Chapter VI: Digestion (3)

=The History of the Foods after Absorption.=—All foods, with the exception of inorganic salts and salts of various vegetable acids, fall into three classes: (1) Proteins—substances of complex chemical constitution, containing nitrogen; (2) carbohydrates—so called because hydrogen and oxygen, in the proportions in which they enter into the formation of water, are united with carbon; (3) fats. Proteins of various kinds are consumed as food. The peptones produced from them by digestion also vary. Yet very little is known as to the differences in physiological value which distinguish the various kinds of protein when absorbed into the fluids of the body (_cf._ p. 134). All carbohydrates after digestion and absorption appear as dextrose. The various fats preserve their individuality until they are taken up by the tissues. When fixed in the tissues, they assume, except under somewhat abnormal conditions, the composition characteristic of the fat of the animal which has eaten them. If a dog which has been severely starved is fed upon mutton-fat, it puts on in the first instance fat which resembles that of a sheep rather than the normal fat of a dog. As soon, however, as it is well nourished (which would never occur unless some protein and carbohydrate were added to the mutton-fat), its fat assumes the usual form.

For practical purposes we are obliged to speak of the three classes of food—proteid, carbohydrate, and fatty—as if there were but one member in each class. And we have abundant evidence that such a simple classification is fully justified. The body has so large a power of altering chemically the nature of the food which it absorbs that it makes little difference in the further history of the food whether the protein supplied to it be an albumin or a globulin; the fat, stearin, palmitin, or olein; the carbohydrate, starch or sugar.

In earlier days it was customary to regard the body as the receiver of a variety of foods which it could break down into simpler substances by oxidation, but could not reconstruct. Plants were regarded as the manufacturers of organic compounds, animals as the destroyers of the complex substances made by plants. The union of molecules, synthesis, was looked upon as the function of the vegetable kingdom. Animals built into their tissues the products elaborated by plants; some of these products they shook to pieces for the purpose of setting their energy free; others slowly disintegrated as the result of tissue “wear and tear.” Gradually it was realized that many chemical changes occur in the body which cannot be viewed as merely exhibitions of its analytical capacity. The tissues were recognized as laboratories in which reactions occur which consist in something more than the splitting of complex into simpler molecules. The instances earliest understood were connected with the history of carbohydrates and fats. In the disease diabetes an enormous quantity of sugar is excreted, amounting in extreme cases to between 1 and 2 pounds _per diem_. When carbohydrates are present in the food, the amount of sugar excreted in diabetes is greater than it is when they are withheld; on an almost exclusively proteid diet the amount of sugar excreted far exceeds the amount of carbohydrates in the food. Another illustration of the power of making sugar possessed by the animal economy is afforded by a dog fed upon lean meat, and nothing else. Sugar is found in its blood, and a store of carbohydrate (glycogen) in its liver. The formation of fat is an instance of constructive metabolism. There is abundant evidence that the quantity of fat produced may greatly exceed the quantity contained in the food. Animals are fattened for the market on a diet which contains less fat than that which accumulates in their bodies. When nursing her young, an animal may secrete in her milk much more fat than she obtains as such in food. It was a great mistake to suppose that the body is dependent upon its tradesmen for fat and sugar. It can make either of these substances out of a mixed diet in which it is relatively deficient. It must, however, be a mixed diet. An animal cannot live exclusively on fat or exclusively on carbohydrate. It is impossible, therefore, for us to determine whether, if given the one alone, it can turn it into the other. Chemists were very unwilling to credit the body with the power of performing even the simpler of these transformations—the conversion of carbohydrate into fat. Proteins are essential constituents of a fattening diet. Their immensely complex molecule has always afforded a tempting field for arithmetical ingenuity. It is easy to remove from it the atoms needed for the composition of fat, and yet to leave such groups of atoms as might reasonably be supposed to constitute its “nitrogenous moiety.” The hypothesis that the metabolic capacity of the body is limited to analytical processes justified the supposition that, when more fat is laid on than the food contains, the balance comes from proteid substances, which split into nitrogenous and fatty moieties. It has been shown, however, that an animal during fattening may put on more fat than is contained as such in the food, or obtainable from its diet, even though all the atoms of carbon and hydrogen in its proteid food were devoted to its formation. The balance must come from carbohydrates. Perhaps a still more striking illustration of constructive capacity is the power of making glycerin. If a dog receive fatty acids in its diet, it accumulates normal fats. The glycerin which, united with fatty acids, constitutes the fat, was not contained in its food. Starch and sugar are sources of fat. As yet there is no evidence that fat can be converted into sugar.

