THE UTILIZATION OF ENERGY. 447 
but we do know that the proportion is materially greater in the 
latter case, since the heat production of a fattening animal is ob- 
viously much less than that of a working animal utilizing the same 
amount of food. ; 
In the following pages the attempt has been made to bring 
together the more important experimental evidence bearing upon 
the utilization of food energy for the production of tissue and of 
work. Before, however, proceeding to a consideration of our present 
knowledge upon the subject, attention should be called once more 
to the fact that we are here dealing with it from the statistical point 
of view of the balance between income and expenditure of energy 
of the body. 
In an animal verforming work, each muscular contraction 
metabolizes a certain quantity of energy, part of which finally 
appears as heat and part as mechanical work. Besides this, how- 
ever, a secondary result is an increase in the activity of the organs 
of circulation and respiration which requires the expenditure of a 
certain amount of energy, this energy ultimately taking the form 
of heat and being added to that resulting directly from the activity 
of the skeletal muscles. When we compare the: actual external 
work done with the total energy metabolized for its performance, 
and so compute the coefficient of utilization, we group all these 
sources of heat production and regard them as, from the economic 
standpoint, a waste of energy, just as in a heat engine the energy 
which escapes conversion into work is regarded as waste energy not- 
withstanding the fact that the loss is inevitable. So, too, in the pro- 
duction of new tissue we look upon total gain of potential energy 
by the body as constituting the net useful result of the feeding, 
and the coefficient of utilization in this case, as in that of muscular 
work, would express the relation which this bears to the net avail- 
able energy supplied in the food. That the effect of abundant 
food may be in some eases to stimulate the metabolism of tissue or 
the ‘incidental’? muscular work (p. 342) is rendered probable by 
Zuntz & Hagemann’s results with the horse (see p. 376). All these 
effects are part of the necessary expenditure of energy by the body, 
and however interesting physiologically are statistically sources of 
loss. 
