232 THE ORIGIN OF THE NERVOUS SYSTEM 



of a lower level, essentially this change occurs (p. 218). 

 The first impulses travel only a short distance from the 

 point of stimulation, but if the stimulation is continued 

 they travel successively farther and farther, until after 

 a time an impulse is produced which reaches the apical 

 end of the row. In this case the continued stimulation 

 and the passage of impulses upward doubtless affect 

 both the condition of the path and the quantitative 

 characteristics of the later impulses. This is obviously 

 a sort of summation. 



