THE EAR 



879 



middle meningeal, whilst fine branches leave the carotid artery as it traverses the 

 carotid canal and pass into the tympanic cavity. The ascending pharyngeal sends 

 branches to the Eustachian tube, some of which reach the tympanum. 



Veins. — The veins empty into the superior petrosal sinus and into the tem2)oro- 

 maxillary vein. 



Nerves. — The tympanic mucous membrane is supplied by the tympanic plexus 

 whicli occu})ies the grooves on the ijromontory and inner wall of the cavity. 



The i)lexus is formed l^y the follcjwing nerves. 



The tympanic branch of the glosso-pharyngeal, which enters the cavity 

 through a foramen in the floor; a communicating branch from the carotid plexus 

 of the sympathetic, which passes tlirough the carotid canal. 



A branch from the great superficial petrosal, Avhich enters by the inner wall 

 close in front of the fenestra ovalis; and one from the small superficial petrosal, 

 which enters near the canal for the tensor tympani. 



The course of the chorda tymj^ani nerve has already been sufhciently indicated 

 (page 762). 



THE INTERNAL EAR, OR LABYRINTH 



The INTERNAL EAR, or LABYRINTH, is the essential part of the organ of 

 hearing, and receives the terminations of the auditory nerve. 



The Osseous Labyrinth is divided into three portions: the vestibule, the 

 semicircular canals, and tlie cochlea — recesses holloAved in the substance of the 

 })etro\is portion of the temporal l>one. 



Fig. 500. — The OssEors Labyrinth of the Right Side. 

 (Modified from Soeminening. Enlarged.) 



Superior semicircular canal 



Posterior semicircular canal 

 External semicircular canal 

 Vestibule and fenestra 



Second turn of cochlea 

 Apez of cochlea 



Ampulla 

 Fenestra rotunda 

 Commenceinent of first turn of the cochlea 



The Vestibule lies between the cochlea and semicircular canals, on the inner 

 side of the tymi)anum (tig. 489a). It is a laterally compressed ovoidal cavity, and 

 is about one-fifth of an inch in horizontal and vertical measurement. 



The outer wall i)resents the fenestra ovalis filled with the foot-piece of the 

 stapes and its annular ligament. On its inner wall there is a small saucer-like 

 depression, the fovea hemispherica, perforated ])elow and in front by several 

 minute orifices f macula cribrosa) for the filaments of the auditory nerve. Imme- 

 diately behind this is a vertical ridge, the i^yramidal eminence or crista vestibuli, 

 behind which is the orifice of the aqueductus vestibuli. 



On the roof there is a second dcjjression, oval in slia|)c, the fovea hemiellip- 

 tica. The two depressions are separated by tlie pyramidal eminence. 



Behind, the semicircular canals open by five orifices; and in front by means 

 of a large oval opening, apertura scalae vestibuli, the vestibular cavity com- 

 municates with the scala vestibuli of the cochlea. 



The Three Semicircular Canals are placed above and behind the vestibule. 

 They are unequal in length, but each forms the greater part of a circle, is laterally 



