THE INTERNAL EAR. 



^511 



portion of the temporal bone. The position of approximately its posterior third 

 is indicated by the transverse ridge that crosses the upper surface of the temporal 

 bone a short distance behind the internal auditory meatus. The irregular cavity of 

 the bony labyrinth, hollowed out in the temporal bone, comprises three subdivis- 



FiG. 1264. 



Tympanic cavity \ •; .\. 



Semicircular canals 



Internal auditory canal 



Right temporal bone, upper part of petrous portion has been removed to show bony 



labyrinth lying in position. 



ions : — a middle one, the vestihilc, an anterior one, the cochlea, and a posterior one, 

 the semicircular canals. Both the front and hind divisions communicate freely with 

 the vestibule, but neither communicates with the membranous labyrinth nor, in the 

 recent condition, with the tympanic cavity. Although corresponding in its general 

 form with the bony compartments of the cochlea and semicircular canals, the 

 membranous labyrinth less accurately agrees in its contour with the bony vestibule, 

 since, instead of presenting a single cavity, it is subdivided into two unequal 

 compartments, known as the saccule and the utricle, which are lodged within the 

 bony vestibule. The divisions of the membranous labyrinth are, therefore, four, 

 which from before backward are : the viembranoiis cochlea, the saccule, the utricle 

 and the vienibranous semicircular canals. 



The Osseous Labyrinth. 



The Vestibule. — The vestibule (vestibulum), the middle division of the bony 



labyrinth, lies between the cochlea in front and the semicircular canals behind and 



communicates freely with both. It is an irregularly elliptical cavity, measuring about 



5 mm. from before back- 



FiG. 1265. 



Superior ampulla 



Common crus 



Lodges utricle 

 Lodges saccule 



Cochlea 



Superior canal 



Horizontal 

 canal 



External 

 ampulla 



Posterior canal 



ward, the same from above 

 downward, and from 3-4 mm. 

 from without inward. The 

 lateral (outer) wall separates 

 it from the tympanic cavity, 

 and contains the oval window 

 with the foot-plate of the 

 stapes. The medial (inner) 

 wall, directed toward the 

 bottom of the internal audi- 

 tory canal, presents two 

 depressions separated by a 

 ridge, the crista vestibuli, 

 the upper pointed end of which forms the pyramidalis vestibuli. The anterior and 

 smaller of these depressions is the spherical recess (recessus sphaericus) and lodges 

 the saccule. In the lower part of this fossa, about a dozen minute peiforations mark 

 the position of the macula cribrosa media for the passage of branches of the vestibu- 

 lar nerve from the bottom of the internal auditory canal to the saccule. The posterior 

 and larger depression is the elliptical recess (recessus ellipticus). Behind the lowei 



Posterior ampulla 

 Cast of right bony labyrinth, mesial aspect. X 2. 



