622 GENERAL METABOLISM [CH. XLI. 



must be derived from the proteins of the body, for there is none 

 coming in, in the way of food. It might be thought if at this time 

 an amount of protein food containing the same quantity of nitrogen 

 as was being lost by the body, was administered, that the loss of 

 nitrogen from the body would be checked, and that the tissues would 

 seize the opportunity of repairing their waste. But this is not the 

 case ; what happens is that the amount of nitrogen lost in the day is 

 almost doubled ; and this is an undoubted proof that nearly all the 

 protein in the food is disintegrated and its nitrogen discharged 

 within the twenty-four hours. In order to get nitrogenous equili- 

 brium, it is necessary to give in the day two and a half times as 

 much protein as is lost during starvation in that period of time.* 

 This was one of the earliest proofs adduced that all the food protein 

 is not used in tissue repair, and it led Voit to formulate his cele- 

 brated theory of the distinction between " tissue protein " and what 

 he termed "circulating protein." The latter expression was coined 

 because Voit believed that the katabolism of this variety of protein 

 occurred in the blood, or at any rate in the tissue juices. In fact, 

 he considered that katabolism occurred only in the "circulating 

 protein," the small amount of "living protein" which dies being 

 dissolved and so added to the " circulating protein " before katabolism 

 occurs. Voit's great opponent was Pniiger, and for many years 

 Pflliger's theory replaced Voit's ; this theory states that all protein 

 must first become assimilated ; that is, must be built into and become 

 part and parcel of living protoplasm before it undergoes katabolism. 

 Pfliiger did good service in emphasising the importance of the cells 

 in metabolic processes, and we certainly do not now believe that 

 respiration or any other metabolic process has its seat in the circulat- 

 ing fluids. But at the same time Voit's theory possesses the correct 

 underlying idea which forms the basis of our present doctrine of 

 metabolism. In every living tissue there exists a framework of 

 what we may call more distinctly living substance, the metabolism 

 of which is constant and does not give rise to massive discharges of 

 energy; in the interstices of this are various kinds of material 

 related in different degrees to this framework ; these materials are 

 less eminently living, and the chief part of the energy set free comes 

 directly from the metabolism of some or other of this material. 

 Both the framework and the intercalated material undergo met- 

 abolism, and have in different degrees their anabolic and katabolic 



* It has recently been stated by Michaud that this amount may be greatly 

 reduced by feeding the animal on proteins which are as near as possible in com- 

 position to the tissue proteins of the animal, for instance, by feeding a dog on dog's 

 flesh. Knowing, however, as we do now that during digestion protein material 

 undergoes almost complete cleavage, it is difficult to accept such a view, and many 

 observers have not confirmed the experiments, or at least have shown that the 

 advantage is far less than Michaud found. 



