HANDBOOK OH' PHYSIOLOGY. ]iY WILLIAM SENHOUSE KIRKES, M.D. ' \ » EDITED BY W. MORRANT BAKER, F.R.C.S., LECTURER ON PHYSIOLOGY, AND ASSISTANT SURGEON TO ST. BARTHOLOMEW’S HOSPITAL ; SURGEON TO THE EVELINA HOSPITAL FOR SICK CHILDREN. WITH TWO HUNDRED AND FORTY-EIGHT ILLUSTRATIONS. A NEW AMERICAN FROM THE EIGHTH ENLARGED ENGLISH EDITION PHILADELPHIA: HENRY C. LEA. 1873. SHERMAN A CO., PRINTERS, PHILADELPHIA. PREFACE TO THE EIGHTH EDITION. The Eighth Edition is the result of an increased demand for this work, involving the necessity for a re- print at an earlier period after the publication of the Seventh Edition than was anticipated. The opportunity has been seized for making corrections and additions where they appeared to be most needed; but the present issue must be regarded as, in great part, a reprint of the Edition of 1869. W. Morrant Baker. The College, St. Bartholomew’s Hospital, October, 1872. CONTENTS. CHAPTER I. TAGE The General and Distinctive Characters of Living Beings, 13 i CHAPTER II. Chemical Composition of the Human Body, ... 18 CHAPTER III. Structural Composition of the Human Body, ... 26 CHAPTER IV. Structure of the Elementary Tissues, .... 34 Epithelium, .......... 34 Areolar, Cellular, or Connective Tissue, . . . . 38 Adipose Tissue, 40 Pigment, 42 Cartilage, ......... 43 Bones and Teeth,. ........ 46 CHAPTER V. The Blood, 55 Quantity of Blood, ........ 56 Coagulation of the Blood, ....... 58 Conditions affecting Coagulation, ..... 62 VI CONTENTS. PAGE Chemical Composition of the Blood, ..... 64 The Blood-Corpuscles, or Blood-cells, ..... 65 Chemical Composition of Red Blood-cells, . . 68 Blood-crystals, ......... 69 The White Corpuscles, ....... 71 The Serum, .......... 72 Variations in the Principal Constituents of the Liquor San- guinis, .......... 73 Variations in Healthy Blood under Different Circumstances, 76 Variations in the Composition of the Blood in Different Parts of the Body, ....... 77 Gases contained in the Blood, ...... 81 Development of the Blood, ....... 81 Uses of the Blood, ........ 85 Uses of the various Constituents of the Blood, ... 85 CHAPTER VI. Circulation of the Blood, 88 The Systemic, Pulmonary, and Portal Circulations, . . 89 The Heart, 91 Structure of the Valves of the Heart,. .... 92 The Action of the Heart, ....... 96 Function of the Valves of the Heart, ..... 99 Sounds of the Heart, 105 Impulse of the Heart, ........ 107 Frequency and Force of the Heart’s Action, . . . 108 Cause of the Rhythmic Action of the Heart, . . . Ill Effects of the Heart’s Action, . . . . . .114 The Arteries, 115 Structure of the Arteries, 115 The Pulse, 123 Sphygmograph, ......... 124 Force of the Blood in the Arteries, ..... 129 Velocity of the Blood in the Arteries, . . . .131 The Capillaries, 131 The Structure and Arrangement of Capillaries, . . . 132 Circulation in the Capillaries, ...... 135 CONTENTS. VII PAGE Thk Veins, 141 Structure, . 141 Agents concerned in the Circulation of the Blood, . . 145 Velocity of Blood in the Veins, . . . . . 147 Velocity of the Circulation,. ...... 147 Peculiarities of the Circulation in Different Parts, 150 Cerebral Circulation, ........ 150 Erectile Structures, ........ 152 CHAPTER VII. Respiration, 155 Position and Structure of the Lungs, 155 Mechanism of Respiration, 161 Respiratory Movements, ....... 162 Respiratory Rhythm, . 165 Respiratory Movements of Glottis, ..... 166 Quantity of Air respired, 166 Movements of the Blood in Respiratory Organs,. . . 172 Changes of the Air in Respiration, 173 Changes produced in the Blood by Respiration, . . . 179 Mechanism of various Respiratory Actions, . . . 180 Influence of the Nervous System in Respiration, . . . 185 Effects of the Suspension and Arrest of Respiration, . . 186 CHAPTER VIII. Animal Heat, 189 Variations in Temperature, ....... 190 Sources and Mode of Production of Heat in the Body, . 192 Regulation of Temperature, ....... 194 Influence of Nervous System, ...... 198 CHAPTER IX. Digestion, 199 Pood, ........... 199 Starvation, 203 VIII CONTENTS. PAGE Passage of Food through the Alimentary Canal, . . 207 The Salivary Glands and the Saliva, ..... 209 Passage of Food into the Stomach, ..... 213 Digestion of Food in the Stomach, . . . . 214 Structure of the Stomach, 214 Secretion and Properties of the Gastric Fluid, . . . 219 Changes of the Food in the Stomach, ..... 225 Movements of the Stomach,....... 231 Influence of the Nervous System on Gastric Digestion, . 234 Digestion of the Stomach after Death, .... 236 Digestion in the Intestines, 238 Structure and Secretion of the Small Intestines, . . . 238 Valvulae Conniventes, ........ 240 Glands of the Small Intestine, ...... 240 The Villi, 246 Structure of the Large Intestine, 248 The Pancreas and its Secretion, 250 Structure of the Liver, ....... 252 Functions of the Liver, ....... 259 The Bile, 259 Glycogenic Function of the Liver, 267 Summary of the Changes which take place in the Food during its Passage through the small Intestine, . . 270 Summary of the Process of Digestion in the large Intestine, 272 Gases contained in the Stomach and Intestines, . . . 274 Movements of the Intestines, ...... 275 CHAPTER X. Absorption, 277 Structure and Office of the Lacteal and Lymphatic Vessels, and Glands, ......... 277 Lymphatic Glands, ...... . 283 Properties of Lymph and Chyle, ..... 286 Absorption by the Lacteal Vessels, . . . . . 290 Absorption by the Lymphatics, ...... 291 Absorption by Bloodvessels, ...... 293 CHAPTER XI. Nutrition and Growth, 299 Nutrition 299 Growth, ....... . 311 CONTENTS. IX CHAPTER XII. PAGE Secretion, 313 Secreting Membranes, . . . . . . . .314 Serous Membranes, 315 Mucous Membranes, . . . . . . . .316 Secreting Glands, ........ 319 Process of Secretion, . . . . . . . . 321 CHAPTER XIII. Vascular Glands, or Glands without Ducts, . . . 325 Structure of the Spleen, ....... 327 Functions of the Vascular Glands, ..... 329 CHAPTER XIV. The Skin and its Secretions, 332 Structure of the Skin, ........ 333 Structure of Hair and Nails, ...... 340 Excretion by the Skin, ....... 344 Absorption by the Skin, ....... 347 CHAPTER XV. The Kidneys and their Secretion, 349 Structure of the Kidneys, 349 Secretion of Urine, ........ 354 The Urine; its General Properties, ..... 356 Chemical Composition of the Urine, 357 CHAPTER XVI. The Nervous System, 367 Elementary Structures of the Nervous System, . . . 368 Functions of Nerve-fibres, ....... 376 Functions of Nerve-centres, 382 Cerebro-spinal Nervous System, 386 Spinal Cord and its Nerves, 386 Functions of the Spinal Cord, 392 X CONTENTS. PAGE The Medulla Oblongata, 402 Its Structure, ......... 402 Distribution of the Fibres of the Medulla Oblongata, . 404 Functions of the Medulla Oblongata, ..... 405 Structure and Physiology of the Pons Yarolii, Crura Cerebri, Corpora Quadrigemina, Corpora Genic- ulata, Optic Thalami, and Corpora Striata, . . 409 Pons Yarolii, ......... 409 Crura Cerebri, ......... 409 Corpora Quadrigemina, ....... 411 The Sensory Ganglia, ........ 413 Structure and Physiology of the Cerebellum, . . 414 Structure and Physiology of the Cerebrum, . . . 419 Physiology of the Cerebral and Spinal Nerves, . . 424 Physiology of the Third, Fourth, and Sixth Cerebral or Cranial Nerves, ........ 425 Physiology of the Fifth or Trigeminal Nerve, . . . 428 Physiology of the Facial Nerve, . ..... 433 Physiology of the Glosso-pharyngeal Nerve, . . . 435 Physiology of the Pneumogastric Nerve, .... 438 Physiology of the Spinal Accessory Nerve, . . . 443 Physiology of the Hypoglossal Nerve, .... 444 Physiology of the Spinal Nerves, 444 Physiology of the Sympathetic Nerve, .... 445 CHAPTER XVII. Causes and Phenomena of Motion, 454 Ciliary Motion, ......... 454 Muscular Motion, 456 Muscular Tissue, ......... 456 Properties of Muscular Tissue, ...... 461 Action of the Voluntary Muscles, 467 Action of the Involuntary Muscles, ..... 473 Source of Muscular Actiop, 473 CONTENTS. XI CHAPTER XVIII. PAGE Of Voice and Speech, 474 Mode of Production of the Human Voice, .... 474 The Larynx, ......... 476 Application of the Voice in Singing and Speaking, . . 483 Speech, 486 CHAPTER XIX. The Senses, 489 The Sense of Smell, 494 The Sense of Sight, ,. 499 Structure of the Eye, ....... 499 Phenomena of Vision, ....... 507 Reciprocal Action of different parts of the Retina, . . 519 Simultaneous Action of the two Eyes, .... 521 The Sense of Hearing, 527 Anatomy of the Organ of Hearing, ..... 527 Physiology of Hearing, ....... 534 Functions of the External Ear. ....... 535 Functions of the Middle Ear; the Tympanum, Ossicula, and Fenestrse, ......... 536 Functions of the Internal Ear, ...... 541 Sensibility of the Auditory Nerve, 543 The Sense of Taste, 547 The Sense of Touch, 554 CHAPTER XX. Generation and Development 559 Generative Organs of the Female, 560 Unimpregnated Ovum, ....... 563 Discharge of the Ovum, ....... 567 Corpus Luteum, 570 XII CONTENTS. PAGE Impregnation of the Ovum, 573 Male Sexual Functions, ....... 573 Development, 578 Changes of the Ovum previous to the Formation of the Embryo, ......... 578 Changes of the Ovum within the Uterus, .... 581 The Umbilical Vesicle, ....... 582 The Amnion and Allantois,....... 585 The Chorion, ......... 588 Changes of the Mucous Membrane of the Uterus and For- mation of the Placenta, ...... 589 Development of Organs, 593 Development of the Vertebral Column and Cranium, . 594 Development of the Face and Visceral Arches, . . . 595 Development of the Extremities, ..... 596 Development of the Vascular System, .... 597 Circulation of Blood in the Foetus, 601 Development of the Nervous System, 603 Development of the Organs of Sense, ..... 603 Development of the Alimentary Canal, .... 606 Development of the Respiratory Apparatus, . . . 608 The Wolffian Bodies, Urinary Apparatus, and Sexual Or- gans, 609 The Mammary Glands, 613 List of Works Referred to, 617 Index, 625 HANDBOOK OF PHYSIOLOGY. CHAPTER I. ON 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 how he dies. Having, then, man as the object of its study, it is unneces- sary to speak here of the laws of life in general, and the means by which they are carried out, further than is requisite for the more clear understanding of those of the life of man in par- ticular. 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 con- sideration of the structure and endowments of the organs and tissues belonging to man. The essentials of life are these,—birth, growth and develop- ment, decline and death ; and an idea of what life is, will be best gained by sketching these events, each in succession, and their relations one to another. The term, birth, when employed in this general sense of one of the conditions essential to life, without reference to any par- ticular kind of living being, may be taken to mean, separation from a parent, with a greater or less power of independent ex- istence as a living being. 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 got by in- heritance. 14 GROWTH. Growth, or inherent power of increasing in size, although essential to our idea of life, is not a property of living beings only. A crystal of sugar or of common salt, or of any other substance, if placed under appropriate conditions for obtaining fresh material, will grow in a fashion as definitely character- istic 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. First, 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 growtli is not super- ficial, but interstitial. In the second place, all living structures are subject to constant decay; and life consists, 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 ex- actly the same particles day after day, although to all intents he remains the same individual. 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 re- spect to those which, though they are formed by living parts, are not themselves alive. Thus, an oyster-shell is formed by the living animal which it incloses, but it is as lifeless as any other mass of saline matter; and in accordance with this cir- cumstance its growth takes place not interstitially, but layer by layer, and it is not subject to the constant decay and recon- struction which belong to the living. The hair and nails are examples of the same fact. Thirdly. In connection with the growth of lifeless masses there is no alteration in composition or properties of the ma- terial 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 changed 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 from this in living beings, DEVELOPMENT. 15 both animal and vegetable. A plant, like a crystal, can only grow when fresh material is presented to it; and this is ab- sorbed by its leaves and roots; and animals for the same pur- pose of getting new matter for growth and nutrition, take food into their stomachs. But in both these cases the materials are much altered before they are finally assimilated by the struc- tures they are destined to nourish. Fourthly. The growth of all living things has a definite limit, and the law which governs this limitation of increase in size is so invariable that we should be as much astonished to find an individual plant or animal without 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, be- fore 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 simply 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 de- velopment 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. The decline of living beings is as definite in its occurrence as growth or development. Death—not by disease or injury— so far from being a violent interruption of the course of life, is but the fulfilment of a purpose in view from the commence- ment. It has been already said that the essential features of life are the same in all living things; in other words, in the mem- bers of both the animal and vegetable kingdoms. It may be well now to notice briefly the distinctions which exist between 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, which are the same in both. I. Perhaps the most essential distinction is the presence or absence of power to live upon inorganic material; in other words, to act chemically on carbonic acid, ammonia, and water, so as to make use of their component elements as food. Indeed one ought probably to say that a question concerning the capa- bility of the lower kinds of animal to live in this way cannot be entertained; and that sqch zpanner of life should decide 16 ANIMALS CONTRASTED at once in favor of a vegetable nature, whatever might be the attributes which seem to point to an opposite conclusion. The power of living upon organic matter would seem to be less de- cisive of an animal nature, for some fungi appear to derive sup- port almost entirely from this source. II. 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 a substance containing carbon, hydrogen, and oxygen only, ar- ranged so as to form a compound closely allied to starch, and called cellulose, the latter are commonly composed in great part of the three elements just named, together with a fourth, nitrogen; the proximate principles formed from these being identical, or nearly so, with albumen. It must not be supposed, however, that either of these typical compounds alone, with its allies, is confined to one kingdom of nature. Nitrogenous or albuminous compounds are freely produced by vegetable structures, although they form an infinitely 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. III. Inherent power of movement is a quality which we so commonly consider an essential indication of animal nature, 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 Crypto- gamia exhibit movements of a like kind to those seen in animal- cules ; and even among the higher orders of plants, many ex- hibit 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 ani- mal nature, is, when taken by itself, no proof of it. Of course, if the movement were such as to indicate any kind of purpose, whether of getting food or any other, the case would be differ- ent, and we should justly call a being exhibiting such motion, an animal. But low down in the scale of life, where alone there exists any difficulty in distinguishing the two classes, movements, although almost always more lively, are scarcely or not at all more purposive in one than the other; and even if we decide on the animal nature of a being, it by no means WITH VEGETABLES. 17 follows that we are bound to acknowledge the presence of sen- sation or volition in the slightest degree. There may be at least no evidence of its possessing a trace of those tissues, ner- vous and muscular, by which, in the higher members of the animal kingdom, these qualities are manifested. Probably there is no more of either of them in the lowest animals than in vegetables. In both, movement is effected by the same means—ciliary action, and hence the greater value, for pur- poses of classification, of the power to live on this or that kind of food—on organic or inorganic matter. As the main purpose of the lowest members of the vegetable kingdom is doubtless to bring to organic shape the elements of the inorganic world around, so the function of the lowest animals is, in like man- ner, to act on degenerating organic matter—“to arrest the fugitive organized particles, and turn them back into the as- cending stream of animal life.” And, because sensation and volition are accompaniments of life in somewhat higher animal forms, it is needless to suppose that these qualities exist under circumstances in which, as we may believe, they could be of no service. It is as needless as to dogmatize on the opposite side, and say that no feeling or voluntary movement is possible without the presence of those tissues which we call nervous and muscular. IV. The presence of a stomach 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 stomach. This distinction then is not a cardinal one. It would be tedious as well as unnecessary to enumerate the chief distinctions between the more highly developed animals and vegetables. They are sufficiently apparent. It is neces- sary to compare, side by side, the lowest members of the two kingdoms, in order to understand rightly how faint are the boundaries between them. 18 CHEMICAL COMPOSITION OF HUMAN BODY. CHAPTER II. CHEMICAL COMPOSITION OF THE HUMAN BODY. The following Elementary Substances may be obtained by chemical analysis from the human body : Oxygen, Hydrogen, Nitrogen, Carbon, Sulphur, Phosphorus, Silicon, Chlorine, Fluorine, Potassium, Sodium, Calcium, Magnesium, Iron, and, probably as accidental constituents, Manganesium, Alumin- ium, Copper, and Lead. Thus, of the sixty-three or more elements of which all known matter is composed, more than one fourth are present in the human body. Only one or two elements, and in very minute amount, are present in the body uncombined with others ; and even these are present much more abundantly in various states of combi- nation. The most simple compounds formed by union in various proportions of these elements are termed proximate principles; while the latter are classified as the organic and the inorganic proximate principles. The term organic was once applied exclusively to those substances which were thought to be beyond the compass of synthetical chemistry and to be formed only by organized or living beings, animal or vegetable ; these being called organ- ized, inasmuch as they are characterized by the possession of different parts called organs. But with advancing knowledge, both distinctions have disappeared; and while the title of living organism is applied to numbers of living things, having no trace of organs in the old sense of the term, and in some, so far as can be now seen, in no other sense, the term organic has long ceased to be applied to substances formed only by living tissues. In other words, substances, once thought to be formed only by living tissues, are still termed organic, al- though they can be now made in the laboratory. The term, indeed, in its old meaning, becomes year by year applicable to fewer substances, as the chemist adds to his conquests over inorganic elements and compounds, and moulds them to more complex forms. Although a large number of so-called organic compounds have long ceased to be peculiar in being formed only by living tissues, the terms organic and inorganic are still commonly used to denote distinct classes of chemical substances, and the classification of the matters of which the human body is com- CHEMICAL COMPOSITION OF HUMAN BODY. 19 posed into the organic and the inorganic is convenient, and will be here employed. No very accurate line of separation can be drawn between organic and inorganic substances, but there are certain pecu- liarities belonging to the former which may be here briefly noted. 1. Organic compounds are composed of a larger number of Elements than are present in the more common kinds of inor- ganic matter. Thus, albumen, fibrin, and gelatin, the most abundant substances of this class, in the more highly organized tissues of animals, are composed of five elements,—carbon, hy- drogen, oxygen, nitrogen, and sulphur. The most abundant inorganic substance, water, has but two elements, hydrogen and oxygen. 2. Not only are a large number of elements usually com- bined in an organic compound, but a large number of equiva- lents or atoms of each of the elements are united to form an equivalent or atom of the compound. In the case of carbon- ate of ammonium, as an example among inorganic substances, one equivalent of carbonic acid is united with two of ammo- nium ; the equivalent or atom of carbonic acid consists of one of carbon with two of oxygen ; and that of ammonium of one of nitrogen with three of hydrogen. But in an equivalent or atom of fibrin, or of albumen, there are of the same elements, respectively, 72, 22, 18, and 112 equivalents. And, together with this union of large numbers of equivalents in the organic compound, it is further observable, that the several numbers stand in no simple arithmetical relation one with another, as the numbers of equivalents combining in an inorganic com- pound do. With these peculiarities in the chemical composition of or- ganic bodies we may connect two other consequent facts; first, the large number of different compounds that are formed out of comparatively few elements ; secondly, their great proneuess to decomposition. For it is a general rule, that the greater the number of equivalents or atoms of an element that enter into the formation of an atom of a compound, the less is the stability of that compound. Thus, for example, among the various oxides of lead and other metals, the least stable in composition are those in which each equivalent has the largest number of equivalents of oxygen. So, water, composed of one equivalent of oxygen and two of hydrogen, is not changed by any slight force; but peroxide of hydrogen, which has two equivalents of oxygen to two of hydrogen, is among the sub- stances most easily decomposed. The instability, on this ground, belonging to organic com- 20 CHEMICAL COMPOSITION OF HUMAN BODY. pounds,, is, in those which are most abundant in the highly- organized tissues of animals, augmented, 1st, by their contain- ing nitrogen, which, among all the elements, may be called the least decided in its affinities, and that which maintains with least tenacity its combinations with other elements; and 2dly, by the quantity of water which, in their natural state, is com- bined with them, and the presence of which furnishes a most favorable condition for the decomposition of nitrogenous com- pounds. Such, indeed, is the instability of animal compounds, arising from these several peculiarities in their constitution, that, in dead and moist animal matter, no more is requisite for the occurrence of decomposition than the presence of at- mospheric air and a moderate temperature ; conditions so com- monly present, that the decomposition of dead animal bodies appears to be, and is generally called, spontaneous. The modes of such decomposition vary according to the nature of the origi- nal compound, the temperature, the access of oxygen, the pres- ence of microscopic organisms, and other circumstances, and constitute the several processes of decay and putrefaction; in the results of which processes the only general rule seems to be, that the several elements of the original compound finally unite to form those substances, whose composition is, under the circumstances, most stable. The organic compounds existing in the human body may be arranged in two classes, namely, the azotized, or nitrogenous, and the non-azotized, or non-nitrogenous principles. The non-azotized principles include the several fatty, oily, or oleaginous substances, as olein, stearin, cholesterin, and others. In the same category of non-nitrogenous substances may be included lactic and formic acids, animal glucose, sugar of milk, &c. The oily or fatty matter which, inclosed in minute cells, forms the essential part of the adipose or fatty tissue of the human body (p. 40), and which is mingled in minute particles in many other tissues and fluids, consists of a mixture of stearin, palmitin, and olein. The mixture forms a clear yellow oil, of which different specimens congeal at from 45° to 35°. Cholesterin, a fatty matter which melts at 293° F., and is, therefore, always solid at the natural temperature of the body, may be obtained in small quantity from blood, bile, and ner- vous matter. It occurs abundautly in many biliary calculi; the pure white crystalline specimens of these concretions being formed of it almost exclusively. Minute rhomboidal scale- like crystals of it are also often found in morbid secretions, as in cysts, the puriform matter of softening and ulcerating tumors, &c. It is soluble in ether and boiling alcohol; but GELATINOUS SUBSTANCES. 21 alkalies do not change it; it is one of those fatty substances which are not saponifiable. The azotized or nitrogenous principles in the human body include what may be called the proper gelatinous and albu- minous substances, besides others of less definite rank and composition, as pepsin and ptyalin, horny matter or keratin, many coloring and extractive matters, &c. The gelatinous substances are contained in several of the tissues, especially those which serve a passive mechanical office in the economy; as the cellular, or fibro-cellular tissue in all parts of the body, the tendons, ligaments, and other fibrous tissues, the cartilages and bones, the skin and serous membranes. These, when boiled in water, yield a material, the solution of which remains liquid while it is hot, but be- comes solid and jelly-like on cooling. Two varieties of these substances are described, gelatin and chondrin, the latter being derived from cartilages, the former from all the other tissues enumerated above, and in its purest state, from isinglass, which is the swimming-bladder of the sturgeon, and which, with the exception of about 7 per cent, of its weight, is wholly reducible into gelatin. The most char- acteristic property of gelatin is that mentioned, of its solution being liquid when warm, and solidifying or setting when it cools. The temperature at which it becomes solid, the proportion of gelatin which must be in solution, and the firm- ness of the jelly when formed, are various, according to the source, the quantity, and the quality of the gelatin; but, as a general rule, one part of dry gelatin dissolved in 100 of water, will become solid when cooled to 60°. The solidified jelly may be again made liquid by heating it, and the transitions from the solid to the liquid state by the alternate abstraction and addition of heat, may be repeated several times; but at length the gelatin is so far altered, and, apparently, oxidized by the process, that it no longer becomes solid on cooling. Gelatin in solutions too wreak to solidify when cold, is dis- tinguished by being precipitable with alcohol, ether, tannic acid, and bichloride of mercury, and not precipitable with the ferrocyanide of potassium. The most delicate and striking of these tests is the tannic acid, which is conveniently supplied in an infusion of oak-bark or gall-nuts; it will detect one part of gelatin in 5000 of water; and if. the solution of gelatin be strong it forms a singularly dense and heavy precipitate, which has been named tanno-gelatin, and is completely insoluble in water. Chondrin, the kind of gelatin obtained from cartilages, agrees with gelatin in most of its characters, but its solution 22 CHEMICAL COMPOSITIOFOF HUMAN BODY. solidifies on cooling much less firmly, and, unlike gelatin, it is precipitable with acetic and the mineral and other acids, and with alum, persulphate of iron, and acetate of lead. Albuminous substances, or proteids, as they are sometimes called, exist abundantly in the human body. The chief among them are albumen, fibrin, casein, syntonin, myosin, and globulin. Albumen exists in most of the tissues of the body, but es- pecially in the nervous, in the lymph, chyle, and blood, and in many morbid fluids, as the serous secretions of dropsy, pus, and others. In the human body it is most abundant, and most nearly pure, in the serum of the blood. In all the forms in which it naturally occurs, it is combined with about six per cent, of fatty matter, phosphate of lime, chloride of sodium, and other saline substances. Its most characteristic property is, that both in solution and in the half-solid state in which it exists in white of egg, it is coagulated by heat, and in thus becoming solid, becomes insoluble in water. The temperature required for the coagulation of albumen is the higher the less the proportion of albumen in the solution submitted to heat. Serum and such strong solutions will begin to coagulate at from 150° to 170°, and these, when the heat is maintained, become almost solid and opaque. But weak solutions require a much higher temperature, even that of boiling, for their coagulation, and either only become milky or opaline, or pro- duce flocculi which are precipitated. Albumen, in the state in which it naturally occurs, appears to be but little soluble in pure water, but is soluble in water containing a small proportion of alkali. In such solutions it is probably combined chemically with the alkali; it is precip- itated from them by alcohol, nitric, and other mineral acids, by ferrocyanide of potassium (if before or after adding it the alkali combined with the albumen be neutralized), by bi- chloride of mercury, acetate of lead, and most metallic salts. Coagulated albumen, i. e., albumen made solid with heat, is soluble in solutions of caustic alkali, and in acetic acid if it be long digested or boiled with it. With the aid of heat, also, strong hydrochloric acid dissolves albumen previously coag- ulated, and the solution has a beautiful purple or blue color. Fibrin is found-most abundantly in the blood and the more perfect portions of the lymph and chyle. It is very doubtful, however, whether fibrin, as such, exists in these fluids—whether, that is to say, it is not itself formed at the moment of coagula- tion. (See chapter on the Blood.) If a common clot of blood be pressed in fine linen while a stream of water flows upon it, the whole of the blood-color is gradually removed, and strings and various pieces remain of CASEIN—SYNTONIN — MYOSIN. 23 a soft, yet tough, elastic, and opaque-white substance, which consist of fibrin, impure, with a mixture of fatty matter, lymph- corpuscles, shreds of the membranes of red blood-corpuscles, and some saline substances. Fibrin somewhat purer than this may be obtained by stirring blood while it coagulates, and collecting the shreds that attach themselves to the instrument, or by retarding the coagulation, and, while the red blood- corpuscles sink, collecting the fibrin unmixed with them. But in neither of these cases is the fibrin perfectly pure. Chemically, fibrin and albumen can scarcely be distin- guished ; the only difference apparently being that fibrin con- tains 1.5 more oxygen in every 100 parts than albumen does. Mr. A. H. Smee has, indeed, apparently converted albumen into fibrin, by exposing a solution to the prolonged influence of oxygen. Nearly all the changes, produced by various agents, in coagulated albumen, may be repeated with coag- ulated fibrin, with no greater differences of result than may be reasonably ascribed to the differences in the mechanical properties of the two substances. Of such differences the prin- cipal are, that fibrin immersed in acetic acid swells up and becomes transparent like gelatin, while albumen undergoes no such apparent change; and that deutoxide of hydrogen is decomposed when in contact with coagulated fibrin, but not with albumen. Casein, which is said to be albumen in combination with soda, exists largely in milk, and forms one of its most im- portant constituents. Syntonin is obtained from muscular tissue, both of the striated and organic kind. It differs from ordinary fibrin in several particulars, especially in being less soluble in nitrate and car- bonate of potash, and more soluble in dilute hydrochloric acid. Myosin is the substance which spontaneously coagulates in the juice of muscle. It is closely allied to syntonin; indeed, in the act of solution in dilute acids, it is converted into it. The percentage composition of albumen, fibrin, gelatin, and chondrin, is thus given by Mulder: Albumen. Fibrin. Gelatin. Chondrin. Carbon, .... 53.5 52 7 50.40 49.97 Hydrogen, 7.0 6.9 6.64 6.63 Nitrogen, .... 15.5 15.4 18.34 14.44 Oxygen, .... 22.0 23.5 \ 94 9fi \ 28.58 Sulphur, .... 1.6 1.2 / 0.38 Phosphorus, . . . 0.4 0.3 100.0 100.0 100.00 100.00 24 CHEMICAL COMPOSITION OF HUMAN BODY. Horny Matter.—The substance of the horny tissues, includ- ing the hair and nails (with whalebone, hoofs, and horns), consists of an albuminous substance, with larger proportions of sulphur than albumen and fibrin contain. Hair contains 10 per cent, and nails 6 to 8 per cent, of sulphur. The horny substances, to which Simon applied the name of keratin, are insoluble in water, alcohol, or ether; soluble in caustic alkalies, and sulphuric, nitric, and hydrochloric acids; and not precipitable from the solution in acids by ferrocyanide of potassium. Mucus, in some of its forms, is related to these horny sub- stances, consisting, in great part, of epithelium detached from the surface of mucous membrane, and floating in a peculiar clear and viscid fluid. But under the name of mucus, several various substances are included of which some are morbid albuminous secretions containing mucus and pus-corpuscles, and others consist of the fluid secretion variously altered, con- centrated, or diluted. Mucus contains an albuminous sub- stance, termed mucin. It differs from albumen chiefly in not containing sulphur. Pepsin and other albuminous ferments, as they are sometimes called, will be described in connection with the secretions of which they are the active principles. And the various color- ing matters, as of the blood, bile, &c., will be also considered with the fluids or tissues to which they belong. Besides the above-mentioned organic nitrogenous compounds, other substances are formed in the living body, chiefly by de- composition of nitrogenous materials of the food and of the tissues, which must be reckoned rather as temporary constitu- ents than essential component parts of the body; although from the continual change, which is a necessary condition of life, they are always to be found in greater or less amount. Examples of these are urea, uric and hippuric acids, creatin, creatinin, leucin, and many others. Such are the chief organic substances of which the human body is composed. It must not be supposed, however, that they exist naturally in a state approaching that of chemical purity. All the fluids and tissues of the body appear to con- sist, chemically speaking, of mixtures of several of these prin- ciples, together with saline matters. Thus, for example, a piece of muscular flesh would yield fibrin, albumen, gelatin, fatty matters, salts of soda, potash, lime, magnesia, iron, and other substances, such as creatin, which appear passing from the organic towards the inorganic state. This mixture of sub- stances may be explained in some measure by the existence of many different structures or tissues in the muscles; the gelatin WATER — POTASH — SODA. 25 may be referred principally to the cellular tissue between the fibres, the fatty matter to the adipose tissue in the same posi- tion, and part of the albumen to the blood and the fluid by which the tissue is kept moist. But, beyond these general statements, little can be said of the mode in which the chemi- cal compounds are united to form an organized structure; or of how, in any organic body, the several incidental substances are combined with those which are essential. The inorganic matters which exist as such in the human body are numerous. Water forms a large proportion, probably more than two- thirds of the weight of the whole body. Phosphorus occurs in combination,—as in the neutral phos- phate of sodium in the blood and saliva, the acid phosphates of the muscles and urine, the basic phosphates of calcium and magnesium in the bones and teeth. Sulphur is present chiefly in the sulphocyanide of potassium of the saliva, and in the sulphates of the urine and sweat. A very small quantity of silica exists, according to Berze- lius, in the urine, and, according to others, in the blood. Traces of it have also been found in bones, in hair, and in some other parts of the body. Chlorine is abundant in combination with sodium, potas- sium, and other bases in all parts, fluid as well as solid, of the body. A minute quantity of fluorine in combination with calcium has been found in the bones, teeth, and urine. Potassium and sodium are constituents of the blood and all the fluids, in various quantities and proportions. They exist in the form of chlorides, sulphates, and phosphates, and prob- ably, also, in combination with albumen, or certain organic acids. Liebig, in his work on the Chemistry of Food, has shown that the juice expressed from muscular flesh always contains a much larger proportion of potash-salts than of soda- salts; while in the blood and other fluids, except the milk, the latter salts always preponderate over the former; so that, for example, for every 100 parts of soda-salts in the blood of the chicken, ox, and horse, there are only 40.8, 5.9, and 9.5 parts of potash-salts; but for every 100 parts of soda-salts in their muscles, there are 381, 279, and 285 parts of potash-salts. The salts of calcium are by far the most abundant of the earthy salts found in the human body. They exist in the lymph, chyle, and blood, in combination with phosphoric acid, the phosphate of calcium being probably held in solution by the presence of phosphate of sodium. Perhaps no tissue is wholly void of phosphate of calcium; but its especial seats are the bones and teeth, in which, together with carbonate and 26 STRUCTURAL COMPOSITION OF HUMAN BODY. fluoride of calcium, it is deposited in minute granules, in a peculiar compound, named bone-earth, containing 51.55 parts of lime, and 48.45 of phosphoric acid. Phosphate of calcium, probably the neutral phosphate, is also found in the saliva, milk, bile, and most other secretions, and acid phosphate in the urine, and, according to Blondlot, in the gastric fluid. Magnesium appears to be always associated with calcium, but its proportion is much smaller, except in the juice ex- pressed from muscles, in the ashes of which magnesia prepon- derates over lime. The especial place of iron is in the hsemoglobin, the color- ing-matter of the blood, of which a further account will be given with the chemistry of the blood. Peroxide of iron is found, in very small quantities, in the ashes of bones, muscles, and many tissues, and in lymph and chyle, albumen of serum, fibrin, bile, and other fluids; and a salt of iron, probably a phosphate, exists in considerable quantity in the hair, black pigment, and other deeply colored epithelial or horny sub- stances. Aluminium, Manganese, Copper, and Lead.—It seems most likely that in the human body, copper, manganesium, alumin- ium, and lead are merely accidental elements, which, being taken in minute quantities with the food, and not excreted at once with the fseces, are absorbed and deposited in some tissue or organ, of which, however, they form no necessary part. In the same manner, arsenic, being absorbed, may be deposited in the liver and other parts. CHAPTER III. STRUCTURAL COMPOSITION OF THE HUMAN BODY. In the investigation of the structural composition of the human body, it will be well to consider in the first place, what are the simplest anatomical elements which enter into its for- mation, and then proceed to examine those more complicated tissues which are produced by their union. It may be premised, that in all the living parts of all living things, animal and vegetable, there is invariably to be dis- covered, entering into the formation of their anatomical ele- ments, a greater or less amount of a substance, which, in chemical composition and general characters, is indistinguish- PROTOPLASM. 27 able from albumen. As it exists in a living tissue or organ, it differs essentially from mere albumen in the fact of its possess- ing the power of growth, development, and the like; but in chemical composition it is identical with it. This albuminous substance has received various names ac- cording to the structures in which it has been found, and the theory of its nature and uses which may have presented itself most strongly to the minds of its observers. In the bodies of the lowest animals, as the Rhizopoda or Gregarinida, of which it forms the greater portion, it has been called “sarcode,” from its chemical resemblance to the flesh of the higher animals. When discovered in vegetable cells, and supposed to be the prime agent in their construction, it was termed “protoplasm.” As the presumed formative matter in animal tissues it was called “ blastema and, with the belief that wherever found, it alone of all matters has to do with generation and nutrition, I)r. Beale has surnamed it “germinal matter.” So far as can be discovered, there is no difference in chemical composition between the protoplasm of one part or organism and that of another. The movements which can be seen in certain vegetable cells apparently belong to a substance which is identical in composition with that which constitutes the greater portion of the bodies of the lowest animals, and which is present in greater or less quantity in all the living parts of the highest. So much appears to be a fact;—that in all living parts there exists an albuminous substance, in which in favor- able cases for observation in vegetable and the lower animal organisms, there can be noticed certain phenomena which are not to be accounted for by physical impressions from without, but are the result of inherent properties we call vital. For example, if a hair of the Tradescantia Virginica, or of many other plants, be examined under the microscope, there is seen in each individual cell a movement of the protoplasmic con- tents in a certain definite direction around the interior of the cell. Each cell is a closed sac or bag, and its contents are therefore quite cut off from the direct influence of any motive power from without. The motion of the particles, moreover, in a circuit around the interior of the cell, precludes the notion of its being due to any other than those molecular changes which we call vital. Again, in the lowest animals, whose bodies resemble more than anything else a minute mass of jelly, and which appear to be made up almost solely of this albuminous protoplasm, there are movements in correspondence with the needs of the organism, whether with respect to seiz- ing food or any other purpose, which are unaccountable accord- ing to any known physical laws, and can only be called vital. 28 STRUCTURAL COMPOSITION OF HUMAN BODY. In many, too, there is a kind of molecular current, exactly resembling that which is seen in a vegetable cell. In the higher animals, phenomena such as these are so sub- ordinate to the more complex manifestations of life that they are apt to be overlooked ; but they exist nevertheless. The mere nutrition of each part of the body in man or in the higher animals, is performed after a fashion which is strictly analogous to that which holds good in the case of a vegetable cell, or a rhizopod ; or, in other words, the life of each anatomi- cal element in a complex structure, like the human body, re- sembles very closely the life of what in the lowest organisms constitutes the whole being. For example, the thin scaly covering or epidermis, which forms the outer part of a man’s skin, is made up of minute cells, which, when living, are com- posed in part of protoplasm, and which are continually wear- ing away and being replaced by new similar elements from beneath ; and this process of quick waste and repair could only take place under the very complex conditions of nutrition which exist in man. One working part of the organism of an animal is so inextricably interwoven with that of another, that any want or defect in one, is soon or immediately felt by the whole ; and the epidermis, which only subserves a mechani- cal function, would be altered very soon by any defect in the more essential parts concerned in circulation, respiration, &c. But if we take simply the life history of one of the small cells which constitute the epidermis, we find that it absorbs nour- ishment from the parts around, grows, and develops in a manner analogous to that which belongs to a cell which con- stitutes part of a vegetable structure, or even a cell which by itself forms an independent being. Remembering, however, the invariable presence of a living albuminous matter or protoplasm of apparently identical com- position in all living tissues, animal and vegetable, we must not forget that its relations to the parts with which it is in- corporated are still very doubtfully known; and all theories concerning it must be considered only tentative and of uncer- tain stability. Among the anatomical elements of the human body, some appear, even with the help of the best microscopic apparatus, perfectly uniform and simple: they show no trace of struc- ture, i. e., of being composed of definitely arranged dissimilar parts. These are named simple, structureless, or amorphous substances. Such is the simple membrane which forms the walls of most primary cells, of the finest gland-ducts, and of the sarcolemma of muscular fibre; and such is the membrane enveloping the vitreous humor of the eye. Such also, having 29 NUCLEI. a dimly granular appearance, but no really granular struc- ture, is the intercellular substance of the so-called hyaline car- tilage. In the parts which present determinate structure, certain primary forms may be distinguished, which, by their various modifications and modes of combination make up the tissues and organs of the body. Such are, 1. Granules or molecules, the simplest and minutest of the primary forms. They are particles of various sizes, from immeasurable minuteness to the 10,000th of an inch in diameter; of various and generally un- certain composition, but usually so affecting light transmitted through them, that at different focal distances their centre, or margin, or whole substance, appears black. From this char- acter, as well as from their low specific gravity (for in micro- scopic examinations they always appear lighter than water), and from their solubility in ether when they can be favorably tested, it is probable that most granules are formed of fatty or oily matter ; or, since they do not coalesce as minute drops of oil would, that they are particles of oil coated over with albumen deposited on them from the fluid in which they float. In any fluid that is not too viscid, they exhibit the phenome- non of molecular motion, shaking and vibrating incessantly, and sometimes moving through the fluid, probably, in great measure, under the influence of external vibration. Granules may be either free, as in milk, chyle, milky serum, yolk-substance, and most tissues containing cells with granules; or inclosed, as are the granules in nerve-corpuscles, gland-cells, and epithelium-cells, the pigment granules in the pigmentum nigrum and medullary substance of the hair; or imbedded, as are the granules of phosphate and carbonate of lime, in bones and teeth. 2. Nuclei, or cytoblasts (Fig. 1, b), appear to be the simplest elementary structures, next to granules. They were thus named in accordance with the hypothesis that they are always connected with cells, or tissues formed from cells, and that in the development of these, each nucleus is the germ or centre around which the cell is formed. The hypothesis is only par- tially true, but the terms based on it are too familiarly ac- cepted to make it advisable to change them till some more exact and comprehensive theory is formed. Of the corpuscles called nuclei some are minute cellules or vesicles, with walls formed of simple membrane, inclosing often one or more particles, like minute granules, called nu- cleoli (Fig. 1, c). Other nuclei, again, appear to be simply small masses of protoplasm, with no trace of vesicular struc- ture. 30 STRUCTURAL COMPOSITION OF HUMAN BODY. One of the most general characters of the nucleus, and the most useful in microscopic examinations, is, that it is neither dissolved nor made transparent by acetic acid, but acquires, when that fluid is in contact with it, a darker and more dis- tinct outline. It is commonly, too, the part of the mature cell which is capable of being stained by an ammoniacal solu- tion of carmine—the test, it may be remarked, by which, ac- cording to Dr. Beale, protoplasm or germinal matter may be always known. Nuclei may be either free or attached. Free nuclei are such as either float in fluid, like those in some of the secretions, which appear to be derived from the secreting cells of the glands, or lie loosely imbedded in solid substance, as in the gray matter of the brain and spinal cord, and most abun- dantly in some quickly-growing tumors. Attached nuclei are either closely imbedded in homogeneous pellucid substance, as in rudimental cellular tissue; or are fixed on the surface of fibres, as on those of organic muscle and organic nerve-fibres; or are inclosed in cells, or in tissues formed by the extension or junction of cells. Nuclei inclosed in cells appear to be at- tached to the inner surface of the cell-wall, projecting into the cavity. Their position in relation to the centre or axis of the cell is uncertain ; often when the cell lies on a flat or broad surface, they appear central, as in blood-corpuscles, epithelium- cells, whether tessellated or cylindrical; but, perhaps, more often their position has no regular relation to the centre of the cell. In most instances, each cell contains only a single nucleus; but in cartilage, especially when it is growing or ossifying, two or more nuclei in each cell are common; and the development of new cells is often effected by a division or multiplication of nuclei in the cavity of a parent cell; as in the primary blood-cells of the embryo, in the germinal vesicle, and others. When cells extend and coalesce, so that their walls form tubes or sheaths, the nuclei commonly remain attached to the inner surface of the wall. Thus they are seen imbedded in the walls of the minutest capillary bloodvessels of, for exam- ple, the retina and brain; in the sarcolemma of transversely striated muscular fibres; and in minute gland-tubes. Nuclei are most commonly oval or round, and do not gen- erally conform themselves to the diverse shapes which the cells assume; they are altogether less variable elements, even in regard to size, than the cells are, of which fact one may see a good example in the uniformity of the nuclei in cells so mul- tiform as those of epithelium. But sometimes they appear to be developed into filaments, elongating themselves and becom- CELLS, 31 ing solid, and uniting end to end for greater length, or by lat- eral branches to form a network. So, according to Henle, are formed the filaments of the striated and fenestrated coats of arteries; and according to Beale, the so-called connective-tis- sue corpuscles are to be considered branched nuclei, formed of protoplasm or germinal matter. 3. Cells.—The word “cell” of course implies strictly a hollow body, and the term was a sufficiently good one when all so- called cells were considered to be small bags with a membra- nous envelope, and more or less liquid contents. Many bodies, however, which are still called cells do not answer to this de- scription, and the term, therefore, if taken in its literal signifi- cation, is very apt to lead astray, and, indeed, very frequently does so. It is too widely used, however, to be given up, at least for the present, and we must therefore consider the term to indicate, either a membranous closed bag with more or less liquid contents, and almost always a nucleus ; or a small semi- solid mass of protoplasm, with no more definite boundary-wall than such as has been formed by a condensation of its outer layers, but with, most commonly, a small granular substance in the centre, called, as in the first place, a nucleus. In both cases the nucleus may contain a nucleolus. Fat-cells (Fig. 11) are examples of the first kind of cells; white blood-corpuscles (Fig. 26) of the second. The cell-wall, when there is one, never presents any appear- ance of structure: it appears sometimes to be an albuminous substance; sometimes a horny matter, as in thick and dried cuticle. In almost all cases (the dry cells of horny tissue, perhaps, alone excepted) the cell-wall is made transparent by acetic acid, which also penetrates into the interior and distends it, so that it can hardly be discerned. But in such cases the cell-wall is usually not dissolved ; it may be brought into view again by merely neutralizing the acid with soda or potash. The simplest shape of cells, and that which is probably the normal shape of the primary cell, is oval or spheroidal, as in cartilage-cells and lymph-corpuscles; but in many instances they are flattened and discoid, as in the red blood-corpuscles (Fig. 26); or scale-like, as in the epidermis and tessellated epithelium (Fig. 2). By mutual pressure they may become many-sided, as are most of the pigment-cells of the choroidal pigmentum nigrum (Fig. 12), and those in close-textured adipose tissue; they may assume a conical or cylindriform or prismatic shape, as in the varieties of cylinder-epithelium (Fig. 4); or be caudate, as in certain bodies in the spleen ; they may send out exceedingly fine processes in the form of 32 STRUCTURAL COMPOSITION OF HUMAN BODY. vibratile cilia (Fig. 6), or larger processes, with which they become stellate, or variously caudate, as in some of the rami- fied pigment-cells of the choroid coat of the eye (Fig. 13). The contents of all living cells, including the nucleus, are formed in a greater or less degree of protoplasm—less as the cell grows older. But, besides, cells contain matters almost infinitely various, according to the position, office, and age of the cell. In adipose tissue they are the oily matter of the fat; in gland-cells, the contents are the proper substance of the secretion, bile, semen, &c., as the case may be ; in pigment-cells they are the pigment-granules that give the color; and in the numerous instances in which the cell-contents can be neither seen because they are pellucid, nor tested because of their minute quantity, they are yet, probably, peculiar in each tissue, and constitute the greater part of the proper substance of each. Commonly, when the contents are pellucid, they contain gran- ules which float in them; and when water is added, and the contents are diluted, the granules display an active molecular movement within the cavity of the cell. Such a movement may be seen by adding water to mucus, or granulation-corpus- cles, or to those of lymph. In a few cases, the whole cavity of the cell is filled with granules: it is so in yolk-cells and milk-corpuscles, in the large diseased corpuscles often found among the products of inflammation, and in some cells when they are the seat of extreme fatty degeneration. All cells containing abundant granules appear to be either lowly organ- ized, as for nutriment, e. g., yolk-cells, or degenerate, e. g., granule-cells of inflammation, or of mucus. The peculiar con- tents of cells may be often observed to accumulate first around or directly over the nuclei, as in the cells of black pigment, in those of melanotic tumors, and in those of the liver during the retention of bile. Intercellular substance is the material in which, in certain tissues, the cells are imbedded. Its quantity is very variable. In the finer epithelia, especially the columnar epithelium on the mucous membrane of the intestines, it can be just seen fill- ing the interstices of the close-set cells; here it has no appear- ance of structure. In cartilage and bone, it forms a large portion of the whole substance of the tissue, and is either hom- ogenous and finely granular (Fig. 14), or osseous, or, as in fibro-cartilage, resembles fine fibrous tissue (Fig. 15). In some cases the cells are very loosely connected with the intercellular substance, and may be nearly separated from it, as in fibro-car- tilage ; but in some their walls seem amalgamated with it. The foregoing may be regarded as the simplest and the near- est to the primary forms assumed in the organization of animal TUBULES. 33 matter; as the states into which this passes in becoming a solid tissue living or capable of life. By the further development of tissue thus far organized, higher or secondary forms are pro- duced, which it will be sufficient in this place merely to enu- merate. Such are, 4. Filaments, or Fibrils.—Threads of exceeding fineness, from an inch upwards. Such filaments are cylindriform, as are those of the striated muscular and the fibro-cellular or areolar tissue (Fig. 8); or flattened, as are those of the organic muscles. Filaments usually lie in parallel fasciculi, as in mus- cular and tendinous tissues; but in some instances are matted or reticular with branches and intercommunication, as are the filaments of the middle coat, and of the longitudinally-fibrous coat of arteries; and, in other instances, are spirally wound, or very tortuous, as in the common fibro-cellular tissue (Fig. 9). 5. Fibres in the instances to which the name is commonly applied are larger than filaments or fibrils, but are by no es- sential general character distinguished from them. The flat- tened band-like fibres of the coarser varieties of organic muscle or elastic tissue (Fig. 10) are the simplest examples of this form; the toothed fibres of the crystalline lens are more com- plex; and more compound, so as hardly to permit of being classed as elementary forms, are the striated muscular fibres, which consist of bundles of filaments inclosed in separate mem- branous sheaths, and the cerebro-spinal nerve-fibres, in which similar sheaths inclose apparently two varieties of nerve-sub- stance. 6. Tubules are formed of simple or structureless membrane, such as the investing sheaths of striated muscular and cerebro- spinal nerve-fibres, and the basement-membrane or proper wall of the fine ducts of secreting glands; or they may be formed, as in the case of the minute capillary lymph and bloodvessels, by the apposition, edge to edge, in a single layer, of variously shaped flattened cells (Fig. 48). With these simple materials, the various parts of the body are built up; the more elementary tissues being, so to speak, first compounded of them; while these again are variously mixed and interwoven to form more intricate combinations. Thus are constructed epithelium and its modifications, con- nective tissue, fat, cartilage, bone, the fibres of muscle and nerve, &c.; and these again, with the more simple structures before mentioned, are used as materials wherewith to form ar- teries, veins, and lymphatics, secreting and vascular glands, lungs, heart, liver, and other parts of the body. 34 ELEMENTARY TISSUES. CHAPTER IV.1 STRUCTURE OF THE ELEMENTARY TISSUES. One of the simplest of the elementary structures of which the human body is made up, is that which has received the name of Epithelium. Composed of nucleated cells which are arranged most commonly in the form of a continuous mem- brane, it lines the free surfaces both of the inside and outside of the body, and its varieties, with one exception, have been named after the shapes which the individual cells in different parts assume. Classified thus, Epithelium presents itself under four principal forms, the characters of each of which are dis- tinct enough in well-marked examples; but when, as frequently happens, a continuous surface possesses at different parts two or more different epithelia, there is a very gradual transition from one to the other. 1. The first and most common variety is the squamous or tessellated epithelium (Figs. 1 and 2) which is composed of flat, Epithelium. Fig. 1. Fig. 2. Fig. 1. Fragment of epithelium from a serous membrane (peritoneum) ; magnified 410 diameters, a, cell; b, nucleus; c, nucleoli (Henle). Fig. 2. Epithelium scales from the inside of the mouth; magnified 260 diameters (Henle). oval, roundish, dr polygonal nucleated cells, of various size, arranged in one, or in many superposed layers. Arranged in 1 The following chapter, containing an outline-description of the elementary tissues, has been inserted for the convenience of students. For a much fuller and better account, the reader may be referred to Dr. Sharpey’s admirable descriptions in Quain’s Anatomy. EPITHELIUM. 35 several superposed layers this form of epithelium covers the skin, where it is called the Epidermis, and is spread over the mouth, pharynx, and oesophagus, the conjunctiva covering the eye, the vagina, and entrance of the urethra in both sexes; while, as a single layer the same kind of epithelium lines the interior of most of the serous and synovial sacs, and of the heart, bloodvessels, and lymphvessels. 2. Another variety of epithelium named spheroidal, from the usually more or less rounded outline of the cells composing it (d, Fig. 3), is found chiefly lining the interior of the ducts of the compound glands, and more or less completely filling the small sacculations or acini, in which they terminate. It commonly indeed occupies the true secreting parts of all glands, and hence is sometimes called glandular epithelium Fig. 3. The gastric glands of the human stomach (magnified), a, deep part of a pyloric gastric gland (from Kolliker); the cylindrical epithelium is traceable to the csecal extremities, b and c, cardiac gastric glands (from Allen Thomson); b, vertical sec- tion of a small portion of the mucous membrane with the glands magnified 30 diame- ters; c, deeper portion of one of the glands, magnified 65 diameters, showing a slight division of the tubes, and a sacculated appearance produced by the large glandular cells within them; d, cellular elements of the cardiac glands magnified 250 diameters. (6, c, and d, Fig. 3). Often, from mutual pressure, the cells acquire a polygonal outline. From the fact, however, of the term spheroidal epithelium being a generic one for almost all gland-cells, the shapes and sizes of the cells composing this variety-of epithelium are, as might be expected, very diverse in different parts of the body. 3. The third variety is the cylindrical or columnar epithelium (Figs. 4 and 5), which extends from the cardiac orifice of the 36 ELEMENTARY TISSUES. stomach along the whole of the digestive canal to the anus, and lines the principal gland-ducts which open upon the mucous Fig. 4. Cylindrical epithelium from intestinal villus of a rabbit; magnified 300 diameters (from Kolliker). surface of this tract, sometimes throughout their whole extent (a, Fig. 3), but in some cases only at the part nearest to the orifice (b and c). It is also found in the gall-bladder and in the greater portion of the urethra, and in some other parts, as the duct of the parotid gland and of the testicle. It is com- posed of oblong cells closely packed, and placed perpendicu- larly to the surface they cover, their deeper or attached ex- tremities being most commonly smaller than those which are free. Each of such cells incloses, at nearly mid distance be- tween its base and apex, a flat nucleus with nucleoli (b, Fig. 5); Fig. 5. Cylinders of the intestinal epithelium (after Henle): b, from the jejunum ; c, cyl- inders of the intestinal epithelium as seen when looking on their free extremities; d, ditto, as seen on a transverse section of a villus. the nuclei being arranged at such heights in contiguous cells as not to interfere with each other by mutual pressure. 4. In the fourth variety of epithelium cells, usually cylin- drical, but occasionally of some other shape, are provided at their free extremities with several fine pellucid pliant processes or cilia (Figs. 6 and 7). This form of epithelium lines the whole respiratory tract of mucous membrane and its prolonga- EPITHELIUM. 37 tions. It occurs also in some parts of the generative apparatus; in the male, lining the vasa efferentia of the testicle, and their prolongations as far as the lower end of the epididymis; and Fig. 6. Fig. 7. Fig. 6. Spheroidal ciliated cells front the mouth of the frog; magnified 300 diameters (Sharpey). Fig. 7. Columnar ciliated epithelium cells from the human nasal membrane ; magnified 300 diameters (Sharpey). in the female commencing about the middle of the neck of the uterus, and extending to the fimbriated extremities of the Fallopian tubes, and for a short distance along the peritoneal surface of the latter. A tessellated epithelium, with scales partly covered with cilia, lines, in great part, the interior of the cerebral ventricles, and of the minute central canal of the spinal cord. If a portion of ciliary mucous membrane from a living or recently dead animal be moistened and examined with a micro- scope, the cilia are observed to be in constant motion, moving continually backwards and forwards, and alternately rising and falling with a lashing or fanning movement. The ap- pearance is not unlike that of the waves in a field of wheat, or swiftly running and rippling water. The general result of their movements is to .produce a continuous current in a de- terminate direction, and this direction is invariably the same on the same surface, being usually in the case of a cavity towards its external orifice. Uses of Epithelium.—The various kinds of epithelium serve one general purpose, namely, that of protecting, and at the same time rendering smooth, the surfaces on which they are placed. But each, also, discharges a special office in relation to the particular function of the membrane on which it is placed. In mucous and synovial membranes it is highly probable that the epithelium cells, whatever be their forms and what- ever their other functions, are the organs in which by a regular process of elaboration and secretion, such as will be afterwards 38 ELEMENTARY TISSUES. described, mucus and synovial fluid are formed and discharged. (See chapter on Secretion.) Ciliated epithelium has another superadded function. By means of the current set up by its cilia in the air or fluid in contact with them, it is enabled to propel the fluids or minute particles of solid matter, which come within the range of its influence, and aid in their expulsion from the body. In the respiratory tract of mucous membrane the current set up in the air may also assist in the diffusion and change of gases, on which the due aeration of the blood depends. In the Fallopian tube the direction of the current excited by the cilia is towards the cavity of the uterus, and may thus be of service in aiding the progress of the ovum. Of the purposes served by the cilia which line the ventricles of the brain nothing is known. The nature of ciliary motion and the circumstances by which it is influenced will be considered hereafter. (See chapter on Motion.) Epithelium is devoid of bloodvessels and lymphatics. The cells composing it are nourished by absorption of nutrient matter from the tissues on which they rest; and as they grow old they are cast off* and replaced by new cells from beneath. Areolar, Cellular, or Connective Tissue. This tissue, which has received various names according to the qualities which seemed most important to the authors who Fig. 8. Filaments of areolar tissue, in larger and smaller bundles, as seen under a magni- fying power of 400 diameters (Sharpey). AREOLAR TISSUE. 39 have described it, is met with in some form or other in every region of the body; the areolar tissue of one district being, directly or indirectly, continuous with that of all others. In most parts of the body this structure contains fat, but the quantity of the latter is very variable, and in some few re- gions it is absent altogether (p. 40). Probably no nerves are distributed to areolar tissue itself, although they pass through it to other structures ; and although bloodvessels are supplied to it, yet they are sparing in quantity, if we except those des- tined for the fat which is held in its meshes. Under the microscope areolar tissue seems composed of a meshwork of fine fibres of two kinds. The first, which makes up the greater part of the tissue, is formed of very fine white structureless fibres, arranged closely in bands and bundles, of wavelike appearance when not stretched out, and crossing and intersecting in all directions (Fig. 8). The second kind, or the yellow elastic fibre (Fig. 10), has a much sharper and Fig. 9. Magnified view of areolar tissues (from different parts) treated with acetic acid. The white filaments are no longer seen, and the yellow or elastic fibres with the nuclei come into view. At c, elastic fibres wind round a bundle of white fibres, which, by the effect of the acid, is swollen out between the turns. Some connective- tissue corpuscles are indistinctly represented in c (Sharpey). darker outline, and is not arranged in bundles, but intimately mingled with the first variety, as more or less separate and well-defined fibres, which twist among and around the bundles of white filaments (Fig. 9). Sometimes the yellow fibres divide at their ends and anastomose with each other by means of the branches. Among the fibrous parts of areolar or connective-tissue are little nuclear bodies of' various shapes, 40 ELEMENTARY TISSUES. called connective-tissue corpuscles (Fig. 9, c), some of which are prolonged at various points of their outline into small pro- cesses which meet and join others like them proceeding from their neighbors. The chief functions of areolar tissue seem to consist in the investment and mechanical support of various parts, and as a connecting bond between such structures as may need it. The connective-tissue corpuscles, which, according to Beale, are small branched particles of ger- minal matter or protoplasm, probably minister to the nutri- tion of the texture in which they are seated. In various parts of the body, each of the two constituents of areolar tissue which have been just mentioned, may exist sepa- rately, or nearly so. Thus ten- dons, fasciae, and the like more or less inelastic structures, are formed almost exclusively of the white fibrous tissue, arranged according to the purpose re- quired, either in parallel bun- dles or membranous meshes; while the yellow elastic fibres are found to make up almost alone such elastic structures as the vocal cords, the ligamenta subfiava, &c., and to enter largely into the composition of the bloodvessels, the trachea, the lungs, and many other parts of the body. Fig. 10. Elastic fibres from the ligamenta subflava, magnified about 200 diame- ters (Sharpey). Adipose Tissue. In almost all regions of the human body a larger or smaller quantity of adipose or fatty tissue is present; the chief excep- tions being the subcutaneous tissue of the eyelids, penis and scrotum, the nymphse, and the cavity of the cranium. Adipose tissue is also absent from the substance of many organs, as the lungs, liver, and others. Fatty matter, not in the form of a distinct tissue, is also widely present in the body, as the fat of the liver and brain, of the blood and chyle, &c. Adipose tissue is almost always found seated in areolar tissue, and forms in its meshes little masses of unequal size AREOLAR TISSUE. 41 and irregular shape, to which the term lobules is commonly applied. Under the microscope it is found to consist essentially Fig. 11. A small cluster of fat-cells; magnified 150 diameters (Sliarpey). of little vesicles or cells about TJ0th or-3-’()tli 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. A nucleus is always present in some part or other of the cell-wyall; but in the ordi- nary condition of the cell it is not easily or always visible. The ultimate cells are held together by capillary bloodvessels ; while the little clusters thus formed are grouped into small masses, and held so, in most cases, by areolar tissue. The only matter contained in the cells is composed chiefly of the com- pounds of fatty acids with glycerin, which are named olein, stearin, and palmitin. It is doubtful whether lymphatics or nerves are supplied to fat, although both pass through it on their way to other structures. Among the uses of fat, these seem to be the chief: 1. It serves as a store of combustible matter which may be reabsorbed into the blood when occasion requires, and being burnt, may help to preserve the heat of the body. 2. 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 sur- face. 3. 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 unen- dangered by pressure. As good examples of situations in which 42 ELEMENTARY TISSUES. fat serves such purposes may be mentioned the palms of the hands, and soles of the feet, and the orbits. 4. In the long bones, fatty tissue, in the form known as marrow, serves to fill up the medullary canal, and to support the small bloodvessels which are distributed from it to the inner part of the substance of the bone. Pigment. In various parts of the body there exists a considerable quantity of dark pigmentary matter, e. g., in the choroid coat of the eye, at the back of the iris, in the skin, &c. In all these cases the dark color is due to the presence of so-called pigment- cells. Pigment-cells are for the most part polyhedral (Fig. 12) or spheroidal, although sometimes they have irregular processes, as shown in Fig. 13. The cell-wall itself is colorless,—the dark tint being produced by small dark granules heaped closely together, and more or less concealing the nucleus, itself color- Fig. 12. Fig. 1?. Fig. 12. Pigment-cells from the choroid; magnified 370 diameters (Henle). A, cells still cohering, seen on their surface; b, nucleus indistinctly seen. In the other cells the nucleus is concealed by the pigment-granules. Fig. 13. Ramified pigment-cells, from the tissue of the choroid coat of the eye ; magnified 350 diameters (after Kolliker). a, cells with pigment; 6, colorless fusi- form cells. less, which each cell contains. The dark tint of the skin, in those of dark complexion and in the colored races, is seated chiefly in the epidermis, and depends on the presence of pig- ment-cells, which, except in the presence of the dark granules in their interior, closely resemble the colorless cells with which they are mingled. The pigment-cells are situate chiefly in the CARTILAGE. 43 deep layer of the epidermis, or the so-called rete mucosum. (See chapter on the Skin.) The pigmentary matter is a very insoluble compound of carbon, hydrogen, nitrogen, and oxygen,—the carbon largely predominating ; besides, there is a small quantity of saline matter. The uses of pigment in most parts of the body are not clear. In the eyeball it is evidently intended for the absorption of superfluous rays of light. Cartilage or gristle exists in different forms in the human body, and has been classified under two chief heads, namely, temporary and permanent cartilage ; the former term being ap- plied to that kind of cartilage which, in the foetus and in young subjects, is destined to be converted into bone. The varieties of permanent cartilage have been arranged in three classes, namely, the cellular, the hyaline, and the fibrous carti- lages,—the last-named, being again capable of subdivision into two kinds, namely, elastic or yellow cartilage, and the so-called fibro-cartilage. Elastic cartilage, however, contains fibres, and fibro-carti- lage is more or less elastic; it will be well, therefore, for dis- tinction’s sake to term those two kinds white fibro-cartilage and yellow fibro-cartilage respectively. The accompanying table represents the classification of the varieties of cartilage: Cartilage. 1. Temporary. 2. Permanent. A Cellular. B. Hyaline. C. Fibrous. White fibro-cartilage. Yellow fibro-cartilage. All kinds of cartilage are composed of cells imbedded in a substance called the matrix: and the apparent differences 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 in- vested by a thin but tough and firm fibrous membrane called the perichrondrium. On the surface of the articular cartilage of the foetus, the perichondrium is represented 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. 44 ELKMENTAHY TISSUES. 1. Cellular cartilage may be readily obtained from the ex- ternal ear of rats, mice, or other small mammals. It is com- posed almost entirely of cells (hence its name), with little or no matrix. The latter, when present, consists of very fine fibres, which twine about the cells in various directions and inclose them in a kind of network. The cells are packed very closely together,—so much so that it is not easy in all cases to make out the fine fibres often encircling them. Cellular cartilage is found in the human subject, only in early foetal life, when it constitutes the Chorda dorsalis. (See chapter on Generation.) 2. Hyaline cartilage is met with largely in the human body,—investing the articular ends of bones, and forming the costal cartilages, the nasal cartilages, and those of the larynx, with the exception of the epiglottis and cornicida laryngis. Like other carti- lages it is composed of cells imbedded in a matrix (Fig. 14). The cells, which contain a nucleus with nucleoli, are irregu- lar in shape, and generally grouped together in patches. The patches are of various shapes and sizes, and placed at unequal distances apart. They generally appear flattened near the free surface of the mass of cartilage in which they are placed, and more or less perpendicular to the surface in the more deeply seated portions. The matrix in which they are imbedded has a dimly granu- lar appearance, like that of ground-glass. 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 ossified in old age, as also do some of those of the larynx. Temporary cartilage 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. Articular hyaline cartilage is reckoned among the so-called Fig. 14. A thin layer peeled off from the sur- face of the cartilage of the head of the humerus, showing flattened groups cff cells. The shrunken cell-bodies are dis- tinctly seen, but the limits of the capsu- lar cavities, where they adjoin one another, are hut faintly indicated. Mag- nified 400 diameters (after Sharpey). CARTILAGE. 45 non-vciscular structures, no bloodvessels being supplied directly to its own substance; it is nourished by those of the bone be- neath. When hyaline cartilage is in thicker masses, as in the case of the cartilages of the ribs, a few bloodvessels traverse its substance. The distinction, however, between all so-called vascular and non-vascular parts, is at the best a very artificial one. (See chapter on Nutrition.) Nerves are probably not supplied to any variety of cartilage. Fibrous cartilage, as before mentioned, occurs under two chief forms, the yellow and the white fibro-cartilage. Yellow fibro-cartilage is found in the external ear, in the epiglottis and cornicula laryngis, and in the eyelid. The cells are rounded or oval, with well-marked nuclei and nucleoli. The matrix in which they are seated is composed almost en- tirely of fine fibres, which form an intricate interlacement about the cells, and in their general characters are allied to the yellow variety of fibrous tissue (Fig. 15). White fibro-cartilage, which is much more widely distributed throughout the body than the foregoing kind, is composed like it, of cells and a matrix; the latter, however, being made up almost entirely of fibres close- ly resembling those of white fibrous tissue. In this kind of fibro-carti- lage it is not unusual to find a great part of its mass composed almost exclusively of fibres, and deserving the name of cartilage only from the fact that in another portion, continuous with it, cartilage-cells may be pretty freely distributed. The different situations in which white fibro-cartilage is formed have given rise to the following classification : 1. Interarticular fibro-cartilage, e. g., the semilunar carti- lages of the knee-joint. 2. Circumferential or marginal, as on the edges of the ace- tabulum and glenoid cavity of the scapula. 3. Connecting, e. g., the intervertebral fibro-cartilages. 4. Fibro-cartilage is found in the sheaths 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 Fig. 15. Section of the epiglottis, magnified 380 diameters (Dr. Baly). 46 ELEMENTARY TISSUES. posticus, in the sole of the foot, and usually in the neighboring tendon of the peroneus longus. The uses of cartilage are the following: in the joints, to form smooth surfaces for easy friction, and to act as a buffer, in shocks; to bind bones together, yet to allow a certain degree of movement, as between the vertebrse; to form a firm frame- work and protection, yet without undue stiffness or weight, as in the larynx and chest-walls; to deepen joint-cavities, as in the acetabulum, yet not so as to restrict the movements of the bones; to be, where such qualities are required, firm, tough, flexible, elastic, and strong. Structure of Bones and Teeth. Bone is composed of earthy and animal matter in the pro- portion of about 67 per cent, of the former to 33 per cent, of the latter. The earthy matter is composed chiefly of phos- phate of lime, but besides there is a small quantity, about 11 of the 67 per cent., of carbonate of lime, with minute quantities of some other salts. The animal matter is resolved into gela- tin by boiling. The earthy and animal constituents of bone are so intimately blended and incorporated 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 by the fact that when by acids the earthy matter is dis- solved out, or, on the other hand, when the animal part is burnt out, the general shape of the bone is alike preserved. To the naked eye there appear two kinds of structure in different bones, and in different parts of the same bone, namely, the dense or compact, and the cancellous tissue. Thus, in making a longitudinal section of a long bone, as the hume- rus or femur, the articular extremities are found capped on their surface by a thin shell of compact bone, while their in- terior is made up of the 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 a central canal, the medullary cavity—so called from its containing the medulla or marrow (p. 42). 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. The spaces in the cancellous tissue are filled by a species of marrow, which differs considerably from BONE 47 that of the shaft of the long bones. It is more fluid, and of a reddish color, and contains very few fat-cells. The surfaces of bones, except the parts covered with articu- lar cartilage, are clothed by a tough fibrous membrane, the periosteum; and it is from the bloodvessels which are distrib- uted first in this membrane, that the bones, especially their Fig. 16. Transverse section of compact tissue (of humerus) magnified about 150 diameters. Three of the Haversian canals are seen, with their concentric rings; also the cor- puscles or lacunae, with the canaliculi extending from them across the direction of the lamellae. The Haversian apertures had got filled with debris in grinding down the section, and therefore appear black in the figure, which represents the object as viewed with transmitted light (after Sharpey). more compact tissue, ai’e in great part supplied with nourish- ment—minute branches from the periosteal vessels entering the little foramina on the surface of the bone, and finding their way to the Haversian canals, to be immediately de- scribed. The long bones are supplied also by a proper nutri- ent artery, which, entering at some part of the shaft so as to reach the medullary canal, breaks up into branches for the supply of the marrow, from which again small vessels are dis- tributed to the interior of the bone. Other small bloodvessels pierce the articular extremities for the supply of the cancellous tissue. Notwithstanding the differences of arrangement just men- 48 ELEMENTARY TISSUES. tioned, the structure of all bone is found, under the microscope, to be essentially the same. Examined with a rather high power its substance is found occupied by a multitude of little spaces, called lacunce, with very minute canals or canaliculi, as they are termed, leading from them, and anastomosing with similar little prolongations from other lacunae (Fig. 16). In very thin layers of bone, no other canals than these may be visible; but on making a transverse section of the compact tissue, e. g., of a long bone, as the humerus or ulna, the arrangement shown in Fig. 16 can be seen. The bone seems mapped out into small circular districts, at or about the centre of each of which is a hole, and around this an appearance as of concentric layers ; the lacunae and canalindi following the same concentric plan of distribution around the small hole in the centre, with which, indeed, they communicate. On making a longitudinal section, the central holes are found to be simply the cut ex- tremities of small canals which run lengthwise through the bone (Fig. 17), 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 bloodvessels, and by means of them blood is conveyed to all, even the densest parts of the bone; the mi- nute canaliculi and lacunse absorbing nutrient matter from the Haversian bloodvessels, and conveying it still more intimately to the very substance of the bone which they traverse. The bloodvessels enter the Haversian canals both from without, by 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. According to Todd and Bowman, the arteries and veins usually occupy separate canals, and the veins, which are the larger, often present, at irregular intervals, small pouch-like dilatations (Fig. 17). The lacunce are occupied by nucleated cells, or, as Hr. Beale expresses it, minute portions of protoplasm or germinal matter; and there is every reason to believe that the lacunar cells are homologous with the corpuscles of the connective tissue, each little particle of protoplasm ministering to the nutrition of the bone immediately surrounding it, and one lacunar particle communicating with another, and with its surrounding district, and with the bloodvessels of the Haversian canals, by means of the minute streams of fluid nutrient matter which occupy the canaliculi. Besides the concentric lamellae of bone-tissue which surround the Haversian canal in the shaft of a long bone, are others, es- BONE 49 pecially near the circumference, which surround the whole bone, and are arranged concentrically with regard to the medullary canal. The ultimate structure of the lamella appears to be reticular. If a thin film be peeled off the surface of a bone from which Fig. 17.- Fig. 18. Fig. 17. Haversian canals, seen in a longitudinal section of the compact tissue of the shaft of one of the long bones. 1. Arterial canal; 2. Venous canal; 3. Dilatation of another venous canal. Fig. 18. Thin layer peeled off from a softened bone, as it appears under a magni- fying power of 400. This figure, which is intended to represent the reticular struc- ture of a lamella, gives a better idea of the object when held rather farther off than usual from the eye (from Sharpey). the earthy matter has been removed by acid, and examined with a high power of the microscope, it will be found composed, according to Sharpey, of a finely reticular structure, formed apparently of very slender fibres decussating obliquely, but coalescing at the points of intersection, as if here the fibres were fused rather than woven together (Fig. 18). In many places these reticular lamellae are perforated by tapering fibres, resembling in character the ordinary white or rarely the elastic fibrous tissue, which bolt the neighboring lamellae together, and may be drawn out when the latter are torn asunder (Fig. 19). Bone is developed after two different fashions. In one, the 50 ELEMENTARY TISSUES. tissue in which the earthy matter is laid down is a membrane, composed mainly of fibres and granular cells, like imperfectly developed connective-tissues. Of this kind of ossification in membrane, the flat bones of the skull are examples. In the other, and much more common case, of which a long bone may be cited as an instance, the ossification takes place in car- tilage. In most bones ossification begins at more than one point; and from these centres of ossification, as they are called, the process of deposition of calcareous matter advances in all directions. Bones grow by constant development of the car- tilage or membrane between these centres of ossification, until by the process of calcification advancing at a quicker rate than the development of the softer structures, the bone becomes im- Fig. 19. Lamellae torn off from a decalcified human parietal bone at some depth from the surface, a, a lamella, showing reticular fibres; b, b, darker part, where several lamellae are superposed; c, c, perforating fibres. Apertures through which perfor- ating fibres had passed, are seen especially in the lower part, a, a, of the figure. Magnitude as seen under a power of 200, but not drawn to a scale (from a drawing by Dr. Allen Thomson). pregnated throughout with calcareous matter, and can grow no more. In the long bones the main centres of ossification are seated at the middle of the shaft, and at each of the ex- tremities. Increase of the length of bones, therefore, occurs at TEETH 51 the part which intervenes between the ossifying centre in the shaft and that at each extremity; while increase in thickness takes place by the formation of layers of osseous tissue beneath the periosteum. The former is an example of ossification in cartilage; the latter of ossification in membrane. Teeth.—A tooth is generally described as possessing a crown, neck, and fang or fangs. The crown is the portion which pro- jects beyond the level of the gum. The neck is that constricted portion just below the crown, which 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. 20 and 21), 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 pidp-cavity, from its containing a very vascular and sensitive little mass com- posed of connective tissue, bloodvessels and nerves, which is called the tooth-pxdp. The pulp is continuous below, through an opening at the end of the fang, with the mucous membrane of the gum. Capping that part of the dentine which projects Fig. 20. Sections of an Incisor and Molar Tooth.—The longitudinal sections show the whole of the pulp-cavity in the incisor and molar teeth, its extension upwards within the crown, and its prolongation downwards into the fangs, with the small aperture at the point of each; these and the cross-section show the relation of the dentine and enamel. 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. At the neck of the tooth the cement is exceedingly thin, but it gradually becomes thicker as it ap- proaches and covers the lower end or apex of the fang. Dentine or ivory in chemical composition closely resembles 52 ELEMENTARY TISSUES. bone. It contains, however, rather less animal matter; the proportion in 100 parts being about 28 of animal matter to 72 of earthy. The former, like the animal matter of bone, may be resolved into gelatin by boil- ing. The earthy matter is made up chiefly of phosphate of lime, with a small portion of the car- bonate, and traces of some other salts. Under the microscope, den- tine is seen to be finely chan- nelled by a multitude of fine tubes, which, by their inner ends, communicate with the pulp-cavity, and by their outer extremities come into contact with the under part of the en- amel and cement, and some- times even penetrate them for a greater or less distance. In their course from the pulp-cavity to the surface of the dentine, these minute tubes form gentle and nearly parallel curves, and divide and subdivide dichotom- ously, but without much lessen- ing of their calibre until they are approaching their peripheral termination. From their sides proceed other exceedingly mi- nute secondary canals, which extend into the dentine between the tubules. The tubules of the dentine, the average diameter of which at their inner and larger ex- tremity is of an inch, con- tain fine prolongations from the tooth-palp, which give the den- tine a certain faint sensitiveness under ordinary circumstances, and without doubt, have to do also with its nutrition. The enamel, which is by far the hardest portion of a tooth, is composed, chemically, of the same elements that enter into the composition of dentine and bone. Its animal matter, how- ever, amounts only to about 2 or 3 per cent. Fig. 21. Magnified Longitudinal Section of a Bicuspid Tooth (after Retzius)—1, the ivory or dentine, showing the direc- tion and primary curves of the dental tubuli; 2, the pulp-cavity, with the small apertures of the tubuli into it; 3, the cement or crusta petrosa, cover- ing the fang as high as the border of the enamel at the neck, exhibiting lacunse; 4, the enamel resting on the dentine; this has been worn away by use from the upper part. TEETH 53 Examined under the microscope, enamel is found composed of fine hexagonal fibres (Figs. 22 and 23), which are set on end on the sur- face of the dentine, and fit into cor- responding 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 towards 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 itself is coated on the outside by a very thin calcified mem- brane, sometimes termed the cuticle of the enamel. The crusta petrosa, or cement, is composed of true bone, and in it are lacunae and canaliculi which some- times communicate with the outer finely-branched ends of the dentine- tubules. Development of Teeth.—The teeth are developed after the following manner: Along the free edge of the toothless gum in the foetus, there ex- tends a groove, or small trench, the primitive dental groove (Goodsir), and from the bottom of this project ten small processes of mucous membrane, or papillae, containing bloodvessels and nerves. As these papillae grow up from below, the edges of the small trench begin to grow in towards each other, and overshadow them, at the same time that each papilla is cut off from its neighbor by the extension of a partition wall from the gum, which grows in from each side to separate the one from the other. Thus closed in above and all around, each dental papilla is at length contained in a separate sac, and gradually assumes the character of a tooth by deposition on its surface of the various hard matters which have been just enumerated as composing the greater part of a tooth’s Fig. 22. Thin section of the enamel and a part of the dentine (from Kolliker) a, cu- ticular pellicle of the enamel; b, enamel fibres, or columns with fissures between them and cross striae; c, larger cavi- ties in the enamel, communi- cating with the extremities of some of the tubuli (d). 54 ELEMENTARY TISSUES. substance. The small vascular papilla is gradually encroached upon and imprisoned by the calcareous deposit, until only a small part of it is left as the tooth-pulp, which remains shut up in the harder substance, with only the before-mentioned small Fig. 23. Enamel fibres (from Kolliker) —y-. A, fragments and single fibres of tbe enamel, isolated by the action of hydrochloric acid. B, surface of a small fragment of enamel, showing the hexagonal ends of the fibres. communication with the outside, through the end of the fang. In this manner the first set of teeth, or the milk teeth, are formed; and each tooth, by degrees developing, presses at length on the wall of the sac inclosing it, and causing its ab- sorption, is cut, to use a familiar phrase. The temporary or milk teeth, having only a very limited term of existence, gradually decay and are shed, while the per- manent teeth push their way from beneath, by gradual increase and development, so as to succeed them. The temporary teeth are ten in each jaw, namely, four in- cisors, two canines, and four molars, and are replaced by ten permanent teeth, each of which is developed from a small sac set by, so to speak, from the sac of the temporary tooth which precedes it and called the cavity of reserve. The num- ber of the permanent teeth is, however, increased to sixteen, 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. The beginning of the development THE BLOOD. 55 of the permanent teeth of course takes place long before the cutting of those which they are to succeed; one of the first acts of the newly-formed little dental sac of a milk-tooth being to set aside a portion of itself as the germ of its successor. The following formula shows, at a glance, the comparative arrangement and number of the temporary and permanent teeth: MO. CA. IN. CA. MO. f Upper, 2 14 12 =10 I _ 20 [ Lower, 2 14 12 =10 Temporary Teeth,. MO. BI. CA. IN. CA. BI. MO. f Upper, 3 2 1 4 1 2 3 = 16 I 32 [Lower, 3214123 = 16' Permanent Teeth,. 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 tem- porary molars having a stronger likeness to the permanent than to their immediate descendants, the so-called bicuspids. The temporary incisors and canines differ but little, except in their smaller size, from their successors. CHAPTER V. THE BLOOD. Although it may seem, in some respects, un advisable to describe the blood before entering upon the physiology of those subservient processes which have for their end or purpose its formation and development, yet there are many reasons for taking such a course, and we may therefore at once proceed to consider the structural and chemical composition of this fluid. Wherever blood can be seen under a moderately high micro- scopic power as it flows in the vessels of a living part, it appears a colorless fluid containing minute colored particles. The greater part of these particles are red, when seen en masse, and they are the source of the color which, so far as the naked eye can see, belongs to every part of the blood alike. The colorless fluid is named liquor sanguinis; the particles are the 56 THE BLOOD. blood-corpuscles or blood-cells. The structural composition of the blood may be thus expressed: Liquid Blood, Corpuscles, . Clot (containing also more or less serum). Liquor Sanguinis or Plasma. Fibrin, Serum. When blood flows from the living body, it is a thickish heavy fluid, of a bright scarlet color when it comes from an artery; deep purple, or nearly black, when it flows from a vein. Its specific gravity at 60° F. is, on an average, 1055, that of water being reckoned as 1000; the extremes consistent with health being 1050 and 1059. Its temperature is generally about 100° F.; but it is not the same in all parts of the body. Thus, while the stream is slightly warmed by passing through the liver and some other parts, it is slightly cooled, according to Bernard, by traversing the capillaries of the skin. The temperature of blood in the left side of the heart is, again 1° or 2° higher than in the right (Savory). The blood has a slight alkaline reaction; and emits an odor similar to that which issues from the skin or breath of the animal from which it flows, but fainter. The alkaline reac- tion appears to be a constant character of blood in all animals and under all circumstances. An exception has been supposed to exist in the case of menstrual blood ; but the acid reaction which this sometimes presents is due to the mixture of an acid mucus from the uterus and vagina. Pure menstrual blood, such as may be obtained with a speculum, or from the uteri of women who die during menstruation, is always alkaline, and resembles ordinary blood. According to Bernard, blood becomes spontaneously acid after removal from the body, owing to conversion of its sugar into lactic acid. The odor of blood is easily perceived in the watery vapor, or halitus as it is called, which rises from blood just drawn : it may also be set free, long afterwards, by adding to the blood a mixture of equal parts of sulphuric acid and water. It is said to be not difficult to tell, by the likeness of the odor to that of the body, the species of domestic animal from which any specimen of blood has been taken : the strong odor of the pig or cat, and the peculiar milky smell of the cow, are es- pecially easy to be thus discerned in their blood (Barruel). Only an imperfect indication of the whole quantity of blood in the body is afforded by measurement of that which escapes, when an animal is rapidly bled to death, inasmuch as a cer- Quantity of Blood. QUANTITY OF BLOOD. 57 tain amount always remains in the bloodvessels. In cases of less rapid bleeding, on the other hand, when life is more pro- longed, and when, therefore, sufficient time elapses before death to allow some absorption into the circulating current of the fluids of the body (p. 76), the whole quantity of blood that escapes may be greater than the whole average amount natur- ally present in the vessels. Various means have been devised, therefore, for obtaining a more accurate estimate than that which results from merely bleeding animals to death. Welcker’s method is the following. An animal is rapidly bled to death, and the blood which escapes is collected and measured. The blood remaining in the smaller vessels is then removed by the injection of water through them, and the mix- ture of blood and water thus obtained, is also collected. The animal is then finely minced, and infused in water, and the infusion is mixed with the combined blood and water pre- viously obtained. Some of this fluid is then brushed on a white ground, and the color compared with that of mixtures of blood and water whose proportions have been previously determined by measurement. In this way the materials are obtained for a fairly exact estimate of the quantity of blood actually existing in the body of the animal experimented on. Another method (that of' Vierordt) consists in estimating the amount of blood expelled from the ventricle, at each beat of the heart, and multiplying this quantity by the number of beats necessary for completing the “round” of the circulation. This method is ingenious, but open to various objections, the most conclusive being the uncertainty of all the premises on which the conclusion is founded. Other methods depend on the results of injecting a known quantity of water (Valentin) or of saline matters (Blake) into the bloodvessels; the calculation being founded in the first case, on the diminution of the specific gravity which ensues, and in the other, on the quantity of the salt found diffused in a cer- tain measured amount of the blood abstracted for experiment. A nearly correct estimate was probably made by Weber and Lehmann, from the following data. A criminal was weighed before and after decapitation; the difference in the weight representing, of course, the quantity of blood which escaped. The bloodvessels of the head and trunk, were then washed out by the injection 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 repre- sented was calculated, by comparing the proportion of solid matter contained in it, with that of the first blood which 58 THE BLOOD. escaped on decapitation. Two experiments of this kind gave precisely similar results. The most reliable of these various means for estimating the quantity of blood in the body yield as nearly similar results as can be expected, when the sources of error unavoidably present in all, are taken into consideration; and it may be stated that in man, the weight of the whole quantity of blood, compared with that of the body, is from about 1 to 8, to 1 to 10. It must be remembered, however, that the whole quantity of blood varies, even in the same animal, very considerably, in correspondence with the different amounts of food and drink, which may have been recently taken in, and the equally vary- ing quantity of matter given out. Bernard found by experi- ment, that the quantity of blood obtainable from a fasting animal is scarcely more than half of that which is present soon after a full meal. The estimate above given, must there- fore be taken to represent only an approximate average. Coagulation of the Blood. When blood is drawn from the body, and left at rest, cer- tain changes ensue, which constitute a kind of rough analysis of it, and are instructive respecting the nature of some of its constitutents. After about ten minutes, taking a general average of many observations, it gradually clots or coagulates, becoming solid like a soft jelly. The clot thus formed has at first the same volume and appearance as the fluid blood had, and, like it, looks quite uniform; the only change seems to be, that the blood which was fluid is now solid. But presently, drops of transparent yellowish fluid begin to ooze from the surface of the solid clot; and these gradually collecting, first on its upper surface, and then all around it, the clot or “ cras- samentum,” diminished in size, but firmer than it was before, floats in a quantity of yellowish fluid, which is named serum, the quantity of which may continually increase for from twenty- four to forty-eight hours after the clotting of the blood. The changes just described may be thus explained. The liquor sanguinis, or liquid part of the blood (p. 55), consists of a thin fluid called serum, holding fibrin in solution.1 The peculiar property of fibrin, as already said, is its tendency to become solid when at rest, and in some other conditions. When, therefore, a quantity of blood is drawn from the vessels, the fibrin coagulates, and the blood-corpuscles, with part of the 1 This statement has been left unaltered in the text; but, as will be seen farther on, it requires modification.—(Ed.) COAGULATION OF BLOOD. 59 serum, are held, or, as it were, entangled in the solid substance which it forms. But after healthy fibrin has thus coagulated, it always con- tracts ; and what is generally described as one process of coagu- lation should rather be regarded as consisting of two parts or stages ; namely, first, the simple act of clotting, coagulating, or becoming solid; and, secondly, the contraction or condensa- tion of the solid clot thus formed. By this second act much of the serum which was soaked in the clot is gradually pressed out; and this collects in the vessel around the contracted clot. Thus, by the observation of blood within the vessels, and of the changes which commonly ensue when it is drawn from them, we may distinguish in it three principal constituents, namely, 1st, the fibrin, or coagulating substance; 2d, the serum; 3d, the corpuscles. That the fibrin is the only spontaneously coagulable material in the blood, may be proved in many ways; and most simply by employing any means whereby a portion of the liquor san- guinis, i. e., the serum and fibrin, can be separated from the red corpuscles before coagulation. Under ordinary circum- stances coagulation occurs before the red corpuscles have had time to subside; and thus, from their being entangled in the meshes of the fibrin, the clot is of a deep dark red color through- out,—somewhat darker, it may be, at the most dependent part, from accumulation of red cells, but not to any very marked degree. If, however, from any cause, the red cells sink more quickly than usual, or the fibrin contracts more slowly, then, in either of these cases, the red corpuscles may be observed, while the blood is yet fluid, to sink below its surface; and the layer beneath which they have sunk, and which has usually an opaline or grayish-white tint, will coagulate without them, and form a white clot consisting of fibrin alone, or of fibrin with entangled white corpuscles; for the white corpuscles, being very light, tend upwards towards the surface of the fluid. The layer of white clot which is thus formed rests on the top of a colored clot of ordinary character, i. e., of one in which the coagulating fibrin has entangled the red corpuscles while they were sinking: and, thus placed, it constitutes what has been called a bujfy coat. When a buffy coat is formed in the manner just described, it commonly contracts more than the rest of the clot does, and, drawing in at its sides, produces a cupped appearance on the top of the clot. In certain conditions of the system, and especially when there exists some local inflammation, this buffed and cupped con- dition of the clot is well marked, and there has been much dis- 60 THE BLOOD. cussion concerning its origin under these circumstances. It is now generally agreed that two causes combine to produce it. In the first place, the tendency of the red corpuscles to form rouleaux (see p. 68) is much exaggerated in inflammatory blood ; and as their rate of sinking increases with their aggre- gation, there is a ready explanation, at least in part, of the colorless condition of the top of the clot. And in the next place, inflammatory blood coagulates less rapidly than usual, and thus there is more time for the already rapidly sinking corpuscles to subside. The colorless or buffed condition of the upper part of the clot is therefore, readily accounted for; while the cupped appearance is easily explained by the greater power of contraction possessed by the fibrin of inflammatory blood and by its contraction being now not interfered with by the presence of red corpuscles in its meshes. Although the appearance just described is commonly the re- sult of a condition of the blood in which there is an increase in the quantity of fibrin, it need not of necessity be so. For a very different state of the blood, such as that which exists in chlorosis, may give rise to the same appearance; but in this case the pale layer is due to a relatively smaller amount of red corpuscles, not to any increase in the quantity of fibrin. It is thus evident that the coagulation of the blood is due to its fibrin. The cause of the coagulation of the fibrin, how- ever, is still a mystery. The theory of Prof. Lister, that fibrin has no natural ten- dency to clot, but that its coagulation 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 tissues, which cause them to act towards it like foreign matter, is insufficient; because even if it be true, it still leaves unex- plained the manner in which the fibrin, fluid in the living bloodvessels, can, by foreign matter, be thus made solid. If it be a fact, it is a very important one, but it is not an expla- nation. The same remark may be applied also to another theory which differs from the last, in that while it admits a natural tendency on the part of the blood to coagulation, it supposes that this tendency in the living body is restrained by some in- hibitory power resident in the walls of the containing vessels. This also may, or may not, be true; but it is only a statement of a possible fact, and leaves unexplained the manner in which living tissue can thus restrain coagulation. Dr. Draper believes that coagulation takes place in the liv- ing body, as out of it, or as in the dead ; but in the one case the fibrin is picked out in the course of the circulation by tis- COAGULATION OF BLOOD. 61 sues which this particular constituent of the blood is destined to nourish; in the others, it remains and becomes evident as a clot. This explanation is ingenious, but requires some kind of proof before it can be adopted. Concerning other theories, as for instance, that coagulation is due to the escape of carbonic acid, or of ammonia, it need only be said that they have been completely disproved. We must therefore, for the present, believe that the cause of the coagulation of the blood has yet to be discovered; but some very interesting observations in connection with the sub- ject have been recently made, and seem not unlikely to lead in time to a solution of this difficult and most vexed question. The observations referred to have been made independently by Alexander Schmidt, although he was forestalled in regard to some of his experiments by Dr. Andrew Buchanan, of Glas- gow, many years ago. When blood-serum, or washed blood-clot, is added to the fluid of hydrocele, or any other serous effusion, it speedily causes coagulation, and the production of true fibrin. And this phenomenon occurs also on the admixture of serous effu- sions from different parts of the body, as that of hydrocele with that of ascites, or of either with fluid from the cavity of the pleura. Other substances also, as muscular or nervous tissue, skin, &c., have been found also able to excite coagulation in serous fluids. Thus, fluids which have little or no tendency to coagulate when left to themselves, can be made to produce a clot, apparently identical with the fibrin of blood by the addi- tion to them of matter which, on its part, was not known to have any special relation*to fibrin. As may be supposed, the coagulation is not alike in extent under all these circumstances. Thus, although it occurs when apparently few or no blood-cells exist in either constituent of the mixture, yet the addition of these very much increases the effect, and their presence evi- dently has a very close connection with the process. From the action of the buffy coat of a clot, in causing the appearance of fibrin in serous effusions, it may be inferred that the pale as well as the red corpuscles are influential in coagulation under these circumstances. Blood-crystals are also found to be effec- tive in producing a clot in serous fluids. The true explanation of these very curious phenomena is, probably, not fully known; but Schmidt supposes that in the act of formation of fibrin there occurs the union of two substances, which he terms fibrinoplastin and fibrinogen. The substance which he terms fibrinoplastin, and which he has obtained, not only from blood, but from many other liquids 62 THE BLOOD. and solids, as the crystalline lens, chyle and lymph, connec- tive tissue, &c., which are found capable of exciting coagula- tion in serous fluids, is probably identical with the globulin of the red corpuscles. The fibrinogenous matter obtained from serous effusions dif- fers but little, chemically, from the fibrinoplastin. Thus iu the experiment before mentioned, the globulin or fibrinoplastic matter of the blood-cells in the clot causes co- agulation by uniting with the fibrinogen present in the hydro- cele-fluid. And whenever there occurs coagulation with the production of fibrin, whether in ordinary bloodclotting, or in the admixture of serous effusions, or in any other way, a like union of these two substances may be supposed to occur. The main result, therefore, of these very interesting experi- ments and observations has been to make it probable that the idea of fibrin existing in a liquid state in the blood is founded on a mistaken notion of its real nature, and that, probably, it does not exist at all in solution as fibrin, but is formed at the moment of coagulation by the union of two substances which, in fluid blood, exist separately. The theories before referred to, concerning the coagulation of the blood, will therefore, if this be true, resolve themselves into theories concerning the causes of the union of fibrino- plastin and fibrinogen ; and whether, on the one hand, it is an inhibitory action of the living bloodvessels that naturally re- strains, or a catalytic action of foreign matter that excites, the union of these two substances. Conditions affecting Coagulation. Although the coagulation of fibriu appears to be sponta- neous, yet it is liable to be modified by the conditions in which it is placed; such as temperature, motion, the access of air, the substances with which it is in contact, the mode of death, &c. All these conditions need to be considered in the study of the coagulation of the blood. The coagulation of the blood is hastened by the following means: 1. Moderate warmth,—from about 100° F. to 120° F. 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, co- agulates very slowly and imperfectly. But rest is not essen- tial to coagulation; for the coagulated fibrin may be quickly CONDITIONS AFFECTING COAGULATION. 63 obtained from blood by stirring it with a bundle of small twigs; and whenever any rough points of earthy matter or foreign bodies are introduced into the bloodvessels, the blood soon coagulates upon them. 3. Contact with foreign matter, and especially multiplica- tion of the points of contact. Thus, when all other conditions are unfavorable, the blood will coagulate upon rough bodies projecting into the vessels ; as, for example, upon threads passed through arteries or aneurismal sacs, or the heart’s valves roughened by inflammatory deposits or calcareous ac- cumulations. And, perhaps, this may explain the quicker co- agulation of blood after death in the heart with walls made irregular by the fleshy columns, than in the simple smooth- walled arteries and veins. 4. The free access of air. 5. Coagulation is quicker in shallow than in tall and nar- row vessels. 6. The addition of less than twice the bulk of water. The blood last drawn is said to coagulate more quickly than that which is first let out. The coagulation of the blood is retarded by the following means: 1. Cold retards the coagulation of blood; and it is said that, so long as blood is kept at a temperature below 40° F., 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. Coagulation is accelerated, but the subsequent contraction of the clot is hindered by a temperature between 100° and 120° : a higher temperature re- tards coagulation, 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 wTith the interior of a living bloodvessel, retards coagulation, although if the blood be at rest it does not prevent it. 4. The addition of the alkaline and earthy salts in the pro- portion of 2 or 3 per cent, and upwards. When added in large proportion most of these saline substances prevent coagulation altogether. Coagulation, however, ensues on dilution with water. The time that blood can be thus preserved in a liquid state and coagulated by the addition of water, is quite in- definite. 64 THE BLOOD. 5. Imperfect aeration,—as in the blood of those who die by asphyxia. 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, &c. In vacuo, the blood coagulates quickly ; but Prof. 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. The coagulation of the blood is prevented altogether by the addition of strong acids and caustic alkalies. It has been believed, and chiefly on the authority of Mr. Hunter, that, after certain modes of death, the blood does not coagulate ; he enumerates the death by lightning, overexertion (as in animals hunted to death), blows on the stomach, fits of anger. He says, “I have seen instances of them all.” Doubt- less he had done so ; but the results of such events are not con- stant. The blood has been often observed coagulated in the bodies of animals killed by lightning or an electric shock ; and Mr. Gulliver has published instances in which he found clots in the hearts of hares and stags hunted to death, and of cocks killed in fighting. Chemical Composition of the Blood. Among the many analyses of the blood that have been pub- lished, some, in which all the constituents are enumerated, are inaccurate in their statements of the proportions of those con- stituents; others, admirably accurate in some particulars, are incomplete. The two following tables, constructed chiefly from the analyses of Denis, Lecauu, Simon, Nasse, Lehmann, Bec- querel, Rodier, and Gavarret, are designed to combine, as far as possible, the advantage of accuracy in numbers with the con- venience of presenting at one view, a list of all the constituents of the blood. Average proportions of the principal constituents of the blood in 1000 parts: Water, .......... 