376 PRINCIPLES OF ANIMAL NUTRITION. 
as in the Introduction, the vital activities are intimately connected 
with the katabolic processes going on in the protoplasm of the 
cells. As was there stated, it is highly probable that the molecules 
of the protoplasm are much more complex than those of the pro- 
teids, fat and carbohydrates of the food (compare pp. 17 and 224), 
To what extent it is necessary that the resorbed nutrients shall be 
synthesized to these more complex compounds before they can 
serve the purposes of the organism we are hardly in position to 
say, but so far as it is required it can be accomplished only by an 
expenditure of energy derived ultimately from the food and con- 
stituting a part, and not impossibly a large part, of the work of 
assimilation. 
Summary.— The considerations of the foregoing paragraphs 
make it plain that the exercise of the function of nutrition, as is the 
case with the other functions of the body, involves the expenditure 
of energy. In general, we may say that this energy is expended for 
the two purposes indicated in the title of this section, viz., for diges- 
tion, or the transformation of the crude materials of the food and 
their transference to the fluids of the body, and for assimilation, or 
the conversion of these resorbed materials into the “potential” of 
the organism. Each of these two general purposes is served by a va- 
riety of processes, and the attempt to assign to each its exact share 
in the increased metabolism brought about by the ingestion of food 
is a physiological problem at once interesting and complicated. 
For our present purpose, however, viz., a consideration from the 
statistical point of view of the income and expenditure of energy 
by the organism, we are concerned primarily with the total ex- 
penditure caused by the ingestion of food rather than with the 
single factors composing it. As a matter of convenience it may be 
permissible to retain the designation above given, viz., the work of 
digestion and assimilation, but it should not be forgotten that other 
processes may conceivably be concerned in the matter. In par- 
ticular, any increased heat production resulting from a direct stimu- 
lation of the metabolic processes or of the incidental muscular 
activity of the animal by the resorbed food, such for example, as 
Zuntz & Hagemann * have observed with the horse as a result of 
abundant feeding, particularly with Indian corn, would be included 
under the term as here used. 
* Landw. Jahbrb., 27, Supp. ITI, 234 and 259. 
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