The vestibular organ of the inner ear helps an animal maintain its posture and keep balanced by monitoring the movement and position of the head. It consists of two structures - the semicircular canals and the otolith organs.
The semicircular canals (see diagram 15.10) respond to movement of the body. They tell an animal whether it is moving up or down, left or right. They consist of three canals set in three different planes at right angles to each other so that movement in any direction can be registered. The canals contain fluid and sense cells with fine hairs that project into the fluid. When the head moves the fluid swirls in the canals and stimulates the hair cells. These send nerve impulses along the vestibular nerveto the cerebellum.
Note that the semicircular canals register acceleration and deceleration as well as changes in direction but do not respond to movement that is at a constant speed.
The otolith organs are sometimes known as gravity receptors. They tell you if your head is tilted or if you are standing on your head. They consist of bulges at the base of the semi circular canals that contain hair cells that are covered by a mass of jelly containing tiny pieces of chalk called otoliths (see diagram 15.10). When the head is tilted, or moved suddenly, the otoliths pull on the hair cells, which produce a nerve impulse. This travels down the vestibular nerve to the cerebellum. By coordinating the nerve impulses from the semicircular canals and otolith organs the cerebellum helps the animal keep its balance.
Diagram 15.10 - Otolith Organs