NET AVAILABLE ENERGY—MAINTENANCE. 407 
the heat production of a quiescent, fasting animal below the critical 
point is made up of— 
1. The heat produced by the internal work. 
2. The heat produced by the processes of “chemical” regula- 
tion. 
The first of these we may regard as substantially constant, while 
the latter varies to meet varying conditions and thus maintain the 
constancy of body temperature. When we give food to such an 
animal we introduce a third source of heat, viz., the work of diges- 
tion and assimilation. Other conditions remaining the same, the 
tendency would be to raise the temperature of the body, and this 
tendency can be overcome either by means of “ chemical” or ‘‘ physi- 
cal” regulation. Recurring to the illustration of the room on 
p. 356, it is as if a second fire were kindled in it. To maintain con- 
stant temperature, either the first fire must be lowered or the win- 
dows must be opened. 
The fact, however, that below the critical point the heat regula- 
tion of the body appears to be largely “chemical” renders it prob- 
able that the regulation is effected by the former method; that is, 
that the heat produced by the work of digestion is utilized to warm 
the body and that correspondingly less energy is withdrawn from 
that stored in the tissues of the body.* Under these circum- 
stances the total heat production of the animal would not be in- 
creased by the ingestion of food, and all the metabolizable energy 
of the food would be apparently available; that is, we should have 
the phenomenon of isodynamic replacement. 
DicEstIvE Work ABove CriticaL Pornt.—The statements of 
the last paragraph refer to conditions below the critical point. 
Above this point no such indirect utilization of the heat resulting 
from digestive work is possible, since the heat production has 
already been reduced to the minimum due, as was concluded on 
p. 356, to internal work. ‘The excess of heat arising from the work 
of digestion is then disposed of by “physical” means. 
Thus Rubner + obtained the following results for the carbon 
* Loewy (Arch. ges. Physiol., 46, 189; quoted by Magnus-Levy, zbid., p. 
116) claims to have shown that such a substitution or compensation does 
not take place in man. 
+ Biologische Gesetze, pp. 17-25. 
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