Behind the vestibule is the third portion of the bony labyrinth known as the semicircular canals. The canals project from the posterior region of the vestibule and are responsible for the detection of head motion in three spatial planes. The anterior duct sense forward and backward motion. The posterior duct detects moving up and down. The lateral duct senses moving left to right. Each canal contains a membranous semicircular duct where angular momentum is sensed. At the base of each duct is an expansion called the ampulla. Within the ampulla, long stereocilia of hair cells are embedded in the cupula, which sticks out into the endolymph. When your head moves, the endolymph moves the cupula and stimulates the stereocilia.
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