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There are many interrelated moving parts to the marvelous eye system. Eye performance is best defined as multiple elements moving in synchrony. People often have questions about their eyes from why the tears, to, what collects in the corners and, why should I get glasses? Learning about the different parts of the eye and how these parts function is a good place to begin.
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This protective covering helps to keep the surface of the eye lubricated. Eyelids are composed of several layers of muscle and supportive tissue. The conjunctiva on the inside of the lids is like a blanket, continuous with the same material on the surface of the eyeball.
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A thin, transparent mucous membrane that lines the inner surface of upper and lower eyelids separates the front half of the eyeball from the back half, earning it the name of fornix. Blood vessels inside nourish the eye, and blood or inflammation can cause the normally clear conjunctiva to appear pink such as when someone contracts “pink eye."
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The tough, white, outer layer of the eyeball made of tightly woven interlocking fibers protects the eye from injury and gives it the characteristic spherical shape. Tiny blood vessels also feed it. Six muscles attach on the sclera and allow you to roll your eyes in the desired direction.
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This central window covers the eyeball and light travels through it to enter the eye. Unlike the other parts of the eye, it does not have a blood supply, but it does contain numerous tiny nerves that make it sensitive to pain when scratched.
The cornea bends and refracts incoming light so that images can focus on the retina and travel to the brain, allowing us to see. Any alterations in the cornea’s curved shape can cause astigmatism and can contribute to nearsightedness and farsightedness.
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The iris divides the front of the eye into two chambers: anterior (front) and posterior (back). Irises are muscles and tissues that adjust the size of the pupil—like the aperture of a camera lens—and regulate the amount of light by dilating or constricting how much light passes through to the retina. Two pigments, melanin and lipochrome, and genetics (family traits) determine eye color. If no pigment is present, the eye is albino or light pink in nature.
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The pupil is the round space in the middle of the iris. Light filters through on its way to the retina, which forms the image. The muscles here also control the amount of light that passes through. A larger pupil lets in more light, good for seeing in a dim room. The reverse is true as well; in bright light, the pupil gets smaller and eyelids (and a brain message) create an involuntary “squint." Since the size of the pupil is regulated by the brain, your reaction to light can be measured as a basic test of brain function.
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In the front or anterior chamber of the eye, the space between the cornea and the front of the iris is filled with aqueous humor, which is a clear fluid produced by the eye to nourish itself.
The rear or posterior chamber is between the back of the iris and the front of the vitreous gel (it surrounds the front of the lens, too) and is transparent. Vitreous gel occupies about 80 percent of the volume of the eyeball and is made up of 99 percent water and 1 percent protein. It becomes more fluid as you age and moves more freely and its attachments to the retina can break free. Some of this free-floating material creates floaters.
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The lens bends or refracts light so that an image can form on the retina. The cornea does about 70 percent of the light bending and the lens picks up the rest at 30 percent. Made from proteins, it forms a crystal-like structure shaped like a lentil. This lens is suspended in a clear bag of fibers called zonules. Ciliary zonules are muscles that hold the lens in place.
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The ciliary body contains muscles that contract and relax as the lens adapts for near vision. When you look at an object close up, the zonules change their tension on the lens, causing it to change shape. This is called accommodation. Incredibly rapid changes in focus are possible with the ciliary muscle. The ciliary body, the choroid and the iris together are known as the uveal tract. As you get older, the lens gets thicker and cannot change its shape as well, prompting you to require reading glasses.
The ciliary body executes three essential functions. It supports the lens, focuses on a near object and produces inner-eye fluid. This eye fluid, aqueous humor, is constantly in production and must be drained or it creates pressure and can lead to an eye condition known as glaucoma.
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A layer of pigmented vascular tissue is between the retina and the sclera. It contains many blood vessels that feed the eye nutrients and oxygen. Besides containing blood vessels for the eye, the uvea is pigmented to absorb excess light as it enters the eye.
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The optic nerve is part of the central nervous system. A special sensory nerve, it carries electrical signals from the retina toward the part of the brain known as the visual cortex. Nerve fibers from the retina come together in a bundle to form the optic nerve, which is connected to the brain. This cable, in simple terms, carries information from the eye to the brain, where that information can be processed into an image that is “seen." Your eye doctor examines the optic nerve for signs of glaucoma, optic nerve swelling, which can be seen with a dilated eye exam.
