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 by the materials in the digestive tube, the blood, the liver, or other organs. This building up of reserve stuff, the complement and counterpart of _functional destruction_, is not chemical synthesis. It is, on the contrary, generally, and on the whole, a simplification of the food that has been introduced. This is true, at least as far as the muscle is concerned. However, to this operation, Claude Bernard has given the name of _organizing synthesis_, but the phrase is not a happy one. But in no case was the eminent physiologist deceived as to the character of the operation. “The organizing synthesis,” says he, “remains internal, silent, hidden in its phenomenal expression, gathering together noiselessly the materials which will be expended.” These considerations enable us to understand the existence of the two great categories into which the eminent physiologist divides the phenomena of animal life: the phenomena of the _destruction of reserve-stuff_ corresponding to _functional facts_—that is to say expenditures of energy; and the _plastic phenomena_ of the _building-up of reserves_ of organic regeneration, corresponding to _functional repose_—_i.e._, to the supply of food to the tissues.