SENSE ORGANS. 181 



of the lymph. All this membranous apparatus is con- 

 nected throughout (Fig. 115). 



Mode of Action of the Whole. — Sound vibrations 

 of the air are gathered by the conch, carried by the 

 meatus to the drumhead, and through the chain of 



7TI.S.C 



Fig. 115. — The whole membranous labyrinth: vs, vestibular sac; s, the 

 sacculus ; msc, membranous semicircular canals ; mc, the membranous 

 cochlea. (After Cleland.) 



bones to the stapes ; the shaking of the stapes communi- 

 cates a vibratory motion to the perilymph, and this to 

 the endolymph of the vestibular sac. The shaking of 

 this causes the otoliths to agitate the nerve terminals 

 exposed on the interior of the sac. The vibratory move- 

 ment now divides in several branches. Three of these 

 go up through the semicircular canals, shaking the hairs 

 of the ampullae in which nerve fibers terminate. Still 

 another branch runs spirally up the scala vestibuli over 

 at the extreme end and down spirally by the scala 

 tympani. These vibrations are communicated to the 

 endolymph of the scala media and impress the rods of 

 Corti. 



The Distinctive Functions of these Parts. — It 

 is believed that there is a distinctive function of each of 

 these several parts. The vestibular sac with its otoliths 

 seems especially adapted to perceive the slightest sound 

 as sound or noise, while the cochlea with its rods of 

 graduated lengths seems specially adapted to the percep- 

 tion of sound as tone or pitch, and therefore for the per- 

 ception of music. These rods might well be supposed 



