THE PROBLEM OF LEARNING I77 



movement a stimulus must reach the cells in cer- 

 tain muscles. Here scientists have looked in vain 

 for some intimate union between the peculiar ter- 

 minal branches (end-plate) of a motor axon and 

 the muscle cell. They are in contact only. 



Because of these three structural arrangements 

 in the nervous system, one may say that there are 

 three types of synapses. First, the type that 

 exists between such a receptor as sound and the 

 dendritic branches of the auditory nerve ; secondly, 

 the relations that exist between the terminal 

 branches of one nerve fiber and the receiving 

 branches of a second; thirdly, the conditions that 

 obtain between a motor end-plate and a muscle 

 cell. There are several modifications which result 

 in stimuli being distributed over several routes or 

 just the opposite, when different stimuli are con- 

 centrated to one point. 



Somewhere between the receptor and the 

 muscle, the stimulus is modified because when light 

 or sound waves are sent directly over a motor 

 nerve, these waves do not cause coordinated move- 

 ment as they do when they enter through receptors 

 for light and sound. Most students of this prob- 

 lem are convinced that important changes are in- 

 troduced in synapsis because of the evidence al- 

 ready cited in connection with the movement of a 

 stimulus along a nerve and the general biological 

 evidence that the main function of the cell body 



