154 PRINCIPLES OF ANIMAL NUTRITION. . , 
of the modifications which later investigaticn has made necessary 
in Rubner’s original conclusions, is possible only in connection 
with a general study of the energy relations of the food, the animal, 
and the environment such as forms the subject of Part II. For 
the present we may content ourselves with accepting the general 
idea that the relative values of the nutrients depend in very large 
measure upon their ability to furnish energy for the vital activi- 
ties, deferring until later the consideration of quantitative rela- 
tions. 
Tur Non-nitroGeNous INGREDIENTS oF FErEDING-sTUFFS.— 
The discussions of the foregoing paragraphs have had reference to 
the effects produced by pure or approximately pure nutrients upon 
the metabolism of carnivora. By reason of the simplicity of con- 
ditions which is possible in such experiments they are indispensa- 
ble in a study of the fundamental laws of nutrition. We must 
presume also that the general principles, established by such 
experiments are applicable to all warm-blooded animals, since 
we know of no radical differences in their vital processes. 
In making such an application to the nutrition of our domestic 
herbivorous animals, however, much caution is necessary to avoid 
unwarranted assumptions and conclusions. Two points need espe- 
cially to be borne in mind: 
First, the food of these animals is, from a chemical point of view, 
very heterogeneous. In addition to true proteids, there are present, 
especially in coarse fodders, various non-proteid nitrogenous sub- 
stances, while the non-nitrogenous nutrients, besides hexose carbo- 
hydrates and true fats, include, on the one hand. pentosans and 
pentoses, lignin, and all the variety of unknown substances com- 
prised under the conventional terms “ nitrogen-free extract” and 
“crude fiber,” and on the other the waxes, resins, coloring matters, 
etc., contained in the ‘crude fat.” 
Second, the process of digestion in herbivora, and especially in 
the ruminants, as was pointed out in Chapter I. differs materially 
from that in carnivora as regards the part played by fermentative 
processes, particularly in the solution of the carbohydrates and 
related bodies which are so abundant in vegetable materials. 
It has been more or less customary to regard the digested por- 
tions of the crude fiber and nitrogen-free extract of feeding-stuffs 
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