98 PRINCIPLES OF ANIMAL NUTRITION. 
rise to a misconception. It is perfectly true that the presence of 
one gram, e.g., of nitrogen in the urine, implies that about six grams 
of protein have yielded up their nitrogen in the form of urea or 
other metabolic products and therefore have ceased to exist as pro- 
tein. It by no means follows from this, however, that this protein 
has been completely oxidized to carbon dioxide and water. We. 
have already seen (Chapter II, p. 48) that the abstraction of the 
elements of urea from protein leaves a non-nitrogenous residue 
equal to nearly two-thirds of the protein, and that there is reason 
to believe that this residue may, according to circumstances, be 
oxidized to supply energy or give rise to a production of glycogen 
or of fat. In other words, the separation of its nitrogen from pro- 
tein and the complete oxidation of its carbon and hydrogen are 
two distinct things. When, therefore, we assert, on the basis of 
the evidence noted above, that the proteid metabolism of the mature 
animal is determined by the supply of proteids in the food, what. 
we really mean is that the cleavage of proteids and the excretion 
of their nitrogen is so determined. 
Rate oF NITROGEN Excretion.—A consideration of the course 
of the nitrogen excretion after a meal of proteids is calculated to 
throw light upon the relations of nitrogen cleavage to the total 
metabolism of the proteids. The early investigations of Becher, 
Voit, Panum, Forster, and Falck showed that when proteids are 
given to a fasting animal the rate of nitrogen excretion shows a 
rapid increase, reaching a maximum within a few hours. 
Feder * observed the maximum rate of nitrogen excretion by 
dogs in different experiments between the fifth and eighth hour 
after a meal of meat. From this point the rate of excretion de- 
creased less rapidly than it had increased and continued to decrease 
until about thirty-six hours after the meal. 
Graffenberger,t} experimenting upon himself, obtained similar 
results after the consumption of fibrin, gelatin, and asparagin, while 
the results with a commercial “meat peptone” were markedly 
different; and Rosemann,{ in studies upon the rate of nitrogen 
excretion by man, traces clearly a similar influence of the ingestion 
of nitrogenous food, while Krummacher’s § results on dogs fully 
* Zeit. f. Biol., 17, 531; Thier Chem. Ber.,12, 402. + Zeit. f. Biol., 28, 318. 
t Arch. ges. Physiol., 65, 343. § Zeit. f. Biol., 35, 481, 
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