*Metabolic Regulations.*--And now we enter the field of regulations in metabolism itself. There are two kinds of outside factors of fundamental importance for all metabolic processes: food is one, and oxygen is the other. And metabolism as a whole is of two different aspects also: it both serves for assimilation proper--that is, building up--and it supplies the energy for driving the functional machine. It is clear that food alone--together of course with the assimilating means of the organism, can account for the first type of metabolism, while both food and oxygen, or some sort of substitute for the latter, as in certain bacteria, supply functional energy. Of course we are not entitled to say that the importance of so-called oxidation or respiration is exhausted by its energetic rôle: it certainly is not, for if it were, the organism would only be stopped in its functions if deprived of oxygen but would not die. It seems that certain substances always arise in the metabolism, in the processes of decomposition, which have to be burnt up in order not to become poisonous. But we shall return to the phenomena of organic oxidation in another chapter of the book, and shall deal with them from a more general point of view.[102] [102] The theory of oxidation we have shortly sketched here was developed in chapter B. 5, of my *Organische Regulationen*. Recent discoveries of Winterstein’s (*Zeitschr. allg. Physiol.* 6, 1907) have given the strongest support to my hypothetic statements, and, in fact, can be said to have brought the doctrine of organic oxidation to a critical point. There can be no doubt that oxygen not only plays the “antipoisonous” rôle I had assigned to it, but that it is not even of such great importance for the supply of functional energy as former times had assumed. No doubt it serves to drive the functional machine, but decomposition of certain chemical constituents of the organism serves this purpose even more. The latter does so in the most fundamental and original manner, so to speak, whilst oxidation only burns up its products. Almost all elemental functions, in nerve-tissue at least, go on very well in the absence of oxygen, provided that certain “poisonous” substances, resulting from this anaërobic metabolism, are constantly removed. In normal conditions that is done by oxygen, and in doing so oxygen certainly assists the supply of energy, but it does not furnish the whole of it. The difference between so-called “aërobic” and “anaërobic” life almost completely disappears under such a view, and many so-called “regulations,” of course, disappear at the same time; there is no more “intramolecular respiration.”