhibits the contraction, the vaso-dilatators or vaso-inhihitors. The vaso-motor nerves of both classes, unlike the ordinary motor nerves, do not pass directly to the muscle-fiber, but indirectly by way of the ganglia of the sympathetic nerve system. In these ganglia the vaso-motor nerves, which come from the central nerve system, terminate, breaking up into tufts, which arborize around the nerve- cells. From the cells new nerve-fibers arise which then pass without interruption to their final destination. The nerve-fibers which emerge from the central nerve system are extremely fine in caliber and medullated; those which emerge from the sympathetic ganglia are equally fine, but non-meduUated. The former are termed pre-ganglionic, the latter post-ganglionic fibers. The ganglion in which the pre-ganglionic fibers end is not necessarily found in the pre- vertebral or lateral chain; it may be found in the collateral or even in the peripheral group of ganglia. The Vaso-constrictor Nerves. — The vaso-constrictor nerves take their origin from nerve-cells located in the anterior horns and lateral gray matter of the spinal cord. They emerge from the cord in company with the fibers which compose the anterior roots of the spinal- nerves from the second thoracic to the second or third lumbar nerves inclusive. A short distance from the cord they leave the anterior roots as the white rami communicantes and enter the pre-vertebral or lateral sympathetic ganglia. From the results of many observations and experiments it is probable that the great majority of the vaso-con- strictor nerves terminate in these ganglia; that is to say, it is here that the pre-ganglionic fibers arborize around the contained nerve-cells. From the nerve-cells new fibers arise, the post-ganglionic, which pass to the blood-vessels of the head, to the upper and lower extremities, and to the thoracic and abdominal viscera. The vaso-constrictors for the head emerge from the spinal cord in the first four thoracic nerves, thence pass successively into and through the ganglion stellatum (the first thoracic), the annulus of Vieussens, the inferior cervical ganglion, the sympathetic cord to the superior cervical ganglion, around the cells of which they arborize. From this ganglion the new fibers follow the carotid artery and its branches to their terminations. The vaso-constrictors for the fore-limbs emerge from the cord in the roots of the fourth to the tenth thoracic nerves inclusive. Through the white rami they pass into the sympathetic chain, after which they take an upward direction and terminate around the cells of the gan- 374 TEXT-BOOK OF PHYSIOLOGY. glion stellatum. From this ganglion the new fibers enter, by way of the gray rami communicantes, the trunks of the cervical nerves which unite to form the brachial plexus and by this route pass to the blood- vessels. The vaso-constrictors for the hind-limbs emerge from the cord in the roots of the eleventh dorsal to the second or third lumbar nerves inclusive. They then pass through the white rami to the lower lumbar and upper sacral ganglia. Thence by way of the gray rami they pass into the nerve-trunks which unite to form the sacral nerves and by this route pass to the blood-vessels. The vaso-constrictors for the viscera of the abdominal cavity pass by way of the splanchnic nerves directly into the collateral ganglia, the semilunar, the superior mesenteric, the inferior mesenteric, and the sacral. From these ganglia an elaborate network of non-medul- lated fibers passes to the blood-vessels of the stomach, intestines, and other viscera. The great splanchnic nerve is one of the most im- portant vaso-constrictor trunks of the body, on account of the large vascular area it controls. The existence, course, distribution, and functions of the vaso- constrictor nerves have been determined by a variety of methods, physiologic and anatomic. Stimulation of the nerve-trunks under appropriate conditions gives rise to a contraction, division to a dilata- tion of the blood-vessels. The physiologic continuity of the pre- ganglionic fibers with the nerve-cells of the sympathetic ganglia has been shown by the intra- vascular injection or the local applica- tion of nicotin. This agent, as shown by Langley, has a selective action on the arborizations of the pre-ganglionic fibers, and when given in sufficient doses suspends their conductivity; hence stimu- lation of the pre-ganglionic fibers is without effect, though stimulation of the post-ganglionic fibers is followed by the usual contraction. The following will serve as illustrations of the functions of vaso- constrictor nerves. Division of the great splanchnic is followed by a marked dilatation of the blood-vessels of the intestinal tract; stimu- lation of the peripheral end by their contraction. Division of the cervical cord of the sympathetic is followed by dilatation of the blood- vessels of the side of the head; stimulation of the peripheral end by their contraction. The Vaso-dilatator Nerves. — The vaso-dilatator nerves have their origin for the most part in nerve-cells situated in the region of the spinal cord included between the origins of the second dorsal to the second lumbar nerves inclusive, though they are not confined to this region. Some vaso-dilatator fibers have their origin in the medulla oblongata, others in the sacral region of the spinal cord. The general course of the dilatator fibers for the intestinal tract is the same as that of the vaso-constrictors, though instead of beconi- ing related to the nerve-cells in the pre-vertebral ganglia, they pass by way of the splanchnics to the collateral ganglia, the semilunar, THE CIRCULATION OF THE BLOOD. 375 the superior and inferior mesenteric, and perhaps to peripheral ganglia in or near the blood-vessels themselves. The vaso-dilatators for the limbs are found in the common nerve- trunks associated with the usual motor and sensor fibers, though the exact route by which they pass from the spinal cord to the peripheral nerves has not in all cases been determined. Their cell stations have not been definitely located. The vaso-dilatator nerves for the blood- vessels of the submaxillary gland arise in the medulla, pass outward in the trunk of the facial nerve, and reach the gland by way of the chorda tympani branch. Their cell station is in the ganglion near the hilum of the gland. The vaso-dilatator nerves for the blood- vessels of the corpora cavernosa of the penis, the nervi erigentes, have their origin in the sacral region of the spinal cord; and emerge in the roots of the second and third sacral nerves. Their cell station is in the ganglion near the organ. The existence, course, distribution, and functions of the vaso- dilatator fibers have been determined by the same methods employed in the investigation of the vaso-constrictors. Thus division and stimulation of the peripheral end of the chorda tympani nerve are at once followed by an active dilatation of the blood-vessels of the submaxillary gland. The inflow of blood is so great that the gland becomes bright red in color. Its tissues being unable to appropriate all the oxygen, the blood emerges in the veins almost arterial in char- acter. Stimulation of the peripheral ends of the divided nervi erigentes is followed by similar effects in the blood-vessels of the corpora caver- nosa. Slow stimulation, once per second, of the peripheral end of a divided sciatic nerve is followed by dilatation of the blood-vessels of the leg. From these and many other facts of a similar character it is highly probable that the blood-vessels of each organ are under the control of two antagonistic classes of nerve-fibers, one augmenting the degree of their contraction, the vaso-constrictors, the other diminishing it through inhibition, the vaso-inhibitors. Through the cooperative antagonism of these two classes of nerves the caliber of the blood- vessels and thereby the volume of the blood is accurately adapted to the needs of each organ both during rest and during activity. It is also to the alternate activity of these nerves that the variations occurring from time to time in the volume of organs are to be attributed. Physiologic Properties. — The vaso-constrictors and the vaso- dilatators differ somewhat in their physiologic properties, as shown by the results of experiment. Thus, when a mixed nerve, i. e., one con- taining both classes of fibers — e. g., the sciatic — is stimulated with frequently repeated induced currents, the constrictor effect is the more pronounced, the dilatator effect being wanting or prevented; when stimulated with slowly repeated induced currents, the dilatator effect is the more pronounced. These different effects are strikingly shown in Fig. 175, A and B. 376 TEXT-BOOK OF PHYSIOLOGY. In the experiment of which these tracings are the result the leg of a cat was enclosed in a plethysmograph and the variations in volume due to dilatation or contraction of the vessels, following stimulation of the sciatic nerve, were recorded by means of a tambour and lever on a slowly revolving cylinder. In A the fall of the curve indicates a diminution of volume, from contraction- of blood-vessels following a rate of stimulation of the sciatic nerve of i6 per second for fifteen seconds. In B the rise of the curve indicates an increase in volume from dilatation of the vessels following a rate of stimulation of i per second for fifteen seconds (Bowditch and Warren). With' different rates of stimulation somewhat difiEerent results are obtained. After division of a mixed nerve the vaso-constrictors degenerate and lose their influence over the blood-vessel in from four to five y^^'*\^ A B Fig. 175.— Plethysmograms of the Kind-leg of the Cat following Stimulation OF the Sciatic Nerve. In A the rate of stimulation was sixteen per second, in B one per second for fifteen seconds. days, the vaso-dilatators in from seven to ten days, as shown by the response to electrical stimulation. When a nerve is cooled, the vaso-constrictors lose their irritability before the vaso-dilatators. Vaso-motor Centers. — The nerve-cells throughout the spinal cord from which the vaso-motor nerves take their origin may be regarded as nerve-centers which through their related nerve-fibers exert from time to time either a constrictor or a dilatator influence over the blood-vessels. Though both the vaso-constrictor and vaso- dilatator centers are in a state of continuous activity, the former decidedly preponderates, as shown by the maintenance of a tonic contraction of the blood-vessels. The activity of both centers may be increased or decreased, augmented or inhibited, by nerve impulses reflected to them from the periphery through afferent nerves or through nerve-fibers descending the cord from higher levels of the nerve system. Experiment has shown that when a definite region of the medulla oblongata is punctured or in anywise destroyed there is an THE CIRCULATION OF THE BLOOD. 377 immediate dilatation of the blood-vessels throughout the body and a fall of blood-pressure below one-half or one-third of the normal value. This region has a width of one and a half millimeters and extends longitudinally for a distance of four or five millimeters, terminating at a point four millimeters above the tip of the calamus scriptorius. A transection of the medulla above the upper limit of this area is without effect on the blood-pressure. A similar section below it, however, is at once followed by vascular dilatation, a loss of vascular tone, and a general fall of blood-pressure. Subsequent stimulation of the peripheral end of the divided medulla, the animal being curar- ized and artificial respiration maintained, will give rise to a marked contraction of the blood-vessels and a rise of blood-pressure up to and far beyond the normal value. If the experimental lesion is limited to the area mentioned in the foregoing paragraph, the vascular dilatation passes away after a time, the blood-vessels regain their normal tone, and the pressure again rises. These facts indicate that there is in the gray matter beneath the floor of the fourth ventricle, a restricted area composed of nerve-cells, which maintains through efferent nerve-fibers the tonus of the blood-vessels by virtue of its dominating influence over the vaso- motor centers in the cord, and which is therefore to be regarded as the general vaso-motor {constrictor) center. The vaso-motor centers through- out the cord are to be regarded as subsidiary centers. The nerve- fibers which transmit the regulative nerve impulses from the general to the subsidiary centers are to be found in the lateral columns of the spinal cord. Since the blood-vessels maintain a more or less constant tone, it is assumed that the vaso-motor center is in a state of continuous activity. In how far, however, this activity is the result of chemic changes between the cells and the surrounding lymph and blood, or the result of constantly arriving nerve impulses reflected from the periphery or from higher regions of the nerve system, is not readily deter- minable. Both factors are probably involved. A general vaso-dilatator center has never been located and there are many reasons for thinking that such a center has no anatomic existence. There are, however, special or local vaso-dilatator centers in the medulla oblongata and in various regions of the spinal cord and especially in the lumbar region. Direct Stimulation of the Vaso-motor Centers. — The general vaso-motor (constrictor) center at least is markedly influenced by the quantity and quality of blood and lymph circulating around and through it. If the blood-supply to the medulla and associated struc- tures be diminished by compression of the carotid arteries, the activity of the center is at once increased, as shown by increased vascular contraction and a rise of pressure. Restoration of the blood-supply is followed by a return of the center to its normal degree of activity. Increased blood-supply, as in cerebral hyperemia, is attended by a 378 TEXT-BOOK OF PHYSIOLOGY. fall in blood-pressure indicating a decrease in the activity of the center. A diminution in the percentage of oxygen or an increase in the per- centage of CO2 in the blood will increase the activity of the center. In asphyxia especially, the center is extremely excitable, as shown by a rise of the arterial tension. The subsidiary centers in the spinal cord are influenced by corresponding conditions. Reflex Stimulation of the Vaso-motor Centers.— The results of experiment make it certain that the degree of vascxilar contraction maintained by the vaso-motor centers can be increased or decreased by nerve impulses reflected to the cord and medulla from the periphery or from the brain. The effect may be general, or local and confined to the area from which the impulses arise. The following experi- ments may be cited as illustrations: Stimulation of the central end of a divided posterior root of a spinal nerve gives rise to increased vascular contraction, as shown by the rise of blood-pressure. Stimulation of the central end of the divided sciatic will give rise to opposite results, according to the strength of the stimulus, weak stimuli producing dilatation, strong stimuli producing contraction of the vessels. Stimulation of the central end of the divided vagus gives rise to dilatation of the vessels of the lips, cheeks, and nasal and palatal mucous membranes. Stimulation of the tongue is followed by dilatation of the vessels of the submaxillary gland. Stimulation of certain branches of the vagus nerve is followed by a passive dilatation of blood-vessels and .a marked fall of pressure. Stimulation of the peripheral terminations of the afferent nerves of the glans penis will give rise to an active dilatation of the blood-vessels of the corpora cavernosa, etc. A satisfactory explanation of these different results is, however, wanting. By some investigators it is believed that the usual variations in the arteriole contraction are the outcome of corresponding varia- tions in the activity of the general vaso-constrictor center the result of nerve impulses coming through afferent nerves. The preceding statements as to the effects on the degree of vascular contraction, and hence on the blood -pressure which follow stimulation of different afferent nerves, has led to the assumption that there are in most afferent nerves two classes of nerve-fibers, though perhaps in varying proportions, one of which when in activity augments, the other of which when in activity inhibits the activity of the vaso-con- strictor center. The former class is generally termed pressor, the latter depressor -fibers. It is possible, therefore, that under physiologic conditions, physio- logic stimuli act on the peripheral terminations of either the one or the other; according as they do will the center be augmented or inhibited in its activity, and attended by either an increase or a decrease in the degree of the previous vascular contraction. Again it may be assumed, from the results of experimentation on afferent nerves, that the physiologic stimuli may act simultaneously THE CIRCULATION OF THE BLOOD. 379 on the peripheral terminations of both classes of fibers and that the vaso-constrictor center is acted on by the two antagonistic influences. In this assumption the resultant effect on the blood-vessels, viz., increased or decreased contraction, will be the resultant of their action on the vaso-constrictor center. If the stimuli act preponderantly on the depressor fibers the center will be depressed and the vessels will dilate; if they act preponderantly on the pressor fibers the center will be stimulated and the vessels will con- tract. Inasmuch as the vas- cular dilatation is often greater than the dilatation which follows division of the vaso-motor fibers them- selves, it has been assumed by some that the general vascular tonus, as well as its variations from time to time, is the resultant of the simultaneous activity and variations in activity of both vaso-constrictor and vaso-dilatator centers; that in the afferent nerves there are two sets of fibers, one of which when stimulated augments the activity of the vaso-constrictor center and inhibits the activity of the vaso-dilatator center; the other of which auguments the activity of the vaso- dilatator center and in- hibits the activity of the vaso-constrictor center. The result, either contrac- tion or dilatation, which follows stimulation of their peripheral terminations will depend on the character of the physiologic stimulus. In those particular instances in which stimulation of the peripheral terminations of afferent nerves, e. g., the nervi erigentes and chorda tympani, is followed by active dilatation of the blood-vessels, it has been assumed that there are afferent nerve-fibers which directly stimulate Fig. 176. — Diagram showing the Origin AND Relation of the Bepressoe. Nerve in the Rabbit. Depr. n., depressor nerve; vag. n., vagus nerve; sup. 1. n., superior laryngeal nerve; inf. e.g., inferior cervical ganglion; sym. n., sympathetic nerve; car. a., carotid artery; dig. m., digastric muscle; hyp. n., hypoglossal nerve; sup. c. g., superior cer- vical ganglion; inf. 1. n., inferior laryngeal nerve. 38o TEXT-BOOK OF PHYSIOLOGY. or augment the activity of a special vaso-dilatator center and for this reason should be termed "reflex vaso-dilatator nerves" (Hunt). The Influence of Emotional States. — The vaso-motor centers are capable of being influenced in their activities by emotional states, doubtless as a result of the arrival of nerve impulses from the cortex of the cerebrum. Thus it is well known that fear causes a contraction of the blood-vessels of the head and face and that shame causes a dilatation of the same vessels. With the cessation or the disappear- ance of the emotional state, the blood-vessels return to their former degree of contraction. The Depressor Nerve. — A striking illustration of the depressor or inhibitor action of afferent nerves upon the vaso-constrictor center is furnished by the result of stimulation of a branch of the vagus, the so- called "depressor nerve." In the rabbit. Fig. 176, there is a small nerve formed by the union of a branch from the trunk of the vagus Fig. 177. — Fall of Blood-prf.ssure from Excitation of the Depressor Nerve. The cylinder was stopped in the middle of the curve and the excitation maintained for seventeen minutes. The line of zero pressure (0,0) should be 30 mm. lower than here shown. — {Bayliss.) with a branch from the superior laryngeal. The peripheral distribu- tion of this nerve is over the wall of the ventricle and perhaps to some extent to the structures of the arota near its origin. A similar ana- tomic arrangement is met with in the horse, pig, and hedge hog. In some other animals, as the dog, it is bound up in the vago-sympathetic. In man it is also present, though shortly after its origin it enters the trunk of the vagus. Division of this nerve is without effect either on the heart or the vessels. Stimulation of the peripheral end has neither an accelerator nor an inhibitor action on the heart. Stimulation of the central end is followed by a fall in blood-pressure, frequently to a level below one-half the normal value; at the same time there is a diminution, brought about reflexly, in the rate of the heart-beat (Fig. 177). The fall in pressure, however, is not due to this cause, for it occurs equally well after division of all the cardiac nerves. For this THE CIRCULATION OF THE BLOOD. 381 reason the nerve was termed the depressor nerve of the vaso-motor center. On exposure of the abdominal cavity, it is observed during stimula- tion of the depressor that there is a notable dilatation of the intestinal vessels. From this fact it was assumed that the action of the depres- sor nerve was to lower the general pressure through reflex dilatation of these vessels. It has been shown by Porter and Beyer that if the splanchnics are divided and the peripheral end stimulated so as to maintain the tonus of the intestinal vessels, and hence the general pres- sure, stimulation of the depressor nerve will nevertheless be followed by a fall of the blood-pressure almost as great as when the splanchnics are intact. From this it is evident that the depressor nerve is related to centers which influence the vascular apparatus in its entirety. It has been supposed that through it the heart can protect itself from injurious results of an excessive rise of arterial pressure. Thus, when the intra-cardiac pressure or the intra-aortic pressure rises beyond a normal amount from increased resistance, the peripheral terminations of this nerve are stimulated with the result that the vaso- motor center is inhibited and the arterioles relaxed. Through this means the pressure falls and the work of the heart is lessened. CHAPTER XV. RESPIRATION. Respiration is a process by which oxygen is introduced into, and carbon dioxid removed from, the body. The assimilation of the former and the evolution of the latter take place in the tissues as a part of the general process of nutrition. Without a constant supply of oxygen and an equally constant removal of the carbon dioxid, those chemic changes which underlie and condition all life phenom- ena could not be maintained. The general process of respiration may be considered under the following headings, viz.: 1. The anatomy and general arrangement of the respiratory appa- ratus. 2. The mechanic movements of the thorax by which an interchange of atmospheric and intra-pulmonary air is accomplished. 3. The chemistry of respiration, the changes in composition under- gone by the air, blood, and tissues. 4. The nerve mechanism by which the respiratory movements are maintained. THE RESPIRATORY APPARATUS. The respiratory apparatus consists essentially of: 1. The lungs and the air-passages leading into them: viz., the nasal chambers, mouth, pharynx, larynx, and trachea. 2. The thorax and its associated structures. The nasal chambers are the natural entrances for the inspired air. Their complicated structure slightly retards the movement of the air, in consequence of which its temperature and moisture are adjusted to the physiologic conditions of the lower respiratory pas- sages. The mouth, though frequently serving as an entrance for air, is not primarily a respiratory passage. Both the nasal chambers and the mouth communicate posteriorly with the pharynx, in which the respiratory and the deglutitory passages cross each other, the former leading directly into the larynx. The larynx is a complicated mechanism serving the widely dif- ferent though related functions of respiration and phonation. It consists of a framework of cartilages, articulating one with another, united by ligaments and moved by muscles; it is covered externally with fibrous tissue and lined with mucous membrane. The superior opening of the larynx, the glottis, is triangular in shape, the base 382 RESPIRATION. 383 being directed upward and forward, the apex downward and back- ward. The inclination of the glottic opening is almost vertical. The cavity of the larynx is partially subdivided by the interposition of the vocal bands into a superior and an inferior portion. The opening, bounded by the vocal bands, is also triangular in shape, Fig. 178. — Trachea and Bronchial Tubes, i, 2. Larynx. 3, 3. Trachea. 4. Bifurcation of trachea. 5. Right bronchus. 6. Left bronchus. 7. Bronchial division to upper lobe of right lung. 8. Division to middle lobe. 9. Division to lower lobe. 10. Division to upper lobe of left lung. 11. Division to lower lobe. 12, 12, 12, 12. Ultimate ramifications of bronchi. 13, 13, 13, 13. Lungs, represented in contour. 14, 14. Summit of lungs. 15, 15. Base of lungs. — {Sappey.) , though in this case the base is directed backward, the apex forward. (See chapter on Voice and Speech.) The introduction of the vocal bands narrows at this level the air- passage and to some extent interferes with the free entrance of air. According to the investigations of Semon, the area of the air-passage above and below the phonatory apparatus is about 200 sq. mm.; while the area bounded by the vocal apparatus is but 155 sq. mm. during quiet respiration. 384 TEXT-BOOK OF PHYSIOLOGY. The trachea is a tube, some 12 centimeters in length, from one- half to three-fourths of a centimeter in breadth, extending from the lower border of the larynx to a point opposite the fifth dorsal verte- bra. It consists of an external fibrous and an internal mucous mem- brane, between which is a series of superposed C-shaped arches or rings of elastic cartilage, some 18 or 20 in number. Between the fibrous and mucous coats posteriorly, and occupying the space be- tween and attached to the free ends of the cartilages, there is a layer of transversely arranged non-striated muscle-fibers, known as the tracheal muscle. The alternate contraction and relaxation of this muscle would by varying the distance between the ends of the cartil- ages, either diminish or increase the caliber of the trachea. The surface of the mucous membrane is covered by a layer of stratified columnar ciliated epithelium (Fig. 179). In the submucous tissue there are a number of glands the ducts of which open on the free surface. Opposite the fifth dorsal verte- bra the trachea divides into a right and a left bronchus. Each bronchus again subdivides into two or three branches, which penetrate the corresponding lung. The lungs, in the physiologic condition, occupy the greater part of the cavity of the thorax. They are separated from each other by the contents of the mediastinal space: viz., the heart, the large blood-vessels, the esophagus, etc. Each lung is somewhat pyramidal in shape with the apex directed upward. The outer surface is con- vex and corresponds to the general conformation of the thorax. The inner surface is concave and accommodates the contents of the me- diastinal space. At about the middle of the inner surface there enter the lung, the bronchi, and blood-vessels. The under surface of the lung is concave and rests on the diaphragm. The posterior border is convex; the anterior border is thin. A histologic analysis of the lung shows it to consist of the branches of the bronchi, their subdivisions and ultimate terminations, blood- vessels, lymphatics and nerves, imbedded in a stroma of fibrous and elastic tissue. The anatomic relations which these structures bear one to another is as follows: — Within the substance of the lung the bronchi divide and subdivide, giving origin to a large number of smaller branches, the bronchial tubes, which penetrate the lung in all directions. With this repeated subdivision the tubes become narrower, their walls thinner, their structure simpler. In passing from the larger to the smaller tubes Fig. 179. — Transverse Section of THE Trachea op a Kitten. — {Stirling.) RESPIRATION. 389 The costo-vertebral and costo-chondral and the chondro-sternal articulations are diarthrodial in character and endow the thoracic walls with a considerable degree of mobility. The costo-vertebral joints are two in number, the first being formed by the beveled head of the rib and the bodies of two adjoining vertebras; the second, by the tubercle of the rib and the transverse process. The costo-chon- dral and the chondro-sternal articulations, as their names imply, are formed by the ribs, cartilages, and sternum. The muscles which complete the formation of the thoracic walls are as follows: the diaphragm, the in- tercostales externi and interni, the levatores costarum, the triangularis sterni, and the in- fra-costales. The diaphragm is the musculo- membranous sheet which closes the inferior opening of the thorax and compl-etely sepa- rates its cavity from that of the ab- domen. It con- sists of two mus- cles which arise from the bodies of the first three or four lumbar verte- brae and neighbor- ing fascia, from the border of the six lower ribs, and from the ensiform cartilage (Fig. 186). From this ex- tensive origin the muscle-fibers pass centrally to be inserted into a common tendon. As the direction of the fibers is from below up- ward and inward, the diaphragm is somewhat dome-shaped. Its inferior border is for a short distance in contact with the sides of the thorax. The intercostales externi, eleven in number on each side, occupy the spaces between the ribs to which they are attached from the tubercle to the anterior extremity (Figs. 187 and 188). Their fibers, which are arranged in parallel bundles, are directed from above downward and from behind forward. The point of attachment, therefore, of any given bundle of fibers to the rib above, lies nearer Fig. 186. — Diaphragm, Infeeior Aspect, i. Anterior and middle leaflet of central tendon. 2. Right leaflet. 3. Left leaflet. 4. Right crus. 5. Left cms. 6, 6. Intervals for phrenic nerves. 7. Muscular fibers, from which the ligamenta arcuata originate. 8. Muscular fibers that arise from the inner surface of the six lower ribs. 9. Fibers that arise from ensiform cartilage. lo. Opening for inferior vena cava. ii. Opening for esophagus. 12. Aortic open- ing. 13, 13. Upper portion of transversalis abdominis, turned upward and outward. 14. Anterior leaflet of trans- versalis aponeurosis. 15, 15. Quadratus lumborum. 16, 16. Psoas magnus. 17. Third lumbar vertebra. 390 TEXT-BOOK OF PHYSIOLOGY. the vertebral column, nearer the fulcrum, than the point of attach- ment below. The intercoslales inlerni, eleven in number, occupy the spaces between, and are attached to, the ribs from the tubercle to the anterior extremity of the cartil- ages. Their fibers, which are also arranged in parallel bundles, are directed from above downward and back- ward (Figs. i88 and 189) . The levatores costa- rum are twelve in num- ber on either side. They arise from the tips of the transverse processes of the last cervical and the thoracic vertebree with the exception of the last. From the point of origin the fibers pass down- ward and outward in a diverging manner to be inserted into the ribs between the tubercle and the angle. Their action, as their name implies, is to elevate the posterior portion of the ribs. The tr i angular is sterni arises from the side of the posterior surface of the lower third of the sternum and is inserted by fleshy slips into the cartilages of the ribs from the second to the sixth. From the fact that the inferior opening of the thorax as well as the intercostal spaces are completely closed by the foregoing muscles, and from the further fact that the superior is closed by fascia except at those points through which pass the trachea, blood-vessels and esophagus, the cavity of the thorax is abso- lutely air-tight. The Pleurse. — Each lung is surrounded by a closed invaginated serous sac, the pleura, of which the inner portion is reflected over Fig. 187. — Sho\vi.\g the Situation, the Points OF Attachment, and Direction of the Intercostal Muscles, i. The intercoslales externi. 2. The inter- coslales interni. 3. The intercartilaginei. — (Denver.) RESPIRATION. 391 and is closely adherent to the surface of the entire lung as far as its root; the outer portion is reflected over the inner wall of the thorax, the superior surface of the diaphragm, and the viscera of the mediasti- num. Under normal conditions these two layers of the pleura, the visceral and parietal, are in contact, or at most separated only by a thin capillary layer of lymph. The presence of this fluid prevents appreciable friction as the two surfaces play against each other in consequence of the movements of the lungs. THE MECHANIC MOVEMENTS OF THE THORAX. The blood receives oxygen from, and yields carbon dioxid to, the alveoli of the lungs, as it flows through the pulmonary capillaries. That this exchange of gases may continue, it is of primary import- ance that the air within the alveoli be removed as rapidly as it is vitiated. This is accomplished by an alternate increase and decrease in the capacity of the thorax, accompanied by corresponding changes in the ca- pacity of the lungs. During the former there is an inflow of atmospheric air (inspiration) , during the latter an out- flow of intra-pulmonary air (expira- tion). The continuous recurrence of these two movements brings about that degree of pulmonary ventilation neces- sary to the normal exchange of gases between the blood and the air. The two movements together constitute a respiratory act or cycle. In the course of the respiratory cycles the thorax presents alternately a short period of rest — viz., between the end of an expiration and the be- ginning of an inspiration — and a relatively long period of activity, in- cluding both inspiration and expiration. The former may be regarded as the static, the latter as the dynamic condition of the thorax. In the static condition, the thorax and its contained and associated organs sustain a definite relation one to another; in the dynamic conditions these relations undergo a change the extent of which is proportional to the extent of the movements.* * It is a matter of dispute as to wliether or not there is an absolute cessation of move- ment of the thoracic walls at the end of expiration. A graphic record of the movement shows that if there is no absolute cessation, the movement is so slight that, for the pur- poses here intended, a pause may be admitted. With this admission it is, however, recog- nized that the forces, both elastic and muscular, which are always acting on the thoracic walls, though in opposite directions, have not ceased to act, but have become so nearly equal that for a brief period they are practically in a condition of equilibrium, during which the thoracic walls are stationary. Fig. 188. — View from behHstd of Four Dorsal Vertebrae and Three Attached Ribs, showing THE Attachment of the Elevator Muscles of- the Ribs and the Intercostals. I. Long and short elevators. •.;. External intercostal. 3. Internal intercostal.^(/I//e« Thomson ) 392 TEXT-BOOK OF PHYSIOLOGY. THE STATIC CONDITION. Relation of the Thoracic Organs. — Intra-pulmonary Pres- sure: Intra-thoracic Pressure. — In the static condition of the thorax the lungs, by virtue of their distensibility, completely fill all parts of the thorax not occupied by the heart and great blood-vessels (Fig. 189). This condition is maintained by the pressure of the air within the lungs, the intra-pulmonary pressure, which with the respira- tory passages open, is that of the atmosphere, 760 mm. Hg. This relation persists so long as the thorax remains air-tight. If the skin and muscles covering an intercostal space be removed the lung can be seen in close contact with the parietal layer of the pleura gliding by with each inspiration and expiration. If, however, an opening be now made in the pleura sufficient to admit air, the lung immediately collapses and a pleural cavity is established. The pressmre of air within and without the lung counterbalancing, at the moment the air is admitted, the elastic tissue at once recoils and forces a large part of the air out of the lung. This is a proof that in the normal condition, the lungs, distended by atmospheric pressure from within, are in a state of elastic tension and ever endeavoring to pull the visceral layer of the pleura away from the parietal layer. That they do not succeed in doing so is due to the fact that the atmospheric pressure from with- out is prevented from acting on the lung by the firm unyielding walls of the thorax. Intra-thoracic Pressure. — As a result of the elastic tension of the lungs a fractional part of the intra-pulmonary pressure, 760 mm. Hg., is counterbalanced or opposed, so that the heart and great vessels and other intra-thoracic viscera are subjected to a pressiure somewhat less than that of the atmosphere; the amount of this pressure will be that of the atmosphere less that exerted by the elastic tissue of the lung in the opposite direction, expressed in terms of millimeters of mercury. In the thorax but outside the limgs, there then prevails a pressure, negative to the pressure inside the lungs and which is known as the intra-thoracic pressure. The amount of this intra-thoracic pressure can be approximately determined in several ways. Thus, if shortly after death a mer- curial manometer be inserted air-tight into the trachea of a human being and the thorax opened, the lungs will recoil and compress their contained air. The mercurial manometer will at once show an excess of pressure in the trachea of about 6 mm. This was taken by Bonders as a measure of the force with which the limgs endeavor to recoil. The intra-thoracic pressure would be, therefore, atmos- pheric pressure, 760 mm., less 6 mm., or 754 mm. Hg. Another method is to insert a rubber catheter through a small opening in an intercostal space into the thoracic cavity. The air which enters through the open extremities of the catheter and leads to a collapse of the lungs may be subsequently aspirated, when the lung returns to RESPIRATION. 393 its normal position. The catheter is then placed in connection with a water manometer. On establishing a communication between them, by the turning of a stopcock, the water will rise in the proximal and fall in the distal limb of the manometer, indicating a pressure in the thorax negative to that in the 'lung. The difference in the level of the water in the two limbs of the manometer, expressed in millimeters of mercury, would also represent the force with which the elastic tissue strives to recoil, and the extent to which it opposes the atmospheric pressure. This subtracted from the atmospheric g Z,CA. Fig. 189. — Section of .Thorax with the Lungs, Heart, and Principal Vessels. 5. Catheter introduced into the pleural space and connected with a manometer. — {After Moral and Doyen.) pressure would give the intra-thoracic pressure. In the living dog this latter is less than the former, to the extent of from 3.5 to 5.5 mm. For the same reason the superior surface of the diaphragm also ex- periences a pressure less than that of the atmosphere. Owing to the soft and yielding character of the abdominal walls the atmospheric pressure is transmitted through the abdominal organs to the inferior surface of the diaphragm. The pressure being greater from below than above, the diaphragm is forced upward until it assumes the dome- like appearance it usually presents. (These relations are shown in Fig. 189.) The cause of the negativity of the intra-thoracic pressure is con- nected with the change in the relation of the lungs to the thorax at- tending the first inspiration. Previous to birth the walls of the alveoli 394 TEXT-BOOK OF PHYSIOLOGY. and bronchioles are collapsed and in apposition. The larger bron- chial tubes in all probability contain fluid. The lungs therefore are devoid of air (atelectatic), and, having a specific gravity greater than water, readily sink when placed in this fluid. The capacity of the thorax does not exceed the volume of the lungs. With the first inspira- tion, however, the thoracic walls take a new position. The air at once rushes into the lungs and distends them. But as the capacity of the thorax even at the end of the expiration is now greater than the volume which the lungs could assume without considerable distention, there at once arises the elastic recoil in the opposite direction, the condition which gives rise to the negativity of the pressure in the thoracic cavity. It is also probable that as the child develops, the thorax grows more rapidly than the lungs, giving rise to a condition which would increase and accentuate the elastic tension and thus increase the negativity of the intra-thoracic pressure. THE DYNAMIC CONDITION. In the dynamic condition as previously stated these relations and pressures change. Thus the diaphragm descends, the ribs ascend, the sternum advances and the lungs expand. The intra- pulmonary pressure varies during both inspiration and expiration. With the enlargement of the thorax through muscle activity, there goes a corresponding increase in the size and capacity of the lungs, in consequence of the expansion and pressure of the air in the pulmonary alveoli. With the expansion of the air its pressure falls; but though it is now less than atmospheric, it is yet much greater than the opposing force of the lung tissue. As a result of the fall of intra-pulmonary pres- sure there is a rapid inflow of air, which continues until atmospheric pressure is restored; that is, at the end of the inspiration. With the diminution of the thorax, through the recoil of the elastic tissue of the thoracic and abdominal walls, there goes a corresponding decrease of lung capacity, in consequence of the recoil of the elastic tissue of the lungs. As a result, the air in the lungs becomes compressed, its pres- sure rises above that of the atmosphere, and a rapid outflow of air takes place, which continues until atmospheric pressure is again re- stored; that is, at the end of the expiration. The cause for the fall of intra-pulmonary pressure during in- spiration and the rise during expiration is to be found in the resist- ance offered by the air-passages to the movement of the air, through- out their entire extent, and especially at the level of the vocal bands. The greater the resistance, from whatever cause, physiologic or patho- logic, the greater the variations of the pressure. In quiet inspiration the fall of pressure, as indicated by a man- ometer inserted into one nostril, seldom amounts to more than 1.5 mm. of Hg., the rise in expiration, 2.5 to 3 mm. of Hg. In forcible inspiratory and expiratory efforts these limits may be largely exceeded. RESPIRATION. 395 Thus it was found by Bonders that with one nostril closed and a mer- curial manometer inserted into the other the pressure by voluntary efforts could be made to fall 57 mm. during inspiration and to rise 87 mm. during expiration. The changes in intra-pulmonary pressure are graphically represented in the upper half of Fig. 190. The intra-thordcic pressure also varies during both inspiration and expiration. As the intra-pulmonary pressure falls, the recoil of the elastic tissue increases, with the result of diminishing the intra-thoracic pressure, though not in a steadily progressive manner. The fall of intra-thoracic pressure at the end of a quiet inspiration amounts to about 9 mm. Hg. In forcible inspiratory efforts this fall in intra- thoracic pressure may amount to 30 or 40 mm. of Hg. As the intra- pulmonary pressure rises above the atmospheric pressure during expiration, the recoil of the elastic tissue is again opposed, with the result of increasing the . Insp- intra-thoracic pressure, though not in a steadily progressive manner. The changes in intra- thoracic pressure are graphically represented in the lower half of Fig. Insp. Intra-pulmonary pressure. B Exp. 760' mm c 760 mm Intra-thoracic pressure. Fig. 190. — Representing the Changes^ i, in THE Intra-pulmonary, and 2, in the Intra-tho- racic Pressures during Inspiration and Ex- piration. 190. Respiratory Move- ments. — As previously stated, the ventilation of the lungs is accom- plished by an alternate increase and decrease in the capacity of the thorax, accompanied by corresponding changes in the lungs, the two movements being known as inspiration and expiration respectively. During the increase in the thoracic capacity, the air passively flows into the lungs; during the decrease in the thora- cic capacity, the air passively flows "out of the lungs. In both move- ments the lungs play an entirely passive part, their movements being determined by the pressure of air within them and by the thoracic walls, with which they are in close contact. 1. Inspiration is an active process, the result of muscle activity. 2. Expiration is a passive process, the result mainly of the recoil of the elastic tissue of the walls of the thorax and abdomen and of the elastic tissue of the lungs. In inspiration the thorax is enlarged in all its diameters: viz., vertical, transverse, and antero-posterior. In expiration these diam- eters are in turn decreased as the thorax returns to its previous con- dition. Inspiratory Muscles. — The muscles which from their origin, direction, and insertion contribute to the enlargement or expansion of 396 TEXT-BOOK OF PHYSIOLOGY. the thorax are quite numerous, and include those muscles which enter into the formation of the thoracic walls (intrinsic muscles), as well as certain muscles which, ha,ving their origin elsewhere, are attached to the thoracic walls at different points (extrinsic muscles), though the extent to which they are called into axrtivity depends on the necessity for either tranquil or energetic inspirations. The gradations between a minimum and a maximum inspiration are very slight, and it is difficult to state at what particular instant any givea muscle begins to act. It is customary, however, to divide the muscles into two groups: (i) Those active in the average or ordinary inspirations, and (2) those active in maximum or extra- ordinary inspirations. Among the muscles active in ordinary inspira- tions may be mentioned the diaphragm, the • intercostales externi, the intercariilagenei, the levatores costarum, the scaleni, and the serratus posticus superior. Among the muscles active in extraordinary inspirations may be mentioned, in addition to the foregoing, the sterno-cleido-mastoideus, the trapezius, and the pectorales minor and major. The vertical diameter is increased by the contraction and descent of the diaphragm, and more especially of its lateral muscular portions. At the end of an expiration the diaphragm is relaxed, and the lower portion closely applied to the walls of the thorax. At the beginning of an inspiration the muscle- fibers contract, shorten, and approximate a straight line, whereby not only is the con- vexity of the diaphragm diminished, but that portion in contact with the thorax is drawn away, thus making a large free space into which the lateral and posterior portions of the The attachment of the central tendon of the diaphragm to the pericardium prevents any marked descent of this portion except in forcible inspiratory efforts (Fig. 191). The vertical diameters are thus enlarged, though unequally in different regions of the thorax. As the diaphragm descends it displaces the abdominal viscera, forcing them downward against the abdominal walls, which advance and become more convex. In forcible inspiration the diaphragm, acting from the central tendon as the more fixed point, would draw the lower portion of the thorax inward were this not prevented by the outward pressure of the displaced viscera. The antero-posterior and transverse diameters are increased by the elevation and outward rotation of the ribs and an advance of the sternum, both movements made possible by the construction and arrangement of the costo-vertebral and costo-chrondral and chrondro- FiG. iQT. — Diagram SHOWING Interval be- tween THE Position of THE Diaphragm in Ex- piration (e, e) AND In- spiration {i, i). The In- crease IN Capacity is Shown by the White Areas. — (Yea.) lungs at once descend. RESPIRATION. 397 sternal articulations. The construction of these articulations is such as to permit at the first a slight elevation and depression of the head of the rib, and at the second a gliding of the tubercle on the transverse process. The axis around which the rib rotates practically coincides with the axis of the rib neck, which in the upper part of the thorax is almost horizontal, in the lower part somewhat sagittal in direction. Hence when the ribs are elevated the upper part of the thorax increases in its antero-posterior, the lower part in its transverse diameters. At the same time, the lower portion of the sternum is pushed forward and upward by the elevation of the anterior extremity of the ribs and the widening of the angle of the costo-chondral articulation. With the elevation of the ribs there goes an eversion-or outward rotation Fig. 192. — Diagram illustrating the Action of A, the External Intercostal AND B, The Internal Intercostal Muscles. V, V. Vertical support. R, R'. Parallel bars. S. Vertical strip, representing respectively the vertebral column, two ribs, and sternum. which gives an additional increase to the transverse diameters. This elevation and outward rotation of the ribs is the resultant of the coop- eration of the following muscles, viz.: the intercostales externi, the intercartilagenei, the levatores costarum, the scaleni and the serratus posticus superior. The action of the external intercostal muscles has been a subject of much discussion. Some investigators have maintained that they are elevators of the ribs, and therefore inspiratory; others that they are depressors of the ribs, and therefore expiratory in function. At the present time the general consensus of opinion is that the former view is the one most in accordance with the facts. The situation of the muscles and the shortness of their fibers render it extremely diffi- cult to obtain myographic tracings of their action. This is supposed, however, to be disclosed by the play of the apparatus suggested orig- inally by Bernouilli, which consists, as shown in Fig. 192, of a vertical support carrying two freely movable parallel bars united at their outer extremities by a short vertical strip, representing respectively the vertebral column, two adjoining ribs, and a piece of the sternum. 398 TEXT-BOOK OF PHYSIOLOGY. The parallel bars are joined to each other by a short elastic band having the direction of and representing the external intercostal muscles. If the bars are depressed, the elastic band is elongated and made tense. On releasing the bars the band at once recoils and elevates them. Although the elastic force is the same in both directions, the bars are. yet elevated for the reason that in accordance with the parallel- ogram of forces the component acting upward on the long arm of the lever preponderates over the component acting downward on the short arm of the lever. The action of the band is supposed to disclose and illustrate the action of the muscle. The intercariilaginei, those portions of the intercostales interni which occupy the space between the costal cartilages from the sternum to their outer extremity, bear the same relation to the cartilages in reference to the sternum that the external intercostals bear to the ribs in reference to the vertebral column; that is, the point of attachment to the cartilage above, lies nearer the sternum, nearer the fulcrum, than the point of attachment below. Hence the same action is at- tributed to them as to the external intercostals: viz., elevation of the cartilages and the anterior extremities of the ribs. The levatores costarum, as is evident from their points of origin and insertion, elevate the ribs posteriorly. The scqleni muscles, anticus, medius, and posticus, arise from the transverse processes of the cervical vertebrae, and after pursuing a downward and forward direction are inserted into the sternal end of the first and second ribs. The action of the first two, at least, is to elevate the first rib and thus establish a fixed point from which the intercostal muscles can act. The posticus has doubtless a similar action on the second rib. The serratus posticus superior, a quadrilateral sheet of muscle- fibers, arises mainly from the spines of the last cervical and first and second thoracic vertebras. The anterior extremity is serrated and attached to the outer surfaces of the second, third, fourth, and fifth ribs beyond the angle. The action of the muscle is the elevation of the ribs to which it is attached. In forcible or extraordinary inspirations, whereby the capacity of the thorax is still further increased, the foregoing muscles are rein- forced by the sterno-cleido-mastoideus, the trapezius, and the pectorales minor and major. Their functions will become apparent from a consideration of their origins and insertions. Expiratory Forces and Muscles. — Expiration, as previously stated, is a passive process brought about by the recoil of the elastic tissues of the thoracic and abdominal walls, and of the lungs, all of which have been stretched and made tense during inspiration. With the cessation of the inspiratory effort the elastic forces, assisted by the weight of the ribs, sternum, and soft tissues, return the thorax to its former condition. The result is a diminution of all the diameters of the thorax. The vertical diameter is diminished by the recoil of RESPIRATION. 399 the tense abdominal walls, the replacement of the abdominal organs and the consequent ascent of the diaphragm to its former position. The transverse and aniero-posterior diameters are diminished by the descent of the ribs, sternum, and lungs. It is somewhat uncertain if a normal expiratory movement necessitates active muscle contrac- tion. If, however, there is any impairment of the elasticity of the lungs or ribs, or any interference with the free exit of the intra-pulmo- nary air, it is highly probable that the elastic forces are assisted by the internal intercostal and triangularis sterni muscles. The action of the internal intercostals is less clearly understood than that of the externals, and for the same reasons. If, however, Bernouilli's model discloses the action of the latter, it equally well discloses the action of the former. Thus, if the parallel bars be joined by an elastic band having the direction of and representing the inter- nal intercostals, and then forcibly elevated, the band is elongated and made tense. On releasing the bars, the elastic band at once recoils and depresses them. Here, again, though the elastic force is the same in both directions, the bars are depressed, for the reason that the component acting downward on the long arm of the lever prepon- derates over that acting upward on the short arm of the lever. The action of the band is supposed to disclose and illustrate the action of the muscle. The triangularis sterni muscle, judging from its anatomic re- lations, in all probability assists in expiration by depressing the car- tilages to which it is attached and as a further result the anterior extremities of the ribs. After the elastic forces have ceased to act and the normal expira- tory movement has been brought to a close, the thorax can be, to a considerable extent, still further diminished in all its diameters by the contraction, through volitional effort, of abdominal and thoracic muscles. To this decrease in the capacity of the thorax, as a result of which a much larger volume of air is expelled from the lungs than during passive expiration, the term forced expiration has been given. With the cessation of muscle activity the elastic forces of the now- compressed thoracic walls, aided by the return of upward displaced abdominal organs, at once restore the thoracic walls to the position they had attained at the end of passive expiration. Of the muscles active in forced expiration in addition to the intercostales interni and the triangularis sterni, the following may be mentioned, viz.: the abdominales, the serratus posticus inferior, and the quadratus lum- borum. The externus abdominis arises by a series of muscle slips from the outer surface of the lower eight ribs. After pursuing an oblique direction downward and forward, the slips blend to form a single muscle, which is inserted mainly into the outer lip of the anterior half of the crest of the ilium and into the central abdominal tendon or aponeurosis. 400 TEXT-BOOK OF PHYSIOLOGY. The internus abdominis arises mainly from the anterior two-thirds of the inner crest of the ilium and the lumbar fascia. Its fibers pass upward and forward to be inserted into the cartilages of the last three ribs and into the central abdominal tendon. The rectus abdominis arises from the crest of the pubes and is inserted above into the cartilages of the fifth, sixth, and seventh ribs, and occasionally into the ensiform cartilage. The transversalis arises from the cartilages of the last six ribs, the lumbar fascia, and the anterior half of the crest of the ilium. After passing transversely across the abdomen, the fibers are inserted mainly into the linea alba. The conjoint action of these muscles is to diminish the convexity of the abdominal walls and to exert a pressure on the abdominal organs. These, taking the line of least resistance, are forced upward against the inferior surface of the diaphragm, which in consequence becomes more strongly curved and ascends higher into the thorax. The vertical diameter of the thorax is thus diminished. Acting from the pelvis as a fixed point, these muscles will also draw downward and inward the lower end of the sternum and the lower ribs and diminish the antero-posterior and transverse diameters. The serratus posticus inferior arises from the spines of the last two thoracic and first two lumbar vertebras. The fibers pass upward to be inserted into the lower border of the last four or five ribs beyond the angle. Their action is to depress the ribs and assist in expira- tion. The quadratus lumborum has a similar action on the last rib. Movements of the Lungs. — As the thorax is enlarging in all its diameters during inspiration, through muscle activity, the lungs are correspondingly enlarging in all their diameters, by virtue of their distensibility, through the pressure of the air within them. The lungs must therefore move downward, outward and forward. That this is the case is made evident both by an examination of the lungs through an intercostal space after removal of the skin and intercostal muscles and by the methods of percussion. The inferior border of each lung descends from the lower border of the sixth to the eleventh rib, insert- ing itself into the space developed between the thorax and diaphragm as the latter contracts and is drawn away from the former. In con- sequence of the lateral expansion the anterior border of each lung advances toward the middle line until the heart is almost covered. With the beginning and continuance of expiration the lungs exhibit a reverse movement which continues until they reach their original position. At all times, however, the movements of the lungs are en- tirely passive and determined by the movements of the thorax. The succession of events in the thorax at the time of a respiratory act may be summarized as follows : During Inspiration. I. Enlargement of the thoracic diameters by muscle action. RESPIRATION. 401 2. Expansion of intra-pulmonary (alveolar) air. 3. Expansion of the lungs. 4. Lowering of the intra-pulmonary air pressure below the atmos- pheric air pressure. 5. Increase in the negativity of the intra- thoracic pressure. 6. Inflow of atmospheric air, in consequence of its higher press- ure, until the intra-pulmonary air pressure rises to that of the atmosphere. During Expiration. 1 . Diminution of the thoracic diameters by the action of elastic forces. 2. Recoil of the lungs. 3. Compression of the intra-pulmonary (alveolar) air. 4. Rise of intra-pulmonary air pressure above the atmospheric air pressure. 5. Decrease in the negativity of the intra- thoracic pressure. 6. Outflow of intra-pulmonary air, in consequence of its higher pressure, until the intra-pulmonary air pressure falls to that of the atmosphere. Respiratory Movements of the Upper Air-passages. — The resistance to the entrance of air into and through the respiratory tract is much diminished by respiratory movements of the nares and larynx which are associated and occur synchronously with the movement of the thorax. The nares at each inspiration are dilated by the outward niove- ment of their alee or wings, the result of muscle activity. At each expiration they are diminished by the return of their cartilages through the play of elastic forces. The larynx, as shown by observation with the laryngoscope, exhibits corresponding movements of the vocal membranes. Their introduction at this level naturally narrows the tract, and would interfere with both the entrance and the exit of air were they not kept widely asunder during the time they are not re- quired for purposes of phonation. This is accomplished by the tonic contraction of the posterior crico-arytenoid muscles, which are entirely respiratory in function. It is not infrequently stated that these membranes exhibit consider- able oscillations, outward and inward, corresponding to the periods of inspiration and expiration. The statements of the majority of laryngologists do not favor this view. During tranquil breathing the membranes are widely separated and almost stationary, seldom moving in either direction more than a few millimeters. In labored respirations these movements are naturally increased in extent. The irregular movements of the membranes occasioned by the unskilful use of the laryngoscope, especially with nervous patients, are not to be re- garded as strictly physiologic. The respiratory space in quiet breath- ing is an isoceles triangle, with a length of 20 mm. and a width at the base of 15.5 mm. with an area of 155 mm. 26 402 TEXT-BOOK OF PHYSIOLOGY. Respiratory Types. — Observation of the respiratory movements in the two sexes shows that while the enlargement of the thoracic cavity is accomplished both by the descent of the diaphragm (as shown by the protrusion of the abdomen) and the elevation of the thoracic walls, the former movement preponderates in the male, the latter in the female, giving rise to what has been termed in the one case the dia- phragmatic or abdominal and in the other the thoracic or costal type of respiration. The cause of this greater mobility and actvity of the tho- rax in the female has been a subject of much discussion. It has been attributed, on the one hand, to the necessity for a physiologic adjust- ment between respiration and child-bearing, and therefore a specific sex peculiarity; on the other hand, it has been attributed to persistent constriction of the waist, in consequence of which the full play of the diaphragm is prevented and the burden of inspiration is thrown on the thoracic muscles. It has been assumed that if inspiration were con- fined in women to the diaphragm, there would arise in the latter stages of gestation such an increase in intra-abdominal pressure that not only would respiratory exchanges be interfered with, but fetal life might be unfavorably influenced, if not endangered. Modern investigations have not confirmed this assumption, but, on the contrary, have corrob- orated the view that the preponderance of thoracic movement is due to the influences of dress restrictions, for with their removal the so-called costal type of breathing entirely disappears. While gestation may lead to a greater activity of the thorax, this is but temporary, for with its termination there is a return to the diaphragmatic type of breathing. Number of Respirations per Minute. — The number of respira- tions which occur in a unit of time varies with a variety of conditions, the most important of which is age. The results of the observations of Quetelet on this point, which are generally accepted, are as follows: Age. Respirations per Minute. Age. Respirations per Minute a- I year, 44 20-25 years, 18.7 5 years, 26 25-30 " 16.0 15-20 " 20 3°~5° " 18.0 From these observations it may be assumed that the average number of respirations in the adult is eighteen per minute, though varying from moment to moment from sixteen to twenty. During sleep, how- ever, the respiratory movements often diminish in number as much as 30 per cent., at the same time diminishing in depth. Rhythjn. — Each respiratory act takes place normally in a regular methodic manner, each event occurring in a definite sequence and occupying the same relative period of time. This rhythm, however, is not infrequently temporarily disturbed by emotions, volitional acts, muscle activity, phonation, changes in the composition of the blood, etc.; with the removal of these disturbing factors, the respiratory mechanism soon returns to its normal condition. A graphic representation of the excursions of the thoracic walls, rhythmic or otherwise, is obtained by fastening to the thorax an appara- RESPIRATION. 403 Fig. 'igs- — Pneitmogeaph. — (Fitz.) tus, a stethometer or a pneumograph, which by means of a tambour takes up and transmits the movement to a second tambour provided with a recording lever. A simple form of pneumograph, suggested by Fitz (Fig. 193), consists of a coil of wire two and a half centimeters in diameter and about 40 centimeters in length, enclosed by thin rubber tubing, one end of which is closed, the other placed in communication with either a tambour and lever or with a piston recorder. By means of an inelastic cord or chain the apparatus is securely fastened to the chest. With each inspiration the spring is elongated, the air within the system is rarefied, ^_^ and as a result the lever f\ 1 \ falls; with each expira- tion the -reverse condi- tions obtain and the lever rises. If the lever be applied to the record- ing surface of a moving cylinder, a curve of the thoracic movement, a pneumatogram, is obtained (Fig. 194), from which it is apparent that inspiration takes place more abruptly and occupies a shorter period of time than expiration; that expiration im- mediately follows inspiration, but that there is a slight pause between the end of the expiration and the beginning of the inspiration. The time relations of the two movements can be obtained by a magnet- signal actuated by an electric current interrupted once a second. The ratio of inspiration to expiration has been represented as 5 to 6, or 6 to 8. Volumes of Air Breathed. — The volumes of air which enter and leave the lungs with each inspiration and expiration naturally vary with the extent of the move- ment, though four at least may be determined: (i) that of an ordinary inspiration; (2) that of an ordinary expiration; (3) that of a forced in- spiration; (4) that of a forced ex- piration. The apparatus employed for the determination of these different volumes is the spirometer, a modification of the gasometer. The form introduced by Jonathan Hutchinson (Fig. 195) consists of two metallic cylinders, one (a) containing water, the other (b) containing air, the latter being inserted into the former. The air cylinder is balanced by weights so accurately that it remains stationary in any position. A tube, penetrating the base of the water cylinder, is con- tinued upward through and above the level of the water. The air- space above is thus placed in free communication with the external air. A stopcock at the outer end of this tube prevents the escape of the air when this is not desirable. To the free end of the tube a rubber tube INSP. Fig. 194. — A Pneumatogram. ■ Marey.) - {Alter 404 TEXT-BOOK OF PHYSIOLOGY. provided with a suitable mouthpiece is attached, through which air can be breathed into or out of the air cylinder. With each inspiration the air cylinder descends; with each expiration it ascends. A scale, on one of the side supports, graduated in cubic inches or centimeters, indicates the volume of air inspired or expired. With this apparatus Hutchinson, from a long series of observa- tions, defined and determined the above-mentioned four volumes as follows: I. The tidal volume, that which flows into and out of the lungs with each inspiration and expiration, which varies from 20 to 30 cubic inches (312 to 468 c.c). 2. The complemental volume, that which flows into the lungs, in addition to the tidal volume, as a result of a forcible inspiration, and which amounts to about no cubic inches (1748 c.c). 3. The reserve volume, that which flows out of the lungs, in addition to the tidal volume, as a result of a forcible expira- tion, and which amounts to about 100 cubic inches (1562 c.c). After the expulsion of the reserve volume there yet remains in the lungs an unknown volume of air which serves the mechanic function of distending the air- cells and alveolar passages, thus main- taining the conditions essential to the free movement of blood through the capillaries and to the exchanges of gases between the blood and alveolar air. As this air can not be displaced by voli- tional effort, but resides permanently in the alveoli and bron- chial tubes though constantly undergoing renewal, it was termed — 4. The residual volume, the amount of which is difficult of determina- tion, but has been estimated by different observers at 914 c.c, 1562 c.c, 1980 c.c. The Vital Capacity of the Lungs. ^From foregoing statements it is clear that the thorax and lungs are capable of. a maximum degree of expansion, at which moment the lungs contain their maximum volume of air. This volume, whatever it may be, represents the entire capacity of the lungs in the physiologic condition, and includes the tidal, the complemental, the reserve, and the residual volumes. Mr. Hutchinson, however, defined the vital or respiratory capacity of the lungs as the amount of air which can be expelled by the most forcible expiration after the most forcible inspiration, and which therefore ex- cludes the residual volume. The vital capacity was supposed to be Fig. 195. — Spirometer. — (JSutchinsnn.) RESPIRATION. 405 Fig. 196. — Pneumatogeaph. — {Gad.) an indication of an individual's respiratory power, not only in phys- iologic but also in pathologic conditions. Though averaging about 230 cubic inches (3593 c.c.) for an individual 5 feet 7 inches in height, the vital capacity varies with a number of conditions, the most im- portant of which is stature. It is found that between 5 and 6 feet the capacity increases 8 inches (125 c.c) for each inch increase in height. The volume changes of the thorax indicated by the volumes of air entering and leaving the lungs can be not only determined but graphically represented by means of an apparatus similar in principle to the spirometer, de- vised by Gad and known as the pneumatograph or aeroplethysmo- graph (Fig. 196). This consists of a quadrangular box with double walls, the space between which is filled with water. The center of the box is an air chamber. A thin- walled mica box sinks into the water. Posteriorly it is attached to and rotates around an axis, which permits of an elevation or depres- sion of the anterior portion. It is also carefully counterpoised. A light lever attached to the mica box records its movements. The interior of the box communicates by a tube with a large reservoir into which the individual breathes, the object being to prevent a too rapid vitiation of the air. Inspiration causes the lever to descend, expira- tion to ascend. Previous gradua- tion of the apparatus is necessary to determine the volumes breathed. A graphic record of the volume changes is shown in Fig. 197. Respiratory Sounds.— On ap- plying the ear over the trachea and bronchi there is heard during both inspiration and expiration a well- defined sound, loud, harsh, and blowing in character, which from its situation is known as the bron- chial sound. It is especially well heard between the scapulee above the fourth dorsal vertebra. This sound is produced in the larynx, for with its separation from the trachea the sound disappears. The cause of the sound is to be found in the narrowing of the air-passage at the level of the vocal membranes, though the mechanism of its pro- duction is uncertain. On applying the ear to almost any portion of the chest-wall, but especially to the infrascapular area, there is heard. --W 20C.1N TIDALVOL Fig. 197. — Representing the Vol- ume Changes of the Thorax and Lungs (Diagrammatic). 4o6 TEXT-BOOK OF PHYSIOLOGY. during both inspiration and expiration a delicate, sighing, rustling sound, which from its supposed seat of origin, the air-vesicles or -cells, is known as the vesicular sound. This sound is supposed to be due to the sudden expansion of the air-cells, during inspiration and to the friction of the air in the alveolar passages. THE CHEMISTRY OF RESPIRATION. The general nutritive process as it takes place in the tissues involves the assimilation of oxygen and the evolution of carbon dioxid. The former is the first, the latter the last, of a series of chemic changes the continuance of which is essential to the maintenance of all life phenomena. A constant supply of oxygen and an equally constant removal of carbon dioxid are necessary conditions for tissue activity. The respiratory movements constitute the means by which the oxygen of the air is broiight into, and the carbon dioxid expelled from, the lungs into the surrounding air. The blood is the medium by which the oxygen is transported from the lungs to the tissues and the carbon dioxid from the tissues to the lungs. The exchanges between blood and tissues constitute internal res- piration, in contradistinction to the thoracic movements by which the air is brought into relation with the blood, and which constitute external respiration. The transfer of the oxygen by the blood from the interior of the lungs to the tissues, and of the carbon dioxid from the tissues to the interior of the lungs, is the outcome of a series of chemic changes which are related to the exchange of gases between the air in the lungs and the blood, on the one hand, and between the blood and tissues, on the other. In consequence of the many and complex chemic changes which attend these gaseous exchanges, there arise changes in composition of: 1. The air breathed. 2. The blood, both arterial and venous. 3. The tissue elements and the lymph by which they are surrounded. The investigation of the nature of these changes, the mechanism of their production, and their quantitative relations constitutes the subject-matter of the chemistry of respiration. CHANGES IN THE COMPOSITION OF THE AIR. Experience teaches that the air during its sojourn in the lungs undergoes such a change in composition that it is rendered unfit for further breathing. Chemic analysis has shown that this change in- volves a loss of oxygen, a gain in carbon dioxid, watery vapor and organic matter. For the correct understanding of the phenomena of respiration it is essential, that not only the character but the extent of these changes be known. This necessitates an analysis of both the inspired and expired airs, from a comparison of which certain deduc- tions can be made. RESPIRATION. 407 The results which have been obtained are represented in the following table: Inspired Air. Expired Air. f Oxygen 20.80. 100 J Carbon dioxid, traces. vols. 1 Nitrogen, 79.20. [ Watery vapor, variable 100 vols. Oxygen, 16.02. Carbon dioxid, . . . 4.38. Nitrogen, 79.60. Watery vapor, . . . .saturated. Organic matter. These analyses indicate that under ordinary conditions the air loses oxygen to the extent of 4.78 per cent, and gains carbon dioxid to the extent of 4.38 per cent.; that it gains in nitrogen to the extent of 0.4 per cent, and in watery vapor from its initial amount to the point of saturation, as well as in organic matter. It is to these changes in their totality that those disturbances of physiologic activity are to be attributed which arise when expired air is re-breathed for any length of time without having undergone renovation. Special forms of apparatus have been devised for the collection and analysis of gases. Their construction as well as the methods of analysis involved are complicated and need not be described in this connection. The presence of the carbon dioxid, however, may be readily shown by breathing through a glass tube into a vessel con- taining barium or calcium hydrate. The turbidity which immediately follows is due to the formation of barium or calcium carbonate, which can be due only to the presence of carbon dioxid. That this turbidity is not due to the carbon dioxid normally present in the air is shown by the fact that the solution remains clear until the passage of the atmos- pheric air has been maintained for some time. From the percentage loss of oxygen and gain in carbon dioxid, the total oxygen absorbed and carbon dioxid exhaled may be approximately calculated. Thus, if the volume of air breathed daily be accepted at either 10,800 or 12,- 240 liters, and the percentage loss of oxygen be 4.78, the total oxygen absorbed may be obtained by the rule of simple proportion, e. g.: 100 : 4.78 :: 10,800 : x = si6 liters Or 100 : 4.78 :: 12,240 : a = 585 liters. By the same method the total carbon dioxid exhaled is found to be either 473 or 526 liters; volumes in both instances which agree very well with volumes obtained by other methods. From the fact that when one volume of oxygen combines with carbon it gives rise to but one volume of carbon dioxid, it is evident that of the oxygen absorbed the greater portion by far is utilized in the oxidation of the carbon, while the smaller portion is utilized in the oxidation of other substances, but especially hydrogen, as shown by the increase in water eliminated beyond that consumed. These amounts, however, are not fixed but variable, and depend on the quality and quantity of the foods, exercise, etc. The ratio of the volume of the carbon dioxid exhaled to the volume of oxygen absorbed is 4o8 TEXT-BOOK OF PHYSIOLOGY. known as the respiratory quotient, and is usually represented by the symbol -^° Thus in the foregoing analysis the respiratory quotient is 0.916. The gain in nitrogen is a variable factor, ranging from zero to 0.9 per cent. This gain is probably of accidental occurrence, due to ab- sorption from the large intestine, in which decomposition of nitrogen- holding compounds is taking place. It is generally believed that free nitrogen plays no part in any phenomenon of combination or decom- position within the body. The gain in watery vapor will depend on the amount previously present in the air. This is conditioned by the temperature. With a rise in temperature the percentage of water increases; with a fall, it decreases. By breathing into a vessel containing pumice stone satu- rated with sulphuric acid, the vapor may be collected. The difference observed between the weight before and after breathing is an indica- tion of the amount by weight of water exhaled during the time of breathing. It has been calculated that the amount of water exhaled daily approximates 500 grams. Though invisible at ordinary temper- atures, it becomes visible at low temperature as soon as it emerges from the respiratory tract. The loss of heat is followed by a condensation of the vapor, which appears at once as a cloudy precipitate. The gain in organic matter is also variable. The amount present is not sufficient to permit of a thorough chemic analysis, but there are reasons for believing that it belongs to the proteid group of bodies. . If it accumulates in the air, especially at high temperatures, it readily undergoes decomposition, with the production of offensive odors. Traces of free ammonia have also been found in the expired air. In addition to these chemic changes, the air experiences physical changes; e. g., a rise in temperature and an increase in volume. The rise in temperature can be shown by breathing through a suitable mouthpiece into a glass tube containing a thermometer. By this means it has been shown that inspired air at 20° C. rises in temperature to 37° C; at 6.3° to 29.8° C. The increase in the temperature will depend upon that of the air inspired and the time it remains in the lungs. If retained a sufficient length of time it will always become that of the body. As a result of the heat absorption the expired air increases in volume about one-ninth over that of the inspired air. When corrected for temperature and pressure and freed from aqueous vapor, the volume of the expired air is less than that of the inspired air by about one-two hundred and fiftieth. The Composition of the Alveolar Air. — The foregoing state- ment of the composition of the expired air, derived in part from the upper air-passages, trachea, and bronchi, does not necessarily repre- sent the composition of the alveolar air. It is very probable that the percentage of carbon dioxid is greater, the percentage of oxygen less, in the latter than in the former. This is made evident by collecting in several portions the expired air as it escapes from the respiratory RESPIRATION. 409 tract and subjecting it to analysis. The last portion always contains a larger amount of carbon dioxid and a smaller amount of oxygen than the first portion. The determination of the composition of the alveolar air is extremely difhcult. It has been estimated to contain from 5 to 6 per cent, of carbon dioxid and from 14 to 18 per cent, of oxygen. Pulmonary Ventilation. — It is owing largely to this inequality of volumes and consequently of the "partial pressures" of these two gases in the trachea and alveoli that the degree of ventilation necessary for the exchange of gases between lungs and air is maintained. Though the respiratory movements doubtless create currents in the air-passages which carry, on the one hand, a portion of the inspired air directly into the alveoli, and, on the other hand, carry a portion of the alveolar air directly out of the body, other portions find their way into and out of the alveoli in accordance with the laws of diffusion. If the pressure of the oxygen in the trachea is 158 mm. Hg. and in the alveoli approxi- mately 122 mm. Hg., diffusion downward will take place. Equilibrium, however, is never established, as the oxygen is continually disappear- ing by passing into the blood. On the contrary, if the carbon dioxid pressure in the alveoli is approximately 28 to 40 mm. Hg., and in the trachea 0.3 mm. Hg., diffusion will take place upward. Equilibrium will never be established, however, as the carbon dioxid is constantly coming out of the blood. Pulmonary ventilation may also be aided by those alternate changes in volume of the heart, great vessels, and lungs occurring as the result of the heart-beat and producing the so- called cardio-pneumatic movements. CHANGES IN THE COMPOSITION OF THE BLOOD. The blood which flows into the lungs through the pulmonary artery is dark bluish-red, that which flows from the lungs into the pulmonary veins is scarlet red, in color. The blood is changed, while flowing through the lung capillaries, from the venous to the arterial condition. As the air in the lungs gains carbon dioxid and loses oxygen, it is fair to assume that what the air gains the blood loses, and what the air loses the blood gains. In other words, the blood, while passing through the lungs, is changed from venous to arterial by the loss of carbon dioxid and the gain of oxygen. The change in color of venous blood from dark bluish to scarlet red is strikingly shown by shaking it in a test-tube with oxygen or atmospheric air. The blood which flows into the tissues through the arteries is red, that which flows from the tissues into the veins is bluish, in color. The blood while flowing through the tissue capillaries is changed from the arterial to the venous condition. Since arterial blood when de- prived of oxygen becomes bluish-red, the indication is that the change in color is associated with, if not entirely due to, the escape of oxygen into the tissues. The constant elimination of carbon dioxid from the 4IO TEXT-BOOK OF PHYSIOLOGY. blood into the lungs indicates that the carbon dioxid is as constantly passing from the tissues through the capillary walls into the blood. These considerations are confirmed by the results of analyses which have been made of both venous and arterial blood. The presence of gas in the blood is demonstrated by subjecting it under appropriate conditions to the vacuum of the mercurial air-pump, into which it at once escapes. From loo volumes, an average of 60 volumes of gas at standard pressure, 760 mm. Hg. and teinperature 0° C, can thus be obtained. Gases of the Blood. — An analysis of the volumes of gas removed from both venous and arterial blood shows that each consists of oxygen, carbon dioxid, and nitrogen; though in different amounts. An aver- age' composition of the gases extracted from dog's blood obtained from the right ventricle and carotid artery is given in the following table: Venous blood 100 vols. Oxygen, 9-12 vols. Arterial blood f '^^S^^' ^° ™'^- Carbon dioxid, 4'; " 1 -^ Carbon dioxid, 40 " TVT-i- 1< I°° vols. --T-i II Nitrogen, .... i- 2 [ Nitrogen, .... i- 2 The changes produced in the blood by respiration, both external and internal, become apparent from a comparison of these analyses. The venous blood while passing through the lungs gains from eight to eleven volumes per cent, of oxygen and loses five volumes per cent, of carbon dioxid. The arterial blood while passing through the tissues loses oxygen and gains carbon dioxid in corresponding amounts. The volume of nitrogen is not appreciably changed. The Relation of the Gases in the Blood. — The mechanism by which the gases become associated with the blood at the moment of their entrance into it, and again become dissociated just prior to their exit from it, as well as their relation to the blood while in transit, will be more readily understood after reference to a few elementary facts relative to the absorption of gases by liquids in general and the conditions of temperature and pressure by which it is influenced. It is well known that liquids will absorb or dissolve at any constant pressure unequal volumes of different gases in accordance with their solubilities and with variations in temperature. Water, for example, will absorb, in accordance with the foregoing conditions, oxygen, carbon dioxid and nitrogen, as well as many other gases. The volume of any gas thus absorbed is known as the coefficient of absorption, and may be defined as the number of cubic centimeters of the gas which one cubic centimeter of water will absorb when the gas, in contact with the water, stands under a pressure of one atmosphere or 760 mm. of mercury and at a temperature of 0° C. The volume absorbed, however, varies inversely as the temperature. Thus at 0° C. the volume of oxygen absorbed by one volume of water is 0.0489 c.c. ; of carbon dioxid 1.713 c.c; of nitrogen 0.0234 c.c. With a rise of tem- perature, the pressure remaining constant, the absorptive power of water for each of these gases diminishes. Thus at 15° C., the volumes RESPIRATION. 411 of oxygen, carbon dioxid and nitrogen absorbed are 0.0310 c.c, 1.0025 c.c. and 0.0168 c.c. respectively. Though the volume of the gas absorbed diminishes as the temperature rises, it is independent of pressure, for no matter to what extent the pressure may vary the volume absorbed is always the same. (Law of Henry.) If the weight of the gas absorbed be considered rather than the volume (that is the product of the volume and the density or the number of molecules in the volume), then the temperature remaining constant, the weight of the volume absorbed increases and decreases proportionately as the pressure rises and falls. Thus at a pressure of 760 mm. of mercury and at a temperature of 0° C, the volume of oxygen absorbed by one volume of water is 0.0489 c.c; at 1520 mm. of mercury, the same volume is absorbed but its weight is doubled. If the pressure falls below 760 mm. of mercury the same volume is ab- sorbed but its weight is diminished. (Law of Dalton.) Because of the foregoing facts, it is necessary in all gaseous determinations to reduce for purposes of comparison the obtained volumes to standard temperature (o° C.) and pressure (760 mm. of mercury). When the liquid is once saturated with a gas at a constant pressure and temperature, there is coincidently with the entrance of the gas into the liquid, an equivalent exit of the gas from it, though the volume retained in the liquid remains constant. The reason for this fact is, that under , the conditions, the volume of the gas dissolved by the liquid though small in amount exerts a pressure in the opposite direction equivalent to the pressure acting upon the liquid. If one cubic centimeter of water absorbs 0.0489 c.c. of oxygen at 760 mm. and 0° C, this volume will exert a pressure opposite in direction of 760 mm. of mercury. For this reason the entrance and exit of the gas are equal and opposite. If water be exposed to atmospheric air consisting of oxygen, carbon dioxid, and nitrogen in the ordinary proportions, at any given tem- perature and pressure, the water will absorb unequal volumes of each of the three gases. The pressure under which each gas is absorbed is a part only, however, of the total atmospheric pressure at the time. The pressure exerted by any one of these gases is known as its "partial pres- sure," and depends on the percentage volume of the gas present. If atmospheric air contains at standard pressure and temperature 79.15 volumes per cent of nitrogen, its partial pressure will be -^1^ of 760, or 601.54 mm. Hg.; if the air contains 0.04 volume per cent, of carbon dioxid and 20.85 volumes per cent, of oxygen, the partial pressure of each will be 0.30 mm. Hg. and 158.46 mm. Hg. respectively. The absorption of each gas is independent of all the rest, and is the same for nitrogen, for example, as if it alone were present at a pressure of 601.54 mm. Hg. Again, if water holding in solution a certain volume of a gas — carbon dioxid, for example — be exposed to an atmosphere containing but 0.04 volume per cent, of carbon dioxid, and having therefore a 412 TEXT-BOOK OF PHYSIOLOGY. pressure of but 0.3 mm. Hg., the gas will at once begin to leave the water, and continue to do so until the pressure of the carbon dioxid in the atmosphere balances the pressure of the gas in the water, at which moment the escape of the gas ceases. The pressure of a gas in a liquid is equal to that pressure in millimeters of mercury of the same gas in the atmosphere which is required to keep it in solution. What is true for the carbon dioxid is true for any other gas that may be in solu- tion. If a liquid has a greater density than water as from the pres- ence of inorganic salts, the absorptive power under standard condi- tions of temperature and pressure becomes less. It is for this reason that blood-plasma contains less oxygen, nitrogen and carbon dioxid than water. It will be recalled that the blood yields up its gases when subjected to the vacuum of the mercurial pump; that is, to a diminution or com- plete removal of the atmospheric pressure. From this it might be in- ferred that the gases are merely held in solution by pressure, and at once escape the moment they are exposed to a space in which there is a very slight or a total absence of pressure. In other words that the absorption of gases by the blood and their escape from it follows the law of pressure as stated in foregoing paragraphs. It is therefore neces- sary to test this supposed condition of the gases in the blood by sub- jecting the latter to gradually diminishing pressures, with a view of determining in how far the discharge of the gases follows the law of falling pressures. For convenience the conditions of each gas will be considered separately. Oxygen.^ — If blood is subjected to a succession of pressures pro- gressively less than the standard, it is found that though oxygen is evolved, its evolution is not in accordance with the law of partial pressures ; that is, in proportion to the diminution of pressure. Within wide limits — e. g., from 760 to 332 mm. atmospheric pressure, to which correspond oxygen pressures of 160 and 70 mm. respectively — there is but a slight increase in the amount of oxygen evolved; and it is not until the pressure of the oxygen falls below this that it begins to be liberated in large amounts. From this on, the oxygen continues to be liberated with decreasing pressures, until the zero point is reached, when all gaseous discharge ceases. Coincidently the blood changes in color from a bright red to a deep bluish-red. It is evident from the results of this procedure that the condition of the oxygen in the blood is but to a slight extent one of physical absorption. The indications are that the union is of the nature of a chemic combination. If the red corpuscles are removed from the blood and the plasma alone treated in the manner above described, it will be found that the oxygen liberated now follows the law of partial pressure. The amount so liberated, however, is small — about one per cent, of the total oxygen of the blood. The agent therefore which holds the oxygen in com- bination is the red corpuscle, and more especially the hemoglobin, which constitutes about 30 per cent, of its volume. This is proved by RESPIRATION. 413 the fact that a solution of gas-free hemoglobin of a strength equivalent to that of the blood (14 per cent.), exposed to oxygen under a gradually increasing pressure from zero up to 50 to 70 mm. pressure, will absorb large quantities of oxygen; beyond this point the amount absorbed is again small in comparison. At 70 mm. pressure the hemoglobin is almost saturated. Coincidently with this absorption the hemo- globin changes in color from dark blue to bright red; changes from hemoglobin to oxyhemoglobin. The reverse method, that of subject- ing oxyhemoglobin to gradually diminishing pressures, yields opposite results, that is the oxygen becomes dissociated and the force by which this is accomplished is known as the force of dissociation. As one gram of hemoglobin combines with 1.59 c.c. of oxygen, and as the per- centage of hemoglobin is 13.50 to 14, it is evident that there is sufficient hemoglobin to combine with practically all the oxygen usually present in the blood. The union of the oxygen with the hemoglobin is therefore largely chemic in character, dependent however on pressure. About one-half of one per cent, is physically absorbed by or dissolved in the plasma; the remainder is chemically combined with the hemoglobin. The association or combination of oxygen is favored by a pressure of at least from 30 to 50 mm. Hg. and upward; the dissociation, by diminution of pressure. In the conversion of hemoglobin into oxy- hemoglobin two antagonistic forces are at work, heat and chemic affinity. The former endeavors to prevent, the latter to favor, the union. Chemic affinity increases with the influence of mass, that is, in proportion to the number of atoms in a unit of volume, with the density and with the partial pressure of the oxygen. Diminution of pressure reduces the mass influence and permits the heat to bring about dissociation (Bunge). The following table by Hiifner shows the relative proportion of hemoglobin and oxyhemoglobin in blood containing 14 per cent, hemoglobin and exposed to air at gradually diminishing pressures: PHERIC Pressure Partial Pressure of Hemoglobin Oxyhemoglobin IN MM. Hg. Oxygen in mm. Hg. Percentage. Percentage. 760 159-3 1.49 98.51 524.8 no :i.i4 97.86 357-8 75 3-II 96.89 238-5 5° 4.60 95-40 "9-3 25 8.79 91.21 47-7 10 19.36 80.64 23.8 5 32-51 67.49 0.0 0.0 100.00 0.00 Carbon Dioxid. — The blood yields up its contained carbon dioxid to the vacuum of the gas-pump as completely as it does its oxygen. The same is not the case, however, if the red corpuscles are first re- moved and the experiment made with either plasma or serum. Even at zero pressure the fluid contains carbon dioxid, as shown by its libera- tion on the addition of some weak acid, as tartaric or phosphoric, an 414 TEXT-BOOK OF PHYSIOLOGY. indication that it exists in a state of firm combination. The same result follows the addition of the red blood-corpuscles, which act in a manner similar to the acids just mentioned. This property of the cor- puscles has been attributed to hemoglobin, and especially when in the state of oxyhemoglobin. It is for this reason that blood yields all its carbon dioxid to the vacuum of the gas-pump. The limit of pressure at which the plasma ceases to physically absorb carbon dioxid and begins to chemically combine it is not very clearly defined. It has been estimated that of the entire amount, 38 to 45 volumes, only about 2,5 volumes are so absorbed, the remain- der being in a condition of both loose and stable combination. An analysis of the serum, and presumably of the plasma, shows the presence of sodium salts, with which the carbon dioxid could enter into combination, viz.: sodium carbonate and dibasic sodium phosphate. The sodium is thus partly divided between carbonic acid and phosphoric acid. The amount of the sodium which falls to carbon dioxid will depend on the mass influence of the latter; that is, its partial pressure. At its origin in the tissues the carbon dioxid acquires a consider- able tension, and its mass influence is correspondingly large. On entering the blood it combines with sodium carbonate, with the forma- tion of sodium bicarbonate, as shown in the following equation : NaaCOj + COr-t- H.O = 2NaHC03. At the same time, having a greater mass influence than the phos- phoric acid, it will withdraw from the dibasic sodium phosphate one- half of its sodium, with the formation of sodium bicarbonate and mono- basic sodium phosphate, as shown in the following equation: Na,HP04 -I- CO2 -I- H,0 = NaHCOj + NaH^PO*. With the diffusion of the carbon dioxid from the blood into the alveoli its tension in the venous blood falls, its mass influence diminishes, while that of the phosphoric acid relatively increases. As a result, the sodium is withdrawn from the sodium bicarbonate, an additional liberation of carbon dioxid takes place and dibasic sodium phosphate is re-formed. The association or combination of the carbon dioxid with the basic salts depends on its partial pressure; its dissociation in the lungs, on a diminution of pressure. Nitrogen. — This gas exists in both arterial and venous blood in a state of solution. There is no evidence that it enters into combination with any other element. Tension of the Gases in the Blood. — It will be recalled that a liquid holding in solution one or more gases will on exposure to an atmosphere composed of the same gases either give up or absorb yol- umes varying in amount and in accordance with their partial pressures until equilibrium is established. If the pressure of any one gas in the atmosphere is greater than the pressure of the same gas in the liquid, RESPIRATION. 