In our brief survey of digestion, and of the methods there followed for the proper utilization of the organic foodstuffs, it was seen that the unorganized ferments or enzymes are the active agents in accomplishing the breaking down of proteids, and the less profound alteration of fats and carbohydrates. Is it not possible that the tissues of the body are likewise supplied with enzymes of various types, and that upon these powerful agents rests the responsibility for the different kinds of decomposition, oxidation and other changes, that take place in the body? Some years ago much interest was aroused by the observation that certain glands in the body, if simply warmed at body temperature with water, in the presence of some germicidal agent sufficient to prevent putrefactive changes, underwent what is now termed autodigestion, i.e., a process of self-digestion, with formation of various products, notably such as would naturally result from the breaking down of proteid material by ordinary proteolytic enzymes. This would seem to imply the presence in the glands of a proteid-splitting enzyme, the products formed being proteoses, peptones, amino-acids, etc., just such products as result from the action of trypsin. To-day, we know that practically all tissues and organs can, under suitable conditions, undergo autolysis, and in many instances the enzymes themselves can be separated from the tissues by appropriate treatment. Liver, muscle, lymph glands, spleen, kidneys, lungs, thymus, etc., all contain what are very appropriately called intracellular enzymes. These enzymes are of various kinds. Especially conspicuous are the hydrolytic, proteid-splitting enzymes, which behave in a manner quite similar to, if not identical with, that of the digestive enzymes of the gastro-intestinal tract, i. e., pepsin, trypsin, and erepsin. Further, there are other hydrolytic cleavages taking place in tissue cells, such as the cleavage of fats, due as we now know to intracellular enzymes of the lipase type, and by which neutral fats are split apart into glycerin and fatty acid. Again, there are in many organs intracellular enzymes which act upon the complex nucleoproteids of the tissue, causing them to break apart into proteid and nucleic acid, the latter being further broken down by other enzymes with liberation of the contained nuclein or purin bases. Many other chemical reactions are brought about by specific enzymes of various kinds, present in the cells of particular glandular organs. Thus, intracellular enzymes have been found, as in the liver, which are able to transform amino-acids into amides, and still others capable of splitting up amides.

Equally important, and even more suggestive, are the data which have been collected recently regarding oxidative processes in the tissues of the body. Specific ferments, known as oxidases, are found widely distributed in many organs and tissues, and it is difficult to escape the conclusion that as intracellular enzymes they have an important part to play in some, at least, of the transformations characteristic of tissue katabolism.1 As a single example, mention may be made of aldehydase, which accomplishes the oxidation of substances having the structure of aldehydes into corresponding acids. Ferments or enzymes of this class are found in the liver, spleen, salivary glands, lungs, brain, kidneys, etc., and they may well be considered as important agents in the chemical transformations going on in the tissues of the body. It would take us too far afield to enter into a detailed consideration of these intracellular enzymes; it must suffice to emphasize the general fact that in all the tissues aiid organs of the body there are present a large number of enzymes of different types, endowed with different lines of activity, and consequently capable of accomplishing a great variety of results in metabolism. Oxidation may still be a dominant feature in nutrition, oxidative changes may characterize more or less every tissue and organ in the body, but the processes are subtle and are not to be defined ia harmony with simple chemical or physical laws. The living cell with its intracellular enzymes, is the guiding and controlling power by which the processes of katabolism are regulated in harmony with the needs of the body. Complex organic matter is broken down step by step in the various tissues, with gradual liberation of the contained energy j processes of hydrolytic cleavage alternate with processes of oxidation, the molecules acted upon growing smaller with each downward step, until at last the final end-products are reached, viz., carbon dioxide, water, and urea, which the body eliminates through various channels as true physiological waste-products.

1 See M. Jacoby: Debar die Bedeutung der intrncellularen Fermenle fur die Physiologle und Pathologic Ergebniaie di-r Physiologie. Enter Jahrgang, 1. Abtbeilung, p. 230.

It will be advisable for us to consider briefly some of these intermediary products of tissue metabolism, since in any discussion of nutritive changes it is quite essential to have some understanding of the chemical relationship existing between the various products which result from the breaking down of proteid and other materials in tissue katabolism. This is especially true of proteid material, since in the gradual disintegration of this substance in tissue metabolism many intermediary bodies are formed, which undoubtedly exercise some physiological influence prior to their transformation into simpler bodies, with ultimate formation of the final product, urea. As has been pointed oat so many times, the proteid foods are peculiar in that they alone contain the necessary nitrogen, and in the peculiar form able to meet the physiological requirements of the body. Variations in the proteid intake are of necessity accompanied by variations in the formation of nitrogenous intermediary products, and both quality and quantity of these substances must be given due attention in any study of nutrition. Further, it is only by an understanding of the general or ground structure of pro-teids that we can hope to attain knowledge of the processes going on in the different tissues and organs in connection with metabolism, while a true appreciation of the chemical peculiarities of the individual proteids will help to explain the different nutritional value of vegetable as contrasted with animal proteids.