COLUMBIA LIBRARIES OFFSITE HEALTH SCIENCES STANDARD HX641 37503 QP38.L51 Physiology: the vita RECAP Mit •- Columbia (Hnituer^ttp intlieCitpottolork College ot $i)i>gfciatts; anb burgeons library Digitized by the Internet Archive in 2010 with funding from Open Knowledge Commons (for the Medical Heritage Library project) http://www.archive.org/details/physiologyvitalpOOIeef PHYSIOLOGY : THE VITAL PROCESSES IN HEALTH BY FREDERIC S. LEE, Ph. D. ADJUNCT PEOFESSOB OF PHYSIOLOGY, COLUMBIA COLLEGE, NEW YORK REPRINTED FROM IN SICKNESS AND IN HEALTH COPYKIGHT, 1896, BY D. APPLETON & Co. II. PHYSIOLOGY: THE VITAL PROCESSES IN HEALTH. By FREDERIC S. LEE, Ph. D. INTRODUCTION. The physiology of an organism treats of the healthy working of the organism. It deals with the living, acting body, with what the body does and how it does it. Anatomy can be studied best upon the dead body. Physiology, however, must be studied chiefly upon the living body, since in death the action ceases. The present sketch is devoted particularly to the physiology of man, but we must bear in mind that physiology is a very broad science and has to do with all living things, animals or plants. Only a limited range of observation and experi- mentation is possible upon the living human being, hence the work of the physiologist consists chiefly of work upon animals other than the human species. Human physiology consists largely of careful, well- guarded inferences from the results of such experimental work. The reasonableness of such a method of inference is apparent when one accepts as a fact what is no longer doubted by scientific men — that the human species is both anatomically and physiologic- „ „ , . . ally a form that has been derived from other and lower oj Avoluhon. J species of animals. Such a belief gives a unity and harmony, not otherwise possible, to the facts of biology. Man is physi- ologically interesting in himself ; he is physiologically more interesting when regarded as the latest and most complex product of a long ancestry reaching back through mammals, reptiles, fishes, and a long line of simpler animals, to the most primitive forms. The study of these ani- mals is a study of the steps by which man has arrived at his present stage. Like other animal bodies, the human body is a complicated machine, adapted for doing a great variety of work. The term " vital energy " is often heard — a term which indicates that the living body is something fundamentally different from other machines that are made of iron, or 85 86 PHYSIOLOGY : THE VITAL PROCESSES IN HEALTH. steel, or brass. During the present century, however, it has been proved that all kinds of energy in inorganic nature — such as mechanical work, heat, light, and electricity — are only different forms of a universal energy. Moreover, it has been shown that " vital energy " is not distinct from other kinds, and that the actions of the living body — walking, swimming, flying, speaking, the circulation of the blood, and probably the activities of gland cells, of brain cells, etc. — are performed according to the same mechanical, physical, and chemical laws that apply to inorganic matter. The task of the physiologist consists, then, largely in a study of the me- chanics, physics, and chemistry of the living body. The goal toward which he is pressing is a full understanding of the nature of life itself. It is unnecessary to say that that goal is still' far distant. A great advance that has taken place during the past sixty years is the proof that life is always associated with a certain visible material sub- stance. This substance is called protoplasm. It occurs ro op asm • muscles, glands, skin, brain, bone, nerves, and all the the Physical Basts ' 5 . . ' ' . . ' ' ' , . of Life organs. It is in fact the living substance of all parts of all living bodies. It is always associated with lifeless substance, such as the fluid parts of a body, the mineral parts of bones, the hard substance of teeth, the nails, the hair, and the microscopic lifeless material that permeates all parts of every body. In its simplest form protoplasm is a colourless, jelly-like, nearly transparent substance, some- what resembling raw white of egg. It is a mixture of several chemical substances ; it has a variable composition, but always contains carbon, hydrogen, nitrogen, oxygen, and sulphur. It differs in appearance and composition in different parts of the body, the protoplasm of muscle being identical neither with that of gland cells nor with that of nervous sub- stance. Dissection shows that a body is composed of definite parts, or organs, each of which, as we shall see, has a definite function to perform. Thus the heart, the stomach, the eye, the liver, and the brain dG 11 are or g ans - Each organ has its own peculiar structure, but each is made up of comparatively few structural substances. These substances are called tissues, and each tissue — such as connective tissue, muscle tissue, fat tissue — has its own work to do. Ex- amination of the tissues with the microscope shows that each is composed partly of lifeless substance, usually in small quantity, and chiefly of minute living particles of protoplasm, the cells (Fig. 1). Cells vary greatly in shape and size and in the work that they do, but in any one tissue they are similar in structure and function. The work of a body is the sum of the work of its individual cells, and the fundamental problems in physi- ology lead back to protoplasm and the cells. Every human body consists at first of a single cell within the body of DIVISION AND DIFFERENTIATION OF CELLS. 87 the mother. In growth the single cell divides into two cells, the two into four, the four into eight, and the process continues until in the adult millions of cells exist. Along with the increase in number there occurs a differentiation in form and a division of labour among the various cells, such that some come to perform the various muscular movements, others prepare digestive substances, others remove waste matter from the blood, others control the breathing, still others do the thinking, some are affected Fig. 1. — Typical cells from the human body. Each cell consists of granular or striated protoplasm, and contains a denser protoplasmic mass, the nucleus. The nucleus is round or oval, and appears darker or lighter than the rest of the cell. A, seven pigment cells from retina (Schultze) : B, four epithelium cells from intestine— one swollen with mucus represents a "goblet cell" (Frey); C, unstriped muscle cell (Arnold); D, nerve cell from brain (Gage) ; E, ciliated epithelium cell — the cilia constitute the brushliko upper end (Rosenthal); F, three "prickle-cells" from skin (Robinson); G, three gland cells from liver— the heavy dark lines represent bile ducts (Kolliker). by waves of light, some by waves of sound, and thus we perceive that in this complicated human machine each part has its own work to do, and the division of labour is far-reaching. Protoplasm is said to be irritable or excitable — that is, it is capable of being thrown into activity by proper stimuli. Every kind of cell, or tissue, or organ has its own natural method of stimulation and its own natural response or function. Every vital action is a response to some kind of stimulus. S3 PHYSIOLOGY : THE VITAL PROCESSES IN HEALTH. The leading functions of the body, together with their chief agents, may be grouped as follows : Functions. Organs and other Agents. f Alimentation \ Alimentary canal: consisting of mouth, oesophagus, stomach, j small intestine and large intestine ; liver ; pancreas. ( Blood system : consisting of heart, arteries, capillaries and v t >■ J Circulation.. \ veins; lymphatic system: consisting of lymphatic vessels Nutrition. < [ and lvmphatic g i ands . blood, lymph. Respiration . . Trachea, lungs. Metabolism.. All living cells. Excretion.. . . Lungs, kidneys and accessory organs, skin. Motion All muscles. Co-ordination Brain, spinal cord, nerves, sympathetic nervous system. Activity of special senses., i ^ e > fl 01 'S ans , of sme11 ;. o£ taste ' of touch > of temp«»ture, " J r a j and of muscular sensations. Support Bones, cartilage, connective tissue. p , ( Female: ovaries and accessory organs. Keproauction -J Male . testeg and accessorv organs . CHAPTER I. NUTRITION. Like any other machine in action, the living human body constantly gives off energy to the outside world in the form of heat and mechanical work, and, moreover, its own material is being constantly used up. It requires food to replace these two losses of energy and substance. The story of nutrition is a long one. It tells how the body prepares for its use the food that is given to it (digestion) ; how the prepared food is taken into the distributing apparatus (absorption) ; how it is carried to the living cells (circulation) ; how it is used there for the production of new protoplasm, and how the protoplasm in action is destroyed, leaving waste matters (metabolism) ; and, finally, how the wastes, being harmful to the body, are removed from it (excretion). These subjects will now be considered in detail. Section I. ALIMENTATION. Alimentation consists of the two processes of digestion and absorp- tion. Digestion comprises the chemical and physical changes which the food undergoes by way of preparation for entrance into Alimentation in ,, , »' ij.it- v i a u c , the tissues, and tor use by the living substance. Ab- sorption is the process of the passage of digested food into the blood and lymph, whence it is carried to the tissues. Alimen- tation takes place in the alimentary canal (Fig. 2), which is a long tube THE HUMAN ALIMENTARY CANAL. 89 extending through the body, opening above at the mouth and below at the anus ; its walls consist of muscle with glands, blood-vessels, and lymph- vessels. The successive parts of the canal are known as mouth, pharynx, oesophagus or gullet, stomach, small intestine and large intestine. To it are joined, by ducts, several large glands — namely, the three pairs of sali- vary glands, the pancreas, and the liver ; all of these in their origin are outgrowths from the canal, but in the adult body they lie outside of its walls. The food enters at the mouth, and is propelled along the canal by the contraction of the muscular walls ; the glands secrete — that is, manufacture and pour into the canal various fluids that differ in nature in various parts of the canal, and whose office it is to digest the food ; the blood-vessels and the lymph-vessels, besides bringing blood and lymph to the walls of the canal, receive the digested food and bear it away to all parts of the body ; the indigestible, the undigested, and the innutritions substances in the canal undergo chemical changes, and finally are expelled from the body as excrement. The common foods, though apparently so different in char- acter from each Food Stuffs. , , other, are found by chemical analysis to be mix- tures of a small number of sub- stances known as food stuffs. The most common food stuffs are proteids (the most important con- stituent of all meats, fish and eggs) ; albuminoids (such as gelatin ; they are closely related to proteids, and hereafter will be included with them) ; Fig. 2. — Human alimentary canal. oesophagus; b, stomach: c, cardiac orifice; f/, pylorus ; e. small intestine ; /', biliary duct and gall bladder; g, pancreatic duct; A, i, j, i, large intestine; h, ascending colon; i, trans- verse colon; j\ descending colon; k, rectum. At junction of e and h is ileo-caaeal valve, be- low which is vermiform appendix. (Dalton.) 90 PHYSIOLOGY: THE VITAL PROCESSES IN HEALTH. carbohydrates (such as starch and sugar, abundant in bread, pastry, and vegetables) ; fats (abundant in meats, butter, and milk) ; water and salts (such as table salt and numerous other salts). For a discussion of the characters and value of these various food stuffs the reader is referred to the article in this volume on Hygiene. Water and the salts that are dissolved in the food are absorbed unchanged into the blood. Of the other food stuffs, proteids and carbo- hydrates are chemically changed by the digestive pro- iges ,omn ce the former into closely related bodies called pro- Cteneral. . . teoses, peptones, and derivatives of the peptones, the carbohydrates mainly into a form of sugar called maltose, together with a small quantity of a starch called dextrin. By reason of the unsettled questions regarding the true nature of some of these digested substances, it will suffice hereafter to speak of all the products of proteid digestion simply as peptone, and of all the products of carbohydrate digestion as sugar. Contrary to what is found in most of the food stuffs, peptone and sugar are soluble, and hence when dissolved in the water of the food are fit for absorption. The digestion of the fats consists in part in changing them chemically into fatty acid, glycerin, and soap, and in part in simply breaking them up mechanically into minute droplets, this latter process being called emulsification ; after these changes the fats are ready for absorption. The digestive fluids are manufactured in the glands of the alimentary canal, and are poured out into it. They are five in number — viz., saliva, gastric juice, intestinal juice, pancreatic juice, and bile. Each consists chiefly of water together with a small quantity of dissolved solids, among which, except in the bile, is one or more of a peculiar class of bodies called enzymes or unorganised fer- ments, to which the digestive property of the fluids is due. The enzymes are peculiar chemical compounds, the exact constitution of which is un- known, but, when mixed with the food stuffs, they produce extensive chemical changes in the latter without being in themselves greatly altered. Saliva is produced by the minute glands in the walls of the mouth and the three pairs of large salivary glands, the parotid, the submaxillary, and the sublingual (see The Anatomy of the Human Body). It is very watery, containing less than 0'6 per cent, of solids. Among the latter are mucin, which gives to the saliva its slightly slimy quality, and assists in the swallowing of the food ; and ptyalin, an enzyme that changes starch into sugar. Saliva is produced in small quantity at all times, but more abundantly when food is in the mouth. Gastric juice is produced by the innumerable small glands lying in the walls of the stomach. It is watery, colonrless, sour, and contains three per cent, of solid matter. Its sourness is due to hydrochloric THE DIGESTIVE FLUIDS AND GLANDS. 91 (muriatic) acid. It contains two enzymes — viz., pepsin, which changes proteid to peptone, and rennin, which curdles milk. The glands of the stomach do not act continuously, but are stimulated to activity by the food that has been swallowed. Intestinal juice is secreted by the glands in the walls of the small intestine. It is not abundant, is a yellowish alkaline fluid, and contains at least two kinds of enzymes, one like the ptyalin of saliva, capable of converting starch into sugar, and the other that changes maltose and other complex sugars into dextrose, a sugar of simpler composition. Pancreatic juice is manufactured by the pancreas ; it resembles saliva in appearance, but contains some thirteen per cent, of solids, among which are sodium carbonate and three powerful enzymes. Of these, trypsin, like pepsin, converts proteids into peptones, and, moreover, splits up some of the peptones into other bodies, the reason for which is not quite clear ; amylopsin, like ptyalin, converts starch into sugar ; steapsin splits up fat into fatty acid and glycerin. The fatty acid thus produced, with the sodium carbonate present, forms soaps that are capable of emulsifying fats. Hence pancreatic juice may digest all kinds of food stuffs. Bile is produced by the liver. It is a yellowish or greenish-yellow, somewhat slimy, alkaline fluid, and contains two to three per cent, of solids that are of great variety, but apparently do not include any enzyme. Its secretion goes on continually, six hundred to eight hundred and fifty cubic centimetres (averaging a pint and three quarters) being produced in twenty-four hours. It is stored up in the gall bladder for a time, and passed out into the intestine during the course of digestion. The con- stituents of bile are largely waste products, thrown off by the living sub- stance throughout the body, removed from the blood by the liver cells, and cast out into the intestine for expulsion. But in addition to its value as an excretion, bile is an important agent in alimentation, aiding greatly, in a manner not yet fully explained, the digestion and absorption of the fats. . The glands that manufacture the digestive fluids consist of chemically active secreting cells that are richly supplied with blood and lymph. They have the power of removing from these fluids Digestive Glands. , . ,. ,. ,. „ . -, ,., some components ot the digestive fluids, like water and salts, and other substances from which they manufacture the remaining components. The resulting secretion passes on into the alimentary canal. The intestinal glands, the simplest of all, consist each of a little pocket opening into the intestine and with walls of secreting cells (Fig. 3). In the more complex pancreas, liver, and salivary glands the secreting pockets, or alveoli, are greatly branched, tortuous channels uniting into one tube, the duct, which conveys the secretion to the alimentary canal. The glands are among the most active of all the living parts of the body, 92 PHYSIOLOGY: THE VITAL PROCESSES IN HEALTH. and are carefully controlled and regulated by the nervous system. The digestive ferments may readily be extracted from glands that have been removed from the bodies of dead animals, and may be used for the manu- facture of artificial digestive fluids. With such fluids the processes of digestion of foods may be carried on under observation in vessels in the laboratory. This has been one of the most fruit- ful methods of studying the subject. Gland ex- tracts form the basis of many substances used by the physician for the relief and cure of dyspepsia. Let us now trace in order the events of diges- tion. Food is put into the mouth, where its pres- ence excites the salivary glands to manufacture and pour out upon it the saliva. Digestion in the The food ■ Qr u tQ b Mouth and D „• ' . Stomach. chewed thoroughly, so as finally to divide it and thus to allow the digestive fluids to permeate it readily. The saliva mixes with it, and the chemical changes of digestion begin by the conversion of some of the carbohydrates present into sugar. By muscular action the food is swallowed — that is, is squeezed along through the pharynx and the gullet into the stomach, where it remains for a time varying from a few minutes to a few hours. "Waves of contraction pass over the muscular walls ; the food, being squeezed here and there, is kept in constant slow motion. The gastric glands are stimulated to activity, and the gastric juice oozing forth mixes with the mass of food and converts it into a half-fluid mass called chyme. The acid of the gastric juice puts a stop to further action of the swallowed saliva. Carbohydrates are hence here unchanged, but the proteids are in great part altered to peptones. At intervals the passageway into the small intestine — the pylorus — opens and allows the chyme to flow out into the duodenum. Thus gradually the stomach completes its work, and transfers its contents to the succeed- ing part of the alimentary canal. In the small intestine the processes of muscular action, glandular secretion, and digestion are continued. The food is moved about and passed slowly along. The gastric juice ceases to act, Diqestion in the .,..,.. , ,. . . ' jjjj.ii c „ T , i- intestinal liuce and pancreatic luice are added to the Small Intestine. J r J - - mass, and the liver adds its contribution of bile. Ihe carbohydrates that were unaffected by the saliva are changed into sugar. Fig. 3. — Gland from the human intestine. The gland is dilated at its closed end ; at the other end it opens into the in- testine. Each of the se- creting cells of which the wall consists contains a deeply shaded nucleus. The clear spaces repre- sent cells laden with mu- cus. (Flemming.) PROCESSES OF DIGESTION. 