Page:Encyclopædia Britannica, Ninth Edition, v. 17.djvu/738

680 N U T R I T I O N plasmic matter of the fat-cell. We are not here guided, as in the case of muscle, by analysis of the blood approach ing the adipose tissue and the blood leaving it. Never theless, there is little doubt that fat may be derived from the proteid elements of food through the activity of living tissue-cells. That proteid matter readily lends itself to a decomposition of which fat is one product is seen in the formation of fat-like " adipocere " in the nitrogenous tissues of dead bodies which have been for some time buried or immersed in water ; and it is likewise seen in the process of " ripening " in cheese, in which the fat increases while the albuminous matter diminishes. But in addition to these general facts, suggestive though they are, there are not wanting proofs of a more particular kind that fat may result from proteid decompositions. If dogs are fed on starch and fat with no proteid food at all, the carbon stored up in the body can all be accounted for by reference to the fat of the food and the proteid matter of the body which, in the state of starvation, came to be disintegrated. In many other cases it seems clear that the fat stored up by animals during fattening unquestionably comes in part from the nitrogenous matters of food. But a considera tion of the relative amount of carbon and nitrogen con tained in proteids and in urea, which is the final product of the oxidation of proteid matter, would alone lead us strongly to suspect such an origin of fat. If all its nitrogen reappeared in the form of urea, 300 grains of proteid would give rise to 100 grains of urea. But 100 grains of urea contain but 20 grains of carbon, whereas 300 grains of proteid matter yield 159 grains of carbon, What has become of the deficit of carbon 1 If it has not been burnt off as carbon dioxide, it must remain within the body in ,some non-nitrogenous form. If fat may be derived from proteids in the body, it is now equally certain that in some animals at least fat may come from carbohydrates of the food. Lawes and Gilbert have conclusively shown that as much as 40 per cent, of the stored-up fat in well-fattened pigs could not possibly have come from the nitrogenous parts of the food, and that it must have been derived from the carbohydrates, All these facts render it very probable that the fat- forming tissues pick out from the blood albuminous or non- nitrogenous matter, transform it, retain the fatty element and discharge the rest. But it must not be for gotten that fat of food may, on finding itself in the blood, be taken up by appropriate tissue -cells and stored up without undergoing further change. Even this, although it seems so probable, may not at once be assumed to occur in such a simple manner. It is true that, if an animal be fed on fat resembling the fat of its own tissues, it might be assumed that the fat which it stores is simply absorbed by the tissue -corpuscles as an amoeba absorbs its food. But if this were really the manner of the process we should expect that allied fats substituted for the natural fat would be equally well taken up and stored, This, how ever, is not at all the case. Dogs fed on a mixture of palmitin and olein, but no stearin, are found on analysis to have stearin in their body- fat; just as dogs fed on palmitin and stearin, but no olein, are found to have an abundance of olein stored up. And, even when fed on spermaceti, a dog was found to present mere traces of it in the fat accumulated in its tissues during the experiment. In short, all facts go to show that the accumulation of fat does not take place without some display of formative or transformative energy. The last tissue whose metabolism we shall discuss is the hepatic tissue of the liver. Hitherto we have considered the liver as a digestive gland secreting the bile for the purposes of digestion ; but the liver has other functions which overshadow entirely its function as a digestive organ. It seems to have the power of accumulating amyloid material very much as adipose tissue accumulates fat, It would probably be a mistake to regard these two functions of the liver as altogether independent of one another, although it is necessary to state that a direct dependence has not yet been established, There can, however, be little doubt that the hepatic tissue draws to itself a variety of matters from the blood flowing into it by the portal vein and hepatic artery, and elaborates these constituents into various products, some of which escape into the blood again, others of which appear as the bile, while a third order remain in store within the substance of the liver. But, although these products are probably associated more or less intimately in their origin, they have, in the natural development of physiology, been studied independently of one another for the most part. If an animal he richly fed on starchy or saccharine food it is found, within a short time of the digestion of the food, that the liver contains a large quantity of a starchy hody called glycogen (C 6 H io 5 ) ?i . In order to obtain glycogen from liver-substance it is necessary to make use of the tissue of an animal just killed, for the starchy body undergoes an extremely rapid post-mortem con version into a sugar, Liver taken quickly from the still warm body of a rabbit is rapidly divided into small pieces and plunged at once into briskly boiling water. The high temperature prevents the post-mortem change and leaves the glycogen in a condition to be separated. The pieces of liver may then be pounded with sand and water, and the mixture acidulated and filtered. From the nitrate the glycogen may be precipitated by alcohol, and the preci pitate may be purified by boiling in caustic potash and afterwards neutralizing and reprecipitating with alcohol. Or the albuminous matters may be thrown down by a solution of mercuric iodide in iodide of potassium before precipitating the glycogen with alcohol. Glycogen so prepared is a white amorphous powder resembling starch in chemical composition ; it is colourless and tasteless. It* aqueous solution is milky, and generally opalescent. Other tissues and organs besides the liver have been found to contain glycogen. For example, muscles contain some glycogen, which diminishes during muscular activity, while it has been de tected in the tissues of embryos and of young animals, as well as in newly-formed pathological growths. There can be little doubt that glycogen is manufactured mainly out of carbohydrates conveyed to it in the portal vein. If a rabbit be starved for some days, the whole of the glycogen disappears from the liver ; but the store is at once, in the course of a lew hours, replenished if starch or sugar of whatever sort (cane, grape, or milk) be introduced into the alimentary canal, The metabolic powers of the liver-cells in respect of glycogen-production are not, however, limited to starches or sugars If glycerin be injected into the intestines the amount of glycogen in the liver is at once in creased. But some preparation of the glycerin is necessary before the liver-cells are able to appropriate it ; and this preparation is accomplished in the true digestive organs, since glycerin introduced into the blood by subcutaneous injection does not bring about any increase in the glycogen of the liver. Even albuminous bodies, such as fibrin and albumin, freed from all trace of amyloid matter, seem in the case of carnivorous animals to yield glycogen in the laboratory of the hepatic tissue. The same may be said of gelatin. Now it is easy to form a chemical conception of the manner in which such a body as grape-sugar leads to the storing of glycogen, for the dehydration of the sugar would be all the change required for the conversion. But we can at present form no picture of the operation in the case of glycerin and of the albuminous matters ; and the fact that these substances also are amenable to the liver- cells should make us hesitate to assume at once that in the case of sugar the process is the simple one of dehydration, We must wait for further light to be shed upon the interdependence of what we may for the moment call the biliary metabolism and the glycogenic metabolism of liver - cell before we speculate as to the exact nature of the changes. The liver has, beyond a doubt, the faculty of manufacturing an amyloid body out of various raw matters brought within the range of its activity, and of storing it up for future use. The use to which it is put has been the subject of much speculation and experimental inquiry. The amyloid matter when drawn upon may leave the liver in the form in which it exists there, or it may be converted into some other modification and afterwards trans ferred to the blood. The fact that other tissues besides the liver contain glycogen and the fact that some, as muscle, seem to make use of glycogen in the act of contraction, have been thought to support the hypothesis that the glycogen is conveyed by the blood to their tissues and then consumed. According to this theory the