THE LABYRINTH. 



6 75 



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. 198.* 



* Fig. 198. Right bony labyrinth, viewed from the outer side (after 

 Sb'mmerring). |i 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. 199. View of the interior of the left labyrinth (from Sommer- 

 ring). |i 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. 



