THE OSSEOUS LABYRINTH, 641 



ovalis. The smaU superficial petrosal nerve (fig. 451, 20) also enters at the fore 

 part of the cavity beneath the canal for the tensor tympani. 



The tensor tympani muscle obtains its nerve from the otic ganghon ; 

 the laxator tympani is said to be supplied by the chorda tympani : and 

 the stapedius is figured by Scimmerring as receiving a filament from the 



facial nerve. . ^ ■, ■,■ ■ 



The chorda tiimpanl is invested by a tubular reflection of the immg 

 membrane of the tympanum ; its course across the cavity has already 

 been described. 



THE nq-TERNAL EAR, OR LABYHITTTH. 



The inner, essential part of the organ of hearing, is contained in the 

 petrous portion of the temporal bone. It consists of a complex cavity— 

 the osseous labyrinth— hollowed out of the bone, and containing the 

 membranous labyrinth. 



The osseous jdhjnnth is incompletely divided into three parts, named 

 the vestibule (fig. 452, 1), the semicircular canals (3, 4, 5), and the 

 €ochlea (G, 7). They are lined throughout by a thin periosteal cover- 

 ing, within which there is a clear fluid named perilymph, or liquor 

 Cotimnii. 



The memlmnous Mi/riiiih being distinctly smaller than the bony 

 labyrinth, a space is left between the two, occupied by the perilymph 

 just referred to. The membranous structure supports minute ramifica- 

 tions of the auditory nerve, and encloses a fluid named the endolymph. 



THE OSSEOUS LABYRINTH. 



The vestiljule forms a central chamber of the labyrinth, which 

 communicates in front with the cochlea, behind with the semicircular 

 canals, on the outer side with the cavity of the tympanuni, and on the 

 inner side with the meatus auditorius interuus. It is irregularly 

 ovoidal in shape from before backwards, and is slightly flattened or 

 compressed from without inwards : except in the last-mentioned direc- 

 tion, in which it is somewhat smaller, it measures about ith of an inch 

 in diameter. 



Tlie outer wall, which separates it from the cavity of the tympanum, 

 is perforated by the fenestra ovahs, 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 

 hemisphcrica (fig. 453, 2), pierced with many small holes, which serve to 

 transmit branches of the auditory nerve from the internal auditory 

 meatus. This fossa is limited behind by a vertical ridge named crista 

 vestiMli, or eminentia pyramidalis. Behind the crest is the small 

 oblique opening of a canal, the aciuecluct of the vestibule (fig. 453, 4), 

 which extends to the posterior surface of the petrous bone. 



In the roof is an oval depression, placed somewhat transversely,/ow« 

 Tmni-clliptka (fig. 453, 1), the inner part of which is separated by the 

 crest from the hemispherical fossa. 



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

 into the semicircular canals : and at the lower and fore part of the cavity 

 is a larger opening, which communicates with the scala vestibuli of the 

 ■cochlea — apertura scalar vestibuli. 



The semicircular canals are three bony tubes, situate above and 



VOL. II. "^ '^ 



