SENSE OF HEARING. 607 



tween the two fenestra} is the promontory, a prominence caused by 

 the projection of the first turn of the cochlea. 



Internal Ear (Figs. 386, 387). This is called also the labyrinth. 

 It consists of a bony part, the osseous labyrinth, in which is con- 

 tained the membranous labyrinth. 



Osseous Labyrinth. This is made up of three parts : The 

 vestibule, the semicircular canals, and the cochlea, all of which 

 are connecting cavities in the petrous bone. These cavities not 

 only communicate with one another, but through the fenestra 

 ovalis and fenestra rotunda the osseous labyrinth is in communica- 

 tion with the tympanum ; and through the meatus auditorius 

 internus with the cranial cavity. 



Vestibule. This cavity is at the inner side of the tympanum, 

 and opens anteriorly into the cochlea and posteriorly into the 

 semicircular canals. It is about 5 mm. in diameter, and on its 

 outer wall communicates with the tympanum through the fenestra 

 ovalis, which is closed by the stapes and its annular ligament. 

 The fovea or fossa hemispherica is a depression on the anterior 

 wall. This is perforated anteriorly, forming the macula cribrosa, 

 and through these perforations the auditory nerves pass to the 

 saccule. Behind the fovea hemispherica is a vertical ridge, the 

 crista vestibuli. The aquceductus vestibuli communicates with the 

 vestibule by an opening at the posterior part of the inner wall. 

 Through this canal pass a vein and the ductus endolymphaticus. 

 The fovea or fossa hemi-elliptica is an oval depression on the roof of 

 the vestibule. Between it and the fovea hemispherica is the crista 

 vestibuli. Posteriorly the semicircular canals open into the vesti- 

 bule, while the opening into the cochlea, apertura scalce vestibuli 

 cochlece, is situated anteriorly. 



Semicircular Canals. These are three in number, situated 

 behind the vestibule and above it. Each has a diameter of about 

 1.5 mm., except at one end, the ampulla, where the diameter 

 is 2.5 mm., and each canal is so arranged as to be at right angles 

 to the others. The superior is vertical and at right angles to 

 the posterior surface of the petrous bone ; its ampullated extremity 

 opens into the vestibule, while its other extremity joins with the 

 non-ampullated extremity of the posterior canal, and these open 

 into the vestibule by one common opening. The posterior canal is 

 also vertical, but parallel with the posterior suriace of the petrous 

 bone. Its ampullated extremity opens into the vestibule. The 

 external canal is horizontal, and both its extremities open into the 

 vestibule. It will be seen that the three canals have but five 

 openings into the vestibule, one opening being common to two 

 canals. 



Cochlea (Fig. 388). The cochlea is situated in front of the 

 vestibule, with its apex directed outward, forward, and downward, 

 its base corresponding to the internal auditory meatus. It is 5 



