[189] _Fr. W. Fröhlich_: “Ueber die scheinbare Steigerung,” etc. Zeitschr. f. allgem. Physiol. Bd. V, 1905. [190] _Keith Lucas_: “On the gradation of activity in a skeletal muscle fiber.” Journ. of Physiology, Vol. XXXIII, 1905–06. The same: “The all or none law of contraction of the skeletal muscle-fiber.” Journ. of Physiology, Vol. XXXIII, 1909. If we assume that the summation of sub-threshold stimulation, by increasing excitation, brings about a state of equilibrium from below, as it were, so also inhibition may be assumed to be the reverse, the level of equilibrium being reached from above, as it were, by decrease of the primary excitation from strong stimulation. This is expressed in our general scheme of the development of summation and inhibition resulting from the effect of a series of stimuli. At the same time the first part of the curve to the fall of irritation to the level of the sub-threshold equilibrium can be shortened to a minimum by strong stimulation or greater frequency of the same, and we have then the type of _inhibition with primary excitation_. As example of this I wish to again recall the strychninized frog which was used in the fundamental experiments for understanding of the theory of inhibition. If we stimulate a sensory nerve of a strychninized frog, in which the refractory period is already lengthened, with rhythmic single induction shocks of slow frequency, the muscle arranged to make a graphic record will show reflex contraction following each stimulus. If, on the other hand, we apply a series of stimuli, consisting of single stimuli rapidly repeated, contraction is produced only by the first, or the first few stimuli (Figures 45 and 46, pages 202, 203). For the succeeding stimuli the centers remain inhibited, because each succeeding stimulus occurs in the refractory period of the former. The origin of this inhibition shows us with particular clearness how excitation produced by each single stimulus depending upon the frequency of the same, falls rapidly or slowly beneath the threshold of perceptible response. In this case, the state of equilibrium is reached which is maintained by the following stimuli. That a single stimulus is not entirely without effect upon this state of equilibrium follows from the fact that during the continuation of the stimulus a recovery to the point of observable response does not occur, whereas such is the case immediately upon the discontinuation of the stimulus. In inhibition, then, the dissimilatory excitation produced by a single stimulus falls to a low level as a result of the reduction of irritability and remains at this level continuously. _Inhibition as well as tonus is based upon the development of a state of equilibrium between excitation and recovery, or disintegration and restitution of the living substance under the continuous effect of a rhythmic series of stimuli. They differentiate themselves essentially by the height of this equilibrium,