THE SPECIAL SENSES. 183 



case of each organ, so that each is adapted to one set of 

 sensations alone, and is incapable of perceiving any other. 

 Thus the nerve of hearing is excited by the undulations of 

 sound, and not by those of light, while the reverse is true 

 of the nerve of sight; and the nerve of smell can appreciate 

 neither of them, being capable only of taking cognizance 

 of the odorous properties of bodies. 



17. By some writers six senses are accorded to man ; the 

 additional one being either the sense of temperature, for as 

 we shall presently see this is not the same as touch; or 

 according to others, the muscular sense by which we are 

 enabled to estimate the weights of bodies. The latter also 

 differs in some respects from the sense of touch. 



18. Organs of Touch. The sense of touch is pos- 

 sessed by nearly all portions of the general. surface of the 

 body, but it finds its highest development in the hands. 

 The human hand is properly regarded as the model organ 

 of touch. The minute structure of the skin fits it admi- 

 rably for this form of sensation : the cuticle, or scarf-skin, 

 is fine and flexible, while the outis, or true skin, contains 

 multitudes of nerve-filaments, arranged in rows of papillm, 

 or cone-like projections, about one-hundredth of an inch in 

 length. It is estimated that there are 20,000 of these papillae 

 in a square inch of the palmar surface of the hand. Now, 

 although the nerves of the cutis are the instruments by 

 which impressions are received and transmitted to the 

 brain, yet the cuticle is essential to the sensation of touch. 

 This is shown by the fact that whenever the true skin is 

 laid bare, as by a burn or blister, the only feeling that it 

 experiences from contact is one of pain, not that of 

 touch. 



19. The office of the cuticle is thus made evident : it is 

 to shield the nerve filaments from direct contact with ex- 



1 7. What, is said in relation to one more than the five senses ? 



18. The sense of touch, how prevalent ? What is said of the hand ? 



19. Office of the cuticle ? Tips of the fingers ? The fingers with thumb ? 