41S it is absorbed; if the pressure is less the gas is discharged. Knowing the pressure of the gases in percentages of an atmosphere, at the be- ginning and the end of an experiment, the original tension or pressure of the gases in the liquid can be easily calculated. On this principle, various forms of apparatus known as aerotonometers have been devised by which the tension of the gases in the blood can be determined. These appliances consist essentially of a glass tube containing oxygen, carbon dioxid and nitrogen in known amounts and tensions. The blood from an animal is then allowed to flow directly from an artery or vein into the tube. As it flows down its sides in a thin layer it pre- sents a large surface to the action of the contained gases. In the aero- tonometer of Fredericq the blood made non-coagulable by the injection of peptone is returned from the opposite extremity of the tube to the adimal. This enables the experiment to be continued for an hour or more. A knowledge of the tensions of the blood gases is of interest, as it affords a clue to the mechanism by which the interchange takes place between the lungs and the blood, on the one hand, and the blood and tissues, on the other. The results, however, of different observers are not sufficiently in accord to permit of positive deductions. In the well-known experiments of Strassburger, the tension of the oxygen. in the arterial blood of the dog was found to be 29.64 mm. Hg., or 3.9 per cent, of an atmosphere, and in the venous blood 22.04 mm. Hg., or 2.9 per cent. The tension of the carbon dioxid in the venous blood was found to be 41.14 mm. Hg., or 5.4 per cent. of an atmosphere, and in the arterial blood 21.8 mm. Hg., or 2.8 per cent. Very different results have been obtained by Fredericq with the aerotonometer devised by him and by the employment of a method different from that of Strassburger. Thus he states that the oxygen tension in the pulmonary alveoli is 136 mm. Hg., or 18 per cent, of an atmosphere while in the arterial blood it is 106 mm. Hg., or 14 per cent.; while the carbon-di- oxid tension in the tissues varies from 38 to 68 mm. Hg., or from 5 to 9 per cent, of an atmosphere; while in the venous blood it varies from 30 to 41 mm. Hg., or from 3.8 to 5.4 per cent, and in the pulmonary alveoli it is about 21 mm. or 2.8 per cent. CHANGES IN THE COMPOSITION OF THE TISSUES AND LYMPH. From previous statements the inferences can be drawn that the oxygen leaves the blood as the latter flows through the capillaries; that it passes through the capillary wall into the surrounding lymph and so to the tissue-cells; that it oxidizes food materials in the tissue- cells whereby the potential energy of the former is liberated as kinetic energy; that the carbon dioxid so evolved passes into the lymph and through the wall of the capillary into the blood. While this is doubtless the case, the presence of free oxygen in the tissues can not be demonstrated by the usual methods of gas analysis. .Only in the saliva and in the blood of the placental umbilical vein can 4i6 TEXT-BOOK OF PHYSIOLOGY. it be shown that oxygen has directly passed through the capillary wall. For this reason it has been claimed by a few investigators that oxygen does not leave the blood, but that the field of its activity as an oxidizing agent is limited to the blood-current, where it meets with and oxidizes easily reducible substances entering from the tissues. On this view the potential energy of the food would be liberated by mere decom- position or cleavage in consequence of cell activity. Nevertheless many facts from the fields of comparative physi- ology and physiologic chemistry combine to support the view that oxygen is absolutely necessary to the maintenance of the life of all tissue-cells. Though they will continue to manifest their character- istic activities — e. g., contraction on the part of a muscle, secretion by a gland, the conduction of a nerve impulse by the nerve, etc. — for -a variable length of time after oxygen is prevented from gaining access to them, nevertheless they will in due time die. The necessity for oxygen on the part of the tissues and the avidity with which they absorb it, is shown by their power of reducing pig- ments such as alizarine blue. If this pigment be injected into the blood-vessels of an animal and the animal killed in about ten minutes, it will be found that while the blood exhibits a deep blue color the tissues present their usual colors. But after exposure to the air or to free oxygen the latter also acquire the characteristic blue color. The explanation offered for this fact is that the tissues in their need for oxygen absolutely extract it from the pigment, reducing it to a color- less compound, which, however, on exposure recombines with oxygen and regains the original color. Though free oxygen can not be shown to be present in the tissues, there are many reasons for believing that it is continually passing into them by way of the lymph-stream. Its rapid disappearance would indicate that it is immediately utilized for the production of carbon dioxid (which is improbable on other grounds), or that the tissues possess a capacity for oxygen storage, of placing it in reserve under some combination or other, by which it can be securely retained until required for oxidation purposes. This is rendered probable from the fact that the carbon dioxid evolved at any given moment is not necessarily dependent on the oxygen just absorbed, for if oxygen be withheld from a nutritive fluid which is being artificially circulated through a recently isolated organ, carbon dioxid will continue to be discharged for some time. A muscle, or even a living animal — e. g., a frog^ — placed in an atmosphere of pure nitrogen will remain active and evolve COj for even several hours. Naturally the absorption of oxygen and the discharge of carbon dioxid and the changes of composition which are incident to nutri- tion will be most marked in those tissues characterized by the greatest degree of physiologic activity. Muscle-tissue exhibits these changes to a greater degree than bone. Tissues with intermediate degrees of activity should exhibit corresponding degrees of respiratory change. RESPIRATION. 417 ATMOSPHERIC AIR. O— ISB MM HGfOR £0.85 f.C CO, 0.3 MM HG OR 0.0+ P.C AUVCOLUS vtNOU5 BLOOD ARTER IAL BLOOD Experiment confirms this view. Thus, 100 grams each of muscle, spleen, and broken bone from a recently living animal exposed to the air for twenty-four hours absorbed respectively 50.8 c.c, 27.3 c.c, and 17.2 c.c. of oxygen, while each discharged during the same period 56.8 c.c, 15.4 c.c, and 8.1 c.c. of carbon dioxid respectively. In another series of experiments by a different observer 100 grams of muscle absorbed in three hours 23 c.c. of oxygen, and 100 grams of bone 5 c.c. of oxygen. Both tissues discharged carbon dioxid in amounts proportional to the oxygen absorbed. The same respiratory changes may be more satisfactorily demon- strated by passing blood through the tis- sues of isolated organs and the tissues of re- cently living animals. The analysis of the blood before and after perfusion shows a loss of oxygen and a gain in carbon dioxid. Tension of the Gases in the Tis- sues. — As the pres- ence of free oxygen can not be demon- strated, its tension there must be re- garded as nil. The tension of the carbon dioxid is quite high, though difficult of exact determination. It has been estimated at from 45 to 68 mm. Hg., or from 6 to 9 per cent, of an atmosphere. The variations of tetision or pressure of these two gases in the lungs, in different parts of the vascular apparatus, and in the tissues, and their relations to each other, are shown in Figure 198, expressed in mm. Hg. and percentages of an atmosphere. The figures inserted are those of Strassburger and are of less value as far as the oxygen tension is concerned than those of Fredericq. The Mechanism of the Gaseous Exchange. — In these pressure differences sufficient cause is found for the exchange of the gases. The oxygen pressure in the alveoli being in excess of that in the blood, the gas passes through the thin alveolo-capillary wall into the plasma. As the oxygen pressure in the plasma rises and approximates that in. 27 aX.04- MM HO Oft 2.0 P.C CO.TeNSION z 4-li04> MM HGOR B.4 P.C. O-'TCNSION 0.00 MM HO CO.-TeNAION O-TEHaiON 28.84- MMHO Oft CO^— TEJISION 21.28 MM MO CM Fig. 198. — Diagram showing the Relative Tension OF Oxygen and Cakbon Dioxid in the Lungs, in the Blood, and in the Tissues. 4i8 TEXT-BOOK OF PHYSIOLOGY. the alveoli, a portion of the oxygen combines with the hemoglobin until the latter is almost saturated. The corpuscle is then carried through the arterial system surrounded by oxygen under a definite pressure which is sufficient to keep the absorbed oxygen in union with the hemoglobin. On passing into the systemic capillaries, the blood enters a region in which the oxygen tension in the surrounding tissues is nil. At once the oxygen dissolved in the plasma passes through the capillary wall into the surrounding tissue-spaces. The pressure removed from the corpuscle, a dissociation of the oxygen and of the hemoglobin takes place, after which the oxygen also passes through the capillary wall into the surrounding lymph and so to the tissue-cells where it is stored and utilized. On passing into iiic venous system the dissociation of the oxygen and the hemoglobin is checked by the rise of oxygen pressure in the plasma. On reaching the lungs the oxygen again passes into the blood until the former con- dition is regained. The sojourn of the blood in the capillaries being short, the oxyhemoglobin can part with but a portion of its oxygen, sufficient, however, to satisfy the needs of the tissues. The carbon dioxid pressure in the tissues being in excess of that in the blood, it passes through the capillary wall into the blood, where it exists in the free and combined states. On passing into the pul- monic capillaries the blood enters a region in which the carbon dioxid in the alveoli is less than in the blood. At once a diffusion and dis- sociation of the carbon dioxid takes place through the alveolo-capillary wall until equilibrium is established. This, however, is of very short duration, for the carbon dioxid so eliminated is rapidly removed from the lungs by the respiratory movements. While diffusion, in response to physical and chemic conditions, thus plays a large part in, and is sufficient to account for, the ex- changes of gases, it is possible that the alveolar or respiratory epithe- lium may also play an essential role. It is believed by some in- vestigators that it is active in both the absorption of oxygen and the excretion of carbon dioxid. This view has been suggested as a means of interpreting the results of the experiments of more recent investi- gators, made with a view of determining the tension of the blood gases. It was found by Bohr that the tension of the oxygen in arterial blood was often as high as loi to 144 mm. Hg., and in many instances higher than the tension of the oxygen in the trachea, while the carbon dioxid tension in the trachea was higher than in the blood. Haldane and Smith by a different method found an oxygen tension in the arterial blood of 200 mm. Hg. If these results should prove to be correct, though they are at present subject to considerable criticism and not generally accepted, some other force than diffusion would have to be found to explain the facts. It would then remain to deter- mine in how far the alveolar epithelium could be regarded as an active agent in both absorption and excretion in opposition to pressure. RESPIRATION. 419 THE TOTAL RESPIRATORY EXCHANGE. The total quantities of oxygen absorbed and carbon dioxid dis- charged by a human being in twenty-four hours are measures of the intensity of the respiratory process, and an indication of the extent and character of the chemic changes attending all life phenomena. Their determination and their relation to each other are matters of interest and importance. The quantities which have been obtained by differ- ent observers are the outcome of calculations based on certain groups of data and of experiments made with special forms of apparatus. Thus from the total air breathed daily, estimated from the amounts obtained during a longer or shorter period, of experiments with spiro- metric apparatus, and from the percentage loss of oxygen and gain of carbon dioxid shown by an analysis of the respired air, it can be cal- culated at least approximately what the total amounts of oxygen ab- sorbed and carbon dioxid exhaled must be. If it be assumed that the minimum daily volume of air breathed is,, 16,800 liters and the maximum volume 12,240 liters, and the percentage loss of oxygen is 4.78, then the total volume of oxygen absorbed is 516 liters (735.17 grams) or 585 liters (836.42 granis). By the same method the total carbon dioxid exhaled daily is found to be either 473 liters (931.8 grams) or 526 liters (1036 grams). The direct experiments which have been made with specially devised forms of apparatus, both on human beings and animals, have yielded similar results. With those forms which are adapted for both human beings and animals — Scharling's, Petten- kofer and Voit's — it is only possible, however, to determine the amount of oxygen absorbed. This is done by deducting the loss in weight by the man or animal during the experiment from the combined weights of the carbon dioxid and water discharged. The difference represents the oxygen absorbed. The Pettenkofer-Voit apparatus (Fig. 199) consists essentially of a chamber large enough to admit a man and capable of being made air-tight with the exception of an inlet for air for breathing purposes. The respired air is drawn through a tube and measured by a large meter turned by a water or gas motor. By means of a side tube a fractional quantity of the main column of air is diverted to an absorption apparatus by a small pump. This air first passes into a vessel containing H^SO^, by which the water is collected; then into long tubes containing barium hydroxid, by which the carbon dioxid is absorbed; thence into a small meter, by which its amount is registered. From the amount of water and carbon dioxid thus ob- tained the amounts of both in the total air breathed are calculated. The water and carbon dioxid previously present in the air are simulta- neously determined by a corresponding absorption apparatus and de- ducted from the amounts obtained from the respired air. As the apparatus is traversed constantly by a column of air of normal com- position and the waste products removed as rapidly as discharged. 420 TEXT-BOOK OF PHYSIOLOGY. the experiment can be continued for periods varying from six to twenty-four hours without detriment to the subject of the experiment. With those forms adapted only for animals — Regnault's and OJ u oj ^ Reiset's, or Jolyet and Regnard's — ^it is possible to determine simul- taneously the absorption of oxygen and the discharge of carbon dioxid. As the apparatus employed is completely closed, the carbon dioxid must be removed as soon as discharged and the oxygen re- RESPIRATION. 421 newed as soon as absorbed. The former is accomplished by the as- piratory action of moving bulbs containing an alkali, the latter by a steadily acting pressure on a reservoir of oxygen. This apparatus (Fig. 200) consists essentially of a bell-jar in which the animal is placed. This is brought into connection by tubes, on the one hand, with the oxygen reservoir, and, on the other hand, with the aspiratory , Fig. 200. — Regnault's and Reiset's Respiration Apparatus. A. Bell-jar for the reception of the animal, surrounded by a compartment, B, containing water. N, N, N. Reservoirs of oxygen communicating, on the one hand, with the animal chamber, and, on the other hand, with pressure botties, P, by which the oxygen is driven into the animal chamber. G, G. Aspiratory bulbs containing sodium hydroxid in solution for the absorp- tion of the carbon dioxid. The bulbs are given an alternate up-and-down movement by a falling weight or electric motor. bulbs, kept in motion by some form of motor. The construction of each of these forms of apparatus is so complex, the conduct of an experiment and the final determination of the results so complicated, that a detailed description would be out of place in a work of this character.* Among the results obtained by these and other methods a few are given in the following table: * Both forms of apparatus are in use in the Physiological Laboratory of the Jefferson Medical College and are fully described by Prof. H. C. Chapman in his text-book on Physiology, to which the reader is directed for further information. 422 TEXT-BOOK OF PHYSIOLOGY. Oxygen Absorbed. Observer. Carbon Dioxid Discharged. 746 grams. \1erordt. 876 grams. 700 " Pettenkofer and Voit. 800 " 663 " Speck. 77° " The amounts of oxygen absorbed in Pettenkofer and Voit's experi- ments varied from 594 to 1072 grams; of carbon dioxid exhaled, from 686 to 1285 grams. In all these results it is evident on examination that the volume of oxygen absorbed is always greater than the volume oi carbon dioxid exhaled, or, what amounts to the same thing, the weight of the oxygen absorbed is always greater than the weight of the oxygen entering into the formation of the carbon dioxid exhaled. The reason for this difference between the amounts of oxygen in the inspired -air aiid in the CO^ exhaled is found in the fact that on a mixed diet — one containing fat — a portion of the oxygen is utilized in the oxidation of the hydrogen of the fat with the formation of water. Under such a diet the respiratory quotient is always less than unity, usually 0.907. On a purely carbohydrate diet — one in which there is no surplus hydrogen — all the oxygen will combine with carbon and be returned as carbon dioxid, and hence the respiratory quotient will be unity. The respiratory quotient therefore indicates the extent to which the oxygen absorbed is utilized in oxidizing carbon, on the one hand, and hydrogen, on the other. Since the total oxygen absorbed and carbon dioxid discharged will vary considerably with the size of the animal, it is customary, for purposes of comparison, to reduce all total results to the unit of body- weight (one kilogram) and to the unit of time (one hour). Respiratory Activity.— The activity or the intensity of the respiratory process may be measured either by the oxygen absorbed or the carbon dioxid discharged. But as the carbon dioxid is more easily estimated than the oxygen, it is usually taken as the index of the activity, though there are reasons for believing that it would be more accurately indicated or represented by the oxygen. Whatever factor may be accepted as the measure, it is certain that the respiratory activity varies in different tissues in accordance with their functional activities, being least in bones and greatest in muscles. This is shown by the relative amounts of oxygen absorbed and carbon dioxid discharged by equal amounts of each of these and other tissues in twenty-four hours, as shown in the following table: '■ QUANTITY OF O AND CO2 ABSORBED AND EXHALED DURING TWENTY-FOUR HOURS, IN CUBIC CENTIMETERS. By TOO Grams of: Oxygen Absorbed. Carbonic Acid Exhaled. Muscle, 50.8 c.c. 56.8 c.c. Brain, 45.8 " 42.8 Kidneys, 37.0 " 15.6 Spleen, 27.3 " 15.4 Testicles, 18.3 " 27.5 Pounded bones 17.2 " 8.1 RESPIRATION. 423 The total respiratory change therefore of the body as a whole is the resultant of the respiratory changes of its individual organs and tissues, and is conditioned by all influences which retard or hasten their activity. Among these influences the more important are the following: Muscle Activity. — As the muscles constitute a large part of the body, about 40 per cent., and as muscle-tissue absorbs and discharges relatively large quantities of oxygen and carbon dioxid, it is readily apparent that an increase in their activity would be followed or attended by an increase in the respiratory exchange". In passing from a con- dition of body repose to one of marked activity there ought to be an increase in the amount of oxygen absorbed and CO^ discharged. Pettenkofer and Voit found that a man in repose w'ho absorbed daily 807.8 grams of oxygen and discharged 930 grams CO^ absorbed during work 1006 grams of oxygen and discharged 113 7 grams of CO^. Edward Smith, who estimated only the CO^, found that a man in repose who discharged carbon dioxid at the rate of 161. 6 c.c. per minute increased the amount while walking at the rate of two and three miles an hour to 569 c.c. and 851 c.c. respectively. Similar results have been obtained by other investigators. Digestive Activity. — The activity of the alimentary canal, involving contraction of its muscle coat through its entire length as well as secretion of its related glands called forth by the ingestion of food, materially influences the absorption of oxygen and discharge of carbon dioxid, independent of the increase due to the oxidation of food materials after absorption. It was found that in a fasting man a dose of sodium sulphate increased the absorption of oxygen as much as 17 per cent, and the discharge of CO^ 24 per cent. (Lowy). It is difficult to determine how much of the increase after a meal is therefore due to food oxidation and how much to functional activity of the canal itself. The consumption of nitrogenized meals, however, has a greater effect than non-nitrogenized meals. Temperature. — A rise in temperature of the surrounding air has as an effect a diminution in the amounts of oxygen consumed and carbon dioxid discharged. A fall in temperature has the opposite effect. Thus a cat at a temperature of — 3.2° C. consumed during a period of six hours 21.39 grams of oxygen and discharged 22 grams of carbon dioxid, while at a temperature of 29.6° C. the correspond- ing amounts for the same period of time were for oxygen 13.9 grams and for carbon dioxid 13.12 grams. Lavoisier and Sequin, having reference only to the oxygen, found that a man at a temperature of 15° C. consumed 38.31 grams of oxygen, while at a temperature of 32.8° C. the corresponding amount was but 35 grams. Similar results have been obtained by other observers with different animals. The explanation of these facts is to be found in the increased activity of all physiologic mechanisms coincident with a fall, and in the de- creased activity, coincident with a rise in temperature. The lower 424 TEXT-BOOK OF PHYSIOLOGY. temperatures act as a stimulus to the peripheral terminations of the nerve system, bringing about reflexly increased activity of the body at large. The muscles especially are not only reflexly but volitionally excited to greater activity. This leads naturally to an increase in the consumption of oxygen and in the production of carbon dioxid and in the evolution of heat. In cold-blooded animals the respiratory exchange is influenced in a manner the reverse of that observed in warm-blooded animals. With a rise of external temperature and a corresponding rise of body- temperature the discharge 'of carbon dioxid steadily increases. Thus a frog in an atmosphere at o° C. with a body-temperature of i° C. discharged per kilogram per hour 4.31 c.c. of carbon dioxid; in an atmosphere of 35° C. with a body-temperature of 34° C. there was discharged 325 c.c. per kilo per hour. Intermediate temperatures were attended by corresponding increases in the amounts of CO^ discharged. The reason for this difference in the two classes of animals is probably to be found in the want, in the cold-blooded animals, of a self-adjusting heat-regulating mechanism. Age. — In early youth, as a result partly of the more pronounced activity of the nutritive energies and partly of a cutaneous surface relatively greater, as compared with the mass of the body, than in adult life, the absorption of oxygen and the discharge of carbon dioxid are greater both absolutely and relatively. Thus, in a boy of nine and a half years with a weight of 22 kilograms it was found that in twenty-four hours there was a discharge of carbon dioxid amounting to 488 grams, or 0.92 gram per kilo per hour, and in man with a weight of 65.5 kilograms there was a discharge of 804.72 grams, or 0.51 gram per kilo per hour. MODIFICATIONS OF THE RESPIRATORY RHYTHM. The character of the respiratory movements is materially modified by a change in the quantitative and qualitative composition of the air and blood as well as by changes of a pathologic nature of the res- piratory apparatus itself. Eupnea. — So long as the air retains its normal composition and the respiratory mechanism its structural integrity, so long do the respiratory movements exhibit a normal rhythm and frequency. To the condition of easy tranquil breathing the term eupnea is given. In this condition the percentages of oxygen and carbon dioxid in the blood are such as to favor at least the rhythmic discharge of nerve impulses to the respiratory muscles, of sufficient energy and frequency for the maintenance of normal respiration. Hyperpnea. — -The normal rate of the respiratory movements is increased by a rise in body-temperature, by active exercise, and by emotional states. Whatever the cause, the increase in rate and prob- ably in depth is termed hyperpnea. RESPIRATION. 425 Febrile states characterized by a rise in the temperature of the blood increase considerably the respiratory activity. This is due in all probability to a warming of the respiratory center, in consequence of which its excitability is heightened; for surrounding the carotid arteries with warm tubes and heating the blood on its way to the medulla has the same effect. It is also possible, however, that the high temperature of febrile conditions may interfere with the absorb- ing power of hemoglobin, and thus by diminishing the quantity of oxygen absorbed lead to more frequent respirations. To the hy- perpnea induced by heat the term ther mo- polypnea is frequently given. Muscle activity, especially if it is violent and indulged in by those unaccustomed to exercise, is generally followed by increased rate and depth of breathing, and not infrequently it is attended with such extreme difficulty that the condition approximates that of dyspnea. This condition is attributed to the production and discharge into the blood of unknown waste products which act as irritants to the respira- tory center and thus increase its activity. As they apparently can not be isolated and their chemic nature determined, it is presumable that they are speedily oxidized or reduced in the blood. Experiment has shown that the increase of carbon dioxid does not account for the increased rate of breathing. Emotional states temporarily increase respiratory activity. With their disappearance the normal condition returns. Apnea. — If the respiratory movements are voluntarily increased in frequency and depth for a short time it will be found on cessation that for a variable length of time the respiratory mechanism remains in a condition of complete rest or inaction. To this complete cessation of activity the term apnea is given. The same phenomenon is wit nessed in animals when the lungs are rapidly inflated with air by means of bellows. At one time this was attributed to an excess of oxygen in the blood (the result of the forced ventilation of the lungs), complete saturation of the plasma and hemoglobin, in consequence of which the respiratory center remained inactive. This has been dis- proved, however, by modern chemic analyses of the blood. The condition is now attributed : 1. To increased ventilation of the lungs and an increased percentage of oxygen in the alveoli, as a result of which the normal percentage of oxygen in the blood can be maintained for a longer period than usual. 2. To a stimulation of the peripheral terminations of the pneumo- gastric nerve whereby the discharge of nerve impulses from the respiratory center is temporarily inhibited. Division of the pneumogastric nerve prevents the development of the apneic condition. Dyspnea. — Excessive and laborious respiratory movements con- stitute a condition known as dyspnea. Movements of this character 426 TEXT-BOOK OF PHYSIOLOGY. indicate that the blood is deficient in oxygen or overcharged with carbon dioxid. In either case the excitabihty of the respiratory center is abnormally heightened. These conditions of the blood may be caused: (i) By all those pathologic conditions of the respiratory apparatus which limit the free entrance of oxygen into and the free exit of carbon dioxid from the blood; (2) by those alterations in the composition of the air and subsequently in the blood which arise when the individual is confined in a space of moderate size with imperfect ventilation. The want of oxygen in the blood gives rise to more forcible inspirations; the presence of CO^ in excess, to more forcible expirations — showing that the former condition affects the inspiratory portion of the respiratory center, the latter condition the expiratory portion. A deficiency in the amount or quality of the hemoglobin is usually attended with dyspnea. Asphyxia. — If the state of the blood observed in dyspnea be ex- aggerated — that is, if the decrease in the percentage of oxygen and the increase in the percentage of carbon dioxid become more marked — the respiratory movements become more laborious. A continuance of this changed composition of the blood eventuates in death. Before this occurs the individual exhibits a succession of phenomena, to the totality of which the term asphyocia is given. Asphyxia may be caused: (i) By a sudden interference with the entrance of oxygen into and the exit of carbon dioxid from the blood, as in drowning, occlusion of the trachea from any cause, double pneumothorax, etc. (2) By confinement in a small space the air of which speedily undergoes a loss of oxygen and an accumulation of carbon dioxid. In the first instance death may occur in a few minutes; in the second instance it may be postponed several hours or more, the time varying with the size of the space. The succession of phenomena presented by an individual in the asphyxiated condition is as follows: Increased rate and depth of the respiratory movements, passing rapidly from hyperpnea to dyspnea, with an active contraction of all the muscles concerned in respira- tion, ordinary and extraordinary; a blue cyanosed condition of the face from the rapid accumulation of carbon dioxid and disappearance of the oxygen of the blood; a diminution in the depth of inspiration and an increase in the force and extent of expiration, followed by general convulsions; collapse, characterized by unconsciousness, loss of the reflexes, relaxation of the muscles, a weak action of the heart, a disappearance of the pulse and death. As shown by observation of the circulatory apparatus in artificially induced asphyxia, there is primarily an increase in the activity of the heart, soon followed by retardation; a rise of blood-pressure in the early stages and a fall to zero after collapse has set in. The retardation and final cessation of the heart, as well as the rise of the blood-pressure, are to be attributed to stimulation of the cardio-inhibitory and vaso-motor centers from the accumulation of the carbon dioxid. With the exhaustion of the RESPIRATION. 427 nerve-centers, there is a general relaxation of the muscles and a fall of the pressure. The.Cheyne-Stokes Respiration. — A modification of the respir- atory movements characterized by periods of rest alternating with periods of activity was described in 1818 and in 1854 by the two writers whose name it bears. The periods of rest vary in duration from twenty to thirty seconds; the periods of activity from thirty to sixty seconds and may include from twenty to thirty respiratory movements. Each period of rest of the respiratory mechanism is closed by the appearance of a slight shallow respiratory movement, which is imme- diately followed by a second, slightly deeper, and this in turn by a Fig. 201. — Tracing Showing the Cheyne-Stokes Form of Respiration. — {Hill.) third, a fourth, a fifth, and so on, each becoming deeper than the preceding until a certain maximum is reached, after which, each succeeding movement gradually diminishes in depth until finally the movement becomes imperceptible and a new period of rest super- venes. A graphic representation of the Cheyne-Stokes type of respira- tion is shown in Fig. 201. This type of respiration is frequently an accompaniment of certain pathologic conditions, e. g., uremic states, cerebral hemorrhage, heart diseases, arteriosclerosis, etc., though no satisfactory explanation of it has yet been presented. A similar though far less marked periodicity in the respiratory movements is frequently observed during normal sleep, especially in children. A periodicity can also be developed by dividing transversely the medulla oblongata just above the calamus scriptorius which either injures the respiratory center or removes from it some cerebral influence. THE NERVE MECHANISM OF RESPIRATION. The nerve mechanism by which the respiratory muscles are ex- cited to action is extremely complex and involves the action of both afferent and efferent nerves and their related nerve-centers in the cen- tral nerve system. For the free introduction of air into the lungs it is 428 TEXT-BOOK OF PHYSIOLOGY. essential that the nasal and laryngeal passages and the cavity of the thorax be simultaneously enlarged. The muscles by which these results are accomplished have already been mentioned and described. Their simultaneous and coordinate contraction implies the coordinate activity of motor nerves and their centers; thus, the nasal and laryn- geal muscles (the dilatatoi: naris and the posterior crico-arytenoid) involve the activity of the facial and inferior laryngeal nerves re- spectively, the centers of origin of which lie in the gray matter beneath the floor of the fourth ventricle; the diaphragm and intercostal muscles involve respectively the activity of the phrenic and intercostal nerves, the centers of origin of which lie in the anterior horn of the gray matter of the spinal cord at a level, for the phrenic, of the fourth, fifth, and sixth cervical nerves, and for the intercostals at the level of the upper thoracic nerves. Division of any one of these nerves is followed by paralysis of its related muscle. Inspiratory Center. — The coordinate contraction of the inspira- tory muscles implies a practically simultaneous discharge of nerve impulses from each of the foregoing nerve-centers, accurately graduated in intensity in accordance with inspiratory needs. This has been supposed to necessitate the existence in the central nerve system of a single group of nerve-cells from which nerve impulses are rhythmically discharged and conducted to the previously mentioned nerve-centers in the medulla oblongata and spinal cord, by which they are in turn excited to activity. To this group of cells the term "inspiratory center" has been given. For the free exit of air from the lungs it is not only essential that the air-passages be open, but that the air in the lungs be compressed until its pressure rises above that of the atmosphere. This is accom- plished by the recoil of the elastic tissue of the lungs and thorax, the return of the displaced abdominal organs aided by atmospheric pres^ sure, and the contraction of the expiratory muscles. In how far muscle action is necessary for expiratory purposes will depend on the resistance offered to the outflow of air and on the degree of efficiency of the elastic forces. Expiratory Center. — The simultaneous and coordinate activity of the expiratory muscles in impeded expirations also involves the action of motor nerves and nerve-centers. The simultaneous and coordinate^ discharge of nerve impulses, also graduated in intensity for expiratory needs, apparently implies the existence in the central nerve system of a single center from which nerve impulses are rhyth- mically discharged which excite and coordinate the lower nerve-centers. To this group of cells the term "expiratory center" has been given. The two centers taken together constitute the so-called "respiratory center." The anatomic existence, however, of a definite group of cells which initiates the respiratory movements has not as yet been demonstrated. Nevertheless there is in the dorsal portion of the medulla oblongata. RESPIRATION. 429 at the level of the sensory end-nucleus of the vagus nerve, a region the sudden destruction of which on one side is followed by a cessation of respiratory movements on the corresponding side, though they con- tinue on the opposite side, a fact which indicates that the area, though acting as a unit, is bilateral. The bilateral character of the area is also shown by the continuance of the respiratory movements on both sides after longitudinal division of the medulla. Destruction of the entire region is followed by a complete cessation of respiratory activity and death of the animal. For this reason the term "nceud vital" was applied to it. In this area the respiratory center was located. It has, however, been shown by Gad that if this area be gradually destroyed by cauterization the respiratory movements do not cease, but continue until the cauterization has reached a point far forward in the f ormatio reticularis, in which the respiratory center was assumed to lie. Though its existence has not been anatomically determined beyond question, it is permissible to speak of the central mechanism as a "center" located in the medulla oblongata. The activity of the inspiratory center has long been described as automatic or autochthonic (Gad) in character, expressive of the idea that the rhythmic discharge of nerve impulses is conditioned by the composition of the blood or lymph by which it is surrounded, though susceptible to the action of nerve impulses reflected to it through afferent nerves. Thus so long as the blood retains its normal com- position the inspiratory movements are normal. If, however, the blood becomes richer in oxygen and relatively poorer in carbon dioxid, the rate of discharge of nerve impulses and hence the inspiratory movements, di- minish until the condition of apnea results. If, on the contrary, the blood becomes poorer in oxygen and richer in carbon dioxid, the reverse condition obtains : viz., an increased rate of discharge of nerve impulses, increased frequency of inspiration, hyperpnea, and dyspnea. This view of the automaticity of the inspiratory center is supported by the fact that the center continues more or less active after division of the medulla oblongata just posterior to the corpora quadrigemina, of the spinal cord at the level of the seventh cervical nerve, of the vagus nerves and the posterior roots of the cervical nerves, channels through which the majority of nerve impulses reach the center. Whether the center would continue active after division of all remaining afferent nerves, e. g., trigeminal and glossopharyngeal, it is impossible to state, since from the nature of the case such an experiment would be most difficult to perform. The first inspiration after birth is supposed to be due to the direct stimulation of the respiratory center by the increase in the carbon dioxid present in the blood, though it may be aided by the cooling of the skin due to vaporization of the amniotic fluid. Reflex Stimulation of the Inspiratory Center. — Whether the inspiratory center is automatic in character or not, it may be 43° TEXT-BOOK OF PHYSIOLOGY. influenced directly by nerve impulses descending from the brain in consequence of volitional acts or emotional states, and indirectly by nerve impulses reflected to it from the general periphery through various afferent nerves, in consequence of agencies acting on their peripheral termination: e. g., cold applied to the skin, irritaing gases to the nasal and bronchial mucous membrane, distention and collapse of the pulmonary alveoli. Of all afferent nerves, the vagus appears to be the most influential in maintaining the rhythmic discharge of nerve impulses from the inspira- tory center. (Fig. 202). Thus, if while the animalis breathing regularly and quietly both vagi are cut, the respiratory movements become much slower, falling perhaps to one-third their original number per minute. If the central end of the divided vagus be stimulated with weak faradic currents, the respiratory movements are increased in frequency and their depth diminished until the normal rate is restored. With the cessation of the stimulation the former condition at once returns. This would indicate that in the physiologic state afferent impulses are ascending the vagus fibers which influence the rate of discharge from the inspiratory center. If, however, the stimulation is increased in strength, the inspiratory movement gradually so exceeds the expi- ratory that the muscles pass into the tetanic state and the chest-walls come to rest in the condition of forced inspiration. The vagus appa- rently contains fibers which augment the inspiratory movement. If, on the other hand, the central end of the divided superior laryngeal nerve be stimulated with faradic currents, the opposite effect is pro- duced: viz., an excess of the expiratory over the inspiratory move- ment until the chest-walls come to rest in the condition of passive expiration. The superior laryngeal nerve apparently contains fibers which inhibit the inspiratory movement. The same result, an expiratory standstill, not infrequently follows strong stimulation of the divided vagus, and always after the admin- istration of large doses of chloral. The vagus apparently contains two classes of nerve-fibers, one ^ of which when stimulated increases the extent of the inspiratory move- ment until the thorax comes to a standstill in the state of forced in- spiration; the other of which when stimulated increases the extent of the expiratory movement at the expense of the inspiratory until the thorax comes to a standstill in a state of deep expiration. The stimulus adequate to the excitation of the nerve-fibers in the physiologic condition was formerly believed to be the chemic action of carbon dioxid; it is now believed to be a mechanic action, the result of the alternate distention and collapse of the walls of the pulmonary alveoli. Thus, it has been shown by Head that if the lungs are actively inflated (positive ventilation) there will be produced an inhibition of the inspiratory and an augmentation of the expiratory movement until the inspiratory muscles are completely relaxed as indicated by the relaxation of the diaphragm, the movements of which are simulta- RESPIRATION. 431 neously recorded (Fig. 203;, a result similar in all respects to that produced by stimulation of the superior laryngeal nerve. On the other hand, if the lungs are collapsed by the artificial withdrawal of I air (negative ventilation) there will be produced an augmentation med. ob. Fic. 202. — Diagram Showing the Relation of the Pulmonaky Fibers oe the Vagus to the Inspiratory Center and the Connections oe the Latter with the Phrenic and Intercostal Nerve Centers and their Related Muscles. (G. Bach- man^ med. oh. Medulla oblongata, sp.c. Spinal cord, p.v.r. Pulmonary vagus nerve, ex- citator and inhibitor, in sp.c. Inspiratory center, phr.c. Phrenic nerve centers, phr.n. Phrenic nerve int.n.c. Intercostal nerve centers, int.c.n. Intercostal nerves, ext.int.c.m. External intercostal muscles. of the inspiratory and an inhibition of the expiratory movements until the inspiratory muscles are in a condition of tetanic contraction as in- dicated by the contraction of the diaphragm (Fig. 204) and by the state of the thorax which is that characteristic of extreme inspiration, 432 TEXT-BOOK- OF PHYSIOLOGY. Positive ventilation. a result similar in all respects to that produced by moderate stimu- lation of the central end of the divided vagus. A satisfactory explanation of the action of the respiratory mechanism is very difficult to present. Theories vary in accordance with the estimate of an investigator as to the degree of automaticity of the inspiratory center, of the effects of vagus stimulation and as to the extent to which the ex- piratory center is in- volved with the activity of the inspiratory center either simultaneously or successively. If it is assumed that the inspiratory center is automatic and in a state of continuous excitation the result of the action of carbon dioxid in the blood circulating around it, then it is only necessary to assume the existence, in the trunk of the vagus of but one set of nerve-fibers, viz., inhibitor fibers, the central terminations of which arborize around the inspiratory center and the function of which is to check or inhibit the action of the inspiratory center and thus permit of an expiratory movement. The inhibitor fibers are supposed to be stimulated peripherally by the expansion of the lungs. With the recoil of the lungs the inhibitor effect gradually dies away, while the inherent excitation of the Fig. 203. — Positive Ventilation {Head). Under the influence of positive ventilation, the inspiratory contractions of the diaphragm become less and less till they disappear completely. Seconds. Fig. 204. — Negative Ventilation (Head). At a negative ventilation was com- menced. The expiratory relaxation of the diaphragm is seen to become more and more incomplete, until it finally enters into continued contraction. inspiratory center again returns, to be followed by another discharge of nerve impulses and a new inspiratory movement, which will in turn be again inhibited as the inhibitor fibers are stimulated by the ex- panding lung. This explanation is in accordance with the results RESPIRATION. 433 which follow stimulation of the superior laryngeal nerve or the trunk of the vagus with strong induced electric currents. If it is assumed, on the contrary, that the inspiratory center is not in a state of constant excitation leading to a continuous discharge of nerve impulses, but requires the arrival of a stimulus to call forth its normal activity, then this theory does not suffice, inasmuch as it leaves out of consideration the presence of nerve-fibers in the vagus which increase or augment the activity of the inspiratory center; and that such fibers are present is apparently indicated by the effects of stimulation of the central end of the vagus nerve with weak and moderately strong induced electric currents and from the experiments of Hering and Breuer, and later of Head. These observers assume, therefore, that in addition to the inhibitor fibers there are also present in the vagus, excitator fibers, the central terminations of which are in relation with the inspiratory center also (Fig. 202; ; and just as the inhib- itor fibers are stimulated by the expansion of the lungs so the excitator fibers are stimulated in turn by the recoil of the lungs. The nerve im- pulses thus developed ascend to the inspiratory center, excite it, and call forth a new inspiration sooner than it would otherwise take place. Ac- cording to this view the respiratory mechanism is self-regulative and maintained by the alternate expansion and recoil of the lungs. Many experimenters, however, find difficulty in accepting the view that the recoil of the lungs should stimulate nerve endings and hence this theory has not received general acceptance. Another explanation which is satisfactory in many respects has been presented by Meltzer. This investigator asserts also the existence in the trunk of the vagus the two classes of nerve-fibers, the inhibitor and the excitator; but that for some reason they do not respond to stimulation at the same time as shown by the effects which follow; the inhibitor fibers respond first and the excitator fibers somewhat later. Therefore when they are stimulated simultaneously the primary effect is an inhibition of the inspiratory center followed Ipy an expiratory movement. The secondary effect is a stimulation of the inspiratory center followed by a new inspiratory movement. In this view ex- pansion of the lungs stimulates both the inhibitor and the excitator fibers, but during the expansion and for a short time after, the effect of the inhibitor stimulation, viz., cessation of inspiration and the advent of expiration, alone manifests itself. With the cessation of expiration, the inhibitor stimulation dies away and the late effect or the long after-effect of the excitator stimulation, viz., a new inspiration, manifests itself. This author assumes the surface of the lung to be the peripheral organ of the respiratory reflexes. When it is assumed that both inspiratory and expiratory centers cooperate in a respiratory movement, as they do in labored respiration either simultaneously or successively, the difficulties of the problem are manifestly much greater. In this case it may be supposed that afferent impulses, developed during tlje expansion of the lung, inhibit the 434 TEXT-BOOK OF PHYSIOLOGY. inspiratory while augmenting the expiratory center, and that impulses developed during the recoil of the lungs inhibit the expiratory while stimulating the inspiratory center. THE EFFECT OF THE RESPIRATORY MOVEMENTS ON THE FLOW OF BLOOD THROUGH THE INTRA-THORACIC VESSELS AND ON THE ARTERIAL PRESSURE. 1. On the Intra-thoracic Vessels. — The forces which cause the air to flow into and out of the lungs will at the same time and in the same way cause the blood of the systemic vessels to flow into, through, and out of the intra-thoracic vessels. From the tendency of the pul- monary elastic tissue to recoil, the blood-vessels in the thorax at the end of an expiration sustain a positive pressure, about six millimeters of mercury less, than that in the lungs, or, in other words, a pressure negative to that of the atmosphere by six millimeters. As a result the blood in the systemic vessels standing under atmospheric pressure will flow steadily toward the intra-thoracic veins, the venae cavae, and the right side of the heart; i. e., from a point of high to a point of low pressure. Since during inspiration, with the increasing tendency to pulmonary recoil, the positive pressure on the veins and heart may diminish by thirty millimeters of mercury, the blood will flow in in- creased volume from the systemic to the intra-thoracic vessels. The right heart, being more generally filled with blood, will discharge a larger volume with each contraction into the pulmonary artery. Coincident with these effects a similar effect is produced in the arterioles and capillaries of the pulmonary alveoli. Situated between the two elastic layers of the alveolar wall, embedded in a meshwork of connective tissue, the pressure to which they are subjected at the end of an expiration will also be a few millimeters less than that of the intra-pulmonary air; and at the end of an inspiration it will be considerably less. With the inspiration there will occur a dilatation of the vessels, a larger flow of blood through them and into the pul- monary veins. The left heart, being in consequence more generously filled with blood, will discharge a larger volume into the aorta at each contraction. During expiration the flow of blood through the intra- thoracic vessels will be diminished for the reverse reasons. 2. On the Arterial Pressure. — An examination of a tracing of the arterial pressure will show that it is characterized by small un- dulations due to the cardiac beat and large undulations due to the respiratory movements, inspiration being accompanied by a rise, expiration by a fall of pressure. These results are readily accounted for by the difference in the volume of blood discharged by the left heart into the aorta during the time of the two movements. If a tracing of the respiratory movements and of the blood-pressure be taken simultaneously, it will be found that the rise of pressure does not exactly correspond with inspiration, nor the fall of pressure with RESPIRATION. 435 expiration; for a certain period after the beginning of an inspiration the pressure continues to fall, and for a certain period after the begin- ning of an expiration the pressure continues to rise. During the remainder of the period, however, inspiration is attended by a rise, expiration by a fall of pressure. The explanation of these results lies in the fact that at the beginning of the inspiration, when the vessels dilate, the blood-flow momentarily slows; the left heart con- tinuing to discharge small volumes into the aorta, the pressure con- tinues to fall. So soon as the left heart begins to be better filled, the pressure at once begins to rise. At the end of an expiration the flow of blood into the left heart continues and the pressure rises, but with the return of the intra-thoracic pressure the vessels diminish in caliber, the volume of blood transmitted by them becomes less, the output of the left heart declines, and the pressure falls. The Traube-Hering Waves. — ^Under certain experimental con- ditions the arterial blood-pressure tracing exhibits, in addition to the usual respiratory variations, certain longer rhythmic variations more or less wave-like in character, which are known as Traube-Hering waves. They can be developed on a blood-pressure tracing by in- jecting magnesium sulphate or morphine into the circulation, by tying the cerebral arteries, etc. These waves indicate a periodic contrac- tion and dilatation of the blood-vessels the result of a stimulation of the vaso-motor centers. CHAPTER XVI. ANIMAL HEAT. The chemic changes which take place in the tissues and organs of the living body and which underlie all manifestations of life are attended by the evolution of heat. In consequence of this each ani- mal acquires a certain body-temperature. In man, as well as in other mammals and in birds, the chemic changes are extremely active and the evolution of heat very great. Through some special heat-regulating mechanism, by which heat- production and heat-dissipation are kept in equilibrium, these animals have acquired and maintain within limits a constant temperature which is independent of and generally above that of the surrounding atmos- phere. As the temperature of these animals is high and perceptible to the sense of touch, they were originally designated "warm-blooded " animals. As this temperature is constant notwithstanding the great variations in external temperature during the summer and winter seasons, they are more appropriately termed constant-temperatured or homoiothermous animals. The intensity of the body-temperature determined by the insertion of a thermometer in the rectum varies in different classes of mammals from 37.2" C. to 40° C. The causes of this variation are doubtless connected with peculiarities of organiza- tion. In birds the rectal temperature is usually higher, varying from 40.9° C. in the pigeon to 44° C. in the titmouse and the swift. In reptiles, amphibians, and fish chemic changes, as a rule, are not very active and heat-production relatively slight. As they are devoid of a sufficiently active heat-regulating mechanism, the tempera- ture of the body is largely dependent on that of the medium in which they live, though it is always one or more degrees above it. In winter the body-temperature of frogs, for example, may decline as low as 8.9° C, the temperature of the surrounding medium being 6.7° C. When subjected to temperatures below zero, the temperature of the body may fall below the freezing-point also, when the lymph and fluids of the body become ice. Though apparently dead, the gradual eleva- tion of the temperature restores their vitality. In summer-time, on the contrary, the body-temperature may attain to 38° C. Similar varia- tions have been observed in other animals. As the temperature of these animals is low and perceptibly below that of our own bodies, they were originally termed "cold-blooded" animals; as their temper- ature is inconstant, varying with the temperature of the surrounding medium, they are more appropriately termed "variable temperatured " or poikilo-thermous animals. 436 ANIMAL HEAT. 437 THE TEMPERATURE OF THE HUMAN BODY. The determination of the temperature of the human body under the changing conditions of life is a matter of the greatest physiologic and clinical interest. The temperature of the superficial portions of the body may be obtained by the introduction of a thermometer into the mouth, the rectum, the vagina, or the axilla. As a result of many observations it has been found that the temperature of the rectum is, on the average, 37.2° C; that of the mouth, 36.8° C; that of the axilla, 36.9° C. Owing to radiation and conduction the surface tem- perature is lower than that of either the mouth or rectum, and varies to a slight extent in different regions of the body: e. g., at a room- temperature of 20° C. the skin of the pectoral region has a temperature of 34.7°; that of the cheek, 34.4°; that of the calf, 33.6°; that of the tip of the ear, only 28.8°, etc. In the interior of the body, especially in organs in which oxidation takes place rapidly, and which at the same time are protected by their anatomic surroundings from rapid radiation, the temperature is higher than that observed in the rectum. From an investigation of the temperature of the blood as it emerges from the liver, the mus- cles, the brain, alimentary canal, etc., it is evident that these organs have a higher temperature than the rectum. As the chemic changes underlying physiologic activity vary in intensity and extent in different regions of the body, there would be marked variations in their temperature were it not that the blood, having a large capacity for heat-absorption, distributes the heat al- most uniformly to all portions of the body, so that at a short distance beneath the surface the temperature does not vary but within a few degrees. In the dog the temperature of the blood in the aorta and in its principal branches is approximately 38.3° C. In passing through the systemic capillaries the temperature falls from radiation and con- duction to surface temperature, to again rise as the venous blood ap- proaches the deeper regions of the body. In the neighborhood of the renal veins and in the superior vena cava the temperature is again that of the aorta. In the portal vein the temperature rises to 40.2° C; in the hepatic vein, to 40.6° C. In the right ventricle, owing to the ad- mixture of blood from different localities having different temperatures, the temperature falls to 38.2° to 40.4°. In passing through the pul- monary capillaries the temperature of the blood again falls, so that in the left ventricle it will register from 38° C. to 40.2° C. There is thus usually a difference between the two sides of the heart of about 0.2° C. Variations ia the Mean Temperature. — The mean tempera- ture of the human body for twenty-four hours, which for the mouth and the rectum may be accepted at 36.7° C. and 37.2° C. respectively, is subject to variations from a variety of circumstances-, such as age, periods of the day, food, exercise, etc. 438 TEXT-BOOK OF PHYSIOLOGY. Age. — At birth the temperature of the infant is slightly higher than that of the mother, registering in the rectum about 37.5° C. In a few hours it rapidly declines to about 36.5°, to be followed in the course of twenty-four hours by a rise to the normal or slightly beyond. During childhood tlie temperature gradually approximates that of the adult. In old age the temperature rises, as a rule, and attains a maximum at eighty years of 37.4° C. Periods of the Day. — The observations of Jiirgensen show that there is a diurnal variation in the mean temperature of from 0.5° C. to 1.5° C, the maximum occurring late in the afternoon, from 5 to 7 o'clock, the minimum early in the morning, from 4 tp 7 o'clock. This diurnal variation in the mean temperature is related to corresponding variations in many other physiologic processes, and its causes are to be found in the ordinary habits of life as regards the time of meals, periods of exercise, sleep, etc. Food and Drink. — The ingestion of a hearty meal increases the temperature but slightly — not more than 0.5° C. Insufl&ciency of food lowers the temperature; total withdrawal of food, as in starva- tion, is followed by a steady though slight decline, until just preceding the death of the animal, when it falls abruptly to from 6° to 8° C. Cold drinks lower, hot drinks raise the temperature. Food and drinks, however, only temporarily change the mean temperature, and after a short period equilibrium is restored through the activity of the heat-regu- lating mechanism. Alcoholic drinks lower the temperature about 0.5° C. In large toxic doses in persons unaccustomed to their use the temperature may be lowered several degrees. This is attributed not to a diminution in heat-production, but rather to an increase in heat-dissipation (Reichert) from, increased action of the heart, dilata- tion of the blood-vessels of the skin, and increased activity of the sweat- glands. Exercise. — The temperature may be raised by active muscular exercise from 1° to 1.5° C. as a result of increased activity in chemic changes in the muscles themselves. A rise beyond this point is pre- vented by the increased activity of the circulatory apparatus, the re- moval of the heat to the surface, and its rapid radiation. External Temperature. — The external temperature influences but slightly the mean temperature of the human body. In the tropic as well as in the arctic regions, notwithstanding the change in the tem- perature of the air, that of the body remains almost constant. The same is true for the seasonal variations in the temperature of the tem- perate regions. THE SOURCE AND TOTAL QUANTITY OF HEAT PRODUCED. The Source of Heat.- — The immediate source of the body-heat is to be found in the chemic changes which take place in all the tissues and organs of the body. Each contraction of a muscle, each act of ANIMAL HEAT. 439 secretion, each exhibition of nerve-force, is accompanied by the evo- lution of heat. The chemic changes are for the most part of the nature of oxidations, the union of oxygen with the elements, carbon and hy- drogen, of the food principles either before or after they have become constituents of the tissues. The ultimate source of the body-heat is the latent or potential energy in the food principles, which was absorbed from the sun's energy and stored up during the growth of the vegetable world. In the metabolism of the animal body the food principles are again reduced through oxidation, directly or indirectly, to relatively simple bodies, such as urea, carbon dioxid, and water, with a liberation of a large portion of their contained energy which manifests itself as heat and mechanic motion. The Total Quantity. — The total quantity of heat liberated in the body daily may be approximately determined in at least two ways : (i) By determining experimentally the heat values of different food principles by direct oxidation; (2) by collecting and measuring with a suitable apparatus, a calorimeter, the heat evolved by the oxidation of the food within, and dissipated from, the body daily. I. Direct Oxidation. — The amount of heat which any given food principle will yield can be determined by burning a definite amount — e. g., I gram — to carbon dioxid and water and ascertaining the extent to which the heat thus liberated will raise the temperature of a given amount of water, e. g., i kilogram. The amount of heat may be expressed in gram or kilogram degrees or calories; a gram calorie or kilogram calorie being the amount of heat required to raise the temperature of a gram or a kilogram (1000 grams) of water 1° C. The apparatus employed for this purpose is termed a calorimeter, which consists essentially of a closed chamber, in which the oxidation takes place, surrounded by a water-jacket. The rise in temperature of the water indicates the amount of heat produced. The results obtained by investigators employing different calor- imeters and different food principles of the same class vary, though within narrow limits: e. g., 1 gram casein yields 5.867 kilogram calor- ies; I gram of lean beef, 5,656; i gram of fat, 9.353, 9.423, 9.686 calor- ies; I gram of starch or sugar, 4. 116, 4.182, 4.479, etc., calories. These results are, however physical values, and indicate the quantity of heat such quantities of foods give rise to when completely oxidized to carbonic acid and water. In the human body the carbohydrates and the fats, with the exception of the small portion which escapes digestion, are reduced to carbon dioxid and water, and hence prac- tically liberate as much heat as they do when oxidized outside the body. The proteids, however, are only reduced to the stage of urea. As this compound is capable of further reduction in the calorimeter to carbon dioxid and water, with the liberation of heat, the quantity of heat it contains must therefore be deducted from the physical heat value of the proteid. According to Rubner, i gram of urea will yield 2.523 kilogram calories. As about one-third of a gram of urea results from the oxi- 440 TEXT-BOOK OF PHYSIOLOGY. dation of i gram of proteid, the amount of heat to be deducted from the heat value of the proteid is ^ of 2.523, or 0.841 calories. It has also been shown by the same investigator that some of the ingested proteid is found in the feces, the heat value of which must also be deter- mined and deducted. This having been done, the physiologic heat value becomes 4.124 calories. The following estimates give approximately the number of kilo- gram calories which should be liberated within the body when the proteid is burned to the stage of urea, and the fat and carbohydrate to the stage of carbon dioxid and water: I gram of proteid 4-124 calories I " fat 9.353 " I " carbohydrate ■ 4. 116 " The total number of kilogram calories yielded by the various diet scales can be readily determined by multiplying the quantities of the food principles consumed by the foregoing factors. The diet scale of Vierordt, for example, yields the following: 120 grams of proteid 494.88 calories 90 " fat 841.77 " 330 " starch 1358.28 Total, 2694.93 " The total calories obtained from other diet scales would be as follows : Rahke's, 2335; Voit's, 3387; Moleschott's, 2984; Atwater's, 3331; Hultgren's, 3436. These numbers indicate theoretically the total heat-production in the body daily. 2. Calorimetric Measurements.— 'By this method the heat dissipated from the body of an animal is directly collected and measured, and the amount so obtained is taken as a measure of the heat evolved by the oxidation of the food. A calorimeter is therefore an apparatus for the direct estimation of the quantity of heat dissipated from the body in a given time. The substance employed for collecting and measuring the heat is either water or air. The calorimeters in general use consist essentially of two metallic boxes placed one within the other, though separated by a space sufSciently large to hold a definite amount of water (Fig. 205). The animal is placed in the inner box, which is also provided with tubes for the entrance of fresh and the exit of expired air. The heat radiated is absorbed by the water and its tem- perature raised. To prevent loss by radiation and to render it inde- pendent of changes in the surrounding temperature the calorimeter is surrounded by a poorly conducting material, such as wool. The temperature of the animal is taken at the beginning and the end of the experiment. If the temperature of the animal remains the same at the end of the experiment, then the heat absorbed by the water repre- sents the amount produced by the animal. If, on the contrary, the temperature of the animal rises or falls, the number of calories so re- ANIMAL HEAT. 441 tained or lost must be added to or subtracted from the amount ab- sorbed by the calorimeter. In the determination of the absolute amount of heat retained or lost by the animal above or below the initial temperature, as well as that absorbed by the materials of the appara- tus in these various instances, the water equivalent of the tissues of the animal and the materials of the calorimeter must be obtained, and then added to or subtracted from, as the case may be, the amount of water in the calorimeter, and the amount thus obtained multiplied by its rise in temperature. In properly conducted experiments in which the sources of error are reduced to a minimum there is a very close correspondence between the total physiologic heat value of the food and the amount col- lected by the calor- imeter. Thus, in an experiment detailed by Rubner, a dog was given during twelve days 228.06 grams of proteid and 340.4 grams of fat the physical heat value of which was estimated at 4419 calories. The urine and feces during this period were col- lected and their heat value determined, which amounted to 305 calories. The heat which theoretic- ally should have been produced was 41 19 calories. During the experiment the calorimeter actually absorbed 3958 calories, a differ- ence between the theoretic and experimental results of 156 calories; thus of the total energy liberated 96 per cent, appeared as heat. Calorimetric experiments on man corresponding to those made by Rubner on dogs have not been successful, owing purely to tech- nical difficulties. Various attempts have been made, however, to determine the daily heat-dissipation. Liebermeister immersed a man in a bath with a temperature lower than that of the man's body. From the rise in temperature of the water it was calculated that the man produced daily 3525 calories. Leyden placed the leg alone of a man in a calorimeter. In one hour 6 calories were absorbed. As- suming that the total superficial area of the body was fifteen times that of the leg, he calculated, taking into consideration various sources of error, that the entire body would produce daily 2376 calories. Ott, Fig. 205. — Water Calorimeter of Dulong. D and D'. Tubes for the entrance and exit of air. T and T'. Thermometers for ascertaining the temperature of the water. S. A mechanic contrivance for stirring the water for the purpose of distributing the absorbed heat uni- formly. To prevent the escape of heat with the expired air, the tube D' is wound many times in the water-space beneath the animal cage. 442 TEXT-BOOK OF PHYSIOLOGY. employing a water calorimeter, found that the body of a man produced 103 calories during an afternoon, or at the rate of 2472 calories daily. These and similar experiments, while not free from many objections, furnish results which indicate that the heat dissipated from the body approximates the physiologic heat values of the foods. HEAT-DISSIPATION AND REGULATION OF THE TEMPERATURE. Heat-dissipation. — ^From the preceding statements it is evident that the body is continually liberating heat in amounts daily far in excess of that necessary for the maintenance of the body-temperature. Should this heat be retained, the temperature of the body would be raised at the end of twenty-four hours an additional 18° or 20° C. — a temperature far in excess of that compatible with the maintenance of physiologic processes. That the body may be kept at the mean temperature of 37.8° C. it is essential that the heat liberated be dissi- pated as fast as it is produced, or to state the problem in another way, the heat dissipated by the body must be replaced by an equal amount liberated, if equilibrium of temperature is to be maintained. The dissipation of the heat is accomplished in several ways: (i) In warm- ing the food and drink to the temperature of the body. (2) In warm- ing, the inspired air to the same temperature. (3) In the evaporation of water from the lungs. (4) In evaporating water from the skin. (S) In radiation and conduction from the skin. The quantities of heat lost to the body by these different processes it is difficult for ob- vious reasons to accurately determine, and the estimates usually given must be regarded only as approximative. Assuming 2500 calories to be an average of heat liberated during a day of repose, the losses, in the ways stated above, may be given as follows : 1. In Warming Food and Drink. — The average temperature of food and drink is about 12° C; the amount of both together is about 3 kilograms; the specific heat of food about 0.8 that of water. The absorption of body-heat therefore by the food amounts approximately to3Xo.8X25° C.=6ocalories = 2.8per cent. With the removal of the end-products of the foods and drink from the body an equal amount of heat is carried out. 2. In Warming the Inspired Air. — The average temperature of the air is 12" C.; the amount of inspired air, about 15 kilograms; the specific heat of air, 0.26. The absorption of body-heat by the air until it attains the temperature of the body will therefore amount to 15 X 0.26 X 25°= 97.5 calories = 3.8 per cent. The expired air removes from the body a corresponding amount. 3. In the Evaporation of Water from the Lungs. — The quantity of water evaporated from the lungs may be estimated at 400 grams; as each gram requires for its evaporation 0.582 calorie, the quantity of heat lost by this channel would be 400X0.582 = 232«8 calories = 9.4 per cent. ANIMAL HEAT. 443 4. In the Evaporation of Water from the Skin. — The quantity of water evaporated from the skin may be estimated at 660 grams, caus- ing a loss of heat by this channel of 660 X 0.582 = 384.1 calories = 15.3 per cent. 5. In Radiation and Conduction from the Skin. — The amount of heat lost by this process can be indirectly determined only by subtracting the total aniount lost by the above-mentioned channels from the total amount produced. Thus, 2500 — 774.4= 1725.6 calories = 69 per cent, would represent the average amount lost by radiation and conduction. Regulation of the Mean Temperature. — In order that the mean temperature of the body may remain practically constant, the heat dissipated must be exactly balanced by the heat liberated. Should there be any want of correspondence between the two processes, there would arise either an increase or a decrease in the mean temperature. As both heat-production and heat-dissipation are variable factors, dependent on a variety of internal and external conditions, their adjustment is accomplished by a complex self-regulating mechanism involving muscular, vascular, and secretory elements, coordinated by the nerve system. Heat-production varies in intensity and amount, in accordance with a number of conditions, but principally with variations in physiologic activity, the quantity and quality of the food, and changes in the external temperature. All physiologic and especially muscle activity is attended by chemic changes and the evolution of heat. The greater the activity, the larger is the volume of heat. Foods have different physiologic heat values. If the food consumed contains much potential energy and the quantity con- sumed be larger than the average daily requirements, there will be an increase in heat-production. A lowering of the external tem- perature, as in the winter season, leads to increased heat-production through stimulation of the nerve-centers. When all these conditions, increased muscular activity, increased amount of food of high poten- tial energy, and a low external temperature coexist, heat-production attains its maximum, amounting to as much as 4726 calories daily (Hultgren). Heat-dissipation varies in rapidity in accordance with variations of a number of factors, but principally with variations in the external temperature and the activity of the perspiratory apparatus. The heat is dissipated mainly by the skin, 69 per cent., in consequence of radiation and conduction and by the evaporation of the sweat. The loss by this channel as well as from the lungs is dependent for the most part on a difference of temperature of the surrounding air and of the body. If the surrounding temperature is high, there is an increase in the activity of both the circulatory and respiratory mechan- isms, brought about by the central nervous system. In addition to an increased action of the heart, the blood-vessels of the skin dilate, and deliver to the surface a larger volume of blood in a given time, 444 TEXT-BOOK OF PHYSIOLOGY. thus increasing the conditions favorable to radiation. The sweat- glands at the same time are stimulated to increased activity, and in consequence of the additional volumes of blood brought to the skin a larger amount of sweat is secreted, which speedily undergoes evap- oration. As each gram of water for its evaporation requires 0.582 of a calorie, it is evident that increased secretion of sweat favors heat- dissipation. The nerve-centers influencing the activity of the sweat- glands may be stimulated not 'only reflexly, but directly by an excess of heat in the blood. If, however, the atmosphere itself possesses a high percentage of moisture, evaporation from the body is much diminished and the value of sweating as a means of lowering the body-temperature is much impaired. Evaporation is hastened by air in motion. Hastened respiratory movements and the dilatation of blood-vessels of the respiratory surface also increase the evaporation of water from the lungs and thus occasion a greater loss of heat. If the external temperature falls there is a decrease in the physio- logic activity of the skin from a contraction of the blood-vessels, a diminution of the blood-supply, and a cessation in the secretion of sweat. The blood, being prevented from coming to the surface, is retained in the deeper portion of the body, and in consequence the conditions for radiation are diminished. These variations in the cutaneous circulation in response to variations in the external tem- perature are brought about by the vaso-motor nerve mechanism; and as they take place with extreme promptness heat-dissipation and heat- production are quickly adjusted and the mean temperature maintained. Radiation from the skin is modified to some extent by clothing. An excess of clothing diminishes, a diminution of clothing increases radiation. The quality of clothing is also an important factor. Wool is a poor conductor of heat but a good absorber and retainer of moisture, and hence is adapted for cold weather. Linen and cotton possess the opposite qualities, and hence are adapted for warm weather. Radia- tion from the skin is somewhat interfered with by subcutaneous fat, the extent of the interference being dependent on its amount. The foregoing estimates as to the amounts of heat produced have reference only to the body in repose. When the body passes into a state of muscle activity, there is at once a notable increase in heat- production in consequence of the increase in the activity of the chemic changes which underlie body activity, as shown by the increase in the consumption of oxygen and the production of carbon dioxid. Not all of the potential energy set free, however, appears as heat; for, if the muscles are engaged in doing work a part of the energy which would otherwise manifest itself as heat is converted into mechanic motion. From the work done during a period of eight hours it has been estimated that about 500 calories are so transformed or utilized. Hirn calculated from an average of five experiments that a man weighitig 67 kilos in repose produced 154.4 calories per hour and absorbed 30.7 grams of oxygen' per hour; but when engaged in active ANIMAL HEAT. 445 muscle movements produced 271.2 calories and absorbed 119.84 grams of oxygen per hour. The increase in heat-production per hour during activity was thus almost doubled, though the sum total pro- duced daily in which there was a working period of eight or ten hours was only about one-third more than during a day of repose. During sleep there is a greatly diminished heat-production, not more than 40 calories per hour being produced. The preceding data may be tabulated as follows (Martin) : Day of Rest. Day of Work. , ■ , , ■ t , Heat units (calor- 1 Rest 16 hrs. Sleep 8 hrs. Rest 8 hrs. Work 8 hrs. Sleep 8 hrs. ies) produced. . / 2470.4 320 1235.2 2169.6 320 2790.4 3724-8 CHAPTER XVII. SECRETION. Secretion. — Secretion is a term applied to a process by which a portion of thfe constituents of the blood are separated from the blood- stream, by the activities of the endothelial cells of the capillary wall, as the blood flows through the capillary blood-vessels. In this process the endothelial cell is aided by the physical forces, diffusion, osmosis and fil- tration. The materials thus separated are collectively termed lymph. This separated or secreted material may be utilized in several ways: 1. For the repair of the tissues, for growth, for the liberation of energy. 2. For the elaboration or production by specialized organs of a variety of complex fluids of widely different application. The fluids thus formed are utilized for the most part to meet some special need of the body. All such fluids are termed secretions. All secretions are products of the activities of epithelial cells covering a flat, or lining a more or less complexly involuted, membrane which in each instance may be termed a secretor organ. As the fluids are poured out on the surface of the body, they have been termed external secretions: e. g., mucus, saliva, gastric juice, milk, sebaceous matter, etc. Within recent years it has been demon- strated that the epithelium of certain organs and particularly of those which do not possess a duct, such as the thyroid, thymus, adrenals, hypophysis, etc., also produces certain specific constituents which are reabsorbed into the blood, and which in some unknown but yet favor- able way influence the general nutrition. To such products of these organs the term internal secretions has been given. The blood, in addition to its nutritive constituents, contains a number of principles, derived from the tissues, which are to be re- garded as waste products, the outcome of the katabolic activity of the tissues and of no further use to the body. If retained, they would seriously if not fatally interfere with the normal physiologic activities of the different tissues. They are therefore removed by specialized organs after their separation from the blood-stream. The waste products in solution thus removed are not capable of being utilized for any special purpose, and are therefore termed excretions: e.- g., urine, perspiration, etc. Excretion, however, is performed by the activities of epithelial cells aided by the physical forces of diffusion, osmosis and filtration; and though a distinction is made between the two classes of fluids, no sharp line can be drawn between the cell processes which take place in secretor and excretor organs. 446 SECRETION. 447 All secretor organs may be divided into — 1. Epithelial. 2. Reticular and vascular, the latter term indicating merely their re- lation to blood-vessels. The Epithelial Secretor Organs. — The epithelial secretor or- gan consists primarily of a thin delicate homogeneous membrane, one side of which is covered with a layer of epithelial cells and the other side of which is closely invested by a network of capillary blood- vessels, lymph- vessels and nerves. Though the epithelial cells have a general histologic resemblance one to another, their physiologic function varies in different situations, in accordance probably with their ulti- mate chemic structure, a fact which determines the difference in the character of the secretions. The epithelial secretor organs may consist of a single layer of cells or a group of cells, and may be subdivided into — 1. Secreting membranes. 2. Secreting glands. The secreting membranes a xe the muc ous .membra nes Unin g the gastro-inteslinal, the pulmonarj, and the_,genito-urinary tracts^ and the serous membranes lining closed cavities, such as the pleural, pericardial, peritoneal, and synovial membranes. The mucous membranes are soft and velvety in character and are composed of a condensed connective tissue forming a basement membrane beneath which is a layer of blood-vessels and muscle-fibers, and on which is a layer of epithelium, the histologic as well as physio- logic characters of which vary in different situations. The mucus secreted by the various epithelial forms will very naturally possess a somewhat different composition, according to the locality in which it is formed. In a general way it may be said that mucus is a pale, semitransparent, alkaline fluid, containing leukocytes and epithelial cells. It is composed chemically of water, mineral salts and an al- buminoid body, mucin, to the presence of which it owes its viscidity. Much of the mucus is secreted by the goblet cells on the surface of the mucous membranes. The principal varieties of mucus are the nasal, bronchial, vaginal, urinary, gastro-intestinal. The serous membranes are composed of thin membrane formed by a condensation of connective tissue and covered by a single layer of large, flat, nucleated cells with irregular margins. These mem- branes enclose what are practically large lymph sacs or spaces, and the fluid they contain resembles lymph in all respects and is prac- tically identical with it. It serves to diminish friction when the viscera they enclose move over one another. The most important of the serous membranes are the pleural, pericardial, and peritoneal. The synovial membranes in and around joints resemble serous membranes. The cells covering them, however, secrete a clear, colorless fluid resembling lymph, but differing from it in containing a mucin-like substance, a nucleo-albumin, which imparts to it con- 448 TEXT-BOOK OF PHYSIOLOGY. siderable viscidity. This synovial fluid serves to diminish friction between the opposing surfaces of the bones as they glide over one another during movement. Other secretions, such as the aqueous and vitreous humors of the eye, the fluid of the internal ear, the cerebrospinal fluid, etc., will be considered in connection with the orga:ns with which they are associated, as have been the digestive secretions. The secreting glands are formed of the same histologic elements as the secreting membranes. They are formed by an involution of the mucous membrane or skin the epithelium of which is variously modified structurally and functionally in the various situations in which they are formed. Like the membranes themselves, the glands are invested by capillary blood-vessels and supplied with lymph- vessels and nerves, of which the latter are in direct connection with the blood- vessels and epithelial cells. The interior of each gland is in com- munication with the free surface by one or more passageways known as ducts. These glands may be classified according as the involution is cylindrical or dilated as — 1. Tubular. The tubular glands may be simple^e. g., sweat- glands, intestinal glands, fundus glands of the stomach; or compound — e. g., kidney, testicle, salivary, and lachrymal glands. 2. Alveolar. The alveolar glands may also be simple — e. g., the sebaceous glands, the ovarian follicles, meibomian glands; or compound, as the mammary glands and salivary glands. For the production of a secretion it is necessary that the plasma of the blood, the common material, be delivered to the lymph-spaces with which the epithelial cells are in close relation. The processes involved in the passage of the plasma across the capillary wall have already been considered in connection with the production of lymph. ' They include the physical processes, difiusion, filtration, and osmosis, combined with a secretory activity of the cells of the capillary wall. The question as to which of these processes is the more active is yet a subject of investigation. As the chemic composition and the chemic features of the organic constituents of all secretions have been demonstrated to be the out- come of metabolic processes going on within the epithelial cells, it must be assumed at least that these differences are correlated with differences in the histologic features and molecular structure of the epithelium. The discharge of the secretion is, as a rule, intermittent; that is, there are periods of activity on the part of the gland followed by periods of inactivity or rest. In rest more especially the epithelial cells, after the assimilation of lymph, accumulate within themselves such characteristic products as globules of mucin, granules which apparently are the antecedents of the digestive enzymes, granules of glycogen, globules of fat, sugar, and proteid, as in tbe case of the mammary gland. In how far all these compounds are the result of SECRETION. 449 secretor activity or of a cell degeneration and disintegration it is impossible to state in the light of present knowledge. During the period of gland rest the blood-supply to the gland is merely sufficient for nutritive purposes. When the occasion arises for gland activity, the blood-vessels, under the influence of the vaso-motor nerves, dilate and deliver to the gland an amount of blood far beyond that required for nutritive purposes. As a result the gland becomes red and vas- cular and the blood emerging by the veins frequently retains its cus- tomary arterial color. The increased blood-supply favors a rapid transudation of water and salts into the lymph-spaces from which they are speedily absorbed and transmitted by the epithelial cells into the interior of the gland lumen. Coincident with the passage of water through the cell, the organic constituents are extruded from the end of the cell bordering the lumen to become dissolved, or in the case of fat to be suspended, in the water. The secretion thus formed accumulates and with the rise of pressure which inevitably follows it at once passes into the ducts to be discharged on the surface of the mucous membrane or skin, as the case may be. Influence of the Nerve System. — The activity of every gland is controlled by nerve-centers situated in the central nerve system. These centers may be excited to activity either by impressions made on the peripheral terminations or by emotional states, or, possibly, by changes in the composition of the blood itself. As a rule, all normal secretion is a reflex act involving the usual mechanism: , viz., a sentient surface (skin, mucous membrane, or sense-organ) , an afferent nerve, an emissive cell from which emerges an efferent nerve to be distributed to a responisve organ, the gland epithelium. For the production of the secretion by the epithelial cell it is believed by some experimenters that two physiologically distinct, efferent nerve-fibers are involved — one stimulating the production of the organic constituents {trophic nerves), the other stimulating the secretion of water and inorganic salts {secretor nerves). The evidence for the influence of the nerve system on secretion and the mode of connection of the nerve-fibers with the gland-cells have been alluded to (page 163 ) and will again be in subsequent chapters. The reticular and vascular glands, though not possessing any common histologic features, are grouped together merely for con- venience, and will be considered in a separate chapter in connection with the problems of internal secretion. MAMMARY GLANDS. The mammary glands, which secrete the milk, are two more or less hemispheric organs situated in the human female on the anterior surface of the thorax. Though rudimentary in childhood, they gradually increase in size as puberty approaches. The gland pre- sents at its convexity a small conical eminence termed the mammilla 29 4SO TEXT-BOOK OF PHYSIOLOGY. or nipple, surrounded by a circular area of pigmented skin, the areola. The glaKid proper is covered by a layer of adipose tissue anteriorly and is attached posteriorly to the pectoral muscles by a network of fibrous tissue. During utero-gestation the mammary glands become larger, firmer,, and more lobulated ; the areola darkens and the blood-vessels, especially the veins, become more prominent. At the period of lactation the gland is the seat of active histologic and physiologic changes correlated vs^ith the production of milk. At the close of lactation these activities cease, the glands diminish in size, undergo involution, and gradually return to their former non-secreting condition. Structure of the Mammary Gland. — Each mammary gland consists of an aggregation of some 15 or 20 irregular pyram- idal lobes, each of which is ,., Fig. 206. — Mammary Gland, i. Lactiferous ducts. 2. Lobuli of the mammary gland. Fig. 207. — Acini of the Mammary Gland of a Sheep During Lactation. u. Membrana propria. b. Secretory epithelium. surrounded by a framework of fibrous tissue. This tissue affords support for blood-vessels, lymphatics, and nerves. Each lobe is pro- vided with a single excretory duct, the lactiferous duct, which as it approaches the areola expands into a fusiform ampulla or reservoir. At the base of the nipple the ampullae contract to form some 16 or 18 narrow ducts, which, ascending the nipple, open by constricted orifices 0.5 mm. in diameter on its apex (Fig. 206). On tracing the lactiferous duct into a lobe, it is found to divide and subdivide into a number of branches, which pass into smaller masses — the lobules. The lobule in turn is composed of a large number of tubular acini or alveoli, the final terminations of the lobu- lar ducts. Each acinus consists of a basement membrane lined by a single layer of low cuboidal epithelial cells (Fig. 207). Externally the acinus is surrounded by blood-vessels, .nerves, and lymphatics. SECRETION. 451 MILK. Milk as obtained during, active lactation is an opaque bluish- white fluid, almost inodorous, with a sweet taste, an alkaline reaction, and a specific gravity of from 1.025 to 1.040. Examined micro- scopically, it is seen to consist of a clear fluid, the milk plasma, hold- ing in suspension an enormous number of small, highly refractive oil-globules, which measure on the average about x"(rB""D""!r o^ ^^ i'^ch in diameter. It has been asserted by some observers that each globule is surrounded by a thin proteid envelope which enables it to maintain the discrete form. This, however, is at present disbelieved. The quantity of milk secreted daily by the human female averages about 1200 c.c. Chemic analysis has shown that the milk of all the mammalia consists of all the different classes of nutritive principles, though in different proportions, which are necessary to the growth and devel- opment of the body. The only exception appears to be an insuffi- cient amount of iron for the formation of the coloring-matter of the blood, the hemoglobin. Caseinogen is the chief proteid constituent of milk. Associated with it, however, are two other proteids, lactalbumin and lactoglobulin, both of which are present in but small quantity. When milk is treated with acetic acid, sodium chlorid, or magnesium sulphate to saturation, the caseinogen is precipitated as such, and after the removal of the fat with which it is entangled may be collected by appropriate chemic methods. On the addition of rennet prepared from the mucous membrane of the calf's stomach, which contains the enzyme rennin or pexin, the caseinogen undergoes cleavage into an insoluble proteid, casein or tyrein, and a small quantity of a new soluble proteid. To this process the term coagulation has been given. The presence of calcium phosphate appears to be essential to this process, inasmuch as it does not take place if the milk be completely decalcified by the addition of potassium oxalate. After coagulation, the more or less solid mass of milk separates into a liquid portion, the serum, and a solid portion, the coagulum. The former, generally termed whey, consists of water, salts, lactalbumin, sugar; the latter, the curd, con- sists of the casein and entangled fat. Boiling the milk retards and even prevents the coagulation by rennet, owing to the precipitation of the calcium phosphate. When milk is taken into the stomach, it is probable that the rennin coagules the caseinogen in a manner similar to, if not identical with, this process, which appears to be essential to the normal digestion of the milk. The fat of milk is more or less solid at ordinary temperatures. It is a compound of olein, palmitin, and stearin with small quantities of butyrin and caproin. The melting-point of butter varies between 31° and 34° C. When milk is allowed to stand for some time, the fat-globules rise to the surface and form a thick layer known as cream. 452 TEXT-BOOK OF PHYSIOLOGY. Churning the milk or cream causes the fat-globules to run together and form a coherent mass termed butter. Lactose is the particular form of sugar characteristic of milk. It belongs to the saccharose group and has the following composition : C12H22O1J. Though incapable of undergoing fermentation by the action of the yeast plant, it is readily reduced by the Bacillus acidi lactici to lactic acid and carbon dioxid, the former of which imparts to milk an acid reaction and a sour taste. With the accumulation of the lactic acid the caseinogen is precipitated as a more or less consistent mass. The inorganic salts of milk are chiefly potassium, sodium, calcium, and magnesium phosphates and chlorids. Iron is also present in small amount. The following table of Bunge gives the quantitative amounts of these constituents in both human and cow's milk: In iooo Parts. Potas- sium. Sodium. Calcium. Magne- sium. Iron OxiD. Phos- phoric Acid. Chlorin. 0.78 1.76 0.25 I. II 0-33 I-S9 0.06 0.21 0.0036 0.0030 0.47 1.97 0.43 1.69 Cow's milk, Mechanism of Milk Secretion. — During the time of lactation the mammary gland exhibits periods of secretory activity which alternate with periods of repose. Coincidently with these periods certain histologic changes take place in the secreting epithelium At the close of a period of active secretion and after the discharge of the milk each acinus presents the following features: The epithelial cells are short, cubical, nucleated, and border a relatively wide lumen, in which is found a variable quantity of milk. After the gland has rested for some time active metabolism again begins. The cells grow and elongate; the nucleus divides into two or three new nuclei; con- striction takes place and the inner portion is detached and discharged into the lumen of the acinus. During the time these changes 3.re taking place oil-globules make their appearance in the cell protoplasm, some of which are discharged separately into the lumen, while others remain for a time associated with the detached portion of the cell (Fig. 208). From these histologic changes it is inferred that the case- inogen and fat are products of the metabolism of the cell protoplasm and not derived directly through the lymph from the blood. The lactose apparently has a similar origin, as appears from the fact that it is not found either in the blood or any other tissue, and that it is formed independently of carbohydrate food. The water, and especially the inorganic salts, are the result of secretory activity rather than of diffusion and filtration. This is rendered probable from the fact that the proportions of the inorganic salts of milk are more closely allied to those of the tissues of the new-born child than to blood. With the SECRETION. 453 passage of the water and salts into the lumen of the acinus the proteids undergo, disintegration and solution and the liquid assumes the charac- teristics of milk. The discharge of milk is occasioned by the suction efforts on the part of the child, aided by atmospheric pressure and the contractions of the non-striated muscle-fibers of the lactiferous ducts. Influence of the Nerve System. — Judging from analogy, it is probable that the secretion of milk is regulated by influences emanating from the nerve system, though the exact nerve-channels for the trans- mission of such influences have not been determined experimentally. Various • at- tempts have been made to isolate and study these nerves, but the results are in- conclusive. It is well known that emo- tional states on the part of the mother modify the quantity as well as quality of milk, indicating a connection between the gland-cells and the central organs of the nerve system. Nerve terminals have been discovered in and around the epithelial Fig. 208. Section of the cells — a fact which supports this view. Mammary Gland of a Cat Colostrum.— Within a day or two after ""^ ™f. Early Stages of Lac- -' . tation. a. Cavity of alveoli parturition the alveoli become filled with a fiUed with granules and glob- fluid which in some respects resembles ules of fat. 1,2,3. Eplthe- milk and which has been termed colostrum. fOTmation!— (Feo.^^^^ ° ™^ This is a watery fluid containing disinte- grated epithelial cells, fat-globules, as well as colostrum corpuscles, which are probably emigrated leukocytes. Colostrum is distinguished from milk in being richer in sugar and inorganic salts. It is said to possess constituents which act as a laxative to the young child. THE LIVER. The liver is a large gland situated in the upper and right side of the abdominal cavity, where it is held in position largely by liga- ments formed by reduplications of the peritoneal investment. In the adult it weighs, freed of blood, from 1300 to 1700 grams. The liver is connected with the duodenal portion of the intestine by the hepatic duct. It receives blood both from the hepatic artery and from the portal vein, and in this respect differs from all other glands in the body. The epithelial structures of the liver are inclosed by a firm fibrous membrane, known as Glisson's capsule. At the trans- verse fissure it invests and follows the blood-vessels, which there enter, in all their ramifications through the gland. Structure of the Liver. — The liver is composed of an enormous number of small masses, rounded, ovoid, or polygonal in shape, called lobules, measuring about one millimeter in diameter and separated 454 TEXT-BOOK OF PHYSIOLOGY. a, % from one another by a narrow space in which are to be found blood- vessels, lymphatics, hepatic ducts, supported by connective tissue. In the pig this space and its contained elements is quite distinct, sharply marking out the border of the lobule (Fig. 209). This is not so apparent in man. Each lobule is made up of irregular or polygonal shaped cells measuring about 30 to 40 micromillimeters in diameter. These cells are arranged in a radial manner from the center to the circumference of the lobule (Fig. 210). Each cell possesses one and at times two nuclei. There is no evidence for the existence of a distinct cell-wall. The cell protoplasm frequently contains globules of fat, granules of a proteid nature, granules of glycogen, pigment. material, etc. The appearance presented by the cell will vary considerably, ac- cording to the time it is observed. Thus there may be a complete ab- sence of these constitu- ents, when the cell may present a series of vacu- oles separated by bands of protoplasm. The cells are the secreting structures of the liver, and hence are in close relation with capillary blood-vessels, lymphatic spaces, nerves, and ir- regular channels or pas- sageways. The latter running between the epithelial cells may be compared to the lumen of other secreting glands. Blood-vessels and Their Distribution. — The blood-vessels which are in relation with the liver are: 1. The portal vein. 2. The hepatic artery. 3. The hepatic vein. The portal vein and the hepatic artery enter the liver at the trans- verse fissure. After penetrating its substance they divide and sub- divide into smaller and smaller branches, which ultimately occupy the space between the lobules, completely surrounding and limiting them. From their situation they are termed interlobular veins and arteries. The interlobular veins give off small capillary vessels which pene- trate the lobule at all points of its surface. These capillaries, though frequently anastomosing, form a radial meshwork which converges toward the center of the lobule. In the meshes of this plexus are Fig. 209. — Section of Liver of Pig, showing VERY DiAGEAMMATiCALLY THE Lobules, a. Interlobu- lar connective tissue, b, c. Branches of portal vein and of hepatic artery, d. Bile- ducts, e. Intralobular vein. — (Piersol.) SECRETION. 