THE UTILIZATION OF ENERGY. 497 
mechanical work, we are perforce, by the very complexity of the 
problem, driven to the statistical method of comparing the total 
income and outgo of energy in the various forms of work. 
THE UTILIZATION OF NET AVAILABLE ENERGY. 
Both the activity of the skeletal muscles in the performance of 
work and the supplementary activity of the muscles concerned in 
circulation, respiration, etc., is carried on at the expense of energy 
stored in the muscles themselves or perhaps in the blood which 
circulates through them. The body thus suffers a loss of energy 
which is replaced from the energy of the food. If, then, we supply 
a working animal, in addition to its maintenance ration, with an 
amount of food exactly sufficient to make good the loss, the total 
energy metabolized in the performance of the work will repre- 
sent the net available energy of the excess food, since this by 
definition is that portion of the gross energy which contributes 
to the maintenance of the store of potential energy in the 
body. 
It is true that in our discussion of the net available energy of 
the food we regarded it as making good the losses that occur below 
the maintenance requirement, and the question may arise whether 
the availability as thus measured is the same as the availability for 
the production of muscular work. In reality, however, the two 
cases are not radically different. Even below the point of mainte- 
nance the internal work of the body consists very largely of muscu- 
lar work, and it is the energy metabolized in the performance of this 
work which appears to constitute the chief demand for available 
food energy. It would appear highly probable, therefore, that the 
net availability of the metabolizable energy of the food will be found 
to be substantially the same whether that energy be employed to 
prevent a loss from the body as a consequence of its internal work 
below maintenance or on account of the performance of external 
work above maintenance. 
If, then, we cause an animal to perform a known amount of 
external work and measure the increase in the amount of energy 
metabolized in the body, we may regard the latter as representing 
net available energy derived from previous food, and a comparison 
