Tom DeMello, age 13, of Greenville, Miss., for his question:

HOW DOES THE EAR GIVE US BALANCE?

We are able to keep our balance because of three fluid filled semicircular canals which are located in our inner ears. These canals enable us to recognize the direction in which our bodies are moving: forward or backward, sideways or up and down.

This balance enables us to ride bikes, sit, stand, walk or run.

At one end of each canal in the inner ear is a sense organ called the crista. Each crista consists of hairs covered by a jellylike cupule (or cuplike structure). Nerves connect the crista to the brain.

One of the semicircular canals is horizontal and the other two are in a vertical position. When the head or body moves up or down, forward or backward, or from side to side, the fluid in the canals moves, affecting the delicate hair cells. These hair cells are supplied with fine fibers from the vestibular nerve.

The brain immediately interprets the impulses as charges in the body's position. Then the brain sends "messages" to the various muscles of the neck, trunk and limbs that maintain the balance of the body.

If the semicircular canals are destroyed, the body loses its ability to maintain balance, or equilibrium. Dizziness and trouble in balancing occur when the semicircular canals become diseased. This happens because the brain has lost an important source of information and cannot direct the muscles that keep the head and other parts of the body in balance.

Seasickness and the dizziness produced by swinging or turning around rapidly result from excessive stimulation of the semicircular canals in one direction.

The inner ear has so many intricate chambers and passageways that it is called a labyrinth. The inner ear is formed partly within the temporal bone of the skull and it has three connected parts: the vestibule, the semicircular canals and the cochlea. While the semicircular canals give us our sense of balance, the cochlea contains the sense organ for hearing.

The semicircular canals are called the superior, posterior and lateral semicircular canals. Tilting the head affects the superior semicircular canal, shaking the head affects the posterior semicircular canal and twisting the head affects the lateral canal.

A sound wave is a vibration in the air. The structure enables sound vibrations to pass from outside the head to the part of the brain that controls hearing.

When the sound waves move through the perilymph, they move the fibers of the basilar membrane, which lies within the cochlea. These fibers cause a vibration of the hair cells, or specialized sense cells, that make up the organ of Corti. There are about 24,000 of these fibers.

The vibrations of the hair cells stimulate nerves attached to them. These nerves send messages through the auditory nerve to the temporal lobe, the center of hearing in the brain.

The sounds are classified and interpreted in the temporal lobe of the brain.