THE INTERNAL EAR. OSSEOUS LABYRINTH. 



753 



fossa. The nerve from the carotid plexus is above and in front of this, and passes 

 through the bone directly from the carotid canal. The branch to the great superficial 

 petrosal nerve is lodged in a canal which opens on the inner wall of the tympanum in 

 front of the fenestra ovalis. The small superficial petrosal nerve also enters at the 

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



Nerves to Muscles. The tensor tympani muscle obtains its nerve from, 

 the otic ganglion (see fig. 410) ; the laxator tympani is said to be supplied 

 by the chorda tympani: and the stapedius is figured by Sommerriug as 

 receiving a filament from the facial nerve. 



The chorda tympani is invested by a tubular reflection of the lining mem- 

 brane of the tympanum ; its course across the cavity has already been 

 described (p. 611). 



THE INTERNAL EAR, OR LABYRINTH. 



The inner, or sensory part of the organ of hearing, is contained in the 

 petrous portion of the temporal bone. It consists of a cavity the osseous 

 labyrinth hollowed out of the bone, and of the membranous labyrinth con- 

 tained within the osseous walls. 



Fig. 507. 



Fig. 507. RIGHT BONY LABYRINTH, VIEWED 

 FROM THE OUTER SIDE (after Sb'mmer- 

 ring). ^ 



The specimen here represented is pre- 

 pared by separating piecemeal the looser 

 substance of the petrous bone from the dense 

 walls which immediately enclose the laby- 

 rinth. 1, the vestibule ; 2, fenestra ovalis ; 

 3, superior semicircular canal ; 4, horizontal 

 or external canal ; 5, posterior canal ; *, am- 

 pullae of the semicircular canals ; 6, first 

 turn of the cochlea ; 7, second turn ; 8, 

 apex ; 9, fenestra rotunda. The smaller 

 figure in outline below shows the natural 

 size. 



The osseous labyrinth is incompletely 

 divided into three parts, named the 

 vestibule, the semicircular canals, and 

 the cochlea. They are lined throughout by a thin membrane, within which 

 there is a clear fluid named perilymph. 



The membranous labyrinth is contained within the bony labyrinth, and, 

 being smaller than it, leaves a space between the two, occupied by the peri- 

 lymph just referred to. The membranous structure supports numerous 

 minute ramifications of the auditory nerve, and encloses a fluid named the 

 endolymph. 



THE OSSEOUS LABYRINTH. 



The vestibule forms a central chamber of the labyrinth, which communi- 

 cates in front with the cochlea, behind with the semicircular canals, on the 

 outer side with the cavity of the tympanum, and on the inner side with the 

 roeatus auditorius interims. The vestibule is irregularly ovoidal in ^hape 

 from before backwards, and is slightly flattened or compressed from without 



3 D 



