purely and simply, the energy brought into play, consumed, or, better still, transformed in the active organ, or tissue. Food as it is introduced, inert food, does not, in fact, take up its place as it is, without undergoing changes in that organ and that tissue, in order to restore the status quo ante.
Before building up the tissue it will have undergone various modifications in the digestive apparatus. It will have also undergone changes in the circulatory apparatus, in the liver, and in the very organ we are considering. It is after all these changes that assimilation takes place. It will find its place and will have then passed into the state of reserve.
The food digested, modified, and finally incorporated as an integral part in the tissue in which it will be expended, is therefore in a new state, differing more or less from its state when it was ingested. It is a part of the living tissue in the state of constitutive reserve. Its potential chemical energy is not the same as that of the food introduced. It may differ from it very remarkably in consequence of sudden alterations.
We do not know for certain at the expense of what category of foods this or that given organ builds up its reserve stuff. There is a belief, for instance, according to M. Chauveau, that the muscle does its work at the expense of the reserve of glycogen which it contains. The potential chemical energy of this substance would be a source of muscular mechanical energy. But we do not know exactly at the expense of what foods, albumenoids, fats, or carbohydrates the muscle builds up the reserve of glycogen expended during its contraction. It is probable that it builds it up at the expense of each of the three categories after the various more or less simple alterations undergone
