Brain controls the motion of eyes
Eyes do not stay still. They are constantly moving, following objects, looking from place to place. Our eyes and the eyes of other mammals, move together. Both of them are always directed toward the thing we wish to see. The delicate control of eye movements depends on nerve impulses from a special centre in the brain. These messages cause the contraction of a set of six muscles which run between the eyeballs and the bony wall which encases it. It is the contraction of one or more of these muscles which moves the eye.
The outer wall of the eyeball is made up of three layers’ the fibrous layer, the pigmented layer and the retina. The fibrous layer is the outermost protective layer of the eyeball. It is thick and tough. Most of this layer is cloudy or opaque. (An object is opaque if you cannot see through it.) But in the centre of exposed part of the eye, the fibrous layer becomes transparent and is called the cornea. Thus the cornea is really a tiny window in an otherwise opaque layer. The rest of the exposed part of the fibrous layer is loosely referred to as the white of the eye.
The cornea and white of the eye are continually moistened and washed by the fluid from the tear glands. These glands located beneath the lids at the outer corners of the eye, have this washing as their primary function. Other animals don’t cry. Only man uses the tear glands, for crying as well as for washing the eyes. The pigmented layer lies beneath the fibrous layer. All of its cells are crowded with tiny colored grains of pigment-hence its name. This dark layer corresponds to the black, inner surface of a camera, and it has the same function-to prevent reflections. The pigmented layer of course does not extend completely around the eyeball. It does not extend under the cornea. If it did, no light could pass into the eye.
The retina is the innermost of the three layers of the eyeball. It is the sensitive layer, the one which reacts when light enters the eye. Thus it may be compared with the film on which one takes a picture with a camera. The retina is a cup-shaped structure lying in the back half of the eyeball. The highly sensitive layer is made up of a large number of nerve cells. Some of these cells are connected with the brain by means of long strands of cytoplasm which extend out from their cell bodies. These long strands are nerve fibers. The fibers from the retina of each eye are all gathered together at the back of the eyeball. Then they pass to the brain in a single, large cable-the optic-nerve.
As light enters the eye, it passes first through the clear cornea. This small circle of transparent tissue lies just in front of the pupil. As you can see by looking at your own eyes in a mirror, the pupil is a small hole in the centre of a ring of colored tissue. The colored tissue ring is the iris. When we say a person has blue eyes, or brown eyes, we really mean that his irises are blue or brown. Iris was the name of the rainbow goddess of the Greeks, and this is the source of the word.
By Rehana khan(111/10)