784. Ked corpuscles (solid residue), ..... 130. Albumen of serum, ....... 70. Saline matters, ........ 6.03 Extractive, fatty, and other matters, .... 7.77 Fibrin, .......... 2.2 1000. 65 COMPOSITION OF BLOOD. Average proportions of all the constituents of the blood in 1000 parts: Water, 784. Albumen, ......... 70. Fibrin, ......... 2.2 Red corpuscles (dry), ...... 130. Fatty matters, ........ 1.4 Inorganic Salts: Chloride of sodium, . . . 3.6 Chloride of potassium, . . . 0.85 Tribasic phosphate of soda, . . 0.2 Carbonate of soda,.... 0.28 Sulphate of soda, .... 0.28 Phosphates of lime and magnesia, 0.25 Oxide and phosphate of iron, . 0.5 Extractive matters, biliary coloring matter, gases, and accidental substances, ..... 6.40 1000. Elementary composition of the dried blood of the ox : Carbon, ......... 57.9 Hydrogen, . . . . . . . . .7.1 Nitrogen, ......... 17.4 Oxygen, ......... 19.2 Ashes, .......... 4.4 These results of the ultimate analysis of ox’s blood afford a remarkable illustration of its general purpose, as supplying the materials for the renovation of all the tissues. For the analysts (Playfair and Boeckmann) have found that the flesh of the ox yields the same elements in so nearly the same proportions that the elementary composition of the organic constituents of the blood and flesh may be considered identical, and may be rep- resented for both by the formula C45H3aN6015. The Blood- Corpuscles or Blood- Cells. It lias been already said that the clot of blood contains, with the fibrin and the portion of the serum that is soaked in it, the blood-corpuscles, or blood-cells. Of these there are two principal forms, the red and the white corpuscles. When coagulation has taken place quickly, both kinds of corpuscles may be uniformly diffused through the clot; but, when it has been slow, the red corpuscles, being the heaviest constituent of the blood, tend by gravitation to accumulate at the bottom of the clot; and the white corpuscles, being among the lightest constituents, collect in the upper part, and contribute to the formation of the huffy coat. 66 THE BLOOD. Mammals. Birds. Reptiles. Fig. 24. Amphibia. Fish The above illustration is somewhat altered from a drawing, by Mr. Gulliver, in the Proceed. Zool. Society, and exhibits the typical characters of the red blood-cells in the main divisions of the Vertebrata. The fractions are those of an inch, and rep- resent the average diameter. In the case of the oval cells, only the long diameter is here given. It is remarkable, that although the size of the red blood-cells varies so much in the different classes of the vertebrate kingdom, that of the w hite corpuscles remains comparatively uniform, and thus they are, in some animals, much greater, in others much less, than the red corpuscles existing side by side with them. It may be here remarked, that the appearance of a nucleus in the red blood-cells of birds, reptiles, amphibia, and fish, has been shown by Mr. Savory to be the result of post-mortem change; no nucleus being visible in the cells as they circulate in the living body, or in those which have just escaped from the bloodvessels. RED BLOOD-CORPUSCLES. 67 The human red blood-cells or blood-corpuscles (Figs. 25 and 29) are circular flattened disks of different sizes, the majority varying in diameter from t° Wdtt °f an inch, and about Totiirf) °i? an iuch in thickness. When viewed singly, they ap- pear 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. Their borders are rounded ; their surfaces, in the perfect and most usual state, slightly concave; but they readily acquire flat or convex surfaces when, the liquor san- guinis being diluted, they are swollen by absorption of fluid. They are composed of a colorless, structureless, and transparent filmy framework or stroma, infiltrated in all parts by a red coloring-matter termed hemoglobin. The stroma is tough and elastic, so that, as the cells circulate, they admit of elongation and other changes of form, in adaptation to the vessels, yet recover their natural shape as soon as they escape from com- pression. The term cell, in the sense of a bag or sac, is inap- plicable to the red blood-corpuscle; and it must be considered, if not solid throughout, yet as having no such variety of con- sistence in different parts as to justify the notion of its being a membranous sac with fluid contents. The stroma exists in all parts of its substance, and the coloring matter uniformly per- vades this, and is not merely surrounded by and mechanically inclosed within the outer wall of the corpuscle. The red cor- puscles have no nuclei, although in their usual state, the un- equal refraction of transmitted light gives the appearance of a central spot, brighter or darker than the border, according as it is viewed in or out of focus. Their specific gravity is about 1088. In examining a number of red corpuscles with the micro- scope, it is easy to observe certain natural diversities among them, though they may have been all taken from the same part. The great majority, indeed, are very uniform; but some are rather larger, and the larger ones generally appear paler and less exactly circular than the rest; their surfaces also are usually flat or slightly convex, they often contain a minute shining particle like a nucleolus, and they are lighter than the rest, floating higher in the fluid in which they are placed. Other deviations from the general characters assigned to the corpuscles depend on changes that occur after they are taken from the body. Very commonly they assume a granulated or mulberry-like form, in consequence, apparently, of a peculiar corrugation of their cell-walls. Sometimes, from the same cause, they present a very irregular, jagged, indented, or star- like appearance. The larger cells are much less liable to this change than the smaller, and the natural shape may be restored by diluting the fluid in which the corpuscles float; by such 68 THE BLOOD. dilution the corpuscles, as already said, may be made to swell up, by absorbing the fluid ; and, if much water be added, they will become spherical and pellucid, their coloring-matter being dissolved, and, as it were, washed out of them. Some of them may thus be burst; the others are made obscure; but many of these latter may be brought into view again by evaporating, or adding saline matter to, the fluid, so as to restore it to its previous density. The changes thus produced by water are more quickly effected by weak acetic acid, which immediately makes the corpuscles pellucid, but dissolves few or none of them, for the addition of an alkali, so as to neutralize the acid, will restore their form though not their color. A peculiar property of the red corpuscles, which is exag- gerated in inflammatory blood, and which appeal's to exist in a marked degree in the blood of horses, may be here noticed. It gives them a great tendency to adhere together in rolls or columns, like piles of coin, and then, very quickly, these rolls fasten together by their ends, and cluster; so that, when the blood is spread out thinly on a glass, they form a kind of ir- regular network, with crowds of corpuscles at the several points corresponding with the knots of the net (Fig. 25). Hence, the clot formed in such a thin layer of blood looks mottled with blotches of pink upon a white ground ; in a larger quantity of such blood, as soon as the corpuscles have clustered and collected in rolls (that is, generally in two or three minutes after the blood is drawn), they begin to sink very quickly; for in the aggregate they present less surface to the re- sistance of the liquor sanguinis than they would if sinking separately. Thus quickly sinking, they leave above them a layer of liquor sanguinis, and this coagulating, forms a buffy coat, as before described, the volume of which is augmented by the white corpuscles, which have no tendency to adhere to the red ones, and by their lightness float up clear of them. Fig. 25. Red corpuscles collected into rolls (after Henle). Chemical Composition of Red Blood-cells. It has been before remarked, that the red blood-corpuscles are formed of a colorless stroma, infiltrated with a coloring matter termed haemoglobin. As they exist in the blood, they contain about three-fourths of their weight of water. The stroma appears to be composed of a nitrogenous prox- BLOOD-CRYSTALS. 69 imate principle termed protagon, combined with albuminous matter (paraglobulin or fibrinoplastin), fatty matters includ- ing cholesterin, and salts, chiefly phosphates, of potash, soda, and lime. Haemoglobin, which enters far more largely into the compo- sition of the red corpuscles than any other of their constituents, is allied to albumen in some respects, but differs remarkably from it in others. One of its most marked distinctive charac- ters is its tendency under certain artificial conditions to crys- tallize ; the so-called blood-crystals being but the natural crys- talline forms assumed by this substance. Haemoglobin can be obtained in a crystalline form, with various degrees of difficulty, from the blood of different ani- mals, that of man holding an intermediate place in this re- spect. Among the animals whose blood-coloring matter crys- tallizes most readily are the guinea-pig and the 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. In many instances, however, a some- what less simple process must be adopted ; as the addition of chloroform or ether, rapid freezing and then thawing, or other means which separate the coloring matter from the other con- stituents of the corpuscles. Different forms of blood-crystals are shown in the accom- panying figures. Fig. 26.1 Prismatic, from human blood. Another and most important character of haemoglobin is its attraction for oxygen, and some other gases, as carbonic and 1 Figs. 26, 27, and 28, illustrate some of the principal forms of blood-crystals. 70 THE BLOOD. nitrous oxides, with all of which it appears to form definite chemical combinations. The combination with oxygen is that which is of most physiological inportance. During the passage of the blood through the lungs, it is constantly formed; while it is as constantly decomposed, in consequence of the readiness with which haemoglobin parts with oxygen, when the latter is Fro. 27. exposed to other attractions in its circulation through the sys- temic capillaries. Thus, the red corpuscles, in virtue of their coloring matter, which readily absorbs oxygen and as readily Tetrahedral, from blood of the guinea-pig. Hexagonal crystals, from blood of squirrel. On these six-sided plates, prismatic crystals, grouped in a stellate manner, not unfrequently occur (after Funke). WHITE CORPUSCLES. 71 gives it up again, are the chief means by which oxygen is carried in the blood (see also p. 75). By heat, mineral and other acids, alkalies, &c., ha3inoglobin is decomposed into an albuminous matter (resembling glob- ulin) and hcematin. The latter, now known to be a product of the decomposition of hsemoglobin, was once thought to be the natural coloring matter of the blood. The White Corpuscles o f the Blood or Blood Leucocytes. The white corpuscles are much less numerous than the red. On an average, in health, there may be one white to 400 or 500 red corpuscles; but in disease, the proportion is often as high as one to ten, and sometimes even much higher. In health, the proportion 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 of food taken ; the number of leucocytes being very considerably in- creased by a meal, and diminished again on fasting. They present greater diversities of form than the red ones do; but the gradations between the extreme forms are so regular, that no sufficient reason can be found for supposing that there is in healthy blood more than one species of white corpuscles. In their most general appearance they ai’e circular and nearly spherical, about of an inch in diameter (Fig. 29). They have a grayish, pearly look, appearing variously shaded or nebulous, the shading being much darker in some than in others. They seem to be formed of protoplasm (p. 26), containing granules which are in some specimens few and very distinct, in_ others (though rarely) so numerous that the whole corpuscle looks like a mass of granules. These corpuscles cannot be said to have any true cell- wall. Iu a few instances an apparent cell-membrane can be traced around them; but, much more commonly, even this is not discernible till after the addition of water or di- lute acetic acid, which pen- etrates the corpuscle, and lifts up and distends what looks like a cell-wall, to the in- terior of which the material, Fig. 29. Red and white blood-eorpuseles. a, White corpuscle of natural aspect; b, Three white corpuscles acted on by weak acetic acid, c, Red blood-corpuscles. 72 THE BLOOD. that before appeared to form the whole corpuscle, remains attached as the nucleus of the cell (Fig. 29). A remarkable property of the white corpuscles, first observed by Mr. Wharton Joues, consists in their capability of assuming different forms, irrespective of any external influence. If a drop of blood be examined with a high microscope power under conditions by which loss of moisture is prevented, at the same time that the temperature is maintained at about the degree natural to the blood as it circulates in the living body, the leucocytes can be seen alternately contracting and dilating very slowly at various parts of their circumference—shooting out irregular processes, and again withdrawing them partially or completely, and thus in succession assuming various irreg- ular forms. These movements, called amoeboid, from their resemblance to the movements exhibited by an animal called the Amoeba, the structure of which is as simple as that of a white blood-cor- puscle, are characteristic of the living leucocyte, and form a good example of the contractile property of protoplasm, before referred to. Indeed, the unchanging rounded form which the corpuscles present in specimens of blood examined in the ordinary manner under the microscope, must be looked upon as the shape natural to a dead corpuscle, or one whose vitality is dormant, rather than as the proper shape of one living and active. Besides the red and white corpuscles, the microscope reveals numerous minute molecules or granules in the blood, circular or spherical, aud varying in size from the most minute visible speck to the -g-y of an inch (Gulliver). These molecules are very similar to those found in the lymph and chyle, and are, some of them, fatty, being soluble in ether, others prob- ably albuminous, being soluble in acetic acid. Generally, also, there may be detected in the blood, especially during the height of digestion, very minute equal-sized fatty particles, similar to those of which the molecular base of chyle is con- stituted (Gulliver). The Serum. The serum is the liquid part of the blood remaining after the coagulation of the fibrin. In the usual mode of coagula- tion, part of the serum remains soaked in the clot, and the rest, squeezed from the clot by its contraction, lies around and over it. The quantity of serum that appears around the clot de- pends partly on the total quantity in the blood, but partly also on the degree to which the clot contracts. This is affected by many circumstances: generally, the faster the coagulation SEBUM OF BLOOD. 73 the less is the amount of contraction; and, therefore, when blood coagulates quickly, it will appear to contain a small proportion of serum. Hence, the serum always appears de- ficient in blood drawn slowly into a shallow vessel, abundant in inflammatory blood drawn into a tall vessel. In all cases, too, it should be remembered, that 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. The serum is an alkaline, slimy or viscid, yellowish fluid, often presenting a slight greenish, or grayish hue, and with a specific gravity of from 1025 to 1030. It is composed of a mixture of various substances dissolved in about nine times their weight of water. It contains, indeed, the greater part of all the substances enumerated as existing in the blood, with the exception of the fibrin and the red corpuscles. Its prin- cipal constituent is albumen, of which it contains about 8 per cent., and the coagulation of which, when heated, converts nearly the whole of the serum into a solid mass. The liquid which remains uncoagulated, and which is often inclosed in little cavities in the coagulated serum, is called serosity; it con- tains, dissolved in water, fatty, extractive, and saline matters. Variations in the principal Constituents of the Liquor Sanguinis. The water of the blood is subject to hourly variations in its quantity, according to the period since the taking of food, the amount of bodily exercise, the state of the atmosphere, and all the other events that may affect either the ingestion or the excretion of fluids. According to these conditions, it may vary from 700 to 790 parts in the thousand. Yet uniformity is on the whole maintained; because nearly all those things which tend to lower the proportion of water in the blood, such as active exercise, or the addition of saline and other solid matter, excite thirst; while, on the other hand, the addition of an ex- cess of water to the blood is quickly followed by its more copious excretion in sweat and urine. And these means for adjusting the proportion of the water find their purpose in maintaining certain important physical conditions in the blood; such as its proper viscidity, and the degree of its ad- hesion to the vessels through which it ought to flow with the least possible resistance from friction. On this also depends, in great measure, the activity of absorption by the bloodves- sels, into which no fluids will quickly penetrate, but such as are of less density than the blood. Again, the quantity of water in the blood determines chiefly its volume, and thereby 74 THE BLOOD. the fulness and tension of the vessels and the quantity of fluid that will exude from them to keep the tissues moist. Finally, the water is the general solvent of all the other materials of the liquor sanguinis. It is remarkable, that the proportion of water in the blood may be sometimes increased even during its abstraction from an artery or vein. Thus Dr. Zimmerman, in bleeding dogs, found the last drawn portion of blood contain 12 or 13 parts more of water in 1000 than the blood first drawn; and Polli noticed a corresponding diminution in the specific gravity of the human blood during venesection, and suggested the only probable explanation of the fact, namely, that, during bleed- ing, the bloodvessels absorb very quickly a part of the serous fluid with which all the tissues are moistened. The albumen may vary, consistently with health, from 60 to 70 parts in the 1000 of blood. The form in which it exists in the blood is not yet certain. It may be that of simple solution as pure albumen ; but it is, more probably, in combin- ation with soda, as an albuminate of soda; for, if serum be much diluted with water, and then neutralized with acetic acid, pure albumen is deposited. Another view entertained by En- derlin is that the albumen is dissolved in the solution of the neutral phosphate of sodium, to which he considers the alkaline reaction of the blood to be due, and solutions of which can dissolve large quantities of albumen and phosphate of lime. The proportion of fibrin in healthy blood may vary between 2 and 3 parts in 1000. In some diseases, such as typhus, and others of low type, it may be as little as 1.034; in other dis- eases, it is said, it may be increased to as much as 7.528 parts in 1000. But, in estimating the quantity of fibrin, chemists have not taken account of the white corpuscles of the blood. These cannot, by any mode of analysis yet invented, be sepa- rated from the fibrin of mammalian blood : their composition is unknown, but their weight is always included in the estimate of the fibrin. In health they may, perhaps, add too little to its weight to merit consideration ; but in many diseases, espe- cially in inflammatory and other blood diseases in which the fibrin is said to be increased, these corpuscles become so numer- ous that a large proportion of the supposed increase of the fibrin must be due to their being weighed with it. On this account all the statements respecting the increase of fibrin in certain diseases need revision. The enumeration of the fatty matters of the blood makes it probable that most of those which are found in the tissues or secretions exist also ready-formed in the blood; for it contains the cholesterin of the bile, the eerebrin and phosphorized fat FATTY MATTERS IN THE BLOOD. 75 of the brain, and the ordinary saponifiable fats, stearin, olein, and palmitin. A volatile fatty acid is that on which the odor of the blood mainly depends; and it is supposed that when sulphuric acid is added (see p. 56), it evolves the odor by com- bining with the base, with which, naturally, this acid is neu- tralized. According to Lehmann, much of the fatty matter of the blood is accumulated in the red corpuscles. These fatty matters are subject to much variation in quan- tity, being commonly increased after every meal in which fat, or starch, or saccharine substances have been taken. At such times, the fatty particles of the chyle, added quickly to the blood, are only gradually assimilated ; and their quantity may be sufficient to make the serum of the blood opaque, or even milk-like. As regards the inorganic constituents of the blood—the sub- stances which remain as ashes after its complete burning—one may observe in general their small quantity in proportion to that of the animal matter contained in it. Those among them of peculiar interest are the phosphate and carbonate of sodium, and the phosphate of calcium. It appears most probable that the blood owes its alkaline reaction to both these salts of sodium. The existence of the neutral phosphate (Na2H,P04) was proved by Enderlin: the presence of carbonate of sodium has been proved by Lehmann and others. In illustration of the characters which the blood may derive from the phosphate of sodium, Liebig points out the large ca- pacity which solutions of that salt have of absorbing carbonic acid gas, and then very readily giving it off again when agitated in atmospheric air, and when the atmospheric pressure is di- minished. It is probably, also, by means of this salt, that the phosphate of calcium is held in solution in the blood in a form in which it is not soluble in water, or in a solution of albu- men. Of the remaining inorganic constituents of the blood, the oxide and phosphate of iron referred to, exist in the liquor sanguinis, independently of the iron in the corpuscles. Schmidt’s investigations have shown that the inorganic con- stituents of the blood-cells somewhat differ from those con- tained in the serum ; the former possessing a considerable pre- ponderance of phosphates and of the salts of potassium, while the chlorides, especially of sodium, with phosphate of sodium, are particularly abundant in the latter. Among the extractive matters of the blood, the most note- worthy are Oreatin and Creatinin. Besides these, other or- ganic principles have been found either constantly or gen- erally in the blood, including casein, especially in women during lactation: glucose, or grape-sugar, found in the blood 76 THE BLOOD. of the hepatic vein, but disappearing during its transit through the lungs (Bernard) ; urea, and in very minute quantities, uric add, (Garrod); hippuric and lactic acids; ammonia (Rich- ardson); and, lastly, certain coloring and odoriferous matters. Variations in healthy Blood under different Circumstances. As the general condition of the body depends so much on the condition of the blood, and as, on the other hand, any- thing that affects the body must sooner or later, and to a greater or less degree, affect the blood also, it might be ex- pected that considerable variations in the qualities of this fluid would be found under different circumstances of disease; and such is found to be the case. Even in health, however, the general composition of the blood varies considerably. The conditions which appear most to influence the compo- sition 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 being of somewhat higher specific gravity, from its containing a relatively larger quantity of red corpuscles. 2. Pregnancy.—The blood of pregnant women has a rather lower specific gravity than the average, from deficiency of red corpuscles. The quantity of white corpuscles, on the other hand, and of fibrin, is increased. 3. Age.—From the analysis of Denis it appears that the blood of the foetus is very rich in solid matter, and especially in red corpuscles; and this condition, gradually diminishing, 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. 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 ab- sorbed with the food and drink, as well as the more lasting changes which must result from generous or poor diet respect- ively, 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 thjt of the blood that flowed first VARIATIONS IN COMPOSITION. 77 (J. Davy and Polli). 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 abstraction from it of watery fluid, as in cholera, diabetes, and the like. For some little time after bleeding, the want of red blood- cells is well marked ; but with this exception, no considerable alteration seems to be produced in the composition of the blood for more than a very short time, the loss of the other constitu- ents, including the pale corpuscles, being very quickly repaired. Valuations in the Composition of the Blood, in 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 throughout the whole course of the systemic arteries, they are not so throughout the venous sys- tem—the blood contained in some veins differing remarkably from that in others. 1. Differences between Arterial and Venous Blood.-—These maybe arranged under two heads,—differences in color, and in general composition. a. Color.—Concerning the cause of the difference in color between arterial and venous blood, there has been much doubt, not to say confusion. For while the scarlet color of the ar- terial blood has been supposed by some observers, and for some reasons, to be due to the chemical action of oxygen, and the purple tint of that in the veins to the action of carbonic acid, there are facts which made it seem probable that the cause was a mechanical one rather than a chemical, and that it de- pended on a difference in the shape of the red corpuscles, by which their power of transmitting and reflecting light was al- tered. Thus, cai'bonic acid was thought to make the blood dark by causing the red cells to assume a biconvex outline, and oxygen was supposed to reverse the effect by contracting them and rendering them biconcave. We may believe, how- ever, that, at least for the present, this vexed question has, by the results of investigations undertaken by Professor Stokes and others, been now set at rest. The coloring matter of the blood, or haemoglobin (p. 69), is capable of existing in two different states of oxidation, and the respective colors of arterial and venous blood are caused by differences in tint between these two varieties—oxidized or scar- 78 THE BLOOD. let haemoglobin and deoxidized or purple haemoglobin. The change of color produced by the passage of the blood through the lungs, and its consequent exposure to oxygen, is due, prob- ably, to the oxidation of purple, and its conversion into scarlet haemoglobin; while the readiness with which the latter is de- oxidized offers a reasonable explanation of the change, in re- gard to tint, of arterial into venous blood,—the transformation being effected py the delivering up of oxygen to the tissues, by the scarlet haemoglobin, during the blood’s passage through the capillaries. The changes of color are more probably due to this cause, namely, a varying quantity of oxygen chemically combined with the haemoglobin, than to any mechanical effect of this gas, or to the influence of carbonic acid, either chemi- cally, on the coloring matter, or mechanically, on the corpuscles which contain it. We are not, perhaps, in a position to deny altogether the possible influence of mechanical conditions of the red corpuscles on the color of arterial and venous blood respectively; but it is probable that this cause alone would be quite insufficient to explain the differences in the color of the two kinds of blood, and therefore if it be an element at all in the change, it must be allowed to take only a subordinate position. The distinction between the two kinds of haemoglobin nat- urally present in the blood, or in other words, the proof that the addition or subtraction of oxygen involves the production respectively of two substances having fundamental differences of chemical constitution, has been made out chiefly hy spectrum- analysis,—the effects produced by placing oxidized and de- oxidized solutions of haemoglobin in the path of a ray of light traversing a spectroscope being different. For while the oxi- dized solution causes the appearance of two absorption bands in the yellow and the green part of the spectrum, these are re- placed by a single band intermediate in position, when the ox- idized or scarlet solution is darkened by deoxidizing agencies, or, in other words, when the change which naturally ensues in the conversion of arterial into venous blood is artificially pro- duced.1 The greater part of the haemoglobin in both arterial and venous blood probably exists in the scarlet or more highly ox- idized condition, and only a small part is deoxidized and made purple in its passage from the arteries into the veins. The differences in regard to color between arterial and 1 The student to whom the terms employed in connection with spectrum analysis are not familiar, is advised to consult, with ref- erence to the preceding paragraph, an elementary treatise on Physics. BLOOD OF PORTAL VEIN. 79 venous blood are sometimes not to be observed. If blood runs very slowly from an artery, as from the bottom of a deep and devious wound, it is often as dark as venous blood. In persons nearly asphyxiated also, and sometimes, under the influence of chloroform or ether,the arterial blood becomes like the venous. In the foetus also both kinds of blood are dark. But, in all these cases, the dark blood becomes bright on exposure to the air. Bernard has shown that venous blood returning from a gland in active secretion is almost as bright as arterial blood. b. General Composition.—The chief differences between ar- terial and ordinary venous blood are these. Arterial blood contains rather more fibrin, and rather less albumen and fat. It coagulates somewhat more quickly. Also, it contains more oxygen, and less carbonic acid. According to Denis, the fibrin of venous blood differs from arterial, in that when it is fresh and has not been much exposed to the air, it may be dissolved in a slightly heated solution of nitrate of potassium. Some of the veins, however, contain blood which differs from the ordinary standard 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 mesenteric veins, which contains the soluble elements of food absorbed from the stomach and intestines during digestion, and that in the splenic vein; it must, there- fore, combine the qualities of the blood from each of these sources. The blood in the gastric and mesenteric veins will vary much according 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, dex- trin, and other soluble matters which may have been absorbed from the alimentary canal, this blood appears to be deficient in solid matters, especially in red corpuscles, owing to dilution by the quantity of water absorbed, to contain an excess of al- bumen, though chiefly of a lower kind than usual, resulting from the digestion of nitrogenized substances, and termed al- buminose, and to yield a less tenacious kind of fibrin than that of blood generally. The blood from the splenic vein is probably more definite in composition, though also liable to alterations according to the stage of the digestive process, and other circumstances. It seems generally to be deficient in red corpuscles, and to con- tain an unusually large proportion of albumen. The fibrin seems to vary in relative amount, but to be almost always above the average. The proportion of colorless corpuscles ap- 80 THE BLOOD. pears also to be 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 albumen, chiefly in the form of albuminose, and yields a less firm clot than that yielded by other blood, owing to the deficient tenacity of its fibrin. These characteristics of portal blood refer to the com- position of the blood itself, and have no reference to the ex- traneous substances, such as the absorbed materials of the food, which it may contain ; neither, indeed, has any complete analysis of these been given. Comparative analyses of blood in the portal vein and blood in the hepatic veins have also been frequently made, with the view of determining the changes which this fluid undergoes in its transit through the liver. Great diversity, however, is ob- servable in the analyses of these two kinds of blood by dif- ferent chemists. Part of this diversity is no doubt attributable to the fact pointed out by Bernard, that unless the portal vein is tied before the liver is removed from the body, hepatic venous blood is very liable to regurgitate into the portal vein, and thus vitiate the result of the analysis. Guarding against this source of error, recent observers seemed to have deter- mined that hepatic venous blood 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, accord- ing to Bernard and others, is one constaut element, namely, grape-sugar, which is found, whether saccharine or farinaceous matter have been present in the food or not. Besides the rather wide difference between the composition of the blood of these veins and of others, it must not be for- gotten that in its passage through every organ and tissue of the body, the blood’s composition must be varying constantly, as each part takes from it or adds to it such matter as it, roughly speaking, wishes either to have or to throw away. Thus the blood of the renal vein has been proved by experi- ment to contain less water than does the blood of the artery, and doubtless its salts are diminished also. The blood in the renal vein is said, moreover, by Bernard and Browrn-S6quard not to coagulate. This then is an example of the change produced in the blood by its passage through a special excretory organ. But all parts of the body, bones, muscles, nerves, &c., must act on the blood as it passes through them, and leave in it some mark DEVELOPMENT OF BLOOD. 81 of their action, too slight though it may be, at any given mo- ment, for analysis by means now at our disposal. On the Gases contained in the Blood. The gases contained in the blood are carbonic acid, oxygen, and nitrogen, 100 volumes of blood containing from 40 to 50 volumes of these gases collectively. Artex’ial blood contains relatively more oxygen and less carbonic acid than venous. But the absolute quantity of car- bonic acid is in both kinds of blood greater than that of the oxygen. The proportion of nitrogen is in both very small. It is most probable that the carbonic acid of the blood is partly in a state of simple solution, and partly in a state of weak chemical combination. That portion of the carbonic acid which is chemically combined, is contained partly in a bicarbonate of soda, and partly is united with phosphate of the same base. The oxygen is combined chemically with the haemoglobin of the red corpuscles (pp. 69 and 77). That the oxygen is absorbed chiefly by the red corpuscles is proved by the fact that while blood is capable of absorbing oxygen in considerable quantity, the serum alone has little or no more power of absorbing this gas than pure water. Development of the Blood. In the development of the blood little more can be traced than the processes by which the corpuscles are formed. The first formed blood-cells of the human embryo differ much in their general characters from those which belong to the latter periods of intra-uterine, and to all periods of extra- uterine life. Their manner of origin differs also, and it will be well perhaps to consider this first. In the process of development of the embryo, the plan, so to speak, of the heart and chief bloodvessels is first laid out in cells. Thus the heart is at first but a solid mass of cells, resembling those which constitute all other parts of the em- bryo ; and continuous with this are tracts of similar cells— the rudiments of the chief bloodvessels. The formation of the first blood-corpuscles is very simple. While the outermost of the embryonic cells, of which the ru- dimentary heart and its attendant vessels are composed, gradu- ally develop into the muscular and other tissues.which form the walls of the heart and bloodvessels, the inner cells simply separate from each other, and form blood-cells; some fluid plasma being at the same time secreted. Thus, by the same 82 DEVELOPMENT OF BLOOD. proeess, blood is formed, and the originally solid heart and bloodvessels are hollowed out. The blood-cells produced in this way, are from about to y-jgo of an inch in diameter, mostly spherical, pellucid, and colorless, with granular contents, and a well-marked nucleus. Gradually, they acquire a red color, at the same time that the nucleus becomes more defined, and the granular matter clears away. Mr. Paget describes them as, at this period, circular, thickly disk-shaped, full-colored, and, on an average, about of an inch in diameter; their nuclei, which are about itn'oir an inch diameter, are central, circular, very little prominent on the surfaces of the cell, and apparently slightly granular or tuberculated. Before the occurrence, however, of this change—from the colorless to the colored state—in many instances, probably, during it, and in many afterwards, a process of multiplication takes place by division of the nucleus and subsequently of the cell, into two, and much more rarely, three or four new cells, which gradually acquire the characters of the original cell from which they sprang. Fig. 30 (b, c, D, e). Fig. 30. Development of the first set of blood-corpuscles in the mammalian embryo, a. A dotted, nucleated embryo-cell in process of conversion into a blood-corpuscle: the nucleus provided with a nucleolus, b. A similar cell with a dividing nucleus ; at c, the division of the nucleus is complete ; at d, the cell also is dividing, e. A blood- corpuscle almost complete, but still containing a few granules, f. Perfect blood- corpuscle. 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, according to Kolliker, and new blood-cells are produced by this organ. Like those just de- scribed, they are at first colorless and nucleated, but afterwards acquire the ordinary blood tinge, and resemble very much those of the first set. Like them they may also multiply by DEVELOPMENT OF BLOOD. 83 division. In whichever way produced, however, whether from the original formative cells of the embryo, or by the liver, these colored nucleated cells begin very early in foetal life to be mingled with colored non-nucleated corpuscles resembling those of the adult, and about the fourth or fifth month of embry- onic existence are completely replaced by them. The manner of origin of these perfect non-nucleated cor- puscles must be now considered. I. Concerning the Cells from, which they arise. a. Before Birth.—It is uncertain whether they are derived only from the cells of the lymph, which, at about the period of their appearance, begins to be poured into the blood ; or whether they are derived also from the nucleated red cells, which they replace, or also from similar nucleated cells, which Kolliker thinks are produced by the liver during the whole time of foetal existence. b. After Birth.—It is generally agreed that after birth the red corpuscles are derived from the smaller of the nucleated lymph or chyle-corpuscles,—the white corpuscles of the blood. II. Concerning the Manner of their Development. There is not perfect agreement among physiologists concern- ing the process by which lymph-globules or white corpuscles (and in the foetus, perhaps the red nucleated cells) are trans- formed into red non-nucleated blood-cells. For while some maintain that the whole cell is changed into a red one by the gradual clearing up of the contents, including the nucleus, it is believed by Mr. Wharton Jones and many others, that only the nucleus becomes the red blood-cell, by escaping from its envelope and acquiring the ordinary blood-tint. Of these two theories, that which supposes the nucleus of the lymph or chyle globule to be the germ of the future red blood- corpuscle is the theory now generally adopted. The development of red blood-cells from the corpuscles of the lymph and chyle continues throughout life, and there is no reason for supposing that after birth they have any other origin. Without doubt, these little bodies 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 performed. Neither the length of their life, however, nor the fashion of their decay, has been yet clearly made out, and we can only surmise that in these things 84 DEVELOPMENT OF BLOOD. they resemble more or less closely those parts of the body which lie more plainly within our observation. From what has been said, it will have appeared that when the blood is once formed, its growth and maintenance are ef- fected by the constant repetition of the development of new portions. In the same proportion that the blood yields its materials for the maintenance and repair of the several solid tissues, and for secretions, so are new materials supplied to it in the lymph and chyle, and by development made like it. The part of the process which relates to the formation of new corpuscles has been described, but it is probably only a small portion of the whole process; for the assimilation of the new materials to the blood must be perfect, in regard to all those immeasurable minute particulars by which the blood is adapted for the nutrition of every tissue, and the maintenance of every peculiarity of each. How precise the assimilation must be for such an adaptation, may be conceived from some of the cases in which the blood is altered by disease, and by assimilation is maintained in its altered state. For example, by the inser- tion of vaccine matter, the blood is for a short time manifestly diseased; however minute the portion of virus, it affects and alters, in some way, the wThole of the blood. And the alteration thus produced, inconceivably slight as it must be, is long main- tained ; for even very long after a successful vaccination, a second insertion of the virus may have no effect, the blood being no longer amenable to its influence, because the new blood, formed after the vaccination, is made like the blood as altered by the vaccine virus; in other wrords,the blood exactly assimilates to its altered self the materials derived from the lymph and chyle. In health we cannot see the precision of the adjustment of the blood to the tissues ; but we may imagine it from the small influences by which, as in vaccination, it is disturbed; and we may be sure that the new blood is as per- fectly assimilated to the healthy standard as in disease it is as- similated to the most minutely altered standard.1 How far the assimilation of the blood is affected by any for- mative power which it may possess in common with the solid tissues, we know not. That this possible formative power is, however, if present, greatly ministered to and assisted by the actions of other parts there can be no doubt; as 1st, by the di- gestive and absorbent systems, and probably by the liver, and all of the so-called vascular glands; and, 2dly, by the excre- tory organs, which separate from the blood refuse materials, 1 Corresponding facts in relation to the maintenance of the tissues by assimilation will be mentioned in the chapter on Nutrition. USES OF THE BLOOD. 85 including in this term not only the waste substance of the tissues, but also such matters as, having been taken with food and drink, may have been absorbed from the digestive canal, and have been subsequently found unfit to remain in the cir- culating current. And, 3dly, the precise constitution of the blood is adjusted by the balance of the nutritive processes for maintaining the several tissues, so that none of the materials appropriate for the maintenance of any part may remain in excess in the blood. Each part, by taking from the blood the materials it requires for its maintenance, is, as has been ob- served, in the relation of an excretory organ to all the rest; inasmuch as by abstracting the matters proper for its nutrition, it prevents excess of such matters as effectually as if they were separated from the blood and cast out altogether by the ex- creting organs specially present for such a purpose. Uses of the Blood. The purposes of the blood, thus developed and maintained, appear, in the perfect state, to be these: 1st, to be a source whence the various parts of the body may abstract the ma- terials necessary for their nutrition and maintenance; and whence the secreting organs may take the materials for their various secretions; 2d, to be a constantly replenished store- house of latent chemical force, which in its expenditure will maintain the heat of the body, or be transformed by the living tissues, and manifested by them in various forms as vital power; 3d, to convey oxygen to the several tissues which may need it, either for the discharge of their functions, or for combination with their refuse matter; 4th, to bring from all parts refuse matters, and convey them to places whence they may be dis- charged ; 5th, to warm and moisten all parts of the body. Regarding the uses of the various constituents of the blood, it may be said that the matter almost resolves itself into an analysis of the different parts of the body, and of the food and drink which are taken for their nutrition, with a subsequent consideration of how far any given constituent of the blood may be supposed to be on its way to the living tissues, to be incorporated with and nourish them ; or, having fulfilled its purpose, to be on its way, in a more or less changed condition, to the excretory organs to be cast out. It must be remem- bered, however, that the blood contains also matters which serve by their combustion to produce heat, and, again, others which possibly subserve only a mechanical, although most im- Uses of the various Constituents of the Blood. 86 USES OF THE BEOOD. portant purpose; as, for instance, the preservation of the due specific gravity of the blood, or some other quality by which it is enabled to maintain its proper relation to the vessels con- taining it, and to the tissues through which it passes. Lastly, among the constituents of the blood, are the gases, oxygen, and carbonic acid, and the substances specially adapted to carry them, which can scarcely be said to take part in the nutrition of the body, but are rather the means and evidence of the combustion before referred to, on which, to a great extent, directly or indirectly, all vitality depends. Albumen.—The albumen, which exists in so large a propor- tion among the chief constituents of the blood, is without doubt mainly for the nourishment of those textures which contain it or other compounds nearly allied to it. Besides its purpose in nutrition, the albumen of the liquor sanguinis is doubtless of importance also in the maintenance of those essen- tial physical properties of the blood to which reference has been already made. Fibrin.—It has been mentioned in a previous part of this chapter, that the idea of fibrin existing in the blood, as fibrin, is probably founded in error; and that it is formed, in the act of coagulation, by the union of two substances, which before existed separately (p. 61). In considering, therefore, the func- tions 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 nutri- tion 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 hemorrhage, for ex- ample, 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 which is produced for the per- manent healing of the injured part, contains a coagulable ma- terial probably 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, at least 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 maintaining the temperature of the body. In certain secretions also, notably the milk and bile, fat is an important constituent. Saline Matter.—The uses of the saline constituents of the blood are, first, to enter into the composition of such textures and secretions as naturally contain them, and, secondly, to assist in preserving the due specific gravity and alkalinity of the blood and, perhaps, also in preventing its decomposition. USES OF THE BLOOD. 87 The phosphate and carbonate of sodium, besides maintaining the alkalinity of the blood, are said especially to preserve the liquidity of its albumen, and to favor its circulation through the capillaries, at the same time that they increase the absorp- tive power of the serum for gases. But although, from the constant presence of a certain quantity of saline matter in the blood, we may believe that it has these last-mentioned impor- tant functions in connection with the blood itself, apart from the nutrition of the body, yet, from the amount which is daily separated by the different excretory organs, and especially by the kidneys, we must also believe that a considerable quantity simply passes through the blood, both from the food and from the tissues, as a temporary and useless constituent, to be ex- creted when opportunity offers. Corpuscles.—The uses of the red corpuscles are probably not yet fully known, but they may be inferred, at least in part, from the composition and properties of their contents. The affinity of haemoglobin for oxygen has been already mentioned; and the main function of the red corpuscles seems to be the absorption of oxygen in the lungs by means of this constitu- ent, and its conveyance to all parts of the body, especially to those tissues, the nervous and muscular, the discharge of whose functions depends in so great a degree upon a rapid and full supply of this element. The readiness with which haemoglo- bin absorbs oxygen, and delivers it up again to a reducing agent, so well shown by the experiments of Prof. Stokes, ad- mirably adapts it for this purpose. How far the red corpus- cles are concerned in the nutrition of the tissues is quite un- known. The relation of the white to the red corpuscles of the blood has been already considered (p. 83); of the functions of the former, other than are concerned in this relationship, nothing is positively known. Recent observations of the migration of the white corpuscles from the interior of the bloodvessels into the surrounding tissues (see section, On the Circulation in the Capillaries) have, however, opened out a large field for inves- tigation of their probable functions in connection with the nu- trition of the textures, in which, even in health, they appear to wander. 88 THE CIRCULATION. CHAPTER VI. CIRCULATION OF THE BLOOD. The body is divided into two chief cavities—the chest or thorax and abdomen, by a curved muscular partition, called the diaphragm (Fig. 31). The chest is almost entirely filled by the lungs and heart; the latter being fitted in, so to speak, between the two lungs, nearer the front than the back of the chest, and partly overlapped by them (Fig. 31). Each of these organs is contained in a distinct bag, called respectively the right and left pleura and the pericardium, the latter being fibrous in the main, but lined on the inner aspect by a smooth shining epithelial covering, on which can glide, with but little friction, the equally smooth surface of the heart enveloped by it. In Fig. 31 the containing bags of pleura and pericardium are supposed to have been removed. Entering the chest from above is a large and long air-tube, called the trachea, which divides into two branches, one for each lung, and through which air passes and repasses in respiration. Springing from the upper part or base of the heart may be seen the large ves- sels, arteries, and veins, which convey blood either to or from this organ. In the living body the heart and lungs are in constant rhythmic movement, the result of which is an unceasing stream of air through the trachea alternately into and out of the lungs, and an unceasing stream of blood into and out of the heart. It is with this last event that we are concerned especially in this chapter,—with the means, that is to say, by which the blood which at one moment is forced out of the heart, is in a few moments more returned to it, again to depart, and again pass through the body in course of what is technically called the circulation. The purposes for which this unceasing cur- rent is maintained, are indicated in the uses of the blood enu- merated in the preceding chapter. The blood is conveyed away from the heart by the arteries, and returned to it 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 capil- laries. The blood, therefore, in its passage from the heart THE CIRCULATION. 89 passes first into the arteries, then into the capillaries, and lastly into the veins, by which it is conveyed back again to the heart,—thus completing a revolution, or circulation. Fig. 31. Larynx. Trachea. Pulmonary Artery. Aorta. Right Lung. Left Lung. View of heart and lungs in situ. The fro-nt portion of the chest-wall, and the outer or parietal layers of the pleurae and pericardium, have been removed. The lungs are partly collapsed. Diaphragm. Heart. As generally described there are two circulations by which all the blood must pass; the one, a shorter circuit from the heart to the lungs and back again ; the other and larger cir- cuit, from the heart to all parts of the body and back again ; but more strictly speaking, there is only one complete circula- tion, which may be diagrammatically represented by a double loop, as in Fig. 32. 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 prin- cipal circulations, the pulmonary and systemic, as they are 90 THE CIRCULATION. named, it will be noticed also in the same figure, that there is another, by which a portion of the stream of blood having been diverted once into the capillaries of the intestinal canal, and some other organs, and gathered up again into a single stream, is a second time divided in its passage through the Fig. 32. Diagram of the circulation. liver, before it finally reaches the heart and completes a revo- lution. This subordinate stream through the liver is called the portal circulation. The principal force provided for constantly moving the blood through this course is that of the muscular substance of the heart; other assistant forces are, (2) those of the elastic walls of the arteries, (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 HEART. 91 the interchange of relations between the blood and the tissues which ensues in the capillary system during the nutritive pro- cesses. The right direction of the blood’s course is determined and maintained by the valves of the heart to be immediately described ; which valves open to permit the movement of the blood in the course described, but close when any force tends to move it in the contrary direction. We shall consider separately each member of the system of organs for the circulation : and first— The Heart. The heart is a hollow muscular organ, the interior of which 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 por- tion called respectively the auricle and ventricle, which freely communicate one with the other; the aperture of communica- tion, however, being guarded by valvular curtains, 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 ventri- cles ; the auricle and ventricle of one side being quite sepa- rate from those of the other. The right auricle communicates, on the one hand, with the veins of the general system, and, on the other, with the right ventricle, while the latter leads directly into the pulmonary artery, the orifice of which is guarded by valves. The left auricle again communicates, on the one hand, with the pulmonary veins, and, on the other, with the left ventricle, while the hitter leads directly into the aorta,—a large artery which conveys blood to the general sys- tem, the orifice of which, like that of the pulmonary artery, is guarded by valves. The arrangement of the heart’s valves is such that the blood can pass only in one definite direction, and this is as follows (Fig. 33): From the right auricle the blood passes into the right ventricle, and thence into the pulmonary artery, by which it is conveyed to the capillaries of the lungs. From the lungs the blood, which is now purified and altered in color, is ga- thered by the pulmonary 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 capil- laries of every portion of the body. The branches of the aorta, from being distributed to the general system, are called sys- temic arteries ; and from these the blood passes into the systemic 92 THE CIRCULATION. 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 in- ferior vena cava, discharge their contents into the right auricle, whence we supposed the blood to start (Fig. 33). Fig. 33. Diagram of the circulation through the heart (after Dalton). «, a. Vena cava, sti- 1 erior and inferior, 6. Right ventricle, e. Pulmonary artery, d. Pulmonary vein. e. Left ventricle. /. Aorta. Structure of the Valves of the Heart It will he well now to consider the structure of the valves of the heart, and the manner in which they perform their func- tion of directing the stream of blood in the course which has been just described. The valve between the right auricle and ventricle is named tricus})id (Fig. 34), because it presents three principal cusps or pointed portions, and that between the left auricle and ventricle bicuspid or mitral, because it has two such portions (Fig. 35). 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 the STRUCTURE OF HEART’S VALVES. 93 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 Fig. 34. The right auricle and ventricle opened, and a part of their right and anterior walls removed, so as to show their interior.—1, 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 narrow portion of the adjacent walls of the auricle and ventricle has been preserved ; 4, 4, cavity of the right ventricle, the upper figure is immediately below the semilunar valves; 4', large columna carnea or musculus papillaris ; 5, 5', 5", tricuspid valve; 6, placed in the interior of the pulmonary artery, a part of the anterior wall of that vessel having been removed, and a narrow portion of it preserved at its commencement where the semilunar valves are attached; 7, concavity of the aortic arch close to the cord of the ductus arteriosus; 8, ascending part or sinus of the arch covered at its com- mencement by the auricular appendix and pulmonary artery; 9, placed between the innominate and left carotid arteries; 10, appendix of the left auricle ; 11,11, the out- side of the left ventricle, the lower figure near the apex. (From Quain’s Anatomy.) 94 THE CIRCULATION. portions, so as to form an annular membrane around the auric- ulo-ventricular opening, 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 portion of the valve may be distinguished a middle- piece, extending from its base to its apex, and including about half its width; this piece is thicker, and much tougher and tighter than the border-pieces, which are attached loose and flapping at its sides. While the bases of the several portions of the valves are fixed to the tendinous rings, their ventricular surfaces and borders are fastened by slender tendinous fibres, the chordae tendinece, to the walls of the ventricles, the muscular fibres of which project into the ventricular cavity in the form of bun- dles or columns—the columnar 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 papillares has been given, are at- tached to the wall of the ventricle by one extremity only, the other projecting, papilla-like, into the cavity of the ventricle (5, Fig. 35), and having attached to it chordae 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 both free and attached, there are some of especial strength, which pass from the same parts to the edges of the mid- dle pieces of the several chief portions of the valve. 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 sur- face 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 may proceed to the adjacent halves of two of the principal divisions, and to one interme- diate or smaller division, of the valve. It has been already said that while the ventricles communi- cate, on the one hand, with the auricles, they communicate, on the other, with the large arteries which convey the blood away from the heart; the right ventricle with the pulmonary artery (6, Fig. 34), which conveys blood to the lungs, and the left ventricle with the aorta, which distributes it to the general system (7, Fig. 35). And as the auriculo-ventricular orifice STRUCTURE OF HEART’S VALVES. 95 is guarded by valves, so are also the mouths of the pulmonary artery and aorta (Figs. 34, 35). Fig. 35. The left auricle and ventricle opened and a part of their anterior and left walls removed so as to show their interior.—y2. The pulmonary artery has been divided at its commencement so as to show the aorta ; the opening into the left ventricle has been carried a short distance into the aorta between two of the segments of the semilunar valves ; the left part of the auricle with its appendix has been removed. The right auricle has been thrown 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 the re- mains of the valve of the foramen ovale, of which the crescentic fold is seen towards the left hand of 1'; 2, a narrow portion of the wall of the auricle and ventricle pre- served round the auriculo-ventrieular orifice; 3, 3', the cut surface of the walls of the ventricle, seen to become very much thinner towards 3", at the apex ; 4, a small part of the anterior wall of the left ventricle which has been preserved with the 96 THE CIRCULATION. The valves, three in number, which guard the orifice of each of these two arteries, are called the semilunar valves. They are nearly alike on both sides of the heart; but those of the aorta are altogether thicker and more strongly constructed than those of the pulmonary artery. Like the tricuspid and mitral valves, they are formed by a duplicature of the lining membrane of the heart, strengthened by fibrous tissue. Each valve is of semilunar shape, its convex margin being attached to a fibrous ring at the place of junction of the artery to the ventricle, and the concave or nearly straight border being free (Fig. 35). In the centre of the free edge of the valve, which contains a fine cord of fibrous tissue, is a small fibrous nodule, the corpus Arantii, and from this and from the attached bor- der, 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 thrown back and flattened by the blood passing out of the ventricle, but which belly out immediately so as to prevent any return (6, Fig. 34). This will be again referred to immediately. The muscular fibres of the heart, unlike those of most in- voluntary muscles, present a striated appearance under the microscope. (See chapter on Motion.) THE ACTION OF THE HEART. The heart’s action in propelling the blood consists in the successive alternate contractions and dilatations of the muscu- lar walls of its two auricles and two ventricles. The auricles contract simultaneously; so do the ventricles; their dilatations also are severally simultaneous; and the contractions of the one pair of cavities are synchronous with the dilatations of the other. The description of the action of the heart may best be com- principal anterior columna carnea or musculus papillaris attached to it; 5, 5, mus- culi papillares; 5', the left side of the septum, between the two ventricles, within the cavity of the left ventricle; 6, 6', 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 sinus; 8, the root of the pulmonary artery and its semilunar valves ; 8', the separated por- tion of the pulmonary artery remaining attached to the aorta by 9, the cord of the ductus arteriosus ; 10, the arteries rising from the summit of the aortic arch. (From Quain’s Anatomy.) ACTION OF THE HEART. 97 menced at that period in each action which immediately pre- cedes the beat of the heart against the side of the chest, and, by a very small interval more, precedes the pulse at the wrist. 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 auricles 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 cor- respond 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; then, in the end of the pause, they contract and empty their contents into the ventricles. The contraction of the auricles is sudden and very quick; it com- mences at the entrance of the great veins into them, and is thence propagated towards the auriculo-ventricular opening; but the last part which contracts is the auricular appendix. The effect of this contraction of the auricles is to propel nearly the whole of their blood into the ventricles. The reflux of blood into the great veins is 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 simultaneous contraction of the mus- cular coats with which the large veins are provided for some distance before their entrance into the auricles; a resistance which, however, is not so complete but that a small quantity of blood does regurgitate, i. e., flow backwards into the veins, at each auricular contraction. The effect of this regurgitation from the right auricle is limited by the valves at the junction of the subclavian and internal jugular veins, beyond which the blood cannot move backwards; and the coronary vein, or vein which brings back to the right auricle the blood which has circulated in the substance of the heart, is preserved from it by a valve at its mouth. 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 the dilatation or diastole of the ven- tricles. Thus distended, they immediately contract: so im- mediately, indeed, that their contraction, or systole, looks as if it were continuous with that of the auricles. This has been graphically described by Harvey in the following passage: “ These two motions, one of the ventricles, another of the au- ricles, take place consecutively, but in such a manner that there is a kind of harmony, or rhythm, present between them, the two concurring in such wise that but one motion is appar- 98 THE CIRCULATION. ent; especially in the warmer-blooded animals, in which the movements in question are rapid. Nor is this for any other reason than it is in a piece of machinery, in which, though one wheel gives motion to another, yet all the wheels seem to move simultaneously; or in that mechanical contrivance which is adapted to fire-arms, where the trigger being touched, down comes the flint, strikes against the steel, elicits a spark which, falling among the powder, it is ignited, upon which the flame extends, enters the barrel, causes the explosion, propels the ball, and the mark is attained—all of which incidents by reason of the celerity with which they happen, seem to take place in the twinkling of an eye.” 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 contrac- tion, a small quantity of blood remains. The form and position of the fleshy columns on the internal walls of the ventricle ap- pear, indeed, especially adapted to produce this obliteration of their cavities during 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. In such a case, only a central fissure may be discernible to the eye in the place of the cavity of each ventricle. At the same time that the walls of the ventricles contract, the fleshy columns, and especially those of them called the musculi papillares, contract also, and assist in bringing the margins of the auriculo-ventricular valves into apposition, so that they close the auriculo-ventricular openings, and prevent the backward passage of the blood into the auricles (p. 100). The whole force of the ventricular contraction is thus directed to the propulsion of the blood through their arterial orifices. During the time which elapses between the end of one con- traction of the ventricles, and the commencement of another, the communication between them and the great arteries—the aorta on the left side, the pulmonary artery on the right—is closed by the three semilunar valves situated at the orifice of each vessel. But the force with which the current of blood is propelled by the contraction of the ventricle separates these valves from contact with each other, and presses them back against the sides of the artery, making a free passage for the stream of blood. Then, as soon as the ventricular contraction ceases, the elastic walls of the distended artery recoil, and by pressing the blood behind the valves, force them down towards the centre of the vessel, and spread them out so as to close the FUNCTION OF THE VALVES. 99 orifice, and prevent any of the blood flowing back into the ventricles (p. 104). As soon as the auricles have completed their contraction they begin again to dilate, and to be refilled with blood, which flows into them in a steady stream through the great venous trunks. They are thus filling during all the time in which the ventricles are contracting; and the contraction of the ventricles being ended, these also again dilate, and receive again the blood that flows into them from the auricles. By the time that the ventricles are thus from one-third to two-thirds full, the auricles are distended ; these, then suddenly contracting, fill up the ventricles, as already described. If we suppose a cardiac revolution, which includes the con- traction of the auricles, the contraction of the ventricles, and their repose, to occupy rather more than a second, the following table will represent, in tenths of a second, the time occupied by the various events we have considered. Contraction of Auricles,. . . 1 -f- Eepose of Auricles, . .10 = 11 “ Ventricles, . . 4 -j- “ Ventricles, . 7=11 Repose (no contraction of eitlier auricles or ventricles), . . 0 -f- Contraction of either — auricles or ventricles, 5=11 11 Action of the Valves of the Heart. 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 dilatation and distension of the ventricles the latter valves are shut, the former open. Each half or side of the heart, through the action of its valves, may be compared with a kind of forcing-pump, like the common enema-syringe with two valves, of which one admits the fluid on raising the piston, but is closed again when the piston is forced down; while the other opens for the escape of the fluid, but closes when the piston is raised, so as to prevent the regurgitation of the fluid already forced through it. The ventricular dilata- tion is here represented by the raising up of the piston; the valve thus admitting fluid represents the auriculo-ventricular valve, which is closed again when the piston is forced down, i. e., when the ventricle contracts, and the other, i. e., the arterial, valve opens. The diagrams on the following page illustrate this very well. 100 THE CI KCU L ATION, During auricular contraction, the force of the blood pro- pelled into the ventricle is transmitted in all directions, but Fig. 36. Diagrams of valves of the heart (after Dalton). being insufficient to raise the semilunar valves, it is expended in distending the ventricle, and in raising and gradually clos- FUNCTION OF THE VALVES. 101 ing the auriculo-ventricular valves, which, when the ventricle is full, form a complete septum between it and the auricle. This elevation of the auriculo-ventricular valves is, no doubt, materially aided by the action of the elastic tissue which Dr. Markham has shown to exist so largely in their structure, es- pecially on the auricular surface. When the ventricle con- tracts, the edges of the valves are maintained in apposition by the simultaneous contraction of the mvsculi papillares, which are enabled thus to act by the arrangement of their tendinous cords just mentioned. In this position the segments of the valves are held secure, even though the form and size of the orifice and the ventricle may change during the continued contraction ; for the border-pieces are held by their mutual apposition and the equal pressure of the blood on their ven- tricular surfaces; and the middle-pieces are secure by their great strength, and by the attachment of the tendinous cords along their margins, these cords being always held tight by the contraction of the musculi papillares. A peculiar advan- tage, derived from the projection of these columns into the cavity of the ventricle, seems to be, that they prevent the valve from being converted into the auricle; for, when the ventricle contracts, and the parts of its walls to Avhich, through the medium of the columns, the tendinous cords are affixed, approach the auriculo-ventricular orifices, there would be a tendency to slackness of the cords, and the valves might be everted, if it were not that while the wall of the ventricle is drawn towards the orifice, the end of the simultaneously con- tracting fleshy column is drawn away from it, and the cords are held tight. What has been said applies equally to the auriculo-ven- tricular valves on both sides of the heart, and of both alike the closure is generally complete every time the ventricles contract. But in some circumstances, the closure of the tri- cuspid valve is not complete, and a certain quantity of blood is forced back into the auricle; and, since this may be advan- tageous, by preventing the overfilling of the vessels of the lungs, it has been called the safety-valve action of this valve (Hunter, Wilkinson King). The circumstances in which it usually happens are those in which the vessels of the lung are already full enough when the right ventricle contracts, as, e. g., in certain pulriionary diseases, in very active exertion, and in great efforts. In these cases, perhaps, because the,right ventricle cannot contract quickly or completely enough, the tricuspid valve does not completely close, and the regurgitation of blood may be indicated by a pulsation in the jugular veins synchronous with that in the carotid arteries. 102 T H E CIRCULATION. The arterial or semilunar valves are, as already said, brought into action by the pressure of the arterial blood forced back towards the ventricles, when the elastic walls of the arteries recoil after being dilated by the blood propelled into them in the previous contraction of the ventricle. The dilatation 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 mus- cular fibres of the ventricle and the elastic fibres of the walls of the artery. The tissue of this ring is tough, 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 in- terwoven cords, and covered with endocardium. Hence, when the ventricle propels blood through the orifice 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 propulsion 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, which had previously lain in contact with the inner surface of the artery (as at A, Fig. 37), are drawn inward towards its centre (Fig. 37, b). Their positions may be ex- Fig. 37. Sections of aorta, to show the action of the semilunar valves, a is intended to show the valves, represented by the dotted lines, in contact with the arterial walls, represented by the continuous outer line, b (after Hunter) shows the arterial wall distended into three pouches (a), and drawn away from the valves, which are straight- ened into the form of an equilateral triangle, as represented by the dotted lines. plained by the foregoing diagrams, in which the continuous lines represent a transverse section of the arterial walls, the FUNCTION OF THE VALVES. 103 dotted ones the edges of the valves, firstly, when the valves are in contact with the walls (a), aud, secondly, when the walls being dilated, the valves are drawn away from them (b). This position of the valves and arterial walls is retained so long as the ventricle continues in contraction ; but, so soon as it relaxes, and the dilated arterial walls can recoil by their elasticity, they press the blood as well towards the ventricles as onwards in the course of the circulation. Part of the blood thus pressed back lies in the pouches (a, Fig. 37, b) between the valves and the arterial walls; aud the valves are by it pressed together till their thin lunated margins meet in three lines radiating from the centre to the circumference of the artery (7 and 8, Fig. 38). Fig. 38. 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, bloodvessels, Ac.; the pulmonary artery and aorta have been removed by a section made immediately beyond the attachment of the semilunar valves, and the auricles 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 artery; 8, placed upon the posterior part of the root of the aorta ; 9, the right, 9', the left coronary artery. (From Quain’s Anatomy.) Mr. Savory has clearly shown that this pressure of the blood is not entirely sustained by the valves alone, but in part by 104 THE CIRCULATION. the muscular substance of the ventricle. Availing himself of a method of dissection hitherto appar- ently overlooked, namely, that of mak- ing vertical sections (Fig. 39) through various parts of the tendinous rings, he has been enabled to show clearly that the aorta and pulmonary artery, expanding towards their termination, are situated upon the outer 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 sup- ported, as it were, on a kind of muscu- lar floor formed by the free border of the ventricle. The result of this ar- rangement will be that the reflux of the blood will be most efficiently sus- tained by the ventricular wall.1 The effect of the blood’s pressure on the valves is, as said, to cause their margins to meet in three lines radiating from the centre to the circumference (7 and 8, Fig. 38). The contact of the valves in this positton, and the complete closure of the arterial orifice, are secured by the pe- culiar construction of their borders before mentioned. Among the cords which are interwoveu in the 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. Each of these cords is attached by its outer extremi- ties to the outer end of the free margin of its valve, and in the middle to the corpus Arantii; they thus inclose 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 luna- ted parts of each, which lie between the free edges and the cords next below them. These parts are firmly pressed to- gether, and the greater the pressure that falls on them, the Fig. 39. Vertical section through the aorta at its junction with the left ventricle. 1. Section of arterial coat. 2. Section of valve. 8. Section of ventricle. 1 Mr. Savory’s preparations, illustrating this and other points in relation to the structure and functions of the valves of the heart, are in the museum of St. Bartholomew’s Hospital. SOUNDS OF THE HEART. 105 closer and more 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 sur- faces is not less secure. Sounds of the Heart. 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 a pause or period of silence. The first sound is dull and prolonged ; its commence- ment 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 respec- tively 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 uncer- tain. The difference may be best appreciated by considering the different forces concerned in the production of the two sounds. In one case there is a strong, comparatively slow, contraction 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 the semilunar valves, as there was to their opening. The difference may be also ex- pressed, as Dr. C. J. B. Williams has remarked, by saying the words lubb—dap. The events which correspond, in point of time, with the first sound, are the contraction of the ventricles, the first part of the dilatation of the auricles, the closure of the auriculo-ven- tricular valves, the opening of the semilunar valves, and the propulsion of blood into the arteries. The sound is succeeded, in about one-thirtieth of a second, by the pulsation of the facial artery, 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 contrac- tion, and corresponds with the closure of the semilunar valves, the continued dilatation of the auricles, the commencing dila- tation of the ventricles, and the opening of the auriculo-ven- 106 THE CIRCULATION. tricular valves. The pause immediately follows the second sound, and corresponds in its first part with the completed dis- tension of the auricles, and in its second with their contraction, and the distension of the ventricles, the auriculo-ventricular valves being all the time open, and the arterial valves closed. The chief cause of the first sound of the heart appears to be the vibration of the auriculo-ventricular valves, 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. This view, long ago advanced by Dr. Billing, is supported by the fact observed by Valentin, that if a portion of a horse’s intestine, tied at one end, be moderately filled with water, without any admixture of air, and have a syringe containing water fitted to the other end, the first sound of the heart is exactly imitated by forcing in more water, and thus suddenly rendering the walls of the intestine more tense. 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 vibration of the semilunar valves when they are pressed down across the orifices of the aorta and pulmonary artery; for, of the other events which take place during the second sound, none is calculated to produce sound. The in- fluence of the valves in producing the sound, is illustrated by the experiment already quoted from Valentin, and from others performed on large animals, 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 the pulmonary artery, below the line of attachment of the semilunar valves, and, after being carried upwards about half an inch, were brought out again through the coats of the re- spective 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, the second sound had ceased to be audible. Disease of these valves, when so extensive as to interfere with their efficient action, also often demonstrates the same fact by modifying or destroying the dis- tinctness of the second sound. One reason for the second sound being a clearer and sharper one than the first may be, that the semilunar valves are not covered in by the thick layer of fibres composing the walls of the heart to such an extent as are the auriculo-ventricular. It might be expected therefore that their vibration 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 end IMPULSE OF THE HEART. 107 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 ventricular contraction, the heart may be felt to beat with a slight shock or impulse against the walls of the chest. This impulse is most evident in the space between the fifth and sixth ribs, between one and two inches to the left of the sternum. The force of the impulse, and the extent to which it may be perceived beyond this point, vary considerably in different in- dividuals, and in the same individuals under different circum- stances. 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 expiration than in a deep inspiration; for, in the one case, the intervention of a thick layer of fat or muscle be- tween the heart and the surface of the chest, and in the other the inflation of the portion of lung which overlaps the heart, pre- vents the impulse from being fully transmitted to the surface. An excited action of the heart, and especially a hypertrophied condition of the ventricles, will increase the impulse, while a depressed condition, or an atrophied state of the ventricular walls, will diminish it. The impulse of the heart is probably the result, in part, of a tilting forwards of the apex, so that it is made to strike against the walls of the chest. This tilting movement is thought to be effected by the contraction of the spiral muscular fibres of the ventricles, and especially of certain of these fibres which, ac- cording to Dr. Reid, arise from the base of the ventricular septum, pass downwards and forwards, forming part of the septum, then emerge and curve spirally around the apex and adjacent portion of the heart. The whole extent of the move- ment thus produced is, however, but slight. The condition, which, no doubt, contributes most to the occurrence and char- acter of the impulse of the heart, is its change of shape; for, during the contraction of the ventricles, and the consequent approximation of the base towards the apex, the heart becomes more globular, and bulges so much, that a distinct impulse is felt when the finger is placed over the bulging portion, either at the front of the chest, or under the diaphragm. The pro- duction of the impulse is, perhaps, further assisted by the ten- dency of the aorta to straighten itself and diminish its curva- ture when distended with the blood impelled by the ventricle; and by the elastic recoil of all the parts about the base of the heart, which according to the experiments of Kurschner, are stretched downward and backward by the blood flowing into 108 THE CIRCULATION. the auricles and ventricles during the dilatation of the latter, but recover themselves when, at the beginning of the contrac- tion of the ventricles, the flowthrough the auriculo-ventricular orifices is stopped. But these last-mentioned conditions can only be accessory in the perfect state of things; for the same tilting movement of the heart ensues when its apex is cut off, and when, therefore, no tension or change of form can be pro- duced by the blood. Although what we generally recognize as the impulse of the heart is produced in the way just mentioned, the beat is not so simple a shock as it may seem when only felt by the finger. By means of an instrument called a cardiograph, it may be shown to be compounded of three or four shocks, of which the finger can only feel the greatest. The cardiograph is a tube, dilated at one end into a cup or funnel, either open-mouthed or closed by an elastic membrane, while at the other it communicates with the interior of a small metal drum, one side of which is formed by an elastic mem- brane, on which rests a finely-balanced lever, like that of the sphvgmograph (Fig. 42). When used, the cup at one end of the tube is placed imme- diately over the part of the chest-wall at which the apex of the heart beats; while the lever on the drum is placed in con- tact with a registering apparatus. (See description of sphyg- mograph, p. 125.) When the heart beats, the shock commu- nicates a series of impulses to the column of air in the now closed tube, with the effect of raising the elastic wall of the drum, and of course the lever which is attached to it. A tracing of the heart’s impulse is thus obtained in the same way as that of the pulse, in the arteries (Figs. 44 and 45). The tracing shows that besides the strong beat which alone the finger recognizes as the impulse of the heart, and which is caused by the contraction of the ventricles, there are other minor shocks which are imperceptible to the touch. The latter, M. Marey, by experiments on the lower animals, has proved to be the results, respectively, of the contraction of the auricles, and of the closure of the auriculo-ventricular and semilunar valves. The frequency with which the heart performs the actions we have described, may be counted by the pulses at the wrist, or in any other artery; for these correspond with the contrac- tions of the ventricles. The heart of a healthy adult man in the middle period of Frequency and Force of the Heart's Action. ACTION OF THE HEART. 109 life, acts from seventy to seventy-five times in a minute. 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: In the embryo the average number of pulses in a minute is ..... 150 Just after birth, ... . . from 140 to 130 During the first year, . “ 130 to 115 During the second year, . ... 11 115 to 100 During the third J'ear, . ... “ 100 to 90 About the seventh year, . . . “ 90 to 85 About the fourteenth year, the average number of pulses in a minute is . “ 85 to 80 In adult age, . . . . . . “ 80 to 70 In old age, . . . . . . . “ 70 to 60 In decrepitude, “ 75 to 65 In persons of sanguine temperament, the heart acts some- what more frequently than in those of the phlegmatic ; and in the female sex more frequently than in the male. After a meal its action is accelerated, and still more so during bodily exertion or mental excitement; it is slower during sleep. The effect of disease in producing temporary increase or diminution of the heart’s action is well known. From the observation of several experimenters, it appears that, in the state of health, 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. It is found, also, that as a general rule, the pulse, especially in the adult male, is more frequent in the standing than in the sitting posture, and in the latter, than in the recumbent position ; the difference being greatest between the standing and the sitting posture. The effect of change of posture is greater as the fre- quency of the pulse is greater, and, accordingly, is more marked in the morning than in the evening. Dr. Guy, by supporting the body in different postures, without the aid of muscular effort of the individual, has proved that the increased frequency of the pulse in the sitting and standing positions is dependent upon the muscular exertion engaged in maintaining them ; the usual effect of these postures on the pulse being almost entirely prevented when the usually attendant muscular exertion was rendered unnecessary. The effect of food, like that of change of posture, is greater in the morning than in the evening. According to Parrot, the frequency of the pulse increases in a corresponding ratio with the elevation above the sea; and Dr. Frankland informed the author, that at the 110 THE CIRCULATION. summit of Mont Blanc, his pulse was about double the ordi- nary standard all the time he was there. After six hours’ perfect rest and sleep at the top, it was 120, on descending to the corridor it fell to 108, at the Grands Mulets it was 88, at Chamounix 56 ; normally, his pulse is 60. In health, there is observed a nearly uniform relation be- tween the frequency of the pulse and of the respirations; the proportion being, on an average, one of the latter to three or four of the former. The same relation is generally main- tained in the cases in which the pulse is naturally accelerated, as after food or exercise; but in disease this relation usually ceases to exist. In many affections accompanied with in- creased frequency of the pulse, the respiration is, indeed, also accelerated, yet the degree of its acceleration bears no definite proportion to the increased number of the heart’s actions, and in many other cases, the pulse becomes more frequent without any accompanying increase in the number of respirations; or, the respiration alone may be accelerated, the number of pul- sations remaining stationary, or even falling below the ordi- nary standard. (On the whole of this subject, the article Pulse, by Dr. Guy, in the Cyclopaedia of Anatomy and Physi- ology, may be advantageously consulted.) The force with which the left ventricle of the heart contracts is about double that exerted by the contraction of the right: being equal (according to Valentin) to about of the weight of the whole body, that of the right being equal only to r^0th °f the same. 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 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 force exercised by the auricles in their contraction has not been determined. Neither is it known with what amount of force either the auricles or the ventricles dilate ; but there is no evidence for the opinion, that in their dilatation they can materially assist the circulation by any such action as that of a sucking-pump, or a caoutchouc bag, in drawing blood into their cavities. That the force which the ventricles exercise in dilatation is very slight, has been proved by Oesterreicher. He removed the heart of a frog from the body, and laid upon it a substance sufficiently heavy to press it flat, and yet so small as not to conceal the heart from view ; he then observed that during the contraction of the heart, the weight was raised ; RHYTHM OF THE HEART. 111 but that during its dilatation, the heart remained flat. And the same was shown by Dr. Clendinning, who, applying the points of a pair of spring callipers to the heart of a live ass, found that their points were separated as often as the heart swelled up in the contraction of the ventricles, but approached each other by the force of the spring when the ventricles di- lated. Seeing how slight the force exerted in the dilatation of the ventricles is, it has been supposed that they are only di- lated by the pressure of the blood impelled from the auricles; but that both ventricles and auricles dilate spontaneously is proved by their continuing their successive contractions and dilatations when the heart is removed, or even when they are separated from one another, and when therefore no such force as the pressure of blood can be exercised to dilate them. By such spontaneous dilatation they at least offer no resistance to the influx of blood, and save the force which would otherwise be required to dilate them. The capacity of the two ventricles is probably exactly the same. It is difficult to determine with certainty how much this may be; but, taking the mean of various estimates, it may be inferred that each ventricle is able to contain on an aver- age, about three ounces of blood, the whole of which is im- pelled into tlieir respective arteries at each contraction. The capacity of the auricles is rather less than that of the ven- tricles : 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 ad- joining ventricles; the former to the circumstance of the ven- tricles being partly filled with blood before the auricles con- tract. Cause of the Rhythmic Action of the Heart. It has beeu attempted in various ways to account for the existence and continuance of* the rhythmic movements of the heart. By some it has been supposed that the contact of blood with the lining membrane of the cavities of the heart, furnishes a stimulus, in answer to which the walls of these cavities con- tract. But the fact that the heart, especially in amphibia and fishes, will continue to contract and dilate regularly and in rhythmic order after if is removed from the body, completely emptied of blood, and even placed in a vacuum where it can- not receive the stimulus of the atmospheric air, is a proof that even if the contact of blood be the ordinary stimulus to the heart’s contraction, it cannot alone be an explanation of its rhythmic motion. The influence of the mind, and of some affections of the 112 THE CIRCULATION. brain and spinal cord upon the action of the heart, proves that it is not altogether, or at all times, independent of the cerebro- spinal nervous system. Yet the numerous experiments insti- tuted for the purpose of determining the exact relation in which the heart stands towards this system, have failed to prove that the action is directly governed under ordinary circum- stances by the power of any portion of the brain or spinal cord. Sudden destruction of either the brain or spinal cord alone, or of both together, produces, immediately, a temporary interrup- tion or cessation of the heart’s action : but this appears to be only an effect of the shock of so severe an injury; for, in some such cases, the movements of the heart are subsequently re- sumed, and if artificial respiration be kept up, may continue for a considerable time; and may then again be arrested by a violent shock applied through an injury of the stomach. While, therefore, we must admit an indirect or occasional in- fluence exercised by, or through, the brain and spinal cord upon the movements of the heart, and may believe this in- fluence to be the greater the more highly the several organs are developed, yet it is clear that we cannot ascribe the regu- lar determination and direction of the movements to these nervous centres. The persistence of the movements of the heart in their regu- lar rhythmic order, after its removal from the body, and their capability of being then re-excited by an ordinary stimulus after they have ceased, prove that the cause of these move- ments must be resident within the heart itself. And it seems probable, from the experiments and observations of various observers, that it is connected with the existence of numerous minute ganglia of the sympathetic nervous system, which, with connecting nerve-fibres, are distributed through the substance of the heart. These ganglia appear to act as so many centres or organs for the production of motor impulses; while the con- necting nerve-fibres unite them into one system, and enable them to act in concert and direct their impulses so as to excite in regular series the successive contractions of the several muscles of the heart. The mode in which ganglia thus act as centres and co-ordinators of nervous power will be described in the chapter on the Nervous System; and it will appear prob- able that the chief peculiarity of the heart, in this respect, is due to the number of its ganglia, and the apparently equal power which they all exercise ; so that there is no one part of the heart whose action, more than another’s, determines the actions of the rest. Thus, if the heart of a reptile be bisected, the rhythmic, successive actions of auricle and ventricle will go on in both halves: we therefore cannot say that the action RHYTHM OF THE HEART. 113 of the right side determines or regulates that of the left, or vice versci; and we must suppose that when they act together in the perfect heart, it is because they are both, as it were, set to the same time. Neither can we say that the auricles deter- mine the action of the ventricles; for, if they are separated, they will both contract and dilate in regular, though not nec- essarily similar, succession. A fact pointed out by Mr. Mal- den shows how the several portions of each cavity are similarly adjusted to act alike, yet independently of each other. If a point of the surface of the ventricle of a turtle’s or frog’s heart be irritated, it will immediately contract, and very quickly afterwards all the rest of the ventricle will contract; but, at the close of this general contraction, the part that was irritated and contracted first, is slightly distended or pouched out, show- ing that it was adjusted to contract in, and for only, a certain time, and that therefore as it began to contract first, so it began to dilate first. The best interpretation, perhaps, yet given of it, and of rhythmic processes in general, is that by Mr. Paget, who re- gards them as dependent on rhythmic nutrition, i. e., on a method of nutrition in which the acting parts are gradually raised, with time-regulated progress, to a certain state of in- stability of composition, which then issues in the discharge of their functions, e. g., of nerve-force in the case of the cardiac ganglia, by which force the muscular walls are excited to con- traction. According to this view, there is in the nervous ganglia of the heart, and in all parts originating rhythmic pro- cesses, the same alternation of periods of action with periods of repose, during which the waste in the structure is repaired, as is observed in most of, if not all, the organic phenomena of life. All organic processes seem to be regulated with exact observance of time; and rhythmic nutrition and action, as ex- hibited in the action of the heart, are but well marked ex- amples of such chronometric arrangement. We may conclude, then, that the nervous ganglia in the heart’s substance are the immediate regulators of the heart’s action, but that they are themselves liable to influences con- veyed from without, through branches of the pneumogastric and sympathetic nerves. The pneumogastric nerves are the media of an inhibitory or restraining influence over the action of the heart; for when by section their influence is withdrawn, the pulsations of the organ are increased in frequency and strength ; while an oppo- site effect is produced by stimulating them,—the transmission of an electric current of even moderate strength diminishing the pulsations, or stopping them altogether. Stimulation of 114 THE CIRCULATION. the sympathetic nerves, on the other hand, accelerates and strengthens the heart’s action. Various theories have been proposed to account for these peculiar results, but none of them are very satisfactory, and it is probable that many more facts must be discovered before any theory on the subject can be permanently maintained. The connection of the action of the heart with the other organs, and the influences to which it is subject through them, are explicable from the connection of its nervous system with the other ganglia of the sympathetic, and with the brain and spinal cord through, chiefly, the pneumogastric nerves. But this influence is proved in a much more striking manner by the phenomena of disease than by any experimental or other physiological observations. The influence of a shock in arrest- ing or modifying the action of the heart,—its very slow action after compression of the brain, or injury to the cervical por- tion of the spinal cord,—its irregularities and palpitations in dyspepsia and hysteria,—are better evidence for the connection of the heart with the other organs through the nervous sys- tem, than are any results obtained by experiments. That the contractions of the heart supply alone a sufficient force for the circulation of the blood, appears to be established by the resvdts of several experiments, of which the following is one of the most conclusive: Dr. Sharpey injected bullock’s blood into the thoracic aorta of a dog recently killed, after tying the abdominal aorta above the renal arteries, and found that, with a force just equal to that by which the ventricle commonly impels the blood in the dog, the blood which he in- jected into the aorta passed in a free stream out of the trunk of the vena cava inferior. It thus traversed both the systemic and hepatic capillaries; and when the aorta was not tied above the renals, blood injected under the same pressure flowed freely through the vessels of the lower extremities. A pressure equal to that of one and a half or two inches of mer- cury was, in the same way, found sufficient to propel blood through the vessels of the lungs. But although it is probably true that the heart’s action alone is sufficient to insure the circulation, yet there exist several other forces which are, as it were, supplementary to the action of the heart, and assist it in maintaining the circu- lation. The principal of these supplemental forces have been already alluded to, and will now be more fully pointed out. Effects of the Heart's Action. STRUCTURE OF ARTERIES. 115 The walls of' the arteries are composed of three principal coats, termed the external or tunica adventitia, the middle, and the internal, while the latter is lined within by a single layer of tessellated epithelium. The external coat or tunica adventitia, the strongest and toughest part of the wall of the artery, is formed of areolar 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 elasth coat. The middle coat is composed of both muscular and elastic fibres. The former, which are of the pale or unstriped variety (see chapter on Motion), are arranged for the most part trans- versely to the long axis of the artery; while the elastic ele- ment, taking also a transverse direction, is disposed in the form of closely interwoven and branching fibres, which inter- sect in all parts the layers of muscular fibre. In arteries of various size there is a difference in the proportion of the mus- cular and elastic element, elastic tissue preponderating in the largest arteries, while this condition is reversed in those of medium and small size. The internal arterial coat is formed by layers of elastic tis- sue, consisting in part of coarse longitudinal branching fibres, and in part of a very thin and brittle membrane which pos- sesses little elasticity, and is thrown into folds or wrinkles when the artery contracts. This latter membrane, the striated or fenestrated coat of Henle, is peculiar in its tendency to curl up, when peeled off from the artery, and in the perforated and streaked appearance which it presents under the microscope. Its inner surface is lined with a delicate layer of epithelium, composed of thin squamous elongated cells, which make it smooth and polished, and furnish a nearly impermeable sur- face, along which the blood may flow with the smallest possible amount of resistance from friction. The w7alls of the arteries, with the possible exception of the epithelial liniug 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 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. Nerve-fibres are also supplied to the walls of the arteries. THE ARTERIES. 116 THE CIRCULATION. The function of the arteries is to convey blood from the heart to all parts of the body, and each tissue which enters into the construction of an artery has a special purpose to serve in this distribution. (1.) The external coat forms a strong and tough investment, Fig. 40. Fig. 41. Fig. 40.—Muscular fibre-cells from human arteries, magnified 350 diameters (Kolliker). o, natural state ; 6, treated with acetic acid. Fig. 41.—Portion of fenestrated membrane from the crural artery, magnified 200 diameters, a, b, c, perforations (from Henle). which, though capable of extension, appears principally de- signed to strengthen the arteries and to guard against their excessive distension from the force of the heart’s action. In it, too, the little vasa vasorum find a suitable tissue in which to subdivide for the supply of the arterial coats. (2.) The purpose of the elastic tissue, which enters so largely into the formation of all the coats of the arteries, is, 1st. 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 forced into the arteries more quickly than they can be discharged into and through the capillaries. The blood therefore being, for an instant, resisted in its onward course, a part of the force with which it was impelled is directed against the sides of the arteries; under this force, which might burst a brittle tube, their elastic walls dilate, stretching enough to receive the blood, and as they stretch, becoming more tense and more resisting. Thus, by yielding, they, as it were, break the shock ELASTICITY OF ARTERIES. 117 of the force impelling the blood, and exhaust it befox-e they ai*e in danger of bursting, through being overstretched. Elas- ticity is thus advantageous in all arteries, but chiefly so in the aorta and its large branches, which are provided, as ali'eady said, with a large proportional quantity of elastic tissue, in adaptation to the great foi’ce of the left ventricle, which falls first on them, and to the increased pressure of the artei'ial blood in violent expii'atory efforts. On the subsidence of the pressure, when the ventricles cease conti’acting, the arteries are able, by the same elasticity, to resume their former calibre; and in thus doing, they manifest, 2d, the chief purpose of their elasticity, that, namely, of equal- izing the current of the blood by maintaining pressure on the blood in the arteries during the periods at which the ventricles are at rest or dilating. If some such method as this had not been adopted—if, for example, the arteries had been rigid tubes, the blood, instead of flowing as it does, in a constant stream, would have been propelled through the arterial system in a series of jerks corresponding to the ventricular contrac- tions, with intervals of almost complete rest during the inaction of the ventricles. But in the actual condition of the arteries, the force of the successive contractions of the ventricles is ex- pended partly in the dii'ect propulsion of the blood, and partly in the dilatation of the elastic arteries; and in the intervals between the contractions of the ventricles, the foi’ce of the re- coiling and contracting ai’teries is employed in continuing the same direct propulsion. Of course, the pressure exercised by the recoiling arteries is equally diffused in every direction through the blood, and the blood would tend to move back- wards as well as onwards, but that all movement backwards is prevented by the closure of the semilunar arterial valves, which 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 i*estored in full, according to that law of action of elastic bodies, by which they return to the state of rest with a force equal to that by which they were disturbed therefrom. There is thus no loss of force; but neither is thei’e any gain, for the elastic walls of the artery cannot oi’iginate any force for the propulsion of the blood—they only restore that which they received from the ventricles; they would not contract had they not first been dilated, any more than a spiral spring would shorten itself unless it were first elongated. The advan- tage of elasticity in this respect is, therefore, not that it in- 118 THE CIECULATION. creases, but that it equalizes or diffuses the force derived from the periodic contractions of, the ventricles. The force with which the arteries are dilated every time the ventricles con- tract, 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 suc- cessive branches of every artery that, at length, the intermit- tent 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 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 the capillaries (p. 136), is a necessary agent. Were there no greater obstacle to the escape of blood from the arteries than exists to its entrance into them from the heart, the stream would be intermittent, notwithstanding the elasticity of the walls of the arteries. It is the resistance which the left ventricle meets with in forcing blood into the arteries that causes part of the force of its contraction to be expended in dilating them, or, as before remarked, in laying up in them a power which will act in the intervals of the ventricle’s contraction. (3.) 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 correspondence with any temporary increase or diminution of the total quantity of blood in the body; and within a certain range of diminution of the quantity, still to exercise due pressure on their contents. The elastic coat, however, not only assists in restoring the normal calibre of an artery after temporary dilatation, but also (4), may assist in restoring it after diminution of the cali- bre, whether this be caused by a temporary contraction of the muscular coat, or the application of a compressing force from without. This action of the elastic tissue in arteries, is well shown in arteries which contract after death, but regain their average patency on the cessation of post-mortem rigidity (p. 119). (5.) By means of their elastic coat the arteries are enabled to adapt themselves to the different movements of the several parts of the body. We have already referred to the fact that the middle coat of the arteries is composed of unstriped muscular fibres, mingled with fine elastic filaments. The evidence for the muscular contractility of arteries may, however, be given briefly for the sake of the physiological facts on which it hinges. (1.) When a small artery in the living subject is exposed CONTRACTILITY OF ARTERIES. 119 to the air or cold, it gradually but manifestly contracts. Hun- ter observed that the posterior tibial artery of a dog when laid bare, became in a short time so much contracted as almost to prevent the transmission of blood ; and the observation has been often and variously confirmed. Simple elasticity could not effect this ; for after death, when the vital muscular power has ceased, and the mechanical elastic one alone operates, the contracted artery dilates again. (2.) When an artery is cut across, its divided ends contract, and the orifices may be completely closed. The rapidity and completeness of this contraction vary in different animals; they are generally greater in young than in old animals; and less, apparently, in man than in animals. In part this con- traction is due to elasticity, but in part, no doubt, 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 picking or twisting them. Such irritation would not be followed by these effects, if the arteries had no other power of contracting than that depending upon elasticity. (3.) The contractile property of arteries continues many hours after death, and thus affords an opportunity of distin- guishing it from elasticity. When a portion of an artery, the splenic, for example, of a recently killed animal, is exposed, 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 per- manently retains the same size. If, while contracted, the ar- tery be forcibly distended, its contractility is destroyed, and it holds a middle or natural size. This persistence of the contractile property after death was well shown in an observation of Hunter, which may be men- tioned as proving, also, the greater degree of contractility pos- sessed by the smaller than by the larger arteries. Haviug injected the uterus of a cow, which had been removed from the animal upwards of twenty-four hours, he found, after 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 results of an experiment which Hunter made with the vessels of an umbilical cord prove still more strikingly the long continuance of the contractile power of arteries after death. In a woman delivered on a Thursday afternoon, the umbilical cord was separated from the foetus, having been first tied in two places, and then cut between, so that the blood 120 THE CIRCULATION. contained in the cord and placenta was confined in them. On the following morning, Hunter tied a string round the cord, about an inch below the other ligature, that the blood might still be confined in the placenta and remaining cord. Having cut off this piece, and allowed all the blood to escape from its vessels, he attentively observed to what size the ends of the cut arteries were brought by the elasticity of their coats, and then laid aside the piece of cord to see the influence of the contractile power of its vessels. On Saturday morning, the day after, the mouths of the arteries were completely closed up. He repeated the experiment the same day with another portion of the same cord, and on the following morning found the results to be precisely similar. On Sunday, he performed the experiment the third time, but the artery then seemed to have lost its contractility, for on Monday morning, the mouths of the cut arteries were found open. In each of these experi- ments there was but little alteration perceived in the orifices of the veins. (4.) The influence of cold in increasing the contraction of a divided artery has been referred to: it has been shown, also, by Schwann, in an experiment on the mesentery of a living toad. Having extended the mesentery under the micro- scope, he placed upon it a few drops of water, the temperature of which was some degrees lower than that of the atmosphere. The contraction of the vessels soon commenced, and gradually increased until, at the expiration of ten or fifteen minutes, the diameter of the canal of an artery, which at first was 0.0724 of an English line, was reduced to 0.0276. The arteries then dilated again, and at the expiration of half an hour had ac- quired nearly their original size. By renewing the application of the water, the contraction was reproduced: in this way the experiment could be performed several times on the same ar- tery. It is thus proved, that cold will excite contraction in the walls of very small, as well as of comparatively large ar- teries : it could not produce such contraction in a merely elastic substance; but it is a stimulus to the organic muscular fibres in many other parts, as well as in the arterial coat; as, e. g., in the skin, the dartos, and the walls of the bronchi. (5.) Lastly, satisfactory evidence of the muscularity of the arterial coats is furnished by the experiments of Ed. and E. H. Weber, and of Professor Kolliker, in which they applied the stimulus of electro-magnetism to small arteries. The ex- periments of the Webers were performed on the small mesen- teric arteries of frogs; and the most striking results were ob- tained when the diameter of the vessels examined did not exceed from \ to y of a Paris line. When a vessel of this FUNCTIONS OF MUSCULAR COAT. 121 size was exposed to the electric current, its diameter in from five to ten seconds, became one-third less, and the area of its section about one-half. On continuing the stimulus, the nar- rowing gradually increased, until the calibre of the tube be- came from three to six times smaller than it was at first, so that only a single row of blood-corpuscles could pass along it at once; and eventually the vessel was closed and the current of blood arrested. With regard to the purpose served by the muscular coat of the arteries, there appears no sufficient reason for supposing that it assists, to more than a very small degree, in propelling the onward current of blood. Its most important office is that of regulating the quantity of blood to be received by each part, and of adjusting it to the requirements of each, accord- ing to various circumstances, but chiefly and most naturally, according to the activity with which the functions of each part are at different 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 exercise of function, such a part requires a larger supply of blood than is sufficient for it during the times when it is comparatively inactive. It is evident that the heart cannot regulate the supply to each part at different periods, neither could this be regulated by any general and uniform contraction 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 di- lating or yielding so as to permit an increase of, the supply of blood, according as the requirements of the part may demand. And thus, while the ventricles of the heart determine the total quantity of blood, to be sent onwards at each contraction, and the force of its propulsion, and while the large and merely elastic arteries distribute it and equalize its stream, the smaller arteries with muscular tissue add to these two purposes, that of regulating and determining, according to its requirements, the proportion of the whole quantity of blood which shall be dis- tributed to each part. It must be remembered, however, that this regulating func- tion of the arteries is itself governed and directed by the ner- vous system. The muscular tissue of arteries is supplied with nerves chiefly, if not entirely, by branches from the sympathetic sys- tem. These so-called vasomotor nerves are again connected, through the medium of ganglia, with the fibres from the sym- 122 THE CIRCULATION. pathetic system supplied to the organs nourished by these same arteries. Thus, any condition in these organs which causes them to need a different amount of blood, whether more or less, produces a certain impression on their nerves, and by these the impression is carried to the ganglia, and thence re- flected along the nerves which supply the arteries. The mus- cular element of these vessels responds in obedience to the impression conveyed to it by the nerves; and, according to its contraction or dilatation, is a larger or smaller quantity of blood allowed to pass. Another function of the muscular element of the middle coat of arteries is, doubtless, to co-operate with the elastic in adapting the calibre of the vessels 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 contained blood would be sometimes very small, and sometimes inordinately great. The presence of a muscu- lar element, however, provides for a certain uniformity in the amount of pressure exercised ; and it is by this adaptive, uni- form, gentle, muscular contraction, that the 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, the artery being cut, it first limits and then, in conjunction with the coag- ulated fibrin, arrests the escape of blood. It is only in conse- quence of such contraction and coagulation that we are free from danger through even very slight wounds; for it is only when the artery is closed that the processes for the more perma- nent and secure prevention of bleeding are established. Mr. Savory has shown that the natural state of all arteries, in regard at least to their length, is one of tension—that they are always more or less stretched, and ever ready to recoil by virtue of their elasticity, whenever the opposing force is re- moved. The extent to which the divided extremities of ar- teries retract is a measure of this tension, not of their elasticity. From what has been said in the preceding pages, it appears that the office of the arteries in the circulation is,—1st, the conveyance and distribution of blood to the several parts of the body; 2d, the equalization of the current, and the con- version of the pulsatile jetting movement given to the blood by the ventricles, into a uniform flow; 3d, the regulation of THE PULSE. 