THE EAR. 



277 



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

 nucleus of bone. 



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

 stapes. 



Blood-vessels. 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 

 ty7npanic 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 2'analion. 



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 divi- 

 sions : the vestibule, the cochlea, 

 and the semicircular canals, and 

 each of these .contains a division 

 of the membranous labyrinth. 



The Vestibule. This is the 

 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 fi^ 

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

 is the fenestra ovalis, closed by the base of the stapes. On its inner 

 wall in front there is a depression — the fovea hemispherica—])\aced 

 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 vesti- 

 buli. The roof of the vestibule shows another depression— the fovea 

 hemi-elliptica. 



Fig. 38. 



diaoram of the membranous labyrinth. 



DC. Ductus cochlearis ; dr. Ductus reiiniens ; 

 S. Saeculus; U. [Jtriculus ; dv. Ductus Testi- 

 buli ; so. Semicircular canals. (Turner after 

 Waldeyer.) 



