OH. LIV.] THE INTERNAL EAR 741 



The Internal Bar. The proper organ of hearing is formed by the 

 distribution of the auditory nerve, within the internal ear, or laby- 

 rinth, a set of cavities within the petrous portion of the temporal 

 bone. The bone which forms the ... 



walls of these cavities is denser 

 than that around it, and forms the 

 osseous labyrinth ; the membrane 

 within the cavities forms the mem- 

 branous labyrinth. The membranous 

 labyrinth contains a fluid called 

 endolymph ; while outside it, be- 3 -" 

 tween it and the osseous labyrinth, 

 is a fluid called perilymph. This 

 fluid is not pure lymph, as it con- 

 tains mucin. 



The OsseOUS Labyrinth COn- Fl - 543- Interior view of the tympanum, with 

 , r , v . * , membrana tympani and bones in natural posi- 



sists or threeprmcipal parts, namely, uon. i, MembAna tympani ; 2, Bustacwan 



fbp wtfilviilp fhp //!/%/*/* anrl fVia tube ; 3, tensor tympani muscle ; 4, lig. mallei 



1110 V6MWWe, I ^ COCmea, anc exter.;5,lig. mallei super. ; 6, chorda-tympani 



Semicircular Canals nerve; a, b, and c, sinuses about ossicles. 



mi ,-7 7 ' ,1 -in (Schwalbe.) 



Ine vestibule is the middle 



cavity of the labyrinth, and the central chamber of the auditory 

 apparatus. It presents, in its inner wall, several openings for the 

 entrance of the divisions of the auditory nerve; in its outer wall, 

 the fenestra ovalis (2, fig. 544a), an opening filled by membrane, in 

 which is inserted the base of the stapes ; in its posterior and superior 

 walls, five openings by which the semicircular canals communicate 

 with it: in its anterior wall, an opening leading into the cochlea. 

 The structure of the semicircular canals is described in Chapter 

 XLIX. 



The cochlea (6, 7, 8, fig. 544a, and 8, fig. 544&) is shaped like a 

 snail-shell, and is situated in front of the vestibule ; its base rests on 

 the bottom of the internal meatus, where some apertures transmit to 

 it the cochlear filaments of the auditory nerve. In its axis, the 

 cochlea is traversed by a conical column, the modiolus, around which 

 a spiral canal winds with two turns and a half from the base to the 

 apex. At the apex of the cochlea the canal is closed ; at the base it 

 presents three openings, of which one, already mentioned, communi- 

 cates with the vestibule ; another, called fenestra rotunda, is separated 

 by a membrane from the cavity of the tympanum ; the third is the 

 orifice of the aquceductus cochlece, a canal leading to the jugular fossa 

 of the petrous bone. The spiral canal is divided into two passages, 

 or scalce (staircases), by a partition formed partly of bone, the lamina 

 spiralis, connected with the modiolus, and partly of a membrane 

 called the basilar membrane. 



The Membranous Labyrinth. The membranous labyrinth 