123 the supply of blood to each part, in accordance with its de- mands. The Pulse. The jetting movement of the blood, which, as just stated, it is one of the offices of the arteries to change into a uni- form motion, is the cause of the pulse, and therefore needs a separate consideration. We have already said, that as the blood is not able to pass through the arteries so quickly as it is forced into them by the ventricle, on account of the resist- ance it experiences in the capillaries, a part of the force with which the heart impels the blood is exercised upon the walls of the vessels which it distends. The distension of each artery increases both its length and its diameter. In their elonga- tion, the arteries change their form, the straight ones becoming curved, or having such a tendency, and those already curved becoming more so ;l but they recover their previous form as well as their diameter when the ventricular contraction ceases, and their elastic walls recoil. The increase of their curves which accompanies the distension of arteries, and the succeeding re- coil, may be well seen in the prominent temporal artery of an old person. The elongation of the artery is in such a case quite manifest. The dilatation or increase of the diameter of the artery is less evident. In several reptiles, it may be seen without aid, in the immediate vicinity of the heart, and it may be watched, with a simple magnifying glass, in the aorta of the tadpole. Its slight amount in the smaller arteries, the difficulty of observing it in opaque parts, and the rapidity with which it takes place, are sufficient to account for its being, in Mam- malia, imperceptible to the eye. But in these also experi- ment has proved its occurrence. Flourens, in evidence of such dilatation, says he encircled a large artery with a thin elastic metallic ring cleft at one point, and that at the moment of pulsation the cleft part became perceptibly widened. This dilatation of an artery, and the elongation producing curvature, or increasing the natural curves, are sensible to the finger placed over the vessel, and produce the pulse. The mind cannot distinguish the sensation produced by the dilata- tion from that produced by the elongation and curving; that which it perceives most plainly, however, is the dilatation^ 1 There is, perhaps, an exception to this in the case of the aorta, of which the curve is by some supposed to be diminished when it is elon- gated ; but if this be so, it is because only one end of the arch is im- movable; the other end, with the heart, may move forward slightly when the ventricles contract. 2 For this fact, which is contrary to the commonly accepted doc- 124 THE CIRCULATION. The pulse—due to any given beat of the heart—is not per- ceptible 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 pro- portion to the distance of the artery from the heart, but the difference in time between the beat of any two arteries never exceeds probably g to of a second. A great deal of light has been thrown on what may be called the form of the pulse by the sphygmograph (Figs. 42 and 43). The principle on which the sphygmograph acts is very simple (see Fig. 42). The small button replaces the finger in the ordinary 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 attached a slight spring, which allows the lever to move up, at the same time that it is just stroDg enough to resist its making any sudden jerk, 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. 43). trine, I am indebted to my friend, Dr. Hensley, who has kindly fur- nished me with the following note on the subject: By determining the conditions of equilibrium of a portion of artery supposed cylindrical and filled with blood at a given pressure, it is easily shown that the transverse tension is double the longitudinal. Also it may be shown experimentally that, if strips of equal breadth, cut in the two directions from one of the larger arteries, be stretched by equal weights, the stretching of the transverse slip is somewhat greater than that of the longitudinal one. [By the word stretching is to be understood amount of stretching, and not increase of length: it may be measured by the ratio which the increase of length bears to the original length: thus things whose natural lengths are 5 and 10 inches are equally stretched, when their lengths are made 6 and 12 inches respectively.) Such experiments also show that, within certain limits, the stretch- ing of each strip varies directly as its tension. Hence it will be seen that the transverse stretching of an artery, when filled with blood, must be somewhat more than double its longi- tudinal stretching. This being true for different blood pressures, the difference between the transverse stretchings for different pressures must be somewhat more than double the difference between the corresponding longitu- dinal stretchings ; and thus we can hardly be justified in saying that the increase of longitudinal stretching which takes place with the pulse is greater than the increase of transverse stretching. It must also be remembered that the arteries are, under all circum- stances, naturally in a state of tension longitudinally, and that their length, therefore, cannot be increased at all until the blood pressure is increased beyond a certain point.—Ed.) PULSE-TEACIUGS. 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, and if the extremity of the latter be inked, it will write Fig. 42. Diagram of the mode of action of the spliygmograph. the effect on the card, which is made to move by clockwork 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. Fig. 43. The spliygmograph applied to the arm. Fig. 44 represents a healthy pulse-tracing of the radial artery, but somewhat deficient in tone. On examination, we see that the up-stroke which represents the beat of the pulse is a nearly vertical line, while the down-stroke is very slanting, and interrupted by a slight reascent. The more vigorous the pulse, if it be healthy, the less is this reascent, and vice versa. Fig. 45 represents the tracing of a healthy pulse in which the tone of the vessel is better than in the last instance, and the down-stroke is therefore less interrupted. Sometimes the up-stroke has a double apex, as in Fig. 46. This will be explained hereafter. 126 THE CIRCULATION. Before proceeding to consider the formation of the pulse, as shown by these tracings, it is necessary to consider what are the elements combined to produce it. Fig. 44. Fig. 45. Fig. 46 Fig. 44. Pulse-tracing of radial artery, somewhat deficient in tone. Fig. 45. Firm and long pulse of vigorous health. Fig. 46. Pulse-tracing of radial artery, with double apex. The above tracings are taken from Dr. Sanderson’s work “ On the Sphygmograph.’- The heart at regular intervals discharges a certain quantity of blood into the arteries and their branches, already filled, though not distended to the utmost, with fluid. This fresh quantity of blood obtains entrance by the yielding of the artery’s elastic walls, and, on the cessation of the propelling force, and when these walls recoil, the blood is prevented from returning into the ventricle whence it is issued, by the shut- ting of the semilunar valves in the manner before described (p. 103). The pressure, therefore, which is exercised on the blood by the contracting arterial walls, will cause it to travel in a direction away from the heart, or, in other words, towards the capillaries and veins. It was formerly supposed that the pulse was caused not by the direct action of the ventricle, but by the propagation of a wave in consequence of the elastic recoil of the large arteries, after their distension; and successive acts of dilatation and recoil, extending along the arteries in the direction of the cir- culation, were supposed to account for the latter appearance of the pulse in the vessels most distant from the heart. The fact, however, that the pulse is perceptible in every part of the arterial system previous to the occurrence of the second sound of the heart, that is, previous to the closure of the aortic PULSE-TRACINGS. 127 valves, is a fatal objection to this theory. For’, if the pulse were the effect of a wave propagated by the alternate dilata- tion and contraction of successive portions of the arterial tube, it ought, in all the arteries except those nearest to the heart, to follow or coincide with, but could uever precede, the second sound of the heart; for the first effect of the elastic recoil of the arteries first dilated is the closure of the aortic valves ; and their closure produces the second sound. The theory which seems to reconcile all the facts of the case, and especially those two which appear most opposed, namely, that the pulse always precedes the second sound of the heart, and yet is later in the arteries far from the heart than in those near it, may be thus stated : It supposes that the blood which is impelled onwards by the left ventricle does not so impart its pressure to that which the arteries already contain, as to dilate the whole arterial system at once; but that it enters the ar- teries, it displaces and propels that which they before con- tained, and flows on with what may be called a head-ivave, like that which is formed when a rapid stream of water overtakes another moving more slowly. The slower stream offers resist- ance to the more rapid one, till their velocities are equalized; and because of such resistance, some of the force of the more rapid stream of blood just expelled from the ventricle, is diverted laterally, and with the rising of the wave the arteries nearest the heart are dilated and elongated. They do not at once recoil, but continue to be distended so long as blood is entering them from the ventricle. The wave at the head of the more rapid stream of blood runs on, propelled and main- tained in its velocity by the continuous contraction of the ven- tricle ; and it thus dilates in succession every portion of the arterial system, and produces the pulse in all. At length, the whole arterial system (wherein a pulse can be felt) is dilated; and at this time, when the wave we have supposed has reached all the smaller arteries, the entire system may be said to be simultaneously dilated ; then it begins to contract, and the contractions of its several parts ensue in the same succession as the dilatations, commencing at the heart. The contraction of the first portion produces the closure of the valves and the second sound of the heart; and both it and the progressive contractions of all the more distant parts maintain, as already said, that pressure on the blood during the inaction of the ventricle, by which the stream of the arterial blood is sustained between the jets, and is finally equalized by the time it reaches the capillaries. It may seem an objection to this theory, that it would prob- ably require a larger quantity of blood to dilate all the ar- 128 THE CIRCULATION. teries than can be discharged by the ventricle at each contrac- tion. But the quantity necessary for such a purpose is less than might be supposed. Injections of the arteries prove that, including all down to those of about one-eighth of a line in diameter, they do not contain on an average more than one and a half pints of fluid, even when distended. There can be no doubt, therefore, that the three or four ounces which the ventricle is supposed to discharge at each contraction, being added to that which already fills the arteries, would be suffi- cient to distend them all. A distinction must be carefully made between the passage of the wave along the arteries, and the velocity of the stream (p. 131) of blood. Both wave and current are present; but the rates at which they travel are very different, that of the wave being twenty or thirty times as great as that of the current. Returning now to the consideration of the pulse-tracings (p. 126), it may be remarked that, in each, the up-stroke cor- responds with the period during which the ventricle is con- tracting ; the down-stroke, with the interval between its con- tractions, or in other words with the recoil, after distension, of the elastic arteries. In the large arteries, when at least there is much loss of tone, the up-stroke is double, the almost instantaneous propagation of the force of contraction of the left ventricle along the column of blood in the arteries, or the percussion impulse, as it is termed by Dr. Sanderson, being sufficiently strong to jerk up the lever for an instant, while the wave of blood, rather more slowly propagated from the ven- tricle, catches it, so to speak, as it begins to fall, and again slightly raises it. In the radial artery tracings, on the other hand, we see that the up-stroke is 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 immedi- ately follows it, and which continues and completes the dis- tension. In cases of feeble arterial tension, however, the per- cussion-impulse may be traced by the sphygmograph, not only in the carotid pulse, but to a less extent in the radial also (as in Fig. 46). In looking now at the down-stroke (Fig. 44) in the tracings, we see that in the case of an artery with deficient tone, it is in- terrupted by a well-marked notch, or in other words, that the descent is interrupted by a slight uprising. There are indica- tions also of slighter irregularities or vibrations during the fall of the lever ; while these are alone to be seen in the pulse of health, or in other words, when the walls of the artery are of good tone (Fig. 45). In some cases of disease the reascent is FORCE OF BLOOD IN ARTERIES. 129 so considerable as to be perceptible to the finger, and this double beat has received the technical name of “dicrotous” pulse. As a diseased condition this has long been recognized, but it is only since the invention of the sphygmograph that it ha? been found to belong in a certain degree to the normal pulse also. Various theories have been framed to account for the dicro- tism of the pulse. By some, it is supposed to be due to the aortic valves, the sudden closure of which stops the incipient regurgitation of blood into the ventricle, and causes a mo- mentary rebound throughout the arterial system ; while Dr. Sanderson considers it to be caused by a kind of rebound from the periphery rather than from the central part of the circu- lating apparatus. Force of the Blood in the Arteries. The force with which the ventricles act in their contraction, and the reasons for believing it sufficient for the circulation of the blood, have been already mentioned. Both calculation and experiment have proved that very little of this force is consumed in the arteries. Dr. Thomas Young calculated that the loss of force in overcoming friction and other hindrances in the arteries would be so slight, that if one tube were introduced into the aorta, and another into any other artery, even into one as tine as hair, the blood would rise in the tube from the small vessel to within two inches of the height to which it would rise from the large vessel. The correctness of the calculation is established by the experiments of Poiseuille, who invented an instrument named a hsemadynamometer, for estimating the statical pressure exercised by the blood upon the walls of the arteries. It consists of a long glass tube, bent so as to have a short horizontal portion (b, Fig. 47), a branch (a) descending at right angles from it, and a long ascend- ing branch (c). Mercury poured into the ascending and descending portions will necessarily have the Fig. 47. 130 THE CIRCULATION. same level in both branches, and in a vertical position the height of its column must be the same in both. If, now, the blood is made to flow from an artery, through the horizontal portion of the tube (which should contain a solution of carbo- nate of potash to prevent coagulation) into the descending branch, it will exert on the mercury a pressure equal to the force by which it is moved in the arteries; and the mercury will, in consequence, descend in this branch, and ascend in the other. The depth to which it sinks in the one branch, added to the height to which it rises in the other, will give the whole height of the column of mercury which balances the pressure exerted by the blood ; the weight of the blood, which takes the place of the mercury in the descending branch, and which is more than ten times less than the same quantity of quicksilver, being subtracted. Poiseuille thus calculated the force with which the blood moves in an artery, according to the laws of hydrostatics, from the diameter of the artery, and the height of the column of quicksilver; that is to say, from the weight of a column of mercury, whose base is a circle of the same diameter as the artery, and whose height is equal to the difference in the levels of the mercury in the two branches of the instrument. He found the blood’s pressure equal in all the arteries examined ; difference in size, and distance from the heart being unattended by any corresponding difference of force in the circulation. The height of the column of mercury displaced by the blood was the same in all the arteries of the same animal. The correctness of these views having been questioned, Poiseuille has recently repeated his observations, and obtained the same results. From the mean result of several observations on horses and dogs, he calculated that the force with which the blood is moved in any large artery, is capable of supporting a column of mercury six inches and one and a half lines in height, or a column of water seven feet one line in height. With these re- sults, the more recent observations of other experimenters closely accord. Poiseuille’s experiments having thus shown to him that the force of the blood’s motion is the same in the most different arteries, he concluded that, to measure the amount of the blood’s pressure in any artery of which the calibre is known, it is necessary merely to multiply the area of a transverse sec- tion of a vessel by the height of the column of mercury which is already known to be supported by the force of the blood in any part of the arterial system. The weight of a column of mercury of the dimensions thus found, will represent the pres- sure exerted by the column of blood. And assuming that the mean of the greatest and least height of the column of mercury THE CAPILLARIES. 131 found, by experiments on different animals, to be supported by the force of the blood in them, is equivalent to the height of the column which the force of the blood in the human aorta would support, he calculated that about 4 lbs. 4 oz. avoirdu- pois would indicate the static force with which the blood is im- pelled into the human aorta. By the same calculation, he estimated the force of the circulation in the aorta of the mare to be about 11 lbs. 9 oz. avoirdupois: and that in the radial artery at the human wrist only 4 drs. We have already seen that the muscular force of the right ventricle is equal to only one-half that of the left, consequently, if Poiseuille’s estimate of the latter be correct, the force with which the blood is pro- pelled into the lungs will only be equal to 2 lbs. 2 oz. avoir- dupois. The amounts above stated indicate the pressure exerted by the blood at the several parts of the arterial system at the time of the ventricular contraction. During the dilatation, this pressure is somewhat diminished. Hales observed, that the column of blood in the tube inserted into an artery, falls an inch, or rather more, after each pulse; Ludwig has observed the same, and recorded it more minutely. The pressure is also influenced by the various circumstances which affect the action of the heart; the diminution or increase of the pressure being proportioned to the weaker or stronger action of this organ. Valentin observed that, on increasing the amount of blood by the injection of a fresh quantity into it, the pressure in the vessels was also increased, while a contrary effect ensued on diminishing the quantity of blood. Velocity of the Blood in the Arteries. The velocity of the stream of blood is greater in the arteries than in auy 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 diastole of the ventricles, and in the parts of the arterial system most distant from the heart. From Volkmann’s ex- periments with the hsemodromometer, it may be concluded that the blood moves in the large arteries near the heart at the rate of about ten or twelve inches per second. Vierordt calculated the rapidity of the stream at about the same rate in the arteries near the heart, and at two and a quarter inches per second in the arteries of the foot. In all organic textures, except some parts of the corpora cavernosa of the penis, and of the uterine placenta, and of the THE CAPILLARIES. 132 THE CIRCULATION. spleen, the transmission of the blood from the minute branches of the arteries to the minute veins is effected through a net- work of microscopic vessels, in the meshes of which the proper substance of the tissue lies (Fig. 48). This maybe seen in all minutely injected preparations; and during life, by the aid of the microscope, in any transparent vascular parts,—such as the web of the frog’s foot, the tail or external branchiae of the tad- pole, or the wing of the bat. The ramifications of the minute arteries form repeated an- astomoses with each other and give off the capillaries which, by their anastomoses, compose a continuous and uniform net- work, from which the venous radicles, on the other hand, take their rise. The reticulated vessels connecting the arteries and veins are called capillary, on account of their minute size ; and intermedi- ate-vessels, on account of their po- sition. The point at which the ar- teries terminate and the minute veins commence, cannot be exactly defined, for the transition is gradual; but the intermediate network has, neverthe- less, this peculiarity, that the small vessels which 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 network that they compose are more uniform in shape and size than those formed by the anastomoses of the minute arteries and veins. The structure of the capillaries is much more simple than that of the arteries or veins. Their walls are composed of a single layer of elon- gated or radiate, flattened and nu- cleated cells, so joined and dovetailed together as to form a continuous transparent membrane (Fig. 49). Outside these cells, in the larger cap- illaries, there is a structureless, or very finely fibrillated membrane, on the inner surface of which they are laid down. The diameter of the capillary vessels varies somewhat in the different textures of the body, the most common size being about -g-o’ovrth of an inch. Among the smallest may be men- Fig. 48. Bloodvessels of an intestinal villus, representing the ar- rangement of capillaries be- tween the ultimate venous and arterial branches ; o, a, the ar- teries; 6, the vein. THE CAPILLARIES. 133 tioned 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 form of the capillary network presents considerable variety in the different textures of the body: the varieties con- sisting principally of modifications of two chief kinds of mesh, the rounded and the elongated. That kind in 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 Fig. 49. Magnified view of capillary vessels from the bladder of the cat.—A, V, an artery and a vein ; i, transitional vessel between them and c c, the capillaries. The muscu- lar coat of the larger vessels is left out in the figure to allow the epithelium to be seen at c', a radiate epithelium scale with four pointed processes, running out upon the four adjoining capillaries (after Chrzonszczewesky, Virch. Arch. 1836)- dense, such as the lungs (Fig. 50), most glands, and mucous membranes, and the cutis. . The meshes of this kind of net- work are not quite circular, but more or less angular, some- times presenting a nearly regular quadrangular or polygonal form, but being more frequently irregular. The capillary net- work with elongated meshes (Fig. 51) is observed in parts in which the vessels are arranged among bundles of fine tubes or fibres, as in muscles and nerves. In such parts, the meshes 134 THE CIRCULATION. usually have the form of a parallelogram, the short sides of which may be from three to eight or ten times less than the long ones; the long sides always corresponding to the axis of Fig. 50. Fig. 51. Fig. 50.—Network of capillary vessels of the air-cells of the horse's lung, magnified. a, a, capillaries proceeding from b, b, terminal branches of the pulmonary artery (after Frey). Fig. 51.—Injected capillary vessels of muscle, seen with a low magnifying power (Sharpey). the fibre or tube, by which it is placed. The appearance of both the rounded and elongated meshes is much varied accord- ing 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. What- ever be the form of the capillary network in any tissue or organ, it is, as a rule, found to prevail in the corresponding parts of all animals. 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 THE CAPILLARIES. 135 themselves. In the human lung they are smaller than the vessels; in the human kidney, and in the kidney of the dog, the diameter of the injected capillaries, 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, com- pared with the long diameter of the meshes, being in the pro- portion of one to eight or ten ; compared with the transverse diameter, in the proportion of one to four or six. In the mu- cous membranes—for example, in the conjunctiva—and in the cntis 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 cellular 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; that is, the closer is its capillary network and the larger its supply of blood. Hence the narrowness of the interspaces in all glandu- lar 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 complete absence of vessels in cartilage, the dense tendons of adults, and such parts as those in which, probably, very little organic change occurs after they are once formed. But the general rule must he modified by the consideration, that some organs, such as the brain, though they have small and not very closely ar- ranged capillaries, may receive large supplies of blood by reason of its more rapid movement. When an organ has large arterial trunks and a comparatively small supply of capillaries, the movement of the blood through it will be so quick, that it may, in a given time, receive as much fresh blood as a more vascular part with smaller trunks, though at any given instant the less vascular part will have in it a smaller quantity of blood. In the Circulation in the Capillaries, as seen in any trans- parent part of a living adult animal by means of the micro- scope (Fig. 52), the blood flows with a constant equable motion. In very young animals, the motion, though continuous, is ac- celerated at intervals corresponding to the pulse in the larger arteries, and a similar motion of the blood is also seen in the capillaries of adult animals when they are feeble: if their exhaustion is so great that the power of the heart is still more diminished, the red corpuscles are observed to have merely 136 THE CIRCULATION. the periodic motion, and to remain stationary in the intervals ; while, if the debility of the animal is extreme, they even re- cede somewhat after each impulse, apparently because of the elastic- ity of the capillaries, and the tis- sues around them. These obser- vations may be added to those already advanced (p. 114) to prove that, even in the state of great debility, the action of the heart is sufficient to impel the blood through the capillary ves- sels. Moreover, Dr. Marshall Hall having placed the pectoral fin of an eel in the field of the microscope and compressed it by the weight of a heavy probe, ob- served that the movement of the blood in the capillaries became obviously pulsatory, the pulsa- tions being synchronous with the contractions of the ventricle. The pulsatory motion of the blood in the capillaries cannot be attributed to an action in these vessels; for, when the animal is tranquil, they present not the slightest change in their diameter. It is in the capillaries, that the chief resistance 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 contact. The velocity of the blood is also in them reduced to its minimum, because of the widening of the stream. Tf, as Professor Midler says, 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. It is quite true, that the force with which the blood is propelled in the arteries, as shown by the quantity of blood which escapes from them in a certain space of time, is greater than that Avith Avhich it moves in the veins; but this force has to overcome all the resistance offered in the arterial and capillary system—the heart, itself, indeed, must overcome this resist- ance ; so that the excess of the force of the blood’s motion in Fig. 52. C'apillaries in the web of the frog’s foot magnified. TIIE CAPILLARIES. 137 the arteries is expended in overcoming this resistance, and the rapidity of the circulation in the arteries, even from the com- mencement of the aorta, would be the same as in the veins and capillaries, if the aggregate capacity of each of the three systems of vessels were the same. But since the aggregate sectional area of the branches is greater than that of the trunk from which they arise, the rapidity of the blood’s motion will necessarily he greater in the trunk, and will diminish in proportion as the aggregate capacity of the vessels increases during their ramification : in the same manner as, other things being equal, the velocity of a stream diminishes as it widens. The observations of Hales, E. H. Weber, and Valentin, agree very closely as to the rate of the blood in the capillaries of the frog; and the mean of their estimates gives the velocity of the systemic capillary circulation at about one inch per minute. Through the pulmonic capillaries, the rate of motion, according to Hales, is about five times that through the sys- temic ones. The velocity in the capillaries of warm-blooded animals is greater, but has not yet been accurately estimated. If it be assumed to be three times as great as in the frog, still the estimate may seem too low, and inconsistent with the facts, which show that the whole circulation is accomplished in about a minute. But the whole length of capillary vessels, through which any given portion of blood has to pass, prob- ably does not exceed of an inch; and therefore the time required for each quantity of blood to traverse its own ap- pointed portion of the general capillary system will scarcely amount to a second ; while in the pulmonic capillary system the length of time required wjjl be much less even than this. The estimates given above are drawn from observations of the movements of the red blood-corpuscles, which move in the centre of the stream. At the circumference of the stream, in contact with the walls of the vessel, and adhering to them, there is a layer of liquor sanguinis which appears to be motion- less. The existence of this still layer, as it is termed, is in- ferred both from the general fact that such a one exists in all fine tubes traversed by fluid, and from what can be seen in watching the movements of the blood-corpuscles. The red corpuscles occupy the middle of the stream and move with comparative rapidity ; the colorless lymph-corpuscles run much more slowly by the walls of the vessel; while next to the wall there is often a transparent space in which the fluid appears to be at rest; for if any of the 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. 138 THE C T FiC ULATIO X. Part of this slow movement of the pale corpuscles and their occasional stoppage may be due, as E. H. Weber has suggested, to their having a natural tendency to adhere to the walls of the vessels. Sometimes, indeed, when the motion of the blood is not strong, many of the white corpuscles collect in a capil- lary vessel, and for a time entirely prevent the passage of the red corpuscles. But there is no doubt that such a still layer of liquor sanguinis exists next the walls of the vessels, and it is between this and the tissues around the vessels that those interchanges of particles take place which ensue in nutrition, secretion, and absorption by the bloodvessels; interchanges which are probably facilitated by the tranquillity of the fluids between which they are effected. Until within the last few years it has been generally sup- posed that the occurrence of any transudation from the inte- rior of the capillaries into the midst of the surrounding tissues was confined, in the absence of injury, strictly to the fluid part of the blood ; in other words, that the corpuscles could not es- cape from the circulating stream, unless the wall of the con- taining bloodvessel were ruptured. It is true that an English physiologist, Dr. Augustus Waller, affirmed in 1846, that he had seen blood-corpuscles, both red and white, pass bodily through the wall of the capillary vessel in which they were contained; and that, as no opening could be seen before their escape, so none could be observed afterwards—so rapidly was the part healed. But 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, was rediscovered by Professor Cohnheim in 1867. Professor Cohnheim’s experiment demonstrating the passage of the corpuscles through the wall of the bloodvessel, is per- formed in the following manner. A frog is eurarized, that is to say, paralysis is produced by injecting under the skin a minute quantity of the poison called curare; 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 con- traction, 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. The process of extrusion of the white corpuscles is thus described by Dr. Burdon Sanderson, and the passage of the red corpuscles oc- curs after much the same fashion. “Simultaneously with the retardation, the leucocytes, in- stead of loitering here and there at the edge of the axial cur- THE CAPILLARIES. 139 rent, begin to crowd in numbers against the vascular wall, as was long ago described by Dr. Williams. 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 (to quote Professor Cohnheim’s words) here and there minute, colorless, button-shaped elevations sjn’ing, 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 corresponding to that of a leucocyte. Eventually the hemisphere is convert- ed into a pear-shaped body, the small end of which is still at- tached to the surface of the vein, while the round part pro- jects 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 transparent 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. The observer has before him an emigrant leucocyte, which in all apprecia- ble respects resembles those which have been already de- scribed in the aqueous humor of the inflamed eye.” Various explanations of these remarkable phenomena have been suggested. Probably the nearest to the truth are those which attribute the chief share in the process to the vital en- dowments with respect to mobility and contractility of the parts concerned—both of the corpuscles (Bastian) and the cap- illary wall (Strieker). Dr. Sanderson remarks, “ The capillary is not a dead conduit, but a tube of living protoplasm. There is no difficulty in understanding how 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 striking peculiari- ties of contractile substance that when two parts of the same 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 bloodvessels 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. The circulation through the capillaries must, of necessity, be largely influenced by that which occurs in the vessels on either side of them—in the arteries or the veins; their in- 140 THE CIRCULATION. termediate position 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 contraction of the capilla- ries, on the application of certain irritating substances, 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 in- fluenced 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 canals for the passage of blood, but as possessing en- dowments of their own, in relation to the circulation. The capillary wall is, according to Strieker, actively living and contractile; and there is no reason to doubt that, as such, it must have an important influence in connection with that nu- tritive exchange which goes on without cessation between the blood within and the tissues outside the capillary vessel; a process which, under the name of vital capillary force, has long been recognized as one of the means concerned in the circulation of the blood. The results of morbid action, as well as the phenomena of health, strongly support the notion of the existence of this so-called vital capillary attraction between the blood and the tissues. For example, when the access of oxygen to the lungs is prevented, the circulation through the pulmonic capillaries is gradually retarded, the blood-corpuscles cluster together, and their movement is eventually almost arrested, even while the action of the heart continues. In inflammation, also, the capillaries of an inflamed part are enlarged and distended with blood, which either moves very slowly or is completely at rest. In both these cases the phenomena are local, and in- dependent of the action of the heart, and appear to result from some alteration in the blood, which increases the adhesion of its particles to one another, and to the walls of the capilla- ries, to an amount which the propelling action of the heart is not able to overcome. It may be concluded then, that the capillaries, which are formed of a simple cellular membrane, can of themselves ex- ercise no such direct influence on the movement of their con- tents as to be at all comparable in degree to that which is ex- ercised by the arteries or veins: yet that the constant inter- change of relations between the blood within and the tissues outside these vessels does in some measure facilitate the move- ment of blood through the capillary system, and constitute one of the assistant forces of the circulation. THE VEI NS. 141 THE VEINS. In structure the coats of veins bear a general resemblance to those of arteries. Thus, they possess an outer, middle, and in- ternal coat. The older coat is constructed of areolar tissue like that of the arteries, but is thicker. In some veins it con- tains muscular fibre-cells. The middle coat is considerably thinner than 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 venae cavce and pulmonary veins, the middle coat is replaced, for some distance from the heart, by circularly arranged striped muscular fibres, continuous with those of the auricles. The internal coat of veins is less brittle than the correspond- ing coat of an artery, but in other respects resembles it closely. The chief influence which the veins have in the circulation, is effected with the help of the valves, which are placed in all veins subject to local pressure from the muscles between or near which they run. The general construction of these valves is similar to that of the semilunar valves of the aorta and pul- monary artery, already described (p. 96) ; but their free mar- gins are turned in the opposite direction, i.e., towards 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. 53). In the smaller veins, single valves are often met with; and three or Fig. 53. Diagrams 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, ex- hibiting a swelling in the situation of a pair of valves. 142 THE C IK C U L A T T O X. 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 unattached edge being in some examples concave, in others straight. They are composed of inextensile fibrous tissue, and are covered with epithelium like that lining the veins. During the period of their inaction, when the venous blood is flowing in its proper direction, they lie by the sides of the veins ; but when in ac- tion, they close together like the valves of the arteries, and offer a complete barrier to any backward movement of the blood (Figs. 54 and 55). Valves are not equally numerous in all veins, and in many they are absent altogether. They are most numerous in the veins of the extremities, 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, 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 supe- rior and inferior vena cava, the trunk and branches of the portal vein, the 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. The principal obstacle to the circulation is already over- come when the blood has traversed the capillaries; and the force of the heart which is not yet consumed, is sufficient to complete its passage through the veins, in which the obstruc- tions to its movement are very slight. For the formidable obstacle supposed to be presented 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 bloodvessels have, of course, to bear the greatest amount of pressure; the pressure on each part being directly proportionate to the height of the column of blood above it: hence their liability to disten- sion. 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 columns of fluid are of equal height in both, and continuous. Their condition may, in this respect, be compared with that of a double bent tube, full of fluid, held vertically; whatever be the height and gravitation PRESSURE IN VEINS. 143 of the columns of fluid, neither of them can move of its own weight, each being supported by the other; yet the least pres- sure on the top of either column will lift up the other; so, when the body is erect, the least pressure on the column of arterial blood may lift up the venous blood, and, were it not for the valves, the least pressure on the venous might lift up the arterial column. In experiments to determine what proportion of the force of the left ventricle remains to propel the blood in the veins, Valen- tin found that the pressure of the blood in the jugular vein of a dog, as estimated by the htemadynamometer, did not amount to more than T’T or of that in the carotid artery of the same animal ; and this estimate is confirmed, in the instances of several other arteries and their corresponding veins, by Mogk. In the upper part of the inferior vena cava, Valentin could scarcely detect the existence of any pressure, nearly the whole force received from the heart having been, apparently, con- sumed during the passage of the blood through the capillaries. But slight as this remaining force might be (and the experi- ment in which it was estimated would reduce the force of the heart below its natural standard), it would be enough to com- plete the circulation of the blood ; for, as already stated, the spontaneous dilatation of the auricles and ventricles, though it may not be forcible enough to assist the movement of blood into them, is adapted to ofler to that movement no obstacle. Very effectual assistance to the flow of blood in the veins is afforded by the action of the muscles capable of pressing on such veins as have valves. The effect of muscular pressure on such veins may be thus ex- plained. When pressure is applied to any part of a vein, and the current of blood in it is obstructed, the portion behind the seat of pressure becomes swollen and distended as far back as to the next pair of valves. These, acting like the arterial valves, and being, like them, inextensile both in themselves and at their margins of attachment, do not follow the vein in its distension, but are drawn out towards the axis of the canal. Then, if the pressure continues on the vein, the compressed blood, tending to move equally 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 distributed partly in pressing the blood onwards in the proper course of the circulation, and partly in pressing it back- wards and closing the valves behind. The circulation might lose as much as it gains by such com- pression of the veins, if it were not for the numerous anas- 144 THE CIRCULATION. tomoses 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 hin- drance 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 by another vein (Figs. 54 and 55). Thus, therefore, the Fig. 54. Fig. 55. Fig. 54.—Vein with valves open (Dalton). Fig. 55.—Vein with valves closed ; stream of blood passing off by lateral channel (Dalton). effect of muscular pressure upon veins which have valves, is turned almost entirely to the advantage of the circulation ; the pressure of the blood onwards is all advantageous, and the pressure of the blood backwards 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 acceleration of the stream of blood when, in venesection, the muscles of the forearm are put in action, and by the general acceleration of the circulation during active exercise; and the numerous movements which are continually 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 circu- lation continues unimpaired in parts at rest, in paralyzed limbs, and in parts in which the veins are not subject to any muscular pressure. EFFECTS OF RESPIRATION. 145 Besides the assistance thus afforded by muscular pressure to the movement of blood along veins possessed of valves, it has been discovered by Mr. Wharton Jones that, in the web of the bat’s wing, the veins are furnished with valves, and possess the remarkable property of rhythmical contraction and a dilatation, whereby the current of blood within them is distinctly accelerated. The contraction occurred, on an aver- age, about ten times in a minute; the existence of valves pre- venting regurgitation, the entire effect of the contractions was auxiliary to the onward current of blood. Analogous phe- nomena have been now frequently observed in other animals. Agents concerned in the Circulation of the Blood. The agents concerned in the circulation of the blood which have been now described, may be thus enumerated : 1. The action of the heart and of the arteries. 2. The vital capillary force exercised in the capillaries. 3. The possible slight action of the muscular coat of veins ; and, much more, the contraction of muscles capable of acting on veins provided with valves. It remains only to consider (4) the influence of the respira- tory movements on the circulation. Although the continuance of the respiratory movements is essential to the circulation of the blood, and although their cessation is followed, within a very few minutes, by that of the heart’s action also, yet their direct mechanical influence on the movement of the current of blood is probably, under ordinary circumstances, but slight. The effect of expiration in increas- ing the pressure of the blood in the arteries is minutely illus- trated by the experiments of Ludwig. It acts as the pressure of contracting muscles does upon the veins, and is advantage- ous to the onward movement of arterial blood, inasmuch as all movement backwards into the heart, which would otherwise occur at the same moment and from the same cause, is pre- vented by the force of the onward stream of blood from the contracting ventricle, and in the intervals of this contraction by the closure of the semilunar valves. Under ordinary cir- cumstances, and with a free passage through the capillaries of the lungs, the effect of expiration on the stream of blood in the veins is also probably to assist, rather than retard its move- ment in the proper direction. For, with no obstruction in front, there is the force of the blood streaming into the heart from behind, to prevent any tendency to a backward flow, even apart from what may be effected by the presence of the valves of the venous system. 146 THE CIRCULATION. It is true that in violent expiratory efforts there is a certain retardation of the circulation in the veins. The effect of such retardation is shown in the swelling up of the veins of the head and neck, and the lividity of the face, during coughing, strain- ing, and similar violent expiratory efforts; the effect shown in these instances being due both to some actual regurgitation of the blood in the great veins, and to the accumulation of blood in all the veins, from their being constantly more and more filled by the influx from the arteries. But strong expiratory efforts, as in straining and the like, are not fairly comparable to ordinary expiration, inasmuch as they are instances of more or less interference with expiration, and involve probably circumstances leading to obstruction of the circulation in the pulmonary capillaries, such as are not present in the ordinary rhythmical exit of air from the lungs. The act of inspiration is favorable to the venous circulation, and its effect is not counterbalanced by its tendency to draw the arterial, as well as the venous, blood towards the cavity of the chest. When the chest is enlarged in inspiration, the ad- ditional space within it is filled chiefly by the fresh quantity of air which passes through the trachea and bronchial passages to the vesicular structure of the lungs. But the blood being, like the air, subject to the atmospheric pressure, some of it also is at the same time pressed towards the expanding cavity of the chest, and therein towards the heart. The effect of this on the arterial current is hindered by the aortic valves, while they are closed, and by the forcible outward stream of blood from the ventricles when they are open ; while, on the other hand, there is nothing to prevent an increased afflux of blood to the auricles through the large veins. Sir David Barry was the first who showTed plainly this effect of inspiration on the venous circulation ; and he mentions the following experiment in proof of it. He introduced one end of a bent glass tube into the jugular vein of an animal, the vein being tied above the point where the tube was in- serted ; the inferior end of the tube was immersed in some colored fluid. He then observed that at the time of each in- spiration the fluid ascended in the tube, while during expiration it either remained stationary, or even sank. Poiseuille con- firmed the truth of this observation, in a more accurate manner, by means of his hsemadynamometer. And a like confirmation has been since furnished by Valentin, and in minute details by Ludwig. The effect of inspiration on the veins is observable only in the large ones near the thorax. Poiseuille could not detect it by means of his instrument in veins more distant from the VELOCITY OF THE CIRCULATION. 147 heart—for example, in the veins of the extremities. And its beneficial effect would be neutralized were it not for the valves ; for he found that, when he repeated Sir D. Barry’s exper- iments, and passed the tube so far along the veins that it went beyond the valves nearest to the heart, as much fluid was forced back into the tube in every expiration as was drawn in through it in every inspiration. Some recent experiments, by Dr. Burdon Sanderson, have proved more directly that inspiration is favorable to the cir- culation, inasmuch as, during it, the tension of the arterial system is increased. And it is only when the respiratory orifice is closed, as by plugging the trachea, that inspiratory efforts are sufficient to produce an opposite effect—to diminish the tension in the arteries. On the whole, therefore, the respiratory movements of the chest are advantageous to the circulation. Velocity of Blood in the Veins. The velocity of the blood is greater in the veins than in the capillaries, but less than in the arteries; and with this fact may be remembered the relative capacities of the arterial and venous systems; for since the veins return to the heart all the blood that they receive from it in a given time through the arteries, their larger size and proportionally greater number must compensate for the slower movement of the blood through them. 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, about twice or three times as great in the arteries as in the veins. Some doubt has, however, been lately expressed regarding the accuracy of this calculation, and the matter, therefore, must be considered not yet settled. The rate at which the blood moves in the veins gradually increases the nearer it approaches the heart, for the sectional area of the venous trunks, compared with that of the branches opening into them, becomes gradually less as the trunks advance to- wards the heart. Velocity of the Circulation. Having now considered the share which each of the circu- latory organs has in the propulsion and direction of the blood, we may speak of their combined effects, especially in regard to 148 THE CIRCULATION. the velocity with which the movement of the blood through the whole round of the circulation is accomplished. As Muller says, the rate of the blood’s motion in the vessels must not be judged of by the rapidity with which it flows from a vessel when divided. In the latter case, the rate of motion is the result of the entire pressure to which the whole mass of blood is subjected in the vascular system, and which at the point of the incision in the vessel meets with no resistance. In the closed vessels, on the contrary, no portion of blood can be moved forwards except by impelling on the whole mass, and by overcoming the resistance arising from friction in the smaller vessels. From the rate at which the blood escapes from opened ves- sels we can only judge, in general, that its velocity is, as already said, greater in arteries than in veins, and in both these greater than in the capillaries. More satisfactory data for the 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. From eighteen such experiments on horses, Hering deduced that the time required for the passage of a solution of ferrocyanide of potassium, mixed with the blood, from one jugular vein (through the right side of the heart, the pulmon- ary circulation, 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 trans- mitted 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 occu- pied 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 heart’s action. Poiseuilie’s observations accord completely with the above, and show, moreover, that when the ferrocyanide is injected into the blood with other substances, such as acetate of am- monia, or nitrate of potash (solutions of which, as other experiments have shown, pass quickly through capillary tubes), the passage from one jugular vein to the other is ef- fected in from eighteen to twenty-four seconds ; while, if instead of these, alcohol is added, the passage is not completed until from forty to forty-five seconds after injection. Still greater rapidity of transit has been observed by Mr. J. Blake, who found that nitrate of baryta injected into the jugular vein of VELOCITY OF THE CIRCULATION. 149 a horse could be detected in blood drawn from the carotid ar- tery of the opposite side in from fifteen to twenty seconds after the injection. In sixteen seconds a solution of nitrate of potash, injected into the jugular vein of a horse, caused com- plete arrest of the heart’s action, by entering and diffusing itself through the coronary arteries. In a dog, the poisonous effects of strychnia on the nervous system were manifested in twelve seconds after injection into the jugular vein ; in a fowl, in six and a half seconds, and in a rabbit in four and a half seconds. In all these experiments, it is assumed that the substance injected moves with the blood, and at the same rate as it, and does not move from one part of the organs of circulation to another by diffusing itself through the blood or tissues more quickly than the blood moves. The assumption is sufficiently probable, to be considered nearly certain, that the times above- mentioned, as occupied in the passage of the injected sub- stances, are those in which the portion of blood, into which each was injected, was carried from one part to another of the vascular system. It would, therefore, 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 one minute, which is the estimate usually adopted of the average time in which the blood completes its entire circuit in man, is rather above than below the actual rate. Another mode of estimating the general velocity of the cir- culating blood, is by calculating it from the quantity of blood supposed to be contained in the body, and from the quantity which can pass through the heart in each of its actions. But the conclusions arrived at by this method are less satisfactory. For the estimates, both of the total quantity of blood, and of the capacity of the cavities of the heart, have as yet only ap- proximated to the truth. Still, the most careful of the esti- mates thus made accord with those already mentioned ; for Valentin has, from these data, calculated that the blood may all pass through the heart in from 43f to 62§ seconds. The estimate for the speed at which the blood may be seen moving in transparent parts, is not opposed to this. For, as already stated (p. 137), though the movement through the capillaries may be very slow, yet the length of capillary vessel through which any portion of blood has to pass is very small. 150 THE CIRCULATION. Even if we estimate that length at the tenth of an inch, and suppose the velocity of the blood therein to be only one inch per minute, then each portion of blood may traverse its own distance of the capillary system in about six seconds. There would thus be plenty of time for the blood to travel through its circuit in the larger vessels, in which the greatest length of tube that it can have to traverse in the human subject does not exceed ten feet. All the estimates here given are averages; but of course the time in which a given portion of blood passes from one side of the heart to the other, varies much according to the organ it has to traverse. The blood which circulates from the left ven- tricle, through the coronary vessels, to the right side of the heart, requires a far shorter time for the completion of its course than the blood which flows from the left side of the heart to the feet, and back again to the right side of the heart; for the circulation from the left to the right cavities of the heart may be represented as forming a number of arches, vary- ing in size, and requiring proportionately various times for the blood to traverse them; the smallest of these arches being formed by the circulation through the coronary vessels of the heart itself. The course of the blood from the right side of the heart, through the lungs to the left, is shorter than most of the arches described by the systemic circulation, and in it the blood flows, cceteris paribus, much quicker than in most of the vessels which belong to the aortic circulation. For although the quantity of blood contained, at any instant, in the greater circulation of the body, is far greater than the quantity within the lesser circulation ; yet, in any given space of time, as much blood must pass through the lungs as passes in the same time through the systemic circulation. If the systemic vessels contain five times as much blood as the pulmonary, the blood in them must move five times as slow as in these; else, the right side of the heart would be either overfilled or not filled enough. Peculiarities of the Circulation in different Parts. The most remarkable peculiarities attending the circulation of blood through different organs are observed in the cases of the lungs, the liver, the brain, and the erectile organs. The pulmonary and portal circulations have been already alluded to (pp. 89, 90), and will be again noticed when considering the functions of the lungs and liver. The chief circumstances requiring notice, in relation to the cerebral circulation, are observed in the arrangement and dis- CEREBRAL CIRCULATION. 151 tribution of the vessels of the brain, and in the conditions attending the amount of blood usually contained within the cranium. The functions of the brain seem to require that it should receive a large supply of blood. This is accomplished through the number and size of its arteries, the two internal carotids, and the two vertebrals. But it appears to be further necessary that the force with which this blood is sent to the brain should be less, or at least, subject to less variation from external cir- cumstances than it is in other parts. This object is effected by several provisions; such as the tortuosity of the large arteries, and their wide anastomoses in the formation of the circle of Wil- lis, which will insure that the supply of blood to the brain maybe uniform, 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 temporal 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 arrangement of the veins within the cranium is also pe- culiar. 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 some 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 condition 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 quantity of blood in the brain must be at all times the same ; and that the quantity of blood received within any given time through the arteries must be always, and at the same time, exactly equal to that re- moved by the veins. In accordance with this supposition, the symptoms commonly referred to either excess or deficiency of blood in the brain, were ascribed to a disturbance in the bal- ance between the quantity of arterial and that of venous blood. 152 THE CIRCULATION. Some experiments performed by Dr. Kellie appeared to estab- lish the correctness of this view. But Dr. Burrows having repeated these experiments, and performed additional ones, obtained different results. He found that in animals bled to death, without any aperture being made in the cranium, the brain became pale and anremic like other parts. And in proof that, during life, the cerebral circulation is influenced by the same general circumstances that influence the circulation elsewhere, he found congestion of the cerebral vessels in rab- bits killed by strangling or drowning ; while in others, killed by prussic acid, he observed that the quantity of blood in the cavity of the cranium was determined by the position in which the animal was placed after death, the cerebral vessels being congested when the animal was suspended with his head down- wards, and comparatively empty when the animal was kept suspended by the ears. He concluded, therefore, that 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 increase in the quantity of the cerebro-spinal fluid, which, by readily admitting of being re- moved 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 uuiformly filled in case of variations in their quantity. And there can be no doubt that, although the arrangements of the bloodvessels, to which reference has been made, insure to the brain an amount of blood which is tolerably uniform, yet, inasmuch as wTith every beat of the heart and every act of respiration, and under many other cir- cumstances, 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 insuffi- cient supply of blood. Hence we may consider that the cere- bro-spinal fluid in the interior of the skull not only subserves the mechanical functions of fat in other parts as a packing material, but by the readiness with which it can be displaced into the spinal canal, provides the means wrhereby undue pres- sure and insufficient supply of blood are equally prevented. Circulation in 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 cer- tain times, receive an unusually large quantity of blood, be- CIRCULATION IN ERECTILE STRUCTURES. 153 come distended and swollen by it, and pass into the state which has been termed erection. Such structures are the cor- pora cavernosa and corpus spongiosum of the penis in the male, and the clitoris in the female; and, to a less degree, the nipple of the mammary gland in both sexes. The corpus cavernosum penis, which is the best example of an erectile structure, has an external fibrous membrane or sheath; and from the inner surface of the latter are prolonged numerous fine lamellae which divide its cavity into small compartments looking like cells when they are inflated. Within these is situated fhe 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 dilatation 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 dis- tension of the vessels, and, during the state of erection, give to the penis its condition 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 vein of the penis; those from the corpus 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 accelerator urinse. 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 in- fluence is communicated to the penis by the pudic nerves, which ramify in its vascular tissue: and Guenther has ob- served, that, after their division in the horse, the penis is no longer capable of erection. It affords a good example of the subjection of the circulation in an individual organ to the 154 THE CIRCULATION. influence of the nerves; but the mode in which they excite a greater influx of blood is not with certainty known. The most probable explanation is that offered by Professor Kolliker, who ascribes the distension of the venous plexuses to the influence of organic muscular fibres, which are found in abundance in the corpora cavernosa of the penis, from the bulb to the glans, also in the clitoris and other parts capable of erection. While erectile organs are flaccid and at rest, these contractile fibres exercise an amount of pressure on the plexuses of vessels distributed amongst them, sufficient to pre- vent their distension with blood. But when through the in- fluence of their nerves, these parts are stimulated to erection, the action of these fibres is suspended, and the plexuses thus liberated from pressure, yield to the distending force of the blood, which, probably, at the same time arrives in greater quantity, owing to a simultaneous dilatation of the arteries of the parts, and thus the plexuses become filled, and remain so until the stimulus to erection subsides, when the organic mus- cular fibres again contract, and so gradually expel the excess of blood from the previously distended vessels. The influence of cold in producing extreme contraction and shrinking of erectile organs, and the opposite effect of warmth in inducing fulness and distension of these parts, are among the arguments used by Kolliker in support of this opinion. The accurate dissections and experiments of Kobelt, extend- ing and confirming those of Le Gros Clark and Krause, have shown, that this influx of the blood, however explained, is the first condition necessary for erection, and that through it alone much enlargement and turgescence of the penis may ensue. But the erection is probably not complete, nor main- tained 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 assist the erection of the nipples, or that amount of turgescence, just falling short of erection, of which the spleen and many other parts are capable. For such tur- gescence nothing more seems necessary than a large plexiform arrangement of the veins, and such arteries as may admit, upon local occasions, augmented quantities of blood. The Influence of the Nervous System on the circulation in the bloodvessels will be considered in Chapter XVII. STRUCTURE OF THE LUNGS. 155 CHAPTER VII. RESPIRATION. As the blood circulates through the various parts of the body, and fulfils its office by nourishing the several tissues, by supplying to secreting organs the materials necessary for their secretions, and by the performance of other duties with which it is charged, it is deprived of part of its nutritive constituents, and receives impurities which need removal from the body. It is, therefore, necessary that fresh supplies of nutriment should be continually added to the blood, and that provision should be made for the removal of the impurities. The first of these objects is accomplished by the processes of digestion and ab- sorption. The second is principally effected by the agency of the various excretory organs, through which are removed the several impurities with which the blood is charged, whether these impurities are derived altogether from the degenerations of tissue, or in part also from the elements of unassimilated food. One of the most important and abundant of the impu- rities is carbonic acid, the removal of which and the introduc- tion of fresh quantities of oxygen, constitute the chief purpose of respiration—a process which, because of its intimate rela- tion to the circulation, may be considered here, rather than with the other excretory functions. Position and Structure of the Lungs. The lungs occupy the greater portion of the chest, or upper- most of the two cavities into which the body is divided by the diaphragm (Fig. 31). 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 their surface of the section, their tubular structure. In fact, however, the lungs are hollow organs, and we may consider them as really two bags containing air, each of which communicates by a separate orifice with a common air-tube (Fig. 31), through the upper portion of which, the larynx, they freely communicate with the external atmosphere. The-orifice 156 RESPIRATION. of the larynx is guarded by muscles, and can be opened or closed at will. It has been said, in the preceding chapter that each lung is enveloped in a distinct fibrous bag, with a smooth, slippery lining, and that the outer smooth surface of the lung glides easily on the inner smooth surface of the bag which envelops Fig. 56. Transverse section of the chest (after Gray). it. This enveloping bag, which is called the pleura, is easily seen in the dead subject; and when it is opened, as in an ordi- nary post-mortem examination, there is a considerable space left, by the elastic recoil of the lung, between the outer sur- face of the lung and the inner surface of the pleura, which is left sticking, so to speak, to the inner surface of the walls and floor of the chest. The space, however, between the lung and the pleura does not exist (except in some cases of disease) so long as the chest is not opened ; and, while considering the subject of normal healthy respiration, we may discard altogether the notion of any space or cavity between the lung and the wall of the chest. So far as the movement of the lung is concerned it might be adherent completely to the chest-wall, inasmuch as they accompany each other in all their movements; only there is a slight gliding of the smooth surface of the lung on the smooth inner surface of the pleura, but no separation, in the slightest degree, of one from the other.1 1 It maybe mentioned, that the smooth covering of the lung is really continuous with the inner smooth lining of the walls and floor of the chest, as will be readily seen in Fig. 56. Hence the mem- STRUCTURE OF THE LUNGS. 157 The trachea, or tube through which air passes to the lungs, divides into two branches—one for each lung; and these primary branches, or bronchi, after entering the substance of the organ, divide and subdivide into a number of smaller and smaller branches, which penetrate to every part of the organ, until at length they end in the smaller subdivisions of the lung called lobules. All the larger branches have walls formed of tough membrane, containing portions of cartilaginous rings, by which Fig. 57. A diagrammatic representation of the heart and great vessels in connection with the lungs—The pericardium has been removed, and the lungs are turned aside- 1, right auricle; 2, vena cava superior, 3, vena cava inferior; 4, right ventricle; 5, stem of the pulmonary artery; a a, its right and left branches; 6, left auricular appendage; 7, left ventricle; 8, aorta; 9, 10, the two lobes of the left lung; 11, 12< 13, the three lobes of the right lung ; b b, right and left bronchi; v v, right and left upper pulmonary veins. they are held open, and unstriped muscular fibres, as well as longitudinal bundles of' elastic tissue. They are lined by mu- cous membrane, the surface of which, like that of the larynx and trachea, is covered with vibratile ciliary epithelium (Fig. 58). As the bronchi divide they become smaller and smaller, and their walls thinner; the cartilaginous rings especially becom- brane which covers the lung is called the visceral layer of the pleura, and that which lines the walls and floor of the chest the parietal]ayer. The appearance of a cavity or space (Fig. 56) between the visceral layer of pleura (covering the lungs) and the parietal layer (covering the inner surface of the wall of the chest and upper part of the dia- phragm) is only inserted for the sake of distinctness. 158 RESPIRATION. ing scarcer and more irregular, until, in the smaller bronchial tubes, they are represented only by minute and scattered car- tilaginous 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 traces of circular muscular fibres; they are still lined, however, by a thin mucous membrane, with ciliated epithe- lium. Each lung is partially subdivided into separate portions, called lobes; the right lung into three lobes, and the left lung into two (Fig. 57). Each of these lobes, again, is composed Fig. 58. Ciliary epithelium of the human trachea magnified 350 diameters, a, layer of longitudinally arranged elastic fibres ; b, basement-membrane ; c, deepest cells, cir- cular in form ; d, intermediate elongated cells; e, outermost layer of cells fully de- veloped and bearing cilia (after Kolliker). of a large number of minute parts, called lobules. Each pul- monary lobule may be considered a lung in miniature, consist- ing, as it does, of a branch of the bronchial tube, of air-cells, bloodvessels, nerves, and lymphatics, with a sparing amount of areolar tissue. On entering a lobule, the small bronchial tube divides and subdivides ; its walls, at the same time, becoming thinner and thinner, until at length they are formed only of a thin mem- brane 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 widening out funnel-wise, and its walls being pouched out ir- regularly into small saccular dilatations, called air-cells (Fig. 59). Such a funnel-shaped terminal branch of the bronchial tube, with its group of pouches or air-cells, has been called an infundibulum (Fig. 59), and the irregular oblong space in its STRUCTURE OF THE LUNGS. 159 centre, with which the air-cells communicate, an intercellular passage. The air-cells may be placed singly, like recesses from the in- tercellular passage, but more often they are arranged in groups or even in rows, like minute sac- culated tubes ; so that a short se- ries of cells, all communicating with one another, open by a common orifice into the tube. The cells are of various forms, according to the mutual pres- sure to which they are subject; their walls are nearly in con- tact, and they vary from to ■Jfj of an inch in diameter. Their walls are formed of fine membrane, similar to that of the intercellular passages, and continuous with it, which mem- brane is folded on itself so as to form a sharp-edged border at each circular orifice of commu- nication between contiguous air- cells, or between the cells and the bronchial passages. Nu- merous fibres of elastic tissue are spread out between con- tiguous air-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 (Fig. 60, b and c). The cells are lined by a layer of squamous or tessellated epithelium, not provided with cilia. Outside the cells, a network of pulmonary capillaries is spread out so densely (Fig. 61), that the interspaces or meshes are even narrower than the vessels, which are, on an average, -g-o’o u °f an inch in diameter. Between the atmo- spheric air in the cells and the blood in these vessels, nothing intervenes but the thin membranes of the cells and capilla- ries and the delicate epithelial lining of the former; 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 same, contain only a single layer of capillaries, both sides of which are thus at once ex- posed to the air. The cells situated nearest to the centre of the lung are Fig. 59. Two small groups of air-cells, or infundibula, a a, with air cells, b b, and the ultimate bronchial tubes, c c, with which the air-cells communicate. From a new-born child (after Kolliker). 160 EESPIRA TION. smaller, and their networks of capillaries are closer than those nearer to the circumference, in adaptation to the more Fig. 60. Air-cells of lung, magnified 350 diameters, a, epithelial lining of the cells; b, fibres of elastic tissue ; c, delicate membrane of which the cell-wall is constructed with elastic fibres attached to it (after Kolliker). Flu. 61. Capillary network of the pulmonary bloodvessels in the human lung (from Kolliker) 6T°. MECHANISM OF RESPIRATION. 161 ready supply of fresh air to the central than the peripheral portion of the lungs. The cells 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. Mechanism of Respiration, For the proper understanding of the mechanism by which air enters and is expelled from the lungs, the following facts must be borne in mind: The lungs form two distinct hollow bags (communicating with the exterior through the trachea and larynx), and are always closely in contact with the inner surface of the chest- walls, while their lower portions are closely in contact with the diaphragm, or muscular partition which separates the chest from the abdomen (Figs. 31 and 65). The lungs follow all movements of the parts in contact with them ; and for the evident reason that the outer surface of the lung-bag not being exposed directly to atmospheric pressure, while the inner sur- face is so exposed, the pressure from within preserves the lungs in close contact with the parts surrounding them, and obliter- ates, practically, the pleural space, and must continue to do so, until from some cause or other—say from an opening for the admission of air through the chest-walls, the pressure on the outside of the lung equals or exceeds that on the interior. Any such artificial condition of things, however, need not here be considered. For the inspiration of air into the lungs it will be evident from the foregoing facts that all that is necessary is such a movement of the side-walls or floor of the chest, or of both, that the capacity of the interior shall be enlarged. By such increase of capacity there will be of course a diminution of the pressure of the air in the lungs, and a fresh quantity will enter through the larynx and trachea to equalize the pressure on the inside and outside of the chest. For the expiration of air, on the other hand, it is also evident that, by an opposite move- ment which shall contract the capacity of the chest, the pres- sure in the interior will be increased, and air. will be expelled, until the pressures within and without the chest are again equal. In both cases the air passes through the trachea and larynx, whether in entering or leaving the lungs, there being no other communication with the exterior, and the lung, for the reason before mentioned, remains under all the circum- 162 RESPIRATION. stances described, closely in contact with the walls and floor of the chest. To speak of expansion of the chest is to speak also of expansion of the lung. We have now to consider the means by which the chest- cavity is alternately enlarged and contracted for the entrance and expulsion of atmospheric air; or, in technical terms, for inspiration and expiration. The chest is a cavity filled by the lungs, heart, and large bloodvessels, &c., and closed everywhere against the entrance of air except by the way of the larynx and trachea. It is bounded behind and at the sides by the spine and ribs, and in front by the sternum and cartilages of the ribs. Its door is formed mainly by the diaphragm. The immediate inner lining of all these parts is the outer or polished layer of the pleura; and this membrane also is stretched continuously across the top of the chest-cavity, and mainly forms its roof. The enlargement of the capacity of the chest in inspiration is a muscular act; the muscles concerned in producing the effect being chiefly the diaphragm and the external intercostal muscles, with that part of the internal intercostal which is be- tween the cartilages of the ribs. These are assisted by the levatores costarum, the serratus posticus superior, and some others. The vertical diameter of the chest is increased by the con- traction and consequent descent of the diaphragm—the sides of the muscle descending most, and the central tendon remain- ing comparatively unmoved, while the intercostal and other muscles just mentioned, by acting at the same time, not only prevent the diaphragm during its contraction from drawing in the sides of the chest, but increase the diameter of the chest in the lateral direction, by elevating the ribs; that is to say, by rotating them, to speak roughly, around an axis passing through their sternal and spinal attachments—somewhat after the fashion of raising the handle of a bucket (Fig. 62). This is not all, however. Another effect of the contraction of the intercostal muscles is to increase the antero-posterior diameter of the chest—by partially straightening out the angle between the rib and its cartilage, and thus lengthening the distance between its spinal and sternal attachments (Fig. 62, a). In this way, at the same time that the ribs are raised, the sternum is pushed forward. This forward movement of the sternum, which is accompanied by a slight upward movement, is in part Respiratory Movements. RESPIRATORY MOVEMENTS. 163 accomplished also by a raising of the anterior extremities of the rib cartilages, which of course, in any movement, carry the sternum with them. The differences in shape and direc- Fig. 62. tion of the upper and lower true ribs, and the more acute angles formed by the junction of the latter with their carti- lages, make the effect much greater at the lower than at the upper part of the chest. The expansion of the chest in inspiration presents some pe- culiarities in different persons and circumstances. In young children, it is effected almost entirely by the diaphragm, which being highly arched in expiration, becomes flatter as it con- tracts, 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 abdominal mode or type of respiration. In adult men, together with the descent of the diaphragm, and the pushing forward of the front wall of the abdomen, the lower part of the chest and the sternum are subject to a wide movement in inspiration. In women, the movement appears less extensive in the lower, and more so in the upper, part of the chest; a mode of breath- ing to which a greater mobility of the first rib is adapted, and which may have for its object the provision of sufficient space for respiration when the lower part of the chest is encroached upon by the pregnant uterus. MM. Beau and Maissiat call the former the inferior costal, and the latter the superior costal, type of respiration; but the annexed diagrams will explain the difference better than the names will, for these imply a 164 RESPIRATION. greater diversity than naturally exists in the modes of in- spiration. From the enlargement produced in inspiration, the chest and lungs return in ordinary tranquil expiration, by their elas- ticity ; the force employed by the inspiratory muscles in dis- Fig. 63. Fig. 64. Fig. 63 (after Hutchinson).—The changes of the thoracic and abdominal walls of the male during respiration. The back is supposed to be fixed in order to throw for- ward the respiratory movement as much as possible. The outer black continuous line in front represents the ordinary breathing movement; the anterior 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 ster- num advances while the abdomen recedes. Fig. 64 (after Hutchinson).—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. tending the 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 rib- cartilages, but also of the lungs themselves, in consequence of the elastic tissue which they contain in considerable quantity, is sufficient, in ordinary quiet breathing, to expel air from the chest in the intervals of inspiration, and no muscular power RESPIRATORY RHYTHM. 165 is required. In all voluntary expiratory efforts, however, as in speaking, singing, blowing, and the like, and in many in- voluntary actions also, as sneezing, coughing, &c., something more than merely passive elastic power is of course necessary, and the proper expiratory muscles are brought into action. By far the chief of these are the abdominal muscles, which, by 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, however, which depress the ribs, must act also as muscles of expiration, and therefore we must conclude that the abdominal muscles are assisted in their ac- tion by the greater part of the internal intercostals, the trian- gularis sterni, the serratus posticus inferior, &e. When by the efforts of the expiratory muscles, the chest has been squeezed to less than its average diameter, it again, on relaxa- tion of the muscles, returns to the normal dimensions by virtue of its elasticity. The construction of the chest-walls, there- fore, admirably adapts them for recoiling against and resisting as well undue contraction as undue dilatation. As before mentioned, the lungs, after distension in the act of inspiration, contract by virtue of the elastic tissue which is present in the bronchial tubes, on and between the air-cells, and in the investing pleura. But in the natural condition of the parts, they 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 able to act only on their interior, 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, and can scarcely be said to be separated from each other by an inter- vening pause. 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 166 RESPIRATION. and the beginning of the next inspiration. The respiratory rhythm may be thus expressed: Inspiration,. . . . . . (1 Expiration, . . . . . . 7 or 8 A very slight pause. During the action of the muscles which directly draw air into the chest, those which guard the opening through which it enters are not passive. In hurried breathing the instinctive dilatation of the nostrils is well seen, although under ordinary conditions it may not be noticeable. The opening at the upper part of the larynx, however, or rima glottidis (Fig. 65), is di- lated at each inspiration, for the more ready passage of air, and collapses somewhat at each expiration, its condition, there- fore, corresponding during respiration with that of the walls of the chest. There is a further likeness between the two acts in that, under ordinary circumstances, the dilatation of the rima glottidis is a muscular act, and its contraction chiefly an elastic recoil; although, under various conditions, to be here- after mentioned, there may be, in the contraction of the glottis, considerable muscular power exercised. Respiratory Movements of the Glottis. Quantity of A ir Respired. The quantity of air that is changed in the lungs in eacli act of ordinary tranquil breathing is variable, and is very difficult to estimate, because it is hardly possible to breathe naturally while, as in an experiment, one is attending to the process. Probably 30 to 35 cubic inches are a fair average in the case of healthy young and middle-aged men; but Bourgery is per- haps right in saying, that old people, even in health, habitu- ally breathe more deeply, and change in each respiration a larger quantity of air than younger persons do. The total quantity of air which passes into and out of the lungs of an adult, at rest, in 24 hours, has been estimated by Dr. E. Smith at about 686,000 cubic inches. This quantity, however, is largely increased by exertion; and the same ob- server has computed the average amount for a hard-working laborer in the same time, at 1,568,390 cubic inches. The quantity which is habitually and almost uniformly changed in each act of breathing, is called by Mr. Hutchinson breathing air. The quantity over and above this which a man can draw into the lungs in the deepest inspiration, he names complemental air: its amount is various, as will be presently QUANTITY OF AIR RESPIRED. 167 shown. After ordinary expiration, such as that which expels the breathing air, a certain quantity of air remains in the lungs, which maybe expelled by a forcible and deeper expiration: this he terms reserve air. But, even after the most violent ex- piratory effort, the lungs are not completely emptied; a certain quantity always remains in them, over which there is no vol- untary control, and which may be called residual air. Its amount depends in great measure on the absolute size of the chest, and has been variously estimated at from forty to two hundred and sixty cubic inches. 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. Mr. Hutchinson names this the vital capacity: it expresses the power which a person has of breathing in the emergencies of active exercise, violence, and disease; and in healthy men it varies according to stature, weight, and age. It is found by Mr. Hutchinson, from whom most of our in- formation on this subject is derived, that at a temperature of 60° F., 225 cubic inches is the average vital capacity of a healthy person, five feet seven inches in height. For every inch of height above this standard the capacity is increased, on an average, by eight cubic inches; and for every inch be- low, it is diminished by the same amount. This relation of capacity to height is quite independent of the absolute capacity of the cavity of the chest; for the cubic contents of the chest do not always, or even generally, increase with the stature of the body; and a person of small absolute capacity of chest may have a large capacity of respiration, and vice versa. The ca- pacity of respiration is determined only by the mobility of the walls of the chest; but why this mobility should increase in a definite ratio with the height of the body is yet unexplained, and must be difficult of solution, seeing that the height of the body is chiefly determined by that of the legs, and not by the height of the trunk or the depth of the chest. But the vast number of observations made by Mr. Hutchinson seem to leave no doubt of the fact as stated above. The influence of weight on the capacity of respiration is less manifest and considerable than that of height: and it is diffi- cult 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 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; so 168 RESPIRATIOX. that, for example, while a man of five feet six inches, and weighing less than 11$ stones, should be able to expire 217 cubic inches, one of the same height, weighing 121 stones, might expire only 203 cubic inches. 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. Mr. Hutchinson’s observations were made almost exclusively on men ; and his conclusions are, perhaps, true of them alone; for women, according to Bourgery, have only half the capacity of breathing that men of the same age have. The number of respirations in a healthy adult person usually ranges from fourteen to eighteen per minute. It is greater in infancy and childhood; and of course varies much according to different circumstances, such as exercise or rest, health or disease, &c. Variations in the number of res- pirations correspond ordinarily with similar variations in the pulsations of the heart. In health the proportion is about one to four, or one to five, and when the rapidity of the heart’s 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 than that of the respirations. According to Mr. Hutchinson, the force with which the in- spiratory 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 stat- ure increases; so that the average of men of six feet can elevate only about two and a half inches of mei’cury. 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 ex- erted 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. QUANTITY OF AIR RESPIRED. 169 The following table expresses the result of numerous exper- iments by Mr. Hutchinson on this subject, the instrument used to gauge the inspiratory and expiratory power being a hsema- dynamometer (see p. 138), to which was attached a tube fitting the nostrils, and through which the inspiratory or expiratory effort was made: Power of Inspiratory Muscles. 1.5 in. . . Weak, .... . Power of Expiratory Muscles. . . 2 0 in. 2 0“ . . Ordinary, . . 2.5 “ 2.5 “ . . Strong, . Very strong, . . . 3.5 “ 3.5 “ . . . . 4.5 “ 4.5 “ . . Remarkable, . . 5.8 “ 5.5 “ . . Very remarkable, . . 7.0 “ 6.0 “ . . Extraordinary, . . 8.5 “ 7.0 “ . . Very extraordinary, . . 10 0 “ Mr. Hutchinson remarks: “Suppose a man to lift by bis inspiratory muscles three inches of mercury, what muscular effort has he used ? The mere quantity of fluid lifted may be very inconsiderable (and as such I have found men wonder they could not elevate more), but not so the power exerted, when we recollect that hydrostatic law, which Mr. Bramah adopted to the construction of a very convenient press. To apply this law here, the diaphragm alone must act under such an effort, with a force equal to the weight of a column of mer- cury 3 inches in height, and whose base is commensurate to the area of the diaphragm. The area of the base of one of the chests now before the Society, is 57 square inches; there- fore, had this man raised 3 inches of mercury by his inspira- tory muscles, his diaphragm alone in this act must have op- posed a resistance equal to more than 23 ounces on every inch of that muscle, and a total weight of more than 83 pounds. Moreover, the sides of his chest would resist a pressure from the atmosphere equal to the weight of a covering of mercury three inches in thickness, or more than 23 ounces on every inch surface, which, if we take at 318 square inches, the chest will be found resisting a pressure of 731 pounds; and allowing the elastic resistance of the ribs as inch of mercury, this will bring the weight resisted by the chest, as follows: Diaphragm, . .... 83 lbs. Walls of the chest, ...... 731 “ Elastic force, ....... 232 “ Total, 1040 “ In round numbers it may be said, that the parietes of the 170 RESPIRATION. thorax resisted 1000 lbs. of atmospheric pressure, and that not counterbalanced,—to say nothing of the elastic power of the lungs, which co-operated with this pressure. “ I would not venture at present to state exactly the distri- bution of muscular fibre over the thorax, which is called into action when resisting this 1046 lbs., but I think I am safe in stating that nine-tenths of the thoracic surface conspire to this act. “ What is here said of the muscular part of the chest resist- ing such a force, must not be confounded with a former state- ment of ‘two-thirds being lifted by the inspiratory muscles, and one-third left dormant,’ under a force equal to 301 lbs. In this case the 301 lbs. are lifted; in the other, nine-tenths of 1046 lbs. are said to be resisted. “ The glass receiver of an air-pump may resist 15 lbs. on the square inch, yet it may be said to lift nothing. This question of the thoracic muscular force and resistance, and muscular distribution, is rendered complicate by the presence of so much osseous matter entering into the composition of the chest, which can scarcely be considered to act the same as muscle.” The great force of the inspiratory efforts during apncea was well shown in some of the experiments performed by the Medico- chirurgical Society’s Committee on Suspended Animation. On inserting a glass tube into the trachea of a dog, and immersing the other end of the tube in a vessel of mercury, the respiratory efforts during apnoea were so great as to draw the mercury four inches up the tube. The influence of the same force was shown in other experiments, in which the heads of animals were im- mersed both in mercury and in liquid plaster of paris. In both cases the material was found, after death, to have been drawn up into all the bronchial tubes, filling the tissue of the lungs. Much of the force exerted in inspiration is employed in over- coming the resistance offered by the elasticity of the walls of the chest and of the lungs. Mr. Hutchinson estimated the amount of this elastic resistance, 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 respiration during life; and he calculated that, in a man capable of breathing 200 cubic inches of air, the muscular power expended upon the elasticity of the walls of the chest, in making the deepest inspiration, would be equal to the rais- ing of at least 301 pounds avoirdupois. To this must be added about 150 lbs. for the elastic resistance of the lungs themselves, so 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. In tranquil respiration, supposing the amount of breathing VITAL CAPACITY. 171 air to be twenty cubic inches, the resistance of the walls of the chest would be equal to lifting more than 100 pounds; and to this must be added about 70 pounds for the elasticity of the lungs. The elastic force overcome in ordinary inspiration must, therefore, be equal to about 170 pounds. It is probable, that in the quiet ordinary respiration, which is performed without consciousness or effort of the will, the only forces engaged are those of the inspiratory muscles, and the elasticity of the walls of the chest and the lungs. It is not known under what circumstances the contractile power which the bronchial tubes possess, by means of their organic muscular fibres, is brought into action. It is possible, as Dr. R. Hall maintained, that it may exist in expiration; but it is more likely that its chief purpose is 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. Another purpose probably served by the muscular fibres of the bronchial tubes is that of contracting upon and gradually ex- pelling collections of mucus, which may have accumulated within the tubes, and cannot be ejected by forced expiratory efforts, owing to collapse or other morbid conditions of the por- tion of lung proceeding from the obstructed tubes (Gairdner). The muscular action in the lungs, morbidly excited, is prob- ably the chief cause of the phenomena of spasmodic asthma. It may be demonstrated by galvanizing the lungs shortly after taking them from the body. Under such a stimulus, they contract so as to lift up water placed in a tube introduced into the trachea (C. J. B. Williams); and Volkmann has shown that they may be made to contract by stimulating their nerves. He tied a glass tube, drawn fine at one end, into the trachea of a beheaded animal; and when the small end was turned to the flame of a candle, he galvanized the pneumo- gastric trunk. Each time he did so the flame was blown, and once it was blown out. The changes of the air in the lungs effected by these respi- ratory movements are assisted by the various conditions of the air itself. According to the law observed in the diffusion of gases, the carbonic acid evolved in the air-cells will, inde- pendently of any respiratory movement, tend to leave the lungs, by diffusing itself into the external air, where it exists in less proportion ; and according to the same law, the oxygen of the atmospheric air will tend of itself towards the air-cells in which its proportion is less than it is in the air in the bron- chial tubes or in that external to the body. But for this ten- dency in the oxygen and carbonic acid to mix uniformly, within and without the lungs, the reserve and residual air 172 RESPIRATION. would, probably, be very injuriously charged with carbonic acid; for the respiratory movements alone are not enough to empty the air-cells, and perhaps expel only the air which lies in the larger bronchial tubes. Probably also the change is assisted by the different temperature of the air within and without the lungs ; and by the action of the cilia on the mu- cous membrane of the bronchial tubes, the continual vibra- tions of which may serve to prevent the adhesion of the air to the moist surface of the membrane. Movement of Blood in the Respiratory Organs. To be exposed 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 through the pulmonary cap- illaries 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 air-cells intervening. The pulmonary circulation is of the simplest kind : for the pulmonary artery branches regularly; its suc- cessive 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 junction, 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 pres- sure, 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. The blood which is conveyed to the lungs by the pulmonary arteries is distributed to these organs to be purified and made fit for the nutrition of all other parts of the body. The capil- laries of the pulmonary vessels are arranged solely with refer- ence to this object, and therefore can have but little to do with the nutrition of the lungs; or, at least, only of those portions of the lungs with which they are in intimate connection for another purpose. For the nutrition of the rest of the lungs, including the pleura, interlobular tissue, bronchial tubes and glands, and the walls of the larger bloodvessels, a special supply of arterial blood is furnished through one or two bron- chial arteries, the branches of which ramify in all these parts. The blood of the bronchial artery, when, having served for the nutrition of these parts, it has become venous, is carried CHANGES OF AIR IN RESPIRATION. 173 partly into the branches of the bronchial vein, and thence to the right auricle, and partly into the small branches of the pulmonary artery, or, more directly, into the pulmonary capil- laries, whence, being with the rest of the blood arterialized, it is carried to the pulmonary veins and left side of the heart. Changes of the Air in Respiration. By their contact in the lungs the composition of both air and blood is changed. The alterations of the former being manifest, simpler than those of the latter, and in some degree illustrative of them, may be considered first. 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, &c. Of every 100 vol- umes of pure atmospheric air, 79 volumes (on an average) consist of nitrogen, the remaining 21 of oxygen. The propor- tion of carbonic acid is extremely small; 10,000 volumes of atmosperic air contain only about 4 or 5 of carbonic acid. The quantity of watery vapor varies greatly, according to the temperature and other circumstances, but the atmosphere is never without some. In this country, the average quantity of watery vapor in the atmosphere is 1.40 per cent. The changes produced by respiration on the atmosjDheric air are, that, 1, it is warmed ; 2, its carbonic acid is increased; 3, its oxygen is diminished; 4, its watery vapory is increased ; 5, a minute amount of organic matter and of free ammonia is added to it. 1. The expired air, heated by its contact with the interior of the lungs, is (at least in most climates) hotter than the in- spired air. Its temperature varies between 97° and the lower temperature being observed when the air has remained but a short time in the lungs, rather than when it is inhaled at a very low temperature ; for whatever the temperature when inhaled may be, the air nearly acquires that of the blood be- fore 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 circumstances. From every volume'of air inspired, about 4? per cent, of oxygen are abstracted ; while a rather smaller quantity of carbonic acid is added in its place. It may be stated, as a general average deduced from the results of experiments by Valentin and Brunner, that, under ordi- nary circumstances, the quantity of carbonic acid exhaled 174 RES PI EAT I ON. into the air breathed by a healthy adult man amounts to 1346 cubic inches, or about 636 grains per hour. According to this estimate, which corresponds very closely with the one furnished by Sir H. Davy, and does not widely differ from those obtained by Allen and Pepys, Lavoisier, and Dr. Ed. Smith, the weight of carbon excreted from the lungs is about 173 grains per hour, or rather more than 8 ounces in the course of twenty-four hours. Discrepancies in the results obtained bjr different experimenters may be due to the variations to which the exhalation of carbonic acid is liable in different circumstances ; for even in health the quantity varies accord- ing to age, sex, diversities in the respiratory movements, ex- ternal temperature, the degree of purity of the respired air, and other circumstances. Each of these deserves a brief notice, because it affords evidence concerning either the sources of carbonic acid exhaled, or the mode in which it is separated from the blood. a. Influence of Age and Sex.—According to Andral and Gavarret the quantity of carbonic acid exhaled into the air breathed by males, regularly increases from eight to thirty years of age; from thirty to forty it is stationary or diminishes a little; from forty to fifty the diminution is greater; and from fifty to extreme age it goes on diminishing, 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 quan- tity abruptly ceases to increase, and remains stationary so long as they continue to menstruate. When, however, men- struation has ceased, either in advancing years or in preg- nancy or morbid amenorrhoea, the exhalation of carbonic acid again augments; but when menstruation ceases naturally, it soon decreases again at the same rate that it does in old men. b. Influence of Respiratory Movements.—According to Vier- ordt, the more quickly the movements of respiration are per- formed, the smaller is the proportionate quantity of carbonic acid contained in each volume of the expired air. Thus he found that, with six respirations per minute, the quantity of expired carbonic acid was 5.528 per cent.; with twelve respi- rations, 4.262 per cent.; with twenty-four, 3.355; with forty- eight, 2.984 ; and with ninety-six, 2.662. 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 air which is breathed in the time. This is the case, whether the respiration be voluntarily accel- TEMPERATURE AND SEASON. 175 erated, or naturally increased in frequency, as it is after feed- ing, active exercise, &c. By diminishing the frequency, and increasing the depth of respiration, the percentage proportion of carbonic acid in the expired air is diminished ; being in the deepest respiration as much as 1.97 per cent, less than in ordi- nary breathing. But for this proportionate diminution also, there is a full compensation in the greater total volume of air which is thus breathed. Finally, the last half of a volume of expired air contains more carbonic acid than the half first expired; a circumstance which is explained by the one por- tion of air coming from the remote part of the lungs, where it has been in more immediate and prolonged contact with the blood than the other has, which comes chiefly from the larger bronchial tubes. c. Influence of External Temperature. — The observations made by Vierordt at various temperatures between 38° F. and 75° F. 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. Letellier, from experiments performed on animals at much higher and lower temperatures than the above, also found that the higher the temperature of the respired air (as far as 104° F.), the less is the amount of carbonic acid exhaled into it, whilst the nearer it approaches zero the more does the carbonic acid increase. The greatest quantity exhaled at the lower temperatures he found to be about twice as much as the smallest exhaled at the higher temperatures. d. Season of the Year.—Dr. Edward Smith has shown that the season of the year, independently of temperature, also ma- terially influences the respiratory phenomena; for with the same temperature, at different seasons, there is a great diversity in the amount of carbonic acid expired. According to his observations, spring is the season of the greatest, and autumn of the least activity of the respiratory and other functions. e. Purity of the Respired Air.—The average quantity of car- bonic acid given out by the lungs constitutes about 4.48 per cent, of the expired air; but if the air which is breathed be previously impregnated with carbonic acid (as is the case when the same air is frequently respired), then the quantity of car- bonic acid exhaled becomes much less. This is shown by the results of two experiments performed by Allen and Pepys. In one, in which fresh air was taken in at each respiration, thirty- two cubic inches of carbonic acid were exhaled in a minute; whilst in the other, in which the same air was respired repeat- edly, the quantity of carbonic acid emitted in the same time was only 9.5 cubic inches. They found also that, however 176 RESPIRATION. often the same air may be respired, even if until it will no longer sustain life, it does not become charged with more than ten per cent, of carbonic acid. The necessity of a constant supply of fresh air, by means of ventilation, through rooms in which many persons are breathing together, or in which, from any other source, much carbonic acid is evolved, is thus rendered obvious; for even when the air is not completely irrespirable, yet in the same proportion as it is already charged with car- bonic acid, does the further extrication of that gas from the lungs suffer hindrance. f. Hygrometric State of Atmosphere.—Lehmann’s observations have shown that the amount of carbonic acid exhaled is con- siderably influenced by the degree of moisture of the atmo- sphere, much more being given off when the air is moist than when it is dry. g. Period of the Day.—The period of day seems to exercise a slight influence on the amount of carbonic acid exhaled in a given time, though beyond the fact that the quantity exhaled is much less by night, we are scarcely yet in a position to state that variations in the amount exhaled occur at uniform periods of .the day, independently of the influence of other circum- stances. h. Food.—By the use of food the quantity is increased, whilst by fasting it is diminished: and, according to Regnault and Reiset, it is greater when animals are fed on farinaceous food than when fed on meat. Spirituous drinks, especially when taken on an empty stomach, are generally believed to pro- duce an immediate and marked diminution in the quantity of this gas exhaled. Recent observations by Dr. Edward Smith, however, furnish some singular results on this subject. Dr. Smith found, for example, that the effects produced by spirituous drinks depend much on the kind of drink taken. Pure alcohol tended rather to increase than to lessen respira- tory changes, and the amount, therefore, of carbonic acid ex- pired : rum, ale, and porter, also sherry, had very similar effects. On the other hand, brandy, whisky, and gin, particularly the latter, almost always lessened the respiratory changes, and consequently the amount of carbonic acid exhaled. i. Exercise and Sleep.—Bodily exercise, in moderation, in- creases 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 tread- wheel, still further increases, according to Dr. E. Smith, the amount of the acid exhaled. EFFEC'I'S OF EXERCISE AND SLEEP. 177 During sleep, on the other hand, there is a considerable diminution in the quantity of this gas evolved ; a result, probably, in great measure dependent on the tranquillity of breathing; directly after walking, there is a great, though quickly transitory, increase in the amount exhaled. A larger quantity is exhaled when the barometer is low than when it is high. 3. The Oxygen in Respired Air is always less than in the same air before respiration, and its diminution is generally proportionate to the increase of the carbonic acid. The ex- periments of Valentin and Brunner seem to show that for every volume of carbonic acid exhaled into the air, 1.17421 volumes of oxygen are absorbed from it; and that when the average quantity of carbonic acid, i. e., 1346 cubic inches, or 636 grains, is exhaled in the hour, the quantity of oxygen ab- sorbed 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 without change of volume; and to this general conclusion, namely, that the volume of air expired in a given time is less than that of the air inspired (allowance being made for the expan- sion 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 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 combining with sulphur and phosphorus to form part of the acids of the sulphates and phosphates excreted in the urine, and probably also, from the experiments of Dr. Bence Jones, with the nitrogen of the de- composing nitrogenous tissues. The quantity of oxygen consumed seems to vary much, not only in different individuals, but in the same individual at different periods; thus it is considerably influenced by food, being greater in dogs when fed on farinaceous than on animal food, and much diminished during fasting, while it varies at different stages of digestion. Animals of small size consume a relatively much greater amount of oxygen than larger ones. The quantity of oxygen in the atmosphere surrounding animals appears 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 (Regnault and Beiset). The Nitrogen of the Atmosphere, in relation to the respira- 178 RESPIRATION. tory process, is supposed to serve only mechanically, by dilut- ing the oxygen, and moderating its action upon the system. This purpose, or the mode of expressing it, has been denied by Liebig, on the ground that if we suppose the nitrogen re- moved, the amount of oxygen in a given space would not be altered. But, although it be true that if all the nitrogen of the atmosphere were removed and not replaced by any other gas, the oxygen might still extend over the whole space at present occupied by the mixture of which the atmosphere is composed; yet since, under ordinary circumstances, oxygen and nitrogen, when mixed together in the ratio of one volume to four, produce a mixture which occupies precisely five vol- umes, with all the properties of atmospheric air, it must result that a given volume of atmosphere drawn into the lungs con- tains four-fifths less weight of oxygen than an equal volume composed entirely of oxygen. The greater rapidity and bril- liancy with which combustion goes on in an atmosphere of oxygen than in one of common air, and the increased rapidity with which the ordinary effects of respiration are produced when oxygen instead of atmospheric air is breathed, seem to leave no doubt that the nitrogen with which the oxygen of the atmosphere is mixed has the effect of diluting this gas, in the same sense and degree as one part of alcohol is diluted when mixed with four parts of water. It has been often discussed whether nitrogen is ever ab- sorbed 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 absorbed. 4. Watery Vapor is, under ordinary circumstances, always exhaled from the lungs in breathing. The quantity emitted is, as a general rule, sufficient to saturate the expired air, or very nearly so. Its absolute amount is, therefore, influenced by the following circumstances. First, by the quantity of air respired ; for the greater this is, the greater also will be the quantity of moisture exhaled. Secondly, 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 saturation of the air. Thirdly, by the temperature of the ex- pired air; for the higher this is, the greater will be the quantity of watery vapor required to saturate the air. Fourthly, by the length of time which each volume of inspired air is allowed to remain in the lungs ; for it seems probable that, although during ordinary respiration the expired air is always saturated CHANGES IN BLOOD. 179 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. For ordinary cases, however, it may be held that the ex- pired air is saturated with watery vapor, and hence is derivable a means of estimating the quantity exhaled in any given time : namely, by subtracting the quantity contained in the air in- spired from the quantity which (at the barometric pressure) would saturate the same air at the temperature of expiration, which is ordinarily about 99°. And, on the other hand, if the quantity of watery vapor in the expired air be estimated, the quantity of air itself may from it be determined, being as much as that quantity of watery vapor would saturate at the ascer- tained temperature and barometric pressure. The quantity of water exhaled from the lungs in twenty- four hours ranges (according to the various modifying circum- stances already mentioned) from about 6 to 27 ounces, the or- dinary quantity being about 9 or 10 ounces. Some of this is probably formed by the combination of the excess of oxygen absorbed in the lungs with the hydrogen of the blood ; but the far larger proportion of it must be the mere exhalation of the water of the blood, taking jfiace from the surfaces of the air- passages and cells, as it does from the free surfaces of all moist animal membranes, particularly at the high temperature of warm-blooded animals. It is exhaled from the lungs what- ever be the gas respired, continuing to be expelled even in hydrogen gas. 5. The Rev. J. B. Reade showed, some years ago, and Dr. Richardson’s experiments confirm the fact, that ammonia is among the ordinary constituents of expired air. It seems probable, however, both from the fact that this substance can- not 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. The quantity of organic matter in the breath has been lately investigated by Dr. Ransome, who calculates that about 3 grains are given off from the lungs of an adult in twenty-four hours. Changes produced in the Blood hy Respiration. The most obvious change which the blood undergoes in its passage through the lungs is that of color, the dark crimson of venous blood beiug exchanged for the bright scarlet of arterial blood. (The circumstances which have been supposed to give 180 RESPIRATION. rise to this change, the conditions capable of effecting it inde- pendent of respiration, and some other differences between arterial and venous blood, were discussed in the chapter on Blood, p. 77): 2d, and in connection with the preceding change, it gains oxygen; 3d, it loses carbonic acid; 4th, it be- comes 1° or 2° F. warmer; 5th, it coagulates sooner and more firmly, and, apparently, contains more fibrin. The oxygen absorbed into the blood from the atmospheric air in the lungs is combined chemically with the haemoglobin of the red blood-corpuscles. In this condition it is carried in the arterial blood to the various parts of the body, and with it is, in the capillary system of vessels, brought into near relation or contact with the elementary parts of the tissues. Herein, co-operating probably in the process of nutrition, or in the re- moval of disintegrated parts of the tissues, a certain portion of the oxygen which the arterial blood contains disappears, and a proportionate quantity of carbonic acid and water is formed. But it is not alone in the disintegrating processes to which all parts of the body are liable, that oxygen is consumed and carbonic acid and water are formed in its consumption. A like process occurs in the blood itself, independently of the decay of the tissues; for on the continuance of such chemical processes depend, directly or indirectly, not only the tempera- ture of the body, but all the forces, the nervous, the muscular, and others, manifested by the living organism. 