6;6 



THE SENSE OF HEARING. 



tympani muscles. The stapes is moveable on the process 

 of the incus, when the stapedius muscle acting draws it 

 backwards. 



The proper organ of hearing is formed by the distribu- 

 tion of the auditory nerve within the internal ear, or 

 labyrinth of the ear, a set of cavities within the petrous 

 portion of the temporal bone. The bone which forms the 

 walls of these cavities is denser than that around it, and 



Fig. I 91.* Fig. 192^ 



* Fig. 191. Eight bony labyrinth, viewed from the outer side (after 

 Somm erring), y. The specimen here represented is prepared by sepa- 

 rating piecemeal the looser substance of the petrous bone from the dense 

 walls which immediately enclose the labyrinth. I, the vestibule; 2, 

 fenestra ovalis ; 3, superior semicircular canal ; 4, horizontal or external 

 canal ; 5, posterior canal ; *, ampullae of the semicircular canals ; 6, first 

 turn of the cochlea ; 7, second turn ; 8, apex ; 9, fenestra rotunda. The 

 smaller figure in outline below shows the natural size. 



f Fig. 192. View of the interior of the left labyrinth (from Sommer- 

 ring. 2 ^ . The bony wall of the labyrinth is removed superiorly and 

 externally. I, fovea hemielliptica ; 2, fovea hemispherica ; 3, common 

 opening of the superior and posterior semicircular canals ; 4, opening 

 of the aqueduct of the vestibule ; 5, the superior, 6, the posterior, and 

 7, the external semicircular canals ; 8, spiral tube of the cochlea (scala 

 tympani) ; 9, opening of the aqueduct of the cochlea ; 10, placed on the 

 lamina spiralis in the scala vestibuli. 