The chemistry of the nitrogen-containing compounds appears to present more difficult problems. Plants build up proteins. Is the animal’s relation to these substances limited to their disintegration? Do proteins inevitably descend from step to step until they reach urea? There are reasons for thinking that, even when dealing with nitrogenous substances, the metabolic power of the body is not exclusively analytical. The liver can make urea from ammonia-salts, such as lactate, or even carbonate, of ammonia—substances more stable, and therefore in the chemical sense simpler, than urea. This is an indication, though a faint one, that the body has a constructive capacity, a power of producing more complex from simpler substances, even in the case of nitrogenous compounds. Beef-tea, mutton broth, meat-extracts have long been regarded as foods of value when the power of assimilation is low. Chemists point out that the nitrogenous substances which these decoctions contain are so near the bottom of the ladder that the energy set free by their further oxidation to urea is scarcely worth consideration. They admit that their ready availability renders them useful as restoratives, but they deny them the status of foods, on the assumption that their further progress must be downward. As was stated when the conversion of peptones into leucin and tyrosin was described, evidence is beginning to accumulate which shows that within certain limits, at present impossible to define, the system can reconstruct its proteins from amides and other simple products of their degradation.

The animal economy receives, and after due digestive preparation absorbs, three classes of food—nitrogenous, fatty, and carbohydrate. If either of the two latter kinds be deficient in the diet, the body can to a certain extent produce it from the other two. What is the special value of each kind of food? What use is made of it? Before attempting to answer these questions, we must endeavour to trace the further history of the foods after they have traversed the wall of the alimentary canal.

After leaving the stomach and intestines, the foods follow two different routes. Proteins and carbohydrates are carried by the portal vein to the liver. Fats are carried by the thoracic duct to the general circulation. An excess of fat is found in the blood in all parts of the body after a meal rich in fat. The eventual destination and fate of fatty foods is unknown. Under certain circumstances they are added to the fatty deposits in connective tissue; but if no additional fat is being laid down, they go to other tissues, in which they are oxidized into carbonic acid and water. When the amount absorbed is excessive, a certain quantity of fat may be stored in the liver. In the cells of this organ it is housed for a time, in order that it may be distributed to the tissues after they have used up the supplies which first reach them through the general blood-stream.

Proteins are completely lost to sight after they are absorbed into the blood. They take part, of course, in the formation of growing tissue, blood-corpuscles, skin, hair, nails. It is also common to speak of them as making good the wear and tear of active tissues, although it is very doubtful whether we can legitimately speak of the wear and tear of tissues. The protoplasm which does the work of the body is not worn out in the same way as the materials of which a machine is made. There is no friction to rub it down. Proteins, like other foods, are used up as sources of muscular energy and heat. Eventually they are reduced to urea, carbonic acid, and water. Chemists naturally seek for substances intermediate in constitution between proteins and urea. They assume that the degradation of proteins will occur in regular steps; complex, partially oxidized, nitrogenous compounds being formed first—in the muscles, for example—to be further oxidized in the glands. The existence in all organs of nitrogenous “extractives,” which can be separated out when the organ is subjected to chemical analysis, seems to justify the search for stages; but hitherto this search has been singularly unsuccessful. Urea is the final product. It is not found in muscle, nor, indeed, in any tissue other than the liver, which, as already said, has the power of making it, even from salts of ammonia. It is therefore clear that if proteins are destroyed in muscle and other tissues, and if all urea is made by the liver, the antecedents of urea must be carried from the muscles to this organ. The substance which is most characteristic of muscular metabolism is lactic acid. It is not impossible that all the nitrogenous portion of the complex proteid molecule is reduced to ammonia (NH₃), which may be regarded as the simplest of all nitrogenous compounds, and that this, combined with lactic acid (C₃H₆O₃) as lactate of ammonia (NH₄C₃H₅O₃), is carried by the general circulation to the liver, where it is converted into urea. A considerable amount of lactate of ammonia may be injected into a vein without any of it overflowing through the kidneys. It is all reduced to the condition of urea, water, and carbonic acid. If the liver is so diseased as to be functionless, or if by operative measures it is thrown out of action, salts of ammonia are excreted by the kidneys instead of urea. In birds and reptiles uric acid takes the place of urea. Their livers yield uric acid on analysis. If lactate of ammonia be injected into their blood, it is converted into uric acid, so long as the liver is intact.

We know nothing of the forms assumed by the proteins absorbed into the blood, of the organs in which they are stored, or of the higher terms of the series of substances through which they pass before they are finally excreted as urea, water, and carbonic acid. No nitrogenous compounds are found in lymph or blood which can be pointed out with confidence as the products of tissue wear and tear. When considering the sources of muscular energy, we shall have something more to say regarding the part that proteins play in the economy.

