Eyes and Vision
Think about all the things you do with your eyes. You watch TV and read books. You surf the Internet. You keep your eye on the ball when you play sports. You see your family and friends. Your eyes are your windows on the world.
Your eyes are like cameras that focus on what is in front of you. Your eyes work together with your brain to create a picture of the world. The process of creating the picture starts when light rays enter your eyeball.
LOOKING AT YOUR EYEBALL
Look at one of your eyes in a mirror. Your eyeball is round. The inside of your eye is a transparent (see-through), jellylike material called the vitreous humor.
The vitreous humor gives your eyeball its shape. You can’t see much of the vitreous humor because it is surrounded by an outer part, or wall. The wall of your eyeball is made up of outer, middle, and inner layers.
THE OUTER WALL
The outside layer of your eye is a protective coating called the sclera and the cornea. The sclera is the white part of your eye. The cornea is clear and goes over the center of your eye, the part you look through. Light rays enter your eye through your cornea.
THE MIDDLE WALL
The middle layer of the wall has three parts called the iris, the ciliary body, and the choroid. What color are your eyes? The color comes from your iris. Your iris is in the center of your eye. It can be shades of brown or blue. The black circle in the center of your iris is called the pupil. It gets bigger or smaller to control how much light comes into your eye.
The ciliary body goes around your iris and connects to the lens of your eye. Muscles in the ciliary body pull on the lens to focus it. The blood vessels that bring blood to your eye are also part of the middle wall. These blood vessels are called the choroid.
THE INNER WALL
The inside layer of your eyeball wall is called the retina. Your cornea and lens focus light rays on your retina just as a camera lens focuses light rays on film. They bend light rays coming into your eye so that they will strike the center of the retina called the macula lutea. This is where you have your sharpest vision.
RODS AND CONES
Your retina has millions of light-sensitive cells called rods and cones. These cells pick up the tiniest dot of light that gets to your retina. There are bits of colored material called pigment in the rods and cones. Pigment in the rods lets you see shades of gray and helps you see at night. Pigment in the cones lets you see colors.
The rods and cones change light rays into electrical signals. Nerves pick up these signals and carry them to your optic nerve.
An optic nerve leads from each of your eyes to your brain. Each eye picks up slightly different images. When these images get put together, you can see in 3-D. You have depth perception that lets you tell how far away things are.
Your optic nerves are like big cables that carry all the signals to a special part of your brain. This “media center” in your brain makes a picture of the world. It gives you sight.
Your eyeballs are set into two holes in your skull called eye sockets. The bones of your skull protect your eyes. Muscles let you turn your eyes in their sockets.
Eyelids and eyelashes also protect your eyes. You can close your eyelids to keep dust or bright light out. Your eyelashes are a fringe of short hairs on each eyelid. They screen out dust when your eyelids are partly closed.
Inside the eyelid is a thin layer called the conjunctiva. It covers part of the sclera. Each eye also has a tear gland that gives off salty liquid to wash small particles out of your eye.
Do you wear contacts or glasses? If not, you probably know someone who does. Many people need contacts or glasses because they are nearsighted. Things far away look blurry. Light rays focus in front of the retina because the eyeball is too long.
Some people have the opposite problem. They are farsighted and can’t see close-up things very well. In farsightedness, light rays focus behind the retina because the eyeball is too short.
Astigmatism is another vision problem. A person has an astigmatism when their cornea is unevenly curved. Older people sometimes need reading glasses because the muscles in their eyes can no longer focus on things that are nearby.
Diseases strike different parts of the eye. A sty is an infection of the eyelid. Conjunctivitis, or pinkeye, is an infection of the thin layers covering the eyelid and outer eyeball.
Many eye diseases are most common in older people. Sometimes the lens of the eye gets cloudy over time. This condition is called cataracts. Retinas can get detached (come loose) from the back of the eye and cause blindness. Glaucoma occurs when fluid gets trapped between the cornea and the lens and puts pressure on the eye. A problem called macular degeneration affects the center of the retina. It can cause blindness in older people.
There are many treatments for eye diseases. Doctors treat infections with drugs. They use laser beams to weld detached retinas back into place. Surgeons can replace clouded lenses with clear plastic ones. They can also replace diseased corneas.
It is very important to protect your eyes. Get regular eye examinations. Wear eye protectors when doing dangerous work or playing rough sports. Wear sunglasses that protect against harmful rays from the Sun. Your eyes are too important to take chances with!
Source: Microsoft ® Encarta