530 ^STHESIOLOGY. 



vestibule, which extends to the posterior surface of the petrosal 

 bone, and gives passage to a diverticulum of the membranous 

 labyrinth. At the lower and anterior part of the vestibule is the 

 large opening, apertura scales vestibuli, or vestibular canal of the 

 cochlea. 



The SEMICIRCULAR CANALS are three half-circular passages 

 communicating with the vestibule, into which they open by both 

 extremities, and having, near one extremity of each, a dilatation 

 termed the ampulla. These canals are distinguished as superior 

 or perpendicular, posterior or oblique, and inferior or hori- 

 zontal. The anterior or dilated end of the superior canal enters 

 the vestibule by a distinct orifice, while its posterior or non- 

 dilated end joins the anterior non-dilated end of the oblique ; 

 thus the two enter the vestibule by one common opening, the 

 posterior end of the oblique canal being dilated, and entering by 

 a distinct opening. The inferior canal has two openings, of 

 which the anterior is dilated. 



The COCHLEA, or SNAIL SHELL, is the anterior division of the 

 labyrinth, and the most complex of the three. It consists of a 

 gradually tapering spiral canal, which takes two and a-half turns 

 round a central axis. This axis, termed the modiolus, is widest 

 at its base, where it is pierced by numerous small openings, 

 through which pass the filaments of the cochlear branch of the 

 auditory nerve. The spiral canal is divided into two compart- 

 ments (scalce} by a partition, partly bony and partly membranous, 

 the lamina spiralis, which is wound spirally around the modio- 

 lus in the direction of the canal, and terminates, at the apex of 

 the cochlea, by a hook-shaped process, the hamulus. The two 

 compartments are thus completely separated, except at the apex, 

 where they communicate by means of an opening, the helicotrema, 

 under the hamulws ; inferiorly, one scala terminates in the vesti- 

 bule, it is hence called the scala vestibuli; the other leads to 

 the tympanum at the feriestra rotunda, and is called the scala 

 tympani. 



The whole of the bony labyrinth is lined by a thin fibro-serous 

 membrane, which closes the openings of both the fenestra rotunda 

 and fenestra ovalis, and, adhering to the bone, acts the part of a 

 periosteum. The inner surface of this membrane is smooth, and 

 secretes a thin limpid fluid, the perilymph or liquor Cotunnii, 

 which, occupying the cochlea, the canals and vestibule, separates 

 the bony from the membranous labyrinth. 



