As has been stated many times in the course of this presentation, the most striking feature of proteid metabolism is the rapidity with which large quantities of proteid consumed as food are broken down, and the contained nitrogen eliminated from the body as urea. A few hours will suffice to accomplish the more or less complete destruction of food proteid; and any theory of proteid metabolism, to be at all satisfactory, must explain this peculiar phenomenon. According to recent investigations, it seems probable that some, at least, of the cleavage products of proteid formed during intestinal digestion are not built up into new proteid, but are at once eliminated mainly in the form of urea, without becoming a part of either the so-called circulating proteid, or the living protoplasm of the body. It will be recalled that under the influence of the digestive enzymes, trypsin and erepsin, proteid foodstuffs may be broken down while undergoing intestinal digestion into monamino- and diamino-acids, such as leucin, tyrosin, arginin, lysin, etc. A certain proportion of these comparatively simple substances may be directly absorbed by the portal circulation and carried to the liver, where they may undergo conversion into urea. In this way, some portion of the nitrogen of the ingested food may be quickly eliminated from the system. As has been stated in another connection, we are not sure at present how far proteid decomposition of the kind indicated takes place normally in the body. We merely know that there are present in the intestine, enzymes capable of splitting up proteid into these small fragments, and that substances of this type when made to circulate through the liver are transformed into urea. These facts, coupled with the well-known tendency of the nitrogen of proteid food to appear in the excretions a few hours after the food in question has been consumed, naturally suggests a direct breaking down of proteid along the lines indicated, with a possible retention of a carbonaceous residue (nitrogen-free) for subsequent oxidation, as a source of energy for heat or work. Obviously, all of the proteid food cannot behave in this manner, for if such were the case there would be no proteid available for making good the normal waste incidental to tissue changes. Either a certain amount of proteid escapes this profound alteration produced by the proteolytic enzymes in question, or else a certain proportion of these simple decomposition products is synthesized in the intestine, or in the tissues of the body, to form new proteid for the regeneration of cell protoplasm. However this may be, we have presented in this view a plausible explanation of the prompt appearance of food nitrogen in the excretions, and without compelling belief in a theory, such as Pflüger’s, which taxes one’s credulity to the utmost. To be sure, as a prominent writer on physiology has recently said, such a view stands opposed to our conceptions of the importance of proteid food;