Endosymbiosis

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Endosymbiosis

The Human Eye

The Eye is the organ of that you gives your sight. Eyes enable people to perform tasks and to learn about the world that around them. Sight, or vision, is a rapidly occurring process that involves interaction between the eye the nervous system and the brain.

When someone looks at an object, what he/she is really seeing is the light that the object reflects, or gives off. This reflected light passes through the lens and falls on to the retina of the eye. Here, the light induces nerve impulses that travel through the optic nerve to the brain, where it makes an image of the object, and then that image is passed on to muscles and glands.

The eye is well protected. It lies within a bony socket of the skull. The eyelids guard it in front. They blink an average of once every six seconds. This washes the eye with the salty secretion from the tear, or lachrymal, glands. Each tear gland is about the size and shape of an almond. These glands are located behind the upper eyelid at the outer corner of the eye. After passing over the eye, the liquid from the gland is drained into the nose through the tear duct at the inner corner of the eye.

Heavy laughter or crying causes muscles in the upper eyelid to squeeze the lachrymal gland. This produces tears that flow too fast to be drained away. The eyelashes catch many flying particles that otherwise would enter the eye. As further protection, the eyelids automatically close when an object suddenly moves close to the eye.

The eye is made of 3 coats, or tunics. The outermost coat consists of the cornea and the sclera. The middle coat contains the main blood supply to the eye and consists of the choroid, the ciliary body, and the Iris. The innermost layer is the retina.

The Sclera, or the white of the eye, is composed of tough fibrous tissue. On the exposed area of the eye the scleral surface is covered with a mucous membrane called the conjunctiva. This protects the eye from becoming dry.

The Cornea, a part of the sclera, is the transparent window of the eye through which light passes. The focusing of the light begins in the cornea. Behind the Cornea is a watery fluid called the aqueous humor. This fluid fills a curved, crescent shaped space, thick in the center and thinner toward the edges. The cornea and the aqueous humor together make an outer lens that refracts, or bends, light and directs it toward the center of the eye.

Behind the aqueous humor is a colored ring called the iris. The color of the iris is inherited and does not affect vision. The iris is like a muscular curtain that opens and closes. It controls the amount of light entering the eye through the pupil, an opening in the iris. The pupil looks like a black spot. Light from everything a person sees must go through the pupil. When more or less light is needed to see better, the pupil becomes larger or smaller through the movement of the muscle in the iris. The aqueous humor flows through the pupil into a small space between the iris and the lens.

A simple way to see how the pupils respond to light is to stand in front of a mirror with your eyes closed covered by your hands for about 10 seconds. When your hands are removed and your eyes open, the pupils begin to get smaller, or contract, in response to the light. When the light is reduced, your pupils expand; when it is increased, they contract.

The choroid is a layer of blood vessels and connective tissue squeezed between the sclera and the retina. It supplies nutrients to the eye. The ciliary body is a muscular structure that changes the shape of the lens.

Behind the pupil and iris are the crystalline lens and the ciliary muscle. The muscle holds the lens in place and changes its shape. The lens is a colorless, nearly transparent double convex structure, similar to an ordinary magnifying glass. Its only function is to focus light rays onto the retina. The lens is made of elongated cells that have no blood supply. These cells obtain nutrients from the surrounding fluids - the aqueous humor in front and the vitreous body, a clear jelly, behind.

The shape of the lens - essentially that of a flattened globe - can be changed by the movement of the ciliary muscles surrounding it. Therefore, the eye can focus clearly on objects at widely varying distances. The ability of the lens to adjust from a distant and a near focus is called accommodation. By contracting, the ciliary muscle pushes the lens to make it thicker in the middle. By relaxing, the muscle pulls the lens and flattens it. To see objects clearly when they are close to the eyes the lens is squeezed together and thickened. To see distant objects clearly it is flattened.

For people with normal vision, the relaxed ciliary muscle flattens the lens enough to bring objects into sharp focus if they are 20 feet or more from the eye. To see closer objects clearly, the ciliary muscle must contract in order to thicken the lens. Young children can see objects clearly at distances as close as 2 1/2 inches. After about age 45 most people must have objects farther and farther away in order to see them clearly. The lens becomes less elastic as a person grows older.

The retina is a soft, transparent layer of nervous tissue made up of millions of light receptors. The retina is connected to the brain by the optic nerve. All of the structures needed to focus light onto the retina and to nourish it are housed in the eye, which is primarily a supporting shell for the retina.

When light enters the eye it passes through the lens and focuses an image onto the retina. The retina has several layers, one of which contains special cells named for their shapes - rods and cones. Light sensitive chemicals in the rods and cones react to specific wavelengths of light and trigger nerve impulses. These impulses are carried through the optic nerve to ...

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