INTERNAL WORK. 339 
in the course of organic evolution those forms have survived in 
which the incidental heat production was sufficient to meet the 
demand of the environment. 
Other physiologists no less eminent hold that at least an ex- 
ceptional demand for heat (low external temperature) may be met 
by a direct combustion of food or body material for that purpose. 
We shall have oceasion later to give further consideration to these 
divergent views. 
Summary.—The following scheme may serve to summarize 
what has been said above regarding the uses to which the energy of 
the food is put in the body, the possible direct heat production being 
considered, for convenience, as part of the physiological work of 
the body in order to include it among the other forms of the 
expenditure of energy: 
Energy of excreta, Work of voluntary 
muscles. 
Gross energy Physiological | Internal work. 
Metabolizable work 
energy Work of digestion and 
assimilation. 
Heat production. 
Storage of energy. 
For the sake of directness of statement, language has been used 
above which seems to imply that the food is directly oxidized some- 
what like the fuel in a locomotive. While statistically the effect is 
the same as if this were the case, it must not be forgotten that the 
body itself constitutes a reservoir of potential energy and that 
the energy liberated in its various activities comes primarily from 
the potential energy stored up in its various tissues, while the func- 
tion of the food is to make good the Joss’ this occasioned. 
The metabolism of matter and energy in the body might be 
compared to the exchange of water in a mill-pond. The water in 
the pond may represent the materials of the body itself, while the 
water running in at the upper end represents the supply of matter 
and energy in the food, and that going down the flume to the mill- 
wheel the metabolism required for the production of physiological 
work as above defined. The water flowing into the pond does not 
immediately turn the wheel, but becomes part of the pond and 
loses its identity. Part of it may be drawn into the main current 
