286 PHYSIOLOGY 



connections within the central nervous system. Stimulation of the optic 

 nerve by any means whatsoever evokes a sensation of light. One and the 

 same stimulus applied to different nerves will evoke different sensations, 

 e.g. a tuning-fork applied to the skin will give a sensation of vibration, 

 to the ear a sensation of sound. We shall have occasion to return to 

 this question of the restricted function of nerve fibres when we deal 

 with Miiller's ' law of specific irritability ' in the chapter on 

 Sensations. 