If there is great difficulty in following fats and proteins after their absorption, it is quite otherwise when we come to deal with sugar. Carbohydrates are the great sources of energy. Muscular work may be generated by the oxidation of either of the three classes of foods, but undoubtedly the carbohydrate glycogen is its most constant source. Provision is therefore made for the storing of glycogen in the liver, and the distribution to the muscles of a regular supply. After a meal the portal blood, on its way from the intestines to the liver, contains a higher percentage of sugar than the blood in the hepatic vein or in any other vessel. If sections of liver be examined after feeding, and compared with those obtained after a period of starvation, it is found that the cells of the well fed liver contain glancing masses of a substance which takes a port-wine colour with iodine. This is glycogen, or animal starch. It has the same empirical formula as starch (C₆H₁₀O₅)ₙ. In the dry state it is a greyish powder, which, unlike starch, forms an opalescent solution in cold water. Like starch, it is non-diffusible. In the animal kingdom it stands to sugar in the same relation as starch to sugar in plants. If a sheep be killed while it is feeding in the paddock, and its liver removed and weighed, it will be found that it is from one-third to one-half heavier than the liver of a sheep of the same weight obtained from a butcher; for butchers have the stupid practice of starving animals before they kill them. It was long ago discovered that it is unnecessary to feed an animal for a day or two before it is killed, and this option has been elevated into a prohibition. A tradition has grown up that it is undesirable to give food for some time before killing. Not only will the liver of a sheep killed during active digestion be found to be heavier than that of a starved sheep, but it will also prove more succulent; for it is loaded with sugar (into which glycogen is rapidly converted after death), as well as with proteins and fats, which are withdrawn from it when the animal fasts. It appears that the liver cannot secure the whole of the sugar which is absorbed after a full meal. Some of it passes into the general circulation, and is stored in the muscles; but the liver always maintains a considerable reserve. Even after prolonged deprivation of food, it holds on to a certain quantity, especially in carnivora. Glycogen is found in the liver of a dog after a long period of starvation. The muscles lose during activity the glycogen which they contain when at rest.

It has already been pointed out that the body is not entirely dependent upon external agencies for the production of the sugar which it needs. When the supply is inadequate, it manufactures glycogen for itself out of the other constituents of the diet. It can, indeed, make it at the expense of its own proteins. If a dog which has been caused to do muscular work, without a sufficiency of carbohydrate food, until (as judged from a control experiment) all glycogen has disappeared from its liver, be placed under the influence of a narcotic drug, which arrests the activity of its muscles, glycogen reappears.

=Dietetics.=—Even those who are most ignorant of the science of physiology flatter themselves that they have one piece of information: “The whole of the body is renewed once in every seven years.” I cannot trace the origin of this sapient apothegm, which for generations has passed current. If seven weeks or seventy years were the period allowed for the renewal of the tissues, the statement would be equally near the truth. Judging from the rate at which they are destroyed, it is unlikely that blood-corpuscles live for more than five or six weeks. Hairs are shed about two years after they first appear above the surface. On attaining this age a hair drops off and a new one takes its place. The superficial cells of the skin are shed in great numbers every day, and their place taken by younger cells which come up from the deeper layers. The cells of many glands would seem to have a comparatively short term of life. On the other hand, some tissue-elements are far more permanent. By the time a child is a year old all its nerve-cells are in position. They last as long as the individual lives. When the statement with regard to the renewal of the tissues is understood as meaning, not that the cells are destroyed and replaced by new ones, but that within a period of seven years all the molecules which enter into their protoplasm are extruded from the body and replaced by molecules received as food, the assertion verges on the transcendental. It is unlikely that we shall ever obtain data against which it can be checked.

The essential part of every living cell is its sponge-work of protoplasm. “Bioplasm” is perhaps a better term to use when we are speaking of protoplasm as a structure, since it does not suggest any prejudice with regard to its chemical constitution. Within the meshes of the bioplasm are nutrient materials, as yet unused, and worked up products in various stages. It has always been taken for granted that when treating of nutrition, we have to consider the repair of the bioplasm, as well as the provision of raw material which it can convert into the specific products of the cell. Suppose that the cell belongs to the class of supporting tissues; let it be a cell of cartilage, for example. The bioplasm manufactures a collagenous substance which remains in and around its meshwork. If it be an epidermal cell, it forms horny substance. If a secreting cell, it accumulates secernable products. If a muscle-cell, it develops a large quantity of material, which by a change in form produces movement. In this last case we suppose that the energy set free as muscular force is due to oxidation. More stable bodies take the place of a less stable substance. After contraction the relatively complex contractile material is renewed from the foods stored in the muscle-cell; or if it be not, in the ordinary sense of the word, destroyed, if it has merely parted with certain oxidizable constituents, it obtains a fresh supply of such constituents from the foods which the muscle-cell contains. Even in the case of cartilage or epidermis, we imagine that, since the matrix is “alive,” it is always undergoing molecular change, and consequently always requiring food. The fact that every tissue, however inert, dies when, owing to the blocking of the bloodvessels which irrigate the part, its supply of nutriment is cut off, justifies this belief that all living tissue is undergoing change.

When we make up a balance-sheet of the body as a whole, placing to the debit side the food which it receives, and to its credit side the work done in external movement and in the production of heat, we again find reason for believing that every part of every cell is constantly undergoing change.

