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Other Terms: Cochlear part of bony labyrinth, Cochlée
The cochlea bears resemblance to a snail shell. It is conical in form and placed almost horizontally in front of the vestibule. It forms the anterior part of the labyrinth. Its apex is directed anterior and lateralward, with a slight inclination inferior, toward the superior and anterior part of the labyrinth wall of the tympanic cavity. Its base corresponds with the inferior part of the internal acoustic meatus and is perforated by numerous apertures for the passage of the cochlear division of the acoustic nerve. It measures about five millimeters from base to apex Its breadth across the base is about nine millimeters. It consists of a conical shaped central axis, the modiolus, the bony canal, and the osseous spiral lamina. The basilar membrane stretches from the free border of its lamina to the outer wall of the bony cochlea and completely separates the canal into two passages. These passages communicate with each other at the apex of the modiolus by a small opening named the helicotrema. The modiolus is the conical central axis of the cochlea. Its base is broad, and appears at the bottom of the internal acoustic meatus, where it corresponds with the area cochleae. It is perforated by numerous orifices which transmit filaments of the cochlear division of the acoustic nerve. The bony canal of the cochlea takes two turns and three-quarters around the modiolus. It is about 30 millimeters in length and diminishes gradually in diameter from the base to the summit, where it terminates in the cupula, which forms the apex of the cochlea. The beginning of this canal is about three millimeters in diameter. It diverges from the modiolus toward the tympanic cavity and vestibule, and presents three openings. One of these openings is the fenestra cochleae. The fenestra cochleae communicates with the tympanic cavity. The secondary tympanic membrane opens into the vestibule. The third is the aperture of the cochlear aqueduct. It leads to a minute funnel-shaped can, which opens on the inferior surface of the petrous part of the temporal bone and transmits a small vein. It also forms a communication between the subarachnoid cavity and the scala tympani. The osseous spiral lamina is a bony shelf which projects from the modiolus into the interior of the canal. It takes turns around the modiolus. It reaches about halfway toward the outer wall of the tube and partially divides its cavity into two passages of scalae. The upper passage is named the scala vestibuli. The lower is the scala tympani. Near the summit of the cochlea, the lamina ends in a hook-shaped process. This assists in forming the boundary of the helicotrema, through which the two scalae communicate with each other. From the spiral canal of the modiolus, numerous canals pass outward through the osseous spiral lamina as far as its free edge. The osseous labyrinth houses the cochlea.