258 THE SPECIAL SENSES 



of special sense terminate varies in the 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 waves 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. 

 (Bead Note 3.) 



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 possessed 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 admirably for this form of sensa- 

 tion ; the cuticle, or scarf-skin, is fine and flexible, while the 

 cutis, or true-skin, contains multitudes of nerve-filaments, 

 arranged in rows of papillce or cone-like projections, about one 

 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 sur- 

 face of the hand. Now, although the nerves of the cutis are 

 the instruments by which impressions are received and trans- 

 mitted 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. 



3. Variation in Structure in the Nerves of Special Sense. "While 

 in the more intellectual senses Sight, Hearing, and Touch the nerves 

 have their protecting arid isolating sheaths corresponding with the dis- 

 tinctness and separateness of the parts of the impression, in Smell the 

 nerves are a plexus of unsheathed fibres, corresponding with the fusion of 

 the odorous impression into one whole, without distinction of parts." 

 Herbert Spencer. 



17. 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 ? 



