FIG. 123. Effect of two successive excitations. The two points of excitation (P] and P 2 ) are marked 

 in each case on the base-line. In A, P : and P.? are sufficiently far apart to give separate curves. 

 In B they are nearer together, and superposition is seen. In C they are sufficiently near to give 

 summation of stimuli. Submaximal stimuli were used throughout ; and the time-tracing in each 

 case shows hundredths of a second. 



