contrary, an _inhibition_ which continues throughout the time during the stimulation of the eighth root. If the stimulation of the eighth root is discontinued, the tetanic response of the ninth root reappears. If, on the other hand, the faradic stimulation of the ninth root is interrupted and the eighth root now again stimulated, one obtains once more, as in the beginning, with each stimulation a contraction of the muscle. This fact is illustrated by the accompanying tracings. (Figure 2.) In this investigation undertaken in the Göttingen laboratory it was further shown that a faradic current of the same strength and the same frequency had at one time an augmenting, at another an inhibitory effect, and these effects could be produced alternately at will. Should the faradic current at one time be called a stimulus, at another not? It is here clearly shown to what absurd consequences it leads if the conception of stimulation is limited solely to the cases in which an external factor has an exciting effect; and yet an immense number of instances of a like nature could be cited to show the untenability of this view. [19] _Max Verworn_: “Physiologisches Prakticum für Medizinen.” Jena 1907. [20] _Julius Vészi_: “Der einfachste Reflexbogen im Rückenmark.” In Zeitschrift f. allgemeine Physiologie Bd. XI, 1910. [Illustration: Fig. 2. Lower thick line shows duration of stimulation of 9th root; upper thick line that of 8th root.] It follows from this, that it is altogether impracticable to define the stimulus itself in relation to the nature of the effects which the stimulus has upon the substances in the living system. One can only appreciate the nature of stimulation in relation to the vital conditions and without considering the nature of the action of the stimuli on the living substance. It is true that every stimulus is followed by an alteration in living processes, but this is to be expected when one clearly understands the nature of vital conditions. A stimulus is in all cases an alteration in vital conditions and, in that each of the vital conditions is necessary for the continuance of life, it follows of necessity that every alteration in the vital conditions, so intimately connected with the living processes, will also be followed by an alteration in the processes occurring in the living system. In short, response is produced. Nevertheless, a definite alteration of an external vital condition, depending upon the state of other vital conditions, that is, according to the state of living substance at the moment, can produce quite opposite effects. Although it may appear expedient to include in the conception of stimulation in given instances, distinctions between stimuli according to the nature of their effects upon the living substance, in all cases the conception must under all circumstances be so formulated that it comprises _all_ alterations in the external vital conditions, either positive or