UNIVERSITY OF ILLINOIS LIBRARY Class Book Volume REMOTE STORAGE r Digitized by the Internet Archive in 2015 https://archive.org/details/kirkeshandbookofOOkirk_0 ABSORPTION SPECTRA 1. Spectrum of ATgaiid-lamTj wth FranTiliofefs lines in position, 2. Blood; I.e. a strong solution of OxyhcBraoglobm ^cieduced. Haimoglo'bm. 5. BIcod more dilute. 4. Reduced HffimoglolDin. 5. Carlson Monoxide compound. 6. Acid tfematin. " 7. A].kaline Hasmatin, 8. Sulphuretted Hydrogen conipouud, 9. Ox-bile acidulated with Acetic acid and colo-urmg matter dissolved in Chloroforia. S/H-ttni (Innv/i /ht/n obst'n'df to/is /ly Jfr.W. /.rpjuiik, /"W. S. KIEKES' HAITD-BOOK OF PHYSIOLOGY '^H-^ HAND-BOOK OF PHYSIOLOGY W. MOEEAISTT BAKEE, F.E.C.S. SURGEON TO ST. BARTHOLOMEW'S HOSPITAL AND CONSULTING SURGEON TO THE EVELINA HOSPITAL FOR SICK CHILDREN; LECTURER ON PHYSIOLOGY AT ST. BARTHOLOMEW'S HOSPITAL, ANP LATE MEMBER OF THE BOARD OF EXAMINERS OF THE ROYAL COLLEGE OF SURGEONS OF ENGLAND. VINCENT DORMEE HARRIS, M.D., Loiro. DEMONSTRATOR OF PHYSIOLOGY AT ST. BARTHOLOMEW'S HOSPITAL. ELEVENTH EDITION WITH NEARLY BOO ILLUSTRATIONS NEW YOEK WILLIAM WOOD & COMPANY 56 & 58 Lafayette Place 18 86 ;^ .G3 The Publishers' Book Composition and Electrotyping Co., 39 AND 41 Park Place, New York. REMOTE STORAGE PREFACE TO THE ELEVENTH EDITION. In the preparation of the present edition of Kirkes' Physiology, we have endeavored to maintain its character as a guide for stu- dents, especially at an early period of their career; and, while incorporating new facts and observations which are fairly estab- lished, we have as far as possible omitted the controvertible matters which should only find a place in a complete treatise or in a work of reference. A large number of new illustrations have been added, for many of which we are indebted to the courtesy of Dr. Klein, Professor Michael Foster, Professor Schaefer, Dr. Mahomed, Mr. Gant, and Messrs. McMillan, who have been so good as to allow various figures to be copied. Our thanks are also due to Mr„ Wm. Lapraik, F.C.S., who has kindly prepared a table of the absorption spectra of the blood and bile, based upon his own observations; as well as to Mr. S. K. Alcock for several careful drawings of microscopical prepara- tions, and for reading several sheets in their passage through the press. Mr. Danielsson, of the firm of Lebon & Co., has executed all the new figures to our entire satisfaction ; and for the skill and labor he has expended upon them we are much indebted to him. We are desirous also of acknowledging the help we have derived from the following works : Klein's Histology ; M. Foster's Text-Book of Physiology; Pavy's Food and Dietetics; Quain's Anatomy, YoL II., Ed. ix. ; Wickham Legg's Bile, Jaundice, and Bilious Diseases; iv PREFACE. Watney's Minute Anatomy of the Thymus ; Rosenthal's Muscles and Nerves ; Cadiat's Traits D'Anatomie G6n^rale ; Ranvier's Traits Technique D'Histologie ; Landois' Lehrbuch der Physiologie des Mensclien, and the Journal of Physiology. \ W. MORRANT BAKER. V. D, HARRIS. WiMPOLE StKEET, August, 1884. CHAPTEE I. PAGK The General and Distinctive Characters op Living Beings . . 1 CHAPTER IL Structural Basis of the Human Body 5 Cells 5 Protoplasm 6 Nucleus 10 Intercellular Substance 17 Fibres 17 Tubules 17 CHAPTER III. Structure of the Elementary Tissues .19 Epithelium 19 Connective Tissues 38 Areolar Tissue 31 White Fibrous Tissue 31 Yellow Elastic Tissue 32 Gelatinous . 33 Retiform or Adenoid • . . . .34 Neuroglia 34 Adipose 35 Cartilage 38 Bone , ... 43 Teeth 55 CHAPTER IV. The Blood 63 Quantity of Blood 63 Coagulation of the Blood 65 Conditions affecting Coagulation . .71 The Blood Corpuscles . . .74 vi CONTENTS. The Blood — Continued. PAGE Physical and Chemical Characters of Red Blood-Cells .... 75 The White Corpuscles, or Blood-Leucocytes 79 Chemical Composition of the Blood . , 83 The Serum . 85 Variations in Healthy Blood under Different Circumstances ... 86 Variations in the Composition of the Blood in Different Parts of the Body 87 Gases contained in the Blood • • .88 Blood-Crystals 91 Development of the Blood 96 Uses of the Blood 99 Uses of the various Constituents of the Blood . . . , . .99 CHAPTEE V. Circulation of the Blood 101 The Systemic, Pulmonary, and Portal Circulations 102 The Forces concerned in the Circulation of the Blood .... 103 The Heart 103 Structure of the Valves of the Heart . . . . . . . . Ill The Action of the Heart Ill Function of the Valves of the Heart . . 112 Sounds of the Heart ........... 117 Impulse of the Heart . 119 The Cardiograph 119 Frequency and Force of the Heart's Action 122 Influence of the Nervous System on the Action of the Heart . . . 124 Effects of the Heart's Action .127 The Arteries, Capillaries, and Veins 128 Structure of the Arteries 129 Structure of Capillaries " . . . . 133 Structure of Veins . .136 Function of the Arteries . 138 The Pulse 142 Sphygmograph . ' 143 Pressure of the Blood in the Arteries, or Arterial Tension . . . 148 The Kymograph 150 Influence of the Nervous System on the Arteries . , . . . 152 Circulation in the Capillaries 158 Diapedesis of Blood-Corpuscles 159 Circulation in the Veins . , IGl TBlood-pressure in the Veins , . . . 162 Velocity of the Circulation 163 Velocity of the Blood in the Arteries 164 Capillaries 165 *' " " " Veins 165 Velocity of the Circulation us a whole ...... ^ 166 CONTENTS. vii PAGE Peculiarities of the Circulation in Different Parts .... 167 Circulation in the Brain . . .167 Circulation in the Erectile Structures 168 Agents concerned in the Circulation 170 Discovery of the Circulation 170 Proofs of the Circulation of the Blood 171 CHAPTER VI. Respiration . . . 173 Position and Structure of the Lungs 173 Structure of the Trachea and Bronchial Tubes 176 Structure of Lobules of the Lungs 178 Mechanism of Respiration . . 183 Respiratory Movements . . 183 Respiratory Rhythm 188 Respiratory Sounds 188 Respiratory Movements of Glottis 188 Quantity of Air Respired 189 Vital or Respiratory Capacity . . 190 Force exerted in Respiration 191 Circulation of Blood in the Respiratory Organs 191 Changes of the Air in Respiration 192 Changes produced in the Blood by Respiration 198 Mechanism of various Respiratory Actions . . . ^ . . 198 Influence of the Nervous System in Respiration . . . * . . . 201 Effects of Vitiated Air— Ventilation 204 Effect of Respiration on the Circulation 205 Apnoea— Dyspnoea— Asphyxia ' . . 209 CHAPTER VII. Foods 212 Classification of Foods 213 Foods containing chiefly Nitrogenous Bodies ..... 214 Carbohydrate Bodies 216 Fatty Bodies. : . . . . .217 Substances supplying the Salts 217 Liquid Foods 217 Effects of Cooking 217 Effects of an Insufficient Diet 218 Starvation 219 Effects of Improper Food 221 Effects of too much Food 222 Diet Scale 223 CHAPTER VIII. Digestion 224 Passage op Food through the Alimentary Canal 224 Mastication 224 Insalivation 226 viii CONTENTS. Passage op Food, '^tc— Continued. PAGE The Salivary Glands and the Saliva 226 Structure of the Salivary Glands 226 The Saliva 229 Influence of the Nervous System on the Secretion of Saliva . . .231 The Pharnyx 236 The Tonsils 236 The (Esophagus or Gullet . . . . 236 Swallowing or Deglutition 238 Digestion op Food in the Stomach . .• . . . , . . 240 Structure of the Stomach 241 Gastric Glands 242 The Gastric Juice 245 Functions of the Gastric Juice 247 Movements of the Stomach 249 Vomiting 251 Influence of the Nervous System on Gastric Digestion .... 252 Digestion of the Stomach after Death 253 Digestion in the Intestines 254 Structure of the Small Intestine 254 Valvulae Conniventes 255 Glands of the Small Intestine ......... 257 The Villi 259 Structure of the Large Intestine 262 The Pancreas and its Secretion . . . 264 Structure of the Liver 268 Functions of the Liver 273 The Bile 273 The Liver as a Blood-elaborating Organ 280 Glycogenic Function of the Liver 280 Summary of the Changes which take place in the Food during its Passage through the Small Intestine 284 Succus Entericus 283 Summary of the Process of. Digestion in the Large Intestine . . . 286 Defeecation 288 Gases contained in the Stomach and Intestines 288 Movements of the Intestines 289 Influence of the Nervous System on Intestinal Digestion .... 290 CHAPTER IX. Absokptton 291 . The Lacteal and Lymphatic Vessels and Glands 291 Lym])hati(; Glands 297 Properties of Lymph and Chyle 301 Absorption by tlie Lacteal Vessels 303 Absorption by tlic Lymphatic Vessels 303 Absorption by Bioodrvesscls 305 CONTENTS. IX CHAPTER X. PAGE Animal Heat 309 Variations in Bodily Temperature 309 Sources of Heat 311 Loss of Heat 313 Production of Heat 315 Inhibitory Heat-centre 316 CHAPTEE XL Secretion 317 Secreting Membranes 319 Serous Membranes 319 Mucous Membranes . . 321 Secreting Glands • . . . . 322 Process of Secretion 324 Circumstances influencing Secretion . . . . . . . 326 Mammary Glands and their Secretion 328 Chemical Composition of Milk 331 CHAPTEE XIL The Skin and its Functions 333 Structure of the Skin • • • .333 Sudoriparous Glands 337 Sebaceous Glands . . " . . . . 339 Structure of Hair . . . .339 Structure of Nails 341 Functions of the Skin 342 CHAPTEE XIII. The Kidneys and Urine 347 Structure of the Kidneys 347 Structure of the Ureter and Urinary Bladder 354 The Urine 355 The Secretion of Urine • .... 365 Micturition . . , 373 ♦ r I i I I ( CHAPTER I. THE GENERAL AND DISTINCTIVE CHARACTERS OF LIVING BEINGS. Human Physiology is the science which treats of the life of man — of the way in which he lives^ and moves, and has his being. It teaches how man is begotten and born; how he attains maturity; and liow he dies. Having, then, man as the object of its study, it is unnecessary to speak here of the laws of life in general, and the means by which they are car- ried out, further than is requisite for the more clear understanding of those of the life of man in particular. Yet it would be impossible to understand rightly the working of a complex machine without some knowledge of its motive power in the simplest form; and it may be well to see first what are the so-called essentials of life — those, namely, which are manifested by all living beings alike, by the lowest vegetable and the highest animal — before proceeding to the consideration of the structure and endowments of the organs and tissue belonging to man. The essentials of life are these, — Birth, Groivtli and Development,, Decline and Death. The term birth, when employed in this general sense of one of the conditions essential to life, without reference to any particular kind of living being, may be taken to mean, separation from a parent, with a. greater or less power of independent life. Taken thus, the term, although not defining any particular stage in development, serves well enough for the expression of the fact, to which no exception has yet been, proved to exist, that the capacity for life in all living beings is obtained, by inheritance. Growth, or inherent power of increasing in size, although essential to our idea of life, is not confined to living beings. A crystal of common salt, or of any other similar substance, if placed under appropriate condi- VoL. I.— 1. 2 HAND-BOOK OF PHYSIOLOGY. tions for obtaining fresh material, will grow in a fashion as definitely char- acteristic and as easily to be foretold as that of a living creature. It is, therefore, necessary to explain the distinctions which exist in this respect between living and lifeless structures; for the manner of growth in the two cases is widely different. Differences between Living and Lifeless Growth. — (1.) The growth of a crystal, to use the same example as before, takes place merely by additions to its outside; the new matter is laid on particle by particle, and layer by layer, and, when once laid on, it remains unchanged. The growth is here said to be superficial. In a living structure, on the other hand, as, for example, a brain or a muscle, where growth occurs, it is by addition of new matter, not to the surface only, but throughout every part of the mass; the growth is not superficial, but interstitial. (2.) All living structures are subject to constant decay; and life con- sists not, as once supposed, in the power of preventing this never-ceasing decay, but rather in making up for the loss attendant on it by never- ceasing repair. Thus, a man's body is not composed of exactly the same particles day after day, although to all intents he remains the same indi- vidual. Almost every part is changed by degrees; but the change is so gradual, and the renewal of that which is lost so exact, that no difference may be noticed, except at long intervals of time. A lifeless structure, -as a crystal, is subject to no such laws; neither decay nor repair is a necessary condition of its existence. That which is true of structures "which never had to do with life is true also with respect to those w^hich, though they are formed by living parts, are not themselves alive. Thus, an oyster-shell is formed by the living animal which it encloses, but it is as lifeless as any other mass of inorganic matter; and in accordance with this circumstance its growth takes place, not inter sf it i ally , but layer by layer, and it is not subject to the constant decay and reconstruction which belong to the living. The hair and nails are examples of the same fact. (3.) In connection with the growth of lifeless masses there is no alter- ation in the chemical constitution of the material which is taken up and added to the previously existing mass. For example, when a crystal of common salt grows on being placed in a fluid which contains the same material, the properties of the salt are not clianged by being taken out of the liquid by the crystal and added to its surface in a solid form. But the case is essentially different in living beings, botli animal and vegeta- ])le. A plant, like a crystal, can only grow wlicn fresh material is pre- sented to it; and this is absorbed by its leaves and roots; and auinuds, for the same purpose of getting new matter for growtli and nutrition, take food into tlieir stomachs. But in both tliese cases the nuitorials are much altered before tliey are finally assimilated by the structures they are destined to nourish. (4.) Tlie growtli of all living things has a dolinile limit, and the law DlSTmCTlVE ClIAKACTEKti OF LIVING BEINGS. 3 which governs this limitation of increase in size is so invariable that we should be as much astonished to find an individual x>iant or animal with- out limit as to growth as without limit to life. Development is as constant an accompaniment of life as growth. The term is used to indicate that change to which, before maturity, all living parts are constantly subject, and by which they are made more and more capable of performing their several functions. For example, a full-grown man is not merely a magnified child ; his tissues and organs have not only grown, or increased in size, they have also developed, or become better in quality. No very accurate limit can be drawn between the end of development and the beginning of decline; and the two processes may be often seen together in the same individual. But after a time all parts alike share in the tendency to degeneration, and this is at length succeeded by death. Differences between Plants and Animals. — It has been already said that the essential features of life are the same in all living things; in other words, in the members of both the animal and vegetable king- doms. It may be well to notice briefly the distinctions which exist be- tween the members of these two kingdoms. It may seem, indeed, a strange notion that it is possible to confound vegetables with animals, but it is true with respect to the lowest of them, in which but little is manifested beyond the essentials of life, whi^^h are the same in both. (1.) Perhaps the most essential distinction is the presence or absence of power to live upon inorganic material. By means of their green color- ing matter, c]iloroj)hyl — a substance almost exclusively confined to the vegetable kingdom, plants are capable of decomposing the carbonic acid, ammonia, and water, which they absorb by their leaves and roots, and thus utilizing them as food. The result of this chemical action, which occurs only under the influence of light, is, so far as the carbonic acid is concerned, the fixation of carbon in the plant structures and the exhala- tion of oxygen. Animals are incapable of thus using inorganic matter, and never exhale oxygen as a product of decomposition. The power of living upon organic as well as inorganic matter is less decisive of an animal nature; inasmuch as fungi and some other plants derive their nourishment in part from the former source. (2.) There is, commonly, a marked difference in general chemical composition between vegetables and animals, even in their lowest forms; for while the former consist mainly of cellulose, a substance closely allied to starch and containing carbon, hydrogen, and oxygen only, the latter are composed in great part of the three elements just named, together with a fourth, nitrogen; the chief proximate principles formed from these being identical, or nearly so, with albumen. It must not be sup- posed, however, that either of these typical compounds alone, with its allies, is <^-onfined to one kingdom of nature. Nitrogenous compounds 4 HAND-BOOK OF PHYSIOLOGY. are freely produced in vegetable structures, although they form a very much smaller proportion of the whole organism than cellulose or starch. And while the presence of the latter in animals is much more rare than is that of the former in vegetables, there are many animals in which traces of it may be discovered, and some, the Ascidians, in which it is found in considerable quantity. (3.) Inherent power of movement is a quality which we so commonly consider an essential indication of animal natare, that it is difficult at first to conceive it existing in any other. The capability of simple motion is now known, however, to exist in so many vegetable forms, that it can no longer be held as an essential distinction between them and animals, and ceases to be a mark by which the one can be distinguished from the other. Thus the zoospores of many of the Cryptogamia exhibit ciliary or amoeboid movements (p. 8) of a like kind to those seen in animalcules; and even among the higher orders of plants, many, e. g., Dioncea Mus- cipula (Venus's fly-trap), and Mimosa Sensitiva (Sensitive plant), exhibit such motion, either at regular times, or on the application of external irritation, as might lead one, were this fact taken by itself, to regard them as sentient beings. Inherent power of movement, then, although especially characteristic of animal nature, is, when taken by itself, no proof of it. (4.) The presence of a digestive canal is a very general mark by which an animal can be distinguished from a vegetable. But the lowest animals are surrounded by material that they can take as food, as a plant is surrounded by an atmosphere that it can use in like manner. And every part of their body being adapted to absorb and digest, they have no need of a special receptacle for nutrient matter, and accordingly have no digestive canal. This distinction then is not a cardinal one. It would be tedious as well as unnecessary to enumerate the chief dis- tinctions between the more highly developed animals and vegetables. They are sufficiently apparent. It is necessary to compare, side by side, the lowest members of the two kingdoms, in order to understand rightly how faint are the boundaries between them. CHAPTER II. STRUCTURAL BASIS OF THE HUMAN BODY. By dissection, the human body can be proved to consist of various dis- similar parts, bones, muscles, brain, heart, lungs, intestines, etc., while, on more minute examination, these are found to be composed of different tissues, such as the connective, epithelial, nervous, muscular, and the like. Cells. — Embryology teaches us that all this complex organization has been developed from a microscopic body about yl-^ in. in diameter (ovum), which consists of a spherical mass of jelly-like matter enclosing a smaller spherical body (germinal vesicle). Further, each individual tissue can be shown largely to consist of bodies essentially similar to an ovum, though often differing from it very widely in external form. They are termed cells : and it must be at once evident that a correct knowledge of the nature and activities of the cell forms the very foundation of physiology. Cells are, in fact, physiological no less than histological units. The prime importance of the cell as an element of structure was first established by the researches of Schleiden, and his conclusions, drawn from the study of vegetable histology, were at once extended by Schwann to the animal kingdom. The earlier observers defined a cell as a more or less spherical body limited by a membrane, and containing a smaller body termed a nucleus, which in its turn encloses one or more nucleoli. Such a definition applied admirably to inost vegetable cells, but the more extended investigation of animal tissues soon showed that in many cases no limiting membrane or cell-wall could be demonstrated. The presence or absence of a cell-wall, therefore, was now regarded as quite a secondary matter, while at the same time the cell-substance came gradually to be recognized as of primary importance. Many of the lower forms of animal life, e.g., the Rhizopoda,were found to consist almost entire- ly of matter very similar in appearance and chemical composition to the cell-substance of higher forms: and this from its chemical resemblance to flesh was termed Sarcode by Dujardin. When recognized in vegetable cells it was called Protoplasm by Mulder, while Remak applied the same name to the substance of animal cells. As the presumed formative mat- ter in animal tissues it was termed Blastema, and in the belief that, wherever found, it alone of all substances has to do with generation and 6 HAT^D-BOOK OF PHYSIOLOGY. nutrition, Beale lias named it Germinal matter or Bioplasm. Of these terms the one most in vogue at the present day is Protoplasm, and inas- much as all life, both in the animal and vegetable kingdoms, is associated with protoplasm, we are justified in describing it, with Huxley, as the ^'physical basis of life.*' A cell may now be defined as a nucleated mass of protoplasm,^ of microscopic size, which possesses sufiicient individuality to have a life- history of its own. Each cell goes through the same cycle of changes as the whole organism, though doubtless in a much shorter time. Begin- ning with its origin from some pre-existing cell, it grows, produces other cells, and finally dies. It is true that several lower forms of life consist of non-nucleated protoplasm, but the above definition holds good for all the higher plants and animals. Hence a summary of the manifestations of cell-life is really an account of the vital activities of protoplasm. Protoplasm. — P/^ ^6- /Vr// characters. — Physically, protoplasm is viscid, varying in consistency from semi-fluid to strongiyc oherent. Chemical characters. — Chemically, living protoplasm is an extremely unstable albu- minoid substance, insoluble in water. It is neutral or weakly alkaline in reaction. It undergoes heat stiffening or coagulation at about 130°F. (54'5'^C.), and hence no organism can live when its own temperature is raised beyond this point, though, of course, many can exist for a time in a much hotter atmosi^here, since they possess the means of regulating their own temperature. Besides the coagulation produced by heat, pro- toplasm is coagulated by all the reagents which produce this change in albumen. If not-living protoplasm be subjected to chemical analysis it is found to be made up of numerous bodies ^ besides albumen, e.g. , of gly- cogen, lecithin, salts and water, so that if living protoplasm be, as some believe, an independent chemical body, when it no longer possesses life, it undergoes a disintegration which is accompanied by the appearance of these new chemical substances. When it is examined under the micro- scope two varieties of protoplasm are recognized — the hyaline, and the granular. Both are alike transparent, but the former is perfectly homo- geneous, while the latter (the more common variety) contains small gran- ules or molecules of various sizes and shapes. Globules of watery fluid are also sometimes found in protoplasm; they look like clear spaces in it, and are hence called vacuoles. Vital or Pliysiological characters. — These may be conveniently treated uuder the three heads of — I. Motion; II. Nutrition; and III. Repro- duction. ' In the Immiin ])()(ly the cells rango from tlio red blood-cell (-^\y^ in.) to the gang- lion-cell ?, „) For jin ticcount of which, reference should be made to the Append|jx. STRUCTURAL BASIS OF THE HUMAN BODY. 7 ■{a) Fluent and (h) I. Motion. — It is probable that the protoplasm of all cells is capable at some time of exhibiting movement; at any rate this phenomenon, which not long ago was regarded as quite a curiosity, has been recently observed in cells of many different kinds. It may be readily studied in the Amoebge, in the colorless blood -cells of all vertebrata, in the branched cornea-cells of the frog, in the hairs of the stinging-nettle and Trades- cantia, and the cells of Vallisneria and Chara. These motions may be divided into two classes- Ciliary. Another variety — the molecular or vibratory — has also been classed by some observers as vital, but it seems exceedingly probable that it is nothing more than the well-known "Brownian^^ molecular movement, a purely mechanical phenomenon which may be observed in any minute particles, e.g., of gamboge, suspended in a fluid of suitable density, such as water. Such particles are seen to oscillate rapidly to and fro, and not to pro- gress in any definite direction. (a.) Fluent. — This movement of protoplasm is rendered perceptible (1) by the motion of the granules, which are nearly always imbedded in it, and (2) by changes in the outline of its mass. If part of a hair of Tradescantia (Fig. 1) be viewed under a high magni- fying power, streams of protoplasm con- taining crowds of granules hurrying along, like the foot passengers in a busy street, are seen flowing steadily in defi- nite directions, some coursing round the film which lines the interior of the cell- wall, and others flowing toward or away from the irregular mass in the centre of the cell-cavity. Many of these streams of protoplasm run together into larger ones, and are lost the central mass, and thus ceaseless variations of form are produced. In the Amoeba, a minute animal consisting of a shapeless and struc- tureless mass of sarcode, an irregular mass of protoplasm is gradually thrust out from the main body and retracted: a second mass is then pro- truded in another direction, and gradually the whole protoplasmic sub- stance is, as it were, drawn into it. The Amoeba thus comes to occupy a new position, and when this is repeated several times we have locomotion in a definite direction, together with a continual change of form. These movements when observed in other cells, such as the colorless blood- corpuscles of higher animals (Fig. 2) are hence termed ammhoid. Colorless blood-corpuscles were first observed to migrate, i.e., pass Fig. 1. — Cell of Tradescantia drawn at successive intervals of two minutes. The cell-contents consist of a central mass con- nected by many irregular processes to a peripheral film: the whole forms a vacuo- lated mass of protoplasm, which is continu- ally changing its shape. (Schofield.) m 8 HAND-BOOK OF PHYSIOLOGY. through the walls of the blood-vessels (p. 159), by Waller, whose obser- vations were confirmed and extended to connective tissue corpuscles by the researches of Recklinghausen, Oohnheim, and others, and thus the phenomenon of migration has been proved to play an important part in many normal, and pathological processes, especially in that of inflam- mation. This amoeboid movement enables many of the lower animals to capture their prey, which they accomplish by simply flowing round and enclosing it. The remarkable motions of pigment-granules observed in the branched pigment-cells of the frog^s skin by Lister are probably due to amoeboid movement. These granules are* seen at one time distributed uniformly through the body and branched processes of the cell, while under the action of various stimuli (e.g., light and electricity) they collect in the central mass, leaving the branches quite colorless. (b.) Ciliary action must be regarded as only a special variety of the general motion with which all protoplasm is endowed. The grounds for this view are the following: In the case of the Infu- soria, which move by the vibration of cilia (microscopic hair-like processes projecting from the surface of their bodies) it has been proved that these are simply processes of their protoplasm protruding through pores of the Fig. 2.— Human colorless blood-corpuscle, showing its successive changes of outline within ten minutes when kept moist on a warm stage. (Schofield.) investing membrane, like the oars of a galley, or the head and legs of a tortoise from its shell: certain reagents cause them to be partially re- tracted. Moreover, in some cases cilia have been observed to develop from, and in others to be transformed into, amoeboid processes. The movements of protoplasm can be very largely modified or even suspended by external conditions, of which the following are the most important. 1. Changes of temperature. — Moderate heat acts as a stimulan^: tliis is readily observed in the activity of the movements of a human colorless blood-corpuscle when placed under conditions in which its normal tem- perature and moisture are preserved. Extremes of heat and cold stop the motions entirely. 2. Meclumical stimnh. — When gently squeezed between a cover and object glass under proper conditions, a colorless blood-corpuscle is stimu- lated to active amoeboid movement. 3. Nerve iiiflnence. — By stimulation of the nerves of tlie frog's cornea, contraction of certain of its branclied cells lias been i)rodueed. 4. Chemical stimuli. — Water generally stojjs anuvboid movement, ami by inil)il)iti()n causes great swelling ;uul fnially bursting of the cells. STRUCTURAL BASIS OF THE HUMAN BODY. 9 In some cases, however, (myxomycetes) protoplasm can he almost enth-ely dried up, and is yet capable of renewing its motions when again moistened. Dilute salt-solution and many dilute acids and alkalies, stimulate the movements temporarily. Ciliary movement is suspended in an atmosphere of hydrogen or car- bonic acid, and resumed on the admission of air or oxygen. 5. Electrical. — Weak currents stimulate the movement, while strong currents cause the corpuscles to assume a spherical form and to become motionless. II. Nutrition. — The nutrition of cells will be more appropriately described in the chapters on Secretion and Nutrition. Before describing the Keproduction of cells it will be necessary to con- sider their structure more at length. Minute Structure of Cells.— (a.) — We have seen (p. 5) that the presence of a limiting-membrane is no essential part of the defini- tion of a cell. In nearly all cells the outer layer of the protoplasm attains a firmer consistency than the deeper portions: the individuality of the cell be- coming more and more clearly marked as this cortical layer becomes more and more differentiated from the deeper portions of cell-substance. Side by side with this physical, there is a gradual chemical differentiation, till at length, as in the case of the fat-cells, we have a definite limiting-mem- brane differing chemically as well as physically from the cell-contents, and remaining as a shriveled-up bladder when they have been removed. Such a membrane is transparent and structureless, flexible, and per- meable to fluids. The cell-substance can, therefore, still be nourished by imbibition thr(fugh the cell- wall. In many cases (especially in fat) a membrane of some toughness is absolutely necessary to give to the tissue the requisite consistency. When these membranes attain a certain degree of thickness and independence they are termed capsules: as examples, we may cite the capsules of cartilage-cells, and the thick, tough envelope of the ovum termed the ' 'primitive chorion. (b.) Cell contents. — In accordance with their respective ages, positions, and functions, the contents of cells are very varied. The original protoplasmic substance may undergo many transforma- tions; thus, in fat-cells we may have oil, or fatty crystals, occupying nearly the whole cell-cavity: in pigment-cells we find granules of pig- ment; in the various gland-cells the elements of their secretions. Moreover, the original protoplasmic contents of the cell may undergo a gradual chemical change with advancing age; thus the protoplasmic cell- substance of the deeper layers of the epidermis becomes gradually con- verted into keratin as the cell approaches the surface. So, too, the orig- 10 HAND-BOOK OF PHYSIOLOGY. inal protoplasm of the embryonic blood-cells is replaced by the haemo- globin of the mature colored blood-corpuscle. The minute structure of cells has lately been made the subject of care- ful investigation, and what was once regarded as homogeneous proto- plasm with a few scattered granules, has been stated to be an exceedingly complex structure. In colorless blood-corpuscles, epithelial cells, con- nective tissue corpuscles, nerve-cells, and many other varieties of cells, an intracellular netiuorlc of very fine fibrils, the meshes of which are occupied by a hyaline interstitial substance, has been demonstrated (Heitzmann^s network) (Fig. 3). At fee nodes, where the fibrils cross, are little swellings, and these are the objects described as granules by the older observers: but in some cells, e.g., colorless blood corpuscles, there are real granules, which appear to be quite free and unconnected with the intra-cellular network. (c.) Nucleus. — Nuclei (Fig. 3) were first pointed out in the year 1833, by Robert Brown, who observed them in vegetable cells. They are either Pi&. 3. — (a). Colorless blood-corpuscle showing intra-cellular network of Heitzmann, and two nuclei with intra-nuclear network. (Klein and Noble Smith.) (B.) Colored blood-corpuscle of newt showing intra-ceUular network of fibrils (Heitzmann). Also oval nucleus composed of hmiting-membrane and fine intra-nuclear network of fibrils, x 800. (lOein and Noble Smith.) small transparent vesicular bodies containing one or more smaller particles (nucleoli), or they are semi-solid masses of protoplasm always in the resting condition bounded by a well-defined envelope. In their relation to the life of the cell they are certainly hardly second in importance to the protoplasm itself, and thus Beale is fully justified in comprising both under the term ^'germinal matter." They exhibit their vitality by ini- tiating the process of division of the cell into two or more cells (fission) by first themselves dividing. Distinct observations have been made show- ing that spontaneous changes of form may occur in nuclei as also in nu- cleoli. Histologists have long recognized nuclei by two important charac- ters: — (1.) Their poAver of resisting the action of various acids and alkalies, particularly acetic acid, by which their outline is more clearly deliiiod, and they are rendered more easily visible. This indicates some chemical STKUCTUKAL BASIS OF THE HUMAN BODY. 11 difference between the protoplasm of the cell and nuclei, as the former is destroyed by these reagents. (2.) Their quality of staining in solutions of carmine, hsematoxylin, etc. Nuclei are most commonly oval or round, and do not generally conform to the diverse shapes of the cells; they are altogether less varia- ble elements than cells, even in regard to size, of which fact one may see a good example in the uniformity of the nuclei in cells so multiform as those of epithelium. But sometimes nuclei appear to occupy the whole of the cell, as is the case in the lymph corpuscles of lymphatic glands and in some small nerve cells. Their position in the cell is very variable. In many cells, especially where active growth is progressing, two or more nuclei are present. The nuclei of many cells have been shown to contain a fine intra- nuclear networh in every respect similar to that described above as intra- cellular (Fig. 3), the interstices of which are occupied by semi-fluid pro- toplasm. III. Reproduction. — The life of individual cells is probably very short in comparison with that of the organism they compose: and their constant decay and death necessitate constant reproduction. The mode in which this takes place has long been the subject of great controversy. In the case of plants, all of whose tissues are either cellular or com- posed of cells which are modified or have coalesced in various ways, the theory that all new cells are derived from pre-existing ones was early ad- vanced and very generally accepted. But in the case of animal tissues Schwann and others maintained a theory of spontaneous or free cell for- mation. According to this view a minute corpuscle (the future nucleolus) springs up spontaneously in a structureless substance (blastema) very much as a crystal is formed in a solution. This nucleolus attracts the surround- ing molecules of matter to form the nucleus, and by a repetition of the process the substance and wall are produced. This theory, once almost universally current, was first disputed and finally overthrown by Eemak and Virchow, whose researches established the truth expressed in the words "Omnis cellula e cellula.-'^ It will be seen that this view is in strict accordance with the truth established much earlier in Vegetable Histology that every cell is de- scended from some pre-existing (mother-) cell. This derivation of cells from cells takes place by (1) gemmation, or {2) fission or divisioji. (1.) Gemmation. — This method has not been observed in the human body or the higher animals, and therefore requires but a passing notice. It consists essentially in the budding off and separating of a portion of the parent cell. (2.) Fission or Divisio7i. — As examples of reproduction by fission, we may select the ovum, the blood cell, and cartilage cells. 12 HAND-BOOK OF PHYSIOLOGY. In the frog's ovum (in which the process can be most readily ob- served) after fertilization has taken place, there is first some amoeboid movement, the oscillation gradually increasing until a permanent dimple appears, which gradually extends into a furrow running completely round the spherical ovum, and deepening until the entire yelk-mass is divided into two hemispheres of protoplasm each containing a nucleus (Fig. 4, h). This process being repeated by the formation of a second furrow at right angles to the first, we have four cells produced (c): this subdivision is Fig. 4.— Diagram of an ovum (a) undergoing segrnentation. In (6) it has divided into two; in (c) into four; in (d) the process has ended in the production of the so-called " mulberry mass." (Frey.) carried on till the ovum has been diyided by segmentation into a mass of cells (mulberry-mass) {d) out of which the embryo is developed. Segmentation is the first step in the development of most animals, and doubtless takes place in man. Multiplication by fission has been observed in the colorless blood-cells of many animals. In some cases (Fig. 5), the process has been seen to commence with the nucleolus which divides within the nucleus. The nucleus then elongates, and soon a well-marked constriction occurs, ren- dering it hour-glass shaped, till finally it is separated into two parts, which gradually recede from each other: the same process is repeated in the cell-substance, and at length we have two cells produced which by Fig. 5.— Blood-corpuscle from a young deer embryo, multiplying by fission. (Frey.) rapid growth soon attain the size of the parent cell {direct division). In some cases there is a primary fission into three instead of the usual two cells. In cartilage (Fig. 6), a process essentially similar occurs, with the ex- ception that (as in the ovum) the cells produced by fission remain in the original capsule, and in their turn undergo division, so that a large num- ber of cells are sometimes observed within a common envelope. This process of fission within a capsule has been by some described as a separate method, under the title * 'endogenous fission," but there seems to bene sufiicient reason for drawing such a distinction. It is important to observe that fission is ofliMi accomplished with great rapidity, the whole process occupying but a few jninutes, hence the com- ])arative rarity with Avhich cells are seen in the act of dividing. STRUCTURAL BASIS OF THE HUMAN BODY. 13 Indirect cell division. — In certain and numerous cases the division of cells does not take place by the simple constriction of their nuclei and surrounding protoplasm into two parts as above described (direct division), but is preceded by complicated changes in their nuclei (karyokinesis). Fig. 6.— Diagram of a cartilage cell undergoing fission within its capsule. The process of divi- sion is represented as commencing in the nucleolus, extending to the nucleus, and at length involving the body of the cell. (Frey.) These changes consist in a gradual re-arrangement of the intranuclear net- work of each nucleus, until two nuclei are formed similar in all respects to the original one. The nucleus in a resting condition, i.e., before any changes preceding division occur, consists of a very close meshwork of fibrils, which stain deeply in carmine, imbedded in protoplasm, which does not possess this property, the whole nucleus being contained in an envelope. The first change consists of a slight enlargement, the disap- pearance of the envelope, and the increased definition and thickness of Fig. 7.— Karyokinesis. a, ordinary nucleus of a columnar epithelial cell; b, c, the same nucleus in the stage of convolution; d, the wreath or rosette form; e, the aster or single star; f, a nuclear spin- dle from the Descemefs endothelium of the frog's cornea: g, h, i, diaster; k, two daughter nuclei. (Klein.) the nuclear fibrils, which are also more separated than they were and stain better. This is the stage of convolution (Fig. 7, b, c). The next step in the process is the arrangement of the fibrils into some definite figure by an alternate looping in and out around a central space, by which means 14 HAISTD-BOOK OF PHYSIOLOGY. the rosette or wreath stage (Fig. 7, d) is reached. The loops of the rosette next become divided at the periphery, and their central points become more angular, so that the fibrils, divided into portions of about equal length, are, as it were, doubled at an acute angle, and radiate V-shaped from the centre, forming a star (aster) or wheel (Fig. 7, e), or perhaps from two centres, in which case a double star (diasterj results (Fig. 7, G, H, and i). After remaining almost unchanged for some time, the V-shaped fibres being first re-arranged in the centre, side by side (angle outward), tend to separate into two bundles, which gradually assume posi- tion at either pole. From these groups of fibrils the two nuclei of the new cells are formed (daughter nuclei) (Fig. 7, k), and the changes they pass through before reaching the resting condition are exactly those through which the original nucleus (mother nucleus) has gone, but in a reverse order, viz., the star, the rosette, and the convolution. During or shortly after the formation of the daughter nuclei the cell itself be- comes constricted, and then divides in a line about midway between them. Functions of Cells. — The functions of cells are almost infinitely varied and make up nearly the whole of Physiology. They will be inore appro- priately considered in the chapters treating of the several organs and sys- tems of organs which the cells compose. Decay and Death of Cells. — There are two chief ways in which the comparatively brief existence of cells is brought to an end. (1) Mechani- cal abrasion, (2) Chemical transformation. 1. The various epithelia furnish abundant examples of mechanical abrasion. As it approaches the free surface the cell becomes more and more flattened and scaly in form and more horny in consistence, till at length it is simply rubbed off. Hence we find epithelial cells in the mucus of the mouth, intestine, and genito-urinary tract. 2. In the case of chemical transformation the cell-contents undergo a degeneration which, though it may be pathological, is very often a normal process. Thus we have (a.) fatty metamorphosis producing oil-globules in the secretion of milk, fatty degeneration of the muscular fibres of the uterus after the birth of the foetus, and of the cells of the Graafian follicle giving rise to the ''corpus luteum.'^ (See chapter on Generation.) (b.) Pigmentary degeneration from deposit of pigment, as in the epi- thelium of the air-vesicles of the lungs. (c.) Calcareous degeneration which is common in the cells of many cartilages. Having thus reviewed the life-history of cells in general, we may now discuss tlie leading van(^tiort of form wliich they ])resent. In passing, it may b(^ well to i)()int out tlie mw'm disl i)u-Ho)is hcfirooi anintaJ (itnl cvijctaldv cells. STKUCTUKAL BASIS OF THE HUMAN BODY. 15 It lias been already mentioned that in animal cells an envelope or cell- wall is by no means always present. In adult vegetable cells, on the other hand, a well-defined cellulose wall is highly characteristic; this, it should be observed, is non-nitrogenous, and tiius differs chemically as well as structurally from the contained mass. Moreover, in vegetable cells (Fig. 8, b), the protoplastic contents of th3 cell fall into two subdivisions: (1) a continuous film which lines the interior of the cellulose wall; and (2) a reticulate mass containing the Fig. 8.— (a). Young vegetable cells, showing cell-cavity entirely filled with granular protoplasm enclosing a large oval nucleus, with one or more nucleoli. (b.) Older cells from the same plant, showing distinct celltdose-wall and vacuolation of proto- plasm, nucleus and occupying the cell-cavity; its interstices are filled with fluid. In young vegetable cells such a distinction does not exist; a finely gran- ular protoplasm occupies the whole cell-cavity (Fig. 8, a). Another striking difference is the frequent presence of a large quan- tity of intercellular substance in animal tissues, while in vegetables it is comparatively rare, the requisite consistency being given to their tissues by the tough cellulose walls, often thickened by deposits of lignin. In animal cells this end is attained by the deposition of lime-salts in a matrix of intercellular substance, as in the process of ossification. Forms of Cells. — Starting with the spherical or spheroidal (Fig. 9, d) as the typical form assumed by a free cell, we find this altered to a poly- hedral shape when the pressure on the cells in all directions is nearly the same (Fig. 9, l). Of this, the primitive segmentation-cells may afford an example. The discoid shape is seen in blood-cells (Fig. 9, c), and the scale-like Fig. 9.— Various forms of cells, a. Spheroidal, showing nucleus and nucleolus. 6. Polyhedral, c. Discoidal (blood-ceUs). d. Scaly or squamous (epithelial ceUs). form in superficial epithelial cells (Fig. 9, Some cells have a jagged outline (prickle-cells) (Fig. 13). Cylindrical, conical, or prismatic cells occur in the deeper layers of laminated epithelium, and the simple cylindrical epithelium of the intes- tine and many gland ducts. Such cells may taper off at one or both 16 HAND-BOOK OF PHYSIOLOGY. ends into fine processes, in the former case being caudate, in the latter fusiform (Fig. 10). They may be greatly elongated so as to become fibres. Ciliated cells (Fig. 10, d) must be noticed as a distinct variety: they possess, but only on their free surfaces, hair-like processes (cilia). These vary immensely in size, and may even exceed in length the cell itself. Finally we have the branched or stellate cells, of which the large Fig. 10.— Various forms of cells, a. Cylindrical or columnar, h. Caudate, c. Fusiform, d. Cilia- ted (from trachea), e. Branched, stellate. nerve-cells of the spinal cord, and the connective tissue corpuscle are typical examples (Fig. 10, e). In these cells the primitive branches by secondary branching may give rise to an intricate network of processes. Classification of Cells. — Cells may be classified in many ways. According to: — (a.) Form : They may be classified into spheroidal or polyhedral, dis- coidal, flat or scaly, cylindrical, caudate, fusiform, ciliated and stellate. (b.) Situation: — we may divide them into blood cells, gland cells, connective tissue cells, etc. (c.) Contents: — fat and pigment cells and the like. (d.) Function: — secreting, protective, contractile, etc. (e.) Origin : — hypoblastic, mesoblastic, and epiblastic cells. (See chapter on Generation.) It remains only to consider the various ways in which cells arc con- nected together to form tissues, and the transformations by which inter- cellular substance, fibres and tubules are produced. Modes of connection. — Cells are connected: — (1) By a cementing intercellular substance. This is probably always present as a transparent, colorless, viscid, albuminous substance, even between the closely apposed cells of cylindrical epithelium, while in the case of cartihigo it forms the main bulk of the tissue, and the cells only appear as imbedded in, not as cemented by, the intercellular substance. This intercellular substance may be either homogeneous or fibrillated. In many ceases (cj/. ci)vm\\) it can l)c shown to contain a number STRUCTURAL BASIS OF THE HUMAN BODY. 17 of irregular branched cavities, which communicate with each other, and in which the branched cells lie: through these branching spaces nutritive fluids can find their v/ay into the very remotest parts of a non-vascular tissue. As a special variety of intercellular substance must be mentioned the basement membrane {memhrana propria) which is found at the base of the epithelial cells in most mucous membranes, and especially as an in- vesting tunic of gland follicles which determines their shape, and which may persist as a hyaline saccule after the gland -cells have all been dis- charged. (2) By anastomosis of their processes. This is the usual way in which stellate cells, e.g., of the cornea, are united: the individuality of each cell is thus to a great extent lost by its connection with its neighbors to form a reticulum : as an example of a net- work so produced, Ave may cite the stroma of lymphatic glands. Sometimes the branched processes breaking up into a maze of minute fibrils, adjoining cells are connected by an intermediate, reticulum: this is the case in the nerve-cells of the spinal cord. Besides the Cell, which may be termed the primary tissue-element, there are materials which may be termed secondary or derived tissue- elements. Such are Intercellular substance. Fibres and Tubules. Intercellular substance is probably in all cases directly derived from the cells themselves. In some cases {e.g. cartilage), by the use of re- agents the cementing intercellular substance is, as it were, analyzed into various masses, each arranged in concentric layers around a cell or group of cells, from which it was probably derived (Fig. 6). Fibres. — In the case of the crystalline lens, and of muscle both stri- ated and non-striated, each fibre is simply a metamorphosed cell: in the case of striped fibre the elongation being accompanied by a multiplication of the nuclei. The various fibres and fibrillse of connective tissue result from a grad- ual transformation of an originally homogeneous intercellular substance. Fibres thus formed may undergo great chemical as well as physical trans- formation: this is notably the case with yellow elastic tissue, in which the sharply defined elastic fibres, possessing great power of resistance to re-agents, contrast strikingly with the homogeneous matter from which they are derived. Tubules which were originally supposed to consist of structureless, membrane, have now been proved to be composed of flat, thin cells, cohering along their edges. (See Capillaries.) With these simple materials the various parts of the body are built up; the more elementary tissues being, so to speak, first compounded of YoL. I.— 3. 18 HAND-BOOK OF PHYSIOLOaY. them; while these again are variously mixed and interwoven to form more intricate combinations. Thus are constructed epithelium and its modifications, connective tissue, fat, cartilage, bone, the fibres of muscle and nerve, etc. ; and these, again, with the more simple structures before mentioned, are used as materials wherewith to form arteries, veins, and lymphatics, secret? ug and vascular glands, lungs, heart, liver, and other parts of the body. CHAPTEK III. STRUCTURE OF THE ELEMENTARY TISSUES. Il^" this chapter the leading characters and chief modifications of two great groups of tissues — the Epithelial and Connective — will be briefly de- scribed; while the Nervous and Muscular, together with several other more highly specialized tissues, will be appropriately considered in the chapters treating of their physiology. Epithelium. Epithelium is composed of cells of various shapes held together by a small quantity of cementing intercellular substance. Epithelium clothes the whole exterior surface of the body, forming the epidermis with its appendages — nails and hairs; becoming continuous at the chief orifices of the body — nose, mouth, anus, and urethra — with the epithelium which lines the whole length of the alimentary and genito- urinary tracts, together with the ducts of their various glands. Epi- ^ thelium also lines the cavities of the brain, and the central canal of the spinal cord, the serous and synovial membranes, and the interior of all blood-vessels and lymphatics. The cells composing it may be arranged in either one or more layers, and thus it may be subdivided into {a) Simple and {h) Stratified or laminated Epithelium. A simple epithelium, for example, lines the whole intestinal mucous membrane from the stomach to the anus: the epidermis on the other hand is laminated throughout its entire extent. Epithelial cells possess an intracellular and an intranuclear network (p. 10). They are held together by a clear, albuminous, cement sub- stance. The viscid semi-fluid consistency both of cells and intercellular substance permits such changes of shape and arrangement in the individ- ual cells as are necessary if the epithelium is to maintain its integrity in organs the area of whose free surface is so constantly changing, as the stomach, lungs, etc. Thus, if there be but a single layer of cells, as in the epithelium lining the air vesicles of the lungs, the stretching of this membrane causes such a thinning out of the cells that they change their shape from spheroidal or short columnar, to squamous, and vice versa, when the membrane shrinks. 20 HAND-BOOK OF PHYSIOLOGY. Classification of Epithelial Cells. Epithelial cells may be conveniently classified as: 1. Sgimmous, scaly, pavement, or tessellated. 2. Spheroidal, glandular, or polyliedral. 3. Columnar, cylindrical, conical, or gohlet-sliaped, 4. Ciliated. ' ' 5. Transitional. Altliougli, for convenience, epithelial cells are thus classified, yet the first three forms of cells are sometimes met with at different depths in Fig. 11.— Vertical section of Rabbit's cornea, a. Anterior epithelium, shoeing the different shapes of the ceUs at vai-ious depths from the free surface, b. Portion of the substance of cornea. (Klein.) che same membrane. As an example of snch a laminated epithelium showing these different cell-forms at various depths, we may select the anterior epithelium of the cornea (Fig. 11). 1. Squamous Epithelium (Fig. 12). — Arranged (a) in several super- posed layers {stratified or laminated), this form of epithelium covers {a) the skin, where it is called the Epidermis, and lines {h) the mouth, pharynx, and oesophagus, {c) the conjunc- tiva, {d) the vagina, and entrance of the urethra in both sexes; while, as (b) a single layer, the same kind of epithelium forms {a) the pigmentary layer of the retina, and lines {h) the interior of the serous and synovial sacs, and (r) of the heart, blood J^? tl.1;l.SrTtreS^^^^^ and lymph-vessels (Eiulotlielium). It con- ^^^""'^•^ sists of cells, which are llattened and scaly, "with an irregular outline: and, when laminated, may form a dense horny investment, as on parts of the palms of the hands and soles of the feet. Tlie nucleus is often not apparent. Tlie really cellular nature of ovon tlie dry and shriveled scales (last off from the surface of the epidermis, can be proved by the application of caustic potash, which causes them ra])i(1ly to s\v(>ll and assume th(Mr original form. STRUCTURE OF THE ELEMENTARY TISSUES. 21 Squamous cells are generally united by an intercellular substance; but in many of the deeper layers of epithelium in the mouth and skin, the outline of the cells is very irregular. Such cells (Fig. 13) are termed * 'ridge and furrow/' ''cogged" or "prickle" cells. These "prickles" are prolongations of the intra-cellular network which run across from cell to cell, thus joining them together, the interstices being filled by, the transparent intercellular cement sub- stance. When this increases in quantity in inflammation, the cells are pushed further apart and the connecting fibrils or "prickles" elongated, and therefore more clearly visible. Squamous epithelium, e.g. the pigment cells of the retina, may have a deposit of pigment in the cell-substance. This pigment consists of minute molecules of melanin, imbedded in the cell-substance and almost concealing the nucleus, which is itself transparent (Fig. 14). In white rabbits and other albino animals, in which the pigment of Fig. 13. Fig. 14. Fig. 13.— Jagged cells of the middle layers of pavement epithelium, from a vertical section of the gum of a new-born infant. (Klein.) Fig. 14.— Pigment cells from the retina. A, cells stjU cohering, seen on their surface; a, nucleus indistinctly seen. In the other ceUs the nucleus is concealed by the pigment granules. B, two cells seen in profile; a, the outer or posterior part containing scarcely any pigment, x 370. (Henle.) the eye is absent, this layer is found to consist of colorless pavement epithelial cells. Endothelium. — The squamous epithelium lining the serous mem- branes, and the interior of blood-vessels, presents so many special features as to demand a special description; it is called by a distinct name — En- dothelium. The main points of distinction above alluded to are, 1. the very flat- tened form of these cells; 2. their constant occurrence in only a single layer; 3. the fact that they are developed from the "mesoblast," while all other epithelial cells are derived from the "epiblast," or "hjrpoblast;^' 4. they line closed ■ cavities not communicating with the exterior of the body. Endothelial cells form an important and well-defined subdivision of squamous epithelial cells, which has been especially studied during the last few years. Their examination has been much facilitated by the adoption of the method of staining serous membranes with silver nitrate. 22 HAND-BOOK OF PHYSIOLOGY. When a small portion of a perfectly fresh serous membrane, as the mesentery or omentum (Fig. 15), is immersed for a few minutes in a quarter per cent, solution of this re-agent, washed with water and exposed to the action of light, the silver oxide is precipitated along the bounda- FiG. 15. — Part of the omentum of a cat, stained in silver nitrate, X 100. The tissue forms a '•'•fenes- trated TJiemftrane," that is to say, one which is studded with holes or windows. In the figiu*e these are of various shapes and sizes, leaving trabeculae, the basis of which is fibrous tissue. The trabecu- Ise are of various sizes, and are covered with endothelial cells, the nuclei of which have been made evident by staining with hsematoxylin after the silver nitrate has outUned the ceUs by staining the intercellular substance. (V. D. Harris.) ries of the cells, and the whole surface is found to be marked out with exquisite delicacy, by fine dark lines, into a number of polygonal spaces (endothelial cells) (Figs. 15 and 16). Endothelium lines, as before mentioned, all the serous cavities of the Fig. 16.— Abdominal surface of centrum tendineum of dlaphrapm of rabbit, showine the general polygonal shape of the endothelial cells; each is nucleated. (Klein.) X 300. body, including tlio anterior chamber of the eye, also the synovial mem- branes of joints, and the interior of the lieart and of all blood-vessels and lymphatics. It forms also a delic^ate investing sheath for nerve-fibres STRUCTURE OF THE ELEMENTARY TISSUES. 23 and peripheral ganglion-cells. The cells are scaly in form, and irregular in outline; those lining the interior of blood-vessels and lymphatics hav- ing a spindle-shape with a very wavy outline. They enclose a clear, oval nucleus, which, when the cell is viewed in profile, is seen to project from its surface. Endothelial cells may be ciliated, e.g., those in the mesentery of frogs, especially about the breeding season. Besides the ordinary endothelial cells above described, there are found on the omentum and parts of the pleura of many animals, little bud-like processes or nodules, consisting of small polyhedral granular cells, round- ed on their free surface, which multiply very rapidly by division (Fig. 17). These constitute what is known as ''germinating endothelium." Fig. 17. — Silver-stained preparation of great omentiuii of dog, which shows, amongst the flat endothehum of the surface, small and large groups of germinating endothehum, between which numbers of stomata are to be seen. (Klein.) x 300. The process of germination doubtless goes on in health, and the small cells which are thrown off in succession are carried into the lymphatics, and contribute to the number of the lymph corpuscles. The buds may be enormously increased both in number and size in certain diseased condi- tions. On those portions of the peritoneum and other serous membranes where lymphatics abound, there are numerous small orifices — stomata — (Fig. 18) between the endothelial cells: these are really the open mouths of lymphatic vessels, and through them lymph-corpuscles, and the serous fluid from the serous cavity, pass into the lymphatic system. 2. Spheroidal epithelial cells are the active secreting agents in most 24 HAND-BOOK OF PHYSIOLOGY. secreting glands, and hence are often termed glandular; they are gener- ally more or less rounded in outline: often polygonal from mutual pres- sure. Fig. 18. — Peritoneal surface of septiim cisternse lymphaticae magnae of frog. The stomata, some of which are open^ som6 collapsed, are surrounded by germinating endotheUum. (Klein.) x 160. Excellent examples are to be found in the liver, the secreting tubes of the kidney, and in the salivary and peptic glands (Fig. 19). 3. Columnar epithelium (Fig. 20, A and b) lines (a.) the mucous mem- brane of the stomach and intestines, from the cardiac orifice of the stomach to the anus, and (b.) wholly or in part the ducts of the glands opening on A Fig. 19.— Glandular epithelium. A, small lobule of a mucous gland of the tongue, showing nu- cleated glandular spheroidal cells. B, Liver cells. X 200. (V. D. HaiTis.) its free surface; also (c.) many gland-ducts in other regions of the body, e.f/.j mammary, salivary, etc.; (d.) the cells which form tlio deeper layers of the epithelial lining of tlie tracliea are approximately columnar. It consists of cells which are cylindrical or prismatic in form, and con- tain a largo oval nucleus. When evenly packed side by side as a single layer, the cells are uniformly columnar; but when occurring in several layers as in the deeper strata of the epithelial lining of the trachea, their STKUCTURE OF THE ELEMENTARY TISSUES. 25 shape is very variable, and often departs very widely from the typical columnar form. GoUet-cells. — Many cylindrical epithelial cells undergo a curious trans- formation, and from the alteration in their shape are termed (Fig. 20, A, c, and b). These are never seen in a perfectly fresh specimen specimen be watched for some time, little knobs are seen Fig. 20.— a. Vertical section of a villus of the small intestine of a cat. a. Striated basilar border of the epithelium, h. Columnar epithelium, c. Goblet cells, d. Central lymph-vessel, e. Smooth muscular fibres. /. Adenoid stroma of the villus in which lymph corpuscles lie. B. Goblet cells. (Klein.) ing on the free surface of the epithelium, and are finally detached; these consist of the cell-cont3nts which are discharged by the open mouth of the goblet, leaving the nucleus surrounded by the remains of the proto- plasm in its narrow stem. Some regard this transformation as a normal process which is continu- ally going on during life, the discharged cell-contents contributing to form mucus, the cells being supposed in many cases to recover their original shape. The columnar epithelial cells of the alimentary canal possess a struc- tureless layer on their free surface: such a layer, appearing striated when viewed in section, is termed the '^striated basilar border" (Fig. 20, a, ci), 4. Ciliated cells are generally cylindrical (Fig. 21, b), but may be spheroidal or even almost squamous in shape (Fig. 21, a). This form of epithelium lines (a.) the whole of the respiratory tract from the larynx to the finest subdivisions of the bronchi, also the lower parts of the nasal passages, and some portions of the generative apparatus — in the male (b.) lining the "vasa efferentia" of the testicle, and their prolongations as far as the lower end of the epididymis; in the female (c.) commencing about the middle of the neck of the uterus, and extend- ing throughout the uterus and Fallopian tubes to their fimbriated ex- tremities, and even for a short distance on the peritoneal surface of the latter, (d.) The ventricles of the brain and the central canal of the 26 HAND-BOOK OF PHYSIOLOGY. spinal cord are clothed with ciliated epithelium in the child, but in the adult it is limited to the central canal of the cord. The Cilia, or fine hair-like processes which give the name to this va- riety of epithelium, vary a good deal in size in different classes of animals, being very much smaller in the higher than among the lower orders, in which they sometimes exceed in length the cell itself. The number of cilia on any one cell ranges from ten to thirty, and those attached to the same cell are often of different lengths. When liv- ing ciliated epithelium, e.g., the gill of a mussel, is examined under the microscope, the cilia are seen to be in constant rapid motion; each cilium being fixed at one end, and swinging or lashing to and fro. The gen- eral impression given to the eye of the observer is very similar to that pro- duced by waves in a field of corn, or swiftly running and rippling water. B Fig. 21.— a. Spheroidal ciliated cells from the mouth of the frog, x 300 diameters. (Sharpey.) B. a. Ciliated colmnnar epithehimi lining a bronchus, h. Branched connective-tissue corpuscles. (Klein and Noble Smith.) and the result of their movement is to produce a continuous current in a definite direction, and this direction is invariably the same on the same surface, being always, in the case of a cavity, toward its external orifice. 5. Transitional Einthelium. — This term has been applied to cells which are neither arranged in a single layer, as is the case with simple epithelium, nor yet in many superimposed strata as in laminated; in other words, the term is employed when epithelial cells are found in two, three, or four superimposed layers. The upper layer may be either columnar, ciliated, or squamous. When the upper layer is columnar or ciliated, the second layer consists of smaller cells fitted into the inequalities of the cells above them, as in the trachea (Fig. 21, b). The epithelium which is met with lining the urinary bladder and ureters is, however, the sitional par cxcelleiice. In this variety there are two or three layers of cells, the upper being more or less flattened according to the full or col- lapsed condition of the organ, their under surface being marked with one or more dejiressions, into which the heads of the next layer of club- shaped cells fit. Between the lower and narrower ])arts of the second row of cells, are fixed the irregular cells which constitute the third row, and in like manner sometimes a fourth row (Fig. 22). It can be easily under- stood, therefore, that if a scraping of the mucous niembnino of the blad- STRUCTURE OF THE ELEMENTARY TISSUES. 27 der be teazed, and examined under the microscope, cells of a great variety of forms may be made out (Fig. 23). Each cell contains a large nucleus, and the larger and superficial cells often possess two. Special Epithelium in Organs of Special Sense. — In addition to the above kinds of epithelium, certain highly specialized forms of epi- thelial cells are found in the organs of smell, sight, and hearing, viz.. Fig. 22.— Epithelium of the bladder; a, one of the cells of the first row; 6, a cell of the second row; c, cells in situ, of first, second, and deepest layers. (Obersteiner.) Fig. 23. — Transitional epithelial cells from a scraping of the mucous membrane of the bladder of the rabbit. (V. D. Harris.) olfactory cells, retinal rods and cones, auditory cells; they will be de- scribed in the chapters which deal with their functions. Functions of Epithelium. — According to function, epithelial cells may be classified as: — (1.) Protective, e.g., in the skin, mouth, blood-vessels, etc. (2.) Protective a7id inoving — ciliated epithelium. (3.) Secreting — glandular epithelium; or. Secreting formed elements — epithelium of testicle secreting spermatozoa. (4.) Protective and secreting, e.g., epithelium of intestine. (5.) Sensorial, e.g., olfactory cells, rods and cones of retina, organ of Epithelium forms a continuous smooth investment over the whole body, being thickened into a hard, horny tissue at the points most ex- posed to pressure, and developing various appendages, such as hairs and nails, whose structure and functions will be considered in a future chapter. Epithelium lines also the sensorial surfaces of the eye, ear, nose, and mouth, and thus serves as the medium through which all impressions from the external world — touch, smell, taste, sight, hearing — reach the delicate nerve-endings, whence they are conveyed to the brain. The ciliated epithelium which lines the air-passages serves not only as a protective investment, but also by the movements of its cilia is en- abled to propel fluids and minute particles of solid matter so as to aid their expulsion from the body. In the case of the Fallopian tube, this Fig. 22. Fig. 23. Corti. 28 HAND-BOOK OF PHYSIOLOGY. agency assists the progress of the ovum toward the cavity of the uterus. Of the purposes served by cilia in the ventricles of the brain, nothing is known. (For an account of the nature and conditions of ciliary motion, see chapter on Motion.) The epithelium of the various glands, and of the whole intestinal tract, has the power of secretion, i.e., of chemically transforming certain materials of the blood; in the case of mucus and saliva this has been proved to involve the transformation of the epithelial cells themselves; the cell-substance of the epithelial cells of the intestine being discharged by the rupture of their envelopes, as mucus. Epithelium is likewise concerned in the processes of transudation, dif- fusion, and absorption. It is constantly being shed at the free surface, and reproduced in the deeper layers. The various stages of its growth and development can be well seen in a section of any laminated epithelium, such as the epidermis. The Coi^nective Tissues. This group of tissues forms the Skeleton with its various connections — ^bones, cartilages, and ligaments — and also affords a supporting frame- work and investmxcnt to various organs composed of nervous, muscular, and glandular tissue. Its chief function is the mechancial one of sup- port, and for this purpose it is so intimately interwoven with nearly all the textures of the body, that if all other tissues could be removed, and the connective tissues left, we should have a wonderfully exact model of almost every organ and tissue in the body, correct even to the smallest minutiffi of structure. Classification of Connective Tissues. — The chief varieties of connective tissues may be thus classified: — I. The Eibrous Coki^ectiye Tissues. A. — Chief Forms. B. — Special Varieties, a. Areolar. a. Gelatinous. I. White fibrous. I. Adenoid or Retiform. c. Elastic. c. Neuroglia. d. Adipose. II. Cartilage. III. Bone. All of the varieties of connective tissue are made up of two parts, namely, cells and intercellular substance. Cells. — The cells are of two kinds. (a.) Fixed. — These are cells of a flattened sluipe, with branched pro- STRUCTURE OF THE ^ELEMENTARY TISSUES. 29 cesses, which are often united together to form a network: they can be most readily observed in the cornea in which they are arranged, layer above layer, parallel to the free surface. They lie in spaces, in the inter- cellular or ground substance, which are of the same shape as the cells they contain but rather larger, and which form by anastomosis a system of branching canals freely communicating (Fig. 24). Fig. 24.— Horizontal preparation of cornea of frog, stained in gold chloride; showing the network of branched cornea corpuscles. The ground-substance is completely colorless. X 400. (Klein.) To this class. of cells belong the flattened tendon corpuscles which are arranged in long lines or rows parallel to the fibres (Fig. 29). These branched cells, in certain situations, contain a number of pig- ment-granules, giving them a dark appearance: they form one variety of pigment-cells. Branched pigment-cells of this kind are found in the outer layers of the choroid (Fig. 25). In many lower animals, such as the frog, they are found widely distributed, not only in the skin, but also in many internal parts, e.g., the mesentery and sheaths of blood-vessels. In the web of the frog^s foot such pigment-cells may be seen, with pigment evenly distributed through the body of the cell and its processes; but under the action of light, electricity, and other stimuli, the pigment-granules become massed in the body of the cell, leaving the processes quite hyaline; if the stimulus be removed, they will gradually be distributed again all over the pro- cesses. Thus the skin in the frog is sometimes uniformly dusky, and sometimes quite light-col- ored, with isolated dark spots. In the choroid and retina the pigment- cells absorb light. (5.) Amcehoid cells, of an approximately spherical shape: they have a great general resemblance to colorless blood corpuscles (Fig. 2), with Fig. 25.— Ramified pig- ment-ceUs from the tissue of the choroid coat of the eye. X 350. a, cell with pigment; 6, colorless fusi- form cells. (KoUiker.) 30 HAND-BOOK OF PHYSIOLOGY. which some of them are probably identical. They consist of finely gran- ular nucleated protoplasm, and have the property, not only of changing their form, but also of moving about, whence they are termed migra- tory. They are readily distinguished from the branched connective-tissue corpuscles by their free condition, and the absence of processes. Some are much larger than others, and are found especially in the sublingual gland of the dog and guinea pig and in the mucous membrane of the intestine. A second variety of these cells called i^lasma cells (Waldeyer) are larger than the amoeboid cells, apparently granular, less active in their movements. They are chiefly to be found in the inter-muscular septa, in the mucous and submucous coats of the intestine, in lymphatic glands, and in the omentum. Intercellular Substance. — This may be fibrillar, as in the fibrous tissues and certain varieties of cartilage; or homogeneous, as in hyaline cartilage. Fig. 26. Fig. 27. Fig. 26.— Flat, pigmented, branched, connective-tissue cells from the sheath of a large blood-ves- sel of frog's mesentery: the pigment is not distributed uniformly through the substance of the larger cell, consequently some parts of the cell look blacker than others (uncontracted state). In the two smaller cells most of the pigment is withdrawn into the cell-body, so that they appear smaller, black- er, and less branched. X 350. (KUein and Noble Smith.) Fig. 27.— Fibrous tissue of cornea, showing bundles of fibres with a few scattered fusiform cells lying in the inter-fascicular spaces, x 400. (Klein and Noble Smith.) The fibres composing the former are of two kinds — (a.) White fibres. (J.) Yellow elastic fibres. («.) Wliite Fibres. — These are arranged parallel to each other in wavy bundles of various sizes: such bundles may either have a parallel arrange- ment (Fig. 27), or may produce quite a felted texture by their interlace- ment. The individual fibres composing these fasciculi are homogeneous, unbranched, and of the same diameter throughout. They can readily be isolated by macerating a portion of white fibrous tissue (e.g., a small piece of tendon) for a short time in lime, or baryta-water, or in a solution of common salt, or potassium permanganate: those roagonts possessing the power of dissolving tlie cementing interfibrillar substance (whicli is nearly allied to syntonin), and thus separating the fibres from each other. (/;.) Yellow Elastic Fibres (Fig. 2S) arc of all sizes, from excossivoly fine (i])i-ils \\\) to fibres of considerable tliickness: tliey are distinguished STRUCTURE OF THE ELEMENTARY TISSUES. 31 from white fibres by the following characters: — (1.) Their great power of resistance even to the prolonged action of chemical reagents, e.g., Caustic Soda, Acetic Acid, etc. (2.) Their well-de- fined outlines. (3.) Their great tendency to branch and form networks by anastomosis. (4. ) They very often have a twisted corkscrew- like appearance, and their free ends usually curl up. (5.) They are of a yellowish tint and very elastic. VARIETIES OF CONNECTIVE TISSUE. I. FiBEOus Connective Tissues. A. — Chief Forms. — {a.) Areolar Tissue. Distribution. — This variety has a very wide distribution, and constitutes the subcutaneous, subserous and submucous tissue. It is found ugame^ta7^bflivI^So!^^(?^^^ in the mucous membranes, in the true skin, in ^^^'^ the outer sheaths of the blood-vessels. It forms sheaths for muscles, nerves, glands, and the internal organs, and, penetrating into their in- terior, supports and connects the finest parts. Structure. — To the naked eye it appears, when stretched out, as a fleecy, white, and soft mesh work of fine fibrils, with here and there wider films joining in it, the whole tissue being evidently elastic. The open- ness of the meshwork varies with the locality from which the specimen is taken. On the addition of acetic acid the tissue swells up, and becomes gelatinous in appearance. Under the microscope it is found to be made up of fine white fibres, which interlace in a most irregular manner, to- gether with a variable number of elastic fibres. These latter resist the action of acetic acid as above mentioned, so that when this reagent is added to a specimen of areolar tissue, although the white fibres swell up and become homogeneous, certain elastic fibres may still be seen arranged in various directions, sometimes even appearing to pass in a more or less circular or in a spiral manner round a small mass of the gelatinous mass of changed white fibres. The cells of the tissue are arranged in no very regular manner, being contained in the spaces (areolse) between the fibres. They communicate, however, with one another by their branched pro- cesses, and also apparently with the cells forming the walls of the capil- lary blood-vessels in their neighborhood, connecting together the fibrils in a certain amount of albuminous cemeiit substance. {h.) White Fibrous Tissue. Distribution. — Typically in tendon; in ligaments, in the periosteum and perichondrium, the dura mater, the pericardium, the sclerotic coat 32 HAOT)-BOOK OF PHYSIOLOGY of the eye, the fibrous sheath of the testicle; in the fasciae and aponeurosis of muscles, and in the sheaths of lymphatic glands. Structure. — To the naked eye, tendons and many of the fibrous mem- branes, when in a fresh state, present an appearance as of watered silk. This is due to the arrangement of the fibres in wavy parallel bundles. Under the microscope, the tissue appears to consist of long, often parallel, wavy bundles of fibres of different sizes. Sometimes the fibres intersect each other. The cells in tendons are arranged in long chains in the ground substance separating the bundles of fibres, and are more or less regularly quadrilateral with large round nuclei containing nucleoli, which are generally placed so as to be contiguous in two cells. Tlie cells consist of a body, which is thick, from which processes pass in various directions into, and partially filling up the spaces between the bundles of fibres. Fig. 29. Fig. 30. Fig. 29.