INTERFERENCE OF EXCITATIONS 215 



ing stimulus is applied before there is complete recovery of exci- 

 tation 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 continu- 

 ance of stimuli of constant intensity. The principles which under- 

 lie 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 inhi- 

 bition. 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 irri- 

 tability of the system, as measured by the threshold of stimula- 

 tion, 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. 



It is known that summation of excitation in the normal nerve 

 does not occur. As already stated, the nerve is a system in which 

 the "all or none law" is operative. Such isobolic systems do not 

 summate, having no power of summation because each individual 

 stimulus brings about a maximum response. But we have seen 

 that the nerve, as a result of depressing factors, such as deficiency 

 of oxygen, narcosis, fatigue, etc., which decrease its irritability, 

 can be transformed from an isobolic into a heterobolic system. 

 In this state the nerve possesses the capability of summat- 



