<P> It is directed superiorly, as per its nomenclature, and posteriorly, nearly parallel to the posterior surface of the petrous bone . The vestibular aqueduct is immediately medial to it . The posterior canal is part of the bony labyrinth and is used by the vestibular system to detect rotations of the head in the coronal plane . It is the longest of the three canals, measuring from 18 to 22 mm . Its lower or ampullated end opens into the lower and back part of the vestibule, its upper into the crus commune . </P> <P> Findings from a 2009 study demonstrated a critical late role for BMP 2b in the morphogenesis of semicircular canals in the zebrafish inner ear . It is suspected that the role of bmp2 in semicircular canal duct outgrowth is likely to be conserved between different vertebrate species . </P> <P> Each canal is filled with a fluid called endolymph and contains motion sensors within the fluids . At the base of each canal, the bony region of the canal is enlarged which opens into the utricle and has a dilated sac at one end called the osseous ampullae . Within the ampulla is a mound of hair cells and supporting cells called crista ampullaris . These hair cells have many cytoplasmic projections on the apical surface called stereocilia which are embedded in a gelatinous structure called the cupula . As the head rotates the duct moves but the endolymph lags behind owing to inertia . This deflects the cupula and bends the stereocilia within . The bending of these stereocilia alters an electric signal that is transmitted to the brain . Within approximately 10 seconds of achieving constant motion, the endolymph catches up with the movement of the duct and the cupula is no longer affected, stopping the sensation of acceleration . The specific gravity of the cupula is comparable to that of the surrounding endolymph . Consequently, the cupula is not displaced by gravity, unlike the otolithic membranes of the utricle and saccule . As with macular hair cells, hair cells of the crista ampullaris will depolarise when the stereocilia deflect towards the kinocilium . Deflection in the opposite direction results in hyperpolarisation and inhibition . In the horizontal canal, ampullopetal flow is necessary for hair - cell stimulation, whereas ampullofugal flow is necessary in the anterior and posterior canals . </P> <P> This adjustment period is in part the cause of an illusion known as "the leans" often experienced by pilots . As a pilot enters a turn, hair cells in the semicircular canals are stimulated, telling the brain that the aircraft, and the pilot, are no longer moving in a straight line but rather making a banked turn . If the pilot were to sustain a constant rate turn, the endolymph would eventually catch up with the ducts and cease to deflect the cupula . The pilot would no longer feel as if the aircraft was in a turn . As the pilot exits the turn, the semicircular canals are stimulated to make the pilot think that they are now turning in the opposite direction rather than flying straight and level . In response to this, the pilot will often lean in the direction of the original turn in an attempt to compensate for this illusion . A more serious form of this is called a graveyard spiral . Rather than the pilot leaning in the direction of the original turn, they may actually reenter the turn . As the endolymph stabilizes, the semicircular canals stop registering the gradual turn and the aircraft slowly loses altitude until impacting with the ground . </P>

The function of the semicircular canals​ is
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