— Caudal tendon of young rat, showing the arrangement, form, and structure of the ten- don eeUs. X 3W. (Klein.) Fig. 30.— Transverse section of tendon from a cross-section of the tail of a rabbit, showing sheath, fibrous septa, and branched comiective-tissue corpuscles. The spaces left white in the dra^\'ing rep- resent the tendinous fibres in transverse section. X 250. (Klein.) The rows of cells are separated from one another by lines of cement sub- stance. The cell spaces can be brought into view by silver nitrate. The cells are generally marked by one or more lines or stripes when viewed longitudinally. This appearance is really produced by the laminar ex- tension either projecting upward or downward, (c.) Yellow Elastic Tissue. Distrihution. — In the ligamentum nuchje of the ox, horse, and many other animals; in the ligamenta subflava of man; in the arteries, consti- tuting the fenestrated coat of Ilenle; in veins; in the lungs and tracliea: in tlie stylo-hyoid, thyro-hyoid, and crico-thyroid ligaments; in the true vocal cords. Structure. — Elastic tissue occurs in various forms, from a structure- less, elastic membrane to a tissue whoso cliicf constituents are bundles of STRUCTURE OF THE ELEMENTARY TISSUES. 33 elastic fibres crossing each other at different angles: these varieties may be classified as follows: — (a.) Fine elastic fibrils, which branch and anastomose to form a net- work: this variety of elastic tissue occurs chiefly in the skin and mucous membranes, in subcutaneous and submucous tissue, in the lungs and true vocal cords. (b.) Thick fibres, sometimes cylindrical, sometimes flattened like tape, which branch and form a network: these are seen most typically in the ligamenta subflava and also in the ligamentum nuchse of such animals as the ox and horse, in which it is largely developed. (c.) Elastic membranes with perforations, e.g., Henle^s fenestrated membrane: this variety is found chiefly in the arteries and veins. (d.) Continuous, homogeneous elastic membranes, e.g., Bowman^s Fig. 31. Fig. 32. Fig. 31.— Tissue of the jelly of Wharton from tunbilical cord. a. connective-tissue corpuscles; 6. fasciculi of connective tissue; c. spherical formative cells. (Frey.) Fig. 32.— Part of a section of a lymphatic gland, from which the corpuscles have been for the most part removed, showing the adenoid recticulum. (Klein and Noble Smith.) anterior elastic lamina, and Descemefs posterior elastic lamina, both in the cornea. A certain number of flat connective tissue cells are found in the ground substance between the elastic fibres constituting this variety of connective tissue. B. — Special Forms. — (a.) Gelatinous Tissue. Distribution. — Gelatinous connective tissue forms the chief part of the bodies of jelly fish; it is found in many parts of the human embryo, but remains in the adult only in the vitreous humor of the eye. It may be best seen in the last-named situation, in the "Whartonian Jelly*' of the umbilical cord, and in the enamel organ of developing teeth. Vol. I.— 3. 34 HAND-BOOK OF PHYSIOLOGY. Structure. — It consists of cells, which in the vitreous humor are rounded, and in the jelly of the enamel organ are stellate, imbedded in a soft jelly-like intercellular substance which forms the bulk of "the tissue, and which contains a considerable quantity of mucin. In the umbilical cord, that part of the jelly immediately surrounding the stellate cells shows marks of obscure fibrillation. (h.) Adenoid or Retiform. Distribution. — It composes the stroma of the spleen and lymphatic glands, and is found also in the thymus, in the tonsils, in the follicular glands of the tongue, in Peyer^s patches and in the solitary glands of the intestines, and in the mucous membranes generally. Structure. — Adenoid or retiform tissue consists of a very delicate net- work of minute fibrils, formed originally by the union of processes of branched connective-tissue corpuscles the nuclei of which, however, are visible only during the early periods of development of the tissue (Eig. 32). The nuclei found on the fibrillar meshwork do not form an essential part of it. The fibrils are neither white fibrous nor elastic tissue, as they are insoluble in boiling water, although readily soluble in hot alkaline solutions. {c.) Neuroglia. — This tissue forms the support of the Kervous ele- ments in the Brain and Spinal cord. It consists of a very fine meshwork of fibrils, said to be elastic, and with nucleated plates which constitute the connective-tissue corpuscles imbedded in it. Fig. 33.— Portion of the submucous tissue of prra^id uterus of sow. a, branched cells, more or less spindle-shaped; 6. bundles of connective tissue. (Klein.) Development of Fibrous Tissues. — In the embryo the place of the fil)rous tissues is at first occupied by a mass of roundish cells, derived from the "inesobhist." These develop either ipto a network of branched cells, or into groups of fusiform cells (Fig. 33). The cells are imbedded in a semi-fluid albuminous substance derived either from the cells themselves or from the neighboring blood-vessels; this afterward forms the cement substance. In it fibres arc developed, STRUCTURE OF THE ELEMENTARY TISSUES. 35 either by part of the cells becoming fibrils, the others remaining as con- nective-tissue corpuscles, or by the fibrils being developed from the out- side layers of the protoplasm of the cells, which grow up again to their original size and remain imbedded among the fibres. This process gives rise to fibres arranged in the one case in interlacing networks (areolar tissue), in the other in parallel bundles (white fibrous tissue). In the mature forms of purely fibrous tissue not only the remnants of the cell- substance, but even the nuclei may disappear. The embryonic tissue, from which elastic &0Yes are developed, is composed of fusiform cells, and a structureless intercellular substance by the gradual fibrillation of which elastic fibres are formed. The fusiform cells dwindle in size and event- ually disappear so completely that in mature elastic tissue hardly a trace of them is to be found: meanwhile the elastic fibres steadily increase in size. Another theory of the development of the connective-tissue fibrils •supposes that they arise from deposits in the intercellular substance and not from the cells themselves; these deposits, in the case of elastic fibres, appearing first of all in the form of rows of granules, which, joining together, form long fibrils. It seems probable that even if this view be correct, the cells themselves have a considerable influence in the produc- tion of the deposits outside them. Functions of Areolar and Fibrous Tissue. — The main function of connective tissue is mechanical rather than vital: it fulfils the subsidi- ary but important use of supporting and connecting the various tissues and organs of the body. In glands the trabeculse of connective tissue form an interstitial frame- work in which the parenchyma or secreting gland-tissue is lodged: in muscles and nerves the septa of connective tissue support the bundles of fibres, which form the essential part of the structure. Elastic tissue, by virtue of its elasticity, has other important uses: these, again, are mechanical rather than vital. Thus the ligamentum nuchae of the horse or ox acts very much as an India-rubber band in the same position would. It maintains the head in a proper position without any muscular exertion; and when the head has been lowered by the action of the flexor muscles of the neck, and the ligamentum nuchge thus stretched, the head is brought up again to its normal position by the relaxation of the flexor muscles which allows the elasticity of the liga- mentum nuchse to come again into play. (a.) Adipose Tissue. Distrihiifion. — In almost all regions of the human body a larger or smaller quantity of adipose or fatty tissue is present; the chief exceptions being the subcutaneous tissue of the eyelids, penis, and scrotum, the nymphae, and the cavity of the cranium. Adipose tissue is also absent from the substance of many organs, as the lungs, liver, and others. 36 HAI^D-BOOK OF PHYSIOLOGY. Fatty matter, not in the form of a distinct tissue, is also widely pres- ent in the body, e.g., in the liver and brain, and in the blood and chyle. Adipose tissue is almost always found seated in areolar tissue, and forms in its meshes little masses of unequal size and irregular shape, to which the term lohules is commonly applied. Structure. — Under the microscope adipose tissue is found to consist essentially of little vesicles or cells which present dark, sharply-defined edges when viewed with transmitted light: they are about ^^-^^ or of an inch in diameter, each composed of a structureless and colorless mem- brane or bag, filled with fatty matter, which is liquid during life, but in part solidified after death (Fig. 34). A nucleus is always present in some part or other of the cell- wall, but in the ordinary condition of the cell it is not easily or always visible. Fig. 34. Fig. 35. Fig. 34.— Ordinary fat-cells of a fat tract in the omentum of a rat. (Kiein.) Fig. 35.— Group of fat-ceUs (fc) with capiUary vessels (c). (Noble Smith.) This membrane and the nucleus can generally be brought into view by staining the tissue: it can be still more satisfactorily demonstrated by ex- tracting the contents of the fat-cells with ether, when the shrunken, shriveled membranes remain behind. By mutual pressure, fat-cells come to assume a polyhedral figure (Fig. 35). The ultimate cells are held -together by capillary blood-vessels (Fig. 35); while the little clusters thus formed are grouped into small masses, and lield so, in most cases, by areolar tissue. Tlie oily matter contained in the cells is composed chiefly of the com- pounds of fatty acids with glycerin, which are named olein, stearin, and ])(dnii,tin. Development of Adipose Tissue. — Fat-cells arc developed from coimoctivo-tissue corpuscles: in the infra-orbital connective-tissue cells may be found ex]iil)iting every intermediate gradation between an ordi- nary ])ranchc(l connective-tissue corpuscle and a mature fat-cell. The process of development is as follows: a few small drops of oil make their STRUCTURE OF THE ELEIMENTARY TISSUES. 37 appearance in the protoplasm: by their confluence a larger drop is pro- duced (Fig. 37): this gradually increases in size at the expense of the orig- inal protoplasm of the cell, which becomes correspondingly diminished in quantity till in the mature cell it only forms a thin crescentic film, closely pressed against the cell-wall, and with a nucleus imbedded in its substance (Figs. 34 and 37). ■ Under certain circumstances this process may be reversed and fat-cells may be changed back into connective-tissue corpuscles. (Kolliker, Vir- chow. ) Fig. 36. Fig. 37. Fig. 36.— Blood-vessels of adipose tissue, a. Minute flattened fat-lobule, in which the vessels only are represented, a, the terminal artery; v, the primitive vein; 6, the fat vesicles of one border of the lobule separately represented. X 100. b. Plan of the arrangement of the capillaries (c) on the exterior of the vesicles: more highly magnified. (Todd and Bowman.) Fig. 37.— a lobule of developing adipose tissue from an eight months' foetus, a. Spherical, or, from pressure, polyhedral cells with large central nucleus, surrounded by a finely reticulated sub- stance staining uniformly with hsematoxylin. b. Similar cells with spaces from which the fat has been removed by oil of cloves, c. Similar cells showing how the nucleus with enclosing protoplasm is being pressed towards periphery, d. Nucleus of endothehimi of investing capillaries. (McCarthy.) Drawn by Treves. Vessels and Nerves. — A large number of blood-vessels are found in adipose tissue, which subdivide until each lobule of fat contains a fine meshwork of capillaries ensheathing each individual fat-globule. Al- though nerve fibres pass through the tissue, no nerves have been demon- strated to terminate in it. The Uses of Adipose Tissue. — Among the uses of adipose tissue, these are the chief: — a. It serves as a store of combustible matter which may be re-ab- sorbed into the blood when occasion requires, and, being burnt, may help to preserve the heat of the body. 1). That part of the fat which is situate beneath the skin must, by its want of conducting power, assist in preventing undue waste of the heat of the body by escape from the surface. 38 HAND-BOOK OF PHYSIOLOGY. c. As a packing material, fat serves very admirably to fill up spaces, to form a soft and yielding yet elastic material wherewith to wrap tender and delicate structures, or form a bed with like qualities on which such structures may lie, not endangered by pressure. As good examples of situations in which fat serves such purposes may be mentioned the palms of the hands and soles of the feet, and the orbits. Fig. 38. — Branched connective-tissue corpuscles, developing into fat-cells. (Klein. ) d. In the long bones, fatty tissue, in the form known as yellow mar- row, fills the medullary canal, and supports the small blood-vessels which are distributed from it to the inner part of the substance of the bone. II. Caetilage. Cartilage or gristle exists in three different forms in the human body, viz., 1, Hyaline cartilage, 2, Yelloiu elastic cartilage, and 3, White fibro- cartilage. Structure of Cartilage. — All kinds of cartilage are composed of cells imbedded in a substance called the matrix : and the apparent differ- ences of structure met with in the various kinds of cartilage are more due to differences in the character of the matrix than of the cells. Among the latter, however, there is also considerable diversity of form and size. With the exception of the articular variety, cartilage is invested by a thin but tough firm fibrous membrane called the pericliojidrinm. On the surface of the articular cartilage of the fa3tus, the perichondrium is rep- resented by a film of epithelium; but this is gradually worn away up to the margin of the articular surfaces, when by use the parts begin to suffer friction. Nerves are probably not supplied to any variety of cartilage. 1. Hyaline Cartilage. I)istril)nlu))i,. — This variety of cartilage is met with largely in the human body — investing the articular ends of bones, and forming the costal cartilages, the nasal carl iliiges, and those of tlio larynx with the ox- STKUCTUKE OF TUB ELEMENTARY TISSUES. 39 ception of the epiglottis and cornicula laryngis. The cartilages of the trachea and bronchi are also hyaline. Structure. — Like other cartilages it is composed of cells imbedded in a matrix. The cells, which contain a nucleus with nucleoli, are irregular in shape, and generally grouped together in patches (Fig. 39). The patches are ' of various shapes and sizes, and placed at unequal distances apart. They gen- erally appear flattened near the free sur- face of the mass of cartilage in which they are placed, and more or less per- pendicular to the surface in the more- deeply seated portions. The matrix of hyaline cartilage has a dimly granular appearance like that of ground glass, and in man and the higher animals has no apparent structure. In some cartilages of the frog, however, even when examined in the fresh state, it is seen to be mapped out into polygo- nal blocks or cell-territories, each con- taining m cell in the centre, and representing what is generally called the capsule of the cartilage cells (Fig. 40). Hyaline cartilage in man has really the same structure, which can be demonstrated by the use of certain reagents. If a piece of human hyaline cartilage be macerated Fig. 39.— Ordinary hyaline cartilage from trachea of a child. The cartilage cells are enclosed singly or in pairs in a capsule of hyaUne substance. X 150 diams. (Klein and Noble •Smith.) Fig. 40.— Fresh cartilage from the Triton. (A. RoUett.) for a long time in dilute acid or in hot water 95"— 113° F. (35° to 45° C), the matrix, which previously appeared quite homogeneous, is found to be resolved into a number of concentric lamellae, like the coats 40 HAND-BOOK OF PHYSIOLOGY. of an onion, arranged round each cell or group of cells. It is thus shown to consist of nothing but a number of large systems of capsules which have become, fused with one another The cavities in the matrix in which the cells lie are connected to- gether by a series of branching canals, very much resembling those in the cornea: through these canals fluids may make their way into the depths of the tissue. In the hyaline cartilage of the ribs, the cells are mostly larger than in the articular variety, and there is a tendency to the development of fibres in the matrix. The costal cartilages also frequently become calcified in old age, as also do some of those of the larynx. Fat-globules may also be seen in many cartilages. In articular cartilage the cells are smaller, and arranged vertically in narrow lines like strings of beads. Temporary Cartilage. — In the foetus, cartilage is the material of which the bones are first constructed; the ^ 'model" of each bone being laid down, so to speak, in this substance. In such cases the cartilage is termed temporary. It closely resembles the ordinary hyaline kind; the cells, however, are not grouped together after the fashion just described, but are more uniformly distributed throughout the matrix. A variety of temporary hyaline cartilage which has scarcely any matrix is found in the human subject only in early foetal life, when it constitutes the chorda dorsalis. Nutrition of Cartilage. — Hyaline cartilage is reckoned among trhcj so-called non-vascular structures, no blood-vessels being supplied directly to its own substance; it is nourished by those of the bone beneath. When hyaline cartilage is in thicker masses, as in the case of the cartila,aes of the ribs, a few blood-vessels traverse its substanc^,. The distinction, however, between all so-called vascular and non-vascvlar parts, is at the best a very artificial one. 2. Yellow Elastic Cartilage. Distrihiition. — In the external ear, in the epiglottis and cornicula laryngis, and in the Eustachian tube. Structure. — The cells are rounded or oval, with well-marked nuclei and nucleoli (Fig. 41). The matrix in which they are seated is composed almost entirely of fine elastic fibres, which form an intricate interlace- ment about the cells, and in their general characters arc allied to the yel- low variety of fibrous tissue: a small and variable quantity of hyaline in- tercellular substance is also usually present. A variety of elastic cartilage, sometimes called cellular, may be obtained from the external ear of rats, mice, or other small mammals. It is com- ])()S(m1 almost entirely of cells (hence the name), which are packed very closely, with little or no matrix. When present the matrix consists of STRUCTURAL BASIS OF THE HUMAN BODY. 41 very fine fibres, which twine about the cells in various directions and enclose them in a kind of network. 3. White Fibro-Cartilage. Distribtttion. — The different situations in which white fibro-cartilage is found have given rise to the following classification: — 1. Inter-articular fibro-cartilage, e.g., the semilunar cartilages of the knee-joint. Fig. 41. Fig. 42. Fig. 41. — Section of the epiglottis. (Baly.) Fig. 42.— Tranverse section through the intervertebral cartilage of the tail of mouse, showing lameUse of fibrous tissue with cartilage ceUs arranged in rows between them. The ceUs are seen in profile, and being flattened, appear staflE-shaped. Each cell lies in a capsule. X 350. (Klein and Noble Smith.) 2. Circumferential or marginal, as on the edges of the acetabulum and glenoid cavity. 3. Connecting , e.g., the inter-vertebral fibro-cartilages. 4. In the sheatlis of tendons, and sometimes in their substance. In the latter situation, the nodule of fibro-cartilage is called a sesamoid fibro- cartilage, of which a specimen may be found in the tendon of the tibialis posti- cus, in the sole of the foot, and usually in the neighboring tendon of the peroneus longus. Structure. — White fibro-cartilage (Fig. 43), which is much more widely distribu- ted throughout the body than the forego- ing kind, is composed, like it, of cells and a matrix; the latter, however, being made up almost entirely of fibres closely resem- bling those of white fibrous tissue. In this kind of fibro-cartilage it is not unusual to find a great part of its mass composed almost exclusively of fibres, and deriving the name of cartilage only from the fact that in another portion, continuous with it, cartilage cells may be pretty freely distributed. Fig. 43.— White fibro-cartilage from an intervertebral ligament. (Klein and Noble Smith.) 42 iia:sd-book of physiology Functions of Cartilage. — Cartilage not only represents in tlie foetus the bones which are to be formed (temporary cartilage), but also offers a firm, but more or less yielding, framework for certain j^arts in the de- yeloped body, possessing at the same time strength and elasticity. It maintains the shape of tubes as in the larynx and trachea. It affords attachment to muscles and ligaments; it binds bones together, yet allows a certain degree of movement, as between the Yertebrte; it forms a firm framework and protection, yet without undue stiffness or weight, as in the pinna, larynx, and chest walls; it deepens joint cavities, as in the acetabulum, without unduly restricting the movements of the bones. Development of Cartilage. — Cartilage is developed out of an em- bryonal tissue, consisting of cells with a very small quantity of intercel- lular substance: the cells multiply by fission within the cell-capsules (Fig. 6); while the capsule of the parent cell becomes gradually fused with the surrounding intercellular substance. A repetition of this process in the young cells causes a rapid growth of the cartilage by the multiplication of its cellular elements and corresponding increase in its matrix. III. BOXE. Chemical Composition. — Bone is composed of eartliy and animal matter in the proportion of about 67 per cent, of the former to 33 per cent, of the latter. The earthy matter is composed chiefly of calcium phosphate, but besides there is a small quantity (about 11 of the 67 per cent.) of calcium carbonate and fluoride, and magnesium ^^hosphate. The animal matter is resolved into gelatin by boiling. The earthy and animal constituents of bone are so intimately blended and incorj^orated the one with the other, that it is only by chemical action, as, for instance, by heat in one case and by the action of acids in another, that they can be separated. Their close union, too, is further shown b}" the fact that when by acids the earthy matter is dissolved out, or, on the other hand, when the animal part is burnt out, tlie shape of the bone is alike preserved. The proportion between these two constituents of bone varies in dif- ferent bones in the same individual, and in the same bone at different ages. Structure. — To the naked eye there appear two kinds of structure in different bones, and in different parts of tlio same bone, namely, the denfte or compact, and the spongy or cancellous tissue. Tlius, in making a longitudinal section of a long bone, as the humerus or femur, tlie articular extremities are found capi)ed on their surface by a thin shell of compact bone, wliile their interior is made up of tlie spongy or cancellous tissue. The shaft, on the other hand, is formed almost entirely of a thick layer of the compact bone, and this surrounds STRUCTURE OF THE ELEMENTARY TISSUES. 43 a central canal, the medullary cavity — so called from its containing the medulla or marrow. In the flat bones, as the parietal bone or the scapula, one layer of the cancellous structure lies between two layers of the compact tissue, and in the short and irregular bones, as those of the carpus and tarsus, the cancellous tissue alone fills the interior, while a thin shell of compact bone forms the outside. Marrow. — There are two distinct varieties of marrow — the red and yelloiu. Red marrow is that variety which occupies the spaces in the cancel- lous tissue; it is highly vascular, and thus maintains the nutrition of the spongy bone, the interstices of which it fills. It contains a few fat-cells and a large number of marrow-cells, many of which are undistinguishable Fig. 44. — Cells of the red marrow of the guinea pig, highly magnified, a, a large cell, the nucleus of wliich appears to be partly divided into three by constrictions; 6, a cell, the nucleus of which shows an appearance of being constricted into a number of smaller nuclei; c, a so-called giant cell, or myeloplaxe,with many nuclei; d, a smaller myeloplaxe, with three nuclei; e — i, proper cells of the marrow. (E. A. Schafer.) from lymphoid corpuscles, and has for a basis a small amount of fibrous tissue. Among the cells are some nucleated cells of very much the same tint as colored blood-corpuscles. There are also a few large cells with many nuclei, termed "giant-cells^^ (myeloplaxes) which are derived from over- growth of the ordinary marrow- cells (Fig. 44). Yelloiv marrow fills the medullary cavity of long bones, and consists chiefly of fat-cells with numerous blood-vessels; many of its cells also are in every respect similar to lymphoid corpuscles. From these marrow-cells, especially those of the red marrow, are derived, as we shall presently show, large quantities of red blood-cor- puscles. Periosteum and Nutrient Blood-vessels. — The surfaces of bones, except the part covered with articular cartilage, are clothed by a tough, fibrous membrane, the periosteum; and it is from the blood-vessels which are distributed in this membrane, that the bones, especially their more compact tissue, are in great part supplied with nourishment, — minute 44 HAND-BOOK OF PHYSIOLOGY. branches from the periosteal vessels entering the little foramina on tlie surface of the bone, and finding their way to the Haversian canals, to be immediately described. The long bones are supplied also by a proper nutrient artery Avhich, entering at some part of the shaft so as to reach the medullary canal, breaks up into branches for the supply of the mar- row, from which again small vessels are distributed to the interior of the bone. Other small blood-vessels pierce the articular extremities for the supply of the cancellous tissue. Microscopic Structure of Bone. — Notwithstanding the differences of arrangement just mentioned, the structure of all bone is found under the microscope to be essentially the same. Fig. 45. — Transverse section of compact bony tissue (of humerus). Three of the Haversian canals are seen, with their concentric rings; also the corpuscles or lacunas, with the canaliculi extend- ing from them across the directi(>n of the lamelljB. The Haversian apertures had got filled with debris in grinding down tlie section, and therefore appear black in the figure, which represents the object as viewed with transmitted light. The Haversian sj-stems are so closely packed in this section, that scarcelj- any interstitial lamellae are visible, x 150. (Sharpe}*.) Examined with a rather high power its substance is found to contain a multitude of little irregular spaces, approximately fusiform in shape, called lacunce, with very minute canals or canaJicnJi, as they are termed, leading from them, and anastomosing with similar little jirolongations from other lacunae (Fig. 45). In very thin layers of bone, no other canals than these may be visible; but on making a transverse section of tlie compact tissue as of a long bone, e.g., the humerus or ulna, the arrange- ment shown in Fig. 45 can be seen. The bone seems mapped out into small circular districts, at or about the centre of each of which is a bole, and around this an appearance as of concentric layers — the Jarnnw and raiutUculi following the same con- centric phm of distribution around the small hole in the centre, with which, indeed, they communicate. STRUCTURE OF THE ELEMENTARY TISSUES. 45 On making a longitudinal section, the central holes are found to be sipiply the cut extremities of small canals which run lengthwise through the bone, anastomosing with each other by lateral branches (Fig. 46), and are called Haversian canals, after the name of the physician, Clopton Havers, who first accurately described them. The Haversian canals, the average diameter of which is -^-^ of an inch, contain blood-vessels, and by means of them blood is conveyed to all, even the densest parts of the bone; the minute canaliculi and lacunae absorbing nutrient matter from the Haversian blood-vessels, and conveying it still more intimately to the very substance of the bone which they traverse. The blood-vessels enter the Haversian canals both from without, by Fig. 46. Fig. 47. Fig. 46. — Longitudinal section of human ulna, showing Haversian canal, lacunae, and canaliculi. (RoUett.) Fig. 47.— Bone corpuscles with their processes as seen in a thin section of human bone. (RoUett.) traversing the small holes which exist on the surface of all bones beneath the periosteum, and from within by means of small channels which extend from the medullary cavity, or from the cancellous tissue. The arteries and veins usually occupy separate canals, and the veins, which are the larger, often present, at irregular intervals, small pouch-like dilatations. The lacuncB are occupied by branched cells (bone-cells, or bone-cor- puscles) (Fig. 47), which very closely resemble the ordinary branched connective-tissue corpuscles; each of these little masses of protoplasm ministering to the nutrition ot the bone immediately surrounding it, and one lacunar corpuscle communicating with another, and with its sur- rounding district, and with the blood-vessels of the Haversian canals, by 46 HAISTD-BOOK OF PHYSIOLOaY. means of the minute streams of fluid nutrient matter which occupy the canaliculi. It will be seen from the above description that bone is essentially con- nective-tissue impregnated with lime salts: it bears a very close resem- blance to what may be termed typical connective-tissue such as the substance of the cornea. The bone-corpuscles Avith their processes, occu- pying the lacunae and canaliculi, correspond exactly to the cornea-cor- puscles lying in branched spaces; while the finely fibrillated structure of the bone-lamellae, to be presently described, resembles the fibrillated sub- stance of the cornea in which the branching spaces lie. Lamellae of Compact Bone. — In the shaft of a long bone three distinct sets of lamellse can be clearly recognized. (1.) General or fundamental lamella; which are most easily traceable just beneath the periosteum, and around the medullary cavity, forming around the latter a series of concentric rings. At a little distance from the medullary and periosteal surfaces (in th§ deeper portions of the bone) they are more or less interrupted by (2.) Sioecial or Haversian lamellae, which are concentrically arranged around the Haversian canals to the number of six to eighteen around each. (3.) Interstitial lamellae, which connect the systems of Haversian lamellae, filling the spaces between them, and consequently attaining their greatest development where the Haversian systems are few, and vice versa. The ultimate structure of the lamellcB appears to be reticular. If a thin film be peeled off the surface of a bone, from which the earthy matter has been removed by acid, and examined with a high power of the microscope, it will be found com- posed of a finely reticular structure, formed appar- ently of very slender fibres decussating obliquely, but coalescing at the points of intersection, as if here the fibres were fused rather than woven Fig. 48.— Thin layer peeled , -o\ /oi \ off from a softened bone, together (Ing. 48). (bharpcy.) This fierure, Avhich is intend- -r ^ xi x-iTn ed to represent the reticular In mauyplaccs thcsc reticular lamellae are T^^^r-:l^Ti^'£^^ perforated by tapering fibres {ClavicuU of Gagli- 7^:n^^lt^"^^ly^^^ ardi), resembling in character the ordinary white 400. (Sharpey.) ^^^^ ^j^^g^j^ fibrous tissuc, whicli bolt the neighboring lamellae together, and may be drawn out when tlie latter are torn asunder (Fig. 49). These perforating fibres originate from ingrow- ing processes of the periosteum, and in the adult still retain their con- nection with it. Development of Bone. — l^^'rom tlie point of view of tlioir dovol(^p- moiit, all b()iH!S may be subdivided into two classes. STRUCTUEE OF THE ELEMENTARY TISSUES. 47 (a.) Those wliich iire ossified directly in 7nembrane, e.g.y the bones forming the vault of the skull, parietal, frontal. {jb.) Those whose form, jjrevious to ossification, is laid down in hyaline cartilage, e.g., humerus, femur. The process of development, pure and simple, may be best studied in bones which are not preceded by cartilage — "membrane-bones" {e.g., parietal); and without a knowledge of this process (ossification in mem- irane), it is impossible to understand the much more complex series of Fig. 49.— Lamellae torn off from a decalcified human parietal bone at some depth from the sur- face, a, a lamella, showing reticular fibres; 6, 5, darker part, where several lameUae are superposed; c, perforating fibres. Apertures through which perforating fibres had passed, are seen especially in the lower part, a, a, of the figm-e. (Allen Thomson.) changes through which such a structure as the cartilaginous femur of the foetus passes in its transformation into the body femur of the adult (ossi- fication in cartilage). Ossification in Membrane. — The membrane or periosteum from which such a bone as the parietal is developed consists of two layers — an external ^5row5, and an internal cellular or osteogenetic. • The external one consists of ordinary connective-tissue, being com- posed of layers of fibrous tissue with branched connective-tissue corpuscles here and there between the bundles of fibres. The internal layer consists of a network of fine fibrils with a large number of nucleated cells, some of which are oval, others drawn out into a long branched process, and others branched: it is more richly supplied with capillaries than the outer layer. The relatively large number of its cellular elements, their varia- bility in size and shape, together with the abundance of its blood-vessels, clearly mark it out as the portion of the periosteum which is imm-ediately concerned in the formation of bone. In such a bone as the parietal, the deposition of bony matter, which is preceded by increased vascularity, takes place in radiating spicul^e. 48 HA]S^D-BOOK OF PHYSIOLOGY. starting from a "centre of ossification/^ and shooting out in all directions toward the peripher}-; while the bone increases in thickness by the depo- sition of successive layers beneath the periosteum. The finely fibrillar network of the deeper or osteogenetic layer of the periosteum becomes transformed into bone-matrix (the minute structure of which has been already (p. 46) described as reticular), and its cells into bone-corpuscles. On the young bone trabeculge thus formed, fresh layers of cells (osteo- blasts) from the osteogenetic layer are developed side by side, lining the irregular spaces like an epithelium (Fig. 50, h). Lime-salts are deposited in the circumferential part of each osteoblast, and thus a ring of osteo- blasts gives rise to a ring of bone with the remaining uncalcified portions of the osteoblasts imbedded in it as bone-corpuscles (Fig. 50). Fig. 50.— Osteoblasts from the parietal bone of a human embryo, thirteen weeks old, a, bony- septa with the ceUs of the lacunse: h. layers of osteoblasts; c, the latter in transition vc bone cor- puscles. Highly magnified. (Gegenbaur.) Thus, the primitive spongy bone is formed, whose irregular branch- ing spaces are occupied by processes from the osteogenetic layer of the periosteum with numerous blood-vessels and osteoblasts. Portions of this primitive spongy bone are re-absorbed; the osteoblasts being arranged in concentric successive layers and thus giving rise to concentric Haversian latfiellae of bone, until the irregular space in the centre is reduced to a well-formed Haversian canal, the portions of the primitive spongy bone between the Haversian systems remaining as interstitial or ground- lamellae (p. 46). The bulk of the primitive spongy bone is thus gradu- ally converted into compact bonj^-tissue with Haversian canals. Tliose portions of the in-growths from the deeper layer of the periosteum which are not converted into bone remain in the spaces of the cancellous tissue as the red marrow. Ossification in Cartilage. — Under this heading, taking the femur as a typical example, we may consider the process by which the solid carti- laginous rod which represents it in the foetus is converted into tlie hollow cylinder of compact bone with expanded ends of cancellous tissue which forms the adult femur; bearing in mind the fact that this fcotal cartilag- STRUCTURE OF THE ELEMENTARY TISSUES. 49 inous femur is many times smaller than the medullary cavity even of the shaft of the mature bone, and, therefore, that not a trace of the original cartilage can be present in the femur of the adult. Its purpose is indeed purely temporary; and, after its calcification, it is gradually and entirely re-absorbed as will be presently explained. Fia. 51. Fig. 52. Fift. 51.— From a transverse section through part of foetal jaw near the extreme periosteum, in the state of spongy bone, p, fibrous layer of periosteum ; b, osteogenetic layer of periosteum; o, osteoblasts; c, osseous substance, containing many bone corpuscles. X 300. (Schofield.) Fig. 52. — Ossifying cartilage showing loops of blood-vessels. The cartilaginous rod which forms the fcBtal femur is sheathed in a, membrane termed the pericliondrium, which so far resembles the perios- teum described above, that it consists of two layers, in the deeper one of which spheroidal cells predominate and blood-vessels abound, while the: outer layer consists mainly of fusiform cells which are in the mature tissue gradually transformed into fibres. Thus, the differences between Vol. I.— 4. 50 HAND-BOOK OF PHYSIOLOGY. the foetal perichondrium and the periosteum of the adult are such as usually exist between the embryonic and mature forms of connective- tissue. Between the hyaline cartilage of which the foetal femur consists and the bony tissue forming the adult femur, two intermediate stages exist — viz., calcified cartilage, and embryonic spongy bone. These tissues, which successively occupy the place of the foetal cartilage, are in suc- cession entirely re-absorbed, and their place taken by true bone. The process by which the cartilaginous is transformed into the bony Fig. 53. Fig. 54. Fig. 53.— Longitudinal section of ossifying cartilage from the humerus of a foetal sheep. Calci- fied trabeculae are seen extending between the columns of cartilage cells, c, cartilage cells. X 140. (Sharpey.) Fig. .54.— Transverse section of a portion of a metacarpal bone of a foetus, showing— 1. fibrous layer of periosteum; 2, osteogenetic layer of ditto; 3, periosteal bone; 4, cartilage with matrix gradu- ally becoming calcified, as at 5, with cells in primary areola?; beyond 5 the calcified matrix is being entirely replaced by spongy bone. X 200. (V, D. Harris.) femur may be dividea for the sake of clearness into the following six stages: Stage I. — Vascularization of the Cartilage. — Processes from the osteogcnetic or cclluhir layer of tlie pericliondrium containing blood- vessels grow into the substance of the cartilage much as ivy insinuates it- self into the cracks and crcvi(H\s of a wall. Thus the substance of the car- tilage, which previously contained no vessels, is traversed by a number of STEUCTUEE OF THE ELEMENTAEY TISSUES. 51 branched anastomosing channels formed by the enlargement and coales- cence of the spaces in which the cartilage-cells lie, and containing loops of blood-vessels (Fig. 52) and spheroidal-cells which will become osteo- blasts. Stage 2. — Calcification of Cartilaginous Matrix. — Lime-salts are next deposited in the form of fine granules in the hyaline matrix of the cartilage, which thus becomes gradually transformed into a number of calcified trabeculse (Fig. 54, forming alveolar spaces {yrimary areolce) containing cartilage cells. By the absorption of some of the trabeculae larger spaces arise, which contain cartilage-cells for a very short time only, their places being taken by the so-called osteogenetic layer of the perichondrium (before referred to in Stage 1) which constitutes the pri- mary marrow. The cartilage-cells, gradually enlarging, become more transparent and finally undergo disintegration. Stage 3. — Substitution of Embryonic Spongy Bone for Car- tilage. — ^The cells of the primary marrow arrange themselves as a con- tinuous layer like epithelium on the calcified trabeculae and deposit a layer of bone, which ensheathes the calcified trabeculae: these calcified trabeculae, encased in their sheaths of young bone, become gradually absorbed, so that finally we have trabeculae composed en- tirely of spongy bone, all trace of the original calcified cartilage having dis- appeared. It is probable that the large multinucleated giant-cells termed ' 'os- teoclasts" by KoUiker, which are de- rived from the osteoblasts by the mul- tiplication of their nuclei, are the agents by which the absorption of cal- cified cartilage, and subsequently of embryonic spongy bone, is carried on (Fig. 55, g). At any rate they are almost always found wherever absorp- tion is in progress. Stages 2 and 3 are precisely similar to what goes on in the growing shaft of a bone which is increasing in length by the advance of the pro- cess of ossification into the intermediary cartilage between the diaphysis and epiphysis. In this case the cartilage-cells become flattened and, multiplying by division, are grouped into regular columns at right angles to the plane of calcification, while the process of calcification extends into the hyaline matrix between them (Figs. 52 and 53). Stage 4.— Substitution of Periosteal Bone for the Primary Fig. 55.— a small isolated mass of bone next the periosteum of the lower jaw of human foetus, a, osteogenetic layer of periosteum. G, multinuclear giant cells, the one on the left acting here probably like an osteoclast. Above c, the osteoblasts are seen to become sur- rounded by an osseous matrix. (Klein and Noble Smith.) 52 HAND-BOOK OF PHYSIOLOGY. Embryonic Spongy Bone. — The embryonic spongy bone, formed as above described, is simply a temporary tissue occupying the place of the foetal rod of cartilage, once representing the femur; and the stages 1, 2, and 3 show the successive changes which occur at the centime of the shaft. Periosteal bone is now deposited in successive layers beneath the perios- teum, i.e., at the circumference of the shaft, exactly as described in the Fig. 56.— Transverse section tlirough the tibia of a foetal kitten semi-diagrammatic. X 60. P, Periostemn. O, osteogenetic layer of the periosteum, showing the ostet>blasts ai-ranged side by side, represented as pear-shaped black dots on tlie surface of the newly-fornietl bone. the periosteal bone deposited in successive layers beneath the periosteum and en'sheatliing E. the spongy endochon- dral bone; represented as more deeply shaded, u'ithin the trabecuhv of endochondral spongy bone are seen the remains of the calcified cartilage trabecular represented its dark wavy lines. 0. the me- dulla, with V, V, veins. In the lower half of the figvu'e the endochondral spongy bone has been com- pletely absorbed. (Klein and Noble Smith.) section on '^ossification in membrane," and thus a casing of periosteal bone is formed around the embryonic endochondral spongy bone: this casing is thickest at the centre, where it is first formed, and thins out toward each end of the shaft. Tlic embryonic^ si)ongy bone is absorbed, its trabeculas becoming gradually thinned and its meshes enlarging, and finally coalescing into one great cavity — the medullary cavity of the shaft. Stage 5. — Absorption of the Inner Layers of the Periosteal STRUCTUKE OF THE ELEMENTARY TISSUES. 53 Bone. — The absorption of the endochondral spongy bone is now complete, and the medullary cavity is bounded by periosteal bone: the inner layers of this periosteal bone are next absorbed, and the medullary cavity is thereby enlarged, while the deposition of bone beneath the periosteum continues as before. The first-formed periosteal bone is spongy in char- acter. Stage 6. — Formation of Compact Bone. — The transformation of spongy periosteal bone into compact bone is effected in a manner exactly similar to that which has been described in connection with ossi- fication in membrane (p. 47). areolae in the spongy bone are absorbed, while the osteoblasts which line them are developed in concentric layers, each layer in turn becoming ossified till the comparatively large space in the centre is reduced to a well- formed Haversian canal (Fig. 57). When once formed, bony tissue grows to some extent in- terstitially, as is evidenced by the fact that the lacunas are rather further apart in fully- formed than in young bone. From the foregoing descrip- tion of the development of bone, it will be seen that the common terms ' 'ossification in cartilage^' and ' 'ossification in membrane'^ are apt to mislead, since they seem to imply two processes radi- cally distinct. The process of ossification, however, is in all cases one and the same, all true bony tissue being formed from membrane (perichondrium or periosteum); but in the development of such a bone as the femur, which may be taken as the type of so-called ''ossification in cartilage," lime-salts are deposited in the cartilage, and this calcified car- tilage is gradually and entirely re-absorbed, being ultimately replaced by bone formed from the periosteum, till in the adult structure nothing but true bone is left. Thus, in the process of "ossification in cartilage," cal- cification of the cartilaginous matrix precedes the real formation of bone. We must, therefore, clearly distinguish between calcification and ossifica- tion. The farmer is simply the infiltration of an animal tissue with lime-salts, and is, therefore, a change of chemical composition rather The irregularities in the walls of the Fig. 57. — Transverse section of femur of a human embryo about eleven weeks old. a, rudimentary Ha- versian canal in cross section ; 6, in longitudinal section ; c, osteoblasts ; ci, newly formed osseous substance of a lighter color ; e, that of greater age ; /, laeunae with their cells; a cell still miited to an osteoblast. (Frey.) 54 HA^s^D-BOOK OF PHYSIOLOGY. than of structure; while ossification is the formation of true bone — a tissue more comr>lex and more liighly organized than that from which it is derived. Centres of Ossification. — In all bones ossification commences at one or more points^ termed "centres of ossification.^^ The long bones, e.g., femur, humerus, etc., have at least three such points — one for the ossification of the shaft or diapJiysis, and one for each articular extremity or epipJiysis. Besides these three primary centres which are always pres- ent in long bones, various secondary centres may be superadded for the ossification of different ^jroce^^es. Growth of Bone. — Bones increase i?i length by the advance of the process of ossification into the cartilage intermediate between the dia- physis and epiphysis. The increase in length indeed is due entirely to Fig. 58.— a. Longitudinal section of a human molar tooth; c, cement; <7, dentine; e, enamel; v, pulp cavity. (Owen.) B. Traiisverse section. The letters indicate the same as in a. growth at the two ends of the shaft. This is proved by inserting two pins into the shaft of a growing bone: after some time their distance ajDart will be found to be unaltered though the bone has gradually in- creased in length, the growth having taken place beyond and not be- tween them. If now one pin be placed in the shaft, and the other in the epiphysis, of a growing bone, their distance apart will increase as the bone grows in length. Thus it is that if the epiphyses with the intermediate cartilage be re- moved from a young bone, growth in length is no longer possible; while tlio natural termination of growth of a bone in lengtli takes place when the epiphyses become united in bony continuity with the shaft. Increase in thickness in the shaft of a long bone, occurs by the depo- sition of successive layers beneath the periosteum. If a tliiii metal plate be Inserted beneath the periosteum of a growing bone, it will soon be covered by osseous deposit, but if it be put betM-een the STRUCTURE OF THE ELEMENTARY TISSUES. 55 fibrous and osteogenetic layers, it will never become enveloijed in bone, for all the bone is formed beneath the latter. Other varieties of connective tissue may become ossified, e.g. tendons in some birds. the Functions of Bones. — Bones form the framework of the body; for this they are fitted by their liardness and solidity together with their com- parative lightness; they serve both to protect internal organs in the trunk and skull, and as levers worked by muscles in the limbs; notwithstanding their hard- ness they possess a considerable degree of elasticity, which often saves them from fractures. Teeth. The principal part of a tooth, viz., den- tine, is called by some a connective tissue, and on this account the structure of the teeth is considered here. A tooth is generally described as pos- sessing a crown, neck, Sbiid fang or fangs. The C7'0wn is the portion which pro- jects beyond the level of the gum. The neck is that constricted portion just below the crown w^hich is embraced by the free edges of the gum, and the fang includes all below this. On making a longitudinal section through the centre of a tooth (Figs. 58, 59), it is found to be principally composed of a hard matter, dentine or ivory; while in the centre this dentine is hollowed out into a cavity resembling in general shape the outline of the tooth, and called the pulp cavity, from its containing a very vascular and sensitive little mass, composed of connective-tissue, blood-vessels, and nerves, w^hich is called the tooth-pulp. The blood-vessels and nerves enter the pulp through a small opening at the extremity of the fang. Capping that part of the dentine which projects beyond the level of the gum, is a layer of very hard calcareous matter, the enamel; while sheathing the portion of dentine which is beneath the level of the gum, is a layer of true bone, called the cement or crusta petrosa. Fig. 59.— Premolar tooth of cat in situ. Vertical section. 1. Enamel with decus- sating and parallel strige. 2. Dentine with Schreger's Unes. 3. Cement. 4. Perios- teum of alveolus. 5. Inferior maxillary bone showing canal for the inferior dental nerve and vessels which appears nearly circular in transverse section. (Waldeyer.) 56 HAND-BOOK OF PHYSIOLOGY. At the neck of the tooth, where the enamel and cement come into contact, each is reduced to an exceedingly thin layer. The covering of enamel becomes thicker as we approach the crown, and the cement as we approach the lower end or apex of the fang. I. — Dentine. Chemical composition. — Dentine or ivory in chemical composition closely resembles bone. It contains, however, rather less animal matter; the proportion in a hundred parts being about twenty-eight animal to seventy-two of earthy. The former, like the animal matter of bone, may be resolved into gelatin by boiling. The earthy matter is made up chiefly of calcium phosphate, with a small portion of the carbonate, and traces of calcium fluoride and magnesium phosphate. Structure. — Under the microscope dentine is seen to be finely chan- neled by a multitude of delicate tubes, which, by their inner ends, com- FiG. 60.— Section of a portion of the dentine and cement from the middle of the root of an incisor tooth, a, dental tubuU ramifying and terminating, some of them in the interglobular spaces 6 and c, which somewhat resemble bone lacunae; d, inner layer of the cement with numerous closely set canahculi; e, outer layer of cement; /, lacunae; (/, canalicuh. x 350. (KoUiker.) municate with the pulp-cavity, and by their outer extremities come into contact with the under part of the enamel and cement and sometimes even penetrate them for a greater or less distance (Fig. 60). In their course from the pulp-cavity to the surface of the dentine, the minute tubes form gentle and nearly parallel curves and divide and sub- divide dichotomously, but without much lessening of their calibre until they are approaching their peripheral termination. From their sides proceed other exceedingly minute secondary canals, which extend into the dentine between the tubules, and anastomose with eacli other. The tubules of the dentine, the average diameter of whicli at tlieir inner and larger extremity is of an inch, contain line pro- longations from the tootli-pul]), which give the dentine a certain faint sensitiveness under ordinary circumstances, and, without doubt, have to do also with its nutrition. These i)r()longations from the tooth-pulp are really processes of the dentine-cells or odontoblasts which arc branched cells lining the ])iili)-cavity; the relation of these processes to the tubules' in STRUCTURE OF THE ELEMENTARY TISSUES. 57 which they lie being precisely similar to that of the processes of the bone- corpuscles to the canaliculi of bone. The outer portion of the dentine, underlying both the cement and enamel, forms a more or less distinct layer termed the granular or inter globular layer. It is characterized by the presence of a number of minute cell-like cavities, much more closely packed than the lacunae in the cement, and communicating with one another and with the ends of the dentine-tubes (Fig. 60), and containing cells like bone-corpuscles. II. — Enamel, Chemical composition. — The enamel, which is by far the hardest por- tion of a tooth, is composed, chemically, of the same elements that enter Fig. 61. Fxg. C2. Fig. 61.— Thin section of the enamel and a part of the dentine, a, cuticular pellicle of the enamel; b, enamel fibres, or columns with fissures between them and cross striae ; c, larger cavities in the enamel, communicating with the extremities of some of the tubuli {d). X 350. (KoUilser.) Fig. 62.— Enamel fibres. A, fragments and single fibres of the enamel, isolated by the action of hydrochloric acid. B, surface of a smaU fragment of enamel, showing the hexagonal ends of the fibres. X 350. (KoUiker.) into the composition of dentine and bone. Its animal matter, however, amounts only to about 2 or 3 per cent. It contains a larger proportion of inorganic matter and is harder than any other tissue in the body. Structure. — Examined under the microscope, enamel is found com- posed of fine hexagonal fibres (Figs. 61, 62) y^Vo diameter. 58 HA^^D-BOOK OF PHYSIOLOGY. which are set on end on the surface of the dentine, and fit into corre- sponding depressions in the same. They radiate in such a manner from the dentine that at the top of the tooth they are more or less vertical, while toward the sides they tend to the horizontal direction. Like the dentine tubules, they are not straight, but disposed in wavy and parallel curves. The fibres are marked by transverse lines, and are mostly solid, but some of them contain a very minute canal. The enamel-prisms are con- nected together by a very minute quantity of hyaline cement-sub- stance. In the deeper part of the enamel, between the prisms, are small lacuncB, which communicate with the ^^interglobular spaces" on the surface of the dentine. The enamel itself is coated on the outside by a very thin calci- fied membrane, sometimes termed the cuticle of the enamel. III. — Crusta Petrosa. The crusta petrosa, or cement (Fig. 60, c, d), is composed of true bone, and in it are lacuna (/ ) and canaliculi (g) which sometimes communicate with the outer fine- ly branched ends of the dentine tubules. Its lamina are as it were bolted together by perforating fibres like those of ordinary bone, but it differs in possessing Haver- sian canals only in the thickest part. Development of Teeth. Fig. 63.— Section of the upper jaAv of a foetal sheep. A.— 1, common enamel-t^erm dipping down into the mucous membrane; 2, palatine jn-oeess of jaw. B.— Section similar to A, but passing through one of the special enamel-germs hen" becoming fhisk-shapcd; c, c', epithelium of mouth; /, neck; /', InuU of special enamel-germ. C— A later st age ; r , on 1 1 ine of cpi 1 1 le- lium of gum; /, neck of enamel-germ; /', enamel organ; papilla; s, dental sax; forming; f j>, the enamel-germ of permanent tooth. (Waldeyer and KoUiker.) Copied from Quaiu's Anatomy. Development of the Teeth. — The first step in the development of the teeth consists in a downward growtli (Fig. G3, A, 1) from tlie stratified epithelium of the mucous membrane of the mouth, now thickened in tlie neiglil)orliood of the miixilla3 wliich are in the course of formation. This process i)asses downward into a recess (enamel groove) of the imperfectly developed tissue of which the chief part of tb jaw consists. The down- STKUCTUKE OF THE ELEMENTARY TISSUES. 59 ward epithelial growth forms the primary enmnd organ or enamel germ, and its position is indicated by a slight groove in the mucous membrane of the jaw. The next step in the process consists in the elongation down- ward of the enamel groove and of the enamel germ and the inclination outward of the deeper part (Fig. 63, b, which is now inclined at an angle with the upper portion or neck (/), and has become bulbous. After this, there is an increased development at certain points corresponding to the situations of the future milk- teeth, and the enamel germ, or com- mon enamel germ, as it may be called, becomes divided at its deeper por- tion, or extended by further growth, into a number of special enamel germs corresponding to each of the above-mentioned milk teeth, and con- nected to the common germ by a narrow neck, each tooth being placed in its own special recess in the embryonic jaw (Fig. 63, b,//'). As these changes proceed, there grows up from the underlying tissue into each enamel germ (Fig. 63, c, jo), a distinct vascular papilla (dental papilla), and upon it the enamel germ becomes moulded and presents the ap- pearance of a cap of two layers of epi- thelium separated by an interval (Fig. 63, c, /). Whilst part of the sub- epithelial tissue is elevated to form the dental papillae, the part which bounds the embryonic teeth forms the dental sacs (Fig. 63, c, s); and the rudiment of the jaw, at first a bony gutter in which the teeth germs lie, sends up processes forming partitions between the teeth. In this way small chambers are produced in which the dental sacs are contained, and thus the sockets of the teeth are formed. The papilla, which is really part of the dental sac, if one thinks of this as the whole of the sub-epithelial tissue surrounding the enamel organ and interposed between the enamel germ and the develop- ing bony jaw, is composed of nucleated cells arranged in a meshwwk, the outer or peripheral part being covered with a layer of columnar nucleated cells called odontoblasts. The odontoblasts form the dentine, while the remainder of the papilla forms the tooth-pulp. The method of the for- mation of the dentine from the odontoblasts is as follows: — The cells elon- gate at their outer part, and these processes are directly converted into the tubules of dentine (Fig. 64). The continued formation of dentine proceeds by the elongation of the odontoblasts, and their subsequent con- version by a process of calcification into dentine tubules. The most recently formed tubules are not immediately calcified. The dentine fibres contained in the tubules are said to be formed from processes of the Fig. 64.— Part of section of developing tooth of a young rat, showing the mode of deposi- tion of the dentine. Highly magnified, a, outer layer of fully formed dentine ; 6, uncal- cified matrix with one or tM^o nodules of cal- careous matter near the calcified parts; c, odontoblasts sending processes into the den- tine; d, pulp. The section is stained in car- mine, which colors the uncalcified matrix but not the calcified part. (E. A. Schafer.) 60 HAND-BOOK OF PHYSIOLOGY. deeper layer of odontoblasts, which are wedged in between the cells of the superficial layer (Fig. 64) which form the tubules only. Since the papillae are to form the main portion of each tooth, i.e., the dentine, each of them early takes the shape of the crown of the tooth it is to form. As the dentine increases in thickness, the papillae diminish, and at last when the tooth is cut, only a small amount of the papilla remains as the dental pulp, and is supplied by vessels and nerves which enter at the end of the fang. The shape of the crown of the tooth is taken by the corresponding papilla, and that of the single or double fang by the subsequent constriction be- low the crown, or by division of the lower part of the papilla. The enamel cap is found later on to consist (Fig. 65) of three parts: {a) an inner membrane, composed of a layer of columnar epithelium in contact with the dentine, called ena- mel cells, and outside of these one or more layers of small polyhedral nu- cleated cells (stratum intermedium of Hannover); {b) an outer mem- brane of several layers of epithelium; (c) a middle membrane formed of a matrix of non-vascular, gelatinous tissue, containing a hyaline intersti- tial substance. The enamel is formed by the enamel cells of the inner membrane, by the elongation of their distal extremities, and the di- rect conversion of these processes into enamel. The calcification of the enamel processes or prisms take? place first at the periphery, the cen- tre remaining for a time transparent. The cells of the stratum interme- dium are used for the regeneration of the enamel cells, but these and the middle membrane after a time disappear. The cells of the outer membrane give origin to the cuticle of the enamel. The cement or crusta j)etrosa is formed from the tissue of the tooth sac, the structure and function of which are identical with those of the ostcogenctic layer of tlie periosteum. In this manner tlio first set of teeth, or the milk-tectli, are formed; and each tooth, by degrees developing, presses at length on the wall of the sac enclosing it, and, causing its absorjition, is cut, to use a familiar phrase. The teinporary ov milk-teeth have only a very limited term of existence. Fig. 65.— Vertical transverse section of the dental sac, pulp, etc., of a kitten, a, dental papilla or pulp; 6, the cap of dentine formed upon the summit; c, its covering of enamel; d, inner layer of epithelium of the enamel organ; e, gelatinous tissue ; /, outer epithelial layer of the enamel organ; g, inner layer, and h, outer layer of dental sac. X 14. (Thiersch.) STRUCTURE OF THE ELEMENTARY TISSUES. 61 This is due to the growth of the permanent teeth, which push their way up from beneath, absorbing in their progress the whole of the fang of each milk-tooth and leaving at length only the crown as a mere shell, which is shed to make way for the eruption of the permanent teeth (Fig. 66). The temporary teeth are ten in each jaw, namely, four incisors, two canines, and four molars, and are replaced by ten permanent teeth, each of which is developed in a way almost exactly similar to the manner of development already described, from a small process or sac set by, so to speak, from the enamel germ of the temporary tooth which precedes it, and called the cavity of reserve. The number of permanent teeth in each jaw is, however, increased to six- teen, by the development of three others on each side of the jaw after much the same fashion as that by which the milk-teeth were themselves formed. Fig. 66. — Part of the lower jaw of a child of three or four years old, showing the relations of the temporary and permanent teeth. The specimen contains all the milk teeth of the right side, to- gether with the incisors of the left; the inner plate of the jaw has been removed, so as to expose the sacs of all the permanent teeth of the right side, except the eighth or wisdom tooth, which is not yet formed. The large sac near the ascending ramus of the jaw is that of the first permanent molar, and above and behind it is the commencing rudiment of the second molar. (Quain.) The beginning of the development of the permanent teeth of course takes place long before the cutting of those which they are to succeed. One of the first steps in the development of a milk-tooth is the out- growth of a lateral process of epithelial cells from its primitive enamel organ (Fig. 63, c, f p). This epithelial outgrowth ultimately becomes the enamel organ of the permanent tooth, and is indented from below by a primitive dental papilla, precisely as described above. The following formula shows, at a glance, the comparative arrange- ment and number of the temporary and permanent teeth: — Temporary Teeth Permanent Teeth Mo. Bi. Ca. In. Ca. Bi. Mo. Upper 3 2 1 4 1 2 3=16 =32 Lower 3 2 1 4 1 2 3 = 16 62 HAND-BOOK OF PHYSIOLOGY. From this formula it will be seen that the two bicuspid teeth in the adult are the successors of the two molars in the child. They differ from them, however, in some respects, the temporary molars having a stronger likeness to the permanent than to their immediate descendants, the so- called bicuspids. The temporary incisors and canines differ from their successors but little except in their smaller size. The following tables show the average times of eruption of the Tem- porary and Permanent teeth. In both cases, the eruption of any given tooth of the lower jaw precedes, as a rule, that of the corresponding tooth of the upper. Temporary or MilJc Teeth. The figures indicate in months the age at which eacQ tooth appears. Molars. Canines, Incisors. Canines. Molars. 24 12 18 9 7 7 9 18 12 24 Permanent Teeth. The age at which each tooth is cut is indicated in this table in years. Molars. Bicuspid. Canines. Incisors. Canines. Bicuspid. Molars. 17 12 to to 6 25 13 10 9 11 to 12 8 7 7 8 11 to 12 9 10 12 17 6 to to 13 25 The times of eruption put down in the above tables are only approxi- mate: the limits of variation being tolerably wide. Some children may cut their first teeth before the age of six months, and others not till nearly the twelfth month. In nearly all cases the two central incisors of the lower jaw are cut first; these being succeeded after a short interval by the four incisors of the upper jaw, next follow the lateral incisors of the lower jaw, and so on as indicated in the table till the completion of the milk dentition at about the age of two years. The milk-teeth usually come through in batches, each period of erup- tion being succeeded by one of quiescence lasting sometimes several months. The milk-teeth are in use from the age of two up to five and a half years: at about this age the first permanent molars (four in number) make their appearance behind the milk-molars, and for a short time the child has four permanent and twenty temporary teeth in position at once. It is worthy of iiotc^ tliat from the age of. five years to the shedding of the first milk-tootli the child has no fewer than forty-eight teeth, twenty milk-teeth and twenty-eight calcified germs of permanent teeth (all in fact except the four wisdom teeth). CHAPTER IV. THE BLOOD. The blood of man, as indeed of the great majority of vertebrate ani- mals, is a more or less viscid fluid, of a red color. The exact shade of red is variable, for whereas that taken from the arteries, from the left side of the heart or from the pulmonary veins, is of a bright scarlet hue, that obtained from the systemic veins, from the right side of the heart, or from the pulmonary artery, is of a much darker color, and varies from bluish-red to reddish-black. To the naked eye, the red color appears to belong to the whole mass of blood, but on examination with the micro- scope it is found that this is not the case. By the aid of this instrument the blood is shown to consist in reality of an almost colorless fluid, called Liquor Sanguinis or Plasma, in which are suspended numerous minute rounded masses of protoplasm, called Blood Corpuscles. The corpuscles are, for the most part, colored, and it is to their presence that the red color of the blood is due. Even when examined in very thin layers blood is opaque, on account of the different refractive powers possessed by its two constituents, viz., the plasma and the corpuscles. On treatment with chloroform and other reagents, however, it becomes transparent, and assumes a lake color, in consequence of the coloring matter of the corpuscles having been, by these means, discharged into the plasma. The average specific gravity of blood at 60° F. (15° 0.) is 1055, the extremes consistent with health being 1045-1062. The reaction of blood is faintly alkaline. Its temper- ature varies within narrow limits, the average being 100° F. (37*8° C). The blood stream is slightly warmed by passing through the muscles, nerve centres, and glands, but is somewhat cooled on traversing the capil- laries of the skin. Eecently drawn blood has a distinct odor, which in many cases is characteristic of the animal from which it has been taken; the odor may be further developed by adding to blood a mixture of equal parts of sulphuric acid and water. Quantity of the Blood. — The quantity of blood in any animal under normal conditions bears a pretty constant relation to the body weight. The methods employed for estimating it are not so simple as might at first sight be thought. For example, it would not be possible to get any accurate information on the point from the amount obtained by rapidly 64 HAND-BOOK OF PHYSIOLOGY. bleeding an animal to death, for then an indefinite quantity would remain in the vessels, as well as in the tissues; nor, on the other hand, would it be possible to obtain a correct estimate by less rapid bleeding, as, since life would be more prolonged, time would be allowed for the passage into the blood of lymph from the lymphatic vessels and from the tissues. In the former case, therefore, we should under-estimate, and in the latter over- estimate the total amount of the blood. Of the several methods which have been employed, the most accurate appears to be the following. A small quantity of blood is taken from an animal by venesection; it is defibrinated and measured, and used to make standard solutions of blood. The animal is then rapidly bled to death, and the blood which escapes is collected. The blood-vessels are next washed out with water or saline solution until the washings are no longer colored, and these are added to the previously withdrawn blood; lastly the whole animal is finely minced with water or saline solution. The fluid obtained from the mincings is carefully filtered, and added to the diluted blood previously obtained, and the whole is measured. Ths next step in the process is the comparison of the color of the diluted blood with that of standard solutions of blood and water of a known strength, until it is discovered to what standard solution the diluted blood corre- sponds. As the amount of blood in the corresponding standard solution is known, as well as the total quantity of diluted blood obtained from the animal, it is easy to calculate the absolute amount of blood which the latter contained, and to this is added the small amount which was with- drawn to make the standard solutions. This gives the total amount of blood which the animal contained. It is contrasted with the weight of the animal, previously known. The result of many experiments shows that the quantity of blood in various animals averages -^j to of the total body weight. An estimate of the quantity in man which corresponded nearly with the above, was made some years ago from the following data. A crim- inal was weighed before and after decapitation; the difference in the weight representing, of course, the quantity of blood which escaped. The blood-vessels of the head and trunk were then washed out by the in- jection of water, until the fluid which escaped had only a pale red or straw color. This fluid was then also weighed; and the amount of blood which it represented was calculated by comparing the proportion of solid matter contained in it with tliat of the first blood which escaped on decapitation. Two experiments of this kind gave precisely similar results. (Weber and Lehmann.) It should 1)0 remembered, however, in connection with these estima- tions, that the quantity of the blood must vary, even in the same animal, very (considerably with the amount of both the ingesta and egesta of the period inunctl lately preceding the experiment; and it has been found. THE BLOOD. 65 indeed, that the quantity of blood obtainable from a fasting animal barely exceeds a half of that which is present soon after a full meal. Coagulation of the Blood. — One of the most characteristic proper- ties which the blood possesses is that of clotting or coagulating, when removed from the body. This phenomenon may be observed under the most favorable conditions in blood which has been drawn into an open vessel. In about two or three minutes, at the ordinary temperature of the air, the surface of the fluid is seen to become semi-solid or jelly-like; this change next taking place, in a minute or two, at the sides of the vessel in which it is contained, and then extending throughout the entire mass. The time which is required for the blood to become solid is about eight or nine minutes. The solid mass occupies exactly the same volume as the previously liquid blood, and adheres so closely to the sides of the contain- Fio. 67.— Reticulum of fibrin, from a drop of human blood, after treatment with rosanilin. (Ranvler.) ing vessel that if it be inverted none of its contents escape. The solid mass is the crassamentum or clot. If the clot be watched for a few min- utes, drops of a light straw-colored fluid, the serum, may be seen to make their appearance on the surface, and, as they become more and more nu- merous, run together, forming a complete superficial stratum above the solid clot. At the same time the fluid begins to transude at the sides and at the under surface of the clot, which in the course of an hour or two floats in the liquid. The first drops of serum appear on the surface about eleven or twelve minutes after the blood has been drawn; and the fluid continues to transude for from thirty-six to forty-eight hours. The clotting of blood is due to the development in it of a substance called fihHn, which appears as a meshwork (Fig. 67) of fine fibrils. This meshwork entangles and encloses within it the blood corpuscles, as clot- ting takes place too quickly to allow them to sink to the bottom of the plasma. The first clot formed, therefore, includes the whole of the con- VoL. I.— 5. 66 HAND-BOOK OF PHYSIOLOGY. stituents of the blood in an apparently solid mass, but soon the fibrinous meshwork begins to contract, and the serum which does not belong to the clot is squeezed out. When the whole of the serum has transuded, the clot is found to be smaller, but firmer and harder, as it is now made up of fibrin and blood corpuscles only. It will be noticed that coagulation rearranges the constituents of the blood according to the following scheme, liquid blood being made up of plasma and blood-corpuscles, and clotted blood of serum and clot. Liquid Blood. Plasma Corpuscles Serum Fibria Clot Clotted Blood Buffy Coat. — Under ordinary circumstances coagulation occurs, as we have mentioned above, before the red corpuscles have had time to sub- side; and thus from their being entangled in the meshes of the fibrin, the clot is of a deep red color throughout, somewhat darker, it may be, at the most dependent part, from accumulation of red corpuscles, but not to any very marked degree. When, however, coagulation is delayed from any cause, as when blood is kept at a temperature of 32° F. (0° C), or when clotting is normally a slow process, as in the case of horse^s blood, or, lastly, in certain diseased conditions of the blood in which clotting is naturally delayed, time is allowed for the colored corpuscles to sink to the bottom of the fluid. When clotting does occur, the upper layers of the blood, being free of colored corpuscles and consisting chiefly of fibrin, form a superficial stratum differing in appearance from the rest of the clot, in that it is of a grayish yellow color. This is known as the huffy coat." Cupped appearance of the Clot. — When the buffy coat has been produced in the manner just described, it commonly contracts more than the rest of the clot, on account of the absence of colored corpuscles from its meshes, and because contraction is less interfered with by adliesion to the interior of tlic containing vessel in the vertical than the horizontal direction. This produces a cup-like appearance of the buffy coat, and the clot is not only buffed but cupped on the surface. The bulled and cu]>ped appearance of the clot is well marked in certain states of the system, especially in inflammation, where the fibrin-forming constituents tu'c in ex(^(\sH, and it is also well marked in chlorosis where the corpuscles are deficient in quantity. THE BLOOD. Formation of Fibrin. — In describing the coagulation of the blood in the preceding paragraplis^ it was stated that this ];)henomenon was duo to the development in the clotting blood of a meshwork of fibrin. Thin may be demonstrated by taking recently-drawn blood, and whipping it with a bundle of twigs; the fibrin is found to adhere to the twigs as a reddish- white, stringy mass, having been thus obtained from the fluid nearly free from colored corpuscles. The defibrinated blood no longer retains the power of spontaneous coagulability. The fibrin which makes its appearance in the blood when it is under- going coagulation is derived chiefly, if not entirely, from the plasma or liquor sanguinis; for although the colorless corpuscles are intimately con- nected with the process in a way which will be presently explained, the colored corpuscles appear to take no active part in it whatever. This may be shown by experimenting with plasma free from colored corpuscles. Such plasma may be procured by delaying coagulation in blood, by keep- ing it at a low temperature, 32° F. (0° C), until the colored corpuscles which are of higher specific gravity than the other constituents of blood, have had time to sink to the bottom of the containing vessel, and to leave an upper stratum of colorless plasma, in the lower layers of which are many colorless corpuscles. The blood of the horse is specially suited for the purposes of this experiment; and the upper stratum of colorless plasma derived from it, if decanted into another vessel and exposed to the ordinary temperature of the air, will coagulate just as though it were the entire blood, producing a clot similar in all respects to blood clot, except that it is almost colorless from the absence of red corpuscles. If some of the plasma be diluted with neutral saline solution,' coagulation is de- layed, and the stages of the gradual formation of fibrin may be more con- veniently watched. The viscidity which precedes the complete coagula- tion may be seen to be due to fibrin fibrils developing in the fluid — first of all at the circumference of the containing vessel, and gradually extend- ing throughout the mass. Again, if plasma be whipped with a bundle of twigs, the fibrin may be obtained as a solid, stringy mass, just in the same way as from the entire blood, and the resulting fluid no longer retains its power of spontaneous coagulability. Evidently, therefore, fibrin is derived from the plasma and not from the colored corpuscles. In these experiments, it is not necessary that the plasma shall have been ob- tained by the process of cooling above described, as plasma obtained in any other way, e.g., by allowing blood to flow direct from the vessels of an animal into a vessel containing a third or a fourth of the bulk of the blood of a saturated solution of a neutral salt (preferably of magnesium sulphate) and mixing carefully, will answer the purpose, and, just as in the other case, the colored corpuscles will subside, leaving the clear super- ^ Neutral saline solution commonly consists of a '75 solution of common salt (sodium chloride) in water. 