The vestibular system has two main parts: a group of three fluid-filled, semicircular canals, set at right angles to one another; and two vestibular sacs, also filled with fluid. Hair cells line the inside of the canals and sacs. As we move about, the fluid sloshes back and forth, pushing the hair cells to and fro. The movement of the hair cells triggers the release of nerve signals that inform the brain that the head is turning clockwise or counter-clockwise or tilting forward, backward, or to the side. Some of the hair cells are highly sensitive to the pull of gravity. Their signals help tell the brain how far we are from an upright position. From the vestibular system, the signals pass along the vestibular nerve to the cerebellum, at the back of the head. It sorts out messages from the inner ear and coordinates them with information from other parts of the body to tell us from moment to moment where we are in space, and whether we are standing, sitting, lying down, running, walking, or standing on our heads.