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 again 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 per- formed is clearly understood. The accompanying diagram (Fig. 65) shows that the cavity of the chest is separated from that of the abdonjen by the diaphragm, which, when acting, will lessen its curve, and thus descending, will push downwards and forwards the abdominal viscera; while the abdominal muscles have the opposite effect, and in acting will push the viscera upwards and backward, and with them the diaphragm, supposing its ascent to be not from any cause interfered with. From the same diagram it will be seen that the lungs commu- nicate with the exterior of the body through the glottis, and further on through the mouth and nostrils—through either of them separately, or through both at the same time, according MECHANISM OF RESPIRATORY ACTIONS. 181 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 Fxo. 65. act independently of each other. The position of the latter is indicated in the diagram. Let us take first the simple act of sighing. In this case there is a rather prolonged inspiratory effort by the diaphragm and other muscles concerned in 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. 182 RESPIRATION. Now, in the first, or inspiratory part of this act, the descent of the diaphragm presses the abdominal viscera downwards, 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, 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 direc- tion, 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, rec- tum, 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 suddenly and spasmodically; and the air, therefore, suddenly rushing through the unprepared rima glottidis, causes vibration of the vocal cords, and the peculiar sound. In the act of coughing, there is most often first an inspira- tion, 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 iu 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 out-streaming current of air. Now it is evident on reference to the diagram (Fig. 65), that pressure exercised by the abdominal muscles in the act of coughing, acts as forcibly 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 con- tents as of the air in the lungs. The instinctive and, if neces- sary, voluntarily increased contraction of the sphincters, how- ever, prevents any escape at the openings guarded by them, and the pressure is effective at one part only, namely, the rima glottidis. The same remarks that apply to coughing, are almost ex- actly applicable to the act of sneezing; but in this instance VOMITING PARTURITION. 183 the blast of air, on escaping from the lungs, is directed by an instinctive contraction of the pillars of the fauces and descent of the soft palate, chiefly through the nose, and any offending matter is thence expelled. In the act of vomiting, as in coughing, there is first an in- spiration ; the glottis is then closed, and immediately after- wards the abdominal muscles strongly act; but here occurs the difference 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. It is fixed, to use a technical phrase. At the same time the cardiac sphincter being relaxed while the pylorus is closed (see Fig. 65), 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, phar- ynx, and mouth. The reversed peristaltic action of the oesophagus probably increases the effect. In the act of voluntary expulsion of urine or feces, there is first an inspiration, as in coughing, sneezing, and vomiting; the glottis is then closed, and the diaphragm fixed as in vom- iting. Now, however, both the rima glottidis and the cardiac opening of the stomach remain closed, and the sphincter of the bladder or rectum, or of both, being relaxed, the evacu- ation of the contents of these viscera takes place accordingly; the effect being, of course, increased by the muscular and elastic contraction of their own walls. As before, there is as much tendency to the escape of the contents of the lungs or stomach as of the rectum or bladder; but the pressure is re- lieved only at the orifice, the sphincter of which instinctively or involuntarily yields. In all these expulsive actions the diaphragm is quite pas- sive ; and when it is fixed, it is in consequence of the closure of the glottis (which by preventing the exit of air from the lungs prevents its upward movement), not from any exertion on its own part. In females, during parturition, almost an exactly similar action occurs, so far as the diaphragm and abdominal walls are concerned, to that which takes place in a straining effort at expulsion of urine or feces. The contraction of the uterus, however, is both relatively and absolutely more powerful than that of the bladder or rectum, although it is greatly assisted by the inspiratory effort, by the fixing of the diaphragm, and by the action of the abdominal muscles, as in the other acts just described. In parturition, as in vomiting, the action of the abdominal muscles is, to a great extent, involuntary—more 184 RESPIRATION. so than it commonly is in the expulsion of faeces or urine; but in these latter instances also, in cases of great pain and diffi- culty, it may cease to be a voluntary act, and be quite beyond the control of the will. In speaking, there is a voluntary expulsion of air through the glottis by means of the abdominal muscles; and the vocal cords are put, by the muscles of the larynx, in a proper posi- tion and state of tension for vibrating as the air passes over them, and thus producing sound. The sound is moulded into words by the tongue, teeth, lips, &c.—the vocal cords produc- ing the sound only, and having nothing to do with articulation. Singing resembles speaking in the manner of its production; 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, &c. Sniffing is produced by a somewhat quick action of the dia- phragm and other inspiratory muscles. The mouth is, how- ever, closed, and by these means the whole stream of air is made to enter by the nostrils. The alse nasi are, commonly, at the same time, instinctively dilated. 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 downwards and backwards the tongue and floor of the mouth, produce a partial vacuum in the latter; and the weight of the atmosphere then acting on all sides tends to pro- duce equilibrium on the inside and outside of the mouth as best it may. The communication between the mouth and pharynx is completely shut off, probably by the contraction of the pillars of the soft palate and descent of the latter so as to touch the back of the tongue ; and the equilibrium, therefore, can be restored only by the entrance of something through the mouth. The action, indeed, of the tongue and floor of the mouth in sucking may be compared to that of the piston in a syringe, and the muscles which pull down the os hyoides and tongue, to the power which draws the handle. In the preceding account of respiratory actions, the dia- phragm and abdominal muscles have been, as the chief muscles engaged, and for the sake of clearness, almost alone referred to. But, of course, in all inspiratory actions, the other muscles of inspiration (p. 162) are also more or less engaged; and in expiration, the abdominal muscles are assisted by others, pre- viously enumerated (p. 165) as grouped in action with them. INFLUENCE OF NERVOUS SYSTEM. 185 Like all other functions of the body, the discharge of which is necessary to life, respiration must be essentially an involun- tary act. Else, life would be in constant danger, and would cease on the loss of consciousness for a few moments, even 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 be impossible to perform those voluntary respira- tory acts which have been just enumerated and explained, as speaking, singing, straining, and the like. The respiratory movements and their regular rhythm, so far as they are involuntary and independent of consciousness (as on all ordinary occasions they are), seem to be under the ab- solute governance of the medulla oblongata, which, as a ner- vous centre, receives the impression of the “ necessity of breathing,” and reflects it to the phrenic and such other motor nerves as will bring into co-ordinate and adapted action the muscles necessary to inspiration. In the cases of voluntary respiratory acts, we may believe that the brain, as well as the medulla oblongata, is engaged in the process; for we have no evidence of the mind exercising either perception or will through any other organ than the brain. But even when the brain is thus in action, it appears to be the medulla oblongata which combines the several re- spiratory muscles to act together. In such acts, for example, as those of coughing and sneezing, the mind first perceives the irritation at the larynx or nose, and may exercise a certain degree of will in determining the actions, as e.g., in the taking of the deep inspiration which always precedes them. But the mode in which the acts are performed, and the combination of muscles to effect them, are determined by the medulla oblon- gata, independently of the will, and have the peculiar char- acter of reflex involuntary movements, in being always, and without practice or experience, precisely adapted to the end or purpose. In these, and in all the other extraordinary respiratory actions, such as are seen in dyspnoea, or in straining, yawning, hiccough, and others, the medulla oblongata brings into adapted combination of action many other muscles besides those commonly exerted in respiration. Almost all the muscles of the body, in violent efforts of dyspnoea, coughing, and the like, may be brought into action at once, or in quick succes- sion ; but more particularly the muscles of the larynx, face, scapula, spine, and abdomen, co-operate in these efforts with the muscles of the chest. These, therefore, are often classed Influence of the Nervous System in Respiration. 186 RESPIRATION. as secondary muscles of respiration ; and the nerves supplying them, including especially the facial, pneumogastric, spinal accessory, and external respiratory nerves, were classed by Sir Charles Bell with the phrenic, as the respiratory system of nerves. There appears, however, no propriety in making a separate system of these nerves, since their mode of action is not peculiar, and many besides them co-operate in the respira- tory acts. That which is peculiar in the nervous influence, directing the extraordinary movements of respiration, is, that so many nerves are combined toward one purpose by the power of a distinct nervous centre, the medulla oblongata. In other than respiratory movements, these nerves may act singly or together, without the medulla oblongata; but after it is de- stroyed, no movement adapted to respiration can be performed by any of the muscles, even though the part of the spinal cord from which they arise be perfect. The phrenic nerves, for example, are unable to excite respiratory movement of the diaphragm when their connection with the medulla oblongata is cut off, though their connection with the spinal cord may be uninjured.1 Effects of the Suspension and Arrest of Respiration. These deserve some consideration, because of the illustra- tion which they afford of the nature of the normal processes of respiration and circulation. When the process of respira- tion is stopped, either by arresting the respiratory movements, or permitting them to continue in an atmosphere deprived of uncombined oxygen, the circulation of blood through the lungs is retarded, and at length stopped. The immediate effect of such retarded circulation is an obstruction to the exit of blood from the right ventricle: this is followed by delay in the re- turn of venous blood to the heart; and to this succeeds venous congestion of the nervous centres and all the other organs of the body. In such retardation, also, an unusually small supply of blood is transmitted through the lungs to the left side of the heart; and this small quantity is venous. The condition, then, in which a suffocated, or asphyxiated animal dies is, commonly, that the left side of the heart is nearly empty, while the lungs, right side of the heart, and other organs, are gorged with venous blood. To this condi- tion many things contribute. 1st. The obstructed passage of 1 The influence of the nervous system in respiration will be again and more particularly considered in the section treating of the me- dulla oblongata and pneumogastric nerves. SUSPENDED ANIMATION. 187 blood through the lungs, which appears to be the first of the events leading to suffocation, seems to depend on the cessation of the interchange of gases, as if blood charged with carbonic acid could not pass freely through the pulmonary capillaries. But the stagnation of blood in the pulmonary capillaries would not, perhaps, be enough to stop entirely the circulation, unless the action of the heart were also weakened. There- fore, 2dly, the fatal result is probably due, in some measure, to the enfeebled action of the right side of the heart, in conse- quence of its overdistension by blood continually flowing into it; this flow, probably, being much increased by the powerful but fruitless efforts continually made at inspiration (Eccles). And 3dly, because of the obstruction at the right side of the heart, there must be venous congestion in the medulla oblon- gata and nervous centres: and this evil is augmented by the left ventricle receiving and propelling none but venous blood. Hence, slowness and disorder of the respiratory movements and of the movements of the heart may be added. Under all these conditions combined, the heart at length ceases to act; the cessation of its action being also in gi'eat measure, proba- bly, brought about, 4thly, by the imperfect supply of oxyge- nated blood to its muscular tissue. In some experiments performed by a committee appointed by the Medico-Chirurgical Society to investigate the subject of Suspended Animation, it was found that, in the dog, during simple apnoea, i. e., simple privation of air, as by plugging the trachea, the average duration-of the respiratory movements after the animal had been deprived of air, was 4 minutes 5 seconds; the extremes being 3 minutes 30 seconds, and 4 minutes 40 seconds. The average duration of the heart’s ac- tion, on the other hand, was 7 minutes 11 seconds; the ex- tremes being 6 minutes 40 seconds, and 7 minutes 45 seconds. It would seem, therefore, that on an average, the heart’s action continues for 3 minutes 15 seconds after the animal has ceased to make respiratory efforts. A very similar relation was ob- served in the rabbit. Recovery never took place after the heart’s action had ceased. The results obtained by the committee on the subject of drowning were very remarkable, especially in this respect, that whereas an animal may recover, after simple deprivation of air for nearly four minutes, yet, after submersion iu water for II minutes, recovery seems to be impossible. This remark- able difference was found to be due, not to the mere submer- sion, nor directly to the struggles of the animal, nor to depres- sion of temperature, but to the two facts, that in drowning, a free passage is allowed to air out of the lungs, and a free en- 188 RESPIRATION. trance of water into them. In proof of the correctness of this explanation it was found that when two dogs of the same size, one, however, having his windpipe plugged, the other not, were submerged at the same moment, and taken out after being under water for 2 minutes, the former recovered on removal of the plug, the latter did not. It is probably to the entrance of water into the lungs that the speedy death in drowning is mainly due. The results of post-mortem examination strongly support this view. On examining the lungs of animals de- prived of air by plugging the trachea, they were found simply congested; but in the animals drowned, not only was the con- gestion much more intense, accompanied with ecchymosed points on the surface and in the substance of the lung, but the air-tubes were completely choked up with a sanious foam, con- sisting of blood, water, and mucus, churned up with the air in the lungs by the respiratory efforts of the animal. The lung- substance, too, appeared to be saturated and sodden with water, which, stained slightly with blood, poured out at any point where a section was made. The lung thus sodden with water was heavy (though it floated), doughy, pitted on pressure, and was incapable of collapsing. It is not difficult to understand how, by such infarction of the tubes, air is debarred from reach- ing the pulmonary cells: indeed the inability of the lungs to collapse on opening the chest is a proof of the obstruction which the froth occupying the air-tubes offers to the transit of air. The entire dependence of the early fatal issue, in apnoea by drowning, upon the open condition of the windpipe, and its results, was also strikingly shown by the following experi- ment. A strong dog had its windpipe plugged, and was then submerged in water for four minutes; in three-quarters of a minute after its release it began to breathe, and in four minutes had fully recovered. This experiment was repeated with sim- ilar results on other dogs. When the entrance of water into the lungs, and its drawing up with the air into the bronchial tubes by means of the respiratory efforts, were diminished, as by rendering the animal insensible by chloroform previously to immersion, and thus depriving it of the power of making violent respiratory efforts, it was found that it could bear im- mersion for a longer period without dying than when not thus rendered insensible. Probably to a like diminution in the respiratory efforts, may also be ascribed the greater length of time persons have been found to bear submersion without being killed, when in a state of intoxication, poisoning by nar- cotics, or during insensibility from syncope. It is to the accumulation of carbonic acid in the blood, and its conveyance into the organs, that we must, in the first place, ANIMAL HEAT. 189 ascribe the phenomena of asphyxia. For when this does not happen, all the other conditions may exist without injury; as they do, for example, in hibernating warm-blooded animals. In these, life is supported for many months in atmospheres in which the same animals, in their full activity, would be speedily suffocated. During the periods of complete torpor, their respi- ration almost entirely ceases; the heart acts very slowly and feebly; the processes of organic life are all but suspended, and the animal may be with impunity completely deprived of atmospheric air for a considerable period. Spallanzani kept a marmot, in this torpid state, immersed for four hours in car- bonic acid gas, without its suffering any apparent inconveni- ence. Dr. Marshall Hall kept a lethargic bat under water for 16 minutes, and a lethargic hedgehog for 22b minutes ; and neither of the animals appeared injured by the experiment. CHAPTER VIII. ANIMAL HEAT. The average temperature of the human body in those in- ternal parts which are more easily accessible, as the mouth and rectum, is from 98.5° to 99.5° F. In different parts of the external surface of the human body the temperature varies only to the extent of two or three de- grees, when all are alike protected from cooling influences ; and the difference which under these circumstances exists, depends chiefly upon the different degrees of blood-supply. In the arm-pit—the most convenient situation, under ordinary circumstances, for examination by the thermometer—the aver- age temperature is 98.63 F. The chief circumstances by which the temperature of the healthy body is influenced are the following: Age.—The average temperature of the new-born child is only about 1° F. above that proper to the adult; and the difference becomes still more trifling during infancy and early childhood. According to Wunderlich, the temperature falls to the extent of about T° to h° F. from early infancy to puberty, and by about the same amount from pubei’ty to fifty or sixty years of age. In old age the temperature again rises, and approaches that of infancy. Although the average temperature of the body, however, 190 ANIMAL HEAT. is not lower than that of younger persons, yet the power of resisting cold is less in them—exposure to a low temperature causing a greater reduction of heat than in young persons. The same rapid diminution of temperature was observed by M. Edwards in the new-born young of most carnivorous and rodent animals when they were removed from the parent, the temperature of the atmosphere being between 50° and 53 £° F.; whereas, while lying close to the body of the mother, their temperature was only 2 or 3 degrees lower than hers. The same law applies to the young of birds. Young sparrows, a week after they were hatched, had a temperature of 95° to 97°, while in the nest; but when taken from it, their temperature fell in one hour to 66£°, the temperature of the atmosphere being at the time 622°. It appears from his in- vestigations, that in respect of the power of generating heat, some Mammalia are born in a less developed condition than others ; and that the young of dogs, cats, and rabbits, for example, are inferior to the young of those animals which are not born blind. The need of external warmth to keep up the temperature of new-born children is well known ; the re- searches of M. Edwards show that the want of it is, as Hunter suggested, a much more frequent cause of death in new-born children than is generally supposed, and furnish a strong argu- ment against the idea, that children, by early exposure to cold, can soon be hardened into resisting its injurious influ- ence. Sex.—The average temperature of the female would appear from observations by Dr. Ogle to be very slightly higher than that of the male. Period of the Day.—The temperature undergoes a gradual alteration, to the extent of about 1° to F. in the course of the day and night; the minimum being at night or in the early morning, the maximum late in the afternoon. Exercise.—Active exercise raises the temperature of the body. This may be partly ascribed to the fact, that every muscular contraction is attended by the development of one or two de- grees of heat in the acting muscle; and that the heat is in- creased according to the number and rapidity of these con- tractions, and is quickly diffused by the blood circulating from the heated muscles. Possibly, also, some heat may be gene- rated in the various movements, stretchings, and recoilings of the other tissues, as the arteries, whose elastic walls, alternately dilated and contracted, may give out some heat, just as caout- chouc alternately stretched and recoiling becomes hot. But the heat thus developed cannot be great. Moreover, the increase of temperature throughout the whole TEMPERATURE OF THE BODY. 191 body, produced by active exercise, is but small; the great ap- parent increase of heat depending, in a great measure, on the increased circulation and quantity of blood, and, therefore, greater heat, in parts of the body (as the skin, and especially the skin of the extremities), which, at the same time that they feel more acutely than others any changes of temperature are, under ordinary conditions, by some degrees colder than organs more centrally situated. That the increased temperature of the skin during exercise is not accompanied by a proportional increase of the heat of other parts, which are naturally much warmer, is well shown by some observations of Dr. J. Davy. Climate and Season.—In passing from a temperate to a hot climate the temperature of the human body rises slightly, the increase rarely exceeding 2° to 3° F. In summer the temperature of the body is a little higher than in winter; the difference amounting to from 4° to F. (Wunderlich.) The same effects are observable in alterations of tempera- ture not depending on season or climate. Food and Drink.—The effect of a meal upon the tempera- ture of a body is but small. A very slight rise usually occurs. Cold alcoholic drinks depress the temperature somewhat to 1° F.). Warm alcoholic drinks, as well as warm tea and coffee, raise the temperature (about F.). In disease the temperature of the body deviates from the normal standard to a greater extent than would be anticipated from the slight effect of external conditions during health. Thus, in some diseases, as pneumonia and typhus, it occasion- ally rises as high as 106° or 107° F.; and considerably higher temperatures have been noted. In a case of malignant fever recently recorded by Mr. Norman Moore, the temperature in the axilla rapidly rose to 1110 F.; when the patient died. The highest temperature recorded in a living man, 112.5° F., was observed by Wunderlich, in a case of idiopathic tetanus, at the time of death. In the morbus cceruleus, in which there is defective arterialization of the blood from malformation of the heart, the temperature of the body may be as low as 79° or 77j° ; in Asiatic cholera a thermometer placed in the mouth sometimes rises only to 77° or 79°; and in a case of tubercular meniugitis, observed by Dr. Gee, the temperature of the rec- tum remained for hours at 79.4° F. The temperature maintained by Mammalia in an active state of life according to the tables of Tiedemann and Rudolphi, averages 101°. The extremes recorded by them were 96° and 106°, the former in the narwhal, the latter in a bat (Vesper- tilio pipistrella). In birds, the average is as high as 107°; 192 ANIMAL HEAT. the highest temperature, 111.25°, being in the small species, the linnets, &c. Among reptiles, Dr. John Davy found, that while the medium they were in was 75°, their average tem- perature was 82.5°. As a general rule, their temperature, though it falls with that of the surrounding medium, is, in temperate media, two or more degrees higher; and though it rises also with that of the medium, yet at very high degrees it ceases to do so, and remains even lower than that of the medium. Fish and Invertebrata present, as a general rule, the same temperature as the medium in which they live, whether that be high or low ; only among fish, the tunny tribe, with strong hearts and red meat-like muscles, and more blood than the average offish have, are generally 7° warmer than the water around them. The difference, therefore, between what are commonly called the warm- and the cold-blooded animals, is not one of abso- lutely higher or lower temperature; for the animals which to us, in a temperate climate feel cold (being like the air or water, colder than the surface of our bodies), would, in an ex- ternal temperature of 100°, have nearly the same temperature and feel hot to us. The real difference is, as Mr. Hunter ex- pressed it, that what we call warm-blooded animals (birds and Mammalia), have a certain “ permanent heat in all atmo- spheres,” while the temperature of the others, which we call cold-blooded, is “variable with every atmosphere.” The power of maintaining a uniform temperature, which Mammalia and birds possess, is combined with the want of power to endure such changes of temperature of their bodies as are harmless to the other classes; and when their power of resisting change of temperature ceases, they suffer serious dis- turbances or die. Sources and Mode of Production of Heat in the Body. In explaining the chemical changes effected in the process of respiration (p. 180), it was stated that the oxygen of the atmosphere taken into the blood is combined, in the course of the circulation, with the carbon and the hydrogen of disin- tegrated and absorbed tissues, and of certain elements of food which have not been converted into tissues. That such a com- bination between the oxygen of the atmosphere and the carbon and hydrogen in the blood, is continually taking place, is made certain by the fact, that a larger amount of carbon and hydrogen is constantly being added to the blood from the food than is required for the ordinary purposes of nutrition, and that a quantity of oxygen is also constantly being absorbed PRODUCTION OF HEAT. 193 from the air in the lungs, of the disposal of which no account can be given except by regarding it as combining, for the most part, with the excess of carbon and hydrogen, and being excreted in the form of carbonic acid and water. In other words, the blood of warm-blooded animals appears to be always receiving from the digestive canal and the lungs more carbon, hydrogen, and oxygen than are consumed in the repair of the tissues, and to be always emitting carbonic acid and water, for which there is no other known source than the combination of these elements.1 By such combination, heat is continually produced in the animal body. The same amount of heat will be evolved in the union of any given quantities of carbon and oxygen, and of hydrogen and oxygen, whether the combina- tion be rapid and evident, as in ordinary combustion, or slow and imperceptible, as in the changes which occur in the living body. And since the heat thus arising will be generated wher- ever the blood is carried, every part of the body will be heated equally, or nearly so. This theory, that the maintenance of the temperature of the living body depends on continual chemical change, chiefly by oxidation, of combustible materials existing in the tissues and in the blood, has long been established by the demonstration that the quantity of carbon and hydrogen which, in a given time, unites in the body with oxygen, is sufficient to account for the amount of heat generated in the animal within the same time; an amount capable of maintaining the temperature of the body at from 98° to 100°, notwithstanding a large loss by radiation and evaporation. Many things observed in the economy and habits of animals are explicable by this theory, and may here briefly be quoted, although no longer required as additional evidence for its truth. Thus, as a general rule, in the various classes of ani- mals, as well as in individual examples of each class, the quantity of heat generated in the body is in direct proportion to the activity of the respiratory process. The highest animal temperature, for example, is found in birds, in whom the func- tion of respiration is most actively performed. In Mammalia, the process of respiration is less active, and the average tem- perature of the body less, than in birds. In reptiles, both the respiration and the heat are at a much lower standard; while in animals below them, in which the function of respiration is at the lowest point, a power of producing heat is, in ordinary 1 Some heat will also be generated in the combination of sulphur and phosphorus with oxygen, to which reference has been made (p. 177) ; but the amount thus produced is but small. 194 ANIMAL HEAT. circumstances, hardly discernible. Among these lowrer ani- mals, however, the observations of Mr. Newport supply con- firmatory evidence. He shows that the larva, in which the respiratory organs are smaller in comparison with the size of the body, has a lower temperature than the perfect insect. Volant insects have the highest temperature, and they have always the largest respiratory organs and breathe the greatest quantity of air; while among terrestrial insects, those also produce the most heat which have the largest respiratory or- gans and breathe the most air. During sleep, hibernation, and other states of inaction, respiration is slower or suspended, and the temperature is proportionately diminished ; while, on the other hand, when the insect is most active and respiring most voluminously, its amount of temperature is at its maxi- mum, and corresponds with the quantity of respiration. Neither the rapidity of the circulation, nor the size of the nervous system, according to Mr. Newport, presents such, a constant relation to the evolution of heat. On the Regulation of the Temperature of the Human Body. The continual production of heat in the body has been already referred to. There is also, of necessity, a continual loss. But in healthy, warm-blooded animals, as already re- marked, the loss and gain of heat are so nearly balanced one by the other, that under all ordinary circumstances, a uni- form temperature, within two or three degrees, is preserved. The loss of heat from the human body takes place chiefly by radiation and conduction from its surface, and by means of' the constant evaporation of water from the same part, and from the air-passages. In each act of respiration, heat is also lost by so much warmth as the expired air acquires (p. 173). All food and drink which enter the body at a lower tempera- ture than itself, abstract a small measure of heat, and the urine and fseces take about a like amount away, when they leave the body. Lastly, some part of the heat of the body is rendered imperceptible, and therefore lost as heat, by being manifested in the form of mechanical motion. By far the most important loss of heat from the body,— probably 80 or 90 per cent, of the whole amount,—is that which proceeds from radiation, conduction, and evaporation from the skin. And it is to this part especially, and in a smaller measure to the air-passages, that we must look for the means by which the temperature is regulated; in other words, by which it is prevented from rising beyond the normal point on the one hand, or sinking below it on the other. The chief REGULATION OF THE TEMPERATURE. 195 indirect means for accomplishing the same end are, variations in the amount and quality of the food and drink taken, varia- tions in clothing, and in exposure to external heat or cold. In order to understand the means by which the heat of the body is regulated, it is necessary to take into consideration the following facts: First, the immediate source of heat in the body is the presence of a large quantity of a warm fluid—the blood, the temperature of which is, in health, about 100° F. In the second place, the blood, while constantly moving in a multitude of different streams, is, every minute or so, gathered up in the heart into one large stream, before being again dis- persed to all parts of the body. In this way, the temperature of the blood remains almost exactly the same in all parts ; for while a portion of it in passing through one organ, as the skin, may become cooler, and through another organ, as the liver, may become warmer, the effect on each separate stream is more or less neutralized when it mingles with another, and an aver- age is struck, so to speak, for all the streams when they form one, in passing through the heart. The means, by which the skin is able to act as one of the most important organs for regulating the temperature of the blood, are—(1) that it offers a large surface for radiation, con- duction, and evaporation; (2) that it contains a large amount of blood ; (3) that the quantity of blood contained in it is the greater under those circumstances which demand a loss of heat from the body, and vice versa. For the circumstance which directly determines the quantity of blood in the skin, is that which governs the supply of blood to all the tissues and organs of the body, namely, the power of the vaso-motor nerves to cause a greater or less tension of the muscular element in the walls of the arteries (see p. 121), and, in correspondence with this, a lessening or increase of the calibre of the vessel accom- panied by a less or greater current of blood. A warm or hot atmosphere so acts on the nerve-fibres of the skin, as to lead them to cause in turn a relaxation of the muscular fibre of the bloodvessels; and, as a result, the skin becomes full-blooded, hot, and sweating; and much heat is lost. With a low tem- perature, ou the other hand, the bloodvessels shrink, and in accordance with the consequently diminished blood supply, the skin becomes pale, and cold, and dry. Thus, by means of a self-regulating apparatus, the skin becomes the most important of the means by which the temperature of the body is regu- lated. In connection with loss of heat by the skin, reference has been made to that which occurs both by radiation and conduc- tion, and by evaporation; and the subject of animal heat has 196 A NIMAL HEAT. been considered almost solely with regard to the ordinary case of man living in a medium colder than his body, and therefore losing heat in all the ways mentioned. The importance of the means, however, adopted, so to speak, by the skin for regulat- ing the temperature of the body, will depend on the conditions by which it is surrounded ; an inverse proportion existing in most cases between the loss by radiation and conduction on the one hand, and by evaporation on the other. Indeed, the small loss of heat by evaporation in cold climates may go far to com- pensate for the greater loss by radiation ; as, on the other hand, the great amount of fluid evaporated in hot air may remove nearly as much heat as is commonly lost by both radiation and evaporation in ordinary temperatures; and thus, it is possible, that the quantities of heat required for the maintenance of a uniform proper temperature in various climates and seasons are not so different as they, at first thought, seem. Many examples might be given of the power which the body possesses of resisting the effects of a high temperature, in virtue of evaporation from the skin. Sir Charles Blagden and others supported a temperature varying between 198° and 211° F., in dry air for several minutes; and in a subsequent experiment he remained eight minutes in a temperature of 260°. But such heats are not tolerable when the air is moist as well as hot, so as to prevent evaporation from the body. M. C. James states, that in the vapor baths of Nero he was almost suffocated in a tempera- ture of 112°, while in the caves of Testaccio, in which the air is dry, he was but little incommoded by a temperature of 176°. In the former, evaporation from the skin was impossible; in the latter, it was, probably, abundant, and the layer of vapor which would rise from all the surface of the body would, by its very slow conducting power, defend it for a time from the full action of the external heat. (The glandular apparatus, by which secretion of fluid from the skin is effected, will be considered in the section on the Skin.) The ways by which the skin may be rendered more efficient as a cooling-apparatus by exposure, by baths, and by other means, which man instinctively adopts for lowering his tem- perature when necessary, are too well known to need more than to be mentioned. As a means for lowering the temperature, the lungs and air- passages are very inferior to the skin; although, by giving heat to the air we breathe, they stand next to the skin in im- portance. As a regulating power, the inferiority is still more marked. The air which is expelled from the lungs leaves the REGULATION OF HEAT. 197 body at about the temperature of the, blood, and is always saturated with moisture. No inverse proportion, therefore, exists between the loss of heat by radiation aud conduction on the one hand, and by evaporation on the other. The colder the air, for example, the greater will be the loss in all ways. Neither is the quantity of blood which is exposed to the cool- ing influence of the air diminished or increased, so far as is known, in accordance with any need in relation to temperature. It is true that by varying the number and depth of the respi- rations, the quantity of heat given off by the lungs may be made, to some extent, to vary also. But the respiratory pas- sages, while they must be considered important means by which heat is lost, are altogether subordinate in the power of regu- lating the temperature, to the skin. It may seem to have been assumed, in the foregoing pages, that the only regulating apparatus for temperature required by the human body is one that shall, more or less, produce a cooling effect; and as if the amount of heat produced were always, therefore, in excess of that which is required. Such an assumption would be incorrect. We have the power of regu- lating the production of heat, as well as its loss. In food we have a means for elevating our temperature. It is the fuel, indeed, on which animal heat ultimately depends altogether. Thus, when more heat is wanted, we instinctively take more food, and take such kinds of it as are good for com- bustion; while everyday experience shows the different power of resisting cold possessed by the well-fed and by the starved. In northern regions, again, and in the colder seasons of more southern climes, the quantity of food consumed is (speaking very generally) greater than that consumed by the same men or animals in opposite conditions of climate and seasons. And the food which appears naturally adapted to the inhabitants of the coldest climates, such as the several fatty and oily sub- stances, abounds in carbon and hydrogen, and is fitted to com- bine with the large quantities of oxygen which, breathing cold dense air, they absorb from their lungs. In exercise, again, we have an important means of raising the temperature of our bodies (p. 190). The influence of external coverings for the body must not be unnoticed. In warm-blooded animals, they are always adapted, among other purposes, to the maintenance of uniform temper- ature; and man adapts for himself such as are, for the same purpose, fitted to the various climates to which he is exposed. By their means, and by his command over food and fire, he maintains his temperature on all accessible parts of the sur- face of the earth. 198 ANIMAL HEAT. The influence of the nervous system, in modifying the produc- tion of heat has been already referred to. The experiments and observations which best illustrate it are those showing, first, that when the supply of nervous influence to a part is cut off, the temperature of that part falls below its ordinary degree ; and, secondly, that when death is caused by severe in- jury to, or removal of, the nervous centres, the temperature of the body rapidly falls, even though artificial respiration be performed, the circulation maintained, and to all appearance the ordinary chemical changes of the body be completely ef- fected. It has been repeatedly noticed, that after division of the nerves of a limb, its temperature falls ; and this diminution of heat has been remarked still more plainly in limbs deprived of nervous influence by paralysis. For example, Mr. Earle found the temperature of the hand of a paralyzed arm to be 70°, while the hand of the sound side had a temperature of 92° F. On electrifying the paralyzed limb, the temperature rose to 77°. In another case, the temperature of the paralyzed finger was 56° F., while that of the unaffected hand was 62°. With equal certainty, though less definitely, the influence of the nervous system on the production of heat, is shown in the rapid and momentary increase of temperature, sometimes general, at other times quite local, which is observed in states of nervous excitement; in the general increase of warmth of the body, sometimes amounting to perspiration, which is ex- cited by passions of the mind ; in the sudden rush of heat to the face, which is not a mere sensation; and in the equally rapid diminution of temperature in the depressing passions. But none of these instances suffices to prove that heat is gen- erated by mere nervous action, independent of any chemical change ; all are explicable, on the supposition that the nervous system alters, by its power of controlling the calibre of the bloodvessels (p. 121), the quantity of blood supplied to apart; while any influence which the nervous system may have in the production of heat, apart from this influence on the blood- vessels, is an indirect one, and is derived from its power of causing nutritive change in the tissues, which may, by involv- ing the necessity of chemical action, involve the production of heat. The existence of nerves, which regulate animal heat otherwise than by their influence in trophic (nutritive) or vaso-motor changes, although by many considered probable, is not yet proven. In connection with the regulation of animal temperature, and its maintenance in health at the normal height, it is in- teresting to note the result of circumstances too powerful, either in raising or lowering the heat of the body, to be controlled by FOOD, 199 the proper regulating apparatus. Walther found that rabbits and dogs, when tied to a board and exposed to a hot sun, reached a temperature of 114.8° F., and then died. Cases of sunstroke furnish us with similar examples in the case of man; for it wTould seem that here death ensues chiefly or solely from elevation of the temperature. In a case related by Dr. Gee, the temperature in the axilla was 109.5° F.; and in many febrile diseases the immediate cause of death appears to be the elevation of the temperature to a point inconsistent with the continuance of life. The effect of mere loss of bodily temperature in man is less well known than the effect of heat. From experiments by Walther, it appears that rabbits can be cooled down to 48° F. before they die, if artificial respira- tion be kept up. Cooled down to 64° F., they cannot recover unless external warmth be applied together with the employ- ment of artificial respiration. Rabbits not cooled below 77° F. recover by external warmth alone. CHAPTER IX. DIGESTION. Digestion is the process by which those parts of our food which may be employed in the formation and repair of the tissues, or in the production of heat, are made fit to be absorbed and added to the blood. Food may be considered in its relation to these two purposes, the nutrition of the tissues and the production of heat. But, under the first of these heads will he included many other allied functions, as, for example, secretion and generation: and under the second, not the production of heat only as such, but of all the other forces correlated with it, which are mani- fested by the living body. The following is a convenient tabular classification of the usual and more necessary kinds of food: Food. Nitrogenous: ProteicU, as Albumen, Casein, Syntonin, Gluten, and their allies, and Gelatin (containing Carbon, Hydrogen, Oxygen, and Nitrogen; some of them, also Sulphur and Phosphorus). 200 DIGESTION. 1STON-N ITROGENOUS : (1.) Amyloids—Starch, Sugar, and their allies (containing Carbon, Hydrogen, and Oxygen). (2.) Oils and Fats (containing Carbon, Hydrogen, and Oxygen; the Oxygen in much smaller proportion than in Starch or Sugar). (3 ) Mineral or Saline Matters, as Chloride of Sodium, Phosphate of Lime, &c. (4 ) Water. Animals cannot subsist on any but organic substances, and these must contain the several elements and compounds which are naturally combined with them: in other words, not even organic compounds are nutritive unless they are supplied in their natural state. Pure fibrin, pure gelatin, and other prin- ciples purified from the substances naturally mingled with them, are incapable of supporting life for more than a brief time. Moreover, health cannot be maintained by any number of substances derived exclusively from one only of the two chief groups of alimentary principles mentioned above. A mixture of nitrogenous and non-nitrogenous organic substances, together with the inorganic principles which are severally contained in them, is essential to the well-being, and, generally, even to the existence of an animal. The truth of this is demonstrated by experiments performed for the purpose, and is illustrated by the composition of the food prepared by nature, as the exclu- sive source of nourishment to the young of Mammalia, namely, milk. Human. Cows. Water, . . 890 858 Solids, . 110 142 1000 1000 Casein, . . 35 68 Butter, . . 25 38 Sugar (with extractives), . 48 30 Salts, . 2 6 110 142 Composition of Milk. In milk, as will be seen from the preceding table, the albu- minous group of aliments is represented by the casein, the oleaginous by the butter, the aqueous by the water, the sac- charine by the sugar of milk. Among the salts of milk are likewise phosphate of lime, alkaline, and other salts, and a trace of iron; so that it may be briefly said to include all the substances which the tissues of the growing animal need for COMPOSITION OF EGGS. 201 their nutrition, and which are required for the production of animal heat. The yolk and albumen of eggs are in the same relation as food for the embryos of oviparous animals, that milk is to the young of Mammalia, and afford another example of mixed food being provided as the most perfect nutrition. White. Yolk. Water, . 80.0 . 53.73 Albumen, . . 15.5 . 17.47 Mucus, 4.5 Yellow Oil, . 28.75 Salts, . . 4.0 6.0 Composition of Fowls’ Eggs. Experiments illustrating the same principle have been per- formed by Magendie and others. Dogs were fed exclusively on sugar and distilled water. During the first seven or eight days they were brisk and active, and took their food and drink as usual; but in the course of the second week, they began to get thin, although their appetite continued good, and they took daily between six and eight ounces of sugar. The emaciation increased during the third week, and they became feeble, and lost their activity and appetite. At the same time an ulcer formed on each cornea, followed by an escape of the humors of the eye: this took place in repeated experiments. The ani- mals still continued to eat three or four ounces of sugar daily, but became at length so feeble as to be incapable of motion, and died on a day varying from the thirty-first to the thirty- fourth. On dissection, their bodies presented all the appear- ances produced by death from starvation; indeed, dogs will live almost the same length of time without any food at all. When dogs were fed exclusively on gum, results almost similar to the above ensued. When they were kept on olive oil and watei', all the phenomena produced were the same, except that no ulceration of the cornea took place: the effects were also the same with butter. Tiedemann and Gmelin ob- tained very similar results. They fed different geese, one with sugar and water, another with gum and water, and a third with starch and water. All gradually lost weight. The one fed with gum died on the sixteenth day; that fed with sugar on the twenty-second; the third, which was fed with starch on the twenty-fourth ; and another on the twenty-seventh day; having lost, during these periods, from one-sixth to one-half of their weight. The experiments of Chossat and Letellier prove the same; and in men, the same is shown by the various dis- eases to which they who consume but little nitrogenous food are liable, and especially, as Dr. Budd has shown by the affec- tion of the cornea which is observed in Hindoos feeding almost 202 DIGESTION. exclusively on rice. But it is not only the non-nitrogenous substances, which, taken alone, are insufficient for the mainte- nance of health. The experiments of the Academies of France and Amsterdam were equally conclusive that gelatin alone soon ceases to be nutritive. Mr. Savory’s observations on food confirm and extend the results obtained by Magendie, Chossat, and others. They show that animals fed exclusively on non-nitrogenous diet speedily emaciate and die, as if from starvation; that a much larger amount of urine is voided by those fed with nitrogenous than by those with non-nitrogenous food; and that animal heat is maintained as well by the former as by the latter—a fact which proves that nitrogenous elements of food, as well as non- nitrogenous, may be regarded as calorifacient. The non-nitro- genous principles, however, he believes to be calorifacient es- sentially, not being first converted into tissue; but of the nitrogenous, he believes that only a part is thus directly cal- orifacient, the rest being employed in the formation of tissue. Contrary to the views of Liebig and Lehmann, Savory has shown that, while animals speedily die when confined to non- nitrogenous diet, they may live long when fed exclusively with nitrogenous food. Man is supported as well by food constituted wholly of ani- mal substances, as by that which is formed entirely of vegeta- ble matters, on the condition, of course, that it contain a mix- ture of the various nitrogeuous and non-nitrogenous substances just shown to be essential for healthy nutrition. In the case of carnivorous animals, the food upon which they exist, con- sisting as it does of the flesh and blood of other animals, not only contains all the elements of which their own blood and tissues are composed, but contains them combined, probably, in the same forms. Therefore, little more may seem requisite, in the preparation of this kind of food for thg nutrition of the body, than that it should be dissolved and conveyed into the blood in a condition capable of being reorganized. But in the case of the herbivorous animals, which feed exclusively upon vegetable substances, it might seem as if there would be greater difficulty in procuring food capable of assimilation into their blood and tissues. But the chief ordinary articles of vegetable food contain substances identical in composition with the albumen, fibrin, and casein, which constitute the principal nutritive materials in animal food. Albumen is abundant in the juices and seeds of nearly all vegetables; the gluten which exists, especially in corn and other seeds of grasses as well as in their juices, is identical in composition with fibrin, and is often named vegetable fibrin ; and the sub- STARVATION. 203 stance named legumen, which is obtained especially from peas, beans, and other seeds of leguminous plants, and from the potato, is identical with the casein of milk. All these vegeta- ble substances are, equally with the corresponding animal principles, and in the same manner, capable of conversion into blood and tissue ; and as the blood and tissues in both classes of animals are alike, so also the nitrogenous food of both may be regarded as, in essential respects, similar. It is in the relative quantities of the nitrogenous and non- nitrogenous compounds in these different foods that the differ- ence lies, rather than in the presence of substances in one of them which do not exist in the other. The only non-nitro- genous compounds in ordinary animal food are the fat, the saline matters, and water, and, in some instances, the vegeta- ble matters which may chance to be in the digestive canals of such animals as are eaten whole. The amount of these, how- ever, is altogether much less than that of the non-nitrogenous substances represented by the starch, sugar, gum, oil, &c., in the vegetable food of herbivorous animals. The effects of total deprivation of food have been made the subject of experiments on the lower animals, and have been but too frequently illustrated in man. (1.) One of the most notable effects of starvation, as might be expected, is loss of weight; the loss being greatest at first, as a rule, but afterwards not varying very much, day by day, until death ensues. Chossat found that the ultimate propor- tional loss was, in different animals experimented on, almost exactly the same; death occurring when the body had lost two-fifths (forty per cent.) of its original weight. Different parts of the body lose weight in very different pro- portions. The following results are taken, in round numbers, from the table given by M. Chossat: Fat loses ...... 93 per cent. Blood, 75 “ Spleen, 71 “ Pancreas, . . . . . .64 “ Liver, ....... 52 “ Heart, ....... 44 “ Intestines, ...... 42 “ Muscles of locomotion, . . .42 “ Stomach loses, . . . . .39 “ Pharynx, (Esophagus, . . .34 “ Skin, 33 “ Kidneys, ...... 31 “ Kespiratory apparatus, . . .22 “ Bones, ....... 16 “ Eyes, ....... 10 “ Nervous system, ..... 2 “ (nearly). 204 DIGESTION. (2.) The effect of starvation on the temperature of the vari- ous animals experimented on by Chossat was very marked. For some time the variation in the daily temperature was more marked than its absolute and continuous diminution, the daily fluctuation amounting to 5° or 6° F., instead of 1° or 2° F., as in health. But a short time before death, the temperature fell very rapidly, and death ensued when the loss had amounted to about 30° F. It has been often said, and with truth, although the statement requires some qualification, that death by starvation is really death by cold; for not only has it been found that differences of time with regard to the period of the fatal result are attended by the same ultimate loss of heat, but the effect of the application of external warmth to animals cold and dying from starvation, is more effectual in reviving them than the administration of food. In other words, an animal exhausted by deprivation of nourishment is unable so to digest food as to use it as fuel, and therefore is dependent for heat on its supply from without. Similar facts are often observed in the treatment of exhaustive diseases in man. (3.) The symptoms produced by starvation in the human subject are hunger, accompanied, or it may be replaced, by pain, referred to the region of the stomach ; insatiable thirst; sleeplessness ; general weakness and emaciation. The exhala- tions both from the lungs and skin are fetid, indicating the tendency to decomposition which belongs to badly-nourished tissues; and death occurs, sometimes after the additional ex- haustion caused by diarrhoea, often with symptoms of nervous disorder, delirium, or convulsions. (4.) In the human subject death commonly occurs within six to ten days after total deprivation of food. But this period may be considerably prolonged by taking a very small quan- tity of food, or even water only. The cases so frequently re- lated of survival after many days, or even some weeks, of abstinence, have been due either to the last-mentioned circum- stances, or to others less effectual, which prevented the loss of heat and moisture. Cases in which life has continued after total abstinence from food and drink for many weeks, or months, exist only in the imagination of the vulgar. (5.) The appearances presented after death from starvation are those of general wasting and bloodlessness, the latter con- dition being least noticeable in the brain. The stomach and intestines are empty and contracted, and the walls of the latter usually appear remarkably thinned and almost transparent. The usual secretions are scanty or absent, with the exception of the bile, which, somewhat concentrated usually fills the gall- bladder. All parts of the body readily decompose. DAILY LOSS OF CARBON AND NITROGEN. 205 It has just been remarked that man can live upon animal matters alone, or upon vegetables. The structure of his teeth, however, as well as experience, seems to declare that he is best fitted for a mixed diet; and the same inference may be readily gathered from other facts and considerations. Thus, the food a man takes into his body daily, represents or ought to repre- sent the quantity and kind of matter necessary for replacing that which is daily cast out by the way of lungs, skin, kidneys, and other organs. To find out, therefore, the quantity and kind of food necessary for a healthy man, it will, evidently, be the best plan to consider in the first place what he loses by excretion. For the sake of example, we may now take only two ele- ments, carbon and nitrogen, and if we discover what amount of these is respectively discharged in a given time from the body, we shall be in a position to judge what kind of food will most readily and economically replace their loss. The quantity of carbon daily lost from the body amounts to about 4500 grains, and of nitrogen 300 grains; and if a man could be fed by these elements, as such, the problem would be a very simple one; a corresponding weight of char- coal, and, allowing for the oxygen in it, of atmospheric air, would be all that is necessary. But, as before remarked, an animal can live only upon these elements when they are ar- ranged in a particular manner with others, in the form of an organic compound, as albumen, starch, and the like; and the relative proportion of carbon to nitrogen in either of these compounds alone, is by no means the proportion required in the diet of man. The amount, 4500 grains of carbon, repre- sents about fifteen times the quantity of nitrogen required in the same period ; and in albumen, the proportion of carbon to nitrogen is only as 3.5 to 1. If therefore, a man took into his body, as food, sufficient albumen to supply him with the needful amount of carbon, he would receive more than four times as much nitrogen as he wanted ; and if he took only sufficient to supply him with nitrogen, he would be starved for want of carbon. It is plain, therefore, that he should take with the albuminous part of his food, which contains so large a relative amount of nitrogen in proportion to the carbon he needs, substances in which the nitrogen exists in much smaller quantities. Food of this kind is provided in such compounds as starch and fat. The latter indeed as it exists for the most part in considerable amount mingled with the flesh of animals, re- moves to a great extent, in a diet of animal food, the difficulty which would otherwise arise from a deficiency of carbon—fat 206 DIGESTION. containing a large relative proportion of this element, and no nitrogen. To take another example; the proportion of carbon to ni- trogen in bread about 30 to 1. If a man’s diet were confined to bread, he would eat, therefore, in order to obtain the requi- site quantity of nitrogen, twice as much carbon as is neces- sary ; and it is evident, that, in this instance, a certain quan- tity of a substance with a large relative amount of nitrogen is the kind of food necessary for redressing the balance. To place the preceding facts in a tabular form, and taking meat as an example instead of pure albumen: meat contains about 10 per cent, of carbon, and rather more than 3 per cent, of nitrogen. Supposing a man to take meat for the supply of the needful carbon, he would require 45,000 grains, or nearly 6£ lbs. containing : Carbon, 4500 grains. Nitrogen, ....... 1350 “ Excess of Nitrogen above the amount required, 1500 “ Bread contains about 30 per cent, of carbon and 1 per cent, of nitrogen. If bread alone, therefore, were taken as food, a man would require, in order to obtain the requisite nitrogen, 30,000 grains, containing: Carbon, ........ 9000 grains. Nitrogen, ....... 300 “ Excess of Carbon above the amount required, 4500 “ But a combination of bread and meat would supply much more economically what was necessary. Thus : Carbon. Nitrogen. 15,000 grains of bread (or rather more than 2 lb), contain ...... 4500 grs. 150 grs. 5,000 grains of meat (or about fib.) contain . 500 “ 150 “ 5000 “ 300 “ So that i lb. of meat, and less than 2 lbs. of bread would supply all the needful carbon and nitrogen with but little waste. From these facts it will be plain that a mixed diet is the best and most economical food for man; and the result of ex- perience entirely coincides with what might have been antici- pated on theoretical grounds only. It must not be forgotten, however, that the value of certain foods may depend quite as much on their digestibility, as on NECESSITY FOR CHANGES OF DIET. 207 the relative quantities of the necessary elements which they contain. In actual practice, moreover, the quantity and kind of food to be taken with most economy and advantage cannot be set- tled for each individual, only by considerations of the exact quantities of certain elements that are required. Much will of necessity depend on the habits and digestive powers of the individual, on the state of his excretory organs, and on many other circumstances. Food which to one person is appropriate enough, may be quite unfit for another; and the changes of diet so instinctively practiced by all to whom they are possi- ble, have much more reliable grounds of justification than any which could be framed on theoretical considerations only. In many of the experiments on the digestibility of various articles of food, disgust at the sameness of the diet may have had as much to do with inability to consume and digest it, as the want of nutritious properties in the substances which were experimented on. And that disease may occur from the want of particular food, is well shown by the occurrence of scurvy when fresh vegetables are deficient, and its rapid cure when they are again eaten : and the disease which is here so re- markably evident in its symptoms, causes, and cure, is matched by numberless other ailments, the causes of which, however, although analogous, are less exactly known, and therefore less easily combated. With regard to the quantity, too, as well as the kind of food necessary, there will be much diversity in different individuals. Dr. Dalton believed, from some experiments which he per- formed, that the quantity of food necessary for a healthy man, taking free exercise in the open air, is as follows: Meat, . . . .16 ounces, or 1.00 lb. avoird. Bread, . . . 19 “ 1.19 “ “ Butter or Fat, . 3£ “ 0.22 “ “ Water,. . . . 52fluidoz. 3.38 “ “ The quantity of meat, however, here given is probably more in proportion to the other articles of diet enumerated than is needful for the majority of individuals under the circum- stances stated. PASSAGE OF FOOD THROUGH THE ALIMENTARY CANAL. The course of the food through the alimentary canal of man will be readily seen from the accompanying diagram (Fig. 66). The food taken into the mouth passes thence through the oesophagus into the stomach, and from this into the small 208 DIGESTION. and large intestine successively; gradually losing, by absorp- tion, the greater portion of its nutritive constituents. The residue, together with such matters as may have been added Fig. 66. Diagram of the alimentary canal. The small intestine of man is from about 3 to 4 times as long as the large intestine. to it in its passage, is discharged from the rectum through the anus. We shall now consider, in detail, the process of digestion, as it takes place in each stage of this journey of the food through the alimentary canal. ‘SALIVARY GLANDS AND SALIVA. 209 The Salivary Glands and the Saliva. The first of a series of changes to which the food is subjected in the digestive canal, takes place in the cavity of the mouth; the solid articles of food are here submitted to the action of the teeth (p. 51), whereby they are divided and crushed, and by being at the same time mixed with the fluids of the mouth, are reduced to a soft pulp, capable of being easily swallowed. The fluids with which the food is mixed in the mouth consist of the secretiou of the salivary glands, and the mucus secreted by the lining membrane of the whole buccal cavity. The glands concerned in the production of saliva, are very extensive, and, in man and Mammalia generally, are presented in the form of four pairs of large glands, the parotid, submax- illary, sublingual, and numerous smaller bodies, of similar structure and with separate ducts, which are scattered thickly beneath the mucous membrane of the lips, cheeks, soft palate, and root of the tongue. The structure of all these glands is essentially the same. Each is composed of several parts, called lobes, which are joined together by areolar tissue; and each of these lobes, again, is made up of a number of smaller parts called lobules, bound together as before by areolar tissue. Each of these small divisions, called lobules, is a miniature represen- tation of the whole gland. It contains a small branch of the duct, which, subdividing, ends in small vesicular pouches, called acini, a group of which may be considered the dilated end of one of the smaller ducts (Fig. 67). Each of the acini Fig. 67, Diagram of a racemose or saccular compound gland : m, entire gland, showing branched duct and lobular structure ; n, a lobule detached, with o, branch of duct proceeding from it (after Sharpey). is about g-J-0 of an inch in diameter, and is formed of a fine structureless membrane, lined on the inner surface and often filled by spheroidal or glandular epithelium; while on the out- side there is a plexus of capillary bloodvessels. The accom- panying diagram is intended to show the typical structure of such glands as the salivary (Fig. 67). 210 DIGESTION. Saliva, as it commonly flows from the mouth, is mixed with the secretion of the mucous membrane, and often with air- bubbles, which, being retained by its viscidity, make it frothy. When obtained from the parotid ducts, and free from mucus, saliva is a transparent watery fluid, the specific gravity of which varies from 1.004 to 1.008, and in which, when examined with the microscope, are found floating a number of minute particles, derived from the secreting ducts and vesicles of the glands. In the impure or mixed saliva are found, besides these particles, numerous epithelial scales separated from the surface of the mucous membrane of the mouth and tongue, and mucus-corpuscles, discharged for the most part from the tonsils, which, when the saliva is collected in a deep vessel, and left at rest, subside in the form of a white opaque matter, leaving the supernatant salivary fluid transparent and colorless, or with a pale bluish-gray tint. In reaction, the saliva, when first secreted, appears to be always alkaline; and that from the parotid gland is said to be more strongly alka- line than that from the other salivary glands. This alkaline condition is most evident when digestion is going on, and ac- cording to Dr. Wright, the degree of alkalinity of the saliva bears a direct proportion to the acidity of the gastric fluid se- creted at the same time. During fasting, the saliva, although secreted alkaline, shortly becomes neutral, and it does so es- pecially when secreted slowly and allowed to mix with the acid mucus of the mouth, by which its alkaline reaction is neutralized. The following analysis of the saliva is by Frerichs: Water, 994.10 Solids, ....... 5.90 Ptyalin, 1.41 Fat, 0.07 Epithelium and Mucus, . . . 2.13 Composition of Saliva. Salts: Sulphocyanide of Potassium, . Phosphate of Soda, “ Lime, “ Magnesia,. Chloride of Sodium, “ Potassium, . 2.29 5.90 The rate at which saliva is secreted is subject to considerable variation. When the tongue and muscles concerned in mas- USES OF SALIVA. 211 ticatiou are at rest, and the nerves of the mouth are subject to no unusual stimulus, the quantity secreted is not more than sufficient, with the mucus, to keep the mouth moist. But the flow is much accelerated when the movements of mastication take place, and especially when they are combined with the presence of food in the mouth. It may be excited also, even when the mouth is at rest, by the mental impressions produced by the sight or thought of food ; also by the introduction of food into the stomach. The influence of the latter circum- stance was well shown in a case mentioned by Dr. Gairdner, of a man whose pharynx had been divided : the injection of a meal of broth into the stomach was followed by the secretion of from six to eight ounces of saliva. Under these varying circumstances, the quantity of saliva secreted in twenty-four hours varies also ; its average amount is probably from two to three pints in twenty-four hours. In a man who had a fistulous opening of the parotid duct, Mits- cherlich found that the quantity of saliva discharged from it during twenty-four hours, was from two to three ounces; and the saliva collected from the mouth during the same period, and derived from the other salivary glands, amounted to six times more than that from the one parotid. The purposes served by saliva are of several kinds. In the first place, acting mechanically in conjunction with mucus, it keeps the mouth in a due condition of moisture, facilitating the movements of the tongue in speaking, and the mastication of food. (2.) It serves also in dissolving sapid substances, and rendering them capable of exciting the nerves of taste. But the principal mechanical purpose of the saliva is (3) that by mixing with the food during mastication, it makes it a soft pulpy mass, such as may be easily swallowed. To this purpose the saliva is adapted both by quantity and quality. For, speaking generally, the quantity secreted during feeding is in direct proportion to the dryness and hardness of the food: as M. Lassaigne has shown, by a table of the quantity produced in the mastication of a hundred parts of each of several kinds of food, thirty parts suffice for a hundred parts of crumb of bread, but not less than 120 for the crusts; 42.5 parts of saliva are produced for the hundred of roast meat; 3.7 for as much of apples; and so on, according to the general rule above stated. The quality of saliva is equally adapted to this end. It is easy to see how much more readily it mixes with most kinds of food than water alone does; and M. Bernard has shown that the saliva from the parotid, labial, and other small glands, being more aqueous than the rest, is that which is chiefly braided and mixed with the food in mastication; while the more viscid 212 DIGESTION. mucoid secretion of the submaxillary, palatine, and tonsillitic glands is spread over the surface of the softened mass, to enable it to slide more easily through the fauces and oesophagus. This view obtains confirmation from the interesting fact pointed out by Professor Owen, that in the great ant-eater, whose enor- mously elongated tongue is kept moist by a large quantity of viscid saliva, the subrnaxillary glands are remarkably devel- oped, while the parotids are not of unusual size. Beyond these, its mechanical purposes, saliva performs (4) a chemical part in the digestion of the food. When saliva, or a portion of a salivary gland, or even a portion of dried ptyalin, is added to starch paste, the starch is very rapidly transformed into dextrin and grape-sugar; and when common raw starch is masticated and mingled with saliva, and kept with it at a temperature of 90° or 100°, the starch grains are cracked or eroded, and their contents are transformed in the same manner as the starch paste. Changes similar to these are effected on the starch of farinaceous food (especially after cooking) in the stomach; and it is reasonable to refer them to the action of the saliva, because the acid of the gastic fluid tends to retard or prevent, rather than favor the transformation of the starch. It may therefore be held, that one purpose served by the saliva in the digestive process is that of assisting in the transforma- tion of the starch which enters so largely into the composition of most articles of vegetable food, and which (being naturally insoluble) is converted into soluble dextrin and grape-sugar, and made fit for absorption. Besides saliva, many azotized substances, especially if in a state of incipient decomposition, may excite the transformation of starch, such as pieces of the mucous membrane of the mouth, bladder, rectum, and other parts, various animal and vegetable tissues, and even morbid products; but the gastric fluid will not produce the same effect. The transformation in question is effected much more rapidly by saliva, however, than by any of the other fluids or substances experimented with, except the pancreatic secretion, which, as will be presently shown, is very analogous to saliva. The actual process by which these changes are effected is still obscure. Probably the azotized substance, ptyalin, acts as a kind of ferment, like diastase in the process of malting, and excites molecular changes in the starch which result in its transformation, first into dextrin and then into sugar. The majority of observers agree that the transformation of starch into sugar ceases on the entrance of the food into the stomach, or on the addition of gastric fluid to it in a test-tube : while others maintain that it still goes on. Probably all are DEGLUTITION. 213 right: for, although gastric fluid added to saliva appears to arrest the action of the latter on starch, yet portions of saliva mingled with food in mastication may, for some time after their entrance into the stomach, remain uuneutralized by the gastric secretion, and continue their influence upon the starchy prin- ciples in contact with them. Starch appears to be the only principle of food upon which saliva acts chemically: it has no apparent influence on any of the other ternary principles, such as sugar, gum, cellulose, or (according to Bernard) on fat, and seems to be equally desti- tute of power over albuminous and gelatinous substances, so that we have as yet no information respecting any purpose it can serve in the digestion of Carnivora, beyond that of soften- ing or macerating the food; though, since such animals masti- cate their food very little, usually “ bolting” it, the saliva has probably but little use even in this respect, in the process of digestion. When properly masticated, the food is transmitted in suc- cessive portions to the stomach by the act of deglutition or swalloiving. This act, for the purpose of description, may be divided into three parts. In the first, particles of food col- lected to a morsel glide between the surface of the tongue and the palatine arch, till they have passed the anterior arch of the fauces; in the second, the morsel is carried through the pharynx ; and in the third, it reaches the stomach through the oesophagus. These three acts follow each other rapidly. The first is performed voluntarily by the muscles of the tongue and cheeks. The second also is effected with the aid of muscles which are in part endued with voluntary motion, such as the muscles of the soft palate and pharynx ; but it is, nevertheless, an involuntary act, and takes place without our being able to prevent it, as soon as a morsel of food, drink, or saliva is carried backwards to a certain point of the tongue’s surface. When we appear to swallow voluntarily, we only convey, through the first act of deglutition, a portion of food or saliva beyond the anterior arch of the palate; then the substance acts as a stimulus, which, in accordance with the laws of reflex movements hereafter to be described, is carried by the sensi- tive nerves to the medulla oblongata, when it is reflected by the motor nerves, and an involuntary adapted action of the muscles of the palate and pharynx ensues. The third act of deglutition takes place in the oesophagus, the muscular fibres of which are entirely beyond the influence of the will. The second act of deglutition is the most complicated, be- Passcige of Food into the Stomach. 214 DIGESTION. cause the food must pass by the posterior orifice of the nose and the upper opening of the larynx without touching them. When it has been brought, by the first act, between the an- terior arches of the palate, it is moved onwards by the tongue being carried backwards, and by the muscles of the anterior arches contracting on it and then behind it. The root of the tongue being retracted, and the larynx being raised with the pharynx and carried forwards under the tongue, the epiglottis is pressed over the upper opening of the larynx, and the mor- sel glides past it; the closure of the glottis being additionally secured by the simultaneous contraction of its own muscles : so that, even when the epiglottis is destroyed, there is little danger of food or drink passing into the larynx so long as its muscles can act freely. At the same time the raising of the soft palate, so that its posterior edge touches the back part of the pharynx, and the approximation of the sides of the posterior palatine arch, which move quickly inwards like side curtains, close the passage into the upper part of the pharynx and the posterior nares, and form an inclined plane, along the under surface of which the morsel descends; then the pharynx, raised up to receive it, in its turn contracts, and forces it onwards into the oesophagus. In the third act, in which the food passes through the oesoph- agus, every part of that tube as it receives the morsel and is dilated by it, is stimulated to contract: hence an undulatory contraction of the oesophagus, which is easily observable in horses while drinking, proceeds rapidly along the tube. It is only when the morsels swallowed are large, or taken too quickly in succession, that the progressive contraction of the oesophagus is slow, and attended with pain. Division of both pneumo- gastric nerves paralyzes the contractile power of the oesophagus, and food accordingly accumulates in the tube (Bernard). DIGESTION OF FOOD IN THE STOMACH. Structure of the Stomach. It appears to be an almost universal character of animals, that they have an internal cavity for the production of a chemical change in the aliment—a cavity for digestion ; and when this cavity is compound, the part in which the food undergoes its principal and most important changes is the stomach. In man and those Mammalia which are provided with a single stomach, its walls consist of three distinct layers or coats, viz., an external peritoneal, an internal mucous, and an inter- STRUCTURE OF THE STOMACH. 215 mediate muscular coat, with bloodvessels, lymphatics, and nerves distributed in and between them. The muscular coat of the stomach consists of three separate layers or sets of fibres, which, according to their several direc- tions, are named the longitudinal, circular, and oblique. The longitudinal set are the most superficial: they are continuous with the longitudinal fibres of the oesophagus, and spread out in a diverging manner over the great end and sides of the stomach. They extend as far as the pylorus, being especially distinct at the lesser or upper curvature of the stomach, along which they pass in several strong bands. The next set are the circular or transverse fibres, which more or less completely en- circle all parts of the stomach ; they are most abundant at the middle and in the pyloric portion of the organ, and form the chief part of the thick projecting ring of the pylorus. Accord- ing to Pettigrew, these fibres are not simple circles, but form double or figure-of-8 loops, the fibres intersecting very obliquely. The next, and consequently deepest set of fibres, are the oblique, continuous with the circular muscular fibres of the oesophagus, and according to Pettigrew, with the same double-looped ar- rangement that prevails in the preceding layer: they are com- paratively few in number, and are placed only at the cardiac orifice and portion of the stomach, over both surfaces of which they are spread, some passing obliquely from left to right, others from right to left, around the cardiac orifice, to which, by their interlacing, they form a kind of sphincter, continu- ous with that around the lower end of the oesophagus. The fibres of which the several muscular layers of the stomach, and of the intestinal canal generally, are composed, belong to the class of organic muscle, being composed of smooth or unstriped, elongated, spindle-shaped fibre-cells; a fuller description of which will be given under the head of Muscular Tissue. The mucous membrane of the stomach, which rests upon a layer of loose cellular membrane, or submucous tissue, is smooth, level, soft, and velvety; of a pale pink color during life, and in the contracted state is thrown into numerous, chiefly longitudinal, folds or rugse, which disappear when the organ is distended. In its general structure the mucous membrane of the stomach resembles that of other parts. (See Structure of Mucous Membrane.) But there are certain peculiarities shared with the mucous membrane of the small and large intestines, which, doubtless, are connected with the peculiar functions, especially those relating to absorption, which these parts of the alimen- tary canal perform. Entering largely into the construction of the mucous mem- 216 DIGESTION. brane, especially in the superficial part of the corium, is a quantity of a very delicate kind of connective tissue, called retiform tissue (Fig. 72), or sometimes lymphoid or adenoid tis- sue, because it so closely resembles that which forms the stroma, or supporting framework of lymphatic glands (see section on Lymphatic Glands); the resemblance being made much closer by the fact that the interspaces of this retiform tissue are filled with corpuscles not to be distinguished from lymph-corpuscles. At the deepest part of the mucous membrane, is a layer of unstriped muscular fibres, called the muscularis mucosae, which must not be confounded with the layers of muscle constituting the proper muscular coat, and from which it is separated by the submucous tissue. The muscularis mucosae is found in the oesophagus, as well as in the stomach and intestines. When examined with a lens, the internal or free surface of the stomach presents a peculiar honeycomb appearance, pro- duced by shallow polygonal depressions or cells (Fig. 68), the diameter of which varies generally from 3^cth to 0th of an inch ; but near the pylorus is as much as of an inch. They are separated by slightly elevated ridges, which some- times, especially in certain morbid states of the stomach, bear minute, narrow, vascular processes, which look like villi, and have given rise to the erroneous supposition that the stomach has absorbing villi, like those of the small intestines. In the bottom of the cells minute openings are visible (Fig. 68), which Fig. 68. Small portion of the surface of the mucous membrane of the stomach (from Ecker) 3 0 The specimen shows the shallow depressions, in each of which the smaller dark spots indicate the orifices of a variable number of the gastric tubular glands. are the orifices of perpendicularly arranged tubular glands (Fig. 69), imbedded side by side in sets or bundles, in the sub- stance of the mucous membrane, and composing nearly the whole structure. The glands which are found in the human stomach may be divided into two classes, the tubular and lenticular. Tubular glands.—The tubular glands may be described as a collection of cylinders with blind extremities, about of an GLANDS OF THE STOMACH 217 Fig. 69. Surface of mucous membrane. Gastric tubes. Dense areolar tissue. Submucous tissue of looser texture. Transverse muscular fibres. Longitudinal muscular fibres. Portion of human stomach (magnified 30 diameters) cut vertically, both in a direc- tion parallel to its long axis, and across it (altered from Brinton). Peritoneum. Fig. 70. The gastric glands of the human stomach (magnified), a, deep part of a pyloric gastric gland (from Kolliker) ; the cylindrical epithelium is traceable to the c'decal extremities, b and c, cardiac gastric glands (from Allen Thompson); b, vertical sec- tion of a small portion of the mucous membrane with the glands magnified 30 diame- ters ; c, deeper portion of one of the glands, magnified 65 diameters, showing a slight division of the tubes, and a sacculated appearance, produced by the large glandular cells within them ; d, cellular elements of the cardiac glands magnified 250 diameters. 218 DIGESTION. inch in length, and Dth in diameter, packed closely together, with their long axis at right angles to the surface of the mucous membrane on which they open, their blind ends rest- ing on the submucous tissue. (See Fig. 69.) They are all composed of basement-membi’ane, and lined by epithelial cells, but they are not all of exactly similar shape; for while some are simple straight tubes, open at one end and closed at the other (Fig. 69), others present at their deeper extremities a varicose, pouched, or in some cases, even a branched ap- pearance (Fig. 70, b and c). The epithelium lining them is not the same throughout. In the upper third or fourth of their length it is cylindrical, and continuous with that which covers the free mucous surface of the rest of the stomach. In their lower part, on the other hand, it is of the variety called glan- dular or spheroidal, the cells being oval or somewhat angular, and about of an inch in diameter. The cells, however do not completely fill up the cavity of the gland which they line, but leave a slight, central, thread-like space, the immediate lining of which is a layer of small angular cells, continuous with the cylindrical epithelium in the upper portion of the tube. This description will become plain on reference to Fig. 71, which represents on a larger scale a longitudinal section of one of the glands depicted in Fig. 69. In the greater number of the glands which are branched at their deeper extremities, the spheroidal epithelium exists in the divisions, while the main duct and the upper part of the branches are lined by the cylin- drical variety (Fig. 70, c). In the human stomach, according to Dr. Brinton, the simple un- divided tubes are the rule, and the branched the exception. The varieties in the epithelial cells lining the different parts of the tubes, correspond probably with differences in the fluid se- creted by their agency—the cyl- inder-epithelium, like that on the free surface of the stomach, Fig. 71. Part of one of the gastric glands highly magnified, to show the ar- rangement of the epithelium in its interior; a, columnar cells lining the upper part of the tube; b, small an- gular cells, into which these merge below to form a central or axial layer within; c, the proper gastric or glandular cells (after Brinton). THE GASTRIC FLUID. 219 being probably engaged in separating the thin alkaline mucus, which is always present in greater or less quantity, while the larger glandular cells probably secrete the proper gastric juice. Near the pylorus there exist glands branched at their deep extremities, which are lined throughout by cylinder-epithelium (Fig. 70, a), and probably serve only for the secretion of mucus. All the tubular glands, while they open by one end into the cavity of the stomach, rest by their blind extremities on a bed or matrix of areolar tissue (Fig. 69), which is prolonged up- wards between them, so as to invest and support them. Lenticular Glands.—Besides the cylindrical glands, there are also small closed sacs beneath the surface of the mucous membrane, resembling exactly the solitary glands of the intes- tine, to be described hereafter. Their number is very variable, and they are found chiefly along the lesser curvature of the stomach, and in the pyloric region, but they may be present in any part of the organ. According to Dr. Brinton they are rarely absent in children. Their function probably resembles that of the intestinal solitary glands, but nothing is certainly known regarding it. The bloodvessels 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 upwards from it, so to speak, is a more superficial network of larger capillaries, which branch densely around the orifices 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 intertubular capil- lary plexus, they open finally into the venous network in the submucous tissue. The nerves of the stomach are derived from the pneumo- gastric and sympathetic. Secretion and Properties of the Gastric Fluid. While 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 foreign substance into the stom- ach, the mucous membrane, previously quite pale, becomes slightly turgid and reddened with the influx of a larger quan- 220 DIGESTION. tity of blood ; the gastric glands commence secreting actively, and an acid fluid is poured out in minute drops, which gradu- ally run together and flow down the walls of the stomach, or soak into the substances introduced. The quantity of this fluid secreted daily has been variously estimated; but the average for a healthy adult has been assumed to range from ten to twenty pints in the twenty-four hours (Brinton). The first accurate analysis of the gastric fluid was made by Dr. Prout; but it does not appear that it was collected in any large quantity, or pure and separate from food, until the time when Dr. 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 open- ing leading directly into the stomach, near the upper extremity of the great curvature, and three inches from the cardiac orifice. The external opening was situate two inches below the left mamma, in a line drawn from that part to the spine of the left ilium. The borders of the opening into the stomach, which was of considerable size, had united, in healing, with the mar- gins of the external wound, but the cavity of the stomach was at last separated from the exterior by a fold of mucous mem- brane, which projected from the upper and back part of the opening, and closed it like a valve, but could be pushed back with the finger. The introduction of any mechanical irritant, such as the bulb of a thermometer, into the stomach, excited at once the secretion of gastric fluid. This could be drawn off with a caoutchouc tube, and could often be obtained to the extent of nearly an ounce. The introduction of alimentary substances caused a much more rapid and abundant secretion of pure gastric fluid than the presence of other mechanical irritants did. No increase of temperature could be detected during the most active secretion; the thermometer introduced into the stomach always stood at 100° Fahr., except during muscular exertion, when the temperature of the stomach, like that of other parts of the body, rose one or two degrees higher. M. Blondlot, and subsequently M. Bernard, and since then, several others, by maintaining fistulous openings into the stomachs of dogs, have confirmed most of the facts discovered by Dr. Beaumont. And the man St. Martin has frequently submitted to renewed experiments on his stomach, by various physiologists. From all these observations it appears, that pepper, salt, and other soluble stimulants, excite a more rapid discharge of gastric fluid than mechanical irritation does; so do alkalies generally, but acids have a contrary effect. When mechanical irritation is carried beyond certain limits so as to produce pain, the secretion, instead of being more abundant, THE GASTRIC FLUID. 221 diminishes or ceases entirely, and a ropy mucus is poured out instead. Very cold water, or small pieces of ice, at first ren- der the mucous membrane pallid, but soon a kind of reaction ensues, the membrane becomes turgid with blood, and a larger quantity of gastric juice is poured out. The application of too much ice is attended by diminution in the quantity of fluid secreted, and by consequent retardation of the process of di- gestion. The quantity of the secretion seems to be influenced also by impressious made on the mouth ; for Blondlot found that when sugar was introduced into the dog’s stomach, either alone, or mixed with human saliva, a very small secretion en- sued : but when the dog had himself masticated and swallowed it, the secretion was abundant. Dr. Beaumont described the secretion of the human stomach as “ a clear transparent fluid, inodorous, a little saltish, and very perceptibly acid. Its taste is similar to that of thin mu- cilaginous water, slightly acidulated with muriatic acid. It is readily diffusible in water, wine, or spirits; slightly effer- vesces with alkalies ; and is an effectual solvent of the materia alimentaria. It possesses the property of coagulating albumen in an eminent degree; is powerfully antiseptic, checking the putrefaction of meat; and effectually restorative of healthy action, when applied to old fetid sores and foul ulcerating surfaces.” The chemical composition of the gastric juice of the human subject has been particularly investigated by Schmidt, a favor- able case for his doing so occurring in the person of a peasant named Catharine Kfitt, aged 35, who for three years had had a gastric fistula under the left mammary gland, between the cartilages of the ninth and tenth ribs. The fluid was obtained by putting into the stomach some hard indigestible matter, as dry peas, and a little water, by which means the stomach was excited to secretion, at the same time that the matter introduced did not complicate the analy- sis by being digested in the fluid secreted. The gastric juice was drawn off through an elastic tube inserted into the fistula. The fluid thus obtained was acid, limpid, and odorless, with a mawkish taste. Its density varied from 1.0022 to 1.0024. Under the microscope a few cells from the gastric glands and some fine granular matter were observable. The following table gives the mean of two analyses of the above-mentioned fluid; and arranged by the side of it, for purposes of comparison, is an analysis of gastric juice from the sheep and dog. 222 DIGESTION. Human Gastric Juice. Sheep’s Gastric Juice. Dog’s Gastric Juice, Water, .... Solid Constituents, 994.40 986.14 971.17 5.59 13.85 28 82 f Ferment, Pepsin (with a trace of Ammonia), 3.19 4.20 17.50 Hydrochloric Acid, . 0.20 1.55 2.70 Solids. • Chloride of Calcium, . 0.06 0.11 1.66 “ Sodium, . 1.46 4.36 3.14 “ Potassium, 0.55 ’ 1.51 1.07 Phosphate of Lime, Magnesia, and Iron, 0.12 2.09 2.73 Composition of Gastric Juice. Iii all the above analyses the amount of water given must be reckoned as rather too much, inasmuch as a certain quan- tity of saliva was mixed with the gastric fluid. The allow- ance, however, to be made on this account is only very small. Considerable difference of opinion has existed concerning the nature of the free acid contained in the gastric juice, chiefly whether it is hydrochloric or lactic. The weight of evidence, however, is in favor of free hydrochloric acid, being that to which, in the human subject, the acidity of the gastric fluid is mainly due; although there is no doubt that others, as lactic, acetic, butyric, are not unfrequently to be found therein. The animal matter mentioned in the analysis of the gastric fluid is named pepsin, from its power in the process of diges- tion. It is an azotized substance, and is best procured by di- gesting portions of the mucous membrane of the stomach in cold water, after they have been macerated for some time in water at a temperature between 80° and 100° F. The warm water dissolves various substances as well as some of the pepsin, but the cold water takes up little else than pepsin, which, on evaporating the cold solution, is obtained in a grayish-brown viscid fluid. The addition of alcohol throws down the pepsin in grayish-white flocculi; and one part of the principle thus prepared, if dissolved in eveu 60,000 parts of water, will digest meat and other alimentary substances. The digestive power of the gastric fluid is manifested in its softening, reducing into pulp, and partially or completely dis- solving various articles of food placed in it at a temperature of from 90° to 100°. This, its peculiar property, requires the presence of both the pepsin and the acid ; neither of them can digest alone, and when they are mixed, either the decomposi- tion of the pepsin, or the neutralization of the acid, at once DIGESTIVE POWER OF GASTRIC FLUID. 223 destroys the digestive property of the fluid. For the perfec- tion of the process also, certain conditions are required, which are all found in the stomach; namely (1), a temperature of about 100° F.; (2), such movements as the food is subjected to by the muscular actions of the stomach, which bring in suc- cession every part of it in contact with the mucous membrane, whence the fresh gastric fluid is being secreted ; (3), the con- stant removal of those portions of food which are already digested, so that what remains undigested may be brought more completely into contact with the solvent fluid; and (4) a state of softness and minute division, such as that to which the food is reduced by mastication previous to its introduction into the stomach. The chief circumstances connected with the mode in which the gastric fluid acts upon food during natural digestion, have been determined by watching its operations when removed from the stomach and placed in conditions as nearly as possi- ble like those under which it acts while within that viscus. The fact that solid food, immersed in gastric fluid out of the body, and kept at a temperature of about 100°, is gradually converted into a thick fluid similar to chyme, was shown by Spallanzani, Dr. Stevens, Tiedemann and Gmelin and others. They used the gastric fluid of dogs, obtained by causing the animals to swallow small pieces of sponge, which were subse- quently withdrawn, soaked with the fluid—and proved nearly as much as the latter experiments of the same kind of gastric fluid by Blondlot, Bernard and others. But these need not be particularly referred to, while we have the more satisfac- tory and instructive observations which Dr. Beaumont made with the fluid obtained from the stomach of St. Martin. After the man had fasted seventeen hours, Dr. Beaumont took one ounce of gastric fluid, put into it a solid piece of boiled recently salted beef weighing three drachms, and placed the vessel which contained them in a water-bath heated to 100°. “ In forty minutes digestion had distinctly commenced over the surface of the meat; in fifty minutes, the fluid had become quite opaque and cloudy, the external texture began to separate and become loose ; and in sixty minutes chyme began to form. At 1 p.m.” (two hours after the commencement of the experi- ment) “ the cellular texture seemed to be entirely destroyed, leaving the muscular fibres loose and unconnected, floating about in small fine shreds, very tender and soft.” In six hours, they were nearly all digested—a few fibres only remaining. After the lapse of ten hours, every part of the meat was com- pletely digested. The gastric juice, which was at first trans- parent, was now about the color of whey, and deposited a fine 224 DIGESTION. sediment of the color of meat. A similar piece of beef was, at the time of the commencement of this experiment, suspended in the stomach by means of a thread : at the expiration of the first hour it was changed in about the same degree as the meat digested artificially ; but at the end of the second hour, it was completely digested and gone. In other experiments, Dr. Beaumont withdrew through the opening of the stomach some of the food which had been taken twenty minutes previously, and which was completely mixed with the gastric juice. He continued the digestion, which had already commenced, by means of artificial heat in a water-bath. In a few hours the food thus treated was completely chymified ; and the artificial seemed in this, as in several other experi- ments, to be exactly similar to, though a little slowTer than, the natural digestion. The apparent identity of the process in- and outside of the stomach thus manifested, while it shows that we may regard digestion as essentially a chemical process, when once the gas- tric fluid is formed, justifies the belief that Dr. Beaumont’s other experiments with the digestive fluid may exactly repre- sent the modifications to which, under similar conditions, its action in the stomach would be liable. He found that, if the mixture of food and gastric fluid were exposed to a temperature of 34° F., the process of digestion was completely arrested. In another experiment, a piece of meat which had been macerated in water at a temperature of 100° for several days, till it ac- quired a strong putrid odor, lost, on the addition of some fresh gastric juice, all signs of putrefaction, and soon began to be digested. From other experiments he obtained the data for estimates of the degrees of digestibility of various articles of food, and of the ways in which the digestion is liable to be af- fected, to which reference will again be made. When natural gastric juice cannot be obtained, many of these experiments may be performed with an artificial digestive fluid, the action of which, probably, very closely resembles that of the fluid secreted by the stomach. It is made by macerat- ing in water portions of fresh or recently dried mucous mem- brane of the stomach of a pig1 or other omnivorous animal, or of the fourth stomach of the calf, and adding to the in- fusion a few drops of hydrochloric acid—about 3.3 grains to half an ounce of the mixture, according to Schwann. Por- tions of food placed in such fluid, and maintained with it * The best portion of the stomach of the pig for this purpose is that between the cardiac and pyloric orifices; the cardiac portion appears to furnish the least active digestive fluid. CHYME 225 at a temperature of about 100°, are, in an hour or more, according to the toughness of the substance, softened and changed in just the same manner as they would be in the stomach. The nature of the action by which the mucous membrane of the stomach and its secretion work these changes in organic matter is exceedingly obscure. The action of the pepsin may be compared with that of a ferment, which at the same time that it undergoes change itself, induces certain changes also in the organic matters with which it is in contact. Or its mode of action may belong to that class of chemical processes termed “ catalytic,” in which a substance excites, by its mere presence, and without itself undergoing change as ordinary ferments do, some chemical action in the substances with which it is in con- tact. So, for example, spongy platinum, or charcoal, placed in a mixture, however voluminous, of oxygen and hydrogen, makes them combine to form water; and diastase makes the starch in grains undergo transformation, and sugar is produced. And that pepsin acts in some such manner appears probable from the very minute quantity capable of exerting the peculiar digestive action on a large quantity of food, and apparently with little diminution in its active power. The process differs from ordinary fermentation, in being unattended with the for- mation of carbonic acid, in not requiring the presence of oxygen, and in being unaccompanied by the production of new quan- tities of the active principle, or ferment. It agrees with the processes of both fermentation and organic catalysis, in that whatever alters the composition of the pepsin (such as heat above 100°, strong alcohol, or strong acids), destroys the diges- tive power of the fluid. Changes of the Food in the Stomach. The general effect of digestion in the stomach is the conver- sion of the food into chyme, a substance of various composition according to the nature of the food, yet always presenting a characteristic thick, pultaceous, grumous consistence, with the undigested portions of the food mixed in a more fluid substance, and a strong, disagreeable acid odor and taste. Its color de- pends on the nature of the food, or on the admixture of yellow or green bile which may, apparently, even in health, pass into the stomach. Reduced into such a substance, all the various materials of a meal may be mingled together, and near the end of the diges- tive process hardly admit of recognition ; but the experiments of artificial digestion, and the examination of stomachs with 226 DIGESTION. fistulse, have illustrated many of the changes through which the chief alimentary principles pass, and the times and modes in which they are severally disposed of. These must now be traced. The readiness with which the gastric fluid acts on the several articles of food is, in some measure, determined by the state of division, and the tenderness and moisture of the substance pre- sented to it. By minute division of the food, the extent of surface with which the digestive fluid can come in contact is increased, and its action proportionably accelerated. Tender and moist substances offer less resistance to the action of the gastric juice than tough, hard, and dry ones do, because they may be thoroughly penetrated by it, and thus be attacked not only at the surface, but at every part at once. The readiness with which a substance is acted upon by the gastric fluid does not, however, necessarily imply the degree of its nutritive property; for a substance may be nutritious, yet, on account of its toughness and other qualities, hard to digest; and many soft, easily digested substances contain comparatively a small amount of nutriment. But for a substance to be nutritive, it must be capable of being assimilated to the blood ; and to find its way *into the blood, it must, if insoluble, be digestible by the gastric fluid or some other secretion in the intestinal canal. There is, therefore, thus far, a necessary connection between the digestibility of a substance and its power of affording nutri- ment. Those portions of food which are liquid when taken into the stomach, or which are easily soluble in the fluids therein, are probably at once absorbed by the bloodvessels in the mucous membrane of the stomach. Magendie’s experiments, and better still, those of Dr. Beaumont, have proved this quick absorption of water, wine, weak saline solutions, and the like ; that they are absorbed without manifest change by the diges- tive fluid, and that, generally, the water of such liquid food as soups is absorbed at once, so that the substances suspended in it are concentrated into a thicker material, like the chyme from solid food, before the digestive fluid acts upon them. The action of the gastric fluid on the several kinds of solid food has been studied in various ways. In the earliest experi- ments, perforated metallic and glass tubes, filled with the ali- mentary substances, were introduced into the stomachs of ani- mals, and after the lapse of a certain time withdrawn, to ob- serve the condition of the contained substances ; but such ex- periments are fallacious, because gastric fluid has not ready access to the food. A better method was practiced in a series of experiments by Tiedemann and Gmelin, who fed dogs with DIGESTION OF FOOD IN THE STOMACH. 227 different substances, and killed them in a certain number of hours afterwards. But the results they obtained are of less interest than those of the experiments of Dr. Beaumont on his patient, St. Martin, and of Dr. Gosse, who had the power of vomiting at will. Dr. Beaumont’s observations show, that the process of di- gestion in the stomach, during health, takes place so rapidly, that a full meal, consisting of animal and vegetable substances, may nearly all be converted into chyme in about an hour, and the stomach left empty in two hours and a half. The details of two days’ experiments will be sufficient examples: Exp. 42.—April 7th, 8 a.m. St. Martin breakfasted on three hard-boiled eggs, pancakes, and coffee. At half-past eight o’clock, Dr. Beaumont examined the stomach, and found a heterogeneous mixture of the several articles slightly digested At a quarter past ten, no part of the break- fast remained in the stomach. Exp. 43.—At eleven o’clock the same day, he ate two roasted eggs and three ripe apples. In half an hour they were in an incipient state of digestion ; and a quarter past twelve no vestige of them remained. Exp. 44.—"At two o’clock p.m. the same day, he*dined on roasted pig and vegetables. At three o’clock they were half chymified, and at half-past four nothing remained but a very little gastric juice. Again, Exp. 46.—April 9th. At thi’ee o’clock p.m. he dined on boiled dried codfish, potatoes, parsnips, bread, and drawn butter. At half-past three o’clock examined, and took out a portion about half digested ; the potatoes the least so. The fish was broken down into small filaments; the bread and parsnips were not to be distinguished. At four o’clock, ex- amined another portion. Very few particles of fish remained entire. Some of the few potatoes were distinctly to be seen. At half-past four o’clock, he took out and examined another portion; all completely chymified. At five o’clock stomach empty. Many circumstances besides the nature of the food are apt to influence the process of chymification. Among them are, the quantity of food taken ; the stomach should be fairly filled, not distended : 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 the meal, gentle exercise being favorable, overexertion in- jurious to digestion; the state of mind—tranquillity of temper being apparently essential to a quick and due digestion : the bodily health : the state of the weather. But under ordinary 228 DIGESTION. circumstances, from three to four hours may be taken as the average time occupied by the digestion of a meal in the stom- ach. Dr. Beaumont constructed a table showing the times required for the digestion of all usual articles of food in St. Martin’s stomach, and in his gastric fluid taken from the stomach. Among the substances most quickly digested were rice and tripe, both of which were chymified in an hour; eggs, salmon, trout, apples, and venison, were digested in an hour and a half; tapioca, barley, milk, liver, fish, in two hours; turkey, lamb, potatoes, pig, in two hours and a half; beef and mutton re- quired from three hours to three and a half, and both were more digestible than veal; fowls were like mutton in their de- gree of digestibility. Animal substances were, in general, con- verted into chyme more rapidly than vegetables. Dr. Beaumont’s experiments were all made on ordinary arti- cles of food. A minuter examination of the changes produced by gastric digestion on various tissues has been made by Dr. Rawitz, who examined microscopically the product of the arti- ficial digestion of different kinds of food, and the contents of the faeces after eating the same kinds of food. The general results of his examinations, as regards animal food, show that muscular tissue breaks up into its constituent fasciculi, aud that these again are divided transversely; gradually the trans- verse striae become indistinct, and then disappear; and finally, the sarcolemma seems to be dissolved, and no trace of the tissue can be found in the chyme, except a few fragments of fibres. These changes ensue most rapidly in the flesh of fish and hares, less rapidly in that of poultry and other animals. The cells of cartilage and fibro-cartilage, except those of fish, pass unchanged through the stomach and intestines, and may be found in the fseces. The interstitial tissues of these structures are converted into pulpy textureless substances in the artificial digestive fluid, and are not discoverable in the fseces. Elastic fibres are un- changed in the digestive fluid. Fat-cells are sometimes found quite unaltered in the fseces; and crystals of cholesterin may usually be obtained from fseces, especially after the use of pork fat. As regards vegetable substances, Dr. Rawitz states, that he frequently found large quantities of cell-membranes unchanged in the fseces; also starch-cells, commonly deprived of only part of their contents. The green coloring principle, chlox*ophyll, was usually unchanged. The walls of the sap-vessels and spiral-vessels were quite unaltered by the digestive fluid, and were usually found in large quantities in the fseces; their con- tents, probably, were removed. DIGESTION IN THE STOMACH. 229 From these experiments, we may understand the structural changes which the chief alimentary substances undergo in their conversion into chyme; and the proportions of each which are not reducible to chyme, nor capable of any further act of di- gestion. The chemical changes undergone in and by the proxi- mate principles are less easily traced. Of the albuminous principles, some, as the casein of milk, are coagulated by the acid of the gastric fluid; and thus, be- fore they are digested, come into the condition of the other solid principles of the food. These, including solid albumen and fibrin, in the same proportion that they are broken up and anatomically disorganized by the gastric fluid, appear to be reduced or lowered in their chemical composition. This chemi- cal change is probaby produced, as suggested by Dr. Prout, by the principles entering into combination with water. It is suf- ficient to conceal nearly all their characteristic properties; the albumen is rendered scarcely coagulable by heat; the gelatin, even when its solution is evaporated, does not congeal in cool- ing; the fibrin and casein cannot be found by their character- istic tests. It would seem, indeed, that all these various sub- stances are converted into one and the same principle, a low form of albumen, not precipitable by nitric acid or heat, and now generally termed albuminose or 'peptone, from which, after being absorbed, they are again raised, in the elaboration of the blood, to which they are ultimately assimilated. The change of molecular constitution suffered by the albu- minous parts of the food, in consequence of the action of the gastric juice, has an important relation to their absorption by the bloodvessels of the stomach. From the condition of “ col- loids,” or substances, so named by Professor Graham, which are absorbed with extreme difficulty, they appear, from ex- periments of Funke, to assume to a great degree the char- acter of “ crystalloids,” which can pass through animal mem- branes with ease.1 . Whatever be the mode in which the gastric secretion affects these principles, it, or something like it, appears essen- tial, in order that they may be assimilated to the blood and tissues. For, when Bernard and Barreswil injected albumen dissolved in water into the jugular veins of dogs, they always in about three hours after, found it in the urine. But if, pre- vious to injection, it was mixed with gastric fluid, no trace of it could be detected in the urine. The influence of the liver seems to be almost as efficacious as that of the gastric fluid, in 1 These terms will be further explained and illustrated in the chapter on Absorption. 230 DIGESTION. rendering albumen assimilable; for Bernard found that, if diluted egg-albumen, unmixed with gastric fluid, is injected into the portal vein, it no longer makes its appearance in the urine, and is, therefore, no doubt, assimilated by the blood. Probably, most of the albuminose, with other soluble and fluid materials, is absorbed directly from the stomach by the minute bloodvessels with which the mucous membrane is so abundantly supplied. The saccharine including the amylaceous principles are at first, probably, only mechanically separated from the vege- table substances within which they are contained, by the action of the gastric fluid. The soluble portions, viz., dextrin and sugar, are probably at once absorbed. The insoluble ones, viz., starch and lignin (or some parts of them), are ren- dered soluble and capable of absorption, by being converted into dextrin or grape-sugar. It is probable that this change is carried on to some extent in the stomach ; but this conver- sion of starch into sugar is effected, not by the gastric fluid, but by the saliva introduced with the food, or subsequently swallowed. The transformation of starch is continued in the intestinal canal, as will be shown, by the secretion of the pan- creas, and perhaps by that of the intestinal glands and mu- cous membrane. The power of digesting uncooked starch is, however, very limited in man and Carnivora, for when starch has been taken raw, as in corn and rice, large quantities of the granules are passed unaltered with the excrements. Cook- ing, by expanding or bursting the envelopes of the granules, renders their interior more amenable to the action of the di- gestive organs; and the abundant nutriment furnished by bread, and the large proportion that is absorbed of the weight consumed, afford proof of the completeness of their power to make its starch soluble and prepare it for absorption. Of the oleaginous principles,—as to their changes in the stomach, no more can be said than that they appear to be reduced to minute particles, and pass into the intestines min- gled with the other constituents of the chyme. In the case of the solid fats, this effect is probably produced by the sol- vent action of the gastric juice on the areolar tissue, albumin- ous cell-walls, &c., which enter into their composition, and by the solution of which the true fat is able to mingle more uni- formly with the other constituents of the chyme. Being fur- ther changed in the intestinal canal, fat is rendered capable of absorption by the lacteals. MOVEMENTS OF THE STOMACH. 