93 The proteids that were unaltered by the gastric juice become peptones, and are even in part decomposed into simpler substances. The fats are for the first time affected, being in small part decomposed into fatty acid and glycerin, in greater part divided mechanically into fine droplets. This em unification is assisted by the soaps that are formed by a union of the fatty acids and the alkalies present in the digestive fluids. Thus all classes of food stuffs are attacked in the small intestine ; no other part of the whole canal forms so active a digestive laboratory as this part. Be- sides the changes already spoken of, due chiefly to the enzymes, the ob- ject of which is to make ready the nutriment for entrance into and use by the protoplasm, fermentative changes of the digested products take place of a destructive character, and due apparently to the agency of living microbes that are always present in the intestine. The significance of this apparent destruction of nutritive substance is not understood. The bacteria are not disease-producing germs; on the contrary, they may pos- sibly be of considerable value to the body, but the role that they play needs investigation. Absorption of the digested food into the blood and lymph is most active in the walls of the small intestine, and by the time the semifluid mass has reached the ileo-csecal valve it is robbed largely of its nutritious components. In the large intestine no new physiological features are added. Feeble digestion, fairly active absorption of digestive products and of water, and marked fermentation by bacteria, are the main r j t ,- events ; and the undigested and the indigestible sub- stances are passed on into the rectum for expulsion from the body. The large intestine appears to be physiologically more impor- tant in herbivorous animals, like cows and sheep, where, in accordance with the enormous quantity of food, it is relatively large. It was doubt- less larger and more important in the ancestors of the human race. But in man it appears to be undergoing degeneration, as is shown especially in the part known as the vermiform appendix. This appendage, probably valuable in the digestive work of the ancestors of man, ' \. seems to be devoid of marked function in man at the Appendix. present time, and is especially prone to inflammatory processes set up by the presence within it of irritating substances brought there by the food. It would seem a blessing if evolution could hasten the disappearance of this apparently useless structure. A word now as to the absorption of the digested food stuffs, and the outline of the story of alimentation will be completed. Within the ali- mentary canal the food is no more a part of the body than if it were upon the outer surface. As has been insisted, digestion within the alimentary canal is merely a preparation of the food for entrance into the actual living substance. The cells that 91 PHYSIOLOGY : THE VITAL PROCESSES IN HEALTH. line the canal are nourished by absorbing food directly from the mass that lies in contact with them. More distant cells require nutriment to be brought to them, and this is one important office of the blood and lymph with which the walls of the alimentary canal are so saturated. Thin-walled blood capillaries and lymph capillaries abound there, and during and after digestion the dissolved food stuffs make their way into them. Absorption takes place very slightly, if at all, in the stomach ; it is at its greatest height in the small intestine, and it is active even throughout the large intestine. The method of absorption is not wholly clear. Probably physical processes play a leading part ; but it is a ques- tion whether the living cells that line the intestine, and through which the food must pass on its way to the capillaries, may not in some manner engage actively in the process. Sugar and fats pass through them un- changed, peptone is mysteriously altered chemically in its passage. This altered peptone, the sugar, and the greater part of the salts and the water go directly into the blood. Fat goes into the lymph and, by way of the lymphatic vessels and the thoracic duct, finally into, the blood system. Thus all nutriment that is not used by the living substance in the wall of the alimentary canal finds its way sooner or later to the blood, the great carrier and distributer of matter and energy. Our next section deals with this circulating mechanism. Section II. CIRCULATION. In an organism that consists of one cell or a few cells, food, when once digested, permeates all parts readily. In larger organisms this is impossible, and hence special mechanisms must exist for the transfer of nutriment from the organs of digestion to the more distant parts. In the growth of all except the simplest animals such a mechanism, some sort of a distributing system, is developed. It is simple Circulation in -, , . ., , , . ,, , „ , , and crude in its beginnings, and in the lower organisms remains always simple and crude. It becomes more complex and perfected as the animal's size increases and his structure becomes more complex, until in the completed circulatory system of the highest animals and man we have an apparatus that is wonderfully adapted to perform its needed work, and responsive in a high degree to the demands of the other physiological systems. The transfer of nutri- ment is not the only important function of the organs of circulation. Of equal value are the transfer of oxygen without which the living substance can not act, and the removal of waste products from the tissues to the organs of excretion. Furthermore, it must be borne in mind that the CIRCULATION IN MAN AND IN ANIMALS. Blood and its Constituents. among other food contains not only material for the manufacture of new protoplasm, but also in a latent form all the energy of which the body makes use ; hence the circulation is the medium for the transference of energy from one part of the body to another. Finally, as we shall discuss later, the 6ame system acts to keep the temperature of the various portions of the body uniform. A review of the circulation comprises a study of the two fluids which serve as carriers of the oxygen, the food, the energy, and the wastes — viz., the blood and the lymph, and a study of the organic mechanisms by which they are made to move. Blood may be regarded as a tissue consisting of a lifeless, slightly yel- lowish liquid, the plasma, and living cells, the corpuscles. The plasma contains in solution the food and the waste matters, hence its composition is complex. Besides water, it contains eight to nine per cent, of solid substances, comprising bodies proteids, sugar, fats, and salts. Some of these con- stituents originate in the di- gested food stuffs. It con- tains also a gas, carbonic acid. The corpuscles are of three kinds, the red, the white or colourless, and the blood-plates, which are also colourless (Fig. 4). The red corpuscles are most numer- ous, it being estimated that in one cubic millimetre of ^ood {yjtts °f a cu °ic inch) there exist five millions in man and about a half million less in woman. The number in the whole body is incon- ceivably great. They are bi- concave round disks about ¥2inr °f an * nCQ * n diameter, and consist of a bit of pro- toplasm coloured by a red- dish pigment called haemo- globin. Haemoglobin gives to the blood its colour, and is one of its most important constituents by reason of having a special attraction for oxygen, the blood in its course passes through the lungs, its haemoglobin absorbs oxygen from the air that has been breathed in, and holds it until the gas Fig. 4.- Ked and white corpuscles of the blood, magnified. .•(, moderately magnified : the red corpuscles are seen lying in rouleaux ; at a and a are seen two white corpuscles ; J3, red corpuscles much more highly magnified, seen in face ; G, ditto, seen in profile ; D, ditto, in rouleaux, rather more highly magnified; J?, a red corpuscle swollen into a sphere by imbibition of water; F, a white corpuscle magnified same as B\ 67, ditto, throw- ing out some blunt processes; A', ditto, treated with acetic acid, and showing nucleus, magnified same as p ; 7/, red corpuscles puckered or crenate all over ; 7, ditto at the edge only. (Huxley.) When 96 PHYSIOLOGY : THE VITAL PROCESSES IN HEALTH. is required by the tissues. The task of the red corpuscles is hence that of carriers of oxygen. In cases of murder or other crimes, where a stain is suspected to be caused by blood, an examination with the microscope, by revealing the red corpuscles, will decide the question. Even if the corpuscles have been destroyed, the presence of haemoglobin can easily be detected by proper chemical methods. Human red blood-corpuscles may be distinguished readily from those of fishes, frogs, reptiles, or birds, but not conclusively from those of higher animals except the camel. Human hasmoglobin is not distinguishable from that of other animals. The white corpuscles are colourless cells of somewhat irregular shape, roughly spherical, about a4 1 00 of an inch in diameter. They are much fewer in number than the red corpuscles. They have the peculiar power of creeping about through the walls of the blood capillaries, and in among the cells of the tissues throughout the body. Their specific work is not fully known. It has been thought that they may be of special benefit to the body, when attacked by germ diseases, by absorbing the bacterial germs into their substance and destroying them. The blood-plates were discovered only recently. They are minute, colourless, spherical, or elliptical bodies, that go to pieces very easily. This happens especially when blood is shed, in which case they seem to aid in the clotting of the blood. Their office within the body is unknown. The quantity of blood in a healthy man of one hundred and fifty ponnds is about five and a half quarts. "When blood is shed it has the peculiar property of clotting or thicken- ing into a jelly-like mass. The special value of this property lies in its use in stopping the loss of blood from wounds ; if it were not for this, the slightest injury to the skin might result in bleeding to death. Clot- ting consists in the formation irom fibrinogen, one of the proteids exist- ing in the plasma, of an insoluble substance called fibrin. Fibrin exists in the clot in the form of fine whitish threads that extend in all directions and form a spongy network. This holds in its meshes the corpuscles, and the whole forms an effectual plug for the wound. By the spontane- ous shrinking of the fibrin threads a yellowish fluid, that is really plasma minus the fibrin and is called serum, is squeezed out. Lymph is a colourless fluid, occurring partly in the lymphatic vessels and partly in the spaces between the cells of the tissues. It thus comes more closely into contact with the living protoplasm than the blood, the latter never leaving its vessels. It is much like blood in composition, but lacks the red corpuscles and the blood-plates. After a meal of fat the lymph in the lymphatics of the intestine is loaded with fat droplets, and is pure white in colour like milk, CIRCULATORY ORGANS OF THE BLOOD. 97 Circulation of the Blood. Th.V. mr. the whiteness of which is due to the contained droplets of hutter. Such lymph is called chyle, and the lymphatics in that region are known as laoteals. The blood is moved through the blood-vessels by the contractions of the heart. The motion was thought formerly to be a sluggish oozing from the heart to the tissues. But in 1628 it was shown by the Englishman, William Harvey, physician to Charles I., that every particle of blood makes a complete circuit of the blood-vessels, and returns ultimately to its place of starting ; that the blood moves, so to speak, in a circle, and since Harvey's time the movement has been spoken of as the circulation of the blood. The circulatory organs of the blood system consist of the heart, the arteries, the capillaries, and the veins. The heart pumps the blood into the main arteries ; along these the liquid courses toward the tissues, passing into smaller and smaller arteries, and finally into the minute capillaries ; in the capillaries it permeates the tissues and courses among the living cells; from the capillaries it passes into the small veins ; these unite into larger and larger vessels, and finally, by a few large venous trunks, the blood returns to the heart again. For the structure and arrangement of the circu- latory organs, and for the general plan of the circulation, the reader is referred to the article on The Anatomy of the Hu- man Body, Sections IV and V, and to the accompanying figure (Fig. 5). In order to make the complete circuit of the circulatory organs, any particle of blood must traverse two sets of arteries, capillaries, and veins, and all four cham- bers of the heart. The path from the Fig. 5. — Diagram of the heart and vessels, with the course of the cir- culation, viewed from behind, so that the proper left of the ob- server corresponds with the left side of the heart in the diagram. L.A., left auricle ; L. V., left ventricle ; Ao., aorta ; A 1 , arteries to the upper part of the body ; A 2 , arteries to the lower part of the body ; H.A., hepatic artery, which supplies the liver with part of its blood ; V 1 , veins of the up- per part of the body ; V 2 , veins of the lower part of the body ; V.P., vena left ventricle through the blood-vessels of portte ; // V., hepatic vein ; V. C.I., in- ferior vena cava; V.C.S., superior vena cava; R.A., right auricle; B. V., right ventricle ; PA., pulmonary artery ; Lg., lung ; P. V., pulmonary vein ; Let., lacteals ; Ly., lymphatics ; Tk. D., thoracic duct ; At., alimentary canal; Lr., liver. The arrows indicate the course of the blood, lymph, and chyle. The vessels which contain arterial blood have dark contours, while those which carry venous blood have light contours. (Huxley.) 98 PHYSIOLOGY : THE VITAL PROCESSES IN HEALTH. the body, except the lungs and to the right auricle, is called the " greater " or " systemic " circulation ; the path from the right ventricle through the vessels of the lungs to the left auricle is called the " lesser " or " pulmo- nary" circulation. The significance of this double circulation lies in the facts that in the capillaries of the tissues of the body the blood delivers up food and oxygen to the cells and receives waste products from them ; and that in those of the lungs the gaseous waste, carbonic acid, is thrown off into the air to be expelled in the breath, while the blood is in return charged highly with oxygen. The left side of the heart hence carries purified blood charged with oxygen, the right side impure blood charged with carbonic acid and other waste matters. These latter wastes are removed from the blood capillaries and from the body by the kidneys and the skin. As has been already said, the food is received into the cir- culation partly in the capillaries of the intestinal wall and partly directly into the veins from the lymphatic ducts. Thus all exchange between blood and living protoplasm takes place through the capillary walls. In harmony with this function these walls consist of a thin membrane made up of flat cells joining Structure one ano t;] ier edge to edge, and allowing a ready diffusion and Function . . .. . , , , n . .„. ,-,, , Al .,, of Vessels °* the liquid blood plasma (rig. b). As the capillaries pass on the one hand into the arteries, and on the other into the veins, the walls become thickened by the addition of a layer of muscle outside outside of the of the muscle. lining membrane and one of connective tissue Both these layers are thicker in the arteries than in the veins, and in the former the con- nective tissue is highly elastic. Thus the arteries are thick-walled, active, elastic struc- tures, capable of altering their calibre greatly, and thus regulating the amount of blood going to the capillaries. If a particular cap- illary area requires a large quantity of blood, the muscles of the adjoining arteries relax and the arteries dilate ; if less blood is de- sired, the muscles contract and constrict the supplying arteries. The veins are thinner walled and passive, and are in brief drainage- tubes for the capillaries and the tissues. The arteries are thus physiologically more interesting than the veins. If an artery be cut, the thickness and stiffness of its walls cause it to stand wide open, and the blood gushes freely out ; if a vein be severed, its walls collapse, and the blood hindered in its flow may clot more readily and less loss may result. Hence wounding an artery is usually a much more serious and dangerous affair than wounding a vein. In this Fig. 6. — Capillaky circulation in the web of the frog's foot. THE HEART AND BLOOD-VESSELS. 99 connection it is interesting to recall the fact that as a rule the arteries lie much farther from the surface of the body than the veins — a most valuable adaptation of Nature. Exceptions to the rule are the radial artery which comes near to the surface at the wrist, the artery at the temple, and a few others. But the veins have one mechan- ism peculiar to themselves — that is, the curi- ous and very numerous little pouch-like valves that project into the tubes and pre- vent any back-flow of blood toward the Cap- Fio. 7.— Diagrammatic section or .,, . i . n 1 VEINS WITH VALVES. manes when any influence, sncli as pressure ... B , ... .,„„, . J r In the upper figure the blood is sup- on the skin, tends to hinder the venous flow posed to be flowing in the nor- /T ^. „. -^ .