In most cases, as, for instance, the nerve impulses which move toward an organ, or better still the electrical stimuli as used for experimental purposes, it is not a question of a permanent but of a temporary alteration in the external vital conditions. The stimulus starts, then ceases after a longer or shorter period. In this way there is added to the deviation at the start also the alteration at its termination. The latter takes place with different degrees of rapidity, in a manner analogous to that of the initial alteration, and can bring about response. With this the curve of the duration of the course of the stimulus becomes somewhat more complicated and in consequence a like effect is observed in the response. The “making,” duration and “breaking” of the constant current furnishes the example of this type. The “making” of the current being a quick alteration calls forth a strong and sudden excitation (in the muscle contraction); the continuation of the current maintains weak excitation of equal intensity (in the muscle a continued contraction) and the “breaking,” being a sudden alteration, is followed again by a stronger excitation (in the muscle a contraction). The duration of the change can, however, be so short that its intensity does not remain at two periods of time at the same height, but instead the ascent of the intensity is immediately followed by its descent to zero. Induction shocks of short duration, the duration of which have been observed more in detail especially by _Grützner_,[39] offer typical examples. Here a single effect of the stimulus results from the rise and fall of the intensity curve. Hence the induction shocks as momentary stimuli are universally used for experimental purposes. [39] _Grützner_: “Über die Reizwirkungen der Stöhrer’schen Maschine auf Nerv und Muskel.” Pflügers Arch. Bd. 41, 1887. In contrast to the single stimuli, which find their ideal in induction shocks, another form of stimulation should receive our attention, namely, the series of stimuli which produce a rhythmical alteration of vital conditions. These show among their complex combination of simultaneous and successive actions of their single stimuli relatively the simplest and most easily understood regularity in their effects. They are of particular interest, because they develop in the normal physiological happenings of the animal body in the form of rhythmical intermittent impulses of the nervous system.