794 HEARING, SMELL, AND TASTE. 



canals, and cochlea and is situated within the petrous portion of the tem- 

 poral bone. Within the osseous labyrinth is a membranous labyrinth to 

 which the auditory nerve is distributed. The vestibule is an irregular 

 chamber which serves as a common means of communication between the 

 tympanum and the semicircular canals and cochlea. On its external wall 

 is the fenestra ovalis, closed by the base of the stapes. On its internal wall 

 is a depression called the fovea hemispherica, which is perforated by minute 

 openings for the passage of auditory nerve- filaments. Above and posterior 

 to this depression is another, the fovea hemielliptica. Posterior to the fovea 

 hemispherica is the orifice of the aqueductus vestibuli. In the posterior 

 wall are five openings leading to the semicircular canals. Anteriorly, it 

 communicates with the cochlea by the aperturce scales vestibuli cochleae. The 

 semicircular canals are three in number superior, posterior or inferior, and 

 horizontal. They form the greater portion of a circle, and communicate 

 with the vestibule by five openings, one of which is common to the superior 

 and horizontal canals. The superior canal is situated vertically and at 

 right angles with the posterior surface of the petrous bone ; the posterior 

 canal is also vertical and parallel with the posterior surface of the petrous 

 bone ; the inferior canal is placed horizontally and at right angles to the 

 others. At the commencement of each of these canals is a dilated portion, 

 called the ampulla. 



700. The cochlea occupies the anterior portion of the labyrinth. Its 

 base, which corresponds to the internal auditory meatus, is perforated by 

 many minute orifices for the passage of filaments of the cochlear branch of 

 the auditory nerve. The cochlea consists of a central axis, or modiolus, 

 which has a spiral canal wound around it. This canal makes two and a 

 half complete turns, and terminates in the apex of the cochlea in an ex- 

 pansion termed the infundibulum. (Fig. 189.) The modiolus is somewhat 



FIG. 189. 



Section through the Cochlea. (Breschet.) a, axis with its canals; 6, infundibulum or en- 

 larged upper end of the axis ; c, septum of the cochlea; d, membrane of Corti ; e, membrane of 

 Reissner ; /, hiatus or hellicotrema ; st, scala tympani ; sv, scala vestibuli. 



cone-shaped, and forms the internal wall of the canal, being perforated in its 

 centre and sides by apertures for the passage of the filaments of the audi- 

 tory nerve. The canal is divided into two passages or scalce by a septum 

 called the lamina spiralis, which is partly osseous and partly membranous. 

 The osseous portion projects from the modiolus, midway across the canal ; 

 it consists of two laminae, between which the nerve-filaments run. The 

 membranous portion extends from the external margin of the osseous lamina 