68 HAKD-BOOK OF PHYSIOLOGY. stratum of (salted) plasma. In order to cause this plasma to coagulate, it is necessary to get rid of the salts by dialysis, or to dilute it with several times its bulk of water. The antecedent of Fibrin. — If plasma be saturated with solid magnesium sulphate or sodium chloride, a white, stick}^ precipitate, called 2:)Iasmi7ie, is thrown down, after the removal of which, by filtration, the plasma will not spontaneously coagulate. This jjJas^niiie is soluble in dilute neutral saline solutions, and the solution of it speedily coagu- lates, producing a clot composed of fibrin. From this we see that blood plasma contains a substance without which it cannot coagulate, and a solution of which, is spontaneously coagulable. This substance is very soluble in dilute saline solutions, and is not, therefore, fibrin, which is insoluble in these fluids. We are, therefore, led to the belief that plas- mine produces or is converted into fibrin, when clotting of fluids contain- ing it takes place. Nature of Plasmine. — There seems distinct evidence that plasmine is a compound body made up of two or more substances, and that it is not mere soluble fibrin. This view is based upon the following observa- tions: — There exists in all the serous cavities of the body in health, e.g., the pericardium, the peritoneum, and the pleura, a certain small amount of transparent fluid, generally of a pale straw color, which in diseased conditions may be greatly increased. It somewhat resembles serum in appearance, but in reality differs from it, and is probably identical with plasma. This serous fluid is not, as a rule, spontaneously coagulable, but may be made to clot on the addition of serum, which is also a fluid which has no tendency of itself to coagulate. The clot produced consists of fibrin, and the clotting is identical with the clotting of plasma. From the serous fluid (that from the inflamed tunica vaginalis testis or hydrocele fluid is mostly used) we may obtain, by saturating it with solid mag- nesium sulphate or sodium chloride, a Avhite viscid substance as a precipi- tate which is called fibrinogen, which may be separated by filtration, and is then capable of being dissolved in water, as a certain amount of the neutral salt is entangled with the precipitate sufficient to produce a dilute saline solution in which it is soluble. Tliis body belongs to the glolmli)i class of proteid substances. Its solution has no tendency to clot of itself. Fibrinogen may also be obtained as a viscid precipitate from hydrocele fluid by diluting it with water, and passing a brisk stream of carbon dioxide gas through the solution. Now if serum be added to a solution of fibrinogen, the mixture clots. From serum may be obtained another globulin very similar in i)ro])er- tics to fi})rinogen, if it be subjected to treatment similar to either of the two methods by whicli fibrinogen is obtained from hydrocele iluid; this substance is called para globulin, and it may be separated by filtration ai d dissolved in a dilute saline solution in a manner similar to fibrinogen. THE BLOOD. G9 If the solutions of fibrinogen and ptiraglobulin be mixed, the mixture cannot be distinguished from a sohition of plasmine, and like that solu- tion (in a great majority of cases) firmly clots; whereas a mixture of the hydrocele fluid and serum, from which they have been respectively taken, no longer does so. In addition to this evidence of the compound nature of plasmine, it may be further shown that, if sufficient care be taken, both fibrinogen and paraglobulin may be obtained from plasma: fibrin- ogen, as a flaky precipitate, by adding carefully 1 3 per cent, of crystalline sodium chloride; and after the removal of fibrinogen from the plasma by filtration, paraglobulin- may be afterward precipitated, on the further addition of the same salt or of magnesium sulphate to the filtrate. It is evident, therefore, that both these substances must be thrown down to- gether when plasma is saturated with sodium chloride or magnesium sul- phate, and that the mixture of the two corresponds with plasmine. Presence of a Fibrin Ferment. — So far it has been shown that plasmine, the antecedent of fibrin in blood, to the possession of which blood owes its power of coagulating, is not a simple body, but is composed of at least two factors — viz., fibrinogen and paraglobulin; there is reason for believing that yet another body is associated with them in plasmine to produce coagulation; this is what is known under the name of fibrin ferment (Schmidt). It was at one time thought that the reason why hydrocele fluid coagulated when serum was added to it was that the latter fluid supplied the paraglobulin which the former lacked; this, however, is not the case, as hydrocele does not lack this body, and if paraglobulin, obtained from serum by the carbonic acid method, be added to it, it will not coagulate, neither will a mixture of solutions of fibrinogen and para- globulin obtained in the same way. But if paraglobulin, obtained by the saturation method, be added to hydrocele fluid, it will clot, as will also, as we have seen above, a mixed solution of fibrinogen and para- globulin, when obtained by the saturation method. From this it is evident that in plasmine there is something more than the two bodies above men- tioned, and that this something is precipitated with the paraglobulin by the saturation method, and is not precipitated by the carbonic acid method. The following experiments show that it is of the nature of a ferment. If defibrinated blood or serum be kept in a stoppered bottle with its own bulk of alcohol for some weeks, all the proteid matter is pre- cipitated in a coagulated form; if the precipitate be then removed by filtration, dried over sulphuric aci'd, finely powdered, and then suspended in water, a watery extract may be obtained by further filtration, contain- ing extremely little, if any, proteid matter. Yet a little of this watery extract will determine coagulation in fluids, e.g., hydrocele fluid or diluted plasma, which are not spontaneously coagulable, or which coagu- late slowly and with difficulty. It will also cause a mixture of fibrinogen and paraglobulin, obtained by the carbonic acid method, to clot. This 70 HAND-BOOK OF PHYSIOLOGY. watery extract appears to contain the body which is precipitated with the paragiobulin by the saturation method. Its active properties are entirely destroyed by boiling. The amount of the extract added does not influ- ence the amount of the clot formed, but only the rapidity of clotting, and moreover the active substance contained in the extract evidently does not form part of the clot, as it may be obtained from the serum after blood has clotted. So that the third factor, which is contained in the aqueous extract of blood, belongs to that class of bodies which promote the union of other bodies, or cause changes in other bodies, without themselves entering into union or undergoing change, i.e. ferments. The third sub- stance has, therefore, received the name fibrin ferment. This ferment is developed in blood soon after it has been shed, and its amount appears to increase for a certain time afterward (p. 74). The part played by Paragiobulin. — So far we have seen that plasmine is a body composed of three substances, viz., fibrinogen, para- giobulin, and fibrin ferment. The question presents itself, are these three bodies actively concerned in the formation of fibrin? Here we come to a point about which two distinct opinions prevail, and wliich it will be necessary to mention. Schmidt holds that fibrin is produced by the interaction of the two proteid bodies, viz., fibrinogen and para- giobulin, brought about by the presence of a special fibrin ferment. Also, that when coagulation does not occur in serum, which contains para- giobulin and the fibrin ferment, the non-coagulation is accounted for by lack of fibrinogen, and when it does not occur in fluids which contain fibrinogen, it is due to the absence of paragiobulin, or of the ferment, or of both. It Avill be seen that, according to this view, paragiobulin has a very important fibrino-plastic property. The other opinion, held by Hammersten, is that paragiobulin is not an essential in coagulation, or at any rate does not take an active part in the process. He believes that paragiobulin possesses the property in common with many other bodies of combining with — or decomposing, and so rendering inert — certain substances which have the power of prev-enting the formation or precipi- tation of fibrin, this power of preventing coagulation being well known to belong to tlie free alkalies, to the alkaline carbonates, and to certain salts; and he looks upon fibrin as formed from -fibrinogen, which is either (1) decomposed into that substance with the production of some other substances; or (2) bodily converted into it uiuler the action of a ferment, which is frequently precipitated with paragiobulin. Influence of Salts on Coagulation. — It is believed that the pres- ence of a certain but small amount of salts, especially of sodium chloride, is necessary for coagulation, and that without it, clotting cannot take ])la(;e. Sources of the Fibrin Generators. — It has been previously re- marked that the colorless corpuscles which arc always present in snuiller THE BLOOD. 71 or greater numbers in the plasma, even when this has been freed from colored corpuscles, have an important share in the production of the clot. The proofs of this maybe briefly summarized as follows: — (1) That all strongly coagulable fluids contain colorless corpuscles almost in direct proportion to their coagulability; (2) That clots formed on foreign bodies, such as needles inserted into the interior of living blood-vessels, are pre- ceded by an aggregation of colorless corpuscles; (3) That plasma in which the colorless corpuscles happen to be scanty, clots feebxy; (4) That if horse's blood be kept in the cold, so that the corpuscles subside, it will be found that the lowest stratum, containing chiefly colored cor- puscles, will, if removed, clot feebly, as it contains little of the fibrin factors; whereas the colorless plasma, especially the lower layers of it in which the colorless corpuscles are most numerous, will clot well, but if filtered in the cold will not clot so well, indicating ^hat when filtered nearly free from colorless corpuscles even the plasma does not contain suffi- cient of all the fibrin factors to produce thorough coagulation; (5) In a drop of coagulating blood, observed under the miscroscope, the fibrin fibrils are seen to start from the colorless corpuscles. Although the intimate connection of the colorless corpuscles with the process of coagulation seems indubitable, for the reasons just given, the exact share which they have in contributing the various fibrin factors remains still uncertain. It is generally believed that the fibrin-ferment at any rate is contributed by them, inasmuch as the quantity of this sub- stance obtainable from plasma bears a direct relation to the numbers of colorless corpuscles which the plasma contains. Many believe that the fibrinogen also is wholly or in part derived from them. Conditions affecting Coagulation. — The coagulation of the blood is hastened by the following means: — 1. Moderate warmth,— from about 100° to 120° F. (37-8—49° C). 2. Rest is favorable to the coagulation of blood. Blood, of which the whole mass is kept in uniform motion, as when a closed vessel completely filled with it is constantly moved, coagulates very slowly and imper- fectly. 3. Contact with foreign matter, and especially multiplication of the points of contact. Thus, coagulated fibrin may be quickly obtained from liquid blood by stirring it with a bundle of small twigs; and even in the living body the blood will coagulate upon rough bodies projecting into the vessels; as, for example, upon threads passed through them, or upon the heart's valves roughened by inflammatory deposits or calcareous accumulations. 1. The free access of air. — Coagulation is quicker in shallow than in tall and narrow vessels. 72 HAND-BOOK OF PHYSIOLOGY. 5. The addition of less than twice the bulk of water. The blood last drawn is said to coagulate more quickly than the first. The coagulation of the blood is retarded, suspended, or prevented by the following means: — 1. Cold retards coagulation; and so long as blood is kept at a tem- perature, 32° F. (0° C), it will not coagulate at all. Freezing the blood, of course, prevents its coagulation; yet it will coagulate, though not firmly, if thawed after being frozen; and it will do so, even after it has been frozen for several months. A higher temperature than 120° F. (49° C. ) retards coag- ulation, or, by coagulating the albumen of the serum, prevents it altogether. 2. The addition of water in greater proportion than twice the bulk of the blood. 3. Contact with living tissues, and especially with the interior of a living blood-vessel. 4. The addition of neutral salts in the proportion of 2 or 3 per cent, and upward. When added in large proportion most of these saline substances prevent coagulation altogether. Coagulation, however, ensues on dilution with water. The time during which blood can be thus pre- served in a liquid state and coagulated by the addition of water, is quite indefinite. 5. Imperfect Aeration, — as in the blood of those who die by as- phyxia. 6. In inflammatory states of the system the blood coagulates more slowly although more firmly. 7. Coagulation is retarded by exclusion of the blood from the air, as by pouring oil on the surface, etc. In vacuo, the blood coagulates quickly; but Lister thinks that the rapidity of the process is due to the bubbling which ensues from the escape of gas, and to the blood being thus brought more freely into contact with the containing vessel. 8. The coagulation of the blood is prevented altogether by the ad- dition of strong acids and caustic alkalies. 9. It has been believed, and cliiefly on the authority of Hunter, that after certain modes of death the blood does not coagulate: he enumerates the death by lightning, over-exertion (as in animals hunted to death), blows on the stomach, fits of anger. He says, "I have seen instances of them all." Doubtless he had done so; but the results of such events are not constant. The blood has been often observed coagulated in the bodies of animals killed by lightning or an electric shock; and Gulliver has published instances in which he found clots in the hearts of liiires and stags Inuitod to death, aiul of corks killed in fighting. Cause of the fluidity of the blood within the living body. — Very closely connected with the problem of the coagulation of the blood arises the question, — why does the blood remain li(]uid within the living body? AVe liave certain pathological and experimental facts, a])parently THE BLOOD. 73 opposed to one another, which bear upon it, and these may be, for the sake of clearness, classed under two heads: — (1) Blood ivill coagulate within the living body under certain condi- tions, — for example, on ligaturing an artery, whereby the inner and mid- dle coats are generally ruptured, a clot will form within it, or by passing a needle through the coats of the vessel into the blood stream a clot will gradually form upon it. Other foreign bodies, e.g. wire, thready etc., produce the same effect. It is a well-known fact that small clots are apt to form upon the roughened edges of the valves of the heart when the roughness has been produced by inflammation, as in endocarditis, and it is also equally true that aneurisms of arteries are sometimes spontaneously cured by the deposition within them, layer by layer, of fibrin from the blood stream, which natural cure it is the aim of the physician or surgeon to imitate. (2) Blood will remain liquid under certain conditions outside the body, without the addition of any re-agent, even if exposed to the air at the ordinary temperature. It is well known that blood remains fluid in the body for some time after death, and it is only after rigor mortis has oc- curred that the blood is found clotted. It has been demonstrated by Hewson, and also by Lister, that if a large vein in the horse or similar animal be ligatured in two places some inches apart, and after some time be opened, the blood contained within it will be found fluid, and that coagulation will occur only after a considerable time. But this is not due to occlusion from the air simply. Lister further showed that if the vein with the blood contained within it be removed from the body and then be carefully opened, the blood might be poured from the vein into another similarly prepared, as from one test-tube into another, thereby suffering free exposure to the air, without coagulation occurring as long as the vessels retain their vitality. If the endothelial lining of the vein, however, be injured, the blood will not remain liquid. Again, blood will remain liquid for days in the heart of a turtle, which continues to beat for a very long time after removal from the body. Any theory which aims at explaining the fluidity under the usual conditions of the blood within the living body must reconcile the above apparently contradictory facts, and must at the same time be made to in- clude all the other known facts concerning the coagulation of the blood. We may therefore dismiss as insufficient the following; — ^that coagulation is due to exposure to the air or oxygen; that it is due to the cessation of the circulatory movement; that it is due to evolution of various gases, or to the loss of heat. Two theories, those of Lister and Briicke, remain. The former sup- poses that the blood has no natural tendency to clot, but that its coagula- tion out of the body is due to the action of foreign matter with which it happens to be brought into contact, and in the body to conditions of the 74 HAND-BOOK OF PHYSIOLOGY. tissues whicli cause them to act toward it like foreign matter. The lat- ter, on the other hand, supposes that there is a natural tendency on the part of the blood to clot, but that this is restrained in the living body by some inhibitory power resident in the walls of the containing vessels. Support was once thought to be given to Briicke's and like theories by cases of injury, in which blood extravasated in the living body has seemed to remain uncoagulated for weeks, or even months, on account of its contact with living tissues. But the supposed facts have been shown to be without foundation. The blood-like fluid in such cases is not uncoag- ulated blood, but a mixture of serum and blood-corpuscles, with a certain proportion of clot in various stages of disintegration. (Morrant Baker.) As the blood must contain the substances from which fibrin is formed, and as the re-arrangement of these substances occurs very quickly when- ever the blood is shed, so that it is somewhat difficult to prevent coagula- tion, it seems more reasonable to hold with Briicke, that the blood has a strong tendency to clot, rather than with Lister, that it has no special tendency thereto. It has been recently suggested that the reason why blood does not coagulate in the living vessels, is that the factors which we have seen are necessary for the formation of fibrin are not in the exact state required for its production, and that the fibrin ferment is not formed or is not, at any rate, free in the living blood, but that it is produced (or set free) at the moment of coagulation by the disintegration of the colorless corpuscles. This supposition is certainly plausible, but if it be a true one, it must be assumed either that the living blood-vessels exert a restraining influence upon the disintegration of the corpuscles in sufficient numbers to form a clot, or that they render inert any small amount of fibrin ferment which may have been set free by the disintegration of a few corpuscles; as it is certain that corpuscles of all kinds must from time to time disintegrate in the blood without causing it to clot; and, secondly, that shed and defibrinated blood which contains blood corpuscles, broken down and dis- integrated, will not, when injected into the vessels of an animal, produce clotting. There must be a distinct difference, therefore, if only in amount, between the normal disintegration of a few colorless corpuscles in the living uninjured blood-vessels and the abnormal disintegration of a large number which occurs whenever tlie blood is shed without suitable precaution, or when coaguhition is unrestrained by the neighborhood of the living uninjured blood-vessels. The I^looi) Corpuscles or Blood-Cells. There are two priiicipMl forms of corpuscles, ihc red and the white, or, as they are now frc(|ii(Mitly named, the colored and the colorless. In the moist state, tlio red corpuscles form abont -15 per cent, by weight. THE BLOOD 75 of the whole mass of the blood. The proportion of colorless corpuscles is only as 1 to 500 or 600 of the colored. Red or Colored Corpuscles. — Human red blood-corpuscles are circular, biconcave disks with rounded edges, from to ^^Vo i^'^h diameter, and t^-^q- i^^^ thickness, becoming flat or convex on addi- tion of water. When viewed singly, they appear of a pale yellowish tinge; the deep red color which they give to the blood being observable in them only when they are seen en masse. They are composed of a colorless, structureless, and transparent filmy framework or stroma, infiltrated in all parts by a red coloring matter termed Immoglolin. The stroma is tough and elastic, so that, as the cells circulate, they admit of elonga,tion and other changes of form, in adaptation to the vessels, yet recover their natural shape as soon as they escape from compression. The term cell, in the sense of a bag or sac, is inapplicable to the red blood corpus- cle; and it must be considered, if not stolid throughout, yet as having no such variety of consistence in different parts -as to justify the notion ol its being a membranous sac with fiuid contents. The stroma exists in all parts of its sub- stance, and the coloring-matter uni- formly pervades this, and is not merely surrounded by and mechanically en- closed within the outer wall of the corpuscle. The red corpuscles have no nuclei, although, in their usual state, the unequal refraction of transmitted fig. 68.-Red corpuscles in rouleaux. At light gives the appearance of a central c., a, are two white corpuscles. spot, brighter or darker than the border, according as it is viewed in or out of focus. Their specific gravity is about 1088. Varieties. — The red corpuscles are not all alike, some being rather larger, paler, and less regular than the majority, and sometimes flat or slightly convex, with a shining particle apparent like a nucleolus. In almost every specimen of blood may be also observed a certain number of corpuscles smaller than the rest. They are termed microcytes, and are probably immature corpuscles. A peculiar property of the red corpuscles, exaggerated in inflammatory blood, may be here again noticed, i.e., their great tendency to adhere to- gether in rolls or columns, like piles of coins. These rolls quickly fasten together by their ends, and cluster; so that, when the blood is spread out thinly on a glass, they form a kind of irregular network, with crowds of corpuscles at the several points corresponding with the knots of the net (Fig. 68). Hence, the clot formed in such a thin layer of blood looks mottled with blotches of pink upon a white ground, and in a larger quan- 76 HAND-BOOK OF PHYSIOLOGY. tity of such blood help, by the consequent rapid subsidence of the cor- puscles, in the formation of the bulfy coat already referred to. This tendency on the part of the red corpuscles, to form rouleaux, is probably only a physical phenomenon, comparable to the collection into somewhat similar rouleaux of discs of corks when they are partially im- mersed in water. (Xorris.) 3Iammals. Birds. Reptiles. Amphibia. Fish. Fig. 69.1 ' The above illustration is somewhat altered from a drawinc; by Gulliver, in the Proceed. Zool. Society, and exhibits the typical characters of the red blood-cells in the main divisions of the Vcrtebrata. The fractions are those of an inch, and represent the averai!;(' diameter. In the case of the oval cells, only the lonii; diameter is here f^iven. It is remarkable, that althoui^h the size of the red blood-cells varies so much in the different classes of the vertebrate kini^dom, that of the ^vhite corpuscles re- mains comparatively uniform, and thus they are, in some animals, nuich greater, in others much l(;ss than the red corpuscles existing side by side with them. THE BLOOD. 77 Action of Reagents. — Considerable light has been thrown on the physical and clioniical constitution of red blood -cells by studying the eifects produced by mechanical means and by various reagents: the fol- lowing is a brief stimmary of these reactions: — Pressure. — If the red blood-cells of a frog or man are gently squeezed, they exhibit a wrinkling of the surface, which clearly indicates that there is a superficial pellicle partly differentiated from the softer mass within; again, if a needle be rapidly drawn across a drop of blood, several cor- puscles will be found cut in two, but this is not accompanied by any es- cape of cell contents; the two halves, on the contrary, assume a rounded form, proving clearly that the corpuscles are not mere membranous sacs with fluid contents like fat-cells. Fluids. — Water. — When water is added gradually to frog's blood, the oval disc-shaped corpuscles become spherical, and gradually discharge their haemoglobin, a pale, transparent stroma being left behind; human red blood-cells change from a discoidal to a spheroidal form, and dis- charge their cell-contents, becoming quite transparent and all but invisible. 8aline solution (dilute) produces no appreciable effect on the red blood-cells of the frog. In the red blood-cells of man the discoid shape is exchanged for a spherical one, with spinous projections, like a horse- chestnut (Fig. 70). Their original forms can be at once restored by the use of carbonic acid. Acetic acid (dilute) causes the nucleus of the red blood cells in the frog to become more clearly defined; if the action is prolonged, the nu- cleus becomes strongly granulated, and. all the coloring matter seems to be concentrated in it, the surrounding cell-substance and outline of the cell becoming almost invisible; after a time the cells lose their color alto- gether. The cells in the figure (Fig. 71) represent the successive stages of the change. A similar loss of color occurs in the red cells of human blood, which, however, from the absence of nuclei, seem to disappear entirely. Alhalies cause the red blood-cells to swell and finally disappear. Chloroform added to the red blood-cells of the frog causes them to part with their haemoglobin; the stroma of the cells becomes gradually broken up. A similar effect is produced on the human red blood-cell. Tannin. — When a 2 per cent, solution of tannic acid is applied to frog's blood it causes the appearance of a sharply-defined little knob, pro- jecting from the free surface: the coloring matter becomes at the same time concentrated in the nucleus, which grows more distinct (Fig. 72). Fig. 70. Fig. 71. Fig. 72. 78 HAND-BOOK OF PHYSIOLOGY. A somewhat similar effect is produced on the human red blood-cell. (Eoberts.) Magenta, when applied to the red blood-cells of the [frog, produces a similar little knob or knobs, at the same time staining the nucleus and causing the discharge of the hsemoglobin. (Eoberts.) The first effect of the magenta is to cause the discharge of the haemoglobin, then the nucleus becomes suddenly stained, and lastly a finely granular matter issues through the wall of the corpuscle, becoming stained by the magenta, and a macula is formed at the point of escape. A similar macula is produced in the human red blood-cell. Boracic acid. — A 2 per cent, solution applied to nucleated red blood- cells (frog) will cause the concentration of all the coloring matter in the nucleus; the colored body thus formed gradually quits its central position, and comes to be partly, sometimes entirely, protruded from the surface of the now colorless cell (Fig. 73). The result of this experiment led Brlicke to distinguish the colored contents of the cell (zooid) from its colorless stroma (oecoid). When applied to the non-nucleated mammalian corpuscle its effect merely resembles that of other dilute acids. Gases — Carlonic acid. — If the red blood-cells of a frog be first exposed Fig. 73. Fig. 74. Fig. 75. Fig. 76. to the action of water-vapor (which renders their outer pellicle more readily permeable to gases), and then acted on by carbonic acid, the nuclei immediately become clearly defined and strongly granulated; when air or oxygen is admitted the original appearance is at once restored. The upper and lower cell in Fig. 74 show the effect of carbonic acid; the middle one the effect of the re-admission of air. These effects can be reproduced five or six times in succession. If, however, the action of the carbonic acid be much prolonged, the granulation of the nucleus becomes permanent; it appears to depend on a coagulation of the paraglobulin. (Strieker.) Ammonia. — Its effects seem to vary according to the degree of con- centration. Sometimes the outline of the corpuscles becomes distinctly crenatcd; at other times the effect resembles that of boracic acid, while in other cases the edges of the corpuscles begin to break up. (Lankester.) The effect of heat up to 120°— 140° F. (50°— 60° 0.) is to cause the formation of a number of bud-like ])rocesses (Fig. 75). ElpciricUy causes the red blood-corpusclos to become crenatcd, and at lengtli mulberry-like. Finally they recover their round form and become quite i)ale. THE BLOOD. 79 The general conclusions to be drawn from these observations have been summed up as follows by Prof. Ray Lankester: — "The red blood-corpuscle of the vertebrata is a viscid, and at the same time elastic disc, oval or round in outline, its surface being differentiated somewhat from the underlying material, and forming a pellicle or mem- brane of great tenuity, not distinguishable with the highest powers (whilst the corpuscle is normal and living), and having no pronounced inner limitation. The viscid mass consists of (or rather yields, since the state of combination of the components is not known) a variety of albuminoid and other bodies, the most easily separable of which is haemoglobin; sec- ondly, the matter which segregates to form Eoberts's macula; and thirdly, a residuary stroma, apparently homogeneous in the mammalia (excepting as far as the outer surface or pellicle may be of a different chemical nature), but containing in the other vertebrata a sharply definable nucleus, this nucleus being already differentiated, but not sharply deline- ated during life, and consisting of, or separable into, at least two com- ponents, one (paraglobulin) precipitable by carbon dioxide, and remov- able by the action of weak ammonia; the other pellucid, and not gran- ulated by acids."' The White or Colorless Corpuscles. — In human olood the white or colorless corpuscles or leucocytes are nearly spherical masses of granular protoplasm without cell wall. The granular appearance, more marked in some than in others {vide infra), is due to the presence of par- ticles probably of a fatty nature. In all cases one or more nuclei exist in each corpuscle. The size of the corpuscle averages g-^^-g- of an inch in diameter. In health, the proportion of white to red corpuscles, which, taking an average, is about 1 to 500 or 600, varies considerably even in the course of the same day. The variations appear to depend chiefly on the amount and probably also on the kind a b of food taken; the number of leuco- cytes being very considerably increased by a meal, and diminished again on fasting. Also in young persons, dur- ing pregnancy, and after great loss fig. 77.-A. Three colored biood-corpi^dls. of blood, there is a larger proportion ?y Sfc^a^SS^lM^^ of colorless blood-corpuscles, which x probably shows that they are more rapidly formed under these circum- stances. In old age, on the other hand, their proportion is diminished. Varieties. — The colorless corpuscles present greater diversities of form than the red ones do. Two chief varieties are to be seen in human blood; one which contains a considerable number of granules, and the other which is paler and less granular. In size the variations are great, for in most specimens of blood it is possible to make out, in addition to 80 HAOT)-BOOK OF PHYSIOLOGY. the full-sized yarieties, a number of smaller corpuscles, consisting of a large spherical nucleus surrounded by a variable amount of more or less granular protoplasm. The small corpuscles are, in all probability, the undeveloped forms of the others, and are derived from the cells of the lymph. Besides the above-mentioned varieties, Schmidt describes another form which he looks upon as intermediate between the colored and the colorless forms, viz., certain corpuscles which contain red granules of hemoglobin in their protoplasm. The different varieties of colorless cor- puscles are especially well seen in the blood of frogs, newts, and other cold-blooded animals. AmcEboid movement. — A remarkable property of the colorless cor- puscles consists in their capability of spontaneously changing their shape. This was first demonstrated by Wharton Jones in the blood of the skate. If a drop of blood be examined with a high power of the microscope on a warm stage, or, in other words, under conditions by which loss of mois- ture is prevented, and at the same time the temperature is maintained at about that of the blood in its natural state within the walls of the living vessels, 100° F. (37 '8° C), the colorless corpuscles will be observed slowly altering their shapes, and sending out processes at various parts of their circumference. This alteration of shape, which can be most conveniently YiG. 78. — Human colorless blood-corpuscle, showing its successive changes of outline within ten minutes when kept moist on a warm stage. (Schofield.) studied in the newt's blood, is called amoeboid, inasmuch as it strongly resembles the movement of the lowly organized amceha. The processes which are sent out are either lengthened or withdrawn. If lengthened, the protoplasm of the whole corpuscle flows as it were into its process, and the corpuscle changes its position; if withdrawn, protrusion of another process at a different point of the circumference speedily follows. The change of position of the corpuscle can also take place by a flowing movement of the whole mass, and in this case the locomotion is compar- atively rapid. The activity both in the processes of change of shape and also of change in position, is much more marked in some corpuscles, viz., in the granular variety, than in others. Klein states that in the newt's blood the changes are especially likely to occur in a variety of the colorless corpuscle, which consists of masses of finely granular protoplasm with Jagged outline, containing three or four nuclei, or of large irregular masses of i)rotoplasm containing from five to twenty nuclei. Another plienomenon may be observed in such a specimen of blood, viz., the divi- sion of the c()rpus(iles, which occurs in the following way. A cleft takes place in the protoplasm at one point, which becomes deeper and deeper, •fllE BLOOD. 81 and then by the lengthening out and attenuation of the connection, and finally by its rupture, two corpuscles result. The nuclei have previously undergone division. The cells so formed are said to be remarkably active in their movements. Thus we see that the rounded form which the colorless corpuscles present in ordinary microscopic specimens must be looked upon as the shape natural to a dead corpuscle or to one whose vitality is dormant rather than as the shape proper to one living and active. Action of re-agents upon the colorless corpuscles. — Feeding the corpuscles. — If some fine pigment granules, e.g., powdered vermilion, be added to a fluid containing colorless blood-corpuscles, on a glass slide, these will be observed, under the microscope, to take up the pigment. In some cases colorless corpuscles have been seen with fragments of colored ones thus embedded in their substance. This property of the colorless corpuscles is especially interesting as helping still further to connect them with the lowest forms of animal life, and to connect both with the organ- ized cells of which the higher animals are composed. The property which the colorless corpuscles possess of passing through the walls of the blood-vessels will be described later on. Enumeration of the Red and White Corpuscles. — Several methods are employed for counting the blood-corpuscles, most of them depending upon the same principle, i.e., the dilution of a minute volume of blood with a given volume of a colorless solution similar in specific gravity to blood serum, so that the size and shape of the corpuscles is altered as little as possible. A minute quantity of the well-mixed solu- tion is then taken, examined under the microscope, either in a flattened capillary tube (Malassez) or in a cell (Hayem & Nachet, Growers) of known capacity, and the number . of corpuscles in a measured length of the tube, or in a given area of the cell is counted. The length of the tube and the area of the cell are ascertained by means of a micrometer scale in the microscope ocular; or in the case of Gowers^ modification, by the division of the cell area into squares of known size. Having ascer- tained the number of corpuscles in the diluted blood, it is easy to find out the number in a given volume of normal blood. Gowers^ modifica- tion of Hayem & !N"achet's instrument, called by him Hcemacytometer," appears to be the most convenient form of instrument for counting the corpuscles, and as such will alone be described (Fig. 79). It consists of a small pipette (a), which, when filled up to a mark on its stem, holds- 995 cubic millimetres. It is furnished with an india-rubber tube and glass mouth-piece to facilitate filling and emptying; a capillary tube (b) marked to hold 5 cubic millimetres, and also furnished with an india- rubber tube and mouthpiece; a small glass jar (d) in which the dilution of the blood is performed; a glass stirrer (e) for mixing the blood thoroughly, (r) a needle, the length of which can be regulated by a Vol. I.— 6. 82 HAND-BOOK OF PHYSIOLOGY. screw; a brass stage plate (c) carrying a glass slide, on which is a cell one-fifth of a millimetre deep, and the bottom of which is divided into one-tenth millimetre squares. On the top of the cell rests the cover glass, which is kept in its place by the pressure of two springs proceeding from the stage plate. A standard saline solution of sodium sulphate, or similar salt, of specific gravity 1025, is made, and 995 cubic millimetres are measured by means of the pipette into the glass jar, and with this five cubic millimetres of blood, obtained by pricking the finger with a needle, and measured in the capillary pipette (b), are thoroughly mixed by the Fig. 79.— Hsemacytometer. glass stirring-rod. A drop of this diluted blood is then placed in the cell and covered with a cover-glass, which is fixed in position by means of the two lateral springs. The preparation is then examined under a micro- scope with a power of about 400 diameters, and focussed until the line-s dividing the cell into squares are visible. After a short delay, the red corpuscles which have sunk to the bottom of the cell, and are resting on the squares, are counted in ten squares, and the number of white corpuscles noted. By adding together the numbers counted in ton (one-tenth millimetre) squares the number of corpuscles in one-cubic millimetre of blood is obtained. The average number of corpuscles per viwh cubic millinu^tro of healthy blood, aecM^rd- ing to Vicrordt and Wolcker, is 5,000,000 in adult men, and rather fewer in women. THE BLOOD. 83 Chemical Composition of the Blood in Bulk. — Water Solids- Corpuscles Proteids {ot serum) Fibrin (of clot) . Fatty matters (of serum) Inorganic salts (of serum) Gases, kreatin, urea and other extractive ) matter, glucose and accidental sub- >• stances ) 784 130 70 2-2 1-4 6 6-4— 216 1,000 Chemical Composition of the Red Corpuscles.— Analysis of a thousand parts of moist blood corpuscles shows the following as the result: — Water . 688 303.88 8.12—312 1,000 Of the solids the most important is HmmogloUn, the substance to which the blood owes its color. It constitutes, as will be seen from the appended Table, more than 90 per cent, of the organic matter of the corpuscles. Besides haemoglobin there are proteid ^ and fatty matters, the former chiefly consisting of globulins, and the latter of cholesterin and lecithin. In 1000 parts organic matter are found: — , Haemoglobin 905*4 Proteids 86*7 Fats 7-9 1,000- Of the inorganic salts of the corpuscles, with the iron omitted- In 1000 parts corpuscles (Schmidt) are found Potassium Chloride Phosphate sulphate Sodium ^' Calcium " Magnesium Soda •679 •343 •132 •633 •094 •060 •341 7^282 ^ An account of the proteid bodies, etc., will be found in the Appendix, and should be referred to for explanation of the terms employed in the text. 84 HAND-BOOK OF PHYSIOLOGY. The properties of hsemoglobin will be considered in relation to the Gases of the blood. Chemical Composition of the Colorless Corpuscles.— In conse- quence of the difficulty of obtaining colorless corpuscles in sufficient num- ber to make an analysis, little is accurately known of their chemical com- position; in all probability, however, the stroma of the corpuscles is made up of proteid matter, and the nucleus of nuclein, a nitrogenous phos- phorus-containing body akin to mucin, capable of resisting the action of the gastric juice. The proteid matter (globulin) is soluble in a ten per cent, solution of sodium chloride, and the solution is precipitated on the addition of water, by heat and by the mineral acids. The stroma con- tains fatty granules, and in it also the presence of glycogen has been demonstrated. The salts of the corpuscles are chiefly potassium, and of these the phosphate is in greatest amount. Chemical Composition of the Plasma or Liquor Sanguinis. — The liquid part of the blood, the plasma or liquor sanguinis in which the corpuscles float, may be obtained in the ways mentioned under the head of the Coagulation of the Blood. In it are the fibrin factors, inasmuch as when exposed to the ordinary temperature of the air it undergoes coag- ulation and splits up into fibrin and serum. It differs from the serum in containing fibrinogen, but in appearance and in reaction it closely resembles that fluid; its alkalinity, however, is less than that of the serum obtained from it. It may be freed from white corpuscles by filtra- tion at a temperature below 41 °F. (5°C.) Fibrin. — The part played by fibrin in the formation of a clot has been already described (p. 66), and it is only necessary to consider here its general properties. It is a stringy elastic substance belonging to the proteid class of bodies. It is insoluble in water and in weak saline solu- tions, it swells up into a transparent jelly when placed in dilute-hydro- chloric acid, but does not dissolve, but in strong acid it dissolves, pro- ducing acid-albumin;' it is also soluble on boiling in strong saline solu- tions. Blood contains only '2 per cent, of fibrin. It can be converted by the gastric or pancreatic juice into peptone. It possesses the power of liberating the oxygen from solutions of hydric peroxide H^O^. This may be shown by dipping a few shreds of fibrin in tincture of guaiacum and tlien immersing them in a solution of hydric peroxide. The fibrin becomes of a bluish color, from its having liberated from the solution oxygen, which oxidizes the resin of guaiacum contained in the tincture and thus produces the coloration. ' Tlio \ise of the two words albumen and albumin mny need explanation. The fornuir is Die (leneric, word which may include several albuminous or i>rotoid bodies, c.r/., albumen of blood; the latter, which requires to be qualitied by another word, is the spccidc form, and is jipjilied to variclics. cj/., ci^g-albumin. serum-albumin. THE BLOOD. 85 Salts of the Plasma. — In 1000 parts plasma there are:— Sodium Chloride 5-540 Soda 1*532 Sodium Phosphate '271 Potassium chloride ....... '359 " sulphate ........ '281 Calcium phosphate ...... '298 Magnesium phosphate . . . . . . '218 8.505 Serum. — The serum is the liquid part of the blood or of the plasma remaining after the separation of the clot. It is an alkaline, yellowish, transparent fluid, with a specific gravity of from 1025 to 1032. In the usual mode of coagulation, part of the serum remains in the clot, and the rest, squeezed from the clot by its contraction, lies around it. Since the contraction of the clot may continue for thirty-six or more hours, the quantity of serum in the blood cannot be even roughly estimated till this period has elapsed. There is nearly as much, by weight, of serum as there is clot in coagulated blood. Chemical Composition of the Serum. — Water about Proteids: a. Serum-albumin . . . . . ) Paraglobulin . . . . . . j Salts. Fats — including fatty acids, cholesterin, lecithin; and some soaps ....... Grape sugar in small amount .... Extractives — kreatrn, kreatinin, urea, etc. . . > Yellow pigment, which is independent of haemo- globin ........ Gases — small amounts of oxygen, nitrogen, and carbonic acid 1000 Water. — The water of the serum varies in amount according to the amount of food, drink, and exercise, and with many other circumstances. Proteids. — a. Serum-albumin is the chief proteid found in serum. It is precipitated on heating the serum to 140° F. (60° C), and entirely coagulates at (167° F. 75° 0.), and also by the addition of strong acids, such as nitric and hydrochloric; by long contact with alcohol it is precipitated. It is not precipitated on addition of .ether, and so differs from the other native albumin, viz., e^/^/-albumin. When dried at 104°F. (40° 0.) serum-albumin is a brittle, yellowish substance, soluble in water, possessing a lasvo-rotary power of — 56^. It is with great difficulty 900 80 20 86 HAND-BOOK OF PHYSIOLOaY. freed from its salts, and is precipitated by solutions of metallic salts, e.g. , of mercuric chloride, copper sulphate, lead acetate, sodium tungstate, etc. If dried at a temperature over 167° F. (75° C.) the residue is insoluble in water, having been changed into coagulated proteid. /?. Paraglobulin can be obtained as a white precipitate from cold serum by adding a considerable excess of water and passing through it a current of carbonic acid gas or by the cautious addition of dilute acetic acid. It can also be obtained by saturating serum with crystallized sulphate mag- nesium or chloride sodium. When obtained in the latter way precipita- tion seems to be much more complete than by means of the former method. Paraglobulin belongs to the class of proteids called globulins. The proportion of serum-albumin to paraglobulin in human blood serum is as 1-511 to 1. The salts of sodium predominate in serum as in plasma, and of these the chloride generally forms by far the largest proportion. Fats are present partly as fatty acids and partly emulsified. The fats are triolein, tristearin, and tripalmitin. The amount of fatty matter varies according to the time after, and the ingredients of, a meal. Of cholesterin and lecithin there are mere traces. Grape sugar is found principally in the blood of the hepatic vein, about one part in a thousand. The extractives vary from time to time; sometimes uric and hip- puric acids are found in addition to urea, kreatin and kreatinin. Urea exists in proportion from '02 to '04 per cent. The yellow pigment of the serum and the odorous matter which gives the blood of each particular animal a peculiar smell, have not yet been properly isolated. Variations in" healthy Blood ukder different Circumstakces. The conditions which appear most to influence the composition of the blood in health are these: Sex, Pregnancy, Age, and Temperament. The composition of the blood is also, of course, much influenced by diet. 1. Sex. — The blood of men differs from that of women, chiefly in be- ing of somewhat higher specific gravity, from its containing a relatively larger quantity of red corpuscles. 2. Pregnancy. — The blood of pregnant women has a ratlier lower (Specific gravity than the average, from deficiency of red corpuscles. The (juantity of white corj)Uscles, on the other hand, and of fibrin, is in- creased. 3. Age. — It appears that the blood of the foetus is very rich in solid matter, and especially in red corpuscles; and this condition, gradually diniinishing, continues for some weeks after birth. The quantity of solid matter then falls during childhood below the average, again rises during adult life, and in old age falls again. THE BLOOD. 87 4. Temperament. — But little more is known concerning the connection of this with the condition of the blood, than that there appears to be a relatively larger quantity of solid matter, and particularly of red corpuscles, in those of a plethoric or sanguineous temperament. 5. Diet. — Such differences in the composition of the blood as are due to the temporary presence of various matters absorbed with the food and drink, as well as the more lasting changes which must result from gener- ous or poor diet respectively, need be here only referred to. Effects of Bleeding. — The result of bleeding is to diminish the specific gravity of the blood; and so quickly, that in a single venesection, the portion of blood last drawn has often a less specific gravity than that of the blood that flowed first. This is, of course, due to absorption of fluid from the tissues of the body. The physiological import of this fact, namely, the instant absorption of liquid from the tissues, is the same as that of the intense thirst which is so common after either loss of blood, or the ab- straction from it of watery fluid, as in cholera, diabetes, and the like. For some littlQ time after bleeding, the want of red corpuscles is well marked; but with this exception, no considerable alteration seems to be produce^ in the composition of the blood for more than a very short time: the loss of the other constituents, including the pale corpuscles, being very quickly repaired. VaEIATIONS IK THE COMPOSITIOI^" OF THE BlOOD, DIFFERENT PaRTS OF THE Body. The composition of the blood, as might be expected, is found to vary in different parts of the body. Thus arterial blood differs from venous; and although its composition and general characters are uniform through- out the whole course of the systemic arteries, they are not so throughout the venous system, — the blood contained in some veins differing remarka- bly from that in others. Differences between Arterial and Venous Blood. — The differ- ences between arterial and venous blood are these: — («.) Arterial blood is bright red, from the fact that almost all its haemoglobin is combined with oxygen (Oxyhgemoglobin, or scarlet haemo- globin), while the purple tint of venous blood is due to the deoxida- tion of a certain quantity of its oxyhsemoglobin, and its consequent reduc- tion to the purple variety (Deoxidized, or purple hemoglobin). (&.) Arterial blood coagulates somewhat more quickly. {c. ) Arterial blood contains more oxygen than venous, and less carbonic acid. Some of the veins contain blood which differs from the ordinary stand- ard considerably. These are the Portal, the Hepatic, and the Splenic veins. Portal vein. — The blood which the portal vein conveys to the liver is supplied from two chief sources; namely, that in the gastric and mesen- teric veins, which contains the soluble elements of food absorbed from the 88 HAND-BOOK OF rHYSlOLOGY. stomach and intestines during digestion, ^md that in the splenic vein; it must, therefore, combine the qualities of the blood from each of these sources. The blood in the gastric and mesenteric veins will vary much accord- ing to the stage of digestion and the nature of the food taken, and can therefore be seldom exactly the same. Speaking generally, and without considering the sugar, dextrin, and other soluble matters which may have been absorbed from the alimentary canal, this blood appears to be defi- cient in solid matters, especially in red corpuscles, owing to dilution by the quantity of water absorbed, to contain an excess of albumin, and to yield a less tenacious kind of fibrin than that of blood generally. The blood from the splenic vein is generally deficient in red corpuscles, and contains an unusually large proportion of proteids. The fibrin ob- tainable from the blood seems to vary in relative amount, but to be almost always above the average. The proportion of colorless corpuscles is also unusually large. The whole quantity of solid matter is decreased, the diminution appearing to be chiefly in the proportion of red corpuscles. The blood of the portal vein, combining the peculiarities of its two factors, the splenic and mesenteric venous blood, is usually of lower specific gravity than blood generally, is more watery, contains fewer red corpuscles, more proteids, and yields a less firm clot than that yielded by other blood, owing to the deficient tenacity of its fibrin. Guarding (by ligature of the portal vein) against the possibility of an error in the analysis from regurgitation of hepatic blood into the portal vein, recent observers have determined that hepatic venous Uoocl contains less water, albumen, and salts, than the blood of the portal vein; but that it yields a much larger amount of extractive matter, in which is one con- stant element, namely, grape-sugar, which is found, whether saccharine or farinaceous matter have been present in the food or not. The Gases of the Blood. The gases contained in the blood are Carbonic acid. Oxygen, and Nitro- gen, 100 volumes of blood containing from 50 to 60 volumes of these gases collectively. Arterial blood contains relatively more ox3^gen and less carbonic acid than venous. But the absolute quantity of carbonic acid is in both kinds of blood greater than that of the oxygen. Oxygen. Carbonic Acid. Nitroo-en. Arterial Blood . . 20 vol. ^ler cent. 39 vol. per cent. 1 to 2 vols. Venous " (from muscles at rest) 8 to 12 " " " 40 " " 1 to 2 vols. The Extraction of tlie Gases from the lUood. — As the ordinary air- ])umps are not sufficiently powerful for the puri)ose, the extraction of the gases from the blood is accomplisiied by means of a mercurial air-i)um]), of wliich there are many varieties, those of Ludwig, Alvcrgnidt, Geissler, and Si)rengel being the chief. The principle of action in all is much the THE BLOOD. 89 same. Ludwig's pump, which may be taken as a type, is represented in the figure. It consists of two fixed globes, C and F, the upper one com- municating by means of the stopcock D, and a stout india-rubber tube with another glass globe, L, which can be raised or lowered by means of a pulley; it also communicates by means of a stop-cock, B, and a bent glass tube. A, with a gas receiver (not represented in the figure), A dip- ping into a bowl of mercury, so that the gas may be received over mercury. The lower globe, F, communicates with C by means of the stopcock, E, with / in which the blood is contained by the ■stopcock G, and with a movable glass globe, M, similar to L. by means of the stopcock, H, and the stout india-rubber tube, K. In order to work the pump, L and M are ■filled with mercury, the blood from which the gases are to be extracted is placed in the bulb I, the stopcocks, H, E, D, and B, being open, and G closed. M is raised by means of the pulley until F is full of mercury, and the air is driven out. E is then closed, and L is raised so that C becomes full of mercury, and the air driven off. B is then closed. On lowering L the mercury runs into it from G, and a vacuum is established in G. On opening E and lower- ing M, a vacuum is similarly established in i^; if G be now opened, the blood in / will enter into ebullition, and the gases will pass off into F and (7, and on raising M and then L, the stopcock B being opened, the gas is driven through A, and is received into the receiver over mercury. By repeating the experiment several times the whole of the gases of the speci- men of blood is obtained, and may be estimated. The Oxygen of the Blood. — It has been found that a very small proportion of the oxygen ^^•~^p^|'^ Mercurial which can be obtained, by the aid of the mer- curial pump, from the blood, exists in a state of simple solution in the plasma. If the gas were in simple solution, the amount of oxygen in any given quantity of blood exposed to any given atmosphere ought to vary with the amount of oxygen contained in the atmosphere. Since, speak- ing generally, the amount of any gas absorbed by a liquid such as plasma would depend upon the proportion of the gas in the atmosphere to which the liquid was exposed — if the proportion were great, the absorption would be great; if small, the absorption would be similarly small. The absorption would continue until the proportion of the gas in the liquid 90 HAND-BOOK OF PHYSIOLOGY, and in the atmosphere became equal. Other things would, of course, in- fluence the absorption, such as the kind of gas employed, nature of the liquid, and the temperature of both, but cmteris paribus, the amount of a gas which a liquid absorbs depends upon the proportion of the gas — the so-called partial pressure — of the gas in the atmosphere to which the liquid is subjected. And conversely, if a liquid containing a gas in solu- tion be exposed to an atmosphere containing none of the gas, the gas will be given up to the atmosphere until its amount in the liquid and in the atmosphere becomes equal. This condition is called a condition of equal tensions. The condition may be understood by a simple illustration. A large amount of carbonic acid gas is dissolved in a bottle of water by ex- posing the liquid to extreme pressure of the gas, and a cork is placed in the bottle and wired down. The gas exists in the water in a condition of extreme tension, and therefore there is a tendency of the gas to escape into the atmosphere, in order that the tension may be relieved ; this causes the violent expulsion of the cork when the wire is removed, and if the water be placed in a glass the gas will continue to be evolved until it is almost all got rid of, and the tension of the gas in the water approximates to that of the atmosphere in which, it should be remembered, the carbon dioxide is, naturally, in very small amount, viz., -04 per cent. Now the oxygen of the blood does not obey this law of pressure. For if blood which contains little or no oxygen be exposed to a succession of atmos- pheres containing more and more of that gas, we find that the absorption is at first very great, but soon becomes relatively very small, not being therefore regularly in proportion to the increased amount (or tension) of the oxygen of the atmospheres, and that conversely, if arterial blood be submitted to regularly diminishing pressures of oxygen, at first very little of the contained oxygen is given ofi to the atmosphere, then suddenly the gas escapes with great rapidity, again disobeying the law of pres- sures. Very little oxygen can be obtained from serum freed from blood cor- puscles, even by the strongest mercurial air-pump, neither can serum be made to absorb a large quantity of that gas; but the small quantity which is so given up or so absorbed follows the laws of absorption according to pressure. It must be, therefore, evident that the chief part of the oxygen is con- tained in the corpuscles, and not in a state of simple solution. The chief solid constituent of the colored corpuscles is hmnioglobin, which consti- tutes more tlian 90 per cent, of their bulk. This body has a very re- \l' markable aftinity for oxygen, absorbing it to a very definite extent under favorable circumstances, and giving it up when subjected to the action of reducing agents, or to a sufficiently low oxygen pressure. From these facts it is inferred tliat tlie oxygen of the blood is combined with hjpmo- globin, and not simply dissolved; but inasmuch as it is comparatively easy THE BLOOD. 91 to cause the haemoglobin to give up its oxygen, it is believed tliat the oxygen is but loosely combined with the substance. Haemoglobin. — Hi^moglobin is a crystallizable body which constitutes by far the largest portion of the colored corpuscles. It is intimately dis- tributed throughout their stroma, and must be dissolved out of it before it will undergo crystallization. Its percentage composition is C. 53-85; H. 7-32; N. 16-17; 0. 21-84; S. -63; Fe. -42; and if the molecule be sup- posed to contain one atom of iron the formula would be 0^^^^, Ilgg^, Nj^^, Fe S3, Oj^g. The most interesting of the properties of haemoglobin are its powers of crystallizing and its attraction for oxygen and other gases. Crystals. — The haemoglobin of the blood of various animals possesses the power of crystallizing to very different extents (blood-crystals). In some animals the formation of crystals is almost spontaneous, whereas in others crystals are formed either with great difficulty or not at all. Among the animals whose blood coloring-matter crystallizes most readily are the guinea-pig, rat, squirrel, and dog; and in these cases to obtain crystals it is generally sufficient to dilute a drop of recently-drawn blood with water and expose it for a few minutes to the air. Light seems to favor the for- mation of the crystals. In many instances oth ^r means must be adopted, e.g., the addition of alcohol, ether, or chloroform, rapid freezing, and then thawing, an electric current, a temperature of 140° F. (60° C), or the addition of sodium sulphate. Human blood crystallizes with difficulty, as does also that of the ox, the pig, the sheep, and the rabbit. Fia. 81.— Crystals of oxy-hsemoglobin— prismatic from human blood. The forms of haemoglobin crystals, as will be seen from the appended figures, differ greatly. Haemogloblin crystals are soluble in water. Both the crystals them- selves and also their solutions have the characteristic color of arterial blood. 92 HAND-BOOK OF PHYSIOLOGY. A dilute solution of liaemoglobin gives a characteristic appearance with the spectroscope. Two absorption bands are seen between the solar lines D and E (see Plate), one toward the red, with its middle line some little way to the blue side of is yery intense, but narrower than the other, which lies near to the red side of e. Each band is darkest in the middle and fades away at the sides. As the strength of the solution increases the bands become broader and deeper, and both the red and the blue ends of the spectrum become encroached upon until the bands coalesce to form one very broad band, and only a slight amount of the green remains un- absolved, and part of the red, and on further increase of strength the former disappears. If the crystals of oxy-hsemoglobin be subjected to a mercurial air-pump they give off a definite amount of oxygen (1 gramme giving off 1-59 Fig. 82.— Oxj'-hsemoglobin crystals — tetrahedral, from blood of the guinea-pig. Fig. 83.— Hexagonal oxy-haemoglobin crystals, from blood of squirrel. On these hexagonal plates, prismatic crystals, grouped in a stellate manner, not unf requently occur (after Fuuke). c.cm. of oxygen), and they become of a purple color; and a solution of oxy- ha?moglobin may be made to give up oxygen and to become purple in a similar manner. This change may be also effected by passing through it liydrogen or nitrogen gas, or by the action of reducing agents, of which Stokes^s fluid' is the most convenient. With the spectroscope a soltition of deoxidized hjemoglobin is found to give an entirely different appearance from that of oxidized luvmoghv bin. Instead of the two bands at D and E we find a single broader but fainter band occupying a position midway between ihe two, and at the ' Sfoken\>< Fluid consists of a solution of fcrrot/K HuljiJutfe, to Avliich ammonia has been added and suflirient tartaric acid to prevent precipitation. Another reducing: a«!^ent ia a solution of stannous chloride, treated in a way similar to the ferrous sulphate, nu(\ a third nmsxent of like nature is an a(iu('ous solution of ammonium sulphide. Fig. 82. Fig. 83. THE BLOOD. 93 same time less of the blue end of the spectrum is absorbed. Even in strong solutions this latter appearance is found, thereby differing from the strong solution of oxidized haemoglobin which lets through only the red and orange rays; accordingly to the naked eye the one (reduced haemoglobin solution) appears purple, the other (oxy-haemoglobin solu- tion) red. The deoxidized crystals or their solutions quickly absorb oxy- gen on exposure to the air, becoming scarlet. If solutions of blood be taken instead of solutions of haemoglobin, results similar to the whole of the foregoing can be obtained. Venous blood neVer, except in the last stages of asphyxia, fails to show the oxy-haemoglobin bands, inasmuch as the greater part of the haemoglobin even in venous blood exists in the more highly oxidized condition. Action of Gases on Haemoglobin. — Carlonic o:?;^(^e, passed through a solution of haemoglobin, causes it to assume a bluish color, and the spec- trum is slightly altered; two bands are still visible, but are somewhat nearer the blue end than those of oxy-haemoglobin (see Plate). The amount of carbonic oxide is equal to the amount of the oxygen displaced. Although the carbonic oxide gas readily displaces oxygen, the reverse is not the case, and upon this property depends the dangerous effect of coal gas poisoning. Coal gas contains much carbonic oxide, and this at once, when breathed, combines with the haemoglobin of the blood, producing a compound which cannot easily be reduced, and since it is by no means an oxygen carrier, death may result from suffocation from want of oxygen notwithstanding the free entry into the lungs of pure air. Crystals of carbonic-oxide haemoglobin closely resemble those of oxyhaemoglobin. Nitric oxide produces a similar compound to the carbonic-oxide haemo- globin, which is even less easily reduced. Nitrous oxide reduces oxyhaemoglobin, and therefore leaves the reduced haemoglobin in a condition to actively take up oxygen. Sulphuretted Hydrogen. — If this gas be passed through a solution of oxyhaemoglobin, the haemoglobin is reduced and an additional band appears in the red. If the solution be then shaken with air, the two bands of oxyhemoglobin replace that of reduced haemoglobin, but the band in the red persists. Pkoducts of the Decomposition of Hemoglobin. Methaemoglobin. — If an aqueous solution of oxyhsemoglobin be exposed to the air for some time, its spectrum undergoes a change; the two D and E bands become faint, and a new line in the red at c is devel- oped. The solution, too, has become brown and acid in reaction, and is precipitable by basic lead acetate. This change is due' to the decomposi- tion of haemoglobin, and to the production of metlKBmoglolin, On add- 94 HAOT)-BOOK OF PHYSIOLOGY. ing ammonium sulphide, reduced haemoglobin is produced, and on shaking this up with air, oxyhgemogiobin is reproduced. Haematin. — By the action of heat, or of acids or alkalies in the pres- ence of oxygen, hasmoglobin can be split up into a substance called Hcematin, which contains all the iron of the haemoglobin from which it was derived, and a proteid residue. Of the latter it is impossible to say more than that it is probably made up of one or more bodies of the globu- lin class. If there be no oxygen present, instead of haematin a body called hcamochromogen is produced, which, however, will speedily undergo oxi- dation into haematin. Haematin is a dark brownish or black non-crystallizable substance of metallic lustre. Its percentage composition is C. 64-30; H. 5*50; N. 9*06; Fe, 8-82; 0. 12-32; which gives the formula C^^, H,,, 1^^, Fe,, (Hoppe- Seyler). It is • insoluble in water, alcohol, and ether; soluble in the caustic alkalies; soluble with difficulty in hot alcohol to which is added sulphuric acid. The iron may be removed from haematin by heating it with fuming hydrochloric acid to 320° F. (160° C), and a new body, Immatoporphyrm, is produced. In acid solution. — If to blood an excess of acetic acid be added, the color alters to brown from decomposition of haemoglobin, and the setting free of haematin; by shaking this solution with ether, solution of the haematin is obtained. The spectrum of the etherial solution shows no less than four absorption bands, viz., one in the red between c and d, one faint and narrow close to D, and then two broader bands, one between d and E, and another nearly midway between l and f. The first band is by far the most distinct, and the acid solution of haematin without ether shows it plainly. In alkaline solution. — The absorption band is still in the red, but nearer to d, and the blue end of the spectrum is partially absorbed to a considerable extent. If a reducing agent be added, two bands resembling those of oxyhemoglobin, but nearer to the blue, appear; this is the spec- trum of reduced Immatin. On shaking the reduced ha?matin with air or oxygen the two bands are replaced by the single band of alkaline haematin. Haematoidin. — This substance is found in tlie form of yellowish crystals in old blood extravasations, and is derived from the ha?nioglobin. Their, crystalline form and the reaction they give witli nitric acid seem to show them to be identical with Biliruhiu. tlie chief coloring matter of the Bile. Haemin. — One of the most important derivatives of na^matin is ITaimiii, It is usually called llydrorhloratc of Ilanuafin (or hydrochlo- ride), but its exact cliemical composition is uncertain. Its formula is C,.^, II,„, N„ Fe,, 0,„, '2 llcl, and it contains 5-18 per cent, of chlorine, but by some it is looked upon as siini)ly crystallized luvmatin. Although THE BLOOD. 95 difficult to obtain in bulk, a specimen may be easily made for the micro- scope in the following way: — A small drop of dried blood is finely powdered with a few crystals of common salt on a glass slide, and spread out; a cover glass is then placed upon it, and glacial acetic acid added by means of a capillary pipette. The blood at once turns of a brownish color. The slide is then heated, and the acid mixture evaporated to dryness at a high tem- perature. The excess of salt is washed away with water from the dried residue, and the specimen may then be mounted. A large number of small, dark, reddish black crystals of a rhombic shape, sometimes ar- ranged in bundles, will be seen if the slide be subjected to microscopic examination. The formation of these haemin crystals is of great interest and impor- tance from a medico-legal point of view, as it constitutes the most cer- tain and delicate test we have for the presence of blood (not of necessity the blood of man) in a stain on clothes, etc. It exceeds in delicacy even the spectroscopic test. Estimation of Haemoglobin. — The most exact method is by the estimation of the amount of iron in a given specimen of blood, but as this is a somewhat complicated process, a method has been proposed which, though not so exact, has the advantage of simplicity. This consists in comparing the color of a given small amount of diluted blood with gly- cerine jelly tinted with carmine and picrocarmine to represent a standard solution of blood diluted one hundred times. The amount of dilution which the given blood requires will thus approximately represent the quantity of haemoglobin it contains. (Gowers.) Distribution of Haemoglobin. — In connection with the ascertained function of haemoglobin as the great oxygen-carrier, the following facts with regard to its distribution are of importance. It occurs not only in the red blood-cells of all Vertebrata (except one fish (leptocephalus) whose blood-cells are all colorless), but also in similar cells in many Worms: moreover, it is found diffused in the vascular fluid of some other worms and certain Crustacea; it also occurs in all the striated muscles of Mammals and Birds. It is generally absent from unstriated Fig. 84.— Haematoidin crystals. (Frey.) Fig. 85. — Hsemin crystals. (Frey.) 96 HAND-BOOK OF PHYSIOLOGY. muscle except that of tlie rectum. It has also been found in Mollusca in certain muscles which are specially active, viz., those which work the rasp- like tongue. In the muscles of Fish it has hitherto only been met with in the very active muscle which moves the dorsal fin of the Hippocampus (Eay Lan- kester). The Carbon Dioxide Gas in the Blood. — Of this gas in the blood, part exists in a state of simple solution in the serum, and the rest in a state of weak chemical combination. It is believed that the latter is combined with the sodium carbonate in a condition of bicarbonate. Some observers consider that part of the gas is associated with the cor- puscles. The Nitrogen in the Blood. — 'It is believed that the whole of the small quantity of the nitrogen contained in the blood is simply dissolved in the fluid plasma. DEYELOPMEIfT OF THE BlOOD. The first formed blood-corpuscles of the human embryo differ much in their general characters from those which belong to the later periods Fift. 86.— Part of the network of developing blood-vessels in the vascular area of a guinea-pig. hi, blood corpuscles becoming free in an enlarged and hollowed out pai-t of the network; a, process of protoplasm. (E. A. Schiifer.) of intra-uterine, and to all periods of extra-uterine life. Their manner of origin is at first very simple. Surrounding the early embryo is a circular area, called the vascular area, in which the first rudiments of the blood-vessels and blood-corpuscles are developed. Here the nucleated embryonal cells of the mesoblast, from which the blood-vessels and corpuscles are to be formed, send out processes in various directions, and these joining together, form an irregular meshwork. "^I'lie nuclei increase in number, and collect chiefly in the larger masses of protoi)lasm, but partly also in the im)cesses. Tlieso nuclei gather around them a certain amount of tlie protoplasm, and be- TlIE BLOOD. 97 coming colored, form tlie red blood corpuscles. The protoplasm of the cells and their branched network in which these corpuscles lie then be- comes hollowed out into a system of canals enclosing fluid, in which the red nucleated corpuscles float. The corpuscles at first are from about ^"gW ^0 TiVo" of i^^^ diameter, mostly spherical, and with granular contents, and a well-marked nucleus. Their nuclei, which are about g-Jy-g- of an inch in diameter, are central, circular, very little prominent on the surfaces of the corpuscle, and apparently slightly granular or tu- berculated. The corpuscles then strongly resemble the colorless corpuscles of the fully developed blood, but are colored. They are capable of amoeboid movement and multiply by division. When, in the progress of embryonic development, the liver begins to be formed, the multiplication of blood-cells in the whole mass of blood ceases, and new blood-cells are produced by this organ, and also by the lymphatic glands, thymus and spleen. These are at first colorless and nucleated, but afterward acquire the ordinary blood- tinge, and resemble very much those of the first set. They also multiply by division. In whichever way produced, however, whether from the original formative cells of the embryo, or by the liver and the other organs mentioned above, these colored nucleated cells begin very early in foetal life to be mingled with colored ??ow-nucleated corpuscles resembling those of the adult, and at about the fourth or fifth month of embryonic existence are completely replaced by them. Origin of the Mature Red Corpuscles. — The non-nucleated red corpuscles may possibly be derived from the nucleated, but in all proba- bility are an entirely new formation, and the methods of their origin are Fig. 87.— Development of red corpuscles in connective-tissue cells. From the subcutaneous tissue of a new-born rat. h, a cell containing haemoglobin in a diffused form in the protoplasm ; h', one con- taining colored globules of varying size and vacuoles; /i", a cell filled with colored globules of nearly- uniform size; /, /', developing fat cells. (E. A. Schafer.) the following: — (1.) During foetal life and possibly in some animals, e.g., the rat, which are born in an immature condition, for some little time after birth, the blood discs arise in the connective tissue cells in the following way. Small globules, of varying size, of coloring matter arise in the protoplasm of the cells, and the cells themselves become branched, their branches joining the branches of similar cells. The cells next become Vol. L— 7. 98 HAND-BOOK OF PHYSIOLOGY. vacuolated, and the red globules are free in a cavity filled with fluid (Fig. 88); by the extension of the cavity of the cells into their processes anas- tomosing vessels are produced, which ultimately join with the previously existing vessels, and the globules, now having the size and appearance of the ordinary red corpuscles, are passed into the general circulation. This method of formation is called intraceTlular (Schafer). Fig. 88.— Further development of blood-corpuscles in connective-tissue cells and transformation of the latter into capillary blood-vessels, a, an elongated cell with a cavity in the protoplasm occu- pied by fluid and by blood-corpuscles which are stiU globular; ft, a hoUow ceU, the nucleus of which has multiplied. The new nuclei ai e arranged arovmd the wall of the cavity, thej^corpuscles in which have now become discord; c, shows the modp of union of a "hsemapoietic" cell, which, in this in- stance, contains only one corpuscle, with the prolongation ( 6Z) of a previously existing vessel; a and c, from the new-born rat; &, from the foetal sheep. (E. A. Schafer.) (2.) From the wliite corpuscles. — The belief that the red corpuscles are derived from the white is still very general, although no new evidence has been recently advanced in favor of this view. It is, however, uncer- tain whether the nucleus of the white corpuscle becomes the red corpus- cle, or whether the whole white corpuscle is bodily converted into the red by the gradual clearing up of its contents with a disappearance of the nucleus. Probably the latter view is the correct one. flfg^gft® ^ % % % Fia. 89.— Colored nucleated corpuscles, from the red marrow of the guinea-pig. (E. A. Schafer.) (3.) From, the medulla of hones. — Red corpuscles are to a very large extent derived during adult life from the large ]xilo cells in the rod mar- row of bones, especially of the ribs (Figs. 44, 89). These cells become colored from the formation of haemoglobin chiefly in one part of their protoplasm. Tliis colored part becomes separated from the rest of the col] and forms a rod corpuscle, being at first cu})-shaped, but soon taking on tlie normal api)earance of the matui-e cor])uscle. It is supposed that the THE BLOOD. 99 protoplasm may grow up again and form a number of red corpuscles in a similar way. (4.) From the tissue of the spleen. — It is probable that red as well as white corpuscles may be produced in the spleen. (5.) From Microcytes. — Hayem describes the small particles (micro- cytes), previously mentioned as contained in the blood (p. 75), and which he calls hsematoblasts, as the precursors of the red corpuscles. They ac- quire color, and enlarge to the normal size of red corpuscles. Without doubt, the red corpuscles have, like all other parts of the organism, a tolerably definite term of existence, and in a like manner die and waste away when the portion of work allotted to them has been per- formed. Neither the length of their life, however, nor the fashion of their decay has been yet clearly made out. It is generally believed that a certain number of the red corpuscles undergo disintegration in the spleen; and indeed corpuscles in various degrees of degeneration have been observed in this organ. Origin of the Colorless Corpuscles.— The colorless corpuscles of the blood are derived from the lymph corpuscles, being, indeed, indistin- guishable from them; and these come chiefly from the lymphatic glands. Their number is increased by division. Colorless corpuscles are also in all probability derived from the spleen and thymus, and also from the germinating endothelium of serous mem- branes, and from connective tissue. The corpuscles are carried into the blood either with the lymph and chyle, or pass directly from the lymphatic tissue in which they have been formed into the neighboring blood-vessels. Uses of the Blood. 1. To be a medium for the reception and storing of matter (ordinary food, drink, and oxygen) from the outer world, and for its conveyance to all parts of the body. 2. To be a source whence the various tissues of the body may take the materials necessary for their nutrition and maintenance; and whence the secreting organs may take the constituents of their various secretions. 