The balance-sheet of the body can be drawn out in either of two ways. We can estimate the quantities of nitrogen, carbon, hydrogen, and oxygen supplied to it in the several foods, and compare them with the amounts of each of these four elements given off in urea, carbonic acid, and water, making, of course, a note of the body’s balance in hand at the beginning and at the end of the period of observation. Or, we may estimate the amount of potential energy contained in the food, and ascertain the use to which this energy is put in doing external work, in maintaining the temperature of the body, and in warming the breath and other excreta.

If we are making up the balance-sheet of a fully-grown man, we may take for granted that he is not making fresh tissue. During the period throughout which he is under observation, care is taken to avoid altering the conditions of his life in such a manner as to lead him to develop additional muscle. If he gains in weight while under observation, he is putting on fat. If he loses in weight, he is sacrificing fat.

The whole of the nitrogen taken in leaves the body in urea, unless, as we have said, growth of tissue is taking place. The body has not the same temptation to store nitrogen as it has to store carbon. Consequently, it is very sensitive to any deficiency of nitrogen in the diet. If food does not contain as much protein as is needed, the deficit is made up at the expense of the tissues. It does not necessarily follow that under these circumstances a man loses in weight. He may be putting on fat, although losing in strength owing to waste of muscle. For observations upon the income and expenditure of the body to be of any value, a condition of “nitrogenous equilibrium” must be established. The nitrogen taken in must equal in amount the nitrogen given out.

Very exact determinations of income and expenditure may be made by placing an animal, or even a man, in a box through which air is drawn. A record is made of the volume of air drawn through the box. The percentages of water vapour and carbonic acid which the air contains are estimated before it enters and after it leaves. The solid food consumed and the urea excreted are also measured.

If it is desired to measure the amount of heat given off, an animal may be placed in a calorimeter.

Even when most passive, the subject under examination, whether an animal or a man, is expending energy in keeping the body warm, in movements of respiration, and in shifting position. If it is desired to ascertain the relation of oxidation to external work, it is easy to devise a form of resistance, such as the turning of a wheel, or the lifting of a weight which can be measured.

In testing diets, it suffices to make sure that nitrogenous equilibrium is maintained, and then to estimate the gain or loss in weight and the output of energy in external work.

=The Relative Value of Foods.=—Dried proteins contain about 15 per cent. nitrogen, 54 per cent. carbon, 7 per cent. hydrogen, 22 per cent. oxygen, a little sulphur, and frequently some phosphorus. A large proportion of their carbon and hydrogen is available for combustion. Fats contain 75 per cent. of carbon, and a considerable quantity of hydrogen available for combustion; carbohydrates, 40 per cent. of carbon, with hydrogen and oxygen in the proportions in which they occur in water. If 1 gramme of protein is oxidized to the condition of urea, carbonic acid, and water, sufficient heat is liberated to raise the temperature of 4,100 grammes of water 1 degree centigrade. Its calorific value is therefore expressed as 4,100 calories, the unit of measurement—a calorie—being the amount of heat needed to raise 1 gramme of water 1°. The calorific value of 1 gramme of fat is 9,300 calories; of 1 gramme of starch, 4,100 calories. Thus, the energy potential in protein and in starch is the same; that in fat more than twice as great as that in either of the other foods.

=A Normal Diet.=—Nitrogenous equilibrium and body-weight can be maintained and work done on diets which vary widely in percentage composition. This is a question which we shall consider at greater length later on. In the meantime, for the sake of illustration, it is necessary to formulate a diet which is fairly representative of the selection of foods made by a man of average weight—say 70 kilogrammes (145 pounds)—who desires to do a moderate day’s work in comfort. It has been found to amount to about 100 grammes of protein, 100 grammes of fat, 240 grammes of carbohydrate, all measured dry and as pure foods. If the several elements of such a diet be multiplied by the figures which represent their calorific value, it will be found that the man is supplied with 2,324,000 calories. The illustration that we have chosen is the diet of a professional man who is not engaged in hard physical work. The pure foods would be found to the amounts stated in 17 ounces lean meat, 4 ounces butter, and 17 ounces bread. The day’s diet would, of course, be much more varied than this, but it is simpler to express it in these terms.

Such a diet would hardly answer the requirements of a man doing hard muscular work. Experience shows that he would expect to receive a more liberal supply of energy, and that to obtain it he would increase slightly his allowance of proteins, and very considerably increase the quantity of carbohydrates that he consumed. The diet of European workmen is remarkably constant in the relative amounts of its several constituents, no matter what their nationality or the exact form of their work may be: Proteins, about 135 grammes; fats, 80 grammes; carbohydrates, 500 to 700 grammes—giving a supply of energy equal to 3,500 to 4,000 kilo-calories.

Speaking generally, carbohydrates are the source of muscular force, and fats of heat. In warm climates men work on carbohydrates. The ’rickshaw men of Japan are said to eat only rice on working days, and to reserve fish for days of leisure. The Japanese, as is well known, consume extremely little fat. The Esquimaux and other inhabitants of high latitudes eat immense quantities of fat. Proteins constitute the luxurious element of a diet. Not only are they more attractive to most palates, and therefore preferred by persons whose dietary is not severely regulated by price, but the body prefers them. It works with greater alacrity when supplied with more protein than, in a strictly physiological sense, it needs.

