CHAPTER XI. 
INTERNAL WORK. 
§ 1. The Expenditure of Energy by the Body. 
Havine considered the food in the light of a supply of energy 
to the animal, it now becomes desirable to take a more general 
view of the subject and inquire into the uses to which the energy 
of the food is applied in the organism. 
We have already. distinguished between that portion of the 
potential energy of the food which is convertible into kinetic energy 
in the body, and which we have here called metabolizable energy, 
and that portion of it which is rejected for one reason or another 
in the potential form in the various excreta. This latter portion 
we may dismiss from consideration for the present. The former 
portion—the metabolizable energy—as common experience informs 
us, is applied to two main purposes. First, it supplies the energy 
for carrying on the various activities of the body. Second, if the 
supply is in excess of the requirements of the body a portion of it 
may temporarily escape conversion into the kinetic form and be 
stored up as gain of tissue, notably of fat. We may say briefly, 
then, that the metabolizable energy of the food is used, first, for 
the production of “ physiological work” and second, for the storage 
of energy. 
Physiological Work.—The term “physiological work” in the 
previous sentence is employed in a somewhat loose and general 
sense to designate all those activities in the body which are sus- 
tained by the metabolizable energy of the food and whose ultimate 
result is the production of heat or motion. A more definite idea 
of what the term includes may be gained by a consideration of the 
chief factors which go to make up the physiological work of the 
body. , 
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