THE UTILIZATION OF ENERGY. 511 
: Conditions Determining Efficiency. 
From the results recorded in the preceding paragraphs it appears 
that, as we were led to expect from a consideration of the efficiency 
of the single muscle, the efficiency of the animal as a converter of 
potential energy into mechanical work varies with the nature of the 
work and the conditions under which it is performed, although the 
variations are perhaps hardly as great as might have been expected. 
In general, we may say that in the neighborhood of one third of the 
potential energy directly consumed in muscular exertion is recov- 
ered as mechanical work. This appears to be a high degree of effi- 
ciency as compared with that of any artificial transformer of poten- 
tial energy yet constructed. The steam-engine, the chief example 
of such transformers, even in its most highly perfected forms, rarely 
utilizes over 15 per cent. of the potential energy of the fuel, while 
in ordinary practice one half of this efficiency is considered a good 
result. 
The comparison is misleading. however, for three reasons: First, 
the figures given in the preceding pages relate to the utilization of 
the net available energy of the food. As we have seen, however, 
a certain expenditure of energy in digestion and assimilation is 
required to render the food energy available, while still another 
portion of the latter is lost in the potential energy of the excreta. 
In the case of herbivorous animals. these two sources of loss very 
materially reduce the percentage utilization when computed upon 
the gross energy of the food. Second, the comparison takes no 
account of the large amount of energy consumed continually 
throughout the twenty-four hours for the internal work of the 
body of the animal, and which continues irrespective of whether 
the animal is used as a motor or not. Third, the expenditure 
of energy in locomotion is not considered in computing the 
efficiency of one third. When these three points are allowed for 
but little remains of the apparent superiority of the animal as a 
prime motor, even omitting from consideration the greater cost of 
his fuel (food). 
It remains now to consider somewhat more specifically the in- 
fluence upon the efficiency of the animal machine of some of the 
more important conditions. 