- ,, . ,, . ,, , mal direction from C (capillary) (.big. 7). Even it the veins are thin -walled to H (heart); in the lower figure .... .1 . , , n ,1 • pressure upon the surface of the and inactive, their valves do not allow their vein has temporarily forced the circulation to be seriously interfered with "°£ b ( a f u Xy.) and olosed the by extraneous pressure. The heart in its embryonic origin is a simple tubular blood-vessel, and in some of the lower and simpler organisms, such as the worms and the tunicates, it retains its tubular character throughout Physiological ]ife But {n ^ wth of al] hi her anima ] s j nclud . Anatomy of the . . : => . a-i- a u v u • j2eart. m S man > lts simple form is early modified by its being curved upon itself, by partitions forming within its cavity, by valves appearing at certain places, and by its walls becoming greatly thickened by muscular tissue. It thus becomes a complicated muscular pump, a part of the circulatory system specially modified by Nature for the purpose of propelling the blood around its circuit. For the details of the anatomy of the heart the reader is referred to the article on The Anatomy of the Human Body, Fig. 27. It will be remembered that the organ is four-chambered, comprising two thin-walled upper cham- bers, the auricles, and two thick-walled lower ones, the ventricles. A partition extends the whole length of the heart, separating the auricle and ventricle of the right side from those of the left side. Each auricle re- ceives blood from veins, and opens below into the corresponding ventricle. The pulmonary veins, bringing blood from the lungs, join the left auricle ; the two great veins that bring blood from the rest of the body, the supe- rior and the inferior vense cavse, enter the right auricle. Each ventricle opens into an artery — the right one into the pulmonary artery which con- veys blood to the lungs, the left one into the aorta, which is the largest artery of all, and supplies with blood all parts of the body except the lungs. The course of the blood stream has been mentioned already, and may be seen readily from the accompanying diagram (Fig. 5). The di- rection of the flow is determined by the valves. Of these there are two kinds : the auriculo- ventricular valves at the opening of each auricle into 100 PHYSIOLOGY : THE VITAL PROCESSES IN HEALTH. its ventricle, and the semilunar valves at the origin of both the aorta and the pulmonary artery. The former allow the blood to pass from the auri- cles to the ventricles, but not to return ; the latter allow it to flow from the ventricles into the arteries, but not to return. The movement of the blood is caused by contractions of the muscle in the walls of the heart, the muscle being so arranged that each contrac- tion diminishes the size of the heart chambers, and the blood is thereby squeezed upon and forced out. The contractions are commonly called " beats," and are performed rhythmic- ally. In the newborn child the heart beats at the rate of one hundred and thirty to one hundred and forty times in the minute. During child- hood and youth the rate gradually diminishes, and throughout the greater part of adult life remains at about seventy-two. The events of each beat in brief and in order are as follows : The gradual flowing of blood from the veins into the auricles and from these into the ventricles ; the sudden short contraction of both auricles for the purpose of overfilling the ven- tricles and causing the auriculo-ventricular valves to rise upward and shut off communication between auricles and ventricles ; sudden contraction of both ventricles by which the contained blood is put under tension, the semilunar valves are pressed open, and the blood is shot out into the aorta and the pulmonary artery with sufficient force to drive it through the arteries, the capillaries, and the veins back again to the opposite auricle. Immediately after contraction the semilunar valves close, each heart cham- ber relaxes and fills with blood from the veins, its muscles rest, and, in a fraction of a second, have recovered energy for a second beat. This fol- lows, and a third, and a fourth, and so the cycle is repeated with alterna- tions of activity and of rest, of systole and diastole, throughout the life of the individual, the order of events never changing unless some form of "heart disease" interferes with the working of this most beautiful of all animal mechanisms. It is apparent that the course of the blood along the arteries is inter- mittent, wave after wave following one another at intervals of less than a second. Each wave or pulse is, of course, the direct result of the ventricular beat, and hence the physician regards the pulse as one of the best indications available to him of the condition of the heart. The practised ear may infer much also from the heart sounds, which may be heard readily by laying the ear on the chest wall over the heart. These are two — a longer, faint, low-pitched tone, due to the contraction of the ventricles, and immediately followed by a short, higher-pitched, abrupt one, caused by the closing of the semilunar valves. If the parts are altered by disease the sounds are altered. The smaller arteries and the capillaries are excessively fine tubes, the diameter of some of the latter being even as small as ^^oo °f an inch, the BEAT OF THE HEART AND HEART SOUNDS. 101 diameter of a red corpuscle. The result is that the resistance to the flow in them is enormous, and the blood tends constantly to accumulate in the arteries. The walls of the arteries are thereby put under great tension, the pressure of the blood within them is great, and their elasticity is brought into play. The result of this combined capillary resistance and arterial elasticity is that by the time the blood has reached the smallest vessels the pulse has disappeared, and the flow is continuous in both the capillaries and the veins. Many more details might be given of the events of the blood movement were there space, for even from the very earliest times, and especially since, in 1628, Harvey gave the right interpretation of the leading facts, and since Malpighi, in 1661, first saw with his micro- scope the exquisite and wonderful picture of the blood corpuscles pick- ing their way through the tortuous capillary channels in the wall of the frog's lung, the circulation has been a favourite study with all schools of physiologists. Many of the once mysterious facts have been shown to be explicable by the common laws of mechanics. Although the heart is a living pump and the vessels living tubes, the circulatory system pre- sents many of the same problems as are presented by any system of closed elastic pipes through which liquid is pumped. Other problems, however, defy the attacks of the mechanical, physical, or chemical physiologists, and prove that the experimental laboratories have still much to do. Of these latter problems, which for lack of a better word may be called " vital," two may here be mentioned — viz., the causation of the heart beat, and the regulation and co-ordination of the various parts of the circulatory apparatus. The beat itself is an excellent example of what physiologists are wont to call spontaneous actions — *. e., actions which originate within the tissue itself, and do not require a stimulus from with- out to set the tissue going. By this is not meant a causeless action ; every action has one or more causes, but the causes of sponta- auseoj neous actions are to be sought within the acting part it- self. The beat of the heart is a spontaneous action. It has been abundantly proved in lower animals and even in the higher quadrupeds that the heart when removed from the body will continue to beat even for hours if it be supplied with proper nourishment and warmth, and this fact no doubt would apply to the human heart, were it possible to test it. The impulse to the beat is then to be sought in the heart itself, but in what tissue ? The heart contains much nervous tissue, con- sisting of nerve cells, which send off filaments, the nerve fibres, to the cardiac muscle cells. In general it may be said that nerve tissue is more inclined to spontaneity than muscle tissue. Within the heart, then, does the impulse to beat originate in the muscle cells that do the contracting, or does it originate in the nerve cells and pass from them along the nerve fibres to the muscle cells ? The question is a fundamental one for physi- 102 PHYSIOLOGY : THE VITAL PROCESSES IN HEALTH. ologists, and its answer would be a valuable contribution to the interest- ing subject of the evolution of muscular and nervous function. In the lower and simpler animals spontaneity is a characteristic of almost all kinds of cells and tissues ; as the evolution of the higher animals has gone on, gradually the tissues have become less spontaneous in their actions and more dependent upon impulses coining to them from the nervous system, until, in the higher animals, the work of the body is largely per- formed as the result of nervous action. In accordance with this the nerv- ous system retains its primitive spontaneity in a high degree. In the hearts of invertebrates, as the snail, and perhaps some low vertebrates, as the frog and the turtle, the impulse to beat appears to originate in the cardiac muscle cells ; in the higher vertebrates it is yet unsettled whether it is nervous or muscular in origin. Why the heart tissue acts rhythmic- ally is another inviting subject, not yet understood, into which we have here not time to go. Although, as we have seen, the heart does not need any impulse from outside to enable it to continue its contractions and do its work, yet its contractions are always regulated and controlled by Nervous Control . , ,. ,-, , . ^ , , , ., „ , nervous impulses coming from the brain. Jbrom the of the Heart. r _ ° part of the brain lying at its base and called the medulla oblongata, situated just within the skull at the back of the neck, nervous impulses go out to the heart along certain nerves. Along the two vagus nerves (see The Anatomy of the Human Body) may go impulses which cause the heart to beat more slowly or more weakly than before. These come from the so-called cardio-inhibitory centre in the medulla, and if sufficiently intense they may cause the heart actually to stop beating. Along the sympathetic nerves may go impulses that cause the heart to beat more rapidly or more strongly than before ; these come also from an augmentor or accelerator centre situated probably in the medulla. These two centres are thus antagonistic in their action on the heart. They are in nervous connection with other parts of the central nervous system and thence with all parts of the body, and through them the heart is delicately controlled constantly as to rate and force of beat, so that its work is adapted to the needs of the body at every moment. As examples of ex- treme activity of these nerve centres may be mentioned, first, the slowing or actual stopping of the heart by a sudden heavy blow in the pit of the stomach, or by a sudden shock caused by a piece of bad news. In both cases the cardio-inhibitory centre is stimulated to activity; in the former through nerves going up from the stomach to the medulla, in the latter through nerve fibres within the brain itself, extending down from the consciously acting brain centres above that have taken cognizance of the bad news. In both cases the result, as stated, is slowing or stopping of the heart ; the fainting that usually accompanies is a secondary result, due NERVOUS CONTROL OF THE HEART AND ARTERIES. 103 to the fact that the weakened heart fails to pump the necessary blood to the consciously acting brain, and unconsciousness results. Second, the very rapid fluttering of the heart accompanying mental excitement is no doubt due to excessive stimulation of the accelerator centre, and thus of the heart, by impulses coming likewise down from the higher brain cells. A little consideration will show that slowing of the heart may result theoretically from activity of inhibitor}' nerves or from the cessation of activity of accelerator nerves ; and the like applies vice versa to accelera- tion. Physiology has not yet unravelled all the mysteries of the inter- actions of these two antagonistic nerve influences. It may be mentioned here incidentally that the rapid pulse present in fever is probably due to the hot blood stimulating directly the heart muscle to excessive activity. The blood supply to the various parts of the body must needs vary constantly, according as any part requires more or less blood at one time than at another. A tissue in action needs more blood Nervous Control ,1,1 , • . 1 ■. j £ 1 , ., , , . than the same tissue at rest, because it needs more food of the Arteries. and more oxygen, and because the injurious waste prod- ucts of its activity must be removed. When the brain thinks, it needs more blood than when it sleeps ; when the digestive glands are secreting, they must have blood in abundance ; when a man works with his muscles, they demand an extra allowance of blood. Obviously, mere alteration of the heart beat affects the general blood supply, but affects all parts equally. Nature has, however, evolved an efficient method of varying the supply according to the needs of the individual parts. The method consists in varying the calibre of the artery that brings blood to each part. Con- striction or narrowing of an artery causes the quantity of blood to be diminished ; dilatation or widening causes it to be increased. The calibre of the arteries, like the action of the heart, is regulated through special nerves by a particular part of the brain. Such nerves are called vaso- motor, from the fact that they supply the muscles or motor part of the arterial walls. The vaso-motor nerves are of two kinds, quite analogous in their functions to the two kinds of cardiac nerves : the vaso-constrictors have the power of causing the muscular coat of the arterial walls to con- tract, and thus a constriction of the artery results (analogous to augmenta- tion of the heart beat) ; the vaso-dilators cause relaxation of the arterial muscle and hence a dilation of the vessel (analogous to cardiac inhibi- tion). These nerves go to the arteries from a vaso-motor centre in the , medulla oblongata. This part of the brain, like the cardio-inhibitory centre near which it lies, is affected by influences coming from all parts of the body, and its actions are determined by the nature of these in- fluences. It is capable of controlling the calibre of each artery, and thus of constricting or dilating small or large vascular areas. Like the case of cardiac augmentation or inhibition, arterial constriction and dilation may 104 PHYSIOLOGY : THE VITAL PROCESSES IN HEALTH. result theoretically not only from direct constricting and dilating im- pulses, but each may also follow from the cessation of impulses leading to the opposite activity. Hence the mutual interactions of these two in- fluences become excessively complicated and present a problem not yet wholly solved. As examples of vaso-motor actions may be mentioned the two cases of blushing and becoming pale, due in the one case to arterial dilation, and in the other to arterial constriction, of the small arteries in the skin of the face. The exciting cause in each case is an unusual thought or emotion originating in the brain and causing nervous impulses to pass down to the vaso-motor centre in the medulla, in the one case decreasing its activity, in the other increasing it. Just why one emotion causes blushing and another paleness is not clear. As might have been expected, of all the blood-vessels the arteries alone are known to be markedly con- trolled by nerves. The vaso-motor nervous apparatus and the cardiac nervous apparatus are connected within the medulla and work in harmony with each other. Together they form a mechanism of remarkable adapta- tion and refinement. As will be seen from the article on Anatomy, the lymphatic system is comparable in a general way with the capillary and the venous systems — i. e., it consists of capillaries uniting to form larger ves- 6 a y , ' sels, and these in turn unite into two laree trunks. The bystem. \ , . . . . capillaries take their origin in irregular spaces among the tissues. (See The Anatomy of the Human Body, Fig. 36.) The lymphatics receive openings from such large cavities as those of the ab- domen, of the chest, and of the pericardium. At intervals they open into the irregular cavities within the lymphatic glands. And finally the two trunks, the thoracic duct and the smaller right lymphatic duct, open into the great veins at the root of the neck. The walls of the lymphatics are not unlike those of the capillaries and veins in structure, but they are ex- cessively thin. Valves, like the venous valves, are very abundant in the vessels. The lymph not only fills the lymphatic organs, but exists also in all cell spaces and interstices of the tissues, and thus bathes the living cells much more intimately than does the blood. The lymph may be regarded as a carrier between the blood and the living cells, all food and all waste matters probably having to pass through it in their passage between the protoplasm and the blood-circulating system. The plasma of the lymph is blood plasma that has escaped through the thin walls of the blood cap- illaries into the spaces in the surrounding tissues ; the corpuscles of lymph are in part escaped white blood-corpuscles and in part new cells that are formed by division of cells within the lymphatic glands. The lymph thus originating constantly in the tissues passes into the lymphatic capillaries, flows constantly along the vessels, and empties itself into the veins. In all vertebrates below the mammals a varying number of lymph hearts THE LYMPHATIC SYSTEM. 105 exist— simple muscular sacs attached to various lymphatic vessels, and capable, like the blood heart, of rhythmic contractions. No such organs are known to exist in man and other mammals. The movement of the lymph is due to several agencies, such as pressure exerted upon the vessels by the muscular movements of the body, the existence of a lower pressure in the veins than in the lymph-vessels themselves, and possibly rhythmic contraction of the walls of the vessels. The numerous valves prevent any possibility of a flow in the wrong direction. Thus both structurally and functionally the lymphatic system is a much less highly specialized appa- ratus than the blood system. The latter, with its efficient means of pro- pulsion and its elaborate nervous mechanism for regulating speed and distribution, is, like the railway system, an efficient 'and rapid carrier. But, just as between the factory and the railway, or between the latter and the consumer, the drayman's cart is indispensable, so in the body, be- tween the place of digestion and the blood, or between the blood and the living cells, the lymph finds its tasks. The lymphatic system and the blood system together form a most efficient distributing and collecting mechanism. We have thus traced the food from outside the body to the living cells. Without oxygen the cells can not utilize it. We have now to consider the source of the oxygen. Section III. RESPIRA TION. The lungs and other respiratory organs have a twofold function — that of bringing to the blood the oxygen that is as essential to life as is the food, and that of removing from Respiration in ^ b]ood and frQm the bo(J water General. . and certain waste and poisonous products, mainly carbonic acid. The great impor- tance of the whole process is indicated by the facts that the respiratory organs occupy so large a space in the body ; that the right ventricle of the heart has as its sole function that of supplying blood to them ; and that during the lifetime of the individ- ual all the blood in the body must pass through Flo 8 _ termination of them once in every twenty or twenty-five seconds ™™ZZ£t beset — the time occupied by the blood in making the ™« AIR SAC3 - (in- complete circuit of the body. To insure rapid and efficient exchange of the two gases, oxygen and carbonic acid, the blood and the air must be brought into as close proximity to each other as possible ; hence we find the lung to consist mainly of innumerable 106 PHYSIOLOGY : THE VITAL PROCESSES IN HEALTH. small air sacs (Fig. 8) with excessively thin walls containing a little elastic connective tissue lined by a layer of flat, thin epithelium cells, and loaded with a rich network of fine blood capillaries. The air sacs are continu- ous with the bronchial tubes, and communicate through the trachea with the outside air (Fig. 9). The blood is separated from the air by the thin epithelial membrane consisting of capillary wall and wall of air sac Fig. 9. — Trachea and lungs, dissected to show bronchial tubes. 1, 2, larynx ; 3, 4, trachea ; 5, 6, bronchi ; 7, 8, 9, 10, 11, bronchial tubes, a few only being shown as far as their terminations ; 12, 13, 14, 15, surface of lungs. (Sappey.) (Fig. 10). Thus the conditions needed for ready diffusion are present. The total amount of air surface exposed in the lungs appears to be more than two hundred square yards, and the amount of capillary surface more than a hundred and fifty square yards. The blood is renewed constantly by the circulation ; the air is exchanged constantly by the respiratory movements ; and the result of the interchange between the two media through the intervening membranous wall is that, while impure blood and pure air have entered the lungs by their respective channels, pure blood and impure air go out from them. RESPIRATORY MOVEMENTS. 