SENSE OF HEARING. 



881 



the vestibule, and, communicating with this, three semicircular canals and 

 the cochlea (Figs. 403 and 404). Lining the internal ear and forming a 

 complete cast of the vestibule and semicircular canals is the so-called 

 membranous labyrinth, while between the walls of the membranous 



\ 



FIG. 404. SCHEME OF THE AUDITORY APPARATUS. (Beaunis.) 



A, external ear; B, middle ear; C, internal ear; 1, auricle: 2, external auditory canal: 3, tympanic 

 cavity; 4, tympanic membrane: 5, Eustachian tube; 6, mastoid cells; 7, malleus; 8, incus; 9, stapes; 

 10, round window; 11, oval window; 12, vestibule; 13, cochlea; 14, scala tympani; 15, scala vestibuli; 

 16, semicircular canal. 



labyrinth and the semicircular canals and vestibule is a fluid called the 

 perilymph, which is also contained in the cochlea. Within the mem- 

 branous labyrinth is a similar fluid termed the endolymph. In .the 

 interior of the membranous lab}^rinth are 

 often found little particles consisting 

 almost entirely of carbonate of lime, 

 called otoliths, or ear-stones (Fig. 405). 

 Sometimes these are attached to the walls 

 of the membranous labyrinth, and some- 

 times they are found tying loosely and 

 are intended to increase the intensity of 

 the sounds. 



The cochlea consists of a spiral 

 canal making two and one-half revo- 

 lutions about a central axis. It is divided 

 by a spiral lamina, partly membranous and partly bony, into two 

 divisions known as scalae, of which one is above the other. The superior 

 at its inferior extremity terminates in the vestibule and is known as the 



56 



FIG. 405. A, OTOLITHS FROM THE 

 HORSE ; B, EPITHELIUM WITH 

 AUDITORY HAIRS FROM THE 

 CRISTA ACOUSTICA OF THE 

 BABBIT. ( Munk. ) 



