278 THE GROWTH OF THK BRAIN. 



In this case the resulting breakdown depends in part upon 

 the force developed by the falling pins, but the amount 

 of this is in turn dependent on the force with which 

 the ball corresponding to the initial stimulus strikes 

 them. The actual disproportion which exists between 

 the stimulus and the reaction is less emphasised by this 

 illustration than by that of the powder magazine, but it 

 possesses the advantage of indicating how the energy of 

 the final disturbance depends i\\ part on that of the 

 stimulus. In view of this relation the possible variations 

 in the strength of the impulse which may cause these 

 changes become important. 



The change set up in the nerve by exciting it is 

 designated the nerve impulse. Of its intimate nature 

 nothing is known. The impulse has been investigated, 

 however, by the aid of the electrical changes in the 

 nerve which accompany it like a shadow, and make the 

 study of it possible. All portions of the nervous 

 system are irritable, which is equivalent to saying that 

 the stimulation of any portion can give rise to nerve 

 impulses ; and it has been further found that when 

 artificially aroused in nerve fibres the impulses travel in 

 both directions from the point of stimulation. 



Attention is here claimed for the important fact that 

 the nerve impulse is normally not aroused in the cell- 

 body, but at the ends of the cell outgrowths, so that 

 however much these outgrowths differ from the cell 

 giving origin to them, irritability and the capacity for 

 transmitting the nerve impulse must be granted to 

 them. There can be little doubt that in a given nerve 

 element the impulses have a predominant direction, but 

 this arrangement does not exclude the possibility of 

 other impulses whose direction is the reverse (see 

 Fig. 62). 



Directly bearing on this suggestion are some ex- 