231 Movements of the Stomach. It has been already said, that 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 di- gestion, by grinding and triturating the hard seeds which con- stitute part of the food. But in the stomachs of man and 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 gizzard ; and the experiments of Reaumur and Spallanzani have demonstrated that substances inclosed 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: first, 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; secondly, to keep the orifices of the stomach closed until the food is digested ; and, thirdly, to perform certain peristaltic movements, whereby the food, as it becomes chymified, is gradually propelled towards, and ultimately through, the py- lorus. In accomplishing this latter end, the movements with- out doubt materially contribute towards 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 stom- ach, and immediately again closes. The pyloric orifice, during the first part of gastric digestion, is usually so com- pletely closed, that even when the stomach is separated from the intestines, none of its contents escape. But towards 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. From the observations of Dr. Beaumont on the man St. Martin, it appears that food, so soon as it enters the stomach, is subjected to a kind of peristaltic action of the muscular coat, whereby the digested portions are gradually approxi- 232 DIGESTION. mated towards 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 towards the pyloric end of the stomach seems to be more energetic and more de- cidedly peristaltic than those of the cardiac portion. Thus, Dr. Beaumont found that when the bulb of the thermometer was placed about three inches from the pylorus, it was tightly embraced from time to time and drawn towards the pyloric orifice for a distance of three or four inches. The object of this movement appears to be, as just said, to carry the food to- wards 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 ac- tion 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 portion forms a dilated sac. Sometimes, by a predominant action of strong circular fibres placed between the cardia and pylorus, the . two por- tions, or ends as they are called, of the stomach, are separated from each other by a kind of hour-glass contraction. The interesting researches of Dr. Brinton have clearly es- tablished that, by means of this peristaltic action of the mus- cular coats of the stomach, not merely is chymified food gradually propelled through the pylorus, but a kind of double current is continually kept up among the contents of the stom- ach, the circumferential parts of the mass being gradually moved onward towards the pylorus by the peristaltic contrac- tion of the muscular fibres, while the central portions are pro- pelled in the opposite direction, namely, towards the cardiac orifice; in this way is kept up a constant circulation of the contents of the viscus, highly conducive to their free mixture with the gastric fluid and to their ready digestion. These actions of the stomach are peculiar to it and indepen- dent. But it is, also, adapted to act in concert with the ab- dominal muscles, in certain circumstances which can hardly be called abnormal, as in vomiting and eructation. It has indeed 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 ca- pacity 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 MOVEMENTS OF THE STOMACH. 233 unresisting bag through the oesophagus; and that, under very abnormal circumstances, the stomach, by itself, cannot or rather does not 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 cer- tainty this action of the stomach in vomiting; but some of the cases of fistulous opening into the organ appear to support the belief that it does take place j1 and the analogy of the case of the stomach with that of the other hollow viscera, as the rec- tum and bladder, may be also cited in confirmation. Besides the influence which it may thus have by its contrac- tion, the stomach also essentially contributes to the act of vomiting, by the contraction of its pyloric orifice at the same time that the oblique fibres around the cardiac orifice are re- laxed. For, until the relaxation of these fibres, no vomiting can ensue; when contracted, they can as well resist all the force of the contracting abdominal and other muscles, as the muscles by which the glottis is closed can resist the same force in the act of straining. Doubtless we may refer many of the acts of retching and ineffectual attempts to vomit, to the want of concord between the relaxation of these muscles and the con- traction of the others. The muscles with which the stomach co-operates in contrac- tion during vomiting, are chiefly and primarily those of the abdomen; the diaphragm also acts, but not as the muscles of the abdominal walls do. They contract and compress the stomach more and more towards the back and upper parts of the diaphragm ; and the diaphragm (which is usually drawn down in the deep inspiration that precedes each act of vomit- ing) holds itself fixed in contraction, and presents an unyield- ing surface against which the stomach may be pressed. It is enabled to act thus, and probably only thus, because the in- spiration which precedes the act of vomiting is terminated by the closure of the glottis; after which the diaphragm can neither descend further, except by expanding the air in the lungs, nor, except by compressing the air, ascend again until, the act of vomiting having ceased, the glottis is opened again (see diagram, p. 181; see also p. 183). Some persons possess the power of vomiting at will, without applying any undue irritation ta the stomach, but simply by a voluntary effort. It seems also, that this power may be ac- 1 A colloction of cases of fistulous communication with the stomach, through the abdominal parietes, has been given by Dr. Murchison in vol. xli of the Medico-Chirurgical Transactions. 234 DIGESTION. quired by those who do not naturally possess it, and by con- tinual 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 re- gurgitate it, piece by piece, into their mouth, remasticate, and again swallow it, exactly as is done by the ruminant order of Mammalia. Influence of the Nervous System on Gastric Digestion. This influence is manifold; and is evidenced, 1st, in the sen- sations which induce to the taking of food ; 2d, in the secretion of the gastric fluid; 3d, in the movements of the food in and from the stomach. The sensation of hunger is manifested in consequence of de- ficiency of food in the system. The mind refers the sensation to the stomach; yet since the sensation is relieved by the in- troduction of food either into the stomach itself, or into the blood through other channels than the stomach, it would ap- pear not to depend on the state of the stomach alone. This view is confirmed by the fact, that the division of both pneu- mogastric nerves, which are the principal channels by which the mind is cognizant of the condition of the stomach, does not appear to allay the sensations of hunger. But that the stomach has some share in this sensation is proved by the relief afforded, though only temporarily, by the introduction of even non-alimentary substances into this organ. It may, therefore, be said that the sensation of hunger is de- rived from the system generally, but chiefly from the condition of the stomach, the nerves of which, we may suppose, are more affected by the state of the insufficiently replenished blood than those of other organs are. The sensation of thirst, indicating the want of fluid, is re- ferred to the fauces, although, as in hunger, this is merely the local declaration of a general condition existing in the system. For thirst is relieved for only a very short time by moistening the dry fauces; but may be relieved completely by the intro- duction of liquids into the blood, either through the stomach, or by injections into the bloodvessels, or by absorption from the surface of the skin or the intestines. The sensation of thirst is perceived most naturally whenever there is a dispro- portionately small quantity of water in the blood; as well, thereto re,, when water has been abstracted from the blood, as when saline or any solid matters have been abundantly added to it. We cau express the fact (even if it be not an explana- tion of it), by saying that the nerves of the mouth and fauces, INFLUENCE OF THE NERVOUS SYSTEM. 235 through which the sense of thirst is chiefly derived, are more sen- sitive to this condition of the blood than other nerves are. And the cases of hunger and thirst are not the only ones in which the mind derives, from certain organs, a peculiar predominant sensation of some condition affecting the whole body. Thus, the sensation of the “necessity of breathing,” is referred es- pecially to the lungs; but, as Volkmann’s experiments show, it depends on the condition of the blood which circulates every- where, and is felt even after the lungs of animals are removed ; for they continue, even then, to gasp and manifest the sensa- tion of want of breath. And, as with respiration when the lungs are removed, the mind may still feel the body’s want of breath; so in hunger and thirst, even when the stomach has been filled with innutritious substances, or the pneumogastric nerves have been divided, and the mouth and fauces are kept moist, the mind is still aware, by the more obscure sensations in other parts, of the whole body’s need of food and water. The influence of the nervous system on the secretion of gastric fluid, is shown plainly enough in the influence of the mind upon digestion in the stomach ; and is, in this regard, well illustrated by several of Dr. Beaumont’s observations. M. Bernard also, watching the act of gastric digestion in dogs which had fistulous openings into their stomachs, saw that on the instant of dividing their pneumogastric 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, however, and the like experiments showing the instant effect of division of the pneumogastric nerves, may prove no more than the effect of a severe shock, and the fact that influences affecting digestion may be conveyed to the stomach through those nerves. From other experiments it may be gathered, that although, as in M. Bernard’s, the division of both pneumogastric nerves always temporarily suspends the secretion of gastric fluid, and so arrests the process of digestion, and is occasionally followed by death from inanition ; yet the digestive powers of the stomach may be completely restored after the operation, and the formation of chyme and the nutri- tion of the animal may be carried on almost as perfectly as in health. In thirty experiments on Mammalia, which M. Wernscheidt performed under Muller’s direction, not the least difference could be perceived in the action of narcotic poisons introduced into the stomach, whether the pneumogastric had been divided on both sides or not, provided the animals were of the same species and size. It appears, however, that such poisons as are capable of being rendered inert by the action of the gastric 236 DIGESTION. fluid, may, if taken into the stomach shortly after division of both pneumogastric nerves, produce their poisonous effects; in consequence, apparently, of the temporary suspension of the secretion of gastric fluid. Thus, in one of his experiments, M. Bernard gave to each of two dogs, in one of which he had di- vided the pneumogastric nerves, a dose of emulsin, and half an hour afterwards a dose of amygdalin, substances which are innocent alone, but when mixed produce hydrocyanic acid. The dog whose nerves were cut, died in a quarter of an hour, the substances being absorbed unaltered and mixing in the blood; in the other, the emulsin was decomposed by the gas- tric fluid before the amygdalin was administered; therefore, hydrocyanic acid was not formed in the blood, and the dog survived. The influence of the pneumogastric nerves over the secretion of gastric fluid has been of late even more decidedly shown by M. Bernard, who 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 nervous system on the movements of the stomach has been often seen in the retardation or arrest of these movements after division of the pneumogastric nerves. The results of irritating the same nerves were ambiguous; but the experiments of Longet and Bischoff have shown that the dif- ferent results depended on whether the stomach were digesting or not at the time of the experiment. In the act of digestion, the nervous system of the stomach appears to participate in the excitement which prevails through the rest of its organiza- tion, and a stimulus applied to the pneumogastric nerves is felt intensely, and active movements of the muscular fibres of the stomach follow; but in the inaction of fasting, the same stimu- lus produces no effect. So, while the stomach is digesting, the pylorus is too irritable to allow anything but chyme to pass; but when digestion is ended, the undigested parts of the food, and even large bodies, coins, and the like, may pass through it. Digestion of the Stomach after Death. If an animal die during 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 fre- quently themselves acted on by their own secretion, and to such an extent, that a perforation of considerable size may be produced, and the contents of the stomach may in part escape POST-MORTEM DIGESTION. 237 into the cavity of the abdomen. This phenomenon is not un- frequently observed in post-mortem examinations of the human body; but, as Dr. Pavy observes, the effect may be rendered, by experiment, more strikingly manifest. “If, for instance,” he remarks, “an animal, as a rabbit, be killed at a period of digestion, and afterwards exposed to artificial warmth to pre- vent its temperature from falling, not only the stomach, but many of the surrounding parts will be found to have been dis- solved. With a rabbit killed in the evening, and placed in a warm situation (100° to 110° Fahr.) during the night, I have seen in the morning, the stomach, diaphragm, part of the liver and lungs, and the intercostal muscles of the side upon which the animal was laid all digested away, with the muscles and skin of the neck and upper extremity on the same side also in a semi-digested state.” 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? John Hunter, who particularly drew attention to the phenomena of post-mortem digestion, explained the immunity from injury of the living stomach, by referring it to the pro- tective influence of the “ vital principle.” But this dictum has been called in question by subsequent observers. It is, indeed, rather a statement of a fact, than an explanation of its cause. It must be confessed, however, that no entirely satisfactory theory has been yet stated as a substitute. It is only necessary to refer to the idea of Bernard, that the living stomach finds protection from its secretion in the pres- ence of epithelium and mucus, which are constantly renewed in the same degree that they are constantly dissolved, in order to remark that this theory has been disproved by experiments of Pavy’s, in which the mucous membrane of the stomachs of dogs was dissected off for a small space, and, on killing the animals some days afterwards, no sign of digestion of the stomach was visible. “ Upon one occasion, after removing the mucous 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 contracted to much less than its original dimensions.” Dr. Pavy believes that the natural alkalinity of the blood, which circulates so freely during life in the walls of the stom- 238 DIGESTION. ach, is sufficient to neutralize the acidity of the gastric juice, were it, so to speak, to make an attempt at digesting parts with which it has no business; and as may be gathered from what has been previously said (p. 228), the neutralization of the acidity of the gastric secretion is quite sufficient to destroy its digestive powers. He also very ingeniously argues that this very alkalinity must, from the conditions of the circula- tion naturally existing in the walls of the stomach, be in- creased in proportion to the need of its protective influence. “ In the arrangement of the vascular supply,” he remarks, “ a doubly effective barrier is, as it were, provided. The vessels pass from below upwards towards the surface: capillaries having this direction ramify between the tubules by which the acid of the gastric juice is secreted ; and being separated by secretion below, must leave the blood that is proceeding upwards correspondingly increased in alkalinity; and thus, at the period when the largest amount of acid is flowing into the stomach, and the greatest protection is required, then is the provision afforded in its highest state of efficiency.” Dr. Pavy’s theory is the best and most ingenious hitherto framed in connection with this subject; but the experiments adduced in its favor are open to many objections, and afford only a negative support to the conclusions they are intended to prove. The matter, therefore, can scarcely be considered finally settled. The intestinal canal is divided into two chief portions, named, from their differences in diameter, the small and large intestine. These are continuous with each other, and com- municate by means of an opening guarded by a valve, the ileo-caecal valve, which allows the passage of the products of digestion from the small into the large bowel, but not, under ordinary circumstances, in the opposite direction. The structure and functions of each organ or tissue con- cerned in intestinal digestion will be first described in detail, and afterwards a summary will be given of the changes which the food undergoes in its passage through the intestines, 1st, from the pylorus to the ileo-caecal valve; and, 2d, from the ileo-caecal valve to the anus. DIGESTION IN THE INTESTINES. Structure and Secretions of the Small Intestine. The small intestine, the average length of which in an adult is about twenty feet, has been divided, for convenience DIGESTION IN THE INTESTINES. 239 of' description, into three portions, viz., the duodenum, which extends for eight or ten inches beyond the pylorus ; the jeju- num, which occupies two-fifths, and the ileum, which occupies three-fifths of the rest of the canal. The small intestine, like the stomach, is constructed of three principal coats, viz., the serous, muscular, and mucous. The serous coat, formed by the visceral layer of the peritoneum, need not be here specially described. The fibres of the mus- cular coat of the small intestine are arranged in two layers; those of the outer layer being disposed longitudinally ; those of the inner layer transversely, or in portions of circles encom- passing the canal. They are composed of the unstriped kind of muscular fibre. Between the mucous and muscular coats, there is a layer of submucous tissue, in which numerous bloodvessels and a rich plexus of nerves and ganglia are imbedded (Meissner). The mucous membrane is the most important coat in relation to the function of digestion. The following structures which enter into the composition of the mucous membrane may be now successively described : the valvulce conniventes; the villi; and the glands. The general structure of the mucous mem- brane of the intestines resembles that of the stomach (p. 215), and, like it, is lined on its inner surface by columnar epithe- lium. Lymphoid or Retiform tissue (Fig. 72) enters largely Fig. 72. The figure represents a cross-section of a small fragment of the mucous mem- brane, including one entire crypt of Lieberkuhn and parts of several others; a, cavity of the tubular glands or crypts ; b, one of the lining epithelial cells ; c, the- lymphoid or retiform spaces, of which some are empty, and others occupied by lymph cells, as at d. into its construction; and on its deep surface is a layer of the muscu,laris mucosce (p. 216). 240 DIGESTION. Valvulce Conniventes. The valvulse conniventes commence 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 the jejunum; then, gradually diminishing in size and number, they cease near the middle of the ileum. In structure they are formed by a doubling inwards of the mu- cous membrane, the crescentic, nearly circular, folds thus formed being arranged transversely with regard to the axis of the intestine, and each individual fold seldom extending around more than I or f of the bowel’s circumference. Un- like the rugse in the stomach, they do not disappear on dis- tension. 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 understand them aright, a piece of gut should be distended either with air or alcohol, and not opened until the tissues have become hardened. On then making a section, it may be seen that instead of disap- pearing, as the rugae in the stomach would under similar circumstances, they stand out at right angles to the general surface of the mucous membrane (Fig. 73). Their functions are probably these: Besides (1) offering a largely increased surface for secretion and ab- sorption, they probably (2) prevent the too rapid passage of the very liquid products of gastric digestion, immedi- ately after their escape from the stom- ach, and (3), by their projection, and consequent interference with a uni- form and untroubled current of the intestinal contents, probably assist in the more perfect mingling of the latter with the secretions poured out to act on them. Glands of the Small Intestine.—The glands are of three prin- cipal kinds, named after their describers, the glands of Lieber- kiihn, of Peyer, and of Brunn. The glands or follicles of LieherJdihn are simple tubular depressions of the intestinal Fig. 73. Piece of small intestine (previously distended and hardened by alcohol) laid open to show the normal position of the valvulse con- niventes. peyee’s glands. 241 mucous membrane, thickly distributed over the whole surface 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 mi- nute 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 intestinal tube; and in the rectum their orifices may be visible to the naked eye. In length they vary from to of a line. Each tubule (Fig. 74) is constructed of the same es- sential parts as the intestinal mucous mem- brane, viz., a fine structureless membrana pro- pria, or basement-membrane, a layer of cylin- drical epithelium lining it, and capillary blood- vessels covering its exterior. 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. At the bottom of the follicle, the contents usually consist of a granular material, in which a few cytoblasts or nuclei are imbedded; these cytoblasts, as they ascend towards the surface, are supposed to be gradually developed into nucleated cells, some of which are discharged into the intestinal cavity. The purpose served by the material secreted by these glands is still doubtful. Their large number and the extent of surface occupied by them, seem, however, to indicate that they are concerned in other and higher offices than the mere production of fluid to moisten the surface of the mucous membrane, although, doubtless, this is one of their functions. The glands of Peyer occur exclusively in the small intestine. They are found in greatest abundance in the lower part of the ileum near to the ileo-caecal valve. They are met with in two conditions, viz., either scattered singly, in which case they are termed glandules solitaries, 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 glandidce agminates, the groups being commonly called Peyer's patches (Fig. 75). The latter are placed almost always opposite the attachment of the mesentery. In structure, and probably in function, there is no essential difference between the solitary glands and the individual bodies of which each group or patch is made up; but the surface of the solitary glands (Fig. 76) is beset with villi, from which those forming the agminate Fig 74. A gland of Liebcrkubn. 242 DIGESTION. patches (Fig. 77) are usually free. In the condition in which they have been most commonly examined, each gland appears as a circular opacpie-white sacculus, from half a line to a line Fig. 75. Agminate follicles, or Peyer’s patch, in a state of distension : magnified about 5 diameters (after Boehm). in diameter, and, according to the degree in which it is de- veloped, either sunk beneath, or more or less prominently raised on, the surface of a depression or fossa in the mucous Fig. 77. Fig.76 Fig. 76.—Solitary glaud of small intestine (after Boelim). Fig. 77.—Part of a patch of the so-called Peyer’s glands magnified, showing the various forms of the sacculi, with their zone of foramina. The rest of the mem- brane marked with Lieberkiihn’s follicles, and sprinkled with villi (after Boehm). peyer’s CtLAiXDS. 243 membrane. Each gland is surrounded by openings like those of Lieberkiihn’s follicles (see Fig. 77) except that they are more elongated; and the direction of the long diameter of each opening is such that the whole produce a radiated ap- pearance around the white sacculus. These openings appear to belong to tubules identical with Lieberkiihn’s follicles: they have no communication with the sacculus, and none of its contents escape through them on pressure. Neither can any permanent opening be detected in the sacculus or Peyer’s gland itself (see Fig. 78). Each gland is an imperfectly closed sac or follicle formed of a tolerably firm membranous capsule of fine connective tissue, imbedded in a rich plexus of minute bloodvessels, many fine branches from which pass through the capsule and enter, chiefly loopwise, the interior of the follicle (Fig. 79). Entering into the formation of the sacculus, moreover, and forming a stroma, or supporting framework throughout its in- terior, is lymphoid or adenoid tissue (Fig. 72), continuous with that which forms a great part of the mucous membrane out- side of it. The contents of each sac consist of a pale grayish Fig. 78. Side view of a portion of intestinal mucous membrane of a cat, showing a Peyer’s gland (a): it is imbedded in the submucous tissue (/), the line of separation between which and the mucous membrane passes across the gland ; 6, one of the tubular fol- licles, the orifices of which form the zone of openings around the gland ; c, the fossa in the mucous membrane ; d, villi; e, follicles of Lieberkuhn (after Bendz). opalescent pulp, formed of albuminous and fatty matter, and a multitude of nucleated corpuscles of various sizes, resembling exactly those found in lymphatic glands. The real office of these Peyerian glands or follicles is still unknown. It was formerly believed that each follicle was a 244 DIGESTION. kind of secreting cell, which, when its contents were fully ma- tured, formed a communication with the cavity of the intes- tine by the absorption or bursting of its own cell-wall, and of the portion of mucous membrane over it, and thus discharged its secretion into the intestinal canal. A small shallow cavity or space was thought to remain, for a time, after this absorp- tion or dehiscence, but shortly to disappear, together with all trace of the previous gland. More recent acquaintance with the real structure of these bodies seems, however, to prove that they are not mere tempo- rary gland-cells which thus discharge their elaborated con- tents into the intestine and then disappear, but that they are rather to be regarded as structures analogous to lymphatic or Fig. 79. Transverse section of injected Peyer’s glands (from Kdlliker). The drawing was taken from a preparation made by Frey: it represents the fine capillary looped net- work spreading from the surrounding bloodvessels into the interior of three of Peyer’s capsules from the intestine of the rabbit. absorbent glands, and that their office is to take up certain materials from the chyle, elaborate and subsequently discharge them into the lacteals, with which vessels they appear to be closely connected, although no direct communication has been proved to exist between them. Moreover, it has been lately suggested that since the mo- brunn’s glands. 245 lecular and cellular contents of the glands are so abundantly traversed by minute bloodvessels, important changes may mu- tually take place between these contents and the blood in the vessels, material being abstracted from the latter, elaborated by the cells, and then restored to the blood, much in the same manner as is believed to be the case in the so-called vascular glands, such as the spleen, thymus, and others; and that thus Peyer’s glands should also be regarded as closely analogous to these vascular glands. Possibly they may combine the func- tions both of lymphatic and vascular glands, absorbing and elaborating material both from the chyle and from the blood within their minute vessels, and transmitting part to the lac- teal system and part direct to the blood. Brunn’s glands (Fig. 80) are confined to the duodenum; they are most abundant and thickly set at the commencement Fig. 80. Enlarged view of one of Brunn’s glands from the human duodenum (from Frey). The main duct is seen superiorly; its branches are elsewhere hidden by the bunches of opaque glandular vesicles. of this portion of the intestine, diminishing gradually as the duodenum advances. Situated beneath the mucous membrane, and imbedded in the submucous tissue, they are minutely lobu- lated bodies, visible to the naked eye, like detached small por- tions of pancreas, and provided with permanent gland-ducts, which pass through the mucous membrane and open on the internal surface of the intestine. As in structure, so probably in function, they resemble the pancreas; or at least stand to it in a similar relation to that which the small labial and buccal 246 DIGESTION. glands occupy in relation to the larger salivary glands, the parotid, and submaxillary. The Villi (Figs. 81, 82) are confined exclusively to the mu- cous membrane of the small intestine. They are minute vas- cular 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. Krauss esti- Fig. 81. (Slightly altered from Teichmann.) a. Villus of sheep, b. Villi of man. mates them at fifty to ninety in number in a square line, at the upper part of the small intestine, and at forty to seventy in the same area at the lower part. They vary in form even in the same animal, and differ according as the lymphatic ves- sels they contain are empty or full of chyle; being usually, in the former case, flat and pointed at their summits, in the latter cylindrical or clavate. Each villus consists of a small projection of mucous mem- brane, and its interior is therefore supported throughout by THE VILLI. 247 fine retiform or adenoid tissue, which forms the framework or stroma in which the other constituents are contained. The surface of the villus is clothed by columnar epithelium, which rests on a fine basement-membrane; while within this Fig. 82. (From Teichmann.) a, lacteals in villi, p, Peyer’s glands, b and D, superficial and deep network of lacteals in submucous tissue, l, Lieberkiihn’s glands, e, small branch of lacteal vessel on its way to mesenteric gland, h and o, muscular fibres of intestine, s, peritoneum. are found, reckoning from without inwards, bloodvessels, fibres of the muscularis mucosa}, and a single lymphatic or lacteal 248 DIGESTION. vessel rarely looped or branched (Fig. 81); besides granular matter, fat-globules, &c. 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 sur- face of the villus (Fig. 81), and their smaller ends resting on the basement-membrane. Some doubt exists concerning the minute structure of these cells, and their relation to the deeper parts of the villus. Beneath the basement or limiting membrane there is a rich supply of bloodvessels. Two or more minute arteries are dis- tributed 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 mucosae in the villus forms a kind of thin hollow cone immediately around the central lacteal, and, is therefore, situate beneath the bloodvessels. The ad- dition of acetic acid to the villus brings out the characteristic nuclei of the muscular fibres, and shows the size and position of the layer most distinctly. Its use is still unknown, although it is impossible to resist the belief, that it is 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. 81.), 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 com- mon in some of the lower animals (a, Fig. 81). The office of the villi is the absorption of chyle from the completely digested food in the intestine. The mode in which they effect this will be considered in the chapter on Absorp- tion. The large intestine, which in an adult is from about 4 to 6 feet long, is subdivided for descriptive purposes into three portions, viz.: the caecum, a short wide pouch, communicating with the lower end of the small intestine through an opening, guarded by the ileo-ccecal valve ; the coion, 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, which, after dilating at its lower part, again contracts, and immediately afterwards opens externally Structure of the Large Intestine. THE LARGE INTESTINE. 249 through the anus. Attached to the caecum is the small appen- dix vermiformis. Like the small intestine, the large is constructed of three principal coats, viz., the serous, muscular, and mucous. The serous coat need not be here particularly described. Connected with it are the small processes of peritoneum containing fat, called appendices epiploicce. The fibres of the muscular coat, like those of the small intestine, are arranged in two layers— the outer longitudinally, the inner circularly. 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, are collected, for the most part, into three strong bands, which being shorter, from end to end, than the other coats of the in- testine, hold the canal in folds, bounding intermediate sacculi. On the division of these bands, the intestine can be drawn out to its full length, and it then assumes, of course, a uniformly cylindrical form. In the rectum, the fasciculi of these longi- tudinal bands spread out and mingle with the other longitudi- nal fibres, forming with them a thicker layer of fibres than exists on any other part of the intestinal canal. The circular muscular fibres are spread over the whole surface of the bowel, but are somewhat more marked in the intervals between the sacculi. Towards 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 pro- jected in the form of valvulte conniventes. Its general micro- scopic structure resembles that of the small intestine. Glands of the Large Intestine.—The glands with which the large intestine is provided are of two kinds, the tubxdar and lenticular. 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. The lenticular glands are most numerous in the csecum and vermiform appendix. They resemble in shape and structure, almost exactly, the solitary glands of the small intestine, and, like them, have no opening. Just over them, however, there is commonly a small depression in the mucous membrane, which has led to the erroneous belief that some of them open on the surface. The functions discharged by the glands found in the large intestine are not known with any certainty, but there is no 250 DIGESTION. reason to doubt that they resemble very nearly those discharged by the glands of like structure in the small intestine. The difficulty of determining the function of any single set of the intestinal glands seems indeed almost insuperable, so many fluids being discharged together into the intestine; for all acting, probably, at once, produce a combined effect upon the food, so that it is almost impossible to discern the share of any one of them in digestion. Ileo-ccecal valve.—The ileo-csecal valve is situate at the place of junction of the small with the large intestine, and guards against any reflux 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 inwards of the mucous mem- brane, and is strengthened on the outside by some of the circu- lar muscular fibres of the intestine, which are contained be- tween the outer surfaces of the two layers of which each fold is composed. The inner surface of the folds is smooth ; the mucous membrane of the ileum being continuous with that of the caecum. That surface of each fold which looks towards the small intestine is covered with villi, while that which looks to the caecum has none. When the caecum is distended, the margins of the folds are stretched, and thus are brought into firm apposition one with the other. While the circular muscular fibrfes of the bowel at the junc- tion of the ileum with the caecum are contained between the outer opposed surfaces of the folds of mucous membrane which form the valve, the longitudinal muscular fibres and the peri- toneum of the small and large intestine respectively are con- tinuous with each other, without dipping in to follow the cir- cular fibres and the mucous membrane. In this manner, therefore, the folding inwards of these two last-named structures is preserved, while on the other hand, by dividing the longi- tudinal muscular 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 Pancreas, and its Secretion. The pancreas is situated within the curve formed by the duodenum; and its main duct opens into that part of the in- testine, either through a small opening or through a duct com- mon to itself and to the liver. The pancreas, in its minute anatomy, closely resembles the salivary glands; and the fluid elaborated by it appears almost identical with saliva. When obtained pure, in all the different animals in which it has been THE PANCREATIC SECRETION. 251 hitherto examined, it has been found colorless, transparent, and slightly viscid. It is alkaline when fresh, and contains a pecu- liar animal matter named pancreatin and certain salts, both of which are very similar to those found in saliva. In pancreatic secretion, however, there is no sulpho-cyanogen. Pancreatin is a substance coagulable by heat, and in many other respects very like albumen : to it the peculiar digestive power of the pancreatic secretion is probably due. Like saliva, the pan- creatic fluid, shortly after its escape, becomes neutral and then acid. The following is the mean of three analyses by Schmidt: Water, ......... 980.45 Solids, 19.55 Pancrealin, . . . . . . 12 71 Inorganic bases and salts, .... 6.84 19.55 Composition of Pancreatic Secretion. The functions of the pancreas are probably as follows: 1. Numerous experiments have shown, that starch is acted upon by the pancreatic secretion, or by portions of pancreas put in starch-paste, in the same manner that it is by saliva and portions of the salivary glands. And although, as before stated (p. 212), many substances besides those glands can ex- cite the transformation of starch into dextrin and grape-sugar, yet it appears probable that the pancreatic fluid, exercising this power of transformation, is largely subservient to the pur- pose of digesting starch. MM. Bouchardat and Sandras have shown that the raw starch-granules which have passed un- changed through the crops and gizzards of granivorous birds, or through the stomachs of herbivorous Mammalia, are, in the small intestine, disorganized, eroded, and finally dissolved, as they are when exposed, in experiment, to the action of the pancreatic fluid. The bile cannot effect such a change in starch; and it is most probable that the pancreatic secretion is the principal agent in the transformation, though it is by no means clear that the office may not be shared by the secretion of the intestinal mucous membrane, which also seems to possess the power of converting starch into sugar. 2. The existence of a pancreas in Carnivora, which have little or no starch in their food, and the results of various ob- servations and experiments, leave very little doubt that the pancreatic secretion also assists largely in the digestion of 252 DIGESTION. fatty matters, by transforming them into a kind of emulsion, and thus rendering them capable of absorption by the lacteals. Several eases have been recorded in which the pancreatic duct being obstructed, so that the secretion could not be discharged, fatty or oily matter was abundantly discharged from the in- testines. In nearly all these cases, indeed, the liver was coin- cidently diseased, and the change or absence of the bile might appear to contribute to the result; yet the frequency of exten- sive disease of the liver, unaccompanied by fatty discharges from the intestines, favors the view that, in these cases, it is to the absence of the pancreatic fluid from the intestines that the excretion or non-absorption of fatty matter should be ascribed. In Bernard’s experiments too, fat always appeared in the evacuations when the pancreas was destroyed or its duct tied. Bernard, indeed, is of opinion that to emulsify fat is the ex- press office of the pancreas, and the evidence that he and ofehess have brought forward in support of this view is very weighty. The power of emulsifying fat, however, although perhaps mainly exercised by the secretion of the pancreas, is evidently possessed to some extent by other secretions poured into the intestines, and especially by the bile. 3. The pancreatic secretion discharges a third function also, namely, that of dissolving albuminous substances ; the peptone produced by the action of the pancreatic secretion on proteids not differing essentially from that formed by the action of the gastric juice (see p. 229). Structure of the Liver. The liver is an extremely vascular organ, and receives its supply of blood from two distinct vessels, the portal vein and hepatic artery, while the blood is returned from it into the vena cava inferior by the hepatic vein. Its secretion, the bile, is conveyed from it by the hepatic duct, either directly into the intestine, or, when digestion is not going on, into the cystic duct, and thence into the gall-bladder, where it accumulates until required. The portal vein, hepatic artery, and hepatic duct branch together throughout the liver, while the hepatic vein and its tributaries run by themselves. On the outside the liver has an incomplete covering of peri- toneum, and beneath this is a very fine coat of areolar tissue, continuous over the whole surface of the organ. It is thickest where the peritoneum is absent, and is continuous on the general surface of the liver with the fine, and, in the human subject, almost imperceptible, areolar tissue investing the STRUCTURE OF THE LIVER. 253 lobules. At the transverse fissure it is merged in the areolar investment called Glisson’s capsule, which surrounding the Fig. 83. Under surface of the liver (from Bonamy). R, right lobe; L, left lobe; Q, lobus quadratus ; S, lobus Spigelii; C, lobus caudatus ; 1, umbilical vein in longitudinal fissure; 2, gall-bladder in its fissure; 3, hepfttic ar- tery in transverse fissure; 4, hepatic duct in transverse fissure; 5, portal vein in transverse fissure; 6, line of reflexion of peritoneum; 7, vena cava; 8, obliterated ductus venosus ; 9, ductus communis choledochus. portal vein, hepatic artery, and hepatic duct, as they enter at this part, accompanies them in their branchings through the sub- stance of the liver. The liver is made up of small roundish or oval portions called lobules, each of which is about of an inch in diameter, and com- posed of the minute branches of the portal vein, hepatic artery, he- patic duct, and hepatic vein ; while the interstices of these vessels are filled by the liver cells. These cells (Fig. 84), which make up a great portion of the substance of the organ, are rounded or polygonal, from about g-J0 to 0 of an inch in diameter, containing well-marked nuclei and granules, and having sometimes a yellowish tinge, especially about their nuclei; fre- quently, they contain also various sized particles of fat (Fig. 84 a). Each lobule is very sparingly invested by areolar tis- sue. To understand the distribution of the bloodvessels in the 254 DIGESTION. liver, it will be well to trace, first, the two bloodvessels and the duct which enter the organ on the under surface at the transverse fissure, viz., the portal vein, hepatic artery, and he- patic duct. As before remarked, all three run in company, and their appearance on longitudinal section is shown in Fig. 85. Running together through the substance of the liver, Fig. 8o. Longitudinal section of a portal canal, containing a portal vein, hepatic artery, and hepatic duct, from the pig (after Kiernan) y. p, branch of vena portee, situated in a portal canal, formed amongst the lobules of the liver, and giving off vaginal branches; there are also seen within the large portal vein numerous orifices of the smallest interlobular veins arising directly from it; a, hepatic artery ; d, hepatic duct. they are contained in small channels, called portal canals, their immediate investment being a sheath of areolar tissue, called Glisson’s capsule. To take the distribution of the portal vein first: In its course through the liver this vessel gives off small branches, which divide and subdivide between the lobules surrounding them and limiting them, and from this circumstance called inter- lobular veins. From these small vessels a dense capillary net- work is prolonged into the substance of the lobule, and this network gradually gathering itself up, so to speak, into larger vessels, converges finally to a single small vein, occupying the centre of the lobule, and hence called mtralobular. This ar- STRUCTURE OF THE LIVER. 255 rangement is well seen in Fig. 86, which represents a trans- verse section of a lobule. The smaller branches of the portal vein being closely surrounded by the lobules, give off directly Kig. 86. Cross-section of a lobule of the human liver, in which the capillary network be- tween the portal and hepatic veins has been fully injected (from Sappey) 6^0_ i. Section of the intralobular vein; 2, its smaller branches collecting blood from the capillary network; 3, interlobular branches of the vena portae with their smaller ramifications passing inwards towards the capillary network in the substance of the lobule. interlobular veins (see Fig. 85) ; but here and there, especially where the hepatic artery and duct intervene, branches called vaginal first arise, and breaking up in the sheath are subse- quently distributed like the others around the lobules and be- come interlobular. The larger trunks of the portal vein being more separated from the lobules by a thicker sheath of Glisson’s capsule, give off vaginal branches alone, which, however, after breaking up in the sheath, are distributed like the others be- tween the lobules, and become interlobular veins. The small intralobular veins discharge their contents into veins called snidobular (Fig. 88), while these again, by their union, form the main branches of the hepatic vein, which leaves the posterior border of the liver to end by two or three prin- cipal trunks in the inferior vena cava, just before its passage through the diaphragm. The snidobular and hepatic veins, unlike the portal vein and its companions, have little or no areolar tissue around them, and their coats being very thin, 256 DIGESTION. they form little more than mere channels in the liver sub stance which closely surrounds them. Fig. 87. Section of a portion of liver passing longitudinally through a considerable hepatic vein, from the pig (after Kiernan) 5.. H, hepatic.venous trunk, against which the sides of the lobules are applied; 6, sublobular hepatic veins, on which the bases of the lobules rest, and through the coats of which they are seen as polygonal figures ; a, a, walls of the hepatic venous canal, formed by the polygonal bases of the lobules. The manner in which the lobules are connected with the sublobular veins by means of the small intralobular veins is well seen in the diagram, Fig. 88, and in Fig. 87, which rep- resent the parts as seen in a longitudinal section. The ap- pearance 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 intralobular veins is that of 1, Fig. 86, while both a transverse and longitudinal section are exhibited in Fig. 89. The hepatic artery, the function of which is to distribute blood for nutrition to Glisson’s capsule, the walls of the ducts and bloodvessels, and other parts of the liver, is distributed in a very similar manner to the portal vein, its blood being re- turned by small branches either into the ramifications of the STRUCTURE OF THE LIVER. 257 portal vein, or into the capillary plexus of the lobules which connects the inter- and mtralobular veins. 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 small polygonal epi- thelium. The exact arrangement of its terminal branches, however, and their relation to the liver-cells have not been clearly made out, or, at least, have not been agreed upon by different observers. The chief theories on the subject are three in number: 1. That the terminal branches of the hepatic duct form an inter- lobular network, which abuts on the outermost cells of a lobule, but does not enter the inside of the lob- ule, or only for a little way. 2. That minute branches begin in the lobules between the cells, not inclosing them. 3. That the ultimate branches begin in the lobules and in- close hepatic cells. Fig. 88. Diagram showing the manner in which the lobules of the liver rest on the sublobular veins (after Kier- nan). Fig. 89. Capillary network of the lobules of the rabbit’s liver (from IColliker) The figure is taken from a very successful injection of the hepatic veins, made hy Hurt- ing : it shows nearly the whole of two lobules, and parts of three others; p, portal branches running in the interlobular spaces; A, hepatic veins penetrating and radi- ating from the centre of the lobules. 258 DIGESTION. The illustrations below’ will show the conflicting theories at a glance. Fio. 90. Diagrams showing the arrangement of the radicles of the hepatic duct, according to different observers. 1. 2, 2, are two branches of the hepatic duct, which is supposed to commence in a plexus situated towards the circumference of the lobule marked 4, 4, called by Kier- nan the biliary plexus. Within this is seen the central part of the lobule, contain- ing branches of the intralobular vein, 1,1. 2. A small fragment of an hepatic lobule, of which the smallest intercellular bili- ary ducts were filled with coloring matter during life, highly magnified (from Chrzonszczewsky). 3. View of some of the smallest biliary ducts illustrating Beale’s view of their relation to the biliary cells (from Kolliker after Beale), £ tJL. The drawing is taken from an injected preparation of the pig’s liver; a, small branch of an interlobular hepatic duct; c, smallest biliary ducts ; 6, portions of the cellular part of the lobule in which the cells are seen within tubes which eommu- nicate with the finest ducts. THE BILE. 259 Functions of the Liver. The Secretioyi of Bile is the most obvious, and one of the chief functions which the liver has to perform; but, as will be presently shown, it is not the only one; for important changes are effected in certain constituents of the blood in its transit through this gland, whereby they are rendered more fit for their subsequent purposes in the animal economy. The Bile. Coynposition of the Bile.—The bile is a somewhat viscid fluid, of a yellow or greenish-yellow color, a strongly bitter taste, and when fresh with a scarcely perceptible odor; it has a neutral or slightly alkaline reaction, and its specific gravity is about 1020. Its color and degree of consistence 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 absorp- tion of its water, but partly also from being mixed with mucus. The following analysis is by Frerichs: Composition of Human Bile. Water, ........ 859.2 Solids, ........ 140.8 1000.0 Biliary acids combined 1 m r with alkalies, } B,1,n’ • • • • 91.6 Fat, ......... 9.2 Cholesterin, ........ 2.6 Mucus and coloring matters, .... 29.8 Salts, ......... 7.7 140.8 The bilin or biliary matter when freed by ether from the fat with which it is combined, is a resinoid substance, soluble in water, alcohol, and alkaline solutions, and giving to the watery solution the taste and general character of bile. It is a com- pound of soda, with two resinous acids, named glycocholic and taurocholic acids. The former consists of cholic acid conjugated with glycin (or sugar of gelatin), the latter of the same acid conjugated with taurin. Fatty substances are found in variable proportions. Besides 260 DIGESTION. the ordinary saponifiable fats, there is a, small quantity of cholesterin (p. 20), which, with the other free fats, is probably held in solution by the tauro-cholate of soda. A peculiar substance, which Dr. Flint has discovered in the faeces, and named stereorin (p. 274), is closely allied to choles- terin ; and Dr. Flint believes that while one great function of the liver is to excrete cholesterin from the blood, as the kidney excretes urea, the stereorin of faeces is the modified form in which cholesterin finally leaves the body. Ten grains and a half of stereorin, he reckons, are excreted daily. The coloring matter of the bile has not yet been obtained pure, owing to the facility with which it is decomposed. It occasionally deposits itself in the gall-bladder as a yellow substance mixed with mucus, and in this state has been frequently examined. It is composed of two coloring matters, called biliverdin and bilifulvin. By oxidizing agencies, as exposure to the air, or the addition of nitric acid, it assumes a dark green color. In cases of biliary obstruction, it is often reabsorbed, circulates with the blood, and gives to the tissues the yellow tint charac- teristic of jaundice. There seems to be some relationship between the coloring matters of the blood and bile, and, it may be added, between these and that of the urine also, so that it is possible they may be, all of them, varieties of the same pigment, or derived from the same source. Nothing, however, is at present certainly known regarding the relation in which one of them stands to the other. The mucus in bile is derived chiefly from the mucous mem- brane of the gall-bladder, but in part also from the hepatic ducts and their branches. 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 this mucus is probably to be ascribed the rapid decomposition undergone by the bilin ; for, according to Berzelius, if the mucus be separated, bile will remain unchanged for many days. The saline or inorganic constituents of the bile are similar to Fig. 91. Crystalline scales of eholesterin. THE BILE. 261 those 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 gen- erally found in healthy bile, and constantly in biliary calculi. Such are the principal chemical constituents of bile ; but its physiology is, perhaps, better illustrated by its ultimate ele- mentary composition. According to Liebig’s analysis, the biliary matter,—consisting of bilin and the products of its spontaneous decomposition—yields, on analysis, 76 atoms of carbon, 66 of hydrogen, 22 of oxygen, 2 of nitrogen, and a cer- tain quantity of sulphur.1 Comparing this with the ultimate composition of the organic parts of blood, which may be stated at C4gH30NBO14, with sulphur and phosphorus—it is evident that bile contains a large preponderance of carbon and hydrogen, and a deficiency of nitrogen. The import of this will pres- ently appear. Tests for Bile.—A common test for the presence of bile consists of the addition of a small quantity of nitric acid, when, if bile be present, a play of colors is produced, beginning with green and passing through various tints to red. This test will detect only the coloring matter of the bile. The best test for the bilin is Pettenkofer’s. To the liquid suspected to contain bile must be added, first, a drop or two of a strong solution of cane-sugar (one part of sugar to four parts of water), and immediately afterwards sulphuric acid, to the extent of about two-thirds of the liquid. On first adding the acid, a whitish precipitate falls; but this redissolves with a slight excess of the acid, and on the further addition of the latter there appears a bright cherry-red color, gradually chang- ing through a lake tint to a dark purple. The process of secreting bile is probably continually going on, but appears to be retarded during fasting, and accelerated on taking food. This was shown by Blondlot, who, having tied the common bile-duct of a dog, and established a fistulous opening between the skin and gall-bladder, whereby all the bile secreted was discharged at the surface, noticed that when the animal was fasting, sometimes not a drop of bile was discharged for several hours; but that, in about ten minutes after the 1 The sulphur is combined with the taurin—one of the substances yielded by the decomposition of bilin. According to Dr. Kemp, the sulphur in the bile of the ox, dried and freed from mucus, coloring matter, and salts, constitutes about 3 per cent. 262 DIGESTION. introduction of food into the stomach, the bile began to flow abundantly, and continued to do so during the whole period of digestion. Bidder and Schmidt’s observations are quite in accordance with this. The bile is probably formed first 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, probably, this hap- pens only while digestion is going on; during fasting it flows from the common bile-duct into the cystic-duct, and thence 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 appears to be its chief or only office, that of a reservoir; for its presence enables bile to be constantly secreted for the purification of the blood, yet insures that it shall all be employed 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 simple. 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 pres- sure behind to force the bile through it. The pressure exer- cised upon the bile secreted during the intervals of digestion appears insufficient to overcome the force with which the orifice 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 peristaltic action of the ducts. The bile is dis- charged 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 probably of the common bile-duct also; for both these organs contain organic muscular fibre-cells. Their contraction is ex- cited 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. Various estimates have been made of the quantity of bile dis- charged in 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 com- putations would give thirty to forty ounces as the quantity daily secreted by man. The purposes served by the secretion of bile may be considered to be of two principal kinds, viz., excrementitiov.s and digestive. As an excrementitious substance, the bile serves especially as a medium for the separation of excess of carbon and hydro- THE BILE MECONIUM. 263 gen from the blood; and its adaptation to this purpose is well illustrated by the peculiarities attending its secretion and dis- posal 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 unnecessary, because of the supply of well-elaborated nutriment received by the vessels of the foetus at the placenta. The liver, during the same time, is proportionally 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 ; for the meconium, or faeces of the foetus, are shown by the analyses of Simon and of Frerichs to contain all the essential principles of bile. Composition of Meconium (Frerichs): Biliary resin, ....... 15.6 Common fat and cholesterin, .... 16.4 Epithelium, mucus, pigment, and salts, . . 69 100. In the foetus, therefore, the main purpose of the secretion of bile must be the purification of the 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 dis- charged, 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 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 lungs. 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. But the analysis of the faeces of both children and adults shows that (except when rapidly discharged in purgation) they con- tain very little of the bile secreted, probably not more than one-sixteenth part of its weight, and that this portion includes only its coloring, and some of its fatty matters, but none of its essential principle, the bilin. All the bilin is again absorbed from the intestines into the blood. But the elementary com- position of bilin (see p. 261) shows such a preponderance of carbon and hydrogen, that it cannot be appropriated to the nutrition of the tissues; therefore, it may be presumed that 264 DIGESTION. after absorption, the carbon and hydrogen of the bilin com- bining with oxygen, are excreted as carbonic acid and water. The destination of the bile is, on this theory, essentially the same in both foetal and extra-uterine life; only, in the former, it is directly excreted, in the latter for the most part indirectly, being, before final ejection, modified in its absorption from the intestines, and mingled with the blood. The change from the direct to the indirect mode of excre- tion of the bile may, with much probability, be connected with a purpose in relation to the development of heat. The tem- perature of the foetus is maintained by that of the parent, and needs no source of heat within the body of the foetus itself; but, in extra-uterine life, there is (as one may say) a waste of material for heat when any excretion is discharged unoxid- ized ; the carbon and hydrogen of the bilin, therefore, instead of being ejected in the faeces, are reabsorbed, in order that they may be combined with oxygen, and that in the combina- tion, heat may be generated. From the peculiar manner in which the liver is supplied with much of the blood that flows through it, it is probable, as Dr. Budd suggest, that this organ is excretory, 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 ex- pelled, 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 into the general current, jealously to guard against their further progress, and turn them back again into an excretory channel. The frequency with which metallic 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. Though one chief purpose of the secretion of bile may thus appear to be the purification of the blood by ultimate excre- tion, yet there 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 secretions are, but is made to pass into the intestinal canal, so FUNCTIONS OF THE LIVER. 265 as to be mingled with the chyme directly after it leaves the stomach ; an arrangement, the constancy of which clearly in- dicates 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 secretion of bile is most active, and the quantity discharged into the intestines much greater, during digestion than at any other time; although, without doubt, this activity of secretion during digestion may, how- ever, 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 reabsorbed, or per- manently. Respecting the functions discharged by the bile in diges- tion, there is little doubt that it assists in emulsifying the fatty portions of the food, and thus rendering them capable of being absorbed by the lacteals. For it has appear iii 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. (2.) It is probable, also, from the result of some experiments by Wistinghausen and Hoffmann, that the moistening of the mucous membrane of the intestines by bile may facilitate ab- sorption of fatty matters through it. (3.) The bile, like the gastric fluid, has a strongly antisep- tic power, and may serve to prevent the decomposition of food during the time of its sojourn in the intestines. The experi- ments of Tiedemann and Gmelin show that the contents of the intestines are much more fetid after the common bile-duct has been tied than at other times; and the experiments of Bidder and Schmidt on animals with an artificial biliary fistula, con- firm this observation; moreover, it is found that the mixture of bile with a fermenting fluid stops or spoils the process of fermentation. (4.) The bile has also been considered to act as a kind of natural purgative, by promoting an increased secretion of the intestinal glands, and by stimulating the intestines to the pro- pulsion 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. Nothing is known with certainty respecting the changes which the reabsorbed portions of the bile undergo, either in 266 DIGESTION. the intestines or in the absorbent vessels. 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 sys- temic vessels. The secretion of bile, as already observed, is only one of the purposes fulfilled by the liver. Another very important func- tion 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 blood generally, and to prepare others for being duly eliminated in the process of respiration. From the labors of M. Bernard, to whom we owe most of what we know on this subject, it appears that the low form of albu- minous matter, or albuminose, conveyed from the alimentary canal by the blood of the portal vein, requires to be submitted to the influence of the liver before it can be assimilated by the blood ; for if such albuminous matter is injected into the jugu- lar 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 probably incorporated with the albuminous part of the blood. An important influence seems also to be exerted by the liver upon the saccharine matters derived from the alimentary canal. The chief purpose of the saccharine and amylaceous princi- ples of food is, probably, in relation to respiration and the production of animal heat; but in order that they may fulfil this, their main office, it seems to be essential that they should undergo some intermediate change, which is effected in the liver, and which consists in their conversion into a peculiar form of saccharine matter, very similar to glucose, or diabetic sugar. That such influence is exerted by the liver seems proved by the fact that when cane sugar is injected into the jugular vein it is speedily thrown out of the system, and ap- pears in the urine ; but when injected into the portal vein, and thus enabled to traverse the liver, it ceases to be excreted at the kidneys; and, what is still more to the point, a very large quantity of glucose may be injected into the venous system without any trace of it appearing in the urine. So that it may be concluded, that the saccharine principles of the food un- dergo, in their passage through the liver, some transformation necessary to the subsequent purpose they have to fulfil in rela- tion to the respiratory process, and without which, such pur- pose probably could not be properly accomplished, and the FORMATION OF SUGAR IN THE LIVER. 267 substances themselves would be eliminated as foreign matters by the kidneys. Then, again, it was discovered by Bernard, and the dis- covery has been amply confirmed, that the liver possesses the remarkable property of forming glucose or grape-sugar (C8H13 Ofi), or a substance readily convertible into sugar, even out of principles in the blood which contain no trace of saccharine or amylaceous matter. In Herbivora and in animals living on mixed diet, a large part of the sugar is derived from the sac- charine and amylaceous principles introduced in their food. But in animals fed exclusively on flesh, and deprived therefore of this source of sugar, the liver furnishes the means whereby it may be obtained. Not only in Carnivora, however, but ap- parently in all classes of animals, the liver is continually en- gaged, during health, in forming sugar, or a substance closely allied to it, in large amount. This substance may always be found in the liver, even when absent from all other parts of the body. 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 sulphate of soda in order to precipitate albu- minous and coloring matters. The decoction is then filtered and may be tested for glucose. The most usual test is Trom- mer’s. To the filtered solution an equal quantity of liquor potassse is added, with a few drops of a solution of sulphate of copper. The mixture is then boiled, when the presence of sugar is indicated by a reddish-brown precipitate of the sub- oxide of copper. The researches of Bernard and others, however, have shown that the sugar is not formed at once at the liver, but that this organ has the power of producing a peculiar substance allied to starch, which is readily convertible into glucose when in contact with any animal ferment. This substance has received the different names of glycogen, glycogenic substance, animal starch, hepatin. Glycogen (C13H10O10) is obtained by taking a portion of liver from a recently killed animal, 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 decoc- tion by the addition of alcohol. When purified, glycogen is a white, amorphous, starch-like substance, odorless and tasteless,soluble in water, but insoluble 268 DIGESTION. in alcohol. It is converted into glucose by boiling with dilute acids, or by contact with any animal ferment. There are two chief theories concerning the immediate desti- nation of glycogen. (1.) According to Bernard and most other physiologists, its conversion into sugar takes place rapidly during life, and the sugar is conveyed away by the blood of the hepatic veins to be consumed in respiration at the lungs. (2.) Pavy and others believe that the conversion into sugar only occurs after death, and that during life no sugar exists in healthy livers,—the amyloid substance or glycogen being pre- vented by some force from undergoing the transformation. The chief arguments advanced by Pavy in support of this view' are, first, 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 immediately after an animal has been suddenly deprived of life, while if the examination be delayed for a little while after death, sugar in abundance may be found in such blood ; secondly, that the liver, like the venous blood in the heart, is, at the moment of death, almost completely free from sugar, although afterwards its tissue speedily becomes saccharine, unless the formation of sugar be prevented by freezing, boiling, or other means calculated to interfere with the action of a ferment on the amyloid substance of the organ. Instead of adopting Bernard’s view, that nor- mally, 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 believe that it may repre- sent an intermediate stage in the formation of fat from ma- terials absorbed from the alimentary canal. For the present we must remain uncertain as to which of these theories contains most truth in it. Whatever be the destination of this peculiar amyloid sub- stance formed at the liver, most recent observers agree that it is formed at, and exists within, the hepatic cells, from which it may be extracted by the process just described. Much doubt exists also respecting the mode in which gly- cogen is formed in the liver, and the materials which furnish its source. Since its quantity is increased after feeding, espe- cially on substances containing much sugar or starch, it is probable that part of it is derived from saccharine principles absorbed from the digestive canal; but since its formation con- tinues even when there is no starch or sugar in the food, the albuminous or fatty principles also have been thought capable of furnishing part of it. Numerous experiments, however, having proved that the liver continues to form sugar in animals after prolonged starvation, and during hibernation, and even FORMATION OF SUGAR IN THE LIVER. 269 after death, its production is clearly independent of the ele- ments of food. One of Bernard’s experiments may be quoted in proof of this: Having fed a healthy dog for many days ex- clusively on flesh, he killed it, removed the liver at once, and before the contained blood could have coagulated, he thor- oughly washed out its tissue by passing a stream of cold water through the portal vein. He continued the injection until the liver was completely exsanguined, until the issuing water con- tained not a trace of sugar or albumen, and until no sugar was yielded by portions of the organ cut into slices and boiled in water. Having thus deprived the liver of all saccharine mat- ter, he left it for twenty-four hours, and on then examining it, found in its tissue a large quantity of soluble sugar, which must clearly have been formed subsequently to the organ being washed, and out of some previously insoluble and non-sac- charine substance. This and other experiments led him and others to the conclusion that the formation of the amyloid sub- stance by the liver is the result of a kind of secretion or elabo- ration out of materials in the solid tissues of the gland—such secretion being probably effected by the hepatic cells, in which, indeed, as already observed, the substance has been detected. According to this view, then, the liver may be regarded as an organ engaged in forming two kinds of secretion, namely, bile and sugar, or rather, glycogen readily convertible into sugar. The former, chiefly excrementitious, passes along the bile-ducts into the intestines, where it may subserve some pur- poses in relation to digestion, and is then for the most part re- absorbed, and ultimately eliminated during the processes con- cerned in the production of animal heat. The latter, namely sugar, being soluble, is, unless Pavy’s view be correct, taken up by the blood in the hepatic vein, conveyed through the right side of the heart to the lungs, where it is probably con- sumed in the respiratory process, and thus contributes to the production of animal heat. The formation of glycogen or of sugar is, like all other pro- cesses in the living body, under the control of the nervous sys- tem. Bernard discovered that by pricking the floor of the fourth ventricle, the quantity of sugar formed was so much in excess of the normal quantity, as to be excreted by the kidney, and thus produce the leading symptom of diabetes. Section of the inferior cervical ganglion of the sympathetic nerve also produces diabetes. The channel by which the influence of the nervous system, is conducted in the preceding and similar experiments is not accurately known ; no theory having been permanently estab- lished, which explains all the facts hitherto observed in con- 270 DIGESTION. nection wTith the influence of the nervous system on the pro- duction of glucose. Summary of the Changes which take place in the Food during its Passage through 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 conversion of starch into sugar, which began in the mouth, has been interfered with, although not stopped altogether. The soluble matters—both those which were so from the first, as sugar and saline matter, and those which have been made so by the action of the saliva and gastric juice—have begun to disappear by absorption into the blood- vessels, and the same thing has befallen such fluids as may have been swallowed,—wine, wTater, &c. 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, con- sists of albuminous matter, broken down, dissolving and half dissolved, fatty matter, broken down, 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 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 digestible and will be finally expelled as part of the fseces. On the entrance of the chyme into the duodenum, it is sub- jected to the influence of the fluid secreted by Lieberkiihn’s and Brunn’s glands, before described, and to that of the bile and pancreatic juice, which are poured into this part of the intestine. Without doubt, that part of digestion which it is a chief duty of the small intestine to perform, 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 respectively to the bile, pancreatic DIGESTION IN SMALL INTESTINE. 271 secretion, and the secretion of the intestinal glands, is still un- certain. It is most probable, however, that the pancreatic secretion and the bile are the main agents in emulsifying the fat, and that they do this by direct admixture with it. They also promote its absorption by moistening the surface of the villi (p. 265). During digestion in the small intestine, the villi become turgid with blood, their epithelial cells become filled, by ab- sorption, with fat-globules, which, after minute division, trans- ude into the granular basis of the villus, and thence into the lacteal vessel in the centre, by which they are conveyed along the mesentery to the lymphatic glands, and thence into the thoracic duct. A part of the fat is also absorbed by the blood- vessels of the intestine. The term chyle is sometimes applied to the emulsified contents of the intestine after their admixture with the bile and pancreatic juice; but more strictly to the fluid contained in the lacteal vessels during digestion, which differs from ordinary lymph contained in the same vessels at other times, chiefly in the greatly increased quantity of fat particles which have been absorbed from the small intestine. Although the most evident function of the small intestine is the digestion of fat, it must not be forgotten that a great part of the other constituents of the food is by no means completely digested when it leaves the stomach. Indeed, its leaving it unabsorbed would, alone, be proof of this fact. The albuminous substances which have been partly dissolved in the stomach continue to be acted on by the gastric juice which passes into the duodenum with them, and the effect of the last-named secretion is assisted or complemented by that of the pancreas and intestinal glands. As the albuminous matters are dissolved, they are absorbed chiefly by the blood- vessels, and only to a small extent, probably, by the lacteals. The starchy, or amylaceous portion of the food, the conver- sion of which into dextrin and sugar was more or less inter- rupted during its stay in the stomach, is now acted on briskly by the secretion of the pancreas, and of Brunn’s glands, and perhaps of Lieberkiihn’s glands also, and the sugar as it is formed dissolves in the intestinal fluids, and afterwards, like the albumen, is absorbed chiefly by the bloodvessels. The liquids, swallowed as such, which may have escaped absorption in the stomach, are absorbed probably very soon after their entrance into the intestine ; the fluidity of the con- tents of the latter being preserved more by the constant secre- tion 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 272 DIGESTION. be gathered that there is a kind of circulation constantly pro- ceeding from the intestines into the blood, and from the blood into the intestines again ; for, as all the fluid, probably 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 reabsorbed into the current of blood—going into the blood charged with nutrient products of digestion, coming out again by secretion through the glands in a comparatively uncharged condition. It has been said before that the contents of the stomach dur- ing gastric digestion have a strongly acid reaction. On the entrance of the chyme into the small intestine, this is gradu- ally neutralized to a greater or less degree by admixture with the bile and other secretions with which it is mixed, and the acid reaction becomes less and less strongly marked as the chyme passes along the canal towards the ileo-cjecal valve. Thus, all the materials of the food are acted on in the small intestine, and a great portion of the nutrient matter is absorbed, the fat chiefly by the ladeals, the other principles, when in a state of solution, chiefly by the bloodvessels, but neither, prob- ably, exclusively by one set of vessels. At the lower end of the small intestine, the chyme, still thin and pultaceous, is of a light yellow color, and has a distinctly fecal odor. In this state it passes through the ileo-csecal opening into the large intestine. Summary of the Process of Digestion in the Large Intestine. The changes which take place in the chyme after its passage from the small into 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 com- pleted in the small intestine, 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 the addition to them, in this region, of some special matter, probably excretory. The acid reaction, which had become less and less distinct in the small bowel, again becomes very manifest in the caecum—prob- ably from acid fermentation processes in some of the materials of the food. DIGESTION IN LARGE INTESTINE. 273 There seems no reason, however, to conclude that any special, “ secondary,” digestive process occurs in the ceecum 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, albu- minous, or other matters, may be gathered from the good effects of nutrient enemata, so frequently given when from any cause there is difficulty in introducing food into the stom- ach. In ordinary healthy digestion, however, the changes which ensue in the chyme after its passage into the large intestine, are mainly the absorption of the more liquid parts, and the addition of the special excretory products which give it the characteristic odor. At the same time, as before said, it is probable that a certain quantity of nutrient matter always escapes digestion in the small intestine, and that this happens more especially when food has been taken in excess, or when it is of such a kind as to be difficult of digestion. Under these circumstances there is no doubt that such changes as were proceeding in it at the lower part of the ileum may go on unchecked in the lai’ge bowel,—the process being as- sisted by the secretion of the numerous tubular glands therein present. By these means the contents of the large intestine, as they proceed towards the rectum, become more and more solid, and losing their more liquid and nutrient parts, gradually acquire the odor and consistence characteristic of feces. After a sojourn of uncertain duration in the rectum, they are finally expelled by the contraction of its muscular coat, aided, under ordinary circumstances, by the contraction of the abdominal muscles. For a description of the mechanism by which the act of defecation is accomplished, see p. 183. The average quantity of solid fecal matter evacuated by the human adult in twenty-four hours is about five ounces; an uncertain proportion of which consists simply of the undi- gested or chemically modified residue of the food and the re- mainder of certain matters which are excreted in the intesti- nal canal. 274 DIGESTION. Water, .......... 783.00 Solids, 267.00 Composition of Fceces. Special excrementitious constituents: Excretin, excretoleic acid (Marcet), and stereorin (Austin Flint). Salts : Chiefly phosphate of magnesia and phos- phate of lime, with small quantities of iron, soda, lime, and silica. Insoluble residue of the food (chiefly starch, grains, woody tissue, particles of cartilage, and fibrous 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, &c. 267.00 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. Dr. Brinton supposes twelve hours to be occupied by the journey of an ordinary meal through the small intestine, and twenty- four to thirty-six hours by the passage through the large bowel. On the Gases contained in the Stomach and Intestines. It need scarcely be remarked that, under ordinary circum- stances, 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 afterwards occupied by the bowels, and by the large degree, therefore, in which the gas, which naturally dis- tends them, contributes to fill the cavity of the abdomen. In- deed, 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 ac- cident, but intended to serve a definite and important purpose, although, probably, a mechanical one. 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 MOVEMENTS OF THE INTESTINES. 275 matter, or of the 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 the gastric and intestinal bloodvessels ; but the conditions of the exchange are not known, and it is very doubtful whether anything like a true and definite secre- tion of gas from the blood into the intestines or stomach ever takes place. There can be no doubt, however, that the intes- tines may be the proper excretory organs for many odorous and other substances, either absorbed from the air taken into the lungs in inspiration, 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 assuming 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 intestines are derived chiefly from the second of the sources which have been enumerated. Tabular Analysis of Gases contained in the Alimentary Canal. Composition by Volume. Whence obtained. Oxygen Nitrog. Carbon. Acid. Ilydrog. Carburet. Hydrogen. Sulphuret. Hydrogen. Stomach, 11 71 14 4 Small Intestine, . 32 30 38 ] Caecum, .... Colon, .... Rectum, .... 66 35 12 57 8 6 13 8 - trace. 46 43 11 J Expelled per anurn 22 41 19 19 * The above tabular analysis of the gases contained in the alimentary canal has been quoted from the analyses of Jurine, Magendie, Marchand, and Chevreul, by Dr. Brinton, from whose work the above enumeration of the sources of the gas has been also taken. 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 in- fluence of fresh portions of intestinal secretion, and as slowly 276 DIGESTION. exposed to the absorbent power of all the villi and blood- vessels of the mucous membrane. The movement of the intes- tines is peristaltic or vermicular, and is effected by the alternate contractions and dilatations of successive portions of the intes- tinal 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 upwards, or, as it were, backwards, over the sub- stance to be propelled, and then the circular fibres of the same portion contracting in succession from above downwards, or, as it were, from behind forwards, press on the substance into the portion next below, in which at once the same succession of actions next ensues. These movements take place slowly, and, in health, are commonly unperceived by the mind ; but they are perceptible when they are accelerated under the in- fluence 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-C£ecal valve. Besides, the orifice of communication between the ileum and csecum (at the bor- ders of which orifice are the folds of mucous membrane which form the valve) is encircled with muscular fibres, the contrac- tion of which prevents the undue dilatation of the orifice. Proceeding from above downwards, 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 fieces, which are gradually becoming firmer. The greatest strength is in the rectum, at the termination 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 holds the orifice close by a constant slight contraction under the influence of the spinal cord. The peculiar condition of the sphincter, in relation to the nervous system, will be again referred to. The remaining portion of the intestinal canal is under the direct influence of the sympathetic or ganglionic system, and, indirectly, or more distantly, is subject to the influence of the brain and spinal cord, which influence appears to be, in some degree, transmitted through the vagus nerve. Experimental irritation of the brain or cord produces no evident or constant effect on the move- ments of the intestines during life; yet in consequence of cer- tain conditions of the mind, the movements are accelerated or LYMPHATIC VESSELS AND GLANDS. 277 retarded ; and in paraplegia the intestines appear after a time much weakeued in their power, and costiveness, with a tym- panitic condition, ensues. Immediately after death, irritation of both the sympathetic and pneumogastric 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 rich ganglionic structure shown by Meissner to exist in the submucous tissue. This regulates and controls the move- ments, and gives to them their peculiar slow, orderly, rhyth- mic, and peristaltic character, both naturally, and when arti- ficially excited. CHAPTER X. ABSORPTION. The process of absorption has, for one of its objects, the in- troduction 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 taking away of parts of the body itself, when, having fulfilled their office, or otherwise requiring removal, they need to be re- newed. In both these offices, i. e., in both absorption from without and absorption from within, the process manifests some variety, and a very wide range of action ; and in both it is probable that two sets of vessels are, or may be, concerned, namely, the bloodvessels, and the lacteals or lymphatics, to which the term absorbents has been especially applied. Structure and Office of the Lacteal and Lymphatic Vessels and Glands. Besides the system of arteries and veins, with their inter- mediate vessels, the capillaries, there is another system of canals in man and other vertebrata, called the lymphatic sys- tem, which contains a fluid called lymph. Both these systems of vessels are concerned in absorption. The principal vessels of the lymphatic system are, in struc- ture and general appearance, like very small and thin-walled veins, and like them are provided with valves. By one ex- tremity they commence by fine microscopic branches, the lym- phatic capillaries or lymph-capillaries, in the organs and tissues 278 ABSORPTION. of nearly every part of the body, and by their other extremi- ties they end directly or indirectly in two trunks which open into the large veins near the heart (Fig. 92). Their contents, the lymph and chyle, unlike the blood, pass only in one diree- Fig. 92. Lymphatics of head and neck, right. Lymphatics of head and neck, left. Right internal ju: ular vein. Right subclavian vein. Thoracic duct. Left subclavian vein. Lymphaticsof right arm. Thoracic duct. Receptaculum chyli. Lac teals. Lymphatics of low- er extremities. Lymphatics of low- er extremities Diagram of the principal groups of lymphatic vessels (from Quain). tion, 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. Remem- bering the course of the fluid in the lymphatic vessels, viz., its passage in the direction only towards the large veins in the neighborhood of the heart, it will be readily seen from Fig. 92 that the greater part of the contents of the lymphatic system of vessels passes through a comparatively large trunk called COURSE OF THE LYMPHATICS. 279 the thoracic duct, which finally empties its contents into the blood-stream at the junction of the internal jugular and sub- clavian 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 Fig. 93. Lymphatic vessels of the head and neck of the upper part of the trunk (from Mas- cagni) -i.—The chest and pericardium have been opened on the left side, and the left mamma detached and thrown outwards 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, axiila, and mediastinum. Between the let} internal jugular vein and the common carotid artery, the upper ascending part of the thoracic duct marked 1, and above this, and descending to 2, the arch and last part of the duct. The termination of the upper lymphatics of the diaphragm in the mediastinal glands as well as the cardiac and the deep mammary lymphatics, are also shown. 280 A BSOEPTION. is no essential distinction, however, between lacteals and lym- phatics. In some part 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 lion-vascular parts, as the nails, cuticle, hair, and the like. The lymphatic capillaries commence most commonly either in closely-meshed networks, or in irregular lacunar spaces between the various structures of which the different organs are composed. The former is the rule of origin with those lymphatics which are placed most superficially, as, for instance, immediately beneath the skin, or under the mucous and serous membranes ; while the latter is most common with those which arise in the substance of organs. In the former instance, their walls are composed of but little more than homogeneous mem- brane, lined by a siugle layer of epithelial cells, very similar to those which line the blood-capillaries (Fig. 49). In the latter instance the small irregular channels and spaces from which the lymphatics take their origin, although they are formed mostly by the chinks and crannies between the blood- vessels, secreting ducts, and other parts which may happen to form the framework of the organ in which they exist, yet have also a layer of epithelial cells to define and bound them. The lacteals apear to offer an illustration of another mode of origin, namely, in blind dilated extremities (Figs. 81, 82); but there is no essential difference in structure between these and the lymphatic capillaries of other parts. Recent discoveries seem likely to put au eud soon to the long-standing discussion whether any direct communications exist between the lymph-capillaries and blood-capillaries; the need for any special intercommunicating channels seeming to disappear in the light of more accurate knowledge of the struc- ture and endowments of the parts concerned. For while, on the one hand, 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, aud occupy the interspaces which exist among their different elements, these same inter- spaces have been shown, as just stated, to form the beginnings of the lymph-capillaries. And while, for many years, the no- tion of the existence of any such channels between the blood- vessels and lymphvessels, as would admit blood-corpuscles, has been given up, recent observations have proved that, for the passage of such corpuscles, it is not necessary to assume the ORIGIN OF LYMPHATICS 281 presence of any special channels at all, inasmuch as blood- corpuscles can pass bodily, without much difficulty, through Fig. 94. Fig. 95. Fig. 94.—Superficial lymphatics of the forearm and palm of the hand, i (after Mascagni). 5. Two small glands at the bend of the arm. 6. Radial lymphatic ves- sels. 7. Ulnar 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. Fig. 95.—Superficial lymphatics of right groin and upper part of thigh, i (after Mascagni). 1.‘Upper inguinal glands. 2'. Lower inguinal or femoral glands. 3,3. Plexus of lymphatics in the course of the long saphenous vein. 282 ABSORPTION. the walls of the blood-capillaries and small veins (p. 138), and could pass with still less trouble, probably, through the com- paratively ill-defined walls of the capillaries which contain lymph. Observations of Recklinghausen have led to the discovery that in certain parts of the body openings exist by which lym- phatic capillaries directly communicate with parts hitherto supposed to be closed cavities. If the peritoneal cavity be ip- jected with milk, an injection is obtained of the plexus of lym- phatic vessels of the central tendon of the diaphragm ; and on removing a small portion of the central tendon, with its peri- toneal surface uninjured, and examining the process of absorp- tion under the microscope, Recklinghausen noticed that the milk-globules ran towards small natural openings or stomata between the epithelial cells, and disappeared by passing vortex- like through them. The stomata, which had a roundish out- line, were only wide enough to admit two or three milk-glob- ules abreast, and never exceeded the size of an epithelial cell. Openings of a similar kind have been found by Dybskowsky in the pleura; and as they may be presumed to exist in other serous membranes, it would seem as if the serous cavities, hitherto supposed closed, form but a large widening out, so to speak, of the lymph-capillary system with which they directly communicate. In structure, the medium-sized and larger lymphatic vessels are very like veins; having, according to Kolliker, an exter- nal coat of fibro-cellular tissue, with elastic filaments; within this, a thin layer of fibro-cellular tissue, with organic muscu- lar 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 epithelium, and numerous valves. The valves, constructed like those of veins, and with the free edges turned towards 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 braided or knotted appearance (Fig.. 99). With the help of the valvular mechanism, all occasional pressure on the exterior of the lymphatic and lacteal vessels propels the lymph towards the heart: thus muscular and other external pressure accelerates the flow of the lymph as it does that of the blood in the veins (see p. 143). The actions of the muscular fibres of the small intestine, and probably the layer of organic muscle present in each intesti- nal villus (p. to assist in propelling the chyle : for, LYMPHATIC GLANDS. 283 in the small intestine of a mouse, Poiseuille saw the chyle moving with intermittent propulsions that appeared to corre- spond with the peristaltic movements of the intestine. But for the general propulsion of the lymph and chyle, it is probable that, together with the vis a tergo resulting from absorption (as in the ascent of sap in a tree), and from external pressure, some of the force may be derived from the contractility of the vessel’s own walls. Kdlliker, after watching the lymphatics in the transparent tail of the tadpole, states that no distinct movements of their walls can ever be seen, but as they are emptied after death they gradually contract, and then, after some time, again dilate to their former size, exactly as the small arteries do under the like circumstances. Thus, also, the larger vessels in the human subject commonly empty themselves after death ; so that, although absorption is proba- bly usually going on just before the time of death, it is not common to see the lymphatic or lacteal vessels full. Their power of contraction under the influence of stimuli has been demonstrated by Kdlliker, who applied the wire of an electro- magnetic apparatus to some well-filled lymphatics on the skin of a boy’s foot, just after the removal of his leg by am- putation, and noticed that the calibre of the vessels diminished at least one half. It is most probable that this contraction of the vessels occurs during life, and that it consists, not in peristaltic or undulatory movements, but in a uniform con- traction of the successive portions of the vessels, by which pressure is steadily exercised upon their contents, and which alternates with their relaxation. Lymphatic Glands. Almost all lymphatic and lacteal vessels in some part of their course pass through one or more small bodies called lym- phatic glands (Fig. 99). A lymphatic gland is covered externally by a capsule of connective tissue, which invests and supports the glandular structure within ; while prolonged from its inner surface are processes or trabeculae which, entering the gland from all sides, and freely communicating, form a fibrous scaffolding or stroma in all parts of the interior. Thus are formed in the outer or cortical part of the glands (Fig. 96), in the intervals of the trabeculse, certain intercommunicating spaces termed alveoli ; while a finer meshwork is formed in the more central or ■medullary part. In the alveoli and the trabecular meshwork the proper gland-substance is contained ; in the form of nod- 284 ABSORPTION. ules in the cortical alveoli, and of rounded cords in the medullary part (Fig. 97). The gland-substance of one part is continuous directly or indirectly with that of all others. Fig. 96. Section of a mesenteric gland from the ox. slightly magnified, a, hilus; b (in the central part of the figure), medullary substance ; c, cortical substance with indis- tinct alveoli; d, capsule (after Kolliker). The essential structure of lymphatic gland-substance resem- bles that which was described as existing, in a simple form, in the interior of the solitary and agminated intestinal follicles Section of medullary substance of an inguinal gland of an ox (magnified 90 diameters), a, a, glandular substance or pulp forming rounded cords joining in a continuous net (dark in the figure); c, c, trabeculae; the space, b, b, between these and the glandular substance is the lymph-sinus, washed clear of corpuscles and traversed by filaments of retiform connective tissue (after Kolliker). LYMPHATIC GLANDS. 285 (p. 242). Pervading all parts of it, and occupying the alveoli and trabecular spaces before referred to, is a network of the variety of connective tissue termed retiform tissue (Fig. 98), the interspaces of which are occupied by lymph-corpuscles. The corpuscles are arranged in such a way, that while in the centre of the alveoli and of each mesh they are so crowded together as to be, with the retiform tissue pervading them, a consistent gland-pulp, continuous in the form of the nodules and cords, before referred to, throughout the whole gland, they are in comparatively small numbers in the outer part of the alveoli and meshes, and leave this portion, as it were, open. Fig. 98. A small portion of medullary substance from a mesenteric gland of the ox (mag- nified 300 diameters), d, d, trabeculee; a, part of a cord of glandular substances from which all but a few of the lymph-corpuscles have been washed out to show its sup- porting meshwork of retiform tissue and its capillary bloodvessels (which have been injected, and are dark in the figure) ; b, b, lymph-sinus, of which the retiform tissue is represented only at c, c (after Kolliker). (See Figs. 97, 98.) This free space between the gland-pulp and the trabecular stroma, occupied only by retiform tissue, is called the lymph-channel or lymph-path, because it is traversed 286 ABSORPTION. by the lymph, which is continually brought to the gland and conveyed away from it by lymphatic vessels; those which bring it being termed afferent vessels, and those which take it awray efferent vessels. The former enter the cortical part of the gland and open into its alveoli, at the same time that they lay aside all their coats except the epithelial lining, which may be said to continue to line the lymph-path into which the contents of the afferent vessels now pass. The efferent vessels begin in the medullary part of the gland, and are continuous with the lymph-path here as the afferent vessels were with the cortical portion ; the epithelium of one is continuous with that of the other. Bloodvessels are freely distributed to the trabecular tissue and to the gland-pulp (Fig. 98). Properties of Lymph and Chyle. The fluid, or lymph, contained in the lymphatic vessels is, under ordinary circumstances, clear, transparent, and color- less, or of a pale yellow tint. It is devoid of smell, is slightly alkaline, and has a saline taste. As seen with the micro- scope in the small transparent vessels of the tail of the tad- pole, the lymph usually contains no corpuscles or particles of any kind; and it is probably only in the larger trunks in which, by a process similar to that to be described in the chyle, the lymph is more elaborated, that any corpuscles are formed. These corpuscles are similar to those in the chyle, but less numerous. The fluid in which the corpuscles float is commonly and in health albuminous, and contains no fatty particles or molecular base; but it is liable to variations ac- cording to the general state of the blood, and that of the organ from which the lymph is derived. As it advances towards the thoracic duct, and passes through the lymphatic glands, it be- comes, like chyle, spontaneously coagulable from the formation of fibrin, and the number of corpuscles is much increased. The fluid contained in the lacteals, or lymphatic vessels of the intestine, is clear and transparent during fasting, and differs in no respect from ordinary lymph ; but during diges- tion, it becomes milky, and is termed chyle. Chyle is an opaque, whitish fluid, resembling milk in ap- pearance, and having a neutral or slightly alkaline reaction. Its whiteness and opacity are due to the presence of innumer- able particles of oily or fatty matter, of exceedingly minute though nearly uniform size, measuring on the average about °f an inch (Gulliver). These constitute what Mr. Gul- CHYLE, 287 liver appropriately terms the molecular base of chyle. Their number, and consequently the opac- ity of the chyle, are dependent upon the quantity of fatty matter con- tained in the food. Hence, as a rule, the chyle is whitish and most turbid in carnivorous animals; less so in Herbivora; while in birds it is usually transparent. The fatty na- ture of the molecules is made mani- fest by their solubility in ether, and, when the ether evaporates, by their being deposited in various-sized drops of oil.1 Yet, since they do not run together and form a larger drop, as particles of oil would, it appears very probable that each molecule consists of oil coated over with albumen, in the manner in which, as Ascherson observed, oil always becomes covered when set free in minute drops in an albuminous solution. And this view is supported by the fact, that when water or dilute acetic acid is added to chyle, many of the molecules are lost sight of, and oil-drops appear in their place, as if the investments of the molecules had been dissolved, and their oily contents had run to- gether. Except these molecules, the chyle taken from the villi or from lacteals near them, contains no other solid or organized bodies. The fluid in which the molecules float is albuminous, and does not spontaneously coagu- late, though coagulable by the addi- tion of ether. But as the chyle passes on towards the thoracic duct, and especially while it traverses one or more of the mesenteric glands (pro- pelled by forces which have been described with the structure of the vessels), it is elaborated. Fig. 99. (Mascagni), a, plan of a lym- phatic gland, 3, with its com- ponent cells tilled with mercury, and having three sets of afferent vessels, 1, 1, 1, leading into it, and one set of efferent vessels, 2, passing out from it. The arrows indicate the course of the lymph in these vessels. The varicose or jointed appearance of the vessels is here shown. 6, a single lym- phatic vessel somewhat enlarged, and cut through, to show the little double valves in its interior. c, lymph-corpuscles, one granu- lar, and three treated with dilute acetic acid, showing the envelope and the pale nucleus; also some finer granules and oil-particles free. Magnified 400 diameters. 1 Some of the molecules may remain undissolved by the ether; but this appears to be due to their being defended from the action of the ether by being entangled within the albumen which it coagulates. 288 ABSORPTION. The quantity of molecules and oily particles gradually di- minishes; cells, to which the name of chyle-corpuscles is given, are developed in it; and by the formation of fibrin, it acquires the property of coagulating spontaneously. The higher in the thoracic duct the chyle advances, the more is it, in all these respects, developed ; the greater is the number of chyle-cor- puscles, and the larger and firmer is the clot which forms in it when withdrawn and left at rest. Such a clot is like one of blood, without the red corpuscles, having the chyle-corpuscles entangled in it, and the fatty matter forming a white creamy film on the surface of the serum. But the clot of chyle is softer and moister than that of blood. Like blood, also, the chyle often remains for a long time in its vessels without coagu- lating, but coagulates rapidly on being removed from them (Bouisson). The existence of fibrin, or of the materials which by their union form it (p. 62 et seq.), is, therefore, certain ; its increase appears to be commensurate with that of the corpus- cles ; and, like them, it is not absorbed as such from the chyme (for no fibrin exists in the chyle in the villi), but is gradually elaborated out of the albumen which chyle in its earliest con- dition contains. The structure of the chyle-corpuscles was described when speaking of the white corpuscles of the blood, with which they are identical. From what has been said, is will appear that perfect chyle and lymph are, in essential characters, nearly similar, and scarcely differ, except in the preponderance of fatty matter in the chyle. The comparative analysis of the two fluids obtained from the lacteals and the lymphatics of a donkey is thus given by Dr. Owen Rees: Water, Albumen, . Fibrin. Animal extractive, Fatty matter, Salts, . Chyle. . 90.237 3.516 0.370 1.565 3 601 0.711 Lymph. 96.536 1.200 0.120 1.559 a trace. 0.585 100 000 100.000 The analyses of Nasse afford an estimate of the relative com- positions of the lymph, chyle, and blood of the horse.1 1 The analysis of the blood differs rather widely from that given at page 72; but if it be erroneous, it is probable that corresponding errors exist in the analysis of the lymph and chyle; and that therefore the tables in the text may represent accurately enough the relation in which the three fluids stand to each other. COMPOSITION OF LYMPH AND CHYLE. 289 Water, Lymph. . 950. Chyle. 935. Blood. 810 Corpuscles,. 4. 92.8 Albumen, . . 39 11 31. 80 Fibrin, 0.75 2.8 Extractive matter, ! 4.88 6.25 5.2 Fatty matter, 0.09 15. 1.55 Alkaline salts, . 5.61 7. 6.7. Phosphate of lime and magnesia, oxide \ n 01 1. 0.95 of iron, &c., 1000. 1000. 1000. The contents of the thoracic duct, including both the lymph and chyle mixed, in an executed criminal, were examined by Dr. Rees, who found them to consist of— Water, ......... 90.48 Albumen and fibrin, ...... 7.08 Extractive matter, ....... 0.108 Fatty “ 0.92 Saline “....... 0.44 From all these analyses of lymph and chyle, it appears that they contain essentially the same organic constituents that are found in the blood, viz., albumen, fibrin, and fatty matter, the same saline substances, and iron. Their composition differs from that of the blood in degree rather than in kind; they contain a less proportion of all the substances dissolved in the water (see Nasse’s analyses, just quoted), and much less fibrin. The fibrin1 of lymph, besides being less in quantity, appears to be in a less elaborated state than that of the blood, coagulating less rapidly and less firmly. According to Virchow, it never coagulates, under ordinary circumstances, within the lymphatic vessels, either during life or after death. These differences gradually diminish, while the lymph and chyle, passing to- wards and through the thoracic duct, gradually approach the place at which they are to be mingled with the blood. For, in the thoracic duct, besides the higher and more abundant de- velopment of the fibrin, the lymph and chyle-corpuscles are found more advanced towards their development into red blood-corpuscles; sometimes even that development is com- pleted, and the lymph has a pinkish tinge from the number of red blood-corpuscles that it contains. The general result, therefore, of both the microscopic and the chemical examinations of the lymph and chyle, demon- strate that they are rudimental blood; their fluid part being, 1 For observations on the nature of fibrin, see p. 62. 290 ABSORPTION. like the liquor sanguinis, diluted, but gradually becoming more concentrated; and their corpuscles being in process of development into red blood-corpuscles. Thus, in quality, the lymph and chyle are adapted to replenish the blood ; and their quantity, so far as it can be estimated, appears ample for this purpose. In one of Magendie’s experiments, half an ounce of chyle was collected in five minutes from the thoracic duct of a middle-sized dog ; Collard de Martigny obtained nine grains of lymph, in ten minutes, from the thoracic duct of a rabbit which had taken no food for twenty-four hours; and Gieger, from three to five pounds of lymph daily from the foot of a horse, from whom the same quantity had been flowing several years without injury to health. Bidder found, on opening the tho- racic duct in cats, immediately after death, that the mingled lymph and chyle continued to flow from one to six minutes; and, from the quantity thus obtained, he estimated that if the contents of the thoracic duct continued to move at the same rate, the quantity which would pass into a cat’s blood in twenty- four hours would be equal to about one-sixth of the weight of the whole body. And, since the estimated weight of the blood in cats is to the weight of their bodies as 1.7, the quantity of lymph daily traversing the thoracic duct would appear to be about equal to the quantity of blood at any time contained in the animals. Schmidt’s observations on foals have yielded very similar results. By another series of experiments, Bidder estimated that the quantity of lymph traversing the thoracic duct of a dog in twenty-four hours is about equal to two-thirds of the blood in the body. If we take these estimates, it will not follow from them that the whole of an animal’s blood is daily replaced by the development of lymph and chyle; for even if the quantity of lymph and chyle daily formed be equal to that of the blood, the solid contents of the blood will be much too great to be replaced by those of the lymph and chyle. According to Nasse’s analyses, the solid matter of a given quantity of blood could not be replaced out of less than three or four times the quantity of lymph and chyle. Absorption by the Lacteal Vessels. During the passage of the chyme along the whole tract of the intestinal canal, its completely digested parts are absorbed by the bloodvessels and lacteals distributed in the mucous membrane. The bloodvessels appear to absorb chiefly the dissolved portions of the food, and these, including especially the albuminous and saccharine, they imbibe without choice; whatever can mix with the blood passes into the vessels, as ABSORPTION BY LYMPHATICS. 291 will be presently described. But the lacteals appear to absorb only certain constituents of the food, including particularly the fatty portions. The absorption by both sets of vessels is carried on most actively, but not exclusively, in the villi of the small intestine; for in these minute processes, both the capillary bloodvessels and the lacteals are brought almost into contact with the intestinal contents. It has been already stated that the villi of the small intestine (Figs. 81 and 82), are minute vascular processes of mucous membrane, each containing a delicate network of bloodvessels and one or more lacteals, and are invested by a sheath of cylin- drical epithelium. In the interspaces of the mucous mem- brane between the villi, as well as over all the rest of the intestinal canal, the lacteals and bloodvessels are also densely distributed in a close network, the lacteals, however, being more sparingly supplied to the large than to the small in- testine. There seems to be no doubt that absorption of fatty matters during digestion, from the contents of the intestines, is effected chiefly by the epithelial cells which line the intestinal tract, and especially by those which clothe the surface of the villi (Fig. 81). From these epithelial cells, again, the fatty parti- cles are passed on into the interior of the lacteal vessels (Figs. 81 and 82), but how they pass, and what laws govern their so doing, are not at present exactly known. It is probable that the process of absorption by the epithe- lial cells, is assisted by the pressure exercised on the contents of the intestines by their contractile walls; and that the ab- sorption of fatty particles is also facilitated by the presence of the bile, the pancreatic and intestinal secretions, which moisten the absorbing surface. For it has been found by experiment, that the passage of oil through an animal membrane is made much easier when the latter is impregnated with an alkaline fluid. The real source of the lymph, and the mode in which its absorption is effected by the lymphatic vessels, were long mat- ters of discussion. But the problem has been much simplified by more accurate knowledge of the anatomical relations of the lymphatic capillaries. It is most probable that the lymph is derived, in great part, from the liquor sanguinis, which, as be- fore remarked, is always exuding from the blood-capillaries into the interstices of the tissues in which they lie; and changes in the character of the lymph correspond very closely with changes in the character of either the whole mass of Absorption by the Lymphatic Vessels. 292 ABSORPTION. blood, or of that in the vessels of the part from which the lymph is examined. Thus Herbst found that the coagula- bility of the lymph is directly proportionate to that of the blood ; and that when fluids are injected into the bloodvessels in sufficient quantity to distend them, the injected substance may be almost directly afterwards found in the lymphatics. It is not improbable, however, that some other matters than those originally contained in the exuded liquor sanguinis may find their way with it into the lymphatic vessels. Parts which, having entered into the composition of a tissue, and, having fulfilled their purpose, require to be removed, may not be altogether excrementitious, but may admit of being reorganized and adapted again for nutrition ; and these may be absorbed by the lymphatics, and elaborated wdth the other contents of the lymph in passing through the glands. Lymph-Hearts.—In reptiles and some birds, an important auxiliary to the movement of the lymph and chyle is supplied in certain muscular sacs, named lymph-hearts (Fig. 100), and Mr. Wharton Jones has lately shown that the caudal heart of the eel is a lymph-heart also. The number and position of Fl«. 100. Lymphatic heart (9 lines long, 4 lines broad) of a large species of serpent, the Python bivittatus (after E. Weber). 4. The external cellular coat. 5. The thick muscular coat. Four muscular columns run across its cavity, which communicates with three lymphatics (1—only one is seen here), with two veins (2,2). 6. The smooth lining membrane of the cavity. 7. A small appendage, or auricle, the cavity of which is eoutinuons with that of the rest of the organ. these organs vary. In frogs and toads there are usually four, two anterior and two posterior; in the frog, the posterior lymph- heart on each side is situated in the isehiatic region, just be- neath the skin; the anterior lies deeper, just over the transverse process of the third vertebra. Into each of these cavities several lymphatics open, the orifices of the vessels being guarded ABSORPTION BY BLOODVESSELS. 