107 The respiratory movements consist of those of inspiration and expira- tion. In both of these acts the lungs are passive organs. The activity re- sides in the muscles of the chest walls. It will be re- espira ory membered that the lungs are inclosed within an air-tight cavity, the thorax or chest, which is bordered at the top and sides by the ribs and the intercostal muscles, and below by the dome- like muscular partition, the diaphragm. Contraction of the external in- tercostal muscles raises the ribs, pushes the sternum or breast bone out- ward, and enlarges the whole chest cavity. Enlargement of the chest may be brought about also by contracting and thereby lowering the diaphragm. The inspiratory act consists in enlarging the chest by simultaneous con- tractions of both these muscles with the assistance of other muscles of the thoracic walls. The walls of the lungs follow passively the movements of the walls of the air-tight and air-empty chest, and the capacity of the lungs is correspondingly increased ; to balance this the air rushes in pas- sively through the nostrils or mouth, pharynx, trachea, and bronchial tubes. Thus we do not breathe air in, as our sensations might mislead us to be- lieve, through any action exerted upon the air by our nostrils or lungs. When we wish air we contract our diaphragm and the muscles of our ribs, and air must come in. Both sexes use both muscles, but in women res- piration by movement of the ribs, or " costal " respiration, predominates ; in men the " diaphragmatic " or " abdominal " method is more promi- nent. It has been greatly discussed and is still undecided whether this is a fundamental sexual difference associated with the func- Sexual Differences .. .,.,,,.. , Al . . , . „ . .. tion of childbearing in woman, or whether it is due to in Hespirahon. c the tightness of woman's dress about the abdominal re- gion and the prevention of the free action of the diaphragm. The expira- tory act is the reverse of that of inspiration : the muscles cease their con- traction, the ribs and the diaphragm return to their former positions, the tension on the lungs is removed ; by their elasticity the lungs return to their former size, and the excess of air is squeezed out. Only when the breathing becomes laboured, as in active exercise, or when from any unusual cause there is danger of suffocation, does expiration become a muscular act, carried FlG - 10.— Diagrammatic view * OF AN AIK SAO. on by various muscles attached to the walls of 8) lies within sac and point3 the chest and the abdomen. Respiration is a to epithelium lining wail; \ 0, partition between two ad- rhythmic, ordinarily unconscious action, repeated jaeent sacs, in which run . , . , capillaries ; c, elastic con- on an average seventeen to eighteen times in the nective tissue. (Huxley.) minute in the adult, but more rapidly in children. At each respiration about thirty cubic inches of air (" tidal air ") passes into and out of the air passages and mixes with the " stationary air " 108 PHYSIOLOGY : THE VITAL PROCESSES IN HEALTH. (about one hundred and eighty cubic inches) that the lungs contain at all times. The actual exchange of gases between the tidal and the sta- tionary air takes place by diffusion. The residt of the espimory exc hanee is that the expired air is saturated with Changes in Air. ° ...... moisture, is warmer than the inspired air, and contains about five per cent, less oxygen, about four per cent, more carbonic acid, and a minute quantity of obscure deleterious substances of unknown nature, to which the odour of the breath is due. The respiratory muscles are not, like the heart, automatic ; they need to be stimulated for each contraction ; and a particular part of the brain has been specialized to originate and send out to them Nervous Control ,, . ■ mi • ■ ,1 n j . .„ ... the necessary impulses. 1 his is the so-called respiratory of Respiration. . . . centre or " vital spot," and it lies in the medulla ob- longata at the base of the brain. Its presence there makes this portion of the brain seem so important, for any serious injury to the centre stops respiration and thus puts an end to life. Hence the fatality in breaking the neck. The nerve cells composing the centre are put into activity, ap- parently, by the impure venous blood circulating about them ; they in- augurate an inspiratory impulse and discharge it along the intercostal and the phrenic nerves to the respiratory muscles, causing the latter to act. Exactly how the regular alternation of inspiration and expiration comes about is not wholly explained, but the centre seems to be regulated in its activity by nerve impulses coming from the lungs. It is, in fact, one of the most sensitive parts of the nervous system, all modifications of breath- ing that take place in laughing, crying, coughing, sneezing, hiccoughing, sighing, " catching one's breath," muscular exercise, talking, singing, step- ping into a cold bath, etc., being due to influences altering the regular working of the respiratory centre. The peculiar facial expressions and characteristic vocal sounds that accompany laughing and crying are to be distinguished from the modified breathing. It is not easy to conceive how and why these peculiarities of facial expression, sounds and breath- ing, which are evidences of pleasure or of grief, and the beginnings of which seem to be found in animals lower than man, have been developed. The changes that the blood undergoes in its passage through the lungs are in harmony with and are no less striking than those of the air. The blood loses to the air contained in the air sacs of the lungs Respiratory g j x ^ g^i^ p er cen t_ f carbonic acid, and gains from it Changes in the , c . Blood eight to twelve per cent, of oxygen ; it comes to the lungs purplish in colour ; it leaves them bright scarlet. The gaseous exchange between air and blood takes place chiefly by the physical process of osmosis through the thin cellular membrane separat- ing them ; but the cells of this membrane may possibly act like gland cells to " secrete " the two gases. EXCHANGE OF OXYGEN AND CARBONIC ACID. 109 The change of colour of the blood is interesting. It will be remem- bered that the red colour is due to the colour of the haemoglobin that exists in the red corpuscles, and also that the red cor- Colour of the Blood. , ,, . ,, „, , , , . puscles are the carriers of oxygen. Ihe haemoglobin exists in the body in two forms — in venous or impure blood, largely as reduced hmmoglobin, and in arterial or pure blood, as oxyhoemoglobin. Reduced haemoglobin contains little oxygen and is purplish in colour ; oxyhemoglobin contains much oxygen and is scarlet in colour. The haemoglobin that is brought to the lungs is in the reduced form ; it greed- ily seizes upon the oxygen that is absorbed through the capillary walls ; it becomes oxidized ; and the colour of the haemoglobin, the red cor- puscles, and the blood changes accordingly to the bright-red tint. Besides the pulmonary respiration, slight exchange of oxygen and car- bonic acid takes place directly through the skin. This method of breath- ing is of great importance to some of the lower animals, Respiration by , „ , , . . . the Skin sl as * ro g s an0 - worms, but in man it is very sub- ordinate. The specific respiratory organs work for the body as a whole. In the broad sense, however, all living cells are respiratory, since they take in oxygen and give out carbonic acid. Such a process is often spoken of as internal or tissue respiration. The lungs and the skin mediate between the air and the blood and lymph. The blood and the lymph are car- riers between the respiratory organs and the living tissues. Charged with oxygen in the lungs, the blood is sent throughout Internal . . „ ■ t - the body, and everywhere in the capillaries it courses among living cells that require oxygen. The oxyhemo- globin is robbed of its contained gas and becomes reduced, while the blood changes to a purplish colour. The cells, on the other hand, are constantly giving off carbonic acid, and this by diffusion passes readily into the blood. The lymph has in tissue respiration a function analogous to that which it has in tissue nourishment : it is the mediator between the blood and the living cells. Hence the result of tissue respiration as re- gards the blood is exactly the reverse of that of pulmonary respiration. The respiratory relations of the pulmonary and the systemic circulatory systems hence appear in a new and striking light. The former deals with the respiratory needs of the body as a whole, the latter with the respira- tory needs of the living particles of which the body is composed. In considering respiration we have unavoidably touched upon the excretion of one waste product — carbonic acid. The other wastes may now be studied. 110 PHYSIOLOGY: THE VITAL PROCESSES IN HEALTH. Section IV. EXCRETION. Leaving for the present the consideration of their sources, we may enumerate the chief waste products of protoplasmic activity as follows : Waste Products. Organs of Excretion. Gaseous. . . .Carbonic acid. Lungs, skin. Liquid Water. Kidneys, lungs, skin. , Urea and other nitro- ( Kidneys> skin . Solid < genous wastes. ( ( Inorganic salts. Kidneys, skin. Protoplasm is unable to extract energy from these substances, and their presence in the body in excessive quantities is harmful. Nature has therefore evolved in the organs of excretion refined mechanisms for re- moving them from the organism. They are cast out from the living cells into the blood, and are transferred by the circulatory organs to the organs of excretion. The latter will now be considered. A. THE KIDNEYS. The kidneys are the most highly specialized of the excretory organs. Their duty is to manufacture and pass out the urine. Urine is a clear yellow or brownish-yellow liquid, consisting of about 96 - 5 per cent, of water and 3"5 per cent, of dissolved solid substances. The solids are numerous, comprising organic bodies such as urea, uric acid, and creatinin, and inorganic salts such as various sulphates, phosphates, and chlorides. Their variety and relatively con- siderable quantity indicate the great importance of the kidneys as purifi- ers of the blood and thus of the body. The greatest interest centres in the organic solids, of which urea is the most abundant, because they represent the characteristic waste products of the destruction of the important proteid substance. The special peculiarity of urea and these other products is the presence within them of nitrogen ; hence the urine is the medium through whicli the all-important nitrogen leaves the body. The average quantity of urine that is passed in twenty-four hours is one and a half quarts, but the quantity is subject to great variations, depend- ing upon the weather, the character of the food, whether much water has been drunk, and the occupation of the individual. The kidneys are highly complicated glands whose structure is spe- cially adapted to the removal from the blood of large quantities of water, together with solid substances. They consist of a mass of minute canals (the uriniferous tubules), and of blood capillaries, inextricably woven together. Each tubule begins in an enlarged cavity (the Malpighian STRUCTURE AND FUNCTION OF THE KIDNEYS. Ill capsule) into which projects a tuft of capillaries, the glomerulus. From the capsule the tubule takes a tortuous course, as shown in the accompany- ing figure (Fig. 11), unites with other tubules, and finally approaches the surface of the kidney, where, together with all the other tubules, it opens into the dilated, funnel-shaped beginning of the duct, the ureter (Fig. 12). The wall of the tubule consists of a single layer of living cells varying in thickness in Structure of the Kidneys. Fig. 11. — Diagrammatic view of course of uriniferous tubules in kidney. /, Malpighian capsule, con- taining glomerulus ; II- VIII, course of tubule ; IX, opening of tubule into pel- vis of kidney at apex of pyramid of Malpighi ; k, outer surface of kidney; r, outer or cortical substance ; ff,p, inner or medullary sub- stance. (Huxley.) Fig. 12. — Vertical section of kidney. (Some- what smaller than natural size.) 1, 2, 3, 4, pyramids of Malpighi, striations repre- senting uriniferous tubules ; 5, apices of pyra- mids, where tubules open ; 6, cortical sub- stance projecting inward between pyramids and containing blood-vessels; 7, dilated end of ureter, called pelvis ; 8, ureter. (Sappey.) different parts. In among the tubules is the very close network of blood capillaries, and lymph permeates all the interstices. The tubules are thus bathed by the circulating fluids. Most of the constituents of the urine exist ready formed in the blood, having been cast into it by the cells outside of the kidney. The process of excretion consists in a discharge of these substances Kidneys through the cells that form the walls of the tubules. Some of the cells apparently have the power of manu- facturing and casting out the few constituents that are not present in the blood. In its mode of action the kidney thus seems to combine the more 112 PHYSIOLOGY : THE VITAL PROCESSES IN HEALTH. clearly physical features of the work of lung cells and the more ob- scure secretory activities of gland cells, as represented by the digestive glands. The urine is secreted constantly, and trickles along the tubules to the dilated end of the ureter ; it then leaves the kidney in the latter tube and passes to the urinary bladder situated in the pelvis. Here it accumulates until the distention of the bladder gives rise to a desire to micturate. Nervous impulses from the brain cause the muscular walls of the bladder to contract, and the urine is discharged from the body through the urethra. B. THE SKIN. The skin performs a variety of duties. It protects the delicate parts within the body ; it contains organs for the senses of touch and of tem- perature ; through its blood-vessels it regulates the tem- Ih" %" S perature of the body ; and it contains important excre- tory organs. The latter are the two kinds of glands known as sudoriferous, or sweat, glands and sebaceous glands ; the former produce the sweat, the latter the oily substance found on the surface of the body. Sweat glands are simple tubes the secreting portion of which, coiled into a knot, lies just be- neath the skin (Fig. 13). The duct passes through the skin and terminates by a minute opening upon the surface. These "pores" Fig. 13. — Vertical section of skin. (Magnified 20 diameters.) 1, outer layer of skin ; 1, 2, cuticle or epidermis ; 3, 4, inner layer of skin or dermis ; 5, subcutaneous tissue ; 6, sweat glands ; 7, masses of fat, consisting of fat cells ; 8, 9, ducts of sweat glands. ( Sappey. ) Fig. 14. — Surface of palm of hand. (Magnified 4 diameters.) 2, ridges in skin bearing (1) openings of ducts of sweat glands. (Sappey.) may readily be seen by a common magnifying glass upon the fine ridges in the palm of the hand (Fig. 14). Sweat is a colourless, salty liquid EXCRETORY FUNCTIONS OF THE LUNGS AND SKIN. 113 consisting of water and slight quantities of urea, inorganic salts (espe- cially common salt), and a few other substances. It is constantly given off. An average quantity is nearly a quart in twenty- four hours, but the amount varies greatly with the weather, the occupation of the individual, and other influences. The perspiration is thus seen to be an important medium of loss of sub- stance from the body. This is especially evident when one exercises vigorously ; it is easily possible in an hour's exercise to diminish one's weight by a pound. The more one perspires, the less the kidneys ex- crete, and vice versa. Hence the skin is more active in summer, the kidneys in winter. The sebaceous glands lie in the deeper part of the skin and open chiefly into the depressions in which lie the roots of Sebaceous Glands. , , . „, .. , , . ,. , , the hairs. Ihey secrete an oily substance of slight value as an excretion, but of use in preventing the skin and the hair from becoming too dry. C. THE LUNGS. The expired air is an important medium of loss of water and the chief one for loss of carbonic acid. The excretory function of the lungs has been considered sufficiently under respiration. We have followed the oxygen and the food — consisting of proteids, fats, carbohydrates, salts, and water — from without the body by way of the digestive organs and the lungs to the living cells. We have seen that waste matters in the forms of carbonic acid, urea, salts, and water go from the living cells by way of the excretory organs to the exterior. What takes place within the living substance? Section V. METABOLISM. If we could answer the question propounded at the end of the last sec- tion we should know what life is ; yet we are unable to point to any one of the millions of cells in the human body and say that we know all the details of the vital process that takes place within it. The difficulties and complexities of the problem are inconceivably great, but perhaps not in- surmountable. We can measure and analyze the income and the outgo of the body ; we can test the effect of different foods upon the general metab- olism ; we can observe how the composition of the different organs changes when food is withheld from the animal for a time, all of which methods are helpful. But, further than this, chemical investigation is revealing to us ever more clearly the steps in the pathway between food and wastes ; 10 114 PHYSIOLOGY : THE VITAL PROCESSES IN HEALTH. we are approaching a knowledge of the structure of protoplasm and of the structural changes that take place in the actively working cell ; and recent studies of the action upon organisms of the environment and of ex- ternal agents — such as light, heat, electricity, and chemical influences — are giving us a deeper insight into the secrets of the physical basis of life. We can conceive the nutritional changes that take place within the cells and that constitute the metabolic process as consisting of a building up and a breaking down. Raw material in the form of food that is rich in energy is brought to and absorbed by the cells. It is Metabolism in ,, , , . n ., ', ,. •, , ., . „ , altered chemically, its atoms are recombmed, and it is General. ■" ' built up, probably by a complex series of steps, into protoplasm, its energy being retained in a latent form ; this is the con- structive phase of metabolism, called andbolism. Later, the protoplasm is changed chemically, its atoms are recoinbined, and it is broken down, probably by a complex series of steps, into wastes, its energy being given off in the form of mechanical work and of heat ; this is the destructive phase of metabolism, called katabolism. Stated in these words, the vital process seems simple enough. But this is not the whole story ; for, in the first place, not all the food is built up into protoplasm before it is broken down into wastes ; some appears to be changed at once after enter- ing the tissues and to be immediately cast out in the excretions ; other food is stored up for a time, to be used subsequently for the manufacture of protoplasm or for other needs of the body. Fat is an excellent example of a substance that is thus stored ; it is not living, but is contained within special living cells — the fat cells. The cells of the liver are likewise loaded with a variety of starch, called glycogen. Fat and glycogen con- stitute a stock of reserve material upon which the cells may draw in time of need. Further, each of the different varieties of cells has its own spe- cial metabolic peculiarities. For example, the digestive glands are pecul- iar in manufacturing in quantity substances (the digestive fluids) that are of the greatest subsequent value to the body ; certain of the brain cells are unique in the fact that their activity is accompanied by phenomena of thought ; the muscles seem to be the greatest producers of the body heat. All these peculiarities complicate the problem of metabolism greatly, and the real difficulty comes when we attempt to learn the details of the matter. The one fact that stands out above all others is the fact that the de- structive or katabolic process is one of oxidation ; that is, the gas, oxygen, is made to unite with the other elements — car- _ ., .. bon, hydrogen, nitrogen, etc. — that exist in the food or Oxidation. ' J ^ ' & ' in protoplasm. Thus, of the various excreted sub- stances, carbonic acid is a compound of oxygen and carbon (in the pro- portions CO,) ; water is a compound of oxygen and hydrogen (in the pro- FOOD STUFFS AND WASTES. 