THE FASTING METABOLISM. gI 
last the metabolism may even become almost entirely proteid in 
its character. We have in these facts the first of the numerous 
illustrations which we shall meet in the course of this discussion 
of the plasticity of the organism in adapting itself to differences 
in the food-supply, and of the. controlling influence exerted upon 
the course of its metabolism by the demand for energy. 
THe InrerMepIaRY MeEtasoxism.—The prime object of the 
metabolism of the quiescent fasting animal is, as already pointed 
out, to supply energy for the performance of the vital functions. 
Mention has already been made in Chapter II of the hypothesis 
that the immediate source of energy to the cells of both muscles 
and glands is the metabolism of carbohydrate material. This 
hypothesis in effect regards the metabolism of the fasting animal as 
divisible into three processes: first, the splitting up of the proteids, 
yielding urea and fat; second, the partial oxidation of fat, whether 
derived from the proteids or from the adipose tissue, yielding dex- 
trose; third, the oxidation of the resulting dextrose in the tissues. 
So far as the kind and amount of excretory products are con- 
cerned, it of course makes no difference whether the metabolism 
takes place in accordance with this hypothesis or whether the 
proteids and fat are oxidized directly in the tissues. In either 
case the fasting animal lives upon its store of proteids and fat, and 
the resulting excretory products, as well as the amount of heat 
produced, are qualitatively and quantitatively the same, so that 
the coincidence observed by Kaufmann * between the observed 
results and those computed from his equations is without special 
significance in this case. 
There is, nevertheless, an important and essential difference in 
the two views. If we regard the proteids and fat as yielding up 
their energy directly for the vital activities, then all the energy 
thus liberated is available for this purpose. If, on the contrary, 
we suppose these substances to be first partially metabolized in the 
liver or elsewhere in the organism, then only that portion of their 
potential energy which is contained in the resulting dextrose is 
available directly for the general purposes of the body. The re- 
mainder of their energy is liberated as heat during the preliminary 
*Archives de Physiologie, 1896, pp. 329 and 352. 
