Ill 11,11 II II ! II Mil siiiti I ijiiiiiiifiiiijjif ittfitfiif - £foui fork Hate (College of Agriculture JU Cornell MnuietBitj} IGihrarij Cornell University Library QP 141.S7 1915 Nutritional physiology. 3 1924 003 567 272 Cornell University Library The original of this book is in the Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/details/cu31924003567272 BOOKS BY PERCY G. STILES Nutritional Physiology i2mo of 287 pages, illustrated. Cloth, #1.2; net. Second Edition. The Nervous System and Its Conservation i2mo of 229 pages, illustrated. Cloth, $1.25 net. NUTRITIONAL PHYSIOLOGY BY PERCY GOLDTHWAIT STILES INSTRUCTOR IN PHYSIOLOGY IN HARVARD UNIVERSITY ; FORMERLY ASSISTANT PROFESSOR OF PHYSIOLOGY IN SIMMONS COLLEGE ; INSTRUCTOR IN PHYSIOLOGY AND PERSONAL HYGIENE IN THE MASSACHUSETTS INSTITUTE OF TECHNOLOGY, BOSTON; INSTRUCTOR IN THE BOSTON SCHOOL OF PHYSICAL EDUCATION SECOND EDITION ILLUSTRATED PHILADELPHIA AND LONDON W. B. SAUNDERS COMPANY 1915 Copyright, 1912, by W B. Saunders Company. Reprinted September, 1914, and June, 1915. Revised, reprinted, and recopyrighted October, 1915. Copyright, 191S, by W B. Saunders Company PRINTED IN AMERICA PRESS OF b. SAUNDERS COMPANY PHILADELPHIA TO ©rabam TLusk WHOSE DELICATE KINDNESS NO LESS THAN HIS SCHOLARLY POWER MADE MEMORABLE THE YEAR OF OUR ASSOCIATION PREFACE TO THE SECOND EDITION The literature of metabolism and nutrition has long been copious, and its volume is still increasing. If a summary is attempted, it is found more and more dan- gerous to make dogmatic and uncompromising state- ments. Almost every assertion has to be qualified. The resulting tone, as the writer knows from long teaching experience, is irritating to the elementary student who craves downright and absolute dictation. But it is well for him to learn that many things are still debatable, and that the power to suspend judgment while awaiting further evidence is a rare and fine one. In the preparation of this edition I have had the con- tinued and invaluable assistance of Miss Ruth Bryant. I have also to acknowledge my indebtedness to Dr. Thome M. Carpenter, of the Carnegie Nutrition Labor- atory, for many illuminating suggestions. P. G. S. Boston, Mass., October, 1915. PREFACE The making of this book has been a study in elimination. It is, therefore, necessary at the outset' to indicate the limits of its scope. It is intended that it shall be used with other books. Supplementary reading upon general biology, human anatomy, food chemistry, and dietetics is greatly to be desired. In the field of physiology itself many fascinating topics are entirely ignored and others treated in bare outline, with the purpose of subordinating all else to the subject of nutrition. Chemical formulae have been excluded from the text and used but sparingly in the notes. A certain preliminary knowledge of elementary science is assumed. The key-word of the following discussion is " energy." The success of the reader in gaining clear con- ceptions of what is presented will depend upon his famil- iarity with the meaning of that term. It is essential that he shall understand that energy is latent or potential in those chemical compounds which are susceptible of oxida- tion. He must have learned to recognize the possibility of its unending transformation. The more readily he thinks in terms of molecules, the more profitably he can read these chapters. Miss Ruth Bryant, Instructor in Biology in Simmons College, has borne a part in the work, which is to be de- scribed as collaboration rather than assistance. P. G. S. Boston, Mass. TABLE OF CONTENTS CHAPTER I page Introduction 11 CHAPTER II The Energy Relations op Plants and Animals 20 CHAPTER III The Nature and the Means of Digestion 28 CHAPTER IV The Work op Muscles and Glands 36 CHAPTER V Reflex Action 46 CHAPTER VI The Alimentary Canal 54 CHAPTER VII The Mouth — Swallowing, Salivary Digestion 63 CHAPTER VIII The Movements op the Stomach 70 CHAPTER IX Gastric Secretion and Digestion 80 CHAPTER X The Small Intestine: Its Movements, Secretions, and Digestive Processes 90 7 8 TABLE OF CONTENTS CHAPTER XI PAGE The Large Intestine 100 CHAPTER XII The Blood 107 CHAPTER XIII The Circulation 118 CHAPTER XIV The Absorption of Food-stuffs 130 CHAPTER XV The Metabolism of Fats and Carbohydrates 137 CHAPTER XVI Nitrogenous Metabolism 150 CHAPTER XVII The Removal of the End-products of Metabolism 166 CHAPTER XVIII The Estimation of Metabolism 175 CHAPTER XIX The Energy of the Metabolism 182 CHAPTER XX The Factors which Modify Metabolism 192 CHAPTER XXI The Maintenance of the Body Temperature 202 CHAPTER XXII The Hygiene of Nutrition 210 TABLE OF CONTENTS 9 CHAPTER XXIII PAGE The Hygiene op Nutrition (Continued) 223 Water; Meat; Sugar. CHAPTER XXIV Food Poisoning 234 CHAPTER XXV Alcohol 242 CHAPTER XXVI Internal Secretion 253 CHAPTER XXVII The Nervous System 261 CHAPTER XXVIII The Nervous System — Its Higher Work 271 INDEX 281 NUTRITIONAL PHYSIOLOGY CHAPTER I INTRODUCTION The Modern Emphasis in Biology. — Living things are transformers of matter and energy. When we say trans- formers rather than generators we indicate the modern as contrasted with the old-time view. When we say that physiology is the physics and chemistry of living matter we suggest the same significant tendency to bring living and lifeless matter into direct comparison and to recog- nize the same laws as operating in both. The teaching that the same laws do hold sway in the living and the non- living is covered by the term " mechanism "; the earlier view that living things are not fairly to be compared with lifeless, and are to some extent exempt from physical principles and limitations, is expressed by the word " vitalism." We have every reason to believe that the principle of the conservation of energy holds as rigidly for the plant or the animal as for the clock or the loco- motive. This is perhaps the most important generaliza- tion of nineteenth century physiology. But while scientific workers are now seeking to analyze the reactions of organisms in accordance with the data furnished by chemists and physicists whose work has been with materials not living, it is probable that the diffi- culties of their problems are better appreciated than was the case a few years ago. Living matter is found to be more complex in structure and more varied in response 11 12 NUTRITIONAL PHYSIOLOGY than had been supposed. Physiologists are bound to be modest in their claims for progress. They are ignorant of many factors at work in even the simplest forms of plant and animal life. And the mystery of consciousness with its relation to nervous systems seems ever to defy approach. Free-living Cells. — About seventy years ago, at a time when investigators were profiting by important im- provements in microscopes, it was found that the larger Fig. 1. — Four types of free-living animal cells: A is the ameba, distinguished for its changeable form; B, the euglena, shows the peculiar feature known as a flagellum, a writhing filament, which is its means of locomotion; C is the paramoecium, or " slipper ani- malcule," which has a ciliated surface; D is the interesting form known as the stentor. plants and animals are made up of structural units as- sembled in vast numbers. These units are generally much too small to be seen without magnification. They are called cells, a term which is not especially appropriate, but not likely to be abandoned. Many microscopic forms, such as the swarming Infusoria of pools and ditches, are cells leading an independent existence. It will be helpful to consider what are the characteristic activities of such cells. They are for the most part equally charac- teristic of the higher forms. INTRODUCTION 13 The free-living animal cell takes something from its en- vironment and returns something to it. It takes into it- self a variety of organic substances together with small quantities of mineral salts. These constitute its food. It receives also a supply of oxygen. This is not ordinarily reckoned as a food and for a good reason. The term food is best restricted to material which can serve constructive purposes or at least be stored in the cell. The function of oxygen is not to promote constructive processes, but to release energy, a process of decomposition in which the stores of the cell are sacrificed. It is hard for the elemen- tary student to realize that a destructive change can be other than disastrous. Yet he has only to reflect that money becomes useful in its expenditure. It is much the same with organic reserves. The process in which oxygen reacts with substances within the cell, giving rise to simple oxidized products in place of complex material rich in potential energy, is called respiration. (The word is, indeed, frequently used as a synonym for breathing, but we shall use it in its chemical sense.) Respiration is often compared with combustion, and while the two are not identical in all their stages, the fact remains that the initial and the final conditions are essentially the same for both. The release of energy is generally just as great in the physiologic change as in the actual burning of like quantities of the cell constituents. The free-living animal cell is thus an accumulator of fuel and a furnace in which it is burnt. But this is a very imperfect comparison, for it has in addition the property of self-repair, and under favorable circumstances capacity for growth and reproduction. Cells multiply by cleaving into two similar parts, and the tendency to do this after a certain increase in size usually limits very definitely the dimensions to which a single cell may attain. When growth is' taking place it is evident that not all the food is serving as fuel; a certain portion is becoming incor- porated with the more permanent substance of the cell and is so changed as to become entirely typical protoplasm. The process through which food becomes an integral part 14 NUTRITIONAL PHYSIOLOGY of the cell is called assimilation. The word emphasizes through its root-meaning the attainment of likeness to the material of the cell and indirectly implies that the food was originally foreign in its nature. We use the expression in much the same sense when we speak of the assimilation of immigrant peoples. The word nutrition is used in about the same way as assimilation, the only distinction being that we speak of the nutrition of cells (or cell- aggregates), while we speak of the assimilation of food, the former term referring to the structure nourished and the latter to the supplies worked over for the purpose. The word digestion is best restricted to the preliminary stages of the assimilation process; its application will be defined later. Respiration has been said to be a process in course of which compounds are decomposed that their potential energy may be made available. The greater part of the released energy appears as heat. A smaller part mani- fests itself in movements through which resistances are overcome. The facts in regard to the production of energy are naturally better known for the larger animals than for the free-living cells, but " the whole is equal to the sum of its parts." The energy from respiration may in exceptional cases become kinetic, in the form of light or electric discharge (firefly, electric eel). The energy which shows itself in movement is of particular interest to us. Motion, when exhibited by animal cells, is almost always the expression of contraction, the word being used in a physiologic sense. So used, contraction does not mean diminution of volume, but does mean diminution of sur- face and active shortening in one or more dimensions. Although an increase in other dimensions attends such changes of form, we do not talk of the " expansion " of cells. It is the contraction which is the positive and forcible element in the movement. When this is said we intentionally leave out of account some types of movement occurring commonly in the plant world, in course of which the cells actually change their volume through gain or loss of water. Among free-living cells the type of move- INTRODUCTION 15 ment may be "ameboid," that is, a flowing of the cell contents to conform to an ever-changing outline. Con- tractile power may be limited in other cases to parts of cells, as in those forms which swim by the lashing of slender projections known as flagella, or by the waving of an animated nap or pile upon their surfaces, the cilia, of which more will be said. In all cases of energetic movement we feel justified in assuming that the source of the power is in destructive chemical reactions, and that a draft is being made upon the fuel stores of the cells. The Association of Cells. — When many cells are massed, as in the body of a worm, the situation of the single unit differs significantly from that of the cell leading an inde- pendent existence. First of all, its environment is made for it to a great extent by other cells. A very small minority are in direct contact with the outside world; the great majority are submerged among their fellows. The typical cell is, therefore, shut in from food supplies of the casual sort on which the free-living cell depends. It is remote from the oxygen of the surrounding air or water. A cell so situated would perish were it not for one of the most striking features of the larger organisms, a moving liquid medium, which bathes the cells and acts as a common carrier. This fluid supplies food and oxygen and removes wastes. The cells composing the body of any animal are of a common descent, but they have taken on widely different characters and have become adapted to particular func- tions. The cell which is in itself a complete living thing must perform all the essential activities for itself — the preparation of crude food, locomotion, etc. In the multicellular animal the individual cells have come to be far more restricted in their powers. Many have become passive structures serving only for mechanical support or surface protection. Such cells may or may not be living. Others, while clearly alive, have ceased to perform cer- tain functions. With limited exceptions movement is exhibited only by those systematically arranged cells 16 NUTRITIONAL PHYSIOLOGY which form the contractile tissues. Almost all the cells require their food to be in solution and of a few standard forms. In other words, the primitive capacity to digest and assimilate every kind of nutriment has been lost, but by a wonderful co-operative activity the internal medium has been made a depot of those particular foods which can still be utilized. d e" " ' Fig. 2. — Drawings like the above are almost always made from tissues which have been prepared and colored by special means to make clear, minute features: a Represents an ovum or egg-cell, the typical cell may be assumed to tend toward this spheric form ; b is a cell from a compact tissue, to show how mutual pressure produces a faceted or polyhedral form; c is a contractile element such as occurs in the walls of the alimentary canal, it illustrates an elon- gated cell; d is an epithelial or lining cell of the order found on the inner surface of blood-vessels; this is an example of extreme flatten- ing; e, from the nervous system, exhibits the possibility of a branch- ing development. As an animal grows larger its directly exposed surface becomes smaller in proportion to its weight. The trans- fers which must take place between the organism and the external world require ample surfaces, and they are secured by infoldings of the body wall at different places. The lining of the alimentary tract is an example of such an infolding and provides a large area for absorption. Among the higher forms the lining of the lungs constitutes a vastly INTRODUCTION 17 extended surface for gaseous exchange. The glands are organs in which are concealed great surfaces through which products of cell activity find an outlet. The specialization which groups the cells of the animal body in a number of classes each with its definite work to do also entails the dependence of each class upon the others. If we compare the life of a savage with that of a civilized man we shall find an analogy not too far fetched to be helpful. We have seen that the free-living cell is self-sufficient, and, indeed, its chances of survival are bet- ter when there are but few of its own kind in the neighbor- hood. Such organisms are competitive rather than co- operative. Somewhat in the same way the solitary savage may be capable of self-maintenance, having the skill to find and prepare his food and after a fashion to shelter and clothe himself. The civilized man is accustomed to look to many other men — and women — to supply his needs. Yet if the man, like the cell, loses something of ruggedness and resourcefulness through becoming a member of a com- plex society, he evidently gains time and opportunity to concentrate his efforts upon a special pursuit. It is very much the same with the cell. Our analogy fails, as such devices are prone to do, when we consider how the civil- ized man may become a hermit or « Robinson Crusoe, whereas no single cell detached from one of the higher forms can exist by itself for any length of time. Co-ordination. — We have emphasized above the services rendered to the organism by its internal medium. The composition of the circulating fluid is influenced by the ever-varying activities of all the organs and tissues. Accordingly, a contribution made to this medium by any group of cells may conceivably modify the conduct of any other group. We shall meet with numerous instances of such influence exerted through the chemical products of one organ upon another or upon the system as a whole. When we speak of an animal as an individual we imply that the parts of the body constantly interact. It is this interaction which makes it appropriate to regard 2 18 NUTRITIOXAL PHYSIOLOGY the body with all its complexity as still forming a unity rather than a colony. The term co-ordination is employed to express this purposeful working together of all parts for the advantage of the whole. The means of co-ordina- tion may be chemical as just now suggested, but a more conspicuous agency in the highly developed types is that of the nervous system. While we must postpone until a later time any detailed description, we ought to indicate at this point the essential contrast between the two modes of co-ordination. One part of the body may affect an- other through the actual despatch of material to it. When the influence is through the nervous system instead of through the circulation there is no transfer of material. The nerves were once supposed to convey a fluid of strange properties, but the fact is established that they transmit a form of energy and not of matter. (The temptation to think of the nerves as conductors of electric currents and to compare the nervous mechanism with a telephone system is strong. Guardedly used, the comparison is valuable, but it is a symbolic rather than a literal repre- sentation of the facts. " Nerve impulses," so called, are not electric currents in the ordinary sense.) In a great nation the prosperity of any section must de- pend to a large degree upon the commercial exchanges taking place between that section and others. Its fac- tories may depend upon the mines of another province for coal and upon still another for raw material. Much in the same way a single organ of the human body is de- pendent upon others. A muscle, for instance, profits by the prepared foods or fuels forwarded to it from the di- gestive tract and by the oxygen borne to it from the lungs. The blood in this case is serving the purpose which is ef- fected by trains and steamers in the case of the nation. Nor do we find lacking in our illustration an analogy for the ner\ous communication between parts of the living body. The type of such intercourse is furnished by the telegraphic messages which pass incessantly from place to place. News, in itself immaterial, may affect the course INTRODUCTION 19 of local events just as surely and much more quickly than can the material exports of another region. ' Reactions produced through the nervous system are correspondingly sharp and prompt in developing. Blood and Lymph. — In the bodies of the higher animals the internal medium may be described as existing in two forms. In direct contact with the majority of the cells there is a comparatively stagnant fluid, the lymph. From this they draw their needful supplies of oxygen and Pig. 3. — B is a blood-vessel of the smallest size — a capillary — with walls of flattened cells like that in Fig. 2, d. The blood flow- ing within- is removed from direct contact with the cells (C, C), but dissolved substances may pass from one to the other through the capillary wall and the medium of the lymph (L). food; into it they discharge their waste. The limited resources of the lymph at a given point would be quickly exhausted were it not that the blood is passing close by in vessels whose delicate walls permit the passage of material in both directions. The blood is in rapid movement and it is constantly renewing the oxygen of the lymph with fresh portions just brought from the lungs. It is at the same time receiving from the lymph the accumulated waste. CHAPTER II THE ENERGY RELATIONS OF PLANTS AND ANIMALS A chemical reaction can usually be assigned to one of two classes. Either it is exothermic, that is, attended by the evolution of heat, or it is endothermic, in which case heat must be supplied to cause its occurrence. When heat is evolved the products of the reaction are generally simpler and more stable than the original material. The most important reactions of this class are the oxidations. Heat, or other forms of energy applied from without, may effect the synthesis of complex substances rich in fuel value from initial material comparatively destitute of potential energy. Broadly speaking, the constructive chemical processes in nature are the work of the higher plants. Animals, as well as those forms of plant life which lack pigment, carry on for the most part reactions of a destructive character. This makes evident the dependence of all other forms of living matter upon the pigmented plants. It is through the agency of light-waves, a form of kinetic energy, that the synthetic reactions resulting in the formation of starch and other energetic compounds are made possible. When light is excluded from the green plant it has no advantage over the animal, but pursues a similar course of decomposi- tion. In fact, there is always going on in the plant, even when its constructive activity is most marked, an undercurrent of an opposite trend. Early writers com- monly exaggerated the supposed contrast between the chemical conduct of plants and that of animals. They were disposed to deny that an animal could execute any 20 ENERGY