115 portions H„0) ; and urea, while more complex in composition (CH.JST^O), is undoubtedly formed by oxidative processes from the still more complex proteids. In this vital process of oxidation heat and mechanical energy are set free for the body's use, hence the body is warm and can do work ; and the excretory substances are therefore largely devoid of energy. Oxidation is nothing more nor less than combustion ; the burning of wood or coal or illuminating gas is oxidation. The burning of these life- less substances, therefore, and the vital processes, are fundamentally of the same chemical nature. So, too, as regards energy, coal, when burned in a furnace, yields heat, mechanical work, and light ; protoplasm, when consumed in a living body, yields heat, mechanical work, and in certain animals, such as the firefly, glowworm, and some marine forms, even brilliant light. Such a parallelism between lifeless and living substance is interesting and suggestive. Another striking fact is that the living tissues do not convert unchanged into their own substance the substance of the corresponding tissues that are eaten in animal food. For example, the SiilteAbZptL. fat of the food does not g° dire ctly to form the fat of the body ; the muscle proteid of beef or mutton that we eat does not form directly our own muscles, nor does the animal starch — glycogen — come directly from the starch of bread and vegetables. On the contrary, all of the food stuffs are worked over by the living substance, are resolved into other substances, and these other substances are recom- bined into the proteids, the fats, and the carbohydrates that are found in living matter. One practical application of this principle is found in the fact that the most certain method of increasing one's weight by fattening is not by eating large quantities of fat, but rather by living largely upon a carbohydrate diet (starch and sugar), since it has been found by experi- ment that such a diet leads directly to the formation of body fat. On the other hand, to reduce the fat of one's body, a diet rich in proteid is most effective. As to the uses of the various food stuffs, it may be said that proteid in some form is always necessary in the food, since it alone of the chief food stuffs contains nitrogen, and nitrogen is an important unive, sm'V constituent of protoplasm. Proteid is the chief source of the elements of new protoplasm, and hence the value of meats, eggs, and milk as articles of diet. Proteid also gives energy to the body, but it is an expensive food. Fat is very rich in energy, and protects the proteid substance within the protoplasm from destruction. It may with advantage take the place of some of the proteid in the food ; " a streak of fat and a streak of lean" is not without physiological justifi- cation. Carbohydrates play a role similar to the fats in supplying energy. They are a cheaper food and are easily digested. The part that is played 116 PHYSIOLOGY: THE VITAL PROCESSES IN HEALTH. in metabolism by inorganic salts, of which common table salt may be taken as the type, is not known. The}' exist in all the tissues, and they are constantly leaving the body in the urine and the sweat. Depriving animals of salt brings on weakness and even paralysis ; yet most salts are not sources of energy. All that is known upon the subject may be summed up in the statement that protoplasm will not continue to do its work without salt. Hence salt is needed in the food to balance the loss through the excreta and thus to maintain a constant supply in the liv- ing substance. Nor is water a source of energy. Its presence every- where in the body facilitates the chemical reactions, and all substances in passing from one part of the organism to another must be dissolved in it. It constantly leaves the body, partly for the purpose of regulating the body temperature, as we shall see later, and partly for the purpose of car- rying off the solid part of the excreta. Without it protoplasm dies, and hence its necessity in the food. Much attention has been given to the determination of a diet most suitable for the average man, and it is now known with reasonable accuracy what such a man requires for his daily needs. A fair division of the various food stuffs is given below : Proteids 4 oz. Fats 2 oz. Carbohydrates 17 oz. Salt 1 oz. Water nearly 3 qts. It is not a simple matter, however, to proceed from such a diet to the preparation of a suitable menu, for, although the proportions of food stuffs in the common foods are known (Fig. 15), it does not follow that all the food stuffs taken into the mouth are absorbed by the tissues. The digestibility of the foods is an important factor too often overlooked by rich and poor alike. A liberal allowance of beefsteak will not yield to the body four ounces of proteid unless it be so prepared that the digestive organs can deal with it. The digestibility of the various foods has not yet been determined with sufficient accuracy ; the problem is a difficult one, complicated as it is by the idiosyncrasies of individuals and by the infinite varieties of method used in the preparation of foods. Scientific investigation and the common experience of mankind point to a combination of animal and vegetable foods as the most suitable diet. Meat and eggs are especially characterized by the presence of proteid in a concentrated form ; to obtain the needed amount of Vegetarianism. . . , . , , , , n ,, proteid from vegetables would require the consumption of an excessively large quantity of food. A strictly vegetarian diet throws upon the digestive organs of man an excessive amount of labour. THE DIGESTIBILITY OF FOODS. 117 This is not so in the case of herbivorous animals, since in them the teeth are specially modified for grinding, the alimentary canal is relatively long and large, and the digestive processes are adapted to their tasks. In man, however, the teeth are evidently degenerating, and the alimentary canal is becoming reduced in capacity, the evident outcome of which in time Beef. Pork. Fowl. Fish. Egg. Cow's milk. Human milk. Animal Foods. Explanation of the signs. U llll l lllllllll'ilill JProteids. Albuminoids. N-free org. bodies. Salts. _.. .. .. ... 55 ■ « ■ ' 73 73.5 89 Vegetable Foods. Explanation of the signs. ,„al°-« 0.4 Proteids Wheaten-bread. Peas. Bice. Potatoes. White Turnip. Cauliflower. Beer. Digestible Non-digestible Salts. N-free organ bodies. is m 00.5 00 6.5: I" J 2.5 ■ " ]1 0.5 0.2 1 0.5 Fig. 15. — Composition of some common foods. Nitrogen-tree organic bodies include both fats and carbohydrates. will be the necessity of less bulky, more easily masticated, and more easily digested food. The misguided vegetarians, in their endeavours to make man herbivorous, forget that that stage in his career was passed ages ago, that his body is no longer fitted for it, either anatomically or physio- logically, and that stemming or turning back the tide of evolution is not without difficulties. 118 PHYSIOLOGY : THE VITAL PEOCESSES IN HEALTH. The two chief modes of manifestation of the energy of the body are through mechanical work and through heat. These may now be consid- ered. Mechanical work is performed by the muscles, nergy of ^^ corri p r j ses a p wor k done by the hands and arms, by Mechanical Work. r _ ... the legs in walking, by the trunk in lifting, by the lar- ynx in speaking, etc. It is comparatively easy to measure approximately the amount of work performed by a man in a given task. It was sup- posed formerly that the energy for muscular work was derived solely from the nitrogen-containing proteid of the food and of the tissues. If this were so, and since urea is produced by the destruction of proteid, the quantity of urea given off from the body would be proportional to the amount of work done. Elaborate investigations made upon the pedes- trian "Weston, and other individuals, in order to test the question, have re- futed the old idea and have shown that the main source of the energy of muscular work is not the proteids, but rather the fats and the carbohydrates. The hunger that follows labour does not, therefore, require for its satis- faction an increase of expensive proteid food. About one fifth of the total energy introduced into the body by the food appears again in the form of mechanical work, the remaining four fifths taking the form of heat. At first thought this would seem to indicate that the muscle is a poorly constructed machine. But a steam engine is able to employ for work only about one tenth of the total energy of the coal, the heat that is lost carrying off the other nine tenths. The human body as a machine for transforming energy is, therefore, much superior to the steam engine. Perhaps no fact in all human physiology is more striking than that the body of man is warm, and constantly warm during all seasons, even though the surrounding temperature may be excessively low. In this re- spect man is like all the mammals and birds and differs from all other animals. The terms " warm-blooded " and " cold-blooded " tell a part of the truth only ; " warm-bodied " and " cold-bodied " are equally applicable. Yet a still more correct designation of the two classes of Body Temperature. , . , ■, „ ... j £ i animals is that of constant temperature and of change- able temperature. An adult man's body has a temperature of about 98"6° Fahr., and normally varies rarely more than a degree above or below that point. A frog's body can not be said to possess a normal temperature. It is always cold to the touch, but varies within very wide limits, depending upon the temperature of the surrounding air or water. Heat is produced, in all animal and plant bodies, but is dissipated at once to the surround- ings in all organisms except the warm-blooded animals. The heat comes from the oxidation or burning of the food and body substance, and is de- rived from all three chief classes of food stuffs. Its production is a fun- damental property ofprotpplasm, and takes place wherever living substance exists. Hence all organs and tissues yield heat ; but the muscles, form- PRODUCTION AND LOSS OF HEAT. 119 ing as they do so large a proportion of the bulk of the body and being actively metabolic organs, are the greatest heat producers. The more active an organ is, the hotter it becomes. The blood, while producing little heat, performs the indispensable role of equalizing the body tem- perature. Receiving its own warmth from the tissues through which it courses, it warms the more sluggish parts, and in turn cools those whose temperature tends to become dangerously high. "While heat is produced constantly, it is as constantly being lost. We warm our clothing and whatever we come in contact with that is of a lower temperature than the body ; the air that we breathe out is warm ; the excretions are warm ; much latent heat goes off in the evaporating sweat. Of all these path- ways of loss, the skin is the chief one, eighty per cent, of the lost heat leaving the body through it. As has been seen, a warm-blooded animal differs from a cold-blooded animal in the fact that the temperature of the body of the latter changes with that of the surroundings, while that of the former Regulation of . ,■■,-, , m, „ , . ,.„. Body Temperature. remains practically constant. The cause of this differ- ence lies in the fact that in the warm-blooded organism both the production and the loss of heat are carefully controlled by the nervous system. As regards production of heat, the lower the tempera- ture of the air the more heat the body produces, partly through invisible, obscure metabolism, partly through visible muscular movements, such as shivering ; on the other hand, the higher the surrounding temperature, the less active are the muscles. As regards loss of heat, the lower the temperature of the air, the less blood goes to the skin, and hence the less heat radiates from the surface ; on the contrary, the higher the tempera- ture of the air, the more the cutaneous vessels dilate and allow the heat of the blood to pass off ; at the same time the body perspires and gives out abundant latent heat in the sweat. Thus the vaso-motor nerves, the se- cretory nerves of the sudoriferous glands, and other nerves are employed for heat regulation, and their various activities are brought into harmony through the central nervous system. In the cold-blooded organism no such regulating mechanism exists. In discussing metabolism the liver must not be overlooked, since it plays so important and such various roles in nutrition. "We shall here enumerate its chief functions. The liver produces bile, the Liver kh e use °^ w hich m the digestion of fats we have dis- cussed. In addition to this digestive property bile con- tains several complex substances which must be regarded as waste prod- ucts of protoplasmic activity, and are passed off from the body with the undigested food matters ; hence the liver is an organ of excretion. The liver cells contain abundant glycogen, or animal starch, which is regarded as a reserve stock ; hence the liver is a storehouse of carbohydrates. 120 PHYSIOLOGY : THE VITAL PROCESSES IN HEALTH. Lastly, the liver seems to be the chief organ in which take place the final processes in the manufacture of urea ; the raw material for excretion comes in the blood from the various organs to the hepatic cells, the cells transform them, and the finished product, urea, is transferred to the kidneys for elimination. With such a multiplicity of functions it is not surprising that " liver complaints " form so large a proportion of human ills. The spleen, the thyroid body, the suprarenal bodies, and the thymus have apparently important metabolic functions, but their exact roles are little known. At present they are being actively studied. Our story oftiutrition is completed. It is but the preliminary to a study of what may be called by some the higher functions of the body. To these we now turn. CHAPTER II. MOTION. Section I. MUSCLE IN GENERAL. Next to producing heat, the chief mode in which the body employs its stock of energy is by doing mechanical work. The physical sign of mechanical work is motion. Its manifestation is most „ 7 evident in the day labourer. But the professional man Creneral. •> r and those who are known technically as brain workers are not simply heat producers. Aside from the ordinarily invisible in- voluntary movements of the organs and the visible voluntary movements of the body, without which no man passes through each succeeding twenty -four hours, the man who thinks gives his thoughts to the world in writing or speaking or acting, all of which are processes of movement. The organs of mechanical work are the muscles. A muscle is made up of muscle tissue and this in turn of muscle cells. Muscle cells always have one axis considerably longer than the other two, from which fact they are often called fibres, and the essence of their activity consists in their power of contracting or shortening in the direction of the long axis ; in this process movement of attached parts is caused. This power of contractility is one of the fundamental attributes of protoplasm ; it is possessed by the living parts of plants and by the one-celled animals. As the evolution of animal life in past ages went on and one-celled animals gave rise to many-celled animals with a variety of functions, the ability to contract became progressively stronger in some cells than in others. THE ORGANS OF MECHANICAL WORK. 121 Structure and Varieties of Muscle. In these contractile structure and function went on developing together, they became more and more perfected as organs of movement, and they gave rise finally to the highly specialized muscle cells that organisms possess to-day. When we look over the whole animal kingdom we recognise a very great variety of muscle tissue, from the simple cells of the jelly fishes, where only a part of the cell can be called muscular, to the highly differentiated muscle fibres of actively moving organs like the wings and legs of insects and the limbs of the higher animals. Notwithstanding this variety, muscle cells, especially in man and other vertebrates, may be grouped into three chief classes that are distin- guished from each other both structurally and functionally. These are known, respectively, as smooth, striped, and cardiac muscle cells or fibres. Smooth or unstriped muscle fibres are so called because, in distinction from the other two varie- ties, they are not cross-striped (Fig. 16). They are usually spindle-shaped ; they are most common in tubular organs, occurring in the walls of the alimentary canal, the arteries and the veins, the ducts of glands, the trachea, and in general in those parts of the body, except the heart, that are capable of involuntary movement only. They are bound together by connective tissue into muscular coats encircling- the tubes in whose walls they lie, and by their contraction they constrict the tubular organs. They are not under the control of the will. They are the most primitive of the three kinds of muscle substance. Their action is slow, as is indicated by the writhing, wavelike movements of the stomach and the intestine during digestion. Striated or striped mus- cle fibres are so-called because, when examined with the microscope, they appear indistinctly cross-striped with alternate lighter and darker bands (Fig. IT). They are very long, delicate, threadlike cells, and their proto- plasm is highly complicated in structure. They are usually under the control of the will, and form the flesh or meat of the body — that is, the muscles of the arms, the legs, the head, and the trunk. Each muscle consists of a mass of innumerable fibres bound together by connective tissue into bundles, and is attached usually to bones by means of tendons. Striped muscle is the most highly specialized kind of muscle tissue. It is capable of very quick action, as is shown by the rapidity with which one can strike a blow or play a piano. Cardiac muscle is intermediate Fig. 16. — Unstriped muscle fibres of man. (Magnified 200 diameters.) 1, nuclei of fibres; 2, fibres in mass; 3, isolated fibres; i, 4, two fibres joined to- gether at 5. (Sappey.) 