Thus, if the head moves forward, the fluid in the two canals that are parallel to that motion sloshes to the back, a motion that is registered by sensitive hairs similar to those used in the ear for detecting sound waves. The otoliths in the appropriate sacs also bounce back against similar hairs. The brain immediately orients the rest of the body with respect to the head. When the body is at rest, gravity takes over from inertia. The otoliths fall to the bottom of the appropriate sacs and lie on and bend the hairs at the base.
Slight motions sway the hairs at the end of the canals to which fluid has flowed.
The most common problem of balance is motion illness, which includes seasickness . Motion illness is caused by the shaking of the balance labyrinth or by contradictions between the visual balance system and the labyrinth system. Astronauts have the same problems as sailors, but for a different reason. In the free-fall conditions of space, gravitational force is almost nonexistent, but inertia is unchanged. Some diseases directly affect the semicircular canals, causing all the effects of motion illness even when movement is normal, as in changing position.