RELATIONS OF PLANTS AND ANIMALS 21 synthesis whatever. It is true that animal cells make no useful application of the energy showered upon them by the sun's rays. Nevertheless they do carry on synthetic reactions, although to a limited extent. Since much energy is released within such cells by the prevailing oxidative changes it is not difficult to see that some portion of it may be applied to promote endothermic reactions. When a hydraulic ram supplies with water a house considerably above the level of the stream which operates the device, we understand that the result is made possible because a great deal of water falls that a little may rise. The prin- ciple of the conservation of energy is not violated here, nor is it when animal cells erect from a portion of their food molecular structures more complex and energetic than anything in their current supply. The formation of fat from sugar is a case in point. Weight for weight, the fat is more highly endowed with potential energy than is the sugar, but we must take into account the fact that the quantity of sugar entering into this common transforma- tion is much greater than the quantity of fat which can be produced. The energy in the product is more concen- trated, but not absolutely larger in amount than it was in the sugar. There are many species of green plants which are uni- cellular, just as there are many single-celled animal forms. It is suggestive to consider the reciprocal relations of one such plant and a solitary animal cell living beside it. A constant and pressing need of the animal is oxygen. Now oxygen is freely produced by plants when they avail themselves of the energy of light to carry on constructive processes. To this extent, then, the proximity of the plant to the animal is advantageous to the latter. This ceases to be true when light is succeeded by darkness. The animal meanwhile is giving off oxidized products, of which carbon dioxid is the most abundant. This, to- gether with water, is the very material out of which the plant can build its stores of starch and sugar. The out- put of the animal includes also various compounds of nitrogen. These, as well as the carbon dioxid, may be of 22 NUTRITIONAL PHYSIOLOGY service to the plant, although to be strictly accurate it must be added that they require some modification, usually effected in nature through the action of bacteria. In view of these exchanges one is tempted to the hasty con- clusion that a single-celled green plant and a single- celled animal form a balanced system capable of con- tinued maintenance — in short, a microcosm. There is, however, a fatal obstacle to the continuance of the part- nership — the animal's need of organic food can only be satisfied by the sacrifice of the plant. To have a truly balanced system of an enduring character we must assume a multiplication of cells descended from the original uni- cellular plant, providing a surplus of vegetable tissue for the animal's consumption. The give and take which has been illustrated for single cells is proceeding on a vast scale everywhere. It is hard to realize that the great harvests which support the races of mankind were formed for the most part from a gas present only very sparingly in the atmosphere, from water, and from the mineral salts of the soil. "The oak is but a foliated atmospheric crystal deposited from the aerial ocean that holds the future vegetable world in solu- tion" (Holmes). While we rely partly upon animal food (meat, milk, and eggs), this does not alter the fact of our absolute dependence upon the green plants, which in turn owe their growth to the translated energy of the sun. It is amusing to note the apparent travesty upon our human life which can be read into the contrasted conduct of green plants and of animals. The plants are conserving, while the animals are spendthrift. Plants create and distribute wealth. They seem like the thrifty and industrious mem- bers of society upon whose charity others less competent depend. One is reminded irresistibly of the parent at home and the son at college. If the father does not liter- ally subsist upon a diet of carbon dioxid and water that the son may have protein and alcohol, the approach to a parallel is too close for complacent attention. The Body and the Diet. — Turning now to the definite problems of human nutrition, let us consider the respective ENERGY RELATIONS OF PLANTS AND ANIMALS 23 make-up of the body and the diet. There must evidently be a degree of similarity between them, inasmuch as the one has been built from the other. Similar compounds are met with in both, but, as we shall find, in quite unlike proportions. The body is mainly water. Water makes up about two-thirds of the total weight and forms even a larger percentage of the most active tissues. No material reduction of its quantity can be tolerated. Even after death from thirst the amount is surprisingly little dimin- ished. In the diet also water occupies the first place. It is likely to constitute fully five-sixths of the daily income. Its most obvious services are in connection with the absorption of food in solution and the removal of dissolved wastes. By its evaporation from the skin and the breath- ing passages it helps to keep the body temperature from rising above its normal level. Second to water among the substances which compose the body we find the group of bewilderingly complex compounds known as the proteins. A protein always yields the five elements — carbon, oxygen, nitrogen, hydrogen, and sulphur 1 — when subjected to analysis. Some members of the group contain phosphorus also. Merely to mention these constituent elements is to give no proper conception of the intricate manner in which they must evidently be combined. To appreciate this we need to consider the very long list of cleavage products, in themselves rather complex, which can be obtained by the decomposition of protein from a single source. The physiologic chemist is somewhat in the position of a person 1 The elements occur in proteins in about the following percentages : C 53, O 22, N 16, H 7, S 1 per cent. Phosphorus when present amounts to 1 per cent, more or less. It is quite impossible to con- vey an adequate impression of the complex fashion in which the five or six elements are combined. Many years ago the following formula was suggested for hemoglobin, the red protein of the blood, which is exceptional in containing iron: C75 8 H 1203 O. M 8N, 9S FeS 3 . It is not seriously maintained that these large numbers are precisely correct, but the order of their magnitude is probably typical. 24 NUTRITIONAL PHYSIOLOGY who sees the various parts of a watch — the wheels, springs, jewels, etc. — lying loose upon the table of the watch- maker. He may gain a fair notion of the intricacy of the watch, though he may be very far from knowing how the parts were related in the time-piece. We do well to use the plural number in speaking of the proteins, for all recent work tends to emphasize the distinctive molecular pat- tern which characterizes each form and makes it differ definitely from every other. " There is one flesh of men and another of beasts and another of fishes and another of birds." This is excellent and altogether modern chemical biology. The presence of the element nitrogen in the proteins distinguishes them sharply from the other promi- nent compounds in both the body and its daily income. Nitrogen makes up about 16 per cent, of protein, and the value of this figure in calculations will be apparent later. It has been said that the proteins are second only to water in their abundance in the body. This is not true of the diet unless we have to do with a carnivorous animal. Among the herbivora, and almost always among men, the second place in the list of supplies is occupied by the carbohydrates. Third in quantity among the constituents of the body we find in most individuals the mineral compounds. These would not make so large a proportion if it were not for the skeleton. Bone is a tissue in which salts of lime are abundantly present. But in all the other tissues and in the fluids too we find a variety of salts, and it is well established that their presence is not accidental, but a matter of moment. Sodium, potassium, calcium, and magnesium at least, perhaps other bases also, must be kept in certain balanced relations if the life processes are to go on. The acids represented are chiefly hydro- chloric, phosphoric, and carbonic. Sodium chlorid, the one salt which we take pains to add to our food, is the one most abundant in blood and lymph. Potas- sium rather than sodium compounds predominate in the cells. ENERGY RELATIONS OF PLANTS AND ANIMALS 25 Next in amount to the salts in the body of average build, and not uncommonly exceeding them, are the fats. 1 The word " fat " is used sometimes in a chemical and some- times in an anatomic sense. In the first case it denotes a compound of carbon, hydrogen, and oxygen, having a formula of a certain type. In the other usage the mean- ing is " adipose tissue," a form of connective tissue rich in such compounds. Fats have familiar physical characters. They are not soluble in water or to any great extent in the fluids of the body. They pass from solid to liquid form at moderate temperatures; the fats of the human body are regarded as in a fluid state when under the influence of its warmth. No other common physiologic compounds have so much latent energy awaiting release by oxidation. Fats are more plentiful in apparently lean individuals than might be judged. A considerable store of adipose tissue is to be found in any condition short of imminent starva- tion. It has been said above that carbohydrates usually have the leading place among the solid matters of the diet. This is owing to the large proportion of vegetable foods generally consumed. In the animal body the occurrence of carbohydrate is rather scant, and it is one of the chief problems of the physiologist to account for the daily dis- appearance of a great quantity of these compounds in the economy of the organism. The reader may already foresee what we shall later explain in detail, that this disappear- ance of carbohydrate is due in part to the fact that it is the fuel most constantly called upon to evolve energy, and in part to the ease with which the tissues transform to fat a surplus of these substances. Under the head of carbo- hydrates we distinguish the starches and the sugars. 1 Fats are compounds which can be resolved into glycerin and organic acids. Those of chief interest in nutrition are the glycerids of palmitic, stearic, and oleic acids. The first mentioned has a com- position indicated by the formula C 3 H5(OOCH 3] C 15 ) s . The others are nearly related. The three common fats differ in their melting- points and in other respects. They are mingled in definite propor- tions to form the body fat of each animal species. 26 NUTRITIONAL PHYSIOLOGY Starches 1 are of high molecular complexity, imperfectly- soluble, and tasteless. Sugars are cleavage products of starches or, if they occur apart from previously existing starches, closely resemble such cleavage products. They are of definite and moderate molecular weight, they are soluble, crystallizable, and sweet. They contain the same elements which are found in fats — carbon, hydrogen, and oxygen — but the percentage composition is wholly differ- ent and the structure of the molecule also. While the carbohydrates are energetic, their fuel value is less than half that of fats. We have now named this list of substances as uniting to form the body — water, proteins, mineral matter, fats, and carbohydrates — the order suggesting their relative abundance. We have said that in the diet water also takes the first place, but that carbohydrates ordinarily take the second. Either proteins or fats may stand third among the constituents of the diet. Often the amounts of the two are found to be about equal. A possible diet comprises 100 grams of protein, 100 grams of fat, and 250 grams of carbohydrate, illustrating this equality. The mineral matter in the ration is not likely to exceed 20 grams a day. Both the body and its income, of course, contain in small amounts substances which do not fall into any of the classes named. Such miscellaneous organic compounds are conveniently grouped as extrac- tives. Many of them are nitrogenous and represent dis- integration products of proteins. Reference to their significance will be made from time to time. At the very outset the double service of food to the 1 The three elements in starch are present in proportions repre- sented by the formula C 6 H 10 O 5 . But the number of atoms in the molecule is not correctly indicated by this formula; it requires to be multiplied throughout by an unknown, but considerable number. Sugars are of two common classes: the disaccharids, with the formula C^H^On, and the monosaccharids or hexoses, with the formula C 6 H 12 6 . Cane-sugar, malt-sugar, and milk-sugar are disaccharids. The hexoses of direct interest in nutrition are glucose (also called dextrose), fructose (or levulose), and galactose (a deriva- tive of milk-sugar). ENERGY RELATIONS OF PLANTS AND ANIMALS 27 organism was indicated. It represents both building mate- rial and fuel. If we liken the living body to a power-house, we see clearly how both kinds of supplies are required. Coal is the most bulky supply of the power-plant and the one on which its operation immediately depends. But there must also be new parts to replace those which wear out. The up-keep of the building calls for new wood-work, for paint, etc. A certain difficulty is encoun- tered in the attempt to show parallel conditions in the case of the body because the separation of the two func- tions is here much less sharp. Protein food, on the whole, has a peculiar title to be regarded as building material, but it is also an entirely available fuel. It is as if planks and beams designed primarily to repair the structure of the power-house were fed into its furnaces. The suggested comparison must not be pressed too far, for it conveys an impression of wanton destructiveness which we cannot assume to be just in the case of the organism. There are some minor supplies brought into the power-house which are not fuels nor precisely materials for repair. The oil is an example. Some of the extractives of the diet occupy an analogous position, being neither sources of energy nor of construction, but nevertheless favorably affecting the course of events. This comes near to our conception of a drug in relation to the processes of life. CHAPTER III THE NATURE AND THE MEANS OF DIGESTION It has been said that one of the results of the specializa- tion of cells is the loss of the primitive power to receive and utilize all kinds of food. The blood brings to the tis- sues of the body food of certain standard forms, and it is only these which can be used. The attempt to add various soluble foods to the blood by direct injection into the circulation has shown that in most cases such foods are offered in vain to the living cells. Milk introduced in this way is not a practical means of nutrition. Cane-sugar added in measured amounts to the blood is excreted promptly and in almost undiminished quantity by the kidneys. Thus it becomes clear that foods introduced directly into the blood are frequently treated like waste products, while the same foods after transformation in the alimentary canal are entirely acceptable to the body cells. The function of the alimentary canal is to work over the many foreign forms of nutriment into a few forms of the native type. From day to day the diet may be of quite variable character, but its variations hardly show them- selves in the composition of the blood. The term digestion is usually applied to those changes in the food-stuffs which precede absorption. To cover sub- sequent changes we use the word metabolism. One of the more evident characteristics of digestion is that it is a refining process. It effects a separation of the useful and the useless portions of the ration. This is a very conspicuous fact with the herbivora, whose food contains much woody material from which the available- nutriment must be laboriously extracted. With man- kind, especially under modern conditions, a great part of 28 THE NATURE AND THE MEANS OF DIGESTION 29 this work of separation is accomplished in the preparation of food, both industrial and domestic. In this way the task of the digestive organs is lightened, and, as we are often told, overeating is made easy.' Some foods are quite devoid of residues when successfully digested and absorbed. The early writers, having little knowledge of chemistry, were naturally led to make much of the mechanical re- duction of food in the alimentary tract. Mastication sub- divides the food, and it was held that the later opera- tions, especially those of the stomach, were essentially further grindings of a similar sort. Such mechanical proc- esses as do occur continue to be of interest, but they are now seen to be preliminary to actual digestion. More- over, we shall see that it is easy to assume that they are of a more positive nature than is really the case. The human stomach is not a mill, though the gizzard of a bird may be fairly described by that word. In the eighteenth century the emphasis passed from mechanical factors to the process of dissolving the food. Solution is plainly one of the features of digestion, but it is a somewhat superficial one. Of course, it is natural to believe that solid food must become liquid before it can penetrate the intestinal wall, but mere solubility, as we have already seen, is not a guarantee of fitness for the use of the cells. Freely soluble foods like cane-sugar and milk-sugar require to undergo digestive changes just as definite as those carried out in the case of fats or coagulated proteins. It is often stated that the object of digestion is to produce diffusible substances. This statement is in- adequate, for diffusibility like solubility does not in itself determine the utility of a food. The sugars mentioned above are sufficiently diffusible, and the changes which they undergo before absorption serve a more fundamental purpose than the mere hastening of their passage through the lining of the intestine. In the light of modern chemical knowledge we can be somewhat specific in regard to the molecular aspects of the 30 NUTRITIONAL PHYSIOLOGY digestive processes. They are probably always cleavages, large molecules giving rise to smaller. When the original molecule is of extraordinary size, as with proteins and starches, these cleavages have a serial character and a number of intermediate products must accordingly be formed. That is to say, the earlier products are in turn subjected to digestion. Such cleavages are generally, if not always, hydrolytic, that is, water enters into the reaction and its elements are found combined in the products. For the simpler instances of digestion, as in the case of fats and of the disaccharids, 1 we can write precise chemical equations. We cannot do this with the same accuracy for the starches, and we are still farther from being able. to express the exact manner in which the protein molecule undergoes hydrolysis. Yet we have sufficient evidence that the digestion is generally of a uni- form type. Some constituents of the diet need no digestion. This is the case with the mineral salts, so far as they are ab- sorbed, with the simple sugars (monosaccharids), and with alcohol. It is hardly necessary to say that water is also ready for reception into the body fluids. The numerous extractives are for the most part absorbed in the form in which they are eaten. A diet entirely predigested seems not to be practicable. If one were prepared it would have to contain advanced decomposition products of the pro- teins, which are bitter to the taste, and an amount of sugar which would be cloying and subject to fermentation. Digestion is anticipated to some extent by changes in our food which precede its actual arrival in the canal. The ripening of fruits and vegetables, as well as the corre- sponding processes in meat, illustrate this point. The influence of cooking is not so constantly of a sort to initiate digestion, yet in many instances it is so. For 1 The following equation illustrates the hydrolysis of a disaccharid: C 12 H 2 Ai + H 2 = 2C6H 12 6 . This means that one molecule of malt-sugar, reacting with one mole- cule of water, gives rise to two molecules of glucose. THE NATURE AND THE MEANS OF DIGESTION 31 example, when meat is boiled the common variety of con- nective tissue in it is converted to gelatin. This change is a typical hydrolysis, and if it were not executed in ad- vance it would be an early task of the gastric juice. When cooking is attended by considerable drying of the food it is less likely to count definitely toward digestion. Most proteins are coagulated by heat, and this change to solid form seems opposed to the general course of events in the alimentary system. The action of microscopic organisms upon food substances is in line more or less with normal digestion; the maturing of cheese is an example. But bacterial action may depart so far from the normal de- composition as to generate products of strongly poisonous properties, the so-called ptomains being among them. The Means of Digestion. — Hydrolytic cleavages closely resembling those of animal digestion may be caused to occur in various ways. Boiling food-stuffs with acids ac- complishes this. So does treatment with alkalis. Similar results follow the application of superheated steam. But the striking fact is that such changes as are brought about in the laboratory by violent reagents, high temperatures, or both in conjunction, are caused to take place in the stomach and the intestine by bland juices acting at the mild temperature of the body. The changes effected by these juices are often modified by the simultaneous activity of bacteria, but the presence of the latter, at least in man, is to be regarded as accidental and non-essential. The power to digest foods has been known for a long time to reside in the secretions which enter the alimentary tract. It was at first necessarily estimated simply by observing the progressive solution of solid food. The intimate nature of the process has become appreciated more recently. Comparison of the juices from different sources shows that they are individual and specific to the extent that each one, as a rule, acts upon certain classes of food and not on all. There is sufficient evidence for the belief that when a juice digests two or more classes of food-stuffs it contains separate and distinct reagents 32 NUTRITIONAL PHYSIOLOGY for the performance of each line of work. We do not know the precise chemical nature of the active bodies, " diges- tive principles " as they were formerly called, but we know a great deal about the conditions of their working. When a digestive secretion has a single well-marked effect upon one sort of food material we say that it contains an enzyme. We are thus naming a body which we know chiefly by its power to promote a certain chemical reaction. It is not many years since an able writer protested against this confident reference to a substance where it is a property rather than a compound which we are observing. It will be admitted that it is doubtful whether anyone has ever seen that which is an enzyme and nothing else. What we see and handle are solutions possessing characteristic powers or dry preparations capable of furnishing such solutions. But it has seemed' altogether reasonable to connect the property with a substance and we shall con- tinue to do so. Acting on this basis, we say of a juice which hydrolyzes starch that it contains a diastatic enzyme, and of one that acts in a parallel fashion on proteins that it contains a proteolytic enzyme. When the action is upon fats we call the enzyme lipolytic, or, using the sub- stantive, we call it a lipase. It is unfortunate that there is much confusion at present in the use of such terms; there are a great many more in use than there need be. The simplest plan is, perhaps, to fall back upon our Saxon and speak of enzymes as starch-splitting, protein-splitting, sugar-splitting, and fat-splitting. We shall take pains, however, in our detailed discussion to introduce various equivalent terms. We shall find especially that enzymes are often named with reference to their sources as well as to their powers. Enzymes are similar in many respects to the catalyzers of inorganic chemistry. Their presence accelerates reac- tions which in their absence might not be appreciable. We do not think of them as contributing either material or energy to the process. They suggest the oil in a machine which lessens the resistance of its parts to the driving force. THE NATURE AND THE MEANS OF DIGESTION 33 The enzyme in a digesting mixture is not forcibly compel- ling the molecules to disintegrate, but it is removing some hindrance to their spontaneous rearrangement. It is not definitely used up in this service. Accordingly, it follows that a limited quantity of a digestive juice, of course containing a still more limited quantity of enzyme, may be responsible for an amount of digestion practically unlimited. Unlimited time would be demanded for such a demonstration. (This form of statement should be qualified. Enzymes are somewhat unstable and liable to deteriorate.) When a process of hydrolysis takes place under the in- fluence of an enzyme and in a glass vessel, there must be a rising percentage of the products and a declining per- centage of the initial substance as the reaction goes on. The velocity of the transformation is found to diminish and at last it seems entirely arrested. A mixture now ex- ists which contains the first and the last members of the chemical system in proportions which have become con- stant. It is an instance of chemical equilibrium. The halting of the reaction does not mean that the enzyme is exhausted. If any means can be devised by which the accumulated end-products can be removed the hydrolysis will be continued. It was specified above that the trial should be made in a glass vessel. The reader will quickly recognize the important difference between such a con- tainer, from which nothing can escape, and the alimentary canal, from which active absorption processes withdraw the products of digestion. A clear field is thus provided for the continuance to substantial completion of the reac- tions which the enzymes are promoting. The contrast between laboratory conditions and those which prevail in the body did not escape the acute mind of Spallanzani, who was a pioneer among students of these matters. As early as 1777 he recorded that the solution of meat by gastric juice could be greatly facilitated by letting the digestive fluid fall drop by drop upon the food and to trickle away, bearing the dissolved products. 34 NUTRITIONAL PHYSIOLOGY Enzymes are exceedingly sensitive to varying degrees of acidity and alkalinity in the medium. Most of them do not keep their efficacy if the solution is far from the neutral point. But they are somewhat individual in this as in other properties, the acid which is highly favorable for gastric digestion, for example, being quite prohibitive of salivary action. They are all destroyed when in solu- tion by temperatures somewhat short of boiling. Cold suspends their activity, but does not prevent its return upon warming. They are most effective at a temperature not far from that of the blood, though in general a few degrees higher. These relations between the enzymes and temperature are much like those established in the case of the simpler living forms. Having this in mind, one easily adopts the common practice of speaking of the killing of enzymes by heat. It must not be forgotten that this is a figurative expression. We are not justified in thinking of enzymes as living. Living organisms when they grow and multiply in a nutrient medium may de- compose it much as suitably assorted enzymes would do, and, in fact, the organisms in question are probably pro- ducing their own enzymes for the purpose. Formerly such living things as the yeasts and the bacteria were de- scribed as " organized ferments," and the detached en- zymes, incapable of self-multiplication, were called " unorganized ferments." These terms are not much used at the present time. Enzymes are assumed to be products of living cells and may be very characteristic fragments of the cell's fabric, but they are not independently living. The digestive changes to which we pay most attention are those which occur in the cavity of the alimentary canal, and which can be observed to take place also when the same mixtures are placed in the flasks and test-tubes of the laboratory. But we must not overlook the prob- able fact that similar changes are constantly occurring within the boundaries of every active cell. Intracellular digestion, presumably made possible by intracellular en- zymes, obviously takes place when a protozoan cell engulfs THE NATURE AND THE MEANS OF DIGESTION 35 a solid food particle, and is probably just as definite a process when a muscle-fiber of the human body nourishes itself at the expense of the surrounding lymph. We owe to the German chemist, Abderhalden, the clear exposition of this second digestion, which is an essential feature of the life-processes of the higher animals. We shall recur to the subject in treating of protein metabolism. While the great majority of enzymes are hydrolytic and favor reactions of the class which we have been discussing, it must be added that there are other enzymes associated with other orders of chemical change. Enzymes which promote oxidations are believed to play a most important part in the activities of the tissues. When reactions are hydrolytic they proceed with but little evolution or absorption of heat. When they are oxidative the release of energy is a most characteristic attendant condition. CHAPTER IV THE WORK OF MUSCLES AND GLANDS We cannot enter upon a description of the alimentary canal and its activities until we have devoted some space to the physiology of contraction and secretion. Movement is the most familiar manifestation of animal life. When visible to the naked eye it is the expression of the shorten- ing of elongated units — cells or fibers — associated to form contractile tissues. In the human body there are three principal kinds of these tissues. The obvious external movements of the limbs and the features, the act of breathing, etc., are produced by what we call the skeletal muscles. Contractile tissue of another order forms the walls of the heart and furnishes the power for its beating. A third kind occurs in the walls of the alimentary tract, in the blood-vessels, and elsewhere. The term skeletal applied to a type of contractile tissue implies relationship to the bones. It is easy to see that external movements are made effective through the connec- tion of the muscles which produce them with bones acting as levers. In some instances the term is a misnomer, for there are some small muscles histologically like the rest which do not act upon bones. The large and conspicuous muscles are attached, usually at both ends, to the bones. We can generally observe that one end is more freely mov- able than the other. The comparatively fixed end is called the origin of the muscle, the end more subject to move- ment is its insertion. What is called a skeletal muscle is a bundle in which we can distinguish an active and a passive part. There are 36 THE WORK OF MUSCLES AND GLANDS 37 the true contractile elements and there is the connective tissue. The inactive substance forms a sheath enclosing the rest and also partitions which subdivide the interior. The arrangement is familiar in the cross-section of a piece of meat. The subdivision which is apparent to the un- aided eye is repeated on a microscopic scale until the finest meshes of the connective tissue enclose the hair-like individual fibers of the muscle. Each of these slender fibers is a miniature muscle in principle. The function of the connective tissue is often overlooked. While this part of the muscle is entirely passive in character, and scarcely to be considered alive excepting for a certain power of renewal after injury, it is quite necessary to the act of contraction. It may fairly be said to constitute a harness through which all the numberless, minute contrac- tile elements are enabled to unite their efforts. As the end of a muscle is approached the connective tissue in- creases in quantity at the expense of the typical contractile material. In most cases there is an extension of the muscle consisting of connective tissue only, and in a dense form, which attaches the whole to the bone. This is the tendon. It may be a long tough cord or it may form a wide thin sheet. A muscle deprived of its connective tis- sue would be simply a mass of unattached living fibers which might slip about among themselves, but which could not apply their combined tension to accomplish any ex- ternal effect. The fiber of skeletal muscle is a modified cell. Its length is exceptionally great for its width, sometimes a thousand times as great. When it shortens it conforms to the general principle laid down in Chapter I, that is, it does not diminish in volume, but only in surface and, therefore, in length. How the chemical process which underlies the forcible shortening is made to contribute energy to carry it out has proved one of the most difficult problems of physiology. It cannot be dealt with here. But the fact is to be emphasized that we are in the presence of a mechanism somewhat like the steam-engine, inasmuch 38 NUTRITIONAL PHYSIOLOGY as it produces motion and does physical work at the cost of fuel destroyed. The resemblance goes farther than this, for both with the engine and with the muscle the ac- complishment of a measured amount of work is attended by seemingly wasteful evolution of heat. An engine is considered economical if it turns 15 per cent, of the energy resident in its fuel into horsepower. Muscles sometimes do better than this, but much of the time they are even less efficient. It is fair to point out that the heat set free as an accompaniment of muscle contraction is often of value to the animal. In our own case the temperature of the body must be kept above that of its usual surroundings. By far the largest part of the heat devoted to this main- tenance of a relatively high body temperature is produced in connection with muscular activity. Muscles are thus seen to be organs of heat production as well as organs to carry out movements. When the external temperature is high or the degree of muscular contrac- tion is greatly above the average, the heat evolved does become distinctly an embarrassment to the organ- ism. The source of the energy displayed in muscular activ- ity is chiefly the disruption of carbohydrate molecules. Sugar appears to be the preferred fuel of the muscular machine, though other foods are known to be available also. When sugar is completely oxidized the only end-products are carbon dioxid and water, the same substances which would be formed by the literal burning of the sugar with an adequate supply of oxygen. These waste-products are very readily removed from the muscle, when its situation is normal, by the circulating blood. The carbon dioxid will almost immediately escape from the blood when it passes through the lungs. The water becomes part of the large total volume which is always passing into and out of the body, and may leave by all the main channels of excretion — the respiratory passages, the kidneys, and the skin. It will be noted that the quantity of water leaving the body is constantly in excess of the income. Ordinar- THE WORK OF MUSCLES AND GLANDS 39 ily the body excretes all the water which it receives plus the water which arises within it by oxidation. A distinction must be borne in mind between the com- pounds which for the most part make up the muscle and those substances which it is generally found to use as sources of energy. The muscle is mainly composed of proteins. But, as just stated, it is most apt to destroy carbohydrates when at work. One is reminded of the fact that a steam-engine is composed chiefly of steel, but burns coal as its fuel. The comparison is somewhat faulty, however, for it suggests a more radical difference between structure and fuel than we can safely infer for the muscle. Under some conditions muscular work may involve some destruction of protein material. All that has been said of contraction up to this time applies equally to all three classes of muscle. Neverthe- less each type is adapted to its particular work by peculiar properties. Skeletal muscle is capable of quick shortening and prompt relaxation. A contraction may occur and the return to an extended condition be accomplished in one-tenth of a second. The trained finger of a pianist may strike a key ten times in a second. Such movements are in strong contrast with those executed by the form of muscle found in the viscera. The contractions of the stomach develop very slowly, are maintained for some time, and are correspondingly slow in fading out. Of course, it is true that skeletal muscles may also make prolonged contractions, as in keeping the body erect, carry- ing a suit-case, and in countless other instances. Experi- mental study has shown that such contractions as these are really compounded of successive brief twitches occurring too rapidly to permit relaxation. In view of this the possibility of having prolonged contractions in skeletal muscle does not invalidate the statement that it is essen- tially a quick-acting tissue. Muscle and Nerve. — The conception that muscular activity is due to the nervous system is probably suffi- ciently familiar. Every skeletal muscle has its own strand 40 NUTRITIONAL PHYSIOLOGY of nerve-fibers placing it in connection with the brain or the spinal cord and under their control. If its connection is severed it becomes paralyzed and remains inactive, unless special local means, like electricity, are employed to excite it. Ordinarily we are justified in saying that skeletal muscle is not automatic, meaning that every move- ment which it makes is an indication of a previous act, or, as we say, a discharge on the part of the nervous system. Fig. 4. — The above represents somewhat diagrammatically a very small fraction of the length of a fiber of skeletal muscle. To include the entire element with the length proportional to the width we should have to extend this drawing to a length of several yards. The fiber is cylindric and enclosed by a more definite membrane than is usual with animal cells. The cross-marking is not a feature of this membrane, but stands for a peculiar organization of the protoplasm inside. Nuclei are seen here and there near the surface. The seg- ment shown is supposed to be the particular one about in the middle of the fiber within which falls the connection with the nervous sys- tem. A nerve-fiber (n.f.) is seen making a junction with the muscle- fiber (M) through the so-called end-plate (e.p.). In somewhat sharp contrast is the behavior of the muscle composing the heart and of the form which is found in the viscera. These two kinds of contractile tissue are described as automatic, in the sense that they show a tendency to rhythmic contraction and relaxation even when deprived of their nervous connections. The automatic property of the heart is the cause of its beating. THE WORK OF MUSCLES AND GLANDS 41 In varying degrees the different portions of the alimentary canal exhibit the same power, not ceasing to shorten and to extend when observed entirely outside the body of the animal. While we emphasize this remarkable tendency to rhythmic activity, we must hasten to add that tissues showing such capacities are nevertheless subject to some nervous control. Thus the heart beats primarily because of the peculiar nature of its own substance, but varia- tions of rate and strength are constantly occurring as a result of the influence of the nervous system. In this connection it must be pointed out that such influence is not necessarily so applied as to excite increased activity, but may be inhibitory, that is, reducing the rate and force of the spontaneous contractions. A large place is now given to the inhibitory functions of the nervous system, and we shall meet with other examples of the restraint which it imposes upon various organs. A little reflection makes us realize that much of the highest work of the brain must be in the line of inhibition. A man is distinguished by the acts from which he refrains quite as much as by those which he performs. Muscular Tone. — It will be well before we go farther to make clear what is meant by tone (tonus, tonicity) in connection with the behavior of contractile tissues. Muscle is said to exhibit tone when it is not completely relaxed. Tone is thus a mild, sustained contraction. It seems rarely to be absent altogether, but may vary much in degree. Tone in the skeletal muscles gives them a cer- tain firmness and maintains a slight, steady pull upon their tendons. This is not likely to result in actual movement, because these muscles usually fall into antagonized groups, one of which opposes another. A heightened tone in the muscles of the arm may not change its position, since the force tending to bend it may be offset by an equal tension adapted to straighten it. Changes of tone in the walls of the hollow viscera, as the stomach, have a much more evident effect, since they alter the size of the cavity. One must discriminate carefully between stretch- 42 NUTRITIONAL PHYSIOLOGY ing and tone change in such a case. A non-living, elastic sac may be distended by increasing its contents, but will react with a pressure proportional to the distention. A living organ which adapts itself to increased contents by a diminution of tone may exert no more pressure when full than when nearly empty. This principle is well illustrated by the urinary bladder. At one time this organ may have a capacity of a pint, and again its cavity may be nearly obliterated, but there is no strict correspondence between its size and the internal pressure. Indeed, a strong degree of tone and a high pressure may exist when the bladder is quite small. Glands and Secretion. — We have taken time and space tq deal with the elements of muscular activity, and we must also give a place to another type of tissue and to its work. Some appreciation of the physiology of glands is as much a prerequisite of the study of the alimentary process as is a knowledge of the mechanism of contraction. Every- one understands that the nervous system throws the skele- tal muscles into their orderly activity, but the fact that the secretions of the body are often produced under nervous influences is not so familiar. Yet we do not have to look far for suggestive examples. The flow of tears as an accompaniment of an emotional experience is clear evidence that the small organs above the eyeballs which elaborate the tears are in connection with the brain and responsive to its changing conditions. A like relationship can be demonstrated for the glands that produce saliva and for those which secrete sweat. Secretion and con- traction are two manifestations of metabolism which are alike regulated by the nervous system. In fact, it is doubtful whether we have any other expression of the working of the nerve-centers than these two, the phe- nomena of consciousness being set aside for the present. What, then, is a gland? The word is used sometimes to designate a large organ like the liver, the pancreas, or a kidney. Sometimes it is used with reference to a micro- scopic affair like an individual sweat-gland or one of the THE WORK OP MUSCLES AND GLANDS 43 minute pits in the inner surface of the stomach or the in- testine. The fundamental structure is the same in both classes. The microscopic gland is a depression of a cel- lular surface — a pocket, one might say — out of which when it is active the secretion wells. The cells which Fig. 5. — The principle of glandular structure. In the upper figure a simple microscopic gland is supposed to be laid open by a section along its vertical axis. The cells are seen to surround a recess into which they discharge their secretion. Below, the same struc- ture is shown in its entirety, and in addition the encircling blood- vessels which contribute to make good the losses suffered by the secreting cells. bound its cavity are the producers of the secretion, and are in turn dependent for renewal upon the lymph which underlies them and the blood which is flowing close by. A superficial view would suggest that such a gland is a filtering device adapted for straining off certain portions of the blood. This is, however, an entirely inadequate con- 44 NUTRITIONAL PHYSIOLOGY ception. Most secretions contain substances which were not present in the blood at all, and which must, therefore, have been elaborated by the cells of the gland. When we remember that the same blood flows through all the glands we cannot fail to be impressed by the variety of the products which are made from the same raw material — products as unlike as milk and bile, urine and saliva. A compound gland, like the pancreas, is an aggregate of numberless units, which are individually like the simple microscopic glands. Within the meshes of an abundant supporting tissue which is shot through with blood- vessels are these small pockets walled around with the characteristic cells of the gland. These ultimate recesses are called alveoli or acini. Each