The supply of food must exceed the apparent demand. The most efficient of motors cannot convert more than 15 per cent. of the energy potential in its fuel into work. If a man endeavours to obtain a better result than this from his muscular system, if he tries to make his machine do more than 15 units of work for every 100 units of energy with which he supplies it, he does it at the expense of his own tissues. First he loses in weight, owing to the consumption of fat; then the excess of nitrogen discharged over nitrogen consumed shows that he is burning up the proteins of his own tissues. It is needless to add that the weakness which results puts a stop to excessive work. Muscles, as we shall find when we consider the relation of their output of work to the energy supplied to them, can produce a much better result than the best of engines; but we are speaking of the body as a whole, which wastes energy in the movements of respiration, masticating food, shifting position, maintaining the body temperature, etc.

Health may be maintained and work done on diets which depart widely from the one which we have selected as a standard. Darwin found the Gauchos of South America living exclusively on meat. Nansen and Johannsen, when seeking the North Pole, lived for months on meat and blubber. Millions of the inhabitants of India abstain from meat and meat-fat, their diet consisting of rice, buttermilk, and a little fruit. In the case of all persons with whom the price of food is an important consideration, carbohydrates are preferred to proteins and fats. Oatmeal is very much cheaper per unit of energy than meat. A man may be a meat-eater or a vegetarian, although he is probably unwise in overlooking the obvious teaching of his teeth and digestive organs, which are those of an omnivorous animal. His prehistoric human ancestors lived chiefly on the harvest of their spears and tomahawks. If we insist upon looking back still farther, we discern a cleavage of the race into the arboreal fruit-eaters, which still retain pre-human characters, and the more enterprising and energetic troglodyte hunters from whom the human race was evolved.

A man may vary his diet within wide limits. Innumerable considerations lead certain individuals to desire to depart from the diet which we have termed “normal”—_i.e._, typical of inhabitants of the temperate zone. One man rebels against the expense of living; he would fain reduce the quantity and the cost of food. Another, having to traverse regions in which food is scarce, wishes to ascertain the lightest, and therefore the most portable, combination of its essential elements. A third—and he belongs to a much larger class—tormented with indigestion or harassed by gout, asks, “Why must I consume things which give the stomach trouble, or produce disagreeable and incapacitating after-effects?” Many circumstances prompt to experiments in diet. Much latitude is undoubtedly allowed. But there are limits within which alone health can be maintained and work done. It is of great interest to ascertain exactly how wide these limits are; and especially important is it to find out the lower limit, the minimum of food, and the minimum of each particular kind of food, which will enable the human machine to work. The problems involved are somewhat complicated. If it were possible to live on a single food, it would be as easy to ascertain the irreducible minimum as it is to find out with how much coal or with how much petrol an engine can be made to turn a wheel. But to support the body several different kinds of food are indispensable. It is therefore necessary to determine, not only the minimum quantity of the combined foods, but also the minimum amount of each kind of food, and the effect upon the total of variations in the relative amount of each of its several factors. The problem is complicated, but certain limits are impassably defined. In the first place, with regard to the total amount, the work which the body does cannot under any circumstances be reduced below a certain level. The food consumed must provide a supply of energy equal, at the least, to the performance of the minimum of work. The body must receive each day food of due caloric value. Then with regard to the amount of each several constituent. Many considerations lead us to wish to increase one of them or to diminish another. Some food is cheap, and other food is dear. Economic reasons are in favour of the cheaper food. Even ethical considerations are not without weight. We have, perhaps, a prejudice against sacrificing life to supply the pot. We have doubts as to whether our system can properly digest, metabolize, and excrete meat. We need an unambiguous answer to the question, To what extent can nitrogen-foods be replaced by carbon-foods, and _vice versa_? A cell, as already said, consists of a framework of bioplasm bathed in cell-juice which contains nutrient substances and manufactured products. The bioplasm is alive; the proteins, carbohydrates, and fats of the cell-juice are the materials with which it is nourished, and upon which it works. Some physiologists incline to the view that non-living substances must enter into the bioplasm before they undergo metabolism. They consider that the molecules of the non-living substance must at the time when they undergo a chemical change be physically and chemically a part of the living substance. Others take the opposite view: that the living substance does not undergo change, but brings about changes in the non-living substance which is in contact with it, enclosed within its meshes. This is a problem which is not likely to be solved, nor is its solution of great importance in relation to the question which we are discussing. Whichever of the two views be justified, we have to distinguish between the bioplasm of the cell—the machine—and its raw materials and manufactured products. The question to which we want an answer is the following: Must the bioplasm undergo change? There seems to be no reason in the nature of things why it should. It is not, as we have already pointed out, subject to wear and tear. A perfect machine would in the absence of friction, which rubs down its steel and brass, continue to turn out its products so long as it was supplied with raw materials and the energy needed to manufacture them. We could imagine the bioplasm as indestructible, receiving energy from a portion of the foods, and expending this energy in the production of chemical change in the remainder. We could imagine that when once the tissues had attained their full growth they would require no more protein for their own nutrition; they would be occupied in producing heat and motion from the non-nitrogenous foods. But observation shows clearly that this is not the case. The force which energizes the bioplasm, enabling it to evoke metabolism in non-living substance, is obtained at the cost of its own destruction. The bioplasm wastes unless constantly supplied with proteid food.