455 ,^ Trabeculas of hepatic cells. ^ Central vein. found, arranged in a corresponding radial manner, the liver cells. The interlobular arteries are distributed to the walls of the portal veiri, to_ the connective tissue, and finally terminate in the portal vein capillaries. The intralobular capillaries thus receive and transmit blood which is an admixture of both arterial and venous blood. In the center of each lobule there is a large vein, formed by the union of the intralobular capillaries, known as the intralobular vein, which collects all the blood of the lobule and transmits it through the lobule to an underlying or sublobular vein (Fig. 211). These latter vessels, uniting and reuniting, ultimately form the hepatic vein, which empties the blood into the inferior vena cava. Bile Capillaries and Hepatic Ducts. — The bile capillaries are narrow channels which penetrate the lobule in all directions and are generally found running along the sides of the cells. These channels, which are devoid of walls, receive from the cells some of the products of their secretory activity, and hence are comparable to the lumen of the alveoli of other secreting glands. At the periphery of the lobules the bile capillaries communicate with larger channels which are the begin- nings of the hepatic or bile-ducts lying in the interlobular spaces. The interlobular bile-ducts possess a dis- tinct wall lined by flattened epithe- lium. There is, however, no distinct line of demarcation between the cells of the interlobular ducts and the secreting cells of the liver proper, as the two blend insensibly, the one into the other. As the hepatic ducts increase in size they gradually acquire the structure characteristic of the main hepatic duct: viz., a mucous, a muscle, and a fibrous coat. Nerves. — Experimental investigations have demonstrated that the liver is supplied with nerves derived from the central nerve system. The route of these nerves is probably by way of the splanchnics and the vagi. Many of the nerves which enter the liver are vaso-motor in function; as to whether others are secretory in character is yet a subject of investigation. It has been asserted that nerve terminals have been demonstrated running between the cells and even pene- trating their substance. This fact would indicate that the metabolic processes of the liver are under the control of the central nerve system. Functions of the Liver. — The anatomic and histologic pecul- Interlobular vein. Hepatic duct. Fig. 210. —Scheme of a Hepatic Lobule, represented in Transverse Section below and, by Partial Lev- eling, IN Longitudinal Section Above. In the left half the blood- vessels are drawn; in the right half .only the cords of hepatic cells. X 20. —(Stiihr.) 456 TEXT-BOOK OF PHYSIOLOGY. iarities of the liver would indicate that it has a variety of relations to the general processes of the body. Experimental investigation has brought some of these relations to liglat. Though its physiologic actions are not yet wholly understood, it may be said that it — 1. Secretes bile. 2. Produces and stores glycogen. 3. Assists in the formation of urea. Secretion of Bile. — The physical properties and chemic com- position of the bile have already been considered (page 202). The characteristic salts of the bile, sodium glycochlate and taurocholate, do not pre-exist in the blood, and therefore must be formed by the liver cells out of materials derived from the blood of the intralobular capil- FiG. 211. — Transverse Section of a Single Hepatic Lobule, x. Intralobular vein, cut across. 2, 2, 2, 2. Afferent branches of the intralobular vein. 3, 3, 3, 3, 3, 3, 3, 3, 3. Interlobular branches of the portal vein, with its capillary branches, forming the lobular plexus, extending to the radicles of the intralobular vein. — {Sappey.) laries. The antecedents of the bile salts, glycocoU and taurin, are crystallizable nitrogenized compounds, and known chemically as amido-acetic and amido-ethylsulphonic acids. Their chemic composi- tion indicates that they are derivatives of the proteids or the albu- minoids, though the intermediate stages in their production are un- known. The origin of the cholalic acid with which they are com- bined is equally obscure. The bile salts as they are found in the bile are produced in the liver cells by metabolic activity. The primary coloring-matter of the bile, bilirubin, has been shown to be a derivative of hematin, a product of the disintegration of hemo- globin. It is supposed that the liver cells bring about this change by combining water with hematin, with the abstraction of iron. The product thus formed is bilirubin, which is excreted, while the iron is for the most part retained. SECRETION. 457 Cholesterin is a waste product derived largely from the nerve- tissue. 'It is brought to the liver and simply excreted by the cells. The remaining constituents of the bile, water and inorganic salts, are secreted here as in all other glands. When once formed, the liver cells discharge these various com- pounds into the channels by which they are surrounded; they then pass into the open mouths of the bile-ducts at the periphery of the lobules. Under the increasing pressure which arises from the secre- tion and accumulation of bile, this fluid flows from the smaller into the larger bile-ducts, and finally is emptied either directly into the in- testine or into the gall-bladder, where it is stored until required for digestive purposes. The secretion of bile, as observed by means of a biliary fistula, is continuous and not intermittent, though the rate of flow is subject to considerable variation. The liver cells, as far as the secretion of bile is concerned, appear to be independent of the nerve system. Their activity, however, is stimulated by the increased blood-supply which arises during di- gestion in consequence of the dilatation of the intestinal vessels, since it is at this period that the rate of discharge is the greatest. The same results have been shown by experiment. Thus, division of the splanchnic nerves is followed by an increased discharge of bile, apparently due to the dilatation of the portal vessels; stimula- tion of their peripheral ends is followed by a decreased discharge of bile in consequence of the contraction of the portal vessels. The bile salts appear to be the most efficient stimulants to the activity of the liver cells, for their administration and absorption is followed by an increase not only in the amount of water, but of the inorganic salts and other solid constituents as well. The flow of bile from the bile capillaries to the main hepatic duct, though primarily dependent on differences of pressure, is aided by the contraction of the muscular walls of the bile-ducts and the inspiratory movements of the diaphragm. Any obstacle to the dis- charge of bile leads to its accumulation, a rise of pressure beyond that of the capillary blood-vessels, and a reabsorption by the lymph- vessels of the bile constituents. After their discharge into the blood from the thoracic duct these constituents are deposited in part in various tissues, giving rise to the phenomena of jaundice, and in part are eliminated in the urine. The Production of Glycogen. — In 1857 Bernard discovered the fact that the liver normally during life produces a sugar-forming substance, analogous in its chemic composition to starch, to which he gave the name glycogen. This substance can be obtained by the following method: Small pieces of the liver of an animal recently killed, preferably after a meal rich in carbohydrates, are placed in acidulated boiling water for a few minutes; then rubbed up in a mortar with sand, again boiled, after which the proteids are removed by filtration. The filtrate thus obtained is opalescent and resembles 458 TEXT-BOOK OF PHYSIOLOGY. a solution of starch. The glycogen may be precipitated from this solution with alcohol as a white amorphous powder, soluble in water. Chemic analysis shows that it consists of CgHioOj, or a multiple of it. When either the original solution obtained by boiling or a solution of this amorphous powder is treated with iodin, it strikes a port-wine color. When digested with saliva, pancreatic juice, or boiled with dilute acids, the solution becomes clear, and testing with Fehling's solution reveals the presence of sugar. If the liver be allowed to remain in the body of an animal for a period of twenty-four hours before the decoction is made as above described, it will be found that the solution contains only a small amount of glycogen but a relatively large amount of sugar. The inference drawn is that after death the glycogen is transformed by some agent, possibly a ferment, into sugar (dextrose). The presence of glycogen in the liver cells can be shown micro- scopically in the form of discrete hyaline and refractive granules. As they are soluble in water they can be readily dissolved out from the cells, leaving small vacuoles separated from one another by strands of cell substance. The amount of glycogen in a well-fed animal varies from 1.5 to 4 per cent, of the total weight of the liver. The production of glycogen is dependent very largely on the consumption of car- bohydrates, the greater the amount of sugar and starch in the food, the greater being the production of glycogen. On a pure proteid diet it is still produced, though in small amounts. Glycogen is also found in muscles, placenta and embryonic tissues generally. Muscles contain from 0.5 per cent, to i per cent, and as they amount to about 40 per cent, of the weight of the body, 70 kilo- grams, they contain from 140 to 280 grams of glycogen. During periods of prolonged activity of the muscles the percentage of glycogen rapidly diminishes, a fact which leads to the inference that it is the source in large part of the energy expended by the muscle. During rest the percentage of glycogen again rapidly increases until the normal percentage is regained. The source of the muscle glycogen is, undoubtedly the liver in which it is temporarily stored. The facts connected with the formation of glycogen, as well as its disposition as at present generally accepted, may be stated as follows: The dextrose into which the carbohydrates are converted by the action of the digestive fluids is absorbed into the blood of the portal vein and carried direct into the liver, where by the action of the cells it is abstracted, dehydrated, and temporarily deposited under the form of the non-diffusible body glycogen. At a subsequent period and in proportion to the needs of the system the liver cells, through the agency of a ferment, transform the glycogen into dextrose, return it to the circulation, by which it is transported to the systemic capillaries, where it disappears. The blood of the hepatic vein therefore contains more sugar than the blood of any other part of the body, and the blood of the arteries more than the blood of the other veins. Should SECRETION. 4SQ there be a failure on the part of the liver cells to abstract the sugar, it would pass through the liver into the general circulation, from which it would be eliminated by the kidneys. The final fate of the sugar is uncertain. It is, however, probable that after its delivery to the muscles, for example, it may be directly oxidized, or stored as glycogen or possibly utilized in the formation of living material. Ultimately, however, through oxidation it yields heat, and contributes to the pro- duction of muscle energy. In opposition to this view. Dr. Pavy, after years of accurate ex- perimentation, states that the blood on the cardiac side of the liver never under normal circumstances contains a larger percentage of sugar than is to be found in any part of the circulation, except in the portal vein. He states that glycogen is never reconverted into sugar, and denies that the liver produces sugar, to be discharged into the blood; that the function of the liver is merely to arrest the passage of sugar, and so to shield the general circulation from an excess; that the sugar which arises in the liver after death is a post-mortem product and not an illustration of what takes place during life. Dr. Pavy, having apparently demonstrated the glucoside constitution of proteid material in general,' accounts for the presence of glycogen in muscles and other tissues on the assumption that during the cleavage of the proteid molecule the carbohydrate element is set free and temporarily stored as glycogen. He thus accounts for the production of sugar in the body, even in the absence of all sugar and starch from the food. Pavy believes that the glycogen produced in the liver is utilized in the formation of fat and the synthesis of complex proteids necessary to the construction of the tissues. The Influence of the Nerve System. — The results of various ex- perimental investigations indicate that the production of sugar from the glycogen in the liver is influenced by the activities of the nerve sys- tem. It was discovered by Bernard that puncture of the floor of the fourth ventricle, at a point between the acoustic and vagus nerves, near the middle line, is followed within an hour or two by the appearance of sugar in the urine, which lasted for from five to six hours in the rab- bit and from two to three or even seven in the dog. For this reason Bernard gave to this area the name of "diabetic area." Coincident with the appearance of sugar in the urine (glycosuria) there is an increase in the percentage of sugar in the blood (hyper- glycemia). The liver at the same time contains a higher percentage of sugar than normally. Apparently the initial step in this series of phenomena is an increased conversion of glycogen into sugar. This supposition receives support from the fact that the degree of the hyper- glycemia, and the subsequent glycosuria, will depend on the amount of glycogen previously in the liver. If the animal has been well fed on carbohydrates, the resulting glycosuria will be pronounced; if, on the contrary, it has been allowed to fast for several days, the glycosuria will be slight. 46o TEXT-BOOK OF PHYSIOLOGY. Assuming that the nerve-cells which constitute the diabetic area influence the conversion of glycogen into sugar, the question arises as to whether the puncture destroys the nerve-cells, or whether it stimulates them to increased activity. The results of experiment lead to the latter supposition. Thus if the vagus nerve is divided in the neck and its central end stimulated there is developed a glycosuria. Stimulation of other sensor nerves has a similar effect. As stimulation of the vagus has the same effect as the puncture, the inference is that the center is normally excited to physiologic activity by impulses reflected from some surface or organ in the peripheral distribution of this nerve. If the nerve- cells in the diabetic area regulate the production of sugar in the liver, the further question arises as to the pathway through which the nerve impulses emanating from them reach the liver, whether by way of the vagi or by way of the spinal cord and splanchnic nerves. That it is not by way of the vagi is shown by the fact that the glyco- suria established by the puncture does not disappear when they are divided ; that it is by way of the spinal cord, as far at least as the first dorsal nerve, and subsequently the splanchnic nerves, is indicated by the fact that a cross-section of the spinal cord above this level, destruc- tion of the upper three dorsal roots as well as division of the splanchnic nerves prevents the development of the glycosuria which follows punc- ture of the medulla. Though stimulation of the upper dorsal (pre- ganglionic) nerve-fibers gives rise to glycosuria, yet, contrary to expectation, stimulation of the splanchnic (post-ganglionic) nerve- fibers does not have the same effect. This may be due, however, to changes in the relation of the capillary blood-vessels to the liver cells or to the character of the stimulus employed. A further question arises as to whether the nerve impulses which pass from the diabetic center to the liver are vaso-motor in character and exerting their effect on the blood-vessels, or whether they are secretor in character and exerting their effect on the liver cell. Bernard was of the opinion that they are vaso-motor in character and that the dia- betic area was a part of the general vaso-motor center. More recent investigators are of the opinion that they are secretor in character, for the reason that whether the blood-pressure rises from a stimulation of the central end of the divided vagus, or falls from a stimulation of the depressor nerve, in each instance there follows a glycosuria. If the production of sugar in the liver is a reflex act as Bernard supposed, taking place through a mechanism consisting of an afferent pathway, the vagus nerve, and an efferent pathway consisting of the spinal cord and splanchnic nerves, the question arises as to the seat of action of the stimulus. This Bernard located in the lungs, for the reasori that though division of the vagus in the neck checks the produc- tion of the sugar, division below the origin of the pulmonary branches had no such effect. Diabetes. — Diabetes is a chronic disease characterized by the appearance of sugar in the urine in variable amounts. This patho- SECRETION. 461 logic condition has usually been associated with derangements of the glycogen function of the liver, though doubtless derangements of other organic functions will produce the same condition. At the present time it is believed that the excretion of sugar by the kidneys depends on two causes: (i) An ineffectual abstraction and storage of sugar due to some impairment in the activity of the liver cells; (2) a rapid cleavage of the proteid constituents of the tissues, in consequence of some profound alteration in the nutritive process, whereby their glucose radicals are liberated in unusual amounts. The physiologic mechanism by which the normal metabolism of the carbohydrates is regulated is unknown. That it is complex in character is shown by the phenomena which follow not only puncture of the medulla, but also removal of the pancreas and the administration of various toxic agents. Removal of the pancreas from the body of a dog or other animal is at once followed by a rise in the percentage of sugar in the blood and its elimination by the kidneys. In a short time acetone, aceto- acetic and oxybutyric acids make their appearance, attended by the usual symptoms characteristic of glycosuria in man. The quantity of sugar excreted and the gravity of the attendant symptoms may be much diminished by allowing a portion of the gland to remain in situ. even though its capacity for the production of pancreatic juice is en- tirely abolished. Transplantation of the pancreas to the subcutaneous tissue or to the abdominal cavity will practically prevent the glycosuria. The explanations which have been offered as to the manner in which the pancreatic tissue prevents and its absence gives rise to the ex- cretion of sugar are purely hypothetical. It has been claimed by some investigators that the pancreas secretes a specific material, which enters the blood and promotes oxidation of the sugar. In the absence of this material the sugar accumulates, and is finally eliminated by the kidneys. Since the discovery of the islands of Langerhans it has been suggested by some investigators that the production of the material which regu- lates carbohydrate metabolism should be attributed to them, rather than to the pancreas as a whole. The sugar excreted doubtless in part comes from the glycogen of the liver, as this disappears in a short time. But as sugar continues to be excreted, even though all carbohydrates be withdrawn from the food, the conclusion is justifiable that it arises in consequence of increased proteid metabolism. This supposition is strengthened by the fact that the quantity of urea excreted rises and falls with the quantity of sugar excreted. Phloridzin, a glucoside obtained from the root bark of the cherry and plum tree, gives rise to the appearance of sugar in the urine, in amounts beyond that which might come from the glucose normally present in the blood or from the glycogen of the liver. As there is a concomitant increase in the amount of urea excreted, the supposition is that phloridzin increases proteid metabolism. Curara, in doses sufficient to paralyze the muscles, also gives rise 462 TEXT-BOOK OF PHYSIOLOGY. to the appearance of sugar in the urine. This is not due, however, to an increased production on the part of the liver, but rather to a want of consumption on the part of the muscles, due to their inac- tivity. The accumulation of the sugar in the blood which takes place for this reason leads very promptly to its removal by the kidneys. The Formation of- Urea.— It is now generally believed that the liver is the most active of all the organs which may be engaged in the production of urea. This belief is based on numerous physiologic and pathologic data. The compounds out of which the hepatic cells construct urea have been for chemic reasons asserted to be the am- monium salts, e. g., the carbonate, lactate, carbamate, which are constantly present in the blood. These salts, which result from proteid metabolism, may be absorbed from the tissues or from the in- testines carried to the liver, and there synthesized to urea. This supposition is supported by an experiment as follows: The liver of an animal recently living is removed from the body and its vessels perfused continuously with blood (the urea content of which is known) containing the ammonium salts. An analysis of this blood shows, after a time, a diminution of these salts, and a large increase in the amount of the urea. After the establishment of an Eck fistula (the union of the portal vein with the ascending vena cave whereby the liver is largely excluded from acting on products absorbed from the intestines) there is a marked diminution in the production of urea while the ammonia content of the urine largely increases. The leucin and tyrosin which result from the prolonged action of pancreatic juice on hemi-peptone are also capable of being converted to urea by the hepatic cells, and in all probability are so disposed of. Destructive diseases of the liver — e. g., acute yellow atrophy, suppuration, cirrhosis — ^largely diminish the production of urea, but increase the quantities of the ammonium salts in the urine. The same is true when the liver cells are destroyed during acute phosphorus-poisoning. VASCULAR OR DUCTLESS GLANDS. INTERNAL SECRETIONS. The metabolism of the body generally, as well as that of individual organs, has been shown to be related not only to the physiologic ac- tivity of such organs as the liver and pancreas, but also to the activity of the so-called vascular or ductless glands. The influence of the pancreas in regulating the production of glycogen by the liver, and the influence of the liver in the maintenance of the general metabo- lism through the production of glycogen and the formation of urea, are now established facts. That the vascular or ductless glands to an equal extent, though perhaps in a different way, assist in the main- tenance of physiologic processes, appears certain from the results of animal experimentation. The explanation given for the 'influence SECRETION. 463 of these glands is that they produce specific substances, which are poured into the blood or lymph and carried direct to the tissues, to the activities of which they appear to be essential; for without these substances the nutrition of the tissues declines and in a short time a fatal termination ensues. Inasmuch as these partly unknown substances are formed by cell activity and are poured into the interstices of the tissues, they have been termed "internal secretions." Though the term internal secre- tions is applicable to all substances which arise in consequence of .tissue metabolism, and which, after being poured into the blood, influence in varying degrees and ways physiologic proc- esses, yet the term in this connection will be applied only to the secretions of the thyroid gland, hypophysis cerebri, and adrenal bodies. Thyroid Gland.— The thyroid gland or body con- sists of two lobes situated on the lateral aspect of the upper part of the trachea (Fig. 212). Each lobe is -pyriform in shape, the base being directed downward and on a level with the fifth or sixth tracheal ring. The lobe is about 50 mm. in length, 20 mm. in breadth, and 25 mm. in thickness. As a rule, the lobes are united by a narrow band or isthmus of the same tissue. The gland is reddish in color, and abundantly supplied with blood-vessels and lymphatics. Microscopic examination shows that the thyroid consists of an enormous number of closed sacs or vesicles, variable in size, the largest not measuring more than o. i mm. in diameter (Fig. 213). Each sac is composed of a thin homogeneous membrane lined by cuboid epithe- lium. The interior of the sac in adult life contains a transparent, viscid, fluid containing albumin and termed "colloid" substance. Externally, the sacs are surrounded by a plexus of capillary blood- vessels and lymphatics. The individual sacs are united and sup- ported by connective tissue, which forms, in addition, a covering for the entire gland. Function of the Thyroid. — The knowledge at present possessed as to the function of the thyroid gland, especially in mammals, is the outcome of a study of the effects which follow its arrested development in the child, its degeneration in the adult, and its extirpation in the human being. as well as in animals. The results, however, which fol- FiG. 212. — View of Thyroid Body. 1. Thyroid isthmus. 2. Median portion of crico- thyroid membrane. 3. Crico-thyroid muscle. 4. Lateral lobe of thyroid body. — {After Morris.) 464 TEXT-BOOK OF PHYSIOLOGY. low its extirpation are not always uniform in all animals, though suf- ficient reasons for the lack of uniformity can not always be assigned. Cretinism, a condition characterized by a want of physical and mental development, is associated with, if not directly dependent on, a congenital absence of the thyroid, or its arrested developm,ent dur- ing the early years of childhood. Myxedema, a condition of the skin in which there is a hyperplasia of the connective tissue, of an embryonic type, rich in mucin, is gener- ally regarded as one of the effects of degenerative processes in the thyroid. Partly in consequence of this change in the skin the face becomes broader, swollen, and flattened, giving rise to a loss of expression. At the same time the mind becomes dull, clouded, even approximat- ing the idiotic type. This supposed infil- tration of the skin with mucin was termed myxedema by Ord, who at the same time associated it with a change in the struc- ture of the thyroid as a result of which it became functionally useless. Extirpation of the thyroid, for relief from symptoms due to grave pathologic changes, has been followed in human beings by symptoms similar to those of myxedema. To this condition the terms operative myxedema and cachexia strumipriva have been applied. After the publication of the history of the myxedema which fol- lowed surgical removal of the thyroid, Schiff, in 1887, repeated his earlier experiments on dogs, and found again that removal of the thyroid was speedily followed by tremors, convulsions, and death. Similar experiments were made by Horsley on monkeys, with results which resembled those characteristic of myxedema. Among the symptoms which developed within a few days after the removal of the gland may be mentioned loss of appetite; fibrillar contractions of muscles; trem- ors and spasms; mucinoid degeneration of the skin, giving rise to puffiness of the eyelids and face and to a swollen condition of the abdomen; hebetude of mind, frequently terminating in idiocy; fall Fig 213. — A Lobule from a Thin Section of the Thyroid Gland of an Adult Man. i. Colloid sub- stance. 2. Epithelium. 3. Tangential section of a tubule, the epithelium viewed from the surface. 4. Tubule in transverse section. 5. Connective tissue. — (Stohr.) SECRETION. 465 of blood-pressure; dyspnea; albuminuria; atrophy of the tissues, followed by death of the animal in the course of from five to eight w6eks. The complexus of symptoms observed in monkeys was divided by Horsley into three stages: viz., the neurotic, the mucinoid, and the atrophic. It is evident that the pressure of the thyroid is essential to the normal activity of the tissues generally. As to the manner in which it exerts its favorable influence, there is some difference of opinion. The view that the gland removes from the blood certain toxic bodies, rendering them innocuous and thus preserving the body from a species of auto-intoxication, is gradually yielding to the more probable view that the epithelium is engaged in the secretion of a specific material, which finds its way into the blood or lymph and in some unknown way influences favorably tissue metabolism. This view of the function of the thyroid is supported by the fact that successful grafting of a portion of the thyroid beneath the skin or in the abdominal cavity will prevent the usual symptoms which follow thyroidectomy. The same result is obtained by the intravenous injection of thyroid juice or by the administration of the raw gland. It was shown by Murray that myxedematous patients could be benefited, and even cured, by feeding them with fresh thyroids or with the dry extract. The chemic features of the material secreted and obtained from the structures of the thyroid indicate that it is a complex proteid con- taining iodin, which, under the influence of various reagents, under- goes cleavage, giving rise to a non-proteid residue, which carries with it the iodin and phosphorus. The amount of iodin in the thyroid varies from 0.33 to i milligram for each gram of tissue. To this com- pound the term thyroiodin has been given. The administration of this compound produces effects similar to those which follow the therapeu- tic administration of the fresh thyroid itself; viz., a diminution of all myxedematous symptoms. In normal states of the body, thyroiodin influences very actively the general metabolism. It gives rise to a de- composition of fats and proteids and to a decline in body-weight. In large doses it may produce toxic symptoms, e. g., increased cardiac action, vertigo, and glycosuria. The conclusions as to the functions of the thyroid gland which have been drawn from the results that have followed its removal from animals by surgical procedures, have been made questionable, since the discovery of the parathyroid glands and a study of the phenomena which follow when they alone are removed. From their situation and close relationship to the thyroid gland it is generally accepted, that in the earlier experiments, especially those made on cats and dogs, and some other carnivorous animals, both sets of glands were removed and hence the symptoms which developed after the removal of the thyroids were due to the loss of function not only of the thyroid but of the parathyroids as well. The Parathyroids. — ^The parathyroids are small bodies, usually 30 466 TEXT-BOOK OF PHYSIOLOGY. four in number, two on each side. They are divided into superior and inferior. The superior are situated internally and on the poste- rior surface in close relation to, and frequently imbedded in, the sub- stance of the thyroid; the inferior are situated externally, sometimes in contact with, and at other times removed a variable distance from the thyroid. Microscopically the parathyroids consist of thick cords of epithelial cells separated by septa of fine connective tissue and sur- rounded by capillary blood-vessels. Chemic analysis shows that they also contain iodin in combination with some organic compound. Effects of Parathyroid Removal. — The surgical removal of the parathyroids is followed in the course of from two to five days by the death of the animal preceded in most instances by a series of symp- toms which are embraced under the general term "tetany." These symptoms are fibrillary contractions of muscles, tremors, spasmodic contractions and paralyses of groups of muscles and not infrequently convulsive seizures and coma. During the convulsion there is an acceleration of the heart-beat, and increase in the respiratory move- ments which frequently become dyspneic in character. There is also a loss of appetite, nausea, mucous vomitings and diarrhea. Death may occur during a convulsion or from coma. (Morat and Doyon.) These results for the most part occur only when all the parathy- roids are removed. It is asserted that even if one gland is retained the animal does not die. The above described symptoms may mani- fest themselves, however, but they are slight in degree. Vincent and Jolly have recently published the results of a series of experiments which seem to negative to some extent the preceding statements. These experimenters state that while it is true, that, as a rule, the removal of both thyroids and parathyroids in the carnivora is a fatal operation, there are nevertheless many exceptions; and in the mammalia generally, e.g., cats, dogs, foxes, guinea-pigs, rats and monkeys, the exception becomes the rule as more than 51 per cent, of animals survived the operation for a prolonged period and of these 68 per cent, showed no specific symptoms of any kind. From the con- tradictory observations it is evident that the subject needs further in- vestigation. The Pituitary Body. — This is a small body lodged in the sella turcica of the sphenoid bone. It consists of an anterior lobe, somewhat red in color, and a posterior lobe, yellowish-gray in color. (Fig. 214.) The former is much the larger and partly embraces the latter. The anterior lobe is developed from an invagination of the epiblast of the mouth cavity, and consists of distinct gland tissue. The posterior lobe is an outgrowth from the brain, and is connected with the in- fundibulum by a short stalk. It has been suggested that the term infundibular body be reserved for the posterior lobe, and the term hypophysis cerebri for the anterior lobe. This distinction appears to be desirable, inasmuch as in their origin and structure they are separate and distinct bodies. SECRETION. 467 Removal of the hypophysis cerebri, or the pituitary body, is always followed by a fatal result, preceded by symptoms not unlike those which follow removal of the thyroid: viz., anorexia, tremors, spasms, etc. Degeneration of the pituitary body has been found in connection with a hypertrophic condition of the bones of the face and extremities to which the term acromegalia has been given. Intravenous injection of an extract of the pituitary increases the force of the heart-beat without any change in its frequency, and causes a rise of blood-pressure from a stimula- tion of the arterioles (Schafer and Oliver) . The material secreted by the pituitary has not been isolated, hence its chemic features are unknown. After its formation it probably passes through a system of ducts into the cerebrospinal fluid, after which it influences the metabolism of the nerve and osseous tissues as well as the force of the heart-muscle. An extract of the anterior lobe itself exerts no appreciable effect on the blood-pressure or on the rate of the heart-beat, nor does it influence the circulatory and respiratory organs. An ex- tract of the infundibular body intravenously injected, however, gives rise to increased blood- pressure and to a slowing of the heart-beat (Howell). Adrenal Bodies, or Suprarenal Capsules. — These are two flattened bodies, somewhat crescentic or triangular in shape, situated each upon the upper extremity of the corresponding kidney, and held in place by connective tissue. They measure about 40 mm. in height, 30 mm. in breadth, and from 6 to 8 mm. in thickness. The weight of each is about 4 gm. Function of the Adrenal Bodies.— It was observed by Addison that a profound disturbance of the nutrition, characterized by a bronze- like discoloration of the skin and of the mucous membranes of the mouth, extreme muscular weakness, and profound anemia, was asso- ciated with, if not dependent on, pathologic conditions of the suprarenal glands. In the progress of the disease the asthenia gradually increases, the heart becomes weak, the pulse small, soft, and feeble, indicating a general loss of tone of the muscular and vascular apparatus. Death ensues from paralysis of the respiratory muscles. The essential nature of the lesion which gives rise to these symptoms has not been deter- mined. Removal of these bodies from various animals is invariably and in a short time followed by death, preceded by some of the symptoms characteristic of Addison's disease. Their development, however. Fig. 214. — Sagittal Section of the Pitu- itary Body and In- fundibulum with ad- JOINING Part of Third Ventricle, a. Ante- rior lobe, a'. A projec- tion from it toward the front of the infundibu- lum. b. Posterior lobe connected by a stalk with the infundibulum, i. I.e. Lamina cinerea. 0. Right optic nerve. ch. S e c t i'o n o f o p t i c chiasm, r.o. Recess of ventricle above the chiasma. cm. Corpus mammillare .-{Schwalbe , from Quain.) 468 TEXT-BOOK OF PHYSIOLOGY. is more acute. From the fact that animals so promptly die after ex- tirpation of these bodies, and the further fact that the blood of such animals is toxic to the subjects of recent extirpation, but not to normal animals, the conclusion was drawn that the function of the adrenal bodies is to remove from the blood some toxic product of muscle metabolism. Its accumulation after extirpation gives rise to death through auto-intoxication. On the supposition that the adrenals might secrete and pour into the blood a specific material which favorably influences general metabolism, Schafer and Oliver injected hypodermically glycerin and water extracts, and observed at once an increased activity of the heart-beats and of the respiratory movements. The effects, however, were only transitory. When these extracts are injected into the veins directly, there follows in a short time a cessation of the auricular contraction of the heart, though the ventricular contraction continues with an independent rhythm. If the vagi are cut previous to the injection or if the inhibition is removed by atropin, the rapidity and vigor of both auricles and ventricles are increased. Whether the inhibitory influence is removed or not, there is a marked increase in the blood-pressure, though it is greater in the former instance. This is attributed to a direct stimulation and contraction of the muscle- fibers of the arterioles themselves, and not to vaso-motor influences, as it occurs also after division of the cord and destruction of the bulb. The contraction of the arterioles is quite general, as shown by plethys- mographic studies of the limbs, spleen, kidney, etc. Applied locally to the mucous membra:nes, adrenal extract produces contraction of the blood-vessels and pallor. The skeletal muscles are affected by the extract very much as they are by veratrin. The duration of a single contraction is very much prolonged, especially in the phase of relaxation or of decreasing energy. It is evident from these experiments that the adrenal bodies are engaged in elaborating and pouring into the blood a specific material which stimulates to increased activity the muscle-fibers of the heart and arteries, and thus assist in maintaining the normal blood-pres- sure as well as the tonicity of the skeletal muscles. An alkaloidal substance was isolated by Abel from extracts of this gland, to which the term epinefihrin was given. A crystallizable substance was iso- lated first by Takamine and later by Aldrich, to which the term adren- alin was given. Both substances are apparently equally efficacious in causing contraction of the blood-vessels and in raising the blood- pressure. The question as to which of these two substances represents the active principle of the gland is as yet a subject of discussion. The Spleen. — The spleen is a soft bluish-red organ, oval in shape, from twelve to fifteen centimeters long by eight broad and four thick. It is situated in the left hypochondrium between the stomach and the diaphragm. In this situation it is held in position by a fold of the perit- oneum which passes from the upper border to the diaphragm. SECRETION. 469 Structure. — A section of the spleen shows that it consists of con- nective tissue, blood-vessels, lymph-corpuscles, and lymphoid tissue. The surface of the spleen is covered by a capsule composed of dense fibrous tissue, from the inner surface of which septa or trabeculae pass inward toward the center of the organ. In their course they give off a series of processes which unite freely, forming a spongy connective- tissue framework. The capsule and the main trabeculae in some animals contain numerous non-striated muscle-fibers. In man they are relatively few in number. The blood-vessels which enter the spleen are supported by the connective-tissue septa. As they pass toward the center of the organ they divide very rapidly and soon diminish in size. In their course small branches are given off, which penetrate the inter- trabecular tissue and be- come encased with spheric or cylindric masses of adenoid tissue known as Malpighian corpuscles. These corpuscles are com- posed largely of leukocytes. In some animals the leuko- cytes, instead of being ar- ranged in masses, are dis- tributed along the walls of the artery as a continuous layer. Within the corpus- cles the arteries pass into capillaries; whether the artery passes directly to the splenic pulp or indirectly by way of the corpuscles, its ultimate branches ter- minate in capillaries which open into the spaces of the splenic pulp. From these spaces a net- work of venules gathers the blood and transmits it to the veins. It is a disputed question as to whether the spaces are lined by epithelium, thus forming a continuous blood channel, or whether they are wanting in this histologic element. The Splenic Pulp. — The spaces of the connective- tissue frame- work are filled with a dark red semifluid mass known as the splenic pulp. When microscopically examined, the pulp presents a fine loose network of adenoid tissue, large numbers of leukocytes or lymph- corpuscles, red corpuscles in various stages of disintegration, and pigment granules. Chemic analysis reveals the presence of a number of nitrogen-holding bodies, e. g., leucin, tyrosin, xanthin, uric acid; organic acids, e. g., acetic, lactic, succinic acids; pigments containing iron, and inorganic salts. The Functions of the Spleen. — Notwithstanding all the ex- FiG. 2 1 5. -^Malpighian Corpuscle of a Cat's Spleen Injected, a. Artery, b. Meshes of the pulp injected, c. The artery of the corpuscle rami- fying in the lymphatic tissue composing it. 470 TEXT-BOOK OF PHYSIOLOGY. periments which have been made to determine the functions of the spleen, it can not be said that any very definite results have been obtained. The fact that the spleen can be removed from the body of an animal without appreciably interfering with the normal metabo- lism would indicate that its function is not very important. The chief changes observed after such a procedure are an enlargement of the lymphatic glands and an increase in the activity of the red marrow of the bones. The presence of large numbers of leukocytes in the splenic pulp and in the blood of the splenic vein suggested the idea that the spleen is engaged in the production of leukocytes, and to this extent contributes to the formation of blood. The presence of disintegrated red blood- corpuscles has suggested the view that the spleen exerts a destructive action on functionally useless red corpuscles. These and other theories as to splenic functions have been offered by different observers, but all are lacking positive con- firmation. Volume Variations of the Spleen. — It was shown some years since by Roy, with the aid of the plethys- mograph, that the spleen undergoes rhythmic varia- tions in volume from mo- ment to moment. In the cat and in the dog the diminution in the volume (the systole) and the increase in volume (the diastole) together occupied about one minute. This fact was determined b,y withdrawing the spleen through an opening in the abdominal wall and enclosing it in a box with rigid walls, the interior of which was connected with a piston record- ing apparatus. The system being filled with oil, each variation in volume was attended by a to-and-fro displacement and a cor- responding movement of the recording lever. The special form of plethysmograph used for this purpose is known as the oncometer or bulk measurer, and the recording apparatus as the oncograph (Fig. 216 and Fig. 222). The cause of these variations in volume Roy attributed to a rhyth- mic contractility of the non-striated muscle-fibers in the capsule and trabeculse, and not to changes in the arterial blood-pressure, as the curve of the pressure taken simultaneously remained practically uniform. The effect of the rhythmic contractions of the splenic muscle tissue is to force the blood through the organ, a condition Fig. 216. — Spleen Oncometer Laid Open. SECRETION. 471 necessitated perhaps by the pressure relations within, though what function is thereby fulfilled is not apparent. It was subsequently shown by Schafer and Moore that the splenic volume is extremely responsive to all fluctuations of the arterial blood- pressure; that though the spleen may passively expand and recoil in response to the rise and fall of the blood-pressure, nevertheless the reverse conditions may obtain: viz., that the splenic volume may diminish as the pressure rises, if the splenic arterioles contract simul- taneously with the contraction of the arterioles generally. On the contrary, the splenic volume may increase coincident with a dilatation of the splenic and systemic arterioles. In addition to the rhythmic variations, the spleen steadily increases in volume for a period of five hours after digestion, and then gradually returns to its former con- dition. Influence of the Nerve System. — The nerves which supply the vascular and visceral muscles in the spleen are derived directly from the semilunar ganglion (post-ganglionic fibers) and pass to it in company with the splenic artery. The nerve-cells from which they arise are in physiologic relation with nerve-fibers (pre-ganglionic fibers) which emerge from the spinal cord in the anterior roots of the third thoracic to the first lumbar nerves inclusive, though they are found most abundantly in the sixth, seventh, and eighth thoracic nerves. Their center of origin is in the medulla oblongata. Stimulation of the nerves in any part of their course gives rise to a diminution in splenic volume; division of the nerves is followed by an increase in the volume. In asphyxia the spleen is small and contracted, a condition attributed to a stimulation of the centers in the medulla by the venosity of the blood. The musculature of the spleen may also be excited to contraction by reflex influences, as shown by the fact that stimulation of the central end of a sensory nerve is attended by a diminution of volume. Inasmuch as the excised spleen will continue to exhibit variations in volume when perfused with blood, it would appear that it possess some mechanism independent to some extent of the nerve system. CHAPTER XVIII. EXCRETION. As stated in the preceding chapter, the term excretion is limited to the process by which the end-products of tissue metabolism are re- moved from the body, the nature of the process, however, differing in no essential particulars from that underlying the process of secre- tion. The histologic structures involved and the forces at work being of the same general character, it is impossible to draw any sharp line of distinction between them. As a general fact it may be stated that in their composition all the characteristic ingredients of the excretions are incapable either of entering into the formation of tissue or of undergoing oxidation for the purpose of heat-production. As the retention of these end-products in the body would exert a deleterious influence on normal metabolism, their prompt removal becomes essential to the maintenance of physiologic activity. The principal excretions of the body — urine, perspiration, and bile — are coinplex fluids in which, with the exception of those given off in the lungs, are to be found in varying proportions the chief end-products of metabolism. THE URINE. Normal urine has a pale yellow or amber color, an aromatic odor, an acid reaction, and a specific gravity of 1.020. As a rule, it is perfectly transparent, though its transparency may be diminished from the presence of mucus, calcium and magnesium phosphates, and mixed urates. The color, which varies within physiologic limits from a pale yellow to a reddish-brown, is due to the presence of the coloring- matters urobilin, urochrome, and uroerythrin, all of which are de- rivatives from the bile pigments absorbed from the liver or the alimen- tary canal. The reaction of the urine is acid, owing to the presence of the acid phosphates of sodium and calcium. The degree of acidity, however, varies at different periods of the day. Urine passed in the morning is strongly acid, while that passed during and after digestion, espe- cially if the food be largely vegetable in character and rich in alkaline salts, is either neutral or alkaline in reaction. The diminished acidity after meals is attributed to the formation of hydrochloric acid by the gastric glands and the consequent liberation of bases which are ex- creted in the urine. The phosphoric acid which enters into com- 472 EXCRETION. 473 bination with sodium and potassium bases is a product of tissue metabolism. The specific gravity is about 1.020, though it varies from 1.015 to 1.025. It will diminish, other things being equal, with increased consumption of water and diminished activity of the skin; it will be increased of course by the opposite conditions. The quantity of urine excreted in twenty-four hours varies from 1200 to 1700 c.c. Amounts both above and below these are fre- quently passed from a variety of causes. The odor of the urine is characteristic and due to the presence of aromatic compounds. COMPOSITION OF URINE. Water, 1500.00 c.c. Total solids, 72.00 grams. Urea, 33.18 " Uric acid (urates) , 0.55 " Hippuric acid (hippurates), 0.40 " Kreatinin, xanthin, hypoxaothin, guanin, arnmo- 1 „ nium salts, pigment, etc. j 11.21 Inorganic salts : sodium and potassium sulphates, ] phosphates, and chlorids; magnesium and cal- | cium phosphates, . \ 27.00 " Organic salts: lactates, acetates, formates in small I amounts, ] Sugar, a trace Gases, nitrogen, and carbonic acid. The estimation of total urinary solids in any given sample of urine is frequently a matter of clinical interest. This may approxi- mately be attained by multiplying the last two figures of the specific gravity by the coefficient of Haeser or Christison, 2.33. The result expresses the total solids in 1000 parts: e. g., urine with a specific gravity of 1.020 would contain 20 X 2.33, or 46.60 grams of solid matter per 1000 c.c. If the amount passed in twenty-four hours be 1500 c.c, the total solids would amount to 69.9 grams. The Water of the Urine. — The amount of urinary water and its ratio to the solid constituents will vary with the amount consumed and the activity of the skin and lungs. In summer the foods, liquid and solid, remaining the same, the quantity of water in the urine is diminished in consequence of increased activity of skin and lungs and the ratio of water to solids decreased. In winter the reverse conditions obtain. The food remaining the same, the consumption of large quantities of water hastens at least the removal of end-products from the tissues and thus increases the urinary solids. Urea is the most abundant of the organic constituents of the urine and is present to the extent of from 2 to 3 per cent. It is a colorless neutral substance, crystallizing under varying conditions in long silky needles or in rhombic prisms. It is soluble in water and alcohol. It is composed of CON^H^. When subjected to pro- longed boiling, it combines with water, giving rise to ammonium 474 TEXT-BOOK OF PHYSIOLOGY. carbonate. The presence of Micrococcus urea in urine will also convert the urea, by combining it with two molecules of water, into ammonium carbonate, CON^H^ + 2HjO = (NHJ^COj. The average amount of urea excreted daily varies from 30 to 34 grams. As urea is now known to be the principal end-product of proteid metabolism within the body, it is evident that the quantity produced and eliminated in the twenty-four hours will depend on the quantity of proteid food consumed and on the extent to which the proteid constituents of the tissues are metabolized. In the condition of nutritive equilibrium, when the proteid ingested is 100 grams and the urea egested 31.5 grams, it is difficult to state the percentage of urea which is derived from the metabolism of the proteid food (circulating proteid) and that derived from the metabolism of the proteids of the tissues (organ proteid). In this condition, however, it is found that if the proteid consumed is varied within limits above or below the standard amount of 100 grams, the quantity of urea excreted rises and falls in practically the same ratio, indicating apparently that the production of urea is directly dependent on the proteid supply. On the contrary, it has been observed in human beings in the fasting condition that for a period of ten days there is a daily excretion of about 21 grams of urea, equivalent to about 70 grams of proteid. Again, contrary to former views, the metabolism of proteid and the production of urea are practically independent of muscular work. Even after severe labor extending over a period of some hours there is no noticeable increase in the urea eliminated. Seat of Urea Formation. — It is quite certain in the light of present knowledge that urea is partly formed in the liver by the action- of the cells out of cleavage products of proteid metabolism. The par- ticular compounds out of which the cells synthetize urea are the ammonium salts, especially the carbamate and carbonate. The experimental reasons for this view have already been stated on page 462. Uric acid is one of the constant ingredients of the urine. It is a crystalline nitrogen-holding body closely resembling urea, its formula being CSH4N4O3. The total quantity excreted daily varies from 0.2 to I gram. It is doubtful if uric acid exists in a free state in the urine, the indications being that it is combined with sodium and potassium in the form of a quadriurate. The urates are frequently deposited when in excess from the urine as a brick-red sediment, the color being due to their combination with the coloring-matter uroerythrin. When pure, uric acid crystallizes in the rhombic form, though it assumes a variety of forms. Uric acid was long regarded as a product of general proteid metabolism and for chemic reasons an antecedent of urea. This view has been abandoned. At present it is believed that it is a cleavage product of nuclein, a constituent of all cell nuclei. In the metabolism of nuclein a proteid and nucleic acid are formed, from the latter of which uric acid is derived. Nu- EXCRETION. 475 cleic acid when decomposed yields a series of bases, such as xanthin, hypoxanthin, adenin, guanin, etc. Because of the fact that these bodies can also be obtained from a synthetized body termed purin they are known collectively as the purin bases. Though there is a close relationship between uric acid and the purin bases, it has been impossible to experimentally derive one from the other. When hy- poxanthin, however, is given internally it is oxidized and converted into uric acid. It is extremely probable, therefore, that uric acid is an oxidation product of one or more of the purin bases. It is probable, however, that not all of the uric acid eliminated is derived from the nuclein of tissue-cells and their decomposition products, the purin bases. Some of it is undoubtedly derived from the nucleins contained in foods. The uric acid eliminated is there- fore partly endogenous and partly exogenous in origin. Xanthin, hypoxanthin, guanin, etc., are also found in urine in small but variable amounts. They are nitrogenized compounds derived mainly from the metabolism of the nuclein bodies. Kreatinin is a crystalline nitrogenous compound closely resem- bling kreatin, one of the constituents of muscular tissue. The amount excreted daily is about i gram. Though kreatinin may arise in conse- quence of proteid metabolism, it is probable that it is largely derived from a transformation of the kreatin contained in the meat consumed as food. Hippuric acid in combination with sodium and potassium is very generally present in urine, though in small amounts. It is more abundant in the urine of the herbivora than the carnivora. In man the amount excreted daily is about 0.7 gram, though the amount may be raised by a diet of asparagus, plums, cranberries, etc., and by the administration of benzoic and cinnamic acids. There is evidence that hippuric acid is formed in the kidney from benzoic acid, its pre- cursors, or related bodies. Various compounds of this class are found in vegetable foods, a fact which may account for the increase in the excretion of hippuric acid on a vegetable diet. Leucin, tyrosin, phenol, cystin, indoxyl, skatoxyl, are found in small amounts even under normal conditions. They arise from putrefactive change in the intestine. Inorganic Salts. — Sodium and potassium phosphates, known as the alkaline phosphates, are found in both blood and urine. The total quantity excreted daily is about 4 grams. Calcium and mag- nesium phosphates, known as the earthy phosphates, are present to the extent of i gram. Though insoluble in water, they are held in solution in the urine by its acid constituents. If the urine be rendered alkaline, they are at once precipitated. Sodium and potassium sulphates are also present to the extent of about 2 grams. The phos- phoric and sulphuric acids which are combined with these bases enter the body for the most part in the foods, though there is evidence that they also arise by oxidation in consequence of the metabolism of proteids 476 TEXT-BOOK OF PHYSIOLOGY. which contain phosphorus and sulphur. Sodium chlorid is the most abundant of the inorganic salts. It is derived mainly from the food. The amount excreted is about 15 grams in twenty-four hours. THE KIDNEYS. The kidneys are the organs engaged in the excretion of the urinary constituents from the blood. They resemble a bean in shape, are from 10 to 12 centi- meters in length, 2 in breadth, and weigh from 144 to 170 grams. They are situ- ated in the lumbar region, one on each side of the vertebral column behind the peritoneum, and ex- tend from the eleventh rib to the crest of the ilium. The anterior surface is convex, the posterior surface con- cave. The latter pre- sents a deep notch — the hilum. The kid- ney is surrounded by a thin smooth mem- brane composed of white fibrous and yellow elastic tissue; though it is attached to the surface of the kidney by minute processes of connec- tive tissue, it can very readily be torn away. The substance of the kidney is dense but friable. Upon making a longitudinal section of the kidney it will be observed that the hilum extends into the interior of the organ and expands to form a cavity known as the sinus, in which are found the blood-vessels, nerves, and duct (Fig. 217). This cavity is mainly occupied by the upper part of the renal duct, the ureter, the interior of which is termed the Fig. 217. — Longitudinal Section through the Kidney, the Pelvis of the Kidney, and a Number OF Renal Calyces. A. Branch of the renal artery. U. Ureter. C. Renal calyx, i. Cortex, i'. Medullary rays. i". Labyrinth, or cortex proper. 2. Medulla: 2'. Papillary portion of medulla, or medulla proper. 2". Border layer of the medulla. .3, 3. Transverse sec- tion through the axes of the tubules of the border layer. 4. Fat of the renal sinus. 5, 5. Arterial branches. *. Transversely coursing medulla rays. — {Tyson, ajter Henle.) EXCRETION. 477 pelvis. The ureter divides into several portions which terminate in small caps of calyces which receive the apices of the pyramids. The parenchyma of the kidney consists of two portions: viz. — 1. An internal or medullary portion, consisting of a series of pyramids or cones, some twelve or fifteen in number, which present a dis- tinctly striated appearance. 2. An external or cortical portion, half an inch in thickness and dis- tinctly friable in character. The Histology of the Kidney.— The kidney is composed of a connective-tissue framework supporting secreting tubules, blood- vessels, lymphatics, and nerves, all of which are directly connected with the removal of the urinary constituents from the blood. The kidney is structurally a compound tubular gland. If the apex of each pyramid be examined with a lens, it will present a number of small orifices which may be regarded as the beginnings of the urinifer- ous tubules. From this point the tubules pass outward in a straight but somewhat diverging manner toward the cortex, giving off at acute angles a number of branches (Fig. 218). From the apex to the base of the pyramids they are known as the tubules of Bellini. In the cortical portion of the kidney the tubule becomes enlarged and twisted, and, after pursuing an extremely convoluted course,- turns backward into the medullary portion for some distance, forming the ascending limb of Henle's loop; it then turns upon itself, forming the descending limb of the loop, reenters the cortex, again expands and becomes convoluted, and finally terminates in an ovoid enlargement known as Miiller's or Bowman's capsule, in which is contained a small tuft of blood-vessels — the glomerulus. Each tubule consists of a basement membrane lined throughout its entire extent by epithelial cells. The epithelium as well as the tubule vary in shape and size in different parts of its course. In the capsule the epithelium is flattened, lining not only the inner surface of the capsule but reflected over the blood-vesseils as well. This is known as the glomerular epi- thelium. In the convoluted portions of the tubules the epithelium is cuboidal, granular, and somewhat striated; in Henle's loop it is more or less flattened. The Blood-vessels of the Kidney.— The renal artery enters the kidney at the hilum behind the ureter; it soon divides into several large branches which penetrate the substance of the kidney between the pyramids and pass outward into the cortex. At the base of the pyramids branches of the arteries form an anastomosing plexus. From this plexus vessels are given off, some of which follow the straight tubules toward the apex of the pyramids, vasa recta, while others enter the cortex and pass to its surface (Fig. 218). In the course of the latter small branches are given off, each of which soon divides and si^bdivides to form a ball of capillary vessels known as the glomer- ulus., These capillaries, however, do not anastomose, but soon re- unite to form an efferent vessel the caliber of. which is less than that 47S TEXT-BOOK OF PHYSIOLOGY. of the afferent arler}-. In consequence of this, there is a greater re- sistance to the outflow of blood than to the inflow, and therefore a higher blood-pressure in the glomerulus than in capillaries generally. The relation of the glomerulus to the tubule is important from a Lobule. Tunica albuginea. Stellate vein. Interlobular artery. Interlobular vein. Arciform artery Arcitorm vein. Papillary du^t '^^ YiQ. 2iS. ^Scheme of the Course of the Uriniferous Tubules and the Rexal Vessels. EXCRETION. 479 physiologic point of view. As stated above, the glomerulus is re- ceived into and surrounded by the terminal expansion or capsule of the tubule. This capsule, formed by an indentation of the terminal portion of the tubule, consists of two walls, an outer one consisting of. an extremely thin basement membrane, covered by flattened epi- thelial cells, and an inner one consisting apparently only of flattened epithelium which is reflected over and closely invests the glomerular blood-vessels (Fig. 219). The blood is thus separated from the interior of the capsule by the epithelial wall of the capillary and the epithelium of the reflected wall of the capsule. During the periods of secretory activity the blood- vessels of the glomerulus are filled with blood to such an extent that the sac cavity is almost obliterated. After its exit from the capsule the effer- ent vessel of the glomerulus soon again divides and sub- divides to form an elaborate capillary plexus which sur- rounds and closely invests the convoluted tubules. From this plexus as well as from the plexus which surrounds the straight tubules veins arise which pass toward and empty into veins at the base of the pyramids. The renal vein formed by the union of these latter veins emerges from the kidney at the hilum and finally empties into the vena cava inferior. The nerves of the kidney are derived from the renal plexus and follow the course of the blood-vessels to their termination. The Renal Duct. — The excretory duct of the kidney, the ureter, is a musculo-membranous tube about 5 mm. in diameter when dis- tended, 30 cm. in length, and extends from the hilum to the base of the bladder. The upper extremity is expanded and within the renal sinus becomes irregularly branched, giving rise to a number of short tubes, called calyces, each of which embraces the apex of a Malpighian pyramid. The interior of the expanded portion of the ureter is known as the pelvis. The wall of the ureter consists of a mucous membrane, a muscle coat, and an external fibrous investment. MECHANISM OF URINE SECRETION. The secretion of urine is a complex process and susceptible of several interpretations. It was originally inferred by Bowman that, Fig. 219. — Scheme of the Renal or Mal- pighian COKPUSCLE. I. Interlobular artery, i!. Afferent vessel. 3. Efferent vessel. 4. Outer vfaW. 5. Inner wall. 6. Glomerulus. 7. Neck of tubule. — {Stohr.) 48o TEXT-BOOK OR PHYSIOLOGY. as the kidney presents anatomically: an apparatus for filtration, the capsule with its enclosed glomerulus, and an apparatus for secretion, the epithelium of the urinary tubules, the elimination of the urinary constituents from the blood is accomplished by the two processes of filtration and secretion; that the water and highly diffusible inorgaiiic salts simply pass by diffusion, under pressure, though the walls of the glomerular capillaries, while the organic constituents are removed by the epithelium lining the tubules. Influenced largely by the facts of blood-pressure Ludwig advanced the view that the factors concerned in the secretion of urine were purely physical; that in consequence of the high pressure in the vessels of the glomeruli, due to the resistance offered by the smaller efferent vessel, all the urinary constituents were filtered off in a state of extreme dilution. In order to account for the higher percentage of the organic constituents in the urine, it was assumed that as the dilute urine passed through the tubules the water was partly reabsorbed, passing by diffusion into the lymph and blood until the urine acquired its normal characteristics. In support of this view, a large number of facts relating to the influence of an increase and a decrease of pressure in the blood-vessels of the glomeruli, the velocity of the blood-stream, etc., in determining the rate of urinary flow were adduced, all of which apparently indicated that the former stood to the latter in the relation of cause and effect, and that the formation of urine was accomplished entirely by physical forces. The progress of physiologic investigation, however, has thrown some doubt on the validity of this physical interpretation, and has rather served to support the view of Bowman that the organic con- stituents at least are removed from the blood by a process of selection on the part of the epithelium of the convoluted part of the urinary tubules; in other words, that the secretion of urine is physiologic rather than physical. Heidenhain has brought forward a series of facts which support this view. As evidence that the cells possess a selective power, he presents the following experiment: The spinal cord of an animal is divided in the neck for the purpose of lowering the blood-pressure in the kidney below the pressure at which the urine is secreted; a solution of indigo-carmine is injected into the blood- vessels; after the lapse of ten minutes the animal is killed, the blood- vessels washed out with alcohol for the purpose of precipitating the indigo-carmine in situ. Section of the kidney shows a uniform blue stain of the cortex alone. Microscopic examination reveals the fact that the blue stain is due to the deposition of the pigment in the lumen and in the lumen border of the cells of the convoluted tubules and the ascending limb of Henle's loop; while the epithelium of Bow- man's capsule as well as the glomerular epithelium present no evi- dence of pigmentation. Nussbaum attempted to establish the secretory power of the epi- thelium in another way. In the frog the kidney receives blood from EXCRETION. 481 two sources: the glomeruli receive their blood from the renal artery, the tubules from the capillaries formed by the anastomosis of branches of the efferent vessel of the glomerulus and the branches of the renal portal vein. Nussbaum believed that by ligating the renal artery all glomerular activity could be abolished and the part played by the epithelium could be established. After so doing the flow of urine was at once checked; the injection of urea at once reestablished it. This fact was taken as a proof that the tubular epithelium not only ex- creted urea, but water and perhaps other constituents as well. It was also found that sugar, peptones, carmine, etc., which are always eliminated from the blood under normal conditions, are not removed after ligation of the renal artery. It was concluded from these ex- periments that the secreting structures of the kidney consist of two distinct systems, the glomerular and the tubular; the former secreting water, salts, sugar, peptone, etc.; the latter urea, uric acid, etc. These and similar facts indicate that the renal epithelium possesses a secretory rather than an absorptive function. Heidenhain and those who agree with him assert that even the water and inorganic salts which pass through the glomerular epithelium do so in consequence of cell selection and cell activity; that the entire process is one of secretion, though conditioned by blood-pressure, blood velocity, etc. Influence of Blood-pressure. — Whether the elimination of the urinary constituents is entirely secretory (physiologic) in character or not there can be no doubt that the whole process is largely deter- mined by the pressure and velocity of the blood in the glomerular capillaries, or, to state it more accurately, on the difference of pres- sure between the blood in the capillaries and the urine in the capsules. As a rule, this latter pressure is at a minimum. If the urine should accumulate in the ureter and tubules either from ligation or mechan- ical obstruction until its pressure approximates that of the blood, the secretion would be diminished if not abolished. It is difficult to determine the average pressure or velocity of the blood in the glomers ular capillaries, though they both must be greater than in capillarie- in other parts of the body, from the fact that the efferent vessel is narrower than the afferent, and therefore offers great resistance to the outflow of blood, a condition most favorable to the production of a high pressure in the glomerulus. The pressure of the blood in the glomeruli may be raised and the velocity increased: 1. By an increase in blood-pressure generally. 2. By an increase in the pressure of the renal artery alone. The first condition may be brought about by an increase in either the force or frequency of the heart's action or by a contraction of the arterioles of vascular areas in any or all parts of the body, excepting, of course, the renal vascular area. The second condition is brought about by a dilatation of the renal artery alone and possibly by a con- traction of the efferent vessels of the glomeruli. 31 482 TEXT-BOOK OF PHYSIOLOGY. The pressure of the blood in the glomeruli may be diminished and the velocity decreased: 1. By a decrease in the blood-pressure generally. 2. By a decrease in the pressure of the renal artery alone. The first condition is brought about by a decrease in either the force or frequency of the heart's action or by a dilatation of the arteri- oles of large vascular areas in any or all parts of the body. The second condition is brought about by contraction of the renal artery alone and possibly by a dilatation of the efferent vessels of the glom- eruli. The effect of the contraction and relaxation of either the afferent or efferent vessels on the pressure within the glomerulus is shown in Figure 220. Coincident with the rise and fall of pressure in the glomerular capillaries there is a rise and fall in the rate of urinary flow. Thus it has been found that an in- crease in the aortic pressure from 127 to 142 mm. of mercury, by ligation of the carotid, femoral, and vertebral arteries, increased the rate of urinary flow from 8.7 grams in thirty minutes to 21.2 grams. On the contrary, a decrease in aortic pressure below 40 mm. of mercury caused by division of the spinal cord is fol- lowed by a total abolition of the urinary flow. These facts serve to indicate the dependence of the secretion on blood-pressure. That there is an increase in •c:,=- ==xg^==^ Fig. 220. — To Illustrate the Efpect or Active Changes in the Vasa Affer- ENIIA AND EffERENTIA ON THE PrESSLTRE IN THE Glomerular Capillaries. A. Re- nal arteries. G. Glomerular capillaries. C. Tubular capillaries. V. Vein. The short thick lines represent the vasa aSerentia and efferentia. The continuous heavy line repre- sents the mean average pressure. If the vas afferens dilates and the vas eflferens contracts separately or conjointly, the pressure will rise, as indicated bytiie upper dotted line. If the vas afferens contracts and the vas efferens dilates separately or conjointly, the pressure will fall, .as indicated by the lower dotted line. — (After Moral.) the volume of the blood flowing through the kidney during its functional activity is apparent from inspection. It is enlarged, swollen, and red in color. The blood in the renal vein is bright red in color and con- tains more oxygen and less carbon dioxid than venous blood generally. During the intervals of activity the kidney diminishes in size^ is pale in color and the blood of the renal vein dark and venous in character. These variations in the volume of the kidney have also been experi- mentally determined and registered by means of the oncometer and oncograph devised by Roy. The oncometer consists of a metallic box (Fig. 221) composed of halves which open and close by means of a hinge. It is connected with a recording apparatus, the oncograph (Fig. 222), through the tube T. The kidney, withdrawn from the body, is placed within the oncometer. Through an opening in the side pass the artery, vein. EXCRETION. 483 and ureter. Between the kidney and the wall of the capsule there is placed a thin membrane. Oil is then poured through the side tube I until the space between the capsule and the kidney, as well as the tube leading to the chamber of the oncograph, are completely filled. When the tube I is closed, the conditions are such that all variations in the^ volume of the kidney are taken up and reproduced by the recording lever attached to the piston of the oncograph. A curve mi % > Fig. 221. — Oncometer. K. Kidney; the thick line is the metallic capsule, h. Hinge. I. [Tube for filling apparatus. T. Tube to connect with T„ a, v, u. Artery, vein, ureter. — {Stirling, after Roy.) Fig. 222. — Oncograph. C. Chamber filled with oil, communicating by T, with T. p. Piston. /. Writing-lever. — {Stirling, after Roy.) of the variations in the volume of the kidney is shown in Figure 223 taken simultaneously with the curve of the blood-pressure. An examination of this curve shows that the volume-changes coincide with changes in the blood-pressure, exhibiting not only the respiratory but also the cardiac undulations. Influence of the Nerve System. — ^The influence of the nerve B.P. Fig. 223. — B. P. Blood-pressure curve. K. Curve of the volume of the kidney. T. Time curve; intervals indicate a quarter of a minute. A. Abscissa. — {Stirling, after Roy.) system in regulating the blood-supply to the kidney is evident from the results of experimentation. If the nerves which accompany the renal artery into the kidney are divided, the artery at once dilates, the kidney enlarges, and a copious flow of urine takes place. If the peripheral ends of these nerves be stimulated with the induced electric 484 TEXT-BOOK OF PHYSIOLOGY. current, the artery contracts, the kidney diminishes in size, and the flow of urine ceases. In addition to these vaso-constrictor nerves, there is evidence that the kidney also receives vaso-dilator nerves which emerge from the spinal cord and are found in the anterior roots of the eleventh, twelfth, and thirteenth dorsal nerves, in the dog. Direct and reflex stimulation of these nerves gives rise to a dilatation of the artery, a swelling of the kidney, and an increase in secretion, independent of any variation in general blood pressure. The route of the vaso-constrictor nerves is, in the dog at least, through the splanchnics. Section of these nerves is followed by a dilatation of the renal vessels and an increase in the flow of urine. Stimulation of the peripheral ends is followed by a constriction of the vessels and a cessation of the flow of urine. The vaso-motor center for the blood-vessels of the kidney is in all probability situated in the medulla oblongata in close proximity to the general vaso-motor centers, though subordinate centers are doubt- less present in the spinal cord. It was found by Bernard that puncture of the medulla was occasionally followed by a profuse secretion of urine without the presence of sugar. The route of the vaso-motor impulses which influence the renal blood-supply is down the cord through the splanchnics and through the renal plexus. Influence of Variations in the Composition of the Blood. — As it is the function of the kidneys to excrete water, inorganic salts, and various end-products from the blood and thus maintain a gen- eral average composition, it is highly probable that as soon as they accumulate beyond a certain percentage they themselves act as stimu- lants to renal activity, either by acting directly on the renal epithelium or by increasing the glomerular pressure. There is evidence at least that urea acts in the former manner. An excess of water in the blood, that from copious drinking or from a sudden checking of the skin from a fall of temperature, will act in the latter way. The introduction into the blood of inorganic salts, such as potassium nitrate, sodium acetate, etc., will in a short time lead to increased activity of the kidneys, as shown by an increase in the quantity of urine excreted. The manner in which these agents and other members of their class, the so-called saline diuretics, increase renal activity is yet a subject of discussion. On the one hand, it is stated that they promote an absorption of water from the tissues to such an extent that a condition of hydremic plethora is produced, which in itself increases not only the general blood- pressure but the local renal pressure as well, and that it is this factor which is the cause of the increased flow of urine. On the other hand, it is asserted that though the salts increase the local pressure and the volume of the kidney, they nevertheless act specifically on the renal epithelium, and therefore may be regarded as secreto-motor agents. An increase in the percentage of sugar or urea in the blood has a similar influence on the kidney. The Storage and Discharge of Urine. — Urination.— The urin- EXCRETION. 485 ary constituents, as soon as they are eliminated from the blood, pass into and through the uriniferous tubules and by them are dis- charged into the pelvis of the kidney. They then enter the ureter by which they are conducted to the bladder. The immediate cause of this movement is undoubtedly a difference of pressure between the terminal portions of the tubules and the terminal portion of the ureter, aided by the peristaltic contraction of the muscle wall of the ureter. The bladder is a reservoir for the temporary reception of the urine prior to its expulsion from the body. When distended it is ovoid in shape and is capable of holding from 600 to 800 cu. cm. The bladder is composed of four coats: viz., serous, muscle, areolar, and mucous. The muscle coat consists of external longitudinal and internal circular and oblique layers of fibers of the non-striated variety which collec- tively encircle the entire or^an. As these fibers by their contraction expel the urine from the bladder, they are known collectively as the detrusor urince muscle. At the exit of the bladder the circular fibers are somewhat increased in number, giving rise to the appearance of a distinct muscle which has been termed the sphincter vesica muscle. The presence of this muscle has, however, been denied and the reten- tion of the urine has been attributed to mechanic conditions at the neck of the bladder. The urethra just beyond the bladder is provided with a distinct circular muscle composed of striated fibers, the sphincter urethrcB muscle. When the urine passes into the bladder it is retained there and prevented from escaping by the contraction oi this latter muscle. Under normal conditions the urine accumulates to a con- siderable extent before the intra-vesic pressure gives rise to a charac- teristic sensation and the desire for urination. The Nerve Mechanism of Urination. — The muscle mechan- isms which retain as well as expel the urine are under the control of the nerve system. The sphincter urethrae muscle, which by the orifice of the bladder is closed, is kept in a state of tonic contraction by nerve impulses coming from the spinal cord through the anterior roots of the third and fourth sacral nerves. The detrusor urinae muscle is excited to contraction by impulses coming likewise through the sacral nerves and through the upper lumbar nerves from the cord. The centers of origin for these two sets of motor nerves are located in the cord in the neighborhood of the fifth lumbar vertebra. The expulsion of the urine is largely a reflex act, though under the con- trol of the will. When the desire to urinate is experienced, nerve impulses are coming through sensory nerves from the mucous mem- brane of the bladder which are reflected to the centers governing the sphincter urethree and detrusor urinae muscles and to the brain. The effect of the reflected impulses is to inhibit the sphincter center and to stimulate the detrusor center. If the act of urination is to be permitted, volitional impulses descend through the spinal cord which have the effect of still further inhibiting the sphincter center and stimulating the detrusor center, the result being a relaxation of the 486 TEXT-BOOK OF PHYSIOLOGY. sphincter muscle and a contraction of the detrusor muscle and the expulsion of the urine. If the act of urination is to be suppressed, volitional impulses inhibit the detrusor center and stimulate the sphincter. PERSPIRATION; SEBUM. The perspiration or sweat, the chief secretion of the skin, is a clear colorless fluid, slightly acid in reaction and saline to the taste. Its specific gravity varies from 1.003 to 1.006. Unless collected from the soles of the feet and the palms of the hand, it is apt to be mixed with epithelial cells and sebum. The total quantity of perspiration secreted daily has been variously estimated at from 700 to 1000 grams; the exact amount, however, is difficult of determination, for the reason that the rate of secretion varies readily with variations in tempera- ture, food, drink, season of the year, etc. Chemic analysis of the sweat shows that it contains but from 0.5 to 2.5 per cent, of solid constituents, the variation in the percentage depending on the quantity of water secreted. The solids consist of traces of urea, neutral fats, lactic and sudoric acids in combination with alkaline bases, and inorganic salts (Fovel). Other observers, however, have not been able to detect the presence of either lactic or sudoric acid. Urea is a constant ingredient, though its percentage is extremely small, possibly not more than o.i percent. The amount, however, may be very much increased in uremic conditions, the result of acute or chronic disease of the kidneys. The inorganic constituents consist mainly of sodium chlorid and alkaline and earthy phosphates. Carbonic acid is also present in the free state as well as in combination with alkaline bases. The very small quantity of the solid constituents in the sweat, taken in connection with the fact that it is excreted most abundantly when the external temperature is high, indicates that it is not so im- portant as an excrementitious fluid as it is as a means for the regulation of the temperature of the body. The sweat is a product of the secretory activity of specialized glands, the sweat-glands, embedded in the skin, to the histologic structures of which they bear a special relation. THE SKIN. The skin is a complexly organized structure investing the entire external surface of the body. Its total area varies from 16 to 20 feet in man and from 12 to 16 feet in woman. It varies in thickness in different localities of the body from ^ to -j-ww of ^^ inch. The skin consists of two principal layers: viz., a deep layer, the derma or corium, and a superficial layer, the epidermis. The derma or corium may be subdivided into a reticulated and a papillary layer. The reticulated layer consists of white fibrous and EXCRETION. 487 h / — I yellow elastic tissue, non-striated muscle-fibers, woven together in every direction and forming an areolar network, in the meshes of which are deposited masses of fat and a structureless amorphous matter; the papillary layer consists mainly of club-shaped elevations or projections of the amorphous matter constituting the papillae. The reticulated layer serves to connect the skin with the underlying structures and to afford support for the blood-vessels, nerves, and lymphatics which are distributed to the papillae (Fig. 224). The epidermis is an extra-vascular structure consist- ing entirely of epi- thelial cells. It may also be sub- divided into two layers — the Mal- pighian or pig- mentary layer, and the corneous or horny layer. The former is closely applied to the pa- pillary layer of the true skin and is composed of large nucleated cells, the lowest layer of which, the "prickle cells," contains the pigment granules which give to the skin its varying hues in different individuals and in different races of men; the corneous layer is composed of flattened cells which from their exposure to the atmosphere, etc., are hard and horny in texture. The Sweat-glands. ^These glands are tubular in shape, the inner extremity of each being coiled upon itself a number of times, forming a little ball situated in the derma or the subcutaneous connective tissue. From this coil the duct passes up in a straight direction to the epidermis, where it makes a few spiral turns, after which it opens obliquely on the surface. The gland consists of a basement membrane lined with epithelial cells. It is supplied abundantly with blood- vessels and nerves. The sweat-glands are extremely numerous all Fig. 224. — Section Perpendicularly Through the Healthy Skin. a. Epidermis or scarfskin. b. Rete mu- cosum, or rete malpighii. c. Papillary layer, d. Derma, corium, or true skin. e. Panniculus adiposus, or fatty tis- sue. /, g, h. Sweat-gland and duct, i, k. Hair, with its follicle and papilla. I. Sebaceous gland. ,,^ 488 TEXT-BOOK OF PHYSIOLOGY. over the cutaneous surface, though they are more thickly disposed in some situations tlian others. They probably average 2500 to the square inch; the total number has been estimated at from 2,000,000 to 2,500,000. The Influence of the Nerve System on the Production of Sweat. — The secretion of sweat, though a product of the activity of epithelial cells and dependent on a variety of conditions, is reg- ulated to a large extent by the nerve system. Here as in other secreting glands the fluid is derived from materials in the lymph-spaces, fur- nished by the blood. Generally the two conditions, increased blood- flow and increased glandular action, coexist. At times, however, a profuse clammy perspiration is secreted with diminished blood-flow. Two sets of nerves are evidently concerned in this process: viz., vaso-motor nerves, which regulate the blood-supply, and secretor nerves, which stimulate the gland cells to activity. The nerve-centers which control the sweat-glands are situated in the spinal cord, though the number of such centers and their exact location for the different regions of the body have not yet been satis- factorily determined. In a general way it may be stated that the centers for the head and face lie in the upper cervical portion of the cord; for the upper extremities, in the lower cervical portion; for the lower extremities, in the lower dorsal and upper lumbar portion. The secretor nerves which emerge from these centers reach the glands of the face and head through the cervical sympathetic; of the arms and legs, through the brachial plexus and the sciatic nerves. It is probable that there is also a general dominating sweat center located in the medulla oblongata. That the sweat-glands are stimulated to activity by nerve impulses is shown by the fact that stimulation of the peripheral end of the divided cervical sympathetic, of the brachial plexus, or of the sciatic nerve is followed in a few seconds by a profuse secretion. Though under physiologic conditions there is a simultaneous dilatation of the blood-vessels and an increased supply of blood, this is merely a con- dition and not a cause of the secretion; for the secretion can be excited and the flow maintained for a period of from ten to fifteen minutes after ligation of the blood-vessels of the limb or even after its ampu- tation, when the corresponding nerve is stimulated. The sweat-glands may be excited to activity by their related nerve- centers, either by central, reflex, or peripheral influences. Among the first may be mentioned mental emotions, venosity of the blood, increased temperature of the blood, hot drinks, violent muscular exercise, etc. Among the second may be mentioned powerful stim- ulation of various afferent or sensor nerves, heightened external temperature, etc. Among the last may be mentioned various drugs. Pilocarpin injected into the blood causes a profuse secretion even when the nerves have been divided. Its action is supposed to be exerted on the terminal branches of the nerves and possibly on the EXCRETION. 489 cells themselves. As in the case of the salivary glands atropin sus- pends the activity of the terminal branches of the secretor nerves. Hairs.— Hairs are found in almost all portions of the body, and can be divided into — 1. Long, soft hairs, on the head. 2. Short, stiff hairs, along the edges of the eyelids and nostrils. 3. Soft, downy hairs on the general cutaneous surface. They consist of a root and a shaft. The shaft is oval in shape and about ^^ of an inch in diameter; it consists of fibrous tissue, covered externally by a layer of imbricated cells, and internally by cells containing granular and pigment material. The root of the hair is embedded in the hair-follicle, formed by a tubular depression of the skin, ex- tending nearly through to the sub- cutaneous tissue; its walls are formed by the layers of the corium, covered by epidermic cells. At the bottom of the follicle there is a papillary projec- tion of amorphous matter, corres- ponding to a papilla of the true skin, containing blood-vessels and nerves, upon which the hair-root rests. The investments of the hair-roots are formed of epithelial cells, constituting the internal and external root-sheaths. The lower portion of the hair- follicle is connected with the upper surface of the derma by bundles of non-striated muscle-fibers which are termed arrectores pilorum muscles. Their inclination and insertion are such that their contraction is followed by erection of the hair-follicle and hair-shaft. These muscles are ex- cited to action by nerves termed pilo-motor nerves. THE SEBUM. The sebum or sebaceous matter is a peculiar oily material produced by specialized glands in the skin. It consists of water, epithelium, proteids, fat, cholesterin, and inorganic salts. The sebaceous glands are simple and compound racemose glands opening by a common excretory duct on the surface of the epidermis or into the shaft of a hair-follicle (Fig. 225). These glands are extremely numerous and found in all portions of the body, with the exception of the palms of the hands and soles of the feet, and most abundantly in the face. They are formed by a delicate structureless membrane lined by polyhedral epithelium. The sebum is not produced by an act of true secretion, but is Fig. 225. — Large Sebaceous Gland, i. Hair in its follicle. 2, 3, 4, 5. Lobules of the gland. 6. Ex- cretory duct traversed by the hair. — (Sappey.) 490 TEXT^BOOK OF PHYSIOLOGY. formed by a proliferation and degeneration of the gland epithelium. When first poured on the surface, the sebum is oily and semiliquid in character, but soon hardens and acquires a cheese-like consistence. It serves to lubricate the hair and skin and prevent them from be- coming dry and harsh. The surface of the fetus is generally covered with a thick layer of sebaceous matter, the vernix caseosa, which possibly keeps the skin in a normal condition by protecting it from the effects of the long- continued action of the amniotic fluid in which the fetus is suspended. CHAPTER XIX. THE CENTRAL ORGANS OF THE NERVE SYSTEM AND THEIR NERVES. The central organs of the nerve system are the encephalon and the spinal cord lodged within the cavity of the cranium and the cavity of the spinal or vertebral column respectively. The general shape of these tvs^o portions of the nerve system corresponds with that of the cavities in which they are contained. The encephalon is broad and ovoid, the spinal cord is narrow and elongated. The encephalon is subdivided by deep fissures into four distinct, though closely related portions: viz., (i) the cerebrum, the large ovoid mass, occupying the entire upper part of the cranial cavity; (2) the cerebellum, the wedge-shaped portion placed beneath the posterior part of the cerebrum and lodged within the cerebellar fossae of the cranium; (3) the isthmus of the encephalon, the more or less pyramidal- shaped portion connecting the cerebrum and cerebellum with each other and both with (4) the medulla oblongata. (Fig. 226.) The spinal cord is narrow and cylindric in shape. It occupies the spinal canal as far as the second or third lumbar vertebra. The central nerve system is bilaterally symmetric, consisting of distinct halves united in the median line. The cerebrum is subdivided by a deep fissure, running antero-posteriorly, into two ovoid masses termed cerebral hemispheres; the cerebellum is also partially subdivided into hemispheres; the isthmus likewise presents in the median line a partial division into halves; the medulla oblongata and spinal cord are sub- divided by an anterior or ventral and a posterior or dorsal fissure into halves, a right and a left. The peripheral organs of the nerve system in anatomic and physiologic relation with the central organs are the encephalic and the spinal nerves. The encephalic nerves, twelve in number on each side of the median line, are in relation with the base of the encephalon, and because of the fact that they pass through foramina in the walls of the cranium they are usually termed cranial nerves. The spinal nerves, thirty-one in number on each side, are in re- lation with the spinal cord, and because of the fact that they pass through foramina in the walls of the spinal column they are termed spinal nerves. As both cranial and spinal nerves are ultimately distributed to the structures of the body — i. e., the general periphery — they collectively constitute the peripheral organs of the nerve system. The central organs of the nerve system are supported and protected by three membranes named, in their order from without inward, the dura mater, the arachnoid, and the pia mater. 491 492 TEXT-BOOK OF PHYSIOLOGY. The dura mater is a tough membrane composed of fibrous tissue. It consists of two layers, the outer of which lines the cranial cavity and forms an internal periosteum; the inner layer is closely attached to the outer except at certain regions where it separates and forms supporting structures, such as the falx cerebri, falx cerebelli, ten- torium cerebelli, etc.; at the margin of the foramen magnum the outer layer becomes con- tinuous with the periosteal tissue, while the inner layer invests the cord dojvn to its ulti- mate termination. (Fig. 227.) The arachnoid is a delicate serous mem- brane. The external surface is smooth and well defined and separated from the dura by a narrow space, the subdural space. The inner surface sends inward fine connective- tissue processes which interlace in every direc- tion, constituting the subarachnoid tissue. This tissue is abundant in the cranium, much less so in the spinal canal. The spaces be- tween the connective tissue, taken collectively, constitute the general subarachnoid space. Around the spinal cord this space is well de- fined, and at the base of the encephalon ex- pands to form large cavities known as the cisterna magna, cisterna pontis, etc. The pia mater is a delicate membrane composed of areolar tissue. It closely invests the encephalon and spinal cord, dipping into the various fissures. It is exceedingly vascular and sends small blood-vessels for some distance into the brain and spinal cord. The Encephalo-spinal Fluid. — The gen- eral subarachnoid space, as well as certain cavities within the encephalon, contain a clear transparent fluid, termed the encephalo-spinal. This fluid has an alkaline reaction and a specific gravity of 1.007 o^ 1.008. It is com- posed of water, proteids (proteoses and serum- globulin), and a compound pyrocatechin, capable of reducing copper salts, though not exhibiting any other of the properties of sugar. In many respects this fluid resembles lymph. The subarachnoid space and the general encephalic cavities, termed ventricles, communicate with one another by an opening in the pia mater (the foramen of Magendie) as it passes over the lower part of the fourth ventricle. Fig. 226. — The Central Organs of the Nerve System, f. t. o. Frontal, temporal, and occipital lobes of the cerebrum, c. Cerebellum, p. Pons. mo. Medulla oblongata. ms., ms. The upper and lower limits of the spinal cord. The remaining letters in- dicate the region and num- ber of the spinal nerves. — {Quain, after Bourgery.) THE ENCEPHALO-SPINAL MEMBRANES. 493 It was stated in Chapter VIII that the entire nerve or neuron system can be resolved into a single morphologic unit, the neuron : the histo- logic features and the physiologic properties of the neuron were there also described; the anatomic relation of the neurons constituting the peripheral organs of the nerve system, to the neurons constituting the central organs of the nerve system were also stated and illustrated in part diagrammatically, page 125. From the statements made regarding the functions of the different neurons in their individual and collective capacity the functions of the nerve system will become apparent. The Functions of the Nerve System.— The functions of the nerve system are twofold: (i) It unites and coordinates the organs and tissues of the body in such a manner that they are enabled to cooperate for the accom- plishment of a definite object. (2) It serves to arouse in the individual a con- sciousness of the existence of an external , world, by virtue of the impressions which it makes on his sense organs, and conse- quently to enable him to adjust himself to his environment. By virtue of the coordination, a stim- ulus, if of sufl&cient intensity, applied to one organ or tissue will call forth activity in one or more organs near or remote from the part stimulated. This coordi- nation is accomplished mainly by the spinal cord and the medulla oblongata. All actions which take place in response to a peripheral stimulus and independ- ently of volition are termed reflex ac- tions. The reflex activities connected with digestion, the circulation of the blood, with respiration, excretion, etc., are illustrations of the coordinating capa- bilities of the nerve-centers located in these portions of the central nerve system. Consciousness of the existence of the external world and of the re- lation existing between it and the individual is associated with the physiologic activities of the encephalon, and more particularly of the cerebral hemispheres. This portion of the nerve system is the chief, though perhaps not the sole, organ of the mind, and its main functions are for the most part mental. The function of a part at least of the peripheral nerve system is'to afford a means of communication between the central nerve system and the remaining structures of the body. The nerve-trunks consti- tuting this part may be divided into two groups, as follows : Fig. 227. — The Membranes of THE Spinal Cord. 1. Dura mater. 