has a way open through which its liquid product may move toward an outlet. Usually there is a single main duct formed by the union of all the fine passages from the alveoli and bearing their combined contributions. A compound gland may have more than one duct. Glandular secretions may be discharged directly upon the surface of the skin, as in the case of the sweat, or they may enter cavities, as happens with the gastric juice, the pancreatic juice, and the secre- tion of the intestinal lining. The bile and the urine are two secretions which accumulate temporarily in special containers, the gall-bladder and the urinary bladder respectively, before they reach their final destination. Internal Secretions. — It may not be premature to add at this point that any organ may yield some peculiar prod- uct of its own life process to the lymph or to the blood as well as to the cavities of the hollow viscera and to the exterior of the body. A product of this kind which merges with the circulating medium instead of appearing distinct and separate from it is called an internal secretion- One may maintain that every organ has such a secretion, for inasmuch as each has its unique chemical composition and its distinctive metabolism, it must give to the blood compounds which no other organ duplicates. As stated before, the actual make-up of the blood is the resultant THE WORK OF MUSCLES AND GLANDS 45 of the action of all the tissues upon it. But we shall find that internal secretion is a function much more clearly attributable to certain organs than toothers and most evident in connection with a few small structures like the thyroid and the adrenal, to which later reference must be made. To have an internal secretion an organ need not be a typical gland. No duct will be required to carry such materials as its cells turn over to the blood-stream. In some cases organs believed to work along these lines are spoken of as ductless glands. A word recently introduced as an equivalent of the term " internal secretion" should be given. It is the word " hormone," meaning a chemical messenger, a very convenient and suggestive expression. Absorption and Secretion. — Gland-cells have been said to draw upon the blood or the lymph for their raw mate- rial and to manufacture their secretions therefrom. In this process something enters the deeper boundary of the cell layer and in a more or less transformed state it is later discharged from the exposed surface. It is helpful to compare this operation with what takes place in the intestine when the products of the digestive cleavage are being removed to the circulation. When absorption is going on it is the exposed ends of the cells which receive dissolved substances and their deeper borders which are discharging to the fluids that underlie them. Such a process has been well called " reversed secretion," and there is the same possibility of an extensive making over of the transferred material in this case as in the other. In other words, the digestive products which are last detected in the intestine are not necessarily those which will be dealt out to the blood by the cells of the absorbing membrane. Both secretion and absorption are phenom- ena which can be completely carried out only by living cells. Each is probably promoted by a definite application of energy on the part of the cells concerned. In either case it is possible that there may be some transfer of material through the clefts between the cells as well as through the cell bodies. CHAPTER V REFLEX ACTION In the previous chapter it was pointed out that all the work done by the skeletal muscles is in response to the discharges of the central nervous system. For the other types of muscle — the cardiac and the visceral — it was shown that there is an inherent tendency to rhythmic activity, but that over these tissues also the nervous sys- tem exercises a regulation. Finally, it was stated that the glands likewise are subject to central government, al- though not to the same degree in all cases. We must now proceed to consider how the nerve-centers are them- selves prompted to throw muscles and glands into action. As we observe the body at work we cannot fail to be impressed with the timeliness of its adjustments. It is constantly meeting with emergencies and adapting itself to new conditions. If we are inclined to attribute all these quick adaptations to intelligent choice of courses to be pursued we shall find that we cannot long defend such an explanation of the facts as they occur. We can- not pretend that we think of each inequality of the pavement as we cross the street, or of each individual in the crowd through which we make our way. The balan- cing of our bodies, standing or walking, is not a matter about which we are given to deliberating. These things seem to take care of themselves. It is such adjustments which " seem to take care of themselves " that are called reflex actions. A reflex is an adaptive change, brought about through the agency of the central nervous system, to meet some new external condition. We may or may not notice the occurrence of a reflex. If consciousness is at all involved, it is incidental and not causal. Often 46 REFLEX ACTION 47 the conscious effort is rather to prevent the reflex from taking place, as is apt to be true when we sneeze. Of some reflexes we are quite unlikely to be aware; this is the case with the narrowing of the pupil in response to in- crease of light. Let us now go into some detail and analyze carefully the reflex process. We have seen that the primary cause is an external change of some sort, the word " external " meaning outside the central nervous system and not necessarily outside the body. The change which is at the root of the reflex is usually referred to as an external stimulus. It would be easy to give a long list of exam- ples. A foreign particle comes in contact with the larynx; its contact is the stimulus which develops the coughing reflex. Slight drying of the exposed surface of the eyeball is a common cause of the winking reflex. Irritation of the lining of the stomach is the most frequent of the many possible stimuli through which vomiting can be excited. External stimuli would fail of any extended effect if it were not for the nervous connections of the parts affected. In the last chapter the nervous system was spoken of as sending its impulses out to muscles and glands. But its work is twofold. It not only acts, but it is acted upon. Its fibers fall into two classes, those which are concerned with transmission of effects outward from the brain and the spinal cord, and those having the opposite function, the carrying inward of impulses started by external causes. The first class of conductors are usually called motor; the second, sensory. Both terms are open to objection, as a little consideration will show. The effects which the ner- vous system produces in the tissues of the body are not solely movements. The word " motor," then, is not in- clusive enough. It is better to substitute the word efferent, 1 which means simply centrifugal, and which im- plies nothing whatever about the nature of the responses evoked. Efferent fibers may be motor, that is, exciting contraction, but they may also inhibit contraction, and 1 Efferre, to bear away. 48 NUTRITIONAL PHYSIOLOGY when they end in connection with the cells of glands they may be secretory. Probably also they may inhibit secretion. Just as we have found the word " motor " inadequate and have agreed to replace it by " efferent," so the word " sensory " does not properly indicate the whole service of the fibers which bear impulses toward the brain and cord. Sensory implies " productive of sensation," and we cannot assign such a property to all the two million fibers or more which assail the centers with their communications. In the great majority of cases we do not feel any consequences of their activity. The term afferent is free from this objection and is the logical complement of efferent. If one hastens to ask what is the significance of afferent fibers which do not arouse sensation, the answer is simple and definite: They produce reflexes. If the first element in the reflex process is the applica- tion of an external stimulus, it is now clear that the next element is the afferent transmission of the impulses. What these impulses are cannot be discussed. It should be recalled that they are not fluid pulses nor electric cur- rents in the usual sense of the expression. They repre- sent energy of some kind in rapid, but not immeasurably rapid, motion. They pass along the nerves at rates in excess of 100 feet in a second, so that the longest paths in the human body are traversed almost instantaneously. The time used in such transits might be quite appreciable if we could observe it closely in a whale. When the afferent impulses reach the central nervous system the third event in the development of the reflex act occurs. This is localized in the brain or the spinal cord, and we may speak of it as "a central process " without committing ourselves as to its exact character. What we actually observe is that the arrival of the afferent impulses is followed by the appearance of efferent ones. It is not necessary to decide whether these efferent im- pulses are the same currents which just entered the in- tricate fabric of the central organ and which have found a path open through its mazes which has led them out REFLEX ACTION 49 again. That is one way of picturing the phenomenon. According to the older and more familiar view the impulses which come out are not those which went in, but a new set generated by an energetic metabolic process, a discharge on the part of cells in the brain or the cord. If this is the true conception the afferent impusles serve to "touch off" irritable nervous elements, much as these elements in their turn may touch off muscle-fibers or gland-cells. Fig. 6. — The principle of reflex action. The subject touches a hot object (H). Afferent nerve-impulses travel the route marked by dots and dashes to the spinal cord (S). Efferent impulses return promptly along the route marked by little crosses to the muscle (M), which co-operates with others not shown to withdraw the finger from the stimulating surface. The situation of the co-ordinating center is left undetermined, whether in the brain or the cord. The fourth step in the evolution of the reflex is the efferent transmission. This may be said always to be more voluminous than the afferent flow which went before. Impulses go out by many channels, where but few were engaged in bringing them in. A great characteristic of the "central process" is the spreading of the initial stimu- lation, so that there seems to be no proportion between the 4 50 NUTRITIONAL PHYSIOLOGY cause and the response. The number of nerve-fibers which can be excited by the slender proboscis of a mosquito as it pierces the skin of a sleeper must be very small. The reflex movement which results may involve a very large share of his skeletal muscles. The fifth and final occurrence completing the reflex is the reaction on the part of the muscles or the gland-tissue in which the efferent fibers end. As already indicated, this may be a movement, an outpouring of secretion, or it may have a negative character, the suppression of movements that would naturally have occurred, or possibly the with- holding of some secretion which would otherwise have been discharged. The illustrations of reflex action most often chosen are those in which an immediate, even abrupt, response is seen. Yet it is quite easy to find examples of gradual adjustment to the new external condition. Changes of color, the outward sign of changes in the blood-supply of the skin, when they occur on account of warming or cooling of the surface, are reflexes of this prolonged and gentle order. If there is any doubt as to whether a certain action is to be classed as a reflex, it may be tested according to the foregoing analysis. There must be an assignable stimulus, external at least as regards the central nervous system, there must be an afferent flow of the impulses resulting from the stimulation, a process within the bounds of the central axis, a return flow of impulses in multiplied volume, and the action itself. The more one thinks of the common course of events, the larger the number of actions which he finds he can place in this class. It becomes appropriate to ask what kinds of bodily activity are outside this depart- ment. To this question it can be replied that automatic actions, such as the beating of the heart, are to be dis- tinguished from reflexes. The nervous system is not re- quired to maintain the heart-beat. There are cases also in which the chemical composition of the blood reaching the centers modifies their behavior and causes them to send out certain impulses. Such cases do not fit our descrip- REFLEX ACTION 51 tion of the reflex, since in them the stimulation is applied centrally and no afferent nervous mechanism is needed. Our breathing movements are determined to a great extent by such chemical conditions, but it is a fact that reflex disturbances of the breathing are so prevalent that it is often difficult to give just recognition to the two factors. We are accustomed to contrast sharply actions which are reflex with those which we regard as strictly volun- tary or deliberate. The distinction is a convenient one and not generally productive of confusion, but sometimes it becomes quite difficult to draw the line. It may be urged that all our conscious, intentional acts are performed in answer to external conditions which have risen to make an occasion for such new adjustments. So it might be argued that the writing of a word from a copy should be considered a reflex in which the retinal image of the copy furnished the external stimulus. Such images printed upon the retina of the trained pianist by the notes that are before him cause his fingers to drop upon the corresponding keys of the instrument. It may be claimed that this is a reflex action. Without denying the force of such reasoning, we shall do well to restrict the term to the class of responses for which we are quite sure that attention is unnecessary, and usually to those for which we have an inborn or at least a very early developed capacity. When we ask ourselves whether any act is really other than the result of external circumstances affecting an organism with its own past history registered in its structure, we find, al- most with a shock, that we are face to face with philo- sophic and ethical problems, responsibility and free will. Most of us like to believe that a place is to be reserved for a type of action,- even though it may be rare and slight, which is not externally caused. The great difficulty encountered by the beginner in physiology lies in the attempt to realize the 1 inevitable character of reflexes and their structural basis. He finds it hard not to read conscious purpose into acts which so constantly prove advantageous to the individual. When a 52 NUTRITIONAL PHYSIOLOGY frog adroitly catches a fly it is natural to assume a desire and a design on the part of the frog. Scientific analysis nevertheless makes it appear far more probable that the fly is entrapped because the frog is a mechanical device adapted to do this thing over and over again. The eye receives the flitting shadow of the insect, the stimulus excites the brain, and the well-directed fling of the tongue follows. The reflex is not done away with when the part of the brain most likely to stand in relation to consciousness has been destroyed. We have to remember that much of the service of the eye is subconscious, as when it makes us turn aside from obstacles in our path. It is in this way that the eyes of the, somnambulist assist in guiding his movements. Conscious attention is no more essential to such a use of the eyes in the waking than in the abnormal sleeping state. In fact, close attention to the balancing of the body is quite as likely to derange as to promote the reflex adjustment. Central Resistance. — Reflexes are not obtainable with the same ease at all times. We express this fact by saying that there are variations of resistance in the central ner- vous system. If reflexes are hard to bring about, we say that the resistance is high; if they occur with unusual free- dom and seem disproportionate to the exciting stimuli, we say that the resistance is low. Narcotics and anesthetics are said to raise the resistance, and their effect can be gaged by observing the degree of difficulty with which certain reflexes can be produced, or whether, indeed, they can be produced at all. Drugs of an opposite order, the true stimulants, make it easy to call out most reflexes. When one is distinctly under the influence of coffee, a noise may cause one to start, with a sharp contraction of many muscles. The auditory stimulus has an undue effect, and it is natural to assume that the conditions in the brain and cord are uncommonly favorable to the penetration and to the multiplication of nervous impulses. In poisoning by strychnin such an extension of conduction may exist that some trifling Cause may precipitate a terrific and exhaust- REFLEX ACTION 53 ing convulsion. Clearly, then, a certain degree of central resistance is the most favorable condition for the activities of life. Any increase will tend to prevent needed adapta- tions to external changes, and any great decrease will make the reflex responses exaggerated, disorderly, and ill suited to their object. There is reason to suppose that the more frequently occurring reflexes become progressively easier to induce through a lowering of resistance in their particu- lar pathways. This brings us close to the subject of habit formation. Our emphasis has been constantly upon the advantage derived by the animal (or by man) from the possession of reflex capacities. When the environment is the accus- tomed one and the changes taking place are such as the species has often experienced, we find that almost every reflex is obviously beneficial. The reactions are such as maintain bodily equilibrium, secure nutriment, evade or defeat enemies, resist changes of temperature, all making for self-preservation. But it must be noted that an unin- telligent mechanism will act amiss in any environment which is sufficiently unlike the accustomed one. It will hardly be claimed that the reflexes exhibited by the novice on first going to sea help him in the struggle for existence. A number of reflex effects can be thought of which can scarcely be of value. Sneezing when going out into bright sunlight is one of these. Hiccups following immoderate laughter do not seem to be of any service, nor does laughter itself when induced by tickling. These instances, which on the whole have little importance, are mentioned simply to enforce the contention that the reflex mechanism, how- ever refined, is not directed in its routine performances by intelligence. Its structure determines its conduct. The finger laid upon hot iron is twitched away before the situa- tion is reasoned out, in fact, before pain is felt. Central connections exist which make the movement sure to occur. If we could rearrange those central connections we can conceive of a luckless subject who would not remove his finger from the stove, but would stand violently coughing while the injury proceeded. CHAPTER VI THE ALIMENTARY CANAL The single-celled animal digests its food within its own protoplasm, sometimes holding it for a while in a temporary cavity filled with fluid, the so-called food vacuole. In such intracellular cavities true digestive secretions contain- ing enzymes are doubtless at work. It is probable that single-celled forms may also secrete enzymes to the exte- rior and so modify food material which is near-by, but not yet enclosed. This appears to be the case with bacte- ria when they dissolve the solid gelatin in which they are growing. Among many-celled animals digestion of this second type, that is, external to the cells, becomes more con- spicuous. Their bodies are so formed as to contain spaces in which food may undergo digestion and from which the hydrolyzed products may be absorbed. In the sea- anemone a round opening or mouth leads to a cavity which is very large in proportion, to the size of the animal. This primitive alimentary tract has no other opening. In the earthworm, a somewhat more highly developed form, a straight canal in the axis of the body leads from a mouth near the anterior end to an anus at the posterior. However much the alimentary systems of the higher ani- mals may be elaborated, each still represents a more or less winding passage between a mouth through which food is received and a vent or anus for the discharge of residues and excretions. The canal may be greatly lengthened through coiling. Some sections may be widened and others nar- rowed; the walls in some places may be thick and elsewhere thin. Local differentiation of this kind causes us to dis- tinguish in the human subject the familiar divisions of the 54 THE ALIMENTARY CANAL 55 tract, as the esophagus, the stomach, the small and the large intestines. The lengthening of the system, it should be noted, does not merely increase its capacity, but multi- plies the surface available for the processes of absorption. A few anatomic expressions may well be denned at this time. Anterior, as we shall use the word, means toward Fig. 7. — I represents a protozoan cell — an ameba — which has enclosed a particle available for food (F). The particle occupies the center of a clear space or vacuole (V). Undoubtedly it is sur- rounded by a fluid having digestive powers. II is a diagrammatic section through the familiar sea-anemone. There is a relatively huge digestive cavity (S) with a single opening to the exterior (M). Ill suggests the type of alimentary system found in the earthworm and in higher animals. Two openings exist, the mouth (M), de- finitely devoted to the reception of food, and the anus (A), used exclusively for the discharge of wastes. the head; posterior, away from the head. Dorsal means toward the back; ventral, toward the front. Right and left have their ordinary use. (In most figures, the subject being viewed from in front, right and left are reversed.) Reference must often be made to the body cavities. These 56 NUTRITIONAL PHYSIOLOGY are the thoracic cavity above the diaphragm and within the cage of the ribs, the abdominal cavity below the dia- phragm, and the much smaller pelvic cavity bounded by the bones of the hip girdle. When we speak of these as cavities we do not mean that they contain any air-filled space. They are completely filled by the organs which they enclose plus a small quantity of fluid. Hence they are only potential cavities in life, becoming actual when their contents are removed in