INTERNAL WORK. 337 
Work oF THE VoLUNTARY MusciEs.—The most obvious form 
of physiological work is that performed by the contraction of the 
voluntary muscles, either in the performance of useful work or in 
the various incidental movements made during the waking hours. 
In a sense the production of muscular work may be said to be the 
chief end of the metabolizable energy of the food, inasmuch as 
all the other activities of the body (apart from the reproductive 
functions and, of course, from mental activities) are accessory to 
this. In amount, however, the energy of muscular work is much 
less than the energy expended in other forms of physiological work 
and consumes a comparatively small percentage of the metaboliz- 
able energy of the food. 
InTERNAL Work.—The body of an animal in what we commonly 
speak of as a state of rest is still performing a large amount of work. 
The most evident forms of this are the work of circulation and res- 
piration. In addition to these, however, there are less obvious kinds 
of work whose total is probably very considerable. The body is an 
aggregate of living cells. The living cell, however, is constantly 
carrying on activities of various sorts, and these activities require 
a supply of energy, although how much of the energy of the food 
is consumed in the various processes of secretion, osmosis, karyoki- 
nesis, etc., it is difficult to say. 
In the numerous varieties of internal work the energy involved 
passes through various forms. Ultimately, however, since it 
accomplishes no work upon the surroundings of the animal, it 
is converted into heat and leaves the body either by radiation and 
conduction, as the latent heat of water vapor or as the sensible 
heat of the excreta. 
Work or DicEsTion AND ASSIMILATION.—Logically the work 
of digestion and assimilation would be classed as part of the internal 
work of the body, but motives of convenience make it desirable to 
consider it separately. 
In a fasting animal, with the digestive tract empty, the various 
forms of internal work indicated above go on with a considerable 
degree of constancy, and the resulting heat production is quite 
uniform under like conditions. If food be given to such an animal 
there results very. promptly an increase in the excretion of carbon 