2. Arachnoid. 3. Poste- rior root of spinal nerve. 4. An- terior root of spinal nerve. 5. Ligamentum dentatum. 6. Linea splendens. — {Morris, after Ellis.) 494 TEXT-BOOK OF PHYSIOLOGY. 1. The first group comprises nerves in connection with the special sense-organs, e. g., eye, ear, nose, tongue, skin, as well as nerves in connection with the general or organic sense-organs, e. g., mu- cous membranes, viscera, etc., which transmit nerve impulses to certain localized areas in the cerebral cortex, where they are translated into conscious sensations. These sensations, both special and general, by their grouping and combinations are the primary elements of intelligence. 2. The second group comprises those nerves which terminate in the muscle apparatus and which transmit nerve impulses, by way of the medulla and spinal cord, from localized areas in the cerebral cortex to the muscles of the face, trunk and extremities, which are in consequence excited to activity. The muscle movements thus become physical expressions of mental states, and if directed in a definite manner to the overcoming of the resistances offered by the external world become capable of modifying it in accordance with the mental states. The first group of nerves, the afferent, especially those connected with the special sense-organs, are excited to activity by impressions made on their peripheral terminations by agencies in the external world, and thus become a means of communication between the physical and the mental worlds. The second group of nerves, the efferent, are excited to activity by those molecular disturbances in their related nerve-cells which accompany volitional efforts, and thus they become a means of com- munication between the mental and the physical worlds. The central nerve system is thus composed of a number of separate though closely related parts, to each of which a separate function has been assigned. In the study of the structure and function of these separate parts it will be found convenient, and conducive to clearness, to consider them in the order of their complexity, beginning with the spinal cord and ending with the cerebrum. THE SPINAL CORD. The spinal cord is the narrow elongated portion of the central nerve system contained within the spinal canal. It is cylindric in shape though presenting an enlargement in both the lower cervical and lower lumbar regions corresponding to the origins of the nerves distributed to the upper and lower extremities. The cord varies in length from 40 to 45 cm., measures 12 mm. in diameter, weighs 42 gms., and extends from the atlas to the second lumbar vertebra, beyond which it is continued as a narrow thread, the filum terminale. (Fig. 228.) It is divided by the anterior and posterior longitudinal fissures into halves, and is therefore bilaterally symmetric. A transverse sec- tion of the cord shows that it is composed of both white and gray mat- ter, the former covering the surface, the latter occupying the center. THE SPINAL CORD. 495 Structure of the Gray Matter.— The gray matter is arranged in the form of two crescents, united in the median line by a trans- verse band or commissure forming a figure resembling the letter H. Though varying in shape in different regions of the cord, the gray matter in all situations presents on either side an anterior or ventral and a posterior or dorsal horn. Between the two horns there is a Superior or Cervical Segment of Spinal Cord. Middle or Dorsal Portion of Cord. Inferior Portion of Cord and Cauda Equina. Fig. 228. — Superior, Middle, and Inferior Portions or Spinai. Cord. j.. Floor of fourth ventricle, ii. Superior cerebellar peduncle. 3. Middle cerebellar peduncle. 4. Inferior cerebellar peduncle. 5. Enlargement at upper extremity of postero-median column. 6. Glosso-pharyngeal nerve. 7. Vagus. 8. Spinal accessory. 9, 9, 9, 9. Ligamentum denticulatum. 10, 10, 10, 10. Posterior roots of spinal nerves. 11, 11, 11, II. Postero-lateral fissure. 12, 12, 12, 12. Ganglia of posterior roots. 13, 13. Anterior roots. 14. Division of united roots into anterior and posterior nerves. 15. Terminal extremity of cord. 16, 16. Filum terminale. 17, 17. Cauda equina. I, VIII. Cervical nerves. I, XII. Dorsal nerves. I, V. Lumbar nerves. I, V. Sacral nerves. — {Sappey.) portion termed the intermediate gray substance. The commissure presents in its center a narrow canal which extends throughout the entire length of the cord. This canal is lined by cylindric epithelium and surrounded by gelatinous material. (Fig. 229.) The anterior horn is short and broad and entirely surrounded by white matter. The posterior horn is narrow and elongated and extends quite up to the surface of the cord, where, it is capped by gelat- 496 TEXT-BOOK OF PHYSIOLOGY. inous matter, the substantia gelatinosa. In the lower cervical and thoracic regions a portion of the intermediate gray substance pro- jects outward and forms the so-called lateral horn. The gray matter fundamentally consists of a framework of fine neuroglia supporting ^ blood-vessels, lymphatics, meduUated and non-medullated nerves, and groups of nerve-cells. The Nerve-cells. — The nerve-cells of the cord are very numerous and pre- sent a variety of shapes and sizes in different regions. They are usually arranged in groups which extend for some distance up and down the cord, forming columns more or less continuous. In the anterior horn two well-marked groups are found, one situated at the anterior and inner angle, known as the antero-median group, the other situated at the posterior and lateral angle and known as the postero-lateral group. In the lower cervical and upper thoracic re- gions, in the region of the lateral horn, another group of cells is found, known as the intermediate group. In the central portion of the horn there is also a central group. The cells of the anterior horns are of large size, nucleated and multipolar. They are the modified descendants of pear-shaped cells, the neuroblasts, which migrated from the medullary tube (see page T05). In the course of their migra- tion they developed dendrites which form an intricate felt-work throughout the anterior horn. One of the processes, the axon, approached the surface of the cord, penetrated it, grew outward, became covered with myelin and neurilemma, and developed into an anterior root- fiber. These nerve-cells, with their dendrites, axons, and terminal branches, form efferent neurons of the first order. The intimate histologic and physiologic relationship existing between the nerve-cell and the axon is revealed by the degenerative changes which arise in the latter when separated from the former. The cell apparently determines the nutrition of the axon and may be regarded as trophic in function. Some of the cells of the D Fig. 229. — Sections through DoTERENT Regions of the Spinal Cord. A. At the level of the sixth cervical nerve. B. At the mid-dorsal region. C. At the center of the lumbar enlargement. D. At the upper part of the conus medullaris. i. Posterior roots. 2. Anterior roots. 3. Posterior fissure. 4. Anterior fissure. 5. Central canal. — {Mor- ris' "Anatomy," ajter Schwalbe.) THE SPINAL CORD. 497 anterior horn send their axons into the immediately surrounding white matter of the same side, after which they divide into two branches, one passing up, the other down, the cord, to re-enter the gray matter at different levels. They are probably associative in function. Other cells send their axons into that portion of the white matter on the same and opposite sides known as Gower's antero-lateral tract. (Fig. 230.) In the posterior horn nerve-cells are also present, though they are not so numerous as in the anterior horn. At the base of the horn and on its inner side there is a well-marked group o.f cells which ex- tends from the seventh or eighth cervical nerves downward to the Donal l/entral Fig. 230. — Scheme or the Structure op the Cord. — {Howell after Lenhossek.) On the right the nerve cells; on the left the entering nerve fibers. Right side: i, Motor j;ells, anterior horn, giving rise to the fibers of the anterior root; 2, tract cells whose axons pass into the white matter of the anterior and lateral columns; 3, commissural cells whose axons pass chiefly through the anterior commissure to reach the anterior columns of the other side; 4, Golgi cells (second type), whose axons do not leave the gray matter; 5, tract cells whose axons pass into the white matter of the posterior column. Left side: i, Enter- ' ing fibers of the posterior root, ending, from within outward, as follows: Clarke's column, posterior horn of opposite side, anterior horn same side (reflex arc), lateral horn of same side, posterior horn of same side; 2, collaterals from fibers in the anterior and lateral columns; 3, collaterals of descending pyramidal fibers ending around motor cells in anterior horn. second or third lumbar nerves, being most prominent in the thoracic region. This column is known as Clarke's vesicular column. From the nerve-cells constituting this column axons pass obliquely outward into that portion of the white matter known as the direct cerebellar tract. Other nerve-cells send their axons into the white matter in the posterior portion of the cord bordering the posterior median fissure. Some of the nerve-cells, their situation and the distribution of their axons are shown in Fig. 230. Classification of Nerve-cells. — The cells of the gray matter may be divided into three riiain groups: viz., intrinsic, efferent, and afferent. 32 498 TEXT-BOOK OF PHYSIOLOGY. The intrinsic cells are associative in function. The axons to which these cells give origin pass more or less horizontally into the white matter, where they divide into two branches, one of which passes upward, the other downward. At various levels they re-enter the gray matter and arborize around other intrinsic cells. The efferent cells, independently of their trophic influence, are also motor in function, inasmuch as the excitation arising in them is transmitted outwardly through their axons to muscles, blood-vessels, glands and viscera, imparting to them motion, either molar or molec- ular. As the efferent fibers in the ventral roots of the spinal nerves are classified (see page 107) in accordance with their physiologic action into motor, vaso-motor, secretor, viscero-motor and pilo-motor nerves, so the nerve-cells of which the nerves are integral parts may be classified physiologically as motor, vaso-motor, secretor, viscero-motor and pilo- motor. Collections or groups of such cells are termed "centers. " The afferent cells are largely sentient or receptive in function, inasmuch as the excitations brought to the spinal cord by the afferent nerves in the dorsal roots from the general periphery are received by them and transmitted by and through their axons to the cortex of the cerebrum, where they are translated into conscious sensations. As the nerve-fibers in the dorsal roots of the spinal nerves are classified, in accordance with the sensations to which they give rise, as sensor, thermal, tactile, etc., so these nerve-cells may be similarly classified according as they transmit their excitations to those specialized areas in the cerebral cortex in which these different sensations arise. Structure of the White Matter. — A transverse section of the cord shows that the white matter completely covers the gray matter except where the posterior horns reach the surface. Anteriorly the white matter of each lateral half is connected by a narrow strip or bridge of white matter, the anterior commissure. Microscopic ex- amination shows that the white matter is composed of vertically dis- posed meduUated nerve-fibers which are devoid of a neurilemma. These fibers are supported partly by a framework of connective tissue, and partly by neuroglia. The white matter of each side of the cord is anatomically divided into an anterior, a lateral, and a pos- terior column by the anterior and posterior roots of the spinal nerves. Classification of the Nerve-fibers. — ^From a study of the em- bryologic development of the white matter and of the degenerative changes which follow its pathologic and experimental destruction, it has been differentiated into a number of specialized tracts which have different origins, destinations, and functions. Some of the more im- portant tracts are shown in Fig. 231. They may be divided, however, into efferent, afferent, and associative fibers. I. The anterior column, comprising that portion between the anterior longitudinal fissure and the anterior roots, has been sub- divided into: (a) The direct pyramidal tract, or column of Tiirck. This tract THE SPINAL CORD. 499 borders the longitudinal fissure and extends from the upper extremity of the cord as far down as the mid-thoracic region. From above downward this tract diminishes in size, for the reason that its fibers or their collaterals cross at successive levels to the opposite side of the cord by way of the anterior commissure to enter the gray matter of the anterior horn. These fibers are the continuations of fibers which take their origin in cells which are located in the cortex of the cerebral hemisphere of the same side. The terminal filaments of these fibers or axons are in physiologic relation either directly or indirectly through intercalated neuron cells with the dendrites of the cornual cells. When divided in any part of their course, these fibers undergo de- scending degeneration. They are therefore efferent neurons and of the second order. Fig. 231. — Transection of the Cervical Spinal Cord Showing Its Chief Sub- divisions. — {After MUls.) (b) The antero-lateral ground bundle or root zone. This tract lies external to the pyramidal tract, surrounds the anterior horn of the gray matter and extends throughout the length of the cord. It is composed of short commissural or associative fibers which come from nerve-cells in the gray matter from the same and opposite sides of the cord. After entering the white matter they divide into two branches, pursue opposite directions, then re-enter the gray matter at higher and lower levels and come into relation with other nerve-cells. 2. The lateral column, comprising that portion between the ventral and dorsal roots, has been divided into : Soo TEXT-BOOK OF PHYSIOLOGY. (a) The antero-lateral tract of Gowers. This tract is somewhat crescentic in shape and situated on the lateral aspect of the cord ex- ternal to the antero-lateral root zone. It extends throughout the entire length of the cord. When divided it undergoes ascending degenera- tion, which would indicate that the axons originate in nerve-cells in the gray matter. This tract is therefore afferent in function. {b) The lateral limiting tract. This tract, which is quite narrow, lies close to the external border of the gray matter. It is composed of fibers which do not degenerate to any considerable extent and are in all probability associative fibers which come from nerve-cells in the gray matter to re-enter at lower and higher levels. It is also believed by some investigators that the anterior portion contains efferent and the posterior portion afferent fibers; for this reason it is frequently termed the mixed lateral tract. (c) The crossed pyramidal tract. This tract occupies the posterior portion of the lateral column, though its exact position varies some- what in different regions of the cord. In the cervical and thoracic regions it is covered by a layer of fibers. In the lumbar region, how- ever, it comes to the surface. From above downward this tract grad- ually diminishes in size, for the reason that its fibers and their col- laterals enter the gray matter at successive levels. The terminal branches of these fibers are in close physiologic relation either directly or indirectly through intercalated neuron cells with the dendrites of the cornual cells. These fibers are the continuations of fibers which take their origin in cells which are located in the cortex of the cerebral hemispheres of the opposite side. When divided in any part of their course, they undergo descending degeneration. They are therefore efferent neurons and of the second order. {d) The direct cerebellar tract, or column of Flechsig. This tract is situated on the surface of the lateral column external to the crossed pyramidal tract. It slightly, increases in size from below upwafd. It is composed of fibers the cells of which are found on the inner side and base of the posterior horn (Clark's vesicular column). From this origin the fibers pass obliquely outward to the surface and then directly upward to terminate, as its name implies, in the cerebellum. Decussation of these fibers takes place in the superior vermiform lobe of the cerebellum. When divided this tract degenerates upward. It is therefore in all probability an afferent tract and of the second order. 3. The posterior column, comprising that portion between the dorsal roots and the posterior longitudinal fissure, has been sub- divided into: (fl) The postero-external tract of Burdach. This tract lies just within the posterior horns. A portion of this tract is composed of ground fibers which, though vertically disposed, have but a short course. They take their origin in cells in the gray matter, and after entering this tract divide into ascending and descending branches, which with their collaterals re-enter the gray matter at different THE SPINAL CORD. 501 levels. Another portion of this tract is made up of nerve-fibers de- rived from the dorsal roots of the spinal nerves, which cross this col- umn toward the median line in an oblique or horizontal direction. The fibers of the upper portion of this tract terminate around the nucleus cuneatus at the medulla oblongata. When divided, these fibers degenerate for but a short distance. The ground fibers are prob- ably associative in function. (b) The postero-internal tract, or column of Goll. This tract is separated from the former by a septum of connective tissue which is most marked above the eleventh thoracic segment. The fibers which compose this tract are long and derived for the most part from the dorsal roots of the spinal nerves of the same side. This is shown by the fact that division of these roots central to the ganglion is followed by ascending degeneration of the column of Goll as far as the nucleus gracilis in the medulla oblongata. Fibers derived from cells in the gray matter are also contained in this column. This tract is largely afferent in function. (c) Lissauer's tract. This tract embraces the tip of the posterior horn and is composed principally of fibers from the dorsal roots of the spinal nerves. After entering the tract the fibers divide into ascend- ing and descending branches, which finally terminate around cells in the posterior horn. In addition to the tracts described in foregoing paragraphs a num- ber of small narrow tracts have been discovered in different regions of the spinal cord the functional significance of which, however, has not been determined. Of these may be mentioned: 1. The antero-lateral tract of Marchi and Lowenthal, situated at the anterior and inner angle of the anterior column, which degenerates downward after removal of one-half of the cerebellum. 2. The comma tract a narrow bundle of fibers situated in the anterior portion of the column of Burdach. When it is divided it degenerates downward. 3. The septo-marginal tract, an oval shaped tract situated along the margin of the posterior longitudinal fissure. 4. The cornu-commissural tract found along the border, of the anterior portion of the posterior column as far forward as the posterior commissure. Both of these tracts are best developed in the lumbo- sacral region. They arise from nerve-cells in the gray matter. They undergo descending degeneration when divided, but not after division of the dorsal roots. The Relation of the Spinal Nerves to the Spinal Cord.— The spinal nerves present near the spinal cord two divisions which from their connection with the anterior or ventral and the posterior or dorsal surfaces are known as the ventral and dorsal roots. The ventral roots are the axons of various groups of nerve-cells situated in the anterior horns of the gray matter. From their origin S02 TEXT-BOOK OF PHYSIOLOGY. these axons pass almost horizontally forward through the anterior column in three distinct bundles. After emerging from the cord they curve downward and backward to join the dorsal roots. The dorsal roots are the centrally directed axons of nerve-cells in the spinal ganglia. After entering the cord they divide into two main groups, a lateral and a mesial. A portion of the lateral group enters the posterior horn directly through the caput cornu; the other portion turns upward and runs through Lissauer's tract and ultimately enters the posterior horn. The mesial group passes into the postero-external column (Burdach), where the fibers divide into descending and as- cending branches. The former constitute the comma tract, the terminal branches of which surround cells in the gray matter; the latter (ascending) cross the column obliquely and enter the postero- internal column (Goll) , in which they pass upward to terminate around the cells of the nucleus gracilis of the same side. As these root fibers pass up and down the cord, collateral branches are given off which enter the gray matter at successive levels and come into physiologic relation with the cells of Clark's vesicular column on the same and opposite sides and with the cells of the anterior horn. The peripherally directed axons of the nerve-cells in the spinal nerve ganglia become associated with the axons of the ventral roots and together they pass as a spinal nerve to peripheral organs. The ventral root axons are distributed to skeletal muscles, blood- vessels, glands and viscera. The dorsal root axons are distributed to skin, mucous membranes and muscles. The classification of the nerve-fibers in the ventral and dorsal roots in accordance with the functions they subserve will be found on pages 107, 108. Though both the efferent and afferent fibers of the spinal nerves are directly connected with nerve-cells in the spinal cord, they are also indirectly connected by efferent arid afferent nerve-tracts with the cerebral cortex. Experimentally, it has been determined that the anterior or ventral roots contain all the efferent fibers, the posterior or dorsal roots all the afferent fibers. The proofs in support of this view are as follows: Stimulation of the ventral roots produces: 1. Tetanic contraction of skeletal muscles. 2. Variations in the degree of the contraction, the tonus, of the muscle walls of the peripheral arteries either in the way of aug- mentation or inhibition. 3. Discharge of secretions from glands. 4. Variations in the degree of the contraction, the tonus, of the muscle walls of certain viscera either in the way of augmentation or inhibition. Division of the ventral roots is followed by: 1. Relaxation of skeletal muscles and loss of movement. 2. Temporary dilatation- and loss of the tonus of blood-vessels. 3. Cessation in the discharge of secretions from glands. Diagram Indicating the Course of the Motor and Sensory Fibers of the Spinal Cord and Medulla. — (Gordinier.) u,, a. Motor cells of the cerebral cotrex. b, b. Arborizations of the fibers of the sensory tract in the cerebral cortex, c. Nucleus of the column of Burdach, showing terminal arborizations of the long sensbry fibers of the cord. d. Nucleus of the column of GoU, showing terminal arborizations of the long sensory fibers of the cord. e. Section of the medulla, showing sensory decussation. /. Section of medulla, showing motor or pyramidal decussation, g, g. Motorial end plates, h. Section through the cervical region of the cord, showing termination in the anterior horn of the motor fibers of the direct pyramidal tract after they have crossed in the anterior commissure; also fiber of crossed pyramidal tract ending about anterior horn cell of same side, i, i. Posterior spinal ganglia, j, k. Sensory fibers of short course. /. Sensory fibers of long course, terminating in medulla, m, m, m. Sensory end organs, n. Section through lumbar cord. PLATE II. THE SPINAL CORD. 503 4. Temporary impairment of the normal activities of the visceral muscles from loss of central nerve control ; the degree of impair- ment depending on the nature of the viscus involved. Peripheral stimulation of the dorsal roots produces: 1. Sensations of touch, temperature, pressure and pain. 2. Reflex excitation of spinal nerve centers in consequence of which there is an increased activity of skeletal muscles, blood-vessels, glands and visceral walls. 3. Reflex inhibition of spinal nerve centers in consequence of which there may be a decrease in the activities of skeletal muscles, blood-vessels, glands and viscera. 4. Sensations of the duration and direction of muscle movements, of the resistance offered and of the position of the body or of its individual parts (muscle sensations). Division of the dorsal roots is followed by : 1. Loss of sensation in all parts to which they are distributed. 2. Loss of the power of exciting or inhibiting reflexly the activities of spinal nerve centers and in consequence a loss of the power of exciting or inhibiting the activities of peripheral organs. The ventral roots are therefore efferent in function transmitting nerve impulses from the spinal cord to the peripheral organs which excite them to activity. The dorsal roots are afferent in function transmitting nerve impulses from the general periphery to (a) the spinal cord where they excite its contained nerve-centers to activity, and (6) to the cerebrum where they excite its centers to activity with the development of sensations. Segmentation of the Spinal Cord. — ^For the elucidation of many problems connected with the physiologic actions of the spinal cord, as well ,i.s of the symptoms which follow its pathologic impairment, it will be found helpful to consider the cord as consisting physiologicly of a series of segments placed one above the other, the number of segments corresponding to the number of spinal nerves. Each spinal segment would therefore comprise that portion of the cord to which is attached a pair of spinal nerves. The nerve-cells in each segment are in histologic and physiologic relation with definite areas of the body, embracing muscles, blood-vessels, glands, skin, etc. If the exact distribution of the nerves of any segment were then known, its function could be readily stated. By virtue of this segmentation it becomes possible for each segment to act inde- pendently, or in cooperation with other segments near or remote, with which they are associated by the intrinsic or associative cells and their axons; and the spinal cord itself is enabled to act as a unit. FUNCTIONS OF THE SPINAL CORD. Physiologic investigation has demonstrated that the spinal cord, by virtue of the presence of nerve-cells and nerve-fibers, may be re- garded as composed of: S04 TEXT-BOOK OF PHYSIOLOGY. 1. Independent nerve-centers, each of which has a special function; and — 2. Conducting paths by which these centers are brought into relation with one another and with the cerebrum and its subordinate or underlying parts, e. g., medulla and pons Varolii. The Spinal Cord Segments as Independent Centers. — The efferent cells of the spinal segments are the immediate sources of the nerve energy which excites activity in muscles, blood-vessels, glands. The discharge of their energy may be caused: 1. By variations in the composition of the blood or lymph by which they are surrounded. The activity of the cell thus occasioned is termed automatic or autochthonic (Gad). 2. By the arrival of nerve impulses coming through afferent nerves from the general periphery, skin, mucous membrane, etc. 3. By the arrival of nerve impulses descending the spinal cord from the cerebrum or subordinate structures. The peripheral ac- tivity in the former instance is said to be reflex or peripheral in origin; in the latter instance, direct or cerebral in origin. In this latter instance, also, the muscle movements are due to volitional, the vascular variations and glandular discharges to emotional, forms of cerebral activity. Each segment of the spinal cord may be regarded, therefore, because of its contained nerve-cells: 1. As a center for automatic activity. 2. As a center for the reception of nerve impulses arising either at the periphery or in the cerebrum, and for their subsequent trans- mission through efferent nerves to various peripheral organs. Automatism. — The growth, the nutrition and multiplication of the cells of various tissues, and their continuous and rhythmic activity, have been attributed to an automatic action of the spinal nerve-cells. By this expression is meant a discharge of energy from the cells oc- casioned by a change in their environment, i. e., in the chemic com- position of the blood or lymph by which they are surrounded, and in- dependent of any excitation coming through afferent nerves. If the cell activity is continuous, though variable in degree from time to time, it gives rise to what is termed tonus, e. g., trophic tonus, vas- cular, muscle tonus, etc. If the cell activity is intermittent, it imparts to muscles a certain rhythmic activity, e. g., the respiratory movements. As no effect arises without a sufhcient cause, the term automatic has been objected to and the term autochthonic has been suggested (Gad), expressive of the idea that the energy originates in the nerve- cell as a result of a reaction between the cell and its ever-changing environment. A center so acting could not be regarded as primarily a center for reflex action, however much it might be influenced or conditioned secondarily by afferent impulses. Though automatic activity of the spinal cord centers is advocated by some physiologists, the fact must be recognized that with increasing knowledge of reflex THE SPINAL CORD. 505 activities some of the phenomena hitherto regarded as automatic have been found to be reflex in origin. Whether this will eventually be found true for all forms of so-called automatic or autochthonic activity remains to be seen. Trophic Tonus. — The normal metabolism of muscle, gland, and connective tissue which underlies the assimilation of food, the storing of energy, and the production of new compounds, is dependent, in the higher animals at least, on the connection of these tissues with the cen- tral nerve system; for if the efferent nerves be divided, not only will they undergo degeneration in their peripheral portions, but the^muscles, glands, and connective tissues to which they are distributed will also undergo similar changes. This is to be attributed not merely to in- activity, but rather to a loss of nerve influence, inasmuch as inactivity leads merely to atrophy and not to degeneration. It would appear from facts of this character that the normal metabolism is dependent for its continuance on nerve influences. There is no evidence, how- ever, as to the existence of special trophic nerves, separate from those which impart to glands and muscles their customary activities. The trophic centers and the motor centers are identical, though the two modes of their activity are separate and distinct. Vascular Tonus. — The state of moderate contraction of the arterioles throughout the body, in consequence of which the average arterial pressure is maintained, has been attributed to constant activity of the vaso-motor centers, this activity being conditioned by variations in the composition of blood, either an increase in the quantity of carbon dioxid or a decrease in the quantity of oxygen. The vaso- motor centers are regarded as primarily automatic, though capable of being influenced secondarily by reflected excitations from the periphery or direct excitations from the cerebrum. Muscle Tonus. — It is well known that if a muscle be divided in the living animal the two portions will contract and separate them- selves to a certain distance. This indicates that the muscle when in a state of rest is in a slight degree of contraction. This condition of the muscle, to which the term muscle tonus is given, was formerly attributed to an. automatic and continuous discharge of energy from the nerve-cells. Brondgeest, however, showed that this tonus is entirely reflex in origin and immediately disappears on division of the posterior roots of the spinal nerves, which would not be the case if the cells in the cord were acting automatically. The afferent nerves in this reflex arise in the muscle or its tendons, and the stimulus is the slight degree of extension to which the muscle is subjected in virtue of its attachments and the ever-varying position of the limbs and trunk. The tonic contraction of the visceral muscles — e. g., the pyloric, the vesical, the anal sphincters — though regarded as automatic by some, is probably reflex in origin, dependent on the arrival of afferent impulses from the periphery. It is probable that future investiga- tion will disclose the existence and pathway of these afferent fibers. So6 TEXT-BOOK OF PHYSIOLOGY. Reflex Actions. — It has already been stated that the nerve-cells in the spinal cord are capable of receiving and transforming afferent nerve impulses, the result of peripheral stimulation into efferent nerve impulses, which are transmitted outward to muscles, exciting contrac- tion; to glands, provoking secretion; to blood-vessels, changing their caliber; and to organs, inhibiting or accelerating their activity. All such actions taking place through the spinal cord and medulla oblon- gata independently of sensation or volition are termed reflex actions. The mechanism involved in every reflex action consists of at least the following structures (Fig. 232): Fig. 232. — Diagram Showing the Structukes Involved in the Production of Reflex Actions, G. Bachman. r.s. Receptive surface; af.n. cifferent nerve; e.c. emissive or motor cells in the anterior horn of the gray matter of the spinal cord, sp.c; ej.n. efferent nerves distributed to responsive organs, e. g., directly to skeletal muscles, sk.m., and indirectly through the intermediation of sympathetic ganglia, sym. g., to blood- vessels, b.v., and to glands, g. The nerves distributed to viscera are not represented. 1. A receptive surface; e. g., skin, mucous membrane, sense organ, etc. 2. An efferent fiber and cell. 3. An emissive cell, from which arises — 4. An efferent nerve, distributed to a responsive organ, as — 5. Muscle, gland, blood-vessel, etc. In this connection the reflex contractions of skeletal muscles only will be considered. If a stimulus of sufhcient intensity be applied to the receptive sur- face, there will be developed in the terminals of the afferent nerve a series of nerve impulses which will be transmitted by the afferent nerve to, and received by, the dendrites of the emissive cell in the anterior horn of the gray matter. With the reception of these impulses there THE SPINAL CORD. 507 will be a disturbance in the equilibrium of the molecules of the cell, a liberation of energy and a transmission of nerve impulses outward through the efferent nerve to the muscle. A reflex mechanism or iarc of this simplicity would subserve but a simple movement. The majority of the reflexes, however, are extremely complex and involve the cooperation and coordination of a number of centers at different levels of the spinal cord and medulla, on the same and opposite sides, and of muscles situated at distances more or less remote from one another. The transference of nerve impulses com- ing from a localized area of a receptive surface, to emissive cells situated at different levels is accomplished by the intermediation of a third neuron situated in the gray matter which is in connection, on the one hand with the central terminals of the afferent nerve, and, on the other hand through collateral branches with the dendrites of the efferent neurons situated at different levels. (Fig. 233.) A histologic and physiologic mechanism of this character readily explains how a localized stimula- tion can give rise to reflex actions ex- tremely complex in character. The reflex contractions of skeletal muscles are best studied after division of the central nerve system at the upper limit of the spinal cord. After this pro- cedure the spinal centers can act inde- pendently of, and uninfluenced by either sensation or volitional efforts on the part of the animal. Though it is possible to provoke reflex contractions under such circumstances in warm-blooded animals, they are, as a rule, incomplete and of short duration, owing to disturbances of the circulation and respiration and the consequent loss of tissue irritability. In frogs and in cold-blooded animals generally, the spinal cord retains its irritability for a long period of time after removal of the brain, and therefore is well adapted for the study of reflex actions. The separation of the spinal cord from the brain is readily effected by destroying the medulla oblongata. This can be done by inserting a pin through the skin, the occipito-atlantal membrane covering the space between the occipital bone and the atlas, until it strikes the bodies of the vertebras below. If the pin is properly directed it passes through the medulla. Care should be taken to avoid injury to the blood- vessels on either side. The brain itself should then be destroyed, so as to remove all consciousness, by inserting the pin into the brain Fig. 233. — Diagram Showing THE Relation of the Third Neuron a, to the Afferent Neuron i, and to the Efferent Neurons c, c, c. — {Ajter KolUker.) 5o8 TEXT-BOOK OF PHYSIOLOGY. cavity through the foramen magnum, and giving it a few rotatory movements. A frog so prepared, and placed on the table and allowed to remain at rest for a few moments until the shock of the operation passes away, will draw the limbs close to the body and assume a position not unlike that of a normal frog. If then the posterior limbs be extended, they will imnjediately be drawn close to the side of the trunk in the usual flexed position. If the toes are pinched with forceps, the foot will execute a series of movements as if it were trying to free itself from the source of irritation. If the frog be suspended, the limbs, through the force of gravity, will be gradually extended and hang down freely. In this, as in the sitting position, the animal will remain perfectly quiet and will not exhibit spontaneous movements. Any stimulus applied to the skin, however, provided it is of sufficient intensity, will be followed by a more or less pronounced movement. Mechanic, chemic and electric stimuli applied to any part of the skin will call forth the characteristic reflex movements. Chemic stimuli such as weak solutions of sulphuric or acetic acid placed on the toes will be followed by feeble flexion of the corresponding leg, to be succeeded in a short time by extension. Stronger solutions will produce more extensive and vigorous move- ments, the foot at the same time being rubbed against the thigh, apparently for the purpose of freeing it from the irritant. Similar phenomena follow the application of the acid to the fingers or the trunk. As a rule, the extent and complexity of the movement is within limits proportional to the strength of the stimulus. By limiting the sphere of action of the stimulus to definite but different areas of the skin a great variety of movements, more or less complex and coordin- ated and apparently purposive and defensive in character, can be produced. The coordinated and purposive character of the move- ments exhibited by a brainless frog led Pfliiger to the assumption that the spinal cord in this as well as in other cold-blooded animals is pos- sessed of sensorial functions, and endowed with rudimentary con- sciousness. This view, however, is not generally accepted, the movement being attributed to specialized mechanisms, in the cord, partially inherited, which permit of one and the same movement with mechanic regularity and precision, so long as the conditions of the experiment remain the same. In warm-blooded animals similar results may be obtained for a short time after division of the cord, especially if artificial respiration is maintained and the circulation of the blood continued. The cord will then retain its irritability for some time. If the conditions of experimentation were favorable, it is highly probable that the human spinal cord would execute similar movements. Thus it was observed by Robin in a man who had been decapitated that reflex muscle con- tractions could be elicited by stimulating the skin after the lapse of an hour after execution. "While the right arm was lying extended by THE SPINAL CORD. 509 the side, with the hand about 25 centimeters distant from the upper part of the thigh, I scratched with the point of a scalpel the skin of the chest at the areola of the nipple, for a space of 10 or 11 centimeters in extent, without making any pressure on the subjacent muscles. We immediately saw a rapid and successive contraction of the great pectoral muscle, the biceps, probably the brachialis anticus, and lastly the muscles covering the internal condyle. The result was a movement by which the whole arm was made to approach the trunk; with rotation inward and half-flexion of the forearm upon the arm; a true defensive movement, which brought the hand toward the chest as far as the pit of the stomach. Neither the thumb, which was partially bent toward the palm of the hand, nor the fingers, which were half bent over the thumb, presented any movements. The arm being replaced in its former position, we saw it again execute a similar movement on scratching the skin, in the same manner as before, a little below the clavicle. This experiment succeeded four times, but each time the movement was less extensive; and at last scratching the skin over the chest produced only contractions in the great pectoral muscle which hardly stirred the limb" (Dalton). Laws of Reflex Action (Pfluger). 1. Law of Unilaterality. — If a feeble irritation be applied to one or more sensory nerves, movement takes place usually on one side only, and that the same side as the irritation. 2. Law of Symmetry. — If the irritation becomes sufficiently intense, motor reaction is manifested, in addition, in corresponding muscles of the opposite side of the body. 3. Law of Intensity. — Reflex movements are usually more intense on the side of irritation; at times the movements of the opposite side equal them in intensity; but they are usually less pronounced. 4. Law of Radiation. — If the excitation still continues to increase, it is propagated upward, and motor reaction takes place through centrifugal nerves coming from segments of the cord higher up. 5. Law of Generalization. — When the irritation becomes very intense, it is propagated in the medulla oblongata; motor reaction then becomes general, and it is propagated up and down the cord, so that all the muscles of the body are thrown into action, the medulla oblongata acting as a focus whence radiate all reflex movements. Special Reflex Movements. — Among the reflexes connected with the more superficial portions of the body there are some which are so frequently either increased or diminished in pathologic con- ditions of the spinal cord that their study affords valuable indications as to the seat and character of the lesions. They may be divided into : 1. The skin or superficial, and 2. The tendon or deep reflexes. 3. The organ reflexes. The skin reflexes, characterized by contraction of underlying mus- Sio TEXT-BOOK OF PHYSIOLOGY. cles, are induced by stimulation of the skin — e. g., pricking, pinching, scratching, etc. The following are the principal skin reflexes: 1. Plantar reflexe, consisting of contraction of the muscles of the foot, induced by stimulation of the sole of the foot; it involves the integrity of the reflex arc through the lower end of the cord. 2. Gluteal reflex, consisting of contraction of the glutei muscles when the skin over the buttock is stimulated; it takes place through the segments giving origin to the fourth and fifth lumbar nerves. 3. Cremasteric reflex, consisting of a contraction of the cremaster muscle and a retraction of the testicle toward the abdominal ring when the skin on the inner side of the thigh is stimulated; it depends upon the integrity of the segments giving origin to the first and second lumbar nerves. 4. Abdominal reflex, consisting of a contraction of the abdominal muscles when the skin upon the side of the abdomen is gently scratched; its production requires the integrity of the spinal segments from the eighth to the twelfth dorsal nerves. 5. Epigastric reflex, consisting of a slight muscular contraction in the neighborhood of the epigastrium when the skin between the fourth aiid sixth ribs is stimulated; it requires the integrity of the cord between the fourth and seventh dorsal nerves. -6. Scapular reflex consisting of a contraction of the scapular muscles when the skin between the scapulae is stimulated; it depends upon the integrity of the cord between the fifth cervical and third dorsal nerves; The skin or superficial reflexes, though variable, are generally present in health. They are increased or exaggerated when the gray matter of the cord is abnormally excited, as in tetanus, strychnin- poisoning, and disease of the lateral columns. The so-called "tendon reflexes," characterized by the contraction of a muscle, are also of much value in the diagnosis of lesions of the cord and are elicited by a sharp tap on a given tendon. The term, tendon reflex, is, however, somewhat inaccurate. The fundamental condition for the production of the tendon reflex is the normal tone of the muscle, which is a true reflex, maintained by afferent nerve impulses developed in the muscle itself in consequence of its extension and hence compression of the end-organs of the afferent nerves, the muscle spindles. When the muscle is passively extended, as it is when the reflex is to be elicited, there is an exaltation of the tonus and an increase in the irritability. To this condition of the muscle due to passive tension, the term myotatic irritability has been given. If the muscle extension be now suddenly increased, as it is when the tendon is sharply tapped, the increased compression of the muscle spindles will develop additional afferent impulses which after transmission to the spinal cord will give rise to contraction of the corresponding muscle. The following are the principal forms of the tendon reflexes: THE SPINAL CORD. 511 1. Patellar reflex or knee-jerk, consisting of a contraction of the ex- tensor muscles of the thigh when the ligamentum patellse is struck between the patella and tibia. This reflex is best ob- served when the legs are freely hanging over the edge of a table. The patella reflex is generally present in health, being absent in only 2 per cent.; it is greatly exaggerated in lateral sclerosis, in descending degeneration of the cord; it is absent in locomotor ataxia and in atrophic lesions of the anterior gray cornua. 2. Ankle-jerk or Ankle Reflex. — If the extensor muscles of the leg be placed on the stretch and the tendo Achillis be sharply struck, a quick extension of the foot will take place. 3. Ankle Clonus. — ^This consists of a series of rhythmic reflex con- tractions of the gastrocnemius muscle, varying in frequency from six to ten per second. To elicit this reflex, pressure is made upon the sole of the foot so as to suddenly and energetically flex the foot at the ankle, thus putting the tendo Achillis and the gas- trocnemius muscle upon the stretch. The rhythmic movements thus produced continue so long as the tension within limits is maintained. Ankle clonus is never present in health, but is very marked in lateral sclerosis of the cord. The toe reflex, peroneal reflex, and wrist reflex are also present in sclerosis of the lateral columns and in the late rigidity of hemiplegia. The organ reflexes, e. g., the activities of the genito-urinary organs, the stomach, intestines, gall-bladder, etc., and which are induced by peripheral stimulation have been considered in connection with the physiologic action of these organs. The genito-urinary center is located in the lumbar region of the spinal cord. In diseased conditions of this region the genito-urinary reflexes are sometimes increased, at other times decreased or even abolished. Reflex Irritability. — The general irritability or quickness of response of the mechanism involved in reflex action can be approxi- mately determined by observation of the length of time that elapses between the application of a minimal stimulus and the appearance of the muscle response. The method of Tiirck is sufficiently accurate for general purposes. This consists ,in suspending a frog, after removal of the brain, and immersing the foot in a 0.2 per cent, solu- tion of sulphuric acid. The time is determined by means of a metro- nome beating one hundred times a minute.' Stimulation of the skin can also be effected by the induced electric current, as suggested by Gaskell. A single shock is, however, ineffective. When the shocks follow each other with sufficient rapidity, they give rise to a summa- tion of effects in the nerve-centers which will soon be followed by a muscle response. It is highly probable that the chemic stimulation gives rise to a similar summation of effects. The period of time thus obtained is distributed over the entire mechanism. The true reflex time, however — i. e., the time occupied in the passage of the nerve impulses across the spinal mechanism — 512 TEXT-BOOK OF PHYSIOLOGY. is shorter and is obtained by subtracting from the whole period the time occupied by the passage of the impulses through the affer- ent and efferent nerves as well as the latent period of muscle con- traction. This corrected period, the true reflex time, has been found to be twelve times longer than the time occupied by the passage of the nerve impulse through the nerves, including the latent period of the muscle. The reflex irritability is increased by: 1. Separation of the Brain from the Cord. — This is at once followed by an increase in reflex irritability, and is taken as evidence that the brain normally exerts an inhibitor influence over the reflex centers of the cord. The same increase is observed upon hemi- section of the cord, though the increase is limited to the same side. 2. The Toxic Action of Drugs. — Strychnin even in small doses in- creases the irritability to such an extent that a minimal stimulus is sufficient to call forth spasmodic contractions of all the skeletal muscles. Under its influence the usual coordinated reflexes disappear and are succeeded by incoordinated reflexes. The explanation of this fact is believed to be a diminution in the resistance offered by the cord to the passage of the afferent im- pulses rather than to a direct stimulation of the efferent cells. So mu^ is this resistance decreased that the nerve impulses, instead of being confined to their accustomed paths, are radiated in all directions. Absolute repose of the animal and the exclu- sion of all external stimuli greatly diminish the tendency to the occurrence of spasms. 3. Degeneration of the Pyramidal Tracts. — In primary lateral scle- rosis, a pathologic condition characterized primarily by a degen- eration of the terminal filaments of the pyramidal tract fibers, the reflex activity of the cord becomes exalted. As the disease progresses the irritability increases to such an extent that violent spasmodic contractions of the arms and legs arise when the skin or tendons are mechanically stimulated. The explanation offered is practically the same as in division of the cord: viz., withdrawal of the inhibitor and controlling influence of the brain. The reflex excitability may be decreased by: I. Stimulation of Certain Regions of the Brain. — It was discovered by Setchenow that when the frog brain is divided just anterior to the optic lobes and the reflex time subsequently determined according to the method of Tiirck, that the time can be consider- ably lengthened by stimulation of the optic lobes. This is read- ily accomplished by placing small crystals of sodium chlorid on the optic lobes. It was concluded from this fact that these lobes contain centers which exert an inhibitor influence over centers in the spinal cord through descending nerve-fibers. This THE SPINAL CORD. 5^-3 conclusion is strengthened by the fact that division of the brain just behind the optic lobes causes a temporary inhibition of the reflexes in consequence of a mechanical irritation of these fibers. It is quite probable that the volitional inhibition of certain reflexes is accomplished through the intermediation of this center localized by Setchenow. (Fig. 234.) 2. Simulation of Sensor Nerves. — If during the application of a stimulus sufficient to call forth a characteristic reaction in a definite period of time, a sensor nerve in a distant region of the body be simultaneously stimu- lated, it will be found that the reflex time will be lengthened or ' the reaction completely in- hibited. 3. Lesions of the spinal cord; e. g., atrophy of the multipolar cells of the anterior horns of the gray matter; degeneration of the terminals of the dorsal root fibers. 4. The toxic action of various drugs — e. ^., chloroform, chloral — which are believed to exert a depressing action on the nerve- cells themselves. The Spinal Cord as a Con- ductor. — The white matter of the spinal cord consists of nerve-fibers the specific function of which is 1. To conduct nerve impulses from one segment of the cord to another. 2. To conduct nerve impulses com- ing to the cord through afferent nerves, directly or indirectly to various areas of the encephalon . 3. To conduct nerve impulses from the encephalon to the spinal cord segments. Intersegmental or Associative Conduction. — The spinal cord consists of a series of physiologic segments each of which has specific functions and is associated through its related spinal nerve with a definite segment of the body. For the harmonious cooperation and coordination of all the spinal segments it is essential that they should be united by commissural or associative fibers. This is, in fact, accomplished by the axons of the intrinsic cells of the gray matter, which constitute such a large part of the antero-lateral and posterior 33 med ok Fig. 234. — Diagram of the Brain of THE Frog. »//. n. olfactory nerves; ol}.l. olfactory lobes ; o.h. cerebral hemispheres ; op. thl. optic thalamus; op. I. optic lobes; c. cerebellum; med. of. medulla oblon- gata; IV. V. fourth ventricle. 514 TEXT-BOOK OF PHYSIOLOGY. root zones. In consequence of this association, the cord becomes ca- pable of complex coordinated and purposive reflex actions. Spino-encephalic or Sensor Conduction. — The nerve impulses that arise in consequence of impressions made on the terminals of the nerves in the cutaneous and mucous surfaces, in the viscera and in the muscles, are transmitted through the dorsal roots of the spinal nerves to the cord. When transmitted through the cord to the cere- bral hemispheres directly or indirectly, they are received by specialized nerve-cells in the cortex and translated into conscious sensations. The sensations thus arising may be divided into special and general sensations. Of the former may be mentioned pain, touch, pressure, temperature; of the latter may be mentioned hunger, thirst, fatigue, well-being, etc. The pathways through the spinal cord that conduct these afferent impulses to the brain are ill defined and imperfectly known, and only for a few sensations can it be said that their pathways have been determined. The reason for this obscurity lies partly in the difficulties of experimentation, partly in the difficulties of interpretation. Clinical observations are for special reasons more or less untrustworthy. Section of one lateral half of the cord, or a lesion involving the one lateral half, as a rule abolishes all forms of cutaneous sensibility on the opposite side below the injury. This would seem to prove that the nerve impulses cross the median line of the cord immediately or very shortly after entering. At the same time, muscle sensibility is abolished on the corresponding side below the injury. This would seem to prove that the fibers of the posterior roots which enter and cross the column of Burdach and ascend in the column of Goll are derived mainly from the muscles. It is, however, believed by some investigators that those fibers which subserve the sense of touch do not decussate at once, but ascend in the column of Goll as far as the medulla oblongata, where they, in common with the fibers coming from the muscles, arborize around the nerve-cells in the gracile and cuneate nuclei. The afferent path is then continued by new nerve- fibers which emerge from these cells, and which, after crossing the median plane and decussating with the fibers coming from the oppo- site side, join the afferent path from the spinal cord. These fibers are known as the internal arcuate fibers and assist in the formation of the lemniscus or fillet. (Fig. 235.) The sensor pathway decus- sates in part at different levels of the spinal cord and in part at the level of the gracile and cuneate nuclei. The former is often termed the lower, the latter the upper sensor decussation. The pathways for the impulses that give rise to the different sen- sation have been variously located by different observers, e. g., in the gray matter, in the limiting layer, and in the antero-lateral tract of Gowers; the pathway for the impulses that give rise to temperature sensations has been located in the gray matter; the pathway for tactile impressions has been located in the posterior columns, though THE SPINAL CORD. SIS this is not beyond dispute. The pathway for pain sensations has been located in Gowers' tract. Encephalo-spinal or Motor Conduction. — At birth the child is capable of performing all the functions of organic life, such as sucking, swallowing, breathing, etc. It is, however, deficient in psychic activity and in volitional control of its muscles. Its movements are therefore largely, if not entirely, reflex in character. Fig. 235. — Diagram of the Sensor PATHWAYfe in the Spinal Cord Augmented ABOVE BY Fibers of the Sensor Cranial Nerves and Nerves of Special Sense. V. The trifacial nerve. VIII. The vestibular branch of the acoustic nerve. IX. The glosso-pharyngeal nerve. X. The pneumogastric nerve. — {Van Gehuchten.) Embryologic and histologic examination of the spinal cord and medulla show that so far as their mechanisms for independent phys- iologic activities are concerned both are fully developed. Similar investigations of the cerebral hemispheres and of the nerve-fibers which bring their nerve-cells into relation with the spinal segments show that the cells of the cortex are not only immature, but that their descending axons are incompletely invested with myelin. With the Si6 TEXT-BOOK OF PHYSIOLOGY. growth of the child, psychic life unfolds and volitional control of mus- cles is acquired. CoincidentLy the cells of the cerebral cortex grow and develop and the fibers become covered with myelin. The nerve-fibers which have their origin in the cells of the cerebral cortex, and which terminate in tufts around the cells in the anterior horns of the gray matter of the spinal segments, are to be regarded as long commissural tracts uniting and associating these two portions of the central nerve system. Experimental investigations and observations of pathologic lesions accord with the view that physiologically these fibers are efferent pathways for the transmission of motor or volitional impulses from the cortex to the spinal segments. The nerve-cells in which the motor impulses originate are located for the most part, as will be fully stated later, in the central portion of the cortex of the cerebral hemi- spheres in the neighborhood of the central or Rolandic fissure. The axons of these cells from each hemisphere descend through the corona radiata to and through the internal capsule, along the inferior sur^ face of the crura cerebri, behind the pons to the medulla, of which they constitute the anterior pyramids. (Fig. 236.) At this point the pyramidal tract* of each side divides into two portions, viz. : 1. A large portion, containing from 80 to 90 per cent, of the fibers, which decussates at the lower border of the medulla and passes downward in the posterior part of the lateral column of the opposite side, constituting the crossed pyramidal trad; as it descends it gradually diminishes in size as its fibers or their collaterals enter the gray matter of each successive segment. 2. A small portion, containing from 20 to 10 per cent, of the fibers, which does not decussate at the medulla but passes downward on the inner side of the anterior column of the same side, consti- tuting the direct pyramidal tract or column of Tiirck. This tract can be traced down, as a rule, only as far as the mid-dorsal region. As it descends it becomes smaller as its fibers cross the anterior commissure to enter the gray matter of the opposite side. Thus all the fibers of the pyramidal tract from each cere- bral hemisphere eventually are brought into relation with the cells of the gray matter of the opposite side of the cord. That the pyramidal tracts are the conductors of volitional impulses throughout the length of the cord to its various segments has been made evident by the results of section, electric stimulation, and disease. Division of the anterior and lateral columns of one side of the cord in any part of its extent is invariably followed by a loss of motion or paral- * From the fact that the region included between the origin of these fibers and the internal capsule presents somewhat the form of a pyramid with four sides, Charcot designated it the pyramidal region and the fibers composing it the pyram- idal tract. The base of the pyramid includes the cortex of the convolutions around the Rolandic fissure. The summit of the pyramid is truncated and covers the pyramidal region of the internal capsule. THE SPINAL CORD. 517 ysis of the muscles below the section, while electric stimulation of the peripheral end of the isolated crossed pyramidal tract is followed by marked characteristic movements of the muscles. Similar results follow division of the pyramidal tract in any part of its course from Fig. 236. — Diagram ofIthe Pyramidal Tract or Motor Path. III. Common oculo-motor nerve. IV. Pathetic nerve. V. Motor division of the trigeminal nerve. VI. The abducens nerve. VII. Facial nerve. IX. and X. Motor divisions of the glosso- pharyngeal and pneumogastric nerves. XI. Spinal accessory nerve. XII. Hypoglossal nerve. — (Van Gehuchten.) the ■ cerebral cortex downward. Electric stimulation of the cortical cells which give origin to the pyramidal tract is also followed by con- traction of the muscles- of the opposite side, while their destruction is attended by paralysis of the same muscles. As the nutrition of the Si8 TEXT-BOOK OF PHYSIOLOGY. fibers is governed by the cells, it follows that when the axon is separated from its cell it degenerates. It has been found that a lesion of the py- ramidal tract in any part of its course is followed by descending degen- eration, which is taken in evidence that it conducts nerve impulses from above downward. Thus experimental investigation and path- ologic observation are in accord in the view that physiologically these nerve-fibers are the pathways for the transmission of motor or volitional impulses from the encephalon to the spinal cord. The relation of the motor and sensor pathways to each other in the spinal cord and brain are shown in Plate II. The afferent fibers which decussate at various levels through the spinal cord are not repre- sented. CHAPTER XX. THE MEDULLA OBLONGATA; THE ISTHMUS OF THE EN- CEPHALON.; THE BASAL GANGLIA. THE MEDULLA OBLONGATA. The medulla oblongata is that portion of the central nerve system immediately superior to and continuous with the spinal cord. It has the shape of a truncated cone, the base of which is directed upward, the truncated apex downward. It is 38 mm. in length, 18 mm. in breadth, and 12 mm. in thickness. By the continuation upward of the anterior and posterior median fissures, the medulla is divided into symmetric halves (Figs. 237 and 238). Like the cord, of which it is a continuation, it is composed of white matter externally and gray matter internally. Structure of the Gray Matter. — The gray matter of the medulla is continuous with that of the cord, though owing to the shifting of position of the different tracts of the white matter it is arranged with much less regularity. The appearance which the gray matter presents on transverse section varies also at different levels. At the level of the first cervical nerve the posterior horns are narrow, elongated, and directed outward. The lateral horns are well devel- oped and present a collection of cells near their bases which can be traced forward and backward for some distance. At the level of the .decussation of the pyramidal tracts the head of the anterior horn be- comes detached from the rest of the gray matter and is pushed backward toward the posterior horn; the bases of the anterior horns become spread out to form a layer of gray matter near the dorsal aspect of the medulla. Transverse sections of the medulla at all levels show a more or less ex- tensive network of nerve-fibers known as the reticular formation. In its meshes are found collections of nerve-cells of varying size. To- ward the dorsal aspect of the medulla special groups of cells are found from which axons arise to become the fibers of various efferent cranial nerves, e. g., the hypoglossal, the efferent fibers of the vagus, and glosso- pharyngeal. Structure of the White Matter. — The white matter is composed of nerve-fibers supported by connective tissue and neuroglia. It is subdivided on either side by grooves into three main columns: viz., an anterior column or pyramid, a lateral column, and a posterior column. The anterior column or pyramid is composed partly of fibers con- tinuous with those of the anterior column of the spinal cord (the direct pyramidal tract), and partly of fibers continuous with those of the lateral column of the cord of the opposite side (the crossed pyram- 519 520 TEXT-BOOK OF PHYSIOLOGY. Fig. 237. — Anterior or Ventral View oe the Medulla Oblongata AND Isthmus. ±. Infundibulum. 2. Tuber cinereum. 3. Corpora albi- cantia. 4. Cerebral peduncle. 5. Tuber annulare. 6. Origin of the middle peduncle of the cerebellum. 7. Anterior pyramids of the medulla oblongata. 8. Decussation of the an- terior p3Tamids. 9. Olivary bodies. 10. Restiform bodies. 11. Arciform fibers. 12. Upper extremity of the spinal cord. 13. Ligamentum dentic- ulatum. 14, 14. Dura mater of the cord. 15. Optic tracts. 16. Chiasm of the optic nerves. 17. Motor oculi communis. 18. Patheti- cus. 19. Fifth nerve. 20. Motor oculi externus. 21. Facial nerve. 22. Auditory nerve. 23. Nerve of Wris- b e r g. 24. Glosso-pharyngeal nerve 25. Pneumogastric. 26, 26. Spinal accessory. 27. Sublingual nerve. 28, 29, 30. Cervical nerves. — {Sappey.) Fig. 238. — Posterior or Dorsal View OF THE Medulla Oblongata, Isthmus, AND Basal Ganglia, i. Corpora quad- rigemina. ^. Corpus quadrigeminum an- terior (pregeminum). 3. Corpus quadri- geminum posterior (post-geminum). 4. Tract of fibers (brachium) passing to the corpus geniculatum externiun. 5. Tract of fibers (brachium) passing to 6, the corpus geniculatum internum. 7. Posterior com- missure. 8. Pineal gland. 9. Superior cere- bellar peduncle. 10, 11, 12. The valve of Vieussens. 13. The pathetic nerve. 14. Lateral groove of the isthmus. 15. Triangu- lar bundle of the isthmus. 16. Superior cerebellar peduncle. 17. Middle cerebellar peduncle. 18. Inferior cerebellar peduncle. 19. Antero-inferior wall of the fourth ventri- cle. 20. Acoustic nerve. 21. Spinal cord. 22. The postero-median column. 23. The posterior pyramids. — {Sappey.) ISTHMUS OF THE ENCEPHALON. 521 idal tract), which decussate at the anterior portion of the medulla. The united fibers can be traced upward to the pons, where they dis- appear from view. The lateral column is composed of fibers continuous with those of the lateral column of the cord. As the fibers pass upward, how- ever, they diverge in several directions. The fibers of the crossed pyramidal tract cross the median line, as previously stated, to enter into the formation of the anterior column; the fibers of the direct cerebellar tract gradually curve backward, and in so doing unite with other iibers to form the restiform body, after which they enter the cerebellum by way of the inferior peduncle. Situated between the anterior pyramid and the restiform body is a small oval mass, the olivary body, composed of both white and gray matter. The posterior column is composed largely of fibers continuous with those of the posterior column of the cord. The subdivision of this column into a postero-external (Burdach) and a postero-internal (Goll) is more marked in the medulla than in the cord. The former is here known as the funiculus cuneatus, the latter as the funiculus gracilis. These two ^strands of fibers are apparently continued into the restiform body. Owing to the divergence of the restiform bodies a V-shaped space is formed, the floor of which is covered with epithe- lium resting on the ependyma. At the upper extremity of the funicu- us cuneatus and funiculus gracilis, two collections of gray matter are found, known respectively as the nucleus cuneatus and nucleus gracilis. Around the cells of these nuclei many of the fibers of the posterior column end in brush-like expansions. The Fillet or Lemniscus. — ^From the ventral surface of the cu- neate and gracile nuclei axons emerge which pass forward and upward through the gray matter and decussate with corresponding fibers com- ing from the opposite nuclei. They then assume a- position just pos- terior to the pyramids and between the olivary bodies. These fibers thus form a new distinct tract, termed the fillet or lemniscus. As this tract ascends toward the brain it receives additional axons from the sensory end-nuclei of all the afferent cranial nerves of the opposite side with the exception of the auditory. From the end-nuclei of the auditory nerve new axons ascend as a distinct tract situated near the lateral aspect of the pons. From their position these two separate tracts have been termed the mesial and lateral fillets respectively. Before proceeding to a consideration of the functions of the medulla oblongata it will be found conducive to clearness to sketch the salient anatomic features of the parts anterior to it and their relations one to another. THE ISTHMUS OF THE ENCEPHALON. The isthmus of the encephalon comprises that portion of the central nerve system connecting the cerebrum above, the cerebellum behind, and the medulla below. Its ventral surface presents below 522 TEXT-BOOK OF PHYSIOLOGY. an enlargement, convex from side to side, the pons Varolii. On each side the fibers of which the pons consists converge to form a com- pact bundle, the middle peduncle, which enters the corresponding half of the cerebellum. Above the pons, this surface presents two large columns of white matter which, diverging somewhat from below up- ward, enter the base of the cerebrum and are known as the crura cerebri. Embracing the crura above are two large bands of white matter, the optic tracts (Fig. 237). The dorsal surface presents below two diverging columns of white matter, the inferior peduncles; above, two converging columns, the superior peduncles of the cerebellum (Fig. 238). At the extreme upper part of this surface there are four small grayish eminences, the corpora quadrigemina. From the disposition of the white matter on the dorsal surface of the isthmus and medulla, there is formed a lozenge-shaped space, the fourth ventricle. The space is merely an expansion of the central cavity of the cord, the result of the changed relations of the white and gray matter in this region of the central nerve system. Above, this ventricle communicates by a narrow canal, the aqueduct of Sylvius, with the third ven- tricle. The floor of the fourth ventricle is covered with a layer of epithelium resting on the ependyma continuous with that lining the central canal of the cord. Beneath this is a layer of gray matter. The pons Varolii comprises in a general way that portion of the central nerve system situated between the me- dulla oblongata and the crura cerebri. The ventral surface is convex from side to side; the lateral surface, owing to the convergence of the fibers of which it is composed, is contracted to form the middle peduncle of the cerebellum; the posterior surface is flat and forms the upper half of the floor of the fourth ven- tricle. The pons consists of white fibers and gray matter supported by connective tissue and neuroglia. Transverse sections of the pons show that it is divided into an anterior or ventral, and a posterior or dorsal portion, the latter being usually termed the tegmentum. The ventral portion consists for the most part of white fibers, ar- ranged longitudinally and transversely (Fig. 239). The longitudinal fibers are largely continuations of the pyramidal tracts, or the fibers composing the anterior pyramid of the medulla. In the lower part of the pons these fibers are compactly arranged, but at higher levels they are separated into a number of bundles by the interlacing of the trans- verse fibers. The transverse fibers are divided into a superficial and a deep set. Among these fibers are groups of nerve-cells which coUec- FiG. 239. — Transection of the Pons through its Middle Por- tion, Showing the Relation OE THE Nerve Tracts of Which it is Composed. D. 1. f. Dorsal longitudinal fasciculus. L.c. and c. Locus ceruleus. L.f. Lateral fillet. ISTHMUS OF THE ENCEPHALON. 523 tively are known as the nucleus pontis. Some of the transverse fibers, especially the superficial ones, are commissural in character — i. e., they connect corresponding parts of the gray matter of the lateral halves of the cerebellum; others coming from the gray matter of the cerebellum cross the median line and terminate around the cells of the nucleus pontis; others again are connected with the gray cells of the same side. Through the intermediation of the nucleus pontis and certain of the longitudinal fibers of the pons, the cerebellum is brought into relation with the cerebrum. The dorsal or tegmental portion consists of: (i) The fillet; (2) the formatio reticularis; (3) the posterior longitudinal bundle; (4) the substantia ferruginosa; (5) groups of nerve-cells from which arise various cranial nerves — e. g., the fifth, sixth, seventh, and eighth. The fillet or lemniscus in this region is divided into a mesial and a lateral portion. The fibers of the mesial portion are partly the axons of the nerve-cells of the gracile and cuneate nuclei of the opposite side of the medulla, and partly of the axons of the sensor nerve-cells of the afferent cranial nerves with the exception of the auditory. The fibers of the lateral portion are mainly the axons of the cells in the floor of the fourth ventricle around which the auditory nerve-fibers end.. They are therefore a continuation of the auditory tract. The formatio reticularis is a continuation of that of the medulla. The posterior longitudinal bundle is triangular in shape and situated behind the formatio reticularis and close to the median line. The fibers composing it are largely derived from the ground fibers of the antero-lateral column of the spinal cord. The superior olive is a cylindric mass of gray matter situated in the pons in the anterior part of the formatio reticularis. It consists of nerve-cells the axons of which pass dorso-laterally, decussate in the median line, and form the lateral fillet of the opposite side. Some few axons go to the lateral fillet of the same side. The substantia ferruginosa is composed mainly of pigmented cells. The groups of nerve-cells lying just beneath the floor of the fourth ventricle give origin to axons of the motor portion of the fifth, the sixth, the seventh cranial nerves. Some of the groups are the sensor end- nuclei of the fifth and eighth cranial nerves. The crura cerebri comprise that portion of the central nerve sys- tem situated between the pons below and the cerebrum above. They are composed of strands of nerve-fibers which are divided, as shown on cross-section, into a ventral and a dorsal portion by a crescentic shaped layer of gray matter, the substantia nigra (Fig. 240). Of the fibers which compose the ventral portion of each crus, the crusta or pes, the larger part is continuous below, through the longitudinal fibers of the pons, with the pyramid of the medulla and the pyramidal tract; above they assist in the formation of the internal capsule. On the inner and on the outer side of each crusta there is a bundle of fibers derived from the frontal, and from the temporal and occipital por- 524 TEXT-BOOK OF PHYSIOLOGY. tions of the cerebrum respectively. These fibers are connected directly with the nuclei pontis and indirectly with the cerebellum of the same and opposite sides. The fibers which compose the dorsal portion, the tegmentum, are continuous with those which pass upward from the medulla and pons, e. g., the fillet, both mesial and lateral, the formatio reticularis, the posterior longitudinal bundle, and, in ad- dition, the fibers of the superior peduncles of the cerebellum. Above, the fibers terminate largely in collections of gray matter at the base of the cere- brum. The aqueduct of Sylvius is a short narrow canal which con- nects the cavity of the fourth with the cavity of the third ventricle. It is lined by the ependyma and surrounded by a layer of gray matter continuous with that forming the floor of the fourth ventricle. In that portion of the gray matter lying beneath or ventral to the aqueduct there are groups of nerve-cells which give origin to axons which unite to form the third and fourth cranial nerves. THE CORPORA QUADRIGEMINA. The corpora quadrigemina are four small grayish eminences situated beneath the posterior border of the corpus callosum and behind the third ventricle. They rest upon the lamina quadrigemina, which forms the roof of the aqueduct of Sylvius. The anterior pair are termed the nates, or the pregemina, the posterior pair the testes, or the post- FiG. 240. — Scheme of Transverse Sec- tion OF THE Cerebral Peduncles. CQ Corpora quadrigemina. Aq. Aqueduct p.l.b. Posterior longitudinal bundle. F Fillet or lemniscus. RN. Red nucleus. SN, Substantia nigra. III. Third nerve. Py. Pyramidal tracts. Fc. Fronto-cerebellar; and TOC, temporo-occipital fibers of the crusta. CC. Caudate-cerebellar fibers in upper part of crusta. — {After Wernicke and Cowers.) From the external surface of each body there pass outward bundles of fibers termed brachia. The fibers which compose the brachium of the pregeminum pass outwaird and enter a small collection of gray matter, the corpus geniculatum externum, and the optic tract. The fibers which compose the brachium of the postgeminum are divided into two bundles, one of which enters a second small collection of gray matter, the corpus geniculatum internum, while the other passes forward beneath this body to enter the internal capsule, beyond which it passes to the cortex of the temporal region of the cerebrum. Though these bodies are closely associated anatomically, they differ in origin, in their relations and in their functions. Microscopic examination of sections of the quadrigeminal bodies BASAL GANGLIA. 525 shows that they are composed of nerve-cells and nerve-fibers, both of which are so intricately arranged that it is difficult to trace their rela- tion one to another and to adjoining structures. Some of the cells of the pregeminum give off axons which course outward and forward, enter the internal capsule, and pass through the optic radiation to the cortex of the occipital region of the cerebrum. Many fibers of the optic tract, axons of the cells of the retina, end in brush-like expansions around these same cells. There is thus formed a connected pathway between the retina and the occipital cortex. The cells of the occipital cortex, however, send axon fibers in the reverse direction through the optic radiation to terminate around the cells of the pregeminum, while axons of pregeminal cells pass for- ward to the retina and to the cells of origin of the third nerve. The cells of the postgeminum give origin to axons which pass up- ward, forward, and outward, enter the internal capsule, and pass by way of the auditory tract to the cortex of the temporo-sphenoidal region of the cerebrum. Many of the fibers of the lateral fillet, a portion of the auditory tract, terminate in brush-like expansions around these same cells. There is thus established a connected path- way between the cochlea and the temporo-sphenoidal cortex. The cells of the temporal cortex, however, send axons in the reverse direc- tion by way of the auditory tract to the cells of the postgeminum. There is thus established a double communication between the occipital and temporal region of the cerebral cortex, and the pregeminal and postgeminal bodies respectively. THE BASAL GANGLIA; THE CORPORA STRIATA AND OPTIC THALAMI. ' The basal ganglia surmount the crura cerebri, but are only made visible by removal of the cerebrum (Fig. 241). The corpora striata are two large ovoid collections of gray and white matter situated at the base of the cerebrum. The larger portion of each body is embedded in the cerebral, white matter, while the smaller portion projects into .the anterior part of the lateral ventricle. A transection of the corpus striatum shows that it is divided by a band of white matter into two portions, viz. : 1. The caudate nucleus, the intra- ventricular portion, convex in shape with its base directed forward, its apex or tail directed backward and downward. 2 . The lenticular nucleus, the extra- ventricular portion, somewhat bicon- vex in shape and embedded largely in the white matter. Each len- ticular nucleus is subdivided by two lamina of white matter into three portions. The two inner, from their pale yellow color, form the globus pallidus, the outer, somewhat darker in color, is the putamen. The Internal Capsule. — The band of white matter separating the caudate from the lenticular nucleus has been termed the internal cap- 526 TEXT-BOOK OF PHYSIOLOGY. sule from the manner in which it embraces the inner surface of the lenticular nucleus. It consists of nerve-fibers which associate histo- logically and physiologically all portions of the cerebral cortex with the optic thalamus, pons, medulla, spinal cord, and cerebellum. The relation of the capsule to the nuclei through which it passes is readily Fig. 241. — Dissection of Brain, from above, Exposing the Lateral Fourth and Fifth Ventricles with the Surroiinding Parts. J. — a. Anterior part, or genu of corpus callosum. b. Corpus striatum, b'. The corpus striatum of left side, dissected so as to expose its gray substance, c. Points by a line to the taenia semicircularis. d. Optic thalamus, e. Anterior pillars of fornix divided; below they are seen descending in front of the third ventricle, and between them is seen part of the anterior commissure; in front , of the letter e is seen the slit-like fifth ventricle, between the two laminae of the septum ucidum. /. Soft or middle commissure; g is placed in the posterior part of the third ventricle; immediately behind the latter are the posterior commissure (just visible) and the pineal gland, the two crura of which extend forward along the inner and upper margins of the optic thalami. h. and i. The corpora quadrigemina. k. Superior crus of cere- bellum. Close to .A is the valve of Vieussens. which has been divided so as to expose the fourth ventricle. I. Hippocampus major and corpus fimbriatum, or tasnia hippocampi. m. Hippocampus minor, n. Eminentia coUateralis. 0. Fourth ventricle, p. Posterior surface of medulla oblongata, r. Section of cerebellum, i. Upper part of left hemisphere of cerebellum exposed by the removal of part of the posterior cerebral lobe. — {Hirschfeld ' and LeveilU.) shown on cross-section (Fig. 242). The appearance which it presents, however, varies considerably at different levels. At a given level it may be said to consist of two segments or limbs, an anterior, situated between the caudate nucleus and the anterior extremity of the lenticu- lar nucleus, and a posterior, situated between the optic thalamus and the posterior extremity of the lenticular nucleus. The two segments BASAL GANGLIA. 527 unite at an obtuse angle, termed the knee, which is directed toward the median line. The optic thalami are two oblong masses of gray matter situated upon the crura cerebri and behind the corpora striata. The anterior and posterior extremities of each thalamus present enlargements known respectively as the anterior tubercle and the posterior tubercle or pul- vinar. The mesial surface of the thalamus forms the lateral wall of the third ventricle and is covered by epithelium resting on a thin layer of ependyma. A transection of the thalamus shows that it is not only covered externally but penetrated by white matter, which- sub- divides its contained gray cells into four more or less distinct masses termed nuclei, viz., an anterior, a lateral, occu- pying the external part of the thalamus, a ventral, close to the entire ventral sur- face, and a posterior, situated beneath the pulvinar. Beneath and somewhat internal to each optic thalamus there is a region, the subthalamic, consisting of an intricate network of nerve-fibers and several nuclei of gray matter, e. g., the red or tegmental nucleus, the subthalamic nucleus, or Luys' body, and the sub- stantia nigra. Though the thalamus has extensive connections with many portions of the central nerve system, the most important are with the cortex, the tegmentum, and the optic tracts. From the cells of these various nuclei axons -emerge which pass into the in- ternal capsule, and through the corona radiata to all portions of the cortex. Those which come from the pulvinar and pass to the occipital lobe constitute a part of the optic radiation; those from the lateral and ventral nuclei ultimately reach the parietal lobe; those from the anterior nucleus pass to the hippocampal and uncinate convolutions. In a similar manner all portions of the cortex are brought into relation with the thalamus, axons from the cortical cells passing downward to terminate in tufts around the thalamic nuclei. The tegmentum is intimately related to the thalamus, though the exact distribution of various strands of fibers is a subject of much dis- cussion. Most of the fibers of the mesial fillet end in tufts around the cells of the ventral and lateral nuclei; other fibers pass directly to the cortex. Fig. 242. — HoEizoNTAi, Sec- tion OP THE I^^TEENAL CAP- SULE SHOWING ITS Relations TO the Caudate Nucleus, Optic Thalamus, and the Lenticular Nucleus. .1. Caudate nucleus. 2. Anterior segment of the internal capsule. 3. External capsule. 4. Len- ticular nucleus. 5. Claustrum. 6. Posterior segment oj internal capsule. 7. Optic thalamus. — {Modified from Landois.) 528 TEXT-BOOK OF PHYSIOLOGY. The optic tract sends fibers directly into the pulvinar, around the cells of which they terminate in brush-like expansions. SUMMARY OF THE STRUCTURE OF THE MEDULLA, ISTHMUS, AND BASAL GANGLIA. Structure of the Central Gray Matter. — Though the general arrangement of the central gray matter has been incidentally alluded to in the foregoing presentation of the anatomic features of the medulla and isthmus, it will be convenient to summarize its arrangement and structure at this point. The gray matter of the cord, of the dorsal aspect of the medulla and pons, of the region surrounding the aqueduct of Sylvius, and of the lining of the third ventricle, constitute practically a continuous system, though presenting modifications in various parts of its extent. In the transition region of the spinal cord and medulla the gray matter of the former becomes much changed in shape owing to the shifting of position of the various tracts of white matter, until in the medulla and pons it is spread out in the form of a thin layer near their dorsal surfaces, where, together with the ependyma, it forms the floor of the fourth ventricle. In the region of the aqueduct of Sylvius the gray matter again converges and ultimately surrounds the canal, to again expand at its anterior extremity, to form the lining of the third ventricle. The Nerve-cells. — The nerve-cells in these different regions do not differ morphologically from those in the gray matter of the spinal cord. The corpus, or body of the cell, presents a number of dendrites as well as the sharply defined axon. As a rule, the cells are arranged in groups, or clusters, or nests, partially surrounded and enclosed by supporting tissue, and situated beneath the floor of the fourth ventricle and the floor of the aqueduct of Sylvius. From some of the cell groups axons pass ventrally through the white matter to merge on the ventral and lateral surfaces of the medulla, pons, and crura, where they are known as efferent or motor cranial nerves. From other groups of cells, axons cross the median line, and after joiniiig the mesial fillet ascend toward the cerebrum. Around these latter cells the terminal filaments of the afferent or sensor cranial nerves arborize. The collection of cells found in the central gray matter may be divided into two groups — efferent and afferent. The efferent cells, like those of the cord independent of a trophic influence, are motor in function, inasmuch as the excitation arising in them is transmitted outward through their related axons to mus- cles, glands, viscera or blood-vessels, imparting to them motion, either molar or molecular. The afferent cells are largely sentient or receptive in function, inasmuch as the excitations brought to them by the afferent cranial nerves from skin and mucous membranes and from sense-organs, MEDULLA AND BASAL GANGLIA. 529 such as the tongue and ear, are received by them and transmitted through their ascending axons to the cortex of the cerebrum, where they are translated into conscious sensations. Structure of the White Matter.— The white matter is com- posed of medullated nerve-fibers, and though arranged in a very complex manner may be divided into longitudinal and transverse fibers. The longitudinal fibers which compose the main portion of the isthmus may be subdivided into (i) a ventral or pedal portion and (2) Fig. 243. — Diagrammatic Arrangement of the Projection Tracts Connecting THE Cerebral Cortex with the Lower Nerve-centers. A. Fronto-cerebellar tract. B. The pyramidal or motor tract. C. Sensory tract. D. Visual tract from optic thalamus (O.T.) to the occipital lobe. E. Central auditory tract. F. Superior cerebellar peduncle. G. Middle cerebellar peduncle. H. Inferior cerebellar peduncle. C.N. Caudate nucleus. C.Q. Corpora quadrigemina. Vt. Fourth ventricle. The numerals refer to cranial nerves. J. Eighth nerve nucleus. — {After Starr.) a dorsal or tegmental portion. The fibers constituting the ventral or pedal portion may for convenience be said to extend from the cere- bral cortex to the pons, medulla, and spinal cord. They may be di- vided into three distinct tracts: e. g., the pyramidal tract, the fronto- cerebellar tract, and the occipito-temporo-cerebellar tract (Fig. 243). The pyramidal tract descends from the cortex of the cerebrum bordering the fissure of Rolando, passes through the posterior one- third of the anterior segment and the anterior two-thirds of the pos- terior segment of the internal capsule, the middle two-fifths of the crusta, behind the transverse fibers of the pons, to become the anterior 53° TEXT-BOOK OF PHYSIOLOGY. pyramids of the medulla, beyond which it divides into the direct and crossed pyramidal tracts of the cord. In its course some of the fibers and their collaterals arborize around efferent cells from the anterior extremity of the aqueduct of Sylvius to the termination of the spinal cord. The fronto-cerebellar tract descends from the cortex of the frontal portion of the anterior lobe, passes through the anterior portion of the anterior segment of the internal capsule, the inner fifth of the crusta to the pons, where its fibers terminate or arborize around the nucleus pontis of the same and opposite sides. The occipito-temporo-cerebellar tract descends from the occipital and temporal lobes, passes to the inner side of the lenticular nucleus, and continues downward on the outer side of the crusta, occupying about one-fifth of its bulk, to the pons, where its fibers also arborize around the nucleus pontis of the same and opposite sides. By means of fibers in the middle peduncle these descending fibers are brought into relation with the cerebellum. The fibers constituting the dorsal or tegmental portion of the longit- udinal system may be said for convenience to extend from the pos- terior portion of the medulla and pons to the optic thalamus and cere- brum. They may be subdivided into several tracts: viz., the fillet, the posterior longitudinal bundle, Gowers' tract, etc. The fillet or lemniscus, consisting of fibers having their origin partly from the cells of the cuneate and gracile nuclei and partly from the cells of the sensor end-nuclei of various sensor cranial nerves, occupies a region in the ventral and mesial portion of the tegmentum throughout its entire extent. Superiorly this mesial fillet divides into two portions, one. of which passes to the thalamus and pregem- inum (anterior corpus quadrigeminum) , the other to the cortex of the parietal and limbic lobes. The fibers coming from the sensor end- nucleus of the auditory nerve (the lateral fillet) lie on the lateral aspect of the pons and crus. Superiorly they terminate in the postgeminum (the posterior corpus quadrigeminum) . The posterior longitudinal bundle, an upward extension of the fibers composing a portion of the ground bundle of the spinal cord, is located on either side of the median line just beneath the floor of the fourth ventricle and the aqueduct of Sylvius. As it passes upward collateral branches are given off, some of which arborize around the cell nuclei of the third, fourth, and sixth cranial nerves of the same side, while others cross the median line and arborize around the corresponding cell nuclei of the opposite side. Superiorly some of the fibers become related to cells in the thalamus and subthalamic region. This bundle of fibers appears to be mainly commissural in character. Gowers' tract, the antero-lateral tract of the spinal cord, occupies a position in the lateral region of the formatio reticularis both in the medulla and pons. Continuing upward, it enters the mesial fillet, and in company with it passes through the. posterior division of the FUNCTIONS Of* THE MEDULLA OBLONGATA. 531 internal capsule and finally terminates around cells in the cortex of the parietal lobe. The transverse fibers of the isthmus are found in the pons. The fibers of the ventral as well as those of the more dorsal regions have their origin in nerve-cells in the cortex of the cerebellum. From their origin they pass through the cerebellar white matter, and through the middle peduncle as far as the median line, where they decussate with fibers coming from the opposite side. Beyond this point they pass to the cerebellar cortex. From their anatomic relations it is prob- able that these transverse fibers are commissural in character,. bring- ing into relation opposite but corresponding regions of the cerebellar cortex. In addition to the commissural fibers other transverse fibers associate the cerebellar cortex with the gray matter in the pons on both the same and opposite sides. In this way the cerebellum is brought into relation ivith longitudinal fibers coming from and going to the cerebrum. FUNCTIONS OF THE MEDULLA OBLONGATA, ISTHMUS, AND BASAL GANGLIA. Microscopic examination of the white and gray matter of these various parts of the central nerve system shows that they are com- posed of nerve-cells and nerve-fibers which morphologically do not differ in essential respects from those found in the spinal cord, though their arrangement is far more complicated and involved. The func- tions of these closely related structures are in consequence equally complex and involved and but imperfectly known. In a general way it may be said that by virtue of the presence of nerve-cells and definite tracts of nerve-fibers these structures col- lectively may be regarded as consisting: 1. Of centers for reflex actions; and — 2. Of conducting paths by which the various parts are brought into relation one with another and with the spinal cord, the cerebel- lum, and the cerebrum. The Medulla Oblongata and Pons. — The gray matter situated in these structures — i. e., just beneath the floor of the fourth ventricle — contains nerve-cells arranged in more or less well-defined groups which may be divided into efferent and afferent. The efferent cells are the immediate sources of nerve impulses which are transmitted through efferent axons to various peripheral organs — muscles, glands, viscera and blood-vessels. Their activity may be ex- cited by the same influences which excite the efferent cells of the spinal cord: e. g., variations in the composition of the blood or lymph; the ar- rival of nerve impulses coming through afferent pathways in the spinal cord and through afferent cranial nerves; the arrival of nerve impulses coming through efferent pathways from the cerebrum. The peripheral activity resulting from their excitation may therefore be automatic or autochthonic, peripheral (reflex) or cerebral (volitional) in origin. 532 TEXT-BOOK OF PHYSIOLOGY. The afferent cells are sentient or receptive in function, inasmuch as they receive nerve energies coming through lower afferent pathways and transmit them through their related axons to the cortex of the cerebrum, where they are translated into conscious sensations. The efferent cells give origin to nerve-fibers which pass ventrally and become the efferent or motor cranial nerves. The afferent cells give origin to fibers which pass to the cerebral cortex. Around both groups of cells, the afferent or sensor cranial nerves terminate in tuft-like expansions. In a subsequent section the origin, course, and distribution of the various cranial nerves will be considered. But as the function of the nerve is but to transmit energy from the cell of which it constitutes a part, the function ascribed to it may without impropriety be transferred to the cell itself. Since it is by means of nerve-cells and their associated fibers that many important functions of organic life are initiated and maintained, it would naturally be expected from its extensive nerve connections that this region of the nerve system plays an extensive r61e in this respect. As the accomplishment of these functions requires the cooperation and coordination of a number of separate but related structures, it is evident that there must exist in the medulla and pons a number of coordinating mechanisms consisting of nerve-cells and nerve-fibers which are associated in various ways for the accomplish- ment of definite functions. To such a coordinating mechanism the term "center" has been given: e. g., respiratory, cardiac, deglutitory, etc.* The Medulla Oblongata and Pons as Centers for Reflex Activ- ities. — Experimentation has shown that the medulla and pons ccmtain a number of such centers, the more important of which are as follows : 1. Cardiac centers, which exert (i) an accelerator action over the heart's pulsations through nerve-fibers emerging from the spinal cord in the roots of the first and second dorsal nerves and reach- ing the heart through the sympathetic nerve; (2) an inhibitor or retarding action on the rate of the heart beat through efferent fibers in the trunk of the pneuniogastric or vagus nerve. (See pages 312, 313.) 2. A vaso-motor center, which regulates the caliber of the blood- vessels throughout the body in accordance with the needs of the organs and tissues for blood, through nerve-fibers passing by way of the spinal nerves to the walls of the blood-vessels. (See page 376.) 3. A respiratory center, which coordinates the muscles concerned in the production of the respiratory movements. (See page 428.) * By the term center as here employed is meant a collection of nerve-cells and nerve- fibers occupying an area of greater or less extent, though its exact anatomic limits may not be accurately defined. That an area may merit the term center, it is. necessary that its stimulation should increase, its destruction should abolish or impair, functional activity. FUNCTIONS OF THE CRURA CEREBRI. 533 4. A mastication center, which excites to activity and coordinates the muscles of mastication. (See page 150.) 5. A deglutition center, which excites and coordinates the muscles concerned in the transference of the food from the mouth to the stomach. (See page 170.) 