Constance Thomas, age 13, of Winston Salem, N.C., for her question:

HOW DOES THE EYE WORK?

Sight is certainly one of our most important senses, and the eye is the organ of sight. Each human has two eyes which are globular in shape and are set into two bony cavities in the front part of the skull. Lids cover the eyes which help protect them and also help to keep them moist.

Your eyeball is made of three coats of tissue. The white portion you can see which is called the sclerotic coat. At the center it becomes transparent and is known as the cornea.

Behind the sclerotic coat is the choroid coat. This layer consists essentially of blood vessels with the opening modified to form the iris, or ring of pigmented muscles which

give the eye color. The iris has an opening in its center known as the pupil.

The innermost coat of the eye is called the retina. It is the nerve tissue of the eye. and is considered an expansion of the optic nerve.

.Where the optic nerve penetrates the eyeball we have what is called the blind spot. Below and slightly to the side is a yellow circular area called the macula. In the center of this area is a depression called the fovea centralis, the point of most distinct vision. .

There is a space between the cornea and the lens which is filled with a liquid called acqueous humor, and behind the lens a larger cavity filled with a gelatin like material called vitreous humor.

To see images clearly, rays of light entering the eye must pass through the cornea and through the pupil. The iris controls the changing of the pupil's size. The rays continue through the lens which thickens or flattens out to allow the light to focus properly on the retina.

The retina is that sensitive inner coat of the eyeball which records the images. The impulse is sent along the optic nerve to the thalamus and the midbrain. And we can see.

In the human eye, in the cell layer of the retina opposite the lens, there are about 125 million cells called rods and 7 million called cones. These cells are the visual receptors for sight. The cones are concerned with vision of bright, colored light while the rods worry about dim, colorless vision.