Cochlear duct
Ductus cochlearis
- Latin synonym: Labyrinthus cochlearis
- Synonym: Cochlear labyrinth
- Eponym: Reissner’s canal
Definition
The cochlear duct (ductus cochlearis; membranous cochlea; scala media) consists of a spirally arranged tube enclosed in the bony canal of the cochlea and lying along its outer wall.
The osseous spiral lamina extends only part of the distance between the modiolus and the outer wall of the cochlea, while the basal lamina stretches from its free edge to the outer wall of the cochlea, and completes the roof of the scala tympani. A second and more delicate membrane, the vestibular membrane (Reissneri) extends from the thickened periosteum covering the osseous spiral lamina to the outer wall of the cochlea, where it is attached at some little distance above the outer edge of the basal lamina. A canal is thus shut off between the scala tympani below and the scala vestibuli above; this is the ductus cochlearis or scala media.
It is triangular on transverse section, its roof being formed by the vestibular membrane, its outer wall by the periosteum lining the bony canal, and its floor by the membrana basilaris and the outer part of the lamina spiralis ossea.
Its extremities are closed; the upper is termed the lagena and is attached to the cupula at the upper part of the helicotrema; the lower is lodged in the recessus cochlearis of the vestibule. Near the lower end the ductus cochlearis is brought into continuity with the saccule by a narrow, short canal, the canalis reuniens of Hensen.
On the membrana basilaris is situated the spiral organ of Corti.
The vestibular membrane is thin and homogeneous, and is covered on its upper and under surfaces by a layer of epithelium.
The periosteum, forming the outer wall of the ductus cochlearis, is greatly thickened and altered in character, and is called the spiral ligament. It projects inward below as a triangular prominence, the basal crest, which gives attachment to the outer edge of the basal lamina; immediately above the crest is a concavity, the spiralis externus. The upper portion of the spiral ligament contains numerous capillary loops and small bloodvessels, and is termed the stria vascularis.
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