6. An articulation center, which coordinates the muscles necessary to the production of articulate speech. In addition, the gray matter contains centers which influence the secretion of saliva, provoke vomiting, coordinate the muscles of the face concerned in expression, and control the secretion of the perspira- tion. As Conducting Pathways. — The anterior pyramids of the medulla and their continuations through the more ventral portions of the pons, being portions of the general pyramidal tract, serve to conduct voli- tional efferent nerve impulses from higher portions of the brain to the spinal cord. Division of either of these pathways is at once followed by a loss of volitional control of the muscles below the section. The dorsal or tegmental portion, containing the fillet and Gowers' tract, serves to transmit afferent nerve impulses from the spinal cord to higher portions of the brain. Transverse division of one-half of the dorsal portion of the pons is followed by complete anesthesia of the opposite half of the body without any impairment of motion. The restiform bodies constitute a pathway between the spinal cord and the cerebellum. The transverse fibers of the pons associate opposite but corresponding portions of the cerebellar hemispheres. The Crura Cerebri. — The crura cerebri consist ventrally of fibers which are largely derived from the pyramidal tracts and are con- tinuous with the longitudinal fibers of the ventral portion of the pons and medulla; and dorsally of fibers continuous with those coming through the lower portions of the tegmentum. Hence they are con- ductors of motor impulses in the former and of sensor impulses in the latter region. It is not definitely known as to whether reflex actions take place through the gray matter, the locus niger, or not. The gray matter beneath the aqueduct of Sylvius contains nerve- cell groups which are centers for reflex actions in connection with ocular movements: e. g., closure of the lids, contraction of the sphinc- ter pupillas, convergence of the eyes, etc. The Corpora Quadrigemina.— From the anatomic relation of the anterior quadrigeminal body (the pregeminum) to the optic tract, on the one hand, and to the optic radiation, on the other, the inference can be drawn that it is in some way essential to the performance of the visual process. Experimental investigations and pathologic changes support the inference. Irritation of the pregeminum in monkeys on one side is followed by dilatation of the pupils first on the opposite side and then almost immediately on the same side. The eyes at the same time are also widely opened and the eyeballs turned upward and to the opposite 534 TEXT-BOOE OF PHYSIOLOGY. side. If the irritation be continued, motor reactions are exhibited in various parts of the body. Destruction of the pregeminum in both monkeys and rabbits is followed by blindness, dilatation and immo- bility of the pupils, with marked disturbance of equilibrium and locomotion (Farrier). From the anatomic relation of the posterior quadrigeminal body (the postgeminum) to the lateral fillet, the basal tract for hearing, the inference may be drawn that it is in some way connected with the auditory process. Stimulation of the postgeminum gives rise to cries and various forms of vocalization. Pathologic states of this body are also attended by impairment of hearing and disorders of the equilibrium. From the foregoing facts it is probable that the cor- pora quadrigemina are as- sociated with station and locomotion. Ferrier as- sumes that in these bodies "sensory impressions, ret- inal and others, are co- ordinated with adaptive motor reactions such as are involved in equilibration and locomotion." The Corpora Striata. — The relation of these bodies to the pyramidal motor tract would indicate that they are in some way connected with motor ac- tivities. Their function, however, is obscure. While stimulation of one corpus produces convulsion of the muscles of the opposite side of the body, and destruction gives rise to paralysis of the corresponding muscles, it is difficult, owing to the intimate association of the white and the gray matter, to state to which the phenomena are to be attributed. The evidence at hand points to the conclusion that if a lesion is limited to the gray matter the paralysis which might result would be but temporary and of short duration. The pathologic evidence is of a similar character. Gowers is of the opinion, that if the lesion is small and at a sufficient distance from the white fibers of the capsule, there may even be no initial hemiplegia; neither motor nor sensory paralysis will arise if the lesion is confined to the gray matter. Fig. 244. — Horizontal Section of the Inter- nal Capsule Showing the Position and Rela- tion OF THE Motor Tracts for the Eye, Head (Hd.), Tongue (Tg.), Mouth (Mth.), Shoulder (Shi.), Elbow (Elb.), Digits of Hand (Dig.), Abdomen (Abd.), Hip, Knee (Kn.), Digits of Foot (Dig.). S. Sensor tract. O. T. Optic tract. A. T.' Auditory tract, i. Caudate nucleus. 2. Anterior segment of internal capsule. 3. Ex- ternal capsule. 4. Island of Reil. 5. Lenticular nucleus. 6. Claustrum. 7. Posterior segment of internal capsule. — {Modified from Landois.) FUNCTIONS OF THE INTERNAL CAPSULE. 535 It is stated by some experimenters that localized injuries, both experimental and pathologic, are followed by a persistent rise of temperature, varying from i° to 2.6° C. The Optic Thalami. — ^From the anatomic relation of the optic thalami to the general and special sense nerve-tracts, on the one hand, and to the cerebral cortex, on the other hand, it is assumed that they are connected with the production of sensations both general and special, and act as intermediates between the peripheral sense-organs and the cortex. The results of experimental stimulation and destruction of the thalami are extremely con- tradictory and fail to throw much light on their func- tions. Ferrier states that destruction of the posterior part of one thalamus pro- duced blindness in the op- posite eye and impairment of the sense of touch and pain in the opposite side of the body. In a patient under the care of Hugh- lings- Jackson there was blindness in the right half of each eye, loss of hearing in the left ear, impairment of taste on the left side of the tongue, and a diminu- tion of the sense of touch on the left side of the body. Postmortem examination showed a patch of soften- ing in the posterior part of the right thalamus, the re- mainder of the organ being normal. It is probable that in the thalamus visual, tactile, and labyrinthine impressions are received, coordinated, and reflected outward, with the result of producing various adaptive motor reactions connected with station and equilibrium. It is also believed by some investigators to act as an intermediate between emotional states and their expres- sion in the muscles of the face, this power being lost in certain patho- logic conditions. The power of regulating the temperature of the body has also been assigned to the thalamus, as destruction of its anterior extremity is usually followed by a rise in temperature. The Internal Capsule. — The internal capsule has been shown by the results both of experiment and of pathologic processes to be, first. Fig. 245. — Vertical Section Through the Right Cerebral Hemisphere in Front or the Gray Commissure, i. Caudate nucleus. 2. Cor- pus callosum. 3. Pillars of the fornix. 4. Internal capsule. 5. Optic thalamus. 6. Gray commissure. 7. External capsule. 8. Claustrum (i, 2, 3, the three divisions of the lenticular nucleus). — {Landois.) 536 TEXT-BOOK OF PHYSIOLOGY. a pathway for the transmission of nerve impulses from the cerebral cortex to the pons, medulla, and spinal cord, which give rise to con- traction of the muscles of the opposite side of the body; and, second, a pathway for the transmission of nerve impulses coming from skin, mucous membrane, muscles, and special sense-organs to the cortex, where they give rise to sensations general. and special. It is therefore the common motor and sensor pathway. For the reason that it trans- mits both motor and sensor impulses, and for the further reason that it is frequently the seat of pathologic lesions which are followed by either a loss of motion or sensation or both, the internal capsule is one of the most important parts of the central nerve system. As shown in Fig. 244, it consists of two segments or limbs united at an obtuse angle, the knee or elbow, which is directed toward the median line. The motor tract is confined to the posterior one-third of the anterior segment and the anterior two-thirds of the posterior segment. The sensor tract is confined to the posterior one-third of the posterior seg- ment, the extreme end of which also contains the optic and auditory tracts. The region of the anterior segment in front of the motor tract contains the fibers of the fronto-cerebellar tract, the function of which is unknown. The motor region contains fibers which descend from the cerebral cortex to nerve-centers situated in the gray, matter beneath the aque- duct of Sylvius, in the gray matter beneath the floor of the fourth ven- tricle, and in the anterior horns of the gray matter of the spinal cord, and which in turn are connected by the cranial and spinal nerves with the muscles of the eye, head, face, trunk, and limbs. The positions occupied by these different tracts are shown in Fig. 244. The relation of the internal capsule to the caudate nucleus and the optic thalamus internally, and to the lenticular nucleus externally, is also shown in a vertical section of the cerebrum made in front of the gray commissure (Fig. 245). From the fact that the internal capsule contains efferent or motor tracts, and afferent or sensor tracts, it is evident that a destructive lesion of the motor tract would be followed by a loss of motion; and of the sensor tract, by a loss of sensation on the opposite side of the body. CHAPTER XXI. THE CEREBRUM. The cerebrum is the largest portion of the encephalon, constitut- ing about 85 per cent, of its total weight. In shape it is ovate, convex on its outer surface, narrow in front and broad behind. It is divided by a deep longitudinal cleft or fissure into halves, known as the cerebral hemispheres. The hemispheres are completely separated anteriorly and posteriorly by this fissure, but in their middle portions are united by a broad white band, the corpus callosum. Each hemisphere or hemi-cerebrum is convex on its outer aspect, and corresponds in a general way with the cavity of the skull; the inner or mesal surface is flat and forms the lateral boundary of the longitudinal fissure. The surface of each hemi-cerebrum presents a series of alternate indentations and elevations, known respectively as fissures or sulci, and convolutions or gyres. A knowledge of the situation and extent of the principal fissures and convolutions, as well as of their relation one to another, is .essential to a clear understanding of many phys- iologic processes, clinical phenomena, and surgical procedures. The general arrangement of the primary fissures and convolutions is rep- resented in Figs. 246 and 247. Fissures. — 1. The fissure of Sylvius, one of the most important of the primary fissures, is found on the side of the cerebrum. It begins at the base and extends upward, outward, and backward to a point corresponding to the eminence of the parietal bone, where it usually terminates in a more or less vertically directed branch, the epi-sylvian branch. Anteriorly a short branch is given off which passes upward and forward into the frontal lobe and known as the pre-sylvian; a horizontal branch is known as the sub-sylvian. The Sylvian fissure is the first to appear in the de- velopment of the fetal brain, becoming visible at the third month. In the adult it is deep and well marked and divides the hemi- cerebrum into a frontal and a temporo-sphenoidal lobe. 2. The fissure of Rolando, or central fissure, equally important, is found on the superior and lateral aspects of the cerebrum. It runs from a point on the convexity of the hemisphere near the median line transversely outward and downward toward the fissure of Sylvius, but as a rule does not pass into it. It divides the frontal from the parietal lobe. The inclination of the central fissure is such as to form with the longitudinal fissure an angle of about 67 degrees. 537 538 TEXT-BOOK OF PHYSIOLOGY. 3. The intra-parietal fissure arises a short distance behind the central fissure. It then runs upward, backward, and downward to terminate near the posterior extremity of the parietal lobe. It divides the parietal lobe into a superior and an inferior portion. 4. The parieto-occipital fissure, situated on the mesal surface of the hemisphere, divides the latter into a parietal and an occipital lobe. It begins as a deep notch on the surface of the hemisphere, and is then continued downward and forward until it enters the calcarine fissure. (Fig. 247.) Fig. 246. — Diagram Showing Fissures and Convolutions on the Lateral Aspect or the Lett Hemi-Cerebrum. F. Frontal. P. Parietal. T. Temporal and O. Occipital lobes. S. Fissure of Sylvius. EPS. Epi-sylvian, PRS. Pre-sylvian, S BS. Sub-sylvian fissures. C. Central fissure or Fissure of Rolando. P R C. Pre-central fis- sure. SPFR. Super-frontal fissure. MEFR. Medi-frontal fissure. SBFR. Sub- frontal fissure. PC. PC. Post-central fissure. PTL. Parietal fissure. P.\ROC. Par-occipital, EXOCC. Ex-occipital fissures. SPTMP. Super-temporal fissure. MTMP Medi-temporal fissure. 5. The calcarine fissure begins on the posterior extremity of the mesal surface of the occipital lobe. From this point it passes downward and forward to unite with the parieto-occipital fissure. 6. The para-central fissure begins at the supero-mesal border of the hemisphere. It then passes downward and forward for a varia- ble distance and then turns upward enveloping a lobule known as the para-central lobule. 7. The super-callosal fissure extends from a point just anterior to the para-central lobule downward and forward below the rostrum of the corpus callosum. Secondary fissures of more or less importance are present in the different lobes, subdividing the surface into convolutions: e. g., in the frontal lobe are found the pre-central, the super-frontal, medi-frontal and sub-frontal fissures; in the temporal lobe the super-temporal and medi-temporal fissures. THE CEREBRUM. 5^9 Convolutions. — The convolutions or gyres are the portions of the cerebral surface comprised between the fissures. The arrange- ment of the surface is such that only the more superficial portions are visible. The depth of the convolution, the portion bordering the fissure, is concealed from view. Each lobe presents a series of such convolutions which differ considerably in their relative physiologic importance. The Frontal Lobe. — The frontal lobe presents on its convex surface four convolutions : viz., the anterior or pre-central convolution, and the super-, medi, and sub-frontal convolutions. I. The anterior or pre-central convolution is situated just in front of the Rolandic or central fissure, with which it corresponds in Fig. 247. — Diagram Showing Fissures and Convolutions on the Mesai, As- pect OP THE Left Hemi-cereerum. C. Upper extremity of the central fissure. PARC. Para-central fissure. SPCL. Super-callosal fissure. CL. Callosal fissure. O C. Oc- cipital fissure. C L C. Calcarine fissure. C L T. Collateral fissure. direction. It is continuous above with the super-frontal and below with the sub-frontal convolution. The super-frontal convolution is bounded internally by the longitudinal fissure and externally by the super-frontal fissure. From the upper end of the pre-central convolution, with which it is continuous, it runs forward and downward to the anterior extremity of the frontal lobe, where it turns backward and rests on the orbital plate of the frontal bone. The medi-frontal convolution is situated on the side of the lobe, between the super-frontal fissure above and the medi-frontal fissure below. Its general direction is downward and forward. The sub-frontal convolution winds around the pre-sylvian branch of the fissure of Sylvius in the anterior and inferior portion of S40 TEXT-BOOK OF PHYSIOLOGY. the frontal lobe. It is continuous posteriorly with the lower end of the pre-central convolution. The Parietal Lobe. — The parietal lobe presents three well- marked convolutions: viz., the posterior or post-central convolution, and the super- and sub-parietal. The latter is again subdivided into the marginal and the angular convolution. 1. The posterior or post-central convolution is situated just beliind the Rolandic or central fissure, with which it corresponds in direction. Above, it is continuous with the super parietal convolution; below, with the marginal and the pre-central con- volutions. 2. The super-parietal convolution is bounded internally by the longit- udinal fissure and externally by the intra-parietal fissure. From the upper end of the post-central convolution, with which it is connected, it runs downward and backward as far as the parieto- occipital fissure. 3. The sub- parietal convolution is connected anteriorly with the post-central convolution. Passing backward, it winds around the superior extremity of the fissure of Sylvius, in which situa- tion it is known as the supra-marginal convolution. Beyond this point it divides into two portions, one of which runs forward into the temporal lobe above the super-temporal fissure, while the other runs downward and backward, following the intra- parietal fissure to its termination. At this point it makes a sharp bend and runs forward into the temporal lobe just beneath the super-temporal fissure. In the neighborhood of the bend it is generally known as the angular convolution or gyrus. The Tempo ro-sphenoidal Lobe. — The temporo-sphenoidal lobe presents on its external surface three well-marked convolutions: viz., the super-, the medi-, and the sub-temporal, separated by the super- and medi- temporal fissures. These three convolutions are in a general way parallel with each other, and pursue a direction from before backward and upward. Anteriorly, they are fused together, but posteriorly their connections are somewhat different. The super- temporal is continuous behind and above with the supra-marginal convolution, and behind and below with the angular convolution or gyre. The medi-temporal blends with the preceding and with the middle occipital. The sub-temporal is continuous with the inferior occipital. The Occipital Lobe. — The occipital lobe is triangular in shape and forms the posterior apex of the hemisphere. Its base on the external surface is formed by an imaginary line drawn from the parieto-occipital fissure to the pre-occipital notch on the inferior and lateral border. The external surface presents three convolutions — the superior, middle, and inferior occipital. The inner or mesal surface of the hemisphere, formed in part THE CEREBRUM. 541 by the frontal, the parietal, the occipital, and the temporal lobes, pre- sents several convolutions of much physiologic interest, viz. : 1. The callosal convolution, situated between the super-callosal fissure and the corpus callosum. From its origin anteriorly at the base of the brain this convolution passes backward, grad- ually increasing in width as it approaches the posterior extremity of the corpus callosum. At this point it again narrows and descends between the calcarine and hippocampal fissures to blend with the hippocampal convolution. 2. The gyrus hippocampus, formed by the union of the posterior extremity of the callosal convolution and the sub-calcar- ine convolution is situated just below the dentate or hippo- campal fissure. Anteriorly it becomes enlarged, and just behind the apex of the temporal lobe turns backward and inward to form a hook-shaped eminence, the uncinate gyrus or uncus. The limbic lobe is the name given to an area of the brain which includes, among other structures, the callosal convolution, the gyrus hippocampus, and the uncus. As forming a part of this general lobe may be mentioned the dentate fascia, the striae and peduncle of the corpus callosum, the septum lucidum, the fornix, and the infracallosal gyrus. 3. The collateral convolution is bounded by the collateral fissure above, and its inferior border extends from the occipital lobe to the anterior pole of the temporal lobe. 4. The quadrate lobule, or precuneus, a square-shaped convolution, is situated between the posterior termination of the para-central fissure and the parieto-occipital fissure. It blends with the callosal convolution, on the one hand, and with the parietal lobule on the other. 5. The cuneus, a triangular or wedge-shaped convolution or lobule, is situated on the mesal surface of the occipital lobe between the parieto-occipital and calcarine fissures. The Insula or Island of Reil. — This anatomic structure con- sists of a triangular shaped cluster of six small convolutions situated at the bifurcation of the Sylvian fissure and concealed from view by the convolutions bordering it, spoken of collectively as the oper- culum. These convolutions are connected with the frontal, the parietal, and the temporal lobes. Structure of the Gray Matter of the Cortex. — The gray matter, the cortex of the cerebrum, varies from two to four millimeters in thickness. When examined with a lens of low power, it presents a laminated appearance, due to differences in color and arrangement of its constituent elements. With higher magnification the cortex is seen to consist of neuroglia cells, nerve-cells with specialized den- drites and axons, medullated and non-meduUated nerve-fibers, blood- vessels, connective ' tissue, etc. — all of which are arranged and inter- 542 TEXT-BOOK OF PHYSIOLOGY. blended in a most intricate manner. Notwithstanding the complexity of its structure, modern histologic methods have enabled Cajal to divide it into four fairly distinct layers or zones, from without inward, as follows (Fig. 248) : I. The Molecular Layer. — The most superficial portion of this layer consists mainly of neuroglia or glia cells, the processes of which interlace in all directions, forming a distinct sheath just beneath the pia. The deeper por- tions of this layer contain a specialized type of nerve-cell (Cajal cells) , of which there are several varieties. These cells give off nerve-fiibers which pursue a horizontal direction for a variable dis- tance, but in their course give ofiE collateral branches which ascend to the outer surface of the layer. Among these structures are to be found, also, dendritic processes of cells situated in the subjacent layer. The terminal filaments of medullated nerve-fibers coming from nerve-cells in lower re- gions of the encephalo-spinal axis are also present, but for the most part they pursue a tangential direction. The. Layer of Small Pyramidal Cells. — This layer consists mainly of nerve- cells, the majority of which are pyra- midal in shape and of small size. Other cells, however, are present, which present a variety of shapes, for which reason the layer was at one time termed the ambiguous layer. The apical proc- ess of the pyramidal cells is broad at the base, but narrows rapidly as it passes upward. It frequently divides into several branches, each of which develops club-shaped processes or gem- mules, which give to it a feathery ap- pearance. Dendrites are also given off from the sides and base of the cell- body. From the base a single axon descends which ultimately becomes the axis-cylinder of a medul- lated nerve. The Layer of Large Pyramidal Cells. — The nerve-cells of this layer, as the name implies, are also pyramidal in shape, but of large size. Each cell presents the same features as the cells of the Fig. 248. — Section of the Cerebral Cortex (Motor Area) OF Child, Stained by Golgi's Silver Method. A. Layer of neuroglia cells. B. Layer of small pyramidal ganglion cells. C. Layer of large pyramidal cells. D. Layer of irregular smaller cells. — {Piersol.) 3- THE CEREBRUM. 543 preceding layer, with the exception that the apical process is larger, better developed, and branches more freely. All the den- drites are extensively provided with gemmules. The axon is well developed, sharply defined, and smooth. After giving off collateral branches, the axon descends into the cerebrum and becomes a meduUated nerve-fiber. 4. The Layer of Polymorphous Cells. — In this layer the nerve-cells present a variety of forms: ■ e. g., spindle, polygonal, pyramidal, etc. The spindle form is the most common. From either end of the spindle a large dendrite emerges which soon branches and becomes gemmulated. The axon is well defined and it soon de- scends into the white matter. The Number of Cortical Cells. — Attempts have been made by various histologists to estimate the total number of functional nerve- cells in the cerebral cortex of man. Though the estimates are widely different, the lowest presents numbers which are beyond compre- hension. Thus, Meynert's estimate is 612 millions; Donaldson's 1200 millions; while Thompson's is 9200 millions. Structure of the White Matter. — The white matter of the cerebrum consists of medullated nerve-fibers which, though intri- cately arranged, may be divided into three systems: viz., the com- missural, the association, and the projection. 1. The commissural system. The fibers which compose this system unite corresponding areas of the cortex of each hemisphere. The fibers from the frontal, parietal, and occipital lobes cross in the median line and form a band of transversely arranged fibers, the corpus callosum. The fibers which unite the corre- sponding areas of the temporo-sphenoidal lobes cross in the an- terior commissure. -All the commissural fibers are the axons of nerve-cells, in the cortex, the terminals of which are to be found in the cortex of the opposite side. 2. The association system. The fibers which compose this system unite neighboring as well as distant parts of the same hemisphere, and may therefore be divided into long and short fibers. They associate the inexcitable or association areas with the excitable or projection areas. 3. The projection system. The fibers composing this system unite certain areas of the cortex of the cerebrum with the basal gan- glia, the pons, medulla oblongata, and spinal cord. They may be divided into: (i) afferent fibers which have their origin in the lower nerve-centers at different levels and thence pass to , the cortex; and (2) efferent fibers which have their origin in the cortex and thence pass to the lower nerve-centers, terminating at different levels. The former are also termed the cortico- afferent or cortico-petal; the latter, cortico-efjerent or cortico-fugal. The afferent fibers, the so-called sensor tract, which transmit nerve impulses coming from the general periphery and the sense- 544 TEXT-BOOK OF PHYSIOLOGY. organs, pass through the tegmentum as the mesial and lateral fillets, and thence to the cortex directly by way of the internal capsule, or indirectly through the intermediation of the thalamic and subthalamic nuclei. The distribution of these fibers to the various areas of the cortex will be found in following paragraphs. The efferent fibers of the so-called motor tract which transmit motor or volitional nerve impulses from the cortex to the pons, medulla, and spinal cord, emerge from the layer of pyramidal cells of the gray •matter of the anterior or the pre-central convolution, the paracentral lobule and immediately adjacent areas. From this origin the axons descend through the white matter of the corona radiata, converging toward the internal capsule, into and through which they pass, occupy- ing the anterior two-thirds of the posterior limb or segment. Be- yond the capsule they continue to descend, occupying the middle three-fifths of the pes or crusta of the crus cerebri, the ventral portion of the pons, and eventually the anterior pyramid of the medulla oblon- gata. At this point the tract divides into two portions, viz. : r. A large portion, containing from ninety-one to ninety-seven per cent, of the fibers, which decussates at the lower border of the medulla and passes down the lateral column of the cord, con- stituting the crossed pyramidal tract. 2. A small portion, containing from three to nine per cent, of the fibers, which does not decussate at the medulla, but passes down the inner side of the anterior column of the same side, constituting the direct pyramidal tract or column of Tiirck. After passing through the internal capsule, and as it descends through the crus, pons, and medulla, the cortico-efferent tract gives off a number of fibers which cross the median line and arborize around the nerve-cells in the gray matter beneath the aqueduct of Sylvius (the nuclei of origin of the third and fourth cranial nerves) , and around the nerve-cells in the gray matter beneath the floor of the fourth ven- tricle (the nuclei of origin of the remainder of the motor cranial nerves). The remaining fibers go to form the crossed and direct pyramidal tracts and arborize around the cells in the anterior horn of the gray matter of the opposite side of the cord at successive levels. By this means the cortex is brought into anatomic and physiologic relation with the general musculature of the body through the various cranial and spinal motor nerves. (See Fig. 236, page 517.) The fronto-cerebellar and the occipito-temparo-cerebellar tracts are also efferent tracts and parts of the projection system. The fronto-cerebellar, originating in the nerve-cells of the cortex of the frontal lobe, passes down to and through the internal capsule, occupy- ing the anterior one-third of the anterior segment. It then descends along the inner side of the crus cerebri to the pons, where its fibers arborize around the cells of the nucleus pontis. Through the inter- mediation of these cells this, tract is brought into relation with the cerebellum of the same but chiefly of the opposite side. The occipito- THE CEREBRUM. 545 temporal tract, originating in the cells of the cortex of both the occip- ital and temporal lobes, passes downward and inward toward the lenticular nucleus, beneath which it passes to enter the outer one-fifth of the crusta. It then enters the pons, and through the nucleus pontis also comes into relation with the cerebellum of both sides. (See Fig. 243, page 529.) THE FUNCTIONS OF THE CEREBRUM. The functions of the cerebrum comprehend, in man at least, all that pertains to sensation, cognition, feeling, and volition. All subjective experiences, which in their totality constitute mind, are dependent on and associated with the anatomic integrity and the physiologic activity of the cerebrum and its related sense-organs, the eye, ear, nose, tongue, etc. From an examination of the anatomic development of the brain in different classes of animals, in different men and races of men, and from a study of the pathologic lesions and the results of experi- mental lesions of the brain, evidence has been obtained which reveals in a striking manner the intimate connection of the cerebrum and all phases of mental activity. 1. Comparative anatomic investigations show that there is a general connection between the size of the brain, its texture, the depth and number of convolutions, and the exhibition of mental power. Throughout the entire animal series an increase in intelligence goes hand in hand with an increase in the development of the brain. In man there is an enormous increase in size over that of the highest animals, the anthropoid apes. The most culti- vated races of men have the greatest cranial capacity, that of the educated European or American being approximately 92.1 cubic inches (1835 c.c); while that of the Australian is but 81.7 cubic inches (1628 c.c). Men distinguished for great mental power usually have large and well-developed brains; e. g., that of Cuvier weighed 64.4 ounces (1830 grams); that of Abercrombie, 63 ounces (1786 grams). A large intelligence, however, is not incom- patible with a much smaller brain weight; thus, the brain of Helmholtz weighed but 50.8 ounces (1440 grams); that of Leidy, 49.9 ounces (1415 grams); that of Liebig, 47.7 ounces (1352 grams) . The average arithmetic brain weight of 96 distinguished men was found to be 51.9 ounces (1473 grams) (Spitzka). 2. Pathologic lesions and mechanic injuries which disorganize the .cerebrum are at once followed by a disturbance or an entire suspension of mental activity. Concussion of the brain or sudden compression from a hemorrhage destroys consciousness. Physical and chemic alterations of the gray matter of the cere- brum have been shown to coexist with insanity, loss of memory, of articulate speech, etc. Congenital defects of organization are accompanied by a deficiency in mental capacity and the higher ,S5 546 TEXT-BOOK OF PHYSIOLOGY. instincts. Under such circumstances no great advance in brain development is possible and the intelligence remains at a low level. In congenital idiocy the brain is small, imperfectly devel- oped, and wanting in proper chemic composition. 3. Experimental lesions of the brain in lower animals are attended by results similar to those observed in disease or after injury in man. Removal of the cerebrum in the pigeon completely abolishes intelligence and destroys the capability of performing volitional movements. The pigeon remains in a state of pro- found stupor, though retaining the capability of executing reflex or instinctive movements. It can temporarily be aroused by loud noises, light placed before the eyes, pinching of the toes, etc., but it soon relapses into a condition of quietude. Coincident with the destruction of the cerebrum there occurs a loss of mem- ory, reason, and judgment, and the animal fails to associate the impressions with any previous train of ideas. The higher the animal in the scale of development, the more striking is the loss of mentality after removal of the cerebrum. 4. Experimental interference with the blood-supply to the cerebrum is followed by a diminished or complete cessation of its activities. There is perhaps no organ of the body that is so directly depend- ent upon its blood-supply for the continuance of its activities as the cerebrum. The supply of blood is furnished by four large blood-vessels: viz., the two carotid and the two vertebral arteries. These vessels, after entering the cavity of the skull, give off branches which unite to form the "circle of Willis!" From this circle, large branches are given off which enter the cerebrum and distribute blood to all its parts. After passing through the capillaries the blood, greatly altered in chemic composition, is returned by large veins. The large volume of blood that is present in the brain and the marked changes in composition that it undergoes while passing through the brain indicate a very active and complex metabolism in this organ. By means of the anatomic arrangement of the blood-vessels at the base of the brain, the blood-supply is equalized. It also explains why, when one, or even two, of the four large vessels are oc- cluded by pathologic deposits or surgical procedures, brain activity continues, though perhaps diminished in degree. Occlu- sion of all four vessels, however, is at once followed by a complete abolition of all forms of cerebral activity. An experiment per- formed by Brown-Sequard illustrates the dependence of cerebral ■activity on the blood-supply. A dog was beheaded at the junction of the neck and chest. After a period of ten minutes all evi- dences of life had entirely ceased. Four tubes connected with a reservoir of warm defibrinated blood were then connected with the four arteries of the head. By means of a pumping apparatus imitating the action of the heart the blood was driven into and THE CEREBRUM. 547 through the brain. After a few minutes cerebral activity returned, as shown by contraction of the muscles of the face and eyes. The character of the contractions were such as to convey the idea that they were directed by the will. These vital manifestations continued for a period of fifteen minutes, when on the cessation of the artificial circulation they disappeared, and the head exhibited once again the usual phenomena observed in dying: viz., con- traction and then dilatation of the pupils and convulsive move- ments of the muscles of the face. Localization of Functions in the Cerebrum. — By the term localization of functions is meant the assignment of definite phys- iologic functions to definite anatomic areas of the cerebral cortex. From experiments made on the brains of animals, by the observation and association of clinical symptoms with pathologic lesions of the central nerve system, and from observation of the developmental stages of the embryonic brain, it has been established in recent years: 1. That the general and special sense-organs of the body are associ- ated through afferent nerve-tracts with definite though perhaps not sharply delimited areas of the cerebral cortex; and — 2. That certain areas of the cortex are associated through efferent nerve-tracts with special groups of skeletal or voluntary muscles. Experimental excitation of a cortical area associated with a sense- organ is undoubtedly attended by the production of a sensation at least similar to that produced by peripheral excitation of the sense- organ itself; destruction of the area is followed by an abolition of all the sensations associated with the sense-organ. For these reasons such areas are termed sensor. Excitation of a cortical area associated with a group of skeletal muscles is attended by their contraction; destruction of the area is followed by their relaxation or paralysis. For these reasons such areas are termed motor. Since the sense-organs are remote from the brain and the impres- sions made upon them by the objective world can be utilized by the mind, only after they have been reproduced in the cortical areas, it may be said that each sense-organ has its special area in the cortex by which it is represented, or, in other words, each sense-organ has a cortical area of representation. Since the muscles are remote from the brain and since they contract in response to the discharge of nerve impulses from the cells of the cortical motor areas, it may be said that the activities of the motor areas are represented by the contractions of the muscles; in other words, that the cortical motor areas have areas of representation in the general skeletal musculature. It is usually stated, however, in the reverse way: viz., that the muscle movements have areas of representation in the cortex. The cortex of the cerebrum may therefore be compared to a mosaic made up, partially at least, of sensor and motor areas which respectively 548 TEXT-BOOK OF PHYSIOLOGY. represent sense-organs and motor organs, and which hear a definite anatomic and physiologic relation one to the other. Their cooperation is essential to the normal performance of all forms of cerebral activity. A knowledge of the situation of these areas, the order of their devel- opment, the effects that arise from their stimulation or follow their destruction, are matters of the highest importance in the study of cere- bral activity and indispensable to the physician in the localization of lesions which manifest themselves in perversions or abolition of sensa- tions and in convulsive seizures or paralyses. The Sensor Areas. — The sensor areas which should theoretically be present in the cortex are primarily those which receive and translate into conscious sensations nerve impulses, developed by changes going on in the body itself; and secondarily those which receive and translate into conscious sensations the nerve impulses developed in the special sense-organs by the impact of the external or objective world. In the former areas, are received the nerve impulses that come from the skin, mucous membranes, muscles, viscera, etc., and give rise to cutaneous, muscle, and visceral sensations. In the latter areas are received the nerve impulses that come from the sense-organs and give rise to tactile, gustatory, olfactory, auditory, and visual sensations. A number of such sense areas may be predicated: e. g., areas of cutaneous and muscle sensibility, of gustatory, olfactory, auditory, and visual sensibility. The Motor Areas. — The motor areas which should theoretically be present in the cortex are those which in consequence of the dis- charge of nerve impulses excite contraction of special groups of mus- cles and which, from their coordinate and purposive character, are conventionally termed volitional. Five such general motor areas may be predicated: e. g., one for the muscles of the head and eyes, one for the muscles of the face and associated organs, and others for the muscles of the arm, leg, and trunk. They are usually designated as head and eye, face, arm, leg, and trunk motor areas. The existence and anatomic location of these areas in the cortex of animals have been determined by the employment of two methods of experimentation: viz., stimulation and destruction or extirpation; the first by means of the rapidly repeated induced electric currents, the second by the electric cautery and the knife. If the stimulation or excitation of any given area is followed by contraction and its de- struction by paralysis of muscles, it is assumed that the area is motor in function — is a center of motion. If the stimulation of a given area is attended by phenomena which indicate that the animal is experienc- ing sensation, and its destruction by a loss of this capability or the loss of a special sense, it is assumed that the area is sensor in function — is an area of special sense. The animals generally employed for ex- periments of this character are dogs and monkeys, though other animals have frequently been employed by different investigators. Of all animals, the monkey is the most frequently selected, as the configura- tion of the brain in its general outlines more closely resembles that THE CEREBRUM. 551 hemiopic in character.* He also found that destruction of the occip- ital lobe together with the angular gyrus gave rise to a more or less enduring hemianopsia, in addition to the transient blindness of the opposite eye. From these and similar facts he concluded that the an- gular gyrus is the area of representation for the macular or central region of the retina, and the occipital lobe for the corresponding halves of the peripheral portions of the retina. It was, however, found by Munk, Schaf er, and others that the angular gyrus was not concerned in any way with vision; that extirpation of the occipital lobe alone, especially if the line of division be carried a little further forward on the mesial and inferior surfaces, was followed by homonymous hemiopia (loss of retinal function on the same side), and therefore homonymous hemianopsia. Additional experiments lead to the conclusion that the area for macular vision is near the an- terior extremity of the calcarine fissure, while the area for peripheral vision is in the posterior portion of the mesial surface and for a variable distance on the outer surface. Moreover, there is reason to believe that the area for macular vision is in relation with homonymous halves of the two maculae luteae. The supposed error, the assignment of macu- lar vision to the angular gyrus, has been attributed to destruction of the fibers of the optic radiation, which in their course to the occipital lobe pass close to this gyrus. The Motor Areas of the Monkey Brain. — ^From experiments made on the brains of monkeys Ferrier mapped out a number of areas stimulation of which give rise to muscle contractions on the opposite side of the body which are so purposive and coordinate in character that they may be regarded as identical with those produced volitionally. Destruction of these areas is followed by paralysis. The results of Ferrier' s earlier work are represented in Fig. 251, the descriptive text to which renders them intelligible. In a general way it may be said that the upper third of the anterior and posterior central convolutions presides over the movements of the leg of the opposite side of the body; * In a consideration of this subject certain facts connected with visual perception, both in physiologic and pathologic conditions, must be kept in mind. Thus, visual sensation may arise from stimulation of either the central portion, the macula, or the peripheral portion of the retina or both. ,In the first instance the vision is termed central or macular; in the second instance, peripheral or retinal. Macular vision is clear, sharp, and distinct ; retinal vision, progressively indistinct from the center toward the periphery. Division of one optic tract is followed, in consequence of the partial decussation of the optic nerve- fibers at the chiasma, by a loss of function in the outer two-thirds of the retina of the same side, both in the central (macular) as well as in its peripheral portions, and the inner one-third of the retina of the opposite side. To this condition the term hemiopia has been applied. As a result of this want of functional activity of these retinal portions on the side of the lesion, rays of light emanating from objects situated in the opposite side of the field of vision will not be perceived when both eyes are directed to the fixation point. To this " blindness " in the opposite half of the field of vision the name hemianopsia is given. In the lesion under consideration (division of one optic tract) the hemianopsia is bilateral, and as it affects the corresponding portions associated in normal vision it is of the homonymous variety. Division of the right optic tract is followed by lejt lateral homon- ymous hemianopsia, indicative of the fact that objects in the field of vision to the left of the binocular fixation point are invisible. 552 TEXT-BOOK OF PHYSIOLOGY. the middle third over the movements of the arm; the inferior third over the movements of the face and tongue. Collectively these areas are known as the motor area or motor zone; and as it is ranged along the Rolandic fissure, it is sometimes termed the Rolandic area. The experiments of Horsley and Schafer added additional facts and enabled them to construct a new diagrammatic representation of the motor area and more ac- curately define the special areas upon the lateral and mesial aspects of the brain of the monkey. The bound- aries of the general and special areas, as determined by these observers, will be readily apparent from an examination of Fig. 249. Their experiments have en- abled them also to subdivide the general into special areas as follows: 1. The head area or area for visual direction into areas excitation of which causes "opening of the eyes, dilatation of the pupils and turning the head to the opposite side with conjugate de- viation of the eyes to that side. " 2. The /e^ area may be sub- divided into (a) an area both on the lateral and mesial surfaces which presides over the move- ments of the hip and thigh; (6) an area in the posterior part which presides over the move- ments- of the legs and toes; (c) an area in the paracentral lobule for the movements of the hallux or great toe. 3. The trunk area, situated largely on the mesial surface, may be sub- divided into an anterior and a posterior area, which respectively preside over the movements of the spinal column as arching and rotation, and the movements of the pelvis and tail. 4. The arm area may be subdivided as follows : (a) an area superiorly Fig. 251. — Left Hemisphere of Monkey, Showing Details or Motor Areas Indicated BY THE Movements Following Stimulation of: I. Superior parietal lobule; exciting advance of the hind limb. 2. Top of ascending frontal and parietal convolutions; flexion and outward rotation of thigh; flexion of toes. 3. On ascending frontal convolution near semilunar sulcus; movements of hind limb, tail and extremity of trunk. 4. On adjacent margins of ascending frontal and parietal convolution; adduction and extension of arm, pronation of hand. 5. Top of ascending frontal near superior frontal convolution ; forward extension of arm. a, b, c, d. On ascending parietal; move- ments of various muscles of the forearm. 6. Ascending frontal convolution; flexion of forearm and supination of hand which is brought toward mouth. 7. Retraction and elevation of corner of mouth. 8. Elevation of nose and lip. 9 and 10. Opening mouth and motions of tongue. 11. Re- traction of angle of mouth, 12. Middle and superior frontal convolutions; movements of head and eyelids. 13 and 13'. Anterior and posterior limbs of angular gyrus; movements of eyeballs. 14. Superior temporo-sphenoidal convolution, ear pricked and head moved. 15. Movement of lip and nostril. — (Ferrier.) THE CEREBRUM. 553 which controls the movements of the shoulder; (b) an area pos- teriorly and below this, which controls the movements of the elbow; (c) an area anteriorly and below the preceding, governing the movements of the wrist and fingers; (d) an area posteriorly and below governing the movements of the thumb. 5. The [ace area may be divided into an upper part, comprising about one-third, and a lower part, comprising the remaining two- thirds. In the upper part are areas governing the movements of the opposite angle of the mouth and of the lower face. In the lower part anteriorly there is an area governing the movements of the vocal membranes or bands (the laryngeal area) ; posteriorly areas governing the opening and closing of the mouth, the protru- sion and retraction of the tongue. Electric stimulation of the sensor areas is attended by certain motor reactions which vary in accordance with the area stimulated. Thus, when the electrodes are applied to different portions of the occipital lobe the eyeballs are conjugately turned upward, downward, or laterally and to the opposite, side; when placed on the upper por- tion of the superior temporal convolution, the ear is pricked up or retracted, the head is turned to the opposite side and the pupils are dilated; when placed on the hippocampal convolution, there is move- ment of torsion of the nostril and lips of the same side. Ferrier assumed that these movements were the result of the origin- ation of subjective sensations and not an evidencet that the area in question is a motor area, in the sense that this term is applied to the areas of the Rolandic region, especially as their destruction is not followed by paralysis of any of the corresponding muscles. This interpretation is supported by the experiments of Schafer, which showed that the contraction of the eye-muscles which followed stimulation of the occipital lobe took place between 0.2 and 0.3 second later than when the frontal lobe was stimulated; and that as the motor reaction takes place after extirpation of the frontal region, the route of the efferent impulse cannot be to and through the frontal lobe, but prob- ably through some lower center. The same facts hold true for the reactions of the ear-muscles following stimulation of the temporal lobe. The view that the cortex of the cerebrum can be divided into separate and independent though physiologically related motor and sensor areas has been questioned in recent years, and a somewhat different interpretation given to the facts. It is believed by many physiologists and neurologists that the so-called motor and sensor areas are so closely related that it is almost impossible to distinguish one from the other either anatomically or physiologically. Thus the Rolandic region is believed to be both motor and sensor in function, the former, however, being more predominant in the pre-central, the latter in the post-central, convolution. As these two functions are so intimately blended and their anatomic substrata so difficult of separation, it is 554 TEXT-BOOK OF PHYSIOLOGY. thought the term sensori-motor should be employed as more descriptive and more in accordance with the facts to the entire Rolandic region. This view has been strengthened by the results of the embryologic investigation of Flechsig, which show that different nerve-tracts be- come meduUated or receive their myelin investment at successively later periods and that the tracts which first become myelinated and are hence first functionally active, belong to the afferent system. Among the first to undergo myelinization are three tracts numbered by Flechsig i, 2 and 3, which arise largely from the median nucleus of the thalamus and the medial lemniscus and pass to the anterior and posterior convolutions, to the para-central lobule and foot of the superior frontal convolution, and to the foot of the third frontal convolution respectively. It is these fibers which convey nerve im- pulses to the cortex and furnish information regarding changes taking place in the body itself and thus lead to the performance of muscle movements. This area is therefore primarily a sensor area, an area for body-feelings, cutaneous, tactile, muscle, and visceral,- and second- arily a motor area. The afferent fibers to this region become myel- inated during the ninth month of intra-uterine life, the efferent fibers from it become myelinated during the third month of extra-uterine life. By the same method of reasoning the gustatory, olfactory, audi- tory, and visual sense areas are to be regarded as sensori-motor in character, for embryologic investigations show that subsequently to the myelinization of the afferent tracts coimecting the sense-organs with the cortex, efferent nerve-tracts arise from or near to the same centers and undergo myelinization. In other words, these areas are primarily sensor and secondarily motor, and therefore should be termed sensori-motor. In Flechsig's own terminology each cortico-petal or afferent tract is accompanied by a cortico-fugal or efferent tract. In this view sensations, or the mental pirocesses the outcome of sensations, are the immediate cause of the movements of the muscles connected with both the sense-organs and skeletal structures. Though this interpretation — viz. , the coincidence of sensor and motor areas — ap- pears more in accordance with the facts than the earlier view, it must be admitted that there are many facts both of a physiologic and pathologic character which it is difficult to harmonize with it. The Motor Area of the Chimpanzee Brain. — In a series of experiments made by Sherrington and Griinbaum on the brain of the chimpanzee it was discovered that the so-called motor area was not so widely distributed as in the monkeys generally, but was confined almost exclusively to the convolution just in front of the fissure of Rolando, as it was impossible to obtain any movement on direct stim- ulation of the convolution just behind it. All points on the surface of the pre-central convolution, including the portion forming the wall of the Rolandic fissure itself, were found to be excitable and productive of movement when stimulated. The sequence of representation from THE CEREBRUM. SS7 the anterior part of the callosal convolution, and the posterior part of the base of the frontal lobe. Lesions in this region are frequently accom- panied by subjective olfactory sensations. The gustatory area has been assigned to the collateral convolu- tion. The auditory area has been assigned to the posterior portion of the super temporal convolution and to the retro-insular convolutions, the island of Reil. Unilateral destruction of this region is followed by only a partial loss of hearing in the opposite ear (owing to the par- tial decussation of the cochlear nerve), which, however, may be re- covered from after a time, owing probably to a compensatory activity of the insular convolutions. Bilateral disease of this region is followed by complete deafness. Within this area there is a smaller region, disease of which is accompanied by word-deafness only, the patient being unable to distinguish the tone intervals between words and syl- lables and therefore hearing only confused noises. Object hearing has also a separate area of representation. The visual area has been assigned to a triangular shaped area on the mesal surface of the occipital lobe, which includes the gray matter above and below the calcarine fissure (the cuneus and upper part of the lingual lobe), and to the gray matter of the first occipital convolution on the lateral aspect of the occipital lobe. Focal lesions of this area on one side are followed by lateral homonymous hemianopsia, which, however, does not involve, as a rule, the fovea or macula. It is, there- fore, the area of homonymous half-retinal representation. The loca- tion of the area for macular or central vision is uncertain. Henschen locates it in the anterior part of the area near the extremity of the cal- carine fissure, and asserts that in each area both maculae are represented. From experiments made on monkeys Schafer locates it in the same region. Beyond the limits of this visual area and on the lateral aspect of the parietal lobe there is a region (the supra-marginal convolution and angular gyrus) in which impressions of words and letters seen have their representation. Destruction of this area by diseases is followed by word- and perhaps letter-blindness, the patient being unable to recognize words and letters seen because of failure to revive the mem- ory images of words and letters. The areas for visual sensations and optic memory pictures are therefore separate, a fact which has led to a division of the visual area into a lower and a higher area. It was stated in a previous paragraph" that electric stimulation of the sensor areas of the monkey brain is attended by certain motor reactions which vary with the area stimulated. Corresponding areas are believed to be present in the human brain and that their stimula- tion would be followed by similar motor reactions. Their location is shown in Figs. 252 and 253, and named visual, auditory, olfactory, and gustatory motor. The stereognostic area or area of stereognostic perception, by which objects are recognized through their form independent of vision and SS8 TEXT-BOOK OF PHYSIOLOGY. by the sense of 'touch alone, has been located in the super parietal convolution and the precuneus (Mills). The existence of such an area is rendered probable by the fact that cases have been recorded in which there was a loss of this power (astereognosis) uaccompanied by either sensor or motor disturbances. Postmortem investigations showed that in these cases there was a destruction of the superior parietal convolution. Equilibratory, intonation, and orientation areas have been pro- visionally located in the sphenotemporal lobe. The Motor Area,— The general motor area (Fig. 254) is represented as occupying the pre-central convolution, the base of the. super-frontal convolution, both on its lateral and mesial aspects, and the paracentral Fig. 254. — Scheme of the Motor Area of the Human Brain and its Subdivisions. -{Ajter Mills.) lobule. The exclusion of the post-central convolution from the motor area is in accordance with the embfyologic researches of Flechsig, which indicate that the efferent fibers which compose the pyramidal tract come from the region anterior to the central fissure, and with the experiments of Sherrington and Griinbaum on the brain of the chimpanzee, which demonstrate that the post-central convolution is absolutely inexcitable to electric! stimulation. It is quite probable that with the growth of the brain in size and complexity, the motor area has come to occupy a posi- tion somewhat farther forward in the human brain than in the monkey brain. This general area is also capable of subdivision into areas of vari- able size, in which the movements of the face and associated structures. THE CEREBRUM. 559 the head and eyes, the arm, trunk, and leg, are represented. (Fig. 254.) The sequence of their representation from below upward is similar to that observed in the monkey and chimpanzee. In each of these five main areas there are yet smaller areas in which the move- ments of localized regions of the body are in part represented and which are indicated in diagram (Fig. 254) by corresponding words. The words in the areas marked, eyes and head, face, arm, trunk, and leg, indicate the location of nerve-cells which through the discharge of nerve impulses excite to contraction the muscles, which impart to the regions indicated by these words, their characteristic movements. A localized irritative lesion of any one of these areas gives rise to convulsive movements of the muscles of the opposite side of the body, similar in character to those resulting from electric stimulation of the corresponding areas of the monkey and ape brains. Destruction of these areas from the growth of tumors, softening, etc., is followed by paralysis of the muscles. Electric stimulation of these areas of the human brain for the purpose of localizing obscure irritative lesions prior to surgical procedures on the brain gives rise to the same con- vulsive movements. Language. — The succession of motor acts by which ideas are expressed, is known as language, which may be divided into (i) artic- ulate or spoken, and (2) written. The expression of ideas both by words and signs depends primarily on the power of reviving the images or memories of words and letters heard and seen; and secondarily on the power of reviving the images or memories of the muscle movements which were previously em- ployed in an effort to imitate or reproduce the words (speech) or the verbal signs (writing) . Clinico-pathologic investigations have shown that words and letters heard and seen have areas of representation in the cortex, the former in the general auditory area, the latter in the supra-marginal- convolution and angular gyrus (Fig. 252). Destruction of these areas is followed by word-deafness and word-blindness respectively. The same methods of investigation have shown that the muscle move- ments employed to reproduce the words and the verbal signs also have areas of representation in the cortex; the former in the sub- frontal convolution (Fig. 252), and probably in the adjacent region, the island of Reil, on the left side in the great majority of people; the latter in front of the arm region of the general motor area. Destruction of these areas is followed in the first instance by a loss of the power of executing the movements of the muscles employed in speech, and in the second instance, of those employed in writing. These different areas are connected with one another by associa- tion fibers, and, taken collectively, constitute the language zone Their situation and relations are shown in Fig. 255. In this figure the dotted lines coming from the ear (a) and the eye (v) represent the auditory and visual tracts through which nerve impulses pass S6o TEXT-BOOK OF PHYSIOLOGY. to the auditory (A) and the visual centers (V) respectively. Similar lines coming from the muscles involved in speech and writing might also be represented to indicate the paths of the nerve impulses to the motor speech (M) and the motor writing center (E). The continuous lines on the surface of the cortex represent nerve-fibers which associate the auditory and visual centers with the speech and writing cen- ters and with higher psychic centers (O O) as well. The dotted lines coming from the speech and writing centers repre- sent the tracts through which nerve impulses pass to th^ muscle of the larynx, tongue, mouth, and lips, and to the muscles of the hand. The anatomic and physi- ologic association of the various areas is essential to the registra- tion of the impressions made on the ear and eye and for the ex- pression of the ideas evolved from them by words (speech) and signs (writing). Their col- lective action is essential to the acquisition of language. De- struction of any part of this cerebral mechanism is attended by an impairment or a total loss either in the power of obtaining auditory images of words heard and visual images of words seen, or in the power of expressing ideas by speech and writing. To this pathologic condition the term aphasia has been gi^'en. Aphasia. ^ — It was discovered by Bouillaud that a destructive lesion of the third frontal convo- lution on the left side was accom- panied by a partial or complete loss of the faculty of articulate speech, the power to express ideas with words. To this condition the term aphasia was given. Though of limited application etymologically, the word is now employed in a wider sense to signify "partial or com- plete loss of the power of expression or comprehension of the con- ventional signs of language," words either spoken or written, due to lesions of different portions of the cortex, and especially on the left side. Fig. 255. — Diagram Showing the Re- lation OF THE Centers of Language and THEIR Principal Associations. A. Audi- tory center. V. Visual center. M. Motor speech center. E. Motor writing center. O O. Intellectual center. — {After Grasset.) THE CEREBRUM. 561 Aphasias are of many degrees and kinds, though they may be in- cluded in the two general divisions, motor and sensor. Motor aphasia may be either ataxic or agraphic. In ataxic aphasia the patient is unable to express or communicate his thoughts by spoken words, owing to an inability to execute those movements of the mouth, tongue, etc., necessary for speech without there being any paralysis of these muscles. The lesion is usually in the third frontal convolution and most frequently associated with right hemiplegia. In agraphic aphasia the patient is unable to communicate his ideas by writing through an inability to execute the necessary movements, though retaining his mental processes. In this form of aphasia the lesion is in the writing area. These two forms of motor aphasia are not infrequently asso- ciated. Sensor aphasia or amnesia may be either visual or auditory. In visual aphasia or amnesia the patient is unable to recognize a letter or word, printed 'or written (though capable of seeing other objects), a condition known as letter^ or word-hlindness. It is usually associated with lesions in the neighborhood of the supra-marginil convolution. In auditory aphasia or amnesia the patient cannot understand articu- late or vocal speech, though capable of hearing and understanding other sounds, through an inability to distinguish tone intervals of words and letters — a condition known as word-deafness. It is associated with lesions of the auditory area. Paraphasia is an inability to recall the proper words to associate with ideas and necessary to their expression. Concept aphasia is the inability to recall the names of objects. It is associated with lesions of the cortex of the mid-temporal or third temporal convolution (Mills). This area is known as the concept or naming area. Bilateral Representation, — Though highly specialized move- ments, such as those perfornied by the arms and hands, legs and feet, have their areas of representation on one side of the cerebrum only, and that, opposite to, the side of the movement, less highly specialized movements, such as the masticatory, phonatory, respiratory and various trunkal movements, which require for their performance the cooperation of muscles on both sides of the body, have their areas of representation on both sides of the cerebrum; the area of either side exciting to action the muscles on both sides of the body. In the case of specialized movements the representation is unilateral; in the case of the more general movements the representation is bilateral. Association Centers. — The sensor and motor areas to which specific functions have been assigned do not constitute more than one-third of the total cerebral cortex. There yet remain large regions to which it has not been possible to assign specific functions based on physiologic experiments. Three or four such regions separated by the sensor and motor centers are to be recognized on the lateral and mesial aspects of the hemisphere. In Fig. 256 the location, extent, 36 562 TEXT-BOOK OF PHYSIOLOGY. and names of these regions are represented. The fibers which are found in these regions belong almost exclusively to the association system, and become medullated at a later period than do the fibers of the projection system; moreover, from the method of their mcdulliza- Motor and tactile area. Parietal associat n ^. M^^'-^. Frontal association area. i Ht 1 i Reil. Occipito-tempora! association area. Auditory area. Motor and tactile area. Parietal associati n irL (Precuneu Visual are£ (cuneus). Occii ito ten i Frontal association area. Olfactory lobe. Olfactory tract. Olfactory area. Gntus hippocampus Fig. 256. — Diagrams to sho\\' the Position and the Relation of the Association AND Projection Areas. The Projection Areas are Dotted. — (After Flechsig.) tion it would appear that many of these fibers grow out directly from the sensor centers into these regions and become related to the nerve- cells of their convolutions, while others grow out from adjacent as well as distant convolutions. From histologic and pathologic evidence these regions were termed by Flechsig association centers or areas, im- plying the idea that through the intervention of their cell mechanisms THE CEREBRUM. 563 the sense areas are indirectly associated anatomically and physiolog- ically, and together constitute a mechanism by which sensations are associated and elaborated into concrete forms of knowledge or related to definite forms of movement. It has been assumed by Flechsig that the frontal association center, from its connections with the sensor and motor areas of the Rolandic region, the olfactory, and perhaps other regions, is engaged in asso- ciating and registering body sensations and volitional acts, and that the knowledge thus gained has reference largely to the personality of the individual; that the par.iefo-occipital association area, from its relation to the visual, auditory, and tactile. sense areas, is engaged in associating and registering visual, auditory, and tactile sensations, and that the knowledge thus gained has reference mainly to the external world. These assumptions in a general way are supported by the phenomena of disease. In certain lesions of the frontal lobe the symptoms indicate a loss or change of ideas regarding personality rather than of the objective world, while the reverse is tru^ in disease of the parieto- occipital lobe. The Intra-cranial Circulation. — The circulation within the cranium presents certain peculiarities which distinguish it from that in other parts of the body. These peculiarities reside in part in the anatomic arrangement of the blood-vessels, in the probable absence of vaso-motor nerves to the blood-vessels, and in greater part in the fact that the brain and its blood-vessels are contained in a box with rigid, unyielding and closed walls. The Blood-supply. — ^As stated in a previous paragraph the arteries supplying the brain with blood are four in number viz. : The two internal carotids and the two vertebrals. These four arteries anastomose very freely at the base of the brain, the anastomosis constituting the circle of Willis. From this circle there arise the anterior, middle and posterior cerebral arteries which are distributed to the cortex and the underlying white matter. The basal ganglia, the capsule and adjacent white matter are supplied by a number of branches which arise from the circle of Willis or from the three cerebral arteries immediately after their origin. From the distribution of these two sets of vessels they have been named the cortical and the central ganglionic respectively. The venous blood is returned by a system of vessels which present characteristics of physiologic interest. These vessels consist of large sinuses formed by folds of the dura mater or, as at the base of the cranium, by the dura mater and the bone. These sinuses, from the very nature of the tissues which enter into their formation, have rigid walls and will therefore withstand any pressure to which they may be subjected under physiologic conditions. The same obtains at their points of exit from the cranium where a free outflow is in consequence always assured; 564 TEXT-BOOK OF PHYSIOLOGY. The various sinuses have opening into them, the veins which return the blood from the cortex and subjacent white matter, and from the inner structures of the brain. Neither sinuses nor veins have valves and most of the veins which empty into the superior longitudinal sinus have their mouths directed forward, hence the blood discharged from these veins must flow against the current in the sinus. The venous blood leaves the cranium mainly by way of the internal jugular veins which are direct continuations of the lateral sinuses. The Intra-cranial Lymph Spaces. — In order to understand the phenomena attending the circulation of blood through the cranium it is necessary to take into consideration an important fact, viz. : that the brain and spinal cord are surrounded on all sides by a relatively large and continuous lymph space. This space which is found between the arachnoid and the pia mater is filled with a liquid, the so-called cerebrospinal fluid, which being interposed between the brain and the skull on the one hand and the spinal cord and the vertebrae on the other hand, acts as a water cushion protecting these delicate organs from the injury which might result from sudden jars. The ventricles of the brain are also filled with cerebrospinal fluid which is in commimication with that in the subarachnoid space through the foramen of Magendie and the foramina of Key and Retzius. The cerebrospinal fluid may also penetrate into the perineural lymph spaces surrounding the cranial and spinal nerves. The quantity of the cerebrospinal fluid is relatively small, amounting to from 60 to 80 c.c. The Mechanism of the Intra-cranial Circulation. — As pre- viously stated, by virtue of the physical relations existing between the blood, the brain, the cerebrospinal fluid and rigid walls of the cranium, the flow of the blood through the brain and cranial cavity, is attended by certain phenomena which are peculiar to this region and present in no other situation. Taking as a point of departure the conditions during the diastole of the arteries, the relations of these structures are somewhat as follows: the cerebrospinal fluid occupies all the available lymph space, but under a pressure approximately equal to that in the large veins and hence not materially above that of the atmosphere; the pressure in the arteries, capillaries and veins presents the usual values in these different regions of the vascular apparatus; the brain presents a volume which may be termed diastolic. With the occurrence of the succeeding cardiac systole, the cerebral vessels, receiving an additional volume of blood, expand and occasion a corresponding increase in the volume of the brain, which is accom- plished by a partial displacement of the cerebrospinal fluid into extra-cranial lymph spaces. Because of the fact that the displacement of the cerebrospinal fluid is insufficient to permit of the complete expansion of the brain, there is developed in the intra-cranial lymph spaces a counter pressure (the so-called intra-cranial pressure) which THE CEREBRUM. 565 would keep pace with and finally equalize the rising pressure in the arteries. In consequence of this, the brain tissue, it is believed, would be subjected to a pressure sufficiently great to interfere with its activi- ties, even to the point of unconsciousness. If this is not to occur the maximum expansion of the arteries, and hence the brain, must be checked and controlled. This is accomplished in the following way: As the brain approaches that degree of expansion permitted by the displacement of the cerebrospinal fluid, it begins to exert a com- pression of the pial veins. This compression by narrowing the lumen of the veins diminishes their capacity and hence increases the pressure of their contained blood until it is equivalent to the pressure exerted by the brain against the veins. At this moment the pressures in the arterioles, capillaries and veins approximate each other in value. From these factors it will be seen that the circulation through the brain approximates a circulation through a system of rigid tubes. The result is an increase in the velocity of the outflow and a diminution of the blood-pressure. As an additional result the pulse wave of the arterial system is transmitted to the blood of the large veins and sinuses which therefore exhibit normally pulsations synchronous with those of the arteries. The rise of the pressure in the cerebral veins is regarded therefore as the factor which, by limiting brain expansion, checks the rise of the intra-cranial pressure beyond physiologic limits. With the diastole of the heart and arteries, the former relation of the blood, brain, cerebrospinal fluid and cranial walls is regained. Be- cause of this change of relation with each heart-beat, the brain pulsates synchronously with the arteries. The brain differs from other organs, also, in that normally its volume is more influenced in a positive direction by the expiratory rise of venous pressure than by the inspiratory rise of general arterial pressure. Thus the rise of pressure in the thoracic veins which occurs with each expiratory act, causes a damming back of the venous blood in the sinuses and pial veins, resulting in a further increase in the volume of the brain and in the intra-cranial pressure. The reverse takes place in inspiration. It has been ascertained experimentally that the intra-cranial pressure may vary considerably and consciousness still be preserved. Hill found it to be 40 to 50 mm. of Hg. in the convulsions of strychnia poisoning and a little less than zero in a patient standing erect. The Regulation of the Volume of Blood Entering the Brain. — It is generally believed that the cerebral vessels are not provided with vaso-motor nerves. Every attempt to prove their existence either by physiologit or histologic methods has thus far failed of convincing proof. In the absence of vaso-motor nerves, the regulation of the circulation in the brain must necessarily be dependent on changes affecting the arterial and venous pressures in other regions of the body. The most effective factor in increasing or decreasing the blood- S66 TEXT-BOOK OF PHYSIOLOGY. supply to the brain resides in the power of the vaso-motor center to cause'a contraction or dilatation of the cutaneous and splanchnic vessels. Thus if the vaso-motor center declines in its tonus from any cause what- ever, there is a relaxatiori of the blood-vessels in one or both of these regions, an increase in the volume of the blood flowing into them, and in consequence, a decrease in the volume of the blood flowing through the brain. If on the contrary the vaso-motor center is increased in its tonus, the reverse conditions prevail in the cutaneous and splanch- nic vessels and the quantity of blood flowing into the brain is increased. Thus in an indirect way the vaso-motor center, by bringing about a rise or a fall in the general arterial pressure, regulates the blood-supply to the brain, and controls its amount in accordance with its needs. . Brain Activity and Brain Repose or Sleep. — Brain activity is characterized by an active consciousness, the development of sensations, ideas, feelings, and the exercise of volitional power (which manifests in muscle movement) and is the result of a physiologic condition of the body at large. For the manifestation of brain activity it is essential, that the irritability of the brain cells and more especially of those com- posing in large measure the cerebral cortex be maintained at a normal physiologic level, so that they may respond in the manner peculiar to them, to the action of nerve impulses reflected through afferent nerves from all regions of the body. Here as elsewhere throughout the body, the irritability depends on, and is maintained by, the presence of blood flowing into and out of the brain in varying quantity from moment to moment, with a given velocity and under a definite pressure. So long as these conditions are maintained in the strictly physiologic condition, so long will the brain respond to stimuli by the development of sensations. The avenues through which nerve impulses pass to the cortical cells are those beginning in the special and general sense organs of the body in contact with the external world, viz. : the eyes, ears, nose, tongue, and skin. The maintenance of these structures in a strictly physiologic condition is also one of the essential conditions for brain activity. Judging from the changes in the character and composition of the blood which occur during its passage through the brain capillaries, there is coincidently with brain activity an active metabolism, which eventuates, at the end of a variable number of hours, in the decline of the irritability, a reduction of functional activity, and the establish- ment of the condition of fatigue. The irritability of the sense organs, especially of the eyes and ears, in all probability declines in a similar manner. These structures pass into the condition of fatigue and become less responsive to external stimuli. The result of all these conditions is a less active stimulation of the brain cells, which in con- nection with other factors predisposes to Brain Repose or Sleep. — Brain repose or sleep is characterized by a greater or less degree of unconsciousness, the non-development of sensations, ideas, feelings and volitional acts, and is the result of a THE CEREBRUM. 567 diminution in the physiologic activities of the body at large and more especially of the brain, sense organs and spinal cord. Coincident with the cessation of brain activity and the onset of sleep, there is a diminution in the rate and force of the heart beat, and in the frequency and depth of the respiratory movements, and a relaxation of the skeletal muscles, especially those employed in voluntary movements. The sense organs are in part protected from the action of external stimuli. The eyeball is so turned, that its anterior pole is directed far upward under the eyelid, while the pupil is markedly diminished in size, and in consequence the entrance of light largely prevented. The ear is protected against the reception of sounds of ordinary pitch by an increased tension of the tympanic membrane. The nose and mouth are less responsive to various stimuli because of the dryness of their mucous membranes from diminished secretion. The skin appears to be less sensitive to mechanic pressure and other forms of stimulation. In addition to the foregoing phenomena, experimental investigations have shown, that there is a shunting of a portion of the blood stream from the brain to other regions of the body, especially to the skin and perhaps to the abdominal viscera as well, whereby it becomes incapable of functionating physiologicly. The fact that the brain receives a lessened quantity of blood during sleep has been shown by trephining the skull and inserting in the orifice a glass plate through which the circulatory conditions of the brain can be observed. In the waking condition the blood-vessels on the surface of the brain are prominent, and turgid with blood and the whole organ completely fills the cranial cavity, indicating that the blood-vessels in the interior of the brain are in a similar condition. With the onset of sleep the larger blood-vessels begin to diminish in size, the smaller vessels disappear from view, the brain tissues become pale and the volume of the brain shrinks. During the continuance of deep sleep, this anemic condition persists. As the period of sleep approaches its termination, the smaller blood- vessels again fill with blood, the surface of the brain flushes, and in a very short time the former circulatory conditions return, the volume of the brain increases and the waking state is reestablished. The fact that the skin receives an increased volume of blood during sleep, has been shown by inserting an arm or leg in a plethysmograph by which means a record of any change in volume can be obtained. Howell thus succeeded in obtaining graphic records in the variations of the volume of the arm during sleep. These records disclosed the fact that with the onset of sleep the volume of the arm gradually increased in size until it attained a maximum which was from one to two hours after the beginning of sleep. After this period the volume remains practically the same for several hours, diminishing as the intensity of sleep diminishes, and the waking state is approached. Just previous to the return of consciousness there is a rapid diminution in the volume of the arm. If it be accepted that the enlargement of 568 TEXT-BOOK OF PHYSIOLOGY. the cutaneous vessels is followed by a diminution in size of the cerebral vessels, it follows that the fornaer condition stands to the latter in the relation of cause and effect, whereby a portion of the blood is diverted from the brain to the skin. It also naturally follows that the with- drawal of the blood from the brain to the skin and possibly other regions as well, is the fundamental condition for brain repose. The Intensity of Sleep. — Observations of individuals during sleep show, that the intensity or the deepth of sleep varies from hour to hour. Attempts have been made to estimate the intensity by measuring the intensity, or the loudness of a sound caused in several ways, that is necessary to awaken the sleeper. Accepting this criterion it may be stated from the results of many experiments, that sleep increases in intensity or depth and reaches its maximum between the first and second hours, after which it rapidly decreases until the end of the third hour, when consciousness is so nearly restored, that but a very slight stimulus is required to awaken the sleeper. It is 'during the latter period when the brain is reviving, that dreams arise the element of which are formed of previous sensations. The Causes of Sleep. — Different theories have been proposed to account for the causes of sleep, none of which have been wholly satis- factory. From all the facts which have been presented it would appear that one cause is a decline in the irritability of the nerve-cells of the brain and associated sense organs, and the development of fatigue conditions, the result of prolonged activity. A second cause is the withdrawal of a large portion of the blood from the brain on the presence of which, here as elsewhere, normal activity depends. As to whether the diminished activity of the brain is the cause of, or the result of the withdrawal of the blood there has been much difference of opinion. Howell has offered a plausible explanation for the withdrawal of the blood from the brain to the cutaneous vessels, based on the activity of the vaso-motor center. He assumes that for a variable number of hours, corresponding to the usual waking state, this center possesses a certain average tonus, due in all probability to reflex influences, by virtue of which it main- tains a certain average contraction of the cutaneous vessels. But at the end of this period it, too, becomes fatigued, declines in irritability, becomes less responsive to reflex influences, and hence loses its control over the vessels. As a result they dilate and thus reduce the amount of blood flowing to the brain to a level insufficient to maintain its activity, after which sleep supervenes. During sleep the irritability and tonus of the center are restored when its control of the blood- vessels is regained. Unless the brain in its functional activities differs from all other organs of the body, it may be inferred that cessation of activity or repose is the result partly of fatigue and partly of a diminu- tion of the blood-supply. CHAPTER XXII. THE CEREBELLUM. The cerebellum is situated in the inferior fossae of the occipital bone, beneath the posterior lobes of the cerebrum, from which it is separated by the tentorium cerebelli, a semilunar fold of the dura mater. It is partially divided into hemispheres by a longitudinal fissure, more apparent on the inferior surface, though united by a central lobe, the vermiform process. Each hemisphere is connected with the cerebrum, the pons, medulla and spinal cord by three bundles of nerve-fibers known respectively as the superior, middle, and injerior peduncles. The surface of the cerebellum presents a series of lobes and fissures of which the former have received more or less fanciful names. A section of the cerebellum shows that it is composed of gray matter externally and white matter internally. The general appearance presented on section is shown in Fig. 257. Structure of the Gray Matter. — The gray matter consists mainly of nerve-cells of varying size and shape, which are arranged in two layers: viz., an outer or molecular and an inner or granular. The molecular layer consists of stellate and multipolar cells of small size, from which dendrites and axons pass horizontally and vertically. The granular layer consists, as its name implies, of granular shaped cells and large stellate cells. These cells are character- ized by the possession of dendrites and axons, the course and relation of which have not been clearly determined. The inner border of the molecular layer presents a series of large cells originally described by Purkinje and known by his name. From the outer end of the cell-body one or more dendrites emerge which soon divide and subdivide into a number of branches which pass toward the cerebellar surface. The general arrangement of these dendrites gives to the entire cell a tree-like appearance (Fig. 258). From the inner end of the cell an axon emerges which passes centrally into the white matter. Structure of the "White Matter. — The white matter consists of nerve-fibers which are arranged in association and projection systems. The Association System. — The fibers which compose this system are of variable lengths and unite adjacent as well as distant regions of the cerebellar cortex. They doubtless associate them both anatom- ically and physiologically. The Projection System.— Tht fibers composing this system con- nect the cerebellar cortex with certain structures in the cerebrum, pons, medulla, and spinal cord. They may be divided into efferent and afferent systems. 56Q 570 TEXT-BOOK OF PHYSIOLOGY. The efferent fibers have their origin in the cells of Purkinje and the dentate nucleus. Some of these fibers emerge from the cere- bellum in the superior peduncles through which they pass toward and beneath the corpora quadrigemina to terminate around the cells of the red nucleus. As they approach this nucleus some of the fibers cross the median line and decussate with those coming from the op- posite side, while others pursue a straight direction, terminating on the same side. Through the intervention of fibers which arise from the red nucleus and ascend to the cerebral cortex, the cortex is thus con- nected with both sides of the cerebellum, though chiefly with the opposite side. . Efferent fibers also leave the cerebellum by the middle peduncle and pass directly to the nu- cleus ponds, around the cells of which their term- inals arborize. Efferent fibers also descend the inferior peduncles and constitute the tract known as the Lowenthal and Marchi tract, situ- ated in the antero-lateral region of the spinal cord in its upper part. The afferent fibers come from a variety of sources. Those found in the superior peduncles come from the red nucleus; those in the middle peduncles from the nucleus pontis of the opposite side, having crossed or decussated at the raphe near the anterior surface of the pons; those contained in the inferior peduncles are the most abundant and important, and are represented by (i) the direct cerebellar tract, which terminates in the superior vermis after decussation; (2) the anterior and posterior arcuate fibers, the former coming from the gracile and cuneate nuclei of the opposite side, the latter from the same side, which also pass to the superior vermis; (3) the acustico- cerebellar tract, composed of fibers the axons of the sensory end nuclei (Deiters) of the vestibular portion of the auditory nerve. It is probable that all these fibers decussate prior to their final termination. Fig. 257. — View of Cerebellum in Section, and OF FotTRTH Ventricle, with the Neighboring Parts. — {From Sappey.) 1. Median grdove fourth ventricle, ending below in the calamus scriptorius, with the longitudinal eminences formed l^y the fas- ciculi teretes, one on each side. ^. The same groove, at the place where the white streaks of the auditory nerve emerge from it to cross the floor of the ventricle. 3. Inferior peduncle of the cerebellum, formed by the restiform body. 4. Posterior pyramid; above this is the calamus scriptorius. 5, 5. Superior peduncle of cerebellum, or processus e cerebello ad testes. 6, 6. Fillet to the side of the crura cerebri. 7, 7. Lateral grooves of the crura cerebri. 8. Corpora quadri- gemina. — {Ajter Hirschjeld and Leveilh.) THE CEREBELLUM. 571 The cerebellum through this system of efferent and afferent fibers is brought into relation with many different regions of the cerebrum, pons, medulla, and spinal cord. Each half of the cerebellum is con- nected with the foregoing structures of the same side, but more espe- cially of the opposite side. THE FUNCTIONS OF THE CEREBELLUM. From the observations of the results of experimental lesions, from analysis of clinico-pathologic facts, and from its comparative anatomic development in different animals, the deduction has been drawn that the cerebellum coordinates and har- monizes the action of those muscles the activities of which are necessary to the maintenance of body equilibrium both during station and progression. By equilibrium of the body is un- derstood a condition which may be maintained for a variable length of time without displacement, and if possible only so long as a line passing through the center of gravity falls within the base of support. The sup- port offered by the earth to the feet neutralizes and counteracts the force of gravity. In station, when the body is in the erect or military position, the arms by the side, the center. of gravity lies between the sacrum and the last lumbar vertebra, and the line of gravity falls between the feet and within the base of support. The en- tire skeleton for the time being is rendered fixed and rigid at all its joints by the combined action of the muscles connected with it. That this position may be maintained all the different groups of antagonistic but cooperative muscles must be accurately coordinated in their ac- tions. Any failure in this respect is at once attended by a disturbance of the equilibrium and displacement. In progression, walking, running, dancing, etc., the body is trans- lated from point to point by the alternate action of the legs. Whether the direction of the translation be linear or curvilinear, as the legs change their position from moment to moment, the center of gravity also changes, and at once the equilibrium is menaced. If it is to be maintained and displacement prevented there must be a prompt readjustment in the relation of all parts of the body so that the line of gravity falls again within the base of support. The more com- plicated the movements of progression, or the narrower the base of support, the greater is the danger to the equilibrium, and hence the Fig. 258. — Section OP Cerebellar Cortex. A. Outer or molecular layer. B. Inner or granular layer. C. White matter, a. Cell of Purk- inje. b. Small cells of inner layer c. Dendrites of these cells, d. A similar cell lying in the white matter. — (Stirling.) 572 TEXT-BOOK OF PHYSIOLOGY. necessity for rapid and compensatory changes in coordinated muscle activity. All movements of this character, in man at least, are pri- marily volitional and require for their performance the constant exercise of the attention. With frequent repetition they gradually come to be performed independently of consciousness and fall into the cate- gory of secondary or acquired reflexes. Though coordinating power is exhibited by the spinal cord, medulla, and basal ganglia, it is only in the cerebellum that this power attains its highest development and differentiation. To it is assigned the power of selecting and grouping muscles, not in any restricted part, but in all parts of the body, and coordinating their actions in such a manner as to preserve the equilibrium. The Results of Experimental Lesions. — If the cerebellum in its totality, coordinates and harmonizes the action of the muscles on the opposite sides of the body, any derangement of its structure or its connections with the cord, medulla, pons, or basal ganglia should at once be followed by incoordination of muscles and a want of har- ms'' ■ \ iv ' " '^"'■^^^'^ "^^ Fig. 259. — Attitude Assumed After Destruction of the Left Half of the Cerebellum. — (Moral and Doyon, ajter Thomas.) mony in their action. Experimental lesions of the cerebellum are attended by such results. The phenomena observed are many and complex. They differ in extent and character in different animals and in accordance with the extent and location of the lesion, though the note of incoordination runs through them all. Removal of one lateral half of the cerebellum in the dog is followed by an inability to maintain the equilibrium necessary to the erect posi- tion. On attempting to stand, the animal at once falls toward the side of the lesion, the muscles of which at the same time contract and give to the body a distinctly curved condition (Fig. 259). The anterior limbs are extended to the opposite side. On making efforts to re- gain the standing position, the animal may roll over around the long axis of its body. Conjugate deviation of the eyes is frequently ob- served as well as nystagmus. After a few days the symptoms partially subside and the animal acquires the power of sitting on the abdomen when the anterior limbs are widely extended (Fig. 260). As the days go by the improve- ment continues, and the animal recovers the power of walking, though THE CEREBELLUM. 573 Fig. 260. — Attitude in Repose after THE Complete Removal of the Cere- bellum BUT DURING THE PERIOD OF RES- TORATION OF Function. — (Moral and Doyon, after Thomas.) each step is attended with tremor and oscillations of the body. Any change in the center of gravity such as results when one leg is lifted may result in a fall toward the side of the lesion, owing to an inability to promptly bring about the necessary compensatory muscle actions. With time the animal continues to improve in its power of adjustment, though it never completely recovers it. Movements of progression are apt to be characterized by stiffness and accompanied by tremor suggestive of volitional . efforts. Total removal of the cerebel- lum is followed by a different train of symptoms. The extensor muscles apparently preponderate in their action, for the limbs are extended and abducted, the head and neck are retracted, and opisthotonos is established. In time these effects also partially subside, though all attempts at walking are permanently accom- panied by tremor and oscillations. The characteristic effect which follows section of the peduncles is again incoordination, manifesting itself in deviation of the head, eyes, inability to walk, tremor on exertion, etc. The effects vary, however, according to the peduncle divided. Section of the middle peduncle gives rise to the most pro- nounced effects. The head and the anterior part of the body are at once drawn toward the pelvis on the side of the section. A voluntary effort on the part of the animal causes it to lose all control of its muscles and the body is rotated in the direction of its longitudinal axis from 40 to 60 times a minute before it comes to rest. According as the lesion is made from behind or before, the rotation is from or to the side of the section. In time these symptoms subside, though the animal never completely recovers. The partial recovery of the power of coordination, observed after removal of a portion or the whole of the cerebellum, indicates that the centers in the cord, medulla, pons, and cerebrum endowed with corresponding though less developed power, develop compensatory activity and acquire to some extent the capabilities of the cerebellum itself (Fig. 261). Fig. 261. — Progression after Destruction OF the Vermis. — {Moral and Doyon, ajler Thomas.) 574 TEXT-BOOK OF PHYSIOLOGY. Clinico- pathologic facts partly corroborate the results of phys- iologic investigations. In various forms of uncomplicated cerebellar disease, vertigo, tremor on making voluntary efforts, difficulty in main- taining the erect position, unsteadiness in walking, opisthotonos, pleurothotonos, are among the symptoms generally observed. Comparative anatomic investigations reveal a remarkable correspond- ence between the development of the cerebellum and the complexity of the movements exhibited by animals. In those animals whose movements are complex and require for their performance the coopera- tion of many groups of muscles the cerebellum attains a much greater development in reference to the rest of the brain than in animals whose movements are relatively simple in character. This relative increase in the development of the cerebellum is found in many animals, such as the kangaroo, the shark, the swallow, and the predaceous birds generally. The Coordinating Mechanism. — Though it is not known how the cerebellum selects and coordinates groups of muscles for the per- formance of any complex movement, it is known that its activity is largely reflex in origin and excited by impulses reflected to it from peripheral organs. In this as in other forms of reflex activity the mechanism involves (i) afferent nerves, e. g., cutaneous, muscle, optic, and vestibular, and their related end-organs, tactile corpuscles, muscle spindles, retina, and semicircular canals, all indirectly connected with (2) the cerebellar centers; (3) efferent nerves indirectly connected with (4) the general musculature of the body. Both station and pro- gression are directly dependent on the development and transmission of afferent impulses from the previously mentioned peripheral sense- organs to the cerebellum. Tactile, muscle, visual, and labyrinthine impressions and sensations not only cooperate in the development and organization of the motor adjustments necessary to the main- tenance of the equilibrium and locomotive coordination, but even after their organization they are necessary to the excitation of cerebellar activity. The manner in which they lead to the development of this capability on the part of the cerebellum is conjectural. Their ever- present influence is shown by the effects which follow their removal, as the following facts indicate. The prevention of the development of tactile impulses by freezing or anesthetizing the soles of the feet, and the blocking of normally de- veloped impulses through destruction of afferent pathways in diseases of the spinal' cord lead at once to marked impairment in the coordinat- ing power. The removal of the skin from the hind legs of the frog, previously deprived of its cerebrum, destroys its coordinating power, which it would otherwise possess in a high degree. The blocking in consequence of destructive lesions of the spinal cord, of the impulses, which come from the muscles^ tendons, etc., and which inform us of the activity and the degree of activity of our muscles, the location of the limbs, the amount of effort necessary THE CEREBELLUM. 