440 PRINCIPLES OF ANIMAL NUTRITION. 
and heat production although the average conditions for the twenty- 
four hours may be such that the necessary production of heat by 
the internal work and the work of digestion and assimilation would 
be more than sufficient for the needs of the animal. 
INFLUENCE OF S1zE or AnrimaL.—The discussion of the heat 
production of the fasting animal in Chapter XI led us to the con- 
clusion that under comparable conditions, at least for the same 
species of animal, the internal work is probably approximately 
proportional to the surface of the body. This, however, is equiva- 
lent to saying that the quantity of net available energy required 
for maintenance is proportional to the body surface. Furthermore, 
if we are right in supposing that the available energy is a linear 
function of the metabolizable energy, the amount of the latter 
required for maintenance will also be proportional to the surface of 
the body. Referring once more to the diagram on p. 410, if OA is 
proportional to the body surface, then OS, which for a given food 
bears a fixed ratio to OA, must also be proportional to the surface. 
If the critical point, K, lies above the maintenance requirement, 
then the metabolizable energy required for maintenance will equal 
the fasting metabolism, and this, as shown on pp. 359-368, is pro- 
portional to the surface. 
Apparently, then, we are justified in concluding that the mainte- 
nance requirements of different normal animals of the same species 
are proportional to their body surface, or, for approximate computa- 
tions, to the two-thirds power of their live weights. It must not be 
overlooked, however, that the results upon which this conclusion is 
based were obtained largely with the dog, an animal which when at. 
rest lies down, and which, therefore, in these experiments was in a 
state of almost complete muscular relaxation. Our common farm 
animals, on the contrary, pass a considerable portion of their time 
standing, which involves an expenditure of energy in muscular 
work. This expenditure we should naturally assume to be pro- 
portional to the mass to be sustained rather than to its surface, 
and if this be true we have here a second determining factor in the 
maintenance requirement. How important this factor is it is diffi- 
cult to say, although the writer’s results with a steer (p. 343) in- 
dicate that it is a large one. Its tendency would be to make the 
maintenance requirement increase more rapidly than the surface. 
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