This excitating effect on the assimilative portion of metabolism is, as we shall see later, a simple manifestation of the law of mass action. [58] _Paul Jensen_: “Das Problem der trophischen Nerven.” Medicinisch-naturwissen-schaftliches Archiv. Bd. II, 1910. As a result manifold effects of excitating stimulation, which seemed possible at a first glance, are already considerably restricted. The great mass of excitating stimuli produce an acceleration of the dissimilative processes of the metabolic chain. But here our former observations have already shown that certain constituent processes are especially responsive and very readily increase as a result of the most varied adequate and inadequate stimuli. These are the “_functional_” members of metabolism. These members are particularly labile, so that they are always affected by every influence to which the system is subjected in the form of a stimulus. The functional portion of metabolism of the muscle, which is particularly labile and is always primarily affected by stimulation, consists as demonstrated in increase of formation of carbon dioxide and water, and in the disintegration of the nitrogen-free groups. The innumerable observations on metabolism during the stage of the activity of the muscle, as those of _Hermann_, _v. Frey_, _Fletcher_, _Johannson_, _Thunberg_, and many others on the individual muscle, and those by _Voit_, _Fick_ and _Wislicenus_, _Pflüger_, _Rubner_, _Zuntz_, _Lehmann_ and _Hagemann_, _Bernstein_ and _Löwy_ and others on the muscle of the entire organisms, have sufficiently proved this fact. However, we should not apply in detail the conditions existing in the _muscle_ to _all_ living substance. Comparative methods show us, rather, that the functional portion of metabolism is very differently involved in various forms of living substance. The formation of carbon dioxide and water is constant in nearly all forms of living substance. We must, however, exclude certain micro-organisms, which have adapted themselves to unusual vital conditions. Further, there appear in some forms manifold special constituent processes consisting in a disintegration of living substance which are in part converted into very complex combinations. In the gland cells this type is represented in an especially high degree. Here the functional disintegration leads to excretion of proteins, glycoproteins, nucleoproteins, cholic acid, enzymes of various kinds, all of which are complex and at the same time nitrogenous organic combinations. This fact must not be lost sight of. The origin of these special members, however, for the present is completely unknown, while on the other hand, it is self-evident that the general and constant constituents of the process of excitation must claim a first place in our interest. It is just at this point, therefore, that we must endeavor to penetrate somewhat more deeply into the mechanism of the excitation process and analyze in greater detail