w a t e r L E V E L

"The semicircular canals, which help maintain balance, are three fluid-filled tubes at right angles to one another [Cartesian xyz?]. Any movement of the head causes the fluid in the canals to move. Depending on the direction of head movement, the fluid movement may be greater in one of the canals than in another. The canals contain hair cells that respond to movement of the fluid. The hair cells initiate nerve impulses that tell the brain which way the head is moving, so appropriate action can be taken to maintain balance. If the semicircular canals become inflamed, as happens in a middle ear infection or the flu, a person may lose the sense of balance or develop vertigo, a whirling sensation."

-The Merck Manual of Medical Information

top: a look inside the ear; the square frames the internal ear. (Merck Manual) bottom: the internal ear laid open, showing semicircular canals and cochlea (Gray's Anatomy)