3. To be a medium for the absorption of refuse matters from all the tissues, and for their conveyance to those organs whose function it is to separate them and cast them out of the body. 4. To warm and moisten all parts of the body. Uses of the Various Constituents of the Blood. Albumen. — Albumen, which exists in so large a proportion among the chief constituents of the blood, is without doubt mainly for the nourish- ment of those textures which contain it or other compounds nearly allied to it. 100 HAI^D-BOOK OF PHYSIOLOGY. Fibrin. — In considering the functions of fibrin, we may exclude the notion of its existence, as such, in the blood in a fluid state, and of its use in the nutrition of certain special textures, and look for the explanation of its functions to those circumstances, whether of health or disease, under which it is produced. In hsemorrhage, for example, the formation of fibrin in the clotting of blood, is the means by which, at least for a time, the bleeding is restrained or stopped; and the material or llastema which is produced for the permanent healing of the injured part, con- tains a coagulable material identical, or very nearly so, with the fibrin of clotted blood. Fatty matters. — The fatty matters of the blood subserve more than one purpose. For while they are the means, in part, by which the fat of the body, so widely distributed in the proper adipose and other textures, is replenished, they also, by their union with oxygen, assist in maintain- ing the temperature of the body. To certain secretions also, notably the milk and bile, fat is contributed. Saline Matter. — The uses of the saline constituents of the blood are, first, to enter into the composition of such textures and secretions as natu- rally contain them, and, secondly, to assist in preserving the due specific gravity and alkalinity of the blood, and in preventing its decomposition. The phosphate and carbonate of sodium, to which the blood owes its alkaline reaction, increase the absorptive power of the serum for gases. Corpuscles. — The important use of the red corpuscles is in relation to the absorption of oxygen in the lungs, and its conveyance to the tissues. How far the red corpuscles are actually concerned in the nutrition of the tissues is quite unknown. The relation of the colorless corpuscles to the coagulation of the blood has been already considered; of their functions, other than are concerned in this phenomenon, and in the regeneration of the red corpuscles, nothing is positively known. CHAPTER V. THE CIRCULATION OF THE BLOOD. The Heart is a hollow muscular organ containing four chambers, two auricles and two ventricles, arranged in pairs. On each side (right and left) of the heart is an auricle joined to and communicating with a ven- tricle, but the chambers on the right side do not directly communicate with those on the left side. The circulation of the blood is chiefly Fig. 90.— Diagram of the Circulation. carried on by the contraction of the muscular walls of these chambers of the heart, the auricles contracting simultaneously, and their contraction being followed by the simultaneous contraction of the ventricles. The blood is conveyed away from the left side of the heart by the arteries, and returned to the right side of the heart by the veins, the arteries and veins being continuous with each other at one end by means of the heart, and at the other by a fine network of vessels called the capillaries. The 102 HAND-BOOK OF PHYSIOLOGY. blood, therefore, in its passage from the heart passes first into the arteries, then into the capillaries, and lastly into the veins, by which it is con- veyed back again to the heart, thus completing a revolution or circulatiori. The right side of the heart does not directly communicate with the left to complete the entire circulation, but the blood has to pass from the right side to the lungs, through the pulmonary artery, then through tlie pulmonary capillary-vessels and through the pulmonary veins to the left side of the heart. Thus there are two circulations by which the blood must pass; the one, a shorter circuit from the right side of the heart to the lungs and back again to the left side of the heart; the other and larger circuit, from the left side of the heart to all parts of the body and back again to the right side; but more strictly speaking, there is only one complete circulation, which may be diagrammatically represented by a double loop, as in the accompanying figure (Fig. 90). Diaphragm. Fig. 91.— View of heart and hings in situ. The front portion of the chest- wall, and the outer or parietal layers of the pleurae and pericardium have been removed. The lungs are partly collapsed. On reference to this figure, and noticing the direction of the arrows, which represent the course of the stream of blood, it will be observed that while there is a smaller and a larger circle, both of which pass through the heart, yet that these are not distinct, one from the other, but are formed really by one continuous stream, the whole of which must, at one part of its course, pass through the lungs. Subordinate to the two principal circulations, the Pulmmiary and Systemic, as they are named, it will be noticed also in the same figure that there is another, by which a ])()rtion of the stream of blood having been diverted once into the cap- illaries of the intestinal canal, and some other organs, aiul gathered uj) again into a single stream, is a second time divided in its passage through CIRCULATION OF THE BLOOD. 103 the liver, before it finally reaches the heart and completes a revolution. This subordinate stream through the liver is called the Portal circulation. The Forces concerned in the Circulation of the Blood.— (1) The principal force provided for constantly moving the blood through the course of the circulation is that of the muscular substance of the heart; other assistant forces are (2) those of the elastic walls of the arte- ries, (3) the pressure of the muscles among which some of the veins run, (4) the movements of the walls of the chest in respiration, and probably, to some extent, (5) the interchange of relations between the blood and the tissues which occurs in the capillary system during the nutritive processes. The Heart. The Pericardium. — The heart is invested by a membranous sac — the pericardium, which is made up of two distinct parts, an external fibrous membrane, composed of closely interlacing fibres, which has its base attached to the diaphragm — both to the central tendon and to the adjoining muscular fibres, while the smaller and upper end is lost on the large blood-vessels by mingling its fibres with that of their external coats; and an iiiternal serous layer, which not only lines the fibrous sac, but also is reflected on to the heart, which it completely invests. The part which lines the fibrous membrane is called the parietal layer, and that enclosing the heart, the visceral layer, and these being continuous for a short distance along the great vessels of the base of the heart, form a closed sac, the cavity of which in health contains just enough fluid to lubricate the two surfaces, and thus enable them to glide smoothly over each other during the movements of the heart. Most of the vessels passing in and out of the heart receive more or less investment from this sac. The heart is situated in the chest behind the sternum and costal car- tilages, being placed obliquely from right to left, quite two-thirds to the left of the mid-sternal line. It is of pyramidal shape, with the apex pointing downward, outward, and toward the left, and the base backward, inward, and toward the right. It rests upon the diaphragm, and ijts pointed apex, formed exclusively of the left side of the heart, is in con- tact with the chest wall, and during life beats against it at a point called the apex teat, situated in the fifth intercostal space, about two inches below the left nipple, and an inch and a half to the sternal side. The heart is suspended in the chest by the large vessels which proceed from its base, but, excepting the base, the organ itself lies free in the sac of the pericardium. The part which rests upon the diaphragm is flattened, and is known as the posterior surface, whilst the free upper part is called the anterior surface. The margin toward the left is thick and obtuse, whilst the lower margin toward the right is thin and acute. 104 HAND-BOOK OF PHYSIOLOGY. On examination of the external surface the division of the heart into parts which correspond to the chambers inside of it may be traced, for a deep transverse groove called the auriculo-ventricular groove divides the auricles which form the base of the heart from the ventricles which form the remainder, including the apex, the ventricular portion being by far the greater; and, again, the inter-ventricular groove runs between the Fig. 92.— The right auricle and ventricle opened, and a part of their right and anterior •*ralls re- moved, so as to show their interior. superior vena cava; 2. inferior vena cava: 2\ hepatic veins cut short; 3, right auricle; 3'. placed in the fossa ovalis, below which is the Eustachian valve; 3", is placed close to the aperture of the coronary vein; +. +, placed in the auriculo-ventricular groove, where a nai'row portion of the adjacent walls of the auricle and ventricle has been preserved; 4, 4, cavity of the right ventricle, the upper figiu-e is immediately below the semilunar valves; 4'. large columna carnea or muscuhis papillaris; 5. 5'. 5", tricuspid valve; G, placed in the interior of the pulmonary artery, a part of tlie anterior wall of that vessel having been removed, and a narrow por- tion of it preserved at its connuencemeut. where the semilunar valves are attached; 7, concavity of the aortic ai'ch close to the cord of the ductus arteriosus; S. ascending part or sinus of the arch cov- ered at its commencement by the auricular appentlix and i)ulmonary arterj-; 1), placed between the innominate and left carotid arteries; 10, appendix of the left auricle; 11, 11, the outside of the left ventricle, the lower figure near the apex. (.Allen Thomson.) ventricles botli front and back, and separates the one from tlie otlior. The anterior groove is nearer the left margin and the posterior nearer the right, as the front surface of tlie heart is made up chiefly of the right ventricle and the posterior surface of tlie loft ventricle. In the furrows run the coronary vessels, which sui)i)ly the tissue of the heart itself with ))l()()d, as well as nerves and lymphatics imbedded in more or less fatty tissue. CIRCULATIOI^ OF THE BLOOD. 105 The Chambers of the Heart. — The interior of the heart is divided by a partition in such a manner as to form two chief chambers or cavities — right and left. Each of these chambers is again subdivided into an upper and a lower portion, called respectively, as already incidentally men- tioned, auricle and ventricle, which freely communicate one with the other; the aperture of communication, however, being guarded by valves, so disposed as to allow blood to pass freely from the auricle into the ven- tricle, but not in the opposite direction. There are thus four cavities altogether in the heart — two auricles and two ventricles; the auricle and ventricle of one side being quite separate from those of the other (Pig. 90). Right Auricle. — The right auricle is situated at the right part of the base of the heart as viewed from the front. It is a thin walled cavity of more or less quadrilateral shape prolonged at one corner into a tongue- shaped portion, the right auricular appendix, which slightly overlaps the exit of the great artery, the aorta, from the heart. The interior is smooth, being lined with the general lining of the heart, the endocardium^ and into it open the superior and inferior venae cavae, or great veins, which convey the blood from all parts of the body to the heart. The former is directed downward and forward, the latter upward and inward; between the entrances of these vessels is a slight tubercle called tubercle of Lower. The opening of the inferior cava is protected and partly covered by a membrane called the Eustachian valve. In the posterior wall of the auricle is a slight depression called the fossa ovalis, which corresponds to an opening between the right and left auricles which exists in foetal life. The right auricular appendix is of oval form, and admits three fingers. Various veins, including the cor- onary sinus, or the dilated portion of the right coronary vein, open into this chamber. In the appendix are closely set elevations of the muscular tissue covered with endocardium, and on the anterior wall of the auricle are similar elevations arranged parallel to one another, called musculi ipectinati. Right Ventricle. — The right ventricle occupies the chief part of the * anterior surface of the heart, as well as a small part of the posterior sur- face: it forms the right margin of the heart. It takes no part in the formation of the apex. On section its cavity, in consequence of the encroachment upon it of the septum ventriculorum, is semilunar or cre- scentic (Fig. 94); into it are two openings, the auriculo-ventricular at the base, and the opening of the pulmonary artery also at the base, but more to the left; the part of the ventricle leading to it is called the comis arteriosus or infundihulum; both orifices are guarded by valves, the former called tricuspid and the latter semilunar or sigmoid. In this ventricle are also the projections of the muscular tissue called columnce carnem (described at length p. 110). 106 HAND-BOOK OF PHYSIOLOGY. Left Auricle. — The left auricle is situated at the left and posterior part of the base of the heart, and is best seen from behind. It is quadri- lateral, and receives on either side two pulmonary veins. The auricular appendix is the only part of the auricle seen from the front, and corre- Fig. 93.— The left auricle and ventricle opened, and a part of their anterior and left walls re- moved. 1/^.— The pulmonary arterj- has been divided at its commencement; the opening into the left ventricle carried a short distance into the aorta between Uvo of the segments of the semilimar valves, and the left part of the auricle with its appendix has been removed. Tlie right auricle is out of view. • 1, the two right pulmonary veins cut short; their openings are seen within the auricle; 1', placed within the cavity of the auricle on the left side of the septum and on the part which forms tne re- mains of the valve of the foramen ovale, of which the crescentic fold is seen toward the left hand of 1'; 2. a narrow portion of the wall of the auricle and ventricle pi eserved round the auriculo-ven- tricular orifice; 3, 3', the cut surface of the walls of the ventricle, seen to become veiy much thinner toward 3". at the apex; 4, a small part of the anterior wall of the left ventricle which has been pre- served with the principal anterior cohunna carneaor musculus papillniis attached to it; 5, ,5, nnisculi papillares; 5', the left side of the septum, between the two ventricles, within the cavity of the left ventricle; 0, (>', the mitral valve; 7, placed in the interior of the aorta near its commencement and above the three segments of its semilunar valve which are hanging loosely together; 7', the exterior of the great aortic siinis; 8, the root of the inilmonary artery and its semilunar valves; 8', the sepa- rated portion of the i)uhiionary art<'ry rcniaining attacluHl to the aorta by it,the cord of the ductus arteriosus; 10, the ai tei ies rising from the sunmiit of the aortic arch. (Allen Thomson.) spoiuls with that on the right side, but is thicker, and tlie interior is more smootli. The loft auricle is only slightly thicker than the right, the dif- ference being as lines to 1 line. The left auriculo-veutricular orifice is oval, and a little smaller than that on the right side of the heart. CIRCULATION OF THE BLOOD. 107 There is a slight vestige of the foramen between the auricles, which exists in fcetal life, on the septum between them. Left Ventricle.— Though taking part to a comparatively slight extent in the anterior surface, the left ventricle occupies the chief part of the posterior surface. In it are two openings very close together, viz. the auriculo-ventricular and the aortic, guarded by the valves corre- sponding to those of the right side of the heart, viz. the bicuspid or mitral and the semilunar or sigmoid. The first opening is at the left and back part of the base of the ventricle, and the aortic in front and toward the right. In this ventricle, as in the right, are the co- lumnae carneae, which are smaller but more closely reticulated. They are Fxg. 94.— Transverse section of bullock^s T heart in a state of cadaveric rigidity, a, chieiiy round near the apex and along cavity of left ventricle. 6, cavity of right , . n •^^ ^ • ventricle. (Dalton.) the posterior wall. They will be again referred to in the description of the valves. The walls of the left ven- tricle, which are nearly half an inch in thickness, are, with the exception of the apex, twice or three times as thick as those of the right. Capacity of the Chambers. — The capacity of the two ventricles is about four to six ounces of blood, the whole of which is impelled into their respective arteries at each contraction. The capacity of the auricles is rather less than that of the ventricles: the thickness of their walls is considerably less. The latter condition is adapted to the small amount of force which the auricles require in order to empty themselves into their adjoining ventricles; the former to the circumstance of the ventricles being partly filled with blood before the auricles contract. Size and Weight of the Heart. — The heart is about 5 inches long, 3^ inches greatest width, and 2^- inches in its extreme thickness. The average weight of the heart in the adult is from 9 to 10 ounces; its weight gradually increasing throughout life till middle age; it diminishes in old age. Structure. — The walls of the heart are constructed almost entirely of layers of muscular fibres; but a ring of connective tissue, to which some of the muscular fibres are attached, is inserted between each auricle and ventricle, and forms the boundary of the auriculo-ventricular opening. Fibrous tissue also exists at the origins of the pulmonary artery and aorta. The muscular fibres of each auricle are in part continuous with those of the other, and partly separate; and the same remark holds true for the ventricles. The fibres of the auricles are, however, quite separate from those of the ventricles, the bond of connection between them being only the fibrous tissue of the auriculo-ventricular openings. The muscular fibres of the heart, unlike those of most of the involun- 108 HAND-BOOK OF PHYSIOLOGY. tary muscles, are striated; but although, in this respect, they resemble the skeletal muscles, they have distinguishing characteristics of their own. The fibres which lie side by side are united at frequent intervals by short branches (Fig. 95). The fibres are smaller than those of the ordinary striated muscles, and their striation is less marked. No sarcolemma can be discerned. The muscle-corpuscles are situate in the middle of the substance of the fibre; and in correspondence with these the fibres appear under certain conditions subdivided into oblong portions or cells, the off-sets from which are the means by which the fibres anastomose one with another (Fig. 96). Endocardium. — As the heart is clothed on the outside by a thin transparent .layer of pericardium, so its cavities are lined by a smooth and Fig. 95. Fig. 96. Fig. 95. — Network of muscular fibres (striated") from the heart of a pig. The nuclei of the mus- cle-corpuscles are well shown, x 4.50. (Klein and Noble Smith.) Fig. 9fj.— Muscular fibre cells from the heart. (E. A. Shiifer.) shining membrane, or endocardium, which is directly continuous with the internal lining of the arteries and veins. The endocardium is composed of connective tissue with a large admixture of elastic fibres; and on its inner surface is laid down a single tessellated layer of flattened endothelial cells. Here and there unstriped muscular fibres are sometimes found in the tis- sue of the endocardium. Course of the Blood through the Heart. — The arrangement of the heart's valves is such that the blood can pass only in one direction, and this is as follows (Fig. 97): — From the riglit auricle the blood passes into the riglit ventricle, and tlience into the pnhuonary artery, by wliich it is conveyed to the capilhiries of the lungs. From tlio Inngs the blood, wliicli is now purified and altered in color, is gathered by the pulmonary CIRCULATION OF THE BLOOD. 109 veins and taken to the left auricle. From the left auricle it passes into the left ventricle, and thence into the aorta, by which it is distributed to the capillaries of every portion of the body. The branches of the aorta, from being distributed to the general system, are called systemic arteries; and from these the blood passes into the systemic capillaries, where it again becomes dark and impure, and thence into the branches of the systemic veins, which, forming by their union two large trunks, called the superior and inferior vena cava, discharge their contents into the right auricle, whence we supposed the blood to start. The Valves of the Heart. — The valve between the right auricle and ventricle is named tricuspid (5, Fig. 99), because it presents three principal cusps or subdivisions, and that between the left auricle and yen- FiG. 97. — Diagram of the circulation through the heart. (Dalton.) tricle bicuspid for mitral), because it has tiuo such portions (6, Fig. 93). But in both valves there is between each two principal portions a smaller one; so that more properly, the tricuspid may be described as consisting of six, and tlie mitral of four, portions. Each portion is of triangular form, its apex and sides lying free in the cavity of the ventricle, and its base, which is continuous with the bases of the neighboring portions, so as to form an annular membrane around the auriculo -ventricular open- ing, being fixed to a tendinous ring which encircles the orifice between the auricle and ventricle and receives the insertions of the muscular fibres of both. In each principal cusp may be distinguished a middle-piece, extending from its base to its apex, and including about half its width, which is thicker, and much tougher and tighter than the border-pieces or edges. While the bases of the several portions of the valves are fixed to the 110 HAND-BOOK OF PHYSIOLOGY. tendinous rings, their yentricular surfaces and borders are fastened by- slender tendinous fibres, the chorclce tendinece, to the walls of the ventri- cles, the muscular fibres of which project into the ventricular cavity in the form of bundles or columns — the cohimncB carnece. These columns are not all of them alike, for while some of them are attached along their whole length on one side and by their extremities, others are attached only by their extremities; and a third set, to which the name musculi 2)cipiUares has been given, are attached to the wall of the ventricle by one extremity only, the other projecting, papilla-like, into the cavity of the ventricle (5, Fig. 93), and having attached to it cliordce tendinece. Of the tendinous cords, besides those which pass from the walls of the ventricle and the musculi papillares to the margins of the valves, there are some of especial strength, which pass from the same parts to the edges of the middle and thicker portions of the cusps before referred to. The ends of these cords are spread out in the substance of the valve, giving its middle piece its peculiar strength and toughness; and from the sides numerous other more slender and branching cords are given off, which are attached all over the ventricular surface of the adjacent border-pieces of the principal portions of the valves, as well as to those smaller portions which have been mentioned as lying between each two principal ones. Moreover, the musculi papillares are so placed that, from the summit of each, tendinous cords i:)roceed to the adjacent halves of two of the prin- cipal divisions, and to one intermediate or smaller division, of the valve. The preceding description applies equally to the mitral and tricuspid valve; but it should be added that the mitral is considerably thicker and stronger than the tricuspid, in accordance with the greater force which it is called upon to resist. It has been already said that while the ventricles communicate, on the one hand, with the auricles, they communicate, on the other, with the large arteries wliich convey the blood away from the heart; the riglit ven- tricle with the pulmonary artery (G, Fig. 93), which conveys blood to the lungs, and the left ventricle with the aorta, which distributes it to the general system (T, Fig. 93). And as the auriculo-ventricular orifice is guarded by valves, so are also the mouths of the pulmonary artery, and aorta (Figs. 9:3, 99). The semilunar valves, three in number, guard the orifice of each of these two arteries. They are nearly alike on both sides of the heart; but those of the aorta are altogether thicker and more strongly constructed than tliose of tlie pulmonary artery, in accordance witli the greater pros- sure which tliey liave to witlistand. Each valve is of semilunar shape, its convex margin being attached to a fibrous ring at tlio place of junction of the artery to the ventricle, and tlie concave or nearly straight border l)eing free, so that eacli valve forms a little pouch like a watch-pocket (7, l^^ig. 93). Ill the centre of the free edge of tlie valve, which contnius CIRCULATION OF THE BLOOD. Ill a fine cord of fibrous tissue, is a small fibrous nodule, the coiyus Arantii, and from this and from the attached border fine fibres extend into every part of the mid substance of the valve, except a small lunated space just within the free edge, on each side of the corpus Arantii. Here the valve is thinnest, and composed of little more than the endocardium. Thus constructed and attached, the three semilunar valves are placed side by side around the arterial orifice of each ventricle, so as to form three little pouches, which can be separated by the blood passing out of the ventricle, but which immediately afterward are pressed together so as to prevent any return (7, Fig. 93, and 7, Fig. 99). This will be again referred to. Opposite each of the semilunar cusps, both in the aorta and pulmonary artery, there is a bulging outward of the wall of the vessel: these bulg- ings are called the sinuses of Valsalva. Structure of the Valves. — The valves of the heart are formed es- sentially of thick layers of closely woven connective and elastic tissue, over which, on every part, is reflected the endocardium. The Actiok of the Heakt. The heart's action in propelling the blood consists in the successive alternate contraction (systole) and relaxation (diastole) of the muscular walls of its two auricles and two ventricles. Action of the Auricles. — The description of the action of the heart may best be commenced at that period in each action which immedi- ately precedes the beat of the heart against the side of the chest. For at this time the whole heart is in a passive state, the walls of both auricles and ventricles are relaxed, and their cavities are being dilated. The auri- cles are gradually filling with blood flowing into them from the veins; and a portion of this blood passes at once through them into the ventricles, the opening between the cavity of each auricle and that of its correspond- ing ventricle being, during all the pause, free and patent. The auricles, however, receiving more blood than at once passes through them to the ventricles, become, near the end of the pause, fully distended; and at the end of the pause, they contract and expel their contents into the ventricles. The contraction of the auricles is sudden and very quick; it commences at the entrance of the great veins into them, and is thence propagated toward the auriculo-ventricular opening; but the last part which contracts is the auricular appendix. The effect of this contraction of the auricles is to quicken the flow of blood from them into the ventricles; the force of their contraction not being sufficient under ordinary circumstances to cause any back-flow into the veins. The reflux of blood into the great veins is, moreover, resisted not only by the mass of blood in the veins and the force with which it streams into the auricles, but also by the simulta- neous contraction of the muscular coats with which the large veins are 112 HAND-BOOK OF PHYSIOLOGY. provided near their entrance into the auricles. Any slight regurgitation from the right auricle is limited also by the valves at the junction of the subclavian and internal jugular veins^ beyond which the blood cannot move backward; and the coronary vein is preserved from it by a valve at its mouth. in birds and reptiles regurgitation from the right auricle is prevented by valves placed at the entrance of the great veins. During the auricular contraction the force of the blood propelled into the ventricle is transmitted in all directions, but being insufficient to separate the semilunar valves, it is expended in distending the ven- tricle, and, by a reflux of the current, in raising and gradually closing the auriculo-ventricular valves, which, when the ventricle is full, form a com- plete septum between it and the auricle. Action of the Ventricles. — The blood which is thus driven, by the contraction of the auricles, into the corresponding ventricles, being added to that which had already flowed into them during the heart's pause, is sufficient to complete their diastole. Thus distended, they immediately contract: so immediately, indeed, that their systole looks as if it were continuous with that of the auricles. The ventricles contract much more slowly than the auricles, and in their contraction probably always thoroughly empty themselves, differing in this respect from the auricles, in which, even after their complete contraction, a small quantity of blood remains. The shape of both ventricles during systole undergoes an alter- ation, the left probably not altering in length but to a certain degree in breadth, the diameters in the plane of the base being diminished. The right ventricle does actually shorten to a small extent. The systole has the effect of diminishing the diameter of the base, especially in the plane of the auriculo-ventricular valves; but the length of the heart as a whole is not altered. (Ludwig.) During the systole of the ventricles, too, the aorta and pulmonary artery, being filled with blood by the force of the ventricular action against considerable resistance, elongate as well as ex- pand, and the whole heart moves slightly toAvard the right and forward, twisting on its long axis, and exposing more of the left ventricle ante- riorly than is usually in front. When the systole ends tlie heart resumes its former position, rotating to the left again as the aorta and pulmonary artery contract. Functions of the Auriculo-Ventricular Valves. — The disten- sion of the ventricles with blood continues throughout the wliole period of tlieir diastole. Tlie auriculo-ventricular valves are gradually brought into play by soine of the blood getting behind the cusps and floating them u}); and ])y the time that the diastole is complete, the valves are no doubt in apposition, the completion of this being brought about by the reflex current caused by the systole of the auricles. This elevation of the au- CIRCULATION OF THE BLOOD. 113 riculo-ventricular valves is, no doubt, materially aided by the action of the elastic tissue which has been shown to exist so largely in their structure, especially on the auricular surface. At any rate at the commencement of the ventricular systole they are completely closed. It should be recol- lected that the diminution in the breadth of the base of the heart in its transverse diameters during ventricular systole is especially marked in the neighborhood of the auriculo- ventricular rings, and thus aids in render- ing the auriculo-ventricular valves competent to close the openings, by greatly diminishing their diameter. The margins of the cusps of the valves are still more secured in apposition with another, by the simulta- neous contraction of the musculi papillares, whose chordae tendineae have a special mode of attachment for this object (p. 110). As in the case of the semilunar valves to be immediately described, the auriculo-ventricular valves meet not by their edges only, but by the opposed surfaces of their thin outer borders. The semilunar valves, on the other hand, which are closed in the intervals of the ventricle's contraction (Fig. 92, 6), are forced apart by the same pressure that tightens the auriculo-ventricular valves; and, thus, the whole force of the contracting ventricles is directed to the expulsion of blood through the aorta and pulmonary artery. The form and position of the fleshy columns on the internal walls of the ventricle no doubt help to produce this obliteration of the cavity dur- ing their contraction; and the completeness of the closure may often be observed on making a transverse section of a heart shortly after death, in any case in which the contraction of the rigor mortis is very marked (Fig. 94). In such a case only a central fissure may be discernible to the eye in the place of the cavity of each ventricle. If there were only circular fibres forming the ventricular wall, it is evident that on systole the ventricle would elongate; if there were only longitudinal fibres the ventricle would shorten on systole; but there are both. The tendency to alter in length is thus counterbalanced, and the whole force of the contraction is expended in diminishing the cavity of the ventricle; or, in other words, in expelling its contents. On the conclusion of the systole the ventricular walls tend to expand by virtue of their elasticity, and a negative pressure is set up, which tends to suck in the blood. This negative or suctional pressure on the left side of the heart is of the highest importance in helping the pulmonary cir- culation. It has been found to be equal to 23 mm. of mercury, and is. quite independent of the aspiration or suction power of the thorax in aid-^ ing the blood-flow to the heart, to be described in the chapter on Eesj)ira- tion. Function of the Musculi Papillares. — The special function of the musculi 2^apiUares is to prevent the auriculo-ventricular valves from being everted into the auricle. For the chordse tendineee might allow the valves to be pressed back into the auricle, were it not that when the Vol. I.— 8. 114 HA™-B00K of PHYSIOLOaY. wall of the ventricle is brought by its contraction nearer the auriculo- ventricular orifice, the musculi papillares more than compensate for this by their own contraction' — holding the cords tight, and, by pulling down the valves, adding slightly to the force with which the blood is expelled. What has been said applies equally to the auriculo-ventricular valves on both sides of the heart, and of both alike the closure is generally com- plete every time the ventricles contract. But in some circumstances the closure of the tricuspid valve is not complete, and a certain quantity of blood is forced back into the auricle. This has been called the safety- valve action of this valve. The circumstances in which it usually hapjDcns are those in which the vessels of the lung are already full enough when the right ventricle contracts, as e.g., in certain pulmonary diseases, in ver} active exertion, and in great efforts. In these cases, the tricuspid vah e does not completely close, and the regurgitation of the blood may be indicated by a pulsation in the jugular veins synchronous with that in the carotid arteries. Function of the Semilunar Valves. — The arterial or semilunar valves are forced apart by the out-streaming blood, with which the con- tracting ventricle dilates the large arteries. The dilation of the arteries is, in a peculiar manner, adapted to bring the valves into action. The lower borders of the semilunar valves are attached to the inner surface of a tendinous ring, which is, as it were, inlaid at the orifice of the artery, between the muscular fibres of the ventricle and the elastic fibres of the walls of the artery. The tissue of this ring is tough, and does not admit of extension under such pressure as it is commonly exposed to; the valves are equally inextensile, being, as already mentioned, formed of tough, close- textured, fibrous tissue, with strong interwoven cords, and covered with endocardium. Hence, when the ventricle propels blood through the ori- fice and into the canal of the artery, the lateral pressure which it exercises is sufficient to dilate the walls of the artery, but not enough to stretch in an equal degree, if at all, the unyielding valves and the ring to which their lower borders are attached. The effect, therefore, of each such propul- sion of blood from the ventricle is, that the wall of the first portion of the artery is dilated into three pouches behind the valves, while the free margins of the valves are draAvn inward toward its centre (Fig. 98, b). Their j)ositions may be explained by the diagrams, in which the continu- ous lines represent a transverse section of tlie arterial walls, the dotted ones the edges of the valves, firstly, when the valves are nearest to the walls (a), and, secondly, when, tlie walls being dilated, the valves are drawn away from them (b). Tliis ])osition of tlu^ valves and arterial walls is retained so long as the ventricle (toiitiuues in contraction: but, as soon as it relaxes, and the di- liiicd jiiicrial walls can recoil by tlieir ehisticity, the blood is forced back- ward lownrd the ventricles as onward in the course of the circnlation. CIRCULATION OF THE BLOOD. 115 Part of the blood thus forced back lies in the pouches (sinuses of Valsalva) (a, Fig. 98, b) between the valves and the arterial walls; and the valves are by it pressed together till their thin lunated margins meet in three Fig. 98.— Sections of aorta, to show the action of the semilunar valves, a is intended to show the valves, represented by the dotted lines, pressed toward the arterial walls, represented by the con- tinuous outer line, b (after Hunter) shows the arterial wall distended into three pouches ( a ), and drawn away from the valves, which are straightened into the form of an equilateral triangle, as rep- resented by the dotted hues. « lines radiating from the centre to the circumference of the artery (7 and 8, Fig. 99). The contact of the valves in this position, and the complete closure of the arterial orifice, are secured by the peculiar construction of their bor- ders before mentioned. Among the cords which are interwoven in the Fig. 99. — View of the base of the ventricular part of the heart, showing the relative position of the arterial and auriculo-ventricular orifices. — %. The muscular fibres of the ventricles are exposed by the removal of the pericardium, fat, blood-vessels, etc. ; the pulmonary artery and aorta have been removed by a section made immediately beyond the attachment of the semilunar valves, and the au- ricles have been removed immediately" above the auriculo-ventricular orifices. The semilunar and auriculo-ventricular valves are in the nearly closed condition. 1, 1, the base of the right ventricle; 1', the conus arteriosus; 2, 2, the base of the left ventricle; 3, 3, the divided wall of the right auricle; 4, that of the left; 5, 5,' 5", the tricuspid valve; 6, 6', the mitral valve. In the angles between these segments are seen the smaller fringes frequently observed; 7, the anterior part of the pulmonary ar- tery ; 8, placed upon the posterior part of the root of the aorta ; 9, the right, 9', the left coronary artery. (Allen Thomson.) substance of the valves, are two of greater strength and prominence than the rest; of which one extends along the free border of each valve, and the other forms a double curve or festoon just below the free border. 116 HAND-BOOK OF PHYSIOLOGY. Each of these cords is attached by its outer extremities to the outer end of the free margin of its valve, and in the middle to the corpus Arantii; they thus enclose a lunated space from a line to a line and a half in width, in which space the substance of the valve is much thinner and more pliant than elsewhere. When the valves are pressed down, all these parts or spaces of their surfaces come into contact, and the closure of the arterial orifice is thus secured by the apposition not of the mere edges of the valves, but of all those thin lunated parts of each which lie between the free edges and the cords next below them. These parts are firmly pressed together, and the greater the pressure that falls on them the closer and ".nore secure is their apposition. The corpora Arantii meet at the centre of the arterial orifice when the valves are down, and they probably assist in the closure; but they are not essential to it, for, not unfrequently, they are wanting in the valves of the pulmonary artery, which are then extended in larger, thin, flapping margins. In valves of this form, also, the inlaid cords are less distinct than in those with corpora Arantii; yet the closure by contact of their surfaces is not less secure. It has been clearly shown that this pressure of the blood is not entirely sustained by the valves alone, but in part by the muscular substance of the ventricle (Savory). By making vertical sections (Fig. 100) through various parts of the tendinous rings it is pos- sible to show clearly that the aorta and pulmonary artery, expanding toward their termination, are sit- uated upon the Older edge of the thick upper border of the ventricles, and that consequently the portion of each semilunar valve adjacent to the vessel passes over and rests upon the muscular substance — being thus supported, as it were, on a kind of muscular floor formed by the upper border of the ventricle. The result of this arrange- ment is that the reflux of the blood is most efficiently sustained by the ventricular wall. * » As soon as the auricles have completed their contraction they begin again to dilate, and to be refilled with blood, which fiows into them in a steady stream tlirougli tlie great venous trunks. They are thus filling- during all the time in which the ventricles are contracting; and the con- traction of tlie ventricles being ended, these also again dilate, and receive again tlie l)lood that flows into them from the auricles. By the time thtit tlie ventricles are thus from one-third to two-thirds full, the auricles are ' Savory's prciuirations, illustralin^; this and oIIut points in relation to the struc- ture and functions of the valves of the heart, are in the Museum of St. Bartholomew's Hospital. Fig. 100.— Vertical sec- tion tlrroug;h the aorta at its junction with the left ventricle, a. Section of aorta, bb. Section of two valves. c\ Section of wall of ventricle, c?. In- ternal surface of ven- tricle. CIRCULATION OF THE BLOOD 117 distended; these, then suddenly contracting, fill up the ventricles, as already described (p. 111). Cardiac Revolution. — If we suppose a cardiac revolution divided into five parts, one of these will be occupied by the contraction of the auricles, two by that of the ventricles, and two by repose of both auricles and ventricles. Contraction of Auricles . . . 1 -|- Eepose of Auricles . . . 4=5 " Ventricles . . % -\- " Ventricles . . 3=5 Eepose (no contraction of either auricles or ventricles) . . . 2 + Contraction (of either auri- — cles or ventricles) . . . 3=5 5 If the speed of the heart be quickened, the time occupied by each cardiac revolution is of course diminished, but the diminution a"ffects only the diastole and pause. The systole of the ventricles occupies very much the same time, about -5%- sec, whatever the pulse-rate. The periods in which the several valves of the heart are in action may be connected with the foregoing table; for the auriculo-ventricular valves are closed, and the arterial valves are open during the whole time of the ventricular contraction, while, during the dilation and distension of the ventricles the latter valves are shut, the former open. Thus whenever the auriculo-ventricular valves are open, the arterial valves are closed and vice versa. SOUN-DS OF THE HeAKT. When the ear is placed over the region of the heart, two sounds may be heard at every beat of the heart, which follow in quick succession, and are succeeded by pause or period of silence. first sound is dull and prolonged; its commencement coincides with the impulse of the heart, and just precedes the pulse at the wrist. The second is a shorter and sharper sound, with a somewhat flapping character, and follows close after the arterial pulse. The period of time occupied respectively by the two sounds taken together, and by the pause, are almost exactly equal. The relative length of time occupied by each sound, as compared with the other, is a little uncertain. The difference may be best appreci- ated by considering the different forces concerned in the production of the two sounds. In one case there is a strong, comparatively slow, con- traction of a large mass of muscular fibres, urging forward a certain quantity of fluid against considerable resistance; while in the other it is a strong but shorter and sharper recoil of the elastic coat of the large arteries, — shorter because there is no resistance to the flapping back of 118 HAND-BOOK OF PHYSIOLOGY. the semilunar valves, as there was to their opening. The sounds may be expressed by saying the words luhh — dup (0. J. B. Williams). The events which correspond, in point of time, with the first sound, are (1) the contraction of the ventricles, (2) the first part of the dilatation of the auricles, (3) the closure of the auriculo-ventricular valves, (4) the opening of the semilunar valves, and (5) the propulsion of blood into the arteries. The sound is succeeded, in about one-thirtieth of a second, by the pulsation of the facial arteries, and in about one-sixth of a second, by the pulsation of the arteries at the wrist. The second sound, in point of time, immediately follows the cessation of the ventricular contraction, and corresponds with [a) the closure of the semilunar valves, {h) the con- tinued dilatation of the auricles, (c) the commencing dilatation of the ventricles, and {d) the opening of the auriculo-ventricular valves. The pause immediately follows the second sound, and corresponds m its first part with the completed distension of the auricles, and in its seco7id with their contraction, and the completed distension of the ventricles; the auriculo-ventricular valves being, all the time of the pause, open, and the arterial valves closed. Causes. — The chief cause of the first sound of the heart appears to be the vibration of the auriculo-ventricular valves, due to their stretch- ing, and also, but to a less extent, of the ventricular walls, and coats of the aorta and pulmonary artery, all of which parts are suddenly put into a state of tension at the moment of ventricular contraction. The effect may be intensified by the muscular soiond produced by contraction of the mass of muscular fibres which form the ventricle. The cause of the second sound is more simple than that of the first. It is probably due entirely to the sudden closure and consequent vihration of the semilunar valves when they are pressed down across the orifices of the aorta and pulmonary artery. The influence of the valves in produc- ing the sound is illustrated by the experiment performed on large ani- mals, such as calves, in which the results could be fully appreciated. In these experiments two delicate curved needles were inserted, one into the aorta, and another into tlie pulmonary artery, below the line of attach- ment of the semilunar valves, and, after being carried upward aboitt half an inch, were brought out again through the coats of the respective vessels, so that in each vessel one valve was included between the arterial walls and the wire. Upon applying the stethoscope to the vessels, after such an operation, tlie second sound had ceased to be audible. Disease of tliesc valves, wlien so extensive as to interfere with their efficient action, also often demonstrates tlie same fact by modifying or destroying the distinctness of tlic secoiul sound. One reason for tlu^ second sound being a clearer and sliar})er one than the first m;iy be, tliat the semihniar valves are not covered in by the thick layer of fibres ('()iiij)()siiig wi.lls of (lie to such an extent as are CIRCULATION OF THE BLOOD. 119 the aitriculo-ventricular. It might be expected therefore thiit their vibra- tion would be more easily heard through a stethoscope applied to the walls of the chest. The contraction of the auricles which takes place in the eiid of the pause is inaudible outside the chest, but may be heard, when the heart is exposed and the stethoscope placed on it, as a slight sound preceding and continued into the louder sound of the ventricular contraction. The Impulse of the Heart. — At the commencement of each ven- tricular contraction, the heart may be felt to beat with a slight shock or impulse against the walls of the chest. The force of the impulse, and the extent to which it may be perceived beyond this point, vary considerably in different individuals, and in the same individual under different cir- cumstances. It is felt more distinctly, and over a larger extent of surface, in emaciated than in fat and robust persons, and more during a forced ex- piration than in a deep inspiration; for, in the one case, the intervention of a thick layer of fat or muscle between the heart and the surface of the chest, and in the other the inflation of the portion of lung which overlaps the heart, prevents the impulse from being fully transmitted to the sur- face. An excited action of the heart, and especially a hypertrophied con- dition of the ventricles, will increase the impulse; while a depressed con- dition, or an atrophied state of the ventricular walls, will diminish it. Cause of the Impulse. — During the period which precedes the ventricular systole, the apex of the heart is situated upon the diaphragm and against the chest-wall in the fifth intercostal space. When the ven- tricles contract, their walls become hard and tense, since to expel their contents into the arteries is a distinctly laborious action, as it is resisted by the tension within the vessels. It is to this sudden hardening that the impulse of the heart against the chest- wall is due, and the shock of the sudden tension may be felt not only externally, but also internally, if the abdomen of an animal be opened and the fiuger be placed upon the under surface of the diaphragm, at a point corresponding to the under surface of the ventricle. The shock is felt, and possibly seen more distinctly, because of the partial rotation of the heart, already spoken of, along its long axis toward the right. The movement produced by the ventricular contraction may be registered by means of an instrument called the cardio- graph, and it -will be found to correspond almost exactly with a tracing obtained by the same instrument applied over the contracting ventricle itself. The Cardiograph (Fig. 101) consists of a cup-shaped metal box, over the open front of which is stretched an elastic membrane, upon which is fixed a small knob of hard wood or ivory. This knob, however, may be attached instead, as in the figure, to the side of the box by means of a spring, and may be made to act upon a metal disc attached to the elastic mem.brane. 120 HAND-BOOK OF PHYSIOLOGY. The knob (a) is for application to the chest-wall over the place of the greatest impulse of the heart. The box or tympanum communicates b}' means of an air-tight elastic tube (/) with the interior of a second tympanum (Fig. 102, h), in connection with which is a long and light lever {a). The shock of the heart's impulse being communicated to the ivory knob, and through it to the first tympanum, the effect is, of course, at once transmitted by the column of air in the elastic tube to the interior of the second tympanum, also closed, and through the elastic and movable lid of the latter to the lever, which is placed in connection with a registering appa- FiG 101 ratus, which consists generally of a cylinder or Cardiograph. (Sanderson's.) cOVercd with Smokcd paper, revolving according to a definite velocity by clockwork. The point of the lever writes upon the paper, and a tracing of the heart's impulse is thus obtained. By placing three small india-rubber air -bags in the interior resjjec- FiG. 102.— Marey's Tambour ( & ), to which the movement of the column of air in the first tym- panmn is conducted by tlie tube, /, and from -wliicli it is communicated by the lever, a, to a revolving cylinder, so that tlie tracing of the movement of the impulse beat is obtained. tively of the right auricle, the right ventricle, and in an intercostal space in front of the heart of living animals (horse), and placing these bags, by means of long narrow tubes, in communication with three levers, arranged Fig. 103.— Tracing of t\w impulse of the heart of man. (IMaivy.) one over the otlier in connection with a registci-ing :i])]):iralns (Fig. 104), MM. Chauveau and Marey have been able to measure willi inucli accuracy tlu^ variations of tlie endocardial pressure and tlu> comparative dnration OIKOULATlOxN^ OF THE BLOOD. 121 of the contractions of the auricles and ventricles. By means of the same apparatus, the synclironism of the impulse with the contraction of the ventricles, is also well shown; and the causes of the several vibrations of which it is really composed, have been discovered. In the tracing (Fig 105), the intervals between the vertical lines rep- resent periods of a tenth of a second. The parts on which any given Fig. 104. — Apparatus of MM. Chauveau and Marey for estimating the variations of endocardial pressure, and production of impulse of the heart. vertical line falls represent, of course, simultaneous events. Thus, — it will be seen that the contraction of the auricle, indicated by the upheaval of the tracing at A in first tracing, causes a slight increase of pressure in the ventricle (a' in second tracing), and produces a tiny impulse (a" in third tracing). So also, the closure of the semilunar valves, while it causes a momen- tarily increased pressure in the ventricle at d', does not fail to affect the pressure in the auri- cle d", and to leave its mark in the tracing of the impulse also, d''. The large upheaval of the ventricular and the impulse tracings, between a' and d', and a" and d", are caused by the ventricular con- traction, while the smaller undulations, between B and c, b' and c', b" and c", are caused by the vibrations consequent on the tightening and closure of the auriculo- ventricular valves. Although, no doubt, the method thus de- scribed may show a perfectly correct view of the endocardiac pressure variations, it should be recollected that the muscular walls may grip the air-bags, even after the complete expulsion of the contents of the chamber, and so the lever might remain for a too long time in the position of extreme tension, and woi:^d Fig. 105.— Tracings of (1), In- tra-auricular, and (2), Intra- ven- tricular pressures, and (3), of the impulse of the heart, to be read from left to right, obtained by Chauveau and Marey 's apparatus. 122 HAND-BOOK OF PHYSIOLOGY represent on the tracing not only, as it ought to do, the auricular or ventricular pressure on the blood, but, also afterward, the muscular pres- sure exerted upon the bags themselves. (M. Foster.) Fkequency and Force of the Heart's Action. The heart of a healthy adult man contracts from seventy to seventy-five times in a minute; but many circumstances cause this rate, which of course corresponds with that of the arterial pulse, to vary even in health. The chief are age, temperament, sex, food and drink, exercise, time of day, posture, atmospheric pressure, temperature. Age. — The frequency of the heart's action gradually diminishes from the commencement to near the end of life, but is said to rise again somewhat in extreme old age, thus: — Before birth the average number of pulses in a minute is 150 . Just after birth from 140 to 130 During the first year .... 130 " 115 During the second year . . . . " 115 100 During the third year . . . . " 100 90 About the seventh year . . . . " 90 85 Abo at the fourteenth year, the average number of pulses in a minute is 85 80 In adult age 80 70 In old age . . . . . . " 70 " 60 In decrepitude . . . . . " 75 65 Temperament and Sex. — In persons of sanguine temperament, the heart acts somewhat more frequently than in those of the phlegmatic; and in the female sex more frequently than in the male. Food and Drink. Exercise. — After a meal its action is accelerated, and still more so during bodily exertion or mental excitement; it is slower during sleep. Diurnal Variation. — It appears that, in the state of healtli, the pulse is most frequent in the morning, and becomes gradually slower as the day advances, and that this diminution of frequency is both more regular and more rapid in the evening than in the morning. Posture. — It is found that, as a general rule, the pulse, especially in the adult male, is more frequent in the standing than in the sitting })os- turc, and in the latter than in the recuinbeiit position; the difiorence being greatest between the standing and the sitting posture. The eifect of (;liange of ])osture is greater jis the frequency of tlio pulse is greater, and, acH'ordiiigly, is more marked in the morning than in the evening. By snp])()rting the body in different postures, without the aid of mus- (;ulaT effort of the individual, it has been proved that the increased fre- (juency of the pulse in the sitting and standing ])()sitions is dependent u])on the muscular exertion engaged in maintaining tlunn; the usual effect of these postures on the ])ulse being almost entirely prevented when the usually at tcnchmt muscular exertion was rcMidered unnecessary, ((luy.) CIRCULATION OF THE BLOOD. 123 Atmospheric Presmre. — The frequency of the pulse increases in a corresponding ratio with the elevation above the sea. Temperature. — The rapidity and force of the heart's contractions are largely influenced by variations of temperature. The frog's heart, when excised, ceases to beat if the temperature be reduced to 32° F. (0° C). When heat is gradually applied to it, both the speed and force of the heart's contractions increase till they reach a maximum. If the tem- perature is still further raised, the beats become irregular and feeble, and the heart at length stands still in a condition of " heat -rigor. Similar effects are produced in warm-blooded animals. In the rabbit, the number of heart-beats is more than doubled when the temperature of the air was maintained at 105° F. (40°.5 C). At 113°— 114° F. (45° C), the rabbit's heart ceases to beat. Relative Frequency of the Pulse to that of Respiration. — In health there is observed a nearly uniform relation between the fre- quency of the pulse and of the respirations; the proportion being, on an average, one respiration to three or four beats of the heart. The same relation is generally maintained in the cases in which the pulse is naturally accelerated, as after food or exercise; but in disease this relation usually ceases. In many affections accompanied with increased frequency of the pulse, the respiration is, indeed, also accelerated, yet the degree of its acceleration may bear no definite proportion to the increased number of the heart's actions: and in many other cases, the pulse becomes more fre- quent without any accompanying increase in the number of respirations; or, the respiration alone may be accelerated, the number of pulsations re- maining stationary, or even falling below the ordinary standard. The Force of the Ventricular Systole and Diastole.— The force of the left ventricular systole is more than double that exerted by the contraction of the right: this difference in the amount of force exerted by the contraction of the two ventricles, results from the walls of the left ventricle being about twice or three times as thick as those of the right. And the difference is adapted to the greater degree of resistance which the left ventricle has to overcome, compared with that to be overcome by the right: the former having to propel blood through every part of the body, the latter only through the lungs. The actual amount of the intra-ventricular pressures during systole in the dog has been found to be 2 '4 inches (60 mm.) of mercury in the right ventricle, and 6 inches (150 mm.) in the left. During diastole there is in the right ventricle a negative or suction pressure of about | of an inch (—17 to —16 mm.), and in the left ventricle from 2 inches to f of an inch (—52 to —20 mm.). Part of this fall in pressure, and possibly the greater part, is to be referred to the influence of respiration; but with- out this the negative pressure of the left ventricle caused by its active dilatation is about | of an inch (23 mm.) of mercury. The right ventricle is undoubtedly aided by this suction power of the 124 HAND-BOOK OF PHYSIOLOGY. left, so that the whole of the work of conducting the pulmonary circula- tion does not fall upon the right side of the heart, but is assisted by the left side. The Force of the Auricular Systole and Diastole. — The maximum pressure within the right auricle is about | of an inch (20 mm.) of mercury, and is probably somewhat less in the left. It has been found that during diastole the pressure within both auricles sinks considerably below that of the atmosphere; and as some fall in pressure takes place, even when the thorax of the animal operated upon has been opened, a certain proportion of the fall must be due to active auricular dilatation independent of respiration. In the right auricle, this negative pressure is about —10 mm. Work Done by the Heart. — In estimating the work done by any machine it is usual to express it in terms of the "unit of work.^^ The unit of work is defined to be the energy expended in raising a unit of weight (1 lb.) through a unit of height (1 ft.). In England, the unit of work is the "foot-pound,'' in France, the ^^Mlogrmmnetre.'' The work done by the heart at each contraction can be readily found by multiplying the weight of blood expelled by the ventricles by the height to which the blood rises in a tube tied into an artery. This height was found to be about 9 ft. in the horse, and the estimate is nearly correct for a large artery in man. Taking tlie weight of blood. expelled from the left ventricle at each systole as 6 oz., i.e., | lb., we have 9 X f = 3-375 foot-pounds as the work done by the left ventricle at each systole; and adding to this the work done by the right ventricle (about one-tliird that of the left) we have 3 "375 X 1'125 = 4*5 foot-pounds as the work done by the heart , at each contraction. Other estimates give ^ kilogrammetre, or about 3 \ foot-pounds. Haughton estimates the total work of the heart in 24 hours as about 124 foot-tons. Influence of the Nervous System on the Action of the Heart. — The hearts of warm-blooded animals cease to beat almost if not quite immediately after removal from the body, and are, therefore, un- favorable for the study of the nervous mechanism which regulates their action. Observations have hitherto, therefore, been principally directed to the lieart of cold-blooded animals, e.g., tlie frog, tortoise, and snake, wliich will continue to beat under favorable conditions for many hours after removal from the body. Of these animals, the frog is the one mostly employed, and, indeed, until recently, it was from the study of tlio frog's lieart that tlie chief })art of our information was obtained. If removed from the body entire, the frog's heart will continue to beat for many hours and even days, and the beat has no a})parent difference from the beat of the heart before removal from the body; it will take place without the ])reseuce of blood or other Ihiid within its chambers. If the bents have become iiifrcqiunit, an additional beat may be induced by stimulating CIRCULATION OF THE BLOOD. 125 the heart by means of a blunt needle; but the time before the stimulus applied produces its result (the latent period) is very prolonged, and as in this way the cardiac beat is like the contraction of unstriped muscle, the method has been likened to a peristaltic contraction. There is much uncertainty about the nervous mechanism of the beat of the frog's heart, but what has just been said shows, at any rate, two things; firstly, that as the heart will beat when removed from the body in a way differing not at all from the normal, it must contain within itself the mechanism of rhythmical contraction; and secondly, that as it can beat without the presence of fluid within its chambers, the movement cannot depend merely on reflex excitation by the entrance of blood. The nervous apparatus existing in the heart itself consists of collections of microscopic ganglia, and of nerve- fibres proceeding from them. These ganglia are Fig. 106. — Heart of frog. (Burdon-Sanderson after Fritsche.) Front view to the left, back view- to the right. A A. Aortse. V. cs. Venae cavae superiores. At s, left auricle. At d, right auricle. Fen., ventricle. B. ar., Bulbus arteriosus. (S. v.. Sinus venosus. V. c. i., Vena cava inferior. V. h., Vense hepaticae. V. p., Venae pulmonales. demonstrable as being collected chiefly into three groups; one is in the wall of the sinus venosus (Kemak's); a second, near the junction between the auricle and ventricle (Bidder^s); and the third in the septum between the auricles. Some very important experiments seem to identify the rhythmical contractions of the frog^s heart with these ganglia. If the heart be re- moved entire from the body, the sequence of the contraction of its several beats will take place with rhythmical regularity, viz., of the sinus veno- sus, the auricles, the ventricle, and bulbus arteriosus, in order. If the heart be removed at the junction of the sinus and auricle, the former will continue to beat, but the removed portion will for a short variable time stop beating, and then resume its beats, but with a rhythm different to that of the sinus: and, further, if the ventricle be removed, it will take a still longer time before recommencing its pulsation after its removal than the larger portion consisting of the auricles and ventricle, and its rhythm is different from that of the unremoved portion, and not so regu- lar, nor will it continue to pulsate so long: during the period of stop- page a contraction will occur if the ventricle be mechanically or otherwise stimulated. If the lower two-thirds or apex of the ventricle be removed, the remainder of the heart will go on beating regularly in the body, but 126 HAND-BOOK OF PHYSIOLOGY. this part will remain motionless, and will not beat spontaneously, although it will respond to stimuli. If the heart be divided lengthwise, its parts will continue to pulsate rhythmically, and the auricles may be cut up into pieces, and the pieces will continue their movements of contraction. It will be thus seen that the rhythmical movements appear to be more marked in the parts supplied by the ganglia, and that the apical portion of the ventricle, in which the ganglia are not found, does not possess the power of automatic movement. Although the theory that the pulsations of the rest of the heart are dependent upon that of the sinus, and to stimuli pro- ceeding from it, when connection is maintained, and only to reflex stim- uli when removal has taken place, cannot be absolutely upheld, yet it is evident that the power of spontaneous contraction is strongest in the sinus, less strong in the auricles, and less so still in the ventricle, and that, therefore, the sinus ganglia are probably important in exciting the rhythmical contraction of the whole heart. This is expressed in the fol- lowing way: — "The power of independent rhythmical contraction de- creases regularly as we pass from the sinus to the ventricles, and "The rhythmical power of each segment of the heart varies inversely as its dis- tance from the sinus." (G-askell.) It has been recently shown that, under appropriate stimuli, even the extreme apex of the ventricle in the tortoise may take on rhythmical contractions, or in other words may be "taught to beat" rhythmically. (Gaskell.) Inhibition of the Heart's Action. — Although, under ordinary conditions, the apparatus of ganglia and nerve-fibi'es in the substance of the heart forms the medium through which its action is excited and rhythmically maintained, yet they, and, through them, the hearths con- tractions, are regulated by nerves which pass to them from the higher nerve-centres. These nerves are branches from the pneumogastric or vagus and the sympathetic. The influence of the vagi nerves over the heart-beat may be shown by stimulating one (especially the right) or both of the nerves when a record is being taken of the beats of the frog's heart. If a single induction shock be sent into the nerve, the heart, after a short interval, ceases beating, but after the suppression of several beats resumes its action. As already mentioned, the effect of the stimulus is not immediately seen, and one beat may occur before the heart stops after the application of the electric-cur- rent. The stoppage of the heart may occur apparently in one of two ways, either by diminution of the strength of the systole or by increas- ing the length of tlie diastole. The stoppage of the heart may be brought about by tlie ai)i)lication of the electrodes to any part of the vagus, but most e1T('(;tiially if thoy are api)licd near the ])osition of l^eniak's ganglia. It is 8iq)p()se(l tliat the flbres of the vagi, tliorefore, terminate there in CIRCULATION OF THE BLOOD. 127 inhihitory ganglia in the lieart-walls, and that tlie inhibition of the heart's beats by means of the vagus, is not a simple action, but that it is pro- duced by stimulating centres in the heart itself. These inhibitory centres are paralyzed by atropin, and then no amount of stimulation of the vagus, or of the heart itself, will produce any effect upon the cardiac beats. Urari in large doses paralyzes the vagjus fibres, but in this case, as the inhibitory action can be produced by direct stimulation of the heart, it is inferred that this drug does not paralyze the ganglia themselves. Mus- carin and pilocarpin appear to produce effects similar to those obtained by stimulating the vagus fibres. If a ligature be tightly tied round the heart over the situation of the ganglia between the sinus and the auricles, the heart stops beating. This experiment (Stannius') would seem to stimulate the inhibitory gan- glia, but for the remarkable fact that atropin does not interfere with its success. If the part (the ventricle) below the ligature be cut off, it will begin and continue to beat rhythmically, this may be explained by sup- posing that the stimulus of section induces pulsation in the part which is removed from the influence of the inhibitory ganglia. So far, the effect of the terminal apparatus of the vagi has been con- sidered; there is, however, reason for believing that the vagi nerves are simply the media of an inliiMtory or restraining influence over the action of the heart, which is conveyed through them from a centre in the me- dulla oblongata which is always in operation, and, because of its restrain- ing the heart's action, is called the cardio-inliibitory centre. For, on dividing these nerves, the pulsations of the heart are increased in fre- quency, an effect opposite to that produced by stimulation of their divided (peripheral) ends. The restraining influence of the centre in the medulla may be increased reflexly, producing slowing or stoppage of the heart, through influence passing from it down the vagi. As an example of the latter, the well-known effect on the heart of a violent blow on the epigastrium may be referred to. The stoppage of the heart's action is due to the conveyance of the stimulus by fibres of the sympathetic to the medulla oblongata, and its subsequent rejiection through the vagi to the inhibitory ganglia of the heart. It is also believed that the power of the medullary inhibitory centre may be reflexly lessened, producing acceler- ated action of the heart. Acceleration of Heart's Action. — Through certain fibres of the sympathetic, the heart receives an accelerating influence from the medulla oblongata. These accelerating nerve-flbres, issuing from the spinal cord in the neck, reach the inferior cervical ganglion, and pass thence to the cardiac plexus, and so to the heart. Their function is shown in the quickened pulsation which follows stimulation of the spinal cord, when the latter has been cut off from all connection with the heart, excepting that which is formed by the accelerating filaments from the inferior cer- 128 HAND-BOOK OF PHYSIOLOGY. vical ganglion. Unlike the inhibitory fibres of the pneumogastric, the accelerating fibres are not continuonsly in action. The accelerator nerves must not, however, be considered as direct antagonists of the vagus; for if at the moment of their maximum stimu- lation, the vagus be stimulated with minimum currents, ijihibition is produced with the same readiness as if these were not acting. The connection of the heart with other organs by means of the nerv- ous system, and the influences to which it is subject through them, are shown in a striking manner by the phenomena of disease. The influence of mental shock in arresting or modifying the action of the heart, the slow pulsation which accompanies compression of the brain, the irregu- larities and palpitations caused by dyspepsia or hysteria, are good evidence of the connection of the heart with other organs through the nervous system. The action of the heart is no doubt also very materially affected by the nutrition of its walls by a sufiicient supply of healthy blood sent to them, and it is not unlikely that the apparently contradictory effect of poisons may be explained by supposing that the influence of some of them is either partially or entirely directed to the muscular tissue itself, and not to the nervous apparatus alone. As will be explained presently, the heart exercises a considerable influence upon the condition of the pressure of blood within the arteries, but in its turn the blood-pressure within the arteries reacts upon the heart, and has a distinct effect upon its contrac- tions, increasing by its increase, and vice versa, the force of the cardiac beat, although the frequency is diminished as the blood-pressure rises. The quantity (and quality?) of the blood contained in each chamber, too, has an influence upon its systole, and within normal limits the larger the quantity the stronger the contraction. Kapidity of systole does not of necessity indicate strength, as two weak contractions often do no more work than one strong and prolonged. In order that the heart may do its maximum work, it must be allowed free space to act; for if obstructed in its action by mechanical outside pressure, as by an excess of fluid within the pericardium, such as is produced by inflammation, or by an over- loaded stomach, or what not, the pulsations become irregular and feeble. The Arteries. Distribution. — The arterial system begins at the left ventricle in a single lai'go trunk, the aorta, which almost immediately after its origin gives off in its course in the thorax three large branches for the supply of the head, neck, and upper extremities; it then traverses the thorax and abdomen, giving off branches, some large and some small, for tlie supply of the various organs and tissues it passes on its way. In the abdomen it divides into two chief branches, for the supply of the lower CIRCULATION OF THE BLOOD. 129 extremities. The arterial branches wherever given off divide and sub- divide, until the calibre of each subdivision becomes very minute, and these minute vessels pass into capillaries. Arteries are, as a rule, placed in situations protected from pressure and other dangers, and are, with few exceptions, straight in their course, and frequently communicate with other arteries (anastomose or inosculate). The branches are usually given off at an acute angle, and the area of the branches of an artery gen- erally exceeds that of the parent trunk; and as the distance from the origin is increased, the area of the combined branches is increased also. After death, arteries are usually found dilated (not collapsed as the veins are) and empty, and it was to this fact that their name was given them, as the ancients believed that they conveyed air to the various parts of the body. As regards the arterial system of the lungs (pulmonary system) it begins at the right ventricle in the pulmonary artery, and is distributed much as the arteries belonging to the general systemic cir- culation. Structure. — The walls of the arteries are composed of three principal coats, termed the external or tunica adventitia, the middle or tunica media, and the internal cosit or tunica intima. The external coat or tunica adventitia (Figs. 107 and 111, t. a.), the strongest and toughest part of the wall of the artery, is formed of areolar Fig. 107.— Minute artery viewed in longitudinal section, e. Nucleated endothelial membrane, vath. faint nuclei in lumen, looked at from above, i. Thin elastic tunica intima. m. Muscular coat; or tunica media, a. Tunica adventitia. (Klein and Noble Smith.) x 250. Fig. 108.— Portion of fenestrated membrane trom the femoral artery, x 200. a, 6, c. Perfo- rations. (Henle.) tissue, with which is mingled throughout a network of elastic fibres.. At the inner part of this outer coat the elastic network forms in most arteries so distinct a layer as to be sometimes called the external elastic coat (Fig. 123, e. e.). The middle coat (Fig. 107, m) is composed of both muscular and Vol. I.— 9 Fig. 107. Fig. 108. 130 HAND-BOOK OF PHYSIOLOGY. elastic fibres, with a certain proportion of areolar tissue. In the larger arteries (Fig. 110) its thickness is comparatively as well as absolutely much greater than in the small, constituting, as it does, the greater part of the arterial wall. The muscular fibres, which are of the unstriped variety (Fig. 109) are arranged for the most part transversely to the long axis of the artery (Fig. 107, m); while the elastic element, taking also a transverse direc- tion, is disposed in the form of closely interwoven and branching fibres, which intersect in all parts the layers of muscular fibre. In arteries of Fig. 109. Fig. 110. Fig. 109.— Muscular fibre-cells from human arteries, magnified 350 diameters. (Kolliker.) a. Nucleus, b. A fibre-cell treated with acetic acid. Fig. 110.— Transverse section of aorta through internal and about half the middle coat. a. Lin- ing endothelium with the nuclei of the cells only shown, b. Subepithelial layer of connective tissue, c, d. Elastic tunica intima proper, with fibrils rumiing circularly or longitudinally, e. f. Middle coat, consisting of elastic fibres arranged longitudinallj', with muscle-fibres cut obliquely, or longitudinallj'. (Klein.) various size there is a difference in the proportion of the muscular and clastic element, elastic tissue preponderating in the largest arteries, while this condition is reversed in those of medium and small size. The internal coat is formed by layers of elastic tissue, consisting in part of coarse longitudinal branching fibres, and in part of a very thin and brittle mem])rane which possesses little elasticity, and is thrown into folds or wrinkles when tlic artery contracts. Tliis latter monibrano, the striated or fenestrated coat of Iloilr (1^'ig. lOS), is peculiar in its ten- d(5ncy to cnrl up, when ])eekHl olT I'roni (he artery, and in the perforated CIRCULATION OF THE BLOOD. 131 and streaked appearance which it presents under the microscope. Its inner surface is lined with a delicate layer of endothelium, composed of elongated cells (Fig. 112, a), which make it smooth and polished, and furnish a nearly impermeable surface, along which the blood may flow with the smallest possible amount of resistance from friction. Immediately external to the endothelial lining of the artery is fine connective tissue, suh-endotlielial layer, with branched corpuscles. Thus the internal coat consists of three parts, {a) an endothelial lining, {h) the sub-endothelial layer, and {c) elastic layers. Vasa Vasorum. — The walls of the arteries, with the possible excep- tion of the endothelial lining and the layers of the internal coat immedi- ately outside it, are not nourished by the blood which they convey, but are, like other parts of the body, supplied with little arteries, ending in Fig. 111. — Transverse section of small artery from soft palate, e, endothelial lining, the nuclei of the cells are shown; i, elastic tissue of the intima, which is a good deal folded; c. m. circular mus- cular coat, showing nuclei of muscle cells; t. a. timica adventitia. X 300. (Schofield.) Fig. 112.