How the Eye Works

The many parts of the eye work together to translate light into nerve impulses that are then transmitted to the brain, where the actual process of sight is performed.

The eye is a spherical organ about one inch in diameter and its wall is composed of three layers of tissue. The outer protective coating, called the scalera, covers all but about one-sixth of the surface of the eye. The middle layer, called the choroids, is a vascular layer full of blood vessels, which lines the rear half of the eye. The inside layer of the eye wall is the light-sensitive retina.

At the center front of the eye is bulging transparent cornea -- a tough five-layered membrane through which light enters the eye. There is a chamber behind the cornea, filled with a clear watery fluid called aqueous humor, which separates the cornea from the lens. The lens is a flattened sphere made up of layers of many trans- parent fibers. Ligaments connect the lens to the ring like muscle surrounding it called the ciliary muscle. This muscle changes the focal length of the eye by pulling on the lens to flatten it or by relaxing to make the lens more spherical.

The pigmented iris -- which is what makes our eyes blue, brown, green etc. -- sits in the aqueous humor between the cornea and lens. In the center of the iris is a circular opening called the pupil. Muscles around the edge of the iris control the size of the pupil. They contract or relax to make the pupil larger in low light situations, to allow more light into the eye, and smaller in bright situations, to limit the light coming into the eye.

Behind the lens, the main body of the eye is a sack called the hyaloid membrane, filled with a transparent jellylike substance called vitreous humor. The pressure of the vitreous humor, within the hyaloid membrane, is what keeps the eye firm and spherical. The retina is the complex light- sensitive interior of the eye. The outer surface of the retina is covered with closely packed light receptor cells called rods and cones. Cones are attached to individual nerve fibers so stimuli to each individual are reproduced, enabling cones to detect fine details. Rods, however, are attached in groups so stimuli over a general area are reproduced. They are therefore more sensitive at lower light levels than cones, but are unable to distinguish small details.

On the back of the eye, directly behind the pupil, is a yellow spot called the macula lutea, and in its center is the fovea centralis. This is the area of highest visual acuity in the eye. The center of the fovea is covered entirely with cones. Although around the fovea there are both rods and cones, the number of cones gradually decreases as you near the edge of the light-sensitive area, until only rods are found at the perimeter. The result is that the visual field of the eye is comprised of a small central portion of strong sharpness, surrounded by a larger area of less sharpness, but greater sensitivity. Below, and slightly to the inside of the fovea, the optic nerve enters they eye. This optic disc forms a blind spot in a small area of the retina where no light sensitive cells are present.