INTERFERENCE OF EXCITATIONS 



209 



Here then we see the possibility of an increase of response 

 resulting from summation. Accordingly the increase of disin- 

 tegration must occur simultaneously with a diminution of irri- 

 tability 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 pro- 

 duced 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 re- 

 sponse and strong stimuli strong response, that is, the response 

 is capable of increase. In systems in which the "all or none 



Fig. 50. 



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 expres- 

 sion 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 summa- 

 tion. 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 