575 to produce a given movement, etc., also gives rise to much incoor- dination. A blocking of both tactile and muscle impulses frequently exists in degeneration or sclerosis of the posterior columns of the spinal cord. The coordinating power is so much impaired in this disease that the patient is unable to maintain, without strained effort, the erect position and especially if the directive power of the eyes be removed by closure of the lids. Walking becomes extremely difficult; the gait is irregular and jerky, and equilibrium is maintained only by keeping the eyes fixed on the ground in front and by artificially increasing the basis of support by the use of canes. An interference with the development of the customary visual impressions which in a measure maintain the sense of relation of the individual to surrounding objects also gives rise to equilibratory dis- turbances. A rapid change in the relation of the individual to sur- rounding objects or the reverse; a change in the direction of one optic axis from the use of a prism or from paralysis of an eye muscle; the destruction of an eye; — these and similar conditions frequently give rise to such marked disturbances of the equilibratory power that displacement is difficult to prevent. An interference with the development of the so-called labyrin- thine impressions by destruction of the semicircular canals gives rise to the most remarkable disturbances in this respect. Section of one horizontal canal* in the pigeon is followed by oscillations of the head in a horizontal plane around a vertical axis. Bilateral section so increases these oscillations that the pigeon is unable to maintain equilibrium and forced to fall and turn continuously around the vertical axis. Bilateral section of the posterior vertical canals gives rise to oscillations around a horizontal axis which frequently become so exaggerated as to eventuate in the turning of backward somersaults, head over heels. Similar phenomena follow division of the superior vertical canals. Bilateral destruction of both sets of canals is attended by extra- ordinary disturbances in the equilibrium. From the moment of the operation the animal, the pigeon, loses all control of its motor mechan- isms. It can neither maintain a fixed attitude nor execute orderly movements of progression; its activity, continuous and uncontrollable, is characterized by spinning around a vertical axis, turning somer- saults, dashing itself against surrounding objects until life is en- dangered. If the animal be protected from injury, these disturbances gradually subside, and in the course of a few months the equilibratory power is so far regained that standing and walking at least become possible. In this condition, however, the coordinating power is directly dependent on visual impulses, for with the closure of the eyes all the previous motor disturbances at once recur. These and similar facts indicate that the semicircular canals are the peripheral *The physiologic anatomy of the semicircular canals is described in the chapter devoted to the ear, to which the reader is referred. 576 TEXT-BOOK OF PHYSIOLOGY. sense-organs from which come the nerve impulses most essential to the excitation of the cerebellar coordinative centers in their control of equilibrium and of progression. The cerebellum may therefore be regarded as the essential, most highly differentiated portion of the coordinating mechanism con- cerned in the maintenance of equilibrium, during both station and progression. The manner in which the cerebellum accomplishes this result is unknown, though it is certain, from the foregoing facts, that its special mode of activity is dependent on the excitatory action of nerve impulses reflected from a variety of peripheral sense-organs. CHAPTER XXIII. THE CRANIAL NERVES. The nerve-trunks which serve as channels of communication between the encephalon and the structures of the head, the face, and in part the organs of the thorax and abdomen, pass through for- amina in the walls of the cranium, and for this reason are termed cranial nerves. According to the classification now generally adopted, there are twelve cranial nerves on either side of the median line, which, enu- merated from before backward, are as follows (Fig. 262) : First or Olfactory. Seventh or Facial. Second or Optic. Eighth or Auditory. Third or Oculo-motor. Ninth or Glosso-pharyngeal. Fourth or Patheticus. Tenth or Pneumogastric or Vagus. Fifth or Trigeminal. Eleventh or Spinal Accessory. Sixth or Abducens. Twelfth or Hypoglossal. The cranial nerves may be classified physiologically in accordance with their functional manifestations into three groups, viz. : I. Nerves of Special Sense: e.g., Olfactory, Optic, Auditory, Gustatory (Glosso- pharyngeal). ■.i. Nerves of General Sensibility: e.g., Large root of the Trigeminal, Glosso- pharyngeal, and Pneumogastric. 3. Nerves of Motion: e. g., Oculo-motor, Patlieticus, the small root of the Trigeminal, .\bducens, Facial, Spinal Accessory, and Hypoglossal. Though this classification in the main holds true, it must be borne in mind that modern investigations have demonstrated that the glosso- pharyngeal and pneumogastric nerves contain even at their junction with the medulla oblongata a number of efferent or motor fibers, and to this extent are mixed nerves. The Origins of the Cranial Nerves.— In accordance with modern views as to the origins of nerves in general, it may be stated that — The nerves of special sense have their origin respectively in the neuro-epithelial cells in the mucous membrane of the olfactory region of the nose, in the ganglion cells of the retina, in the cells of the spiral ganglion of the cochlea and the ganglion of Scarpa, and in the cells of the petrous and jugular ganglia. From the cells of these ganglia dendrites pass peripherally to become associated with specialized end-organs, while axons pass centrally in well-defined bundles ' to become related by means of their end-tufts with primary basal ganglia. The nerves of general sensibility have their origin in the ganglia on their trunks, and in this respect resemble the spinal nerves. From the ganglion cell there emerges a short axon process which soon divides into a central and a peripheral branch. The former passes 37 577 578 TEXT-BOOK OF PHYSIOLOGY. toward and into the gray matter located beneath the floor of the fourth ventricle, where its end-tufts arborize about nerve-cells. The latter (the peripheral branch) passes toward the general periphery to be distributed to skin and mucous membranes (Fig. 263). The nerves of motion have their origin in the nerve-cells in the gray matter beneath the aqueduct of Sylvius and beneath the floor of the fourth ventricle (Fig. 264). The axons emerging from these cells course peripherally to be distributed to skeletal muscles. In some of the motor nerves, and in some sen- sory nerves as well, there are to be found efferent fibers of smaller size which have a similar origin and which become related through the intervention of sympathetic ganglia (peripheral neurons) with visceral muscles and glands. These nerves have been termed autonomic nerves. The Cortical Connections of the Cranial Nerves. — Each of these three groups of cranial nerves has special connections with the cerebral cortex. The nerves of special sense for the most part terminate in primary basal ganglia, around the cells of which their central end-tufts arbor- ize. From these cells axons arise which pass upward and directly or indirectly come into physiologic re- lation with sensor nerve-cells in the cerebral cortex. The tierves of general sensibility terminate in the gray matter be- neath the floor of the fourth ventri- cle, around the nerve-cells of which their end-tufts arborize. These groups of nerve-cells are known as sensor end-nuclei. Though once regarded as the centers of origin of the sensor nerves, they are now regarded as the centers of origin of axons which pass upward to the cortex of the cerebrum, where they also come into physiologic relation with sensor nerve-cells. The axons in both of these classes of nerves thus originate in the cells of the central nerve system a,nd continue upward to the cere- brum, the primary afferent path. The motor nerves which have their origin in the cells of the gray matter beneath the aqueduct of Sylvius and beneath the floor of the fourth ventricle are in physiologic relation with nerve-cells in the Fig. 262. — Superficial Origin of THE Cranial Nerves from the Base OF THE Encephalon. I. Olfactory. 2 Optic. 3. Motor oculi. 4. Patheticus. 5. Trigeminal. 6. Abducens. 7. Facial. 7'. Nerve of Wiisberg. 8. Auditory. 9. Glosso-pharyngeal. 10. Pneumogastric. II. Spinal accessory. 12. Hypoglossal. — {Moral and Doyon.) THE CRANIAL NERVES. S79 motor region of the cortex through descending axons contained in the pyramidal tract, the end-tufts of which arborize around the nerve- cells. The efferent path beginning in the cerebral cortex is thus continued by the motor nerves to the general periphery. The three groups of nerves, those of special sense, of general sensibility, and the motor nerves, are neurons of the first order; the :,' Fig. 263. — Ganglia op Origin of the Sensor Cranial Nerves, i. Trigem- inal (ganglion of Gasser).. .i. Nerve of Wrisberg. 3. Auditory (ganglion of Scarpa). 4. Glosso-pharyngeal (gang- lion of Andersch). J. Pneumogastric (ganglion plexiformis). — {After Moral and Doyon.) Fig. 264. — Nuclei of Origin of the Motor Cranial Nerves, i. Motor oculi. 2. Patheticus. 3. Motor root of the trigeminal. 4. Abducens. 5. Fa- cial. 6. Mixed nucleus for efferent fibers of the glosso-pharyngeal vagus and spinal accessory. 7. Hypoglossus. 8. Spinal accessory, g. Spinal nerves. — (After Moral and Doyon.) nerve-cells and fibers which constitute the cerebral connections are neurons of the second order. It is probable that the sensor cells in the cerebral cortex are neurons of a third order. FIRST PAIR. THE OLFACTORY. The first cranial nerve, the olfactory, is situated in the upper third of the nasal fossa, in the regio olfactoria. It consists of from 20 to 30 branches, the fibers of which are non-meduUated. Origin. — The olfactory nerve is composed of centrally coursing S8o TEXT-BOOK OF PHYSIOLOGY. axons which have their origin in the central ends of bipolar, rod- shaped, or spindle-shaped nerve-cells interspersed among the epithelial cells covering the mucous membrane in the regio olfactoria; the peripheral ends of these cells give off a number of dendrites which are spread out to form a delicate feltwork over the surface of the mucous membrane. From their origin the axons gradually converge to form bundles which ascend to the cribriform plate of the ethmoid bone, through the foramina of which they pass to become related by their end-tufts with structures in ■■ — ^ the gray matter of the olfac- tory bulb (Fig. 265). Cortical Connections. — The olfactory bulb and olfactory tract', formerly called the olfactory nerve, are portions of the cere- brum (the olfactory lobe) which arise embryologically by a protrusion of the walls of the cerebral cavity. The bulb is oval-shaped and consists of both gray and white matter. It rests on the cribriform plate of the ethmoid bone and is em- braced by the olfactory nerves. As seen on sagittal section, there is just be- neath the surface a layer of large pyramidal and spin- dle-shaped cells (termed also mitral cells) , each pro- vided with an apical and two lateral dendrites. The apical dendrite passes toward the surface and ends in a brush- or basket-like expansion which interlaces with the end-tufts of the olfactory nerves, forming what are known as the olfactory glomerules. The lateral dendrites end free. The axons of the pyramidal cells pass toward the center of the bulb and bend at right angles, after which they pursue a horizontal direction toward and into the olfactory tract. This tract is about five centimeters in length, prismatic in shape on cross-section and divisible into a ventral and a dorsal portion. It emerges from the posterior extremity of the bulb, passes backward to the posterior part of the anterior lobe, where it divides into three roots: viz., a lateral or external, a mesial or internal, a middle or dorsal. The fibers of the lateral and mesial foots are derived almost exclusively from the ventral portion of the tract, the fibers of which come from the mitral cells in the bulb. Fig. 265. — The Relation of the Olpactoey Nerves to the Olfactory Tract. 1. Ol- factory nerve-cell. 2. Axon process. 3. Epi- thelial cells. 4. Glomerulus. 5. Mitral cells. 6. Centrally coursing axons of the olfactory tract. — {Morat and Doyon.) THE CRANIAL NERVES. S8i The lateral root-fibers pass outward into the fossa of Sylvius and come into relation with nerve-cells in the inferior extremity of the gyrus hippocampus and the gyrus uncinatus. The mesial fibers pass inward and come into relation with nerve-cells in the pre-callosal part at least of the gyrus fornicatus. The fibers thus far considered are undoubt- edly true olfactory fibers, pursuing a centripetal direction, carrying nerve impulses from the olfactory cells to the cerebrum (Fig. 266). Histologic and embryologic inethods of research have shown that some of the fibers in the olfactory tract are cen- ^^ | flilRiliit' trifugal in direction. They originate in the olfactory cortical areas, pass toward the peri- phery as far as the an- terior commissure, where they cross to become the dorsal root, enter the olfactory tract, and finally terminate in the bulb. This tract serves to connect the cortex with the bulb of the op- posite side, and carries impulses from the cortex to the bulb. The two opposite cerebral olfac- tory areas are also united by commissural fibers which decussate at the anterior commissure. Function. — The function of the olfactory system in its entirety, is the transmission of nerve impulses from its origin in the olfactory region of the nose to the cerebral cortex, where they evoke sensations of odor. The stimulus to its excitation is the impact and chemic action of gaseous or volatile organic matter on the dendrites of the olfactory cells. The sensitiveness of the olfactory end-organ to the action of many sub- stances is remarkable, responding, for example, to the xT^-ninro" of a gram of oil of roses and to the ^re^Tinnr of a gram of mercaptan. Division or destruction of the olfactory path at any point is followed by an abolition of the sense of smell on the corresponding side. De- structive lesions of the hippocampal and uncinate gyri are followed by similar results. -P olf. Fig. 266. — Olfactory Lobe of the Human Brain. —Bu. Olfactory bulb. T. Tract. Tr.o. Trigone. R. Rostrum of corpus callosum. p. Peduncle of corpus callosum, passing into G. s., gyrus subcallosus (diagonal tract, Broca). Br. Broca's area. F.p. Fissura prima. F.s. Fissura serotina. C.a. Position of anterior com- missure. L.t. Lamina terminalis. Ch. Optic chiasma. T.o. Optic tract, p. olf. Posterior olfactory lobule (or anterior perforated space), m.r. Mesial root. l.r. Lateral root of tract. — (His.) — {After Quain.) S82 TEXT-BOOK OF PHYSIOLOGY. SECOND PAIR. THE OPTIC. The second cranial nerve, the optic, consists of centrally coursing axons of neurons, which connect tie essential part of the organ of vision, the retina, with sensory end-nuclei or ganglia situated at the base of the cerebrum. Origin. — The axons which constitute the optic nerve have their origin in the ganglionic cells in the anterior part of the retina. Through their dendrites these cells are brought into relation posteriorly with successive layers of cells which collectively constitute the retina. Though the retina is said to consist of ten or eleven layers, it may be reduced practically to three, viz. (Fig. 267) : 1. The layer of visual cells. 2. The layer of bipolar cells. 3. The layer of ganglionic cells. The visual cells present peripherally modified dendrites, known as the rods and cones; centrally they give off an axon which after a short course terminates in an end-tuft. The bipolar cells also possess dendrites and an axon; the former interlace with the end-tufts of the visual cell axon, the latter with the dendrites of the ganglion cell. The retina may be regarded therefore as the peripheral end- organ in which the optic nerve originates. From their origin the axons turn backward, at the same time converging to form a distinct bundle which passes through the chorioid coat and sclera. After emerging from the eyeball the nerve-bundle (the optic nerve) passes backward as far as the sella turcica, traversing in its course the orbit cavity and the optic foramen. At the sella turcica there is a union and partial decussation in man and other mammals of the two nerves, forming the optic chiasm.* Decussation of the Optic Nerves. — The ex- tent to which the fibers from each eye decussate at the chiasm is a subject of dispute but the results of various methods of research would seem to indicate, that the fibers from the nasal third of the retina of the left eye cross in the chiasm, to unite with the fibers from the tem- poral two-thirds of the retina of the right eye. In a similar manner the fibers from the nasal third of the retina of the right eye cross in the chiasm, and unite with the fibers from the temporal two-thirds of * Though the foregoing is the usual method of stating the origin and course of the optic nerve, nevertheless morphologically the true optic nerve lies wholly within the retina and is composed of the visual cells there found. The remainder of the visual system from and including the ganglion cells of the retina to the optic basal gainglia, is the optic tract, there being no anatomic or physiologic distinction between the optic nerve so called and the optic tract. Both are outgrowths from the brain and hence possess properties which differentiate them from other cranial nerves. Fig. 267. — Reti- nal Cells, s', z'. Visual cells with their peripheral ter- minations, s. Rods. z. Cones, b. Bi- polar cells, g. Gan- glion cells from which arise the axons of the optic nerve. THE CRANIAL NERVES. 583 the retina of the left eye (Fig. 268). Posterior to the chiasm the crossed and uncrossed fibers form the so-called optic tracts, which after winding around the crura cerebri enter the optic basal ganglia. Transection of the optic nerve shows that it is composed of an enormous number of non-medullated nerve-fibers, estimated by Salzer at from 450,00c to 800,000, enclosed in a sheath of the dura mater. The visual fibers comprising the optic nerve may be physiologicly divided into two classes, (a) those coming from the peripheral portion of the retina, and (b) those com- ing from that central area Vismimw VISUM HEW known as the macula lutea. The retinal fibers are by far the more abundant, and make up the major portion of the nerve; the macular fibers are less abundant. An examina- tion of a cross-section of the optic nerve shows the presence of a wedge-shaped tract occu- pying the center of the nerve and which is regarded as com- posed of the macular fibers At the chiasm this bundle of fibers undergoes a partial de- cussation similar to that of the fibers coming from the more peripheral portions of the retina. In the left optic tract, therefore, fibers from at least four different regions are to be found: viz., the two-thirds of the temporal side of the left retina, the temporal half of the left macula, the nasal third of the right retina, and the nasal half of the right macula. Corresponding fibers are to be found in the right optic tract. As the optic tract passes around the crus cerebri it divides into a lateral or outer, and a mesial or inner bundle, which then terminate in the optic basal ganglia. The fibers of the lateral bundle are traceable into the lateral or external geniculate body (the pre-geniculum), the pulvinar of the optic thalamus, and the anterior quadrigeminal body (the pre-geminum). These are in all probability the true visual fibers. The fibers of the mesial bundle are traceable into the internal geniculate body (the post-geniculum) and the posterior quadrigeminal body (the post-geminum). Fig. 268. — Diagram Illustrating Left Homonymous Lateral Hemianopsia from A Lesion op the Right Optic Tract or THE Right Cuneus. The Shaded Lines IN the Visual Fields Indicate the Darkened Area. 584 TEXT-BOOK OF PHYSIOLOGY. Cortical Connections.— After entering the basal ganglia the visual fibers terminate in end-tufts which arborize around nerve-cells. From these cells new axons arise which ascend through the posterior part of the internal capsule, at the same time curving backward to form the optic radiation of Gratiolet, and terminate finally around nerve- cells in the gray matter of the cuneus and in the gray matter bordering the calcarine fissure, both situated on the mesal aspect of the occipital lobe. Centrifugal Fibers of the Optic Nerve. — All the fibers previously alluded to have been afferent or centripetal in direction; but the optic nerve also contains efferent or centrifugal fibers which come from nerve-cells in the basal ganglia and ramify around special cells, the amacrine cells, in the retina. Their function is unknown. It has been suggested that they regulate the vascular supply to the retina. Centrifugally coursing fibers also connect the visual areas of the cortex with the basal ganglia. Function. — The function of the visual apparatus in its entirety is the transmission of nerve impulses from the retina to the cerebral cortex where they evoke the sensations of light and its different qualities — colors. The specific physiologic stimulus to the retinal visual cells is the impact of the undulations of the ether. In general it may be said that, at least for the same color, the intensity of the objective undulation or vibration, determines the intensity of the sen- sation. Pupillary Fibers. — The optic nerve also contains nerve-fibers somewhat larger in caliber than the usual visual fibers, which are supposed to form the afferent path for those nerve impulses which excite reflexly a contraction of the sphincter pupillm muscle, thus vary- ing the size of the pupil. These fibers, termed pupillary fibers, come from all portions of the retina but most abundantly from the posterior pole in and around the macula. The existence of these fibers is confirmed by pathologic findings. In a manner similar to that of the visual fibers they, too, undergo a decussation in the optic chiasm, so that in the optic tract there are pupillary fibers which come from the temporal side of the eye of the corresponding side, and fibers which come from the nasal side of the eye of the opposite side (Fig. 272). The central termination of these fibers is not positively known. Hemiopia and Hemianopsia. — Division of the optic nerve between the eyeball and the optic chiasm is followed by complete blindness in the eye of the corresponding side. Owing to the partial decussation of the fibers in the chiasm, division of an optic tract is fol- lowed by a loss of sight in the outer two-thirds of the eye of the same side and in the inner third of the eye of the opposite side. To this loss of visual power in the retina the term hemiopia is given. In consequence of this loss of visual power in the retina there is a corresponding obscura- tion or total obliteration of nearly one-half of the visual field, to which the term hemianopsia is given. If, for example, the right optic tract THE 'CRANIAL NERVES. S8S is divided there will be hemiopia in the outer two-thirds of the right eye and the inner third of the left eye, with left lateral hemianopsia, and as the portions of the retina which are affected are associated in vision the loss of the visual fields is spoken of as homonymous hemi- anopsia (Fig. 268). A destructive lesion of the cerebral visual area, the cuneus and the adjacent gray matter on the right side, is also followed by left lateral hemianopsia.* The existence of an homonymous hemianopsia becomes evident when the individual is directed to focus the vision on an object placed directly in front and with its center in the median plane of the body, when if the lesion be on the right side, the left half of the object will be invisible. The reason for this will be ap- parent on reference to Fig. 269. All the light rays emanating from the left half of the object fall on the retina on the side of the injury, and hence there will be no sensation. If, however, the object be moved to the right without change in the position of the head, the entire object will be visible, as all the rays fall on the normal side. If, on the contrary, the object be moved to the left, it will be -in- visible for the opposite reason. Hemianopsia may be the result of either destruction of the optic tract or of the cortical visual area. The seat of lesion in any given case is indicated by a peculiarity of the iris reflex pointed out by Wernicke, which will be referred to in connection with the consideration of the oculo-motor Fig. 269. — Diagram to Show the Exist- ence OF Hemianopsia. The lesion is sup- posed to be in the right optic tract. nerve. THIRD PAIR. THE OCULO-MOTOR. The third cranial nerve, the oculo-motor, consists of some 15,000 peripherally coursing nerve-fibers which serve to bring the nerve-cells from which they arise into relation with a large portion of the general musculature of the eye. Origin. — The axons composing the third nerve arise from a series * It should be borne in mind tliat in both instances the retina itself is unaffected. The impact of Hght generates, as usual, nerve impulses which proceed as far backward as the point of division or destruction. In consequence those portions of the cerebral cortex stimulation of which evokes the sensation of light remain unaffected and the individual does not become aware through sensation, of the presence of a luminous body in the left side of the visual field. S86 TEXT-BOOK OF PHYSIOLOGY. of seven or eight groups of nerve-cells, located in the gray matter beneath the floor of the aqueduct of Sylvius. From each of these groups or nuclei, bundles of axons emerge, which after a short course unite to form the common trunk. The large majority of the fibers in the nerve come directly from the nuclei of the same side; the remainder come from a, group of cells on the opposite side of the median line. There is thus a partial decussation of its fibers (Fig. 270). The different groups of cells, the nuclei of origin, are arranged in a serial manner. The anatomic arrangement of these nuclei would indicate that each nucleus is related to an individual member of the eye-group of muscles. Clinical observation and the investiga- tion of the results of pathologic pro- cesses have not only shown that this is the case, but have succeeded in locating the position of the nucleus for any given muscle. Though there is some difference of opinion in regard to the exact location of one or two of .>^'^^s«!''S^ ^^^ nuclei, the tabulation subjoined is ^,»'J^^^j^^^^^^^ S- Crico- thyroid articulations. 6. Union of^the cricoid cartilage and of the trachea. 7. Epiglottis. 8. Ligament uniting it to the reentering angle of the thyroid car- tilage. — {Sappey.) shape. Each cartilage is a triangular pyramid, the apex of which is recurved, and directed backward and inward. The base presents three angles — an anterior, an external, and an internal. The anterior angle is long and pointed and projects forward in a horizontal plane. .It serves for the attachment of the vocal membranes and is therefore termed the vocal process. The external angle is short, rounded, and prominent, and serves for the attachment of muscles. The internal angle- affords a point of insertion for a ligament. The inferior surface of the arytenoid is concave for articulation with the convex surface of the cricoid facet. Its long axis, however, is directed from before PHONATION; ARTICULATE SPEECH. 629 backward and almost at right arjgles to the long axis of the cricoid facet. The cornicula laryngis and the cuneiform cartilages are small nodules of yellow elastic cartilage embedded in a fold of membrane which unites the arytenoid and the epiglottis. They are fragments of a ring of cartilage which in some animals — e. g., anteater — extends between these two cartilages. The crico-thyroid articulation is formed by the opposition of the tip of the inferior cornu of the thyroid cartilage and an articular facet on the side of the cricoid. The joint is provided with a synovial membrane and enclosed by a capsular ligament. The movements permitted at this joint take place around a horizontal axis and consist of an upward and downward movement of both the thyroid and cricoid, combined with a sliding movement of the latter upward and backward. The crico-arytenoid articulation is formed by the apposition of the articulating surfaces of the cricoid and arytenoid cartilages. This joint is provided with a synovial membrane and enclosed by a loose capsular ligament which would permit of an extensive sliding of the arytenoid cartilage downward and outward were it not prevented by the posterior crico-arytenoid ligament, which is attached, on the one hand, to the cricoid, and, on, the other, to the inner angle of the arytenoid. The movements permitted at this joint are : (i) Rotation of the arytenoid around a vertical axis which lies close to its inner surface. (2) A sliding motion inward and forward with inward rotation of the vocal process, or a sliding motion outward and back- ward with outward rotation of the vocal process. In either case the process describes an arc of a circle. (3) A sliding movement towards the median line in consequence of which the inner surfaces of the arytenoids are brought almost in contact. The crico-thyroid membrane is composed mainly of elastic tissue. It may be divided into a mesial and two lateral portions. The mesial portion is well developed, triangular in shape, and unites the con- tiguous borders of the cricoid and thyroid cartilages. The lateral portion is attached below to the superior border of the cricoid. From this attachment it passes upward and inward under cover of the thyroid. As it ascends it elongates and becomes thinner, and is finally attached anteriorly to the thyroid near the median line, and posteriorly to the vocal process of the arytenoid, thus constituting the inferior thyro-arytenoid ligament. It is covered internally by mucous membrane and externally by the internal thyro-arytenoid muscle. The free edge of this ligament forms the basis of the true vocal band. A superior thyro-arytenoid ligament forms the basis of the false vocal band. The thyro-hyoid membrane, composed of elastic tissue, uni^tes the superior border of the thyroid to the hyoid bone. The mucous membrane lining the larynx is thin and pale. As 630 TEXT-BOOK OF PHYSIOLOGY. it passes downward it is reflected over the superior thyro-arytenoid ligament, and assists in the formation of the false vocal band; it then passes into and lines the ventriclCj after which it is reflected inward over the superior border of the thyro-arytenoid muscle and ligament, and assists in the formation of the true vocal band; it then returns upon itself and passes downward over the lateral portion of the crico- thyroid membrane into the trachea. Fig. 290. — Posterior View of the Muscles of the Larynx, i. Posterior crico-arytenoid muscle. 2, 3, 4. Differ- ent fasciculi of the arytenoid muscle. 5. Aryteno-epiglottidean muscle. — {Sap- pey.) The thin, free, reduplicated edge of the mucous membrane constitutes the true vocal band. The surface of the mucous mem- brane is covered by ciliated epithelium except in the im- mediate neighborhood of the vocal bands. The vocal bands are attached anteriorly to the thyroid cartilage near the receding angle and posteriorly to the vocal processes of the arytenoid cartilages. They vary in length in the male from 20 to 25 mm. and in the female from 15 to 20 mm. The Muscles of the Larynx. — The muscles which have a direct action on the cartilages of the larynx and determine the position of the vocal bands both for respiratory and phonatory purposes, and which Fig. 291. — Lateral View of the Muscles of the Larynx, i. Body of the hyoid bone. ^. Vertical section of the thyroid cartilage. 3. Horizontal sec- tion of the thyroid cartilage turned down- ward to show the deep attachment of the crico-thyroid muscle. 4. Facet of articulation of the small cornu of the thyroid cartilage with the cricoid cartilage. 5. Facet on the cricoid cartilage. 6. Superior attachment of the crico-thyroid muscle. 7. Posterior crico-arytenoid muscle. 8, 10. Arytenoid muscle. 9. Thyro-arytenoid muscle, ii. Aryteno- epiglottidean muscle. 12. Middle thyro- hyoid ligament. 13. Lateral thyro-hyoid hgament. — {Sappey.) PHONATION; ARTICULATE SPEECH. 631 regulate their tension as well, are nine in number and take their names from their points of origin and insertion: viz., two posterior crico- arytenoids, two lateral crico-ar.ytenoids, two thyro-arytenoids, one arytenoid, and two crico-thyroids (Figs. 290 and 291). The posterior crico-arytenoid muscle lies on the posterior surface of the quadrate plate of the cricoid cartilage, on either side of the median line, from which it takes its origin. The fibers of the muscle pass upward and outward and in their course converge to be inserted into the external angle of the arytenoid cartilage. The superior and more horizontally directed fibers rotate the arytenoid around its vertical axis; the inferior and obliquely directed fibers draw the cartilage downward and inward. As a result of the action of the muscle in its entirety, the vocal process is turned upward and outward, and as the vocal band is carried with it the glottis is widened, a condition nec- FiG. 292. — Glottis Widely Opened FROM Simultaneous Conteaction op Both Crico-arytenoid Muscles, b. Epiglottis, rs. False vocal band. ri. True vocal band. ar. Arytenoid car- tilages, a. Space between the arytenoids. c. Cuneiform cartilages, ir. Interarytenoid fold. rap. Aryepiglottic fold. cr. Car- tilage rings. — (Mandl.) Fig. 293. — Position op the Vocal Bands Due to the Simultaneous Contraction of Both Lateral Crico- arytenoid Muscles and Both Thyro- arytenoid Muscles, b. Epiglottis, rs. False vocal band. ri. True vocal band. or. Space between the arytenoid cartil- ages, the glottis respiratoria. ar. Ary- tenoid cartilages, c. Cuneiform carti- lages, rap. Aryepiglottic fold. ir. In- terarytenoid fold. — {Mandl.) essary to the free entrance of air into the lungs (Fig 292). Since the contraction of the crico-arytenoid has this result, it is generally spoken of as the abductor or respiratory muscle. The lateral crico-arytenoid muscle arises from the side of the cricoid cartilage. From this point its fibers are directed upward and back- ward to be inserted into the external process of the arytenoid. Its action is to draw the arytenoid cartilage forward and inward, thus approximating and relaxing the vocal band. The thyro-arytenoid muscle arises from the inferior two-thirds of the inner surface of the thyroid cartilage just external to the median line. From this origin the fibers pass backward and outward, to be inserted into the anterior surface and external angle of the arytenoid cartilage. The inner portion of the muscle lies close to and supports, if it does not constitute a part of, the vocal band. The action of the thyro-arytenoid muscle in conjunction with the lateral crico-arytenoid is to rotate the arytenoid cartilage around the vertical axis and to 632 TEXT-BOOK OF PHYSIOLOGY. draw the vocal process forward and inward, thus carrying the vocal cord toward the median line. When the muscles of the two sides simultaneously contract, the vocal bands are closely approximated and the space between them, the rima vocalis, reduced to a mere slit, one of the conditions essential to phonation (Fig. 293). The arytenoid muscle consists (i) of transversely arranged fibers which arise from and are inserted into the outer surface of the oppo- site arytenoid cartilages, and (2) of obliquely directed fibers which arise from the outer angle of one arytenoid to be inserted into the apex of the other. In their course they decussate in the median line. The action of this muscle is to approximate the arytenoid cartilages and thus obliterate that portion of the glottis between the vocal proc- esses, the rima respiratoria, and so direct the expiratory blast of air toward and through the rima vocalis. The collective actions of the three foregoing muscles is to close or constrict the glottis, and for this reason they are spoken of as the adductor or phonatory muscles. The crico-thyroid muscle arises from the side and front of the cricoid cartilage and is inserted above into the lower border of the thyroid cartilage. The action of this muscle is to draw up the an- terior part of the cricoid cartilage toward the thyroid, which remains stationary, and to swing the quadrate plate of the cricoid and the arytenoid cartilages downward and backward. This movement has the result of tensing the vocal bands. The cricoid is at the same time drawn backward by the action of the more longitudinally dis- posed fibers. Nerves of the Larynx. — The nerves which innervate the muscles of the larynx and endow the mucous membrane with sensibility are derived from the vagus trunk. The superior laryngeal is for the most part sensor and distributed to the mucous membrane, though it contains motor fibers for the crico-thyroid muscle. The inferior laryngeal' is purely motor and is distributed to all the muscles with the exception of the crico-thyroid. THE MECHANISM OF PHONATION. Phonation, the production of vocal sounds in the larynx, is the result of the vibration of the vocal bands caused by an expiratory blast of air from the lungs. That a sound may arise it is essential that the glottis be approximately closed and the vocal bands be made more or less tense. The closure of the glottis — the approximation of the vocal proc- esses and the vocal bands — is accomplished, it will be recalled, by the contraction of the lateral crico-arytenoid, the arytenoid, and the thyro-arytenoid muscles. The increase in tension is accomplished by the contraction of the crico-thyroid and the thyro-arytenoid muscles, the former by the backward displacement of the cricoid and arytenoid PHONATION; ARTICULATE SPEECH. 633 cartilages, the latter by converting the natural concave edge of the vocal band to a straight line. The lengthening and tensing of the vocal bands by the crico-thyroid muscle is regarded by some investi- gators as a coarse means, the approximation of the free edges by the thyro-arytenoid, as a finer means, of adjustment for the production of slight changes in the pitch of sounds. The extent to which the glottis is closed and the membranes tensed will depend, however, on the pitch of the sound to be emitted. The appearance presented by the glottis just previous to the emission of a note of medium pitch, as determined by laryngologic examination, is shown in Fig. 294. When the foregoing conditions in the glottis are realized, the air stored or collected in the lungs is forced by the contraction of the expiratory muscles, through the narrow space between the bands. As a result of the resistance offered by this narrow outlet and the force of the expiratory muscles the air within the lungs and trachea is subjected to pressure, and as soon as the pressure attains a certain level the vocal Fig. 294. — Position of the Vocal Bands Previous to the Emission of a Sound, b. Epiglottis, rs. False vocal band. ri. True vocal band. ar. Ary- tenoid cartilages. — {Mandl.) Fig. 295. — Position of the Vocal Bands in the Production of Notes of Low Pitch. /. Epiglottis, or. Glottis. . ns. False vocal cord. ni. True vocal cord. ar. Arytenoid cartilages. — {Mandl.) bands are thrown into vibrations, which in turn impart to the column of air in the upper air-passages a corresponding series of vibrations by which the laryngeal vibrations are reinforced. The degree of pressure to which the air in the lungs and trachea is subjected was determined by Latour to vary from 160 mm. of water for sounds of moderate, to 940 mm. of water for sounds of highest intensity. .With the escape of the air or the separation of the vocal bands the vibration ceases and the sound dies away. The Characteristics of Vocal Sounds. — In common with the sounds produced by all other music instruments, all vocal sounds are characterized by intensity, pitch and quality, tone or color. The intensity or loudness of a sound depends on the extent or am- plitude of the upTand-down vibration or the extent of the excursion of the vocal band on either side of the position of equilibrium or rest; and this in turn depends on the force with which the blast of air strikes the band. The more forceful the blast of air, the larger, other things being equal, will be the primary vibrations of the bands, and hence the secondary vibrations of the air in the upper air-passages. 634 TEXT-BOOK OF PHYSIOLOGY. The pitch of the voice depends on the number of vibrations in a unit of time, a second. This will be conditioned by the length of the bands in vibration or the length and width of the aperture through which the air passes and the degree of tension to which the bands are subjected. In the emission of soimds of highest pitch the tension of the vocal bands and the narrowing of the glottis attain their maxi- mum. In the emission of sounds of lowest pitch the reverse conditions obtain. In passing from the lowest to the highest pitched sounds in the range of the voice peculiar to any one individual, there is a pro- gressive increase in both the tension of the vocal bands and the narrow- ing of the glottic aperture. In the production of low-pitched notes of men, those due to vibrations lying be- tween 80 and 240 per second, the tension is regulated by the crico-thyroid muscle; the aperture of the glottis during this time being elliptic in shape and relatively wide (Fig. 295). In the production of notes due to vibrations lying between 240 and 512 vibrations per second, the anterior fibers of the cricothyroid muscle relax and the thyro- arytenoid muscle comes into play; by its action the vocal bands are more closely approximated and the vocal aperture re- duced to a linear slit. In the high-pitched notes emitted by soprano singers the vocal bands are so closely applied to each other that only a very small portion in front, bounding a small oval aperture, is capable of vibrating (Fig. 296). The difference in the pitch of the voice in men and women is due largely to the greater size and develop- ment of the vocal bands in the former than in the latter. The quality of the voice, the timbre or color, depends on the form combined with the intensity and pitch of the vibration. As with sounds produced by music instruments, the primary or fundamental vibration of the vocal band is complicated by the superposition of secondary or partial vibrations (overtones) . The form of the vibration will therefore be a resultant of the blending of a number of different vibrations. The quality of the sound produced in the larynx is, however, modified by the resonance of the mouth and nasal cavities; certain of the overtones being reinforced by changes in the shape of the mouth cavity more especially, thus giving to the voice a soiiiewhat different quality. The Varieties of Voice. — The region of the music scale, com- prising all vibrations between 32 and 2048 per second, with which laryngeal sounds are in accord will vary in the two sexes and in different individuals of the same sex. It is customary to classify voices, es- FiG. 296. — Glottis Seen WITH THE Laryngoscope dur- ing THE Emission of High- pitched- Sounds, i, 2. Base of the tongue. 3, 4. Epiglot- tis. S) 6- Pharynx. 7. Ary- tenoid cartilages. 8. Opening between the true vocal cords. 9. Aryteno-epiglottidean folds. 10. Cartilage of Santorini. 11. Cuneiform cartilage. 12. Su- perior vocal cords. 13. In- ferior vocal cords. — {Le Bon.) PHONATION; ARTICULATE SPEECH. 635 pecially those of singers, into bass, baritone, tenor, contralto, mezzo- soprano, and soprano, in accordance with the regions of the music scale with which they correspond. Thus the succession of notes characteristic of the bass voice vary in pitch from F, fa', to C, doj, or from 87 to 256 vibrations per second; those of the baritone from A, la, to F', faj, or from 106 to 341 vibrations per second; those of the tenor from C, do^, to a', kj, or from 128 to 435 vibrations per second; those of the contralto from e, mi^, to C", do^, or from 160 to 512 vibrations per second; those of the mezzo-soprano from g, sol^, to e", mi^, or from 192 to 640 vibrations per second; those of the soprano from b, si^, to g", S0I4, or from 240 to 768 vibrations per second. The range of the voice is thus seen to embrace from one and three- quarters to two octaves. Some few individual singers have far ex- ceeded this range, but they are exceptional. Speech is the expression of ideas by means of articulate sound?. These sounds may be divided into vowel and consonant sounds. The vowel sounds, a, e, i, 0, u, are laryngeal sounds modified by the superposition and reinforcement of certain overtones developed in the mouth and pharynx by changes in their shapes. The number of vibrations underlying the production of each vowel sound is a matter of dispute. According to Konig, the sound of a is the result of 940 vibrations; of e, 1880 vibrations; of i, 3760 vibrations; of o, 470 vibrations; of ou, 235 vibrations. Consonant sounds are produced by the more or less complete in- terruption of the vowel sounds during their passage through the organs of speech. These may be divided into: 1. Labials, p, b, m. 2. Labio-dentals, /, v. 3. Linguo-dentals, s, z. 4. Anterior linguo-palatals, t, d, I, n. 5. Posterior linguo-palatals, k, g, h, y, r. The names of these different groups of consonants indicate the region of the mouth in which they are produced and the means by which the air blast is interrupted. THE NERVE MECHANISM OF THE LARYNX. The nerve mechanism by which the musculature of the larynx is excited to action and coordinated so as to subserve both respiration and phonation involves the fibers contained in the superior and inferior laryngeal nerves (both branches of the vagus) and their related nerve- centers in the central nerve system. For respiratory purposes it is essential that the lumen of the glottis shall be sufficiently large to permit the entrance and exit of air without hindrance. Laryngoscopic examination of the larynx in the human being shows that during quiet respiration the vocal bands are widely separated and almost stationary, moving but slightly during either 636 TEXT-BOOK OF PHYSIOLOGY. inspiration or expiration. At this time, according to the investigations of Semon, the area of the glottis is approximately 160 sq. mm., some- what less than the area of either the supraglottic or infraglottic regions, which is about 200 sq. mm. This condition of the glottis is maintained by the steady continuous contraction of the posterior crico-arytenoid muscles, the abductors of the vocal bands. For phonatory purposes it is essential that the respiratory function be temporarily suspended and the vocal bands closely approximated. This is accomplished by the contraction of the remaining muscles of the larynx, with the exception of the crico-thyroid, which are collectively known as the adductors of the vocal bands. During phonation the adductor muscles overcome the activity of the abductors. With the cessation of phonation the abductors immediately restore the vocal bands to their former respiratory position. The activities of these two antagonistic groups of muscles are under the control of the central nerve system. The only pathway for the excitatory lierve impulses is through the fibers of the inferior or re- current laryngeal nerve. The relation of these nerve-fibers both centrally and peripherally, as well as their physiologic action, has been the subject of much experimentation. The results have not always been in accord, owing to the choice of animal, the use of anesthetics, strength of stimulus, etc. As the outcome of many investigations it is believed that each muscle group is innervated by its own bundle of nerve-fibers, both of which are contained in the inferior laryngeal, though coming from two separate centers in the medulla oblongata. Russell succeeded in separating the fibers for the abductors from the fibers for the ad- ductors in the inferior laryngeal, and in tracing them to their termi- nations. So completely was this done that it became possible to produce at will, through stimulation, either abduction or adduction, without contraction of the muscle of opposite function. The laryngeal respiratory center was located by Semon and Horsley, in the cat, in the upper part of the floor of the fourth ventricle. Stim- ulation of this area during etherization was followed by abduction of the vocal bands. The efferent fibers of this center are believed by some investigators to leave the central nerve system in the spinal accessory nerve, by others in the lower roots of the vagus. From the continuous activity of the abductor muscle, and the stationary position of the vocal bands, it is probable that the medullary center is in a state of continuous activity or tonus, the result probably of reflex influences. A cortical representation for laryngeal respiratory movements has been determined by Semon and Horsley in different classes of animals. In the cat especially, stimulation of the border of the olfactory sulcus gives rise to complete abduction of the vocal bands on both sides. The representation is therefore bilateral. The phonatory center was located by the same investigators in PHONATION; ARTICULATE SPEECH. 637 the medulla near the ala cinerea and the upper border of the calamus scriptorius. Stimulation of this area was invariably followed by bilateral adduction of the vocal bands and closure of the glottis. A cortical representation for phonatory movements also was located in the lower portion of the precentral convolution, near the anterior border. Stimulation of this area gives rise to marked ad- duction of both vocal bands, indicating that the representation is also bilateral. Faradic stimulation of the inferior laryngeal nerve during slight ether anesthetization gives rise to closure of the glottis; the same stimulation, however, during deeper anesthetization gives rise to opening or dilatation of the glottis, a fact indicating that either the adductor muscles or their nerve terminals are depressed by the action of the ether before the muscles and nerves of opposite function. The superior laryngeal nerves contain motor fibers for the crico-thyroid muscles. Stimulation of the nerve gives rise to contraction of the muscle and increased tension of the vocal bands. It is believed that these fibers are derived originally from the efferent fibers of the glosso- pharyngeal nerve. The remaining fibers of the superior laryngeal endow the upper portion of the larynx with extreme sensibility which to a certain extent protects the air-passages against the entrance of foreign bodies. Irritation of the terminal filaments of this nerve by particles of food, solid or liquid, gives rise to marked reflex spasm of the adductor muscles and closure of the glottis, followed by a strong expiration blast of air from the lungs by which the offending particles are removed. Division of this nerve on both sides is followed by a paralysis of the crico-thyroid muscles, a lowering of the tension of the vocal bands, and a loss of sensibility of the laryngeal mucous mem- brane. CHAPTER XXVI. THE SPECIAL SENSES. It is one of the functions of the nerve system to bring the individual into conscious relation with the external world. This is accomplished in part through the intermediation of afferent nerves, connected per- ipherally, with highly specialized terminal organs, and centrally, with specialized areas in the cerebral cortex. Excitation of the terminal organs by material changes in the en- vironment develops nerve impulses which, transmitted to the cortical areas, evoke sensations. These sensations, differing in character from those vague ill-defined sensations — e. g., fatigue, well-being, discomfort, etc. — caused by material changes occurring within the body, are termed special sensations — e. g., touch; pressure; pain; temperature; taste; smell; light and its varying qualities, intensity, hue, and tint; sound and its varying qualities, intensity, pitch, and timbre. The terminal organs which receive the impress of the external world are the skin, tongue, nose, eye, and ear, and collectively con- stitute the special sense-organs. The physiologic mechanisms which underlie and develop these special sensations are known respectively as the tactile, gustatory, olfactory, optic, and auditory. Each mechan- ism responds to but a single form of stimulus and to no other. Thus, the stimulus for .the skin is mechanic pressure; for the tongue, soluble organic and inorganic matter; for the nose, volatile or gaseous matter; for the eye, ether vibrations; for the ear, atmospheric undulations. These stimuli alone are adequate to the physiologic excitation of the different mechanisms. The factors involved in the production of the sensations include (i) a special physical stimulus; (2) a specialized terminal organ; (3) an afferent nerve pathway, and (4) a specialized receptive sensor cell in the cerebral cortex. Though the resulting sensations in each instance differ widely in their characteristics, it is difficult to present a satisfactory explanation for these differences. If it be assumed that the nerve impulses which ascend the different nerves of special sense are alike in quality, then it must be admitted that the character of the sensation is the expression of a specialization and organization of the cortical area. If, on the other hand, specialization of the cortex is denied, then there must be admitted a specialization of the peripheral organ — with a resulting difference in quality or rapidity of the nerve impulses which would impress or excite the non-specialized cortex in such a way as to call forth the characteristic sensation. It is possible, however, that neither 638 THE SENSE OF TOUCH. 639 supposition is wholly correct, and that the character of the sensation depends on the construction and adaptation of the entire sense appara- tus to the character of the stimulus. Whatever the conditions for their origin and whatever their char- acteristics, sensations in themselves do not constitute knowledge; they are but elementary states of consciousness, raw materials out of which the mind elaborates conceptions and forms judgments as to the character of any given object in comparison with former ex- periences. THE SENSE OF TOUCH. The physiologic mechanism involved in the sense of touch in- cludes the skin and the mucous membrane lining the mouth, the afferent nerves, their cortical connections, and nerve-cells in the cortex of the parietal lobe. Peripheral excitation of this mechanism develops nerve impulses which, transmitted to the cortex, evoke sensations of touch and temperature. To the skin, therefore, is ascribed a touch sense and a temperature sense. Of the touch sensations two kinds may be dis- tinguished: viz., pressure sensations and place sensations. With the contact of an external body there arises the perception not only of the pressure, but also the perception of the place or locality of the contact. In accordance with this, it is customary to attribute to the skin a pres- sure sense and a location sense. The specific physiologic stimuli to the terminal organs in the skin and oral mucous membrane are mechanic pressure and thermic vibrations. The Skin. — The skin, which constitutes the basis for the sense of touch, covers and closely invests the entire body. It varies in thickness and delicacy in different regions, though its structure is everywhere essentially the same. As the physiologic anatomy of the skin has elsewhere been detailed (page 486), it is only necessary to state here that it is divided into a deep and a superficial layer. The former, known as the derma, consists of an inner layer of rather loose connective tissue and an outer layer of condensed connective tissue. The latter, known as the epidermis, consists of an inner layer of pig- ment cells and a thick outer layer of epithelial cells. The derma is characterized by the presence of elevations (papillee) which are every- where extremely abundant. Throughout the derma ramify blood- vessels and nerves. The Peripheral or Terminal Organs. — Between the contact surface and the afferent nerves specialized structures are found which serve as intermediates between the stimulus, on the one hand, and the afferent nerves, on the other hand. By virtue of their structure they are far more irritable than the nerve-fibers and hence respond more quickly to the physiologic stimulus than the nerve-fiber itself. To 640 TEXT-BOOK OF PHYSIOLOGY. these specialized organs, found not only in the skin but in other sense- organs as well, the term peripheral or terminal organ is given. It is these structures that are primarily excited to activity by the physiologic stimulus, and that in turn arouse the nerve to activity. Peripheral organs are to be regarded as special modes of termination of afferent nerves and adapted for the impress of a specific stimulus. The periph- eral organs of afferent nerves found in the skin and oral mucous mem- brane present a variety of forms, some of which are as follows: 1. Free Endings. — These are pointed or club-shaped processes, the ultimate terminations of afferent nerve-fibrils, found in and among epidermic cells. 2. Tactile Cells. — These are oval nucleated bodies found in the deeper layers of the epidermis. They rest upon or are embraced . by a crescentic shaped body, the tactile meniscus, which in' turn is directly connected with the nerve- fibril and probably a modification of it (Fig. 297). The Corpuscles of Meissner and Wagner. — In the papillae of the derma, especially in the palm of the hand and in the finger-tips, are found elliptical bodies con- sisting of a connective-tissue cap- sule containing a number of tactile discs with which the nerve-fibrils are connected. If Fig. 297.-TACTILE Cells from Snout ^he afferent nerve is traced to the OF Pig. a. Tactile cell. m. Tactile disc. , ... i . 1 i ^i n. il. e. Plate ending. (All these are probably sensor in function.) — {Starling's "Physiology.") The sensor fiber loses its external investments as it approaches the capsule. The naked axis-cylinder then penetrates the capsule, and after dividing several times terminates in a ribbon-like or spiral manner around the intra-fusal muscle fiber. This ending was described by and is known as Rufifini's " annulo-spiral ribbon." The motor nerve also penetrates the capsule and terminates in the polar extremities of the intra-fusal fiber. Sensor end-organs supposed to be connected with the muscle sense are also found in the tendons of muscles. Afferent Nerves. — That muscles are abundantly supplied with afferent nerves has been proved by different methods of investigation. With histologic methods Sherrington has traced afferent fibers from the muscle spindles directly into the spinal nerve ganglia. The con- tractions of muscles from electric stimulation as well as the con- tractions known as muscle cramp, due to unknown agents, give rise to sensations of pain, a fact which indicates the presence in muscles of afferent or sensor nerves. Cortical Area. — Pathologic findings have shown that an im- pairment or a loss of the muscle sense is associated with destructive 646 TEXT-BOOK OF PHYSIOLOGY. lesions of perhaps the super and sub-parietal convolutions (Figs. 252, 253). In a case reported by Starr the removal of a small tumor in the pia mater situated over the junction of the superior and inferior parietal lobules was followed by a loss of the muscle sense and marked ataxia in the right hand for a period of six weeks, after which recovery took place. These symptoms were attributed to injury of the cortex from unavoidable surgical procedures. The muscle sensations, as stated in foregoing paragraphs, form the basis of the perception not only of the direction and the duration of a body movement and the resistance experienced, but also of the position and the tension of the muscle groups. The latter fact more especially makes it possible for the mind to direct the muscles and to graduate the energy necessary to the accomplishment of a definite purpose. Active Touch. — Active touch or the application of the fingers to the surfaces of external objects implies the cooperation of the skin and the muscles. The sensations which are evoked are combina- tions of contact and muscle sensations. The union of these sensa- tions forms the basis of the perception of hardness, softness, smooth- ness, and roughness of bodies. THE SENSE OF TASTE. The physiologic mechanism involved in the sense of taste includes the tongue, the gustatory nerves (the chorda tympani and the glosso- pharyngeal), their cortical connections and nerve-cells in the gray matter of the fourth temporal convolutions. The peripheral ex- citation of this apparatus gives rise to nerve impulses which transmitted to the brain evoke the sensations of taste. The specific physio- logic stimulus is matter, organic and inorganic, in a state of solution. The Tongue. — The tongue consists of both intrinsic and extrinsic muscles, in virtue of which it is susceptible of both a change in shape and position. The movements of the tongue, though not essential to taste, are made use of in the finer discrimination of tastes. The tongue is covered over by mucous membrane continuous with that lining the oral cavity. The dorsum of the tongue presents a series of papillae richly supplied with blood- vessels and nerves. Of these there are three varieties, the filiform, the fungiform, and the circumvallate (Fig. 302). I. The filiform papilla, the most numerous, cover the anterior two- thirds of the tongue; they are conical or filiform in shape and Fig. 302. — The Tongue. . Papillfe circumvallatae. . Papillfe fungiformes. THE SENSE OF TASTE. 647 covered with horny epithelium which is often prolonged into filamentous tufts. 2. The fungiform papillcB, found chiefly at the tip and sides of the tongue, are less numerous but larger than the preceding and of a deep red color. 3. The circumvallate papillm, from eight to ten in number, are situated at the base of the tongue arranged in the form of the letter A They consist of a central projection surrounded by a wall or circumvallation from which they take their name. The Peripheral End-organs. The Taste-buds.— Embedded in the epithelium covering the mucous membrane not only of the tongue but of the palate and posterior surface of the epiglottis are small ovoid bodies which from their relation to the gustatory nerves are regarded as their peripheral end-organs and known as taste- buds or taste-beakers. Each bud is ovoid in shape (Fig. 303). Its base rests on the tunica propria; its apex comes up to the epithelium, where it presents a narrow funnel-shaped opening, the taste-pore. The wall of the bud is composed of elongated curved epithelium. The interior contains narrow spindle- shaped neuro-epithelial cells provided at their outer extremity with stiff hair-like filaments which pro- ject into the taste-pore. The neuro-epithelial cells are in physiologic re- lation with the nerves of taste. . The terminal branches, after entering the bud at its base, develop fine tufts which come into contact with the cells. That the taste-buds are connected with the nerves of taste is rendered probable from the fact of their degeneration after division of the nerves. The Taste Area. — The taste area, though con- fined for the most part to the tongue, extends itself in different individuals to the mucous membrane of the hard palate, to the anterior surface of the soft palate, to the uvula, the anterior and posterior half arches, the tonsils, the posterior wall of the pharynx, and the epiglottis. The Taste Sensations. — The sensations which arise in conse- quence of impressions made by different substances on the peripheral apparatus of this area are in so many instances combinations of taste, touch, temperature, and smell that they are extremely difficult of classification. Nevertheless four primary tastes can be recognized: viz., bitter, sweet, acid or sour, salt or saline. Though the contact of any bittet, sweet, acid, or salt substance with any part of the tongue will, if the substance be present in sufficient quantity or concentration, develop a corresponding sensation, some regions of the tongue are more sensitive and responsive than others. Thus, the posterior Fig. 303. — Taste- bud FROM Circum- vallate Papilla op A Chlld. The oval structure is limited to the epithelium (e) lining . the furrow, encroaching slightly upon the adjacent connective tissue (/) ; 0, taste-pore through which the taste-cells communicate with the mucous surface. — {Ajier Piersol.) 648 TEXT-BOOK OF PHYSIOLOGY. portion is more sensitive to bitter substances than the anterior; the reverse is true for sweet substances and perhaps for acids and salines. The intensity of the resulting sensation in any given instance will depend on the degree of concentration of the substance, while its massiveness will depend on the area affected. THE SENSE OF SMELL. The physiologic mechanism involved in the sense of smell in- cludes the nasal fossae, the olfactory nerves, the olfactory tracts, and nerve-cells in those areas of the cortex known as the uncinate con- volution and anterior part of the gyrus fornicatus. Peripheral stimu- lation of this mechanism develops nerve impulses which, transmitted to the cortex, evoke the sensations of odor. The specific physiologic stimulus is matter in the gaseous or volatile state. The Nasal Fossae. — The nasal fossse are irregularly shaped cavities separated by a vertical septum formed by the perpendicular plate of the ethmoid bone, the vomer, and the triangular cartilage. The outer wall presents three recesses separated by the projection inward of the turbinated bones. Each fossa opens anteriorly and posteriorly by the anterior and posterior nares, the latter communicat- ing with the pharynx. Both fossae are lined throughout by mucous membrane. The upper part of the fossa is known as the olfactory, the lower portion as the respiratory region. In the former, the mucous membrane over the septum and superior turbinated bone is somewhat thicker than elsewhere and covered with a neuro-epithelium which constitutes — The Peripheral End-organ. — This consists of a basement mem- brane supporting two kinds of cells, the olfactory and the sustentacular. The olfactory cells are bipolar nerve-cells, the center of which contains a large spheric nucleus. The peripheral pole is cylindric or conic in shape and provided at its extremity with several hair-like processes. The central pole becomes the axon process and passes directly to the olfactory bulb. The sustentacular cells are epithelial in character and, as their name implies, support or sustain the olfactory cells. For the appreciation of odorous particles the air must be drawn through the nasal fossae with a certain degree of velocity. If the particles are widely diffused in the air, they must be drawn not only more quickly but more forcibly into contact with the olfactory hairs, as in the act of sniffing, the result of short energetic inspira- tions. To many substances the olfactory apparatus is extremely sensitive. Thus, it has been shown that a particle of mercaptan the actual weight of which was calculated to be 460006000 ^^ ^ milli- gram gave rise to a distinct sensation. The Olfactory Sensations. — The sensations which arise in conse- quence of the excitation of the olfactory apparatus are very numerous THE SENSE OF SMELL. 649 and their classification is extremely difficult. For this reason it is customary to divide them into two groups: viz., agreeable and dis- agreeable, in accordance with the feelings they excite in the individual. As the. olfactory sensations give rise to feelings rather than ideas, this sense plays in man a subordinate part in the acquisition of knowl- edge. In lower animals this sense is employed for the purpose of discovering and securing food, for detecting enemies and friends, and for sexual purposes. In land animals the entire olfactory appa- ratus is well developed and the sense keen; in some aquatic animals, as the dolphin, whale, and seal, the apparatus is poorly developed and the sense dull. CHAPTER XXVII. THE SENSE OF SIGHT. The physiologic mechanism involved in the sense of sight includes the eyeball, the optic nerve, the optic tracts, their cortical connections, and nerve-cells in the cuneus and adjacent gray matter. Peripheral stimulation of this mechanism develops nerve impulses which trans- mitted to the cortex evoke (i) the sensation of light and its different qualities — colors; (2) the perception of light and color under the form of pictures of external objects; and (3) in connection with the ocular muscles, the production of muscle sensations by which the size, dis- tance, and direction of objects may be judged. The specific physiologic stimulus to the terminal end-organ, the retina, is the impact of ether vibrations. In general, it may be said that, at least for the same color, the intensity of the objective vibration determines the intensity of the sensation. THE PHYSIOLOGIC ANATOMY OF THE EYEBALL. The eyeball is situated at the fore part of the orbit cavity, and in such a position as to permit of an extensive range of vision. It is loosely held in position by a fibrous membrane, the capsule of Tenon, which is attached, on the one hand, to the eyeball itself, and, on the other, to the walls of the orbit cavity. Thus suspended, the eyeball is susceptible of being moved in any direction by the contraction of the muscles attached to it. The ball is spheroid in shape, measuring about 24 millimeters in its anterio-posterior diameter and a little less in its transverse and vertical diameters. When viewed in profile, it is seen to consist of the segments of two spheres, of which the posterior is the larger, occupying five-sixths, and the anterior is the smaller, occupying one- sixth of the ball. It is composed of several concentrically arranged membranes enclosing various refracting media essential to vision. The membranes, enumerating them from without inward, are as follows : the sclera and cornea, the chorioid and iris, and the retina. The refracting media are the aqueous humor, the crystalline lens, and the vitreous humor. The Sclera and Cornea. — The sclera is the thick opaque mem- brane covering the posterior five-sixths of the ball. It is composed of layers of connective tissue which are arranged transversely and longitudinally. It is pierced posteriorly by the optic nerve about 650 THE SENSE OF SIGHT. 651 3 or 4 millimeters internal to the optic axis. By virtue of its tirmness and density the sclera gives form to the eyeball, protects delicate structures enclosed by it, and serves for the attachment of the muscles by which the ball is moved (Figs. 304, 309.) The cornea is the transparent membrane forming the anterior one-sixth of the ball. It is nearly circular in shape, measuring in its horizontal meridian 12 mm., in its vertical meridian 11 mm. The curvature is therefore sharper in the latter than in the former. The radius of curvature of the anterior surface at that central portion ordinarily used in vision is 7.829 mm.; that of the posterior surface about 6 mm. The substance of the cornea is made up of thin layers of delicate transparent hbrils of connecti\-e tissue continuous Avith those found in the sclera. Lvmph- spaces are present ^ ./l^^s^^^^Aas* -'^'^ throughout the cor- nea, in which are to be found lymph-cor- puscles. The anterior surface of the cornea is covered with several la_\-ers of nucleated epithelium supported by a structureless membrane, the an- terior clastic lamina. The posterior surface ' also is covered by a layer of ejnthelium supported b}' a similar membrane, the mem- brane of Descemet, which at its periphery becomes continuous with the iris. At the junction of the cornea and sclera there is a circular g r o o \- e , known as the canal of Schlemm. The Chorioid, Iris, Ciliary Muscle, and Ciliary Processes.— The chorioid is the dark brown membrane which extends forward nearly to the cornea, where it terminates in a series of folds, the ciliary processes. Posteriorly, it is pierced by the optic nerve. It is com- posed largely of blood-\-essels, arteries, capillaries, and \eins, sup- ported by connective tissue. Externally it is loosely connected to the sclera; internally it is lined by a layer of hexagonal cells containing black pigment which, though usually described as a part of the chorioid. Fic. 304. — C'HOkioiD Coat of the Eye. 1. Optic ncr\c. 2, 2, 2, 2, 3, 3, 3, 4. Selenitic coat di\"iticd and turned back to slmw the chorioid. 5, 5, 5, 5. The cornea, di\'ided into four portions and turned l^ack. 6, 6. Canal of Schlemna. 7. External surface of the chorioid, traversed by the ciliary nerves and one of the long cihary arteries. S. Central vessel into which open the vasa vorticosa. q, 0, jo, 10. Chorioid zone. II, II. Ciliary nerves. 12. Long ciliary artery. 13, 13, 13, 13. .Anterior ciliar)- arteries. 14. Iris. 15, 13. A'ascular circle of the iris. 16. Pufiil. — (Sappey.) 6s2 TEXT-BOOK OF PHYSIOLOGY. is now known to belong, embryologicly and physiologicly, to the retina. Lying within the outer layer of arteries and veins there is a thick layer of small arterioles and capillaries, known as the chorio- capillaris. The chorioid with its contained blood-vessels bears an important relation to the nutrition and function of the eye. It provides a free supply of lymph and presents a uniform temperature to the retina in contact with it. The iris is the circular, variously colored membrane in the anterior part of the eye just behind the cornea. It presents a little to the nasal side of the center a circular opening, the pupil. The outer or circumferential border is united by connective tissue to the cornea, sclera, and ciliary muscle; the inner border forms the boundary of the pupil. The iris consists of d framework of connective tissue sup- porting blood-vessels, muscle-fibers, and pigmented connective-tissue cells. The anterior surface is covered by a layer of cells continuous with those covering the posterior surface of the cornea. The pos- terior surface is formed by a thin structureless membrane supporting a layer of pigment cells continuous with those lining the chorioid. The color which the iris presents in different individuals depends on the relative amoimt of pigment in the connective-tissue corpuscles. In blue eyes the pigment is wanting. In gray, brown, and black eyes the pigment is present in progressively increasing amounts. The blood-vessels are connected with those of the chorioid coat. The muscle-fibers are of the non-striated variety and arranged in two sets, one circularly, the other radially, disposed. The circular fibers are found close to the pupil near the posterior surface of the iris. Contraction of this band of fibers diminishes, relaxation increases, the size of the pupil. This muscle is known as the sphincter pupillcB or sphincter iridis. The radial fibers form a more or less continuous layer in the posterior part of the iris, extending from the margin of the pupil, where they blend with the circular fibers, to the outer border. Con- traction of the fibers enlarges the size of the pupil. The muscle is known as the dilatator pupilla. The nerves exciting the sphincter pupillce to action are the ciliary nerves, axons of nerve-cells located in the ciliary or ophthalmic gang- lion. Stimulation of these fibers gives rise to contraction of the sphincter and diminution in the size of the pupil. The nerves excit- ing the dilatator pupilla to action are axons of nerve-cells located in the superior cervical ganglion. They reach the iris by way of the cervical sympathetic, the ophthalmic division of the fifth, and the long ciliary nerve. Stimulation of these nerves is followed by contraction of the dilatator and enlargement in the size of the pupil. Both the ciliary and superior cervical ganglia are in relation with pre-ganglionic fibers coming from the central nerve system. (See page 589.) The ciliary muscle is a gray circular band about two millimeters in width, consisting of non-striated muscle-fibers. The majority of THE SENSE OF SIGHT. 653 these fibers pursue a radial or meridional direction. Taking their origin from the junction of the sclera, cornea, and iris, they pass backward to be inserted into the chorioid coat opposite the ciliary processes. The inner portion of the muscle is interrupted by bundles of fibers which pursue a circular direction. (Fig. 305.) They collec- tively constitute the annular or ring muscle of Miiller. The ciliary muscle in common with the circular fibers of the iris receives its nerve- supply direct from the nerve-cells in the ciliary ganglion. Contraction of the ciliary muscle tenses the chorioid coat, and for this reason it is frequently termed the tensor chorioidece. The Retina. — The retina is the internal coat of the eye, extending forward almost to the ciliary processes, where it terminates in an indented border, known as the ora serrata. In the living condition it is clear, transparent, and pink in color. After death it becomes Fig. 305. — Section through the Ciliary Region or the Human Eye. a. Radi- ating bundles of the ciliary muscle, b. Deeper bundles, c. Circular network, d. Annular muscle of Miiller. e. Tendon of ciliary muscle. /. Muscle-fibers on posterior side of the iris. g. Muscles on the ciliary border of the same. h. Ligamentum pectin- atum. — {After Iwanoff.) opkque. The retina is abundantly supplied with blood-vessels, de- rived from the arteria centralis retina, a branch of the ophthalmic, which pierces the optic nerve near the sclera, runs forward in its center, to the retina, in which its terminal branches are distributed. The veins arising from the capillary plexus leave the retina by the same route. In the posterior portion of the retina, at a point corresponding with the axis of vision, there is a small oval area bout 2 mm. in its transverse and about 0.8 mm. in its vertical diameter. From the fact that it presents a yellow appearance, it is known as the macula lutea. This area presents in its center a depression with sloping sides, known as the ]ovea centralis. About 3.5 mm. to the nasal side of the macula is the point of entrance of the optic nerve. The retina is remarkably complex in structure, presenting an appearance when viewed microscopically, something like that repre- sented in Fig. 306, indicating that it is composed of different cellular 6S4 TEXT-BOOK OF PHYSIOLOGY. elements arranged in layers. These have been named, from behind forward, as follows: 1. The layer of pigment cells. 2. The layer of rods and cones, or Jacobson's layer. 3. The external limiting membrane. 4. The outer nuclear or granular layer. 5. The outer molecular or reticular layer. 6. The inner nuclear or granular layer. 7. The inner molecular or reticular layer. 8. The layer of ganglion cells. 9. The layer of nerve-fibers. Modern histologic methods of research have made it possible to reduce the retina, exclusive of the pigment cells, to three successive layers of nerve-cells, supported by a highly developed neuroglia, forming what has been termed the fibers of Miiller. These nerve- cells are as follows: 1. The visual cells. 2. The bipolar cells. 3. The ganglion cells. The relation of these nerve-cells one to another and to the supporting neuroglia tissue and the manner in which 'they unite to form the above-mentioned layers are schematicly shown in Fig. 307. The pigment layer is composed of hexagonal cells. Though formerly described as forming a part (the inner layer) of the chorioid, these cells belong embryologicly to the retina. From their retinal surface delicate pigmented processes ex- tend into and between the rods and cones. On exposure to light these processes elongate and push themselves between the rods. In the dark they retract and with- draw into the cell-body. The visual cells which form the layer of rods and cones are of two varieties, the rod-shaped and the cone-shaped. The rod-shaped visual cells consist of a straight elongated cylinder extending through the entire thickness of Jacobson's membrane and J.. Pignient-layer (not shown). z. Layer of rods and cones. 3. External limiting membrane. 4. Outer nuclear layer. 5. Outer molecular layer. 6. Inner nuclear layer. 7. Inner molecular layer. 8. Layer of ganglion cells. 9. Layer of nerve-fibers. Fig. 306. — Vertical Section of Human Retina. — [Schaper.) THE SENSE OF SIGHT. 65s a fine fiber containing a nucleus, which, after piercing the external limiting membrane, passes into the outer molecular layer, where it terminates in a spheric enlargement. The outer portion of the rod is clear and homogeneous, though containing a pigment known as visual purple or rhodopsin; the inner portion of the rod is slightly granular. The cone-shaped visual cells also consist of two portions, a conic portion situated in Jacobson's membrane between the rods, and a fine fiber, containing a nucleus, which, after piercing the external limiting membrane, passes into the outer molecular layer, where it terminates in a fine tuft. The inner portion of the cone is thicker than the rod and rests on the limiting membrane; the outer portion tapers to a fine point and is known as the cone-style. The cones, as a rule, are shorter than the rods. The proportion of rods to cones varies in different parts of the retina, though there are on the average about four- teen rods to one cone. In the macula the rods are almost en- tirely absent, cones alone being present. The layer of visual cells to- gether with the neuroglia consti- tute the first three layers of the retina proper. The external limiting membrane is formed by the blending of the ends of neu- roglia cells. The bipolar cells consist of a central portion, found in the inner nuclear layer, from which are given off two processes which pass in opposite directions, one toward the visual cells, the other toward the ganglion cells. The former terminate in tufts which arborize around the tufts and spheric enlargements of the visual cells, and assist in the formation of the outer molecular layer; the latter terminate in similar tufts in the inner molecular layer. The ganglion cells are arranged in a single layer, as a rule. They are large and nucleated. From the inner side of each cell there is given off a single axon which passes toward the center of the retina (forming the nerve-fiber layer), where it enters and assists in forming Fig. 307. — Cross-section of the Retina FROM A Mammal. A. Layer of rods and cones. B. Visual cells (outer granules). C. Outer molecular layer. E. Bipolar cells (inner granules). F. Inner molecular layer. G. Ganglion cells. H. Layer of nerve- fibers, a. Rods. b. Cones, c. Bipolar rod. f. Bipolar cone. r. Lower ramifica- tion of a bipolar rod. f. Lower ramification of a bipolar cone, g, h, i, j, k. Ganglion cells in various stages, branching from F. X, z. Bipolar contact of rods and cones, t. Miiller's supporting fibers. S. Centrifugal nerve-fibers. — {After Ramon y Cajal.) 6s6 TEXT-BOOK OF PHYSIOLOGY. the optic nerve. From the outer side of the ganglion cell dendrites pass into and assist in forming the inner molecular layer. These dendrites come into physiologic relation with those of the inner proc- esses of the bipolar cells. Horizontally disposed nerve-cells are also present in the outer molecular layer in relation with the visual cells. Spongioblasts or amacrine cells are also present at the border of and in the inner molec- ular layer. From the relation of the ganglion cells, from which the optic nerve- fibers take their origin, to the visual cells and the bipolar cells, the latter may be regarded as the terminal visual organ, the intermediate Fig. 308. — Horizontal Section through the Macula and Fovea of a Man Sixty Years Old. The section is not through the exact center of the fovea, for there are only cone visual cells and no remnants of the confluence of the inner granule and ganglion cell layers are present, i. Cones. 2. External limiting membrane. 3. Outer nuclear layer. 4. Henle's fiber layer. 5. Outer molecular or reticular layer. 6. Inner nuclear layer. 7. Inner molecular or reticular layer. 8. Layer of ganglion cells. 9. Nerve- fiber layer. — {After Schaper, Stohr's "Histology.") between the ether vibrations and the ganglion cell. The visual cells are directed toward the chorioid, away from the entering light, dip- ping into the pigment cells. They, with the pigment layer, are the elements by which the ether vibrations are transformed into nerve energy. In the fovea most of the retinal elements are wanting or are re- duced in thickness. The cones alone are present. The cone-fibers with their nuclei are directed obliquely upward and outward along the slope of the fovea, to end in tufts which come into physiologic relation- with the dendrites of the ganglion cells which at the top of the fovea are generally increased in number (Fig. 308). It is estimated that the optic nerve contains about 500,000 nerve- fibers, and that for each fiber there are about 7 cones, 100 rods, and THE SENSE OF SIGHT. 657 7 pigment cells. In accordance with this estimate there would be about 3,500,000 cones, 50,000,000 rods, and 3,500,000 pigment cells. The distance between the centers of two adjacent cones in the fovea is 4 micromillimeters. The vitreous humor is the largest of the refracting media and occupies by far the largest portion of the interior of the eyeball. From Its position it gives support to the retina. Anferiorlv it presents a concavity, in which the crystalline lens is lodged.' The vitreous humor consists of water (97 per cent.), organic matter and salts, en- closed in a transparent membrane, the /uiiua hyaloidca. The mass of the vitreous humor is penetrated by a species of connective tissue. Fio. 309. — Horizontal Section of the Eyeball, i. Sclera. 2. Cornc-a. 3. Choridid. 4. Iris. 5. Ciliary mu.scle. 6. Retina. 7. Lens. S. Suspensory ligament, g. Canal of Schlemni. to. Canal of Petit. 11. Optic nerve. — (Denver.) The aqueous humor is small in amount in comparison with the vitreous and is found in the space bounded by the cornea, the ciliary body, the suspensory ligament, and the lens. The projection of the iris into this space partially di\ides into an anterior and posterior [lortion or chamber. The aqueous humor is a clear, watery, alkaline fluid derived from or secreted by the capillary blood-vessels of the ciliary body. From this origin it passes through the pupil into the anterior chamber. It serves to keep the cornea tense and smooth. The ocular tension partly dei.)ends on the presence of this fluid in the eyeball. There is every reason for believing that there is a constant stream of fluid from the blood-vessels into the eye and from the eye through the spaces of Fontana at IJie base of the iris into the canal of 658 TEXT-BOOK OF PHYSIOLOGY. Schlemm, and so into the blood. Any interference with the exit of this fluid rapidly increases the ocular tension. The lens is the transparent biconvex body situated just behind the iris, in the concavity of the vitreous. The thickness of the lens is 3.6 mm., the diameter about 9 mm. It consists of a transparent capsule containing elongated hexagonal fibers which, having their origin near the anterior central portion of the lens, pass out toward the margin, where they bend around to terminate in a triradiate figure on the opposite side. Chemicly the lens consists of water, a globulin body (crystallin), and salts. The Suspensory Ligament. — ^The lens is held in position by the suspensory ligament, formed in part by the hyaloid membrane and in part by fibers derived from the ciliary processes. The former be- comes attached to the posterior siurface, the latter to the anterior surface of the lens near the equator. The space between the two layers of the ligament is the canal of Petit. The anterior surface of the ligament presents a series of plications conforming to correspond- ing plications on the surface of the ciliary processes. The relations of all the parts entering into the structure of the eye are shown in Fig. 309. THE PHYSIOLOGY OF VISION. The Retinal Image. — The general function of the eye is the formation of images of external objects on the free ends of the per- cipient elements of the retina, the rods and cones. The existence of an image on the retina can be readily seen in the excised eye of an albino rabbit, when placed between a lighted candle and the eye of an observer. Its presence in the human eye can be demonstrated with the ophthalmoscope. It is this image, composed of focal points of luminous rays, which stimulates the rods and cones, which is the basis of our sight-perceptions, and out of which the mind constructs space relations of external objects. In only two essential respects does the image on the retina differ from the object, aside from the fact that the object has usually three, the image only two, dimensions — viz., in size and relative arrangement of its parts. Whatever the dis- tance, the image is generally smaller than the object; it is also reversed, the upper part of the object becoming the lower part of the image, and the right side of the object the left of the image. The Dioptric Apparatus. — The formation of an image is made possible by the introduction of a complex refracting apparatus con- sisting of the cornea, aqueous humor, lens, and vitreous humor. Without these agencies the ether vibrations would give rise only to a sensation of diffused luminosity. Rays of light emanating from any one point — that is, homocentric rays — arriving at the eye must traverse successively the different refracting media. In their passage from one to the other, they undergo at the surfaces changes in direc- tion before they are finally converged to a focal point. In order to THE SENSE OF SIGHT. 659 mathematically follow the rays in all their deviations through the media, to determine their focal points and to construct an image, a knowledge of the form of the refracting surfaces, the refractive indices of the different media, and the distances of the surfaces from one another must be known. The following constants are now accepted: The radius of curva- ture of that portion of each refracting surface used for distinct vision is " for the cornea 7.829 mm., for the anterior and posterior surfaces of the lens 10 and 6 mm., respectively. The indices of refraction of the different media are as follows: cornea and aqueous humor, 1.3365; lens, 1. 4371; vitreous body, 1.3365. The distance from the vertex of the cornea to the lens is 3.6 mm.; the thickness of the lens, 3.6 mm.; the distance from the posterior surface of the lens to the retina, 15 mm. As the two surfaces of the cornea are practically parallel, and as the index of refraction of the aqueous humor is the same as that of the cornea, they may be re- garded as but one medium. The refracting surfaces may therefore be reduced to the anterior surface of the cornea, the anterior surface of the lens, and the posterior surface of the lens.* Parallel rays of light pntfHno- tViP PVP riflss frnm ^^°- 31°-— REFRACTION OF HoMOCENTRIC entering tne eye pass trom j^^^g ^^ ^^^ Formation of an Image. air, with an index of re- fraction of 1.00025, i^to the cornea, with an index of refraction of 1.3365. In passing from the rarer into the denser medium they undergo refraction in accordance with the laws of optics and are rendered somewhat convergent. The extent of this first refraction and convergence is sufficiently great to bring parallel rays, if con- tinued, to a focus about lo mm. behind the retina. This would be the condition in aphakia whether the lens is congenitally absent or has been removed by surgical procedures. Perfect vision, however, requires that the convergence of the light must be great enough to bring the focal point, the image, on the retina. This is accomplished by the introduction of an additional refracting body, the lens. On entering the lens the rays are for the same reason — i. e., the passage from a rarer into a denser medium — again refracted and converged, and if continued would come to a focus about 6.5 mm. behind the retina. On passing from the lens .into the vitreous — i. e., from a denser into a rarer medium — the rays are once more converged and to an extent sufficient to focalize them on the retina (Fig. 310). * Strictly speaking, the posterior surface of the cornea is not parallel to the anterior surface, and the index of refraction of the cornea is a trifle greater than that of the aqueous humor, viz., 1.377. But as the increase in the corneal refraction due to a higher index is almost exactly counteracted by a decrease in refraction, due to the higher curva- ture of the posterior corneal surface, the usual assumptions furnish quite accurate results. 66o TEXT-BOOK OF PHYSIOLOGY. While it is thus possible to geometricly follow the rays through these media by means of the above-mentioned factors, the procedure is attended with many difficulties. Moreover, as the relations all change when rays enter the eye from objects situated progressively nearer the eye, a separate calciilation is necessitated for each distance for the determination of the size of the image. A method by which these difficulties are much reduced was sug- gested by Gauss and developed by Listing. It was demonstrated by Gauss that in every complicated system of refracting media separated by centered spheric surfaces there may be assumed certain ideal or cardinal points, to which the system may be reduced, and which, if their relative position and properties be known, permit of the de- termination, either by calculation or geometric construction, of the path of the refracted ray, and the position and size of the image in the last medium, of the object in the first. Every dioptric system can be replaced, as Gauss showed, by a single system composed of six cardinal points and six planes per- pendicular to the common axis — e. g., two focal points, two principal ^/ jr, fp^TW Jk Fig. 311. — Diagram showing the Position and Relation of the Cardinal Points. points, two nodal points, two focal planes, two principal planes, and two nodal planes. Properties of the Cardinal Points. — The f,rst local point, F^, in Fig. 311, has the property that every ray which before refraction passes through it, after refraction is parallel to the axis. The second jocal point, F^, has the property that every ray which before refraction is parallel to the axis, passes after refraction through it. The second principal point, H^, is the image of the f,rst, H^; that is, rays in the first medium which go through the first principal point pass after the last refraction through the second. Planes at right angles to the axis at these points are principal planes. The second principal plane is the image of the first. Every point in the first principal plane has its image after refraction at a corresponding point in the second principal plane at the same distance from the axis and on the same side. The second nodal point, N^, is the image of the f,rst, N^: a ray which in the first medium is directed to the first nodal point passes after refraction through the second nodal point, and the directions of the rays before and after refraction are parallel to each other. In THE SENSE OF SIGHT. 66 r Fig. 311 let 4 5 represent the axis. The distance of the first focal point, F^, from the first principal plane, H^, is the anterior focal distance. The distance of the posterior focal point, F^, from the second principal plane, H„ is the posterior focal distance. The dis- tance of the first nodal point, N„ from the first focal point is equal to the second focal distance. The distance of the second nodal pomt, A^„ from the posterior focal point is equal to the anterior focal distance. It is evident, therefore, that the distance of the correspond- FiG. 312.— Diagram to Find the Image in Last Medium or a Luminous Point in THE First. ing principal and nodal points from each other is equal to the differ- ences_ between the two focal distances. Also the distance of the two principal points from each other is equal to the distance of the two nodal points from each other. Finally, the focal distances are pro- portional to the refractive indices of the first and last media. Planes passing through the focal points vertically to the axis are known as focal planes. From these properties of the cardinal points the position of an image in the last medium of a luminous point in the first may be Fig. 313. — Diagram to Find the Repracted Ray in the Last Medium of a Given Ray in the First Medium. determined, and the course of a refracted ray in the last medium be constructed if its direction in the first be given according to the fol- lowing rules: I. To find the image in the last medium of a luminous point in the first: Let A (Fig. 312) be this given point. Draw A B parallel to the axis until it meets the second principal plane in B; then BF^ will be this ray after refraction. Draw a second ray from A to the first nodal point; then draw another ray, D E, from the 662 TEXT-BOOK OF PHYSIOLOGY. second nodal point parallel to A C. This will be the refracted ray in the last medium. Where the two refracted rays, BF^ and D E, intersect, the image of A will be A j.* 2. To find the refracted ray in the last medium of a given ray in the first medium: Let A B (Fig. 313) be the given ray. Continue this ray until it meets the first principal plane in C. Draw C D parallel to the axis. Now assume any point, such as E, in the given ray, and find its image E., by the Rule i. Then D £, becomes the course of the refracted ray. The Schematic Eye. — Accepting the system of cardinal points, Fig. 314. — Diagram showing the Position of the Cardinal Points in the "Sche- matic Eye." The continuous lines in the upper half of the figure show their position in the passive emmetropic eye. The dotted lines indicate the change in their position in an eye accommodated for the object A at the distance a from the cornea, or 152 mm. The lower half of the figure shows the formation of a distinct image on the retina of an eye accommodated for the object A at the distance a from the cornea. Listing, Bonders, and v. Helmholtz have constructed "schematic" eyes to be substituted for the refracting system of the natural eye. For this purpose it is necessary to make use of the various esti- mates of the indices of refraction of the different media, of the radii of curvatures of the different refracting surfaces, and of the distances separating them, to deduce an average eye as a basis for calculation. The most widely accepted attempt is that of v. Helmholtz. The data he assumed are as follows: The refractive index of air = i; of the cornea and aqueous humor, 1.3365; of the lens, 1.4371; of the vitreous * If the point A is infinitely far from the eye, all the rays striking the eye wiU be paral- lel to each other. The nodal ray must therefore be drawn, and the point where this nodal ray meets the second focal plane will be the image of A=Ai where all rays parallel to the nodal ray will meet. THE SENSE OF SIGHT. 663 humor, 1.3365; the radius of curvature of the cornea, 7.829 mm.; of the anterior surface of the lens, 10 mm.; of the posterior surface, 6 m m .; the distance from the apex of the cornea to the anterior surface of the lens, 3.6 mm.; thickness of lens, 3.6 mm. From the above- mentioned data V. Helmholtz calculated the position of the cardinal points for the eye as follows (see Fig. 314): The first focal point is situated 13.745 mm. before the anterior surface of the cornea; the posterior focal point is situated 15.619 mm. behind the posterior surface of the lens; the first principal point, 1.753 "ini- behind the cornea; the second principal point, 2.106 mm. behind the cornea; the first and second nodal points, 6.968 and 7.321 mm. behind the apex of the cornea, respectively. The anterior focal distance of this schematic eye, the distance between F^ and Hj, therefore amounts to 15.498 mm., and the posterior focal distance, H^ to F^, to 20.713 mm. When the eye, however, is accommodated for near vision, the relations of the cardinal points are changed and will be as follows, if the point accommodated for, lies 152 .