This is a cone-shaped cavity lying with its base upon the internal auditory meatus, and the apex directed outwards. It measures about five millimeters in length, and the diameter of its base is about the same. The center of this cavity is occupied by a column of bone - the modiolus - around which a delicate bony lamella appears to be wound. This lamella is the osseous spiral lamina, which gives attachment to the structures which form collectively the membranous cochlea. The lamina makes two and a half turns in all. The first turn is the largest, and forms a bulging, the promontory, on the inner wall of the tympanum . The lamina terminates at the apex of the cochlea in a hooldike process - the hamulus.
The modiolus is traversed by a central canal, and presents many canaliculi for the transmission of the twigs of the cochlear division of the auditory nerve. There is also a canal which winds round the modiolus at the base of the spiral lamina, known as the spiral canal of the modiolus.
The portion of the cochlea above the lamina is the scala vestibuli ; the part below, that is, on the basal aspect of the lamina, is the scala tympani ; it opens into the tympanum by way of the fenestra rotunda. Near the commencement of the scala tympani, and close to the fenestra rotunda, is the cochlear orifice of the aqueductus cochleae (ductus perilymphaticus). In the adult this opens on the inferior surface of the petrosal near the apex, and transmits a small vein from the cochlea to the jugular fossa.