The reactions to catalytic stimuli of short duration, which produce merely an alteration of rapidity in the specific phenomena of a living organism, show on a closer analysis the interesting fact, that it is not always the _entire_ metabolic processes of the cell which are perceptibly quickened, but that only certain constituent processes of the same are affected by the action of excitation. This is the _more_ noticeable, as, considering the close correlation which all the individual links of the chain of metabolism bear to each other, it is to be expected that the alteration in rapidity of _one_ would be followed at once by a corresponding change in all the others. An example of the case in question, in which a special constituent process may be predominately affected, is that of the specific activity of a muscle which is repeatedly stimulated by nervous impulses. Since the classical investigation of _Fick_ and _Wislicenus_[47] on themselves, and of _Voit_[48] on the dog, we know that the nitrogen metabolism is practically unaltered by the functional use of the muscle and there is a remarkable increase only in the breaking down of the nitrogen-free groups of the living substance. Sufficient importance has not as yet been attached to this knowledge. This fact not only has a particular interest for the much-discussed question of the source of muscle energy, but also affords a deeper insight into the metabolic activity of the living substance. It shows us that we must not imagine a purely linear linking of the individual constituent metabolic processes, but rather, at least at certain points, a branching formation, the individual members spreading in various directions. An alteration in an individual member can occur without an immediate change in the other branches. This _would not_ be the case if there were only a linear connection of the constituent processes, for the breaking of a single member of the chain would be followed by a change in all the following members. [47] _Fick und Wislicenus_: “Ueber die Entstehung der Muskelkraft.” Vierteljahresschrift d. Züricher Naturforschenden Gesellschaft. Bd. 10, 1865. [48] _Voit_: “Ueber die Entwicklung der Lehre der Quelle der Muskelkraft and einiger Theile der Ernährung seit 25 Jahren.” Zeitschrift f. Biologie Bd. VI, 1870. Derselbe: Physiologie des allgemeinen Stoffwechsels u. d. Ernährung. In Hermanns Handbuch d. Physiologie, Bd. VI, 1881.