222 The Vertebrate Organ Systems 



part, separated from the other two by means of a bony shelf which ex- 

 tends partially across from the central axis. The vestibular and 

 cochlear canals are separated from one another by a membrane. The 

 vestibular and tympanic canals are filled with perilymph, while the 

 cochlear canal is filled with endolymph. The vestibular canal is sep- 

 arated from the midde ear by the membrane of the oval window, while 

 the tympanic canal is separated by the membrane of the round window. 



TECTORIAL __ 



MEMBRANC \\>^ 



HAIR CELI I .\ \. 



BASILAR l>^<5i 



MEMBRANE 



Fig. 70. — A diagrammatic cross section of the cochlea. 



The bony shelf which partly separates the canals is completed in 

 the region of the cochlear canal by means of a membrane, the basilar 

 membrane. Located on this membrane is the organ of Corti (Fig. 71), 

 a series of sensory hair cells projecting into the endolymph of the 

 cochlear canal. From the basilar membrane, the tectoral membrane 

 projects into the cochlear canal above the sensory endings of the hair 

 cells. 



Sound vibrations are transmitted from the tympanic membrane 

 to the malleus, incus, and finally to the stapes. The stapes by means 

 of its attachment to the membrane of the oval window sets the peri- 

 lymph of the vestibular canal into motion. These vibrations pass into 

 the tympanic canal which now sets up vibrations in the basilar mem- 

 brane. The basilar membrane in turn sets the membrane separating 

 the vestibular and cochlear canals into motion. The vibrations thus set 

 up in the endolymph cause the tectoral membrane to vibrate and afifect 

 the hair cells of the organ of Corti. The spent vibrations are absorbed 

 by the membrane of the oval window. 



