152 PRINCIPLES OF ANIMAL NUTRITION. 
meat used to have contained 3.4 per cent. of nitrogen and 12.51 
per cent. of carbon, and the fat 76.5 per cent. of carbon, we have: 
Food per Day. Gain or Loss by Animal, 
Meat, Fat, Dextrose, Nitrogen, Carbon, 
Grms. Grms. Grms. Grms. Grms. 
300 Vn ree +1.81 +1.27 
Meat and fat......... 300 50 | 1, 40.10 49 31 
300 oe 63.7 +1.78 —7.44 
Meat and dextrose.... 300 ee 79.7 +2.28 —8.15 
300 on 115.5 +1.98 +6.21 
The averages of Pettenkofer & Voit’s results as tabulated on 
p. 150 may likewise be regarded in this light. 
ReELaTIvE VaLurEs.—The close similarity in the functions of the 
several non-nitrogenous nutrients is too obvious to have escaped 
early notice, and the investigations of the Munich school of physi- 
ologists served both to emphasize the similarity and to follow it 
into details. To Rubner, a pupil of Voit, is generally ascribed 
the credit of having first placed in a clear light the quantitative 
relations of the subject, although v. Hésslin* and Danilewsky t+ 
enunciated similar ideas at about the same time, which, however, 
were not based on their own experiments. 
As the result of his investigations upon the replacement values 
of the nutrients,t Rubner announced the law of “isodynamic re- 
placement.” This law is, in brief, that the several nutrients can 
replace each other in amounts inversely proportional to their physi- 
ological heat values, that is, to the amounts of heat which they 
would liberate if oxidized to the same final products which 
result from their metabolism in the body. In other words, aside 
from the minimum of proteids the nutrients are of value to the 
organism in proportion to the amount of energy which their meta- 
bolism liberates—they are “the fuel of the body.” One gram of 
fat, for example, when oxidized to carbon dioxide and water, liber- 
ates about 9.5 Cals. of energy, while one gram of starch similarly 
* Arch. path. Anat. u. Physiol., 89, 333. 
+ Die Kraftvorriite der Nahrungsst offe 
t Zeit. f. Biol., 19, 313. 
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