174 NUTRITION, DIETETICS AND ANIMAL HEAT 



broken down into simpler but more stable ones. These end 

 products are carbon dioxide, water and urea, together with 

 some sulphates and phosphates, the production of which is 

 comparatively immaterial. The urea is distinctive. Heat, 

 which is equivalent to so much energy, is evolved in the oxi- 

 dation process. 



It is probable that not all the proteid, under the ordinary 

 diet, is actually built up into cell substance. A part of it 

 seems to be destroyed without being transformed into pro- 

 toplasmic material, but the destruction always takes place 

 through the agency of the cells, and the end products are al- 

 ways the same whether disassimilation of the proteid occurs 

 with or without its becoming an intrinsic part of the cell. 



Nitrogenous Equilibrium Circulating and Tissue Pro- 

 teids. The fact, however, that the characteristic function of 

 the nitrogenous foods is to furnish protoplasmic material 

 should not be lost sight of. A certain amount is necessary 

 to maintain "nitrogenous equilibrium" ; that is, to keep the 

 intake of nitrogen up to the output. When nitrogenous food 

 is withdrawn there continues to be a discharge of urea, which 

 is the chief nitrogenous excretion and the amount of which 

 represents the amount of nitrogenous disassimilation in the 

 body. The urea eliminated under these conditions must rep- 

 resent the actual destruction of cell substance, and, since the 

 supply is zero and the output is considerable, there is not a 

 state of nitrogenous equilibrium; the animal is suffering 

 destruction of its protoplasm without a compensatory con- 

 structive process. On the other hand, the supply of nitro- 

 genous material may be, and usually is, in excess of the de- 

 mands of the cells for the actual regeneration of their sub- 

 stance. This excess may be termed "circulating proteid," 

 and is that just referred to as being oxidized under the in- 

 fluence of the cells, but without being transformed into pro- 

 toplasm. That part of the nitrogenous supply which is 

 built up into a part of the cell has been called "tissue pro- 

 teid" Whether any given molecule of proteid food pass 



