but it seems possible, in the light of accumulating knowledge, that our conceptions of the part played by proteid foods in the nutrition of man have not been strictly logical, or quite in accord with true physiological reasoning. Again, in this connection, we may ask the question, why is it that the body provides such an effective method for the speedy breaking down of proteid food and the prompt elimination of the contained nitrogen? Whatever the means made use of by the organism in accomplishing this, the result is the same; the nitrogen of the ingested food is, in large measure, quickly gotten rid of. We clearly recognize the all-important position of proteid foods in the nutrition of the body, but there appears a certain inconsistency in this prompt removal of the nitrogen-containing portion of the proteid molecule. The nitrogenous part of the proteid food is, physiologically considered, the all-important part. It is the only source of nitrogen available to the system, and yet apparently the larger proportion of this nitrogenous material is not utilized in any recognizable way, but is eliminated as quickly as possible. Is it not within the limits of possibility that these methods, whatever may be the exact mechanism involved, are merely a means of getting rid of a surplus of proteid for which the body has no real need? This question I shall try to answer later on in another connection, but we may advantageously keep this possibility in mind while we are discussing these theories of proteid metabolism. It is obvious, in the light of present knowledge, that there must be a certain amount of true tissue proteid broken down each day, independent of that larger metabolism coincident with the intake of proteid food. However much this more voluminous proteid katabolism may fluctuate, owing to variations in the intake of proteid, and whatever the significance of this latter phase of metabolism, it is self-evident that there must be a steady, constant metabolism, upon which the life of the various tissues and organs of the body depends, and by which the proteid integrity of the tissue cells is maintained. This implies a certain degree of true tissue change, in which definite amounts of proteid material are broken down and the resultant loss made good from the proteid intake. No matter what specific name be applied to this form of proteid katabolism, its existence is clearly recognized. It is obviously a form of metabolism distinct, and probably quite different, from that form, more variable in extent, which is associated with the intake of proteid food. Plainly, if there is truth in these statements, there should be some data available by means of which these two lines of proteid katabolism can be more or less sharply differentiated.