Spiral canal of cochlea

The spiral canal of cochlea is the bony canal of the cochlea and takes two turns and three-quarters around the modiolus.

It is about 30 mm. 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 3 mm. in diameter; it diverges from the modiolus toward the tympanic cavity and vestibule, and presents three openings :

One, the fenestra cochleæ, communicates with the tympanic cavity—in the fresh state this aperture is closed by the secondary tympanic membrane; another, of an elliptical form, opens into the vestibule. The third is the aperture of the aquæductus cochleæ, leading to a minute funnel-shaped canal, which opens on the inferior surface of the petrous part of the temporal bone and transmits a small vein, and also forms a communication between the subarachnoid cavity and the scala tympani.

The inner wall of spiral canal of cochlea is formed by the central axis, wound spirally around it for two turns and three-quarters, from the base to the apex; and of a delicate lamina, the osseous spiral lamina, which projects from the modiolus, and, following the windings of the canal, partially subdivides it into two. In the recent state a membrane, the basal lamina stretches from the free border of this lamina to the outer wall of the bony cochlea and completely separates the canal into two passages, which, however, communicate with each other at the apex of the modiolus by a small opening named the helicotrema.