86 PRINCIPLES OF ANIMAL NUTRITION. 
METABOLISM PROPORTIONAL TO AcTIVE TissuE.—In a critical 
discussion of these and other results on fasting animals, to which 
we shall have occasion to refer again in Part II, E. Voit * shows 
that a still more constant relation is obtained when either the pro- 
teid or the total metabolism is compared with the total mass of 
proteid tissue estimated to be contained in the body on the several 
days of the experiment. The total protein of the body, however, 
may be regarded as at least an approximate measure of the active 
cell mass, as distinguished from the relatively inactive cells of 
adipose tissue. It is the vital activities of the former, in the fast- 
ing animal, that mainly determine the amount of the total meta- 
bolism, the energy liberated being supplied in part by the relatively 
small amount of proteid metabolism which goes on in the cells of 
the fasting animal, but largely by the metabolism of fat supplied 
to the active cells from the adipose tissue. 
Ratio oF Prorerip to TotaL Metaporism.—In the preceding 
paragraph it was implied that the proteid metabolism constitutes 
but a small portion of the total metabolism of the fasting animal, 
the remainder of the necessary energy being supplied, after the 
small store of glycogen in the body is exhausted, by the metabo- 
lism of body fat. Rubner + appears to have been the first to call 
specific attention to this aspect of the question. In his investiga- 
tions upon the relation of size of animal to total metabolism he 
adduces experimental results to prove that this ratio is not mate- 
rially different in large and in small animals. The question has, 
however, been more recently discussed by E. Voit{ from a general 
point of view, the results of numerous investigators being summa- 
rized. In discussing these results, Voit has computed from the 
nitrogen and carbon balance, when these data were available, in 
substantially the manner described in Chapter VIII, the amount of 
energy liberated by the metabolism of the protein and fat lost by 
the body. In those instances in which only the nitrogen balance 
was determined, he estimates the amount of energy liberated in 
the body from the computed surface on the basis of average results 
with similar animals. (Compare Chapter XI, § 2.) Taking this 
amount, expressed in calories, as the measure of the total meta- 
bolism, and including only experiments in which the animals — 
* Zeit. f. Biol., 41, 113. + Ibid., 19, 557. + Ibid., 41, 167. 