Under ordinary circumstances the amount of urea excreted varies directly as the quantity of nitrogen contained in the food. Since urea contains 45 per cent. of nitrogen, and protein 15 per cent., every gramme of urea excreted represents 3 grammes of dry protein consumed; or, in terms of nitrogen, every gramme of nitrogen excreted represents 6·25 grammes of protein consumed. If all food is withheld, the excretion of nitrogen falls, but it never reaches zero. Many observations have been made on fasting men. On the second day of fasting the nitrogen excreted falls to about 13 grammes, representing 80 grammes of protein used up. It is generally thought that by the second day all “floating proteins” are exhausted, and that therefore nitrogenous metabolism is reduced, as it were, to a business basis. So long as the supply of food is abundant, the body has a luxurious habit of using proteins in preference to non-nitrogenous food. But after a day’s starvation there is no longer any fancy metabolism, no consumption of proteins as fuel when cheaper fats and sugar would answer equally well. In the case of Succi, who fasted for thirty days, the nitrogen excreted fell to 6·7 grammes on the tenth day, to 4·3 grammes on the twentieth, and to 3·2 grammes on the last day. Clearly, we have to make a distinction, when all food is cut off, between the oxidation of the protein which, failing all other material, is withdrawn from the tissues for the purpose of supplying the force absolutely necessary to maintain respiration and such other movements as are inevitable, and to keep up the temperature of the body—force which under other circumstances might be supplied by non-nitrogenous food—and the oxidation to which bioplasm is inevitably subject, so long as it is alive. The oxidation of bioplasm under ordinary circumstances of course supplies force; but it does not follow that this is sufficient to maintain the respiratory movements and the contraction of the heart. When a herbivorous animal is starved, it not infrequently excretes more urea at the commencement of the starvation period than it was excreting when well fed. Its activities did not come to a standstill when carbohydrate food was cut off. For a time they were maintained at the expense of its own tissues. On the other hand, the results obtained from the observation of the man who went without food for thirty days show that Nature is able to economize force by reducing the metabolism of living substance below the normal. It might be supposed that the irreducible metabolism could be ascertained by giving a nitrogen-starved animal non-nitrogenous food, but it is found that this scarcely affects the tissue-waste. Becoming more active, the tissues, while saved from the necessity of supplying fuel for the production of heat and motion, suffer more waste. Again, it might be expected that if to an animal which had been starved for a few days, until its urea had fallen to the starvation limit, exactly sufficient protein were given to supply this amount, the tissues would be saved. It is found, on the contrary, that nearly twice as much urea is excreted as before. If the quantity of protein be steadily increased, equilibrium is at last established, but not until the amount of nitrogen in the protein given is two and a half times as great as the amount excreted during the starvation period. Additional food at once gives rise to additional waste. The tissues which during the period of scarcity had reduced their oxidation to a minimum become more active at the first hint of returning plenty.

This last experiment illustrates a general law. An increase of proteid food within certain limits increases the metabolic activity of the tissues—provokes them to extravagance. It is possible, by adding protein to a mixed diet which sufficed for the maintenance of body-weight and nitrogenous equilibrium, to bring about a nitrogen deficit and to reduce the body-weight. Or, if the body is gaining in weight, owing to the accumulation of fat, the substitution of protein for carbohydrate (weight for weight, since their caloric values are the same) will lead to its reduction. It is difficult to avoid the use of fanciful language in accounting for these results. The animal economy is like an over-careful housekeeper, who, when meat is scarce, doles out porridge also with a thrifty hand. When meat is plentiful she is prodigal with every article of diet. Protein is the most costly of foods. Any indication that it is scarce leads to a shutting-down of activity. On the other hand, no other food is so readily absorbed (unless the digestive organs be protein-sick); none is so quickly incorporated in the bioplasm; none is so easy to decompose. When fed with protein the machinery hums. The insatiable appetite for beef and eggs which overtakes a man of sedentary habits after a long morning in a boat or on a bicycle does not indicate that his muscular tissue is suffering from wear and tear. It does not prove that he is setting free energy by oxidizing proteid food. It shows that he is asking certain tissues which are accustomed to a quiet life to exhibit prodigious energy. They will not shake off their customary sloth unless he stimulates them with sumptuous fare. At the end of a week he finds that proteins are not the best fuel for steady work. If he consumes sufficient to supply all the energy needed by his muscles, he is hampered by a quantity of nitrogenous residues which have to be reduced to urea and eliminated by the kidneys. He goes back approximately to his old regimen, so far as proteins are concerned, and consumes more carbohydrates for the supply of the force which his increased muscular activity demands.

