234 THE POCKET ANATOMY 



The stapedius is lodged in the aquaeductus Fallopii and 

 pyramid. The tendon issues from the apex of the pyramid, 

 and is inserted in the posterior part of the neck of the stapes. 



The mastoid antrum is a large recess placed behind and 

 rather above the tympanum, with which it is connected by 

 the large opening (aditus) before mentioned on the upper part 

 of the posterior tympanic wall ; it is developed with the 

 tympanum and lined by a continuation of its mucous mem- 

 brane. Into it open the mastoid pneumatic cells. 



THE INTERNAL EAR OR LABYRINTH. 



Within the internal ear are the terminal ramifications oi 

 the auditory nerve. The internal ear or labyrinth is divided 

 into osseous and membranous parts, the former enclosing the 

 latter. Within the membranous labyrinth is a fluid, the 

 endolymph ; arid outside, between the membranous and osseous 

 labyrinths, is a fluid, theperilymph or liquor Cotunnii. 



The OSSEOUS LABYRINTH consists of the vestibule, the 

 cochlea, and the semicircular canals. 



The vestibule is the central part of the labyrinth. Its 

 outer wall corresponds to the inner wall of the tympanum, 

 and in it is thefenestra ovalis, closed by the base of the stapes ; 

 on its inner wall is a depression, the fovea hemispherica, per- 

 forated by several holes for the branches of the auditory 

 nerve; behind this is a ridge, the crista vestibuli ; and still 

 further back is the internal opening of the aqueduct of the 

 vestibule. On the roof is a depression, the fovea hemi-elliptica. 

 At the posterior part are the five openings of the semicircular 

 canals, and at the anterior part is the apertura scales vestibuli. 



The semicircular canals are three arched osseous canals 

 placed above and behind the vestibule, opening into that 

 chamber by five rounded apertures, two adjacent canals 

 having a common opening. Each canal forms about two- 

 thirds of a circle, and presents at one end a dilated part, the 

 ampulla. Two of the canals are vertical, and the third is 

 horizontal. 



The siip erior canal is vertical, and forms a projection on the 

 upper surface of the petrous part of the temporal bone. The 

 inner extremity joins the opening of the posterior canal. 



The posterior canal is vertical, its upper end being joined to 

 the lower opening of the superior canal. 



The horizontal or external canal is the smallest of the three, 

 and lies horizontally just above the level of the fenestra 

 ovalis. 



The cochlea is cone-shaped, with the base turned to the 



