STRUCTURE OF THE EAK. 361 



shape and opposite the preceding, the foot-plate of the stapes, as has been 

 said, being placed upon it ; it is also sometimes called fenestra vestibuli ; 

 3d. Fenestra rotunda, which is below the preceding, and separated from 

 it by the promontory. From the circumstance that it leads from the 

 tympanum to the cochlea, it is also called fenestra cochlea? : like the pre- 

 ceding, it is closed by a double membrane ; 4th. The Eustachian tube, 

 which extends from the anterior of the tympanum to the pharynx ; and, 

 5th. The mastoid cells. The smaller openings are for the passage of va- 

 rious nerves and muscles. 



The labyrinth, called likewise the internal ear, consists of three parts, 

 .the vestibule, the semicircular canals, and the cochlea. 



The vestibule has three corners, an anterior, a superior, and a poste- 

 rior, termed its ventricles. There open into it the fenestra oftheiaby- 

 ovalis, the scala vestibuli, and the five openings of the three sem- rinth - 

 icircular canals. Besides these there are some smaller ones, as the aque- 

 duct of the vestibule, and foramina for small arteries, and for the branch- 

 es of the auditory nerve. The semicircular canals are three bony semi- 

 circles opening into the vestibule : upon one of the branches of each there 

 is a dilatation, the ampulla. The three canals are respectively placed in 

 planes at right angles to each other. The cochlea is a spiral bony canal 

 raised upon a central axis, the modiolus : its interior is divided into two 

 passages or scalar by the lamina spiralis. These communicate at the 

 apex of the cochlea through a small aperture, their other extremities 

 opening differently ; the scala vestibuli into the anterior ventricle of the 

 vestibule, and the scala tympani through the fenestra rotunda into the 

 tympanum. The labyrinth contains interiorly a membrane, the mem- 

 branous labyrinth. Between the membranous labyrinth and the bony, 

 a liquid, the perilymph, intervenes ; the membranous labyrinth being 

 also filled with liquid, the endolymph. There is no perilymph in the 

 cochlea. 



Of the three portions of the ear, the external canal is, of course, full 

 of air, as is also the tympanic cavity or drum ; but the labyrinth, as we 

 have seen, is filled with a liquid, and in this the terminal filaments of 

 the auditory nerve are placed. 



The essential part of the mechanism of hearing is the auditory nerve, 

 which arises from the anterior wall of the fourth ventricle, of the audi- 

 and then, joining the facial, passes forward upon the crus cer- ' tor J -nerve. 

 ebelli ; reaching tha meatus, it divides into two portions, the cochlear and 

 vestibular nerves, which subdivide again, and are distributed to the ves- 

 tibule and cochlea respectively in the manner hereafter explained. 



VIEW OF EXTERNAL, MIDDLE, AND INTERNAL EAR. 



Fig. 183 : #, #, pavilion and external auditory canal, or external ear ; 



