278 SPECIAL ANATOMY. 



1. Fenestra ovalis on the external wall ; it is tightly closed by the basis 

 stapedis. 



2. The five openings of the three semicircular canals. 



3. Aditus ad cochleam, before and below the fenestra ovalis. 



b. Four smaller openings : 



1. Ostium aquceductus vestibuli, on the posterior wall, internal to the 

 common opening of the superior and inferior semicircular canals (leads 

 to the posterior, the cerebral surface of the petrous bone). 



2. Maculae cribrosce, three of many small holes on the perforated part of 

 the posterior wall, at the base of meatus auditorius interims, through 

 which vessels and nervous twigs enter ; there is a superior, for n. sac- 

 cularis major ; an inferior for n. ampullaris inferior; and a.hemisph<r- 

 rica for nerv. saccularis minor. 



c. Depressions, on the floor of the Vestibule, nearer to the inner wall : 



1. Recessus hemisphcericus, anterior, smaller fossa, situated inwards to- 

 wards the cochlea, opposite to fenestra ovalis. 



2. Recessus hemiellipticus , oval, larger fossa, rather behind and above, at 

 the openings of the semicircular canals. Through the foramina on 

 their base the nerves pass. The two are separated from one another by 

 a crista pyramidalis. 



513. 2. The semicircular canals, canales semicircular es, 



three curved, rather elliptical cylinders, of half a line in diameter, 

 lying behind the vestibule, two vertically and one horizontally be- 

 tween them. They open with the two extremities, which are rather 

 wider than the rest of the canals, into the vestibule, as one extre- 

 mity is expanded like a flask (ampulla), but with only five orifices. 



a. The superior (vertical) semicircular canal, about six lines long, at the 

 most superior part of the labyrinth, in the transverse diameter of oss.petrosi ; 

 is convex above, and projects rather below the superior surface of the petrous 

 bone. Its anterior crus expands and opens alone, on the superior wall of the 

 vestibule ; its posterior crus in common with the superior of the inferior semi- 

 circular canal, and without expanding on the superior and internal wall of the 

 vestibule. 



b. The inferior (vertical) semicircular canal, seven lines long, narrower 

 than the other two, describes almost a complete circle, is situated parallel with 

 the posterior surface of the petrous bone, with its convexity directed back- 

 wards and outwards towards proc. mastoideus. Its superior crus opens in 

 common with a ; its inferior expanded crus, one line distant from that on the 

 inferior wall of the vestibule. 



c. The external (horizontal) semicircular canal, four lines long, but wider than 

 the other two, also lies in the long diameter of the petrous bone, with the con- 

 vexity directed outwards. Its anterior expanded crus commences close above 

 the fenestra ovalis, below the crus of the superior semicircular canal; its pos- 



