canals of the inner ear of vertebrate animals and man that help regulate equilibrium and body orientation. Fish, terrestrial vertebrates, and man have three semicircular canals arranged in three mutually perpendicular planes. There is a horizontal (in man, lateral) semicircular canal and two vertical ones—the anterior and the posterior. In fossil Agnatha and extant cyclostomes and petromyzons there are only two vertical semicircular canals, anterior and posterior. In Myxini there is one posterior canal.

The membranous labyrinth of the ear is filled with fluid endolymph and is located within osseous sheaths; the space between the membranous and the osseous labyrinth is filled with perilymph. Each semicircular canal ends in two branches, one of which is dilated to form an ampulla. The two adjacent branches of the vertical semicircular canals are united into a single common branch in most vertebrates; the semicircular canals open into the vestibule by five openings.

Each ampulla of the semicircular canals has sensory hair cells, grouped in a crista, or crest. Over the crista is a gelatinous cupula, into which these hair cells enter. The bases of the sensory cells are entwined with fibers of the vestibular nerve, which transmit excitation to the brain. Changes in the position of the head or body, accompanied by shifting of the cupulae and endolymph, and by displacement of the otoliths suspended in the endolymph, cause stimulation of the hairs and of the sensory cells of the semicircular canals. This leads to the transmission of an electrical discharge along the nerve and into the brain. From the brain the nerve impulses are directed to the muscles and the other organs that regulate the body’s orientation.