336 THK SENSORIAL FUNCTIONS. 



sion, between the action of the external object on 

 the organ of sense, and the voluntary movement of 

 the limb which it excites. 



The external agents, which are capable of affect- 

 ing the difterent parts of the nervous system, 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 structure of that portion 

 of the nervous system which receives visual im- 

 pressions, 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 delicate 

 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 functions 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 of the nervous system, but the 



