The sense organ in vertebrates that is concerned with the maintenance of physical equilibrium (sense of balance). It occurs in the inner ear and consists of three looped canals set at right angles to each other and attached to the utriculus. The canals contain a fluid (endolymph) that flows in response to movements of the head and body. A swelling (ampulla) at one attachment point of each canal contains sensory cells that respond to movement of the endolymph in any of the three planes. These sensory cells initiate nervous impulses to the brain.