course of dissection. The thoracic cavity contains the lungs, nearly surrounding the heart, and is traversed by the esophagus. The abdominal cavity is filled almost entirely by the organs of digestion — the stomach, the small and large intestines, the liver, and the pancreas. The spleen at the left of the stomach is less certainly connected in its functions with the alimentary system. The kidneys lie in the dorsal body wall rather than in the abdomen. In the small pelvic cavity are the urinary bladder, the terminal part of the large intestine, and the reproductive organs. The mouth, the first division of the alimentary canal, scarcely calls for detailed description. Above, a bony par- tition separates it from the intricate spaces of the nasal passages. At the back this "roof" is prolonged as a mobile, muscular curtain — the soft palate. Below the edge of the soft palate a region is reached which is common to the alimentary and respiratory systems. This segment of the canal is known as the pharynx, though the term is extended also to the space behind the soft palate, which is above the normal course of food. The teeth and the tongue with its wonderful muscular development are suffi- ciently obvious. Ducts from the salivary glands open into the mouth. We are rarely conscious of the situation of these openings, though in the dentist's chair we may notice the rapid flow of saliva from one which is opposite the upper molars. This is the place of entrance of the secretion of the parotid gland, situated before and below the ear, the gland usually affected in mumps. Under the tongue and within the sweep of the lower jaw-bone there Fig. S. — The human alimentary canal shown diagrammatically : O is the esophagus; S is the stomach; S.I. suggests the small in- testine; C is the colon (see Fig. 14) ; R is the rectum. The connection between the stomach and the small intestine occurs behind the transverse colon, which also hides the pancreas. Fig. 9. — Relations of the mouth and nose. This is a vertical sec- tion through one nostril, and therefore slightly away from the midplane of the head. The convoluted character of the lateral wall of the nasal cavity is suggested. The connection between the nose and the throat will be seen behind the soft palate (P); L is placed in the larynx, above which is shown the spur of the epi- glottis; O indicates the course of the esophagus. It will be noted that the course taken by the food crosses the route of the breathing in the pharynx. THE ALIMENTARY CANAL 57 are, on either side, two other glands, the submaxillary and the sublingual, with ducts opening in the floor of the mouth. Below the root of the tongue there is a leaf-like pro- jection, the epiglottis, which juts backward and guards the entrance to the larynx. Through the larynx a way is open to the trachea and the lungs. At this point, therefore, the courses taken by the food and by the breath part com- pany. From here the esophagus extends through the neck and the thorax, lying at first behind the trachea, and lower down passing back of the heart. Perforating the dia- phragm slightly to the left of the midline it opens into the stomach. The stomach is the most expanded part of the aliment- ary canal. Its position is higher up than is generally as- sumed, so that it is well within the embrace of the lower ribs on the left side. It has a capacity varying greatly with the degree of its distention and with its variations of tone. After a full meal it may contain more than a quart. The form of the stomach also changes consider- ably from time to time, but we distinguish a large, rounded portion toward the left and a more conical region tapering off toward the right and joining the small intestine. The opening from the esophagus into the stomach is called the cardia, and that from the stomach to the small intestine is the pylorus. The pylorus is a trifle to the right of the mid- line. The upper border from the cardia to the pylorus is the "lesser curvature" of the stomach; a line drawn from the cardia around the convex left-hand side and thence along the lower margin to the pylorus is said to follow the "greater curvature." Leading away from the pylorus the small intestine de- scribes a short turn, within which is the pancreas. This first curve is called the duodenum. The remainder of the small intestine is a slender tube about 20 feet in length, coiled upon itself in a confusing manner. Two divisions are recognized, the jejunum, continuous with the duodenum and the ileum, extending onward to join the large in- 58 NUTRITIONAL PHYSIOLOGY testine. No sharp line of demarcation exists between these sections, but the latter is regarded as constituting somewhat more than half the whole. The ileum finally arrives at a point not far from the crest of the right hip- bone and there enters the large intestine. The large intestine is so called from its diameter, which is two or three times that of the small. It is quite as often Fig. 10. — The stomach with the pancreas and duodenum: C is placed at the cardiac opening of the stomach, while P is at the pylorus. A dotted line is used to complete the form of the pancreas, which discharges to the intestine near W. The shape of the stomach is of one moderately filled; with further distention the lower border of the organ would sag and the pylorus would cease to be the lowest point. B is the common bile-duct, which reaches the intestine be- hind at the same point at which the pancreas delivers its secretion. called the colon. Beginning with a small. rounded pouch, the cecum, from which hangs the infamous appendix vermi- formis, it may be followed upward on the right side of the body to the level of the lower ribs. This part is the ascending colon. From here it bends sharply to the left and crosses the full width of the abdominal cavity. This horizontal segment is known as the transverse colon and lies close to the ventral body wall. Thus it passes in front of the duodenum and is in practical contact with the THE ALIMENTARY CANAL 59 stomach. At the left side of the body and near the spleen the descending colon begins. Its course is downward and backward, so that it passes behind the coils of the small intestine. Following the dorsal boundary of the cavity around to the middle line, the colon forms the short, curved region called the sigmoid flexure. The remaining section is the rectum, situated directly in front of the lower extremity of the spinal column within the pelvis and ter- minating at the anus. Almost everywhere the lining of the alimentary canal is pitted with microscopic glands. Those in the stomach furnish the gastric juice; those in the intestine, the intesti- nal juice. Besides these small glands and the salivary glands already mentioned, there are the pancreas and the liver, contributing secretions to the cavity of the digestive tract. The pancreas has been said to lie in the turn of the duodenum. It is thus under the pyloric portion of the stomach and behind the transverse colon. Its main duct discharges into the small intestine about 3 inches below the pylorus. A second, but very small, duct may open close by. The liver, which is the largest gland in the body, is fitted to the concave under surface of the diaphragm and is mainly to the right of the midplane. It is cleft into several lobes, from which ducts converge and unite as they ap- proach the duodenum. A single duct is finally formed and it enters the intestine at the same point as the chief pan- creatic duct. The arrangement serves to blend the two secretions, and is somewhat suggestive of the devices used with bath-tubs for mingling hot and cold water. The liver produces bile, and its channel of discharge to the duodenum is accordingly known as the bile-duct. This duct has a side branch which leads to a contractile sac embedded in the under surface of the liver, this reser- voir being the gall-bladder. Bile, as it flows down from the liver, may either find its way directly to the intestine or it may turn aside into the gall-bladder. The course taken will depend on the contraction and relaxation of the muscular walls of the ducts. Active contraction of the 60 NUTRITIONAL PHYSIOLOGY gall-bladder when it is full may send a considerable amount of bile at one time into the intestine. The relation of the gall-bladder to the liver is like that of the urinary bladder to the kidneys, at least to the extent that its existence makes possible a continuous production of the secretion with an intermittent emptying. It has been shown, how- ever, that the bile is concentrated and otherwise modified Fig. 11. — This is an entirely schematic section across the human body in the mid-abdominal region: S indicates the spine; K, the kidneys; P is the peritoneum, the lining of the abdominal wall. It is prolonged from the back to form the mesentery (M), which extends to and around the loop of intestine (/). The large unoc- cupied space shown does not really exist, for successive portions of the alimentary canal together with other organs completely fill the cavity. during its stay in the gall-bladder. The urine does not change its character distinctly while it is in storage. When the abdominal wall of an animal is cut through and laid back from the organs within, one's first impression is that the viscera are lying unattached in the cavity. They are, in fact, not adherent to the ventral or lateral portions of the wall. But if we take a loop of the small intestine at random and attempt to lift it from its resting- place, we find it attached to the middle of the back by a tough, transparent membrane, the mesentery. In the THE ALIMENTARY CANAL 61 mesentery can be seen blood-vessels, lymphatics, and nerves. This suspending sheet thus serves not merely for mechanical support, but also establishes connection be- tween the intestine and the circulatory and nervous sys- tems. The student is apt to find it hard to visualize the mesentery in its actual form; he is to imagine a membrane which at one edge extends to the entire length of the small intestine and to much of the large, while its other edge is condensed to be inserted into the space of a few inches before the spinal column. What results from these condi- tions has been described as "a ruffle or flounce." Al- though the mesentery is thin it is really a doubled sheet enveloping the intestine. This will be made clear by the diagram, which also shows how the mesentery is continu- ous with the exquisitely smooth, lustrous lining of the ab- dominal cavity, to which is given the name of peritoneum. Dissection of a small animal will give a comprehension of these anatomic facts which can scarcely be gained by reading. The stomach has a supporting membrane attached to it along its lesser curvature and uniting it to the liver, which is, in turn, anchored to the dorsal body wall. This mem- brane is, in effect, a mesentery for the stomach, but is called the lesser omentum. An extension of similar tissue hangs from the greater curvature like an apron over the intestinal coils and is called the great omentum. It may become a ponderous appendage from the fat which it sometimes accumulates. The continuation of the mesentery over the external surface of the intestine and the identical covering of the stomach form for these organs what is spoken of as their serous coat. The Finer Structure of the Alimentary Organs. — We have said that the internal surface of the stomach and of both intestines is provided with glands. The inner layer of the wall of the canal in which these glands occur is called the mucous coat or mucous membrane. This is in reference to the fact that its exposed cells produce the slimy substance, mucus, more familiarly associated with nasal 62 NUTRITIONAL PHYSIOLOGY discharges. It probably acts as a lubricant in the digestive tract and also protects the lining cells from harsh contacts, both physical and chemical. The mucous membrane is depressed to form the recesses of the glands, and in the small intestine is raised into the microscopic prominences referred to as villi. Underlying it blood-vessels and nerve- fibers run thickly. Between the mucous layer and the serous coat on the outside of the intestine there is a development of muscle of the order described in a previous chapter as characteristic of most internal organs, that is to say, slow acting, more or less automatic, and much given to tone changes. In the small intestine there are two distinct muscular coats: the inner and thicker has its fibers at right angles to the axis of the canal, while the outer has them set parallel to this axis. The inner coat is hence spoken of as circular and the outer as longitudinal. There is no doubt of the superior prominence of the circular coat in the production of intes- tinal movements. In the stomach the muscular organiza- tion is less simple and there are oblique elements in addi- tion to those which can be classed as circular and longi- tudinal. The colon has the circular coat, but instead of a complete covering of longitudinal muscle it has three bands of contractile tissue extending along its wall. At any point along the course of the intestine temporary closure may be effected by the contraction of the circular muscle. But there are certain places where such closure is far more frequent or, indeed, the usual condition. Where the esophagus joins the stomach an irritable band, the cardiac sphincter, is much of the time firmly contracted. There is, similarly, a pyloric sphincter guarding the open- ing between the stomach and the duodenum. Where the ileum enters the cecum a valve exists which is adapted to prevent the reflux of material from the colon to the small intestine. This, the ileocecal valve, is probably reinforced in its mechanical action by muscular support. Finally, the short anal canal is closed by an inner sphincter which is essentially a thickening of its own wall, and an external one composed of skeletal muscle. CHAPTER VII THE MOUTH— SWALLOWING; SALIVARY DIGESTION Mastication. — The hygienic importance of thorough mastication is undoubted, but there is little occasion for any extended analysis of a process so obvious. It is to be observed that the lower jaw does not have merely an up- and-down movement, but that it glides backward and forward and has some lateral play at the same time. The teeth, therefore, do not simply chop the food, but rub and grind it. In the work of mechanical reduction a larger part is borne by the tongue than is commonly recognized. The little member seems to be everywhere at once, thrust- ing food between the teeth, withdrawing it again, bruising and rasping it against the roof of the mouth. While this action is going on an intimate mixture with the saliva is accomplished. We must now proceed to a discussion of this the first of the digestive secretions. Mention has been made of the three pairs of glands which supply the saliva. Their united product is estimated to reach an amount of about 3 pints a day, equalling the vol- ume of the urine. If one finds it hard to credit such a statement, attention may be called to the copious character of the flow which is noted when one is interrupted at the moment of taking food. There is little secretion apart from eating unless it is excited by chewing sundry things. At mealtime a large part of what is swallowed is saliva, and the proportion must be greatly raised by the practice of prolonged mastication, so-called Fletcherism. The formation of saliva is to be regarded as a reflex in which the primary stimulation is furnished by food in the mouth 63 64 NUTRITIONAL PHYSIOLOGY acting upon the endings of nerves excitable by its chemical ingredients and by its temperature more than by its mere contact. We have to do with something more than the typical reflex, however, because it is a familiar fact that the appeal to consciousness has much influence upon the flow of saliva. The "watering of the mouth" at the approach of acceptable food is a hard phenomenon to classify. It is a reflex, but it is one which would not occur in an unconscious subject. For such actions the term psychoreflex is often used. The saliva is a bland fluid which one would hardly sup- pose to be endowed with active powers of digestion. In some animals it does not have any apparent chemical ac- tion. Still, it has valuable properties which we shall do well to recognize. Whether it is a digestive juice or not, its physical effect is useful in mastication, since it softens the food, makes it cohere into the pellets which are pre- pared for swallowing, and later lubricates their transit to the stomach. Moreover, it has a defensive use, protecting the mouth from injury when food or fluid is taken too hot or when some corrosive liquid calls for dilution. As it issues fresh from the glands it is slightly alkaline in reaction. If it stagnates for a long time in the by-places of the mouth, as happens during sleep, and if it contains at the same time traces of carbohydrate food in solution, bacterial fermenta- tion may make it acid and the effect upon the teeth may be injurious. The value of an alkaline mouth-wash, like milk of magnesia, used at bedtime is evident. Human saliva contains various salts. Attention need be called only to its lime compounds, which are always de- posited more or less upon the back surfaces of the teeth, a process which reminds one of the formation of stalactites and stalagmites in caverns. The hard crust that results is the tartar. It is not very unlike the original substance of the teeth in its chemical composition, and its occurrence might seem to indicate a mode of making good the wearing away of the teeth. Unfortunately we cannot regard it in this favorable light, for the lime salts are always contami- THE MOUTH — swallowing; salivary digestion 65 nated with food particles and bacteria. The deposit should be removed by the dentist at regular intervals. The three glands furnish slightly different varieties of saliva. Mucin, the essential compound in mucus, is pres- ent in the secretion of the two lower glands and not in that of the parotid. It gives to the saliva from the sub- maxillary and sublingual glands a ropy, mucilaginous character, which, of course, becomes more apparent when evaporation has concentrated the solution. This is illus- trated when the mouth is dried by rapid breathing during exercise and becomes furred with the residue of salivary mucin. This constituent of the saliva probably makes it superior to water as an agent for molding the food into pellets. The most interesting property of the secretion, its power to hydrolyze starch, may be discussed to more advantage after we have followed the food to the stomach. Clearly, there is not time enough for much digestive change in the mouth of a person of average habits. Swallowing. — The transfer to the stomach is a more complex matter than is likely to be realized. It involves an interruption of breathing and the protection of the nasal passages and the larynx against the intrusion of food. The first purpose is effected by the retraction of the soft palate against the back of the pharynx. The second is accom- plished by the drawing forward of the larynx toward the chin, a movement which can be plainly felt. By it the larynx is tucked under the root of the tongue and overlaid by the epiglottis. The same motion serves to widen the upper part of the esophagus, which is not at other times ap- preciably open. With the parts in this position the bolus of food is crowded back from its original seat upon the tongue and urged through the pharynx by the successive contraction of the bands of muscle which surround it. As soon as it is fairly within the esophagus the soft palate is lowered, the larynx is allowed to emerge from its covert, and the breathing can be resumed. Such quick and well- ordered adjustments give evidence of co-ordinated reflex action, the contact of the food morsel with one spot after 5 66 NUTRITIONAL PHYSIOLOGY another furnishing the requisite stimuli. We cannot go through the series of movements' unless there is at least a little saliva to be swallowed, and we cannot arrest the march of events when it is once begun. The contractile tissue in the upper part of the esophagus is of a skeletal variety. Lower down this gives place to typical visceral muscle. Hence it is not strange to find that the advance of the bolus becomes progressively slower as it descends. The movement which is here taking place Fig. 12. — An exaggerated representation of peristalsis. I and II are successive views of the same portion of the alimentary tube: P is the zone of contraction shifting downward and always pre- ceded by the zone of unusual relaxation (N). Ill is an imaginary section through II, showing the food bolus (6) slipping along in advance of the contracting region, its advance being facilitated by the relaxation below. is what is known as a peristalsis, and it is highly important that its principle should be understood. The most ob- vious feature is a ring of contraction setting in above the enclosed pellet, causing it to slide onward, and following it down by involving in succession each level of the tube. The mechanical application can be simply illustrated by propelling a glass bead through a soft-rubber tube by pinching repeatedly behind it with thumb and finger. A strict analysis of what occurs in the esophagus obliges us to recognize that the process is not so simple as it first appears. THE MOUTH — SWALLOWING; SALIVARY DIGESTION 67 There seem to be two phases in what is called the peristal- tic wave. The eye detects chiefly the traveling contrac- tion, but this is apparently preceded by a zone of unusual relaxation, a region of inhibition. The peristaltic wave which is necessary for the propul- sion of solid food does not seem to be required to send liquid to the stomach. A swallow of water is shot swiftly from the mouth to the cardiac sphincter and arrives there dis- tinctly in advance of the plodding peristalsis. When one drinks a glass of water, the swallows following in rapid suc- cession, a single peristaltic wave ends the series. Of course, when fluid is carried up-grade in the esophagus, as when a horse is drinking from a pool at his feet, active peristalsis is as necessary as though solid food were being moved, and one may plainly see the passing of each swallow along the extended neck. We shall find that the small intestine exhibits movements which are approximately the same in principle as those of the esophagus, but far slower and usually less energetic. Sensibility of the Esophagus. — It may be laid down as a general principle that the sensory equipment of the ali- mentary tract is most complete for the mouth and less and less developed as the course of the canal is followed. The lining of the mouth is manifestly sensitive to contact, heat, and cold, besides possessing the particular sense organs for taste. When any point in the mouth is stimulated, we can localize it with great accuracy. Interesting observations, which have been made upon the esophagus, show that its mucous membrane is quite responsive to heat and cold, but indifferent to pressure unless it is of such a degree as to stretch the underlying muscular coat. 1 Salivary Digestion. — Within the stomach the accumu- lated food with a large admixture of saliva lies for some time with little motion. Here then salivary digestion must take place. The statement has been made that in some animals the saliva has only mechanical and protective functions. 