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 out 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 proteids 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. Our understanding of the chemical structure of any organic substance is based primarily upon a study of the decomposition products which result from its breaking down, under the influence of various chemical agencies. Simple proteid substances when acted upon by pancreatic juice reinforced by the enzyme erepsin, or when boiled with dilute acids, undergo hydrolytic cleavage with ultimate formation of a large number of relatively simple bodies, mostly amino-acids, the chemical structure of which throws some light upon the nature of the proteid. Thus, in the pancreatic digestion of proteid in the intestine we may adopt the following scheme as showing in a general way the progressive transformation that occurs, understanding at the same time that like transformations may be accomplished by corresponding intracellular enzymes in the tissues and organs of the body; and further, that by the long-continued action of hydrolytic agents there is a complete breaking down into amino-acids and other simple products. Native Proteid /\ / \ / \ Protoproteose Heteroproteose : Primary proteoses | | Deuteroproteose Deuteroproteose : Secondary proteoses | | Peptone Peptone | | Amino-acids Amino-acids