266 THE SENSORIAL FUNCTIONS. 



the impressions made l)y these agents, and must be modi- 

 fied in exact conformity with the physical laws they obey. 

 Thus, the structure of that portion of the nervous system 

 which receives visual impressions, and which is termed the 

 retina, must Ix; adapted to the action of light; and the eye, 

 throusili which the ravs 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 imj)ressions 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 ap- 

 propriate 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 ani- 

 mal system. No nerve, but the optic nerve, and no part 

 of that nerve, except the retina, is capable, however im- 

 pressed, of giving rise to the sensation of light: no part of 

 the nervous system, but the auditory nerve, can convey that 

 of sound; and so of the rest. The credulity of the public 

 has sometimes been imposed upon by persons who pretend- 

 ed to see by means of their fingers: thus, at Liverpool, the 

 celebrated Miss M'Avoy contrived for a long time to per- 

 suade a great number of persons that she really possessed 

 this miraculous power. Equally unworthy of credit are all 

 the stories of persons, under the influence of animal mag- 

 netism, hearing sounds addressed to the pit of the stomach, 

 and reading the pages of a book applied to the skin over 



that organ. 



In almost every case the impression made upon the sen- 

 tient extremity of the nerve which is appropriated to sen- 

 sation, is not the direct effect of the external body, but re- 

 sults from the agency of some intervening medium. There 

 is always a portion of the organ of sense interposed hetwcen 



