NET AVAILABLE ENERGY—MAINTENANCE. 441 
Moreover, so far as we can judge from the accounts of Rubner’s 
experiments, it would seem likely that what were designated on 
p. 342 as incidental muscular movements are a more important 
factor in determining the maintenance requirements of farm ani- 
mals than they are in fixing that of the dog. 
While, therefore, we are probably justified in retaining pro- 
visionally the computation of the maintenance requirement in 
proportion to the real or estimated surface, it should be with a clear 
understanding that it is at present a deduction from experiments 
on other species and under more or less different conditions. 
Effect of Fattening on Maintenance Requirement.—An interesting 
question, and one of practical importance, is what effect the pro- 
gressive change in weight of the same animal as it is fattened has 
upon its maintenance requirement. We can hardly suppose that 
the internal work of the body will be materially increased by such a 
gain. The increased mass of tissue must involve, of course, some 
increase in metabolism, but all that we know of metabolism of adi- 
pose tissue indicates that it is very sluggish. The most important 
effect might be anticipated to be an increase in the muscular ex- 
ertion required in standing, perhaps counterbalanced to a greater or 
less extent by the tendency of the fat animal to pass more of its 
time in a recumbent position. 
Zuntz & Hagemann* have investigated the effect of a load 
carried on the back upon the metabolism of the horse, and. have 
found the latter to be proportional to the total mass (horse plus 
load), but the applicability of this result to another species of ani- 
mal and to an increase of weight caused by fattening may perhaps 
be questioned. The only experiments upon cattle bearing on this 
point are those of Kellner,t who has compared the maintenance 
requirements of fattened and unfattened cattle. It being impossible 
to hit upon exactly the maintenance ration, it is computed from the 
actual results. In case there was a loss of tissue the maintenance 
requirement of the animal is computed by subtracting the poten- 
tial energy of the excreta from the potential energy of food plus 
tissue lost; in other words, the replacement of energy claimed by 
Rubner is assumed to occur. When there was a gain of tissue, on 
* Landw. Jahrb., 27, Supp. III, 269. 
+ Landw. Vers. Stat., 50, 245; 538, 14. 
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