IN HUMAN PHYSIOLOGY. 193 



enlarges the field of thought, it gives birth to precise no- 

 tions of light, of form, of extent ; and it permits the com- 

 munication of thought by conventional signs. Hearing is 

 a necessary condition of articulate language ; without it 

 man lives alone, affection and confidence lose their most 

 precious forms of expression, and friendship cannot exist. 

 Auditory sensations act upon the nervous system with 

 more force than visual sensations. We are carried away 

 by rhythm, or it adapts itself to our ideas and our passions ; 

 music plunges us into an ideal world, and holds us by an 

 indefinable charm ; in a word, if sight speaks more es- 

 pecially to the intellect, hearing addresses itself to the 

 affections. Sight is certainly more necessary to man than 

 hearing, but still the blind are generally gay and commu- 

 nicative, while the deaf seem inclined to melancholy. As 

 to the relative influence of these two senses on the devel- 

 opment of the intellect, we know that the education of the 

 deaf is slow, but may be complete, while that of the blind 

 is, on the contrary, rather rapid, but is almost always very 

 limited ; many ideas cannot be acquired by them, and, as 

 has been remarked by M. Longet, their minds rarely 

 attain maturity." 



17. Which coat is the white of the eye ? 

 The sclerotic. 



1 8. What makes the difference in the color of eyes ? 



The varying shade of the pigment deposited in the iris 

 of the eye. 



19. Why do we snuff the air when we wish to obtain a 

 distinct smell? 



As muscular actions are called into play to aid the 

 sense of taste, as in smacking the tongue and lips, so the 