122 PHYSIOLOGY: THE VITAL PROCESSES IN HEALTH. Fig (Magnified structurally between the other two. It consists of short, compact, indis- tinctly striped cells. It occurs only in the heart, and, as we have learned in studying the heart beat, contracts spontaneously, involuntarily, and rhythmically. For the past fifty years muscle has been a fascinating and fruitful field of physiological research. Many of the most interesting and funda- „ mental problems of general proto- plasmic action have been and may be studied here more successfully than in other tissues ; not only the laws of vital movement, but funda- mental questions such as antoma- ticity, irritability, rhythm, animal electricity, and the chemical phe- nomena of life. A large variety of delicate and valuable apparatus has been devised for the exact investi- gation of these various problems. The study of striped muscle has yielded the most information, and our present discussion will be lim- ited to this. As has been stated, the essence of muscular action con- sists in the power of the muscle fibres to contract. In cold-blooded animals, such as the frog, the muscles retain their contractile power — that is, remain living — long after the animal has been hilled, hence it is easy in such animals to study muscular action. During life the muscles are made to contract through impulses coming to them along the nerves from the brain or spinal cord. After death in cold-blooded animals they may be stimulated to activity by electric shocks, heat, pinching, or by certain chemicals applied either to the muscles directly or to their nerves. For each stimulation the muscle gives a single twitch or contraction, during which it shortens and becomes thicker and harder, and then immediately relaxes into its former state. The whole period of activity occupies only about one tenth of a second, yet during this moment the muscle undergoes profound molecular changes. Besides the mechanical changes spoken of, it produces heat and becomes warmer, produces carbonic and lactic acids, and develops a considerable electric current. All these phenomena indicate what great metabolic changes muscle protoplasm is subjected to during activity. It would be interesting to trace these further, but it would take us beyond our present space. In life it is probable that voluntary muscle rarely, if ever, gives -Striped muscle fibres. 250 diameters.) A, piece of a fibre showing cross-striations and nuclei ; B, piece of a fibre with cell-wall (sar- colemma) ruptured, showing tendency of fibre to split into fibrillar. (Sappey.) Action of Muscle. MUSCULAR MECHANISMS, ACTION AND TONE. 123 single isolated twitches. Each contraction, however quick, consists of numerous single contractions following one another at the rate of about twelve in the second, and becoming fused into a compound contraction called tetanus. These rapid contractions give rise to a dull booming sound which is emitted by the muscle and may readily be heard by in- serting the tips of one's fingers into the ears and contracting strongly the muscles of the arms. In health the muscles always seem to be in a state , „ of slight contraction, or "tone," which accounts in jjlUSCUlct'F TOTVB great part probably for the elasticity, springiness, and ready muscular response of the athlete. This healthy tone appears to be due to nervous impulses coming constantly to the muscles from the spinal cord. It is noticeably absent in ill health. Many muscles are so placed as to antagonize the actions of others. For example, the flexors, which bend the arms, legs, fingers, and toes, act in opposition to the extensors which straighten the same parts ; the eye is closed by the orbicularis and opened by its antagonist, the levator of the upper lid ; and the delicate adjustments of the parts of the larynx in speaking and singing are due to refined balancing of opposing muscles. Suction II. SPECIAL MUSCULAR MECHANISMS. As instances of special motor phenomena we have already mentioned the digestive movements, the beat of the heart, arterial constriction and dilation, and the movements of respiration. We may now notice briefly a few others. A. LOCOMOTION. The erect posture in standing or sitting requires the co-ordinated action of numerous muscles of the trunk and the legs. That this is so is evident from the fact that the body collapses whenever, as in fainting, the muscles fail to receive their customary stimulating impulses from the central nervous system. Walking is a complicated muscular act, participated in by a large number of muscles of the legs, the trunk and the arms, that contract in regular sequence (Fig. 18). Each leg is flexed as it is swung forward like a pendulum ; it is then straightened and serves as a support for the swinging trunk. The movements of the trunk are peculiar. It alter- nately rises and sinks, in falling forward, and sways from side to side as the centre of gravity of the body comes now over the right, now over the left foot ; at no time is the body wholly free from contact with the ground, and for a brief interval, when the feet are farthest apart, both feet touch it at the same time. In running, the sequence of events is so 124 PHYSIOLOGY : THE VITAL PROCESSES IN HEALTH. far different from that in walking that at one time the body is entirely free from support. The recent advances in the art of instantaneous 2 Fig. 18. — Series of figures from instantaneous photographs to illustrate movements in slow walking. All phases of movement of the right arm and leg are shown in figures I to VI. Arabic numerals indicate the corresponding positions of the left arm and leg ; thus position III of the right side is simultaneous with position 6 of the left side. (Marey.) photography have added much to our knowledge of the mechanism of bodily movements. B. FACIAL EXPRESSION. Changes in facial expression are muscular phenomena, due to contrac- tion of the facial muscles in combinations varying with the various emo- tions. This is shown by the fact that it is easy by electrical stimulation of the muscles through the skin to produce arti- ficially in an individual at will a desired emotional expression (Fig. 19). The anatomical peculiarities of the face con- stitute the features and give a certain set to every countenance. The expres- sions are physiological, and are produced in much the same manner in different individuals. In his book, entitled The Expression of the Emotions in Man and Animals, Darwin has given the results of a careful study of expression. He analyzes into their various muscular components the changes accompanying joy, grief, despair, love, hatred, anger, disdain, contempt, pride, surprise, fear, horror, etc. He finds the origin of many of these expressions in the lower animals, and shows how they have gradually become habitual and \ X Fig. 19. — Expression of extreme terror. Produced artificially by stimulating with electricity the muscles of the forehead and the "jaws. The four curved rods are the stimulating electrodes laid upon the skin over the muscles. (From Darwin, after Duchenne.) CHANGES IN FACIAL EXPRESSION. 125 innate in man. Their primary purpose was not to reveal emotional states of mind, but rather they were either of some direct bodily benefit to the _ - „ : '"■'"■ Fig. 20. — Expression of various emotions, showing characteristic muscular contractions. A, pride and defiance ; B, helplessness ; C, childish joy ; D, childish grief. (A and B from Darwin after Duchenne.) individual or were indirectly the effect of a general excitement of the nervous system. (See Fig. 20.) 126 PHYSIOLOGY : THE VITAL PROCESSES IN HEALTH. C. VOICE. The production of voice is the most delicate of all muscular acts of which the human body is capable. The larynx, the organ of voice, is the modified upper end of the trachea. However much the voice may seem to arise at the lips, or in the mouth, or in the chest, it is only modified by these parts, and the sound is produced in the larynx Physiological only. Considered as a musical instrument, the larynx is L r nx most nearly like a reed instrument, of which the clario- net is an example ; but the resemblance is not close. In the larynx the parts that correspond to the reeds, by the vibration of which voice is produced, are the two vocal cords (Fig. 22). They are elas- tic membranes that extend from each side horizontally toward each other into the cavity of the hollow larynx, and are stretched more or less from before backward. They do not meet in the middle line, but have between them a chink of variable width, the glottis, extending across the larynx from front to back. Each cord is thickened with muscle at its outer part attached to the walls, but its free edge at the glottis is thin, and consists of white, tough, elastic connective tissue. Thus the air-passage to and from the lungs is obstructed at its upper end by this horizontal mem- branous partition, with a passageway for air between its two halves. During ordinary silent breathing this obstruction is slight, for then the vocal cords recede to the side walls of the larynx and the glottis is wide open. During speaking or singing the cords are extended in toward each other and the glottis is reduced to a mere slit. Voice is the sound produced by the rapid vibration of the thin edges of the cords as the air rushes between them in expiration. The essential conditions of the production of voice are that the cords Conditions of Voice. . , , ., . , , , . , must be taut and their edges must be approximately parallel. These conditions are fulfilled through the various delicate muscles acting upon the cartilages to which the cords are attached, or even upon the cords directly. The natural pitch of a voice depends upon the natural length and ten- sion of the cords. Variations in pitch are produced by varying either the degree of tension or the length of the vibrating ' ,.. ' cord, or by varying both together. Thus, for the low and Quality. ' •' J ° m n tones, the more tightly the cords are stretched the higher the note, just as is the case in tuning a violin. For the high tones, the cords do not usually vibrate along their whole length ; a portion, usually the posterior, is " stopped " by the cords being brought into con- tact with each other, and the anterior part only is capable of acting (Fig. 22, B), just as in playing the violin the pitch is regulated by placing the finger upon the string. Loudness of voice is determined by the strength QUALITY, RANGE, AND PITCH OF THE HUMAN VOICE. 127 of the outgoing current of air. The quality of the voice — that by which we distinguish one voice from another and recognise the voices of our friends — depends upon the make and the age of the individual larynx, its size, the quality of the cords, and the shape, the size, and the mutual re- lations of the accessory vocal organs, such as the mouth and its parts, the nose, the pharynx, and the chest ; in like manner the quality of tone 256 Soprano. 1024 171 Contralto. 684 EFGAB edefigab c' d' e' f r g' a' b' £ 80 c" d" e" f" g" a" b" a" Bass. 342 Control of the Voice. 128 Tenor. 512 Fig. 21. — Tub average range of human voices. c' to f is common to all voices. The figures indicate the number of vibrations per second in the corresponding tones. (Landois and Stirling.) of a violin depends upon age, the peculiarities of the grain of its wood, and the shape, size, and connections of the various parts of the instru- ment. The muscular adjustments necessary in producing all the wonderful variations in tone and in quality of which the human voice is capable are inconceivably delicate. Not only the muscles of the larynx, but those of the various accessory vocal organs, the tongue, the lips, the palate, and the pharynx, and the respiratory muscles, contribute their share in the process. All of these muscles are under the most careful nervous control, and, as we shall see later, a par- ticular area of the brain has as its special duty the management of the vocal organs. The training of the voice is a training of these nervous and muscular mechanisms. That which determines whether a voice shall be called soprano, contralto, tenor, or bass, is partly the natural length of the vocal cords and partly the general nature of the vocal mechanism. The average range of the individual voice is two to two and a half oc- taves ; the relative pitches of the four kinds of voice are shown in the accompanying table (Fig. 21). In singing a scale we are conscious of the necessity at certain notes of rearranging our vocal organs if we wish to produce well-rounded tones and prevent the voice from breaking. Such adjust- ments take place at different notes for different indi- viduals. The compass that is possible for each adjustment constitutes Vocal Registers. 128 PHYSIOLOGY : THE VITAL PROCESSES IN HEALTH. the so-called " register." Vocal teachers detect several registers, but physiologists recognise commonly two — the chest register or voice, and the head register or voice. The former, employed for low notes, is charac- terized by richness and fulness of tone ; the latter, employed for high notes, is thinner. The difference in the mechanism of the two is not fully understood, and is perhaps not the same for all individuals. The appearance of the vocal cords during the production of voice, as shown by the laryngoscope (a small mirror placed in the back of the mouth and reflecting a bright light down into the larynx), is presented in the accom- B ■J.. A Fig. 22. — Inteeiob of larynx, as seen by laryngoscope, during production of (A) chest- voice (Mandl and Griitzner), (B) head-voice (Mills). The glottis is represented as a black longitudinal slit in the middle of the figures, long in A, short in B ; the vocal cords are shaded in A, white in B ; the curved body at the upper part of the figures is the epiglottis ; the rounded elevations at the lower part of the figures are cartilages. panying figure (Fig. 22). The mechanism of the falsetto voice in man is also in dispute. The breaking of the voice in boys at puberty is caused by the rapid growth of the larynx and the constant congested condition of the vocal cords. Speech is voice modified by changes in the accessory vocal organs, especially the resonance cavities, the pharynx, the nasal cavities, and the mouth. The sounds of speech are classified into vowels and consonants. All vowels have the same laryngeal sound as their basis ; but for each vowel, by changes in the shape of the resonance cavities, different over- tones are added to the fundamental laryngeal tone, hence the difference in the sounds. Consonants are noises produced mainly in the mouth by modifications of the outgoing current of air. Some are and some are not accompanied by vocal sounds. In gutturals (K, G) the modification is produced by the soft palate and the root of the tongue ; in dentals (T, D, S, L, Z, 1ST, R) by the tip of the tongue near the teeth ; in labials (P, B, F, V, M) by the lips. The details of the mechanisms of the consonants must be omitted. FUNCTIONS OF AMOEBOID AND CILIATED CELLS. 129 Section III. NON-MUSCULAR MOTOR MECHANISMS. Muscle is not the only motor tissue that is found in the human body. There are two other varieties of contractile cells that in a very unostenta- tious way perform mechanical work and are indispensable to the body's welfare. These are amoeboid cells and ciliated cells. Amoeboid cells comprise the colourless corpuscles of blood and of lymph (Fig. 4, G), and are so called because they resemble and are capable of moving about from place to place like the simple one- Amoeboid Cells. ,,, • t A 7 t> /• ^ ■< -ii celled animal, Amceba. Keterence has already been made to their function. Ciliated cells are epithelial cells, and are fixed in position with one end exposed to the cavity which they line. This uncovered end bears a tuft of minute, delicate, hairlike filaments (the cilia) Ciliated Cells. ,, ■,•.,-, ., m . -, -i-in -^ • -,.,. -, that project into the cavity (rig. 1, L). During life the cilia are in constant, rapid, wavelike motion, sweeping along whatever substances come in contact with them. They are especially useful in car- rying from the lungs toward the mouth and the nose mucus, and with it inhaled particles of dust. They line not only the bronchial tubes, the trachea, the larynx, and the nasal cavities, but the ducts of certain other organs, and, being in incessant action throughout the lifetime, they ac- complish a large amount of labour. CHAPTER III. THE NERVOUS SYSTEM. It would be a sorry community of people wherein every indi- vidual worked for himself alone, regardless of the wants and the welfare of others, and wherein there existed between individuals Nervous System , . ~ • i i • i • . r, , and between professions no social and no commercial in- tra tfenerai. L tercourse. A continuance of such a state of things would be impossible unless the community were composed of few indi- viduals and such as were content to remain low in the scale of civilization. The same principles apply to a community of protoplasmic cells and organs ; if there be no intercellular and no interorganic comity and ex- change there is no rising in the scale of organisms. We have seen that in the human body the mutual interactions of the various parts are exces- sively complex, and this complexity is carried so far that no part is able to live when separated from the body. In both the community of people 11 130 PHYSIOLOGY: THE VITAL PROCESSES IN HEALTH. and the organic community an agent is needed to control the relations of individuals. Such an agent exists in the system of government of the one and the nervous system of the other. In every body of men that has taken rank above the lowest a government exists, while in every protoplasmic animal or- ganism above the simplest there is a nervous system. The subordinate posi- tion in the organic world that is ac- corded to plants is due more than all else to their lack of nervous organs. The nervous system is at once the servant and the master of all the other systems ; it responds to the needs of one by control- ling the work of another ; thus it co- ordinates and harmonizes, and makes one of many. But it attends not only to internal affairs : it keeps the organism apprised of what goes on without, and thus enables the body to adapt itself to its environment. It is, finally, the medi- um of all mental life. To accomplish all this it must of necessity be complicated both in its anatomy and in its mode of working. No system in the body is more complicated. None is more difficult to investigate and to understand. The nerv- ous system of man comprises the central nervous system, consisting of the brain and the spinal cord, and the peripheral nervous system, consisting of the nerves and the ganglia. A portion of the pe- ripheral system is known as the sympa- thetic system, though this can not be re- garded as physiologically independent of the rest. To this enumeration must be added the organs of the special senses; these are so unique as to justify treat- ment in a separate chapter. The differ- ent parts of the nervous system have very different structures and functions, such that the whole may be regarded as a union of numerous complex organs ; but notwithstanding the complexity, the elements of structure Fig. 23. — Diagram of a typical necron. 6, cell body ; d, dendrites, or protoplasmic processes ; a, axis-cylinder process, or nerve fibre. STRUCTURE AND FUNCTION OF THE NERVOUS SYSTEM. 131 and of function are fundamentally the same throughout all parts. "Within the past ten years remarkable advances in our knowledge of nervous structures have been made. The elements of nervous structure are the nerve cells, or neurons, as they are now coming to be called. Neurons vary in shape, but each con- sists of a cell body and processes extending from it Neii'ZftruLre. ^ 23 > The cel1 ho ^ consists of Protoplasm and a large nucleus. The processes may be of two kinds, called dendrites, or protoj)lasmic processes, and axis-cylinder processes. The dendrites are much-branched, short filaments. The axis-cylinder process, usually one in number for each cell, is the most highly specialized part of the neuron. It has few branches, and may be very long (three to four feet). Near its end it splits into numerous fine filaments that terminate in the vicinity of the cells supplied by the neuron, whether they be muscle cells, gland cells, sense cells, or other neurons. The brain and the spinal cord are masses of neurons bound together by connective tissue and richly permeated by blood-vessels. It was formerly supposed that the processes are joined together into an inextricable network, and that nervous impulses, in pass- ing from one part of the nervous system to another, traverse this maze. But recent discoveries have made it reason- ably certain that there is no network what- ever ; that, on the other hand, every neuron is independent of every other, and that a nerv- ous impulse passes from one to another through contact, and not through continuity of their respective pro- cesses. In some parts of the brain and the spinal cord cell bodies and dendrites predomi- nate and constitute the gray matter ; in other parts axis-cylinder pro- cesses only exist, constituting the white matter. Nervous ganglia are masses of cell bodies. Nerves are bundles of axis-cylinder processes ar- ranged parallel to each other, each process being ensheathed usually in a fatty covering called the medullary sheath. The axis-cylinder process Fig. 24. — Psychic brain cells in different stages of evo- lution. A, frog ; B, newt : C, mouse ; D, man. Series a to e shows the stages that a single brain cell of a higher vertebrate passes through in its growth. (Baker, after BamGn y Cajal.) 