1 Carlson and Braafladt, Amer. Jour. Phys., 1915, xxxvi, 153; Boring, Amer. Jour. Psy., 1915, xxvi, 1. DO NUTRITIONAL PHYSIOLOGY More frequently, however, it has the power to hydrolyze starch, forming malt-sugar as the chief end-product. This seems to justify the assumption that an enzyme is present, and it is variously named ptyalin, salivary amylase, or salivary diastase. Such an enzyme probably plays an im- portant part in the digestive processes of ruminants, ani- mals which chew the cud. Human saliva acts upon starch with surprising energy. A simple demonstration of the fact may be had by holding a bread-crumb in the mouth longer than is habitual, when it will gradually develop a mildly sweet taste. The prevailing opinion in regard to the amount of diges- tion accomplished by the saliva in man has undergone a change during the last few years. It is allowed a larger place than was formerly granted to it. The enzyme is extremely sensitive to acid. Inasmuch as the gastric juice is decidedly acid, it used to be claimed that salivary diges- tion could not proceed in the stomach. But it has come to be recognized that when a large mass of food is intro- duced into the stomach within a short time the gastric juice penetrates it rather slowly. A few minutes after the com- pletion of a meal we may picture the stomach-contents as being acidified near the surface, the acid slowly making its way inward, but having a neutral or even alkaline central portion. Salivary digestion will be continued in the stead- ily diminishing region not yet reached by the acid, and will cease only when the gastric secretion from one wall of the stomach meets that from the other. Any rotation of the contents would probably bring about an earlier distribution of the acid and arrest of starch digestion. No such rota- tion seems normally to occur. A factor which operates to postpone the destruction of ptyalin is the power of the proteins of the diet to engage hydrochloric acid in com- bination. Since proteins are almost always present, the gastric glands must secrete acid enough to satisfy their capacity before there can be the excess of strictly free acid which will put an end to salivary digestion. If the mixed food is quite acid at the outset, it is hard to THE MOUTH — SWALLOWING; SALIVARY DIGESTION 69 see how there can be any hydrolysis of starch brought about by the saliva. Yet we constantly eat acid fruits before our breakfast cereal and notice no ill effects. Starch which escapes digestion at this stage is destined to be acted upon by the pancreatic juice, and the final result may be entirely satisfactory. Still it is reasonable to as- sume that the greater the work done by the saliva, the lighter will be the task remaining for the other secretions and the greater the probability of its complete accomplish- ment. The power of saliva to convert raw starch to sugar is almost incomparably smaller than its capacity to digest starch which has been cooked. Raw starch exists in very dense grains which have to be dissolved from the surface inward. Cooking, especially boiling, utterly destroys these grains, and permits a reaction between the enzyme and the separated molecules of the carbohydrate. The change from starch to sugar seems not to be effected by a single reaction, but by stages. Physiologic chemists have studied extensively the numerous intermediate bodies which have a fugitive existence in the process. Most of these are covered by the term dextrins. It is sufficient for our present purpose to regard them as carbohydrates, simpler in their molecular structure than the original starch, but complex as compared with the familiar sugars. We have said that the chief product of salivary hydrolysis is malt-sugar or maltose. This is one of several sugars classed as disaccharids. By various means it can be hydro- lyzed further to form dextrose, a sugar of the simplest type, and one which is ready to be absorbed and to minister to the living tissues. Some dextrose is said to be formed in prolonged salivary digestion, but the cleavage lags when the maltose stage has been reached. CHAPTER VIII THE MOVEMENTS OF THE STOMACH It will be recalled (Chapter VI) that the stomach consists of a main rounded portion from which a much smaller conical segment extends to the right to join the duodenum at the pylorus. The larger part is called the fundus, the tapering region is the antrum. The muscular coats of the antrum are somewhat thicker than those of the fundus and show an especially conspicuous development of the circu- lar elements. There is no such contrast between the two parts of the human stomach as between the thin-walled crop and massive gizzard of the bird, but there is a faint suggestion of an analogous difference. The fundus is, indeed, primarily a place for the storage of food ; the an- trum, while not a crushing mechanism, has distinctly greater motor properties. The antrum is considered to be set off from the fun- dus by the so-called transverse band. This is an irritable ring of the circular muscle which is often contracted enough to indent the outline of the stomach at this point, and which may occasionally create a temporary division of the gas- tric cavity into two parts. It has been called the sphinc- ter of the antrum, but it cannot fairly be compared with the cardiac and the pyloric sphincters, since these are habitually closed, while closure at the transverse band is rare. Regulation of the Cardiac Sphincter. — Food and drink entering the stomach pass the cardiac sphincter. The guardian muscle is usually more or less contracted. It relaxes upon the arrival of the peristaltic wave in the esophagus. If it is recalled that the peristalsis consists of a wave of inhibition running before a contraction, it is 70 Fig. 13. — To suggest the probable appearance of the distended and active human stomach. Three marked waves of contraction are seen in the antral region. These are to be conceived of as pass- ing onward toward the pylorus. THE MOVEMENTS OP THE STOMACH 71 easy to see how the cardia may be opened at the moment when its muscular walls fall under the influence of the phase of relaxation. Closure will follow immediately, as the second or positive part of the peristalsis involves the sphincter. Attention has been called to the fact that liq- uids may outrun a pursuing peristalsis and arrive several seconds in advance of it at the cardiac opening. Under such circumstances it is said that the fluid remains at the bottom of the esophagus until overtaken by the wave, when the relaxation occurs which permits it to pass into the stomach. A person drinking with ill-advised haste may have the disagreeable experience of filling the esoph- agus enough to produce a painful distention. Relief comes abruptly when the peristalsis has made its way to the cardiac sphincter and secured an entrance for the liquid. While this has been the generally accepted description of the facts, it has been shown recently that the cardia is not always so firmly contracted. Observations by x-ray methods, to be described presently, have shown that for some time after a meal there may be a reflux from the stomach of a cat into the esophagus. Each escape of food evokes a local peristaltic wave which returns it to the stomach. Such an incident does not entail any movement of the throat muscles and is probably subconscious. As the period of digestion continues the sphincter becomes more tightly set and no longer allows any such return of stomach-contents. The increased tension has a simple explanation, which, like many another point about the stomach, we owe to W. B. Cannon. He has shown that the tension is developed in response to the rise of acidity in the liquid just within the cardia. Since the acid appears normally after each filling of the stomach, we have here an automatic provision for the establishment of the requi- site guard over this opening. The influence of the ner- vous system seems capable of nullifying the local effect of acid, since the sphincter may be relaxed to permit vomit- ing at times when the gastric contents are excessively acid. 72 NUTRITIONAL PHYSIOLOGY The Fundus. — An important service of the stomach is to store food in relatively large quantities at mealtimes and to deliver it gradually to the intestine. A person who has been deprived of the stomach, or of most of it, by sur- gery is made aware of this when he finds it impossible to eat "a square meal," and is compelled to take small por- tions of food at short intervals. He is then serving his intestines somewhat as they are normally treated by the stomach. Storage is not the sole function of the stomach, but we do well to emphasize it. The fundus accommodates itself to its contents by tone changes, relaxing when food is being swallowed and afterward exerting a steady, mod- erate pressure which insures the filling of the antrum after every discharge at the pylorus. A lack of this tonic re- action may be a cause of serious disorders. The earlier writers often claimed that they could observe a definite and regular overturning of the contents of the fundus. In the light of more recent observations this does not seem to be usual. A German investigator fed to a rat three courses of food of contrasted colors. The animal was then killed and frozen. A section made through the hard- ened mass within the stomach showed distinct stratifica- tion. The food first taken was in the antrum and the lower part of the fundus, the second instalment was above the first, and the third was just under the cardia. It seems hardly probable that entirely liquid food could re- main thus stratified when one considers the extent to which the stomach is subjected to the influence of bodily move- ments. Hunger. — Very recently it has been established that there is a connection between certain contractions of the stomach — particularly of the fundus — and the sensation of hunger. The experimenters who have demonstrated this fact point out that we must distinguish between hunger and appetite in all discussions of this kind. Hunger, as they intend to have the word understood, is a recurring pang readily referred to the gastric region. It is experi- enced alike by adults and infants, by human beings and THE MOVEMENTS OF THE STOMACH 73 animals. Appetite, on the other hand, depends on psycho- logic factors (recollections of favorite foods, etc.). It will be worth while to recount the ingenious experi- ment by which it was made clear that the intermittent hunger sensation corresponds with a motor process among the contractile elements of the stomach. Washburn, working under the direction of Cannon, devised a way to record the contractions of his own stomach. For this purpose a stomach-tube, bearing a collapsed balloon at the lower end, was swallowed by the self-sacrificing investi- gator. The balloon could then be inflated, and the tube brought into connection with a gauge consisting of a water column carrying a float and writing-point. Thus, if the stomach squeezed the balloon the tracing showed an ele- vation; as the stomach relaxed the curve declined. Washburn fasted until he could rely upon the occurrence of hunger pangs in spite of the discomfort to which he was subjected. The revolving drum which received the record was out of his sight. By pressing an electric key he could cause a mark to be made upon it at any time. He pressed this key whenever he experienced the sensation of hunger. The tracing showed that strong periodic contractions of the stomach coincided with the recorded gnawings. This result has been confirmed by Carlson, who has made similar experiments upon a man with an opening through the body wall to the interior of the stomach. It was observed in Washburn's experiments and in those of Carlson that the hunger contractions ceased when food was tasted and chewed. This indicates a reflex inhibition of the gastric musculature. It had already been noticed by Cannon that the tone of the fundus is temporarily lowered whenever food is swallowed. This constitutes what is called the "receptive relaxation," and it has an evident value since it facilitates the transfer of material from the esophagus to the stomach. We are told that when a long fast is undertaken the characteristic hunger pangs are not experienced after two or three days. It is probably true that insufficient feeding 74 , NUTRITIONAL PHYSIOLOGY is more distressing than downright fasting. Such suffer- ings as those endured by the Greeley party at Cape Sabine afford a confirmation of this opinion. Certain statements have previously been made concern- ing the sensibility of the esophagus to common forms of stimuli. Carlson, and others have reported corresponding facts with reference to the mucosa of the stomach. They find that it is relatively difficult to evoke sensations from this surface. Pressure, to be effective, must be of such in- tensity that its transmission to deeper tissues has to be assumed. Heat and cold are productive of recognizable sensory impressions, but it has long been an unsettled question whether the mucous membrane itself is respon- sible for the effect or whether this depends on temperature changes in the skin. Carlson's subject gives such a prompt reaction when a small piece of ice is pressed against the lining of his stomach that the investigator believes there must be a true local sensitiveness to this kind of stimulation. sc-Ray Studies of the Stomach. — While much can be learned of the behavior of the stomach through experi- ments involving its exposure by surgical procedures, the ideal method is clearly one which leaves the animal in its normal condition. Such a method became available when the x-ray was first turned to account to observe visceral movements. The image of any part of the body projected by means of the x-ray shows the bones in clear contrast with the softer parts, but scarcely outlines the organs. If, however, any harmless substance opaque to the x-ray is introduced into the contents of the alimentary canal, it becomes possible to recognize the situation of this sub- stance so long as it remains sufficiently concentrated. More than this, if the cavity is well filled its outline is, of course, identical with that of the included material. The x-ray picture will then show the changing contour of the organ in silhouette. The compounds most used to secure opacity to the x-ray are the salts of bismuth, generally the subnitrate or the subcarbonate. THE MOVEMENTS OF THE STOMACH 75 The most numerous experiments of this sort are those of Cannon, and the cat has been the favorite subject. When the animal has had a full meal of bread-crumbs and milk with the bismuth salt evenly mixed in the mass the x-ray shows the entire form of the stomach. The fundus has an even outline and preserves it unchanged from hour to hour, except that a very gradual contraction takes place. The antrum is traversed by deep but slow-moving peris- taltic waves, which originate near the transverse band and pass to the pylorus. The tendency of such waves must be to force successive portions of the food into the intestine, but in the great majority of cases the waves bear down upon a tightly closed pyloric sphincter. The only possible result is then an eddying movement, the contents advanc- ing only to rebound from what is, for the time, a blind pouch. This favors the reduction of the larger morsels and helps to secure at length the formation of the smooth, creamy "chyme." But it is probable that most people have an exaggerated notion of the mechanical powers of the stomach. The waves which pass over the antrum arise in the cat with strange regularity at intervals of ten seconds. Each wave takes about half a minute to make its way to the pylorus, so there are commonly three creases to be seen, all shifting with a motion of clock-like slowness toward the outlet. During the prolonged period required for the emptying of the stomach of the cat — eight hours or more — it is evident that the total number of the waves may be over two thousand. When the pyloric sphincter momen- tarily relaxes, under influences to be discussed presently, the peristalsis of the antrum naturally drives more or less of its contents into the duodenum. Nervous Control of the Gastric Movements. — Muscular elements of the order found in the stomach have been said to have an automatic property. We have insisted, however, that this fact does not exclude the influence of the central nervous system. There is abundant evidence so far as the stomach is concerned that the musculature of 76 NUTRITIONAL PHYSIOLOGY the organ is played upon by efferent impulses. If it is separated from the central nervous system many of its reactions take place in a nearly normal manner, but we cannot assume that its adjustments are as well timed and decisive as they were before. Laboratory trials show that the impulses which are sent to the stomach may either accentuate or abate its spontaneous movements. In other words, they may either excite or inhibit the contrac- tile elements. Of the two types of nervous control, the inhibitory seems to be of particular significance. Complete arrest of the peristalsis of the antrum may be brought about. This happens in the cat when the animal is enraged or terrified, and, indeed, when it seems merely to be restless. Cannon has again and again seen the peristaltic notches fading away from the x-ray profile of the antrum when the animal has wearied of being kept under restraint, and is manifest- ing its feeling by switching its tail and struggling. He has seen the regular activity resumed when the cat has been pacified. Similar facts have been demonstrated for the rabbit. Since we generally believe that the higher the grade of an animal's development, the more extensive the command of the nervous system over its organs, we have every reason to think that unpleasant emotions may be accompanied in man also by inhibition of the gastric move- ments. We shall have occasion to enlarge upon this matter in connection with the Hygiene of Nutrition. The Pyloric Sphincter. — The regulation of the escape of the stomach-contents to the intestine has long been a subject of interest. More than eighty years ago William Beaumont published his observations upon the stomach of Alexis St. Martin, a young Canadian trapper, who had suffered a gunshot wound in the left side, in consequence of which he had a permanent gastric fistula. The impres- sion which went abroad from this celebrated case has seemed to convey to the less scientific writers the idea that the pylorus has a power almost akin to intelligent inspec- tion whereby it permits the passage of certain portions of THE MOVEMENTS OP THE STOMACH 77 the chyme and refuses egress to other portions. This notion recalls amusingly the teaching of Van Helmont, in the seventeenth century, that the soul of man resides in the pylorus. It cannot be said that all the conditions affecting the discharge from the stomach are entirely clear, but much progress has been made in this direction. The sphincter is influenced both by the physical consist- ency and by the chemical reaction of the gastric contents. The contact of coarse, angular particles with the adjacent mucous membrane seems to reinforce its contraction, so that such material tends to be kept longer in the stomach. A more important factor with this sphincter, as with the cardiac, is the acidity of the chyme. It was stated above that when the stomach-contents becomes distinctly acid- ified, the tone of the cardiac sphincter is increased. The acid in this case is acting upon the lining below the irritable ring. Comparison of the two sphincters shows it to be a principle applicable to both — that acid acting immediately above favors their relaxation, while acid below causes them to tighten. If this is the main factor in regulating the pylorus, the first opening will occur when the acidity in the antrum has reached a certain point. The next peristaltic wave will transfer a little chyme to the duodenum. At this instant there will be acid material both above and below the sphincter. The action from below appears to predom- inate, so that closure will be established and maintained until the acid in the duodenum is either neutralized or re- moved somewhat from the pylorus. When the stimula- tion from below is no longer effective the acid above will cause a second gaping of the sphincter, followed as before by prompt and decisive contraction. A more efficient mechanism to insure gradual delivery to the intestine without distending it locally can scarcely be conceived. When the latest portions of a meal are leaving the stomach, the first which went out may have reached the colon, and intermediate fractions may be undergoing digestion in numerous loops of the intervening small intestine. Our meals are usually of a mixed character, including 78 NUTRITIONAL PHYSIOLOGY proteins, fats, and carbohydrates. For purposes of experi- ment single food-stuffs may be fed to an animal and the rate of departure from the stomach noted for each. The a;-ray has been employed for this purpose. Carbohydrates have a striking tendency to escape rapidly to the intestine. The discharge of proteins and of fats is relatively much de- layed. We must be content with stating the fact without undertaking to discuss its somewhat complex causes. Vomiting. — The occasional expulsion of the stomach- contents through the cardia and esophagus is accomplished as the result of a reflex movement in which the chief mus- cles involved are not the coats of the stomach, but the contractile tissue of the diaphragm and abdominal wall. When these contract simultaneously a high pressure is thrown upon the stomach. Such a pressure may accom- pany the act of straining or bracing the body for lifting, but does not ordinarily result in vomiting, it would appear, because of the resistance offered by the cardiac sphincter. In the crisis of nausea, however, convulsive movements of this kind take place with inhibition of the sphincter. With the passage open, each application of intense pressure to the stomach may drive a portion of its contents to the exterior. The palate meanwhile has assumed the same position as for swallowing; the larynx is drawn forward and is shielded