THE OSSEOUS LABYRINTH. 



99 



THE OSSEOUS LABYRINTH. 



The vestibule forms a central chamber of the labyrinth, which communicates 

 in front with the cochlea, and behind with the semicircular canals. It is irregularly 

 ovoidal in shape, measuring about 5mm. from above down and from before back but 

 slightly less from without inwards. 



The outer wall, which separates it from the cavity of the tympanum, is perforated 

 by the fenestra ovalis, which in the recent state is closed by the base of the stapes. 



At the fore part of the inner wall is a small round pit, the fovea hemwpherica 

 (fig. 108, 2 ; fig. Ill), pierced with many minute holes, which serve to transmit 

 branches of the auditory nerve from the internal auditory meatus to the saccule. 

 This fossa is limited behind by a vertical ridge named crista vestibuli, the anterior 



C. tympkl Pi: 



Fig. 106. THE RIGHT TEMPORAL BONE SAWN ACROSS IN A CORONAL PLANE ; THE CUT PASSING THROUGH 

 THE FENESTRA OVALIS. Natural size. (Testut.) 



A, anterior, B, posterior segment, showing the cut surfaces. 



m.e., meatus externus ; m.tymp., groove for membrana tympani ; c.ti/mp., tympanic cavity; 

 antr.mast., antrum mastoideum ; pr., promontory ; or.Eu.t., orifice of Eustachian tube ; /.o., fenestra 

 ovalis ; /.?*., fenestra rotunda ; cock. , commencement of cochlea ; m.i., meatus audit, internus ; art.car., 

 carotid artery. 



extremity of which has been termed the pyramid of the vestibule, and merges on to 

 the roof. The crista passes backwards and downwards and bifurcates behind the 

 fovea hemispherica ; the fork encloses a small fossa, which was termed recessus 

 cochlearis by Reichert : it receives the beginning of the ductus cochlearis and is 

 pierced with a number of holes for the passage of nerve-fibres (fig. Ill, fossa 

 cochlearis}. Behind the lower part of the crest is the small oblique groove which 

 deepens into a fine canal, the aqueduct of tJie vestibule (fig. 108, 4). This extends 

 to the posterior surface of the petrous bone and transmits the ductus endo- 

 lymphaticus (p. 105) and a small vein. 



In the roof is an oval depression, placed somewhat transversely, fovea hemi- 

 elliptica (fig. 108, 1 ; fig. ] 11), which is separated by the crest from the hemispherical 

 fossa. The crest and pyramid close to this fossa are pierced with fine holes for the 

 passage of nerve-fibres, those in the crest itself being destined for the utricle : those 

 in the pyramid for the ampullae of the superior and external semicircular canals. 



At the back part of the vestibule are five round apertures, leading into the semi- 

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