293 by valves, which prevent the retrograde passage of the lymph. From each heart a single vein proceeds and conveys the lymph directly into the venous system. In the frog, the inferior lym- phatic heart, on each side, pours its lymph into a branch of the ischiatic vein ; by the superior, the lymph is forced into a branch of the jugular vein, which issues from its anterior sur- face, and which becomes turgid each time that the sac contracts. Blood is prevented from passing from the vein into the lym- phatic heart by a valve at its orifice. The muscular coat of these hearts is of variable thickness; in some cases it can only be discovered by means of the micro- scope ; but in every case it is composed of transversely-striated fibres. The contractions of the hearts are rhythmical, occur- ring about sixty times in a minute, slowly, and, in comparison with those of the blood-hearts, feebly. The pulsations of the cervical pair are not always synchronous with those of the pair in the ischiatic region, and even the corresponding sacs of opposite sides are not always synchronous in their action. Unlike the contractions of the blood-heart, those of the lymph-heart appear to be directly dependent upon a certain limited portion of the spinal cord. For Volkmann found that so long as the portion of spinal cord corresponding to the third vertebra of the frog was uninjured, the cervical pair of lym- phatic hearts continued pulsating after all the rest of the spinal cord and the brain was destroyed; while destruction of this portion, even though all other parts of the nervous centres were uninjured, instantly arrested the heart’s movements. The posterior or ischiatic pair of lymph-hearts were found to be governed, in like manner, by the portion of spinal cord cor- responding to the eighth vertebra. Division of the posterior spinal roots did not arrest the movements; but division of the anterior roots caused them to cease at once. Absorption by Bloodvessels. The process thus named is that which has been commonly called absorption by the veins; but the term here employed seems preferable, since, though the materials absorbed are commonly found in the veins, this is only because they are carried into them with the circulating blood, after being ab- sorbed by all the bloodvessels (but chiefly by the capillaries) with which they were placed in contact. There is nothing in the mode of absorption by bloodvessels, or in the structure of veins, which can make the latter more active than arteries of the same size, or so active as the capillaries, in the process. In the absorption by the lymphatics or lacteal vessels just 294 ABSORPTION. described, there appears something like the exercise of choice in the materials admitted into them. But the absorption by bloodvessels presents no such appearance of selection of ma- terials; rather, it appears, that every substance, whether gaseous, liquid, or a soluble or minutely divided solid, may be absorbed by the bloodvessels, provided it is capable of per- meating their walls, and of mixing with the blood ; and that of all such substances, the mode and measure of absorption are determined solely by their physical or chemical properties and conditions, and by those of the blood and the walls of the bloodvessels. The phenomena are, indeed, exactly comparable to that passage of fluids through membrane, which occurs quite inde- pendently of vital conditions, and the earliest and best scien- tific investigation of which was made by Dutrochet. The in- strument which he employed in his experiments was named an endosmometer. It may consist of a graduated tube expanded into an open-mouthed bell at one end, over which a portion of mem- brane is tied (Fig. 101). If now the bell be filled with a solution of a salt, say chloride of sodium, and be immersed in water, the water will pass into the solution, and part of the salt will pass out into the water; the water will pass into the solution much more rapidly than the salt will pass out into the water, and the diluted solution will rise in the tube. To this passage of fluids through membrane the term Osmosis is ap- plied. The natux-e of the membrane used as a septum, and its affinity for the fluids subjected to ex- periment, have an important influence, as might be anticipated, on the rapidity and duration of the osmotic current. Thus, if a piece of ordinary bladder be used as the septum between water and alcohol, the current is almost solely from the water to the alcohol, on account of the much greater affinity of water for this kind of mem- brane ; while, on the other hand, in the case of a membrane of caoutchouc, the alcohol, from its greater affinity for this sub- stance, would pass freely into the water. Various opinions have been advanced in regard to the na- ture of the force by which fluids of different chemical compo- sition thus tend to mix through an intervening membrane. According to some, this power is the result of the different de- grees of capillary attraction exerted by the pores of the mem- Fig. 101. COLLOIDS AND CRYSTALLOIDS. 295 brane upon the two fluids. Prof. Graham, however, believes that the passage or osmose of water through membrane may be explained by supposing that it combines with the membra- nous septum, which thus becomes hydrated, and that on reach- ing the other side it partly leaves the membrane, which thus becomes to a certain degree dehydrated. For example, a membrane such as that used in the endosmometer, is hydrated to a higher degree if placed in pure water than in a neutral saline solution. Hence, in the case of the endosmometer filled with the saline solution and placed in water, the equilibrium of hydration is different on the two sides; the outer surface being in contact with pure water tends to hydrate itself in a higher degree than the inner surface does. “ When the full hydration of the outer surface extends through the thickness of the membrane, and reaches the inner surface, it there re- ceives a check. The degree of hydration is lowered, and water must be given up by the inner layer of the membrane.” Thus the osmose or current of water through the membrane is caused. The passage outwards of the saline solution, on the other hand, is not due, probably, to any actual fluid current; but to a solution of the salt in successive layers of the water contained in the pores of the membrane, until it reaches the outer surface and diffuses in the water there situate. Thus, “ the water movement in osmose is an affair of hydra- tion and of dehydration in the substance of the membrane or other colloid septum, and the diffusion of the saline solution placed within the osmometer has little or nothing to do with the osmotic result, otherwise than as it affects the state of hy- dration of the septum.” Prof. Graham has classed various substances according to the degree in which they possess this property of passing, when in a state of solution in water, through membrane ; those which pass freely being termed crystalloids, and those which pass with difficulty, colloids. This distinction, however, between colloids and crystalloids, which is made the basis of their classification, is by no means the only difference between them. The colloids, besides the absence of power to assume a crystalline form, are character- ized by their inertness as acids or bases,' and feebleness in all ordinary chemical relations. Examples of them are found in albumen, gelatin, starch, hydrated alumina, hydrated silicic acid, &c.; while the wystalloids are characterized by qualities the reverse of those just mentioned as belonging to colloids. Alcohol, sugar, and ordinary saline substances are examples of crystalloids. Absorption by bloodvessels is the consequence of their walls 296 ABSORPTION. being, like the membranous septum of the endosmometer, por- ous and capable of imbibing fluids, and of the blood being so composed that most fluids will mingle with it. The process of absorption, in an instructive, though very imperfect degree, may be observed in any portion of vascular tissue removed from the body. If such a one be placed in a vessel of water, it will shortly swell, and become heavier and moister, through the quantity of water imbibed or soaked into it; and if now, the blood contained in any of its vessels be let out, it will be found diluted with water, which has been absorbed by the bloodvessels and mingled with the blood. The water round the piece of tissue also will become bloodstained ; and if all be kept at perfect rest, the stain derived from the solution of the coloring matter of the blood (together w ith which chemistry would detect some of the albumen and other parts of the liquor sanguinis) will spread more widely every day. The same will happen if the piece of tissue be placed in a saline solution in- stead of water, or in a solution of coloring or odorous matter, either of which will give their tinge or smell to the blood, and receive, in exchange, the color of the blood. Even so simple an experiment will illustrate the absorption by bloodvessels during life; the process it shows is imitated, but with these differences: that, during life, as soon as water or any other substance is admitted into the blood, it is carried from the place at which it was absorbed into the general cur- rent of the circulation, and that the coloring matter of the blood is not dissolved so as to ooze out of the bloodvessels into the fluid which they are absorbing. The absorption of gases by the blood may be thus simply imitated. If venous blood be suspended in a moist bladder in the air, its surface will be reddened by the contact of oxygen, which is first dissolved in the fluid that moistens the bladder, and is then carried in the fluid to the surface of the blood : while, on the other hand, watery vapor and carbonic acid will pass through the membrane, and be exhaled into the air. In all these cases alike there is a mutual interchange be- tween the substances ; wdiile the blood is receiving water, it is giving out its coloring matter and other constituents : or, while it is receiving oxygen, it is giving out carbonic acid and water; so that, at the end of the experiment, the two substances em- ployed in it are mixed; and if, instead of a piece of tissue, one had taken a single bloodvessel full of blood and placed it in the water, both blood and water wfould, after a time, have been found both inside and outside the vessel. In such a case, more- over, if one were to determine accurately the quantity of water that passed to the blood, and of blood that passed to the water, RAPIDITY OF ABSORPTION. 297 it would be found that the former was always greater than the latter. And so with other substances ; it almost always hap- pens, that if the two liquids placed on opposite sides of a mem- brane be of different densities or specific gravities, a larger quantity of the less dense fluid passes into the more dense, than of the latter into the former. The rapidity with which matters may be absorbed from the stomach probably by the bloodvessels chiefly, and diffused through the textures of the body, may be gathered from the history of some experiments by Dr. Bence Jones. From these it appears that even in a quarter of an hour after being given on an empty stomach, chloride of lithium may be diffused into all the vascular textures of the body, and into some of the non- vascular, as the cartilage of the hip-joint, as well as into the aqueous humor of the eye. Into the outer part of the crystal- line lens it may pass after a time, varying from half an hour to an hour and a half. Carbonate of lithia, when taken in five or ten-grain doses on an empty stomach, may be detected in the urine in 5 or 10 minutes; or, if the stomach be full at the time of taking the dose, in 20 minutes. It may sometimes be detected in the urine, moreover, for six, seven, or even eight days. Some experiments on the absorption of various mineral and vegetable poisons, by Mr. Savory, have brought to light the singular fact, that, in some cases, absorption takes place more rapidly from the rectum than from the stomach. Strychnia, for example, when in solution, produces its poisonous effects much more speedily when introduced into the rectum than into the stomach. When introduced in the solid form, how- ever, it is absorbed more rapidly from the stomach than from the rectum, doubtless because of the greater solvent property of the secretion of the former than of that of the latter. With regard to the degree of absorption by living blood- vessels, much depends on the facility with which the substance to be absorbed can penetrate the membrane or tissue which lies between it and the bloodvessels; for, naturally, the blood- vessels are not bare to absorb. Thus absorption will hardly take place through the epidermis, but is quick when the epi- dermis is removed, and the same vessels are covered with only the surface of the cutis, or with granulations. In general, the absorption through membranes is in an inverse proportion to the thickness of their epithelia; so Muller found the urinary bladder of a frog traversed in less than a second ; and the ab- sorption of poisons by the stomach or lungs appears sometimes accomplished in an immeasurably small time. The substance to be absorbed must, as a general rule, be in 298 ABSORPTION. the liquid or gaseous state, or, if a solid, must be soluble in the fluids with which it is brought in contact. Hence the marks of tattooing, and the discoloration produced by nitrate of silver taken internally, remain. Mercury may be absorbed even in the metallic state; aud in that state may pass into and remain in the bloodvessels, or be deposited from them (Oesterlen) ; and such substances as exceedingly finely-divided charcoal, when taken into the alimentary canal, have been found in the mesenteric veins (Oesterlen) ; the insoluble materials of oint- ments may also be rubbed into the bloodvessels; but there are no facts to determine how these various substances effect their passage. Oil, minutely divided, as in an emulsion, will pass slowly into bloodvessels, as it will through a filter moistened with water (Vogel); and, without doubt, fatty matters find their way into the bloodvessels as well as the lymphvessels of the intestinal canal, although the latter seem to be specially intended for their absorption. As in the experiments before referred to, the less dense the fluid to be absorbed, the more speedy, as a general rule, is its absorption by the living bloodvessels. Hence the rapid ab- sorption of water from the stomach; also of weak saline solu- tions ; but with strong solutions, there appears less absorption into, than effusion from, the bloodvessels. The absorption is the less rapid the fuller aud tenser the bloodvessels are ; and the tension may be so great as to hinder altogether the entrance of more fluid. Thus, Magendie found that when he injected water into a dog’s veins to repletion, poison was absorbed very slowly; but when he diminished the tension of the vessels by bleeding, the poison acted quickly. So, when cupping-glasses are placed over a poisoned wound, they retard the absorption of the poison, not only by diminish- ing the velocity of the circulation in the part, but by filling all its vessels too full to admit more. On the same ground, absorption is the quicker the more rapid the circulation of the blood; not because the fluid to be absorbed is more quickly imbibed into the tissues, or mingled with the blood, but because as fast as it enters the blood, it is carried away from the part, and the blood, being constantly renewed, is constantly as fit as at the first for the reception of the substance to be absorbed. NUTRIT ION. 299 CHAPTER XI. NUTRITION AND GROWTH. Nutrition or nutritive assimilation is that modification of the formative process peculiar to living bodies by which tissues and organs already formed maintain their integrity. By the incorporation of fresh nutritive principles into their substance, the loss consequent on the waste and natural decay of the com- ponent particles of the tissues is repaired ; and each elementary particle seems to have the power not only of attracting ma- terials from the blood, but of causing them to assume its struc- ture, and participate in its vital properties. The relations between development and growth have been already stated (Chap. I) ; under the head of Nutrition will be now considered the process by which parts are maintained in the same general conditions of form, size, and composition, which they have already, by development and growth, at- tained ; and this, notwithstanding continual changes in their component particles. It is by this process that an adult per- son, in health, is maintained, through a series of some years, with the same general outline of features, the same size and form, and perhaps even the same weight; although, during all this time, the several portions of his body are continually changing: their particles decaying and being removed, and then replaced by the formation of new ones, which, in their turn, also die and pass away. Neither is it only a general similarity of the whole body which is thus maintained. Every organ or part of the body, as much as the whole, exactly main- tains its form and composition, as the issue of the changes con- tinually taking place among its particles. The change of component particles, in which the nutrition of organs consists, is most evidently shown when, in growth, they maintain their form and other general characters, but increase in size. When, for example, a long bone increases in circumference, and in the thickness of its walls, while, at the same time, its medullary cavity enlarges, it can only be by the addition of materials to its exterior, and a coincident removal of them from the interior of its wall; and so it must be with the growth of even the minutest portions of a tissue. And that a similar change of particles takes place, even while 300 NUTRITION. parts retain a perfect uniformity, may be proved, if it can be shown that all the parts of the body are subject to waste and impairment. In many parts, the removal of particles is evident. Thus, as will be shown when speaking of secretion, the elementary structures composing glands are the parts of which the secre- tions are composed : each gland is constantly casting off its cells, or their contents, in the secretion which it forms: yet each gland maintains its size and proper composition, because for every cell cast off a new one is produced. So also the epidermis and all such tissues are maintained. In the mus- cles, it seems nearly certain, that each act of contraction is accompanied with a change in the composition of the con- tracting tissue, although the change from this cause is less rapid and extensive than was once supposed. Thence, the development of heat in acting muscles, and then the discharge of urea, carbonic acid, and water—the ordinary products of the decomposition of the animal tissues—which follows all active muscular exercise. Indeed, the researches of Helm- holtz almost demonstrate the chemical change that muscles undergo after long-repeated contractions; yet the muscles retain their structure and composition, because the particles thus changed are replaced by new ones resembling those which preceded them. So again, the increase of alkaline phosphates discharged with the urine after great mental exertion, seems to prove that the various acts of the nervous system are at- tended with change in the composition of the nervous tissue; yet the condition of that tissue is maintained. In short, for every tissue there is sufficient evidence of impairment in the discharge of its functions: without such change, the produc- tion or resistance of physical force is hardly conceivable: and the proof as well as the purpose of the nutritive process ap- pears in the repair or replacement of the changed particles; so that, notwithstanding its losses, each tissue is maintained unchanged. But besides the impairment and change of composition to which all parts are subject in the discharge of their natural functions, an amount of impairment which will be in direct proportion to their activity, they are all liable to decay and degeneration of their particles, even while their natural actions are not called forth. It may be proved, as Dr. Carpenter first clearly showed, that every particle of the body is formed for a certain period of existence in the ordinary condition of active life ; at the end of which period, if not previously destroyed by outward force or exercise, it degenerates and is absorbed, or dies and is cast out. NUTRITION OF HAIR. 301 The simplest examples that can be adduced of this are in the hair and teeth; and it may be observed, that, in the pro- cess which will now be described, all the great features of the process of nutrition seem to be represented.1 An eyelash which naturally falls, or which can be drawn out without pain, is one that has lived its natural time, and has died, and been separated from the living parts. In its bulb such a one will be found different from those that are Fig. 102. intended to represent the changes undergone by a hair towards the close of its period of existence. At a, its activity of growth is diminishing, as shown by the small quantity of pigment contained in the cells of the pulp, and by the interrupted line of dark medullary substance. At b, provision is being made for the formation of a new hair, by the growth of a new pulp connected with the pulp or capsule of the old hair. c. A hair at the end of its period of life, deprived of its sheath and of the mass of cells composing the pulp of a living hair. still living in any period of' their age. In the early period of the growth of a dark eyelash, the medullary substance appears like an interior cylinder of darker granular substance, con- 1 These and other instances are related more in detail in Mr. Paget’s Lectures on Surgical Pathology, from which this chapter was originally written. 302 NUTRITION. tinued down to the deepest part, where the hair enlarges to form the bulb. This enlargement, which is of nearly like form, appears to depend on the accumulation of nucleated cells, whose nuclei, according to their position, are either, by narrowing and elongation, to form the fibrous substance of the outer part of the growing and further protruding hair, or are to be transformed into the granular matter of its medullary portion. At the time of early and most active growth, all the cells and nuclei contain abundant pigment-matter, and the whole bulb looks nearly black. The sources of the material out of which the cells form themselves are at least two; the inner surface of the sheath or capsule, which dips into the skin, enveloping the hair, and the surface of a vascular pulp which fits in a conical cavity in the bottom of the hair-bulb. Such is the stateof parts so longas the growinghair is all dark. But as the hair approaches the end of its existence, instead of the almost sudden enlargement at its bulb, it only swells a little, and then tapers nearly to a point; the conical cavity in its base is contracted; and the cells produced on the inner surface of the capsule contain no pigment. Still, for some time, it continues thus to live and grow; and the vigor of the pulp lasts rather longer than that of the sheath or capsule, for it continues to produce pigment-matter for the medullary sub- stance of the hair after the cortical substance has become white. Thus the column of dark medullary substance appears paler and more slender, and perhaps interrupted, down to the point of the conical pulp, which, though smaller, is still dis- tinct, because of the pigment-cells covering its surface. At length the pulp can be no longer discerned, and un- colored cells are alone produced, and maintain the latest growth of the hair. With these it appears to grow yet some further distance; for traces of the elongation of their nuclei into fibres appear in lines running from the inner surface of the capsule inwards and along the surface of the hair ; and the column of dark medullary substance ceases at some distance above the lower end of the contracted hair-bulb. The end of all is the complete closure of the conical cavity in which the hair-pulp was lodged, the cessation of the production of new cells from the inner surface of the capsule, and the detach- ment of the hair, which, as a dead part, is separated and falls. Such is the life of a hair, and such its death ; which death is spontaneous, independent of exercise, or of any mechanical external force—the natural termination of a certain period of life. Yet, before the hair dies, provision is made for its suc- cessor : for when its growth is failing, there appears below its base a dark spot, the germ or young pulp of the new hair MAINTENANCE BY NUTRITION. 303 covered with cells containing pigment, and often connected by a series of pigment-cells with the old pulp or capsule (Fig. 102, b). Probably there is an intimate analogy between the process of successive life and death, and life communicated to a suc- cessor, which is here shown, and that which constitutes the ordinary nutrition of a part. It may be objected, that the death and casting out of the hair cannot be imitated in inter- nal parts; therefore, for an example in which the assumed absorption of the worn-out or degenerate internal particles is imitated in larger organs at the end of their appointed period of life, the instance of the deciduous or milk-teeth may be adduced. Each milk-tooth is developed from its germ; and in the course of its own development, separates a portion of itself to be the germ of its successor; and each, having reached its perfec- tion, retains for a time its perfect state, and still lives, though it does not grow. But at length, as the new tooth comes, the de- ciduous tooth dies ; or rather its crown dies, and is cast out like the dead hair, while its fang, with its bony sheathing, and vas- cular and nervous pulp, degen- erates and is absorbed (Fig. 103). The degeneration is accompanied by some unknown spontaneous decomposition of the fang ; for it could not be absorbed unless it was first so changed as to be solu- ble. And it is degeneration, not death, which precedes its re- moval ; for when a tooth-fang dies, as that of the second tooth does in old age, then it is not absorbed, but cast out entire, as a dead part. Such, or generally such, it seems almost certain, is the pro- cess of maintenance by nutrition; the hair and teeth may be fairly taken as types of what occurs in other parts, for they are parts of complex organic structure and composition, and the teeth-pulps, which are absorbed as well as the fangs, are very vascular and sensitive. Nor are they the only instances that might be adduced. The like development, persistence for a time in the perfect state, death, and discharge, appear in all the varieties of cuti- Fig. 103. Section of a portion of the upper jaw of a child, showing a new tooth in process of formation, the fang of the corresponding deciduous tooth being absorbed. 304 NUTRITION. cles and gland-cells; and in the epidermis, as in the teeth, there is evidence of decomposition of the old cells, in the fact of the different influence which acetic acid and potash exer- cise on them and on the young cells. Seeing, then, that the process of nutrition, as thus displayed, both in active organs and in elementary cells, appears in these respects similar, the general conclusion may be that, in nutrition, the ordinary course of each complete elementary organ in the body, after the attainment of its perfect state by development and growth, is to remain in that state for a time; then, independently of the death or decay of the whole body, and in some measure, independently of its own exercise, or exposure to external violence, to die or to degenerate; and then, being cast out or absorbed, to make way for its successor. It appears, moreover, that the length of life which each part is to enjoy is fixed and determinate, though in some degree subject to accidents and to the expenditure of life in exercise. It is not likely that all parts are made to last a certain and equal time, and then all need to be changed. The bones, for instance, when once completely formed, must last longer than the muscles and other softer tissues. But when we see that the life of certain parts is of determined length, whether they be used or not, we may assume, from analogy, the same of nearly all. Now, the deciduous human teeth have an appointed average duration of life. So have the deciduous teeth of all other animals ; and in all the numerous instances of moulting, shed- ding of antlers, of desquamation, change of plumage in birds, and of hair in Mammalia, the only explanation is that these organs have their severally appointed times of living, at the ends of which they degenerate, die, are cast away, and in due time are replaced by others, which, in their turn, are to be de- veloped to perfection, to live their life in the mature state, and in their turn to be cast off. So also, in some elementary struc- tures, we may discern the same laws of determinate period of life, death, or degeneration, and replacement. They are evi- dent in the history of the blood-corpuscles, both in the super- seding of the first set of them by the second at a definite period in the life of the embryo, and in the replacement of those that degenerate by others new-formed from lymph-corpuscles. (See p. 83.) And if we could suppose the blood-corpuscles grouped together in a tissue instead of floating, we might have in the changes they present an image of the nutrition of the elements of the tissues. The duration of life in each particle is, however, liable to be modified; especially by the exercise of the function of the PROCESS OF NUTRITION. 305 part The less a part is exercised the longer do its component particles appear to live: the more active its functions are, the less prolonged is the existence of its individual particles. So in the case of single cells; if the general development of the tadpole be retarded by keeping it in a cold, dark place, and if hereby the function of the blood-corpuscles be slowly and im- perfectly discharged, they will maintain their embryonic state for even several weeks later than usual, the development of the second set of corpuscles will be proportionally postponed, and the individual life of the corpuscles of the first set will be, by the same time, prolonged. Such being the mode in which the necessity for the process of nutritive maintenance is created, such the sources of impair- ment and waste of the tissues, the next consideration may be the manner in which the perfect state of a part is maintained by the insertion of new particles in the place of those that are absorbed or cast off. The process by which a new particle is formed in the place of the old one is probably always a process of development; that is, the cell or fibre, or other element of tissue, passes in its formation through the same stages of development as those elements of the same tissue did which were first formed in the embryo. This is probable from the analogy of the hair, the teeth, the epidermis, and all the tissues that can be observed : in all, the process of repair or replacement is effected through development of the new parts. The existence of nuclei or cyto- blasts in nearly all parts that are the seats of active nutritiop makes the same probable. For these nuclei, such as are seen so abundant in strong, active muscles, are not remnants of the embryonic tissue, but germs or organs of power for new forma- tion, and their abundance often appears directly proportionate to the activity of growth. Thus, they are always abundant in the foetal tissues, and those of the young animal; and they are peculiarly numerous in the muscles and the brain, and their disappearance from a part in which they usually exist is a sure accompaniment and sign of degeneration. A difference may be drawn between what may be called nutritive reproduction and nutritive repetition. The former is shown in the case of the human teeth. As the deciduous tooth is being developed, a part of its productive capsule is detached, and serves as a germ for the formation of the second tooth ; in which second tooth, therefore, the first may be said to be re- produced, in the same sense as that in which we speak of the organs by which new individuals are formed, as the reproduc- tive organs. But in the shark’s jaws, and others, in which we see row after row of teeth succeeding each other, the row be- 306 NUTRITION. hind is not formed of germs derived from the row before: the front row is simply repeated in the second one, the second in the third, and so on. So, in cuticle, the deepest layer of epi- dermis-cells derives no germs from the layer above : their de- velopment is not like a reproduction of the cells that have gone on towards the surface before them : it is only a repetition. It is not improbable that much of the difference in the degree of repair, of which the several tissues are capable alter injuries or diseases, may be connected with these differences in their ordinary mode of nutrition. In order that the process of nutrition may be perfectly ac- complished, certain conditions are necessary. Of these, the most important are: 1. A right state and composition of the blood, from which the materials for nutrition are derived. 2. A regular and not far distant supply of such blood. 3. A cer- tain influence of the nervous system. 4. A natural state of the part to be nourished. 1. This right condition of the blood does not necessarily im- ply its accordance with any known standard of composition, common to all kinds of healthy blood, but rather the existence of a certain adaptation between the blood and the tissues, and even the several portions of each tissue. Such an adaptation, peculiar to each individual, is determined in its’first formation, and is maintained in the concurrent development and increase of both blood and tissues; and upon its maintenance in adult life appears to depend the continuance of a healthy process of nutrition, or, at least, the preservation of that exact sameness of the whole body and its parts, which constitutes the perfec- tion of nutrition. Some notice of the maintenance of this sameness in the blood has been given already (p. 84), in speaking of the power of assimilation which the blood exer- cises, a power exactly comparable with this of maintenance by nutrition in the tissues. And evidence of the adaptation be- tween the blood and the tissues, and of the exceeding fineness of the adjustment by which it is maintained, is afforded by the phenomena of diseases, in which, after the introduction of cer- tain animal poisons, even in very minute quantities, the whole mass of the blood is altered in composition, and the solid tis- sues are perverted in their nutrition. It is necessary to refer only to such diseases as syphilis, small-pox, and other erup- tive fevers, in illustration. And when the absolute dependence of all the tissues on the blood for their very existence is re- membered, on the one hand, and, on the other, the rapidity with which substances introduced into the blood are diffused into all, even non-vascular textures (p. 297), it need be no source of wonder that any, even the slightest alteration, from CONDITIONS NECESSARY FOR NUTRITION. 307 the normal constitution of the blood, should be immediately reflected, so to speak, as a change in the nutrition of the solid tissues and organs which it is destined to nourish. 2. The necessity of an adequate supply of appropriate blood in or near the part to be nourished, in order that its nutrition may be perfect, is shown in the frequent examples of atrophy of parts to which too little blood is sent, of mortification or ar- rested nutrition when the supply of blood is entirely cut off, and of defective nutrition when the blood is stagnant in a part. That the nutrition of a part may be perfect, it is also neces- sary that the blood should be brought sufficiently near to it for the elements of the tissue to imbibe, through the walls of the bloodvessels, the nutritive materials which they require. The bloodvessels themselves take no share in the process of nutrition, except as carriers of the nutritive matter. There- fore, provided they come so near that this nutritive matter may pass by imbibition into the part to be nourished, it is comparatively immaterial whether they ramify within the substance of the tissue, or are distributed only on its surface or border. The bloodvessels serve alike for the nutrition of the vascular and the non-vascular parts, the difference between which, in regard to nutrition, is less than it may seem. For the vascu- lar, the nutritive fluid is carried in streams into the interior; for the non-vascular, it flows on the surface ; but in both alike, the parts themselves imbibe the fluid ; and although the pas- sage through the walls of the bloodvessels may effect some change in the materials, yet all the process of formation is, in both alike, outside the vessels. Thus, in muscular tissue, the fibrils in the very centre of the fibre nourish themselves: yet these are distant from all bloodvessels, and can only by imbibition receive their nutriment. So, in bones, the spaces between the bloodvessels are wider than in muscle; yet the parts in the meshes nourish themselves, imbibing materials from the nearest source. The non-vascular epidermis, though no vessels pass into its substance, yet imbibes nutritive matter from the vessels of the immediately subjacent cutis, and main- tains itself, and grows. The instances of the cornea and vitre- ous humor are stronger, yet similar; and sometimes even the same tissue is in one case vascular, in the other not, as the osseous tissue, which, when it is in masses or thick layers, has bloodvessels running into it; but when it is in thin layers, as in the lachrymal and turbinated bones, has not. These bones subsist on the blood flowing in the minute vessels of the mucous membrane, from which the epithelium derives nutriment on one side, the bone on the other, and the tissue of the membrane 308 NUTRITION. itself on every side: a striking instance how, from the same source, many tissues maintain themselves, each exercising its peculiar assimilative and self-formative power. 3. The third condition said to be essential to a healthy nu- trition, is a certain influence o f the nervous system. It has been held that the nervous system cannot be essential to a healthy course of nutrition, because in plants and the early embryo, and in the lowest animals, in which no nervous system is developed, nutrition goes on without it. But this is no proof that in animals which have a nervous system, nutri- tion may be independent of it; rather it may be assumed, that in ascending development, as one system after another is added or increased, so the highest (and, highest of all, the nervous system) will always be inserted and blended in a more and more intimate relation with all the rest: according to the general law, that the interdependence of parts augments with their development. The reasonableness of this assumption is proved by many facts showing the influence of the nervous system on nutrition, and by the most striking of these facts being observed in the higher animals, and especially in man. The influence of the mind in the production, aggravation, and cure of organic dis- eases is matter of daily observation, and a sufficient proof of influence exercised on nutrition through the nervous system. Independently of mental influence, injuries either to por- tions of the nervous centres, or to individual nerves, are fre- quently followed by defective nutrition of the parts supplied by the injured nerves, or deriving their nervous influence from the damaged portions of the nervous centres. Thus, lesions of the spinal cord are sometimes followed by mortification of por- tions of the paralyzed parts; and this may take place very quickly, as in a case by Sir B. C. Brodie, in which the ankle sloughed within twenty-four hours after an injury of the spine. After such lesions also, the repair of injuries in the paralyzed parts may take place less completely than in others; so, Mr. Travers mentions a case in which paraplegia was produced by fracture of the lumbar vertebrce, and, in the same accident, the humerus and tibia were fractured. The former in due time united ; the latter did not. The same fact was illustrated by some experiments of Dr. Baly, in which having, in salaman- ders, cut off the end of the tail, and then thrust a thin wire some distance up the spinal canal, so as to destroy the cord, he found that the end of the tail was reproduced more slowly than in other salamanders in whom the spinal cord was left unin- jured above the point at which the tail was amputated. Illus- trations of the same kind are furnished by the several cases in INFLUENCE OF NERVOUS SYSTEM. 309 which division or destruction of the trunk of the trigeminal nerve has been followed by incomplete and morbid nutrition of the corresponding side of the face; ulqeration of the cornea being often directly or indirectly one of the consequences of such imperfect nutrition. Part of the wasting and slow de- generation of tissue in paralyzed limbs is probably referable also to the withdrawal of nervous influence from them; though, perhaps, more is due to the want of use of the tissues. Undue irritation of the trunks of nerves, as well as their division or destruction, is sometimes followed by defective or morbid nutrition. To this may be referred the cases in which ulceration of the parts supplied by the irritated nerves occurs frequently, and continues so long as the irritation lasts. Further evidence of the influence of the nervous system upon nutrition is furnished by those cases in which, from mental an- guish, or in severe neuralgic headaches, the hair becomes gray very quickly, or even in a few hours. So many and various facts leave little doubt that the ner- vous system exercises an influence over nutrition as over other organic processes; and they cannot be explained by supposing that the changes in the nutritive processes are only due to the variations in the size of the bloodvessels supplying the affected parts. The question remains, through what class of nerves is the influence exerted ? When defective nutrition occurs in parts rendered inactive by injury of the motor nerve alone, as in the muscles and other tissues of a paralyzed face or limb, it may appear as if the atrophy were the direct consequence of the loss of power in the motor nerves ; but it is more probable that the atrophy is the consequence of the want of exercise of the parts; for if the muscles be exercised by artificial irritation of their nerves their nutrition will be less defective (J. Reid). The defect of the nutritive process which ensues in the face and other parts, moreover, in consequence of destruction of the trigeminal nerve, cannot be referred to loss of influence of any motor nerves; for the motor nerves of the face and eye, as well as the olfactory and optic, have no share in the defective nu- trition which follows injury of the trigeminal nerve; and one or all of them may be destroyed without any direct disturbance of the nutrition of the parts they severally supply. It must be concluded, therefore, that the influence which is exercised by nerves over the nutrition of parts to which they are distributed is to be referred either to those among their branches which conduct impressions to the brain and spinal cord, namely, the nerves of common sensation, or, as it is by some supposed, by nerve-fibres which preside specially over 310 NUTRITION. the nutrition of the tissues and organs to which they are sup- plied. Such special nerves are called trophic nerves (see chap- ter on the Nervous System). It is not at present possible to say whether the influence on nutrition is exercised through the cerebro-spinal or through the sympathetic nerves, which, in the parts on which the observa- tion has been made, ai’e generally combined in the same sheath. The truth perhaps is, that it may be exerted through either or both of these nerves. The defect of nutrition which ensues after lesion of the spinal cord alone, the sympathetic nerves being uninjured, and the general atrophy which sometimes occurs in consequence of diseases of the brain, seem to prove the influence of the cerebro-spinal system : while the observa- tion of Magendie and Mayer, that inflammation of the eye is a constant result of ligature of the sympathetic nerve in the neck, and many other observations of a similar kind, exhibit very well the influence of the latter nerve in nutrition. 4. The fourth condition necessary to healthy nutrition is a healthy state of the part to be nourished. This seems proved by the very nature of the process, which consists in the forma- tion of new parts like those already existing; for, unless the latter are healthy, the former cannot be so. Whatever be the condition of a part, it is apt to be perpetuated by assimilating exactly to itself, and endowing with all its peculiarities, the new particles which it forms to replace those that degenerate. So long as a part is healthy, and the other conditions of healthy nutrition exist, it maintains its healthy condition. But, ac- cording to the same law, if the structure of a part be diseased or in any way altered from its natural condition, the alteration is maintained; the altered, like the healthy structure, is per- petuated. The same exactness of the assimilation of the new parts to the old, which is seen in the nutrition of the healthy tissues, may be observed also in those that are formed in disease. By it, the exact form and relative size of a cicatrix are preserved from year to year; by it, the thickening and induration to which inflammation gives rise are kept up, and the various morbid states of the blood in struma, syphilis, and other chronic diseases are maintained, notwithstanding all diversities of diet. By this precision of the assimilating process, may be explained the law that certain diseases occur only once in the same per- son, and that certain others are apt to recur frequently; because in both cases alike, the alteration produced by the first attack of the disease is maintained by the exact likeness which the new parts bear to the old ones. The period, however, during which an alteration of structure GROWTH. 311 may be exactly maintained by nutrition, is not unlimited; lor in nearly all altered parts there appears to exist a tendency to recover the perfect state; and, in many cases, this state is, in time, attained. To this we may attribute the possibility of re- vaccination after the lapse of some years ; the occasional recur- rence of small-pox, scarlet-fever, and the like diseases, in the same person; the wearing out of scars, and the complete restor- ation of tissues that have been altered by injury or disease. Such are some of the more important conditions which ap- pear to be essential to healthy nutrition. Absence or defect of any one of them is liable to be followed by disarrangement of the process; and the various diseases resulting from defective nutrition appear to be due to the failure of these conditions, more often than to imperfection of the process itself. GROWTH. Growth, as has been already observed, consists in the increase of a part in bulk and weight by the addition to its substance of particles similar to its own, but more than sufficient to re- place those which it loses by the waste or natural decay of its tissue. The structure and composition of the part remain the same; but the increase of healthy tissue which it receives is attended with the capability of discharging a larger amount of its ordinary function. While development is in progress, growth frequently pro- ceeds with it in the same part, as in the formation of the various organs and tissues of the embryo, in which parts, while they grow larger, are also gradually more developed until they attain their perfect state. But, commonly, growth continues after development is completed, and in some parts, continues even after the full stature of the body is attained, and after nearly every portion of it has gained its perfect state in both size and composition. In certain conditions, this continuance or a renewal of growth may be observed in nearly every part of the body. When parts have attained the full size which in the ordinary process of growth they reach, and are then kept in a moderate exercise of their functions, they commonly (as already stated) retain almost exactly the same dimensions through the adult period of life. But when, from any cause, a part already full- grown in proportion to the rest of the body, is called upon to discharge an unusual amount of its ordinary function, the demand is met by a corresponding increase or growth of the part. Illustrations of this are afforded by the increased thick- ening of cuticle at parts where it is subjected to an unusual 312 NUTRITION. degree of occasional pressure or friction, as in the palms of the hands of persons employed in rough manual labor ; by the enlargement and increased hardness of muscles that are largely exercised ; and by many other facts of a like kind. The increased power of nutrition put forth in such growth is greater than might be supposed ; for the immediate effect of increased exercise of a part must be a greater using of its tissue, and might be expected to entail a permanent thinning or diminution of the substance of the part. But the energy with which fresh particles are formed is sufficient not only to replace completely those that are worn away, but to cause an increase in the substance of the part—the amount of this in- crease being proportioned to the more than usual degree in which its functions are exercised. The growth of a part from undue exercise of its functions is always, in itself, a healthy process; and the increased size which results from it must be distinguished from the various kind of enlargement to which the same part may be subject from disease. In the former case, the enlargement is due to an increased quantity of healthy tissue, providing more than the previous power to meet a particular emergency; the other may be the result of a deposit of morbid material within the natural structure of the part, diminishing, instead of augment- ing, its fitness for its office. Such a healthy process of growth in a part, attended with increased power and activity of its functions, may, however, occur as the consequence of disease in some other part; in which case it is commonly called Hy- pertrophy, i. e., excess of nutrition. The most familiar ex- amples of this are in the increased thickness and robustness of the muscular walls of the cavities of the heart in cases of continued obstruction to the circulation; and in the increased development of the muscular coat of the urinary bladder when, from any cause, the free discharge of urine from it is interfered with. In both these cases, though the origin of the growth is the consequence of disease, yet the growth itself is natural, and its end is the benefit of the economy; it is only common growth renewed or exercised in a part which had attained its size in due proportion to the rest of the body. It may be further mentioned, in relation to the physiology of this subject, that when the increase of function, which is requisite in the cases from which hypertrophy results, cannot be efficiently discharged by mere increase of the ordinary tissue of the part, the development of a new and higher kind of tissue is frequently combined with this growth. An exam- ple of this is furnished by the uterus, in the walls of which, when it becomes enlarged by pregnancy, or by the growth of SECRETION. 313 fibrous tumors, organic muscular fibres, found in a very ill- developed condition in its quiescent state, are then enormously developed, and provide for the expulsion of the foetus or the foreign body. Other examples of the same kind are furnished by cases in which, from obstruction to the discharge of their contents and a consequently increased necessity for propulsive power, the coats of reservoirs and of ducts become the seat of development of organic muscular fibres, which could be said only just to exist in them before, or were present in a very imperfectly developed condition. Respecting the mode and conditions of the process of growth, it need only be said, that its mode seems to differ only in de- gree from that of common maintenance of a part; more par- ticles are removed from, and many more added to a growing tissue, than to one which only maintains itself. But so far as can be ascertained, the mode of removal, the disposition of the removed parts, and the insertion of the new particles, are as in simple maintenance. The conditions also of growth are the same as those of com- mon nutrition, and are equally or more necessary to its occur- rence. When they are very favorable or in excess, growth may occur in the place of common nutrition. Thus hair may grow profusely in the neighborhood of old ulcers, in consequence, apparently, of the excessive supply of blood to the hair-bulbs and pulps; bones may increase in length when disease brings much blood to them; and cocks’ spurs transplanted from their legs into their combs grow to an unnatural length; the conditions common to all these cases being both an increased supply of blood, and the capability, on the part of the growing tissue, of availing itself of the opportunity of increased absorption and nutrition thus afforded to it. In the absence of the last-named condition, increased supply of blood will not lead to increased nutrition. CHAPTER XII. SECRETION. Secretion is the process by which materials are separated from the blood, and from the organs in which they are formed, for the purpose either of serving some ulterior office in the economy, or being discharged from the body as excrement. In the former case, both the separated materials and the processes 314 SECRETION. for their separation are termed secretions; in the latter, they are named excretions. Most of the secretions consist of substances which, probably, do not pre-exist in the same form in the blood, but require special organs and a process of elaboration for their formation, e. g., the liver for the formation of bile, the mammary gland for the formation of milk. The excretions, on the other hand, commonly or chiefly consist of substances which, as urea, car- bonic acid, and probably uric acid, exist ready-formed in the blood, and are merely abstracted therefrom. If from any cause, such as extensive disease or extirpation of an excretory organ, the separation of an excretion is prevented, and an ac- cumulation of it in the blood ensues, it frequently escapes through other organs, and may be detected in various fluids of the body. But this is never the case with secretions; at least with those that are most elaborated; for after the removal of the special organs by which any of them is elaborated, it is no longer formed. Cases sometimes occur in which the secretion con- tinues to be formed by the natural organ, but not being able to escape towards the exterior, on account of some obstruction, is reabsorbed into the blood, and afterwards discharged from it by exudation in other ways; but these are not instances of true vicarious secretion, and must not be thus regarded. These circumstances, and their final destination, are, how- ever, the only particulars in which secretions and excretions can be distinguished ; for, in general, the structure of the parts engaged in eliminating excretions, e. g., the kidneys, is as com- plex as that of the parts concerned in the formation of secre- tions. And since the dilfetences of the two processes of sepa- ration, corresponding with those in the several purposes and destinations of the fluids, are not yet ascertained, it will be sufficient to speak in general terms of the process of separation or secretion. Every secreting apparatus possesses, as essential parts of its structure, a simple and apparently textureless membrane, named the primary or basement-membrane; certain cells; and bloodvessels. These three structural elements are arranged together in various ways; but all the varieties may be classed under one or other of two principal divisions, namely, mem- branes and glands. SECRETING MEMBRANES. The principal secreting membranes are the serous and syno- vial membranes, the mucous membranes, and the skin.1 1 The skin will be described in a subsequent chapter. 315 SEROUS MEMBRANES. The serous membranes are formed of fibro-cellular tissue, interwoven so as to constitute a membrane, the free surface of which is covered with a single layer of flattened cells, forming, in most instances, a simple tessellated epithelium. Between the epithelium and the subjacent layer of fibro-cellular tissue, is situated the primary or basement-membrane (Bowman). Fig. 104. Plan of a secreting membrane: a, menibrana propria, or basement-membrane; b, epithelium composed of secreting nucleated cells ; c, layer of capillary bloodvessels (after Sharpey). In relation to the process of secretion, the layer of fibro- cellular tissue serves as a groundwork for the ramification of bloodvessels, lymphatics, and nerves. But in its usual form it is absent in some instances, as in the arachnoid covering the dura mater, and in the interior of the ventricles of the brain. The primary membrane and epithelium are probably always present, and are concerned in the formation of the fluid by which the free surface of the membrane is moistened. The serous membranes are of two principal kinds: 1st. Those which line visceral cavities,—the arachnoid, pericar- dium, pleurae, peritoneum, and tuuicse vaginales. 2d. The synovial membranes lining the joints, and the sheaths of ten- dons and ligaments, with which, also, are usually included the synovial bursae, or bursae mucosae, whether these be subcuta- neous, or situated beneath tendons that glide over bones. The serous membranes form closed sacs, and exist wherever the free surfaces of viscera come into contact with each other, or lie in cavities unattached to surrounding parts. The viscera, which are invested by a serous membrane, are, as it were, pressed into the shut sac which it forms, carrying before them a portion of the membrane, which serves as their investment. To the law that serous membranes form shut sacs, there is, in the human subject, one exception, viz.: the opening of- the Fallopian tubes into the abdominal cavity,—an arrangement which exists in man and all Vertebfata, with the exception of a few fishes. The principal purpose of the serous and synovial membranes is to furnish a smooth, moist surface, to facilitate the move- ments of the invested organ, and to prevent the injurious effects of friction. This purpose is especially manifested in 316 SECRETION. joints, in which free and extensive movements take place ; and in the stomach and intestines, which, from the varying quan- tity and movements of their contents, are in almost constant motion upon one another and the walls of the abdomen. The fluid secreted from the free surface of the serous mem- branes is, in health, rarely more than sufficient to insure the maintenance of their moisture. The opposed surfaces of each serous sac, are at every point in contact with each other, and leave no space in which fluid can collect. After death, a larger quantity of fluid is usually found in each serous sac; but this, if not the product of manifest disease, is probably such as has transuded after death, or in the last hours of life. An excess of such fluid in any of the serous sacs constitutes dropsy of the sac. The fluid naturally secreted by the serous membranes appears to be identical, in general and chemical characters, with the serum of the blood, or with very dilute liquor sanguinis. It is of a pale yellow or straw-color, slightly viscid, alkaline, and, because of the presence of albumen, coagulable by heat. The presence of a minute quantity of fibrin, at least in the dropsical fluids effused into the serous cavities, is shown by their partial coagulation into a jelly-like mass, on the addition of certain animal substances, or on mixture with certain fluids, especially such as contain cells (p. 70 etseq.). This similarity of the serous fluid to the liquid part of blood, and to the fluid with which most animal tissues are moistened, renders it probable that it is, in great measure, separated by simple transudation through the walls of the bloodvessels. The probability is increased by the fact that, in jaundice, the fluid in the serous sacs is, equally with the serum of the blood, colored with the bile. But there is reason for supposing that the fluid of the cerebral ventricles and of the arachnoid sac are exceptions to this rule; for they differ from the fluids of the other serous sacs not only in being pellucid, colorless, and of much less specific gravity, but in that they seldom receive the tinge of bile in the blood, and are not colored by madder, or other similar substances introduced abundantly into the blood. It is also probable that the formation of synovial fluid is a process of more genuine and elaborate secretion, by means of the epithelial cells on the surface of the membrane, and espe- cially of those which are Accumulated on the edges and pro- cesses of the synovial fringes; for, in its peculiar density, vis- cidity, and abundance of albumen, synovia differs alike from the serum of blood and from the fluid of any of the serous cavities. The mucous membranes line all those passages by which in- MUCOUS MEMBRANES. 317 ternal parts communicate with the exterior, and by which either matters are eliminated from the body or foreign substances taken into it. They are soft and velvety, and extremely vas- cular. Their general structure resembles that of serous mem- branes. It consists of epithelium, basement-membrane, and fibro-cellular or areolar tissue containing bloodvessels, lym- phatics, and nerves. The structure of mucous membranes is less uniform, especially as regards their epithelium, than that of serous membranes ; but the varieties of structure in different parts are described in connection with the organs in which mucous membranes are present, and need not be here noticed in detail. The external surfaces of mucous membranes are attached to various other tissues ; in the tongue, for example, to muscle; on cartilaginous parts, to perichondrium; in the cells of the ethmoid bone, in the frontal and sphenoid sinuses, as well as in the tympanum, to periosteum ; in the intestinal canal, it is connected with a firm submucous membrane, which on its exterior gives attachment to the fibres of the muscular coat. The mucous membranes are described as lining certain prin- cipal tracts. 1. The digestive tract commences in the cavity of the mouth, from which prolongations pass into the ducts of the salivary glands. From the mouth it passes through the fauces, pharynx, and oesophagus, to the stomach, and is thence con- tinued along the whole tract of the intestinal canal to the ter- mination of the rectum, being in its course arranged in the various folds and depressions already described, and prolonged into the ducts of the pancreas and liver and into the gall-blad- der. 2. The respiratory tract includes the mucous membrane lining the cavity of the nose, and the various sinuses commu- nicating with it, the lachrymal canal and sac, the conjunctiva of the eye and eyelids, and the prolongation which passes along the Eustachian tubes and lines the tympanum and the inner surface of the membrana tympani. Crossing the pharynx, and lining that part of it which is above the soft palate, the respi- ratory tract leads into the glottis, whence it is continued, through the larynx and ti’achea, to the bronchi and their divisions, which it lines as far as the branches of about -fa of an inch in diameter, and continuous with it is a layer of delicate epithelial membrane which extends into the pulmonary cells. 3. The genito-urmary tract, which lines the whole of the urinary pas- sages, from their external orifice to the termination of the tubuli uriniferi of the kidneys, extends into and through the organs of generation in both sexes, into the ducts of the glands connected with them ; and in the female becomes continuous 318 SECRETION. with the serous membrane of the abdomen at the fimbriae of the Fallopian tubes. Along each of the above tracts, and in different portions of each of them, the mucous membrane presents certain struc- tural peculiarities adapted to the functions which each part has to discharge; yet in some essential characters mucous membrane is the same, from whatever part it is obtained. In all the principal and larger parts of the several tracts, it pre- sents, as just remarked, an external layer of epithelium, situated upon basement-membrane, and beneath this, a stratum of vascular tissue of variable thickness, which in different cases presents either outgrowths in the form of papillae and villi, or depressions or involutions in the form of glands. But in the prolongations of the tracts, where they pass into gland-ducts, these constituents are reduced in the finest branches of the ducts to the epithelium, the primary or basement-membrane, and the capillary bloodvessels spread over the outer surface of the latter in a single layer. The primary or basement-membrane is a thin transparent layer, simple, homogeneous, and with no discernible structure, which on the larger mucous membranes that have a layer of vascular fibro-cellular tissue, may appear to be only the blastema or formative substance, out of which successive layers of epithelium-cells are formed. But in the minuter di- visions of the mucous membranes, and in the ducts of glands, it is the layer continuous and correspondent with this basement- membrane that forms the proper walls of the tubes. The cells also which, lining the larger and coarser mucous membranes, constitute their epithelium, are continuous with and often similar to those which, lining the gland-ducts, are called gland-cells, rather than epithelium. Indeed, no certain dis- tinction can be drawn between the epithelium-cells of mucous membranes and gland-cells. In reference to their position, as covering surfaces, they might all be called epithelium-cells, whether they lie on open mucous membranes, or in gland- ducts ; and in reference to the process of secretion, they might all be called gland-cells, or at least secreting-cells, since they probably all fulfil a secretory office by separating certain definite materials from the blood and from the part on which they are seated. It is only an artificial distinction which makes them epithelial cells in one place, and gland-cells in another. It thus appears, that the tissues essential to the production of a secretion are, in their simplest form, a simple membrane, having on one surface bloodvessels, and on the other a layer of cells, which may be called either epithelium-cells or gland- SECRETING GLANDS. 319 cells. Glands are provided also with lymphatic vessels and nerves. The distribution of the former is not peculiar, and need not be here considered. Nerve-fibres are distributed both to the bloodvessels of the gland and to its ducts; and, in some glands, it is said, to the secreting cells also. The structure of the elementary portions of a secreting ap- paratus, namely, epithelium, simple membrane, and blood- vessels, having been already described in this and previous chapters, we may proceed to consider the manner in which they are arranged to form the varieties of secreting glands. SECRETING GLANDS. The secreting glands are the organs to which the office of secreting is more especially ascribed: for they appear to be occupied with it alone. They present, amid manifold diversi- ties of form and composition, a general plan of structure, by which they are distinguished from all other textures of the body; especially, all contain, and appear constructed with particular regard to, the arrangement of the cells, which as already expressed, both line their tubes or cavities as an epi- thelium, and elaborate, as secreting cells, the substances to be discharged from them. For convenience of description, they may be divided into three principal groups, the characters of each of which are de- termined by the different modes in which the sacculi or tubes containing the secreting cells are grouped : 1. The simple tubule or tubular gland (a, Fig. 105), exam- ples of which are furnished by the several tubular follicles in mucous membranes: especially by the follicles of Lieberkiihn in the mucous membrane of the intestinal canal (p. 241), and the tubular or gastric glands of the stomach (p. 217). These appear to be simple tubular depressions of the mucous mem- brane on which they open, each consisting of an elongated gland-vesicle, the wall of which is formed of primary mem- brane, and is lined with secreting cells arranged as an epithe- lium. To the same class may be referred the elongated and tortuous sudoriparous glands of the skin (p. 338), and the Meibomian follicles beneath the palpebral conjunctiva; though the latter are made more complex by the presence of small pouches along their sides (b, Fig. 105), and form a connecting link between the members of this division and the next, as the former by their length and tortuosity do between the first di- vision and the third (d, Fig. 105). 2. The aggregated glands, including those that used to be called conglomerate, in which a number of vesicles or acini are 320 S E C It E TI O N. arranged in groups or lobules (c, Fig. 105). Such are all' those commonly called mucous glands, as those of the tra- chea, vagina, and the minute salivary glands. Such, also, are Fig. 105 Plans of extension of secreting membrane by inversion or recession in form of cavities. A, simple glands, viz., g, straight tube; h, sac ; i, coiled tube. B, multi- locular crypts; k, of tubular form ; l, saccular. C, racemose, or saccular compound gland; m, entire gland, showing branched duct and lobular structure; n, a lobule, detached with o, branch of duct proceeding from it. D, compound tubular gland (after Sharpey). the lachrymal, the large salivary and mammary glands, Brunn’s, Cowper’s, and Duverney’s glands, the pancreas and PROCESS OF SECRETION. 321 prostate. These various organs differ from each other only in secondary points of structure ; such as, chiefly, the arrange- ment of their excretory ducts, the grouping of the acini and lobules, their connection by fibro-cellular tissue, and supply of bloodvessels. The acini commonly appear to be formed by a kind of fusion of the walls of several vesicles, which thus com- bine to form one cavity lined or filled with secreting cells which also occupy recesses from the main cavity. The small- est branches of the gland-ducts sometimes open into the cen- tres of these cavities; sometimes the acini are clustered round the extremities, or by the sides of the ducts: but, whatever secondary arrangement there may be, all have the same essen- tial character of rounded groups of vesicles containing gland- cells, and opening, either occasionally or permanently, by a common central cavity into minute ducts, which ducts in the large glands converge and unite to form larger and larger branches, and at length, by one common trunk, open on a free surface of membrane. 3. The convoluted tubular glands (d, Fig. 105), such as the kidney and testis, form another division. These consist of tu- bules of membrane, lined with secreting cells arranged like an epithelium. Through nearly the whole of their long course, the tubules present an almost uniform size and structure; ultimately they terminate either in a cul-de-sac, or by dilating, as in the Malpighian capsules of the kidney, or by forming a simple loop and returning, as in the testicle. Among these varieties of structure, all the permanent glands are alike in some essential points, besides those which they have in common with all truly secreting structures. They agree in presenting a large extent of secreting surface within a comparatively small space; in the circumstance that while one end of the gland-duct opens on a free surface, the oppo- site end is always closed, having no direct communication with bloodvessels, or any other canal; and in uniform arrange- ment of capillary bloodvessels, ramifying and forming a net- work around the walls and in the interstices of the ducts and acini. PROCESS OF SECRETION. From what has been said, it will have already appeared that the modes in which secretions are produced are at least two. Some fluids, such as the secretions of serous membranes, appear to be simply exudations or oozings from the bloodves- sels, whose qualities are determined by those of the liquor san- guinis, while the quantities are liable to variation, or are chiefly dependent on the pressure of the blood on the interior 322 SECRETION. of the bloodvessels. But, in the production of the other se- cretions, such as those of mucous membranes and all glands, other besides these mechanical forces are in operation. Most of the secretions are indeed liable to be modified by the cir- cumstances which affect the simple exudation from the blood- vessels, and the products of such exudations, when excessive, are apt to be mixed with the more proper products of all the se- creting organs. But the act of secretion in all glands is the result of the vital processes of cells or nuclei, which, as they develop themselves and grow, form in their interior the proper materials of the secretion, and then discharge them. The best evidence for this view is: 1st. That cells and nuclei are constituents of all glands, however diverse their outer forms and other characters, and are in all glands placed on the surface or in the cavity whence the secretion is poured. 2d. That many secretions which are visible with the micro- scope may be seen in the cells of their glands before they are discharged. Thus, bile may be often discerned by its yellow tinge in the gland-cells of the liver; spermatozoids in the cells of the tubules of the testicles; granules of uric acid in those of the kidneys of fish ; fatty particles, like those of milk, in the cells of the mammary gland. The process of secretion might, therefore, be said to be accomplished in, and by the life of, these gland-cells. They appear, like the cells or other elements of any other organ, to develop themselves, grow, and attain their individual perfec- tion by appropriating the nutriment from the adjacent blood- vessels and elaborating it into the materials of their walls and the contents of their cavities. In this perfected state, they subsist for some brief time, and when that period is over they appear to dissolve or burst and yield themselves and their con- tents to the peculiar material of the secretion. And this ap- pears to be the case in every part of the gland that contains the appropriate gland-cells ; therefore not in the extremities of the ducts or in the acini alone, but in great part of their length. In these things there is the closest resemblance between secretion and nutrition ; for if the purpose which the secreting glands are to serve in the economy be disregarded, their for- mation might be considered as only the process of nutrition of organs, whose size and other conditions are maintained in, and by means of, the continual succession of cells developing themselves and passing away. In other wTords, glands are maintained by the development of the cells, and their con- tinuance in the perfect state; and the secretions are discharged as the constituent gland-cells degenerate and are set free. The processes of nutrition and secretion are similar, also, in DISCHARGE OF SECRETIONS. 323 their obscurity: there is the same difficulty in saying why, out of apparently the same materials, the cells of one gland elaborate the components of bile, while those of another form the components of milk, and of a third those of saliva, as there is in determining why one tissue forms cartilage, another bone, a third muscle, or any other tissue. In nutrition, also, as in secretion, some elements of tissues, such as the gelatinous tis- sues, are different in their chemical properties from any of the constituents ready-formed in the blood. Of these differences, also, no account can be rendered ; but, obscure as the cause of these diversities may be, they are not objections to the ex- planation of secretion as a process similar to nutrition; an explanation with which all the facts of the case are recon- cilable. It may be observed that the diversities presented by the other constituents of glands afford no explanation of the dif- ferences or peculiarities of their several products. There are many differences in the arrangements of the bloodvessels in different glands and mucous membranes; and, in accordance with these, much diversity in the rapidity with which the blood traverses them. But there is no reason for believing that these things do more than influence the rate of the pro- cess and the quantity of the material secreted. Cceteris pari- bus, the greater the vascularity of a secreting organ, and the larger the supply of blood traversing its vessels in a given time, the larger is the amount of secretion; but there is no evidence that the quantity or mode of movement of the blood can directly determine the quality of the secretion. The discharge of secretions from glands may take place as soon as they are formed; or the secretion may be long re- tained within the gland or its ducts. The secretion of glands which are continually in active function for the purification of the blood, such as the kidneys, are generally discharged from the gland as rapidly as they are formed. But the secre- tions of those whose activity of function is only occasional, such as the testicle, are usually retained in the ducts during the periods of the gland’s inaction. And there are glands which are like both these classes, such as the lachrymal and salivary, which constantly secrete small portions of fluid, and on occa- sions of greater excitement discharge it more abundantly. When discharged into the ducts, the further course of secre- tions is effected partly by the pressure from behind; the fresh quantities of secretion propelling those that were formed before. In the larger ducts, its propulsion is assisted by the contraction of their walls. All the larger ducts, such as the ureter and common bile-duct, possess in their coats organic muscular 324 SECRETION. fibres; they contract when irritated, and sometimes manifest peristaltic movements. Bernard and Brown-Sequard, indeed, have observed rhythmic contractions in the pancreatic and bile-ducts, and also in the ureters and vasa deferentia. It is probable that the contractile power extends along the ducts to a considerable distance within the substance of the glands whose secretions can be rapidly expelled. Saliva and milk, for in- stance, are sometimes ejected with much force; doubtless by the energetic and simultaneous contraction of many of the ducts of their respective glands. The contraction of the ducts can only expel the fluid they contain through their main trunk; for at their opposite ends all the ducts are closed. Circumstances influencing Secretion.—The influence of exter- nal conditions on the functions of glands, is manifested chiefly in alterations of the quantity of secretion ; and among the prin- cipal of these conditions are variations in the quantity of blood, in the quantity of the peculiar materials for any secretion that it may contain, and in the conditions of the nerves of the glands. In general, an increase in the quantity of blood traversing a gland, coincides with an augmentation of its secretion. Thus, the mucous membrane of the stomach becomes florid when, on the introduction of food, its glands begin to secrete; the mam- mary gland becomes much more vascular during lactation; and it appears that all circumstances which give rise to an in- crease in the quantity of material secreted by an organ, pro- duce, coincidently, an increased supply of blood. In most cases, the increased supply of blood rather follows than pre- cedes the increase of secretion; as, in the nutritive processes, the increased nutrition of a part just precedes and determines the increased supply of blood ; but, as also in the nutritive process, an increased supply of blood may have, for a conse- quence, an increased secretion from the glands to which it is sent. Glands also secrete with increased activity when the blood contains more than usual of the materials they are designed to separate. Thus, when an excess of urea is in the blood, whether from excessive exercise, or from destruction of one kidney, a healthy kidney will excrete more than it did before. It will, at the same time, grow larger: an interesting fact, as proving both that secretion and nutrition in glands are identical, and that the presence of certain materials in the blood may lead to the formation of structures in which they may be incorporated. The process of secretion is, also, largely influenced by the condition of the nervous system. The exact mode in which the nervous system influences THE DUCTLESS GLANDS. 325 secretion must be still regarded as somewhat obscure. In part, it exerts its influence by increasing or diminishing the quantity of blood supplied to the secreting gland, in virtue of the power which it exercises over the contractility of the smaller blood- vessels ; while it also has a more direct influence analogous to the trophic influence referred to in the chapter on Nutrition. Its influence over secretion, as well as over other functions of the body, may be excited by causes acting directly upon the nervous centres, upon the nerves going to the secreting organ, or upon the nerves of other parts. In the latter case, a reflex action is produced: thus the impression- produced upon the nervous centres by the contact of food in the mouth, is reflected upon the nerves supplying the salivary glands, and produces, through these, a more abundant secretion of saliva. Through the nerves, various conditions of the mind also in- fluence the secretions. Thus, the thought of food may be suf- ficient to excite an abundant flow of saliva. And, probably, it is the mental state which excites the abundant secretion of urine in hysterical paroxysms, as well as the perspirations and, occasionally, diarrhoea, which ensue under the influence of terror, and the tears excited by sorrow or excess of'joy. The quality of a secretion may also be affected by the mind; as in the cases in which, through grief or passion, the secretion of milk is altered, and is sometimes so changed as to produce irritation in the alimentary canal of the child, or even death (Carpenter). The secretions of some of the glands seem to bear a certain relation or antagonism to each other, by which an increased activity of one is usually followed by diminished activity of one or more of the others; and a deranged condition of one is apt to entail a disordered state in the others. Such relations appear to exist among the various mucous membranes: and the close relation between the secretion of the kidney and that of the skin is a subject of constant observation. CHAPTER XIII. THE VASCULAR GLANDS ; OR GLANDS WITHOUT DUCTS. The materials separated from the blood by the ordinary process of secretion by glands, are always discharged from the organ in which they are formed, and either straightway ex- 326 THE DUCTLESS GLANDS. pelled from the body, or if they are again received into the blood, it js only after they have been altered from their original condition, as in the cases of the saliva and bile. There ap- pears, however, to be a modification of the process of secre- tion, in which certain materials are abstracted from the blood, undergo some change, and are added to the lymph or restored to the blood, without being previously discharged from the secreting organ, or made use of for any secondary purpose. The bodies in which this modified form of secretion takes place, are usually described as vascular glands, or glands without ducts, and include the spleen, the thymus and thyroid glands, the supra-renal capsules, and, according to OEsterlin and Ecker and Gull, the pineal gland and pituitary body; possibly, also the tonsils. The solitary and agminate glands of the intestine (p. 242), and lymph-glands in general, also closely resemble them ; in- deed, both in structure and function, the vascular glands bear a close relation, on the one hand, to the true secreting glands, and on the other, to the lymphatic glands. The evidence in favor of the view that these organs exercise a function analogous to that of secreting glands, has been Fig. 106. Vesicles from the thyroid gland of a child (from Kolliker) 250 a, connective tissue between the vesicles ; 6, capsule of the vesicles ; c, their epithelial lining. chiefly obtained from investigations into their structure, which have shown that most of the glands without ducts contain the same essential structures as the secreting glands, except the ducts. They are mainly composed of vesicles, or sacculi, either simple and closed, as in the thyroid (Fig. 106\ and supra- THE DUCTLESS GLAXD S. 327 renal capsules, or variously branched, and with the cavities of the several branches communicating in and by common canals, as in the thymus (Fig. 107). These vesicles, like the acini of secreting glands, are formed of a delicate homogeneous mem- brane, are surrounded with and often traversed by a vascular plexus, and are filled with finely molecular albuminous fluid, suspended in which are either granules of fat, or cytoblasts, or nuclei, or nucleated cells, or a mixture of all these. Structure of the Spleen.—The spleen is covered externally almost completely by a serous coat derived from the peri- toneum, while within this is the proper fibrous coat or capsule of the organ. The latter, composed of connective tissue, with Fig. 107. Transverse section of a lobule of an injected infantile thymus gland (after Kol- liker) (magnified 30 diameters), a, capsule of connective tissue surrounding the lobule; 6, membrane of the glandular vesicles ; c, cavity of the lobule, from which the larger bloodvessels are seen to extend towards and ramify in the spheroidal masses of the lobule. a large preponderance of elastic fibres, forms the immediate investment of the spleen. Prolonged from its inner surface are fibrous processes or trabeculae, which enter the interior of the organ, and, dividing and anastomosing in all parts, form a kind of supporting framework or stroma, in the interstices of which the proper substance of the spleen, or the spleen-pulp, is contained. At the hilus of the spleen, or the part at which 328 THE DUCTLESS GLANDS. the bloodvessels, nerves, and lymphatics enter, the fibrous coat is prolonged into the spleen-substance in the form of investing sheaths for the arteries and veins, which sheaths again are con- nected with the trabeculce before referred to. The spleen-pulp, which is of a dark red or reddish-brown color, is composed chiefly of cells. Of these, some are granular corpuscles resembling the lymph-corpuscles, both in general appearance and in being able to perform amoeboid movements; others are red blood-corpuscles of normal appearance or vari- ously changed; while there are also large cells containing either pigment allied to the coloring matter of the blood, or rounded corpuscles like red blood-cells. The splenic artery, which enters the spleen by its concave surface or hilus, divides and subdivides, with but little anas- tomosis between its branches, in the midst of the spleen-pulp, at the same time that its branches are sheathed, as before said, by the fibrous coat, which they, so to speak, carry into the spleen with them. Ending in capillaries, they either com- municate, as in other parts of the body, wfith the radicles of the veins, or end in lacunar spaces in the spleen-pulp, from which veins arise (Gray). On the face of a section of the spleen can be usually seen, readily with the naked eye, minute, scattered, rounded or oval whitish spots, mostly from to inch in diameter. These are the Malpighian corpuscles of the spleen, and are situated on the sheaths of the minute splenic arteries, of which, indeed, they may be said to be outgrowths (Fig. 108). For while the sheaths of the larger arteries are constructed of ordinary con- nective tissue, this has become modified where it forms an in- vestment for the smaller vessels, so as to be a fine retiform tissue, with abundance of corpuscles, like lymph-corpuscles, contained in its meshes; and the Malpighian corpuscles are but small outgrowths of this cytogenous or cell-bearing connec- tive tissue. They are composed of masses of corpuscles, inter- sected in all parts by a delicate fibrillar tissue, which, though it invests the Malpighian bodies, does not form a complete capsule. Blood-capillaries traverse the Malpighian corpuscles and form a plexus in their interior. The structure of a Mal- pighian corpuscle of the spleen is, therefore, very similar to that of lymphatic-gland substance (p. 284). The general resemblances in structure between certain of the vascular glands and the true glands lead to the supposition that both sets of organs pursue, up to a certain point, a similar course in the discharge of their functions. It is assumed that certain principles in an inferior state of organization are effused FUNCTIONS OF DUCTLESS GLANDS. 329 from the vessels into the sacculi, and gradually develop into nuclei or cytoblasts, which may be further developed into cells ; that in the growth of these nuclei and cells, the materials de- Fig. 108. Tlie figure shows a portion of a small artery, to one of the twigs of which the Malpighian corpuscles are attached. rived from the blood are elaborated into a higher condition of organization ; and that when liberated by the dissolution of these cells, they pass into the lymphatics, or are again received into the blood, whose aptness for nutrition they contribute to maintain. The opinion that the vascular glands thus serve for the higher organization of the blood, is supported by their being all especially active in the discharge of their functions during foetal life and childhood, when, for the development and growth of the body, the most abundant supply of highly or- ganized blood is necessary. The bulk of the thymus gland, in proportion to that of the body, appears to bear almost a direct proportion to the activity of the body’s development and growth, and when, at the period of puberty, the development of the body may be said to be complete, the gland wastes, and finally disappears. The thyroid gland and supra-renal cap- sules, also, though they probably never cease to discharge some 330 THE DUCTLESS GLANDS. amount of function, yet are proportionally much smaller in childhood than in foetal life and infancy; and with the years advancing to the adult period, they diminish yet more in pro- portionate size and apparent activity of function. The spleen more nearly retains its proportionate size, and enlarges nearly as the whole body does. The function of the vascular glands seems not essential to life, at least not in the adult. The thymus wastes and dis- appears ; no signs of illness attend some of the diseases which wholly destroy the structure of the thyroid gland ; and the spleen has been often removed in animals, and in a few in- stances in men, without any evident ill-consequence. It is possible that, fn such cases, some compensation for the loss of one of the organs may be afforded by an increased activity of function in those that remain. The experiment, to be com- plete, should include the removal of all these organs, an opera tion of course not possible without immediate danger to life. Nor, indeed, would this be certainly sufficient, since there is reason to suppose that the duties of the spleen, after its re- moval, might be performed by lymphatic glands, between whose structure and that of the vascular glands there is much resemblance, and which, it is said, have been found peculiarly enlarged when the spleen has been removed (Meyer). Although the functions of all the vascular glands may be similar, in so far as they may all alike serve for the elabora- tion and maintenance of the blood, yet each of them probably discharges a peculiar office, in relation either to the whole economy, or to that of some other organ. Respecting the special office of the thyroid gland, nothing reasonable can be suggested ; nor is there any certain evidence concerning that of the supra-renal capsules.1 Respecting the thymus gland, the observations of Mr. Simon, confirmed by those of Friedle- ben and others, have shown that in the hibernating animals, in which it exists throughout life, as each successive period of hibernation approaches, the thymus greatly enlarges and be- comes laden with fat, which accumulates in it and in fat- glands connected with it, in even larger proportions than it does in the ordinary seats of adipose tissue. Hence it appears 1 Mr. J. Hutchinson, and more recently, Dr. Wilks, following out Dr. Addison’s discovery, have, by the collection of a large and valua- ble serie* of cases in which the supra-renal capsules were diseased, demonstrated most satisfactorily the very close relation subsisting be- tween disease of these organs and brown discoloration of the skin ; but the explanation of this relation is still involved in obscurity, and consequently does not aid much in determining the functions of the supra-renal capsules. FUNCTIONS OF SPLEEN. 331 to serve for the storing up of materials which, being reabsorbed in the inactivity of the hibernating period, may maintain the respiration and the temperature of the body in the reduced state to which they fall during that time. With respect to the office of the spleen, we have somewhat more definite information. In the first place, the large size which it gradually acquires towards the termination of the digestive process, and the great increase observed about this period in the amount of the finely-granular albuminous plasma within its parenchyma, and the subsequent gradual decrease of this material, seem to indicate that this organ is concerned in elaborating the albuminous or formative materials of food, and for a time storing them up, to be gradually introduced into the blood, according to the demands of the general system. The small amount of fatty matter in such plasma, leads to the inference that the gland has little to do in regard to the prepa- ration of material for the respiratory process. Then again, it seems not improbable that, as Hewson origi- nally suggested, the spleen, and perhaps to some extent the other vascular glands, are, like the lymphatic glands, engaged in the formation of the germs of subsequent blood-corpuscles. For it seems quite certain, that the blood of the splenic vein contains an unusually large amount of white corpuscles; and in the disease termed leucocythoemia, in which the pale cor- puscles of the blood are remarkably increased in number, there is almost always found an hypertrophied state of the spleen or thyroid body, or some of the lymphatic glands. Accordingly there seems to be a close analogy in function be- tween the so-called vascular and the lymphatic glands: the former elaborating albuminous principles, and forming the germs of new blood-corpuscles out of alimentary materials absorbed by the bloodvessels ; the latter discharging the like office on nutritive materials taken up by the general absorbent system. In Kolliker’s opinion, the development of colorless and also colored corpuscles of the blood is one of the essential functions of the spleen, into the veins of which the new-formed corpuscles pass, and are thus conveyed into the general cur- rent of the circulation. There is reason to believe, too, that in the spleen many of the red corpuscles of the blood, those probably which have discharged their office and are worn out, undergo disintegra- tion ; for in the colored portion of the spleen-pulp an abun- dance of such corpuscles, in various stages of degeneration, are found, while the red corpuscles in the splenic venous blood are said to be relatively diminished. According to Kolliker’s description of this process of disintegration, the blood-corpus- 332 THE SKIN. cles, becoming smaller and darker, collect together in roundish heaps, which may remain in this condition, or become each surrounded by a cell-wall. The cells thus produced may con- tain from one to twenty blood-corpuscles in their interior. These corpuscles become smaller and smaller; exchange their red for a golden yellow, brown, or black color; and, at length are converted into pigment-granules, which by degrees become paler and paler, until all color is lost. The corpuscles undergo these changes whether the heaps of them are enveloped by a cell-wall or not. Besides these, its supposed direct offices, the spleen is be- lieved to fulfil some purpose in regard to the portal circula- tion, with which it is in close connection. From the readiness with which it admits of being distended, and from the fact that it is generally small while gastric digestion is going on, and enlarges when that act is concluded, it is supposed to act as a kind of vascular reservoir, or diverticulum to the portal system, or more particularly to the vessels of the stomach. That it may serve such a purpose is also made probable by the enlargement which it undergoes in certain affections of the heart and liver, attended with obstruction to the passage of blood through the latter organ, and by its diminution when the congestion of the portal system is relieved by discharges from the bowels, er by the effusion of blood into the stomach. This mechanical influence on the circulation, however, can hardly be supposed to be more than a very subordinate part of the office of an organ of so great complexity as the spleen, and containing so many other structures besides bloodvessels. The same may also be said with regard to the opinion that the thyroid gland is important as a diverticulum for the cerebral circulation, or the thymus for the pulmonary in childhood. These, like the spleen, must have peculiar and higher, though as yet ill-understood, offices. CHAPTER XIV. THE SKIN AND ITS SECRETIONS. To complete the consideration of the processes of organic life, and especially of those which, by separating materials from the blood, maintain it in the state necessary for the nutrition of the body, the structure and functions of the skin must be now considered: for besides the purposes which it serves—(1), as an internal integument for the protection of EPIDERMIS. 333 the deeper tissues, and (2), as a sensitive organ in the exercise of touch, it is also (3), an important excretory, and (4) an absorbing organ ; while it plays a most important part in (5) the regulation of the temperature of the body. Structure of the Skin. The skin consists, principally, of a layer of vascular tissue, named the coriwn, derma, or cutis vera, and an external cover- ing of epithelium termed the cuticle or epidermis. Within and beneath the corium are imbedded several organs with special functions, namely, sudoriparous glands, sebaceous glands, and hair-follicles; and on its surface are sensitive papillce. The so-called appendages of the skin—the hair and nails—are modi- fications of the epidermis. Epidermis.—The epidermis is composed of several layers of epithelial cells of the squamous kind (p. 34), the deeper cells, however, being rounded or elongated, and in the latter in- stance having their long axis arranged vertically as regards the general surface of the skin, while the more superficial cells are flattened and scaly (Fig. 109). The deeper part of the epidermis, which is softer and more opaque than the super- ficial, is called the rete mucosum. Many of the epidermal cells contain pigment, and the varying quantity of this is the source of the different shades of tint in the skin, both of individuals and races. The coloring mat- ter is contained chiefly in the deeper cells composing the rete mucosum, and be- comes less evident in them as they are gradually pushed up by those under them, and become, like their predecessors, flat- tened and scale-like (Fig. 109). It is by this pro- cess of production from beneath, to make up for the waste at the surface, that the growth of the cuticle is effected. Fig. 109. Skin of the negro, in a vertical section, mag- nified 250 diameters, a, a, cutaneous papillae ; b, undermost and dark-colored layer of oblong vertical epidermis cells: c. mucous or Malpig- hian layer ; d, horny layer (from Sharpey). 334 THE SKIN. The thickness of the epidermis on different portions of the skin is directly proportioned to the friction, pressure, and other sources of injury to which it is exposed; and the more it is subjected to such injury, within certain limits, the more does it grow, and the thicker and more horny does it become; for it serves as well to protect the sensitive and vascular cutis from injury from without, as to limit the evaporation of fluid from the bloodvessels. The adaptation of the epidermis to the latter purposes may be well shown by exposing to the air two dead hands or feet, of which one has its epidermis perfect, and the other is deprived of it; in a day, the skin of the lat- ter will become brown, dry, and horn-like, while that of the former will almost retain its natural moisture. Cutis vera.—The corium or cutis, which rests upon a layer of adipose and cellular tissue of varying thickness, is a dense and tough, but yielding and highly elastic structure, composed of fasciculi of fibro-cellular tissue, interwoven in all directions, and forming, by their interlacements, numerous spaces or areolae. These areolae are large in the deeper layers of the cutis, and are there usually filled with little masses of fat (Fig. 112): but, in the more superficial parts, they are exceedingly small or entirely obliterated. By means of its toughness, flexibility, and elasticity, the skin is eminently qualified to serve as the general integument of the body, for defending the internal parts from external violence, and readily yielding and adapting itself to their various move- ments and changes of position. But, from the abundant sup- ply of sensitive nerve-fibres which it receives, it is enabled to fulfil a not less important purpose in serving as the principal organ of the sense of touch. The entire surface of the skin is extremely sensitive, but its tactile properties are due chiefly to the abundant papilke with which it is studded. These papillae are conical elevations of the corium, with a single or divided free extremity, more prominent and more densely set at some parts than at others (Figs. 110 and 111). The parts on which they are most abundant and most prominent are the palmar surface of the hands and fingers, and the soles of the feet— parts, therefore, in which the sense of touch is most acute. On these parts they are disposed in double rows, in parallel curved lines, separated from each other by depressions (Fig. 112). Thus they may be seen easily on the palm, whereon each raised line is composeed of a double row of papillae, and is intei*sected by short transverse lines or furrows corresponding with the interspaces between.the successive pairs of papillae. Over other parts of the skin they are more or less thinly scat- tered, and are scarcely elevated above the surface. Their THE CORIUM OR CUTIS VERA. 335 average length is about too °f au inch, and at their base they measure about of an inch in diameter. Each pa- * Fig. 110. Fig. 111. Fig. 110.—Papillae, as seen with a microscope, on a portion of the true skin, from which the cuticle has been removed (after Breschet). Fig. 111.—Compound papillae from the palm of the hand, magnified 60 diameters ; a. basis of a papilla ; 6, 6, divisions or branches of the same; c, c, branches belonging to papillae, of which the bases are hidden from view (after Kollikcr). Fig. 112. Vertical section of the skin and subcutaneous tissue, from end of the thumb, across the ridges and furrows, magnified 20 diameters (from KSlliker): a, horny, and b, mucous layer of the epidermis; c, corium; d, panniculus adiposus; c, papillae on the ridges; /, fat clusters; g, sweat-glands; h, sweat-ducts; i, their openings on the surface. 336 THE SKIN. pilla is abundantly supplied with blood, receiving from the vascular plexus in the cutis one or more minute arterial twigs, which divide into capillary loops in its substance, and then reunite into a minute vein, which passes out at its base. The abundant supply of blood which the papillse thus receive ex- plains the turgescence or kind of erection which they undergo when the circulation through the skin is active. The majority, but not all, of the papillse contain also one or more terminal nerve-fibres, from the. ultimate ramifications of the cutaneous plexus on which their exquisite sensibility depends. The exact mode in which these nerve-fibres terminate is not yet satisfac- torily determined. In some parts, especially those in which the sense of touch is highly developed, as, for example, the palm of the hand and the lips, the fibres appear to terminate, in many of the papillse, by one or more free ends in the sub- stance of a dilated oval-shaped body, not unlike a Pacinian corpuscle (Figs. 136, 137), occupying the principal part of the interior of the papillse, and termed a touch-corpuscle (Fig. 113). Fig. 113. Papillae from the skin of the hand, freed from the cuticle and exhibiting the tac- tile corpuscles. Magnified 350 diameters, a. Simple papilla with four nerve-fibres: a, tactile corpuscle ; b, nerves, b. Papilla treated with acetic acid: a, cortical layer with cells and fine elastic filaments; b, tactile corpuscle with transverse nuclei; c, entering nerve with neurilemma or perineurium ; d, nerve-fibres winding round the corpuscle, c. Papilla viewed from above so as to appear as a cross-section: a, corti- cal layer; 6, nerve-fibre; c, sheath of the tactile corpuscle containing nuclei; d, core (after KCSlliker). The nature of this body is obscure. Kolliker, Huxley, and others, regard it as little else than a mass of fibrous or con- nective tissue, surrounded by elastic fibres, and formed, accord- ing to Huxley, by an increased development of the neurilemma TOUCH-CORPUSCLES END-BULBS. 337 of the nerve-fibres entering the papillae. Wagner, however, to whom seems to belong the merit of first fully describing these bodies, believes that, instead of thus consisting of a homogene- ous mass of connective-tissue, they are special and peculiar bodies of laminated structure, directly concerned in the sense of touch. They do not occur in all the papillae of the parts where they are found, and, as a rule, in the papillae in which they are present there are no bloodvessels. Since these pecu- liar bodies in which the nerve-fibres end are only met with in the papillae of highly sensitive parts, it may be inferred that they are specially concerned in the sense of touch, yet their absence from the papillae of other tactile parts shows that they are not essential to this sense. Closely allied in structure to the Pacinian corpuscles and touch-corpuscles are some little bodies about B-^0 of an inch in diameter, first particularly described by Krause, and named by him “ end-bulbs.” They are generally oval or spheroidal, and composed externally of a coat of connective tissue inclos- ing a softer matter, in which the extremity of a nerve termin- ates. These bodies have been found chiefly in the lips, tongue, palate, and the skin of the glans penis (Fig. 114). Fig. 114. Eud-bulbs in papillae (magnified) treated with acetic acid, a, from the lips ; the white loops in one of them are capillaries, b, from the tongue. Two end-bulbs seen in the midst of the simple papillae: a, a, nerves (from Kolliker). Although destined especially for the sense of touch, the papillae are not so placed as to come into direct contact with external objects; but, like the rest of the surface of the skin, 338 THE SKIN. are covered by one or more layers of epithelium, forming the cuticle or epidermis. The papillse adhere very intimately to the cuticle, which is thickest in the spaces between them, but tolerably level on its outer surface: hence, when stripped off from the cutis, as after maceration, its internal surface presents a series of pits and elevations corresponding to the papillse and their interspaces, of which it thus forms a kind of mould. Besides affording by its impermeability a check to undue evaporation from the skin, and providing the sensitive cutis with a protecting investment, the cuticle is of service in rela- tion to the sense of touch. For, by being thickest in the spaces between the papillse, and only thinly spread over the summits of these processes, it may serve to subdivide the sen- tient surface of the skin into a number of isolated points, each of which is capable of receiving a distinct impression from an external bodies. By covering the papillse it renders the sensa- tion produced by external bodies more obtuse, and in this manner also is subservient to touch : for unless the very sensi- tive papillse were thus defended, the contact of substances would give rise to pain, instead of the ordinary impressions of touch. This is shown in the extreme sensitiveness and loss of tactile power in a part of the skin when deprived of its epi- dermis. If the cuticle is very thick, however, as on the heel, touch becomes imperfect, or is lost, through the inability of the tactile papillse to receive impressions through the dense and horny layer covering them. Sudoriparous Glands.—In the middle of each of the trans- verse furrows between the papillse, and irregularly scattered between the bases of the papillse in those parts of the surface of the body in which there are no furrows between them, are the orifices of ducts of the sudoriparous or sweat glands, by which it is probable that a large portion of the aqueous and gaseous materials excreted by the skin are separated. Each of these glands consists of a small lobular mass, which appears formed of a coil of tubular gland-duct, surrounded by blood- vessels and imbedded in the subcutaneous adipose tissue (Fig. 112). From this mass, the duct ascends, for a short distance, in a spiral manner through the deeper part of the cutis, then pass- ing straight, and then sometimes again becoming spiral, it passes through the cuticle and opens by an oblique valve- like aperture. In the parts where the epidermis is thin, the ducts themselves are thinner and more nearly straight in their course (Fig. 115). The duct, which maintains nearly the same diameter throughout, is lined with a layer of epithelium con- tinuous with the epidermis ; while the part which passes through the epidermis is composed of the latter structure only ; the SEBACEOUS GLANDS. 339 cells which immediately form the boundary of the canal in this part being somewhat differently arranged from those of the adjacent cuticle. The sudoriparous glands are abundantly distributed over the whole surface of the body ; but are especially numerous, as well as very large, in the skin of the palm of the hand, where, according to Krause, they amount to 2736 in each su- perficial square inch, and according to Mr. Erasmus Wilson, to as many as 3528. They are almost equally abundant and large in the skin of the sole. The glands by which the pecu- liar odorous matter of the axillae is secreted form a nearly complete layer under the cutis, and are like the ordinary su- doriparous glands, except in being larger and having very short ducts. In the neck and back, where they are least numerous, the glands amount to 417 on the square inch (Krause). Their total number Krause estimates at 2,381,248; and, supposing the orifice of each gland to present a surface of -j-'gth of a line in diameter (and regarding a line as equal to ygth of an inch), he reckons that the whole of the glands would present an evaporating surface of about eight square inches.1 Sebaceous Glands.—Besides the perspiration, the skin se- cretes a peculiar fatty matter, and for this purpose is provided with another set of special organs, termed sebaceous glands (Fig. 115), which, like the sudoriparous glands, are abun- dantly distributed over most parts of the body. They are most numerous in parts largely supplied with hair, as the scalp and face, and are thickly distributed about the entrances of the various passages into the body, as the anus, nose, lips, and external ear. They are entirely absent from the palmar surface of the hands and the plantar surfaces of the feet. They are minutely lobulated glands, composed of an aggregate of small vesicles or sacculi filled with opaque white substances, like soft ointment. Minute capillary vessels overspread them ; and their ducts, which have a bearded appearance, as if formed of rows of shells, open either on the surface of the skin, close to a hair, or, which is more usual, dii’ectly into the follicle of the hair. In the latter case, there are generally two glands to each hair (Fig. 115). 1 The peculiar bitter yellow substance secreted by the skin of the external auditory passage is named cerumen, and the glands them- selves ceruminous glands ; but they do not much differ in structure from the ordinary sudoriparous glands. 340 THE SKIN. Structure of Hair and Nails. Hair.—A hair is produced by a peculiar growth and modi- fication of the epidermis. Externally it is covered by a layer Fro. 115. Fig- 115a. Fig. 115.—Sebaceous glands of the skin, after Guilt: a, a, sebaceous glands opening into the follicle of the hair by efferent ducts; b, a hair on its follicle. Fig. U5cr.—Sweat-gland and the commencement of its duct. a. Venous radicles on the wall of the cell in which the gland rests. This vein anastomoses writh others in the vicinity, b. Capillaries of the gland separately represented, arising from their arteries, which also anastomose. The bloodvessels are all situated on the out- side or deep surface of the tube, in contact with the basement-membrane.—Magn. 35 diam. of fine scales closely imbricated, or overlapping like the tiles of a house, but with the free edges turned upwards (Fig. 116, a). It is called the cuticle of the hair. Beneath this is a much thicker layer of elongated horny cells, closely packed together so as to resemble a fibrous structure. This, very commonly, in the human subject, occupies the whole of the inside of the hair; but in some cases there is left a small cen- tral space tilled by a substance called the medulla or pith, composed of small collections of irregularly shaped cells, con- taining fat- and pigment-granules. The follicle, in which the root of each hair is contained (Fig. 117), forms a tubular depression from the surface of the STRUCTURE OF HAIR. 341 skin,—descending into the subcutaneous fat, generally to a greater depth than the sudoriparous glands, and at its deepest part enlarging in a bulbous form, and often curving from its Fig. 116. A, surface of a white hair, magnified 160 diameters. The wave lines mark the upper or free edges of the cortical scales. B, separated scales, magnified 350 diame- ters (after Kolliker). previous rectilinear course. It is lined throughout by cells of epithelium, continuous with those of the epidermis, and its walls are formed of pellucid membrane, which commonly, in the follicles of the largest hairs, has the structure of vascular fibro-cellular tissue. At the bottom of the follicle is a small papilla, or projection of true skin, and it is by the production and outgrowth of epidermal cells from the surface of this pa- pilla that the hair is formed. The inner wall of the follicle is lined by epidermal cells continuous with those covering the general surface of the skin; as if indeed the follicle had been formed by a simple thrusting in of the surface of the integu- ment (Figs. 117, 118). This epidermal lining of the hair- follicle, or root-sheath of the hair, is composed of two layers, the inner one of which is so moulded on the imbricated scaly cuticle of the hair, that its inner surface becomes imbricated also, but of course in the opposite direction. When a hair is pulled out, the inner layer of the root-sheath and part of the outer layer also are commonly pulled out with it. Nails.—A nail, like a hair, is a peculiar arrangement of epidermal cells, the undermost of which, like those of the general surface of the integument, are rounded or elongated, while the superficial are flattened, and of more horny consist- ence. That specially modified portion of the corium, or true skin, by which the nail is secreted, is called the matrix. The back edge of the nail, or the root as it is termed, is received into a shallow crescentic groove in the matrix, while the front part is free, and projects beyond the extremity of the digit. The intermediate portion of the nail rests by its broad 342 THE SKIN. under surface on the front part of the matrix, which is here called the bed of the nail. This part of the matrix is not uni- Fig. 117. Fig. 118. Fig. 117.—Medium-sized hair in its follicle, magnified 50 diameters (from Kolli- ker). a, stem cut short; b, root; c, knob ; <1, hair cuticle ; e, internal, and/, external root-sheath ; g, h, dermic coat of follicle ; i, papilla ; k, k ducts of sebaceous glands; l, corium; m, mucous layer of epidermis ; o, upper limit of internal root-sheath (from Kolliker). Fig. 118 —Magnified view of the root of a hair (after Kohlrausch). a, stem or shaft of hair cut across ; b, inner, and c, outer layer of the epidermal lining of the hair-follicle, called also the inner and outer root-sheath ; d, dermal or external coat of the hair-follicle, shown in part; e, imbricated scales about to form a cortical layer on the surface of the hair. The adjacent cuticle of the root-sheatli is not repre- sented, and the papilla is hidden in the lower part of the knob where that is rep- resented lighter. formly smooth on the surface, but is raised in the form of lon- gitudinal and nearly parallel ridges or laminae, on which are STRUCTURE OF NAILS. 343 moulded the epidermal cells of which the nail is made up (Fig. 119). The growth of the nail, like that of a hair, or of the epi- dermis generally, is effected by a constant production of cells from beneath and behind, to take the place of those which are worn or cut away. Inasmuch, however, as the posterior edge of the nail, from its being lodged in a groove of the skin, can- Fig. 119. Vertical transverse section through a small portion of the nail and matrix largely magnified (after Kolliker). A, corium of the nail-bed, raised into ridges or laminae a, fitting in between cor- responding laminae 6, of the nail. B, Malpighian, and C, horny layer of nail; d, deepest and vertical cells ; e, upper flattened cells of Malpighian layer. not grow backwards, on additions being made to it, so easily as it can pass in the opposite direction, any growth at its hinder part pushes the whole forwards. At the same time fresh cells are added to its under surface, and thus each por- tion of the nail becomes gradually thicker as it moves to the front, until, projecting beyond the surface of the matrix, it can receive no fresh addition from beneath, and is simply moved forwards by the growth at its root, to be at last worn away or cut off. 344 THE SKIN. Excretion by the Skin. The skin, as already stated, is the seat of a twofold excre- tion ; of that formed by the sebaceous glands and hair-follicles, and of the more watery fluid, the sweat or perspiration, elimi- nated by the sudoriparous glands. The secretion of the sebaceous glands and hair-follicles (for their products cannot be separated) consists of cast-off epithe- lium-cells, with nuclei and granules, together with an oily matter, extractive matter, and stearin ; in certain parts, also, it is mixed with a peculiar odorous principle, which is said by Dr. Fischer to contain caproic, butyric, and rutic acids. It is, perhaps, nearly similar in composition to the unctuous coat- ing, or vernix caseosa, which is formed on the body of the foetus while in the uterus, and which contains large quantities both of olein and margarin (J. Davy). Its purpose seems to be that of keeping the skin moist and supple, and, by its oily nature, of both hindering the evaporation from the surface, and guarding the skin from the effects of the long-continued ac- tion of moisture. But while it thus serves local purposes, its removal from the body entitles it to be reckoned among the excretions of the skin; though the share it has in the purify- ing of the blood cannot be discerned. The fluid secreted by the sudoriparous glands is usually formed so gradually, that the watery portion of it escapes by evaporation as fast as it reaches the surface. But, during strong exercise, exposure to great external warmth, in some diseases, and when evaporation is prevented by the application of oiled silk or plaster, the secretion becomes more sensible and collects on the skin in the form of drops of fluid. A good analysis of the secretion of these glands, unmixed with other fluids secreted from the skin, can scarcely be made; for the quantity that can be collected pure is very small. Krause in a few drops from the palm of the hand, found an acid reac- tion, oily matter, and margarin, with water. The perspiration of the skin, as the term is sometimes em- ployed in physiology, includes all that portion of the secre- tions and exudations from the skin which passes off by evap- oration ; the sweat includes that which may be collected only in drops of fluid on the surface of the skin. The two terms are, however, most often used synonymously; and for distinc- tion, the former is called insensible perspiration : the latter, sensible perspiration. The fluids are the same, except that the sweat is commonly mingled with various substances lying on the surface of the skin. The contents of the sweat are, in part, matters capable of assuming the form of vapor, such as car- THE SWEAT. 