-««=»^^ mm. from the cornea: Anterior focal « /f ^^ distance, 13.990 mm.; posterior focal dis- ^/C^/^ \ tance, 18.689 mm.; distance from cornea ' ' y \ „ „ y /- '¥• ■^ , \\ 20.713 mjn. m of the first and second principal points, \ J 1.858 and 2.257 ^^- respectively; dis- ^»« te. ^ tance of the posterior focus, 20.955 mm. fkj. 315.— the Reduced Eye. from cornea. Given this schematic eye in the accommodated state, the course of the rays and the determina- tion of the position of an image in the last medium of a luminous point in the first can easily be determined by the rules already given. The Reduced Eye. — As suggested by Listing, this schematic eye may be yet further simplified or reduced to a single refracting surface bounded anteriorly by air and posteriorly only by aqueous or vitreous humor. Without introducing any noticeable error in the determination of the size of the retinal image, the anterior principal and the anterior nodal points may be disregarded, owing to the minuteness of the distances (0.39 mm.) separating the two systems of points. There is thus obtained one principal point and one nodal point, which latter becomes the center of curvature of the single refracting surface. The dimensions of this "reduced" eye are as follows (see Fig. 315). From the anterior surface of the cornea, corresponding to the principal plane H, to the nodal point N , 5.215 mm., from the anterior focal point F^, to the principal plane H, i. e., the anterior focal distance /', 15.498 mm.; from the principal plane H to the posterior focal point F^, i. e., the posterior focal distance /", 20.713 mm.; the index of refraction is 1.3365. There is thus sulastituted for the natural eye a single refracting surface with a radius of curvature, r, of 5.215 mm. In such an eye luminous rays emanating from the anterior focal point are parallel to the axis after refraction in the interior of the eye. 664 TEXT-BOOK OF PHYSIOLOGY. Also rays parallel to the axis before refraction unite at the posterior focal point. By means of this reduced eye the construction of the refracted ray, the various calculations as to the size of the image, the size of diffusion circles, etc., are greatly facilitated: e. g., In Fig. 316 let A B represent an object. From A homocentric rays fall on the single refracting surface. One of the rays, the nodal ^_^ ray, falling on the surface '^^^^:;:;---___^^ ^'^ ^'^v perpendicularly, passes ^~~^-^^^;^^-^^^II^;^~-^^C^ __-^* unrefracted through the -^ ,.v-— ^-.--,^^JSa - ^:^!!^....!L single nodal point, N, to I ^ — ^ ^^^H |g the posterior focal plane. ^L— — ""^ >«^^^,s«<^. The remaining rays, par- tially represented in the Fig. 316. — The Formation of an Image in the ficrure falling on this sur- Reduced Eye. r'^ ' j ■ j face under varymg de- grees of incidence, undergo corresponding degrees of refraction, by which they form a converging cone of rays which unite -at a point situated on the nodal ray. These two points, A, a, are known as conjugate foci. The same holds true for homocentric rays emanating from B or any other point of the object. The Size of the Retinal Image. — The size of the retinal image, / (in Fig. 316 a &), may now be easily calculated, when the size of the object, O (in Fig. 316 A B), and its distance, D, from the refracting surface with radius of curvature, r, are known, by the following formula: O: I = D -(- r:f"— r. For, as the triangles A N B and a N b are similar, we have A B ^ N e A B:ah = fN:Ng, or ab = -^ — K Independent of the foregoing method, the size of the retinal image may be calculated if it is remembered that the eye, like any optic system, has a point of such a quality that a ray of light which before entering the eye was directed toward it, after refraction continues as if it came from this point. In other words, there is in the eye a point which allows a ray of light to pass unrefracted. This point, termed the nodal point of the eye, determines the size of the image; for if a line be drawn from both the upper and lower ends of an object through this nodal point, it is clear that the images of the respective points must lie on these two rays where they intersect the retina. The dis- tance of this nodal point from the retina is 15.498 mm. It is clear, therefore, that the size of the object is to the size of the image, as the distance of the object from the nodal point is to the distance of the nodal point from the retina; or, in other words, to find the size of the retinal image: multiply the size of the object by 15.5 mm. and divide by the distance of the object from the eye. The visual angle is defined as the angle formed by the intersection THE SENSE OF SIGHT. 66s of two lines drawn from the extremities of an object to the nodal point of the eye. Beyond the nodal point, however, the lines again diverge and form an inverted or reversed image of the object on the retina. The size of the visual angle increases with the nearness and decreases with the remoteness of the object; the retinal image correspondingly . increases and decreases in size. B' Fig. 317. — Drawing Designed to show HOW THE Visual Angle and Size of. Retinal Image Varies with the Distance of an Object of Given Size. For the distant position of A-B the visual angle is a; for the near position (dotted lines) /3. (From Stewart.) These facts will become ap- parent from an examination of Fig. 317. As the size of the retinal image diminishes as the visual angle diminishes either as a result of the re- moval of a given object from the eye, or of a diminution of the size of the object, there comes a limit in the size of the visual angle, beyond which it is impossible to see the two end points (A and B) of the object separately. When this limit is reached the size of the angle expressed in degrees of the circle^ may be determined if the distance between the two points and their distance from the eye be known. Thus it has been experimentally determined that at a distance of 5 meters, the smallest object or the smallest interval between two points which permits the eye to distinguish them as such, is about 1.454 mm. Lines drawn from the extremities of such an object or interval, to the nodal point, subtend an angle of 60 seconds.* Beyond this the two points are indistinguish- able. In other words the emmetropic eye possesses the power of distin- guishing the correspondingly small interval between the two images on the retina of the two objective points. The size of the image or the inter- val between the two retinal points, determined from the foregoing factors by the formulae on page 664 is 0.004 mm., and which would correspond * The size of the visual angle, under which an object of this size and situated at a distance of s meters is distinctly seen, can be determined from the following Fig. 318, in which A B, represent the size of the object 1 .454 mm. ; A', the nodal point; CN, the line which bisects the object, represent the distance of the object from the nodal point; a, the visual angle subtended and whose value it is desired to know, and b one-half of the angle a. By trigo- nometry the size of the angle a can be deter- mined in the following way: one-half the size of the object A B, is divided by its distance from the nodal point; the quotient is the tangent of half the angle. Thus 0.727 -h 5000 = 0.0001454. By reference to logarithmic tables it will be found that the angle or fraction of the circle corresponding to this tangent is 30 seconds, and that therefore the whole angle is 60 seconds. Fig. 318. — Figure showing the Method of Obtaining THE Visual Angle Expressed in Degrees or Fraction OF a Degree of the Circle. 666 TEXT-BOOK OF PHYSIOLOGY. to a visual angle of 60 seconds. If the retinal distance is less than this the tWo sensations fuse into one. The reason assigned for this is, that the distance between the centers of two adjoining cones in the macula is 0.004 mm- With a visual angle not less than 60 seconds, the two foci fall on separate cones. With a smaller visual angle the two foci fall on, and excite but a single cone and hence there arises the sensation of but a single point. The acuteness of vision, therefore, of the emmetropic eye depends on its power of distinguishing the smallest retinal image or the smallest interval between two points on the retina corresponding to a visual angle of 60 seconds. In ophthalmic practice it is customary in testing the acuteness of vision to employ test letters of specific sizes for specific distances. The letters are so proportioned that when they are placed at the specified distances, the extremities of the letters subtend an angle of 5 minutes. The letters have been constructed on the following basis: Since to an angle of 60 seconds there corresponds an object of 1.454 mm. at the distance of 5 meters as shown before and as the object decreases in proportion to the distance (for the same visual angle) it is evident E T E L A. B. Fig. 319. — Standard Test Letters. that the object would have to be one-fifth of 1.454 mm. or 0.2908 mm. in order to subtend an angle of 60 seconds at one meter. From this the size for any other distance in meters is found simply by multiplying 0.2908 mm. with the distance. The standard letters are so constructed that each meridian is composed of five squares, each of which at a specified distance, sub- tends an angle of i minute, and hence the entire letter an angle of 5 minutes. The letter that could be distinctly seen at a distance of 5 meters, would have, therefore, a vertical and a horizontal meridian of 5 times 1.454 mm. or 7.27 mm. (Fig. 319 A), and 'at 10 meters corresponding meridians of 14.54 mm., etc. (Fig. 319 B.) If with the accommodation suspended, the emmetropic eye could clearly distinguish a letter 7.27 mm. in size at a distance of 5 meters and which would, therefore, subtend an angle of 5 minutes, the acuity of the vision would be normal and could be expressed as follows: V=| or V = i. If on the contrary at this distance the smallest letter that could be clearly seen is one that subtends an angle of 5 minutes at a distance of 10 meters then the visual acuity would be only one-half the normal and could be expressed as follows V=^ or V = i, etc. The acuity of vision is expressed, therefore, by a fraction the enumerator of which is the distance at which the test is made, and THE SENSE OF SIGHT. 667 whose denominator is the distance at which the smallest letters dis- tmguished by the patient, subtend an angle of 5 minutes, but which in the instance cited above subtends an angle of 10 minutes; or in other words the distance at which the patient reads divided by the distance at which he ought to read the smallest letters seen by him were his visual acuity normal. Accommodation. — Accommodation may be defined as the power which the eye possesses of adjusting itself to vision at different dis- tances; or in other words, the power of focusing rays of light on the retina, which come from different distances at different times. That such a power is a necessity is apparent from the fact that it can not focus rays coniing from a distant and a near object at the same time. Thus, if an object is held before the eye at a distance of 22 centimeters, for example, and the vision is directed to a distant object it is evi- dent that the near ob- ject is indistinctly seen; but if the vision is then directed to the near ob- ject, it in turn becomes clear and distinct, while the distant object be- comes blurred and indis- tinct. It is evident, therefore, that rays of light coming from a dis- tant and a near object can not be simultane- ously, but only alter- nately, focused on the retina. The observer at the same time becomes conscious, as the vision is directed from the distant to the near object, of a change in the eye itself, a change that involves time and effort. The reasons for these facts will become apparent from a consideration of the following facts: In a normal or emmetropic eye, homocentric parallel rays of light (Fig. 320, a, h) after passing through the optic media are converged and brought to a focus on the retina, /. Rays, however, which come from a luminous point situated near the eye, P, and are therefore divergent, passing through the optic media at the same time, are inter- cepted by the retina before they are focused, and give rise to the for- mation of diffusion-circles and indistinctness of vision. The reverse is also true. When the eye is adjusted for the refraction and focusing of divergent rays (Fig. 320, P) parallel rays will be brought to a focus before reaching the retina, and, again diverging, will form diffusion- FiG. 320. — The Refraction of Parallel and Divergent Rays in the Emmetropic Eye in the Passive and in the Active or Accommodated Condition. 668 TEXT-BOOK OF PHYSIOLOGY. Distance of the Focal Point behind the Posterior Surface o£ the Retina. Diameter of the Diffusion-circle o.o mm. 0.0 mm. 0.005 O.OOII ' 0.012 0.0027 ' 0.025 0.0056 ' 0.050 0.OEI2 ' 0.1 00 0.0222 ' 0.20 0.40 0.80 0.0443 ' 0.0825 ' 0.1616 ' 1.60 0.3122 ■ 3.20 3-42 0.5768 ' 0.6484 ' circles. It is evident, therefore, that it is impossible to simultaneously focus both parallel and divergent rays, and to see distinctly at the salne time, two objects which are situated at different distances. The eye must be alternately adjusted first to one object and then to another. To this adjustment the term accommodation has been given. The following table of Listing shows the size of the diffusion- circles formed of objects situated at different distances when the accommodative power is suspended in an emmetropic eye: Distance of Luminous Point. 00 65 m. 25 12 " 6 3 1.500 " 0.750 " °-37S " 0.188 " 0.094 " 0.088 " The normal eye when adjusted for distant vision is in a passi\'e condition, and hence vision of distant objects is unattended with fatigue. In the act of adjustment, however, for near vision the eye passes into an active state, the result of a muscle effort, the energy of which is proportional to the nearness of the object toward which the eye is directed. From the foregoing table it is evident that between infinity and 65 meters, the diffusion-circles are so slight that no perceptible ac- commodative effort is required to eliminate them. From 65 meters to 6 meters the diffusion-circles gradually become larger, though they are yet so faint as to require for their correction an accommodative effort which is scarcely measurable. From 6 meters up to 6 centi- meters, however, a progressive increase in accommodative power is demanded for distinct vision. Mechanism of Accommodation. — Inasmuch as neither the corneal curvature nor the shape of the eyeball undergoes any change during accommodation, the necessary change, whatever it may be, is to be sought for in the lens. As to the character of the changes in this body, two views are held, based largely on the fact and its interpretation, that images of a luminous point reflected from the anterior surface of the cornea, the anterior and posterior surfaces of the lens, change their relative positions during accommodation. Thus, if in a darkened room a lighted candle be placed in front of and to the side of an individual whose eye is directed to a distant object, an observer placed in the same relative position as the candle will observe three images in the eye, one at the surface of the cornea, two at the pupillary margin (Fig. 321). Of the two latter, one is THE SENSE OF SIGHT. 669 quite large and situated apparently in front of the third, which is faint, small, and inverted. The middle image is reflected from the convex surface of the lens, the last from the concave surface. These images of reflection are known as catoptric images. If now the individual be directed to fix the gaze on a near object, the second image changes its position, advances toward the corneal image and at the same time becomes smaller, a change which, in accordance with the laws of optics, could only be due to an increase in the convexity of the anterior surface of the lens. A slight displacement of the third image some- times observed indicates a possible increase in the convexity of the posterior surface lens. According to Helmholtz, during accommodation the entire anterior surface of the lens becomes more convex, while at the same time it slightly advances, possibly as much as 0.4 mm. in extreme efforts. This change is represented in Fig. 322. According to Tscherning, the increase in convexity of the anterior surface is confined to the central portion, the remainder of the surface becoming somewhat flattened. There is, moreover, no evidence that there is any advance of the surface or any increase in the thickness of the lens. A p^^ _catop- series of new and ingenious experiments lend sup- tric Images in the port to Tscherning's view. The radius of curvature ?''^- ?■■ Upright T .,, • J y • . rr image of reflection, m either case approximates mm. in extreme efforts from the cornea, h. of accommodation. The increase in convexity Upright image from naturally increases the refracting power. of^ the 'lens ^c'^ln- Whichever view is accepted, the nearer the ob- verted image, from ject — that is, the greater the degree of divergence the posterior surface of the light rays — the more pronounced must be the 'hoitz) ^"^'"^ ^ " increase in convexity in order that they may be sufficiently converged and focalized on the retinal. surface. Changes in the convexity of the lens, either of increase or decrease, are attended by changes in the distinctness of images. Coincident with the lens change, the pupillary margin advances and the pupil itself becomes smaller. By this means an indistinctness of the image is prevented by cutting off the rays which would give rise, owing to the angle at which they fall on the surface, to diffusion-circles, from spheric aber- ration. The Function of the Ciliary Muscle,— Though it is generally admitted that the increase in the convexity of the lens is caused by the contraction of the ciliary muscle, the exact manner in which this is accomplished is not clearly understood. According to Helmholtz, when the eye is in repose and directed to a distant object the lens is somewhat flattened from a traction exerted by the suspensory liga- ment. When the eye is directed to a near object, the ciliary muscle contracts, thereby relaxing the ligament, as a result of which the lens, by virtue of an inherent elasticity, bulges forward and becomes more 670 TEXT-BOOK OF PHYSIOLOGY. convex. In consequence of this latter fact the refracting power is proportionally increased. In extreme efforts of accommodation it is believed by some observers that the circularly arranged fibers, the so-called annular muscle, contract and exert a pressure on the periphery of the lens and thus aid other mechanisms in relaxing the ligament and in increasing the convexity. This view appears to be supported by the fact that in hypermetropia, where a constant effort is required to obtain a distinct image of even distant objects, the annular muscle becomes very much hypertrophied, thus reinforcing the meridional fibers. In myopia, on the contrary, where the accommodative effort is at a minimum, the entire muscle possesses less than its average size and development. According to Tscherning, a different explanation of the action of the ciliary muscle must be given. Thus, when it contracts, the antero- internal angle, that portion in close relation with the suspensory ligament, recedes and exerts on the ligament a pressure which in turn exerts a traction on the peripheral portions of the anterior surface of Fig. 322. — The Left Half Represents the Eye in a State of Rest. The Right Half in State of Accommodation. the lens, which produces the deformation observed. At the same time the postero-external portion of the muscle exerts traction on the chorioid, thus sustaining the vitreous and indirectly the lens. The reason for the flattening of the periphery of the lens from zonnjlar compression and the sharpening of the central convexity is to be found in the fact that the convexity of the more solid central portion, the nucleus, is greater than that of the lens itself. Hence it is easily understood why a zonular traction would give rise to peripheral flattening. There is, however, one point which seems difficult to harmonize with Tscherning's view; that is, the fact that during accommodation the lens appears to be slightly tremulous, thus showing relaxation, and not increased tension, of the suspensory ligament. Range of Accommodation. — It has been stated that rays of light coming from a luminous point situated at -any distance beyond 65 meters are so nearly parallel that no accommodative effort is re- quired for their focalization. So long as the luminous point remains between infinity and 65 meters, the eye, directed toward it, remains THE SENSE OF SIGHT. 671 completely relaxed. The point at which the object can be distinctly seen without accommodation is termed the far point or the puncium remotum. This for the normal eye is at a distance of 65 meters or beyond.* If the luminous point gradually approaches the eye from a point 65 meters distant, the accommodative power comes into play and gradually increases until it attains its maximum. The nearest point up to which the eye is able to form distinct images of objects is called its near point or punctum proximum. This near point in a healthy boy of twelve years will lie at 2f inches from the eye, while the same point lies only at 8 inches or 20 cm. in a man of forty years. Of objects which lie nearer than the punctum proximum the eye cannot form distinct images. The distance between the punctum ■ remotum and the punctum proximum is termed the range of accommo- dation. Force of Accommodation.— The increase in curvature of the lens necessary to focalize rays when the eye is directed from' the far to the near point necessitates the expenditure of energy on the part of the ciliary muscle. The energy expended in the act of accommo- dation may be measured by a lens, the refracting power of which is such as to enable it to produce the same result — that is, to give the diverging rays coming from the near point, e. g., 20 cm., a parallel direction. A lens, therefore, which has for a near point a focal dis- tance of 20 cm. would be a measure of the force expended, for such a lens placed in front of the crystalline lens, when in a state of repose, would, with the assistance of the latter, bring diverging rays coming from the near point to a focus on the retina. A lens of this character is said to .have a refracting power of 5 dioptrics. Since lenses of the same curvature made from different materials have different refracting powers, it becomes necessary to have, for purposes of comparison, some unit of measurement. The unit now accepted is the refracting power of a glass lens which is sufficient to focalize parallel rays at a distance of 100 cm. or i meter. This amount of refracting power is termed a dioptry. Lenses which would focalize parallel rays at a distance of 50, 20, or 10 cm. are said to have a refractive power of 2, 5, or 10 dioptrics, respectively, obtained by dividing into 100 cm. the focal distance. The refracting power of a biconcave lens is determined by prolonging backward in the direction the parallel rays have come, the rays which have been rendered divergent by the lens. The refracting media of the human eye in repose have collectively a refracting power of about 64 dioptrics, the reciprocal of its focal length. The refracting power of the corneal surface alone is equiva- lent to 42 dioptrics. The crystalline lens could in the schematic eye be replaced by a lens of about 13 dioptrics in front of the eye, as is done after the extraction of a cataract. But owing to its position in a * In practical ophthalmic work a point six meters distant is taken as the far point for the reason that the rays at this distance are practically parallel. 672 TEXT-BOOK OF PHYSIOLOGY. medium denser than air, it has been calculated that its refracting power is about 20 dioptries. The capability of the lens to increase its refraction during accom- modative efforts iaeyond the 20 dioptries varies considerably at differ- ent periods of life. At ten years the increase is 14 dioptries, as the near point is 7 cm.; at thirty years the increase is but 7 dioptries, as the near point is 14 cm.; at sixty the increase is but i dioptry and the near point 100 cm.; at sevfenty it is zero. From youth to old age, the elasticity of the lens steadily declines, and the range of accom- modation diminishes from the recession of the near point. Convergence of the Eyes during Accommodation. — In binocu- lar vision of near objects the eyes are turned inward and the optic axis of each — a line passing through the center of the cornea and the center of the eye— turned toward the median line during accom- modation. So long as the eyes are directed toward the far point, 65 meters or beyond, the optic axes are parallel. When the eyes are directed to any point within 65 meters the optic axes are converged, the convergence increasing steadily as the near point is approached. In this way the fovea of each eye is directed to the same point and single vision made possible. Were this not the case, double vision would result. Functions of the Iris. — For purposes of distinct vision it is essen- tial that the quantity of light entering the interior of the eye shall be so adjusted that the. formation and subsequent perception of the image shall be sharp and distinct. This is accomplished by the iris, the circular fibers of which alternately contract and relax with in- creasing and decreasing intensities of the light. The size of .the pupil, therefore, through which the light passes, will vary from moment to moment and in accordance with variation in the light intensity. The quantity of light necessary to distinct vision is thus regulated. In the total absence of light the sphincter pupillas muscle is relaxed and the pupil widely dilated. With the appearance of light and an increase in its intensity the muscle again contracts and the pupil progressively narrows. With a given intensity in the light, the sphinc- ter contraction is greater when the light falls directly into the fovea. Contraction of this muscle also occurs as an associated movement in the convergence of the eyes during accommodation and in consensus with the other eye. In addition to this function of the iris, it constitutes, by virtue of the sphincter muscle contraction, an important corrective apparatus. Being non-transparent, it serves as a diaphragm intercepting those rays which would otherwise pass through the peripheral portions of the lens and by spheric aberration give rise to indistinctness of the image. The movements of the iris by which the size of the pupil is determined are caused by the contractions and relaxations of the sphincter pupillcR and dilatator pupillcB muscles. The contraction of the sphincter is entirely reflex and involves those structures necessary THE SENSE OF SIGHT. 673 to the performance of any reflex act, viz.: a receptive surface, the retina; afferent nerves, the pupillary fibers of the optic nerve; a central emissive center situated in the gray matter beneath the aqueduct of Sylvius; and efferent nerves, the motor oculi and the ciliary nerves. The stimulus requisite to the excitation of this mechanism is the impact of light waves or ether vibrations on the rods and cones. According to the intensity of these vibrations will be the resulting contraction of the muscle. The contraction of the dilatator pupillae muscle is deter- mined by the activity of a continuously active nerve-center in the medulla oblongata which transmits its nerve impulses through the spinal cord, along the first and second dorsal nerves to the superior cervical ganglion, and thence to the iris by way of the fifth nerve. (See Fig. 272, page 589.) These two muscles appear to bear an an- tagonistic relation to each other, for section of the motor oculi is fol- lowed by relaxation of the sphincter muscle and dilatation of the pupil. Stimulation of the sympathetic is followed by a more pronounced dilatation. The size of the pupil is the resultant of a balancing of these two forces. OPTIC DEFECTS. Presbyopia. — This is a condition of the eye characterized by a defective or diminished accommodative power. As age advances the lens loses its elasticity and the power to increase its refraction, and vision at the normal reading distance becomes impossible. The near point, therefore, advances toward the far point, or recedes from the indi- vidual. The range of accommodation is also diminished. At forty years the near point is about 22 cm.; at forty-five years it has receded to 28 cm. This would indicate that the lens in these five years has lost I dioptry of refracting power; at fifty years the near point recedes to 43 cm., and at sixty to 200 cm., indicating a loss in refracting power on the part of the lens of 2 and 4 dioptries respectively. Convex lenses placed before the eyes having a refracting power of i, 2, and 4 dioptries would in the three instances return the near point to its normal position. At the age of seventy the lens is incapable of any increase during an accommodative effort. A lens of 4 dioptries would therefore be required by such a man, for near vision at 10 inches. Myopia. — This is a condition of the eye characterized by an in- crease in the antero-posterior diameter or a hypernormal refracting power of the lens. The former is the usual condition; Parallel rays of light brought to a focus in front of the retina again diverge, giving rise to diffusion-circles and indistinctness of the image. Divergent rays alone are capable of being focalized on the retina in its new position. The punctum remotum is always at a definite distance, but approaches the eye as the myopia increases. The near point is usually much nearer the eye than 20 cm. For this reason the condition is termed near sight. (Fig. 323.) The increase in the length of the antero-posterior diameter may 43 67^- TEXT-BOOK OF PHYSIOLOGY. range from a fraction of a millimeter up to lo mm. With an increase of 0.16 mm. the far point is but 200 cm. distant; and with an increase of 3.2 mm. it is but 10 cm. distant. Inasmuch as only divergent rays can be focalized by the myopic eye normal vision can be restored by the use of a biconcave lens with a diverging power in the first instance of 0.5 dioptry and the second of 10 dioptries. (Fig. 324.) Hypermetropia. — This is a condition of the eye characterized by decrease of the normal antero-posterior diameter or by a subnormal refracting power of the lens. The former is the usual condition. Fig. 323. — Myopia. Parallel rays focus at F, cross and form diffusion- circles; divergent rays from A focus on the retina. — {Hansell and Sweet.) Fig. 324. — Correction of Myopia BY A Concave Lens. — {Hansell and Sweet.) Parallel rays of light do not, therefore, come to a focus when the accommodation is suspended. Falling on the retina previous to focalization, they give rise to diffusion-circles and indistinctness of the image. As no object can be seen distinctly no matter how remote, there is no positive far point. The near point is abnormally distant — sometimes as far as 200 cm. For this reason the condition is termed far sight. A hypermetropic eye without accommodative effort can focalize only converging rays on the retina. If rays of light were to come from the retina of such an eye, they would, on emerging, take Fig. 325. — The Hypermetropic Eye. Parallel rays {A, B) can be focused only at a point behind the eye, as at /; rays of light coming from the retina take, on"emerging from the eye, a divergent direction, C, D. K. The negative punctum remotum. ' a divergent direction, as shown in Fig. 325, dotted line C and D. If these same rays were to be prolonged backward, they would meet at the point K, which is the punctum remotum; and as it is behind the eye, it is termed negative. Since rays coming from the retina take a divergent direction on emerging from the eye, it is evident that only converging rays can be focalized by a passive hyperme- tropic eye. As there are no convergent rays in nature, it is necessary for distinct vision that all rays, parallel and divergent, shall be given THE SENSE OF SIGHT. 67s a convergent direction before entering the eye. This is done by placing before the eye convex lenses the converging power of which is proportional to the degree of hypermetropia (Figs. 326, 327). Astigmatism. — This is a condition of the eye characterized by an inequality of curvature of its refracting surfaces in consequence of which not all of a homocentric bundle of rays are brought to the same focus. The inequality may be either in the cornea or lens, or both, though usually in the cornea. Fig. 326. — Hypermetropia. Par- allel Rays Focused behind the Retina. — {Hansell and Sweet.) Fig. 327. — Correction of Hyper- metropia BY A Convex Lens. — {Hansell ani Sweet.) In the normal cornea the radius -of curvature in the vertical meridian is a trifle shorter, 7.6 mm., than that of the horizontal, 7.8 mm., and hence its focal distance is slightly shorter. The difference, however, in the focal distances is so slight that the error in the formation of the image is scarcely noticeable. A transection of a cone of light coming from the cornea is practically a circle. If, however, the vertical cur- vature exceeds the normal to any marked extent, the rays passing through this meridian will be more sharply refracted and brought to a focus much sooner than the rays passing through the horizontal Fig. 328. — Refraction by an Astigmatic Surface. — {Hansell and Sweet.) meridian. The result will be that the cone of light will be no longer circular, but more or less elliptic. The variations of the shape of this cone are shown in Fig. 328, which represents the appearances presented on cross-section both before and after focalization of each set of rays. Though the vertical meridian has usually the sharper curvature, it not infrequently happens that the reverse is true. For the reason that the rays from one point do not all come to the same focus or point, the condition is termed astigmatism. 676 TEXT-BOOK OF PHYSIOLOGY. Spheric Aberration.— When the rays of light which emanate from a point fall upon a spheric lens, they do not after passing through it reunite at one point because of the fact that the more peripheral rays have a shorter focus than the central rays. To this condition the term spheric aberration is given. Spheric aberration can be dem- onstrated in the human eye. That this condition is present to but a slight extent in the normal eye is due to the presence of the iris, which intercepts those rays which would otherwise pass through the marginal portions of the refracting media. In widely dilated eyes the spheric aberration of the peripheral parts may amount to as much as 4.5 dioptrics. Chromatic Aberration. — When a beam of white light is made to pass through a prism, it is decomposed into the primary colors owing to a difference in the refrangibility of the rays. In passing through the refracting media of the eye the different rays composing white light also undergo unequal refraction and those rays which give rise to one color are brought to a focus at a point somewhat different from, those which give rise to other colors. If the eye is accommodated for one set of rays, it is not for another, and the result is a fringe of colors around the image. This defect in the normal eye is so slight that the mind fails to take cognizance of it. That the eye is incapable of simultaneously focalizing rays of widely different refrangibility, as those which give rise to the blue and red colors, is shown by the following experiment: The eye being directed to a luminous point, a plate of cobalt-glass is placed between the light and the observer close to the eye. This substance has the property of intercepting all rays but the red and the blue and hence these alone will be seen. The center of the image produced will be red and clearly defined, the periphery blue and ill defined. The reason for this is clear. The eye more readily accommodates itself for the red rays, and hence their focal point is distinct. The blue rays, having a higher degree of refrangibility, come to a focus, cross and diverge, and give rise to diffusion-circles. If a biconcave glass be placed before the cobalt, the blue rays can be focalized on the retina, while the red will fall on the retina without focalization. The image will now be blue and distinct in the center, the periphery red and ill defined. With the removal of the minus glass the reverse condition again obtains. Imperfect Centering.— From a purely physical point of view, the eye is not a perfect optic instrument. In addition to the defects noticed in the foregoing paragraphs, there is yet another, viz.: an imperfect centering of the refracting surfaces. In first-class optic instruments the lenses are centered — that is, their exact centers are situated on the same axis. In viewing an object through such a system the visual line corresponds with the axis of the lens system. This is not the case with the refracting system of the eye. A line passing through the center of the cornea and the center of the eye, the optic axis (O A in Fig. 329), does not pass exactly through the THE SENSE OF SIGHT. 677 2. center of the lens and does not fall into the point for most distinct vision, the fovea. This has led to the recognition of other lines the relations of which must be kept in mind in all optic discussions, viz. : I. The visual axis or visual line (F L), the line connecting the point viewed, the nodal point and the fovea centralis. The^ line^ of fixation or line of regard (VC), the line connecting the point viewed with the center of rotation, the latter being situated 6 mm. behind the nodal point of the eye and 9 mm. before the retina. The relation of these lines and certain angles connected with them are shown in Fig. 329. The angle included between the line D D (the major axis of the corneal ellipse) and the visual line is the angle alpha, amounting on the average to 5°. The angle included between the optic axis and the line of fixation or 2efnfitinziiyic€e Fig. 329. — DiAGEAM showing the Corneal Axis D D, the Optic Axis O A, THE Visual Axis V L, and the Line as Fixation V C; also the Three Adoels, a, ^, 7, regard is the angle gamma, while the angle between the optic axis and the line of vision is the angle beta. In emmetropia the angle alpha is about 5°. In hypermetropia it is greater, amount- ing to 7° or 8°, giving to the eye an appearance of divergence. In myopia it is much smaller — 2° — or in extreme cases may be abolished, the line of vision corresponding with the optic axis or even passing beyond it. The angle gamma is of value in de termining the 'actual deviation of the eye in squint. Functions of the Retina. — Of all the layers of the retina, the rods and cones appear to be the most essential to vision. It is only this layer that is capable of receiving the light stimulus and of trans- forming it into some specific form of energy, which in turn arouses in the fibers of the optic nerve the characteristic nerve impulses. A ray of light entering the eye passes entirely through the various layers of the retina, and is arrested only upon reaching the pigmentary epithelium in which the rods and cones are embedded. As to the manner in which the objective stimuli — light and color, so called — 678 TEXT-BOOK OF PHYSIOLOGY. are transformed into nerve impulses, but little is known. It is prob- able that the ether vibrations are transformed into heat, which excites the rods and cones. These, acting as highly specialized end-organs of the optic nerve, start the impulses on their way to the brain, where the seeing process takes place. As to the relative function of the rods and cones, it has been suggested, from the study of the facts of comparative anatomy, that the rods are impressed only by differ- ences in the intensity of light, while the cones, in addition, are im- pressed by qualitative differences in color. The nerve-fibers them- selves are insensible to the impact of the ether vibrations, and require for their excitation some intermediate form of energy. That this is the case was shown by Bonders, who reflected a beam of light on the optic nerve at its entrance without the individual experiencing any sensation of light. This region, occupied only by the optic-nerve fibers and devoid of any special retinal elements, is therefore an insensitive or blind spot. The diameter of this spot is about 1.5 mm., and occupies in the field of vision a space of about 6°. It is situated about 3.5 mm. to the nasal side of the visual axis. Its existence can Fig. 330. — Diagram for Observing the Situation of the Blind Spot. — (HelmhoUz.) be demonstrated by the familiar experiment of Mariotte, which con- sists in placing before the eye two objects having the relation to each other as in Fig. 330. With the left eye closed and the right eye directed to the cross, both objects may be visible. But by moving the figure away from or toward the eye, there will be found a distance, about 30 cm., when the circle will be invisible. This occurs when the image falls on the optic nerve at its entrance. The experiment of Purkinje as described in the following paragraph demonstrates also the fact that the sensitive portion of the retina is to be found only in the layer of rods and cones. It is well known that the blood-vessels of the retina are situated in its innermost layers a short distance behind the optic-nerve fibers. Owing to this anatomic arrangement, a portion of the light coming through the pupil will be intercepted by the vessels and a shadow projected on the layer of rods and cones. Ordinarily, these shadows are not perceived, for the reason that the shaded parts are more sen- sitive, so that the small amount of light passing through the vessels produces as strong an impression on this part as does the full amount of light on the unshaded parts of the retina, and perhaps because the mind has learned to disregard them. But if light be made to enter THE SENSE OF SIGHT. 679 the eye obliquely, the position of the shadows will be changed, when at once they become apparent. This can be shown in the following way: If in a darkened room a lighted candle be held several inches to the side and to the front of the eye, and then moved up and down, there will be perceived, apparently in the field of vision, an arborescent figure corresponding to the retinal blood-vessels. This is due to the falling of the shadows on unusual portions of the layer of rods and cones. Excitability of the Retina. — The retina is not equally excitable in all parts of its extent. The maximum degree of sensibility is found in the macula lutea, and especially in its central portion, the fovea. In this region the layers of the retina almost entirely disappear, the layer of rods and cones alone remaining, and in the fovea only the latter are present. That this area is the point of most distinct vision is shown by the observation that when the eye is directed to any given point of light, its image always falls in the fovea. Any pathologic change in the fovea is attended by marked indistinctness of vision. The sensibility of the retina gradually but irregularly diminishes from the macula toward the periphery. This diminution in sensibility holds true for monochromatic as well as white light. As stated above, the nature of the molecular processes which, take place in the retinal tissue, caused on one hand by the light vibrations, and on the other hand developing nerve impulses, is entirely un- known. The discovery of the visual purple in the outer segment of the rods gave promise of some explanation of the process, especially when it was shown to undergo changes when exposed to the action of light. But as the pigment is wanting in the cones, and especially in the fovea, it cannot be considered essential to distinct vision, al- though that it plays some important role in the visual process is highly probable. It was observed by Van Genderen Stort, that when an animal is kept in darkness some time before death, the cones are long and filiform; but if the animal has been exposed to light, they are short and swollen. It was discovered by Boll that if an animal is kept in darkness an hour or two before death the pigment is massed at the ends of the rods and cones, but after exposure to light it becomes displaced and extends over and between the rods almost to the external limiting membrane. These conditions are represenied in Fig. 331. The Eye a Living Camera. — In its construction, in the arrange- ment of its various parts, and in their mode of action the eye may be compared to a camera obscura. Though the comparison may not be absolutely exact, yet in a general way it is true that there are many striking points of similarity between them; e. g., the sclera and chorioid may be compared to the walls of the camera; the combined refracting media to the single lens, the action of which results in the focusing of the light rays; the retina to the sensitive plate receiving the image formed at the focal point; the iris to the diaphragm for the regulation 68o TEXT-BOOK OF PHYSIOLOGY. of the amount of light to be admitted, and for the partial exclusion of those marginal rays which give rise to spheric aberration; the ciliary muscle to the adjusting screw, by means of which the image is brought to a focus on the sensitive plate, notwithstanding the varying distances of the object from the lens. The presence of the visual purple in the rods of the retina capable of being altered by light makes the com- parison still more striking. Kiihne even succeeded in obtaining a fixed image or an optogram of an external object in a manner similar to that by which an image is fixed on the sensitive plate of a camera. An animal is kept in the dark for about ten minutes in order to permit the retinal pigment to be completely regenerated. The animal, with the eyes covered, Fig. 331. — Section op the Retina of a Frog. A. In darkness. B. In light. — {After Van Genderen Start, from Tscheming's "Physiologic Optics.") is then brought into a room with a single window. While the head is steadily directed to the window, the eye is exposed for several minutes. The eyes are again covered, the animal killed, and the eyes removed by the light of a sodium flame. The retina is then placed in a 4 per cent, solution of alum. In a short time the image of the window, the optogram, will be fixed (Fig. 332). That portion of the retina corresponding to the image is quite bleached in appear- ance from the action of the light on the pigment. During life the regeneration of the visual purple must take place with extreme rapidity. It is believed to be derived from a pigment secreted by the layer of pigment cells. Bino,cular Vision. — Though two images are formed, one on each retina, when the eyes are directed to a given object, there results but one sensation. If the direction of either visual axis be changed by THE SENSE OF SIGHT. 68i Fig. 332. — Retina of a Rabbit. Optogram OF a Window Four Meters Distant a. Yellow spot. b, b. White streak of nerve- fibers. — (Kuhne.) pressure on the eyeball, there arise two sensations, and the object appears to be doubled. The reason assigned for this, in the first instance, is that the two images fall into the fovese, two corresponding points; while in the second instance they fall on non-corresponding points. It would appear, therefore, that for the purpose of seeing an object singly when the eyes are directed toward it, the rays eman- ating from it must fall on corresponding parts of the retina. As all portions of the retina are sensitive to light, though in varying degrees, it is not essential that the images always fall in the fovcce. The parts of the retinae which cor- respond physiologicly are shown in Fig. 333. In this figure the retinal area is divided into quadrants by vertical and horizontal lines of separation, as they are termed. If one retina is placed in front of or over the other, it will be found that the quadrants bearing similar letters cover each other. So long as the rays of light, entering 1 he eye, fall on corresponding areas the sensation of but one object arises. If, however, they fall on non-corresponding areas, two sensa- tions arise. Normal binocular vision enlarges very considerably the area of the visual field, permits of a better estima- tion of the size and distance of objects, enables the mind to form more readily a perception of depth, increases the intensity of sensations and makes sensation more uniform by off-setting retinal rivalry. The Horopter. — When the eyes are in the so-called secondary position — ^that is, in a position in which the visual axes are con- verged and directed to a point in front of and in the middle plane of the body — it will be found on examination that rays of light from a number of other objects enter the eye, pass through the nodal point, and fall on corres- ponding parts of the two retinte • and give rise to but single im- ages. All such points lie, for the horizontal line of separation, on a line termed the horopter. The form of this line is that of a circle which passes through the fixation point and the two nodal points. Any object on the horopter will give rise to but a single image. This is shown in Fig. 334, in which the objects I, II, III project their rays into both eyes which fall on corresponding areas. ■ In addition to the horopter for the horizontal line of separation, there is also an horopter for the vertical line of separation. At a distance of two meters the vertical horopter is a plane. Within this distance it is concave to the face; beyond this distance it is convex. An object which lies either in front of or behind the fixation point Fig. 333. — Corresponding Areas of the Retina. 682 TEXT-BOOK OF PHYSIOLOGY. will project its rays on parts of the retinae which do not correspond, and hence give rise to double images. This is evident from examina- tion of Fig. 335. While the eyes are direcited to figure 2, of which there is but a single image, the objects B and A give rise to double images, for reasons already given. If the eyes are now directed to B, double images will be formed of 2 and A. At all times, therefore, double images are formed on the retinae the existence of which is scarcely noticed unless the attention is directed to them. This is due to the fact that many of the images fall on the peripheral, less sensitive parts of the retinae. At the same time, from a want of accommodation and the formation of diffusion-circles, they are indistinct. For these reasons they are readily neglected. B Fig. 334. — Horopter for the Secondary Position, with Con- vergence OP THE Visual Axes. — {Landois.) Fig. 335. — Scheme of Identical and Non-identical Points of the Retina. — {Landois.) In the primary position of the eyes — that is, a position in which the visual axes are parallel — the horopter is a plane in infinity. In the tertiary position the horopter is a curve of complex form. Movements of the Eyeball. — The almost spheric eyeball lies in the correspondingly shaped cavity of the orbit, like a ball placed in a socket, and is capable of being moved to a considerable extent by the six muscles which are attached to it. These muscles are the superior and inferior recti, the external and internal recti, and the superior and inferior obliqui (Fig. 336). The four recti muscles arise from the apex of the orbit cavity, from which point they pass forward to be inserted into the sclera about 7 to 8 mm. from the corneal border. The superior oblique muscle having a similar origin THE SENSE OF SIGHT. 683 passes forward to the upper and inner angle of the orbit cavity, at which point its tendon passes through a cartilaginous pulley, after which it is reflected backward to be inserted into the superior sur- face of the sclera about 16 mm. behind the corneal border. The inferior oblique muscle arises from the inner and inferior angle of the orbit cavity. It then passes outward, upward, and backward, to be inserted into the upper, posterior and temporal portion of the sclera about 4 or 5 mm. from the optic nerve entrance. The movements of each eye are referred to three fixed lines or axes, which have their origin at the point of rotation of the eyeball, this point lying about 1.7 mm. behind the center of the globe. If the eye looks straight forward in the horizontal plane (the head being erect) , the line joining the center of rotation with the object looked at is the line of fixation or line of regard. Around this antero-posterior axis the eye may be regarded as performing its circular rotation or torsion. At right angles to this line, and joining the center of rotation of both eyes, is the horizontal or trans- verse axis, around which the movements of eleva- tion (up to 34 degrees) and depression (down to 57 degrees) take place. At right angles to both , of these lines there is the vertical axis, around wtiich the movements of adduction (toward the nose lip to 45 degrees) and abduction (toward the temple up to 42 degrees) occur. The six muscles may be divided into three pairs, each of which has a common axis around which it tends to move the eyeball. But only the common axis of the internal and external recti coincides with one of three axes before mentioned — namely, with the vertical axis — thus moving the ball only inwardly or outwardly — respec- tively. The other two pairs, however, have their own axes of action, and their movements of the ball must be, therefore, analyzed with regard to all the three axes, each of these four muscles producing rotation, elevation, and depression, and abduction or adduction. The superior and inferior recti muscles, forming one pair, move the eye around a horizontal axis which intersects the median plane of the body in front of the eyes at an angle of 63 degrees, and the superior and Fig. 336. — MtrscLES of the Eye. Tendon or Ligament of Zinn. i. Tendon of Zinn. 2. Ex- ternal rectus divided. 3; Internal rectus. 4. Inferior rectus. 5. Superior rectus. 6. Superior obJique. 7. Pulley for superior oblique. 8. In- ferior oblique. 9. Levator palpebrse superioris. 10, 10. Its anterior expansion, ir. Optic nerve. — {Sappey.) 684 TEXT-BOOK OP PHYSIOLOGY. Inward and Rectus intemus. downward, Rectus inferior. Outward and Obliquus superior Rectus extemus. upward, Outward and Rectus superior. Obliquus inferior. Rectus extemus. downward, .... Rectus inferior. Obliquus superior. inferior oblique muscles forming the third pair rotate the globe around a horizontal axis which cuts the median plane of the body behind the eyes at an angle of 39 degrees. Thus it is that each muscle moves the eye as follows, the movement for practical purposes being referred to the cornea: The rectus externus draws the cornea simply to the temporal side, the rectus intemus simply to the nose; the superior rectus displaces the cornea upward, slightly inward, and turns the upper part toward the nose (medial torsion) ; the inferior rectus moves the cornea downward, slightly inward, and twists the upper part away from the nose (lateral torsion) ; the superior oblique displaces the cornea downward, slightly outward, and produces medial torsion; while the inferior oblique moves the cornea upward, slightly outward, and produces lateral torsion. These facts show that for certain move- ments of the eye at least three muscles are necessary (see following table) : Inward, Rectus intemus. Outward, Rectus extemus. upward I '^^^^ ^"Pf °^- '^ ' I Obliquus uuerior. Downward, .... 1 ]^f ^?"= intenor. ' I Obliquus superior. Inward and ( Rectus intemus. upward, •) Rectus superior. [ Obliquus inferior. If both eyes have their line of vision in the horizontal plane parallel with each other and with the median plane of the body, they are said to be in the primary position. All other positions are called secondary. Both eyes always move simultaneously, which is called the associated movement of the eyes. There are three forms of asso- ciated movements: (i) movement of both eyes in the same direction; (2) movements of convergence by which the visual lines are con- verged on a point in the middle line of the body; (3) movements of divergence, by which the eyes are brought back from convergence to parallelism, or even to divergence, as in certain stereoscopic exercises. A combination of (i) and (2) or of (i) and (3) takes place for certain positions of the object looked at. Color-perception. — A beam of sunlight passed through a glass prism is decomposed into a series of colors — -red, orange, yellow, green, blue, and violet — the so-called spectral colors, so well exem- plified in the rainbow. The spectral colors are termed simple colors, because they can not be. any further decomposed by a prism. Ob- jectively, the spectral colors consist of very rapid transverse vibrations of the ether, from about 400 millions of millions per second for red to about 760 millions of millions for violet, but subjectively they are sensations caused by the impact of the ether-waves on the percipient layer of the retina. It is possible to mix or blend these spectral color-sensations in the eye by stimulating the same area of the retina by different spectral THE SENSE OF SIGHT- 68s colors, either at the same time or in rapid succession. The following table shows the results of such experiments as performed by v. Helm- holtz (Dk. = dark; Wh. = whitish): Red. Orange. Yellow. Gr. -yellow. Green. Bluish-green, Cyan- Blue. Violet. Purple. Dk.-rose. Wh.-rose. White. White-blue. Water-blue. Indigo. Indigo. Dk.-rose. Wh.-rose. White. Wh. -green. Water-blue. Water-blue. Cyan- blue. Wh.-rose. White. Wh. -green. Wh. -green. Bl. -green. Bluish- green. White. Wh.-yellow. Wh. -yellow. Green. Green. Wh.-yellow. Yellow. Gr.-yellow. Greenish- yellow. Yel- low. Gold-yellow. Orange. Yellow. These are the mixed colors. But it is to be observed that only two new color-sensations can be produced, white and purple, the remain- ing mixed colors already finding their equivalent in the spectrum. White and purple, therefore, are color-sensations which have no objective equivalent in a simple number of ether-vibrations like the spectral colors. Two spectral colors which by their mixture produce the sensation of white are called complementary colors. Such are red and green- blue, golden yellow and blue, green and violet. The mixture of all the spectral colors produces white again. This is the result of adding two or more color-sensations. Different results are obtained, however, by adding color pigments. Yellow and blue, for example, produce in the eye white, but on the painter's palette green. The colors of nature are usually mixtures of simple colors, as can be shown by spectroscopic analysis or by a synthesis of spectral colors. In all color-sensations we must distinguish three primary qualities : (i) hue; (2) purity or tint; (3) brightness or luminosity. The first quality gives the main name to the color — e. g., red or blue — this de- pending on the spectral color or the mixture of two spectral colors with which it can be matched. The second quality, the tint, depends on the admixture of white with the ground color; and the third quality, brightness, depends on the objective intensity of the light and the subjective sensitiveness of the retina. Color-perception thus far refers only to the most sensitive part of the retina. At the more peripheral parts of the retina the colors are seen somewhat differently, as is shown by the following table giving the limits up to which the colors are recognized: White. Blue. Red. Green. Externally 90° '80° 65° 50° Internally 60° 55° 5°° 40° Superiorly 45° 4°° 35° 3°° Inferiorly 7°° ^°° 45° 35° Theories of Color-perception.— The theory of v. Helmholtz, originated by Thomas Young (1807), assumes in its latest form the existence in the human retina of three different kinds of end-organs, 686 TEXT-BOOK OF PHYSIOLOGY. each of which is loaded with its own photo-chemical substance capable of being decomposed by a certain color, and thus exciting the fiber of the optic nerve. In the first group these end-organs are loaded with a red-sensitive substance, which is affected mainly by the red part of the spectrum; the second group has its end-organs provided with a green-sensitive substance, which is mainly excited by the green color; while the third group is provided with a blue-sensitive substance, this latter being mainly affected and decomposed by the blue-violet portion of the spectrum. All these three different end-organs are present in every part of the most sensitive area of the retina, and are connected by separate nerve-fibers with special parts of the brain, in the cells of which each calls up its separate sensation of red or green or blue. Out of these three primary color-sensations all other color-sensa- tions arise. If a light mainly excites the red- or green- or blue -sensi- tive substance of a retinal area, we term it red, green, or blue, re- spectively. But if two of these photo-chemicaL substances are stimu- lated simultaneously, quite different sensations arise. Thus simul- taneous stimulation of the red and green substances gives rise to the sensation of yellow, that of red and blue to the sensation of purple, and that of blue and green to the sensation of blue-green. Simul- taneous stimulation of all three substances of a certain area produces the sensation of white. According to this theory, complementary colors are all those which together excite all the three substances. Color-blindness is explained by this theory, on the assumption that two of the photo-chemical substances have become similar or equal in composition to each other. The theory of Hering, brought forward in 1874, has the under- lying assumption that the process of restitution in a nerve-element is capable of exciting a sensation. This theory asserts that there are three visual substances in the retina — a white-black, a red-green, and a yellow-blue visual substance. A destructive process in the white- black substance, such as is induced not only by white light, but also by any other simple or mixed color, produces the sensation of white, while the process of restitution or assimilation in this substance pro- duces the sensation of black. Similarly, red light produces dis- assimilation or decomposition in the red-green substance, and this, again, the sensation of red. Green light, however, favors the process of restitution or assimilation in the red-green substances, and thus gives rise to the sensation of green. In the same way the sensation of yellow has its cause in the decomposition of yellow-blue substance induced by yellow light, while the sensation of blue is produced by an assimilative process in the same substance. Simultaneous processes of disassimilation and assimilation in the same visual substance an- tagonize each other, and consequently produce no color-sensation by means of this substance, but only the sensation of white, by reason of decomposition, by both colors, in the white-black substance. Thus, THE SENSE OF SIGHT. 687 yellow and blue, impinging on the same retinal area, have no effect on the yellow-blue substance, because they are antagonistic in their action on this substance, but only produce the sensation of white, as both yellow and blue decompose the white-black material. Color- blindness is explained by the assumption of the absence of either the red-green or the yellow-blue visual substance in the retina. Accessory Structures.— The eyeball is protected anteriorly by the eyelids and their associated structures, the Meibomian glands, the lachrymal glands, and tears. The eyelids consist of a central framework of connective tissue supporting muscle tissue (the orbicularis palpebrarum muscle) and glands, and covered externally by skin and internally by a modified skin, the conjunctiva. The free border of each lid is strengthened Fig. 337. — The Lacrimal and Meibomian Glands, and Adjacent Organs of the Eye. I, I. Inner wall of orbit. 2, 2. Inner portion of orbicularis palpebrarum. 3, 3. Attachment to circumference of base of orbit. 4. Orifice for transmission of nasal artery. 5., Muscle of Horner (tensor tarsi). 6, 6. Meibomian glands. 7, 7. Orbital portion of lacrimal gland. 8,9,10. Palpebral portion. 11, 11. Mouths of excretory ducts. 12,13. Lacrimal puncta. — [Sappey.) by a semilunar plate of dense fibrous tissue, the tarsus. The cuta- neous edge of the lid is bordered with short stifE hairs. At the inner extremity each eyelid presents a small opening, the punctum lacrimale, the beginning of the lachrymal duct. The two ducts after uniting open into the nasal duct. The Meibomian glands are modified sebaceous glands imbedded in the posterior portion of the lids (Fig. 337). Their ducts open on the free border of the lid. These glands secrete an oleaginous ma- terial resembling sebaceous matter which accumulates along the margin of the lid and prevents the tears from flowing down the cheek. The lachrymal gland is situated at the upper and outer part of the orbit cavity. It consists of a series of compound tubules lined by 688 TEXT-BOOK OF PHYSIOLOGY. epithelium. The' secretion (the tears) is conducted from the gland to the outer part of the conjunctiva by seven or eight ducts. The lachrymal secretion consists of water and inorganic salts. It is dis- tributed over the corneal surface during the act of winking, thus keeping it moist and free from foreign particles. It eventually passes into the lachrymal ducts and then into the nose. The lachrymal glands receive secretory fibers by way of the fifth nerve and the cervical sympathetic. The secretion can be excited reflexly from stimulation of sensor nerves as well as by emotional states. CHAPTER XXVIII. THE SENSE OF HEARING. The physiologic mechanism involved in the sense of hearing in- cludes the ear, the auditory nerve, its cortical connections, and nerve- cells in the cortex of the temporal lobe. Peripheral excitation of this mechanism develops nerve impulses which, transmitted to the cortex, evoke the sensation of sound and its varying qualities— intensity, pitch, and timbre. The specific physiologic stimulus to the terminal organ, the organ of Corti, is the impact of atmospheric undulations of varying energy and rapidity. THE PHYSIOLOGIC ANATOMY OF THE EAR. The ear, the organ of hearing, is lodged within the petrous portion of the temporal bone. It may, for convenience of description, be divided into three portions; viz., the external, the middle, and the internal portions (Fig. 338). The external ear consists of the pinna or auricle and the external auditory canal. The pinna is composed of a thin layer of cartilage which presents a series of elevations and depressions. It is attached by fibrous tissue to the outer edge of the auditory canal and covered by a layer of skin continuous with that covering adjacent structures. The general shape of the pinna is concave. Its anterior surface pre- sents, a little below the center, a deep depression — the concha. The external auditory canal extends from the concha inward for a distance of from 25 to 30 mm. It is directed at first upward, for- ward, inward, and then downward to its termination. It is composed partly of bone and partly of cartilage and lined by a reflection of the skin covering the pinna. At the external portion of the canal the skin contains a npmber of tubular glands, the ceruminous glands, which resemble in their conformation the perspiratory glands. They secrete cerumen or ear-wax. The middle ear, or tympanum, is an irregularly shaped cavity hollowed out of the temporal bone and situated between the external auditory canal and the internal ear. It is narrow from side to side, though wider above than below. It is relatively long in its antero- posterior and vertical diameters. The upper portion is known as the attic. The middle ear is in communication posteriorly with the mastoid cells, anteriorly with the pharynx through the Eustachian tube. The Eustachian Tube. — The passageway between the tympanic 44 689 690 TEXT-BOOK OF PHYSIOLOGY. cavity and the naso-pharynx is known from its discoverer as the Eustachian tube. It is composed internally of bone, externally of cartilage, and is lined by mucous membrane covered with ciliated epithelium. Near the middle of its course the tube is contracted, though expanded at either extremity (Fig. 338). It measures about 40 mm. in length. Its general direction from the pharyngeal orifice is outward, backward, and upward at an angle of about 45 degrees. The middle ear cavity is separated from the external ear by a membrane — the membrana tympani — and from the internal ear by an osseo-membranous partition which forms a common wall for both Fig. 338. — The Ear. i. Pinna, or auricle. 2. Concha. 3. External auditory canal. 4. Membrana tympani. 5. Incus. 6. Malleus. 7. Manubrium mallei. 8. Tensor tym- pani. 9. Tympanic cavity. 10. Eustachian tube. 11. Superior semicircular canal. 12. Posterior semicircular canal. 13. External semicircular canal. 14. Cochlea. 15. Internal auditory canal. 16. Facial nerve. 17. Large petrosal nerve. 18. Vestibular branch of auditory nerve. 19. Cochlear branch. — (Sappey.) cavities. The interior of the cavity is crossed from side to side by a chain of bones and lined by a mucous membrane continuous with that lining the pharynx. The membrana tympani is a thin, translucent, nearly circular membrane, measuring about lo mm. in dianieter, placed at the inner termination of the external auditory canal. It is inclosed in a ring of bone which in the fetal condition can be easily removed, but in the adult condition can not be removed, owing to its consolidation with the surrounding bone. This membrane consists primarily of a layer of fibrous tissue which is covered externally by a thin layer of skin continuous with that lining the auditory canal, and internally THE SENSE OF HEARING. 691 by a thin mucous membrane. The tympanic membrane is placed obliquely at the bottom of the auditory canal, inclining from above and behind downward and forward at an angle of about forty-five degrees. The external surface of this membrane presents a funnel- shaped depression, the sides of which are slightly convex. The Ear-hones. — Running across the tympanic cavity and form- ing an irregular line of joined levers is a chain of bones, which articu- late one with another at their extremities. These bones are known as the malleus, incus, and stapes. The form and arrangement of these bones are shown in Figs. 339, 340. The malleus, or hammer bone, consists of a head, neck, and handle, of which the latter is attached to the inner surface of the membrana tympani. The incus or anvil bone presents a con- cave articular surface which receives the head of the malleus. The stapes, or stirrup-bone, articulates ex- ternally with the long pro- cess of the incus, and in- ternally, by its oval base, with the edges of an oval opening, the foramen ovale. The entire chain is partially supported by a ligament attached to the short pro- cess of the incus and to the walls of the tympanic cavity. The Tensor Tympani Muscle. — This is a d&licate muscle, about 15 mm. in length, situated in a nar- row groove just above the Eustachian tube (Fig. 341). It arises from the cartila- ginous portion of the Eusta- chian tube and the adjacent portion of the sphenoid bone. From this origin it passes nearly horizontally backward to the tympanic cavity; just opposite to the foramen ovale, its tendon bends at a right angle over the processus cochleariformis and then passes outward across the tympanic cavity to be inserted into the handle of the malleus near tha neck. The stapedius muscle emerges from the cavity of a pyramid of bone which projects from the posterior wall of the tympanum. Its ten- don passes forward to be inserted into the neck of the stapes bone near its point of articulation with the incus. The internal ear, or labyrinth, is located within the petrous Fig. 339. — Tympanic Membrane and the Audi- tory Ossicles (Left) seen from within, i. c, from: the Tympanic Cavity. M. Manubrium or handle of the malleus. T. Insertion of the tensor tympani. h. Head. IF. Long process of the malleus, a. Tncus, with the short {K) and the long (/) process. S. Plate of the stapes. Ax, Ax, is the common axis of rotation of the auditory ossicles. 5'. The pinion-wheel arrangement be- tween the malleus and ircus. — (Landois.) 692 TEXT-BOOK OF PHYSIOLOGY. Fig. 340. — Audi- tory Ossicles, i. Head of malleus. 2. Processus brevis. 3. Processus gracilis. 4. Manubrium. 5. Long process of in- cus. 6. Articulation between incus and stapes. 7. Stapes. —{Sappey.) portion of the temporal bone. It consists of an osseous and a mem- branous portion, the latter contained within the former. The osseous labyrinth is subdivided into vestibule, semicircular canals, and cochlea. The vestibule is a small, triangular-shaped cavity between the semicircular canals and the cochlea. It is separated from the cavity of the middle ear by an osseous partition which presents near its center an oval opening, the foramen ovale. In the living con- dition this opening is closed by the base of the stapes bone, which is held in position by an annular ligament. The inner wall presents a number of openings for the passage of nerve-fibers (Fig. 342). The semicircular canals are three in number, a superior vertical, an inferior vertical, and a hori- zontal, each of which opens by two orifices into the cavity of the vestibule, with the exception of the two vertical, which unite at one extremity and then open by a single orifice. Each canal near its vestibular orifice is enlarged to almost twice the size of the rest of the canal, forming what is known as the ampulla. The cochlea, the anterior portion of the labyrinth, is a gradually tapering canal, about 35 mm. in length, wound spirally two and a half times around a central bony axis, the modiolus. The cavity of the cochlea is partially subdivided into two cavities by a thin spiral plate of bone which projects from the inner wall, known as the lamina osseous spiralis. In the natural condition this partition is completed by a connective-tissue mem- brane, so that the two passages are com- pletely separated from each other. The upper passage or scala is in free com- munication with the vestibule, and is known as the scala vestibuli; the lower passage or scala in the dead condition communicates with the tympanum by means of a round opening, the foramen rotundum, and is therefore known as the scala tympani. In the living condition this opening is completely closed by a membrane, a second membrana tympani. Both the scalae vestibuli and tympani communicate at the apex of the cochlea by a small opening, the helicotrema. The modiolus, the central bony axis, is perforated from base to apex by a canal for the passage of the auditory nerve-fibers; lateral canals, diverging from the central canal, pass through the osseous lamina spiralis and transmit fibers of the auditory nerve. The interior of the bony labyrinth is Fig. 341. — M, The Tensor Tympani Muscle — the Eus- tachian Tube (Left). — {Landois.) THE SENSE OF HEARING. 693 Fig. 342. — Bony Cochlea, i. Ampulla of superior semicircular canal. 2. Horizontal canal. 3. Junction of superior and posterior semicircular .canals. 4. The pos- terior semicircular canal. 5. Fora- men rotundum. 6. Foramen ovale. 7. Cochlea. lined by periosteum covered by epithelium and in communication with lymph-spaces at the base of the skull by means of the aqueduct of the vestibule. The membranous labyrinth, lying within the osseous labyrinth, corresponds with it in form, though it is smaller in size. It may be subdivided into vestibule, semicircular canals, and cochlea (Fig. 343). The vestibular portion consists of two small sacs, the utricle and the saccule, which communicate with each other by means of the two branches of a duct passing through the aqueduct of the vestibule — the ductus endolymphaticus. The semicircular canals communicate with the utricle in the same manner as the bony canals communicate with the vestibule. The saccule communicates with the membranous cochlea by a short canal, the canalis reuniens. The walls of the utricle, saccule, and semicircular canals are composed of connective tissue lined by epithelium. At the points of entrance of the auditory nerve, the macules acusticce, in all three structures, the epithelium undergoes a marked change in appearance and structure. It becomes columnar in shape and provided with stiff hair-like processes or threads, which projecl into the cavity. In the saccule and utricle the hair-like processes are covered by a layer of small crystals of calcium carbonate held together by a gelatinous material. The crystals are known as otoliths (Fig. 344). The fibers of the vestibular nerve, arising from the cells of the ganglion of Scarpa in the internal auditory meatus, send their peripherally directed branches through the foramina in the inner wall of the vestibule, through the walls of the utricle and semicircular canals near the ampulla. As the fibers approach the maculae acusticas they subdivide into delicate fibrillse, which ultimately become histologically and physiologically related to the neuro-epithe- lium. From the relation of the nerve-fibers to the epithelium, the latter must be re- garded as the highly specialized terminal organ of the vestibular portion of the auditory nerve. The cochlea is a closed membranous tube situated between the osseous lamina spiralis and the outer bony wall. A transection of the entire cochlea shows the relation of the osseous and membranous portions (Fig. 345). The cochlear tube is triangular in shape. The base is attached to the bony wall, the apex to the edge of osseous Fig. 343. — I. Utricle. 2. Saccule. 3. Vestibular end of cochlea. 4. Canalis reuniens. 5. Membranous cochlea. 6. Membranous semicircular canal. — {Potter's "Anatomy.") 694 TEXT-BOOK OF PHYSIOLOGY. Fig. 344. — Section of Wall of Utricle of the Internal Ear, through macular region, from rabbit, showing otoliths (0), em- bedded within granular substance (g). h. Ciliated cells with proc- esses, {p), extending between sustentacular elements (j). m. Basement membrane, n. Nerve- fibers within fibrous tissue {t) passing toward hair-cells and becoming non-meduUated at base- ment-membrane. — {After Piersol.) lamina spiralis. One side of the tube forms in part the membrane of Reissner, the other side forms in part the basilar membrane. The sides of the cochlea toward the scala vestibuli and scala tympani are covered with epithelium. The triangular cavity of the cochlear tube is known as the scala media. The inner surface of the cochlear tube is lined by epithelium, which becomes extraordi- narily modified and specialized along the surface of the basilar membrane, to con- stitute what is known as — • The Organ of Corti.— In Fig. 345 this organ is represented as it appears on cross-section of the cochlea. It con- sists primarily of an arch composed of two modified epithelial cells known as the rods or pillars of Corti, which rest below on the basilar membrane, but meet and interlock above; it consists second- arily of a series of columnar epithelial cells provided with hair-like processes which rest upon and are supported by the rods both on the inner and outer aspects of the arch. The space beneath the arch is known as the tunnel. The inner hair cells are not nearly so numerous as the outer hair cells The epithelial cells external to the outer and inner hair cells are sup porting or sustentacular in character, The organ of Corti extends the entire length of the cochlea. The number of rods which, standing side by side, form the inner limb of the arch is estimated at 5600; the number which form the outer limb is estimated at 3850. The outer rods are broader than the inner and at some places articu- late with two or three inner rods. The upper edges of the rods are flattened, elongated, and project outward, forming a reticulated membrane through the meshes of which the hair-like processes of the cells project. From the connective-tissue thickening on the upper surface of the osseous lamina spiralis there extends outward over the organ of Corti a thin membrane, the membrana tectoria. The common cavity between the walls of the osseous and membranous labyrinth in the Fig. 345. — A Transverse Section of a Turn of the Cochlea. THE SENSE OF HEARING. 695 vestibule, the semicircular canals, in the scala vestibuli and scala tympani of the cochlea, is filled with a clear fluid— the perilymph; the common cavity within the walls of the entire membranous labyrinth is also filled with a similar fluid — the endolymph. The hair-like proc- esses of the epithelial cells covering the maculae acusticae and the rods of Corti are consequently bathed by endolymph. Both fluids are in relation with the subarachnoid lymph-spaces at the base of the brain, the perilymph through the aqueduct of the vestibule, the endo- lymph Through the endolymphatic duct. The fibers of the cochlear nerve, arising from the ganglion cells of the spiral ganglion situated in the osseous lamina spiralis near the modiolus, send their peripheral branches to the saccule and to the organ of Corti. As they approach this structure they lose their medullary sheath and become naked axis-cylinders. The fibers then divide into two parts, of which one passes to the inner hair cells; the other passes between the inner rods and crosses the tunnel between the outer rods to the outer hair cells. The exact method of termina- tion of these fibers in the hair cells is unknown, but doubtless it is both histologic and physiologic. From the relation of the nerve-fibers to the organ of Corti the latter must be regarded as the highly specialized terminal organ of the cochlear division of the auditory nerve. THE PHYSIOLOGY OF AUDITION. The general function of the ear is the reception and transmission of atmospheric vibrations from the concha to the percipient elements —the hair cells — of the organ of Corti. The vibratory excitation of these end-organs thus caused, is the basis of auditory perceptions. The atmospheric vibrations are collected by the pinna and concha, conveyed by the auditory canal to the tympanic membrane, trans- mitted by the chain of bones to the labyrinth to pass successively thrQ,ugh the perilymph, the membranous walls, the endolymph, to the hair cells. The nerve impulses generated by these vibrations are then transmitted by the cochlear nerve to the auditory centers of the cerebrum, where the sensations of sound are evoked. In order to appreciate the function of the individual structures concerned in this general function there must be kept in mind a few of the character- istics of atmospheric vibrations. Atmospheric Vibrations. — The vibrations of the atmosphere which are the objective causes of the sensations of sound are com- municated to it by the vibrations of elastic bodies such as tuning- forks, rods, strings, membranes, etc. These produce in the air a to-and-fro movement of its particles, resulting in a succession of alternate condensations and rarefactions which are propagated .in all directions. The impact of a rhythmic succession of such con- densations on the ear gives rise to musical sounds; the impact of an arrhythmic or irregular succession gives rise to noises. 696 TEXT-BOOK OF PHYSIOLOGY. If a writing point attached to a tuning-fork in vibration be placed in contact with a traveling recording surface, each vibration will be recorded in the form of a wave. For this reason atmospheric vibra- tions are generally spoken of as sound-waves. A line drawn hori- zontally through such a curve indicates the position of rest of the fork; the extent of the curve on each side of this line indicates the excursion of the fork or the amplitude of its movement. The sounds which physiologically result from the impact and transmission of the effects of sound-waves, possess intensity, pitch, and quality or tone. The intensity or loudness of a sound depends on the amplitude of the vibration which causes it. The greater the amplitude or swing of the vibrating body, the greater is the energy with which it strikes the ear. The pitch of a sound depends on the number of vibrations which strike the ear in a unit of time — a second. The greater the number, the higher the pitch. Thus while the pitch of the soimd caused by the note C, on the first leger line below, of the music scale, corre- sponds, to 256 vibrations, the pitch of the sound caused by the note C an octave above, corresponds to 512 vibrations. The lowest rate of vibration which .can produce a distinct sound varies in different individuals from 14 to 18; the highest rate varies from 35,000 to 40,000 per second. Between these two extremes lies the range of audibility, which embraces about 11 octaves. Vibrations less than 14 per second can not be perceived as a continuous sound; vibrations beyond 40,000 also fail to be- so perceived. In the ascent of the music scale from the lowest to the highest regions there is a gradual increase in the vibration frequency. The quality of a sound depends on the jorm of the vibration. It is this feature which gives rise to those differences in sensations which permit one to distinguish one instrument from another when both are emitting the same note. The form of the sound-wave in any given instance is the resultant of a combination of a fundamental vibration and certain secondary vibrations of subdivisions of the vibrating body. These secondary vibrations give rise to what is known as overtones. By their union with and modification of the fundamental . vibration there is produced a special form of vibration which gives rise not to a simple but a composite sensation. It is for this reason that the same note of the piano, the violin, and the human voice varies in quality. The Function of the Pinna and External Auditory Canal.— In those animals possessing movable ears the pinna plays an im- portant part in the collection of sound-waves. In man it is doubtful if it plays a part at all necessary for hearing. Nevertheless an indi- vidual with defective hearing may have the perception of sound increased by placing the pinna at an angle of 90 degrees to the side of the head or by placing the hand behind it. The external auditory THE SENSE OF HEARING. 697 canal transmits the sonorous vibrations to the tympanic membrane- From the obliquity of this canal it has been supposed that the vibra- tions, after passing the concha, undergo a series of reflections on their way to the tympanic membrane, which, owing to its inclination, would be struck by them in a much more effective manner. The Function of the Tympanic Membrane.— The function of the tympanic membrane is the reception of the atmospheric vibrations which are transmitted to it. This it does by vibrating in unison with them. The vibrations which the membrane exhibits correspond in amplitude, in frequency, and in form to those of the atmosphere. That this membrane actually reproduces all vibrations within the range of audibility has been experimentally demonstrated. The membrane not being fixed, as far as its tension is concerned, does not possess a fixed fundamental note, like a stationary fixed mem- brane, and is therefore just as well adapted for the reception of one set of vibrations as another. This is made possible by variations in its tension in accordance with the pitch of frequency of the atmos- pheric vibrations. In the absence of vibration the membrane is in a condition of relaxation; with the advent of sound-waves possessing a gradual increase of pitch, as in the ascent of the music scale, the tension of the membrane increases until its maximum is reached at the upper limit of the range of audibility. By this change in tension certain tones become perceptible and distinct, while others become imperceptible and indistinct. The Function of the Tensor Tympani Muscle. — The function of this muscle is, as its name indicates, to change and to fix the tension of the tympanic membrane, so that it can most readily vibrate in unison with vibrations of varying degrees of rapidity. The tendon of this muscle playing around the processus cochleariformis is attached almost at a right angle to the handle of the malleus. Hence as the muscle contracts it exerts its traction from the process and draws the handle of the malleus inward, thus increasing the convexity of the tympanic membrane and at the same time its tension. With the relaxation of the muscle the handle of the malleus passes outward, and the convexity and tension diminish. In the ascent of the music scale, each note corresponding to an increase in vibration frequency, requires for its perception an increase in tension and an increase in the force of the contraction of the tensor muscle. In the descent of the music scale the reverse conditions obtain. The contraction of the muscle is of the nature of a single twitch, and of just sufficient force and duration to tense the membrane for a given rate of vibration. The contraction of the muscle is excited reflexly. The afferent path is through fibers of the trigeminal nerve distributed to the tym- panic membrane; the efferent path is through fibers in the small root of the trigeminal. The stimulus is sudden pressure on the tympanic membrane. The more frequently and forcibly the stimulus is applied. 698 TEXT-BOOK OF PHYSIOLOGY. the greater is the muscle response. The tensor tympani muscle may therefore be regarded as an accommodative apparatus by which the tympanic membrane is adjusted for the reception of vibrations of varying degress of frequency. The Function of the Chain of Bones. — ^The function of the chain of bones is to transmit the effects of the atmospheric vibrations to the fluid of the labyrinth. The manner in which this is accom- plished becomes evident from the relation which the bones of this chain bear to one another and to the tympanic membrane on the one hand and to the fluid of the labyrinth on the other. When pressure is made on the outer surface of the tympanic mem- brane it is at once pushed inward, carrying with it the handle of the malleus, the head at the same time rotating outward around an axis corresponding to its ligamentous attachments. As the handle moves inward a small ledge of bone just below the malleo-incudal joint locks with, and hence pushes inward, the long process of the incus. Since this process is united at almost a right angle to the stapes bone, the latter is forced toward and into the foramen ovale, thus producing a pressure on the perilymph. With the cessation of the pressure the elastic forces of the membrane and of the ligaments return the handle of the malleus to its former position; by the im- locking of the malleo-incudal joint the entire chain also returns to its former position without exerting undue traction on the basal attach- ment of the stapes. As the long process of the incus is shorter than the handle of the malleus, and as the movement between them takes place aroimd an axis from before backward, it follows that the excursion of the incus • and stapes will be less than that of the malleus, while the force will be greater. Hence as the vibrations are transferred from the tym- panic membrane of large area to the base of the stapes of small area (20 to 1.5), they lose in amplitude but increase in force. Their pres- sure on the perilymph is therefore thirty times greater than on the membrana tympani. In addition to its function as a transmitter of vibrations, the chain of bones serves as a point of attachment for muscles which regulate the tension of the tympanic membrane and the pressure on the labyrinth. The Function of the Stapedius Muscle. — The function of the stapedius muscle is a subject of much discussion. According to Henle, its function is to so adjust the stapes hope that it will be pre- vented from exerting an undue pressure on the perilymph during the inward excursions of the incus process. According to Toynbee, its function is to press the posterior part of the stapes inward, make it a fixed point, and place the anterior part in such a position that it will vibrate freely and accurately. The Function of the Eustachian Tube. — In order that the tym- panic membrane may vibrate freely it is essential that the air pressure on both sides shall be equal at all times. This is made possible by THE SENSE OF HEARING. 699 the Eustachian tube. Were it not for this passageway, with each inward swing of the membrane the air in the tympanic cavity would be condensed and its pressure raised, in consequence of which the movement of the membrane would be retarded; with each outward swing, the air would be rarefied and its pressure lowered below that of the atmosphere, and in consequence the movement outward would be retarded; the maximum response, therefore, of the membrane to a given vibration could not be attained and the resulting sound would be mufHed and indistinct. But as with each vibration of the mem- brane the air can pass into and out of the tympanum through this tube, inequalities of pressure are prevented and a free vibration per- mitted. The impairment in the acuteness of hearing which is caused by either a rise or fall of pressure in the middle ear can be shown — 1. By closing the mouth and nose and then forcing air from the lungs through the Eustachian tube into the tympanum, thus in- creasing the pressure. 2. By closing the mouth and nose and then making an effort of deglutition. As this act is attended by an opening of the phar- yngeal end of the Eustachian tube, the air in the tympanum is partly withdrawn and the pressure lowered. In each instance hearing is impaired. After either experiment the normal con- dition is restored by swallowing with the nasal passages open. The Functions of the Internal Ear.— From the anatomic arrangement of the structures of the internal ear it is evident that if the vibrations of the stapes bone are to reach the peripheral organs — the hair cells — of both the vestibular and cochlear nerves, they must traverse successively the perilymph, the membranous walls^ and the endolymph. As the perilymph is incompressible, the inward move- ment of the stapes would be prevented were it not for the elastic character of the membrane closing the foramen rotundum. The pressure wave- occasioned by each inward movement of the stapes is transmitted through the scala vestibuli, the helicotrema, the scala tympani, to this membrane, which by virtue of its elasticity is pressed into the tympanic cavity. With the outward movement of the stapes, equilibrium is at once restored. The Functions of the Cochlea.— The cochlea is the portion of the internal ear which is concerned in the perception of tones. The arrangement of the histologic elements of the organ of Corti indicates that they in some way respond to the vibrations of varying frequency and form, and through the development of nerve impulses, evoke the sensations of pitch and quality. The manner in which this is accomplished is largely a matter of speculation. While many theories have been offered in explanation of the power to distinguish the pitch and the quality or timbre of a tone, most physiologists prefer that of Helmholtz, who regardeji the transverse fibers of the basilar membrane as the elements immediately concerned, and compared 700 TEXT-BOOK OF PHYSIOLOGY. them, both in their arrangement and power of sympathetic vibration, with the strings of a piano. He said: "If we could so connect every string of a piano with a nerve-fiber that the nerve-fiber would be excited as often as the string vibrated, then, as is actually the case in the ear, every musical note which affected the instrument would excite a series of sensations exactly corresponding to the pendulum- like vibrations into which the original movements of the air can be resolved; and thus the existence of each individual overtone would be exactly perceived, as is actually the case with the ear. The per- ception of tones of different pitch would, under these circumstances, depend upon different nerve-fibers, and hence would occur quite independently of each other. Microscopic investigation shows that there are somewhat similar structures in the ear. The free ends of all the nerve-fibers are connected with small elastic particles which we must assume are set into sympathetic vibration by sound-waves." (Stirling.) The mechanism might be regarded, therefore, somewhat as follows: The sound-waves received by the membrana tympani and transmitted by the chain of bones to the fenestra ovalis produce variable pressures in the fluids of the internal ear; these pressures vary in intensity, in number, and in quality, and correspond with the intensity, pitch, and quality of the tones. Tf, therefore, a com- pound wave of pressure be communicated by the base of the stapes, it will be resolved into its constituents by the different transverse fibers of the basilar membrane, each picking out its peculiar portion of the wave and thus stimulating corresponding nerve filaments. Thus different nerve impulses are transmitted to the brain, where they are fused in such a manner as to give rise to a sensation of a particular quality, but still so imperfectly fused that each constituent, by a strong effort of attention, may be still recognized. The transverse fibers of the basilar membrane vary in length from 0.04155 mm. at the base of the cochlea to 0.495 ™™- ^^ the apex, and, according to Retzius, are about 24,000 in number. As the human ear usually cannot distinguish more than 11,064 tones, it is evident that there is a sufficient anatomic basis for this theory. The functions of the semicircular canals have already been stated in connection with the chapter relating to the functions of the cerebellum. CHAPTER XXIX. REPRODUCTION. Reproduction is the process by which a new individual is initiated and developed and the species to which it belongs is preserved. Re- production is the result of the union and subsequent development of germ- and sperm-cells. These cells are produced and their union accomplished by the cooperation of the reproductive organs charac- teristic of the two sexes. Embryology is a department of anatomic science which has for its object the investigation of the successive stages that the new being passes through during its gradual development prior to birth. THE REPRODUCTIVE ORGANS OF THE FEMALE. The reproductive organs of the female . comprise the ovaries, Fallopian tubes, uterus, and vagina (Fig. 346). The Ovaries. — The ovaries are two small, flattened bodies, measuring about 40 mm. in length and 20 in breadth. They are situated in the cavity of the pelvis, one on either side, and embedded in a fold of the peritoneum, known as the broad ligament. A section of the ovary shows that it consists externally of a thin, firm, connective- tissue membrane and internally of a fine connective-tissue stroma, supporting blood-vessels, non-striated muscle-fibers and nerves, and containing in its meshes a very large number of spheric sacs named after their discoverer, de Graaf , the Graafian sacs or follicles. These follicles are very numerous and are present in all portions of the ovary, though they are most abundant toward its peripheral portions. It is estimated that the human ovary contains from 20,000 to 40,000 follicles. The follicles vary considerably in size; while many are visible to the unaided eye, others require for their detection high powers of the microscope. Although the follicles are present in the ovary at the time of birth, it is not until the period of puberty that they assume functional activity. From this time on to the catamenial period there is a constant growth and development of these follicles. Each follicle consists of an external investment of fibrous tissue and blood-vessels, and an internal investment of cells, the membrana granulosa. At the lower portion of this membrane there is an accumulation of cells, the pro- ligerous disc (Fig. 347). The cavity of the follicle contains a slightly yellowish, alkaline, albuminous fluid, a transudate in all probability from the blood-vessels. The Graafian follicle is of especial interest, 701 702 TEXT-BOOK OF PHYSIOLOGY. for it is in this structure, and more especially in the proligerous disc, that the true germ-cell or ovum is developed. The ovum is a spheric body measuring about 0.3 mm. in diameter. It consists of a mass of living, protoplasmic material, cytoplasm, a nucleus or germinal vesicle, and a nucleolus or germinal spot. The cytoplasm presents toward its central portion a quantity of granular material, partly fatty in character, the deutoplasm or vitellus. The peripheral portion of the cytoplasm is surrounded by a delicate radially striated border, the zona pellucida or radiata (Fig. 348). The nucleus consists of a nuclear membrane enclosing contents. The latter consist of an amorphous material in which is embedded a network, some of the threads of which have a strong affinity for cer- FiG. 346. — Uterus, Fallopian Tubes AND Ovaries; Posterior View. 1,1. Ovaries. 2, 2. Fallopian tubes. 3, 3. Fimbriated extremity of the left Fallopian tube seen from its concavity. 4. Opening of the left tube, s- Fimbriated extremity of the right tube, posterior view. 6, 6. Fimbrise which attach the extremity of each tube to the ovary. 7, 7. Ligaments of the ovary. 8, 8, 9, 9. Broad ligament. 10. Uterus. 