It is possible to live on meat alone, but the quantity required is very great, involving the digestive organs, the liver, and the kidneys in an excessive amount of work. On the other hand, it is possible to reduce the consumption of proteins to a minimum by substituting for them fats and carbohydrates. But, again, after the proper balance is disturbed, the substitution ceases to be a simple problem in arithmetic. The carbon-food has to be increased out of all proportion to the protein which it replaces. If a dog which is being fed on a diet natural to it—chiefly meat—is in a condition of nitrogenous equilibrium, carbohydrate may be substituted for some of the meat. But from the very beginning it is found that, if nitrogenous equilibrium is not to be disturbed (if the dog is not to be induced to consume its own tissues), a weight of carbohydrate must be given considerably greater than the weight of the protein withdrawn. The disproportion increases as the experiment proceeds, until perhaps 12 to 15 grammes of carbohydrate have to be substituted for every gramme of protein. The proteid food has now come down to 1·5 gramme per kilogramme of the animal’s weight. Owing to the increase of carbohydrate, the caloric value of the total food, nitrogenous and non-nitrogenous, is several times as great as the animal requires. The surplus is oxidized without any equivalent in work. At about this point the experiment is brought to an end, owing to the failure of the digestive organs to deal with so large a mass of food.

The value of gelatin as an article of diet is of interest in this connection. Gelatin is not, strictly speaking, a protein, and it cannot be built up into the tissues. It does not prevent, nor even delay, starvation. Yet up to a certain point it can be used as a substitute for proteid food. In the observation just referred to, protein might be withdrawn at any stage, without disturbing nitrogenous equilibrium, by substituting about 2 grammes of gelatin for every gramme of protein withdrawn. It spares protein, although it does not take its place. It is said that the minimum of protein necessary for the maintenance of nitrogenous equilibrium may be reduced to about one-half by the substitution of gelatin. This has been interpreted as indicating that when we have reduced the oxidation of nitrogenous substance to its smallest amount the nitrogen comes from two sources in about equal proportions—(_a_) the bioplasm; (_b_) the food-proteins in contact with it. It is inferred that gelatin, although it cannot be built up into bioplasm, may take the place of proteins present in the cell-juice. It appears to be impossible to starve the cell until it consists of a bioplasm framework bathed in nitrogen-free cell-juice. As the non-living proteins of cell-juice are removed, they are, if no nitrogenous food be given, renewed by the breaking down of bioplasm. When gelatin is absorbed, it takes its place in the cell-juice, and the breaking down of bioplasm is no longer necessary. When digestion is impaired, or vitality lowered, decoctions of meat which contain extractives of low calorific value, useless, without synthesis (_cf._ p. 144), for the purposes of tissue-repair, may to a certain extent save tissue-waste. In the same way, gelatin, which is very rapidly digested in the stomach, may cover the consumption of proteins, although it cannot take their place.

To sum up: The requisite daily income of energy must come from both nitrogenous and non-nitrogenous food. It is impossible to reduce the nitrogenous factor below a certain minimum. From this minimum upwards, until a certain level is reached, every additional unit of nitrogenous food enables the system to dispense with more than its equivalent of non-nitrogenous food. When the proper balance of foods is attained, there is no waste either of labour involved in digestion, or of labour involved in metabolism and excretion.

=The Liver.=—The liver weighs from 3 to 3½ pounds. It lies beneath the diaphragm, more on the right side than on the left. Its posterior border, which rests against the last three ribs (separated from them by the diaphragm), is about 3 inches thick. Its anterior border is thin, and keeps close along the line of the ribs. If the organ is neither unduly enlarged nor squeezed out of its place owing to the use of a tight corset, it does not project below the ribs, save where it crosses the space between the rib-cartilages below the end of the breast-bone.

In its centre is a space, the intralobular vein,
through which the blood falls into a branch of
the hepatic vein, on its way to the heart. An
interlobular branch of the portal vein, which
brings the blood from the digestive organs, pours
it by many smaller vessels over the surface of
the lobule. It filters into the lobule through
innumerable pseudo-capillary vessels, or spaces,
between the radiating columns of liver-cells.
Arterial blood is brought to the lobule by a
twig of the hepatic artery. Bile is drained away
from it by an affluent of the hepatic duct. In
the lower part of the diagram seven liver-cells
are shown, forming a divided column, magnified
about 300 diameters. The cells are loaded with
glycogen, and contain minute globules of fat.
Red blood-corpuscles and two leucocytes are seen
between the columns of liver-cells. One of the
leucocytes has ingested two blood-corpuscles.]