345 bonic acid and water, and in part, other matters which are deposited on the skin, and mixed with the sebaceous secretion. Thenard collected the perspiration in a flannel shirt which had been washed in distilled water, and found in it chloride of sodium, acetic acid, some phosphate of soda, traces of phos- phate of lime, and oxide of iron, together with an animal sub- stance. In sweat which had run from the forehead in drops, Berzelius found lactic acid, chloride of sodium, and chloride of ammonium. Anselmino placed his arm in a glass cylinder, and closed the opening around it with oiled silk, taking care that the arm touched the glass at no point. The cutaneous exhalations collected on the interior of the glass, and ran down as a fluid : on analyzing this, he found water, acetate of ammonia, and carbonic acid ; and in the ashes of the dried residue of sweat he found carbonate, sulphate, and phosphate of soda, and some potash, with chloride of sodium, phosphate and carbonate of lime, and traces of oxide of iron. Urea has also been shown to be an ordinary constituent of the fluid of perspiration. The ordinary constituents of perspiration, may, therefore, according to Gorup-Besanez, be thus summed up: water, fat, acetic, butyric and formic acid, urea, and salts. The princi- pal salts are the chlorides of sodium and potassium, together with, in small quantity, alkaline and earthy phosphates and sulphates; and, lastly, some oxide of iron. Of these several substances, none, however, need particular consideration, ex- cept the carbonic acid and water. The quantity of watery vapor excreted from the skin was estimated very carefully by Lavoisier and Sequin. The latter chemist inclosed his body in an air-tight bag, with a mouth- piece. The bag being closed by a strong band above, and the mouth-piece adjusted and gummed to the skin around the mouth, he was weighed, and then remained quiet for several hours, after which time he was again weighed. The differ- ence in the two weights indicated the amount of loss by pul- monary exhalation. Having taken off the air-tight dress, he was immediately weighed again, and a fourth time after a cer- tain interval. The difference between the two weights last ascertained gave the amount of the cutaneous and pulmonary exhalation together; by subtracting from this the loss by pul- monary exhalation alone, while he was in the air-tight dress, he ascertained the amount of cutaneous transpiration. The repetition of these experiments during a long period, showed that, during a state of rest, the average loss by cutaneous and pulmonary exhalation in a minute, is from seventeen to eigh- teen grains,—the minimum eleven grains, the maximum 346 THE S K I X. thirty-two grains ; and that of the eighteen grains, eleven pass off by the skiu, and seven by the lungs. The maximum loss by exhalation, cutaneous and pulmonary, in twenty-four hours, is about 3! lb.; the minimum about 1£ lb. Valentin found the whole quantity lost by exhalation from the cutaneous and respiratory surfaces of a healthy man who consumed daily 40,000 grains of food and drink, to be 19,000 grains or lb. Subtracting from this, for the pulmonary exhalation, 5000 grains, and, for the excess of the weight of the exhaled car- bonic acid over that of the equal volume of the inspired oxy- gen, 2256 grains, the remainder, 11,744 grains, or nearly 1-f lb., may represent an average amount of cutaneous exhalation in the day. The large quantity of watery vapor thus exhaled from the skin, will prove that the amount excreted by simple transuda- tion through the cuticle must be very large, if we may take Krause’s estimate of about eight square inches for the total evaporating surface of the sudoriparous glands; for not more than about 3365 grains could be evaporated from such a sur- face in twenty-four hours, under the ordinary circumstances in which the surface of the skin is placed. This estimate is not an improbable one, for it agrees very closely with that of Milne-Edwards, who calculated that when the temperature of the atmosphere is not above 68° F., the glandular secretion of the skin contributes only £th to the total sum of cutaneous exhalation. The quantity of watery vapor lost by transpiration, is of course influenced by all external circumstances which affect the exhalation from other evaporating surfaces, such as the temperature, the hygrometric state, and the stillness of the atmosphere. But, of the variations to which it is subject un- der the influence of these conditions, no calculation has been exactly made. Neither, until recently, has there been any estimate of the quantity of carbonic acid exhaled by the skin on an average, or in various circumstances. Regnault and Reiset attempted to supply this defect, and concluded, from some careful exper- iments, that the quantity of carbonic acid exhaled from the skin of a warm-blooded animal is about of that furnished by the pulmonary respiration. Dr. Edward Smith’s calcula- tion is somewhat less than this. The cutaneous exhalation is most abundant in the lower classes of animals, more particu- larly the naked Amphibia, as frogs and toads, whose skin is thin and moist, and readily permits an interchange of gases between the blood circulating in it and the surrounding atmos- phere. Bischoff found that, after the lungs of frogs had been ABSORPTION BY THE SKIN. 347 tied and cut out, about a quarter of a cubic inch of carbonic acid gas was exhaled by the skin in eight hours. And this quantity is very large, when it is remembered that a full-sized frog will generate only about half a cubic inch of carbonic acid by his lungs and skin together in six hours (Milne- Edwards and Miilier). That the respiratory function of the skin is, perhaps, even more considerable in the higher animals than appears to be the case from the experiments of Regnault and Reiset just alluded to, seemed probable by the fact ob- served by Magendie and others, that if the skin of animals is covered with an impermeable varnish, or the body inclosed, all but the head, in a caoutchouc dress, animals soon die, as if asphyxiated ; their heart and lungs being gorged with blood, and their temperatures, during life, gradually falling many degrees, and sometimes as much as 36° F. below the ordinary standard (Magendie). Some recent experiments of Lashke- witzch appear, however, to confirm the opinion of Valentin, that loss of temperature is the immediate cause of death in these cases. A varnished animal is said to have suffered no harm when surrounded by cotton wadding, but it died when the wadding was removed. Absorption by the skin has been already mentioned, as an instance in which that process is most actively accomplished. Metallic preparations rubbed into the skin have the same action as when given internally, only in a less degree. Mer- cury applied in this manner exerts its specific influence upon syphilis, and excites salivation ; potassio-tartrate of antimony may excite vomiting, or an eruption extending over the whole body; and arsenic may produce poisonous effects. Vegetable matters, also, if soluble, or already in solution, give rise to their peculiar effects, as cathartics, narcotics, and the like, when rubbed into the skin. The effect of rubbing is probably to convey the particles of the matter into the orifices of the glands whence they are more readily absorbed than they would be through the epidermis. When simply left in Con- tact with the skin, substances, unless in a fluid state, are sel- dom absorbed. It has long been a contested question whether the skin covered with the epidermis has the power of absorbing water ; and it is a point the more difficult to determine because the skin loses water by evaporation. But, from the result of many experiments, it may now be regarded as a well-ascer- tained fact that such absorption really occurs. M. Edwards has proved that the absorption of water by the surface of the body may take place in the lower animals very rapidly. Not only frogs, which have a thin skin, but lizards, in which the 348 THE SKIN. cuticle is thicker than in man, after having lust weight by being kept for some time in a dry atmosphere, were found to recover both their weight and plumpness very rapidily when immersed in water. When merely the tail, posterior extremi- ties, and posterior part of the body of the lizard were im- mersed, the water absorbed was distributed throughout the system. And a like absorption through the skin, though to a less extent, may take place also in man. Dr. Madden, haying ascertained the loss of weight, by cutaneous and pulmonary transpiration, that occurred during half an hour in the air, entered the bath, and remained im- mersed during the same period of time breathing through a tube which communicated with the air exterior to the room. He was then carefully dried and again weighed. Twelve experiments were performed in this manner; and in ten there was a gain of weight, varying from 2 scruples to 5 drachms and 4 scruples, or a mean gain of 1 drachm 2 scruples and 13 grains. The loss in the air during the same length of time (half an hour) varied in ten experiments from drachms to 1 ounce scruples, or in the mean was about drachms. So that, admitting the supposition that the cutaneous trans- piration was entirely suspended, and estimating the loss by pulmonary exhalation at 3 drachms, there was, in these ten experiments of Dr. Madden, an average absorption of 4 drachms 1 scruple, and 3 grains, by the surface of the body, during half an hour. In four experiments performed by M. Berthold, the gain in weight was greater than in those of Dr. Madden. In severe cases of dysphagia, when not even fluids can be taken into the stomach, immersion in a bath of warm water or of milk and water may assuage the thirst; and it has been found in such cases that the weight of the body is increased by the immersion. Sailors also, when destitute of fresh water, find their urgent thirst allayed by soaking their clothes in salt water and wearing them in that state; but these effects may be in part due to the hindrance to the evaporation of water from the skin. The absorption, also, of different kinds of gas by the skin is proved by the experiments of Abernethy, Cruikshank, Beddoes, and others. In these cases, of course, the absorbed gases com- bine with the fluids, and lose the gaseous form. Several phys- iologists have observed an absorption of nitrogen by the skin. Beddoes says, that he saw the arm of a negro become pale for a short time when immersed in chlorine; and Abernethy ob- served that when he held his hands in oxygen, nitrogen, car- STRUCTURE OF THE KIDNEY. 349 bonic acid, and other gases contained in jars, over mercury, the volume of the gases became considerably diminished. The share which the evaporation from the skin has in the maintenance of the uniform temperature of the body, and the necessary adaptation thereto of the production of heat, have been already mentioned (p. 195). CHAPTER XV. THE KIDNEYS AND THEIR SECRETION. Structure of the Kidney. The kidney is covered on the outside by a rather tough fibrous capsule, which is slightly attached by its inner surface to the proper substance of the organ by means of very fine fibres of areolar tissue and minute bloodvessels. From the healthy kidney, therefore, it may be easily torn oft* without Fig. 120. Plan of a longitudinal section through the pelvis and substance of the right kid- ney, %; a, the cortical substance; b, b, broad part of the pyramids of Malpighi; c, c, the divisions of the pelvis named calyces, laid open ; c', one of these unopened; d, summit of the pyramids or papillae projecting into calyces; e, e, section of the narrow part of two pyramids near the calyces ; p, pelvis or enlarged divisions of the ureter within the kidney; u, the ureter ; s, the sinus ; A, the hilus. 350 THE KIDNEYS AND THEIR SECRETION. injury to the subjacent cortical portion of the organ. At the lulus or notch of the kidney, it becomes continuous with the external coat of the upper and dilated part of the ureter. On making a section lengthwise through the kidney (Fig. 120) the main part of its substance is seen to be composed of two chief portions, called respectively the cortical and the medullary portion, the latter being also sometimes called the pyramidal portion, from the fact of its being composed of about a dozen conical bundles of urine-tubes, each bundle being called a pyramid. The upper part of the duct of the organ, or the ureter, is dilated into what is called the pelvis of the kidney; and this, again, after separating into two or three principal divisions, is finally subdivided into still smaller portions, vary- ing in number from about 8 to 12, or even more, and called calyces. Each of these little calyces or cups, again receives the pointed extremity or papilla of a pyramid. Sometimes, how- ever, more than one papilla is received by a calyx. The kidney is a gland of the class called tubular, and both its cortical and medullary portions are composed essentially of secreting tubes, the tubuli uriniferi, which by one extremity, in the cortical portion, end commonly in little saccules con- taining bloodvessels, called Malpighian bodies, and by the other open through the papillae into the pelvis of the kidney, and thus discharge the urine which flows through them. In the pyramids they are chiefly straight—dividing and diverging as they ascend through these into the cortical por- tion ; while in the latter region they spread out more irregu- larly, and become much branched and convoluted. The tubuli uriniferi (Fig. 121) are composed of a nearly homogeneous membrane, lined internally by spheroidal epithe- lium, and for the greater part of their extent are about gJ<j of an inch in diameter,— becoming somewhat larger than this immediately before they open through the papillce. On trac- ing these tubules upwards from the papillae, they are found to divide dichotomously as they ascend through the pyramids, and on reaching the bases of the latter, they begin to branch and diverge more widely, and to form by their branches and convolutions the essential part of the cortical portion of the organ. At their extremities they become dilated into the Malpighian capsides. Until recently, it was believed that the straight tubules in the pyramids branch out and become con- voluted immediately on reaching the bases of the pyramids; but between the straight tubes in the pyramids and the convo- luted tubes in the cortical portion, there has been shown to be a system of tubules of smaller diameter than either, which form intercommunications between the two varieties formerly STRUCTURE OF THE KIDNEY. 351 recognized. These intervening tubules, called the looped tubes of Henle, arising from the straight tubes in some part of their course, or being continued from their extremities at the bases of the pyramids, pass down loopwise in the pyramids for a Fig. 122. Fig. 121. Fig. 121.—a. Portion of a secreting canal from the cortical substance of the kid- ney. b. The epithelium or gland-cells, more highly magnified (700 times). Fig. 122.—Diagram of the looped uriniferous tubes and their connection with the capsules of the glomeruli (from Southey, after Ludwig). In the lower part of the figure one of the large branching tubes is shown opening on a papilla; in the mid- dle part two of the looped small tubes are seen descending to form their loops, and reascending in the medullary substance; while in the upper or cortical part, these tubes, after some enlargement, are represented as becoming convoluted and dilated in the capsules of glomeruli. longer or shorter distance, and then, again turning up, end in the convoluted tubes whose extremities are dilated into the Malpighian capsules before referred to (Fig. 122). On a transverse section of a pyramid (Fig. 123), these looped tubes 352 THE KIDNEYS AND THEIR SECRETION. are seen to be of much smaller calibre than the straight ones, which are passing down to open through the papillae. The Malpighian bodies are fouud only in the cortical part of the kidney. On a section of the organ, some of them are just visible to the naked eye as minute red points ; others are too small to be thus seen. Their average diameter is about 0 of an inch. Each of them is composed of the dilated extremity of a urinary tube, or Malpighian capsule, inclosing a tuft of bloodvessels. In connection with these little bodies the general distribu- tion of bloodvessels to the kidney may be here considered. The renal artery divides into several branches, which, pass- ing in at the hilus of the kidney, and covered by a fine sheath of areolar tissue derived from the capsule, enter the substance of the organ chiefly in the intervals between the papillse, and penetrate the cortical substance, where this dips down between the bases of the pyramids. Here they form a tolerably dense Fig. 123. Transverse section of a renal papilla (from Kolhker) -y a, larger tubes or papil- lary ducts; 6, smaller tubes of Henle; c, bloodvessels, distinguished by their flatter epithelium , d, nuclei of the stroma. plexus of an arched form, and from this are given off smaller arteries which ultimately supply the Malpighian bodies. The small afferent artery (Fig. 124), which enters the Mal- pighian body by perforating the capsule, breaks up in the in- terior into a dense and convoluted and looped capillary plexus, which is ultimately gathered up again into a single small effer- STRUCTURE OF THE KIDNEY. 353 ent vessel, comparable to a minute vein, which leaves the Malpighian capsule just by the point at which the afferent artery enters it. On leaving, it does not immediately join other small veins as might have been expected, but again breaking up into a network of capillary vessels, is distributed on the exterior of the tubule, from whose dilated end it had just emerged. After this second breaking up it is finally col- lected into a small vein, which, by union with others like it, helps to form the radicles of the renal vein. The Malpighian capsule is lined by a layer of fine squamous epithelial cells; but whether the small glomerulus or tuft of capillaries in the interior is covered by a similar layer is un- Fig. 124 Fig. 125. Fig. 124.—Plan of the renal circulation in man anil the Mammalia, a, terminal branch of the artery, giving the terminal twig 1, to the Malpighian tuft m, from which emerges the efferent or portal vessel, 2. Other efferent vessels, 2, are seen entering the plexus of capillaries, surrounding the uriniferous tube, t. From the plexus, the emulgent vein, v, springs. Fig. 125.—Semidiagrammatic representation of a Malpighian body in its relation to the uriniferous tube (from Kolliker) —y-. a, capsule of the Malpighian body; d, epithelium of the uriniferous tube; e, detached epithelium ; /, afferent vessel; g, efferent vessel; h, convoluted vessels of the glomerulus. certain. Kolliker believes that such a covering, although ex- ceedingly thin, is present, and has delineated the appearance in the accompanying diagram (Fig. 125). Besides the small afferent arteries of the Malpighian bodies, there are, of course, others which are distributed in the ordi- nary manner, for nutrition’s sake, to the different parts of the organ; and in the pyramids, between the tubes, there are nu- 354 THE KIDNEYS AND THEIR SECRETION. inerous straight vessels, the vasa recta, supposed by some ob- servers to be branches of vasa efferentia from Malpighian bodies, and therefore comparable to the venous plexus around the tubules in the cortical portion, while others think that they arise directly from small branches of the renal arteries. Between the tubes, vessels, &c., which make up the main substance of the kidney, there exists in small quantity a fine matrix of areolar tissue. The nerves of the kidney are derived from the renal plexus.1 The separation from the blood of the solids in a state of so- lution in the urine is probably effected, like other secretions, by the agency of the gland-cells, and equally in all parts of the urine-tubes. The urea and uric acid, and perhaps some of the other constituents existing ready formed in the blood, may need only separation, that is, they may pass from the blood to the urine without further elaboration ; but this is not the case with some of the other principles of the urine, such as the acid phosphates and the sulphates, for these salts do not exist as such in the blood, and must be formed by the chemi- cal agency of the cells. The watery part of the urine is probably in part separated by the same structures that secrete the solids, but the ingeni- ous suggestion of Mr. Bowman that the water of the urine is mainly strained off, so to speak, by the Malpighian bodies, from the blood which circulates in their capillary tufts, is ex- ceedingly probable; although if, as Kolliker and others main- tain, there is an epithelial covering to these tufts or glomeruli, it is very likely that the solids of the urine may be in part se- creted here also. We may, therefore, conclude that all parts of the tubular system of the kidney take part in the secretion of the urine as a whole, but that there is a provision also in the arrangement of the vessels in the Malpighian bodies for a more simple draining off of water from the blood when re- quired. The large size of the renal arteries and veins permits so rapid a transit of the blood through the kidneys, that the whole of the blood is purified by them. The secretion of urine is rapid in comparison with other secretions, and as each por- Secretion of Urine. 1 For a more detailed account of the structure of the kidney and a summary of the various opinions on the subject, the student may be referred especially to Quain’s Anatomy, 7th ed., and to a paper by Dr. Reginald Southey, in vol. i of the St. Bartholomew’s Hospital .Reports. PASSAGE OF URINE INTO THE BLADDER. 355 tion is secreted, it propels that which is already in the tubes onwards into the pelvis of the kidney. Thence through the ureter the urine passes into the bladder, into which its rate and mode of entrance has been watched in cases of ectopia vesicse, i. e., of such fissures in the anterior and lower part of the walls of the abdomen, aud of the front wall of the bladder, as exposed to view its hinder wall together with the orifices of the ureters. Some good observations on such cases were made by Mr. Erichsen. The urine does not enter the bladder at any regular rate, nor is there a synchronism in its movement through the two ureters. During fasting, two or three drops enter the bladder every minute, each drop as it enters first raising up the little papilla on which, in these cases, the ureter opens, and then passing slowly through its orifice, which at once again closes like a sphincter. In the recumbent posture, the urine collects for a little time in the ureters, then flows gently, and, if the body be raised, runs from them in a stream till they are empty. Its flow is increased in deep inspiration, or straining, and in active exercise, and in fifteen or twenty minutes after a meal. The same observations, also, showed how fast some substances pass from the stomach through the circulation, and through the vessels of the kidneys. Ferrocyanide of potassium so passed on one occasion in a minute: vegetable substances, such as rhubarb, occupied from sixteen to thirty-five minutes ; neutral alkaline salts with vegetable acids, which were generally de- composed in transitu, made the urine alkaline in from twenty- eight to forty-seven minutes. But the times of passage varied much; and the transit was always slow when the substances were taken during digestion. The urine collecting in the urinary bladder is prevented from regurgitation into the ureters by the mode in which these pass through the walls of the bladder, namely, by their lying for between half and three-quarters of an inch between the muscular and mucous coats, and then turning rather abruptly forwards, and opening through the latter, it collects till the distension of the bladder is felt either by direct sensation, or, in ordinary cases, by a transferred sensation at and near the orifice of the urethra. Then, the effort of the will being di- rected primarily to the muscles of the abdomen, and through them (by reason of its tendency to act with them) to the urinary bladder, the latter, though its muscular walls are really com- posed of involuntary muscle, contracts, and expels the urine. (See also p. 183.) 356 THE URINE. The Urine: its General Properties. Healthy urine is a clear limpid fluid, of a pale yellow or amber color, with a peculiar faint aromatic odor, which be- comes pungent and ammoniacal when decomposition takes place. The urine, though usually clear and transparent at flrst, often becomes as it cools opaque and turbid from the de- position of part of its constituents previously held in solution ; and this may be consistent with health, though it is only in disease that, in the temperature of 98° or 100°, at which it is voided, the urine is turbid even when first expelled. Although ordinarily of pale amber color, yet, consistently with health, the urine may be nearly colorless, or of a brownish or deep orange tint, and, between these extremes, it may present every shade of color. When secreted, and most commonly when first voided, the urine has a distinctly acid reaction in man and all carnivorous animals, and it thus remains till it is neutralized or made alka- line by the ammonia developed in it by decomposition. In most herbivorous animals, on the contrary, the urine is alka- line and turbid. The difference depends, not on any peculi- arity in the mode of secretion, but on the differences in the food on which the two classes subsist: for when carnivorous animals, such as dogs, are restricted to a vegetable diet, their urine becomes pale, turbid, and alkaline, like that of an her- bivorous animal, but resumes its former acidity on the return to an animal diet; while the urine voided by herbivorous ani- mals, e. g., rabbits, fed for some time exclusively upon animal substances, presents the acid reaction and other qualities of the urine of Carnivora, its ordinary alkalinity being restored only on the substitution of a vegetable for the animal diet (Bernard). Human urine is not usually rendered alkaline by vegetable diet, but it becomes so after the free use of alkaline medicines, or of the alkaline salts with carbonic or vegetable acids; for these latter are changed into alkaline carbonates previous to elimination by the kidneys. Except in these cases, it is very rarely alkaline, unless ammonia has been developed in it by decomposition commencing before it is evacuated from the bladder. The average specific gravity of the human urine is about 1020. Probably no other animal fluid presents so many va- rieties in density within twenty-four hours as the urine does; for the relative quantity of water and of solid constituents of which it is composed is materially influenced by the condition and occupation of the body during the time at which it is se- creted, by the length of time which has elapsed since the last COMPOSITION OF URINE. 357 meal, and by several other accidental circumstances. The ex- istence of these causes of difference in the composition of the urine has led to the secretion being described under the three heads of urina sanguinis, urina potus, and urina cibi. The first of these names signifies the urine, or that part of it which is secreted from the blood at times in which neither food nor drink has been recently taken, and is applied especially to the urine which is evacuated in the morning before breakfast. The urina potus indicates the urine secreted shortly after the intro- duction of any considerable quantity of fluid into the body; and the urina cibi the portions secreted during the period im- mediately succeeding a meal of solid food. The last kind con- tains a larger quantity of solid matter than either of the others ; the first or second, being largely diluted with water, possesses a comparatively low specific gravity. Of these three kinds, the morning urine is the best calculated for analysis, since it represents the simple secretion unmixed with the ele- ments of food or drink; if it be not used, the whole of the urine passed during a period of twenty-four hours should be taken. In accordance with the various circumstances above- mentioned, the specific gravity of the urine may, consistently with health, range widely on both sides of the usual average. The average healthy range may be stated at from 1015 in the winter to 1025 in the summer, and variations of diet and ex- ercise may make as great a difference. In disease, the varia- tion may be greater; sometimes descending, in albuminuria, to 1004, and frequently ascending in diabetes, when the urine is loaded with sugar, to 1050, or even to 1060. The whole quantity of urine secreted in twenty-four hours is subject to variation according to the amount of fluid drunk, and the proportion of the latter passing off from the skin, lungs, and alimentary canal. It is because the secretion of the skin is more active in summer than in winter, that the quantity of urine is smaller, and its specific gravity propor- tionately higher. On taking the mean of numerous observa- tions by several experimenters, Dr. Parkes found that the average quantity voided in twenty-four hours by healthy male adults from twenty to forty years of age, amounted to fluid ounces. Chemical Composition of the Urine. The urine consists of water, holding in solution certain ani- mal and saline matters as its ordinary constituents, and occa- sionally various matters taken into the stomach as food— salts, coloring matter, and the like. The quantities of the several natural and constant ingredients of the urine are 358 THE URINE. stated somewhat differently by the different chemists who have analyzed it; but many of the differences are not important, and the well-known accuracy of the several chemists renders it almost immaterial which of the analyses is adopted. The analyses by A. Becquerel being adopted by Dr. Prout, and by Dr. Golding Bird, will be here employed. (Table I.) Table II has been compiled from the observations of Dr. Parkes, and of numerous other authors quoted in his admira- ble work on the urine. Table I. Average quantity of each constituent of the Urine in 1000 parts. Water, 907. Urea, ........... 14.230 Uric acid, .......... .468 Coloring matter, .... Mucus, and animal extractive matter, inseparable from each other, 10.107 Sulphates, Bi-phosphates, Chlorides, [ Soda, L Potash, Lime, Soda, Magnesia, Ammonia, Salts, 8.135 / Sodium, ( Potassium, Hippurate of soda, Eluoride of potassium, Silica, traces. Table II. 1000.000 Average quantity of the chief constituents of the Urine excreted in ' 24 hours by healthy male adults. Water, ........ 52. fluid ounces. Urea, ......... 512.4 grains. Uric acid, 8.5 “ Hippuric acid, uncertain, probably 10 to 15. “ Sulphuric acid, ....... 31.11 “ Phosphoric acid, ....... 45. “ Chlorine, ........ 105.0 “ Chloride of Ammonium, , . . . . 35.25 “ Potash, . . . . . . . 58. . “ Soda, . . ...... 125. “ Lime, 3.5 “ Magnesia, ........ 3. “ Mucus, ........ 7. “ Creatin, Creatinin, Pigment, Xanthin, Hvpoxanthin, Extractives, 154.0 Resinous matter, &c. UREA 359 From these proportions, however, most of* the constituents are, even in health, liable to variations. Especially the water is so. Its variations in different seasons, and according to the quantity of drink and exercise, have already been mentioned. It is also liable to be influenced by the condition of the ner- vous system, being sometimes greatly increased in hysteria, and some other nervous affections; and at other times diminished. In some diseases it is enormously increased ; and its increase may be either attended with an augmented quantity of solid matter, as in ordinary diabetes, or may be nearly the sole change, as in the affection termed diabetes insipidus. In other diseases, e. g., the various forms of albuminuria, the quantity may be considerably diminished. A febrile condition almost always diminishes the quantity of water; and a like diminu- tion is caused by any affection which draws off a large quantity of fluid from the body through any other channel than that of the kidneys, e. g., the bowels and the skin. Urea.—Urea is the principal solid constituent of the urine, forming nearly one-half of the whole quantity of solid matter. It is also the most important ingredient, since it is the chief substance by which the nitrogen of decomposed tissue and superfluous food is excreted from the body. For its re- moval, the secretion of urine seems especially provided; and by its retention in the blood the most pernicious effects are produced. Urea, like the other solid constituents of the urine, ex- ists in a state of solution. But it may be procured in the solid state, and then appears in the form of delicate silvery acicu- lar crystals, which under the microscope, appear as four- sided prisms (Fig. 126). It is obtained in this state by evapo- rating urine carefully to the consistence of honey, acting on the inspissated mass with four parts of alcohol, then evaporat- ing the alcoholic solution, and purifying the residue by repeated solution in water or alcohol, and-finally allowing it to crystallize. It readily combines with an acid, like a weak base; and may thus be conveniently procured in the form of a nitrate, by add- ing about half a drachm of pure nitric acid to double that quan- tity of urine in a watch-glass. The crystals of nitrate of urea are Fig. 126. Crystals of urea. 360 THE URINE. formed more rapidly if the urine have been previously concen- trated by evaporation. Urea is colorless when pure; when impure, yellow or brown ; without smell, and of a cooling, nitre-like taste; has neither an acid nor an alkaline reaction, and deliquesces in a moist and warm atmosphere. At 59° F. it requires for its solution less than its weight of water; it is dissolved in all proportions by boiling water; but it requires five times its weight of cold alcohol for its solution. At 248° F. it melts without under- going decomposition ; at a still higher temperature ebullition takes place, and carbonate of ammonia sublimes ; the melting mass gradually acquires a pulpy consistence; and, if the heat is carefully regulated, leaves a gray-white powder, cyanic acid. Urea is identical in composition with cyanate of ammonia, and was first artificially produced by Wohler from this sub- stance. Thus: Cyanate of Ammonia. Urea. CHNO. H3N = CH4N20. The action of heat upon urea in evolving carbonate of am- monia, and leaving cyanic acid, is thus explained. A similar decomposition of the urea with development of carbonate of ammonia ensues spontaneously when urine is kept for some days after being voided, and explains the ammoniacal odor then evolved. It is probable that this spontaneous decom- position is accelerated by the mucus and other animal matters in the urine, which, by becoming putrid, act the part of a ferment and excite a change of composition in the surrounding compounds. It is chiefly thus that the urea is sometimes de- composed before it leaves the bladder, when the mucous mem- brane is diseased, and the mucus secreted by it is both more abundant and, probably, more prone than usual to become putrid. The same occurs also in some affections of the nervous system, particularly in paraplegia. The quantity of urea excreted is, like that of the urine itself, subject to considerable variation. It is materially influenced by diet, being greater when animal food is exclusively used, less when the diet is mixed, and least of all with a vegetable diet. As a rule, men excrete a larger quantity than women, and persons in the middle periods of life a larger quantity than infants or old people (Lecanu). The quantity of urea does not necessarily increase and decrease with that of the urine, though on the whole it would seem that whenever the amount of urine is much augmented, the quantity of urea also is usually in- creased (Becquerel); and it appears from observations of Genth, that the quantity of urea, as of urine, may be especially in- UREA 361 creased by drinking large quantities of water. In various dis- eases, as albuminuria, the quantity is reduced considerably be- low the healthy standard, while in other affections it is above it. The urea appears to be derived from two different sources. That it is derived in part from the unassimilated elements of nitrogenous food, circulating with the blood, is shown in the increase which ensues on substituting an animal or highly nitrogenous for a vegetable diet; in the much larger amount, nearly double, excreted by Carnivora than Herbivora, inde- pendent of exercise; and in its diminution to about one-half during starvation, or during the exclusion of non-nitrogenous principles of food. But that it is in larger part derived from the disintegration of the azotized animal tissues, is shown by the fact that it continues to be excreted, though in smaller quantity than usual, when all nitrogenous substances are strictly excluded from the food, as when the diet consists for several days of sugar, starch, gum, oil, and similar non-azotized vegetable substances (Lehmann). It is excreted, also, even though no food at all be taken for a considerable time; thus it is found in the urine of reptiles which have fasted for months; and in the urine of a madman, who had fasted eigh- teen days, Lassaigne found both urea and all the components of healthy urine. Probably all the nitrogenous tissues furnish a share of urea by their decomposition. It has been commonly taken for granted that the quantity of urea in the urine is greatly increased by active exercise; but numerous observers have failed to detect more than a slight increase under such circumstances; and our notions concern- ing the relation of this product to the destruction of muscular fibre, consequent on the exercise of the latter, have lately undergone considerable modification. There is no doubt, of course, that like all parts of the body, the muscles have but a limited term of existence, and are being constantly renewed, at the same time that a part of the products of their disintegration appears in the urine in the form of urea. But the waste is not so fast as it has been frequently supposed to be ; and the theory that the amount of work done by the muscle is expressed by the quantity of urea excreted in the urine, and that each act of contraction corresponds to an equivalent waste of muscle-structure, is founded on error. (See also chapter on Motion.) Urea exists ready-formed in the blood, and is simply ab- stracted therefrom by the kidneys. It may be detected in small quantity in the blood, and in some other parts of the body, e. g., the humors of the eye (Millon), even while the func- tions of the kidneys are unimpaired : but when from any cause, 362 THE URINE. especially extensive disease or extirpation of the kidneys, the separation of urine is imperfect, the urea is found largely in the blood and in most other fluids of the body. Uric Add.—This, which is another nitrogenous animal sub- stance, with the formula C5N4 H403, and was formerly termed lithic acid, on account of its existence in many forms of urinary calculi, is rarely ab- sent from the urine of man or animals, though in the feline tribe it seems to be sometimes entirely replaced by urea (G. Bird). Its proportionate quan- tity varies considerably in dif- ferent animals. In man, and Mammalia generally, especially the Herbivora, it is compara- tively small. In the whole tribe of birds and of serpents, on the other hand, the quantity is very large, greatly exceed- ing that of the urea. In the urine of granivorous birds, in- deed, urea is rarely if ever found, its place being entirely sup- plied by uric acid. The quantity of uric acid, like that of urea, in human urine, is increased by the use of animal food, and decreased by the use of food free from nitrogeu, or by an exclusively vegetable diet. In most febrile diseases, and in plethora, it is formed in unnaturally large quantities ; and in gout it is deposited in, and in the tissues around, joints, in the form of urate of soda, of which the so-called chalkstones of this disease are principally composed. The condition in which uric acid exists in solution in the urine has formed the subject of some discussion, because of its difficult solubility in water. According to Liebig the uric acid exists as urate of soda, produced, he supposes, by the uric acid, as soon as it is formed, combining with part of the base of the alkaline phosphate of soda of the blood. Hippuric acid, which exists in human urine also, he believes, acts upon the alkaline phosphate in the same way, and increases still more the quantity of acid phosphate, on the presence of which it is probable that a part of the natural acidity of the urine depends. It is scarcely possible to say whether the union of uric acid with the base soda and probably ammonia, takes place in the blood, or in the act of secretion in the kidney ; the latter is the more probable opinion ; but the quantity of either uric acid or Fi<;. 127. Various forms of uric acid crystals. HIPPURIC ACID. 363 urates in the blood is probably too small to allow of this ques- tion being solved. The source of uric acid is probably in the disintegrated ele- ments of albuminous tissues. The relation which uric acid and urea bear to each other is, however, still obscure. The fact that they often exist together in the same urine, makes it seem probable that they have different origins or different offices to perform; but the entire replacement of either by the other, as of urea by uric acid in the urine of birds, serpents, and many insects, and of uric acid by urea, in the urine of the feline tribe of Mammalia, shows that each alone may dis- charge all the important functions of the two. Owing to its existence in combination in healthy urine, uric acid for examination must generally be precipitated from its bases by a stronger acid. Frequently, however, when ex- creted in excess, it is deposited in a crystalline form (Fig. 127), mixed with large quantities of urate of ammonia or soda (Fig. 130). In such cases it may be procured for microscopic exam- ination, by gently warming the portion of urine containing the sediment; this dissolves urate of ammonia and soda, while the comparatively insoluble crystals of uric acid subside to the bottom. The most common form in which uric acid is deposited in urine, is that of a brownish or yellowish powdery substance, consisting of granules of urate of ammonia or soda. When deposited in crystals, it is most frequently in rhombic or dia- mond-shaped laminae, but other forms are not uncommon (Fig. 127). When deposited from urine, the crystals are generally more or less deeply colored, by being combined with the color- ing principles of the urine. Hippuric Acid has long been known to exist in the urine of herbivorous animals in combi- nation with soda. Liebig has shown that it also exists nat- urally in the urine of man, in quantity equal to the uric acid, and Weismann’s obser- vations agree with this. It is a nitrogenous compound with the formula C0HalSrO3. It is closely allied to benzoic acid ; and this substance when in- troduced into the system, is excreted by the kidneys as hippuric acid (Ure). Its source is not satisfactorily de- Fig. 128. Crystals of hippuric acid. 364 THE URINE. terrained: in part it is probably derived from some constitu- ents of vegetable diet, though man has no hippuric acid in his food, nor, commonly, any benzoic acid that might be converted into it; in part from the natural disintegration of tissues, inde- pendent of vegetable food, for Weismann constantly found an appreciable quantity, even when living on an exclusively ani- mal diet. The nature and composition of the coloring matter of urine are involved in some obscurity. It is probably closely related to the coloring matter of the blood. The mucus in the urine consists principally of the epithelial debris of the mucous surface of the urinary passages. Particles of epithelium, in greater or less abundance, may be detected in most samples of urine, especi- ally if it has remained at rest for some time, and the lower strata are then examined (Fig. 129). As urine cools, the mu- cus is sometimes seen suspended in it as a delicate opaque cloud, but generally it falls. In in- flammatory affections of the urinary passages, especially ot the bladder, mucus in large quantities is poured forth, and speedily undergoes decomposi- tion. The presence of the de- composing mucus excites (as already stated) chemical changes in the urea, whereby ammonia, or carbonate of ammonia, is formed, which, combining with the excess of acid in the super- phosphates in the urine, produces insoluble neutral or alkaline phosphates of lime and magnesia, and phosphate of ammonia and magnesia. These, mixing with the mucus, constitute the peculiar white, viscid, mortar-like substance which collects upon the mucous surface of the bladder, and is often passed with the urine, forming a thick, tenacious sediment. Besides mucus and coloring matter, urine contains a consid- erable quantity of animal matter, usually described under the obscure name of animal extractive. The investigations of Lie- big, Heintz, and others, have shown that some of this ill-defined substance consists of Creatin and Oreatinin, two crystallizable substances derived, probably, from the metamorphosis of mus- cular tissue. These substances appear to be intermediate be- tween the proper elements of the muscles, and, perhaps, ot other azotized tissues and urea: the first products of the dis- Fig. 129. Mucus deposited from urine. ALKALINE AND EARTHY PHOSPHATES. 365 integrating tissues probably consisting not of urea, but of cre- atin and creatinin, which subsequently are partly resolved into urea, partly discharged, without change, in the urine. The names of some other substances of which there are commonly traces in the urine, will be found in Table II, p. 358. It has been shown by Scherer that much of the substance classed as extractive matter of the urine, is the peculiar coloring matter, probably derived from the haemoglobin of the blood. Saline Matter.—The sulphuric acid in the urine is combined chiefly or entirely with soda and potash: forming salts which are taken in very small quantity with the food, and are scarcely found in other fluids or tissues of the body; for the sulphates commonly enumerated among the constituents of the ashes of the tissues and fluids are, for the most part or entirely, pro- duced by the changes that take place in the burning. Dr. Parkes, indeed, considers that only about one-third of the sul- phuric acid found in the urine is derived directly from the food. Hence the greater part of the sulphuric acid which the sulphates in the urine contain, must be formed in the blood, or in the act of secretion of urine; the sulphur of which the acid is formed, being probably derived from the decomposing nitro- genous tissues, the other elements of which are resolved into urea and uric acid. It may be in part derived also, as Dr. Parkes observes, from the sulphur-holding taurin and cystin which can be found in the liver, lungs, and other parts of the body, but not generally in the excretions; and which, therefore, must be broken up. The oxygen is supplied through the lungs, and the heat generated during combination with the sulphur, is one of the subordinate means by which the animal tempera- ture is maintained. Besides the sulphur in these salts, some also appears to be in the urine, uncombined with oxygen; for after all the sul- phates have been removed from urine, sulphuric acid may be formed by drying and burning it with nitre. Mr. Ronalds believes that from three to five grains of sulphur are thus daily excreted. The combination in which it exists is certain : possibly it is in some compound analogous to cystin or cystic oxide (p. 367). The phosphoric acid in the urine is combined partly with the alkalies, partly with the alkaline earths—about four or five times as much with the former as with the latter. In blood, saliva, and other alkaline fluids of the body, phosphates exist in the form of alkaline or neutral acid salts. In the urine they are acid salts, viz., the phosphates of sodium, am- monium, calcium, and magnesium, the excess of acid being, according to Liebig, due to the appropriation of the alkali 366 THE URINE. with which the phosphoric acid in the blood is combined, by the several new acids which are formed or discharged at the kidneys, namely, the uric, hippuric, and sulphuric acids, all of which he supposes to be neutralized with soda. The presence of the acid phosphates accounts, in great measure, or, according to Liebig, entirely, for the acidity of the urine. The phosphates are taken largely in both vege- table and animal food ; some thus taken, are excreted at once ; others, after being transformed and incorporated with the tis- sues. Phosphate of calcium forms the principal earthy con- stituent of bone, and from the decomposition of the osseous tissue the urine derives a large quantity of this salt. The de- composition of other tissues also, but especially of the brain and nerve-substance, furnishes large supplies of phosphorus to the urine, which phosphorus is supposed, like the sulphur, to be united with oxygen, and then combined with bases. This quantity is, however, liable to considerable variation. Any undue exercise of the mind, and all circumstances produc- ing nervous exhaustion, in- crease it. The earthy phos- phates are more abundant af- ter meals, whether on animal or vegetable food, and are diminished after long fasting. The alkaline phosphates are increased after animal food, diminished after vegetable food. Exercise increases the alkaline, but not the earthy phosphates (Bence Jones). Phosphorus uncombined with oxygen appears, like sulphur, to be excreted in the urine (Ronalds). When the urine undergoes alkaline fermentation, phosphates are deposited in the form of a urinary sediment consisting chiefly of phosphate of ammonia and magnesia (triple phosphate) (Fig. 130.) This compound does not, as such, exist in healthy urine. The ammonia is chiefly or wholly derived from the decomposition of urea (p. 360). The chlorine of the urine occurs chiefly in combination with sodium, but slightly also with ammonium, and, perhaps, potassium. As the chlorides exist largely in food, and in most of the animal fluids, their occurrence in the urine is easily understood. Fig. 130. Urinary sediment of triple phosphates (large prismatic crystals) and urate of ammonia, from urine which had under- gone alkaline fermentation. THE NERVOUS SYSTEM. 367 Cystin (Fig. 132) is an occasional constituent of urine. It resembles taurin in containing a large quantity of sulphur— more than 25 per cent. It does not exist in healthy urine. Another common morbid constituent of the urine is oxalic acid, which is frequently deposited in combination with lime Fig. 131. Fig. 132. Crystals of oxalate of lime. Crystals of cystin. (Fig. 131) as a urinary sediment. Like cystin, but much more commonly, it is the chief constituent of certain calculi. A small quantity of gas is naturally present in the urine in a state of solution. It consists chiefly of carbonic acid and nitrogen. CHAPTER XVI. THE NERVOUS SYSTEM. The nervous system consists of two portions or systems, the cerebro-spinal and the sympathetic or ganglionic, each of which (though they have many things in common) possesses certain peculiarities in structure, mode of action, and range of influ- ence. The cerebro-spinal system includes the brain and spinal cord, with the nerves proceeding from them, and the several ganglia seated upon these nerves, or forming part of the sub- stance of the brain. It was denominated by Bichat the ner- vous system of animal life; and includes all the nervous organs in and through which are performed the several functions 368 THE NERVOUS SYSTEM. with which the mind is more immediately connected, namely, those relating to sensation and volition, and the mental acts connected with sensible things. The sympathetic or ganglionic portion of the nervous system, which Bichat named the nervous system of organic1 life, con- sists essentially of a chain of ganglia connected by nervous cords, which extend from the cranium to the pelvis, along each side of the vertebral column, and from which, nerves with ganglia proceed to the viscera in the thoracic, abdomi- nal, and pelvic cavities. By its distribution, as well as by its peculiar mode of action, this system is less immediately con- nected with the mind, either as conducting sensations or the impulses of the will; it is more closely connected than the cerebro-spinal system is with the processes of organic life. The differences however, between these two systems, are not essential: their actions differ in degree and object more than in kind or mode. Elementary Structures of the Nervous System. The organs of the nervous system or systems are composed essentially of two kinds of structure, vesicular and fibrous ; both of which appear esssential to the construction of even the simplest nervous system. The vesicular structure is usually collected in masses, and mingled with the fibrous structure, as in the brain, spinal cord, and the several ganglia; and these masses constitute what are termed nerve-centres, being the organs in which it is supposed that nervous force may be gen- erated, and in which are accomplished all the various reflec- tions and other modes of disposing of impressions when they are not simply conducted along nerve-fibres. The fibrous nerve- substance, besides entering into the composition of the nervous centres, forms alone the nerves, or cords of communication, which connect the various nervous centres, and are distributed in the several parts of the body, for the purpose of conveying nervous force to them, or of transmitting to the nervous cen- tres the impressions made by stimuli. 1 The term organic is often used in connection with a function, such as digestion or secretion, which belongs to all organized beings alike; while the term animal function, or animal life, is used in con- nection with such qualities as volition or motion, which seem alto- gether or in great part to belong only to animals. The terms which have been thus used in this general way, are often loosely applied to special tissues. Thus organic nerve-fibres are those which are dis- tributed especially to organs concerned in the discharge of the func- tions of organic, as distinguished from animal life ; and the term is still more commonly applied to one kind of muscular fibre. STRUCTURE OF NERVE-FIBRES. 369 Along the nerve-fibres impressions or conditions of excite- ment are simply conducted: in the nervous centres they may be made to deviate from their direct course, and be variously diffused, reflected, or otherwise disposed of. Nerves are constructed of minute fibres or tubules full of nervous matter, arranged in parallel or interlacing bundles, which bundles are connected by intervening connective tissue, in which their principal bloodvessels ramify. A layer of the areolar, or of strong fibrous tissue, also surrounds the whole nerve, and forms a sheath or neurilemma for it. In most nerves, two kinds of fibres are mingled; those of one kind being most numerous in, and charac- teristic of, nerves of the cerebro- spinal system ; those of the other, most numerous in nerves of the sympathetic system. The fibres of the first kind appear to consist of tubules of a pellucid simple membrane, within which is contained the proper nerve sub- stance, consisting of transparent oil- like, and apparently homogeneous material, which gives to each fibre the appearance of a fine glass tube filled with a clear transparent fluid (Fig. 133, a). This simplicity of composition is, however, only ap- parent in the fibres of a perfectly fresh nerve ; for shortly after death, they undergo changes which make it probable that their contents are composed of two different materials. The internal or central part, occu- pying the axis of the tube, becomes grayish, while the outer, or cortical portion, becomes opaque and dimly granular or grumous, as if from a kind of coagulation. At the same time, the fine outline of the pre- viously transparent cylindrical tube is exchanged for a dark double con- tour (Fig. 133, b), the outer line being formed by the sheath of the fibre, the inner by the margin of curdled or coagulated medullary substance. The gi-anular Fig. 133. Primitive nerve-tubules, a. A perfectly fresh tubule with a single dark outline, b. A tubule or fibre with a double contour from commencing post-mortem change. C. The changes further advanced, producing a varicose or beaded appearance, d. A tubule or fibre, the central part of which, in consequence of still further changes, has accumulated in separate portions within the sheath (after Wagner). 370 THE NERVOUS SYSTEM.* material shortly collects into little masses, which distend por- tions of the tubular membrane, while the intermediate spaces collapse, giving the fibres a varicose, or beaded appearance (Fig. 133 c and d), instead of the previous cylindrical form. The difference produced in the contents of the nerve-fibres when exposed to the same conditions, has, with other facts, led to the opinion now generally adopted, that the central part or axis-cylinder of each nerve-fibre differs from the outer portion. The outer portion is usually called the medullary or white sub- stance of Schwann, being that to which the peculiar white aspect of cerebro-spinal nerves is principally due. The whole contents of the nerve-tubules appear to be extremely soft, for when subjected to pressure they readily pass from one part of the tubular sheath to another, and often cause a bulging at the side of the membrane. They also readily escape, on pressure, from the extremities of the tubule, in the form of a grumous or granular material. That there is an essential difference in chemical composition between the central and circumferential parts of the nerve- fibre, i. e., between the axis-cylinder and the medullary sheath, has of late been clearly shown by Messrs. Lister and Turner. Their observations, founded on Mr. Lockhart Clarke’s method of investigating nervous substance by means of chromic acid and carmine, have shown that the axis-cylinder of the nerve- fibre is unaffected by chromic acid, but imbibes carmine with great facility, while the medullary sheath is rendered opaque and brown and laminated by chromic acid, but is entirely un- tinged by the carmine. From this difference in their chemi- cal behavior, the central and circumferential portions of the nerve-fibres are readily distinguished on microscopic examina- tion, the former being indicated by a bright red carmine- colored point, the latter by a pale ring surrounding it. The laminated character of the medullary sheath after treatment with chromic acid is believed by Mr. Lockhart Clarke to be due to corrugations effected by the acid, and not to its having a fibrous structure, as maintained by Stilling. The size of the nerve-fibres varies, and the same fibres do not preserve the same diameter through their whole length, being largest in their course within the trunks and branches of the nerves, in which the majority measure from 2doo t° gy/ary of an inch in diameter. As they approach the brain or spinal cord, and generally also in the tissues in which they are dis- tributed, they gradually become smaller. In the gray or vesic- ular substance of the brain or spinal cord, they generally do not measure more than from TOJof, to °f an inch. The fibres of the second kind (Fig. 134), which constitute COURSE OF NERVE-FIBRES. 371 the whole of the branches of the olfactory nerves, the principal part of the trunk and branches of the sympathetic nerves, and are mingled in various proportions in the cerebro-spinal nerves, differ from the preceding, chiefly in their fineness, being only about l or as large in their course within the trunks and Fig. 134. Gray, pale, or gelatinous nerve-fibres (from Max Schultze), magnified between 400 and 500 diameters. A. From a branch of the olfactory nerve of the sheep; a, a, two dark-bordered or white fibres from the fifth pair, associated with the pale olfactory fibres. B. From the sympathetic nerve. branches of the nerves; in the absence of the double contour; in their contents being apparently uniform; and in their having, when in bundles, a yellowish-gray hue instead of the whiteness of the cerebro-spinal nerves. These peculiarities make it probable that they differ from the other nerve-fibres in not possessing the outer layer of white or medullary nerve- substance ; and that their contents are composed exclusively of the substance corresponding with the central portion, or axis-cylinder of the larger fibres. Yet since many nerve-fibres may be found which appear intermediate in character between these two kinds, and since the large fibres, as they approach both their central and their peripheral end, gradually diminish in size, and assume many of the other characters of the fine fibres of the sympathetic system, it is not necessary to suppose that there must be a material difference in the office or mode of action of the two kinds of fibres. Every nerve-fibre in its course proceeds uninterruptedly from its origin at a nervous centre to near its destination, whether this be the periphery of the body, another nervous centre, or the same centre whence it issued. Bundles, or fasciculi of fibres, run together in the nerves, 372 THE NERVOUS SYSTEM. but merely lie in apposition with each other; they do not unite; even when the fasciculi anastomose, there is no union of fibres, but only an interchange of fibres between the anastomosing fasciculi. Although each nerve-fibre is thus single and undi- vided through nearly its whole course, yet as it approaches the region in which it terminates, individual fibres break up into several subdivisions (Fig. 135) before their final ending in the Fig. 135. Small branch of a muscular nerve of the frog, near its termination, showing divi- sions of the fibres, o, into two ; b, into three ; magnified 350 diameters (from Kol- liker) different fashions to be immediately described. The white or medullated nerve-fibres (Fig. 133), moreover, lose their medul- lary sheath or white substance of Schwann before their final distribution, and acquire the characters more or less of the pale or gray fibres (Fig. 134). At certain parts of their course, nerves form plexuses, in which they anastomose with each other, and interchange fas- ciculi, as in the case of the brachial and lumbar plexuses. The object of such interchange of fibres is, probably, to give to each nerve passing off from the plexus, a wider connection PACINIAN BODIES. 373 with the spinal cord than it would have if it proceeded to its destination without such communication with other nerves. Thus, each nerve by the wideness of its connections, is less de- pendent on the integrity of any single portion, whether of nerve-centre or of nerve-trunk, from which it may spring. By this means, also, each part supplied from a plexus has wider relations with the nerve-centres, and more extensive sympa- thies; and, by means of the same arrangement, as Dr. Gull suggests, groups of muscles may be associated for combined actions; every member of the group receiving motor filaments from the same parts of the nerve-centre. The terminations of nerve-fibres are their modes of distribu- tion and connection in the nerve-centres, and in the parts which they supply: the former are called their central, the latter their peripheral terminations. The peripheral termination of nerve-fibres has been always the subject of considerable discussion and doubt. The follow- ing appear to be the chief modes of ending of nerve fibres in the parts they supply : 1. In fine networks or plexuses; examples of this are found in the distribution of nerves in muscles, and in mucous and serous membranes. 2. In special terminal organs, called touch-corpuscles (Fig. 113), end-bulbs (Fig. 114), and Pacinian bodies (Figs. 136, 137). 3. In cells; as in the eye and inter- nal ear, and some other parts. 4. In free ends; as from the fine plexuses in muscles, according to Kolliker. 5. In mus- cles, a peculiar termination of nerves in small bodies called motorial end-plates, has been described by Rouget and others. These small bodies, vaiying from 1° wko °f an inch in diameter, and placed by different observers outside and inside the sarcolemma, are fixed to the muscular fibres, one for each, and to them the extremity of a minute branch of nerve-fibre is attached. These little plates appear to be formed of an expansion of the end of a nerve-fibre with a small quantity of connective tissue. The Pacinian bodies or corpuscles (Figs. 136 and 137), to which reference has been just made, are little elongated oval bodies, situated on some of the cerebro-spinal and sympathetic nerves, especially the cutaneous nerves of the hands and feet; and on branches of the large sympathetic plexus about the abdominal aorta (Kolliker). They often occur also on the nerves of the mesentery, and are especially well seen in the mesentery of the cat. They are named Pacinian, after their discoverer Pacini. Each corpuscle is attached by a narrow pedicle to the nerve on which it is situated; it is formed of several concentric layers of fine membrane, with intervening 374 THE NERVOUS SYSTEM. spaces containing fluid ; through its pedicle passes a single nerve-fibre, which, after traversing the several concentric layers and their immediate spaces, enters a central cavity, and, gradually losing its dark border, and becoming smaller, ter- Flo. 137. Flo. 136. Fig. 136.—Extremities of a nerve of the finger with Pacinian corpuscles attached. a. Nerve from the finger, natural size; showing the Pacinian corpuscles, b. Ditto, magnified two diameters, showing their different size and shape. Fig. 137.—Pacinian corpuscles from the mesentery of a cat; intended to show the general construction of these bodies. The stalk and body, the outer and inner system of capsules with the central cavity are seen. a. Arterial twig, ending in capillaries, which form loops in some of the intercapsular spaces, and one penetrates to the central capsule, b. The fibrous tissue of the stalk, prolonged from the neuri- lemma. n. Nerve-tube advancing to the central capsule, there losing its white sub- stance, and stretching along the axis to the opposite end, where it is fixed by a tubercular enlargement. minates at or near tlie distal end of the cavity, in a knob-like enlargement, or in a bifurcation. The enlargement commonly found at the end of the fibre, is said by Pacini to resemble a STRUCTURE OF NERVE-CENTRES. 375 ganglion-corpuscle ; but this observation has not been con- firmed. The physiological import of these bodies seems to be still quite obscure. The central termination of nerve-fibres can be better con- sidered after the account of the vesicular nerve-substance. The vesicular nervous substance contains, as its name implies, vesicles or corpuscles, in addition to fibres; and a structure, thus composed of corpuscles and intercommunicating fibres, usually constitutes a nerve-centre: the chief nerve-centres being the gray matter of the brain and spinal cord, and the various so-called ganglia. In the brain and spinal cord a fine stroma of retiform tissue called the neuroglia extends throughout both the fibrous and vesicular ner- vous substance, and forms a sup- porting and investing frame- work for the whole. The nerve-corpuscles, which give to the ganglia and to cer- tain parts of the brain and spi- nal cord the peculiar grayish or reddish-gray aspect by which these parts are characterized, are large, nucleated cells, filled with a finely granular material, some of which is often dark like pigment: the nucleus, which is vesicular, contains a nucleolus (Fig. 138). Besides varying much in shape, partly in conse- quence of mutual pressure, they present such other varieties as make it probable either that there are two different kinds, or that, in the stages of their development, they pass through very different forms. Some of them are small, generally spherical or ovoid, and have a regular uninterrupted outline (Fig. 138). These simple nerve-corpuscles are most numerous in the sympathetic ganglia. Others, which are called caudate or stellate nerve-corpuscles (Fig. 139), are larger, and have one, two, or more long processes issuing from them, the cells being called respectively unipolar, bipolar, or multipolar; which pro- cesses often divide and subdivide, and appear tubular, and filled with the same kind of granular material that is contained within the corpuscle. Of these processes some appear to taper to a point and terminate at a greater or less distance from the corpuscle; some appear to anastomose with similar offsets from other corpuscles ; while others are believed to become continuous with nerve-fibres, the prolongation from the cell Fig. 138. Nerve-corpuscles from a ganglion (after Valentin). In one a second nu- cleus is visible. In several the nu- cleus contains one or two nucleoli. 376 THE NERVOUS SYSTEM. by degrees assuming the characters of the nerve-fibre with which it is continuous. Fig. 139. Various forms of ganglionic vesicles: A, b, large stellate cells, with their prolon- gations, from the anterior horn of the gray matter of the spinal cord; c, nerve-cell with its connected fibre, from the anastomosis of the facial and auditory nerves in the meatus auditorius internus of the ox ; a, cell-wall; 6, cell-contents; c, pigment; d, nucleus; e, prolongation forming the sheath of the fibre; /, nerve-fibre ; d, nerve- cell from the substantia ferruginea of man; k, smaller cell from the spinal cord, magnified 350 diameters. Functions of Nerve-Fibres. The office of the nerves as simple conveyers or conductors of nervous impressions is of a twofold kind. First, they serve to convey to the nervous centres the impressions made upon their peripheral extremities, or parts of their course. Sec- ondly, they serve to transmit impressions from the brain and other nervous centres to the parts to which the nerves are distributed. For this twofold office of the nerves, two distinct sets of nerve-fibres are provided, in both the cerebro-spinal and sym- pathetic systems. Those which convey impressions from the periphery to the centre are classed together as centripetal or afferent nerves. Those fibres, on the other hand, which are employed to transmit central impulses to the periphery are classed as centrifugal or efferent nerves. Centripetal or afferent nerve-fibres may (a) convey to the nerve-centres with which they are connected impressions which FUNCTIONS OF NERVE-FIBRES. 377 will give rise to sensation (sensitive nerves), or (b) they may convey an impression which travels out again from the nerve- centre by an efferent nerve-fibre, and produces some effect where the latter is distributed (see section on Reflex Action), or (c) they may convey an impression which will produce a restraining or inhibitory action in the nerve-centre (inhibitory nerves, p. 113). Centrifugal or efferent nerves may be (a) for the convey- ance of impulses to the voluntary and involuntary muscles, (motor nerves), or (6) they may influence nutrition (trophic nerves), (p. 310), or (c) they may influence secretion (some- times called secretory nerves) (p. 325). With this difference in the functions of nerves, there is no apparent difference in the structure of the nerve-fibres by which it might be explained. Among the cerebro-spinal nerves, the fibres of the optic and auditory nerves are finer than those of the nerves of common sensation; but, with these exceptions, no centripetal fibres can be distinguished in their microscopic or general characters from those of centrifugal nerves. Nerve-fibres possess no power of generating force in them- selves, or of originating impulses to action : for the manifesta- tion of their peculiar endowments they require to be stimu- lated. They possess a certain property of conducting impres- sions, a property which has been named excitability; but this is never manifested till some stimulus is applied. Thus, under ordinary circumstances, nerves of sensation are stimulated by external objects acting upon their extremities; and nerves of motion by the will, or by some force generated in the nervous centres. But almost all things that can disturb the nerves from their passive state act as stimuli, and agents the most dissimilar produce the same kind, though not the same degree of effect, because that on which they act possesses but one kind of excitable force. Thus all stimuli—chemical, me- chanical, and electric,—when applied to parts endowed with sensation, or to sensitive nerves (the connection of the latter with the brain and spinal cord being uninjured) produce sen- sations ; and when applied to the nerves of muscles excite con- tractions. Muscular contraction is produced by such stimuli as well when the motor nerve is still in connection with the brain, as when its communication with the nervous centres is cut off' by dividing it; nerves, therefore, have, by virtue of their excitability, the property of exciting contractions in muscles to which they are distributed; and the part of the divided motor nerve which is connected with the muscle will still retain this power, however much we may curtail it. Mechanical irritation, when so violent as to injure the tex- 378 THE NERVOUS SYSTEM. ture of the primitive nerve-fibres, deprives the centripetal nerves of their power of producing sensations when irritation is again applied at a point more distant from the brain than the injured spot; and in the same way, no irritation of a motor nerve will excite contraction of the muscle to which it is distributed, if the nerve has been compressed and bruised between the point of irritation and the muscle ; the effect oj' such an injury being the same as that of division. The action of nerves is also excited by temperature. Thus, when heat is applied to the nerve going to a muscle, or to the muscle itself, contractions are produced. These contractions are very violent when the flame of a candle is applied to the nerve, while less elevated degrees of heat,—for example, that of a piece of iron merely warmed,—do not irritate sufficiently to excite action of the muscles. The application of cold has the same effect as that of heat. The effect of the local action of excessive or long-continued cold or heat on the nerves is the same as that of destructive mechanical irritation. The sensitive and motor power in the part is destroyed, but the other parts of the nerve retain their excitability; and, after the extremity of a divided nerve going to a muscle has been burnt, contractions of the muscle may be excited by irritating the nerve below the burnt part. Chemical Stimuli excite the action of both afferent and ef- ferent nerves as mechanical irritants do; provided their effect is not so strong as to destroy the structure of the nerve to which they are applied. A like manifestation of nervous power is produced by electricity and by magnetism. Some of these laws regulating the excitability of nerves, and their power of manifesting their functions, require further notice, with several others which have not yet been alluded to. Certain of the laws and conditions of actions relate to nerves both centrifugal and centripetal, being dependent on properties common to all nerve-fibres; while of others, some are peculiar to nerves of motion, some to nerves of sensation. It is a law of action in all nerve-fibres, and corresponds with the continuity and simplicity of their course, that an im- pression made on any fibre, is simply and uninterruptedly transmitted along it, without being imparted or diffused to any of the fibres lying near it. In other words, all nerve-fibres are mere conductors of impressions. Their adaptation to this pur- pose is, perhaps, due to the contents of each fibre being com- pletely isolated from those of adjacent fibres by the membrane or sheath in which each is inclosed, and which acts, it may be supposed, just as silk, or other non-conductors of electricity do, FUNCTIONS OF NERVE-FIBRES. 379 which, when covering a wire, prevent the electric condition of the wire from being conducted into the surrounding medium. Nervous force travels along nerve-fibres with considerable velocity. Helmholtz and Baxt have estimated the average rate of conduction of electrical impressions in human motor nerves at 111 feet per second: this result agreeing very closely with that previously obtained by Hirsch. Dr. Rutherford’s observations agree with those of Von Wittieh, that the rate of transmission in sensory nerves is about 140 feet per second. Nerve-fibres convey only one kind of impression. Thus, a motor fibre conveys only motor impulses, that is, such as may produce movements in contractile parts: a sensitive fibre trans- mits none but such as may produce sensation, if they are propa- gated to the brain. Moreover, the fibres of a nerve of special sense, as the optic or auditory, convey only such impressions as may produce a peculiar sensation, e. g., that of light or sound. While the rays of light and the sonorous vibrations of the air, are without influence on the nerves of common sen- sation, the other stimuli, which may produce pain when applied to them, produce when applied to these nerves of special sense, only morbid sensations of light, or sound, or taste, according to the nerve impressed. Of the laws of action peculiar to nerves of sensation and of motion respectively, many can be ascertained only by experi- ments on the roots of the nerves. For it is only at their origin that the nerves of sensation and of motion are distinct; their filaments, shortly alter their departure from the nervous cen- tres, are mingled together, so that nearly all nerves, except those of the special senses, consist of both sensitive and motor filaments, and are hence termed mixed nerves. Nerves of sensation appear able to convey impressions only from the parts in which they are distributed, towards the nerve- centre from which they arise, or to which they tend. Thus, when a sensitive nerve is divided, and irritation is applied to the end of the proximal portion, i. e., of the portion still con- nected with the nervous centre, sensation is perceived, or a reflex action ensues; but, when the end of the distal portion of the divided nerve is irritated, no effect appears. When an impression is made upon any part of the course qf a sensitive nerve, the mind may perceive it as if it were made not only upon the point to which the stimulus is applied, but also upon all the points in which the fibres of the irritated nerve are distributed: in other words, the effect is the same as if the irritation were applied to the parts supplied by the branches of the nerve. When the whole trunk of the nerve 380 THE NERVOUS SYSTEM. is irritated, the sensation is felt at all the parts which receive branches from it; but when only individual portions of the trunk are irritated, the sensation is perceived at those parts only which are supplied by the several portions. Thus, if we compress the ulnar nerve where it lies at the inner side of the elbow-joint, behind the internal condyle, we have the sensation of “pins and needles,” or of a shock, in the parts to which its fibres are distributed, namely, in the palm and back of the hand, and in the fifth and ulnar half of the fourth finger. When stronger pressure is made, the sensations are felt in the forearm also; and if the mode and direction of the pressure be varied, the sensation is felt by turns in the fourth finger, in the fifth, and in the palm of the hand, or in the back of the hand, according as different fibres or fasciculi of fibres are more pressed upon than others. It is in accordance with this law, that when parts are de- prived of sensibility by compression or division of the nerve supplying them, irritation of the portion of the nerve connected with the brain still excites sensations which are felt as if de- rived from the parts to which the peripheral extremities of the nerve-fibres are distributed. Thus, there are cases of paralysis in which the limbs are totally insensible to external stimuli, yet are the seat of most violent pain, resulting apparently from irritation of the sound part of the trunk of the nerve still in connection with the brain, or from irritation of those parts of the nervous centre from which the sensitive nerve or nerves which supply the paralyzed limbs originate. An illustration of the same law is also afforded by the cases in which division of a nerve for the cure of neuralgic pain is found useless, and in which the pain continues or returns, though portions of the nerve be removed. In such cases, the disease is probably seated nearer the nervous centre than the part at which the division of the nerve is made, or it may be in the nervous centre itself. When the cause of the neuralgia is seated in the trunk of the nerve—for example, of the facial or infraorbital nerve—division of the branches can be of no service; for the stump remaining in connection with the brain, and containing all the fibres distributed in the branches of the nerve to the skin, continues to give rise, when irritated, to the same sensations as are felt when the peripheral parts themselves are affected. Division of a nerve prevents the possibility of external impressions on the cutaneous extremities of its fibre being felt; for these impressions can no longer be communi- cated to the brain: but the same sensations which were before produced by external impressions may arise from internal causes. In the same way may be explained the fact, that when FUNCTIONS OF NEEVE-FIBRES. 381 part of a limb has been removed by amputation, the remaining portions of the nerves which ramified in it may give rise to sensations which the mind refers to the lost part. When the stump and the divided nerves are inflamed, or pressed, the patient complains of pain felt as if in the part which has been removed. When the stump is healed, the sensations which we are accustomed to have in a sound limb are still felt; and tingling and pains are referred to the parts that are lost, or to particular portions of them, as to single toes, to the sole of the foot, to the dorsum of the foot, &c. But (as Yolkmann shows) it must not be assumed, as it often has been, from these examples, that the mind has no power of discriminating the very point in the length of any nerve-fibre to which an irritation is applied. Even in the in- stances referred to, the mind perceives the pressure of a nerve at the point of pressure, as well as in the seeming sensations derived from the extremities of the fibres; and in stumps, pain is felt in the stump, as well as, seemingly, in the parts removed. It is not quite certain whether those sensations are perceived by the nerve-fibres which are on their way to be distributed elsewhere, or by the sentient extremities of nerves which are themselves distributed to the many trunks of the nerves, the nervi nervorum. The latter is the more probable supposition. The habit of the mind to refer impressions received through the sensitive nerves to the parts from which impressions through those nerves are, or were, commonly received, is fur- ther exemplified when the relative position of the peripheral extremities of sensitive nerves is changed artificially, as in the transposition of portions of skin. When in the restoration of a nose, a flap of skin is turned down from the forehead and made to unite with the stump of the nose, the new nose thus formed has, as long as the isthmus of skin by which it maintains its original connections remains undivided, the same sensations as if it were still on the forehead ; in other words, when the nose is touched, the patient feels the impression as if it were made on the forehead. When the communication of the nervous fibres of the new nose with those of the forehead is cut off by division of the isthmus of skin, the sensations are no longer referred to the forehead; the sensibility of the nose is at first absent, but is gradually developed. When, in a part of the body which receives two sensitive nerves, one is paralyzed, the other may or may not be inadequate to maintain the sensibility of the entire part; the extent to which the sensibility is preserved corresponding probably with the number of the fibres unaffected by the paralysis. Thus 382 THE NERVOUS SYSTEM. when the ulnar nerve, which supplies the fifth and a part of the fourth finger, is divided, the sensibility of those parts is not preserved through the medium of the branches which the ulnar derives from the median nerve; but the fourth aud fifth fingers are permanently deprived of sensibility. On the other hand, there are instances in which the trunk of the chief sen- sitive nerve supplied to a part having been divided, the sensi- bility of the part is still preserved by intercommunicating fibres from a neighboring nerve-trunk. Thus, a case is related by Mr. Savory in which, after excision of a portion of the musculo-spiral nerve, the sensibility of some of the parts sup- plied by it, although impaired, was not altogether lost, prob- ably on account of those fibres from the external cutaneous nerve which are mingled with the radial branch of the mus- culo-spiral. One of the uses of a nervous plexus (p. 372) is here well illustrated. Several of the laws of action in viotor nerves correspond with the foregoing. Thus, the motor influence is propagated only in the direction of the fibres going to the muscles; by irritation of a motor nerve, contractions are excited in all the muscles supplied by the branches given off by the nerve below the point irritated, and in those muscles alone: the muscles supplied by the branches which come off from the nerve at a higher point than that irritated, are never directly excited to contraction. No contraction, for instance, is pro- duced in the frontal muscle by irritating the branches of the facial nerve that ramify upon the face; because that muscle derives its motor nerves from the trunk of the facial previous to these branches. So, again, because the isolation of motor nerve-fibres is as complete as that of sensitive ones, the irrita- tion of a part of the fibres of the motor nerve does not affect the motor power of the whole trunk, but only that of the por- tion to which the stimulus is applied. And it is from the same fact that, when a motor nerve enters a plexus and contributes with other nerves to the formation of a nervous trunk pro- ceeding from the plexus, it does not impart motor power to the whole of that trunk, but only retains it isolated in the fibres which form its continuation in the branches of that trunk. Functions of Nerve-Centres. As already observed (p. 375), the term nerve-centre is applied to all those parts of the nervous system which contain gan- glion corpuscles, or vesicular nerve-substance, i. e., the brain, spinal cord, and the several ganglia which belong to the cere- bro-spinal and the sympathetic systems. Each of these nervous FUNCTIONS OF NERVE-CENTRES. 383 ceutres has a proper range of functions, the extent of which bears a direct proportion to the number of nerve-fibres that connect it with the various organs of the body, and with other nervous centres; but they all have certain general properties and modes of action common to them as nervous centres. It is generally regarded as the property of nervous centres that they originate the impulses by which muscles may be ex- cited to action, and by which the several functions of organic life may be maintained. Hence, they are often called sources or originators of nervous power or force. But the instances in which these expressions can be used are very few, and, strictly speaking, do not exist at all. The brain does not issue any force, except when itself impressed by some force from within, or stimulated by an impression from without; neither without such previous impressions do the other nerve-centres produce or issue motor impulses. The intestinal ganglia, for example, do not give out the nervous force necessary to the contractions of the intestines, except when they receive, through their cen- tripetal nerves, the stimuli of substances in the intestinal canal. So, also, the spinal cord; for a decapitated animal lies motion- less so long as no irritation is applied to its centripetal nerves, though the moment they are touched movements ensue. The more certain and general office of all the nervous centres is that of variously disposing and transferring the impressions that reach them through the several centripetal nerve-fibres. In nerve-fibres, as already said, impressions are only conducted in the simple isolated course of the fibre; in all the nervous centres an impression may be not only conducted, but also communicated ; in the brain alone it may be perceived. Conduction in or through nerve-centres may be thus simply illustrated. The food in a given portion of the intestines, acting as a stimulus, produces a certain impression on the nerves in the mucous membrane, which impression is conveyed through them to the adjacent ganglia of the sympathetic. In ordinary cases, the consequence of such an impression of the ganglia is the movement of the muscular coat of that and the adjacent part of the canal. But if irritant substances be mingled with the food, the sharper stimulus produces a stronger impression, and this is conducted through the nearest ganglia to others more and more distant; and from all these, motor impulses issuing, excite a wide-extended and more forcible action of the intestines. Or even through all the sympathetic ganglia, the impression may be further conducted to the gan- glia of the spinal nerves, and through them to the spinal cord, whence may issue motor impulses to the abdominal and other muscles, producing cramp. And yet further, the same morbid 384 THE NERVOUS SYSTEM. impression may be conducted through the spinal cord to the brain, where the mind may perceive it. In the opposite direc- tion, mental influence may be conducted from the brain through a succession of nervous centres—the spinal cord and ganglia, and one or more ganglia of the sympathetic—to produce the influence of the mind on the digestive and other organic func- tions. In short, in all cases in which the mind either has cognizance of, or exercises influence on, the processes carried on in any part supplied with sympathetic nerves, there must be a conduction of impressions through all the nervous centres between the brain and that part. It is probable that in this conduction through nervous centres the impression is not prop- agated through uninterrupted nerve-fibres, but is conveyed through successive nerve-vesicles and connecting nerve-fila- ments ; and in some instances, and when the stimulus is ex- ceedingly powerful, the conduction may be effected as quickly as through continuous nerve-fibres. But instead of, or as well as, being conducted, impressions made on nervous centres may be communicated from the fibres that brought them, to others; and in this communication may be either transferred, diffused, or reflected. The transference of impressions may be illustrated by the f ain in the knee, which is a common sign of disease of the hip. n this case the impression made by the disease on the nerves of the hip-joint is conveyed to the spinal cord; there it is traiis- ferred to the central ends or connections of the nerve-fibres distributed about the knee. Through these the transferred impression is conducted to the brain, and the mind, referring the sensation to the part from which it usually through these fibres receives impressions, feels as if the disease and the source of pain were in the knee. At the same time that it is trans- ferred, the primary impression may be also conducted; and in this case the pain is felt in both the hip and the knee. So, not unfrequently, if one touches a small pimple, that may be seated in the trunk, a pain will be felt in as small a spot on the arm, or some other part of the trunk. And so, in whatever part of the respiratory organs an irritation may be seated, the impres- sion it produces is transferred to the nerves of the larynx; and then the mind perceives the peculiar sensation of tickling in the glottis, which best, or almost alone, excites the act of cough- ing. Or, again, when the sun’s light falls strongly on the eye, a tickling may be felt in the nose, exciting sneezing. In all these cases, the primary impression may be conducted as well as transferred; and in all it is transferred to a certain set of nerves which generally appear to be in some purposive rela- tion with the nerves first impressed. REFLECTION OF IMPRESSION. 385 The diffusion or radiation of impressions is shown when an impression received at a nervous centre is diffused to many- other fibres in the same centre, and produces sensations extend- ing far beyond or in an indefinite area around the part from which the primary impression was derived. Hence, as in the former cases, result various kinds of what have been denomi- nated sympathetic sensations. Sometimes such sensations are referred to almost every part of the body: as in the shock and tingling of the skin produced by some startling noise. Some- times only the parts immediately surrounding the point first irritated participate in the effects of the irritation; thus, the aching of a tooth may be accompanied by pain in the adjoin- ing teeth, and in all the surrounding parts of the face; the ex- planation of such a case being, that the irritation conveyed to the brain by the nerve-fibres of the diseased tooth is radiated to the central ends of adjoining fibres, and that the mind per- ceives this secondary impression as if it were derived from the peripheral ends of the fibres. Thus, also, the pain of a calculus in the ureter is diffused far and wide. All the preceding examples represent impressions communi- cated from one sensitive fibre to others of the same kind; or from fibres of special sense to those of common sensation. A similar communication of impressions from sensitive to motor fibres, constitutes reflection of impressions, displays the impor- tant functions common to all nervous centres as reflectors, and produces reflex movements. In the extent and direction of such communications, also, phenomena corresponding to those of transference and diffusion to sensitive nerves, are observed in the phenomena of reflection. For, as in transference, the re- flection may take place from a certain limited set of sensitive nerves to a corresponding and related set of motor nerves; as when in consequence of the impression of light on the retina, the iris contracts, but no other muscle moves. Or, as in diffu- sion or radiation, the reflection may bring widely-extended muscles into action: as when an irritation in the larynx brings all the muscles engaged in expiration into coincident move- ment. It will be necessary, hereafter, to consider in detail so many of the instances of the reflecting power of the several nervous centres, that it may be sufficient here to mention only the most general rules of reflex action : 1. For the manifestation of every reflex muscular action, three things are necessary : (1), one or more perfect centrip- etal nerve-fibres, to convey an impression; (2), a nervous centre to which this impression may be conveyed, and by 386 THE NERVOUS SYSTEM. which it may be reflected ; (3), one or more centrifugal nerve- flbres, upon which this impression may be reflected, and by which it may be conducted to the contracting tissue. In the absence of any one of these three conditions, a proper reflex movement could not take place; and whenever impressions made by external stimuli on sensitive nerves give rise to mo- tions, these are never the result of the direct reaction of the sensitive and motor fibres of the nerves on each other; in all such cases the impression is conveyed by the sensitive fibres to a nervous centre, and is therein communicated to the motor fibres. 2. All reflex actions are essentially involuntary, and may be accomplished independently of the will, though most of them admit of being modified, controlled, or prevented by a voluntary effort. 3. Reflex actions performed in health have, for the most part, a distinct purpose, and are adapted to secure some end desirable for the well-being of the body; but, in disease, many of them are irregular and purposeless. As an illlustration of the first point, may be mentioned movements of the diges- tive canal, the respiratory movements, and the contraction of the eyelids and the pupil to exclude many rays of light, when the retina is exposed to a bright glare. These and all other normal reflex acts afford also examples of the mode in which the nervous centres combine and arrange co-ordinately the actions of the nerve-fibres, so that many muscles may act together for the common end. Another instance of the same kind is furnished by the spasmodic contractions of the glottis on the contact of carbonic acid, or any foreign substance, with the internal substance of the epiglottis or larynx. Examples of the purposeless irregular nature of morbid reflex action are seen in the convulsive movements of epilepsy, and in the spasms of tetanus and hydrophobia. 4. Reflex muscular acts are often more sustained than those produced by the direct stimulus of muscular nerves. As Yolkmann relates, the irritation of a muscular organ, or its motor nerve, produces contraction lasting only so long as the irritation continues ; but irritation applied to a nervous centre through one of its centripetal nerves, may excite reflex and harmonious contractions, which last some time after the with- drawal of the stimulus. The physiology of the cerebro-spinal nervous system in- cludes that of the spinal cord, medulla oblongata, and brain, of the several nerves given off from each, and of the ganglia CEREBRO-SPINAL NERVOUS SYSTEM. THE CEREBRO-SPINAL AXIS. 387 Fig. 140. View of the eerebro-spinal axis of the nervous system (after Bourgery). The right half of the cranium and trunk of the body has been removed by a vertical section; the membranes of the brain and spinal marrow have also been removed, and tlio roots and first part of the fifth and ninth cranial, and of all the spinal nerves of the 388 THE NERVOUS SYSTEM. on those nerves. It will be convenient to speak first of the spinal cord and its nerves. Spinal Cord and its Nerves. The spinal cord is a cylindriform column of nerve-substance, connected above with the brain through the medium of the medulla oblongata, terminating below, about the lower border of the first lumbar vertebra, in a slender filament of gray or vesicular substance, the filum terminate, which lies in the midst of the roots of many nerves forming the cauda equina. The cord is composed of fibrous and vesicular nervous substance, of which the former is situated externally, and constitutes its chief portion, while the latter occupies its central or axial por- tion, and is so arranged, that on the surface of a transverse section of the cord it appears like two somewhat crescentic masses connected together by a narrower portion or isthmus (Fig. 141). Passing through the centre of this isthmus in a longitudinal direction is a minute canal, which is continued through the whole length of the cord, and opens above into the space at the back of the medulla oblongata and pons Varolii, called the fourth ventricle. It is lined by a layer of cylindrical ciliated epithelium. The spinal cord consists of twro exactly symmetrical halves united in the middle line by a commissure, but separated an- teriorly and posteriorly by a vertical fissure; the posterior fis- sure being deeper, but less wide and distinct than the anterior. Each half of the spinal cord is marked on the sides (obscurely at the lower part, but distinctly above) by two longitudinal furrows, which divide it into three portions, columns, or tracts, an anterior, middle or lateral, and posterior. From the groove between the anterior and lateral columns spring the anterior roots of the spinal nerves ; and just in front of the groove be- tween the lateral and posterior column arise the posterior roots of the same; a pair of roots on each side corresponding to each vertebra (Fig. 141). The fibrous part of the cord contains continuations of the innumerable fibres of the spinal nerves issuing from it, or en- tering it; but it is, probably, not formed of them exclusively; nor is it a mere trunk, like a great nerve, through which they may pass to the brain. It is, indeed, among the most difficult right side, have been dissected out and laid separately on the wall of the skull and on the several vertebrae opposite to the place of their natural exit from the cranio- spinal cavity. STRUCTURE OF THE SPINAL CORD. 389 things in structural anatomy to determine the course of indi- vidual nerve-fibres, or even of fasciculi of fibres, through even a short distance of the spinal cord ; and it is only by the exam- Fig. 141. Different views of a portion of the spinal cord from the cervical region, with the roots of the nerves slightly enlarged (from Quain). In a, the anterior surface of the specimen is shown, the anterior nerve-root of its right side being divided; in b, a view of the right side is given ; in c the upper surface is shown ; in d, the nerve-roots and ganglion are shown from below. 1, the anterior median fissure; 2, posterior median fissure; 3, anterior lateral depression, over which the anterior nerve-roots are seen to spread; 4, posterior lateral groove, into which the posterior roots are seen to sink; 5, anterior roots passing the ganglion : 5', in a, the anterior root divided ; 6, the posterior roots, the fibres of which pass into the ganglion 6'; 7, the united or compound nerve; 7', the posterior primary branch, seen in a and d to he derived in part from the anterior and in part from the posterior root. ination of transverse and longitudinal sections through the substance of the cord, such as those so successfully made by Mr. Lockhart Clarke, that we can obtain anything like a cor- rect idea of the direction taken by the fibres of the roots of the spinal nerves within the cord. From the information afforded by such sections it would appear, that of the root-fibres of the nerve which enter the cord, some assume a transverse, others a longitudinal direction : the fibres of the former pass hori- zontally or obliquely into the substance of the cord, in which 390 THE NERVOUS SYSTEM. many of them appear to become continuous with fibres enter- ing the cord from other roots; others pass into the columns of the cord, while some perhaps terminate at or near the part which they enter: of the fibres of the second set, which usually first tra verse a portion of the gray substance, some pass up- wards, and others, at least of the posterior roots, turn down- wards, but how far they proceed in either direction, or in what manner they terminate, are questions still undetermined. It is probable that of these latter, many constitute longitudinal commissures, connecting different segments of the cord with each other; while others, probably, pass directly to the brain. The general rule respecting the size of different parts of the cord appears to be, that the size of each part bears a direct proportion to the size and number of nerve-roots given off from itself, and has but little relation to the size or number of those given off below it. Thus the cord is very large in the middle and lower part of its cervical portion, whence arise the large nerve-roots for the formation of the brachial plexuses and the supply of the upper extremities, and again enlarges at the lowest part of its dorsal portion and the upper part of its lum- bar, at the origins of the large nerves, which, after forming the lumbar and sacral plexuses, are distributed to the lower ex- tremities. The chief cause of the greater size at these parts of the spinal cord is increase in the quantity of gray matter; for there seems reason to believe that the white or fibrous part of the cord becomes gradually and progressively larger from be- low upwards, doubtless from the addition of a certain number of upward-passing fibres from each pair of nerves. It may be added, however, that there is no sufficient evi- dence for the supposition that an uninterrupted continuity of nerve-fibres is essential to the conduction of impressions on the spinal nerves to and from the brain: such impressions may be as well transmitted through the nerve-vesicles of the cord as by the nerve-fibres; and the experiments of again to be alluded to, make it probable that the gray sub- stance of the cord is the only channel through which sensitive impressions are conveyed to the brain. The Nerves of the Spinal Cord consist of thirty-one pairs, issuing from the sides of the whole length of the cord, their num- ber corresponding with the intervertebral foramina through which they pass. Each nerve arises by two roots, an anterior and posterior, the latter being the larger. The roots emerge through separate apertures of the sheath of dura mater sur- rounding the cord ; and directly after their emergence, where the roots lie in the intervertebral foramen, a ganglion is found ORIGIN OF THE SPINAL NERVES. 391 on the posterior root. The anterior root lies in contact with the anterior surface of the ganglion, but none of its fibres in- termingle with those in the ganglion. But immediately be- yond the ganglion the two roots coalesce, and by the mingling of their fibres form a compound or mixed spinal nerve, which, after issuing from the intervertebral canal, divides into an an- terior and posterior branch, each containing fibres from both the roots (Fig. 141). According to Kolliker the posterior root-fibres of the cord enter into no connection with the nerve-corpuscles in the gan- glion, but pass directly through, in one or more bundles, which are collected into a trunk beyond the ganglion, and then join the motor root. From most, if not all, of the ganglionic cor- puscles, one or two, rarely more, nerve-fibres arise and pass out of the ganglion, in a peripheral direction, in company with the posterior root-fibres of the cord. Each spinal ganglion, there- fore, is to be regarded as a source of new nerve-fibres, which Kolliker names ganglionic fibres. The destination of these fibres is not yet determined: probably they pass especially into the vascular branches of the nerves which they accompany. The anterior root of each spinal nerve arises by numerous separate and converging fasciculi from the anterior column of the cord ; the posterior root by more numerous parallel fasciculi, from the posterior column, or, rather, from the posterior part of the lateral column ; for if a fissure be directed inwards from the groove between the middle and posterior columns, the pos- terior roots will remain attached to the former. The anterior roots of each spinal nerve consist exclusively of motor fibres; the posterior as exclusively of sensitive fibres. For the knowl- edge of this important fact, and much of the consequent prog- ress of the physiology of the nervous system, science is in- debted to Sir Charles Bell. The fact is proved in various ways. Division of the anterior roots of one or more nerves is followed by complete loss of motion in the parts supplied by the fibres of such roots; but the sensation of the same parts remains perfect. Division of the posterior roots destroys the sensibility of the parts supplied by their fibres, while the power of motion continues unimpaired. Moreover, irritation of the ends of the distal portions of the divided anterior roots of a nerve excites muscular movements; irritation of the ends of the proximal portions, which are still in connection with the cord, is followed by no effect. Irritation of the distal portions of the divided posterior roots, on the other hand, produces no muscular movements and no manifestation of pain; for, as al- ready stated, sensitive nerves convey impressions only towards the nervous centres: but irritation of the proximal portions of 392 THE NERVOUS SYSTEM. these roots elicit signs of intense suffering. Occasionally, under this last irritation, muscular movements also ensue; but these are either voluntary, or the result of the irritation being re- flected from the sensitive to the motor fibres. Occasionally, too, irritation of the distal ends of divided anterior roots elicits signs of pain, as well as producing muscular movements: the pain thus excited is probably the result of cramp (Brown-Se- quard). As an example of the experiments of which the preceding paragraph gives a summary account, this may be mentioned: If in a frog the three posterior roots of the nerves going to the hinder extremity be divided on the left side, and the three an- terior roots of the corresponding nerves on the right side, the left extremity will be deprived of sensation, the right of motion. If the foot of the right leg, which is still endowed with sensa- tion but not with the power of motion, be cut off, the frog will give evidence of feeling pain by movements of all parts of the body except the right leg itself, in which he feels the pain. If, on the contrary, the foot of the left leg, which has the power of motion, but is deprived of sensation, is cut off, the frog does not feel it, and no movement follows, except the twitching of the muscles irritated by cutting them or their tendons. Functions of the Spinal Cord. The spinal cord manifests all the properties already assigned to nerve centres (see p. 382). 1. It is capable of conducting impressions, or states of ner- vous excitement. Through it, the impressions made upon the peripheral extremities or other parts of the spinal sensitive nerves are conducted to the brain, where alone they can be perceived by the mind. Through it, also, the stimulus of the will, applied to the brain, is capable of exciting the action of the muscles supplied from it with motor nerves. And for all these conductions of impressions to and fro between the brain and the spinal nerves, the perfect state of the cord is necessary; for when any part of it is destroyed, and its communication with the brain is interrupted, impressions on the sensitive nerves given off from it below the seat of injury, cease to be propagated to the brain, and the mind loses the power of vol- untarily exciting the motor nerves proceeding from the por- tion of cord isolated from the brain. Illustrations of this are furnished by various examples of paralysis, but by none better than by the common paraplegia, or loss of sensation and voluntary motion in the lower part of the body, in consequence of destructive disease or injury of a FUNCTIONS OF THE SPINAL CORD, 393 portion, including the whole thickness, of the spinal cord. Such lesions destroy the communication between the brain and all parts of the spinal cord below the seat of injury, and con- sequently cut off from their connection with the mind the various organs supplied with nerves issuing from those parts of the cord. But if this lower portion of the cord preserves its integrity, the various parts of the body supplied with nerves from it, though cut off from the brain, will nevertheless be sub- ject to the influence of the cord, and, as presently to be shown, will indicate its other powers as a nervous centre. From what has been already said, it will appear probable that the conduction of impressions along the cord is effected (at least, for the most part) through the gray substance, i. e., through the nerve-corpuscles and filaments connecting them. But there is reason to believe that all parts of the cord are not alike able to conduct all impressions; and that, rather, as there are separate nerve-fibres for motor and for sensitive im- pressions, so in the cord, separate and determinate parts serve to conduct always the same kind of impression. The importantand philosophical laborsof Dr. Brown-S6quard have cast much new light on all relating to the functions of the spinal cord. It is not possible to do justice to these inves- tigations in any summary, however lengthy and complete: the whole series (delivered in lectures at the College of Surgeons) must be read and studied. An attempt will be made here to point out only the principal conclusions deducible from them. a. Sensitive impressions, conveyed to the spinal cord by root- fibres of the posterior nerves are not conducted to the brain by the posterior columns of the cord, as hitherto has been gener- ally supposed, but pass through them into the central gray substance, by which thev are transmitted to the brain (Fig. 142). b. The impressions thus conveyed to the gray substance do not pass up to the brain along that half of the cord correspond- ing to the side from which they have been received, but, almost immediately after entering the cord, cross over to the other side, and along it are transmitted to the brain. There is thus, in the cord itself, a complete decussation of sensitive impressions brought to it; so that division or disease of one posterior half of the cord is followed by lost sensation, not in parts on the corresponding, but in those of the opposite side of the body. c. The various sensations of touch, pain, temperature, and muscular contraction, are probably conducted along separate and distinct sets of fibres. All, however, with the exception of the last named, undergo decussation in the spinal cord, and along it are transmitted to the brain by the gray matter. 394 THE NERVOUS SYSTEM. d. The posterior columns of the cord appear to have a great share in reflex movements, and this is the principal cause of Fig. 142. The above diagram (after Brown-S6quard) represents the decussation of the con- ductors for voluntary movements, and those for sensation: a, r, anterior roots and their continuations in the spinal cord, and decussation at the lower part of the medulla oblongata, m, o; p, r, the posterior roots and their continuation and decus- sation in the spinal cord ; g g, the ganglions of the roots. The arrows indicate the direction of the nervous action; r, the right side; l, the left side. 1, 2, 3, indicate places of alteration in a lateral half of the spino-cerebral axis, to show the influence on the two kinds of conductors, resulting from section of the cord at any one of these three places. the peculiar kind of paralysis so often observed in disease of these columns. e. Impulses of the will, leading to voluntary contractions of muscles, appear to be transmitted principally along the anterior columns, and the contiguous gray matter of the cord. /. Decussation of motor impulses occurs, not in the spinal cord, as is the case with sensitive impressions, but, as hitherto admitted, at the anterior part of the medulla oblongata. This decussation, however, does not take place, as generally sup- CONDITION OF THE SPINAL CORD. 395 posed, all along the median line, at the base of the enceph- alon, but only at that portion of the anterior pyramids, which is continuous with the lateral columns of the cord. Hence, the mandates of the will, having made their decussation, first enter the cord by the lateral tracts and adjoining gray matter, and then pass to the anterior columns and to the gray matter associated with them. Accordingly, division of the anterior pyramids, at the point of decussation, is followed by paralysis of motion in all parts below; while division of the olivary bodies, which constitute the true continuations of the anterior columns of the cord, appears to produce very little paralysis. Disease or division of any part of the cerebro-spinal axis above the seat of decussation is followed, as well known, by impaired or lost power of motion on the opposite side of the body ; while a like injury inflicted below this part, induces similar paralysis on the corresponding side. 2. In the second place, the spinal cord as a nerve-centre, or rather as an aggregate of many nervous centres, has the power of communicating impressions in the several ways already men- tioned (p. 384). Examples of the transference and radiation of impressions in the cord have been given; and that the transference at least takes place in the cord, and not in the brain, is nearly proved by the case of pain felt in the knee and not in the hip, in diseases of the hip; of pain felt in the urethra or glans penis, and not in the bladder, in calculus; for, if both the primary and the secondary or transferred impressions were in the brain, both should be always felt. Of radiations of im- pressions, there are, perhaps, no means of deciding whether they take place in the spinal cord or in the brain; but the analogy of the cases of transference makes it probable that the communication is, in this also, effected in the cord. The power, as a nerve-centre, of comnmicating impressions from sensitive to motor, or, more strictly, from centripetal to centrifugal nerve-fibres, is what is usually discussed as the reflex function of the spinal cord. Its general mode of action, its general though incomplete independence of consciousness and of the will, and the conditions necessary for its perfection, have been already stated. These points, and the extent to which the power operates in the production of the natural reflex movements of the body, have now to be further illustra- ted. They will be described in terms adapted to the general rules of reflection of impressions in nervous centres, avoiding all such terms as might seem to imply that the power of the spinal cord in reflecting, is different in kind from that of all other nervous centres. 