In this conception of proteid metabolism, we picture the different organs and tissues of the body as being permeated by a fluid which carries variable amounts of nutritive material, the quantity of the latter determining in a way the extent of the proteid katabolism which shall take place. As the proteid of the food passes into the blood and lymph, the fluids bathing the cells are correspondingly enriched, and as a result, proteid katabolism is accelerated in parallel degree. During hunger, on the other hand, the organized proteid of the tissue cells is gradually liquefied and passes out into the current of the circulating fluids. As before stated, the organized proteid as such is never decomposed; it must first enter into solution, and then under the influence of the living cells it undergoes disruption in the same manner as the circulating proteid. It is thus evident that the tissue cells and the circulating fluids permeating them bear an ever changing relationship to each other. Excess of circulating proteid will be attended by increased katabolism, while at the same time there may be some accumulation of proteid in the cells, and indeed some conversion into organized proteid. During fasting, hunger, or with an insufficient intake of proteid food, the current will naturally be in the opposite direction, and organized proteid will slowly, but surely, be drawn upon. Again, we may ask in view of these facts, of what real use to the body is this large katabolism of circulating proteid? We can easily understand the need of proteid to supply the loss incidental to the breaking down of organized or true tissue proteid, but this we are led to believe is very small in amount. Is there any real need for proteid beyond this requirement? The physiological fuel value of proteid is no greater than that of carbohydrate and considerably less than half that of fat, consequently there is on the surface no apparent reason why proteid should be used for its energy value in preference to the non-nitrogenous foodstuffs. Further, as we have seen, the energy of muscle work comes mainly, at least, from the breaking down of fat and carbohydrate; proteid, in the case of the well-nourished individual, ordinarily playing no part in this important line of energy exchange. Lastly, in the katabolism of proteid there is the large proportion of nitrogenous matter to be split off and disposed of before the carbon moiety of the molecule can be rendered available. Here, we have involved not only a loss of energy, but in addition a certain amount of what appears to be useless labor thrown upon the liver, kidneys, and other organs. Is there any wonder that the thoughtful physiologist, looking at the facts and theories presented by the Voit conception of proteid katabolism, should ask wherein lies the value to the body of this high rate of metabolism of circulating proteid, a rate of metabolism which is seemingly governed primarily by the amount of proteid food ingested?