132 PHYSIOLOGY : THE VITAL PROCESSES IN HEALTH. Elements of Nervous Function. and its protecting sheath form a nerve fibre, and every nerve is composed of numerous nerve fibres. In the embryo nerve cells arise as compact bodies. The processes appear as outgrowths from them, and continue to grow in length and complexity during embryonic life and adolescence. It is a significant fact that the neurons are more complex the higher they are in the animal scale (Fig. 24). The elements of nervous function comprise the functions of the body of the neuron and those of the processes. The cell body is the central organ of nervous energy. It receives, originates, and gives out nervous impulses. In most neurons the activ- ity of the cell body is wholly unconscious, but in those existing in the superficial layers, the cortex of the cerebrum, mental phe- nomena accompany the nervous actions, hence such neurons are called psychic. The processes of the nerve cell are specialized to conduct nerve impulses, the den- drites conducting probably toward the cell body, the axis- cylinder process in some cells away from, in others toward, the cell body. Nerve REFLEX CENTRE - AUTOMATIC CENTRE — o- sensorv cell and cells are usually said AFFERENT NERVE to act either auto- MOTOR CELL AND EFFERENT NERVE ' MOTOR CELL WITH EFFERENT NERVE matically or reflexly. An automatic action is one in which the impulse originates in the cell body as the result of chemical or other changes, and passes thence along the axis-cylinder pro- cess to the end organ (Fig. 25). The re- spiratory centre is said to act automatically. The psychic cells are called automatic. It is a question whether automatism in this sense is at all a common phenomenon. A reflex action is one in which the nervous im- pulse originates outside of the nerve cell, passes to the latter, is there elaborated, and then passes on as before to the end organ (Fig. 25). A typical example of a reflex action is that of winking : a foreign body touches the eyelashes or the eyeball ; this causes nervous impulses to go to the nerve cells that control the muscles of the lids ; return impidses Fig. 25.— Diagram to illustrate nervous mechanism in (1) auto- matic ACTION, (2) REFLEX ACTION', (3) PASSAGE OF SENSORY IM- PULSE UPWARD AND OF MOTOR IMPULSE DOWNWARD WITHIN CEN- TRAL NERVOUS SYSTEM. (Mills.) THE NATURE OF REFLEX ACTIONS. 133 Nerve Centres mid Nerve Conductors. come back to the muscles, and the lids close. Keflex actions are invol- untary. The greater part of the body's actions are reflex ; not only the unconscious movements that we are making constantly by means of our skeletal muscles, but also the muscular movements of the viscera and secretion in glands. The term "reflex arc" signifies the anatomical apparatus required for a reflex action. It consists of (1) a sensory end organ ; (2) an afferent nerve fibre ; (3) its associated nerve-cell body ; (4) a second nerve-cell body in functional connection with (3), and giving rise to (5) an efferent nerve fibre ; (6) a motor end organ, usually a muscle. Thus we see that, physiologically, the nervous system consists of innumerable nerve centres and nerve conduct- ors. The bodies of the nerve cells are the centres ; the processes, especial }j the axis-cylinder processes, are the conductors. Considered en masse and roughly, the gray matter of the central nervous system and the ganglia outside of it have the functions of centres ; the white matter of the central nervous sys- tem and the nerves are conducting in func- tion. No fibre conducts in more than one direction. The nerve fibres outside of the brain and the spinal cord may be divided into two great classes, according as they conduct impulses toward the central nervous system or away from it ; accordingly, they are known either as centripetal or afferent, or as centrifugal or efferent fibres. Most nerves are composed of both kinds. In the case of the spinal nerves a separation of the two kinds takes place at the junction of the nerve and the spinal cord, such that the posterior or dorsal root consists of afferent, the anterior or ventral root of efferent fibres. An afferent impulse, upon arriving at its centre, may simply give rise at once to an unconscious efferent impulse, pro- ducing a reflex action, or, with or without doing this, it may pass up- ward to the brain and give rise to a sensation (Figs. 25 and 26). Corre- spondingly, an efferent impulse may either arise in a lower reflex centre, as the direct result of an afferent impulse, or it may arise high up, even in the psychic part of the brain, and pass downward and outward, giving rise to a voluntary act. Hence, in harmony with the classification of periph- eral fibres into afferent and efferent, there occurs within the brain and the cord a distinction between such fibres as conduct upward toward the cere- brum and such as conduct downward from the cerebrum. The former are really paths for the continuation of the afferent impulses coming Fig. 26. — Diagram intended to show the relations of the brain, the spinal cokd, and the peripheral organs. sensory end organ ; C, spinal cord ; M, motor end organ (mus- cle); H, hemisphere of brain. (James.) s, 134 PHYSIOLOGY: THE VITAL PROCESSES IN HEALTH. from tlie outside to the psychic cells, where the impulses may give rise to sensations; hence afferent nerve fibres and those that conduct upward within the brain and the spinal cord are often called sensory. On the other hand, the downward impulses within the central Sensory and Motor. . . , . , . ,. nervous system are destined in large part for the mus- cles, and pass to them along the efferent nerve fibres, hence such conduct- ing paths are called motor. The same terms apply to the cell bodies which the fibres join. The distinction between sensory cells and motor cells, sensory fibres and motor fibres, sensory centres and motor centres, and sensation and motion as nervous functions, is one of the most funda- mental distinctions in the physiology of the nervous system. It is a curious and as yet not explained fact that the sensory parts of the brain and the spinal cord lie, in general, dorsal or posterior to the motor parts. We have now presented the elements of nervous action. The central nervous system is a collection of central stations for the receipt, trans- formation, and transmission of nervous energy. Each , T . , . . of these stations has its own specific function, but they Jyervous Action. m r ' . ■' are joined with each other in the most intricate manner, and they are continually modifying each other's work. In ascending from the lower to the higher parts of the nervous system there is a pro- gressive correlation of functions and a supervision of the lower by the higher centres. At the top in the cortex of the cerebrum lie the psychic cells, which are the physical media of mental life, the seat of the sensa- tions and the place of origin of voluntary acts, and which are able to con- trol the acts of all the lower centres (Fig. 26). To this elaborate mechan- ism stream constantly through sensory nerves impulses from all parts of the body and from the organs of the special senses, giving information re- garding the condition and the needs of the various organs and tissues and the occurrences of the outside world. The ultimate possible goal of these impulses is the cortex of the cerebrum, there to give rise to sensations. But very few of the impulses reach this goal. Those to which the atten- tion is directed, those which are unusually or excessively strong, or which for other reasons require consideration by the mind, pass to the cortex. The great majority, however, are dealt with by the lower centres. Wherever the impulses terminate they act upon sensory centres ; these in turn stimulate motor centres ; and, largely as the result of the incoming stream, there is as constant an outgoing stream of motor and other im- pulses through efferent nerves to the tissues and the organs. These out- going nervous impulses regulate the actions of the various parts and of the body as a whole. Let us now consider the parts of the central nervous system more in detail. As we ascend through the series of vertebrate animals in the order, THE BRAIN AND SPINAL CORD. 135 fish, reptile, bird, mammal, man, we find that there is a progressive in- crease in the weight of the brain as compared with the Comparative Im- . 1 , * , -, ■, , „, . . , . , , t f D ■ weight of the body. I Ins is shown in round numbers portance of Brain. & J in the accompanying table (Waller) : Weight of brain. Weight of body. Pish Bird 1 1 1 1 1 1 5,000 1,500 220 180 120 Man. . . 50 Moreover, there is a progressive increase in the size of the brain as compared with the spinal cord ; within the brain there is a progressive increase in the size and complexity of the higher parts as compared with the lower ; and, lastly, there is a progressive increase in the size and relative importance of the cerebral cortex. This last fact is to be corre- lated with the gradual evolution and perfecting of mind, while the facts together mean that in general there is a progressive subordination of lower to higher centres. Accordingly we find, in ascending the series, a gradual limiting of the work of the lower parts. A fish or a frog will continue to live for days after the brain is wholly destroyed if the spinal cord be left intact. This is impossible in the case of man. The human spinal cord is pre-eminently the Spinal Cord. , £ ,, a ,. . , . , central nervous organ for the reflex actions in which the spinal nerves take part, hence for the actions of the limbs and the trunk (Fig. 27). The " tone " of the voluntary muscles depends upon it. It contains respiratory, vaso-motor, and other centres which are accessory to more powerful ones in the medulla oblongata. Obviously it is also the path of conduction of impulses between the spinal nerves and the brain. The majority of these impulses ascend and descend upon the side of the cord from which their nerves arise. A few cross over to the opposite side. The medulla oblongata and the rest of the brain stem contain the reflex centres of the cranial nerves and various other automatic or reflex centres ; they regulate the movements of the eyes, the face, the tongue, the alimentary canal, the heart, res- piration, the arteries, the larynx in speaking, the pharynx and oesophagus in swallowing ; they also control the secretion of saliva and other digest- ive fluids. The brain stem serves also, like the spinal cord, as a path of upward and downward impulses. It is a curious fact that both sensory and motor impulses here cross over from one side of the brain to the other ; hence the left side of the cerebrum deals with the right half of the body, and vice versa. Brain Stem. 136 PHYSIOLOGY : THE VITAL PROCESSES IN HEALTH. The function of the cerebellum is commonly believed to be that of harmonizing or co-ordinating the actions of the muscles. Injury to this part of the Cerebellum. , . ,, brain results in irregularity and uncertain- ty of bodily movements. But experimental evidence has not made it clear exactly how the organ acts. Exact knowledge regard- ing the functions of the optic thalamus and the corpus stri- atum is quite wanting. The cortex of the cere- brum is spoken of as the "seat" or Cortex of Cerebrum. the "organ' of consciousness and of intel- ligence ; its cells form the physical basis of mental phe- nomena. When a man thinks, his cortical cells act; and if the latter be destroyed, men- tal phenomena seem to cease so far as that individual is concerned. We can only speculate as to the kind of relation that exists between the cerebral and the mental processes ; so far as we know, the latter are always accom- panied by the former. As experimental physiologists we search for the cerebral pro- cesses, and we find that apparently they do not differ from those that take place in any mass of nerve cells whose activity is unaccompanied by consciousness. Structurally the cortex is unique, being characterized by the presence of the large, much-branched pyramidal cells that occur nowhere else in the organism (Fig. 28) ; in details it differs in different parts. Formerly it was believed that the cortex acts as a whole, but modern research has shown the untenability of this view, which, indeed, seems now opposed to common sense. In 1870 two German physiologists, Fritsch and Hitzig, found that if Fig. 27. — Diagram to illustrate reflex actions. A, skin; F, F', muscle fibres; large column at left represents a piece of the spinal cord with gray mat- ter within and white matter without; B, dorsal nerve roots : C, ventral nerve roots ; N i, N' 1, N 2, N' 2, N 3, N' 3, neurons. A simple reflex path com- prises N 1 and N 2, which include origin of axis- cylinder process in end bulb (a) or between cells (A), afferent fibre (f its mother. The blood and the blood-vessels of the two are quite distinct from each other; the child has its own intrinsic blood system. But in addition to this the large umbilical artery passes from its body along the umbilical cord, there to end in the placenta in thin-walled 13 162 PHYSIOLOGY : THE VITAL PROCESSES IN HEALTH. tufts that penetrate into large spaces filled with the mother's blood. The embryonic and the maternal blood are separated by a thin mem- brane only, that allows ready diffusion of food, oxygen, carbonic acid, and wastes. The embryonic blood is purified and refreshed at the ex- Fig. 51. — Embryo within the uterus."* (Diagrammatic.) c, embryo ; ?', alimentary canal of embryo; r,o, m\ m, various structures composing the umbilical cord; q* n\ q, a, placenta; m,a\q\ s, membranes surrounding embryo; ft, ft, lining membrane of uterus : at each side extends off a Fallopian tube, and below is the neck of the uterus. (Liegeois.) pense of the mother, and is returned by the umbilical vein to the body of the child. The usual duration of pregnancy is about forty weeks. Toward its close the presence of rhythmically repeated pains in the uterus heralds the birth of the child. These pains of labour are accompaniments of wavelike contractions that pass over the muscular uterine walls. With each succeeding contraction the walls press closer and closer upon the foetus, the opening of the womb and the vagina relax, and the child is slowly and painfully forced upon the world. The first breath is drawn into the lungs, and the infant announces its arrival by cries of distress. The umbilical cord is tied and cut ; a few minutes after birth the placenta is expelled ; and after this the wounded uterus gradually heals. PHYSIOLOGICAL LIFE OF THE INDIVIDUAL. 163 Usually for some time after birth the child continues to depend upon the mother for its sustenance. During pregnancy the mammary glands of the mother, which form the breasts, increase in size and in functional power, and at the time of birth they are capable of secreting the milk that the child requires. Human milk is essentially not unlike the milk of other female animals. It contains, how- ever, somewhat more sugar, and is more watery than cow's milk. We can not bring this chapter to a close more fittingly than by a brief review of the physiological life of the individual and a reference to some of its more special features. A lifetime may be di- Individual vi ded roughly into three periods : those of youth, of middle life, and of old age. Youth is the period of growth, middle life that of maturity, and old age that of de- cline. A sharp distinction between these three is, however, impossible ; the coming of age of the individual at twenty-one years is a convenient legal fact, but not a principle of nature. Growth in height may con- tinue until about twenty-five years of age, and after fifty years a dimi- nution of stature may follow. Weight may continue to increase until forty years, and after sixty years it may decrease. From about ten to fifteen years of age girls grow more rapidly than boys, the year of most ac- tive growth in girls being, in Europe and the United States, the thirteenth. After the fifteenth year boys surpass girls in rate of growth, their most active year being the sixteenth. Girls reach puberty before boys, and attain their complete growth at an age three or four years younger than boys. The metabolism during the period of youth is of necessity largely constructive, during middle life the constructive and destructive phases balance each other, and in old age destructive metabolism, with possible fatty or calcareous degeneration of tissues, becomes more prominent. In comparison with the young of most other animals, the human infant comes into the world very immature and helpless. Hence the period of youth is largely devoted to a perfecting of the various functions, and es- pecially to the forming of associations and the laying out of paths of greater and less resistance within the central nervous system — in a word, to the formation of habits. Many functions vary in their activity peri- odically. As examples of such variations may be mentioned the common increase of weight in winter and decrease in summer, the monthly men- strual flow, a regular daily variation in temperature, such that the highest temperature occurs between 9 a. m. and 6 p. m., the lowest between 11 p. m. and 3 a. m., and a similar daily variation in the pulse rate and in the rate and depth of respiration. Of all the daily rhythms, the alternation of sleep and waking is the most striking. Sleep is, in brief, a profound depression of the activities 164 PHYSIOLOGY : THE VITAL PROCESSES IN HEALTH. of the central nervous system. The muscular reflexes, the activities of the sense-organs, respiration, secretion by the glands, and general meta- bolism are all depressed, perhaps as the result of the cen- tral depression ; but the absence of consciousness— that is, the depression of the activities of the psychic cells — is the most pro- nounced feature of sleep. That the psychic activity is not always wholly eliminated is evidenced by the occurrence of dreams, but the fragmentary and grotesque character of dreams indicates that but a few brain-cells are engaged in their production. The reason for this periodic depression of brain activity has been long sought, but with little success. Perhaps the accumulation within the body of the waste products of protoplasmic activity causes a temporary paralysis of the brain-cells, lasting until the wastes are removed. However this may be, the amount of blood in the brain is probably considerably less during the sleeping than during the waking hours, and this may of itself cause cerebral depression. Sooner or later, even if disease and accident are safely overcome, the vital machine is doomed to wear out, and death