366 HUMAN ANATOMY. 



ing a membranous sac filled also with fluid, the endolymph, which 

 receives the distributions of the auditory nerve. 

 Its divisions are : 



Vestibule, 



Osseous portion, -{ Semicircular canals, 



Cochlea ; 



Membranous portion, - 



" Utricle, 

 Saccule, 



Semicircular canals, 

 Cochlea. 



VESTIBULE, is a common cavity of communication between 

 the bony parts of the internal ear. It is situated between the 

 tympanum without, the cochlea behind, and the semicircular 

 canals in front, and measures one-fifth of an inch in its antero- 

 posterior and vertical diameters, and less from without inward. 

 It presents: 



Fenestra ovalis on its outer wall, closed by the stapes and 

 its ligament; 



Fovea liemispli erica, a circular depression on its inner wall 

 for the saccule, and perforated by the macula cribrosa, for the 

 vestibular filaments of the auditory nerve ; 



Orifice of the aquwductus vestibulce, on the inner wall for 

 the transmission of a small vein; 



Eminencia pyramidalis , a vertical ridge on the inner wall 

 separating the two fora; 



Fovea semi-elli plica, in the roof lodges the utricle. 



SEMICIRCULAR CANALS are three canals, forming each two- 

 thirds of a circle one-twentieth of an inch in diameter, and 

 named, from their position, the superior, posterior, and external. 

 They are placed nearly at right angles to each other, and open 

 into the vestibule by five apertures two extremities uniting to 

 form one. 



The superior and posterior are both vertical, the former 

 being more anterior. 



The external is placed horizontally, its arch directed back- 

 ward. 



Ampullce, are the dilated, flask-shaped extremities of the 

 tubes, and are about one-tenth of an inch in diameter. 



THE COCHLEA resembles closely a common snail's shell, 

 placed with the base corresponding to the bottom of the meatus 

 auditorius internus, and its apex directed outward and forward. 

 It consists of two parallel tubes one and one-half inches in length, 

 one-tenth of an inch in diameter, wound spirally for two and one- 

 half turns around a central pillar the modiolus. 



