94 PRINCIPLES OF ANIMAL NUTRITION. 
food and of the body separately from those of the non-nitrogenous 
nutrients. 
§ x. The Proteid Supply. 
The effects of the proteid supply upon metabolism may be most 
readily and clearly traced in experiments in which the food consists 
solely, or nearly so, of proteids, deferring to the next section a 
consideration of the modifications introduced by the presence of 
non-nitrogenous nutrients in the food. 
Effects on Proteid Metabolism. 
Our knowledge of the relations between proteid supply and 
proteid metabolism in the animal body is based upon the funda~ 
mental investigations of Bischoff & Voit,* Carl Voit,t and Petten- 
kofer & Voit,t at Munich. The results of these researches have 
been so fully confirmed by subsequent investigators and have 
become so much the common property of science that it is unneces- 
sary to do more than summarize them here, with the addition of 
such examples as may seem best adapted to illustrate them. 
Amount Requirep To Reacw Nirrocen Equitiprium.—As 
we have seen, the proteid metabolism of a fasting animal speedily 
reaches a minimum which we may probably regard as representing, 
at least approximately, the amount of proteids necessarily broken 
down and oxidized in the vital activities of the tissues of the body. 
If we supply proteid food to such an animal, we might naturally 
be inclined to expect that the first use to which the proteids of 
the food would be put would be to stop the loss of proteid tissue, 
and that if as much proteid was supplied in the food as was being 
metabolized in the body, nitrogen equilibrium would be reached. 
Experiment shows, however, that this is very far from being 
the case. Even the least amount of proteids causes a prompt 
increase in the urinary nitrogen, and each successive addition of 
proteids results in a further increase, so that it is not until the food 
proteids largely exceed the amount metabolized during fasting 
that nitrogen equilibrium is reached. Thus Bischoff & Voit,} 
* Gesetze der Erniéhrung des Fleischfressers, 1860. 
+ Published chiefly in the Annalen der Chemie und Pharmacie and the 
Zeitschrift ftir Biologie. See also Voit, ‘Physiologie des Stoffwechsels,” in 
Herman’s Handbuch der Physiologie. 
t Zeit. f. Biol., 8, 29 and 33. 