follows. Physiologists recognise two kinds of death : death of the individual and death of the tissues. The individual dies when the heart ceases to beat and to supply the whole body with food and oxygen. But the individual tissues vary very greatly in the time of their death. The nervous tissues die almost immediately, but the muscles are capable of contraction upon artificial stimulation for a considerable time after the thinking man has forever ceased his activities. Probably not for several hours do all the chemical changes take place within the protoplasm that are the evidence of the passing away of that mysterious, unexplained condition that we are wont to call life. CHAPTEK VI. HEREDITY. In the vast world of life nothing seems more marvellous than the fact of inheritance. Like produces like ; a child resembles its parents. This fact is easily comprehensible in the one-celled organisms, where the parent cell, after a period of activity, puts an end to its individuality by an equal division of its whole body into two similar Biological Fact offspring, and nothing is lost. But in the higher and larger animals, composed of countless millions of cells that are specialized in a great variety of ways, the vital material con- tributed by the parents consists solely of the two microscopic germ cells, INHERITANCE A BIOLOGICAL FACT. 165 and from this infinitesimal beginning there comes an organism that resembles, often in minute and unimportant details, the parents, the grandparents, and even remote ancestors, and that does not resemble other races. Here, if anywhere in our search after natural causes of things, it seems necessary to bring into causal relation the supernatural, and to hide our helplessness in an assumption of some mysterious guiding force that is distinct from vital matter and that we know not of. Truly, we know little of natural law, but the restless scientific spirit of man is ill content with a supernatural explanation of heredity. Inheritance is a biological fact, and must be explained in accordance with biological laws. Before proceeding, however, to the explanation, let us review in some detail the facts that must be explained. We have said that like produces like. This statement must be accepted with certain acsoj modifications. Obviously, no two individuals are like Inheritance. w _ . . each other in all points ; twins, indeed, may be "as like as two peas," and yet one is not the mirrored image of the other. As a general law, a child resembles its parents more closely than it resembles any other individual. Between its parents, the resemblance is in some cases more strongly in favour of the father, in other cases in favour of the mother ; in still others, both paternal and maternal characteristics seem to be present in approximately equal proportions ; rarely is the child " the exact image " of either parent. At times, however, the off- spring seems to possess almost or quite none of the parental qualities, but to show likeness to grandparents or great-grandparents, either as regards general or as regards particular features or qualities. The reappearance of an ancestral quality thus, after having lapsed in one or more genera- tions, is called atavism or reversion, and the subject forms one of the in- teresting chapters in the story of heredity. Lastly, in rare cases a child seems to be a veritable " black sheep," and to possess no qualities what- ever that ally it to its progenitors. In speaking of inherited resemblances, we do not mean to confine ourselves to mere anatomical matters ; likeness of feature, of form, of size, of structure, is most obvious, and is the kind of resemblance most commonly sought for. But likeness in things physiological and things psychological, in the mode of working of the body and of the mind, in things moral, in peculiarities of temperament, is as common, if not so readily perceived. In fact it is difficult, if not quite impossible, to draw the line between features and qualities of the parent that are heritable and those that are not so. The question of the inheritance of disease has been much debated. It seems to be a fact that the germs of certain infectious diseases, such as syphilis, may be conveyed to the child's organism within the parent's germ cells, or even directly from the mother's body to the growing embryo. It seems also probable that predisposition to certain diseases, in the form 166 PHYSIOLOGY : THE VITAL PKOCESSES IN HEALTH. of constitutional weakness and diminished power of resistance to the dis- ease germs, is heritable. Probably the apparent transmission of con- sumption is thus to be explained. Inherited predispo- n en a e oj gition and the fact that the child is usually brought up Diseases. , j o i in the home of its consumptive parents, with every opportunity of acquiring the very germ that it can not successfully resist, are probably responsible for many attacks of this fatal malady. Predis- position to nervous diseases, to insanity, to suicide, and to crime, are certainly transmissible. A second much-debated question is that as to whether new character- istics acquired by a parent may be transmitted to his child. To the mind of the layman, and at first thought the question seems Inheritance of Ac- j j. , -, , 1 ' ui ■ . , ,,, ' superfluous, for apparently there are innumerable m- quired Characters. r ' . . ™ stances of such transmission about us — the affirmative side of the question goes without saying. But when we examine the evi- dence carefully we are surprised at its inconclusiveness. In one case the peculiarity is found, after all, to be not one acquired by the parent, but one possessed also by other ancestors — to be inherent in the race ; in another case, what is inherited may easily be a tendency, a strength or weakness of mind or character, that with a suitable environment may show in the child in the same striking manner as in the parent, and yet that is in no sense an inheritance of anything acquired by the parent — such, for ex- ample, as a taste for strong drink ; in another case, the parent's peculiarity may prove, upon examination, to have been obtained subsequent to the birth of the child ! And thus, one by one, innumerable cases of seeming inheritance of acquired characters have been disproved. Experimental investigation has proved equally inconclusive. For centuries the Chinese have compressed and distorted the feet of their girls ; yet the feet of Chinese girls born at the present time are apparently not different from those of a generation a thousand years ago. Numerous researches made in recent years upon the dehorning of cattle and the removal of the tails of mice for a long series of generations have resulted in no reduction of the horns in the one case or of the tails in the other. Hence, though there are still many seeming arguments in favour of use and disuse as important factors in evolution and in heredity, there are many persons that exclude these utterly from their creed, and so the matter of the inheritance of acquired characters must be left for the present undecided. In view of the above facts it seems idle, and it is certainly misleading, to talk of the equality of men, for Nature has established classes that are much more firmly ordained, than are those of society. i y pouerju ^^ environment of the child and the man has much to do with what the individual accomplishes in the world, but the environ- ment works upon material that already is moulded in great part by past THEORIES OF HEREDITY. 167 generations. The inherited qualities of one person place him at once well in advance in the struggle for existence ; those of another too often prove a serious handicap. Heredity is a powerful factor in human progress. It is to be hoped that a greater knowledge of its facts and principles will gradually modify existing educational systems for the young and penal systems for criminals. How, now, may the above facts of inheritance be explained ? Leav- ing aside all metaphysical and theological theories as unbiological and inadequate, we must start with the belief, incredible as it may seem, that the bits of protoplasm that constitute the ovum and the spermatozoon contain within themselves in some form the qualities of the mother and the father respectively, and to some ex- tent of more distant progenitors. Probably we can go even further than this, for there is much evidence that the real carrier of the hereditary qualities, the germ-plasm, is confined to the nuclei alone of the two germ cells. We have already seen that the tail of the spermatozoon is a loco- motor organ, and dies after conveying the head to the ovum ; the head consists chiefly of nucleus, and it alone enters the egg. Likewise, within the ovum the mass of egg substance seems to subserve nutritive and other purposes, while the nucleus alone is probably essentially hereditary. The fusion of the spermatozoon and the ovum is a fusion of their nuclei, and the segmentation of the egg is primarily a nuclear phenomenon. But how is it possible for the minute germ-plasm to obtain and to hold the parental qualities ? This problem has been the subject of much spec- ulation, especially since the writings of Darwin set men Darwin's Theory , , . •, . <■ , ,' ■ • r • j . i 1 • „ „ .to thinking of the origin of species and the mechanism of Pangenesis. ° . of descent. In his book entitled The Variation of Animals and Plants under Domestication, published in 1868, Darwin gave to the world his own hypothesis of inheritance in a chapter entitled Provisional Hypothesis of Pangenesis. In brief, this theory supposes that all parts of the body are giving off at all times excessively minute particles, the germs of the various cells, or gemmules, as Darwin called them ; these pass into the circulating blood, are carried to the repro- ductive organs, and become a part of the germ cells. Hence every germ cell is a mass of germs of the body cells of the parent, together with some gemmules of more remote ancestors, and hence the fertilized ovum contains the potentialities of the image of either parent and of some more ancient ancestral characteristics. Darwin's gemmules are somewhat of the nature of Herbert Spencer's hypothetical " physiological units," which, before Darwin wrote, were used by Spencer to explain heredity. Dar- win's hypothesis allows the transmission of characteristics that are ac- quired by the parent ; for an alteration of any part by use, disuse, loss, or injury might cause altered gemmules to go to the germ cells, and the 168 PHYSIOLOGY : THE VITAL PROCESSES IN HEALTH. child might then show in the corresponding part of his body the effects of use, disuse, loss, or injury in the parent. Darwin's theory is purely speculative. Neither he nor later biologists have known, as an observed fact, any such giving off of germinal units as he assumes. The theory has not been generally accepted ; for, apart from the inconceivability of the presence of so vast a number of germs within a single minute cell, many facts render it improbable. But it has served a useful purpose, as doubtless Darwin intended it should • serve, in stimulating thought and investigation. Since the appearance of Darwin's provisional hypothesis a large lit- erature upon heredity has appeared, and several theories have been pro- posed. Some of these follow Darwin and attempt to Weismann's Theory. ,,,.■.. n . , n j. ■-, supply lacks in Ins explanation ; others are tar removed from his speculations. We have here space to mention but one of these, which has attracted wider attention than any other, and which has been the subject of widespread and earnest discussion. This is the theory of Prof. Weismann, of the German University of Freiburg, who since 1881 has published several volumes of essays upon this and kindred subjects. Weismann is a careful and conscientious thinker. His theory has been de- veloped gradually, and, as the significance of his main conceptions has broadened with continued thought and investigation, his later writings contain modifications of his earlier views. The nucleus of Weismann's theor}* is expressed by his own phrase, " the continuity of the germ-plasm." This phrase means that, according to his idea, the germinal or hereditary substance that is The Continuity of , . ,, ,, , .. » . ,. „ 7 present -in the ovum or the spermatozoon of any indi- vidual, whether man, lower animal, or plant, has come directly from the germinal substance of the parent of the individual ; that the germinal substance of the parent likewise has come directly from that of the parent's parent ; that the germinal substance of any individual may be traced through parent, grandparent, great-grandparent, and so backward to the remotest ancestors ; that, in a word, germinal substance, arising far back in the lowly and primitive predecessors of existing organisms, has been and is continuous in each line of descent throughout all generations. Hence, germinal substance does not arise de novo ; it does not originate within a body by the concourse of multitudinous minute "gemmules" that are given off from all parts of the body, but it is derived solely from pre-existing germinal substance ; in this sense all individuals are powerfully and equally " blue-blooded." Every individual, therefore, begins life as a minute mass of germ-plasm, that consists partly of male germ-plasm and partly of female germ-plasm, and that in the case of the human being is destined to grow and develop into the child within the mother's womb. As growth and development go on, much of WEISMANN'S THEORY. 169 the germ-plasm differentiates, and produces the various -cells, tissues, and organs of the child's body. A small portion, however, remains undiffer- entiated, and takes up its residence in the essential reproductive organ of the child — in the ovary, if the child be a girl, in the testis if a boy ; and this undifferentiated residue which, as has been seen, has come directly from the parents, is the germ of the future progeny of the child. Hence, according to Weismann's theory, the body of every individual consists of two kinds of protoplasm, viz., germ-plasm, or hereditary sub- stance, which in its undifferentiated form resides within Germ-plasm and . , , . , , ,.,,,.. Somatoplasm essential reproductive organ, and is the derivative of past and the progenitor of future races ; and body- plasm, or somatoplasm, which is the protoplasm of the rest of the body, the muscles, the glands, the heart, and the brain. The latter serves the daily needs of the individual and dies when the individual dies. The former subserves reproduction alone ; if the individual reproduces its kind, some of the germ-plasm is passed on to the descendant ; if the indi- vidual does not reproduce, the germ-plasm dies with him, and that par- ticular line of descent is forever ended. Thus far Weismann's theory seems readily comprehensible and reason- able. But let us carry it, as its author does, a little farther. What relation within an individual body do these two kinds of proto- e a wn of plasm — the germ-plasm and the somatoplasm — bear to Germ-plasm and *■ ° _^ J . r Somatoplasm. eacn other { Do changes in the one necessarily affect the other ? Weismann believes that the two are quite independent of and distinct from each other. They are nourished by the same nutrient blood and lymph, it is true, and any general alteration of the nutrient fluids alters the nutrition of the two alike ; but this is of comparative unimportance. The important consideration is that any alteration of a particular part of the body-protoplasm does not affect the germ-plasm ; in the origin of the latter from the parent's germ-plasm instead of from the individual's own somatoplasm, and in the absence of gemmules passing constantly from somatoplasm to germ-plasm, the latter can not reflect the condition of the former. Hence, the loss of a limb by accident, the gain of strength in particular muscles by athletic exercise, the acquisition of an art, like piano-playing, which consists of delicate and intricate muscular and nervous adjustments, the long-continued mental development of the man or the woman which carries with it molecular adjustments of brain substance, the practice of a trade or pro- fession which develops abnormally certain organs, or tissues, or cells, and allows other organs, tissues, or cells to degenerate — all these affect the germ-plasm in no wise, and can not be transmitted to the descendants of the individual. Characteristics acquired by the parent are, therefore, not inherited by the child. The non-inheritance of acquired characters is 170 PHYSIOLOGY : THE VITAL PROCESSES IN HEALTH. the second fundamental postulate of Weismann's theory, and is to be placed beside that of the continuity of germ-plasm. Why, then, are not all children alike mentally, morally, and physi- cally ? From a common origin in the early history of organic beings, with a descent through numberless generations by a Variation continuity of germ-plasm that leads a charmed life of a certain degree of independence of environmental changes, how is varia- tion possible, and how may the differences of individuals and of races be accounted for ? In the first place, germ-plasm is not wholly independent of environmental changes. It is capable of, and is constantly undergoing, alteration as the result of general nutritional alterations taking place in the body in which it is housed. The germ-plasm of one line of descent must therefore necessarily differ somewhat from that of another line of descent, since the environment of the one differs from that of the other. In the second place — and here we have the chief source of variation — the individual is the result of a fusion of two varieties of germ-plasm, that of the father and that of the mother, each with a history and capabilities differing from those of the other. Therefore the resultant organism, while possessing ancestral qualities, must possess them in a combination that has never before existed ; it must differ from any organism that has preceded it ; and hence it follows that no two individuals, or two species, or two races can be alike. These, then, are the elements of Weismann's theory : continuity of germ-plasm and non-inheritance of acquired characters tending to pre- serve the uniformity of the race ; slight germinal changes and sexual re- production tending to destroy that uniformity.* During the past ten years and more the theory in its manifold details has been fought over with great vigour by its friends and its foes. The contest has been especially warm over the question of the inheritance of acquired characters. We have thus touched very incompletely upon some of the principles and problems and attempted explanations of the mystery of inheritance. To the scientist a mystery is always inviting, and to the modern biologist that of heredity is especially so. What is needed most is careful, truth- ful observation and recording of the facts that are all about us, and well- directed, painstaking and unprejudiced physiological experimentation as to hereditary possibilities. Darwin, Galton, Weismann, and others have done much of the former service ; the latter belongs largely to the science of the future. * Essays upon Heredity and Kindred Biological Problems. By August Weismann. Authorized translation. Vol. I, 1889 ; Vol. II, 1892. Oxford, The Clarendon Press. The Germ-plasm : A Theory of Heredity. By August Weismann. Authorized trans- lation. 1893. New York, Charles Scribner's Sons. COLUMBIA UNIVERSITY LIBRARIES This book is due on the date indicated below, or at the expiration of a definite period after the date of borrowing, as provided by the rules of the Library or by special arrange- ment with the Librarian jn charge. DATE BORROWED DATE DUE DATE BORROWED DATE DUE C2e(ll4l)M100 I , -..-■v m .