The Human Eye and Its Parts
August 18, 1999
Web posted at: 12:16 PM EDT (1616 GMT)
The eye is an extension of the brain, connected by the optic nerve, which consists of millions of nerve fibers. The nerve carries visual messages, in the form of nerve impulses, from the retina to the brain, which interprets the impulses as images.
The eyeball, set in a protective fatty socket, is about 1 inch in diameter and is made up of layers within layers, covering what is essentially a hollow core. The individual parts of the eye work in much the same fashion as a camera.
LAYERS OF THE EYE
Sclera: the outer protective white coating
Choroid: the middle layer, which contains nourishing blood vessels
Retina: the inner layer, which is imbedded with nerves that sense light
THE OUTER LAYER
Cornea: Curving out from the sclera, the cornea is the "front window" of the eye itself. The cornea's half-sphere shape enables it to bend light as it enters the eye, allowing the eye to focus on an image. The cornea is as strong and durable as plastic and as smooth and clear as glass, which means it is also an excellent shield against dust, germs and other debris. While the cornea contains no blood vessels, it is packed with nerve fibers, making it highly sensitive to pain.
THE MIDDLE LAYER
Iris: Regulating the amount of light entering the eye, the iris functions much like the shutter of a camera. The iris has three layers and is distinctive because of its genetically determined coloring.
Pupil: The iris works in conjunction with the pupil, the dark opening at the center of the iris. As the amount of available light decreases, the iris muscles pull away from the center and make the pupil dilate, allowing more light to enter. When there's too much light, the pupil contracts. When the lighting changes quickly, say while you're watching television, the pupil contracts and dilates rapidly, and you could find yourself with a headache.
Lens: Immediately behind the iris is the lens, which actually changes shape as it makes delicate adjustments to the path of incoming light rays and sends them on to a precise location on the retina. In younger people, the lens is soft and pliable, so that it can easily fine-tune focusing, providing sharp vision at many distances. But as people age, the lens can harden, decreasing focusing ability.
THE INNER LAYER
Retina: Acting as the film in a camera, this nerve layer that lines the back of the eye contains cells that sense light, creating an upside-down image on the outermost retinal layer. The light is reflected back to a second layer that contains cells called rods and cones, which create impulses that are sent through the optic nerve to the brain to create the images we see -- right-side up.
Macula: At the center of the retina is the macula, which contains the cones that sharpen vision. If the macula is healthy, you'll have 20/20 vision.
Fovea centralis: In the center of the macula is the fovea centralis, which gives your eye the ability to sharpen an image. This is the main portion of the retina used to distinguish colors.
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