At the base of the cochlea, each "duct" ends in a membranous portal that faces the middle ear cavity: The "vestibular duct" ends at the oval window, where the footplate of the stapes sits. The footplate vibrates when the pressure is transmitted via the ossicular chain. The wave in the perilymph moves away from the footplate and towards the helicotrema. Since those fluid waves move the cochlear partition that separates the ducts up and down, the waves have a corresponding symmetric part in perilymph of the "tympanic duct", which ends at the round window, bulging out when the oval window bulges in.