736 



SPECIAL SENSES. 



The vestibule is the central chamber of the labyrinth, communicating 

 with the tympanic cavity by the fenestra ovalis, which is closed in the nat- 

 ural state by the base of the stapes. This is the central, ovoid opening 

 shown in Fig. 266. The inner wall of the vestibule presents a round depres- 

 sion, the fovea hemispherica, perforated by a number of small foramina, 

 through which pass nervous filaments from the internal auditory meatus. 

 Behind this depression is the opening of the aqueduct of the vestibule. In 

 the posterior wall of the vestibule are five small, round openings leading to 

 the semicircular canals, with a larger opening below, leading to the cochlea. 



The general arrangement of the semicircular canals is shown in Fig. 266 

 (6, 7, 8, 9, 10, 11, 12). 



The arrangement of the cochlea, the anterior division of the labyrinth, is 

 shown in Fig. 266 (1, 3, 4). This is a spiral canal, about an inch and a half 

 (38*1 mm.) long, and one-tenth of an inch (2*5 mm.) wide at its beginning, 

 gradually tapering to the apex, and making in its course, two and a half 

 turns. Its anterior presents a central pillar, around which winds a spiral 

 lamina of bone. The fenestra rotunda (2, Fig. 266), closed in the natural 

 state by a membrane (the secondary membrana tympani), lies between the 

 lower portion of the cochlea and the cavity of the tympanum. 



FIG. 266. The left bony labyrinth of a new-born child, forward and outward view. From a photograph 



(Rtidinger). 



1, the wide canal, the beginning of the spiral canal of the cochlea ; 2, the fenestra rotunda ; 3. the sec- 

 ond turn of the cochlea ; 4, the final half-turn of the cochlea : 5, the border of the bony wall of the 

 vestibule, situated between the cochlea and the semicircular canals ; 6, the superior, or sagittal 

 semicircular canal: 7, the portion of the semicircular canal bent outward: 8, the posterior, or trans- 

 verse semicircular canal : 9, the portion of the posterior connected with the superior semicircular 

 canal ; 10. point of junction of the superior and the posterior semicircular canals : 11, the ampulla 

 ossea externa : 12, the hori/xmtal, or external semicircular canal. The explanation of this figure 

 has been modified and condensed from Rildinger. 



What is called the membranous labyrinth is contained within the bony 

 parts just described. Some of the anatomical points connected with its 

 structure and the distribution and connections of the auditory nerve have 



