37 o NORMAL HISTOLOGY. 



tympanic cavity and contains the oval window with the foot-plate of the stapes. 

 The margin of the window and the foot-plate are covered with hyaline carti- 

 lage and connected by fibro-elastic tissue, thus preventing the escape of the 

 perilymph from the vestibule. The medial (inner) wall of the vestibule 

 presents two depressions separated by a ridge, the crista vestibuli. The 

 anterior wall of the vestibule is pierced by the narrow opening leading into 

 saccule. The posterior and larger depression is the elliptical recess. The 

 anterior wall of the vestibule is pierced by the large opening leading into 



the scala vestibuli of the coch- 



Superior ampulla ^^--Superior canal j ea Posteriorly the vestibule 



Common crus \ /J directly communicates with the 



v \/y j semicircular canals by five round 



Lodges^* ^\/Ny dt "^^^.Externai The three bony semicir- 



saccuie "^<^^ y^ ^vJ^L-^lF ~" i"'"" cular canals the superior, 



f MK&\L SEJ^^m p -or fa& posterior, and the horizontal 



CochieaTl^g^t ^ ,m~~ canal lie behind the vestibule, their 



^I^^I^^^^^^H^^ disposition being such that the 



o v/ planes of the three canals cor- 



Postenor ampulla , . , . . . . . 



respond with the sides of the 



FIG. 418. Cast of right bony labyrinth, mesial aspect. X 2. . . _ , 



corner of a cube. Each canal 



possesses at one end a dilatation, called the osseous ampulla. The semi- 

 circular canals open into the posterior part of the vestibule by five apertures, 

 the undilated ends of the superior and posterior canals joining to form a 

 common limb (Fig. 418). The horizontal canal alone communicates with 

 the vestibule by two independent openings. 



The vestibule and the bony semicircular canals are lined by a very thin 

 periosteum composed of a feltwork of a resistent fibrous tissue, containing 

 flattened connective tissue cells. Endothelium everywhere lines the peri- 

 lymphatic space between the membranous and osseous canals, covering the 

 free inner surface of the periosteum, the fibrous trabeculse, and the outer or 

 perilymphatic surface of this part of the membranous labyrinth. 



The bony cochlea constitutes the anterior part of the labyrinth and 

 appears as a short blunt cone, about 5 mm. in height, whose base forms the 

 anterior wall of the inner end of the internal auditory meatus. Its apex is 

 directed horizontally outwards. The bony cochlea consists essentially of a 

 tapering central column, the modiolus, around which the bony canal, about 

 30 mm. long, makes something more than two and a half spiral turns, the 

 basal, middle, and apical. The conical modiolus has a broad concave base, 

 which forms part of the base of the cochlea, and a small apex, which extends 

 nearly to the apex of the cochlea or cupola. It is much thicker within the 

 lowest turn of the canal than above, and is pierced by many small canals for 

 the nerves and vessels to the spiral lamina. The axis of the modiolus, from 

 base to apex, is traversed by the central canal, while a more peripherally 

 situated channel, the canalis spiralis, encircles the modiolus and contains the 

 spiral ganglion and a spiral vein. Projecting at a right angle from the 

 modiolus into the canal of the bony cochlea is a thin shelf of bone, the 

 lamina spiralis ossea, which is made up of two delicate bony plates between 

 which are fine canals containing the branches of the cochlear nerve. The 

 partial division of the canal of the bony cochlea effected by the osseous 

 spiral lamina is completed by the membranous spiral lamina, which stretches 

 from the free edge of the osseous lamina to the outer wall of the canal (Fig. 

 422). The upper division of the canal is called the scala vestibuli and com- 



