Cochlea

The cochlea is the shell-shaped part of the bony labyrinth that contains the cochlear duct, the part of the internal ear concerned with hearing. The spiral canal of the cochlea begins at the vestibule and makes 2.5 turns around a bony core, the modiolus, the cone-shaped core of spongy bone about which the spiral canal of the cochlea turns. The modiolus contains canals for blood vessels and for distribution of the branches of the cochlear nerve. The apex of the cone-shaped modiolus, like the axis of the tympanic membrane, is directed laterally, anteriorly, and inferiorly. The large basal turn of the cochlea produces the promontory of the labyrinthine wall of the tympanic cavity. At the basal turn, the bony labyrinth communicates with the subarachnoid space superior to the jugular foramen through the cochlear aqueduct. It also features the round window (Latin fenestra cochleae), closed by the secondary tympanic membrane.