— Two blood-vessels from a frog's mesentery, injected with nitrate of silver, showing the outlines of the endothelial cells, a. Artery. The endothelial cells are long and narrow; the trans- verse markings indicate the muscular coat. t. a. Tunica adventitia. v. Vein, showing the shorter and wider endothehal ceUs with which it is lined, c, c. Two capillaries entering the vein. (Schofield.) capillaries and veins, which, branching throughout the external coat, extend for some distance into the middle, but do not reach the internal coat. These nutrient vessels are called vasa vasorum. Lymphatics of Arteries and Veins. — Lymphatic spaces are pres- ent in the coats of both arteries and veins; but in the tunica adventitia or external coat of large vessels they form a distinct plexus of more or less tubular vessels. In smaller vessels they appear as sinous spaces lined by endothelium. Sometimes, as in the arteries of the omentum, mesentery, and membranes of the brain, in the pulmonary, hepatic, and splenic arteries, the spaces are continuous with vessels which distinctly ensheath Fig. 111. Fig. 112, 132 HAND-BOOK OF PHYSIOLOGY. them— perivascular lympliatic sheaths (Fig. 121). Lymph channels are said to be present also in the tunica media. Nervi Vasorum. — Most of the arteries are surrounded by a plexus of sympathetic nerves^ which twine around the vessel very much like ivy round a tree: and ganglia are found at frequent intervals. The smallest Fig. 113.— Blood-vessels from mesocolon of rabbit, a. Artery, with two branches, showing tr. n. nuclei of transverse muscular fibres; I. n. nuclei of endothelial lining; t. a. tunica adventitia. v. Vein. Here the transverse nuclei are more oval than those of the artery. The vein receives a small branch at the lower end of the drawing; it is distinguished from the artery among other things by its straighter course and larger calibre, c. Capillary, showing nuclei of endotheUal cells. X 300. (Schofleld.) arteries and capillaries are also surrounded by a very delicate network of similar nerve-fibres, many of which appear to end in the nuclei of the transverse muscular fibres (Fig. 122). It is through these plexuses that the calibre of the vessels is regulated by the nervous system (p. 152). The Capillaries. Distribution. — In all vascular textures, except some parts of the corpora cavernosa of the penis, and of the uterine placenta, and of the spleen, the transmission of the blood from the minute branches of the arteries to the minute veins is effected through a network of ')nicroscoj){e vessels, called capillaries. These may be seen in nil minutely injected preparations; and during life, in any trans})arent vascular parts, — such as the web of the frog's foot, the tail or external bruiichiiv of the tadpole, or the wing of the bat. Tlic branches of the minute arteries form repeated anastomoses witli CIKC UL ATION OF THE HLOOD. 133 each other, and give off tlie capillaries which, hy their anastomoses, com- pose a continuous and uniform network, from which the venous radicles take their rise (Fig. 114). The i)oint at wliich the arteries terminate and the minute veins commence, cannot be exactly defined, for the transition is gradual; but the capillary network has, neverthe- less, this peculiarity, tliat the small vessels wliich compose it maintain the same diameter throughout: they do not diminish in diameter in one direction, like arteries and veins; and the meshes of the net- work that they compose are more uniform in shape and size than those formed by the anastomoses of the minute arteries and veins. Structure. — This is much more simple than that of the arteries or veins. Their walls are com- posed of a single layer of elongated or radiate, flat- tened and nucleated cells, so joined and dovetailed together as to form a continuous transparent mem- brane (Fig. 115). Outside these cells, in the larger capillaries, there is a structureless, or very finely fibrillated membrane, on the inner surface of which they are laid down. In some cases this external membrane is nu- cleated, and may then be regarded as a miniature representative of the tunica adventitia of arteries. Here and there, at the junction of two or more of the delicate endo- thelial cells which compose the capillary wall, ^Jseudo-sto^naf a may be seen Fig. 114.— Blood-vessels of an intestinal villus, repre- senting the arrangement of capillaries between the ulti- mate venous and arterial branches; a, a, the arteries; b, the vein. Fig. 115.— Capillary blood-vessels from the omentum of rabbit, showing the nucleated endothe- lial membrane of which they are composed. (Klein and Noble Smith.) resembling those in serous membranes (p. 296). The endothelial cells are often continuous at various points with processes of adjacent connective- tissue corpuscles. 134 HAND-BOOK OF PHYSIOLOGY. Capillaries are surrounded by a delicate nerve-plexus resembling, in miniature, that of the larger blood-vessels. The diameter of the capillary vessels varies somewhat in the different textures of the body, the most common size being about -g-o-Fo^^^ inch. Among the smallest may be mentioned those of the brain, and of the follicles of the mucous membrane of the intestines ; among the largest, those of the skin, and especially those of the medulla of bones. The size of capillaries varies necessarily in different animals in relation to the size of their blood corpuscles: thus, in the Proteus, the capillary circulation can Just be discerned with the naked eye. The form of the capillary network presents considerable variety in the different textures of the body: the varieties consisting principally of modi- fications of two chief kinds of mesh, the rounded and the elongated. That kind of which the meshes or interspaces have a roundish form is the most common, and prevails in those parts in which the capillary network is most dense, such as the lungs (Fig. IIG), most glands, and mucous mem- branes, and the cutis. The meshes of this kind of network are not quite circular but more or less angular, sometimes presenting a nearly regular quadrangular or polygonal form, but being more frequently irregular. The capillary network witli elongated meshes (Fig. 117) is observed in parts in which the vessels are arranged among bundles of fine tubes or fibres, as in muscles aiul nerves. In such parts, the meshes usually have the form of a parallelogram, the short sides of whicli may be from three to eiglit or ten times less than tlie long ones; the long sides always corre- sponding to the axis of the fibre or tube, by whicli it is ])laced. The ap- pearaii{'(^ of both Ihc rouiuh'd mul (doiigatcd iiH'sh(>s is much varied CIRCULATION OF THE BLOOD. 135 according as the vessels composing them have a straight or tortuous form. Sometimes the capillaries have a looped arrangement, a single capillary projecting from the common network into some prominent organ, and returning after forming one or more loops, as in the papillae of the tongue and skin. The number of the capillaries and the size of the meshes in different parts determine in general the degree of vascularity of those parts. The parts in which the network of capillaries is closest, that is, in which the meshes or interspaces are the smallest, are the lungs and the choroid membrane of the eye. In the iris and ciliary body, the interspaces are somewhat wider, yet very small. In the human liver the interspaces are of the same size or even smaller than the capillary vessels themselves. In the human lung tliey are smaller than the vessels; in the human kidney, and in the kidney of the dog, the diameter of the injected capil- laries, compared with that of the interspaces, is in the proportion of one to four, or of one to three. The brain receives a very large quantity of blood; but the capillaries in which the blood is distributed through its substance are very minute, and less numerous than in some other parts. Their diameter, according to E. H. Weber, compared with the long diam- eter of the meshes, being in the proportion of one to eight or ten ; com- pared with the transverse diameter, in the proportion of one to four or six. In the mucous membranes — for example in the conjunctiva and in the cutis vera, the capillary vessels are much larger than in the brain, and the interspaces narrower, — namely, not more than three or four times wider than the vessels. In the periosteum the meshes are much larger. In the external coat of arteries, the width of the meshes is ten times that of the vessels (Henle). It may be held as a general rule, that the more active the functions of an organ are, the more vascular it is. Hence the narrowness of the inter- spaces in all glandular organs, in mucous membranes, and in growing parts; their much greater width in bones, ligaments, and other very tough and comparatively inactive tissues; and the usually complete absence of vessels in cartilage, and such parts as those in which, prob- ably, very little vital change occurs after they are once formed. The Veiks. Distribution. — The venous system begins in small vessels which are slightly larger than the capillaries from which they spring. These vessels are gathered up into larger and larger trunks until they terminate (as regards the systemic circulation) in the two venae cavae and the coronary veins, which enter the right auricle, and (as regards the pulmonary circu- lation) in four pulmonary veins, which enter the left auricle. The capac- ity of the veins diminishes as they approach the heart; but, as a rule. 136 HAND-BOOK OF PHYSIOLOGY. the capacity of the veins exceeds by several times (twice or three times) that of their corresponding arteries. The pulmonary veins, however, are an exception to this rule, as they do not exceed in capacity the pulmonary arteries. The veins are found after death as a rule to be more or less collapsed, and often to contain blood. The veins are usually dis- tributed in a superficial and a deep set which communicate frequently in their course. Structure. — In structure the coats of veins bear a general resem- blance to those of arteries (Fig. 118). Thus, they possess an outer, Fig. 118.— Transverse section through a small artery and vein of the mucous membrane of a child's epiglottis: the contrast between the thick--s\ alled artery and the thin-walled vein is well shown. A. Artery, the letter is placed in the lumen of the vessel, e. Endothelial cells with nuclei clearly vis- ible: these cells appear xevy thick from the contracted state of the vessel. Outside it a double wavy hne marks the elastic tunica intima. )n. Tunica media forming the chief part of arterial wall and consisting of unstriped muscular fibres circularly arranged: their nuclei are well seen, a. Part of the tunica adventitia showing bundles of connective-tissue fibres in section, with the circidar nuclei of the connective-tissue corpuscles. This ccxit gi-adually merges into the surrounding connective- tissue, v. In the lumen of the vein. The other letters indicate the same as in the artery. The mus- cular coat of the vein {ta) is seen to be much thimier than that of the artery, x 350. (^Kleiii and Noble Smith.) middle, and infernal coat. The outer coat is constructed of areolar tissue like that of the arteries, but is thicker. In some veins it contains mus- cular fibre-cells, which are arranged longitudinally. The middle coat is considenibly thinner tlian that of the arteries; and, although it contains circular unstriped muscular fibres or fibre-cells, these are mingled with a larger proportion of yellow elastic and white fibrous tissue. In the large veins, near the heart, namely the ronv cava' and ptilmonary veins, tlie middle coat is replaced, for some distance from the lu^art, by circularly arranged stri])ed muscular libres, continuous with those of the aui'icles. CIRCULATION OF THE BLOOD. The internal coat of veins is less brittle than the corresponding coat of an artery, but in other respects resembles it closely. Valves. — The chief influence which the veins have in the circulation, is effected with the help of the valves^ which are placed in all veins sub- ject to local pressure from the muscles between or near which they run. The general construction of these valves is similar to that of the semi- lunar valves of the aorta and pulmonary artery, already described;- but their free margins are turned in the opposite direction, i.e., toward the heart, so as to stop any movement of blood backward in the veins. They are commonly placed in pairs, at various distances in different veins, but almost uniformly in each (Fig. 119). In the smaller veins, single valves are often met with; and three or four are sometimes placed together, or near one another, in the largest veins, such as the subclavian, and at their junction with the jugular veins. The valves are semilunar; the A B C Fig. 119.— Diagram showing valves of veins, a, part of a vein laid open and spread out. with two pairs of valves, b. Longitudinal section of a vein, showing the apposition of the edges of the valves in their closed state, c, portion of a distended vein, exhibiting a sweUing in the situation of a pair of valves. unattached edge being in some examples concave, in others straight. They are composed of inextensile fibrous tissue, and are covered with endothelium like that lining the veins. During the period of their in- action, when the venous blood is flowing in its proper direction, they lie by the sides of the veins; but when in action, they close together like the valves of the arteries, and offer a complete barrier to any backward movement of the blood (Figs. 119 and 120). Their situation in the superficial veins of the forearm is readily discovered by pressing along its surface, in a direction opposite to the venous current, i.e., from the elbow toward the wrist; when little swellings (Fig. 119, c) appear in the position of each pair of valves. These swellings at once disappear when the pressure is relaxed. Valves are not equally numerous in all veins, and in many they are absent altogether. They are most numerous in the veins of the extremi- ties, and more so in those of the leg than the arm. They are commonly absent in veins of less than a line in diameter, and, as a general rule. 138 HAND-BOOK OF PHYSIOLOGY. there are few or none in those which are not subject to muscular pressure. Among those veins which have no valves may be mentioned the superior and inferior vena cava, the trunk and branches of the portal vein, the A B Fig. 120. — A, vein with valves open, b, vein with valves closed: stream of blood passing off by lateral channel. (Dalton.) hepatic and renal veins, and the pulmonary veins; those in the interior of the cranium and vertebral column, those of the bones, and the trunk and branches of the umbilical vein are also destitute of valves. Circulation- iiq- the Arteries. Functions of the External Coat of Arteries. — The external coat forms a strong and tough investment, which, though capiible* of exten- sion, appears principally designed to strengthen the arteries and to guard against their excessive distension by the force of the heart's action. It is this coat which alone prevents the complete severance of an artery Avlien a ligature is tightly applied; the internal and middle coats being divided. In it, too, the little vasa vasorum (p. 131) find a suitable tissue in which to subdivide for tlie supply of tlie arterial coats. Functions of the Elastic Tissue in Arteries. — Tlic purpose of the elastic tissue, which enters so largely into tlie formation of all the coats of tlie arteries, is, (a) to guard the arteries from the suddenly exerted pressure to which they are subjected at each contraction of the ventricles. In every such contraction, the contents of the ventricles are for(;ed into the arteries more quickly than they can bo discharged into and through the capillaries. The blood therefore, being, for an instant, resisted in its onward (bourse, a part of the force with which it was im- CIRCULATION OF THE JiLOOD. 139 pelled is directed against the sides of the arteries; under tliis force their elastic walls dilate, stretching enough to receive the blood, and as they stretch, becoming more tense and more resisting. Thus, by yielding, they break the shock of the force impelling the blood. On the subsidence of the pressure, when the ventricles cease contracting, the arte- ries are able, by the same elasticity, to resume their former calibre; (b) It equalizes the cur- rent of the blood by maintaining pressure on it in the arteries during the periods at which the ventricles are at rest or dilating. If the arteries had been rigid tubes, the blood, in- stead of flowing, as it does, in a constant stream, would have been propelled through the arterial system in a series of jerks corre- sponding to the ventricular contractions, with intervals of almost complete rest during the inaction of the ventricles. -But in the actual condition of the arteries, the force of the suc- cessive contractions of the ventricles is ex- pended partly in the direct propulsion of the blood, and partly in the dilatation of the elastic arteries; and in the intervals between the con- tractions of the ventricles, the force of the re- coil is employed in continuing the same direct propulsion. Of course, the pressure they ex- ercise is equally diffused in every direction, and the blood tends to move backward as well as onward, but all movement backward is pre- vented by^ the closure of the semilunar arterial valves (p. 114), Avhich takes place at the very commencement of the recoil of the arterial walls. By this exercise of the elasticity of the arteries, all the force of the ventricles is made advantageous to the circulation; for that part of their force which is expended in dilating the arteries, is restored in full when they recoil. There is thus no loss of force; but neither is there any gain, for the elastic walls of the artery cannot originate any force for the ^^ropul- sion of the blood — they only restore that which they received from the ventricles. The force v/ith which the arteries are dilated every time the ventricles contract, might be said to be received by them in store, to be all given out again in the next succeeding period of dilatation of the ventricles. It is by this equalizing influence of the successive branches of every artery that, at length, the intermittent accelerations produced in the arterial current by the action of the heart, cease to be observable, and the jetting stream is converted into the continuous and equable movement of the Fig. 121. — Surface view of an artery from the mesentery of a f rog,'ensheathed in a perivascular lyrnphatic vessel, a. The artery, with its circular muscular coat (media) indicated by broad trans- verse markings, with an indication of the adventitia outside. I. Lym- phatic vessel ; its wall is a simple endothelial membrane. (Klein and Noble Smith.) 140 HAND-BOOK OF PHYSIOLOGY. blood which we see in the capillaries and veins. In the production of a continuous stream of blood in the smaller arteries and capillaries, the resistance which is offered to the blood-stream in these vessels (p. 158), is a necessary agent. Were there no greater obstacle to the escape of blood from the larger arteries than exists to its entrance into them from the heart, the stream would be intermittent, notwithstanding the elas- ticity of the walls of the arteries. (c.) By means of the elastic tissue in their walls (and of the muscular tissue also), the arteries are enabled to dilate and contract readily in cor- respondence with any temporary increase or diminution of the total quantity of blood in the body; and within a certain range of diminution Fig. 122. Fig. 123. Fig. 122.— Ramification of nerves and termination in the muscvilar coat of a small arterj- of the frog. (Arnold.) Fig. 123.— Transverse section through a large branch of the inferior mesenteric aj'teiy of a pig. e, endothelial membrane; i, tunica elastica interna, no subendothehal layer is seen; >», muscular tu- nica media, containing only a few wavy elastic fibres; ee. tunica elastica externa, dividing the media from the connective tissue adventitia, a. (Klein and Noble Smith.) x 350. of the quantity, still to exercise due pressure on their contents; {d.) The elastic tissue assists in restoring the normal state after dimimition of its calibre, whether this has been caused by a contraction of the muscular coat, or the temporary application of a compressing force from without. This action is well shown in arteries which, having contracted by means of their muscular clement, after death, regain their average patency on the cessation of post-mortem rigidity (p. 14:2). (e.) By means of their elastic coat the arteries are enabled to adapt themselves to the different movemcTits of the several ])arts of the body. Tc)is{())i. of Arteries. — Tlio natural state of all arteries, in regard at least to theii- l(»ngth, is onc^ of tension — they are always more or less stretched, and (>ver ready to recoil by virtue of tluMr (>last icily, whenever the opjios- CIRCULATION OF THE liLOOD. 141 ing force is removed. The extent to which the divided extremities of arteries retract is a measure of this tension, not of their elasticity. (Savory. ) Functions of the Muscular Coat. — The most important office of the muscular coat is, (1) that of regulating the quantity of blood to be received by each part or organ, and of adjusting it to the requirements of each, according to various circumstances, but, chiefly, according to the activity with which the functions of each are at dilferent times performed. The amount of work done by each organ of the body varies at different times, and the variations often quickly succeed each other, so that, as in the brain, for example, during sleep and waking, within the same hour a part may be now very active and then inactive. In all its active exer- cise of function, such a part requires a larger supply of blood than is suffi- cient for it during the times when it is comparatively inactive. It is evi dent that the heart cannot regulate the supply to each part at different periods; neither could this be regulated by any general and uniform con- traction of the arteries; but it may be regulated by the power which the arteries of each part have, in their muscular tissue, of contracting so as to diminish, and of passively dilating or yielding so as to permit an in- crease of the supply of blood, according to the requirements of the part to which they are distributed. And thus, while the ventricles of the heart determine the total quantity of blood, to be sent onward at each contrac- tion, and the force of its propulsion, and while the large and merely elastic arteries distribute it and equalize its stream, the smaller arteries, in addi- tion, regulate and determine, by means of their muscular tissue, the propor- tion of the whole quantity of blood which shall be distributed to each part. It must be remembered, however, that this regulating function of the arteries is itself governed and directed by the nervous system (vaso-motor centres and fibres). Another function of the muscular element of the middle coat of arteries is (2), to co-operate with the elastic in adapting the calibre of the ves- sels to the quantity of blood which they contain. For the amount of fluid in the blood-vessels varies very considerably even from hour to hour, and can never be quite constant; and were the elastic tissue only present, the pressure exercised by the walls of the containing vessels on the con- tained blood would be sometimes very small, and sometimes inordinately great. The presence of a muscular element, however, provides for a certain uniformity in the amount of pressure exercised; and it is by this adaptive, uniform, gentle, muscular contraction, that the normal tone of the blood-vessels is maintained. Deficiency of this tone is the cause of the soft and yielding pulse, and its unnatural excess, of the hard and tense one. The elastic and muscular contraction of an artery may also be regarded as fulfilling a natural purpose when (3), the artery being cut, it first limits and then, in conjunction with the coagulated fibrin, arrests the escape of blood. It is only in consequence of such contraction and coagulation that 142 HAXD-BOOK OF PHYSIOLOGY. we are free from danger througli even very slight wounds; for it is only when the artery is closed that the processes for the more permanent and secure j)reYention of bleeding are established. (4) There appears no reason for supposing that the muscular coat assists, to more than a very small degi'ee, in ^Dropelling the onward current of blood. (1.) When a small artery in the living subject is exposed to the air or cold, it gradually but manifestly contracts. Hunter observed that the posterior tibial artery of a dog when laid bare, became in a short time so much contracted as almost to jorevent the transmission of blood; and the observation has been often and variously confirmed. Simple elasticity could not effect this. (2.) When an artery is cut across, its divided ends contract, and the orifices may be com^Dletely closed. The rapidity and completeness of this contraction vary in different animals; they are generally greater in young than in old animals; and less, a^D^^arently, in man than in the lower ani- mals. This contraction is due in part to elasticity, but in part, also, to muscular action; for it is generally increased by the application of cold, or of any simple stimulating substances, or by mechanically irritating the cut ends of the artery, as by pickinsj or twisting them. (3.) The contractile property of arteries continues many hours after death, and thus affords an opportunity of distinguishing it from their elasticity. When a portion of an artery of a recently killed animal is ex- posed, it gradually contracts, and its canal may be thus completely closed: in this contracted state it remains for a time, varying from a few hours to two days: then it dilates again, and i^ermanently retains the same size. This persistence of the contractile j^roperty after death was well shown in an observation of Hunter, which may be mentioned as proving, also, the greater degree of contractility possessed by the smaller than by the larger arteries. Having injected the uterus of a cow, which had been removed from the animal upward of twenty-four hours, he found, aftei the lapse of another day, that the larger, vessels had become much more turgid than when he injected them, and that the smaller arteries had contracted so as to force the injection back into the larger ones. The Pulse. If one extremity of an elastic tube be fastened to a syringe, and the other be so constricted as to present an obstacle to the escape of fluid, we shall have a rough model of what is present in the living body: — The syringe representing the heart, the elastic tube the arteries, and the con- tracted oritice the arterioles (smallest arteries) and capillaries. If the apparatus be filled with water, aVid if a fiugcr-lip be placed on any part of the elastic tube, there will be felt with every action of the syringe, an impulse or beat, which corresponds exactly with what we feel in the arteries of the living body witii every contraction of the heart, and call the pulse. Thv. ])ulse is essentially caused by an expansion ware, wliich is due to the injection of blood into an already full aorta: which blood CIRCULATION OF THE BLOOD. 143 expanding the vessel produces the pulse in it, almost coincidently with the systole of the left ventricle. As the force of the left ventricle, however, is not expended in dilating the aorta only, the wave of blood jjasses on, expanding the arteries as it goes, running as it were on the surface of the more slowly traveling blood already contained in them, and producing the pulse as it proceeds. The distension of each artery increases both its length and its diameter. In their elongation, the arteries change their form, the straight ones be- coming slightly curved, and those already curved becoming more so, but they recover their previous form as well as their diameter when the ven- tricular contraction ceases, and their elastic walls recoil. The increase of their curves which accompanies the distension of arteries, and the succeed- ing recoil, may be well seen in the prominent temporal artery of an old person. In feeling the pulse, the finger cannot distinguish the sensation produced by the dilatation from that produced by the elongation and BUTTON Fig. 124.— Diagram of the mode of action of the Sphygmograph, curving; that which it perceives most plainly, however, is the dilatation, or return, more or less, to the cylindrical form, of the artery which has been partially flattened by the finger. The pulse — due to any given beat of the heart — is not perceptible at the same moment in all the arteries of the body. Thus, — it can be felt in the carotid a very short time before it is perceptible in the radial artery, and in this vessel again before the dorsal artery of the foot. The delay in the beat is in proportion to the distance of the artery from the heart, but the difference in time between the beat of any two arteries never exceeds probably ^ to -J^ of a second. A distinction must be carefully made between the passage of the loave along the arteries and the velocity of the stream (p. 165) of blood. Both wave and current are present; but the rates at which they trawl are very different; that of the wave 16*5 to 33 feet per second (5 to 10 metres) being twenty or thirty times as great as that of the current. The Sphygmograph. — A great deal of light has been thrown on what may be called the form of the pulse by the sphygmograph (Figs. 124 and 125). The principle oi] which the sphygmograph acts is very lU HAND-BOOK OF PHYSIOLOGY. simple (see Fig. 124). The small button replaces the finger in the act of taking the pulse^, and is made to rest lightly on the artery, the pulsations of which it is desired to investigate. The up-and-down movement of the button is communicated to the lever, to the hinder end of which is at- tached a slight spring, which allows the lever to move up, at the same time that it is just strong enough to resist its making any sudden jerk. Fig. 125.— The Sphygmograph applied to the arm. and in the interval of the beats also to assist in bringing it back to its original position. For ordinary purposes the instrument is bound on the wrist (Fig. 125). It is evident that the beating of the pulse with the reaction of the spring will cause an up-and-down movement of the lever, the pen of which will write the effect on a smoked card, which is made to move by clock- work in the direction of the arrow. Thus a tracing of the pulse is ob- tained, and in this way much more delicate effects can be seen, than can be felt on the application of the finger. The pulse-tracing differs somewhat according to the artery upon which the sphygmograph is applied, but its general characters are much the same in all cases. It consists of: — A sudden upstroke (Fig. 126, a), which is somewhat higher and more abrupt in the pulse of the carotid and of other arteries near the heart than in the radial and other arteries more remote; and a gradual decline (b), less abrupt, and therefore taking a longer time than (a). It is seldom, however, that the decline is an uninterrupted fall: it is usually marked about half-way by a distinct notch (c), Fig. VSk- Diagram of pulse tracing. called the dlCrottC notch, whicll is caUSCd A. upstroke; b, down-stroke; c, predi- ^ i l erotic wave; D, dicrotic; E, post dicrotic by a sccoud morc or loss markoa ascent ^*^^* of the lever at that point by a second wave called the dicrotic wave (d); not unfrequently (in which case the tracing is said to liave a double apex) there is also soon after the commencement of the descent a sliglit ascent previous to the dicrotic notcli, this is called the predicrotic wave (c), and in addition there may be one or more slight ascents after the dicrotic, coModpost dicrotic (e). CIllCULATION OF THE liJ.OOD. 145 TlLe explanation of these tracings presents some difficulties, not, how- ever, as regards tlie two primary factors, viz., the upstroke and down- stroke, because they are universally taken to mean the sudden injection of blood into the already full arteries, and that this passes through the artery as a wave and expands them, the gradual fall of the lever signify- ing the recovery of the arteries by their recoil. It may be demonstrated on a system of elastic tubes, such as was described above, where a syringe pumps in water at regular intervals, just as well as on the radial artery, or on a more complicated system of tubes in which the heart, the arteries, the capillaries and veins are represented, which is known as an arterial schema. If we place two or more sphygmographs upon such a system of tubes at increasing distances from the pump, we may demonstrate Fig. 127.— Diagram of the formation of the pulse-tracing. A, percussion wave; B, tidal ware; C, dicrotic wave. (Mahomed.) that the rise of the lever commences first in that nearest the pump, and is higher and more sudden, while at a longer distance from the pump the wave is less marked, and a little later. So in the arteries of the body the wave of blood gradually gets less and less as we approach the periphery of the arterial system, and is lost in the capillaries. By the sudden in- jection of blood two distinct waves are produced, which are called the tidal and percussion waves. The tidal wave occurs whenever fluid is injected into an elastic tube (Fig. 127, b), and is due to the expansion of the tube and its more gradual collapse. The percussion wave occurs (Fig. 127, a) when the impulse imparted to the fluid is more sudden; this causes an abrupt upstroke of the lever, which then falls until it is again caught up perhaps by the tidal wave which begins at the same time but is not so quick. Vol. I.— 10. 146 HAND-BOOK OF PHYSIOLOGY. In this way, generally speaking, the apex of the upstroke is double, the second upstroke, the so-called predicrotic elevation of the lever, representing the tidal wave. The double apex is most marked in tracings from large arteries, especially when their tone is deficient. In tracings. Fig. 128,— Pulse-tracing of radial artery, somewhat deficient in tone. (Sanderson.) on the other hand, from arteries of medium size, e.g., the radial, the upstroke is usually single. In this case the percussion-impulse is not sufficiently strong to jerk up the lever and produce an effect distinct from that of the systolic wave which immediately follows it, and which Fig. 129.— Pulse-tracing of radial artery, with double apex. (Sanderson.) continues and completes the distension. In cases of feeble arterial ten- sion, however, the percussion-impulse may be traced by the spliygmo- graph, not only in the carotid pulse, but to a less extent in the radial also (Fig. 129). The interruptions in the downstroke are called the hatacrotic waves, to distinguish them from an interruption in the upstroke, called the an- acrotic wave, which is occasionally met with in cases in which the predi- crotic or tidal wave is higher than the percussion wave. K A -A ■\ ! Fig. 130.— Anacrotic pulse from a case of aortic aneurism. A, anacrotic wave (or percussion wave). B, tidal or predicrotic wave, continued rise in tension (or higher tidal wave). There is considerable difference of opinion as to Avhether the dicrotic wave is present in health generally, and also as to its cause. The balance of opinion ap[)ears to be in favor of the belief of its presence in health, although it may be very faint; while, at any rate, in certain conditions not necessarily diseased, it becomes so marked as to be quite plain to the unaided finger. Such a ])ulse is called dirrotic. Sometimes the dicrotic rise exceeds the iiiitiid u])stn)la', and the ])ulso is ihen called hypordicroiic. As to the cause of d icroi isui, o\w oj-inion is lliat it, is duo to a nn'overv CIRCULATION OF THE BLOOD. 147 of pressure during the elastic recoil, in consequence of a rebound from the periphery, and it may indeed be produced on a schema by obstructing the tube at a little distance beyond the spot where the sphygmograph is placed. Against this view, however, is the fact that the notch appears at about the same point in the downstroke in tracings from the carotid and from the radial, and not first in the radial tracing, as it should do, since that artery is nearer the periphery than the carotid, and as it does in the cor- responding experiment with the arterial schema when the tube is obstructed. The generally accepted notion among clinical observers, is that the dicrotic wave is due to the rebound from the aortic valves causing a second wave; but the question cannot be considered set- tled, and the presence of marked dicro- tism in cases of haemorrhage, of anaemia, and of other weakening conditions, as well as its presence in cases of dimin- ished pressure within the arteries, woiild imply that it might, at any rate some- times, be due to the altered specific gravity of the blood within the vessels, either directly or through the indirect effect of these conditions on the tone of the arterial walls. Waves may be produced in any elastic tube when a fluid is being driven through it with an intermittent force, such waves being called luaves of oscillation (M. Eoster). They have received various explana- tions. In an arterial schema they vary Tvith the specific gravity of the fluid used, and with the kind of tubing, and may be therefore supposed to vary in the body with the condition of the blood and of the arteries. Some consider the secondary waves in the downstroke of a normal wave to be due to oscillation; but, as just mentioned, even if this be the case, as is most likely, with post-dicrotic waves, the dicrotic wave itself is almost certainly due to the rebound from the aortic valves. The anacrotic notch is usually associated with disease of the arteries, e.g., in atheroma and aneurism. The dicrotic notch is called diastolic or aortic, and indicates closure of the aortic valves. Fig. 131. — Diagrams of pulse curves with exaggeration of one or other of the three waves. A. percussion; B, tidal: C. dicrotic. 1, percussion wave very marked; 2, tidal wave sudden; 3. dicrotic pulse curve; 4 and 5, the tidal wave very exaggerated, from high tension. (Mahomed.) 148 HAND-BOOK OF PHYSIOLOGY. Of the three main parts then of a pulse-tracing, viz., the percussion wave, the tidal, and the dicrotic, the percussion wave is produced by sudden and forcible contraction of the heart, perhaps exaggerated by an excited action, and may be transmitted much more rapidly than the tidal wave, and so the tAvo may be distinct; frequently, however, they are in- separable. The dicrotic wave may be as great or greater than the other two. According to Mahomed, the distinctness of the three waves depends upon the following conditions: — The 2^ercussio?i wave is increased by: — 1. Forcible contraction of the Heart; 2. Sudden contraction of the Heart; 3. Large volume of blood; 4. Fulness of vessel; and diminished by the reversed conditions. The tided wave is increased by: — 1. Slow and prolonged contraction of the Heart; 2. Large volume of blood; 3. Comparative emptiness of vessels; 4. Diminished outflow or slow capillary circu- lation; and diminished by the reversed conditions. The dicrotic wave is increased by: — 1. Sudden con- traction of the Heart; 2. Comparative emptiness of vessels; 3. Increased outflow or rapid capillary circu- lation; 4. Elasticity of the aorta; 5. Eelaxation of mus- cular coat; and diminished by the reversed conditions. One very important precaution in the use of the sphygmograph lies in the careful regulation of the pres- sure. If the pressure be too great, the characters of the pulse may be almost entirely obscured, or the artery may be entirely obstructed, and no tracing is obtained; and on the other hand, if the pressure be too slight, a very small part of the characters may be represented on the tracing. The Pkessure of the Blood within the Arteeies (PRODUCIKG arterial TEXSIOJs). It will be understood from the foregoing that the arteries in a normal condition, are continually on the of mereuSarmLSl?^- strctcli during life, and in consequence of the injection of more blood at each systole of the venti-icle into the elastic aorta, this stretched condition is exaggerated each time the ventricle empties itself. This condition of the arteries js due to the pressure of blood within them, because of the resistance presented by the smaller ar- teries and capillaries (i)eriphcral resistance) to the emptying of the arterial system in the intervals betwcMMi the contractions of tlu^ ventricle, and is called tlie condition of arlcrial tension. On the otlier hand, it must be equally clear thai, as the blood is forcibly injected into the already full CIRCULATION OF THE liLOOD. 141) arteries against their elasticity, it must be subjected to the pressure of the arterial walls, the elastic recoil sending on the blood after the imme- diate effect of the systole has passed; so that, when an artery is cut across, the blood is projected forward by this force for a considerable distance; at each ventricular systole, a jet of blood escaping, although the stream does not cease flowing during the diastole. The relations which exist between the arteries and their contained blood are obviously of the utmost importance to the carrying on of the circulation, and it therefore becomes necessary to be able to gauge the Fig. 133.— Diagram of mercurial kymograph, a, revolving cylinder, worked by a clockwork ar- rangement contained in the box (b), the speed being regulated by a fan above the box; cylinder sup- ported by an upright (6), and capable of being raised or lowered by a screw (a), by a handle attached to it; D, c, E, represent mercm-ial manometer, a somewhat different form of which is shown in next figure. alterations in blood-pressure very accurately. This may be done by means of a mercurial manometer in the following way: — The short hori- zontal limb of this (Fig. 132, 1) is connected, by means of an elastic tube and cannula, with the interior of an artery; a solution of sodium or po- tassium carbonate being previously introduced into this part of the appa- ratus to prevent coagulation of the blood. The blood-pressure is thus communicated to the upper part of the mercurial column (2); and the depth to which the latter sinks, added to the height to which it rises in the other (3), will give the height of the mercurial column which the 150 HAND-BOOK OF PHYSIOLOGY. blood-pressure balances; the weight of the soda solution being sub- tracted. For the estimation of the arterial tension at any given moment, no further apparatus than this, which is called Poiseuille^s hcBmadynamometer, is necessary; but for noting the variatioiis of pressure in the arterial sys- tem, as well as its absolute amount, the instrument is usually combined with a registering apparatus and in this form is called a kymogrcqih. The kymograph, invented by Ludwig, is composed of a hsemadynamometer, the open mercurial column of which supports a floating piston and vertical rod, with short horizontal pen (Fig. 134). The pen is adjusted in con- tact with a sheet of paper, which is caused to move at a uniform rate by clockwork; and thus the up-and-down movements of the mer- curial column, which are communicated to the rod and pen, are marked or registered on the moving paper, as in the registering apparatus of the sphygmograph, and minute variations cire graphically recorded (Fig. 135). For some purposes the spring kymograph of Fick (Fig. 136) is preferable to the mercurial kymograph. It consists of a hollow C-shaped spring, filled with fluid, the interior of wdiich is brought into connection with the interior of an artery, by means of a flexible metallic tube and cannula. In response to the pressure transmitted to its interior, the spring, c, tends to straighten itself, and the movement thus produced is communicated by means of a lever, h, to a writing-needle and registering apparatus. Fig. 134. — Diagram of mercu- rial manometer, a. Floatiag rod and pen. 6. Tube, which commu- nicates with a bottle containing an alkaline solution, c'. Elastic tube and cannula, the latter being Intended for insertion in an artery. Fig. 135.— Normal tracing of arterial pressure in the rabbit obtained with the mercurial kymo- graph. The smaller undulations correspond with the heart beats; the'lai'ger curves with the respir- atory movements. (Burdon-Sanderson.) Fig. 137 exhibits an ordinary arterial pulse-tracing, as obtained by the 8pring-kymograi)h. From obscrvnl ions Avlii(^h liavo been made by iucmiis of the mercurial manometer, it has been found that tlie })resRuro of l)h)0(l in the carotici of a rabbit is capable of supporting a column of 2 to \)l inches (50 to 90 CIRCULATION OF THE BLOOD. 151 mm.) of mercury, in the dog 4 to 7 inches (100 to 175 mm.), in the horse 5 to 8 inches (150 to 200 mm.), and in man about the same. To measure the absolute amount of this pressure in any artery, it is necessary merely to multiply the area of its transverse section by the height of the column of mercury which is already known to be supported by the blood-pressure in any part of the arterial system. The weight of a column of mercury thus found will represent the pressure of the blood. Calculated in this way, the blood-pressure in the human aorta is Fig. 136,— a form of Fick's Spring Kymograph, a, tube to be connected with artery; c, hollow spring, the movement of which moves b, the writing lever; e, screw to regulate height of 6; d, out- side protective spring; screw to fix on the upright of the support. equal to 4 lb. 4 oz. avoirdupois; that in the aorta of the horse being 11 lb. 9 oz. ; and that in the radial artery at the human wrist only 4 drs. Supposing the muscular power of the right ventricle to be only one- half that of the left, the blood-pressure in the pulmonary artery will be only 2 lb. 2 oz. avoirdupois. The amounts above stated represent the arterial tension at the time of the ventricular contraction. The blood-pressure is greatest in the left ventricle and at the begin- ning of the aorta, and decreases toward the capillaries. It is greatest in the arteries at the period of the ventricular systole, and is least in the auricles, during diastole, when the pressure there and in the great veins becomes, as we have seen, negative. The mean arterial pressure equals the average of the pressures in all the arteries. The pressure in the veins is never more than one-tenth of the pressure in the corresponding 152 HAND-BOOK OF PHYSIOLOGY. arteries and is greatest at the time of auricular systole. There is no peri- odic variation in venous pressure, as there is in the arterial, except in the great veins. Fig. 137.— Normal arterial tracing obtained vrith Fick"s kymograph in the dog. (Burdon- Sanderson.) Variations of Blood Pressure. — Many circumstances cause con- siderable variations in the amount of the blood-pressure. The following are the chief: — (1) Changes in tJie beat of the Heart; (2) Changes in the Arteries and Capillaries; (3) Changes due to Nerve Action; (4) Clianges in the Blood; (5) Respiratory Changes. 1. Changes in the Beat of the Heart. — The systole and diastole of the muscular chambers. The arterial tension increases during systole and diminishes during diastole. The greater the frequency, moreover^ of the heart's contractions, the greater is the blood- j)ressure, cceteris paribus; although this effect is not constant, as it may be compensated for by the delivery into the arteries at each beat of a comparatively small quantity of blood. The greater the quantity of blood expelled from the heart at each contraction the greater is the blood-pressure. The quantity and quality of the blood nourishing the heart's substance through the coronary arteries must exercise also a very considerable influence upon its action, and therefore upon the blood-pressure. 2. Changes in the Arteries and Capillaries. — Variations in the degree of contraction of the smaller arteries modify the blood-pressure by favor- ing or impeding the accumulation of blood in the arterial system which follows every contraction of the heart; the contraction of the arterial walls increasing the blood-pressure, while their relaxation lowers it. 3. Changes due to Nerve Action. — As with the heart, so with the blood-vessels, the action of the nervous system is very important in rela- tion to the blood-pressure; regulating, as it does, not only the force, fre- quency, and length of the heart's systole, but also the condition of the arteries, both through the central and ])eripheral vaso-motor centres. As this subject has not yet been fully considered it will be as well to treat of it liere. It is upon the muscular coat of the arteries that the nervous system exercises its infhience; tlie elastic element possessing, as must be obvious, rather physical than vital properties. Tlie muscular tissue in the walls of (lie vessels increases relatively to the other coats as the arteries grow smaller, so that in the smallest arteries it is develo])ed out of all propor- CIRCULATION OF THE P>LOOJ). tion to the other elements; in fact, in passing from cai)illary vessels, made up as we have seen of endothelial cells with a ground substance, the first change which occurs as the vessels become larger (on the side of the arteries) is the appearance of muscular fibres. Thus the nervous system is more powerful in regulating the calibre of the smaller than of the larger arteries. It has been shown that if the cervical sympathetic nerve be divided in a rabbit, the blood-vessels of the corresponding side become dilated. The effect is best seen in the ear, which if held up to the light is seen to become redder, and the arteries to become larger. The whole ear is dis- tinctly warmer than the opposite one. This effect is produced by remov- ing the arteries from the influence of the central nervous system, which Fig. 138.— Plethysmograph. By means of this apparatus, the alteration in volume of the arm. E, which is enclosed in a glass tube, a, fiUed with fluid, the opening through which it passes being firmly closed by a thick gutta percha band, f, is communicated to the lever, d, and registered by a recording apparatus. The fluid in a communicates with that in b, the upper limit of which is above that in a. The chief alterations in volume are due to alteration in the blood contained in the arm. When the volume is increased, fluid passes out of the glass cyhnder, and the lever, d, also is raised, and when a decrease takes place the fluid returns again from b to a. It will therefore be evident that the apparatus is capable of recording alterations of blood-pressure in the arm. Apparatus founded upon the same principle have been used for recording alterations in the volume of the spleen ai.d kidney. influence usually passes down the divided nerve; for if the peripheral end of the divided nerve {i.e., that farthest from the brain) be stimulated, the arteries which were before dilated return to their natural size, and the parts regain their primitive condition. And, besides this, if the stimulus which is applied be too strong or too long continued, the point of normal constriction is passed, and the vessels become much more con- tracted than normal. The natural condition, which is somewhere about midway between extreme contraction and extreme dilatation, is called the natural tone of an artery, and if this be not maintained, the vessel is said to have lost tone, or if it be exaggerated, the tone is said to be too great. The influence of the nervous system upon the vessels consists in maintain- ing a natural tone. The effects described as having been produced by section of the cervical sympathetic and by subsequent stimulation are not peculiar to that nerve, as it has been found that for every j)art of the 154 HAND-BOOK OF PHYSIOLOGY. body tliere exists a nerve the division of which produces the same effects, viz., dilatation of the arteries; such may be cited as the case with the sciatic, the splanchnic nerves, and the nerves of the brachial plexus: when divided, dilatation of the blood-vessels in the parts supplied by them taking place. It appears, therefore, that nerves exist which have a distinct control over the vascular supply of a part. These nerves are called vaso-motor; or, since they seem to run now in cerebro-spinal nerves, now in the sympathetic, we speak of those nerves as containing vaso-motor fibres, in addition to the fibres which have other functions. Vaso-motor centres. — Experiments by Ludwig and others show that the vaso-motor fibres come primarily from grey matter (vaso-motor centre) in the interior of the medulla oblongata, between the calamus scriptorius and the corpora quadrigemina. Thence the vaso-motor fibres pass down in the interior of the spinal cord, and issuing with the anterior roots of the spinal nerves, traverse the various ganglia on the pra3 -vertebral cord of the sympathetic, and, accompanied by branches from these ganglia, pass to their destination. Secondary or subordinate centres exist in the spinal cord, and local centres in various regions of the body, and through these, directly under ordinary circumstances, vaso-motor changes are also effected. The influence exerted by the chief vaso-motor centre is called into play in several ways, but chiefly by afferent ^sensory) stimuli, and it may be exerted in two ways, either to increase its usual action which main- tains a medium tone of the arteries or to diminish such action. This afferent influence upon the centre may be extremely well shown by the action of a nerve the existence of which was demonstrated by Cyon and Ludwig, and which is called the depressor, because of its characteristic influence on the blood-pressare. Depressor Xerve. — This small nerve arises, in the rabbit, from the superior laryngeal branch, or from this and the trunk of the pneumogas- tric nerve, and after communicating with filaments of the inferior cervical ganglion proceeds to the heart. If during an observation of the blood-pressure of a rabbit this nerve be divided, and the central end (i.e., that nearest the brain) be stimu- lated, a remarkable fall of blood-jn-essure ensues (Fig. 130). Tlie cause of the fall of blood-pressure is found to proceed from the dilatation of the vascular district supplied by tlie splanchnic nerves, in consequence of wliich it; holds a mucli larger quantity of blood than usual, and tliis very greatly diminislios the blood in the vessels elsewhere, and so materially affects the blood-})r(*8Sure. This effect of the depressor nerve is presumed to prove that the nerve is a means of conveying to the vaso- motor centre indications of such conditions of the heart as require a diminution of the tension in the blood-vessels; us, for exam})le, when the CIRCULATION OV THE BLOOD. 155 heart cannot, with siilliciont ease, proi)el blood into the ah'eacly too full or too tense arteries. The action of the depressor nerve illustrates the effect of afferent im- pulses in causing an inhibition of the vaso-motor centre as regards its action upon certain -arteries. There exist other nerves, however, the stimulation of the central end of which causes a Teverse action of the centre, or, in other words, increases its tonic influence, and by causing Fig. 139. — Tracing showing the effect on blood pressure of stimulating the central end of the De- pressor nerve in the rabbit. To be read from right to left. T, indicates the rate at which the re- cording-surf ace was traveling, the intervals correspond to seconds : C, the moment of entrance of current; O, moment at which it was shut off. The effect is some time in developing and lasts after the current has been taken off. The larger undulations are the respiratory nerves; the pulse oscilla- tions are very small. (M. Foster.) considerable constriction of certain arterioles, either locally or generally, increases the blood-pressure. Moreover, the effect of stimulating an afferent nerve may be to dilate or constrict the arteries either generally or in the part supplied by the afferent nerve; and it is said that stimula- tion of an afferent nerve may produce a kind of paradoxical effect, causing general vascular constriction and so general increase of blood-j)ressure but at the same time local dilatation. This must evidently have an immense influence in increasing the flow of blood through a part. Not only may the vaso-motor centre be reflexly affected, but it may also be affected by impulses proceeding to it from the cerebrum, as in 'the case of blushing from mind disturbance, or of pallor from sudden fear. It will be shown, too, in the chapter on Eespiration that the circulation of deoxygenated blood may directly stimulate the centre itself. Local Tonic Centres. — Although the tone of the arteries is influ- enced by the centres in the cerebro-spinal axis, certain experiments point out that this is not the only way in which it may be affected. Thus the dilatation which occurs after section of the cervical sympathetic in the first experiment cited above, only remains for a short time, and is soon followed — although a portion of the nerve may have been removed entirely — by the vessels regaining their ordinary calibre; and afterward 156 HAND-BOOK OF PHYSIOLOGY. local stimulation, e.g., the application of lieat or cold, will cause dilatation or constriction. From this it is probable that there exists a local mechanism distinct for each vascular area, and that the effect produced by the central nervous s}- stem acts through it much in the same way as the cardio-inhibitory centre in the medulla acts upon the heart through the ganglia contained within its muscular substance. Central impulses may inhibit or increase the action of these local centres, which may be considered to be sufficient under ordinary circum- stances to maintain the local tone of the vessels. The observations upon the functions of the vaso-motor nerves appear to divide them into four classes: (1) those on division of which dilatation occurs for some time, and which on stimulation of their peripheral end produce constriction; (2) those on division of which momentary dilatation followed by constric- tion occurs, with dilatation on stimulation; (3) those on division of which dilatation is caused, which lasts for a limited time, with constriction if stimulated at once, but dilatation if some time is allowed to elapse before the stimulation is applied; (4) a class, division of which produces no effect but which, on stimulation, cause according to their function either dilatation or constriction. A good examj^le of this fourth class is afforded by the nerves supplying the submaxillary gland, viz., the chorda tympani and the sympathetic. When either of these nerves is simply divided, no change takes place in the vessels of the gland; but on stimulating the chorda tympani the vessels dilate, and, on the other hand, when the sympathetic is stimulated the vessels contract. The nerves acting like the chorda tympani in this case are called vaso-dUators, and those like the sympathetic vaso-consfrictors. The third class, which produce at one time dilatation, at another time constriction, are believed to contain both kinds of vaso-motor nerve-fibres, or to act as dilators or contractors according to the condition of the locaL apparatus. It is probable that these nerves act bv inliibitino- or auo-mentinor the action of the local nerv- o o o ous mechanism already referred to; and as they are in connection Avith the central nervous system, it is through this arrangement that that sys- tem is capable of influencing or of maintaining the normal local tone. It may also be supposed that the local nerve-centres themselves may be directly affected by the condition of blood nourishing them. 'J'he following table may serve as a summary of the effect of the nerv- ous system upon the arteries and so upon tlie blood-pressure: — A. An increase of the blood-pressure may be produced: — (1.) \\y st iiiuilnl ion of tlie vaso-motor centre in niodnlla, either (v. DirvrHii, as by carbonated oi* deoxygenated blood. liuUrccthi, \)\ iin])ressi()ns descending from the cerebrum, rjj., in sudiliMi i):illor. licjh'A'Uj, by stimulation of sensory nerves anywhere. CIRCULATION OF THE BLOOD. 157 (2.) By stimulation of the centres in spinal cord. Possibly directly or indirectly, certainly reflexly. (3.) By stimulation of the local centres for each vascular area, by the vaso-constrictor nerves, or directly by means of altered blood. B. A decrease of the blood pressure may be produced: — (1.) By stimulation of the vaso-motor centre in medulla, either (a.) Directly, as by oxygenated or aerated blood. (fi.) Indirectly, by impressions descending from the cerebrum — e.g., in blushing. {y.) Reflexly, by stimulation of the depressor nerve, and consequent dilatation of vessels of splanchnic area, and possibly by stimulation of other sensory nerves, the sen- sory impulse being interpreted as an indication for diminished blood-pressure. (2.) By stimulation of the centres in spinal cord. Possibly directly, indirectly, or reflexly. (3.) By stimulation of local centres for each vascular area by the vaso-dilator nerve, or directly by means of altered blood. 4. Changes in the Mood. — a. As regards quantity. At first sight it would appear that one of the easiest ways to diminish the blood-pressure would be to remove blood from the vessels by bleeding; it has been found by experiment, however, that although the blood-pressure sinks whilst large abstractions of blood are taking place, as soon as the bleeding ceases it rises rapidly, and speedily becomes normal; that is to say, unless so large an amount of blood has been taken as to be positively dangerous to life, abstraction of blood has little effect upon the blood-pressure. The rapid return to the normal pressure is due not so much to the withdraival of lymph and other fluids from the body into the blood, as was formerly supposed, as to the regulation of the peripheral resistance by the vaso- motor nerves; in other words, the small arteries contract, and in so doing maintain pressure on the blood and favor its accumulation in the arterial system. This is due to th6 stimulation of the vaso-motor centre from diminution of the supply of blood, and therefore of oxygen. The failure of the blood-pressure to return to normal in the too great abstraction must be taken to indicate a condition of exhaustion of the centre, and consequently of want of regulation of the peripheral resistance. In the same way it might be thought that injection of blood into the already pretty full vessels would be at once followed by rise in the blood-pressure, and this is indeed the case up to a certain point — the pressure does rise, but there is a limit to the rise. Until the amount of blood injected equals about 2 to 3 per cent, of the body weight the pressure continues to« rise gradually; but if the amount exceed this proportion, the rise does not continue. In this case therefore, as in the opposite when blood is ab- 158 HAND-BOOK OF PHYSIOLOG^Y. stracted, the vaso- motor apparatus must counteract the great increase of pressure by dilating tlie small vessels, and so diminishing the peripheral resistance, for after each rise there is a partial fall of pressure; and after the limit is reached the whole of the injected blood displaces, as it were, an equal quantity which passes into the small veins, and remains within them. It should be remembered that the veins are capable of holding the whole of the blood of the body. The amount of blood supplied to the heart both to its substance and to its chambers, has a marked effect upon the blood-pressure. l. As regards qucdity. The quality of the blood supplied to the heart . has a distinct effect upon its contraction, as too watery or too little oxy- genated blood must interfere with its action. Thus it appears that blood containing certain substances affects the peripheral resistance by acting upon the muscular fibres of tlie arterioles themselves or upon the local centres, and so altering directly, as it were, the calibre of the vessels. 5. Resjyiratory changes affecting the blood-pressure will be considered in the next Chapter. OlRCULATIOlT m THE CAPILLARIES. mm When seen in any transparent part of a living adult animal by means of the microscope (Fig. 140) the blood flows with a constant equable mo- tion; the red blood-corpuscles moving along, mostly in single file, and bending in various ways to accommodate themselves to the tortuous course of the capillary, but instantly recovering their normal outline on reaching a wider vessel. It is in the capillaries that the chief resist- ance is offered to the progress of the blood; for in them the friction of the blood is greatly increased by the enormous multiplication of the surface with which it is brought in con- tact." At the circumference of the stream in the larger capillaries, but chiefly in the small arte- ries and veins, in contact with the walls of the vessel, and adhering to them, there is a layer of liquor sanguinis which appears to be motionless. The existence of this ,*^^/7/ Jayer, as it is termed, is inferred both from tlie general fact that such an one exists in all fine tubes traversed by fluid, and from what can be seen in watching the move- ments of tlie blood -corpuscles. The red corpuscles occupy the middle of the stroain and mow. witli comparative rapidity; the colorless lymph-cor- puscles run inucli nioi'c slowly by the walls of the vessel; while next to tho Willi tluM'c is often a transparent S2)ace in whicli the fluid appears to Fia. 140.— Capillaries (C) in the web of the frop^'s foot connecting: a small artery (A) with a small vein V (after Allen Thomson). CIRCULATION OF THE BLOOD. 159 be at rest; for if any of tne corpuscles happen to be forced within it, they move more slowly than before, rolling lazily along the side of the vessel, and often adhering to its wall. Part of this slow movement of the pale corpuscles and their occasional stoppage may be due to their having a natural tendency to adhere to the walls of the vessels. Sometimes, in- deed, when the motion of the blood is not strong, many of the white cor- puscles collect in a capillary vessel, and for a time entirely prevent the passage of the red corpuscles. Intermittent flow in the Capillaries. — When the peripheral re- sistance is greatly diminished by the dilatation of the small arteries and capillaries, so much blood passes on from the arteries into the capillaries at each stroke of the heart, that there is not sufficient remaining in the arteries to distend them, ^hus, the intermittent current of the ventric- ular systole is not converted into a continuous stream by the elasticity of the arteries before the capillaries are reached; and so inter mittency of the flow occurs in capillaries and veins and a pulse is produced. The same phenomenon may occur when the arteries become rigid from disease, and when the beat of the heart is so slow or so feeble that the blood at each cardiac systole has time to pass on to the capillaries before the next stroke occurs, the amount of blood sent at each stroke being insufficient to properly distend the elastic arteries. Diapedesis of Blood Corpuscles. — Until with- in the last few years it has been generally supposed that the occurrence of any transudation from the in- terior of the capillaries into the midst of the sur- rounding tissues was confined, in the absence of injury, strictly to the fluid part of the blood; in other words, that the corpuscles could not escape from the circulating stream, unless the wall of the containing blood-vessel were ruptured. It is true that an Eng- lish physiologist, Augustus Waller, affirmed, in 1846, that he had seen blood-corpuscles, both red and white, . Fig.141.— a i^rge cap- pass bodily through the wall of the capillary vessel mesentery eight hours T-i.T , ' 1 / ,^ r> ' T, after irritation had been m which they were contamed (thus confirming what set up, showing emigra- had been stated a short time previously by Addison); Cells in the act^of trav- and that, as no opening could be seen before their wSi^ s/some^lSead^ escape, so none could be observed afterward— so ^'^^^y-^ rapidly was the part healed, feut these observations did not attract much notice until the phenomena of escape of the blood-corpuscles from the capillaries and minute veins, apart from mechanical injury, were re- discovered by Professor Cohnheim in 1867. Cohnheim^s experiment demonstrating the passage of the corpuscles through the wall of the blood-vessel, is performed in the following man- 160 HAra-BOOK OF PHYSIOLOGY. ner. A frog is urarized, that is to say, paral3^sis is produced by inject- ing under the skin a minute quantity of tlie poison called urari; and the abdomen having been opened, a portion of small intestine is drawn out, and its transparent mesentery spread out under a microscope. After a variable time, occupied by dilatation, following contraction of the minute vessels and accompanying quickening of the blood-stream, there ensues a retardation of the current, and blood-corpuscles, both red and white, begin to make their way through the capillaries and small veins. * 'Simultaneously with the retardation of the blood-stream, the leu- cocytes, instead of loitering here and there at the edge of the axial cur- rent, begin to crowd in numbers against the vascular wall. In this way the vein becomes lined with a continuous pavement of these bodies, which remain almost motionless, notwithstanding that the axial current sweeps by them as continuously as before, though with abated velocity. Now is the moment at which the eye must be fixed on the outer contour of the vessel, from which, here and there, minute, colorless, button-shaped ele- vations spring, just as if they were produced by budding out of the wall of the vessel itself. The buds increase gradually and slowly in size, until each assumes the form of a hemispherical projection, of width correspond- ing to that of the leucocyte. Eventually the hemisphere is converted into a pear-shaped body, the small end of which is still attached to the surface of the vein, while the round part projects freely. Gradually the little mass of protoplasm removes itself further and further away, and, as it does so, begins to shoot out delicate prongs of transpai'ent protoplasm from its surface, in nowise differing in their aspect from the slender thread by which it is still moored to the vessel. Finally the thread is severed and the process is complete." (Burdon Sanderson.) The process of diapedesis of the red corpuscles, which occurs under circumstances of impeded venous circulation, and consequently in- creased blood-pressure, resembles closely the migration of the leuco- cytes, with the exception that they are squeezed through the wall of the vessel, and do not, like them, work their way through by amoeboid movement. Various explanations of these remarkable phenomena have been sug- gested. Some believe that minute openings {sfig?))af(7 or pscu do .^fo/uafa) between contiguous endothelial cells (p. 133) provide the means of escape for the blood-corpuscles. But the chief share in the process is to be found in the vital endowments witli respect to uu')bility and contraction of the parts concerned —both of the corpuscles (Bastian) and the capillary wall (Strieker). Burdon-Sanderson remarks, "the capillary is not a dead conduit, but a tube of living protoplasm. There is no difficulty in un- derstanding liow the membrane may open to allow the escape of leucocytes, and close again after they have passed out; for it is one of the most strik- ing peculiarities of contractile substance that when two parts of the same CIRCULATION OF THE BLOOD. 161 mass are separated, and again brought into contact, they melt together as if they had not been severed." Hitherto, the escape of the corpuscles from the interior of the blood- vessels into the surrounding tissues has been studied chiefly in connection with pathology. But it is impossible to say, at present, to what degree the discovery may not influence all present notions regarding the nutrition of the tissues, even in health. Vital Capillary Force. — The circulation through the capillaries must, of necessity, b ^ largely influenced by that which occurs in the vssssls on either side of them — in the arteries or the veins; their intermediate posi- tion causing them to feel at once, so to speak, any alteration in the size or rate of the arterial or venous blood-stream. Thus, the apparent con- traction of the capillaries, on the application of certain irritating sub- stances, and during fear, and their dilatation in blushing, may be referred to the action of the small arteries, rather than to that of the capillaries themselves. But largely as the capillaries are influenced by these, and by the conditions of the parts which surround and support them, their own endowments must not be disregarded. They must be looked upon, not as mere passive channels for the passage of blood, but as possessing endow- ments of their own (vital capillary force), in relation to the circulation. The capillary wall is actively living and contractile; and there is no reason to doubt that, as such, it must have an important influence in connection with the blood-current. Blood-Pressure in the Capillaries. — From observations upon the web of the frog's foot, the tongue and mesentery of the frog, the tails of newts, and small fishes (Roy and Brown), as well as upon the skin of the finger behind the nail (Kries), by careful estimation of the amount of pressure required to empty the vessels of blood under various conditions, it appears that the blood-pressure is subject to variations in the capillaries, ajjparently following the variations of that of the arteries; and that up to a certain point, as the extravascular pressure is increased, so does the pulse in the arterioles, capillaries, and venules become more and more evident. The pressure in the first case (web of the frog's foot) has been found to be equal to about 14 to 20 mm. of mercury; in other experiments to be, equal to about J- to \- of the ordinary arterial pressure. The CiRCULATioiNT ix the Veiks, The blood-current in the veins is maintained by the slight vis a tergo remaining of the contraction of the left ventricle. Very effectual assist- ance, however, to the flow of blood is afforded by the action of the muscles capable of pressing on such veins as have valves. The effect of such muscular pressure may be thus explained. When pressure is applied to any part of a vein, and the current of blood in it is Vol. I.— 11. 162 HAND-BOOK OF PHYSIOLOGY. obstructed, the portion behind the seat of pressure becomes swollen and distended as far back as to the next pair of valves. These, acting Hke the semilunar valves of the heart, and being, like them, inextensible both in themselves and at their margins of attachment, do not follow the vein in its distension, but are drawn out toward the axis of the canal. Then, if the pressure continues on the vein, the compressed blood, tending to move equall}^ in all directions, presses the valves down into contact at their free edges, and they close the vein and prevent regurgitation of the blood. Thus, whatever force is exercised by the pressure of the muscles on the veins, is distribtited partly in pressing the blood onward in the proper course of the circulation, and partly in pressing it backward and closing the valves behind (Fig. 128, A and B). The circulation might lose as much as it gains by such compression of the veins, if it were not for the numerous anastomoses by which they communicate, one with another; for through these, the closing up of the venous channel by the backward pressure is prevented from being any serious hindrance to the circulation, since the blood, of which the onward course is arrested by the closed valves, can at once pass through some anastomosing channel, and proceed on its way b}^ another vein. Thus, therefore, the effect of muscular pressure upon veins which have valves, is turned almost entirely to the advantage of the circulation; the pressure of the blood onward is all advantageous, and the pressure of the blood back- ward is prevented from being a hindrance by the closure of the valves and the anastomoses of the veins. The effects of such muscular pressure are well shown by the accelera- tion of the stream of blood when, in venesection, the muscles of the fore- arm are put in action, and by the general acceleration of the circulation during active exercise: and the numerous movements which are continu- ally taking place in the body while awake, though their single effects may be less striking, must be an important auxiliary to the venous circulation. Yet they are not essential; for the venous circulation continues unim- paired in parts at rest, in paralyzed limbs, and in parts in which the veins are not subject to any muscular pressure. Rhythmical Contraction of Veins.— In the web of the bat's wing, the veins are furnished Avitli valves, and possess the remarkable lU'operty of rhythmical contraction and dilatation, whereby the current of blood within them is distinctly accelerated. (Wharton Jones.) The contraction occurs, on an average, about ten times in a minute; the existence of valves ]ire- vcnting regurgitation, the entire effect of tlie contractions was auxiliary to the onward current of blood. Analogous iihonomena liave boon fre- quently observed in other animals. Blood-Pressure in the Veins. — 'l'li(0)loo(l-prossuro gradually falls as we proceed from tlie lieart to the arteries, from tliese to the oajiillarios, and tlu'iice along the veins to the right auricle. The blood-jiressure in CIRCULATION OF THE BLOOD. 163 the veins is nowhere very great, but is greatest in the small veins, while in the large veins toward the heart the pressure becomes negative, or, in other words, when a vein is put in connection with a mea*curial manometer the mercury will fall in the area furthest away from the vein and will rise in the area nearest the vein, having a tendency to suck in rather than to push forward. In the veins in the neck this tendency to suck in air is especially marked, and is the cause of death in some operations in that region. The amount of pressure in the brachial vein is said to support 9 mm. of mercury, whereas the pressure in the veins of the neck is about equal to a negative pressure of —3 to —8 mm. The variations of venous pressure during systole and diastole of the heart are very slight, and a distinct pulse is seldom seen in veins except under very extraordinary circumstances. The formidable obstacle to the upward current of the blood in the veins of the trunk and extremities in the erect posture supposed to be pre- sented by the gravitation of the blood, has no real existence, since the pressure exercised by the column of blood in the arteries, will be always sufficient to support a column of venous blood of the same height as itself: the two columns mutually balancing each other. Indeed, so long as both arteries and veins contain continuous columns of blood, the force of gravitation, whatever be the position of the body, can have no power to move or resist the motion of any part of the blood in any direction. The lowest blood-vessels have, of course, to bear the greatest amount of pres- sure; the pressure on each part being directly proportionate to the height of the column of blood above it: hence their liability to distension. But this pressure bears equally on both arteries and veins, and cannot either move, or resist the motion of, the fluid they contain, so long as the col- umns of fluid are of equal height in both, and continuous. Velocity of the Circulation. The velocity of the blood-current at any given point in the various divisions of the circulatory system is inversely proportional to their sectional area at that point. If the sectional area of all the branches of a vessel united were always the same as that of the vessel from which they arise, and if the aggregate sectional area of the capillary vessels were equal to that of the aorta, the mean rapidity of the blood^s motion in the capillaries would be the same as in the aorta and largest arteries; and if a similar correspondence of capacity existed in the veins and arteries, there would be an equal correspondence in the rapidity of the circulation in them. But the arterial and venous systems may be re]3- resented by two truncated cones with their apices directed toward the heart; the area of their united base (the sectional area of the capillaries) being 400 — 800 times as great as that of the truncated apex representing 164 HAND-BOOK OF PHYSIOLOGY. the aorta. Thus the velocity of blood in the capillaries is at least of that in the aorta. Velocity in the Arteries. — The velocity of the stream of blood is greater in the arteries than in any other part of the circulatory system, and in them it is greatest in the neighborhood of the heart, and during the ventricular systole; the rate of movement diminishing during the dias- tole of the ventricles, and in the parts of the arterial system most distant from the heart. Ohauveau has estimated the rapidity of the blood- stream in the carotid of the horse at over 20 inches per second during the hearths systole, and nearly 6 inches during the diastole (520 — 150 mm.). Estimation of the Velocity. — Various instruments have been devised for measuring the velocity of the blood-stream in the arteries. Ludwig's ^^Stromuhr'- (Fig. 142) consists of a U-shaped glass tube dilated at a and a', and whose extremities, h and i, are of known calibre. The bulbs can be filled by a common opening at Ic. The instrument is so contrived that at !