In mammals, otoliths are small particles, composed of a combination of a gelatinous matrix and calcium carbonate in the viscous fluid of the saccule and utricle. The inertia of these small particles causes them to stimulate hair cells when the head moves. The hair cells are made up of 40 to 70 stereocilia and one kinocilium, which is connected to an afferent nerve. When the body changes position or begins a movement, the weight of the membrane bends the stereocilia and stimulates the hair cells. Hair cells send signals down sensory nerve fibers, which are interpreted by the brain as motion. The brain interprets the orientation of the head by comparing the input from the utricles and saccules from both ears to the input from the eyes, allowing the brain to discriminate a tilted head from the movement of the entire body. When the head is in a normal upright position, the otolith presses on the sensory hair cell receptors. This pushes the hair cell processes down and prevents them from moving side to side. However, when the head is tilted, the pull of gravity on otoliths shift the hair cell processes to the side, distorting them and sending a message to the central nervous system that the head is no longer level but now tilted. (see: Benign paroxysmal positional vertigo)