The structure that separates the cochlear duct from the tympanic duct is the

the basilar membrane

Explanation:

The tympanic duct or scala tympani is one of the perilymph-filled cavities in the inner ear of humans. It is separated from the cochlear duct by the basilar membrane, and it extends from the round window to the helicotrema, where it continues as vestibular duct.

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