374 PRINCIPLES OF ANIMAL NUTRITION. 
On the whole, however, and in view of the patent fact that the 
activity of the digestive apparatus consequent upon the consump- 
tion of food must lead to an expenditure of energy, the results of 
Zuntz & v. Mehring appear to have been generally accepted as proof 
that it is this influence rather than any direct effect of the resorbed 
food upon the metabolism to which the increase of the latter after 
a meal is to be ascribed. This increased expenditure is often, 
although rather loosely, spoken of as the “ work of digestion.” 
Factors oF Work oF Dicrstion.—In the process of digestion 
we are probably safe in assuming that the muscular work of pre- 
hension, mastication, deglutition, rumination, peristalsis, etc., con- 
stitutes an important source of heat production. A secondary 
source of heat production, which we may designate as glandular 
metabolism, is the activity of the various secretory glands which 
provide the digestive juices, to which may be added also the work 
of the resorptive mechanisms. Furthermore, the various processes 
of solution, hydration, cleavage, etc., which the nutrients undergo 
during digestion contribute their share to the general thermic effect. 
Fermentations.—To the above general sources of heat produc- 
tion during the digestive process, there is to be added as a very 
important one in the case of ruminating animals the extensive fer- 
mentation which the carbohydrates of the food undergo. We have 
already seen that a considerable fraction of the gross energy of these 
bodies is carried off in the potential form in the combustible gases 
produced. A further portion is liberated as heat of fermentation. 
This latter portion forms a part of the metabolizable energy of the 
food as defined in the preceding chapter, since it assumes the kinetic 
form in the body. Since, however, it appears immediately as heat, 
it can be of use to the body only indirectly, as an aid in maintaining 
its temperature. While, therefore, it does not constitute work in 
the strict sense of the term, the heat produced by fermentation 
constitutes a part of the expenditure of metabolizable energy in 
digestion, and therefore is included under the term “ work of diges- 
tion” in the general sense in which the term is frequently used. 
Warming Ingesta.—The food, and particularly the water, con- 
sumed by an animal have to be warmed to the temperature of the 
body. To the extent that this warming of the ingesta is accom- 
plished at the expense of the heat generated by the muscular, gland- 
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