Curve showing the general development of the effect produced by interference of the stimuli of the same series in an heterobolic system. The effect is first summation and then inhibition. _R_ indicates the intensity of the stimuli, _S_ the level of the threshold of perceptible effect. ] We will first direct our attention to the simplest case brought about by an interference between the individual effects of stimuli in the same series. We will study the effect, which here occurs, in the accompanying diagram, which shows the facts involved in the interference of _two_ stimuli of a _series_ of stimuli. (Figure 53.) The curve shows the development of summation and inhibition. The single stimuli of equal intensity follow at the same intervals, so that the succeeding stimuli meet with an incomplete recovery of excitation and accordingly a decreased state of irritability. In spite of the diminution of the relative response to each stimulus the summation of excitation brings about an absolute increase of the same. At the same time the irritability decreases more and more, for after each stimulation the oxydative disintegration as well as restitution require a progressively greater time and a relative fatigue must, therefore, necessarily develop. The summation, consequently, reaches its limit very soon and then decreases progressively, for, as a result of the increase of fatigue, the oxydative decomposition which occurs at the instant of every stimulation reduces and with this the energy production becomes less and less. The system is relatively refractory for the given intensity of stimulus. Accordingly the response to stimulation falls below the threshold of perceptible response (dotted line S) and finally an equilibrium between disintegration and restitution occurs, wherein the small amount of material used at each stimulation by oxydative decomposition is again replaced before the next stimulus. In other words, the irritability is reduced at each stimulation to an amount equal to that of the recovery in the interval. If this all takes place beneath the threshold of perceptible response, the system during the continuance of the stimulation seems responseless, that is, inhibited. The _inhibition_ consists then of a reduction of irritability below the perceptible threshold of response of the stimulus concerned. It depends upon a continued lessening of dissimilative excitation to a low level through the delay of the oxydative decomposition processes. The inhibition is according to this a relative fatigue, which is conditioned, as is true of every fatigue, by a lengthening of the refractory period following a relative deficiency of oxygen. _The processes of inhibition are simply and solely an expression of a refractory period persisting as a result of dissimilatory excitating stimuli._ Accordingly the general conditions requisite for summation on the one side and inhibition on the other may be formulated as follows: