272 PRINCIPLES OF ANIMAL NUTRITION. 
§ 1. Experiments on Carnivora. 
The comparative simplicity and completeness of the digestive 
processes of carnivora, together with the great variations which can 
be made in their diet, have made them favorite subjects for physio- 
logical experiments. It is possible to feed a dog or cat on what are 
close approximations to simple nutrients for a sufficient length of 
time to permit an accurate determination of the waste products, 
while with herbivora this is impracticable for obvious reasons. 
While earlier experimenters, among whom may be mentioned 
Frankland,* Traube,t and Zuntz,{ have concerned themselves with 
the question of the energy values of foods and nutrients, it is to the 
fundamental researches of Rubner that we owe not merely more 
accurate determinations of metabolizable energy, but in particular 
a clearer conception of its actual significance in nutrition. Rubner’s 
experiments § were made chiefly with dogs and were directed 
toward the determination of what he designates as the physiological 
heat value of the more important proteid foods, corresponding 
substantially to what is here called the metabolizable energy. 
Proterps.—As regards the non-nitrogenous ingredients of the 
food, Rubner assumes that, so far as they are digested, their metab- 
olizable energy is the same as their gross energy, or, in other words, 
that there are no waste products. For example, if a dog is given a 
certain amount of starch and none appears in the feces it is assumed 
that the starch has simply undergone hydration and solution in the 
digestive tract without material loss of energy and that conse- 
quently the full amount of energy contained in the starch is avail- 
able in the resorbed sugar for the metabolism of the body. In 
herbivora we know that there is a considerable production of gas- 
eous hydrocarbons by fermentation in the digestive tract. The 
respiration experiments of Pettenkofer & Voit on dogs, however 
(compare p. 72), showed but a small excretion of such gases, while 
Tappeiner || denies the presence of methane in any part of the dog’s 
alimentary canal. In the case of carnivora, then, the above 
* Phil. Mag. (4), 32, 182. 
ft Virchow’s Archiv., 29, 414. 
{ Landw. Jahrb., 8, 65. 
§ Zeit. f. Biol., 21, 250 and 337. 
|| Quoted by Rubner, <bid., 19, 318. 
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