semicircular canals
a set of three looped tubular channels in the inner ear that detect movements of the head and provide the sense of dynamic equilibrium that is essential for maintaining balance. They form part of the vestibular apparatus. The channels are filled with fluid (endolymph) and are oriented roughly at right angles to each other. Hence, they can monitor movements in each of three different planes. Each canal has an enlarged portion, the ampulla, inside which is a sensory structure called a crista. This consists of hair cells whose fine extensions are embedded in a gelatinous cap (the cupula). When the head moves in a certain plane, endolymph flows through the corresponding canal, displacing the cupula and causing the hairs to bend. This triggers the hair cells to fire nerve impulses, thus sending messages to the brain about the direction and rate of movement.