374 THE SENSORIAL FUNCTIONS. 



tern, so as to produce sensation, are of different 

 kinds, and are governed by laws peculiar to 

 themselves. The structure of the organs must, 

 accordingly, be adapted, in each particular case, 

 to receive the impressions made by these agents, 

 and must be modified in exact conformity with 

 the physical laws they obey. Thus the struc- 

 ture of that portion of the nervous system 

 which receives visual impressions, and which is 

 termed the retina, must be adapted to the action 

 of light ; and the eye, through which the rays 

 are made to pass before reaching the retina, 

 must be constructed with strict reference to the 

 laws of optics. The ear must, in like manner, 

 be formed to receive delicate impressions from 

 those vibrations of the air which occasion sound. 

 The extremities of the nerves, in these and other 

 organs of the senses, are spread out into a deli- 

 cate expansion of surface, having a softer and 

 more uniform texture than the rest of the nerve, 

 whereby they acquire a susceptibility of being 

 affected by their own appropriate agents, and 

 by no other. The function of each nerve of 

 sense is determinate, and can be executed by no 

 other part of the nervous system. These func- 

 tions are not interchangeable, as is the case with 

 many others in the animal system. No nerve, 

 but the optic nerve, and no part of that nerve, 

 except the retina, is capable, however impressed, 

 of giving rise to the sensation of light : no part 