> and i' the glass part is firmly fixed into metal cylinders, which are fixed into a circular horizontal table, c c' , capa- ble of horizontal movement on a similar table d d' about the vertical axis marked in figure by a dotted line. The opening in c c', when the instrument is in position, as in Fig., corresponds exactly with those in d d'\ but if c be turned at right angles to its present position, there is no communication between li and a, and i and but li communicates directly with i\ and if turned through two right angles c' communicates with d, and c with d' , and there is no direct connection between li and i. The experiment is performed in the following- way: — The artery to be experimented upon is divided and connected with two cannulae and tubes which fit it accurately with li and i — li the central end, and % the peripheral; the bulb a is filled with olive oil up to a point rather lower than h, and a' and the remainder of a is filled with defibri- nated blood; the tube on Tc is then carefully clamped; the tubes 6? and d' are also filled with defibrinated blood. When everything is ready, the blood is allowed to flow into a through //, and it pushes before it the oil, and that the defibrinated blood into the artery through /, and replaces it in a'\ when the blood reaches the former level of the oil in a, the disc c 6'' is turned rapidly through two right angles, and the blood flowing through d into a* again displaces the oil which is driven into a. This is repeated several times, and the duration of the experiment noted. Tlic capacity of a and a' is kiuown; the diameter of the artery is also known by its corresponding with the cannuliB of known diameter, and us the number of times a has been tilled in a given time is known, tho velocity of the current can bo calculated. Fig. 142.— Ludwig's Stromuhr. CIRCULATION OF THE BLOOD. 1G5 Chauveau's instrument (Fig. 143) consists of a thin brass tube, a, in one side of which is a small perforation closed by thin vulcanized india- rubber. Passing through the rubber is a fine lever, one end of which, slightly flattened, extends into the lumen of the tube, while the other moves over the face of a dial. The tube is inserted into the interior of Fig. 143.— Diagram of Chauveau's Instrument, a. Brass tube for introduction into the lumen ot the artery, and containing an index-needle, which passes through the elastic membrane in its side, and moves by the impulse of the blood-current, c. Graduated scale, for measuring the extent of the oscillations of the needle. an artery, and ligatures applied to fix it, so that the movement of the blood may, in flowing through the tube, be indicated by the movement of the outer extremity of the lever on the face of the dial. The HcBmatocliometer of Vierordt, and the instrument of Lortet, resemble in principle that of Chauveau. Velocity in the Capillaries. — The observations of Hales, E. H. AVeber, and Valentin agree very closely as to the rate of the blood-current in the capillaries of the frog; and the mean of their estimates gives the velocity of the systemic capillary circulation at about one inch (25 mm.) per minute. The velocity in the capillaries of warm-blooded animals is greater. In the dog ^ to jIq- inch (-5 to '75 mm.) a second. This may seem inconsistent with the facts which show that the whole circulation is accomplished in about half a minute. But the whole length of capillary vessels, through which any given portion of blood has to pass, probably does not exceed from -g^th to -^V^h of an inch ( -5 mm. ) ; and therefore the time required for each quantity of blood to traverse its own appointed portion of the general capillary system will scarcely amount to a second. Velocity in the Veins. — The velocity of the blood is greater in the veins than in the capillaries, but less than in the arteries: this fact depending upon the relative capacities of the arterial and venous systems. If an accurate estimate of the proportionate areas of arteries and the veins corresponding to them could be made, we might, from the velocity of the arterial current, calculate that of the venous. A usual estimate is, that the capacity of the veins is about twice or three times as great as that of the arteries, and that the velocity of the blood^s motion is, therefore. 166 HAND-BOOK OF PHYSIOLOGY. about twice or three times as great in the arteries as in the veins, 8 inches (about 200 mm.) a second. The rate at which the blood moves in the veins gradually increases the nearer it approaches the heart, for the sec- tional area of the venous trunks, compared with that of the branches opening into them, becomes gradually less as the trunks advance toward the heart. Velocity of the Circulation as a whole.— It would appear that a portion of blood can traverse the entire course of the circulation, in the horse, in half a minute. Of course it would require longer to traverse the vessels of the most distant part of the extremities than to go through those of the neck: but taking an average length of vessels to be traversed, and assuming, as we may, that the movement of blood in the human subject is not slower than in the horse, it may be concluded that half a minute represents the average rate. Satisfactory data for these estimates are afforded by the results of experiments to ascertain the rapidity with which poisons introduced into the blood are transmitted from one part of the vascular system to another. The time required for the passage of a solution of potassium ferrocyanide, mixed with the blood, from one jugular vein (through the right side of the heart, the pulmonary vessels, the left cavities of the heart, and the general circulation) to the jugular vein of the opposite side, varies from twenty to thirty seconds. The same substance was transmitted from the jugular vein to the great saphena in twenty seconds; from the jugular vein to the masseteric artery, in between fifteen and thirty seconds; to the facial artery, in one experiment, in between ten and fifteen seconds; in another experiment in between twenty and twenty- five seconds; in its transit from the jugular vein to the metatarsal artery, it occupied between twenty and thirty seconds, and in one instance more than forty seconds. The result was nearly the same whatever was the rate of the hearths action. In all these experiments, it is assumed that the substance injected moves with the blood, and at the same rate, and does not move from one part of the organs of circulation to another by diffusing itself through the blood or tissues more quickly tlian the blood moves. The assumption is sufficiently probable, to be considered nearly certain, that the times above mentioned, as occupied in tlie passage of the injected substances, are those in which tlie portion of blood, into whicli each was injected, was carried from one part to another of the vascular system. Another mode of estimating the general velocity of the circulating blood, is by calculating it from the quantity of blood su})posed to bo con- tained in the body, and from the quantity whicli can })ass through the heart in each of its actions. But the conclusions arrived at by this metliod are less satisfactory. For the estimates both of the total (]uantity of blood, and of the capacity of the cavities of the heart, have as yet only CIRCULATION OF THE BLOOD. 167 approximated to the truth. Still the most careful of the estimates thus made accord very nearly with those already mentioned; and it may be assumed that the blood may all pass through the heart in from twenty- five to fifty seconds. Peculiarities of the Circulation in Different Parts. — The most remarkable peculiarities attending the circulation of blood through differ- ent organs are observed in the cases of the hrain, the erectile organs, the lungs, the liver, and the kidney. 1. In the Brain. — For the due performance of its functions, the brain requires a large supply of blood. This object is effected through the number and size of its arteries, the two internal carotids, and the two vertehrals. It is further necessary that the force with which this blood is sent to the brain should be less, or at least should be subject to less vari- ation from external circumstances than it is in other parts, and so the large arteries are very tortuous and anastomose freely in the circle of Willis, which thus insures that the supply of blood to the brain is uni- form, though it may by an accident be diminished, or in some way changed, through one or more of the principal arteries. The transit of the large arteries through bone, especially, the carotid canal of the tem- poral bone, may prevent any undue distension; and uniformity of supply is further insured by the arrangement of the vessels in the pia mater, in which, previous to their distribution to the substance of the brain, the large arteries break up and divide into innumerable minute branches ending in capillaries, which, after frequent communications with one another, enter the brain, and carry into nearly every part of it uniform and equable streams of blood. The arteries are also enveloped in a special lymphatic sheath. The arrangement of the veins within the cranium is also peculiar. The large venous trunks or sinuses are formed so as to be scarcely capable of change of size; and composed, as they are, of the tough tissue of the dura mater, and, in somo instances, bounded on one side by the bony cranium, they are not compressible by any force which the fulness of the arteries might exercise through the substance of the brain; nor do they admit of distension when the flow of venous blood from the brain is obstructed. The general uniformity in the supply of blood to the brain, which is thus secured, is well adapted, not only to its functions, but also to its con- dition as a mass of nearly incompressible substance placed in a cavity with unyielding walls. These conditions of the brain and skull have appeared, indeed, to some, enough to justify the opinion that the quan- tity of blood in the brain must be at all times the same. It was found that in animals bled to death, without any aperture being made in the cranium, the brain became pale and anaemic like other parts. And in death from strangling or drowning, congestion of the cerebral vessels; while in death by prussic acid, the quantity of blood in the cavity of the 168 HAISTD-BOOK OF PHYSIOLOGY. cranium was determined by the position in which the animal was placed after death, the cerebral vessels being congested when the animal was sus- pended with its head downward, and comparatively empty when the animal was kept suspended by the ears. That, it was concluded, although the total volume of the contents of the cranium is probably nearly always the same, yet the quantity of blood in it is liable to variation, its increase or diminution being accompanied by a simultaneous diminution or in- crease in the quantity of the cerebro-spinal fluid, which, by readily admitting of being removed from one part of the brain and spinal cord to another, and of being rapidly absorbed, and as readily effused, would serve as a kind of supplemental fluid to the other contents of the cranium, to keep it uniformly filled in case of variations in their quantity (Bur- rows). And there can be no doubt that, although the arrangements of the blood-vessels, to which reference has been made, ensure to the brain an amount of blood which is tolerably uniform, yet, inasmuch as with every beat of the heart and every act of respiration, and under many other circumstances, the quantity of blood in the cavity of the cranium is constantly varying, it is plain that, were there not provision made for the possible displacement of some of the contents of the unyielding bony case in which the brain is contained, there would be often alternations of excessive pressure with insufficient supply of blood. Hence we may con- sider that the cerebro-spinal fluid in the interior of the skull not only subserves the mechanical functions of fat in other parts as 'd. paching material, but by the readiness with which it can be displaced into the spinal canal, provides the means whereby undue pressure and insufficient supply of blood are equally prevented. Chemical Composition of Cerebro-spinal Fluid. — The cerebro-spinal fluid is transparent, colorless, not viscid, with a saline taste and alkaline reaction, and is not affected by heat or acids. It contains 981-984 parts water, sodium chloride, traces of potassium chloride, of sulphates, car- l)onates, alkaline and earthy phosphates, minute traces of urea, sugar, sodium lactate, fatty matter, cholesterin, and albumen (Flint). 2. Li Erectile Structures. — The instances of greatest variation in the quantity of blood contained, at different times, in the same organs, are found in certain structures which, under ordinary circumstances, are soft and flaccid, but, at certain times, receive an unusually large quantity of blood, become distended and swollen by it, and pass into the state which has been termed erection. Such structures are the corpora cavernosa and corpus sporujiositm, of the penis in the male, and the clitoris in the female: tmd, to a less degree, the nipple of tlie mammary gland in botli sexes. The corpus cavei-nosum penis, which is the best example of an erectile stru(;tiire, has an external lil)rous nuMnbrane or sheath; and from the inner surface of the latter are ])rolonged numerous lino lamelli\3 which CIRCULATION OF THE liLOOD. 1G9 divide its cavity into small compartments looking like cells when they are inflated. Within these is situated the plexus of veins upon which the peculiar erectile property of the organ mainly depends. It consists of short veins which very closely interlace and anastomose with each other in all directions, and admit of great variation of size, collapsing in the passive state of the organ, but, for erection, capable of an amount of dila- tation which exceeds beyond comparison that of the arteries and veins which convey the blood to and from them. The strong fibrous tissue lying in the intervals of the venous plexuses, and the external fibrous membrane or sheath with which it is connected, limit the distension of the vessels, and, during the state of erection, give to the penis its con- dition of tension and firmness. The same general condition of vessels exists in the corpus spongiosum urethrae, but around the urethra the fibrous tissue is much weaker than around the body of the penis, and around the glans there is none. The venous blood is returned from the plexuses by comparatively small veins; those from the glans and the fore part of the urethra empty themselves into the dorsal veins of the penis; those from the cavernosum pass into deeper veins which issue from the corpora cavernosa at the crura penis; and those from the rest of the urethra and bulb pass more directly into the plexus of the veins about the prostate. For all these veins one condition is the same; namely, that they are liable to the pressure of muscles when they leave the penis. The muscles chiefly concerned in this action are the erector penis and acceler- ator urinae. Erection results from the distension of the venous plexuses with blood. The principal exciting cause in the erection of the penis is nervous irritation, originating in the part itself, or derived from the brain and spinal cord. The nervous influence is communicated to the j)enis by the pudic nerves, which ramify in its vascular tissue: and after their division in the horse, the penis is no longer capable of erection. This influx of the blood is the first condition necessary for erection, and through it alone much enlargement and turgescence of the penis may ensue. But the erection is probably not complete, nor maintained for any time except when, together with this influx, the muscles already mentioned contract, and by compressing the veins, stop the efflux of blood, or prevent it from being as great as the influx. It appears to be only the most perfect kind of erection that needs the help of muscles to compress the veins; and none such can materially as- sist the erection of the nippies, or that amount of turgescence, just falling short of erection, of which the spleen and many other parts are capable. For such turgescence nothing more seems necessary than a large plexiform arrangement of the veins, and such arteries as may admit, upon occasion, augmented quantities of blood. (3, 4, 5.) The circulation in the Lungs, Liver, and Kidneys will be described under those heads. 170 HAND-BOOK OF PHYSIOLOGY. Agents concerned in the circulation. — Before quitting this sub- ject it will be as well to bring together in a tabular form the various agencies concerned in maintaijiing the circulation. 1. The Systole and Diastole of the Heart, the former pumping into the aorta and so into the arterial system a certain amount of blood, and the latter to some extent sucking in the blood from the veins. 2. Tlie elastic and muscular coats of the arteries, which serve to keep up an equable and continuous stream. 3. The so-called vital cajnllary force. 4. The pressure of the muscles on veins with valves, and the slight rhythmic contraction of the veins. 5. Aspiration of the Thorax during inspiration, by means of which the blood is drawn from the large veins into the thorax (to be treated of in next Chapter). DiSCOVEEY OF THE ClECULATIO^T. Up to nearly the close of the sixteenth century it was generally be- lieved that the blood passed from one ventricle to the other through fora- mina in the "septum ventriculorum."^ These foramina are of course purely imaginary, but no one ventured to dispute their existence till Ser- vetus boldly stated that he could not succeed in finding them. He fur- ther asserted that the blood passed from the Eight to the Left side of the heart by way of the lungs, and also advanced the hypothesis that it is thus "revivified,^" remarking that the Pulmonary Artery is too large to serve merely for the nutrition of the lungs (a theory then generally accepted). Eealdus, Columbo, and Csesalpinus added several important observa- tions. The latter showed that the blood is slightly cooled by passing through the lungs, also that the veins swell up on the distal side of a liga- ture. The existence of valves in the veins had previously been discovered by Fabricius of Aquapendente, the teacher of Harvey. The honor of first demonstrating the general course of the circulation belongs by right to Harvey, who made his grand discovery about 1G18. He was the first to establish the muscular structure of the heart, which had been denied by many of his predecessors; and by careful study of its action both in the body and when excised, ascertained the order of con- traction of its cavities. He did not content himself with inferences from the anatomy of the parts, bat employed the experimental method of injection, and made an extensive and accurate series of observations on the circulation in cold-blooded animals. He forced water through the Pulmonary Artery till it trickled out through the Left Ventricle, the tip of which had been cut off. Another of his experiments was to fill the Riglit side of the heart with water, tie the Pulmonary Artery and the Ventv Cava? and then squeeze the Right ventricle: not a drop could be forced throiigli into the Left ventricle, and thus lie conclusively disproved the existence of foramina in the septum ventriculorum. "I have suffi- ciently proved," says lie, "that by the beatino; of the heart the blood passes from the veins into tlie arteries tlirougli the ventricles, and is dis- tributed over X\\i\ wliole body." "In the warmer aninuils, sucli as man, the blood pusses from the Eight CIKCULATION OF TJIE BLOOD. 171 Ventricle of the Heart through the Pulmonary Artery into the Lungs, and thence through the Pulmonary Veins into the Left Auricle, thence into the Left Ventricle." Proofs of the Circulation of the Blood. — The following are the main arguments by which Harvey established the fact of the circulation: — 1. The heart in half an hour propels more blood than the whole mass of blood in the body. 2. The great force and jetting manner with which the blood spurts from an opened artery, such as the carotid, with every beat of the heart. 3. If true, the normal course of the circulation explains why after death the arteries are commonly found empty and the veins full. 4. If the large veins near the heart were tied in a fish or snake, the heart became pale, flaccid, and bloodless; on removing the ligature, the blood again flowed into the heart. If the artery were tied, the heart be- came distended; the distension lasting until the ligature was removed. 5. The evidence to be derived from a ligature round a limb. If it be drawn very tight, no blood can enter the limb, and it becomes pale and cold. If the ligature be somewhat relaxed, blood can enter but cannot leave the limb; hence it becomes swollen and congested. If the ligature be removed, the limb soon regains its natural appearance. 6. The existence of valves in the veins which only permit the blood to flow toward the heart. 7. The general constitutional disturbance resulting from the introduc- tion of a poison at a sijagle point, e. g., snake poison. To these may now be added many further proofs which have accumu- lated since the time of Harvey, e. g. : — 8. Wounds of arteries and veins. In the former case haemorrhage may be almost stopped by pressure between the heart and the wound, in the latter by pressure beyond the seat of injury. 9. The direct observation of the passage of blood corpuscles from small arteries through capillaries into veins in all transparent vascular parts, as the mesentery, tongue or web of the frog, the tail or gills of a tadpole, etc. 10. The results of injecting certain substances into the blood. Further, it is obvious that the mere fact of the existence of a hollow muscular organ (the heart) with valves so arranged as to permit the blood to pass only in one direction, of itself suggests the course of the circula- tion. The only part of the circulation which Harvey could not follow is that through the capillaries, for the simple reason that he had no lenses sufficiently powerful to enable him to see it. Malpighi (1661) and Leeu- wenhoek (1668) demonstrated it in the tail of the tadpole and lung of the frog. CHAPTEE YI. RESPIRATION. The maintenance of animal life necessitates the continual absorption of oxygen and excretion of carbonic acid; the blood being, in all animals which possess a well developed blood-vascular system, the medium by which these gases are carried. By the blood, oxygen is absorbed from without and conveyed to all parts of the organism, and, by the blood, carbonic acid, which comes from within, is carried to those parts by which it may escape from the body. The two processes, — absorption of oxygen and excretion of carbonic acid, — are complementary, and their sum is termed the process of Respiration. In all Yertebrata, and in a large number of Invertebrata, certain parts, either lungs or gills, are specially constructed for bringing the blood into proximity with the aerating medium (atmospheric air, or water contain- ing air in solution). In some of the lower Vertebrata (frogs and other naked Amphibia) the skin is important as a respiratory organ, and is capable of supplementing, to some extent, the functions of the j!?rojt;er hreatldng apparatus; but in all the higher animals, including man, the respiratory capacity of the skin is so infinitesimal that it may be practi- cally disregarded. Essentially, a lung or gill is constructed of a fine transparent mem- brane, one surface of which is exposed to the air or water, as the case ma}^ be, while, on the other, is a network of blood-vessels, — the only separation between the blood and aerating medium being the thin wall of the blood- vessels, and the fine membrane on one side of which vessels are distributed. The difference between the simplest and the most complicated respiratory membrane is one of degree only. The various complexity of the respiratory membrane, and the kind of aerating medium, are not, however, the only conditions which cause a difference in the respiratory capacity of different animals. The number and size of the red blood-corpuscles, the mechanism of the breatliing ap- pji.ratus, the i)resence or absence of a pnlmonarii heart, physiologically distinct from the systemic, are, all of them, conditions scarcely second in importance. In the heart of man and all other l\ranim;]lia, the r////// side from which the blood is propelled into and through the lungs may be termed the RESPIRATION. 175 •'puiiryoTiary^^ heart; while the left side is " systemic in function. In many ui' the lower animals, however, no such distinction can be drawn. Thus, m Fish the heart propels the blood to the respiratory organ (gills); but tiiere is no contractile sac corresponding to tlie left side of the heart, to propel the blood directly into the systemic vessels. It may be well to state here that the lungs are only the medium for the exchange, on the part of the blood, of carbonic acid for oxygen. They are not the seat, in any special manner, of those combustion-processes of which the production of carbonic acid is the final result. These occur in all parts of the body — more in one part, less in another: chiefly in the substance of the tissues, but in part in the capillary blood-vessels contained in them. The Kespiratory Passages akd Tissues. The object of respiration is the interchange of gases in the lungs; for this purpose it is necessary that the atmospheric air shall pass into them Fig. 144. and be expelled, from them. The lungs are contained in the chest or thorax, which is a closed cavity having no communication with the out- 174 HAl^D-BOOK OF PHYSIOLOGY. side^ except by means of the respiratory passages. The air enters these passages through the nostrils or through the mouthy thence it passes through the larynx into the trachea or windpipe^ which about the middle of the chest divides into two tubes, hroncM, one to each (right and left) lung. The Larynx is the upper part of the passage whicn leaas exclusively to the lung; it is formed by the thyroid, cricoid, and arytenoid cartilages (Fig. 145), and contains the vocal cords, by the vibration of which the voice is chiefly produced. These vocal cords are ligamentous bands at- tached to certain cartilages capable of movement by muscles. By their approximation the cords can entirely close the entrance into the larynx; but under the ordinary conditions, the entrance of the larynx is formed by a more or less triangular chink between them, called the rima glot- tidis. Projecting at an acute angle between the base of the tongue and the larynx to which it is attached, is a leaf-shaped cartilage, with its larger extremity free, called the epiglottis (Fig. 145, e). The whole of the larynx is lined by mucous membrane, which, however, is extremely thin over the cords. At its lower extremity the larynx joins the trachea. ^ With the exception of the epiglottis and the so-called cornicula laryngis, the cartilages of the larynx are of the hyaline variety. Structure of Epiglottis. — The supporting cartilage is composed of yellow elastic cartilage, enclosed in a fibrous sheath (perichondrium), and covered on both sides with mucous membrane. The anterior surface, which looks toward the base of the tongue, is covered with mucous mem- brane, the basis of which is fibrous tissue, elevated toward both surfaces in the form of rudimentary papillae, and covered with several layers of squamous epithelium. In it ramify capillary blood-vessels, and in its meshes are a large number of lymphatic channels. Under the mucous membrane, in the less dense fibrous tissue of which it is composed, are a number of tubular glands. The j)osterior or laryngeal surface of the epiglottis is covered by a mucous membrane, similar in structure to that on the other surface, but that the epithelial coat is thinner, the number of strata of cells being less, and the papillae few and less distinct. The fibrous tissue which constitutes the mucous membrane is in great part of the adenoid variety, and this is here and there collected into distinct masses or follicles. The glands of the posterior surface are smaller but more numerous than those on the other surface. In many places the glands wliicli are situated nearest to the perichondrium are directly continuous through apertures in the cartilage witli those on the other side, and often the ducts of the glands from one side of the cartilage pass through and open on tlie mucous surface of the other side, l^fsfe goblets have been ' A dctaiUul uccount of the structure and function of the Larynx will be found in Chapler XVI. RESPIRATION. 175 found in the epithelium of the posterior surface of the epiglottis, and in several other situations in the laryngeal mucous membrane. The Trachea and Bronchial Tubes.— The trachea or wind-pipe extends from the cricoid cartilage, which is on a level with the fifth cervi- FiG. 145. Fig. 146. Fig. 145.— Outline showing the general form of the larynx, trachea, and bronchi, as seen from "before, the great cornu of the hyoid bone; e, epiglottis; t, superior, and inferior cornu of the thyroid cartilage; c, middle of the cricoid cartilage: ti\ the trachea, showing sixteen cartilaginous rings; ft, the right, and 5', the left bronchus. (Allen Thomson.) X Fig. 146.— Outline showing the general form of the larynx, trachea, and bronchi, as seen from be- hind. 7i, great cornu of the hyoid bone; ^, superior, and t', the inferior cornu of the thyroid cartilage; e, the epiglottis; a, points to the back of both the arytenoid cartilages which are surmounted by the cornicula ; c, the middle ridge on the back of the cricoid cartilage ; ir, the posterior membranous part of the trachea; 6, 6', right and left bronchi. (Allen Thomson.) ^. cal vertebra, to a point opposite the third dorsal vertebra, where it divides into the two bronchi, one for each lung (Fig. 146). It measures, on an average, four or four-and-a-half inches in length, and from three-quarters of an inch to an inch in diameter. 176 HAXD-BOOK OF PHYSIOLOGY. Structure. — Tlie trachea is essentiall}" a tube of fibro-elastic membrane, within the layers of which are enclosed a series of cartilaginous rings, from sixteen to twenty in number. These rings extend only around the front and sides of the trachea (about two-thirds of its circumference), and are deficient behind; the interval between their posterior extremities being bridged over by a continuation of the fibrous membrane in which they are enclosed (Fig. 145). The cartilages of the trachea and bronchial tubes are of the hyaline variety. Fig 147.— Section of trachea, a. columnar ciliated epithelium: h and c. proper structnre of the mucou.s nembrane. containing elastic fibres cut across transversely; rf. subnuicous tissue contaiTiing mucous glands, e. separated from the hyaline cartilage, r/, by a fine fibrous tissue, /; /i, external in- vestment of fine fibrous tissue. (S. K. Alcock.) Immediately within this tube, at the back, is "a layer of unstriped muscular fibres, which extends, transverseh/, l)etween the ends of the car- tilaginous rings to whicli tliey are attached, and opposite the intervals between them, also; their evident function being to diminish, when re- quired, the calibre of the trachea by approximating the ends of the car- tilages. Outside these are a few Jonfiitndinal bundles of muscular tissue which, like the preceding, are attached both to the fibrous and cartilagi- nous framework. KESri RATIO >r. 177 The mucous membrane consists of adenoid tissue, separated from the stratified columnar epithelium which lines it by a homogeneous basement membrane. This is penetrated here and there by channels which connect the adenoid tissue of the mucosa with the intercellular substance of the epithelium. The stratified columnar epithelium is formed of several layers of cells (Fig. 147), of which the most superficial layer is ciliated, and is often branched downward to join connective-tissue corpuscles; while between these branched cells are smaller elongated cells prolonged up toward the surface and down to the basement membrane. Beneath these are one or more layers of more irregularly shaped cells. In the deeper part of the mucosa are many elastic fibres between which lie con- nective-tissue corpuscles and capillary blood-vessels. Numerous mucous glands are situate on the exterior and in the substance of the fibrous framework of the trachea; their ducts perfora- ting the various structures which form the wall of the trachea, and opening through the mucous membrane into the interior. The two bronchi into which the trachea divides, of which the right is shorter, broader, and more horizontal than the left (Fig. 145), resemble the trachea exactly in structure, and in the arrangement of their carti- laginous rings. On entering the substance of the lungs, however, the rings, although they still form only larger or smaller segments of a circle, are no longer confined to the front and sides of the tubes, but are dis- tributed impartially to all parts of their circumference. The bronchi divide and subdivide, in the substance of the lungs, into a number of smaller and smaller branches, which penetrate into every part of the organ, until at length they end in the smaller subdivisions of the lungs, called lobules. All the larger branches still have walls formed of tough membrane, containing portions of cartilaginous rings, by which they are held open, and unstriped muscular fibres, as well as longitudinal bundles of elastic tissue. They are lined by mucous membrane, the surface of which, like that of the larynx and trachea, is covered with ciliated epithelium (Fig. 148). The mucous membrane is abundantly provided with mucous glands. As the bronchi become smaller and smaller, and their walls thinner, the cartilaginous rings become scarcer and more irregular, until, in th& smaller bronchial tubes, they are represented only by minute and scattered cartilaginous flakes. And when the bronchi, by successive branches, are reduced to about -^^ of an inch in diameter, they lose their cartilaginous- element altogether, and their walls are formed only of a tough fibrous, elastic membrane, with circular muscular fibres; they are still lined, how- ever, by a thin mucous membrane, with ciliated epithelium., the length of the cells bearing the cilia having become so far diminished, that the cells are now almost cubical. In the smaller bronchi the circular muscular Vol. I.— 10. 178 HAND-BOOK OF PHYSIOLOGY fibres are more abundaiit than in the trachea and larger bronchi, and form a distinct circular coat. The Lungs and Pleura. — The Lungs occupy the greater portion of the thorax. They are of a spongy elastic texture, and on section appear to the naked eye as if they were in great part solid organs, except here and there, at certain points, where branches of the bronchi or air-tubes may have been cut across, and show, on the surface of the section, their Fig. 148. — Transverse section of a bronchus, about one-fourth of an inch in diameter, e, Epithe- lium (ciliated) ; immediately beneath it is the mucous membrane or internal fibrous layer, of vari,-ing thickness; m, muscular layer; s, m, submucous tissue; /, fibrous tissue; c, cartilage enclosed within the layers of fibrous tissue; mucous gland. (F. E. Schiilze.) tubular structure. In fact, however, the lungs are hollow organs, each of which communicates by a separate orifice with a common air-tube, the trachea. The Pleura, — Each lung is enveloped by a serous membrane — the pleura, one layer of which adheres closely to the surface of the lung. 1 !!». — Transverse section of the chi>st uvfter Gray). and provides it with its smooth and sli])i)ory covering, while the other adlnu-es to tlio iimor surfjuH' of the chest-wall. The oontinnity of the two layers, which form m ciostMl sac, as in the case of oilier serous mom- brancs, will Ix^ l)t\s( inuhM-siood by n^t'criMU'c io l^jg. 14'.). The ji]>poar;ince RESriRATION. 179 of a space, however, between the pleura which covers the lung {visceral layer), and that which lines the inner surface of the chest {parietal layer), is inserted in the drawing only for the sake of distinctness. These layers are, in health, everywhere in contact, one with the other; and between them is only just so much fluid as will ensure the lungs gliding easily, in their expansion and contraction, on the inner surface of the parietal layer, which lines the chest-wall. While considering the subject of normal respiration, we may discard altogether the notion of the existence of any space or cavity between the lungs and the wall of the chest. If, however, ' an opening be made so as to permit air or fluid to enter the pleural sac, the lung, in virtue of its elasticity, recoils, and a consid- erable space is left between the lung and the chest- wall. In other words, the natural elasticity of the lungs would cause them at all times to con- tract away from the ribs, were it not that the contraction is resisted by atmospheric pressure which bears only on the inner surface of the air- tubes and air-cells. On the admission of air into the pleural sac, atmos- pheric pressure bears alike on the inner and outer surfaces of the lung, and their elastic recoil is thus no longer prevented. Structure of the Pleura and Lung. — The pulmonary pleura consists of an outer or denser layer and an inner looser tissue. The former or pleura proper consists of dense fibrous tissue with elastic fibres, covered by endothelium, the cells of which are large, flat, hyaline, and transpar- ent when the lung is expanded, but become smaller, thicker, and gran- ular when the lung collapses. In the pleura is a lymph-canalicular system; and connective tissue corpuscles are found in the fibres and tissue which forms its groundworr.. The inner, looser, or subpleural tissue contains lamellae of fibrous connective tissue and connective tissue cor- puscles between them. Numerous lymphatics are to be met with, which form a dense plexus of vessels, many of which contain valves. They are simple endothelial tubes, and take origin in the lymph-canalicular system of the pleura proper. Scattered bundles of unstriped muscular fibre occur in the pulmonary pleura. They are especially strongly developed on those parts (anterior and internal surfaces of lungs) which move most freely in respiration: their function is doubtless to aid in expiration. The structure of the parietal portion of the pleura is very similar to that of the visceral layer. Each lung is partially subdivided into separate portions called lohes; the right lung into three lobes, and the left into two. Each of these lobes, again, is composed of a large number of minute parts, called lolules. Each pulmonary lobule may be considered a lung in miniature, consist- ing, as it does, of a branch of the bronchial tube, of air-cells, blood vessels, nerves, and lymphatics, with a sparing amount of areolar tissue. On entering a lobule, the small bronchial tube, the structure of which 180 HAND-BOOK OF PHYSIOLOGY. has been just described {a, Fig. 150), divides and subdivides; its walls at the same time becoming thinner and thinner, until at length they are formed only of a thin membrane of areolar and elastic tissue, lined by a layer of squamous epithelium, not provided with cilia. At the same time, they are altered in shape; each of the minute terminal branches Fig. 150.— Ciliary epitheliiim of the human trachea, a, Layer of longitudinally arranged elastic fibres; &, basement membrane; c, deepest ceUs, circular in form; d, intermediate elongated cells; e, outermost layer of ceUs fuUy developed and bearing ciUa. x 350. (Kolliker.) widening out funnel-wise, and its walls being pouched out irregularly into small saccular dilatations, called air-cells (Fig. 151, h). Such a. funnel-shaped terminal branch of the bronchial tube, with its group of pouches or air-cells, has been called an mftindihulum (Figs. 151, 152), Fig. 151. Fig. 153. Fig, 151.— Terminal branch of a bronchial tube, with its infnndibiila and air-cells, from the mar- gin of the lung of a monkey, injected with quicksilver, a, terminal bronchial twig; b 6, infundibula and air-cells, X 10. (F. Shulze.) Vm. l.V-J.— Two small infimdibula or groups of air-cells, a o, with air-cells, b b, and the ultimate bronchial tubes, c c, with which the air-cells comnmuicate. From a ncw-boru child. (.Kiillikcr.) and tlic irregular oblcmg space in its centre, Avitli which tlio air-cells com- municate, an intercellular passage. The air-cells, or air-vesicles, may be placed singly, like recesses from tlu5 intorcelluhir i)assage, but more often they are arranged in groups or RESPIRATION. 181 even in rows, like minute sacculated tubes; so that a short series of vesicles, all communicating with one another, open by a common orifice into the tube. The vesicles are of various forms, according to the mutual pressure to which they are subject; their walls are nearly in contact, and they vary from -^j^ to of an inch in diameter. Their walls are formed of fine membrane, similar to that of the intercellular passages, and con- tinuous with it, which membrane is folded on itself so as to form a sharp- edged border at each circular orifice of communication between con- tiguous air- vesicles, or between the vesicles and the bronchial passages. Numerous fibres of elastic tissue are spread out between contiguous air- Fig. 153. — From a section of lung of a cat, stained with silver nitrate. A. D. Alveolar duct or in- tercellular passage. S. Alveolar septa. N. Alveoli or air-ceUs, lined with large flat, nucleated Cells, with some smaller polyhedral nucleated cells. Circular mviscular fibres are seen surrounding the in- terior of the alveolar duct, and at one part is seen a group of small polyhedral ceUs continued from the bronchus. (Klein and Noble Smith.) cells, and many of these are attached to the outer surface of the fine membrane of which each cell is composed, imparting to it additional Strength, and the power of recoil after distension. The cells are lined by a layer of epithelium (Fig. 153), not provided with cilia. Outside the cells, a network of pulmonary capillaries is spread out so densely (Fig. 154), that the interspaces or meshes are even narrower than the vessels, which are, on an average, inch, in diameter. . Between the atmospheric air in the cells and the blood in these vessels, nothing inter- venes but the thin walls of the cells and capillaries; and the exposure of the blood to the air is the more complete, because the folds of membrane between contiguous cells, and often the spaces between the walls of the 182 HAND-BOOK OF PHYSIOLOGY. same, contain only a single layer of capillaries, both sides of which are thus at once exposed to the air. The air-vesicles situated nearest to the- centre of the lung are smaller and their networks of capillaries are closer than those nearer to the cir- cumference. The vesicles of adjacent lobules do not communicate; and those of the same lobule or proceeding from the same intercellular passage, do so as a general rule only near angles of bifurcation; so that, when any bronchial tube is closed or obstructed, the supply of air is lost for all the cells opening into it or its branches. Blood-supply. — The lungs receive blood from two sources, {a) the pul- monary artery, (b) the bronchial arteries. The former conveys venous blood to the lungs for its arterialization, and this blood takes no share in the nutrition of the pulmonary tissues through which it passes, {h) The Fig. 154.— Capillary network of the pulmonary blood-vessels in the human lung, x 60. (Kolliker.) branches of the bronchial arteries ramify for nutrition's sake in the walls of the bronchi, of the larger pulmonary vessels, in the interlobular con- nective tissue, etc.; the blood of the bronchial vessels being returned chiefly through the bronchial and partly through the pulmonary veins. Lymphatics. — The lymphatics are arranged in three sets: — 1. Irreg- ular lacunae in the walls of the alveoli or air-cells. Tlie lymphatic vessels Avhich lead from these accompany tlic i)ulmonary vessels toward the root of the lung. 2. Irregular anastomosing s})aces in the walls of the bronchi. 3. Lymph-spaces in the })ulmonary pleura. 1'he lymphatic vessels from all these irregular sinuses pass in toward the root of the lung to reach tlio ])r()n('lnal glands. Nerves. — l^lie lunwcs of the lung are to be traced from the anterior and ])()sterior pulmonary ])lexuses, which are formed by branches of the vagus anva1i()n of the ribs is accompanied by a slight opening out of the RESPIRATION". 185 angle which the bony part forms with its cartilage (Fig. 156, A); and thus an additional means is provided for increasing the antero-posterior diameter of the chest. The muscles by which the ribs are raised, in ordinary quiet inspiration, are the external intercostals, and that portion of the internal intercostals which is situate between the costal cartilages; and these are assisted by the levatores costarum, and the serratus posticus siq^erior. The action of the levatores and the serratus is very simple. Their fibres, arising from the spine as a fixed point, pass obliquely downward and forward to the ribs, and necessarily raise the latter when they contract. The action of the intercostal muscles is not quite so simple, inasmuch as, passing merely from rib to rib, they seem at first sight to have no fixed point toward which they can pull the bones to which they are attached. A very simple apparatus will explain this apparent anomaly and make their action plain. Such an apparatus is shown in Fig. 157. A B is an upright bar, representing the spine, with which are jointed two parallel bars, 0 and D, which represent two of the ribs, and are connected in front by movable joints with another upright, representing the sternum. Fig. 157. Fig. 158. Fig. 157.— Diagram of apparatus showing the action of the external intercostal muscles. Fig. 158.— Diagram of apparatus showing the action of the internal intercostal muscles. If with such an apparatus elastic bands be connected in imitation of the intercostal muscles, it will be found that when stretched on the bars after the fashion of the external intercostal fibres (Fig. 157, C D), i.e., passing downward and forward, they raise them (Fig. 157, C D'); while on the other hand, if placed in imitation of the position of the internal intercostals (Fig. 158, E F), i.e., passing downward and backward, they depress them (Fig. 158, E' F'). The explanation of the foregoing facts is very simple. The intercostal muscles, in contracting, merely do that which all other contracting fibres 186 HAND-BOOK OF PHYSIOLOGY. do, viz., bring nearer together the points to which they are attached; and in order to do this, the external intercostals must raise the ribs, the points C and D (Fig. 15T) being nearer to each other when the parallel bars are in the position of the dotted lines. The limit of the movement in the apparatus is reached when tlie elastic band extends at right angles to the two bars which it connects — the points of attachment C' and D' being then at the smallest possible distance one from the other. The infernal intercostals (excepting those fibres which are attached to the cartilages of the ribs), have an opposite action to that of the exter- nal. In contracting they must pull down the ribs, because the points E and F (Fig. 158) can only be brought nearer one to another (Fig. 158, E' F') by such an alteration in their position. On account of the oblique position of the cartilages of the ribs with reference to the sternum, the action of the tnfer-cartilaginous fibres of the internal intercostals must, of course, on the foregoing principles, re- semble that of the external intercostals. In tranquil breathing, the expansive movements of the lower part of the chest are greater than those of the upper. In forced inspiration, on the other hand, the greatest extent of movement appears to be in the upper antero-j)osterior diameter. Muscles of Extraordinary Inspiration. — In extraordinary or forced inspiration, as in violent exercise, or in cases in Avhich there is some interference with the due entrance of air into the chest, and in which, therefore, strong efforts are necessary, other muscles than those just enumerated, are pressed into the service. It is very difficult or im- possible to separate by a hard and fast line, the so-called muscles of ordi- nary from those of extraordinary inspiration; but there is no doubt that the following are but little used as reBpiratory agents, except in cases in which unusual efforts are required — the scaleni muscles, the sternomas- toid, the serratus magnns, the pectorales, and the trapezius.. Types of Respiration. — The expansion of the chest in inspiration presents some peculiarities in different persons. In young children, it is effected chiefly by the diaphragm, which being highly arched in expiration, becomes flatter as it contracts, and, descending, presses on the abdominal viscera, and pushes forward the front walls of the abdomen. The move- ment of the abdominal walls being here more manifest than that of any other part, it is usual to call this the abdomuial type of respiration. In men, together with the descent of the diaphragm, and the pushing for- Avard of the front wall of the abdomen, the chest and the sternum are subject to a wide movement in inspiration (inferior costal t3^pe). In women, the movement appears less extensive in tlie lower, and more so in the upper, part of the chest (superior costal type). (See Figs. 159, IGO.) B. Expiration. — From tlie enlargement produced in inspiration, the chest and lungs return in ordinary tranquil expiration, by their elas- ticity; the force enii)loyed by the ins])irat()ry muscles in distending the EESPIRATION. 187 chest and overcoming the elastic resistance of the lungs and chest-walls, being returned as an expiratory effort when the muscles are relaxed. This elastic recoil of the lungs is sufficient, in ordinary quiet breathing, to expel air from the chest in the intervals of inspiration, and no muscular power is required. In all voluntary expiratory efforts, however, as in speak- ing, singing, blowing, and the like, and in many involuntary actions also, as sneezing, coughing, etc., something more than merely passive elastic power is necessary, and the proper expiratory muscles are brought into action. By far the chief of these are the abdominal muscles, which, by Fig. 159. Fig. 160, Fig. 159.— The changes of the thoracic and abdominal walls of the male during respiration. The back is supposed to be fixed, in order to throw forward the respiratory movement as much as possi- ble. The outer black continuous line in front represents the ordinary breathing movement: the ante- rior margin of it being the boundary of inspiration, the posterior margin the limit of expiration. The line is thicker over the abdomen, since the ordinary respiratory movement is chiefly abdominal ; thin over the chest, for there is less movement over that region. The dotted line indicates the movement on deep inspiration, during which the sternum advances while the abdomen recedes. Fig. 160.— The respiratory movement in the female. The lines indicate the same changes as in the last figure. The thickness of the continuous line over the sternum shows the larger extent of the ordinary breathing movement over that region in the female than in the male. (John Hutchinson.) The posterior continuous line represents in both figures the limit of forced expiration. pressing on the viscera of the abdomen, push up the floor of the chest formed by the diaphragm, and by thus making pressure on the lungs, expel air from them through the trachea and larynx. All muscles, how- ever, which depress the ribs, must act also as muscles of expiration, and therefore we must conclude that the abdominal- muscles are assisted in their action by the greater part of the ivternal intercostals, the triangu- laris sterni, the serratus posticus inferior, and qnadratus lumhorum. When by the efforts of the expiratory muscles, the chest has been squeezed to less than its average diameter, it again, on relaxation of the muscles, returns to the normal dimensions by virtue of its elasticity. The con- 188 HATO-BOOK OF PHYSIOLOGY. struction of the chest- walls, therefore, admirably adapts them for recoiling against and resisting as well undue contraction as undue dilatation. In the natural condition of the parts, the lungs can never contract to the utmost, but are always more or less ''on the stretch,^' being kept closely in contact with the inner surface of the walls of the chest by atmospheric pressure, and can contract away from these only when, by some means or other, as by making an opening into the pleural cavity, or by the effusion of fluid there, the pressure on the exterior and interior of the lungs becomes equal. Thus, under ordinary circumstances, the degree of contraction or dilatation of the lungs is dependent on that of the boundary walls of the chest, the outer surface of the one being in close contact with the inner surface of the other, and obliged to follow it in all its movements. Respiratory Rhythm. — The acts of expansion and contraction of the chest, take up, under ordinary circumstances, a nearly equal time. The act of inspiring air, however, especially in women and children, is a little shorter than that of expelling it, and there is commonly a very slight pause between the end of expiration and the beginning of the next inspiration. The respiratory rhythm may be thus expressed: — Inspiration 6 Expiration 7 or 8 A very slight pause. Respiratory Sounds. — If the ear be placed in contact with the wall of the chest, or be separated from it only by a good conductor of sound, a faint respiratory murmur is heard during inspiration. This sound varies somewhat in different parts — being loudest or coarsest in the neigh- borhood of the trachea and large bronchi (tracheal and bronchial breath- ing), and fading off into a faint sighing as the ear is placed at a distance from these (vesicular breathing). It is best heard in children, and in them a faint murmur is heard in expiration also. The cause of the vesic- ular murmur has received various explanations. Most observers hold that the sound is produced by the friction of the air against the walls of the alveoli of the lungs when they are undergoing distension (Laennec, Skoda), others that it is due to an oscillation of the current of air as it enters the alveoli (Ohauveau), whilst otliers believe that the sound is pro- duced in the glottis, but that it is modified in its passage to the pulmo- ]iary alveoli (Beau, Geo). Respiratory Movements of the Nostrils and of the Glottis. — During the actioii of tlie muscles wliicli directly dr;iw air into the chest, those wliicli <^u;M'd tlio opening through wliich it enters are not passive. In liurried ])i-cathing tlie instinctive dilatation of the nostrils is well seen, although under ordinary conditions it may not be noticeable. The o])en- ing at the upper ])art of the larynx, howev(M-, or r\)na (jJoitulix (Fig. ^5^7), RESPIRATION. 189 is dilated at each inspiration, for the more ready passage of air, and be- comes smaller at each expiration; its condition, therefore, corresponding during respiration with that of the walls of the chest. There is a further likeness between the f wo acts in that, under ordinary circumstances, the dilatation of the rima glottidis is a muscular act, and itc contraction chiefly an elastic recoil; although, under various conditions, to be here- after mentioned, there may be, in the contraction of the glottis, consider- able muscular power exercised. t-w " Terms used to express Quantity of Air breathed.— Breathing or tidal air, is the quantity of air which is habitually and almost uni- formly changed in each act of breathing. In a healthy adult man it is about 30 cubic inches. Complemental air, is the quantity over and above this which can be drawn into the lungs in the deepest inspiration; its amount is various, as will be presently shown. Reserve air. After ordinary expiration, such as that which expels the breathing or tidal air, a certain quantity of air remains in the lungs, which may be expelled by a forcible and deeper expiration. This is termed reserve air. Residual air is the quantity which still remains in the lungs after the most violent expiratory effort. Its amount depends in great measure on the absolute size of the chest, but may be estimated at about 100 cubic inches. The total quantity of air which passes into and out of the lungs of an adult, at rest, in 24 hours, is about 686,000 cubic inches. This quantity, however, is largely increased by exertion; the average amount for a hard- working laborer in the same time, being 1,568,390 cubic inches. Respiratory Capacity. — The greatest respiratory capacity of the chest is indicated by the quantity of air which a person can expel from his lungs by a forcible expiration after the deepest inspiration that he can make; it expresses the power which a person has of breathing in the emergencies of active exercise, violence, and disease. The average capacity of an adult (at 60° F. or 15-4° C.) is about 225 cubic inches. The respiratory capacity, or as Hutchinson called it, vital capacity, is usually measured by a modified gasometer {spirometer of Hutchinson), into which the experimenter breathes, — making the most prolonged ex- piration possible after the deepest possible inspiration. The quantity of air which is thus expelled from the lungs is indicated by the height to which the air chamber of the spirometer rises; and by means of a scale placed in connection with this, the number of cubic inches is read off. In healthy men, the respiratory capacity varies chiefly with the stature, weight, and age. It was found by Hutchinson, from whom most of our information on 190 HAND-BOOK OF PHYSIOLOGY. this subject is derived, that at a temperature of 60° F., 225 cubic inches is the average vital or respiratory capacity of a healthy person, five feet seven inches in height Circumstances affecting the amount of respiratory capacity. — For every inch of height above this standard the capacity is*^ increased, on an average, by eight cubic inches; and for every inch below, it is diminished by the same amount. The influence of weight on the capacity of respiration is less manifest and considerable than that of height; and it is difficult to arrive at any definite conclusions on this point, because the natural average weight of a healthy man in relation to stature has not yet been determined. As a general statement, however, it may be said that the capacity of respiration is not affected by weights under 161 pounds, or Hi stones; but that, above this point, it is diminished at the rate of one cubic inch for every additional pound up to 196 pounds, or 14 stones. By age, the capacity appears to be increased from about the fifteenth to the thirty-fifth year, at the rate of five cubic inches per year; from thirty-five to sixty-five it diminishes at the rate of about one and a half cubic inch per year; so that the capacity of respiration of a man of sixty years old would be about 30 cubic inches less than that of a man forty years old, of the same height and weight. (John Hutchinson.) Number of Respirations, and Relation to the Pulse. — The numler of respirations in a healthy adult person usually ranges from fourteen to eighteen per minute. It is greater in infancy and childhood. It varies also much according to different circumstances, such as exercise or rest, health, or disease, etc. Variations in the number of respirations correspond ordinarily with similar variations in the pulsations of the heart. In health the proportion is about 1 to 4, or 1 to 5, and when the rapidity of the hearths action is increased, that of the chest movement is commonly increased also; but not in every case in equal proportion. It happens occasionally in disease, especially of the lungs or air-passages, that the number of respiratory acts increases in quicker proportion than the beats of the pulse; and, in other affections, much more commonly, that the number of the pulses is greater in proportion tlian that of the respirations. There can be no doubt that the number of respirations of any given animal is largely affected by its size. Thus, comparing animals of the same kind, in a tiger (lying quietly) the number of respirations was 20 per minute, while in a small leopard (lying quietly) the number was 30. In a small monkey 40 per ininnte; in a large baboon, 20. Tlie rai)id, panting respiration of mice, even when quite still, is familiar, and contrasts strongly with the slow breathing of a large animal such as the elephant (eight or nine times per minute). These facts may be explained as folloAvs: — '^l^he heat-])roducing power of any given animal depends largely on its bulk, while its loss of heat depends to a great extent upon the surface area of its body. If of two animals of similar shape, one be ten times as long as the other, the area of the large animal RESPIRATION. (representing its loss of heat) is 100 times that of the small one, while its bulk (representing production of heat) is about 1000 times as great. Thus in order to balance its much greater relative loss of heat, the smaller animal must have all its vital functions, circulation, respiration, etc., carried on much more rapidly. Force of Inspiratory and Expiratory Muscles. — The force with which the inspiratory muscles are capable of acting is greatest in individ- uals of the height of from five feet seven inches to five feet eight inches^ and will elevate a column of three inches of mercury. Above this height, the force decreases as the stature increases; so that the average of men of six feet can elevate only about two and a half inches of mercury. The force manifested in the strongest expiratory acts is, on the average, one- third greater than that exercised in inspiration. But this difference is in great measure due to the power exerted by the elastic reaction of the walls of the chest; and it is also much influenced by the disproportionate strength which the expiratory muscles attain, from their being called into use for other purposes than that of simple expiration. The force of the inspiratory act is, therefore, better adapted than that of the expiratory for testing the muscular strength of the body. (Jolin Hutchinson.) The instrument used by Hutchinson to gauge the inspiratory and ex- piratory pow^r was a mercurial manometer, to which was attached a tube fitting the nostrils, and through which the inspiratory or expiratory effort was made. The following table represents the results of numerous experiments: Power of Power of Inspiratory Muscles. Expiratory Muscles. 1- 5 in Weak . . . 2*0 in. 2- 0 . . . . Ordinary . . 2-5 " 2-5 .... Strong . . .3-5 3*5 " .... Very strong . . 4*5 " 4-5 " . . . . Eemarkable , . 5-8 5'5 . . . . Very remarkable . 7-0" 6- 0 . . • . Extraordinary . 8-5 7- 0 " . . . .. Very extraordinary . lO'O The greater part of the force exerted in deep inspiration is employed in overcoming the resistance offered by the elasticity of the walls of the chest and of the lungs. The amount of this elastic resistance was estimated by observing the elevation of a column of mercury raised by the return of air forced, after death, into the lungs, in quantity equal to the known capacity of respira- tion during life; and Hutchinson calculated, according to the well-known hydrostatic law of equality of pressures (as shown in the Bramah press), that the total force to be overcome by the muscles in the act of inspiring 200 cubic inches of air is more than 450 lbs. 192 HAND-BOOK OF PHYSIOLOGY. The elastic force overcome in ordinary inspiration is, according to the same authority, equal to about 170 lbs. Douglas Powell has shown that within the limits of ordinary tranquil respiration, the elastic resilience of the walls of the chest favors inspira- tion; and that it is only in deep inspiration that the ribs and rib-cartilages offer an opposing force to their dilatation. In other words, the elastic resilience of the lungs, at the end of an act of ordinary breathing, has drawn the chest -walls within the limits of their normal degree of expan- sion. Under all circumstances, of course, the elastic tissue of the lungs opposes inspiration, and favors expiration. Functions of Muscular Tissue of Lungs. — It is possible that the contractile power which the bronchial tubes and air-vesicles possess, by means of their muscular fibres may (1) assist in expiration; but it is more likely that its chief purpose is (2) to regulate and adapt, in some measure, the quantity of air admitted to the lungs, and to each part of them, according to the supply of blood; (3) the muscular tissue contracts upon and gradually expels collections of mucus, which may have accumulated within tiie tubes, and cannot be ejected by forced expiratory efforts, owing to collapse or other morbid conditions of the portion of lung connected with the obstructed tubes (Gairdner). (4) Apart from any of the before- mentioned functions, the presence of muscular fibre in the walls of a hol- low viscus, such as a lung, is only what might be expected from analogy with other organs. Subject as the lungs are to such great variation in size it might be anticipated that the elastic tissue, which enters so largely into their composition, would be supplemented by the presence of much muscular fibre also. Eespiratoey Changes in the Air and in the Blood. A. In the Air. Compositioji of the Atmosphere. — The atmosphere we breathe has, in every situation in which it has been examined in its natural state, a nearly uniform composition. It is a mixture of oxygen, nitrogen, carbonic acid, and watery vapor, with, commonly, traces of other gases, as ammonia, sulphuretted hydrogen, etc. Of every 100 volumes of pure atmospheric air, 79 volumes (on an average) consist of nitrogen, the remaining 21 of oxygen. By weight the proportion is N. 75, 0. 25. The proportion of carbonic acid is extremely small; 10,000 volumes of atmospheric air con- tain only about 4 or 5 of carbonic acid. The quantity of watery vapor varies greatly according to the temper- ature and other circumstances, but the atmospliere is never without some. In this country, the average quantity of watery vapor in the atmosphere is 1 -40 })er cent. KESPIRATIOW. 193 Composition of Air which has heen hreathed. — The changes effected by respiration in the atmospheric air are: 1, an increase of temperature; 2, an increase in the quantity of carbonic acid; 3, a diminution in the quan- tity of oyxgen; 4, a diminution of volume; 5, an increase in the amount of watery vapor; Q, the addition of a minute amount of organic matter and of free ammonia. 1. The expired air, heated by its contact with the interior of the lungs, is (at least in most climates) hotter than the inspired air. Its temperature varies between 97° and 99.5° F. (36°— 37-5° C), the lower temperature being observed when the air has remained but a short time in the lungs. Whatever may be the temperature of the air when inhaled, it nearly acquires that of the blood before it is expelled from the chest. 2. The Carbonic Acid in respired air is always increased; but the quantity exhaled in a given time is subject to change from various cir- cumstances. From every volume of air inspired, about 4*8 per cent, of oxygen is abstracted; while a rather smaller quantity, 4 "3, of carbonic acid is added in its place: the air will contain, therefore, 434 vols, of car- bonic acid in 10,000. Under ordinary circumstances, the quantity of carbonic acid exhaled into the air breathed by a healthy adult man amounts to 1346 cubic inches, or about 636 grains per hour. According to this esti- mate, the weight of carbon excreted from the lungs is about 173 grains per hour, or rather more than 8 ounces in twenty-four hours. These quantities must be considered approximate only, inasmuch as various cir- cumstances, even in health, influence the amount of carbonic acid ex- creted, and, correlatively, the amount of oxygen absorbed. Circumstances injluencing the amount o f carbonic acid excreted, — The following are the cifiief: — Age and sex. Respiratory movements. Ex- ternal temperature. Season of year. Condition of respired air. Atmos- pheric conditions. Period of the day. Food and drink. Exercise and sleep. a. Age and Sex. — The quantity of carbonic acid exhaled into the air breathed by males, regularly increases from eight to thirty years of age; from thirty to fifty the quantity, after remaining stationary for awhile, gradually diminishes, and from fifty to extreme age it goes on diminish- ing, till it scarcely exceeds the quantity exhaled at ten years old. In females (in whom the quantity exhaled is always less than in males of the same age) the same regular increase in quantity goes on from the eighth, year to the age of puberty, when the quantity abruptly ceases to increase, and remains stationary so long as they continue to menstruate. When menstruation has ceased, it soon decreases at the same rate as it does in old men. h. Respiratory Movements. — The more quickly the movements of respiration are performed, the smaller is the proportionate quantity of carbonic acid contained in each volume of the expired air. Although, however, the proportionate quantity of carbonic acid is thus diminished during frequent respiration, yet the absolute amount exhaled into the air within a given time is increased thereby, owing to the larger quantity of Vol. I.— 13. 194 TTATO-BOOK OF PHYSIOLOGY. air which is breathed in the time. The last half of a volume of expired air contains more carbonic acid than the half first exi)ired; a circumstance which is explained by the one portion of air coming from the remote part of the lungs, where it ]ias been in more immediate and prolonged contact with the blood than the other has, which comes chiefly from the larger bronchial tubes. c. External temjoerature. — The observation made by Vierordt at vari- ous temperatures between 38" F. and 75° F. (3-4°— 23*8° C.) show, for warm-blooded animals, that within this range, every rise equal to 10° F. causes a diminution of about two cubic inches in the quantity of carbonic acid exhaled per minute. d. Season of the Year. — The season of the year, independently of temperature^ materially influences the respiratory phenomena; spring being the season of the greatest, and autumn of the least activity of the res- piratory and other functions. (Edward Smith.) e. Purity of the Respired Air. — The average quantity of carbonic acid given out by the lungs constitutes about 4*3 per cent, of the expired air; but if the air which is breathed be previously impregnated with car- bonic acid (as is the case when the same air is frequently respired), then the quantity of carbonic acid exhaled becomes much less. /. Hygrometric State of Atmosphere. — The amount of carbonic acid exhaled is considerably influenced by the degree of moisture of the atmos- phere, much more being given off when the air is moist than when it is dry. (Lehmann.) g. Period of the Day. — During the daytime more carbonic acid is ex- haled than corresponds to the oxygen absorbed; while, on the other hand, at night very much more oxygen is absorbed than is exhaled in carbonic acid. There is, thus, a reserve fund of. oxygen absorbed by night to meet the requirements of the day. If the total quantity of carbonic acid ex- haled in 24 hours be represented by 100, 52 parts are exhaled during the day. and 48 at nigh't. While, similarly, 33 parts of the oxygen are ab- sorbed during the dav, and the remaining 67 by night. (Pettenkofer and Voit.) \ h. Food and Drinlc. — By the use of food the quantity is increased, whilst by fasting it is diminished; it is greater when animals are fed on farinaceous food than when fed on meat. The efi'ects produced by spiritu- ous drinks depend much on the kind of drink taken. Pure alcohol tends rather to increase than to lessen respiratory changes, and the amount therefore of carbonic acid expired; rum, ale. and porter, also sherry, have very similar eft'ects. On the other hand, brandy, whisky, and gin, par- ticularly the latter, almost always lessened the resj^iratory changes, and consequently the amount of carbonic acid exhaled. (Edward Smith.) i. Exercise — Bodily exercise, in moderation, increases the quantity to about one-third more than it is during rest: and for about an hour after exercise the volume of the air expired in the minute is increased about 118 cubic inches: and the quantity of carbonic acid about 7*8 cubic inches per minute. Violent exercise, such as full labor on the treadwheel, still further increases the amount of the acid exhaled. (Edward Smith.) A larger quantity is exhaled when the barometer is low than when it is high. 3. The oxygen is diminished, and its diminution is generally propor- tionate to the increase of the carbonic acid. RESPIRATION". 195 For every volume of carbonic acid exhaled into the air, 1*17421 volumes of oxygen are absorbed from it, and 134G cubic inches, or 636 grains, be- ing exhaled in the hour, the quantity of oxygen absorbed in the same time is 1584 cubic inches, or 542 grains. According to this estimate, there is more oxygen absorbed than is exhaled with carbon to form carbonic acid. 4. The volume of air expired in a given time is less than that of the air inspired (allowance being made for the expansion in being heated), and that the loss is due to a portion of oxygen absorbed and not returned in the exhaled carbonic acid, all observers agree, though as to the actual quantity of oxygen so absorbed, they differ even widely. The amount of oxygen absorbed is on an average 4-8 per cent., so that the expired air contains 10*2 volumes per cent, of that gas. The quantity of oxygen that does not combine with the carbon given off in carbonic acid from the lungs is probably disposed of in forming some of the carbonic acid and water given off from the skin, and in com- bining with sulphur and phosphorus to form part of the acids of the sul- phates and phosphates excreted in the urine, and probably also, with the nitrogen of the decomposing nitrogenous tissues. (Bence Jones.) The quantity of oxygen in the atmosphere surrounding animals, ap- pears to have very little influence on the amount of this gas absorbed by them, for the quantity consumed is not greater even though an excess of oxygen be added to the atmosphere experimented with. It has often been discussed whether Nitrogen is absorbed by or exhaled from the lungs during respiration. At present, all, that can be said on the subject is that, under most circumstances, animals appear to expire a very small quantity above that which exists in the inspired air. During prolonged fasting, on the contrary, a small quantity appears to be ab- sorbed. 5. The watery vapor is increased. The quantity emitted is, as a gen- eral rule, sufficient to saturate the expired air, or very nearly so. Its abso- lute amount is, therefore, influenced by the following circumstances, (1), by the quantity of air respired; for the greater this is, the greater also will be the quantity of moisture exhaled. (2), by the quantity of watery vapor contained in the air previous to its being inspired; because the greater this is, the less will be the amount required to complete the satu- ration of the air; (3), by the temperature of the expired air; for the higher this is, the greater will be the quantity of watery vapor required to saturate the air; (4), by the length of time which each volume of in- spired air is allowed to remain in the lungs; for although, during ordinary respiration, the expired air is always saturated with watery vapor, yet when respiration is performed very rapidly the air has scarcely time to be raised to the highest temperature, or be fully charged with moisture ere it is expelled. 196 HAND-BOOK OF PHYSIOLOGY. The quantity of water exhaled from the lungs in twenty-four hours ranges (according to the various modifying circumstances already men- tioned) from about 6 to 27 ounces, the ordinary quantity being about 9 or 10 ounces. Some of this is probably formed by the chemical combina- tion of oxygen with hydrogen in the system; but the far larger propor- tion of it is water which has been absorbed, as such, into the blood from the alimentary canal, and which is exhaled from the surface of the air- passages and cells, as it is from the free surfaces of all moist animal mem- branes, particularly at the high temperature of warm-blooded animals. 6. A small quantity of ammonia is added to the ordinary constituents of expired air. It seems probable, however, both from the fact that this substance cannot be always detected, and from its minute amount when present, that the whole of it may be derived from decomposing particles of food left in the mouth, or from carious teeth or the like; and that it is, therefore, only an accidental constituent of expired air. 7. The quantity of organic matter in the breath is about 3 grains in twenty -four hours. (Ransome.) The following represents the kind of experiment by which the fore- going facts regarding the excretion of carbonic acid, water, and organic matter, have been established. A bird or mouse is placed in a large bottle, through the stopper of which two tubes pass, one to supply fresh air, and the other to carry oft that which has been expired. Before entering the bottle, the air is made to bubble through a strong solution of caustic potash, which absorbs the carbonic acid, and then through lime-water, which by remaining limpid, proves the absence of carbonic acid. The air which has been breathed by the animal is made to bubble through lime water, which at once becomes turbid and soon quite milky from the precipitation of cal- cium carbonate; and it finally passes through strong sulphuric acid, which, by turning brown, indicates the presence of organic matter. The watery vapor in the expired air will condense inside the bottle if the sur- face be kept cool. By means of an apparatus sufficiently large and well constructed, experiments of the kind have been made extensively on man. Methods by which the Respiratory Changes in" the Air are effected. The method by which fresh air is inhaled and expelled from the lungs has been considered. It remains to consider liow it is that the blood absorbs oxygen from, and gives up carbonic acid to, tlio air of the alveoli. In the first place, it must be remembered that the tidal air only amounts to about 25 — 30 cubic inches at each inspiration, and that this is of course insufficient to fill the lungs, but it mixes with the stationary air by diffx- si(m, and so supplies to it now oxygon. Tlio amount of oxygen in ox])irod air, wliicli may be taken as tlio average composition of the mixed air in RESPIRATION. 