CHAPTER IV. 
THE FASTING METABOLISM. 
Tue matter which the animal organism derives from its food is 
applied substantially in three general directions: first, to the main- 
tenance of those vital activities, such as circulation, respiration, 
secretion, the metabolic activity of the various tissues, etc., and 
probably to some extent the direct production of heat, which in 
their entirety make up the physical life of the organism; second, 
to the support of those functions by which the crude materials 
ingested are prepared to nourish the body, that is, to the work of 
digestion and assimilation; third, to the production of external 
mechanical work or to the storage of surplus material in the form 
of growth of tissue. 
Of these three general functions of the food, the one first named 
is obviously of fundamental significance, and a determination of 
the nature and amount of its demands constitutes the natural first 
step in a study of the laws of nutrition. For this purpose we can 
eliminate the influence of the other two factors by keeping the ani- 
mal as nearly as possible in a state of absolute rest and by with- 
holding food. Under these circumstances the expenditure of matter 
from the tissues of the body may be taken as representing the 
miminum demands of the vitalfunctions. It will therefore be both 
logical and convenient to consider first, in the present chapter, the 
fasting metabolism of the quiescent animal, while in succeeding chap- 
ters we take up the influence respectively of the food-supply and of 
external work upon metabolism. The protein of the food has such 
peculiar and distinct functions in the animal economy that it will 
be a matter of practical convenience to follow the historical order 
of investigation and consider first the proteid metabolism by itself 
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