THE EAB. 269 



the back of the tympanum. Issuing from the pyramid, it is inserted 

 into the neck of the stapes. Its tendon of insertion contains a small 

 nucleus of bone. 



Action. — To regulate (diminish the excursions of) the movements of 

 the stapes. 



Bloodvessels. The arteries of the tympanum are derived from the 

 tympanic artery, a branch of the internal maxillary artery. 



Nerves. The chorda tympani branch of the 7th nerve enters the 

 cavity of the tympanum from the aqueduct of Fallopius; and passing 

 across the membrana tympani it leaves the cavity by the styloid fora- 

 men. The sensory nerves of the tympanum are derived from the 

 tympanic branch (Jacobson's nerve) of the glosso-pharyngeal. 

 The Nerve to the Stapedius is a branch of the 7th. 

 The Nerve to the Tensor Tympani comes from the 5th, through the 

 otic ganglion. 



THE INTERNAL EAR. 



The Internal Ear, called also, from its complexity, the Labyrinth, 

 consists of a series of chambers, or passages, in the petrous temporal 

 bone, and of certain fluids and soft 

 textures contained within these 

 passages. The chambers, with the 

 wall of condensed bone tissue which 

 immediately surrounds them, con- 

 stitute the osseous labyrinth ; the 

 contained soft structures form the 

 membranous labyrinth. The osseous 

 labyrinth consists of three divisions; 

 — ^the vestibule, the cochlea, and the 

 semicircular canals, and each of these 



contains a division of the mem- Diagram oi' the Membeaiious Laeybinih. 



, DC. Ductus coclilearis ; dr. Ductus reuniens ; 



branOUS labyrinth. S. Saoculus ; U. Utriculus ; dv. Ductus vesti- 



„, -rr rri,;„ Jc +V,q bull; SC. Semicii-cular canals. (Turner, uttei 



The Vestibule. This is the ^„j^,j^er). 

 central division of the labyrinth. It 



lies between the inner wall of the tympanum and the internal audi- 

 tory meatus. In front it communicates with the scala vestibuli of the 

 cochlea, and the semicircular canals open into it behind by five 

 openings. On its outer wall, which separates it from the tym- 

 panum, is the fenestra ovahs, closed by the base of the stapes. On 

 its inner wall in front there is a depression — the fovea, hemispherica — 

 placed over the meatus auditorius internus, and pierced by minute 

 foramina for the passage of the filaments of the auditory nerve. 

 Behind the fovea hemispherica is a small slit which leads into the 

 aqueductus vestibuli. The roof of the vestibule shows another depres- 

 sion the fovea hemi-elliptica. 



