organ which is part of the ear, responsible for maintenance of equilibrium
and sensations of motion
. The three semicircular canals are oriented in three perpendicular planes (XZ, XY and YZ, if you will). As a result, motion in any direction can be detected by breaking it down into these three components.
Each canal is full of a fluid called the endolymph. Motion causes movement of the canal with respect to the liquid (due to the inertia of the liquid). This liquid moves against a gelatinous projection known as the cupola. At the base of the cupola are hair cells which sense mechanical motion and convert the information into a nerve impulse.
Below the semicircular canals is a bony chamber containing more hair cells and mineralized particles called otoliths. The otoliths are allowed to roll around freely, allowing the organism to detect the direction of gravity. This in combination with proprioception and the semicircular canals, is integrated by the brain to give us our sense of equilibrium.