96 PRINCIPLES OF ANIMAL NUTRITION. 
The proteid supply gradually overtakes the proteid metabolism, 
but when only proteids are fed the supply must largely exceed the 
fasting metabolism in order to attain nitrogen equilibrium. E. 
Voit has endeavored to obtain a numerical expression for this 
relation by taking as the basis of comparison the fasting meta- 
bolism. He estimates (loc. cit., p. 101) that of the total nitro- 
gen excretion of a fasting animal 81.55 per cent. is derived from 
true proteids and 18.45 per cent. from the extractives of the 
muscles. Since the food in his experiments consisted substan- 
tially of true proteids, he compares its nitrogen with 81.55 per 
cent. of the nitrogen of the excreta and thus finds that the mini- 
mum supply of proteid nitrogen required to reach nitrogen equi- 
librium was between 3.67 and 4.18 times that metabolized during - 
fasting, the true value being estimated at 3.68. Five other less 
exact experiments gave confirmatory results and similar confirma- 
tion is found in the experimental results of C. Voit. 
Errect or Excrss oF Protrips.—If the supply of proteids to 
a mature animal be still further increased after nitrogen equilib- 
rium is reached, the excess of proteids is promptly metabolized, 
its nitrogen reappearing in the excreta. In other words, the ex- 
cretory nitrogen keeps pace with the supply of nitrogen in the food. 
The experiments by Bischoff & Voit just cited serve to illustrate 
this fact also. Approximate nitrogen equilibrium was reached on 
1200-1500 grams of meat, but in other trials even double this 
supply caused but a slight apparent gain of nitrogen, and it is 
probable that if the total urinary nitrogen had been determined 
instead of the urea, and account taken of the nitrogen of the 
feces, even this small difference would have disappeared. 
It is needless to multiply examples of. this perfectly well-estab- 
lished fact. The animal body puts itself very promptly into equi- 
librium with its nitrogen supply and no considerable or long-con- 
tinued gain of proteid tissue can be produced in the mature animal 
by even the most liberal supply of proteid food. 
TRANSITORY STORAGE OF PROTEIDS.—But while no continued 
gain of protein by the body can be brought about by additions to 
the proteid food, nevertheless, during the first few days following 
such an increase in the proteid supply a transitory storage of nitro- 
gen takes place. Conversely, too, a decrease in the proteid supply 