by the root of the tongue, which is depressed and grooved. At every descent of the diaphragm the capacity of the thorax is increased, and as no air is permitted to enter the lungs the esophagus is dilated. During the act of vomiting the fundus is said to contract steadily upon its diminishing contents. This is not a movement powerful enough to secure the emptying of the stomach, but adapts it to be gripped effectively by the muscles of the body wall. The transverse band is at the same time strongly contracted and the antrum has a very small volume. The pyloric sphincter is said to be closed, but it is a familiar fact that bile from the duodenum may be pressed backward into the stomach under the stress of violent vomiting. Profuse salivation precedes and accom- THE MOVEMENTS OF THE STOMACH 79 panies the act. Such a reflex has a manifest value when it serves to remove from the stomach material which might prove poisonous. It occurs, however, under many circum- stances when it seems not to have any advantageous re- sult. Its apparent uselessness in seasickness has already been alluded to. It appears equally illogical when, in pregnancy, it is excited by irritation of the pelvic nerves. CHAPTER IX GASTRIC SECRETION AND DIGESTION It was in the eighteenth century that the chemical factors in digestion were first clearly separated from the mechanical. The accounts which have been preserved of the experiments of Reaumur (1752) and of Spallanzani (1777) are of extraordinary interest. An entertaining summary is to be found in Foster's "Lectures on the His- tory of Physiology," Chapter VIII. These ingenious investigators were the first to show that digestive changes may be caused to take place outside the body and in the absence of any mechanical process whatever. They obtained small quantities of gastric juice from various animals, mixed it with food in flasks and test-tubes, and watched for signs of alteration. Spallanzani, in particular, succeeded in bringing about considerable solution of his samples. From that time to the present studies of digestion have continued. Where the pioneers were forced to be content with observing the dissolving of solid food, their successors are drawing inferences regarding the transformations of unseen molecules. A deeper insight into the meanings of digestion became possible as the new science of organic chemistry was swiftly advanced by the researches of Wohler, Berzelius, and Liebig. As was pointed out in Chapter III, it is the molecular change which is significant and not the physical. Experiments in which material is introduced into the alimentary canals of animals and later withdrawn for analysis are constantly compared with trials in which the digestion takes place throughout in the thermostats of the laboratory. The stomach is popularly supposed to have a very large share in the total work of digestion. It cannot, however, GASTRIC SECRETION AND DIGESTION 81 be claimed that it is indispensable to man. It forms, as already indicated, a convenient place of deposit for food and a gradual feeder of the small intestine, but it is mean- while the seat of preliminary digestive changes which greatly facilitate the further advance of the process. Attention has been called to the fact that salivary diges- tion is continued for a time in the almost stationary con- tents of the fundus. When this is stopped by the penetra- tion of the acid gastric juice it is superseded by a new type of digestion in which the proteins are the food-stuffs acted upon. The gastric hydrolysis of proteins is generally re- ferred to as peptic digestion. Before we discuss it in detail we must consider the nature and the circumstances of formation of the gastric juice. This secretion is the product of the numerous, relatively simple glands with which the mucous coat of the stomach is provided. Beaumont has vividly described the appearance presented by the lining of St. Martin's stomach directly after a meal. The surface, usually of a pale gray, flushed deeply, and the gastric juice welled up in glistening beads from the invisible mouths of the glands. Its manner of breaking out resembled the rising of perspiration from the pores of the skin. The empty stomach may have a well- marked film of mucus upon its walls, but its active secre- tion is limpid and free flowing. The volume which the human stomach produces in twenty-four hours is ap- parently very large; if we have a right to judge from what is known of the dog, it may be 3 or 4 quarts. A dog, being carnivorous, probably secretes a disproportionate quan- tity, and our estimate for man should very likely be re- duced. The Acid of the Gastric Juice. — Repeated reference has been made to the acidity of the stomach-contents. The early investigators were surprised and puzzled when they were forced to recognize that the acid in question is largely free hydrochloric. When we consider that this acid can- not be made industrially except by decomposing a chlorid with the still stronger and more corrosive sulphuric acid 82 NUTRITIONAL PHYSIOLOGY and in an earthen container, we can appreciate their feeling. For here is what we call a strong mineral acid proceeding from delicate living cells and from fluids of neutral reaction. The formation of hydrochloric acid by the cells of the gastric glands has become much more intelligible in the light of modern chemical teachings. The current theories cannot be presented here. It is evident that when the elements of an acid are withdrawn from a neutral fluid it must, theoretically at least, be rendered alkaline. We have a capital illustration of the refinement of the mechanism by which the body preserves uniform internal conditions in the fact that when gastric juice is being secreted, the urine, usually acid to common indicators, becomes alkaline. Thus the normal chemical equilibrium of the blood is maintained sacred from disturb- ance. The juice secreted into the antrum is said not to be acid. Mention has been made of some ways in which the acid of the stomach modifies local conditions. We have seen that it gradually checks salivary digestion. It is the most important controlling factor for the two sphincters. Other features of its action must now be presented. Among these is its distinct antiseptic infleunce. Spallan- zani noticed that pieces of meat undergoing digestion in gastric juice passed into solution without putrefying. Similar pieces kept for an equal time in water had radically spoiled. This was a significant observation at the time, because digestion and putrefaction had been regarded by many as identical. We know now that putrefactive de- composition of protein is due to the influence of swarming micro-organisms, and that hydrochloric acid in the con- centration usual in the gastric juice restrains the develop- ment of such forms. The average strength of the acid in man is given as 0.2 to 0.3 per cent., rising sometimes to 0.5. Such a concen- tration by no means suffices to sterilize the stomach-con- tents, but in all probability it destroys many kinds of bac- teria, including some which might become the cause of dis- GASTRIC SECRETION AND DIGESTION 83 ease. Others it undoubtedly weakens, so that they multi- ply less rapidly when the chyme passes on to the small intestine, where the conditions for bacterial growth are more favorable. It is not surprising to find that the species of organisms which thrive most in the stomach are those which themselves produce acid. The "sour stomach" com- monly referred to is a stomach in which the generation of lactic acid from sugars is actively taking place. This is a process very like the familiar souring of milk. Indeed, milk is one source of such acid fermentation in the stomach. Excessive acidity, whether due to the native juice or to the activity of bacteria, may be a cause of discomfort and a hindrance to digestion. Cannon has lately shown that acidity above a certain degree delays the departure of food from the stomach, and this is easily comprehensible when it is recalled that after each brief relaxation the pylorus re- mains closed until the acid which has just passed has been neutralized or dispersed. Acidity within limits is a neces- sary condition of gastric digestion, and this will be dis- cussed later. The Secretion of the Gastric Juice. — The glands of the empty stomach seem to be quite inactive. The natural supposition that they begin to secrete when food comes in contact with the mucous membrane is not borne out by the results of experiments. It is most important to note that the juice may start somewhat in advance of the arrival of food. The stomach as well as the mouth may be said to water at the contemplation of a meal. Abundant evi- dence of this fact has been furnished by the extraordinary experiments of the Russian physiologist Pawlow upon dogs. When a permanent opening has been made to the interior of a dog's stomach a little gastric juice may issue when the dog is merely shown food which he likes. That it is unnecessary to have actual contact with the stomach wall is still better shown in the case now to be described. A dog having a gastric fistula is subjected to a second operation, by which the esophagus is severed and the portion connected with the pharynx made to open 84 NUTRITIONAL PHYSIOLOGY through the skin of the neck. Whatever is swallowed by the dog is now returned to the exterior. The pleasure of eating is not impaired. To maintain nutrition suitable food may be introduced directly into the stomach. When the dog chews and swallows a meal he is quaintly said to have a "Scheinfutterung" — a fictitious feeding. This proceeding is accompanied by a steady flow of the secre- tion from the gastric fistula. The secretion obtained under circumstances like the above is called the psychic secretion. This term serves to emphasize the fact that a mental element is a necessary incident of the reaction. The conditions governing gastric secretion seem to be quite parallel with those which regu- late the movements of the stomach, and their importance in hygiene is equally evident. There are a number of individuals who have in various ways lost the power to swallow food, commonly because of the closure of the esophagus. Their lives are preserved by feeding through gastric fistulas established by surgery. These unfortu- nates find it to their advantage to attend to the idea of eating, and to taste and chew portions of their food at the time when it is being introduced into their stomachs. After a brief period of fictitious feeding the flow of gastric juice into the stomach of a dog may continue for two or three hours. In view of this we must conclude that, however important the mental state may be for the initiation of the process, it need not be its accompaniment throughout. We cannot pretend that a meal receives our constant attention during any such interval. Neverthe- less we are hardly likely to overestimate the necessity of securing a normal start. If the initial circumstances are not favorable the secretion may be long delayed or even lacking. Now that we have reserved a due place for the psychic element, we must pass on to consider the other factors which modify the activity of the glands of the stomach. Much has been learned from the surprising operations of Pawlow and others, who have succeeded in dividing the GASTRIC SECRETION AND DIGESTION 85 stomach of the dog into two parts without robbing either of its connection with the circulatory and nervous systems. When the dog has recovered from the immediate effects it may be said to have two stomachs. Either or both may communicate with the exterior by a fistula, while one still retains its normal relations with the esophagus and small intestine. This arrangement makes it possible to place food in one stomach and to obtain and measure the un- mixed juice secreted into the other. The evidence goes to show that when the glands in the lining of one sac are active there is corresponding activity on the part of those in the other. A most significant fact is at once noted when such a dog is under observation. There are kinds of food, perfectly appropriate for the animal's nutrition, which may lie in the stomach without exciting any flow of the juice. This is said to be true of bread, white of egg, starch, and some sugars. The same articles of diet would be met by an abundant gastric secretion if they had been eaten with enjoyment by the hungry dog. It makes a radical differ- ence, then, whether these materials enter the stomach through the mouth and attract the favorable notice of the animal, or whether they are slipped through a fistula, a proceeding which would probably not be recognized by him as a mode of feeding. On the other hand, there are some things which do cause an outbreak of gastric juice by their mere presence in the stomach and in the absence of any psychic factor. To a somewhat limited extent this is the case with water, though only, it appears, when there is enough of it to dis- tend the stomach slightly. The best-known excitant of the secretion is meat, and the property is said to belong to the extractives or minor substances in this food, and not to the proteins of which it is chiefly composed. Meat causes a considerable flow of the juice, but there is a much longer initial delay than when the psychic element has its normal place. In fact, the total quantity produced when meat is introduced into a dog's stomach without attracting his 86 NUTRITIONAL PHYSIOLOGY attention is decidedly less than when it is eaten in the natural way, and when the psychic and chemical agencies are combined. It is unfortunate that our knowledge pf this matter has been drawn so largely from a carnivorous animal. Meat might be expected to stimulate the stomach of the dog more surely than other foods. How far the secretion may be elicited by placing other compounds in the stomach is imperfectly known. Milk is credited with some power to call it forth, but its superiority to water seems doubtful. Alcohol is said to have a positive action of the same kind. It is claimed that the dextrins, the intermediate bodies produced in salivary digestion of starch, have the property of starting the gastric flow. If this is true it is interesting as establishing a connecting link between the two successive processes, and making it. apparent how one may tend to insure the setting in of the other in due time. A link of this sort exists between gastric digestion and pancreatic secretion, as we shall have occasion to point out. The condiments, such as pepper and spices, have a reputation for stimulating the discharge of gastric juice, and un- doubtedly do so when they favorably affect the flavor of the. food. They are known to increase the blood flow in the lining of the stomach, which would perhaps help to con- tinue the secretion process when once under way, but whether they can actually initiate it apart from their psychic effect remains uncertain. When gastric secretion is well started there is provision for its maintenance as long as the stomach contains food. It appears that some of the early products of digestion act after the manner of the extractive substances of meat and excite the glands to continued activity. The acid itself has been found to be absorbed by the cells lining the antrum and to set in motion a train of events leading to the same result. This form of stimulation will evidently cease only with the departure of the last portions of the acid chyme. The flow of gastric juice is retarded by fats and by alkaline mixtures. GASTRIC SECRETION AND DIGESTION 87 Digestion in the Stomach. — The gastric juice is usually- said to contain two enzymes. Recent work indicates the presence of a third. The two familiar ones are pepsin and rennin. The third is the gastric lipase. Certain writers have questioned whether we ought to speak of pepsin and rennin as distinct individuals, suggesting rather that there is in the secretion a single body having two sets of proper- ties. We need not enter into such a discussion; we shall for the present continue the convenient usage of speaking of pepsin and rennin as two substances. Rennin. — The fact that extracts of the stomach wall cause the curdling or coagulation of milk has been known from very early times. Such extracts, usually derived from the stomach of the calf, have long been in use in the manufacture of cheese. Rennet is the industrial term for an extract with this property; rennin is the scientific term for the supposed enzyme contained in it. Cheese curd consists of the bulk of the protein of milk which has undergone an obscure chemical change and has passed into an insoluble form. From a physical standpoint this is an anomaly among the digestive processes. We look to see solids becoming liquids, while in this curious instance a liquid becomes a solid. No very convincing explanation of this occurrence has been offered. It may be suggested that it prevents an unduly rapid passage to the small in- testine, but so far as we know the mechanism of the pylorus is entirely competent, even for liquid food. The curd when formed has to undergo solution like any other solid. The action of rennin becomes the more enigmatic when it is noted that it is found in the stomachs of animals which do not have milk in their normal diets. Milk is curdled by extracts of various organs other than the digest- ive glands and by some vegetable juices. In the human stomach a very firm curd may be formed when a large quantity of cows' milk is taken at one time. The dense mass may be slow to digest. Human milk is said never to set into such a tenacious coagulum, and this is natural, 88 NUTRITIONAL PHYSIOLOGY since, regarded as a solution of proteins, it is much more dilute than the milk of the cow. Peptic Digestion. — The chief enzyme of the gastric juice is the one commonly called pepsin. Its relations with the acid of the stomach are so close that many writers urge that we should speak rather of "pepsin-hydrochloric acid," the term suggesting the existence of a compound of the two which is responsible for the action on the food. The power to digest proteins is manifested only with an acid reaction, and is permanently lost when the mixture is made distinctly alkaline. The conditions which permit peptic digestion to take place are, therefore, precisely those which exclude the action of the saliva. When protein in solid form, such as boiled white of egg, is subjected to the influence of gastric juice the pieces swell and become softened. Later they are dissolved. When the trial is made with protein which is originally in solution, such as unboiled white of egg, there is no visible evidence of change. But there are physical and chemical tests which can be employed to show that digestion is as definite a change in this case as in the other. An early manifestation of this fact is the loss of the property of coagulation on heating. Later there are indications that the molecules are undergoing cleavage. At each successive stage there is a gain in the power of diffusion, a reduction of viscosity, and a diminution in the number of precipitants which can be employed to throw the protein out of solution. Physiologic chemists have studied minutely the charac- teristics of the hydrolytic products during the advance of peptic digestion. They have attempted to identify nu- merous compounds, each of which has a transient existence and is then itself hydrolyzed. For our present purposes it would be unprofitable to dwell upon such questions of detail. Certain of the earlier cleavage products are included under the general name of proteoses or albumoses; others, arising later and of a simpler character, are called peptones. Roughly speaking, there is a parallel between salivary and peptic digestion. In either case, molecules of great size — GASTRIC SECRETION AND DIGESTION 89 of starch and protein respectively — are subdivided pro- gressively, first with the formation of somewhat complex bodies — dextrins and proteoses — later to form maltose in the first instance and peptones in the second. The corre- spondence is imperfect in several ways; for example, mal- tose is a single substance, while there appear to be a number of peptones. It will be remembered that maltose itself is slowly transformed by long-continued action of saliva. Quite similarly, the gastric juice will in time effect a fur- ther digestion of the peptones, but this advanced diges- tion seems normally to be postponed until the intestine is reached. Gastric Lipase. — Down to a recent time it was held that fat underwent no true digestion in the stomach. It was recognized that some forms of fat, butter, for example, must be melted, and that the fat in the adipose tissue of meat might be released from the enclosing cells when their protein portion was dissolved. These, however, are mere physical changes. Attentive study has brought out the fact that there is, after all, some hydrolysis of fat in the stomach, though it is probably slight. An enzyme with this action is accordingly assumed and is spoken of as gastric lipase. It is only the most finely divided (emulsi- fied) fats which seem to be appreciably affected. The products of this decomposition as well as of the later fat digestion in the intestine are glycerin and free fatty acids. Summary. — The material passing the pylorus is com- paratively dilute and normally free from coarse particles. It is acid in reaction, both the native hydrochloric acid and the acids formed by fermentation contributing. Much of the food is as yet practically undigested. On the other hand, some progress has been made in the transformation of cooked starch into sugar. The proteins are partially peptonized. If milk has formed a part of the diet, it will have been curdled and redissolved. Fats may have been liquefied and scattered, but are not likely to have been extensively hydrolyzed. On the whole, gastric digestion may fairly be described as preliminary in character. CHAPTER X THE SMALL INTESTINE: ITS MOVEMENTS, SECRETIONS, AND DIGESTIVE PROCESSES The intestinal content is propelled on its winding way by peristaltic movements. These are similar in their mechanical principle to the waves of muscular contraction passing down the esophagus in the act of swallowing. They are, however, much slower and gentler in character. As a rule they do not run an uninterrupted course from the pylorus to the ileocecal valve, though such a phenomenon is an occasional possibility. More commonly a wave will travel a limited distance and then fade out, leaving the material which it was moving to rest for a time in some depending loop. If this is true, the progress of the food is intermittent; x-ray observations upon human subjects have led to the estimate that it takes four or five hours for the passage through the whole length of the small intestine. This time may be assumed to vary widely with the indi- vidual and the diet. Reduced to an average rate the data quoted above give us about one inch per minute. Intestinal peristalsis, like the movements of the stom- ach, is governed mainly by local mechanisms. Yet in this case as in the other the central nervous system may exert an influence tending to accelerate or to suppress the ac- tivity of the muscular coats. The second or inhibitory action is the more marked. A question much discussed is whether the direction of peristalsis in the small intestine is at all subject to reversal. The sum of the evidence at present supports the view that such a reversal (antiperis- talsis) is possible, but only under conditions which are clearly abnormal. Ordinarily it is plain that the intestine is distinctly specialized to act in one way rather than the other. 