11. Cervix uteri. 12. Os externum. 13, 13. Vagina. tain staining materials, and hence are known as chromatin, while others stain less deeply and are known as achromatin. The Fallopian Tubes. — The Fallopian tubes are about 12 centi- meters in length and extend from the upper angles of the uterus to the ovaries. Each tube is somewhat trumpet-shaped, the narrow portion being close to the uterus, the wide portion close to the ovary. The outer extremity of the tube is expanded and subdivided, and presents a series of processes termed iimbriee, one of which is attached to the ovary. The tube consists of three coats^ — an external or serous; a middle or muscular, the fibers of which are arranged longitudinally and transversely; and an internal or mucous. The surface of the mucous coat is covered with a layer of ciliated epithelial cells, the motion of which is toward the uterus. f ' The Uterus. — The uterus is pyriform in shape and divided into a body and neck. It measures, before the first pregnancy, about PLATK III. -flcccentcL, DIAGRAM OF FCETAL CIRCULATION, W.Preyer del. REPRODUCTION. 703 7 cm. in length, 5 cm. in breadth and 2^ cm. in thickness. A frontal section of the uterus shows a central cavity which in the body is tri- angular in shape, in the neck oval or fusiform (Fig. 349). At the upper angles of the uterus the cavity is continuous with the cavity of each Fallopian tube. At the junction of the body and the neck, the cavity presents a constriction, the internal os. The constriction at the end of the neck is known as the external os. The walls of the uterus are extremely thick and composed of non-striated muscle- fibers arranged in a very complicated manner. The interior of the uterus is lined by mucous mem- brane covered with cylin- dric ciliated epithelial cells, the motion of which is toward the external os. Tubular glands are found in large numbers in the mucous membrane lin- ing the cavity, while racemose glands are found in the mucous membrane lining the neck. Owing to the flattening of the uterus from before backward the walls are almost in contact and the cavity almost obliterated. The Vagina.— The vagina is a musculo- membranous canal, from 12 to 18 cm. in length, situated between the rec- tum and bladder. It extends from the surface of the body to the brim of the pelvis, and em- braces at its upper extremity the neck of the uterus. Ovulation. — After the establishment of puberty a Graafian follicle develops and ripens or matures periodically, usually every twenty- eight days. During the time of maturation the follicle increases in size, from an 'augmentation of its fluid contents, and approaches the surface of the ovary, where it forms a projection varying from 6 to 12 mm. in size. When maturation is complete the vesicle ruptures, and the ovum and liquid contents are discharged. The ovum, by a mechanism not fully understood, is received by the fimbriated ex- FiG. 347. — Section of Cortex of Cat's Ovary, Exhibiting Large Graafian Follicles, a. Per- ipheral zone of condensed stroma, b. Groups of im- mature follicles, o. Theca of follicle, d. Membrana granulosa, c. Discus proHgerus. /. Zona pellucida. g. Vitellus. k. Germinal vesicle, i. Germinal spot. h. Cavity of liquor folliculi. — {After Piersol.) 704 TEXT-BOOK OF PHYSIOLOGY. tremity of the Fallopian tube and enters its cavity. The ovum is then transferred through the tube by the peristaltic contraction of its muscle-fibers and by the action of the cilia of its lining epithelium. The time occupied in the transference of the ovum from the ovary to the interior of the uterus has been estimated to be from four to ten days. Either at the time, or very shortly after, its discharge from the follicle, the ovum, and more especially the nucleus, undergoes a series of histologic changes which eventuates in an extrusion of a portion of the chromatin material. The extruded portions are known as the polar bodies. The non-extruded portion of the chromatin material Fig. 348. — Ovum op a Cow. i. Zona pellucida. 2. Cjftoplasm, vitellus. 3. Nu- cleus, germinal vesicle. 4. Nucleolus, germ- inal spot. 5. Corona radiata. The radial striation of the zona pellucida can not be seen.— (5to'^r.) Fig. 349. — Frontal Sec- tion OF THE Uterus, i. Cav- ity of the body. 2, 3. Lateral walls. 4,4. Comua. 5. Os internum. 6. Cavity of the cervix. 7. Arbor vitse of the cervix. 8. Os externum. 9. Vagina. — {Sappey.) is known as the female pronucleus. The succession of changes which the nucleus undergoes is termed maturation. As the nucleus is regarded as the part of the ovum which transmits parental character- istics it is assumed that the extrusion of a portion of the nuclear material is a means by which an excess of inherited substance it prevented. Menstruation. — Menstruation is a periodic discharge of blood and mucus from the surface of the mucous membrane of the uterus, and occurs about every twenty-eight days. The duration of the menstrual period extends over four or five days and the amount of blood discharged varies from i8o c.c. to 200 c.c. Menstruation is REPRODUCTION. 7°S usually an accompaniment of ovulation, though the latter process may take place independently of the former. It is characterized by both local and systemic changes. The local changes are most marked in the uterus, the mucous membrane of which increases in thickness from a proliferation of the connective tissue and a hyperemic condi- tion of the blood-vessels. Subsequently to these changes the epithe- lial surface, as well as the more superficial portions of the connective tissue, undergo degeneration and exfoliation, after which the finer blood-vessels rupture and permit of an escape of blood into the uterine cavity. At the end of the menstrual period regenerative changes set in which continue until the normal condition of the mucous mem- brane is reestablished. The Corpus Luteum. — With the rupture of the Graafian follicle there is an effusion of blood into the follicular cavity which soon coagulates, loses its color and assumes the characteristics of fibrin. The walls of the follicle, which have become thickened from the deposition of a reddish-yellow glutinous substance, now become con- voluted and undergo a still further hypertrophy, until they encroach upon and almost obliterate the follicular cavity. In a few weeks the mass loses its red color and becomes decidedly yellow, when it is known as the corpus luteum. With the continuance of reparative changes this body gradually disappears until at the end of two months nothing remains but a small cicatrix on the surface of the ovary. Such are the changes in the follicle if the ovum has not been impreg- nated. The corpus luteum, after impregnation has taken place, undergoes a much slower development, becomes larger, and continues during the entire period of gestation. The difference between the corpus luteum of the unimpregnated and pregnant condition is expressed in the following table by Dalton: Corpus Luteum of Menstruation. Corpus Luteum of Pregnancy. At the end of weeks. One month. Two months Four months. Six months. Nine months. 4t three Three-quarters of an inch in diameter; central clot reddish; convoluted wall pale. Larger; convoluted wall bright yel- low; clot still reddish. Smaller; convoluted wall bright yellow; clot still reddish. Reduced to the condition of an insignificant cicatrix. Absent or unnoticeable. Absent. Absent. Seven-eighths of an inch in di- ameter; convoluted wall bright yellow; clot perfectly decolorized. Seven-eighths of an inch in diame- ter; clot pale and fibrinous; convo- luted wall dull yellow. Still as large as at the end of second month; clot fibrinous; convoluted wall paler. Half an inch in diameter; central clot converted into a radiating cicatrix ; external wall tolerably thick and con- voluted, but without any bright yellow color. 7o6 TEXT-BOOK OF PHYSIOLOGY. THE REPRODUCTIVE ORGANS OF THE MALE. The reproductive organs of the male comprise the testicles, vasa deferentia, vesiculae seminales, and penis. The Testicles. — The testicles are oblong glands, about 40 mm. in length, 30 mm. in breadth and 20 mm. in thickness, and contained within the cavity of the scrotum. A section of the testicle (Fig. 350) reveals the presence externally of a dense fibrous membrane, the tunica albuginea, and internally a connective-tissue framework consisting mainly of septa, which enter the organ on its posterior aspect at the mediastinum testis, passing inward in a diverging manner. The spaces between the septa are occupied by the true gland substance, the seminiferous tubules. The seminiferous tubules are very numerous, the estimate as to their number varying from 800 to 1000. When unraveled they measure from 30 to 40 cm. in length and 0.3 mm. in diameter. At their peripheral extremities the tubules are very much convoluted, but as they pass toward the mediastinum testis, the convolutions disappear, and after xmiting with one another terminate in from twenty to thirty straight tubes, the vasa recta, which pass through the mediastinum and form the rete testis. At the upper part of the mediastinum the tubules unite to form from nine to thirty small ducts, the vasa efferentia, which soon become very much convo- luted. After a short course they unite to form a single tortuous tube, about 7 meters in length and 0.4 mm. in diameter, which descends behind the testicle to its lower border. This tube is known as the epididymis. The seminal tubule con- sists of a basement membrane lined by granular nucleated epithelium. The vas deferens, the excretory duct of the testicle, is about 60 cm. in length and from 2 to 3 mm. in diameter, and extends upward from the epididymis to the inguinal canal, through which it passes into the abdominal cavity and then to the under surface of the base of the bladder, where it unites with the duct of the vesicula seminalis to form the ejaculatory duct The vesiculae seminales are two lobulated pyriform bodies, about 40 mm. in length, situated on the under surface of the bladder. Each vesicula seminalis consists of an external fibrous coat, a middle, mus- cular coat, and an external mucous coat. The mucous coat contains a number of small tubular albumin-producing glands which, secrete a characteristic fluid. Fig. 350. — Diagram of a \er- TicAL Section through a Tes- ticle. I. Mediastinum testis. :i, 2. Trabeculse. 3. One of the lobules. 4, 4. Vasa recta. 5. Globus major of the epidid)'inis. 6. Globus minor. 7. Vas def- erens. — (Holden.) REPRODUCTION. 707 The ejaculatory duct, formed by the union of the vas deferens and the duct of the vesicula seminalis, opens into the prostatic portion of the urethra (Fig. 351). The prostate gland is a musculo-glandular mass situated at the posterior extremity of the urethra. It contains a large number of tubules, more or less branched and convoluted, and lined by columnar epithelium. They secrete a fluid which is poured into the urethra at the time of the ejaculation of semen. The penis consists of three parts: the corpus spongiosum below, through which passes the urethra, and the two corpora cavernosa, one on either side and above. The corpus spongiosum termi- nates anteriorly in a conic-shaped structure, the glans penis; the corpora cavernosa consist ex- ternally of a fibrous investment and internally of erectile tissue. These bodies are abundantly supplied with blood, which after entering their substance by the arteries, passes into sinuses or reservoirs, from which it is car- ried away by veins. These ves- sels pass to the dorsum of the penis and unite to form a large vein by which the blood is re- turned to the general circulation. By virtue, of the erectile tissue in the corpora cavernosa the penis becomes erect and rigid when the blood supply is in- creased. This takes place in response to peripheral stimula- tion or emotional states, or both combined. When these conditions are established nerve impulses pass outward through nerves, the nervi erigentes, which have their origin in the lumbar region of the spinal cord, and bring about an active dilatation of the arteries and a relaxation of the non-striated muscle-fibers in the corpora cavernosa. With these events there is a rapid influx of blood and a distention and an erection of the organ. This condition is furthered and maintained by a partial compression of the dorsal vein by the fibrous capsule. Semen. — -The semen is a complex fluid composed of the secretions of the testicles, the vesiculae seminales, the prostatic tubules, and urethral glands. It is grayish-white in color, mucilaginous in con- sistence, characteristic in odor, and somewhat heavier than water. In response to appropriate stimulation the muscle-fibers in the walls Fig. 351. — Vas Deferens, Vesicul.s .Seminales, and Ejaculatory Ducts. a. Vas deferens, b. Seminal vesicle. c. Ejaculatory duct. d. Termination of the ejaculatory duct. e. Opening of the prostatic utricle. /, g. Veru montanum. h, I. Prostate. — (Liegeois.) 7o8 TEXT-BOOK OF PHYSIOLOGY. of the vasa deferentia, vesiculas seminales, and prostatic tubules contract and discharge their contents into the urethra, from which they are forcibly ejected by the rhythmic contraction of the ejaculatory muscles, the ischio- and bulho cavernosi. The amount of semen dis- charged at each ejaculation varies from i to 5 c.c. Spermatozoa.— The spermatozoa are, peculiar morphologic ele- ments whichjarise within the seminiferous tubules as a result of com- plex histologic changes in the lining epithelium. An adult spermatozoon consists of a conoid slightly flattened head, from the posterior part of which there projects a short straight rod, provided with a long filamentous tail or cilium and an end-piece (Fig. 352). The head contains a nucleus of chro- matin material. The total length of a spermato- zoon varies from 50 to 80 micromillimeters. The characteristic physiologic feature of spermatozoa is incessant locomotion when in a suitable medium. So long as they are confined to the vas deferens they are quiescent, but with their advent into the vesicula seminalis and dissemination in its contained fluid, they become extremely active and move around with considerable rapidity. The power of locomo- tion depends on the possession of the tail which, by lashing the surrounding fluid now in this and now in that direction, propels the head from place to place. The vitality of spermatozoa is such as to enable them to retain their physiologic activities in the uterus for more than eight days. The development of spermatozoa from testicular cells as observed in lower animals indicates that each cell gives rise to four embryonic forms — sper- matids — ^which subsequently develop into adult spermatozoa. In this process the primary nuclear chromatin undergoes a division, so that each sper- matozoon receives but a fractional amount. The changes by which this condition is brought about are comparable to the changes exhibited by the ovum, and have for their result a reduction in the quantity of hereditary substance to be transmitted. Fecundation. — ^Fecundation is the union of the spermatozoon (the sperm-cell) with the ovum (the germ-cell) and takes place in the great majority of instances in the Fallopian tube. After the intro- duction of the spermatozoa into the vagina during the act of copulation, they soon begin to pass upward, into and through, the uterine cavity and out into the Fallopian tube, where they accumulate in large numbers and retain their vitality for some days. The migration is effected by the propelhng power of the filamentous tail. From observations made on the behavior of the spermatozoa Fig. 352. — Human Spermatozoon, i. Front view, 2, side view, of the head. k. Head. m. mid- dle piece. /. Tail. e. Terminal fila- ment. — (vi/ter Ret- zius.) REPRODUCTION. 709 toward the ovum in lower animals, and on the manner by which their union is effected, the inference may be drawn that a similar procedure takes place in mammals. In lower animals the spermatozoa on approaching an ovum take on increased activity, swimming around it in all directions and apparently seeking a point of entrance. In fish and molluscs the zona pellucida presents a distinct opening, the micropyle, through which the spermatozoon passes. Inasmuch as the mammalian ovum is devoid of such an opening, the mechanism of entrance of the spermatozoon is not clearly understood. Notwith- standing their enormous numbers it is generally believed that but a single spermatozoon effects an entrance into the ovum. With the accomplishment of this, however, the spermatozoon loses its vitality, after which the body and tail disappear. The head, which in this instance also is the transmitter of the inherited material, advances to Fig. 353. — Impregnated Uterus, WITH Folds of Decidua Growing up Around the Egg. The narrow open- ing, where the folds approach each other, is seen over the most prominent portion of the egg. — {Dalton.) Fig. 354. — Impregnated Uterus; showing the connection between the villosities of the chorion and the decidual membranes. — {Dalton.) meet and unite with the nucleus of the ovum. A series of histologic changes now arise, which eventuate in the production of a new cell, a parent cell, possessing all the features of cell structure and the physio- logic activities and characteristics of both ancestral cells. From this parent cell the new being develops through successive division, multi- plication, and differentiation of cells. The Fixation of the Ovum. — If the ovum is to develop into a new being it is essential that it be retained within the cavity of the uterus. This is accomplished by the development of specialized structures on the surface of the uterine mucosa and on the surface of the ovum. With the fertilization of the ovum, the mucous mem- brane of the uterus takes on an increased growth; it becomes hyper- trophied and vascular, and develops small elevations known as villi. Inasmuch as this membrane is detached and discharged at the birth of the fetus, it is known as the decidua vera. With the fertilization 7IO TEXT-BOOK OF PHYSIOLOGY. of the ovum, the zona pellucida or radiata also develops villosities, and as it passes from the Fallopian tube into the uterus the villi inter- digitate, and its further progress is retarded. (Figs. 353 and 354.) In a short time a portion of the decidua vera grows up on all sides and encloses the ovum. Its retention is thus secured. That portion of the decidua which grows around the ovum is termed the decidua reflexa; while the portion to which the ovum attaches itself is termed the decidua serotina, and is of interest for the reason that it becomes the seat of development of the placenta, the organ by which the fetus is nourished. As development advances the decidua reflexa also increases in size and extent, and about the end of the fourth month comes into contact with the decidua vera, with which it ultimately fuses. DEVELOPMENT OF FETAL ACCESSORY STRUCTURES. Segmentation of the Ovum. — Shortly after the formation of the parent cell, segmentation of the nucleus and cytoplasm takes place in accordance with karyokinetic methods. The two new cells thus formed undergo a similar division into four, the four into eight, the eight into sixteen, and so on until the space within the zona pellucida is completely filled with a large number of small cells, each possessing the char- acteristic cell struc- tures. The peri- pheral cells then ar- range themselves in the form of a mem- brane, and as they are, at the same time, subjected to mutual pressure they assume a poly- hedral shape, and give to the mem- brane a mosaic ap- pearance (Fig. 3Ss). The central cells then undergo liquefaction. At some point on the inner surface of the membrane, cells accumulate which by their division and multiplication form a second membrane. The two together are known as the external and internal blastodermic membranes. Germinal Area. — At about this period there is an accumulation of cells at a certain spot in the substance of the blastodermic mem- FiG. 355. — Primitive Trace of the Embryo, a. Primitive trace, h. Area pellucida. <,. Area opaca. d. Blastodermic cells, e. Villi beginning to appear on the surface of the zona pellucida. — (Lugois.) REPRODUCTION. 711 branes which marks the position of the future embryo. This spot, at first circular, soon becomes elongated. A slight indentation now develops into what is known as the primitive trace. Around this area there is a clear space, the area pellucida, which is in turn surrounded by a darker region, the area opaca. The primitive trace soon dis- appears and the area pellucida becomes guitar-shaped. A second groove, the medullary groove, is now formed, which develops from before backward and becomes the neural medullary canal. Blastodermic Membranes.— The embyro, at this period, con- sists of three layers — viz., the external and the internal blastodermic membranes and a middle membrane formed by a genesis of cells from their internal surfaces. These layers are known from without inward as ■■'t the epiblast, mesoblast, and hypoblast. The epiblast gives rise to the central nerve system, the epidermis and its ap- pendages, and the primitive kidneys. The mesoblast gives rise to the dermis, muscles, bones, nerves, blood-vessels, sympathetic nerve system, connective tissue, the urinary and reproductive ap- paratus, and the walls of the alimentary canal. The hypoblast gives rise to the epi- thelial lining of the alimentary canal and its glandular appendages, the liver and pancreas, and the epithelium of the respiratory tract. Dorsal Laminae. — As development advances, the true medullary groove deepens, and there arise two longitudinal elevations of the epiblast^ — ^the dorsal lamincB, one on either side of the groove — which grow up, arch over, and unite so as to form a closed tube, the primitive central nerve system. The Chorda Dorsalis. — Just beneath the neural canal there arises a group of hypoblastic cells which arrange themselves in the form of a cylindric rod, which marks out the position of the future bony axis of the body. This rod is known as the chorda dorsalis or notochord. Primitive Vertebrae.— On either side of the neural canal the cells of the mesoblast undergo a longitudinal thickening, which develops and extends around the neural canal and the chorda dorsalis, and forms the arches and the bodies of vertebrae. They become divided transversely into segments. The mesoblast now separates into two layers: the external, joining with the epiblast, forms the somatopleura; the internal, joining with -—i>p ys Fig. 356. — Diagram Repre- senting THE Relation of Pri MARY Structures in a Develop- ing Chicken; Vertical Trans- verse Section. The medullary groove and chorda dorsalis are seen in section; the aUmentary canal pinched off from the yolk-sac is completely closed, a. Amnion, a, i.. Amniotic cavity filled With amniotic fluid, pp. Space be- tween amnion and chorion con- tinuous with the pleuro-peritoneal cavity inside the body. vt. Vitel- line membrane, or zona pellucida. ys. Yolk-sac, or umbilical vesicle. — {Foster and Balfour.) 712 TEXT-BOOK OF PHYSIOLOGY/ the hypoblast, forms the splanchno pleura; the space between them constitutes the pleuro- peritoneal cavity (Fig. 356). Visceral Laminae. — ^The walls of the pleuro-peritoneal cavity are formed by a downward prolongation of the somatopleura (the visceral lamince), which, as they extend around in front, pinch off a portion of ■ the yolk-sac (formed by the splanchnopleura) , which becomes the primitive alimentary canal; the lower portion, remaining outside of the body cavity, forms the umbilical vesicle. The Fetal or Embryonic Membranes.— With the appearance of the visceral laminae two membranes develop in succession, both of which play an important part in the subsequent life of the embryo. These are known as the amnion and the allantois. The amnion is formed by folds of the epiblast and the external layer of the mesoblast rising up in front, behind, and at the sides. These folds gradually extend over the back of the embryo to a certain Fig. 357. — Diagram OF Fecdndated Egg. a. Umbfflcal vesicle, b. Amniotic cavity, c. Allantois. — {Dalton.) Fig. 358.^FEcnNDATED Egg vitth Allantois neailly Complete, a. Inner layer of amniotic fold. b. Outer layer of ditt6. c. Point where the amniotic folds come in contact. The allantois is seen penetrating between the outer and inner layers of the amniotic folds. — {Dalton.) point where they meet, coalesce, and enclose a cavity known as the amniotic cavity. The membranous partition between the folds is absorbed, after which the outer layer recedes and becomes blended with the primitive enveloping membrane of the ovum and thus assists in the formation of the chorion — the external covering of the embryo. The cavity enclosed by the amnion is at first quite small, but soon enlarges from the accumulation of a clear, transparent fluid, the amniotic fluid. It gradually increases in amount up to the latter period of gestation, when its volume reaches about one liter. This fluid is derived mainly from the blood, as it contains albumin, sugar, fatty matter, and inorganic salts. Traces of urea indicate that some of its constituents are derived from the embryo itself. The allantois is primarily a pouch or diverticulum which develops from the posterior portion of the alimentary canal. As it develops it enlarges, and in its growth inserts itself between the two layers of the amnion, coming into contact more especially with the external layer. REPRODUCTION. 713 It finally completely surrounds the embryo, after which its edges become fused together (Figs. 357 and 358). The allantois is of especial interest and importance, as it is the means by which the blood of the embryo is brought into relation with the blood of the mother. As it develops, two arteries, the hypo- gastrics, one from each internal iliac, pass out of the abdominal cavity within the walls of the allantois, and follow it in its course around the embryo. The ultimate branches of these arteries penetrate the villous processes which develop on the- surface of the chorion and which take part in the formation of the placenta. A single large vein emerges from the placenta and returns the blood to the embryo. In its course it winds around the arteries in a spiral manner a number of times. These vessels — the umbilical arteries and vein — are en- closed by the walls of the allantois and amnion, and together constitute the umbilical cord which at the end of gestation is about 60 cm. in length. (Figs. 359, 360). The Chorion.— The chorion, the external investment of the embryo, is formed by the fusion of the primitive egg membrane — the zona pellucida — the ex- ternal layer of the amnion, and the allantois. Very early in de- velopment its external surface becomes covered with homogen- eous, granular, club-shaped proc- esses, which by continued bud- ding and growth, give to the membrane a shaggy appearance. At about the end of the second month these processes begin to atrophy and disappear from the surface of the chorion, with the exception of that portion which is in contact with the decidua serotina. At this point the processes or villi continue to grow and develop, and insert themselves more deeply into the mucous membrane. Corresponding processes from the mucous membrane insert themselves between the villi of the chorion, which by their growth and fusion secure, among other things, the retention of the embryo. The Nutrition of the Embryo. — Coincidently with the develop- ment of the amnion, allantois, and chorion, there arises within the body of the embryo the early forms of many, if not all, of the future viscera. The nutritive material required for their growth is partly contained within the umbilical vesicle lying without the body cavity. That this material may be utilized, blood-vessels emerge from the body and ramify within the walls of the vesicle. The capillaries to which these vessels give rise come into close relation with and absorb the food material, after which it is carried by veins to the heart, by Fig. 359. — Human Embryo and its En- velopes AT THE End of the Third Month. 714 TEXT-BOOK OF PHYSIOLOGY. which it is distributed to all parts of the embryo. These vessels are collectively known as the omphalo-mesenteric arteries and veins. This primitive or vitelline circulation is of short duration in mam- mals, as the nutritive material in the vesicle is small in amount and is soon exhausted. In birds, however, it is of primary importance. The main supply of nutritive material, however, is derived from the mother by means of a highly developed and specialized organ — The Placenta. — Of all the embryonic structures the placenta is the most important. It is formed by the end of the third month, after which it gradually increases in size up to the end of the eighth month, by which time it is fully developed. It then measures from i8 to 24 cm. in diameter and weighs from 400 to 600 grams. It is most frequently situated at the upper and back part of the uterine cavity. Though exceedingly complex in structure it consists essen- tially of two portions, a fetal and a maternal. The fetal portion con- sists primarily of those villi on the chorion in relation with the decidua serotina. These structures gradually increase in size and num- ber, and receive the ulti- mate branches of the um- bilical arteries. The ma- ternal portion consists primarily of the decidua serotina. As gestation ad- vances the chorionic villi rapidly increase in size and number, and receive the branches of the umbilical arteries. At the same time the decidua serotina becomes hyper- trophied and vascular. With the continued growth and development of these two structures they gradually fuse together and finally become inseparable. In accordance with the needs of the embryo, the decidua serotina and • its contained blood-vessels undergo certain histologic changes which result in the formation of large cavities, sinuses, or lakes, into which the blood of the uterine vessels is emptied. Coin- cidently the villi of the chorion grow and give oflf numerous branches, which project themselves in all directions into the blood of uterine sinuses (Figs. 361, 362). As the placenta develops, the structures separating the blood of the mother from that of the child gradually become modified until they are represented by a thin cellular or Fig. 360. — Human Embryo, with Amnion and Allantois, in the Third Week. There axe as yet no limbs; the embryo and its appendages are surrounded by the tufted chorion. — {Haeckel.) REPRODUCTION. 715 homogeneous membrane. The conditions now are such as to permit of a free exchange of material between the mother and child . Whether by osmosis or by an act of secretion, the nutritive materials of the maternal blood pass through the intervening membrane into the fetal blood on the one hand, whUe waste products pass in the reverse direction into the maternal blood on the other hand. Inasmuch as oxygen is absorbed and carbon dioxid e:!dialed by the same struc- tures, the placenta is to be regarded as both a digestive and a respira- tory organ. So long as these exchanges are permitted to take place in a normal manner the nutrition of the embryo is secured. Fig. 361. — Diagram showing the Relations of the Fetal Membranes. Am. Amnion. Ck. Chorion. M. Muscle wall of uterus. R. Decidua reflexa. 5. Serotina. V. Decidua vera. Y. Yolk stalk. — {McMurrich.) The Fetal Circulation. — The composition of the blood as well as the course it pursues through the heart and vascular apparatus presents peculiarities which have arisen in consequence of the neces- sity of obtaining nutritive material through the placenta and the almost impervious condition of the pulmonary capillaries. On re- turning from the placenta, the blood in the umbilical vein is relatively rich in nutritive material and scarlet red in color from the presence of oxygen. As it passes into the abdominal cavity a portion of the blood is directed by the ductus venosus into the vena cava, while another portion is emptied into the branches of the portal vein, by which it is distributed to the liver and from which it emerges by the 7i6 TEXT-BOOK OF PHYSIOLOGY. hepatic veins and poured into the vena cava. The blood in the vena cava is thus a mixture of venous blood from the lower extremities and liver, and oxygenated blood from the placenta. Aft'er its dis- charge into the right auricle the blood is directed by a fold of the lining membrane, the Eustachian valve, through an opening in the interauricular septum, the foramen ovale, into the left auricle. It then flows through the auriculo-ventricular opening into the left ventricle, thence into the aorta, and by its branches is distributed to all parts of the body. The blood from the head and upper extremities is emptied by the superior vena cava into the right auricle, but as it passes in front of the Eustachian valve, it flows directly into the right ventricle and then into the pulmonary artery. On account of the unexpanded \ Chorionic villi. Intervillous spaces. Floating villus. \ Attached villi. Vein. Spiral artery. Gland. Vein. Muscularis. Fig. 362. — Diagram of Human Placenta at the Close of Pregnancy. — {Schaper.) condition of the lungs and the almost impervious condition of the pulmonary capillaries, but a small portion of the blood passes through them, while the larger portion by far passes into the aorta directly through a duct, the ductus arteriosus, which enters at a point below the origin of the left carotid and subclavian arteries. A comparison of the blood distributed to the head and upper extremities, with that distributed to the lower extremities, will show a larger percentage of nutritive material and oxygen in the former than in the latter, a fact which has been offered as an explanation of the more rapid growth of the upper half of the body. As the blood passes through the aorta, a portion is directed from the main ciu-rent by the hypogas- tric and umbilical arteries to the placenta, where it loses carbon REPRODUCTION. 717 dioxid and gains oxygen,- and changes in color from a bluish red to a scarlet red. Parturition. — At the end of gestation — approximately 280 days from the time of conception — a series of changes occur in the uterine structures which lead to an expulsion of the child, the placenta, and decidua vera. To this process in its entirety the term parturition is given. At this time, from causes not clearly defined, the uterine walls begin to exhibit throughout their extent a series of slight con- tractions which are somewhat peristaltic in character; these con- tractions, which gradually increase in frequency and vigor, bring about a dilatation of the internal os and a descent of the membranes into the cervical canal. The pressure exerted by these membranes during the time of the contraction materially assists in the relaxation of the circular fibers and a dilatation of the external os. When the dilatation has so far advanced that the diameter of the external os attains a measure of 7 or 8 cm., the tension of the membranes becomes sufficiently great to lead to their rupture and to a partial escape of the amniotic fluid. With this event, the "presenting part of the child, usually the head, descends into the cervical canal. After a short period of rest the uterine contractions return and rapidly increase in vigor and duration. . As a result of the pressure thus exerted from all sides on the body of the child, the head gradually descends into the vagina and finally emerges through the vulva to be followed in a short time by the expulsion of the trunk and limbs, and a discharge of the remaining amniotic fluid. With the expulsion of the child the uterine contractions cease for a period bf ten or fifteen minutes, when they again recur, with the result of detaching the placenta and expelling it into the vagina. It is then removed by the cooperative action of the abdominal and perineal muscles. The hemorrhage which would naturally occur with the detachment of the placenta and the laceration of the maternal vessels is prevented by the firm continuous contraction of the uterine walls, by which the vessels are compressed and perma- nently closed. The Establishment of Inspiration and the Adult Circulation. — After the birth of the child and the detachment of the placenta, there speedily occurs a decrease in the quantity of oxygen and an increase in the quantity of carbon dioxid in the blood, a condition which causes a discharge of nerve energy from the inspiratory center, a contraction of the inspiratory muscles, an expansion of the thorax, and an inflow of air into the lungs. In the later months of intrauterine life the vascular apparatus undergoes certain anatomic changes which favor the transition from the placental to the adult circulation. Thus the ductus venosus con- tracts, and shunts a larger volume of blood into and through the liver; the Eustachian valve diminishes in size and at the time of birth has almost disappeared; a membranous fold grows upward and backward from the edge of the foramen ovale on the left side; the 7r8 TEXT-BOOK OF PHYSIOLOGY. ductus arteriosus also contracts. With the first inspiration and the expansion of the lungs, the blood which enters the pulmonary artery passes through the pulmonary capillaries in large volume and is returned by the pulmonary veins to the left auricle. The entrance of the blood into this cavity presses the membranous fold against the margins of the foramen ovale and thus prevents the further flow of blood from the right auricle. The blood entering the right auricle by the inferior vena cava now flows into the right ventricle, which is favored by the small size of the Eustachian valve. The foramen ovale is permanently closed at the end of a week or ten days; the ductus arteriosus at the end of four days. The umbilical vein and ductus venosus, at the end of four or five days, have also become almost impervious from the contraction of their walls. The hypo- gastric arteries remain open and carry blood to the walls of the bladder. Lactation. — As pregnancy advances the mammary glands in- crease in size, partly from a deposition of fat and connective tissue and partly from a multiplication of the secreting acini. The lining epithelial cells at the same time increase in size, and toward the end of pregnancy begin to exhibit functional activity. At the time of birth, or within a day or so after birth, the acini are filled with a fluid which in its qualitative composition resembles milk and is known as colostrum. It is distinguished from milk more especially in the fact that it contains in large quantity a proteid which coagulates on boiling, and certain inorganic salts which have a laxative effect on the new- born child. Normal lactation and the phenomena which accompany it are fully established by the end of the second or third day. The composition of milk and the mechanism of its production have been stated in the chapter on Secretion. Physiologic Activities of the Embryo. — During intrauterine life the evolution of structure is accompanied by an evolution of function. The relatively simple and uniform metabolism of the undifferentiated blastodermic membranes gradually increases in complexity and variety, as the individual tissues and organs make their appearance and assume even a slight degree of functional activity. As to the periods at which different organs begin to functionate, but little is positively known. The primitive heart, in all probability, begins to pulsate very early, as in an embryo from fifteen to eighteen days old and measuring but 2.2 mm. in length, Coste found the amnion, the allantois, the omphalo-mesenteric vessels, and the two primitive aortae developed. In the earlier weeks, all products of metabolism are doubtless elimi- nated by the placental structures; but as metabolism increases in complexity the liver and kidney assume excretory activity. Thus, at the end of the third month the intestine contains a dark, greenish, viscid material — meconium — composed of bile pigments, bile salts, and desquamated epithelium; the amniotic fluid, as well as the fluid within the bladder, contains urea at the end of the sixth month. REPRODUCTION. 719 indicating the establishment of both hepatic and renal activity. Con- tractions of the skeletal muscles of the limbs begin about the fifth month, from which it may be inferred that the mechanism for muscle activity, viz., muscles, efferent nerves, and spinal centers, has become anatomically developed and associated, and capable of coordinate activity. These contractions are, in all probability, automatic or autochthonic in character due to stimuli arising within the spinal centers. The remaining organs remain more or less inactive. After birth, with the first inspiration and the introduction of food into the alimentary canal, the physiologic mechanisms which sub- serve general metabolism begin to functionate and in the course of a week are fully established. At this time the cardiac pulsation averages about 135 a minute; the respiratory movements vary from 30 to 35 a minute, and are diaphragmatic in type; the urine, which was at first scanty, is now abundant and proportional to the food consumed; the digestive glands are elaborating their respective enzymes, digestion proceeding as in the adult. The hepatic secretion is active and the lower bowel is emptied of its contents; the coordinate activities of the nerve-, muscle-, and gland- mechanisms are entirely reflex in character. Psychic activities are in abeyance by reason of the incomplete develop- ment of the cerebral mechanisms. APPENDIX. PHYSIOLOGIC APPARATUS. The study of the physical and physiologic properties of muscles and nerves necessitates the employment of some stimulus which, when applied to either tissue, will call forth a contraction of the muscle, or the development of a nerve impulse in the nerve. The most convenient stimulus is electricity, for the reason that, with appropriate apparatus, its intensity and duration can be graduated with the utmost nicety. Moreover, it does not destroy the tissues, as do many chemic, physical, and mechanic stimuli. It is therefore necessary that the student should have a practical acquaintance with those appliances by means of which electricity is generated, applied and controlled. The electric cell is an apparatus composed of different elements, which, by virtue of chemic actions taking place among them, generate and conduct elec- tricity. In its simplest form an electric cell consists of two metals — zinc and copper, or carbon, or platinum, etc., immersed in an exciting fluid, usually dilute sul- phuric acid (Fig. 363). The zinc element is the one acted on chemically by the sulphuric acid, and at the expense of which the electricity is maintained. It is known as the gener- Fig. 363.— An ating element. The copper is the collecting and con- lecteic ell. ducting element. With the immersion of these elements in a solution of H^SO^ a chemic action at once takes place between the zinc and the acid, with the formation of zinc sulphate and the liberation of hydrogen, as expressed in the following formula: Zn 4- H.SO4 = ZnS04 -|- Hj. The zinc sulphate passes into the solution, while the hydrogen accu- mulates on the surface of the copper element. As all chemic action is accompanied by the development of elec- tricity, it can be shown by appropriate means that this is the case at the surface of the zinc. Such a combination is the means of establish- ing a difference of potential between two points; the point of highest potential being the surface of the zinc or the positive element, the point of lowest potential being the copper or the negative element. 46 721 722 TEXT-BOOK OF PHYSIOLOGY. So long as the elements remain unconnected there is no movement of electricity, no current. If the ends of the elements projecting beyond the fluid are connected by a copper wire, a pathway or circuit is established, and a movement of the electricity takes place. As electricity flows from the point of high to the point of low potential, it follows that inside the cell the current flows from the zinc to the copper, and outside the cell from the copper to the zinc. Such a current is termed a continuous, a galvanic or a voltaic current. Inasmuch as there is a progressive fall in potential between the highest and lowest points, it follows that any two points in the circuit will exhibit a similar difference of potential. For this reason the projecting end of the copper element is at a higher potential than the projecting end of the zinc element. The end of the copper is, therefore, termed the positive, + pole or anode, the end of the zinc the negative, — pole or kathode. Electric Units. — Owing to the difference of the electric potential in the cell, the electricity leaves the cell under a certain degree of pressure, termed the " electro- motive force." As it passes through the circuit it meets with resistance, the amount of which will depend on the nature of the circuit material, its length, and the area of its cross-sec- tion. In accordance with the resistance will depend the quantity of electricity that a given electro-motive force will press through in a unit of time. The strength of the current will there- fore not depend entirely on the ratio between the T-l- FiG. 364. — Two Simple Electric Cells Joined IN Series. C. Copper. Z. Zinc. on the electro-motive force, but, rather, electro-motive force and the resistance. For the measurement of electric quantities, a system of units has been devised. The unit of electro- motive force is the volt; the unit of resistance is the ohm, i. e., the resistance offered by a column of mer- cury 106.3 cm. long and i sq. mm. in section at 0° C; the unit of quantity is the coulomb; the unit of time is one second. One volt is the electro-motive force which, when steadily applied, will press through a resistance of one ohm, one coulomb of electricity in one second of time yielding a current strength of one 3.mpere. This relation may be expressed in the following formula. Ohm's law: „, ^ , ,,v Electro-motive force (E.M.F.) „, .„^„^ Volt C (current strength) = Resistance (R) -°' ^"^"^''^ = OKE In practical work it is often necessary to increase the strength of the current. This is done by uniting two or more cells in series, i. e.. PHYSIOLOGIC APPARATUS. 723 uniting the copper of one cell to the zinc of a second, and so on (Fig. 364). If the resistance remains the same the total voltage is that of one cell multiplied by the number of cells united. The cell as above described cannot maintain a current of constant strength for any length of time, for the following reasons: 1. The sulphuric acid solution, in consequerice of its chemic action, soon becomes nothing more than a saturated solution of zinc sulphate, after which its chemic activity ceases. The current, therefore, soon diminishes in strength. 2. The accumulation of hydrogen bubbles on the surface of the copper hinders the passage of the electricity. In a short time they develop a current in the opposite direction, which also tends to weaken the original current. This action is termed polarization of the elements. Cells of this character are not suited for physiologic work, in which constancy in the strength of the current is absolutely necessary. To overcome these disadvantages, cells have been devised which are less violent in action, which prevent polarization, and which maintain a current of constant strength for a long period of time. One of the most generally used for physi- ologic purposes is — The Daniell cell. This consists of a porous cup con- taining a saturated solution Fig. 36s.-Daniell Cell. of CuSO^, copper sulphate, in which is immersed a copper plate or rod. This combination is placed in a glass vessel containing a solution of H2SO4 (1:15). In this solution is immersed a roll of sheet zinc (Fig. 365). Each of the plates is provided with a binding screw. When the cell is in action the sulphuric acid attacks the zinc, forming zinc sulphate, and liberates hydrogen; the cup being porous, the hydrogen passes into the copper sulphate solution, where it combines with the sulphuric acid radicle, and liberates metallic copper. Polarization of the copper is thus prevented. The metallic copper is deposited on the copper plate, which is thus kept bright. The copper sulphate solution is kept at the point of saturation by packing around the copper cylinder a quantity of the crystals of the salt. The sulphuric acid passes back into the porous cup, to take the place of that used. This cell is remarkably constant for these reasons, and well adapted for physiologic as well as other purposes where a current of uniform strength is necessary. 724 TEXT-BOOK OF PHYSIOLOGY. The projecting ends of the copper and zinc plates are termed respec- tively the positive pole or anode, and the negative pole or kathode. The electro- motive force of a Daniell cell is practically i volt ; bjit when the two poles are connected by a wire of i ohm resistance, the current strength will be less than i ampere, possibly only 0.7, owing to the resistance offered to the flow of electricity by the fluids between the zinc and the copper. In all measurements, the internal resistance of the cell must be taken into consideration. The Dry Cell. — The commercial dry cell is a convenient source of electricity for general laboratory work. It consists of a cup of zinc, the inner surface of which is covered over with a thick layer of a paste of plaster-of-Paris, saturated with ammonium chlorid. In the center of the cup there is a rod of carbon. Surrounding this rod and occupying the space between it and the plaster- of-Paris paste, is a mixture of man- ganese dioxid and charcoal. The upper surface of the cell is sealed to prevent evaporation. The electricity is generated at the surface of the zinc cup by the chemic action of the chlorin which arises from the dissociation of the ammonium chlorid. When the plates are united by a conjunctive wire the current within the cell flows from the zinc (the positive element) to the carbon (the negative element), and without the cell from the carbon (the positive pole) to the zinc (the nega- tive pole). Leads. — By means of insulated wires attached to the poles of a cell, the electricity may be conducted from the cell and used for exciting or stimulating purpose. As the wires thus become practically prolongations of the plates their ends become the corresponding poles. In experimental work the ends of the wires are provided with special devices, termed — Non-polarizable electrodes. The necessity for the employmen of such electrodes arises from the fact that when the ends of the wires from a cell are placed in direct contact with the tissues chemic changes are produced in a short time, which lead to their polarization. As a result, a current opposite in direction to that of the cell is developed, which tends to weaken or neutralize it. This polarization current vitiates the result of many experiments made with highly irritable tissue such as nerve-tissue. Whether for stimulating purposes or for the purpose of detecting the existence of electric currents in living Fig. 366. — NoN-POLAEizABLE Elec- trodes. I. Du Bois-Reymond's. 2. Von Fleischl's. 3. d'Arsonval's. PHYSIOLOGIC APPARATUS. 725 tissues, it is essential that the electrodes used shall be non-polarizable. The earliest electrodes of this character were made by du Bois-Rey- mond and were based on the fact discovered by Regnault that a strip of chemically pure zinc or amalgamated zinc (Matteucci) immersed in a saturated solution of zinc sulphate would not polarize. One form made by du Bois-Reymond is shown in Fig. 366. It consists of a flattened glass tube attached to a universal joint and supported by an insulated brass stand. The lower end of the tube is closed with kaolin or China clay made into a paste with a 0.6 per cent, solution of sodium chlorid. It can be moulded into any desired shape. The interior of the tube is partially filled with a saturated solution of sulphate of zinc in which is immersed the strip of amalga- mated zinc. To the upper end of the zinc the conducting wire is attached. The V. Fleischl brush electrode is similar to the preceding except that the end of the tube is closed by the brush of a camel's-hair pencil. The d'Arsonval electrode consists of a glass tube containing a silver rod coated with fused silver chlorid. The interior of the tube is filled with normal salt solution 0.6 per cent, and the end closed with a thread or plug of asbestos which is made to project beyond the tube for a short dis- tance. Any one of these three electrodes is suitable for physiologic experimentation, as their free ends neither corrode the tissues nor develop electric currents. Keys. — Muscle and nerve-tissues are conductors of electricity. When, therefore, the terminals (the non-polarizable elec- trodes) of the wires of a cell are placed in. contact with either a muscle or a nerve a circuit is made through which a current of electricity flows; when one or both are removed, the circuit is broken and the current ceases. In practical work it is often necessary to keep the electrodes in contact with the tissues for a variable length of time. The circuit, however, may be alternately made and broken at will by interposing along the return wire a mechanic contrivance known as a key, of which there are many forms. The du Bois-Reymond Friction Key. — This consists of a plate of vulcanite attached to a screw clamp by which it can be fastened to the edge of a table (Fig. 367). The surface of the vulcanite plate carries two rectangular blocks of brass, each of which has two holes Fig. 367. — Du Bois-Rey- mond Friction Key. 726 TEXT-BOOK OF PHYSIOLOGY. drilled through it, for the insertion of wires, which are held in position by small screws. A movable bridge of brass, provided with an ebonite handle, serves to make connection between the blocks. There are two ways of interposing this key in the circuit. 1. As a Simple Key. — ^For this purpose one of the wires, usually the negative, is carried from the cell to one block and then continued from the second block. When the bridge is down, the circuit is made and the current passes; when it is up, the circuit is broken. 2. As; a Short-circuiting isCej'.— When used for this purpose, the wires of the cell are carried to the inner holes of each block and then continued from the outer holes to the tissues or to some form of apparatus which it is desired to actuate. When the key is closed i. e., when the bridge is down, the current on reaching the key, will divide, one portion passing across the bridge and so back to the cell, the other portion passing to the tissue or ap- paratus. The amount of the current which is returned to the cell through the short circuit will be proportional to the resistance of the longer circuit. As the latter is usually great in compar- ison with the former; prac- tically all the current is short-circuited. When the Fig. .368.-A Mercury Key. bridge is lowered, therefore, the current is short-circuited ; when it is raised, the current flows into the longer circuit through the tissue or apparatus. The Mercury Key. — In this form the connection is established by means of mercury. It consists of a circular block in the center of which there is a cup containing mercury (Fig. 368). At opposite points there are binding posts, one of which is provided with a rigid fixed copper rod passing into the mercury; the other is provided with a movable bent rod which may be made to dip into or be withdrawn from the mercury by the ebonite handle. The effect of a constant or galvanic current on a muscle or nerve will depend to some extent on its strength. This may be accurately regulated by means of an apparatus known as — The Rheocord. With this apparatus an electric current may be divided, one portion continuing through a conductor back to the battery, the other portion being sent off through the nerve. The strengths of these two currents are inversely proportional to the resistances of their circuits. A simple form of rheocord (Fig. 369) consists of a long wire arranged for convenience in parallel lines on a small wooden base and connected at its two ends with binding posts PHYSIOLOGIC APPARATUS. 727 A and B. The resistance of this wire, 1.6 ohms, can be increased by the introduction of small resistance coils, between D and B, varying from 5 to 20 ohms. The two binding posts A and B are connected with the positive and negative poles of an electric cell respectively. A simple key is placed in the circuit. From A, a wire passes to one of the electrodes on which the muscle or nerve rests. A second wire passes from the second electrode to a clamp S, by way of the binding post C, which can be fastened to the long wire at any given point. The current, on reaching A, will divide into two branches, one of which will pass along the wire A, B, and thence back to the cell; the other will pass through the nerve and back to S and thence also to the cell. The amount of current passing through the nerve circuit will be inversely pro- portional to the resistance of the nerve and directly propor- tional to the difference of potential between A and S. If S is close to A, the differ- ence of potential is slight. If S is removed from A to- ward B, the difference of potential is increased and the current sent through the nerve circuit is increased. In many experiments it is necessary to reverse the direc- tion of the current, in other experiments to deflect it, with- out changing the position of the electrodes. Both thSse results may be accomplished by the use of— Pohl's commutator. This is a round block of wood with six cups, each of which is in connection with a binding post (Fig. 370). In each of the two cups marked i and 2, + and — , is inserted one end of a copper wire bent at right angles. The other ends of the wires are supported and insulated by a hard-rubber handle. To the top of each wire is soldered a semicircular copper wire. This arrange- ment permits of a rocking movement, whereby the opposite ends of the semicircular wires can be made to dip into cups 3 and 4, and into cups 5 and 6 alternately. Two wires crossed in the middle of the block serve to connect opposite pairs of cups. When in use, the cups are filled with clean mercury. The method of using the com- mutator is as follows: I. As a Current Reverser. — The positive and negative poles of the Fig. 369. — Rheocoed. 728 TEXT-BOOK OF PHYSIOLOGY. electric cell are connected by wires with binding posts i and 2 respectively. A key is interposed in the circuit. Wires are then carried from binding posts 3 and 4 to the electrodes in connection with the muscle or nerve. The rocker of the commutator is so turiied that the ends of the semicircular wires dip into cups 3 and 4. The direction of the current will be on the closure of the circuit from I to 3, then from 3 along a wire to and through the tissue and back to 4, and thence to the cell. If the position of the rocker be now reversed so that the opposite ends of the semi- circular wires dip into cups 5 and 6, the direction of the current through the tissue will be reversed. The positive current, after entering binding post i, will flow to 5; then along one of the cross wires to 4; then along a wire to and through the tissue and back to 3, along the opposite cross wire to 6, thence to 2 and so back to the cell. «ir-->^^^ Fig. 370. — Pohl's Commutator. A. Arranged as a current reverser; B, as a cur- rent deflector. 2. As a Current Deflector. — When it is desirable to deflect the current to two pairs of electrodes differently situated, wires are carried from binding posts 3 and 4 to one pair, and from 5 and 6 to the other pair. The cross wires are then removed. According to the position of the rocker the current will be deflected to one or the other. The Inductorium. — This is an apparatus designed for the purpose of obtaining single or rapidly succeeding electric currents by induction. Its construction is based on facts discovered by Faraday, some of which are the following: If two circuits, a primary and a secondary, are placed parallel to each other, the former connected with a galvanic cell, the latter with a galvanometer, it is found that, at the moment the primary circuit is made, and at the moment it is broken, a current is induced in the secondary circuit, as shown by a momentary deflection of the galvano- meter needle. During the continuous flow of the current through the primary circuit there is no evidence of a current in the secondary PHYSIOLOGIC APPARATUS. 731 the make as well as on the break of the circuit, though it will be less pronounced. The explanation offered for this difference in the strengtli of the two induced currents is as follows: With the make of the circuit and the passage of the battery current through the primary coil there is induced in the neighboring and parallel turns of the wire an extra current opposite in direction to the primary current. This extra or self -induced current antagonizes and prevents the current from attaining its maximum development as quickly as it otherwise would, and therefore its efl&ciency as an inducer of a current in the secondary is diminished. On the break of the circuit the primary current disappears quickly, and as there is nothing to retard its disappearance its efficiency as an inducer of a current in the secondary coil is not diminished. It is not infrequendy stated that the disappearance of the primary current induces in the neighboring coils a break extra current corresponding in direction which assists in die development of the induced current. This is not the case, however, as no break extra current is developed in the inductorium as ordinarily used when actuated by a battery current of moderate strength. As it is not so much the intensity of the current as it is rapid variations in intensity that produce effects, it is readily apparent why the induced current developed at the break of the primary is more effective as a stimulus than the induced current developed at the make of the primary circuit. The quantity of the electricity is, however, the same in both cases. If the secondary be pushed further along the slideway until it largely covers the primary coil, a position will be reached when the make induced current equals in its ef&ciency as a stimulus the break induced current; and if the secondary be yet further advanced, a position is reached when the inake induced current becomes more powerful and efficient than the break induced current, as shown by the greater contraction of the muscle. This result is explained by the fact that the make extra current is now able of itself to induce a current in the secondary coil, on account of its proximity, which, added to that induced by the battery current, produces a current, greater than that induced on the break of the circuit.* Rapidly Repeated Induced Currents. — As the single induced current is of extremely short duration, it is inefficient as a stimulus in the con- duct of many experiments. It is necessary, therefore, to develop it with a frequency that is sufficient to give rise to a summation of effects. The duration of the stimulation may be thus considerably prolonged. Tliis is accomplished by introducing in the primary circuit close to the primary coil an automatic interrupter, usually Neef's modification of Wagner's hammer (Fig. 371). This consists of a vertical post, P', to the top of which is fastened a metallic spring carrying at its opposite end a steel or iron hammer, H, which hangs over, but does not touch, the two vertical bars of soft iron around which the wire of the primary coil is wound. About the middle of the spring on its upper surface there is a small plate of platinum which is in contact with an adjustable, platinum-tipped screw, S', carried by a plate of brass in connection with binding post S". For the purpose of interrupting the primary circuit frequently in a unit of time, and thus developing induced currents in quick succession, the apparatus is arranged in the following way: The positive and * " On certain peculiarities of the inductorium," Prof. Colin C. Stewart, " Univ. Pa. Medical Bulletin," Feb., 1904. 732 TEXT-BOOK OF PHYSIOLOGY. negative poles of the electric cell are connected by wires with binding posts P' and P", a key being interposed in the circuit. If the screw S' is in contact with the spring, the current on the closure of the circuit will enter P', pass along the spring to S', thence into and through the primary coil R', to the coils surrounding the vertical bars B', then to P", and so back to the cell. As the current passes around the vertical bars, they are magnetized. The magnetization draws down the hammer, and, in so doing, breaks the circuit at the tip of the screw, S'. The vertical bars are at once demag- netized, and the hammer is restored to its original position by the elasticity of the spring. The circuit is thus re-established, the current flows through the coils, the bars are again magnetized, the hammer is drawn down, to be followed by a second break of the circuit. The number of times the circuit is thus made and broken per second will vary with the length of the spring. As each interruption of the primary circuit develops an induced current, it follows that the latter must succeed each other with a frequency corres- ponding with the frequency of the former. If while the primary circuit is thus being interrupted the wires of the secondary coil be placed in con- tact with a muscle, the induced current will give rise to contractions which will succeed each other so rapidly that they fuse together, producing a spasm or tetanus of the muscle. For this reason these currents are frequently spoken of as tetanizing currents, and the pro- cedure as tetanization or Faradization. These currents also increase in strength as the secondary approaches the primary. Fig. 372. — Helmholtz's Modih- CATiON OP Neep's Hammer. As long as c is not in contact with d, g h remains magnetic; thus c is at- tracted to d and a secondary circuit, a, b, c, d, e, is formed; c then springs back, again, and thus the process goes on. A new wire is introduced to connect a with /. K. Battery. Helmholtz's Modification of the Inductorium. — With a view of equalizing the strengths of the induced currents, Helmholtz suggested a device the adoption of which accomplishes this to a certain extent. It consists (Fig. 372) in connecting with a wire binding posts P' and S", and in providing binding post P" with an adjustable screw which can be raised until the spring comes in contact with it, when the hammer is drawn down by the electromagnet B'. This latter arrangement is practically a short-circuiting key by which a portion of the current is returned to the cell withoilt ever entering the primary coil . The same arrangement, though differently lettered, is shown in Fig. 371. By the use of the entire device the changes in the primary coil are made not by making and breaking the primary current, but by alternately long- and short-circuiting the current. "When the short-circuiting key is opened, the full volume of the primary current flows through the pri- mary coil. When the short-circuiting key is closed, most of the current fails to enter the coil, taking the easier path through the key. Some of the current, however, always flows through the coil and is never diverted. The cycle of' changes in the electric condition of the primary coil is thus altered for two reasons : PHYSIOLOGIC APPARATUS. 733 "First, we no longer have an alternation between a full primary current and none at al —rather an alternation between a full primary current and a weaker one. The difference in the phases is thus lessened, the extent of the change on making and breaking is lessened, and correspondingly the ef&ciency of the make and break currents induced in the secondary coil is slightly decreased. "Second, on making the primary current, as in the ordinary coil, the sudden appearance of the primary current is antagonized by the opposing make extra current, with the result that the make induced current is still further reduced; while on breaking the current the break extra current can now flow through the primary coil across the short-circuiting key. This current, trailing behind the disappearing primary current in the same direction, pro- duces the same effect as if the primary current itself were to disappear slowly. As a result the disappearance of the primary current loses its former efficiency as an inducer of second- ary currents, and the break induction current is reduced to about the efficiency of the make. "This so-called 'equalizing' of the make and break induced currents is never perfect, if for no other reason, because the make extra current must take the long circuit through the battery, while the break extra current has an easier path through the short-circuiting key, and is thus greater than the make extra current." (C. C. Stewart.') THE GRAPHIC METHOD. The terni graphic is applied to a method by which curves or tracings are obtained which represent the extent, duration, and time relations of the movements accompanying physiologic processes. If these move- ments can be trans- lated in one direc- tion, they may be recorded in different ways: 1. By attaching the moving struc- ture — e. g., heart, muscle, etc. — to a deli- cate lever the free extremity of which is pro- vided with a writing point. 2. By transmitting the movement through a column of air enclosed in a rubber tube the two ends of which are attached to a metallic capsule, covered by a rubber membrane, termed a drum or tam- bour. When the membrane of the first tambour is pressed or driven inward, the air is forced through the rubber tube into the second tambour and its membrane is pushed outward. As soon as the primary pressure is removed, the membranes return to their former condition. If the membrane of the first tambour is drawn outward, the air in the system is rarefied and the mem- brane of the second tambour is pressed inward. For the purpose of registering the movement transmitted by the column of air, the second tambour is provided with a light lever supported by a vertical bearing resting on a small metallic disk. The mem- brane of the first tambour is frequently provided with a button, Fig. 373. — A Receiving Tambour. 734 TEXT-BOOK OF PHYSIOLOGY. which is placed over the moving structure. The inward move- ment of the membrane of the first tambour produces an outward movement of the membrane of the second tambour, indicated, though magnified, by the rise of the free end of the lever. The reverse movement of the membrane is attended by a fall of the lever. The first tambour is termed the receiving, the second the recording tambour (Figs. 373, 374). 3. By enclosing an organ — e. g., kidney, spleen, arm, finger, etc. — in a rigid glass or metal vessel which at one point is in communic- ation with a recording apparatus — e. g., (i) a piston provided with a lever (page 483); or (2) a tambour and lever (page 336); or (3) a mercurial manometer carrying a float and pen (page 341). The space between the part investigated and the vessel is filled with fluid. The variations in volume of the organ cause a dis- placement of the fluid and give rise to a to-and-fro movement which is taken up and reproduced by the recording apparatus. The writing point may be (i) some form of pen carrying ink which records the movement on a white paper surface, or (2) a piece of metal, glass, or paper which records the movement on smoked paper or glass. The Recording Surface. — The sur- face which receives and records the move- FiG. 374. — A Recording Tambour. — {Marey.) ments of a pen or lever is usually that of a cylinder which is covered with glazed paper and coated with a thin layer of soot, obtained by passing the cylinder through the flame of a gas burner. The axis of the cylinder is supported by a metal frame- work. If the writing point of the lever be placed against the cylinder and a movement be imparted to it, a portion of the soot is rubbed off, leaving a white line behind. If the cylinder be stationary, the rise and fall of the lever are recorded as a vertical line. Such a record shows only the extent of a movement. If the cylinder is traveling, however, at a uniform rate, the rise and fall of the lever are recorded in the form of a curve the width of the two arms of which will depend partly on the rapidity of the movement of the lever and partly on the rate of movement of the cylinder. The cylinder movement is initiated and maintained by clock-work or by the transmission of power by belting to a system of pulleys in connection with its axis. As the tracing is wave-like in form, the cylinder is frequently spoken of as a kymograph or wave recorder (Fig. 375). From the record thus olatained it is possible to determine not only the extent but also the duration, the form, and the rate of recurrence of any given movement. PHYSIOLOGIC APPARATUS. 739 be raised or lowered and centered in the copper chamber. Deflections of the mirror from currents of air are prevented by inclosing it with a brass cover provided with a glass window. The coils are placed on each side of the copper chamber, and supported by a rod, on which they slide. By this arrangement they can be approximated until they meet and completely conceal the cylinder. By varying the position of the coils the influence of the current upon the needle can be increased or diminished. An advantage which this galvanometer possesses is the damping of the oscillation of the needle, so that it quickly comes to rest after deflection. This is accomplished by the development of induction currents in the copper cylinder, the direction of which is opposite to that of the movement of the needle. The instrument, therefore, is aperiodic — that is to say, when the needle is influenced by a current it moves comparatively slowly until the maximum deflection is reached, when it comes to rest without oscilla- tions. When the circuit is broken the needle swings slowly back to zero, and again comes to rest without oscillations. Inasmuch as the needle is not astatic, it is rendered so by the use of an accessory magnet — the so-called Hauy's bar. This magnet, supported by a rod directed perpendicular to the coils, is placed in the magnetic meridian, horizontal to the needle, with its north pole point- ing north. By sliding the magnet toward the needle the directive influence of the earth's magnetism is gradually diminished, and when it is reduced to a minimum the needle acquires its highest degree of instability. By means of a pulley an angular movement can be im- parted to the end of the accessory magnet in the direction of the magnetic meridian, which serves to keep the needle on the zero of the scale. The deflections of the needle are observed by means of an astronomic telescope, above which is placed a scale divided into centimeters and millimeters, and distant from the galvanometer about six or eight feet. As the numbers on the scale are reversed, they will be seen in the mirror in their natural position, and with the deflection of the needle the numbers will appear as if drawn across the mirror. The extent of the deflection is readily determined when the needle comes to rest. The reflecting galvanometer of Sir William Thompson is also used for the same purposes. The Capillary Electrometer.— Notwithstanding the extreme sensitiveness of the modern galvanometer, it has been found desirable, in the investigation of many physiologic processes, to possess some means which will respond even more promptly to slight variations in electro-motive force. This has been realized in the construction by Lippmann of the capillary electrometer. The principle of this appa- ratus rests upon the fact that the capillary constant or the surface- tension of mercury undergoes a change upon the passage of an electric current, in consequence of a polarization by hydrogen taking place at its surface. If a capillary glass tube be filled with mercury and 740 TEXT-BOOK OF PHYSIOLOGY. its lower end inserted into a solution of sulphuric acid, and the former connected with the positive and the latter with the nega- tive electrode, it will be observed, upon the passage of the current, that a definite movement of the mercury takes place, in the direction of the negative electrode, in consequence of the diminution of its capillary constant or the tension of its surface in contact with the acid. As a reverse movement follows a cessation of the current, a series of oscillations will follow a rapid making and breaking of the current. If the direction of the current is reversed, the capillary constant is increased and the mercury ascends the tube toward the negative pole. From facts such as these Lippmann constructed the capillary electrometer, a con- venient modification of which devised by M. v. Frey, is shown in Fig. 380. This consists of a glass tube, A, forty millimeters in length, three millimeters in diameter, the lower end of which is drawn out to a fine capillary point. The tube is filled with mercury and its capillary point immersed in a 10 per cent, solution of sul- phuric acid. The vessel containing the acid is filled to the extent of several millimeters with mercury also. The mer- cury in the tube is put in connection with a platinum wire (a), and the acid in the vessel with a second wire (&). When a constant current passes into the apparatus in the direction from h to a the mercury is pushed up the tube, and, upon the breaking of the current, it may or may not return to the zero-point. For the purpose of measuring in millimeters of mercury the pressure necessary to compensate this change in the capillary constant produced by the electro-motive force of polarization, the apparatus is provided with a pressure-vessel, H, and a manometer, B. This electrometer can be applied to any microscope having a reversible stage. The oscillations of the mercury can then be observed with the microscope provided with an ocular micrometer (Fig. 381). The special advantage of the electrometer is, that it will respond instantly to any variation in the electro-motive force, and indicate a Fig. 380. — Von Frey's Capillary Electrometer. PHYSIOLOGIC APPARATUS. 741 difference of potential, according to Lippmann's observation, as slight as the t-b-bttt of a Daniell. These rapid oscillations can be re- corded by photographic methods. In using either the galvanometer or the electrometer for detecting the existence of electric currents or differences of potential in living tissues, it is absolutely essential that non-polarizable electrodes be employed in connection with it. DISSECTION OF THE HIND-LEG OF THE FROG. Much of our knowledge of the physiologic properties of muscles and nerves has been derived from the study of the muscles and nerves of the cold-blooded animals, especially of the frog, for the reason that in these animals the tissues retain their vitality under appropriate conditions for a considerable period of time after death or removal from the body. The muscles generally employed for ex- perimental purposes are the gastrocnemius, the sartorius, the semimembranosus, the gracilis, and the hyoglossus. The nerve generally employed is the sciatic. Both niuscle and nerve may be studied independently of each other, or they may be studied together, as when in their usual physi- ologic relation. For this latter purpose the gas- trocnemius muscle and sciatic nerve are em- ployed, constituting the so-called "nerve-muscle preparation." For these, and many other reasons, the stu- dent should familiarize himself with the general anatomy of the frog, and especially with the anatomy of the posterior extremities. Preparation of the Frog. — Destroy the frog by plunging a pin through the skin and soft tissues covering the space between the occipital bone and the first vertebra until the point is stopped by the vertebra. Turn the pin toward the head and push it into the brain cavity; move it from side to side and destroy the brain. Pass the pin into the spinal canal and destroy the spinal cord. With a stout pair of scissors cut off the body behind the fore-limbs. Remove the viscera and the abdominal walls. Draw the hind-legs out of the skin. Place the legs on a glass plate, back uppermost, and moisten them freely with normal saline solution. Observe on the outer side of the dorsal surface of the thigh the follow- ing muscles (Figs. 382, 383). The triceps femoris (tr), made up of the rectus anticus (ra), the vastus externus (ve), and the vastus internus (vi), not seen from behind; on the inner side, the semimembranosus Fig. 3S1. — Capillary Electrometer. R. Mercury in tube; capil- lary tube. s. Sulphuric acid. g. Hg. B. Ob- server. M. Microscope. 742 TEXT-BOOK OF PHYSIOLOGY. (sm) and the rectus internus minor or gracilis (ri"). Between'' these two groups, note the biceps femoris (b). Above the thigh observe the gluteus (gl), the ileococcygeus (ci), and the pyriformis (p). In the leg observe the gastrocnemius (g) with its tendon (the tendo Achillis), the tibialis anticus (ta), and the peroneus (pe). Turn the frog on its back and note the muscles on the ventral surface of the thigh, the rectus internus major (ri') , and minor (ri") , the ad- ductor magnus (ad"), the sartorius (s), the adductor longus (ad'), and the vastus internus (vi). In the leg, in addition to those, already seen from behind, note the tibialis posticus (tp) and the extensor cruris (ec). cc Fig. 382. — Leg Muscles oe the Frog. Ventral Surface. — {Ecker.) Fig. 383. — Leg Muscles of the Frog. Dorsal Surface. — {Ecker.) Note in the abdominal cavity the three large spinal nerves, the seventh, eighth, and ninth. Dissection of the Sciatic Nerve. — The sciatic nerve is composed of the seventh, eighth, and ninth spinal nerves. After its emergence from the pelvic cavity, it passes down the thigh between the semi- membranosus and the biceps muscles, in company with the femoral blood-vessels. Below the knee it divides into the tibialis and peroneus nerves; the former sending branches into the gastrocnemius. In its course, the sciatic sends branches to the muscles of the entire leg. Carefully separate the biceps and semimembranosus by tearing the connective tissue uniting them. The sciatic nerve and femoral blood-vessels come into view; with a bent glass rod gently separate the PHYSIOLOGIC APPARATUS. 743 nerve from its surroundings from the knee to the thigh. Begin at the knee. In order to expose the nerve at the pelvis, it will be necessary to divide the pyriformis and the ileo-coccygeus muscles. Care must here be exercised, so as not to injure the nerve which lies immediately beneath. Lift up the uro-style with the forceps and separate it from the last vertebra. With the scissors cut off the vertebral column above the seventh vertebra. Place the legs on the dorsal surface and then divide the seventh, eighth, and ninth vertebrae lengthwise. With, the forceps lift up one lateral half of the vertebras and free the nerve as far as the knee by dividing connective tissue and nerve branches. Be careful not to injure the nerve with scissors or forceps. The Nerve-Muscle Preparation. — Divide the tendo Achillis just below its fibro-cartilaginous thickening at the heel, and detach the gastrocnemius up to the knee. Cut through the tibio-fibular bone just below the knee-joint. Cut the femur transversely near its middle and remove the muscles from the lower end, carefully avoiding injury to the nerve. The completed preparation consists of the gastroc- nemius muscle, the sciatic nerve, with half of the seventh, eighth, and ninth vertebrae and the lower half of the femur. INDEX Abducens nerve, 597 Aberration, chromatic, 676 spheric, 676 Absorption, 213 by epithelium of villi, 224 of foods, 223 of fat, 227 of proteins, 226 of sugar, 225 of water, 225 of lymph, 222 spectra of blood, 256 Accommodation of the eye, 667 convergence of eyes during, 672 force of, 671 mechanism of, 668 range, 670 Action currents of muscles, 86 of nerves, 115 reflex, 123 of medulla oblongata, 532 of spinal cord, 506 Adrenal bodies, 467 Agraphia, 561 Albuminoids, 17 Albumins, 16 Alcohol, effects of, 134 Alimentary canal, 146 Allantois, 712 Animo-acids, 14 Amnion, 712 Amylopsin, 197 Amyloses, 8 Animal body, structure of, 3 heat, 437 Ankle clonus, 511 jerk, 511 Aphasia, 560 ataxic, 561 amnesic, 561 Apnea, 425 Arterial circulation, 326 pressure, 341 Arteries, structure and properties of, 326 Articulate speech, 635 Asphyxia, 426 Association centers of cerebrum, 561 Astigmatism, 675 Auditory area, 557 nerve, 603 Basal gangha, 525 Bile, 201 composition of, 202 Bile, mode of secretion, 204 physiologic action, 205 pigments, 203 salts, 203 Bilirubin, 203 Biliverdin, 203 Bioplasm, 30 physiologic properties, 4 Blastodermic membranes, 711 Bhnd spot, 678 Blood, 236 changes in, during respiration, 409 circulation of, 271 coagulation of, 238 chemistry of, 267 extravascular, 268 intravascular, 269 constituents of, 236 corpuscles, 242, 261, 265 defibrinated, 240 general composition of, 267 physical properties of, 237 plates, 265 pressure, 339 arterial, 341 capillary, 345 causes of, 347 determination of, in man, 352 methods of estimation, 344, 352 variations in, 348 venous, 346 quantity of, 266 serum, 240 velocity of, in arteries, 358 of, in capillaries, 360 of, in veins, 361 Burdach, column of, 466 Calcium salts of the body, 22 Calorimeter, 441 Capillary blood-vessels, 328 functions of, 329 circulation, 367 electrometer, 739 Capsule, internal, 525 functions of, 536 Carbohydrates, 8 Carbon monoxid hemoglobin, 269 Cardiac cycle, 285 Cardio-accelerator center, 320 factors which determine its activity, 320 Cardio-inhibitor center, 321 factors which determine its activity; 321 Cardio-pulmonary vessels, 275 745 746 INDEX. Caseinogen, i8, 451 Caudate nucleus, 525 Cells, structure of, 26 chemic composition, 27 manifestatations of life b}', 28 reproduction of, 31 Central organs of the nerve system, 491 Cerebellar tract, 405 Cerebellum, 569 functions of, 571 results of experimental lesions, 572 Cerebrum, 537 convolutions of, 539 fissures of, 537 functions of, S4S localization of function in, 547 motor area of the chimpanzee brain, SS4 motor area of the human brain, 558 motor area of the monkey's brain, 551 sensor areas of the human brain, 555 sensor areas of the monkey's brain, 549 structure of the gray matter, 541 structure of the white matter, 543 Chemic composition of the body, 7 Chimpanzee brain, motor area of, 554 Cholesterin, 203 Chorda tympani nerve, 161 Chorion, 713 Chromo-proteins, ig Chyle, 227 Ciliary movement, 94 muscle, 625 function of, 669 Circulation of blood, 271 forces concerned, 370 Clark's vesicular column, 455 Clzissification of food principles, 130 Coagulated proteins, 20 Cochlea, 693 functions of, 699 Colostrum, 453 Commutator, 727 Complemental air, 404 Conjugated proteins, 18 Connective tissues, 35 physical and physiologic (properties of, 41 Corpora quadrigemina, 524 functions of, 533 striata, 525 functions of, 534 Corpus luteum, 705 Cranial nerves, 577 Crura cerebri, 523 functions of, 533 origins of, 577 Crystalline lens, 658 Daily ration of U. S. soldier, 144 Decidual membrane, 710 Defecation, 211 nerve mechanism of, 211 Deglutition, 163 nerve mechanism of, 170 Demarcation current, 85 Depressor nerve, 323, 378, 380 Dextrin, 9 ■ Dextroses, 9 Diabetes, 460 Diapedesis of leucocytes, 369 Diaphragm, 389 Dietaries, 143 Diffusion, 230 Digestion, 145 Digestive apparatus, 145 Dilatator pupillae muscle, 652 Direct cerebellar tract, 500 pyramidal tract, 498 DucUess glands, 462 Ductus arteriosus, 718 venosus, 717 Dyspnea, 425 Electrodes, non-polarizable, 724 Electrotonic alterations in excitability of nerves, 117 current, 116 Electrotonus, 116 Encephalo-spinal fluid, 492 Endocardium, 274 Enterokiuose, 200 Epidermis, 487 Epididymis, 706 Epinephrin, 468 Epithelial tissues, functions of, 33, 34 Equilibration, mechanism of, 574 Erepsin, 199 Erlanger's sphygmomanometer, 356 Erythrocytes, 242 Eupnea, 424 Eustachian tube, 689, 698 Excretion, 472 Expiratory forces and muscles, 398 Expired air, composition of, 407 Eye, cardinal points of, 660 dioptric apparatus of, 658 muscles of, 683 physiologic anatomy of, 650 reduced, 663 schematic, 662 Facial nerve, 598 paralysis of, 601 Fallopian tube, 702 Fat, 12 absorption of, 227 digestion of, 199 emulsification of, 13 saponification of, 13 Feces, 210 Fecundation, 708 Fehling's solution, 9 Fetal circulation, 715 membranes, 712 structures, 712 Fibrin, 20 Fibrinogen, 241 FiUet, 521 Filtration, 234 INDEX. 747 Follicle, Graafian, 701 Food, 127 animal, 139 cereal, 141 compoation of, 139 disposition of, 130 heat value of, 134 principles, 130 quantities required daily, 128 vegetable, 142 Forces aiding the movement of lymph and chyle, 228 Fovea, 653, 656 Galactose, 11 Gall-bladder, 201 Galvanic current, effect of, on nerves, 116 Galvanometer, 738 Ganglia, cephalic, 624 Gaseous exchange in lungs, 407 in tissues, 415 Gases of blood, relation of, 410 tension of, 414 carbon dioxid, 413 oxygen, 412 Gastric digestion, 171 fistulae, 176 glands, 174 juice, 177 mode of secretion, 178 physiologic action of, 182 Globulins, 17 Glossopharyngeal nerve, 605 Glycogen, 10, 458 Glycogenic function of the liver, 457 Gluco-proteins, 19 Gmelin's test for bile pigments, 204 Goll, columns of, 501 Gowers' antero-lateral tract, 500 Graafian follicle, 701 Graphic method, 733 Green vegetables, 142 Hairs, 489 Hearing, sense of, 689 Heart, 271 action of sympathetic nerve on, 312, 316 of vagus nerve on, 314, 317 auriculo-ventricular bundle, 280 beat, nature of the stimulus, 302 action of inorganic salts, 303 frequency of, 285 of the excised heart, 296 blood-supply, 294 causation of, 305 causes of the variations of, 322 course of blood through, 276 cycle of, 285 intracardiac nerve-cells, 310 pressure, 289 intraventricular pressure curve, 290 mechanics of, 282 modifications of beat due to the action of drugs, 323 muscle-band of His, 280 Heart, muscle-fibers of, 279 negative pressure of, 292 nerve, mechanism of, 308 orifices and valves, 277, 278 origin and distribution of the sympa- thetic nerves to, 310 origin and distribution of the vagus nerve to, 311 physiologic anatomy of, 271 relative function of auricles and ven- tricles, 288 sounds, 293 synchronism of the two sides, 288 valves, action of, 286 work done by, 371 Heart-muscle, properties of, 296 automaticity, 302 conductivity, 297 irritability, 296 response to action of a stimulus, 306 rhythmicity, 301 tonicity, 301 Heat dissipation, 442 income, 438 relation to work, 444 rigor, 70 Helmholtz's theory of color perception, 685 Hematin, 260 Hemianopsia, 584 Hemoglobin, 252 absorption spectra, 256 chemic composition of, 253 compounds of, 258 quantity of, 254 Hemoglobinometer, Gowers', 255 Hemometer, v. Fleischl's, 256 Bering's theory of color perception, 686 Histons, 16 Horopter, 681 Hypermetropia, 674 Hyperpnea, 424 Hypoglossal nerve, 615 Incus, 691 Induced currents, 730, 731 Inductorium, 729 Infra-proteins, 20 Insalivation, 151 nerve mechanism of, 155 Inspiration; 395 movements of thorax, 391 muscles, 395 Insula, 541 Intercostal muscles, 389. Internal capsule, 525 functions of, 536 secretion, 462 Intestinal digestion, 191 fermentation, 210 juice, 193 physiologic action of, 200 movements, 206 nerve mechanism of, 208 Intracardiac presssure, 289 748 INDEX. Intracranial circulation, 563 mechanism of, 564 Intrapulmonary pressure, 392 Intrathoracic pressure, 392 Intravascular coagulation, 269 Invertin, 200 Iris, 652 functions of, 672 nerve mechanism of, 588, 673 Iron of the body, 24, 254 Irritability of muscles, 59 of nerves, ro9 Island of Langerhans, 195 of Reil, 541 Isometric myogram, 72 Isotonic myogram, 67 Isthmus of encephalan, 521 functions of, 527 Jacobsen's nerve, 605 Joints, 49 classification of, 49 Kidney, 476 ' histology of, 477 Knee-jerk, 475 Kymograph, 734, 735 Labyrinth of ear, 691 Lacrimal glands, 687 Lactation, 718 Lacteals, 227 Lactose, 11 Language, 559 Large intestine, 208 Larynx, 626 nerve mechanism of, 635 structure of, 627 Lateral columns of the spinal cord, 499 Law of contraction, iig Lecithin, 204 Lemniscus, 521 Lens, crystalline, 658 Lenticular nucleus, 526 Leukocytes, 26T chemic composition of, 262 classification of, 263 functions of, 265 number of, 262 origin of, 265 physiologic properties, 263 Levers, 88 Levulose, 10 Limbic lobe, 506 Liver, 201, 483 formation of urea in, 462 functions of, 455 influence of the nerve system on, 459 production of glycogen, 457 secretion of bile, 456 Localization of functions in cerebrum, 547 Lungs, structure of the, 384 Lymph, 218 absorption of, 222 composition of, 219 Lymph, functions of, 221 movement of, 228 physical properties, 219 production of, 220 Lymph capillaries, 2r4 Lymph-glands, 215 Lymph-vessels, 214 Lymphocytes, 219, 264 Macula lutea, 653 , Malleus, 691 Maltose, 11 Mammary gland, 449 Mastication, 147 muscles of, 149 nerve mechanism of, 150 Meats, composition of, 139 Medulla oblongata, 519 reflex activities of, 532 Meibomian glands, 687 Membrana tympani, 690 functions of, 697 Menstruation, 704 Metabolism on protein diet, 138 on fat and carbohydrate diet, 139 Methemoglobin, 260 Migration of leukocytes, 264, 369 Milk, 451 composition of, 139, 452 mechanism of secretion, 452 Moist chamber, 736 Mosso's plethysmograph, 366 spygmomanometer, 353 Motor area of chimpanzee brain, 554 of human brain, 558 of monkey brain, 549 oculi nerve, 585 Mouth digestion, 147 Movements of the eyeball, 682 of the intestines, 206 of the lower jaw, 149 of the lungs, 400 of the stomach, 186 Muscle action currents, 86 contraction, 65 chemic phenomena of, 80 electric phenomena of, 83 graphic record of, 66 modifying influences of, 68 physical phenomena of, 63 rigor mortis, 89 summation effect, 74 tetanus, 75 thermic phenomena of, 82 electric currents from, 85 electric currents, negative variation of, 8s energy, source of, 8r fatigue, 70 groups, special action of, 87 sense, 644 sound, 80 spindle, 645 stimuli, 60 tissue, 53 INDEX. 749 Muscle tissue, chemic composition of, 56 elasticity, 58, 64 histology of, 54, 91 irritability, 59 physical properties of, 57 physiologic properties of, 60 tonicity, 59 Myopia, 673 Myosinogen, 17, 57 Myxedema, 427 Nerve, abducens, 597 auditory, 603 facial, 598 glossopharyngeal, 605 hypoglossal, 615 irritability, 109 motor oculi, 585 olfactory, 579 optic, 582 patheticus, 591 pneumogastric, 606 spinal accessory, 612 stimuli, iro tissue, histology of, 96 trigeminal, 592 Nerve impulse, no Nerve-muscle preparation, 112, 743 Nerve system, functions of, 493 Nerve tissue, 96 histology of, 96 Nerves, chemic composition and metab" olism of, loi classification of, rc7 degeneration of, 105 development of, 104 effects of galvanic current on, 116 electric currents of, 114 electric currents of, negative varia- tion of, 113 electric excitation of, 112 electric phenomena of, 113 action currents, 115 diphasic action currents, 115 peripheral endings of, 103 physiologic properties of, 109 pilo-motor, 108 polar stimulation of, 119, 121 relation of, to central nerve system, 102 stimuli of, 109 Neuron, 96 Nicotin, actions of, 324 Nucleo-proteins, 19 Nucleus caudatus, 525 cuneatus, 501 gracilis, 501 lenticularis, 526 Nutrition of the embryo, 713 Oculo-motor nerve, 585 Ohm's law, 722 Olein, 13 Olfactory nerve, 579 Oncograph, 483 Oncometer, 483 Ophthalmic ganglion, 624 Optic constants, 659 thalamus, 527 functions of, 535 Optic nerve, 582 Optogram, 680 Organ of Corti, 694 Osazones, 12 Osmometer, 231 Osmosis, 230 Osmotic pressure, 231 Ossicles of ear, 691 Otic ganglion, 625 Ovary, 701- Ovulation, 703 Ovum, 702 Oxygen in blood, 412 in tissues, 415 quantity absorbed daily, 422 Oxyhemoglobin, 258 Pacinian corpuscle, 640 Palmitin, 13 Pancreas, 194 Pancreatic juice, 196 mode of secretion, 196 physiologic action of, 197 Parathyroids, 465 Partial pressure of gases, 411 j-Parturition, 7r7 Pathetic nerve, 591 Pepsin, 178 Peptones, 184 Perspiration, 486 Peripheral organs of the nerve system, 100, 491 Petrosal nerves, 600 Pettenkofer-Voit respiration apparatus, 420 Pexin, 178 Phagocytosis, 265 Phloridzin diabetes, 461 Phonation, 626 mechanism of, 632 Phospho-proteins, 18 Physiology of the cell, 26 of movement, 43 Pilo-motor nerves, 454 Pituitary body, 466 Placenta, 714 Plasma of blood, composition of, 240 Pleura, 390 Pneumatograph, 405 Pneumogastric nerve, 606 Pneumograph, 403 Polar stimulation, iig of human nerves, 121 Pons varolii, 522 functions of, 523 Portal vein, 227 Postures, 89 Presbyopia, 673 Prosecretin, 197 Protamins, 16 75° INDEX. Proteins, 14 chemic composition, 14 color reactions, 21 physical properties, 15 precipitation tests, 21 structure of, 14 Ptyalin, 159 Pulmonary vascular apparatus, 370 ventilation, 409 Pulse, 362 frequency, 363 wave, velocity of, 364 Punctum proximum, 671 remotum, 671 Pyramidal tracts of spinal cord, 498, 501 Reaction of degeneration, 124 Red corpuscles, 242 chemic composition of, 252 effects of reagents, 248 function of, 251 life history of, 251 number of, 246 of vertebrated animals, 249 Reduced hemoglobin, 258 Reflex action, 125, 506 laws of, 509 Refractory period of the heart, 307 Regnault's and Reisset's respiration ap- paratus, 421 Relation of gases in the blood, 410 Rennin, 178 Reproduction, 701 Reproductive organs of the female, 701 Reproductive organs of the male, 706 Reserve air, 404 Residual air, 404 Respiration, 382 changes in composition of air during, 406 changes in composition of blood, 409 changes in tissues, 415 chemistry of, 406 expiratory forces and muscles, 398 frequency of, 402 mechanism of gaseous exchange, 417 nerve mechanism of, 427 Respiration, total respiratory exchange, 419 volumes of air breathed, 403 Respiratory apparatus, 382 movements, 395 muscles, 395 effects of, on arterial pressure, 434 effects of, on the flow- of blood through the thoracic vessel, 434 of upper air passages, 401 pressures, 392 quotient, 408, 422 rhythm, 402 modification of, 429 Cheyne-Stokes, 427 sounds, 405 types, 402 Retina, 653 functions of, 677 Retinal image, 658 size of, 664 Rheocord, 726 Rigor mortis, 80 Rima glottidis, 626 respiratoria, 632 vocalis, 632 Routes of the absorbed food, 227 Saccharose, 11 Saliva, 154 physiologic action of, 158 Salivary glands, 152 histologic changes in, during secre- tion, 157 nerve mechanism of, 160 Sebaceous glands, 489 Sdero-proteins, 17 Sebum, 489 Secretin, 197 Secretion, 446 internal, 462 Semen, 707 Semicircular canals, 575 Sensor areas of human brain, 555 of monkey brain, 549 Serum, 240 Setchenow's center, 477 Sight, sense of, 650 Skeleton, physiology of, 48 Skin, 486 nerve endings in, 603 reflexes, 509 Sleep, 565 Smell, sense of, 648 Sodium glycocholate, 203 Sodium taurocholate, 203 Spectroscope, 115 Speech, 635 Spermatozoa, 708 Spheno-palatine ganglion, 624 Sphygmograph, 364 Sphygmomanometer, 355, 356 Spinal accessory nerve, 6r2 cord, 494 encephalo-spinal conduction, 515 functions of, 503 as a conductor, 513 as an independent center, 504 nerve-cells, classification of, 497 nerve fibres of, 498 classification of, 463, 498 reflex actions of, 506 reflex irritabiHty of, 511 relation of spinal nerves to, 501 segmentation of, 503 spinal nerve roots, functions of, 502 spino-encephalic conduction, 514 structure of gray matter, 495 structure of white matter, 498 tracts of, 499 INDEX. 7SI Spirometer, 404 Splanchnic nerves, 622 Spleen, 468 functions of, 469 Stanton's sphygmomanometer, 355 Stapes, 691 Starch, 8 digestion of, 158 Starvation, 136 Stearin, 12 Stereognostic area, 557 Stomach, 171 movements of, 186 nerve mechanism of, 189 Suprarenal capsules, 467 Sweat-glands, 487 Sweat, influence of nerve system on pro- duction of, 488 Sympathetic nerve system, 616 cephalic ganglia of, 624 functions of the cervical por- tions, 621 functions of the lumbosacral portions, 623 functions of the thoracic por- tion, 622 Taste buds, 647 nerve of, 646 sense of, 646 Teeth, 147 Tegmentum, 488 Temperature of body, 437 regulation of, 443 < sense, 643 Tendon reflexes, 510 Tension of gases in blood, 414 tissues, 417 Tensor tympani muscle, 691 functions of, 697 Testicles, 706 Tetanus, 75 experimental, 79 pathologic, 79 pharmacologic, 79 physiologic, 78 Thoracic duct, 217 Thorax, 388 dynamic condition of, 394 mechanic movements of, 391 static condition of, 392 Thyroid gland, 463 functions of, 463 Tidal air, 404 Tissue spaces, 213 Tongue, 646 Total carbon-dioxid exhaled, 422 oxygen absorbed, 422 respiratory exchange, 419 Touch, sense of, 639 Trachea, 384 Tracts of spinal cord, 465 Traube-Hering waves, 435 Trigeminal nerve, 592 Trypsin, 198 Tiirck, column of, 498 Tympanum, 689 Umbilical cord, 713 Upper air-passages, respiratory move- ments of, 401 Urea, 473 seat of formation, 462, 474 Uric acid, 474 Urine, 472 composition of, 473 mechanism of secretion, 479 influence of blood composition, 484 influence of nerve system, 483 relation of blood-pressure to, 48 Urination, 484 nerve mechanism of, 485 Uterus, 702 Vagus nerve, 606 influence on heart, 314, 317 Valves of heart, 278 ' Vasa deferentia, 706 Vascular apparatus, 326 glands, 462 hydrodynamic considerations, 330, 333 stream bed, 336 nerve mechanism of, 372 Vaso-motor center, 376 direct stimulation, 377 nerves, 373 reflex stimulation, 378 Veins, 313 structure and function, 329 Velocity of blood, 357, 359 Venous circulation, 369 Vertebral column, 51 Vesiculae seminales, 706 Villi, 223 functions of, 224 Visceral muscle, 90 functions of, 93 properties of, gi Vision, 650 accommodation, 667 astigmatism, 675 binocular, 680 color perception, 684 functions of retina, 677 hypermetropia, 674 myopia, 673 presbyopia, 673 Visual angle, 664 Vital capacity of lungs, 404 Vocal bands, 630 sounds, 633 Voice and speech, 635 Volume pulse, 366 Walking, 90 Wallerian degeneration, 106 Water, amount of, in the body, 22 Watery vapor in breath, 408 752 INDEX. Wernicke's pupiUary reaction, 590 YeUow spot, 6