Muscle tissue and nerve tissue, the large secreting glands, such as the liver, stomach, and pancreas, all are the seat of oxidative and other changes which we class under the broad term of nutritional. To these cells, therefore, we must look for an explanation of the causes of oxidation, and the other transformations of a kindred nature that take place in the body. 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