A _summation_ may develop in a heterobolic system and by the use of submaximal stimuli. It always develops when the following stimulus is applied before there is complete recovery of excitation from the previous stimulus. The absolute increase of excitation as a result of summation is, however, limited by the diminution of irritability. By continuation of the series of stimuli the state of equilibrium between the amount of excitation and the irritability will be established on a higher or lower level. There occurs then, depending on whether the feeble persistent excitation remains above or below the level of perceptible effect, either a tonus or an inhibition. Summation can be transformed into inhibition by the continuance of stimuli of constant intensity. The principles which underlie both processes are in no way antagonistic and indeed are not separated by distinct boundaries. The diagram here shown (Figure 53) illustrates this development of summation and inhibition. The time required for this development is in manifold ways influenced by variations of the above-stated factors which control the occurrence of interference. Thereby results an immense number of special cases which differentiate themselves in characteristic manner depending on whether an isobolic or heterobolic system is involved, depending on whether the irritability of the system, as measured by the threshold of stimulation, is high or low, depending on whether fatigability is great or small, depending upon the intensity and frequency of the stimuli, etc. Analysis of every instance shows us different combinations of the interaction of the individual factors. It is, therefore, self-evident that we cannot here analyze a greater number of these cases of summation and inhibition. I wish only to refer to a few typical examples at this time.