THE EAR 235 



internal meatus, nd the apex opposite the canal for the 

 tensor tympani, and consists of a tapering spiral canal of 

 2^ turns, with the inner wall formed by its axis or modiolus : 

 the canal is divided into two scalae by a partition of bone and 

 membrane, the lamina spiralis. The enclosed arched extremity 

 of the cochlea is called the cupola, and the first turn of the 

 canal bulging into the tympanum forms the promontory. 

 The lamina spiralis ossea ends at the apex of the cochlea in a 

 small point, the hamulus, between which and the modiolus is 

 a small opening, the helicotrema, by which the two scalae 

 communicate. 



The modiolus is pierced by small canals for the passage of 

 nerves, and one larger one, the central canal of the modiolus, 

 passes from the base to the last half-turn of the cochlea. At 

 the base of the lamina spiralis is a small canal, the spiral 

 canal of the modiolus, which winds round the axis, and contains 

 a spiral ganglionated cord, the ganglion spirale. 



The scalse are known respectively as the scala tympani 

 and the scala vestibuli. 



The scala tympani is the lower one , it commences at the 

 fenestra rotunda. 



The scala vestibuli commences at the cavity of the vestibule, 

 and communicates at the apex of the modiolus with the scala 

 tympani, by the helicotrema. 



The MEMBRANOUS LABYRINTH consists of sacs containing 

 fluid (endolymph). The ramifications of the auditory nerve 

 are distributed on the wall. These membranous sacs com- 

 plete the septum between the scala tympani and the scala 

 vestibuli, besides enclosing a third space, the scala media or 

 membranous canal of the cochlea. 



The membranous vestibule consists of two sacs, the utricle 

 and the saccule. 



The utricle or common sinus is larger than the saccule, 

 and is situated in the posterior and upper part of the vestibule, 

 being contained in the fovea hemi-elliptica. The apertures 

 of the membranous semicircular canals open in the posterior 

 part. At the anterior part is a thickened spot, the macula 

 acustica, where the nerves enter. The interior contains, 

 opposite the macula, attached to the wall, a small mass of 

 calcareous grains, known as otoliths, otoconia, or ear sand. 

 Below there is a canal, which, joined to a similar tube from 

 the saccule, extends along the aqueduct of the vestibule, and 

 ends in a dilated pouch, the saccus endo-lymphaticus. 



The saccule is smaller and rounder than the utricle, and 

 lies in the fovea hemispherica. Like the utricle, it contains 