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The retina is a thin layer of complex nerve tissue that lines the inside back wall of the eyeball. It is located between the vitreous and the choroid. This vascular tissue contains its own blood vessels with nutrients and oxygen. The back edge borders the optic nerve. The small central area is called the macula and is responsible for detailed central vision.
The retina has 10 cell layers that work together to receive images made of light rays. It transfers those images into electrical signals and sends them to the brain. Two types of retinal cells called rod and cone photoreceptors play an especially important role in the visual process. These rods and cones actually change the incoming light rays from a visual image into electrical signals. The rods process black-and-white vision while the cones process color vision. The electrical signals they produce travel through to specialized nerve cells in the retina, which all come together to form the optic nerve.
Interesting fact: your eyes can see about two million different shades of color.
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Vision Issues and Other Junk
Have you ever wondered why some people need glasses while others can see perfectly without them? Several conditions can cause vision problems. Learning about some of the most common iissues including myopia, hyperopia and astigmatism can help you better understand how our eyes work.
In addition, your eyes come with built-in protection that you may have been taking for granted.
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Nearsighted and Farsighted
Nearsightedness or myopia is the ability to see clearly up close but not a distance without glasses or contact lenses. The cause is unknown, but it tends to run in families and affects more than 25 percent of the United States population.
There are two types of nearsightedness: axial myopia and refractive myopia. Don’t let the names scare you; it just means that in axial myopia the eyeball is elongated rather than spherical. Because of the shape, light rays enter the eye and focus in front of the retina instead of right on it. In refractive myopia, the refractive power of the cornea or lens is too strong, causing the same effect.
Since your vision is blurred when looking at a distance, you will need correction by wearing prescription glasses or contact lenses. Some people can benefit from refractive surgery.
Farsightedness or hyperopia is a condition in which distant objects are usually seen clearly, but close objects are blurred. The image that falls on the retina is blurred because light entering the eye is focused behind the retina instead of directly on it. This condition is caused by a cornea that is flatter than normal, a lens in the eye that is not strong enough or an eyeball that is shorter than average. Approximately 25 percent of the population in the U.S. is farsighted.
The treatment depends upon age and daily activities. Young people may not require glasses if their eyes compensate for their farsightedness with accommodation. Older patients usually require glasses, contact lenses or may benefit from refractive surgery.
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This is an irregular curve of the corneal surface of the eye. Instead of being round or spherical like a basketball, it is more like a football, so light rays are not bent normally. It prevents the formation of one image at a single point (but most people have some degree of astigmatism). Patients will be able to wear eyeglasses or contact lenses to correct the problem; surgery is another option.
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The Protectors: Eyebrows and Lashes
Eyebrows are more than just the mantle above your eyes. Brows are engineered to protect and help your eyes by keeping droplets of rain out. They also trap sweat and keep it from trickling into your vision. The arched shape of your eyebrows is meant to force rain and sweat to run out and down the sides of your face. Pretty cool.
A little known fact: a boney ridge with prominent eyebrows protects a camel’s eyes. In addition, camels have a third eyelid—a haw or nictitating third membrane. This translucent membrane moves in many directions to act as a windshield wiper and protect his eyes in a dust storm. A double row of thick, extra-long camel eyelashes keep out sand and dust as well.
Your eyelashes flick off bits of dust, dirt and debris every time you blink, and supply fresh oil and tears that wash across your eyes and eyelid. You blink about five times a second or 10,000 times a day and more than 10 million times a year. Blinking less often—like when you are playing a video game on a computer—makes your eyes burn and feel tired.
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You don’t blink while you sleep, so small amounts of oil, sweat and tears build up in the corners. If bacteria get in your eye and your sleeping eyelids warm up, which they do normally, bacteria grow faster and reproduce and your body will send an army of bacteria-destroying cells to your eyes. Eyelids may be pasted close with eye goop when you wake up.
- Rochester Institute of Technology: Parts of the Eye
- "Three Main Layers of the Eye" by Artwork by Holly Fischer - http://open.umich.edu/education/med/resources/second-look-series/materials - Eye Slide 3. Licensed under CC BY 3.0 via Commons - https://commons.wikimedia.org/wiki/File:Three_Main_Layers_of_the_Eye.png#/media/File:Three_Main_Layers_of_the_Eye.png
- Fekrat, Sharon, M.D., et al. All about Your Eyes. Durham and London: Duke University Press. 2006. Book.
- Stewart, Melissa. The Eyes Have It. New York: Marshall Cavendish Benchmark, 2010. Book.