174 NUTRITION, DIETETICS AND ANIMAL HEAT 



ents of that fluid, and are offered to the tissues through the me- 

 dium of the lymph. The complex proteid molecule is broken 

 down into simpler but more stable ones. These end products 

 are carbon dioxide, water and urea, together with some sul- 

 phates 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 oxidation 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 protoplasmic ma- 

 terial, but the destruction always takes place through the agency 

 of the cells, and the end products are always the same, whether 

 disassimilation of the proteid occurs with or without its becoming 

 an intrinsic part of the cell. 



Nitrogenous Equilibrium Circulating and Tissue Proteids. 

 The fact, however, that the characteristic function of the ni- 

 trogenous foods is to furnish protoplasmic material should not 

 be lost sight of. A certain amount is necessary to maintain "ni- 

 trogenous equilibrium"; that is, to keep the intake of nitrogen up 

 to the output. When nitrogenous food is withdrawn there con- 

 tinues 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 represent 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 com- 

 pensatory constructive process. On the other hand, the supply 

 of nitrogenous material may be, and usually is, in excess of the 

 demands of the cells for the actual regeneration of their substance. 

 This excess may be termed " circulating proteid" and is that just 

 referred to as being oxidized under the influence of the cells, but 

 without being transformed into protoplasm. That part of the 



