Otolith organ
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Bigotolith.jpg
The otolith organs (the utricle and the saccule) are structures in the inner ear that are sensitive to gravity and linear acceleration. Because of their orientation in the head, the utricle is sensitive to a change in horizontal movement, and the saccule gives information about vertical acceleration (such as when in an elevator).
Otoliths are small particles of 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 send signals down sensory nerve fibres, which are interpreted by the brain as motion.
In 1991, Martin Lenhardt of the University of Virginia discovered that the saccule has an additional function. As published in journal Science, Vol. 253, 5, 1991, 82, Lenhardt demonstrated that the saccule is also a hearing organ for ultrasonic sound.