On the other hand, the conditions for the production of _inhibition_ are favored when the intensity of the series of stimuli is weak. Here it is a question of the development of a relative refractory period for the weak stimuli by increase in their frequency. A relative fatigue of the motor ganglion cells for weak stimuli rapidly occurs, and there develops a state of equilibrium beneath that of the threshold of perceptible effect throughout the continuation of stimulation. _Vészi_ succeeded in isolating these types of summation and inhibition in the spinal cord. His method consisted in cutting the posterior roots of the spinal cord of the frog and stimulating faradically the central ends, and at the same time graphically recording the response of the gastrocnemius muscle. Upon faradic stimulation of the ninth posterior root, one obtains tetanic reflex contraction of this muscle. When the tenth posterior root is then stimulated, tetanus is also produced but of somewhat shorter duration. If, while obtaining tetanus reflexly by stimulation of the ninth root, a faradic current of short duration and not too weak is applied to the tenth root, then a summation of excitation occurs, an increase in the reflex contraction. (Figure 57, A and B.) When, on the other hand, the tenth root is stimulated with weak shocks, one can obtain an increase of the tetanus of short duration followed by inhibition. Here, as the result of interference, we have an instance of inhibition with primary tetanus. (Figure 58.) When the tenth root is stimulated with very weak shocks, inhibition of the tetanus produced simultaneously from the ninth root occurs without primary summation. (Figure 59.) The fact that two series of stimuli, both of which produce dissimilative excitation, bring about an inhibition by their combined action, is sufficient to show the untenability of the _Gaskell-Hering_ hypothesis, that inhibitory processes result from assimilatory excitation. It would be impossible to understand how two dissimilatory exciting stimuli, by their simultaneous action, could bring about assimilatory excitation. When the eighth or the seventh root is stimulated with stronger faradic shocks during the time when tetanus is produced reflexly by faradic stimulation of the ninth, an inhibition is practically always obtained. Indeed, faradic currents that are so weak as to be _far_ below the threshold of perceptible response bring about when applied to the seventh or eighth root a decided inhibition of the tetanus, brought about by simultaneous stimulation of the ninth root. The inhibitory effect of weak sub-threshold excitations are here particularly apparent. This inhibition resulting from excitation far below that of the threshold of perceptible response is a common occurrence in the functional activities of the central nervous system. In various parts of the nervous system, the excitation in its conduction is weakened