82 PRINCIPLES OF ANIMAL NUTRITION. 
comparatively short time it sank to a minimum which was practi- 
cally the same in all the experiments upon this particular animal, 
viz., the equivalent of about 12 grams of urea per day. This mini- 
mum we may fairly regard as representing the necessary and inev- 
itable destruction of proteids involved in the vital processes of 
the organism, and therefore may consider as taking place also 
when the animal was fed. If, now, we subtract from the total 
urea excreted the 12 grams corresponding to the minimum de- 
mand of the body, there is revealed the second and variable factor 
of the proteid metabolism, which is large in the well-fed animal but 
rapidly disappears during fasting, as the following table shows: 
Previous Food per Day. 
1800 Grms. 
wagon ars Sm" GS) on. 
Fat. 
Urea per day: ; Grms. Grms. Grms. Grms. Grms. 
Last day of feeding...... 168.8 | 118.0 | 98.8 98.8 | 12.7 
Ist “ ‘* fasting...... 48.1 25.5 17.7 14.5 7.6 
7: ess Oe TFs hac 12.9 11.3 6.2 6.6 3.6 
sd an es 7.1 4.7 5.5 3.7 2.9 
4th “ “ Me ag eof 5.3 2.8 2.9 2.9 1.2 
5th “ Hes Satta as 0.3 0.6 22 2.8 0.7 
6th “ “ Sak ees 1.3 0.8 1.0 0.8 1.0 
ae is ‘- Sacek ies 0.5 0.0 0.1 0.9 
pice ee ee end 
Math RES AE oe et el og pera eanseealeankoceal 0G 
ORGANIZED AND CIRCULATORY PRoTEIN.—It is evident from 
the above results, and will appear still more clearly when we come 
to consider the influence of the food-supply upon proteid meta- 
bolism, that in addition to the great mass of proteid tissue in the 
body, whose metabolism results in the excretion of a relatively 
small and constant amount of nitrogenous products, the well- 
nourished organism may also contain variable amounts of nitrogen- 
ous matter which is subject to rapid metabolism and which speed- 
ily disappears during fasting. Voit employed the term circula- 
tory protein (Zirkulationseiweiss) to designate this variable store 
of rapidly metabolized nitrogenous matter, which he regards as. 
being substantially the dissolved protein which penetrates from 
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