338 PRINCIPLES OF ANIMAL NUTRITION. 
dioxide and the absorption of oxygen and in the amount of heat 
produced. In general terms this is brought about in four ways: 
First, the muscular work required for masticating and swallow- 
ing the food and moving it through the digestive apparatus involves 
an expenditure of energy which finally gives rise to the evolution 
of heat. 
Second, the activity of the various secreting glands of the diges- 
tive tract is stimulated, again making a demand for energy and 
giving rise to an increased heat production. 
Third, the work of the resorbing cells likewise makes demand 
for energy. 
Fourth, the various fermentations, cleavages, hydrations, and 
syntheses which the food ingredients undergo in the course of diges- 
tion, resorption, and assimilation may occasion in individual cases 
either an evolution or an absorption of energy, but taken as a whole 
result in the production of a greater or less amount of heat and con- 
sume a corresponding amount of the metabolizable energy of the food. 
Propuction or Hrat.—The body temperature of the healthy 
warm-blooded animal is practically constant, any considerable 
variation from the average indicating some serious disturbance of 
the animal economy. Since this temperature is ordinarily higher 
than that of the environment, a continual production of heat is 
necessary to maintain it. 
As stated above, the various forms of internal work, including 
the work of digestion and assimilation, give rise in the aggregate 
to the evolution of a large amount of heat, and this heat is of course 
available for the maintenance of the body temperature. 
Whether its amount is sufficient for this purpose, or whether 
under any or all circumstances there is a production of heat for its 
own sake, simply to keep the animal warm, is still a debatable 
question. Many eminent physiologists, notably Chauveau and 
his associates, hold that the primary function of metabolism is to. 
furnish energy for the physiological processes going on in the body. 
They hold that the potential energy of the food is converted imme- 
diately into some form of physiological energy, which in its turn, 
in fulfilling its functions, is converted into heat which serves inci- 
dentally to maintain the body temperature. In other words, they 
regard heat as substantially an excretion and would consider that 
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