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PHONETICS: Snail-like shaped organ in the inner ear. The cochlea is divided into three regions along most of its length by a membranous structure called the cochlear partition. The interior of the partition, the scala media, is filled with fluid. The scala vestibuli lies on one side of the partition, and the scala tympani on the other. The helicotrema, an opening in the partition at the far, or apical, end of the cochlea, allows fluid to pass freely between the two cavities. The oval window, an opening between scala vestibuli and the middle ear, lies at the end of the cochlea nearest the middle ear, the basal end. The oval window is covered by the footplate at the stapes in the middle ear. The round window, a membrane-covered opening between the scala tympani and the middle ear also lies at the basal end of the cochlea. The cochlear structure is excited through the oval window by motions of the stapes footplate. When the window moves inward, fluid is displaced toward the apical end of the cochlea. If the motion is slow, fluid passes through the helicotrema and back along the other side to the basal end of the cochlea, where the round window moves outward to accommodate the flow.

Anatomy of the Auditory System