174 MAN, — THE ANIMAL 



directly or indirectly through a special receptor 

 such as a sound receptor, the changes which occur 

 in the transmitting nerve fiber (axon, dendrite) 

 are the same. The stimulus travels along the 

 fiber accompanied by a series of vital changes re- 

 sulting in the formation of the waste substance, 

 carbon dioxide. There is no justification for the 

 conclusion that the nerves of sight, hearing or 

 taste are distinct in the way that a stimulus travels 

 over them. The fact that is important is that 

 each one of these sensory nerves is connected with 

 a receptor that is stimulated by one form of 

 energy change, light, sound, etc. If it were pos- 

 sible to stimulate the receptor cells in the eye by 

 sound vibrations, the resulting stimulus in passing 

 over the nerve fibers of the optic nerve would 

 cause just the same changes in this nerve that the 

 regular light stimulus produces. 



This is really one of the very important conclus- 

 ions which biology contributes to our problem. It 

 is made more emphatic as one tries to recall his 

 own attempts to understand how the inverted 

 image due to the shape of the lens in the eye 

 travels along the optic nerve. The explanation 

 which science offers Is that, so far as the stimulus 

 Is concerned, the Inverted Image Is just like any 

 other sensory stimulus passing along a nerve fiber. 



Each type of receptor and the nerve connected 

 with It work together as a unit. This relation is 