396 THE NERVOUS SYSTEM. The occurrence of movements under the influence of the spinal cord, and independent of the will, is -well exemplified in the acts of swallowing, in which a portion of food carried by voluntary efforts into the fauces, is conveyed by successive involuntary contractions of the constrictors of the pharynx and muscular walls of the oesophagus into the stomach. These contractions are excited by the stimulus of the food on the centripetal nerves of the pharynx and oesophagus being first conducted to the spinal cord and medulla oblongata, and thence reflected through the motor nerves of these parts. All these movements of the pharynx and oesophagus are involun- tary ; the will cannot arrest them or modify them ; and though the mind has a certain consciousness of the food passing, which becomes less as the food passes further, yet that this is not necessary to the act of deglutition, is shown by its occurring when the influence of the mind is completely removed; as when food is introduced into the fauces or pharynx during a state of complete coma, or in a brainless animal. So also, for example, under the influence of the spinal cord, the involuntary and unfelt muscular contraction of the sphinc- ter ani is maintained when the mind is completely inactive, as in deep sleep, but ceases when the lower part of the cord is destroyed, and cannot be maintained by the will. The independence of the mind manifested by the reflecting power of the cord, is further shown in the perfect occurrence of the reflex movements when the spinal cord and the brain are disconnected, as in decapitated animals, and in cases of injuries or diseases so affecting the spinal cord as to divide or disorganize its whole thickness at any part whose perfection is not essential to life. Thus, when the head of a lizard is cut off, the trunk remains standing on the feet, and the body writhes when the skin is irritated. If the animal be cut in two, the lower portion can be excited to motion as well as the upper portion ; the tail may be divided into several segments, and each segment, in which any portion of spinal cord is con- tained, contracts on the slightest touch; even the extremity of the tail moves as before, as soon as it is touched. All the portions of the animal in which these movements can be ex- cited, contain some part of the spinal cord ; and it is evidently the cause of the motions excited by touching the surface ; for they cannot be excited in parts of the animal, however large, if no part of the cord is contained in them. Mechanical irri- tation of the skin excites not the slightest motion in the leg when it is separated from the body; yet the extremity of the tail moves as soon as it is touched. The same power of the spinal cord in reflecting impressions will cause an eel, or a REFLEX FUNCTION OF THE SPINAL CORD. 397 frog, or any other cold-blooded animal, to move along after it is deprived of its head, and when, however much the move- ments may indicate purpose, it is not probable that conscious- ness or will has any share in them. And so, in the human subject, or any warm-blooded animal, when the cord is com- pletely divided across, or so diseased at some part that the in- fluence of the mind cannot be conveyed to the parts below it, the irritation of any part of the surface supplied by nerves given off from the cord below the seat of injury, is commonly followed by spasmodic and irregular reflex movements, even though in the healthy state of the cord, such involuntary movements could not be excited when the attention of the mind was directed to the irritating cause. In the fact last mentioned, is an illustration of an impor- tant difference between the warm-blooded and the lower ani- mals, in regard to the reflecting power of the spinal cord (or its homologue in the Invertebrata), and the share which it and the brain have, respectively, in determining the several natural movements of the body. When, for example, a frog’s head is cut off*, the limbs remain in, or assume, a natural posi- tion ; resume it when disturbed ; and when the abdomen or back is irritated, the feet are moved with the manifest purpose of pushing away the irritation. It is as if the mind of the animal were still engaged in the acts.1 But, in division of the human spinal cord, the lower extremities fall into any position that their weight and the resistance of surrounding objects combine to give them ; if the body is irritated, they do not move towards the irritation ; and if themselves are touched, the consequent movements are disorderly and purposeless. Now, if we are justified by analogy in assuming that the will of the frog cannot act more than the will of man, through the spinal cord separated from the brain, then it must be admitted that many more of the natural and purposive movements of the body can be performed under the sole influence of the cord in the frog than in man; and what is true in the instance of these two species, is generally true also of the whole class of cold-blooded, as distinguished from warm-blooded, animals. It may not, indeed, be assumed that the acts of standing, leap- ing, and other movements, which decapitated cold-blooded 1 The evident adaptation and purpose in the movements of the cold- blooded animals, have led some to think that they must be conscious and capable of will without their brains. But purposive movements are no proof of consciousness or will in the creature manifesting them. The movements of the limbs of headless frogs are not more purposive than the movements of our own respiratory muscles are ; in which wo know that neither will nor consciousness is at all times concerned. 398 THE NERVOUS SYSTEM. animals can perform, are also always, in the entire and healthy state, performed involuntarily, and under the sole influence of the cord; but it is probable that such acts may be, and com- monly are, so performed, the higher nerve-centres of the ani- mal having only the same kind of influence in modifying and directing them, that those of man have in modifying and di- recting the movements of the respiratory muscles. The fact that such movements as are produced by irritating the skin of the lower extremities in the human subject, after division or disorganization of a part of the spinal cord, do not follow the same irritation when the mind is active and con- nected with the cord through the brain, is, probably, due to the mind ordinarily perceiving the irritation and instantly controlling the muscles of the irritated and other parts; for, even when the cord is perfect, such involuntary movements will often follow irritation, if it be applied when the mind is wholly occupied. When, for example, one is anxiously think- ing, even slight stimuli will produce involuntary and reflex movements. So, also, during sleep, such reflex movements may be observed when the skin is touched or tickled; for ex- ample, when one touches with the finger the palm of the hand of a sleeping child, the finger is grasped—the impression on the skin of the palm producing a reflex movement of the muscles which close the hand. But when the child is awake, no such effect is produced by a similar touch. On the whole, it may, from these and like facts, be concluded that the proper reflex acts, performed under the influence of the reflecting power of the spinal cord, are essentially inde- pendent of the brain, and may be performed perfectly when the brain is separated from the cord d that these include a much larger number of the natural and purposive movements of the lower animals than of the warm-blooded animals and man: and that over nearly all of them the mind may exer- cise, through the brain, some control; determining, directing, hindering, or modifying them, either by direct action or by its power over associated muscles. In this fact, that the reflex movements from the cord may be perfectly performed without the intervention of conscious- ness or will, yet are amenable to the control of the will, we may see their admirable adaptation to the well-being of the body. Thus, for example, the respiratory movements may be performed while the mind is, in other things, fully occupied, 1 Keflex movements, occurring quite independently of sensation, are generally called excito-motor; those which are guided or accom- panied by sensation, but not to the extent of a distinct perception or intellectual process, are termed sensori-mutor. REFLEX FUNCTION OF THE SPINAL CORD. 399 or in sleep powerless; yet in an emergency, the mind can direct and strengthen them : and it can adapt them to the several acts of speech, effort, &c. Being, for ordinary pur- poses, independent of the will and consciousness, they are per- formed perfectly, without experience or education of the mind; yet they may be employed for other and extraordinary uses when the mind wills, and so far as it acquires power over them. Being commonly independent of the brain, their constant con- tinuance does not produce weariness ; for it is only in the brain that it or any other sensation can be perceived. The subjection of the muscles to both the spinal cord and the brain, makes it difficult to determine in man what move- ments or what share in any of them can be assigned to the re- flecting power of the cord. The fact that after division or disorganization of a part of the cord, movements, and even forcible though purposeless ones, are produced in the lower limbs when the skin is irritated, proves that the spinal cord can reflect a stimulus to the action of the muscles that are, naturally, most under the control of the will; and it is, therefore, not improbable that, for even the involuntary action of those muscles, when the cord is perfect, it may supply the nervous stimulus, and the will the direction. As instances in Avhich it supplies both stimulus and direction, that is, both ex- cites and determines the combination of muscles, may be men- tioned the acts of the abdominal muscles in vomiting and voiding the contents of the bladder and rectum : in both of which, though, after the period of infancy, the mind may have the power of postponing or modifying the act, there are all the evidences of. reflex action; namely, the necessary prece- dence of a stimulus, the independence of the will, and, some- times, of consciousness, the combination of many muscles, the perfection of the act without the help of education or experi- ence, and its failure or imperfection in disease of the lower part of the cord. The emission of semen is equally a reflex act governed by the spinal cord; the irritation of the glans penis conducted to the spinal cord, and thence reflected, ex- cites the successive and co-ordinate contractions of the muscu- lar fibres of the vasa deferentia and vesiculse seminales, and of the accelerator urinse and other muscles of the urethra; and a forcible expulsion of semen takes place, over which the mind has little or no control, and which, in cases of paraplegia, may be unfelt. The erection of the penis, also, as already ex- plained (p. 153), appears to be in part the result of a reflex contraction of the muscles by which the veins returning the blood from the penis are compressed. Irritation of the vagina in sexual intercourse appears also to be propagated to the 400 THE NERVOUS SYSTEM. spinal cord, and thence reflected to the motor nerves supplying the Fallopian tubes. The involuntary action of the uterus in expelling its contents during parturition, is also of a purely reflex kind, dependent in part upon the spinal cord, though in part also upon the sympathetic system : its independence of the brain being proved by cases of delivery in paraplegic women, and now more abundantly shown in the use of chloro- form. Besides these acts regularly performed under the influence of the reflecting power of the spinal cord, others are manifested in accidents, such as the movement of the limbs and other parts to guard the body against the effects of sudden danger. When, for example, a limb is pricked or struck, it is instantly and involuntarily withdrawn from the instrument of injury; and the same preservative tendency of the reflex power of the cord is shown in the outstretched arms when falling forwards, and their reversed position when falling backwards; the action, although apparently voluntary, being really, in most cases, only an instance of reflex action. To these instances of spinal reflex action, some add yet many more, including nearly all the acts which seem to be performed unconsciously, such as those of walking, running, writing, and the like : for these are really involuntary acts. It is true that at their first performances they are voluntary, that they require education for their perfection, and are at all times so constantly performed in obedience to a mandate of the will, that it is difficult to believe in their essentially in- voluntary nature. But the will really has only a controlling power over their performance ; it can hasten .or stay them, but it has little or nothing to do with the actual carrying out of the effect. And this is proved by the circumstance that these acts can be performed with complete mental abstraction : and, more than this, that the endeavor to carry them out entirely by the exercise of the will is not only not beneficial, but posi- tively interferes with their harmonious aud perfect perform- ance. Any one may convince himself of this fact by trying to take each step as a voluntary act in walking down stairs, or to form each letter or word in writing by a distinct exercise of the will. These actions, however, will be again referred to, when treating of their possible connection with the functions of the so-called sensory ganglia (p. 413). The phenomena of spinal reflex actions in man are much more striking and unmixed in cases of disease. In some of these, the effect of a morbid irritation, or a morbid irritability of the cord, is very simple; as when the local irritation of REFLEX FUNCTION OF THE SPINAL CORD. 401 sensitive fibres, being propagated to the spinal cord, excites merely local spasms,—spasms, namely, of those muscles, the motor fibres of which arise from the same part of the spinal cord as the sensitive fibres that are irritated. Of such a case we have instances in the involuntary spasmodic contraction of muscles in the immediate neighborhood of inflamed joints; and numerous other examples of a like kind might be quoted. In other instances, in which we must assume that the cord is morbidly more irritable, i. e., apt to issue more nervous force than is proportionate to the stimulus applied to it, a slight impression on a sensitive nerve produces extensive reflex movements. This appears to be the condition in teta- nus, in which a slight touch on the skin may throw the whole body into convulsion. A similar state is induced by the in- troduction of strychnia, and, in frogs, of opium, into the blood ; and numerous experiments on frogs thus made tetanic, have shown that the tetanus is wholly unconnected with the brain, and depends on the state induced in the spinal cord. It may seem to have been implied that the spinal cord, as a single nervous centre, reflects alike from all parts all the im- pressions conducted to it. But it is more probable that it should be regarded as a collection of nervous centres united in a continuous column. This is made probable by the fact that segments of the cord may act as distinct nervous centres, and excite motions in the parts supplied with nerves given off from them; as well as by the analogy of certain cases in which the muscular movements of single organs are under the control of certain circumscribed portions of the cord. Thus Volkmann has shown that the rhythmical movements of the anterior pair of lymphatic hearts in the frog depend upon nervous influ- ence derived from the portion of spinal cord corresponding to the third vertebra, and those of the posterior pair on influence supplied by the portion of cord opposite the eighth vertebra. The movements of the heart continue, though the whole of the cord, except the above portions, be destroyed; but on the in- stant of destroying either of these portions, though all the rest of the cord be untouched, the movements of the correspond- ing hearts cease. What appears to be thus proved in regard to two portions of the cord, may be inferred to prevail in other portions also; and the inference is reconcilable with most of the facts known concerning the physiology and comparative anatomy of the cord. The influence of the spinal cord on the sphincter ani has been already mentioned (p. 396). It maintains this muscle 402 THE NERVOUS SYSTEM. in permanent contraction, so that, except in the act of defeca- tion, the orifice of the anus is always closed. This influence of the cord resembles its common reflex action in being invol- untary, although the will can act on the muscle to make it contract more or to permit its dilatation, and in that the con- stant action of the muscle is not felt, nor diminished in sleep, nor productive of fatigue. But the act is different from ordi- nary reflex acts in being nearly constant. In this respect it resembles that condition of muscles which has been called tone,* or passive contraction; in a state in which they always appear to be when not active in health, and in which, though called inactive, they appear to be in slight contraction, and certainly are not relaxed, as they are long after death, or when the spinal cord is destroyed. This tone of all the mus- cles of the trunk and limbs seems to depend on the spinal cord, as the contraction of the sphincter ani does. If an ani- mal be killed by injury or removal of the brain, the tone of the muscles may be felt, and the limbs feel firm as during sleep; but if the spinal cord be destroyed, the sphincter ani relaxes, and all the muscles feel loose, and flabby, and atonic, and remain so till the rigor mortis commences. THE MEDULLA OBLONGATA. Its Structure. The medulla oblongata is a mass of gray and white nervous substance partly contained within the cavity of the cranium, forming a portion of the cephalic prolongation of the spinal cord and connecting it with the brain. The gray substance which it contains is situated in the interior, and variously di- vided into masses and laminae by the white or fibrous substance which is arranged partly in external columns, and partly in fasciculi traversing the central gray matter. The medulla oblongata is larger than any part of the spinal cord. Its columns are pyriform, enlarging as they proceed towards the brain, and are continuous with those of the spinal cord. Each half of the medulla, therefore, may be divided into three columns or tracts of fibres, continuous with the three 1 This kind of tone must be distinguished from that mere firmness and tension which it is customary to ascribe, under the name of tone, to all tissues that feel robust and not flabby, as well as to muscles. The tone peculiar to muscles has in it a degree of vital contraction : that of other tissues is only due to their being well nourished, and therefore compact and tense. STRUCTURE OF THE MEDULLA OBLONGATA. 403 tracts of which each half of the spinal cord is made up. The columns are more prominent than those of the spinal cord, and separated from each other by deeper grooves. The anterior, continuous with the anterior columns of the cord, are called the anterior pyramids; the posterior, continuous with the pos- terior columns of the cord, are called the restiform bodies; and Fig. 143. Fig. 144. Fig. 143.—View of the anterior surface of the pons varolii, and medulla oblongata, a, a, anterior pyramids, b, their decussation; c, c, olivary bodies; d, d, restiform bodies ; e, arciform fibres ; /, fibres described by Solly as passing from the anterior column of the cord to the cerebellum; g, anterior column of the spinal cord; h, lateral column ; p, pons varolii; i, its upper fibres ; 5, 5, roots of the fifth pair of nerves. Fig. 144.—View of the posterior surface of the pons varolii, corpora quadrigemina, and medulla oblongata. The peduncles of the cerebellum are cut short at the side. a, a, the upper pair of corpora quadrigemina; 6, b, the lowersuperior peduncles of the cerebellum; c, eminence connected with the nucleus of the hypoglossal nerve; e, that of the glosso-pharyngeal nerve ; i, that of the vagus nerve ; d, d, resti- form bodies; p, p, posterior pyramids ; v, v, groove in the middle of the fourth ven- tricle, ending below in the calamus scriptorius; 7, 7, roots of the auditory nerves. the lateral, continuous with the lateral columns of the cord, are named simply from their position. On the fibres of the lateral column of each side, near its upper part, is a small oval mass, containing gray matter, and named the olivary body; and at the posterior part of the restiform column, immediately on each side of the posterior median groove, a small tract is marked off by a slight groove from the remainder of the resti- 404 THE NERVOUS SYSTEM. form body, and called the posterior pyramid. The restiform columns, instead of remaining parallel with each other through- out the whole of the medulla oblongata, diverge near its upper part, and by thus diverging, lay open, so to speak, a space called the fourth ventricle, the floor of which is formed by the gray matter of the interior of the medulla, by this divergence exposed. On separating the anterior pyramids, and looking into the groove between them, some decussating fibres can be plainly seen. Distribution of the Fibres of the Medulla Oblongata. The anterior pyramid of each side, although mainly com- posed of continuations of the fibres of the anterior columns of the spinal cord, receives fibres from the lateral columns, both of its own and the opposite side; the latter fibres forming al- most entirely those decussating strands before mentioned, which are seen in the groove between the anterior pyramids. Thus composed, the anterior pyramidal fibres proceeding onwards to the brain are distributed in the following manner : 1. The greater part pass on through the pons to the cerebrum.1 A portion of the fibres, however, running apart from the others, joins some fibres from the olivary body, and unites with them to form what is called the olivary fasciculus or fillet. 2. A small tract of fibres proceeds to the cerebellum. The lateral column on each side of the medulla, iu proceed- ing upwards, divides into three parts, outer, inner, and middle, which are thus disposed of: 1. The outer fibres go with the restiform tract to the cerebellum. 2. The middle decussate across the middle line with their fellows, and form a part of the anterior pyramid of the opposite side. 3. The inner pass on to the cerebrum along the floor of the fourth ventricle, on each side, under the name of the fasciculus teres. The fibres of the restiform body receive some small contribu- tions from both the lateral and anterior columns of the me- 1 The expressions “continuous fibres,” and the like, appear to be usually understood as meaning that certain primitive nerve-fibres pass without interruption from one part to another. But such con- tinuity of primitive fibres through long distances in the nervous centres is very far from proved. The apparent continuity of fasciculi (which is all that dissection can yet trace) is explicable on the suppo- sition that many comparatively short fibres lie parallel, with the ends of each inlaid among many others. In such a case, there would be an apparent continuity of fibres; just as there is, for example, when one untwists and picks out a long cord of silk or wool, in which each fibre is short, and yet each fasciculus appears to be continued through the whole cord. FUNCTIONS OF THE MEDULLA OBLONGATA. 405 dulla, and proceed chiefly to the cerebellum, but that small part behind, called posterior pyramid, is continued on with the fasciculus teres of each side along the floor of the fourth ven- tricle to the cerebrum. As in structure, so also in the general endowments of their several parts, there is, probably, the closest analogy between the medulla oblongata and the spinal cord. The difference between them in size and form appears due, chiefly, first, to the divergence, enlargement, and decussation of the several columns, as they pass to be connected with the cerebellum or the cerebrum; and, secondly, to the insertion of new quantities of gray matter in the olivary bodies and other parts, in adap- tation to the higher office and wider range of influence which the medulla oblongata as a nervous centre exercises. Functions of the Medulla Oblongata. In its functions the medulla oblongata differs from the spinal cord chiefly in the importance and extent of the actions that it governs. Like the cord, it may be regarded, first, as conducting impressions, in which office it has a wider extent of function than any other part of the nervous system, since it is obvious that all impressions passing to and fro between the brain and the spinal cord and all nerves arising below the pons, must be transmitted through it. The decussation of part of the fibres of the anterior pyramids of the medulla oblongata explains the phenomena of cross-paralysis, as it is termed, i. e., of the loss of motion in cerebral apoplexy, being always on the side opposite to that on which the effusion of blood has taken place. Looking only to the anatomy of the medulla oblongata, it was not possible to explain why the loss of sensation also is on the side opposite the injury or disease of the brain; for there is no evidence of a decussation of posterior fibres like that which ensues among the anterior fibres of the medulla oblongata. But the discoveries of Brown-Sequard have shown that the crossing of sensitive impressions occurs in the spinal cord (see p. 393). The functions of the medulla oblongata as a nerve-centre seem to be more immediately important to the maintenance of life than those of any other part of the nervous system, since from it alone, or in chief measure, appears to be reflected the nervous force necessary for the performance of respiration and deglutition. It has been proved by repeated experiments on the lower animals that the entire brain may be gradually cut away in successive portions, and yet life may continue for a considerable time, and the respiratory movements be uninter- 406 THE NERVOUS SYSTEM. rupted. Life may also continue when the spinal cord is cut away in successive portions from below upwards as high as the point of origin of the phrenic nerve, or in animals without a diaphragm, such as birds or reptiles, even as high as the me- dulla oblongata. In Amphibia, these two experiments have been combined; the brain being all removed from above, and the cord from below; and so long as the medulla oblongata was intact, respiration and life were maintained. But if, in any animal, the medulla oblongata is wounded, particularly if it is wounded in its central part, opposite the origin of the pneumogastric nerves, the respiratory movements cease, and the animal dies as if asphyxiated. And this effect ensues even when all parts of the nervous system, except the medulla ob- longata, are left intact. Injury and disease in men prove the same as these experi- ments on animals. Numerous instances are recorded in Avhich injury to the human medulla oblongata has produced instanta- neous death; and, indeed, it is through injury of it, or of the part of the cord connecting it with the origin of the phrenic nerve, that death is commonly produced in fractures and diseases with sudden displacement of the upper cervical vertebra;. The centre whence the nervous force for the production of combined respiratory movements appears to issue is in the interior of that part of the medulla oblongata from which the pneumogastric nerves arise; for with care the medulla ob- longata may be divided to within a few lines of this part, and its exterior may be removed without the stoppage of respira- tion; but it immediately ceases when this part is invaded. This is not because the integrity of the pneumogastric nerves is essential to the respiratory movements ; for both these nerves may be divided without more immediate effect than a retarda- tion of these movements. The conclusion, therefore, may safely be, that this part of the medulla oblongata is the nervous centre whereby the impulses producing the respiratory move- ments are reflected. The power by which the medulla oblongata governs and combines the action of various muscles for the respiratory movements, is an instance of the power of reflexion, which it possesses in common with all nervous centres. Its general mode of action, as well as the degree to which the mind may take part in respiration, and the number of nerves and mus- cles which, under the governance of the medulla oblongata, may be combined in the forcible respiratory movements, have been already briefly described (see p. 184, et seq.). That which seems most peculiar in this centre of respiratory action is its wide range of connection, the number of nerves by which the FUNCTIONS OF THE MEDULLA OBLONGATA. 407 centripetal impression to excite motion may be conducted, and the number and distance of those through which the motor impulse may be directed. The principal centripetal nerves engaged in respiration are the pneumogastic, whose branches supplying the lungs appear to convey the most acute impres- sion of the “ necessity of breathing.” When they are both divided, the respiration becomes slower (J. Reid), as if the necessity were less acutely felt: but it does not cease, and therefore other nerves besides them must have the power of conducting the like impression. The experiments of Volk- mann make it probable that all centripetal nerves possess it in some degree, and that the existence of imperfectly aerated blood in contact with any of them acts as a stimulus, which, being conveyed to the medulla oblongata, is reflected to the nerves of the respiratory muscles : so that respiratory move- ments do not wholly cease so long as any centripetal nerves, and any nerve supplying muscles of respiration, are both in continuous connection with the respiratory centre of the medulla oblongata. The circulation of imperfectly aerated blood in the medulla oblongata itself may also act as a stimu- lus, and react through this nerve-centre on the nerves which supply the inspiratory muscles. The wide extent of connection which belongs to the medulla oblongata as the centre of the respiratory movements, is further shown by the fact that impressions by mechanical and other ordinary stimuli, made on many parts of the external or inter- nal surface of the body, may induce respiratory movements. Thus involuntary respirations are induced by the sudden con- tact of cold with any part of the skin, as in dashing cold water into the face. Irritation of the mucous membrane of the nose produces sneezing. Irritation in the pharynx, oesophagus, stom- ach, or intestines, excites the concurrence of the respiratory movements to produce vomiting. Violent irritation in the rec- tum, bladder, or uterus, gives rise to a concurrent action of the respiratory muscles, so as to effect the expulsion of the faeces, urine, or foetus. The medulla oblongata appears to be the centre whence are derived the motor impulses enabling the muscles of the palate, pharynx, and oesophagus, to produce the successive co-ordinate and adapted movements necessary to the act of deglutition (see p. 213). This is proved by the persistence of swallowing in some of the lower animals after destruction of the cerebral hemispheres and cerebellum; its existence in anencephalous monsters; the power of swallowing possessed by marsupial embryos before the brain is developed ; and by the complete arrest of the power of swallowing when the medulla oblongata 408 THE NERVOUS SYSTEM. is injured in experiments. But the reflecting power herein exercised by the medulla oblongata is of a much simpler and more restricted kind than that exercised in respiration; it is, indeed, not more than a simple instance of reflex action by a segment of the spinal axis, receiving impressions for this pur- pose from only a few centripetal nerves, and reflecting them to the motor nerves of the same organ. The incident or cen- tripetal nerves in this case are the branches of the glosso- pharyngeal, and, in a subordinate degree, those of the fifth nerve, some of the branches of the superior laryngeal nerve, which are distributed to the pharynx; and the nerves through which the motor impressions to the fauces and pharynx are reflected, are the pharyngeal branches of the vagus, and, in sub- ordinate degrees, or as supplying muscles accessory to the move- ments of the pharynx, the branches of the hypoglossal, facial, cervical, recurrent, and fifth nerves. For the oesophageal move- ments, so far as they are connected with the medulla oblon- gata, the filaments of the pneumogastric nerve alone, which contain both afferent and efferent fibres, appear to be sufficient (John Reid). Though respiration and life continue while the medulla oblongata is perfect and in connection with respiratory nerves, yet, when all the brain above it is removed, there is no more appearance of sensation, or will, or of any mental act in the animal, the subject of the experiment, than there is when only a spinal cord is left. The movements are all involuntary and unfelt; and the medulla oblongata has, therefore, no claim to be considered as an organ of the mind, or as the seat of sensa- tion or voluntary power. These are connected with parts next to be described. It would appear that much of the reflecting power of the medulla oblongata may be destroyed; and yet its power in the respiratory movements may remain. Thus, in patients completely affected with chloroform, the winking of the eye- lids ceases, and irritation of the pharynx will not produce the usual movements of swallowdng, or the closure of the glottis (so that blood may run quietly into the stomach, or even into the lungs) ; yet, with all this, they may breathe steadily, and show that the power of the medulla oblongata to combine in action all the nerves of the respiratory muscles is perfect. In addition to its influence over the functions of respiration and deglutition, the medulla oblongata appears to be largely concerned also in the faculty of speech. In the medulla oblongata appears to be seated also the chief vaso-viotor nerve-centre (p. 452). From this arise fibres which, passing down the spinal cord, issue with the anterior THE PONS VAROLII. 409 roots of the spinal nerves, and enter the ganglia and branches of the sympathetic, by which they are conducted to the blood- vessels. The influence which is exercised by the medulla oblongata, or, at least, by its irritation, on the formation of sugar in the liver, has been referred to (p. 269). STRUCTURE AND PHYSIOLOGY OF THE PONS VAROLII, CRURA CEREBRI, CORPORA QUADRIGEMINA, CORPORA GENICU- LATA, OPTIC THALAMI, AND CORPORA STRIATA. Pons Varolii.—The mesocephalon, or pons (o, Fig. 145), is composed principally of transverse fibres connecting the two hemispheres of the cerebellum, and forming its principal com- missure. But it includes, interlacing with these, numerous longitudinal fibres which connect the medulla oblongata with the cerebrum, and transverse fibres which connect it with the cerebellum. Among the fasciculi of nerve-fibres by which these several parts are connected, the pons also contains abun- dant gray or vesicular substance, which appears irregularly placed among the fibres, and fills up all the interstices. The anatomical distribution of the fibres, both transverse and longitudinal, of which the pons is composed, is sufficient evidence of its functions as a conductor of impressions from one part of the cerebro-spinal axis to another. Concerning its functions as a nerve-centre, little or nothing is certainly known. Crura Cerebri.—The crura cerebri (t, Fig. 145), are prin- cipally formed of nerve-fibres, of which the inferior or more superficial are continuous with those of the anterior py- ramidal tracts of the medulla oblongata, and the superior or deeper fibres with the lateral and posterior pyramidal tracts, and with the olivary fasciculus. Besides these fibres from the medulla oblongata, are others from the cerebellum ; and some of the latter as well as a part of the fibres derived from the lateral tract of the medulla oblongata, decussate across the middle line. On their upper part, the crura cerebri bear three pairs of small ganglia, or masses of mingled gray and white nerve- substance, namely, the corpora geniculata externa and interna, and the corpora quadrigemina, or nates and testes. And in their onward course to the cerebrum, the fibres of each crus cerebri pass through two large ganglia, the optic thalamus and corpus striatum, and in their substance come into connection with variously-shaped masses and layers of gray substance. Whether all the fibres of the crura cerebri end in the gray 410 TIIE NERVOUS SYSTEM. matter of these two ganglia, while others start afresh from them to enter the cerebral hemispheres; or whether some of the fibres of the crura pass through them, while only a portion Fig. 145. Shows the under surface or base of the encephalon freed from its membranes—a, anterior, b, middle, and c, posterior lobe of cerebrum.—a. The fore part of the great longitudinal fissure, b. Notch between hemispheres of the cerebellum, c. Optic commissure, d. Left peduncle of cerebrum, e. Posterior perforated space, e to i. Interpeduncular space. //'. Convolution of Sylvian fissure, h. Termination of gyrus fornicatus behind the Sylvian fissure, i. Infundibulum. 1. Right middle crus or peduncle of cerebellum, m m. Hemispheres of cerebellum, n. Corpora albicantia. o. Pons varolii, continuous at each side w7ith middle crura of cerebel- lum. p. Anterior perforated space, q. Horizontal fissure of cerebellum, r. Tuber cinereum. s s'. Sylvian fissure, t. Left peduncle or crus of cerebrum, u u. Optic tracts, v. Medulla oblongata, x. Marginal convolution of the longitudinal fissure. 1 to 9 indicate the several pairs of cerebral nerves, numbered according to the usual notation, viz. 1. Olfactory nerve. 2. Optic. 3. Motor nerve of eye. 4. Pathetic. 5. Trifacial. 6. Abducent nerve of eye. 7. Auditory, and 7'. Facial. 8. Glosso- pharyngeal, 8'. Vagus, and 8". Spinal accessory nerve. can be strictly said to have their termination there, must re- main at present undecided, the difficulties in the way of solv- ing such an anatomical doubt being at present insuperable. CORPORA QUADRIGEMINA. 411 Each crus cerebri contains among its fibres a mass of vesic- ular substance, the locus niger, the nerve-corpuscles of which abound in pigment-granules, and afford some of the best in- stances of the caudate structure. With regard to their functions, the crura cerebri may be regarded as, principally, conducting organs. As nerve-centres they are probably connected with the functions of the third cerebral nerve, which arises from the locus niger, and through which are directed the chief of the numerous and complicated movements of the eyeball and iris. From the result of vivisection it appears that when one of the crura cerebri is cut across, the animal moves round and round, rotating around a vertical axis from the injured towards the sound side. Such movements, however, attend the sections of other parts than the crura cerebri; and as indications of the functions of these parts, the results of such experiments have been hitherto almost valueless. Corpora Quadrigemina.—The corpora quadrigemina (from which, in function, the corpora geniculata are not distinguished), are the homologues of the optic lobes in birds, amphibia, and fishes, and may be regarded as the principal nervous centres for the sense of sight. The experiments of Flourens, Longet, and Hertwig, show that removal of the corpora quadrigemina wholly destroys the power of seeing; and diseases in which they are disorganized are usually accompanied with blindness. Atrophy of them is also often a consequence of atrophy of the eyes. Destruction of one of the corpora quadrigemina (or of one optic lobe in birds), produces blindness of the opposite eye. This loss of sight is the only apparent injury of sensibility sustained by the removal of the corpora quadrigemina. The removal of one of them affects the movements of the body, so that animals rotate, as after division of the crus cerebri, only more slowly: but this is probably due to giddiness and partial loss of sight. The more evident and direct influence is that produced on the iris. It contracts when the corpora quadri- gemina are irritated : it is always dilated when they are re- moved : so that they may be regarded, in some measure at least, as the nervous centres governing its movements, and adapting them to the impressions derived from the retina through the optic nerves and tracts. Concerning the functions, taken as a whole, discharged by the olfactory and optic lobes, the gray substance of the pons, the corpora striata and optic thalami (6, cl, Fig. 146), with 412 THE NERVOUS SYSTEM. some other centres of gray matter not so distinct, such as the gray matter on the floor of the fourth ventricle with which the auditory nerve is connected, the most philosophical theory is Fig. 146. Dissection of brain, from above, exposing the lateral, fourth, and fifth ventricles, with the surrounding parts (from Hirschfeld and Leveille). a, anterior part, or genu of corpus callosum ; 6, corpus striatum; b\ the corpus striatum of left side, dis- sected so as to expose its gray substance ; c, points by a line to the tsenia semicircu- laris; d, optic thalamus; e, anterior pillars of fornix divided; below they are seen descending in front of the third ventricle, and between them is seen part of the an- terior commissure; in front of the letter e is seen the slit-like fifth ventricle, between the two lamin;e of the septum lucidum; /, soft or middle commissure; g is placed in the posterior part of the third ventricle; immediately behind the latter are the posterior commissure (just visible) and the pineal gland, the two crura of which extend forwards along the inner and upper margins of the optic thalami; h and i, the corpora quadrigemina ; k, superior crus of cerebellum; close to k is the valve of Vieussens, which has been divided so as to expose the fourth ventricle ; l, hippo- campus major and corpus fimbriatum, or tsenia hippocampi; m, hippocampus minor ; n, eminentia collaterals; o, fourth ventricle ; p, posterior surface of medulla oblon- gata ; r, section of cerebellum ; s, upper part of left hemisphere of cerebellum exposed by the removal of part of the posterior cerebral lobe. undoubtedly that which has been so ably enunciated by Dr. Carpenter. He supposes these ganglia to constitute the real SENSORY GANGLIA. 413 sensoriura ; that is to say, it is by means of them that the mind becomes conscious of impressions made on the organs or tissues with which (by means of nerve-fibres) they are in com- munication. Thus impressions made on the optic nerve, or its expansion in the retina, are conducted by the fibres of the optic nerve to the corpora quadrigemina, and through the medium of these ganglia the mind becomes conscious of the impression made. And impressions on the filaments of the olfactory or auditory nerve are in the same way perceived through the medium of the olfactory or auditory ganglia, to which they are first conveyed. The optic thalami and corpora striata probably have some function of a like kind—perhaps in relation to ordinary sensation, but nothing is certainly known regarding them. Besides their functions, however, as media of communica- tion between the mind and external objects, these sensory ganglia, as they are termed, are probably the nerve-centres by means of which those reflex acts are performed which require either a higher combination of muscular acts than can be directed by means of the medulla oblongata or spinal cord alone, or, on the other hand, such reflex actions as require for their right performance the guidance of sensation. Under this head are included various acts, as walking, reading, writ- ing, and the like, which we are accustomed to consider volun- tary, but which really are as incapable of being performed by distinct and definite acts of the will as are those more simple movements of which we are not conscious, and which, per- formed under the guidance of the spinal cord or medulla oblongata alone, we call simple reflex actions. It is true that, in the performance of such acts as those just mentioned, a certain exercise of the will is required at the commencement, but that the carrying out of its mandates is essentially reflex and involuntary, any one may convince himself by trying to perform each individual movement concerned, strictly as a voluntary act. That such movements are reflex and essentially independent —as regards their mere production—of the will, there is no doubt; that the nerve-centres through which such reflex actions are performed are the so-called sensory ganglia, is, of course, only a theory which may or not be confirmed by future investigations. Besides their possible functions in the manner just men- tioned, it is supposed that these sensory ganglia may be the means of transmitting the impulses of the will to the muscles, which act in obedience to it, and thus be the centres of reflex action as well for impressions conveyed downwards to them 414 THE NERVOUS SYSTEM. from the cerebral hemispheres, as for impressions carried up- wards to them by the different nerves which preserve their connection with the organs of the various senses. The cerebellum (7, 8, 9, 10, Fig. 147) is composed of an elongated central portion called the vermiform processes, and two hemispheres. Each hemisphere is connected with its fel- low, not only by means of the vermiform processes, but also by a bundle of fibres called the middle crus or peduncle (the latter forming the greater part of the pons Yarolii), while a superior crus with the valve of Vieussens, connects it with the cerebrum (Fig. 147, 5), and an inferior crus (formed by the STRUCTURE AND PHYSIOLOGY OF THE CEREBELLUM. Fig. 147. View of cerebellum in section and of fourth ventricle, with the neighboring parts (from Sappey after Hirschfeld and Leveillfi). 1, median groove of fourth ventricle, ending below in the calamus scriplorius, with the longitudinal eminences formed by the fasciculi tereles, one on each side; 2, the same groove, at the place where the white streaks of the auditory nerve emerge from it to cross the floor of the ventricle; 3, inferior crus or peduncle of the cerebellum, formed by the restiform body; 4, posterior pyramid; above this is the calamus scriptorius ; 5, superior crus of cere- bellum, or processus a cerebello ad cerebrum (or ad testes)6, 6, fillet to the side of the crura cerebri ; 7, 7, lateral grooves of the crura cerebri ; 8, corpora quadrigemina. prolonged restiform body) connects it with the medulla ob- longata (3, Fig. 147). The cerebellum is composed of white and gray matter like FUNCTIONS OF THE CEREBELLUM. 415 that of the cerebrum, but arranged after a different fashion, as shown in Fig. 147. Besides the gray substances on the surface, however, there is near the centre of the white substance of each hemisphere, a small capsule of gray matter called the corpus dentatum (Fig. 148, c d), resembling very closely the corpus dentatum of the olivary body of the medulla oblongata (Fig. 148, o). The physiology of the cerebellum may be considered in its relation to sensation, voluntary motion, and the instincts or higher faculties of the mind. It is itself insensible to irrita- tion, and may be all cut away without eliciting signs of pain (Longet). Yet, if any of its crura be touched, pain is indi- cated ; and, if the restiform tracts of the medulla oblongata be irritated, the most acute suffering appears to be produced. Its removal or disorganization by disease is also generally un- accompanied with loss or disorder of sensibility ; animals from which it is removed can smell, see, hear, and feel pain, to all appearance, as perfectly as before (Flourens ; Magendie). So Fig. 148. Outline sketch of a section of the cerebellum showing the corpus dentatum (from Quain). The section has been carried through the left lateral part of the pons, so as to divide the superior peduncle and pass nearly through the middle of the left cerebellar hemisphere. The olivary body has also been divided longitudinally so as to expose in section its corpus dentatum. c r, crus cerebri; /, fillet; q, corpora quadrigemina ; s p, superior peduncle of the cerebellum divided ; m p, middle pe- duncle or lateral part of the pons Varolii, with fibres passing from it into the white stem; a v, continuation of the white stem radiating towards the arbor vita1 of the folia; c d, corpus dentatum; o, olivary body with its corpus dentatum; p, anterior pyramid. that, although the restiform tracts of the medulla oblongata, which themselves appear so sensitive, enter the cerebellum, it cannot be regarded as a principal organ of sensibility. In reference to motion, the experiments of Longet and most others agree that no irritation of the cerebellum produces movement of any kind. Remarkable results, however, are produced by removing parts of its substance. Flourens 416 THE NERVOUS SYSTEM. (whose experiments have been abundantly confirmed by those of Bouillaud, Longet, and others) extirpated the cerebellum in birds by successive layers. Feebleness and want of har- mony of the movements were the consequence of removing the superficial layers. When he reached the middle layers, the animals became restless without being convulsed ; their move- ments were violent and irregular, but their sight and hearing were perfect. By the time that the last portion of the organ was cut away, the animals had entirely lost the powers of springing, flying, walking, standing, and preserving their equi- librium. When an animal in this state was laid upon its back, it could not recover its former posture ; but it fluttered its wings, and did not lie in a state of stupor; it saw the blow that threatened it, and endeavored to avoid it. Volition, sen- sation, and memory, therefore, were not lost, but merely the faculty of combining the actions of the muscles; and the en- deavors of the animal to maintan its balance were like those of a drunken man. The experiments afforded the same results when repeated on all classes of animals; and, from them and the others be- fore referred to, Flourens inferred that the cerebellum belongs neither to the sensitive nor the intellectual apparatus; and that it is not the source of voluntary movements, although it belongs to the motor-apparatus; but is the organ for the co- ordination of the voluntary movements, or for the excitement of the combined action of muscles. Such evidence as can be obtained from cases of disease of this organ confirms the view taken by Flourens; and, on the whole, it gains support from comparative anatomy ; animals whose natural movements require most frequent and exact combinations of muscular actions being those whose cerebella are most developed in proportion to the spinal cord. M. Foville holds that the cerebellum is the organ of muscu- lar sense, i. e., the organ by which the mind acquires that knowledge of the actual state and position of the muscles which is essential to the exercise of the will upon them ; and it must be admitted that all the facts just referred to are as well ex- plained on this hypothesis as on that of the cerebellum being the organ for combining movements. A harmonious combina- tion of muscular actions must depend as much on the capa- bility of appreciating the condition of the muscles with regard to their tension, and to the force with which they are con- tracting, as on the power which any special nerve-centre may possess of exciting them to contraction. And it is because the power of such harmonious movement would be equally lost, whether the injury to the cerebellum involved injury to FUNCTIONS OF THE CEREBELLUM. 417 the seat of muscular sense, or to the centre for combining mus- cular actions, that experiments on the subject afford no proof in one direction more than the other. Gall was led to believe, that the cerebellum is the organ of physical love, or, as Spurzheim called it, of amativeness; and this view7 is generally received by phrenologists. The facts favoring it are, first, several cases in which atrophy of the testes and loss of sexual passion have been the consequence of blows over the cerebellum, or wounds of its substance; sec- ondly, cases in which disease of the cerebellum has been at- tended with almost constant erection of the penis, and frequent seminal emissions; and thirdly, that it has seemed possible to estimate the degree of sexual passion in different persons by an external examination of the region of the cerebellum. The cases of disease of the cerebellum do not prove much; for the same affections of the genital organs are more gener- ally observed in diseases, and in experimental irritations of the medulla oblongata and upper part of the spinal cord (Longet). The facts drawn from craniological examination will receive the credit given to the system of vrhich they are a principal evidence. But, in opposition to them, it must be stated that there has been a case of complete disorganization or absence of the cerebellum without loss of sexual passion (Combiette, Longet, and Cruveilhier); that the cocks from whom M. Fiourens removed the cerebellum showed sexual desire, though they were incapable of gratifying it; and that among animals there is no proportion observable between the size of the cere- bellum and the development of the sexual passion. On the contrary, many instances may be mentioned in which a larger sexual appetite coexists with a smaller cerebellum; as e. g., that rays and eels, which are among the fish that copulate, have not laminse on their almost rudimental cerebella; and that cod-fish, which do not copulate, but deposit their genera- tive fluids in the water, have comparatively well-developed cerebella. Among the Amphibia, the sexual passion is ap- parently very strong in frogs and toads; yet the cerebellum is only a narrow bar of nervous substance. Among birds there is no enlargement of the cerebellum in the males that are polyg- amous ; the domestic cock’s cerebellum is not larger than the hen’s, though his sexual passion must be estimated at many times greater than hers. Among Mammalia the same rule holds; and in this class the experiments of M. Lassaigne have plainly shown that the abolition of the sexual passion by re- moval of the testes in early life is not followed by any diminu- tion of the cerebellum; for in mares and stallions the average 418 THE NERVOUS SYSTEM. absolute weight of the cerebellum is 61 grains, and in geldings 70 grains; and its proportionate weight, compared with that of the cerebrum, is, onaverage, as 1: 6.59 in mares ; as 1: 5.97 in geldings, and only as 1: 7.07 in stallions. On the whole, therefore, it appears advisable to wait for more evidence before concluding that there is any peculiar and direct connection between the cerebellum and the sexual in- stinct or sexual passion. From all that has been observed, no other office is manifest in it than that of regulating and com- bining muscular movements, or of enabling them to be regu- lated and combined by so informing the mind of the state and position of the muscles that the will may be definitely and aptly directed to them. The influence of each half of the cerebellum is directed to muscles on the opposite side of the body; and it would appear that for the right ordering of movements, the actions of its two halves must be always mutually balanced and adjusted. For if one of its crura, or if the pons on either side of the middle line, be divided, so as to cut off from the medulla oblongata and spinal cord the influence of one of the hemispheres of the cerebellum, strangely disordered movements ensue. The ani- mals fall down on the side opposite to that on which the crus cerebelli has been divided, and then roll over continuously and repeatedly; the rotation being always round the long axis of their bodies, and from the side on which the injury has been inflicted.1 The rotations sometimes take place with much ra- pidity ; as often, according to M. Magendie, as sixty times in a minute, and may last for several days. Similar movements have been observed in men ; as by M. Serres in a man in whom there was apoplectic effusion in the right crus cerebelli; and by M. Belhomme in a woman, in whom an exostosis pressed on the left crus.2 They may, perhaps, be explained by assuming that the division or injury of the crus cerebelli produces paral- ysis or imperfect and disorderly movements of the opposite side of the body; the animal falls, and then, struggling with the disordered side on the ground, and striving to rise with the 1 Magendie and Muller, and others following them, say the rotation is towards the injured side ; but Longet and others more correctly give the statement as in the text. The difference has probably arisen from using the words right and left: without saying whose right and left are meant, whether those of the observer or those of the observed. When, for example, an animal’s right crus cerebelli is divided, he rolls from his own right to his own left, but from the left to the right of one who is standing in front of him. 2 See such cases collected and recorded by Dr. Paget in the Ed. Med. and Surg. Journal for 1847. STRUCTURE OF THE CEREBRUM. 419 other, pushes itself over; and so, again and again, with the same act, rotates itself. Such movements cease when the other crus cerebelli is divided ; but probably only because the paral- ysis of the body is thus made almost complete. STRUCTURE AND PHYSIOLOGY OF THE CEREBRUM. The cerebrum is placed in connection with the pons and medulla oblongata by its two crura or ped,uncles (Fig. 149) : it is connected with the cerebellum, by the processes called su- Fig. 149. Plan in outline of the encephalon, as seen from the right side. (From Quain.) The parts are represented as separated from one another somewhat more than natural, so as to show their connections. A, cerebrum ; /, g, h, its anterior, middle, and posterior lobes; e, fissure of Sylvias; B, cerebellum; C, pons varolii; D, me- dulla oblongata ; a, peduncles of the cerebrum ; b, c, d, superior, middle, and inferior peduncles of the cerebellum. perior crura of the cerebellum, or processus a cerebello ad testes, and by a layer of gray matter called the valve of Vieussens, which lies between these processes, and extends from the in- ferior vermiform process of the cerebellum to the corpora quad- rigemina of the cerebrum. These parts, which thus connect the cerebrum with the other principal divisions of the cerebro- spinal nervous centre, form parts of the walls of a cavity (the fourth ventricle) and a canal (the iter a tertio ad quartum ven- 420 THE NERVOUS SYSTEM. triculum.), which are the continuation of the canal that in the foetus extended through the whole length of the spinal cord and brain. They may, therefore, be regarded as the contin- uation of the cerebro-spinal axis or column ; on which, as a de- velopment from the simple type, the cerebellum is placed; and, on the further continuation of which, structures both larger and more numerous are raised, to form the cerebrum (Fig. 142). The cerebral convolutions appear to be formed of nearly parallel plates of fibres, the ends of which are turned towards the surface of the brain, and are overlaid and mingled with successive layers of gray nerve-substance. The external gray matter is so arranged in layers, that a vertical section of a convolution, according to Mr. Lockhart Clarke, generally presents the appearance of seven layers of pale and dark ner- vous substance. The structure of the gray matter is that which belongs to vesicular nervous substance (p. 375). It is nearly certain that the cerebral hemispheres are the organ by which,—1st, we perceive those clear and more im- pressive sensations which we can retain, and according to which we can judge; 2dly, by which are performed those acts of will, each of which requires a deliberate, however quick, de- termination; 3dly, they are the means of retaining impressions of sensible things, and reproducing them in subjective sensa- tions and ideas; 4thly, they are the medium of the higher emotions and feelings, and of the faculties of judgment, under- standing, memory, reflection, induction, and imagination, and others of a like class. The evidences that the cerebral hemispheres have the func- tions indicated above, are chiefly these : 1. That any severe injury of them, such as a general concussion, or sudden pres- sure by apoplexy, may instantly deprive a man of all power of manifesting externally any mental faculty. 2. That in the same general proportion as the higher sensuous mental facul- ties are developed in the vertebrate animals, and in man at different ages, the more is the size of the cerebral hemispheres developed in comparison with the rest of the cerebro-spinal system. 3. That no other part of the nervous system bears a corresponding proportion to the development of the mental faculties. 4. That congenital and other morbid defects of the cerebral hemisphere are, in general, accompanied with corre- sponding deficiency in the range or power of the intellectual faculties and the higher instincts. Respecting the mode in which the brain discharges its func- tions, there is no evidence whatever. But it appears that, for all but its highest intellectual acts, one of the cerebral hemi- spheres is sufficient. For numerous cases are recorded in FUNCTIONS OF THE CEREBRUM. 421 which no mental defect was observed, although one cerebral hemisphere was so disorganized or atrophied that it could not be supposed capable of discharging its functions. The re- maining hemisphere was, in these cases, adequate to the func- tions generally discharged by both; but the mind does not seem in any of these cases to have been tested in very high intellectual exercises; so that it is not certain that one hemi- sphere will suffice for these. In general, the mind combines, as one sensation, the impressions which it derives from one object, through both hemispheres, and the ideas to which the two such impressions give rise are single. In relation to common sensation and the effort of the will, the impressions to and from the hemispheres of the brain are carried across the middle line: so that in destruction or com- pression of either hemisphere, whatever effects are produced in loss of sensation or voluntary motion, are observed on the side of the body opposite to that on which the brain is injured. In speaking of the cerebral hemispheres as the so-called organs of the mind, they have been regarded as if they were single organs, of which all parts are equally appropriate for the exercise of each of the mental faculties. But it is pos- sible that each faculty has a special portion of the brain ap- propriated to it as its proper organ. For this theory the principal evidences are as follows: 1. That it is in accordance with the physiology of the other compound organs or systems in the body, in which each part has its special function; as, for example, of the digestive system, in which the stomach, liver, and other organs perform each their separate share in the general process of the digestion of the food. 2. That in different individuals the several mental functions are mani- fested in very different degrees. Even in early childhood, before education can be imagined to have exercised any in- fluence on the mind, children exhibit various dispositions— each presents some predominant propensity, or evinces a sin- gular aptness in some study or pursuit; and it is a matter of daily observation that everyone has his peculiar talent or pro- pensity. But it is difficult to imagine how this could be the case, if the manifestation of each faculty depended on the whole of the brain : different conditions of the whole mass might affect the mind generally, depressing or exalting all its functions in an equal degree, but could not permit one faculty to be strongly and another weakly manifested. 3. The plu- rality of organs in the brain is supported by the phenomena of some forms of mental derangement. It is not usual for all the mental faculties in an insane person to be equally disor- dered; it often happens that the strength of some is increased, 422 THE NERVOUS SYSTEM. while that of others is diminished ; and in many cases one function only of the mind is deranged, while all the rest are performed in a natural manner. 4. The same opinion is sup- ported by the fact that the several mental faculties are devel- oped to their greatest strength at different periods of life, some being exercised with great energy in childhood, others only in adult age; and that, as their energy decreases in old age, there is not a gradual and equal diminution of power in all of them at once, but, on the contrary, a diminution in one or more, while others retain their full strength, or even increase in power. 5. The plurality of cerebral organs appears to be indicated by the phenomena of dreams, in which only a part of the mental faculties are at rest or asleep, while the others are awake, and, it is presumed, are exercised through the me- dium of the parts of the brain appropriated to them. These facts have been so illustrated and adapted by phren- ologists, that the theory of the plurality of organs in the cere- brum, thus made probable, has been commonly regarded as peculiar to phrenology, and as so essentially connected with it, that if the system of Gall and Spurzheim be untrue, this theory cannot be maintained. But it is plain that all the system of phrenology built upon the theory may be false, and the theory itself true ; for phrenologists assume not only this theory, but also that they have determined all the primitive faculties, of which the mind consists, i. e., all the faculties to which special organs must be assigned, and the places of all those organs in the cerebral hemispheres and the cerebellum. That this is a system of error there need be no doubt, but it is possibly founded on a true theory: the cerebrum may have many organs, and the mind as many faculties; but what are the faculties that require separate organs, and where those organs are situate, are subjects of which only the most general and rudimentary knowledge has been yet attained. From the apparently greater frequency of interference with the faculty of speech in disease of the left than of the right half of the cerebrum, it has been thought that the nerve-centre for language, including in this term all intellectual expression of ideas, is situated in the left cerebral hemisphere. It cannot be said, however, that the existing evidence for this theory is at present sufficient to have established it. Of the physiology of the other parts of the brain, little or nothing can be said. Of the offices of the corpus callosum, or great transverse and oblique commissure of the brain, nothing positive is known. But instances in which it was absent, or very deficient, either without any evident mental defect, or with only such as might THE CORPUS CALLOSUM. 423 be ascribed to coincident affections of other parts, make it probable that the office which is commonly assigned to it, of enabling the two sides of the brain to act in concord, is exer- cised only in the highest acts of which the mind is capable. And this view is confirmed by the very late period of its de- velopment, and by its absence in all but the placental Mam- malia.1 Fig. 150. View of the corpus callosum from above (from Sappey after Foville). —The upper surface of the corpus callosum has been fully exposed by separating the cere- bral hemispheres and throwing them to the side; the gyrus fornicatus has been detached, and the transverse fibres of the corpus callosum traced for some distance into the cerebral medullary substance. 1, the upper surface of the corpus callosum; 2, median furrow or raphe ; 3, longitudinal strise bounding the furrow ; 4, swelling formed by the transverse bands as they pass into the cerebrum ; 5, anterior extrem- ity or knee of the corpus callosum ; 8, posterior extremity ; 7, anterior, and, 8, pos- terior part of the mass of fibres proceeding from the corpus callosum ; 9, margin of the swelling ; 10, anterior part of the convolution of the corpus callosum ; 11, hem or band of union of this convolution; 12, internal convolutions of the parietal lobe ; 13, upper surface of the cerebellum. 1 See eases of congenital deficiency of the corpus callosum, by Mr. Paget and Mr. Henry, in the twenty-ninth and thirty-first volumes of the Medico-Chirurgical Transactions. 424 THE NERVOUS SYSTEM. To the fornix and other commissures no special function can be assigned ; but it is a reasonable hypothesis that they con- nect the action of the parts between which they are severally placed. As little is known of the function of the pineal and pitu- itary glands. The latter has been supposed, from its micro- scopic structure, to be rather a ductless gland (p. 325) than a nervous organ. PHYSIOLOGY OF THE ‘CEREBRAL AND SPINAL NERVES. The cerebral nerves are commonly enumerated as nine pairs; but the number is in reality twelve, the seventh nerve consist- ing, as it does, of two nerves, and the eighth of three. These and the spinal nerves, of which there are thirty-one pairs, symmetrically arranged on each side of what, reduced to its simplest form, may be regarded as a column or axis of nervous matter, extending from the olfactory bulbs on the ethmoid bone to the filum terminate of the spinal cord in the lumbar and sacral portions of the vertebral canal. The spinal nerves all present certain characters in common, such as their-double roots; the isolation of the fibres of sensation in the posterior roots, and those of motion in the anterior roots; the formation of the ganglia on the posterior root; and the subsequent min- gling of the fibres in trunks and branches of mixed functions. Similar characters probably belong essentially to the cerebral nerves ; but even when one includes the nerves of special sense, it is not possible to discern a conformity of arrangement in any besides the fifth, or trifacial, which, from its many anal- ogies to the spinal nerves, Sir Charles Bell designated as a spinal nerve of the head. According to their several functions, the cerebral or cranial nerves may be thus arranged: Nerves of special sense, . . Olfactory, optic, auditory, part of the glosso-pharyngeal, and the lingual branch of the fifth. Nerves of common sensation, The greater portion of the fifth, and part of the glosso-pharyngeal. Nerves of motion, .... Third, fourth, lesser division of the fifth, sixth, facial, and hypoglossal. Mixed nerves, Pneumogastric and accessory. The physiology of the several nerves of the special senses will be considered with the organs of those senses. THE CEREBRAL NERVES. 425 Physiology of the Third, Fourth, and Sixth Cerebral or Cranial Nerves. The physiology of these nerves may be in some degree com- bined, because of their intimate connection with each other in the actions of the muscles of the eyeball, which they supply. They are probably all formed exclusively of motor fibres: some pain is indicated when the trunk of the third nerve is ir- ritated near its origin ; but this may be because of some fila- ments of the fifth nerve running backwards to the brain in the trunk of the third, or because adjacent sensitive parts are in- volved in the irritation. The third nerve, or motor oculi, supplies the levator palpe- brse superioris muscle, and, of the muscles of the eyeball, all but the superior oblique or trochlearis, to which the fourth nerve is appropriated, and the rectus externus, which receives the sixth nerve. Through the medium of the ophthalmic or lenticular ganglion, of' which it forms what is called the short root, it also supplies the motor filaments to the iris. When the third nerve is irritated within the skull, all those muscles to which it is distributed are convulsed. When it is paralyzed or divided, the following effects ensue: first, the upper eyelid can be no longer raised by the levator palpebrse, but drops and remains gently closed over the eye, under the unbalanced influence of the orbicularis palpebrarum, which is supplied by the facial nerve: secondly, the eye is turned out- wards by the unbalanced action of the rectus externus, to which the sixth nerve is appropriated : and hence, from the irregularity of the axes of the eyes, double-sight is often ex- perienced when a single object is within view of both the eyes : thirdly, the eye cannot be moved either upwards, downwards, or inwards; fourthly, the pupil is dilated. The relation of the third nerve to the iris is of peculiar in- terest. In ordinary circumstances the contraction of the iris is a reflex action, which may be explained as produced by the stimulus of light on the retina being conveyed by the optic nerve to the brain (probably to the corpora quadrigemina), and thence reflected through the third nerve to the iris. Hence the iris ceases to act when either the optic or the third nerve is divided or destroyed, or when the corpora quadri- gemina are destroyed or much compressed. But when the optic nerve is divided, the contraction of the iris may be excited by irritating that portion of the nerve which is connected with the brain; and when the third nerve is divided, the irritation of its distal portion will still excite contraction of the iris, in which its fibres are distributed. 426 THE NERVOUS SYSTEM. The contraction of the iris thus shows all the character of a reflex act, and in ordinary cases requires the concurrent ac- tion of the optic nerve, corpora quadrigemina, and third nerve ; and, probably also, considering the peculiarities of its perfect mode of action, the ophthalmic ganglion. But, besides, both irides will contract their pupils under the reflected stimulus of light falling only on one retina or under irritation of one optic nerve. Thus, in amaurosis of one eye, its pupil may contract when the other eye is exposed to a stronger light: and gener- ally the contraction of each of the pupils appears to be in di- rect proportion to the total quantity of light which stimulates either one or both retinae, according as one or both eyes are open. The iris acts also in association with certain other muscles supplied by the third nerve: thus, when the eye is directed inwards, or upwards and inwards, by the action of the third nerve distributed in the rectus interims and rectus superior, the iris contracts, as if under direct voluntary influence. The will cannot, however, act on the iris alone through the third nerve; but this aptness to contract in association with the other muscles supplied by the third, may be sufficient to make it act even in total blindness and insensibility of the retina, whenever these muscles are contracted. The contraction of the pupils, when the eyes are moved inwards, as in looking at a near object, has probably the purpose of excluding those outermost rays of light which would be too far divergent to be refracted to a clear image on the retina; and the dilatation in looking straight forwards, as in looking at a distant object, per- mits the admission of the largest number of rays, of which none are too divergent to be so refracted. The fourth nerve, or Nervus trochlearis or patheticus, is ex- clusively motor, and supplies only the trochlearis or obliquus superior muscle of the eyeball. The sixth nerve, Nervus abducens or ocularis externus is also, like the fourth, exclusively motor, and supplies only the rectus externus muscle.' The rectus externus is, therefore, convulsed, and the eye is turned outwards, when the sixth nerve is irri- tated ; and the muscle paralyzed when the nerve is disorganized, compressed, or divided. In all such cases of paralysis, the eye squints inwards, and cannot be moved outwards. 1 In several animals it sends filaments to the iris (Radelyffe Hall) ; and it has probably done so in man, in some instances in which the iris has not been paralyzed, while all the other parts supplied by the third nerve were (Grant). THE CEREBRAL NERVES. 427 In its course through the cavernous sinus, the sixth nerve forms larger communications with the sympathetic nerve than any other nerve within the cavity of the skull does. But the import of these communications with the sympathetic, and the subsequent distribution of its filaments after joining the sixth nerve, are quite unknown; and there is no reason to believe that the sixth nerve is, in function, more closely connected with the sympathetic than any other cerebral nerve is. The question has often suggested itself why the six muscles of the eyeball should be supplied by three motor nerves when all of them are within reach of the branches of one nerve; and the true explanation would have more interest than attaches to the movements of the eye alone; since it is probable that we have, in this instance, within a small space, an example of some general rule according to which associate or antagonist muscles are supplied with motor nerves. Now, in the several movements of the eyes, we sometimes have to act with symmetrically placed muscles, as when both eyes are turned upwards or downwards, inwards or outwards.1 All the symmetrically placed muscles are supplied with sym- metrical nerves, i.e., with corresponding branches of the same nerves on the two sides; and the action of these symmetrical muscles is easy and natural, as we have a natural tendency to symmetrical movement in most parts. But because of this tendency to symmetrical movements of muscles supplied by symmetrical nerves, it would appear as if, when the two eyes are to be moved otherwise than symmetrically, the muscles to effect such a movement must be supplied with different nerves. So, when the two eyes are to be turned towards one side, say the right, by the action of the rectus externus of the right eye and the rectus internus of the left, it appears as if the tendency to action through the similar branches of corresponding nerves (which would move both eyes inwards or outwards) were cor- rected by one of these muscles being supplied by the sixth, and the other by the third nerve. So with the oblique muscles: the simplest and easiest actions would be through branches of the corresponding nerves, acting similarly as symmetrical muscles; but the necessary movements of the two eyes require the contraction of the superior oblique of one side, to be asso- ciated with the contraction of the inferior oblique, and the re- laxation of the superior oblique, of the opposite side. For this, the fourth nerve of one side is made to act with a branch of 1 It is sometimes said that the external recti cannot be put in action simultaneously: yet they are so when the eyes, having been both di- rected inwards, are restored to the position which they have in looking straight forwards. 428 THE NERVOUS SYSTEM. the third nerve of the other; as if thus the tendency to simul- taneous action through the similar nerves of the two sides were prevented. At any rate, the rule of distribution of nerves here seems to be, that when in frequent and necessary move- ments any muscle has to act with the antagonist of its fellow on the opposite side, it and its fellow’s antagonist are supplied from different nerves. Physiology of the Fifth or Trigeminal Nerve. The fifth or trigeminal nerve resembles, as already stated, the spinal nerves, in that its branches are derived through two roots; namely, the larger or sensitive, in connection with which is the Gasserian ganglion, and the smaller or motor root, which has no ganglion, and which passes under the gan- glion of the sensitive root to join the third branch or division which issues from it. The first and second divisions of the nerve, which arise wholly from the larger root, are purely sensitive. The third division being joined, as before said, by the motor root of the nerve, is of course both motor and sen- sitive. Through the branches of the greater or ganglionic portion of the fifth nerve, all the anterior and antero-lateral parts of the face and head, with the exception of the skin of the parotid region (which derives branches from the cervical spinal nerves), acquire common sensibility ; and among these parts may be in- cluded the organs of special sense, from which common sensa- tions are conveyed through the fifth nerve, and their peculiar sensation through their several nerves of special sense. The muscles, also, of the face and lower jaw acquire muscular sen- sibility through the filaments of the ganglionic portion of the fifth nerve distributed to them with their proper motor nerves. Through branches of the lesser or non-ganglionic portion of the fifth, the muscles of mastication, namely, the temporal, masseter, two pterygoid, anterior part of the digastric, and mylo-hyoid, derive their motor nerves. The motor function of these branches is proved by the violent contraction of all the muscles of mastication in experimental irritation of the third, or inferior maxillary, division of the nerve; by paralysis of the same muscles, when it is divided or disorganized, or from any reason deprived of power; and by the retention of the power of these muscles, when all those supplied by the facial nerve lose their power through paralysis of that nerve. The last instance proves best, that though the buccinator mus- cle gives passage to, and receives some filaments from, a buccal branch of the inferior division of the fifth nerve, yet it derives THE FIFTH NERVE. 429 its motor power from the facial, for it is paralyzed together with the other muscles that are supplied by the facial, but retains its power when the other muscles of mastication are paralyzed. Whether, however, the branch of the fifth nerve which is supplied to the buccinator muscle is entirely sensi- tive, or in part motor also, must remain for the present doubt- ful. From the fact that this muscle, besides its other func- tions, acts in concert or harmony with the muscles of mastica- tion, in keeping the food between the teeth, it might be sup- posed from analogy, that it would have a motor branch from the same nerve that supplies them. There can be no doubt, however, that the so-called buccal branch of the fifth, is, in the main, sensitive; although it is not quite certain that it may not give a few motor filaments to the buccinator muscle. The sensitive function of the branches of the greater divi- sion of the fifth nerve is proved by all the usual evidences, such as their distribution in parts that are sensitive and not capable of muscular contraction, the exceeding sensibility of some of these parts, their loss of sensation when the nerve is paralyzed or divided, the pain without convulsions produced by morbid or experimental irritation of the trunk or branches of the nerve, and the analogy of this portion of the fifth to the posterior root of the spinal nerve. But although formed of sensitive filaments exclusively, the branches of the greater or ganglionic portion of the fifth nerve exercise a manifold influence on the movements of the mus- cles of the head and face, and other parts in which they are distributed. They do so, in the first place, by providing the muscles themselves with that sensibility without which the mind, being unconscious of their position and state, cannot voluntarily exercise them. It is, probably, for conferring this sensibility on the muscles, that the branches of the fifth nerve communicate so frequently with those of the facial and hypoglossal, and the nerves of the muscles of the eye; and it is because of the loss of this sensibility that when the fifth nerve is divided, animals are always slow and awkward in the movement of the muscles of the face and head, or hold them still, or guide their movements by the sight of the objects towards which they wish to move. Again, the fifth nerve has an indirect influence on the mus- cular movements, by conveying sensations of the state and position of the skin and other parts: which the mind perceiv- ing, is enabled to determine appropriate acts. Thus, when the fifth nerve or its infra-orbital branch is divided, the move- ments of the lips in feeding may cease, or be imperfect; a fact which led Sir Charles Bell into one of the verv few errors of his 430 THE NERVOUS SYSTEM. physiology of the nerves. He supposed that the motion of the upper lip, in grasping food, depended directly on the infra-orbital nerve; for he found that, after he had divided that nerve on both sides in an ass, it no longer seized the food with its lips, but merely pressed them against the ground, and used the tongue for the prehension of the food. Mr. Mayo cor- rected this error. He found, indeed, that after the infra-or- bital nerve had been divided, the animal did not seize its food with the lip, and could not use it well during mastication, but that it could open the lips. He, therefore, justly attributed the phenomena in Sir C. Bell’s experiments to the loss of sen- sation in the lips ; the animal not being able to feel the food, and, therefore, although it had the power to seize it, not know- ing how or where to use that power. Lastly, the fifth nerve has an intimate connection with muscular movements through the many reflex acts of muscles of which it is the necessary excitant. Hence, when it is divided, and can no longer convey impressions to the nervous centres to be thence reflected, the irritation of the conjunctiva produces no closure of the eye, the mechanical irritation of the nose excites no sneezing, that of the tongue no flowing of saliva ; and although tears and saliva may flow naturally, their afflux is not increased by the mechanical or chemical or other stimuli, to the indirect or reflected influence of which it is liable in the perfect state of this nerve. The fifth nerve, through its ciliary branches and the branch which forms the long root of the ciliary or ophthalmic gan- glion, exercises also some influence on the movement of the iris. When the trunk of the ophthalmic portion is divided, the pupil becomes, according to Valentin, contracted in men and rabbits, and dilated in cats and dogs .; but in all cases, becomes immovable, even under all the varieties of the stimu- lus of light. How the fifth nerve thus affects the iris is unex- plained ; the same effects are produced by destruction of the superior cervical ganglion of the sympathetic, so that, possibly, they are due to the injury of those filaments of the sympathetic which, after joining the trunk of the fifth, at and beyond the Gasserian ganglion, proceed with the branches of its oph- thalmic division to the iris ; or, as Dr. B. Hall ingeniously suggests, the influence of the fifth nerve on the movements of the iris may be ascribed to the affection of vision in conse- quence of the disturbed circulation or nutrition in the retina, when the normal influence of the fifth nerve and ciliary gan- glion is disturbed. In such disturbance, increased circulation making the retina more irritable might induce extreme con- traction of the iris ; or, under moderate stimulus of light, pro- THE FIFTH NERVE. 431 ducing partial blindness, might induce dilatation : but it does not appear why, if this be the true explanation, the iris should in either case be immovable and unaffected by the various degrees of light. Furthermore, the morbid effects which division of the fifth nerve produces in the organs of special sense, make it prob- able that, in the normal state, the fifth nerve exercises some indirect influence on all these organs or their functions. Thus, after such division, within a period varying from twenty-four hours to a week, the cornea begins to be opaque ; then it grows completely white; a low destructive inflammatory process en- sues in the conjunctiva, sclerotica, and interior parts of the eye; and within one or a few weeks, the whole eye may be quite disorganized, and the cornea may slough or be penetrated by a large ulcer. The sense of smell (and not merely that of mechanical irritation of the nose), may be at the same time lost, or gravely impaired ; so may the hearing, and commonly, whenever the fifth nerve is paralyzed, the tongue loses the sense of taste in its anterior and lateral parts, i. e., in the por- tion in which the lingual or gustatory branch of the inferior maxillary division of the fifth is distributed.1 The loss of the sense of taste is no doubt chiefly due to the lingual branch of the fifth nerve being a nerve of special sense; partly, also, perhaps, it is due to the fact that this branch supplies, in the anterior and lateral parts of the tongue, a nec- essary condition for the proper nutrition of that part. But, deferring this question until the glosso-pharyngeal nerve is to be considered, it may be observed that in some brief time after complete paralysis or division of the'fifth nerve, the power of all the organs of the special senses may be lost; they may lose not merely their sensibility to common impressions, for which they all depend directly on the fifth nerve, but also their sen- sibility to the several peculiar impressions for the reception and conduction of which they are purposely constructed and sup- plied with special nerves besides the fifth. The facts observed in these cases2 can, perhaps, be only explained by the influence which the fifth nerve exercises on the nutritive processes in the organs of the special senses. It is not unreasonable to believe, that, in paralysis of the fifth nerve, their tissues may be the 1 That complete paralysis of the fifth nerve may, however, be un- accompanied, at least, for a considerable period, by injury to the or- gans of special sense, with the exception of that portion of the tongue which is supplied by its gustatory branch, is well illustrated by a valu- able case lately recorded by Dr. Althaus. 2 Two of the best cases are published, with analyses of others, by Mr. Dixon, in the Medico-Chirurgical Transactions, vol. xxviii. 432 THE NERVOUS SYSTEM. seats of such changes as are seen in the laxity, the vascular congestion, oedema, and other affections of the skin of the face and other tegumentary parts which also accompany the pa- ralysis ; and that these changes, which may appear unimpor- tant when they affect external parts, are sufficient to destroy that refinement of structure by which the organs of the special senses are adapted to their functions. According to Magendie and Longet, destruction of the eye ensues more quickly after division of the trunk of the fifth beyond the Gasserian ganglion, or after division of the oph- thalmic branch, than after division of the roots of the fifth between the brain and the ganglion. Hence it would appear as if the influence on nutrition were conveyed through the fila- ments of the sympathetic, which join the branches of the fifth nerve at and beyond the Gasserian ganglion, rather than through the filaments of the fifth itself; and this is confirmed by experiments in which extirpation of the superior cervical ganglion of the sympathetic produced the same destructive disease of the eye that commonly follows the division of the fifth nerve. And yet, that the filaments of the fifth nerve, as well as those of the sympathetic, may conduct such influence, appears certain from the cases, including that by Mr. Stanley, in which the source of the paralysis of the fifth nerve was near the brain, or at its very origin, before it receives any commu- nication from the sympathetic nerve. The existence of gan- glia of the sympathetic in connection with all the principal divisions of the fifth nerve where it gives off those branches which supply the organs of special sense—for example, the connection of the ophthalmic ganglion with the ophthalmic nerve at the origin of the ciliary nerves; of the spheno-pala- tine ganglion with the superior maxillary division, where it gives its branches to the nose and the palate; of the otic gan- glion with the inferior maxillary near the giving off of fila- ments to the internal ear; and of the submaxillary ganglion with the lingual branch of the fifth—all these connections suggest that a peculiar and probably conjoint influence of the sympathetic and fifth nerves is exercised in the nutrition of the organs of the special senses ; and the results of experiment and disease confirm this, by showing that the nutrition of the organs may be impaired in consequence of impairment of the power of either of the nerves. A possible connection between the fifth nerve and the sense of sight, is shown in cases of no unfrequent occurrence, in which blows or other injuries implicating the frontal nerve as it passes over the brow, are followed by total blindness in the THE FACIAL NERVE. 433 corresponding eye. The blindness appears to be the conse- quence of defective nutrition of the retina; for although, in some cases, it has ensued immediately, as if from concussion of the retina, yet in some it has come on gradually like slowly progressive amaurosis, and in some with inflammatory disor- ganization, followed by atrophy of the whole eye.1 Physiology of the Facial Nerve. The facial, or portio dura of the seventh pair of nerves, is the motor nerve of all the muscles of the face, including the platysma, but not including any of the muscles of mastication already enumerated (p. 428); it supplies, also, the parotid gland, and through the connection of its trunk with the Vidian nerve, by the petrosal nerves, some of the muscles of the soft palate, most probably the levator palati and azygos uvulae ; by its tympanic branches it supplies the stapedius and laxator tympani, and, through the otic ganglion, the ten- sor tympani; through the chorda tympani it sends branches to the submaxillary gland and to the lingualis and some other muscular fibres of the tongue; and by branches given off be- fore it comes upon the face, it supplies the muscles of the external ear, the posterior part of the digastricus, and the stylo-hyoideus. To the greater number of the muscles to which it is dis- tributed it is the sole motor nerve. No pain is produced by irritating it near its origin (Valentin), and the indications of pain which are elicited when any of its branches are irritated may be explained by the abundant communications which, in all parts of its course, it forms with sensitive nerves, whose filaments being mingled with its own are the true source of the pain. Besides its motor influence, the facial is also, by means of the fibres which are supplied to the submaxillary and parotid glands, a so-called secretory nerve (p. 377). For through the last-named branches impressions may be conveyed which excite increased secretion of saliva. For example, if, in a dog, the submaxillary gland be exposed, and the chorda tympani be divided, it will be seen that on stimulating the distal end of the nerve by a weak electric current, the gland becomes ex- ceedingly vascular, and saliva is secreted in largely increased amount. Under ordinary circumstances of increased secretion of saliva by the submaxillary gland, as from the presence of 1 Such a case is recorded by Snabilie in the Nederlandsch Lancet, August, 1846. 434 THE NERVOUS SYSTEM. food in the mouth, the stimulus is conveyed by the same channel, the chorda tympani being the efferent nerve in a reflex action, iu which the afferent fibres are branches of the fifth and glosso-pharyngeal nerves. When the facial nerve is divided, or in any other way par- alyzed, the loss of power in the muscles which it supplies, while proving the nature and extent of its functions, displays also the necessity of its perfection for the perfect exercise of all the organs of the special senses. Thus, in paralysis of the facial nerve, the orbicularis palpebrarum being powerless, the eye remains open through the unbalanced action of the levator palpebrae; and the conjunctiva, thus continually exposed to the air and the contact of dust, is liable to repeated inflamma- tion, which may end in thickening and opacity of both its own tissue and that of the cornea. These changes, however, ensue much more slowdy than those which follow paralysis of the fifth nerve, and never bear the same destructive character. In paralysis of the facial nerve, also, tears are apt to flow con- stantly over the face, apparently because of the paralysis of the tensor tarsi muscle, and the loss of the proper direction and form of the orifices of the puncta lachrymalia. From these circumstances, the sense of sight is impaired. The sense of hearing, also, is impaired in many cases of paralysis of the facial nerve; not only in such as are instances of simultaneous disease in the auditory nerves, but in such as may be explained by the loss of power in the muscles of the internal ear. The sense of smell is commonly at the same time impaired through the inability to draw air briskly to- wards the upper part of the nasal cavities, in which part alone the olfactory nerve is distributed; because, to draw the air per- fectly in this direction, the action of the dilators and com- pressors of the nostrils should be perfect. Lastly, the sense of taste is impaired, or maybe wholly lost, in paralysis of the facial nerve, provided the source of the paralysis be in some part of the nerve between its origin and the giving off of the chorda tympani. This result, wThich has been observed in many instances of disease of the facial nerve in man, appears explicable only by the influence which, through the chorda tympani, it exercises on the movements of the lingualis and the adjacent muscular fibres of the tongue; and, according to some, or probably in some animals, on the move- ments of the stylo-glossus. We may therefore suppose that the accurate movement of these muscles in the tongue is in some way connected with the proper exercise of taste. Together with these effects of paralysis of the facial nerve the muscles of the face being all powerless, the countenance THE GLOSSO-PHARYNGEAL NERVE. 435 acquires on the paralyzed side a characteristic, vacant look, from the absence of all expression: the angle of the mouth is lower, and the paralyzed half of the mouth looks longer than that on the other side : the eye has an unmeaning stare. All these peculiarities increase, the longer the paralysis lasts; and their appearance is exaggerated when at any time the muscles of the opposite side of the face are made active in any expres- sion, or in any of their ordinary functions. In an attempt to blow or whistle, one side of the mouth and cheek acts prop- erly, but the other side is motionless, or flaps loosely at the impulse of the expired air; so in trying to suck, one side only of the mouth acts; in feeding, the lips and cheek are powerless, and food lodges between the cheek and gum. As a nerve of expression, the seventh nerve must not be considered independent of the fifth nerve, with which it forms so many communications ; for, although it is through the facial nerve alone that all the muscles of the face are put into their naturally expressive actions, yet the power which the mind has of suppressing or controlling all these expressions can only be exercised by voluntary and well-educated actions directed through the facial nerve with the guidance of the knowledge of the state and position of every muscle, and this knowledge is acquired only through the fifth nerve, which confers sensi- bility on the muscles, and appears, for this purpose, to be more abundantly supplied to the muscles of the face than any other sensitive nerve is to those of other parts. Physiology of the Glosso-Pharyngeal Nerve. The glosso-pharyngeal nerves (4, Fig. 151), in the enume- ration of the cerebral nerves by numbers according to the po- sition in which they leave the cranium, are considered as di- visions of the eighth pair of nerves, in which term are included with them the pneumogastric and accessory nerves. But the union of the nerves under one term is inconvenient, although in some parts the glosso-pharyngeal and pneumogastric are so combined in their distribution that it is impossible to separate them in either anatomy or physiology. The glosso-pharyngeal nerve appears to give filaments through its tympanic branch (Jacobson’s nerve), to the fenestra ovalis, and fenestra rotunda, and the Eustachian tube; also, to the carotid plexus, and, through the petrosal nerve, to the spheno-palatine ganglion. After communicating, either within or without the cranium, with the pneumogastric, and soon after it leaves the cranium, with the sympathetic, digastric branch of the facial, and the accessory nerve, the glosso-pharyngeal 436 THE NERVOUS SYSTEM. nerve parts into the two principal divisions indicated by its name, and supplies the mucous membrane of the posterior and lateral walls of the upper part of the pharynx, the Eustachian tube, the arches of the palate, the tonsils and their mucous membrane, and the tongue as far forwards as the foramen csecum in the middle line, and to near the tip at the sides and inferior part. Some experiments make it probable that the glosso-pharyn- geal nerve contains, even at its origin, some motor fibres, to- gether Avith those of common sensation and the sense of taste. Whatever motor influence, however, is conveyed directly through the branches of the glosso-pharyngeal, may be as- cribed to the filaments of the pneumogastric or accessory that are mingled with it. The experiments of Dr. John Reid, confirming those of Panizza and Longet, tend to the same conclusions; and their results probably express nearly all the truth regarding the part of the glosso-pharyngeal nerve which is distributed to the pharynx. These results were that,—1. Pain was produced when the nerve, particularly its pharyngeal branch, Avas irri- tated. 2. Irritation of the nerve before the origin of its pharyngeal, or of any of these branches, gave rise to extensive muscular motions of the throat and lower part of the face: but w7hen the nerve was divided, these motions were excited by irritating the upper or cranial portion, Avhile irritation of the lower end, or that in connection Avith the muscles, Avas folloAved by no movement; so that these motions must have depended on a reflex influence transmitted to the muscles through other nerves by the intervention of the nervous centres. 3. When the functions of the brain and medulla oblongata Avere arrested by poisoning the animal Avith prussic acid, irritation of the glosso-pharyngeal nerve, before it was joined by any branches of the pneumogastric, gave rise to no movements of the muscles of the pharynx or other parts to which it Avas distributed; while, on irritating the pharyngeal branch of the pneumogastric, or the glosso-pharyngeal nerve, after it had received the com- municating branches just alluded to, vigorous movements of all the pharyngeal muscles and of the upper part of the oesoph- agus folloAved. The most probable conclusion, therefore, may be that what motor influence the glosso-pharyngeal nerve may seem to exer- cise, is due either to the filaments of the pneumogastric or ac- cessory that are mingled Avith it, or to impressions conveyed through it to the medulla oblongata, and thence reflected to muscles through motor nerves, especially the pneumogastric, accessory, and facial. Thus, the glosso-pharyngeal nerve ex- THE GLOSSO-PHARYNGEAL NERVE. 437 cites, through the medium of the medulla oblongata, the ac- tions of the muscles of deglutition. It is the chief centripetal nerve engaged in these actions; yet not the only one, for, as Dr. John Reid has shown, the acts are scarcely disturbed or retarded when both the glosso-pharyngeal nerves are divided. But besides being thus a nerve of common sensation in the parts which it supplies, and a centripetal nerve through which impressions are conveyed to be reflected to the adjacent muscles, the glosso-pharyngeal is also a nerve of special sensation ; being the gustatory nerve, or nerve of taste, in all the parts of the tongue to which it is distributed. After many discussions, the question, which is the nerve of taste?—the lingual branch of the fifth, or the glosso-pharyngeal ?—may be most probably answered by stating that they are both nerves of this special function. For very numerous experiments and cases have shown that when the trunk of the fifth nerve or its lingual branch is paralyzed or divided, the sense of taste is completely lost in the superior surface of the anterior and lateral parts of the tongue. The loss is instantaneous after division of the nerve; and, therefore, cannot be ascribed to the defective nu- trition of the part, though to this, perhaps, may be ascribed the more complete and general loss of the sense of taste when the whole of the fifth nerve has been paralyzed. But, on the other hand, while the loss of taste in the part of the tongue to which the lingual branch of the fifth nerve is distributed proves that to be a gustatory nerve, the fact that the sense of taste is at the same time retained in the posterior and postero-lateral parts of the tongue, and in the soft palate and its anterior ai'ch, to which (and to some parts of which exclusively) the glosso-pharyngeal is distributed, proves that this also must be a gustatory nerve. In a female patient at St. Bartholomew’s Hospital, the left lingual branch of the fifth nerve was divided in removing a portion of the lower jaw : she lost both common sensation and the sensation of taste in the tip and the anterior parts of the left half of the tongue, but retained both in all the rest of the tongue. M. Lisfranc and others have noted similar cases; and the phenomena in them are so simple and clear, that there can scarcely be any fallacy in the conclusion that the lingual branches of both the fifth and the glosso-pharyngeal nerves are gustatory nerves in the parts of the tongue which they severally supply. This conclusion is confirmed by some experiments on ani- mals, and, perhaps, more satisfactorily as concerns the sense of taste in man, by observation of the parts of the tongue and fauces, in which the sense is most acute. According to Valen- tin’s experiments made on thirty students, the parts of the 438 THE NERVOUS SYSTEM. tongue from which the clearest sensations of taste are derived,, are the base, as far as the foramen caecum and lines diverging forwards on each side from it; the posterior palatine arches down to the epiglottis; the tonsils and upper part of the pharynx over the root of the tongue. These are the seats of the distribution of the glosso-pharyngeal nerve. The anterior dorsal surface, and a portion of the anterior and inferior sur- face of the tongue, in which the lingual branch of the fifth is alone distributed, conveyed no sense of taste in the majority of the subjects of Valentin’s experiments ; but even if this were generally the case, it would not invalidate the conclusion that, in those who have the sense of taste in the anterior and upper part of the tongue, the lingual branch of the fifth is the nerve by which it is exercised. Physiology of the Pneumogastric Nerve. The pneumogastric nerve, nervus vagus, or par vagum (Fig. 151), has, of all the cranial and spinal nerves, the most various distribution, and influences the most various functions, either through its own filaments, or those which, derived from other nerves, are mingled in its branches. The parts supplied by the branches of the pneumogastric nerve are as follows : By its pharyngeal branches, which enter the pharyngeal plexus, a large portion of the mucous mem- brane, and, probably, all the muscles of the pharynx; by the superior laryngeal nerve, the mucous membrane of the under surface of the epiglottis, the glottis, and the greater part of the larynx, and the crico-thyroid muscle; by the inferior laryngeal nerve, the mucous membrane and muscular fibres of the trachea, the lower part of the pharynx and larynx, and all the muscles of the larynx, except the crico-thyroid ; by oesophageal branches, the mucous membrane and muscular coats of the oesophagus. Moreover, the branches of the pneu- mogastric uerve form a large portion of the supply of nerves to the heart and the great arteries through the cardiac nerves, derived from both the trunk and the recurrent nerve ; to the lungs, through both the anterior and the posterior pulmonary plexuses ; and to the stomach, by its terminal branches pass- ing over the walls of that organ; while branches are also dis- tributed to the liver and to the spleen. From the parts thus enumerated as receiving nerves from the pneumogastric, it might be assumed that this latter is a nerve of mixed function, both sensitive and motor. Experi- ments prove that it is so from its origin, for the irritation of its roots, even within the cranial cavity, produces both pain T H E P N E U M O G A S T R IC NEEV E. 439 and convulsive movements of the larynx and pharynx ; and when it is divided within the skull, the same movements follow the irritation of the distal portion, showing that they are not due to reflex action. Similar experiments prove that, through its whole course, it contains both sensitive and motor fibres, but after it has emerged from the skull, and, in some instances even sooner, it enters into so many anastomoses that it is hard to say whether the filaments it contains are, from their origin, its own, or whether they are derived from other nerves com- bining with it. This is particularly the case with the filaments of the sympathetic nerve, which are abundantly added to nearly all the branches of the pneumogastric. The likeness to the sympathetic which it thus acquires is further increased by its containing many filaments derived, not from the brain, but from its own peti’osal ganglia, in which filaments originate, in the same manner as in the ganglia of the sympathetic, so abundantly that the trunk of the nerve is visibly larger below the ganglia than above them (Bidder and Volkmann). Next to the sympathetic nerve, that which most importantly commu- nicates with the pneumogastric is the accessory nerve, whose internal branch joins its trunk, and is lost in it. Properly, therefore, the pneumogastric might be regarded as a triple-mixed nerve, having out of its own sources, motor, sensitive, and sympathetic or ganglionic nerve-fibres ; and to this natural complexity it adds that which it derives from the reception of filaments from the sympathetic, accessory, and cervical nerves, and, probably, the glosso-pharyngeal and facial. The most probable account of the particular functions which the branches of the jmeumogastric nerve discharge in the sev- eral parts to which they are distributed, may be drawn from Dr. John Reid’s experiments on dogs. They show that: 1. The pharyngeal branch is the principal, if not the sole motor nerve of the pharynx and soft palate, and is most probably wholly motor ; a part of its motor fibres being derived from the internal branch of the accessory nerve. 2. The inferior laryngeal nerve is the motor nerve of the larynx, irritation of it producing vigorous movements of the arytenoid cartilages ; while irritation of the superior laryngeal nerve gives rise to no action in any of the muscles attached to the arytenoid carti- lages, but merely to contractions of the crico-thyroid muscle. 3. The superior laryngeal nerve is chiefly sensitive; the in- ferior, for the most part, motor; for division of the recurrent nerves puts an end to the motions of the glottis, but without lessening the sensibility of the mucous membrane ; and division of the superior laryngeal nerves leaves the movements of the 440 THE NERVOUS SYSTEM. glottis unaffected, but deprives it of its sensibility. 4. The motions of the oesophagus are dependent on motor fibres of the pneumogastric, and are probably excited by impressions made upon sensitive fibres of the same ; for irritation of its trunk excites motions of the oesophagus, which extend over the cardiac portions of the stomach ; and division of the trunk paralyzes the oesophagus, which then becomes distended with the food. 5. The cardiac branches of the pneumogastric nerve are one, but not the sole channel through which the in- fluence of the central organs and of mental emotions is trans- mitted to the heart. 6. The pulmonary branches form the principal, but not the sole channel by which the impressions on the mucous surface of the lungs that excite respiration, are transmitted to the medulla oblongata. Dr. Reid was unable to determine whether they contain motor fibres. From these results, and by referring to what has been said in former chapters, the share which the pneumogastric nerve takes in the functions of the several parts to which it sends branches may be understood : 1. In deglutition, the motions of the pharynx are of the reflex kind. The stimulus of the food or other substance to be swallowed, acting on the filaments of the glosso-pharyngeal nerve as well as the filaments of the superior laryngeal given to the pharynx, and of some other nerves, perhaps, with which these communicate, is conducted to the medulla oblongata, whence it is reflected, chiefly through the pneumogastric, to the muscles of the pharynx. 2. In the functions of the larynx, the sensitive filaments of the pneumogastric supply that acute sensibility by which the glottis is guarded against the ingress of foreign bodies, or of irrespirable gases. The contact of these stimulates the fila- ments of the superior laryngeal branch of the pneumogastric ; and the impression conveyed to the medulla oblongata, whe- ther it produce sensation or not, is reflected to the filaments of the recurrent or inferior laryngeal branch, and excites con- traction of the muscles that close the glottis. Both these branches of the pneumogastric co-operate also in the produc- tion and regulation of the voice; the inferior laryngeal deter- mining the contraction of the muscles that vary the tension of the vocal cords, and the superior laryngeal conveying to the mind the sensations of the state of these muscles necessary for their continuous guidance. And both the branches co-operate in the actions of the larynx in the ordinary slight dilatation and contraction of the glottis in the acts of expiration and inspiration, and more evidently in those of coughing and other forcible respiratory movements (p. 182). THE PNEUMOGASTRIC NERVE. 441 3. It is partly through their influence on the sensibility and muscular movements in the larynx, that the pneumogastric nerves exercise so great an influence on the respiratory pro- cess, and that the division of both the nerves is commonly fatal. To determine how death is in these cases produced, has been the object of innumerable, and often contradictory, ex- periments. It is probably produced differently in different cases, and in many is the result of several co-operating causes. Thus, after division of both the nerves, the respiration at once becomes slower, the number of respirations in a given time being commonly diminished to one-half, probably because the pneumogastric nerves are the principal conductors of the im- pression of the necessity of breathing to the medulla oblon- gata. Respiration does not cease; for it is probable that the impression may be conveyed to the medulla oblongata through the sensitive nerves of all parts in which the imperfectly aerated blood flows (see p. 407): yet the respiration being re- tarded, adds to the other injurious effects of division of the nerves. Again, division of both pneumogastric trunks, or of both their recurrent branches, is often very quickly fatal in young animals; but in old animals the division of the recurrent nerve is not generally fatal, and that of both the pneumogastric trunks is not always fatal (J. Reid), and, when it is so, the death ensues slowly. This difference is, probably, because the yielding of the cartilages of the larynx in young animals per- mits the glottis to be closed by the atmospheric pressure in in- spiration, and they are thus quickly suffocated unless trache- otomy be performed (Legallois). In old animals, the rigidity and prominence of the arytenoid cartilages prevent the glottis from being completely closed by the atmospheric pressure; even when all the muscles are paralyzed, a portion at its posterior part remains open, and through this the animal continues to breathe. Yet the diminution of the orifice for respiration may add to the difficulty of maintaining life. In the case of slower death, after division of both the pneu- mogastric nerves, the lungs are commonly found gorged with blood, oedematous, or nearly solid, or with a kind of low pneu- monia, and with their bronchial tubes full of frothy bloody fluid and mucus, changes to which, in general, the death may be proximately ascribed. These changes are due, perhaps in part, to the influence which the pneumogastric nerves exercise on the movements of the air-cells and bronchi; yet, since they are not always produced in one lung when its pneumogastric nerve is divided, they cannot be ascribed wholly to the suspension of organic nervous influence (J. Reid). Rather, they may be 442 THE NERVOUS SYSTEM. ascribed to the hindrance to the passage of blood through the lungs, in consequence of the diminished supply of air and the excess of carbonic acid in the air-cells and in the pulmonary capillaries (see p. 187); in part, perhaps, to paralysis of the bloodvessels, leading to congestion; and in part, also, as the experiments of Trail be especially show, they appear due to the passage of food and of the various secretions of the mouth and fauces through the glottis, which, being deprived of its sensi- bility, is no longer stimulated or closed in consequence of their contact. He says, that if the trachea be divided and separated from the oesophagus, or if only the oesophagus be tied, so that no food or secretion from above can pass down the trachea, no degeneration of the tissue of the lungs will follow the division of the pneumogastric nerves. So that, on the whole, death after division of the pneumogastric nerves may be ascribed, when it occurs quickly in young animals, to suffocation through mechanical closure of the paralyzed glottis: and, when it occurs more slowly, to the congestion and pneumonia produced by the diminished supply of air, by paralysis of the bloodvessels, and by the passage of foreign fluids into the bronchi; and ag- gravated by the diminished frequency of respiration, the in- sensibility to the diseased state of the lungs, the diminished aperture of the glottis, and the loss of the due nervous influ- ence upon the process of respiration. 4. Respecting the influence of the pneumogastric nerves on the movements of the oesophagus and stomach, the secretion of gastric fluid, the sensation of hunger, absorption by the stomach, and the action of the heart,1 former pages may be referred to. Cyon and Ludwig have discovered that a remarkable power appears to be exercised on the dilatation of the bloodvessels by a small nerve, which arises, in the rabbit, from the superior laryngeal branch, or from this and the trunk of the pneumo- gastric nerve, and after communicating with filaments of the inferior cervical ganglion proceeds to the heart. If this nerve be divided, and its upper extremity be stimulated by a weak interrupted current, an inhibitory influence is conveyed to the vaso-motor centre in the medulla oblongata (p. 452), so as to cause, by reflex action, dilatation of the principal bloodvessels,- with diminution of the force and frequency of the heart’s action. From the remarkable lowering of the blood-pressure in the vessels, thus produced, this branch of the vagus is called the depressor nerve; and it is presumed, as an afferent nerve of the heart, to be the means of conveying to the vaso-motor 1 See foot-note, p. 453. THE SPINAL ACCESSORY NERVE. 443 centre in the medulla indications of such conditions of the heart as require a lowering of the blood pressure in the vessels; as, for example, when the heart cannot, with sufficient ease, propel blood into the already too full or too tense arteries. Physiology of the Spinal Accessory Nerve. In the preceding pages it is implied that all the motor in- fluence which the pneumogastric nerves exercise, is conveyed through filaments, which, from their origin, belong to them; and this is, perhaps, true. Yet a question, which has been often discussed, may still be entertained, whether a part of the motor filaments that appear to belong to the pneumogastric nerves are not given to them from the accessory nerves ? The principal branch of the accessory nerve, its external branch, supplies the sterno-inastoid and trapezius muscles; and though pain is produced by irritating it, is composed almost exclusively of motor fibres. It might appear very probable, therefore, that the internal branch, which is added to the trunk of the pneumogastric just before the giving off of the pharyngeal branch, is also motor; and that through it the pneumogastric nerve derives part of the motor fibres which it supplies to the muscles enumerated above. And further, since the pneumogastric nerve has a ganglion just above the part at which the internal brauch of the accessory nerve joins its trunk, a close analogy may seem to exist between these two nerves and the spinal nerves Avith their anterior and posterior roots. In this vieAV, Arnold and several later physiologists have regarded the accessory nerve as constituting a motor root of the vagus nerve; and although this view cannot now be maintained, yet it is very probable that the accessory nerve gives some motor filaments to the pneumogastric. For, among the experiments made on this point, many have shoAvn that Avhen the accessory nerve is irritated within the skull, convul- sive movements ensue in some of the muscles of the larynx; all of which, as already stated, are supplied, apparently, by branches of the pneumogastric; and (which is a very signifi- cant fact) Vrolik states that in the chimpa