It is from the beginning apparent that the response to the second stimulus is determined by the intensity of the second stimulus in relation to the degree of irritability which exists at the moment when this is effective. This relation is dependent first upon the absolute intensity of the second stimulus. In the following diagram the intensity of the existing threshold value is fixed for convenience as ordinates beneath the abscissa. If, for example, at the time point _x_, a stimulus of weak intensity R_{1} acts, this stimulus being under the existing threshold, produces no perceptible effect. (Figure 48.) If now instead of a weak stimulus, one of stronger intensity acts at the time point _x_, this stimulus will produce an appreciable response. (Figure 49.) If the second stimulus is of the same strength as the first, this second stimulus will bring about relatively less disintegration, because the system is then in a state in which irritability is still reduced. But this lessened disintegration in that it summates the excitation still existing as the result of the first stimulus can produce an absolute increase of the height above that of the abscissa. Here then we see the possibility of an increase of response resulting from summation. Accordingly the increase of disintegration must occur simultaneously with a diminution of irritability and this must fall below the level of the reduction of irritability produced by the first stimulus. This augmentation of the response through summation above the level of that produced by the first stimulus acting upon an unexcitated system is, however, connected with another condition. The above example refers to systems in which weak stimuli bring about weak response and strong stimuli strong response, that is, the response is capable of increase. In systems in which the “all or none law” is applicable, such an alteration in the absolute height of excitation, as results in summation, is not possible. In order to characterize these two types of living systems by a short expression rather than by a long sentence, we will call the first a “_heterobolic system_,” the latter in which the “all or none law” is operative an “_isobolic system_.” The former term expresses various degrees of discharge depending upon the intensity of the stimulus, the latter term refers to the constancy of discharge following stimuli of various intensities. Isobolic systems are in contradistinction to the heterobolic systems not capable of summation. The response to the second stimulus of equal intensity cannot be greater than that of the first, it may be equal to the first (Figure 50) or be less in extent, but it can never be greater than that resulting when a single stimulus is applied. These facts have been known for a long time in the case of the heart muscle. A word is necessary, however, concerning the effect of stimuli beneath the threshold in heterobolic systems. We must here distinguish between