THE ORGANS OF COKTI AND SOUND IMPULSES 475 



reach the ear they will stimulate the auditory nerve and 

 produce the sensation of sound. When these waves of air 

 reach the ear they are first collected by the external ear 

 and concentrated upon the tympanic membrane. This 

 membrane is thus set in vibration in unison with the vibrat- 

 ing body, winch caused the waves. Its vibrations are in 

 turn communicated by the bones and air of the middle 

 ear to the fenestra ovalis of the inner ear. The mem- 

 brane covering this in turn sets in vibration the peri- 

 lymph which fills the spaces of the bony labyrinth and 

 thus distributes these vibrations to all parts of the inner 

 ear. The rotunda by bulging out when the ovalis moves 

 in, and vice versa, permits the vibration of the other- 

 wise incompressible perilymph. Finally, the vibrations 

 of the perilymph are communicated to the membranous 

 labyrinth and its endolymph and thus reach the fibers of 

 Corti where they start the auditory impulses. 



The organs of Corti and sound impulses. — While the 

 auditory nerves send branches to all parts of the mem- 

 branous labyrinth, the impulses of hearing appear to 

 originate almost exclusively in the cochlear division of this 

 nerve. This division gives off many fibers which ter- 

 minate in the sensory cells of the organ of Corti. 



This organ of Corti is located inside the membranous 

 cochlea. It consists of three parts, the rods, the sensory 

 cells, and the supporting cells. The arrangement of these 

 parts is seen in the accompanying figure. (See Fig. 225.) 

 Notice that the rods are arranged in pairs, three thousand 

 to five thousand in all, and are so constructed as to inclose 

 a passage, their bases being expanded while the bodies 

 lean toward one another. Against these rods on either 

 side lean the sensory cells with hairlike extensions reach- 