197 the lungs, is about 16 to 17 per cent.; in the pulmonary alveoli it may be rather less than this. From this air the venous blood has to take up 0x3'- gen in the proportion of 8 to 12 vols, in every hundred volumes of blood, as the difference between the amount of oxygen in arterial and venous blood is no less than that. It seems therefore somewhat difficult to un- derstand how this can be accomplished at the low oxygen tension of the pulmonary air. But as was pointed out in a previous Chapter (IV.), the oxygen is not simply dissolved in the blood, but is to a great extent chemically combined with the haemoglobin of the red corpuscles; and when a fluid contains a body which enters into loose chemical combination in this way with a gas, the tension of the gas in the fluid is not directly pro- portional to the total quantity of the gas taken up by the fluid, but to the excess above the total quantity which the substance dissolved in the fluid is capable of taking up (a known quantity in the case of haemoglobin, viz., 1*59 cm. for one grm. haemoglobin). On the other hand, if the sub- stance be not saturated, i.e., if it be not combined with as much of the gas as it is capable of taking up, further combination leads to no increase of its tension. However, there is a point at which the haemoglobin gives up its oxygen when it is exposed to a low partial pressure of oxygen, and there is also a point at which it neither takes up nor gives out oxygen; in the case of arterial blood of the dog, this is found to be when the oxy- gen tension of the atmosphere is equal to 3*9 per cent, (or 29-6 mm. of mercury), which is equivalent to saying that the oxygen tension of arterial blood is 3 '9 per cent. ; venous blood, in a similar manner, has been found to have an oxygen tension of 2*8 per cent. At a higher temperature, the tension is raised, as there is a greater tendency at a high temperature for the chemical compound to undergo dissociation. It is therefore easy to see that the oxygen tension of the air of the pulmonary alveoli is quite sufficient, even supposing it much less than that of the expired air, to enable the venous blood to take up oxygen, and what is more, it will take it up until the haemoglobin is very nearly saturated with the gas. As regards the elimination of carbonic acid from the blood, there is evidence to show that it is given up by a process of simple diffusion, the only condition necessary for the process being that the tension of the car- bonic acid of the air in the pulmonary alveoli should be less than the ten- sion of the carbonic acid in venous blood. The carbonic acid tension of the alveolar air probably does not exceed in the dog 3 or 4 per cent., while that of the venous blood is 5*4 per cent., or equal to 41 mm. of mercury. B. Respiratory Changes in the Blood. Circulation of Blood in the Respiratory Organs. — To be ex- posed to the air thus alternately moved into and out of the air cells and minute bronchial tubes, the blood is propelled from the right ventricle 198 HAND-BOOK OF PHYSIOLOGY. through the pulmonary capillaries in steady streams, and slowly enough to permit every minute portion of it to be for a few seconds exposed to the air, with only the thin walls of the capillary vessels and the air-cells intervening. The pulmonary circulation is of the simplest kind: for the pulmonary artery branches regularly; its successive branches run in straight lines, and do not anastomose: the capillary plexus is uniformly spread over the air-cells and intercellular passages; and the veins derived from it proceed in a course as simple and uniform as that of the arteries, their branches- converging but not anastomosing. The veins have no valves, or only small imperfect ones prolonged from their angles of junc- tion, and incapable of closing the orifice of either of the veins between which they are placed. The pulmonary circulation also is unaffected by changes of atmospheric pressure, and is not exposed to the influence of the pressure of muscles: the force by which it is accomplished, and the course of the blood, are alike simple. Changes produced in the Blood by Respiration. — The most obvious change which the blood of the pulmonary artery undergoes in its passage through the lungs is 1^^, that of color, the dark crimson of venous blood being exchanged for the bright scarlet of arterial blood; 'ind, and in connection with the preceding change, it gains oxygen; 3r^?, it loses carbonic acid; Uhy it becomes slightly cooler (p. 193); bih, it coagu- lates sooner and more firmly, and, apparently, contains more fibrin (see p. 87). The oxygen absorbed into the blood from the atmospheric air in the lungs is combined chemically with the hsemoglobin of the red blood-corpuscles. In this condition it is carried in the arterial blood to the various parts of the body, and brought into near relation or contact with the tissues. In these tissues, and in the blood which circulates in them, a certain portion of the oxygen, which the arterial blood contains, disappears, and a proportionate quantity of carbonic acid and water is formed. The venous blood, containing the new-formed carbonic acid, returns to the lungs, where a portion of the carbonic acid is exhaled, and a fresh supply of oxygen is taken in. Mechanism of Various Respiratory Actions. — It will be well here, perhaps, to explain some respiratory acts, which appear at first sight somewhat complicated, but cease to be so when the mechanism by which they are performed is clearly understood. The accompanying dia- gram (Fig. IGl) shows that the cavity of the chest is separated from that of the abdomen by the diaphragm, which, when acting, will lessen its curve, and thus descending, will push downward and forward the ab- dominal viscera; while the abdominal muscles have the opposite effect, and in actiiig will push tlie viscera vpward and hacl-ward, and with them the diaphragm, supposing its ascent to be not from any clause inter- fered with. From the same diagram it will be seen that the lungs com- municate with the exterior of the body through the glottis, and further KESPIRATION. 199 on through the month and nostrils — through either of them separately, or through both at the same time, according to the position of the soft palate. The stomach communicates with the exterior of the body through the oesophagus, pharynx, and mouth; while below the rectum opens at the anus, and the bladder through the urethra. All these openings, through which the hollow viscera communicate with the exterior of the body, are guarded by muscles, called sphincters, which can act independ- ently of each other. The position of the latter is indicated in the dia- gram. Fig. 161. Sighing. — In sighing there is a rather prolonged inspiration; the air almost noiselessly passing in through the glottis, and by the elastic recoil of the lungs and chest -walls, and probably also of the abdominal walls, being rather suddenly expelled again. Now, in the first, or inspiratory part of this act, the descent of the diaphragm presses the abdominal yiscera dowuAvard, and. of course this pressure tends to evacuate the contents of such as communicate with the exterior of the body. Inasmuch, however, as their various openings are guarded by sphincter muscles, in a state of constant tonic contraction. 200 HAND-BOOK OF PHYSIOLOGY. there is no escape of their contents, and air simply enters the lungs. In the second, or expiratory part of the act of sighing, there is also pressure made on the abdominal viscera in the opposite direction, by the elastic or muscular recoil of the abdominal walls; but the pressure is relieved by the escape of air through the open glottis, and the relaxed diaphragm is pushed up again into its original position. The sphincters of the stomach, rectum, and bladder, act as before. Hiccough resembles sighing in that it is an inspiratory act; but the inspiration is sudden instead of gradual, from the diaphragm acting sud- denly and spasmodically; and the air, therefore, suddenly rushing through the unprepared rima glottidis, causes vibration of the vocal cords, and the peculiar sound. Coughing. — In the act of coughing, there is most often first an in- spiration, and this is followed by an expiration; but when the lungs have been filled by the preliminary inspiration, instead of the air being easily let out again through the glottis, the latter is momentarily closed by the approximation of the vocal cords, and then the abdominal muscles, strongly acting, push up the viscera against the diaphragm, and thus make pressure on the air in the lungs until its tension is sufficient to burst open noisily the vocal cords which oppose its outward passage. In this way a considerable force is exercised, and mucus or any other matter that may need expulsion from the lungs or trachea is quickly and sharply expelled by the outstreaming current of air. Kow it is evident on reference to the" diagram (Fig. 161), that pressure exercised by the abdominal muscles in the act of coughing, acts as for- cibly on the abdominal viscera as on the lungs, inasmuch as the viscera form the medium by which the upward pressure on the diaphragm is made, and of necessity there is quite as great a tendency to the expulsion of their contents as of the air in the lungs. The instinctive, and if necessary, voluhtarily increased contraction of the sphincters, howevei", prevents any escape at the opepnings guarded by them, and the pressure is effective at one part only, namely, the rima glottidis. Sneezing. — The same remarks that apply to coughing, are almost exactly applicable to the act of "sneezing; but in this instance the blast of air, on escaping from the lungs, is directed, by an instinctive con- •traction of the pillars of the fauces and descent of the soft palate, chiefly through the nose, and any offending matter is thence expelled. Speaking. — In speaking, there is a voluntary expulsion of air through (lie glottis by means of the expiratory mu?cles; and the vocal cords are ])ut, ])y the muscles of the larynx, in a proper position and state of tension for vibrating as tlie air passes over them, and thus producing sound. The sound is moulded into words by the tongue, teeth, lips, etc. — the vocal cords prodiuiing the sound only, and having nothing to do with articu- lation. RESPIRATION. 201 Singing. — Singing resembles speaking in the manner of its produc- tion; the laryngeal muscles, by variously altering the position and degree of tension of the vocal cords, producing the different notes. Words used in the act of singing are of course framed, as in speaking, by the tongue, teeth, lips, etc. Sniffing. — Sniffing is produced by a somewhat quick action of the diaphragm and other inspiratory muscles. The mouth is, however, closed, and by these means the whole stream of air is made to enter by the nostrils. The alae nasi are, commonly, at the same time, instinctively dilated. Sobbing. — Sobbing consists in a series of convulsive inspirations, at the moment of which the glottis is usually more or less closed. Laughing. — Laughing is a series of short and rapid expirations. Yawning. — Yawning is an act of inspiration, but is unlike most of the preceding actions in being always more or less involuntary. It is attended by a stretching of various muscles about the palate and lower jaw, which is probably analogous to the stretching of the muscles of the limbs in whicli a weary man finds relief, as a voluntary act, when they have been some time out of action. The involuntary and reflex character of yawn- ing depends probably on the fact that the muscles concerned are them- selves at all times more or less involuntary, and require, therefore, something beyond the exercise of the will to set them in action. For the same reason, yawning, like sneezing, cannot be well performed voluntarily. Sucking. — Sucking is not properly a respiratory act, but it may be most conveniently considered in this place. It is caused chiefly by the depressor muscles of the os hyoides. These, by drawing downward and backward the tongue and floor of the mouth, produce a partial vacuum in the latter: and the weight of the atmosphere then actin g ^ i; )n all sides tends to prod^uce equilibrium on the inside and outsid^ best it may. The communication betweefn the moui completely shut off by the contraction of the pillars offfhe sofjb^palate' descent of the latter so as to touch the back of the toi librium, therefore, can be restored only by the entr^V-e^ of sO^^^thfng through the mouth. The action, indeed, of the tongur mouth in sucking may be compared to that of the and the muscles which pull down the os hyoides and tongue^'^l^^^h^ power which draws the handle. Influence of the Nervous System in Respiration.- other functions of the body, the discharge of which is necessary to life, respiration must be essentially an involuntary act. Else, life would be in constant danger, and would cease on the loss of consciousness for a few moments, as in sleep. But it is also necessary that respiration should be to some extent under the control of the will. For were it not so, it would 202 HAND-BOOK OF PHYSIOLOGY. be impossible to perform those voluntary respiratory acts which have been just enumerated and explained, as speaking, singing, and the like. The respiratory movements and their rhythm^ so far as they are invol- untary and independent of consciousness (as on all ordinary occasions) are under the governance of a nerve-centre in the medulla oblongata correspond- ing with the origin of the pneumogastric nerves; that is to say, the motor nerves^ and through them the muscles concerned in the respiratory move- ments, are excited by a stimulus which issues from this part of the nerv- ous system. How far the medulla acts automatically, i.e., how far the stimulus originates in it, or how far it is merely a nerve-centre for reflex action, is not certainly known. Probably, as will be seen, both events happen; and, in both cases, the stimulus is the result of the condition of the blood. The respiratory centre is bilateral or double, since the respiratory movements continue after the medulla at this point is divided in the mid- dle line. As regards its supposed automatic action, it has been shown that if the spinal cord be divided below the medulla, and both vagi be divided so that no afferent impulses can reach it from below, the nasal and laryn- geal respiration continues, and the only possible course of the afferent im- pulses would be through the cranial nerves; and when the cord and me- dulla are intact the division of these produces no effect upon respiration, so that it appears evident that the afferent stimuli are not absolutely necessary for maintaining the respiratory movements. But although au- tomatic in its action the respiratory centre may be reflexly excited, and the chief channel of this reflex influence is the vagus nerve; for when the nerve of one side is divided, respiration is slowed, and if both vagi be cut the respiratory action is still slower. The influence of the vagus trunk upon it is twofold, for if the nerve be divided below the origin of the superior laryngeal branch and the cen- tral end be stimulated, respiratory movements are increased in rapidity, and indeed follow one another so quickly if the stimuli be increased in number, that after a time cessation of respiration in inspiration follows from a tetanus of the respiratory muscles (diaphragm). Whereas if the superior laryngeal branch be divided, although no effect, or scarcely any, follows the mere division, on stimulation of the central end respiration is slowed, and after a time, if the stimulus be increased, stops, but not in inspiration as in the other case, but in expiration. Thus the vagus trunk contains fibres which slow and fibres which accelerate respiration. If we adopt the theory of a doubly acting respiratory centre in the floor of the medulla, one tending to produce inspiration and the other expiration, and acting in antagonism as it were, so that there is a gradual increase in tlie tendency to produce respiratory action, until it culminates in an in- spiratory effort, wliich is followed by a similar action of the expiratory RESPIRATION. 203 part of the centre, producing an expiration, we must look upon the main trunk of the vagus as aiding the inspiratory, and of the superior laryngeal as aiding the expiratory part of the centre, the first nerve possibly in- hibiting the action of the expiratory centre, whilst it aids the inspiratory, and the latter nerve having the very opposite effect. But inasmuch as the respiration is slowed on division of the vagi, and not quickened or affected manifestly on simple division of the superior laryngeal, it must be supposed that the vagi fibres are always in action, whereas the superior laryngeal fibres are not. It appears, however, that there are, in some animals at all events, subordinate centres in the spinal cord which are able, under certain con- ditions, to discharge the function of the chief medullary centre. The centre in the medulla may be influenced not only by afferent im- pulses proceeding along the vagus and laryngeal nerves but also by those proceeding from the cerebrum, as well as by impressions made upon the nerves of the skin, or upon part of the fifth nerve distributed to the nasal mucous membrane, or upon other sensory nerves, as is exemplified by the deep inspiration which follows the application of cold to the surface of the skin, and by the sneezing which follows the slightest irritation of the nasal mucous membrane. At the time of birth, the separation of the placenta, and the conse- quent non-oxygenation of the foetal blood, are the circumstances which immediately lead to the issue of automatic impulses to action from the respiratory centre in the medulla oblongata. But the quickened action which ensues on the application of cold air or water, or other sudden stimulus, to the skin, shows well the intimate connection which exists between this centre and other parts which are not ordinarily connected with the function of respiration. Methods of Stimulation of the Respiratory Centre.— It is now necessary to consider the method by which the centre or centres are stim- ulated themselves, as well as the manner in which the afferent vagi impulses are produced. The more venous the blood, the more marked are the inspiratory im- pulses, and if the air is prevented from entering the chest, in a short time the respiration becomes very labored. Its cessation is followed by an abnormal rapidity of the inspiratory acts, which make up even in depth for the previous stoppage. The condition caused by obstruction to the entrance of air, or by any circumstance by which the oxygen of the blood is used up in an abnormally quick manner, is known as dyspyioea, and as the aeration of the blood becomes more and more interfered with, not only are the ordinary respiratory muscles employed, but also those extra- ordinary muscles which have been previously enumerated (p. 186), so that as the blood becomes more and more venous the action of the medullary centre becomes more and more active. The question arises as to what 204 HAND-BOOK OF PHYSIOLOGY. condition of the venous bbod causes this increased activity, whether it is due to deficiency of oxygen or excess of carbonic acid in the blood. This has been answered by the experiments, which show on the one hand that dyspnoea occurs when there is no obstruction to the exit of carbonic acid, as when an animal is placed in an atmosphere of nitrogen, and therefore cannot be due to the accumulation of carbonic acid, and sec- ondly, that if plenty of oxygen be supplied, dyspnoea proper does not occur, although the carbonic acid of the blood is in excess. The respir- atory centre is evidently stimulated to action by the absence of sufficient oxygen in the blood circulating in it. The method by which the vagus is stimulated to conduct afferent im- pulses, influencing the action of the respiratory centre, appears to be by the venous blood circulating in the lungs, or as some say by the condition of the air in the pulmonary alveoli. And if either of these be the stimuli it Avill be evident that as the condition of venous blood stimulates the peripheral endings of the vagus in the lungs, the vagus action which tends to help on the discharge of inspiratory impulses from the centre, must tend also to increase the activity of the centre, Avhen the blood in the lungs becomes more and more venous. Xo doubt the venous condition of the blood will affect all the sensory nerves in a similar manner, but it has been shown that the circulation of too little blood through the centre is quite sufficient by itself for the purpose; as when its blood sup- ply is cut off increased inspiratory actions ensue. Effects of Vitiated Air. — Ventilation. — T\^e have seen that the air expired from the lungs contains a large proportion of carbonic acid and a minute amount of organic putrescible matter. Hence it is obvious that if the same air be breathed again and again, the proportion of carbonic acid and organic matter will constantly increase till fatal results are produced; but long before this point is reached, uneasy sensations occur, such as headache, languor, and a sense of oppres- sion. It is a remarkable fact that the organism after a time adapts itself to such a vitiated atmosphere, and that a person soon comes to breathe, without sensible inconvenience, an atuiosphere which, when he first entered it, felt intolerable. Such an adaptation, however, can only take ]"»lare at the expense of a depression of all the vital functions, which must be injurious if long continued or often repeated. This power of adaptation is well illustrated by the experiments of Claude Bernard. A sparrow is placed under a bell-glass of such a size that it will live for three hours. If now at the end of the second hour (when it could have survived another liour) it be taken out and a fresh healthy sj)arrow introduced, the latter will ]HM'ish instantly. The ada])tation above spoken of is a gradual and continuous one: thus a bird which will live one hour in ii })int of air will live three hours in two pints; and it! two birds of the same species, age, and size, be placed RESPIRATION. 205 in a quantity of air in which either, separately, would survive thrco hours, they will not live I j- hour, but only 11 hour. From what has been said it must be evident that provision for a con- stant and plentiful supply of fresh air, and the removal of that which is vitiated, is of far greater importance than the actual cubic space per head of occupants. Not less than 2000 cubic feet per head should be allowed in sleeping apartments (barracks, hospitals, etc.), and with this allow- ance the air can only be maintained at the proper standard of purity by such a system of ventilation as provides for the supply of 1500 to 2000 cubic feet of fresh air per head per hour. (Parkes.) The Effect of Respiratiok on the Cieculatiok. Inasmuch as the heart and great vessels are situated in the air-tight thorax, they are exposed to a certain alteration of pressure when the Fi<5. 162.— Diagram of an apparatus illustrating the effect of inspiration upon the heart and great vessek within the thorax. — I, the thorax at rest; II, during inspiration; d, represents the diaphragm when relaxed; d' when contracted (it must be remembered that this position is a mere diagram), i. e., when the capacity of the thorax is enlarged; h, the heart; v, the veins entering it, and a, the aorta; rI, U, the right and left lung; t, the trachea; m, mercurial manometer in connection with the pleura. The increase in the capacity of the box representing the thorax is seen to dilate the heart as well as the lungs, and so to pump in blood through v, whereas the valve prevents reflex through a. The position of the mercury in m shows also the suction which is taking place. (Landois.) capacity of the latter is inc?'eased; for although the expansion of the lungs during inspiration tends to counterbalance this increase of area, it never quite does so, since part of the pressure of the air which is drawn 206 HAND-BOOK OF PHYSIOLOGY. into the cliest tlirongli the trachea is expended in overcoming the elas- ticity of the Inngs themselves. The amount thus used up increases as the lungs become more and more expanded, so that the pressure inside the thorax during inspiration as far as the heart and great vessels are con- cerned, never quite equals that outside, and at the conclusion of inspira- tion is considerably less than the atmospheric pressure. It has been ascer- tained that the amount of the pressure used up in the way above described, varies from 5 or 7 mm. of mercury during the pause, and to 30 mm. of mercury when the lungs are expanded at the end of a deep inspiration, so that it will be understood that the pressure to which the heart and great vessels are subjected diminishes as inspiration progresses. It will be understood from the accompanying diagram how, if there were no lungs in the chest, but if its capacity were increased, the effect of the increase would be expended in pumping blood into the heart from the veins, but even Avith the lungs placed as they are, during inspiration the f)ressure outside the heart and great vessels is diminished, and they have therefore a tendency to expand and to diminish the intra-vascular pres- sure. The diminution of pressure within the veins passing to the right auricle and within the right auricle itself, will draw the blood into the thorax, and so assist the circulation: this suction action aiding, though independently, the suction power of the diastole of the auricle about which we have previously spoken (p. 124). The effect of sucking more blood into the right auricle will, cceteris parihus, increase the amount passing through the right ventricle, which also exerts a similar suction action, and through the lungs into the left auricle and ventricle and thus into the aorta, and this tends to increase the arterial tension. The effect of the diminished pressure upon the pulmonary vessels will also help toward the same end, i.e., an increased flow through the lungs, so that as far as the heart and its veins are concerned inspiration increases the blood pressure in the arteries. The effect of inspiration upon the aorta and its branches within the thorax would be, however, contrary; for as the pressure outside is diminished the vessels would tend to expand, and thus to diminish the tension of the blood within them, but inasmuch as the large arteries are capable of little expansion beyond their natural calibre, the diminution of the arterial tension caused by this means would be in- sufficient to counteract the increase of arterial tension produced by the effect of inspiration upon the veins of the chest, and the balance of the whole action would be in favor of an increase of arterial tension during the ins2)iratory period, l^ut if a tracing of the variation be taken at the same time that the respiratory movements are recorded, it will be found tliat, although speaking generally, the arterial tension is increased during inspiration, the nuiximum of arterial tension does not correspond with the acme of inspiration (Fig. liV.V). As regards the ctTect of ex]uration, tlun^apacity of the clu^st is dimin- RESPIRATION. 207 ished, and the intra-thoracic prossiu'o returns to tlie normal, which is not exactly equal to the atmospheric, pressure. The effect of this on the veins is to increase their intra-vascular pressure, and so to diminish the flow of blood into the left side of tlie heart, and with it the arterial ten- sion, but this is almost exactly balanced by the necessary increase of arterial tension caused by the increase of the extra-vascular pressure of the aorta and large arteries, so that the arterial tension is not much affected during expiration either way. Thus, ordinary expiration does ( i - K ■ ■\ b a 1 ,■ \r\AA y ■ . 1 '"■.-'■■-■^ ■ ■■• .V Fig. 163.— Comparison of blood-pressure curve with curve of intra-thoracic pressure. (To be read from left to right.) a is a curve of olood-pressure with its respiratory undulations, the slower beats on the descent being very marked; 6 is the curve of intra-thoracic pressure obtained by connecting one Umb of a manometer with the pleural cavity. Inspiration begins at i and expiration at e. The intra-thoracic pressure rises very rapidly after the cessation of the inspiratory effort, and then slow- ly falls as the air issues from the chest; at the beginning of the inspiratory effort the fall becomes more rapid. (M. Foster.) not produce a distinct obstruction to the circulation, as even when the expiration is at an end the intra-thoracic pressure is less than the extra- thoracic. The effect of violent expiratory efforts, however, has a distinct action in preventing the current of blood through the lungs, as seen in the blueness of the face from congestion in straining; this condition being produced by pressure on the small pulmonary vessels. We may summarize this mechanical effect, therefore, and say that in- spiration aids the circulation and so increases the arterial tension, and that although expiration does not materially aid the circulation, yet under ordinary conditions neither does it obstruct. Under extraordinary con- ditions, as in violent expirations, the circulation is decidedly obstructed. But we have seen that there is no exact correspondence between the points of extreme arterial tension and the end of inspiration, and we must look to the nervous system for an explanation of this apparently contra- dictory result. The effect of the. nervous system in producing a rhythmical alteration of the blood pressure is twofold. In the first place the cardio-inJiihitory centre is believed to be stimulated during the fall of blood pressure, pro- 208 HAND-BOOK OF PHYSIOLOGY. ducing a slower rate of heart-beats during expiration, which will be noticed in the tracing (Fig. 163), the undulations during the decline of blood-pressure being longer but less frequent. This effect disappears when, by section of the vagi, the eifect of the centre is cut off from the heart. In the second place, the vaso-motor centre is also believed to send out rhythmical impulses, by which undulations of blood pressure are pro- duced independently of the mechanical effects of respiration. Fig. 164.— Traube-Hering's curves. (To be read from left to right.) The curves 1, 2, 3, 4, and 5 are portions selected from one continuous tracing forming the record of a prolonged observation, so that the several curves represent successive stages of the same expei-iment. Each cm-ve is placed in its proper position relative to the base Hne, which is omitted; the blood-pressure rises in stages from 1, to 2, 3, and 4, but falls again in stage .5. Curve 1 is taken from a period when artificial respiration was being kept up, but the vagi having been divided, tlie pulsations on the ascent and descent of the undulations do not differ; when artificial resiTiratioii ceased these undula! ions for a while disappeared, and the blood-pressure rose steadily while the heart-beats heeanie slo^\ (^r. SQon, as at 0. new un- dulations apjK'ared; a little later, tlie, blood-pressun" was st.ill i-isin;;-. the heart-beats still slower, hut the undulations still more obvious (3): still later (4), the pi-essure was still higher, hut the heart-heatvS were (iuic;ker, and the undulations fiatter, tlie i)ressure tluni Im gan to fall rapidly- (,6), and continued to fall until some time after artificial respiration was resumed. (IM. Foster.) The action of the vaso-motor centre in taking part in producing rhythmical changes of blood-pressure which are called respiratory, is shown in the following way: — In an animal under the influence of imiri, record of whose blood -pressure is being taken, and where artificial respi- ration has been st()])ped, and botli vagi cut, the blood-pressure curve rises at first jilmost in a sf raiglit line; but jiftiM- a lime iumv rhythmical undula- tions occur very like llir original res])irat()ry undulations, only somewhat I RESPIRATION. 209 larger. These are called Traitles or Trauhe-Hering's curves. They con- tinue whilst the blood-pressure continues to rise, and only cease when the vaso-motor centre and the heart are exhausted, when the pressure speedily falls. These curves must be dependent upon the vaso-motor centre, as the mechanical effects of respiration have been eliminated by the poison and by the cessation of artificial respiration, and the effect of the cardio- inhibitory centre be the division of the vagi. It may be presumed there- fore that the vaso-motor centre, as well as the cardio-inhibitory, must be considered to take part with the mechanical changes of inspiration and expiration in producing the so-called respiratory undulations of blood- pressure. Cheyne-8tohes's 'breathing. — This is a rhythmical irregularity in respi- rations which has been observed in various diseases, and is especially con- nected with fatty degeneration of the heart. Eespirations occur in groups, at the beginning of each group the inspirations are very shallow, but each successive breath is deeper than the preceding until a climax is reached, then comes in a prolonged sighing expiration, succeeded by a pause, after which the next group begins. Apkcea. — Dyspkcea. — Asphyxia. As blood which contains a normal proportion of oxygen excites the respiratory centre (p. 204), and as the excitement and consequent respir- atory muscular movements are greater (dyspnoea) in proportion to the deficiency of this gas, so an abnormally large proportion of oxygen in the blood leads to diminished breathing movements, and, if the proportion be large enough, to their temporary cessation. This condition of absence of breathing is termed apnoea,"- and it can be demonstrated, in one of the lower animals, by performing artificial respiration to the extent of satura- ting the blood with oxygen. When, on the other hand, the respiration is stopped, by, e.g., interference with the passage of air to the lungs, or by supplying air devoid of oxygen, a condition ensues, which passes rapidly from the state of dyspnoea (difficult breathing) to what is termed asphyxia; and the latter quickly ends in death. The ways by which this condition of asphyxia may be produced are very numerous; as, for example, by the prevention of the due entry of oxygen into the blood, either by direct obstruction of the trachea or other part of the respiratory passages, or by introducing instead of ordinary air a gas devoid of oxygen, or, again, by interference with the due inter- change of gases between the air and the blood. Symptoms of Asphyxia. — The most evident symptoms of asphyxia or suffocation are well known. Violent action of the respiratory muscles ' This term has been, unfortunately, often applied to conditions of dyspnoea or aspJiyxia; but the modern application of the term, as in the text, is the more convenient. Vol. I.— 14. 210 HAND-BOOK OF PHYSIOLOGY. and, more or less, of all the muscles of the body; lividity of the skin and all other vascular parts, while the veins are also distended, and the tissues seem generally gorged with blood; convulsions, quickly followed by in- sensibility, and death. The conditions which accompany these symptoms are — (1) More or less interference with the passage of the blood through the pulmonary blood-vessels. (2) Accumulation of blood in the right side of the heart and in the systemic veins. (3) Circulation of impure (non-aerated) blood in all parts of the body. Cause of Death from Asphyxia. — The causes of these conditions and the manner in which they act, so as to be incompatible with life, may be here briefly considered. (1) The obstruction to the passage of blood through the lungs is not so great as it was once supposed to be; and such as there is occurs chiefly in the later stages of asphyxia, when, by the violent and convulsive action of the expiratory muscles, pressure is indirectly made on the lungs, and the circulation through them is proportionately interfered with. (2) Accumulation of blood, with consequent distension of the right side of the heart and systemic veins, is the direct result, at least in part, of the obstruction to the pulmonary circulation just referred to. Other causes, however, are in operation, (a) The vaso-motor centres stimu- lated by blood deficient in oxygen, causes contraction of all the small arteries with increase of arterial tension, and as an immediate conse- quence the filling of the systemic veins, (b) The increased arterial ten- sion is followed by inhibition of the action of the heart, and, thus, the latter, contracting less frequently, and gradually enfeebled also by defi- cient supply of oxygen, becomes over-distended by blood which it cannot expel. At this stage the left as well as the right cavities are distended with blood. The ill eifects of these conditions are to be looked for partly in the heart, the muscular fibres of which, like those of the urinary bladder or any other hollow muscular organ, may be paralyzed by over-stretching; and partly in the venous congestion, and consequent interference with the function of the higher nerve-centres, especially the medulla oblongata. (3) The passage of non-aerated blood through the lungs and its dis- tribution over the body are events incompatible with life, in one of the higher animals, for more than a few minutes; tlie rapidity with which deatli ensues in asphyxia being due, more particularly, to the ofl'ect of iion-oxygcnized blood on the medulla oblongata, and, through the coro- nary arteries, on the muscular substance of the heart. The excitability of both n(n-v()us aiul muscular tissue is dependent on n constant and large supply of oxygen, and, wheu this is interfered with, is rapidly lost. The (liiiiinuiion of oxygen, it may be here remnrked, has a move direct in- RESPIRATION. 211 fluence in the production of the usual symptoms of asphyxia than the increased amount of carbonic acid. Indeed, the fatal effect of a gradual accumulation of the latter in the blood, if a due supply of oxygen be maintained, resembles rather that of a narcotic poison. In some experiments performed by a committee appointed by the Medico-Chirurgical Society to investigate the subject of Su!])er l)ecomes shorter, and occasionally branclied at the fundus. DIGESTION. 243 (b) Pyloric Glands.— These glands (Fig. 179) have much longer ducts than the peptic glands. Into each duct two or three tubes open by very short and narrow necks, and the body of each tube is branched, wavy, and convoluted. The lumen is very large. The ducts are lined with columnar epithelium, and the neck and body with shorter and more gran- FiG. 177.— From a vertical section through the mucous membrane of the cardiac end of stomach. Two peptic glands are shown with a duct common to both, one gland only in part, a, duct with col- immar epithelium becoming shorter as the cells are traced downward; n, neck of gland tubes, with central and parietal or so-called peptic ceUs; 6, fundus with curved cwecal extremity— the parietal ceUs are not so numerous here, x 400. (Klein and Noble Smith.) Fig. 178.— Transverse section through lower part of peptic glands of a cat. a, peptic cells; b, smaU spheroidal or cubical cells; c, transverse section of capillaries. (Frey.) Fig. 179.— Section showing the pyloric glands, s, free surface ; d, ducts of pyloric glands ; n, neck of same; m, the gland alveoU; m m, muscularis mucosae. (Klein and Noble Smith.) ular cubical cells, which correspond with the central cells of the peptic glands. During secretion the cells become, as in the case of the peptic glands, larger and the granules restricted to the inner zone of the cell. As they approach the duodenum the pyloric glands become larger, more 244 HAND-BOOK OF PHYSIOLOGY. convoluted and more deeply situated. They are directly continuous with Brunner's glands in the duodenum. (Watney.) Changes in the gland cells during secretion. — The chief or cubical cells of the peptic glands^ and the corresponding cells of the pyloric glands during the early stage of digestion, if hardened in alcohol, appear swollen and granular, and stain readily. At a later stage the cells become smaller, but more granular and stain even more readily. The parietal cells swell up, but are otherwise not altered during digestion. The gran- ules, however, in the alcohol-hardened specimen, are believed not to exist in the living cells, but to have been precipitated by the hardening re- agent; for if examined during life they appear to be confined to the inner zone of the cells, and the outer zone is free from granules, whereas during rest the cell is granular throughout. These granules are thought to be Fig. 180.— Plan of the blood-vessels of the stomach, as they would be seen in a vertical section, a, arteries, passing up from the vessels of submucous coat; b, capillaries branching between and around the tubes; c, superficial plexus of capillaries occup3-ing the ridges of the mucous membrane; d, vein formed by the union of veins which, having collected the blood of the superficial capillary plexus, are seen passing down between the tubes. (Brinton.) pepsin, or the substance from which pepsin is formed, pepsinogen, which is during rest stored chiefly in the inner zone of the cells and discharged into the lumen of the tube during secretion. (Langley.) Lymphatics. — Lymphatic vessels surround the gland tubes to a greater or less extent. Toward the fundus of the peptic glands are found masses of lymphoid tissue, which may appear as distinct follicles, somewhat like the solitary glands of the small intestine. Blood-vessels. — Tlie blood-vessels of the stomach, which first break up in the submucous tissue, send branches upward between the closely packed glandular tubes, anastomosing around them by means of a fine capillary network, with oblong meshes. Continuous with this deeper plexus, or prolonged u])ward from it, so to s})eak, is a more superficial network of larger cai)illaries, which branch densely around the orifices I DIGESTION. 245 of the tubes, and form the framework on which are moulded the small elevated ridges of mucous membrane bounding the minute, polygonal pits before referred to. From this superficial network the veins chiefly take their origin. Thence passing down between the tubes, with no very free connection with the deeper inter-tuhular capillary plexus, they open finally into the venous network in the submucous tissue. Nerves. — The nerves of the stomach are derived from the pneumo- gastric and sympathetic, and form a plexus in the submucous and mus- cular coats, containing many ganglia (Remak, Meissner). Digestion" iit the Stomach. Gastric Juice. — The functions of the stomach are to secrete a diges- tive fluid (gastric juice), to the action of which the food is next subjected after it has entered the cavity of the stomach from the oesophagus; to thoroughly incorporate the fluid with the food by means of its muscular movements; and to absorb such substances as are capable of absorption. AVhile the stomach contains no food, and is inactive, no gastric fluid is secreted; and mucus, which is either neutral or slightly alkaline, covers its surface. But immediately on the introduction of food or other sub- stance the mucous membrane, previously quite pale, becomes slightly turgid and reddened with the influx of a larger quantity of blood; the gastric glands commence secreting actively, and an acid fluid is poured out in minute drops, which gradually run together and flow down the walls of the stomach, or soak into the substances within it. Chemical Composition of Gastric Juice. — The first accurate analysis of gastric juice was made by Prout: but it does not appear to have been collected in any large quantity, or pure and separate from food, until the time when Beaumont was enabled, by a fortunate circumstance, to obtain it from the stomach of a man named St. Martin, in whom there existed, as the result of a gunshot wound, an opening leading directly into the stomach, near the upper extremity of the great curvature, and three inches from the cardiac orifice. The introduction of any m.echanical irritant, such as the bulb of a thermometer, into the stomach, excited at once the secretion of gastric fluid. This was drawn off, and was often obtained to the extent of nearly an ounce. The introduction of aliment- ary substances caused a much more rapid and abundant secretion than did other mechanical irritants. Ko increase of temperature could be detected during the most active secretion; the thermometer introducjed into the stomach always stood at 100° F. (37*8° C.) except during muscu- lar exertion, when the temperature of the stomach, like that of other parts of the body, rose one or two degrees higher. The chemical composition of human gastric juice has been also in- vestigated by Schmidt. The fluid in this case was obtained by means of an 246 HAND-BOOK OF PHYSIOLOGY. accidental gastric fistula, which existed for several years below the left manimar}^ region of a patient between the cartilages of the ninth and tenth ribs. The mucous membrane was excited to action b}^ the introduc- tion of some hard matter, such as dry peas, and the secretion was removed by means of an elastic tube. The fluid thus obtained was found to be acid, limpid, odorless, with a mawkish taste — with a specific gravity of 1002, or a little more. It contained a few cells, seen with the microscope, and some fine granular matter. The analysis of the fluid obtained in this is given below. The gastric juice of dogs and other animals obtained by the introduction into the stomach of a clean sponge through an artifi- cially made gastric fistula, shows a decided difference in composition, but possibly this is due, at least in part, to admixture with food. Chemical CoMPOsmoi^" of Gastkic Juice. Dogs. Human. Water 971-17 994*4 Solids 28-82 5-39 Solids- Ferment— Pepsin . . . . 17*5 3-19 Hydrochloric acid (free^ .... 2-7 -2 Salts- Calcium, sodium, and potassium, chlorides; and calcium, magnesium, and iron, phosphates . 8*57 2*19 The quantity of gastric juice secreted daily has been variously esti- mated; but the average for a healthy adult may be assumed to range from ten to twenty pints in the twenty-four hours. The acidity of the fluid is due to free liydrocliloric acid, although other acids, e.g., lactic, acetic, butyric, are not unfrequently to be found therein as products of gastric digestion. The amount of hydrochloric acid varies from 2 to -2 per 1000 parts. In healthy gastric juice the amount of free acid may be as much as -2 per cent. As regards the formation of pepsin and acid, the former is produced by the central or chief cells of the peptic glands, and also most likely by the similar cells in the pyloric glands; the acid is chiefly found at the surface of the mucous membrane, but is in all probability formed by the secreting action of the parietal cells of the peptic glands, as no acid is formed by the pyloric glands in wliich this variety of cell is absent. The ferment Pepsin (p. 24()) can be procured by digesting portions of the mucous membrane of tlu^ stomacli in cold water, after they liave been macerated for some time in water at a temperature 80° — lOO*-"* F. (27-0 — 37 '8° C). The warm water dissolves various substances as well as some of the pepsin, but the cold water takes up little else than pepsin, which is contained in a greyish-brown viscid fluid, on eva])orating the DIGESTION. cold solution. The addition of alcohol throws down the pepsin in greyish- white flocculi. Glycerine also has the property of dissolving out the fer- ment; and if the mucous membrane be finely minced and the moisture removed by absolute alcohol, a powerful extract may be obtained by throwing into glycerine. Functions. — The digestive power of the gastric juice depends on the pepsin and acid contained in it, both of which are, under ordinary cir- cumstances, necessary for the process. The general effect of digestion in the stomach, is the conversion of the food into cliyme, a substance of various composition according to the nature of the food, yet always presenting a characteristic thick, pultace- ous, grumous consistence, with the undigested portions of the food mixed in a more fluid substance, and a strong, disagreeable acid odor and taste. The chief function of the gastric juice is to convert ])Toteids into pep- tones. This action may be shown by adding a little gastric juice (natural or artificial) to some diluted egg-albumin, and keeping the mixture at a temperature of about 100° F. (37 '8° C); it is soon found that the albu- min cannot be precipitated on boiling, but that if the solution be neutral- ized with an alkali, a precipitate of acid- albumin is thrown down. After a while the proportion of acid-albumin gradually diminishes, so that at last scarcely any precipitate results on neutralization, and finally it is found that all the albumin has been changed into another proteid sub- stance which is not precipitated on boiling or on neutralization. This is called peptone. Characteristics of Peptones. — Peptones have certain characteristics which distinguish them from other proteids. 1. They are diffusihUj i.e., they possess the property of passing through animal membranes. 2. They cannot be precipitated by heat, nitric, or acetic acid, or potassium ferrocyanide and acetic acid. They are, however, thrown down by tannic acid, by mercuric chloride and by picric acid. 3. They are very soluble in water and in neutral saline solutions. In their ditfusibility peptones differ remarkably from egg-albumin, and on this diffusibility depends one of their chief uses. Egg-albumin as such, even in a state of solution, would be of little service as food, inas- much as its indiffusibility w^ould effectually prevent its passing by absorp- tion into the blood-vessels of the stomach and intestinal canal. Changed, however, by the action of the gastric juice into peptones, albuminous matters diffuse readily, and are thus quickly absorbed. After entering the blood the peptones are very soon again modified, so as to re-assume the chemical characters of albumin, a change as neces- sary for preventing their diffusing out of the blood-vessels, as the previous change was for enabling them to pass in. This is effected, probably, in great part by the agency of the liver. Products of Gastric Digestion. — The chief product of gastric 248 HAND-BOOK OF PHYSIOLOGY. t. digestion is undoubtedly peptone. We have seen^ however, in the above experiment that there is a by-product, and this is almost identical with syntonin or acid albumin. This body is probably not exactly identical, however, with syntonin, and its old name of parajjeptone had better be retained. The conversion of native albumin into acid albumin may be effected by the hydrochloric acid alone, but the further action is undoubt- edly due to the ferment and the acid together, as although under high pressure any acid solution may, it is said, if strong enough, produce the entire conversion into peptone, under the condition of digestion in the stomach this would be quite impossible; and, on the other hand, pepsin will not act without the presence of acid. The production of two forms of peptone is usually recognized, called respectively anti-^e.^ionQ and Aem^-peptone. Their differences in chemical properties have not yet been made out, but they are distinguished by this remarkable fact, that the pancreatic juice, while possessing no action over the former, is able to convert the latter into leucin and ty rosin. Pepsin acts the part of a hydrolytic ferment (proteolytic), and appears to cause hydration of albu- min, peptone being a highly hydrated form of albumin. Circumstances favoring Gastric Digestion. — 1. A temperature of about 100° F. (37 '8° C); at 32° F. (0° 0.) it is delayed, and by boil- ing is altogether stopped. 2. An acid medium is necessary. Hydro- chloric is the best acid for the purpose. Excess of acid or neutralization stops the process. 3. The removal of the products of digestion. Excess of peptone delays the action. Action of the Gastric Juice on Bodies other than Proteids. — ^All proteids are converted by the gastric juice into peptones, and, there- fore, whether they be taken into the body in meat, eggs, milk, bread, or other foods, the resultant still is peptone. Milh is curdled, the casein being precipitated, and then dissolved. The curdling is due to a special ferment of the gastric juice (curdling ferment), and is not due to the action of the free acid only. The effect of rennet, which is a decoction of the fourth stomach of a calf in brine, has long been known, as it is used extensively to cause precipitation of casein in cheese manufacture. The ferment which produces this curdling action is distinct from pepsin. Gelatin is dissolved and changed into peptone, as are also cliondrin and elastin; but tmiciu, and the Jiorny tissues, keratin generally are un- affected. On the amylaceotis articles of food, and upon pure oleaginous prin- ciples the gastric juice has no action. Tu the case of adipose tissue, its effo(;t is to dissolve the aroohir tissue, {ilhiiniinous cell-walls, etc., wliich enter into its comi)osition, by wliich means the fat is able to mingle more unifornily with the other constituents of the chyme. DIGESTION. 249 The gastric fluid acts as a general solvent for some of the mime con- stituents of the food, as, for example, particles of common salt, which may happen to have escaped solution in the saliva; while its acid may enable it to dissolve some other salts which are insoluble in the latter or in water* It also dissolves cane sugar, and by the aid of its mucus causes its conversion in part into grape sugar. The action of the gastric juice in preventing and checking putrefac- tion has been often directly demonstrated. Indeed, that the secretions which the food meets with in the alimentary canal are antiseptic in their action, is what might be anticipated, not only from the proneness to de- composition of organic matters, such as those used as food, especially under the influence of warmth and moisture, but also from the well- known fact that decomposing flesh (e.g., high game) may be eaten with impunity, while it would certainly cause disease were it allowed to enter the blood by any other route than that formed by the organs of digestion. Time occupied in Gastric Digestion. — Under ordinary condi- tions, from three to four hours may be taken as the average time occupied by the digestion of a meal in the stomach. But many circumstances v/ill modify the rate of gastric digestion. The chief are: the nature of the food taken and its quantity (the stomach should be fairly filled — not dis- tended); the time that has elapsed since the last meal, which should be at least enough for the stomach to be quite clear of food; the amount of exercise previous and subsequent to a meal (gentle exercise being favor- able, over-exertion injurious to digestion); the state of mind (tranquillity of temper being essential, in most cases, to a quick and due digestion); the bodily health; and some others. Movements of the Stomach. — The gastric fluid is assisted in accomplishing its share in digestion by the movements of the stomach. In granivorous birds, for example, the contraction of the strong muscular gizzard affords a necessary aid to digestion, by grinding and triturating the hard seeds which constitute part of the food. But in the stomachs of man and other Mammalia the motions of the muscular coat are too feeble to exercise any such mechanical force on the food; neither are they needed, for mastication has already done the mechanical work of a giz- zard; and experiments have demonstrated that substances enclosed in perforated tubes, and consequently protected from mechanical .influence, are yet digested. The normal actions of the muscular fibres of the human stomach appear to have a threefold purpose; (1) to adapt the stomach to the quantity of food in it, so that its walls may be in contact with the food on all sides, and, at the same time, may exercise a certain amount of compression upon it; (2) to keep the orifices of the- stomach closed until the food is digested; and (3) to perform certain peristaltic movements, . Avhereby the food, as it becomes chymified, is gradually propelled toward. 250 HAND-BOOK OF PHYSIOLOGY. and ultimately through, the pylorus. In accomplishing this latter end, the movements without doubt materially contribute toward effecting a thorough intermingling of the food and the gastric fluid. When digestion is not going on, the stomach is uniformly contracted, its orifices not more firmly than the rest of its walls; but, if examined shortly after the introduction of food, it is found closely encircling its contents, and its orifices are firmly closed like sphincters. The cardiac orifice, every time food is swallowed, opens to admit its passage to the stomach, and immediately again closes. The pyloric orifice, during the first part of gastric digestion, is usually so completely closed, that even when the stomach is separated from the intestines, none of its contents escape. But toward the termination of the digestive process, the pylorus seems to offer less resistance to the passage of substances from the stomach; first it yields to allow the successively digested portions to go through it; and then it allows the transit of even undigested substances. It appears that food, so soon as it enters the stomach, is subjected to a kind of peristaltic action oi the muscular coat, whereby the digested por- tions are gradually moved toward the pylorus. The movements were observed to increase in rapidity as the process of chymification advanced, and were continued until it was completed. The contraction of the fibres situated toward the pyloric end of the stomach seems to be more energetic and more decidedly peristaltic than those of the cardiac portion. Thus, it was found in the case of St. Martin, that when the bulb of the thermometer was placed about three inches from the pylorus, through the gastric fistula, it was tightly em- braced from, time to time, and drawn toward the pyloric orifice for a dis- tance of three or four inches. The object of this movement appears to be, as just said, to carry the food toward the pylorus as fast as it is formed into chyme, and to propel the chyme into the duodenum; the undigested portions of food being kept back until they are also reduced into chyme, or until all that is digestible has passed out. The action of these fibres is often seen in the contracted state of the pyloric portion of the stomach after death, when it alone is contracted and firm, while the cardiac por- tion forms a dilated sac. Sometimes, by a predominant action of strong circular fibres placed between tlie cardia and pjdorus, the two portions, or ends ag they are called, of the stomach, are partially separated from each other by a kind of hour-glass contraction. By means of the peri- staltic action of tlie muscular coats of the stonuxcli, not merely is chymitied food gradually propelled through the pylorus, but a kind of double cur- rent is continually kept up among the contents of the stomach, the cir- cumferential parts of the mass being gradually moved ouAvard toward the pylorus by the contraction of the muscular libres, while the central por- tions are propelled in the opposite direction, namely, toward the cardiac orifice; in this way is kept up a constant circulation of the contents of DIGESTION. 251 the viscus, highly conducive to their free mixture with the gastric fluid {ind to their ready digestion. Vomiting. — The expulsion of the contents of the stomach in vomit- ing, like that of mucous or other matter from the lungs in coughing, is preceded by an inspiration; the glottis is then closed, and immediately afterward the abdominal muscles strongly act; but here occurs the dif- ference in the two actions. Instead of the vocal cords yielding to the action of the abdominal muscles, they remain tightly closed. Thus the diaphragm being unable to go up, forms an unyielding surface against which the stomach can be pressed. In this way, as well as by its own contraction, it infixed, to use a technical phrase. At the same time the cardiac sphincter-muscle being relaxed, and the orifice which it naturally guards being actively dilated, while the pylorus is closed, and the stomach itself also contracting, the action of the abdominal muscles, by these means assisted, expels the contents of the organ through the oesophagus, pharynx, and mouth. The reversed peristaltic action of the oesophagus probably increases the effect. It has been frequently stated that the stomach itself is quite passive during vomiting, and that the expulsion of its contents is effected solely by the pressure exerted upon it when the capacity of. the abdomen is diminished by the contraction of the diaphragm, and subsequently of the abdominal muscles. The experiments and observations, however, which are supposed to confirm this statement, only show that the contraction of the abdominal muscles alone is sufficient to expel matters from an unre- sisting bag through the 'oesophagus; and that, under very abnormal circumstances, the stomach, by itself, cannot expel its contents. They by no means show that in ordinary vomiting the stomach is passive; and, on the other hand, there are good reasons for believing the contrary. It is true that facts are wanting to demonstrate with certainty this action of the stomach in vomiting; but some of the cases of fistulous open- ing into the organ appear to support the belief that it does take place; and the analogy of the case of the stomach with that of the other hollow viscera, as the rectum and bladder, may be also cited in confirmation. The muscles concerned in the act of vomiting, are chiefly and pri- marily those of the abdomen; the diaphragm also acts, but usually not as the muscles of the abdominal walls do. They contract and compress the stomach more and more toward the diaphragm; and the diaphragm (which is usually drawn down in the deep inspiration that precedes each act of vomiting) is fixed, and presents an unyielding surface against which the stomach may be pressed. The diaphragm is, therefore, as a rule, passive during the actual expulsion of the contents of the stomach. But there are grounds for believing that sometimes this muscle actively contracts, so that the stomach is, so to speak, squeezed between the descending diaphragm and the retracting abdominal walls. 252 HAND-BOOK OF PHYSIOLOGY. Some persons possess the power of vomiting at will, without applying any undue irritation to the stomach, but simply by a voluntary effort. It seems also, that this power may be acquired by those who do not naturally possess it, and by continual practice may become a habit. There are cases also of rare occurrence in which persons habitually swallow their food hastily, and nearly unmasticated, and then at their leisure regurgi- tate it, piece by piece, into their mouth, remasticate, and again swallow it, like members of the ruminant order of Mammalia. The various nerve- act ions concerned in vomiting are governed by a nerve-centre situate in the medulla oblongata. The sensory nerves are the fifth, glosso-pharyngeal and vagus princi- pally; but, as well, vomiting may occur from stimulation of sensory nerves from many organs, e.g., kidney, testicle, etc. The centre may also be stimulated by impressions from the cerebrum and cerebellum, so called central vomiting occurring in disease of those parts. The efferent im- pulses are carried by the phrenics and the spinal nerves. Influence of the Nervous System on Gastric Digestion. — Th( normal movements of the stomach during gastric digestion are directlj connected with the plexus of nerves and ganglia contained in its walls, the presence of food acting as a stimulus which is conveyed to the gan- glia and reflected to the muscular fibres. The stomach is, however, also directly connected with the higher nerve-centres by means of branches of the vagus and solar plexus of the sympathetic. The vaso-motor fibres of the latter are derived, probably, from tlie splanchnic nerves. The exact function of the vagi in connection with the movements of the stomach is not certainly known. Irritation of the vagi produces co]i- traction of the stomach, if digestion is proceeding; while, on the other hand, peristaltic action is retarded or stopped, when these nerves are divided. Bernard, watching the act of gastric digestion in dogs which had fis- tulous openings into their stomachs, saw that on the instant of dividing their vagic nerves, the process of digestion was stopped, and the mucous membrane of the stomach, previously turgid with blood, became pale, and ceased to secrete. These facts may be explained by the theory that the vagi are the media by which, during digestion, an inliiliforg impulse is conducted to the vaso-motor centre in the medulla; such impulse being reflected along the splanchnic nerves to the blood-vessels of the stomach, and causing their dilatation ( Rutherford). From other experiments it ma^ be gathered, that although division of both vagi always temporarily sus- pends the secretion of gastric fluid, and so arrests the process of digestion, being occasionally followed by death from inanition; yet the digestive power5 of the stomach may be completely restored after the operation, jind the formation of chyme and the nutrition of the animal may be can-iod on almost as perfectly as in health. This Avould indicate the DIGESTION. 253 existence of a special local nervous mechanism which controls the secretion. Bernard found that galvanic stimulus of these nerves excited an active secretion of the fluid, while a like stimulus applied to the sympathetic nerves issuing from the semilunar ganglia, caused a diminution and even complete arrest of the secretion. The influence of the higher nerve-centres on gastric digestion, as in the case of mental emotion, is too well known to need more than a ref- erence. Digestion of the Stomach after Death. — If an animal die dur- ing the process of gastric digestion, and when, therefore, a quantity of gastric juice is present in the interior of the stomach, the walls of this organ itself are frequently themselves acted on by their own secretion, and to such an extent, that a perforation of considerable size may be pro- duced, and the contents of the stomach may in part escape into the cavity of the abdomen. This phenomenon is not unfreqiiently observed in post-mortem examinations of the human body. If a rabbit be killed during a period of digestion, and afterward exposed to artificial warmth to prevent its temperature from falling, not only the stomach, but many of the surrounding parts, will be found to have been dissolved (Pavy). From these facts, it becomes an interesting question why, during life, the stomach is free from liability to injury from a secretion which, after death, is capable of such destructive effects? It is only necessary to refer to the idea of Bernard, that the living stomach finds protection from its secretion in the presence of epithelium and mucus, which are constantly renewed in the same degree that they are constantly dissolved, in order to remark that, although the gastric mucus is probably protective, this theory, so far as the epithelium is con- cerned, has been disproved by experiments of Pavy\ in which the mucous membrane of the stomachs of dogs was dissected off for a small space, and, on killing the animals some days afterward, no sign of digestion of the stomach was visible. ''Upon one occasion, after removing the mu- cous membrane, and exposing the muscular fibres over a space of about an inch and a half in diameter, the animal was allowed to live for ten days. It ate food every day, and seemed scarcely affected by the operation. Life was destroyed whilst digestion was being carried on, and the lesion in the stomach was found very nearly repaired: new matter had been deposited in the place of what had been removed, and the denuded spot had con- tracted to much less than its original dimensions. Pavy believes that the natural alkalinity of the blood, which circulates so freely during life in the walls of the stomach, is sufficient to neutralize the acidity of the gastric juice; and as may be gathered from what has been previously said, the neutralization of the acidity of the gastric secre- tion is quite sufficient to destroy its digestive powers; but the experi- 254 HAND-BOOK OF PHYSIOLOGY. ments adduced in favor of this theory are open to many objections, and afford only a negative support to the conclusions they are intended to prove. Again, the pancreatic secretion acts best on proteids in an alka.' Fig. 181.— Auerbach's nerve-plexus in small intestine. The plexus consists of flbrillated substance, and is made up of trabeculae of various thicknesses. Nucleus-like elements and gangUon-cells are im- bedded in the plexus, the whole of which is enclosed in a nucleated sheath. (Kllein.) line medium; but it has no digestive action on the living intestine. It must be confessed that no entirely satisfactory theory has been yet stated. The Iktestines. The Intestinal Canal is divided into two chief portions, named from their differences in diameter, the (I.) small and (II.) large intestine (Fig. 165). These are continuous with each other, and communicate by means of an opening guarded by a valve, the ileo-ccecal valve, vrhich allows the passage of the products of digestion from the small into the large bowel, but not, under ordinary circumstances, in the opposite direction. /. Tlie Small Intestine. — The Small Intestine, the average length of which in an adult is about twenty feet, has been divided, for convenience of description, into three portions, viz., the duodenum, which extends for eight or ten inches beyond the pylorus; the jejunum, which forms two- fifths, and the ileum, which forms three-fifths of the rest of the canal. Structure. — The small intestine, like the stomach, is constructed of four principal coats, viz. , the serous, muscular, submucous, and mucous. (1) The sei'ous coat, formed by the visceral layer of the peritoneum, and has the structure of serous membranes in general. {%) The muscular- coats consist of an internal circular and an external longitudinal layei*: the former is usually considerably the thicker. Both DIGESTION. 255 alike consist of bundles of unstriped muscular tissue supported by con- nective tissue. They are well provided with lymphatic vessels, which form a set distinct from- those of the mucous membrane. Between the two muscular coats is a nerve-plexus (Auerbach^s plexus, plexos myentericus) (Pig. 181) similar in structure to Meissner's (in the submucous tissue), but with more numerous ganglia. This plexus regu- lates the peristaltic movements of the muscular coats of the intestines. (3) Between the mucous and muscular coats, is the submucous coat, which consists of connective tissue, in which numerous blood-vessels and lymphatics ramify. A fine plexus, consisting mainly of non-medullated nerve-fibres, "Meissner's plexus," with ganglion cells at its nodeS, occurs Fig. 182. — Horizontal section of a small fragment of the mucous membrane, including one entire crypt of Lieberkuhn and parts of several others: a, cavity of the tubular glands or crypts; 6, one of the hning epithelial cells; c. the lymphoid or retiform spaces, of which some are empty, and others occupied by lymph cells, as at d. in the submucous tissue from the stomach to the anus. From the posi- tion of this plexus and the distribution of its branches, it seems highly probable that it is the local centre for regulating the calibre of the blood- vessels supplying the intestinal mucous membrane, and presiding over the processes of secretion and absorption. (4) The mucous membrane is the most important coat in relation to the function of digestion. The following structures, Avhich enter into its composition, may now be successively described; — the valvules conniventes; the villi; and the glands. The general structure of the mucous mem- brane of the intestines resembles that of the stomach (p. 241), and, like it, is lined on its inner surface by columnar epithelium. Adenoid tissue (Fig. 182, c and cl) enters largely into its construction; and on its deep surface is the muscularis mucoscB {m m, Fig. 183), the fibres of which are arranged in two layers: the outer longitudinal and the inner circular. Valvulae Conniventes. — The valvulce conniventes (Fig. 184) com- mence in the duodenum, about one or two inches beyond the pylorus, and becoming larger and more numerous immediately beyond the entrance of the bile duct, continue thickly arranged and well developed throughout 256 HAND-BOOK OF PIIYSIOLOGY. the jejunum; then, gradually dimmishing in size and number, they cease near the middle of the ileum. They are formed by a doubling inward of the mucous membrane; the crescentic, nearly circular, folds thus formed being arranged transversely to the axis of the intestine, and each indi- vidual fold seldom extending around more than J~ or | of the bowel's cir- cumference. Unlike the rugae in the oesophagus and stomach, they do not disappear on distension of the canal. Only an imperfect notion of their natural position and function can be obtained by looking at them after the intestine has been laid open in the usual manner. To under- Fig. 183. Fig. 184. Fig. 183.— Vertical section through portion of small intestine of dog. two villi showing e, epithe- lium; g, goblet cells. The free surface is seen to be formed by the "striated basilar border," while inside the villus the adenoid tissue and unstriped muscle-cells are seen; Z/, Lieberkiihn's folhcles: ?>i m, muscularis mucosae, sending up fibr-es between the follicles into the vilh; sm, submucous tissue; containing (gm), ganglion cells of Meissner's plexus. (Schofield.) Fig. 184.— Piece of small intestine (previously distended and hardened by alcohol) laid open to show the normal position of the valvulse conniventes. stand them aright, a piece of gut should be distended either Avith air or alcohol, and not opened until the tissues have become hardened. On then making a section it will be seen that, instead of disappearing, they stand out at right angles to the general surface of the mucous membrane (Fig. 184). Their functions arc probably less — Besides (1) offering a largely increased surface for secretion and absorption, they probably (2) prevent the too rapid passage of the very liquid products of gastric diges- tion, immediately after their escape from the stomach, and (3), by their projection, and consequent interference with a uniform and untroubled current of the intestinal contents, probably assist in the more perfect mingling of the latter witli the secretions })ourcd out to act on thorn. DIGESTION. 257 Glands of the Small Intestine.— The glands are of three princi- pal kinds: — viz., those of (1) Lieberkiihn, (2) Brunner, and (3) Peyer. (1.) The glands or crypts of Lieherkuhn are simple tubular depressions of the intestinal mucous membrane, thickly distributed over the whole sur- face both of the large and small intestines. In the small intestine they are visible only with the aid of a lens; and their orifices appear as minute dots scattered between the villi. They are larger in the large intestine, and increase in size the nearer they approach the anal end of the intes- tinal tube; and in the rectum their orifices may be visible to the naked eye. In length they vary from 3V to of a line. Each tubule (Fig. 186) is constructed of the same essential parts as the intestinal mucous membrane, viz., a fine membrana propria, or basement membrane, a « Fig. 185. Fig. 186. Fig. 185.— Transverse section through four crypts of Lieberkiihn from the large intestine of the pig. They are hned by columnar epithelial cells, the nuclei being placed in the outer part of the cells. The divisions between the cells are seen as lines radiating from L, the lumen of the crypt; G, epithehal cells, which have become transformed into goblet cells. X 350. (IQein and Noble Smith.) Fig. 186.— a gland of Lieberkiihn in longitudinal section. (Brinton.) layer of cylindrical epithelium lining it, and capillary blood-vessels cover- ing its exterior, the free surface of the columnar cells -presenting an appearance precisely similar to the '^striated basilar border" which covers the villi. Their contents appear to vary, even in health; the varieties being dependent, probably, on the period of time in relation to digestion at which they are examined. Among the columnar cells of Lieberkiihn's follicles, goblet-cells fre- quently occur (Fig. 185). (2.) Brunner' s glands (Fig. 188) are confined to the duodenum; the}^ are most abundant and thickly set at the commencement of this portion of the intestine, diminishing gradually as the duodenum advances. They are situated beneath the mucous membrane, and imbedded in the submu- cous tissue, each gland is a branched and convoluted tube, lined with columnar epithelium. As before said, in structure they are very similar to the pyloric glands of the stomach, and their epithelium undergoes a Vol. I.— 17. 258 HAND-BOOK OF PHYSIOLOGY. similar cluiiige during secretion; but they are more branched and convo- hited and their ducts are longer. (AVatney.) The duct of each gland passes through the muscularis mucosae, and opens on the surface of the mucous membrane. (3.) The glands of Peyer occur chiefly but not exclusively in the small intestine. They are found in greatest abundance in the lower part of the Fig. 187. Fig. 188. Fig. 187.— Transverse section of injected Peyer''s glands (from Kolliker). The drawing was taken from a preparation made byFrey: it represents the fine capillary -looped network spreading from the surroimding blood-vessels into the interior of three of Peyser's capsules from the intestine of the rabbit. Fig. 188.— Vertical section of duodenum, sho^^'ing a, viUi; &, crypts of Lieberkiihn, and c, Bi-uu- ner's glands in the submucosa s, with ducts, d: muscularis mucosae, m\ and circular muscular coat/. (Schofield.) ileum near to the ileo-cgecal valve. They are met with in two conditions, viz., either scattered singly, in which case they are termed glandular soli- taricB, or aggregated in groups varying from one to three inches in length and about half-an-inch in width, chiefly of an oval form, their long axis parallel with that of the intestine. In this state, they are named glandulw agminatcB, the groups being commonly called Peyefs patches (Fig. 189), and almost always placed opposite the attachment of the mesentery. In structure, and in function, there is no essential difference between the solitary glands and the individual bodies of which each §roup or patch is made up. They are really single or aggregated masses of adenoid tissue DIGESTION. 25'J forming lymph-follicles. In the condition in which they have been most commonly examined, each gland appears as a circular opaque-white rounded body, from to -^^ i^^^ diameter, according to the degree in which it is developed. They are principally contained in the submucous coat, but sometimes project through the musciilaris mucosce into the mucous membrane. In the agminate glands, each follicle reaches the free surface of the intestine, and is covered Avith columnar eioithelium. Each gland is surrounded by the openings of Lieberkuhn^s follicles. The adjacent glands of a Peyer s patch are connected together by ade- noid tissue. Sometimes the lymphoid tissue reaches the free surface, replacing the epithelium, as is also the case with some of the lymphoid follicles of the tonsil (p. 236). Peyer^s glands are surrounded by lymphatic sinuses which do not penetrate into their interior; the interior is, however, traversed by a very rich blood capillary plexus. If tlie vermiform appendix of a rabbit, which consists largely of Peyer's glands, be injected with blue, by pressing the , . ■ ....V Fig. 189.— Agminate follicles, or Peyer's patch, in a state of distension, x 5. (Boehm.) point of a fine syringe into one of the lymphatic sinuses, the Peyer's glands will appear as greyish white spaces surrounded by blue; if now the arteries of the same be injected with red, the greyish patches will change to red, thus proving that they are surrounded by lymphatic spaces, but penetrated by blood-vessels. The lacteals passing out of the villi commu- nicate with the lymph sinuses round Peyer^s glands. It is to be noted that they are largest and most prominent in children and young persons. Villi.— The Villi (Figs. 183, 188, 190, and 191), are confined exclu- sively to the mucous membrane of the small intestine. They are minute vascular processes, from a quarter of a line to a line and two-thirds in length, covering the surface of the mucous membrane, and giving it a peculiar velvety, fleecy appearance. Krause estimates them at fifty to ninety in number in a square line, at the upper part of the small intes- 260 HAND-BOOK OF PHYSIOLOGY. tine, and at forty to seventy in the same area at the lower part. They vary in form even in the same animal, and dilfer according as the lym- phatic vessels they contain are empty or fnll of chyle; being usually, in the former case, flat and pointed at their summits, in the latter cylindri- cal or cleavate. Each villus consists of a small projection of mucous membrane, and its interior is therefore supported throughout by fine adenoid tissue, which forms the framework or stroma in which the other constituents are con- tained. The surface of the villus is clothed by columnar epithelium, which rests on a fine basement membrane; while within this are found, reckon- ing from without inward, blood-vessels, fibres of the muscularis ^nucosce, and a single lymphatic or lacteal vessel rarely looped or branched (Fig. 192); besides granular matter, fat-globules, etc. Fig. 190. Fig. 191. Fig. 190.— Section of small intestine showing villi, Lieberkiihn's glands and a Peyer's solitary gland, m, m, muscularis mucosa. (Klein and Noble Smith.) Fig. 1»91. — Vertical section of a viUus of the small intestine of a cat. a, striated basilar border of the epithelium ; h. columnar epithelium ; c, goblet cells ; , central lymph-vessel ; e, smooth muscular fibres; /, adenoid stroma of the villus in which lymph corpuscles lie. (Klein.) The epithelium, is of the columnar kind, and continuous with that lining the other parts of the mucous membrane. The cells are arranged with their long axis radiating from the surface of the villus (Fig. 191), and their smaller ends resting on the basement membrane. The free surface of the epithelial cells of the villi, like that of the cells which cover the general surface of the mucous membrane, is covered by a fine border which exhibits very delicate striations, whence it derives its name, ''stria- ted basilar border." Beneath the basement or limiting membrane there is a ricli supply of blood-vessels. Two or more minute arteries are distributed within each villus; and from their capillaries, which form a dense network, proceed one or two small veins, which pass out at the base of the villus. The layer of the muscularis mucosm in the villus forms a kind of thin hollow cone immediately around the central lacteal, and is, therefore, DIGESTION. 