Scheme of the simplest unilateral reflex arc of the spinal cord.] [Illustration: Fig. 61. Scheme of the simplest reflex arc from one to the other side, and from a higher to a lower level.] Finally, I wish to conclude this discussion on the origin of central inhibition and its dependence upon the strength of the stimulus by referring to a point which apparently is contradictory. We have already met with the fact that series of stimuli by their interference in the nervous system may have different effects depending upon their intensity; if this is strong, we obtain summation of excitation, if weak an inhibition. The question may be asked, how is it possible that a weak stimulus can have a different effect when it is believed that the nerve as an isobolic system responds to intensities of all gradations to the same extent, namely, with maximum excitation? If the “all or none law” is applicable, then the same intensity of excitation is always carried to the centers and yet we see that various kinds of responses follow various intensities of stimulation. Here, indeed, is a difficulty which has not as yet been explained. Naturally between the two facts there can be no contradiction. But the question arises, how are we to bring them into harmony? Two entirely different possibilities present themselves. If the various intensities of stimulation always bring about excitation of the same strength and we see in spite of this that various intensities of stimulation produce various kinds of effects, then we must think of the possibility that various intensities of stimulation bring about some other effect than that of variations in intensity in the course of the wave of excitation. In this connection variations in the time involved must be taken into consideration. One might think that _strong_ stimuli may develop a longer wave of excitation than such of _weak_ intensity. _Gotch_[199] tested these questions experimentally with completely negative results. A single strong stimulus does not result in an excitation differing in its course from that of a weak stimulus. But there is another possibility that requires testing. This was brought to light by the investigation of _Thörner_[200] on the fatigue of the nerve. His investigations showed that in a normal nerve in air the first typical beginning of fatigue resulting from faradic stimulation can be demonstrated in the characteristic summation of excitations. This is shown by the nerve after fifteen minutes of stimulation with faradic shocks applied for short intervals. The irritability, when tested with single induction shocks, is at the same time reduced. Thereby the amount of fatigue of the nerve, that is, the amount of the reduction of irritability, is dependent upon the strength and frequency of stimulation producing fatigue. When the nerve is stimulated with weak faradic shocks of a slow rate of frequency, there is a slight or a complete absence of the reduction of irritability.