THE RELATIONS OF METABOLISM TO FOOD-SUPPLY. 155 
as consisting essentially of carbohydrates. The basis for this 
assumption is the demonstration by Henneberg that the ultimate 
composition of that portion of these two groups of substances 
which is not recovered in the feces is substantially that of starch 
or cellulose, while Kellner * has more recently demonstrated. their 
equality in energy value. This fact of itself, however, does not 
justify the inference of equal nutritive value, as may be readily 
seen in the case of starch. It is obviously not a matter of indiffer- 
ence whether a given amount of this substance is resorbed from the 
digestive canal of a steer in the form of sugar or whether, as in some 
of Kihn’s experiments, 65 per cent. of it is converted into methane, 
carbon dioxide, and organic acids, yet the elementary composition 
of the “ digested ” portion would be the same in either case. 
The fact is that while the resorbed food of herbivora contains 
proteids, carbohydrates, and fats, whose functions in nutrition must 
be assumed to be the same as in the carnivora, it is very far from 
consisting entirely of them, but contains also a variety of other 
substances of whose exact nature and proportions we are compara- 
tively ignorant. We know, of course, that the digested non-nitrog- 
enous ingredients of feeding-stuffs, taken as a whole, do serve as 
sources of energy. When an ox or a sheep is fed exclusively on 
ordinary coarse fodders such as hay, straw, or corn stover, the small 
supply of proteids that he receives is likely to be little if any in ex- 
‘cess of the minimum demand, and the requirements of the body for 
energy must be satisfied very largely by the non-nitrogenous mate- 
rials. Moreover, the supply of such substances as starch, sugars, 
and true fats in such a case is so small relatively that it appears 
difficult to suppose that these alone are sufficient for the needs of 
the organism, and one is forced to the conclusion that the ill-knowh 
ingredients of the “crude fiber” and “nitrogen-free extract” are 
also utilized. 
The separation and identification of these various substances 
and the study of their physiological effects presents a problem at 
once attractive and laborious and one whose complete solution we 
cannot hope soon to reach. Some few data as to certain classes, 
however, are available. 
* Compare Part II, Chapter X. 
Digitized by Microsoft® 