90 THE SMALL INTESTINE 91 Not all the muscular contractions exhibited by the small intestine have a progressive character. Frequently a loop which contains food will become creased at short intervals by rings of constriction which do not shift their position, but remain stationary for a little. The internal effect is to create a series of small pouches holding separate portions of the chyme. After this condition has persisted for a mo- ment the regions originally contracted become relaxed, and new contractions set in at points midway between them. Under the influence of such movements the food is con- stantly shifted about and subdivided, but it is not driven steadily in one direction. This "marking time" on the part of the small intestine is referred to as rhythmic seg- mentation. Inasmuch as it serves to alter the contact relations between the intestinal contents and the lining, it probably favors absorption. Some writers have made much of the effect which these contractions may be as- sumed to have upon the flow of blood and lymph in the walls of the canal. When pressure is applied at brief intervals to tissue containing these fluids the result may be described as a massaging action, hastening the circulation and crowding out some of the lymph. This again must tend to promote the absorption process. A longitudinal movement of individual loops is often described. Two neighboring turns of the intestine may be' seen to glide the one upon the other, coming to rest after slipping a short distance, and presently reversing the direc- tion of their travel. This form of activity cannot defi- nitely further the progress of the contents, but when it affects segments enclosing liquid material we may suppose that the food and juices are tilted about and brought into relation with the largest possible area of absorbing surface. The Secretions Entering the Small Intestine. — In Chapter VI it was stated that this part of the canal re- ceives the contributions of the liver and the pancreas, as well as of the microscopic glands in its own mucous mem- brane. The bile and the pancreatic juice, it will be re- membered, enter just below the pylorus. The intestinal 92 NUTRITIONAL PHYSIOLOGY juice is produced by all parts of the extensive lining, but more abundantly in the upper than in the lower segments. The three secretions have some characters in common. They are all alkaline in reaction, owing to the presence in them of sodium carbonate. This confers upon them in a considerable degree the power to neutralize acids. As the acid chyme from the stomach meets the alkaline secre- tions in the duodenum there must be more or less carbonic acid gas evolved. This may be helpful to the digestive process, since the tendency will be to lighten the texture of the food particles, much as dough is lightened by the agency of yeast. It is not merely the acid from the stomach which may be combated by the alkali of the juices below; there are two other sources of acid to be taken into account. One of these is found in the bacterial fermentation, chiefly of sugars, which goes on in the intestine. The second is the entirely normal formation of free acids occurring in the course of fat digestion. So far as the first class of acids are neutralized the products are mainly lactates and buty- rates; the fatty acids may be converted into soaps. There is no guarantee of an exact proportionality between the acids and the alkali, and it is impossible to say which will be in excess in a particular part of the canal. Generally, however, the resulting reaction of the mixture is not far from the neutral point. The united volume of the three secretions is held to be very large, but any estimate tends to mislead, since throughout the length of the intestine we have the with- drawal of water keeping pace approximately with its in- flow. In a certain section selected for observation the bulk of the contents may show little change during a long period, and yet there may have been profuse secretion entirely disguised by counterbalancing absorption. In this consideration we see indicated an important service shared by all the digestive secretions, that of supplying liberal quantities of water to act as the solvent and carrier of food-stuffs destined for absorption. If absorption were THE SMALL INTESTINE 93 to continue without compensatory secretion, a final stage might be reached in which the cavity of the intestine would be drained of all fluid, while the walls would be crusted with a dry residue suggestive of boiler scale. The Pancreatic Juice. — In considering the causes of gastric secretion the importance of the central nervous system called for emphasis. This is not true to the same extent of the government of the pancreas, though here also it is held that the nervous system plays a part. A chemical means of control is better known. As has been hinted before, there is an intimate relation between the gastric activity and the later awakening of the pancreas from its state of repose. The arrival of acid from the stomach in the duodenum causes a timely outflow of pan- creatic juice. This might be supposed to be an instance of reflex action, like the production of saliva when acid is taken into the mouth. It has been shown, however, that it has another explanation. The acid, striking into the lining membrane of the duodenum, initiates a series of reactions which have been studied in detail, and which lead at last to the for- mation of a substance of quite definite chemical properties called secretin. This finds its way into the circulation, and, like a drug, is swept far and wide. It stimulates the pancreas to produce its secretion, augments the formation of bile by the liver, and probably excites the glands in the wall of the intestine to greater activity. In the light of these facts it becomes clear that a vigorous gastric digestion with the strongly acid chyme which results goes far to insure a normal intestinal process. The pancreatic juice in man is an abundant secretion — a pint or more daily — and, in marked contrast with the saliva and the gastric juice, it has relations with all three principal classes of food-stuffs. It contains an enzyme which some have thought to be identical with the ptyalin of the saliva, but generally called amylopsin or pancreatic amylase. Thus the progress of starch digestion, inter- rupted for a time in the stomach, is now renewed. The slight acidity which may exist in the intestine is not likely 94 NUTRITIONAL PHYSIOLOGY to prevent this type of digestion from going on to com- pletion. Beside acting on starches, the pancreatic juice continues and greatly accelerates the hydrolysis of fats which has been barely begun in the stomach. The enzyme concerned is called steapsin in the older books, but by more recent writers lipase. The immediate products are glycerin and fatty acids. A secondary formation of soaps is a possibil- ity already indicated. When an oil undergoes digestion it breaks up at an early stage into microscopic drops and is said to be emulsified. This subdivision clearly multiplies the surface of contact between the food and the digestive juice, and has an effect corresponding to that of mastication upon solids. It is, therefore, most helpful to digestion, but it is not to be confused with digestion itself. The statement is commonly made that the pancreatic juice continues the work of pepsin upon proteins by virtue of an enzyme called trypsin. While this is approximately true, it calls for a certain qualification. If the juice is carefully collected as it comes from the main duct of the pancreas without being allowed to mingle with other se- cretions or even to touch the lining of the intestine, it is reported that it is usually incapable of hydrolyzing pro- teins. This power it gains in a striking degree when it has been mixed with ever so little of the intestinal juice, the succus entericus, as it is sometimes called. The interaction of the two secretions is described as resulting in the "activating" of the pancreatic juice. The natural infer- ence is that the inactive fluid contains in solution a body, not yet deserving the name of enzyme, but ready to be- come one by a quick transformation. An inactive ante- cedent body of this kind is termed a zymogen; in this specific instance, trypsinogen. Acting on the assumption that there is a definite substance in the intestinal juice capable of changing trypsinogen to trypsin, physiologists have given the name of enterokinase to the agent concerned. Tryptic digestion differs in characteristic ways from the peptic process which has been described. Acid which is essential to digestion in the stomach is antagonistic to the THE SMALL INTESTINE 95 pancreatic type. Trypsin does its work best in a nearly neutral mixture. In the normal course of events it acts upon material already partly hydrolyzed, but it has the power to carry through all its stages the digestion of native protein. If the food is a solid, mere inspection reveals a difference between peptic and tryptic solution. In the former case there is marked swelling, as previously stated ; in the latter there is progressive corrosion, a shredding or honeycombing of the specimen. When chemical methods are employed in the study of tryptic digestion, it is found to run a course roughly parallel with that of the gastric digestion of proteins. Correspond- ing intermediate bodies — proteoses — are described in both instances, though it is said that some stages in the tryptic process are passed so rapidly as to seem almost to be omit- ted. What distinguishes the pancreatic proteolysis most radically from the peptic is the facility with which the peptones are broken down into still simpler bodies. We have made the statement that these late cleavages take place only very slowly under the influence of gastric juice. So it becomes clear that trypsin is distinctly adapted to fol- low after pepsin in the accomplishment of protein digestion. Peptones are compounds which are simple by comparison with standard proteins, but which are still too complex to be given precise chemical formulas. When they are hydrolyzed, most of the products come within the knowl- edge of the organic chemist so definitely that their molecu- lar structure can be confidently expressed. To the student it must be admitted that such formulas do not appear simple, but if he is disposed to resent the use of the word he is to reflect that these molecules stand in some such rela- tion to the original protein complex as the bits of the mosaic bear to the whole design. It is with some such an idea that the Germans have called them Bausteine, that is, the building-sfones, from which a new architecture can be constructed. The simplest products of the tryptic process are conveniently called amino-acids. The Intestinal Juice. — This copious secretion was for- merly regarded as having little to do with digestion. The 96 NUTRITIONAL PHYSIOLOGY present disposition is to credit it with a very considerable share. When the pancreatic juice is prevented from enter- ing the intestine it remains possible to keep up the nutri- tion of the animal, and one must conclude that the intes- tinal juice is successfully preparing more than one kind of food for absorption. Samples of the secretion have often been obtained from loops of the intestine disconnected from the remainder of the canal. Different workers give vary- ing accounts of its properties. The feature of its digestive action concerning which there is the most general agreement is the hydrolysis of the more complex sugars, the disaccharids. Of these sugars, three are commonly present, and there appear to be three en- zymes adapted to act upon them. Maltose arises princi- pally from the salivary and pancreatic digestion of starch. It is hydrolyzed to dextrose by an enzyme, which is best called maltose. Lactose, or milk-sugar, is similarly con- verted into equal parts of dextrose, and the less familiar sugar galactose by the enzyme lactase. Saccharose, or cane-sugar, gives rise to dextrose, and a sugar of different properties, levulose (fructose), under the influence of the enzyme, invertase. When an extract is prepared from the thoroughly minced lining of the small intestine it can be shown to have the power to cause proteoses and peptones to undergo hydro- lysis, though it is said not to act upon the original unmodi- fied protein. This is equivalent to saying that such an extract can parallel the later work of trypsin, though lack- ing its power to initiate digestion. The enzyme implied has been named erepsin. It is regarded as an open ques- tion whether this enzyme normally enters the cavity of the intestine or does its work within the confines of the cells from which it can be extracted. We may conceive that when an animal is deprived of its pancreatic secretion it is still able to digest proteins, pepsin beginning the digestion and carrying it to a stage at which the products are suscep- tible to the action of erepsin. The presence of enteroki- nase in the intestinal juice has just been noted. The Bile. — The secretion of the liver cannot be regarded THE SMALL INTESTINE 97 in the same light as the digestive juices mentioned hereto- fore. It is not secreted merely after meals, but is always flowing through the ducts which converge from the several lobes of the liver. It is not necessarily entering the in- testine at all times, since the gall-bladder provides a place for its temporary storage, as described in Chapter VI. While the production of bile never ceases, it does show an acceleration during the digestive periods, and this is be- lieved to be in response to the stimulating effect of secretin. Bile attracted the attention of physicians in very early times, its conspicuous color and intensely bitter taste giving it a certain distinction. It entered largely into ancient theories of disease and of medicine. We have traces of these facts in the root-meaning of such words as bilious, choleric, and melancholy. In popular estimation bile is a poison arising now and then in the system and causing digestive disturbances. Patient study has shown that the bile is a complex mixture, and that it numbers among its constituents some which are waste-products and others which have a favorable effect upon the progress of diges- tion and absorption. It stands, therefore, in a position intermediate between that of the gastric juice, which is formed solely to advance digestion, and that of the urine, which is composed of material useless to the body. The pigments of the bile are counted as waste substances. A red one predominates in carnivorous animals. The bile of the herbivora is green; human bile may be green, yellow, or orange. These pigments show in their chemical nature a close relationship with the red coloring-matter of the blood, the important compound hemoglobin. All the evi- dence goes to show that the bile-pigments are modified fractions of the great hemoglobin molecule, and that their abundance is an indication of the amount of destruction suffered by the red corpuscles of the blood. They do not contain the iron previously present in the hemoglobin; this element seems to be conserved. These pigments are rela- tively insoluble and are not always successfully carried to the intestine by the bile. When they deposit in solid form 7 98 NUTRITIONAL PHYSIOLOGY in the gall-bladder they contribute to the formation of "gall-stones," aggregations in which another compound, the waxy cholesterin, may be included. The pigments are usually more or less altered by bacterial action in the course of their journey through the canal, and eventually become the chief coloring-matter of the feces. The bitter taste of bile is due to two organic salts of high molecular weight. These seem to have a totally dif- ferent significance from that of the pigments. They are not lost to the body, but are absorbed from the lower part of the small intestine and are presumably secreted again and again. This phenomenon has been spoken of as the "circulation of the bile-salts." The withholding of these bodies from the alimentary tract tends to derange digestion and, in particular, to diminish the absorption of fats. When bile is tested by itself it shows only the feeblest digestive powers, yet pancreatic digestion is greatly promoted by its presence, and it may be likewise an ally of the intestinal juice. Light is thrown on the properties of the bile by observing the condition of jaundice. This disorder is commonly caused by the more or less general plugging of the bile- ducts with mucus. The secretion cannot make its escape from the liver in the normal way and some of it enters the circulation. Bile-pigments make their appearance in the white of the eye and in the skin. The urinary pigment, which is always closely related to the pigments of the bile, is much increased. The ill feeling which usually attends the condition may be due in part to the mildly poisonous effect of the abnormally retained bile constituents. It is likely to be aggravated by indigestion. The bile-salts are lacking and the capacity to digest the food and promptly absorb the end-products is greatly reduced. Bacterial action in the intestine may become pronounced. This last fact has suggested that the bile may be an antiseptic. It cannot be shown to have anything like a universal action of this kind, but it is very probable that it has a selective one, favoring one type of organism and restraining another. Even though it had no such influence, the intestinal THE SMALL INTESTINE 99 bacteria might be expected to multiply in its absence, for the simple fact of delayed absorption would suffice to bring this about. Our best defense against excessive fermenta- tion and putrefaction is in the early and complete removal of the food from the sphere of action of micro-organisms. Summary. — The secretions flowing into the small intes- tine supply enzymes in sufficient number and variety to accomplish the digestion of all common foods. The trans- formation of starch to maltose begun in the stomach is completed by the pancreatic amylase. The resolution of proteins into the simple structural units from which their molecules are built is carried out under the influence of trypsin and perhaps of erepsin. The last-named enzyme is supposed by many to work upon the tryptic products as they pass through the lining cells on their way to the blood. Fats are hydrolyzed by the pancreatic lipase, with the formation of glycerin and fatty acids, the latter being in some measure converted to soaps. The disaccharids are changed to monosaccharids, a work attributed to the in- testinal juice. Fermentation caused by bacteria has been taking place along with the strictly normal processes. The most evident products are organic acids, which may or may not be fully neutralized by the alkaline secretions. Protection Against Self-digestion. — There has been much discussion of the fact that the proteolytic enzymes in the digestive tract do not ordinarily attack its mucous membrane. They may do so after death and when the tissues are in an abnormal condition, as in case of gastric ulcer, the juices may strongly antagonize the healing pro- cess. It is often asserted that there is a definite chemical difference between the proteins of living and of lifeless matter. A recent explanation of the resistance which living cells offer to digestion is based on the apparent fact that such cells form bodies which have the capacity to neu- tralize enzymes in fixed proportions. The name of anti- enzymes has been applied to such protective substances. The ability of the tissues to withstand digestive agents is thus made closely comparable with immunity to the toxins of diseases. CHAPTER XI THE LARGE INTESTINE The material passing into the colon is dilute and much reduced in volume as compared with the chyme passing out of the stomach. In the lower part of the small intestine the absorption of water more than counterbalances the secretion, hence the shrinkage of the contents. But since the end-products of digestion are being absorbed also, there is no tendency toward extreme concentration. In the large intestine there is but little secretion, and the con- tinuation of the absorption of water reduces the contents at last to a nearly solid consistency. The colon appears to be of very unequal value to animals of different classes. In the carnivora the work of diges- tion and absorption is so nearly finished by the small in- testine that very little remains to be done. The small quantity of matter having a potential food value may be accompanied into the large intestine by enzymes, which may there carry further their digestive action. Such food, however, is likely to be a negligible amount, and the digest- ive powers of the mixture at this point are unreliable. In the herbivora, the food being bulky and refractory, a considerable portion may arrive undigested in the colon. These animals have very capacious ceca, in which great masses of contents seem to be held for long intervals. The digestion occurring there may be partly effected by the native secretions, but it is believed to be largely the work of bacteria. The average human being resembles the carnivorous type rather than the other. Numerous cases have been observed in which no use was made of the colon, and it was never difficult to maintain nutrition. When the large in- 100 THE LARGE INTESTINE 101 testine is no longer traversed the discharges are watery and rather voluminous, but they contain only small percentages Fig. 14. — The colon with special reference to its movements: T is placed near the seat of frequent sustained or tonic contraction. From here backward to the cecum (C) antiperistalsis is of common occurrence, but this segment is swept also by occasional waves in the opposite direction ; V indicates the position of the ileocecal valve, which prevents reflux to the small intestine under the influence of antiperistalsis; beyond T the infrequent movements which take place have always a progressive character; 8 and