§ 4. CHARACTERISTICS OF NUTRITION. _Definition of Nutrition._—As we have just seen, the organism is the scene of chemical reactions of two kinds, the one destructive and simplifying, the other synthetic, constructive, or assimilating. This totality of reactions constitutes nutrition. Hence the two phases that it is convenient to consider in this function—_assimilation_ and _disassimilation_. This twofold chemical movement or _metabolism_ corresponding to the two categories of vital phenomena, of destruction (catabolism) and of synthesis (anabolism) is therefore the chemical sign of vitality in all its forms. But it is clear that disassimilation or organic destruction, which is destined to furnish energy to the organism for its different operations, reappears in the plan of the general phenomena of nature. It is not specifically vital in its principle. Assimilation, on the other hand, is in this respect much more characteristic. To some physiologists nutrition is only assimilation. Of the two aspects of metabolism they consider only one, the most typical, _Ad-similare_, to assimilate, to restore the substance borrowed from the ambient medium, the alimentary substances, _similar_ to living matter, to make living matter of them, to increase active protoplasm—this is indeed the most striking phenomenon of vitality. To grow, to increase, to expand, to invade, is the law of living matter. Assimilation, nutrition in its essentials, is, according to the definition of Ch. Robin, “the production by the living being of a substance identical with its own.” It is the act by which the living matter, the protoplasm of a given being, is created. _Permanence in Nutrition._—Nutrition presents one quite remarkable character—permanence. It is a vital manifestation, a property if we look at it in the cell, in the living substance, a function if we consider it in the animal or in the plant as a whole, which is never arrested. Its suspension involves _ipso facto_ the suspension of life itself. It is, in the words of Claude Bernard, that property of nutrition “which, as long as it exists in an element, compels us to believe that this element is alive, and which, when it is absent, compels us to believe that it is dead. It dominates all others by its generality and its importance. In a word, it is the absolute test of vitality.” _Biological Energetics shows the Importance of Nutrition._—We have indicated in advance the reason of its importance, showing that its two phases, disassimilation and assimilation, are the energetic condition of the two kinds of vital phenomena which we can distinguish.