Chap. XLI.] THE INTERNAL EAR. 321 



fenestra ovalis is, in the fresh condition, filled out by a 

 membrane, in which the basis of the stapes is fixed, 

 the circumference of this being nearly as great as that 

 of the fenestra. 



460. The osseous labyrinth in all parts consists of 

 ordinary osseous substance, with the usual periosteum 

 lining its outer surface and its inner cavities. These 

 cavities contain the albuminous fluid called perilymph. 

 But they are not filled out by this, since, in each of 

 the two divisions of the vestibule, in each of the 

 semicircular canals, and in the cochlea, is a mem- 

 branous structure, analogous in shape to the corre- 

 sponding division of the labyrinth. These membranous 

 structures possess a cavity filled with the same albu- 

 minous fluid as above, called the endolymph. These 

 structures are disposed thus in the fovea hemispherica 

 is a spherical sac, called the saccule ; in the fovea 

 hemielliptica is an elliptical sac, the utricle; in each of 

 the three semicircular canals is a membranous semi- 

 circular tube, which possesses also an ampulla corre- 

 sponding to the ampulla of the bony canal. 



461. In the cochlea is a membranous canal, tri- 

 angular in cross-section the scala media or cochlear 

 duct which also twists two and half times from the 

 basis to the apex. of the cochlea, and is placed against 

 the end of the lamina spiralis ossea so as to occupy a 

 position between the peripheral part of the scala vesti- 

 buli and scala tympani. 



462. The different divisions of the membranous 

 labyrinth are connected with one another in this 

 manner : the three semicircular (membranous) canals 

 open into the utricle ; this does not form a direct con- 

 tinuity with the saccule, but a narrow canal comes off 

 both from the saccule and utricle ; the two canals 

 join into one minute membranous tube situated in the 

 aqueductus vestibuli. At its distal end it enlarges 

 into the saccus endolymphaticus, situated in a cleft of 



T 1 



