THE PROBLEM OF LEARNING 1 75 



similar to the following: If one takes a brass rod 

 and strikes it, sound waves travel along the length 

 of the rod; or if one were to apply heat to one end 

 of this same rod, in a short time the heat would 

 be carried along the whole length; or, this same 

 brass rod may be attached to an electric current 

 and the current will flow through it. The kind of 

 energy change will depend upon the will of the ex- 

 perimenter but the brass rod does not change dur- 

 ing the passage of the sound wave, heat wave, or 

 electric current. If one wished to carry on all 

 three of these experiments at once, and leave the 

 results distinct, he would need three brass rods. 

 This is what Nature requires in animals and man, 

 • — a separate nerve for each receptor. While 

 these several nerves have each an independent 

 history, their present service seems to be identical, 

 i.e., to carry stimuli from sense organs on the 

 surface of the body to the spinal cord and brain. 



Secondly, the main business of a receptor is to 

 be sensitive to specific energy changes in the en- 

 vironment and the sole work of the nerves im- 

 mediately associated with these receptors is to 

 transmit the respective stimuli received from each 

 receptor. Thus far the analysis is clear and the 

 evidence conclusive. But how does the stimulus 

 pass from a receptor to a nerve fiber; from one 

 nerve fiber to another; and finally from a nerve 

 fiber to a muscle or gland. 



