This is the name given to a conoid cavity which is separated from the semicircular canals by the vestibule, with which it communicates, and terminates at the fenestra rotunda. The cavity of the cochlea is a spiral, describing two turns and a half round its columella or axis; it is divided transversely into two portions by a partitionóthe lamina spiralis, throughout its entire length That portion opening into the vestibule is called the scala vestibuli; and that opening into the fenestra rotunda the scala tympanióby which it would, if it were not for the membrane which closes it, communicate with the cavity of the tympanum.

The lamina spiralis is divided lengthwise into a bony portion, which corresponds at its internal border to the axis; and a membranous portion, which attaches the osseous portion to the external wall of the cochlea. This wall is formed by the spiral plate. The cochlea is lined by a fibro-mucous membrane, which appears to be a continuation of the periosteum of the other two cavities of the labyrinth; the membranous portion of the spiral plate may be considered as a prolongation of the membranous labyrinth. Lastly, the vascular canal, called the canal of the cochlea, analogous to that of the vestibule, communicates also with the cavity of the skull. The base of the cochlea rests on the bottom of the internal auditory canal, by which the auditory nerve enters the organ of hearing.