56 NERVES OF COMMON AND [CHAP. I. 



sensibility ; whilst the skin, which is largely supplied with nerves, 

 is highly sensitive. 



Light, sound, and the sapid and odoriferous qualities of bodies, 

 are capable of stimulating certain nerves, and exciting appropriate 

 sensations in the mind. The nerves which respond to such stimuli, 

 are called nerves of proper or special sensation ; and this name 

 seems appropriate, because these nerves, when otherwise stimulated, 

 excite only their peculiar sensations. If the optic nerve be me- 

 chanically irritated, a flash of light is produced; as sometimes 

 occurs if the retina be touched by the needle in the operation for 

 cataract. If the auditory nerves be stimulated by a galvanic 

 shock, a sound is produced. Volta, who tried the experiment on 

 his own person, perceived a hissing and pulsatory sound, which he 

 compared to that of a viscid substance boiling : and Hitter relates, 

 that, upon closing the circle when both his ears were included in it, 

 he was -sensible of the sound of G treble; if but one ear was in the 

 circuit, and the positive pole applied to it, the sound was lower 

 than G ; if the negative pole was applied to the ear, the sound was 

 higher.*" These peculiarities of the nerves of proper sensation are 

 due to the fact, that at their periphery they are so organized as 

 to be admirably adapted for receiving the impressions of their 

 special stimuli, and at their centres they are connected with those 

 parts of the brain which take cognizance of these special agents. 

 Thus the optic nerve is admirably disposed in the eye for the 

 reception of luminous impressions, and the auditory nerve is beau- 

 tifully adapted to receive the pulsations of sound, whilst each is 

 connected with a different part of the brain ; and what are called 

 subjective phenomena of vision or hearing are often the result of 

 local congestions of blood affecting the respective nerves of these 

 senses, and producing mechanical irritation of them. 



In the manifestation of the vital properties, under the influence 

 of appropriate stimuli, it cannot be doubted that an organic mole- 

 cular change is produced in nerve as well as in muscle. This may 

 be considered as a polar state, in which the ultimate particles of the 

 tissue assume a polarized condition, which may be fairly compared 

 to that which friction or other means can produce in various sub- 

 stances by which they may be rendered mutually attractive or 

 repulsive. In muscle, it becomes at once evident by the powerful 

 attraction which is exerted between its particles, by which the 

 shortening is effected. In nerve, it is shewn by the rapidity with 



* Muller's Physiology, translated by Baly. 