261 situate beneath the blood-vessels. It is without doubt instrumental in the propulsion of chyle along the lacteal. The lacteal vessel enters the base of each villus, and passing up in the middle of it, extends nearly to the tip, where it ends commonly by a closed and somewhat dilated extremity. In the larger villi there may be two small lacteal vessels which end by a loop (Fig. 192), or the lacteals may form a kind of network in the villus. The last method of ending, however, is rarely or never seen in the human subject, although common in some of the lower animals (a. Fig. 192). Fig. 192.— a. Villus of sheep. B. Villi of man. (Slightly altered from Teichmann.) The office of the villi is the absorption of chyle and other liquids from the intestine. The mode in which they affect this will be considered in the Chapter on Absorption. II. The Large Intesitine. — The Large Intestine, which in an adult is from about 4 to 6 feet long, is subdivided for descriptive purposes into three portions (Fig. 165), viz. : — the cmcum, a short wide pouch, commu- nicating with the lower end of the small intestine through an opening, guarded by the ileo-cmcal valve; the colon, continuous with the caecum, which forms the principal part of the large intestine, and is divided into an ascending, transverse and descending portion: and the rectum, Avhich, after dilating at its lower part, again contracts, and immediately afterward 262 HAND-BOOK OF PHYSIOLOGY. opens externally through the emus. Attached to the csecum is the small appendix vermiformis. Structure. — Like tlie swall intestine, the large is constructed of four principal coats, viz., the serous, muscular, submucous, and mucous. The serous coat need not be here particularly described. Connected with it are the small processes of peritoneum, containing fat, called ap)pendices epiploiccB. The fibres of tlie muscular coat, like those of the small in- testine, are arranged in two layers — the outer longitudinal, the inner circu- lar. In the caecum and colon, the longitudinal fibres, besides being, as in the small intestine, thinly disposed in all parts of the wall of the bowel. Fig. 193.— Diagram of lacteal vessels in small intestine, a, lacteals in villi ; p, Payer's glands; b and D, superficial and deep network of lacteals in submucous tissue; l, Lieberkuhn's glands ; e, small branch of lacteal vessel on its way to mesenteric gland; h and o, muscular fibres of intestine; s, peri- toneum. (Teichmann.) are collected, for the most part, into three strong bands, which being shorter, from end to end, than the other coats of the intestine, hold the canal in folds, bounding intermedia,te sacculi. On the division of these bands, the intestine can be drawn out to its full length, and it then ns- sumes, of course, a uniformly cylindrical form. In tlie rectum, the fas- ci(;uli of these longitudinal bands spread out and mingle with the other longitudinal fibres, forming with them a thicker layer of libres than exists on any other part of the intestinal canal. The circular uniscular fibres are spread over the whole surface of tlie bowel, but are somewhat more DIGESTION. marked in the intervals between the sacculi. Toward the lower end of the rectum they become more numerous, and at the anus they form a strong band called the internal sphincter muscle. The mucous 'membrane of the large, like that of the small intestine, is lined throughout by columnar epithelium, but, unlike it, is quite smooth and destitute of villi, and is not projected in the form of valvules conni- ventes. Its general microscopic structure resembles that of the small in- testine: and it is bounded below by the muscularis mucosce. The general arrangement of ganglia and nerve-fibres in the large in- testine resembles that in the small (p. 255). Glands of the Large Intestine. — The glands with which the large intestine is provided are of two kinds, (1) the tubular and (2) the lymphoid. I a Fig. 194.— Horizontal section through a portion of the mucous membrane of the large intestine, showing Lieberkiihn''s glands in transverse section, a, lumen of gland— Uning of columnar cells with c, goblet cells, 6, supporting connective tissue. Highly magnified. (V. D. Harris.) (1.) The tubular glands, or glands of Lieberkiihn, resemble those of the small intestine, but are somewhat larger and more numerous. They are also more uniformly distributed. (2.) Follicles of adenoid or lymphoid tissue are most numerous in the caecum and vermiform appendix. They resemble in shape and structure^ almost exactly, the solitary glands of the small intestine. Peyer^s patches are not found in the large intestine. Ileo-Caecal Valve. — The ileo-caecal valve is situate at the place of junction of the small with the large intestine, and guards against any re- flex of the contents of the latter into the ileum. It is composed of two semilunar folds of mucous membrane* Each fold is formed by a doubling inward of the mucous membrane, and is strengthened on the outside by 264 HAND-BOOK OF PHYSIOLOGY. some of the circular muscular fibres of the intestine, which are contained between the cuter surfaces of the two layers of which each fold is composed. While the circular muscular fibres, however, of the bowel at the junction of the ileum witli the csecum are contained between the outer opposed surfaces of the folds of mucous membrane which form the valve, the longitudinal muscular fibres and the peritoneum of the small and large intestine respectively are continuous with each other, without dipping in to follow the circular fibres and the mucous membrane. In this man- ner, therefore, the folding inward of these two last-named structures is preserved, while, on the other hand, by dividing the longitudinal muscu- lar fibres and the peritoneum, the valve can be made to disappear, just as the constrictions between the sacculi of the large intestine can be made to disappear by performing a similar operation. The inner surface of the folds is smooth; the mucous membrane of the ileum being con- tinuous with that of the caecum. That surface of each fold which looks toward the small intestine is covered with villi, while that which looks to the caecum has none. "When the caecum is distended, the margin of the folds are stretched, and thus are brought into firm apposition one with the other. DlGESTIOl^" 11^" THE InTESTIHTES. After the food has been duly acted upon by the stomach, such as has not been absorbed passes into the duodenum, and is there subjected to the action of the secretions of the pancreas and liver, which enter that portion of the small intestine. Before considering the changes which the food undergoes in consequence, attention should be directed to the structure and secretion of these glands, and to the secretion (succus en- tericus) w^iich is poured out into the intestines from the glands lining them. The Pajtckeas, akd its Secketion". The Pancreas is situated within the curve formed by the duodenum; and its main duct opens into that part of the small intestine, through a small opening, or through a duct common to it and to the liver, about two and a half inches from the pylorus. Structure. — In structure the pancreas bears some resemblance to the salivary glands. Its capsule and septa, as well as the blood-vessels and lymphatics, are similarly distributed. It is, however, looser and softer, the lobes and lobules being less compactly arranged. The main duct divides into branches (lobar ducts), one for each lobe, and these branches subdivide into intralobular ducts, and these again by their division and branching form the gland tissue proper. The intralobular ducts corre- DIGESTION. 265 spond to a lobule, while between them and the secreting tubes or alveoli are longer or shorter intermediary ducts. The larger ducts possess a very distinct lumen and a membrana propria lined with columnar epi- thelium, the cells of which are longitudinally striated, but are shorter than those found in the ducts of the salivary glands. In the intralobular ducts the epithelium is short and the lumen is smaller. The intermediary ducts opening into the alveoli possess a distinct lumen, with a membrana propria lined with a single layer of flattened elongated cells. The alveoli are branched and convoluted tubes, with a membrana propria lined with a single layer of columnar cells. They have no distinct lumen, its place being taken by fusiform or branched cells. Heidenhain has observed that the alveoli cells in the pancreas of a fasting dog consist of two zones, an inner or central zone, which is finely granular, and which stains feebly. Fig. 195. — Section of the pancreas of a dog during digestion, a, alveoli lined with ceUs, the outer zone of which is well stained with haematoxyUn ; d, intermediary duct lined with squamous epithehum. X 350. (Klein and Noble Smith.) and a smaller parietal zone of finely striated protoplasm, which stains easily. The nucleus is partly in one, partly in the other zone. During digestion, it is found that the outer zone increases in size, and the central zone diminishes; the cell itself becoming smaller from the discharge of the secretion. At the end of digestion the first condition again appears, the inner zone enlarging at the expense of the outer. It appears that the granules are formed by the protoplasm of the cells, from material supplied to it by the blood. The granules are thought to be not the ferment itself, but material from which, under certain conditions, the ferments of the gland are made, and therefore called Zymogen. Pancreatic Secretion. — The secretion of the pancreas has been ob- tained for purposes of experiment from the lower animals, especially the dog, by opening the abdomen and exposing the duct of the gland, which is then made to communicate with the exterior. A pancreatic fistula is thus established. 266 HAND-BOOK OF PHYSIOLOGY. An extract of pancreas made from the gland, which has been removed from an animal killed during digestion, possesses the active properties of pancreatic secretion; It is made by first dehj^drating the gland, which has been cut up into small pieces, by keeping it for some days in absolute alcohol, and then, after the entire removal of the alcohol, placing it in strong glycerin. A glycerin extract is thus obtained. It is a remarkable fact, however, that the amount of the ferment tri/psin greatly increases if the gland be exposed to the air for twenty-four hours before placing in alcohol; indeed, a glycerin extract made from the gland immediately upon removal from the body often appears to contain none of that fer- ment. This seems to indicate that the conversion of zymogen in the gland into the ferment only takes place during the act of secretion, and that the gland, although it always contains in its cells the materials (tryp- sinogen) out of which trypsin is formed, yet the conversion of the One into the other only takes place by degrees. Dilute acid appears to assist and accelerate the conversion, and if a recent pancreas be rubbed up with dilute acid before dehydration, a glycerin extract made afterward, even though the gland may have been only recently removed from the body, is very active. Properties. — Pancreatic juice is colorless, transparent, and slightly viscid, alkaline in reaction. It varies in specific gravity from 1010 to 1015, according to whether 'it is obtained from, a permanent fistula — then more watery — or from a newly-opened duct. The solids vary in a tempo- rary fistula from 80 to 100 parts per thousand, and in a permanent one from 16 to 50 per thousand. Chemical Composition op the Pancreatic Secretion. From a permanent fistula. (Bernstein.) Water .975 Solids — Ferments : Proteids, including Serum — Albumin, Casein, ) ^ Leucin and Tyrosin, Fats and Soaps . ) Inorganic residue, especially Sodium Carbonate . 8 25 1000 Funcfinnfi. — (1.) It converts ^??*o?'^/V/.-a-lobular vein; 2, its smaller branches collecting blood from the capillary network; 3, infe?--lobular branches of the vena portae with their smaller ramifications passing inward toward the capillary network in the substance of the lobule, x 60. (Sappey.) Fia. 200.— Section of a portion of liver passing longitudinally through a considerable hepatic vein, from the pig. h, hepatic venous trunk, jigaiiist which the sides of the lobules (/) are applied: h, /i. /i, sublobular hepatic veins, ou which tlie bases of the lobules rest, and through the coats of which they are seen as polygoual figtuvs; /, mouth of the intralobular veins, ojiening into the sublobular veins; i' , intralobular veins shown passmg up the centre of some divideil lobules; /, /, cut surface of the liver; c, c, walls of the hepatic venous canal, formed by the polygonal bases or the lobules. X 5. (Kiernan.) tlie main branches of the hepatic veins, which leave the ])osterior border of the liver to end by two or tlireo principal ti-imks in the interior vena DIGESTIOl^T. 271 cava, just before its passage through the diaphragm. The sub-lohuVdr and hepatic veins, unlike the portal vein and its companions, have little or no areolar tissue around them, and their coats being very thin, they form little more than mere channels in the liver substance which closely surrounds them. The manner in which the lobules are connected with the sul-lohular veins by means of the small intra-lobidar veins is well seen in the diagram (Fig. 200 and in Fig. 201), which represent the parts as seen in a longitudinal section. The appearance has been likened to a twig having leaves without footstalks — the lobules representing the leaves, and the sub-lobular vein the small branch from which it springs. On a transverse section, the appearance of the intra-lohular veins is that of 1, Fig. 199, while both a transverse and longitudinal sec- tion are exhibited in Fig. 176. The hepatic artery, the function of which is to distribute blood for nutrition to Glisson^s capsule, the walls of the ducts and blood- vessels, and other parts of the liver, is distrib- uted in a very similar manner to the portal vein, its blood being returned by small branches either into the rami- fications of the portal vein, or into the capillary plexus of the lobules which connects the inter and intra lobular veins. Irobuiss £obul( Fig Diagram showing the manner in which the lobules of the liver rest on the sublobiilar veins. (After Kiernan.) Fig. 202.— Capillary network of the lobules of the rabbit's liver. The figure is taken from a very- successful injection of the hepatic veins, made by Harting: it shows nearly the whole of two lobules, and parts of three others; jo, portal branches running in the interlobular spaces; h, hepatic veins pen- etrating and radiating from the centre of the lobules. X 45. (Kolliker.) The hepatic duct divides and subdivides in a manner very like that of the portal vein and hepatic artery, the larger branches being lined by cylindrical, and the smaller by ^md,]]. polygonal epithelium. 272 HAND-BOOK OF PHYSIOLOGY. The bile-capillaries commence between the hepatic cells, and are bounded by a delicate membranous wall of their own. They appear to be always bounded by hepatic cells on all sides, and are thus separated from the nearest blood-capillary by at least the breadth of one cell (Figs. 203 and 204). The Gail-Bladder.— The Gall-bladder (g, B, Fig. 196) is a pyriform bag, attached to the under surface of the liver, and supported also by the peritoneum, which passes below it. The larger end or fundus, projects beyond the front margin of the liver; while the smaller end contracts into the cystic duct. Structure. — The walls of the gall-bladder are constructed of three principal coats. (1) Externally (excepting that part which is in contact with the liver), is the serous coat, which has the same structure as the peritoneum with which it is continuous. Within this is (2) the fibrous or areolar coat, constructed of tough fibrous and elastic tissue, with which is mingled a considerable number of plain muscu- lar fibres, both longitudinal and circular. (3) .Internally the gall-bladder is lined by mucous membrane, and a layer of columnar epithelium. The surface of the mucous membrane presents to the naked eye a minutely honeycombed appearance from a number of tiny polygonal depressions with intervening ridges, by which its surface is mapped out. In the cystic Fig. 203.— Portion of a lobule of liyer. o, bile capillaries between liver-cells, the network in which is weU seen; 6, blood capillaries. X 350. (Klein and Noble Smith.) Fig. 204.— Hepatic cells and bile capillaries, from the liver of a child three months old. Both fig- ures i-cjiresent fraf^iuciits of a section carried throufih the periphery of a lobule. The red corpuscles of the blood aic rcc-oKiii/.cd by their circular c-ontom-; vp, corresponds to an interlobular vein in im- mediate proximity with which are the epithelial cells of I lie biliary ducts, to which, at the lower v>art of the figures, the nmch larger hepatic cells suddenly succeed. (,E. Hering.) duct the mucous membrane is raised up in the form of crescentic folds, which together appear like a spiral valve, and which minister to tho function of tho gall-bladder in retaining the bile during the intervals of digestion. DIGESTION. 273 The gall-bladder and all the main biliary ducts are provided " with mucous glands, which open on their internal surface. Functions of the Liver. — The functions of the Liver may be classified under the following heads: — 1. The Secretion of Bile. 2. The Elaboration of Blood; under this head may be included the Glycogenic Function. 1. The Secretioj^" of Bile. Properties of the Bile. — The bile is a somewhat viscid fluid, of a yellow or reddish-yellow color, a strongly bitter taste, and, when fresh, with a scarcely perceptible odor: it has a neutral or slightly alkaline reac- tion, and its specific gravity is about 1020. Its color and degree of con- sistence vary much, apparently independent of disease; but, as a rule, it becomes gradually more deeply colored and thicker as it advances along its ducts, or when it remains long in the gall-bladder, wherein, at the same time, it becomes more viscid and ropy, of a darker color, and more bitter taste, mainly from its greater degree of concentration, on account of partial absorption of its water, but partly also from being mixed with mucus. Chemical Composition of Human Bile. (Frerichs.) Water 859-2 Solids 140-8 1000-0 Bile salts or Bilin .91*5 Fat 9-2 Cholesterin 2.6 Mucus and coloring matters 29.8 Salts 7-7 140-8 Bile salts, or Bilin, can be obtained as colorless, exceedingly deliques- cent crystals, soluble in water, alcohol, and alkaline solutions, giving to the watery solution the taste and general characters of bile. They consist of sodium salts of glycocholic and taurocholic acids. The former salt is composed of cholic acid conjugated with glycin (see Appendix), the latter of the same acid conjugated with taurin. The proportion of these two salts in the bile of different animals varies, e.g., in ox bile the glycocho- late is in great excess, whereas the bile of the dog, cat, bear, and other carnivora contains taurocholate alone; in human bile both are present in about the same amount (glycocholate in excess?). Preparation of Bile Salt.— Bile salts may be prepared in the fol- VoL. I.— 18. 274 HAND-BOOK OF PHYSIOLOGY. lowing manner: mix bile which has been evaporated to a quarter of its bulk with animal charcoal, and evaporate to perfect drj-ness in a water bath. Next extract the mass whilst still warm with absolute alcohol. Separate the alcoholic extract by filtration, and to it add perfectly anhy- drous ether as long as a precipitate is thrown down. The solution and precipitate should be set aside in a closely stoppered bottle for some days, when crystals of the bile salts or bilin will have separated out. The giy- cocholate may be separated from the taurocholate by dissolving bilin in water, and adding to it a solution of neutral lead acetate, and then a little basic lead acetate, when lead glycocholate separates out. Filter and add to the filtrate lead acetate and ammonia, a precipitate of lead taurocho- late will be formed, which may be filtered off. In both cases, the lead may be got rid of by suspending or dissolving in hot alcohol, adding hydrogen sulphate, filtering and allowing the acids to separate out by the addition of water. The test for bile salts is known as Pettenkofer's. If to an aqueous solution of the salts strong sulphuric acid be added, the bile acids are first of all precipitated, but on the further addition of the acid are re-dissolved. If to the solution a drop of solution of cane sugar be added, a fine purple color is developed. The re-action will also occur on the addition of grape or fruit sugar instead of cane sugar, slowly with the first, quickly with the last; and a color similar to the above is produced by the action of sulphuric acid and sugar on albumen, the crystalline lens, nerve tissue, oleic acid, pure ether, cholesterin, morphia, codeia and amylic alcohol. The spectrum of Pettenkofer's reaction, when the fluid is moderately diluted, shows four bands — the most marked and largest at E, and a little to the left; another at F; a third between D and E, nearer to D; and the fourth near D. The yellow coloring matter of the bile of man and the Carnivora is termed BiliruMn or Bilifulvin (CjgHjgj^^Og) crystallizable and insoluble in Avater, soluble in chloroform or carbon disulphate; a green coloring matter, Biliverdin (Cj^h^qX^oJ, which always exists in large amount in the bile of Herbivora, being formed from bilirubin on exposure to the air, or by sub- jecting the bile to any other oxidizing agency, as by adding nitric acid. When the bile has been long in the gall-bladder, a third pigment, Bih'jrra- sin, may be also found in small amount. In cases of biliary obstruction, the coloring matter of the bile is re- absorbed, and circulates with the blood, giving to the tissues the yellow tint characteristic of jaundice. 'i'he coloring matters of human bile do not appear to give characteristic absorption spectra; but the bile of the guinea pig, rabbit, mouse, sheep, ox, and crow do so, the most constant of which appears to be a band at DIGESTIOIT. 275 F. The bile of the sheep and ox give three bands in a thick layer, and four or five bands with a thinner layer, one on each side of D, one near E, and a faint line at F. (McMunn.) There seems to be a close relationship between the color-matter of the blood and of the bile, and it may be added, between these and that of the urine (urobilin), and of the faeces (stercobilin) also; it is probable they are, all of them, varieties of the same pigment, or derived from the same source. Indeed it is maintained that UroUUn is identical with Hydro- UliruUn, a substance which is obtained from bilirubin by the action of sodium amalgam, or by the action of sodium amalgam on alkaline hsema- tin; both urobilin and hydrobilirubin giving a characteristic absorption band between b and F. They are also identical with stercobilin, which is formed in the alimentary canal from bile pigments. A common test (Gmelin's) for the presence of bile-pigment consists of the addition of a small quantity of nitric acid, yellow with nitrous acid; if bile be present, a play of colors is produced, beginning with green and passing through blue and violet to red, and lastly to yellow. The spec- trum of Gmelin's test gives a black ^5and extending from near b to beyond F. Fatty substances are found in variable proportions in the bile. Besides the ordinary saponifiable fats, there is a small quantity of Cholesterin, a so-called non-saponifiable fat, which, with the other free fats, is prob- ably held in solution by the bile salts. It is a body belonging to the class of mon- atomic alcohols (c^eH^^o), and crystallizes in rhombic plates (Fig. 205). It is insoluble in water and cold alcohol, but dissolves easily ^^^--^gSterin^ in boiling alcohol or ether. It gives a red color with strong sulphuric acid, and with nitric acid and ammonia; also a play of colors beginning with blood red and ending with green on the addition of sulphuric acid and chloroform. Lecitliin (c^^HgoNPOj, a phosphorus-containing body and Neurin (c^Hj^koJ, are also found in bile, the latter probably as a decomposition product of the former. The Mucus in bile is derived from the mucous membrane and glands of the gall-bladder, and of the hepatic ducts. It constitutes the residue after bile is treated with alcohol. The epithelium with which it is mixed may be detected in the bile with the microscope in the form of cylindrical cells, either scattered or still held together in layers. To the presence of the mucus is probably to be ascribed the rapid decomposition undergone by the bilin; for, according to Berzelius, if the mucus be separated, bile w\\\ remain unchanged for many days. The Saline or inorganic constituents of the bile are similar to those 276 HAND-BOOK OF PHYSIOLOGY. found in most other secreted fluids. It is possible that the carbonate and neutral phosphate of sodium and potassium, found in the ashes of bile, are formed in the incineration, and do not exist as such in the fluid. Oxide of iron is said to be a common constituent of the ashes of bile, and copper is generally found in healthy bile, and constantly in biliary calculi. Gas — A certain small amount of carbonic acid, oxygen, and nitrogen, may be extracted from bile. Mode of Secretion and Discharge. — The process of secreting bile is continually going on, but appears to be retarded during fasting, and accelerated on taking food. This has been shown by tying the common bile-duct of a dog, and establishing a fistulous opening between the skin and gall-bladder, whereby all the bile secreted was discharged at the sur- face. It was noticed that when the animal was fasting, sometimes not a drop of bile was discharged for several hours; but that, in about ten min- utes after the introduction of food into the stomach, the bile began to- flow abundantly, and continued to do so during the whole period of diges- tion. (Blondlot, Bidder and Schmidt.) The bile is formed in the hepatic cells; then, being discharged into- the minute hepatic ducts, it passes into the larger trunks, and from the main hepatic duct may be carried at once into the duodenum. But, prob- ably, this happens only while digestion is going on; during fasting, it regurgitates from the common bile-duct through the cystic duct, into the gall-bladder, where it accumulates till, in the next period of digestion, it is discharged into the intestine. The gall-bladder thus fulfils what ap- pears to be its chief or only office, that of a reservoir; for its presence enables bile to be constantly secreted, yet insures its employment in the service of digestion, although digestion is periodic, and the secretion of bile constant. The mechanism by which the bile passes into the gall-bladder is sim- ple. The orifice through which the common bile-duct communicates with the duodenum is narrower than the duct, and appears to be closed, except when there is sufficient pressure behind to force the bile through it. The pressure exercised upon the bile secreted during the intervals of digestion appears insufficient to overcome the force with which the ori- fice of the duct is closed; and the bile in the common duct, finding no exit in the intestine, traverses the cystic duct, and so passes into the gall- bladder, being probably aided in this retrograde course by the peristaltia action of the ducts. The bile is discharged from the gall-bladder and enters the duodenum on the introduction of food into the small intestine: being pressed on by the contraction of the coats of the gall-bladder, and of the common bile-duct also; for both these organs contain unstriped muscular fibre-cells. Their contraction is excited by the stimulus of the food in the duodenum acting so as to produce a reflex movement, the force of which is sufficient to open the orifice of the common bile-duct. DIGESTION. 277 Bile, as such, is not pre-formed in the blood. As just observed, it is formed by the hepatic cells, although some of the material may be brought to them almost in the condition for immediate secretion. When it is, however, prevented by an obstruction of some kind, from escaping into the intestine (as by the passage of a gall-stone along the hepatic duct) it is absorbed in great excess into the blood, and, circulating with it, gives rise to the well-known phenomena of jaundice. This is explained by the fact that the pressure of secretion in the ducts is normally very low, and if it exceeds f inch of mercury (16 mm.) the secretion ceases to be poured out, and if the opposing force be increased, the bile finds its way into the blood. Quantity. — Various estimates have been made of the quantity of bile discharged into the intestines in twenty-four hours: the quantity doubtless varying, like that of the gastric fluid, in proportion to the amount of food taken. A fair average of several computations would give 20 to 40 oz. (600 — 900 cc.) as the quantity daily secreted by man. Uses. — (1) As an excrementitious substance, the bile may serve especially as a medium for the separation of excess of carbon and hydrogen from the blood; and its adaptation to this purpose is well illustrated by the peculiarities attending its secretion and disposal in the foetus. During intra-uterine life, the lungs and the intestinal canal are almost inactive; there is no respiration of open air or digestion of food; these are unneces- sary, on account of the supply of well elaborated nutriment received by the vessels of the foetus at the placenta. The liver, during the same time, is proportionately larger than it is after birth, and the secretion of bile is active, although there is no food in the intestinal canal upon which it can exercise any digestive property. At birth, the intestinal canal is full of thick bile, mixed with intestinal secretion; the meconium, or fasces of the foetus, containing all the essential principles of bile. Composition of Meconium (Frerichs) : Biliary resin 15.6 Common fat and cholesterin . . . ,15.4 Epithelium, mucus, pigment, and salts . .69.0 100.0 In the foetus, therefore, the main purpose of the secretion of bile must be the purification of blood by direct excretion, i.e., by separation from the blood, and ejection from the body without further change. Probably all the bile secreted in foetal life is incorporated in the meconium, and with it discharged, and thus the liver may be said to discharge a function in some sense vicarious of that of the lungs. For, in the foetus, nearly all the blood coming from the placenta passes through the liver, previous to its distribution to the several organs of the body; and the abstraction of 278 HAND-BOOK OF PHYSIOLOGY. carbon, hydrogen, and other elements of bile will purify it, as in extra- uterine life it is purified by the separation of carbonic acid and water at the Inngs. The evident disposal of the foetal bile by excretion, makes it highly probable that the bile in extra-uterine life is also, at least in part, destined to be discharged as excrementitious. The analysis of the faeces of both children and adults shows that (except when rapidly discharged in pur- gation) they contain very little of the bile secreted, probably not more than one-sixteenth part of its weight, and that this portion includes chiefly its coloring, and some of its fatty matters, and to only a very* sliglit degree, its salts, almost all of which have been re-absorbed from the intestines into the blood. The elementary composition of bile salts shows, however, such a pre- ponderance of carbon and hydrogen, that probably, after absorption, it combines with oxygen, and is excreted in the form of carbonic acid and water. The change after birth, from the direct to the indirect mode of excretion of the bile, may, with much probability, be connected with a purpose in relation to the development of heat. The temperature of the foetus is maintained by that of the parent, and needs no source of heat within itself; but, in extra-uterine life, there is (as one may say) a waste of material for heat when any excretion is discharged unoxidized; the carbon and hydrogen of the bilin, therefore, instead of being ejected in the faeces, are re-absorbed, in order that they may be combined with oxygen, and that in the combination heat may be generated. A substance, which has been discovered in the faeces, and named ster- corin is closely allied to cholesterin; and it has been suggested that while one great function of the liver is to excrete cholesterin from the blood, as the kidney excretes urea, the stercorin of f^ces is the modified form in which cholesterin finally leaves the body. Ten grains and a half of ster- corin are excreted daily (A. Flint). From the peculiar manner in which the liver is supplied with much of the blood that flows through it, it is probable that this organ is excre- tory, not only for such hydro-carbonaceous matters as may need expulsion from any portion of the blood, but that it serves for the direct purification of the stream which, arriving by the portal vein, has just gathered up various substances in its course through the digestive organs — substances which may need to be expelled, almost immediately after their absorption. For it is. easily conceivable that many things may be taken up during digestion, which not only are unfit for purposes of nutrition, but which would be positively injurious if allowed to mingle with the general mass of the blood. The liver, therefore, may be supposed placed in the only road by which such matters can pass unchanged into the general current, jealously to guard against their further i)rogress, and turn them back again into an excretory channel. The frequency with which metallic DIGESTIOJS^. 279 poisons are either excreted by the liver, or intercepted and retained, often for a considerable time, in its own substance, may be adduced as evidence for the probable truth of this supposition. (2). As a digestive fluid. — Though one chief purpose of the secretion of bile may thus appear to be the purification of the blood by ultimate excretion, yet thgre are many reasons for believing that, while it is in the intestines, it performs an important part in the process of digestion. In nearly all animals, for example, the bile is discharged, not through an excretory duct communicating with the external surface or with a simple reservoir, as most excretions are, but is made to pass into the intestinal canal, so as to be mingled with the chyme directly after it leaves the stomach; an arrangement, the constancy of which clearly indicates that the bile has some important relations to the food with which it is thus mixed. A similar indication is furnished also by the fact that the secre- tion of bile is most active, and the quantity discharged into the intestines much greater, during digestion than at any other time; although, with- out doubt, this activity of secretion during digestion may, however, be in part ascribed to the fact that a greater quantity of blood is sent through the portal vein to the liver at this time, and that this blood contains some of the materials of the food absorbed from the stomach and intestines, which may need to be excreted, either temporarily (to be afterward reab- sorbed) or permanently. Eespecting the functions discharged by the bile in digestion there is little doubt that it, (a.) assists in emulsifying the fatty portions of the food, and thus rendering them capable of being absorbed by the lacteals. For it has appeared in some experiments in which the common bile-duct was tied, that, although the process of digestion in the stomach was un- affected, chyle was no longer well formed; the contents of the lacteals consisting of clear, colorless fluid, instead of being opaque and white, as they ordinarily are, after feeding. {b.) It is probable, also, that the moistening of the mucous memhrane of the intestines by bile facilitates absorption of fatty matters through it. {c.) The bile, like the gastric fluid, has a considerable antiseptic power, and may serve to prevent the decomposition of food during the time of its sojourn in the intestines. Experiments show that the con- tents of the intestines are much more foetid after the common bile-duct has been tied than at other times; moreover, it is found that the mixture of bile with a fermenting fluid stops or spoils the process of fermentation. {d.) The bile has also been considered to act as a natural purgative, by promoting an increased secretion of the intestinal glands, and by stimulating the intestines to the propulsion of their contents. This view receives support from the constipation which ordinarily exists in jaundice, from the diarrhoea which accompanies excessive secretion of bile, and from the purgative properties of ox-gall. 280 HAND-BOOK OF PHYSIOLOGY. (e.) The bile appears to have the power of 2^'>^ecipitating the gastric parapeptones and peptones, together ^uith the pepsin which is mixed up with them, as soon as the contents of the stomach meet it in the duo- denum. The purpose of this operation is probably both to delay any change in the parapeptones until the pancreatic juice can act upon them, and also to prevent the pepsin from exercising its solvent action on the ferments of the pancreatic juice. • Nothing is known with certainty respecting the changes which the re- absorbed portions of the bile undergo. That they are much changed appears from the impossibility of detecting them in the blood; and that part of this change is effected in the liver is probable from an experiment of Magendie, who found that when he injected bile into the portal vein, a dog was unharmed, but was killed when he injected the bile into one of the systemic vessels. II. The Liter as a BLOOD-ELABOKATiis-G Gla^s"©. The secretion of bile, as already observed, is only one of the purposes fulfilled by the liver. Another very important function appears to be that of so acting upon certain constituents of the blood passing through it, as to render some of them capable of assimilation with the blood gen- erally, and to prepare others for being duly eliminated in the process of respiration. It appears that the peptones, conveyed from the alimentary canal by the blood of the portal vein, require to be submitted to the influ- ence of the liver before they can be assimilated by the blood; for if such albumi]ious matter is injected into the jugular vein, it speedily appears in the urine; but if introduced into the portal vein, and thus allowed to traverse the liver, it is no longer ejected as a foreign substance, but is incorporated with the albuminous pai-t of the blood. Albuminous mat- ters are also subject to decomposition by the liver in another way to be immediately noticed (p. 281). The formation of urea by the liver will be again referred to (p. 371). Glycogenic Function. — One of the chief uses of the liver in connec- tion with elaboration of the blood is comprised in what is known as its glycogenic function. The important fact that the liver normally forms glucose or grape sugar, or a substance readily convertible into it, was dis- covered by Claude Bernard in the course of some experiments which he undertook for the purpose of finding out in what part of the circulatory system the saccharine matter disappeared, which was absorbed from the alimentary canal. With this purpose he fed a dog for seven days with food containing a large quantity of sugar and starch; and, as might be expect found sugar in both the portal and hepatic veins. He then fed :i (log with meat only, and, to his surprise, still found sugar in the DIGESTION. 281 hepatic veins. Eepeated experiments gave invariably the same result; no sugar being found, under a meat diet, in the portal vein, if care were taken, by applying a ligature on it at the transverse fissure, to prevent reflux of blood from the hepatic venous system. Bernard found sugar also in the substance of the liver. It thus seemed certain that the liver formed sugar, even when, from the absence of saccharine and amyloid matters in the food, none could be brought directly to it from the stomach or intestines. Excepting cases in which large quantities of starch and sugar were taken as food, no sugar was found in the blood after it had passed through the lungs; the sugar formed by the liver, having presumably disa^^peared by combustion, in the course of the pulmonary circulation. Bernard found, subsequently to the before-mentioned experiments, that a liver, removed from the body, and from which all sugar had been completely washed away by injecting a stream of water through its blood- vessels, will be found, after the lapse of a few hours, to contain sugar in abundance. This post-mortem production of sugar was a fact which could only be explained in the supposition that the liver contained a substance, readily convertible into sugar in the course merely of post-mortem decom- position; and this theory was proved correct by the discovery of a sub- stance in the liver allied to starch, and now generally termed glycogen. We may believe, therefore, that the liver does not form sugar directly from the materials brought to it by the blood, but that glycogen is first formed and stored in its substance; and that the sugar, when present, is the result of the transformation of the latter. Quantity of Glycogen formed. — Although, as before mentioned, glyco- gen is produced by the liver when neither starch nor sugar is present in the food, its amount is much less under such a diet. Average amount of Glycogen in the Liver of Dogs tinder various Diets. (Pavy.) Diet. . Amount of Glycogen in Liver. Animal food 7*19 per cent. Animal food with sugar (about \ lb. of sugar daily) 14*5 Vegetable diet (potatoes, with bread or barley-meal) 17-23 The dependence of the formation of glycogen on the food taken is also well shown by the following results, obtained by the same experimenter: Average quantity of Glycogen found in the Liver of RaMits after Fasting and after a diet of Starch and Sugar respectively. Average amount of Glycogen in Liver. After fasting for three days .... Practically absent. " diet of starch and grape-sugar . . .15*4 per cent. " " cane-sugar 16-9 " 282 HAND-BOOK OF PHYSIOLOGY. Regardiug these facts there is no dispute. . All are agreed that glyco- gen is formed, and laid up in store, temporarily, by the liver-cells; and that it is not formed exclusively from saccharine and amylaceous foods, but from albuminous substances also; the albumen, in the latter case, being probably split up into glycogen, which is temporarily stored in the liver, and urea, which is excreted by the kidneys. Destination of Glycogen. — There are two chief theories on the sub- ject of the destination of glycogen. (1.) That the conversion of glycogen into sugar takes place rapidly during life by the agency of a ferment also formed in the liver: and the sugar is conveyed away by the blood of the hepatic veins, and soon undergoes combustion. (2.) That the conver- sion into sugar only occurs after death, and that during life no sugar exists in healthy livers; glycogen not undergoing this transformation. The chief arguments advanced in support of this view are, (a) that scarcely a trace of sugar is found in blood drawn during life from the right ventricle, or in blood collected from the right side of the heart im- 7necliateJy after an animal has been killed; while if the examination be delayed for a very short time after death, sugar in abundance may be found in such blood; (&), that the liver, like the venous blood in the heart, is, at the moment of death, completely free from sugar, although afterward its tissue speedily becomes saccharine, unless the formation of sugar be prevented by freezing, boiling, or other means calculated to in- terfere with the action of a ferment on the amyloid substance of the organ. Instead of adopting Bernard^s view, that normally, during life, glycogen passes as sugar into the hepatic venous blood, and thereby is conveyed to the lungs to be further disposed of, Pavy inclines to the belief that it may represent an intermediate stage in the formation of fat from materials absorbed from the alimentary canal. Liver-sugar and Glycogen. — To demonstrate the presence of sugar in the liver, a portion of this organ, after being cut into small pieces, is bruised in a mortar to a pulp with a small quantity of water, and the pulp is boiled with sodium-sulphate in order to precipitate albuminous and coloring matters. The decoction is then filtered and may be tested for glucose (p. 230). Glycogen (CgHj^o.) is an amorphous, starch-like substance, odorless and tasteless, soluble in water, insoluble in alcohol. It is converted into glu- cose by boiling with dilute acids, or by contact with any animal ferment. It may be obtained by taking a portion of liver from a recently killed rabbit, and, after cutting it into small pieces, placing it for a short time in boiling water. It is then bruised in a mortar, until it forms a pulpy mass, and subsequently boiled in distilled water for about a quarter of an hour. The glycogen is precipitated from the filtered decoction by the addition of alcohol. Glycogen has been found in many other structures than the liver. (See Ai)pendix.) DIGESTION. 283 Glycosuria. — The facility with which the glycogen of the liver is transformed into sugar would lead to the expectation that this chemical change, under many circumstances, would occur to such an extent that sugar would be present not only in the hepatic veins, but in the blood generally. Such is frequently the case; the sugar when in excess in the blood being secreted by the kidneys, and thus appearing in variable quan- tities in the urine (Glycosuria). Influence of the Nervous System in producing Glycosuria. — Glycosuria may be experimentally produced by puncture of the medulla oblongata in the region of the vaso-motor centre. The better fed the animal the larger is the amount of sugar found in the urine; whereas in the case of a starving animal no sugar appears. It is, therefore, highly probable that the sugar comes from the hepatic glycogen, since in the one case glycogen is in excess, and in the other it is almost absent. The nature of the influence is uncertain. It may be exercised in dilating the hepatic vessels, or possibly on the liver cells themselves. The whole course of the nervous stimulus cannot be traced to the liver, but at first it passes from the medulla down the spinal cord as far as — in rabbits — the fourth dorsal vertebra, and thence to the first thoracic ganglion. Many other circumstances will cause glycosuria. It has been observed after the administration of various drugs, after the injection of urari, poisoning with carbonic oxide gas, the inhalation of ether, chloroform, etc., the injection of oxygenated blood into the portal venous system. It has been observed in man after injuries to the head, and in the course of various diseases. The well-known disease, tUahetus melUtus, in which a large quantity of sugar is persistently secreted daily with the urine, has, doubtless, some close relation to the normal glycogenic function of the li\^er; but the nature of the relationship is at present quite unknown. The Intestinal Secretion, or Succus Entericus. — On account of the difiiculty in isolating the secretion of the glands in the wall of the intestine (Brunner^s and Lieberkiihn's) from other secretions poured into the canal (gastric juice, bile, and pancreatic secretion), but little is known regarding the composition of the former fluid (intestinal juice, succus en- tericus). It is said to be a yellowish . alkaline fluid with a specific gravity of 1011, and to contain about 2*5 per cent, of solid matters (Thiry). Functions. — The secretion of Brunner's glands is said to be able to convert proteids into peptones, and that of Lieberkiihn^s is believed to convert starch into sugar. To these functions of the succus entericus the powers of converting cane into grape sugar, and of turning cane sugar into lactic, and afterward into butyric acid, are added by some physiologists. It also probably contains a milk-curdling ferment (W. Roberts). 284 HAND-BOOK OF PHYSIOLOGY. The reaction which represents tlie conversion of cane sugar into grape sugar may be represented thus: — 8C„H„0„ + 2H,0 = C„H„0., + C„ H,, 0„ Saccharose Water Dextrose Laevulose The conversion is probably effected by means of a hydrolytic ferment. (Inversive ferment, Bernard.) The length and complexity of the digestive tract seem to be closely connected with the character of the food on which an animal lives. Thus, in all carnivorous animals, such as the cat and dog, and pre-eminently in carnivorous birds, as hawks and herons, it is exceedingly short. The seals, which, though carnivorous, possess a very long intestine, appear to furnish an exception; but this is doubtless to be explained as an adaptation to their aquatic habits: their constant exposure to cold requiring that they should absorb as much as possible from their intestines. Herbivorous animals, on the other hand, and the ruminants especially, have very long intestines (in the sheep 30 times the length of the body) wiiich is no doubt to be connected with their lowly nutritious diet. In others, such as the rabbit, though the intestines are not excessively long, this is compensated by the great length and capacity of the caecum. In man, the length of the intestines is intermediate between the extremes of the carnivora and herbivora, and his diet also is intermediate. Summary of the Digestive Changes in the Small Intestine. In order to understand the changes in the food which occur during its passage through the small intestine, it will be well to refer briefly to the state in which it leaves the stomach through the pylorus. It has been said before, that the chief office of the stomach is not only to mix into a uniform mass all the varieties of food that reach it through the oesophagus, but especially to dissolve the nitrogenous portion by means of the gastric juice. The fatty matters, during their sojourn in the stomach, become more thoroughly mingled with the other constituents of the food taken, but are not yet in a state fit for absorption. The con- version of starch into sugar, which began in the mouth, has been inter- fered with, if not altogether stopped. The soluble matters — both those which were so from the first, as sugar and saline matter, and the gastric peptones — have begun to disappear by absorption into the blood-vessels, and the same thing has befallen such fluids as may have been swallowed, — wine, water, etc. The thin pultaceous chyme, therefore, which during the whole period of gastric digestion, is being constantly squeezed or strained through the pyloric orifice into the duodenum, consists of albuminous matter, broken down, dissolving and half dissolved; fatty matter broken down and molted, but not dissolved at all; starch very slowly in process of conversion into sugar, and as it becomes sugar, also dissolving in the fluids with which DIGESTION. 285 it IS mixed; while, with these are mingled gastric fluid, and fluid that has been swallowed, together with such portions of the food as are not digest- ible, and will be finally expelled as part of the faeces. On the entrance of the chyme into the duodenum, it is subjected to the influence of the bile and pancreatic juice, which are then poured out, and also to that of the succus entericus. All these secretions have a more or less alkaline reaction, and by their admixture with the gastric chyme its acidity becomes less and less until at length, at about the middle of the small intestine, the reaction becomes alkaline and continues so as far as the ileo-csecal valve. The special digestive functions of the small intestine may be taken in the following order: — (1.) One important duty of the small intestine is the alteration of the fat in such a manner as to make it fit for absorption; and there is no doubt that this change is chiefly effected in the upper part of the small intestine. What is the exact share of the process, however, allotted re- spectively to the bile, to the pancreatic secretion, and to the intestinal juice, is still uncertain, — probably the pancreatic juice is the most impor- tant. The fat is changed in two ways. {a). To a slight extent it is chemically decomposed by the alkaline secretions with which it is mingled, and a soap is the result, {l). It is emulsionized, i.e., its particles are minutely subdivided and diffused, so that the mixture assumes the condi- tion of a milky fluid, or emulsion. As will be seen in the next Chapter, most of the fat is absorbed by the lacteals of the intestine, but a small part, which is saponified, is also absorbed by the blood-vessels. (2.) ThQ albuminous substances which have been partly dissolved in the stomach, and have not been absorbed, are subjected to the action of the pancreatic and intestinal secretions. The pepsin is rendered inert by being precipitated together with the gastric peptones and parapeptones, as soon as the chyme meets with bile. By these means the pancreatic fer- ment trypsin is enabled to proceed with the further conversion of the parapeptones into peptones, and of part of the peptones (hemipeptone, Kiihne) into leucin and tyrosin. Albuminous substances, which are chemically altered in the process of digestion (peptones), and gelatinous matters similarly changed, are absorbed by both the blood-vessels and lymphatics of the intestinal mucous membrane. Albuminous matters, in a state of solution, which have not undergone the peptonic change, are probably, from the difficulty with which they diffuse, absorbed, if at all, almost solely by the lymphatics. (3.) The starchy, or amyloid portions of the food, the conversion of which into dextrin and sugar was more or less interrupted during its stay in the stomach, is now acted on briskly by the pancreatic juice and the succus entericus; and the sugar, as it is formed, is dissolved in the intes- tinal fluids, and is absorbed chiefly by the blood-vessels. 286 HAND-BOOK OF PHYSIOLOGY. (4.) Saline and saccharine matters, as common salt, or cane sugar, if not in a state of solution beforehand in the saliva or other fluids which ma}^ have been swallowed with them, are at once dissolved in the stomach, and if not here absorbed, are soon taken up in the small intestine; the blood-vessels, as in the last case, being chiefly concerned in the absorp- tion. Cane sugar is in part or wholly converted into grape-sugar before its absorption. This is accomplished partially in the stomach, but also by a ferment in the succus entericus. (5.) The liquids, including in this term the ordinary drinks, as water, wine, ale, tea, etc., which may have escaped absorption in the stomach, are absorbed probably very soon after their entrance into the intestine; the fluidity of the contents of the latter being preserved more by the con- stant secretion of fluid by the intestinal glands, pancreas, and liver, than by any given portion of fluid, whether swallowed or secreted, remaining long unabsorbed. From this fact, therefore, it may be gathered that there is a kind of circulation constantly proceeding from the intestines into the blood, and from the blood into the intestines again; for as all the fluid — a very large amount — secreted by the intestinal glands, must come from the blood, the latter would be too much drained, were it not that the same fluid after secretion is again re-absorbed into the current of blood — going into the blood charged with nutrient products of digestion — com- ing out again by secretion through the glands in a comparatively un- charged condition. At the lower end of the small intestine, the chyme, still thin and pul- taceous, is of a light yellow color, and has -a distinctly faecal odor. This odor depends upon the formation of indol. In this state it passes through the ileo-csecal opening into the large intestine. SUMMAKY OF THE DIGESTIVE CHANGES 11^" THE LaKGE IkTESTII^E. The changes which take place in the chyme in the large intestine are probably only the continuation of the same changes that occur in the course of the food's passage through the upper part of the intestinal canal. From the absence of villi, however, we may conclude that absorption, especially of fatty matter, is in great part completed in the small intes- tine; while, from the still half -liquid, pultaceous consistence of the chyme when it first enters the caecum, there can be no doubt that the absorption of liquid is not by any means concluded. The peculiar odor, moreover, which is acquired after a short time by the contents of the large bowel, would seem to indicate a further chemical change in the alimentary mat- ters or in the digestive fluids, or both. The acid reaction, which had dis- appeared in tlie small bowel, again becomes very manifest in the caecum — probably from acid fermentation-processes in some of the materials of the food. DIGESTION. 287 There seems no reason to conclude that any special "secondary diges- tive" process occurs in the caecum or in any other part of the large intestine. Probably any constituent of the food which has escaped digestion and absorption in the small bowel may be digested in the large intestine; and the power of this part of the intestinal canal to digest fatty, albuminous, or other matters, may be gathered from the good effects of nutrient ene- mata, so frequently given when from any cause there is difficulty in intro- ducing food into the stomach. In ordinary healthy digestion, however, the changes which ensue in the chyme after its passage into the large in- testine, are mainly the absorption of the more liquid parts, and the com- pletion of the changes which were proceeding in the small intestine,^ — the process being assisted by the secretion of the numerous tubular glands therein present. Faeces. — By these means the contents of the large intestine, as they proceed toward the rectum, become more and more solid, and losing their more liquid and nutrient parts, gradually acquire the odor and consist- ence characteristic of fceces. After a sojourn of uncertain duration in the sigmoid flexure of the colon, or in the rectum, they are finally ex- pelled by the act of def ascation. The average quantity of solid faecal matter evacuated by the human adult in twenty-four hours is about six or eight ounces. Composition of F^ces. Water 733-00 Solids 267-00 Special excrementitious constituents: — Excretin, ^ excretoleic acid (Marcet), and stercorin (Aus- tin Flint). Salts: — Chiefly phosphate of magnesium and phos- phate of calcium, with small quantities of iron, soda, lime, and silica. Insoluble residue of the food (chiefly starch grains, woody tissue, particles of cartilage and fibrous )■ 267-00 tissue, undigested muscular fibres or fat, and the like, with insoluble substances accidentally introduced with the food). Mucus, epithelium, altered coloring matter of bile, fatty acids, etc. Varying quantities of other constituents of bile, and derivatives from them. Length of Intestinal Digestive Period. — The time occupied by the journey of a given portion of food from the stomach to the anus, varies considerably even in health, and on this account, probably, it is that such different opinions have been expressed in regard to the subject. About twelve hours are occupied by the journey of an ordinary meal 288 HAND-BOOK OF PHYSIOLOGY. through the small intestine, and twenty-four to thirty-six hours by the passage through the large bowel. (Brinton.) Defaecation. — Immediately before the act of voluntary expulsion of faeces {clef cecat ion) there is usually, first an inspiration, as in the case of coughing, sneezing, and vomiting; the glottis is then closed, and the diaphragm fixed. The abdominal muscles are contracted as in expira- tion; but as the glottis is closed, the whole of their pressure is exercised on the abdominal contents. The sphincter of the rectum being relaxed, the evacuation of its contents takes place accordingly; the effect being, of course, increased by the peristaltic action of the intestine. As in the other actions just referred to, there is as much tendency to the escape of the contents of the lungs or stomach as of the rectum; but the pressure is relieved only at the orifice, the sphincter of which instinctively or in- voluntarily yields (see Fig. 144). Nervous Mechanism of Defaecation. — The anal sphincter muscle is normally in a state of tonic contraction. The nervous centre which governs this contraction is probably situated in the lumbar region of the spinal cord, inasmuch as in cases of division of the cord above this region the sphincter regains, after a time, to some extent the tonicity which is lost immediately after the operation. By an effort of the will, acting through the centre, the contraction may be relaxed or increased. In ordi- nary cases the apparatus is set in action by the gradual accumulation of faeces in the sigmoid flexure and rectum pressing against the sphincter and causing its relaxation; this sensory impulse acting through the brain and reflexly through the spinal centre. Peristaltic action, especially of the sigmoid flexure in pressing onward the faeces against the sphincter, is a very important part of the act. The Gases contained in the Stomach and Intestines. — Under ordinary circumstances, the alimentary canal contains a considerable quantity of gaseous matter. Any one who has had occasion, in a post- mortem examination, either to lay open the intestines, or to let out the gas which they contain, must have been struck by the small space after- ward occupied by the bowels, and by the large degree, therefore, in which the gas, which naturally distends them, contributes to fill the cavity of the abdomen. Indeed, the presence of air in the intestines is so constant, and, within certain limits, the amount in health so uniform, that there can be no doubt that its existence here is not a mere accident, but in- tended to serve a definite and important purpose, although, probably, a mechanical one. Sources. — The sources of the gas contained in the stomach and bowels may be thus enumerated: — 1. Air introduced in the act of swallowing either food or saliva; 2. Gases developed by the decomposition of alimentary nuittcr or of the DIGESTION. 289 secretions and excretions mingled with it in the stomach and intestines; 3. It is probable that a certain mutual interchange occurs between the gases contained in the alimentary canal, and those present in the blood of these gastric and intestinal blood-vessels; but the conditions of the exchange are not known, and it is very doubtful whether anything like a true and definite secretion of gas from the blood into the intestines or stomach ever takes place. There can be no doubt, however, that the in- testines may be the proper excretory organs for many odorous and other substances, either absorbed from the air taken into the lungs in inspira- tion, or absorbed in the upper part of the alimentary canal, again to be excreted at a portion of the same tract lower down — in either case as- suming rapidly a gaseous form after their excretion, and in this way, perhaps, obtaining a more ready egress from the body. It is probable that, under ordinary circumstances, the gases of the stomach and intes- tines are derived chiefly from the second of the sources which have been enumerated (Brinton). Composition of Gases coktaiited ix the Alimentary Canal. (tabulated from various authorities by brinton.) Whence obtained. Composition by Volume. Oxygen. Nitrog. Carbon. Acid. Hydrog. Carburet. Hydrogen. Sulphuret. Hydrogen. 11 71 14 4 Small Intestines . . . 32 30 38 1 " 66 12 8 • 13 35 57 6 8 j> trace 46 43 11 J Expelled ])er anum . . 22 41 19 19 2 Movements of the Intestines. — It remains only to consider the manner in which the food and the several secretions mingled with it are moved through the intestinal canal, so as to be slowly subjected to the influence of fresh portions of intestinal secretion, and as slowly exposed to the absorbent power of aU the villi and blood-vessels of the mucous membrane. The movement of the intestines is peristaltic or vermicular, and is effected by the alternate contractions and dilatations of successive portions of the intestinal coats. The contractions, which may commence^ at any point of the intestine, extend in a wave-like manner along the tube. In any given portion, the longitudinal muscular fibres contract first, or more than the circular; they draw a portion of the intestine upward, or, as it were, backward, over the substance to be propelled, and then the^ circular fibres of the same portion contracting in succession from abovo downward, or, as it were, from behind forward, press on the substance into the portion next below, in which at once the same succession of action next ensues. These movements take place slowly, and, in health, are com- VoL. I.— 19. 290 HAND-BOOK OF PHYSIOLOGY. monly unperceived by the mind; but they are perceptible when they are accelerated under the influence of any irritant. The movements of the intestines are sometimes retrograde; and there is no hindrance to the backward movement of the contents of the small intestine. But almost complete security is afforded against the passage of the contents of the large into the small intestine by the ileo-caecal valve. Besides, — the orifice of communication between the ileum and caecum (at the borders of which orifice are the folds of mucous membrane which form the valve) is encircled with muscular fibres^ the contraction of which prevents the undue dilatation of the orifice. Proceeding from above downward, the muscular fibres of the large intestine become, on the whole, stronger in direct proportion to the greater strength required for the onward moving of the faeces, which are gradually becoming firmer. The greatest strength is in the rectum, at the termi- nation of which the circular unstriped muscular fibres form a strong band called the internal sphincter; while an external sphincter muscle with striped fibres is placed rather lower down, and more externally, and as we have seen above, holds the orifice close by a constant slight tonic con- traction. Experimental irritation of the brain or cord produces no evident or con- stant effect on the movements of the intestines during life; yet in conse- quence of certain conditions of the mind the movements are accelerated or retarded; and in paraplegia the intestines appear after a time much weak- ened in their power, and costiveness, with a tympanitic condition, ensues. Immediately after death, irritation of both the symjDathetic and pneumo- gastric nerves, if not too strong, induces genuine peristaltic movements of the intestines. Violent irritation stops the movements. These stimuli act, no doubt, not directly on the muscular tissue of the intestine, but on the ganglionic plexus before referred to. Influence of the Nervous System on Intestinal Digestion. — As in the case of the oesophagus and stomach, the peristaltic movements of the intestines are directly due to reflex action through the ganglia and nerve fibres distributed so abundantly in their walls (p. 255); the presence of chyme acting as the stimulus, and few or no movements occurring when the intestines are empty. The intestines are, moreover, connected with the higher nerve-centres by the splanchnic nerves, as well as other branches of the sympathetic which come to them from the caliac and other abdominal plexuses. The splanchnic nerves are in relation to the intestinal movements, inhibitory — these movements being retarded or stopped wlien the splancli- nics are irritated. As the vaso-motor nerves of the intestines, the splanch- nics are also much concerned in intestinal digestion. CHAPTER IX. ABSORPTION. The process of Absorption has, for one of its objects, the introduction into the blood of fresh materials from the food and air, and of whatever comes into contact with the external or internal surfaces of the body; and, for another, the gradual removal of parts of the body itself, when they need to be renewed. In both these offices, i.e., in both absorption from without and absorption from within, the process manifests some variety, and a very wide rai^ge of action; and in both two sets of vessels are, or may be, concerned, namely, the Blood-vessels, and the Lymph- vessels or Lymphatics to which the term Absorbents has been also applied. The Lymphatic Vessels akd Glai^ds. Distribution. — The principal vessels of the lymphatic system are, in structure and general appearance, like very small and thin-walled veins, and like them are provided with valves. By one extremity they com- mence by fine microscopic branches, the lymphatic capillaries or lymph- capillaries, in the organs and tissues of the body, and by their other ex- tremities they end directly or indirectly in two trunks which open into the large veins near the heart (Fig. 206). Their contents, the lymph and chyle, unlike the blood, pass only in one direction, namely, from the fine branches to the trunk and so to the large veins, on entering which they are mingled with the stream of blood, and form part of its constituents. Remembering the course of the fluid in the lymphatic vessels, viz., its passage in the direction only toimrd the large veins in the neighborhood of the heart, it will readily be seen from Fig. 206 that the greater part of the contents of the lymphatic system of vessels passes through a com- paratively large trunk called the thoracic duct, which finally empties its contents into the blood-stream, at the junction of the internal jugular and subclavian veins of the left side. There is a smaller duct on the right side. The lymphatic vessels of the intestinal canal are called lacteals; because, during digestion, the fluid contained in them resembles milk in appearance; and the lymph in the lacteals during the period of digestion is called chyle. There is no essential distinction, however, between lac- 292 HAND-BOOK OF PHYSIOLOGY. teals and lymphatics. In some parts of their course all lymphatic vessels pass through certain bodies called lymphatic glands. Lymphatic vessels are distributed in nearly all parts of the body. Their existence, however, has not yet been determined in the placenta, the umbilical cord, the membranes of the ovum, or in any of the non- vascular parts, as the nails, cuticle, hair and the like. Lymphatics of head and neck, right. Right internal jugular vein. Right subclavian vein Lymphatics of right arm. Receptaculum chyli. Lymphatics of lower ex- tremities. Lymphatics of head and neck, left. Thoracic duct. Left subclavian vein. Thoracic duct. LacteaJs. Lymphatics of lower ex- tremities. Fig. 206.— Diagram of the principal groups of lymphatic vessels (from Quain). Origin of Lymph Capillaries. — The lymphatic capiUaries com- mence most commonly either in closely-meshed networks, or in irregular lacunar spaces between the various structures of which the different organs are composed. Such irregular spaces, forming what is now termed the lymph-can alicidar system, have been shown to exist in many tissues. In serous membranes, such as the omentum and mesentery, they occur as a connected system of very irregular branched spaces partly occu- pied by connective-tissue corpuscles, and both in tliese and in many other tissues are found to communicate freely with reguhir lymphatic vessels. In many cases, though they are formed mostly by the chinks and crannies between the blood-vessels, secreting ducts, and other parts which may ABSORPTION. 293 happen to form the framework of the organ in which they exist, they are lined by a distinct layer of endothelium. The lacteals offer an illustration of another mode of origin, namely, in blind dilated extremities (Figs. 192 and 193); but there is no essen- tial difference in structure between these and the lymphatic capillaries of other parts. Structure of Lymph Capillaries.— The structure of lymphatic capillaries is very similar to that of blood-capillaries: their walls consist of a single layer of endothelial cells of an elongated form and sinuous outline, which cohere along their edges to form a delicate membrane. Fig. 207.— Lymphatics of central tendon of rabbit's diaphragm, stained with silver nitrate. The ground substance has been shaded diagrammatically to bring out the lymphatics clearly. I. Lym- phatics hned by long narrow endothelial cells, and showing v. valves at frequent intervals (Schofield). They differ from blood capillaries mainly in their larger and very varia- ble calibre, and in their numerous communications with the spaces of the lymph-canalicular system. Communications of the Lymphatics. — The fluid part of the blood constantly exudes or is strained through the walls of the blood-capillaries, so as to moisten all the surrounding tissues, and occupies the interspaces which exist among their different elements. These same interspaces have been shown, as just stated, to form the beginnings of the lymph-capilla- ries; and the latter, therefore, are the means of collecting the exuded blood-plasma, and returning that part which is not directly absorbed by the tissues into the blood-stream. For many years, the notion of the existence of any such channels between the blood-vessels and lymph-ves- sels as would admit blood-corpuscles, has been given up; observations having proved that, for the passage of such corpuscles, it is not necessary 294 HAND-BOOK OF PHYSIOLOGY. to assume the presence of any special channels at all, inasmuch as blood- corpuscles can pass bodily, without much difficulty, through the walls of the blood-capillaries and small veins (p. 159), and could pass with still less trouble, probably, through the comparatively ill-defined walls of the capillaries which contain lymph. Fig. 208.— Lymphatic vessels of the head and neck and the upper part of the trunk (Mascagni). 1-6.— The chest and pericardium have been opened on the left side, and the left mamma detached and thrown outward over the left arm, so as to expose a great part of its deep surface. The principal lymphatic vessels and glands are shown on the side of the head and face, and in the neck, axilla, and mediastinum. Between the left internal jugular vein and the common carotid artery, the upper as- cending part of the thoracic dvict marked 1, and above this, and descending to 2, the arch and last part of the duct. The termination of the upper lymjihatics of the diaphragm in the mediastinal glands, as well as the cardiac and the deep mammary lymphatics, is also shoAra. It is worthy of note that, in many animals, both arteries and veins, especially the latter, are often found to be more or less completely en- sheathed in large lymphatic channels. In turtles, crocodiles, and many other animals, the abdominal aorta is enclosed in a large lymphatic vessel. Stomata, — In certain parts of the body openings exist by which lymphatic capillaries directly communicate with parts hitherto supposed to be closed cavities. If the peritoneal cavity be injected with milk, an injection is obtained of the plexus of lymphatic vessels of the central tendon of the diaphragm (Fig. 207); and on removing a small portion of the central tendon, with its peritoneal surface uninjured, and examining ABSORPTION. 295 the process of absorption under the microscope, the milk-globules run toward small natural openings or stomata between the epithelial cells, and disappear by passing vortex-like through them. The stomata, which have a roundish outline, are only wide enough to admit two or three milk- globules abreast, and never exceed the size of an epithelial cell. Fig. 209. Fig. 210. Fig. 209.— Superficial lymphatics of the forearm and palm of the hand, 1-5. 5. Two small glands at the bend of the arm. 6. Radial lymphatic vessels. 7. IJlnar lymphatic vessels. 8, 8. Palmar arch of lymphatics. 9, 9. Outer and inner sets of vessels, b. Cephalic vein. d. Radial vein. e. Median vein. /. Ulnar vein. The lymphatics are represented as lying on the deep fascia. (Mascagni.) Fig. 210.— Superficial lymphatics of right groin and upper part of thigh, 1-6. 1, upper inguinal glands. 2. 2', Lower inguinal or femoral glands. 3, 3'. Plexus of lymphatics in the course of the long sai^henous vein. (Mascagni.) Pseudostomata. — When absorption into the lymphatic system takes place in membranes covered by epithelium or endothelium through the interstitial or intercellular cement-substance, it is said to take place through pseudostomata. 29G HAND-BOOK OF PHYSIOLOGY. Demonstration of Lymphatics of Diaphragyn. — The stomata on the peritoneal surface of the diaphragm are the openings of short vertical canals which lead up into the lymphatics, and are lined by cells like those of germinating endothelium (p. 23). By introducing a solution of Berlin blue into the peritoneal cavity of an animal shortly after death, and sus- pending it, head downward, an injection of the lymphatic vessels of the diaphragm, through the stomata on its peritoneal surface, may readily be obtained, if artificial respiration be carried on for about half an hour. In this way it has been found that in the rabbit the lymphatics are arranged between the tendon bundles of the centrum tendineuai; and they are heiice termed interfascicular. The centrum tendineum is coated by endothelium on its pleural and peritoneal surfaces, and its substance con- sists of tendon bundles arranged in concentric rings toward the pleural side and in radiating bundles toward the peritoneal side. Fig. 211.— Peritoneal surface of septum cisternae lymphaticae magmse of frog. The stomata, some of which are open, some coUapsed, are surrounded by germinating endotheUum. x 160. (Klein.) The lymphatics of the anterior half of the diaphragm open into those of the anterior mediastinum, while those of the posterior half pass into a lymphatic vessel in the posterior mediastinum, which soon enters the tho- racic duct. Both these sets of vessels, and the glands into which they pass, are readily injected by the method above described; and there can be little doubt that during life the flow of lymph along these channels is chiefly caused by the action of the diaphragm during respiration. As it descends in inspiration, the spaces between the radiating tendon bun- dles dilate, and lymph is sucked from the peritoneal cavity, through the widely open stomata, into the interfascicular lymphatics. During expira- tion, the spaces betAveen the concentric tendon bundles dilate, and the lymph is squeezed into the lymphatics toward the pleural surface. (Klein. ) It thus appears probable that during health there is a continued sucking in of lymph from the .peritoneum into the lymphatics by the "pumping'' action of the diaphragm; and there is doubtless an equally continuous exudation of fluid from the genei-al serous surface of the })eritoneum. When this balance of transudation and absorption is disturbed, either by increased transudation or some impediment to absorption, an accumula- tion of fluid necessarily takes place (ascites). Stomata have been found in the pleura; and as they may be presumed to exist in other serous membranes, it would seem as if the serous cavities, ABSOEPTION. 297 hitherto supposed closed, form but a large lymph-sinus, or widening out, so to speak, of the lymph-capillary system with which they directly com- municate. Structure of Lymphatic Vessels.— The larger vessels are very like veins, having an external coat of fibro-cellular tissue, with elastic fila- ments; within this, a thin layer of fibro-cellular tissue, with plain mus- cular fibres, which have, principally, a circular direction, and are much more abundant in the small than in the larger vessels; and again, within this, an inner elastic layer of longitudinal fibres, and a lining of epithe- lium; and numerous valves. The valves, constructed like those of veins, and with the free edges turned toward the heart, are usually arranged in pairs, and, in the small vessels, are so closely placed, that when the vessels are full, the valves constricting them where their edges are attached, give them a peculiar beaded or knotted appearance. Current of the Lymph. — With the help of the valvular mechanism (1) all occasional pressure on the exterior of the lymphatic and lacteal vessels propels the lymph toward the heart: thus muscular and other external pressure accelerates th6 flow of the lymph as it does that of the blood in the veins. The actions of (2) the muscular fibres of the small intestine, and probably the layer of organic muscle present in each intes- tinal villus, seem to assist in propelling the chyle: for, in the small intes- tine of a mouse, the chyle has been seen moving with intermittent pro- pulsions that appeared to correspond with the peristaltic movements of the intestine. But for the general propulsion of the lymph and chyle, it is probable that, together with (3) the vis a tergo resulting from absorp- tion (as in the ascent of sap in a tree), and from external pressure, some of the force may be derived (4) from the contractility of the vessels own walls. The respiratory movements, also, (5) favor the current of lymph through the thoracic duct as they do the current of blood in the thoracic veins (p. 20G). Lymphatic Glands are small round or oval compact bodies varying in size from a hempseed to a bean, interposed in the course of the lym- phatic vessels, and through which the chief part of the lymph passes in *ts course to be discharged into the blood-vessels. They are found in