THE RELATIONS OF METABOLISM TO FOOD-SUPPLY. 97 
causes at first a loss of nitrogen from the body, which, however, 
unless the new supply of proteids falls below a certain minimum is 
as transitory as the gain in the other case. In other words, while 
the nitrogen excretion of the mature animal is in the long run 
equal to the supply in the food, when the amount of the latter 
is changed the full effect on the excretion is not realized at once. 
This fact is well illustrated by the following selection from 
C. Voit’s investigations upon the dog,* the results being expressed 
in terms of ‘ flesh”: 
‘* Flesh’? Metabolized per Day. 
Previous, New Fi 
Ration. | Ration. On On New Ration: 
Grms. | Grms. | Previous 
Meat. Meat. | Ration. |1st Day./2d Day./3d Day.|4th Day./5th Day./6th Day.|7th Day. 
Grms. Grms. | Grms. | Grms. | Grms. | Grms. | Grms. | Grms. 
1800 | 2500 1800 | 2153 | 2480 | 2532 ; 
500 | 1500 547 | 1222 | 1310 ; 1390 | 1410 | 1440 1450 | 1500 
0 | 1500 176 | 1267 | 1393 | 1404 
2500 | 2000 | 2500 | 2229 | 1970 
1500 | 1000} 1500 | 1153’; 1086 | 1088 ; 1080 | 1027 
1000 5000 1000 | 706; 610) 623 | 560 
An example of the same fact is found in the experiments cited 
on p. 81, in which a:. proteid food was withdrawn, the nitrogen 
excretion falling rapidly, but reaching its minimum only after three 
or four days. 
Voit explained the facts just adduced as the consequence of 
the difference between organized and circulatory proteids already 
noted on p. 82. According to this hypothesis, the amount of 
the proteid metabolism is chiefly determined by the store of circu- 
latory. proteids in the body. The ingestion of additional proteids 
increases the amount of these circulatory proteids in the body, and 
as a consequence the proteid metabolism increases until the nitrogen 
excretion overtakes the supply. Similarly, a decrease in the pro- 
teid food has the converse effect. 
Prorerp METABOLISM AND NirROGEN ExcrETION.—Up to this 
point, following common usage, the terms nitrogen excretion and 
proteid metabolism have been employed as practically synony- 
mous. In one sense this usage is correct, but it is liable to give 
* E. v. Wolff, Ernahrung Landw. Nutzthiere, p. 271. 