The liver is supplied with blood by the hepatic artery. This vessel is small for so large an organ. Although responsible for the nutrition of the liver, it does not bring it the materials which are stored in its cells. A much larger supply of blood is derived from the portal vein, which breaks into capillaries, or, to speak more accurately, into sinuses, or pseudo-capillaries, in the liver. The blood, whether conveyed to the organ by the hepatic artery or by the portal vein, is drained away by the hepatic veins. The plan of structure of the liver is best understood when viewed with reference to the hepatic veins. These, if traced backwards, are found to break up into fairly straight vessels, each of which has a large number of lateral branches. Each of the lateral branches is in the centre of a mass of cells, which are packed round it in radiating columns. These masses, which have a diameter of from 1 to 2 millimetres, are termed “lobules.” By mutual pressure the lobules are squeezed into a pentagonal or hexagonal form. The vein in the centre of the lobule is the intralobular vein. Turn now to the portal vein; this is seen to break up into branches which run between the lobules, and are therefore termed “interlobular veins.” The branches of the hepatic artery also run between the lobules, as do the radicles of the bile-duct. Each lobule is a liver in miniature. The blood of the portal vein, which has come from the spleen, in which red blood-corpuscles are destroyed, and from the stomach and intestines, from which it has absorbed the products of digestion, is poured over the surface of the lobule, to be filtered through into its central intralobular vein. In its passage from the interlobular veins (and branches of the hepatic artery) to the intralobular vein the blood is confined to radiating capillary channels; but since these merely prevent the escape of red blood-corpuscles without imposing any restrictions upon the exudation of blood-plasma, the portal blood is to all intents and purposes filtered through the columns of liver-cells. The body-substance of the liver-cells is soft, destitute of envelope, and capable, when free on the (warmed) stage of a microscope, of changing in form, somewhat after the manner of a leucocyte. Such cells have a great capacity for taking up the products of digestion. Possibly they take up and store fats and proteins, but undoubtedly it is their chief business to absorb sugar which accumulates as glycogen in their substance. The glycogen is handed out to the hepatic blood as required. The pigment which results from the disintegration of red blood-corpuscles in the spleen is secreted, along with the bile-salts, into minute channels, or canaliculi, which groove the flat surfaces of adjacent liver-cells. These canaliculi converge to the bile-ducts. The liver is therefore at the same time the storehouse of sugar which it takes up from the blood when it is in excess, and passes out to the blood when it is deficient, and an excretory organ which eliminates the refuse of hæmoglobin. The iron derived from hæmoglobin it stores, and returns to the blood.

Another function of the liver has been referred to already. It is the organ, and, as far as we know, the only organ, in which urea is made in mammals, and uric acid in birds. If the liver of a freshly killed animal be excised and a stream of blood passed through it, the blood which leaves the organ contains urea. If a salt of ammonia, even the carbonate, be added to the blood, it is converted by the liver into urea. When a bird’s liver is made the subject of the same experiment, uric acid appears instead of urea. The liver can convert many nitrogenous substances into urea, but it seems probable that, normally, the salt with which it has chiefly to deal is lactate of ammonia (_cf._ p. 146).

A few words must be added with regard to the functions of the liver during prenatal life, obscure though these functions are. The liver develops very early, and attains a relatively enormous size. At the third month it weighs as much as the whole of the rest of the body (_cf._ p. 34). Yet it cannot, one must suppose, have to do much of the work which falls to its share in postnatal life. Food is reaching the embryo in a constant stream, and not as the result of intermittent meals. The embryo has no need to store glycogen; nor does its liver, on analysis, yield much of this substance. In the embryo glycogen is widely distributed throughout the tissues, not specially accumulated in the liver. No digestion is occurring in the alimentary canal. Bile is not needed to aid the hydrolysis and absorption of fats. A small quantity of cholesterin and less lecithin is being eliminated, but not much bile is needed to facilitate this process.

A process which is proceeding at a great rate in the embryo, in various situations, is the formation of red blood-corpuscles. In this the liver takes part. But its duty in regard to blood-formation is not sufficiently onerous to account for its size. The formation of blood-corpuscles in the liver is observed with difficulty in microscopic sections. It is therefore impossible to speak with certainty as to the extent to which it is going on, but it may be safely asserted that this function by itself cannot be held to account for the great size of the organ in embryonic life. What other office it fills at this period is a question which still awaits an answer.

There is no more curious chapter in medical history than the story of the views held at various periods with regard to the functions of the liver. From being a mere mass of “parenchyma” serving as packing for the abdominal viscera, it was elevated to the rank of Grand Purifier of the “humours” of the body. Next, its excessive activity became the cause of that form of dyspepsia known as “biliousness.” Still later its want of activity was its chief vice. A “sluggish” liver was held responsible for mental perversity and moral dulness. Calomel, podophyllin, and other drugs were used as whips to stir it up; and the increased secretions of the alimentary canal were mistaken for bile. Poor patient organ! It is the still-room of the body, in which the day’s supplies are stored, and from which they are served out, without haste and without delay. And it makes urea. What else it does we have yet to find out; and it is not impossible that when physiologists have quite shaken themselves free from the explanations based upon conjecture, which their predecessors have handed down, they may discover that it has other duties which are not obvious, but of great importance.

FOOTNOTE: