The part played by respiration has often been misunderstood. It consists in the burning off of some of the carbon and hydrogen of the shattered protoplasm molecules, by means of the oxygen of the air, which finds its way into the fluids around the protoplasm, and when it is active every act of combustion--which is here an explosion--leads to the shattering of more protoplasm molecules, and consequently to more respiratory combustion of the products. If the supply of oxygen is limited the breaking down of the molecules of protoplasm does not cease, but the carbon and hydrogen which would otherwise have been oxidised are now in part left to form other compounds in the surrounding liquid, and thus incompletely oxidised bodies, such as vegetable acids, alcohols, etc., accumulate. Even in the complete absence of atmospheric oxygen the protoplasm may go on breaking down and accumulating various compounds containing relatively much carbon and hydrogen--so-called intramolecular respiration; but in ordinary plants this process soon comes to an end, because the blocking up of the molecular plexus leads to obstruction and interferes with the normal assimilation and dis-assimilation, and, if prolonged, leads to pathological conditions, and eventually death. Here, then, we meet with a cause of disease, or of predisposition to disease. The deprivation of oxygen interferes with the normal processes of building up and breaking down of the protoplasmic molecules, and bodies we term poisonous accumulate and may lower the vitality or even bring life to an end. During normal life other products of the disruption of the protoplasm molecules are nitrogenous bodies, such as proteids, and these we have reason to believe are used up again, acting as the nuclei, so to speak, of the new molecules, and so being built up again with fresh food-materials into the plexus, to be again set free, and again used up, and so on. Others are the carbohydrates, such as cellulose, which pass out of the molecule into an insoluble form, and are accumulated outside the protoplasm in the form of cellulose membranes, and so forth. It is these formed products of metabolism (Metabolites), especially cellulose and bodies which result from its subsequent transformation, which constitute the main permanent mass of the ordinary plant. We are now in a position to see how another fundamental cause of disease or predisposition to disease exists in the deprivation of the protoplasm of any of the elements needed to supply--in the food-materials--the place of those which have been permanently put aside in the form of cell-walls, or burnt off in respiration, passed out as excretions, or in other ways lost. It is clear that the indispensability of an element must mean that the protoplasmic molecule cannot be completed without it: the same conclusion is supported by the experimental proof that these elements cannot be replaced by chemically similar elements.