THE SENSE OF HEARING 411 



draws the head of the stapes backward, thus pressing 

 the base against the fenestra ovalis and compressing 

 the contents of the vestibule. Its nerve is the tympanic 

 branch of the facial. 



The Internal Ear. This is the essential part of the 

 hearing apparatus, since here the auditory nerve is 

 distributed. It is contained in a cavity in the petrous 

 bone, and is made up of the osseous labyrinth and 

 the membranous labyrinth. 



The osseous labyrinth contains the membranous 

 labyrinth, and is divided into three parts, the vesti- 

 bule, semicircular canals, and cochlea. It communi- 

 cates in the dry state with the tympanum by means 

 of the fenestrse. Between the osseous and mem- 

 branous labyrinth is a space occupied by a clear fluid, 

 the perilymph, and within the membranous labyrinth 

 is the endolymph. 



The vestibule is the central cavity lying between 

 the cochlea in front and the semicircular canal behind, 

 the tympanum being external. Its outer or tympanic 

 wall presents the fenestra ovalis. 



Its inner wall has in front a depression, the fovea 

 hemispherica, pierced by several minute holes for 

 the auditory filaments, and behind this a ridge, the 

 crista vestibuli. Behind this ridge is the opening of 

 the aqueductus vestibuli. In the roof is a depression, 

 the fovea hemielliptwa. 



Behind, the vestibule presents five foramina leading 

 into the semicircular - canals, and in front a larger 

 foramen leading into the scala vestibuli of the cochlea. 



The semicircular canals are three bony tubes of 

 unequal length lying above and behind the vestibule, 

 each forming about two-thirds of a circle. Their 

 general diameter is -%$ inch, but at one end is a dila- 

 tation, the ampulla, T V inch in diameter. They empty 

 into the vestibule by five apertures, in one of which 

 two tubes join. 



