THE SENSES. 5T1 



paratus. It presents, in its inner wall, several openings for the entrance 

 of the divisions of the auditory nerve; in its outer wall, the fenestra 

 ovalis (2, Fig. 384), an opening filled by the base of the stapes, one of 

 the small bones of the ear; in its posterior and superior walls, five open- 

 ings by which the semicircular canals communicate with it: in its ante- 

 rior wall, an opening leading into the cochlea. The hinder part of the 

 inner wall of the vestibule also presents an opening, the orifice of the 

 ciqu&ductus vestibuli, a canal leading to the posterior margin of the 

 petrous bone, with uncertain contents and unknown purpose. 



The semicircular canals (Figs. 384, 385), are three arched cylin- 

 driform bony canals, set in the substance of the petrous bone. They all 

 open at both ends into the vestibule (two of them first coalescing). The 

 ends of each are dilated just before opening into the vestibule; and one 

 end of each being more dilated than the other is called an ampulla. 

 Two of the canals form nearly vertical arches; of these the superior is 

 also anterior; the posterior is inferior; the third canal is horizontal, and 

 lower and shorter than the others. 



FIG. 384. FIG. 385. 



FIG. 384. Right bony labyrinth, viewed from the outer side. The specimen here represented is 

 prepared by separating piecemeal the looser substance of the petrous bone from the dense walls 

 which immediately inclose the labyrinth. 1, the vestibule; 2, fenestra ovalis; 3, superior semicir- 

 cular 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 fig- 

 ure in outline below shows the natural size. 2^/1. (Sommering. ) 



FIG. 385. View of the interior of the left labyrinth. The bony wall of the labyrinth is removed 

 superiorly and externally. 1, 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. 2>^/l. (Summering/) 



The cochlea (6. 7, 8, Figs. 384 and 385), a small organ, shaped like 

 a common snail-shell, is seated in front of the vestibule, its base resting 

 on the bottom of the internal meatus, where some apertures transmit to 

 it the cochlear filaments of the auditory nerve. In its axis, the cochlea 

 is traversed by a conical column, the modiolus, around which a spiral 

 canal winds with about two turns and a half from the base to the apex. 

 At the apex of the cochlea the canal is closed; at the base it presents 

 three openings, of which one, already mentioned, communicates with the 

 vestibule; another called fenestra rotunda, is separated by a mem- 

 brane from the cavity of the tympanum; the third is the orifice of the 



